Synergistic Enhancement of Ion Transport and Cycling Stability in Composite Solid Electrolytes via Inert/Active Dual-Ceramic Fillers

Poly(ethylene oxide) (PEO)-based solid electrolytes are promising candidates for solid-state lithium metal batteries because of their flexibility and ease of processing. However, their practical application is limited by insufficient mechanical strength and poor interfacial stability. Conventional single-filler strategies typically improve either ionic conductivity or mechanical robustness, making it challenging to simultaneously optimize both properties. In this work, a dual-ceramic strategy is proposed that integrates inert and active ceramic fillers with complementary roles to construct a polymer electrolyte that is both mechanically robust and ionically conductive. The inert ceramic filler promotes lithium-salt dissociation and Li⁺ transport, whereas the active ceramic filler enhances structural integrity and suppresses lithium dendrite growth, enabling a synergistic balance between ionic transport and cycling stability. As a representative implementation, paraelectric SrTiO₃ and Li⁺-conducting Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO) are incorporated into the PEO/LiTFSI matrix to construct a composite solid electrolyte (PLLS). The optimized PLLS electrolyte, containing 8 wt% STO and 5 wt% LLZTO, exhibits a high ionic conductivity of 4.48×10-4Scm-1, an increased Li⁺ transference number of 0.20, and a wide electrochemical stability window of 5.165 V versus Li/Li⁺ at 60 °C. Li/Li symmetric cells demonstrate stable lithium plating/stripping for nearly 2000 h at a current density of0.2mAcm-2. Furthermore, LiFePO₄/Li full cells retain 92.1% of their initial capacity after 500 cycles at 1 C, and stable cycling performance is also achieved with high-voltage LiCoO₂ cathodes. These results demonstrate that the proposed dual-ceramic synergistic strategy offers an effective and potentially generalizable approach to enhancing the durability of PEO-based solid electrolytes for long-life solid-state lithium metal batteries.