FR2633609A1 - Electrochromic system - Google Patents

Abstract

An electrochromic system employing an ion conductor containing lithium as electrolyte. This system is characterised by the presence of a counterelectrode made up of a rare-earth oxide, in particular cerium oxide, symmetrical with the layer of electrochromic material. The invention applies especially to the production of glazing for building or for motor vehicles.

Description

ELECTROCHROME SYSTEM
The present invention relates to an electrochromic system preferably an electrochromic glazing operating in transmission or in reflection, the color change of which is due to the reversible insertion of lithium ions in a layer of tungsten trioxide. Glazing or other electrochromic systems are systems whose color is modified by the action of a difference in electrical potential. The electrochromic character is obtained by a layer of an oxide of a transition metal, in particular a tungsten oxide layer placed between two conductive layers of the electric current, an electrolytic layer being interposed between the electrochromic layer and one of the two conductive layers.

 Most of the ion-conducting materials in the first column of the table in MendeleTev are suitable as electrolytes. Thus, electrochromic systems have been described using sulfuric acid or any other strong acid placed in contact with a layer of tungsten oxide. However, the major drawback of proton-conducting electrolytes is their acidic nature, which leads to the rapid degradation of the other layers of the system, the longevity of which is therefore extremely short.

 This defect is less with less corrosive alkali ion conductive materials; lithium electrolytes are more particularly preferred because of the high diffusion rate of lithium ions which allows rapid coloring and discoloration of the system.

 On the other hand, to improve the contrast between the colored state and the discolored state of the system, it is known from US Pat. No. 4,350,414 to use a symmetrical counter-electrode of the electrode formed by the layer of electrochromic material able to insert ions in the bleaching phase and release them in the coloring phase.

 The technical problem which then arises is to find a suitable material for the counter electrode, that is to say perfectly compatible with the electrolyte used, not decomposing under the action of the electric current. and above all not presenting any coloration - Or in the discolored state when the layer of electrochromic material is itself in the discolored state - this last condition being essential for the production of a glazing operating in transmission susceptible for example of 'be used as building glazing.

 A more specific subject of the invention is an electro-chromium glazing capable of operating in transmission comprising a transparent substrate, in particular made of glass coated with a transparent electroconductive layer, of a layer of an electrochromic material formed of an oxide of a transition metal, in particular of tungsten trioxide, of a layer of an ionic conductive material with lithium, of a counterelectrode and of a second conductive layer preferably also transparent.

 The counter electrode according to the invention consists of a layer of an oxide of a rare earth having at least two stable degrees of oxidation. Satisfactory results have been more particularly obtained with a layer of cerium oxide. This layer is for example deposited by thermal evaporation or by reactive sputtering assisted or not with a magnetic field from a target of metallic cerium or by any other technique of depositing a thin layer of metallic oxide. Preferably, the cerium oxide layer has a thickness of between 100 and 500 nanometers. Its reversibility with regard to Li + ions is total. In addition, the diffusion coefficient of Li + ions in cerium oxide is of the same order of magnitude as that measured in tungsten trioxide so that the times of coloring and discoloration are neighbors.

 According to a first embodiment of the invention, the ionic conductive material used as electrolyte is a liquid solution of lithium perchlorate in propylene carbonate.

 According to a second more particularly preferred embodiment of the invention, the ion-conducting material is an organic lithium-conducting polymer. Patent EP 13,199 gives examples of polymers which are suitable for implementing the invention. Good results have been obtained more particularly with a solid solution of lithium perchlorate in polyethylene oxide. This organic polymer preferably serves as an adhesive material for the assembly of laminated glazing, the first sheet of glass being provided with a transparent electroconductive layer and a tungsten trioxide layer and the second glass sheet with an electroconductive layer and the counter electrode.

 Other details and advantageous characteristics of the invention are described below on the basis of exemplary embodiments of the invention.

Characteristics of cerium oxide:
On a silicate glass substrate was deposited by reactive spraying assisted by a magnetic field (magnetron) a layer of 350 manometers of indium oxide doped with tin oxide having a square resistance of 5 Ohms. By vacuum evaporation is then deposited a layer of 160 nanometers of cerium oxide.

