AU2181692A - Immobilized alkaline zinc anode for rechargeable cells with improved conductivity and cumulative capacity - Google Patents

Description

IMMOBILIZED ALKALINE ZINC ANODE FOR RECHARGEABLE

CELLS WITH IMPROVED CONDUCTIVITY AND

CUMULATIVE CAPACITY

BACKGROUND OF THE INVENTION:

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 exhausted a soon as the amount of metallic zinc in the anode gel decreases to about 305 by weight [see P.G. Cheesman and M.G. Stock, Power Sources 10, L.J. Pearce ed., Int. Power Sources Sy p. Comm., 1985, pp. 217, and A. Hynes, L. Binder and K. Kordesch, Progress in Batteries & Solar Cells, Vol. 8 (1989), 39.].

Taking into account that the original anode gel contains about 60% by eight zinc, only one half of the zinc is consumed as an active electrode material. The remaining half of the zinc powder is disposed of, or collected for recycling, with the used battery.

With primary cells this may not be a big problem. Zinc powder is not expensive and the rejected fraction has done its work as an electronically -conductive component of the anode mass.

The rechargeable version of the zinc/ manganese dioxide system requires a capacity limiting zinc anode, therefore the amount of zinc usable in the anode is determined by the active cathode mass i.e. the over dosage of zinc is not a possible way of providing the required mass after a number of charge-discharge cycles.

The increase of the effectivity of zinc utilization has long been an objective of research activity. In U.S. patent 4,091,178 issued to K.V . Kordesch two sug gestions were made for increasing the uniformity of the discharge capacity of an alkaline manganese dioxide rechargeable cell over its useful cycle life i.e.: to provide a charge reserve mass adjacent to the zinc anode, and to provide a perforated metal carrier formed by a non- corroding electrically conductive material like amalgamated copper, silver, lead, etc. coated by amalgamated zinc particles of the anode.

The use of the perforated metal carrier has, however, its limitations. It occupies a substantial portion (about 30%) of the anode mass decreasing thereby the utilization of the available anode space. A further drawback lies in that this technical solution requires the use of amalgamated zinc particles. owing to environmental load caused by the use of mercury, there is recently a tendency of decreasing the amount of mercury or completely eKiϊ-Lnating its use by producing mercury free cells.

In use the oxidation and reduction of zinc particles in the anode belong to the normal electrode process. A particular zinc particle can only then be regarded as taking an active part in the anode process if it continues to stay in electric contact with the other adjacent zinc particles and with the current collector. The oxidation and reduction process has the side effect of insulating the zinc particles from one another, and the insulated zinc isles cannot participate in the electrochemical process anymore, therefore the ■discharge capacity decreases after each cycle. The reduction of the amount of mercury decreases the quality of the contacts among the anode particles i.e. the conductivity of the anode decreases as well.

OBJECTS OF THE INVENTION:

The primary object of the present invention lies in providing an immobilized alkaline zinc anode structure for rechargeable cells, in which there is a slower decrease in the utilization of the zinc mass during the cycle life of the cell using the anode.

This primary object indirectly includes the need for increasing the conductivity of the anode.

A further objective lies in aintεiining the conductivity of the anode even if the amount of mercury is decreased or fully eliminated. A still further objective lies in providing an anode structure, in which the utilization of available space is increased compared to those ones using a perforated metal carrier.

According to the invention it has been discovered that the conductivity of the zinc gell anode is increased, and the capacity fade from cycle to cycle is slowed down if a conductive fiber structure is admixed to the anode mass in an amount of at least 0.15% weight. The performance will not be improved if the fiber structure is added in an amount exceeding about 2 to 5%, and in case of higher amounts of fiber the effective loss in zinc volume will be remarkable and form an economic upper limit. During discharge cycles of the cell the conductive fiber structure prevents the formation is isolated zinc particles without contact to the current collector w hich is particularly important in mercury -free cells. On the other hand, during charge cycle the fiber structure serves as a three- dimensional substrate for the zinc deposition.

The fibers themselves do not have to be made of electrically conductive materials, they can be made of non-conductive materials such as glass, polymers, etc., and can be coated by the conductive material either by electroless plating or by vacuum deposition such as cathode sputtering.

The optimum range for the length to diameter ratio was found to be between 100:1 and 1000:1, the fiber diameter lies in the micron range, typically between about 5 and 20 microns. Preferable fiber materials are polyimide, polyester and polyvinyl alcohol.

The coating material can be copper, silver, gold, nickel and similar metals or alloys of these metals. In case of mercury-free cells the use of copper should be avoided, since it tends to corrode the non-amalgamated zinc.

