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.