Alternating current impedance and Raman spectroscopic study on electrochromic a-WO3 films

The chemical diffusion of lithium ions in $\mathrm{a-}{\mathrm{Li}}_x{\mathrm{WO}}_{\mathrm{3}}$ films is investigated using alternating current impedance spectroscopy and Raman scattering measurements. The diffusion coefficients increase with increasing $x$ in $\mathrm{a-}{\mathrm{Li}}_x{\mathrm{WO}}_{\mathrm{3}}$ up to $\text{x=0.072}$ and then decrease. Raman measurements show that the ${\mathrm{W}}^{\mathrm{6+}}{\mathrm{=O/O–W}}^{\mathrm{6+}}\mathrm{–O}$ ratio also increases at the early stage of lithium insertion and then decreases with further lithium insertion. We conclude that the diffusion kinetics of lithium ions in $\mathrm{a-}{\mathrm{Li}}_x{\mathrm{WO}}_{\mathrm{3}}$ films is very closely related to the ${\mathrm{W}}^{\mathrm{6+}}{\mathrm{=O/O–W}}^{\mathrm{6+}}\mathrm{–O}$ ratio.