Molten‐Salt Synthesis of O3‐Type Layered Oxide Single Crystal Cathodes with Controlled Morphology towards Long‐Life Sodium‐Ion Batteries

Abstract Sodium layered oxides show great promise as affordable alternatives to lithium layered oxides, but their poor cycle life and air stability limit their practical potential. Micron‐scale single crystals with greater packing density and lower surface area can overcome these challenges and improve performance compared to the traditional polycrystalline morphology. Herein, the authors present the synthesis of layered O3‐type Na(Ni 0.3 Fe 0.4 Mn 0.3 )O 2 single‐crystals with greatly improved cycle life and air stability. A molten‐salt synthesis technique is adopted with excess sodium hydroxide to obtain platelet‐like single crystals. Because the main mechanisms of both capacity fade and air degradation occur as a result of surface reactions at the opening of the sodium diffusion channels, particle morphology is found to be a critical metric for materials performance. More important than particle size or total surface area, the smaller proportion of exposed edge planes in the platelet morphology greatly reduces the amount of harmful surface reactions. Furthermore, the molten‐salt method is found to eliminate the need for coprecipitated precursors and even form better morphology, starting from metal oxides instead of coprecipitated hydroxides.