Study On Synthesis Of LiFePO₄ Cathode Material For Lithium Ion Batteries By Quenching Treatment

Intensive research and development work is being conducted to further improve the performance of lithium ion batteries and reduce the cost of electrode materials.Based on this trend and reviewing the development of cathode materials for lithium ion batteries,this dissertation focusing on olivine LiFePO₄ cathode materials.The synthesis and modification of LiFePO₄ cathode materials were studied by water quenching treatment in details.The effect of common cooling mode and water quenching(WQ) mode on the microstructure and electrochemical performance of LiFePO₄ were studied.In contrast to the common cooling mode,LiFePO₄ synthesized by water quenching treatment had more fine grain size. Crystal defect existence of the sample prepared by water quenching treatment was found by Rietveld refinement,which produced the crystal volume contraction.The working mechanism of water quenching treatment was discussed from chemical thermodynamics,chemical dynamics and crystallography.And the existences of vacancy and diffusion channel were brought on in the sample by water quenching method.These vacancies were helpful to improve the electrochemical performance of LiFePO₄ for offering the diffusion path of lithium ion in the electrode process.The synthetic conditions of pure LiFePO₄ were optimized by orthonormal experiment.The pure LiFePO₄ prepared under optimum condition had the highest reversible discharge specific capacity of 151.1 mAh·g^-1 at 0.1C rate and 132.0 mAh·g^-1 at 0.5C rate.LiFePO₄/C composite was prepared by water quenching treatment and the working mechanism of carbon in the LiFePO₄/C composite was studied by Raman spectrum method.The results showed that three necessary conditions determine the improvement of LiFePO₄/C.(â…°).The particles of LiFePO₄ need be coated by sp² type carbon;(â…±).The good electronic conductive network among particles need be formed;(â…²).The uniformity of carbon on the surface of LiFePO₄ particle is also necessary. The synthesis of LiFePO₄/C with low carbon content and good activity by water quenching treatment was optimized by orthonormal experiment. The optimum sample with good cyclic capability displayed the highest reversible discharge specific capacity of 163.8 mAh·g^-1 at 0.1C rate and 143.4 mAh·g^-1 at 1C rate and 124.7 mAh·g^-1 at 2C rate.And the in situ carbon content of the sample was only 0.98%.The specific surface area was 8.9 m²Â·g^-1.Hence the physical property of the sample was improved greatly.The Fe sites(M2) doping and Li sites(M1) doping LiFePO₄ compound were synthesized by WQ method and the mechanism of the improvement of electronic conductivity was also discussed by four-probe method,powder microelectrode cyclic voltammetry and electrochemical impedance spectroscopy measurements.The study indicated:(â…°) the Fe sites doping Mg²⁺ and Ni²⁺ did not only improve the electronic conducivity by 2～3 order of magnitude but also weakened the bound of oxygen to lithium,which is propitious to the transport of Li ions;(â…±) the electronic conducivity of doping samples in Li sites by Ti⁴⁺ and Nb⁵⁺ was improved by 100 times.Meanwhile,structure change due to phase transformation was inhibited to some extent.But electrochemical performance was poor with doping excess atoms in Li sites.The synthesis and capability of doping type sample with low carbon content were investigated in order to further improve the electronic conductivity of LiFePO₄ and optimize the performance of LiFePO₄/C. The electronic conductivity was enhanced to 10^-2 S·cm^-1 by this method. The electrochemical property especially the rate capability was improved greatly.The sample LiFe_0.99Mg_0.01PO₄/C showed 133.9 mAh·g^-1 at 2C discharge rate and almost no loss after 50^th cycles.And the in situ carbon in the sample was only 1.05%.The ranges of exchange current density(i₀) in Li_1-xFePO₄ and Li_1-xFe_0.99Mg_0.01PO₄/C(x=0～0.8) were 0.04～0.10 and 0.1～0.42 mA·cm^-2,respectively.The exchange current density were changed with lithium content in the electrode,and the max value was obtained near at x=0.4～0.5.The synthetic conditions of spherical LiFePO₄ with high tap density were studied by spray drying(SD) method.The results showed dense spherical precursor was prepared at the low solid content(20%) and high speed of centrifugal atomizer for 18000 r·min^-1.The product with dense particle was synthesized by using dense precursor.And the tap density was 1.51 g·cm^-3.The sample with good cycle capability could reach 150.2, 144.0 and 129.8 mAh·g^-1 at the discharge rate of 0.2C,1C and 2C, respectively.