CS211894B1 - A method for producing plastic bonded electrodes - Google Patents

Abstract

Method for manufacturing plastic-bonded electrodes for alkaline cells. An electrode mass containing metals and plastic particles, which are enriched in a solvent or a mixture of solvent and non-solvent, is deposited on a metal substrate and the solvent is removed from the electrode under such conditions that at no stage of the process there is complete dissolution of the plastic and thus destruction of the porous structure of the electrode.

Description

The invention relates to the manufacture of electrodes for alkaline cells connected by a sheath,

Recently, electrodes for alkaline cells have been increasingly reinforced with the addition of plastics. These electrodes can achieve high durability and mechanical strength, thereby increasing the cell capacity.

The difficulty in preparing highly active plastic-bonded electrodes lies in the fact that the production must ensure such a final structure of the material, which is formed by a basic plastic skeleton with accessible solid active particles of the electrode material, enabling, in addition to perfect access of the electrolyte to the electrode material, above all, a perfectly conductive connection of the particles of this active electrode material.

Therefore, it is not possible to start from a plastic melt and homogenize it with the electrode material when producing these electrodes. In such a case, an undesirable system results in which the particles of the active electrode material are encapsulated by a plastic binder, which makes their electrochemical use impossible.

The procedure in which the preparation of a plastic-bonded electrode is based on the principle of pressing at elevated temperature a well-homogenized electrode active mixture is also not fully satisfactory. Increased temperature and pressure achieve a similar final effect on the plastic as in the preparation from a solution, in addition, heat treatment is usually accompanied by undesirable chemical processes, which together with the action of pressure cause the destruction of the electrode material particles, a decrease in their porosity and surface.

The technology of electrode preparation, based on soaking electrodes prepared from active electrode material and honeycomb in solvents, is also very difficult and does not achieve good results. The difficulties lie in the fact that the solvent absorption rate competes with the plastic particle dissolution rate, resulting in an uneven state between the particles inside and on the surface of the material, and in the fact that it is impossible to prevent the formation of a solution of low-molecular plastic, which is a natural part of native polymers, in this method. Low-viscosity gels in the material flow, limit the permeability and isolate parts of the active electrode material.

From the above, it can be seen that the problems associated with the preparation of plastic-bonded electrodes for electrochemical sources are considerable and have remained essentially unresolved to this day.

It has now been found, within the framework of the studies on the preparation of plastic-bonded electrodes, that a very simple and undemanding preparation of plastic-bonded electrodes can be realized if the plastic particles are swollen during pressing.

The subject of the invention is a method of producing plastic-bonded electrodes for alkaline cells by pressing an electrode mass containing metals and their oxides and plastic particles. The essence of the invention lies in the fact that the plastic particles are softened in a solvent or a mixture of solvent and non-solvent before mixing with the electrode mass or during mixing and pressing, and the solvent is removed from the electrode mass by evaporation or extraction at a temperature at which the plastic does not completely dissolve.

The advantage of the production method according to the invention is that it is possible to use the geometric shape and particle size distribution in the construction of the plastic-bonded electrode. By choosing the weight ratio of the plastic, the size of the particles and their distribution, a significant number of necessarily maintained electrode parameters can be managed in a wide range, especially the mechanical strength of the electrode, the access of the electrolyte to the active mass, the contact between the particles of the active mass and the most suitable electrode structure.

The production of electrodes according to the present invention is carried out by swelling plastic particles with the help of a solvent - or a mixture of solvent and non-solvent - (solvent with anti-solvent), while several basic conditions must be observed to achieve the desired properties of the electrode.

The choice of solvent or mixture of solvent with anti-solvent and their ratio must be such that only the polymer is enriched, not its dissolution. The softened plastic particles, during further suitable processing of the plastic of the electrode material, stick together and form a basic soft skeleton, which, upon evaporation of the solvents, forms a solid skeleton with well-conductively connected particles of the active electrode material.

Another condition (follows from the first condition). Its essence is the necessary requirement that the solvent removal be carried out in such a way that the mixture added to the electrode mass remains during the removal of solvents by evaporation or extraction either in a constant ratio of solvent to antisolvent or is gradually enriched with antisolvent.

This second condition ensures the transition from the remaining plastic parts in the electrode mass to the solid skeleton formed by the plastic used and is achievable when removing the solvent by evaporation in essentially three ways from the point of view of the solvent components:

I. The components form an azeotrope and flow together.

II. The components form an azeotrope and the antisolvent is in excess.

III. The components do not form an azeotrope, but the antisolvent has a higher boiling point.

Conditions II. and III. It is necessary to extend the requirement of a low vapor pressure of the antisolvent during evaporation in relation to the quantitative ratio of the solvent, so that during evaporation of the solvent, even if it has a lower boiling point, there is no withdrawal of the antisolvent or a significant decrease in the concentration of the antisolvent in relation to the solvent.

