GB2028569A - Improvements in negative electrodes for use in electrochemical cells - Google Patents

Abstract

According to the invention there are provided negative electrodes for use in zinc based electrochemical cells, comprising as additive incorporated in the electrode, chemically stable electrically conductive fine particles, which are substantially electrically insulated from each other, which create a substantial ohmic drop from the exterior of the electrode to the current collector, said additive being in a form providing a substantially increased effective surface area at a small increase of weight. The preferred form of the additive is foil particles, ribbon, flakes, fibers and platelets. Preferred material of the additive is steel, copper, brass, carbon or graphite, plated with silver, cadmium bismuth or nickel.

Description

SPECIFICATION Improvements in negative electrodes for use in electrochemical cells According to the present invention there are provided negative electrodes for use in zinc alkaline secondary electric cells hereinafter "negatives".

The novel electrodes are of use in all secondary alkaline batteries having zinc negatives, such as Ni-Zn, Ag-Zn, MnO2-Zn, Zn-02 and Zn-air batteries. According to the invention there are provided improved negatives for such cells, having an increased effective surface area and suffering less from the usual drawbacks of conventional zinc electrodes, such as dendrite formation and shape change degradation during repeated cycling. More uniform deposits of zinc are obtained, which are compact and which adhere well to the negative matrix.

The novel electrodes are also of use in zinc based primary cells such as Zn-MnO2 and Zn-air cells in which their improved mechanical strength and high current capabilities are of advantage.

One of the main problems with secondary electric cells of the zinc-alkaline type is dendrite formation during cycling and shape change of the negative as zinc is deposited thereon, resulting in fading capacities of such systems. The problem has been tackled before, but no satisfactory solution has been found. Amongst improved negatives dealt with in the prior art there may be mentioned: Firstly the use of battery organic or inorganic additives, added to the negative and which are intended to maintain good zinc morphology on charging. This is exemplified by U.S. Patent No.

3,816,178 (1974) of Hitachi Ltd., according to which a calcium hydroxide/lead mixture is incorporated into the zinc oxide negative paste.

Unfortunately on prolonged cycling lead is plated out and capacity fades. A second approach is based on the vibration of the zinc electrode in order to prevent formation of dendrites (see U.S.

Patent 3,923,550 of AGA, Sweden 1975). This requires complicated mechanical means and bulky, weighty construction and is of limited application. A third solution is based on the use of preshaped negatives which take into account the tendency of zinc to plate out preferably on certain parts of the plate (see U.S. Patent 1,214,285 of Yardley, 1 970). This only delays the deleterious effects of shape change degradation, especially at rapid charge rates and deep discharge levels. The latter patent also discloses the use of PTFE as binder of the negative active material and reports 150 shape change-free cycles at 60 % depth for SAH size Ag/Zn cells.

A substantially different approach was taken by Langer and co-workers (U.S Patent 3,262,81 5 for Westinghouse, 1966). These workers described an electrode suitable for a secondary battery comprising a plate formed from a compact body of fine metal fibres, with active material distributed on and disposed within the body of the fibres. The majority of the fibres extended the full length of the plate and a small proportion extended transversely; the fibres had a generally parallel lined orientation in one direction, and most of them were directly connected to the plate current collector. The invention was applicable to a variety of secondary battery electrodes, including zinc negatives and resulted in high active surface and energy per unit weight, and also good conductivity characteristics for the element A similar approach is described in U.S.Patent 3,660,167 (Ching and co-workers, 1 972) in which stainless steel filaments are added to the cathode mix (the positive) of alkaline dry cells. As in the Langer patent the importance of a continuous high conductivity path via the metal additive from current collector to active material was emphasized.

According to the present invention a simple and inexpensive solution of the problem of shape change in secondary zinc-alkaline cells is provided, giving a considerably improved performance. The electrodes shou!d also be of use in primary zinc cells.

According to the present invention there are provided improved negatives for use in alkaline type zinc secondary electrical cells characterized by a substantially increased effective surface area for.

the plating out of zinc, said increased surface area being provided according to a preferred embodiment by solids which result in an appreciable ohmic drop from the exterior of the electrode to the current collector, thereby ensuring that the zinc plating occurs from the inner portions of the plate outwards, without blocking the fine structure. The zinc deposits are of satisfactory morphology, they are compact, adhere well to the substrate and little or no shape change is observed during repeated heavy-duty cycling.

According to conventional practice good deposits are obtained when low overpotential (lower than about 100 mV) are used for the deposit of the zinc, thus making necessary the use of currents lower than about 5 mA-cm2 of electrode surface. This results in unacceptable charge times, and when higher charge rates (10-20 mA/cm2) are used, problems of shape change and dendrite formation result.

