Quantification and Optimization of Interfacial Ion Transport in Polymer/Ceramic Composite Electrolytes for Solid‐State Batteries

Solid polymer/ceramic composite electrolytes have emerged as promising candidates for solid‐state batteries owing to their superior mechano‐chemical compatibility, oxidation stability, and high ionic conductivity. While extensive studies confirm that the newly formed interphase critically enhances ionic conductivity, its quantitative contribution remains experimentally unverified for any composite electrolyte. This knowledge gap lacks a key guideline for designing commercially viable solid electrolytes thereby hinders the development of solid‐state batteries. A key challenge arises from the conventional low‐dimensional fillers used in the composite electrolytes that tend to aggregate to create non‐uniform interphase distribution thus complicating the determination of critical carrier transport parameters. To address this, we fabricate three‐dimensional Li6.4Al0.1La3Zr1.7Ta0.3O12 self‐supported porous skeletons as fillers, in which 1,3‐dioxolane is in situ polymerized to establish a composite model system. Using advanced characterization techniques, we determine the geometric parameters governing carrier transport and develop a corresponding model to estimate interphase conductivity. Remarkably, the interphase exhibits a room‐temperature conductivity of 2.5 mS cm−1, 33‐fold higher than that of the bulk composite electrolyte. We attribute this enhancement to Lewis acid–base interactions that increase initiator concentration at the interphase, producing short‐chain interfacial poly(1,3‐dioxolane) with enlarged free volume for rapid Li‐ion conduction. By applying this mechanistic understanding and coating the Li6.4Al0.1La3Zr1.7Ta0.3O12 skeleton with a stronger Lewis base (Li6PS5Cl), we further optimize interphase conductivity to 12 mS cm−1. The applicability of the composite electrolytes is demonstrated in high‐energy solid‐state batteries with both sulfur and LiNi0.8Co0.1Mn0.1O2 cathodes paired with lithium metal anodes. This work establishes fundamental design principles for engineering high‐conductivity interphases in polymer/ceramic composite electrolytes.