Process for charging and discharging zinc/bromine batteries

A process for charging and discharging zinc/bromine batteries with a plurality of anode and cathode chambers into and from which anolyte or catholyte fluids are fed and drained. Metallic zinc is deposited on the anode and atomic and/or molecular bromine is deposited at the cathode, and bromine is bonded in an only slightly water soluble complex with a complex-forming agent from the aqueous phase. The anolyte and/or catholyte fluids are circulated periodically. The oleophilic cathode phase is separated from the hydrophilic cathode phase. Only the aqueous phase of the catholyte is circulated through the cathode chambers, and the times at which the fluid is circulated is dependent upon the temperature of the anolyte in the anode chambers.

The present invention relates to a process for charging and discharging 
zinc/bromine batteries, having a plurality of electrodes, bipolar ones in 
particular, and a plurality of anode and cathode spaces into which 
optionally temperature-stabilized catholyte or catolyte fluid is 
introduced. 
Voltaic cells based on the zinc/bromine pairing can be particularly 
efficient. The high reactivity of bromine, though, requires a particular 
choice of materials; plastics, for example polyethylene or polypropylene, 
are preferred materials, because of their high chemical resistance to 
bromine. The construction of zinc/bromine batteries with plastic is done 
in such a way that on the one hand, bipolar electrodes are provided, which 
are constructed with plastic-bonded carbon, for example graphite, 
activated charcoal, and the like. Between the electrodes, diaphragms are 
disposed, which are likewise comprised of plastic, in particular 
polypropylene or polyethylene. The diaphragms or electrodes have 
thickenings on their edges so that packets of electrodes, between each of 
which diaphragms are disposed, create electrode spaces, specifically anode 
and cathode spaces. These spaces are supplied and emptied via separate 
lines. The electrolytes, specifically the catholyte and catholyte, each 
have at their disposal separate loops with separate reservoirs and pumps 
associated with each of these. If need be, the pumps can be driven by a 
common electromotor. 
The storage of the zinc is effected in metallic form on the surface of the 
electrode. If the electrolyte composition is inadequate, or if there are 
excessive current densities, dendrite formation can occur, which on the 
one hand can cause damage to the diaphragms; on the other hand, for 
example upon discharging, zinc fragments can be formed, which are no 
longer connected to the electrode via a first-class electrical conductor. 
These zinc fragments cause malfunctions, which among other things can lead 
to individual electrode spaces not being supplied with the electrolyte 
fluid, since the supply lines or also the drain lines are clogged with the 
zinc fragments. These malfunctions can be solved, according to European 
Patent Application Serial No. EP-A-0149448, by introducing 
bromine-containing electrolyte into the electrolyte space which has the 
zinc, so that the zinc attains dissolution not electrochemically, but 
purely chemically, and hence there is no need to connect zinc fragments to 
the electrode. This introduction of bromine-containing electrolyte fluid 
into the electrode spaces, in which zinc is precipitated, can take place 
for example at the end of a discharging process; bromine-containing 
electrolyte is supplied, by means of which the zinc can be chemically 
dissolved. 
Although zinc/bromine batteries have been developed in which the bromine is 
also deposited on the electrode surface, nevertheless if a larger capacity 
is to be achieved, then the bromine must be stored outside the electrode. 
A preferred process for binding of free bromine is comprised in that a 
complexing agent is dissolved in the aqueous electrolyte, which agent 
forms a complex with the bromine, which dissolves in water only with 
difficulty, so that the electrolyte has an aqueous phase and a hydrophobic 
phase. The hydrophobic phase can be stored together with the aqueous phase 
in an electrolyte vessel. The hydrophobic phase is also on the surface of 
the cathode, next to the aqueous phase, during both charging and 
discharging. Since the cathode surface as a rule has an increased surface 
area, for example by means of the embedding of carbon fibers, carbon 
particles, or the like, the hydrophobic phase additionally adheres to the 
surface of the cathode. Consequently, after a charging or discharging 
process, there is a bromine concentration in the cathode space, which is 
required for carrying out the electrochemical process. In order to 
decrease the bromine concentration, in the internal prior art, after an 
electrochemical reaction, the electrode space is rinsed out with the 
aqueous phase; care is taken that there be no hydrophobic phase with a 
high bromine concentration in the electrode space. Despite the relatively 
large size of bromine molecule, the bromine which is in the cathode space 
seeps through the diaphragm into the anode space and dissolves metallic 
zinc there. Hence, on the one hand, the capacity of the battery is reduced 
and on the other hand, the chemical reaction is strongly exothermic, so 
that the battery experiences a powerful temperature increase in the 
resting phase. This temperature increase is of particular significance if 
the battery has an additional covering to prevent the escape of bromine, 
even when electrolyte cells are damaged. The thermic strain on the anode 
or cathnode spaces causes distortion of both the diaphragms and the 
electrodes, which as a rule are made of plastic. Since both the anode 
space and the cathode space have a particularly low thickness normal to 
the surface of the electrodes, distortions of this kind can lead to 
irregular through flows of the electrode spaces and with these to 
undesired resistance- and capacitance changes of the battery. 
