~6937 1 BACKGROUND OF THE INVENTION 2 ield o~ the Invention 3 This invention relates to metal halogen cells having an aqueous solution of a metal halide as the electro lyte. In particular, the present invention relates to 6 improved cells and batteries employ~ a zinc or cadmium anode, a bromine cathode and an aqueous metal halide elec- ! 8 trolyte in which the metal of the metal halide is the same 9 as the metal of the anode. The Prior Art - 11 As is well known in the art, electrochemical cells 12 have been proposed which have an electrode with a high posi- 13 tive oxidizing potentlal and another electrode with a strong 14 negative or reducing potential. Typical of such cells is the metal halogen cell in which the anode material most 16 commonly employed is zinc and the most commonly employed 17 cathodic halogen is bromine. Among the advantages of such 18 cells is their extremely high theoretical energy density. 19 For example, a zinc bromine cell has a theoretical energy density of 200 wh/lb (i.e. watt hours per pound) and an j 21 electric potential of about l.85 volts per cell. 3 22 In such a cell the surface of the metal anode, for ! 23 example, zinc, oxidizes thereby undergoing a positive 24 increase in valence. As a result thereof, zinc atoms are converted to zinc ions which enter the electrolyte according Z 26 to the equation: 27 Zn > Zn~ +2e 28 The chemical reaction occurring at the cathode is expressed 29 by the following equation: j 30 Br2 + 2e ~ 2Br- 31 Thus, the overall chemical reaction can be written as follows: 32 Zn + Br2 < > Zn~+ ~ 2Br~ ~e~ - 2 - ~x' ~ ,, , .~ . 6~37 . 1 The arrow to the right indicates the direction of the 2 chemical reaction occurring during cell discharge and the 3 arrow to the left indicates the chemical reaction occurring 4 during charging of the cell. The electrochemical cells of the foregoing type 6 are known to suffer from a number of disadvantages. Most 7 of these disadvantages are associated with side reactions 8 which may occur in such cells. For example, during the q charging process free bromine is produced in the cell. This ree bromine is available for chemical reaction with the ll metal anode thereby resul~ing in auto dlscharge of the cell. 12 Additionally, there is a tendency for hydrogen gas to be 13 generated when considerable amounts of free bromine are 14 present in the aqueous phase. The art is replete with efforts on the part of 16 many inventors ~o overcome the above-mentioned disadvanta- 7 ges. In U~S. Patent 2,566,114, for example, the use of 18 tetraethyl and tetramethyl ammonium bromide for combining 19 with bromine generated during charging of the cell is dis- closed. The tetramethyl ammonium salt is added ~o the 21 powdered carbon surrounding the cathode. 22 In U.S. Patent 3,738,870 the use of a solid mix- 23 ture of alkyl ammonium perchlorate and conducti~e materials 24 such as graphite to form solid addition products with 2s halogen released during charging of such cells is disclosed. 26 In U.S. Patent 3,811,945 the use of certain alkyl 27 ammonium perchlorates, diamine bromides and diamine per- 28 chlorates which are capable of forming solid addition pro- 29 ducts with cathodic bromine and which are substant~ally insoluble in water is disclosed. 31 In contra~t to those references which suggest 32 forming solid addition products with bromine, U.S. Patent .. ... . . , , . ., .. . ... ~ ~ . . . . . ., .. . .. . . . . .. ~ . . ~ ZJg9~37 1 3,816,177 discloses the use of a quaternary am~onium halide 2 and a depolarizer in the electrolyte. The depolarizer 3 functions as an organic complexing solvent which dissolves Z 4 in water and is not reactive toward the halogen or halogens s in the cell. The function of the depolarizer apparently is 6 to formZ water insoluble complex in the presence of quater- 7 nary ammonium halides. 8 As will be readily appreciatedJ however, even with 9 the use of the aforementioned complexing techniques self- o discharge of metal halogen cells will not be totally elimin- ll ated since some of the cathodic bromine will remain in the 12 aqueous phase notwithstanding the use of these complexing 13 agenZts. Indeed, the presence of some halogen is desirable Z 14 particularly when current is being withdrawn from the cell. 15 Thus, while the many references cited above show 16 a continuing effort on the part of numerous inventors to '17 overcome the disadvantages associated with metal halogen 18 cells of the-type referred to herein, the methods proposed 19 have not adequately overcoZme the problems encountered in Zl 20 such systems. Consequently, there remains a need for more 21 effective methods for preventing loss of cell capacity ln 22 aqueous metal halogen cells. 