Systhesis Of LiFePO₄ Cathode Materials Via Microwave Hydrothermal/Plasma And Its Modification & Performance

New energy vehicles promote the development of the lithium ion battery,research and development of lithium ion battery cathode material with high performance,low cost is very realistic significance.Olivine lithium iron phosphate（LiFePO₄）with advantages of high security,cheap and non-toxic is the leader of lithium ion battery cathode materials,but its low electronic conductivity and lithium ion diffusion coefficient will constraint its high rate and fast charge and discharge performance.In this work,the microwave-assisted hydrothermal/microwave plasma chemical vapor deposition（MPCVD）synthesis,surface modifications and ion doping of LiFePO₄ were investigated systematically.Olivine LiFePO₄ nanoplates,with a short b-axis,were successfully synthesized using microwave-assisted hydrothermal method.The effects of microwave irradiation temperature and time on the structure,morphology and electrochemical performance were investigated systematically.The LiFePO₄ nanoplates were prepared at 160℃ in20 min.Microwave irradiation can lower the synthesis temperature and shorten the synthesis time dramatically.LiFePO₄ nanoplates with short b-axis,can shorten diffusion routes for ionic transfer,and improve the electrochemical performance.Its initial discharge capacity reaches 133.8 mAhg^-11 at 0.1 C rate.In addition,the nucleation and growth processes of LiFePO₄ nanoplates in the microwave field mainly follows the dissolving-precipitation mechanism,the microwave field can induce and promote the reaction and the formation of crystalline morphology.Surface decoration of LiFePO₄ nanoplates was adopted to improve its electrochemical performance.The LiFePO₄/C composite materials were prepared by ex situ carbon coating with the glucose.The effects of calcination temperature and carbon content on the structure,morphology and electrochemical performance were investigated.The products prepared at 700℃ show the excellent electrochemical performance,this is ascribe to the high crystallization and high graphitization degree of carbon layer.Carbon content can affect particle size and coating,with the increase of carbon content,the electrochemical performance increase firstly and then decrease.The discharge capacity of LiFePO₄/C（7 wt.%）sample are 132.1 mAhg^-1and 111.8mAhg^-11 at 10 C and 20 C rates.The high rate performance of LiFePO₄/C sample needs to be improved.Surface decoration of LiFePO₄ nanoplates with reduced graphene oxide（rGO）coating were prepared by in situ microwave-assisted hydrothermal method.The effects of graphene content on the structure,morphology and electrochemical performance were investigated systematically.The right amount of graphene can wrap on the surface LiFePO₄ particles completely and uniformly.The introduction of graphene improved the material discharge specific capacity,high performance and cycle stability significantly.Therefore,controlling the graphene of content in the product is significant to the increase of the electrochemical properties.The LiFePO₄/graphene（5 wt.%）sample shows an excellent high rate performanc,the discharge capacity are 148.2 mAh/g,132.6 mAh/g and 114.4 mAh/g at 10 C,20 C and 30 C rates.The LiFePO₄/carbon nanoparticles were synthesized successfully using a microwave plasma chemical vapor deposition（MPCVD）method.Electrochemical impedance spectroscopy（EIS）is carried out to collect information about the kinetic behavior of lithium diffusion in LiFePO₄/carbon nanoparticles during the charging and discharging processes.The variation of charge transfer resistance（Rct）shows aninverted-U-shape,which indicate the electrical conductivity of single solid-solution phase is higher than that of the two-phase.The diffusion coefficients of lithium ions(D_Li)get a V-like shape,and the calculated values of D_Lii in the solid-solution regions are two or three orders of magnitude greater than those in the two-phase region.Therefore,narrowing two-phase region and widening solid-solution regions,which can enhance the Li-ion diffusion.F-doped LiFePO₄/C materials were synthesized using a co-precipitation followed by MPCVD method with hydrofluoric acid source.The structure,morphology and electrochemical performance of F-doped LiFePO₄/C materials were investigated systematically.Analysis of the structure indicates that F doping can increase the length of Li-O bonds and weaken the interaction of Li-O bonds,which are beneficial to the diffusion of Li ions between LiFePO₄ and FePO₄ phases.F doping can also decrease the length of the P-O bonds and enhance the interaction of P-O bonds,which will lead to a more stable structure;thus F doping can improve the cycling stability even at high rate.F doping can improve the inherent demerits of LiFePO₄ materials,enhance the electronic conductivity,accelerate the Li ions diffusion coefficient,and improve the structure stability.For LiFePO4-x Fx/C（x=0.15）sample,its discharge capacity is 126 mAh·g^-1and 107.4 mAh·g^-11 at 20 C and 30 C rates after 50 cycles.A novel,facile,and high-energy efficient synthetic technology has been developed for synthesis of LiFePO₄/CNTs composite via MPCVD method.A stainless steel mesh works as a green catalyst for the in situ growth of carbon nanotubes during the reaction,which can avoid the pollution of the impurity of ions.The structure,morphology,valence state,electrochemical performance and the growth mechanism of LiFePO₄/CNTs materials were investigated.The LiFePO₄/CNTs composite materials display remarkably improved specific capacity,rate capability and cycling stability.The excellent electrochemical performances are ascribed to the3D highly conductive networks of CNTs,which can greatly improve the connectivity between the LiFePO₄ particles,the CNTs can provide abundant transport paths of electrons and increase the diffusion of the lithium ions,leading to the enhanced lithium-ion insertion kinetics.