Unveiling the Structure and Diffusion Kinetics at the Composite Electrolyte Interface in Solid‐State Batteries

Abstract The “interface” between polymer and oxide within the polymer‐oxide composite electrolytes is widely acknowledged as a crucial factor influencing ionic conduction. However, a fundamental understanding of the precise composition and/or micro‐structure, and the ionic conduction mechanism at the complex interface has remained elusive, primarily due to a dearth of compelling experimental evidence. In this study, the intricate correlation between morphology and composition in composite electrolytes is discerned by leveraging advanced 1D and 2D exchange nuclear magnetic resonance spectroscopy (1D and 2D EXSY NMR) techniques. Notably, this research represents the inaugural elucidation of the microstructure of the interface. The findings underscore the pivotal role of the preparation conditions for polymer‐oxide composite electrolytes, particularly the solvent selection, in determining the formation of the interface structure. Direct insights into the lithium‐deficient surface of Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO) are provided and elucidate the timescales of Li‐ion exchange processes among various components. Furthermore, a comprehensive investigation into the roles of individual components within the composite electrolyte on the Li‐ion conduction mechanism is conducted through the ⁶Li→⁷Li isotope tracer technique as a function of current density.