Effects of local Joule heating on the reduction of contact resistance between carbon nanotubes and metal electrodes

We report here a practical application of known local Joule heating processes to reduce the contact resistance between carbon nanotubes and metallic electrical contacts. The results presented in this study were obtained from a series of systematic Joule heating experiments on 289 single-walled carbon nanotubes (SWCNTs) and 107 multiwalled carbon nanotubes (MWCNTs). Our experimental results demonstrate that the Joule heating process decreases the contact resistances of SWCNTs and MWCNTs to 70.4% and 77.9% of their initial resistances, respectively. The I-V characteristics of metallic nanotubes become more linear and eventually become independent of the gate voltages (Vgs). For semiconducting nanotubes, the contact resistance has a similar decreasing tendency but the dependency of source-drain current (Ids) on Vgs does not change with the Joule heating process. This suggests that the reduction of the contact resistance and the decrease of the transport potential barrier are largely attributed to the thermal-energy-induced desorption of adsorbates such as water and oxygen molecules from the nanotube surface and the interface region, as well as thermal-energy-enhanced bonding between the nanotubes and electrode surfaces. In comparison to several other methods including rapid thermal annealing, e-beam lithography patterning of the top metal layer, and focused ion beam induced metal deposition of the top layer, the Joule heating process not only effectively reduces the contact resistance but also simultaneously measures the resistance and monitors the change in the transport potential barrier at the interface region

Preparation and characterization of LiNi0.5Co 0.5O2 and LiNi0.5Co0.4Al 0.1O2 by molten salt synthesis for Li ion batteries

10.1021/jp0676890Journal of Physical Chemistry C1113111712-1172

Facile conversion of commercial coarse-type LiCoO2 to nanocomposite-separated nanolayer architectures as a way for electrode performance enhancement

Coarse-type LiCoO2 is the state-of-the-art cathode material in small-scale lithium-ion batteries (LIBs); however, poor rate performance and cycling stability limit its large-scale applications. Here we report the modification of coarse-type LiCoO2 (LCO) with nanosized lithium lanthanum titanate (Li3xLa2/3–xTiO3, LLTO) through a facile sol–gel process, the electrochemical performance of commercial LiCoO2 is improved effectively, in particular at high rates. The crystalline structure of pristine LiCoO2 is not affected by the introduction of the LLTO phase, while nanosized LLTO particles are likely incorporated into the space of the LiCoO2 layers to form a LCO-LLTO nanocomposite, which separate the LCO layers with the increase of layer spacing to ∼100 nm. The LLTO incorporation through the facile post-treatment effectively reduces the charge-transfer resistance and increases the electrode reactions; consequently, the LLTO-incorporated LCO electrode shows higher capacity than LiCoO2 at a higher rate and prolonging cycling stability in both potential ranges of 2.7–4.2 V and 2.7–4.5 V, making it also suitable for high-rate operation. This novel concept is general, which may also be applicable to other electrode materials. It thus introduces a new way for the development of high rate-performance electrodes for LIBs for large scale applications such as electric vehicles and electrochemical energy storage for smart grids

Nonrigid Band Behavior of the Electronic Structure of LiCoO2 Thin Film during Electrochemical Li Deintercalation

In this study, a comprehensive experimental in situ analysis of the evolution of the occupied and unoccupied density of states as a function of the charging state of the Lix≤1CoO2 films has been done by using synchrotron X-ray photoelectron spectroscopy (SXPS), X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and O K- and Co L3,2-edges XANES. Our experimental data demonstrate the change of the Fermi level position and the Co3d–O2p hybridization under the Li+ removal and provide the evidence for the involvement of the oxygen states in the charge compensation. Thus, the rigid band model fails to describe the observed changes of the electronic structure. The Co site is involved in a Co3+ → Co4+ oxidation at the period of the Li deintercalation (x ∼ 0.5), while the electronic configuration at the oxygen site is stable up to 4.2 V. Further lowering of the Fermi level promoted by Li+ extraction leads to a deviation of the electronic density of states due to structural distortions, and the top of the O2p bands overlaps the Co3d state which is accompanied by a hole transfer to the O2p states. The intrinsic voltage limit of LiCoO2 has been determined, and the energy band diagram of Lix≤1CoO2 vs the evolution of the Fermi level has been built. It was concluded that LixCoO2 cannot be stabilized at the deep Li deintercalation even with chemically compatible solid electrolytes