FR2671231A1 - Electrochemical cell protected against self-discharge - Google Patents

Abstract

The cell comprises a chemically active material 8 and a gas-diffusion electrode 10 in a casing formed by a box 2 and a lid 4 separated from each other by an insulating seal 6. An orifice 28 is provided in the box 2 to allow the gas absorbed or produced by the gas-diffusion electrode to pass. <??>According to the invention, a sheet 26, which is leaktight with respect to this gas, is arranged in the cell near the bottom of the box, to prevent self-discharge of the cell. <??>Application to gas cells, in particular in the medical field, and to zinc/air cells (batteries).

Description

i

ELECTROCHEMICAL CELL PROTECTED AGAINST SELF-DISCHARGE

The present invention relates to an electrochemical cell, such as a gas cell or a zinc-air battery, protected against self-discharge. Gas electrochemical cells are cells containing a chemically active material associated with an aqueous electrolyte and

a gas diffusion electrode which, when the cell is traversed

street by an electric current (for example, if the poles of the cell are electrically connected with each other), react together to produce a quantity of hydrogen, proportional to this electric current This operation is only possible under anaerobic conditions. The gas emitted is used in particular as a propellant gas, for example in the medical field to push the piston of a syringe at constant speed and thus deliver continuously and

regular medication under a very low flow.

Zinc-air batteries are identical in construction but func-

work in the opposite way, that is, the oxygen in the air reacts with the chemically active material to produce an electric current. These two types of cells are generally in the form of a button cell battery.The box and the cover are isolated from each other by a gasket and together form a housing containing the

chemically active material and the gas diffusion electrode.

The box, in electrical contact with the gas diffusion electrode, is provided with at least one orifice for the passage of gas through the cell (input of 02 in the case of the zinc-air battery;

H 2 outlet in the case of the gas cell).

If no special precautions are taken for the storage of an electrochemical cell after its manufacture, the interior of the cell is not protected from the ambient air The oxygen from the air enters the cell through the orifice of the box and causes a self-discharge of this cell, and the carbon dioxide causes a

zinc passivation.

The humidity is also harmful during the storage of the cell.If it enters, the pressure in the cell increases and the aqueous electrolyte is expelled from the cell On the contrary, if the ambient air is dry, the cell dries up (expulsion water) and part of the aqueous electrolyte disappears, reducing the autonomy of

the cell.

A first way to protect such a cell during storage is to place it under an inert atmosphere in a sealed container. These containers can be formed from a container molded from a plastic sheet and closed by a glued metal sheet When it is desired to use cell, all you have to do is tear the metal foil to remove the cell from its container. A second way to protect a cell from self-discharge during storage consists of covering the openings of the housing by means of a protective pad placed on the external face of the cell, as is commonly done for the protection of zinc batteries. -air The use of protective pellets made of polymer material for zinc-air batteries is mentioned in particular in the

documents JP-63/224161, JP-62/172668 and JP-58/164153 It is also

known to use a protective patch formed from a thin aluminum foil covered with a polymeric material

(see JP-55/043765).

These protective pellets naturally adhere to the cell due to the respective natures of the materials of the cell (metal, generally stainless steel) and of the protective pellet (polymer). However, it has been observed in use that the protection offered by these pellets was not perfect In particular, the polymer materials not being perfectly impervious to oxygen (or, more generally, to gases) as well as to humidity, there is always a certain self-discharge of the cell during storage This is a bearable drawback in certain cases (battery for hearing aids) where a slight decrease in battery life can be accepted, but a major drawback in the case of gas cells used in the medical field for injection of medication o it is very important that

the autonomy of the cell is not degraded before use.

It should also be noted that the degree of self-discharge of a cell is not known, we can only affirm that it increases

with the duration of storage, which is inconvenient in all cases.

The object of the invention is to remedy the drawbacks of the techniques

known features of protection of electrochemical cells of the

mentioned above, in particular the self-discharge.

To this end, the invention relates to an electrochemical cell.

mique comprising a housing formed of a first electrical part

conductive and a second electrically conductive part separated from each other by an insulating gasket and containing a

chemically active material associated with an electrolyte and an electric

gas diffusion trode, said first part of the housing comprising

both at least one orifice through which said gas enters or escapes from the cell, which is characterized in that it comprises a sheet impervious to said gas disposed inside said housing

between the gas diffusion electrode and said orifices.

