A Quasi‐Double‐Layer Solid Electrolyte with Adjustable Interphases Enabling High‐Voltage Solid‐State Batteries

Abstract Increasing the energy density and long‐term cycling stability of lithium‐ion batteries necessitates the stability of electrolytes under high/low voltage application and stable electrode/electrolyte interfacial contact. However, neither a single polymer nor liquid electrolyte can realize this due to their limited internal energy gap, which cannot avoid lithium‐metal deposition and electrolyte oxidation simultaneously. Herein, a novel type of quasi‐double‐layer composite polymer electrolytes (QDL‐CPEs) is proposed by using plasticizers with high oxidation stability (propylene carbonate) and high reduction stability (diethylene glycol dimethyl ether) in a poly(vinylidene fluoride) (PVDF)‐based electrolyte composites. In‐situ‐polymerized propylene carbonate can function as a cathode electrolyte interface (CEI) film, which can enhance the antioxidant ability. The nucleophilic substitution reaction between diethylene glycol dimethyl ether and PVDF increases the reduction stability of the electrolyte on the anodic side, without the formation of lithium dendrites. The QDL‐CPEs has high ionic conductivity, an enhanced electrochemical reaction window, adjustable electrode/electrolyte interphases, and no additional electrolyte–electrolyte interfacial resistance. Thus, this ingenious design of the QDL‐CPEs improves the cycling performance of a fabricated LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811)//QDL‐CPEs//hard carbon full cell at room temperature, paving a new way for designing solid‐state battery systems accessible for practical applications.