Unraveling the electronic mechanisms of transition metal and fluorine co-doping for enhanced electrochemical performance in sodium lithium manganese oxide cathodes

Abstract

Enhancing the structural stability and electronic and electrochemical properties of sodium-ion cathode materials is essential for developing the next generation of sodium-ion batteries. In this study, we carry out first-principles calculations to assess the performance of P2-type sodium lithium manganese oxide (NLM) cathodes co-doped with transition metals X (X = Sc, Ti, V, Cr, Fe, Co, Ni, Cu, and Zn) and fluorine. Our findings reveal that among the investigated transition metals and fluorine, codoping cobalt and fluorine atoms helps mitigate Jahn–Teller distortions, thus stabilizing the NLM cathodes. The stability of this most promising cobalt–fluorine co-doped heterostructure is further validated by experimental X-ray diffraction patterns synthesized by the standard sol–gel method. Regarding electronic properties, the pristine NLM systems exhibit ferromagnetic metallic behavior. However, cobalt–fluorine co-doped systems display ferromagnetic semi-metallic characteristics, featuring a mix of free holes and electrons. The doped system shows a higher carrier density and lower activation energies, leading to improved transport properties compared to the undoped systems. These results highlight the significant role of co-doping cobalt and fluorine atoms in creating high-performance sodium-ion cathodes.