23 SUMMARY OF THR INVENTION 24 Accordingly, in one aspect of the present inven- -Z 25 tion an improved metal bromide cell is provided. Broadly ! 26 stated, the cell of the invention comprises a housing, a 27 zinc or cadmium anode and a chemically nonreactive electrode. 28 The cell is also provided with an aqueous electrolyte con- 29 taining metal ions of the same metal as the metal of the i 3Z~ anode. The aqueous electroly~e also contains bromine com- Z 31 ple~ing substituents which substituents are soluble in the 32 aqueous electrolyte but which form substantially water ~ ~ ~ 6 ~ 3 insoluble liquids in the presence of the cathodic bromine~ This cell is also provided with means for circulating the electrolyte containing complexing substituents to at least the surface of the chemically nonreactive electrode and preferably between the anode and cathode such that during cell charging the substantially insoluble bromine-containing complexes are removed from the cell and stored in a separa- tion zone. During discharging of the cell the substantially water insoluble bromine complex is returned to the chemi- cally nonreactive electrode so as to be in contact with thesurface of that electrode In the cell of the present invention, it is pre- ferred to also have a separator in the form of porous material or an ion exchange membrane between the anode and the inert electrode; moreover, if the separator means is an ion exchange membrane it is also preferred to provide separate means for circula~ing electrolytei i.e. anolyte, through the anolyte compartment and ca~holyte through the catholyte compartment. 2~ In other words,the invention provides an electrochemical cell having a metal bromine couple comprising: all electro(le structure on wh;ch to deposit t~le metal of the metal bromine couple; an inert coulterelectrode structurc at which to gener~te tlle bromine of the met~l bromine couple; an aqueous electrolyte containing a metal bromide and a bromine complexing agent, the metal of said metal bromide being selected from zinc and cadmium, said bromine complexing agent being selected from water soluble organic --5-- B 11~ 37 quaternary ammonium compounds and mixtures of quaternary ammonium compounds which in the presence of bromine at 'temperatures in the range of about 10C to about 60C form a substantially water immiscible liquid; means for circulating said electrolyte between said electrode structures during charging of the cell; a separation zone communicating with said cell whereby bromine complex formed during charging of the cell .is separated from said aqueous electrolyte; and, means for circulating separated bromine complex from said separation zone to said cell during discharge of said cell. In another aspect of the present invention, there is provided a method of operating a metal bromine secondary cell of the type which includes metal anode of zinc or cadmium, a chemically nonreactive electrode and an electrolyte comprising an aqueous solution of metal bromide, the metal of the metal bromide being th-e same as the metal of the anode and including complexlng substituents which will form a substantially water immiscible liquid complex with bromine, which method comprises contacting the chemically nonreactive electrode with the metal bromide solution while impressing an electric current on the cell thereby generating bromine while circulating said electrolyte from said chemically nonreactive electrode to a separation zone -5a- ~6~37 whereby the ~romine which has been generated at the chemically nonreactive electrode separates from the aqueous phase in the form of a su~stantially water immiscible complex with the complexing substituents present in said aqueous phase. During cell discharge the said electrolyte is circulated along with the substantially water immiscible liquid complex of bromine through the cell and in contact with the chemically nonreact- ive electrode. In another embodiment, the invention provides a battery system comprising at least one cell, an electrolyte containing halogen therein and at