CA1129945A

CA1129945A - Complexing agents for zinc bromine storage systems

Info

Publication number: CA1129945A
Application number: CA311,588A
Authority: CA
Inventors: Henry F. Gibbard
Original assignee: Gould Inc
Current assignee: Gould Inc
Priority date: 1977-09-30
Filing date: 1978-09-19
Publication date: 1982-08-17
Also published as: JPS5461640A; GB2005064B; FR2404928A1; GB2005064A; DE2842500C2; JPS6313310B2; FR2404928B1; DE2842500A1

Abstract

ABSTRACT OF THE DISCLOSURE
Complexing Agents for Zinc-Bromine Storage Systems Quaternary ammonium salts for complexing bromine in battery systems such as, fro example, zinc-bromine, to form liquid, low viscosity polybromide oils over a range of bromination levels of at least 3 to 7 (i.e., the bromination level of the oil is equal to 1 + 2 times the number of moles of Br2 present per mole of the tetraalkyl salt) are disclosed.

Description

~ his invention relates to battery systems including a bromine positive electrode, such as zinc-bromine, and to an improved technique for reversibly complexing bromine in such systems.
Conceptually, the zinc-bromine battery can be con-sidered to be among the simplest of electrochemical systems in that only zinc,bromine and zinc bromide pa~ticipate actively in the basic cell reaction. In its most elementary form, a re-chargeable zinc bromine battery could be constructed with two inert, electronically conductive electrode substrates and an aqueous solution of zinc bromide. A charge/discharge cycle would thus involve the electrodeposition and the subsequent anodic dissolution of zinc at the ne~ative electrode and the liberation of bromine into the aqueous solution with its sub-sequent cathodic reduction at the positive electrode.
This inherent simplicity together with the fact that both of the electrode reactions occur with good electro-chemical reversibility was recognized rather early. The early incentive was to develop a simpler alternative to the lead-acid battery. A strong commercial interest in this system has now been created by the emerging need for large-scale batteries for application in off-peak energy storage. The requirements for this application differ considerably from those of other re-chargeable battery applications. The most important criteria which these batteries must satisfy are capital cost, cycle life, safety efficiency and maintainabilityO In order to be com--' petiti~e for the load levelling application, batteries face very stringent cost and cycle life re~uirements.
The early investigations of the zinc-bromine battery system pinpointed two major problems in the system.
First of all, this type of system has a relatively high self-discharge rate ~viz., a low coulombic efficiency) resulting . .
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94~ii from the reaction for the zinc ,to electrodeposit dendritically, which can ultimately result in the short-circuiting of the cell.
Subsequent attempts to exploit the zinc-bromide system have accordingly focused on developing solutions to these two problems. Perhaps the major effort has been expended in various attempts to improve the coulombic efficiency by lower-ing the concentration of free bromine in the aqueous zinc bro-mide electrolyte. An early approach, described in U.S.
1,006,49~, involved the use of porous carbon as the positive electrode structure. The usefulness of this techni~ue was limited by the significant amount of carbon required to absorb the bromine together with the reluctance of the material to re-lease the bromine for cell discharge.
A further approach utilized has been to employ a complexing agent incorporated into the electrode structure.
The use of quaternary ammonium bromides, such as tetramethyl-,ammonium bromide, is described in U.S. 2,566,114. Such mate-rials form a series of polybromides up to a maximum bromination level of Brg. In other words, four molecules of bromine per molecule of the organic compound could be utilized. ~oth solid and li~uid polybromides were formed as the bromination level changed, creating difficulties in retaining the complexed com-pounds within the electrode structure as well as causing excessive electrode polarization.
Subsequently, as is described in U.S. 3,738,870, it is claimed that the performance of the bromine electrode can be significantly improved if only a single solid bromine complex is formed in the electrode structure. Apparently, to achieve the single solid bromine complex, it was necessary to add the bromine extremely slowly to solutions containing a bromide ion and a fine suspension of the complexing agent in particulate form. Thus, rapid addition of the bromine resulted in the for-~2~S
mation of what were termed ~mixed polybromide-bromine lumps".
More recently, it was noted that the presence of an aprotic dipole in the electrolyte such as propylene carbonate together with a selected quaternary ammonium bromide resulted in the formation of insoluble polybromide oils over a range of bromination levels. This is described in U.S. 3,816,177.
Even more recently, U.S. Patents 4,038,459 and 4,038,460 describe a large number of compounds which may be added to the electrolyte of halogen cells to'complex the halo-gen. The '459 patent discloses various alcohols and nitriles which form insoluble oil-like complexes with halogens, which compounds may also include tetraalkylammonium moieties. The '460 patent shows various halogen complexing ethers, likewise including tetraalkylammonium moieties, which form insoluble, oil like complexes with the halogens. A serious disadvantage with these alcohols, ethers, and nitriles is the quick chemical and electrochemical degradation of the compounds in the presence of bromine and an active bromine/bromide electrode, making such compounds w~olly unsuitable for use in the zinc-bromine system.
It is accordingly a principal object of the present invention to provide a zinc-bromine battery system characteri-zed by relatively high coulombic efficiency. A related and more specific object provides a reliable means of storing bro-mine to maintain the concentration of bromine in the electro-lyte at an acceptable level.
A further object lies in the provision o complex-, ~ ing agents for bromine which will form water-insoluble, low viscosity oils over a signi~icant range of bromination levels.
Yet another object of this invention is to provide a bromine-complexing agent that is stable in the environment of a zinc-bromine battery.

