Synergistic Reduction and Oxidation Resistant Interface Modifier for High‐Voltage and High‐Loading Solid‐State Lithium Batteries

Abstract Solid‐state batteries (SSBs) with high‐voltage cathodes and Li‐anodes offer promising energy density and safety for next‐generation batteries. However, poor contact and electrochemical instability of solid electrolyte interfaces hinder their long‐term performance. Traditional rigid solidification interlayers possess restricted capability to address these issues. Herein, a composite buffer interlayer (CBI) with localized high‐concentration electrolytes (LHCEs) in a flexible polymer scaffold, tackling contact and stability problems and ensuring a perfect interface is developed. The extended electrochemical window provides it with synergistic antioxidation and antireduction capabilities, making it compatible with high‐voltage cathodes and Li anodes, while an in situ formed LiF‐Li 3 N rich inorganic interface ensures uniform lithium deposition and prevents dendrite formation. This CBI enables lithium symmetric cells to achieve a super high critical current density of 7.2 mA cm −2 . Most impressively, coupled with a high‐voltage LiNi 0.83 Co 0.12 Mn 0.05 O 2 cathode (NCM83), the full cell achieves 94.1% capacity retention after 125 cycles (coulombic efficiency >99.8%) at a mass loading of 14.6 mg cm −2 and a high voltage of 4.45 V. Additionally, a pouch cell with 17.2 mg cm −2 NCM83 achieves an initial discharge capacity of 3.82 mAh cm −2 an superior cycling stability (75 cycles, 89% capacity retention), showcasing the practical potential of LHCE‐CBI enabled SSBs.