Mg−O Bond Enables Fast Sodium-Ion Insertion/Extraction in Fe0.97Mg0.03PO4: Achieving Low Voltage Hysteresis and High-Capacity Cathodes

Abstract

Olivine-type FePO₄ garner significant research interest due to its remarkable sodium storage capacity of 177.70 mAh g⁻¹ and an appropriate discharge voltage of 2.8 V. However, existing synthesis methods often require complex processes or toxic raw materials, which hinder its further development. Additionally, significant voltage hysteresis, resulting from volume mismatches during phase transitions in the Na⁺ insertion/extraction process, decreases discharge voltage and energy density. To overcome these issues, this study utilizes an environmentally friendly and cost-effective aqueous ion exchange method, incorporating a small amount of Mg²⁺. The volume effect caused by doping and the stabilizing effect of Mg−O bonds alleviate the voltage hysteresis phenomenon. Kinetic analysis reveals that Mg doping widens Na⁺ transport channels, with Fe_0.97Mg_0.03PO₄/C exhibiting the highest Na⁺ diffusion. Furthermore, DFT analysis uncovers changes in the band gap and electrostatic field around the MO₆ octahedra, elucidating the improved conductivity and Na⁺ kinetic. Fe_0.97Mg_0.03PO₄/C demonstrates a satisfactory initial capacity (170.54 mAh g⁻¹ at 0.2 C) and excellent rate performance (80.60 mAh g⁻¹ at 5 C), maintaining a specific capacity of 86.39 mAh g⁻¹ after 300 cycles at a 2 C rate. This study approaches from a new direction and presents a novel strategy for advancing the synthesis and modification of high-performance FePO₄/NaFePO₄.