Enabling Safe Sodium Metal Batteries by Solid Electrolyte Interphase

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Review

Enabling Safe Sodium Metal Batteries by Solid Electrolyte Interphase Engineering: A Review Edward Matios, Huan Wang, Chuanlong Wang, and Weiyang Li Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 23 May 2019 Downloaded from http://pubs.acs.org on May 28, 2019

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Industrial & Engineering Chemistry Research

Enabling Safe Sodium Metal Batteries by Solid Electrolyte Interphase Engineering: A Review Edward Matios,† Huan Wang,† Chuanlong Wang,† Weiyang Li†,*

†Thayer

School of Engineering, Dartmouth College, 14 Engineering Drive, Hanover, New

Hampshire 03755, USA *To

whom correspondence may be addressed. E-mail: [email protected]

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract With the merits of naturally abundant sodium (Na) resource and similar electrochemical characteristics to that of lithium-ion batteries, Na-based batteries have been widely studied as the next-generation economical and practical energy storage choices. Particularly, Na metal anode possesses high theoretical specific capacity of 1166 mAh/g and low electrochemical potential of −2.71 V (vs. standard hydrogen electrode), and it is therefore considered as the ultimate anode material for Na-based batteries. Nevertheless, the commercialization of Na metal anode is still largely hindered by several long-lasting challenges, namely metallic Na dendrite growth and unstable solid electrolyte interphase (SEI) formation. In this review, we first go over the fundamental mechanisms associated with these challenges. Then, we provide in-depth discussion on the recent key advancements from the perspectives of liquid electrolyte optimization, artificial SEI fabrication and solid-state electrolyte implementation. Lastly, we highlight the promising aspects from each strategy for the future development of Na-based batteries.

KEYWORDS: Sodium metal anode, solid electrolyte interphase, sodium dendrite suppression, interface engineering, solid-state electrolyte

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1. INTRODUCTION There is an urgent need to accelerate the advent of renewable energy and sustainable development to minimize the catastrophic consequences of ongoing climate change.1-3 Renewable energy generation from wind and solar remains to be the practical solution towards the curtailing of greenhouse gas emissions while meeting global energy demand. Meanwhile, the intermittency nature of these renewable energy sources requires reliable coupling with energy storage system to effectively supply electricity that would otherwise be produced by fossil fuel consumption. Rechargeable battery is the key enabling technology for renewable energy implementation because of its high energy conversion efficiency, long cycling life, low maintenance and scalability. Specifically, lithium-ion batteries (LIBs) have already dominated the market with their applications ranging from consumer electronics, electric vehicles and gridscale energy storage.4-10 The price of LIBs has been decreasing substantially in the past decade, with the current production cost reaching as low as $100/KWh at cell level.11-13 Moving forward, metallic Li is regarded as the ideal choice for anode owing to its high specific capacity, low density, and the lowest electrochemical potential.14-18 However, considering the natural scarcity, geographical constraint of Li resource as well as increasing material cost associated with Li derived electrodes, Li metal anode can be too expensive to be adapted by the market. In the quest of searching for lower cost and high performance alternatives, many of the recent studies focus on developing battery chemistries based on sodium (Na) material.19-30 With the merits of naturally abundant Na resource and similar electrochemical characteristics to that of LIBs, Na-based batteries have been widely studied for enabling more economical and practical battery systems. The early studies on Na-based batteries began with high temperature sodium-sulfur (Na-S) systems that have been commercialized as grid-scale energy storage. Unfortunately, the high operating temperature of 300 °C and corrosion problems hindered further developments of high temperature Na-S batteries. The research focus was then shifted to develop Na-ion batteries (SIBs), which exhibit similar electrochemical properties to that of LIBs. Various insertion-type cathodes for SIBs have been developed in the past decade, including layered transition metal oxides (i.e. NaFeO2), and polyanionic compounds (i.e. Na3V2(PO4)3).19-21 On the anode side, the first carbon based anode for SIBs was realized by Dahn’s group, demonstrating electrochemical reversibility of Na+ into hard carbon at room temperature.22,23 Since then, a number of nanostructured Na-alloy anodes (i.e. Na-Sn, Na-Sb, Na-

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P, etc) for SIBs have been developed by various sophisticated approaches.24-30 However, the specific capacities and cycling performances of these anode materials are still unsatisfactory. Similar to the role of Li metal anode, metallic Na is considered as the ultimate anode choice for high energy density Na metal batteries (SMBs). Na metal anode possesses high theoretical specific capacity of 1166 mAh/g, low electrochemical potential of −2.71 V (vs. standard hydrogen electrode) and exhibits overall similar electrochemical behavior as Li metal anode.31-34 The pairing of Na metal anode with high capacity S cathode or oxygen (O2) cathode leads to NaS and Na-O2 batteries with much higher theoretical energy density than the state-of-the-art LIBs.35-37 As a result, these high energy density batteries based on Na metal anode quickly spur intense research interests, leading to meaningful advancements on SMBs.38-44 Nevertheless, the commercialization of Na metal anode is still largely hindered by a number of challenges, including metallic Na dendrite growth, unstable solid electrolyte interphase (SEI) formation and large volume change.45 These challenges can be intensified when pairing Na metal anode with S and O2 cathodes. Na-S battery is highly promising because of the high theoretical capacity of S cathode (1672 mAh/g), which can deliver about 1200 Wh/kg specific energy based on the discharge product of Na2S. However, the low electrochemical utilization of S cathode material and polysulfide dissolution lead to low Coulombic efficiency and poor cycling performance.46-48 Na-O2 battery possesses even more impressive theoretical specific energy of around 1600 Wh/kg based on the discharge product of Na2O2. Nevertheless, Na-O2 chemistry suffers from undesirable discharge product formation, electrolyte instability and slow electrochemical reaction rate on the cathode side.49-55 So far, a variety of strategies have been developed to tackle each of the abovementioned challenges associated with Na metal anode. Minimizing Na dendrite growth requires sophisticated interface engineering to facilitate uniform Na+ deposition and therefore homogenous Na metal plating.29,32 Electrochemically stable SEI can be obtained through the modification of electrolyte system.33,34 Furthermore, the challenge of relatively infinite volume expansion can be resolved by stable Na host.56-58 In this review, we first go through a systematic overview of the major challenges for Na metal anode, namely electrochemically unstable SEI formation and metallic dendrite growth. Next, we provide in-depth discussion on the recent research progresses that aim to enable safe and practical SMBs with a specific focus on the state-of-the-art strategies tailored to resolve the challenges of metallic Na dendrite and unstable SEI formation from the perspectives of liquid

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electrolyte optimization, artificial SEI fabrication and solid-state electrolyte development. Lastly, we highlight the promising aspects from each of the solutions for the future development of Na metal anode and SMBs.

2. SODIUM METAL ANODE CHALLENGES The extremely reactive nature of Na metal anode leads to uncontrollable parasitic reaction with organic solvent liquid electrolyte, resulting in the formation of unstable SEI upon repeated Na stripping/plating, followed by SEI breakage that exposed fresh Na metal surface for subsequent preferential plating that promotes dendrite growth. The continuous breaking of SEI on Na dendrite structures leads to the unnecessary consumption of electrolyte, along with Na dendrite propagation that can penetrate separator that altogether result in low Coulombic efficiency and battery short-circuit. Meanwhile, the “host-less” nature of Na metal anode gives rise to large volume change during repeated Na stripping/plating. Overall, the three major challenges for Na metal anode are denoted in Figure 1: (1) unstable SEI formation, (2) metallic Na dendrite growth and (3) relatively infinite volume change. Herein, detail discussion on the fundamental for each challenge is presented. 2.1 Unstable Solid Electrolyte Interphase Achieving stable SEI formation on Na metal anode remains to be the major hurdle for realizing a safe and high performance SMB. Ideally, SEI should be highly conductive for ion transport while serving as a passivating layer to prevent further parasitic reactions between liquid electrolyte and electrodes. The early relevant research began with the investigation of SEI formation mechanism by Peled, who elucidated that SEI arises from the decomposition of liquid electrolyte induced by the highly reactive alkali metals.59 Goodenough et al. then proposed to describe the origin of SEI based on molecular orbital theory that involved the electrochemical potential of anode (μA) and cathode (μC), as well as the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) of a typical carbonate liquid electrolyte.60 As denoted in Figure 2a, carbonate solvent can be electrochemically reduced by anode and oxidized by cathode when the μA and μC lie outside the LUMO and HOMO energy levels, respectively. The spontaneous SEI formation gradually passivates the surfaces of the

