Cathode Properties of Na3FePO4CO3 Prepared By Mechanical Milling Method for Na-Ion Battery

Abstract

Introduction In rechargeable batteries for grid storage, the cost performance is more important than the energy density. For large-scale batteries, the minor metal-free cathode active materials are attractive for Na-ion batteries because of their natural abundance and low cost. In addition, polyanionic cathodes are also attractive for the high thermal and chemical stability. Among the iron-based phosphate family, the theoretical capacity of Olivine-type LiFePO4 is restricted to 170 mAh/g and the Na counterpart, Maricite-type NaFePO4 is electrochemically inactive. On the other hand, a new polyanionic intercalation cathode Na3FePO4CO3 has high theoretical capacity of 190 mAh/g based on Fe2+/Fe4+ redox 1). Nevertheless, Na2.24FePO4CO3 synthesized by hydrothermal method can just deliver discharge capacity of 112 mAh/g at 10 mA/g, corresponding to 1.18 e- reaction, because of the oxidation of Fe2+ during hydrothermal process and intrinsic property of a poor ionic conductivity2). In order to improve electrochemical performance of Na3FePO4CO3, mechanochemical synthesis method by ball milling was tried in a manner similar to NaMF3 3). In addition, we investigated the cathode properties of Na3FePO4CO3 against Na metal anode and confirmed the reversible multi electron reaction in it. Experimental NaFePO4 + Na2CO3 /C composite was prepared by three-step mechanical ball milling. First, the mixture of Na2CO3 and maricite-NaFePO4 obtained by solid state method 4) with the molar ratio of Na2CO3 : NaFePO4 = 1 : 1 was put in the Ar-filled container with 3f-ZrO2 balls. This mixture was ball-milled using a planetary mill (Pulverisette7, Fritsch Japan Co., Ltd.) with a rotation speed of 600 rpm for 12 h. Second, the obtained mixture was ball-milled with 15 wt% acetylene black (AB, Denka Co., Ltd.) at a rotation speed of 500 rpm in Ar. And then, the above mixture was ball-milled again with 15 wt% AB with a rotation speed of 400 rpm in Ar. The cathode pellets were fabricated by mixing of the NaFePO4 + Na2CO3 /C composite powder with a 10 wt% polytetrafluoroethylene (PTFE) Teflon binder (Polyflon PTFE F-104, Daikin Industries, Ltd.) and punched into disks (ca. 30 mg weight and 10 mm diameter). The electrochemical performance of the NaFePO4 + Na2CO3 /C composite was evaluated by a 2032 coin-type cell with a nonaqueous electrolyte (1 M NaPF6/EC:DMC(1:1 in vol.) Tomiyama Pure Chemical Industries, Ltd.) and a glass filter separator (GA-55, ADVANTEC) against Na metal (Sigma-Aldrich). All cells were assembled in an Ar-filled glove box. The charge-discharge measurements were performed in galvanostatic mode at 25°C. Results and discussion The initial and second charge-discharge profiles of the mechanical milling-synthesized NaFePO4 + Na2CO3 /C composite cathode was obtained between 1.5 and 4.5 V at a rate of 0.4 mA/cm2 at room temperature (Fig. 1). During the first cycle, the obtained sample deliver a discharge capacity of 125 mAh/g, and even after 30 cycles, a reversible discharge capacity of 113 mAh/g, corresponding to 1.19 e- reaction per mole. In order to investigate the valence state of Fe in initial sample, Mössbauer spectroscopy experiment performed on initial sample. The result suggests that the 34% Fe in the sample is oxidized from 2+ to 3+. In Fig. 1, two plateaus of 2.6 and 4.2 V in the 2nd charged profile were observed, which correspond to the redox voltage of Fe2+/Fe3+ and Fe3+/Fe4+ in the reported value3). We will report about the electrochemical performance of NaFePO4 + Na2CO3 /C composite at RT and elevated temperature. The charge-discharge mechanism of NaFePO4 + Na2CO3 /C composite cathode will be also discussed at the presentation. References 1) H. Chen, et al., J. Am. Chem. Soc., 134, 19619 (2012). 2) A. Kitajou, et al., Electrochim. Acta., 245, 424 (2017). 3) W. F. Huang, et al., Sci. Rep., 4, 4188 (2014). 4) M. Avdeev, et al., Inorg.Chem., 52, 8685 (2013) Figure 1