WO1995018468A1

WO1995018468A1 - Solid rechargeable zinc halide electrolyte electrochemical cell

Info

Publication number: WO1995018468A1
Application number: PCT/US1994/014780
Authority: WO
Inventors: Joost Manassen; Baruch Ittah
Original assignee: Yeda Research And Development Co., Ltd.; Rycus, Avigail
Priority date: 1993-12-28
Filing date: 1994-12-23
Publication date: 1995-07-06
Also published as: IL108253A0; AU1725095A

Abstract

A rechargeable zinc halide electrolyte electrochemical cell and meethod of making the same is disclosed. The cell preferably includes an electrolyte solution containing a bromine complexing agent. The cell further includes a polymer matrix preferably made by the method of polymerizing a solution comprising an acrylic monomer in the presence of a crosslinking agent, a redox initiator and a zinc halide electrolyte to form the polymer matrix.

Description

SOLID RECHARGEABLE ZINC HALIDE ELECTROLYTE ELECTROCHEMICAL CELL
TECHNICAL FIELD
The present invention relates to rechargeable
zinc halide cells and methods of making the same. Such
cells can be used individually or connected electrically
in series to form a non-flow zinc halide storage battery.

BACKGROUND OF THE INVENTION
A thin ribbon like flexible rechargeable zinc
halide cell is disclosed in U. S. Patent 5,011,749 to
applicant, issued April 30,1991, the disclosure of which
being incorporated herein by reference. The'749 patent
specifically discloses a rechargeable zinc halide
electrolyte electrochemical cell comprising at least a
three layer laminate. A first layer includes an
electrically conducting chemically inert material. A
second layer includes a membrane matrix formed of a
hydrophilic polymer matrix in which is immobilized a zinc
halide electrolyte. A third layer includes an
electrically conducting layer adapted to absorb and
adsorb halogen.

The disclosed cells, having various
embodiments in addition thereto, provided a quite
satisfactory cell having reduced hydrogen evolution and
dendrite formation while providing sufficient energy
density without requiring an overdesign in zinc capacity.

The above mentioned'749 patent showed that a
non-flow zinc halide cell is greatly improved by the use
of a polymer matrix, which takes up several times its
weight of electrolyte solution and swells in the process.

A problem arose when it appeared to be not feasible to
prepare the swollen matrix in the presence of the electrolyte solution, which would make the swelling step superfluous. In other words, if the matrix could be formed in the presence of electrolyte solution, then the step of adding electrolyte to a polymer matrix and causing swelling would be eliminated. Additionally, a precisely sized polymer matrix could be constructed in situ, not requiring the swelling step and concommitent increase in size of the polymer matrix made by prior art methods.

It has been known that an aqueous solution of acrylamide in water in the presence of NN'methylene diacrylamide as cross linker readily polymerizes with a persulfate initiator. However, if electrolytes are also dissolved in the mixture, no polymerization will occur.

Accordingly, known chemistry does not provide means for preparing an acrylamide polymer matrix with persulfate initiator in the presence of electrolytes as electrolytes are known to inhibit polymerization.

Applicants have found an unexpected solution to the above problem which provides for a method of in situ matrix formation.

Contrary to the solution to the problems of rechargeable zinc halide cells presented above, others have almost exclusively developed flow cells, where both anolyte and catholyte are circulated continuously and both compartments are separated by a separator or a cationic exchange membrane.

The development of the above mentioned flow cells has been possible only because of the development of bromine complexing agents which lower the vapor pressure of dissolved bromine. In principle, these agents are quaternary ammonium bases which are very soluble in water and form complexes with the polybromide ions (Br) n. For lower values of n, these complexes are soluble. With higher values of n, these complexes precipitate out of solution.

In the non-flow cell, wherein bromine is bound to an electrically conducting layer adapted to absorb and adsorb bromine, this layer takes the place of dissolved complexing agent and no such agent need be added for the purpose of lowering vapor pressure. Accordingly, the non-flow cell, as disclosed in the above mentioned'749 patent, does not at all require the complexing agents for the purposes that the complexing agents have provided in the flow cells.

