CH671304A5 - - Google Patents

Description

DESCRIPTION The present invention relates to a method for producing an electrochemical cell with reduced gas formation, the cell having an anode made of polycrystalline metal which is subject to gas formation. In the cell mentioned is also the amount of mercury,

required for anode amalgamation of such cells.

Metals, such as zinc, have been commonly used as anodes in electrochemical cells, particularly in cells with aqueous alkaline electrolytes. In such cells, the zinc is amalgamated with mercury. to prevent or reduce the extent of the reaction of the zinc with the aqueous electrolyte and the associated harmful evolution of hydrogen gas. In the past, it was necessary to use about 6 to 7% by weight of mercury amalgam in the anode to reduce the amount of "gassing" to acceptable levels. From an environmental point of view, however, it has been desirable to eliminate or at least reduce the amount of mercury used in such cells without, however, increasing the cell's gassing. Various tools have been used to achieve such a reduction in mercury, such as special treatment of zinc, the use of additives and unusual amalgamation methods. However, such methods either brought economic setbacks or had limited success.

It is therefore an object of the present invention to provide a method for producing an electrochemical cell with reduced gas formation. The method according to the invention is characterized in that a) the number of grains in the polycrystalline metal is reduced to a third or less of the number of grains originally present;

b) forming the metal thus obtained with a reduced number of grains into an anode for the cell; and c) introducing the shaped metal anode into the cell.

The method according to the invention is economical. When carrying out the method according to the invention, it is simultaneously possible for the amount of mercury used in the amalgamation of metals in aqueous electrochemical anodes to be reduced without simultaneously increasing the gas formation in the cell or simultaneously reducing the cell quality.

These and other objects, features, and advantages of the present invention will become more apparent from the following discussion and drawings, in which:

Figure 1 is a microphotograph of polycrystalline zinc particles in cross section; and

FIG. 2 shows a microphotograph of polycrystalline zinc that has been treated according to the invention in cross section.

The method according to the invention thus comprises reducing the number of grains in the polycrystalline anode material to a third or less of the original number of grains. The resulting anode metal with a reduced grain number is then formed into an anode, for example by compressing the powder particles either on a support or in a cavity. Alternatively, the anode metal can be in the form of a sheet or a sheet, the anode being wound over one another in the form of a “jelly roll” and rolled up together with the cell separator and the cathode. The sheet or tabular metal can also be used in a prismatic cell without winding. If desired, the anode material (especially zinc) can be amalgamated with mercury after the grain has been reduced and before the anode metal has been introduced into the cell. In all of the above-mentioned embodiments, with such a degree of grain reduction, there is a simultaneous reduction in the grain size.

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limits and a reduction in gassing in such places.

In order to further reduce the amount of gas generation, a small amount of a surface active heteropolar substance (surface active agent) of a type that hinders the evolution of hydrogen can additionally be added to the cell. Because of the heteropolar nature of the surfactant, it should be at least slightly soluble in the cell electrolyte and have a polar affinity for the surface of the anode metal particles, thereby forming a coating. Such an affinity is particularly pronounced with regard to zinc particles which are usually used in anodes of cells with alkaline electrolytes. The surfactant can be effectively incorporated into the cell in a number of ways. For example, it can be added to the anode or incorporated into the electrolyte or into the separator by wetting or impregnating the separator with the additive beforehand. The surfactant can even be added to the cathode. In all of these cases, the surfactant migrates to the surface of the anode metal particles and forms the required layer to prevent hydrogen generation. The surfactant is usually added to the anodic material by direct addition to the powdered metal (amalgamated or not amalgamated) to form a surface coating for the anode metal. Alternatively, the surfactant can be added to the electrolyte which is then mixed with the anode metal particles; this results in the migration of the surfactant to the surface of the anode metal particles. Migration of the surfactant to the anode metal particles can also be effected by adding the surfactant to the separator or to the cathode.

Alternatively or in addition, the anode material particles, such as zinc, may be previously alloyed with a small amount of indium, cadmium, gallium, thallium, bismuth, tin and / or lead and then converted or reduced to particles with a reduced number of grains discrete single crystal particles, which are then amalgamated with mercury.

