Na-ion batteries (NIBs) have a great potential as next-generation large-scale energy storage devices. Among various cathode materials for NIBs, P2-type layered transition metal oxides have attracted much attention because of their excellent kinetics and easy processability. In P2-type layered oxides, Na ion occupies two different sites, i.e., Na_e and Na_f. The triangular prism polyhedron of Na_f site shares faces with two TmO6 octahedrons, while Na_e shares edges with six TmO6. The migration behavior of Na ions at different positions is diverse due to the distinction of distance and electrostatic force between Na⁺-Tmⁿ⁺. The migration of Na ions at Na_e site has a lower migration energy barrier, compared to that of Na_f position. Namely, more Na ions at Na_e site and increasing the occupation ratio of Na_e/Na_f can effectively improve the electrochemical performance of P2 layered oxides. Previous research indicates that the ratio of Na ions at different sites can be modulated by introducing inactive ions to transition metal layers. However, the introduction inevitably results in the capacity sacrifice. The ration of Na_e/Na_f is mainly affected by two aspects, i.e., the interaction between Tmⁿ⁺ and Na⁺, and the interaction between Na⁺ and Na⁺. In the case with the same transition metal component, the design of Na content is expected to achieve the regulation of Na_e/Na_f ratio, mainly from the following three reasons: 1) Na content affects the valence state of transition metals, and thus modulates the interaction between Tmⁿ⁺ and Na⁺; 2) Na content changes the Na interlayer distance, and then affects the distance between Na⁺ and Tmⁿ⁺ and the corresponding interaction between Na⁺ and Tmⁿ⁺; and 3) Na content regulates the distance between Na⁺ and Na⁺. In this paper, we proposed a novel strategy to optimize the Na_e/Na_f occupancy ratio with the same transition metal composition via adjusting Na content in the material. This strategy could open up new opportunities to design high-performance cathode materials for NIBs.

Na_xNi_0.1Mn_0.9O₂ (x = 0.45, 0.55, and 0.65, denoted as Na45, Na55, Na65) were prepared by a simple high-temperature solid-phase method. The precursors were Na₂CO₃, NiO and Mn₂O₃ and weighed according to a required stoichiometry ratio with a 5% excess of Na₂CO₃. After grinding, they were heatd in an alumina crucible in a muffle furnace at an elevated rate of 5 ℃/min up to 950 ℃. After heated at 950 ℃ for 15 h, the powdered materials were obtained by natural cooling and finally transferred to an argon-filled glove box. The working electrodes were prepared via spreading the slurry of active materials, acetylene black and polyvinylidene difluoride with a mass percent of 75∶15∶10 on Al foil. After drying in vacuum at 80 ℃ for 12 h, the electrodes were transferred to an argon-filled glove box and assembled into CR2032-type button cells. The counter electrode was metal sodium, the separator was a glass fiber, and the electrolyte was 1 M NaClO₄ in propylene carbonate (PC) solution with 5% fluoroethylene carbonate (FEC) additive.

The effect of Na content on the occupancy ratio of Na_e/Na_f positions and the corresponding evolution of structure and electrochemical properties was investigated. All the Nax compounds have a hexagonal P2 structure with P6₃/mmc space group. According to the XRD refinement and eletrochemical results, Na55 material has the maximum Na_e/Na_f occupancy ratio and specific discharge capacity, mainly originating from the better mobility of Na ions at Na_e sites. Na55 exhibits a prominent capacity of 174.5 mAh·g^-1 at 0.2 C, which is much higher than that of Na45 and Na65. Benefited from the optimized Na_e/Na_f ration, Na55 displays 10^-10-10^-12 cm²·s^-1 of Na⁺ diffusion coefficient, which is much higher than 10^-12-10^-14 cm²·s^-1 of the usual transition metal layered oxides cathodes. Also, Na55 exhibits an excellent rate performance with a capacity of 83.6 mAh·g^-1 at 10 C rate, and a great cycling stability with a 92.95% capacity retention after 200 cycles.

Na_e/Na_f occupancy ratio of P2-type layered oxides was modulated by an extremely simple strategy for Na content optimization. The combined results of XRD refinement, electrochemical curve, transmission electron microscopy, cyclic voltammogram, ex-situ XRD and X-ray photoelectron spectroscopy showed that a higher Na_e/Na_f ratio in Na55 materials effectively enhanced the Na⁺ diffusion rate, consequently resulting in a high specific capacity (174.5 mAh·g^-1 at 0.2 C), an outstanding kinetic performance (87 mAh·g^-1 at 10 C) with an excellent cycling performance (92.95% capacity retention after 200 cycles). Compared to conventional P2 materials with common Na content, Na55 had a smaller volume change without any phase transition during electrochemical process. The simple and effective strategy could offer an insight into the rational design of high-performance layered cathode materials and also enhance the practical application of NIBs.