Electrochromism in lithiated Sn oxide: Mössbauer spectroscopy data on valence state changes

Lithiated Sn oxide films, denoted LixSnO2, were produced by reactive rf magnetron sputtering of Sn and electrochemical post-treatment. Optical transmittance and Mössbauer spectra were recorded for progressively increased lithiation. Increasing x from zero to ∼0.1 did not have any significant effect on the optical data or Mössbauer spectra, and it is likely that the lithium is located in internal double layers in the film. Further increasing x from ∼0.1 to ∼0.2 yielded significant transmittance drops and Mössbauer spectra unambiguously showing a conversion Sn4+→Sn2+. Hence the optical absorption can be reconciled with intervalency transitions as in other cathodically coloring electrochromic oxides. Electrocrystallization appeared to dominate the electrochemistry at x≳0.2.