The Preperation And Properties Investigation Of LiFePO₄ Modified By Nano-carbon Materials As Lithium Ion Battery Cathode Materials

As countries have announced the ban of traditional energy（gasoline and diesel）vehicle schedule,Chinese also launched a related study to stop production and sales of traditional energy vehicles schedule.China is the largest auto market in the world,and it will also be the largest market for new energy vehicles.As one of the main driving forces of new energy vehicles,lithium ion batteries are becoming more and more demanding.People are eager to develop a lithium ion battery with higher energy density.As one of the most potential cathode materials for lithium ion power batteries,LiFePO₄ is still a hot spot of research.In this paper,LiFePO₄ was modified with nano carbon materials:First,LiFePO₄ was synthesized by high temperature solid-state method;few layer graphene oxide and modified carbon nanotubes were obtained respectively by micro adjustment and control processing,and then were used for the preparation of the LiFePO₄/graphene composite,LiFePO₄/CNTs composite and LiFePO₄/graphene/CNTs composite by sol-gel method.The electrochemical performance of the materials were systematically researched by electrochemical measurement methods of charge and discharge performance,cyclic voltammetry（CV）,electrochemical impedance spectroscopy（EIS）;and their microstructures and morphologies were studied by X-ray diffraction（XRD）,fourier transform infrared spectrometer（FT-IR）,sanning electron microscope（SEM）and transmission electron microscope（TEM）.The effects of dif-ferent additive amount of graphene oxide（0.2%,0.4%,0.6%,0.8%,1.0%）on the structures.morphologies and electrochemical performance of LiFePO₄/graphene composites were researched.The crystal structure of LiFePO₄ was intact,and the distribution of graphene in the composite was uniform.Graphene were dispersed between LiFePO₄ particles to form a three-dimensional conductive network structure.The electrochemical performance test results showed that the optimum additive amount of graphene oxide is 0.4%.The rates performance of LiFePO₄/graphene composites were better than that of LiFePO₄.The LiFePO₄/graphene composite which was prepared with 0.4%additive amount of graphene oxide showed a initial discharge specific capacity of 114.8 mAh/g at the rate of 1 C,and there was almost no attenuation in capacity after 60 cycles.The cycle performance of composites is more stable.The polarization and the charge transfer impedance of LiFePO₄/graphene composite was smaller than that of LiFePO₄ and showed a better electrochemical reversibility.LiFePO₄/CNTs composites were prepared by different additive amount of carbon nanotubes（1%,2%,3%,4%,5%）,and their electrochemical performance were researched.The carbon nanotubes of composites were uniformly distributed,the interlaced "carbon nanotube network" were dispersed between LiFePO₄ particles to form a three-dimensional conductive network structure of the composites.Carbon nanotubes improved the rates performance,discharge platform voltage,cycle performance of LiFePO₄,and the optimum additive amount of carbon nanotubes was 4%.LiFePO₄/4%CNTs composite showed a first discharge specific capacity of 120.48 mAh/g and a capacity retention rate of 99.33%with the discharge specific capacity of 119.67 mAh/g after 100 cycles at 1 C rate.The polarization and the charge transfer resistance of LiFePO₄/CNTs composites were decreased,and showed a better electrochemical reversibility and a faster electrochemical reaction rate.LiFePO₄/graphene/CNTs composites were prepared with different additive amount of carbon nanotubes（1%,2%,3%）on the basis of the optimum additive amount of graphene oxide（0.4%）.Graphene were dispersed between LiFePO₄ particles to form a primary network structure,and the carbon nanotubes formed a secondary,both of which formed a more efficient three-dimensional composite conductive network structure of the LiFePO₄/graphene/CNTs composites.The results of electrochemical performance study showed that the rate performance of LiFePO₄/graphene/CNTs composites are further improved than that of LiFePO₄/graphene composites,and a smaller △Ep and lower Rct than that of LiFePO₄/graphene composites.LiFePO₄/graphene/2%CNTs showed the excellent cycle performance with a first discharge specific capacity of 139.33 mAh/g and the discharge specific capacity of 136.17 mAh/g after 80 cycles at 1 C rate,the loss of discharge capacity was only 3.16 mAh/g,and the capacity retention rate was 97.73%.