Bifunctional urchin-like WO3@PANI electrodes for superior...

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Journal of Materials Science: Materials in Electronics https://doi.org/10.1007/s10854-018-9617-8

Bifunctional urchin-like WO3@PANI electrodes for superior electrochromic behavior and lithium-ion battery

Kun Zhang1 · Na Li1 · Yi Wang2 · Xiaoxuan Ma1 · Jiupeng Zhao1 · Liangsheng Qiang1 · Shuai Hou2 · Junyi Ji1 · Yao Li2

Received: 12 March 2018 / Accepted: 6 July 2018 © Springer Science+Business Media, LLC, part of Springer Nature 2018

AbstractA improved bifunctional nanostructure composite was designed and fabricated as reversible electrodes for electrochromic film and lithium-ion battery. This unique optic-electro responsive urchin-like architecture assembled from WO3@PANI nanobelts were prepared by solvothermal and electropolymerization methods. As expected, the composite exhibits the superior optical-electrochemical performances. The composite owns quick switching time ranging from purple, green, yellow, gray to blue via voltage regulation, better rate performance and long-term cycling stability in the galvanostatic charge/discharge process. Benefited from stable structure and short diffusion path, it also demonstrates a distinct optical modulation (△T = 45%) and excellent durability after long-term cycles (1200 cycles). The as-prepared electrode exhibits outstanding cycling stability (capacity retention of 516 mAh g−1 after 1200 cycles with a low average fading capacity of ca. 0.103 mAh g−1 and fading cyclic rate of ca. 0.02% per cycle). The long-term stability studies reveal that urchin-like composite has much more excellent optical and electrochemical durability. The morphology and structure of the composite were carried out by characterization equipment. This effective synthesis strategy will have profound implications for developing the other inorganic–organic nanocomposites in optical and electrochemical field.

1 Introduction

Switchable optic-electro materials can be widely used in military camouflages, smart windows, rear-view mirrors for automobiles, large area displays, lithium-ion batteries (LIBs), and electronic paper [1–4]. Nowadays, new genera-tion materials owed visual-comfort, adjustable multicolor and energy-saving are likely to revolutionize our future life. Dynamical control the reversible transmission in long-term cycles is central to achieving large-scale application in

optic-electro materials. Besides, these ideal materials should exhibit further remarkable multifunctional properties such as low open circuit condition, multicolor, rapid switching time, large optical modulation and long-term cyclability [5, 6]. However, ideal technologies in fabricated such multi-functional materials still remain stagnant by single material. So, numerous researches on the new generation adjustable bifunctional materials are focus on mixtures, composites or hybrids [7–9].

Widely accepted as the most promising candidate for electrochromic materials, WO3 is superior to other inor-ganic transition metal oxides attributed to its cost effective-ness, outstanding chromic nature, large spectrum responsive range and environmental friendliness [10–15]. Meanwhile WO3 widely used for LIBs, due to its desirable properties such as a high specific capacity and practically ubiquitous [16, 17]. Nevertheless, the electrochemical performances of bulk WO3 suffers from the practical difficulty due to long diffusion distance, slow switching speed, poor electrochro-mic stability and low diffusion coefficient [18–20]. Opera-tionally, in order to conquer these deficiencies, an effective strategy is to design various stable morphologies. The crys-tal structure and designed morphologies are critical to the long-life durability, fast switching time and broad optical

Electronic supplementary material The online version of this article (https ://doi.org/10.1007/s1085 4-018-9617-8) contains supplementary material, which is available to authorized users.

