Electrochemical characterization of carbon nanotube/nanohoneycomb diamond composite electrodes for a hybrid anode of Li-ion battery and super capacitor

Carbon nanotube/nanohoneycomb diamond (CNT-NANO) composite electrodes were fabricated by introducing multiwalled carbon nanotubes into the pores of nanohoneycomb diamond of 400 nm diam using the chemical vapor deposition method. The electrochemical behavior of these electrodes was examined with cyclic voltammetry, electrochemical impedance, and galvanostatic measurements in LiClO4/propylene carbonate electrolyte. The behavior of Li+ insertion into CNTs was observed in the cathodic sweep at -3.3 V (vs. Ag/Ag+) in CV. AC impedance measurements have indicated that at the nanohoneycomb diamond densely deposited CNTs (HD CNT-NANO), only the Li+ intercalation process was observed. In contrast, the nanohoneycomb diamond modified with CNTs in low-density (LD CNT-NANO) exhibited the combination behavior of Li+ intercalation at CNTs and the electrochemical double-layer discharging on the diamond surface. In galvanostatic measurements, HD CNT-NANO behaved as a pure Li+ ion battery anode, and the specific capacity (per 1 g of activated material) was found to be 894 mAh g(-1), which is higher than that obtained for mesophase carbon materials. For LD CNT-NANO, in the initial time following the start of discharging, the behavior of the double-layer discharging was observed in addition to Li+ deintercalation. Suppression of the potential drops associated with Li+ deintercalation by rapid discharging from the electrical double-layer could increase the specific power for LD CNT-NANO. The combination function of the super capacitor and the Li+-ion battery that work simultaneously supporting each other in one electrochemical cell suggests the possible realization of a hybrid electrode material with high energy density and high specific power. (C) 2004 The Electrochemical Society.