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electrodes from further reacting with carbonate solvent. It is worth noting that conventional graphite anode used in LIBs has an electrochemical potential that is above the LUMO energy level of typical carbonate solvent (Figure 2b), and SEI formation in LIBs is the essential process during battery manufacturing. As a result, graphite anode reached a major commercial success partly because of the high quality SEI formation that is capable of sustaining thousands of charging and discharging cycles.61 SEI formation is a complex function of electrolyte components and electrochemical conditions, with the average thickness at around a few nanometers. The SEI thickness (T) can be approximated from a parallel capacitor model:15

where A = electrode area, ɛ = dielectric constant and C = capacitance. For SMBs, an ideal SEI should therefore be (1) Na+ conductive and electronically insulated across the surface to facilitate uniform Na+ deposition and minimize preferential Na plating; (2) electrochemically and chemically stable to prevent further reaction with electrolyte and (3) structurally robust to withstand volume change and dendrite propagation.32-34 Nevertheless, SEI formation is largely an uncontrollable process with spatial variation in ionic conductivity, therefore leading to non-uniform Na+ flux.62 Furthermore, the as-formed SEI serves to be a favorable site with higher surface energy that promotes dendrite growth.63 Repeated preferential Na plating results in unstable SEI and mechanically unstable dendrite that can detach from the Na metal surface and becomes “dead” Na. The newly exposed Na surfaces then further aggravate dendrite formation at the expenses of working ions, organic solvent and Na metal depletion.64 Continuous electrolyte decomposition and Na metal detachment lead to a thick layer of high impedance SEI.65 Consequently, SMBs with undesirable SEI properties inevitably suffer from subpar electrochemical cycling performance and low Coulombic efficiency. 2.2 Metallic Dendrite Similar to metallic Li anode, Na metal anode is plagued by dendrite growth upon repeated stripping and plating that leads to severe battery safety issue. There are a few existing models that describe the fundamental mechanism of dendrite growth. The mosaic model states that metallic alkali metal anode and its SEI exhibits surface roughness and non-uniform spatial

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distribution, resulting in locally intensified current densities.63 Moreover, high surface area dendrite tips can self-amplify preferential Na plating, and they are susceptible to gradual dissolution due to the fragile nature. This inevitably leads to the exposure of fresh Na surface, resulting in uneven spatial distribution of SEI and therefore high cell overpotential.66,67 In-situ microscopic observations of randomly propagated dendrite growth have visually confirmed the high surface area needle-like structure.68 Subsequently, Chazalviel model was proposed to describe metallic dendrite growth as the direct result of space charge upon anion depletion in close proximity to the electrode.69 With this model, the ramified dendrite deposition can be predicted to advance at a speed just equal to the velocity of the anions in the applied electric field, and the presence of this space charge ahead of the deposition front is associated with a potential drop. It was subsequently discovered that Na nucleation regimes can be revealed by in-situ Na nuclear magnetic resonance (NMR) to experimentally confirm that Na plating behavior is current density dependent.70 As expected, dendrite formation behaves quite differently at various current densities. Under a relatively low current density, the number of observed nucleation sites increase as a function of time. However, all nucleation sites were activated simultaneously on Na surface at high current densities, hence forming dendritic morphology with high specific surface area. Additionally, dendrite growth is also largely dependent upon system temperature. Generally, lower system temperature increases ion diffusion resistance and decreases surface film thickness, thereby favoring dendrite growth during electrodeposition.71 Battery charging style also plays a significant role in dendrite formation. Given the fact that dendrite growth rate being proportional to the magnitude of current density, faster charging rate can therefore accelerate dendrite propagation.72 The relationship between dendrite formation and current density can be modeled by the Sand’s behavior.14,15 For Na metal anode that undergoes a high plating current density in electrolyte, Na+ is expected to be rapidly depleted, and therefore its concentration near the anode surface decreases to zero at time τs, known as the Sand’s time. Subsequently, the strong negative electric field attracted Na+ within a very short time, leading to dendrite growth. This is known as the Sand’s behavior, and the corresponding Sand’s time (τs) is defined as the initiation time of dendrite growth:73

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here c = cationic mobility, a = anionic mobility, e = electron charge, cationic charge number,

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= current density,

=

= initial cation concentration and D = ambipolar diffusion coefficient.

The nucleation mechanism and surface chemistry of Li dendrite have been extensively investigated over the past decade for the effort of enabling safe and practical Li metal batteries.7482

However, there had been a lack of in-depth comparison study on Li and Na dendrites until

Hong et al. recently published a comprehensive work about this topic.83 In the study, in operando experiments were designed to reveal the chemical, mechanical and electrochemical stability of Na and Li dendrites under quasi-zero electrochemical field (QZEF), which is defined as a battery system with its external electrochemical process been halted. Despite the fact that both Li and Na are part of the alkali metal group, Na metal intrinsically exhibits higher chemically reactivity and weaker mechanical structures due to larger atomic radius and weaker metallic bonding, as shown in Figure 3a. Li and Na dendrites were prepared by Li+ and Na+ depositions on the respective electrodes under the identical electrochemical condition. In order to study the electrochemical and chemical stabilities of the newly formed dendrites and SEIs, a comparison experiment was carried out by settling the as-deposited Na and Li dendrites under QZEF condition for 6 hours in the similar electrolytes of 1 M LiPF6 in dimethyl carbonate (DMC)/ethylene carbonate (EC) and 1 M NaPF6 in DMC/EC, respectively (Figure 3b). It can be observed by scanning electron microscope (SEM) that Na dendrite gradually shrunk and dissolved into the electrolyte while Li dendrite remained largely intact during the settling process, suggesting that Na dendrite is more chemically vulnerable than that of Li dendrite. The chemical stabilities of pristine Na and Li metals were further studied by placing them into the respective electrolytes for various time periods for real-time observation, as shown in Figure 3c. The results revealed that soaked pristine Na metal surface was full of rough pits, a clear indication of parasitic reaction with electrolyte during soaking. Meanwhile, soaked pristine Li metal surface remained smooth, suggesting a stable SEI formed between metallic Li and electrolyte during soaking that inhibited further reaction. Subsequently, the mechanical rigidity of both Na and Li dendrites were investigated by applying mechanical forces. The fluid shear force removed a relatively small portion of the asdeposited Li dendrite, while Na dendrite was completed detached from the metallic Na. Hence, it can be concluded that Li dendrite is more mechanically robust than that of Na. Lastly, Na/Na and Li/Li symmetric cells were assembled to study the respective electrochemical cycling

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performances (Figure 3d). As expected from the abovementioned observations, Li/Li symmetric cell exhibited relatively less fluctuating voltage hysteresis during the stripping and plating cycles at various current densities. The exacerbated voltage polarizations associated with Na/Na symmetric cell indicated unstable SEI formation, which is consistent with the previous conclusions from in-situ observations.84,85 These valuable findings on Na metal characteristics are critical to the understanding of dendrite and SEI behaviors. 2.3 Volume Expansion Conventional intercalation type anode such as graphite undergoes about 10% volume change to accommodate ions within the specific sites. In comparison, Na metal undergoes much greater volume expansion and contraction during the electrochemical plating and stripping process as the Na+ deposition on anode surface is largely an uncontrollable process, which can in turn destroy the anode structure upon repeated cycles.86 As a result, the volume change of metallic Na anode is considered relatively infinite. Not only that the anode structure is under tremendous stress upon the large volume change, the as-formed SEI can be easily ruptured and thus exacerbate the dendrite challenge. The apparent strategy to overcome the volume change challenge is by employing a highly stable host to accommodate Na+. Nevertheless, the implementation of Na host on the anode side increases the overall weight and therefore reduces the total energy density of SMBs. As a number of great review papers on Na metal hosts already exist,19-25,56-58 this review will focus on the solutions tailored to overcome the challenges of unstable SEI and Na dendrite formation.

