Transition-metal high-entropy oxides for Li- and Na-ion batteries: Design rules, interfaces, and device-relevant metrics

Transition-metal high-entropy oxides (TM-HEOs) are emerging as a versatile platform across anodes, cathodes, and solid-state electrolytes (SSEs) for Li- and Na-ion batteries. This review establishes a comprehensive “synthesis-structure-properties-devices” framework integrating 2019-2025 advances to link processing routes with morphology, defects, and crystallography for enhanced battery performance and safety. The review distills actionable design rules: balancing entropy stabilization with charge compensation and uniform cation mixing, controlling oxygen vacancies and local chemistry for cathodes, and applying aliovalent multi-cation strategies alongside rapid densification for solid-state electrolytes (SSEs). An interface-centric perspective is emphasized, highlighting coatings, graded or compliant interlayer, and mixed ionic-electronic networks to suppress side reactions and reduce impedance. A data-driven roadmap is proposed that couples operando characterization with automated, closed-loop processing to accelerate discovery and scale-up. Together these elements provide a coherent, cross-component playbook to convert TM-HEO promise into manufacturable LIB and NIB technologies.