The Synergy Between In Situ Gradient Polymerization and Phase Separation Enables Practical Solid‐State Ni‐Rich Lithium‐Ion Batteries

Abstract Solid polymer electrolytes (SPEs) have garnered significant attention due to their exceptional safety property. However, most of the previously reported SPEs cannot well match with high‐loading and high‐voltage cathodes due to their low ionic conductivity and limited anodic stability. Herein, a SPE with superior compatibility with high‐loading Ni‐rich cathodes is generated by in situ gradient polymerization of a deep eutectic electrolyte. Besides, a polymerization‐induced petaloid phase separation structure enhances interfacial ion transport, resulting in a high room temperature ionic conductivity of 1.5 × 10 ⁻³ S cm ⁻¹ . As a result, the as‐assembled high‐loading (19.5 mg cm ⁻² ) NCM811||graphite full battery exhibites a high capacity retention of 85.3% after 200 cycles and outstanding rate performance (1 C). Industrial 1.2 Ah NCM811||SiO _x pouch cell demonstrates unprecefented energy density of 382 Wh kg ⁻¹ . Moreover, this SPE also exhibits significantly enhanced safety characteristics, delaying the onset temperature of heat release from 157 °C to 266 °C and thermal runaway temperature from 198 °C to 312 °C. This study provides a general and practical avenue to high‐energy‐density lithium‐ion batteries.