Crystallinity Regulation of Ionic Liquid‐Based Quasi‐Solid‐State Polymer Electrolytes for Long‐Lifespan Flexible Transparent Lithium‐Ion Batteries

ABSTRACT Ionic liquid‐based quasi‐solid polymer electrolytes (ILQSPEs) are promising for flexible and transparent energy storage devices. Nevertheless, owing to the lack of ILQSPEs combining high transparency with efficient ionic transport, their application in flexible transparent lithium‐ion batteries (FTLIBs) remains unexplored. To break through this bottleneck, herein we propose a crystallinity regulation strategy for the co‐optimization of the electrolyte's crystalline‐amorphous phase distribution and ionic transport channels, thereby simultaneously achieving high optical transparency and ionic transport performance. Specifically, the amorphous polymer polymethyl methacrylate (PMMA) is incorporated into poly(vinylidene fluoride‐co‐hexafluoropropylene) (PVDF‐HFP)‐based ILQSPEs, and its doping ratio is precisely tuned to modulate the crystallinity of the resulting electrolyte membranes. The optimal membrane (PVDF‐HFP/PMMA mass ratio of 4:3) exhibits an outstanding optical transmittance of 88.2%, a satisfactory ionic conductivity of 1.91 × 10 −4 S cm −1 , and a high lithium‐ion transference number of 0.55. The optimized electrolyte membranes are integrated into FTLIBs; the devices achieve an unprecedented cycle life of 300 cycles at 5 µA cm −2 with 79.9% capacity retention, along with a high specific capacity of 17.9 µAh cm −2 , a high transparency of 61.2% at 550 nm, and a desirable energy density of 44.8 µWh cm −2 . This work provides a novel ILQSPE design strategy for high‐performance FTLIBs.