Poly(ionic liquid)-Based Hybrid Hierarchical Free-Standing Electrolytes with Enhanced Ion Transport and Fire Retardancy Towards Long-Cycle-Life and Safe Lithium Batteries

Ionic liquid electrolytes (ILEs), owing to their wide electrochemical window, high ionic conductivity, and low volatility, have been extensively applied in the field of energy storage devices. However, it is extremely significant and greatly challenging to seek the counterbalance of good mechanical properties and excellent electrochemical performances, including a high transference number of ILEs for safe flexible lithium batteries. In this work, a novel hybrid poly(ionic liquid)-based quasi-solid electrolyte containing silica nanoparticles (SiO2-PIL-QSE) is fabricated through the one-step in situ polymerization of the ionic liquid covalently tethered to silica nanoparticles (SiO2-IL-TFSI), which fills a poly(vinylidene fluoride-hexafluoropropylene) (PVdF-HFP) porous membrane prepared through phase inversion. The hybrid hierarchical porous architecture endows the SiO2-PIL-QSE with excellent thermal stability, good fire safety (i. e. nonflammable and low shrinkage), improved mechanical strength, and excellent electrochemical performance, including a high room-temperature ionic conductivity of 1.69 mS cm(-1), a superior electrochemical window up to 5.5 V, as well as a high lithium-ion transference of 0.46. Accordingly, the LiFePO4|SiO2-PIL-QSE|Li cell can maintain a discharge specific capacity of 133.7 mAh g(-1) at 0.1 C at 25 degrees C with a capacity retention ratio of 91.9 % after 400 cycles. The excellent performances and good applicability in the pouch-type battery of SiO2-PIL-QSE indicate a good prospect in the field of flexible solid-state polymer batteries (SSPB).