High-rate Mg doped NaNi1/3Fe1/3Mn1/3O2 cathode with excellent low-temperature stability improved by one-step coprecipitation for sodium ion battery applications

The progress in layered oxide cathode for sodium-ion batteries, while encouraging due to higher capacity and favorable operating potential, has been hindered by two critical issues: structural collapse during cycling and insufficient ionic conductivity, which collectively degrade electrochemical performance. In this work, we successfully developed Na[Ni_1/3Fe_1/3Mn_1/3]O₂ cathodes with Mg-doped (Mg_x-NFM, x = 0.01, 0.03, and 0.05) through the one-step coprecipitation method, achieving uniform Mg²⁺ distribution within transition metal layers. Systematic characterization reveals that Mg doping effectively mitigates structural deformation during Na⁺ insertion/extraction, stabilizes the crystalline framework, and reduces electrode polarization, thereby significantly enhancing Na⁺ diffusion kinetics. Exceptional enhancement is achieved by the Mg_0.03-NFM cathode, demonstrating 91.00 mAh g⁻¹ even at 15C. Cycling tests reveal 60.26 % capacity preservation after 400 cycles (10C) and 80.29 % after 200 cycles (1C), significantly exceeding the 4.03 % and 42.94 % values of the undoped NFM under equivalent testing conditions. Density functional theory shows that the Mg doping strategy substantially enhances the conductivity of O3-type NFM cathode. Moreover, the fabricated Mg_0.03-NFM//HC reaches an energy density of 368.65 Wh kg⁻¹. Finally, ≈90 % capacity retention is preserved after 200 cycles even under −7 °C. This study demonstrates an effective materials design approach for sodium-ion battery cathodes, providing a pathway to develop energy storage technology with enhanced performance.