Sodium Metal Anodes: Emerging Solutions to Dendrite Growth

Review Cite This: Chem. Rev. XXXX, XXX, XXX−XXX

pubs.acs.org/CR

Sodium Metal Anodes: Emerging Solutions to Dendrite Growth Byeongyong Lee,† Eunsu Paek,§ David Mitlin,*,§ and Seung Woo Lee*,† †

George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States Chemical & Biomolecular Engineering, Clarkson University, Potsdam, New York 13699, United States

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§

ABSTRACT: This comprehensive Review focuses on the key challenges and recent progress regarding sodium-metal anodes employed in sodium-metal batteries (SMBs). The metal anode is the essential component of emerging energy storage systems such as sodium sulfur and sodium selenium, which are discussed as example full-cell applications. We begin with a description of the differences in the chemical and physical properties of Na metal versus the oft-studied Li metal, and a corresponding discussion regarding the number of ways in which Na does not follow Li-inherited paradigms in its electrochemical behavior. We detail the major challenges for Na-metal systems that at this time limit the feasibility of SMBs. The core Na anode problems are the following interrelated degradation mechanisms: An unstable solid electrolyte interphase with most organic electrolytes, “mossy” and “lathlike” metal dendrite growth for liquid systems, poor Coulombic efficiency, and gas evolution. Even solid-state Na batteries are not immune, with metal dendrites being reported. The solutions may be subdivided into the following interrelated taxonomy: Improved electrolytes and electrolyte additives tailored for Na-metal anodes, interfacial engineering between the metal and the liquid or solid electrolyte, electrode architectures that both reduce the current density during plating−stripping and serve as effective hosts that shield the Na metal from excessive reactions, and alloy design to tune the bulk properties of the metal per se. For instance, stable plating−stripping of Na is extremely difficult with conventional carbonate solvents but has been reported with ethers and glymes. Solid-state electrolytes (SSEs) such as beta-alumina solid electrolyte (BASE), sodium superionic conductor (NASICON), and sodium thiophosphate (75Na2S·25P2S5) present highly exciting opportunities for SMBs that avoid the dangers of flammable liquids. Even SSEs are not immune to dendrites, however, which grow through the defects in the bulk pellet, but may be controlled through interfacial energy modification. We conclude with a discussion of the key research areas that we feel are the most fruitful for further pursuit. In our opinion, greatly improved understanding and control of the SEI structure is the key to cycling stability. A holistic approach involving complementary post-mortem, in situ, and operando analyses to elucidate full battery cell level structure−performance relations is advocated.

CONTENTS 1. Introduction 2. Resurgence of Sodium-Metal Batteries 2.1. Brief History of Lithium- and Sodium-Metal Batteries 2.2. Motivation for SMBs: Techno-economics and Performance 2.3. Full Sodium-Metal Battery: S, Se, and SSe Systems 3. Sodium Dendrite Growth Is Not a Direct Li Analogue 4. Key Challenges for Sodium-Metal Batteries 4.1. Understanding the Na Solid Electrolyte Interphase 4.2. The SEI Layer Is Less Stable with Na than with Li 4.3. Dendrite Growth and Related Issues 4.4. Gas Evolution 5. Electrolytes for Sodium-Metal Anodes 5.1. Liquid Electrolytes 5.2. Electrolyte Additives 5.3. Solid-State Electrolytes

© XXXX American Chemical Society

6. Sodium Metal−Electrolyte Interfacial Engineering 7. Electrode Engineering 7.1. Sodium-Metal Hosts, Templates, and Membranes 7.2. Room-Temperature Liquid-Metal Anodes 8. Future Outlook and Promising Research Directions Author Information Corresponding Authors ORCID Notes Biographies Acknowledgments References

A B B C C F I I L M O P P R R

X AA AA AF AF AG AG AG AG AG AG AG

1. INTRODUCTION The metal anode is the essential component for roomtemperature sodium-metal batteries (SMBs) such as Na-S, NaReceived: October 24, 2018

A

DOI: 10.1021/acs.chemrev.8b00642 Chem. Rev. XXXX, XXX, XXX−XXX

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Figure 1. Summary of the aspects covered in this sodium-metal battery anode review.

for advancing both the science and the technology. The overall structure of the Review is summarized in a schematic in Figure 1.

Se, and Na-O2. It is also the key component of laboratory halfcells employed for quantifying ceramic cathodes, alloy and conversion anodes, etc. Originally SMBs were based on molten sodium sulfur “Na-S”, operating around 300 °C. Due to the emergence of lithium-ion batteries (LIBs) and the safety issues associated with the high operation temperature, this technology has generally taken a back seat. In view of concerns regarding long-term lithium precursor fluctuations and the growing demand for a diverse range of high-energy storage systems, room-temperature SMBs are gaining scientific interest. To date, however, there have been no attempts to commercialize room-temperature SMBs in any form, at least in part due to the significant technical issues discussed in this manuscript. Before entering the technical discussion, we first overview the history of the metal battery, the techno-economic motivation for sodium, and the recent emergence of the full SMB systems based on Na-S, Na-Se, or Na-SSe. This is the focus of section 2. In section 3, we provide an overview of the key physical and chemical characteristics of Na-metal anodes and relate them to the key problems with Na anode performance. While for proper comparison, Li-metal anodes are also discussed, it is not the focus of this Review to comprehensively describe the work that has been done on the Li-metal systems. For this, we refer the reader to several excellent recent review articles directly on the subject.1−10 In section 4, we go through in detail: the three directly interrelated problems for Na-metal anodesan unstable solid electrolyte interphase (SEI), dendrite growth, and gas evolutionall of which lead to accelerated cell energy fading and safety problems. In sections 5−7, various approaches that attempt to resolve these issues are described, providing cases where approaches employed for Li do not necessarily achieve the same efficacy for Na. In section 5, we discuss the electrolyte additives, liquid electrolytes, and solid state electrolytes (SSEs). Section 6 addresses the issues of the unstable SEI and dendrite growth through interfacial engineering, and section 7 concerns Na-metal anodes which utilize host systems for Na plating and liquid-phase Na-metal anodes. In section 8, we provide concluding perspectives, discussing aspects that are insufficiently explored, but that are important

2. RESURGENCE OF SODIUM-METAL BATTERIES 2.1. Brief History of Lithium- and Sodium-Metal Batteries

Room-temperature lithium-metal batteries (LMBs) consisting of a Li-metal anode, an organic electrolyte, and an intercalation cathode (TiS2) were pioneered by Stanley Whittingham in the 1970s.11,12 Later, in the 1980s, Moli Energy commercialized LMBs using a MoS2 cathode.5 However, LMBs soon faced severe challenges due to explosion hazards caused by dendritic Li growth, resulting in product recalls. To overcome the safety issue originating from the Li-metal anode, rocking-chair batteries (e.g., LIBs consisting of two different intercalation electrodes) were investigated by Murphy et al. and Scrosati et al.13−15 With the progress of lithium intercalation technologies based on lithium cobalt oxide by Goodenough and graphite by Armand and Touzain,16,17 Sony demonstrated a reliable LIB using a carbonaceous anode and LiCoO2 cathode in 1991. Since this technology was established as one of the commercial LIBs, development of Li-metal anode seemed to be halted.8,18 Oxide-based cathodes and graphite anodes possess capacities around 250 and 350 mAh g−1, respectively, limiting the gravimetric energy of LIBs.19 As such, the high-energy Li-S system is widely attracting attention as a “Beyond Li” system.19−21 Sulfur cathode follows a two-electron conversion reaction: S8 + 16Li ↔ 8Li2S, thus providing a high theoretical specific capacity of 1673 mAh g−1.22 The theoretical capacity of Li-metal anode is 3860 mAh g−1 based on Li ↔ e− + Li+. With an average cell voltage of 2.15 V, the theoretical gravimetric energy of Li-S is ∼2600 Wh kg−1 (further discussed in Section 2.3).23 In practical terms, Li-S is estimated to be about 2−3 times higher in energy than the state-of-theart lithium ion, which is currently in the 225 Wh kg−1 range. High-temperature sodium metal−sulfur systems were developed in the 1960s.24−26 This early embodiment of SMBs operated at 300 °C using a liquid Na anode, liquid S, and a beta-alumina solid electrolyte (BASE) that possessed a B


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Figure 2. (a) BatPaC software package calculated gravimetric energy for sodium ion battery (SIB) versus a sodium-metal battery (SMB), using the same NayMnO2 cathode. (b) Theoretical volumetric and gravimetric energy of alkali metal−S systems based on M2S (M = Li, Na, K). Theoretical gravimetric energy of a LIB based on standard graphite anode and LiCoO2 cathode. Figure data from refs 46, 61−63. Panel a adapted with permission from ref 46. Copyright 2013 John Wiley & Sons, Inc.

relatively high ion conductivity of 0.1 S cm−1. These systems required an external energy source to maintain the high operating temperature, with the resultant overall efficiency being around 87%.27,28 The sophisticated system design, high manufacturing costs, and safety issues limited the market share of these high-temperature systems.29 Because of these issues, it is doubtful that the high-temperature Na-S cell will make a comeback in the foreseeable future. However, as detailed next, the room-temperature Na system remains promising with many techno-economics working in its favor.

view of the current collector material.48 Since aluminum reacts with Li at the low anode potential, a copper current collector is employed for LIB anodes. On the other hand, Al does not react with Na,49 saving cost in allowing Al current collectors for both the anode and the cathode. Further discussion of the Al current collector will be provided in section 7.1. Due to these techno-economic advantages, sodium-ion batteries (SIBs) have received intensive scientific attention over the past 10 years.50,51 For instance, Komaba et al. recently reported a high-performance metal oxide cathode (P2-type Nax[Fe0.5Mn0.5]O2), showing the nearly comparable gravimetric energy of ∼500 Wh kg−1 (based on electrode), to the wellknown Li analogue LiFePO4.52,53 At the anode side, nongraphitic carbons have been extensively studied since they reversibly intercalate Na ions.54−59 While their total capacity is on par or even higher than that of LIB graphite, this capacity is achieved over wide voltage window, starting at ∼1.5 V vs Na/ Na+. This gives a lower overall cell voltage and is a major contributor to the lower energy of most (not all) SIBs as compared to LIBs.60 Therefore, it appears that the Na cathode materials are further along than the Na anode materials, and that metal anodes are quite timely. To increase the energy of a full Na battery, the capacity of the anode should increase while its average voltage should decrease.60 Figure 2a shows the energy of SIBs calculated using the BatPaC model software package. Based on a NayMnO2 cathode with a capacity of 200 mAh g−1 and a carbon anode with a capacity of 250 mAh g−1, the calculated gravimetric energy of the ion battery is 150 Wh kg−1. Conversely, the gravimetric energy of a sodium cell with the same ceramic cathode but with a Na-metal anode is over 275 Wh kg−1.46

2.2. Motivation for SMBs: Techno-economics and Performance

LIBs are becoming the commercially dominant electrochemical energy storage technology for transport systems and other applications.30−33 However, there is a concern about the sustainability of Li carbonate precursor.34 If the supplies are not sufficient to meet the growing demand, even for brief periods, battery production costs may sharply spike.35,36 Assuming that annual demand for Li grows at 5%, the existing mineable resources are estimated to support the current LIB market for only about 65 years,36,37 with Li recycling being able to extend this approaching shortage to perhaps 2100.38 The cheapest Li extraction is made from the salt lake brines, which represents the majority of the currently produced lithium carbonate. Since the salt lake brines are geographically concentrated in South America, it can cause geostrategic and geo-economic bottlenecks.39 In addition, the processing time of the salt lake brine extraction and treatment is on the order of years, so it is difficult to adjust the production in the short term to adapt to increasing Li demands. This may to lead large and possible ongoing significant fluctuations of Li prices, akin to the well-known “oil shocks”, that would in turn diminish the market prospects for electric vehicles etc.40 In terms of material availability, Na possesses a major advantage over Li.41,42 It is much more abundant than Li in the Earth’s crust and may be found in almost every part of the globe,43−45 both in the economically developed regions and in regions that are emerging. For instance, it is estimated that 23 billion tons of soda ash (Na2CO3) is located in the United States alone.46 The main resource of soda ash, trona (Na2CO3· NaHCO3·2H2O), is low cost, being at $135−165/ton.46,47 Na resources can also be obtained through several different methods, such as hard rock mining, brine, and seawater. In addition, Na provides an additional cost advantage over Li in

2.3. Full Sodium-Metal Battery: S, Se, and SSe Systems

In this section, we briefly summarize the state-of-the-art and the known issues regarding S, Se, and SeS-Na systems, which are the first and perhaps the most exciting embodiment of fullcell SMBs. The theoretical gravimetric energy of LIB, Li-S, NaS, and K-S systems is shown in Figure 2b. In terms of gravimetric energy, the Li-S system dominates. However, considering the techno-economic merit of Na versus Li, the Na-S system can be another fruitful research and possible commercialization direction:64−67 The theoretical energy of Na-S (∼1274 Wh kg−1) is still much higher than that of the current LIB (∼387 Wh kg−1) (Figure 2b). Instead or in addition to S, selenium can be employed as a cathode material C


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Figure 3. (a) Theoretical phases formed and practical discharge capacity of Na-S. (b) Calculated equilibrium sodiation voltage profile of Se and the associated reactions. Panel a adapted with permission from ref 71. Copyright 2014 John Wiley & Sons, Inc. Panel b adapted with permission from ref 72. Copyright 2016 American Chemical Society.

Figure 4. (a) Schematic illustration of Na-ion migration in a SexNa occupied nanotube pore, and the proposed electrochemistry of sodiation to terminal Na2Se. (b) Schematic illustration of Na-S (or Se) system, the electrochemical issues, and emerging solutions. (c) Various carbon host structures to incorporate S and/or Se through physical and/or chemical interactions. Panel a adapted with permission from ref 70. Copyright 2015 American Chemical Society.

opposing a Na-metal anode. Although the gravimetric capacity of Se is lower than that of S (Na2Se = 678 mAh g−1, Na2S = 1675 mAh g−1), its volumetric capacity is on par (Na2Se = 3250 mAh cm−3, Na2S = 3470 mAh cm−3) since Se is dense (4.81 g cm−3).68 Typical S-Na and Se-Na cells consist of a Nametal anode, a carbonate, ether, or ionic liquid hybrid electrolyte, and S or/and Se active phase embedded in a carbon matrix host. During the discharge process, S and Se are

reduced by Na. The discharge reactions at the S and Se cathodes can be expressed as follows:69,70 S cathode:

nS + 2Na + + 2e− ↔ Na 2Sn

(4 ≤ n ≤ 8) (1)

Se cathode:

Se + 2Na + + 2e− ↔ Na 2Se

(2)

The final reduced forms of S and Se are recognized to be similar in stoichiometry and equilibrium structure (faceD


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centered cubic Na2S and Na2Se). However, the intermediate sodiation−desodiation processes are quite complicated and appear different for Se vs S. The presence of equilibrium crystalline phases in the terminal sodiated state is not always confirmed, especially in nanoconfined architectures. For Na-S, Manthiram et al. showed that there are four reaction regions in the S−Na system (Figure 3a).71 Region I. A high-voltage-plateau at ∼2.20 V, corresponding to a solid−liquid transition from elemental S to polysulfide of Na2S8: S8 + 2Na + + 2e− → Na 2S8

Li2Se. Se undergoes non-reconstituted chain-shortening reaction with Na. This yields highly active small Se molecules during initial discharge, showing multiple peaks in the cyclic voltammetry (CV) plot (Figure 4a). On the contrary, Se reacts with Li through a one-step reaction, showing a single pair of redox peaks in CV.70 In a way, this study also examined a model system since it is not possible to create practical electrodes based on Se inside carbon nanotubes. More analytical and modeling studies on the sodiation (and lithiation) phase transformations are needed, specifically where the Se is confined inside micro and/or mesopores of conventional carbon hosts such as various activated carbons. Unlike the Na-S and Na-Se systems, the Na-SeS system is virtually unexplored in terms of the solid−solid and/or solid− liquid sodiation phase transformations. More studies there are clearly needed. Several concerns are known to exist for the S-Na, Se-Na, or their alloy SeS-Na systems: (i) dissolution of polysulfides and polyselenides into the electrolyte during cycling, (ii) the electrically insulating nature of polysulfides and polyselenides when precipitated on the anode due to crossover, and (iii) large volume changes of S, Se, and SeS upon terminal sodiation to solid phase, leading to localized electrode pulverization (Figure 4b).73 The intermediate polysulfide and polyselenide anions are highly soluble in most organic electrolytes. They can crossover from the cathode to coat the Na-metal anode, leading to surface “poisoning”.74 This reaction may also have unexpected effects on the SEI formation, which need to be further understood. The lower-ordered polysulfides can also shuttle back to the cathode and be oxidized again, leading to parasitic self-discharge and Coulombic efficiency (CE) loss. Unoccupied pore space in the host may buffer a portion of the volume changes associated with Na2Se, Na2S, etc. However, how much “free” space is required and the role of pore size distribution (are mesopores more effective than micropores, or vice versa?) remain to be determined. Such complex questions are highly fertile for studies based on advanced post-mortem, in situ, and operando synchrotron and electron scattering methods, as summarized in the conclusions section of this paper. To practically tackle the above issues, Li-S inherited strategies have been employed with varying success. These include various carbon hosts with tuned pore size distributions and/or tuned surface chemistries to bond with the Na-active Se, S phases. Examples include N- and S-rich carbons with strong chemical interactions such as pyrolyzed polyacrylonitrile, poly-ion blocking interlayers that reduce crossover, tuned electrolytes and additives, etc. (Figure 4c).72,75−88 Table 1 shows the specific capacities and cycling stability of state-ofthe-art S, Se, and SexSy cathodes. Considering that the energy of a cell will also depend on the mass of the various “inactive” carbon frameworks, increasing the mass loading of the active materials while maintaining rate capability and cyclability is a key target for future research. Incorporating S and Se into various carbon hosts has been a nearly universal approach to address the above issues, although with varying levels of effectiveness. Researchers employed various one, two, three-dimensional carbon host structures (e.g., carbon nanofiber, carbon nanosheets, and porous carbon sphere; Figure 4b). The hollow space (or pores) within the carbon hosts can accommodate the large volume changes of S or Se. Additionally, the shuttle effect can be tackled by trapping intermediate species inside the micropores and small

