The Synthesis And Electrochemical Performance Of Li₃V₂（PO₄）₃Cathode Material With NASICON Structure For Lithium Ion Batteries

The synthisis and modification of the Li₃V₂（PO₄）₃cathode material wasinvestaged in this paper. A novel complexometric titration sel-gel method was firstdeveloped to prepared Li₃V₂（PO₄）₃/C. The synthesizing mechanism of the precursorand the optimal synthesizing process of Li₃V₂（PO₄）₃/C were studied. Using Li₂CO₃,NH₄H₂PO₄and V₂O₅as raw materials, citric acid as carbon source and chelatingagent, we found that Li₃V₂（PO₄）₃/C with7.54wt%carbon content calcining at800°Cfor8h possessed the best electrochemical performance. The sample obtained at abovecondition had the monoclinic structure, small material particles size and uniformmorphology; at the rates of1C and5C, the sample delievered the discharge capacitiesof123.1mAh g^-1and117.0mAh g^-1respectively，and even at10C,15C and20C rates,the discharge capacities still kept at111.2mAh g^-1,100.1mAh g^-1and87.2mAh g^-1.Especially at10C and15C rates, the discharge capacities of111.1mAh g^-1and84.1mAh g^-1could be obtained after150cycles.In order to simplify the synthesizing process of the complexometric titrationsel-gel method, and to meet the requirements of large scale production, we haveameliorated the whole process to prepare high performance of Li₃V₂（PO₄）₃/C throughintroducing a fast sol-gel method based on spontaneous chemical reactions. Thismethod is low energy consumption, easy for operation and sutiable for large scaleproduction. The investigation results showed that the sample containg3.34wt%residual carbon via pre-calcining at350°C for4h and sintering at750°C for8h hadthe best electrochemical performance. After100cycles at1C and5C rates, theoptimal sample exhibited the discharge capacities of127.9mAh g^-1and124.0mAhg^-1respectively; besides, the prepared Li₃V₂（PO₄）₃/C composite showed excellenthigh rate capability, the discharge capacities of117.6mAh g^-1,114.9mAh g^-1and83.6mAh g^-1still kept at the rates of10C,15C and20C rates after150cycles. Weattributed the high performance of Li₃V₂（PO₄）₃/C to the carbon coating on the surfaceof the material and the loose porous microstructure.In addition, with the purpose of further improving the high rate cycle stability（especially at20C rate）, we first prepared the Mo-doped Li₃V₂（PO₄）₃/C cathodematerial, and the effects of Mo-doping on crystal structure, morphology and electrochemical performance of Li₃V₂（PO₄）₃/C had been carefully studied. The XRDpatterns of the doped samples showed that Mo-doping had no effects on NASICONstructure, SEM and electrochemical measurements proved that the morphology of theLi₃V₂（PO₄）₃/C had no obvious change after doping, but Li₃Mo_0.03V_1.97（PO₄）3/C hadthe smallest particle size, the most uniform morphology and the best electrochemicalperformance. A capacity of117.6mAh g^-1was delievered at the rate20C, and after150cycles, the discharge capacity of the Li₃Mo_0.03V_1.97（PO₄）₃/C was110.1mAh g^-1,which were much better that that of the un-doped one. The EIS measurement showedthat the electronic conductivity of the doped sample decreased, and the ionicconductivity increased, which illustrated that the solid state diffusion of lithium ionsis the rate-determining step of the extraction/insertion process.