In order to carry out the grain number reduction, polycrystalline anode materials such as zinc are preferably heat treated at a temperature below its melting point for a sufficient time, thereby reducing the number of grains of the polycrystalline material to one third or less of the original material.

Although the anode material remains polycrystalline after this heat treatment, the amount of grain boundaries decreases with the decrease in the grain number. As a result, the size of the gassing in the cell with the treated particles is considerably reduced because the range of grain boundaries contributes most to the high chemical activity and gas generation. In addition, mercury readily penetrates the grain boundaries. The reduction in the grain boundaries is accompanied by a reduction in the amount of mercury required for amalgamation with the anode material. With the reduced grain anode material, the amount of mercury required for amalgamation can be effectively reduced from about 6 to 7% to about 4%.

The preferred heat treatment of the anode material depends on the factors of purity of the polycrystalline starting material, temperature at which the heat treatment is carried out and duration of such a heat treatment. Of course, the heat treatment of powder particles of different mass can be as long as required

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Time may be different, since the inside of the unit is somewhat insulated by the outer material and does not "see" the same amount of heat as the outer material, which absorbs the heat directly. In practice, heating is most effective in a continuously rotating calciner; as a result, if the burner is correctly designed, less than 10 minutes are sufficient. at temperatures above 370 ~ C to achieve a sufficient grain reduction. The recrystallization and grain enlargement depend on many factors, such as temperature, time, tension energy within the material, and the purity. As a result, exact parameters for the heat treatment are set in accordance with the specific heat treatment equipment used. To make it clear, the effective heat and temperature mentioned below refer to the direct application of heat to the material. In any case, a reduction in the number of grains in the material to one third or less, based on the original material, is the desired result.

The preferred heat treatment of the polycrystalline anode material is both with powdery material, which is generally used to form pressed anodes in cells with a coil structure, as well as with regard to the treatment of metal strips or sheets or foils which are wound in prismatic or one above the other Cell structures are used effectively.

The purity of the original polycrystalline anode material partially determines the amount of time it takes to achieve the required grain size reduction or the temperature at which the material needs to be heated for a given amount of time; the lower the purity, the higher the temperature or the longer the time required. The most common anode material for electrochemical cells is zinc, with the most common contaminant in it being lead. Other less common anode materials include cadmium, nickel, magnesium, aluminum, manganese, calcium, copper, iron, lead, tin, and mixtures thereof, including mixtures with zinc.

The alkaline electrolyte solution in which the anode material is usually placed and which is generally a factor in gas generation (generally the anode reacts with the electrolyte to produce gas) is preferably an aqueous solution of a hydroxide of alkali or alkaline earth metals such as NaOH and KOH. Common cathodes for the alkaline cells include manganese dioxide, cadmium oxide and hydroxide, mercury oxide, lead oxide, nickel oxide and hydroxide, silver oxide and air. However, the reduced grain number anodes according to the present invention are also useful in cells with other electrolytes where anode gassing is problematic, such as acid type electrolytes.

Common basic type cells contain pressed polycrystalline zinc particles with an average particle size of about 100 microns. Each of these particles has about 16 or more grains; In accordance with the present invention, the grain number of each particle is preferably reduced by heating to an effective temperature between 50 and 419.5 C (the melting point of zinc) for a minimum period of 2 hours at 50 C to 5 minutes at 419.5 C to reduce the grain number reduced to an average of about three to five grains per particle. Zinc particles with lead contaminants generally require a temperature of about 100 ° C for at least two hours to achieve a similar reduction in grain number.

Useful surface-active agents which can be added in the process according to the invention in order to further reduce the extent of the gas include polymers containing ethylene oxide, such as, for example, those which contain phosphate groups, saturated or unsaturated monocarboxylic acid with at least two ethylene amide groups. pings; Tridecyloxypoly (ethyleneoxy) ethanol; and, most preferably, organic phosphate esters. The preferred organic phosphate esters are generally monoesters or diesters of the following formula:

in the: ^

x + y = 3

M = H, ammonia, amino, or an alkali or alkaline earth metal,

R = phenyl or alkyl or alkylaryl with 6 to 28 carbon atoms.

