Fig. 4: Various states of WO₃ during Li ion intercalation, trapping and detrapping.

a Schematic illustration of the formation of independent W⁵⁺ and W⁴⁺, coupled W⁴⁺-Li₂WO₄ and coupled W⁴⁺-Li₂O in amorphous WO₃ host matrix. The arrows illustrate the possible migrate paths for oxygen. The white cubes, denote as V_o⁰, indicate the oxygen vacancies. b In-situ optical transmittance during the ion trapping and detrapping process. The current density profile upon potentiostatic detrapping shows a peak when the sharp increase of the optical transmittance occurs. Insets mark the 5 selected points to be tested by XPS and Raman spectroscopy. c XPS results for the selected points as well as during potentiostatic ion insertion. Yellow circles represent the raw data. Purple, blue and black lines, with associated shaded regions, stand for the signals from W⁶⁺, W⁵⁺ and W⁴⁺, respectively. d Proportions of each W state of the selected points. e Raman results for the selected points as well as during potentiostatic ion insertion. Indium tin oxide, denotes as ITO, served as the substrate for film deposition. The two yellow dash lines represent the Raman signals from ITO; the red, purple and blue dash lines indicate the peak position for W⁺⁴-O stretching, Li₂WO₄ vibrating and W⁺⁶ = O stretching, respectively. Shaded green and yellow regions illustrate the W⁺⁵-O and W⁺⁵ = O, as well as W⁺⁶-O stretching.