Charge-Discharge Mechanism of Nonstoichiometric Lithium Iron Silicate
Authors: Matsui Ryoji , Furutani Junya , Yamanaka Keisuke , Nakanishi Koji , Katayama Misaki , Inada Yasuhiro , Ohta Toshiaki , Orikasa Yuki 
Published: February 2020

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 materials3-6
). 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
x
Fe2+
(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
x
Fe2+
(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
x
Fe2+
(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
x
Fe2+
(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
x
Fe2+
(4-x)/2SiO4 samples, their crystal structures are almost similar from XRD measurements. Considering the Fe2+/Fe3+ redox reaction in Li
x
Fe2+
(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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