This waterproof sheet preferably comprises at least one metal sheet It can be arranged, for example, directly

on the internal face of the first part or in a diffused layer

health between this first part and the diffusion electrode

gas.

In the case of a gas cell, the thickness of the sealed sheet can be chosen relatively small, so that it can be torn automatically by the pressure prevailing in the cell when the cell starts to deliver, or relatively

high, so that it can only be torn by use.

lizer using a tip (nail, pin or other) introduced

in one of the openings of the cell.

In the case of a zinc-air battery, the waterproof foil must always

days be torn by the user.

The characteristics and advantages of the invention will emerge

better of the description which will follow given by way of illustration,

but not limiting, with reference to the accompanying drawings, in which: FIG. 1 shows in section an electrochemical cell provided with a protective pellet according to the prior art,

Figure 2 shows in section a first embodiment

sation of an electrochemical gas cell provided with a protection against self-discharge according to the invention, Figure 3 shows the shape of the sealed sheet when the cell of Figure 2 has partially self-discharged, the FIG. 4 is a graph representing the internal pressure as a function of time in a typical gas electrochemical cell the orifices of which are closed, and

Figure 5 shows in section a second embodiment

tion of an electrochemical gas cell fitted with a protection

against self-discharge according to the invention.

The cell shown in Figure 1 comprises two electrically conductive parts, namely a box 2 and a cover 4, separated from each other by an insulating gasket 6, generally made of plastic Parts 2 and 4 are generally made of stainless steel or in steel

covered on their outer face with a nickel film.

This cell contains a chemically active material 8, comprising

nant Zn and an aqueous electrolyte such as KOH or Na OH, which occupies the

greater part of the volume of the cell and which forms the anode.

In this case, to avoid a reaction (corrosion) between Zn and the cover, the latter is covered on its internal face with a layer of Cu The cell also contains a gas diffusion electrode 10, forming the cathode This electrode is formed a layer 12 formed from a mixture of a PTFE powder and a Raney metal; a stainless steel screen 14 forming the current collector; and at least one sheet 16 of a material porous to gases and impermeable to the electrolyte, such as PTFE A separator film 18, porous to ions, is arranged between the cathode and the anode and a diffusing layer, for example made of non-woven material, is placed between the electrode

10 and box 2.

Finally, note that the box 2 is provided with a plurality of ori-

fices 22, the diameter of which is of the order of approximately 0.4 mm, which allow the hydrogen formed in the cell to escape (in the case of the gas cell) or the oxygen in the air d 'reach the electrode 10

(case of the zinc-air battery).

The chemical reactions that take place in the cell are at the anode: Zn + 2 OH + Zn (OH 2) + 2 e at the cathode: 2 H 20 + 2 e H 2 + 2 OH (gas cell)

or 02 + 2 H 20 + 4 e 4 OH (zinc-air battery).

The orifices are sealed, in a conventional manner, by a pellet.

the protector 24 made of polymer material The disadvantages of this protective pellet have been underlined above.Remember that, in particular, this construction does not make it possible to completely prevent self-discharge of the battery and does not allow the value to be known.

of this self-discharge.

FIG. 2 illustrates a first embodiment of an electrochemical cell according to the invention By way of example, a gas cell is considered here. The numerical references common with those of FIG. 1 designate the same elements In this cell, the protection against self-discharge is obtained, according to the invention, by a sheet 26 gas-tight produced by the cell (here, hydrogen) placed inside the cell, and more exactly, in this mode of embodiment, between the box 2 and the diffusing layer 20 This sheet 26 is preferably made of metal, for example aluminum (which can be protected against alkaline attack), copper or gold Its external face can be covered, at least in the part located opposite the orifices, by a layer of polypropylene or another polymer material, so

to prevent corrosion of the metal of this sheet 26.

Furthermore, the box is provided with an orifice 28 (but there could be several) having a sufficiently large diameter for the sealed sheet 26 to be able to deform under the influence of the pressure prevailing in the gas cell when it delivers This diameter is of the order of at least 2 mm. For a gas cell of 11 mm in diameter, it is for example of the order of mm. The edge of the orifice 28 located on the internal side of the cell, that is to say the edge with which the sheet 26 is in contact, is preferably rounded. Indeed, a pointed or sharp edge risks unexpectedly tearing the sheet. sheet 26 as soon as the internal pressure rises. 35 With this structure, the cell is protected against self-discharge due to ambient air The gas cell can however discharge during storage if the poles of the button cell battery are

accidentally brought into electrical contact with each other.