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9~3~5 A still further object provides a complexing agent for bromine which forms polybromide oils of sufficiently low viscosity to allow a satisfactory rate of transfer of bromine between the oil and the electrolyte.
Yet another object of this invention is to provide a zinc-bromine battery that allows a wide variety of materials to be used for construction in battery systems of this type due to the less corrosive system which this invention provides.
~ further object provides a battery system of potentially reduced toxicity due to the lower bromine activity achieved.
Other objects and advantages of the present invention will become apparent from the ensuing description.
While the invention is susceptible of various modi-fications and alternative forms, specific embodiments thereof are described in detail herein. It should be understoodt how-, ever, that it is not intended to limit the invention to theparticular forms disclosed, but, on the contrary, the intention is to cover all modifications, e~uivalents, and alternatives falling within the spirit and scope of ~he invention as expressed in the appended claims. Thus, for example, while the present invention will be principally described in con-nection with a zinc-bromine rechargeable cell or battery system, it should be appreciated that the present invention may like-wise be employed in any system in which a bromine positive electrode i5 utilized. As an illustration, the present invention may be used in various redox systems. Indeed, the present invention may be useful simply as a facile technique for complexing bromine for purposes outside the electro-chemistry field, as in the storage of bromine prior to trans-port or the like. Still further, the present invention will be described in connection with the use of certain tetraalkyl-.
~ 4 --~9~4~i a~nonium compounds. However, while the utilization of suchammonium compounds is preferred, it should be appreciated that similar phosphonium or sulphonium compounds could also be employedr Of course, the use of such alternative compounds must meet the crlteria for the ammonium compounds set foxth herein.
The present invention is, in general, predicated on the discovery that particular tetraalkylammonium salts from polybromide oils that are essentially insoluble in aqueous solutions, are of sufficiently low viscosity to allow a satis-factory rate of transport between the electrolyte and the oil, are stable in the environment of a zinc bromine battery, and possess the other characteristics which make such compounds suitable to achieve and maintain the desired concentrations of bromine in the aqueous zinc bromide electrolytes used. The particular salts that may be employed and the manner of forming , the polybromide oils will be discussed as the description pro-ceeds.
The general construction of the particular cells utilized is well known. Thus, basically, the cell comprises a positive electrode (the bromine electrode), a negative elect-rode (the zinc electrode), a separator therebetween, and suit-ably electrolyte. On charge, zinc metal is plated on the face of the negative electrode ~ubstrate, remaining there until dis-charged. The positive electrode, on the other hand, is a redox : electrode.
The separator used must be capable of satisfying, in general, two functions. The separator must be capable of minimizing the bromine transport rate from the positive elect -rode to the negative electrode. Additionally, the materialselected for the separator should be chemically inert in the cell environment and should have a low resistance to the passage of ions. Various microporous and cation-exchange mate-rials may be utilized and are known in the art. As one illust-rative example, a conventional "Daramic"* microporous poly-ethylene separator (W. R. Grace) has been found suitable.
It is preferred to employ a battery system which utilizes a circulating electrolyte system of some sort to mini-mize formation of zinc dendrites. Since the electrolyte on the ne~ative electrode side of the separator~ must typically have a much lower bromine concentration than the electrolyte on the pos1tive electrode side, two separate flow loops are desirably provided. Generally, in a system of this type, the electrolyte will be under constant circulation, making many passes through the cell during a single charge-discharge cycle.
While any technique may be used for complexing the bromine to form the oils in accordance with the present invention, it is preferred to utilize an external storage of , the complexing agents and a liquid-liquid contacting device to achie-ve this, as is disclosed in U.S. Patent 4,162,351, issued July 24, 197~, Ronald A. Putt et al. In this fashion, bromine is partitioned between the electrolyte and the bromine oils .
formed. The electrolyte solution, thus str1pped of the excess bromine by the oils formed pursuant to this invention, may either be passed to storage or pumped again 1nto this cell.
Further details of suitable devices are set forth in the Putt Patent.
In accordance with the present invention, the com-ple~ing agents utilized are salts which form liquid polybromide oils upon contact with bromine over a significant and contin-uous range of bromination levels. Thus, the salts employed must Eorm liquid oils at the operating conditions that will be en-countered over bromination levels of at least 3 to 7, prefer-* trade mark l~Z~gL5 ably 3 to 9, and have a viscosity of no more than about 30 centipoise at the temperature of the cell in use (which may be up to about 50 to 60C. for a zinc-bromine cell3, preferably less than about 20 centipoise, most preferably no more than 10 to 15 centiposes.
According to a preferred aspect of the present invention, it has been discovered that certain unsymmetrical tetraaLkylammonium salts form liquid oils upon being com-plexed with bromine over the range of bromination levels desired. These salts and the resulting polybromide oils possess the necessary re~uisites for use in a zinc-bromine battery system, finding particular utility where a circulat-ing electrolyte is employed. The tetraalkyl moieties which form liquids over the 3 to 9 range of the bromi~ation levels at room temperature (i.e., 23C.) include diethyldimethyl, ethyltrimethyl, ethyldimethylpropyl, diethylmethylpropyl, ethylmethyldipropyl and diethyldipropyl. In addition, a methyltriethylammonium salt may be used as it forms a poly bromide oil over a 3 to 7 range of bromination levels.
' While the bromine partition achieved by the pre-viously described complexing agents may be satisfactory, it could be desirable in some cases to raise or, more typically, lower the bromine concentration in the electrolyte from that which can be achieved by using a single complexing agent.` T~is may, of course, be accomplished to some extent by using a mixture of two or more of the previously described agents.
However, further to an additional feature of this invention, the bromine partition can be also, ln effect, fine tuned by forming a mixture of a previously described complexing agent or agents with a supplemental complexing agent in appropriate amounts. This expands the ability to achi~ve the results desired by allowing utilization of a mixture including any f~

