Thin, Highly Ionic Conductive, and Mechanically Robust Frame‐Based Solid Electrolyte Membrane for All‐Solid‐State Li Batteries

Abstract A thin but robust solid electrolyte layer is crucial for realizing the theoretical energy density of all‐solid‐state batteries (ASSBs) beyond state‐of‐the‐art Li‐ion batteries (LIBs). This study proposes a simple but practical strategy for fabricating thin solid electrolyte membranes using 5‐µm perforated polyethylene separators with 35% open areas as the supporting component, which ensures mechanical robustness for commercial‐level cell assembly. The thickness of this frame‐based solid electrolyte (f‐SE) membrane can be reduced to ≈45 µm, even after coating the Li₆PS₅Cl (LPSCl) solid electrolyte composite. Despite a slightly lower ionic conductivity compared to that of thick LPSCl pellets, the f‐SE membranes show high conductance and low overpotential in Li||Li symmetric cells. Their incorporation into LiNi_0.7Co_0.15Mn_0.15O₂ full cells increases the reversible capacity and rate capability compared to those of cells with conventional LPSCl pellets. The f‐SE membrane cells exhibit excellent cycling stability over 250 cycles, while maintaining high‐capacity retention and Coulombic efficiency. Notably, the f‐SE membranes significantly increase the energy density of ASSBs (314 Wh kg⁻¹), exceeding the values reported for sulfide‐based cells. These results highlight the crucial role of f‐SE membranes in improving the mechanical properties and energy density of ASSBs, thereby contributing to the development of next‐generation Li battery technologies.