However, it is possible to visually determine whether the cell has self-discharged or not.It suffices to look if the waterproof sheet is not deformed (as shown in Figure 2), which indicates that the cell has gas did not self-discharge; if it is inflated outward (as shown in Figure 3), which denotes some self-discharge; or if she is

torn, indicating significant or even total self-discharge.

The thickness of the sheet 26 is chosen so that it tears for a pressure in the gas cell, or more exactly a pressure difference between the inside and the outside of the gas cell included, for example, between 0.5 and 5 bars This is a pressure much lower than the resistance of the cell which can withstand internal pressures much greater than 15 bars, as shown in the graph of figure 4 In this figure, we have indicates the internal pressure as a function of time in a cell of the RENATA GZ 13-18 type carrying a current of 3 m A and whose

orifices in the cell are plugged.

It is also possible to provide that the user himself tears the sheet 26 by means of a point (nail, pin or other) when he wishes to use the gas cell.In this case, the tear resistance of the sheet 26 is indifferent.We can therefore use a sheet having the same tear resistance as above, but it is also possible to choose a much thicker sheet 26 so that it is much more resistant to tearing, for example so that the pressure difference required between the inside and the outside of the gas cell to

tearing the sheet 26 is of the order of at least 10 bars.

Of course, when the user is expected to have to pierce the sheet 26 himself, the diameter of the orifices is smaller. It is then possible to use larger orifices.

small, as in the cell in Figure 1.

The solution consisting in tearing the waterproof sheet 26 automatically

tically by the internal pressure of the gas cell is preferable to that consisting in manually tearing the sealed sheet In fact, a gas cell is necessarily used in an anaerobic atmosphere, failing which oxygen enters the cell and it works in gold zinc-air battery mode, if the waterproof foil is

manually torn before being placed in its environment.

anaerobic, it is necessarily in contact with the ambient air for a certain period of time This drawback is avoided if the gas cell is first placed in its anaerobic environment, then started up so that the sealed sheet is sealed.

automatically rips.

In the case of a zinc-air battery, o the cell must receive oxygen from the outside to start functioning, the user

must always pierce the waterproof sheet 26 himself.

A second embodiment of the invention is shown in Figure 5 The essential difference from the embodiment of Figure 2 lies in the location of the sheet

26 which is here in the diffusing layer 20.

In this embodiment, it is preferable to provide a sheet 26 which can be torn automatically by the pressure of the gas in the gas cell, which means that it is preferable to have an orifice 28 of large diameter, as in the embodiment shown in Figure 2 In fact, the sheet 26 is not directly visible through the orifice 28, the user can

difficult to control the piercing of the sheet 26.

Claims (8)

1 Electrochemical cell comprising a casing formed of a first electrically conductive part (2) and of a second electrically conductive part (4) separated from each other by an insulating gasket (6) and containing a material (8) chemically active associated with an electrolyte, and a gas diffusion electrode (10), said first part of the housing comprising at least one orifice (28) through which said gas enters or escapes from the cell, characterized by that said cell comprises a waterproof sheet (26) said gas disposed inside said housing between the electrical gas diffusion trode and said orifices. 2 Cell according to claim 1, characterized in that said waterproof sheet (26) comprises at least one metal sheet. 3 Cell according to claim 2, characterized in that said waterproof sheet (26) is arranged directly on the face internal part of the first part (2). 4 Cell according to claim 3, characterized in that at least the parts of said metal sheet located opposite the orifices of the housing are covered with a protective layer of the metal against corrosion. 5 Cell according to any one of claims 3 and 4, characterized in that said waterproof sheet tears for a pressure difference on its faces greater than 10 bars. 6 Cell according to claim 2, comprising a diffusing layer (20) between said first part (2) and said gas diffusion electrode (10), characterized in that said sheet sealed (26) is disposed in said diffusing layer (20). 7 gas cell according to any one of claims 4 and 6, characterized in that the housing has an orifice (28) having a diameter at least equal to 2 mm and in that said waterproof sheet tears for a minimum pressure difference on its faces between 0.5 and 5 bars. 8 gas cell according to claim 7, characterized in that said sealed sheet is torn by the pressure of the gas formed in the cell when the two parts of the housing are connected. between them electrically.