Ionocovalency of the Central Metal Halide Bond-Dependent Chemical Compatibility of Halide Solid Electrolytes with Li₆PS₅Cl

Halide solid electrolytes (HSEs), owing to their high oxidative stability, are used with high-voltage cathodes for all-solid-state batteries (ASSBs). However, their limited reduction stability against Li metal enforces a bilayer separator configuration with sulfide solid electrolytes (SSEs) in ASSBs. However, there are indications of electrochemical incompatibility between the HSE and SSE. Here, we study the chemical reactivity between SSE Li 6 PS 5 Cl with HSEs by varying the central metal and their impact on cell performance in bilayer configuration. We performed operando electrochemical impedance spectroscopy, temperature-dependent X-ray diffraction, ex situ X-ray photoelectron spectroscopy, and differential scanning calorimetry to quantify the reactivity between the SSE and HSE. We infer that the reaction kinetics is indeed determined by the central metal of the HSE, with Li 3 InCl 6 and Li 2 ZrCl 6 being highly reactive with Li 6 PS 5 Cl, whereas Li 3 YCl 6, Li 3 ScCl 6, and Li 3 ErCl 6 are stable. Our work provides a guideline for selecting HSE and SSE pairs for bilayer ASSBs.