High-entropy oxides (HEOs) have attracted significant attention in rechargeable batteries due to their unique structural features, customizable compositions, entropy-driven stabilization, and superionic conductivity. This review provides a comprehensive overview of HEOs, covering their fundamental concepts, structures, properties, synthesis methods, and applications in lithium-ion (LIBs), sodium-ion (SIBs), and lithium-sulfur (Li–S) batteries. HEOs are single-phase compounds with diverse cation species and oxide anions, offering structural flexibility, high stability, and tunable electrochemical performance. They are promising candidates for anode, cathode, electrolyte, and electrocatalyst materials in rechargeable batteries. HEOs are characterized by high configurational entropy, which enhances phase stability and suppresses phase separation. The entropy stabilization effect is crucial for achieving single-phase structures under high-temperature sintering conditions. HEOs exhibit diverse crystal structures, including rock salt, spinel, fluorite, perovskite, and layered structures, each with unique properties and applications. For example, rock salt HEOs have a simple structure with high configurational entropy, while spinel HEOs accommodate trivalent ions and offer superior electrochemical performance. The ionic conductivity of HEOs is enhanced by oxygen vacancies, which facilitate ion transport and improve both ionic and electronic conductivity. Strategies such as doping, nanostructuring, and creating oxygen vacancies are employed to enhance ionic conductivity. HEOs also show potential in reducing reliance on rare elements, offering sustainable alternatives to conventional cathode materials. High-entropy cathodes, such as those with layered structures, exhibit enhanced oxygen redox activity and improved lattice oxygen stability, addressing issues like irreversible oxygen loss and gas production. Synthesis methods for HEOs include solid-state sintering, solvothermal, sol-gel, flame spray, and electrospinning. Each method offers advantages in terms of composition control, morphology, and scalability. The choice of synthesis method depends on the specific application and desired properties of the HEO material. In applications, HEOs are used as anode and cathode materials, electrolytes, and electrocatalysts in rechargeable batteries. For example, HEO anodes exhibit high reversible capacity and cycle stability, while HEO cathodes offer enhanced energy density and long-cycle life. The unique properties of HEOs, such as high configurational entropy and structural flexibility, make them promising candidates for next-generation energy storage systems. This review highlights the current state of HEO research, challenges, and future directions for their application in rechargeable batteries.High-entropy oxides (HEOs) have attracted significant attention in rechargeable batteries due to their unique structural features, customizable compositions, entropy-driven stabilization, and superionic conductivity. This review provides a comprehensive overview of HEOs, covering their fundamental concepts, structures, properties, synthesis methods, and applications in lithium-ion (LIBs), sodium-ion (SIBs), and lithium-sulfur (Li–S) batteries. HEOs are single-phase compounds with diverse cation species and oxide anions, offering structural flexibility, high stability, and tunable electrochemical performance. They are promising candidates for anode, cathode, electrolyte, and electrocatalyst materials in rechargeable batteries. HEOs are characterized by high configurational entropy, which enhances phase stability and suppresses phase separation. The entropy stabilization effect is crucial for achieving single-phase structures under high-temperature sintering conditions. HEOs exhibit diverse crystal structures, including rock salt, spinel, fluorite, perovskite, and layered structures, each with unique properties and applications. For example, rock salt HEOs have a simple structure with high configurational entropy, while spinel HEOs accommodate trivalent ions and offer superior electrochemical performance. The ionic conductivity of HEOs is enhanced by oxygen vacancies, which facilitate ion transport and improve both ionic and electronic conductivity. Strategies such as doping, nanostructuring, and creating oxygen vacancies are employed to enhance ionic conductivity. HEOs also show potential in reducing reliance on rare elements, offering sustainable alternatives to conventional cathode materials. High-entropy cathodes, such as those with layered structures, exhibit enhanced oxygen redox activity and improved lattice oxygen stability, addressing issues like irreversible oxygen loss and gas production. Synthesis methods for HEOs include solid-state sintering, solvothermal, sol-gel, flame spray, and electrospinning. Each method offers advantages in terms of composition control, morphology, and scalability. The choice of synthesis method depends on the specific application and desired properties of the HEO material. In applications, HEOs are used as anode and cathode materials, electrolytes, and electrocatalysts in rechargeable batteries. For example, HEO anodes exhibit high reversible capacity and cycle stability, while HEO cathodes offer enhanced energy density and long-cycle life. The unique properties of HEOs, such as high configurational entropy and structural flexibility, make them promising candidates for next-generation energy storage systems. This review highlights the current state of HEO research, challenges, and future directions for their application in rechargeable batteries.