Design of Symmetric TiO2/LiI/TiO2 Electrochromic Devices for Gradient Shaded Smart Windows with Enhanced Switching and Cycling Properties

Li+ insertion and extraction inherently impact the switching dynamics and cycling stability of inorganic electrochromic (EC) electrodes. Herein, the extraction of Li+ out of the TiO2 lattice to release the electron is replaced by the charge recombination of the electron with I3− at the TiO2‐electrolyte interface, which avoids the mechanical breakdown of the electrodes and renders self‐bleaching with no applied voltages. A device design of the same two TiO2 nanocrystal (NC) electrodes combined with the redox lithium salt (LiI) electrolyte confers symmetric electrochromism. By applying a forward or reverse bias, the two TiO2 electrodes alternately serve as the electrochromic electrode and exhibit voltage‐controlled gradient coloration, a maximum optical modulation of 90% at 700 nm, and a doubled cycling performance. The microstructure of the TiO2 NC film is characterized by transmission electron microscopy, X‐ray diffraction, X‐ray photoelectron spectroscopy, and Brunauer–Emmett–Teller methods. The electrochemical and electrochromic properties of the device are investigated using cyclic voltammetry, chronoamperometry, electrochemical impedance spectroscopy, and Mott–Schottky method.


Introduction
Electrochromic (EC) materials reversibly change their colors and optical states in response to an applied electric potential.[3][4][5] EC materials can be roughly categorized into organic and inorganic ones.The former, including enhanced LSPR absorption to the NIR range, and therefore are discovered to be an effective dual-band electrochromic material with high optical contrast (95.5% at 633 nm and 90.5% at 1200 nm), which draws the attention back to single-component CE materials. [24,25]lthough high optical contrast is achieved in TiO 2 -based EC devices, the switching kinetics of the EC electrode are significantly limited by the ion diffusion rate during the intercalation and extraction of Li + ions.The sluggish Li + diffusion often leads to undesired long coloring time of up to minutes in labscale electrochromic devices or more than 10 min in large-scale electrochromic smart windows. [26]On the other hand, ions are intercalated and deintercalated frequently, potentially straining the EC material and causing mechanical breakdown over time.With time, the device may remain irreversibly tinted or fail to darken sufficiently when charged. [27]For application as smart windows in buildings, EC devices should withstand more than 5000 cycles without much degradation, which has still been a challenge to the Li + -based EC electrodes. [28]Although considerable strategies (such as doping, nanostructure modification, etc.) have been employed to improve cycling stability, the decay of electrochromic performance is still inevitable.Moreover, for most EC devices based on ions insertion and extraction, an applied voltage is needed to fully extract the Li + from the lithiated matrix and return the device to the fully transparent state. [29]This extra energy-input-bleaching consumes additional energy, which is undesirable for large-scale EC window applications.
The insertion and extraction of Li + are in nature for reversible electron injection and release in the EC electrode.A typical TiO 2based EC device usually consists of an electrochromic TiO 2 layer, an ion transport layer (electrolyte), and an ion storage layer. [30]hen a voltage is applied, a double injection of Li + and electrons occurs in TiO 2 , which produces a visible extinction resulting from the localization of injected electrons on titanium cations creating a polaronic lattice distortion. [31]When the injected electrons are released from the lattice of TiO 2 , they transport through the external circuit onto the counter electrode with an ion storage layer.Meanwhile, Li + ions, under an applied opposite voltage, transfer through the electrolyte and back to the ion storage layer, causing the TiO 2 layer to bleach.For coloring, the Li + insertion is necessary for efficient electron injection.For bleaching, on the other hand, except for the Li + extraction by an applied voltage, there is another approach for electron release, which is the charge recombination with oxidation species in the electrolyte.However, the use of electrolytes containing redox couples in inorganic EC devices is controversial.34] Because redox couples in electrolytes could cause potentially an internal short circuit between the two electrodes creating a continuous current (called "loss current"), which has to be maintained at a low level to keep a constant potential and a uniform coloration. [35]On the contrary, several researchers reported that the faradaic current from the redox reaction of 3I − ↔ I − 3 + 2e − contributes to the electron injection into EC materials and the conductivity of electrolytes, leading to an overall improvement in electrochromic performance. [36,37]Herein, we propose a design of TiO 2 electrochromic devices based on an iodide/triiodide (I 3 − / I − ) redox electrolyte with the configuration of TiO 2 /LiI/TiO 2 and compare it with the device based on a lithium electrolyte without I − , for example, TiO 2 /LiBF 4 /TiO 2 .The electrochemical and optical modulation properties are investigated to reveal the impacts of charge recombination at the electrode-electrolyte interface on electrochromic performances.Particular emphasis is placed on the electrochromism in the vis range to investigate the color switching, the optical modulation, and the cycling properties.The mechanism and regime for the performance enhancement of TiO 2 -base EC devices for smart windows are further explored and revealed.

