Interlayer spacing expansion for V₂O₅ towards ultra-stable zinc anode-based flexible electrochromic displays in Zn²⁺/Li⁺-PC organic electrolyte

Abstract

To enhance the electrochemical stability and kinetics of V₂O₅, 3,4-ethylenedioxythiophene (EDOT) was in situ polymerized within V₂O₅ interlayers, rendering PEDOT@V₂O₅ with enlarged layer spacing, increased stability in organic electrolyte, and superior electrochemical kinetics (bleaching/coloration times 10.9/9.1 s) and cyclability (98.5% ΔT and 85.6% capacity retention after 1000 cycles) in a PEDOT@V₂O₅‖Zn electrochromic device.

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With the rapid development of modern optoelectronics, electrochromic (EC) technology has made significant breakthroughs, exhibiting compelling potential for applications spanning non-emissive displays,^1,2 smart windows that offer energy saving in buildings via light and heat regulation,^3,4 integration with energy storage technology,^5,6 and so on.

The performance of electrochromic devices (ECDs) is highly dependent on the electrochromic electrodes. Compared with organic and molecular electrochromic materials, inorganic electrochromic materials have the advantages of simple preparation, low cost and long service life.^7,8 Transition metal oxides are state-of-the-art electrode materials in intercalation chemistry,⁵ and typical inorganic electrode materials for ECDs. Among them, V₂O₅ with a typical layered structure is considered as one of the most promising candidates for constructing ECDs,⁹ compared to other inorganic electrochromic materials, especially for EC displays due to its low cost, abundant reserves, and multi-color characteristics at different voltages.¹⁰ The inorganic multi-color EC V₂O₅ has harvested widespread attention for EC displays, yet still suffers from limited electrochemical stability and slow switching kinetics. In order to improve the electrochemical performance of V₂O₅, methods such as expanding the layer spacing,^11–15 morphology engineering, and designing new electrolyte systems^16–19 have been attempted. In addition, using salts with larger anions can alter the solvation structure of cations, and formulating electrolytes with metal salt additives can improve the dissolution equilibrium of the V element,^16,20 achieving a stabilized V₂O₅ electrode.

In this work, poly(3,4-ethylenedioxythiophene)@V₂O₅ composite nanofibers with an enlarged interlayer spacing were realized by in situ polymerization of 3,4-ethylenedioxythiophene (EDOT) within the V₂O₅ interlayers via an improved solid–liquid stirring method. Remarkably, the as-prepared PEDOT@V₂O₅ achieved enhanced structural stability, rapid switching times (t_b = 9.6 s, t_c = 9.2 s), and ultra-stable cycling performance (85.6% capacity retention and 98.5% optical contrast after 1000 cycles) in organic propylene carbonate (PC) solvent-based electrolytes, allowing the assembly of advanced Zn anode-based electrochromic devices (ZECDs). Our work sheds light on the crystal structural engineering and electrolyte formulation for inorganic electrode materials towards next generation EC displays.

As shown in Fig. 1a, the diffraction peaks of bare V₂O₅ correspond to the orthorhombic V₂O₅ phase (JCPDS No. 77-2418), while PEDOT@V₂O₅ was transformed into a more stable monoclinic phase.¹⁴ The 2θ diffraction angle of the (001) crystal plane was downshifted from 20.31° of V₂O₅ to 6.76° for 3-PEDOT@V₂O₅, indicating the enlarged interlayer spacing from 4.37 Å of V₂O₅ to 13.1 Å of 3-PEDOT@V₂O₅ according to Bragg's law (Fig. 1b and Formula S1, ESI†). The formation of PEDOT@V₂O₅ can be described by the Ostwald ripening mechanism as follows.²¹ Commercial V₂O₅ exhibits slight solubility in water, resulting in the production of vanadium oxide species. As the stirring time increases, the concentration of vanadium species gradually rises until all V₂O₅ is completely dissolved and reassembled into nanobelts. During this process, due to the high valence state of V⁵⁺, V₂O₅ itself acts as an oxidant. Consequently, under prolonged stirring conditions, EDOT slowly polymerizes through oxidation by V₂O₅ to form PEDOT layers inserted between the V₂O₅ layers.¹⁴ Remarkably, such an enlarged interlayer spacing is beneficial for facilitated ion transport and elevated electrochemical kinetics.²²

Fig. 1 Crystal structure comparison of bare V₂O₅ and PEDOT@V₂O₅. (a) XRD patterns of the as-prepared V₂O₅ and PEDOT@V₂O₅. (b) Schematic illustration of the V₂O₅ and PEDOT@V₂O₅ crystal structures.

