Salt‐Segregated Solid Polymer Electrolytes for High‐Rate Solid‐State Lithium Batteries

Solid-polymer electrolytes (SPEs) demonstrate great potential for solid-state lithium batteries (SSLBs), however, interfacial instability and sluggish ion transport at the interface critically hinder their high-rate capability and long-term stability. Here, a novel salt-segregation methodology with spatial salt grade for SPEs is introduced. This approach leverages the differential solubility of lithium salts and PVDF matrix in a commercially available fluoroethylene carbonate during fabrication, which drives the formation of an ion-enriched surface layer. The strategy simultaneously enhances interfacial and bulk ionic conductivity while effectively mitigating parasitic reactions. These advancements optimize Li⁺ flux at the lithium metal interphase, promoting a spherical Li growth with minimized surface area and leading to dense lithium deposition. Consequently, the engineered SPE achieves a remarkable cycling of 500 h in Li||Li cells at 2 mA cm^-2. Solid-state Li||LiFePO₄ cells exhibit a record stability for 20 000 cycles at 1.12 A g^-1 (2 mg cm^-2 LiFePO₄ cathode), and a high capacity of 147 mAh g^-1 over 300 cycles at 0.84 mA cm^-2 under a high-loading 2 mAh cm^-2 cathode. The strategy addresses interfacial limitations in SPEs and further introduces a paradigm shift by emphasizing the critical role of spatial salt-graded engineering at the surface over uniform ion distribution for stabilizing high-rate SSLBs.