Interfacial Enhancement of an Iodide–Oxide–Sulfide Solid Electrolyte for Long-Life All-Solid-State Lithium Metal Batteries

Interfacial instability against lithium metal remains a major challenge for sulfide-based solid electrolytes in all-solid-state lithium metal batteries (ASSLMBs). In this study, the previously reported iodide–oxide–sulfide solid electrolyte 10LPSOI was prepared by mechanical ball milling (BM) and a modified liquid-phase shaking (LS) method to investigate the influence of the synthesis route on structural characteristics, particle morphology, ionic transport, and lithium–metal interfacial behavior. Structural and spectroscopic analyses revealed differences in phase characteristics and local phosphorus environments between 10LPSOI-BM and 10LPSOI-LS, while SEM analysis showed that the LS method produced substantially finer and more homogeneous particles. Although 10LPSOI-LS exhibited a lower room-temperature ionic conductivity of 0.7 mS cm–1 than 10LPSOI-BM at 3.4 mS cm–1, it achieved a higher critical current density of 0.925 mA cm–2 compared with 0.30 mA cm–2 for 10LPSOI-BM. The Li | 10LPSOI-LS | Li symmetric cell maintained relatively stable polarization for about 3092 h at 0.30 mA cm–2 before a sustained decrease in overpotential was observed. In addition, 10LPSOI-LS showed improved cycling stability in NMC111-based cells compared with 10LPSOI-BM under the tested conditions. These results indicate that the modified LS route improves lithium–metal interfacial performance despite the lower ionic conductivity. This improved behavior is correlated with synthesis-dependent differences in particle morphology, phase characteristics, local structure, and surface chemistry. The modified liquid-phase process may also offer potential advantages for scalable preparation of sulfide-based solid electrolytes.