Applicants have found unexpected additional benefits by the addition of bromine complexing agents to non-flow cells.

SUMMARY OF THE INVENTION
In accordance with the present invention, there is provided a method of making a polymer matrix of rechargeable zinc halide electrolyte electrochemical cells by polymerizing a solution comprising an acrylic monomer in the presence of crosslinking agent, redox initiator and zinc halide electrolyte to form the polymer matrix.

The present invention further provides a rechargeable zinc halide electrolyte electrochemical cell comprising a laminate of sequence:
a. electrically conducting chemically inert material;
b. a membrane matrix formed of a hydrophilic polymer matrix in which is immobilized a zinc halide electrolyte containing a bromine complexing agent, and
c. an electrically conducting layer to absorb and adsorb hydrogen.

DETAILED DESCRIPTION OF THE INVENTION
A rechargeable zinc halide electrolyte electrochemical cell constructed in accordance with the present invention generally includes a piece of electrically conducting cloth, for instance made of carbon or graphite fibers, which is covered with a mixture of activated carbon and electrolyte solution.

Carbon blacks of high surface area, such Ketjen black or
Black Pearls are preferred. A preferred composition of electrolyte solution is 3.5M zinc halide and 3M potassium chloride.

On this composition, a sheet of hydrophilic polymer in either a crosslink or non-crosslinked form is deposited. A suitable polymer which is preferred for the present invention is polyacrylamide.

A battery separator is then laid, preferably carrying acidic groups such as Permion 1010. On this again, a piece of polymer sheet is deposited and on that another piece of conducting graphite cloth. This sandwich is thoroughly wetted with electrolyte solution.

The thus obtained rechargeable cell can be pressed between additional current collectors or may be enclosed in a plastic envelope with pieces of the graphite cloth protruding for making electrical contact.

Such a cell is approximately two millimeters in thickness and can be charged for two hours at approximately 8mA/cm'and will discharge for about two hours at about 80% current efficiency and about 95% voltage efficiency. Cycling can be pursued for hundreds of times without any change in the current voltage behavior as a function of time. Energy density conforms to 120mWH per ml.

As discussed below a wide choice of polymers and separators can be used and the choice is made according to considerations of price and performance.

The greater part of the halogen formed is retained in the activated carbon layer. The activated carbon can be mixed with anion exchange material. The remainder of the cell is only in contact with a very small amount of halogen and corrosion problems are much smaller than those encountered in flow cells.

Preferably, the electrolyte includes a bromine complexing agent. The addition of the bromine complexing agent unexpected-provides the beneficial effect of decreasing corrosion after prolonged cycling and decreasing cell selfdischarge.

More specifically, one of the advantages of the non-flow cell as compared to the flow cell is the absence of corrosion because there is no flowing bromine containing stream. But even with the non-flowing system, after prolonged cycling there is some corrosion that can be found at the bromine electrode. The addition of complexing agent unexpectedly prevents this corrosion entirely. That is, the bromine complexing agent unexpectedly functions in a manner which is totally different than that expected from its use in prior art flow cells. Hence, the otherwise unnecessary additions of the coupling agents unexpectedly decreases electrode corrosion and slightly provides additional protection against unwanted self-discharge.

With specific regard to the bromine complexing agent, the bromine complexing agent can be included in the range 0.001 to 4 moles/liter. The bromine complexing agent is selected from the group consisting essentially of N-ethyl, N-methylmorpholinebromide, N-ethyl,
N-methylpyrrolidiumbrom.de, N-methoxymethyl,
N-methylpiperidiniumbromide, N-chloromethyl, Nmethylpyrrolidiniumbromide, all belonging to the group of unsymmetrically substituted quaternary ammonium salts.

Other complexing agents capable of functioning in accordance with the present invention can be used.

As stated above, a further aspect of the present invention relates to the preparation of the ç polymer matrix in the non-flow zinc halide cell. The problem addressed is a fact that an aqueous solution of the acrylamide in water and in the presence of crosslinker readily polymerizes with a persulfate initiator but polymeration will not occur if electrolytes are also dissolved in the mixture. This chemistry prevents the in si tu preparation of the polymer matrix and requires preparation of polymer matrix and then the addition of the electrolyte after polymerization and drying has occurred. Swelling results from the addition of electrolytes which causes a loss of precision in sizing of the matrix.