 Analysis of the cerium layer indicates a Cet2.3 stoichiometry which can be attributed to a slight hydration.

 By the so-called complex impedance method, a diffusion coefficient of lithium in this layer of cerium oxide was measured equal to 10-13 cm2 / s. Under the same measurement conditions, the diffusion coefficient of lithium in tungsten trioxide is 10-12 cm2 / s.

Insertion of lithium
By secondary ion mass spectrometry, the depth concentration profile of the lithium ions in the cerium oxide layer of 320 nanometers was determined. The profile obtained is shown in Figure 1. we see that the amount of lithium absorbed by the layer increases with the amount of current; there is indeed insertion.

 On the other hand, by the technique of cyclic sweeping voltamperometry, it is verified that the quantity of current inserted in a go cycle is completely recovered in the return cycle.

 The cerium oxide layer is therefore suitable for perfectly reversible insertion of the cerium oxide.

Electrochromic cell
On a silicate glass substrate was deposited a layer of indium oxide doped with tin oxide of 350 nanometers having a resistivity of 5 Ohms / c #. On this layer is deposited a layer of tungsten trioxide by thermal evaporation of a tungsten trioxide powder of 300 nanometers.

 The 2 cm 2 plate coated with a layer of cerium oxide and the glass plate coated with a layer of tungsten trioxide are then assembled in the autoclave by means of a 50 micron polyoxyethylene film within which is dissolved lithium lithium perchlorate. The visible light transmission of the system obtained is greater than 70%.

 The cell is placed in a hot enclosure whose temperature is 80-C. 1300 staining / discoloration cycles were carried out without observing degradation, with a voltage of -2 volts for coloring and + 1 volt for discoloration. For each cycle, the system response time is less than 30 seconds, which is well suited to the production of building glazing or a sunroof for a motor vehicle.

 No change in performance could be observed If the thickness of the cerium oxide layer is doubled.

 The present invention relates more particularly to electrochromic systems operating in transmission, but it goes without saying that it can also be applied to the production of electrochromic systems operating in reflection. It suffices for this to insert at the rear of the system, for example between the layer of cerium oxide and the transparent conductive electrode - or in substitution of the latter - a reflective metallic layer such as a silver layer . This system can then be used in particular for the production of a rear-view mirror. day and night for motor vehicles.

Claims (8)

 1. Electrochromic system comprising a transparent substrate in particular made of glass coated with a transparent electroconductive layer, a layer of electrochromic material formed of a transition metal oxide, in particular of tungsten trioxide, of a layer of a material riau lithium ion conductor, a counter electrode and another transparent conductive layer, characterized in that the counterelectrode consists of a layer of a rare earth oxide, preferably cerium oxide.  2. An electrochromic system according to claim 1, characterized in that the cerium oxide layer has a thickness of between 100 and 500 nanometers.  3. Electrochromic system according to claim 1 or 2, characterized in that the cerium oxide layer is deposited under vacuum.  4. Electrochromic system according to one of claims 1 to 3, characterized in that the layer of cerium oxide has a stoichiometry of 1 atom of cerium for 2, 3 atoms of oxygen.  5. Electrochromic system according to one of claims 1 to 4, characterized in that the ionic conductive material of lithium is a solution of lithium perchlorate in propylene carbonate.  6. Electrochromic system according to one of claims 1 0 4, characterized in that the ionic conductive material is a solid solution of lithium perchlorate in polyethylene oxide.  7. Electrochromic glazing consisting of two glass sheets coated with transparent electroconductive layers assembled by a film of polyethylene oxide in which lithium perchlorate is dissolved, one of the sheets being further coated with tungsten trioxide and the other sheet of a layer of cerium oxide.  8. Electrochromic glazing according to claim 7 consisting of - a glass sheet, - a layer of indium oxide doped with tin oxide, 350 nanometers thick, - a layer of tungsten trioxide 300 nanometers thick, - a film of 50 micrometers of polyethylene oxide in which lithium perchlorate is dissolved, - a layer of cerium oxide of 160 nanometers, - a layer of indium oxide doped with tin oxide, 350 nanometers thick, - of a glass sheet.