Interfacial Phase Separation Governs the Chemomechanics of Polymer Electrolytes in High-Voltage, Solid-State Lithium Batteries

Polymer electrolytes hold great promise for safe and high-energy solid-state batteries. Multiphase polymer electrolytes, consisting of mobile and rigid phases, exhibit fast ion conduction and desired mechanical properties. However, fundamental challenges exist in understanding and regulating intricate chemomechanical interactions at the electrode-electrolyte interface, especially when using a high-voltage layered cathode. Here, we report that depletion of the mobile conductive phase at the interface contributes to battery performance degradation. Molecular ionic composite electrolytes, composed of a rigid-rod ionic polymer with small mobile cations and anions, serve as a multiphase platform to investigate the evolution of conductive domains at the interface. Synchrotron chemical and structural analyses enable the direct visualization of concentration polarization and spatially resolve the interfacial chemical states over a statistically significant field of view for buried interfaces. We discover that concentration and chemical heterogeneities prevail at electrode-electrolyte interfaces, leading to interfacial chemomechanical degradation. We further demonstrate an interphase tailoring strategy based on electrolyte additives to mitigate such interfacial heterogeneity and improve battery stability. While polymer electrolytes have been considered “deformable” for conformal interfaces, our work shows the hidden roles of interfacial chemomechanics in polymer electrolytes and provides an effective mitigation approach.