Applicants have found that it is possible to perform the polymerization of the matrix in the presence of redox initiator. Generally, this aspect of the present invention provides a method of making the polymer matrix of the rechargeable zinc halide electrolyte electrochemical cell by polymerizing a solution comprising an acrylic monomer in the presence of a redox initiator and a zinc halide electrolyte to form the polymer matrix.

More specifically, it is well known that acrylic monomer can be polymerized by redox initiators.

In accordance with the present invention, however, the polymerization occurs readily in the presence of electrolytes but fails when only monomer, initiator and crosslinker are present in the absence of the electrolyte. Accordingly, applicant has discovered a novel and inexpensive synergy between the monomer, redox initiator, and electrolyte.

The present method lends itself to in situ preparation of the polymer matrix in a cell. That is, the acrylic monomer, redox initiator, and zinc halide electrolyte solution can be poured into a cell and polymerization of the solution take place in situ to totally conform with the cell dimensions. This was not achieved by prior art methods of addition of the electrolyte to a polymer matrix resulting in swelling of the matrix.

Acidification of the solution of the acrylic monomer, redox inhibitor, and zinc halide is beneficial.

Acidification is accomplished by bringing the pH of the solution a range of pH 1 to 5.

Preferably, the solution is brought to a pH of about 2. Preferably acids for acidifying the solution are selected from the group consisting essentially of hydrochloric acid, sulfuric acid, phosphoric acid, trichloroacetic acid and trifluoro acetic acid.

Acrylic monomers which are preferred for the present invention are selected from the group consisting essentially of acrylamide, acrylic acid, methacrylamide, vinylpyrrolidone, and polyethyleneglycolacrylates.

The oxidative part of the redox initiators which are preferred for use with the present invention are selected from the group consisting essentially ofpersulfates of sodium, potassium, or aluminum; t-butyl peroxide, hydrogen peroxide; benzoyl peroxide; and methylethylketone peroxide and the reductive parts from bisulfate, metabisulfate (pyrosulfite); dithionate; ascorbic acid; sodium thiosulfate; and sodium formaldehyde sulfoxylate.

Preferably, the solution which is used to form the polymer matrix of the present invention comprises
a. 1 to 10 molar acrylamide,
b. 0.0001 to 1 molar redox initiator, and
c. 0.1 to 6 molar zinc halide, and
d. 0.1 to 6 molar complexing agent.

The solution for preparing the polymer preferably includes an additional halide salt such as potassium chloride and ammonium chloride preferable 0.1 to 6 molar potassium chloride is used.

Polymerization preferably occurs at a temperature of 0 to 90 C and can be accomplished in about 30 minutes or less.

As discussed above, the polymer matrix can be laminated between the electrically conducting chemically inert material and an electrically conducting layer adapted to adsorb and absorb halogen to form the electrochemical cell. Such cells can be connected in series to form a rechargeable zinc halide electrolyte chemical battery.

In the preferred embodiment of the present invention, it has been found that self-discharge can be greatly reduced by the use of two instead of one polymer matrix. The cell includes a laminate having a bi-layer membrane matrix formed of a pair of hydrophilic polymer matrices in each of which is immobilized the zinc halide electrolyte. Critically, a material capable of reversibly adsorbing and releasing bromine is disposed between the membrane matrices for reducing self discharge in the cell, such as an anion exchanging material. The anion exchange layer preferably includes at least one layer of anion exchange beads separating the membrane matrices. It is critical that the layer clearly separate between the membrane matrices.

It has been found that if the anion exchange beads are not forming a separate layer but are homogeneously distributed inside the matrices, almost no effect is observed. On the other hand, by the use of the layer of anion exchange beads or other materials in accordance with the present invention, self discharge has been reduced to a decrease of 15% of total discharge within 48 hours, which is a value accepted by the guidelines of the U. S. battery consortium.

