HUT63513A - Immobilized alkali-zinc anode of improved conductivity and cumulative capacity for rechargeable cells - Google Patents

Abstract

Immobilized alkaline zinc anode for rechargeable cells which comprises zinc particles, zinc oxide, alkaline electrolyte, gelling agent. These components are mixed together and they are formed to an anode mass with predetermined shape. A conductive fiber structure is admixed to the anode mass in an amount of at least 0.15 % weight consisting of fibers in the micron range with a length to diameter ratio between 1:100 and 1:1000 and a diameter of about 5 microns to about 20 microns. The fibers are made preferably of a non-conductive material such as polyimide and a conductive coating of copper, silver, gold or nickel is provided thereon. The fibers interconnect zinc particles and their capillary structure acts as an additional electrolyte reservoir. The increased conductivity decreases the drop in discharge capacity at higher cycle numbers. The alkaline zinc anode can be used at the first place in rechargeable alkaline manganese dioxide-zinc cells.

Description

The present invention relates to immobilized alkaline zinc anodes for rechargeable cells with improved conductivity and cumulative capacity.

In rechargeable alkaline manganese dioxide zinc cells, the usable cell capacity is depleted as the metal zinc content in the anode gel decreases to about 30% by weight. P. G. Cheesman and M. G. Stock, Power Sources 10, L. J. Pearce Ed., Int. Power Sources Symp. Comm., 1985, p. 217, and A. Haynes, L. Binder and K. Kordesch, Progress in Batteries & Solar Cells, Vol. 8 (1989), 39].

Considering that the original anode gel contains 60% by weight of zinc, only one half of the available amount of zinc can be used as active electrode material. The remaining half of the zinc powder is discarded or recycled.

For primary batteries, this loss is not a particular problem. Zinc powder is not expensive, and the used part has already served its purpose as the electrically conductive component of the anode material.

However, a refillable version of the zinc / manganese dioxide system requires a capacity-limiting zinc anode, so the amount of zinc that can be used in the anode is determined by the active weight of the cathode, so that the required zinc weights over a given charge-discharge cycle cannot be provided.

Increasing zinc recovery efficiency has long been the subject of research activities. U.S. Pat. No. 4,091,178. There are two proposals in the patent for rechargeable alkaline manganese

To increase the homogeneity of discharge capacity of all-carbon dioxide elements over the lifecycle, ie:

a charge reserve mass must be established near the zinc anode, and

- a perforated metal support should be made of a non-corrosive, electrically conductive material, such as amalgamated copper, silver, lead, etc., coated with anode amalgamated zinc particles.

However, there are limitations to the use of a perforated metal support. A substantial part (about 30%) of the weight of the anode is made by the carrier, reducing the usability of the available anode space. A further disadvantage of this solution is the use of amalgamated zinc particles. When mercury is used, the battery becomes pollutant, and today the amount of mercury in battery production is reduced or completely eliminated.

During use, the oxidation and reduction of zinc particles in the anode are part of the normal electrode process. A given zinc particle can only be considered as an active part of the anode process if it remains electrically in contact with other adjacent zinc particles and the current collector. As a result of the side effects of oxidation and reduction processes, the bond between the zinc particles is broken and the zinc particles, which are isolated from one another, can no longer participate in the electrochemical process, which causes the discharge capacity to start to decrease per cycle. Reducing the amount of mercury impairs the quality of the anode particle bond, that is, the anode

-4 also decreases its conductivity.

It is a primary object of the present invention to provide an immobilized alkaline zinc anode for use in rechargeable batteries which has a slower utilization of its zinc content during use over the life of the battery than previously known.

This primary objective implicitly involves the need to increase the conductivity of the anode.

Another object of the present invention is to maintain the conductivity of the anode, even when the amount of mercury is reduced or the anode is mercury-free.

Yet another object of the present invention is to provide an anode structure in which the utilization of available space is more advantageous compared to solutions using perforated metal substrates.

