Sn-Gradient Sulfide Solid Electrolytes with Superior Air-Stability for Practical All-Solid-State Batteries

Sulfide solid electrolytes (SSEs) with high ionic conductivity and mechanical flexibility are considered promising Li+ transport media for all-solid-state batteries (ASSBs). However, susceptibility to moisture originating from their crystal structures degrades their inherent superior properties. In this study, we synthesized core-shell structured SSEs by inducing the growth of compounds with moisture-stable SnS44- units on the surface of Li6PS5Cl (LPSC). This Li10SnP2S12 (LSPS)@LPSC showed > 30 times higher Li+ conductivity than LPSC after exposure to dry room environment for 2 h. Additionally, the hydrolysis reaction was effectively inhibited in LSPS@LPSC, resulting in not only significant reduction of hydrogen sulfide (H2S) gas release, but the onset of its generation was also more delayed than in LPSC. Also, in LSPS@LPSC, the inhibition of P-O bond formation after moisture exposure contributes to retention of mechanical properties, as demonstrated by nano-indentation measurements: hardness changes from 1.42 GPa to 1.50 GPa for LSPS@LPSC versus from 1.21 GPa to 1.62 GPa for LPSC (dew point of -7.5 °C, 5 min). Furthermore, the symmetric cell with LSPS@LPSC exhibited excellent cycling stability comparable to that of LPSC under typical external pressure (30 MPa). And more remarkably, the Li(Ni0.8Co0.1Mn0.1)O2 half cell showed superior cycle retention than LPSC cell under ultra-low external pressure (~0.3 MPa).