Long-life alkaline primary battery

were:

FR-PS2274143

Nr.355647

The invention relates to a long-life alkaline primary battery having a negative electrode containing cadmium or amalgamated zinc and a positive electrode containing monovalent silver oxide, mercury oxide or γ-multi-dioxide.

As is known, self-discharge processes which reduce the stored capacity occur in the case of very long storage or service life of alkaline primary batteries. The self-discharge processes are chemical processes which take place at the electrodes as so-called secondary reactions.

At negative electrodes hydrogen evolution may be such a minor process. It occurs when the potential of the negative electrode is deeper (ie, more negative) than that of a hydrogen electrode, as well as electrolytes. It is therefore observed for example on Zinkelektroden and can be formulated as follows;

z

Zn + Η O -> ZnO + H

2

Hydrogen evolution results in a loss of metallic Zn, that is, negative active material. It can be slowed down by keeping the water overvoltage of the Zn electrodes very high, for example by carefully keeping away impurities which reduce the overvoltage, or by additives to the electrode which increase the overvoltage, such as amalgamation.

On positive electrodes, self-discharge can be caused by the solubility of the active material. Even if this solubility in the electrolyte is small, noticeable capacity losses occur over time. Such a loss occurs, for example, in alkaline primary batteries with positive electrodes containing monovalent silheroxide or mercury oxide. Monovalent silver and mercury oxides have a solubility of about 4.5 × 10 5 in 5 MKOH^-* Mol Ag (OH) - per liter or 3.10 "<MolHg (OH)₂ per liter. The dissolved silver oxide or mercury oxide diffuses through the separator layer, which is arranged between the electrodes, and finally reaches the negative electrode, which consists for example of zinc or cadmium. At the negative electrode, the dissolved silver or mercury oxides are reduced, according to

Zn + 2 Ag (OH) "-> ZnO + 2 Ag + Η O + 2 (OH)^-

Zn + Hg (OH)₂ -> ZnO + Hg + H₂O

Cd + 2 Ag (OH) '-> Cd (OH)₂ + 2 Ag + 2 (OH) -

Cd + Hg (OH)₂ -> Cd (OH)₂ + Hg

There is a corresponding loss of negative active material, Zn or Cd.

The dissolved silver or mercury oxides, instead of reacting with the negative electrode, may also react with organic matter of the separator system, also reducing to the corresponding metals. As a result, short circuits can be brought about, which accelerate the self-discharge in a catastrophic manner.

The subject of this invention is an arrangement which reduces this type of self-discharge, which is caused by the solubility of the positive active material of the battery, without adversely affecting the internal resistance of the battery.

According to the invention, this is accomplished by covering the entire electrochemically active geometric surface of the positive electrode facing the negative electrode with an electronically conductive microporous, electrolyte-impregnated, optically opaque filter electrode, such that the filter electrode has no or containing less than a hundred times smaller amount of monovalent silver oxide or mercury oxide per unit volume than the positive electrode, that the filter electrode contains electronically conductive material, such as graphite, nickel or nickel alloys, and that the filter electrode with the positive electrode of the cell is in electrical contact.

It is also possible to arrange a plurality of filter electrodes alternating with separator layers.

In the following, embodiments of the invention will be described with reference to the drawings. FIG. 1 shows a cross section through an alkaline button cell. FIGS. 2 to 4 show cross sections through the outer zone of alkaline button cells.

The invention can be applied mutatis mutandis to other geometric configurations.

In the cup 1 of nickel, nickel-plated steel sheet or of a stainless nickel alloy, for example according to the Swiss Patent No. 5,121,00 , the tablet -shaped , positive electrode -2-, which einwer3

No. 355647 tiges silver oxide or mercury oxide contained housed.

The positive electrode -2- is from the negative electrode -4--, which consists of amalgamated zinc powder or fine cadmium powder, through the polypropylene-felt separator layer - 3--

Polyethylene-methacrylic acid polymer, cellophane, cotton or from a similar combination of commercial Separator materials, separated.

