Synergetic effects of Mo doping and Mn site vacancies on electrochemical performance of Na4MnV(PO4)3/C cathode material for sodium-ion batteries

Na₄MnV(PO₄)₃ (NMVP) has become an auspicious substitute for Na₃V₂(PO₄)₃ on account of its lower environmental toxicity, reduced production costs and elevated working voltage. Nevertheless, sodium storage performance of NMVP is compromised arising from Mn³⁺-induced Jahn-Teller distortion and low electronic conductivity. Herein, a strategy combining cation doping with transition metal site vacancies was employed to enhance the cyclability and rate capability of NMVP/C. The synergistic effects of Mo⁶⁺ doping into Mn sites and Mn vacancies in NMVP simultaneously alleviate Jahn-Teller distortion, upgrade electrochemical reaction reversibility, bulk electronic conductivity and Na⁺ diffusion coefficients in electroactive material, and reduce charge transfer resistance, leading to the improved electrochemical performance. The optimal Mo-doped NMVP/C with Mn vacancies (Na₄Mn_0.91Mo_0.03V(PO₄)₃/C, NMVP-0.03Mo) outperforms NMVP/C and the Mo-doped NMVP/C without Mn vacancies (Na_3.88Mn_0.97Mo_0.03V(PO₄)₃/C) in rate capability, cyclability and capacity. NMVP-0.03Mo shows superior electrochemical performance with high initial discharge capacities of 98.8 mAh g⁻¹ (0.2C) and 82.1 mAh g⁻¹ (10C), maintaining capacity retention rates of 89 % and 90.4 % after 100 cycles. By comparison, NMVP/C delivers lower initial capacities (94.2 mAh g⁻¹ at 0.2C, 70.5 mAh g⁻¹ at 10C) with the reduced capacity retention rates of 67 % and 87.4 %, correspondingly. While Na_3.88Mn_0.97Mo_0.03V(PO₄)₃/C presents the lowest initial discharge capacities (85 mAh g⁻¹ at 0.2C, 66 mAh g⁻¹ at 10C) with intermediate retention rates of 85.9 % and 88.5 %, respectively. These findings suggest that cation doping/transition metal site vacancies may be one of the effective strategies for electrochemical improvement of Mn-based polyanionic electrodes.