Synthesis And Modification Of High-performance Olivine-type Cathode Materials For Lithium Ion Battery

The cathode materials with olivine structures,which have many advantages,such as stable structure,flat charging and discharging platform,abundant raw materials,and environmental friendliness,have attracted wide attention as cathode for lithium ion batteries.However,the intrinsic electronic structure and crystal structure of olivine-type cathode materials lead to a large energy band gap for the internal electronic transition,and one-dimensional lithium ions diffusion pathway along the b-axis in the bulk phase,resulting in low electronic conductivity and ion diffusion rate.In view of the poor conductivity of olivine-type cathode materials,this work takes olivine-type LiFePO₄ and Li Mn_0.5Fe_0.5PO₄ cathode materials as the research objects,followed by exploring the mechanism of regulating the material band structure and morphologic structure with ion doping strategy.The main research contents are as follows:（1）Supported by theoretical calculations,the thermodynamic energy generation of Li sites and Fe sites in LiFePO₄ replaced by Ti⁴⁺were calculated by constructing two thermodynamic models of Li sites and Fe sites in LiFePO₄ replaced by Ti⁴⁺,respectively.The results exhibit that the lowest energy of the system is with Li sites replaced Ti⁴⁺,indicating that Ti⁴⁺is more likely to replace Li sites in LiFePO₄.Meanwhile,the influence of Ti⁴⁺doping on the band structure of LiFePO₄ system was analyzed by DFT calculation.The results show that the band structure of LiFePO₄ is modulated by Ti⁴⁺doping,giving rise to an effective reduction of the band gap and the electrons effective mass,so that the electrons transition can be motivated more easily and the electronic conductivity of the material can be significantly enhanced.In addition,Ti⁴⁺-doped LiFePO₄/C materials were prepared by mechanical ball-milling assisted carbothermic reduction method.Three groups of samples with different Ti⁴⁺doping levels（0,0.5at.%,1 at.%）were compared and analyzed.The results show that proper Ti⁴⁺doping can reduce the particle size of the samples and accelerate the Li⁺migration inside the particles.Among them,the 0.5%Ti⁴⁺-LFP/C sample has the most excellent rate performances,which can achieve the specific discharge capacity of 160.3,157.0,152.8,143.7,136.1 and 126.4m Ah g^-1 at the current density of 0.1 C,1 C,2 C,5 C,10 C and 20 C,respectively.（2）Based on the synthesis process of LiFePO₄/C material,a series of Li Mn_0.5-x/2Fe_0.5-x/2Ti_xPO₄/C（x=0,0.01,0.02,0.03）materials were successfully synthesized by mechanical ball-milling assisted high-temperature solid-state method.The effects of different Ti⁴⁺doping amounts on the morphology,structure and electrochemical properties of the products were investigated.The results show that with the addition of Ti⁴⁺,the large particles are gradually broken,which can effectively shorten the transport distance of Li⁺.However,when the doping amount of Ti⁴⁺reaches 3%,the small particles of the sample will agglomerate,which is not conducive to the diffusion of Li⁺.The electrochemical analysis results show that the LMFP-3 sample doped with 2%Ti⁴⁺has the best electrochemical performance.At the current density of 5 C,the initial specific capacity of the LMFP-3 sample was 115.8 m Ah g^-1,giving a capacity retention of 83.4%after 3000 cycles.