  Using a combination of experimental techniques and density functional theory
(DFT) calculations, the influence of lithium insertion on the electronic and
electrochemical properties of fluourine-doped SnO$_2$ (FTO) is assessed. For
this purpose, we investigate the electrochromic behavior of commercial FTO
electrode embedded in a solution of lithium perclorate (LiClO$_4$). The
electrochromic properties are evaluated by UV-VIS spectroscopy, cyclic
voltammetry, and chronoamperometry. These tests show that FTO exhibits
electrochromism with a respectable coloration efficiency ($\eta=47.9$ cm$^2$/C
@ 637 nm). DFT study indicates that lithium remains ionized in the lattice,
raising the Fermi level about 0.7 eV deep into the conduction band. X-ray
photoelectron spectroscopy (XPS) is used to study chemical bonding and
oxidation states. XPS analysis of the Sn 3d core levels reveals that lithium
insertion in FTO induces a shift of 350 meV in the Sn 3d states suggesting that
lithium is incorporated into the SnO$_2$ lattice.