These drawbacks are overcome by providing an effective surface area of the zinc electrode which is at least twice that of the geometrical surface area of the said electrodes. The increased effective surface is obtained by incorporating into the zinc negative high surface area constituents of low weight with the electrode exhibiting a substantial ohmic drop from the exterior to the current collector. The resulting composite electrode has a weight and a geometric configuration similar to that of conventional negatives, but much higher charge rates may be used and considerably reduced shapechange or dendrite formation take place. The term "increased effective area" means essentially an increase of at least two times that of the geometric dimensions of the plate. The material added is preferably used in discrete small particles of preferably at least one small dimension.Thus, for example, the additive can be any suitable metal, such as steel, copper, nickel, brass, cadmium, or plated materials such as nickel plated copper or the like, in the form of fibers, ribbons, flakes, platelets, foil or the like. The material is chopped to suitable dimensions, if required, and used in such form. It is incorporated into the electrode matrix, preferably by means of a suitable binder. There may also be added a further additive adapted to reduce overvoltage for zinc plating.

Low-carbon steel wool of fine fiber diameter (about 0.01 mm to 0.1 mm) chopped to linear dimensions of less than 1 mm gave good results.

Such fibers provide about 100 cm2/gram fibers, and thus incorporation of about 2 grams of such fibers into a zinc oxide negative plate of 1 5 cm x 6 cm of 15 grams weight for 3 Ah, having a geometric surface area of 180 cm2 (both sides and thickness 10 mm after pressing at 100 kg/cm2, results in doubling the effective surface area, at an added weight of about 10%.

According to a preferred embodiment fibers are bonded by a suitable binder, such as PTFE polyphenylene oxide, PVA, carboxymethyl cellulose or the like, with conventional active zinc oxide material and applied in conventional manner (pressing into, rolling on) a suitable current collector matrix, such as nickel, nickel plated steel, copper, silver or brass, the current collector being in mesh or foil form. Thus a composite porous and flexible electrode structure is obtained, which is robust Undesired hydrogen evolution (due to low overvoltage on one or more of the electrode constituents) is prevented by incorporating into the mixture a conventional additive such as mercury, lead, cadmium, tin, indium or thallium or their compounds, or a suitable organic compound known for their ability to suppress hydrogen evolution. The latter may also be incorporated into the electrolyte.The finished plate generally has a fairly high ohmic resistance (of the order of 1 to 10 ohms for the plates described above) from the surface to the current collector. In equivalent terms the active material mix, after pressing at 100 kg/cm2 had a specific resistance of 1 to 10 ohm.cm. Such an ohmic drop is desirable since it ensures that zinc plating will take place from the inner portion of the plate outwards and thus will not block the fine structure. This implies that the additive particles are relatively well electrically isolated from each other, and the binder material is quite effective in ensuring this. The exact ohmic drop which can be varied at will by altering the weight ratios of zinc oxide, additive and binder in the plate, or the pressing conditions, should be consistent with the current density requirements for the plate and the overall dimensions of the plate.

It is clear that there may be used various types of surface area extenders; requirements for these are that they are inert and insoluble in the electrolyte system (or electrochemically reversible), with good zinc deposition properties and little tendency for evolution of hydrogen.

There may be used metals, as set out above; semiconducting oxides, sulfides, carbides, borides, nitrides, hydrides, carbon or graphite, and the like, which are substantially inert in the system.

According to the present invention the surface area extender is provided in the form of fine fibers or other small particles, which are relatively well electrically insulated from each other resulting in a substantial ohmic drop from the electrode exterior to the current collector. In the case of zinc plating from solutions, only thus is it possible to make sure that zinc plating occurs starting from the inner portions of the plate outwards, without blocking the fine high surface area structure. It may be that the Linger structure is suitable for electrode systems, like NiOOH, Cd. Fe, AgO in which both charge and discharge products are essentially insoluble in the alkaline electrolyte and hence remain permanently locked in the plate.

This is quite different from the zinc system of the present invention. According to the Langer patent no binder is required. The beneficial effects of binders is apparent from the description of the present invention. It helps maintain a stable structure over many cycles and as it is electrochemically inert it cannot give rise to parasitic hydrogen evolution, giving also the required ohmic drop from the exterior to the current collector of the plate.

The Ching patent deals with the provision of stainless steel filaments for use in primary cell positive elements. This would not be suitable for secondary zinc cells as zinc plates poorly on the passive material. Their configuration of continuous conductive filaments is quite different in performance from the present invention.

The invention is illustrated with reference to the following specific examples, of Zn-Ni and Ag-Zn systems, to which the invention is not restricted.

