Design Strategies, Practical Considerations, And New Solution Processes Of Sulfide Solid Electrolytes For All‐Solid‐State Batteries

DOI: 10.1002/aenm.201800035 Since the first demonstration of prototype Li batteries (TiS2/Li) in 1976,[1] the develo­ pment of LIBs to date has been strongly affected by safety issues. One of the major technical breakthroughs for the commer­ cialization of LIBs was the replacement of Li metal with carbonaceous materials as the anode.[2–4] It is well known that the use of Li metal was challenged by serious safety concerns associated with internal short circuit by the dendritic growth of Li metal.[5–7] The ever­rising requirements for higher energy density of LIBs have raised more serious safety concerns. Raising the upper cutoff voltages leads to poorer sta­ bility at electrode–electrolyte interfaces.[8,9] Ultrathinning the polymeric separators to less than 10 μm, despite the reinforce­ ments using ceramic materials,[10–12] result in more vulnerability toward internal short circuits. These may also be related to degassing, fire, and explosion accidents of LIBs in recent years. Further­ more, large­scale applications of LIBs, such as battery­driven electric vehicles and grid­scale energy storages, face unprecedented challenges in terms of safety requirements.[13–15] In this regard, solidification of conventional flammable organic liquid electrolytes with inorganic materials, such as superionic conductor solid electrolytes (SEs), is an ideal solution.[16–25] Another strong motivation in the development of SEs is to unleash the harness of limited energy density for con­ ventional LIBs by using SEs to stabilize and enable alternative high­capacity electrode materials, such as Li metal anode and sulfur cathode.[15,23] Additionally, the design of all­solid­state Li or Li­ion batteries (ALSBs) by stacking bipolar electrodes allows the minimization of inactive encasing materials, thereby increasing cell­level energy density.[22,26] The first superionic conductors PbF2 and Ag2S were discov­ ered by Michael Faraday in 1838.[27] Since then, several notable progresses in the field of solid­state superionic conductors and their newly enabled electrochemical devices had occurred;[27] the development of oxygen­ion conductors (Y­doped ZrO2) applied to solid oxide fuel cells, the discoveries of Ag+ superionic conduc­ tors (e.g., RbAg4I5), and the development of Na­ion conducting sodium beta alumina (β′′­Al2O3). Currently, it is a promi sing opportunity for Li­ion SEs to revolutionize LIB technologies Owing to the ever-increasing safety concerns about conventional lithium-ion batteries, whose applications have expanded to include electric vehicles and grid-scale energy storage, batteries with solidified electrolytes that utilize nonflammable inorganic materials are attracting considerable attention. In particular, owing to their superionic conductivities (as high as ≈10−2 S cm−1) and deformability, sulfide materials as the solid electrolytes (SEs) are considered the enabling material for high-energy bulk-type all-solid-state batteries. Herein the authors provide a brief review on recent progress in sulfide Liand Na-ion SEs for all-solid-state batteries. After the basic principles in designing SEs are considered, the experimental exploration of multicomponent systems and ab initio calculations that accelerate the search for stronger candidates are discussed. Next, other issues and challenges that are critical for practical applications, such as instability in air, electrochemical stability, and compatibility with active materials, are discussed. Then, an emerging progress in liquid-phase synthesis and solution process of SEs and its relevant prospects in ensuring intimate ionic contacts and fabricating sheet-type electrodes is highlighted. Finally, an outlook on the future research directions for all-solid-state batteries employing sulfide superionic conductors is provided. Solid-State Batteries