Mechanistic Control of Cathode-Electrolyte Interphase Formation in Gel Polymer Electrolyte for Lithium-Ion Batteries

In-situ gel polymer electrolytes (GPEs) have attracted attention as promising candidates for next-generation lithium-ion batteries, offering superior interfacial compatibility and enhanced safety compared with conventional liquid electrolytes. However, the issue of residual monomers remaining after polymerization has not been adequately addressed. They readily decompose at the high-Ni cathode (NCM) surface during the early stage of formation, generating an organic-rich layer within the inner cathode-electrolyte interphase (CEI) and disrupting the normal CEI-forming pathway. Here, we present a strategy that combines a molecular additive, tris(4-fluorophenyl)phosphine (TFPP), with a tailored formation protocol to mitigate monomer-induced degradation. Prior to oxidative decomposition, TFPP modifies the interfacial environment of NCM through early surface interaction and partial coverage, reducing the accessibility of residual monomers to the cathode interface. The formation protocol then promotes preferential TFPP decomposition before extensive monomer oxidation, enabling a protective CEI with a LiF-rich inner region. This sequential interfacial regulation suppresses monomer-driven side reactions, reduces electrolyte consumption, stabilizes Li-ion transport, and mitigates structural degradation of the cathode. Consequently, a 2 Ah-level pouch cell delivering 303 Wh kg^-1 retained 82.8% capacity after 150 cycles under lean electrolyte conditions (2 g Ah^-1). This work thus provides a practical strategy for developing more durable high-energy-density GPE-based batteries.