Multi-walled carbon nanotubes functionalized by carboxylic groups: Activation of TiO2 (anatase) and phosphate olivines (LiMnPO4; LiFePO4) for electrochemical Li-storage

Multi-walled carbon nanotubes functionalized by carboxylic groups, exhibit better affinity towards TiO2 (P90, Degussa) as compared to that of pristine nanotubes. Also the electrochemical performance of TiO2 is improved by nanotube networking, but the Li-storage capacity of TiO2 is unchanged. Whereas the composite of TiO2 with non-functionalized nanotubes demonstrates simple superposition of the behavior of pure components, the composite with functionalized nanotubes shows unique faradaic pseudocapacitance which is specific for this composite only. The surface functionalization of nanotubes enhances charge storage capacity and reversibility of a composite with LiMnPO4 (olivine), but mediates also the electrolyte breakdown at potentials >4.2 V. Whereas the electrochemical activation of LiMnPO4 (olivine) by functionalized nanotubes is quite modest, excellent performance was found for LiFePO4 (olivine) in composite materials containing only 2 wt% of functionalized nanotubes.