3. SOLUTIONS FOR SODIUM METAL STABILIZATION SEI is the critical component of SMBs that dictates the interfacial properties, electrochemical stability, cycling efficiency and Na plating morphology. Realizing a desirable SEI with both electrochemically stability and mechanically rigidity through liquid electrolyte optimization is therefore an effective solution.87 Modifying liquid electrolyte composition to enable safe SMBs through stable SEI formation is also a standard practice and attractive strategy from the commercial viewpoint because it requires little or no change to cell manufacturing process nor pre-treatment of electrodes. Another effective method to stabilize Na metal anode is

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by constructing an artificial SEI on Na metal anode prior to cell assembly.88-91 There are two major advantages of an artificial SEI. Firstly, an artificial SEI can eliminate any direct contact between the highly reactive Na metal anode and liquid electrolyte, thus preventing unnecessary parasitic reaction and liquid electrolyte consumption.92 Secondly, it is possible to tune the properties of an artificial SEI for maximizing the performance of a particular cell configuration without affecting the overall electrochemical process. Meanwhile, the challenge of Na dendrite formation from the uncontrollable parasitic reactions between metallic Na anode and liquid electrolyte can be effectively resolved by employing non-flammable solid Na+ conductor as an electrolyte into SMBs. In fact, solid electrolytes not only can eliminate the severe safety concerns of SMBs, they can also enhance electrochemical stability and prolong cycling life.93-99 In the following content, an in-depth review is given on liquid electrolyte modification, artificial SEI and solid electrolyte implementation to enable safe and high performance SMBs. 3.1 Liquid Electrolyte Optimization 3.1.1 Additives in Carbonate Electrolyte Carbonate electrolyte is predominantly used in commercialized LIBs. Thus, incorporating additives into carbonate electrolyte for enhanced SEI properties and improved battery performance can have profound positive impact on the battery industry.100-102 Despite its successful application in LIBs, carbonate electrolyte implemented in SMBs generally delivers unsatisfactory performance as a result of the parasitic reactions between the carbonate solvents and Na metal anode. Upon contact with carbonate solvent, undesirable SEI composed of mainly Na2CO3, NaO2CH and NaO2COR forms on Na anode. This SEI is permeable to carbonate solvents and can self-trigger further dendrite growth that results in high voltage overpotential even at a relatively low current density of 0.1 mA/cm2.103 Empirical evidences prove that a SEI composed of mainly inorganic NaF can effectively protect Na metal from further reacting with the electrolyte, therefore minimizes the irreversible capacity loss of SMBs.104-106 Archer’s group demonstrated experimentally that incorporating a simple metal fluoride additive into carbonate electrolyte can indeed improve the cycling performance and electrochemical stability drastically.104 Dugas et al. discovered that fluoroethylene carbonate (FEC) additive can spontaneously form a thin yet mechanically robust and homogenous NaF protective layer on metallic Na.105 Meanwhile, Lee et al. demonstrated a

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novel electrolyte consists of FEC with 1 M sodium bis(fluorosulfonyl)imide (NaFSI) salt that can deliver stable and dense deposition of Na metal during electrochemical cycling.106 Figure 4a compares the Coulombic efficiency of Na/Cu cells in 1 M NaFSI–EC/propylene carbonate (PC) (v:v=1:1), 1 M NaFSI–EC/PC (v:v=1:1) + 1 wt% FEC, and 1 M NaFSI–FEC at 0.28 mA/cm2 current density. FEC-based Na/copper (Cu) cell retained 94% Coulombic efficiency over 100 cycles at 0.28 mA/cm2 current density with consistently low overpotential, far superior than that of EC/PC-based cell. This result suggests that dendritic Na deposition can be effectively addressed by robust and Na+ permeable SEI containing NaF and sodium alkylcarbonates that come from FEC–NaFSI electrolyte. Very recently, Luo’s group incorporated SnCl2 additive into carbonate electrolyte for forming NaCl and Na-Sn alloy inorganic interface to substantially improve Na metal anode performance.107 Impressively, the Na metal anode cell with Na/SnCl2 incorporated electrolyte and Na3V2(PO4)3 cathode exhibited high capacity retention over cycling and excellent rate capability (101 mAh/g at 10 C), as presented in Figure 4b. To sum up, SEI with inorganic components that arise from FEC solvent or relevant inorganic additives can be highly desirable in carbonate electrolyte as it can successfully passivate Na metal. 3.1.2 Ether Electrolytes Compared to carbonate solvents, ether solvents exhibit relatively higher LUMO energy level and it is therefore less reactive towards Na metal anode.108,109 Additionally, higher LUMO energy level facilitates the preferential decomposition of salts rather than solvents, thereby forming Na halide SEI with lower surface energy barrier and higher Na+ conductivity.110 While ether solvent is electrochemically unstable beyond 4 V, it is still a promising solvent for SMBs that typically operate below 4 V. Tetraethylene glycol dimethyl ether (TEGDME) solvent has been demonstrated to deliver stable Na stripping/plating performance with extremely low overpotential for hundreds of cycles.111 Meanwhile, dimethyoxyethane (DME) solvents exhibited excellent compatibility with Na metal compared to conventional carbonate electrolyte.112 A systematic study conducted by Cui’s group revealed that an ideal SEI should consist of inorganic Na2O and NaF in order to be impermeable to organic solvents and while being ionically conductive.113 Importantly, this study showcased that Na/Cu cells cycling with 1 M NaPF6 in glyme electrolytes that enable an exceptional reversibility (99.9% Coulombic efficiency) at 0.5 mA/cm2 because of the mechanically compact and solvent impermeable SEI (Figure 4c). After-

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cycled analysis with X-ray photoelectron spectroscopy (XPS) revealed that the decomposition of NaPF6 in glyme electrolyte produced desirable SEI composed of inorganic Na2O and NaF.113 Additive also plays an important role in ether electrolyte to provide further stabilization of Na metal. Our group recently discovered that sodium polysulfide (Na2S6) can serve as an effective additive or pre-passivation agent in ether electrolyte to improve the cycling stability and reversibility of metallic Na, as illustrated in Figure 4d.39 Not only does the Na2S6 additive greatly enhanced the stripping/plating performance of Na metal, the after-cycled Na metal electrodes from cells with 0.033 M Na2S6 additive exhibited a smooth morphology, while severe dendritic/mossy structures were observed for the case without any additive (Figure 4e). XPS analysis concluded that the SEI formed with Na2S6 as an additive is mainly composed of Na2O, Na2S2, and Na2S, which is robust enough to protect the Na surface from further reacting with the electrolyte components. Na metal anode can also be effectively stabilized by bi-functional electrolyte additive with potassium bis(trifluoromethylsulfonyl)imide (KTFSI).114 K+ were observed to preferentially adsorbed onto the plated Na, leading to electrostatic shielding for more homogenous Na+ flux. As a result, this bi-functional electrolyte additive promotes stable Na stripping/plating at a high capacity of 10 mAh/cm2 for hundreds of cycles. Na salt also plays a relevant role in SEI formation. The inner layer of Na metal SEI consists of various inorganic components that are largely influenced by the specific Na salt decomposition. For instance, SEI composed of NaF, NaCl and Na2CO3 can be observed with NaPF6 salt, NaClO4 salt and NaTFSI salt, respectively.115 Amongst these salt choices, NaPF6 salt allows for the highest SMB efficiency and performance because of the beneficial SEI composed of NaF. Overall, an ether electrolyte is less reactive towards Na metal anode thanks to the relatively higher LUMO level. 3.1.3 High Concentration Electrolytes The stripping/plating performance of Na metal anode is also predominantly determined by the electrolyte concentration. Conventional electrolytes are prepared at ~1 M concentration because the conductivity tends to peak at this level. However, recent studies found that more concentrated electrolytes can result in a far superior high rate performance.116,117 As shown in Figure 5a, higher salt concentration contains more cations per unit volume such that cations proximity is significantly closer, resulting in much shorter ionic flux transport distance.117 Also, more ions can be effectively coordinated with reactive solvent, thus decreasing the