Specific useful organic phosphate ester surfactants include materials with the trade designation GAFAC RA600 (an anionic organic phosphate ester supplied by GAF Corporation as a free acid based on a linear primary alcohol that is an unneutralized portion of esters of phosphoric acid), GAFAC RA610 (an anionic complex organic phosphate ester supplied by GAF Corporation as a free acid with an aromatic hydrophobic portion which is an unneutralized portion of esters of phosphoric acid); and KLEARFAC AA-040 (an anionic monosubstituted orthophosphate ester supplied by BASF Wyandotte Corporation).

It has been found that the addition of a surface-active additive of the type described here to a cell in an amount of 0.001 to 5%, preferably 0.005 to 1%, in particular 0.001 to 0.3%, based on the weight of the active anode component of the cell, prevents or at least considerably hinders the development of hydrogen within the cell, thereby increasing its lifespan and useful life.

The addition of the surfactant to cells having a reduced number of anode metal grains or single crystals of such anode metals results in a further synergistic reduction in cell gassing.

While the use of single crystal anode material and the use of organic phosphate esters as surfactants (US Pat. Nos. 4,487,651 and 4,195,120, the assignee of which is the assignee of the present application) are each individually effective in reducing cell gassing or known a certain reduction in the mercury content in the anode without harmful increase in gas formation; the combination effect surprisingly turns out to be considerably greater than the sum of the individual effects. Thus, in cells with amalgamated single crystal zinc anodes, the amount of mercury in the amalgam can be effectively reduced from about 6 to 7% to about 4%, or in other words, the amount of polycrystalline zinc amalgam containing 1.5% mercury formed in the unit time Gases can be reduced approximately two-fold by using single crystal zinc.

Similarly, the use of an organic phosphate ester as a surfactant, such as. B. GAFAC RA600, with polycrystalline zinc amalgam anodes results in an approximately 4-fold reduction in the gas, e.g. 0.1% GAFAC RA600. However, according to a preferred embodiment of the present invention

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the combination of the two measures, i.e. a single crystal zinc amalgam with a surfactant, surprisingly the effective reduction in mercury to about 1.5% with about a 20-fold reduction or prevention of gas generation rate or about twice the expected level. Of course, the combination of chemical agents to reduce gas formation usually does not produce an additional effect, as does the excessive use of such additives. The preferred use of the reduced grain number zinc surfactant as the anode material provides an economically synergistic reduction in cell gassing that is greater than that obtained with single crystal anode material, but significantly less than that achieved with high grain number polycrystalline zinc .

The zinc single crystals are preferably produced as in the aforementioned US Pat. No. 4,487,651, the disclosure of which is also made by reference to the disclosure of the present patent application. Such procedural measures include e.g. the formation of a crucible with thin skin on each zinc part by oxidation in air at a temperature below the melting point (419 C) of zinc, heating the zinc-coated skin particles in an inert atmosphere above the melting point of zinc and then slowly cooling and the Removal of the oxide skins. The sizes of zinc particles for use in electrochemical cells are generally between 80 and 60 microns; such a method is an effective means of producing single crystal particles of such small dimensions.

The amount of mercury in the anode amalgam can range from 0 to 4%, depending on the cell use and the extent of the tolerated gas.

The amalgamated grains in reduced number or the single-crystal metal parts with surface-active additives, such as. B. GARFAC RA600 can be formed into anodes for electrochemical cells, especially alkaline electrochemical cells. Alternatively, the anodes can be formed from the reduced number of grains or the single crystal particles and the surfactant migrates from other cell components such as e.g. the electrolyte, separator or cathode to which the surfactants were originally added into the anodes. Other anode metals that can be formed into reduced grain number powders or single crystal powders and that are useful in electrochemical cells include, in particular, Al, Cd, Ca, Cu, Pb, Mg, Ni, and Sn.

Another additional or alternative preferred means for reducing gassing is alloying polycrystalline anode metal parts with indium or other additives prior to anode metal grain reduction or prior to formation of the single crystal metal particles, generally in amounts in the range of about 25 to 5000 ppm, preferably between 100 and 1000 ppm. The amount of mercury in the anode amalgam can range from 0 to 4% depending on the cell use and the extent of the tolerated gas.