The lid -5- carries in the case of a negative electrode-4-made of amalgamated zinc powder on the inside of a layer on copper, bronze or other amalgamable metal high hydrogen overvoltage and has a structure as described in CH-PS no. 508283. The sealing ring -6- of polyamide, neoprene or other plastic, which is not attacked by potash or caustic soda, and has the smallest possible cold flow, is clamped between the cup -1- and lid -5, the lid, according to CH -PS No. 508283, is elastically deformed.

As shown in Figs. 1 to 4, the galvanic cell according to the invention is provided with a metallic conductive, microporous filter electrode -7-. This additional electrode is in electrical contact with the cup -1- or the positive electrode support ring -8- accommodated therein at the edge 8-.

The filter electrode -7- contains electronically conductive material which is not attacked in the electrolyte used at the potential of the positive electrode, oxidized or dissolved. For alkaline primary batteries which contain monovalent silver oxide, mercury oxide or their mixtures with manganese dioxide in the positive electrode, suitable graphite, nickel, nickel alloys are used as the conductive material for the filter electrode.

The filter electrode can also consist of a porous body of organic or inorganic materials, the inner surfaces or pore walls are metallized in a corresponding manner. Thus, the filter electrode made of a metallized felt made of plastic fibers, such as nickel-plated polyester fibers exist. The filter electrode can also be a pressed or sintered body of inert metal oxide powder, eg Cd (OH)₂, Äl₂O₃ or ZrO₂ be whose inner pore surface is coated with an electronically conductive layer.

The filter electrode may contain, in addition to the electronically conductive portion, other non-conductive or semiconductive inert fillers which are insoluble in the electrolyte and affect the pore structure in a manner that impedes the diffusion of the monovalent silver oxide or mercury oxide dissolved in the electrolyte. Such additives include carbon black, manganese dioxide, thermally stabilized or degraded manganese dioxide, activated carbon, cadmium hydroxide, magnesium oxide, etc.

Finally, the filter electrode may contain organic or inorganic binders which bind the electronically conductive portions, such as polyvinylpyrrolidone, and further fillers which thicken the electrolyte, such as carboxymethylcellulose.

The filter electrode has a thickness of 0.1 to 2 mm and a porosity, which will be explained in more detail below, the application is adapted. The porosity can be between 5 and 85% (porosity 0.05 to 0.85). The mean pore diameter can be between 0, 05 and 50 gm.

In any case, the filter electrode should optically have no translucent holes or pores. This clearly distinguishes the filter electrode from the structure of an electrically conductive network.

The filter electrode also has a completely different composition, structure and function than the divalent silver oxide electrode assembly described in US Pat. No. 3,920,478 which discloses an oxidizable open mesh of a metal such as zinc which is intended to to lower the voltage of the bivalent silver oxide electrode.

The purpose of this oxidizable sieve lies precisely in the fact that it is to be attacked and thereby reduces the divalent silver oxide. This sieve is in no way intended as a diffusion-inhibiting filter.

The same is to say for the DE-OS 2525360, filing 6.6.1975, disclosure 18. 12. 1975. It refers to the use of an oxidizing metal such as zinc, cadmium, lead, copper or silver, which is applied to a carrier grid is to lower the voltage of the divalent silver electrode. It should be noted that silver is considered to be oxidizable here because it is exposed to the electric potential of divalent silver oxide.

DE-OS 2506399, filing date 15.2.1975, disclosure date 26.8.1976, also relates to a galvanic cell with a positive electrode of divalent silver oxide and the purpose of the arrangement is to lower the potential of the electrode to the value of the monovalent silver electrode. This is achieved by reducing the positive electrode of divalent silver oxide on its surface and isolating it from the positive arrester by an electronically non-conductive plastic layer and isolating the electronically non-conductive plastic layer and making electronic contact with the arrester via a porous silver layer. The porous silver layer described is not inert, but by the divalent silver oxide oxidizable, and thus also serves to lower the voltage. It should be remembered that

In any case, in the discharge of silver oxide electrodes, deliberate or not, metallic porous silver rods are formed. In the electrochemical reduction, the porous silver layer is formed mainly at the edges of the contact ring.

The resulting porous silver layer therefore does not fulfill the purpose of a stable, non-oxidizable, diffusion-inhibiting filter electrode for a long storage time in accordance with the invention.

