Sodium Superionic Conductors Research Articles

Fast Ion Transport Mechanism and Electrochemical Stability of Trivalent Metal Iodide-based Na Superionic Conductors Na3XI6 (X = Sc, Y, La, and In).

The exploration of solid-state sodium superionic conductors with high sodium-ion conductivity, structural and electrochemical stability, and grand interface compatibility has become the key to the next-generation energy storage applications with high energy density and long cycling life. Among them, halide-based compounds exhibit great potential with the higher electronegativity of halogens than that of the sulfur element. In this work, combined with first-principles calculation and ab initio molecular dynamic simulation, the investigation of trivalent metal iodide-based Na superionic conductors C2/m-Na3XI6 (X = Sc, Y, La, and In) was conducted, including the fast ion transport mechanism, structural stability, and interface electrochemical compatibility with electrode materials. Along with the tetrahedral-center saddle site-predominant three-dimensional octahedral-tetrahedral-octahedral diffusion network, C2/m-Na3XI6 possesses the merits of high Na ionic conductivities of 0.36, 0.35, and 0.20 mS cm-1 for Na3ScI6, Na3YI6, and Na3LaI6, respectively. Benefiting from its structural stabilities, C2/m-Na3XI6 exhibits lower interface reaction energy and better electrochemical compatibility in contact with both Na metal and high-voltage poly-anion (fluoro)phosphate cathode materials than those of sulfide-based ones. Our theoretical work provides rational design principles for screening and guiding iodide-based C2/m-Na3XI6 (X = Sc, Y, La, and In) as promising Na superionic conductor candidates used in all-solid-state energy storage applications.

Soft-phonon anharmonicity, floppy modes, and Na diffusion in Na3FY (Y=S,Se,Te) : Ab initio and machine-learned molecular dynamics simulations

A class of Na-based antiperovskites ${\mathrm{Na}}_{3}XY$ $(X=\mathrm{F},\mathrm{H};Y=\mathrm{S},\mathrm{Se},\mathrm{Te})$ was recently reported with a remarkably high ionic conductivity \ensuremath{\sim}0.1 mS/cm near room temperature. Herein, we report comprehensive atomic dynamics investigations using ab initio and large-scale machine-learned molecular dynamics (MLMD) simulation on these sodium superionic conductors. Previous studies identified the role of soft phonons involving rotations of ${\mathrm{FNa}}_{6}$ octahedral units in Na diffusion. In contrast, our MLMD simulations show that the Na diffusion pathways are essentially uncorrelated and do not involve the collective dynamics of octahedral rotations or reorientations. Moreover, while the soft phonons do not involve any $Y$ atomic dynamics, the diffusion pathways are sterically hindered unless facilitated by $Y$ displacements and Na vacancies. However, we do find that the anharmonic low-energy phonon modes of wave vectors along the $M\text{\ensuremath{-}}R$ line at the Brillouin zone (BZ) boundary are important precursors of Na diffusion. These modes involve vibrations perpendicular to the F-Na ionic bond, along $\ensuremath{\langle}100\ensuremath{\rangle}$ directions, which provide the initial pathways for the Na diffusion. We have calculated the branch-resolved phonon spectral energy density (SED) in the entire BZ using the MLMD simulations. The SED results as a function of temperature reveal the large anharmonicity of the soft phonon modes, which leads to floppy dynamics of Na atoms. The volume of cubic-${\mathrm{Na}}_{3}\mathrm{FS}$ is the smallest among the three antiperovskites and has the largest mean-phonon energy. Our calculated Na diffusion coefficient at 700 K in ${\mathrm{Na}}_{3}\mathrm{FS}$ of $\ensuremath{\sim}0.63\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}/\mathrm{s}$ is significantly larger than $\ensuremath{\sim}0.16\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}/\mathrm{s}$ in ${\mathrm{Na}}_{3}\mathrm{FTe}$, which clearly shows contrast with the hypothesis of a softer lattice leading to faster diffusion. The estimated diffusion barrier energy for ${\mathrm{Na}}_{3}\mathrm{FS}$ with 2% Na vacancy is \ensuremath{\sim}0.44 eV, which is in fair agreement with the reported value of 0.44 eV estimated from the total conductivity measurement with Na-vacant iodine-doped ${\mathrm{Na}}_{3}\mathrm{FS}$. Similarly, for ${\mathrm{Na}}_{3}\mathrm{FSe}$ and ${\mathrm{Na}}_{3}\mathrm{FTe}$, the barrier energies have been estimated to be 0.50 and 0.55 eV, respectively. Our large-scale MLMD-based theoretical study provides a comprehensive understanding of the role of soft phonons, host dynamics, and vacancies in Na diffusion in ${\mathrm{Na}}_{3}\mathrm{F}Y$ $(Y=\mathrm{S},\mathrm{Se},\mathrm{Te})$ and other materials of this class, which will be helpful in designing materials for application in solid-state batteries.