The amalgamated single crystal metal particles with pre-alloyed inclusions of materials such as indium can then be formed into anodes for electrochemical cells, in particular alkaline electrochemical cells. Other anode metals that can be formed into single crystal powders and that are useful in electrochemical cells include Al, Cd, Ca, Cu, Pb, Mg, Ni, and Sn. It is understood that for anodes made from these metals, the master alloy material is not the same as the active anode material, but is less active electrochemically.

In order to clarify the effectiveness of the present invention in reducing cell gassing, the following comparative examples are given. These examples are for illustration only; details contained therein are not intended to limit the present invention. Unless otherwise stated, all parts in the present description mean parts by weight.

example 1

Three batches of polycrystalline zinc with an average particle size of about 100 micrometers are of different lengths and are heat-treated at different temperatures and then amalgamated with about 4% by weight of mercury. A fourth batch of 4% mercury amalgamated polycrystalline zinc is not heat treated and serves as a control. Two grams of each batch are placed in 37% KOH solutions (similar to the electrolyte of alkaline cells) at 90 ° C, the heating parameters and amounts of gas per unit time are given in Table 1:

Table 1

Zinc treatment ml gas (24 hours) ml gas (93 hours)

Comparison not warmed 0.62 3.77

114 hrs at 400 ° C 0.25 2.07

235 hrs at 400 ° C 0.28 2.78

70 hrs at 419 ° C 0.28 2.18

From the above values it can be seen that the heat treatment according to the present invention leads to more than doubling the gas rate of the amalgamated zinc. It can also be seen that continued long-term heating does not appreciably affect gassing rates and is generally economically undesirable.

Example 2

New Jersey Zinc Corporation (NJZ) polycrystalline zinc powder (average particle size 100 micrometers) is heat treated at 370 ° C by tumbling it in a rotating calciner for one hour. The powder, as obtained from New Jersey Zinc, has the crystalline structure shown in FIG. 1. After the heat treatment, the powder has the crystalline structure shown in Fig. 2, in which the grain size is remarkably increased, the number of grains is reduced and the amount of the grain boundaries is reduced at the same time. The polycrystalline zinc is amalgamated as it is obtained and after heat treatment with 4% mercury and 2 gram samples of each sample are tested for gas generation as in Example 1. An additional 2 gram sample of zinc from Royce Zinc Corporation, amalgamated with 7% mercury and having a similar polycrystalline grain structure and average particle size, is also tested for gases as an additional control (and represents amalgamated zinc according to the prior art), the Gassing results are given in Table 2:

Table 2

Zinc type gas formation (ml) after 24 h at 90 ° C

As obtained from NJZ, 4% Hg 0.85 heated at 370 ° C, 4% Hg 0.4

7% Hg, Royce 0.25

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The heat treatment, as described, results in an anode material with markedly improved gassing properties compared to untreated polycrystalline zinc, and slightly worse than prior art amalgamated zinc with considerably more mercury in the amalgam.

Example 3

Two grams of the various amalgamated zinc materials of Example 2 are similarly tested for gas formation at 71 ° C after periods of 7 and 14 days. The results are shown in Tables 3 and 4:

Table 3

Zinc type 7 days (ml gas) 14 days (ml gas)

As sourced from NJZ,

4% Hg 0.98 1.95 heated at 370 ° C for 1 hour, 4% HG 0.50 1.19

7% Hg, Royce 0.46 0.95

Table 4

Zinc type gas formation rate (nl / g / day)

0-7 days 7-14 days 0-14 days

As sourced from NJZ,

4% Hg

70

69

70

at 370 ° C for 1 hour

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43

heated, 4% HG

7% Hg, Royce

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Both the total amount of gas evolved and the gas formation rates of heat-treated zinc powders after extended periods of time are comparable to those of zinc powders that are amalgamated with considerably more mercury.

It can be seen from the photomicrographs of Figures 1 and 2 that the numerous polycrystalline grain boundaries have been reduced in number with a simultaneous reduction in the number of polycrystalline grains per particle without a general change in shape of the individual particles. The number of grains in the heat-treated particles is one third or less based on the number of the original particles.

