From Low-to High-Entropy Layered Transition Metal Oxide Cathodes: Recent Progress on Spray-Drying Technologies in Materials Design for Sodium-Ion Batteries

Abstract

Sodium-ion batteries (SIBs) have emerged as a cost-effective and sustainable alternative to lithium-ion-based energy technologies, intensifying the search for high-performance cathode materials. Among the available candidates, layered transition-metal oxides with the general formula NaxTMO2 stand out due to their high energy density and structural versatility. Recently, entropy engineering, ranging from low-to high-entropy oxide designs (LEOs, MEOs, and HEOs), has been reported as an effective strategy to enhance structural stability, ionic transport, and electrochemical performance. In parallel, spraydrying has gained increasing attention as a scalable, industrially relevant synthesis route that ensures homogeneous cation distribution, controlled particle morphology, and reproducible microstructures. This review provides a critical and systematic assessment of spray-drying technologies applied to the synthesis of layered NaxTMO2 cathodes for SIBs, with particular emphasis on entropy-driven material design. Fundamental aspects of the spray-drying process, including atomization methods, are discussed in detail. In addition, statistical information was obtained based on the type of precursor used in the spray-drying synthesis of NaxTMO2 and the post-thermal processing treatments applied.Recent advances in spray-dried LEOs, MEOs, and HEOs are analyzed, correlating composition trends and strategies with structural features and electrochemical performance. Moreover, common misclassifications of entropy levels in the literature are addressed through a rigorous discussion of configurational entropy criteria. Finally, current challenges and future perspectives for spray-drying-assisted entropy engineering of layered cathodes are outlined, highlighting its potential for large-scale manufacturing of next-generation SIBs.