Role of tin doping in Na4VMn(PO4)3 for sodium-ion batteries

Na₄VMn(PO₄)₃ (NVMP) is a high-voltage sodium storage cathode material that has been favorably studied lately. However, it still faces lower conductivity and a more serious Jahn-Teller effect plague, leading to unsatisfactory electrochemical performance. In this work, tin (Sn) trace doping at vanadium (V) sites is systematically investigated for its effect on the electrochemical properties of NVMP. The results show that Sn doping can significantly stimulate the electrochemical activity of the cathode material thanks to the enlargement of the sodium ion diffusion channel and the stabilization of the manganese (Mn) environment. Excessive doping (e.g., up to 0.07) generates Na₄SnO₄ on the surface of the active material, which leads to a partial loss of capacity but still comes out on top in terms of cycling stability and rate performance. When the Sn doping amount is 0.03, the best overall performance specimen, Na_3.97V_0.97MnSn_0.03(PO₄)₃ having a discharge capacity of 103.1 mAh g⁻¹ at 0.2 C and a capacity retention of 87.3 % at 2 C as compared to 0.1 C can be obtained. In addition, it has 89.8 % capacity retention after 300 cycles at 1 C and remains as high as 89.4 % capacity retention after 1000 cycles at 10 C, which is significantly better than that of the undoped system. As an application demonstration, the utility of the resulting materials is further evaluated using coin-type and pouch cells, respectively, e.g., the pouch cells feature a capacity of 14 mAh at 0.2 C and a capacity retention of 60 % for 150 cycles at 1 C. It is expected that this work will provide another idea for the modification of Mn-containing cathode materials.