http://pubs.acs.org/journal/aelccp
Engineering Materials for Progressive AllSolid-State Na Batteries Chengtian Zhou, Sourav Bag, and Venkataraman Thangadurai*
ACS Energy Lett. 2018.3:2181-2198. Downloaded from pubs.acs.org by LULEA UNIV OF TECHNOLOGY on 09/14/18. For personal use only.
Department of Chemistry, University of Calgary, 2500 University Drive Northwest, Calgary, Alberta T2N 1N4, Canada ABSTRACT: Following the prevalence of the Li-ion battery for electrical energy storage systems (EESs), the world is looking toward alternative, cost-effective, electrical EESs 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 is discussed together with historically important Na-β-alumina and Na superionic conductors (NASICONs). Furthermore, we report on engineering a ceramic Na-ion electrolyte and electrode interface, which is considered a bottleneck for practical applications of solid-state electrolytes in AS3Bs. A soft Na-ion conducting interlayer is critical to suppress the interfacial 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.
E
NIB full cells were developed in the 1980s, their inferior performances compared to the LIBs prohibit practical utilization.7 A significant effort for development of highcapacity electrodes and electrolyte materials for NIBs is being made to respond to the increasing global energy crisis in the 21st century. Recent studies on insertion electrodes for NIBs show comparable energy density and relatively higher rate performances than Li-based materials due to the low Lewis acidic nature of Na. Few companies have started fabricating prototype NIBs.7 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 as a solvent for liquid organic electrolyte in NIBs, decomposes the polypropylene membrane. The dendrite formation, noncompatibility with the separator, safety issues, and expensive nature of liquid electrolyte prohibit realization of NIBs based on liquid electrolytes. Introduction of a solid-state electrolyte (SSE) instead of liquid electrolytes in NIBs improves the energy density, recyclability, and safety issues as SSEs are noncombustible, leak-proof, and internal short circuit free.8 To date, interests in electrification of vehicles, and consequently the launch of Tesla’s models S/X and Nissan leaf, have triggered the battery industry to design more efficient and safer batteries. The vehicle technologies office of the U.S. Department of Energy
nergy has been recognized as the fundamental enabler for socioeconomic and technological development of a modern society. Combustion of fossil fuel as the major component of today’s energy supply raises environmental concerns, including global warming, air quality deterioration, acid rain, and oil spills. 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 consumed.1 Batteries are deemed a promising energy storage method because of their high energy efficiency and flexible designs.2−4 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 densities.5 LIBs are being widely employed to power portable electronics and 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 suggests that all of the reserved Li will likely be consumed in next 28 years, if it is not recycled.6 Additionally, the pressing demand for electric vehicle manufacturing would be evident in Li shortage and further price increase. Thus, alternative costeffective energy storage is required without compromising the energy density and efficiency. Sodium ion batteries (NIBs) have been considered to replace LIBs due to their low cost compared to Li. NIB technology is very promising for next-generation energy storage as they have similar battery chemistry as LIBs. Though © 2018 American Chemical Society
Received: June 6, 2018 Accepted: August 3, 2018 Published: August 3, 2018 2181
DOI: 10.1021/acsenergylett.8b00948 ACS Energy Lett. 2018, 3, 2181−2198
Review
Cite This: ACS Energy Lett. 2018, 3, 2181−2198
ACS Energy Letters
Review
Figure 1. Evaluation of solid Na-ion electrolytes for AS3Bs.11,14−20
Figure 2. (A) Schematic illustration of a typical Na−S cell containing a molten Na anode, Na-β″-alumina solid electrolyte, and molten S as the 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 the c-axis. Red: O2−; yellow: Na+; and blue: Al3+. NASICON with (C) 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 split the M2 sites into M2α and M2β (adopted from ref 33, Copyright 2017 with permission from Elsevier).
