Two Phase Electrolytes Used as Halogen Traps in Metal Halogen Secondary Cells and Batteries

This invention relates to water insoluble organic electrolytes useful for complexing halogens and more particularly to two-phase electrolytes useful as an electrolyte for electrochemical cells and batteries of the type which utilize a halogen as its electrochemically active agent.

Voltaic cells which include an aqueous solution of zinc or cadmium halide as an electrolyte are known, but are frequently characterized by a relatively high self-discharge rate, low capacity, and high internal resistance. Since elemental halogen is soluble in the aqueous electrolyte, it is difficult to keep metallic zinc or cadmium and elemental halogen apart while simultaneously achieving a system in which a good percentage of the theoretical energy storage capacity can be realized.

Recently, various attempts have been made to prevent elemental halogen from migrating to the zinc or cadmium electrode. For example, U.S. Pat. No. 3,352,720 to G. R. Wilson et al teaches the use of water insoluble polymeric amine halogen complexes in place of the elemental halogen. While the structures disclosed in the Wilson et al patent function as halogen cells, they nevertheless suffer from less than optimum capacities and self-discharge rates because of the low stability of the polyhalogens employed.

Still another improvement in halogen electrochemical cells is disclosed in U.S. patent application Ser. No. 652,780 to Myles A. Walsh, entitled Halogen Electrode, filed Jan. 27, 1976. The novel structure disclosed in patent application Ser. No. 652,780 is an electrode which comprises a current collecting matrix which has an extremely stable polymer containing quaternary ammonium, phosphonium, or sulfonium sites incorporated into its structure. Since the polymer is formed in the presence of a high surface area, porous, current collecting matrix, the electrodes are capable of storing halogens in a nonchemically active but highly electrochemically active state in such a manner that uniform, intimate electrical contact is maintained between the halogen-rich sites of the polymer and the current collector, and interaction between halides in the electrolyte and the quaternary sites is greatly facilitated.

U.S. Pat. No. 3,408,232 to R. D. Blue et al teaches the use of organic solvents for extracting bromine from an aqueous electrolyte. While this represents a two-phase approach to halogen storage, the organic phase disclosed therein lacks ionic conductivity; and therefore, the halogen dissolved in the organic phase cannot be discharged at a significant rate and must be back extracted into the aqueous phase before discharge can occur. This means that the aqueous electrolyte of this cell must partially cover both electrodes.

U.S. Pat. No. 3,816,177, to Myles A. Walsh, teaches the use of soluble quaternary ammonium halides and the like which may be dissolved in the electrolyte together with a water soluble depolarizer. When elemental halogen is released into the electrolyte, it combines with the quaternary halide to form a quaternary polyhalide which complexes with the depolarizer to form an insoluble, halogen rich, oil-like complex. If an inert electrode made of a material which absorbs the insoluble complex is employed, an improved cell is provided, since the complex is relatively stable and since the halogen molecules, being concentrated about the current collector, are available for electrochemical reaction to an improved degree.

The instant invention constitutes a further improvement in the halogen cell art and provides a large number of water insoluble organic electrolytes any one of which may be used in conjunction with the aqueous electrolyte of metal halogen cells of the type described. The organic electrolytes of the invention eliminate the necessity of a depolarizer or a specially fabricated electrode, yet provide increased halogen complexing ability, enhancing the shelf-life and capacity of the cells and batteries in which they are used.

In general, the invention features water insoluble organic electrolytes useful when used in conjunction with the aqueous electrolyte of halogen cells and batteries for complexing halogen. The organic electrolyte has two components, one of which is an organic solvent which is relatively insoluble in water. The second component is an organic salt which is very soluble in the organic solvent, relatively insoluble in an aqueous metal halide solution, forms complexes with halogens which do not crystallize out of the solvent at any point during halogen addition, and which do not react irreversibly with free halogens.

Solvents useful in this invention include but are not limited to solvents such as:

dichloromethane (methylene chloride)

chloroform

carbon tetrachloride

dichloroethane

trichloroethane

tetrachloroethane

pentachloroethane

dibromomethane (methylene bromide)

bromoform

or any solvent which does not react with halogens.

Organic salts useful in this invention include: ##STR1## where all R's may be but need not be equal and R's may be hydrogens, aliphatic radicals, O-alkyl radicals, functionalized aliphatic or O-alkyl radicals, aryl radicals, and/or functionalized aryl radicals. Functionalities on these aliphatic or aryl radicals may include but are not limited to amides, carbocycles, carboxylates, halides, hydroxyls, ethers, esters, nitriles, phosphonates, siloxanes, sulfonates, sulfones, and sultones. Counter ions (X.sup.-) associated with these cations may include but are not limited to F.sup.-, Cl.sup.-, Br.sup.-, and I.sup.-.