tetraalkylammonium moiety regardless of whether it would, by itself, provide the necessary liquid oil. The sheer nurnber of such moieties in relation to the seven specie which themselves form liquid oils over the necessary range of bromination levels illustrate the versatility of this approach.
Still further, and pursuant to yet another feature of this invention, the polybromide oils described herein may be formed by employing a mixture of salts of two or more tetra-alkylammonium moieties, each of which by itself would not form a satisfactory liquid polybromide oil. 'rhe flexibility involved should be apparent.
In either embodiment, ~hen a salt (or salts) o~ a tetraalkylammonium moiety is used that does not itself form a liquid polybromide oil, the amount used in admixture with the other salt or salts employed should, of course, provide a liquid polybromide oil. ~ppropriate amounts can be most readily determined simply by complexing with bromine and ascertaining the bromine partition, viscosity and other characteristics of the mixture that are desired. !-It should also be appreciated that, in all embodi-ments, the tetraalkylammonium moiety used need not be unsub-stituted. Accordingly, as used herein, the term "tetraalkyl"
should be considered as embracing both substituted and unsub-stituted moieties. Any groups substituted must result in a stable material in the intended environment. Chlorine and bro-mine may thus be used as substituents on the tetraalkyl moiety.
Suitable unsubstituted and substituted tetraalkyl moieties thus include N,N-diethyl-N-propyl-N-methyl; N,N-diethyl-N, N-di-methyl; ~,N,N-triethyl-~-methyl; N-ethyl, ~,N,N-trimethyl and their chlorine and bromine substituted counterparts, including chloro substituted N-ethyl-N,~,N-trimethyl, for example, ~-2-chloroethyl-N,~,N-trimethyl and chloro substituted N,N-diethyl-_ ~ _ .
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N,N-dimethyl, for example, N,N-diethyl-~-methyl-N-chloromethyl.
The viscosity of the polybromide oil phase is in zinc-bromine battery system designed for high efficiency is a critical factor in selecting the compound or compounds for complexing bromine. In the transport of bromine between the aqueous electrolyte phase and the polybromide oil phase, the rate limiting step typically occurs within the oil phase because of its higher viscosity. This is apparent from the equations defining the mass transfer coefficients in liquid-liquid extraction operations. Under typical operating conditions, the mass transfer coefficient for the oil phase is as small as that for the aqueous phase only for an oil phase viscosity approach-ing zero. The performance of a zinc-bromine battery (for given operating conditions) thus becomes progressively better ; as the oil phase viscosity becomes lower.
A further important consideration concerning the oil phase viscosity is that of the power required to provide effective mixing of the oil and aqueous phases. ~s an example, for a contacting column through which the oil phase is circulated via a pump, the pumping power for constant oil flow rate is lineraly proportional to the viscosity of the oil. For ~, a more viscous oil, more power is re~uired to drive the pump, ; resulting in a lower overall energy efficiency of the battery.
It has thus been found highly preferable to main-tain an oil phase viscosity of 10 to 15 centipoise or less at the temperature range of the cell or battery in use. As the viscosity increases to 30 centipoise, the performance becomes considerably less efficient. Further increases in viscosity will cause further deterioration in performance, eventually reaching a point where the battery or cell would be essentially inoperable.
The importance of the low viscosity oils of this f .