paradigm for solid-state battery research from improving electrolyte ionic conductivity to alleviate impedance between SSEs and electrodes through interfacial engineering. The poor interfacial contact between the electrode and electrolyte, manifested as a high interface impedance, becomes the biggest bottleneck for all-solid-state batteries. Several strategies have been carried out recently to improve interfacial 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
(DOE) set a target cost for an electric vehicle battery pack of $125/kWh by 2020, which is 75% less than that in 2012. The DOE has also emphasized development of high-voltage safe solid-state (ceramic) electrolytes beyond LIBs.9 Toyota has developed a solid superionic conductor based on sulfide, achieving a conductivity equal to that of a liquid electrolyte.10 This remarkable achievement triggers practical realization of all-solid-state batteries. The room-temperature all-solid-state sodium battery (RTAS3B) was successfully demonstrated with cubic Na3PS4 electrolyte.11 Ongoing extensive research on allsolid-state sodium batteries (AS3Bs) suggests a change of the 2182
DOI: 10.1021/acsenergylett.8b00948 ACS Energy Lett. 2018, 3, 2181−2198
ACS Energy Letters
Review
with polymer and liquid electrolyte.13 This review emphasizes ceramic Na-ion electrolyte/electrode interfacial modification strategies in AS3Bs and also provides an 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 AS3Bs: • total (bulk + grain boundary) ionic conductivity of ∼10−2 S cm−1 with negligible electronic and anionic conductivities at the operating temperature, preferably from −50 to 100 °C • chemically stable against reaction with high-voltage Nabased cathodes • chemically stable against reaction with elemental Na and Na−metal alloy anodes • mechanically stable, dense and avoid growth of Na dendrite at high current densities • wide range of the electrochemical potential stability window (>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 • easy to scale up the synthesis, amiable for cost-effective fabrication methods, environmentally safe, and chemically stable under ambient conditions Figure 1 shows a brief overview of solid-state Na-ion conduction known since 1960. None of the presently known ceramic Na-ion electrolytes simultaneously meet all of the
above-mentioned requirements. In the following sections, we discuss 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 the reported period. First-Generation Solid Na-Ion Electrolytes; Na-β-alumina. Yao and Kummer first realized the extraordinarily high Na-ion mobility in Na-β-alumina in 1967,21 which led to the development of high-temperature Na batteries (NBs). Figure 2A shows the schematic of a NB containing a Na anode, a Naβ″-alumina electrolyte, and S cathode. There are two different crystal structures known for Na-β-alumina: β-Al2O3 (hexagonal; space group P63/mmc, with a formula of Na2O·8−
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. 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 a 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 a weaker attraction with the 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.
Figure 3. Periodic table illustrating the elements that were doped in the NASICON (NaZr2P3O12) structure (yellow: Na-site; green: Zr-site; and orange: P-site).34−41 2183
DOI: 10.1021/acsenergylett.8b00948 ACS Energy Lett. 2018, 3, 2181−2198
ACS Energy Letters
Review
at 75 °C, evidenced by variation of the P+/PO+ intensity ratio, attributed 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 glovebox and treatment above 600 °C can remove the water and CO2 uptake of the sample. The synthesizing requirements of NASICON materials are not as stringent as those of Na-β″-alumina, but still, high-temperature sintering (∼1200 °C) is needed to prepare phase-pure highly ionic conductive NASICONs.