(3)

Region II. Sloping region at 2.20−1.65 V, corresponding to a liquid−liquid reaction: Na 2S8 + 2Na + + 2e− → 2Na 2S4

(4)

Region III. A low-voltage-plateau at ∼1.65, corresponding to a liquid−solid transition: Na 2S4 + 2 3 Na + + 2 3 e− → 4 3 Na 2S3 +

−

(5)

Na 2S4 + 2Na + 2e → 2Na 2S2

(6)

Na 2S4 + 6Na + + 6e− → 4Na 2S

(7)

Region IV. Sloping region at 1.65−1.20 V, corresponding to a solid−solid reaction: Na 2S2 + 2Na + + 2e− → 2Na 2S

(8)

The capacity and discharge voltage of Region III depend on the competition of species in eqs 5−7. Due to the insulating nature of Na2S2 and Na2S, Region IV is kinetically sluggish.71 Dravid et al. studied free-standing crystalline Se nanowires, employing in situ transmission electron microscopy combined with density functional theory (DFT) calculations. This is illustrated in Figure 3b.72 It should be noted that the same reaction sequence has not been confirmed for amorphous Se confined in nanopores of a host, which is the usual electrode architecture employed for high-performance SMBs. Step 1:

Se + x Na + + x e− → NaxSe

(9)

Step 2: 2NaxSe + 2(1 − x)Na + + 2(1 − x)e− → Na 2Se2 (10)

Step 3:

+

−

Na 2Se2 + 2Na + 2e → 2Na 2Se

(11)

The first reaction step refers to a transformation of a Se single crystal into a solid-solution amorphous phase up to Na0.5Se stoichiometry. In the second step, the Na0.5Se amorphous phase crystallizes to form a hexagonal Na2Se2 phase with a distinct two-phase voltage plateau. The step 3 is the transformation of Na2Se2 into the Na2Se phase, yielding the second voltage plateau. The authors examined Se−Li in an identical manner and observed a distinct absence of the intermediate Li2Se2. Rather, lithiation of Se followed a single plateau transformation of amorphous solid solution to the terminal fcc Li2Se. A nearly opposite architecture was studied by Goodenough et al., who confined amorphous Se chains into the slit pores of carbon nanotubes. The Se-Na and Se-Li reacted differently here as well, in neither case displaying well-defined crystalline structure despite reaching a capacity associated with Na2Se and E


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is effective in suppressing the initial capacity drop.71,92 In addition, cation-selective membranes such as Nafion can play the same role as the carbon-based interlayer in the S−Na system.93,94 Based on the cation selective feature of Nafion, Manthiram et al. was able to stably cycle polysulfide (Na2S6) catholyte with the addition of solid S powder up to 100 cycles with a high capacity of ∼600 mAh g−1.95 The authors’ (D.M.) own unpublished work, however, points to Nafion separators reducing the cycling life on Na-metal anodes as compared to glass frit separators. Khalil et al. proposed SexSy as a cathode material for roomtemperature LMB and SMBs.96 Later, authors reported that Se0.6S0.4/carbon nanofiber composite has superior cycling stability as compared to S/carbon nanofiber, which was attributed to the inhibition of SexSey dissolution by chemical bonding between S and Se.97 Crystalline Se possesses 20 orders of higher electrical conductivity than S (10−5 vs 5 × 10−30 S cm−1).96 The electrical conductivity of amorphous Se is also much higher, being at 10−11 S cm−1. Employing SexSy as a cathode is a synergistic approach since the S improves the gravimetric capacity while the Se improves the electrical conductivity.98 Lou et al. reported excellent electrochemical performance of SeS2/pyrolyzed polyacrylonitrile, showing a high reversible capacity over 800 mAh g−1 after 400 cycles.98

Table 1. Specific Capacity of State-of-the-Art Na-S, Na-Se, and Na-SexSy Cathodes material Se-mesoporous carbon Se/porous carbon nanofibers Se-monolithic carbon Se-slit microporous carbon Se/porous carbon nanofibers Se/porous carbon particle

material S/CNTmicroporous carbon Na2S6 catholyte with sodiated Nafion Na2S-activati carbon nanofiber sulfur/microporous carbon polyhedron S@N,S-doped hierarchical porous carbon S-infiltrated carbon sphere S@multiporous carbon fiber pyrolyzed polyacrylonitrile/ SeS2

active material wt%

volumetric capacity (mAh cm−3)

capacity retention (cycles)

ref

30

−

95% (150)

76

52.3

−

87% (80)

85

70

848

98% (150)

89

50.2

659

93% (100)

70

35

−

98% (80)

99

54

−

50% (50)

100

active material wt%

gravimetric capacity (mAh g−1)

capacity retentiona(cycles)

ref

40

∼950

∼64% (200)

86

−

∼700

∼65% (100)

95

−

∼600

∼96% (100)

65

65

866

69% (100)

87

22

∼386

98% (350)

91

35

∼370

∼82% (1500)

82

61

1200

∼78% (200)

101

63

1043

∼78 (400)

98

3. SODIUM DENDRITE GROWTH IS NOT A DIRECT LI ANALOGUE Sodium metal holds considerable promise as an anode material for Na-based batteries, considering its high capacity of 1166 mA h g−1 and low redox potential of −2.71 V vs SHE.102,103 Dendrite growth is ubiquitous when electroplating a wide range of metals in a similarly wide range of electrolytes. It is well recognized that dendrite growth is the major impediment toward implementation of metal anodes for both Na and Li batteries.104 Na-metal anodes are hampered by somewhat analogous issues as Li anodes: the electrodeposition of Na is accompanied by uncontrolled mossy and dendritic growth that leads to low CE due to SEI growth, rapid electrolyte exhaustion and rise in impedance, and premature cell failure. It is known that metallic Na reacts with carbonate electrolytes in fundamentally different ways than does Li metal,105 so there is a limit to how many parallels may be drawn between the two materials sets. The morphology of Na dendrites is varied, but is often either “needle-like” or “mossy”, as shown by representative examples in Figure 5.106,107 For the case of Li, mossy and needle-like shapes are associated with defect catalyzed base growth conditions at currents low enough where ion diffusional limitations do not yet dominate.108 With Na metal, the origin of the dendrites’ mossy and needle structure is not established, although it may also be reaction-limited defect-catalyzed. For the purpose of brevity, we will refer to such complex structures as simply “dendrites”, with an understanding that there is a range of morphologies and that growth may be at base or at tip. Some differences between Na and Li properties of importance for battery performance are provided in Table 2. It should be pointed out that the plastic properties of Na metal and its bonding with carbon may be significantly altered by various impurities. These may come from the surrounding electrolyte and SEI or be purposefully introduced as dopants, as will be discussed. Moreover, any porosity in the anode would impact (reduce) its elastic stiffness and plastic strength. Na possesses 55% larger ionic radius than Li109,110 and has a

a

When the capacity value was not specified, the capacity and capacity retention were estimated from figures in the cited reference. The capacity retention was calculated on the basis of the discharge capacity of the second cycle.

mesopores. However, the loading amount of active material within the pores is limited, resulting in a lowering of the overall capacity, especially by volume. Impregnation of Se into a monolithic carbon matrix has been reported to partially overcome this issue, with a high areal capacity of 3.39 mAh cm−2 and 98% capacity retention after 150 cycles.89 Heteroatom doping of the carbon matrix with N is an established strategy to modify the electron density distribution so as to increase the binding energy with the polysulfide ions.90 Some authors have shown that a high concentration N within the carbon effectively traps polysulfides in the nanopores through electrostatic interaction, and that the Na−N bonds inhibit side reactions between the polysulfides and the carbonate electrolyte.91 Modifying the separator to trap polysulfides has been shown to be effective in preventing polysulfides from crossing over to the metal anode. For example, Manthiram et al. employed carbon-based interlayers (carbon nanofiber, carbon nanotube, carbon foam) between the S cathode and a glass fiber separator, demonstrating that it F


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surface self-diffusion at ambient conditions, opening up the possibilities for facile self-healing reactions at the metal−SEI interface. The order of magnitude lower hardness of Na vs Li indicates the propensity for plastic flow under the mildest shear conditions,116 while its significantly lower modulus implies a greater elastic compliance of the dendrites. Further understanding of these key differences will become essential when considering Na metal−SEI interactions, as to be discussed. In liquid battery electrolytes, Na-metal dendrite growth is qualitatively recognized as a series of interrelated steps, involving an interplay between metal−support interactions and SEI growth/stability. During the first charge, there is the formation of SEI layer at approximately below 1 V vs Na/Na+. The SEI layer is well-recognized to be chemically and geometrically heterogeneous. The underlying Al or Cu current collectors are also geometrically and structurally non-uniform, containing mechanical scratches from processing, dislocation terminations in the form of steps, grain boundaries of various orientations, etc. This in turn leads to heterogeneous nucleation and growth kinetics of the plated/stripped Na. Over a number of cycles, the result is poor CE due to additional SEI growth and “dead Na”. This leads to an increase in the plating−stripping overpotential, SEI-induced electrolyte depletion,107,117 and in some cases dendrite growth through the battery separator.118 In carbonate-based solvents, Na metal is more reactive than Li metal when tested under identical conditions. These include various combinations of ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), and dimethyl carbonate (DMC).119 For instance, Figure 6a,b shows the cycling behavior of symmetric Na-Na and Li-Li cells, employing 1 M NaClO4 or 1 M LiFePO4 salts in EC/DEC/ PC solvents [Mitlin et al., unpublished]. It may be immediately observed that at identical current density, the overpotential for the Na-Na case is significantly higher and noisier from the onset, indicating unstable SEI growth. Employing glyme-based solvents, either by themselves or combined with carbonates, the SEI structure is stabilized. The layer possess a more inorganic-based composition of Na2O and NaF, versus the usual sodium carbonate-rich content in EC/PC/DEC/DMC, etc.120 This is illustrated in Figure 6c,d, which compares the Na-metal cycling behavior of several electrolyte combinations and shows a top-down SEM image of a favorable diglyme solvent. It is essential to emphasize that the plating−stripping scenario with Na-metal cells is not just a Li analogue that needs updating, but rather a process that is fundamentally different in every respect: (i) The Na-metal nucleation thermodynamics and kinetics differ since Na would bond differently with inorganic (e.g., Na2O and NaF), metallic (e.g., cathode species crossover), and host carbon structures. (ii) Na-metal growth kinetics differ since at room temperature the Na cells are at a higher homologous temperature than Li, e.g., T/Tm = 0.8 vs 0.65. (iii) Dendrites of Na metal are an order magnitude more plastically complaint than ones of Li metal and possess roughly half the elastic rigidity. For instance, Chen et al. examined Na dendrite stability employing operando microscopy under quasi-zero electrochemical fields.121 It was observed that in a quasi-zero electric field, the Na dendrites were gradually dissolved in a standard battery organic electrolyte (EC/DMC) and broken under external stress.121 Conversely, under the same conditions, the Li dendrites were relatively stable.

Figure 5. (a) Morphologies of “lath-like” Na metal deposited from 1 M NaPF6 in EC/DEC, EC/FEC, and FEC/DEC upon passage of 4 mAh cm−2 at 1 mA. (b) Visualization of the electrochemical Na deposition. Light microscopy images of electrochemical Na deposition on the Na electrode in 1 M NaClO4 in EC/PC. The images were taken at Na deposition at 0, 10, 20, and 30 min at a current density of 1 mA cm−2. Panel a adapted with permission from ref 106. Copyright 2017 American Chemical Society. Panel b adapted with permission from ref 107. Copyright 2017 John Wiley & Sons, Inc.

Table 2. Comparison of Various Physical Properties of Lithium and Sodium Atomic Properties

Li Na

atomic radius (pm)

covalent radius (pm)

van der Waals radius (pm)

ionization energy (kJ mol−1)

152 186

128 ± 7 166 ± 9

182 227

520.2 495.8

melting point (K)

boiling point (K)

Physical Properties

Li Na

454 371 shear modulus (GPa)

critical point (K)

1603 3220 1156 2573 Mechanical Properties bulk modulus (GPa)

Mosh hardness

heat of vaporization (kJ mol−1) 136 97 Brinell hardness ( MPa)

Li 4.2 11 0.6 5 Na 3.3 6.3 0.5 0.69 Bonding to Carbon, Metal Underpotential Deposition on Carbon Surfaces Li Na

strong, yes weaker, no, or maybe?

notably weaker bonding with solid carbon, resulting in less exothermic adsorption/intercalation.111,112 While this results in Na not being able to intercalate into graphite (the standard LIB anode),112,113 it still reversibly intercalates into amorphous carbons, and importantly does not appear to readily underpotential deposit on carbon surfaces inside nanopores.114,115 The 98 °C melting point of Na indicates appreciable bulk and G


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Figure 6. Voltage versus time profiles for cycling of (a) Li-Li and (b) Na-Na metal cells in comparable carbonate electrolytes and 1 M LiPF6 or NaClO4 [Mitlin et al., unpublished]. (c) Plating−stripping CEs of Na-metal anodes cycled using 1 M NaPF6 in various electrolyte solvents. (d) SEM images of the Na-metal surface after one cycle at 0.5 mA cm−2 and 1 mAh cm−2 using 1 M NaN(SO2CF3)2 in diglyme. Panels c,d adapted with permission from ref 120. Copyright 2015 American Chemical Society.

Figure 7. (a) Schematic illustration of Li-metal deposition being catalyzed by transition-metal species crossed over from the cathode. (b) Cathode failure triggered by chemical crossover from the Li-metal anode. Panel a adapted with permission from ref 133. Copyright 2017 American Chemical Society. Panel b adapted with permission from ref 134. Copyright 2018 American Chemical Society.

In addition: (iv) SEI chemistry, structure, and stability in Na-based systems do not follow the known trends observed with Li and do not respond to well-known Li additives. For instance, there are surprising reports that the well-accepted Li SEI stabilizing additive fluoroethylene carbonate (FEC) will induce significant voltage polarization when employed with Na metal.105,122 The SEI composition of a Na-metal anode tested in the same carbonate (EC/DMC) electrolyte with analogous salt as Li metal (1 M LiPF6 and 1 M NaPF6) displayed a much higher inorganic (Na-F to Na-O) content, implying core

differences in the electrochemical/chemical passivation behavior.121 The dominant presence of Na2O (shear modulus 49.7 MPa) and NaF (31.4 MPa) is encouraging since both are potentially elastically stiff enough to block Na dendrites. Yet, in general, SEI growth with Na is relatively unstable. (v) The low melting point of Na and the possibility of suppressing it further with eutectic and off-eutectic solutions123 offer a unique possibility to fully prevent dendrite growth through alloy design. Finally, it is becoming recognized in the LMB and LIB (half-cells studies) literature that cathode crossover species are H


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Figure 8. (a) Schematic illustrating energy states of the electrodes and of the electrolyte, leading to the formation of the SEI and CEI (also labeled “SEI”) layers. (b) SEI film formation processes for recently proposed “near-shore aggregation” versus classical “surface growth” mechanism. (c) Polyhetero microphase (mosaic structure) of SEI on Li or carbon electrode. Panel a adapted with permission from ref 142. Copyright 2010 American Chemical Society. Panel b adapted with permission from ref 144. Panel c adapted with permission from ref 158. Copyright 1997 The Electrochemical Society.