EXAMPLE 1: Ni-Zn CELL In order to demonstrate that a high surface area bundle of metallic fibres with good electrical contact to the current collector, is no basis for a good recyclable secondary zinc electrode, a Ni-Zn cell was constructed, using as negative a 15 x > c6 cm steel mesh which had spot welded to it at many points; a pad of steel wool (000 grade, fiber diameter 0.03 mm) on each side over the whole face. Each pad weighed 2 g, had an apparent surface of about 200 cm2/pad and the plate on light compression was 3 mm thick; the plata weight was 7 g. The electrode was then Zn-plated until about 15 g. Zn had been picked up, and after rinsing and drying the ohmic drop from plate surface to current collector was very low, of the order of a milliohm. The plate was confined between two positive 3 AH nickel sintered electrodes of dimensions 15 x 6 x 0.1 cm, each wrapped in a zinc dendrite - resistant separator envelope, finally inserting the whole stack into a perspex cell and adding electrolyte (rO ml of lithiated, zincated KOH containing a hydrogen evolution suppressant). After charging, the cell delivered 5 AH at an average discharge voltage of 1.6 V (2 hour rate). The cell weight was 1 50 g and the energy density about 60 WH/kg (without case). However, on cycling the cell at 50 % depth, based on 2 hour charge and discharge cycles, less than 20 cycles were obtained before capacity fading to below 50 % of the initial value.Post mortem showed the main failure mode to be zinc plating only on the outer regions of the steel wool and not within the pads. An attempt to extend cycle life by pressing 20 g PTFE-bonded zinc oxide into the pads in place of the 15 g Zn plating before cell assembly met with limited success (50 cycles to the same capacity fade level); again, the inner portions of the pads rapidly became inactive. The latter experiment was repeated but this time there was no spot welding of steel wool to the current collector, and the 4 g steel wool was chopped into 1 mm lengths and mixed carefully with the 20 g PTFE-bonded ZnO before pressing onto the mesh.

A sample of the pressed mix had a resistivity of 2 ohm cm and this was adequate to provide an ohmic drop from the current collector to the plate extremities. Under the same test conditions after 500 cycles the full initial capacity of the cell could be regained (5 AH), and post mortem showed adequate zinc plating evenly throughout the whole negative mix.

EXAMPLE 2: Ni-Zn CELL A negative electrodewas made as follows: 12 g of zinc oxide containing 2 1/2 % PTFE binder, 2 % of a conventional inorganic additive (for suppressing hydrogen evolution in alkaline solution) were homogeneously mixed. Carefully, to avoid clumping, 1.5 g of steel wool fibers (0.03 mm average diameter), cut to lengths below 1 mm, were stirred in uniformly. The mixture was applied to both sides of a nickel plated mesh current collector (linear dimensions 1 5 x 6 cm) by pressing. The plate was 1 mm thick, it was robust and flexible, and the resistance from surface to collector tab was about 5 ohms. The added fibers increased the effective surface area to about 200 cm2.The plate was sandwiched between two sintered nickel positives of 15 cm x 6 cm x 0.8 mm, each wrapped in a conventional zincdendrite-resistant separator. The 3 elements were inserted into a perspex container, and the cell filled with 60 ml electrolyte (KOH 600 g/l, LiOH 10 g/1, saturated with zincate).

The cell, when fully charged, gave 5 Ah at an average discharge voltage of 1.6 V. Cell weight was 150 g (without case) and its energy density about 60 WH/kg. The cell was cycled at 50 % depth, at 2 1/2 Afor one hour on charge and 2 A discharge to 1 V. Cell capacity decreased by less than 10 % after 1000 cycles, and still provided 5 AH on periodical full charge tests. After 1000 cycles the cell was disassembled revealing some shape change effects with excessive hydrogen evolution on charging, as main reason of the fading. In a control test without steel fibers 50 % capacity fading occurred in the first 100 cycles.

Post mortem showed the thickness in the lower portion of the negative to have doubled compared to its upper sections. Self discharge was less than 1 %perday.

EXAMPLE 3: Ni-Zn CELL A Ni-Zn cell was constructed, comprising 3 negatives of 1 5 x 6 cm of the steel fiber bonded type of Example 2 and four sintered nickel positives of identical dimensions interleaved with them. Each positive was inserted into a zinc dendrite resistant separator. The cell was positive limiting, the net capacity of the positives being 12 AH, whilst enough zinc oxide was present for 18 AH. About 120 ml of lithiated zincated KOHelectrolyte was used to cover the plates. The fully charged cell gave 12 AH at 1.65 V average discharge voltage, and the energy density was as in Example 2. Accelerated testing with high charging, deep discharge rates and prolonged overcharge was used to examine shape change degradation.In one series of drastic tests involving 1-3 hour charge rates to 100--150 % overcharge, and deep discharges of from 7S100 0O% depth, capacity decrease over 300 continuous cycles was below 20 %.