Solid-state reaction at 1600 °C is typically carried out using α-Al2O3 and Na2CO3 to prepare β″-alumina.24 Despite the high ionic conductivity and excellent chemical/electrochemical stability, the complicated process and harsh sintering conditions hamper application of β-alumina. Several synthesizing methods such as sol−gel25,26 coprecipitation,27 sprayfreeze/freeze-drying,25,26 flame spray pyrolysis,23,28 and vaporphase processes29 have developed to reduce the sintering temperature, decrease grain boundary resistance, and 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 it is difficult to build a 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 application.30 Second-Generation Solid Na-Ion Electrolytes: NASICONs. In 1976, Goodenough and Hong developed a 3D framework structure for fast Na superionic conduction (NASICON), which exhibits isotropic conduction.31,32 The originally reported NASICON chemical formula is Na1+xZr2P3−xSixO12 (0 ≤ x ≤ 3), which consist of corner-sharing PO4, SiO4 and ZrO6 creating a 3D 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+ migrating through paths in a NASICON structure. A rhombohedral structure for NASICON, with space group R3c, 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 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 conductivity of 6.7 × 10−4 S cm−1 at RT.15 Figure 3 shows a periodic table of elements doped in the NASICON structure to understand the crystal structure, chemical composition, and ionic conductivity property relationship. NASICON materials generally show a huge grain boundary resistance (Rgb) to total conductivity at RT.42 Attempts have been made to improve the total ionic conductivity (bulk + grain boundary) by applying different sintering techniques. Naqash et al.43 prepared Na3Zr2Si2PO12 with a total ionic conductivity of 1 mS cm−1 through a solution-assisted solidstate 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 with different elements changes the Na-ion diffusion bottleneck, enables extra Na-ions in the structure, or minimizes 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. A solid-state Na−S cell has been successfully fabricated using Na3.1Zr1.95Mg0.05Si2PO12 electrolyte. Several NASICON-type materials also possess a stable structural framework and good thermal stability.46 Na3Zr2Si2PO12 is confirmed to gradually react with molten Na at ∼300 °C.47,48 Wang et al.49 conducted a TOF-SIMS study on the interface of Na and Na3Zr2Si2PO12, showing that reactions between Na and electrolyte follow the diffusion of Na into the NASICON
High grain boundary and solid electrolyte−electrode interface impedance of NASICONs impede their use in advanced RT all-solid-state sodium batteries. Third-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 quasispherical architectures with high symmetry (Figure 4A,B).17,18 Large closo-borate anions remain translationally rigid and possess disordered lattices with a cation vacancy, which enables facile Na+ hopping in the structure.18,52 Several hydrides anion, including [BH4]−, [NH2]−, [AlH4]−, and [AlH6]3−, form complexes with several alkali ions that exhibit unique applications. LiBH4 is a typical example for a hydrogen storage compound53 and shows fast Li+ conductivity of ∼1 mS cm−1 due to structural transition at ∼117 °C.54 Na(BH4)0.5(NH2)0.5 exhibits Na-ion conductivity of 2 × 10−6 S cm−1 at 27 °C, which is 4 orders of magnitude higher than that of the host materials NaBH4 and NaNH2.55 Subsequently, Li and Na compounds with large polyhedral closo-borate anion architectures were discovered, which showed entropy-driven order−disorder transitions,56 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 disorderinduced superionic conductivity at elevated temperature.56−58 The extraordinary Na-ion conductivity and comparatively low activation energy 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 °C. Introducing extrinsic vacancies was suggested to further improve the conductivity.59 Na2B10H10 remains airstable at RT with no noticeable degradation up to 227 °C and exhibits an electrochemical stability window (ESW) up to 5 V at 120 °C.56 First-principle study showed that sodium borohydrides tend to be oxidized at relatively low voltages, but the corresponding decomposition products, such as B12H122−-containing phases, have wide ESWs, which seem to protect the electrolyte, leading to a large ESW of 5 V.60 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 2184
DOI: 10.1021/acsenergylett.8b00948 ACS Energy Lett. 2018, 3, 2181−2198
ACS Energy Letters
Review
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; gray: H). Orientationally disordered CB9H10− and CB11H12− anions (superimposed) are shown in spherical shells (Reprinted from ref 18). (C) Unit cells 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). (E) Structure of Na11Sn2PS12 from single-crystal data; the Na1/Na2-ions having sites with fractional occupancy are represented by rose ellipsoids, and the 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).
