Improving the Cyclic Reversibility of Layered Li-Rich Cathodes by Combining Oxygen Vacancies and Surface Fluorination

Layered-type Li-rich cathode materials have attracted significant attention for next-generation Li-ion batteries, but the advantage of their high capacity is eclipsed by their poor reversibility upon cycling. Irreversible oxygen redox activity and surface degradation have been deemed as the root cause and direct cause for their poor performance, respectively. We attempted to suppress surface degradation by inserting fluoride ions up to some depth on the surface. By fluorination with NH₄HF₂ after introducing a significant amount of oxygen vacancies in layered Li_1.2Ni_0.2Co_0.2Mn_0.4O₂ by using CaH₂ as a reducing agent, the reversible capacity reached 268 mAh/g, and the capacity retention after 100 cycles was about 99%. The scanning transmission electron microscopy-electron energy loss spectroscopy (STEM-EELS) technique revealed that, in contrast to directly fluorinated samples, our materials exhibit deeper fluorine signals besides surface signals, and hard X-ray photoelectron spectroscopy (HAXPES) patterns show ionic and covalent fluorine coordination. These results indicate that the combination of oxygen deficiency introduction and surface fluorination allows some F^- ions to occupy near-surface oxygen vacancy sites rather than forming only a LiF layer on the surface, suggesting a new strategy to modify cathode materials for lithium-ion batteries.