Accordingly, it is an object of this invention to provide a halogen complexing electrolyte, which is insoluble in an aqueous metal halide solution and which is useful in the cells and batteries of the type described which, by complexing elemental halogen, is capable of greatly improving the performance of halogen utilizing electrochemical systems.

Another object of the invention is to provide a large number of electrolytes, any one of which may be used in zinc or cadmium halide cells to complex and hold halogens more efficiently than the aforementioned polymeric amine halogen complexes and the quaternary ammonium polyhalide-depolarizer systems, thereby increasing shelf-life and decreasing self-discharge of the cells and batteries in which the complexing electrolyte is used.

Still another object of the invention is to provide electrolytes which when complexed with halogen comprise water insoluble liquid phases in aqueous metal halide solutions which provide a source of halogen that may be utilized in a halogen cell or battery.

Yet another object of the invention is to provide electrolytes which when complexed with halogen, comprise reasonably conductive liquids allowing the halogens to undergo rapid electrochemical reduction on the electrode of a halogen cell or battery which is in contact with these electrolytes.

Still another object of the invention is to provide electrolytes which when complexed with halogen comprise liquids which can be pumped, stored indefinitely, and thereby used in a regenerative fuel cell or battery and which when complexed with halogen do not structurally alter carbon electrodes or those plastics which uncomplexed elemental halogens quickly degrade.

FIG. 1 is a cross-sectional view of a cell of the type in which the complexing electrolytes of the present invention are useful;

FIG. 2 is a graph which illustrates ionic conductivities;

FIG. 3 is a schematic of a regenerative fuel cell in which a recirculating electrolyte of the present invention is useful; and,

FIG. 4 is a cross-sectional view of a battery of the type in which the complexing electrolytes of the present invention are useful.

At the outset, it is desirable to comment on certain terms that are used throughout this specification and claims. For example, the terms "zinc electrode" and "bromide electrode" are not intended to imply that the electrodes are formed from these material exclusively. As is conventional practice, the terms are used merely to indicate the electrochemically active element reacting. The metal and halogen electrodes of the cells with which the electrolytes of the present invention are employed, are electrically conductive, and preferably porous and noncorrosive. Carbon in its various forms is a preferred material. The carbon of the metal electrode serves as a plating surface for zinc or cadmium.

Furthermore, although reference is made to one counter electrode and one metal electrode, it is obvious that the compounds of the invention are useful in cells which include more than one pair of electrodes and with batteries of two or more cells constructed in accordance with methods known to those skilled in the art. Because cadmium is electrochemically similar to zinc, it is also obvious that the zinc of the electrode and the electrolyte can be replaced by cadmium. Although bromine is used throughout this specification and claims, it will be apparent to those skilled in the art that chlorine, iodine, or any combination of bromine, chlorine, and iodine may be substituted therefore.

During discharge of the well-known zinc bromine cell the following half reactions occur:

On charging, the reactions are reversed:

The zinc bromide of the electrolyte is hence consumed during charging and produced during discharging. In order for the cell to function properly, Br.sub.2, which obviously must be present near the cathode when the cell is undergoing discharge, must somehow be stored. Since Br.sub.2 is soluble in water containing dissolved metal halides, precautions must be taken to prevent it from migrating through the aqueous electrolyte and reacting directly with the metallic zinc at the anode. This goal has been reached by incorporating a porous, conducting, preferably carbon matrix of the type disclosed in U.S. Pat. No. 3,816,177 to M. A. Walsh, entitled Secondary Cells and Batteries, the disclosure of which is incorporated herein by reference, and one of the electrolytes of the invention, into a halogen cell.

At this point it should be noted that the organic phase of the two-phase electrolyte need not be denser than the aqueous phae of the two-phase electrolyte. If the organic phae is denser than the aqueous phase then its use is best illustrated in FIG. 1 where the bromide electrode 16 is shown positioned in the bottom of the cell and is covered by the organic phase of the two-phase electrolyte. By locating the bromide electrode at the bottom of the cell and by utilizing an organic solvent in the two-phase electrolyte which is heavier or denser than the aqueous phase, the organic phase of the two-phase electrolyte is absorbed into the porous electrode 16 and halogen produced during charging is complexed by the organic salt dissolved in the organic phase and is prevented from solubilizing in the aqueous phase of the electrolyte.

