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Possibility of Composite Cathodes with Sacrificial Salts

Tuesday, 21 June 2016: 16:30
Grand Ballroom (Hyatt Regency)
S. Okada, D. Tsunoe (Kyushu University), A. Kitajou (IMCE, Kyushu University), H. Hori, N. Dimov (Kyushu University), and P. Barpanda (Indian Institute of Science)
Recently, Na3V2(PO4)2F3 [1] and alluaudite-type Na2Fe2(SO4)3 [2] have been reported as high voltage cathode materials for sodium-ion battery. Both of them, the rechargeable capacities are more than 100 mAh/g and the discharge voltages are almost 4 V against Na. So, the cathode performance of the Li counterparts against Li should be also interesting. Nevertheless, there is no report about the direct synthesis of Li3V2(PO4)2F3 and Li2Fe2(SO4)3. As for the alternatives, 3LiF-2VPO4 and Li2SO4-2FeSO4, which consist of the same compositions with Li3V2(PO4)2F3 and Li2Fe2(SO4)3 were synthesized by ball-milling method at room temperature.

The 3LiF-2VPO4/C and Li2SO4-2FeSO4/C were prepared by three-step mechanical ball-milling. First, the mixture of LiF (Wako) and VPO4 with the molar ratio of LiF:VPO4 = 3:2 was placed in the teflon-lined stainless container with T3-ZrO2 balls. As for Li2SO4-2FeSO4/C, the molar ratio of Li2SO4 and FeSO4 was 1:2. These mixtures were ball-milled using a planetary mill (Fritsch, Pulverisette7) with a rotation speed of 600 rpm for 6 h. Second, the obtained mixtures were ball-milled with 5 wt% acetylene black (AB, Denki Kagaku) with a rotation speed of 600 rpm for 3 h and then the resulting mixtures were ball-milled again with 20 wt% AB. All the procedures were carried out under argon atmosphere. The title compounds prepared by mechanical ball-milling were characterized by powder X-ray diffractometer (Cu-Kα, 50kV, 300mA) and Field emission electron microscope (FE-TEM, JEM-2100F). Electrode pellets were manually prepared by mixing 3LiF-2VPO4/C and Li2SO4-2FeSO4/C with 5% polytetrafluoroethylene (PTFE) and punched together with Teflon binder (Polyflon PTFE F-104, Daikin Industry) into disks (ca. 20 mg in weight and 10 mm in diameter). The electrochemical performance of the obtained composite cathodes were evaluated versus Li metal (Honjo Metal Co., Ltd.) in 2032 coin-type cells using nonaqueous electrolyte (1M LiPF6/EC : DMC = 1 : 1 in volume, Tomiyama Pure Chemical Industries) and polypropylene separators (Celgard 3501).

These materials showed an excellent reversible capacity corresponding to 2 Li reaction. The energy density of the obtained 3LiF-2VPO4 and Li2SO4-2FeSO4/C were 680 Wh/kg and 420 Wh/kg, respectively. In addition, the feasibility of these composite cathodes with sacrificial salts were also confirmed by the Li-ion type cell with graphite anode.

The initial discharge and charge efficiencies were more than 80% as shown in Fig. 1 and 2. They suggest that the cell reactions in the Li ion-type cells were actually attributed to the insertion and extraction of Li ions come from sacrificial salts such as LiF and Li2SO4.

References

[1] K. Chihara, A. Kitajou, I.D. Gocheva, S. Okada, and J. Yamaki, J. Power Sources, 227(2013) 80.

[2] P. Barpanda, G. Oyama, S. Nishimura, S. C. Chung, and A. Yamada, Nature Commun., 5, (2014) 4358.