In situ Raman study on single- and double-walled carbon nanotubes as a function of lithium insertion

We investigated the electrochemical lithium ion (Li⁺) insertion/desertion behavior on highly pure and bundled single- and double-walled carbon nanotubes (SWNTs and DWNTs) using an in situ Roman technique. In general, two storage sites could host Li⁺ in SWNT and DWNT bundles when varying an external potential: a) the outer surface sites, and b) the interstitial spaces within the bundles. The most sensitive changes in the tangential mode (TM) of the Raman spectra upon doping with Li⁺ can be divided into two regions. The first region was found from 2.8 to 1.0 V (the coverage of Li⁺ on the outer surface of a bundled nanotube) and was characterized by the loss of resonant conditions via partial charge transfer, where the G⁺ line of the SWNT and the TM of the outer tube of DWNTs experienced a highly depressed intensity, but remained almost constant in frequency. The appearance of a Breit-Wigner-Fano (BWF) profile provided strong evidence of metallic inner tubes within DWNTs. The second region was observed when the applied potentials ranged from 0.9 to 0 V and was characterized by Li⁺ diffusion into the interstitial sites of the bundled nanotube material. This phenomenon invoked a large downshift of the G^- band in SWNTs, and a small downshift of the TM of the inner tube of DWNTs caused by expansion of the C-C bonds due to the charge transferred to the nanotubes, and the disappearance of the BWF profile through the screening effect of the interstitial Li⁺ layers.