DE2006499A1 - Separation electrode for metals - Google Patents

Abstract

An electrochemical element having at least one separation electrode for non-dendritic metal separation, partic. for deposition of Zn. is made of a conductive material while surrounded by a magnetic field pref. parallel to the largest dimensions of the electrode. Specif. the magnetic field is established outside the separation electrode which may be made of conductive ferromagnetic material of high electrical permeability. An active electrode may be a support column for the separation electrodes and may be made of Ni or Fe or a high Ni ratio Ni/Fe alloy.

Description

Chargeable electrochemical cell and method of making one Deposition electrode for this purpose. The invention relates to a chargeable electrochemical one Cell with an electrode designed to deposit zinc.

It also becomes an advantageous manufacturing method for the deposition electrode specified.

Because of its favorable position within the series of voltages and the At a relatively low price, zinc is used as an electrode material for primary and secondary elements widely used. While the application to primary elements with depletion the zinc cathode does not encounter any particular difficulties is the use of Zinc electrodes in chargeable electrochemical cells not yet satisfactory been solved. There are; Already alkaline batteries with silver-zinc ~ mercury-zinc and Nickel-zinc electrodes and chargeable air-zinc elements are known in which zinc is used as a negative electrode material. Here it is in comparison with the known alkaline batteries with cadmium # admiumhydroxid electrodes disadvantageous, that when discharging such alkaline batteries with zinc electrodes primarily soluble products that tend to be over-saturated. The solid phase which separates out can consist of zinc oxide or zinc hydroxide in one there exist a large number of modifications which are distinguished by their solubility product and can only be converted into one another very slowly. The resulting fluctuations The zinc concentration in the electrolyte of the battery favors, among other things, the charging process the appearance of non-adherent or moss-like or crystalline dendrites.

The crystalline zinc dendrites tend to grow needle-shaped perpendicular to the electrode surface and lead by direct short circuit at the Touching the neighboring electrode or through loss of capacity when falling off of electrode material to a destruction of the alkaline accumulator.

A well-known task is therefore in secondary Zinc elements when charging the appearance of moss-like or crystalline, pointed Prevent dendrites.

To solve this problem, additions of lead are already or tin ions in the electrolyte have been suggested.

However, these additives are not sufficient in all cases Protective effect and also limit the choice of positive electrode.

The use of separators which use the zinc electrode is also known surround. Swellable separators, see B. Cellophane, applicable when using kept smaller Electrolyte amount is worked at overpressure. However, such a structure also limits the selection of the positive electrode and is otherwise not applicable to air-zinc elements. With the equally well-known Ion exchange membranes disrupt the mechanical sensitivity and the associated high manufacturing costs.

To suppress the dendrite growth is also the generation a mechanical electrolyte movement known, i.e. a complex technical solution, the special cases must be reserved.

After all, it can be that which occurs and which causes the formation of dendrites Zincate in a known manner into skin-soluble compounds, e.g. potassium-calcium-zincate be convicted.

This, however, makes the regression more difficult during the discharge process and the capacity of the battery is reduced.

The invention is based on the object, a chargeable electrochemical Indicate a cell with a separation electrode intended for the separation of zinc, which has a particularly favorable electrochemical behavior, with high currents can be charged and in particular effectively suppresses the formation of dendrites.

The characteristic of the invention can be seen in the fact that the deposition electrode made of a ferromagnetic, electrically conductive material of high magnetic strength Permeability exists, which essentially has a stable preferred magnetic direction parallel to the largest electrode extension having. As suitable Ferromagnetic materials with high magnetic permeability are nickel-iron alloys with a predominant nickel content, especially those under the protected product description PERIJIAUOY well-known alloy with 81% by weight of mainly nickel and 19% by weight iron, also pure iron and pure nickel as well as cobalt can be used. The investigations carried out have shown that the use of electrically conductive materials is highly magnetic Permeability with a stable preferred magnetic direction parallel to the plate surface an effective suppression of dendrite growth occurs, and that in particular, do not form surface areas of lower activity where the electrochemical reaction with the electrolyte is reduced.

Such a deposition electrode can be made from the specified materials be formed in either wire, tape or plate form; but it can also be used in advantageous further development these specified materials as a coating of a support structure contain. Ferromagnetic ones are also particularly suitable for the support structure Materials, the purity requirements in contrast to the effective electrode material are not too high. For example, iron, nickel, swabs, but also non-conductive materials can be used. The dimensions of the full electrodes or the electrode layer effective on the support structure are in any case like this to choose large that a stable preferred magnetic direction can be generated.

