Unveiling the Dilution‐Shielding Effect as a Universal Strategy for Interfacial Regulation in Composite Polymer Electrolytes

ABSTRACT Composite solid electrolytes integrating poly(vinylidene fluoride‐co‐hexafluoropropylene) (PVDF‐HFP) with garnet‐type ceramic fillers offer a balance of mechanical flexibility and ionic conductivity, yet parasitic interfacial reactions compromise long‐term chemical and electrochemical stabilities. PVDF‐HFP undergoes dehydrofluorination when lanthanum (La) sites on garnet fillers coordinate with polar solvents, creating alkaline microenvironments that accelerate polymer degradation. Here, we unravel a dilution‐shielding effect that offers a universal strategy for promoting interfacial passivation of composite polymer electrolytes (CPEs) and enabling uniform Li + transport. Specifically, incorporating fluoroethylene carbonate (FEC) with an ultrahigh dielectric constant effectively attenuates the activity of strongly basic coordinating solvents and electrostatically shields La sites, thereby effectively preventing the formation of alkaline microenvironments and alleviating dehydrofluorination. This simple strategy also concurrently drives in situ formation of a LiF‐rich protective interphase. Comprehensive in situ characterizations substantiate that the resulting FEC‐modified CPE enables enhanced interfacial passivation and stable Li + transport. Consequently, Li|Li symmetric cells achieve stable cycling for over 700 h at 0.1 mA cm −2 , and LiFePO 4 |Li full cells deliver more than 1200 cycles at 1C with 90.2% capacity retention. This work establishes the dilution‐shielding effect as a transferable interfacial regulation strategy, providing new insights into the design of chemically and electrochemically stable solid‐state lithium metal batteries (ssLMBs).