The one-dimensional nanofiber morphology of PEDOT@V₂O₅ was captured by both scanning electron microscopy (SEM, Fig. 2a and b) and transmission electron microscopy (TEM, Fig. 2c and d). The high-resolution TEM image (Fig. 2e) also revealed the lattice spacing of 1.31 nm, corresponding to the (001) diffraction peak located at 6.86° in the XRD pattern (Fig. 1a).^23,24 Energy dispersive X-ray (EDX) spectroscopy mapping affirmed the presence and uniform distribution of S, O, and V elements within the PEDOT@V₂O₅ nanofibers (Fig. 2f). The above results indicate that PEDOT was successfully polymerized within the interlayer of V₂O₅, forming PEDOT@V₂O₅ composites.

Fig. 2 Morphological characterization of PEDOT@V₂O₅. (a) and (b) High magnification and low magnification SEM images of the PEDOT@V₂O₅ nanofibers. (c) and (d) High magnification and low magnification TEM images of the PEDOT@V₂O₅ nanofibers. (e) High-resolution TEM image of a PEDOT@V₂O₅ nanofiber depicting the lattice spacings. (f) Dark-field (DF) TEM image of PEDOT@V₂O₅ nanofibers and the corresponding elemental mappings of S, O, and V.

The chemical structure of PEDOT@V₂O₅ was further analyzed by Raman spectroscopy (Fig. S1a, ESI†). The peaks at 101, 165, 178, 264, 305, 406, 531, 704, and 986 cm⁻¹ correspond to characteristic vibrations of V₂O₅ crystals. PEDOT-related peaks include 1364 cm⁻¹ (C_β–C_β stretch), 1430 cm⁻¹ (symmetric C_α=C_β (–O) stretch), and 1557 cm⁻¹ (asymmetric C_α=C_β stretch).²⁵ The peak at 1100 cm⁻¹ is attributed to SiOH groups originating from the indium-doped tin oxide (ITO) glass substrate.²⁶ In a nutshell, the Raman spectra confirm the existence of PEDOT and V₂O₅, validating the formation of PEDOT@V₂O₅ composites. The chemical structure of PEDOT@V₂O₅ was further analyzed by Fourier transform infrared spectroscopy (Fig. S1b and c, ESI†). Fig. S1b (ESI†) shows the FTIR spectrum of commercial V₂O₅, where the peaks at 596 and 828 cm⁻¹ correspond to the V–O–V symmetric and asymmetric stretching modes and the peak at 1017 cm⁻¹ is associated with the V=O stretching vibration. In comparison, the position and shape of the vibration peak of PEDOT@V₂O₅ show obvious changes (Fig. S1c, ESI†), in which the V–O–V stretching mode and asymmetric stretching mode shift to 521 cm⁻¹ and 827 cm⁻¹, respectively, and the V=O stretching mode shifts to 978 cm⁻¹. This may be due to the presence of more V⁴⁺ centers in the nanocomposites. The peaks at 1522 and 1391 cm⁻¹ are caused by the aromatic C=C and C–C stretching in the thiophene ring. The peaks at 1203, 1140 and 1097 cm⁻¹ are assigned to C–O–C bond stretching. The peaks at 921, 770 and 689 cm⁻¹ are related to C–S bond stretching vibrations in the thiophene ring.¹⁴ The presence of PEDOT in PEDOT@V₂O₅ is thus confirmed via FTIR characterization.