In accordance with the present invention, it has been found that the addition of a conductive fiber system to the anode material in an amount of at least 0.15% by weight increases the conductivity per cycle! slowing down. The addition of more than 2-5% by weight of the fiber system no longer improves the function of the anode, and even with larger volumes, there is a loss of zinc due to volume loss, so there is an economic limit to the addition of fibers. For mercury-free batteries, it is particularly important that the conductive fiber system prevents the formation of isolated zinc particles that are unrelated to the current collector during the battery discharge cycle. In contrast, in the charge cycle, the leading fiber system acts as a three-dimensional substrate for zinc deposition.

The fibers themselves are not necessarily made of electrically conductive material such as glass, polymers, etc.

-5 can be used and then coated with conductive material by electrodeposition or vacuum deposition, e.g. by cathode atomization.

The optimum ratio of fiber length to diameter is in the range of 100: 1 to 1000: 1, where the diameter of the fiber is in the range of microns, typically 5-20 microns. Preferably, the fiber material is polyimide, polyester and polyvinyl alcohol.

The coating may be based on copper, silver, gold, nickel and the like or on their alloys. For mercury-free batteries, the use of copper should be avoided as it promotes corrosion of non-amalgamated zinc.

The invention will now be described in more detail with reference to the drawings, in which: In the drawing it is

Figure 1: Measuring assembly for conducting measurement, a

Figure 2 is a graph of the individual discharge capacity and the cumulative cycle capacity versus cycle number, charge: 0.3% silver-plated polyimide fiber,

Fig. 3 is a graph of Fig. 2, filling: 0.3% copper-filled polyimide fiber,

Figure 4 is a graph of Figure 2, filling: 0.3% silver-plated polyimide fiber, no copper shielding; the

Fig. 5 is a graph of Fig. 2, filling: 0.6% copper-plated polyimide fiber, no copper shielding; the

Figure 6 is a graph of Figure 2, filling: 0.6% silver-plated polyimide fiber, no copper shielding;

The coated fiber structure is made of a non-current metal coating.

The non-conductive substrate fiber is degreased with potassium hydroxide solution, rinsed with water and

-6 I (II) chloride in water / hydrochloric acid. The most commonly used catalyst, palladium (II) chloride, is not applicable in this case as this noble metal may be present as an impurity in the anode mixture. Thus, a complex solution of silver ions can be used to catalyze the substrates. Subsequently, the materials are immersed in a solution containing a metal salt of the casing, a complexing agent, formaldehyde and water. These operations are performed in a hot chamber equipped with a paddle stirrer.

First, the uniformity and luminosity of the layer applied to the materials were examined with a 63: 1 stereo microscope. Adherence of the metal coating to the substrate was also tested.

To determine the metal content of the final product, a portion of the run-through fiber was treated with nitric acid to dissolve the metal component. After sufficient dilution, the metal concentration was measured. As a result of the low polymer densities, up to 81% metal content was found.

In the case of polyimide fibers a very high quality metal coating was observed, whereas in the case of polyester and polyvinyl alcohol fibers it was only of good quality.

The common anode mixture is added with 0.1-2% run-in fibers. The composition of the starting anode mixture was: Zinc (Hoboken-Overpelt L 305 F / 64, 3% Hg) + Additive = 74%, Zinc Oxide = 4%, Manganese Oxide = 0.7%, Carbopol = 1.3%, potassium hydroxide (saturated with zinc oxide) = 20%. The conductivity of the resulting slurry was measured using the measurement composition shown in Figure 1.

The anode mixture 1 has a predetermined length

Pressed into a glass tube 2 with a cross-section and a single piston-type contact electrode 3, 4 is connected to the two free ends of the mixture 1 in the tube 2. The two electrodes 3, 4 are connected to a measuring bridge 5 (RLC meter). The conductivity of the paste containing the conductive fill was compared to that of the normal mixture. One part of the measurements was made before discharge and the other part after discharge. Measurements were made in a less favorable manner, that is, after the addition of minimal fiber, to detect the significant improvement due to the presence of run-in fibers.

Table 1 shows the results of these measurements, where the conductivity of the anode mixture containing only 0.16% copper-free polyimide fibers was measured after 35 cycles, since measurements on an anhydrous silver plating should result in higher values.

