Study On The Synthesis And Modification Of Li₃V₂（PO₄）₃@C Cathode For Lithium-ion Batteries

Polyanion cathode material Li₃V₂（PO₄）₃（lithium vanadium phosphate）is a very promising cathode material for lithium-ion battery because of its stable structure,excellent electrochemical performances,favourable safety performance,good environmental compatibility and so on.This material has been widely concerned.However,the conductivity of Li₃V₂（PO₄）₃ is relatively low,which limits its large-scale application.In this paper,Li₃V₂（PO₄）₃@C cathode material was prepared by conventional carbothermal reduction method,and the optimum calcination temperature,calcination time and carbon content were explored.What’s more,the effect on the phase and electrochemical performance of these optimum parameters is also explored.Furthermore,the Li₃V₂（PO₄）₃@C synthesis process was optimized,and the Li₃V₂（PO₄）₃@C cathode was prepared by modified carbon thermal entrapment reduction method.To verify the feasibility of the improved method,the morphological characteristics and electrochemical properties of the samples were studied.The main results of this paper are as followed.1.The samples were prepared by the conventional carbothermal reduction method,and the effect on the phase and electrochemical performance of technological parameter was explored.The results show that,when the sintering temperature is lower than 950℃,LiVP₂O₇ can be found in the Li₃V₂（PO₄）₃@C sample,and when the sintering temperature is higher than 1000℃,Li₃PO₄ exists.When the sintering temperature is between 950℃ and1000℃,the Li₃V₂（PO₄）₃@C sample is pure.The results of constant current charge and discharge test show that the specific capacity of the sample containing pure phase is the best.When the sintering temperature is 950℃,the particle size of Li₃V₂（PO₄）₃@C is larger as the sintering time is longer.The influence of carbon content on the agglomeration of particles is relatively large.When the carbon content is lower than 2.77%,it’s unable to form a coating layer on the surface of the positive electrode.When the carbon content is more than 2.77%,agglomeration exists.Therefore,the optimum carbon content is 2.77%.In general,the optimum process parameters are as follows:sintering temperature 950℃,sintering time 12 h,carbon content 2.77%.The sample is prepared under the optimum process parameters,and the constant current charge and discharge test is carried out.Under the charge and discharge voltage of 3.0⁴.3 V and the current density of 0.1C,the initial discharge specific capacity is 127.7 mAh·g^-1.After 100 cycles,the discharge specific capacity is 125.7 mAh·g^-1.Under the current density of 10C,the initial discharge specific capacity is102.8 mAh·g^-1.After 500 cycles,the discharge specific capacity is 98.5 mAh·g^-1,which is almost 96.6%of the initial discharge specific capacity.2.The Li₃V₂（PO₄）₃@C composites were parapered by carbon entrapment method under the sintering temperature of 950℃ and the sintering time of 12 h.The morphological characteristics and electrochemical properties were analyzed.The test results of the morphological characteristics show that Li₃V₂（PO₄）₃@C composites have a particle size ranging from 500 nm to 3μm with uniform carbon coating,and the thickness of carbon coating is about 7 nm.The constant current charge and discharge test is carried out under the voltage of 3.0⁴.3 V and the current density of 0.1C,the initial discharge specific capacity is130.9 mAh·g^-1,which is very close to the theoretical specific capacity of 133 mAh·g^-1.Even under the current density of 10C,the initial discharge specific capacity is 102.8 mAh·g^-1.The constant current charge and discharge tests under a variety of current density show that the decline of specific capacity under high current density is temporary.That’s to say,once the current density is back to be small,the specific capacity and reversibility will be restored.When the Li₃V₂（PO₄）₃@C composites are prepared by the carbon entrapment method,no reduction gas is required to provide protective atmosphere,the operation is very simple and the muffle furnace can be used directly.In addition,Li₃V₂（PO₄）₃@C can be produced in large quantities by this improved carbothermal reduction process.What’s more,this carbon entrapment process can also be applied to other cathode materials,such as LiFePO₄,etc.