4.7 Article

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

Journal

APPLIED SURFACE SCIENCE
Volume 568, Issue -, Pages -

Publisher

ELSEVIER
DOI: 10.1016/j.apsusc.2021.150898

Keywords

LixWO3; Hard X-ray photoelectron spectroscopy; Li+ trapping; Li+ solid electrolyte; Li+ insertion

Funding

  1. JSPS KAKENHI [JP20H05301, JP19K05279, JP19J22244]

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The combination of in situ HAXPES and electrochemical measurements allows for the separation and quantitative evaluation of reversible Li+, irreversible Li2WO4 formation, and irreversible Li+ trapping. The inserted Li+ conversion ratios of reversible Li+, irreversible Li2WO4 formation, and irreversible Li+ trapping were clarified to be 41.4%, 50.9%, and 7.7%, respectively.
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).

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