Due to the tendency of EC cathode materials to dissolve in aqueous solvents and the inevitable side reactions with water, organic solvent PC was employed to address these challenges, due to their wide electrochemical stability window and formation of the stable chemical–electrochemical interface.²⁷ Zn(ClO₄)₂ (0.1 M) and LiClO₄ (0.8 M) (total positive charge of 1 M) were added to PC solvent and aqueous solvent respectively to obtain two types of hybrid electrolytes, denoted as Zn²⁺/Li⁺-PC and Zn²⁺/Li⁺-H₂O, respectively. Two identical PEDOT@V₂O₅ electrodes were soaked in these two hybrid electrolytes for several weeks, and their appearance and transmittance (T%) spectra variations were recorded (Fig. S2a and b, ESI†).

Due to the successful dissolution inhibition, the EC performance of PEDOT@V₂O₅ was then investigated in the Zn²⁺/Li⁺-PC hybrid electrolyte, with a two-electrode configuration where zinc foil was used as the anode (Fig. 3a) and a voltage window of 0.1–2.2 V. PEDOT@V₂O₅ with different amounts of PEDOT was prepared and bare V₂O₅ without EDOT was also evaluated for comparison. 3-PEDOT@V₂O₅ electrodes can operate stably with a pair of redox peaks at 0.41 V and 1.71 V in the cyclic voltammetry (CV) curve (Fig. 3b), which correspond to the Zn²⁺ insertion and extraction processes (as will be confirmed by X-ray photoelectron spectroscopy (XPS) analysis later). Subsequently, the T% spectra of the PEDOT@V₂O₅ electrode were tested under 0.1 V (reduced state) and 2.2 V (oxidized state) in the wavelength range of 400–800 nm. As shown in Fig. 3c, the 3-PEDOT@V₂O₅ electrode had a maximum optical contrast of 27.1% at a wavelength of 400 nm, compared to only 18.2% and 18.1% for 1-PEDOT@V₂O₅ and 5-PEDOT@V₂O₅ (Fig. S3a and b, ESI†). Digital photos of the electrode at 0.1 V, 2.2 V, and the intermediate state of 1.2 V were taken, corresponding to points A, C, and B in Fig. 3d, respectively. This Commission Internationale d'Eclairage (CIE) color coordinate shows the color transition of the PEDOT@V₂O₅ film between gray and olive. In addition, as shown in Fig. S4a (ESI†), at voltages of 0.1 and 2.2 V, the bare V₂O₅ electrode exhibits only 13.3% optical contrast at a wavelength of 400 nm and 15.7% at 800 nm.

Fig. 3 Electrochemical and EC properties of the PEDOT@V₂O₅ electrode in Zn²⁺/Li⁺-PC. (a) Schematic diagram of the Zn-PEDOT@V₂O₅ configuration. (b) Cyclic voltammograms (CV). (c) Transmittance spectra and corresponding digital photos at different applied voltages. (d) Two-dimensional CIE color coordinates corresponding to different applied voltages. (e) The dynamic T% test at 400 nm switched between 0.1–2.2 V. (f) CV cycling stability.

To investigate the EC mechanism of the PEDOT@V₂O₅ film, XPS measurements were conducted at different states (as-prepared, discharged to 0.1 V and charged to 2.2 V). The full survey spectrum (Fig. S5a, ESI†) and the high-resolution spectrum of Zn 2p (Fig. S5b, ESI†) indicate that, compared with the as-prepared electrode, zinc ions are inserted into the PEDOT@V₂O₅ film when reduced to 0.1 V and extracted when charged at 2.2 V. High-resolution XPS measurements of S, V and O were also conducted. As shown in Fig. S5c (ESI†), the peak near 164 eV coincides with S 2p and originates from the thiophene ring of PEDOT,²⁸ while the peak near 169 eV originates from oxidized S (S–O bond).²⁹ The high-resolution V 2p XPS spectra (Fig. S5d–f, ESI†) indicate two pairs of peaks; the peaks located at 517.2 and 524.6 eV are attributed to V⁵⁺, and the peaks centered at 516.2 and 523.2 eV correspond to V⁴⁺.³⁰ The atomic ratio of V⁵⁺/V⁴⁺ increases with increasing voltage between 0.1–2.2 V (Table S1, ESI†). High-resolution C 1s XPS spectra were also collected at different states (Fig. S5g–i, ESI†), and the results confirmed the presence of peaks attributed to C–C/C=C, C–S, and C–O bonds located near 284.5, 286, and 289 eV in each state. The XPS analysis has re-confirmed the successful preparation of PEDOT@V₂O₅, and the color change is due to the altered V valence states during the insertion/extraction of zinc ions, which is similar to other vanadium oxide or vanadate films.^16,31–33

