Effect of 3D metal on electrochemical properties of sodium intercalation cathode P2-Na_xMe_1/3Mn_2/3O₂ (M = Co, Ni, or Fe)

This research aims to evaluate the influence of different 3D metals (Fe, Co, and Ni) substituted to Mn on the electrochemical performance of P2-Na_xMe_1/3Mn_2/3O₂ material, which was synthesized by the coprecipitation process followed by calcination at high temperature. X-ray diffraction (XRD) results revealed that the synthesized Mn-rich materials possessed a P2-type structure with a negligible amount of oxide impurities. The materials possessed their typical cyclic voltammogram and charge-discharge profiles; indeed, a high reversible redox reaction was obtained by Na_xCo_1/3Mn_2/3O₂ sample. Both Na_xCo_1/3Mn_2/3O₂ and Na_xFe_{1/ 3}Mn_2/3O₂ provided a high specific capacity of above 140 mAh·g^-1; however, the former showed better cycling performance with 83% capacity retention after 50 cycles at C/10 and high rate capability. Meanwhile, the Ni-sub Na_xNi_1/3Mn_2/3O₂ exhibited excellent cycling stability but a low specific capacity of 110 mAh·g^-1 and inferior rate capability. The diffusion coefficient of Na⁺ ions into the structure tended to decrease with a depth of discharge; those values were in the range of 10^-10-10^-9 cm²·s^-1 and 10^-11-10^-10 cm²·s^-1 in the solid solution region and biphasic region, respectively.