4. KEY CHALLENGES FOR SODIUM-METAL BATTERIES

important for either stabilizing or destabilizing SEI growth of Li-metal anodes.124−133 For instance, metallic Li deposition on graphite is promoted by transition-metal ions that migrate from a ceramic cathode into the SEI layer. The unstable SEI growth process is then self-amplified, with both the Li metal and the transition elements leading to accelerated growth (Figure 7a).133 Besides the deterioration of anode by the cathode species, the cathode can also be damaged by the species crossing over from the metal anode. Zhang et al. reported that detrimental cathode− electrolyte interface (CEI) growth occurs due to species from the anode, leading to capacity fade. The cathode degradation appears more severe in LMBs than in LIBs due to the high reducing power of the Li metal (Figure 7b).134 One would then infer that due to bidirectional crossover, ion-selective separators may be beneficial for Na-metal anode batteries, especially ones based on a ceramic cathode. Yet, as indicated prior, experimental evidence shows Na-metal cells with glass frit separators give more robust cycling than with Nafion membranes. This is another example where knowledge and expertise gained from optimizing LMBs is not directly transferable to NMBs. This is especially true since the NMB vs LMB ceramic cathode materials typically contain different elements. For example, cobalt, which is the staple of Li cathodes, is not widely employed in Na systems.135−139

4.1. Understanding the Na Solid Electrolyte Interphase

The solid electrolyte interphase (SEI) is regarded as one of the key elements of non-aqueous secondary batteries since the report on its formation on anode surfaces in the 1970s.140,141 Figure 8a illustrates the energy states of the electrodes and of the electrolyte.142 The μA and μC of thermodynamically stable cells are placed within Eg, while μA and μC being outside Eg leads to SEI and CEI (labeled “SEI” in Figure 8a) formation, respectively. To relate these energy states to electrode and electrolyte voltage windows one would flip the figure upside down (ΔG = −nFE), achieving an identical set of conclusions. In the schematic, Eg is the gap between the energy of the lowest unoccupied molecular orbital (LUMO) and that of the highest occupied molecular orbital (HOMO) of the electrolyte. It is the electrolyte’s thermodynamic window. The μA and μC are the chemical potentials of the anode and cathode, respectively. An anode with μA higher than LUMO will spontaneously transfer available electrons to the electrolyte. This means that the Na metal, with its Na/Na+ voltage about 1 V below the stability of a typical carbonate electrolyte, will spontaneously reduce the electrolyte during the initial charge. The Na anode may also reduce the electrolyte prior to charging, through parasitic corrosion reactions where the source of electrons is the metal itself. A cathode with μC lower I


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NaPF6 in ether-based solvents and NaAlCl4·2SO2, respectively.120,157 Understanding the ion transport at the electrolyte−electrode interface is another key underexplored aspect of SMBs. Ion migration mechanisms in ionically bound materials are schematically illustrated in Figure 9a. The ion mobility in a

than HOMO will extract electrons from the electrolyte, spontaneously oxidizing, i.e., corroding it. Hence, many highvoltage ceramic cathodes will spontaneously form a CEI layer on their surface. Since the voltage of S and Se cathodes is relatively low, being at 2−2.5 vs Na/Na+, they are not expected to spontaneously form CEI. However, some secondary chemical reactions of the cathode surface with residual moisture in the electrolyte are likely, especially in the case of Se. The route for SEI film formation is the “surface growth mechanism” in which the insoluble inorganic and organic products (e.g., Li2CO3, Na2CO3, Li2O, Na2O, LiF, NaF, ROCO2Li, ROCO2Na, etc.) from electrolyte decomposition form on the anode surface. Growth stops when the SEI film is thick and continuous enough to impede further electrolyte decomposition (Figure 8b).143,144 Even for the case of the much better known Li, the fundamental understanding of SEI is still evolving. For example, in a recent study, an alternative “near-shore aggregation mechanism” has been proposed for SEI growth.144 In this mechanism, the decomposed electrolyte components on the anode surface desorb into the electrolyte and form aggregates. The agglomerates then coalesce and readsorb onto the anode surface (Figure 8b).144,145 In our opinion, more evidence is needed to support this intriguing alternative scenario. The SEI layer is known to form instantaneously below approximately 1 V vs Na/Na+ or Li/Li+ on metals, carbons, semiconductors, and oxides when they are in contact with carbonate-based electrolytes.146 The SEI layer consists of insoluble and partially soluble products, which are generated by a reductive decomposition of the electrolyte and the reaction of the electrolyte (including residual moisture) with the anode.147 This layer acts like a solid electrolyte between the metal and the liquid electrolytes, being ionically conductive but electrically insulating.148 The instantaneously formed surface layer blocks direct electron transfer from the metal to the solution, thereby preventing the continuous corrosion of the metal electrode. In this regard, the SEI layer on the metal surface is correctly termed a “passivation layer”.149 Passivation by the SEI is also critical because it prevents the further reduction of the organic electrolytes at the anode, enabling operation of battery cell beyond the Eg of the electrolyte.150 Analysis of the SEI composition was pioneered by Nazri and Muller.151 They showed that lithium carbonate (Li2CO3) is the main inorganic component of SEI and that it is present in the inner region of the SEI layer, closest to the anode. The organic polymer reduction products constitute the outer portion of the SEI, facing the electrolyte. Aurbach et al. revealed that alkyl carbonates are the principal SEI components.152 Peled et al. proposed an SEI model that is a mosaic of heteropoly microphases (Figure 8c) and converted it into an equivalent circuit. It is now generally believed that in most cases, the SEI consists of intermixed layers of organic and inorganic phases.153 The organic components of SEI are mainly determined by electrolyte solvent decomposition products.5 In the carbonate-based electrolytes, Na-metal anodes show SEI components (i.e., HCOO-Na, ROCO2-Na and inorganic Na2CO3) similar to those of the Li metal (ROCO2-Li and Li2CO3).152,154,155 Other inorganic components of the SEI come from the reactions of the salts and solvents,5 as well as from residual electrolyte dissolved species such as H2O and HF.156 For example, inorganic NaF and NaCl SEI films on the Na metal are formed under the electrolyte environment of 1 M

Figure 9. Schematic illustration of ion diffusion in crystalline materials through (a) direct-hopping, (b) knock-off, and (c) concertedmigration (versus single-ion migration) mechanism. Panels a,b adapted with permission from ref 164. Copyright 2015 American Chemical Society. Panel c adapted with permission from ref 160. Published 2017 Open Access under Creative Commons Attribution 4.0 International License.

solid is described by the well-known Arrhenius equation, with the highest barrier along the diffusion path being rate determining.159 One of the representative diffusion models of ion transport is direct hopping, where a Na+ or Li+ diffuses from one interstitial site to a neighboring interstitial site. Another mechanism is “knock-off”, in which an interstitial ion such as Na+ displaces a different ion located at an adjacent lattice site, knocking it off the lattice position and into an interstice (Figure 9b). There is also concerted migration, which accounts for fast ion diffusion in super-ionic conductors. In this mechanism, due to strong ion−ion interactions, the concerted motion of multiple ions gives a lower diffusion barrier than that of a single ion (Figure 9c).160 A typical model of ionic transport through SEI (dual layer of the porous organic layer and dense inorganic layer) is shown in Figure 10a.161 In the outer layer the cations gradually experience a desolvation process, migrating toward the electrode. However, they cannot easily diffuse through the dense inorganic layer due to their large size.5,161 Using time-of-flight secondary ion mass spectroscopy (TOF-SIMS) and density functional theory (DFT) calculation, the authors argued that Li ions in crystalline Li2CO3 diffuse by the knock-off mechanism rather J


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With both Na and Li, the SEI is a complex composite with a rich array of homophase and heterophase interfaces. Therefore, one would expect that interfacial ion diffusion to be significant and perhaps dominant over any bulk lattice diffusion processes. In our opinion, more studies are needed to specifically focus on interfacial transport in model SEI systems. Akin to classical diffusion studies, representative structures consisting of relevant heterophases may be constructed and analyzed, for instance by creating a Na2O−Na2CO3 composite. Moreover, one would definitely expect cracks and pores within the SEI layer, which means Na+ surface diffusion must be understood. The average content of pores and cracks within the SEI may be to some extent controlled, in principle creating experimentally relevant conditions for performing experimental diffusion studies. Conversely, modeling can readily create a wide array of inorganic SEI interfaces and surfaces, where ions may be explored through simulation. Ion diffusion in the inorganic layer (e.g., alkali metal carbonate or fluoride) of the SEI is regarded as a ratedetermining bulk diffusion step.161−163 As such, further understanding of ion migration through these bulk layers is important. The ionic conductivity in the inorganic layer of the SEI should be highly influenced by defects. In a recent study, Greeley et al. calculated the activation barriers for diffusion in LiF and NaF layers, considering various defects such as vacancy, interstitial, and Li-F and Na-F pairs.164 Despite the same rock salt structure of NaF and LiF (Fm3m), in almost all the cases the barrier energy for NaF was higher by about 200 meV. The ionic conductivity in NaF was several orders of magnitude smaller than that in LiF (∼10−7−10−13 S cm−1). Figure 10b summarizes the lowest activation energy barriers for Li- and Na-ion diffusion through the various SEI components.165 Na ion exhibits higher energy barriers in both the alkali carbonate and the fluoride layer. This can be attributed to the larger ion size of Na than Li, indicating that Na ions perturb the lattice to a greater extent and further increase the barrier for diffusion.164,166

Figure 10. (a) Schematic illustration of ionic diffusion through various SEI phases, including through outer organic and inner inorganic layers. Blue solid lines represent channels which Li+ in the electrolyte transport with anions via pore diffusion. Red arrows show Li+ diffusion in the dense inorganic layers. (b) The calculated lowest energy barriers for Li- and Na-ion diffusion through crystalline inorganic SEI components. Panel a adapted with permission from ref 161. Copyright 2012 American Chemical Society. Panel b adapted with permission from ref 165. Copyright 2017 John Wiley & Sons, Inc.

than by direct-hopping through the empty lattice sites.161 Although ion diffusion in Na2CO3 has not been explored in similar detail, one would expect an analogous mechanism to be dominant in the bulk lattice.

Figure 11. Comparison of stability of Li- and Na-metal anode interfaces in carbonate-based electrolytes using symmetric Li/Li and Na/Na cells at room temperature. (a,d) Nyquist plot of impedance as a function of time at OCV. (b,e) SEM images of pristine Li and Na surface. (c,f) SEM images of Li and Na surface after 24 h at OCV. Adapted with permission from ref 154. Copyright 2015 The Electrochemical Society. K


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4.2. The SEI Layer Is Less Stable with Na than with Li

The plating−stripping volume changes are actually far less than for Na alloy anodes, such as Sn- and Sb-based,178−181 where the entire electrode expands and contracts up to 400% at every cycle. Nevertheless, the repeated plating−stripping related expansion−contraction in the direction normal to the Na foil is bound to create significant internal stresses in the polymer-rich SEI layer. Due to the stresses generated, the Na metal should yield and may rapidly creep at ambient due to its high homologous temperature (in metals, creep becomes significant at T/Tm = 0.4). Therefore, it is feasible that not only will the plating metal deform the SEI, but also vice versa, where the SEI extrudes Na through cracks, pores, and regions of otherwise weak mechanical strength. This is one potential which to our knowledge has not been considered either through experiment or by modeling. Stresses on the Na metal due to the SEI may also drive what would otherwise be a planar SEI−Na interface to assume an irregular “ruffled” morphology. Stress-induced and diffusion driven instabilities of surfaces and of interfaces is a well-known phenomenon in classic thin films.182,183 Given the self-diffusion of Na metal at room temperature, it is also conceivable that the plating stresses in sum may cause grain boundary migration and/or localized dynamic recrystallization. Such metallurgical heterogeneities combined with dislocation surface steps due to flow would impart a highly anisotropic energy to the Na metal−SEI interface and may be another contributor to its instability. To date, little experimental or modeling results are available on these topics. In our opinion, the role of intrinsic and extrinsic stresses, and of metallurgical defects, is especially important to pursue for Na due to its elastic, plastic, and diffusional characteristics. Monroe and Newman showed that based on elasticity theory the SEI should have a shear modulus on the order of a gigapascal (GPa) to stop growing Li dendrites.184,185 The LiSEI film formed by the decomposition of carbonate-based electrolytes exhibits a much lower Young’s modulus of 50−400 MPa.186−188 The elastic modulus of SEI can be measured experimentally by atomic force microscopy (AFM). Hu et al. found that the modulus of the Na-SEI on the Cu foil varied in the range of 50−500 MPa depending on the measuring position (Figure 12).189 This provides direct evidence of lateral inhomogeneity in the mechanical properties. Despite the order of magnitude lower elastic properties of Na metal, the Na-SEI is likely not strong or stiff enough to be an effective blocking layer for the entire metal−electrolyte interface. Locally the SEI may block or slow down dendrite growth, e.g., in regions that

In conventional carbonate-based electrolytes, metal anodes suffer from chemical and mechanical instability of the SEI formed on their surfaces.167−169 This is perhaps the key underlying issue that drives the remainder of associated problems with both Na- and Li-metal anodes, including dendrite growth in various forms. The instability of the SEI layer on the Na-metal surface appears to be more pronounced than on a Li-metal surface; hence, the associated problems are often worse. A systematic comparison of SEI solubility in Li and Na cells has been recently performed.170 Those authors reported that the solubility of SEI in the Na electrolyte is higher than in the Li electrolyte. As compared to Na, Li forms relatively more stable organometallic compounds which can act as an anionic polymerization initiator. Thus, there are more species ready to polymerize in the Li cells, whereas the nonpolymerized organic SEI components in the Na cells continuously dissolve.170 Ponrouch et al. compared the stability of SEI on Li- and on Na-metal surfaces in a mixture of EC and DMC with salts of LiPF6 or NaPF6.154 The authors measured the interfacial resistance and investigated the surface morphology changes of the metal anodes at open-circuit voltage (OCV). While the Limetal anode exhibited negligible changes in the interfacial resistance and surface morphology for 24 h, the Na-metal anode showed much higher interfacial resistance that also gradually increased with time (Figure 11).154 Additionally, granular spots appeared on the Na-metal surface. Since this experiment was performed at OCV, it indicates that ongoing chemical and/or electrochemical reactions on the electrode surface occur even at static conditions where there is no external source of electrons. The ongoing increase in the interfacial resistance and the associated surface morphology changes of the Na metal over time indicate that the formed SEI cannot be stabilized over practical time frames. The Na metal continues to consume the electrolyte, boding ill for direct utilization of analogous salt−carbonate solvent combinations that are currently employed for Li applications. Recent work by Mitlin et al., however, demonstrated that, with a pristine graphene-based protection membrane that effectively separates most of the Na or Li metal from the electrolyte, dendrite-free cycling is possible in a conventional carbonate solvent.171,172 A “hostless” plating−stripping process of Na/Li metal results in large interfacial fluctuations between the bulk metal anode and the SEI film.173−177 While one may assume that the internal stresses within the Na metal are relieved in the process, the stress on the SEI layer during plating−stripping may be significant. If all the Na metal is stripped during discharge of a cell and then is plated during subsequent charging, the associated volume change is infinite. While it is popular to state this point when discussing the volume changes in SMB and LMB anodes during charging−discharging, it is only true if all of the anode material is dissolved and then reprecipitated. Otherwise, the volume changes may be quite small. For instance, stripping and plating the top 5 μm of a 100 μm Na foil, gives 5% change in height and volume. This is in fact the scenario one would expect from laboratory testing where a relatively thick hand-cut Na foil is often utilized. One would expect that as the field matures, the available Na foils would remain oversized relative to the capacity required to balance a cathode one-to-one. This has been the case with LMBs, which are relatively far along in development.

Figure 12. (a) Schematic illustration of AFM-based approach for measuring elastic properties of Na SEI on a Cu current collector, in 1 M NaPF6-EC/DEC electrolyte. (b) Distribution of measured Young’s modulus over the SEI surface. Adapted with permission from ref 189. Copyright 2013 Elsevier. L


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Figure 13. Dendrite formation at the Na-metal surface. (a) Schematic illustration of Na dendrite formation on plating−stripping. (b) Crosssectional SEM images of separators before (top) and after (bottom) cycling in Na-O2 batteries. The dotted yellow lines present the laminated separator layers (polypropylene−polyethylene−polypropylene). (c) Digital images of Na-metal anode and separator surface under different discharge cutoff capacity in Na-O2 batteries. Panel a adapted with permission from ref 204. Panel b adapted with permission from ref 205. Copyright 2012 PCCP Owner Societies.

clear examples of cycling-induced cell shorting. Long before dendrites can reach the separator, pierce it, and grow to contact the cathode, the cells achieve unacceptably high impedance due to SEI and potentially “dry out” (depending on electrolyte relative volume). Therefore, we strongly argue that the achievement of a stable SEI layer is the first and foremost priority for advancing the Na anode field. This must be done prior to attempting other improvements such as stronger separators or cell charging design,192 etc. which are useful for Li-metal cells due to their more stable SEI layer.

are primarily inorganic. This would further promote the irregular Na-metal interface observed experimentally. The SEI layer is highly heterogeneous and dynamically changes during multiple charge−discharges, likely evolving with voltage, cycle number, and calendar time. Even an elastically stiff SEI layer will not be able to resist the plating induced expansion of the underlying metal, else by definition the plating process will be curtailed. As will be discussed, with cycling the growing Na dendrites are able to penetrate even much elastically stiffer solid-state electrolytes, simply by diffusing along defect interfaces. Therefore, we do not consider elastic considerations of the SEI layer as important aspects in stabilizing Na-metal growth. The heterogeneous mechanical proprieties of the SEI may play a deleterious role by promoting preferential metal growth at elastically soft points. Archer et al. found that a failure of Na-metal cell is evidenced by a diverging voltage rather than by shorting, indicating that SEI build-up (increased overpotential) and associated electrolyte depletion will induce the cell failure.107,190 Considering this observation along with the generally observed low CE for cycled Na-metal anodes, it is reasonable to argue that in general the dominant failure mode for SMBs is the excessive SEI buildup.107 In flooded button cells, this may not directly lead to electrolyte depletion. In more relevant pouch cells with a more limited electrolyte volume relative to the electrode mass, electrolyte depletion would occur in parallel to SEI buildup. This is distinct from and will occur prior to dendrite-induced short circuits, which is the most oft perceived failure mode for metal anodes.191 Surveying all Na-metal anode literature, it is difficult to find

