Interfacial O‐Cl Exchange via Transition Metal Oxide Fillers Boosts Performance and Cost‐Efficiency in Zr‐Based Chloride Li‐Ion Conductors

ABSTRACT Halide solid‐state electrolytes (HSSEs), exemplified by low‐cost Li ₂ ZrCl ₆ , exhibit high ionic conductivity and a wide electrochemical stability window, making them promising for all‐solid‐state batteries (ASSBs). Yet, optimizing HSSEs by elemental doping is either inefficient or costly, necessitating alternative approaches. Inspired by the filler‐modified LiI‐Al ₂ O ₃ system with fast‐conductive interfacial percolation layer (IPL) and with regard to the underlying insufficient interfacial filler/electrolyte contact with Li ₂ ZrCl ₆ matrix, which hinders the IPL formation, the use of transition metal oxide Fe ₂ O ₃ is pioneered as a filler to generate oxygen vacancies and foster benign filler/electrolyte interfaces. At the Fe ₂ O ₃ /electrolyte interface, synchrotron X‐ray adsorption spectra and electron energy loss spectra mapping reveal a distinctive O‐Cl exchange reaction and improved interfacial contact. Density function theory, bond‐valence site energy calculations, and finite‐element simulations validate improved Li ⁺ migration via mitigated electrostatic confinement, reduced migration energy barriers, and spatially confined electric‐field‐accelerated IPLs. Benefiting from these merits, the ionic conductivity of the composite electrolyte surges from 0.4 to 2.15 mS cm ⁻¹ , with Li ⁺ migration activation energy reduced to 0.28 eV. Paired with the NCM811 cathode, the ASSB delivers 90% capacity retention over 1000 cycles at 0.5 C. This work offers a novel, cost‐effective strategy for high‐performance halide‐oxide composite electrolytes from the counterintuitive perspective of nanofiller integration.