Low‐Ion‐Electron Transport Strategy Minimizes Electrolyte Oxidation for Stable All‐Solid‐State Batteries Without Cathode Modification

ABSTRACT Sulfide solid electrolytes (e.g., Li 6 PS 5 Cl) are pivotal for high‐energy all‐solid‐state batteries but suffer from severe oxidative decomposition at high voltages. Here, we identify that this degradation is governed by an Ion‐Electron Coupling Transport (IECT) mechanism, dictated by the simultaneous availability of electronic and ionic transport pathways at the carbon interface. We propose a carbon‐targeted “Low‐Ion‐Electron Transport” (LIET) strategy by engineering an N‐heterocyclic conjugated polymer layer on the conductive additive. This architecture simultaneously impedes electron tunneling through its conjugated backbone and anchors interfacial lithium ions via abundant pyridine‐nitrogen sites, decoupling the transport synergy and freezing the electrolyte degradation kinetics. Consequently, without requiring modification of the cathode active material, the engineered battery delivers robust high‐rate cycling, maintaining a 99.4% capacity retention over 2000 cycles at a 5C rate. Demonstrating broad universality across diverse solid electrolytes and carbon morphologies, this LIET principle establishes a mechanism‐driven paradigm for stabilizing composite cathodes.