Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Separator – retainer – spacer or materials for use therewith
Reexamination Certificate
1999-07-21
2001-08-07
Weiner, Laura (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Separator, retainer, spacer or materials for use therewith
C429S251000, C429S129000, C429S254000, C429S126000, C429S309000, C029S623100, C029S623400, C029S623500
Reexamination Certificate
active
06270928
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention concerns a polymeric separator for an organic electrolyte electrochemical system, and a process for its production.
It also concerns an electric cell comprising the separator of the invention and a process for the production of an electric cell comprising that separator.
2. Description of the Prior Art
“Solid” organic electrolyte electrochemical systems, in particular cells and supercapacitors, include electrodes around a layer of porous electrically insulative material impregnated with electrolyte, termed a separator.
The separator must have good intrinsic mechanical behavior and a good affinity for the organic electrolyte. Polymeric materials, in particular polyvinylidene fluoride (PVDF) satisfy those two criteria and are conventionally used for the separator.
Polyvinylidene fluoride (PVDF) in particular is widely used. However, as it dries, a PVDF based separator tends to undergo large dimensional variants which can result in the electrode and separator, or even the electrode and the metallic current collector, separating.
The present invention proposes a polymeric separator for an organic electrolyte electrochemical system in which dimensional variants are minimized, by replacing some or all of the PVDF by an elastomeric polymer or by an alloy of an elastomeric polymer with a polymer which swells in the electrolyte.
SUMMARY OF THE INVENTION
The present invention provides a polymeric separator for an organic electrolyte electrochemical system comprising:
a) an elastomeric polymer;
b) optionally, a polymer which swells in the organic electrolyte and with which the elastomeric polymer forms an alloy; and
c) optionally, an inorganic compound; the polymeric separator having a microporous structure characterized by a porosity in the range 30% to 95% and pores with an average diameter in the range 0.1 &mgr;m to 5 &mgr;m.
The average pore diameter is preferably of the order of 1 &mgr;m.
The elastomeric polymer improves the mechanical properties of the separator by limiting dimensional variants.
The elastomeric polymer can be selected from polyurethanes, an acrylonitrile-butadiene copolymer, a styrene-butadiene-styrene copolymer, a styrene-isoprene-styrene copolymer, polyesters and block amide polyethers.
The elastomeric polymer, the main function of which is to improve the mechanical properties of the separator, can advantageously swell in the organic electrolyte. An example of such an elastomeric polymer is polycarbonate-polyurethane.
The polymer which swells in the organic electrolyte must have a certain affinity with the electrolyte but without dissolving in the electrolyte at temperatures in the range 50° C. to 80° C.
The polymer which swells in the organic electrolyte can be selected from polyvinylidene fluoride and its copolymers, polyacrylonitrile, polymethylmethacrylate, polyvinyl formal, polybutylmethacrylate and polyvinyl chloride.
The inorganic compound, such as silica, can improve the mechanical properties and the absorption properties of the separator.
The polymeric separator of the invention advantageously contains 40% to 100% of elastomeric polymer, 0 to 60% of the polymer which swells in the electrolyte and 0 to 20% of the inorganic compound.
The present invention also concerns a process for the production of the polymeric separator described above.
The process of the invention is advantageously based on phase inversion of the polymer or polymers selected for the separator.
A first or “immersion” variant of the invention comprises the following steps:
forming a solution comprising the elastomeric polymer, optionally the polymer which swells in the organic electrolyte, and optionally the inorganic compound, dissolved in a solvent which is common to the two polymers;
spreading the solution on a support in the form of a film;
immersing the film in a non-solvent which is miscible with the solvent; and
drying the film to eliminate the solvent and the non-solvent.
A second or “evaporation” variant of the process of the invention comprises the following steps:
forming a solution comprising the elastomeric polymer, optionally the polymer which swells in the organic electrolyte, and optionally the inorganic compound, dissolved in a solvent which is common to the two polymers, to which is added a non-solvent which is miscible with the solvent in a proportion which is insufficient to cause precipitation of the polymer or polymers;
spreading the solution on a support in the form of a film; and
drying the film to eliminate the solvent and the non-solvent.
The term “solvent” means an organic solvent in which the polymer or polymers dissolve without difficulty and which can be readily eliminated by heating to a moderate temperature.
The term “non -solvent” means a liquid in which the polymer or polymers are not soluble (strong non-solvent) or are only very slightly soluble (weak non-solvent) at the operating temperature. When the selected non-solvent is water, either pure or as a mixture, this temperature is in the range 5° C. to 80° C.
In the second variant of the process for the production of the separator of the invention the solution can be heated before being spread over the support.
The boiling point of the solvent is preferably lower than the boiling point of the non-solvent. Thus during evaporation the solution becomes richer in non-solvent until the polymeric phase becomes insoluble in the liquid and precipitates out.
The solution is spread over the support using a known method such as dipping, coating or spraying.
In the first and second variants of the process of the invention the solvent is selected so that it dissolves the elastomeric polymer, the polymer which swells in the organic electrolyte, when present, and 15% to 30% of the polymer alloy.
The solvent is an organic solvent selected from cyclohexanone, dichloromethane, dimethylacetamide (DMA), dimethylformamide (DMF), hexamethylphosphoramide (HMPA), dimethylsulfoxide (DMSO), triethylphosphate (TEP), N-methylpyrrolidone (NMP), and mixtures thereof.
The non-solvent is selected from water, ethanol, ethylene glycol, glycerol, acetone, propylene carbonate, dichloromethane, ethyl acetate, butanol, pentanol, acetonitrile, and mixtures thereof.
If the selected non-solvent is water the process of the invention has the advantage of not contaminating the environment and facilitating solvent recycling.
The solution of polymer or polymers in the solvent is a concentrated solution. The concentration of polymer(s) is one of the parameters which conditions the porosity of the film. This concentration must not be too high. The solution preferably contains at least 50% solvent.
The film is preferably dried in two stages, firstly at a temperature which is in the range 20° C. to 60° C. and then at a temperature in the range 80° C. to 140° C.
In a third variant of the process for the production of the separator of the invention phase inversion is carried out by cryoprecipitation.
In the first and second variants of the phase inversion process for the production of the separator the support is advantageously a sheet of a chemically inert material which is separated from the film once it has dried.
When the organic electrolyte electrochemical system comprises at least two electrodes each comprising a porous layer containing an electrochemically active material and a binder the support is advantageously the porous layer of one of the electrodes or the porous layer of each of the electrodes.
When the support is the porous layer of one of the electrodes a bifunctional electrode is obtained. The electrode comprises a first electronically conductive porous layer coated with a second microporous layer constituting the separator.
When the support is the porous layer of each of the electrodes two bifunctional electrodes are obtained. Each comprises a first porous layer which is electronically conductive coated with a second microporous layer constituting one half of the separator.
The present invention also concerns an organic electrolyte electric cell comprising the
Andrieu Xavier
Boudin François
Olsen Ib Ingemann
Alcatel
Sughrue Mion Zinn Macpeak & Seas, PLLC
Weiner Laura
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