Enhanced Rate Performance and Available Specific Capacity of the O3-NaNi0.5Mn0.3Ti0.2O2 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-NaNi0.5Mn0.3Ti0.2O2 materials. A series of K-doped materials, KxNa1-xNi0.5Mn0.3Ti0.2 (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 K0.05Na0.95Ni0.5Mn0.3Ti0.2 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.