Effect of copper substitution on the electrochemical properties of high entropy layered oxides cathode materials for sodium-ion batteries

Layered oxide cathode materials in batteries usually experience complex phase transition during intercalating-deintercalating reaction, leading to poor cycle stability. However, high entropy oxides (HEOs) which can suppress the intermediate phase change have been reported with stable crystal structure and improved cycle stability as cathode materials for sodium-ion batteries (SIBs). Here we synthesize a non-toxic Cu substituted HEOs NaNi0.4-xCuxFe0.2Mn0.2Ti0.2O2 (x = 0, 0.1, 0.2, 0.3, 0.4) via high temperature solid-state reaction, and the effect of Cu substitution is analyzed in detail. NaNi0.3Cu0.1Fe0.2Mn0.2Ti0.2O2 delivers an initial specific capacity of 123.6 mA h g−1 at 0.1C rate corresponding to a capacity retention of 90.05 % after 50 cycles, while NaNi0.4Fe0.2Mn0.2Ti0.2O2 delivers a reversible an initial specific capacity of 129.1 mA h g-1corresponding to a capacity retention of 74.83 %. Cu substitution inhibits the phase transition and improve cycle stability of the material during cycling without sacrificing much capacity. This result constitutes a step forward in increasing the practicality of SIBs.