Electrochromic window, based on a polymeric polyepoxy electrolyte

Abstract: An electrochromic window comprises: (a) a tungsten oxide (WO₃) electrode on a transparent, conductive glass sheet; (b) a nickel oxide (NiO) counterelectrode activated by electrochemical intercalation of lithium, on a transparent, conductive glass sheet; and (c) a solid, polymeric electrolyte interposed between said electrode (a) and said counterelectrode (b), with the polymeric electrolyte being formed by a solid solution of an ionic lithium compound in a solid, crosslinked polyepoxide, obtained by copolymerization of a monoepoxide with a diepoxide.

Description: The present invention relates to an electrochromic window, based on a polymeric polyepoxy electrolyte, which electrochromic window is endowed with improved characteristics. Optical devices which, following the application of an electric signal, are capable of switching from a condition of maximum transparency into the opposite condition of minimal transmittance, are known in the art. These devices are considerably interesting from the viewpoint of energy saving, in the field of building industry (smart window) and in automobile industry. In these devices, the optical modulation can be suitably induced by electrochemical way by means of the use of electrochromic materials. By «electrochromic materials», those compounds are meant which are capable of undergoing reversible changes in colour following suitable electrochemical processes. The example is typical of tungsten oxide, WO₃, which turns from transparent into blue-coloured owing to an electrochemical intercalation process of the type: which takes place in a cell containing a transparent electrolyte capable of supplying the intercalating ion M⁺ (normally, an alcali metal ion). If the cell contains another, optically passive, material (i.e., a material which is transparent irrespective of whether the signal applied to it is an anodic or a cathodic one), or a material which is electrochromic with complementary characteristics to WO₃, a device is obtained which, when a signal (e.g. a cathodic signal) is applied to it, becomes of dark colour (due to the formation of blue-coloured M x WO₃), whilst, when the opposite signal (i.e., an anodic signal) is applied to it, turns back into transparent (owing to the restoration of light-coloured WO₃). Therefore, by the application of a square-wave signal, the optical transparency of the device can be modulated, with said device being hence given the characteristics of an electrochromic window, with the important technological implications as above mentioned. For this known art, reference is made in particular to the paper by B. Scrosati in Chimicaoggi, June 1989, pages 41-45. In the sector to which the present invention is directed, a continuous need exists, due to application reasons, of improving both the laminar configuration of the electrochromic devices in the solid state, and the electrochromic materials, so as to accomplish fast and reversible electrochromic processes. In co-pending Patent Application EP-A-0 454 240, to the same Applicant's name, an electrochromic window is disclosed, which has a laminar configuration and is endowed with improved performance, by means of the use of a polymeric electrolyte in the solid state based on crosslinked polyether, inserted between a layer of tungsten oxide and a layer of nickel oxide, with said layers being deposited on a glass support made conductive by films of indium and tin oxides. A similar window is disclosed in the article by S. Passerini published in the Journal of the Electrochemical Society, Vol. (1989), pages 3394-5. The present Applicant has found now, according to the instant finding, that the introduction of a polyepoxy-based, solid, polymeric electrolyte in the electrochromic window makes it possible for the performance thereof to be further improved, in particular as regards the speed of the electrochromic process. In accordance therewith, the present invention relates to an electrochromic window comprising: (a) a first transparent glass sheet bearing on its inner face a first layer of conductive material of tin oxide, or of tin and indium oxides, and on said first layer an electrode of tungsten trioxide WO₃; (b) a second transparent glass sheet bearing, on its inner face, a second layer of a conductive material of tin oxide, or of tin and indium oxides, and on said second layer a counterelectrode of nickel oxide NiO z , with z being comprised within the range of from 1 to 1.66, activated by intercalation of lithium metal; and (c) a solid, polymeric electrolyte interposed between said electrode (a) and said counterelectrode (b), with said polymeric electrolyte being a solid solution of an ionic lithium compound in a solid, crosslinked polyepoxy, obtainable by copolymerization of a monoepoxide having the formula: wherein: R represents a methyl or ethyl radical; and n is an integer comprised within the range of from 1 to 6; with a diepoxide having the formula: wherein: m is an integer comprised within the range of from 1 to 6; with a molar ratio of the diepoxide (II), to the monoepoxide (I) comprised within the range of from 1 : 100 to 10 : 100; with said solid crosslinked polyepoxide having a weight average molecular weight of at least 10,000 AMU, and a glass transition temperature (Tg) comprised within the range of from -60°C to -80°C. In the electrochromic window according to the present invention, the