Carbon-coated LixFe2+(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, LixFe2+(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 LixFe2+(4-x)/2SiO4 samples, their crystal structures are almost similar from XRD measurements. Considering the Fe2+/Fe3+ redox reaction in LixFe2+(4-x)/2SiO4 system, the maximum charge-discharge capacity is 203 mAh g-1 in Li1.33Fe1.335SiO4. 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.
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