Medium- and high-entropy materials as positive electrodes for sodium-ion batteries: Quo Vadis?

Medium- and High-entropy materials (MEMs/HEMs) have garnered growing global research attention due to their distinctive structural characteristics and the correlated opportunities for customizing functional properties, which hold the potential to drive advancements in various energy conversion and storage technologies. Such a new class of materials breaks through the traditional design concept of low entropy-based materials. Recently, the library of MEMs and HEMs was further expanded, encompassing positive electrode materials for sodium-ion batteries (SIBs) such as layered transition metal oxides, polyanionic compounds (NASICON-type, Alluaudite polyphosphates, fluorophosphates, mixed phosphates, etc.) and Prussian blue analogues. Taking into account such significant progress, herein, the recently developed MEMs and HEMs for SIBs, including synthesis methods and their performances as cathode materials are reviewed. In addition, the effective strategies and promising trends for rationally designing MEMs and HEMs, as well as the roles of multielement and their synergistic effects on the electrochemical performance are discussed. Finally, the challenges, prospects, and Quo Vadis? in designing MEMs and HEMs provide guidance for the development of new and improved cathode materials to pave the way for future energy storage advancements.