Many cathode materials for lithium-ion battery, LIB, have shown the high capacity over 200 mAh g-1, such as LixMO2, Li2MSiO4 and Li1+xM1-xO2 (M: transition metals) compounds. Recently there has been considerable interest in thermally stable materials for LIB in charge states because of the use of large battery. Since the polyanion-type cathodes exhibited the stable structure, the LiFePO4 and Li2FeSiO4 have attracted attention from the viewpoint of the capacity and thermal stability. The first report of the electrochemical and structural property of the pyroxene-type LiFeSi2O6 was recently described by Zhou et al. [1]. In the previous paper they revealed a reversible electrochemical reaction centered at 2 V for LiFeSi2O6/Li cell. The low potential for the redox of Fe2+ to Fe3+ was attributed by the low inductive effect such as Li2FeSiO4. 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 from 1.5 V to 4.0 V showed 78 mAh·g-1 which corresponded to the 60% of theoretical value.
For further improvement, the enhancements of the potential and capacity are required. We carried out he substitution of Fe for Co, Ni or Mn and the electrochemical measurements at high temperature where the present study found the 3.3 V plateau and the reversible capacity of 60 mAh·g-1 for 10 cycles. To reveal the mechanism of the enhancement, we performed the synchrotron XRD and XAFS study for the electrodes during charge-discharge process.
2. Experimental
The stoichiometric amounts of LiOH·H2O, SiO2, FeC2O4·2H2O, Co(NO3)2·6H2O, Ni(NO3)2·6H2O and MnC2O4·4H2O were added in 25 mL of distilled water. The resultant sol was sealed in a Teflon-lined portable stainless steel autoclave and heated at 200 ºC for 72 hours. After cooling, the resulting powder was washed in vacuum filtration and with deionized 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 LiFeSi2O6-carbon composites were obtained by ball-milling in a planetary mill at a rotation rate of 300 rpm for several hours.
The prepared powders were characterized by X-ray diffraction (XRD). The synchrotron X-ray diffractions for the samples and cathodes after charge or discharge were measured at BL02B2 beam line (SPring-8, JAPAN) to refine the crystal structures by the Rietveld analysis (Rietan-FP). The 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 or 4.8-2.2 V vs. Li/Li+ with constant current of 12.5 mA·g-1 at room temperature or 60 °C.
3. Results and discussion
The XRD patterns for obtained materials and cathodes after charge or discharge showed a pyroxene-type structure Li(Fe,Ni,Co,Mn)Si2O6 with a C2/c space group. The crystal structures for obtained powders were refined by Rietveld analysis including two sub phases. Since the cathodes after electrochemical measurements showed broad peaks due to conductive carbon and binders, it was difficult to refine the structural parameters. The cell volume was compressed by substitution of Fe to 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 confirmed during charge-discharge cycles. In addition, the XANES results supported the redox mechanism. The electrochemical properties at high temperature showed the abnormal behavior at only first charge. It was the first time, however, subsequently discharge-charge curves expressed the new plateau at 3.3 V. The reversible capacity over 60 mAh·g-1 for LiFeSi2O6 in the voltage range of 4.8-2.2 V was obtained by carbon coating and measuring at 60 °C for 10 cycles. More investigation about the chemical diffusion coefficients of Li needs to be done.
Reference
[1] S. Zhou, G. King, D. O. Scanlon, M. T. Sougrati and B. C. Melot, Journal of The Electrochemical Society, 161, A1642-A1647 (2014)