A Thin Sandwich‐Structured Solid Polymer Electrolyte Enabling High‐Rate Lithium Metal Batteries

ABSTRACT Solid polymer electrolytes (SPEs) provide inherent safety and processing advantages for solid‐state lithium metal batteries (SSLMBs), but their application under high current densities remains limited by sluggish ion transport and significant polarization phenomenon. In this work, a thin BN‐reinforced sandwich‐structured solid polymer electrolyte (BSPE) with a thickness of 27 µm is designed to address these challenges. By combining the mechanically robust BN‐polyvinylidene fluoride (BN‐PVDF) supporting core layer with ion‐conductive poly(iBMA‐co‐PEGDA) outer layers, the BSPE achieves enhanced room temperature ionic conductivity and increased Li + transference number, accompanied by suppressed polarization. COMSOL simulations show that BSPE enables a more uniform Li + concentration distribution and a stabilized internal electric field distribution, effectively inhibiting the formation of lithium dendrites. Thus, symmetric Li||Li cell demonstrates stable Li plating/stripping for more than 5500 h. When used in Li||LiFePO 4 (Li||LFP) batteries, the BSPE delivers significantly improved cycling performance, and maintains long‐term cycling stability even at high C‐rates. In detail, the Li|BSPE|LFP battery sustains 81% capacity retention after 2000 cycles at 5 C. Overall, this study establishes that the rational design of electrolyte architecture plays a key role in regulating ion transport and interfacial behavior, which suggests a practical strategy for high‐rate SSLMBs.