Enhancing electrochemical performance of sodium-ion batteries: Optimal Ca2+ doping in O3-NaNi0.33Mn0.33Fe0.33O2 layered oxide cathode materials

Current sodium-ion battery technologies are hindered by limitations in cathode material performance, including insufficient cycle stability and low energy density, which impede their wider application and competitiveness with lithium-ion counterparts. Addressing these challenges requires innovative material engineering to improve electrochemical properties and operational durability. In this study, we synthesized a series of Na1-2xCax(Ni0.33Mn0.33Fe0.33)O2 (x = 0, 0.015, 0.025, 0.04, 0.05), by doping with electrochemically inactive Ca2+. Our findings demonstrate that doping with Ca2+ significantly enhances the electrochemical performance, as evidenced by comparisons with undoped materials. Specifically, the material Na0.95Ca0.025(Ni0.33Mn0.33Fe0.33)O2, synthesized at 900 °C, showed the best electrochemical performance, achieving an initial discharge specific capacity of 137 mAh/g within a voltage range of 2–4 V, maintaining a capacity of 117 mAh/g at a 5C discharge rate, and exhibiting a capacity retention rate of 96.2 % after 50 cycles at 1C. Additionally, Ca2+ doping was found to enhance the air stability, with negligible change in initial discharge capacity after exposure to air for 24 and 72 h. This doping strategy not only improves the performance of sodium-ion batteries but also supports the development of high-energy–density industrial batteries, offering a viable approach to designing other layered oxide materials.