A particularly advantageous embodiment of a single deposition electrode can be constructed in such a way that a large number of parallel lying wires or tapes is provided, at least at the ends, optionally also on sections of their length by electrically conductive, non-ferromagnetic ones Guide bars are connected and which have a coating made of the effective electrode material, wear in particular made of a nickel-iron alloy. Such a structure enables at low weight a good utilization of the by creating a magnetic Preferred direction occurring advantages.

It has also been shown that the electrochemical effect and especially the suppression of dendrite formation in an alkaline accumulator with a deposition electrode that has a stable preferred magnetic direction, can additionally be supported by the fact that at least one additive in the electrolyte a metal hydroxide of a trivalent metal which is soluble in the electrolyte is provided is. The tests carried out leave aluminum, gallium, indium and scandium appear to be advantageously applicable.

For the production of such a deposition electrode with magnetic In the preferred direction, it appears expedient to place the effective electrode material in the magnetic field to shape or

to separate. The deposition can expediently electrolytically by chemical reaction or by high vacuum evaporation. To achieve a flawless one magnetic preferred direction, essentially parallel to the upper surface of the plate, it can also be useful, the main component the magnetic field strength during the forming or deposition process parallel to the largest To lay longitudinal extension of the electrode. Any additional stray field components that may be present do not interfere with the manufacturing process, provided there is a pronounced preferred direction can be achieved substantially parallel to the plate surface, carried out Studies have also shown that one prepared according to the foregoing The deposition electrode is charged particularly quickly, i.e. with a high charging current can be if the deposition electrode is in a during charging external stabilization magnetic field is located, which is impressed during manufacture preferred magnetic direction. For this purpose, the accumulator can be built-in or have externally attached additional devices that during the charging process generate the desired stabilization magnetic field. It is also possible to adjust the charging current to use for the generation of this magnetic field.

Such a deposition electrode can already be used during its manufacture have a zinc coating on the active electrode material; however, it can may be produced without this coating, which is then when inserted builds up in the electrolyte from zinc compounds present in it.

In the drawing, an embodiment of the invention is discrepant shown; show it; Fig. 1 is a sectional side view of a chargeable electrochemical Cell with two deposition electrodes according to the invention, Fig. 2 is a perspective view Representation of a deposition electrode for a chargeable electrochemical cell according to Fig. 1 in an enlarged view.

The cell shown in Fig. 1 consists of a housing 1 with two Deposition electrodes 2,3 for zinc with a common support frame 4 'made of ferromagnetic conductive material between the two deposition electrodes 2,3 in a Insulation means 5 is embedded, and three positive electrodes 6,7,8, which in immerse an alkaline electrolyte solution 9. On the two deposition electrodes 2.3 eïn horseshoe magnet 10 can be put on, softer the training during the charging process a stabilizing magnetic field causes. The main field directions in one Support frame 4 forming magnetic yoke are indicated by arrows in the drawing H displayed.

In Fig. 2 is an enlarged view of a deposition electrode shown, in which a large number of parallel wires are used as the support structure 11 is stretched between two copper guide bars 12, 13. Located on wires 11 themselves a layer of effective ferromagnetic electrode material, i.e. of a ferromagnetic, electrically conductive material with high magnetic permeability, which has a stable preferred magnetic direction essentially parallel to the wire axis having.

The special features of the invention are to be added below are explained with examples.

Example 1: A nickel hydroxide-zinc cell was used, whose Electrolyte in the charged state from an aqueous, thirty percent potassium hydroxide solution consists. It contained three positive electrodes connected in parallel, each consisting of three Tubes (diameter 0.7 cm, length 10 cm) made of a close-meshed nickel mesh and a layer of cotton cloth. These tubes were made with a 1: 1 volume mixture cobalt-containing nickel hydroxide and nickel powder according to a maximum loading capacity filled by 4 Ah. Between these three positive electrodes became two negative ones Zinc deposition electrodes spaced 2.5 cm apart, prepared as follows were: Twenty iron wires with a diameter of 0.8 mm and 22.5 cm in length were made arranged parallel to each other and at their ends and at a distance of 10 cm from this with 2.5 cm long and 0.4 cm wide guide bars made of 0.5 mm thick copper sheet soldered. The iron wires attached in this way were angled into a U-shaped structure, so that its 10 cm long free legs are 2.5 cm apart. The soldering points and the connecting pieces of the legs were insulated, whereby the support frame for two series-connected deposition electrodes with a total surface area of 1 dm2 was obtained. The one> part one on this A deposition electrode produced in this manner is shown schematically in FIG.

