Batteries with lithium metal as anodes and gel polymer membranes as electrolytes are promising, because of their high energy densities and improved safety. To achieve metal battery energy densities that are comparable to liquid‐electrolyte batteries, ultrathin and lightweight gel electrolytes with wide electrochemical stability windows are desired. However, it is challenging to make gel polymer electrolyte membranes with comparable thicknesses to commercial polymer electrolyte separators (<20 μm), because of the porous configuration and increased risk of short‐circuiting cells. Here, we report on a 10 μm‐thick compacted gel polymer electrolyte (TCGPE), demonstrated to have a robust chemical interaction between SiO2 nanoparticles and poly(vinylidene fluoride‐co‐hexafluoropropylene) (PVDF‐HFP) chains that are activated via an in situ sol‐gel reaction. The developed TCGPE exhibits an extremely wide electrochemical stability window of 5.5 V vs. Li/Li+ and prevents the batteries from short‐circuiting even at a high current density of 1.0 mA cm−2. Moreover, Li/TCGPE/LiNi0.5Co0.2Mn0.3O2 shows an excellent capacity retention of 99.8 % after 180 cycles at 1 C rate.

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To provide molecular-scale insight into the structural evolution from tris(methylamino)borane to boron nitride (BN) fiber during the chemical thermal-treating process, polymeric green fiber is cured in hot synthetic air at 300 • C and then treats to 400, 600, 800, and 1000 • C in ammonia. The chemical composition and structure of the volatile compounds and residual products are analyzed during the pyrolysis process for the polymeric green fiber. It is demonstrated that oxygen can be used to cure polymeric green fiber rapidly under the premise of ensuring a final fiber content of less than 1 wt% carbon and 2 wt% oxygen while maintaining the fiber tensile strength at 1000 • C. The molecular structure evolution during the pyrolysis process for polymeric green fiber after oxygen curing is determined. Specifically, in hot synthetic air, introducing oxygen and releasing methylamine generates a B-O six-membered ring structure in the polymer in the first stage. Then, the removal of methyl results in the formation of a B-N-O network in hot ammonia. Afterward, nitridation of the B-O six-membered ring promotes the evolution of the B-N six-membered ring structure with the release of water and carbon dioxide. Finally, the growth and rearrangement of the BN structure are achieved.