Results and Discussion
The colloidal TiO 2 NCs were synthesized by the hydrothermal treatment of the precursor under 120 °C for 6 h.The precursor was a transparent sol (Figure 1a) obtained by peptizing the hydrolyzed product resulting from tetrabutyl titanate (TBT) reacting with a dilute nitric acid aqueous solution.The resultant colloidal TiO 2 NCs were mixed with surfactants to prepare a paste (Figure 1b) for film coating.The paste was blade coated onto an FTO conductive glass and sintered at 450 °C for 30 min to prepare a transparent TiO 2 electrode (Figure 1c).Scanning electron microscopy (SEM) (Figure 1d,e) and EDS element mapping (Figure 1f) show a uniform TiO 2 mesoporous film with a thickness of about 1.96 μm on top of an FTO conductive glass.X-ray diffraction (XRD) pattern (the inset in Figure 1d) shows a wellcrystallized anatase structure of the synthesized TiO 2 NCs.The transmission electron microscopy (TEM) images show the particle size is around ≈5 to 20 nm (Figure S1, Supporting Information).The Ti 2p X-ray photoelectron spectroscopy (XPS) spectra of the synthesized TiO 2 NCs (Figure 1g) show the presence of a small amount of Ti 3+ , indicating the existence of oxygen vacancies in the TiO 2 lattice. [38]Figure 1h gives N 2 adsorptiondesorption isotherms of the synthesized TiO 2 NCs.[41] The surface area (S BET ) and average pore size determined using the Brunauer-Emmett-Teller (BET) method are 201.68 m 2 g −1 and 3.26 nm.
The same two TiO 2 NC electrodes were used as the electrochromic layer and the ion storage layer to fabricate the EC device.A lithium-based liquid electrolyte without redox species, that is, 0.1 M lithium tetrafluoroborate (LiBF 4 ) in acetonitrile (ACN), was first used for the device, denoted as TiO 2 /LiBF 4 /TiO 2 .The cyclic voltammogram (CV) of the device was tested in different potential windows ranging from approximately −0.5 to +0.5 to −4 to +4 V (Figure 2a).With the potential range increasing over approximately −3 to +3 V, the well-developed mirror-symmetrical positive and negative current peaks emerged, which could be attributed to the insertion/extraction of Li + into/out of the TiO 2 NC lattice triggering the electron injection.An average of 80% of the contribution of the pseudo-capacitive capacity suggested a good electrode-reaction status of the device (Figure S2, Supporting Information).It was a typical overpotential-charging of Li + -based devices, as the device was scanned oppositely from approximately −4 to +4 to 0.5 to +0.5 V, the symmetrical redox peaks emerged at the potential ranges lower than approximately −3 to +3 V (Figure 2b).The optical transmittance spectra of TiO 2 /LiBF 4 /TiO 2 were recorded with respect to the applied voltage from 0 to 4 V in the wavelength range of ≈300 to 800 nm, as shown in Figure 2c.In agreement with the CV tests, the transmittance in the vis region began to decrease under an applied voltage over 3 V.A gradient coloration occurred under the voltage ranging from 3 to 4 V, while failed bleaching with no voltage applied (Figure 2d,e).Under −1.5 V, the device turned light blue, but not completely transparent (Figure 2f).It resulted from the fact that the ion storage layer on the counter electrode was also the TiO 2 NCs, which caused it to be lightly colored due to Li + insertion under a low voltage of −1.5 V.
The EC device with a redox electrolyte and the double-TiO 2electrodes configuration was investigated for comparison.The electrolyte was 0.1 M LiI (providing Li + ions and I 3 − /I − redox couples) in acetonitrile (ACN) with tetrabutylammonium hexafluorophosphate (TBAPF6) as a buffer solution.The device was denoted as TiO 2 /LiI/TiO 2 .The CV curves showed the disappearance of Li + insertion/extraction peaks and the enhancement of electrode polarizations (Figure 3a).The optical transmittance spectra of TiO 2 /LiI/TiO 2 demonstrated the vis extinction under an applied voltage (Figure 3b).The device exhibited a gradient coloration under the voltage ranging from 2 to 4 V and self-bleaching with no voltage applied (Figure 3c,d).The bright yellow color from the oxidization of I − into I 3 − in the electrolyte caused the colored TiO 2 electrode in green visually.
The current-voltage characteristics of TiO 2 /LiI/TiO 2 differed from those of TiO 2 /LiBF 4 /TiO 2 .On the CV curves, the redox current peaks (Figure 2a) between −1.5 and +1.5 V were replaced by a zero-current region (Figure 3a), suggesting that no constant Li + insertion/extraction into/out of the TiO 2 NC lattice occurred.Moreover, although the current increased significantly with a potential over 1.5 V, the device did not undergo an internal short circuit between the two electrodes.If so, the current should linearly increase from 0 V.The impact of LiI on the electrochemical and electrochromic properties of the device was investigated by changing the amount of LiI in the electrolyte.TBAPF6 in acetonitrile was used as the blank electrolyte, in which the concentration of LiI was increased from 0 to 0.149 M. Figure 4a,b shows the impact of the LiI concentration on the current-voltage characteristics of the device.In the low LiI concentration of less than 0.0074 M, the Li + insertion/extraction was predominant, by which the device generated the symmetric redox current peaks in the potential range of approximately −1.5 to +1.5 V and colored in light blue.However, without an applied voltage, the electrode was incapable of bleaching.With the LiI concentration increasing, the electrode polarization became predominant leading to considerable electrons accumulating on the surface of electrodes.The TiO 2 electrode thus turned green and completed self-bleaching without an applied voltage (Figure 4c).
An electrochemical polarization basically causes by the dynamic superiority of electronic processes over ionic processes occurring at the electrode-electrolyte interface. [42]In order to obtain a reasonable understanding of the present observations, we carried out Mott-Schottky (MS) measurements to make a reasonable evaluation for relative V fb values of the TiO 2 electrode in the electrolytes with different concentrations of LiI.5] 1 where V fb is the flat band potential, V is the applied voltage, A is the area of the device, N D is the carrier concentration,  is the relative dielectric constant of the semiconductor,  0 is the vacuum permittivity, and e is the electronic charge, respectively.k and T are Boltzmann's constant and absolute temperature, respectively.C −2 versus V yields a straight line with slope = 2  0 A 2 eN D and the intercept on the x-axis is (V fb + kT e ).The slope gives the conducting type of the semiconductor with positive for n-type and negative for p-type.The relative V fb of the TiO 2 -electrolyte interface is determined by the intercept.
Figure 4d shows the MS plots of the TiO 2 electrode in the electrolyte with different LiI concentrations.When the LiI concentration was higher than 0.00747 M, the MS plots showed typical n-type semiconductor character.As the potential increased from −2 to +4 V, three regions of increasing, flat, and decreasing on the C −2 versus V curve were evident.The three regions are called the accumulation region, the depletion region, and the inversion region, which results from the accumulation of electrons, the depletion of electrons, and the domination of hole density over electron density at the TiO 2 -electrolyte interface. [46]The calculated V fb value decreased from −0.55 to −0.09 V with the LiI concentration increasing from 0.00747 to 0.149 M (Figure 4e).It could be easily presumed that the existence of I 3 − /I − put E CB of TiO 2 toward favorable levels to allow smooth electron transfer from TiO 2 to the electrolyte, which led to the Li + extraction being screened due to the recombination of electrons accumulated in TiO 2 with I 3 − .A self-bleaching without an applied voltage thus occurred in the colored TiO 2 electrode.In addition, when the concentration of LiI in the electrolyte was too low (< 0.00747 M), the shape of the MS plot was too weak to analyze.