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amount of available un-coordinated solvent that can parasitically react with freshly plated alkali metal anode.118-120 For SMBs, high salt concentration can alter Na nucleation with nanostructured Na deposition and decrease interfacial impedance, leading to increased mass transport-limited Na+ plating current that consequently promotes dendrite growth.121,122 The concentration effect of NaFSI in N-propyl-N-methylpyrrolidinium bis(fluorosulfonyl) imide has been investigated by molecular dynamics (MD) simulation on the coordination environments with various salt concentrations.85 The findings suggested that significant clustering between Na+ and FSI- can was created under high salt concentration, where each FSI- coordinates with 4 or 5 Na+.85 Lee et al. experimentally confirmed the effectiveness of an ultraconcentrated electrolyte composed of 5 M sodium bis(fluorosulfonyl)imide in DME solvent that exhibited outstanding cycling performances for symmetric Na/Na cells compared with the conventional dilute electrolyte (Figure 5b).123 Furthermore, this ultraconcentrated electrolyte is highly compatible towards high-voltage Na cathodes with outstanding full cell cycling stability. Nevertheless, increasing electrolyte concentration comes with the disadvantages of high viscosity, poor wettability, and expensive salt cost. To address these shortcomings, Zhang’s group recently developed a localized high-concentration electrolyte by utilizing hydrofluoroether as an “inert” diluent to maintain the solvation structures of ultraconcentrated electrolyte, as schematically illustrated in Figure 5c.124 This localized high-concentration electrolyte consists of 2.1 M NaFSI/(DME)–bis(2,2,2-trifluoroethyl) ether (BTFE) with solvent molar ratio 1:2 can deliver dendrite-less Na stripping/plating with >99% Coulombic efficiency at ultra-high 20C current density for 40000 cycles. As shown in Figure 5d, the localized high-concentration electrolyte has the merit of measurably low viscosity compared to the high concentration electrolyte. Moreover, Na/Na3V2(PO4)3 cells with localized high-concentration electrolytes yielded significantly higher discharge specific capacity than the highly concentrated electrolyte (Figure 5e). This superior performance can be attributed to the inertness of bis(2,2,2trifluoroethyl) ether diluent that does not interfere with the localized Na+−FSI-−DME solvation structures while improving the interfacial reaction kinetics and stability of Na metal anode. Therefore, the superior performances in rate capability and stability enabled by high concentration electrolyte can be attributed to the increased cations proximity and decreased availability of reactive solvent.

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3.1.4 Ionic liquids Ionic liquids (ILs) are room temperature molten salts consist of organic cation and organic/inorganic anions with inherently desirable properties, such as high thermal stability, low vapor pressure and wide electrochemical window.125-128 Temperature and moisture content are two critical factors that dictate IL-based battery performances.129 Higher temperature leads to increasing free moving Na+ for more rapid ion-transportation as a result of weaker coordination between Na+ and the solvent anion,130 as the coordination interaction is particularly sensitive to temperature. Chen et al. reported that elevated operation temperatures significantly improves the performance of SMBs contain IL with bis(fluorosulfonyl)amide (FSA) anion and sodium manganese orthosilicate cathode, as depicted in Figure 6a.131 At the same time, increased moisture content in ILs can cause rapid cell degradation and high impedance SEI formation due to the parasitic reaction between traceable amount of water and metallic Na. Hence, Na/Na symmetric cells cycled with ILs contain various high moisture content consistently lead to higher overpotential and more profound cathodic peak that indicates impeded Na+ deposition.132-134 In terms of compatibility with Na metal anode, ILs with FSI- anion has shown to be highly stable by reductively generate radical anions to passivate Na metal, hence it is regarded as a “magic anion” for stable SEI formation and capable of delivering superior SMBs cycling perfornamnce.135 Meanwhile, ILs contain bis(trifluoromethanesulfonyl)imide (TFSI-) also demonstrated desirable properties as coupled with Na metal anode.136,137 Remarkably, the cation coordination and electrolyte safety tests suggested that IL with TFSI- exhibits non-flammability.128 Moreover, the SEI induced by IL with TFSI- possesses both better mechanical and electrochemical stability than that derived from conventional liquid electrolytes. Compared to the thinner SEI formation induced by FSI-, the relatively thicker SEI formed with TFSI--based IL has also demonstrated to stabilize alkali metal anode.138,139 Overall, ILs with TFSI- are ideal for battery applications with stringent safety requirement because of their exceptional thermal stability, non-flammability, as well as the ability to effectively suppress Na dendrite formation.140-143 Archer’s group reported a room-temperature Na-S cell that utilizes microporous carbon–sulfur composite cathode coupled by carbonate electrolyte with 1-methyl-3-propylimidazolium-chlorate IL tethered to SiO2 nanoparticles (Figure 6b).143 Electrochemical analysis revealed that these functionalized SiO2 nanoparticles formed a Na+ conductive film on the anode surface that stabilizes Na deposition, enabling these Na-S cells to deliver stable cycling at a rate of 0.5C with 600 mAh/g reversible

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capacity and nearly 100% Coulombic efficiency (Figure 6c). Meanwhile, ILs contain immobilized anions have been proven to improve the stability of alkali metal plating by decreasing the electric field on the metal electrode.144 To conclude, IL with FSI- anion can enable chemically stable SEI formation on Na metal, while IL with TFSI- anion can induce mechanically robust and thicker SEI with exceptional thermal stability, thus both FSI- anion and TFSI- anion can lead to improve SEI quality and minimizing dendrite growth. 3.2 Artificial Solid Electrolyte Interphase 3.2.1 Free-Standing Protective Layer Modifying Na metal anode surface with a pre-fabricated protective thin film can serve as an effective artificial SEI to greatly enhance the electrochemical performance of SMBs. Firstprinciples computations on the interaction mechanism between Na+ and protective film materials at the atomic level indicated that layered materials with higher structural defects can lead to higher bond strength and proximity effect for improved Na+ conductivity.145 Notably, Open Quantum Materials Database has identified around 120 coating materials exhibiting chemical equilibrium with Na metal anode as promising artificial SEI.146 Our group have recently presented a highly stable and dendrite-free Na metal anode over a wide current density range and long-term cycling via directly applying free-standing graphene films synthesized by chemical vapor deposition (CVD) with tunable thickness on Na metal surface (Figure 7a).38 Our findings revealed that only a few nanometers differences in the graphene thickness can have decisive influence on the stability and rate capability of Na anodes, where the high density of defects within graphene layers serve as the uniform Na+ diffusion channels and thereby greatly suppress dendrite formation. To achieve the optimal performance, the thickness of the graphene film covered on Na surface needs to be meticulously controlled based on the applied current density. We demonstrated that with only a few layers of graphene film (~2.3 nm in thickness) as a protective layer, stable Na cycling behavior can be achieved in carbonate electrolyte without any additives over 100 cycles at a current density of 1 mA/cm2 with a capacity of 1 mAh/cm2, as presented in Figure 7b. Composite protective layer consists of structurally rigid Al2O3 inorganic particles and liquid electrolyte-swollen poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) polymers has been recently demonstrated by Kim et al.147 As schematically illustrated in Figure

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7c, this highly Na+ conductive composite protective layer can be directly laminated onto Na metal to mechanically suppressed dendrite growth and increase the electrochemical cyclability. The enhanced cycling performance can be attributed to the higher shear modulus of the composite protective layer that is critical to the mechanical suppression of dendritic metal growth based on the linear elastic model (Figure 7d), and the increased volume fraction of Al2O3 strengthens the mechanical structure of the composite protective layer. Another straightforward approach towards artificial SEI was proposed by Li et al. through the directly use of commercialized carbon paper as a protective layer on Na metal anode, leading to improved electrochemical stability and significantly decreased stripping/plating overpotential (Figure 7e).148 The positive effect is due to the large surface area of carbon paper that covers on Na metal anode, promoting homogenous Na+ flux across electrolyte/electrode interface. Experimental evidence showcased the substantially improved cycling stability of Na metal anode with the carbon paper protection (Figure 7f). The symmetric cell with carbon paper protected Na electrodes cycling in a typical ether electrolyte at 0.5 mA/cm2 current density and 1 mAh/cm2 capacity revealed that its overpotential gradually stabilized during initial few cycles and eventually maintained at a small but non-zero value (10-20 mV) for more than 100 cycles, while the overpotential of pristine Na symmetric cell quickly overshot to above 1.4 V after 40 cycles. Overall, free-standing protective layer can be an effective strategy to eliminating detrimental reactions between the interface of Na anode and liquid electrolyte. 3.2.2 Chemical Reaction with Sodium Na metal can be used as a substrate for chemical reaction to directly form a stable artificial SEI on its surface. Inspired by the natural tendency of magnesium (Mg) to form smooth deposition morphology because of its low surface diffusion barrier,149 Archer’s group proposed a facile strategy for subduing Na dendrites with NaBr coating that possesses comparable ionic transport energy barrier to Mg (Figure 8a).110 Metallic Na can undergo Wurtz reaction with 1Bromopropane to yield a homogenous 2-10 μm thick NaBr coating. The resulting NaBr coating creates an exceptionally low energy barrier to Na+ transport, and the enhanced surface diffusion at the electrode/electrolyte interface drastically smooths the deposition morphology. As illustrated in Figure 8b, the Na/Na symmetric cell with NaBr coating delivered 250 cycles at 0.5 mA/cm2 current density and 0.5 mAh/cm2 capacity with considerably more stable voltage

