Metal-halogen secondary battery

In a metal-halogen secondary battery a sheet-shaped article containing porous carbon fibers is joined to the surface of the positive electrode.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a metal-halogen battery, and more 
particularly it relates to a secondary battery in which zinc is the active 
material of the negative electrode and bromine is the active material of 
the positive electrode and in which a particular porous carbon fiber 
sheet-shaped article is applied onto the positive electrode surface. 
2. Description of the Prior Art 
Since the energy crisis in 1973, the importance of the energy problem has 
become widely acknowledged in various fields. While it has become 
important to develop new energy sources, it has also become important to 
develop systems which utilize the generated energy effectively, including 
those for the conversion, storage, transportation, utilization, etc. of 
energy. Taking the storage as an example, in the large scale generation of 
electricity such as atomic power generation, coal thermal power 
generation, etc. which are expected to constitute a large percentage of 
the electric energy sources in the future, it is necessary for maintaining 
a high efficiency to generate electricity with a constant output. 
Therefore a strong requirement for developing a technique of electric 
energy storage which makes it possible to suitably store the surplus 
electric energy during the night and release it during daytime in 
accordance with the variations of energy demand. For example, at present, 
the yearly working ratio of the main electric power plants is less than 
60% in Japan, and there is a continuing decline. Although as a method of 
storage of electric energy pumping-up hydraulic power generation has been 
put into practical use, it involves a loss of energy due to transmission, 
and is becoming limited in the location. Therefore, various other methods, 
such as new type secondary battery, fly wheel, compressed air, super 
conduction, etc. are being studied. 
Among others, the electrochemical operations using new type battery systems 
are promising, and these are thought to be the method most realizable for 
some time to come which replaces the pumping-up hydraulic power 
generation. And these systems will solve the problems of location and 
transmission. Also, the new type secondary battery is expected as a backup 
apparatus for the electric power generation utilizing natural energies 
such as solar light, wind power, wave power, etc., and it is also expected 
as the battery for use in electric automobiles. As the secondary battery 
applicable for the above-mentioned purposes, there have been developed 
lead storage battery, sodium-sulfur battery, lithium-iron sulfide battery, 
metal-halogen battery, redox flow type battery, etc. Among these 
batteries, metal-halogen battery (for example, zinc-chlorine, or 
zinc-bromine secondary battery) is being developed rapidly in recent 
years, because of its excellent characteristics such that the battery 
output is easily regulatable since it is of liquid circulation type; its 
maintenance control is easy since it is an aqueous solution type battery 
which operates at low temperatures; the battery capacity can be easily 
regulated by the electrolyte reserver volume; the active materials of both 
electrodes are abundant in natural resources and are low-priced; its 
theoretical energy density is high; and since the battery reaction is 
simple, the battery is simple in structure and can be made with cheap 
materials. For example, in the zinc-bromine secondary battery, the active 
material at the negative electrode; is zinc and the active material at the 
positive electrode is bromine, and an aqueous ZnBr.sub.2 solution is used 
as the electrolyte. The electric charge and discharge reactions proceed as 
follows: 
##STR1## 
However, in order to put the metal-halogen battery into practical use, 
there are several problems to be solved. Among others, it is the most 
important technical subject how rapidly and effectively the reduction 
reaction of halogen at the positive electrode should be caused, because it 
influences directly to the energy efficiency of the battery. Examples of 
inexpensive positive electrodes replacing the conventional Pt plate 
include a carbon-plastic electrode in sheet-form produced by the 
heat-press shaping of a mixture of an electroconductive carbon powder and 
a resin powder, and a sintered carbon plate. With these electrodes, when 
the discharge proceeds and the concentration of the active material 
(halogen) at the positive electrode is lowered, it has been usual that the 
potential drop is remarkable and the energy efficiency of the battery 
remains at a low value. Especially, upon the electric discharge at a high 
current density, a marked potential drop has been observed. 
STATEMENT OF THE INVENTION 
We conducted research to find a remedy for the various disadvantages 
attendant upon the conventional carbon-plastic electrodes and sintered 
carbon plates, and as a result we have reached the present invention. 
The present invention is a metal-halogen secondary battery in which is used 
positive electrode produced so that, on the surface of the electrode 
substrate a material (electricity collector) such as the above-mentioned 
carbon-plastic electrode plate or sintered carbon plate, is joined a 
sheet-shaped article containing porous carbon fibers, of which the pore 
volume of the pores having a diameter in the range of from 30 to 100 .ANG. 
is more than 0.1 cc/g. 