The switching kinetics of the PEDOT@V₂O₅ electrode were quantified by an electrochemical workstation combined with a UV-visible-NIR spectrophotometer, to measure the dynamic T% changes (Fig. 3e). Due to the larger interlayer spacing of 3-PEDOT@V₂O₅ than V₂O₅ (Fig. 1b), sodium vanadium oxide (SVO)³³ and layered potassium vanadate (K₂V₆O₁₆·1.5H₂O, KVO),³¹ Zn²⁺ in the electrolyte can be more easily diffused in the electrodes, resulting in a fast response (9.6/9.2 s for bleaching and coloration). In comparison, the 1-PEDOT@V₂O₅ and 5-PEDOT@V₂O₅ showed prolonged response of 12.2/22.4 s and 22.1/23.4 s for bleaching and coloration (Fig. S3a and b, ESI†), and the V₂O₅ without PEDOT showed prolonged response of 31.7/18.2 s for bleaching and coloration (Fig. S4b, ESI†). Meanwhile, the coloration efficiency of 3-PEDOT@V₂O₅ is calculated to be 30.28 cm² C⁻¹, significantly higher than the 17.96 cm² C⁻¹ of the V₂O₅ electrode (Fig. S6a, ESI†).

The 3-PEDOT@V₂O₅ electrode also exhibits extraordinary cycling stability in the Zn²⁺/Li⁺-PC electrolyte, surpassing all reported Zn-vanadium-based EC systems in organic electrolyte (Table S2, ESI†). After 1000 CV cycles, the electrode surprisingly sustained a capacity retention of 85.6% (Fig. 3f), a ΔT retention of over 98.5% (Fig. S6b, ESI,† the ΔT at 400 nm is 26.7%), and the minimum change in response times (Fig. S6c, ESI,† the bleaching/coloration times are 10.9/9.1 s, respectively). The 3-PEDOT@V₂O₅ showed greatly enhanced performance compared to 1-PEDOT@V₂O₅ (Fig. S3c, ESI,† capacity retention of 74.3% after only 1000 CV cycles), 5-PEDOT@V₂O₅ (Fig. S3c, ESI,† capacity retention of 28.9% after only 1000 CV cycles), and bare V₂O₅ without PEDOT (Fig. S4c, ESI,† capacity retention of 76.7% after only 100 CV cycles). The electrochemical impedance spectroscopy (EIS) of 3-PEDOT@V₂O₅ thin films in a Zn²⁺/Li⁺-PC hybrid electrolyte system was investigated to understand the charge transfer process. As shown in Fig. S7 (ESI†), the Nyquist plots of V₂O₅ film and 3-PEDOT@V₂O₅ films are semicircular in both the high-frequency region and the middle-frequency region, and an oblique line in the low-frequency region. Compared to V₂O₅ film, the 3-PEDOT@V₂O₅ film has higher linear slope in the low-frequency region and smaller semicircle diameter in the mid-frequency region, which indicates the smaller ion diffusion impedance and smaller charge transfer resistance R_CT, thus showing superior EC performance.

The multi-color transition of PEDOT@V₂O₅ inspired the assembly of flexible Zn anode-based EC displays. To achieve color visualization while employing zinc foil as the anode or counter electrode,³⁴ a zinc frame was pasted onto an ITO-polyethylene terephthalate (PET) transparent conductive substrate, as shown in Fig. 4a. Polymethyl methacrylate (PMMA) was added to Zn²⁺/Li⁺-PC to obtain a gel electrolyte, which was injected between the 3-PEDOT@V₂O₅ cathode and zinc anode, allowing the assembly of a flexible Zn-PEDOT@V₂O₅ EC display. Subsequently, the dynamic changes in absorbance of the EC display in the 0.1 and 2.2 V states were tested (Fig. 4b), corresponding to points A and B in the CIE color coordinate map (Fig. 4c). At the same time, as shown in Fig. 4d and e, the digital photos of the EC display at 0.1 and 2.2 V were also captured. At 0.1 V, the pineapple patterns on the display are clearly visualized due to the darker color and higher absorbance of the patterns on the display, while the display exhibits augmented bleaching effects when changed to 2.2 V. As shown in Fig. 4f and g, the display also exhibits favorable flexibility.