-81. spreadsheet

Conductivity of anode mixture

Management (10 " ² 0hm ^_1 cm ^-1 ) Mix Initial values Anode data after 35 cycles Stuffed Dried Normal 4.3 2.9 0.6

0.16% Toe threads 15.1 8.9 4.2 0.16% BREAK threads 13.6 8.9 4.2

FEATURES: uncharged silver-plated polyimide fibers

BROWN: non-charged copper-plated polyimide fibers

The effect of adding a conductive component to the mixture is most pronounced at the discharged anode, where the conductivity by the zinc particles is significantly (from 4.3

Down to -90.6). With the addition of 0.16% run-in fiber during the discharge period, the conductivity increased in seven cases and thus the value obtained was approximately equal to the initial conductivity of the normal mixture.

There are two main reasons for improving conductivity. The first reason is the direct contact between the fibers and the zinc particles, the second is the capillary action of the fiber structure, whereby the electrolyte is distributed more evenly within the anode mixture.

Before performing the measurements, it was expected that by improving the conductivity of the zinc particles, a more favorable utilization of zinc would be achieved over the life of the battery, i.e., the capacity of the cycle would be drastically increased over a higher number of cycles.

To understand the longer term behavior of the element, cyclic measurements of the conductive fiber anode mixture were performed under the following conditions.

Cylindrical LR-14 (size C) alkaline manganese dioxide cells used by Y. Sharma, A. Haynes, L. Binder, and K. Kordesch, J. Power Sources 27 (1989) 145, p. we were trained as found in his writing.

Cycle measurements were limited to a period of 40 days.

The complete measurement program was carried out with computer control, which allowed the simultaneous operation of 48 elements. The constant resistance discharge method was used for the cycle measurements. Each cell was charged with a 3.9 ohm resistor until the cut-off voltage reached 0.9 V. Charging was carried out for a period of 20 hours with a 1.72 V constant voltage dropper.

Charging and discharging currents and battery voltages - 10 - were measured every minute, and then the output / input charge was calculated. The charge / discharge data was printed when the battery was switched from charge to discharge or discharge to charge.

2-6. Figures 3A and 4B show the results of these measurements, i.e., the individual and cumulative discharge capacitance curves as a function of the number of cycles. These figures also show the curves of conventional elements without a conductive charge having similar design.

Tables 2 and 3 contain data for percentage increases in cycle capacities and cumulative capacities, each of which illustrates more clearly the advantageous properties of the fiber-containing anode over the anode of the conventional design element.

First results were obtained with the addition of 0.16% silver or copper polyimide fibers. The metals used to coat the polymer do not appear to be influencing factors.

Doubling the amount of silver-coated fibers containing the conductive charge, i.e. 0.3%, resulted in a slight improvement over the values obtained at 0.16%.

The use of 0.3% copper-plated fibers showed a more pronounced improvement. The curves for the element containing 0.3% copper-plated fibers are shown in Figure 2 and for the element containing 0.3% silver-plated fibers are shown in Figure 3.

Encouraged by these observations, we have tried, like a current collector, a conventional copper-shielded element, a conventional contactor connected to a central contact.

-11anode. It should be noted that when copper shielding was applied, the conductivity of the anode appeared to have improved and, with increasing number of cycles, the extremely large increase in discharge capacity was offset. The anode space of the element without copper shading is about 20% larger as the presence of shielding did not cause volume loss ·

The behavior of the element having an anode without copper shading and having an anode containing 0.3% silver in polyimide fibers is shown in Figure 4. Although such elements are significantly better than the standard elements used for comparison, the 0.3% conductive fiber does not fully compensate for the lack of anode shading.

The best results were obtained when the amount of metal-filled polymer (polyimide) fibers was increased to 0.6% and the complex copper-shielded collector became obsolete. Figures 5 and 6 show the discharge curves of such elements, i.e., those having anodes containing 0.6% copper or silver-wound fibers and not a copper shield. It should be noted that the normal cells used for comparison were the same as the measured cells, the only difference being that the anode gel of the normal cells did not contain conductive charge.

