Method of operating metal-halogen battery

A method of operating a metal-halogen battery such as a zinc-bromine battery, intended to prevent dendrite from formation on an electrode during charging the battery. The method comprises the step of carrying out an operation of discharging the battery at a predetermined constant current having a first current value throughout charging the battery; and the step of initiating an operation of charging the battery at a second current value of two times the first current value and maintaining the changing operation at a third current value which linearly decreases from the second current value to a zero value toward a termination of charging the battery.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to improvements in a method of operating a 
metal-halogen battery such as a zinc-bromine battery, and more 
particularly to a technique for preventing a dendrite from formation 
during changing the battery. 
2. Description of the Prior Art 
A metal-halogen battery such as a zinc-bromine battery includes a plurality 
of cells which are electrically connected in series with each other in 
order to generate a high voltage to be picked up. The respective cells are 
separated from each other by separator plates. Each cell includes a 
bipolar electrode plate which defines positive and negative electrode 
chambers on the opposite sides thereof. Two kinds of electrolyte solutions 
are respectively circulated through the positive and negative electrode 
chambers under the action of pumps thereby to accomplish charging and 
discharging the battery. 
In such a zinc-bromine battery, metallic zinc is electrodeposited on the 
surface of the negative electrode during the charging while the 
electrodeposited metallic zinc is dissolved in the electrolyte solution 
during the discharging, thus developing an electromotive force. It is 
known that the electrodeposition of zinc takes the form of a dendrite, 
which has considerably shortens a battery service life. In order to 
improve such shortening of the battery service life due to the dendrite, a 
variety of countermeasures have been hitherto taken. For example, 
inhibitors are added to the electrolyte solutions; or a complete 
discharging is once carried out in one charging and discharging cycle in 
order to electrically remove the electrodeposited zinc. 
However, difficulties have been encountered in such dendrite prevention 
measures. Concerning addition of the inhibitors, if organic inhibitors are 
employed, they are short in life and therefore cannot provide a stable 
dendrite formation preventing effect throughout a long period of time. If 
inorganic inhibitors are used, they can suppress the formation of dendrite 
upon forming a codeposition between them and zinc; however, they forms a 
segregation so that the dendrite formation suppressing effect is unstable 
throughout a long period of time. Concerning carrying out the complete 
discharging, it needs several hours and is unavoidably required in each 
charging and discharging cycle. This is very inconvenient and requires a 
device for causing the complete discharging. 
SUMMARY OF THE INVENTION 
It is an object of the present invention is to provide an improved method 
of operating a metal-halogen battery, which can effectively overcome 
drawbacks encountered in conventional operating methods of a metal-halogen 
battery. 
Another object of the present invention is to provide an improved method of 
operating a metal-halogen battery, by which electrodeposition of a metal 
on the surface of an electrode can be effectively prevented from taking 
the form of dendrite while prolonging the battery service life. 
A method of operating a metal-halogen battery, according to the present 
invention comprises the step of carrying out an operation of discharging 
the battery at a predetermined constant current having a first current 
value throughout charging the battery; and the step of initiating an 
operation of charging the battery at a second current value higher than 
said first current value and maintaining the charging operation at a third 
current value which linearly decreases from said second current value to a 
zero value toward a termination of charging the battery. 
During charging the battery, a metal is electrodeposited on the surface of 
an electrode of the battery. By virtue of the above-mentioned operational 
method of the battery, crystal of the electrodeposited metal does not take 
a grain form and takes a flat form, thereby effectively preventing 
formation of dendrite. Thus, according to the operational method of the 
present invention, a high dendrite formation suppressing effect can be 
always obtained without lowering the efficiency of the battery. 
Additionally, no complete discharging is required and therefore the 
battery is highly convenient in maintenance while rendering unnecessary a 
device for causing the complete discharging of the battery.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to FIG. 1 of the drawing, there is shown a metal-halogen 
battery or cell, in the form of principle, to which the principle of the 
present invention is applied. In this instance, the metal-halogen battery 
is a zinc-bromine battery or cell and designated by the reference numeral 
10. The zinc-bromine battery 10 comprises a battery main body 12 which 
includes a plurality of battery cells 14. The respective battery cells 14 
are separated from each other by separator plates 16, and electrically 
connected in series with each other. Each battery cell includes positive 
and negative electrode chambers 20 on either side of the bipolar electrode 
plate 18, the bipolar electrode being formed by the positive electrode 
chamber 20 of one cell and the negative electrode chamber of an adjacent 
cell. Each positive electrode chamber 20 is supplied with a positive 
electrolyte solution 26 and a bromine complex compound 28 in a positive 
electrolyte storage tank 30 through upper and lower positive electrolyte 
solution manifolds 24. The bromine complex compound 28 is located on the 
bottom of the positive electrolyte solution storage tank 30. The 
electrolyte solution 26 and the bromine complex compound 28 are circulated 
through the positive electrode chamber 20 under the action of a pump 32 
and a four-way cock 34. The bromine complex compound 28 flows down through 
a valve 30a. The pump 32 and the cock 34 are disposed in a piping system 
(no numeral) for connecting the tank 30 and the manifolds 24. 