Based on the observations and analysis above, the electrochromic mechanism of TiO 2 /LiI/TiO 2 was reasonably described and schematically illustrated in Figure 5.For coloring, with a forward bias applied on the device, Li + ions migrated and inserted into the TiO 2 NC layer on the cathode and the electrons simultaneously injected into TiO 2 forming a positive current (Figure 5a).At the same time, the I − ions migrated and adsorbed onto the anode.As the mesoporous TiO 2 NC film on the anode provided huge surfaces (201.6823m 2 g −1 ), considerable I − ions were oxidized into I 3 − , leading to a bright yellow color of the electrolyte.The resultant I 3 − ions were attracted to the anode too due to its negative charge.For bleaching, without applying a voltage, the Li + ions failed to extract out of TiO 2 to release the electrons.However, driven by a concentration gradient, the I 3 − ions diffused to the cathode and recombined with the electrons by the reduction reaction of I − 3 + 2e − → 3I − , which formed a positive current as well (Figure 5b).The TiO 2 NC film on the cathode was bleached owing to the electron release.It suggested that in the configuration of TiO 2 /LiI/TiO 2 since Li + ions were not extracted out of the lattice of TiO 2 on the cathode, the TiO 2 on the anode served actually as a conducting layer but not an ion storage layer.The cathode TiO 2 somehow worked independently for coloration/bleaching behaviors.Certainly, the anode electrode has to serve as a conducting electrode.If the applied voltage switched oppositely from positive to negative, the same electrochromic reactions could happen to the other TiO 2 electrode.In other words, the two TiO 2 electrodes could alternately work as cathode and anode determined by applying a forward bias or reverse one.
On the Nyquist plots of TiO 2 /LiI/TiO 2 , the radii of the two semicircles, emerging under the low applied potential of ≈1.3 to 1.6 V, were quite close with the ratio of R 1 /R ct around 1.38, demonstrating that the coloring-related electron transport within   the TiO 2 electrode was dynamically comparable to the bleachingrelated charge recombination at the TiO 2 /electrolyte interface.It indicated that, under this potential range, the electron injection and release occurred at an equivalent rate, therefore no considerable electrons accumulated to color the TiO 2 electrode, which agreed with the observations in Figure 3b,c.From another angle, when the potential decreased from a high value to 1.6 V below, the charge recombination became a highly efficient electron-releasing route to cause the TiO 2 electrode to bleach quickly.
However, if the potential went up to 1.8 V, the Nyquist plot changed into one semicircle and a straight line (Figure 6c), indicating that the electron transfer at the TiO 2 -electrolyte interface was screened.The electron accumulation became dominant, the TiO 2 electrode thus being colored, as shown in Figure 3c.It can be explained by the electrostatic attraction of the anode to the negatively charged I 3 − ions when a forward bias applies to the device.As illustrated in Figure 5a, when a forward bias was applied to the device, I − ions migrated toward the anode where considerable I − ions were oxidized into I 3 − .The resultant I 3 − ions could diffuse toward the cathode driven by the concentration gradient or be attracted to the anode due to their negative charge.At a low applied potential, the diffusion triumphed and I − ions arrived at the cathode to recombine with the electrons accumulated in TiO 2 .With the potential increasing, the electrostatic attraction triumphed, and more and more I 3 − ions stayed at the anode side thus the charge recombination at the cathode side was screened.
The comparison of electrochromic performances of TiO 2 /LiI/TiO 2 and TiO 2 /LiBF 4 /TiO 2 is shown in Figure 7. Chronoamperometry (CA) was used to measure current-time response during the periodic coloration and bleaching processes by applying a step voltage from −1.5 to 3.6 V for TiO 2 /LiBF 4 /TiO 2 (Figure 7a) and 0 to 3.6 V for TiO 2 /LiI/TiO 2 (Figure 7b).The coloration efficiency (CE) is a measure of the optical density change (ΔOD) in response per unit time and per unit area of charge (ΔQ) inserted into (or extracted from) the electrochromic materials at a specific wavelength.0] where T b and T c are the transmittances of an electrochromic film in the bleached and colored states at a given wavelength, respectively.A is the active area of the working electrode.CE can be extrapolated according to the slope of the linear region of the curve (in the plot of ΔOD as a function of ΔQ).Based on the transmittance spectra in Figures 2c and 3c as well as the CA results in Figure 7a,b, the ΔOD-ΔQ plots of TiO 2 /LiBF 4 /TiO 2 and TiO 2 /LiI/TiO 2 at a wavelength of 700 nm under a potential range of ≈3 to 4.2 V were illustrated in Figure 7c,d, respectively.The CE values of TiO 2 /LiI/TiO 2 were calculated to be 205.1 cm 2 C −1 , much higher than 31.9cm 2 C −1 of TiO 2 /LiBF 4 /TiO 2 , which exhibited a significant enhancement in optical modulation efficiency.Moreover, on the ΔOD-ΔQ plot of TiO 2 /LiI/TiO 2 , ΔOD, and ΔQ were quite linearly related, indicating a constant electrochromic mechanism of the device over the entire coloringbleaching processes.
Switching speed is a description for the time required to switch from a bleached mode to a colored mode and vice versa (denoted as t c and t b ).The t c and t b are defined as the time calculated for 90% changes between colored and bleached states, which is shown in Figures S3 and S4, Supporting Information.Figure 7e gives the dependence of t c on the applied voltage and the corresponding t b .The t b of TiO 2 /LiBF 4 /TiO 2 was recorded by applying a voltage of −1.5 V and that of TiO 2 /LiI/TiO 2 was recorded without an applied voltage after coloration.The switching speed of TiO 2 /LiBF 4 /TiO 2 was related reciprocally to the voltage.With the applied voltage increasing from 3 to 4 V, t c increased from 1.1 to 6.3 s and t b was from 2.9 to 26.2 s.For TiO 2 /LiI/TiO 2 , t c increased from 5.8 to 13.0 s as the applied voltage increased from 3 to 4.2 V.And t b , independent of the coloring voltage, stayed around 12.15 s.The optical contrast (ΔT/%) was calculated based on the transmittance spectra in Figures 2c and 3c by the difference between T b and T c . Figure 7f shows the ΔT with respect to the applied voltage at a wavelength of 700 nm.The maximums of ΔT, obtained under 4 V, were 73% for TiO 2 /LiBF 4 /TiO 2 and 85% for TiO 2 /LiI/TiO 2 .TiO 2 /LiBF 4 /TiO 2 needs a potential over 3.6 V for an optical contrast higher than 60%.It is noticeable in Figure 7e that under the applied voltage higher than 3.6 V, TiO 2 /LiI/TiO 2 has a much shorter t b than TiO 2 /LiBF 4 /TiO 2 .It demonstrates that for highly efficient optical modulation, the redox couple I 3 − / I − in electrolyte makes remarkable improvements in bleaching properties.
To investigate the cyclic stability of the EC devices, chronoamperometry tests with in situ switching behaviors were performed.The voltage step switched from 3.6 to −1.5 V for TiO 2 /LiBF 4 /TiO 2 and 3.6 to 0 V for TiO 2 /LiI/TiO 2 , respectively, with one coloringbleaching cycle length for 160 s.During about 300 cycles, the coloration current of TiO 2 /LiBF 4 /TiO 2 decayed constantly (Figure 7g), while that of TiO 2 /LiI/TiO 2 was stable over 350 cycles (Figure 7h).The insets in Figure 7g,h are the pictures of the devices after the cyclic test.The TiO 2 film in TiO 2 /LiBF 4 /TiO 2 was obviously cracked, while that in TiO 2 /LiI/TiO 2 was intact.It indicates that the TiO 2 -electrolyte interfacial charge recombination is a promising regime for electron release, which significantly improves the cyclic stability of Li + -based EC devices.
Although the high ionic mobility of acetonitrile (ACN) benefits fast switching speed and high coloration efficiency, its volatility is undesirable to an EC device.Therefore, the nonvolatile propylene carbonate (PC) replaced ACN as the solvent in the LiI electrolyte to fabricate the TiO 2 -based EC device denoted as TiO 2 /LiI (PC)/TiO 2 .The electrochemical and electrochromic properties of TiO 2 /LiI (PC)/TiO 2 were estimated.
