Liquid-Phase Synthesis of Highly Deformable and Air-Stable Sn-Substituted Li₃PS₄ for All-Solid-State Batteries Fabricated and Operated under Low Pressures

The liquid-phase synthesis (LS) of sulfide solid electrolytes (SEs) has promising potential for mass production of practical all-solid-state Li batteries (ASLBs). However, their accessible SE compositions are mostly metal-free. Moreover, liquid-phase-synthesized-SEs (LS-SEs) suffer from high electronic conductivity due to carbon impurities, resulting in below-par electrochemical performance of ASLBs. Here, the LS of highly deformable and air-stable Li_3+xP_1-xSn_xS₄ (0.19 mS cm⁻¹) using 1,2-ethylene diamine-1,2-ethanedithiol with tetrahydrofuran is reported. A low heat-treatment temperature (260 °C) prevents the carbonization of organic residues. Importantly, a remarkable enhancement in the deformability of LS-SEs compared to that of conventional solid-state-synthesized SEs (SS-SEs) is identified for the first time. LiNi_0.7Co_0.15Mn_0.15O₂ electrodes employing LS-SEs in ASLBs significantly outperform those using SS-SEs, notably when assembled under a low fabricating pressure (148 vs 370 MPa, e.g., capacity loss: 2 vs 41 mA h g⁻¹) or tested under a low operating pressure (12 or 3 MPa), which is attributed to reduced electrochemo-mechanical effects. Finally, when employing SEs that are exposed to air (dew point of −20 °C), LiNi_0.7Co_0.15Mn_0.15O₂ electrodes employing SEs with Sn-substituted composition or prepared by LS exhibit significantly better capacity retention than conventional SEs with Sn-free composition or prepared by SS (e.g., 92.2% for LS-Li_3.2P_0.8Sn_0.2S₄ vs 32.5% for SS-Li₃PS₄).