Improvement on the high-rate performance of Mn-doped Na3V2(PO4)3/C as a cathode material for sodium ion batteries

Abstract

Mn²⁺ doped Na₃V_2−xMn_x(PO₄)₃/C (x = 0, 0.015, 0.025 and 0.035) samples were synthesized by a facile sol–gel method and doping effects on the crystal structure and electrical conductivity were investigated by Rietveld refinement of XRD and a RTS-4 linear four-point probe. The results show that moderate doping of Mn²⁺ does not alter the structure of Na₃V₂(PO₄)₃, and Mn²⁺ successfully substituted partial V³⁺ sites. Due to the larger ionic radius of Mn²⁺ (0.91 Å) as compared to V³⁺ (0.64 Å), the lattice volume of Mn²⁺ doped Na₃V_2−xMn_x(PO₄)₃/C noticeably increased, which could significantly accelerate Na⁺ transport in the material. Moreover, moderate Mn doping is in favour of increasing the electronic conductivity of Na₃V_2−xMn_x(PO₄)₃/C samples. As a result, the Mn²⁺ doped Na₃V_2−xMn_x(PO₄)₃/C samples show obvious improvements on the electrochemical performance in terms of the high-rate performance and cycling stability, particularly for the Na₃V_1.875Mn_0.025(PO₄)₃/C sample. As an example, when the discharging rate is 15C, it can deliver an initial discharge capacity of 86.7 mA h g⁻¹, and after 100 cycles, 79.4 mA h g⁻¹ can still be achieved.