A comparative study of electrochemical properties of two kinds of carbon nanotubes as anode materials for lithium ion batteries

Two kinds of carbon nanotubes (CNTs), i.e., short carbon nanotubes (CNTs-1) synthesized by co-pyrolysis method and long carbon nanotubes (CNTs-2) produced using common CVD technique were comparatively investigated as anode materials for lithium ion batteries via transmission electron microscope (TEM), high-resolution TEM and a variety of electrochemical testing techniques. The test results showed that the reversible capacities of CNTs-1 electrode were 266 and 170 mAh g−1 at the current densities of 0.2 and 0.8 mA cm−2, respectively, which were almost twice those of CNTs-2 electrode. The larger voltage hysteresis in CNTs-2 electrode was not only related to the surface functional groups on CNTs, but also to the surface resistance of CNTs, which results in greater hindrance and higher overvoltage during lithium extraction from electrode. The kinetics properties of these two CNTs electrodes were compared by AC impedance measurements. It was found that, both the surface film and charge-transfer resistances of CNTs-1 were significantly lower than those of CNTs-2; the lithium diffusion coefficient (DLi) of both CNTs electrodes decreased as the drop of voltage, but the magnitude of the DLi variation of CNTs-1 electrode was smaller than that of CNTs-2 electrode, indicating CNTs-1 exhibited higher electrochemical activity and more favorable kinetic properties during charge and discharge process.