Enhancing structural stability and conductivity of O3-type NaNi1/3Fe1/3Mn1/3O2 cathodes for sodium-ion batteries through Mg substitution for Ni

The complex phase transition of O3-type NaNi 1/3 Fe 1/3 Mn 1/3 O 2 (NFM) during charge/discharge process results in limited cycling stability and rate performance, significantly impeding its application in sodium-ion batteries. In this study, a series of NaNi 1/3-x Fe 1/3 Mn 1/3 Mg x O 2 ( x  = 0, 0.03, 0.05) samples with varying concentrations of Mg substitution were synthesized using the solid-state method. The impact of Mg substitution on the crystal structure and the element ratio were investigated by XRD and ICP. It was found that Mg substitution not only reduces the content of Mn 3+ to suppress the Jahn–Teller effect in the material but also broadens the voltage range (from 3.3 V → 4.0 V → 2.74 V) of the pure P3 phase compared to the NFM (from 3.48 V → 4.0 V → 2.85 V), which significantly improve cycling performance. In addition, the DFT calculations indicate that the bandgap reduction and the electronic conductivity enhancement occur after Mg substitution, which results in significant improvement in rate performance. After 200 cycles at 1C, the NFM-Mg0.03 sample maintained a discharge-specific capacity as high as 104 mAhg −1 with a capacity retention rate reaching up to 87.5%. The discharge-specific capacity increases from 99.7 mAhg −1 (NFM) to 114.5 mAhg −1 (NFM-Mg0.03) at 2 C. Therefore, Mg substitution effectively improves the electrochemical performance and shows great potential for large-scale energy storage applications.