4.3. Dendrite Growth and Related Issues

Dendrite growth is recognized as the bottleneck for the safe and stable operation of SMBs well as of LMBs.193 The classic view of dendrites is that they form during the electroplating of metals due to diffusional limitations within the electrolyte.194,195 This understanding, i.e., that dendrites are directly related to Sand’s time, is in our opinion not fully applicable to the Na-metal system. Sand’s time model accurately describes the high current density behavior for a range of plating− stripping systems. However, a review of the Na-metal literature indicates that dendrites form at a range of rates, including moderate and even low currents.196,197 Existing explanations of dendrite growth in Na-metal anodes are based on a sequential growth mechanism (Figure 13a). When the Na metal having a non-uniform surface contacts the organic electrolytes, an uneven SEI is spontaneously generated along the surface, with the uneven SEI leading to non-uniform Na deposition during initial plating. Then, the ion flux becomes more concentrated to the protuberance, eventually generating a dendrite.107 M


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Figure 14. Needle-like Na dendrite formation. (a) Cross-sectional views of Na/separator/Na assembly before and after Na deposition. (b) Light microscopy images of electrochemical Na deposition. Adapted with permission from ref 199. Copyright 2016 Scientific Reports.

batteries and Na/Na symmetric cells (Figure 13b,c).204,205 Guo et al. revealed that the penetration of dendrite is more severe with the increase of the discharge depth (amount of Naplating).205 In most cases reported in literature, the Na-metal system will fade before this occurs due to SEI-induced impedance rise and potentially due to electrolyte loss. This is one point that we believe is worth re-emphasizing, in existing literature there is less evidence for Na-metal cells cycling long enough to short out catastrophically. In commercially representative pouch or cylindrical cells, where the system is not flooded with electrolyte, the drying scenario would occur far faster. During the stripping process, the Na near the base of the Na dendrite may be preferentially dissolved. It has been argued that roots of the Na dendrites tend to accept electrons early, giving rise to faster dissolution rates.199 This may indeed be the case; however, stochastic variations in surrounding SEI structure may likewise result in preferential metal dissolution at certain locations, some of them being near the base. Dendrites which electrically separate from the current collector may do so because of preferential ion flux or due to mechanical stress, the two being not mutually exclusive. This creates “dead Na”, which gives sharp drops in the CE and helps to contribute to cell failure by impedance rise (and capacity fade if the Na supply is limited).206 It should be noted that while dead Na is no longer electrochemically active, it still may remain chemically active further forming electrolyte decomposition products on its surface. Therefore, dead Na would have a negative effect on the SEI at cycles beyond at which it occurred, or during cell storage. The formation of electrically isolated dead Na also creates new metal−electrolyte interfaces in the electrically connected remaining metal foil, leading to accelerated electrochemical SEI formation.107,207 It is known that Li-metal anodes with dendrites possess a non-uniform porous structure, inducing large polarization due to SEI overgrowth.208−210 The Na-metal anodes appear to possess more severe porous surface morphologies, along with increasing polarization, i.e. overpotential during cycling.118 The formation of Li dendrites is typically explained by standard nucleation and growth scenarios.5 The growing Li dendrites have various morphological patterns, such as needle-

We point out that certain aspects of the above scenario are indeed correct. But a geometrically non-uniform metal surface is actually not required. The SEI is intrinsically heterogeneous,161 being a composite of multiple organic and inorganic phases that change with cycling and time.198 The actual physical and chemical heterogeneity of the SEI layer should promote the preferential growth of Na metal at certain locations even if it was initially fully isotropic. In the presence of a non-uniform SEI, Na-metal heterogeneities develop, which would not require enhanced ion flux to keep growing. Preferential growth may be driven by the surrounding SEI structure, for example, if the local SEI was elastically softer, contained cracks and/or pores, contained preferential nucleation sites such as inorganic Na2O and NaF nanoparticles, or was simply thinner in sections (shorter Na+ diffusion distance). Considering the usual dense distribution Na dendrites, rather than the more isolated dendrites observed with Li, it is conceivable that various SEI heterogeneities are quite effective in promoting non-isotropic growth. The process would be self-amplifying, since mossy dendrite growth is known to be accompanied by accelerated SEI formation.199 Another difference between Na and Li is that dendrite growth for Na is considered to occur at the base (“root”) rather than at the tip. This is inferred from the observed dense mossy and needle-like dendrite morphologies, although we are not aware of experimental evidence proving it directly. The implication of all base growth is that an accelerated ion flux to a dendrite tip is not required: Like weeds sprouting through cracked pavement, points in the SEI which give the least resistance are where Na-metal moss grows. If a dendrite reaches the cathode and short-circuits, rapid and highly exothermic failure may result.200 The temperature rise associated with shorting of conventional LIB cells is known. For a short-circuit condition of a sealed cell consisting of LiCoO2 and graphite, the internal temperature of the cell can reach up to 132 °C and generates flammable gases.200,201 This temperature is high enough to cause catastrophic thermal runaway such as fire or explosion.202,203 The case for a Na cell would be analogous or potentially worse, since the metal is more reactive than Li. Several groups observed that Na dendrite can penetrate the separators upon cycling in Na-O2 N


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Figure 15. In situ light optical microscope analysis of gas evolution on Na with (a) pure PC, (b) 1 M NaClO4-PC, (c) pure DME, (d) 1 M NaClO4 DME, (e) pure TEGDME, and (f) 1 M NaClO4-TEGDME electrolyte. Adapted with permission from ref 221. Copyright 2018 John Wiley & Sons, Inc.

like, moss-like, and tree-like.211−213 Needle-like Li dendrites grow without branches, exhibiting linear elongation in both the longitudinal and radial directions.5,214 Moss-like dendrites show 3D omnidirectional growth, consuming more Li to form the SEI layer due to the higher surface area.5 Tree-like Li dendrites grow in all directions with differentiated branches and are not as commonly observed as the needle and moss.5,215−217 There have been some observations for the needle-like dendrite on the Na-metal anodes in carbonatebased electrolytes (Figure 14).106,199 Yuhki et al. argued that the needle-like Na dendrite formed by the granular nucleation of Na at a pit on a Na-metal electrode and the continuous linear growth of the deposited Na.199 This is a certainly a feasible scenario, although as we discussed other mechanisms are possible too. For instance, the needle dendrite may be extruded through a preferential portion of the SEI. Over the past several decades, extensive research has been conducted to understand the Li dendrites.215−220 Much less work has been performed on the Na-metal system, adding a certain level of excitement and novelty to all new Na studies. To date, many fundamental questions regarding Na dendrite have not been answered. For instance, is the nucleation and growth model of Li dendrite applicable to Na? We believe that it is in the most general terms only, since the mechanical, ion transport, and catalytic (toward SEI formation) properties of the metals are dissimilar. For instance, it is unclear what is the structure of “dead Na” (metal, oxide, composite?) or the exact

reason why it occurs. Broadly speaking, progress on the Li system is much further along than it is for the Na. A valid nucleation model(s) for Na is still in need, and the points of dendrite nucleation are indeterminate. Researchers are yet to directly relate dominant Na dendrite growth patterns to factors such as current density, cycle number, electrolyte and additive type, etc. 4.4. Gas Evolution

Considering that Na has higher reactivity than Li in organic electrolytes, the gas evolution should be considered as one of the important design factors in SMBs. The high reactivity of Na metal with organic electrolytes will cause gas evolution in the process of SEI formation.221 Zhang et al. showed that when the organic solvent, such as such as propylene carbonate (PC), is complexed with Na-ions (Na+-PC), the decomposition of organic electrolytes and gas evolution are facilitated as compared to pure solvent without Na ions.221 Such facilitated gas evolution in Na+-PC solution can be attributed to its lower level of LUMO (the lowest unoccupied molecular orbital) energy than pure PC solvent, per first principles calculations.221 Gas evolution occurs even in ether-based electrolytes of dimethoxyethane (DME) and tetraethylene glycol dimethyl ether (TEGDME) (Figure 15). Even non-flammable gas evolution will pose major engineering and safety problems for scaled-up devices, since cell swelling is considered unacceptable and cell rupture is a catastrophic event. The situation is not that different from Li-metal or even convention O


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These represent a key way forward for not only SMBs, but also for SIBs, which risk cycling-induced dendrite growth on the ion insertion anode. While arguments regarding cost are important, one has to keep in mind at laboratory scales, i.e., purchased from a scientific supplier, all battery chemicals are not industrially feasible. Once a given electrolyte chemistry is demonstrated to offer true technological utility, it is reasonable to expect that economies of scale will kick in, with vendors offering much lower prices for mass produced products. An unstable SEI on the Na-metal anode is primarily responsible for mossy Na dendrite growth and the associated low CE. Since the SEI is formed by electrolyte decomposition, various attempts have been made to obtain a more stable SEI though the design of the electrolytes to go beyond the standard carbonate-based systems. In the case of LMBs, numerous salts (e.g., Li halides, LiBF4, LiPF6, LiSO3CF3, LiTFSI, LiClO4, and LiAsF6) in PC solvent have been studied and less reactive salts to Li (e.g., LiAsF6) showed better cycling efficiency by forming a more stable thin SEI film. For instance, Naoi and co-workers reported that LiN(C2F5SO2)2 in PC solvent can form a very thin and compact film on the Li-metal anode that mainly consists of LiF, resulting in improved cycling performance.230 Early studies on Na plating−stripping at room temperature using chloroaluminate ionic liquid electrolytes showed limited reversibility.231−235 More recently, Cui et al. reported that simple electrolytes of 1 M NaPF6 in glymes allowed long-term non-dendritic plating−stripping operation of the Na-metal anode at room temperature.120 The authors found that stable inorganic Na2O- and NaF-rich SEI films are formed in the NaPF6-glyme (mono-, di-, and tetraglyme), whereas those inorganic component-rich films were not formed in NaPF6EC/DMC or NaPF6-EC/DEC.120 This indicates that NaPF6 with ether-based solvents is capable of forming the highly stable passivating inorganic layers on the electrode surfaces. Stable SEI formation on Na-metal anodes was also achieved using sodium bis(fluorosulfonyl) imide (NaFSI) salt with DME solvent.236 Freunberger attributed the formation of stable SEI from the NaFSI salt systems to the film-forming properties of FSI anion.236,237 For Li-metal anodes, the concentration of salts has been shown to influence cycling performance.238,239 Dendritic Li deposition can be accelerated at high current densities. A concentration gradient of the electrolyte ions occurs in the electrolyte during fast electrodeposition.240,241 When a critical current density is reached, ion flux through the electrolyte can only last for a certain period of time, after which there is a diffusional concentration gradient to the electrode. This time period is termed “Sand’s time”. Thereafter, the electrical neutrality is broken at the surface of the working electrode, leading to an excess of charge and corresponding voltage polarization. The local space charge then drives nucleation and growth of metal dendrites at geometrically preferential locations, i.e., the stochastic protrusions from a planar front.8,240,241 Sand’s time depends on the concentration of ions in the electrolyte, which means that a higher electrolyte concentration will extend the currents at which dendrites are not formed.242 For Li metal, Sand’s time has been shown to be relevant at certain conditions, though even dendrites are routinely reported at lower currents. However, as discussed, for Na metal there is less evidence that the limitation of ion diffusion through the electrolyte is the driver for dendrite growth. The preferential sites are probably not only geometric

graphite-based anodes, where gas (CO2, O2, other) is also evolved,222,223 except that the unstable SEI with Na-metal dendrites can cause more copious gas evolution that is ongoing with cycling. In most scientific studies, for both Na and Li, rigid stainless steel 2032-type coin cells are exclusively employed for asymmetric full device and symmetric metal−metal cell testing. Except in extreme cases, any evolved gas largely goes unnoticed due to the thick walls and the strong mechanical seal of the coin cell. Researchers and engineers working with flexible pouch cells are much more sensitive to the gas evolution issue. Even with conventional graphite−ceramic Li cathode architectures, a pre-conditioning step is usually required which allows the cycle 1 SEI byproduct gas to escape. This solution is impossible if the cells continue to generate gas during cycling, another reason why high surface area mossy Na dendrite growth will ruin a device probably long before it can short the anode to the cathode. The porous dendrite structure catalyzes SEI formation and the associated release of byproduct gas.106 Therefore, it is important to holistically consider gas evolution, SEI formation, and dendrite growth. A key unknown for Na metal includes the quantification of gases evolved at cycle 1 versus during subsequent cycling, as well as the role of electrolyte type and additive chemistry on gas content. For instance, are the evolved gases the same at cycle 1 as at cycle 10, and if not, how is that related to the SEI structure and chemistry? Research on SMBs has generally overlooked this significant issue, which in our opinion requires in-depth analysis. Compared to Na cells, gas evolution in Li cells is relatively well investigated. For instance, C2H4, H2, and other hydrocarbons are the main gaseous components generated on a graphite anode.224−227 Gasteiger et al. reported more H2 generation in a graphite-NMC full cell when there is no diffusion barrier between the anode and cathode. They proposed that the diffusion of protic electrolyte oxidation species (R-H+) from the cathode to anode and its subsequent reduction as the origin of enhanced H2 gassing. To minimize the gas generation, film-forming type additives (e.g., vinylene carbonate and fluoroethylene carbonate) are often employed.222,227−229 In a seminal study, Mullins et al. compared gas evolution rates of EC/DEC, PC/FEC, and FEC/DEC on Na-metal anode and found that gas evolution can be partially mitigated using FEC as a cosolvent.106 However, dendritic porous structure was observed in all the electrolytes, independent of FEC presence. We are not aware of other gas evolution studies for the Na-metal system. This important topic constitutes a key “blind spot” in the emerging body of literature and requires more attention. Especially useful would be direct one-to-one comparisons between the better understood Li-metal anodes and Na-metal anodes with analogous electrolytes.

5. ELECTROLYTES FOR SODIUM-METAL ANODES 5.1. Liquid Electrolytes

Looking ahead, we believe that new solvent−salt combinations should be a key research focus for achieving stable Na-metal cycling. Many “conventional” SMB electrolytes are direct descendants of prior art on LMBs, and there is now overwhelming evidence that the two are not directly interchangeable. Efforts such as the ones described below, which are directly targeted toward Na, are highly welcome. P


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Figure 16. (a) Digital photographs of a pristine Na metal and Cu electrode after 25 cycles of plating and stripping using electrolytes 1 M NaPF6EC/DEC, 1 M NaFSI-DME, and 4 M NaFSI-DME. The Na/Cu cells were cycled at 0.5 mA cm−2 for 2 h while keeping the upper cutoff voltage at 1 V vs Na/Na+. (b) CE of Na plating−stripping in Na/stainless steel (SS) cells with different electrolytes. Na was plated on SS at a current density of 0.056 mA cm−2 for 1 mAh cm−2. The cutoff voltage during the stripping process was kept at 1 V vs Na/Na+. Panel a adapted with permission from ref 243. Copyright 2016 Elsevier. Panel b adapted with permission from ref 244. Copyright 2017 American Chemical Society.