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overpotential than the control cell. Meanwhile, Archer’s group also developed an electrochemical constructed artificial SEI via in-situ electro-polymerization of 1,3-diallyl imidazolium perchlorate (DAIM) IL monomer to form a rubber-like film and open morphology on Na metal anode (Figure 8c).150 This chemically robust membrane markedly improved cycling performance of SMBs even at a relatively high current density. Furthermore, the ionic membrane served as a tethered supporting electrolyte to reduce the electric field at Na anode, which can in turn promote homogenous deposition of Na and minimize dendrite growth. Na/Na symmetric cells with DAIM additive increased the cycle life by three times at a relatively high current density of 1 mA/cm2. This IL membrane protected Na anode was further evaluated in Na-S chemistry, as shown in Figure 8d. As expected, the protected Na-S cell delivered nearly 100% Coulombic efficiency with a reversible discharge capacity of ~400 mAh/g after 200 cycles at 0.2C. In contrast, the control cell exhibited fluctuated Coulombic efficiency with the discharge capacity went below 200 mAh/g after only 100 cycles. Therefore, these works showcased the effectiveness of constructing an artificial SEI layer on Na anode surface by directly using Na metal as a substrate for suitable chemical reactions. 3.2.3 Inorganic Thin Film Deposition Atomic layer deposition (ALD) is a subclass of CVD with the sequential use of a gas phase chemical process for highly uniform thin film deposition with atomic scale percision.151-153 Owing to these merits, ALD has been extensively used in electrode surface coating and interface modification to improve the electrochemical stability and minimize parasitic reaction.154-156 Inorganic Al2O3 thin film coating on Li electrodes via ALD have been thoroughly investigated to validate its effectiveness as an artificial SEI.157-159 Subsequently, Sun’s group applied ALD to fabricate < 10 nm thick homogenous and dense Al2O3 coating on Na metal enabled by two half reactions between trimethylaluminum (TMA) and H2O, and this nanoscale Al2O3 coating was confirmed by the lack of Na signal through Rutherford backscattering spectrometry on Al2O3coated Na.160 The subsequent symmetric cell cycling test of Al2O3 coated Na vs. pristine Na at a current density of 5 mA/cm2 and 1 mAh/cm2 capacity revealed that Al2O3 coated Na symmetric cell demonstrated much lower initial overpotential at ~40 mV that remained extremely stable after 100 cycles with flat voltage plateau and without obvious increases in voltage hysteresis, while the control cell had a sudden voltage drop after only 20 cycles due to short circuit and

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progressed with fluctuating voltage in the following cycles. For SMBs, Al2O3 coating is particularly desirable because it can be sodiated during the deposition process and form highly Na+ conductive NaAlOx.161 Low-temperature plasma-enhanced ALD (PEALD) technology presented by Hu’s group demonstrated the operation below the melting point Na to form a thin Al2O3 layer for passivating Na metal anode from reactive solvent.162 Figure 9a depicts the Al2O3 coating process via PEALD and the proposed surface reaction that involves the sequential pulsing of TMA and O2 plasma with an extensive Ar purge in between. Na metal anode can be successfully isolated from liquid electrolyte as a result of the conformal Al2O3 coating, thus significantly suppressed dendritic mossy Na formation and prolonged cell cycle life in carbonate electrolytes.162 3D high surface area Na dendrites can be clearly observed on after-cycled Na without Al2O3 coating. For comparison, the Al2O3-coated Na metal remained smooth with no obvious 3D Na dendrite growth observed after cycling for 450 hours, which suggests a more uniform deposition/stripping process of the Al2O3-coated Na metal (Figure 9b). To summarize, the ability of utilizing ALD to deposit ultrathin and uniform inorganic coating on Na metal anode can be highly beneficial to the longevity and cycling performance of SMBs. 3.2.4 Inorganic-Organic Layer Deposition While ALD is limited to forming inorganic artificial SEI, molecular layer deposition (MLD) is a vapor phase thin film deposition technique based on self-limiting surface reactions for producing organic and organic-inorganic artificial SEI.163,164 Notably, artificial SEI formed by MLD exhibits improved mechanical robustness while maintaining structural flexibility because of the single covalent bonding from C-C and C-O within the organic linkers.165-167 As shown in Figure 9c, controllable inorganic-organic alucone coating via MLD has been demonstrated by Sun’s group to effectively protect Na anode from dendritic and mossy Na formation and stabilize electrochemical cycling in carbonate electrolyte.168 The coating of alucone via advanced MLD as a protective layer on Na metal anode was carried out by sequential exposures of TMA and ethylene glycol (EG). This mechanically flexible inorganic–organic coating passivated metallic Na as a stable SEI layer upon plating and stripping cycling, thereby significantly enhancing electrochemical performance under various current densities. Figure 9d compares the symmetric cell performances with alucone coated Na electrodes and pristine Na electrodes at a current

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density of 1 mA/cm2 with a capacity of 1 mAh/cm2. In conclusion, it can be evidently observed that alucone coated Na electrodes delivered far superior voltage stability with insignificant hysteresis increase for more than 250 hours. 3.3 Solid-State Electrolytes 3.3.1 Fundamental There are generally three types of solid-state electrolyte (SSE): inorganic ceramic SSE, inorganic sulfide SSE, and organic polymer SSE. Although they can greatly enhance battery safety and electrochemical stability, SSEs are still plagued by the long-lasting challenges of low ionic conductivity and high interfacial impedance. Na+ conductivity of an inorganic SSE is determined by the number of free moving Na+ per unit volume, as well as the quantity of structural defects within SSE.169,170 Based on the Schottky and Frenkel defects model, Na+ transport occurs through random hoping among common structural defects consist of interstitial ions as well as ion vacancy sites. Overall. Na+ conductivity of an inorganic SSE (σ) can be expressed by Arrhenius equation:99

with

= Arrhenius pre-exponential factor, EA = activation energy of diffusion, kB = Boltzmann

constant, and T = absolute temperature in Kelvin. As indicated by the Arrhenius equation, the movement of Na+ within the inorganic SSE structural skeletons is highly dependent on temperature,

as

higher

temperature

leads

to

considerably

faster

Na+ diffusion.171

Polymer SSE, on the other hand, conducts Na+ through its segmental motion. Specifically, Na+ in polymer SSE coordinates with the polar groups on segmental chains, such that Na+ sequentially hops from one coordinating site to another through the polymer segmental motion. Vogel-Tammann-Fulcher equation can be used to describe the ionic conductivity of polymer SSE (σ):99

here

= Arrhenius pre-exponential factor, T = working temperature in Kelvin, T0 = reference

temperature (about 50 K below the experimental glass transition temperature), and B = pseudo-

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activation energy for ion conduction. Na+ conductivity of polymer SSE is therefore exponentially correlated to the working temperature. Electrochemical impedance spectroscopy (EIS) is frequently used to quantify the ionic conductivity of SSE, with a typical setup with SSE in between two ion-blocking electrodes. The ionic conductivity (σ) can be calculated by:

S = cross-sectional area of SSE, l = thickness of SSE, and Rt = total resistance of electrolyte. 3.3.2 Ceramic Electrolytes Among the various Na+ conducting ceramic SSEs, Na+ superionic conductor (NASICON) Na1+xZr2SixP3-xO12 (0 ≤ x ≤ 3) first reported by Goodenough et al. remains to be the most promising material because of its relatively high ionic conductivity, excellent electrochemical and thermal stability.172-176 Na3Zr2Si2PO12 (x=2) exhibits the highest room temperature Na+ conductivity at about 5 10-4 S/cm.177 Traditionally, NASICON is synthesized by solid-state reaction at around 1200 °C with excess Na and P precursors to minimize the evaporations of Na and P resources and impurity formation such as ZrO2 during the high temperature treatment.178181

