Engineering Materials for Progressive All-Solid-State Na Batteries

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Engineering Materials for Progressive All-Solid-State Na Batteries Chengtian Zhou, Sourav Bag, and Venkataraman Thangadurai ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b00948 • Publication Date (Web): 03 Aug 2018 Downloaded from http://pubs.acs.org on August 3, 2018

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Engineering Materials for Progressive All-Solid-State Na Batteries Chengtian Zhou, Sourav Bag and Venkataraman Thangadurai * Department of Chemistry, University of Calgary, 2500 University Drive North West, Calgary, Alberta, T2N 1N4, Canada. * Corresponding author Email: [email protected]

ABSTRACT. Following prevailing the electrical energy storage system (EES) by Li-ion battery, the

world is looking towards alternative, cost-effective electrical EES for portable electronics, electric

vehicles and grid storage from renewable sources. Na-based batteries are the most promising candidates and show similar chemistry as Li-based batteries. All-solid-state-sodium batteries (AS3Bs)

have attracted great attention due to safe operation, high energy density and wide operational

temperature. Herein, current development of solid-state crystalline borate and chalcogenide-based

Na-ion conductors are discussed together with historically important Na-β-alumina and Na super-ionic

conductors (NASICONs). Furthermore, we report on engineering ceramic Na-ion electrolyte and

electrode interface, which are considered as bottlenecks for practical applications of solid-state electrolytes in AS3Bs. Soft Na-ion conducting interlayer is critical to suppress the interface Na-ion

charge transfer resistance between the solid electrolyte and electrode. Several Na-ion conducting ionic

liquids, polymers, gels, crystalline plastics interlayers and other interfacial modification strategies have been effectively employed in advanced AS3Bs.

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TOC figure only


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Energy has been recognized as the fundamental enabler for socio-economic and technological

development of a modern society. Combustion of fossil fuel as the major component of today’s energy

supply raises environmental concern including global warming, air quality deterioration, acid rain and

oil spill. Energy storage attracts tremendous attention as the critical component for utilizing renewable

energy as well as balancing the grid during peak power demand. Unfortunately, the current energy storage capacity is only around 1% of the total energy consumed1. Among the energy storage methods,

batteries are deemed as promising candidates because of their high-energy efficiency and flexible designs2-4. Among the commercialized secondary batteries, including Pb-acid, Ni-Cd, and Ni-metal hydride, lithium-ion batteries (LIBs) exhibit the highest volumetric and gravimetric energy densities5.

LIBs are being widely employed to power portable electronics, electric and hybrid electric vehicles

due to their high practical energy density. A recent U.S Geological survey claims that only 13 million

tons of Li is present globally. The global demand of Li2CO3 increased annually 7-10%, which suggest all the reserved Li will likely be consumed in next 28 years, if it is not recycled6. Additionally, the

pressing demand for electric vehicle manufacturing would be evident into Li shortage and further price

increase. Thus, alternative cost-effective energy storage is required without compromising the energy

density and efficiency.



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Sodium ion batteries (NIB) have been considered to replace LIBs due to their low cost than Li.

NIB technology is very promising for next generation energy storage demand as they have similar

battery chemistry of LIBs. Though NIB full cells were developed in 1980s, their inferior performances than the LIBs prohibit the practical utilization7. A significant effort on development of high capacity electrodes and electrolyte materials for NIBs is being made with increase of global energy crisis in 21st

century. Recent studies on insertion electrodes for NIBs show comparable energy density and

relatively higher rate performances than Li-based material due to low Lewis acidic nature of Na. Few companies have started fabricating prototype NIBs7. The liquid organic electrolyte presently used in

NIBs has some issues related to safety. The liquid organic electrolytes are flammable and restrict the

use of high capacity metallic Na due to dendrite formation. Propylene carbonate, most commonly used

solvent for liquid organic electrolyte in NIB decomposes the polypropylene membrane. The dendrite

formation, non-compatibility with separator, safety issue and expensive nature of liquid electrolyte

prohibit the realization of NIBs based on liquid electrolytes.

Introduction of solid-state electrolyte (SSE) instead of liquid electrolytes in NIBs improve the

energy density, recyclability and safety issues. The main advantages of solid ceramic electrolytes are non-combustible, leak-proof and internal short circuit free8. To date interests in electrification of


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vehicle, consequently the launch of Tesla’s model S and X, Nissan leaf, trigger the battery industry to

design more efficient, and safe batteries. The vehicle technologies office of US Department of Energy

(DOE) also set a target cost for an electric vehicle battery pack of $125/kWh by 2020, which is 75%

less than 2012. DOE has also emphasized in development of high voltage safe solid-state (ceramic) electrolyte for beyond LIBs9. Toyota has developed a solid super-ionic conductor based on sulfide achieving a conductivity equal to liquid electrolyte10. This remarkable achievement triggers for

practical realization of all-solid-state batteries. The room-temperature all-solid-state sodium battery (RTAS3B) was successfully demonstrated with cubic Na3PS4 electrolyte11. Ongoing extensive research on AS3B suggests the change of paradigm for solid-state battery researches from improving

electrolyte’s ionic conductivity to alleviate impedance between SSEs and electrodes through

interfacial engineering. The poor interfacial contact between electrode and electrolyte manifested as

high interface impedance, become the biggest bottleneck for all-solid-state batteries. Several strategies have been carried out in the recently to alleviate interface contact for facile permeation of Na+ through

the interface. Total impedance of the solid-state Na cell can be reduced from ~ 4 kΩ to ~ 0.8 kΩ by introduction of a plastic-crystal interface12 and from ~ 9 kΩ to ~ 1 kΩ by blending ceramic electrolyte with polymer and liquid electrolyte13. This article emphases on the ceramic Na-ion



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electrolyte/electrode interfacial modification strategies in AS3B and also provides overview of recent

developments in solid-state Na-ion conductors.

Solid-state Na-ion conductors. In order to cover the challenges related to the stability of organic

electrolytes, attempts have been made to develop chemically stable ceramic Na-ion conductors. The

practical solid-state Na-ion electrolytes should exhibit the following desired physical and chemical properties to be used in advanced AS3B:

•

total (bulk + grain-boundary) ionic conductivity of ~ 10-2 S cm-1 with negligibly electronic and anionic conductivities at the operating temperature, preferably from -50 to 100 oC;

•

chemically stable against reaction with high voltage Na-based cathodes;

•

chemically stable against reaction with high elemental Na and sodium-metal alloy anodes;

•

mechanically stable, dense, and avoid growth of Na dendrite at high current densities;

•

wide range of electrochemical potential window (> 5.5 V vs. Na+/Na);

•

negligible charge transfer area-specific polarization resistance with Na-based anodes and

cathodes;

•

able to prepare thin-films using current ceramic processing technologies, and ceramic

thin-film should retain desired ionic conductivity with chemical and electrochemical stability;


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•

easy to scale up the synthesis, amiable for cost effective fabrication method, environmentally

safe, and chemically stable under ambient condition.

Figure 1 shows the brief overview of solid-state Na-ion conductions known since 1960. None of

the presently known ceramic Na-ion electrolytes simultaneously meet all the above-mentioned

requirements. In the following sections, we discuss the some of the known ceramic Na-ion

electrolytes that are developed for application in all-solid-state batteries. We have classified the

solid Na-ion electrolytes based on reported period.

Figure 1. Evaluation of solid Na-ion electrolytes for all-solid-state Na batteries11, 14-20.



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1st generation solid Na-ion electrolytes-Na-β-alumina. Yao and Kummer first realized the extraordinarily high Na-ion mobility in Na-β-alumina in 196721, which led to the discovery of

high-temperature Na batteries (NB). Figure 2A shows the schematic of NB containing Na anode, a

Na-β″-alumina electrolyte and S cathode. There’re two different crystal structures known for

Na-β-alumina: β-Al2O3 (hexagonal; space group P63/mmc; with a formula of Na2O⋅⋅8–11Al2O3) and β″-Al2O3 (rhombohedral; space group R3m; with a formula of Na2O⋅⋅5–7Al2O3)22,23. The ion-transport properties of β- and β″-alumina result from unique layered structure and mobile Na-ions are loosely

packed in conduction planes between the spinel blocks. Al-O-Al bonds connect the spinel blocks where O2- in the conduction plane forms a bridge between adjacent spinel blocks (Figure 2B). For

β″-alumina, bridging oxygen has weaker attraction with surrounding Na-ion and its conduction planes

allow accommodation of more Na-ions (Figure 2Bb), therefore, it exhibits higher ionic conductivity

than β-alumina. Both β″-alumina and β-alumina structures exhibit anisotropic Na-ion conductivity.

