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

Three kinds of components are known in the insertion of Li+ into electrochromic WO3 (LixWO3): (i) reversible Li+, (ii) irreversible Li2WO4 formation and (iii) irreversible Li+ trapping. To develop LixWO3-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).