Synthesis and Electrochemical Properties of I4-Type Li1+2xZn1–xPS₄ Solid Electrolyte

I 4̅-type LiZnPS 4 solid electrolyte was synthesized starting from Li 2 S, P 2 S 5, 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+2 x Zn 1– x PS 4 ( x = 0.125, 0.25, 0.375, 0.5, 0.625, 0.75, 0.8, 0.9) were synthesized from mixtures of LiZnPS 4 and amorphous Li 3 PS 4 (a-LPS). The samples with x ≤ 0.8 were confirmed by means of XRD and Raman spectroscopy to have the I 4̅ structure. The maximum σ 25°C was 5.7 × 10 –4 S/cm at x = 0.625. NMR measurement revealed that Li 1+2 x Zn 1– x PS 4 ( x ≤ 0.625) is almost a single phase material and not a simple mixture of LiZnPS 4 and a-LPS, confirming that its crystal structure is responsible for the improved ionic conductivity. Li 2.25 Zn 0.375 PS 4 ( x = 0.625) was reactive to graphite and indium anodes but stable against a Li 4 Ti 5 O 12 anode and a 4 V class cathode. We fabricated an all-solid-state battery in the form of graphite|a-LPS|Li 2.25 Zn 0.375 PS 4 |Li 2 O-ZrO 2 coated Li(Ni, Mn, Co)O 2 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.