The support frame was electroplated after cleaning and degreasing Plated with a nickel-iron alloy with 81 Vo nickel, with during the entire Deposition time the two legs of the support structure in contact with the legs of a small horseshoe magnet. Between his two 1.4 cm from each other removed magnetic poles with an area of 0.8 x 1.9 cm and at a distance of 0.5 cm from the poles, the magnetic field was 500 Gauss. In contact with the Poles a Hall probe indicated a magnetic field of 600 Gauss.

The bath for depositing the alloy contained: 200 g / l nickel sulfate (Ni (S04) .6 H20) 50 g / l Nfökelchlorid (NiGl2.6 H20) 8 g / l ammonium iron (II) sulphate (NH4) 2Fe (S04) 2.6 H20) 30 g / l boric acid 2.5 g / l tartaric acid 1 g / l saccharine Pure nickel electrodes used. # The iron content was corrected by adding iron salt. The working temperature was 200.degree. C. and the pH was 3.5.

The deposition time was one hour at a current density of 5 mA / cm2.

Before installing these deposition electrodes in the cell, the Required zinc from a proprietary potassium hydroxide solution saturated with zinc oxide deposited using pure zinc anodes. The nickel hydroxide electrodes were in the cell with a galvanostatic charging current of 0.2 A and a discharge current formed by 0.1 A. The formed cell was then each five hours with 0.8 A charged and then discharged with 1 A. During 30 charge and discharge cycles the zinc coating of the deposition electrode remained free of dendrites.

Improving the quality of zinc coatings with the help of here The deposition electrodes described was determined by the properties of these electrodes itself causes how different electrolyte compositions by the Comparison of corresponding zinc coatings showed which two at the same time in a circuit of the same cells connected in series with uniform deposition electrodes were won. If with this series connection the only difference between the two Cells consisted in that only one of the two had the deposition electrode according to the invention then zinc coatings of better quality could always be found in this cell than in the comparison cell can be obtained. This also applies to all of the following Examples.

Example 2: A nickel hydroxide-zinc cell was used for easier observation built into a glass cylinder with a clear width of 2.6 cm and a height of 10 cm. The positive one Electrode was filled as an axially arranged tube made of nickel mesh in Example 1, but additionally covered with a layer of cotton fabric. Your exterior The diameter was 1.5 cm. Their maximum charge capacity was 1.4 Ah.

The support structure for the undersized beer deposition electrode consisted of seven 9 cm long iron wires 0.8 cm in diameter, which were in the same Distance with their ends were attached to a ring made of copper wire.

Wake up to the galvanizing of the support frame in a bath as in example 1 and in a magnetic field with a main component parallel to Wire direction of 700 gauss, the deposition electrode was over the positive electrode put inside. The individual wires of the deposition electrode were spaced apart by approx. 1 mm from the positive electrode. In the case of the electrolyte, 25 ml of an aqueous Solution of 30% potassium hydroxide and 3% (weight percent) ZnO and 3 g solid pure Zinc oxide added. First the cell was charged with 0.05A and discharged with 0.4A. With 20 charge and discharge cycles with a charge current of 0.2 A and a discharge current of 0.4 Å, no dendrites were observed. The same result was even found for charging currents up to 0.5 A. A disruptive degree of dendrite formation only became apparent observed at 1 A discharge current (corresponding to 7 A / dm2), but the dendrites formed in the process do not lead to short-circuit formation.

After a rest period of four weeks, the cyclic loading and Discharge continued without any change in the behavior of the deposition electrode could be observed.

Example 3: The rechargeable cell was according to Example 1 with the Difference built up that in the electrolyte an addition of 3 g of aluminum hydroxide per 1 liter of 30% potassium hydroxide solution was present. Loading and unloading processes corresponded to this Example 1. The condition was even better after 30 charge and discharge cycles the electrode surface.

Example 4: As in Example 3, except that 1 liter of 30% potassium hydroxide solution 0.5 g gallium hydroxide was provided.

The specified investigations showed essentially the same result.