least one negative electrode and one halogen positive electrode positioned in said cell, a storage means for halogen in the liquid phase located externally of the cell, means for circulating electrolyte between said storage means and said cell, and means providing sufficient liquid-liquid contact between the electrolyte and the halogen in said storage means to maintain the desired concentration of halogen in the el-ectrolyte. Also included is a method of operating a battery system including at least one halogen positive electrode and an electrolyte containing halogen which comprises providing ; a storage for halogen in a liquid phase external of the cell, circulating electrolyte between the halogen storage and the cell, and providing sufficient liquid-liquid contact in the storage between the electrolyte and the halogen to maintain the desired concentration of halogen in the electrolyte. These and other features of the present invention will be better understood in view of the following detailed description and accompanying drawings which form a part of the specification and wherein: g37 BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic view of one embodiment of the electric cell of the present invention. Figure 2 is a schematic diagram of a second preferred embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFER~ED EMBODIMENTS Turning first to Figure 1, there is shown one embodiment of an electrolytic cell of the present invention. As illustrated in the figure, an electrochemical cell of the invention comprises a metal anode 10 disposed in a container or hbusing 12. The metal anode in accordance with the present invention is seIected from zinc and cadmium. It should be noted, however, that it is not absolutely essential that the metal anode be formed solely of zinc or cadmium. Indeed, inert wire mesh or various forms of porous carbon materials upon which zinc or cadmium may be plated can serve very well in forming zinc or cadmium electrodes. Spaced apart from the anode 10 and within housing 12 is a chemically nonreactive or inert electrode 14. Inert electrode 14 is disposed within housing 12 so as to define with the enclosing walls of container 12 and anode 10 an electrolyte chamber. Inert or chemically nonreactive electrode 14 can be formed from a wide range of nonreactive materials such as various forms of electrically conductive and non-corrosive materials including porous carbon, graphite and felt. Indeed high surface area materials contain- ing carbon are particularly effective as inert electrodes in the cells of the invention. Optionally, the cell is provided with a separator 11. This separator can be any porous material typically used to prevent physical contact of the two electrodes such as fiberglass mats, fiberglass felt, and microporous polymeric ., ,. ,~ . 15~96937 materials such as porous polyethylene and the li~e. The separator 11, in the cell of Figure 1, merely prevents physical contact of the two electrodes but does not restrict movement of ionic materials in the electrolyte. As can be seen in Figure 1, communicating with the electrolyte chamber of the cell is a separation zone. In the Figure 1 embodiment of the present invention it also can be seen that separation zone consists of a container 18 communicating with the electrolyte compartment of the cell via line 19 and line 21. Lines 19 and 21 are divided so as to assure a good flow of electrolyte through the electrolyte ; chamber past electrodes 10 and 14; however, it is not essential that the lines 19 and 21 be divided as shown. Additionally and preferably, a dam or baffle 20 is located within container 18 for purposes which will be explained hereinafter. Circulating means such as pump 22 is provided. Turning now to the embodiment shown in Figure 2, the electrolytic cell of this invention is provided with an anode 100 within a housing 120. The cell also is provided -7a- ~ ~i 3~ 1 with an inert counterelectrode i40. The electrodes of the 2 cell in this em~odiment are fabricated in the same manner 3 as the corresponding electrodes in the Figure 1 embodlment. 