Coupled Antisite/Interstitial Li Defects Regulate Lithium Migration in Li5AlO4 Antifluorite Oxide

Antifluorite oxides containing ordered tetrahedral voids provide a structurally flexible platform for Li-ion transport; however, experimentally identifying and controlling the defect configurations that activate these voids remains challenging. Here, Li-excess defect engineering is demonstrated in orthorhombic Li5AlO4 to regulate its Li-ion transport behavior. High-resolution time-of-flight neutron powder diffraction (TOF-NPD), combined with Rietveld refinement and Fourier difference nuclear-density mapping, resolves the underlying Li-related defect architecture. The refinement results reveal that excess Li is accommodated through the coupled formation of Al/Li antisite occupancy at the 2a Wyckoff site and newly occupied interstitial Li at the intrinsically vacant 2b site, accompanied by systematic lattice expansion and local structural relaxation. BVSE-guided CI-NEB calculations reveal a multistep Li-ion migration network, in which the three-dimensional bottleneck decreases from 0.43 eV in pristine Li5AlO4 to 0.40 eV in the coupled-defect model, while the local migration segments directly involving the occupied 2b site exhibit larger barrier reductions. Consistently, electrochemical impedance spectroscopy shows an increase in the room-temperature bulk ionic conductivity to 3.33 × 10-7 S cm-1 for the Li-excess sample. These findings establish an experimentally validated correlation between Li-related defect chemistry and ion transport in Li5AlO4, providing guidance for defect-enabled transport design in antifluorite oxide conductors.