Study On Preparation And Electrochemical Properties Of LiFe_0.98V_0.02Po₄/3DG/C Composites

As more and more fossil fuels are used,the deterioration of the natural environment has become increasingly serious.Therefore,the development of new energy batteries is very important.Lithium-ion battery is an extremely important new energy battery.Among them,olivine-type lithium iron phosphate?LiFePO₄?has the advantages of good cycle performance,high theoretical specific capacity,abundant source,low price and environmental friendliness.It is a very competitive cathode material in the power battery.However,its low electron conductivity(1 x 10^-10 S/m)and low Li⁺diffusion coefficient(1 x 10^-14 cm²/s)limit the practical application of lithium iron phosphate.In order to improve the electrochemical performance of lithium iron phosphate materials,hydrothermal synthesis,freeze-drying and two-step sintering techniques were used.Ascorbic acid and grapevine were used as carbon sources.Three-dimensional graphene?3DG?coating and vanadium doping were used.The material was modified to prepare a LiFePO₄ composite cathode material with high specific capacity,good cycle performance and high rate performance.Firstly,a spherical FePO₄/3DG precursor with good dispersibility,uniform morphology and particle size of about 1?m was prepared by hydrothermal and freeze-drying.Based on this,a chemical reagent such as LiOH,NH₄VO₃,and ascorbic acid were added,and after being uniformly ground by a wet method,it is subjected to high-temperature calcination in an argon atmosphere.After calcination at 350?for 5 h,it was naturally cooled,then glucose was added and uniformly ground by wet grinding.After undergoing secondary sintering at 650?and natural cooling,a LiFePO₄composite positive electrode material was prepared.In this paper,3DG-coated and V-doped are used to make full use of their respective advantages to achieve synergistic enhancement.3DG has higher mechanical strength,electrical conductivity and specific surface area than 2DG.The choice of 3DG coating can not only effectively enhance the electronic conductivity of the material,but also limit the growth of particles and increase the tap density of materials,thereby increasing the energy density of the material.The effect of V ion doping on the electrochemical performance of lithium iron phosphate material is mainly reflected in:low concentration doping can form V₂O₃ and VO₂ nanocrystals with high conductivity;when the doping concentration is above the critical concentration,it will generate electrochemical Active Li₃V₂?PO₄?₃ and promote the degree of graphitization of the coated carbon layer.The morphology and structure of the materials were characterized by XRD,Raman,XPS,SEM,EDS and TGA.The electrochemical properties of the materials were tested by means of first charge and discharge,cycle performance,cyclic voltammetry and electrochemical impedance.The results show that LiFe_0.98V_0.02PO₄/3DG/C is the best composite material.The initial discharge specific capacity of this material is up to 159.1 mAh/g at 0.2 C,and the capacity retention rate is 99.4%after100 cycles.The Li⁺diffusion coefficients of the oxidation peak and the reduction peak of LiFe_0.98V_0.02PO₄/3DG/C were calculated to be 8.555×10^-12 cm²/s and 6.689×10^-12cm²/s,respectively,and the charge transfer impedance was 113.6?.Therefore,the lithium iron phosphate composite cathode material prepared by using 3DG-coated and V-doped simultaneously with respect to lithium iron phosphate.Its electrochemical performance has been significantly improved,not only has a higher discharge specific capacity,but also has good rate performance and cycle performance.