The use of halogenated solvents to store bromine in secondary cells was reported by Little, U.S. Pat. No. 870,973 (1907). The use of solid quaternary ammonium perhalogen salts to store bromine in a secondary cell was reported by Bloch, U.S. Pat. No. 2,566,114 (1951). Both of these methods suffer from a slowed rate of cell discharge due to a lack of conductivity in the halogenated phase. This problem was solved by the use of a water soluble organic solvent in U.S. Pat. No. 3,816,177 to Myles A. Walsh. The solvent was shown to keep the halogen complex liquid and thereby conductive. The instant invention discloses the use of a water insoluble organic solvent to accomplish the same purpose with an added organic salt to complex the halogen and increase the conductivity of th insoluble organic solvent in the electrolyte. The organic phase of the two-phase electrolyte of this invention forms a multi-component layer which separates from the aqueous metal halide solutions which comprise the aqueous phase.

As is stated above, one of the components of halogen complexing organic phase of the two-phase electrolyte is an organic solvent which is relatively insoluble in water. The solvents useful as a component in the organic phase include but are not limited to:

dichloromethane (methylene chloride)

chloroform

carbon tetrachloride

dichloroethane

trichloroethane

tetrachloroethane

pentachloroethane

dibromomethane (methylene bromide)

bromoform

or any water immiscible solvent which does not react with bromine.

The second component of the halogen complexing organic phase of the two-phase electrolyte is an organic salt which is very soluble in the organic solvent, relatively insoluble in an aqueous metal halide solution, forms complexes with halogens which do not crystallize out of the solvent at any point during halogen addition, and which do not react irreversibly with free halogens.

Organic salts useful in this invention include: ##STR2## where all R's may be but need not be equal and R's may be hydrogens, aliphatic radicals, O-alkyl radicals, functionalized aliphatic or O-alkyl radials, aryl radicals, and/or functionalized aryl radicals. Functionalities on these aliphatic or aryl radicals may include but are not limited to amides, carbocycles, carboxylates, halides, hydroxyls, ethers, esters, nitriles, phosphonates, siloxanes, sulfonates, sulfones, and sultones. Counter ions (X.sup.-) associated with these cations may include but are not limited to F.sup.-, Cl.sup.-, Br.sup.-, and I.sup.-.

The synthesis of these cations involves the addition of an alkyl halide to the divalent (S) or trivalent (N or P) base which is in the pure form or diluted in any one of a number of common solvents.

In a 250 ml round-bottomed flask, tri-n-propylamine (14.3 g) in 1:9 methanol-nitromethane (75 ml) was treated with ethyl bromide (16 g), added in one portion, and warmed at 65.degree. for 36 hours below a water cooled condensor. The methanolnitromethane mixture was removed at reduced pressure and the residue slurried with 1:9 acetone-hexane to complete crystallization. After refrigeration for 2 hours at 0.degree.-4.degree., the crystalline product was filtered and desiccated yielding 23.5 g (94%) of coarse white needles, m.p. 290.degree.-291.degree. d., according to the reaction: ##STR3##

In a 250 ml round-bottomed flask, triethylamine (10.5 g) in 50 ml nitromethane was treated with 1-bromobutane, added in one portion and left stoppered at room temperature for 72 hours. Removal of the nitromethane under reduced pressure afforded a solid residue which was recrystallized from acetone-hexane yielding 19.5 g (92%) of white crystalline quaternary salt, m.p. 208.degree.-211.degree. d., according to the reaction: ##STR4##

The amount of halogen complexing agent (salt) dissolved in the organic solvent phase is important both for conductivity and the bromine partion coefficient with the aqueous electrolyte phase. Solubilities of a number of quaternary ammonium bromides in methylene chloride are given in Table 1 below.

Since the liquidity of the organic phase must be maintained, the ammonium polyhalides must be soluble in this phase. The only cation disclosed in which the polybromide is less soluble than the bromide is tetrabutyl ammonium cation. Ionic conductivities given in Table 2 and plotted in FIG. 2 show that as theory dictates the smaller the cation the higher the conductivity.

The increased ionic conductivity of polybromide anions is probably due to transfer diffusion (I. Ruff and V. J. Friedrich, J. Phys. Chem., 76, 2957 (1972)). Partition of the bromine between the two electrolyte phases has been evaluated titrimetrically and is given in Table 3 below.

Bromine used was 0.504 ml per test corresponding to Br.sub.7.sup.-. Aqueous zinc bromide (5 ml, 2M) per test was used. Each test was stirred vigorously for 45 minutes at 25.degree. before an aliquot was titrated for bromine.

