Li insertion into WO 3: introduction of a new electrochemical analysis method and comparison with impedance spectroscopy and the galvanostatic intermittent titration technique

Abstract

The chemical diffusion coefficient, D ̃ , of Li in sputtered disordered and crystalline WO 3 samples has been studied using electrochemical impedance spectroscopy, the galvanostatic intermittent titration technique, and a new electrochemical analysis method based on chronoamperometry. The new method is founded on a theory [G. Greeuw, B.J. Hoenders, J. Appl. Phys. 55 (1984) 3371] developed to account for the release of alkali ions from interface traps in a metal-oxide–silicon structure. D ̃ was found to drop about two orders of magnitude in the regions where crystalline WO 3 changes structure. Outside these regions, D ̃ remains constant at about 10 −12 cm 2/s. This behavior of D ̃ could be explained within thermodynamics by the net force between intercalated ions being attractive in the regions were structural changes take place, while this force is close to zero in other composition regions. For disordered WO 3, D ̃ was almost potential independent. The new electrochemical method was shown to be a very good tool for investigating materials to be used in applications were the electrochemical system is far from equilibrium, such as in electrochromic devices. The method could model the transversal of Li ions from the electrolyte into the WO 3 intercalation host, and an effective Li chemical diffusion coefficient relevant for device applications could be obtained.