For the production of plastic-bonded electrodes according to this invention, most plastics currently in production can be used. The choice of plastic will of course depend on the conditions for which the plastic-bonded electrode is intended. For example, for the construction of plastic-bonded electrodes that are to operate in an alkaline environment, it will be appropriate to use polyethers, polyacetals, PVC, polypropylenes, polyarylenoxides, polyarylenes, polyethylenes, polycarbonates, etc.

An important advantage in the production of electrodes according to the present invention is the fact that the molecular weight of the polymer used does not have a significant effect on the production of electrodes in the described manner. The insignificant importance of molecular weight can be compensated by a suitable choice of temperature or by changing the ratio of solvent:antisolvent in the mixture intended for swelling the particles.

The advantage of this fact is clearly visible when comparing the production of a plastic-bonded electrode according to the present invention with the production based on a plastic solution. In the latter case, the molecular weight of the polymer significantly affects the production, since the value of the molecular weight determines the viscosity of the plastic solution, which has a decisive influence on the surface of the active particles, low-viscosity solutions easily soak into the particles and perfectly insulate their surface. The mixtures are well homogenized, but separation easily occurs during standing, and the resulting electrode is inhomogeneous in terms of the distribution of plastic in the mass of the electrode.

Highly viscous solutions form undesirable thick layers on the surface of the active electrode particles, and the mixture is difficult to homogenize.

The choice of solvent and non-solvent is primarily governed by the type of filler selected and the degree of desired swelling of the plastic particles.

Most often, aromatic or aliphatic organic compounds such as alcohols, ketones, ethers, esters, amines, nitriles, and chlorinated hydrocarbons are considered as solvents for honeycombs; the choice of solvents is considerably wider.

The most preferred anti-solvent is water, organic compounds based on simple hydrocarbons, or alcohols.

It is clear that the division of compounds into antisolvents and solvents may not be universally valid. A substance with the character of a solvent when used in one plastic may have the character of an antisolvent in the case of another plastic and can also be used as an antisolvent.

From the point of view of economy and simplification of the entire technology of production of plastic-bonded electrodes, it is necessary to draw attention in particular to the possibility of using water as an antisolvent. From this point of view, it is advantageous to divide solvents for the preparation of mixtures intended for swelling of a part of the plastic into water-miscible and immiscible solvents. It is obvious that for the preparation of swollen particles, those solvents that are miscible with water will be preferentially selected; this will save not only the price of the antisolvent, the regeneration device for the used antisolvent, but also other energy that we would otherwise have to spend on covering the evaporation heat of the antisolvent. In the case that we use water as an antisolvent, it is not necessary to remove it from the electrode mass by drying and other costs are of course also eliminated.

Another advantage of the preparation method according to this invention, which can be achieved by high strength of the prepared electrode, is the possibility of reducing the proportion of metals that form grids or other supporting structures in the electrodes.

The formation of a solid plastic skeleton does not insulate the metal particles, so that a collector with a smaller area is sufficient for the electrode prepared according to this invention.

The method of manufacturing plastic-bonded electrodes according to this invention is technologically undemanding and economically efficient. Electrodes prepared in this way are characterized by high efficiency, high mechanical strength and long service life.

Production can be carried out in two ways:

By separately preparing the swollen particles, placing them with the other electrode components and applying (pasting) this mixture onto the collector and finally drying.

By mixing all the electrode components including the plastic and by applying a mixture of solvent and antisolvent to the mixture to achieve swelling of the honeycomb particles, and further processing is identical to the first method.

Both methods have their advantages and disadvantages, especially in the first case the homogenization is more difficult, compared to the second method, because the swollen plastic particles stick together before complete penetration. In the second case, the disadvantage is the increased consumption of the solvent and antisolvent mixture compared to the separate swelling of the plastic particles.

The invention will be illustrated by the following examples. The percentages given in the examples represent the concentration by weight unless otherwise expressly stated. Example 1

A negative cadmium electrode was prepared by the following procedure: 9 g of active cadmium mass was mixed with 12% of polyvinyl butyral mass with a polymerization degree of about 1100 and a content of 68% vinyl butyral groups. The mixture was thoroughly homogenized dry in a mortar. After thorough homogenization of the powder mixture, 60% of the total weight of the mixture of ethanol and water (in a mutual volume ratio of 50:50) was gradually added and the mixture was thoroughly homogenized again. After 4 hours of aging, the resulting well-spreadable paste was applied to an area of 45 ^cm2 of a plate made of steel sheet 0.2 mm thick (ČSN 13 2121).