Referring to FIG. 1, a cell 10 is shown which utilizes the two-phase electrolyte of the present invention in which the organic phase is denser than the aqueous electrolyte and which comprises a case 12, a pair of graphite electrodes 14, 16, a graphite-felt mass 18, and a porous polymeric separator 20. Graphite electrode 16 acts as the bromine electrode; electrode 14 acts as the zinc electrode. Electric leads (not shown) are connected to the electrode 14 by means of clips. A lead is also connected through case 12 to electrode 16. The area of case 12 around the lead is sealed to prevent leakage. Electrodes 14, 16 have a porosity of 26%. Suitable graphite felt is available commercially from Union Carbide Corporation (VWF grade), and a suitable separator may be purchased from W. R. Grace, Inc. under the tradename DARAMIC. The electrolyte of the cell comprises inorganic salts, such as zinc halide, or halide, or cadmium halide, and may include 0 to 30% of one or more of various additives for improved metal plating. The cell case 12 is fitted with non-wetting porous plugs 13 for venting any hydrogen gas formed on the zinc electrode 14. The heavier organic phase is held by gravity in the graphite felt mass 18 during charge and discharge of the cell. The lighter aqueous phase is held by gravity against the zinc electrode 14 during charge and discharge of the cell.

Referring to FIG. 3, a cell 21 is shown with a recirculating two-phase electrolyte as described herein. The cell 21 comprises a case 22, a pair of graphite electrodes 23, 24, a graphite felt mass 25, and a porous polymeric separator 26. The equipment for storing the halogen complex phase 27, and providing fresh electrolyte comprises pumps 28, 28', and electrolyte reservoir 29, a halogen complex phase reservoir 30, and a pair of valves 31 and 32 for storing and releasing the halogen complex phase 27. During charge, the halogen complex phase 27 is formed in the felt 25 from the organic phase of the electrolyte 33 of the present invention. The resulting halogen complex phase 27 is pumped to reservoir 30 where it can be pumped back into felt 25 for discharge or from which it can supply the halogen at a controlled rate into the electrolyte. The metal is plated out on electrode 23 during charge with the quality of the plate dependent on flow rate. The organic phase of the electrolyte 33 is held in a storage reservoir 34 with a pair of valves 35 and 36 for storing and releasing organic electrolyte 33. The aqueous electrolyte of the cell comprises inorganic salts such as zinc halide or cadmium halide, and may include 0 to 30% of one or more of various additives for improved plating.

Those skilled in the art will appreciate that complex 27 may be used to drive cells other than the one illustrated in FIG. 3. For example, in addition to zinc and cadmium cells, the complexes will be useful in other halogen utilizing cells which may employ, for example, titanium, iron, chromium, hydrogen or organic redox couples as an anode.

To demonstrate the effect of the compounds of the invention, a cell with a recirculating two-phase electrolyte as described above containing ECO-1 was charged at current densities ranging typically from 10 to 60 amperes per square foot for periods of time ranging from 1 to 3 hr. The cell was tested at a constant current drain for a variety of current densities and was demonstrated to have a coulombic efficiency of 50 to 80% and an energy efficiency of 40 to 60%. Without a two phase electrolyte of the present invention no halogen complex 27 is formed and the bromine, which dissolves in the electrolyte, reacts with the zinc plate on electrode 23 resulting in a coulombic and energy efficiency of less than 5%.

FIG. 4 depicts a bipolar, multi-celled battery 37 of the type which utilizes the two phase electrolyte of the invention. It comprises a glass water-tight case 39 which encloses a pair of graphite plate current collectors 41, 43, to which electrical leads (not shown) are connected. Plate 43 serves as a metal electrode. Plate 41 serves as a halide electrode. Interposed between the current collectors 41, 43 is an array of graphite felt masses 45 which serves as counter electrodes in conjunction with a bipolar electrode which is formed by a porous polymeric separator 47 (Daramic, approximately 0.125 in. thick) and a bipolar electrode 49 (conductive polymer film made by Conductive Polymer Corporation, Marblehead, Massachusetts, approximately 0.020 in. thick. As is shown, separator 47 and bipolar electrode 49 are interposed between each graphite felt mass 45. The cell case 39 is fitted with non-wetting porous plugs 51 for venting any hydrogen gas formed on the zinc electrodes 43 and 49. The number of cells in the battery, of course, may be increased or decreased as desired.

For the case in which the organic phase is denser than the aqueous phase the heavier organic phase is held by gravity in the graphite felt masses 45 during charge and discharge of the cell. The lighter aqueous phase is held by gravity against the zinc electrodes 43 and 49 during charge and discharge of the cell. As is apparent, the faces of the bipolar electrodes facing metal electrode 43 serve as halide electrodes while the face facing the halide electrode 41 serves as metal electrodes.

Those skilled in the art will appreciate that organic phases less dense than the electrolyte can be used by inverting the cells described. When cells with recirculating electrolytes are used with electrodes in a vertical configuration, the density of the organic phase is not important; in such cells the organic phase is allowed to flow through the porous graphite-felt mass 18.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.