Exploring Li + and Na+ substitution for defect engineering in high-entropy oxides for lithium storage behavior

High-entropy oxides (HEOs) are promising electrode materials for electrochemical energy storage due to their structural tunability and defect driven functionalities. Here, aliovalent substitution of Li + and Na + into rock-salt (MgCoNiCuZn)O is investigated. Single-phase HEOs with 15 mol% aliovalent incorporation were synthesized and confirmed by XRD and Rietveld refinement. XPS analysis revealed higher oxygen vacancy concentrations in Na-containing samples compared to Li-containing ones. Both systems exhibited comparable electrochemical behavior with high reversible capacities, while Na + containing HEOs demonstrated slightly better rate capability and cycling stability. These results highlight that Na, owing to its abundance and low cost, can serve as a sustainable alternative to Li for defect engineering and performance optimization in HEO-based anodes. This study also opens new opportunities for designing HEO-based anodes and extending these concepts to other functional HEOs through compositional engineering.