Enabling the Electrochemical Performance of Maricite-NaMnPO4 and Maricite-NaFePO4 Cathode Materials in Sodium-Ion Batteries

NaMnPO4 and NaFePO4, polyanion cathode materials, exist in two different phases maricite/natrophilite and maricite/olivine, respectively. Both natrophilite NaMnPO4 and olivine NaFePO4 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)-NaMnPO4 and maricite (m)-NaFePO4 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-NaMnPO4 and m-NaFePO4 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 NaMnPO4 and NaFePO4 has been thoroughly investigated and compared. The phase transition of NaFePO4 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.