An analysis of F-doping in Li-rich cathodes

Li-rich materials, due to their high capacity (> 250 mAh·g⁻¹), have recently been considered as an alternative to the current generation of cathode materials for Li-ion batteries (LIBs). However, their inferior cycling stability limits their practical applicability. Doping is a common technique to solve this problem. However, anion doping remains relatively underexplored. Fluorine (F) is one of the most effective anion dopants owning to the improved capacity, cycling stability, and rate performance in batteries. The explanations and experimental results, however, vary significantly from study to study. Herein, we find that bulk F-doping significantly improves both rate performance and cycling stability, likely driven by charge compensation and greater electronegativity. Additionally, bulk F-doping occasionally improves capacity via enhanced activation and occasionally decreases capacity by preventing activation from occurring. Surface F-doping has similar effects to bulk F-doping on capacity and stability, while significantly hindering the rate performance. Furthermore, the improvements in surface-doped materials do not appear to be a result of specific surface modification, and instead can be ascribed to the effect of fluorine on the near-surface bulk material. Greater understanding of fluorine’s influence on activation, in particular, is required to unlock the full potential of synergistic cation/anion co-doping. Graphical abstract: [Figure not available: see fulltext.]