4 In the Figure 2 embodiment, however, the separator 110 which prevents physical con~ac~ of ~he anode 100 and the inert 6 electrode 140 preferably is an ion exchange membrane of the 7 cationic type. Thus separa~or 110 operates to prevent 8 movement of anions such as bromide ion from the cathode 9 compartment to the anode compartment. Separator 110 thereby o provides for separate cham~ers or CGmpartmen~s for electro~ ll lyte. Indeedg separate anolyte and separate catholyte czn l2 be fed to the cell as will be fur~her explained hereinafter. l3 Cation exchange membr~nes suit~ble for separa~or 110 are l4 well known commercially available materials that consist typically of a ma~rix crosslinked polymer to which are l6 a~tached charged radicals such as -S03~, ~C00 , -P03 , ~l7 -HP02 and the li~e. ~he matrix poly~ler, for example, can l8 be one of any ~umber of polymeric materials such as poly- l9 ethylene, polys~yrene and polyformaldehyde resins. In any event, the cell is provided with catholyte compart- 21 ment and an anolyte compartment. The holding tank 180 is 22 divided into an anolyt2 storage zone 180a and a catholyte 23 storage zone 180c. Anolyte storage zone 180a communicates 24 with the anolyte chamber or compartment of the cell via linesl90a and 210a while catholyte storage zone 180c commu- 26 nicates with the cath~lyte chamber or compar~ment of the 27 cell ~ia line l90c and 210c~ As c~n be sePn in Figure 2, 28 pump means 220~ is provided for circulating anolyte from ~ the anolyte storage ~one 180a to the anoly~e compar~ment of the cell. Similarly, pump means 220c is provided for cir- 3l culating catholyte from the catholyte storage zone 180c to 32 the catholyte compartment of the cell. Also, lt should be g37 noted that a baffle 200c optionally but preferably is provided in the catholyte storage zone 180c. The cell of the present invention employs an aqueous solution of a metal bromide as the electrolyte. As indicated hereinabove, it also is particularly preferred in the practice of the present invention that the metal of the metal bromide be the same metal as the anode. Thus, if a zinc anode is employed thé metal bromide is preferably zinc bromide. However, it will be appreciated that when a cell is divided into an anolyte compartment and a catholyte com- - partment by a cationic exchange membrane separator 110 as shown in Figure 2 different aqueous salt solutions can be employed for the anolyte and the catholyte. For example, the anolyte may be a metal salt other than a metal bromide, the metal of the salt, of course, being the same metal as the metal of the anode and the catholyte can be a bromide salt of a different metal than the metal of the anode such as an alkali metal. It is preferred in the practice of the present 2~ invention that the separator merely be a physical contacting barrier sùch as described in relation to separator 11 of Figure 1. In this mode, a single aqueous metal halide electrolyte is employed. Generally, the concentration of such salt in the aqueous phase, whether separate anolyte and catholytes are used or a single electrolyte is used, will be in the range of about 0.5 moles/liter to 6.0 moles/liter and preferably between about 2.5 moles/liter to about 3.5 moles/liter prior to charging of the cell. Thus, when a cationic exchange membrane separator such as separator 100 is employed, the concentration of the salt in the anolyte will be such as to provide the requisite metal anions in the anolyte in the _g _ .~ ~ g37 1 range of about 0.5 moles/liter to 6.0 moles/liter and pre- 2 ferably 2.5 moles/liter to 3.5 moles/liter; and the concen- 3 tration of the alkali metal bromide in the catholyte will be 4 such that to provide bromide ions in the range of about 0.5 S moles/liter to about 6.0 moles/liter and preferably ahout 6 2.5 moles/liter to about 3.5 moles/liter. t - 7 The electrolyte of the Figure 1 embodiment of this ; 8 invention and at least the catholyte of the Figure 2 embodi- 9 ment of the present invention also contains a halogen com- 10 plexing agentO This complexing agent must be one ! 11 which is soluble in water and readily forms a complex with 12 bromine, which bromine complex alone or in combination with 13 another material is substantially a water immiscible liquid 14 at temperatures in the range of about 10C. to about 60C. 15 The types of halogen complexing agents contemplated by the .. 