A viscosity of 2.5 mPa s is the major drawback of PC, much higher than that of acetonitrile (0.35 mPa s), which decreases the diffusion rate of I 3 − ions.A slow ion diffusion could lead to a lag of the charge recombination of electrons accumulated in TiO 2 with I 3 − thus the decrease of self-bleaching rate.The CV tests demonstrated the electrode reaction characteristics of TiO 2 /LiI (PC)/TiO 2 .In the potential window lower than approximately −2.5 to 2.5 V, the typical CV curves were with significant electrode polarization and no Li + insertion/extraction peaks showed up (Figure 8a).However, when the potential window was over approximately −2.5 to 2.5 V, the mirror-symmetrical positive and negative current peaks began to develop (Figure 8b).It implied that a relatively slow ionic diffusion in the PC-based electrolyte could affect the electrode reactions, especially the bleaching process.Under a relatively low applied potential, the electric double layer at the anode side was thick, which caused a short distance for I 3 − to diffuse to the neighborhood of the cathode.Therefore, even though its diffusion was slow when bleaching, I 3 − ions could reach the surface of the cathode timely to com-bine with the electrons, and no negative currents were generated (Figure 8c).With the applied potential increasing, the thickness of the electric double layer at the anode side decreased, which enlarged the distance of I 3 − ions diffusing toward the anode during the self-bleaching process.A few of the I 3 − ions reached the surface of the cathode and the extraction of Li + out of the lattice of TiO 2 could occur to release the electrons to the external circuit, thus forming a negative current (Figure 8d).It suggested that for electron release through charge recombination, an applied potential higher than 2.5 V was undesired to TiO 2 /LiI (PC)/TiO 2 .This limitation narrowed the potential window for electrochromism.Therefore, a nonvolatile solvent with relatively low viscosity is desired for the EC devices based on the charge-recombination regime.
The electrochemical impedance spectroscopy (EIS) measurements were carried out under different applied potentials.With the potential increasing, TiO 2 /LiI (PC)/TiO 2 shows a similar evolution of Nyquist plots to the analogue device with electrolyte using ACN as the solvent.From 0.9 to 1.2 V, the Nyquist plot exhibits the two semicircles (Figure 8e) with the ratio of R 1 /R ct around 1.3 (Figure 8g), indicating the competition of the charge recombination at TiO 2 -electrolyte interface and the bulk electron transport of TiO 2 electrode.From 1.3 to 1.8 V, the Nyquist plot changes into one semicircle and a straight line (Figure 8f), indicating that the electron injection into TiO 2 becomes predominant.Moreover, the MS measurement of the device TiO 2 /LiI (PC)/TiO 2 was undertaken.It is intriguing that the MS plot shows two independent active regions in negative and positive potential areas, respectively.The symmetric-like linear regions corresponding to both n and p conducting types are shown on the MS plots.The calculated V fb values were 0.47 and −0.69 V, respectively.It suggested that the two TiO 2 electrodes could serve as the cathode (the electrochromic electrode) and anode (the counter electrode) alternately, under an opposite applied potential, which was in good agreement with the mechanism analysis above.
To validate the symmetric electrochromism, the optical transmittance spectra and the chronoamperometry (CA) spectra of TiO 2 /LiI (PC)/TiO 2 were tested under the potential range of 1.5 to 2.5 and −1.5 to −2.5 V, respectively.The device exhibited gradient coloration under either positive or negative voltage and selfbleaching with no voltage applied (Figure 9a).The evolution of the transmittances in the vis region under positive voltages and negative voltages were almost the same and obtained a minimum of 0.8% and 1.3% at 2.5 and −2.5 V (Figure 9b).Under positive and negative voltages, the dependences of switching time and optical contrast on applied voltage show symmetrical characteristics.Based on the CA spectra, the coloring time (t c ) and bleaching time (t b ) were calculated (Figure S5, Supporting Information).Compared to TiO 2 /LiI/TiO 2 , TiO 2 /LiI (PC)/TiO 2 has an equivalent t c around 12 s while a prolonged self-bleaching time (t b ).With the applied voltage (for coloration) increasing, the t b increased from 7.6 to 48.1 s (Figure 9c), validating the impact of slow ion diffusion on electron release analyzed above.When ACN was the solvent, the t b was around 12 s and independent of the coloring voltage.The maximum optical contrast (ΔT) of 90% was obtained (Figure 9d), higher than 85% of the device using ACN as the solvent in electrolyte.The ΔOD-ΔQ plots obtained under positive and negative voltages were quite similar too (Figure 9e).The relationship between ΔOD and ΔQ was linear   interfacial electron process of the TiO 2 -base EC devices with and without redox couples.The EIS was measured in a frequency range of 0.01 to 100 000 Hz at different potentials.The Nyquist plots of TiO 2 /LiI/TiO 2 show a semicircle at high-frequency region attributing to bulk transport processes (i.e., the electron transport within the TiO 2 NC electrode) and a second semicircle at lower frequencies correlated to interfacial effects [47] (i.e., the charge recombination at the TiO 2 /electrolyte interface) (Figure 6a).The former relates to the electron injection into TiO 2 electrodes, thus the coloring.The latter relates to the electron release from TiO 2 electrodes, thus the bleaching.The Nyquist plots of TiO 2 /LiBF 4 /TiO 2 show, like conventional Li + -based EC devices, one semicircle at high-frequency corresponding to the bulk electron transport processes and a straight line at midfrequency region from the diffusion of ions in the electrolyte (Figure 6b).The absence of the second semicircle suggests no electron transfer occurs at the electrode-electrolyte interface.The equivalent circuit models in the insets are used to fit the Nyquist plots and the calculated values of R CT , R 1 , and R s are listed in Table 1.To investigate the long-term stability of the symmetric TiO 2 /LiI (PC)/TiO 2 device, chronoamperometry was used to control the in situ coloring-bleaching behavior with one cycle length lasting for 160s.A positive voltage step of 0 to 2.3 V was used for the first 2300 cycles to test the cycling performance of one TiO 2 electrode.And then a negative voltage step of 0 to −2.3 V was used to run the second 2300 cycles to test the other TiO 2 electrode.The optical transmittance under +2.5 or −2.5 V was tested to see how the cyclic switching affected the maximum optical modulation of the device, as shown in Figure 10a,b.In coloration, compared with the first cycle performance, after 4600 cycles, the optical transmittances under +2.5 and −2.5 V were nearly no change at 700 nm while slightly increased at 516 nm.When the forward bias was applied to the device, the transmittance at 516 nm varied from 6% to 9% after 2300 cycles, while nearly no change after 4600 cycles.When the reverse bias was applied to the device, the transmittance was not changed after 2300 cycles, while varied from 7.7% to 11.4% after 4600 cycles.It was in good agreement with the alternate work of the two TiO 2 electrodes.The optical transmittances of the device in self-bleaching status after 4600 cycles, on the other hand, were maintained at 91% at 700 nm and 92% at 516 nm, although a slight decrease at 2300 cycles.The corresponding digital photos of the device after different cycles are shown in Figure 10c.Owing to the alternate work for the switching cycle by each TiO 2 electrode, the device can easily withstand 4600 cycles of the coloration-bleaching switch with a slight optical-modulation degradation of 1.2% at 700 nm and 4.7% at 516 nm.It indicates that the symmetric configuration based on double TiO 2 electrodes and redox electrolytes has a remarkable enhancement in the cycling performance of EC devices.