Solid-state reaction at 1600 °C is typically carried out using α-Al2O3 and Na2CO3 to prepare β″-alumina24. Despite of high ionic conductivity and excellent chemical/electrochemical stability, the

complicated process and harsh sintering condition hamper the application of β-alumina. Several synthesizing methods such as sol–gel25, 26 co-precipitation27, spray-freeze/freeze-drying25, 26, flame


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spray pyrolysis23, 28 and vapor phase process29 have developed to reduce the sintering temperature,

decrease grain-boundary resistance, increase the ratio of β″-alumina over β-alumina. Yet, solid-state

reaction is the most widely adopted to prepare Na-β-alumina electrolytes. β″-alumina has low

mechanical strength and difficult to build battery using a single crystal. Therefore, either a mixture of

polycrystalline β- and β″-alumina, or β″-alumina mixed with zirconia which increases the fracture strength is preferred in practical electrolyte application30.

(QUOTE 1)

2st generation solid Na-ion electrolytes-NASICONs. In 1976, Goodenough and Hong developed a 3D

framework structure for fast Na super-ionic conduction (NASICON), unlike layered β-alumina, which

exhibits

isotropic

conduction31,32.

Originally

reported

NASICON

chemical

formula

is

Na1+xZr2P3−xSixO12 (0 ≤ x ≤ 3), which consist of corner sharing PO4, SiO4 and ZrO6 creating a 3-D covalent network enabling Na-ions to hop among interstitial sites. The conduction pathways of Na+ are of a zigzag geometry: M1 → M2 → M1. Figure 2C shows Na+ migrate through paths in NASICON

structure. Rhombohedral structure NASICON, with a space group R-3c, was found for all

compositions in Na1+xZr2P3−xSixO12 except for those in the range of 1.8 ≤ x ≤ 2.2, which is monoclinic with a space group C2/c (Figure 2D).31-33 The monoclinic distortion splits the M2 sites into M2α and M2β sites. NASICON with monoclinic structure, particularly Na3Zr2Si2PO12, shows the highest Na-ion



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conductivity of 6.7 × 10-4 S cm-1 at RT15. Figure 3 shows periodic table of elements doped in

NASICON structure to understand the crystal structure, chemical composition and ionic conductivity

property relationship.

Figure 2. (A) Schematic illustration of a typical Na–S cell containing molten Na anode, Na-β″-alumina solid electrolyte and molten S as cathode. (B) Crystal structures of (a) β-alumina and (b) β″-alumina showing Na-ion conducting layers denoted as dashed lines which are perpendicular to c-Axis. Red: O2-, yellow: Na+; and blue: Al3+. (C) NASICON with rhombohedral structure and (D) monoclinic structure. The rhombohedral structure has two distinct Na sites: M1 and M2. The local Si/P environment and the monoclinic distortion splits the M2 sites into M2α and M2β (Adopted from ref. 33, Copyright 2017 with permission from Elsevier).


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Figure 3. Periodic table illustrates the elements have been doped in NASICON (NaZr2P3O12) structure (Yellow: Na-site; Green: Zr-site and Orange: P-site)34-41.

NASICON materials generally show a huge grain-boundary resistance (Rgb) to total conductivity at RT42. Attempts have been made to improve the total ionic conductivity (bulk + grain-boundary) through applying different sintering techniques. Naqash et al.43 prepared Na3Zr2Si2PO12 with total ionic conductivity of 1 mS cm-1 through a solution-assisted solid-state reaction. Porkodi et al.44

employed a molecular precursor process to prepare highly conducting NASICON with conductivity of 2.2 mS cm-1 at RT. Zhang et al.45 reported a new strategy of self-forming a composite material by introduction of La3+ which yields an ionic conductivity of 3.4 mS cm-1 at RT. Numerous possible ionic substitutions make NASICON a versatile family of solids (Figure 3)34-41. Doping the Zr-site and P-site



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with different elements changes Na-ion diffusion bottleneck, enables extra Na-ions in the structure or minimize the Coulombic interactions between Na+ and surrounding cations. Song and Lu et al.41 reported Na3.1Zr1.95Mg0.05Si2PO12 with a high Na-ion conductivity of 3.5 mS cm-1 at RT. Solid-state

Na-S cell has been successfully fabricated using Na3.1Zr1.95Mg0.05Si2PO12 electrolyte. Several NASICON-type materials also possess stable structural framework and good thermal stability46. Na3Zr2Si2PO12 is confirmed to gradually react with molten Na at ~ 300 °C47, 48. Wang et al.49

conducted a TOF-SIMS study on the interface of Na and Na3Zr2Si2PO12, showing reactions between

Na and electrolyte follows the diffusion of Na into the NASICON at 75 °C, evidenced by variation of P+/PO+ intensity ratio, that attributed due to growing interfacial resistance. Zhou et al.50 claimed that

Na3Zr2Si2PO12 is stable against Na up to 175 °C. They also found that a black interlayer was formed when the pellet and Na were heated to 380 °C. Guillon et al.51 studied the stability of NASICON

Na3.4Sc2Si0.4P2.6O12 against water and CO2. They recommend that Na3.4Sc2Si0.4P2.6O12 as well as other

NASICON samples should be stored in a glove box and treatment above 600 °C can remove the water

and CO2 uptake of the sample. The synthesizing requirements of NASICON materials are not so

stringent as Na-β″-alumina, but still, high-temperature sintering (~ 1200 °C) is needed to prepare

phase pure highly Na-ion conducting NASICONs. (QUOTE 2)


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3st generation solid Na-ion electrolyte - borate- and chalcogenide-based SSEs. Sodium closo-borates with polyhedral BnHn2- anions are a new family of solid-state Na-ion conductors very recently

discovered. Unlike conventional oxide-based Na-ion conductors such as Na-β-alumina and

NASICON, closo-borate anions have large cage-like quasi-spherical architectures with high symmetry (Figure 4A, B)17,18. Large closo-borate anions remain translationally rigid, possess disordered lattices with cation-vacancy, which enables facile Na+ hopping in the structure18,52. Several hydrides anions, including [BH4]−, [NH2]−, [AlH4]−, and [AlH6]3− form complex with several alkali ions that exhibit unique applications. For example, LiBH4 is a typical example for hydrogen storage compound53 and shows a fast Li+ conductivity of ~ 1mS cm-1 due to structural transition at ~ 117

o 54

C .

Na(BH4)0.5(NH2)0.5 exhibits Na-ion conductivity of 2 x 10-6 S cm-1 at 27 oC, which is 4 orders of magnitude higher than the host materials NaBH4 and NaNH255. Subsequently, Li and Na compounds

with large polyhedral closo-borate anion architectures were discovered, which showed entropy-driven order-disorder transitions56, leading to superionic conductivity (e.g., 0.07 S cm-1 at RT for Na2(CB9H10)(CB11H12)18).

Na2B12H12 and Na2B10H10 were found to exhibit disorder-induced super-ionic conductivity at elevated temperature56-58. Extraordinary Na-ion conductivity and comparatively low activation energy



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were attributed to the Na+ transport corridors afforded by unusually large quasi-spherical anions compared with the substantially smaller BH4- anions (Figure 4A). Na2B10H10 possesses high ionic conductivity of 0.01 S cm-1 at 110 oC. Introducing extrinsic vacancies were suggested to further improve the conductivity59. Na2B10H10 remains air-stable at RT with no noticeable degradation up to 227 oC and exhibits electrochemical stability window up to 5 V at 120 oC56. First-principle study

showed that sodium boro-hydrides tend to be oxidized at relatively low voltages, but the corresponding decomposition products, such as B12H122−-containing phases, have wide

electrochemical stability windows (ESWs) which seem to protect the electrolyte, leading to large ESW of 5 V60. In addition, borate-based SSEs are soft and ductile suggesting intimate contact with electrode

materials. However, these materials are impaired by their reverse transformation to the non-conducting phase when temperature decrease61. In order to stabilize these super-ionic phases

down to RT, strategies have been proposed including mechanical milling to reduce the particle size and cation/anion substitutions to enhance disorder18, 61-63. Na3BH4B12H12, (Li0.7Na0.3)3BH4B12H12 and ball-milled Na2B12H12 all reach RT conductivity values close to 10-3 Scm-1. Duchene et al.52 reported a AS3B using Na2(B12H12)0.5(B10H10)0.5 electrolyte, Na anode and NaCrO2 cathode, which exhibit reversible and stable cycling with a capacity of 85 mAh g-1 at 0.05C. Na2(B12H12)0.5(B10H10)0.5 shows


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Na-ion conductivity of 0.9 mS cm-1 at 20 °C which is several orders of magnitude higher compared to

that of Na2B12H12 and Na2B10H10, and slightly lower than 1 M NaClO4 dissolved in ethylene and

propylene carbonate (EC:PC). It also possesses other advantages such as thermal stability up to 300 °C, ESW of 3 V vs. Na+/Na, and solution processability. However, Na closo-borates tend to be incapable of blocking dendrite due to low shear modulus60. Carbon-substituted NaCB11H12 and NaCB9H10 were developed by Tang et al.18, 63-65, where carbon

is found to modify the orientation preferences of the closo-borate anions and aid rotational mobility,

which enhances disorder in the crystal structure (Figure 4B). The disordered structure enables more

possible favorable cation sites than available cation quantity, making the structure intrinsically cation-vacant. Furthermore, CB11H12- and CB9H10- anions cause less Coulombic attraction to the cations than the multiple-charged B12H122- and B10H102- anions, allowing facile Na+ mobility.