Table 3. Cycling Stability of Na-Metal Anodes with Various Electrolytes working electrode

counter electrode

electrolyte

current density(mA cm−2)

capacity (mAh cm−2)

overpotentiala (mV) (cycles)

ref

Na Na Na Na/Cu

Na Na Na Na piece

NaAlCl4·2SO2 5 M NaFSI-DME 5.2 M NaFSI−DME NaBF4·2.5NH3

0.3 0.0028 2 10

0.75 0.0014 1 −

∼500 (83) >10 (600) ∼100 (900) ∼45 (100)

157 244 245 246

working electrode

counter electrode

electrolyte

current density (mA cm2)

capacity (mAh cm2)

Cu Cu Al Cu

Na Na Na Na

1 M NaPF6-diglyme 4 M NaFSI-DME 4 M NaFSI-DME 0.5% NaFSI-DME

0.5 0.5 0.5 0.2

1 1 1 0.5

CEb(cycles)

ref

99.9% 99% 99% 97.7%

120 243 243 236

(300) (300) (300) (250)

a

When the overpotential is not specified, the overpotential (stripping process) was estimated. bWhen the CE is not specified, it was estimated from figures in the cited references.

concentration of Na ion in excess of 7 M in the ammoniabased electrolytes. Despite the ultrahigh Na concentration and the high conductivity of 0.1 S cm−1, these electrolytes showed low density and low viscosity. In the test of Na plating− stripping on Cu foil using NaBF4−2.5NH3, a low-voltage hysteresis of 4.5 mV for at 100 cycles was reported. The use of these ammoniate-based electrolytes does not incur a high cost, since Na salts are generally cheaper than Li ones and liquid ammonia is also inexpensive.246 The cycling stability of Nametal anodes in various combination of salts and solvents is shown in Table 3. To date, the stable operation of Na-metal anodes is mostly based on ether electrolytes such as DME and glymes rather than carbonate electrolytes (e.g., EC, PC, DEC, DMC, etc.; Table 3). Considering there are no reports of dendrite-free Na anodes in carbonate electrolytes regardless of salt concentration, one may argue that it is the solvent chemistry that dominates stability. Ionic liquids (ILs) are molten salts that inherently do not require solvents,247 are chemically and electrochemically stable.248 ILs’ disadvantage is that many are too viscous at room temperature, and almost all are highly hygroscopic, requiring extensive purification before used in SMB and LMB cells.249−252 The current price for scientific research-grade ILs is quite high,253 although widespread industrial-scale demand is likely to bring it down significantly. Commercially, to our knowledge there are no IL-based energy storage devices, be it batteries or electrochemical capacitors. In scientific literature, however, ILs are widespread in energy storage and conversion applications due to their low volatility, wide electrochemical windows, and acceptable ionic conductivity.254−258 Due to their high chemical and thermal stability, ILs can be used to

features of the plated metal, but also (and perhaps more importantly) preferential “weak points” in the SEI. Concentrated electrolytes of NaFSI-DME (e.g., higher than 4 M) were explored for SMBs, effectively suppressing the dendrite Na growth, although this is probably not directly linked to the Sand’s time diffusion issue.243,244 Choi et al. noted that the stable SEI formation in the highly concentrate electrolyte can be attributed to a minimal amount of free (uncoordinated) DME, thus minimizing the formation of the resistive SEI by the uncoordinated DME with reactive Na metal.244 At a glance, the high-concentration salts seem to work for the enhancement of the cycling stability. It is noteworthy highly concentrated salts were only effective in DME solvents, whereas they were not effective in the carbonate-based electrolytes (Figure 16). For instance, 6 M NaFSI in EC/PC showed cell failure in less than 20 cycles. This indicates that the cycling stability of Na-metal anodes in the NaFSI salt system is critically influenced by the solvent.244 As a complementary strategy, the dilution of the highly concentrated electrolyte with an “inert” additive has been proposed.245 The diluent has little or no effect on the solvation structure of the cation−anion aggregates, but lowers the overall salt concentration and the viscosity. It was demonstrated that the use of 1.5 M NaFSI/DME with the diluent of bis(2,2,2trifluoroethyl) ether (BTFE) can improve the CE for Na plating−stripping as compared to 5.2 or 1.7 M NaFSI/ DME.245 The importance of highly concentrated electrolytes was also demonstrated in liquid ammonia-based electrolytes (i.e., NaY· xNH3, Y = I, BF4, BH4, and x indicates the molar ratio of ammonia to Na salt).246 Those authors utilized an ultrahigh Q


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suppress metal anode shorts and thermal runaway issues in alkali metal batteries.259,260 An excellent early study on ILs as an additive for SMBs has been published.107 However, the feasibility of employing a Na-containing salt and an IL, without other solvents, is not yet demonstrated. It should be considered that Li salts and Na salts may have different solubility and thermodynamic stability in ILs as compared to in conventional liquid solvents.261 Therefore, it may be necessary to use an IL having a strong coordination ability.261 Certainly, more scientific efforts on IL solvents for Na salts are needed, as the field is very young. A secondary issue to consider, however, is the hygroscopic nature and poor low-temperature conductivity of most ILs.262 Unless both factors are addressed, employing ILs as secondary additives is probably more promising. 5.2. Electrolyte Additives

Electrolyte additives are generally employed to form a more stable SEI layer during the initial activation cycles.263−266 It is known that film-forming electrolyte additives, such as FEC,267 vinylene carbonate (VC),268 and ethylene sulfide (ES),269 are beneficial for enhancing interfacial stability between the electrode and the electrolyte. In particular, FEC has been widely used as an additive to SIBs.270,271 Komaba and coworkers reported that the addition of FEC suppresses the undesirable side reactions on the hard carbon surface, such as the formation of sodium propyl carbonate between Na and PC.272 For Li-metal anodes, FEC in the PC electrolyte is beneficial for improving CE, whereas ES and VC are not effective.273 The role of FEC on Na-metal anodes in carbonatebased electrolytes was recently investigated by Mullins et al.106 The work revealed that FEC can lower the levels of organic and salt anion reduction products by forming the insoluble NaF-rich SEI, resulting in improved cycling performance. However, porous dendrite and gas evolution were observed regardless of the presence of FEC in the electrolyte (Figure 17a), indicating that the overall benefit is limited.106 FEC in an ultra-concentrated ether-based electrolyte (5 M NaFSI-DME) can be detrimental to SMBs (Figure 17b).244 In the highly concentrated electrolyte systems, NaF formed by FEC decomposition acts as a resistive layer for Na plating− stripping, increasing the overpotential.244,274 These contradictory results of using FEC in LMBs and SMBs indicate that more studies on FEC are warranted, especially in conjunction with new electrolyte development. In our opinion, one of the key reasons that FEC does not work as well with Na as it does with Li is that it simply becomes exhausted at a quicker pace. The additive by its very nature is consumed when it reacts to form NaF or other surface species. Since Na is highly catalytic to new SEI growth whenever fresh metal is exposed to the electrolyte, the electrolyte will become FEC depleted sooner than it would with Li. One potentially interesting study which has not been performed is a side-to-side comparison of the time/cycle number it takes to exhaust FEC from similar electrolytes with Li vs Na metal. Another variation of this would be to monitor the NaF vs LiF content in the SEI layer during the stages of cycling. Employing depth-sensitive surface since techniques such as depth profiling XPS or TOF-SIMS,275 one should be able to identify the point where the NaF and LiF phases became less pronounced; i.e., there is unmodified SEI overgrowth due to FEC exhaustion. Electrolytes additives for Li-metal anodes are largely classified into film-forming and ion-plating types.276 The ion-

Figure 17. Effect of FEC additive on cycling CE of Na-metal anodes. (a) Light optical images of Na metal in electrolytes EC/DEC, PC/ FEC, and FEC/DEC, after plating Na. Na was deposited at the rate of 1.0 mA cm−2 for 4 h. (b) CE of Na/stainless steel (SS) in 5 M NaFSIDME with and without 1% of FEC additive. The Na/SS cells were cycled at 0.56 mA cm−2 with a capacity of 1 mAh. Panel a adapted with permission ref 106. Panel b adapted with permission from ref 244. Copyright 2017 American Chemical Society.

plating additives, such as alkali metal ions and halide ions, are distinguished from the film-forming additives in that they do not directly participate in the SEI structure during the initial activation cycles. Instead, these ion-plating additives have been found to influence ion-diffusion and plating behavior, which can improve electrochemical performance.277−279 For instance, the positively charged electrostatic shield around the initial growth tips of Li dendrites was achieved using metal ions (i.e., Cs+).279 Having a lower reduction potential than the Li ions forces their further deposition in adjacent regions outside the dendrite tip. Conversely, studies on electrolyte additives for SMBs have exclusively focused on the film-forming types. Since the ion-plating type additives have had minimal attention, this is another fruitful and important future research area. 5.3. Solid-State Electrolytes

Solid-state electrolytes (SSEs) present an exciting opportunity for SMBs since in principle they are free from electrolyte leakage, volatilization, and gas evolution risks.280−283 It was initially believed that SSEs are intrinsically resistant to dendrite growth, although this has been shown not to be the case. While an argument may put forward that SMBs will have to be based on SSEs to operate safely, to date there is no Na-based SSE that allows for sufficient rate capability for most applications. It has been argued that an ideal SSEs would possess four key features: The obvious first is the necessary ionic conductivity (>10−2 S cm−1). The next three are a sufficient mechanical strength to suppress dendrite growth, chemical and electrochemical stability, and chemical compatibility with various cathode materials.284 Contravening the need for elastic stiffness as to resist dendrite penetration, the SSE still need to maintain an intimate bond to various cathodes. Unlike the elastically and plastically compliant Na metal, the usual ceramic or sulfur/selenium-based cathodes are fairly brittle. Therefore, R


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Table 4. Ionic Conductivity and Diffusion Activation Energy of Various Single-Phase and Hybrid SSEs at Room Temperature type

ionic conductivity (mS cm−1)

solid electrolyte

diffusion activation energy (eV)

ref

liquid electrolyte

LP30 (1 M LiPF6−EC/DMC) 1 M NaClO4 in carbonate solvents

10−12 ∼12

− −

303, 304 303

sulfide

Na3PS4 (cubic) Na3PS4 (tetragonal) 75Na2S·25P2S5 Na3PS4−Na4SiS4 Na3PSe4 Na2(Ga0.1Ge0.9)2Se4.95 Na3SbS4 Na3.6Sn0.6Sb0.4S4 Na3.1Sn0.1P0.9S4 Na10SnP2S12 Na11Sn2PS12 Na3P0.62As0.38S4 Na2.9375PS3.9375Cl0.0625

0.2 0.46 0.2 0.74 1.16 ∼10−2 1.1 0.3 0.25 0.4 1.4 1.46 1.38

0.28 0.20 0.28 − 0.21 0.63 0.2 0.39 0.18 0.36 0.25 0.26 0.23

284 372 370 375 384 404 312 405 374 406 407 377 387

oxide (beta-alumina)

single-crystal β-Al2O3 polycrystalline β-Al2O3 single-crystal β″-Al2O3 polycrystalline β″-Al2O3

14 1.2 14 2.1

0.16 0.27 0.31 0.31

320 320 320 320

oxide (NASICON)

(Na2O + NaF)-TiO2−B2O3−P2O5−ZrF4 Na3Zr2Si2PO12 Na3+0.4Sc2Si0.4P3−0.4O12 Na3.3Zr1.7La0.3Si2PO12 Na3.1Zr1.95Mg0.05Si2PO12

0.03 0.67 0.69 3.4 3.5

0.2 − − 0.29 0.25

319 340 352 353 354

oxide (anti-perovskite)

Na2.99Ba0.005OCl1‑x(OH)x

10

99%), and humidity-sensitive NaAlO2 is formed along grain boundaries.135,327,328 β″Alumina is brittle, which means that it may be easily cracked, including during cell fabrication. Polycrystalline β″-alumina possesses a relatively high conductivity of 2.1 × 10−3 S cm−1 at room temperature,310 it is still significantly inferior to liquid electrolytes. Although single crystal β″-alumina is much more conductive,320 there do not seem to be practical paths for creating single-crystal-based SSEs. Inspired by its relatively high ionic conductivity, many efforts have been devoted to develop room-temperature SIBs or SMBs using β″-alumina.329−331 For example, the authors investigated β″-alumina as SSE for a room-temperature battery consisting of a Na-metal anode and a Na0.66Ni0.33Mn0.67O2 cathode (Figure 18b).331 To make intimate contact between the cathode and SSE, the Na0.66Ni0.33Mn0.67O2(NMM) slurry was pasted directly onto the β″-alumina. The NMM delivered a comparable reversible capacity of 72 mAh g−1 to its theoretical capacity (88 mAh g−1). When the current rate increased to 0.5 C, its capacity decreased only slightly. Despite the attractive ionic conductivity of β″-alumina, its application with Na metal remains limited. Extended cycling behavior, especially the evolution and possible control of the structure− chemistry of the interface, is not well understood. We believe that this is a research topic that should be intensely pursued in the future: Maintaining a chemically and mechanically stable electrode−electrolyte interface is the essential for extended cycling lifetime of SSE-based SMBs. NASICON materials (e.g., NaZr2(PO4)3, NaTi2(PO4)3, and NaGe2(PO4)3) were first studied in 1960s.332 A major

(cathode electrolyte interface, CEI) remains a concern.310−313 Ion transport in ceramic electrolytes is governed by solid-state diffusion through the highest flux path, which may be bulk, surface, or grain boundaries depending on their availability.314,315 These characteristics are described by the Arrhenius formula or equation given in eq 13.316 σ=

σ0 −EA / kBT e T

(13)

where EA is the activation energy of diffusion. In the case of inorganic ceramic electrolytes, it is therefore important to lower EA to achieve high conductivity at room temperature.317 As shown in Table 4, various inorganic SSEs exhibit roughly an order of magnitude lower ionic conductivity as compared to liquid electrolytes. For instance, oxide-based (sodium superionic conductor and beta-alumina solid electrolyte) and chalcogenide (S and Se)-based electrolytes show roomtemperature ionic conductivities of (1.1−3.5) × 10−3 S cm−1.318,319 Inorganic SSEs can be subdivided into oxide-based systems (e.g., BASE, NASICON, and anti-perovskite) and sulfide-based systems (e.g., Na3PS4). Figure 18a shows structures of βalumina (hexagonal, P63/mmc, a0 = 0.559 nm, c0 = 2.261 nm) and β″-alumina (rhombohedral, R3m, a0 = 0.560 nm, c0 = 3.395 nm) which contain the spinel blocks and conduction slabs.320,321 The spinel block consists of four layers of oxygen with aluminum ions both in the octahedral and tetrahedral sites. The conduction plane consists of loosely packed oxygen and sodium ions.322,323 β-alumina has the formula of Na2O· 11Al2O3 or NaAl11O17, whereas β″-alumina is Na2O·5Al2O3 or NaAl5O8.324 Due to the stacking sequence, the unit cell of β″alumina is 50% larger than β-alumina and, β″-alumina possesses higher Na-ion concentration in the conduction slab than those of β-alumina. Thus, β″-alumina is more conductive than β-alumina.323 T


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Figure 19. (a) Crystal structures of rhombohedral and monoclinic NASICON (Na3Zr2Si2PO12). (b) Cycling stability of SMB consisting of a Nametal anode, Na3V2(PO4)3 cathode, and Na3.3Zr1.7La0.3Si2PO12 solid electrolyte with an addition of ionic liquid (N-methyl-N-propylpiperidiniumbis(fluorosulfonyl) imide). Tests were performed at room temperature with a window of 2.5−3.7 V. (c) Crystal structure of anti-perovskite (Na3OX). (d) Room-temperature voltage profiles for an SMB with a Na-metal anode, ferrocene molecule cathode, and anti-perovskite solid electrolyte (Na2.99Ba0.005O1+xCl1−2x). Panel a adapted with permission from ref 339. Copyright 2018 John Wiley & Sons, Inc. Panel b adapted with permission from ref 353. Copyright 2016 John Wiley & Sons, Inc. Panel c adapted with permission from ref 361. Copyright 2015 Elsevier. Panel d adapted with permission from ref 362. Copyright 2017 Royal Society of Chemistry.

air environment.347,348 This may not be a major issue for SMB cell fabrication since Na metal requires highly inert conditions (more than Li) to keep from oxidizing. For Na1+xZr2SixP3‑xO12-based SSEs, a relatively high ionic conductivity of ∼10−4 S cm−1 was obtained for x = 1.667−2 at room temperature.349,350 To increase the ionic conductivity, partial substitution of Zr4+ by a trivalent cation (e.g., Sc3+) was investigated.351,352 The Na3+xScxZr2‑x(SiO4)2(PO4) showed a high ionic conductivity of 4 × 10−3 S cm−1 at 25 °C. The strategy of the partial substitution by a trivalent cation can be explained by that trivalent cation generates a deficiency of positive charge in crystal structure, and the deficiency is compensated by additional Na+ ions, thus enhancing ionic conductivity. Analogous to the substitution of Zr4+ with Sc3+, La substitution to Zr sites (Na3+xZr2‑xLaxSi2PO12) also showed an improved ionic conductivity of 3.4 × 10−3 S cm−1.353 To our best knowledge, there have been limited reports of SMB using NASICON-type SSE at room temperature.353,354 A composite system based on Na3.3Zr1.7La0.3Si2PO12 SSE with an addition of IL (N-methyl-N-propylpiperidinium-bis(fluorosulfonyl) imide; PP13FSI) was shown promising in terms of rate capability and cycling stability (Figure 19b).353 The cell configuration was based on a Na-metal anode and Na3V2(PO4)3 cathode that is known for its stability during cycling. This study points to another critical research direction, which we believe may be highly fruitful: In our opinion, neither β″-alumina nor NASICON will likely work by themselves due to their elastic rigidity and possible brittleness, which cannot accommodate the volume changes in either the anode or the cathode. However, combined with a secondary liquid or a semiliquid phase, a synergy may be achieved where the electrode−electrolyte interface remains stable due to the

advancement to these materials occurred when Goodenough et al. developed Nal+xZr2P3‑xSixO12 through partial replacement of P by Si in 1976.333,334 NASCIONs basically have the stoichiometry AM2P3O12, where alkali metal ions (e.g., Li+ or Na+) occupy A sites, the M sites are occupied by various divalent (M II ), trivalent (M III ), tetravalent (M IV ), or pentavalent transition metal ions (MV), and Si or As partially substitutes P.323,334−338 NASICON, Nal+xZr2P3‑xSixO12, has a rhombohedral structure in 0 ≤ x ≤ 3 (except for 1.8 ≤ x ≤ 2.2) and monoclinic phase (C2/c) in 1.8 ≤ x ≤ 2.2.339 Both the rhombohedral and monoclinic NASICON structures are three-dimensional frameworks consisting of PO4 and SiO4 tetrahedra and MO6 octahedra. The tetrahedra and octahedra share their corners with each other (Figure 19a). In rhombohedral NASICON structure, there are two distinct Na sites of Na1 and Na2, which are six-fold coordinated to the oxygen ions of three [Si/PO4] tetrahedral.340,341 In monoclinic NASICON structure, there is an addition Na site (Na3) which is 3-fold coordinated to oxygen ions of three [ZrO6] octahera.339,341 During diffusion, the Na+ ions jump from Na1 to Na2 or Na3. NASICONs show a wide range of ionic conductivity (∼10−3−10−9 S cm−1) with different compositions.340,342,343 From a conductivity vantage, they are clearly inferior to β″alumina. A secondary potential issue with NASICON is that it is thermodynamically unstable when in contact with elementary Na.344,345 However, reactions were not observed at low temperatures (100 °C), indicating slow kinetics.159,345,346 It is unclear whether significant reaction layers would be formed after extended cycling and storage. Additionally, NASICON is sensitive to moisture. For instance, Na3Zr2Si2PO12 reacts with water, and thus it requires a dried U


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Figure 20. (a) Crystal structure of cubic phase (left) and tetragonal phase (right) Na3PS4 projected on the a−c plane. (b) Cycling stability of roomtemperature all-solid-state SIBs which have a configuration of Na2+2δFe2‑δ(SO4)3|Na3.1Sn0.1P0.9S4|Na2Ti3O7) in the voltage window of 1.5−4.0 V. (c) Cycling performance of a cell consisting of Na2.9PS3.95Se0.05(NPS)-coated Fe1‑xS cathode, Na-metal anode, and Na2.9PS3.95Se0.05 solid electrolyte. Panels a,b adapted with permission from ref 374. Copyright 2017 Royal Society of Chemistry. Panel c adapted with permission from ref 381. Copyright 2018 American Chemical Society.

