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

Halide solid-state electrolytes (HSSEs), exemplified by low-cost Li₂ZrCl₆, 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-Al₂O₃ system with fast-conductive interfacial percolation layer (IPL) and with regard to the underlying insufficient interfacial filler/electrolyte contact with Li₂ZrCl₆ matrix, which hinders the IPL formation, the use of transition metal oxide Fe₂O₃ is pioneered as a filler to generate oxygen vacancies and foster benign filler/electrolyte interfaces. At the Fe₂O₃/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.