electrode of tungsten oxide can be prepared, according to technologies presently known in the art, by depositing, by sputtering or evaporation techniques, a thin layer of tungsten oxide (WO₃) on a glass sheet made conductive by means of a film of tin oxide, or of tin and indium oxides. The layer of tungsten oxide will typically have a thickness of the order of 300nm (3,000 Å). The conductor glasses are products available from the market. Also the counterelectrode of nickel oxide can be prepared by deposition on conductive glass by means of techniques known in the art. However, in this initial phase , nickel oxide is not capable of operating as the counterlectrode in an electrochromic window. This condition requires an activation treatment, which essentially consists in a pre-intercalation of lithium. For that purpose, the electrode is charged to an electrochemical cell containing a solution of a lithium salt (e.g., lithium perchlorate) in an aprotic solvent (e.g., propylene carbonate) and a counterelectrode of lithium metal. Owing to the cathodic polarization, the intercalation of lithium inside nickel oxide is promoted. This process modifies the optical properties of said nickel oxide layer. In fact, owing to lithium entering inside its structure, nickel oxide becomes transparent and capable of reversibly donating and retaking said lithium, either without loosing its transparency, or turning into its coloured state with an electrochromic process complementary to the process of (WO₃). In the electrochromic window according to the present invention, the solid polymeric electrolyte interposed between the electrode (a) and the counterelectrode (b), is constituted by a solid solution of an ionic lithium compound in a solid, crosslinked polyepoxide. The ionic compounds of lithium can be advantageously selected from the group consisting of lithium perchlorate, borate, fluoroborate, thiocyanate, hexafluoroarsenate, trifluoroacetate and trifluoromethane-sulphonate. Lithium perchlorate is preferred. The solid, crosslinked polyepoxide useful for the purposes according to the present invention preferably is the product of copolymerization of a diepoxide (II) and a monoepoxide (I) in which R is a methyl radical and n and m are integers comprised within the range of from 1 to 6, in a mutual molar ratio of the order of from 1 : 100 to 6 : 100, with a weight average molecular weight comprised within the range of from 10,000 to 100,000 AMU (Atomic Mass Units). In the solid polymeric electrolyte, the atomic ratio of oxygen in said polyepoxide to lithium in the ionic compound may be comprised within the range of from 6 : 1 to 24 : 1, and preferably is of the order of 14 : 1. In Figure 1 of the accompanying drawing tables a typical form of practical embodiment is shown of the window according to the present invention. In particular, in this Figure: with (1) the glass support is indicated, which bears the layer (2) of tin and indium oxide, onto which a layer (3) of tungsten oxide (having a thickness of approximately 300nm (3,000 Å)) was deposited by sputtering; with (4) the solid polymeric electrolyte is indicated, which has the shape of a film having a thickness of approximately 100 microns, constituted by a solid, crosslinked polyepoxide containing lithium perchlorate. More particularly, said crosslinked polyepoxide is the product obtained by copolymerization of the diepoxide having the formula with the monoepoxide having the formula in a mutual molar ratio of 2 : 100, having a weight average molecular weight of approximately 25,000 AMU and a glass transition temperature (Tg) of -77°C. In this polyepoxide, lithium perchlorate was dissolved in such an amount, as to have an atomic ration of oxigen (contained in said polyepoxide) to lithium (contained in lithium perchlorate) of about 14 : 1; with (5) the glass support is indicated, which bears the layer (6) of tin and indium oxide, onto which a layer (7) of nickel oxide, activated by intercalation with lithium, was deposited, by operating in the way as described hereinabove; with (8) a sealant is indicated; with (9) a spacer is indicated; with (10) an external generator of square-wave voltage is indicated; and with (11) the leads are indicated, which connects the voltage source (10) to two terminals connected to the conductor layer of the electrode and the conductor layer of the counterelectrode. The electrochromic window of Figure 1 is controlled by a square-wave signal generally variable within the range of from -2 volts to +2 volts. During the negative impulse (cathodic WO₃), the window is dark (intercalation of lithium into WO₃) and its transmittance is low, whilst during the following positive impulse (anodic WO₃), the window turns back into transparent (de-intercalation of lithium from WO₃), and transmittance is high. This behaviour, which can be repeated a very large number of times, is bound to the following process: This behaviour is schematically shown in Figure 2 of the accompanying drawing tables. The time interval necessary for the electrochromic processes (i.e., the cathodic and anodic processes) to take place up to completeness is of the order of tens of seconds. This time varies with varying values of the signal which controls the cell. In case of a square wave of from +2.5 volts to -3 volts, the change in transmittance between the minimum and the maximum values thereof, and vice-versa, is obtained within a time of approximately 20 seconds.

