Synthesis And Electrochemical Properties Of LiFePo₄ Cathode Material For Lithium Ion Batteries

In the face of increasingly severe energy shortages and environmental pollution,the development of sustainable clean energy conversion and storage technologies are effective strategies to promote energy and sustainable socio-economic development.Lithium-ion batteries（LIBs）have the advantages of large specific capacity,small self-discharge,no memory effect,high energy density,long cycle life,safety and no pollution.They have been widely used in electrical equipments such as electronic equipment and electric vehicles.Among the key cathode materials of LIBs,lithium iron phosphate（LiFePO₄,LFP）has good application prospects in energy storage equipment,electric buses and electric ships due to its low price,large theoretical specific capacity,good cycle stability and excellent safety performance.However,poor electron conductivity and low lithium ion diffusion coefficients limit the further application of LFP in the new energy industry.Although lots of research work（carbon coating,refinement of material size and doping of certain elements）has been devoted to improving the electrochemical performance of LFP,it is urgent to further improve its large rate performance.Among them,carbon coating can not only increase the electron conduction between the particles,but also avoid the pollution of Fe³⁺,which leads to increasing the capacity,rate performance and cycle life of the LFP.However,the carbon content and its type have a great influence on the electrochemical performance of LFP.Therefore,it is very necessary to choose the suitable carbon source and optimize the carbon content to improve the electrochemical performance of LFP.In addition,reducing the size of the LFP particles can shorten the diffusion path length of the lithium ions and lower the charge transfer resistance of the entire electrode.In order to solve the problem of poor electronic conductivity and low lithium ion diffusion coefficient of LFP,a series of carbon-coated LFP（LFP/C）composite cathode materials were prepared by wet ball milling and in-situ carbothermal reduction in this paper and study the effect of the content and sintering temperature on its electrochemical properties;then,carbon nanotubes（CNTs）and graphene（G）were introduced into LFP/C to further improve their electrochemical performance.In addition,we also explored the effect of transition metal catalyzed amorphous carbon graphitization on the electrochemical performance of LFP/C.In this paper,the main research work is following three aspects:（1）Synthesis of LFP/C Cathode Materials and electrochemical performance studyLiFePO₄/C cathode materials were prepared by carbothermal reduction method.The effects of glucose content and sintering temperature on the microstructure and electrochemical properties of LFP/C were investigated.The structural characterization indicates that the diffraction peak of the prepared LiFePO₄/C sample is completely consistent with the characteristic peak of the LiFePO₄ standard card without impurities;the electrochemical performance test shows that the LFP/C-700 has best electrochemical performance when the glucose content of FePO₄ is 20 wt.% and the sintering temperature is 700 C.The initial discharge specific capacity at 0.1 C is 160m Ah/g;at a current density of 5 C,a specific capacity of 105.2 m Ah/g can still be released.Moreover,the capacity of the LFP/C-700 after the 300 cycles of 1C cycle（144.8 mAh/g）is as high as 99.9%,showing excellent cycle stability.（2）Preparation and Electrochemical Properties of 3D Network Structure LFP/C/G/CNTsConsidering that both CNTs and graphene have high conductivity,they are introduced into LFP/C,LFP/C/G,LFP/C/CNTs and LFP/C/G/CNTs composite prepared by wet sanding method as scheme（1）.The structural characterization indicates that the prepared composite cathode electrode material belongs to pure olivine structure LiFePO₄ and has no extra impurities.Raman spectroscopy shows that the addition of CNTs and graphene significantly increased the degree of graphitization of the material;SEM/TEM characterization confirms that the amorphous carbon,graphene and CNTs and LFP nanoparticles in the material formed a good three-dimensional conductive network.Electrochemical performance tests show that the addition of CNTs and graphene can greatly improve the conductivity of the material.At lower current density（0.1C）,the discharge specific capacities of LFP/C,LFP/C/CNTs,LFP/C/G and LFP/C/G/CNTs were 150.3 m Ah/g,155.7 m Ah/g and 159.7 m Ah/g and 164.5 m Ah/g,respectively.Their discharge specific capacities were 78.1 m Ah/g,85.7 m Ah/g,92.5 m Ah/g,and 99.5 m Ah/g at a current density of 5C.Among these four cathode materials,LFP/C/G/CNTs showed the best electrochemical performance:LFP/C/G/CNTs had the highest discharge specific capacity（154.4 m Ah/g）at 1C current density,and the discharge specific capacity was152.9 m Ah/g after 200 cycles,showing good cycle stability of the material.（3）Transition metal catalyzed graphitization improves electrochemical performance of LFP/CFe,Co,Ni transition metal precursors were introduced during the preparation of LFP/C to catalyze graphitization of amorphous carbon produced by glucose pyrolysis,and LFP/C-M cathode material was obtained（M=Fe,Co,Ni）.Raman tests indicate that the introduction of transition metals can increase the degree of graphitization of carbon in LFP/C materials.Electrochemical performance test results show that Ni has the best catalytic graphitization effect,and the obtained LFP/C-Ni（3%）shows the best electrochemical performance.Further study on the catalytic effect of Ni content on LiFePO₄/C cathode material found that LFP/C-Ni（10%）obtained the best crystallinity and dispersibility when the Ni content was 10 wt.%of Fe PO₄.The electrochemical performance test showed that the specific capacity of LFP/C-Ni（10%）at 0.2C,5C was 163.7 m Ah/g and 101.5 m Ah/g;the specific capacity and capacity retention after 300 cycle of 1C current density was 144.8 m Ah/g and 99.9%,showing better rate and cycle performance.