Fluorinated boron nitride nanosheet enhanced ultrathin and conductive polymer electrolyte for high‐rate solid‐state lithium metal batteries

Abstract Polyethylene oxide (PEO)‐based polymer solid electrolytes (PSE) have been pursued for the next‐generation extremely safe and high‐energy‐density lithium metal batteries due to their exceptional flexibility, manufacturability, and lightweight nature. However, the practical application of PEO‐PSE has been hindered by low ionic conductivity, limited lithium‐ion transfer number ( t Li+ ), and inferior stability with lithium metal. Herein, an ultrathin composite solid‐state electrolyte (CSSE) film with a thickness of 20 μm, incorporating uniformly dispersed two‐dimensional fluorinated boron nitride (F‐BN) nanosheet fillers (F‐BN CSSE) is fabricated via a solution‐casting process. The integration of F‐BN effectively reduces the crystallinity of the PEO polymer matrix, creating additional channels that facilitate lithium‐ion transport. Moreover, the presence of F‐BN promotes an inorganic phase‐dominated electrolyte interface film dominated by LiF, Li 2 O, and Li 3 N on the lithium anode surface, greatly enhancing the stability of the electrode‐electrolyte interface. Consequently, the F‐BN CSSE exhibits a high ionic conductivity of 0.11 mS cm −1 at 30°C, high t Li+ of 0.56, and large electrochemical window of 4.78 V, and demonstrates stable lithium plating/striping behavior with a voltage of 20 mV for 640 h, effectively mitigating the formation of lithium dendrites. When coupled with LiFePO 4 , the as‐assembled LiFePO 4 |F‐BN CSSE|Li solid‐state battery achieves a high capacity of 142 mAh g −1 with an impressive retention rate of 82.4% after 500 cycles at 5 C. Furthermore, even at an ultrahigh rate of 50 C, a capacity of 37 mAh g −1 is achieved. This study provides a novel and reliable strategy for the design of advanced solid‐state electrolytes for high‐rate and long‐life lithium metal batteries.