The Preparation And Coating Modification Of LiVPO₄F Cathode Materials For Lithium-ion Batteries

With the development of electric vehicles,lithium-ion batteries have become a hot topic in the field of new energy.At present,there are several limitations in the commonly used cathode materials for lithium ion batteries.LiVPO4F cathode material contains not only high plateau at 4.2 V,but also delivers better thermal stability than other mainstream materials.However,its conductivity is relatively low.The tranditional cathode prepared by two-step carbothermal reduction contains excessive carbon powder.This will affects the mass and volumetric energy densities of material.For the purpose of improving the electrical conductivity and electrochemical performance cathode of the paper,a novel one-step method was used to prepare the LiVP04F/C cathode material.The reaction characteristic was discovered through the thermodynamics and kinetics analysis.The Influence mechanisms of sintering temperature on the microstructures was explored,the effects of carbon sources phenolic resin,polyvinylidene fluoride(PVDF),glucose,sucrose on the microstructure and electrochemical performance of the cathode materials were discussed.Finally,cathode materials with high conductivity and excellent performance were prepared.The main results are as follows:(1)Through the analysis of reaction process.The V5+ in V2O5 can be reduced to V3-in the preparation of precursor by navel one-step method.The effects of sintering temperature on the structure,morphology and electrochemical performance of the materials were investigated,under the guidance of the reaction thermodynamics and kinetics.Researces show that Li3V2(PO4)3 is the accompanying phase in the synthesis of LiVPO4F.The generation temperature of Li3V2(PO4)3 is also low.Calculation on the Arrhenius equcation indicates that the reaction is in thr third order with the activation energy of 208.9 kJ/mol.It is found that the cathode material obtains the lowest content of Li3V2(PO4)3,uniform particle size and excellent specific discharge capacity and rate performance when the sintering temperature is 800℃.The specific discharge capacity reaches 130.3 mAh/g and 116.5 mAh/g at 1 C and 5 C.And the formation reasons of Li3V2(PO4)3 is investigated.(2)The carbon coating on LiVPO4F was carried out by using phenolic resin,PVDF,glucose and sucrose as carbon source.It is found that the phenolic resin with three-dimensional carbon chain structure can be in situ coated with cathode.The improvement of conductivity is significant.The content of the pure phase LiVPO4F in the coated sample with PVDF as carbon source is higher.The material delivers the best specific discharge capacity and rate performance.(3)The addition of phenolic resin by high speed ball milling shows that the precursor and carbon source can be effectively mixed by ball milling at room temperature.It can avoid the dropping of fluidity of phenolic resin in the heating process.And it can also refine the particles and improve the electrochemical performance of cathode.(4)The influence of the cantent of PVDF on LiVPO4F/C cathode was explored.The mechanism of the preparation of LiVP04F/C with pure phase material was also discovered.It is found that the polycondensation reaction will take place and HF gas will be released in the process pyrolysis.This can effectively reduce the lost fluorine element in LiVPO4F/C at high temperature and inhibit the phase transition from LiVPO4F to Li3V2(PO4)3.When the carbon coating amount is 8 wt.%,the discharge capacity of materials reach 147.6 mAh/g and 135.8 mAh/g at 0.1 C and 5 C,respectively.It is the highest value compered with reported results.The capacity retention rate is 91.2%at 1 C,after 200 cycles.