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invention can likewise be appreciated in considering the functions involved in the charge-discharge cycle. Thus, in the charge phase, increasing oil viscosity lowers the coulombic efficiency since the rate of transport of the bromine into the oil phase becomes too slow, resulting in self-discharge. On the other hand, during the discharge phase, too slow bromine transport from the oil to the electrolyte results in a constant voltage not being sustained.
As to the salt used, conceptually, any anion may be employed which is not oxidized at a lower potential than is bro-mide and does not form an insoluble specie in the electro-chemical environment. While the use of a bromide is preferred, chlorides may likewise be used. In addition, when the environ-ment does not contain potassium ions, the salts may also be in the form of perchlorates.
The polybromide oils of the present invention may ,be prepared in any desired fashion. As an example, the oils may be prepared by dissolving the desired bromide or other salt in water and then adding liquid bromine in at least an equimolar amount. The oil will form instantly and may be collected at-the bottom of the vessel, either by pipetting or by use of a separatory funnel. Alternatively, if desired, bro-mine could be added directly to the salt or to an aqueous solu-tion containing the salt. The ma~imum yield will be typically achieved when the oil is formed in water.
Organic complexation of the bromine allows independent control of its aqueous phase concentration. m e relative amount of the oil or salt that should be used may be determined by working backwards from the bromine partition for the polybromide oil having the highest bromination level desired in use. The number of moles of bromine that will be produced in the particular size cell being used and the bromi-~L~Z~4S

