Deciphering Failure Mechanisms of High-Ni Cathodes Via Residual Monomer-Driven Interphase Evolution in Gel Polymer Electrolytes

The integration of gel polymer electrolytes (GPEs) with high-Ni layered cathodes (NCM) has emerged as a promising strategy for realizing high-energy lithium-ion batteries, offering a potential solution to the failure modes commonly observed in liquid electrolyte systems. Yet the impact of residual monomers from incomplete curing on capacity fade has been largely overlooked. In this work, we present a comprehensive investigation of side reactions between unreacted ETPTA (ethoxylated trimethylolpropane triacrylate) monomers and NCM cathode surfaces within representative GPE systems, employing a combination of multiscale spectroscopic analyses and theoretical calculations. By systematically tuning monomer conversion during curing and linking it to interphase chemistry, transport, and electrode integrity, we reveal a direct conversion-structure-performance relationship. Density functional theory (DFT) and linear sweep voltammetry (LSV) indicate that ETPTA is oxidatively less stable than carbonate solvents. In thermally cured GPEs (TC-GPEs) with higher monomer content, initial charging triggers oxidative decomposition of residual monomers, producing organic ether/ester byproducts that drive interphase reconstruction toward an organic-rich, resistive CEI (cathode electrolyte interphase). In situ EIS reveals a sharp increase in interfacial resistance above the ETPTA oxidation threshold for low-conversion GPEs, whereas well-cured GPEs show more reversible impedance evolution. X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS) depth profiling confirm thicker organic CEI and suppressed LiF formation when residual monomers are abundant. This reconstructed interphase exhibits retarded ion transport and uneven growth across the cathode microstructure, which amplifies polarization and accelerates the onset of characteristic NCM failure modes. Structural degradation of NCM cathodes including transition metal reduction/dissolution, surface reconstruction toward rock-salt-like motifs, cation mixing, lattice distortion, and intergranular crack formation was elucidated by high-resolution transmission electron microscopy (HR-TEM) and X-ray diffraction (XRD). These chemical and structural changes manifest poorer rate capability, larger overpotentials, and inferior capacity retention compared with well-cured counterparts. Overall, this work identifies residual-monomer-initiated interphase reconstruction as a primary driver of capacity fade in NCM||GPE cells. The findings provide mechanistic insights into the multiscale fading processes in NCM‖GPE systems, highlighting the critical role of residual monomers in interphase instability, and guiding the design of more durable NCM‖GPE configurations for practical battery applications. Figure 1