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 the practical application. In this work, oxygen-deficient rock-salt-type (Co_0.2Cu_0.2Mg_0.2Ni_0.2Zn_0.2)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 (Co_0.2Cu_0.2Mg_0.2Ni_0.2Zn_0.2)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.