DEVELOPMENT OF ELECTROCHROMIC DEVICES WORKING WITH HYDROPHOBIC LITHIUM ELECTROLYTE

This manuscript report on a new lithium electrolyte, allowing the 
manufacturing of electrochemical systems, such as electrochromic 
devices, in ambient atmosphere. It is based on lithium 
bis-trifluoromethane sulfonimide dissolved in the following 
hydrophobic salt, which was first prepared by M. Gratzel et 
al.: 1-ethyl. 3-methyllimidazolium bis-trifluoromethane 
sulfonimide. We have also successfully tested the compatibility of 
this electrolyte with WO3 and TiO2-CeO2 films acting respectively 
as efficient electrochromic electrode and counter electrode for 
smart-window working with Li


INTRODUCTION
Thin film electrochromic (EC) materials (ECM) and electrochromic devices (ECD) present a basis for light modulators and other electro- optic devices [1][2][3][4][5][6][7][8][9][10][11].The ECD can be presumed as a system, *Corresponding author.containing an electrochromic material (ECM1), a lithium ion conducting electrolyte (Li/-IC), and a counter electrode (EMC2).This assembly is sandwiched between two identical transparent electronic conductors (ITO Indium tin oxide) serving as ohmic contacts.EMC1 switches from the transparent to the colored state during the cathodic process when electrons and Li +ions are injected into it.These electrons and cations which are provided by ECM2, transit via the external circuit and the electrolyte respectively.The process must be reversible.ECM2 can either optically switch in a complementary way of ECM2 or can remain transparent in both the inserted and deinserted states.
The most important problem to solve is the fabrication of efficient Li +-IC, EMC1 and ECM2 components, which allow the manufacturing process of ECD in open air, i.e., outside the inert atmosphere of the dry- box.The use of the dry-box is indeed unrealistic for obvious practical reasons; it means, for instance, that hydrophobic Li+-IC have to be used.This paper report on Li+-IC, ECM1 and ECM2 components allowing this important aim to be attained.
(i) Li +-IC is a hydrophobic electrolyte of high ionic conductivity.
Recently, new hydrophobic ionic liquids with low melting tempera- ture, low vapor pressure and high conductivity have been investigated: they are based on hydrophobic ions.Among these hydrophobic ionic liquids we have focused on the following ionic liquids which was first prepared by M. Gr/ietzel et al. [12]: We have indeed successfully used this hydrophobic liquid salt as a solvent for the solid salt: Li(CF3SO2)2N.That allows us to obtain a new hydrophobic lithium electrolyte of high conductivity, recently patented by some of us [13].
(ii) ECM1 is a WO 3 film coated, using the sputtering technique, on ITO glass (ITO glass =_ transparent and conducting Indium Tin Oxide deposited in a glass substrate).
(iii) ECM2 is a TiO2-CeO2 film deposited on ITO glass using RF sputtering.Some of us have established that cerium doped titanium oxide (TiO2-CeO2) is an efficient transparent counter electrode [14][15]: it has been shown that Ce 4 + acts, indeed, as a deep acceptor center for the electrons.It implies that the Ce4+---Ce3+reduction-oxidation will take place during the Li + insertion-deinsertion process and will, therefore, inhibit the Ti4/reduction, thereby maintaining the film transparency.
The schematic synthesis route is depicted below: 10% by weight hydrophobic Li+-IC (gel type) In order to synthesize the hydrophobic Li+-IC, we first mixed 136 cm 3 of 1, 1, trichloroethane (Prolabo 94%) with 24, 3 cm 3 of 1-methylimidazole (Aldrich 99%) under vigorous stirring. 100g of bromoethane (Aldrich 99+ %) were then dropped out, into the solution, for hour.Afterwards, the solution was refluxed for 2 hrs at around 80C, allowing the reaction to proceed.The so-obtained liquid molten salt was immediately washed 3 times with 100 cm 3 of 1, 1, trichloroethane.Finally, 19 g of the crystallized product.1, 3-  EtMelm+Br-(whose melting point is 76C) was obtained.After drying under primary vacuum for hour, it was dissolved in 45.5cm3of distilled water.We separately dissolved 28 g of Li(CF3 SO2)2N in 90 cm 3 of distilled water.The two solutions were intimately mixed under vigorous stirring, at 70C for 30 mins.Finally the 2 phases were separated and, after heating at 70C under primary va- cuum for hour, the hydrophobic salt 1,3-EtMelm(CF3SO2)2N, was obtained (35 g).In order to get the expected hydrophobic Li /-IC, we dissolved 3.5 g of Li(CF3SO2)2N into the 35 g of the hydrophobic salt.
(ii) The Electrochromic Material ECMI W03 The WO3 films were either provided by Dr. A. Richardt, President of the Society Inland Europe-Paris (France), or deposited in our laboratory using planar magnetron Balzer BAS 450 PM apparatus.
We have optimized the sputtering experimental conditions which are listed in Table I. (iii) The Counter Electrode Material ECM TiO-CeO The sputtering conditions for various compositions are listed in Table II.
Film Characterizations X-ray diffraction measurements were carried out, using CuKa radiation, to investigate the film structure.A JEOLJSM-840 A apparatus, allowing scanning or transmission electron microscopy (SEM or TEM) was used to determine the film texture, thickness and composition.

