Preparation And Electrochemical Performance Of Graphene Coated Lithium Iron Phosphate Cathode

Lithium iron phosphate is an ideal cathode material for lithium ion battery. As a new generation of cathode material, it has gained widely attention and research for the advantages like good thermal stability, high theoretical capacity, good cycle performance and excellent environmental friendly. But the electronic conductivity of the material is extremely low, restricting its application in high-rate appliance like electric cars. To improve conductivity, this paper adopted different coating methods to synthesize LiFe PO₄/graphene. The structure and electrochemical properties of the composites were examined. The research contents and results are as followed:（1） The facile solid state method was adopted to synthesize LiFe PO₄/graphene/C composites. FeC2O₄, NH₄H₂PO₄, Li2CO3 were used as raw materials, glucose and graphene oxide were chosen as carbon sources. The optimal content of these two carbon sources was discussed. Research shows that the double coated composites delivered higher discharge capacity than sample solely coated with graphene. When the content of organic carbon source is 15 wt.%, graphene oxide content is 5 wt.%, the composites performed the best electrochemical performance. The current impedance test confirmed that the charge transfer impedance of the cathode decreased effectively.（2） In order to strengthen the contact of graphene and LiFe PO₄ grains, this paper applied the precipitation method, using FeSO₄·7H₂O, NH₄H₂PO₄ and graphene oxide as raw materials to synthesize FePO₄·xH₂O/GO precursor. After mixing with the Li source and followed by carbon thermal reduction, graphene tightly coated LiFe PO₄/graphene composites were obtained. The as-prepared particles had small particle sizes. When the rate was 0.1 C, the discharge specific capacity was up to 163.4 mAh/g, which was 96.11% of the theoretical capacity. As the rate raised to 1 C, 2 C, 5 C and 10 C, discharge specificcapacity were 150.9 mAh/g, 144.9 mAh/g, 130.8 mAh/g and 103.4 mAh/g, respectively.These results show that this preparation process is reliable to prepare LiFe PO₄ with excellent electrochemical performances.