Example 5: In this example. was in the electrolyte on 1 1 30% but Potash lye an addition of 1 g indium hydroxide is available. When loading became lighter ones and obtained glossier zinc coatings than in the previous examples.

No deterioration was observed after 30 charge and discharge cycles.

Example 6: There was an additive for 1 liter of goat potassium hydroxide solution as the electrolyte of 0.2 g of scandium hydroxide provided. There was an effect according to the example 3.

Example 7: A cell according to Example 1 was chosen, with the difference that instead of the electroplated nickel-iron alloy as a ferromagnetic one Material of the deposition electrode Nickel by chemical reduction with sodium hypophosphite was deposited from a -buffered nickel sulfate solution. The support frame was during the two-hour deposition of the nickel layer in contact with that in example 1 mentioned magnets. The desired behavior of the deposition electrode presented only fully engages after 3 to 4 charging and discharging cycles. In the following Cycles, the result was the same as in Example -1, but the dendritic formation appeared compared to example 1 to be used even at somewhat lower discharge currents.

Example 8: The rechargeable cell specified in Example 1 was used the charging current increased so that, despite the improved training, an irregular Activity and formation of dendrites on the electrode surface of the deposition electrode entered. With a charging current of # 1; 5 ## is around 2 A, the dendrites grew predominantly in the plane of the deposition electrode between its individual wires Charging currents around 2 A began parallel to the direction of current against the dendrites positive electrodes to grow. In a parallel experiment, the same structure was used the flipper magnet used to make the deposition electrode in the same # Orientation placed on both deposition electrodes. As it turned out that now no dendrites were formed when charged at 2 A for two hours. now 80 U / a charge with 4 A was possible in one hour.

Claims (14)

Expectations == Laabare electrochemical cell with one for the separation of zinc certain deposition electrode, d u r c g e k e n n n n e i n e t that the deposition electrode made of a ferromagnetic, electrically conductive material high magnetic permeability, which has a stable preferred magnetic direction has essentially parallel to the largest electrode extension. 2. Cell according to claim 1, d a d u r c h g e k e n n -z e i c h n e t that the effective electrode material is formed as a coating of a support structure is. 3. Cell according to claim 1 or 2, d a d u r c h g e -k e n n z e i c h n e t that the effective electrode material consists of a nickel-iron alloy with a predominant nickel content. 4. Cell according to claim 1 or 2, d a d u r c h g e -k e n n z e i c h n e t that the effective electrode material consists of pure iron. 5. Cell according to claim 1 or 2, d a d u r c h g e -k e n n z e i c h n e t that the effective electrode material consists of pure nickel. 6. The cell of claim 2, d a d u r c h g e k e n n -z e 1 c h n e t that the support frame consists of ferromagnetic material. 7. Cell according to one of claims 2 to 6, d a d u r c h g e k e n n z e i c h n e t that a multitude of parallel wires are used as the support structure or bands are provided, which are at least at the ends by electrically highly conductive, non-ferromagnetic guide bars are connected and a coating of the effective Wear electrode material. 8. Cell according to one of claims 1 to 7, d a d u r c h g e k e n n ze i c h n e t that when using alkaline electrolytes the additive is at least a metal hydroxide of a III-valent metal which is soluble in the electrolyte is provided is. 9. A method of making a deposition electrode for a Cell according to one of claims 1 to 8, d a -d u r c h g e k e n ii z e i c h n e t that the effective electrode material is deposited in the magnetic field. 10. The method according to claim 9, d a d u r c h g e k e n II -z e i c It should be noted that the effective electrode material is deposited electrolytically. 11. The method according to claim 9, d a d u r c h g e k e n n -z e i c It should be noted that the electrode material is deposited by chemical reaction. 12. The method according to claim 9, d a d u r c h g e k e n n -z e i c h n e t that the deposition of the effective electrode material by high vacuum vapor deposition he follows. 13, the method according to claim 9, d a d u r c h g e k e n n -z e i c That is to say, the main component in the manufacture of framework or plate electrodes the magnetic field strength in the manufacturing process parallel to the greatest longitudinal extent the deposition electrode. 14. A method for rapid charging of a cell according to any one of the claims 1 to 8 ## d a d u r c h g e n n n z e i c h -n e t that in the rechargeable cell a stabilizing magnetic field in the area of the deposition electrode during the charging process is generated, the main component of which is essentially parallel to the greatest longitudinal extent the deposition electrode.