CA1174272A - Electrolytes circulation type metal-halogen secondary battery - Google Patents

Abstract

Abstract of the Disclosure.
The invention relates to an electrolyte circulation type metal-halogen secondary battery in which a bormine complexing agent comprising a tertiary amine combined with an alkyl is added to the positive electrolyte.

Description

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Background of the Invention.
The present invention relates to electrolytes circulation type secondary batteries, particularly electrolytes circulation type metal-halogen secondary batteries. More particularly, the invention relates to a metal-bromine secondary battery which employs a complexing agent such that the complexing agent cornbines with the bromine molecules in the positive electrolyte so that the bromine molecules separated from the electrolyte form a bromine com-plex compound in an oily form and precipitates ;n the lower part of the positive electrolyte storage tank.
The embodiments of the invention and prior art, by way of example only, will be described by reference to the accompanying drawings in which:
Fig. 1 is a schematic diagram showing the construction of a prior art metal-halogen secondary battery.
Fig. 2 is a characteristic diagram showing the relation between the concentration of complexing agents and the concentration of bromine molecules in the supernatant electrolyte.
Fig. 3 is a characteristic diagram showing the variations of pH behavior due to the addition of hydrochloric acid to the positive electrode electrolyte.
Fig. 4 is a characteristic diagram showing the variations of pH behavior due to the addition of hydrobromic acid in place of hydrochloric acid.
Fig. 5 shows characteristic diagrams showing the behaviors of the amount, density and viscosity of the formed bromic compound in relation to the dropping of bromine into the electrolyte.
Fig. 6, which is on the same sheet of drawings as Figs.
3 and 4, shows the electrolyte storage tank of Fig. 1 in which a filter is arranged in the lower part of the storage tank.
The electrolytes circulation type metal-halogen secondary batteries known in the art are basically constructea as shown in Fig. 1 of the accompanying drawings. In the Figure, numeral 1 mab/ ;~' 4~7~

designates a negative electrode, 2 a negative electrode chamber, 3 a positive electrode, 4 a positive electrode chamber, S a separator arranged substantially midway between the negative electrode 1 and the positive elec-trode 3, 6 a negative electrolyte storage tank, 7 a positive electrolyte storage tank, 8 a positive electro-lyte including a bromic complex compound, 9 electrolyte circulation pumps, 10 valves, and 11 a bypass for circulating the positive electrolyte including no bromic LO complex compound.
While the metal used may be zinc, cobalt, cadmium, or copper, if zinc is selected, for example, during the charging a reaction of Zn + + 2e -~ Zn takes place in the cathode chamber and a reaction of 2Br -~ Br2 +
2e takes place in the positive electrode chamber.
During the discharging, the reverse reactions proceed.
Note that the negative electrolyte consists of an aqueous solution of ~nX2 (X is a halogen atom) ana the positive electrolyte consists of an aqueous solutîon ~0 of ZnBr2, Br2 and a bromic complexing agent.
The separator for dividing the two electrolyte chambers generally comprises a perforated membrane of polyolefine type or ion exchange membrane.
When this type of secondary battery is charged, the bromine deposits onto the positive electrode and the metal deposits onto the negative electrode.
While the metal deposited onto the negative electrode is electrodeposited on the electrode plate, the bromine deposited onto the positive electrode is dissolved into the electrolyte and circulated along with the electrolyte.
There is another disadvantage that during the operation of the battery a part of the deposited mab/' -3 ~'7~7Z
bromine is passed through the separator and reaches the metal side of the pairing electrode thus causing a self-discharge thereat. Also, the bromine itself is a highly corrosive substance and thus limitations are imposed on the selection of mat.erials for forming the battery.
In view oE these restrictions, no electrolytes circulation type metal-halogen secondary battery has been put in practical use.
Summary of the Invention.
According to an aspect of the invention there is provided in an electrolyte circulation type metal-halogen secondary battery in ~hich a cell chamber diyided into a negative electrode chamber and a positive electrode chamber by a separator, a negative electrolyte storage tank and a .positive electrolyte storage tank are connected to one another by pipes, the improvement wherein a bromi`ne complexing agent comprising a tertiary amine combined to an alkyl is added to the positive electrolyte.

