Engineering oxygen vacancies via liquid reduction for superior lithium storage in rock-salt high-entropy oxide

Abstract High-entropy oxides (HEOs) have attracted considerable attention for energy storage applications due to their structural stability and chemical versatility. However, their intrinsically low electrical conductivity remains a major obstacle to the practical application. In this work, oxygen-deficient rock-salt-type (Co0.2Cu0.2Mg0.2Ni0.2Zn0.2)O HEOs were synthesized via a solution combustion method and subsequently reduced with H2O2 and NaBH4 solution. The introduction of oxygen vacancies effectively accelerates charge transfer, enhances electron/Li+ transport kinetics, and provides a higher pseudocapacitive contribution, all of which lead to improved electrochemical properties. As a result, NaBH4-reduced (Co0.2Cu0.2Mg0.2Ni0.2Zn0.2)O (HEO–NaBH4) delivers an exceptional reversible capacity of 802 mAh·g−1 after 300 cycles at 0.2 A·g−1, which is ~2.3 times that of the pristine sample. Even after 500 cycles at 1 A·g−1, it retains 319 mAh·g−1, a 45% improvement. Further insight into the lithium storage mechanism shows that the inherent lattice stability of HEO–NaBH4 greatly hinders structural degradation and facilitates reversible redox reactions. This defect engineering route suggests potential applicability to other analogous materials.