Electrochemical Testing and Benchmarking of Compositionally Complex Lithium Argyrodite Electrolytes for All-Solid-State Battery Application

Ceramic ion conductors play a pivotal role as electrolytes in solid-state batteries (SSBs). Aside from the ionic conductivity, their (electro)chemical stability has a profound effect on the performance. Lithium thiophosphates represent a widely used class of superionic materials, yet they suffer from limited stability and are known to undergo interfacial degradation upon battery cycling. Knowledge of composition-dependent properties is essential to improving upon the stability of thiophosphate solid electrolytes (SEs). In recent years, compositionally complex (multicomponent) and high-entropy lithium argyrodite SEs have been reported, having room-temperature ionic conductivities of σ_ion>10 mS cm⁻¹. In this work, various multi-cationic and -anionic substituted argyrodite SEs are electrochemically tested via cyclic voltammetry and impedance spectroscopy, as well as under operating conditions in SSB cells with layered Ni-rich oxide cathode and indium-lithium anode. Cation substitution is found to negatively affect the electrochemical stability, while anion substitution (introducing Cl⁻/Br⁻ and increasing halide content) has a beneficial effect on the cyclability, especially at high current rates.