Promoting the Reversible Oxygen Redox Reaction of Li-Excess Layered Cathode Materials with Surface Vanadium Cation Doping.

Abstract

Li‐excess layered cathode (LLC) materials have a high theoretical specific capacity of 250 mAh g−1 induced by transition metal (cationic) and oxygen (anionic) redox activity. Especially, the oxygen redox reaction related to the activation of the Li2MnO3 domain plays the crucial role of providing a high specific capacity. However, it also induces an irreversible oxygen release and accelerates the layered‐to‐spinel phase transformation and capacity fading. Here, it is shown that surface doping of vanadium (V5+) cations into LLC material suppresses both the irreversible oxygen release and undesirable phase transformation, resulting in the improvement of capacity retention. The V‐doped LLC shows a high discharge capacity of 244.3 ± 0.8 mAh g−1 with 92% retention after 100 cycles, whereas LLC delivers 233.6 ± 1.1 mAh g−1 with 74% retention. Furthermore, the average discharge voltage of V‐doped LLC drops by only 0.33 V after 100 cycles, while LLC exhibits 0.43 V of average discharge voltage drop. Experimental and theoretical investigations indicate that doped V‐doping increase the transition metal–oxygen (TM—O) covalency and affect the oxidation state of peroxo‐like (O2)n − species during the delithiation process. The role of V‐doping to make the oxygen redox reversible in LLC materials for high‐energy density Li‐ion batteries is illustrated here.