Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
1999-05-26
2001-06-12
Ben, Loha (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S265000, C429S303000, C429S317000, C429S307000
Reexamination Certificate
active
06246508
ABSTRACT:
The present invention relates to high voltage electrochromic devices, and particularly to the electrolyte used therein.
Electrochromic devices are electrochemical devices the colour and/or the radiation transmission of which are changed upon changing the device potential applied.
The recent development within electrochemical technology has revealed systems like primary and secondary batteries, capacitors and electrochromic devices of high cell voltages, e.g. the systems based on lithium known in the art. These high voltage electrochemical devices are devices having an operational positive electrode potential above 2 V vs. Li/Li
+
.
For the above mentioned systems one of the limiting factors for their performance has been their low electrolyte electrochemical stability, i.e. that the components of the electrolyte system are irreversibly oxidised, reduced or decomposed as a result of the strong oxidation/reduction-forces implied by the high cell voltage. This subject is referred to as voltage stability, and the voltage range in which the an electrolyte system is stable is referred to as the voltage stability window.
When the electrolyte system of an electrochemical device is damaged by irreversible oxidation or reduction the performance of the system is reduced accordingly and the system is eventually rendered unusable.
As a number of these systems are meant for continued use, involving multiple charging/discharging cycles and because the effect of irreversible degradation is cumulative, it is crucial that the electrolyte system provides excellent voltage stability to ensure acceptably long life times for these systems.
In e.g. lithium based systems electrode potentials for the positive electrode of 4.5V vs. Li/Li
+
are observed for a number of the above mentioned applications, and, when combined with Li-based negative electrode structures of approx. 0 V vs. Li/Li
+
, a voltage stability window in excess of 4.5V is required to prevent the electrolyte system from deteriorating.
Electrochromic devices, which are also referred to as variable transmission windows or “smart windows”, are as mentioned above transparent electrochemical systems, the colour of which can be controlled upon variation of the applied potential. Upon change of colour the transmission properties of the system components are changed, for visible light as well as for other solar radiation frequencies.
Electrochromic devices may be based on combinations of a number of materials. The transmission variation may be based on the colour variation of a single component, however, potentially two or more components may change colour simultaneously upon potential variation.
The colour and transmission variation should be fast. In practical use, e.g. for its application in buildings the response time should be in the seconds or tens of seconds range. Also the colour and transmission variation should be uniform for the entire surface area of the electrochromic device. Further, the variation should be fully reversible; variable transmission windows should be coloured and bleached thousands of times without loss of performance.
The common electrochromic device is based on a working glass electrode of tungsten trioxide, WO
3
, which is colourless at high potentials and blue at low potentials. The working potential range for tungsten trioxide is approx. 1-4 V vs. Li/Li
+
. At 1-2 V vs. Li/Li
+
counterions like Li
+
are intercalated into the structure, which in this potential range is blue. At higher potentials, say in the range 2-4 V vs. Li/Li
+
, those counterions are desintercalated from the structure. The “empty” structure of tungsten trioxide is colourless.
Traditionally, a glass electrode based on nickel oxide, NiO
x
, is used as counter electrode. Being light brown at high potentials, nickel oxide is transparent at low potentials, say when combined with the bleached tungsten trioxide. The operational potential range for the nickel oxide electrode is similar to the range of the tungsten trioxide, say 1-4 V vs. Li/Li
+
.
In the electrochromic device both of these electrodes are coated onto thin layers of a transparent current collector. Such current collectors are traditionally based on indium tin oxide (ITO) films, which are indium oxide films doped with approx. 10% of tin.
Sandwiched between those electrode-current collector systems is the electrolyte. The complete electrochromic device is therefore formed from one working electrode-current collector unit, one counter electrode-current collector unit and the electrolyte. In a final window assembly, the electrochromic device may be covered with a transparent protective layer and placed on top of one layer of glass or be confined between two layers of glass.
The operational voltage range of electrochromic devices may be (−2.3 V)-(+2.3 V), or may be as broad as (−3.0 V)-(+3.0 V), corresponding to the full stability window of the electrodes.
The electrode operational potentials reflect the stability demands for the electrolyte of the electrochromic device. Upon polarisation, the maximum operation potential of the working electrode is in the range of 4-4.5V vs. Li/Li
+
. At such high potentials, electrolyte components may be oxidised. Similar, at low potentials, components may be reduced. Both of these processes may lead to irreversible decomposition of the electrolyte components resulting in gassing and eventually system failure.
The voltage stability requirements are valid in the short term as well as in the long term. In the short term, decomposition of unstable components may lead to cell failure. However, also long term stability is required, as electrochromic devices should be charged and discharged thousands of times. Under such conditions, even the smallest instability will lead to slow decomposition, which over time will lead to functional failure of the electrochromic device.
Traditionally, the electrolytes of high voltage electrochemical devices have been organic carbonate-based electrolytes, which provide electrochemical stability of approximately 4 V. Such electrochemical device is e.g. described in WO patent application no. 9500502, which describes batteries, capacitors and electrochromic devices having an acceptable stability against oxidation as well as against reduction. However, as indicated above there exists a need for alternative electrochromic devices being stable at even higher potentials.
U.S. Pat. No. 5,266,422 describes an electrolyte for use in batteries comprising a polyhydroxyalkanoate polymer and a salt. Optionally the electrolyte may comprise a plasticizer, e.g. dibutyl phthalate, for improving the film forming process and to increase the conductivity.
JP-A-03,084,808 of Yuasa Battery Co Ltd. describes a high stability solid polymer electrolyte based on a salt dissolved in one or more solvents selected from tetrahydrofuran, 2-methyl tetrahydrofuran, 1,3-dioxalane, 4,4-dimethyl-1,3-dioxalane, &ggr;-butyrolactone, ethylene carbonate, propylene carbonate, methyl bis(3-methoxyethyl)ether a.o. The electrolyte is used for the manufacture of primary and secondary batteries, capacitors, electrochromic devices and electrochemical sensors.
For long term operation, however, the electrolyte solvents of JP-A-03,084,808 do not provide electrochemical stability beyond 4 V vs. Li/Li
+
.
Japanese laid open application No. 6-267589 discloses a rechargeable non-aqueous battery comprising an electrolyte system based on an electrolyte salt dissolved in ethylene carbonate and an organic solvent having a melting point below −20° C. and a boiling point above 160° C., said electrolyte system having improved charging and discharging characteristics in terms of improved thermal stability. The organic solvent is mentioned as e.g. being diethyl phthalate.
However, no-one has so far suggested an electrolytic composition suitable for use in an electrochromic device and having the desired properties regarding voltage stability.
Accordingly, it is an object of the present invention to provide high
Consigny Marine
Yde-Andersen Steen
Ben Loha
Danionics A/S
Darby & Darby
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