Hydrothermal Synthesis and the Cathode Properties of Na3MPO4CO3 (M = Fe, Mn, Ni, Co) with Highly Concentrated Aqueous Electrolyte

Abstract

Na-ion batteries (NiBs) have drawn much interest as low cost and low environmental impact power source for large-scale grid storage system, because of the abundant resources of sodium. For the past decades, various iron-based polyanionic compounds have been extensively studied as cathode materials for NiBs. 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.Recently, our group confirmed that 17m NaClO4 aqueous electrolyte showed high operating voltage up to 2.78V in both Na and Li systems [3]. By using 17 m NaClO4 aqueous electrolyte, it was found that Na3FePO4CO3 prepared by mechanical ball milling provided an initial discharge capacity of 161 mAh g-1 between -1.2 and 1.3 V with a reference electrode of Ag/AgCl at 2 mA cm-2 [4].In order to make full use of the resource and cost advantages of carbonophosphate, the raw materials used in the synthesis and the synthesis method are also important. In this presentation, we report the direct synthesis of Na3MPO4CO3 (M = Fe, Mn, Co, Ni) by the hydrothermal method and the cathode properties.Figure 1 shows charge-discharge profiles of Na3FePO4CO3 obtained by hydrothermal synthesis at 180 ° C for 16 hours using iron sulfate (II) (FeSO4·7H2O), trisodium phosphate (Na3PO4·12H2O), and sodium carbonate (Na2CO3) as raw materials. This hydrothermal synthesis Na3FePO4CO3 delivered initial discharge capacity of 112 mAh g-1.In the presentation, we will be reported the details of cathode properties of the hydrothermal synthesized Na3MPO4CO3 (M = Fe, Mn, Co, Ni).