Na2.99Ba0.005OCl1−x(OH)x electrolyte, the authors stably operated the cell consisting of Na-metal anode and ferrocene cathode for 200 cycles (Figure 19d). Considering that Na3OX (X = Br and Cl) has a high shear modulus of 23.6−24.6 GPa,363 it is thought that anti-perovskite-type could effectively suppress Na dendrite growth. The anti-perovskites are highly hygroscopic;360,364 thus, they are needed to be handled inside dry gloveboxes.365 However, this is not a major downside for Na-metal applications since the anode also requires an inert or in the very least a dry atmosphere. In our opinion, antiperovskite-based SSE are highly scientifically interesting and should be further explored. A key point to focus on may be the chemical and physical stability of the SSE−Na metal interface. Sulfide-based SSEs have received intensive attention due to their high ionic conductivity.366,367 These materials are highly water sensitive, though, and almost always need to be handled inside an inert glovebox rather than just a dry room.368,369 While this is the case for Na metal and for oxide-based electrolytes, the sulfides are uniquely problematic since the can react with water to form highly toxic H2S gas. In a research laboratory, this is manageable. However, doubts persist whether a scaled system with the propensity to generate H2S upon rupture would be feasible commercially or even at pilot scale. Tatsumisago et al. reported high ionic conductivity of inorganic sulfide-based electrolytes [(2−4.6) × 10−4 S cm−1 Na3PS4 and 75Na2S·25P2S5) at room temperature, which can be attributed to facile Na-ion diffusion through the threedimensional pathways in the structure.284,370−372 The composite of 75Na2S·25P2S5 displayed improved electrochemical stability against Na deposition/dissolution.370 It has reported that Na3PS4 have cubic and tetragonal phase (Figure 20a).372−374 In both phases, PS43− occupies sites with the body-centered cubic arrangement. In the cubic phase, there are two distinctive Na sites; 6b site Na(1) with the partial

wetting liquid. More research on combining stable ILs with ceramics for composite SSEs is warranted. Recently, perovskite-type electrolytes have drawn some attention.317,355−357 However, they possess low compatibility with alkaline metals. For instance, when lithium lanthanum titanate is directly contacted to Li metal, Ti4+ in lithium lanthanum titanate is reduced to Ti3+, resulting in the increase of electrical conductivity.358 To our best knowledge, the perovskite-type Na-ion conductor has not been reported yet. Anti-perovskite electrolytes have attractive features for SSEs due to their relatively high ionic conductivity (∼10−3 S cm−1) at room temperature and large voltage window (>5 V vs Li/ Li+).314,359 Different from perovskite structure, anti-perovskite has a structure where the cation and anion position are switched (e.g., B3OA, B = Li or Na).360 The crystal structure of anti-perovskite is shown in Figure 19c. ONa6 octahedrons share their corners with each other. There are two possible diffusion pathways for Na+ ions along [101] and [001], directions marked as J1 and J2 (Figure 19c).361 The authors prepared Na2.9Sr0.05OBrI0.4 by partially substituting A-site with larger halogen ions and doping divalent alkali-earth metal to Na sites based on Na3OBr. The Na2.9Sr0.05OBrI0.4 showed ionic conductivity of 10−5−10−5 S cm−1 with activation energy of 0.62 eV at 50 °C. Very recently, Goodenough et al. reported anti-perovskite-based Na+ conductive SSE (Na2.99Ba0.005OCl1−x(OH)x) with a high ionic conductivity of (10−2 S cm−1) and low activation energy less than 0.1 eV. Those authors prepared the sodium superionic conductors by adding a small amount of water into a mixture of precursors (NaCl, NaOH, Ba(OH)2·8H2, etc.) and then drying HCl. The formation of the Na-ion conductors involves the separation of H2O into OH− and H+ ions. OH− react with one another, 2(OH)− = O2− + H2O. The O2− attacks Na+ to form dipoles; the remaining Na are mobile. 3 1 8 , 3 6 2 Using the V


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Figure 21. Schematic illustration of the interface between a ceramic pellet solid electrolyte Na1+3xZr2(P1‑xSixO4)3 and the sodium metal, showing the possibility of Na dendrite growth through grain boundaries, nanopores, etc. Adapted with permission from ref 104. Published 2017 ACS AuthorChoice License.

occupancy of 0.8 and 12d site Na(2) with the occupancy of 0.1. Na(1) and Na(2) have distorted octahedral coordination and tetragonal coordination by PS43−, respectively.374 Ionic conductivity was further improved to 7.4 × 10−4 S cm−1 by substituting Si at the P sites.370,375 Ong et al. revealed that Si4+ doping induces Na disorder created by excessive Na ions in the crystal structure and the Na disorder enhances the Na-ion conductivity in cubic Na3PS4.376 The authors showed that the other aliovalent doping of M4+ (e.g., Ge and Sn) also works for enhancing ionic conductivity of cubic Na3PS4.376 It was reported that all-solid-state SIB, which has a configuration of Na2+2δFe2−δ(SO4)3|Na3.1Sn0.1P0.9S4|Na2Ti3O7, can achieve a high capacity of 94 mAh g−1 close to theoretical capacity of Na2+2δFe2−δ(SO4)3 (113 mAh g−1) at the 10th cycle (Figure 20b).374 The substitution of trivalent doping at the P sites (Na3SbS4) was also investigated with the improved ionic conductivities of (1.1−1.46) × 10 −3 S cm −1 being reported.312,377 Researchers investigated cycling stability of room-temperature all-solid SIB (Na-Sn|Na3SbS4| Na3SbS4coated NCO) in the voltage window of 1.2−3.7 V at 30 °C.312 Despite the high ionic conductivity of the solid electrolyte (1.1 × 10−3 S cm−1), the cell showed poor cycling stability due to electrochemical instability of sulfide-based SSEs and chemical reaction of NCO to Na3SbS4.378−380 More improved cycling stability of room-temperature SMB was achieved by Na3.1Sn0.1P0.9S4(NPS) SSE with Na-metal anode and Fe1−x cathode (Figure 20c).381 The cell showed a capacity retention of 75.45% at the 100th cycle. However, this highcapacity retention was not obtainable when cathode was not coated with SSE, indicating the importance of intimate contact between SSE and cathode side. Promisingly, the authors reported that no side reaction occurs at anode sides.381 Meanwhile, authors showed that Na vacancies in tetragonal Na3SbS4 contribute to the enhancement of ionic conductivity.382 In the study of Li10+1M2P2X12 for Li-ion conductor, Ceder et al. showed that anion substitution has a great effect on the improvement of ionic conductivity.383 Motivated by the anion substitution, researchers substituted Se at S site (Na3PSe4), achieving a high Na-ion conductivity of 1.16 × 10−3 S cm−1.312,384 Selenide-based electrolyte can have two advantages: the larger atomic radius of Se can expand the lattice, and the high polarizability of Se2− can weaken the binding energy between the Na-ion and the anion framework.119,384

A core advantage of sulfide materials is their relative softness as compared to ceramics, which in principle will allow some level of reversible deformation of the SSE−metal interface during cycling. In practice, the oxide-based SSEs (except for BASE) are indeed stable, whereas the sulfide ones are very sensitive to air exposure and offer the possibility for evolving gas during oxidation, including H2S.339,385−387 Another major downside to sulfides as applied to SMBs, versus to SIBs, is that Na metal thermodynamically reacts with most thiophosphates (e.g., Na3PS4 type) to form poorly ion conducting Na2S and other reaction products. While approaches based on composite thiophosphate SSEs have been kinetically effective, the decomposition is thermodynamically driven and hence bound to happen eventually. In our opinion, improved oxide-based SSEs may represent a more promising approach for Na-metal anodes, while the sulfides are better coupled with high-capacity alloying systems such as Sn, Sb, and their combinations. In general, the relatively low ionic conductivity, combined with high interface resistance due to reactions and loss of contact, limits the utility of single-phase SSEs.135,388 We already discussed one variation of a hybrid system meant to overcome these issues, based on an oxide combined with an IL. A specific class termed hybrid solid electrolytes (HSEs) have been proposed, combining inorganic electrolytes with polymer electrolytes.389,390 Recently, the HSEs based on the mixture of NASICON (Na3Zr2Si2PO12) powder and polymer precursor (PVDF-HFP) have been reported.119,391 Kim et al. achieved an ionic conductivity of 1.2 × 10−4 S cm−1 at 0 °C from NaCF3SO3-TEGDME-soaked NASICON-PVDF-HFP matrix.391 The HSE showed a wide voltage window of 5 V.391 This indicates that Na-metal anodes could be applied to highvoltage cells in combination with HSE and high-voltage ceramic cathodes. If aiming for a Na-S and a Na-Se cell, however, high-voltage stability is not required. These cells operate at a maximum of 2 V, which still creates all the concerns regarding the SEI layer on the anode but eliminates the CEI formation issues. While ceramic electrolytes present minimal danger of selfignition, dendrite penetration from the anode to the cathode nevertheless still represents catastrophic even that ends the life of a cell. One example is the Na superionic conductor (NASICON) with the Na1+3xZr2(P1−xSixO4)3 structure, which reaches an-ionic conductivity of ∼10−3 S cm−1 at T ≳ 65 W


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°C.104,135,392 NASICON is plagued by both large interfacial impedance, presumably due to a secondary reaction at the anode−electrolyte interface, and anode dendrite formation through the grain boundaries and possibly pores of the sintered material (Figure 21).393,394 The elastic moduli of ceramic electrolytes are in the range of tens to hundreds of GPa.314 For instance, beta-alumina solid electrolyte (BASE), sodium superionic conductor (NASICON), and sodium-thiophosphate (75Na2S·25P2S5) have the elastic modulus values of ∼290, ∼75, and 18 GPa, respectively.320,363,395 Monroe and Newman predicted that a 1.8 higher shear modulus of the protection layer is required to mechanically suppress Li-metal dendrites.184,185 This would in principle mean only 5.2 GPa is required to suppress Na dendrite growth. However, Na dendrites are known to propagate along pre-existing SSE defects, including pores and grain boundaries. This renders the SSE elastic modulus, yield strength, etc. not a very good predictor for dendrite resistance. We argue that other SSE features, e.g., Na-metal wetting behavior, may be better indicators of resistance to Na dendrite penetration. A quantitative method for predicting dendrite growth through SSEs does not exist to date, and clearly more work is needed on that topic. It is known that sintered ceramics are never fully dense, possessing residual porosity.396 Sintered SSEs are polycrystalline with a fine nanometer-scale grain size and ample grain boundary interfacial area. This indicates that there is always the possibility of dendrite growth in a wide range of oxide and sulfide SSE materials. In the next section, we will discuss approaches that have been successful in addressing dendrite growth, at least for specific SSE systems. Likely a similar methodology will be effective for other SSEs, although we emphasize again that currently there is not enough experimental or modeling evidence to state this conclusively. In the case of liquid electrolyte SMBs, intimate interfacial contact with the cathode is not an issue since the electrolyte wets the ceramic or sulfur particles. However, with SSEs interfacial contact may be a critical issue since the electrolyte is elastically stiff and does not flow around either electrode.397−399 Even with an initially good electrode−electrolyte interface, cycling-induced chemical/electrochemical reactions and volume changes may cause separation. A cycling-induced increase of interfacial resistance is reported in virtually all solid state sodium batteries.399−402 In addition, if the Na-metal anode and the SSE are not in fully uniform contact, there will be preferential-ion-flux to regions of better contact, which will cause uneven stripping/plating, leading to dendrites.403 A high mechanical strength of ceramic SSEs may actually be a negative, since it does not allow the electrolyte to “flow” during cycling to maintain uniform interfacial contact. Hence, metal− SSE interfacial chemical and mechanical integrity is another key research topic that needs further exploration. It may turn out that, in general, unmodified SSEs are not a satisfactory solution to dendrite growth, and further interfacial design is required, just as it is with liquid electrolyte systems. The problem of solid state dendrite growth then becomes one of achieving low and isotropic Na-SSE interfacial energy. If the interfacial energy is high, wetting is poor and non-uniform. Dendrites will then propagate through the regions where the interfacial energy is the lowest. With poor wetting by the Na metal, the interfacial impedance is also unacceptably high, which means the cell will be even more resistive than usual for solid systems.

Other outstanding questions concerning SSEs and Na metal include the following: How do the mechanical properties of the dendrite affect its penetration into the ceramic? Could a dendrite actually “push” its way into separating the ceramic from the primary metal foil, or do dendrites always follow a path of pre-existing porosity or grain boundaries? Is there solid−state crossover of elements from the various ceramic, S, or Se cathodes through the electrolyte into the Na metal? If so, what is their effect on interfacial impedance and on dendrite nucleation, growth? Because of potential issues of crossover, even in the solid state, strongly advocate treating the dendrite problem holistically. What we mean by “holistically” is that the optimum cell architecture for understanding Na dendrites includes a fully functional cathode and a limited amount (i.e., thinner foil) of Na. Studies based on symmetric cells with oversized yet effectively limitless Na sources are scientifically interesting. However, even for SSEs, they may not parallel the growth phenomenology of full battery cells. Of course, the same argument applies to liquid electrolytes: Studies of dendrite growth should ideally take account of cathode effects as well.

6. SODIUM METAL−ELECTROLYTE INTERFACIAL ENGINEERING Designing a robust and stable SEI is one of the key enabling technologies for SMBs.9 For it to function, the SEI layer would have to be electrically insulating, chemically and electrochemically stable over the needed voltage range.141,207 An ideal SEI for SMBs should also possess a high and uniform Na-ion permeability through a wide range of temperatures, charge rates, and voltage conditions. The SEI must possess mechanical toughness (combination of strength and ductility) so as to tolerate the volume changes during plating and stripping. We believe that a key emphasis should be on uniformity of elastic and plastic properties over the entire SEI film surface, as local weak points are a likely contributor to dendrite growth. The natural SEI formed on the Na metal in standard carbonate electrolytes does not satisfy the above conditions, as dendrite growth is nearly universally observed. Other electrolytes discussed earlier have been shown to be more effective. As will now be discussed, tuning the metal− electrolyte interface represents a complementary strategy for addressing dendrite growth that may be applied to virtually any liquid or solid electrolyte system. Since Na metal is highly catalytic toward unstable SEI growth, a promising method to stabilize the interface is to introduce artificial barriers between the metal and the electrolyte, which minimizes or altogether prevents their contact.408,409 Of course, the SEI layer still forms, but now it is located on top of a nominally much less reactive secondary material. It is recognized that any artificial layer for the metal anodes should be mechanically robust. Over many cycles, it needs to withstand the underlying plating and stripping volume changes of the Na metal and the stresses associated with SEI formation. Of course, it also needs to resist the penetration of dendrites, where such structures can nucleate beneath it.410 There is also the implicit requirement that the process of introducing the interlayer not be too technologically difficult, as otherwise the benefit of Na versus Li is lost.185,403 The artificial interface approach has been widely employed for Li-ion anode materials such as Si,411−413 as well as for Limetal anodes.414−417 With Li-ion anodes, artificial nanometerscale ceramic layers (e.g., Al2O3, Cu3N, Li3PO4,) have shown X


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Figure 22. (a) Thickness of sodium bromide layer on sodium metal at various nominal reaction times. (b) Visualization of sodium electrodeposition on pristine Na (left) and NaBr-coated Na (right). Na was deposited at 1 mA cm−2 in the electrolyte of 1 M NaPF6-EC/PC. (c) Diffusion energy barriers computed for Na adatoms on various surfaces associated with SEI components. The red bars denote surface in contact with vacuum; blue bars indicate contact with acetonitrile. The NaBr layer was introduced by 15 μL of bromo-propane onto Na-metal electrode at varied time. Adapted with permission from ref 207. Published 2017 Open Access under Creative Commons Attribution 4.0 International License.

