Breaking the Upper Limit of Substitution Concentration in Li Argyrodite Solid Electrolytes Using a Single-Solvent-Mediated Approach

Abstract

Although raising the substitution concentration of aliovalent cations in Li argyrodite solid electrolytes could boost solid-state battery performance, surpassing the known substitution limit has not been attempted. In this study, the upper substitution limit of a Li_6+xP_1−xSi_xS₅Br solid electrolyte is increased using a single-solvent-mediated approach. The limit attained through this method is ≈40%, whereas that achieved through solid-state ball milling is ≈30%. This result is validated by monitoring variations in the interplanar distance, Raman shift, and ionic conductivity with respect to the substitution level. The ionic conductivity of Li_6.4P_0.6Si_0.4S₅Br is as high as ≈3.1 mS cm⁻¹, exceeding that accomplished through ball milling. The enhanced limit is ascribed to the reduced particle size, which leads to an increased surface-area-to-volume ratio of the particles. This interpretation is supported by a theoretical formalism developed based on substituent accumulation within the space-charge layers, which predicts how the technical limit depends on the surface-volume fraction. A Li// Li_6.4P_0.6Si_0.4S₅Br//Li symmetric cell demonstrates excellent Li plating and stripping over extended cycling. A full cell incorporating Li_6.4P_0.6Si_0.4S₅Br retains ≈67% (96 mAh g⁻¹) of its initial capacity (143 mAh g⁻¹) after 50 cycles at 0.2 C, and delivers 76 mAh g⁻¹ at 1 C.