* Jiupeng Zhao [email protected]

* Yao Li [email protected]

1 School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, People’s Republic of China

2 Center for Composite Materials and Structures, Harbin Institute of Technology, Harbin 150001, People’s Republic of China

http://orcid.org/0000-0001-5271-7562http://crossmark.crossref.org/dialog/?doi=10.1007/s10854-018-9617-8&domain=pdfhttps://doi.org/10.1007/s10854-018-9617-8

Journal of Materials Science: Materials in Electronics

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modulation [21]. Nevertheless, self-aggregation and pul-verization will induce conductivity deteriorating and struc-ture collapsing during long-term cycles, further resulting in the optic-electrochemical performance and Li+ storage behaviors fading. In this regard, another effective strategy is to encapsulate conducting polymer on the surface of WO3. Furthermore, WO3 and conducting polymer composite dis-plays both p- and n-type photo-electrochemical activity and broads the working potential window [8, 22–24]. But the hybrid degradation and long ion diffusion distance seriously limit its long-term stability. One-dimensional (1D) nano-structures, including nanobelts, nanowires and nanofibers, have been regarded as one of the most effective ways to pro-vide abundant active sites on high surface area and shorten diffusion path for ion insertion/extraction [25–27]. But its deficiencies can be mainly attributed to structural fracture and particulate aggregation in long-term cycles. The urchin-like morphology can avoid nanolayers stacking during long cycles and effectively increase the material reversibility in ion insertion/extraction process [28, 29]. Although these strategies could achieve enhanced optical-electrochemical performance, the shortcomings have still not been entirely overcome, and new directions for bifunctional materials are still need to explore.

Inspired by these precious design strategies, an attractive strategy is to synthesis stable WO3@PANI composite. In this work, the bifunctional urchin-like WO3@PANI com-posite were prepared successfully by facile solvothermal and electropolymerization processes. Owing to inherent material features and their synergistic effect, this composite shows complementary dual electrochromic coloration both of WO3 and PANI, ranging from purple, green, yellow then gray to blue. And the material gives faster response speed and more excellent cycling stability than those of pure WO3. For the storage of Li+ ions, the resultant electrode delivers high reversible capacity of 516 mAh g−1 after 1200 cycles with outstanding cycling stability (an average capacity decay of ca. 0.103 mAh g−1 and fading cyclic rate of ca. 0.02% per cycle). The structure and morphology of the composite electrode were studied by characterization equipment. The outstanding optical modulation, excellent electrochemical cycling stability and colored mechanism of the composite material were discussed detailed.

2 Experimental section

2.1 Materials

All chemicals used were analytical grade. Aniline (C6H7N), tungsten hexachloride (WCl6 99.9% purity) and lithium perchlorate (LiClO4) were purchased from Aladdin. propyl-ene carbonate (PC) was gotten by Sigma–Aldrich. Aniline

monomer was freshly purified under reduced pressure. Absolute ethyl alcohol (99.8%) was purchased from the Sinopharm Chemical Reagent.

2.2 Material characterization

The phase structure of samples was measured using pow-ders with a wide-angle diffractometer (Rigaku D/max-rB, Cu Ka radiation, l = 0.1542 nm, 40 kV, 100 mA). The mor-phology of the electrodes was characterized on a scanning electron microscopy (SEM, ZEISS EVO18) at an accelera-tion voltage of 15 kV. Fourier Transform Infrared (FTIR) spectra were collected by using an IR spectrophotometer (PerkinElmer spectrum) using the KBr-pellet method. X-Ray photoelectron spectroscopic (XPS) analysis was carried out on a PHI-5000 VersaProbe (Kratos Axis Ultra Al at 14 kV). Thermogravimetric (TGA) curves were recorded on a Perki-nElmer Pyris 6, and the samples were characterized from 40 to 800 °C in an air atmosphere. The morphology was tested by using using scanning electron microscopy (SEM, ZEISS EVO18) and high-resolution transmission electron micros-copy (HRTEM, JEM 2100F) analysis and energy dispersive X-ray (EDX) spectroscopy.

2.3 Synthesis and preparation of the urchin‑like WO3@PANI composite electrodes

The urchin-like WO3 nanobelts were prepared by solvother-mal. Briefly, 1 mmol of tungsten hexachloride was dissolved in 100 mL of absolute ethyl alcohol and stirred at room tem-perature, added a few drops H2O2 and 0.5 mL oleylamine, then transferred to a 120 mL Teflon-lined stainless steel autoclave and heated at 220 °C for 16 h. The obtained prod-uct was washed with ethanol and ultrapure water and dried for 12 h at 80 °C. Subsequently, the WO3 nanobelts were placed on ITO glass substrate after annealing at 475 °C for 1 h.