nation level of the oil or salt desired for use in the startupof the operation will then dictate the amount of the oil or salt that should be added. Thus, if it is desired to end up with the bromine partition and other characteristics provided by an oil having a bromination level of 7 and if 10 moles of bromine will theoretically be produced, 5 moles of the oil at a bromination level of 3 should be utilized.
As to the considerations which apply-in setting the particular concentration range, the bromine concentration should desirably be kept as low as possible so as to minimize the severity of materials degradation that could be caused by the bromine in addition to of course increasing the coulombic efficiency. On the other hand, an operational lower limit arises from the re~uirement of mass transport of dissolved bromine to the positive electrode substrate during discharge.
~s may be appreciated, an unacceptable concentration polari-, zation will result if this transport is retarded.
Likewise, the bromine concentration on the nega-tive electrode side of the separator should be ~ept extremely low (viz., about 0.005 molar) so as to minimize the non fara-daic zinc-bromine reaction. ~ile theoretically the separator should keep this concentration low, no separator will realisti-cally totally prevent bromine transport, each particular type having a limit to the concentration gradient which it can sus-tain before allowing excessive bromine flux. For this reason, there will exist a distinct operational upper limit on the positive side electrode bromine concentration, based upon the performance of the separator.
Suitably, when an a~ueous zinc bromide electrolyte of one molar concentration is utiliæed with a porous, flow-throug~ electrode, it is preferred to maintain the bromine con-centration in the range of from about 5 to 15 grams per liter , -- 11 --34~i of electrslyte solution, most preferably 5 to 10. This type of electrode is generally preferred as the coulombic efficiency will be maximized. However, when flow-through electrodes are not utilized, the bromine content in the electrolyte which is maintained must necessarily be increased due to the differences in the mass transport characteristics that will result.
It will also be found generally desirable to main-tain the pH of the electrolyte within certain limit~ to pro-vide satisfactory performance at the negative electrode. Thus, as is known, at a pH of about 4 or greater, the zinc plated out during the charge phase will be somewhat mossy and not very adherent. This can result in a restriction of electrolyte flow, with the poor adherence likewise causing low coulombic efficiencies. On the other hand, however, at very low pH (i.e., less than 1), the acid corrosion of the zinc plates presents a problem, again resulting in poor coulombic efficiencies. For , these reasons, the utilization of a moderate pH in the range of from 1-3 is preferred, this range achievin~ tightly adherent zinc platings. This will be typlcally inherently achieved in a zinc-bromine battery as there are no reactions causing any rapid pH drifts and zinc bromide, when dissolved in water, pro-viding a pH of about 4. The hydronium ion concentration can then be increased through addition of either hydr~chloric or hydrobromic acid.
The density and viscosity characteristics of the oils formed by using the six complexing agents which result in polybromide oils over the entire and preferred range of brsmination levels are set forth in Tables 1 and 2.

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~2~45 As can be seen, the liquid oils formed have satis-factorily low viscosities to provide the necessary bromine transport characteristics between the oils and the electro-lyte.
Table 3 sets forth the concentration of dissolved bromine in a l.0 molar zinc bromide electrolyte which is in equilibrium with the oils of the present invention.

Dissolved Bromine in_Electrolyte g/l at~25~C.~~~~
_ Tetraalkyl Moiety of Species Bromination Level Ethyltrimethyl lL.415.8 25.3 39~8 Diethyldimethyl 5.710.8 20.5 35.1 ~thyldimethylpropyl 3.07.6 17.4 32.9 Diethylmethylpropyl 3.36.6 18,5 31.8 Dimethyldipropyl 1.76.4 18.9 34.2 Ethylmethyldipropyl 1.44.9 16.3 30.9 As can be seen, in quilibrium conditions, the various oils achieve satisfactory low bromine concentrations in the electrolyte.
As may be appreciated, àctive zinc-bromine batteries particularly of the size required for load levelling applic-ations, are exothermic, necessitating operation at elevated temperatures, temperatures up to 60C. or so being thus encoun-tered. Tables 4 through 6 show the characteristics of the ethyltrimethyl species at temperatures ranging from 25C. to 60~C.

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Dissolved Bromine in Electrolyte g/l Temp.,C. Bromination Level 25*l 11 6 175.4 277.2 41.1 13.2 19.9 23.2 28.7 21.2 19.3 20.5 43.8 *The bromine partitions at this temperature represents the same information as in Table 3 for this species, but run at a dif~erent time. The differences in the concentrations noted provide a~ indication of the experimental error involved.

