Electrochemical Doping

Electrochemical intercalation of Li ions changes the electronic, magnetic and structural properties of VOx-NT. Interestingly, it was found that ferromagnetism evolves upon doping making VOx-NT a self-assembled nanoscaled ferromagnet [1].
Our efforts to electrochemically treat VOx-NT apply a Swagelok-type cell, which contains positive and negative current collectors made by stainless still or aluminum. The cells were assembled in an argon-filled dry box with vanadium oxides nanotubes used as a positive electrode, separated by the negative one by two sheets of Whatman GF/D borosilicate glass fiber disks saturated with electrolyte (Merck, LP 30).
In the case of Li ion batteries the electrolyte used is a liquid solution of a salt (LiPF₆ or LiClO₄) in a organic solvent mixture (Etylene carbonate (EC) and Dimethyl Carbonate (DMC) for LiPF₆ or Propylene Carbonate (PC) for LiClO₄). Positive electrodes were prepared by mixing 80 wt % of vanadium oxide nanotubes with 20 wt % SP Carbon, as electronically conductive additive.
The discharge mechanism of vanadium oxide nanotubes in order to obtain the final product Li_xVO-NT is presented in the following picture. The process is based on a redox reaction, this means transfer of electrons between the Li metal and vanadium oxide nanotubes. The discharge process involves the dissolution of lithium ions at the anode, their migration across the electrolyte and their insertion within the crystal structure of the host compound, while the compensating electrons travel in the external circuit to be injected into the electronic band structure of the same host.

The Galvanostatic Intermittent Titration Technique (GITT- Weppner and Huggins, 1977) and Potentiostatic Intermittent Titration Techinuque (PITT- Wen et al., 1979) methods are standard experimental techniques used for measuring Li transport in Li batteries. In the case of GITT we obtained successive charge increments by applying from a galvanostat a constant current for a given time, then switching to open circuit for determine the corresponding equilibrium potential, and measure the potential as a function of time. In the case of PITT we impose to the system using a potentiostat a potential equal to initial equilibrium potential, and we measure the current as a function of time.