Ca pillar effect on the electrochemistry and stability of P2-NaxCayFe0.5Mn0.5O2 for sodium-ion batteries

Cationic substitution at the Na site in the layered transition-metal oxides creates a pillar effect that enhances the stability and electrochemical performance of sodium-ion battery cathodes. However, the conventional solid-state synthesis method for such substitution is often plagued by nonuniform elemental distribution or phase segregation, limiting the application and understanding of the pillaring effect. In this study, we synthesized a series of P2-type NaxCayFe0.5Mn0.5O2 materials via a solid-state ion exchange method, achieving controlled incorporation of Ca2+ at the Na sites. Structural analysis confirms successful Ca substitution and uniform distribution. Ca substitution reduces the lattice mismatch for the phase transitions during electrochemical cycling. Electrochemical testing reveals that Ca substitution decreases reversible capacity without significant improvement in capacity retention. Furthermore, Ca-substituted samples demonstrate enhanced resistance to degradation under air, water, and moist CO2 exposure. These findings highlight the pillaring effects induced by Ca, which provide insights into designing more durable cathode materials for sodium-ion batteries.