Improvement of Electrochemical Properties of Pyroxene-Type LiFeSi2O6
Many cathode materials for Li-ion battery, LIB, have been identified, such as LixMO2, LiM2O4, LiMPO4, Li2MSiO4 and Li3M2(PO4)3 (M: transition metals) compounds from a viewpoint of the high energy density. Recent research about the cost and safety of LIB exhibited the difficulty to apply to large battery like electric car and stationary battery. Recently there has been considerable interested in more thermally stable cathode materials. Since the large polyanions appear to stabilize the structure, the LiFePO4 and Li2FeSiO4 have noted in the capacity and thermal stability.
The first indication that the electrochemical activity in the pyroxene-type LiFeSi2O6 was reported by Zhou et al., who noted that the reversible electrochemical discharge-charge behavior was succeeded in the around 2 V vs. Li/Li+. Although the theoretical capacity of 125 mAh·g-1 for LiFeSi2O6 was estimated by assuming the redox of Fe2+ to Fe3+, the obtained capacity in the voltage range 1.5-4.0 V showed 78 mAh·g-1 which corresponded to the 60% of theoretical value . Therefore there is a residual in capacity to further study to improve the electrochemical properties. Furthermore more stable structure is expected in the pyroxene-type LiFeSi2O6 than LiFePO4 and Li2FeSiO4, especially in air and moisture atmosphere. The purpose of this study is to improve the capacity of LiFe2O6 by substitution of Co, Ni or Mn to Fe and to explore the electrochemical properties at high temperature as well as carbon coating technique.
The appropriate amounts of LiOH·H2O, SiO2, FeC2O4·2H2O, Co(NO3)2·6H2O, Ni(NO3)2·6H2O and MnC2O4·4H2O were combined in 25 mL of deionized water. The resultant sol was placed in a Teflon-lined portable stainless steel autoclave, sealed and maintained at 200 °C for 72 hours. After cooling, the resulting product was collected by vacuum filtration and washed with distilled water for several times. This gel was subsequently preheated to 500°C for 6 hours in air, and then annealed at 950°C for 12 hours in air. The product-graphite composites were obtained by ball-milling in a planetary mill at a rotation rate of 300 rpm.
The prepared materials were characterized by X-ray diffraction (XRD). The synchrotron X-ray diffractions for the prepared samples were measured at BL02B2 beam line (SPring-8, JAPAN) to refine the crystal structures by the Rietveld analysis (Rietan-FP). The HS cells of Li/LiMSi2O6 were assembled with the electrolyte of 1 M LiPF6 in EC/DMC solution. Charging and discharging were performed in the voltage range 4.8-1.5 V vs. Li/Li+ with constant current of 12.5 mA·g-1 at room temperature or 60 °C.
Results and discussion
The XRD patterns for calcined compounds and cathodes after charge and discharge showed a main phase of the pyroxene-type Li(Fe,Ni,Co,Mn)Si2O6 with a C2/c space group. The crystal structures of the samples were refined by Rietveld analysis. The cell volume was compressed by substituting the Co, Ni or Mn. The bond valence sums, BVS, were calculated from the refined bond lengths. Since the BVS of Fe site increased after charge and decreased after discharge, the redox from Fe2+ to Fe3+ was clearly confirmed during charge-discharge cycles. The electrochemical properties for the substituted LiFe0.9M0.1Si2O6, M was Ni or Co materials showed slight higher capacity than unsubstituted LiFeSi2O6. The reversible capacity over 100 mAh·g-1 for LiFeSi2O6 was obtained by carbon coating and measuring at 60 °C in the voltage range between 1.5 and 4.8 V (Fig. 1). As the low diffusivity of Li in LiFeSi2O6 was prospected by the previous work , the substitution of high conductive elements such as Ni and Co and higher temperature resulted in the good kinetics to be available to the theoretical capacity.
 S. Zhou, G. King, D. O. Scanlon, M. T. Sougrati and B. C. Melot, Journal of The Electrochemical Society, 161, A1642-A1647 (2014)