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)₃were 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.