nonconducting phase when the temperature decreases.61 In order to stabilize these superionic phases down to RT, strategies have been proposed including mechanical milling to reduce the particle size and cation/anion substitutions to enhance disorder.18,61−63 Na3BH4B12H12, (Li0.7Na0.3)3BH4B12H12, and ball-milled Na2B12H12 all reach RT conductivity values close to 10−3 S cm−1. Duchene et al.52 reported an AS3B using a Na2(B12H12)0.5(B10H10)0.5 electrolyte, Na anode, and NaCrO2 cathode, which exhibits reversible and stable cycling with a capacity of 85 mAh g−1 at 0.05C. Na2(B12H12)0.5(B10H10)0.5 shows 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, an 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 modulus.60 Carbon-substituted NaCB 11H12 and NaCB9 H10 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. Compared with other borate compounds, C-incorporated closo-borates have much lower transition temperatures: ∼400 K for NaCB11H1264 and ∼RT for NaCB9H10.65 Na2(CB9H10)(CB11H12) possesses the highest Na-ion conductivity up to 0.07 S cm−1 at RT,18 unmatched by any other polycrystalline 2185
DOI: 10.1021/acsenergylett.8b00948 ACS Energy Lett. 2018, 3, 2181−2198
ACS Energy Letters
Review
ceramics,79 when x = 0.06; Na3.06P0.94Si0.06S4 possesses the highest Na-ion conductivity of 7.4 × 10−4 S cm−1 at RT.80 The effect of Si-doping and the associated increase in ionic conductivity was explained by Zhu et al. using first-principles calculation. Na disorder in the structure was induced by a small amount of excess Na, which attributed to enhanced Na-ion conductivity.81 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 AS 3 B comprised of Na 2+2x Fe 2x (SO 4 ) 3 |Na 3.1 Sn 0.1 P 0.9 S 4 | Na2Ti3O7 was demonstrated with the first discharge capacity near the theoretical value 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 sulfidebased Na-ion electrolytes. Full cell performance was carried out at 80 °C with a 0.02 C rate for TiS2|Na3P0.62As0.38S4|Na− Sn, and an irreversible sharp capacity decay from 118 to 103 mAh g−1 was observed during the first nine cycles. In addition, it should be mentioned that Na3P0.62As0.38S4 reacts with water and eventually generates a toxic arsenic compound. Zhang et al.75 demonstrated a substitution of S with Se, resulting in cubic Na3PSe4 that showed a Na-ion conductivity of 1.2 mS cm−1 at RT. The higher ionic conductivity of the Sesubstituted product was mainly attributed to the larger atomic radius of Se than S, which could result in larger diffusion bottlenecks and a weaker binding energy between Se and Na compared to that between the S and Na. However, Na3PSe4 processes a significantly narrower ESW than Na3PS4,84 which makes it impractical for battery applications. The phosphorus site of Na3PS4 has also been successfully substituted with Sb to obtain high Na-ion conductivity. On the basis of the hard and soft acid and base theory, Liang and co-workers attempted to improve the stability of the sulfide by introducing elements that act as soft acids that can bind strongly with S.16 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, the 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, a Na3SbS4-coated NaCrO2 cathode was demonstrated to provide intimate ionic contact with SSE, resulting in dramatic improvement in the cycling performance of the battery. In Zhang’s work,85 2.5 mol % Na deficiency in tetragonal Na3SbS4 was revealed, and the experimental value was verified using theoretical study. This material shows a total ionic conductivity of 1 mS cm−1 at RT. However, it is instable against metallic Na, which is a common problem for most chalcogenide-based electrolytes.11,73,86 Table 1 shows the electrical conductivity, activation