Comparing with other borate compounds, C-incorporated closo-borates have much lower transition temperatures: ~ 400 K for NaCB11H1264 and ~ RT for NaCB9H1065. Na2(CB9H10)(CB11H12) possesses the highest Na-ion conductivity up to 0.07 S cm-1 at RT18, unmatched by any other polycrystalline

materials. However, electrochemical stability window and battery performance remains to be tested

for carbon-based closo-borates.



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Figure 4. (A) Crystal structure of Na2(B12H12)0.5(B10H10)0.5. (Reproduced with permission from ref. 17, Copyright 2017 The Royal Society of Chemistry). (B) Hexagonal structure of Na-based C-incorporated closo-borates (orange: Na, large green: B/C and grey: H). Orientationally disordered CB9H10− and CB11H12− anions (superimposed) are shown in spherical shells. (Reprinted from ref. 18.) (C) Unit cell of cubic Na3PS4, Na1-, and Na2-sites are represented by red and orange, respectively. (Reprinted from ref. 66.), (D) Crystal structure of Na3SbS4 with the unit cell outlined. (Reproduced with permission from ref. 67, Copyright 2016 Wiley-VCH Verlag GmbH & Co. KGaA) and (E) Structure of Na11Sn2PS12 from single crystal data, the Na1/Na2-ions having sites with fractional occupancy are represented by rose ellipsoids and the


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17 Na3/Na4/Na5-ions with almost fully occupied sites are shown as red ellipsoids. (Reproduced with permission from ref. 19, Copyright 2018 The Royal Society of Chemistry). Chalcogenide-based Na-ion electrolytes. Although the oxide-based NASICON and Na-β-alumina

have been well-developed to achieve a high ionic conductivity as well as good electrochemical stability34, they suffer from harsh sintering conditions and poor electrolyte–electrode contact at low

temperature. Recently, chalcogenide-based SSEs have drawn much attention due to their appreciable ionic conductivity > 10-4 S cm-1 at RT11, 68-70, low synthesizing temperature and superior mechanical properties. Chalcogenide-based SSEs are generally softer than oxides71, which allows intimate contact between electrode and SSE to be achieved via cold pressing72,

73

instead of high-temperature

co-sintering. However, sulfide-based SSEs could undergo hydrolysis when exposed to moisture, releasing noxious H2S gas16. Therefore, extra caution is suggested when dealing with this group of

materials. Hayashi et al11 first reported the glass–ceramic electrolyte with Na3PS4 phase crystallized from

75Na2S-25P2S5 glass at 270 °C and 400 °C, corresponding to cubic (P421c) and tetragonal phases

(I43m) respectively. Both phases exhibit higher conductivities than glassy phase, while cubic Na3PS4 possesses slightly higher Na-ion conductivity of ~ 2 × 10-4 S cm-1 at RT. The positions of Na in cubic

phase split into lower energy Na1 sites with partial occupancy of 0.8, and higher energy Na2 sites with



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partial occupancy of 0.166. The 3D migration pathways are believed to lead towards higher conductivity of cubic Na3PS4 (Figure 4C)34, 66, 74. Hayashi et al. also fabricated RT AS3B battery: Na–Sn|Na3PS4|TiS211 and shows a few cycles with a low current density 0.013 mA cm-2. The highest conductivity of 4.6 × 10-4 S cm-1 was obtained by ball milling of stoichiometric amount of precursor for 1.5 h and consecutive heat treatment at 270 °C for 1 h68. Furthermore, through substitution of

P-site with Sb or S-site with Se, the total ionic conductivity of chalcogenide-based electrolyte exceed 1 mS cm-1 at RT67, 75 However, the decomposition potential of Na3PS4 is reported to be as low as 2.7 V vs. Na+/Na76. (QUOTE 3) Molecular dynamics simulation studies proposed that introduction of Na+ vacancies66,

77

in

Na3PS4 could improve the Na-ion conductivity. Chu et al.78 successfully prepared Cl-doped tetragonal

t-Na2.9375PS3.9375Cl0.0625 through mixing small quantity of NaCl with stoichiometric Na2S and P2S5

precursor followed by heat treatment, quenching, and spark plasma sintering. Ionic conductivity of 1.1 mS cm-1 at RT was observed for Na2.9375PS3.9375Cl0.0625, which is in remarkable agreement with the predicted value (1.4 mS cm-1) using computational method. This electrolyte material was also successfully applied to an all-solid-state battery, which delivered a capacity of 80 mAh g-1 at 0.1C for

10 cycles at RT. Na vacancies with halide-doping would result in slight lattice expansion and promote


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the formation of tetragonal phase, which led to unsuccessful attempt to synthesis Cl-doped cubic Na3PS4. Hayashi and co-workers studied several compositions of Na3PS4·xNa4SiS4 glass–ceramics79 where x = 6 member, Na3.06P0.94Si0.06S4 possesses the highest Na-ion conductivity of 7.4 × 10-4 S cm-1 at RT80. The effect of Si-doping and associated increase in ionic conductivity was explained by Zhu et

al. using first-principle calculation. Na disorder in the structure was induced by small extent of Na excess, which attributed to enhanced Na-ion conductivity81. They also suggested that Ge4+ and Sn4+ are also viable dopants to introduce similar disorder in cubic Na3PS4. Rao et al.82 systematically studied the doping of Na3PS4 with Ge4+, Sn4+ and Ti4+. A maximum total ionic conductivity of 2.5 × 10-4 S

cm-1

was

achieved

for

Sn-doped

Na3.1Sn0.1P0.9S4.

An

AS3B

comprised

of

Na2+2xFe2x(SO4)3|Na3.1Sn0.1P0.9S4|Na2Ti3O7 was demonstrated with first discharge capacity near the theoretical value of 113 mAh g-1 at RT (0.1 C rate) and at 80 °C (2 C rate). Yu et al.83 partially replaced P with As in Na3PS4, which led to an excellent conductivity of 1.5 mS cm-1 at RT for the nominal

chemical composition Na3P0.62As0.38S4. It is the highest ionic conductivity value reported, so far, in

sulfide-based Na-ion electrolytes. Full cell performance was carried out at 80 °C with 0.02 C rate for TiS2|Na3P0.62As0.38S4|Na-Sn and an irreversible sharp capacity decay from 118 mAh g-1 to 103 mAh g-1



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was observed during the first nine cycles. It should be mentioned that Na3P0.62As0.38S4 reacts with

water and eventually generating toxic arsenic compound. Zhang et al.75 demonstrated a substitution of S with Se, resulting cubic Na3PSe4 that showed a Na-ion conductivity of 1.2 mS cm-1 at RT. Higher ionic conductivity of Se substituted product was

mainly attributed to larger atomic radius of Se than S, which could result in larger diffusion

bottlenecks and the weaker binding energy between Se and Na than that of the S and Na. However; Na3PSe4 processes significantly narrower ESW than Na3PS484 which makes it unpractical for battery

applications. Phosphorous site of Na3PS4 has also been successfully substituted with Sb to obtain high

Na-ion conductivity. Based on the hard and soft acid and base theory, Liang and coworkers attempted

to improve the stability of the sulfide by introducing elements that act as soft acids which can bind strongly with S16. Pristine Na3SbS4·9H2O has a poor ionic conductivity of 5 × 10-7 S cm-1 at RT. After a simple heat treatment at 150 °C to remove H2O, anhydrous Na3SbS4 phase (Figure 4D)67 showed RT conductivity of 1.1 mS cm-1. Subsequently, Banerjee et al.67 developed a scalable solution-based

synthesizing process using methanol or water. Taking advantage of its solubility, Na3SbS4 coated

NaCrO2 cathode was demonstrated to provide intimate ionic contact with SSE, resulting in dramatic improvement in the cycling performance of NaCrO2. In Zhang’s work85, 2.5 mol.% Na deficiency in


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tetragonal Na3SbS4 was revealed, experimental value was verified using theoretical study. This material shows a total ionic conductivity of 1 mS cm-1 at RT. Although, exhibiting a broad ESW up to 5 V vs. Na+/Na, Na3SbS4 shows instability against metallic Na which is the common problem for most chalcogenide-based electrolyte11, 73, 86. Table 1 shows electrical conductivity, activation energy and

ESW of some chalcogenide and borate-based Na-ion electrolytes. Most of these electrolytes show a

wide ESW ≥ 5 V from cyclic voltammetry, however, stability window seems to vary with experimental condition.17

Inspired by the discovery of fast Li-ion conducting Li10GeP2S12 (Thio-LISICON) with room-temperature conductivity of ~ 1.0 × 10−2 S cm-1, researchers predicted that analogous Na10GeP2S12 would exhibit a high Na-ion conductivity87. Richards et al.88 studied the family of

Na10MP2S12 (M = Sn, Ge, and Si) based on first-principles simulations. Among them, tetragonal

Na10SnP2S12 with P42/nmc space group was successfully synthesized and characterized. This material possesses RT ionic conductivity of 4.0 × 10−4 S cm-1 and is comparable to that of the predicted value (9.4 × 10−4 S cm-1).88 This conductivity was achieved in spite of a small fraction of impurity phases of

P2S5 and Na3PS4. The author also suggested that within the Na10MP2S12 family, the highest predicted conductivity is 10 mS cm-1 for Na10SiP2S12; however, .it hasn’t been successfully prepared.