The object of the present invention is to create a process in which thermic 
strain on the battery during the resting state can be prevented and the 
consumption of additional energy, electrical energy in particular, is 
especially easily avoided, so that deformations of the battery, which are 
ascribed both to thermic causes and to reductions in capacitance, can be 
prevented with particular ease. 
The process according to the invention for charging and discharging 
zinc/bromine batteries, having a plurality of electrodes, bipolar ones in 
particular, and a plurality of anode and cathode spaces into which 
optionally temperature-stabilized anolyte or catolyte fluids respectively 
are separately supplied to and drained from reservoirs, wherein in 
charging, metallic zinc is precipitated out at the anode (negative 
electrode) and atomic and/or molecular bromine is precipitated out at the 
cathode, (positive electrode) and bromine is bound in a complex, which is 
poorly soluble in water, with a complexing agent from the aqueous phase, 
and the cathode spaces are connected to the anode spaces via diaphragms, 
and wherein in charging and discharging, the catholyte and/or anolyte 
fluid is kept circulating, at least intermittently, and wherein in the 
reservoir for the catholyte fluid, separation of an oleophilic catholyte 
phase from a hydrophilic catholyte phase, with drain- and supply lines 
available, is effected, is defined essentially in that an exclusively 
aqueous phase of the catholyte fluid is passed through the cathode spaces, 
chronologically after the supply and/or draining of electrical energy, and 
then a temporal interruption occurs, whereupon the aqueous phase is once 
again intermittently passed (circulated) through the cathode spaces. 
Since only the aqueous phase is employed or used, any additional increase 
in the bromine concentration can be prevented; thus at the same time, 
rinsing of the cathode surface can be carried out in a particularly 
effective manner. By maintaining pauses between the individual rinsing 
phases, on the one hand, the consumption of electrical energy, such as for 
pumped recirculation, is kept low, and on the other hand, even in the 
resting state of the electrolyte, a separation of the bromine complex from 
the cathode surface can take place by means of dissolution in the aqueous 
phase. This dissolving process is very time-dependent and is adequately 
fast, even when the electrolyte is in a resting state. The concentration 
of the free bromine in the cathode space is consequently sharply increased 
so that the penetration speed of the bromine through the diaphragm to the 
anode space would be significant if the electrolyte in the cathode space 
were exchanged in order to prevent this crossing over. 
If a temporary pause is provided after a current drain and after the pumped 
recirculation that follows of the purely aqueous phase of the electrolyte, 
then in this pause a dissolving of the bromine complex, which is still 
adhering to the fiber or the like, into the aqueous phase can take place. 
This aqueous phase, which at this point is enriched with bromine, can 
easily be replaced by a pure aqueous phase, which is to a large extent 
bromine-free, by means of a further rinsing process. 
If a diversion (circulation) of the aqueous phase of the electrolyte occurs 
at least twice, then an essential reduction of the bromine concentration 
in the cathode spaces is possible, with a particularly low energy 
expenditure for the diversion of the electrolyte. 
If temporary pauses are maintained that are 10-30 times, preferably 15-20 
times, as long as the diversion time of the aqueous electrolyte, then the 
dissolving process in the aqueous phase, which proceeds slowly, is 
particularly favorably taken into account. 
If the diversion of the aqueous phase is initiated depending on the 
temperature, in particular of the anode fluid in the anode space, then the 
control can be carried out merely by means of a thermal switch without 
providing a separate time function element. The temperature control can 
also be made to depend for example on the temperature in the anode space 
at the cell output, since the exothermic chemical reaction takes place in 
this space, so that the heat conduction by means of the diaphragm does not 
act as a time delay element. 
