Lithium-Ion Capacitor with Three-Dimensional Porous HAC/SP/PVDF as Positive Electrode

Abstract

Activated carbon (AC) was acidified with dilute sulfuric acid (H2SO4) as an acidifying agent to obtain acidified activated carbon (HAC). The positive electrode was prepared by mixing HAC with a conductive agent (SP) or polyvinylidene fluoride. Carbon nanotubes and mesocarbon microbeads, as negative materials, were prelithiated and used as the negative eletrode. The positive electrode and negative electrode were assembled into the lithium ion capacitor. Materials and electrodes were characterized by scanning electron microscopy; elemental analysis, chemical bond analysis and specific surface area analysis of acidified activated carbon were made by x-ray energy spectrum analysis, an intelligent Fourier transform infrared spectrometer and specific surface space as well as a porosimetry analyzer (BET); The electrochemical capability of lithium ion capacitors was tested by constant current charge and discharge as well as electrochemical impedance. From the results, it can be concluded that the activated carbon acidified successfully grafted with hydroxyl (OH) and carboxyl (COOH) functional groups has increased the surface area by about 60% compared with AC. Under this situation, the charge and discharge current density is 50 mA/g. Also the capacitor has a mass ratio of 40.17 F/g, whose maximum power density is 0.98 kW/kg (700 mA/g) and maximum energy density is 52.34 Wh/kg (50 mA/g). Impedance tests showed that it has low impedance characteristics; the capacitance retention rate is above 60% after 2000 cycles of charge and discharge; the energy density can still reach 21.68 Wh/kg with a power density of 0.98 kW/kg. The electrochemical capability of the lithium ion capacitor was improved with acidified activated carbon as positive electrode.