Conclusion
In summary, this study presents a symmetric EC device using the same two TiO 2 NC electrodes as cathode and anode, filling with a redox lithium salt (LiI) electrolyte.The charge recombina-tion of the electrons accumulated in TiO 2 with oxidized species (I 3 − ) in the electrolyte is validated to be an efficient route to replace Li + extraction for electron release and triggers the fast self-bleaching behavior of electrodes.The I 3 − /I − redox shuttle decreases the reaction resistance of the device, resulting in the coloration efficiency (CE) increasing to a high level of 205.1 cm 2 C −1 .EIS measurements demonstrate that the bulk and interface electron transport properties within the device are related to the potential, causing a gradient-colored behavior controlled by the applied voltage.Owing to the symmetric configuration and the charge recombination-bleaching-regime, the two TiO 2 electrodes can alternately work as the electrochromic electrode determined by applying a forward bias or reverse one.The cyclic performance thus can be doubled, which renders the device withstand 4600 cycles of coloring-bleaching without much degradation.However, the diffusion dynamics of I 3 − ions in the electrolyte, which is sluggish in a nonvolatile solvent with high viscosity, make significant impacts on the coloration efficiency and bleaching speed.Therefore, a nonvolatile electrolyte with relatively low viscosity is desired for the EC device based on the charge-recombination regime.The results demonstrated here help inorganic EC devices to achieve high optical contrast, gradient coloration, low energy consumption, long-term cycling stability, and fast switching speed, which are desired for smart windows on demands of architectural aesthetics, optical modulation, and large-scale application.