If the diversion of the aqueous phase is switched on and/or off by means of 
a time switch element, in particular depending upon the duration of 
diversion, then a predetermined control for fixed sizes of batteries can 
be carried out, by means of which the control of the thermal economy of a 
battery is embodied in a particularly simple and effective manner. 
If the diversion of the aqueous phase of the electrolyte after the draining 
or supply of electrical energy, is carried out at a low flow velocity, in 
particular at 1/3 to 1/2 the flow velocity of the one during the charging 
and/or discharging, then turbulent flow phenomena such as eddies and the 
like are prevented in the reservoir, so that the formation of a suspension 
can be considered unlikely. 
If the anolyte is diverted at the same time as the aqueous phase of the 
catholyte fluid, then a depletion of the bromine concentration in the 
anode space takes place; identical fluid pressures can be maintained in 
the cathode space and in the anode space as well. 
The invention is further explained below from the examples and the drawing.

EXAMPLE 1 
In a zinc/bromine battery with bipolar electrodes, which each had an area 
of 1200 cm.sup.2 and which were separated from one another via diaphragms, 
by means of which anode and cathode spaces were formed, which were each 
separately supplied with the anolyte or catolyte. The flow velocity during 
charging and discharging was approximately 1 cm/sec at the electrode 
surface. The flow velocity was approximately 0.3-0.5 cm/sec when the 
electrolyte fluid was being diverted. 
The aqueous electrolyte had the following chemical composition: 
2.3 mol/l zinc bromide 
1 mol/l potassium chloride 
1 mol/l methylethyl morpholinium bromide 
The complexing agent, specifically methylethyl morpholinium bromide, forms 
a complex with bromine that is poorly soluble in water; specifically, 2-3 
g per liter of the bromine complex dissolves in the aqueous electrolyte, 
depending upon the temperature. The battery had 32 electrodes, so that a 
total voltage of 48 volts could nominally be achieved. The temperature 
measurement was carried out in the anode space of a centrally disposed 
anode; heat radiation was kept particularly low by a further covering of 
the battery, in order to prevent any escape of electrolyte fluid from the 
battery. 
As can be inferred from FIG. 1, after the end of the charging procedure, 
the temperature was increased to approximately 70.degree., and in fact the 
battery was charged up to 80% of its capacity without corresponding 
rinsing procedures being provided. This temperature increase occurred 8 
hours after the end of the charging procedure. A temperature decrease then 
took place. This process of long-term temperature increase can be 
explained by the crossover of bromine from the cathode space through the 
diaphragm into the anode space, by means of which an exothermic reaction 
between bromine and zinc can take place. 
EXAMPLE 2 
In a battery according to Example 1, after predetermined time intervals of 
60 minutes each, a 4 minute rinsing with the aqueous electrolyte was 
carried out. There was a wait until precipitation of the bromine complex 
in the reservoir took place; then the catholyte was removed by suction so 
that no suspension, but only the catholyte exclusively, could be pumped 
into the cathode spaces. As is apparent, it was possible to prevent 
excessive thermic strain on the battery by means of the rinsing 
procedures, which are chronologically preset with the control over time, 
there is also a dependency on the outside temperature and hence various 
maximum temperatures can occur inside the battery. The course of 
temperature in the anode space is described in FIG. 2; the brief 
temperature drop, which occurs three times, indicates the rinse 
procedures. The maximum temperature comes to approximately 48.degree.. 
EXAMPLE 3 
The control of the diversion of the anode fluid was carried out via the 
temperature of the anolyte in the anode space; each time the temperature 
reached 50.degree., a pumped recirculation of the cathode fluid in the 
cathode space was carried out. This type of control offers the advantage 
that preset temperatures are not ever exceeded and consequently an 
inadmissible thermic strain on the battery is prevented in any case. The 
temperature course is shown in FIG. 3. 
In the examples, the anode fluid was diverted together with the cathode 
fluid. 
The embodiment of the battery can be inferred from EP-A-0149.448, which 
comprises a component of the present invention. 
All ionogenic compounds that are neither precipitated nor decomposed during 
the charging or discharging process can be used as the conducting salt, 
such as KCl or NaCl. 
All kinds of compounds can serve as the complexing agent for the bromine, 
for example methylethyl morpholinium bromide, or methylethyl pyrolinium 
bromide.