Claims: Electrochromic window comprising: (a) a first transparent glass sheet (1) bearing on its inner face a first layer (2) of conductive material of tin oxide, or tin and indium oxides, and on said first layer an electrode (3) of tungsten trioxide WO₃; (b) a second transparent glass sheet (5) bearing, on its inner face, a second layer (6) of a conductive material of tin oxide, or of tin and indium oxides, and on said second layer a counterelectrode (7) of nickel oxide NiO z , with z comprised within the range of from 1 to 1.66, activated by electrochemical intercalation of lithium; and (c) a solid, polymeric electrolyte (4) interposed between said electrode (a) and said counterelectrode (b), characterized in that said polymeric electrolyte is a solid solution of an ionic lithium compound in a solid, crosslinked polyepoxide, obtainable by copolymerization of a monoepoxide having the formula: wherein: R represents a methyl or ethyl radical; and n is an integer comprised within the range of from 1 to 6; with a diepoxide having the formula: wherein: m is an integer comprised within the range of from 1 to 6; with a molar ratio of the diepoxide (II) to the monoepoxide (I) comprised within the range of from 1 : 100 to 10 : 100; with said solid crosslinked polyepoxy having a weight average molecular weight of at least 10,000 AMU, and a glass transition temperature (Tg) comprised within the range of from -60°C to -80°C. Electrochromic window according to claim 1, characterized in that the electrode of tungsten trioxide is obtainable by depositing, by sputtering or by evaporation, a layer of about 300 nm (3,000 Å) of tungsten trioxide on the first conductive layer. Electrochromic window according to claim 1, characterized in that the counterelectrode of nickel oxide is obtainable by depositing nickel oxide onto the second conductive layer, with a following activation treatment by electrochemical intercalation of lithium. Electrochromic window according to claim 1, characterized in that the ionic compounds of lithium are selected from among lithium perchlorate, borate, fluoroborate, thiocyanate, hexafluoroarsenate, trifluoroacetate and trifluoromethane-sulphonate. Electrochromic window according to claim 4, characterized in that the ionic compound of lithium is lithium perchlorate. Electrochromic window according to claim 1, characterized in that the solid, crosslinked polyepoxy is the product of copolymerization of a diepoxide (II) and a monoepoxide (I), in which R is the methyl radical, with their mutual molar ratio being comprised within the range of from 1 : 100 to 6 : 100, with a weight average molecular weight comprised within the range of from 10,000 to 100,000 AMU. Electrochromic window according to claim 1, characterized in that in the solid, polymeric electrolyte the atomic ratio of oxygen of said polyepoxide to lithium of said ionic compound is comprised within the range of from 6 : 1 to 24 : 1, and preferably is of the order of 14 : 1. Electrochromic window according to claim 1, characterized in that it furthermore comprises a source of square-wave voltage and leads which connect said voltage source to two terminals respectively connected to the two conductive layers of the electrode and of the counterelectrode.

Inventors: ANDREI MARIA, MARCHESE LUCA, PASSERINI STEFANO, ROGGERO ARNALDO, SCROSATI BRUNO, ANDREI, MARIA, MARCHESE, LUCA, PASSERINI, STEFANO, ROGGERO, ARNALDO, SCROSATI, BRUNO
Applicants: ENIRICERCHE SPA, ENIRICERCHE S.P.A.
Application year: 1991
Main classification: G02F1
IPC codes: G02F1