New Study On Modified Lithium Iron Phosphate Cathode Materials For Lithium Ion Batteries

Lithium iron phosphate?LiFePO₄?has been recognized as a promising candidate for cathode material of lithium secondary batteries,because of its high theoretical capacity of 170 mAh g^-1 low cost,cycling stability and environmental compatibility.Recently,many researchers have been conducted to improve the properties of LiFePO₄.In this paper,the further studies on modified LiFePO₄ using new carbon materials have been done.Firstly,LiFePO₄/phenolic resin nanocomposite particles with diameter of 50-80 nm have been synthesized successfully by in-situ polymerization depending on the theory of miniemulsion polymerization.Polyacenic semiconductor?PAS?from the phrolysis of phenolic resin formed good networks on the surface of particles.This nanocomposite cathode materials revealed excellent electrochemical properties,which include stable discharge plateau at 3.4 V,high specific capacity of 160 mAh·g^-1?95%of the theoretical values?at 0.2C.The discharge capacity of LiFePO₄/PAS is about 160,146,136,120 and 109 mAh·g^-1 at 0.2,1.0,2.0,5.0 and 10C,respectively.After 80 cycles under various conditions,the test cells deliver about 97%of initial capacity.These results are due to the good conductivity of PAS and the excellent conductive networks formed among particles.Secondly,graphene modified LiFePO₄ materials were further studied.The graphene and its ramifications as new kinds of carbon materials can be applied to electrode materials because of their remarkable properties,such as high conductivity,specific surface area(2630 m2·g^-1)and so on.Graphene was synthesized from natural graphite powders by Hummers method.The FTIR,XRD and SEM tests proved that the graphite has been exfoliated excellently.LiFePO₄/graphene composite materials were prepared by co-precipitation method.Different from the above method of preparing graphene,in this part,oxidation time was extended to 120 h.TEM images presented the good dispersion of LiFePO₄ particles into the network of graphene.Also,LiFePO₄/graphene composites have good electrochemical properties.This kind of material showed flat discharge plateau and good rate performance.At the range of 2.5-4.2 V,the discharge capacities are 155 mAh·g^-1,140 mAh·g^-1,122 mAh·g^-1 at0.1C,1C,5C respectively with only 1.5 wt%of graphene.Depending on the good properties of graphene which have been mentioned above,the electrochemical properties of LiFePO₄/graphene should be better than that of this experiment.This result mainly results from the oxygen-containing groups even though after reduction and the defects on the graphene surface from the process of deep oxidation.