Density, q/cm Temp., C. Bromination Level

2.084 2.158 2.240 2.318 2~080 2.13~ 2.206 2.~75 2.077 2.114 2.191 2.149 _.

iscosi~, Centipoise Temp., C. Bromination Level

3 5 _ 7 11.1 10.5 8.7 7.g 6.0 5.6 5,8 4.8

4.4 4.1 3.9 3.9 As can be seen, the characteristics of this species at the elevated temperatures are satisfactory. The signifi-cant decrease in viscosity with increased temperatures is particularly import,ant as regards the increased rate of bromine transport that will result.
Table 7-9 show the characteristic~ of the diethyl-dimethyl species ~ver the 25~ to 60C. temperature range.

Dissolved Bromine in Electrolyte Temp., C. ¦ Bromination Level 25* .7 10.4 22.2 34.2 6.5 13.1 26.0 42.5 8.3 23.6 j 26.6 41.7 - *The values should be contrasted with those in Table 3 for this species, for the reason noted ;n the footnote to Table 4 Density, g/cm3 Temp., C. Bromintation Level l.980 --5 2.170 2.260 1.966 2.032 2.119 2.199 ___ 2.027 2.103 2.182 Viscosity, Centipoise ~= - `_ _ Temp., C. 3 IBromina tion Level 12.6 11.3 9.9 9.0 -7.3 7.0 6.4 5.6 ___ 4.9 4.7 4.6 ~ _ _ As can be seen, this species is somewhat more effective than , the ethyltrimethyl specles in keeplng the aqueous phase bromine concentration low at a still satisfactory low viscosity.
The following Examples are intended to be merely illustrative of the use of the present invention and not in limitation thereof.

:

This illustrates the use of ethyltrimethylammonium-bromide species as a complexing agent in a rechargeable 32 watt hour, zinc-bromine battery system utilizing circulating electrolyte and a porous, flow through electrode.
The negative electrode su~s~rateis a flat, non-porous titanium plate, and the positive electrode substrate is a flat sheet of porous titanium catalyzed with a ruthenium-containing coating. A microporous "Daramic"* separator (W. R. Grace3 is used, consisting of a flat backweb with vertical ribs at regular intervals on each side to provide electrolyte flow channels.
The composition of theel~ctrolyte in the fully discharged condition is as follows:

zinc bromide concentration - 1.5 molar Potassium chloride concentration - 4.0 molar lead chloride concentration - 8.0 x 10-4 molar pH 1.0 - 3.0 The bromine oil at full discharge comprises 57 grams of ethyltrimethylammonium bromine oil and 51 grams of bromine (Bromination level of 3). During the course of the charge, the oil increases in the amount of bromine complexed and in total volume.
The cell operates satisfactorily when exposed to the following charge-discharge cycle: (a) charge at 3 Amps (galvanostatic) for 2 hours. The charge voltage should initially be about 1.90 volts at 27C., rising some 50 milli-volts during the course of charge, (b~ open circuit stand for 15 minutes, (c) discharge at 3 Amps (galvanostatic) until the cell voltage falls off sharply, and (d~ cell shortcut for 30 minutes.

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EX~P~E 2 This shows the formation of a liquid polybromide oil by utilizing a mixture or two salts each of which by themselves form a solid complex with bromine.
About 0.02 moles of a mixture comprising equimolar amounts of tetraethylammonium bromide and diethyldipropyl-ammonium bromide were added to a 200,ml. beaker, along with 100 ml. of a 1 molar ZnBr2 solution. Four equal additions of bromine (i.e., 0.02 moles each) were made. The solution was allowed to equilibrate with stirring between additions.
The results below in Table 10 compare the properties of the equimolar mixture with those obtained using the individual salts.

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34~i As can be seen, the mixture provides a useful complexing agent although the individual salts are unsuitable.

