Architecturally Confined PEO-Based Composite Solid Electrolytes with Transport-Active Polymer–Ceramic Interphases for Stable Lithium Metal Batteries

Composite solid electrolytes are promising for solid-state lithium metal batteries because ceramic fillers can enhance ionic transport and mechanical integrity in polymer matrices. However, direct exposure of ceramic-rich domains to lithium metal can cause heterogeneous ion transport, local current concentration, and unstable interfacial evolution. Herein, we propose an architecturally confined composite solid electrolyte (AC-CSE) that decouples bulk transport enhancement from lithium-metal interfacial instability. The electrolyte consists of a transport-active composite core based on polyethylene oxide (PEO), LiTFSI, and surface-functionalized Li_0.33La_0.56TiO₃ (TPMP@LLTO), sandwiched between thin filler-free PEO/LiTFSI buffer layers. TPMP functionalization improves filler dispersion, polymer–ceramic compatibility, and Li⁺ coordination through phosphonate-derived interfacial sites, forming an efficient Li⁺ transport network within the composite core. Meanwhile, the filler-free buffer layers spatially separate ceramic-rich domains from lithium metal, reducing interfacial heterogeneity and polarization amplification.

The optimized AC-CSE exhibited an ionic conductivity of 1.46 × 10^-3 S cm^-1 at 60 °C and a Li⁺ transference number of 0.77. In Li|Li symmetric cells, it enabled stable lithium plating/stripping for over 400 h. Post-mortem analysis confirmed uniform lithium deposition and the formation of a LiF-rich solid electrolyte interphase. As a result, Li||LiFePO₄ cells retained approximately 72% of their capacity after 1,000 cycles at 60 °C and 1 C, and 98% after 300 cycles at 40 °C and 0.5 C. These results demonstrate that electrolyte-scale spatial confinement is an effective strategy for achieving both fast Li⁺ transport and stable lithium-metal interfaces in polymer-based solid-state batteries.