Dual-interfacial engineering via LiPF ₆ treatment: eradicating Li ₂ CO ₃ on LLZTO and regulating SEI for high-voltage solid-state batteries

The performance of composite polymer electrolytes (CPEs) in all-solid-state lithium batteries (ASSLBs) is critically limited by two interfacial challenges: incompatibility between ceramic fillers and the polymer matrix, and an unstable solid electrolyte interphase (SEI) against lithium metal. Garnet-type Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO) is a promising inorganic filler, yet air exposure induces an insulating surface Li₂CO₃ layer, causing PVDF dehydrofluorination and hindering ion transport. Herein, we developed a facile surface modification strategy using LiPF₆ solution treatment. This approach efficiently eliminated Li₂CO₃via an acid-base reaction with LiPF₆ hydrolysis products, simultaneously in situ generating a Li₃PO₄-LiF composite coating that compensated for lithium loss. Moreover, this modification modulated the structure and composition of the solid electrolyte interphase (SEI) formed at the electrolyte/lithium metal interface, inducing the formation of a uniform, stable inorganic SEI layer enriched with LiF, Li₃PO₄, and Li₃N. The resulting modified-LLZTO based PVDF CPE (LiPF₆-CPE) achieved a high room-temperature ionic conductivity of 8.74 × 10^-4 S cm^-1, a Li⁺ transference number of 0.750, and enabled stable cycling for over 1000 h in a Li symmetric cell with suppressed lithium dendrite growth. When paired with an NCM811 cathode, the solid-state battery delivered a capacity retention of 88.6% after 100 cycles at 0.5 C (2.7-4.3 V) and maintained 80.3% retention at a high voltage of 4.5 V. This efficient and economical interfacial optimization strategy, which synchronously improved filler-polymer compatibility and regulated SEI layer properties, provided a general and practical solution for interface engineering in the fabrication of high-performance PVDF-based CPEs and ASSLBs.