Preferably, the anion exchange beads are macroreticular resins. Stongly basic types such as
Amberlite IRA 900 with quaternary ammonium functionality work well, but also the weaker types with polyamine functionality like Amberlite IRA 93 appeared to be quite satisfactory. These are Rohm and Haas products.

Equivalent resins can be obtained from other manufacturers like the Dowex resins from Dow chemical, which work as well. Preferred bead sizes are from 14 to 400 mesh. As other materials can be mentioned phosphinic and Arsinic compounds. Triphenylphosphine for instance could be shown to prevent selfdischarge as well as the
IRA 900 resin. An especially active and resistant example of such exchangers is Amberlite IRA 93 resin and similar resins made by other companies.

A bi-polar non-flow zinc halide storage battery can be made by a plurality of cells constructed as above being electrically connected in series, each of the cells being a rechargeable zinc halide electrolyte electrochemical cell as described above.

In batteries such as discussed above, it is not advisable to use metal current carriers because of corrosion problems and it is better to use a carbon/plastic composite material. In non-flow zinc halogen batteries such as those constructed in accordance with the present invention, many cells are coupled in bipolar fashion and it is important that the separating charge carrier be as thin and conductive as possible.

The preferred embodiment of the present invention includes a novel current carrier comprising a first layer which is electrically conductive and a pair of plastic foil sheets which sandwich the first layer therebetween. Such layers can be made to be a few hundred microns thick thereby making them thin and light and therefore preferable for use as current carriers for storage batteries conducted in accordance with the present invention. Since there are no great demands on the dimensional stability of the carrier, such extremely thin and light current carriers are more than sufficiently functional in systems constructed in accordance with the present invention.

The first layer which is sandwiched between the plastic foil sheets can be a material selected from the group consisting essentially of carbon fiber paper, carbon felt, carbon cloth, carbon paper, and carbon fibers, wherein carbon is used to mean carbon in all of its forms, such as graphitic, amorphous, microcrystalline, diamond or fullerene. The thickness of the material can be 10 to 10000 microns. Preferably, carbon paper is used which is only a few hundred microns thick and is electrically conductive.

The plastic foil sheets comprise a material selected from the group consisting essentially of polyethylene, high as well as low density, polypropylene, polyvinylacetate, polyvinylchloride, chlorinated polyethylene, and mixtures or copolymers thereof.

A preferred method of making the current carrier described above includes the steps of disposing the first layer of material which is electrically conductive and between a pair of plastic foil sheets and ¯ pressing the foil sheets together to sandwich a first layer therebetween. This process is accomplished at a preferred temperature for a defined period of time. The pressing step can be accomplished at a temperature of 25 to 400 C for one to 600 minutes at a pressure of 14 to 50000 psi. Preferably, the temperature and time for pressing will be adjusted with respect to the thickness of the plastic foil sheets and first layer of material as well to the composition of the materials. Such adjustments can be accomplished by those skilled in the art having an understanding of the present invention as described above.

The pressing of the materials can be done in a static arrangement using a single press or can be continuously done by passing the aligned layers between heated rolling drums of a pinch press.

In use in a bipolar battery there are two aspects of the current carrier of critical importance.

The current carrier must conduct current from cell to cell through the carrier, this property being expressed as a resistance in ohms per square centimeter. This can be referred to perpendicular conductivity. This perpendicular conductivity must be less than 1 ohm/cm2 for there not to be caused unnecessary losses. A second critical feature is the lateral conductivity, which is important in order to make a charging or discharging correction on one cell in a battery stack. This lateral conductivity is expressed as a resistance in ohms/square.

There is no unit for the square because every square for a certain material has the same lateral resistance when measured along the entire side of the square. Less than 10 ohms per square is a reasonable value for this resistance.

With regard to the specific current carrier of the present invention, a conducting material is pressed between foil layers. If a metal sheet is pressed between foils, there would never be perpendicular conductivity because the foils insulate the metal effectively. With regard to the present invention, it is presumed that the hairy or rough surface of the carbon paper or felt penetrates the plastic foils during the hot pressing step and are exposed enough at the surface to provide the electrical contact. With a prior art system of hot pressing, a mixture of carbon powder and plastic, conductivity is caused because the carbon particles touch each other and apparently also penetrate through a bit from the plastic surface after hot pressing so that they can take up the current.

