Interphase activators for continuous Li⁺ transport in garnet-polymer composite solid electrolytes at room temperature

Ta-doped garnet (LLZTO) composite solid electrolytes (CSEs) with poly(vinylidene fluoride) (PVDF) coatings are attractive for solid-state Li-metal batteries (SSLMBs), but are limited by the absence of continuous, low-barrier Li⁺ percolation pathways across ceramic-polymer interfaces. Surface contaminants on LLZTO and sluggish Li⁺ coordination exchange within semicrystalline PVDF collectively disrupt interfacial continuity, resulting in high transport resistance and dendrite growth. Here, we introduce an interphase activator (IA) strategy that reconstructs the LLZTO surface by converting native Li₂CO₃/LiOH species while simultaneously modulating Li⁺ transport within PVDF. Using SbF₃ as a representative IA, native Li₂CO₃/LiOH species are transformed into a spatially coupled Sb₂O₃/LiF interphase. Density functional theory calculations reveal a low Li⁺ migration barrier (0.22 eV) at the Sb₂O₃(220)/LiF(001) boundary, supporting enhanced interfacial transport kinetics. In the presence of ion-conducting cellulose (ICC), which provides additional oxygen-containing coordination sites, SbF₃ further establishes a multicomponent noncovalent interaction environment that suppresses PVDF crystallinity and accelerates Li⁺ coordination exchange. Owing to the dual-function role of SbF₃, the resulting CSEs achieve an ionic conductivity of 6.7 × 10^-4 S cm^-1, a Li⁺ transfer number of 0.84, and a critical current density of up to 3 mA cm^-2. LiFePO₄ (LFP)|Li, LiNi_0.5Co_0.2Mn_0.3O₂ (NCM523)|Li, and Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ (LNCMO)|Li cells exhibit stable cycling at 30 ℃. This work provides a foundation for developing IA strategies in garnet-polymer CSEs.