Figure 1 .
Figure 1.Digital photos of a) the peptized transparent hydrolysate of tetrabutyl titanate as the precursor for hydrothermal treatment, b) the resultant colloidal TiO 2 NCs paste for coating, and c) the sintered TiO 2 NC film on FTO glass; Characterization of the TiO 2 NC film on an FTO glass: d) surface (the inset is the XRD patterns together with the anatase TiO 2 reference JCPDS #21-1272) and e) cross-sectional SEM images; f) SEM-EDS elemental mapping of the cross-section of the TiO 2 NC film; g) Ti 2p XPS spectra and h) N 2 adsorption isotherms of the TiO 2 NCs.The inset is the pore size distribution of the TiO 2 NCs.

Figure 2 .
Figure 2. Cyclic voltammograms of the TiO 2 /LiBF 4 /TiO 2 device scanned at 20 mV s −1 in different potential windows ranging a) from approximately −0.5 to 0.5 to approximately −4 to 4 V and b) oppositely from approximately −4 to 4 to approximately −0.5 to 0.5 V; Electrochromism of the TiO 2 /LiBF 4 /TiO 2 device under the applied voltage from 2.5 to 4 V: c) Optical transmittance spectra and d) digital photos of the coloration of TiO 2 /LiBF 4 /TiO 2 under different applied voltages (the insets show the bleaching results with no applied voltage); Chronoamperometry (CA) spectra obtained by applying a voltage with a step from e) 0 to 3.6 and f) −1.5 to 3.6 V to control the periodic coloration and bleaching of the device (the insets are the digital photos of the colored or bleached device).