In the present invention, the hairs or projections from the carbon or felt paper presumably function in a similar manner. The exact opposite occurs with lateral conductivity. If for perpendicular conductivity the resistance decreases with thinner materials, the perpendicular conductivity prefers a thicker material to provide a lower resistivity. In the classical material with the carbon powder, lateral conductivity mostly is not sufficient and some metal screen is incorporated which will take care of the lateral conductivity. Unexpectedly provided by the present invention is in the lateral direction there is a continuous layer of carbon paper or felt and this layer is conductive enough to provide low resistance also in the lateral direction.

In view of the above, only conducting foils which have a certain roughness, so that after pressing parts of the conducting material such as fibers or the like are still penetrating out through the plastic layers, will show perpendicular conductivity. On the other hand, the fact that there is a continuous conducting foil in the lateral direction provides good lateral conductivity.

The following examples illustrate a usefulness and unexpected results of the present invention.

EXAMPLES:
1. Preparation of polymer matrix.

A solution of 10% in acrylamide and 0.05% crosslinker is brought to a pH of 2 by addition of a few drops of hydrochloric acid. To one ml of this solution, one drop of a 10% sodium thiosulfate solution is added followed by one drop of a 5% hydrogen peroxide solution.

No polymerizatoin occurs. After a night standing at room temperature only the smallest trace of a polymerized product is detected. With addition of a persulfate initiator, polymerization occurs within half an hour.

A second solution is prepared containing 10% acrylamide, 0.05% crosslinker, 3 molar zincbromide, and 3 molar potassium chloride. The solution is brought to a pH of two by the addition of hydrochloric acid. This solution, which does not react with the persulfate initiator, polymerizes within half an hour at room temperature after the addition of one drop of thiosulfate and one drop of hydrogen peroxide solution. After the polymerization is finished, a nice elastic dry homogeneous medium is formed which works in an excellent way as the ionic conductor in a non-flow zincbromide storage cell.

2. Use of Bromine Complexing Agent.
a. No complexing agent added.

A thin high self-discharge cell is made by having a polymer layer of one mm thickness between two activated carbon layers also of one mm thickness. The electrolyte is a solution of 3.5 molar zinc bromide and 3 molar potassium chloride at pH 2. This is a cell having a charge density of 100 mAh per ml and an energy density of 160 mWh per ml and can reach a power density of 310 mW/ml. Its self-discharge is high however and reaches 70% loss of charge in two days. If after a few hundred cycles at the full range of current and voltage the cell is opened some small corrosion pits can be discerned at the bromine electrode.
b. Addition of complexing agent.

The same cell as in Example 2a is constructed, but now with an electrolyte solution which in addition is one molar in N-ethyl, N-methylmorpholinebromide. The high current, energy and power densities are preserved, but the self-discharge is brought down to 55% in two days and no signs of corrosion can be found after the same amount of cycles as in Example 2a.

The invention has been described in an illustrative manner, and it is to be understood that the terminology which has been used is intended to be in the nature of words of description rather than of limitation.

Obviously many modifications and variations of the present invention are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

Claims

What is claimed is: 1. A method of making a polymer matrix for a rechargeable zinc halide electrolyte electrochemical cell by polymerizing a solution comprising an acrylic monomer in the presence of a redox initiator and a zinc halide electrolyte to form the polymer matrix.

2. A method as set forth in claim 1 further including the steps of combining the acrylic monomer, redox initiator and zinc halide electrolyte solution in a cell surface and polymerizing the solution in situ to totally conform with the cell dimensions.

3. A method as set forth in claim 1 further defined as combining as a solution the acrylic monomer, redox initiator and zinc halide and then acidifying the solution.

4. A method as set forth in claim 3 wherein said acidifying step is further defined as bringing the pH of the solution to a range of pH 1 to 5.

