Mechanochemically Prepared Li₂S-P₂S₅-LiBH₄ Solid Electrolytes with an Argyrodite Structure

Solid electrolytes with compositions of (100 - x)(0.75Li₂S·0.25P₂S₅)·xLiBH₄ (mol %, 0 ≤ x ≤ 100) were mechanochemically prepared from the 75Li₂S·25P₂S₅ (mol %) glass and LiBH₄ crystal. The samples with x ≥ 43 have crystalline phases and those with x ≤ 33 formed a glassy phase. The crystalline phase was identified as argyrodite Li₆PS₅(BH₄). The x = 50 sample formed a crystalline phase and demonstrated a high lithium-ion conductivity of 1.8 × 10^-3 S cm^-1 at 25 °C with an activation energy of 16 kJ·mol^-1. The argyrodite-type crystal with a BH₄ ^- anion that occupies the halide site is a novel and promising solid electrolyte.