When using higher concentrations of conductive fibers, there was no significant improvement in battery performance.

Table 2 ···:

Percentage increase in cycle capacity (Delta Ah / Ah)

Polyimide fibers wound with silver or copper

Silver Copper Silver Silver Copper 0.3% 0.3% 0.3% 0.6% 0.6% shadows shadows shadows shadows -shielded cycles look to look to No Seeing No Seeing Noise without % % % % %

1 -13 -12 -22 -8 -13 5 - 4 - 5 -15 0 0 10 20 25 10 10 5 20 25 35 23 23 30 30 28 35 25 45 50 40 10 30 30 53 66

Table 3.

:: ...... x r «· 4 · · ·« - · · · ·

Cumulative capacity (Ah)

Running in silver or copper Change percent polyimide threads Normal B Cu-fibers 0.6% C Ag-fibers 0.6% B / A C / A cycles 5 14.8 15.2 14.6 3.0 -1.4 10 23.6 25.0 24.2 6.0 2.4 15 30.9 35.1 32.1 13.3 3.8 20 37.3 40.6 39.8 8.7 6.6 25 43.0 47.6 47.0 10.8 9.3 30 48.2 54.9 54.6 13.8 13.1 35 53.3 62.1 62.0 16.4 16.3 40 57.8 68.8 67.0 19.1 16.0

It should be noted that the decrease in capacity values is the result of the volume occupied by the threads in the first few cycles (moving away from the initial capacity value), which, however, improves the power in the subsequent cycle. · * • ·· 4 · ··· • · · · · «··· · ··«

-14radt zinc efficiency, forming conductive bridges between the zinc particles remaining after partial discharge. This fact is supported by the conductivity data.

The initial loss of capacity, that is, the change in the slope of the cycle curve, is an inherent feature that results from a more modest reduction in capacity of the initial manganese dioxide discharges which improve the cycle life. In other words, the intensity of the discharge is shifted, which is particularly significant for manganese dioxide.

The use of conductive bridges between zinc particles in the manufacture of mercury-free elements is very significant. The conductivity of the amalgamated zinc particles is lower, so it is desirable to add conductive fibers to them. The use of conductive fibers alone does not solve the complex problem of mercury-free rechargeable batteries, but it contributes significantly to their complete solution. Publications to solve such problems are outside the scope of the present invention.

Claims (9)

Claims An immobilized alkaline zinc anode for rechargeable elements having improved conductivity and cumulative capacity, comprising a mixture of zinc particles, zinc oxide, alkaline electrolyte, gelling agent formed into a predetermined anode mass and electrically coupled to a current collector, wherein the mixture is at least 0, Contains 15% by weight of an electrically conductive fiber system having a fiber diameter in the micron size range. 2. An alkaline zinc anode according to claim 1, characterized in that the fibers in the fiber system consist of a non-conductive material, the surface of which is coated with conductive metal. The alkaline zinc anode of claim 1, wherein the filament has a length to diameter ratio of between 100: 1 and 1000: 1. An alkaline zinc anode according to claim 1, characterized in that the fibers have a surface made of copper, silver, gold, nickel or an alloy of these metals. 5. An alkaline zinc anode according to claim 2, characterized in that the fibers have a metal plating formed by a current-free metal coating. 6. An alkaline zinc anode according to claim 2, characterized in that the fibers have a coating formed by vacuum metal deposition. 7. An alkaline zinc anode according to claim 1, wherein: 4 · ··· · ··· -16 characterized in that the fibers are at least of the anode material 0.5% by weight, and the current collector is formed by a central metal contact connected to the fiber system. 8. An alkaline zinc anode according to claim 2, characterized in that the fibers are made of polyimide, polyester or polyvinyl alcohol. The alkaline zinc anode for rechargeable, mercury-free alkaline manganese dioxide / zinc according to claim 1, characterized in that the conductive surface of the fibers is made of silver, gold, nickel or an alloy of these metals.