The composite film was fabricated by electrodepositing PANI on the WO3 nanobelts electrode. The PANI film was deposited by cyclic voltammograms (CV) between − 0.2 and 1.2 V at a scan rate of 50 mV s−1 for four cycles. The electrochromic test used 1 M LiClO4/PC solution in three electrodes.

2.4 Cell assembly and testing

The rating and cycling properties were determined in 2032 coin-type cells. The electrodes were produced by casting a slurry of active materials (80 wt%), 1 acetylene black (20 wt%) and sodium carboxymethyl cellulose binder (10 wt%) on a copper foil. The mass loading of the active materials on copper foil is about 1.7–1.9 mA cm−2. 2032 coin-type cells were fabricated in an Ar-filled glovebox (O2 ≤ 0.05 ppm,


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H2O ≤ 2 ppm). The as-prepared electrode was dried at 115 °C in vacuum for 20 h. The charge–discharge charac-teristics of the samples were tested at various given cur-rent densities in the voltage range from 0.01 to 3 V using computer-controlled NEWARE BTS-610 systems (Newware Technology Co. Ltd. China).

3 Results and discussion

3.1 Morphology of the samples

Figure 1 illustrates the fabrication of the urchin-like WO3@PANI composite via electrodepositing PANI on the urchin-like WO3 film. During the polymer coating process, the ani-linium ions are adsorbed on the surface of WO3 by sweep-ing the potential between − 0.2 and 1.2 V at a scan rate of 50 mV s−1 in aqueous solution containing 0.1 M aniline. The urchin-like WO3 were synthesized by facile solvothermal process. The crystalline structure of WO3 is illustrated in Fig. 1. This structural framework consists of corner-sharing WO6 octahedral by one W and encircling six O atoms.

The microstructure and morphology of the electrodes were examined in Fig. 2. As presented in Fig. 2a, typi-cal nanobelts is characterized by SEM in the range of 200–300 nm in length and 20 nm in width. To obtain the composite, surface of WO3 nanobelts is uniformly wrapped by coarse PANI in electropolymerization process in Fig. 2b. It can be seen that urchin-like morphology are shown in Fig. S1. TEM and HRTEM images are displayed in Fig. 2c, g. In Fig. 2c, it can be known that the nanobelts structure and sizes is good agreement with the SEM image. In Fig. 2d, the HR-EDX line profile of the urchin-like WO3@PANI composite were taken across, which confirms the existence and homo-geneous distribution of W, O, C and N. The lattice-resolved HRTEM image of urchin-like WO3 is depicted in Fig. 2e, and lattice fringe spacing nearly 0.38 nm corresponding the

growth direction along the (002) direction. This is similar to findings reported in other papers [30, 31]. The PANI coat-ing WO3 nanobelts is labeled by orange lines and it gives the thickness of the PANI within 5–10 nm in Fig. 2f. As shown in Fig. 2g, the composition of the urchin-like WO3@PANI composite is clearly demonstrated by EDX elemen-tal mappings of W, O, C and N, respectively. TEM image of the urchin-like WO3 and corresponding EDX mappings are given in Fig. S2. Therefore, the urchin-like WO3@PANI composite was successful prepared.

