Ion doping boosts the electrochemical performance of Na ₂ MnPO ₄ F/C cathode materials

Abstract Sodium manganese fluorophosphate (Na ₂ MnPO ₄ F), as a novel fluorophosphate-based cathode material, has attracted widespread attention in the scientific community in recent years due to its high theoretical specific capacity, low cost, and eco-friendliness. However, this material has problems such as poor electronic conductivity and obvious capacity fading during cycling, which limit its practical application. Transition metal cation doping and carbon coating modification represent common yet effective strategies for enhancing the electrochemical properties of phosphate-based materials.​ In this study, Na ₂ MnPO ₄ F/C composites and Na ₂ Mn _0.98 M _0.02 PO ₄ F/C composites (where M = Fe ²⁺ , Co ²⁺ , Ni ²⁺ ) were synthesized via a wet ball milling and in situ pyrolytic carbon coating procedure to investigate the influence of transition metal cation doping on the structural and electrochemical properties of Na ₂ MnPO ₄ F as a cathode in lithium-ion batteries. The phase constituents, morphological structure, and electrochemical performance of the as-obtained materials were examined using various characterization techniques. The results indicate that the introduction of a small amount of transition metal cations does not alter the crystal structure of Na ₂ MnPO ₄ F; all samples maintain a well-crystallized monoclinic phase with the P2 _1/n space group. The samples exhibit a micro-scale aggregate architecture with varying degrees of aggregation, composed of primary nanoparticles measuring approximately 10 to 50 nm. When utilized as cathode materials in lithium-ion batteries, the electrochemical performances of the metal ion-doped Na ₂ MnPO ₄ F/C composites is enhanced compared to the undoped variant. Notably, Na ₂ Mn _0.98 Ni _0.02 PO ₄ F/C demonstrates the best comprehensive electrochemical performance, achieving a high initial specific discharge capacity of 116.9 mAh·g ⁻¹ with no significant capacity decay observed after 50 cycles at a charge–discharge current density of 10 mA·g ⁻¹ within a voltage range of 1.5 V to 4.8 V.