The reason why the reduction reaction of halogen does not proceed 
sufficiently on the conventional carbon-plastic or sintered carbon plate 
electrode, is supposed to be that, since the electrode surface is smooth 
and the real reaction surface area is small, when the halogen 
concentration lowers, the amounts of diffusion and adsorption of halogen 
to the electrode surface decrease, that is to say, the so-called 
polarization is caused. Then we tried etching of the surface of the 
carbon-plastic electrode by various methods to increase the surface area, 
or we made by way of trial an electrode in which activated carbon powder 
was used in place of carbon powder. But the effect was not satisfactory 
enough. Thereupon, we made an electrode such that a sheet-shaped article 
composed of porous carbon fibers is bonded to the surface of the positive 
electrode substrate material composed, for example, of the above-mentioned 
carbon-plastic plate or sintered carbon plate. When this electrode was 
used in the metal-halogen secondary battery, the positive electrode 
potential was very high even when the halogen concentration was lowered, 
and also the energy efficiency of the battery was markedly improved. When 
using a sheet-shaped article composed of porous carbon fibers, of which 
the pore volume of the pores having a diameter from 30 to 1000 .ANG. is 
more than 0.1 cc/g-carbon fiber, it has been found that an excellent value 
is obtained for both valtaic and coulombic efficiency, and an electrode 
performance not inferior to that of precious platinum plate was exhibited. 
If the distribution of the pores is composed mainly of a diameter less 
than 30 .ANG., the diffusion coefficient in the pores of the halogen 
dissolved in the electrolyte is small because of the small pore diameter 
and these pores do not act effectively on the electrode reaction. On the 
contrary, if the distribution of pore diameters exceeding 1000 .ANG. is 
predominant, the whole surface area of the porous carbon fibers becomes 
small and therefore such a pore diameter is not desirable. Futhermore, in 
the case of a sheet-shaped articles composed of porous carbon fibers, of 
which the pore volume composed of the pores having a diameter within the 
range of 30 to 1000 .ANG. is less than 0.1 cc/g, the surface area per unit 
volume becomes small and the effect of the present invention cannot be 
obtained. Also, the fiber density of the sheet-shaped article is 
preferably more than 0.1 gg/cc. A fiber density less than 0.1 g/cc makes 
the contact between the fibers insufficient, increases the electric 
resistance, induces an increases of the internal resistance of the battery 
and lowers the voltaic efficiency. Therefore such a fiber density is not 
favorable. Furthermore, when the fiber density is less than 0.1 g/cc, upon 
producing the electrode, falling-off of the fibers is liable to occur and 
there may be a problem in respect of processing. 
DESCRIPTION OF PREFERRED EMBODIMENT 
The raw material fibers used in the present invention may be any that can 
be carbonized. However, cellulosic, acrylic, phenolic fibers or fibers 
from petroleum or coal pitch can be advantageously used because of their 
ease of carbonization, ease of causing the porosity to develop, strength 
and elongatation of the porous carbon fibers, etc. 
For the sheet-shaped articles employed in the present invention, woven 
fabrics, knit fabrics, paper-like articles, etc. containing at least 30 
weight % of porous carbon fibers are advantageously used. 
The term "woven fabric composed of porous carbon fibers" means a 
cloth-shaped article interwoven lengthwise and breadthwise with porous 
carbon fiber yarns composed of a plurality of single porous carbon fiber. 
For example, by using as the starting material a cloth produced by 
interweaving lengthwise and breadthwise spun yarns or filament yarns 
composed of carbonizable organic material, and by carbonizing it and 
developing its porosity (i.e. activation), it is possible to produce a 
woven fabric composed of porous carbon fibers; or it can be also produced 
by weaving into a cloth the yarns after carbonization or after activation. 
The construction of weaving may be any that are usually used. For example, 
it can be selected from plain weave, twill weave, crape weave or satin 
weave. 
The term "knit fabric composed of porous carbon fibers" means a porous 
carbon fiber cloth obtained by preparing a tubular knit fabric or a warp 
knit fabric (having for example a construction of double denbigh, double 
cord, half tricot, half back, interlock, jacquard stitch, mock loading, 
rib, etc.) from a spun yarn or a filament yarn composed of a carbonizable 
organic material, and carbonizing and activating it to produce the porous 
carbon fiber fabric which retains the initial geometrical construction. 