energy, and ESW of some chalcogenide- and boratebased Na-ion electrolytes. Most of these electrolytes show a wide ESW ≥ 5 V from cyclic voltammetry; however, the stability window seems to vary with experimental conditions.17 Inspired by the discovery of a fast Li-ion conducting Li10GeP2S12 (Thio-LISICON) with a conductivity of ∼1.0 × 10−2 S cm−1 at RT, researchers predicted that analogous Na10GeP2S12 would exhibit a high Na-ion conductivity.87 Richards et al.88 studied the family of Na10MP2S12 (M = Sn, Ge, and Si) based on first-principles simulations. Among them,
materials. However, ESW and battery performance remains to be tested for carbon-based closo-borates. Chalcogenide-Based Na-Ion Electrolytes. Although the oxidebased NASICON and Na-β-alumina have been well-developed to achieve a high ionic conductivity as well as good electrochemical stability,34 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 RT,11,68−70 low synthesizing temperature, and superior mechanical properties. Chalcogenide-based SSEs are generally softer than oxides,71 which allows intimate contact between the 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 gas.16 Therefore, extra caution is suggested when dealing with this group of materials. Hayashi et al.11 first reported the glass−ceramic electrolyte with the Na3PS4 phase crystallized from 75Na2S-25P2S5 glass at 270 and 400 °C, corresponding to cubic (P421c) and tetragonal phases (I43m), respectively. Both phases exhibit higher conductivities than the glassy phase, while cubic Na3PS4 possesses a slightly higher Na-ion conductivity of ∼2 × 10−4 S cm−1 at RT. The positions of Na in the cubic phase split into lower-energy Na1-sites with partial occupancy of 0.8 and higher energy Na2-sites with partial occupancy of 0.1.66 The 3D migration pathways are believed to lead toward higher conductivity of cubic Na3PS4 (Figure 4C).34,66,74 Hayashi et al. also fabricated a RT AS3B, Na−Sn|Na3PS4|TiS211 and showed a few cycles with a low current density of 0.013 mA cm−2. The highest conductivity of 4.6 × 10−4 S cm−1 was obtained by ballmilling of a stoichiometric amount of precursor for 1.5 h and consecutive heat treatment at 270 °C for 1 h.68 Furthermore, through substitution of the P-site with Sb or the S-site with Se, the total ionic conductivity of the chalcogenide-based electrolyte exceeded 1 mS cm −1 at RT 67,75 However, the decomposition potential of Na3PS4 is reported to be as low as 2.7 V vs Na+/Na.76
AS3Bs with borate- and chalcogenidebased SSEs can operate at a low power density due to their limited electrochemical stability. 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 Cldoped tetragonal t-Na2.9375PS3.9375Cl0.0625 by mixing a 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 methods. 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. Unfortunately, Na vacancies with halide-doping resulted in slight lattice expansion and promoted formation of the tetragonal phase, which led to an unsuccessful attempt to synthesize Cl-doped cubic Na3PS4. Hayashi and co-workers studied several compositions of Na3PS4·xNa4SiS4 glass− 2186
DOI: 10.1021/acsenergylett.8b00948 ACS Energy Lett. 2018, 3, 2181−2198
ACS Energy Letters
Review
Table 1. Ionic Conductivity Property of some ThirdGeneration Solid Na-ion Electrolytes solid electrolytes
σRT (S cm−1)
Ea (eV)
ESW vs Na+/Na (V)
Na3PS468,76 Na3.1Sn0.1P0.9S482 Na3.06P0.94Si0.06S479 Na3SbS416 Na3PSe475 Na3P0.62As0.38S483 Na10SnP2S1288 Na11Sn2PS1219 Na(BH4)0.5(NH2)0.555 Na2B10H1056 Na2B12H1258 Na3BH4B12H1261 ball-milled Na2B12H1263 NaCB9H1065 Na2(CB9H10)(CB11H12)18 Na2(B12H12)0.5(B10H10)0.517
4.6 × 10−4 2.5 × 10−4 7.4 × 10−4 1.0 × 10−3 1.2 × 10−3 1.5 × 10−3 4 × 10−4 1.4 × 10−3 2 × 10−6
