Intergranular amorphous film in GeO₂-enriched Li_1.5Al_0.5Ti_1.5(PO₄)₃ composite electrolytes for high-performance solid-state lithium-ion batteries

Solid-state electrolytes have emerged as a key area of development in the field of Li-ion batteries owing to safety concerns surrounding liquid electrolytes. Among solid-state electrolytes, Li_1.5Al_0.5Ti_1.5(PO₄)₃ (LATP), a NASICON-type material, is a leading candidate owing to its promising ionic conductivity, chemical and environmental stability, and cost-effectiveness. However, its ionic conductivity is limited by grain-boundary scattering, which hinders its broader adoption. Herein, we introduce a novel grain-boundary engineering strategy for the LATP electrolyte system using typical solid-state method, wherein a Ge-rich liquid phase spontaneously forms at the grain boundaries of GeO₂-enriched LATP during synthesis, producing an intergranular amorphous film in the final material that significantly enhances Li-ion transport at the grain boundaries. With an optimal content of 4 wt% GeO₂, the ionic conductivity reaches 8.92 × 10⁻⁴ S cm⁻¹—an eightfold increase compared to that of pristine LATP. This high ionic conductivity also bestows 4 wt% GeO₂-LATP with excellent cell performance, with a symmetric Li/4 wt% GeO₂-LATP/Li cell exhibiting stable operation for over 500 h with low overpotentials. Our findings underscore the importance of grain-boundary engineering in advancing solid-state electrolytes and pave the way for the commercialization of next-generation all-solid-state Li-ion batteries.