Na-Rich Na3+xV2–xNix(PO4)3/C for Sodium Ion Batteries: Controlling the Doping Site and Improving the Electrochemical Performances

In order to get an element substituted into Na₃V₂(PO₄)₃/C in an appointed V site, the simple sol-gel method is used to design and prepare a series of Na-rich Na_3+xV_2-xNi_x(PO₄)₃/C (x = 0-0.07) compounds. To get a charge balance, the ratio of Na, V, and Ni would be changed if Ni goes into a different site. Hence, ICP is applied to probe the real stoichiometry of the as-prepared Na_3+xV_2-xNi_x(PO₄)₃/C (x = 0-0.07). According to the Na/V ratio from the ICP result, it indicates that Ni²⁺ goes to a V site, and more Na⁺ will be introduced into the crystal to keep the charge balance. In addition, the crystal structure changes are explored by XRD and Rietveld refinement, it is indicated from the results that Ni²⁺ doping does not destroy the lattice structure of Na₃V₂(PO₄)₃. When applied as Na-storage material, the electrochemical property of all Ni²⁺ doped Na_3+xV_2-xNi_x(PO₄)₃/C composites have been significantly improved, especially for the Na_3.03V_1.97Ni_0.03(PO₄)₃/C sample. For example, 107.1 mAh g^-1 can be obtained at the first cycle; after 100 cycles, the capacity retention is as high as 95.5%. Moreover, when charging/discharging at a higher rate of 5 C, the capacity still remains 88.9 mAh g^-1, displaying good rate performance. The good electrochemical performance is ascribed to the optimized morphology, stable crystal structure, and improved ionic conductivity.