Sustainable synthesis of Ni, Mn co-doped FePO₄@C cathode material for Na-ion batteries

Olivine FePO₄ is widely regarded as an optimal cathode material for sodium-ion batteries due to its impressive theoretical capacity of 177.7 mAh g^-1. Nonetheless, the material's limited application stems from its intrinsic low electronic and ionic conductivities and ion diffusion rate. Previously, most modifications of olivine FePO₄ are conducted through electrochemical or ion exchange processes in organic solvents, which severely restricted its potential for large-scale applications. In this research, a novel water-based ion exchange method is proposed for the synthesis of Ni-doped, Mn-doped, and Ni, Mn co-doped FePO₄@C, which is non-toxic, cost-effective, and demonstrating promising prospects for various applications. Fe_2.7Mn_0.2Ni_0.1PO₄@C (0.2Mn0.1Ni-FP@C) is synthesized by a straightforward ion exchange method in aqueous media. The material exhibits a discharge capacity of 154.4 mAh g^-1 at 0.1C rate. After 300 cycles at 1C, the capacity retention rate remains at 70.7 %. Numerous tests and calculations conducted in this study demonstrate that 0.2Mn0.1Ni-FP@C, modified through Mn³⁺ and Ni³⁺ co-doping, exhibits superior electrochemical performance due to its enhanced electronic conductivity and ion diffusion rate.