3.2 Structure of the urchin‑like WO3@PANI composite

To demonstrate the chemical structure of samples, we tested FTIR spectroscopic studies. In Fig. 3a shows the FTIR spectra of pure PANI, WO3 and WO3@PANI in the region 1800–500 cm−1, respectively. The peak at 796 cm−1 is attributed to the stretching vibrations of W=O bands, and the peaks at 653 and 601 cm−1 are ascribed to the bridging oxygen of W–O–W bands in WO3 [32].The spec-trum of pure PANI shows the main peaks at 1568 and 1482 cm−1, corresponding to the stretching the C=N and C=C stretching modes of the quinoid and benzenoid units, respectively. The bands at 1297 and 1245 cm−1 are attrib-uted to the C–N stretching mode of the benzenoid ring. The peak at 1137 cm−1 can be due to in plane C–H stretch-ing vibrations in the quinoid ring of PANI [33, 34]. The spectrum of the WO3@PANI composite electrode exhibits the characteristic peaks of WO3 and PANI. There is a red shift of the bands of WO3 corresponding to the stretching vibrations in comparison with the FTIR spectra of WO3 and WO3@PANI, while the peaks of the PANI show a blue shift. These shifts indicate the chemical bonding between the PANI and WO3. The N atoms in PANI are inferred to form coordinated compounds with the exposed W atoms on the surface of WO3. The strong chemical bond

Fig. 1 Schematic illustration of the formation of the urchin-like WO3@PANI composite


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W–N cause the observed peak shifts. The results verify the fact that the WO3@PANI composite is successfully synthesized.

TGA curves of Fig. 3b were carried out to determine the WO3 content in the composite. The curve of WO3@PANI displays obvious two-step decomposition stage. The first degradation stage of TGA curve exhibits mass loss about 10.1% below 370 °C, mainly due to the elimination of the small molecule. With temperature rising to 650 °C, curve of WO3@PANI records the second weight loss nearly 18.1% is attributed to the degradation of the skeletal PANI chain structures. It can be seen that the pure WO3 at high tempera-ture shows no obvious mass loss of ablation in Fig. 3b. With the ablation of PANI, the curve of WO3@PANI also stably stands above 650 °C and leaves residual WO3. Compared the decomposition temperature mass loss process, the percent-age of WO3 is calculated to be 71.8% in WO3@PANI.

The as-prepared samples were identified by X-ray dif-fraction (XRD) and shown in Fig. 3c. All of the characteris-tic diffraction peaks of WO3 backbone could be indexed to orthorhombic structure in a Pmnb (62) space group (JCPDS-ICDD Card No. 71-0131), indicating that the WO3 are high purity [22, 35]. The WO3@PANI composite has a consist-ent position with pure WO3 and displays weaker diffraction peaks than the sample without coated PANI. The consistent peak position explains that the existent PANI on WO3 sur-face does not dramatically destroy the structure of WO3, and the aniline molecules have not intercalated into WO3 interlayers. The decreased peaks intensity of the composite reveals that the PANI are successfully coated on the surface of WO3 via electropolymerization.

The chemical compositions and states of the as-synthe-sized samples were investigated by XPS. The W4f XPS spectrum of composite electrode displays two asymmetrical

Fig. 2 SEM images of a the urchin-like WO3 and b the urchin-like WO3@PANI composite (inset the high magnification SEM image), TEM images of c the urchin-like WO3, HRTEM images of d the WO3@PANI composite (inset the HR-EDX line profile), e the urchin-

like WO3 and f the urchin-like WO3@PANI composite and g TEM image of the urchin-like WO3@PANI composite (the corresponding EDX element mappings of W, O, C and N, respectively)


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peaks located at 35.6 and 37.7 eV corresponding to the existence of W6+ oxidation state, while two separate peaks observed at 34.6 and 36.7 eV corresponding to the W5+ oxidation state in Fig. 3d. The W 4f spectrum of WO3@PANI composite shows two additional peaks attributed to the W5+ oxidation state than that of WO3 in Fig. S3a, indicating electron transfer occurs between W–N of WO3 and PANI in WO3@PANI composites [36, 37]. The O1s core level spectra of WO3 and WO3@PANI are displayed in Figs. S3b and 3e. The sharp peak at 530.6 eV is assigned to O2− ions of W=O bonding mode, and another peak at 532.3 eV could be assigned to the absorbed oxygen, water or CO2 molecules on the sample surface. The calculated areas of absorbed oxygen for WO3 is higher than that of WO3@PANI. It confirms that the weaker absorbing oxy-gen is formed by coating PANI on the composite surface. From N 1s spectrum, the spectrum could be decomposed in Fig. 3f. The binding energy of 397.8 eV are associated with quinoid imine (–N=). The peaks at 398.8 eV is assigned to benzenoid amine (–NH–). The peak located at 400.2 eV is ascribed to positively charged nitrogen (–N+–) [38].