NASICONs‐type solid‐state electrolytes: The history, physicochemical properties, and challenges

AbstractSolid‐state electrolytes are critical for the development of next‐generation high‐energy and high‐safety rechargeable batteries. Among all the candidates, sodium (Na) superionic conductors (NASICONs) are highly promising because of their evident advantages in high ionic conductivity and high chemical/electrochemical stability. The concept of NASICONs was proposed by Hong and Goodenough et al. in 1976 by reporting the synthesis and characterization of Na1+xZr2(SixP3−x)O12 (0 ≤ x ≤ 3), which has attracted tremendous attention on the NASICONs‐type solid‐state electrolytes. In this review, we are committed to describing the development history of NASICONs‐type solid‐state electrolytes and elucidating the contribution of Goodenough as a tribute to him. We summarize the correlations and differences between lithium‐based and sodium‐based NASICONs electrolytes, such as their preparation methods, structures, ionic conductivities, and the mechanisms of ion transportation. Critical challenges of NASICONs‐structured electrolytes are discussed, and several research directions are proposed to tackle the obstacles toward practical applications.

Ionic conductivity and ion transport mechanisms of solid‐state lithium‐ion battery electrolytes: A review

AbstractThis review article deals with the ionic conductivity of solid‐state electrolytes for lithium batteries. It has discussed the mechanisms of ion conduction in ceramics, polymers, and ceramic‐polymer composite electrolytes. In ceramic electrolytes, ion transport is accomplished with mobile point defects in a crystal. Li+ ions migrate mainly via the vacancy mechanism, interstitial mechanism, or interstitial‐substitutional exchange mechanism. In solid polymer electrolytes, Li+ ions are transported mainly via the segment motion, ion hopping (Grotthuss mechanism), or vehicle mechanism (mass diffusion). This study has also introduced various electrolyte materials including perovskite oxides, garnet oxides, sodium superionic conductors, phosphates, sulfides, halides, cross‐linked polymers, block‐copolymers, metal‐organic frameworks, covalent organic frameworks, as well as ceramic‐polymer composites. In addition, it has highlighted some strategies to improve the ionic conductivity of solid‐state electrolytes, such as doping, defect engineering, microstructure tuning, and interface modification.

Improving the stability of NASICON-type electrolyte with Li metal anode by interfacial modification

Sodium super-ionic conductors (NASICON)-type electrolyte Li1.4Al0.4Ti1.6(PO4)3 (LATP), with high ionic conductivity and low cost, is considered as one of the most attractive alternatives to liquid electrolytes. However, the poor interfacial compatibilities of LATP electrolyte in lithium batteries lead to the failure, which hinders its further development. Herein, a MoS2 coating layer as an artificial solid electrolyte interphase (ASEI) is used for modifying the surface of LATP (MCLATP) via an economical and uncomplicated spin coating method, which not only effectively inhibits the decomposition of LATP, but also in-situ forms a conversion layer consisting of Li2S and Mo metal during cycling. The conversion layer can improve the interfacial charge transfer kinetics and decrease the charge transfer resistance. According to interfacial modification of MoS2, the symmetric cells show slight polarization, and the Li/MCLATP/LFP cells demonstrate excellent cycling performance over 300 cycles at 1 C. The enhanced batteries performance is ascribed to interfacial modification of MoS2 as an ASEI layer, which reduces interfacial concentration polarization caused by the formed microcracks around the surface during decomposition of LATP. This work provides a promising strategy to construct the interface between Li metal and solid-state electrolytes for other unstable electrolytes beyond NASICON.