16 present invention are water soluble quaternary ammonium 17 salts, particularly halides in which the halide is selected 18 from chloride, bromide and iodide. The substituents of the 19 nitrogen of the quaternary ammonium halides are alkyl, halo- 20 alkyl, cycloalkyl or aryl groups or any combination of these 21 functionalities. Quaternary ammonium compounds having the 22 following structural formulas are par~icularly preferred in 23 the practice of the present invention: 24 Rl - ~ - R4 X~ ; lN)~ X- ; ~\+ X- 26 R3 R~l ~R2 C~2C02H 27 R3 / R4 28 N ~ X~ ; N X~ / \ / \ 29 Rl CH2C02H Rl CH2CO2H 10 - ~ 37 1 wherein Rl, R~, R3 and R4 are different alkyl groups or 2 haloalkyl groups of frorn about 1 to 8 carbon atoms and X~ 3 is selected rom Cl-, Br~ and I-. 4 As indicated hereinbefore~ the chief characteris~ tics of the complexing agent is that they are formed from 6 organo-substituted nitrogen compounds which are water soluble 7 and which are capable of combining with halogen~ i.e. 8 bromine. Additionally, the resultant l~aiogen co.mplex must 9 be substan~ially a water insoluble liquid at nor~al cell lo operating temperature, e.g. about 10C. to about 60C. ll Many of the foregoing prererred nitrogen compounds do, in 12 fact, form substantially water insoluble liquid complexes 13 with halogen. Some, however, form solid quaternary ammonium 14 polyhalides which are further complexed by use of suitable complexing solvents thereby resulting in the formation o 16 halogen complexes that are substantially water insoluble 17 and liquid at about 10C. to about 60C. Examples of 18 suitable organic complexing solvents are propylene carbonate, l9 dimetnyl carbonate, triethyl phosphate, dimethyl sulfate, sulfolane, 1,4-butanesulfane and the like. IndeedJ in 2l those instances where the quaternary ammonium compound 22 forms a substantially water insoluble liquid phase with 23 halogen relatively small amounts of the foregoing solvents 24 can be added nonetheless to increase the fluidity of the halogen-containing water insoluble liquid phase. 26 Operation of the cell of the present invention 27 will now be described using the zinc-bromine couple for the 28 purposes of illustration. Referring first to Figure 1J an 29 aqueous solution of zinc bromide containlng the water soluble complexing agent is circulated by pump 22 through 31 lines 19) 21 and 23 so as to pas.s through the electrolyte 32 chamber between the electrodes 10 and 14. I~hile the - 1 1 - 37 electrolyte is being circulated through the cell an electric potential is impressed between the electrodes 10 and 14. This electromotive force (direct current) operates to deposite metallic zinc onto the anode 10 while generating molecular bromine at the chemically inert eIectrode 14. The bromine generated reacts with the complexing agent to form a sub- stantially water insoluble oil. Since the ~romine rich oil is heavier than water it tends to settle on the bottom of the tank 18 and is, therefore, not recirculated, at least in any substantial amount, through the cell during charging. Indeed, baffle 2a in the holding tank helps with the separat- ion of the bromine-containing aqueous insoluble complex. Consequently, substantially an aqueous phase is recirculated through the cell during the charging period. On discharging, however, the oil is pumped back to the cathode after first emulsifying or dispersing it in the aqueous phase to provide sufficient liquid-liquid contact between the electrolyte and the halogen to maintain the desired concentration of halogen in the electrolyte oil. This can be accomplished by mixing means (not shown). For example, a high shear mixer or ultrasonic mixing device can be incorporated within the gravity separator tank. In this instance, activation of the mixing mechanism will be initiated prior to discharge of the cell. Optionally, pipe means (also not shown) for drawing substantially the water insoluble oil from the bottom of the separator tank can be provided. In any event, the bromine phase will be distributed as an emulsion in the aqueous phase and recirculated through electrolyte chamber between eIectrodes 10 and 14 during cell discharge. In the operation of the cell shown in Figure 2, the anolyte containing zinc anions such as an aqueous zinc nitrate solution is circulated through the cell by means of -12- ~96g37 pump 220a. Additionally, the catholyte containing bxomide cations such as an aqueous potassium bromide solution is circulated through the catholyte compartment via pump means 220c. The catholyte, of course, also contains a complexing agent for the bromine that is generated. An electric potential is impressed upon the cell. Bromine is