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

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 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, the 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 its potential applicability to other analogous materials.