  A novel non-destructive methodology for operando observation of ion
intercalation and the state of charge on battery electrodes is presented based
on spectroscopic ellipsometry (SE). The potentiality of this technique for
performing time-resolved measurements of (de-)lithiation processes on electrode
materials has been demonstrated using thin film spinel LiMn$_2$O$_4$ as a
cathode for Li-ion batteries. The chemical diffusivity of Li+ ions in this
material has been determined by the time evolution of the Li insertion into the
film, and in the light of these results, it has been possible to relate the
controversial pseudo-capacitive behavior observed in this material to the
nanostructure of the layer. The results presented here establish a new way for
operando characterization of battery materials and devices providing a powerful
tool for the understanding of ion diffusion mechanisms in a collection of
electrochemical devices.
