Synthesis and the Cathode Properties of Na3Fe3(PO4)4 for Sodium-Ion Battery

Abstract

Na-ion batteries (NiBs) are very attractive and promising for large scale electric power storage system because of the abundance for sodium source, the high cost-effectiveness and the long lifetime. Among the candidate of cathode active materials for NiBs, practical use of iron phosphate such as NaFePO4, Na2FePO4F, and Na3Fe3(PO4)4 [1] or carbonophosphate such as Na3FePO4CO3 [2] is expected from the viewpoint of cost and safety. In order to make full use of the resource and cost advantages of these materials, the raw materials used in the synthesis and the synthesis method are also important. In this presentation, we report the direct synthesis of Na3Fe3(PO4)4 by the hydrothermal method with mass productivity and the cathode properties. Synthesis was carried out using ferrous sulfate hydrate (FeSO4·nH2O), phosphoric acid, and various sodium phosphate salts as sodium sources with various the raw material composition, hydrothermal treatment temperature, and reaction time. For the purpose of controlling the reaction, synthesis was performed under the condition that an oxidizing agent or a reducing agent was added. The obtained compounds were identified by powder X-ray diffraction measurement and Rietveld analysis. Iron (Ⅱ) sulfate (FeSO4·7H2O), phosphoric acid (H3PO4), sodium dihydrogen phosphate (NaH2PO4·2H2O) and sodium persulfate (Na2S2O8) were used as raw materials, and hydrothermal synthesis was performed at 200 ° C for 16 hours. From the Rietveld analysis, the obtained compound was assigned to the layered structure Na3Fe3(PO4)4. This hydrothermal synthesis Na3Fe3(PO4)4 delivered initial discharge capacity of 160mAh g-1 between -1.2 and 1.3 V with a reference electrode of Ag/AgCl at 2 mA cm-2 in the 17 m NaClO4 aqueous solution (Figure 1). In the presentation, we will be reported the details of cathode properties of Na3Fe3(PO4)4.