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Descrip-tion of the Preferred Embodi.ments.
In the past, the use of quaternary ammonium salt as -the complexing agent for achieving the above-mentioned objec-t of effec-t has been proposed. The electrolyte used in the above type of secondary battery system generally consists of an aqueous solution of metal bromide and the heretofore proposed quaternary ammonium salt dissolves into the elec-trolyte. Thus, the bromine deposited during -the charging combines with the quaternary ammonium salt and the resulting compound pre-cipita-tes in the lower part of the positive electrode storage tank. However, there is a disadvantage that -the dissolution of t.he quaternary ammonium sal-t into the electrolyte decreases its electric conductivity and the in-ternal resistance of the battery is increased.
During the discharging,:the ~eci.pi:tated bromic complex compound and the upper electrolyte containing no bromic complex compound are mixed and transported to the electrode chamber and the bromic complex compound contribute to the electrode reaction.
The heretofore proposed quaternary ammonium salts are mab/ ' --J

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represented by the general formulas oE R4N X, R3R'N-X
and R2R'2N X (in the formulas, ~ and R' are alkyl groups of C~ to C3 and X is a halogen, particularly bromine ion or chlorine ion). Other us~ble quaternary ammonium salts include for example a derivative of morpholine in which nitrogen atom is a member of the cyclic structure.

On the other hand, the complexing agents used in this invention consist of tertiary amines which are given by the general formula R R'R"N (where R, R' and R" are alkyl groups f C1 to C4 and these groups may be either the same or different from one another).

Generally, the tertiary amines are not soluble in water and these tertiary amines are of course not solved completely in the electrolyte of this invention thus separating the elec-trolyte into two phases. As a result, the electric resistance of the aqueous electrolyte solution is not increased and more-over the tertiary amine is combined with the bromine deposited during the charging to form a bromine complex compound. The resulting bromine complex compound dissolves into the complex-ing agent (the tertiary amine) region and they form a single phase condition. Then, during the discharge the separated two phases, that is, the upper electrolyte and the lower bromine complex compound region (or a single region which is a mixed region of the complexing agent and the bromine complex compound) are transported to the electrochemical reaction chamber (cell chamber) so that the bromine complex compound contacts with the electrode surface, causes an electrochemical 427~

reaction and contributes as the active material.

The tertiary amines which can be used in this invention include trimethyiamine, triethylamine, tripropylamine and tributylamine (including normal and iso) as ~ell as those tertiary amines formed by hydrocarbon residues of different kinds.

I~here a solution in which an electrolyte and a complex- -ing agent are separated into two phases as mentioned above is used in the metal-haiogen secondary battery, the battery construction shown in figure hereinafter is generally used.
The bromine electrolyte storage tank 7 serves simultaneously as an electrolyte storage tank and a reaction chamber for the bromine and the complexing agent in the diphase electrolytes.
As a result, a satisfactory contact must be ensured between the bromine dissolved in the upper region and the complexing agent in the lower region.

Complexing -1 Agent K(mho cm ) Electrolyte Composition (C2H5)4 NBr - 0.106 ZnBr2(3M)~(C2H5)4NBr(0-2M) (CH3)3N 0.081 ZnBr2(3M)+(CH3i3N(0-9M) ( 2 5)3 . 75 ZnBr2(3M)+(C2H5)3N(0-9M) (C3H7~3N 0.110 ZnBr2(3M)+(C3~17)3N(0.9M) The above table shows the electrolyte conductivities obtained by using the quaternary ammonium salt cited as the comparative example and the three tertiary amines cited as ~L~t~ Z~
the examples according to the invention as the complexing agents. Since the aqueous zinc bromide solution used has a electric conductivity of 0.124 mho cm (only for the electrolytic solution oE ZnBr2) at room temperature, the use of tripropylamine is also advantageous from the standpoint of the battery internal resistance since the variation of the conductivity of the supernatant electro-lyte is not large. Fig. 2 shows the concentrations of the unabsorbed bromine molecules which were left in the upper electrolyte when a given amount of bromine was dropped into the electrolytes containing the complexing agents of vary~ing concentrations and bromic complex compounds were formed. ~s will be seen from Fig. 2, among the complexing agents according to the present invention the addition of tripropylamine, though small in quantity, had the effect of absorbing the bromine satisfactorily and the addition of 2.5 mols showea no trace of the bromine molecules in the supernatant electrolyte.
The use of acids in combination with the tertiary amines used has the effect of eliminating a certain pro-perty, i.e., the insolubility of these amines.
The acids whi~ch can be used for this purpose include inorganic acids such as hydrochloric acid, nitric acid, sulfuric acid and hydrobromic acid and organic acids having at least one carboxyl group such as acetic acid, maleic acid and succinic acid.
The solubility of the tertiary amines with the electrolyte varies considerably with the proportions of the aclds used. For example, where olle mole of the tertiary ~; mab/

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amines is to be added to the electrolyte, theoretically it is only necessary to add one mol of the acid.
The following table shows the nu~ber of mols of hydrochloric acid or hydrobromic acid at the salting-out null point in cases where one mol of.the tertiary amine (trimethylamine or triethylamine) is to be added to the aqueous zinc bromide solutions containing the varying numbers of mols of the zinc bromide.

