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

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, Li₂O, and Li₃N 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 LiFePO₄, the as-assembled LiFePO₄|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.