Li₂FeSiO₄ is one of interesting cathode material for lithium ion batteries. The two lithium composition per one iron atom and one poly-anion unit, in principle, exhibits a multi-electron charge transfer with Fe²⁺/Fe⁴⁺redox couple, which enables much high theoretical capacity of 331 mAhg^–1. This value is approximately twice as high as the commercialized cathode materials, such as LiCoO₂ and LiFePO₄. However, the accessible charge-discharge capacity of Li₂FeSiO₄ was limited to one-electron reaction with Fe²⁺/Fe³⁺ redox couple in the early research ^{1, 2)}. Recently, some research groups have reported more than one-electron reaction by using nanostructured materials^3-6). Unfortunately, their high capacity is not stable during charge-discharge cycle and there is a large polarization at high voltage reaction. Therefore, the utilization of Fe²⁺/Fe³⁺ redox couple in this system is preferred for the stable battery operation. To maximize this Fe²⁺/Fe³⁺ redox reaction in lithium iron silicate system, we investigate composition dependency of Li_xFe²⁺_(4-x)/2SiO₄ on charge-discharge capacity. Although the nonstoichiometric lithium iron silicate has been reported in the previous conference by the other group⁷⁾, we cannot access enough data to discuss the possibility of the nonstoichiometric system. We synthesized various Li_xFe²⁺_(4-x)/2SiO₄ samples and the charge-discharge measurements were performed. Their reaction mechanisms are discussed by using X-ray absorption spectroscopic data.

Carbon-coated Li_xFe²⁺_(4-x)/2SiO₄ samples were synthesized by the solid-state reaction. A given amounts of SiO₂, FeC₂O₄・2H₂O and Li₂CO₃ powders were weighed and 10 wt% of carbon (acetylene black) was added prior to mixing. These powders were mixed in a planetary ball mill at 400 rpm for 6 hours. The mixture was calcined at 700°C for 6 hours with a fixed Ar flux. The products were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM).

For electrochemical measurements, Li_xFe²⁺_(4-x)/2SiO₄ samples, carbon black, and polyvinylidene fluoride were mixed at a ratio of 80:10:10 with 1-methyl-2-pyrrolidone. The slurry was coated onto an aluminum foil current collector and dried in a vacuum oven at 80°C. For the charge-discharge measurements, the prepared electrode, lithium metal, and electrolyte-soaked separator (Celgard #2500) were constructed into a stainless steel flat cell. The electrolyte was a 1 mol dm^–3 solution of LiPF₆ in ethylene carbonate/ethyl methyl carbonate (3:7 volume ratio, Kishida). The cell construction process was performed in an Ar-atmosphere glove box. Galvanostatic charge-discharge measurements were performed at 55°C.

For the prepared Li_xFe²⁺_(4-x)/2SiO₄ samples, their crystal structures are almost similar from XRD measurements. Considering the Fe²⁺/Fe³⁺ redox reaction in Li_xFe²⁺_(4-x)/2SiO₄ system, the maximum charge-discharge capacity is 203 mAh g^-1 in Li_1.33Fe_1.335SiO₄. However, the observed discharge capacity was smaller than the stoichiometric system. The diffusion path of lithium ion might be blocked by the occupation of iron ion in the lithium site. We will discuss the charge-discharge mechanism in this system by using X-ray absorption spectroscopy data.

1) A. Nyten, A. Abouimrane, M. Armand, T. Gustafsson, J.O. Thomas, Electrochem. Commun., 7, 156-160 (2005).

2) A. Nyten, S. Kamali, L. Haggstrom, T. Gustafsson, J.O. Thomas, J. Mater. Chem., 16, 2266-2272 (2006).

3) D. Rangappa, K.D. Murukanahally, T. Tomai, A. Unemoto, I. Honma, Nano Lett., 12, 1146-1151 (2012).

4) Z.X. Chen, S. Qiu, Y.L. Cao, J.F. Qian, X.P. Ai, K. Xie, X.B. Hong, H.X. Yang, J. Mater. Chem. A, 1, 4988-4992 (2013).

5) D.P. Lv, J.Y. Bai, P. Zhang, S.Q. Wu, Y.X. Li, W. Wen, Z. Jiang, J.X. Mi, Z.Z. Zhu, Y. Yang, Chem. Mat., 25, 2014-2020 (2013).

6) T. Masese, C. Tassel, Y. Orikasa, Y. Koyama, H. Arai, N. Hayashi, J. Kim, T. Mori, K. Yamamoto, Y. Kobayashi, H. Kageyama, Z. Ogumi, Y. Uchimoto, J. Phys. Chem. C, 119, 10206-10211 (2015).

7) K. Kam, A. Liivat, D. Ensling, T. Gustafsson, J. Thomas, ECS Meeting Abstracts, MA2009-02, 390 (2009).