This Example illustrates the use of a supplemental complexing agent, that would itself yield a normally solid polybromide, with a species giving a liquid polybromide oil to further adjust the bromine partition.
Ethyltrimethylammonium bromide was accordingly mixed in various amounts with ethyltri-n-propylammonium bromide, the latter compound yielding a normally solid polybromide but having superior bromine partition properties. Liquid oils were formed over the range of bromination levels of 3 to 9.
In a 1 molar zinc bromide electrolyte, the addition of the ethyltri-n-propylammonium bromide tended to increase the amount of dissolved bromine. However, in a 2 M zinc bromide electrolyte, the addition resulted in, for the most part, lower concentrations of dissolved bromine.
Thus, as has been seen, the present invention provides complexing agents that are stable in the chemical and electrochemical environment, satisfactorily increase the coulombic efficiency of the system by reducing and maintaining the level of the bromine at the concentration desired. The ability of the polybromide oils formed to remain as low j viscosity liquids over a useful range of bromination levels allows wide freedom in operation without the apprehension that malfunctions could occur, as would be the case with a complexing agent varying from a liquid to a solid depending upon the level of bromination. The relatively low viscosi-ties of the oil.s of this invention are essential to achieve satis~actorily rapid transport of bromine between the oil and aqueous phases.

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Claims

The embodiments of the invention in which an exclusive proprerty or privilege is claimed are defined as follows:

1. In a cell including a bromine electrode and a complexing agent for bromine, the improvement wherein said complexing agent comprises at least one quaternary ammonium salt, said complexing agent forming a liquid oil at a range of bromination levels of at least 3 to 7, and said liquid oil having a viscosity of less than about 30 centipoise at the temperature range of said cell when in use.

2. The cell of claim 1 wherein the cell is zinc-bromine.

3. The zinc-bromine cell of claim 2 wherein said complexing agent is a liquid at bromination levels of from 3 to 9.

4. The zinc-bromine cell of claim 2 wherein the quaternary moiety of the quaternary ammonium salt is a member selected from the group consisting of ethyltrimethyl, dimethyldiethyl, ethylpropyldimethyl, dimethyldipropyl, methyltriethyl, methylpropyldiethyl and methylethyldipropyl.

5. The zinc-bromine cell of claim 4 wherein the quaternary moiety is ethyltrimethyl.

6. The zinc-bromine cell of claim 4 wherein the quaternary moiety is diethyldimethyl.

7. The zinc-bromine cell of claim 2 wherein the viscosity of said oil is less than about 20 centipoise.

8. The zinc-bromine cell of claim 2 wherein the viscosity of said oil is less than about 15 centipoise.

9. The zinc-bromine cell of claim 4, which further includes a quaternary ammonium salt itself forming a normally solid complex with bromine.

10. The zinc-bromine cell of claim 2, which includes a mixture of at least two quaternary ammonium salts, each of said salts itself forming a normally solid complex with bromine.

11. A method of complexing bromine in a cell, having a bromine electrode and a aqueous electrolyte, which comprises adding to said electrolyte at least one quaternary ammonium salt and bromine in a molar amount at least equal to the molar amount of said added salt, said salt being added in an amount sufficient to form with said bromine a liquid polybromide oil at an initial bromination level of at least 3 and ranging up to a bromination level of 9 during the charging of said cell, and said oil having a viscosity of less than about 30 centipoise at the operating temperature range of said cell.

12. A liquid polybromide which provides bromine to the electrolyte of a cell containing a bromine electrode, wherein said polybromide is substantially insoluble in said electrolyte and comprises at least one quaternary ammonium salt and bromine in at least an equimolar amount to the amount of said salt, said salt forming with said bromine a liquid polybromide oil at a range of bromination levels of at least 3 to 7, and said oil having a viscosity of less than about 30 centipoise at the operating temperature of the cell.

13. A process for improving the performance of current delivering electrochemical systems of the type which utilize bromine as the electro-chemically active agent, said process comprising adding to the electrolyte of said system at least one bromine complexing compound which, in the presence of at least an equimolar amount of bromine, exists as a substantially water insoluble liquid at the operating temperature of the system, said compound being selected from the group consisting of:
N, N-diethyl-N-propyl-N-methyl ammonium bromide;
N-2-chloroethyl N, N, N-trimethyl ammonium bromide;
N, N-diethyl-N-methyl-N-chloromethyl ammonium bromide;
N, N-diethyl-N, N-dimethyl ammonium bromide; and mixtures thereof.