BRIEF DESCRIPTION OF THE DRAWINGS:

The in vention will no w be described in connection with examples in which reference will be made to the attached drawings. In the drawing: Figure 1 shows the test set up for conductivity measurements;

Figure 2 shows individual discharge capacity and cumulative cycle capacity versus cycle number curves, filler: 0.3% silver plated polyimide fibers;

Figure 3 shows curves similar to Figure 2, filler: 0.3% copper plated polyimide fibers;

Figure 4 shows curves similar to Figure 2, filler: 0.3% silver plated polyimide fibers, no copper cage;

Figure 5 shows curves similar to Figure 2, filler: 0.6% copper plated polyimide fibers, no copper cage;

Figure 6 shows curves similar to Figure 2, filler: 0.6% silver plated polyimide fibers, no copper cage;

DESCRIPTION OF THE PREFERRED EMBODIMENTS :

The coated fiber structure was made by electroless plating. The non-conducting substrate fiber material was degreased with potassium hydroxide solution, rinsed with water and activated with a solution of tin(II) chloride in a water/hydrochloric acid mixture. The most common catalyst — palladium(II)chloride — was not applicable because of a possible contamination of the anode mixture with traces of this noble metal. Thus a complex solution of silver ions was used to catalyze the substrates. After this step the materials were immersed in a solution consisting of a salt of the covering metal, a complexing agent, formaldehyde and water. All these operations were performed in a beaker-cell equipped with a paddle starrer.

The first inspection of the plated materials was done by checking the uniformity and brilliance of the deposition under a "stereo" microscope (magnification up to 63:1). The adherence of the metal coating to the substrate was also examined.

To determine the amount of metal in the final product, small fractions of the coated fibers were treated with nitric acid to dissolve the metal component. After adequate dilution the metal concentration was measured . As a result of the low polymer densities, meted, content up to 81% was found. Very good quality metal deposition was observed in case of fibers made from polyimide, while good quality metal deposition was observed in case of polyester and polyvinyl alcohol fibers.

The coated fibers were added to the commonly used anode mixture in amounts ranging from 0.1% to 2%. The composition of the starting anode mixture was: zinc (Hoboken-Over-pel L305F/64, up to 3% Hg) + additive = 74%, zinc oxide = 4%, magnesium oxide ^*** 0.7%, Carbopol = 1.3%, potassium hydroxide (saturated with zinc oxide) = 20%. The conductivity of the paste obtained was measured using the test set up shown in Figure 1.

Here the anode mix 1 w as pressed in a glass tube 2 of predetermined length and cross-section, and a pair of piston type contact electrodes 3, 4 were attached to both ends of the mix in the glass tube 2. A resistance brid ge 5 using an LRC meter was connected to the electrodes 3, 4. The conductivities of pastes containing a conductive filler were compared with the conductivity of the standard mixture. One series of tests was carried out using anode mixtures before discharge and another series after discharge. The tests were carried out in the less favourable case, i.e., when the addition of the fiber is about its minimum to demonstrate thereby the significant improvement offered by the presence of the coated fibers.

Table 1 sho w s the results of these tests , in w hich the conductivity after 35 cycles was measured. In the case of the anode mix comprising 0.16% by weight of the anode mass polyimide fibers with electroless copper coating only the measurements were slightly lo wer , since the electroless silver coatin g provides higher conductivity values. Tabl e 1 : Conductivity of the anode mix

ELS : el ectrol ess sil ver coating on polyimide fibers ELC : el ectrol ess copper coating on polyimide fibers

The effect of adding a conductive component to the mixture is most ap parent in the case of discharged anodes, where the contribution of zinc particles to the conductivity is dramatically reduced (from 4.3 to 0.6) . The presence of 0.16% coated fibers resulted in a seven times increase in this conductivity which is almost equal to the initial one of the standard mix.

The increase in conductivity has two main reasons. The first one is the direct contact between the fibers and the zinc particles while the second one is the capillary effect of the fibrous structure, whereby the electrolyte is more evenly distributed in the anode mix.

Before carrying out the tests we expected that the establishment of a better conductivity between the zinc particles would result in a better zinc utilization during the cycle life of the cell i.e., the cycle capacity increases drastically in higher cycle numbers compared to the standard cell.

To learn the long term behaviour of the cells using conductive fibers in the anode mix a cycling test series was carried out under the following conditions.

Cylindrical LR-14 (C-size) alkaline manganese dioxide cells were taken which were designed as disclosed in the paper of Y. Sharma, A. Haynes, L. Binder and K. Kordesch: J. Power Sources 27 (1989) 145. The cycling tests were limited to a period of 40 days. The whole test program was done by a computer controlled device which was capable of operating 48 batteries simultaneously. In the cycling tests the constant resistor discharge method was used, each cell was loaded by a 3.9 ohm resistor until a cut-off voltage of 0.9 V was reached. The charging occurred by a trickle charger which provided 1.72 V constant voltage through 20 hours.

The charge and discharge currents and the cell voltages were measured every minute and the charge input/output was calculated subsequently. Any time when a battery was switched from charge to discharge or vice versa, a record of data concernin g the charging/discharging procedure was printed.