The origin of high Na+ ion conductivity in Na1+xZr2SixP3-xO12 NASICON materials.

Due to the high sodium ion conductivity, sodium super ionic conductors (NASICONs) are among the most promising candidates as solid electrolytes in solid state batteries and have therefore gained enormous attention in recent years. Previous experimental and computational investigations show excellent sodium ion conductivity for Na1+xZr2SixP3-xO12 with x = 2-2.5. In order to elucidate the conductivity maximum at high substitution levels, we investigate the influence of the cation environment on the site energies of sodium ions and the correlated migration using density functional theory. The results reveal that the site energy strongly depends on electrostatic effects. In addition, an increasing fraction of sodium and silicon ions enhances the correlated migration due to the increasing coulombic repulsion of sodium ions and opening of the bottleneck along the migration path. Based on the results, we generate an energy model to predict configurational and migration energy in subsequent Kinetic Monte Carlo simulations. We show that sodium ions are trapped due to introduced silicon ions but percolate in the system for x≥2.0, which greatly increases the ionic conductivity.

Sodium Superionic Conductors (NASICONs) as Cathode Materials for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) have developed rapidly owing to the high natural abundance, wide distribution, and low cost of sodium. Among the various materials used in SIBs, sodium superion conductor (NASICON)-based electrode materials with remarkable structural stability and high ionic conductivity are one of the most promising candidates for sodium storage electrodes. Nevertheless, the relatively low electronic conductivity of these materials makes them display poor electrochemical performance, significantly limiting their practical application. In recent years, the strategies of enhancing the inherent conductivity of NASICON-based cathode materials have been extensively studied through coating the active material with a conductive carbon layer, reducing the size of the cathode material, combining the cathode material with various carbon materials, and doping elements in the bulk phase. In this paper, we review the recent progress in the development of NASICON-based cathode materials for SIBs in terms of their synthesis, characterization, functional mechanisms, and performance validation/optimization. The advantages and disadvantages of such SIB cathode materials are analyzed, and the relationship between electrode structures and electrochemical performance as well as the strategies for enhancing their electrical conductivity and structural stability is highlighted. Some technical challenges of NASICON-based cathode materials with respect to SIB performance are analyzed, and several future research directions are also proposed for overcoming the challenges toward practical applications. Graphic Abstract

Co-Sintering Study of Na0.67[Ni0.1Fe0.1Mn0.8]O2 and NaSICON Electrolyte–Paving the way to High Energy Density All-Solid-State Batteries

Sodium is a promising candidate for stationary storage applications, especially when the demand for lithium-ion batteries increases due to electromobility applications. Even though its energy density is lower, Na-ion technology is estimated to lead to a cost reduction of 30% compared to Li-ion technology. To improve safety as well as energy density, Na-based all-solid-state-batteries featuring solid electrolytes such as beta-alumina and sodium superionic conductors and cathode materials such as Na3V2(PO4)3 and NaxCoO2 have been developed over the past years. However, the biggest challenge are mixed cathodes with highly conductive interfaces, especially when co-sintering the materials. For example, a promising sodium superionic conductor type Na3Zr2Si2PO12 electrolyte sinters at 1,250°C, whereas the corresponding Na3V2PO12 cathode decomposes at temperatures higher than 900°C, posing a bottleneck. Thus in this paper, we synthesized Na0.62 [Ni0.10Fe0.10Mn0.80]O2 as cathode material for all-solid-state sodium-ion batteries via a relatively cheap and easy solution-assisted solid state reaction processing route. The thermal investigations of the pure cathode material found no degradation up to 1,260°C, making it a perfect match for Na3.4Zr2Si2.4P0.6O12 electrolyte. In our aim to produce a co-sintered mixed cathode, electron microscopy investigation showed a highly dense microstructure and the elemental mapping performed via energy dispersive X-ray spectroscopy and secondary ion mass spectrometry confirm that Na3.4Zr2Si2.4P0.6O12 and Na0.62 [Ni0.10Fe0.10Mn0.80]O2 do not react during sintering. However, the active cathode material forms a sodium rich and a sodium deficient phase which needs further investigation to understand the origin and its impact on the electrochemical performance.

