The Interaction between Cu and Fe in P2-Type Na_xTMO₂Cathodes for Advanced Battery Performance

Recently, Cu element has been introduced into layered sodium transition metal oxides (NaxTMO2) as cathode materials for sodium ion batteries to engineer rate and cycling performance. To study the unique role provided by Cu, we designed, synthesized and tested four different compositions of P2-type Nax(MnyFezCo1-y-z)O2 and three compositions of P2-type Nax(MnyFezCu1-y-z)O2 cathode materials. When cycled in the full voltage range of 1.5∼4.5 V under different rates 0.1 C, 1 C and 10 C, the cyclability of MnFeCu-based compounds is better than that of MnFeCo-based ones. Using X-ray diffraction, we observed the P2 to O2-like phase transition of MnFeCu-based materials upon charging and studied its influence on battery performance. Limiting the P2-O2 phase transition delivers less capacity, but improves cyclability. By DFT simulations, we showed that different Na diffusivity and site preference in the high voltage phase contribute to the difference in the electrochemical performances of these cathode materials.