In situ hard X-ray photoelectron spectroscopy on the origin of irreversibility in electrochromic LixWO3 thin films

Highlights: • In situ hard X-ray photoelectron spectroscopy was performed to investigate the origin of irreversibility in electrochromic Li_xWO₃ thin films. • The appearance of irreversibility was observed in Li_xWO₃ during operation. • Reversible/irreversible components generated during operation were distinguished and evaluated quantitatively. Three kinds of components are known in the insertion of Li⁺ into electrochromic WO₃ (Li_xWO₃): (i) reversible Li⁺, (ii) irreversible Li₂WO₄ formation and (iii) irreversible Li⁺ trapping. To develop Li_xWO₃-based electrochromic devices, in situ hard X-ray photoelectron spectroscopy (HAXPES), in situ Raman spectroscopy and electrochemical measurements were performed. In situ HAXPES can quantitatively distinguish (i) reversible Li⁺ and (iii) irreversible Li⁺ trapping as the variation in the oxidation state of the W ion whereas the electrochemical measurements can evaluate (i) reversible Li⁺ and the sum of (ii) irreversible Li₂WO₄ formation and (iii) irreversible Li⁺ trapping. Furthermore, in situ Raman spectroscopy detected the enhancement of crystallinity due to Li⁺ insertion with high sensitivity. The combination of the in situ HAXPES and the electrochemical measurements enables the separation and quantitative evaluation of (i), (ii) and (iii). The inserted Li⁺ conversion ratios of (i) reversible Li⁺, (ii) irreversible Li₂WO₄ formation and (iii) irreversible Li⁺ trapping to the entire inserted Li⁺ were clarified. (i.e., 41.4 %, 50.9 %, and 7.7 %, respectively).