Figures 2 to 6 comprise the results of these tests in the form of curves showing the individual and cumulative discharge capacities as a function of cycle number. In each of these figures the curves of the corresponding standard cell i.e., which does not have any conductive filler having also been illustrated.

Tables 2 and 3 show the percentual increase of cycle capacities and the cumulative capacity data, respectively which demonstrate more clearly the improved behaviour of the fiber containing anodes compared to that of the standard cells.

The first positive results were obtained by adding about 0.16% silver or copper plated polyimide fibers. The nature of the metal used for metallization of the polymer seemed to be without visible influence.

Doubling the amount of the conductive filler comprising sliver coated fibers to 0.3% lead to a slight improvement over the values obtained in case of using 0.16%. The use of 0.3% copper coated fibers was connected with a more definite improvement. The cell curves with 0.3% copper coated fibers are shown in Figure 2 and the ones with 0.3 silver coated fibers are shown in Figure 3.

Encouraged by these observations an attempt was made to build a cell without the usual copper cage as the current collector i.e., to have a conventional anode contacted by a central pin. It should be noted that the use of the copper cage has been suggested to increase the conductivity of the anode and to partially compensate the steep decrease in discharge capacity at increased cycle numbers. The anode space of the cells without copper cage is by about 20% higher since there is no volume loss due to the presence of the cage.

Figure 4 shows the behaviour of a cell without copper cage and having 0.3% silver plated polyimide fibers in the anode. Although such a cell is much better than the standard one used for the comparison, the 0.3% conductive fibers could not fully compensate the lack of the anode cage.

As soon as the concentration of the metallized polymer (polyimide) fiber in the anode gel was raised to 0.6% the best results were achieved and the complicated copper screen collector became obsolete. Figures 5 and 6 show the discharge curves of such cells i.e., with anodes comprising 0.6% copper resp. silver coated fibers and no copper screen (cage). It should be noted that the standard cells used for comparison were identical with the tested ones, the only difference being in that the anode gel of the standard cells did not comprise the conductive finings.

The cell performance showed no significant improvements in case of using higher concentration of the conductive fibers.

Table 2: Percentage Increase of Cycle Capacities (Delta Ah/Ah) Polyimide Fibers Coated with Silver or Copper

Table 3 : Cumulative Capacity (Ah)

It should be noted that the cross-over of the capacities in the first few number of cycles is due to the volume occupied by the plated fibers (taking away initial capacity) but improving the efficiency of the remaining zinc in later cycles, simply by providing conductive bridges between the zinc particles left after partial discharge. This fact is supported by the conductivity data as well.

The initial loss of capacity, actually a "changing of the slope" of the cycle curve is an additional desired feature, because it takes less capacity out at the initial manganese dioxide discharges and thereby improves the cycle life. In other words this is a depth of discharge shift which is especially helpful with manganese dioxide.

The use of conductive bridges between the zinc particles will be more significant if mercury free batteries are made. The conductivity of non -amalgamated zinc particles is lower, therefore the presence of the conductive fibers alone cannot solve the complex problems of mercury free rechargeable alkaline batteries but represents a significant contribution towards the solution. The issues connected with such problems fall beyond the scope of the present invention.

Claims (9)

WHAT IS CLAIMED IS:

1. An immobilized alkaline zinc anode for use in rechargeable cells, where said zinc anode comprises zinc particles, zinc oxide, alkaline electrolyte, and gelling agent; said components being mixed to gether and formed to an anode mass having a predetermined shape; and a current collector means in electrical contact with the anode mass; c h a r a c t e r i z e d b y: said zinc anode further comprising a conductive fiber structure admixed to the anode mass in an amount of at least 0.15% weight, wherein said conductive fibers have a length to diameter ratio between 100:1 and 1000:1, and a diameter in the range of from about 5 to about 20 microns.

2. The alkaline zinc anode as claimed in claim 1, wherein the fibers in said structure are made of a non-conductive material having a conductive coating thereon.

3. The alkaline zinc anode as claimed in claim 1, wherein said fibers have a conductive surface of copper, silver, gold, nickel or alloys of these metals.

4. The alkaline zinc anode as claimed in claim 2, wherein said conductive coating is provided by electroless plating.

5. The alkaline zinc anode as claimed in claim 2, wherein said conductive coating is provided by vacuum deposition.

6. The alkaline zinc anode as claimed in claim 1, wherein the amount of said fiber structure in said anode mass is at least 0.15% and said current collector means is a central pin.

7. The alkaline zinc anode as claimed in claim 2, wherein said fiber is made of polyimide, polyester or polyvinyl alcohol.

8. The alkaline zinc anode as claimed in claim 1, when used in mercury free alkaline manganese dioxide-zinc rechargeable cells, wherein said fibers have a conductive surface of silver, gold, nickel or alloys of these metals.

9. The alkaline zinc anode as claimed in claim 8, wherein said predetermined shape of said zinc anode is cylindrical.