Enhanced conduction and phase transition reversibility of O3-type NaNi1/3Fe1/3Mn1/3O2 cathode via Ta doping for high-performance sodium-ion battery

Abstract

The complex phase transition that occurs during the charge/discharge process leads to poor rate and cycling performance of O3-type NaNi1/3Fe1/3Mn1/3O2 (NFM). In this paper, we propose a novel modification strategy for NFM that involves doping its transition metal layer with Ta5+ ions of higher valence and stronger binding energy to oxygen than the transition metal. The effects of Ta doping on the crystal structure, transition metal valence states, activation barrier energy, and bandgap of NFM were investigated, and the modification mechanisms were explored. The capacity retention of NFM-Ta0.01 (80.15 %) was higher than that of NFM (35.44 %) after 200 cycles. This improvement is attributable to the strong bonding energy effect of TaO, which stabilized the crystal structure of NFM, delayed the transition of the material from the hexagonal O3 phase to the hexagonal P3 phase beyond 3.2 V, and enhanced the phase transition reversibility of NFM. Moreover, Ta introduction increased the sodium layer spacing and reduced the Na+ activation barrier energy of the material from 0.773 to 0.692 eV, and the band gap from 1.7416 to 1.5951 eV, leading to a significant improvement in electronic conductivity. Owing to the synergistic effect of these factors, the discharge capacity of NFM-Ta0.01 at a 5C rate (100.6 mAh g−1) was considerably higher than that of NFM (79.3 mAh g−1). This work presents a proven modification strategy for achieving structurally stable and improved rate performance for NFM.