## 4 Conclusions
**Supplementary Materials:** The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app12052327/s1](https://www.mdpi.com/article/10.3390/app12052327/s1)]([https://www.mdpi.com/article/10.3390/app12052327/s1](https://www.mdpi.com/article/10.3390/app12052327/s1)), Figure S1: SEM of WO31 omp and WO31 up400 thin films on FTO; Figure S2: GLXRD patterns of WO31 omp, WO31 up400, WO31 mp and WO31 up thin films calcined at different temperatures; Figure S3: Normalized Raman spectra of thin films; Figures S4-S7: XPS data for WO31 omp, WO31 up400, WO31 mp and WO31 up thin films; Figure S8: Amount of W in different oxidation states in the thin films obtained by XPS; Figure S9: BET plots of thin films; Figure S10: Cyclic voltammograms for thin films of WO31 omp, WO31 up400, WO31 up600 and WO31 mp; Figure S11: Transmittance spectra of WO31 up400, WO31 comp, WO31 mp and commWO3 thin films measured in contact to LiClO4 in PC; Table S1: Effective diffusion coefficient \(D_{\mathrm{a,CV}}\) determined from CV, transmittance modulation \(\Delta T_{\mathrm{710 nm}}\) and coloration efficiency \(CE_{\mathrm{710 nm}}\) obtained from CV of the films prepared with or without PEG 600 and annealed at different temperatures; Figure S12: Optical transmittance at 710 nm of WO31 up400, WO31 up600, WO31 mp and commWO3 in contact to LiClO4 in PC, recorded during chronomomperometry for 30 min; Figure S13: Optical transmittance at 710 nm of WO31 mp, WO31 up400 and WO31 up in contact to LiClO4 in PC, recorded during chronomomperometry for more than two hours; Figure S14: Changes of transmittance spectra of WO31 comp and WO31 up600 after subsequent switching cycles; Figure S15: Transmittance spectra of WO31 comp and WO31 up600 at different applied potentials; Figure S16: W 4f XPS spectra of intercalated thin films; Figure S17: Depth profiles of the Li- signal of thin films intercalated for different time spans obtained from ToF-SIMS analysis; Figure S18: Transmittance spectra of WO31 comp intercalated for various time spans; Table S2: Fitting parameters and results of impedance analysis. **Author Contributions:** Conceptualization, T.H.Q.N., D.S. and B.M.S. ; Methodology, T.H.Q.N. and P.C. ; Validation, T.H.Q.N., D.S. and B.M.S. ; Formal Analysis, T.H.Q.N. and D.S., Investigation, T.H.Q.N., F.E., S.G., P.C., M.E., T.P.S. and L.G., Resources, D.S. ; Data Curation, T.H.Q.N., F.E., S.G., P.C., M.E., T.P.S. and L.G. ; Writing--Original Draft Preparation, T.H.Q.N. ; Writing--Review & Editing, T.H.Q.N., D.S. and B.M.S. ; Visualization, T.H.Q.N. ; Supervision, D.S. ; Project Administration, D.S. ; Funding Acquisition, D.S. and B.M.S. All authors have read and agreed to the published version of the manuscript. **Funding:** This research was funded by the Deutsche Forschungsgemeinschaft (DFG) via the GRK 2204 \"Substitute Materials for sustainable Energy Technologies\". **Institutional Review Board Statement:** Not applicable. **Informed Consent Statement:** Not applicable. **Data Availability Statement:** The authors confirm that the data supporting the findings of this study are available within the article and its supplementary materials. **Acknowledgments:** The authors are grateful to [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] (all at ZfM, JLU Giessen) and [PERSON] (Sorbonne Universite Paris) for fruitful discussions and valuable assistance in the one or the other experiment. **Conflicts of Interest:** The authors declare no conflict of interest.