Surface Phase Conversion in a High-Entropy Layered Oxide Cathode Material

High-entropy transition-metal oxides are potentially interesting cathode materials for lithium-ion batteries, among which high-entropy layered oxides are considered highly promising because there exist two-dimensional ion transport channels that may, in principle, enable fast ion transport. However, high-entropy layered oxides reported to date exhibit fast capacity fading in initial cycles and thus are hardly of any practical value. Here, we investigate the structural and property changes of a five-element layered oxide, LiNi_0.2Co_0.2Mn_0.2Fe_0.2Al_0.2O₂, using electrochemical and physical characterization techniques. It is revealed that the M₃O₄ phase formed at the surface of LiNi_0.2Co_0.2Mn_0.2Fe_0.2Al_0.2O₂ due to the migration of metal ions from octahedral sites of the transition-metal layer to tetrahedral 8a and octahedral sites of the lithium layer hinders the intercalation of lithium ion, which leads to the low initial Coulombic efficiency and fast decay of reversible capacity. This mechanism could be generally applicable to other high-entropy layered oxides with different elemental compositions.