Enhanced Rate Performance and Available Specific Capacity of the O3-NaNi_0.5Mn_0.3Ti_0.2O₂ Cathode by K Substitution for Sodium-Ion Batteries

O3-type layered oxide materials are regarded as the most application-prospective cathode materials for sodium-ion batteries owing to the high theoretical specific capacity and good full-cell matching. However, there are still problems, such as poor rate performance and low available specific capacity. In this paper, we propose to overcome the above-mentioned problems by using K to replace the Na sites of O3-NaNi 0.5 Mn 0.3 Ti 0.2 O 2 materials. A series of K-doped materials, K x Na 1– x Ni 0.5 Mn 0.3 Ti 0.2 ( x = 0, 0.025, 0.05, 0.075), were successfully synthesized. When the doping amount is 0.05, the available specific capacity and rate performance are significantly improved. The K 0.05 Na 0.95 Ni 0.5 Mn 0.3 Ti 0.2 sample exhibits an initial discharge specific capacity of 151.6 mAh g –1 at 0.1C (1C = 200 mA g –1 ), in the voltage window of 2.0–4.2 V, while the pristine one only delivers 135.8 mAh g –1 at 0.1C. It still delivers an initial discharge specific capacity of 119.8 mAh g –1 at 5C and 70.2 mAh g –1 after 200 cycles, while the pristine one only has a specific capacity of 36 mAh g –1 after 200 cycles. The X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), electrochemical impedance spectroscopy (EIS), and galvanostatic intermittent titration technique (GITT) tests were conducted to investigate the effect of different K doping amounts on the crystal structure and electrochemical properties. The results indicate that the expanded Na layer spacing and improved Na + diffusion coefficient are the primary reasons for the improved electrochemical properties. This finding could provide insights into improving the rate performance and available specific capacity of the O3-type layered oxide materials.