Synergistic Na-site Ca and TM-site Cu doping in an O3-type NaNi1/3Fe1/3Mn1/3O2 cathode towards fast kinetics and long-life sodium-ion batteries

Abstract

The O3-type layered oxide of NaNi_1/3Fe_1/3Mn_1/3O₂ (NFM111) is a potential cathode for sodium-ion batteries owing to its high theoretical capacity and cobalt-free composition. However, its practical application is limited by sluggish Na⁺ diffusion, irreversible structure evolution during cycling, and air sensitivity. To resolve these challenges, a Ca²⁺/Cu²⁺ dual-site doping strategy is adopted to modify NFM111. Ca²⁺ acts as a kind of structural ‘pillar’ in the alkali-layer to enhance the transport behavior of Na⁺ during cycling and reduces Ni to a lower valence state (Ni²⁺) to stabilize the structure and contribute to better capacity retention, while Cu²⁺ substitution in the transition-metal layer reduces the charge transfer resistance of the material and improves air stability. The synergistic modification via Ca²⁺/Cu²⁺ dual-site doping can not only improve the rate capability and long cycle performance, but also enhance the structure stability upon air exposure and reversibility of the electrode. Therefore, CaCu-NFM111 exhibits 83.0% capacity retention after 500 cycles at 1C (47.6% improvement over pristine NFM111) and 69.3% retention after 200 cycles at 2C. This work offers a feasible modification approach to improve the electrochemical performance of O3-type layered oxide cathodes.