Layered NaxCoO2-based cathodes for advanced Na-ion batteries: review on challenges and advancements

Global interest in the development of sodium-ion batteries (SIBs) continues, largely due to the advantage of the affordable cost of sodium resources (compared to lithium), which could produce cost-effective rechargeable batteries for large-scale applications. However, the discovery of reliable cathodes, tailored amorphous carbon anodes, and compatible electrolytes is required to yield safer, longer lasting SIBs with wide operating temperature. Among various cathodes systems, layered oxide cathodes are of great interest due to 230–245 mAhg−1 theoretical capacity with facile structure forming ability. Among the various phases of NaxCoO2 cathode, the P2 is the most favored, because of low polarization with enhanced structural stability and high theoretical capacity of 183 mAhg−1 for the empirical formula of Na0.74CoO2. Charge/discharge profiles of these systems exhibit plateaus, continuous changes in voltage and voltage drop, which impacts the electrochemical performance. This review discusses recent advancement in NaxCoO2 cathode in terms of the effect of particle morphology, size, crystal structure, electronic structure, cation and anion doping, sacrificial salt, Na deficiency, effect of electrolyte salts and solvents, and thermal safety. We present a comprehensive analysis of the recent developments in NaxCoO2 and its derivative cathode materials and propose various approaches to mitigate the challenges for the near future successful commercialization of SIBs.