Uncovering Interfacial Instability: How Phase Separation in Polymer Electrolytes Undermines Battery Performance?
Solid-state batteries (SSBs) pairing lithium metal anodes with high-voltage cathodes promise higher energy density and safety than liquid-electrolyte lithium-ion batteries. Polymer electrolytes (PEs) are pivotal due to flexibility, processability, and conformal electrode contact. However, high interfacial resistance, lithium dendrite growth, inactive dead lithium, and parasitic side reactions—especially with high-voltage cathodes—severely limit PE-based SSBs. These failures stem from concentration heterogeneity (concentration polarization) and chemical heterogeneity at the electrode/PE interface during operation. Concentration gradients produce non-uniform Li-ion flux, creating localized hot spots that trigger dendrite formation and electrolyte decomposition. Chemical heterogeneity from spatially non-uniform side reactions dictates the nature and effectiveness of the solid electrolyte interphase (SEI) or cathode electrolyte interphase (CEI). Probing the evolution of these heterogeneities at buried solid/solid interfaces with chemical specificity and spatial resolution in functional cells remains a significant challenge. Lin et al. (Nat Nanotechnol, 2025, 20, 787–797) reported a molecular ionic composite (MIC) electrolyte—a rigid-rod ionic polymer (PBDT) network, an ionic liquid (Pyr14TFSI), and LiTFSI—as a model multiphase system. Using X-ray fluorescence (XRF) microscopy and X-ray absorption spectroscopy (XAS), they mapped sulfur distribution across Li|PE|NCM811 cross-sections. After 200 cycles, sulfur-depleted regions emerged near both electrodes, more severe at the Li metal side, indicating TFSI− anion depletion and local phase separation that compromises structural integrity and ionic transport. This work establishes interfacial chemomechanics as a governing factor for PE-based SSB stability.