Example 4

Zinc powder amalgams containing 1.5% mercury are produced with polycrystalline zinc with standard grain alone, polycrystalline zinc with standard grain with 0.1% RA600 as an additional element, single crystal zinc and single crystal zinc with 0.1% RA600 as an additional element. Equal amounts of the amalgam powder are then added to equal amounts of 37% KOH solution (typical electrolytic solution of alkaline cells) and tested for gas formation at a temperature of 71 ° C. The 0.1% GAFAC RA600 is added to the alkaline solution; stirring the zinc in such solutions results in the surfactant settling on the zinc. The amount of gas formation, measured in microliters / gram per day (jil / g day), and the factors of the reduction in gas formation rate (with the polycrystalline zinc comparison sample being 1) are shown in Table 5:

Table 5

Anode material gas formation rate factor of

Decrease rate

5

Polycrystalline zinc,

1.5% Hg

295

1

Polycrystalline zinc,

1.5% Hg + 0.1% RA600

80

3.7

Single crystal zinc,

1.5% Hg

140

2.1

Single crystal zinc,

1.5% Hg + 0.1% RA600

15

19.7

15 The reduction in gas formation (if any) would be expected at most at about 7.8 (3.7 x 2.1) for a combined use of single crystal zinc and RA600 with a reduction in the gas rate to about 38 pl / g-day. However, the combination 20 synergistically reduces gas generation to about twice the expected reduction.

Example 5

Zinc powder amalgams of polycrystalline zinc and single crystal zinc with and without 0.1% GAFAC RA600 addition are tested as in Example 3, but with 0.5% mercury amalgams. The amount of gas formation, measured in microliters / gram-day ((tl / g-day) and the factors of the reduction in the gas formation rate (polycrystalline so being zinc as comparative sample 1) are shown in Table .6:

Table 6

35 anode material gas formation rate factor of

Reduction of the rate of gas formation

Polycrystalline zinc,

«0.5% Hg

720

1

Polycrystalline zinc,

0.5% Hg, 0.1% RA600

130

5.5

Single crystal zinc,

0.5% Hg

265

2.7

4s single crystal zinc,

0.5% Hg, 0.1% RA600

26

28

The factor of reducing the gas generation rate (if any) would at most be about 14.9 (5.5 x 2.7) for such a combined use of single crystal zinc and RA600 with a reduction of the gas generation rate to about 48 ^ 1 / g- Expected day. However, the combination synergistically reduces gas generation to almost double the expected reduction.

It can be seen from the examples and tables above that the single crystal zinc with one or more additives according to the invention noticeably effectively allows for significant reductions in mercury without increasing the cell gassing.

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Example 6

Polycrystalline zinc is pre-alloyed with 550 ppm gallium and 100 ppm indium. A first sample from this is then amalgamated with 1.5% mercury. A second sample 65 is made into individual single crystal alloy particles as described above before being amalgamated with mercury. Two grams of each sample are placed in 37% KOH electrolyte solution, the gas bil

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at the end of 24 and 48 hours at 90 "C as typical for corrosion. As a control, an amalgam with polycrystalline zinc with 7% mercury, similar to that commonly used in alkaline type cells, is made. The results of the tests are shown in Table 7.

Table 7

Sample gas volume (ml). 90 "C.

24 h 48 h

Polycrystalline

Alloy 0.7 1.9

Single crystal alloy 0.3 1.0

Control (7% Hg) 0.2 0.5

Example 7

A first portion of polycrystalline zinc powder with 0.04% lead is amalgamated with 2% Hg and a second portion is converted into individual single crystal particles before amalgamation. The amalgams are then subjected to corrosion rates in 10M KOH. containing 2% ZnO. The gas formation rates at 71 ~ C are 225 | il / g-day and 80 pl / g-day.

It is apparent that the corrosion reduction of anode metals, such as zinc, by the master alloy with anti-corrosion additional material is greatly enhanced by the formation of single crystals from the anode metal additive alloy.

It goes without saying that the examples given above are intended to illustrate the invention and that changes in material treatment, material proportions, specific material, cell construction and the like are within the scope of the present invention as defined by the following claims.

