Synthesis and Electrochemical Properties of <i>I</i>4-Type Li_1+2<i>x</i>Zn_1–<i>x</i>PS₄ Solid Electrolyte

I4̅-type LiZnPS4 solid electrolyte was synthesized starting from Li2S, P2S5, and ZnS, and its room temperature ionic conductivity, σ25°C, was ∼10–8 S/cm. To improve σ25°C, compositions of Li1+2xZn1–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 I4̅ structure. The maximum σ25°C was 5.7 × 10–4 S/cm at x = 0.625. NMR measurement revealed that Li1+2xZn1–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. Li2.25Zn0.375PS4 (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|Li2.25Zn0.375PS4|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.