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Na11SnP2S12 was reported by Zhang et al.19 with a RT total ionic conductivity up to 1.4 mScm-1. With

a single crystal-favored synthesis protocol, rectangular Na11SnP2S12 single crystal with tetragonal space group I41/acd (Figure 4E)19 was acquired after slowly cooling down from molten precursors.

Notably, Na–Na distances are effectively equivalent, and all Na-ions are in the same coordination

environment allowing the material to possess Na-ion conduction in three dimensions with very low

activation energy of 0.24 eV. However, the stability of Na10MP2S12 and Na11SnP2S12 compounds and

their electrochemical performances remain to be tested. Ionic conductivity of the three generations of

solid Na-ion electrolytes is compared with liquid electrolyte (sodium salts dissolved in organic solvents) in Figure 5.11, 16-19, 23, 41, 83, 89.


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23

Figure 5. Na-ion conductivity of some first, second and third generation solid-state electrolytes. Table 1. Ionic Conductivity Property of some 3rd Generation Solid Na-ion Electrolytes σRT

Ea

ESW vs. Na+/Na

(S cm-1)

(eV)

(V)

Na3PS468, 76

4.6 × 10-4

0.20

2.7

Na3.1Sn0.1P0.9S482

2.5 × 10-4

0.18

Not reported

Na3.06P0.94Si0.06S479

7.4 × 10-4

0.26

5

Na3SbS416

1.0 × 10-3

0.22

5

Na3PSe475

1.2 × 10-3

0.21

Not reported

Na3P0.62As0.38S483

1.5 × 10-3

0.26

5

Na10SnP2S1288

4 × 10-4

0.36

Not reported

Na11Sn2PS1219

1.4 × 10-3

0.25

5

Na(BH4)0.5(NH2)0.555

2 × 10-6

0.62

6

Na2B10H1056

< 10-6

N/A

5

Na2B12H1258

< 10-6

N/A

Not reported

Na3BH4B12H1261

1 × 10-3

0.34

10

Ball-milled Na2B12H1263

3 × 10-4

0.21

5

Solid electrolytes



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NaCB9H1065

3 × 10-2

0.20

5

Na2(CB9H10)(CB11H12)18

7 × 10-2

0.22

Not reported

Na2(B12H12)0.5(B10H10)0.517

1 × 10-3

N/A

3

* ESW: Electrochemical stability window

Ceramic Na-ion conductors possess has been plagued by significant interfacial impedance, which hampers their practical realization in AS3Bs. Elemental Na is usually desired as anode because of its

high capacity. Na is well known to have a poor contact with ceramic Na-ion electrolytes and result in a large interfacial charge transfer resistance90, 91. A proper interlayer phase between solid electrolyte and Na anode either formed in-situ or ex-situ is necessary for long-term good performance of AS3B. At the

cathode, it’s difficult to achieve a good contact of hard ceramic electrolyte with conventional cathodes without high temperature co-sintering45, 52. Three-dimensional volume change of cathode particles during charge-discharge also provides challenges to the structural stability12. The performance of a

solid-state battery is highly dependent on a good electrolyte/electrode interface with chemical stability,

intimate contact, allowing fast charge-transfer and inhibiting dendritic growth.

Solid electrolyte and cathode interface engineering. It’s a common practice to employ co-sintering of solid electrolyte and electrode particles to improve contact and interfacial bonding92. The


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25

high-temperature process can induce inter-diffusion of elements with the formation of a high-resistance interlayer93,

94

. Therefore, interfacial modifications apart from high temperature

sintering which can improve the electrode/electrolyte contact and buffer the volume change are ideal

approaches to effectively improve the all-solid-state battery performance. A soft interlayer consisting of gel, solid polymer or plastic–crystal. Zhao et al.95 assembled Na-β″-Al2O3-based RT AS3B with a gel composite cathode layer. A mixture of NaTi2(PO4)3/graphite,

PVdF-HFP and NaPF6 dissolved in EC/DMC was used as cathode. Cathode layer was screen-printed

on the ceramic electrolyte and the solvent was then evaporated, leaving a solid gel composite layer.

This cell with Na anode, exhibited a stable voltage of 2.1 V along with a reversible discharge capacity of approximately 100 to 130 mAh g-1 for 50 cycles. However, without complete elimination of the

flammable liquid electrolyte, there’s still risk of thermal runaway for the battery. Goodenough and co-workers50,

96

were used cross-linked poly-ethylene-glycol methyl ether

acrylate (CPMEA) interlayer between Na3Si2Zr2PO12 and NaTi2(PO4)3. The solid CPMEA polymer

framework functioned as a flexible Na-ion conducting binder with enhanced ionic conductivity and

thermal

stability

up

to

270

°C.

Solid-state

cell

with

configuration

of

NaTi2(PO4)3|CPMEA|NASICON|CPMEA|Na exhibited excellent cycling performance at 65 °C for 70



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cycles. Following this study, a solid-state Na battery was successfully fabricated with cathode

composed of Na2MnFe(CN)6 (Prussian blue), carbon black, and polyethylene oxide polymer electrolyte (PEO-NaClO4)97. The cathode was directly compressed onto the Na3Si2Zr2PO12 electrolyte

pellet and the dissolution effect of Prussian blue was completely eliminated with the employment of

SSE. Incorporation of Na-ion conducting polymer between SSE and cathode, as a soft interlayer was

demonstrated to provide the necessary plasticity and the ionic conduction pathways between

electrolyte and electrode interface. Gao et al.12 introduced a plastic–crystal electrolyte interphase between solid NASICON

electrolyte and cathode. The plastic-crystal electrolyte consists of CN(CH2)2CN and NaClO4 with a

molar ratio of 20:1 that penetrate into the cathode to access a much larger fraction of the area and deform reversibly with electrode volume change (Figure 6).12 Solid-state cells were tested with Na

anode, Na3Zr2Si2PO12 electrolyte and Na3V2(PO4)3 cathode mixed with either NASICON particles or

the plastic–crystal electrolyte. The latter one exhibits much lower total resistance and a dramatically

improved electrochemical performance at 50 °C with retention of capacity for over 100 cycles. They

suggested that the application of a plastic–crystal electrolyte interphase between SSE and a solid cathode could be extended to other AS3Bs.