The unpasted electrode was dried in a drying oven with warm air at a temperature of 318 K for 60 minutes. The dried electrode was framed with a U-profile made of unplasticized PVC and in this configuration was built into the Slank together with a pair of classic positive (pocket) nickel-oxide electrodes. The cell was filled with 100 ml of 5N potassium hydroxide and after 24 hours of soaking the capacity and life of the thus assembled cell were measured.

To verify the service life, the cells were periodically charged and discharged on a cycling table with suitable currents. In the first three cycles, the cells were formed with a five-hour current, i.e. a current corresponding to about 0.2 of the nominal capacity. After the third cycle, the electrolyte was replaced. From the fourth cycle onwards, the cells were automatically cycled and in selected cycles the actual capacity of the negative electrodes was determined by discharging them with a constant current to a final voltage value of 0.8 V.

The actual ampere-hour capacity was determined using the relationship:

Mdle I is the current in Á, it is the time in minutes.

The theoretical capacity C _T (Cd] was determined from the cadmium content in the active material according to Faraday's laws. In 9 g of the active material used, 54.30% cadmium was achieved. The measurement results are given in Table 1, where the results obtained under the same arrangement and conditions on a cell with a negative pocket electrode of commercial design, containing 64.2% cadmium, are also given for comparison.

Table 1: Lifespan of plastic-bonded and pocket electrodes

Number of cycles Plastic electrode Pocket electrode ^C 0.8 C A>,s C ^TÍCd) • ^Τ(Οά) A.h A.h 1 2 3 1.08 1.27 1.37 0.464 0.545 0.588 1.87 1.73 1.60 0.800 0.742 0.685

After the third cycle, replace the electrolyte with a solution of potassium hydroxide with a density of 1.2 g. cm ^-3 and lithium hydroxide 15 g. dm ³

4 1.44 0.619 1.52 0.652 5- 1.53 0.657 1.49 0.638 22 1.50 0.644 1.43 . 0.612 100 1.53 0.657 1.39 0.598 150 1.44 0.619 1.36 0.583

^c o.8 ' capacity obtained by discharging the cell to the final voltage value of 0.8 V

C_ .

C _T(cd) — capacity utilization C _og related to the theoretical capacity of cadmium C _T(Ca) .

Example 2

Under the same conditions as in Example 1, an electrode was manufactured using low molecular weight polyethylene swollen in a mixture of carbon tetrachloride:octane (6:4) by volume. This electrode was compared with an electrode bonded with low molecular weight polyethylene pressed and sintered for 1 hour at 165°C.

Table 2: Service life of plastic-bonded electrodes prepared by polymer swelling technology and polymer sintering technology

Number Polymer swollen Polymer sintered cycles . C p C ^at 0.8 C-T(Cd) ^at 0.8 ^T(Cd) A.h . A.h 1 0.91 0.392 1.00 0.431 2 1.12 0.483 0.96 0.412 3 1.25 0.539 0.93 0.40.1 Mon 3, c ykiu electrical exchange electrolyte for solution potassium hydroxide o density 1.2 g. cm ^-3 and lithium im hýdlrqxiid at 15 g. dm' ³ 4 1.34 0.578 0.89 0.382 5 1.37 0.590 0.83 0.356 10 1.39 0.599 0.77 . 0.332 24 1.41 0.607 0.74 0.318 104 1.37 0.590 0.72 0.309 180 1.40 0.603 0.70 0.302

Example 3

Under the same conditions as in Example 1, plastic-bonded negative electrodes were produced based on polyvinyl chloride, polyvinylidene fluoride, polycarbonate and polyphenylene oxide.

The following volumetric mixtures of solvent and antisolvent were used for individual plastics. The required degree of swelling was achieved at a temperature of 130 to 150 °C.

Polyvinyl chloride — trichloroethylene : propanol (8:2)

Polyvinylidene fluoride — trichlorethylene : octane (7:3)

Boilycarbonate — tetrahydrofuran: water (8:2)

Polycarbonate — methylene chloride: ethanol (7:3)

Polyphenylene oxide - azeotrope benzene-ethanol:propanol (9:1)

For all electrodes, an electrode mass utilization efficiency of 0.55 to 0.65 was achieved after the tenth cycle.

Claims (1)

A method for producing plastic bonded electrodes for alkaline cells by molding an electrode composition comprising metals and their oxides and plastic particles, characterized in that the plastic particles are mixed with the electrode composition before mixing with the electrode composition. OF THE INVENTION during mixing and molding, the solvent or non-solvent swells in the solvent or mixture, and the solvent is removed from the electrode mass by evaporation or extraction at a temperature at which the plastic does not completely dissolve.