The essential features of the invention are that the filter electrode contains no (or at least substantially reduced) proportion of monovalent silver oxide or mercury oxide, that it is electronically conductive, and not chemically or electrochemically attacked under the present conditions of electrolyte composition and potential is oxidized or dissolved, and that it covers the entire, free, the negative electrode facing geometric surface of the positive electrode. The invention also includes an arrangement in which an additional separator layer is interposed between the filter electrode -7 and the positive electrode -2-. The filter electrode must be in electrically conductive contact with the positive electrode. The additional separator layer of organic or inorganic material is said to be permeable to OH ions, but is intended to slow the diffusion of dissolved monovalent silver oxide or mercury oxide into the filter electrode. The additional separator layer can consist of oxidation-resistant, commercially available separator material, for example of polypropylene felt and methacrylic acid-polyethylene polymer, etc.

The following are examples of how the filter electrode can be made. The examples refer to an alkaline mercury-cadmium button cell 17.4 mm in diameter and 7.5 mm in height. The positive electrode was mercury oxide mixed with 5% graphite powder and 9% manganese dioxide. The negative electrode consisted of cadmium sponge, the separators of the commercial, the following combination.

Example 1: A nickel wire mesh with a mesh size of 0.4 mm, wire gauge 0.1 mm, was in a viscous paste of carbonyl-nickel powder, average particle size 2.6 to 3.4 gm, water and thickening agents such as gelatin, Methyl cellulose, starch, etc., dipped, dried and sintered at 900 ϋ for 15 minutes. The result was a highly porous (degree of porosity 0.85) sintered nickel plate of 0.4 mm thickness, were punched out of which rondelles of 16, 0 mm in diameter. The rondels were pressed as filter electrodes -7- in support ring -9- as shown in Fig. 1, wherein the edge portion -8- compressed. Subsequently, the tablet 2- was inserted from mercuric oxide and the whole pressed into the cup -1-. About the sintered, porous filter electrode -7- were the commercially available, microporous separators made of plastic fleece, Polyethylene-methacrylic acid polymer, cellulose and cotton inlaid. A plastic rondelle -10- made of polytetrafluoroethylene covered the edge of the filter electrode. The cell was discharged at 75 ° C through resistors of 1200 Ω. It was found that the separators were much less oxidized and less dissolved mercury oxide diffused to the negative electrode than in cells without a filter electrode.

Example 2 A mixture of 99% fine graphite powder and 1% polyvinylpyrrolidone as binder was pressed into a tablet of 13 mm diameter and 0.4 mm thickness and placed in a cell as a filter electrode -7-, as shown in FIG is. The filter electrode -7- was in mechanical and electrical contact with the inner edge -8- of the support ring -9-. Above the filter electrode -7-, the separator layers -3- were arranged as in Example 1. Again after discharge at high temperature (75 ° C) it was found that the separators were practically unaffected. There was no trace of metallic mercury on the negative electrode. The filter electrode had prevented the diffusion of dissolved mercury oxide.

Example 3: A mixture of 50% fine graphite powder, 49% manganese dioxide, which was previously stabilized at 400 ° C for 4 h and 1% Polyvinylpyrrolidone as a binder was pressed into a tablet and arranged as shown in Fig. 3.

The manganese dioxide caused a very fine-pored structure of the filter electrode, which wets very well with alkaline electrolyte. After discharge at 75 ° C., metallic mercury was found neither on the separator layers nor in the cadmium electrode.

Example 4: As shown in Fig. 4, here another separator layer -11- of electronically nonconducting material was installed between the filter electrode and the positive electrode. It consisted of a polypropylene felt which was supported on the positive electrode and a membrane of polyethylene-methacrylic acid polymer. The additional separator layer between the filter electrode and the positive electrode served to further slow the diffusion of mercury oxide dissolved in the electrolyte. The filter electrode contained as thickening agent for the alkaline electrolyte 0.5% carboxymethylcellulose. Their composition was 50% graphite powder, 48.5% manganese dioxide, 1% polyvinyl pyrrolidone, 0.5% CMC. After discharging at 75 ° C, on the surface of the filter electrode facing the negative electrode,

These examples show that it is possible with the aid of the described filter electrode, the diffusion of

No.355647 dissolved positive active material to the negative electrode and the separator layers largely prevent. Thus, the above-described loss of negative active material as well as the destruction of the separators by oxidation can be avoided.

