Carbon Nano-materials Doping Modification And Research Of The Composite Electrode Materials Of Power Lithium-ion Battery

Lithium-ion battery as an energy form in chemical power supply,with high working voltage,large energy density,light weight,small volume,good safety,and the advantages of green environmental protection,has become the best type of high energy of green battery.It is an ideal power source for electric vehicles and hybrid electric vehicles.In recent years,the rapid development of electric vehicles has put forward a further requirement for lithium ion battery,which requires further research and development of its electrode materials.Li₄Ti₅O₁₂ and LiFePO₄ were studied for lithium-ion battery in this paper.Li₄Ti₅O₁₂ with "zero strain material" maintains high structural stability during the process of lithium ion removal,and has a good charging and discharging platform,electrochemical reversibility,cyclic stability and safety.However,the lithium ion battery,which used Li₄Ti₅O₁₂ as electrode material,can only display the performance advantages of its materials under the condition of low magnification,which can not meet the power requirements of large equipment.Moreover,the electronic conductivity of pure Li₄Ti₅O₁₂ materials is low,which is not conducive to the capacity of electrode materials and the performance of cyclic performance.With many characteristic,such as stable structure,high working voltage,high theoretical discharge specific capacity,long cycle life,rich raw material resources,cheap and environmental protection,LiFePO₄ become one of athe most potential and new generation anode materials applied in electric vehicle and hybrid electric vehicle is of lithium ion battery.But its structure characteristics,its conductivity and ionic conductivity are low,which determine its large current discharge performance is poor,limited the practical application.Aiming at the shortcomings of Li₄Ti₅O₁₂ and LiFePO₄,combining carbon nanotubes and graphene with excellent electronic conductivity and special one-dimensional or two-dimensional flexible structures,the paper focuses on the study of carbon doping modification of Li4TisO12 and LiFePO₄.The crystalline Li₄Ti₅O₁₂ materials with good crystallization performance were prepared both by high temperature solid state method and microwave method.Li₄Ti₅O₁₂ materials prepared by microwave method has obvious advantage in particle size and particle size distribution,with smaller polarization,better capacity and invertibility.Because microwave heating is carried out simultaneously from the inside and outside of the material,so that the material is heated evenly.The Li₄Ti₅O₁₂/CNT composite materials with different doping amount?1%?3%?5%?7%?were prepared by high energy ball-milled and one-step sintering under different temperature?700??800??850??900??.Before mixing,in order to disperse CNTs in Li₄Ti₅O₁₂ particles preferably,the CNTs were first treated with acidification and ultrasonic shear dispersion.The composite electrodes have steady discharge platform of 1.54V and large specific capacity,initial discharge capacities are 168,200,196 and 176mAh g-1 in different CNTs doping amount of 1%,3%,5%,7%respectively at 0.1 C rate for the Li₄Ti₅O₁₂ synthesized in an optimized heat treatment temperature of 800?.In our experimental range,the composite electrode on an CNTs doping amount of 3%shows the best performance such as the minimum impedance,the initial capacity is 176mAh·g-1 with discharge capacities retention rate of nearly100%.Its capacity is about 151mAh·g-1 under 20C rate discharge condition with excellent high-rate performance.There is almost no decline after 20th cycles under 10C discharge condition.This is due to an appropriate amount of CNTs doping can form a good conductive network in the Li₄Ti₅O₁₂ anode material.With a special tubular structure,CNTs can absorb more electrolyte,that provides a better channel for the intercalation/deintercalation of lithium-ions and electron migration,thus improving the conductivity and cycling performance of electrode materials.The incorporation of CNTs is too low to form an effective conductive network,which can not improve the electrochemical properties of the electrode.The high incorporation of CNTs is not conducive to the electrochemical performance of electrode materials either.On the one hand,it is easy to cause agglomeration,dispersing inhomogeneity,and on the other hand,the proportion of active substances is reduced.A three-dimensional Li₄Ti₅O₁₂/CNT/Graphene composite was prepared by ball-milling method,followed by microwave heating.And compared with pure Li;Ti5O12 electrode,Li₄Ti₅O₁₂/CNT and LiaTi5O12/Graphene composite electrode materials.CNTs and graphene were used to construct 3D conducting networks,leading to faster electron transfer and lower resistance during the lithium-ion reversible reaction,more conducive to the penetration of the electrolyte in a variety of directions and easier contact between active material and the electrolyte,which significantly enhanced the electrochemical activity of Li₄Ti₅O₁₂/CNT/graphen composite.The Li₄Ti₅O₁₂/CNT/Graphene composite exhibited a high initial discharge capacity of 172mAh·g-1 at 0.2C and 132mAh·g-1 at 20C,as well as an excellent cycling stability.The electrochemical impedance spectroscopy demonstrated that the Li₄Ti₅O₁₂/CNT/Graphene composite has the smallest charge-transfer resistance with only 74.13? compared with other composites,indicating that the fast electron transfer from the electrolyte to the Li₄Ti₅O₁₂/CNT/Graphene active materials during the lithium-ion intercalation/deintercalation.The LiFePO₄/CNT composite electrode materials with different doping amount?5%?10%?15%?20%?were prepared by ball milled and hydrothermal method.LiFePO₄ material particles prepared by hydrothermal method were finer and more evenly distributed.LiFePO₄ particles and CNTs were evenly distributed and intersected in the composite prepared by both methods.The CNTs formed a network structure between LiFePO₄ particles,providing a conductive network.The composite electrode on an CNTs doping amount of 10%shows the best performance such as the minimum impedance with only 56? and the best chemical reversibillity,which exhibited a high initial discharge capacity of 148mAh·g-1 and 148mAh·g-1 at 0.1C and 131mAh·g-1 at 0.5C,as well as an excellent cycling stability under a high temperature of 60?.The capacity of The LiFePO₄/CNT composite electrode is almost no attenuation after 30 cycles at 0.1C,and its retention rate of the cycle is still more than 94 percent at 0.5C.A nanocrystalline LiFePO₄/CNT/Graphene composite has been successfully synthesized by a hydrothermal method followed by heat-treatment.At the same time,pure LiFePO₄,LiFePO₄/CNT and LiFePO₄/Graphene positive electrode materials were prepared for comparison.The LiFePO₄/CNT/Graphene nanoparticles were wrapped homogeneously and loosely within a 3D conducting network of Graphene-CNTs.The conducting networks provided highly conductive pathways for electron transfer during the intercalation/deintercalation process,facilitated electron migration throughout the secondary particles,accelerated the penetration of the liquid electrolyte into the LiFePO₄/CNT/Graphene composite in all directions and enhanced the diffusion of Li-ions.The results indicated that the LiFePO₄/CNT/Graphene composite significantly enhanced the electrochemical activity,which has a minimum impedance of only 20.984?and a longer and smoother charge and discharge platform.In particular,the LiFePO₄/CNT/Graphene composite with a low content of graphene and carbon nanotubes exhibited a high initial discharge capacity of 168.4mAh g-1 at 0.1C and 103.7 mAh g-1 at 40 C and an excellent cycling stability.