Electrochemical and Spectro Photometric Measurements
Electrochemical experiments were performed with a computer- controlled potentiostat/galvanostat (TACUSSEL, PGS 201 T model)  for the electrochemical cell of Pt/LiTFSI + 1,3-EtMelmTFSI/WO3 (or TiOE-CeO2).All the measurements were advantageously performed at room temperature in air owing to the hydrophobic character for the lithium electrolyte (The Pt voltage was standardized versus a Li electrode).The electrochemical lithium insertion/deinsertion was processed under various charge densities which were repeated automatically.Optical properties of the colored and bleached states were investigated using a UV-Vis-NIR spectrophotometer (Varian Cary 2415 Spectrophotometer equipped with DS-15 Data Station).
The ionic conductivity of the electrolyte was determined from impedance measurements using a Solartron impedancemeter.

RESULTS
Most interestingly, the hydrophobic Li +-IC, -whose water content is less than 3% (Karl Fisher analysis)-, is electrochemically stable over a large electrochemical window (Fig. 1).Moreover, its room tempera- I, potential vs Li FIGURE Cyclic voltammograms of the hydrophobic Li +-IC (anode lithium metal; cathode-stainless steel).
The Figure 3 shows the TEM photographs of the WO3 films prepared at 70C (A) and 150C (B).The photo A evidences crysta- llites of about 10 nm diameter in a rather porous texture.A more dense film is observed on the photo B, and the crystallites have an average diameter of'30 nm.When the substrate temperature is higher than 150C, 300C for instance, the films are highly dense with an average crystallite size of 100 nm.We are then dealing with the orthorhombic variety of WO3.
The color changes are less marked in these film due to the too high crystallinity.That is the reason why we have finally used the two substrate temperatures which are quoted in the table (70C and 150C).
The Figure 4 represents the evolution of the inserted charge (Li /) as a function of the film thickness for the two types of films.A formating process, accounting for the inserted charge difference between the first  A FIGURE 3 1 cm= 10nm TEM photographs of the films prepared at 70C (A) and 150C (B).
insertion step (i) and the following insertion-deinsertion steps (ii), is more pronounced for the more porous films, A. This formating process corresponds, very likely, to the reduction of the adsorbed oxygen such as Oin the nanoscale structure (surface contamina- tion): WO3 + 2Li + + 0.5022-+ 2e----WO3 + Li20 The Figure 5 gives the evolution of the change in optical density (AOD), defined here as ln(Tbeach/Tcoor), as a function of the reversible inserted charge (Li +), for the films A and B. We see that Thickness (nm) FIGURE 4 Evolution of Q, which is the inserted charge (Li+)-, as a function of the film thickness (WO3 films A and B; V 21/" vs. Li, (i) first insertion, (ii) following insertions and deinsertions).
AOD and therefore the optical coulombic efficiency AOD/AQ, i.e., coloration efficiency, is higher for the film A. Consequently, we believe that the film A is the most interesting one (for an application point of view) in spite of the above mentioned formatting process.Moreover, the cyclic voltammogram of the film A, reported on the Figure 6, (mA) shows that the insertion potential is larger than 2V" it is, hopefully, within the stability domain of the hydrophobic Li-IC (Fig. 1).
Concerning the TiO2-CeO2 films, the achievement of a good transparency in both inserted and deinserted states is illustrated on the Figure 7 for the composition (TiO2)0.9(CeO2)o.l:indeed, the absorption coefficient does not change in the whole visible range upon Li + insertion.The reversible Ce 4 + 4-e Ce 3 + redox process occuring during the Li + insertion process is evidenced on the Figure 8. showing the voltammogram for the composition (TiO2)o.9(CeO2)0.1;as shown in this figure, the electrode potential must remain larger than ,,2 V vs. Li, in order to avoid the Ti 4 + reduction and thereby maintain the film transparency.

CONCLUSIONS
All the above mentioned results allow us to manufacture cheap and new electrochromic displays, ECD, which are based on the compo- nents presented in this manuscript.Let us quote that the ECD use Li + ions and not H + ones, so that they will have an efficient memory effect (over several days).The are easily manufactured in "open-air", and not in inert and costly atmosphere of a dry box as it generally occurs for other ECD(Li+), those which are very sensitive to water contamination due to the hydrophilic character of the ionic conduct- ing electrolyte, Li +-IC.We have solved this problem by manufactur- ing new hydrophobic LI+-IC having also a high ionic conductivity and a low vapor pressure.

FIGURE 2
FIGURE 2 Impedance data (from Hz to MHz) of the hydrophobic Li+-IC (the electrolyte is sandwiched between two stainless steel electrodes).

FIGURE 5
FIGURE 5 Evolution of the optical density, AOD, as a function of Q., which is the reversible inserted charge (WO3 films A and B; film thickness 3000 A; A 550 nm).

FIGURE 7
FIGURE 7 Variation of the absorption coefficient, as a function of the inserted charge of a (TiO2)o.9(CeO2)0.1 film (film thickness 3000 A S

TABLE Sputtering Experimental
Condition for WO film