Moving forward, sol-gel method has also been widely applied to NASICON synthesis as it

requires lower processing temperature and yields more homogenous mixing of precursors at molecular level.182 Notably, a liquid-phase sintering method has been reported as well, capable of synthesizing NASICON at significantly lower processing temperature of 700 °C with the presence of 9 wt% Na3BO3.183 There are a number of bottleneck regions in NASICON structure, including two in the Na1-Na2 and the other two in the Na1-Na3 channels as depicted by Park et al. in Figure 10a.177 The low ionic conductivity can be largely due to the small size of bottleneck regions in NASICON structure that impede Na+ movement. Therefore, constructing ceramic SSE with greater number of free moving Na+ with sufficiently large bottleneck regions can effectively improve conductivity.184 Recent studies revealed that increasing ionic conductivity can be achieved by increasing bottleneck areas by substituting Zr sites of NASICON octahedrally coordinated alkaline earth cations with larger ionic radius.185,186 Trivalent cations are highly

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promising substitution ion for NASICON to increase bottleneck areas, especially Sc3+ with similar ionic radius as Zr4+.187 Blending other trivalent cations such as Nd3+ or Y3+ into NASICON can also improve the Na+ transport of NASICON.188 Altering NASICON structure with low solid solubility dopants can lead to the formation of a conducting secondary phase that enhances grain boundary conductivity.189 Meanwhile, higher ionic conductivities can also be achieved with higher annealing temperature or lower silicon content that result in larger crystallite sizes.190 The development of solid-state Na batteries is also hindered by the large interfacial charge transfer resistance between electrodes and solid electrolyte. Our group recently reported the direct growth of ultrathin graphene-like layer on NASICON by CVD to significantly decrease the interfacial resistance, thereby improving all-solid-state Na plating/stripping cycling stability at a high current density of 1 mA/cm2 with uniform Na plating morphology (Figure 10b).44 The lattice spacing of NASICON ceramic particle and graphene-like interlayer with 3−4 nm thickness can be clearly observed by transmission electron microscope (TEM) in Figure 10c, and the boundary between NASICON and graphene-like interlayer can be easily identified. Figure 10d presents the Nyquist plots of Na/Na symmetric cells with and without graphene-like coated NASICON. Evidently, the graphene modified NASICON effectively decreases the interfacial impedance of Na/Na symmetric cell by more than 10-fold. Besides CVD method, a thin interlayer can also be physically coated on NASICON to improve the interfacial contact. Goodenough’s group proposed the use of melted Na metal or poly(ethylene glycol) methyl ether acrylate (CPMEA) as a wetting agent to improve the interfacial properties between Na electrodes and NASICON.191 In their study, it was shown that NASICON remains stable toward Na metal at lower temperatures (175 °C), and gradually reacts with molten Na at temperatures over 300 °C as a stable interlayer on NASICON surface was formed (Figure 10e). Likewise, CPMEA-coated NASICON electrolyte with NaTi2(PO4)3/Na full cells demonstrated a stable capacity of around 102 mAh/g and 99.7% Coulombic efficiency at 0.2 C under 65 °C for 70 cycles, suggesting enhanced interfaces stability during the charge/discharge process (Figure 10f). To sum up, NASICON can a highly reliable ceramic SSE that can deliver stable SMB cycling performance and physically suppress Na dendrite growth. 3.3.3 Sulfide Electrolytes

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Sulfide SSE has the advantages of relatively high Na+ conductivity (~10-3 S/cm).192-195 The first promising sulfide SSE developed was a glass-ceramic Na3PS4 with high room temperature Na+ conductivity at 2 10-4 S/cm.196 The high Na+ conductivity of glass-ceramic sulfide arises from the stabilization of a high-temperature phase through glassy state crystallization. Na3PS4 SSE synthesis involved the mechanochemical ball milling of 75 mol% Na2S with 25 mol% P2S5. Notably, utilizing raw Na2S with high purity can further increase the Na+ conductivity of glass-ceramic Na3PS4.197 Similar to ceramic SSE, Na+ conductivity of sulfide SSE can be effectively enhanced by introducing interstitial Na+ and Na+ vacancies. Both firstprinciples studies and experimental results verified the enhanced Na+ conductivity as a result of interstitial Na+ introduction from aliovalent cation doping.169,198 Bo et al. worked on relevant MD simulations and proposed that additional Na+ vacancies can increase the number of mobile Na+ and thus greatly influences the overall Na+ conductivity.199 Specifically, a perfect stoichiometric Na3PSe4 with additional 2.1% Na+ vacancies reaching an exceptionally high theoretical room temperature Na+ conductivity of 3 10-2 S/cm was successfully synthesized based on the MD simulation data. The structural parameters of the as-prepared Na3PSe4 was confirmed by the Rietveld fitting of the synchrotron XRD data that matched with the MD simulations (Figure 11a). Moreover, inspired by their Li-conducting counterparts of Li7P3S11 and Li10GeP2S12, the development of Na7P3S11 and Na10GeP2S12 that can provide high Na+ conductivities of ~1 10-3 S/cm based on the computational simulation data is an ongoing research effort.200,201 However, it has been experimentally challenging to produce pure Na7P3S11 and Na10GeP2S12 due to the incomplete mechanochemical reaction from Na2S and P2S5 starting materials.196 Hence, there is yet to be a report on the successful synthesis of these sulfide SSEs with high purity due to the measurable amount of impurity phases containing Na2S and P2S5. Recently, pure Na11Sn2PS12 has been successfully fabricated by Duchardt et al., which is capable of achieving an unprecedented 3.7 10-3 S/cm Na+ conductivity.202 Bonding valence site energy calculations indicated that the ultra-high Na+ conductivity of Na11Sn2PS12 can be linked to lattice vacancies that interconnect the Na+ transport pathway in a 3D manner, as presented in Figure 11b. Although they are generally equipped with the highest Na+ conductivity amongst all SSE types, sulfides SSEs are plagued by their chemically instability in air and extremely sensitivity to moisture.203 The reason behind this detrimental instability is the relatively weak P-S bonding that is easily reacted with both O2 and H2O to form toxic H2S gas.204 Therefore, it is necessary to

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improve the chemical stability of sulfide SSE. Chemically weak P-S bonding can be partially replaced by relatively stronger As-S bonding through As5+ doping, and the resulted Na3P0.62As0.38S4 has been empirically proven to be more stable against H2O.205 Meanwhile, Na3SbS4 has been successfully synthesized by replacing P-S bonding with Sb-S bonding to further improve chemical stability against H2O, while exhibit a high room temperature Na+ conductivity of 3 10-3 S/cm.206 On the other hand, chemical encapsulation can effectively minimize instability of sulfide. For example, recent study showed that Na2Se-Ga2Se3-GeSe2 with crystal phase encapsulated in a glass matrix was relatively inert towards moisture.207 Very recently, Tian et al. employed a reactivity-driven strategy to improve the interfacial stability between Na3SbS4 and Na metal anode by forming a protective hydrate coating through the air exposure of Na3SbS4 (Figure 11c).208 Both first-principles calculations and experimental characterizations identified the new hydrated phase to be Na3SbS48H2O with adequate Na+ conductivity, and this hydrated phase partially reacts with metallic Na metal to forming NaH and Na2O as a practical passivating interface. Figure 11d compares the cycling performances of Na/Na symmetric cells with surface-hydrated Na3SbS4 and non-hydrated counterpart. Clearly, a substantially smaller voltage hysteresis can be observed during the cycling of surface-hydrated Na3SbS4 symmetric cells at 0.1 mA/cm2 current density, indicating the enhanced interfacial stability and effective passivation enabled by the hydrate protective layer. Together, these sulfide materials demonstrated the performance capability of electrochemically stable and safe all-solidstate Na-based batteries. 3.3.4 Solid Polymer Electrolytes Na+ conducting solid polymer is generally composed of a polymer host and corresponding Na salt. Common polymer hosts include polyethylene oxide (PEO), polyacrylonitrile (PAN), poly(methyl methacrylate) (PMMA) and polyvinylidene fluoride (PVDF).209-211 Because of the sluggish polymer segmental motion at room temperature, most polymer electrolytes have relatively low room temperature ionic conductivity at around 10-4 S/cm or less. Despite the drawback of low ionic conductivity, polymer materials hold great potential for commercialization due to the ease of manufacturing. Na+ conductivity of polymer SSE can be improved by increasing the amorphous region within the polymer host and the concentration of dissociated Na+. Adding ceramic filler or co-polymerization, can effectively

