Enabling the Electrochemical Performance of Maricite‐NaMnPO 4 and Maricite‐NaFePO 4 Cathode Materials in Sodium‐Ion Batteries

NaMnPO 4 and NaFePO 4 , polyanion cathode materials, exist in two different phases maricite/natrophilite and maricite/olivine, respectively. Both natrophilite NaMnPO 4 and olivine NaFePO 4 are electrochemically active and possess a one‐dimensional tunnel for sodium‐ion migration; however, these two phases are thermodynamically unstable. Therefore, they can be synthesized through an electrochemical route. On the contrary, maricite (m)‐NaMnPO 4 and maricite (m)‐NaFePO 4 are thermodynamically stable forms but have a huge activation energy of their diffusion pathways for sodium extraction and insertion in the crystal structure, which hinders electrochemical reactions. Therefore, the electrochemical behaviour of commercial m‐NaMnPO 4 and m‐NaFePO 4 has been studied to find a way for enabling them electrochemically. Ball milling and thermal/mechanical carbon coating are employed to reduce the particle size to enhance the electrochemical performance and shorten the diffusion pathway. Moreover, ball milling leads to defects and partial phase transformation. The electrochemical performance of milled‐coated NaMnPO 4 and NaFePO 4 has been thoroughly investigated and compared. The phase transition of NaFePO 4 is revealed by a differential scanning calorimeter. As a result, the achievable capacities of both cathode materials are significantly enhanced up to ∼50 mAh.g −1 via the particle size reduction as well as by carbon coating. However, the side reactions and agglomeration problems in such materials need to be minimized and must be considered to enable them for applications.