Cathode Properties of Amorphous Xllif-FeSO4 (x = 1.0, 1.2 and 1.5) Composite for Li-Ion Batteries

Abstract

The iron-based cathode materials such as LiFePO4 have attracted attention due to its low cost and low environmental impact. LiFePO4 has an operating voltage of 3.3 V corresponding to Fe2+/Fe3+ redox. On the other hand, lithium iron sulfate Li2Fe(SO4)2 shows the higher operating voltage of 3.8 V (vs. Li+/Li) by the inductive effect of SO4 [1]. However, the theoretical capacity is restricted by the large molecular weight. Moving from Li2Fe(SO4)2 into LiFeSO4F, its theoretical capacity can increase from 120 to 150 mAh/g. LiFeSO4F has been reported to have two-type crystal structures, triplite-LiFeSO4F and tavorite-LiFeSO4F, which showed operating voltages of 3.6 V and 3.75 V, respectively [2]. However, the synthesis route to LiFeSO4F requires pressures of 0.2-0.3 MPa and FeSO4 hydrate. Therefore, it is necessary to develop a simple synthetic process. In this work, to develop a simple and low-cost synthetic process of LiFeSO4F, LiF-FeSO4 composite which has the same chemical composition as LiFeSO4F was prepared using mechanical milling method, and its cathode properties were investigated against Li metal. xLiF-FeSO4/C (x = 1.0, 1.2, 1.5) cathodes were prepared by the following three-step mechanical ball-milling. To obtain FeSO4 as a starting material, FeSO4･7H2O was sintered at 300oC for 12 h under Ar. The mixtures of LiF and FeSO4 with the molar ratio of LiF:FeSO4 = 1:1, 1.2:1 and 1.5:1 were ball-milled using a planetary mill with a rotation speed of 600 rpm under Ar for 6 h. The obtained mixtures were ball-milled with 5 wt% acetylene black at a rotation speed of 600 rpm for 6 h. And then the above mixtures were ball-milled again with 20 wt% AB in Ar. Cathode pellets were fabricated by mixing the obtained xLiF-FeSO4/C powder with a 5 wt% polytetrafluoroethylene Teflon binder and punched into disks (ca. 30 mg in weight and 10 mm in diameter). The electrochemical performance of the LiFeSO4F/C was evaluated with a 2032 coin-type cell using nonaqueous electrolyte and a polypropylene separator against Li metal. All cells were assembled in an Ar-filled glove box. The charge-discharge measurements were performed in galvanostatic mode at 25oC. The XRD profiles of the obtained xLiF-FeSO4/C composites showed typical diffuse scattering pattern with a halo peak in the amorphous or nano particulate materials. We observed the particle surfaces of the obtained amorphous xLiF-FeSO4/C by TEM, and confirmed the uniform dispersion state of each Fe, F, S and O element by STEM-EDS. The particle size of the obtained product was approximately 60 nm. Figure 1 shows the initial and the second charge-discharge curves of the obtained amorphous xLiF-FeSO4/C cathode at a rate of 0.2 mA/cm2. These electrochemical measurements were carried out in a potential window between 2.5 and 4.3 V. The initial discharge capacities for LiF-FeSO4/C, 1.2LiF-FeSO4/C and 1.5LiF-FeSO4/C were 100 mAh/g, 113 mAh/g and 124 mAh/g, respectivily. In addition, their average voltage was ca. 3.5 V. It well agrees with previously reported voltage value of trpilite-LiFeSO4F prepared by mechanical milling method [2]. Moreover, the obtained amorphous xLiF-FeSO4/C composites had relatively good cyclability. After 30 cycles, the discharge capacities were kept to 100 mAh/g (x = 1.0), 111 mAh/g (x = 1.2) and 122 mAh/g (x = 1.5), respectively.