3.3 Electrochemical properties

The CV curves of WO3 and WO3@PANI films were car-ried out with different scan rates in 1 M LiClO4/PC solution (Fig. 4a, c). When the scan rate increases, the reduction peak shifts toward a more negative potential and the oxidation peak slightly shifts toward a more positive potential. The

peak currents (ip) are plotted against the square root of scan rates (v1/2) in Fig. 4b, d, approximately a linear relationship can be obtained, indicating that the redox reaction of Li+ insertion/extraction is diffusion-controlled. Furthermore, the Li+ diffusion coefficient can be interpreted from a lin-ear relationship between ip and v1/2 according to the Ran-dles–Sevcik Eq. (1):

where n is the number of charge transfer during electro-chemical reaction, A is the electrode area, D is the Li+ dif-fusion coefficient, C is the concentration of active ion (Li+). It can be seen that the D is determined by the slope, because the n, A, and C value is constant. WO3@PANI film exhibits a much higher slope in both cathodic and anodic scan than the corresponding WO3 film, suggesting that redox reactions within WO3 are promoted due to coated conductivity PANI.

The CV curves of WO3, PANI and WO3@PANI films were listed at a scanning rate of 20 mV s−1 in Fig. 4e. The WO3@PANI film shows three typical pairs redox peaks of PANI, marked by A/A′, B/B′ and C/C′ from leucoemeral-dine salt (LS), emeraldine salt (ES), emeraldine salt (ES) to pernigraniline salt (PS) of PANI with anion doping/dedop-ing processes, respectively. The main molecular structures of LB, ES, EB, and PS are given in Fig. S4. Another redox peaks D/D′ of WO3 are also found in WO3@PANI film, which proves that the WO3@PANI composite was prepared successfully.

(1)ip = 2.69 × 105n3∕2AD1∕2

Cv1∕2

Fig. 3 a FTIR spectra, b TGA curves, c XRD patterns, and XPS spectra of the urchin-like WO3@PANI composite d W 4f, e O 1s, and f N 1s


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To further understand the electrochemical behavior of the samples, electrochemical impedance spectroscopy (EIS) of three electrodes were tested in the frequency range of 0.01 Hz to 100 kHz in Fig. 4f. Nyquist plots are composed of the semicircle ascribed to the charge transfer resistance (Rct) in the high-frequency region and the sloped lines in the low-frequency region. It is well understood that a smaller semi-circle means a lower Rct and a steeper slope. The WO3@PANI composite film shows the lowest Rct (15.9 Ω) than

those of WO3 (25.6 Ω) and PANI (36.4 Ω) electrodes. It is considered that WO3@PANI composite film owns good ion diffusion behavior and electrical conductivity, leading to higher charge transfer kinetics and reactivity.

The lithium storage behaviors of the composites were investigated using half cells as anode materials. Galvano-static charge–discharge measurements were carried out at 1st, 2nd, 10th, 100th, 500th and 1200th cycles in a cut-off voltage range of 0.01–3.0 V at 0.2 A g−1 in Figs. 5a

Fig. 4 CV curves of a the urchin-like WO3 film and c the urchin-like WO3@PANI composite film at different scan rates, fitting plots between peak current i and square root of scan rate v1/2 of b the urchin-like WO3 film and d the urchin-like WO3@PANI composite

film, e CV curves of WO3, PANI and WO3@PANI films at a scanning rate of 20 mV s−1 and f nyquist plots of WO3, PANI and WO3@PANI films


