Cathode Properties of Perovskite-type NaMF₃ (M = Fe, Mn, and Co) Prepared by Mechanical Ball Milling for Sodium-ion Battery

NaFeF₃ and NaMnF₃ are promising cathode materials for sodium-ion batteries due to their low cost, low environmental impact and large theoretical capacity. NaCoF₃ also has attracted attention as the high voltage cathode material for sodium-ion batteries. But, these ionic NaMF₃ (M = Fe, Mn and Co) compounds have insulating character. To improve the lower electronic conductivity, two-step ball-milling with carbon was performed to obtain a uniform NaMF₃ and carbon composite. The rechargeable capacity of NaFeF₃ was 169 mAh g⁻¹, and its energy density was over 500 Wh kg⁻¹, which is the highest among the reported iron-based cathode materials for sodium-ion batteries. In addition, the experimental voltage for NaMnF₃ was ca. 3.4 V, and the voltages for NaMF₃ (M = Fe and Mn) calculated by the DFT method corresponded well with the experimental results for NaFeF₃ and NaMnF₃.