In order to understand the reason why polyiodide ions encapsulated in single-walled carbon nanotubes (SWCNTs) drastically improve the electric conductivity and the water dispersibility of SWCNTs at low temperature, we performed in situ Raman measurements of polyiodide ions encapsulated in three kinds of SWCNTs having different mean tube diameters under low temperature down to −100 °C. It was found that, for all the three samples, the Raman peak intensities of polyiodide ions increase and the G-band peak position of SWCNTs shifts toward the higher-wavenumber side with decreasing temperature. It means that the charge-transfer from SWCNTs to encapsulated iodine molecules increases with decreasing temperature and that hole-doping level of SWCNTs increases at low temperature. Furthermore, the peak profiles changed with temperature drastically for polyiodide ions encapsulated in the SWCNT sample having the largest mean tube diameter of 2.5 nm. This change indicates the structural transformation of polyiodide ions in SWCNTs. These experimental results can be explained by the promotion of the chain-like polyiodide ion formation at low temperature. It was discussed with the control experiment using amylose that the promotion of the polyiodide ion formation at low temperature is characteristic for iodine molecules encapsulated in SWCNTs.

We have performed Li and Na ion charge-discharge experiments of 9,10-phenanthrene quinone (PhQ) molecules encapsulated in single-walled carbon nanotubes (SWCNTs) with mean tube diameters of 1.5 and 2.5 nm at room temperature and also at low temperatures. The Na ion reversible capacity of PhQ encapsulated in the larger diameter SWCNTs, measured at a low temperature of 0 °C, remained as high as that measured at room temperature (RT), while the capacity of PhQ in the smaller diameter SWCNTs at 0 °C was about a half of that at RT. The diameter dependence of the capacity should be attributed to the difference in the interactions between the encapsulated PhQ molecules and the host SWCNTs, which was revealed by Raman peak profile analysis. Charge-transfer reaction from metallic tubes to PhQ molecules encapsulated in the smaller diameter SWCNTs was detected by Raman measurements. The electrostatic interaction between charged SWCNTs and PhQ molecules, induced by the charge-transfer reaction, would partly contribute to the stabilization of PhQ molecules in the smaller diameter SWCNTs, while only van der Waals interaction stabilizes PhQ molecules in the larger diameter SWCNTs. The difference in stability was confirmed by thermogravimetric, x-ray photoelectron spectroscopy, and Raman measurements. Charge-discharge curves of PhQ encapsulated in SWCNTs were also discussed based on the stability difference.

We developed a new concept aqueous electrolyte secondary battery by combining redox reactions of iodide ions in single-walled carbon nanotubes (SWCNTs) and alkali metal ions with quinone molecules. The new concept battery consisting only of abundant elements utilizes the fast ion movements of both the cation and anion in a safe aqueous electrolyte medium. As active quinone molecules, anthraquinone (AQ) and 9,10-phenanthrene quinone (PhQ) were used. AQ and PhQ molecules were inserted in SWCNTs because the electric conductivities of AQ and PhQ molecules are insufficient as battery electrode materials. Both redox reactions of the cation and anion of an alkali metal halide aqueous electrolyte were investigated by electrochemical measurements such as cyclic voltammetry and chronopotentiometry. The redox reaction of iodide ion was observed by chronopotentiometry as a very flat charge/discharge potential plateau at about 0.45 V vs. Ag/AgCl. On the other hand, PhQ redox reactions showed gradient potential plateau. It was also found that the discharge plateau potential of PhQ electrode measured with NaI aqueous electrolyte is about 0.1 V lower than that with LiI.

9,10-Anthraquinone and 9,10-phenanthrenequinone (PhQ) were grafted onto two kinds of single-walled carbon nanotube (SWCNT) samples having different mean tube diameters by diazo-coupling reactions. The structural details of PhQ-grafted SWCNT (PhQ/SWCNT) samples were analyzed by X-ray diffraction and Raman measurements. It was discussed that a few-nanometer-thick layer of polymerized PhQs covers the outside of SWCNT bundles. The obtained PhQ/SWCNT works very well as lithium-ion battery and sodium-ion battery electrodes, not only at room temperature but also at 0 °C. It should be noted that the cycle performance of the PhQ/SWCNT electrode is much better than that of PhQ encapsulated in SWCNT (PhQ@SWCNT). We also calculated molecular base reaction energies by density functional theory calculations to gain a qualitative insight into the observed discharge potentials of the PhQ/SWCNT electrode.

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