Residual Monomer‐Induced Side Reactions in Gel Polymer Electrolytes: Unveiled High‐Ni Cathode Failure in Lithium Batteries

Coupling gel polymer electrolytes (GPEs) with high‐Ni cathodes (NCM) has emerged as a compelling approach for high‐energy lithium‐ion batteries, capable of circumventing NCM failure modes in liquid electrolytes. However, a detailed origin of capacity decay caused by residual monomers from an uncontrollable curing process has been largely ignored. Here, we report an in‐depth investigation into the side reactions of unreacted monomers within typical GPEs at the NCM cathode interfaces by utilizing multiscale spectroscopy combined with theoretical calculations. We evaluate conversion rate‐interphase structure correlation, revealing that interfacial evolution is highly dependent on residual monomer content. Specifically, the degradation chemistry in NCM cathodes with thermally cured gel polymer electrolytes (TC‐GPEs) is governed by monomer‐initiated interphase reconstruction, leading to an imbalanced interphase growth mode with organic‐rich species and retarded diffusion kinetics through the electrode. We further reveal that organic ether/ester‐based byproducts, caused by the oxidative decomposition of unreacted monomers during the initial charging step, are the key factor for the acceleration of NCM failure modes. This study elucidates the multiscale fading mechanism in the NCM||GPE system, providing improved insights into interphase issues in typical GPEs and facilitating the further development of long‐life NCM||GPE prototypes for commercial applications.