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and S5. The WO3 electrode exhibits initial discharge and charge specific capacities of the 871 and 668 mAh g−1, resulting in initial Coulombic efficiency of 76.69%. In contrast, WO3@PANI with relatively initial lower dis-charge and charge specific capacities are found to be 831 and 652 mAh g−1, corresponding to enhanced ini-tial Coulombic efficiency of 78.46%. The WO3@PANI composite electrode indicates good reversibility, attrib-uted to the Coulombic efficiency remained at exceeding 99.9% after the first several cycles. A comparison for the rate capabilities of WO3 and WO3@PANI are shown in Fig. 5b. The capacities of WO3@PANI at current densi-ties of 0.2, 0.5, 1.0, 2.0, 5.0, 10.0, 0.2, 0.5, 1.0, 2.0, 5.0, 10.0 and 0.2 A g−1 are 617, 519, 404, 328, 209, 135, 566, 490, 365, 303, 181, 121 and 506 mAh g−1, respectively. Even at the high current density up to 10.0 A g−1, a rela-tively higher reversible capacity of WO3@PANI is still retained at 135 and 121 mAh g−1. When the current rate is reset back to 0.2 A g−1, it still can recover up to 566 and 506 mAh g−1 and keep steady in the following 70 cycles

after testing at various rates. By comparison, the revers-ible capacity of WO3 obtains 54 and 32 mAh g−1 at the high current density up to 10.0 A g−1. It just can recover back to 467 and 320 mAh g−1 after the high current pulse, while rapidly decreases to 130 mAh g−1 in the following 70 cycles. It is clear that the WO3@PANI composite has better rate performance than WO3. To further investigate the long-term cyclability of the composites, the electrodes are tested at 0.2 A g−1. The two electrodes exhibit similar specific capacities in the initial few cycles. The capac-ity of the WO3 decreases rapidly and drop to 58 mAh g−1 after 1200 cycles in the subsequent cycles in Figs. 5c and S6. Meanwhile, the capacity of the WO3@PANI presents excellent capacity retention capability and finally main-tains at 516 mAh g−1 after 1200 cycles with an average capacity decay of 0.103 mAh g−1 and fading cyclic rate of ca. 0.02% per cycle at 0.2 A g−1 (Figs. 5c and S7). Com-paring the WO3, WO3@PANI electrode exhibits better rate performance and long-term cycling stability, indicating WO3@PANI composite owns electrochemical superiority.

Fig. 5 a The galvanostatic charge–discharge profiles of the urchin-like WO3@PANI electrode at 1st, 2nd, 10th, 100th, 500th and 1200th cycles in the voltage range of 0.01–3.0 V vs. Li+/Li at a current density of 0.2 A g−1, b multi-rate testing of WO3 and WO3@PANI

at discharge current densities of 0.2, 0.5, 1.0, 2.0, 5.0, 10.0 0.2, 0.5, 1.0, 2.0, 5.0, 10.0 and 0.2 A g−1, c cycling performance of WO3 and WO3@PANI electrodes at a current density of 0.2 A g−1


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3.4 Optical performance

The optical transmittance spectra of WO3 and WO3@PANI films are recorded in Fig. 6a, b at different poten-tials. The transmittance spectrum of WO3 film obviously decreases from 0 to − 0.6 V. When a negative bias potential is applied on WO3 film, the insertion of Li+ into the film causes color changes from transparent to blue. So, WO3 is cathodic coloration in negative potentials. As the applied potential increases from − 0.6 to 1.0 V, the WO3@PANI film show multiple colors, range from dark blue (− 0.6 V), gray (− 0.4 V), yellow (0.2 V) then green (0.4 V) to purple (1.0 V). In addition, PANI is anodic coloration in positive potentials. Moreover, the WO3@PANI composite film pos-sesses a complementary dual electrochromic effect due to the non-intersect of the colored and bleached states, which WO3 is attributed to n-type and PANI attributed to p-type electrochromism. The dual electrochromic mechanism of the WO3@PANI composite film is listed Fig. S8.