Figure 3 .
Figure 3. a) Cyclic voltammograms of TiO 2 /LiI/TiO 2 scanned at 20 mV s −1 in different potential windows ranging from approximately −2.5 to 2.5 to approximately −4.5 to 4.5 V; Electrochromism of TiO 2 /LiBF 4 /TiO 2 under the applied voltage from 1.5 to 4.5 V: b) Optical transmittance spectra and c) digital photos of the coloration of TiO 2 /LiI/TiO 2 under different applied voltages (the insets show the bleaching results with no applied voltage); d) Chronoamperometry (CA) spectra obtained by applying a voltage with a step from 0 to 3.6 V to control the periodic coloration and bleaching of the device (the insets are the digital photos of the colored or bleached device).

Figure 4 .
Figure 4. a) Cyclic voltammograms of the TiO 2 NC electrode in the electrolyte with different LiI concentrations varying from 0 to 0.149 M in the approximately −4 to 4 V potential window scanned at 20 mV s −1 and b) the enlarged part within approximately −2 to 2 V; c) Digital photos of the devices with different LiI concentration in colored state (the insets show the corresponding bleaching results with no applied voltage); d) Mott-Schottky plots of the TiO 2 NC electrode in the electrolyte with different LiI concentration and e) the corresponding linear segment on the plots with the calculated V fb values.

