Electrolyte

Details Electrolyte Electrolyte

1999-04-06

2001-06-12

Niland, Patrick D. (Department: 1714)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

At least one aryl ring which is part of a fused or bridged...

C204S194000, C204S410000, C359S265000, C359S270000, C361S500000, C361S523000, C361S527000, C429S303000, C429S304000, C429S307000, C524S099000, C524S104000, C524S408000

Reexamination Certificate

active

06245847

ABSTRACT:

This invention relates to an electrolyte, in particular a polymer electrolyte suitable for batteries, supercapacitors, electrochromic windows and displays.
Conventional electrolytes for high power rechargeable batteries such as lithium ion batteries are mixtures of organic liquids. However, these liquids tend to be highly inflammable giving rise to concern about safety and also degrade with time, possibly due to adverse electrochemical reactions. Solid electrolytes have been proposed which are prepared by dissolving, or complexing, lithium salts within a polymer matrix to form a material which can be deposited as a thin film onto suitable electrode materials. In another example, a low melting temperature mix of lithium salts is dispersed within a polymer to give a film with enhanced conductivity. However, these electrolytes suffer from low ionic conductivities, whether they are plasticised polymers or immobilised liquids in polymers, because the ionic conductivity of the liquid fraction is low. This low ionic conductivity leads to poor performance at high currents. Such electrolytes are thermally stable at room temperature, but degrade easily if heated.
In accordance with a first aspect of the present invention, an electrolyte comprises a composite of a polymer and molten salt electrolyte immobilised within the polymer, wherein the composite is stable and ionically conductive at a temperature substantially equal to the melting point of the molten salt electrolyte.
The electrolyte of the present invention suitably has a solid form, remains stable at elevated temperatures, has good ionic conductivity and the liquid molten salt electrolyte does not leak from the composite.
The term “molten salt electrolyte” refers to any salt which conducts electricity when in a liquid or molten state. These salts are well known in the art.
Preferably, the molten salt electrolyte is a liquid at a temperature in the range from room temperature to 80° C. above room temperature.
This is convenient for many applications where batteries are used at room temperature, because no additional heat source is required before they can be used, but the benefits of the invention still apply with molten salts which operate at higher temperatures and an electrolyte using these molten salts has the capacity to be held for extended periods in a reserve state.
Preferably, the molten salt electrolyte incorporates a nitrogen containing cation, such as imidazolium or pyridinium.
Preferably, the molten salt electrolyte incorporates an anion comprising one of a sulphonate, sulphide, borate, phosphate or halide. For example alkyl sulphonate, optionally substituted with halide.
Examples of molten salts include 1-ethyl-3-methylimidazolium, and other dialkylsubstituted imidazolium or pyridinium salts.
As used herein the term “alkyl” means a straight or chain alkyl group, for example of from 1 to 20, more preferably from 1 to 6 carbon atoms in length. The term “halide” includes fluoride, chloride, iodide and bromide.
Preferably, the polymers include acrylate polymers; nitrile polymers; styrene polymers; polyalkenes, e.g. where the alkene includes from 2 to 8 carbon atoms and where the carbon atoms are optionally substituted, for example with hydroxy or C-6 alkoxy; polyethers, polyfluorinated compounds or copolymers thereof.
Specific examples of polymers which are suitable include polyethylene oxide, polymethylmethacrylate, polyacrylonitrile, polypropylene, polystyrene-polylbutadiene, polyethylene glycol, polyvinyldenefluoride, polyvlnyidene-hexafluoropropylene copolymers, or ethylene propylene diene monomer.
Preferably, the electrolyte further comprises a metal salt dissolved in the molten salt electrolyte.
This increases ionic conductivity.
Suitably, the metal salt comprises an alkali metal or alkaline earth metal salt, such as lithium, sodium, potassium, magnesium and calcium salts.
Preferably, the anions of the metal salt comprise borates, such as tetrafluoroborate; phosphates, such as hexafluorophosphate; imides; arsenates, such as hexafluoroarsenate; sulphonamides, such as trifluorosulphonamide; or sulphonates, such as trifluoromethane sulphonate.
Preferably, the electrolyte further comprises an organic solvent. The addition and solvation of organic species decreases the viscosity of the liquid molten salt electrolyte so that there is a further increase in ionic conductivity of the polymer electrolyte.
Examples of organic solvents include methanol, dimethylformamide, tetrahydrofuran, ethanol, propan-2-ol, propanone and N-methyl pyrollidinone.
The electrolyte alternatively comprises an organic plasticiser in order to reduce the viscosity of the molten salt electrolyte and increase the ionic conductivity of the polymer electrolyte.
Examples of organic plasticisers include, cyclic carbonates, cyclic and alkyl esters and ethers, such as propylene carbonate, diethyl carbonate, dimethylcarbonate and ethylene carbonate.
In accordance with a second aspect of the present invention, an electrochemical cell comprises a pair of electrodes; and an electrolyte according to the first aspect between the electrodes.
In accordance with a third aspect of the present invention, a supercapacitor comprises a pair of conducting electrodes; and an electrolyte according to the first aspect.
Preferably, the conducting electrodes comprise one of metal foil, high surface area carbons, or electrically conducting polymers.
In accordance with a fourth aspect of the present invention, an electrochromic window comprises first and second electrically conducting layers; first and second metal oxides coated on the respective layers; and an electrolyte according to any of claims
1
to
11
in contact with each of the metal oxide layers; wherein a change in potential difference between the conducting layers gives rise to a change in transparency of the metal oxide layers.
In accordance with a fifth aspect of the present invention, an electrochromic display comprises a plurality of display elements; each display element comprising first and second electrically conducting layers; and an electrolyte according to the first aspect between the first and second layers; wherein application of a potential difference between the conducting layers causes a change in transparency of the second electrically conducting layer.


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patent: 5693432 (1997-12-01), Matsumoto
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Pyati et al, “Voltammetry and conductivity of a polyether-pyridinium room temperature molten salt electrolyte and of its polymer electrolyte solutions in polydimethylsiloxane”, Journal of the Electrochemical Society, vol. 143, No. 2, Feb. 1, 1996, Manchester, New Hampshire, pp. 401-405.
Watanabe et al, “Ionic conductivity of polymer electrolytes containing room temperature molten salts based on pyridinium halide and aluminum chloride”, Electrochimica Acta, vol. 40, No. 13 14, Oct. 1, 1995.

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