ZnBr2 concen- HCl (mol) HCl (mol) HBr (mol) HBr tmol) tration (mol/l) (CH3)3N(mol) (C2H5)3N (mol) (CH3)3N (mol) (C2H5)3N (mol) 4 0.80 0.95 0.78 0.95 3 0.85 1.00 0.88 O~g7

2 0.89 0.95 O.g5 0.98 As will be seen from the above table, it can be recognized that the acids having substantially the equal numbers of mols as the tertiary amines are required.
Fig. 3 shows the behavior of pH variation of the positive electrolyte due to the addition of hydrochloric acid. In the Figure, the abscissa represents the added amount in ml of 10~ aqueous hydrochloric solution to 50 ml of the positive electrolyte and the ordinate represents the corresponding pH value. The characteristic curves a, b and c show respectively at d the salting-out null points in the cases where -the molar ratios between the ~inc bromide and the tertiary amines are 4:1, 3:1 and 2.1, respectively.

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E`ig. 4 shows the variaticns in the pH behavior o the positive electrolyte due to the addition of aqueous hydrobromic acid solution. The abscissa represents the addition in ml of 10% aqueous hydrobromic acid solution to 50 ml of the positive electrolyte and -the ordinate represents the corresponding pH value. In the E'igure, the characteristic curves a, _ and c respectively correspond to the cases where the molar ratios between the zinc bromide and the tertiary amine are 4:1, 3:1 and 2:1, respectively, and the saltiny-out null points are indicated at d in the like manner as in Fig. 3.
In either of the cases of Figs. 3 and ~, the pH
of the electrolyte decreased suddenly when the aqueous aci~
solution was added in amounts exceed ng those corresp~ndin~
to the salting-ou-t null points. While this decrease in pH
value can be modified to describe a gentle curve by decreasing the concentration o the acid use~, it is desirable to adjust the pEI o the electrolyte to a value intermediate between the pH value indicated by the electro-lyte itself and the pH value corresponding to the salting-out null point. If the adjustment of the pH value results in an excessively small value, the difference wlth respect to the zinc side electrolyte increases considerably and this causes a vigorous movement of the electrolytes through the separator due to the proton concentration dif~erence.
Figs. 5(a), (b) and (c) show respectively the amount of absorbed bromine or the amount of bromic complex compound formed, its speciic weight and its viscosity which were obtained with the posi-tive electroly-te in which the aqueous zinc bromide _ 9 _ .~
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solution, triethylamine and hydrochloric acid (equal in mol with the triethylamine) were adjusted by the above-mentioned method. In each of these Figures, the abscissa represents the amount of bromine (Br2) added by dropping to 50 ml of the aqueous zinc bromide solution. Wi-th respect to the brominic complex compound formed, the amount is indicated in ml by the ordinate in Fig. 5 (a), the density is indicated in g1ml by the ordinate in (b) and the viscosity is indicated c P
in ~ by the ordinate in (c). In Figs~ 5(a), (b) and (c), the curves A show the cases where 10g of tri~thylamine was added to 50 ml o~ an aqueous solution containing 3 mols of zinc bromide (ZnBr2), the curves B show the cases where 5g of triethylamine was added to 50 ml of an aqueous solution containing 1 mol of ZnBr2 and the curves c show the cases where 5g of triethylamine was added to 50 ml of an aqueous solution containing 3 mols of ZnBr2.

Further, when Sg of trimethylamine and the equimolar of hydrochloric acid were added to 50 ml of an aqueous solution containing 3 mols of ZnBr2 and Br2 was dropped into the resulting mixed aqueous solution, the formation behavior of bromic complex compound was confirmed as a tendency such as shown in Fig~ 5, and the same results were obtained when hydrobromic acid was used in place of the hydrochloric acid.

