Tailoring grain boundary resistance in Li-ion conducting polymer–ceramic hybrid electrolytes based on polyether and Li _1.5 Al _0.5 Ge _1.5 (PO ₄ ) ₃

Composite solid electrolytes comprising ceramic and polymer components have garnered significant attention as promising materials for next-generation all-solid-state lithium batteries owing to the combination of high ionic conductivity and enhanced interfacial stability. In this study, we systematically investigate the effects of incorporating either crystalline or amorphous Li_1+<i>x</i> Al _<i>x</i> Ge_2-<i>x</i> (PO₄)₃ (LAGP) into a polyether-based polymer matrix. Differential scanning calorimetry reveals that the addition of LAGP does not markedly influence the thermal transitions of the host polymer, suggesting minimal disruption of polymer chain dynamics. Ionic conductivity measurements indicate that crystalline LAGP slightly reduces overall conductivity, whereas amorphous LAGP effectively mitigates the conductivity drop at lower temperatures, potentially providing alternative Li⁺ conduction pathways through the amorphous phase. Impedance spectroscopy shows significant grain boundary resistance in composites with crystalline LAGP, whereas those with amorphous LAGP exhibit improved interfacial ion transport, particularly under non-blocking electrode conditions. High-energy X-ray diffraction using synchrotron radiation and pair distribution function analysis further confirms homogeneous structural integration between the polymer and amorphous LAGP. These findings demonstrate that the microstructure of ceramic fillers, particularly their amorphous nature, plays a pivotal role in dictating ion transport behavior, providing valuable insights for the design of high-performance composite electrolytes.