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27

Figure 6. Schematic illustrations of AS3B: (A) conventional solid-state sodium battery with SSE particles in the cathode, and (B) solid-state sodium battery with plastic–crystal electrolyte in the cathode. (C) Photographs of the mixture of CN(CH2)2CN and sodium salt at 65 °C (left) and at room temperature (right) (D) The first charge–discharge profile at 50 °C of (a) the solid-state cells made of Na3Zr2Si2PO12 and (b) the plastic–crystal electrolyte at a rate of 0.1 C. (Adopted with permission from ref. 12, Copyright 2017 Wiley-VCH Verlag GmbH & Co. KGaA). Ceramic and polymer hybrid electrolyte interface. Solid polymer electrolytes (SPE) possess relatively low ionic conductivity of 10−8 -10−6 S cm-1 at RT. However, with its flexibility, SPE is able to

construct facile ionic conduction path to electrode particles. Incorporating ceramic filler (especially

ceramic Na-ion conductors) into SPE can dramatically enhance the ionic conductivity, and therefore, combine the merits of both polymer and ceramic electrolytes. Kim et al.13 proposed a NASICON

ceramic-based hybrid solid electrolyte (HSE) to decrease interfacial resistance. The HSE is composed

of NASICON, PVdF–HFP polymer, and NaCF3SO3 dissolved in TEGDME liquid electrolyte in



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70:15:15 wt.%. The PVdF–HFP polymer binder adds flexibility to the HSE enabling compensation for

the volume change of electrodes and the inclusion of the small amount of liquid electrolyte improves

the ceramic–ceramic, ceramic–polymer, and electrolyte–electrode interface stabilities. The total

resistance of the HSE is significantly lower than that of the NASICON + PVdF–HFP film but higher

than liquid electrolyte (Figure 7A). The preparation process involved a doctor-blade method and phase

inversion shown in Figure 7B. The full cell with bare NASICON ceramic SSE is not able to display

reversible charge-discharge. It was believed due to the electrodes volume change during extraction/insertion of Na+ for which cannot be compensated by NASICON electrolyte. For the cell

with HSE, the discharge voltage is continually maintained during cycling because of the stable contact

between solid electrolyte and electrode. Zhang et al.98 reported a similar approach utilizing ceramic/polymer HSE for AS3B but without

the incorporation of liquid organic solvent. The HSE was prepared using PEO, sodium

bis(fluorosulfonyl) imide NaFSI, and Na3.4Zr1.8Mg0.2Si2PO12 ceramic powder by a solution-casting

technique. The solid-state Na3V2(PO4)3|HSE|Na battery using HSE exhibits an initial reversible capacity of 106.1 mAh g-1 and excellent cycle performance with negligible capacity loss over 120

cycles at 80 °C. They ascribed this excellent performance to the high ionic conductivity of the


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ACS Energy Letters

29

composite electrolyte and flexible interfacial contact between the composite electrolyte and electrode

materials.

Figure 7. (A) EIS for composite solid film (SM), hybrid solid electrolyte (HSE), and ether-based liquid electrolyte (inset). (B) Scheme illustrating HSE preparation process, involving NASICON powder, PVdF–HFP, and 1 M NaCF3SO3/TEGDME (Reproduced from ref. 13, with permission, Copyright 2015 The Royal Society of Chemistry). Ionic liquid interface. The ionic liquid (IL) has the benefits of nonflammable and nonvolatile comparing to liquid electrolyte, which can be utilized in AS3B providing ionic migration path with a favorable interface kinetic. Zhang et al.45 fabricated an AS3B with ultralong life cycle based on a

self-forming NASICON composite SSE. Apart from the novel SSE with high ionic conductivity up to 3.4 mS cm-1, they demonstrated an interface modification strategy by addition of IL (PP13FSI, ~ 5 µL cm−2) at the cathode side. Figure 8 shows the schematic of AS3B with and without IL45. In contrast to

all-solid-state battery, with the addition of a small amount of IL at the cathode, even without Na salt,

the rate capability and cycling performance was surprisingly improved. NVP|IL|SSE|Na solid-state sodium battery exhibited a specific capacity of ~ 90 mAh g-1 up to 10000 cycles without capacity



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decay at 10 C rate at RT (Figure 8B)45. The superior performance was attributed to the enhanced

interfacial contact between cathode/SSE as well as a soft buffer space for the volume change provided

by IL. Notably, liquid electrolyte was also attempted as wetting agent but didn’t exhibit performance, as good as IL, because of its volatilization and or decomposition. Liu et al.14 designed a toothpaste-like

electrode, which could easily adhere on SSE by pasting. Figure 8C shows the traditional cathode/SSE

interface configuration and the interface modified with IL. A non-flammable IL (PY14FSI) together

with conductive additive super P was coated around cathode active material to obtain mixed ionic and electronic conducting network. An AS3B Na|Na-β″-Al2O3|Na0.66Ni0.33Mn0.67O2 with toothpaste-like cathode was constructed, where the active material mass loading reached 6 mg cm-2 without any

binders and electrolyte particles. It exhibited high current rate of 6 C, an ultra-long cycle life of 10k cycles with a capacity retention of 90% at 70 °C (Figure 8D)14.


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31

Figure 8. (A) Schematic representation of the NVP|SSE|Na and NVP|IL|SSE|Na solid-state batteries. (B) Rate performance of the NVP|IL|SSE|Na solid-state battery at RT with charge-discharge profiles of the 1st and 5th cycle at a current rate of 0.2C (inset) of left side. Cycling performance and Coulombic efficiency of the NVP|IL|SSE|Na solid-state battery with a current rate of 10C for 10000 cycles is also demonstrated at the right side (Reproduced with permission from ref. 45, Copyright 2016 Wiley-VCH Verlag GmbH & Co. KGaA), (C) Schematic diagrams of a ceramic-based solid-state battery with conventional sintering and new “toothpaste-like” cathode. (D) Long-term cycling performance at 6C rate for the AS3B with “toothpaste-like” cathode at 70°C. (Reprinted from ref. 14.)



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Battery with NASICON structure. NASICON-type material can function as a cathode, an anode and an

electrolyte and offering “an interface free” monolithic battery. As a proof-of-the-concept,

all-NASICON solid-state battery was developed. Solid-solution electrolyte buffer layers are expected to form between NASICON electrodes and electrolyte99, and volume change can be buffered by the

symmetric electrodes. Okada’s group fabricated a symmetrical solid-state battery using Na3V2(PO4)3 as both cathode/anode material and Na3Zr2Si2PO12 electrolyte100. The battery was assembled via

screen-printing technique using electrode material with a 10-20 µm thickness on both side of

electrolyte and dried at 100 °C for 40 min. Fabricated cell was hot pressed at 700 °C for 5 h under 65 MPa. The battery showed a discharge capacity of 68 mAh g-1 at a current density of 1.2 µA cm-2, which was 58 % of the Na3V2(PO4)3 theoretical capacity (117 mAh g-1). Inspired by Li10GeP2S12 single-material battery101, Inoishi et al.102 further exploited this idea by

fabricating a single-phase all-solid-state battery using a Na3V2(PO4)3-based material as the cathode

and anode, as well as the electrolyte. This single-phase cell exhibits lower total resistance comparing

with previous all-NASICON cell with Na3Zr2Si2PO12, because of the low interfacial resistance, even

though the electrolyte resistance of Na3Zr2Si2PO12 is much lower. This result underpinned the change


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33

of paradigm in designing solid-state batteries, shifting from conductivity enhancement to interface

optimization.

Composite electrolyte/electrode mixture. Highly ionic conductive Na3PS4 glass-ceramic is a promising electrolyte candidate since it is softer than oxides71, which allows better contact with electrode materials. The first RT AS3B Na–Sn|Na3PS4|TiS2 was fabricated by Hayashi et al.11. The working

electrode was a composite of TiS2 and SSE powders, and only 40% of the theoretical capacity of TiS2

have been realized. Reducing particle size of chalcogenide-based SSE and mixing with active

materials to form a composite cathode is an efficient way to increase the physical contact area at the interface. Rao et al.82 demonstrated a discharge capacity close to the theoretical cathode capacity of 113 mAh g-1 at RT (0.1C rate) and 80 °C (2C rate) for a AS3B using Na3+xMxP1-xS4 (M = Ge4+, Ti4+, Sn4+; x = 0, 0.1) SSE and Na2Ti3O7 cathode. In this study, the composite cathode was prepared by

ball-milling the composite of active material, SSE particles and activated carbon in the ratio of 6:3:1, followed by pressing onto SSE pellet. Chi et al.76 developed an organic electrode material (Na4C6O6)

tetrahydroxy-benzoquinone with hydroxyl functional group. A solid-state cell was constructed using Na4C6O6|Na3PS4|Na15Sn4 delivered a high specific capacity of 184 mAh g-1 and the highest specific energy of 395 Wh kg-1, among AS3Bs based on intercalation mechanism. Figure 9A shows schematic



Page 34 of 75

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of the Na4C6O6|Na3PS4|Na15Sn4 AS3B76. Cross sectional SEM and EDX mapping analysis in Figure

9B shows the elemental distribution of cathode/electrolyte interface. After 400 cycles with 0.2C rate, the cell remains capacity of 107 mAh g-1 (Figure 9C)76. For the composite cathode fabrication,

Na4C6O6, Na3PS4 SSE and conductive carbon were well mixed at a mass ratio of 4:5:1 in an agate

mortar and attached to the electrolyte pellet by cold pressing. To explore novel anode materials for

chalcogenide-based SSE besides Na-Sn, they also investigated the possibility of Na4C6O6 as an anode.

A symmetric solid-state cell using Na4C6O6 as electrodes was successfully demonstrated with a

capacity retention of 66% after 50 cycles.