Processing thin but robust electrolytes for solid-state batteries

The widespread adoption of high-energy-density solid-state batteries (SSBs) requires cost-effective processing and the integration of solid electrolytes of about the same thickness as the polymer-membrane separators found in conventional lithium-ion batteries. In this Review, we critically discuss the current status of research on SSB processing as well as recent cost calculations, and compare SSB oxide electrolyte material and processing options in terms of performance parameters for thick versus thin ceramics. We identify as critical for future SSB design the need to capture the thermal processing budget and the stability of the phase of interest for oxide solid electrolytes, namely lithium phosphorus oxynitride, sodium superionic conductors, perovskites and garnets, in addition to the classic plots of Arrhenius lithium transport and the electrochemical stability window. Transitioning to SSB oxide electrolyte films with thicknesses close to the range for lithium-ion battery separators could provide ample opportunities for low-temperature ceramic manufacture and potential cost reduction. High-performance solid-state electrolytes are key to enabling solid-state batteries that hold great promise for future energy storage. The authors survey the fabrication process of thin-film versus thick oxide-based solid-state electrolytes and discuss their material design and processing options.

Design Principles for Cation‐Mixed Sodium Solid Electrolytes

AbstractAll‐solid‐state sodium‐ion batteries are highly promising for next generation grid energy storage with improved safety. Among the known sodium superionic conductors, the Na3PnS4 family and the recently discovered Na11Sn2PnS12 (Pn = P, Sb) have garnered major interest due to their extremely high ionic conductivities. In this work, comprehensive investigation of the Na3PnS4‐Na4TtS4 (Pn = P/As/Sb, Tt = Si/Ge/Sn) phase space of superionic conductors using density functional theory calculations, as well as AIMD simulations on the promising new Na11Sn2PnS12 (Pn=P/As/Sb) structures are presented. Crucial design rules on the effect of cation mixing are extracted on relative phase stability, electrochemical stability, moisture stability, and ionic conductivity. In particular, it is shown that while larger cations can substantially improve the ionic conductivity and moisture stability in these structures, there is an inherent trade‐off in terms of electrochemical stability. Na11Sn2AsS12 is also identified as a hitherto unexplored stable sodium superionic conductor with higher Na+ conductivity and better moisture stability than the Na11Sn2PS12 and Na11Sn2SbS12 phases already reported experimentally.

Recent progress and future prospects of atomic layer deposition to prepare/modify solid-state electrolytes and interfaces between electrodes for next-generation lithium batteries.

Lithium ion batteries (LIBs) are encouraging electrochemical devices with remarkable properties including a high energy/power density, fast charging capability, and low self-discharge rate. Further increase in energy density as well as safe usage is needed for next-generation LIBs in electric transportation vehicles. Solid-state electrolytes (SSEs) are very promising for high-performance LIBs since they enable improved safety along with increased energy density compared to flammable liquid organic electrolytes. However, utilizing SSEs with a Li metal anode is very challenging due to the possibility of undesired side reactions and the formation of an unstable solid-electrolyte interphase. Therefore, it is critical to enhance the stability of SSEs against the Li anode. One feasible approach is to form a thin and conductive interlayer between the Li anode and solid-state electrolyte. Atomic layer deposition (ALD) is a unique technique for conformal coating of complex 3D structures with finely controlled film thickness (at the atomic scale). ALD coating on the surface of SSEs can be adopted for engineering solid-electrolyte interfaces with desired attributes and improved stability. In this review paper, we have discussed recent progress in implementing the ALD technique for depositing thin layers on various SSE configurations including lithium phosphorus oxynitride (LiPON), garnets, oxides, perovskites, sulphides, Li3BO3–Li2CO3 (LBCO), and sodium super ionic conductors (NASICON). We have also highlighted the major areas for future research and development in the field. We believe that this review will be very helpful for directing future research on implementing ALD for synthesizing stable and high-performance SSEs with an engineered solid-electrolyte interface for next-generation electrochemical devices (e.g., Li-ion batteries, supercapacitors, and flow batteries).

