Study On Preparation And Modification Of Li₃V₂（PO₄）₃as Cathode Material For Lithium-ion Battery

Lithium vanadium phosphate, a kind of polyanion cathode material based onphosphate anions, has been intensively attractive in recent years for its merits such ashigh theoretical capacity, high discharge voltage and extraordinarily stable structure.H8owever, the low electronic conductivity （2><10" S*1cm_） and ionic conductivity ofLi₃V₂（PC>4）₃result in its poor electrochemical performance, hence unfavorable forpractical applications. This dissertation aims to overcome the above mentionedshortcomings of Li₃V₂（PC>4）3by improving the synthesis methods, which combinewith doping and coating techniques. The cathode materials of Li₃V₂（P0₄）₃/C,Li3_xNaxV₂（P0₄）3/C （0<x<0.7） and Li2N.95a0A.05V2（P04）3^2O3were prepared and theirphysical properties and electrochemical performances were investigated by X-raydiffraction （XRD）, scanning electron microscopy （SEM）, transmission electronmicroscopy （TEM） and electrochemical tests. The effects of synthesis conditions onthe physical properties and electrochemical performances of the above-mentionedcathode materials were discussed in detail and the prepare conditions were optimized.Li3V2（PC>4）3/C composite was synthesized by a sol-gel method using NH4VO3,NH4H2PO4, LiOH and citric acid as raw materials and the effects of calcinationtemperature, calcination time and the molar ratio of citric acid to vanadium on theelectrochemical performance of the composite were investigated. The results showthat optimized prepare conditions are as foll°ows: the calcination temperature is800C,the calcination time8h，and the molar ratio of citric acid to vanadium is1.Comparative investigation of electrochemical performance of Li3V2（PC>4）3/C cathodematerials prepared by sol-gel method using NH4VO3and V2O5as vanadium source,respectively, was carried out and the results indicate that V2O5is an optimal vanadiumsource for preparation of Li₃V₂（P0₄）₃/C.Li3_xNaxV2（P04）3/C （x=0.03，0.05and0.07） composite materials were preparedby a sol-gel method using V2O5，LiOH, NaNC>3，NH4H2PO4, H2O2and citric acid asraw materials and the effects of the content of doped Na+on the electrochemicalperformance of Li3_xNaxV2（P04）3/C was investigated. The results show that theelectrochemical performances of Li3_xNaxV2（P04）3/C composites are better than thatof Li3V2（P〇4）3/C composite in the x range of0.03to0.07and Li2N.95a0.05V2（PO4）3/Cexhibits the best electrochemical performance amongst all Li3_xNaxV2（P04）3/Ccomposites. In the voltage range of3.0to4.8V，Li2PO.95Na0.05V2（4）3/C displays initial di1scharge capacities of187and173mAh"-g at0.2C and1C，respectively, and thecorresponding capacity retentions are95%and91%, respectively, atfer30cycles.While Li3V2（PC>4）3/C exhibits lower initial discharge capacities of175and153mAh"-1g at0.2C and1C，respectively, and shows the corresponding capacityretentions of90%and87%，respectively, atfer30cycles. The Li+diffusioncoeiffcients for Li2C.95Na0.05V2（PO4）3/C and Li₃V₂（P0₄）₃/were determined bygalvanostatic intermittent titration technique （GITT） measurement. The GITT resultsreveal that the Li+diiffision coeiffcient of Li2N>.95ao.o5V2（PC4）3/C is much higher thanthat of Li₃V₂（P0₄）₃/C, indicating that a small amount of doped Na+in Li sites ofLi3V2（PC>4）3could signiifcantly improve the Li+diffusion coeiffcient and hence toenhance the electrochemical performance of Li3V2（PC>4）3.Li3_xNaxV2（P04）3（x=0,0.03，0.05and0.07） cathode materials were synthesizedby a solid state method using hydrogen as a reductant and Li2N.95a0.05V2（PO4）3displays the best electrochemical performance amongst all Li3_xNaxV2（P04）3cathodematerials. Based on the above results, V2O3coated Li2.95Na0.05V2（PO4）3compositeswere prepared by calcination of the mixture of Li2VOd NH.95Na0.052（P4）3an4VO3athigh temperature, and the effects of V2O3content in Li2.95Na0V.052（PO4）3Ar2O3composite on the electrochemical performance of Li2PO.95Na0.05V2（4）3Ar2O3werestudied. The results show that the electrochemical performances ofLi2PO.95Na0.05V2（4）3/V2O3composites are much higher than that ofLi2.95Na0V.052（PO4）3in the voltage range of3.0to4.3V and the optimal weight ratioof V2O3to Li2N.95Na0V.052（PO4）3is5%. This optimal Li2.95a0/V.05V2（PO4）32O3composite displays initial discharge capacity of125mAh"-1g at0.2C and capacityretention of99%atfer30cycles. While Li2Vit.95Na0.052（PO4）3exhibs much loweri1nitial discharge capacity of110mAh"-g at0.2C and capacity retention of91%atfer30cycles. The Li2Nth hi.95a0.05V2（PO4）3coaed by V2O3, which is of mucgher densitythan carbon, results in the tap density of Li2N.95a0.05V2（PO4）3/V2O3much higher thanthat of Li2POit.95Na0.05V2（4）3/C compose.