Synthesis and Electrochemical Properties of i 4-Type Li_1+2xZn_1-xPS₄ Solid Electrolyte

I4-type LiZnPS₄ solid electrolyte was synthesized starting from Li₂S, P₂S₅, and ZnS, and its room temperature ionic conductivity, σ_25°C, was ∼10^-8 S/cm. To improve σ_25°C, compositions of Li_1+2xZn_1-xPS₄ (x = 0.125, 0.25, 0.375, 0.5, 0.625, 0.75, 0.8, 0.9) were synthesized from mixtures of LiZnPS₄ and amorphous Li₃PS₄ (a-LPS). The samples with x ≤ 0.8 were confirmed by means of XRD and Raman spectroscopy to have the I4Ì structure. The maximum σ_25°C was 5.7 × 10^-4 S/cm at x = 0.625. NMR measurement revealed that Li_1+2xZn_1-xPS₄ (x ≤ 0.625) is almost a single phase material and not a simple mixture of LiZnPS₄ and a-LPS, confirming that its crystal structure is responsible for the improved ionic conductivity. Li_2.25Zn_0.375PS₄ (x = 0.625) was reactive to graphite and indium anodes but stable against a Li₄Ti₅O₁₂ anode and a 4 V class cathode. We fabricated an all-solid-state battery in the form of graphite|a-LPS|Li_2.25Zn_0.375PS₄|Li₂O-ZrO₂ coated Li(Ni,※Mn,※Co)O₂ and examined its performance. It showed a first-cycle discharge capacity of ∼144 mAh/g_-NMC with a capacity retention of 83% after 100 cycles.