High entropy metal oxides as next-generation electrolytes, electrodes, and interfacial engineering for solid-state batteries: Opportunities, challenges, and future directions

High entropy metal oxides (HEMOs), characterized by the incorporation of five or more cations in near-equiatomic ratios, have recently emerged as promising multifunctional materials for solid-state battery (SSB) technologies. Their unique configurational entropy stabilizes single-phase crystal structures simultaneously ensuring tunable ionic and electronic transport, wide electrochemical stability windows, and improved mechanical rigidity, which are the vital requirements for next-generation solid electrolytes and electrode materials. This review critically examines recent advances in the design, synthesis, and functional performance of HEMOs as solid electrolytes, cathodes, anodes, and interfacial layers in SSB systems. With emphasis placed on structure–property relationships governing ionic conductivity, redox activity, and interfacial compatibility with lithium and sodium metal anodes. On phase formation and defect chemistry, established fabrication routes, such as mechanochemical synthesis, solid-state reactions, spark plasma sintering, and additive manufacturing, are discussed alongside their impact. Moreover, some challenges like limited room-temperature ionic conductivity, phase instability under cycling, and interfacial resistance are analyzed. Lastly, Future research opportunities are identified, focusing on entropy-driven defect engineering, compositional optimization, and scalable processing strategies to elevate the deployment of HEMOs in high-performance solid-state batteries.