Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
1998-02-27
2002-04-02
Brouillette, Gabrielle (Department: 1745)
Metal working
Method of mechanical manufacture
Electrical device making
C029S623500, C429S145000, C429S250000
Reexamination Certificate
active
06364916
ABSTRACT:
The present invention relates to a method of manufacturing an organic electrolyte electrochemical cell of structure that is unitary, i.e. in with the various component elements are connected together so as to form a single whole.
BACKGROUND OF THE INVENTION
Organic electrolyte cells, in particular lithium cells are presently the subject of rapid and major development because of the high energy density they make available and because of their long lifetime. They are particularly suitable for use in objects of small dimensions.
Traditional cells are made up of solid electrodes sandwiching a porous separator containing liquid electrolyte. Because of the risk of leakage, such cells are ill-suited to consumer portable equipment.
As a result cells have been proposed that include an electrolyte that is made solid by being immobilized in a polymer matrix. However, over time electrical contact between the various components of the generator degrades, particularly if gas is given off. In order to limit that phenomenon, the cell is held in a compressed state by a rigid metal container, thereby increasing the weight of the cell. Attempts have therefore been made to interconnect the various components of the generator so that electrical contact between them is guaranteed under all circumstances.
One method of making an electrochemical cell in which the various layers are bonded together is proposed in U.S. Pat. No. 5,540,741. To constitute a first electrode, a paste is deposited on a conductive support, the paste being formed of an electrochemically active material and a polymer solution. After it has dried, the electrode is covered in a layer of a solution of a plasticizer and a copolymer of a vinylidene fluoride and of hexafluoropropylene (VDF-HFP) to form the separator. The resulting assembly is covered in a second electrode constituted by a sheet of lithium made elsewhere. Pressing is performed at a temperature of at least 150° C., causing the copolymer to melt completely and thus leading to a non-porous material. The plasticizer is subsequently extracted by means of a solvent that is inert relative to the polymer. On being used, the cell is impregnated by the electrolyte which occupies the void left between the polymer chains by removal of the plasticizer.
That method suffers from the drawback of giving rise to large changes in dimensions during manufacture of the cell. During extraction, the structure collapses, thus leading to a dense material. Consequently impregnation with the electrolyte takes place slowly. Thereafter impregnation leads to the VDF-HFP copolymer swelling in the presence of the electrolyte solvent. Such changes in dimensions give rise to tightening and unsticking phenomena that are harmful to the electrical continuity of the resulting cell.
OBJECTS AND SUMMARY OF THE INVENTION
An object of the present invention is to provide a method of making an electrochemical cell of unitary structure which minimizes variations in dimensions.
The present invention provides a method of manufacturing an organic electrolyte electrochemical cell comprising at least one electrochemical couple made up of two electrodes sandwiching a solid film of porous polymer containing said electrolyte, each electrode comprising a porous layer containing an electrochemically active material and a binder, the method comprising the following steps:
a polymer is put into solution in a solvent;
said solution is spread in the form of a film on a support;
said film of solution is immersed in a volatile non-solvent that is miscible with said solvent in order to precipitate said polymer;
said polymer film is dried to eliminate said non-solvent; and
said couple made up of said polymer film placed between said electrodes and in contact therewith, and impregnated with said electrolyte, is pressed while being heated to a temperature less than or equal to the temperature at which said polymer film starts to melt so as to obtain incomplete melting of said polymer, said electrodes becoming uneeparable after cooling.
The invention presents numerous advantages over known methods. Because the polymer film is impregnated with electrolyte, its porosity is conserved during the sticking step. The mean size of the pores lies in the range 0.1 &mgr;m to 1 &mgr;m. The porous volume is large, constituting 30% to 95% of the volume of the film.
The pressing temperature is selected so that melting of the polymer film remains incomplete and restricted mainly to the surface. The film adheres on the electrodes as soon as the sticking surface has softened sufficiently Consequently, dimensions do not change during sticking, and the porous volume is not significantly altered. In addition, when electrolyte impregnation takes place after sticking, the film is observed to swell, and this is avoided by the method of the invention.
In an implementation of the method of the invention, the support used is an inert support.
The polymer film has good mechanical strength since the polymer itself contains less than 30% of the electrolyte solvent, with the major portion being contained in the pores.
In a first variant, said polymer film and each of said electrodes is impregnated with said electrolyte, and then said polymer film is placed between electrodes and in contact therewith to form said couple.
In a second variant, said polymer film is placed between said electrodes and in contact therewith to form said couple, and said couple is then impregnated with said electrolyte.
In another implementation of the method of the invention, at least one of said electrodes is used as the support as follows:
said solution is spread on the surface of said porous layer of one of said electrodes in the form of a film;
said film is immersed in a volatile non-solvent that is miscible with said solvent;
said film is dried to eliminate said non-solvent;
said electrode including said film is impregnated with said electrolyte;
the film side of said electrode is covered in an electrode impregnated with said electrolyte to form an electrochemical couple; and
said couple is pressed while being heated to a temperature less than or equal to the temperature at which said film begins to melt, so as to obtain incomplete melting of said polymer, said electrodes becoming unseparable after cooling.
In yet another implementation of the method of the invention, both of said electrodes are used as the support, as follows:
said solution is spread in the form of a film on the surface of said porous layer of each of said electrodes;
each of said films is immersed in a volatile non-solvent that is miscible with said solvent;
said films are dried to eliminate said non-solvent;
said electrodes including said films are impregnated with said electrolyte;
said electrodes are placed together so that said films are in contact to form an electrochemical couple; and
said couple is pressed while being heated to a temperature lower than or equal to the temperature at which said film begins to melt so as to obtain incomplete melting of said polymer, said electrodes becoming uneeparable after cooling.
Using the electrodes as the support for making the polymer film makes it possible to avoid subsequent handling, and thus makes it possible to deposit films that are thinner. The method is thus simplified and made more reliable, and the final product has better performance.
Said polymer is selected from: polyvinylidene fluoride; polyvinyl chloride; polymethylmethacrylate;
cellulose acetate; a polysulfone; a polyether; a polyolefin; and from an alloy of polyvinylidene fluoride with a polymer selected from a polysulfone, polymethylmethacrylate, polyvinylpyrolidone, a copolymer of vinylidene fluoride and ethane tetrafluoride, and a copolymer of vinylacetate and of vinylalcohol.
The preferred one of said polymers is polyvinylidene fluoride (PVDF). PVDF has the advantage of exhibiting very little swelling in the presence of solvent, thus limiting dimensional changes during the manufacture of the electrochemical cell.
Said solvent is an organic solvent selected from: cyclohexanone; dichloromethane; dime
Andrieu Xavier
Boudin François
Alcatel
Brouillette Gabrielle
Sughrue & Mion, PLLC
Tsang Susy
LandOfFree
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