Improving thermal stability of sulfide solid electrolytes: An intrinsic theoretical paradigm

All-solid-state batteries (ASSBs) have been widely acknowledged as the key next-generation energy storage technology/device, due to their high safety and energy density. Among all solid electrolytes (SEs) that have been studied for ASSBs, sulfide SEs represent the most promising technical route due to their ultra-high ionic conductivity and desirable mechanical property. However, few results have been reported to study the thermal stability/safety issue of sulfide SEs and ASSBs. Herein, we develop the first-of-its-kind theoretical paradigm and a new conceptual parameter 7th to quantitatively calculate/predict the essential thermal stability of sulfide SEs. This theoretical paradigm takes all types of parameters (e.g. crystal structure, localized polyhedra configuration, bond energy, bond type, bond number, normalization factor, and the energy correction factor) into consideration, and more importantly, can be simplified into one straightforward equation for its convenient application in any crystalline systems. To prove its functionality, the typical experimental strategies (stoichiometric ratio control and elemental doping) are adopted for typical sulfide SEs (Li7P3S11, Li3PS4) to improve their thermal stabilities, based on the predictions obtained from the derived theory and equation. Moreover, the potential doping elements to improve thermal stability of sulfide SEs are screened throughout the whole periodic table, and the theoretically predicted trends correspond well with experimental evidence. This work may represent the most critical breakthroughs in the research field of thermal stability for sulfide SEs, not only because it fills the gap of this field, but also due to its precise and quantitative prediction based on a complete consideration of all parameters that determine their thermal stabilities. The handy model developed herein can also be applied to any crystalline materials.