- While the heretofore proposed quaternary ammonium salt has the disadvantage of increasing the amount of absorbed bromine (bromic complex compound) and thereby tending to solidify the oily compleY~ compound, it has been confirmed that there was no solidified region in the cases where the ' 9L 2r~
_ertiary amines and acids according to the invention were used.
The bromic comple~ compound according to the invention does not solidify and moreover if the amount of absorbed bromine increases, the viscosity decreases as shown in (c) of Fig. 5 with the result that during the charging or discharging periods the positive electrolyte is circulated easily, is easy to handle, reduces the head loss in the circulation system includiny the pumps, valves and liquid transfer pipes, flows unifor~lly within the positive electrode chamber and ensures a satisfactory reaction at the electrode.
Where 2.5 mols of triethylamlne was dissolved with hydrochloric acid in 50 ml Or an aqueous solution containing 0.5 mols of ZnBr2 and 6.4 ml of Br2 was dropped into the resulting solution, it was confirmed that the concentration of the bromine in the electrolyte was sub-stantially zero, that is, practically all of Br2 was converted to a bromic complex compound. This dropped amount of Br2 corresponds to an 80% charged level of the me-tal-bromine battery.
From the foregoing detailed description it will be seen that in accordance with the invention there is -provided the electrolytes circulation type metal-bromine secondary battery in which the positive electrolyte using bromine as the positive active material is prepared by adding a tertiary amine represented by the general formula of R3N, R2R'N, RR'2N or RR'R"N and an inorganic acid or organic acid to an aqueous solution oE a metal bromide -- 11 -- .

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2~2 (e.g., zinc bromide), whereby during the charying period the bromine molecules formed in the positive electrode cham~er are absorbed by the bromic complexing agent consisting of the tertiary amine and the acid and a bromic complex compound is formed which practically has no danger of solidification and is low in viscosity. Thus, the present invention can realize the electrolytes circulation type metal-halogen secondary battery capable of easily realizing a positive electrolyte circulation system, ensuring a uniform flow distribution and eliminating the deficiencies due to free bromine.
Where a complexing agent is used in accordance with the invention, the rate of reaction between the free bromine and the complexing agent can be increased by arranginy in a relatively lower complexing agent retaining portion of the positive electrolyte storage tank, preferably adjacent to the electrolyte exiting port a filter which is designed so that when the electrolyte is circulated, the complexing agent is prevented from flowing out and the contact area with the electrolyte is increased.
In order to effect the reaction efficiently~ it is desirable to use a filter having a large number (innumerable) of fine perforations and practically equal in area with the bottom surface of the storage tank. ~
preferred exemplary filter may be a glass fiber filter of a suitable thickness.
Fig. 6 shows an example of the filter. In the Figure, numeral 7 designates the positive electrolyte storage tank (the bromine electrolyte tank) of Fig. l, 8 the bromic complex compound kht ~- .

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region formed by the complexing agent comprising the tertiary amine, 13 the filter, and 12 separated particles in the electrolyte.

On discharge, the valve 10 is opened so that the electrolyte circulated from the cell chamber 4 to the elec-trolyte storage tank 7 and returned again to cell chamber 4 from the electrolyte storage tank 7 via the filter 13.
On charging, the electrolytes is circulated in the reverse order.

It will thus be seen from the foregoing description that in.accordance with the present invention the bromine deposited during the operation of the battery has no danger of reaching the metal side pairing electrode and.causing a self-discharge and also there are no danger of any decrease in the electrlc conductlvity of the electrolyte and increase in the battery internal resistance, thereby ensuring excellent performance.

Claims (6)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An electrolyte circulation type metal-halogen secondary battery, which comprises: a cell chamber divided into a negative electrode chamber and a positive electrode chamber by a separator, a negative electrolyte storage tank, and a positive electrolyte storage tank connected to one another by pipes, wherein said positive electrolyte is formed by adding a tertiary amine combined with an alkyl group and an acid to an aqueous solution of a metal bromide.

2. A secondary battery according to claim 1, wherein said tertiary amine has the general formula RR'R"N, wherein R, R' and R", independently, represent an alkyl group selected from methyl, ethyl, propyl and butyl, with the proviso that at least one of R, R' and R" is a higher alkyl than propyl.

3. A secondary battery according to claim 1, wherein said metal bromide is zinc bromide.

4. A secondary battery according to claim 1, 2 or 3, wherein said acid is selected from the group consisting of hydrochloric acid, sulfuric acid, nitric acid, hydrobromic acid, acetic acid, oxalic acid, maleic acid, succinic acid and mixtures thereof.

5. A secondary battery according to claim 1, 2 or 3, wherein said tertiary amine and said acid are equimolar.

6. A secondary battery according to claim 1, 2 or 3, wherein a filter is arranged in said positive electrolyte storage tank.