Stabilizing Solid‐State Li Metal Batteries via a Poly(ethylene oxide)‐LiTFSI@Boron Nitride Interfacial Layer

Na super‐ionic conductor (NASICON)‐structured solid electrolyte Li _1.3 Al _0.3 Ti _1.7 (PO ₄ ) ₃ (LATP) excels in mechanical strength, ionic conductivity, and commercialization prospect, yet its incompatibility against Li metal anode typically leads to interfacial failure, significantly reducing the overall electrochemical performance of fabricated solid‐state Li metal batteries (SSLMBs). Here, this issue is tackled by interposing an in situ cured poly(ethylene oxide) (PEO)‐LiTFSI@BN interfacial layer between LATP and Li metal. This layer exhibits an ionic conductivity of 7.76 × 10 ⁻⁵ S cm ⁻¹ at room temperature, robust thermostability, as well as satisfactory stability against Li metal (critical current density: 1.5 mA cm ⁻² ). A Li|LATP|Li symmetric cell employing the interfacial layer demonstrates long cyclic stability over 1200 cycles with a low overpotential of ± 100 mV. Furthermore, this interfacial layer enables a LiNi _0.5 Co _0.2 Mn _0.3 O ₂ (NCM523)|LATP|Li metal SSLMB to sustain 120 cycles of operation at a cutoff voltage of 4.2 V. This work provides a practical strategy for integrating the LATP solid electrolyte into SSLMBs.