Synthesis And Performances Of Porous Spherical LiFePO₄/C Cathode Materials Prepared By Coprecipitation-Spray Drying Mathods

Since the lithium iron phosphate（LiFePO₄）was reported in 1997,it has been considered to be one of the most potential cathode materials for secondary lithium ion battery,due to its good safety,rich raw material sources,high theoretical capacity and excellent cycle performance.Nevertheless,the practical application is seriously limited by the low electronic conductivity and Li-ion diffusion coefficient.Thus researchers developed several methods,such as carbon coating,ion doping and morphology optimization,to overcome these defects.In recent years,they proposed a combined synthesis method instead of a single way to improve the performance of the material.Herein,LiFePO₄/C composite materials with consistent morphology and refined particle size were synthesized by coprecipitation-spray drying method.The Li-ion diffusion coefficient was improved by morphology optimization resulted from combining the coprecipitation method and spray drying method.And electronic conductivity was enhanced by carbon coating.The main details as follows:（1）LiFePO₄/C cathode material was prepared via coprecipitation method with glucose as carbon source.We emphatically discussed the effect of ascorbic acid,coprecipitation process conditions and glucose on the phase morphology and electrochemical properties.Studies showed that the pure LiFePO₄/C was prepared under 700℃ for 10h,when the ratio of iron source and ascorbic acid was 4:1,iron source and glucose was 1:0.2.Electrochemical tests revealed that this sample had better electrochemical performance,for the initial discharge capacity was 140.8mAh/g at 0.1C.However,SEM results exhibited that the particle morphology was abnormity and particle size distribution was uneven causing by glucose.（2）Spherical LiFePO₄/C cathode material was synthesized coprecipitation-spray drying method.The problems of irregular particle morphology and particle size distribution caused by glucose were effectively relieved.Experimental results showed that the spray drying method could shorten the holding time,to reduce the energy consumption,effectively.When the holding time reduced to 2h,the uniform morphology LiFePO₄/C sample was obtained.The sample particles,stacked by smaller primary particles,appeared a small quantity of porosity,which increased the contact area between materials and the electrolyte,to improve the electrochemical performance.（3）We researched the porosity formation mechanism on surface of spherical particle via changing spray concentration.We also discussed the influence of porosity on electrochemical properties.Results showed the porous spherical LiFePO₄/C material was synthesized with concentration of 0.028mol/L,and sintering on 700℃ for 4h.Observed by SEM and TEM,the porous spherical particle was stacked by smaller primary particles,which coated by a layer of carbon.The electrochemical tests indicated that the porous spherical material showed better properties.The Li-ion diffusion coefficient was 1.065×10-11 cm2/s,and the initial discharge capacitie was 149mAh/g at 0.1C.After 50 cycles,the capacity retention was still 97.1%.