generated at the inert electrode 140 and i5 complexed by the complexing agent forming a water insoluble oil. The water insoluble liquid phase tends to separate at the bottom of the separation tank 180c. During discharge of the cell the water insoluble bromine-containing complex is dispersed in the catholyte by mixing means (not shown~ such as a high shear mixer or an ultrasonic mixing device and recirculated through the cell while simultaneously an anolyte is circulated through the anolyte compartment of the cell and the electric current produced is withdrawn. It will be appreciated that while the present invention is described in connection with a single cell a plurality of cells may be employed in battery fashion. While not wishing to be bound by any theory, it would appear that dendrite formation that occurs during the charging of zinc-bromine batteries is a function of composit- ional ingredients in the electrolyte. By flowing the electrolyte or anolyte past the anode, in accordance with the practice of the present invention, the concentration gradients apparently are minimized. In any event, dendrite formation of zinc is substantially avoided. Additionally, it should be appreciated that self-discharge losses that normally occur in zinc-bromine batteries are substantially avoided as well since only the aqueous phase is recirculated through the cell during the -13- ;~3 ~ .'. ,.~ ' 1096~37 charging period, and on storage, most of the bromine is kept outside of the cell in a holding or separation tank. Also, as will be readily appreciated, the state -13a- 3~ 1 of charge of the cell of the pre3ent in~ention can be 2 readily ascertained by a measure of tlle volume of liquid 3 bromine complex. This is simply achieved by using a gradu- 4 ated holding tank. S '~he following examples further illustrate the 6 invention. 7 EXAMPLE 1 8 An a~ueous electrolyte sys~em was prepared having 9 3.0M ZnBr2, l.OM N~ethyl,N~mDthyl morpholinium bromide, 0.59M sulfolane and 0.2M ZnSO4. The electrolyte was circu- 11 lated be~ween an anode and a counterelectrode each con~ 12 sisting o ca~bon powder ln a plas~ic binder impressed upon 13 a silver screen current collec~or. Ihe area of each 14 electrode w~s 100 cm2. A microporous polye~hylene sheet material was used as electrode separator. The elec~rolyte 16 was circulated ~rough ~he cell and ~he cell was charged to 17 80% of the theoretical capaclty (5,8 A~hr)~ The bromine 18 complex during charglng was separated outside the cell in 19 the holding tan~O T~e zinc plated on the anode was smoo~h and in dendrite~free conditicn. During discharge the oil 21 phase was drawn from th2 separ~ion ~ank wi~h some of ~he 22 aqueous electroly~e and was c~rculated between the electrodes. 23 Some emulsification of the complex and the aqueous phase 24 was achieved by ~he mixing action of the circulating pump. The cell perfonmance da~a is given in Table I ~elow. 26 ~ABLE I 27 State of charge9 Gurrent D~nsity Cell Potential,volts 28 ~/0 of ~cheore~:ical ~oem 7~ Cha~scharge _ 29 60 1~ 1 . 9 7 1 . 42 2 O 01 1 . 32 31 30 2 . 07 1 . 19 32 80 10 1.97 1.42 33 20 2 . 0~. 1 . 34 3~ 30 2 . 08 1 . 20 - 14 - 3'~ ~ 1 EXA~E 2 ~ - 2 In ~hi,s test a battery of elght ce~ls cormected 3 in series was constructed. The cells had bipo'lar plates 4 consistlng o~ carbon powder in a polytetrafluoroet'nylene binder. The electrode area was lOV cm20 An ion exchange 6 membrane, sold under the tradename Permicn~1010 by RAI 7 Research Co~p,O was used as the electrode separator. The 8 electrolyte, anolyte and ca~holy~e w~s an aqueous solution 9 o 3.0M ZnBr2J l.OM N~methyl,N~e~hyl morpholinium bromide and Or 2M ZnS04O The electrolyte wa; c;rculated between the 11 electrodes and the cell was ~ha~ged at 20 mA~cm~2 to 74~/O of 12 the theoretica'l c~p~ci~,~ (5.4 Aohr~. The oil that formed 13 d~ring charglng of th~ cell separ~ed ~n tlle c~tholy~e 14 separa~ion t~nkO During discharge of ~he cell~ the oil was passed wi~h c~holyte through a bladeless vortex mixer 16 before being fed thro~gh ~he cellO The coulombic eficien- 17 cies or ~his batte~y were be~ween 82 and g5%. Good, 18 smooth, dendrite~ree deposits o~ zinc were achieved. 19 Additional cell performance da~a is g~ven in T~ble II below TABLE II 21 State of Charge~ ~urrent Dens~ty Cell Po~ential~volts 22D/o o ~heoretlc~l (rnA~cm~2~ Dischar~e ~ . . ~3 74 20 13.20 24 30 12.67 12.10 26 50 11.65 27 60 11.15 .