The Synthesis And Performance Study Of Borates And Phosphate Compounds As Cathode Materials For Lithium Ion Batteries

The main content of this thesis includes: the synthesis of hexagonal lithium manganese borate,LiMnBO₃,by one-step solid state reaction;studies on the mechanism of solid-state reactions for LiMnBO₃, optimized the temperature of synthesis and studies the effect of temperature on the shape and performance;low temperature solid-state synthesis for LiZnPO4 using LiF as lithium precursor; the synthesis of LiFe_1-xZn_xPO₄(0.1?x<1.0) compounds and studies on the solid solubility of them and the electrochemical performance;the Li9V3-xCox (P2O7)3(PO4)2/C(x=0.0,0.02,0.05)compounds were prepared by sol-gel method;the investigation on Co-doping in V sites;the synthesis of LiCoBO₃ by one-step solid-state method.We studied the mechanism of solid-state reactions by using TG-DTA on the precursor mixture of the LiMnBO₃ and the optimizing of solid-state routine for LiMnBO₃. The hexagonal LiMnBO₃ was prepared by one-step solid state reaction when sintering temperatures are higher than 850?. The SEM images show that sample particles with small uniform particle size and good crystallization can be obtained at 850?. The electronic conductivities of LiMnBO₃ is obviously higher than that of LiFePO₄ and Li3V2(PO4)3.The initial specific discharge capacity was 75.5mAh/g at the current density of 5 mA/g and the mean fade of capacity was 0.09% per cycle except for the first cycle. Even when the current density is increased to 50mA/g, the initial charge and discharge capacity of 46.8mAh/g can still be held and the capacity fade per cycle was only 0.2% during 40 cycles.LiZnPO4 and LiFe_1-xZn_xPO₄ (0.0?x<1.0) compounds have been synthesized using LiF as lithium source at 750?. We discussed the effect of Zn-doping content in LiFe_1-xZn_xPO₄ on the phase ingredient, lattice constant, crystal structure and electrochemical performance. The single-phase region of the LiFe_1-xZn_xPO₄ with orthorhombic and monoclinic structure determined by XRD can be expressed as 0.0? x?0.31 and 0.89?x?1.0 respectively. The two-phase region of the LiFe_1-xZn_xPO₄ involve the Zn-poor region with orthorhombic and Fe-poor region with monoclinic structure can be expressed as 0.31<x<0.89. Our experimental results show that the initial capacity of LiFe_1-xZn_xPO₄(0.0?x<0.31) compounds has a Zn-doping dependence. As the Zn-doping contents increased there is a concurrent decrease in the initial specific capacity, which may be due to a decreased theoretical specific capacity during Zn-doping, where Zn-doping can raise the molecular mass, however the charge/discharge voltage flats are still very flat and cycle performance has improvement. It should be noted that the higher capacity conservation rate of 93.7% can be held for LiFe_0.8Zn_0.2PO₄ after 20 cycles although its initial discharge specific capacity is lower than that of LiFe_1-xZn_xPO₄ compounds with x?0.10.Li₉V_3-xCo_x (P₂O₇)₃(PO₄)₂/C(x=0.0,0.02,0.05) compounds were prepared at 750?by sol-gel method under H2Ar reduction atmosphere and were characterized as pure phase by XRD; The SEM images show that Li₉V_2.98Co_0.02 (P₂O₇)₃(PO₄)₂/C sample particles have smaller uniform particle size,better crystallization and larger specific surface area than others. Li₉V_2.98Co_0.02 (P₂O₇)₃(PO₄)₂/C exhibits good electrochemical performance, especially in the high current density. The initial discharge capacity is 136.2mAh/g at the current density of 10mA/g and still can be held in 134.1mAh/g after 10 cycles. When the current density is increased to 50mA/g, the discharge capacity is 123.6mAh/g and can be obtained in 102.6mAh/g after 30 cycles. We have also successfully synthesized the LiCoBO₃ by one-step solid state method. The sample having monoclinic structure and C12/c1 space group was charactered by XRD. The SEM images show that sample has uniform particle size and good crystallization, the particle size is about 200-600nm.