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Claims (14)

671 304 PATENT CLAIMS 1. A method for producing an electrochemical cell with reduced gas formation, the cell having an anode made of polycrystalline metal which is subject to gas formation, characterized in that a) the number of grains in the polycrystalline metal to one third or less of that originally present Number of grains reduced; b) forming the metal thus obtained with a reduced number of grains into an anode for the cell; and c) introducing the shaped metal anode into the cell. 2. The method according to claim 1, characterized in that the polycrystalline metal is heated at an elevated temperature below the melting point of the metal for a sufficiently long time to reduce the number of grains in the polycrystalline metal to one third or less of the number of grains originally present to reduce. 3. The method according to claim 2, characterized in that the polycrystalline metal is zinc, cadmium, nickel, magnesium, aluminum, manganese, calcium, copper, iron, lead, tin and / or a mixture thereof. 4. The method according to claim 3, wherein the polycrystalline metal is zinc, characterized in that the zinc is heated to a temperature of 50 to 419.5 ° C for at least five minutes to two hours. 5. The method according to claim 1, characterized in that one additionally adds a surface-active, heteropolar material as an additive with a polar affinity to the anode of the cell. 6. The method according to claim 5, characterized in that the polycrystalline metal is converted to single crystals. 7. The method according to claim 5 and 6, characterized in that the surface-active heteropolar material as an additive an organic phosphate ester of the formula [RO (EtO) 3 - P = n x (OM) y is, m is x + y = 3 M = H, ammonia, amino or an alkali or alkaline earth metal and R = phenyl or alkyl or alkylaryl having 6 to 28 carbon atoms. 8. The method according to any one of claims 1 to 5, characterized in that the polycrystalline anode metal is alloyed with indium, gallium, thallium, cadmium, bismuth, tin and / or lead before reducing the number of grains. 9. The method according to claim 8, characterized in that the polycrystalline anode material is converted into discrete single crystal particles, the alloy metal forming part of the single crystal. 10. Electrochemical cell, produced by the process according to claim 1, comprising an anode, a cathode and an aqueous electrolyte, characterized in that the anode consists of single-crystal anode metal particles and that the cell contains a surface-active heteropolar material which has a polar affinity for the anode owns, contains. 11. Electrochemical cell according to claim 10, characterized in that the surface-active heteropolar material as an additive is an organic phosphate ester of the formula [RO (EtO) 3 n x - P = O ■ * ■ ^ '(OM) 5, in which y x + y = 3 M = H, ammonia, amino or an alkali or alkaline earth metal and R = phenyl or alkyl or alkylaryl with 6 to 28 carbon atoms io lenstoffatomen mean. 12. Electrochemical cell according to claim 11, characterized in that the organic phosphate ester has the following formula 15 [RO (EtO) 3 - P = O n x \ (OM) 2o y wherein M, R, n, x and y are defined in claim 11 and that said ester contains the free acid of an anionic organic phosphate ester based on a linear primary alcohol and an unneutralized par- 25 tialester of phosphoric acid. 13. Electrochemical cell according to claim 10, containing an aqueous alkaline electrolyte, characterized in that the anode is amalgamated with mercury single crystal zinc particles and an organic phosphate Contains 30 esters, the mercury making up up to 4 wt .-% of the anode and the organic phosphate ester has the following formula EtO) 3 - P = 0 n x [RO (] 35 (OM) wherein R, M, n, x and g are defined in claim 11 and 40 said ester contains the free acid of an anionic organic phosphate ester based on a linear primary alcohol which is a non-neutralized partial ester of phosphoric acid with the organic Is phosphate ester and makes up 0.01 to 3% by weight of the anode. 45 14. Electrochemical cell according to claim 13, characterized in that the mercury comprises up to 1.5% by weight of the anode, the cathode comprises manganese dioxide and the aqueous electrolyte comprises a potassium hydroxide solution. 15. Electrochemical cell according to claim 10, characterized in that the anode contains particles of separate single crystals of anode metal and indium, cadmium, gallium, thallium, bismuth, tin and / or lead as further metal, the or the further metals in the Anode is in a range of 25 to 5000 ppm or 55 and the other metal or metals is (are) alloyed with the anode metal before the formation of the discrete single crystal particles, whereby the metal (metals) form part of the single crystal. 60