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.