Revealing critical roles of alkaline passivation layer on garnet surface toward poly(vinylidene fluoride)-based composite electrolytes for solid-state lithium batteries

The poly(vinylidene fluoride) (PVDF) has been deemed as an appealing matrix for solid polymer electrolytes due to its wide electrochemical window and excellent thermal stability. Further incorporation with garnet filler endows PVDF-based electrolyte with increased ionic conductivity and mechanical strength. However, the spontaneous formation of alkaline layer containing LiOH/Li₂CO₃ on garnet surface cannot be neglected, concerning its low ionic conductivity combined with the destructive effect on electrochemical performance of PVDF-based composite electrolytes. Herein, it is found that in organic solutions, LiOH rather than Li₂CO₃ on garnet surface triggers the dehydrofluorinaton of PVDF chains, forming conjugated C = C bonds in local regions. To overcome this issue, the alkali-free garnet fillers by acetic-acid pretreatment are coupled with PVDF to diminish the undesired dehydrofluorinaton. Benefited from the enhanced ionic transfer along PVDF chains, percolation channels and garnet phase, the PVDF electrolyte with alkali-free garnet delivers an enhanced ionic conductivity of 1.14 × 10^-4 S cm^-1 at 30 °C. Besides, the absence of C = C bond further contributes to an extended electrochemical window of 4.86 V. The corresponding solid-state LiNi_0.6Co_0.2Mn_0.2O₂/Li batteries exhibit an improved discharge capacity of 128.9 mAh/g at 1 C (i.e., 73.8 % of 0.1 C), and 128.3 mAh/g (i.e., 75.8 % of initial capacity) after 100 cycles at 0.2 C. This work clarifies the key factor of dehydrofluorinaton for PVDF electrolytes, which provides a fundamental guidance to construct high-performance PVDF-based solid-state lithium batteries.