The long-term stability of EC films is a key factor that limits film application. Figure 6c carried out the durabil-ity test of the WO3@PANI composite film. The WO3@

PANI composite film exhibits excellent cycling stability for 1200 cycles at a wavelength at 700 nm. The electrochromic response time of the WO3@PANI composite film is char-acterized by continuously stepping the voltage from 1.0 V (10 s) to − 0.2 V (10 s) in situ transmittance at 700 nm in Fig. 6d. The response times for the oxidation and reduction are 1.5 and 1.9 s, respectively, which are faster speed than those of WO3 film (1.7 and 2.1 s in Fig. S9). The response times for colored states are different from bleached states, which should be owing to ionic dispersion dynamics result-ing from the different surface images during the oxidation and reduction process [39]. Moreover, the change of trans-mittance (△T%) the for WO3@PANI composite film is 45%, which is wider than those of WO3 film (△T = 40%) and PANI film (△T = 33% in Fig. S10). The result of the composite reveals that urchin-like WO3@PANI composite film possesses long-term stability and outstanding optical and electrochemical durability in cyclic test.

The synergistic effects between urchin-like WO3 and PANI is critical to understand the remarkable electro-chromic performances. The backbone of urchin-like WO3 could provide firm skeleton for PANI, avoiding undesirable

Fig. 6 Transmittance spectra of a the urchin-like WO3 film and b the urchin-like WO3@PANI composite film under different voltages (the inset the photographs of under different voltages), c the durability test

of the urchin-like WO3@PANI composite film for 1200 cycles at a wavelength and d electrochromic response of the urchin-like WO3@PANI composite film from 1.0 V (10 s) to − 0.2 V (10 s) at 700 nm


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agglomeration or irreversible inertness. Simultaneously, the presence of PANI offers a protective layer for urchin-like WO3 matrix, preventing WO3 pulverization or inactivation of photo-electroactivity. Meanwhile, the two components of composite materials share the dual electrochromism syner-gistic effect, due to the nonoverlapping of the coloration and bleaching state. The WO3 and the encapsulation of PANI can shorten the transportation path and guarantee the fast kinet-ics for ion insertion/exsertion. Thereby, this unique structure enhances the synergistic effects for composite material and decreases the ion trapping by dynamic factors. Moreover, the stable urchin-like morphology effective alleviate the physical strain and inhibit the self-aggregation of WO3 in the long-term cycles.

4 Conclusion

In summary, the bifunctional urchin-like WO3@PANI com-posite was fabricated successfully via solvothermal and elec-tropolymerization. The obtained composite offers urchin-like morphology, large heterogeneous interface areas and composite nanobelts. It exhibits highly optical transmittance change (△T = 45%), long-term stability (1200 cycles) and complementary dual electrochromic effect (ranging from purple, green, yellow then gray to blue). The composite also shows high reversible capacity retention (516 mAh g−1), bet-ter rate performance and long cycle-life for LIBs. This study paves a novel way for many potential applications in electro-chromic devices, smart windows, lithium-ion batteries and photoelectric fields.

Acknowledgements We thank National Natural Science Founda-tion of China (Nos. 51572058, 51502057), National Key Rerch & Development Program (2016YFB0303903, 2016YFE0201600), the International Science & Technology Cooperation Program of China (2013DFR10630, 2015DFE52770), and Foundation of Equipment Development (6220914010901).

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Bifunctional urchin-like WO3@PANI electrodes for superior electrochromic behavior and lithium-ion batteryAbstract1 Introduction2 Experimental section2.1 Materials2.2 Material characterization2.3 Synthesis and preparation of the urchin-like WO3@PANI composite electrodes2.4 Cell assembly and testing

3 Results and discussion3.1 Morphology of the samples3.2 Structure of the urchin-like WO3@PANI composite3.3 Electrochemical properties3.4 Optical performance

4 ConclusionAcknowledgements References

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