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increase the Na+ conductivity by decreasing the crystallinity regions of polymer SSE.212-214 There are two types of ceramic filler: active filler such as NASICON that can selfconduct Na+, and inactive filler such as SiO2 that does not participate in Na+ conduction. A composite hybrid solid electrolyte consists of PVDF-hexafluoropropylene (HFP) polymer, NASICON additive and a small amount of ether electrolyte proposed by Kim et al. exhibited a relatively high room temperature conductivity of 3.6 10-4 S/cm (Figure 12a).212 The EIS analyses in Figure 12b quantified the total resistance of the NASICON + PVDF-HFP composite film, liquid electrolyte, and hybrid solid electrolyte to be 9170, 148, and 1215 ohms, respectively. The total resistance of the hybrid solid electrolyte is significantly lower than those of the NASICON + PVDFHFP film and the previously reported solid electrolytes due to the inclusion of the small amount of liquid electrolyte in the hybrid solid electrolyte, which improves the ceramic–ceramic, ceramic– polymer, and electrolyte–electrode interface stability as well as interfacial contact. Interestingly, this proposed hybrid solid electrolyte exhibits higher Na+ conductivity at a temperature below -10 °C (Figure 12c). This is because the crystallization of ether electrolyte at low temperature causes decreased ionic conductivity while the incorporation of the NASICON and PVDF-HFP degrades the phase transition and crystallization of the ether electrolyte, resulting in improved low-temperature ionic conductivity. Zhang et al. also showcased the composite polymer consists of PEO host and Mgdoped NASICON (Na3.4Zr1.8Mg0.2Si2PO12) with a high ionic conductivity of 2.4 10-3 S/cm at 80 °C, and delivered electrochemically stable cycling with Na3V2(PO4)3/Na cells.213 Meanwhile, Chen’s group published the work of quasi-solid state polymer electrolyte (with ~30 wt% liquid electrolyte trapped in the polymer) by utilizing inactive SiO2 filler into PVDF-HFP polymer host to effectively disrupt its polymer chain, therefore increasing the amorphous region and resulting in high Na+ conductivity of 1 10-3 S/cm (comparable to that of liquid electrolyte) (Figure 12d).214 The Na+ conductivity measurements in Figure 12e for composite PVDF-HFP electrolyte with various contents of fumed SiO2 reveal that 4 wt% SiO2 yields the highest ion conductivity, as higher ceramic nanoparticles can impede the mobile Na+ movement within the polymer chain. All in all, given the advantage of the versatility of polymer electrolyte, other inorganic SSE can be seamlessly incorporated into it to realize high performance composite polymer SSE.

4. CONCLUSIONS AND OUTLOOKS

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Na metal anode is considered as the “holy-grail” material for enabling an economically viable high performance energy storage solution, especially for the emerging Na-S and Na-O2 batteries with energy densities comparable to that of fossil fuels. Meanwhile, thanks to the ongoing research efforts on Li metal anode stabilization research, some of the proposed strategies toward a safe and reliable Li metal anode can be extended to Na metal anode. Indeed, with the promising aspects of Na-based batteries, startups such as Faradion, Natron Energy and AGM Batteries are working to commercialize cost-effective Na+ ion energy storage technology. Nevertheless, in order to engineer methods that can efficiently protect Na metal anode from unstable SEI formation and dendrite growth, there still lacks in-depth computational and experimental investigations to fully understand the relevant mechanisms. Although significant progress has been made on interface engineering and solid electrolyte development for enabling SMBs with stable cycling performance, most of the works done so far were conducted on relatively small current densities and areal capacities, as well as low active material mass loading. In order to support practical energy storage solutions, it is necessary to shift some of the research focus to the development of new cell chemistries that can sustain real world requirements in terms of current density and ramp rate. Also, highly effective solutions for a practical SMB sometimes required the use of expensive materials or costly experimental procedure. Therefore, it will be worthwhile developing new strategies with the specific aim of driving down the costs of implementation in order to bring significant research findings out to the world. Liquid electrolyte modification can be a promising near-term solution to address the challenges associated with unstable Na metal anode because it is a relatively straightforward method that can be readily implemented into the existing battery configuration and manufacturing line. Organic solvent and salt choices must be thoroughly examined in industrial standard as they dictate the quality of the SEI on Na metal anode. Additive in carbonate electrolyte shows promising results, but more rigorous tests under various industrial standard conditions are needed to quantify the performance in real world situations. Ether electrolyte consistently delivers top-notch performance in SMB, but it is not widely used in the industry due to its more flammable nature than that of carbonate electrolyte. Both IL-based electrolyte and high concentration electrolyte are highly effective in improving SMB cycling performance. However, for practical reasons, the cost of IL-based electrolyte and high salt concentration electrolyte should be further reduced by exploring more sustainable and cost-effective synthesis

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methods. Artificial SEI fabrication provides the significant merits of completely eliminating the contact between reactive Na metal anode and organic solvent, and therefore can fundamentally solve the long-standing unstable SEI formation and dendrite growth challenges. Although highly promising, constructing a free-standing protective layer, forming in-situ passivation coating with Na metal as well as thin layer deposition onto Na metal anode can be very costly methods. Therefore, future research focus on this topic can be devoted to reduce the operating and material expenses associated with artificial SEI engineering. In order to lower the relevant cost, it requires the efforts to improve the instrumental technology that can provide facile and consistent artificial SEI coating as well. Overall, artificial SEI can be a long-term solution towards safety SMBs. SSEs can be the ultimate solution for a safe SMB by replacing the flammable organic solvent. Polymer SSE has the advantages of easy manufacturability, mechanical flexibility, lower interfacial impedance compared to ceramic and sulfide SSE. However, the low bulk conductivity with polymer SSE hinders it from being commercially useful unless a small amount of liquid electrolyte is still retained in the polymer SSE. At the same time, some of the most recently developed inorganic ceramic and sulfide materials exhibit high conductivities that are nearly on a par with typical liquid electrolytes, but with the shortcoming of manufacturing difficulty, low throughput, high material cost as well as the severe reactivity of sulfide SSE. Therefore, future research targets on SSE can be designing and fabricating hybrid SSE that incorporates both stateof-the-art polymer material and inorganic components in order to maximize the advantages of superior interfacial properties from polymer SSE and high bulk ionic conductivity from inorganic SSE.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] Notes The authors declare no competing financial interests.

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ACKNOWLEDGMENT This contribution was identified by Veronica Augustyn (North Carolina State University) as the Best Presentation in the “ENFL: Battery Technology: Vehicle to Grid. The authors greatly acknowledge the support from the start-up funds at Thayer School of Engineering, Dartmouth College.

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Figure 1. Schematic diagram presents the high energy Na metal batteries with various cathode choices, and depicts the three major challenges upon repeated stripping and plating cycles of Na metal anode.

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Figure 2. (a) The open-circuit energy diagram of a typical carbonate electrolyte. ΦA and ΦC are the anode and cathode work functions. Eg is the electrolyte thermodynamic stability window. A μA > LUMO and/or a μC < HOMO requires a kinetic stability by the formation of an SEI layer. (b) Voltage versus capacity of various electrode materials relative to the thermodynamic stability window of typical 1 M LiPF6 in EC/diethyl carbonate (DEC) (v:v=1:1) electrolyte. Reprinted with permission from ref 60 (Copyright 2010, American Chemical Society).

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Figure 3. (a) A comparison of different properties of Na and Li. (b) In situ top-view observations of dendritic formation on the surface of Na and Li anodes during the plating at 0.4 mA/cm2 current density; with the schematic depicting the respective observations on Na and Li dendrites. The scale bars in the photographs are 1 mm. (c) Chemical stability of Na and Li anodes progressed from initial pristine form to after settled for 2 days. Schematic of the observed morphologies are presented at cross-section views. (d) Electrochemical performance of Li/Li symmetric cells (top) and Na/Na symmetric cells (bottom) with added quasi-zero electrochemical field resting period upon cycles at 5 mA/cm2 current density. Reprinted with permission from ref 83 (Copyright 2018, Elsevier).