14. An electrochemical cell comprising a case, a porous conductive halogen electrode, an aqueous electrolyte containing zinc bromide, an additive in said electrolyte consisting of at least one bromine complexing compound which, in the presence of at least an equimolar amount of bromine, exists as a substantially water insoluble liquid at the operating temperature of the cell, said compound being selected from the group consisting of:
N, N-diethyl-N-propyl-N-methyl ammonium bromide;
N, N-diethyl-N, N-dimethyl ammonium bromide;
N, N-diethyl-N-methyl-N-chlorome-thyl ammonium bromide;
N-2-chloroethyl-N, N, N-trimethyl ammonium bromide; and mixtures thereof.

15. A bromine rich, substantially water-insoluble, liquid complex for use in electrochemical cells of the type which utilize bromine as the electrochemically active agent, said complex consisting essentially of elemental bromine and at least one complexing agent selected from the group consisting of:
N, N-diethyl-N-propyl-N-methyl ammonium bromide;
N, N-diethyl-N, N-dimethyl ammonium bromide;
N, N, N-triethyl-N-methyl ammonium bromide;
N, N-diethyl-N-metllyl-N-chloromethyl ammonium bromide, N-2-chloroethyl-N, N-trimethyl ammonium bromide; and mixtures thereof.

16. A process for improving the performance of current delivering electrochemical systems of the type which utilize bromine as the electrochemically active agent, said process comprising adding to the electrolyte of said system at least one bromine complexing compound which, in the presence of at least an equimolar amount of bromine, exists as a substantially water insoluble liquid at the operating temperature of the system, said compound being a tetraalkyl ammonium bromide in which each alkyl group is unsubstituted or substituted by a substituent selected from chlorine and bromine.

17. An electrochemical cell comprising a case, a porous conductive halogen electrode, an aqueous electrolyte containing zinc bromide, an additive in said electrolyte consisting of at least one bromine complexing compound which, in the presence of at least an equimolar amount of bromine, exists as a substantially water insoluble liquid at the operating temperature of the cell, said compound being a tetraalkyl ammonium bromide in which each alkyl group is unsubstituted or substituted by a substituent selected from chloride and bromine.

18. A bromine rich, substantially water-insoluble, liquid complex for use in electrochemical cells of the type which utilize bromine as the electrochemically active agent, said complex consisting essentially of elemental bromine and at least one complexing agent comprising a tetraalkyl ammonium bromide in which each alkyl group is unsubstituted or sub-stituted by a substituent selected from chlorine and bromine.

19. A process for improving the performance of current delivering electrochemical systems of the type which utilize bromine as the electro-chemically active agent, said process comprising adding to the electrolyte of said system at least one bromine complexing compound which, in the presence of at least an equimolar amount of bromine, exists as a substantially water insoluble liquid at the operating temperature of the system, said compound being selected from the group consisting of:
N,N-diethyl-N-propyl-N-methylammonium bromide;
chloro substituted N-ethyl-N,N,N-trimethyl ammonium bromide;
chloro substituted N,N-diethyl-N,N,N-dimethyl ammonium bromide;
N,N-diethyl-N,N-dimethyl ammonium bromide; and mixtures thereof.

20. An electrochemical cell comprising a case, a porous conductive halogen electrode, an aqueous electrolyte contain-ing zinc bromide, an additive in said electrolyte consisting of at least one bromine complexing compound which, in the pre-sence of at least an equimolar amount of bromine, exists as a substantially water insoluble liquid at the operating temperature of the cell, said compound being selected from the group consisting of:
N,N-diethyl-N-propyl-N-methyl ammonium bromide, N,N-diethyl-N, N-dimethyl ammonium bromide;
chloro substituted N-ethyl-N,N,N-trimethyl ammonium bromide, chloro substituted N,N-diethyl-N-N-dimethyl ammonium bromide, and mixtures thereof.

21. A bromine rich, substantially water-insoluble, liquid complex for use in electrochemical cells of the type which utilize bromine as the electrochemically active agent, said complex consisting essentially of elemental bromine and at least one complexing agent selected from the group consisting of:
N,N-diethyl-N-propyl-N-methyl ammonium bromide:
N,N-diethyl-N,N-dimethyl ammonium bromide N,N,N-triethyl-N-methyl ammonium bromide, chloro substituted N-ethyl-N,N,N-trimethyl ammonium bromide;
chloro substituted N,N-diethyl-N,N-dimethyl ammonium bromide, and mixtures thereof.