Uncovering Interfacial Instability: How Phase Separation in Polymer Electrolytes Undermines Battery Performance?

• • After 200 cycles, XRF mapping revealed distinct sulfur-depleted regions near both the cathode and anode interfaces, with greater severity at the Li metal side, directly evidencing TFSI− anion depletion and local phase separation that degrades ionic percolation pathways. • • The MIC electrolyte comprises 90 wt% mobile ions (ionic liquid Pyr14TFSI plus LiTFSI) within a rigid-rod PBDT network, a composition that overcomes the traditional ionic conductivity–mechanical stiffness trade-off seen in homogeneous poly(ethylene oxide) electrolytes. • • Sulfur signal served as a proxy for TFSI− distribution because the PBDT backbone contributes only a minor fraction of total sulfur via sulfonate groups, enabling quantitative tracking of anion concentration heterogeneity across the buried Li|PE|NCM811 interface. • • Concentration heterogeneity triggers local phase separation of the polymer electrolyte, compromising structural integrity and ionic transport; the severity correlates with upper cut-off potential, indicating that high-voltage operation accelerates interfacial chemomechanical failure.