The test was repeated with negatives of even larger surface area and increased robustness, containing 40 g zinc oxide (with 4 % PTFE and 2 % inorganic lead compound as additive for the suppressing of hydrogen evolution) and containing 10 g chopped steel fibers of 0.01 mm diameter containing 40 % PTFE. Under the same conditions set out above, fading of 20 % was delayed until the 500 th cycle.

EXAMPLE 4: NiZn CELL A negative was prepared as in Example 2 containing in place of the steel fibers, the same weight of copper fibers which were plated with about 1 to 5 microns of tin so as to be suitable as substrate for zinc plating. Two such elements flanking a 3 AH positive sintered nickel electrode of the same dimensions, encased in a zinc dendrite resistant separator were assembled in a perspex case containing 40 ml electrolyte (lithiated zincated KOH). The cell, when fully charged, delivered 3 AH at 1.6 V average discharge voltage. On continuous cycling at the 2 hour rate and 70 % depth, the cell withstood 300 cycles with no excessive shape charge effects. The negatives retained their structural integrity and chemical analysis of the electrolyte revealed minimal iron content ( < 5 mg/1) at commencement and end of the test.As a control, a parallel test was run omitting the inorganic hydrogen evolution suppressant from the negative mix. After 20 cycles capacity fell to less than 50 % of the initial value. Post mortem showed that the negative had disintegrated due to hydrogen evolution.

EXAMPLE 5: Ag-Zn CELL WITH NEGATIVE INCORPORATING COPPER FIBRES A negative element was fabricated as in Example 1, for an electrode of dimensions 8 x 5 cm; only 6 g of the PTFE-bonded, additive doped zinc oxide mix was required, and the chopped copper fibre content was only 10 % of the total zinc oxide. The element, sandwiched between two pasted silver oxide positives, each encased in a zinc dendrite resistant envelope, was assembled in a perspex cell requiring 1 5 ml electrolyte (40 wt % KOH). The cell when fully charged, provided 3 AH at 1.5 V average discharge voltage, the energy density for cell weight 50 gm was 90 WH/kg. The cell, when cycled at 50 % depth for a 2 hour charge and discharge rate gave 300 cycles with minimal capacity fading.

Claims (16)

1. A negative electrode for use in zinc based electrochemical cells, comprising as additive incorporated in the electrode chemically stable, conductive fine particles substantially electrically insulated from each other, said additive creating a substantial ohmic drop from the exterior of the electrode to the current collector said additive being in a form providing a substantially increased effective surface area at a small increase of weight.

2. A negative according to Claim 1, wherein the additive is in the form of discrete particles of at least one small dimension.

3. A negative according to claim 1 or 2, wherein the additive is in the form of a foil, ribbons, flakes, fibers or platelets.

4. A negative according to any of Claims 1 to 3, wherein the active material mix after pressing at 100 kg/cm2 has a specific resistance from 0.1-20 ohm.cm.

5. A negative according to any of Claims 1 to 4, wherein the particles have a low overvoltage for zinc deposition from alkaline zincate solution.

6. A negative according to any of Claims 1 to 4, wherein the additive is selected from steel, copper, brass, carbon or graphite, alone or in combination, optionally plated with silver, cadmium bismuth or nickel.

7. A negative according to any of Claims 1 to 5, wherein the additive is a semiconducting oxide, sulfide, carbide, boride, nitride or hydride substantially inert and insoluble in the battery system.

8. A negative according to any of Claims 1 to 7, wherein the additive is used in the form of cut foil or cut fibers and the increment of effective surface area is about 50 cm2 to 200 cm2 per gram of additive.

9. A negative according to Claim 1, wherein the additive is cut-up steel wool, steel foil or steel fiber, if desired plated with nickel, copper silver, zinc or cadmium.

10. A negative according to any of Claims 1 to 8, in which the additive is covered with a sparingly soluble material such as lead, tin, indium, antimony and their compounds, said material providing beneficial ions for zinc plating from alkaline zincate solution.

11. A negative according to any of Claims 1 to 10, wherein the additive is incorporated into the electrode zinc oxide active material in combination with a suitable binder.

12. A negative according to Claim 11, wherein the binder is PTFE, polyphenylene oxide, polyvinyl acetate or carboxymethyl cellulose and the resulting composition is applied to a current collector by pasting or rolling.

1 3. A negative according to any of Claims 1 to 12, containing a conventional additive to suppress hydrogen evolution during zinc plating.

14. A battery containing a negative according to any of Claims 1 to 13, wherein the suppressor of hydrogen evolution is added to the electrolyte.

15. Negative electrodes substantially as hereinbefore described and with reference to the Examples.

16. Zinc based electrochemical cells comprising at least one negative according to any of Claims 1 to 13 or 15.