Study On Performance Of Lithium-ion Capacitor Using Graphite As Negative Electrode

Lithium-ion capacitor (LIC), a new electrochemical storage device, has become aresearch hotspot in recent years. LIC can deliver high energy in much short time thanlithium-ion battery (LIB) and store more energy than electric double layer capacitor(EDLC). In this paper we used direct and indirect method to pre-dope Li+into LICusing graphitization mesophase microbeads (MCMB) as negative electrode, activedcarbon (AC) as positive electrode, and researched the effects of preliminary Li+intercalation on the capacitors.The direct pre-dope Li+method was that getting the pre-lithium intercalationgraphite (LMCMB) by handling of lithium intercalation into MCMB battery, thenfabricated LIC with AC positive electrode. Using LMCMB electrode as negativeelectrode, a stable working potential is obtained at lower voltage, and the potentialrange of the positive electrode is extended to a lower range, energy densities as highas97.9Wh/kg were obtained. The indirect pre-doping Li+method was that using(AC+LiFePO4) hybrid materials for the positive electrode, and then fabricated LICwith MCMB electrode. In small current density, the LIC had both capacitancecharacteristics and the characteristics of the battery. The lithium-rich compoundsLiFePO4compensated lithium source and improved the LIC cycle performance. Atthe rate of4C, energy density of capacitor was obtained as high as69.02Wh/kg.Mild expanded graphite (e-MCMB) was prepared by oxidation andheat-treatment of MCMB. The charge cut-off voltage of MCMB/e-MCMB was ashigh as4.6V. Using e-MCMB materials instead of traditional activated carbon as thepositive electrode, LIC would obtain higher energy density. The internal structure ofe-MCMB materials was the small size of graphite crystallites and disordered structurecoexistence, the average interlayer spacing d002was0.411-0.418nm. There was"electrochemical activation" phenomenon in the first charging process of e-MCMB atabout4.5-4.8V voltage range.