Tailoring planar strain for robust structural stability in high-entropy layered sodium oxide cathode materials

High-entropy oxides have expanded the potential for high-performance Na-ion battery cathodes due to their vast compositional space and entropy-driven stabilization. However, a rational design approach for optimizing their composition is still lacking. Here, we develop an O3-type oxide cathode composed of all-3d transition metals, NaNi0.3Cu0.1Fe0.2Mn0.3Ti0.1O2 (NCFMT), which exhibits improved reversible specific capacity and exceptional cycling stability. Replacing Ti4+ with Sn4+ ions (NaNi0.3Cu0.1Fe0.2Mn0.3Sn0.1O2; NCFMS) results in poor structural reversibility and diminished cycling stability. Our investigations suggest that the structural integrity of the layered cathode is affected by the compatibility of constituent elements within the transition metal layers (TMO2). In NCFMS, planar strain induced by metal-ion displacement triggers elemental segregation and crack formation during repeated cycling. In contrast, NCFMT demonstrates a robust structural framework for stable Na+ storage due to its high mechanochemical compatibility among constituent elements. This understanding provides insights for designing outstanding layered high-entropy cathode materials. High-entropy oxides offer potential for high-performance battery cathodes due to their broad compositional space. The authors present a design approach showing that 3d-compatible elements in O3-type Na-ion batteries reduce lattice strain and ion migration, enhancing structural integrity.