Figure 23. (a) Light optical image of Na-metal surface before and after Al2O3 coating. (b) Cycling behavior of Al2O3-coated Na-metal electrode, at 3 mA cm−2 in 1 M NaSO3CF3-DEGDME. Cross-sectional view of (c,d) pristine Na-metal and (e,f) Alucone (aluminum alkoxide)-coated Na-metal anode after 10 cycles at 1 mA cm−2 with the capacity of 1 mAh cm−2, 1 M NaPF6-EC/PC. Panel a adapted with permission ref 118. Copyright 2016 John Wiley & Sons, Inc. Panel b adapted with permission from ref 430. Copyright 2017 John Wiley & Sons, Inc. Panels c−f adapted with permission from ref 432. Copyright 2017 American Chemical Society.

to yield an intrinsic trade-off between ion flux (which decreases with thickness) and mechanical stability (rigidity, ductility).169,417,418 We suspect that similar concerns may be encountered with Na metal as with alloying electrodes: Due to the large volume expansion of the underlying anode, the thinner coatings cannot remain intact while thicker coatings create unacceptable Na-ion diffusion resistance. As it takes only a single mossy dendrite penetrating through the barrier to catalyze unstable SEI growth, the integrity of the interlayer is

essential for long-term lifetime. However, if the ion diffusion kinetics are sufficiently poor through the barrier, it loses its effectiveness as well. One could expect that thick enough barriers would introduce enough diffusional limitations to begin growing dendrites due to Sand’s time considerations. It remains to be determined whether any coating can survive, e.g., 2000 cycles, an aggressive duty requirement for a secondary battery. Y


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Figure 24. (a) Schematic of the free-standing composite protective layer for Na-metal electrode. (b) Illustration of transferring free-standing graphene film onto a Na-metal surface. (c) High-temperature reaction interlayer (or dry polymer interlayer) that promotes isotropic wetting of Na on NASICON and thereby suppresses dendrite growth. (d) Schematic illustration of the polymeric ionic liquid film formation on the metal anode. Panel a adapted with permission from ref 403. Copyright 2017 American Chemical Society. Panel b adapted with permission from ref 408. Copyright 2017 American Chemical Society. Panel c adapted with permission from ref 104. Published 2017 under ACS AuthorChoice License. Panel d adapted with permission from ref 107. Copyright 2017 John Wiley & Sons, Inc.

To date, there have been various types of protective films for a range of Li electrodes, such as Al2O3, amorphous carbon, organic polymer, Nafion, etc.5,419−423 Gaseous precursors such as N2 and acetylene424−426 have been employed. Much less work has been done of the Na-metal system. Archer et al. utilized a NaBr layer as the artificial interface between the Nametal anodes and the electrolyte.207 The motivation for this was the rationale that an inorganic NaF-rich SEI layer would be more tolerant for the plating−stripping interfacial volume changes than the standard carbonate-based SEI. The artificial inorganic NaBr layer was formed by immersing the Na-metal anode into 1-bromopropane prior to cell assembly. The advantage of this strategy is that the thickness of the artificial SEI can be controlled by adjusting the exposure time (Figure 22a).207 The introduced NaBr layer was effective in a carbonate electrolyte, suppressing the formation of porous dendrite (Figure 22b).207 Calculations demonstrated that as the size of the anion in the halide increased (F→ Cl → Br), the Na-ion diffusion barrier energy decreased (Figure 22c).207 These findings bring up a key requirement for an artificial barrier, which is that while mechanical integrity is essential, a high Na-ion diffusivity is also necessary.409 An ultrathin Al2O3 layer was first employed to stabilize Limetal anodes.427,428 A study using molecular dynamics simulations revealed that Al2O3 is first lithiated to its thermodynamically stable phase (Li3.4Al2O3), which is then the active barrier material during lithiation−delithiation.429 The role of Al 2 O 3 coating on Na-metal anode was subsequently investigated by several groups (Figure 23).118,430 Hu et al. prepared an Al2O3-coated Na-metal anode using atomic layer deposition (ALD).118 A core advantage of ALD is that the coatings are tunable to be conformal and of nearly exact single nanometer-scale thickness.431−433 Due to the low melting temperature of the Na metal, deposition was performed using low-temperature (75 °C) plasma-enhanced atomic layer deposition (PEALD).118

The Al2O3-coated Na-metal anode showed stable voltage hysteresis in the Na plating−stripping process for 450 h.118 Sun’s group demonstrated the stable operation of the Al2O3coated (3.5−7 nm) Na-metal anode during 750 continuous Na plating−stripping cycles.430 In addition, Han and co-workers predicted via modeling that, unlike the Li case (forming a stable Li−Al−O glass tube),429 a thermodynamically stable NaxAl2O3 glass does not form.434 However, studies of the Al2O3-coated Na show the formation of a Na−Al−O complex during the initial cycles.430,431 Interestingly, Han et al. predicted that Na-ion diffusivity in NaxAl2O3 is actually higher than Li-ion diffusivity in LixAl2O3, even though there is a threshold in the Na-ion content for Al2O3.434 This implies that an optimized Al2O3 coating may be effective at high charging rates, which is where dendrite growth is expected to be the most severe. A core advantage of ALD layers is also their drawback: Being on the order of only several nanometers thick, they provide facile solid state diffusion but are liable to fracture under applied stress. Additionally, it remains to be determined whether any coating layer that is applied only once (ALD or other methods) is able to withstand prolonged cycling at various rates and temperatures. Other authors have physically “mounted” the artificial protective layer on the Na-metal surface.403,408 For instance, Kim et al. physically mixed PVdF-HFP (poly(vinylidene fluoride-co-hexafluoropropylene)) and Al2O3 powder in organic solvents. Then, they pasted the mixture on the glass substrate. After peeling of the Al2O3−polymer hybrid films, they intimately mounted the hybrid film on the Na-metal anode by roll-processing (Figure 24a).403 In the study, they revealed that the optimized inorganic−organic hybrid composite layer is elastically stiff (shear modulus >6 GPa) and is effective in promoting uniform Na plating−stripping.403 In a similar concept, graphene can be physically transferred onto Na metal after peeling off CVD-grown graphene from a substrate (Figure 24b).408 This strategy is electrolyte agnostic, Z


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Table 5. Various Interlayer Barriers for Na Metal and the Associated Cycling Overpotential interlayer/Na

counter electrode

NaBr/Na graphene/Na Al2O3/Na Al2O3/Na

NaBr/Na graphene/Na Al2O3/Na Al2O3/Na

Alucone/Na Al2O3-PVDF-HFP/Na in situ formed interlayer/Na CPMEA/Na polymerized IL/Na

Alucone/Na Al2O3-PVDF-HFP/Na in situ formed interlayer/Na CPMEA/Na stainless steel

electrolyte M NaPF6-EC/PC M NaPF6-EC/Pc M NaPF6-EC/DEC M NaSO3CF3DEGDME 1 M NaPF6-EC/PC 1 M NaClO4-EC/PC NASICON 1 1 1 1

NASICON 1 M NaClO4-EC/PC + 20% IL

current density (mA cm−2)

stripping capacity (mAh cm−2)

1 2 0.25 3

0.5 3 0.125 1

3 0.5 0.15

1 1 0.15

0.2 1

0.2 0.167

overpotentiala (mV) (cycles) ∼150 ∼450 100 20

ref

(250) (100) (900) (750)

207 408 118 430

400 (150) >1000 (127) ∼50(167)

432 403 104

∼100(192) ∼50 (100)

104 107

When the overpotential is not specified, the overpotential (stripping process) was estimated from the figures.

a

adding to its overall utility.185 It would also allow for precise control of the layer thickness, perhaps better than what is possible by wet chemistry methods etc. Graphene possesses excellent chemical stability in various organic electrolytes, showing the potential as a protective layer material for metal anodes.435 Despite the interesting chemical properties, graphene coatings on Li metal have shown varying levels of effectiveness.436,437 For example, the effect of graphene as an interfacial layer to stabilize Li metal was not sufficient unless Cs ions were also added.437 In contrast, a graphene coating worked effectively for Na-metal anodes.408 Those authors were able to achieve a dendrite-free Na-metal anode using the graphene coating at a high current density (2 mA cm−2) with a capacity of 3 mAh cm−2 for 300 cycles.408 Although graphite, graphene, and CNTs are very elastically stiff along their basal planes, in practice graphene sheets are highly compliant. Therefore, we do not consider the possibility of a wet or dry deposited graphene layers being effective in mechanically constraining dendrites. This is one core difference between graphene coatings and ceramic systems such Al2O3. By acting as a Na-ion porous chemical barrier, graphene layers would prevent excessive reaction of the metal with the electrolyte.438 We argue that most of the efficacy of graphene coatings come from this enhanced chemical/electrochemical stability rather due to mechanical constraint effects. If the graphene layer is able to create an ionically porous barrier between the electrolyte and the Na metal, the SEI would be formed on the much less catalytic carbon surface. The SEI would be more stable and would lead to less mossy and lathlike dendrite growth. Goodenough et al. introduced a high-temperature reaction layer or a dry polymer interlayer between Na metal and solid electrolyte (NASICON) (Figure 24c).104 As discussed prior, without such interlayer, Na dendrites were able to readily penetrate the sintered NASICON SSE by diffusing along defects, presumably grain boundaries. At low temperatures, the ceramic is poorly wetted by the Na. A polycrystalline ceramic will intrinsically contain a distribution of grain boundary energies leading to anisotropic wetting by the Na metal, which is more severe when the interfacial energies are high. To obtain acceptable wetting and interfacial impedance, the NaNASICON-Na trilayer had to be heated at 380 °C, forming a secondary reaction layer at the interface that was wetted uniformly. In a separate experiment, a polymer interlayer was employed to achieve comparable isotropic wetting without the high-temperature anneal. With the dry polymer, NASICON

grain boundaries do not contact directly with Na metal, and it is rather the ion-conducting polymer that is wetted. The polymer layer possesses an isotropic surface energy, which leads to homogeneous wetting and no dendrites during cycling. Some authors reported that an ionic polymer membrane coating, formed by in situ by electrochemical polymerization of IL additive, can effectively protect the Na-metal anodes (Figure 24d).107 While it is recognized that ILs are currently too expensive to function as the main electrolyte components in batteries (maybe not in the future), small additions are fully feasible. Per Table 5, it may be observed that a number on interlayers are to a varying extent, although more work is needed to substantiate their long-term cycling utility at various temperatures, rates, calendar times, etc.118,403,430,432 It should also be noted that for an actual battery operation, an electrode needs to deliver the areal capacity of at least 3 mAh cm−2,8 which is often higher than what is reported in scientific studies. This is another key area that needs further exploration: Are the artificial or in situ formed barriers that are employed at relatively low currents similarly effective near 3 mAh cm−2? Is there a critical current density where existing barrier solutions stop serving their function? Could barriers be tailored for fast charging, and what are the associated microstructural design rules?

7. ELECTRODE ENGINEERING 7.1. Sodium-Metal Hosts, Templates, and Membranes

It is well-known that in some cases, dendrite growth can be delayed by lowering the effective current density.1,439,440 With Na there is evidence of rate dependence of the plating process as well, with the less formation of high-surface-area dendritic structure at lower current densities.441 A high surface area electrically conductive host could have this effect, since the metal is now being plated onto a support that may be up to several orders of magnitude higher surface area than the underlying planar current collector.442,443 The improved electrochemical performance of the metal anode with the porous scaffold can therefore partly be attributed to the large surface area that impedes concentration polarization driven dendrite growth based per Sand’s model.1,62,444 A high surface area support will also act as dense template of heterogeneous nucleation sites for the plating Na metal. This would also refine the growth front and reduce the extent of dendrites since there would be more metal nuclei and a finer grain size.445−447 The observed improvement in plating−stripping CE and reduction AA


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Figure 25. (a) Comparison of Na deposition on planar versus porous Al current collector with a capacity of 1 mAh cm−2 in 1 M NaPF6-diglyme. (b) Schematic of the Na deposition process on a honeycomb-like 3D Ni@Cu. The hierarchical 3D Ni branch was created by electroplating Ni on the Cu current collector. Panel a adapted with permission from ref 448. Copyright 2017 American Chemical Society. Panel b adapted with permission from ref 449. Copyright 2018 Elsevier.

Table 6. Various Hosts for Na Metal and the Associated Cycling Overpotential host/Na

counter electrode

electrolyte

current density (mA cm−2)

stripping capacity (mAh cm−2)

overpotentiala (mV) (cycles)

.ref

porous Al/Na 3D Ni@Cu/Na r-GO/Na carbon felt/Na carbonized wood/Na carbon fiber/Na

Na Na same same same same

1 M NaPF6-diglyme 1 M NaPF6-DEGDME 1 M NaPF6-diglyme 1 M NaClO4-EC/PC M NaClO4-EC/DEC 1 M CF3SO3Na-diglyme

0.5 1 1 5 1 1

0.5 0.5 1 2 1 1

∼20 (1000) ∼10 (100) ∼20 (300) ∼100 (125) >200 (250) ∼20 (500)

448 449 116 450 451 458

When the overpotential was not specified, the overpotential (stripping process) was estimated from the figures.

a

been also employed for Na-metal anodes (Figure 25b).445,449 Overall, nanostructured metal hosts demonstrate a performance improvement, for instance, having the Na anode survive over 100 cycles at 0.5 mAh cm−2 (Table 6). One key advantage of carbon hosts over metals such as Cu and Ni is their light weight. While the weight of the host is never included in the gravimetric energy calculation, it is still there, adding to the overall weight of the current collector and of the system. Some authors have correctly argued that an additional benefit of a flexible carbon host is the ability to buffer the plating−stripping volume expansion,450,451 akin to the established approach for alloy and conversion anode materials. Carbon-based 3D materials, such as carbon felt, carbonized wood, and graphene foam, have also been explored with this rationale.450−453 Mostly, the Na is allowed to electrochemically plate onto the carbon support during the first and the subsequent charging cycles. Some carbon hosts may also be pre-sodiated by dipping them into molten Na.450,451

of dendrites with high surface area supports is a combination of these two effects. In some cases, supports may also prevent the electrolyte from contacting the Na metal directly, although fully open structures would not be able to achieve this. Relatively high surface area metal supports have received attention in literature, having the key advantage of being readily directly integrated into the current collector, or actually being the current collector itself. For example, authors showed that the 3D porous Al current collector can provide sufficient surface area for Na nucleation and improved Na plating− stripping homogeneity.448 After plating Na at 1 mAh cm−2, the planar Al foil showed spotty Na-metal surface whereas the 3D Al current collector displayed even Na deposition (Figure 25a).448 The advantage of this 3D Al current collector is effective even in the carbonate-based electrolyte of 1 M NaClO4-EC/DEC, showing the small overpotential less than 0.05 V for 600 cycles of plating−stripping.448 Non-reactive metallic hosts based on porous Ni@Cu and porous Cu have AB


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Figure 26. Illustration of Na infusion into (a) carbon felts and (b) carbonized wood. In Na/C composite anodes, Na is infused by immersing carbon hosts into molten Na. Panel a adapted with permission from ref 450. Copyright 2018 John Wiley & Sons, Inc. Panel b adapted with permission from ref 451. Copyright 2017 American Chemical Society.

Figure 27. (a) Schematic illustration of the flow-aided sonochemistry (FAS) exfoliation method for fabricating graphene with tuned structure and chemistry. (b) Raman spectra of “defect-free” graphene AES-G and chemically/structurally defective graphene IPS-G. (c,d) X-ray photoelectron spectroscopy of AES-G and IES-G. (e,f) Coulombic efficiency of AES-G and IPS-G during Na plating−stripping cycles. Adapted with permission from ref 172. Copyright 2018 American Chemical Society.