Fig. 4 Zn-PEDOT@V₂O₅ flexible EC displays. (a) Schematic diagram. (b) The absorbance of the display at 0.1 and 2.2 V corresponds to points A and B in the (c) CIE color coordinate map, respectively. (d) and (e) Digital photos of the display at 0.1 and 2.2 V states. (f) and (g) Bending experiments of the flexible EC display.

In summary, this study demonstrates the successful in situ polymerization and intercalation of EDOT within the V₂O₅ interlayers, obtaining PEDOT@V₂O₅ nanofibers through improved simple solid–liquid stirring. The intercalation of PEDOT greatly broadens the interlayer spacing of commercial V₂O₅, significantly elevating the structural stability and electrochemical kinetics. In the meantime, a hybrid organic electrolyte system effectively inhibited the dissolution of the PEDOT@V₂O₅ cathode, allowing significantly ameliorated cycling stability (capacity retention of 85.6% after 1000 CV cycles) and notably enhanced switching kinetics (a ΔT retention of over 98.5%, the bleaching/coloring times are 10.9/9.1 s, respectively), thus enabling the successful assembly of a flexible Zn anode-based EC display. The advances in this study are expected to facilitate and accelerate the development of flexible Zn anode-based EC displays and multifunctional electronics.

Zhe Li: writing – original draft, visualization, data curation. Zhaoyang Song: data curation. Linhua Liu: data curation. William W, Yu: data curation, supervision. Jingwei Chen: writing – review & editing, supervision, methodology, investigation. Qianqian Zhu: writing – review & editing, validation, supervision, methodology, investigation, funding acquisition. Haizeng Li: writing – review & editing, supervision, investigation.

The authors acknowledge the support from the National Natural Science Foundation of China (52002196, 62105185, 62375157) and Natural Science Foundation of Shandong Province (ZR2020QF084).

