Stabilization, Characterization, and Electrochemical Applications of High-Entropy Oxides: Critical Assessment of Crystal Phase–Properties Relationship

High-entropy oxides (HEOs), a class of newly emerging energy conversion and storage technology materials, have gained significant interest due to their unique structure, complex stoichiometry, and corresponding synergetic effect. Despite the increasing number of reported studies related to HEOs in recent years, details of their structural properties and electrochemical activities are still lacking. Herein, the exciting developments of HEOs regarding their design, synthesis, characterization, theoretical calculations, and electrochemical performances are outlined. The fundamentals of HEOs, including their strict definition, main features, and four-core aspect effects are presented. The different synthetic methods of HEOs are categorized to highlight the significance of parameter optimization to ensure the single-phase stability of HEOs. The advances in characterization techniques on the local lattice and atomic distribution and the basic principles of combinatorial screening methods based on computational techniques are also elaborated. Recent HEO-based electrode/electrocatalysts toward Li-ion batteries and oxygen catalysis are reviewed to assess the potential applications of HEOs. This review draws attention to the critical challenges of HEOs that are worth more extensive explorations in the future.