Investigation of sodium content on the electrochemical performance of the Na-x(Fe0.35Mn0.35Co0.3)O-2 (x=0.5, 0.6, 0.7, 0.8, 0.9) for sodium-ion batteries

A series of different Na content P2- and O3-type Na-x(Fe0.35Mn0.35Co0.3)O-2 (x = 0.5, 0.6, 0.7, 0.8, 0.9) were successfully synthesized by sol-gel method. The newly prepared materials were examined by means of X-ray diffraction. They were characterized by scanning electron microscopy (SEM-EDS) and inductively coupled plasma mass spectrometry (ICP-OES). The electrochemical performances of these materials were studied to understand the effect of the Na content. The initial discharge capacities of P2-type Na-0.5(Fe0.35Mn0.35Co0.3)O-2 is 122.5 mAh g(-1), Na-0.6(Fe0.35Mn0.35Co0.3)O-2 is 114.7 mAh g(-1), and Na-0.7(Fe0.35Mn0.35Co0.3)O-2 is 136.5 mAh g(-1) at 0.05 C. After 110 cycles of these materials, the discharge capacities dropped to 55.6 mAh g(-1), 71.2 mAh g(-1), and 89.0 mAh g(-1) at 0.05 C, respectively. The initial discharge capacities of O3-type Na-0.8(Fe0.35Mn0.35Co0.3)O-2 and Na-0.9(Fe0.35Mn0.35Co0.3)O-2 are 145.6 mAh g(-1) and 120.2 mAh g(-1) at 0.05 C, respectively. After 110 cycles of these materials, the discharge capacities dropped to 94.2 mAh g(-1) and 24.95 mAh g(-1) at 0.05 C, respectively. Given these results, P2-type Na-x(Fe0.35Mn0.35Co0.3)O-2 (x = 0.5, 0.6, and 0.7) have higher cycling stability than the O3-type Na-x(Fe0.35Mn0.35Co0.3)O-2 (x = 0.8 and 0.9).