Figure 9. (A) Schematic of the Na4C6O6|Na3PS4|Na15Sn4 AS3B; (B) Cross sectional SEM image of cathode/electrolyte interface (left top) and cathode surface (right top) and corresponding EDX elemental mapping for O and S (bottom). (C) Capacity and coulombic efficiency of


Page 35 of 75 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

ACS Energy Letters

35 Na4C6O6|Na3PS4|Na15Sn4 AS3B is demonstrated against cycle number at 60°C with a constant C rate of 0.2 (Adopted with permission from ref. 76, Copyright 2018 Wiley-VCH Verlag GmbH & Co. KGaA).

Figure 10. Elemental mapping of composite sulfur cathode prepared by (A) mechanical milling S-AB-Na3PS4 or (B) hand mixing S+AB+Na3PS4 and corresponding (C) charge-discharge curves for the first cycle of the AS3B using the composite sulfur electrodes. (D) Cyclic performance of discharge capacities and Coulombic efficiencies at various current densities of AS3B using the sulfur composite electrodes prepared by mechanical milling S−AB−Na3PS4. (Reprinted from ref. 72.) For solid-state Na-S batteries, the electronic and ionic insulating nature of the S and its discharge products further plague the charge transfer at the interface. Yue et al.73 proposed a Na3PS4/Na2S/C

nanocomposite as cathode for all-solid-state Na-S batteries. This nanocomposite cathode was prepared

by mechanical milling of Na3PS4, commercial micron-sized Na2S and acetylene black. The



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homogeneous distribution of nanosized Na2S, Na3PS4, and carbon in the nanocomposite cathode

ensures a mixed high conductivity leading to significantly improved electrochemical performances. Tanibata et al.72 compared the composite S electrode prepared by hand-mixing and mechanically

milling. SEM and EDX results indicated the contact area of S with acetylene black and Na3PS4 was

effectively increased by the mechanical milling treatment (Figure 10A, B). They fabricated a

bulk-type Na-S cell with mechanical-milled S, acetylene black and Na3PS4 composite cathode and Na3PS4 SSE, which exhibited a high rate of utilization of S active material over 1100 mAh g-1 and no

large capacity degradation within 25 cycles at RT, while the cell with hand-mixed S, acetylene black,

and Na3PS4 electrode can barely deliver any capacity (Figure 10C). The rate of utilization is about two times more than Na-S batteries operating at high temperatures103, but exhibited a dramatic decay of

capacity with increasing current density (Figure 10D), which need to be solved for practical

application. All these studies indicated that intimate contact of S active materials with carbon and

Na3PS4 electrolyte could lead to reversible high capacity of sulfur. Overall, chalcogenide-based SSEs

have been successfully demonstrated to provide intimate contact with different electrode materials by facile treatments such as milling and cold pressing. However, AS3Bs with chalcogenide-based SSE


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37

can only operate at a low current density for now and its limited electrochemical stability window

renders to only few choices of electrode materials.

Figure 11. (A) Schematic illustration of cross section and SEM image of the cell architecture investigated. (Reproduced with permission from ref. 52, Copyright 2017 The Royal Society of Chemistry) (B) HRTEM image of FIB-sectioned Na3SbS4-coated NaCrO2. (C) Cross sectional elemental mapping of Na3SbS4-coated NaCrO2 and its corresponding EDX elemental maps. (D) Initial charge–discharge profiles with a current density of 0.05 mA cm-2 at 30 °C and (E) EIS of AS3Bs made of mixed electrode and Na3SbS4-coated NCO electrode are compared. (Reproduced from ref. 67, Copyright 2016 Wiley-VCH Verlag GmbH & Co. KGaA) (Fa) Schematic illustration of the fabrication of Fe1−[email protected] nanorods and corresponding SEM images of (Fb) Fe1−xS and (Fc) Fe1−[email protected] nanocomposites. (G) Cycling performance of the AS3Bs under a current density of 100 mA g-1. (Reprinted from ref. 104.) Solution impregnation of the electrolyte onto the cathode particles. Some SSEs with exceptional

advantages of solution process can be coated by infiltration on cathode materials to achieve high

contact

surface

area.

Na2(B12H12)0.5(B10H10)0.517,

A 52

recently

reported

AS3B

using

borate-based

electrolyte

showed enhanced performance by modifying cathode/electrolyte



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interface with solution impregnation of electrolyte particles on cathode surface (Figure 11A). The

coating was obtained by dissolving the electrolyte in methanol and dispersing NaCrO2 cathode

particles. The recrystallized electrolyte on the cathode particle was obtained by vacuum drying

followed by heating. The electrolyte-coated cathode was again grounded with SSE and carbon additive,

followed by pressing together with another SSE layer. The fabricated cell using an impregnated

NaCrO2 cathode mixture, SSE and Na anode exhibited high cycling stability with a reversible capacity of 80 mAh g-1 at 0.2C and 85% capacity retention after 250 cycles. Notably, this cell can only operate

at slightly elevated temperature of 60 °C to prevent dendrite growth which required interface

engineering on the anode side in the future. Another recently discovered ceramic electrolyte Na3SbS4 was coated on cathode particles in a similar manner (Figure 11 B, C)67. The solution-processed

solidified 13 wt% Na3SbS4 coating on NaCrO2 provided intimate ionic contact with the SSE, resulting

in a dramatic improvement in the utilization of the cathode active materials. The full cell with

Na3SbS4-coated NaCrO2 possessed much lower total resistance comparing with mixed electrode (Figure 11E). It shows a discharge capacity of 108 mAh g-1 (Figure 11D), which is close to the value of cells with liquid electrolyte. AS3B with Na3SbS4-coated FeS2 as cathode demonstrated a high discharge capacity of 324 mAh g-1 at 30 °C105. However, gradual capacity fading during cycling


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appeared in both cases of sulfide-based SSEs, a protective coating layer is suggested to solve this problem. Wang et al.104 reported an in-situ liquid-phase deposition method to coat Na2.9PS3.95Se0.05

SSE on Fe1−xS cathode. Fe1−xS, Na2S, P2S5, and Se were added into acetonitrile solvent followed by

stirring at 50 °C for 24 h. The obtained product was further annealed at 270 °C to transfer the deposited

electrolyte precursor to Na2.9PS3.95Se0.05, yielding core−shell structured Fe1−[email protected] nanorods104.

Figure

11F(a)

shows

the

schematic

illustration

of

the

fabrication

of

Fe1−[email protected] nanorods. Figure 10F(b) and (c) present SEM images of Fe1−xS and

Fe1−[email protected] nanocomposites, respectively. 10 wt% of acetylene black was added to the

composite cathode as electronic conductor. The Fe1−[email protected]|Na2.9PS3.95Se0.05|Na battery was tested under a current density of 100 mA g-1 (Figure 11G). The discharge capacity can be maintained at ~ 500 mAh g-1, with a capacity retention of 75.45% after 100 cycles, showing superior performance than the batteries with simply mixing the SSE and cathode materials104. Solid electrolyte and anode interface engineering. Na metal has the capacity of 1165 mAh g-1 and the

redox potential of -2.71 V versus standard hydrogen electrode; it becomes the ultimate choice for

anode in sodium-based batteries. The assembly of solid-state cell with metallic Na anode in lab scale is done by pressing Na foils/granules on one side of the SSE in an Ar filled glove box12, 45. Huge



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interfacial resistance between Na and ceramics as well as dendritic formation is a major problem

hindering the utilization of elemental Na anode. Several strategies have been developed to improve the

Na wetting, enhance stability and decrease the interface charge transfer resistance, as well as to

alleviate the Na dendritic growth.

Alloy. Surface energy and viscosity of the molten Na can be tuned by alloying, which enables improvement of the wettability against ceramic electrolytes. Hu and his coworkers91 demonstrated that

molten Na forms a bead (Figure 12A a) and Na–Sn alloy can be coated on alumina substrate (Figure 12A (b)). Liu et al.90 discovered that the addition of Cs to Na remarkably improves its wettability on

the surface of β-alumina, which allows the batteries to be operated at 95 °C. The improvement was

attributed to lower surface tension of liquid Cs, and stronger interactions between Cs and β″-alumina

atoms compared against those with bare Na.


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Figure 12. (A) Digital images of (a) molten Na on an alumina substrate and (b) molten Na–Sn alloy wetted alumina substrate. (Reproduced from ref. 91, Copyright 2017 with permission from Wiley-VCH Verlag GmbH & Co. KGaA). (B-E) Simulated structures of liquid Na–Cs alloy droplets on BASE at 100 °C. (B) Na, (C) Na–Cs (molar ratio of 4:1), (D) Na–Cs (molar ratio of 1:4) and (E) Cs liquid. The Al, O, Na and Cs are in purple, red, orange and dark green, respectively. (F) Wetting behavior of Na and Na alloys on BASE surface. Liquid Na drops on untreated BASE (a,e,i). Liquid Na–K alloy (molar ratio of 3:7) on untreated BASE (b,f,j). Liquid Na–Rb alloy (molar ratio of 1:4) on untreated BASE (c,g,k). Liquid Na–Cs alloy (molar ratio of 1:4) drops on untreated BASE (d,h,l). (G) Voltage profiles for cell Na–Cs|untreated BASE|S. during the 1st, 5th, 10th, 20th and 40th cycles at 95 °C. (Adopted with permission from ref. 90, Nature publishing group Copyright 2014).



