A Study On The Synthesis Of Phosphates As Cathode Materials By Glycine-nitrate Process

With the increasing applications of lithium ion batteries, the cathode materialsfor lithium ion battery are intensively researched. Most of commercial lithiumbatteries utilize LiCoO₂as the cathode material, but the high cost and pollutionalproperties of LiCoO₂prohibit its intensive use in large-scale applications. Recently,lithium transition metal phosphates were considered as one of the most potentialcathode materials for lithium ion batteries, due to its advantages such as the friendlyenvironment, low cost, stable structure, high safety and good electrochemicalperformance.In this study, two cathode materials Li₃V₂（PO₄）₃å'ŒLiVOPO4were synthesizedby glycine-nitrate process（GNP）, using glycine as a complexing agent and the fuel,nitrate as oxidizer, and sucrose as the carbon sources, first for the synthesis of productprecursor body, then for the final heat treatment products. By XRD, SEM and othermeans for the final product test analysis, and found that the purity of the product,morphology and electrical property has very big effect by the different heat treatmenttemperature, glycine content and adding amount of sucrose. Through the comparisonand the better synthesis technics were optimized.The Li₃V₂（PO₄）₃/C cathode material was synthesized by GNP using LiNO3,V2O5and NH4H2PO4as the starting materials, were dissolved in distilled water andheated with stirring on a hot plate to get a homogenous solution. Glycine solution andsucrose solution were added to the hot solution in turn and stirred further until a fluffymass was formed. According to XRD and SEM analysis found that the powder ofLi₃V₂（PO₄）₃/C can be synthesized in750℃with the inert gas protection and heatpreservation6h. When Gly: LVP=2.0:1, the content of sucrose was25%, powderswas comprised of ruleless flake or granular particles, grain size distribution is relatively uniform, and the thickness of the flake ranged about50-150nm, lengthranged about1-2μm. The material in the voltage of3.0-4.8V, the charge curve hasfour obvious platform, which located in3.6,3.7,4.1,4.5-4.8V near, the initial chargeand discharge capacity of the sample at0.5C was203.8mAh·g^-1and166.1mAh·g^-1respectively, the efficiency of charge and discharge is81.5%. The secondcharge-discharge curve are four platform, too, but the charge/discharge capacitydeclined obviously, and were122.0mAh·g^-1and113.2mAh·g^-1, respectively.The LiVOPO₄/C cathode material was synthesized by GNP method using LiNO3,V₂O₅and NH₄H₂PO₄as the starting materials, were dissolved in distilled water andheated with stirring on a hot plate to get a homogenous solution. Nitrate, glycinesolution and sucrose solution were added to the hot solution in turn and stirred furtheruntil a fluffy mass was formed. According to XRD and SEM analysis found that: thepowder of LiVOPO₄/C can be synthesized in800℃with the inert gas protection andheat preservation6h. When Gly: LVP=2.0:1, the content of sucrose was10%, theshape of samples was bar and at the end of the bar the thickness was0.3-0.5μm, thelength is about2μm. This shows that the introduction of a carbon effectively restrainthe sample size, particle size of the sample greatly decreased. But reunionphenomenon was serious, this will go against Li⁺diffusion, and need to add somedispersant in order to disperse products and improve the electrochemical properties ofmaterials.