Such an approach is facile for laboratory-scale research and has good potential for scalability. In principle, a metal or even a conductive ceramic host would be similarly amendable to such a molten impregnation methodology, as long as they are sufficiently wetted. Highly pure porous carbon hosts are not wetted by molten Li or Na,454 and heteroatom functional groups are needed to facilitate penetration. Such carbon hosts, with a tuned surface chemistry to promote a low wetting angle, were classified as a “sodiophilic matrix”.445,450,455

Figure 26 shows Na-metal electrodes with a 3D carbon host. These electrodes are prepared by dipping 3D carbon host into molten Na to fill the void space or coat Na on the host surface. Such an approach makes high Na loading possible, an advantage for systems such as Na-S where the cathode does not contain its own Na source. Such hosts allow cells to be operated at high current densities with minimal dendrite formation. For instance, Chen et al. showed that a Na-metal anode with 3D graphene foam can operate even at 16.5 mA cm−2 without noticeable dendrite formation.456 Pint and coAC


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Figure 28. SEM top-down images of the post-100 cycles electrode surfaces in the fully plated condition: (a,b) defect-free graphene host AES-G, (d,e) highly defective graphene host IPS-G, and (g,h) bare Cu current collector tested as a baseline. Dendrite growth−SEI schematics are shown to their right. Panels c, f, and i adapted with permission from ref 172. Copyright 2018 American Chemical Society.

workers designed the nanocarbon-coated Al current collector, where the carbon coating served as a nucleation layer for Nametal plating. 457 The resultant architectures exhibited significantly improved cycling stability, demonstrating over 1000 cycles with a low-voltage hysteresis of 14 mV. The authors demonstrated a high Na loading of 12 mAh cm−2 and a current density of up to 4 mA cm−2. While numerous carbon and non-carbon hosts improve cycling to a varying degree, there does not seem to be a fully systematic study comparing various classes of materials in terms of their basic properties. For instance, how does carbon support surface chemistry, bulk defect content−degree of graphitization, pore size distribution, elastic compliance, the ability to store Na in its bulk, etc. affect the plating−stripping behavior? We believe that there is a need for basic studies Na plating on carbon that much akin to what has been performed over the last several years on Na storage in carbon. For instance, surface oxygen moieties and microporosity profoundly increase SEI formation in carbons tested down to 10 mV vs Na/Na+. An SEI layer would cover similar carbons when they are employed as Na plating supports at 0 V. Since SEI has a profound influence on Na dendrite growth, one may speculate that a highly oxygen-rich carbon may make a less effective host than a purer carbon of comparable geometry. However, how does one balance this intuitive guess with the established observation that a carbon requires surface oxygen to be thermally wetted by molten Na during impregnation? Are the two effects similar, i.e., electrochemical wetting also require oxygen functionalities, or are they unrelated? Similarly, the role of the metal host surface chemistry and its propensity to form SEI are not understood in relation to its effectiveness as a host. If Cu is

more catalytic toward SEI formation that the naturally passivated Al, does this mean that it would be less of an effective host for Na plating? Even with the open supports, which nominally do not have the function of preventing the Na metal from contacting the electrolyte, we still argue that SEI formation is the key for dendrite control. Of course, much more work is needed in this area to turn such hypothesis into a broadly applicable materials design principle, and researchers are encouraged to pursue this direction. A carbon host serves a dual role as Na nucleation template and as a protective layer to keep the metal from severely reacting with the electrolyte. As discussed, graphene and related carbons are widely employed for templates and protection layers to improve metal plating behavior, albeit with efficacy that varies substantially from study to study. Mitlin and Liu have recently explored the poorly understood relationship between the host structure/chemistry, SEI growth, and metal dendrites.171,172 The authors employed a novel directional flow-aided sonochemistry (FAS) exfoliation method to synthesize graphene with highly ordered structure and minimal oxygen content (Figure 27a). Graphite flakes were exfoliated into single-nanameter-scale thickness graphene that is “defect-free”, termed at-edge sonication graphene, AESG. The same graphite flakes were exfoliated into highly structurally and chemically defective graphene, termed in-plane sonication graphene, IPS-G. The AES-G had a Raman G/D band intensity ratio of 14.3 and an XPS-derived oxygen content of 1.3 at.%, while IPS-G was at 1.6 and 6.2 at.%, respectively (Figure 27b−d). The two carbons were used to examine the role of structure and chemistry of graphene supports in promoting efficient NaAD


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Figure 29. Wetting behavior of (a) Na alloys at varied temperature and (b) Na dissolved in biphenyl and dimethoxyethane (Na-BP-DME) at room temperature on BASE surface. (c) Reversible redox reaction of Na-BP-DME, the charge/discharge capacity being limited to 5 mAh. (d) Cycling performance of Na2S8|BASE|Na-BP-TEGDME cell at a current density of 1.1 mA g−1 from 1.8 to 2.5 V. Panel a adapted with permission from ref 461. Panels b−d adapted with permission from ref 463. Published 2017 under Creative Commons Attribution 4.0 International License.

metal cycling in a standard carbonate electrolyte. It was demonstrated that graphene defects are actually quite harmful to efficient Na plating and stripping. AES-G yielded state-ofthe-art Na performance, with stable cycling at 2 mA cm−2 at 100% Coulombic efficiency (CE) and an areal capacity of 1 mAh cm−2 (Figure 27e). Meanwhile IPS-G performed on-par with the baseline Cu support in terms of severe charging instability (Figure 27f). The explanation put fourth was that the defective graphene demonstrates much more copious SEI formation due to its defects and oxygen groups being catalytic. A thicker SEI results in a larger overpotential and worse CE loss during subsequent Na plating−stripping, manifesting in severe mossy metal dendrite growth and periodic electrical shorts. The authors therefore proposed the following design rule for next-generation supports for Na metal: An ideal

architecture will not only possess a large surface area for copious preferred heterogeneous nucleation, but itself will be non-catalytic for SEI formation. More research is needed to substantiate this design rule for a broad range of hosts including non-carbon systems, as well as for various electrolytes. Figure 28 shows top-downSEM images of the fully Na plated electrode surfaces after the electrodes were cycled 100 times. Figure 28g,h shows the results of the Na metal deposited on baseline host-free Cu foil current collector. The Na metal is covered by SEI and shows a coarse “mossy” dendrite morphology with branches that are 1−2 μm in diameter. The true metal dendrites are finer, since the covering SEI layer is relatively thick. In general, it is taken that mossy dendrites form by base growth rather than by tip growth, although this AE


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8. FUTURE OUTLOOK AND PROMISING RESEARCH DIRECTIONS Since room-temperature SMBs are both technologically and scientifically new, the basic understanding of Na-metal SEI growth, structure, and chemistry is in its infancy. While SEI is qualitatively recognized to be a dynamic structure that grows and shrinks at every charge−discharge cycle, its time-/ratedependent phenomenology is not quantified either at the isolated electrode or at the full-cell level. Since the SEI−metal interface is by its nature a buried one, achieving new insight on process dynamics requires methods that can potentially “see through” model half-cells as well as realistic anode−electrolyte−cathode full batteries. Dynamic neutron scattering techniques focused on in situ and operando analysis are ideally suited to this mission, offering the needed beam penetration at the full cell level. Driving the experimental effort could also be an array of multiscale modeling. Modeling may include atomicscale ab initio for simulations of adsorption of Na on various defect sites in the composite SEI layer and metal supports. Approaches such as molecular dynamics simulations may address Na transport through the various phases of the SEI layer and their solid−solid and solid−pore interfaces. Continuum models could address stress evolution, elastic response, and plastic flow in both the SEI and the underlying Na-metal dendrites of various morphologies. In contrast to Li deposition, most fundamental questions about the dynamics of Na deposition remain unanswered. Considering the differences in size, ionicity, and reactivity of Na+ and Li+ ions, are the reversibility and efficiency of Na plating distinctly different from those of Li? If so, what are the differences at the atomic, meso, and macro scales? Could the approaches have developed for enabling reversible Li plating be trivially extended for Na? Perhaps the most important factor that determines the stability of Li deposition is the SEI formed. Which SEI components are the most effective in stabilizing Na deposition, and what are their properties? Such fundamental questions regarding the specific aspects of SEI interfacial chemistry and Na-metal microstructures (dendrites) remain unanswered. In the future, studies should determine the rate of parasitic Na loss and hence the cycling CE. In parallel, researchers should address solid-state electrolyte−Na metal interactions. The aim would be to identify the aspects of local chemistry and microstructure that control the transfer of Na ions from the solid electrolyte to the metal. The ideal approach would be based on a close connection between experiment and theory, with modeling and experimental experts working closely and in an iterative manner. More research is needed on coupled ex situ, in situ, and operando studies to probe bulk structural changes in the Nametal anode and SEI structure, chemistry, and morphology, e.g., growth and porosity. Post-mortem analysis should be coupled with dynamic studies on half-cells (in situ) and on full cells (operando) that include functional cathodes. Neutron scattering is ideally suited for studies on light elements such as Na, since these materials possess high neutron scattering cross sections due to nuclear interaction, as compared to their low X-ray scattering cross sections due to electron interactions. For instance, the element Na possesses a positive cross section of 3.38 barn, as compared to 5.551 barn for C, and 4.232 barn for O. This gives substantially higher detection resolution for metallic Na nanoparticles, as well as allows for improved local structure determination. The mean atomic nearest neighbors

needs further quantification for the case of Na metal. Figure 28a,b shows the same surface with an AES-G host, while Figure 28d,e shows the surface with an IPS-G host. The surface morphology post 100 cycles AES-G hosted Na electrode is uniform, while that of IPS-G is rough with dendrite growth being evident. AES-G graphene is highly ordered with much less structural defects. It also contains minimal nanopores and oxygen groups. A separate set of galvanostatic experiments was performed above the Na plating potential, with the lower cutoff voltage being 10 mV vs Na/Na+. It was demonstrated that employing a pristine vs a defective carbon structure leads to a major reduction in SEI formation, even prior to the onset of Na-metal plating. This was rationalized by the fact that SEI forms below about 1 V vs Na/Na+ and is therefore expected to be already present at the onset of the first plating cycle. A schematic cross-section of the defect-free AES-G and defective IPS-G carbon hosts and of the baseline Cu current collector is provided in Figure 28c,f,i. Per panel f, the excessive SEI in IPSG probably also leads to increased levels of electrically isolated “dead Na” entrapped in the SEI layer. 7.2. Room-Temperature Liquid-Metal Anodes

While most studies on metal batteries have focused on solid Na- or Li-metal anodes, room-temperature liquid anodes represent a novel underexplored approach. While it remains to be determined whether such an approach is viable for scaledup systems, where calendar life and safety are key concerns, the preliminary laboratory findings have been promising. Alkalimetal alloys, such as Na-Cs, Na-Rb, and Na-K, have eutectic points below room temperature.123,459,460 Goodenough et al. showed the utility of liquid Na-K with the acceptable stripping overpotential of 0.2 V, in a conventional carbonate-based electrolyte.123 Since plating−stripping stability in carbonate electrolytes is most difficult, we believe that using Na-K is a major finding that should be followed by additional research. Liu and co-workers investigated the liquid phase Na-metal anode for the liquid Na/BASE (β″-Al2O3 solid electrolyte)/S system and the Na-Cs anode, showing a superior life cycle compared to the pure solid Na.461 However, the cycling results were taken at 95−150 °C and may not be directly transferable for room-temperature SMBs. The authors also investigated the wettability of liquid Na-Cs, Na-K, and Na-Rb to BASE.461 In the systems using BASE, the wettability of Na to BASE is one of the most challenging issues to improve the electrochemical performance. The contact angle range of Na-Cs and Na-Rb to BASE is 62.4°−88.4° at 100 °C, which is promising (Figure 29a).461 This indicates that these liquid phase Na alloys may be applicable to intermediate-temperature Na-NiCl2 or NaFeCl2 batteries.462 However, room-temperature wetting is essential for SMBs, and for practical applications, even lower temperature thermodynamic wettability is needed. Recently, room-temperature dendrite-free liquid Na-metal anode has been reported by Chen et al.463 The liquid Na anode was prepared by dissolving Na metal into a solution of aromatic hydrocarbons (biphenyl, BP) and ethers (DME or TEGDME),463,464 showing a stable solution of Na-BP-ether at room temperature (Figure 29b). Figure 29c shows the reversible Na removal/uptake from Na-BP-DME electrode for 40 cycles with the capacity of 5 mAh. Na-BP-DME not only maintained its liquid phase, but also exhibited good wettability to BASE at room temperature, thus demonstrating a very long life cycle up to 3500 cycles in a cell configuration of Na2S8| BASE|Na-BP-TEGDME (Figure 29d).463 AF


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in amorphous SEI compounds, the structure of the nanocrystalline oxides, fluorides, etc. may be ascertained much more accurately than what has been done in the past with conventional X-rays, etc. Another key advantage of neutron diffraction is its ability to access much higher scattering vector Q range as compared to X-rays, yielding finer spatial resolution. Since neutrons are readily able to penetrate full cells, operando analysis on assembled Na-metal anode− electrolyte−ceramic or S−Se cathode systems is possible. Through combined experiments and modeling, researchers should elucidate the physical stress−strain and the chemical reactions of the Na metal with the dynamic SEI. How much do the evolved volatile species in the electrolyte and in the SEI influence the structure and chemistry of the Na metal? What if the Na dendrites, which by their nature possess a high surfaceto-volume ratio, actually act to trap and react with SEI evolved species? How do the starting electrolyte chemistry and the residual/evolved species affect the dynamics of Na growth? The existing literature reports on Na-metal morphology and growth offer a disparate phenomenology, even when similar carbonate or carbonate−glyme electrolytes are employed. It is recognized that SEI growth is associated with an increase in the interfacial resistance of the electrode, which manifests itself as an overpotential that increases with cycling. During cycling, this polarization is dissipated as localized heat. For the lowmelting Na metal and the associated polymer-rich SEI, there should be significant localized thermal effects, which to date have been minimally explored.

patents, all of which are or were previously licensed, and has presented over 110 invited, keynote, or plenary talks. Dr. Mitlin is an Associate Editor for Sustainable Energy and Fuels, a Royal Society of Chemistry Journal focused on renewables. He received a Doctorate in Materials Science from the University of California, Berkeley, in 2000, M.S. from The Pennsylvania State University in 1996, and B.S. from Rensselaer Polytechnic Institute in 1995.

AUTHOR INFORMATION

(1) Xu, W.; Wang, J. L.; Ding, F.; Chen, X. L.; Nasybulin, E.; Zhang, Y. H.; Zhang, J. G. Lithium metal anodes for rechargeable batteries. Energy Environ. Sci. 2014, 7, 513−537. (2) Cheng, X. B.; Zhang, R.; Zhao, C. Z.; Wei, F.; Zhang, J. G.; Zhang, Q. A Review of Solid Electrolyte Interphases on Lithium Metal Anode. Adv. Sci. 2016, 3, 1500213. (3) Wood, K. N.; Noked, M.; Dasgupta, N. P. Lithium Metal Anodes: Toward an Improved Understanding of Coupled Morphological, Electrochemical, and Mechanical Behavior. ACS Energy Lett. 2017, 2, 664−672. (4) Xin, S.; You, Y.; Wang, S.; Gao, H. C.; Yin, Y. X.; Guo, Y. G. Solid-State Lithium Metal Batteries Promoted by Nanotechnology: Progress and Prospects. ACS Energy Lett. 2017, 2, 1385−1394. (5) Cheng, X. B.; Zhang, R.; Zhao, C. Z.; Zhang, Q. Toward Safe Lithium Metal Anode in Rechargeable Batteries: A Review. Chem. Rev. 2017, 117, 10403−10473. (6) Mauger, A.; Armand, M.; Julien, C. M.; Zaghib, K. Challenges and issues facing lithium metal for solid-state rechargeable batteries. J. Power Sources 2017, 353, 333−342. (7) Lin, D. C.; Liu, Y. Y.; Pei, A.; Cui, Y. Nanoscale perspective: Materials designs and understandings in lithium metal anodes. Nano Res. 2017, 10, 4003−4026. (8) Lin, D. C.; Liu, Y. Y.; Cui, Y. Reviving the lithium metal anode for high-energy batteries. Nat. Nanotechnol. 2017, 12, 194−206. (9) Tikekar, M. D.; Choudhury, S.; Tu, Z. Y.; Archer, L. A. Design principles for electrolytes and interfaces for stable lithium-metal batteries. Nat. Energy 2016, 1, 16114. (10) Li, B.; Wang, Y.; Yang, S. B. A Material Perspective of Rechargeable Metallic Lithium Anodes. Adv. Energy Mater. 2018, 8, 1702296. (11) Whittingham, M. S. Electrical Energy-Storage and Intercalation Chemistry. Science 1976, 192 (4244), 1126−1127. (12) Whittingham, S., Chalcogenide battery. U.S. Patent US4009052A1977. (13) Lazzari, M.; Scrosati, B. Cyclable Lithium Organic Electrolyte Cell Based on Two Intercalation Electrodes. J. Electrochem. Soc. 1980, 127, 773−774.

Seung Woo Lee received his B.S. degree in Chemical Engineering from Seoul National University summa cum laude in 2004, and Ph.D. in Chemical Engineering from Massachusetts Institute of Technology in 2010. He joined the Woodruff School of Mechanical Engineering at Georgia Institute of Technology as an Assistant Professor in January 2013. Dr. Lee is an expert in electrochemical energy storage and conversion systems, which are the key enabling technologies to support fast-evolving consumer electronics and electric vehicles. Dr. Lee is the recipient of the National Science Foundation CAREER Award, the National Aeronautics and Space Administration Early Career Faculty Award, the Hanwha Advanced Materials Non-Tenure Faculty Award, and the Korean-American Scientists and Engineers Association (KSEA) Young Investigator Grant Award.

ACKNOWLEDGMENTS This work is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under Award No. DE-SC0018074. REFERENCES

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Byeongyong Lee: 0000-0002-3548-3642 Notes

The authors declare no competing financial interest. Biographies Byeongyong Lee received his B.S. degree from Seoul National University in 2006, and Ph.D. from Georgia Institute of Technology in 2018. Now he is a postdoctoral fellow at Georgia Institute of Technology. His current research interests mainly focus on highenergy-density materials, including Li- and Na-metal anodes. Eunsu Paek is an Assistant Professor at Clarkson University. in the department of Chemical & Biomolecular Engineering. Dr. Paek’s research focuses on developing theoretical foundations for guiding the rational design and synthesis of novel nanomaterials for energy and environmental applications. The Paek research group utilizes molecular-level computer simulations to investigate fundamental properties and processes in various nanomaterials, such as interfacial phenomena, solution dynamics, chemical reactions, and phase transformations. Dr. Paek received his B.S. and M.S. degrees from Seoul National University, Korea, and Ph.D. from the University of Texas at Austin. David Mitlin is a Professor and General Electric Chair at Clarkson University, in the Department of Chemical & Biomolecular Engineering. Prior to that, Dr. Mitlin was an Assistant, Associate, and full Professor at the University of Alberta, Canada. Dr. Mitlin has published about 140 peer-reviewed journal articles on various aspects of energy storage and conversion materials. He holds 6 granted AG


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Sodium Metal Anodes: Emerging Solutions to Dendrite Growth

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