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The wetting property of the β -alumina was studied using metallic Na and alkali metal-based alloys (K,

Rb, and Cs with Na). Computational model shows that the height of 50-atom metal cluster decreased

while width increased with higher Cs content in the droplets (Figure 12B-E), which indicated that the

wetting gradually improved with increasing ratio of Cs in the Na-Cs alloys. The Na-Cs alloy showed

excellent wettability over the β-alumina (Figure 12Fd, h, l). The Na-β-alumina samples coated with

Na–K and Na–Rb alloys were fractured (Figure 12Fj, k) after heating at 200° C, probably due to ion exchange of Na+ by K+ and Rb+. The Na–S cell with Na–Cs alloy anodes could be cycled at 95 °C with a high capacity of 330 mAh g-1 and shows excellent stability with no obvious degradation after initial few cycles (Figure 12G)90. Na3PS4 electrolyte is a promising candidate for AS3B but it’s proven to be not electrochemically stable with Na and it form reaction products such as Na2S and Na3P with Na78. Na-Sn alloy with interfacial stability is the preferred choice11, 72. Acetylene black can also be added to

the Na-Sn alloy functioning as a buffer for the huge volume change of the Na−Sn anode during Na

charging/discharging, thus leading to a better interfacial stability during long-term charge-discharge cycles73. In situ/ex situ interlayer products. Chu et al.78 developed a Cl-doped Na2.9375PS3.9375Cl0.0625 electrolyte and constructed TiS2|t-Na2.9375PS3.9375Cl0.0625|Na AS3B cell. At a current density of 0.149 mA cm−2,


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capacity of ~ 80 mAh g-1 was achieved over 10 cycles at RT. The superior performance was attributed

to the formation of electronically insulating, ionically conducting passivation layer during the first cycle at the electrode-SSE interface due to the presence of the Cl−. Zhou et al.50 reported a dry

cross-linked poly-ethylene-glycol methyl ether acrylate (CPMEA) polymer as interlayer between the

NASICON/Na interface to ameliorate the wetting and suppress dendritic growth. In another approach,

NASICON electrolyte pellet was reduced in the presence of Na at 380 °C, which formed interlayer product enabling the wetting of Na metal50.

Thin films of metal oxides can be coated on the electrolyte surface using various deposition methods such as atomic layer deposition106, 107. The application of Al2O3 layer on SSE has shown positive effect on wetting of the metal electrode/SSE interface for Li anode. For AS3B, oxides coating hasn’t got as much attention yet. Hu et al.108 demonstrated porous iron oxide coatings to improve the wettability of the molten Na on β″-alumina through a wet chemistry method. Tang et al.84 applied

first-principle calculation to screen combinations of buffer oxides with Na electrodes/electrolytes for electrochemical and chemical compatibility. Table 2 lists battery performances of some AS3Bs.



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Table 2. Battery performances and interfacial modifications of some AS3Bs Electrolyte

Na3PS411

Method

Mixing SSE and

Cathode

TiS2

Anode

Capacity

Discharge

C-rate

(mAh g-1) (# cycles)

(V)

(T, oC)

100(10)

1.2-1.8

0.013

Na-Sn

mA cm-2

cathode

(RT)

Na3Zr2Si2PO12

Hybrid

NaFePO4

C

131 (200)

1.5-3.0

-polymer13

0.2C

(RT)

Na3SbS467

Solution-coating

NaCrO2

Na-Sn

108

2.5

-

Na3.4Zr1.8Mg0.2Si2

Hybrid

Na3V2(PO4)3

Na

106 (120)

3.3

0.2C

PO12-polymer98

Na-β″-Al2O395

(80°C)

Gel-like interlayer

NaTi2(PO4)3

Na

115 (50)

2.1

0.1C

(RT)

Na3.1Zr1.95Mg0.05S

Mechanochemical

i2PO1241

milling

La-composite-Na

Ionic liquid

Sulfur

Na

527 (decade to 160

0.8-2.8

after 10 cycles)

Na3V2(PO4)3

Na

45 3Zr2Si2PO12


90 (10000)

0.01C

(RT)

3.4

10C (RT)

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Na3Zr2Si2PO1212

Plastic-crystal

Na3V2(PO4)3

Na

77 (100)

3.4

interface

Na2.9375PS3.9375Cl0

78 .0625

Na3.1Sn0.1P0.9S482

Na-β″ -Al2O314

In-situ forming of a

(50°C)

TiS2

Na

80 (10)

2.0

passivation layer

Mixing SSE and

Na2+2xFe2-x(SO4

cathode

)3

Ionic liquid

Na0.66Ni0.3

Polymer interlayer

0.1C

(RT)

Na2Ti3O7

109 (100)

1.5-3.0

NaTi2(PO4)3

2C

(80°C)

Na

60 (100000)

3.0-3.7

Mn0.67O2

Na3Zr2Si2PO1250

5C

6C

(90°C)

Na

110 (70)

2

0.2C

(65°C)

Na2(B12H12)0.5(B1

Solution-coating SSE

52 0 H10 )0.5

on cathode

Na3PS472

Mechanical milling

NaCrO2

Na

80 (20)

3

0.2C

(65°C)

Sulfur

Na-Sn

1100 (25)

1.6-2.1

0.013

mA cm-2

(RT)

Na3PS473

Mechanical milling

Na2S

Na-Sn

869 (decade to 438


0.5-2.0

50 mA


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SSE, Na2S and

g-1

after 50 cycles)

carbon

Na3PS476

Mixing SSE and

(60°C)

Na4C6O6

Na-Sn

184 (400)

1.6-2.4

cathode

Na2Zn1.9Ga0.1Te

Gel-like interlayer

(60°C)

Na3V2(PO4)3

Na

70 (10)

3.4

O620

Na3SbS4105

0.2C

0.2C

(80°C)

Solution-coating SSE

FeS2

Na-Sn

on cathode

250 (decade to 62%

0.6-2.0

0.05 mA

cm-2

after 50 cycles)

(30°C)

Na3Zr2Si2PO1297

Polymer interlayer

Na2MnFe(CN)6

Na

125 (200)

3.0-3.6

0.5C

(60°C)

Na2.9PS3.95Se0.0510

Liquid-phase

4

deposition of SSE on

mA g-1

cathode

(RT)

Fe1-xS

Na


494 (100)

0.5-2.2

0.013

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Computational studies on ion pathway and interface. Computational methods are fascinating tools to

understand the structure-property relationship and design of new functional materials. The role of defects in the crystal structure and Na+ diffusion mechanism in SSEs has been explained using density functional theory and molecular dynamics simulation66, 109-111. The recent trend in the solid-state

battery research is to identify and resolve the interfacial issues of the solid electrolytes and electrodes.

Besides experimental study, computational studies on the chemical and electrochemical stability of

SSEs with electrode materials and interfacial kinetics have been calculated to understand the interface

issues.

Selection of a cathode and anode materials for a specific SSE is very crucial for the practical realization of the AS3Bs. Tian et al. investigated the chemical stability of cathode materials (NaxMO2; M = Cr, Mn, Fe, Co, Ni) with SSEs (Na3PS4 and Na3PSe4) using DFT calculations86. Alike to their

previous studies on Li-based systems, oxide-based cathode materials react with chalcogenide-based SSEs and produce transition metal sulfides, selenides, NaMS2 and NaPO4112, 113. Reaction products of

NaxMO2 (M = Cr, Mn, Fe, Co, Ni) cathode with Na3PY4 (Y = S, Se) SSE were calculated and experimentally verified. NaCrO2/Na3PS4/Na-Sn was found to be the most stable AS3B system86. The

better contact and wettability of Na-based alkali alloy (Na-Cs) anode on Na-β-alumina electrolyte was



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explained using molecular dynamics simulations and calculating the work of adhesion90. These results were in agreement with experimental observations (Figure 12F(d, h, l)). Tang et al.84 systematically

studied a series of metal oxide buffer coating over several oxide and chalcogenides electrolytes and

their compatibility with Na and Na2Ti3O7 anodes, NASICON and olivine-type cathodes using first

principle calculations. The electrochemical and chemical stability of different metal oxides buffer

layer combined with charge and discharged electrodes and SSEs were studied by calculating reaction

energy and volume changes. Sc2O3, SiO2, TiO2, ZrO2, and HfO2 were estimated to be compatible with

most electrodes and SSEs.

