Abstract:

Sodium-ion batteries have promising potential in large-scale electric storage applications due to their low cost, environmental friendliness, and similar working principles to lithium-ion batteries. O3-type layered oxides are used as a cathode material that determines the energy density of SIBs and stand out from other cathodes due to their high capacity and ease of synthesis. However, the migration of Na⁺ between octahedral positions in the O3 phase must overcome a large energy barrier, which results in complex reaction phase transitions and rapid capacity decay. Therefore, the Na⁺ de-intercalation behavior and the structural evolution of O3-type structure should be explored before developing high-performance cathodes. Herein, we systematically investigated the electrochemical properties, Na⁺ transport kinetics, and phase transition mechanism of O3-NaNi_0.4Fe_0.2Mn_0.4O₂ (O3-NFM). The O3-NFM cathode delivered a capacity about 201.9 mAh/g (corresponding to 0.84 mol of Na⁺ extraction) when charged to 4.3 V. After the cut-off voltage is set to 4.0 V, O3-NFM can achieve a stable reversible cycle. The improved cycling performance between 2.0 V and 4.0 V can be ascribed to the reversible transformation of O3-P3/O3-P3-P3/O3-O3 structural evolution, as determined by in situ X-ray diffraction. A fast kinetics of the Na⁺ diffusion in the O3 structure was revealed by cyclic voltammetry and galvanostatic intermittent titration technique techniques, which contributes to a good rate performance. This work provides a theoretical basis for investigating the structure modification and material design based on O3-NFM cathodes.

Key words: sodium batteries, layered oxide, O3-NaNi_0.4Fe_0.2Mn_0.4O₂, in-situ XRD