LayeredNa‐Ion Transition‐Metal Oxide Electrodes for Sodium‐Ion Batteries

Alternative to lithium-ion batteries (LIBs), sodium-ion batteries (SIBs) can be a suitable energy-storage device for stationary large-scale energy-storage applications, e.g. smart grid. Performance characteristics and costs of the SIBs largely depend on positive electrode materials. Layered transition-metal oxides (Na x MO 2 , M = 3d transition metal and their combinations) are promising positive electrode materials for SIBs. Layered transition-metal oxides have the advantages of high theoretical capacity, ease of synthesis, suitable operating voltage, and large-scale production. Some of the layered oxide positive electrodes have been applied in commercial SIB prototypes. However, the realization of low-cost, air-stable, and practical positive electrode materials are still challenging. Optimization of cation composition, doping of cations, surface coating, and understanding the anionic redox activity are prominent factors for enhancing the electrochemical performances of the layered oxides. This review will address recent advancements of layered transition-metal oxide electrode materials for SIBs. Further, structural classifications of sodium metal oxide (Na x MO 2 ), influence of cation compositions, electrochemical performance, structural changes during battery operation and anionic redox properties are reviewed and discussed.