5. A method as set forth in claim 4 wherein the pH of the solution is brought to about pH 2.

6. A method as set forth in claim 3 wherein the solution is acidified by the addition of an acid selected from the group consisting essentially of hydrochloric, sulfuric acid, phosphoric acit, trichloroacetic acid and trifluoroacetic acid.

7. A method as set forth in claim 1 wherein the acrylic monomer is selected from the group consisting essentially of acrylamide, acrylic acid, methacrylamide, vinylpyrrolidonc, and polyethyleneglycoacrylates.

8. A method as set forth in claim 1 wherein the oxidative part of the redox initiator is selected from the group consisting essentially of persulfates of sodium, potassium, or aluminum; t-butyl peroxide, hydrogen peroxide; benzoyl peroxide; and methylethylketone peroxide and the reductive parts from bisulfate, metabisulfate (pyrosulfite); dithionate; ascorbic acid; sodium thiosulfate; and sodium formaldehyde sulfoxylate.

9. A method as set forth in claim 1 wherein the solution comprises: 1 to 10 molar acrylamide, 0.0001 to 1 molar redox initiator, 0.1 to 6 molar zinc halide, and 0.1 to 6 molar complexing agent.

10. A method as set forth in claim 9 wherein the solution further includes an additional halide salt.

11. A method as set forth in claim 10 wherein the additional halide salt is selected from the group consisting essentially of potassium chloride, of ammonium chloride.

12. A method as set forth in claim 1 wherein said polymerizing step is further defined as polymerizing the solution within about 30 minutes or less.

13. A method as set forth in claim 1 further including the step of laminating the polymer matrix between an electrically conducting chemically inert material and an electrically conducting layer adapted to adsorb and absorb halogen to form the electrochemical cell.

14. A method as set forth in claim 13 further including the step of assembling a plurality of the cells in series to form a rechargeable zinc halide electrolyte electrochemical battery.

15. A method as set forth in claims 1-14 wherein the solution further includes a bromine complexing agent.

16. A method as set forth in claim 15 wherein the solution contains a bromine complexing agent in the range of 0.001 to 4 molar.

17. A method as set forth in claim 16 wherein the bromine complexing agent is selected from the group consisting essentially of N-ethyl, N-methylmorpholinebromide, N-ethyl, N-methylpyrrolidiumbromide, N-methoxymethyl, N-methylpiperidiniumbromide, N-chloromethyl, N-methylpyrrolidiniumbromide, all belonging to the group of unsymmetrically substituted quaternary ammonium salts.

18. A rechargeable zinc halide electrolyte electrochemical cell comprising a laminate of the sequence: a. an electrically conducting chemically inert material; b. a membrane matrix formed of an hydrophylic polymer matrix in which is immobilized a zinc halide electrolyte including a bromine complexing agent; and c. an electrically conducting layer adapted to adsorb and absorb halogen.

19. A cell as set forth in claim 18 containing said bromine complexing agent in the range of 0.001 to 4 molar.

20. A cell as set forth in claim 19 wherein the bromine complexing agent is selected from the group consisting essentially of N-ethyl, N-methylmorpholinebromide, N-ethyl, N-methylpyrrolidiumbromide, N-methoxymethyl, N-methylpiperidiniumbromide, N-chloromethyl, N-methylpyrrolidiniumbromide, all belonging to the group of unsymmetrically substituted quaternary ammonium salts.

21. A battery including a plurality of said cells as set forth in claim 18 connected in series toform a rechargeable zinc halide electrolyte electrochemical battery.

22. A cell as set forth in claim 18 wherein said layer (b) is a bilayer membrane matrix, said laminate further including a layer (d) comprising a material capable of reversably absorbing and releasing bromine in the form of at least one layer disposed between said membrane matrices for reducing selfdischarge in said cell.

23. A bipolar non-flow zinc halide storage battery comprising a plurality of cells electrically connected in series, each of said cells having a rechargeable zinc halide electrolyte electrochemical cell as set forth in claim 22.

24. A battery as set forth in claims 21 or 23 further including a current carrier operatively connected thereto obtained by hot pressing a first layer being electrically conductive and a pair of plastic foils sheets which sandwich said first layer therebetween.