Each negative electrode chamber 22 is supplied with a negative electrolyte 
(ZnBr.sub.2) solution 36 in a negative electrolyte solution storage tank 
38 through upper and lower negative electrolyte solution manifolds 40. The 
electrolyte solution 36 is circulated through the negative electrode 
chamber 22 under the action of a pump 42. The pump 42 is disposed in a 
piping system (no numeral) for connecting the tank 38 and the manifolds 
40. It will be understood that charging and discharging the battery is 
carried out by circulating the electrolyte solutions 26, 36 respectively 
through the positive and negative electrode chambers 20, 22. 
According to the principle of the present invention, the metal-halogen 
battery such as the zinc-bromine battery is operated in such a manner that 
an operation of discharging the battery is carried out at a predetermined 
constant current having a first current value throughout charging the 
battery; and additionally an operation of charging the battery is 
initiated at a second current value higher than the first current value 
and maintaining the charging operation at a third current value which 
linearly decreases from the second current value to a zero value toward a 
termination of charging the battery. 
It will be understood that, in conventional methods of operating the 
metal-halogen battery, the charging operation of the battery is carried 
out at a constant current as same as in the discharging operation. 
However, according to the operating method of the present invention, the 
current value is increased to the high value and linearly decreased so as 
to take the zero value at the terminal period of the charging. 
In order to evaluate the effect of the operating method according to the 
present invention, the following experiments were conducted: 
EXPERIMENT 1 
A mixture of a solution of 2.75 mol/l of Zn.sup.2+, a solution of 4.5 mol/l 
of Br.sup.-, a solution of 1 mol/l of Cl.sup.- and a solution of 1 mol/l 
of quaternary ammonium salt containing N-ethyl-N-methyl-morpholinium 
bromide and N-ethyl-N-methyl-pyrrolidium bromide (in a ratio of 1:1) as a 
bromine complex compound was supplied into a small cell. A carbon-plastic 
(composite of carbon and plastic) electrode and a zinc (99.99%) plate as 
the opposite electrode were dipped in the mixture solution in the cell, 
thereby preparing a battery cell. Then, electrodeposition was carried out 
at a current value of 20 mA/cm.sup.2 for 4.5 hours so that zinc was 
electrodeposited on the carbon-plastic electrode. Thereafter, charging and 
discharging of the cell were carried out by the following methods: 
(1) A conventional method 
Discharging the cell was carried out at a current value of 20 mA/cm.sup.2 
(constant current) for 2 hours to remove the electrodeposited zinc on the 
carbon-plastic electrode. Then, charging the cell was carried out at the 
current value of 20 mA/cm.sup.2 (constant current) for 2 hours. Such a one 
cycle of discharging and charging was repeated 5 times, upon which the 
surface of the carbon-plastic electrode was observed. 
(2) A method of the present invention 
Discharging the cell was carried out at a current value of 20 mA/cm.sup.2 
(constant current) for 2 hours to remove the electrodeposited zinc on the 
carbon-plastic electrode. Then, charging the cell was carried out at a 
current value which linearly decreased from 40 mA/cm.sup.2 (at the initial 
time of the charging) to 0 mA/cm.sup.2 during a time period of 2 hours. 
Such a one cycle of discharging and charging was repeated 5 times, upon 
which the surface of the carbon-plastic electrode was observed. 
EXPERIMENT 2 
A zinc-bromine battery of the electrolyte solution circulation type was 
produced to take a construction as same as that shown in FIG. 1 with the 
exception that 8 cells (14) were arranged side by side. Each of bipolar 
electrodes (18) had a surface area of 830 cm.sup.2. 
This zinc-bromine battery was operated in a conventional method which was 
the same as that in EXPERIMENT 1 except for the fact that zinc was 
electrodeposited on and removed from the bipolar electrode (18), and in a 
method of the present invention which method was the same as that in 
EXPERIMENT 1 except for the fact that zinc was electrodeposited on and 
removed from the bipolar electrode (18). In these operations in which 5 
times of the discharging and charging cycle was repeated, discharging was 
finally made to 0 V and then a Coulombic efficiency (quantity of discharge 
electricity/quantity of charge electricity) of the battery was measured. 
TEST RESULTS 
According to the conventional method in EXPERIMENT 1, the electrodeposited 
zinc on the surface of the carbon-plastic electrode took a crystal grain 
form shown in FIGS. 3A and 3B. As shown in FIGS. 3A and 3B, most crystal 
grains of the electrodeposited zinc had a diameter ranging from 0.1 to 0.2 
mm. It is to be noted that such crystal grains of zinc serve as nuclei of 
dendrite and therefore promotes the formation of the dendrite. 
According to the method of the present invention in EXPERIMENT 1, the 
electrodeposited zinc on the surface of the carbon-plastic electrode took 
a flat crystal form as shown in FIGS. 2A and 2B. As seen from FIGS. 2A and 
2B, the crystals of zinc were not in the form of grain and flatly extend 
on the surface of the electrode so as to take a flat electrodeposited zinc 
state. It will be understood that such a flat electrodeposited zinc state 
can effectively suppress formation of the dendrite. 
According to the conventional method in EXPERIMENT 1, the measured 
Coulombic efficiency was 62%. According to the method of the present 
invention in EXPERIMENT 1, the measured Coulombic efficiency was 78%. 
Thus, the Coulombic efficiency was considerably lowered when the 
zinc-bromine battery was operated under the conventional method. On the 
contrary, the same efficiency was maintained at a high level when the 
zinc-bromine battery was operated under the above-mentioned method of the 
present invention, by virtue of improved electrodeposited state of zinc on 
the electrode.