Temperature-Dependent Atomistic Dynamics Correlated to Cation Transport in Cluster-Ion Anti-Perovskites

As fast-ion conductors, cluster-ion anti-perovskites (APs) are attractive for potential new mechanisms of conduction whereby cation and anion motion is highly correlated [1,2]. Cluster-ion APs, like perovskites, have a framework that accommodates diverse atomic substitutions. Furthermore, the potential for cluster-ion substitution onto one or both anion sites (A and B in X3AB) provides additional compositional degrees of freedom. To identify model cluster-ion APs for understanding such conduction mechanism, we have synthesized and characterized ~20 cluster-ion APs compositions, from which Na3-x(NH2)xO1-x(BH4) has been identified as an ideal model system for understanding the correlation between cluster-ion rotation and cation transport as shown in Figure 1. Particularly, Na2(NH2)(BH4) exhibits an unusual positive deviation from ideal Arrhenius behavior indicative of a flattening of the energy landscape with increasing temperature. Concurrently, the sodium conductivity increases by 104 to 105, which is most likely due to some combination of the effects of Na vacancy introduction and NH2 and/or BH4 anion mobility. Herein, synthesis, property, and characterization of the cluster-ion APs, aimed at understanding the temperature-dependent atomistic dynamics and its effects on cation transport, will be discussed in the presentation.This work was supported as part of the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. Reference s : [1] Hong Fang and Puru Jena. Li-rich antiperovskite superionic conductors based on cluster ions. Proceedings of the National Academy of Sciences, 2017, 114, 110461.[2] Hong Fang and Puru Jena. Sodium Superionic Conductors Based on Clusters. ACS Appl. Mater. Interfaces. 2019, 11, 963-972. Figure 1

Anion and cation co-doping of Na4SnS4 as sodium superionic conductors

Na based all-solid-state batteries are one of the most promising candidates for large-scale electrochemical energy storage owing to their low cost and outstanding safety properties. The key enabler of this technology is solid electrolyte with sufficiently high ionic conductivity at room temperature. In this work, the design, synthesis, characterization, and testing of a series of novel fast Na-ion conductors, Na4-x-ySn1-xAxS4-yXy with aliovalent cation (P5+/Sb5+) and anion (Cl−/Br−) substitutions of the parent compound Na4SnS4 are reported. The doped samples crystalize in a new phase with space group I41/acd and show ionic conductivities that are one to four orders of magnitude higher than that of undoped Na4SnS4. R. t. conductivity of 0.64 mS cm−1 is achieved in Na3.7Sn0.8Sb0.2S3.9Cl0.1 with a low activation energy of 0.26 eV. Rietveld refinement against high resolution synchrotron X-ray powder diffraction data reveals that the distribution of the doping ions and the resulted Na vacancies are underlying causes of the enhanced conductivity.

Deciphering the Structure-Property Relationship in Sulfide-Based Solid Electrolytes for All-Solid-State Na-Ion Batteries

Safety concerns on conventional lithium ion batteries are attracting a lot of attention from both academia and industry with the rapid growth of the electric automobile market. Replacing flammable organic liquid electrolytes with nonflammable inorganic solid electrolytes can potentially eliminate the fire hazards. On the other hand, low-cost electrochemical energy storage technologies beyond lithium chemistry are also highly desired for large-scale energy storage applications. All-solid-state Na ion batteries are one of the promising candidates for grid-level energy storage owing to their low cost and outstanding safety properties. The key enabler of this technology is solid electrolyte with sufficiently high ionic conductivity at room temperature and good electrochemical stability. This work presents a group of novel sodium superionic conductors to address the foregoing issues, which is a topic currently receiving extensive attention in the field of electrochemistry and energy storage. Through a rational design approach and with using facile solid-state reactions, a series of novel Na4SnS4-Na3SbS4 solid-solution-based compounds with cation and anion substitutions were obtained. These sulfides show room temperature conductivities of 0.1~1 mS/cm, which is among the highest values reported for sodium ion conductors to date. The underlying structural factors, including lattice parameters, concentration of defects, local ordering, etc., of these compounds are investigated with a variety of characterization methods, including synchrotron X-ray diffraction, neutron diffraction, and pair distribution function analysis. Interesting structure-property relationships in these compounds were revealed. All-solid-state Na-ion battery cells using these as-synthesized materials as electrolyte was successfully assembled and cycled, demonstrating the promise of this group of sulfide-based compounds as solid electrolytes and implying new opportunities for materials and device development in solid-state batteries.

