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

Abstract

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 Th 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.

Thermal Stability between Sulfide Solid Electrolytes and Oxide Cathode

“…Previous work has reported that sulfide SEs undergo different decomposition reactions during heating in detail, the onset temperature of which is far below that of oxide SEs. For example, Li 7 P 3 S 11 showed obvious sulfur precipitation at approximately 300 °C, with preliminary decomposition products (Li 3 PS 4 , Li 2 PS 3 , and S).…”

“…As a consequence, the decomposition products are speculated to play an important role in the reaction of these typical sulfide SEs and LiCoO 2 . According to the previous experiments, it can be inferred that the reactivity of S with the oxide cathode is higher than that of Li 2 S. Therefore, the reaction of Li 7 P 3 S 11 and LiCoO 2 at 400 °C is speculated to be initiated by the poor thermal stability of Li 7 P 3 S 11 itself and its decomposition product S . In addition, since Li 6 PS 5 Cl has good thermal stability and its decomposition product is mainly Li 2 S, the initial reaction temperature of Li 6 PS 5 Cl and LiCoO 2 is higher (approximately 500 °C).…”

“…This is because sulfide SEs react severely with characterization facilities and generate toxic gases (H 2 S, SO 2 ), leading to severe corrosion and environmental pollution. Our group systematically studied the thermal stability of typical sulfide SEs by developing an intrinsic theoretical thermodynamic paradigm/model and corroborating experimental evidence. Results show that sulfide SEs would decompose and release volatile/flammable sulfur during the heating process.…”

“…It can be concluded that Li 6 PS 5 Cl is also flammable in air, but produces no obvious volatile/flammable components during heating/ combustion, which is consistent with the experimental results reported previously by our group. 14 Through the above combustion experiments, it can be inferred that sulfide ASSBs have a certain degree of thermal safety risks, especially at high ambient or internal temperatures.…”

“…Compared with polymer and oxide SEs, sulfide SEs have good interfacial contact with electrodes, desirable machinability, and high room-temperature ionic conductivity. , However, research on the thermal stability of sulfide SEs/ASSBs has been rarely reported, because of their high corrosiveness on instruments, demanding requirements in test environments, instability in air/water, and easy release of toxic gases (e.g., H 2 S, SO 2 ). , Nevertheless, our group systematically studied the thermal stability of typical sulfide SEs by developing a suitable theoretical paradigm and parameter Th and revealed the thermal stability order of typical sulfide SEs to be Li 6 PS 5 Cl > Li 4 SnS 4 > Li 9.54 Si 1.74 P 1.44 S 11.7 Cl 0.3 (LSPSCl) > Li 3 PS 4 > Li 7 P 3 S 11 . Because the LIBs’ cathode materials generally have high oxidation potential, the thermal stability between sulfide SEs and oxide cathode materials should have attracted more attention.…”