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Notes and references

1. W. Zhang, H. Li and A. Y. Elezzabi, Adv. Funct. Mater., 2021, 32, 2108341.
2. H. Zhang and Y. Tian, Matter, 2023, 6, 2591–2594.
3. P. Lei, J. Wang, Y. Gao, C. Hu, S. Zhang, X. Tong, Z. Wang, Y. Gao and G. Cai, Nano-Micro Lett., 2023, 15, 34.
4. T. Bai, W. Li, G. Fu, Q. Zhang, K. Zhou and H. Wang, Sol. Energy Mater. Sol. Cells, 2023, 256, 112320.
5. Z. Tong, X. Zhu, H. Xu, Z. Li, S. Li, F. Xi, T. Kang, W. Ma and C. S. Lee, Adv. Funct. Mater., 2024, 202308989.
6. Q. Ma, J. Chen, H. Zhang, Y. Su, Y. Jiang and S. Dong, ACS Energy Lett., 2022, 8, 306–313.
7. X. Luo, R. Wan, Z. Zhang, M. Song, L. Yan, J. Xu, H. Yang and B. Lu, Adv. Sci., 2024, 202404679.
8. C. Gu, S. Wang, J. He, Y.-M. Zhang and S. X.-A. Zhang, Chem, 2023, 9, 2841–2854.
9. J. W. Chen, B. Xu, Y. X. Zhang, W. Zhang, H. L. Wang, A. Y. Elezzabi, L. H. Liu, W. W. Yu and H. Z. Li, Appl. Phys. Rev., 2024, 11, 0113116.
10. Z. Wang, X. Wang, S. Cong, J. Chen, H. Sun, Z. Chen, G. Song, F. Geng, Q. Chen and Z. Zhao, Nat. Commun., 2020, 11, 302.
11. C. Yin, C. Pan, X. Liao, Y. Pan and L. Yuan, ACS Appl. Mater. Interfaces, 2021, 13, 39347–39354.
12. B. Wang, F. Zhao, W. Zhang, C. Li, K. Hu, B. N. Carnio, L. Liu, W. W. Yu, A. Y. Elezzabi and H. Li, Small, 2023, 3, 2300046.
13. S. Liu, H. Zhu, B. Zhang, G. Li, H. Zhu, Y. Ren, H. Geng, Y. Yang, Q. Liu and C. C. Li, Adv. Mater., 2020, 32, 2001113.
14. H. Sun, W. Wang, Y. Xiong, Z. Jian and W. Chen, Chin. Chem. Lett., 2024, 35, 109213.
15. S. Bi, S. Wang, F. Yue, Z. Tie and Z. Niu, Nat. Commun., 2021, 12, 6991.
16. Z. Song, B. Wang, W. Zhang, Q. Zhu, A. Y. Elezzabi, L. Liu, W. W. Yu and H. Li, Nano-Micro Lett., 2023, 15, 229.
17. Y. Wang, Z. Wang, W. K. Pang, W. Lie, J. A. Yuwono, G. Liang, S. Liu, A. M. D. Angelo, J. Deng, Y. Fan, K. Davey, B. Li and Z. Guo, Nat. Commun., 2023, 14, 2720.
18. Q. Zhang, Y. Ma, Y. Lu, X. Zhou, L. Lin, L. Li, Z. Yan, Q. Zhao, K. Zhang and J. Chen, Angew. Chem., Int. Ed., 2021, 60, 23357–23364.
19. J. Cao, D. Zhang, X. Zhang, Z. Zeng, J. Qin and Y. Huang, Energy Environ. Sci., 2022, 15, 499–528.
20. F. Wan, L. Zhang, X. Dai, X. Wang, Z. Niu and J. Chen, Nat. Commun., 2018, 9, 1656.
21. C. X. Guo, K. Sun, J. Ouyang and X. Lu, Chem. Mater., 2015, 27, 5813–5819.
22. R. Sun, X. Ji, C. Luo, S. Hou, P. Hu, X. Pu, L. Cao, L. Mai and C. Wang, Small, 2020, 16, 2000741.
23. J. Zeng, Z. Zhang, X. Guo and G. Li, J. Mater. Chem. A, 2019, 7, 21079–21084.
24. S. Li, X. Wei, C. Wu, B. Zhang, S. Wu and Z. Lin, ACS Appl. Energy Mater., 2021, 4, 4208–4216.
25. L. Wang, T. Shu, S. Guo, Y. Lu, M. Li, J. Nzabahimana and X. Hu, Energy Storage Mater., 2020, 27, 150–158.
26. A. Alessi, S. Agnello, G. Buscarino and F. M. Gelardi, J. Non-Cryst. Solids, 2013, 362, 20–24.
27. M. Li, R. P. Hicks, Z. Chen, C. Luo, J. Guo, C. Wang and Y. Xu, Chem. Rev., 2023, 123, 1712–1773.
28. Z. Liu, J. Xu, R. Yue, T. Yang and L. Gao, Electrochim. Acta, 2016, 196, 1–12.
29. D. Wu, W. Zhang, Y. Feng and J. Ma, J. Mater. Chem. A, 2020, 8, 2618–2626.
30. G. Silversmit, D. Depla, H. Poelman, G. B. Marin and R. De Gryse, J. Electron Spectrosc. Relat. Phenom., 2004, 135, 167–175.
31. B. Wang, F. Zhao, W. Zhang, C. Li, K. Hu, B. N. Carnio, L. Liu, W. W. Yu, A. Y. Elezzabi and H. Li, Small, 2023, 3, 2300046.
32. W. Zhang, H. Li, M. Al-Hussein and A. Y. Elezzabi, Adv. Opt. Mater., 2020, 8, 1901224.
33. W. Zhang, H. Z. Li, W. W. Yu and A. Y. Elezzabi, Light: Sci. Appl., 2020, 9, 11.
34. W. C. Poh, X. Gong, F. Yu and P. S. Lee, Adv. Sci., 2021, 8, 2101944.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4cc04974h

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