Figure 5 .
Figure 5. Schematics of mechanisms of a) coloration and b) self-bleaching of the TiO 2 /LiI/TiO 2 EC device.

Figure 6 .
Figure 6.Impedance spectroscopies (Nyquist plots) of (a) TiO 2 /LiI/TiO 2 in different potentials ranging from 1.3 to 1.6 V, b) TiO 2 /LiBF 4 /TiO 2 from 2 to 3.5 V, and c) TiO 2 /LiI/TiO 2 from 1.8 to 2.5 V (the inset is equivalent circuit model.Here, R CT is the bulk charge transfer resistance, R 1 is the interfacial charge transfer resistance, CPE1 and CPE2 are the electric double-layer capacitances, W s is the Warburg component due to the diffusion of ions and R s is the resistance of the conducting substrate.);d) Dependence of R ct on the applied potential.

Figure 7 .
Figure 7. Chronoamperometry (CA) spectra obtained by applying a voltage with a step from a) −1.5 to 3.6 V for TiO 2 /LiBF 4 /TiO 2 and b) 0 to 3.6 V for TiO 2 /LiI/TiO 2 to control the periodic coloration and bleaching.Comparison of electrochromic performances of TiO 2 /LiBF 4 /TiO 2 and TiO 2 /LiI/TiO 2 : c,d) Optical density (Δ[OD]) at 700 nm as a function of charge density (CE values were determined by fitting the linear region of the plot); e) Dependence of the coloration time (t c ) on the applied voltage, and the corresponding bleaching time (t b ); f) Variation of optical contrast (T%) against the applied voltage; g,h) CA spectra of in situ coloration-bleaching switching cycles (the insets are the digital photos of TiO 2 /LiBF 4 /TiO 2 and TiO 2 /LiI/TiO 2 after cyclic tests).

Figure 8 .
Figure 8. Cyclic voltammograms of the TiO 2 /LiI(PC)/TiO 2 device scanned at 20 mV s −1 in different potential windows ranging from a) approximately −1.5 to 1.5 to approximately −2.5 to 2.5 and b) approximately −2.6 to 2.6 to approximately -3 to 3 V; Schematic illustration of the electrode reactions at an applied potential c) lower than 2.5 V and d) higher than 2.5 V; Impedance spectroscopies (Nyquist plots) of the TiO 2 /LiI(PC)/TiO 2 device in different potential ranging from e) 0.9 to 1.2 and f) 1.3 to 1.8 V; g) Dependence of the fitting values of R ct and R 1 (with the ratio of R 1 /R ct marked on top of the columns) on the applied potential (the inset is equivalent circuit model.Here, R CT is the bulk charge transfer resistance, R 1 is the interfacial charge transfer resistance, CPE1 and CPE2 are the electric double-layer capacitances, W s is the Warburg component due to the diffusion of ions and R s is the resistance of the conducting substrate.);h) Mott-Schottky plots of the TiO 2 /LiI(PC)/TiO 2 device and the calculated V fb values.
from 1.5 to 2.1 V (or −1.5 to −2.1 V) and the corresponding calculated CE values were 51.5 cm 2 C −1 (or 41.2 cm 2 C −1 ), which was much lower than that when using ACN as the solvent in the electrolyte.The non-linear parts on the ΔOD-ΔQ plots might result from the mutual interference of the two-electron release The recombination of the electrons accumulated in TiO 2 with I 3 − ions plays an important role in the electrochromism, which changes the interfacial electron transport properties of conventional TiO 2 -base EC devices.Electrochemical impedance spectroscopy (EIS) shows the distinction of

Figure 9 .
Figure 9. Symmetrical electrochromism of TiO 2 /LiI(PC)/TiO 2 under the positive and negative potentials: a) Digital photos of the coloration (the insets are the bleaching results with no applied voltage); b) Optical transmittance spectra; Dependence of c) the coloration (t c ) and bleaching time (t b ) and d) the optical contrast on the applied potential; e) Optical density (Δ[OD]) at 700 nm as a function of charge density and the calculated CE values.

Figure 10 .
Figure 10.Optical transmittances of TiO2/LiI (PC)/TiO2 after the first cycle, 2300 and 4600 cycles under a) 2.5 V and b) −2.5 V for coloration, as well as those under open circuit for self-bleaching; c) Digital photos of the device after the corresponding cycles.

Table 1 .
EIS fitting parameters and calculated values.