First-Principles Investigations on Sodium Superionic Conductor Na11Sn2PS12

Sodium superionic conductors are key components of solid-state sodium ion batteries, which are regarded as promising alternative energy storage options for large-scale application. Recently, a new ...

Group 14 element based sodium chalcogenide Na4Sn0.67Si0.33S4 as structure template for exploring sodium superionic conductors

Sodium superionic conductors are keys to realize room-temperature and high safety all-solid-state sodium batteries. Sulfide based sodium superionic conductors are promising but most reported examples are based on group 15 elements. To extend the horizon, we demonstrate a development route from group 14 elements based sodium chalcogenide. Na4Sn0.67Si0.33S4 shows a distinctly new structure with space group I41/acd and 3D diffusion pathway by silicon doping in Na4SnS4, which exhibits 2 orders of magnitude conductivity improvement than that of the mother compound Na4SnS4. And the Na4Sn0.67Si0.33S4 can be taken as a high tolerance structural template for chemical manipulation. Based on the structural template, aliovalent substitution of P5+ for M4+ leads to a wide range of solid solutions of Na4-x[Sn0.67Si0.33]1-xPxS4 (0 ≤ x ≤ 0.4) with enhanced sodium conductivity. The highest ionic conductivity reaches 1.6 × 10−3 S cm−1 with x = 0.25. Change of vacancy population and weak Na-S interaction induced from P doping are main reasons for the improvement of ionic conductivity. In addition, excellent air stability is demonstrated. This work expands the exploration opportunities and provides further understanding of structure-property relationship for sodium superionic conductors.

Rotational Cluster Anion Enabling Superionic Conductivity in Sodium-Rich Antiperovskite Na3OBH4.

Sodium superionic conductors are keys to develop high safety and low cost all-solid-state sodium batteries. Among developed sodium ionic conductors, antiperovskite-type ionic conductors have attracted vast interest due to their high structural tolerance and good formability. Herein, we successfully synthesize Na3OBH4 with cubic antiperovskite structure by solid-state reaction from Na2O and NaBH4. Na3OBH4 exhibits ionic conductivity of 4.4 × 10-3 S cm-1 at room temperature (1.1 × 10-2 S cm-1 at 328 K) and activation energy of 0.25 eV. The ionic conductivity is 4 orders of magnitude higher than the existing antiperovskite Na3OX (X = Cl, Br, I). It is shown that such enhancement is not only due to the specific cubic antiperovskite structure of Na3OBH4 but also because of the rotation of BH4 cluster anion. This work deepens the understanding of the antiperovskite structure and the role of cluster anions for superionic conduction.

(Invited) Stable and High Conductivity Sodium Superionic Conductors Via Cation Mixing