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Figure 4. (a) Coulombic efficiency of Na/Cu cells in 1 M NaFSI–EC/PC (v:v=1:1), 1 M NaFSI– EC/PC (v:v=1:1) + 1 wt% FEC, and 1 M NaFSI–FEC at 0.28 mA/cm2 current density. (b) Rated

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cycling performance of Na/Na3V2(PO4)3 full cells with blank or 50 mM SnCl2-added electrolyte. (c) Plating–stripping Coulombic efficiency of Na metal anodes cycled using 1 M NaPF6 in various electrolyte. (d) The role of Na2S6 additive and Na2S6‐NaNO3 co‐additives in affecting Na stability in diglyme electrolyte. (e) SEM images of the Na electrode surface after 30 cycles at 2 mA/cm2 current density and 1 mAh/cm2 capacity with 0.033 M Na2S6 additive (left) vs. no additive (right). (a) is reprinted with permission from ref 106 (Copyright 2018, American Chemical Society). (b) is reprinted with permission from ref 107 (Copyright 2019, American Chemical Society) (c) is reprinted with permission from ref 113 (Copyright 2015, American Chemical Society). (d) and (e) are reprinted with permission from ref 39 (Copyright 2018, John Wiley and Sons).

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Figure 5. (a) MD simulation of 1 M LiFSI-DME electrolyte and high concentration 4 M LiFSI-DME electrolyte. The color for various elements: Li-purple, O-red, N-blue, S-yellow and F-green. The uncoordinated DME solvent molecules are in light grey. (b) Cycling curve of Na/Na symmetric cells for 600 cycles with time cutoff condition of 0.5 h at 0.0028 mA/cm2 current density. (c) Schematic illustration of dilution from a high concentration electrolyte to a localized high-concentration electrolyte (HCE). (d) Viscosity of HCE, localized highconcentration electrolyte (LHCE) and conventional electrolyte. (e) Cycling performance of Na/Na3V2(PO4)3 batteries at 20C. (a) is reprinted with permission from ref 117 (Copyright 2015, Springer Nature). (b) is reprinted with permission from ref 123 (Copyright 2017, American Chemical Society). (c), (d) and (e) are reprinted with permission from ref 124 (Copyright 2018, American Chemical Society).

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Figure 6. (a) The schematic illustrates that increasing operational temperatures improves the performance of SMBs contain IL with FSA anion. (b) Schematic drawing of the Na-S cell during electrochemical cycling. The electrolyte consists of 1-methyl-3-propylimidazoliumchlorate ionic liquid tethered silica nanoparticle (SiO2–IL–ClO4) as additive in 1 M NaClO4 in a mixture of ethylene carbonate and propylene carbonate (EC/PC) (v:v=1:1). The scale bar is 30 nm. (c) Coulombic efficiency and capacity versus cycle number for the Na-S cells with various amounts of SiO2–IL–ClO4 in the electrolytes, respectively, at 0.5C current density. (a) is reprinted with permission from ref 131 (Copyright 2014, Elsevier). (b) and (c) is reprinted with permission from ref 143 (Copyright 2016, Springer Nature).

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Figure 7. (a) Illustration of (i to iii) transferring free-standing graphene film onto Na metal surface, and (iv to vi) the high stability of graphene-coated Na anode during stripping/plating without the formation of Na dendrites. (b) Galvanostatic cycling of symmetric Na/Na cells made

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from FL-G/Na (red line) and bare Na (blue line) electrode at a current density and a cycling capacity of 1 mA/cm2 and 1 mAh/cm2, respectively. (c) Schematic of the FCPL and FCPL–Na metal electrode integration fabrication. (d) Shear modulus of the as-prepared FCPLs with various ratios of PC and PVDF-HFP. (e) Schematic drawing of the surface morphology progression with cycling on pristine alkali metal anode vs. metal anode protected by carbon paper. (f) Cycling performances of Na and Na/CP at 0.5 mA/cm2 in the DGME electrolyte. (a) and (b) are reprinted with permission from ref 38 (Copyright 2017, American Chemical Society). (c) and (d) are reprinted with permission from ref 147 (Copyright 2017, American Chemical Society). (e) and (f) are reprinted with permission from ref 148 (Copyright 2018, Elsevier).

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Figure 8. (a) Schematic showing the procedure for coating Na with NaBr. (b) Symmetry cells cycling curve showcases voltage hysteresis represented by the mid-voltage values for NaBr coated Na and pristine Na. (c) The schematic depicts the formation of polymeric ionic liquid film. (d) Cycling performance of the Na-S cells with and without protected Na metal anode at 0.2C. (a) and (b) are reprinted with permission from ref 110 (Copyright 2017, Springer Nature). (c) and (d) are reprinted with permission from ref 150 (Copyright 2017, John Wiley and Sons).

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Figure 9. (a) An illustration of the PEALD process that involves TMA and O2 plasma, allowing highly precise layer‐by‐layer formation of Al2O3 on Na metal. (b) SEM imaging of pristine Na metal after long‐term cycling (450 h) with observed 3D Na dendrites vs. Al2O3 coated Na metal 54


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with a smooth surface. (c) Schematic diagrams of Na stripping/plating on Na metal with and without MLD alucone coating. (d) Comparison of the cycling stability of the alucone coated Na and the pristine Na anode at a current density of 1 mA/cm2, with potential profiles of alucone coated Na and pristine Na anode at the three different stages. (a) and (b) are reprinted with permission from ref 162 (Copyright 2016, John Wiley and Sons). (c) and (d) are reprinted with permission from ref 168 (Copyright 2017, American Chemical Society).

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Figure 10. (a) The drawing of 4 different types of bottlenecks (A–D) in Na+ conducting pathways in the monoclinic NASICON structure. (b) Schematic diagram illustrates the formation of non-uniform and dendrite-like morphology on Na anode after repeated stripping and plating cycles with pristine NASICON solid ceramic electrolyte vs. the uniform plated Na anode with CVD-grown graphene-like layer coated NASICON (G-NASICON) that effectively suppressed

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the dendrite-like Na formation. (c) TEM image of G-NASICON revealing the graphene-like layer coating on NASICON electrolyte. (d) The Nyquist plots of Au/G-NASICON/Au, Na/GNASICON/Na, and Na/NASICON/Na symmetric cells. (e) Optical photos of Na metal on a NASICON pellet at 175 °C (top) and at 380 °C (bottom). (f) Cycling performance of a NaTi2(PO4)3/Na cell with CPMEA/NASICON as the electrolyte at 0.2 C and 65 °C. (a) is reprinted with permission from ref 177 (Copyright 2016, American Chemical Society). (b), (c) and (d) are reprinted with permission from ref 44 (Copyright 2019, American Chemical Society). (e) and (f) are reprinted with permission from ref 191 (Copyright 2017, American Chemical Society).

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Figure 11. (a) Rietveld fitting of the diffraction pattern for the as-prepared Na3PSe4 (left) with the observation, calculation and difference curves displayed in the color of blue, red and dark gray, respectively. The refined structure is shown on the right. (b) Illustration of Na-Na interatomic contacts in the crystal structures of Na11Sn2PS12 (left) and Na3PS4 (right) for comparison. (c) Schematic illustration of solid electrolyte and Na metal interface: a mixed conductive interface layer grew upon cycling of the non-hydrated Na3SbS4, whereas a passivating interface was formed on the surface-hydrated Na3SbS4 SSE. (d) The cycling of

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Na/Na3SbS4/Na symmetric cells with (orange) and without (gray) surface hydration pretreatment at a current density of 0.1 mA/cm2. (a) is reprinted with permission from ref 199 (Copyright 2016, American Chemical Society). (b) is reprinted with permission from ref 202 (Copyright 2017, John Wiley and Sons). (c) and (d) are reprinted with permission from ref 208 (Copyright 2019, Elsevier).

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Figure 12. (a) The schematic showing hybrid SSE preparation process, involving NASICON powder, PVDF–HFP, and 1 M NaCF3SO3/TEGDME. (b) EIS for composite solid film, hybrid SSE, and ether-based liquid electrolyte (inset), respectively. (c) Ionic conductivity vs. temperature profile for composite solid film (SM), hybrid SSE, and ether-based liquid electrolyte, respectively. (d) The composition of quasi-solid state polymer electrolyte. Inset: TEM image of fumed SiO2. (e) Ionic conductivity of composite polymer electrolyte with various contents of fumed SiO2. (a), (b) and (c) are reprinted with permission from ref 212 (Copyright 2015, Royal Society of Chemistry). (d) and (e) are reprinted with permission from ref 214 (Copyright 2017, American Association for the Advancement of Science).

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Enabling Safe Sodium Metal Batteries by Solid Electrolyte Interphase

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