Improving Ionic Conductivity with Bimodal-Sized Li₇La₃Zr₂O₁₂ Fillers for Composite Polymer Electrolytes

Ceramic-polymer composite electrolytes (CPEs) are being explored to achieve both high ionic conductivity and mechanical flexibility. Here, we show that, by incorporating 10 wt % (3 vol %) mixed-sized fillers of Li₇La₃Zr₂O₁₂ (LLZO) doped with Nb/Al, the room-temperature ionic conductivity of a polyvinylidene fluoride (PVDF)-LiClO₄-based composite can be as high as 2.6 × 10^-4 S/cm, which is 1 order of magnitude higher than that with nano- or micrometer-sized LLZO particles as fillers. The CPE also shows a high lithium-ion transference number of 0.682, a stable and low Li/CPE interfacial resistance, and good mechanical properties favorable for all-solid-state lithium-ion battery applications. X-ray photoelectron spectroscopy and Raman analysis demonstrate that the LLZO fillers of all sizes interact with PVDF and LiClO₄. High packing density (i.e., lower porosity) and long conducting pathways are believed responsible for the excellent performance of the composite electrolyte filled with mixed-sized ionically conducting ceramic particles.