Such filters are advantageous in HgO-Cd, HgO-Zn, Ag₂O-Cd and Ag₂O-Zn cells.

The operation of the additional filter electrode can be described physically in the following way. During discharge, the current distribution in the positive electrode adjusts to first reduce the material to which the lowest electrical resistance leads. The electrical resistance is composed of the resistance of the ion flux in the electrolyte and the resistance of the electron flow in the electronically conductive part of the positive electrode. By introducing the metallically conducting filter electrode between the positive and negative electrodes, it is preferable to first reduce that positive active material which diffuses into the filter electrode, since the ohmic voltage drop is the smallest at this reaction location.

Using the example of a positive electrode on mercuric oxide and an electrolyte of 5M KOH, it should be shown how the filter electrode can be dimensioned such that no dissolved mercury oxide reaches the negative electrode.

The quantity of dissolved mercury oxide, H (OH), diffusing into the filter electrode per unit of time₂, is roughly approximated by m = D. (dc / dx). p. (l / t). r²x and the corresponding reduction current needed to reach this number of moles of Hg (OH)₂/ s in the

To reduce the filter electrode is approximately i = 2 F. D. (Δ c / Δχ). p. (l / t). r²% where F = 96500 Coulombs, D the diffusion coefficient (cm²/ s) of dissolved Hg (OH)₂, Δο / Δχ is the linearized concentration gradient in (mol / cm³) / cm, p is the porosity (fraction of pore volume), t is a tortuosity factor of the porous filter electrode, r is the radius of the filter electrode. With

D = 10 "5 cm²/ s, Δο = 3. 10 "T moles / cm³, Δχ = 0.1 cm, p = 0.5, (l / t) = 0.2 and r = 0.65 cm results in a reduction flow of ~ 8.10 ~? Amp "

If the battery is continuously charged with currents above this limit, theoretically, according to this simplified calculation, no dissolved mercury oxide would reach the negative electrode or separator and the solubility of the mercury oxide contained in the positive electrode could cause no loss of capacity.

By taking into account the electrolyte resistance in the pores of the filter electrode, as well as the speed of the electrochemical reaction

Hg (OH)₂ + 2 e "- * Hg + 2 (OH)" as a function of the local potential in the filter electrode, which is determined by the ohmic voltage drop in the electrolyte-filled pores, differential equations could be established by means of which, a more accurate current distribution However, the approximate calculation already shows that the filter electrode effectively reduces the self-discharge when batteries are charged with small currents over a long period of use.

The smaller the pore diameter and the smaller the porosity, the more effective the filter electrode. As a result, the electrolyte resistance in the pores increases and the reduction takes place at a lower potential in a more preferred mass in the filter electrode. Of course, one must then accept a higher internal resistance of the cell.

When dimensioning the filter electrode, it must also be considered that the reduction of dissolved oxide causes metal deposition, which can reduce the porosity. In such cases it may be advantageous not to rigidly bind the particles of the filter electrode but to arrange them in the form of a more or less flexible or flexible bed. The porosity should initially be large enough to account for this factor.

Even with open circuit storage, the filter electrode helps to reduce the diffusion of dissolved monovalent silver oxide or mercury oxide, since the concentration gradient must form over a prolonged distance, and the diffusion rate is slowed by the described measures in the filter electrode. The result of the self-discharge of negative zinc electrodes ent6

No.355647 hydrogen can diffuse to the filter electrode and serve there as an electrochemical reducing agent for the dissolved monovalent silver oxide or dissolved mercury oxide. Organic components of the separator can also dissolve in traces in the electrolyte and penetrate into the filter electrode, where they can serve as a reducing agent for the dissolved silver or mercury oxides. These side reactions may contribute to less dissolved silver oxide or mercury oxide to the negative electrode during open circuit storage where they would cause the described self-discharge.