Study On The Performance Of Lithium Iron Phosphate/carbon Composite Electrode Materials And Their Supercapacitors

LiFePO₄/carbon ?LiFePO₄/C? composites were prepared with LiH2PO₄ and FeC6H5O7 as the precursors and the expanded graphite ?EG? and polyacrylonitrile?PAN?-based carbon fiber as the carrier by sol-gel in-situ composite method, and the Li-ion capacitors were assembled with LiFePO₄/C composite electrode materials as anodes. These were characterized by the methods of SEM, TEM, XRD, cyclic voltammetry, galvanostatic charge-discharge and electrochemical impedance spectroscopy. The influence of carbonization condition, the content of EG and the composite technology on the morphology, microstructure and electrochemistry performances of LiFePO₄/C composites was studied. The influence of the mass ratio of cathode to anode and the current density on the electrochemistry performance of the Li-ion capacitors was further studied.The result shows that when the EG as the carrier, the LiFePO₄ with the particle size of 100-200nm has been composited with EG successfully. The specific capacitance?Cpe? of the optimized LiFePO/C composites reached 326.2 F/g measured by the cyclic voltammetry method at the scanning rate of 5 mV/s when, the carbonization temperature is 550?, the carbonization time is 6 h and the EG content is 5wt.%. The Li-ion capacitors were assembled with LiFePO₄/C composites as the anode and active carbon as the cathode and 1 mol/L LiNO3 aqueous solution as the electrolyte. When the mass ratio of the cathode to anode was 2: 1 and the current density was 200 mA/g, the specific capacitance?Cps? of the Li-ion capacitors was 53.31 F/g.When the polyacrylonitrile ?PAN?-based carbon fiber as the carrier, the LiFePO₄/C composites were prepared by the method of forming firstly and then compositing of the carbon fiber have the best electrochemical performances. The Cpe of the optimized LiFePO₄/C composites reached 375.00 F/g when the carbonization temperature was 800?, the carbonization time was 8 h. When the mass ratio of the cathode to anode was 2: 1 and the current density was 200mA/g, the Cps of the Li-ion capacitors was 57.14 F/g. And when the current density varied from 200mA/g and 1000 mA/g the change of Cps, specific energy and specific power was studied.