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

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) ^- 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.