Study Of Polyanion Orthosilicates As Cathode Materials For Lithium Ion Batteries

Lithium metal orthosilicates (Li₂MSiO₄, M = Fe, Mn, Co) are a new class of 'polyanion' compounds containing compact tetrahedral 'anion' structural units (SiO₄)^4- with strong covalent bonding. Orthosilicates are promising candidates for next generation of lithium ion batteries, due to their high theoretical capacity and excellent safety performance. Polyanion cathode materials are attractive for low-cost, good cyclic stability, and excellent thermal stability. However, their low electronic conductivity impedes their use as electrode materials in high-power batteries. In this work, Li₂FeSiO₄/C, Li₂Mn_xFe_1-xSiO₄/C and Li₂CoSiO₄ electrode materials were prepared by modified solid state reaction, sol-gel method and hydrothermal reaction. An in-situ carbon coating method by adding sucrose to the synthetic precursor with ball-milling techniques was introduced in order to improve the low electronic conductivity of these materials. More important, a new synthesis route (hydrothermal assisted sol-gel method) has been developed for the preparation of these materials. The structure character, physico-chemical properties and electrochemical performance of the prepared materials are studied in detail by various methods and techniques, including structural analysis, surface analysis, magnetization measurements and electrochemical techniques. The factors affecting the electrochemical performance of the materials were investigated.The studies of Li₂FeSiO₄/C composite materials prepared by different methods show that Li₂FeSiO₄ powders possess an antiferromagnetic ordering below T_N = 20 K due to long range Fe-O-Li-O-Fe interactions. Analysis of the paramagnetic region giving the Curie-Weiss parametersθ_p = - 38.1 K and C_p = 3.06 emuK·mol^-1, show the divalent state of Fe cations. The morphology, micro-structure and phase purity are the main factors affecting the electrochemical performance of the material. The Li₂FeSiO₄/C composite material prepared through hydrothermal assisted sol-gel process displays a large discharge capacity of ca. 160 mAhg^-1 at C/16 rate and shows superior charge and discharge capabilities under high rate conditions, which could be, at least in part, attributed to the high phase purity, porous aggregate nano-structure, and improved electronic conductivity through carbon connection. The high rate capability and excellent capacity retention of the Li₂FeSiO₄ material shows high potential for cathode materials for high-power lithium-ion batteries.The studies of Li₂Mn_xFe_1-xSiO₄/C composite materials prepared by different methods show that Li₂Mn_xFe_1-xSiO₄ solid solutions can be achieved in a wide compositional range. Magnetic susceptibility experiments give evidence that the effective momentμ_eff increases with the increase in Mn content x, consistent with the stoichiometric. Electrochemical tests show that more than one Li reversible exchange can be achieved for Li₂Mn_xFe_1-xSiO₄. An optimized capacity and energy density for Li₂Mn_xFe_1-xSiO₄ was achieved at x = 0.5. The Li₂Mn_0.5Fe_0.5SiO₄/C composite material prepared through hydrothermal assisted sol-gel process shows a capacity as high as 235 mAhg^-1. The studies of the possible fading mechanism of Li₂Mn_xFe_1-xSiO₄ (x > 0) materials show that the poor cyclic performance of Li₂Mn_xFe_1-xSiO₄ (x > 0) is its intrinsic property. The poor cycling performance of the materials is associated with the Jahn-Teller effect of Mn³⁺, which cause the volumetric effect and destroys the structure of the materials.The studies of Li₂CoSiO₄ materials prepared by different methods show that two modifications of Li₂CoSiO₄ were prepared by different methods at various synthesis conditions. Magnetic susceptibility experiments give evidence that Li₂CoSiO₄ powers posses an antiferromagnetic ordering below T_N = 18 K due to long range Co-O-Li-O-Co interactions. Analysis of the paramagnetic region giving the Curie-Weiss parametersθ_p = - 31.49 K and C_p= 2.31 emu·K·mol^-1. The low conductivity of this compound and the difficulty of performing in-situ carbon coating to improve its low electronic conductivity result in its poor electrochemical performance. Through improving the synthesis methods, optimizing the synthesis conditions and coating with carbon by ball milling process, we successfully synthesized Li₂CoSiO₄ material with uniform nanoparticles and high phase purity, which shows high electrochemical activity for the first time. Reversible extraction and insertion of the first lithium from and into Li₂CoSiO₄ at -4.1 V vs. lithium have shown that this material is a potential candidate for new high-voltage cathodes in lithium-ion batteries. After coated with carbon nanotubes (CNTs) the Li₂CoSiO₄ prepared through hydrothermal assisted sol-gel process shows a discharge capacity of 101 rnAhg^-1 in the first cycle. The low electronic conductivity of Li₂CoSiO₄ and the parasitic reaction with the electrolytes contribute to the irreversible capacity loss of the material in the first charge-discharge cycle.