Comparative impact of surface and bulk fluoride anion doping on the electrochemical performance of co-free Li-rich...

Li-rich Mn-based (LMR) layered oxides are considered promising cathode materials for high energy-density Li-ion batteries. Nevertheless, challenges such as irreversible oxygen loss at the surface during the initial charge, alteration of the bulk structure, and poor rate performance impede their path to commercialisation. Most modification methods focus on specific layers, making the overall impact of modifications at various depths on the properties of materials unclear. This research presents an approach by using doping to adjust both surface and bulk properties; the materials with surface and bulk fluoride anion doping are synthesised to explore the connection between doping depth, structural and electrochemical stability. The surface-doped material significantly improves the initial Coulombic efficiency (ICE) from 77.85% to 85.12% and limits phase transitions, yet it does not enhance rate performance. Conversely, doping in bulk stands out by improving both rate performance and cyclic stability: it increases the specific discharge capacity by around 60 mAh g^-1 and enhances capacity retention from 57.69% to 82.26% after 300 cycles at 5C. These results highlight a notable dependence of material properties on depth, providing essential insights into the mechanisms of surface and bulk modifications.