All-solid-state Na cell manufacture. Though the ionic conductivity of SSEs has been significantly

improved in past few decades, scientists are still struggling with the poor mechanical strength and chemical stability of SSEs, and their interfacial issues for commercialization of AS3B. In spite of, development of prototype AS3Bs by several groups14, 45, 52, 76, implementation for commercial market is still far away. To achieve a low cost high energy density AS3Bs, researchers should focus on

development of mixed electronic and ionic conducting cathodes, layer formation for electrode and electrolyte, and cell stack designing. For commercialization, AS3Bs should also compete with the

energy density of rapidly falling price of current LIBs. Though Na-ore is omnipresent and inexpensive,


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but the technological progress to reduce the cost for overall Na-ion battery and concerned

commercialization is still far behind. For practical NIBs in electric vehicles, it should meet the target specific energy of about 300 Wh kg-1 and a cost of $125 (kWh)-1 by 20259, 114. Further understanding

on solid electrolyte and electrode synthesis and processing, layer formation of electrode and

electrolyte, and cell architecture, stack design and cell assembly to compete with the conventional LIBs are needed to realize the practical AS3B. (QUOTE 4)

Material synthesis and processing: In the past few years several attempts have been made on the synthesis of novel electrode material and electrolyte for NIBs11, 17,18, 76. Na-β-alumina and NASICON

based materials have been studied for few decades and several ceramic-processing methods are used to develop thin-film and tubular shape membranes for all-solid-state ionic devices22,30. As

chalcogenide-based electrolytes are moisture and air sensitive, the synthesis is carried out at inert and

dry environment. Though precursor materials for most of the recently studied Na-ion conductor are

inexpensive, their bulk scale synthetic strategy and storage environment may enhance the price of the

electrolyte. Recently explored borate-based SSEs should be further studied for practical applications.

A detailed cost-analysis and life cycle analysis for up scaling of the novel materials are required for



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practical implementations. Further improvement of total (bulk + grain-boundary) ionic conductivity of SSEs to realize RT AS3B system is also very important.

Layer formation of electrode and electrolyte. Preparing a thin layer of solid electrolyte is one of the most crucial steps for fabricating RT AS3B. The solid ceramic powder must be mixed with suitable

solvents and then could be tape casted or screen-printed. SSE powder and solvents can be mixed in

planetary ball-mill to prepare a slurry and casted over suitable substrate. Layer formation of SEs

powder are important to fabricate a dense, highly ionic conductive with excellent mechanical strength to make it safe and as thick (~ 20-30 µm) as conventional Li-ion battery separator116. The densification

or compaction process may be carried out using pressing or sintering. The technique for pressing is

good for maintaining appropriate porosity but still suffer from less ionic conductivity. Though

sintering process increase the conductivity but make the SSEs rigid which is detrimental for the

interface. To improve the interfacial contact between the electrolyte and electrode, metal oxide or

polymer coating on SSEs surface could be carried out. There are plenty of rooms to explore in this area to realize the AS3B commercialization. A very popular and successful technique used in solid oxide

fuel cell was used to fabricate a bi/tri-layer electrolyte. In the triple layer design, a dense membrane is


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sandwiched between two porous electrolyte layers where the electrode material is infiltrated115. This

process is shown to improve the interfacial contact with the electrode and SSEs.

Processing of the electrodes has been carried out using mainly two strategies to evaluate the

performance of cathode material with solid electrolyte. The first strategy is coating the conventional

cathode/carbon/polymer mixed slurry over current collector and connecting with the electrolyte pellet.

Most of the reported solid-state Li batteries with SSEs still use drops of liquid electrolyte for better contact at cathode and SSEs106. The second strategy is to fabricate all-solid-state interface between

cathode and electrolyte by sintering both the material together to obtain low interfacial resistance. The

cathode material could be mixed with electrolyte powder to obtain a mixed electronic and ionic

conductivity. This mixture powder would be kept on the top of electrolyte powder layer and hot

pressed followed by sintering to obtain a good electrochemical interface between cathode and

electrolyte. The interface can be modified using a few nanometer coatings of metal oxides. The

interlayer reduces the interfacial resistance and improves the ionic transport between the electrode and

electrolyte interface.

Cell architecture, stack design and cell assembly. Cell architecture and design has significant role to

enhance the energy density and overall performance of all-solid-state battery. Especially, the bipolar



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stack design could improve the volumetric energy density than conventional liquid electrolyte. Not much detailed studies have been carried out yet in this area116. Furthermore, computational studies

could help to design an efficient stack and cell. Volume expansion and contraction during the charging

and discharging process may lead to crack and damage the interface. Innovative stack design and cell

assembly are important to overcome this obstacle.

Summary and perspective. Sodium-based energy storage systems are promising for applications in

portable electronics, electric vehicles to grid-level storage because of the material abundance, economic viability and versatile chemistry. AS3B with ceramic SSE attract wide attention because of

their non-corrosive, non-combustible, leak-proof and internal short circuit free properties. Among

different ceramic SSEs, Na-β-alumina and NASICON possess high ionic conductivity and a wide electrochemical stability window. AS3Bs based on Na-β-alumina and NASICON have demonstrated

more than 10000 cycles. However, with rigid structures and large Young’s modulus, Na-β-alumina

and NASICON are difficult to have sufficient contact with cathodes. A soft Na-ion conducting

interlayer such as ionic liquid, polymer, gel, or composite is necessary for the operation of Na-β-alumina and NASICON-based AS3B. Na-β-alumina and NASICON SSEs require usually high

temperature sintering, above 1200 °C and hence controlling the chemical composition is remaining


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challenging due to Na evaporation. Comparing with oxide-based SSEs, chalcogenide-based and

borate-based SSEs possess facile production requirement and high ionic conductivity equal to or better

than Na-β-alumina and NASICON. Chalcogenide-based and borate-based SSEs can be easily

engineered to achieve a good contact with electrode materials. Mechanical milling of Na3PS4 with

cathode materials to make a composite cathode was proven to effective for both intercalation type and Na-S type AS3B. Na2(B12H12)0.5(B10H10)0.5 and Na3SbS4 are solution processable which allows

solution impregnation of the electrolyte onto the cathode particles which provides high surface-coverage coating and greatly ameliorates interfacial contact. AS3B full cells have been

successfully assembled employing chalcogenide-based and borate-based SSEs. However, most of

them are unable to persist long-cycle performance (< 50 cycles). Further studies about their

degradation mechanism are needed and imperative modification should be performed to improve their stability. Although many challenges remain to be solved, we envision that AS3B with ceramic

electrolyte would have a great impact on energy storage market in the near future. QUOTES (1) High sintering temperature, anisotropic conduction, large grain-boundary resistance at low

temperature, and chemical reaction with moisture and CO2 warrant Na-β-alumina’s application in RT AS3Bs.



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(2) High grain-boundary and solid electrolyte-electrode interface impedance of NASICONs

imped their use in advanced solid-state Na batteries. (3) AS3Bs with borate and chalcogenide-based SSEs can operate at low energy density due to

their limited electrochemical stability. (4) To achieve the practical high performance and reliable AS3B, extensive interdisciplinary

research should be focused on discovery novel solid Na-ion electrolytes, deeper

understanding of electrode and electrolyte interface reaction products, and developing cost

effective electrolyte and electrode processing methods, together with cell architecture – stack

design. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Notes

The authors declare no competing financial interest.

BIOGRAPHIES

Chengtian Zhou is a Ph.D. student in Department of Chemistry at the University of Calgary under the

supervision of Prof. V. Thangadurai. He received his dual BSc degree from Changzhou University,


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China and St. Francis Xavier University, Canada in 2017. His graduate studies involve development of

solid-state (ceramic) electrolyte for energy storage system.

Sourav Bag is a postdoctoral associate in Department of Chemistry at the University of Calgary under

the supervision of Prof. V. Thangadurai. He obtained his PhD degree from Indian Institute of

Technology (IIT) Kharagpur in 2016. His research interests are in solid-state metal-based energy

storage devices.

Venkataraman Thangadurai is full professor of chemistry at the University of Calgary, Canada. He

received his Ph.D. from the Indian Institute of Science, Bangalore, India in 1999. He received his

Habilitation degree from the University of Kiel in 2004. His research interests include discovery of fast

ion conducting solids and mixed conductors for all-solid state ionic devices such as batteries, fuel cells,

electrolysis cells and gas sensors. www.ucalgary.ca/vthangad

ACKNOWLEDGMENTS

The Natural Sciences and Engineering Research Council of Canada (NSERC) have supported this

work through discovery grants to one of us (V. T.) (Award number: RGPIN-2016-03853). Authors

also thank the University of Calgary for the support.



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