Ultrathin and Robust Composite Electrolyte for Stable Solid-State Lithium Metal Batteries

Solid-state polymer electrolytes (SPEs) are considered as one of the most promising candidates for the next-generation lithium metal batteries (LMBs). However, the large thickness and severe interfacial side reactions with electrodes seriously restrict the application of SPEs. Herein, we developed an ultrathin and robust poly(vinylidene fluoride) (PVDF)-based composite polymer electrolyte (PPSE) by introducing polyethylene (PE) separators and SiO₂ nanoparticles with rich silicon hydroxyl (Si-OH) groups (nano-SiO₂). The thickness of the PPSE is only 20 μm but possesses a quite high mechanical strength of 64 MPa. The introduction of nano-SiO₂ fillers can tightly anchor the essential N,N-dimethylformamide (DMF) to reinforce the ion-transport ability of PVDF and suppress the side reactions of DMF with Li metal, which can significantly enhance the electrochemical stability of the PPSE. Meanwhile, the Si-OH groups on the surface of nano-SiO₂ as a Lewis acid promote the dissociation of the lithium bis(fluorosulfonyl)imide (LiFSI) and immobilize the FSI^- anions, achieving a high lithium transference number (0.59) and an ideal ionic conductivity (4.81 × 10^-4 S cm^-1) for the PPSE. The assembled Li/PPSE/Li battery can stably cycle for a record of 11,000 h, and the LiNi_0.8Co_0.1Mn_0.1O₂/PPSE/Li battery presents an initial specific capacity of 173.3 mA h g^-1 at 0.5 C, which can stably cycle 300 times. This work provides a new strategy for designing composite solid-state electrolytes with high mechanical strength and ionic conductivity by modulating their framework.