LITHIUM INTERCALATION INTO WO3 AND THE PHASE-DIAGRAM OF LIXWO3

We report studies of the electrochemical intercalation of lithium in WO3 powder and in polycrystalline evaporated WO3 films. Electrochemical cells with beryllium x-ray windows were used to study the structure changes in the WO3 host in situ. Rietveld profile refinement was used to analyze the x-ray-diffraction patterns of Li(x)WO3 after correcting for the absorption of the beryllium window. Upon the intercalation of Li, the monoclinic structure of WO3 proceeds to higher and higher symmetry. First a first-order phase transition to a tetragonal phase with x approximately 0.1 in Li(x)WO3 occurs. Then another first-order phase transition to a cubic phase with x approximately 0.36 occurs. The cubic phase can accommodate further Li, up to a maximum of about x = 0.5. All the phase transitions can be reversed, although some hysteresis is observed; upon complete deintercalation, monoclinic WO3 is recovered. We report the structural changes, prove that the transitions are first order, and give a phase diagram for Li(x)WO3 as a function of x. Our results suggest that the formation of the tetragonal phase upon intercalation of Li in WO3 is responsible for the electrochromatic properties of crystalline Li(x)WO3, and that conventional interpretations that ignore the existence of this phase are most likely in error.