All-solid-state sodium-ion batteries utilizing a Na superionic conductor solid electrolyte (SE) are promising candidates for cheap and safe large-scale energy storage. Among the most promising family of Na SEs are the Na3PnS4 (Pn = P, Sb, As) family1–5 and the recently discovered Na11Sn2PnS12 (Pn=P, Sb). 6,7 In this talk, we will present a comprehensive investigation of cation mixing in the Na4TtS4-Na3PnS4 (Tt = Ge, Si, Sn) composition space using first principles calculations. This composition space, which includes Na3PnS4 and Na11Sn2PnS12 as a subset, provides several levers to tune Na concentration and cation types to simultaneously achieve the twin goals of high ionic conductivity and good electrochemical and chemical stability. We will demonstrate that isovalent-mixed Na3Pn’Pn’’S4 compositions have low enthalpies of mixing, indicating that they are likely to be highly miscible. In the aliovalent mixed Na4-xSn1-xPxS4 system, we will show that the experimentally-reported Na11Sn2PS12 prototype with space group I41/acd leads to not only lower formation energies, but also larger 3D diffusion channels. The predicted ionic conductivities from ab initio molecular dynamics simulations are also in excellent agreement with experiments. Finally, we will propose new mixed compositions with potentially higher conductivities and better chemical stability than those already reported experimentally. Reference: Hayashi, A., Noi, K., Sakuda, A. & Tatsumisago, M. Superionic glass-ceramic electrolytes for room-temperature rechargeable sodium batteries. Nat. Commun. 3, 856 (2012).Zhang, L. et al. Na3PSe4 : A Novel Chalcogenide Solid Electrolyte with High Ionic Conductivity. Adv. Energy Mater. 5, 1501294 (2015).Chu, I.-H. et al. Room-Temperature All-solid-state Rechargeable Sodium-ion Batteries with a Cl-doped Na3PS4 Superionic Conductor. Sci. Rep. 6, 33733 (2016).Banerjee, A. et al. Na3SbS4 : A Solution Processable Sodium Superionic Conductor for All-Solid-State Sodium-Ion Batteries. Angew. Chemie 128, 9786–9790 (2016).Zhu, Z., Chu, I.-H., Deng, Z. & Ong, S. P. Role of Na+ Interstitials and Dopants in Enhancing the Na+ Conductivity of the Cubic Na3PS4 Superionic Conductor. Chem. Mater. 27, 8318–8325 (2015).Zhang, Z. et al. Na11Sn2PS12: A new solid state sodium superionic conductor. Energy Environ. Sci. 11, 87–93 (2018).Heo, J. W., Banerjee, A., Park, K. H., Jung, Y. S. & Hong, S. T. New Na-Ion Solid Electrolytes Na4-xSn1-xSbxS4(0.02 ≤ x ≤ 0.33) for All-Solid-State Na-Ion Batteries. Adv. Energy Mater. 4, 4–9 (2018).

A First-Principles Study on Sodium Superionic Conductor Na11Sn2PS12

Sodium superionic conductors are key components for solid-state sodium batteries which are regarded as promising energy storage devices for large-scale application due to their safety, possibility of increasing energy density, and design flexibility. Recently, a new crystalline sodium superionic conductor Na11Sn2PS12 was reported with a high ionic conductivity over 1 mS/cm at room temperature. Herein, we present first-principles investigations on the new sodium superionic conductor. Our ab initio molecular dynamics (AIMD) simulations confirm the isotropic diffusion of Na11Sn2PS12 and show that all Na sites participate in the diffusion comparably. This study also reveals that the presence of Na vacancy/interstitial does not affect the ionic conductivity, while isovalent substitutions of Sn site by Si or Ge significantly impede the diffusion by reducing free volumes of Na diffusion path as demonstrated by our systematic AIMD simulations. Thermodynamic calculations based on phase diagram analyses indicate that Na11Sn2PS12 phase is stable even at zero kelvin but possesses relatively narrow electrochemical window of 1.16–1.92 V. It is predicted that Na11Sn2PS12 forms a compatible interface with TiS2 but reacts with Na metal to produce undesirable byproducts. We believe that our theoretical study provides a broader understanding on Na11Sn2PS12 and helps guide the optimal design for application in solid-state batteries.

Sodium Superionic Conductors Based on Clusters.

Owing to the high abundance and low cost of sodium (Na), Na-based rechargeable batteries hold great potential for large-scale applications in the future energy industry. However, as key component of the battery electrolyte, only a few Na-based superionic conductors can reach the ionic conductivity comparable to that of liquid or gel electrolytes. Here, we provide a guideline for the development of cluster-based Na-rich antiperovskite superionic conductors using computational studies. With a selected cluster ion BCl4-, we are able to achieve high-room-temperature Na-ionic conductivity over 10-3 S/cm and low activation energies below 0.2 eV in the antiperovskite crystals Na3S(BCl4) and Na3S(BCl4)0.5I0.5. In addition, these materials have large bandgaps and favorable mechanical properties. A comprehensive study of the stability and formation energy of these materials further illustrates possible routes for their synthesis. New insights into the conduction mechanism of these cluster-based superionic conductors are provided, including the cooperative motion of Na ions and the significant reduction of the migration barrier due to the changing orientation of the cluster ion.