The paper-like articles composed of porous carbon fibers can be produced 
from porous carbon fibers by the following method: porous carbon fibers 
are mixed with other organic or inorganic fibers, if necessary with a 
binder, and the mixture is made into a paper-like article. This paper-like 
article may be further carbonized or activated. In the present invention, 
non-woven fabrics may be used, but in this case it is desirable to employ 
those having a relatively high fiber density (for example, higher than 
0.10 g/cc). 
For the carbonization of the above mentioned organic fibers or sheet-shaped 
articles, a suitable method should be selected depending on the 
characteristics of the organic substance which composes respective fibers. 
For the activation of the fibers, any method may be employed which can 
make the fibers finally have a pore volume more than 0.1 cc/g of the pores 
of 30-1000 .ANG. diameter. The sheet-shaped articles of porous carbon 
fibers may be obtained by carbonizing yarns, cloths of paper-like articles 
composed of single porous carbon fibers. The activating method at a 
temperature between 400.degree. C. and 1100.degree. C. in an atmosphere of 
steam, carbon dioxide or oxygen for obtaining activated carbonaceous 
fibers is effective as a simplest method. Especially advantageously used 
for this purpose is a method wherein carbonaceous fibers, of which the 
volume of pores of 30-300 .ANG. diameter is less than 0.1 cc/g, are caused 
to carry at least one compound selected from compounds of II A Group 
elements of the Periodic Table and transition metals, and then the fibers 
are subjected to reactivation treatment to bring the volume of pores of 
30-1000 .ANG. diameter to more than 0.1 cc/g. 
To reduce the internal resistance of the battery and to increase the 
oxidation-reduction reaction speed of halogen at the positive electrode by 
bringing the specific electric resistance of the porous carbon fibers to 
less than 5.times.10.sup.-2 .OMEGA..cm, the carbon fibers may be subjected 
to the activation after a high temperature treatment between 1100.degree. 
C. and 3000.degree. C. in an inert gas, or conversely the suitable high 
temperature treatment may be carried out after the activation. 
To produce a positive electrode by using the sheet-shaped article 
containing porous carbon fibers thus produced, the sheet-shaped article is 
joined or bonded to the surface of the electrode substrate material: The 
sheet-shaped article is placed on the bottom of a metallic mold, and a 
uniform mixture of an electroconductive carbon powder and a resin powder 
(for example an olefin resin powder) is added in an even thickness, the 
temperature of the metallic mold being set so that it is by 10.degree. C. 
higher than the softening point of the resin. The mixture is then 
heat-pressed to produce a positive electrode which is composed of a 
carbon-plastic plate, on which surface is joined the sheet-shaped article 
of porous carbon fibers. Alternatively, the same positive electrode can be 
produced by first placing a mixture of an electroconductive carbon powder 
and a resin powder in an even thickness on the bottom of a metallic mold, 
of which the temperature is set by 10.degree. C. higher than the softening 
point of the resin, heat-pressing the mixture to prepare a carbon-plastic 
plate beforehand, then placing the sheet-shaped article on the 
carbon-plastic plate, and heat-pressing the whole body. 
As for the pore diameter and the pore volume of the porous carbon fibers in 
the present invention, the pore volume of the pores in the range of 30 to 
300 .ANG. diameter was obtained after the calculating method by 
Cranston-Inkley using the nitrogen gas adsorption isothermal line on the 
adsorption side at the boiling point of liquid nitrogen under atmospheric 
pressure; the pore volume of the pores in the range of 300 to 1000 .ANG. 
diameter was determined by the measurement with a mercury porosimeter; and 
the transitional pore volume of the pores having a diameter from 30 to 
1000 .ANG. (hereinafter abbreviated as TPV.sub.30.sup.1000) was obtained 
by the sum of the above-mentioned two pore volumes. As for the relation 
between the thickness (t) of nitrogen multilayer at adsorption and the 
relative pressure (P/Ps), the formula of Frenkel-Halsey: 
EQU t(A)=3.54[5/ln(Ps/P)].sup.1/3 
was employed. 
In the following, the present invention will be explained in further detail 
by way of Examples, but it is to be understood that the invention is not 
limited to these Examples.

COMATIVE EXAMPLE 1 
A polyolefin resin powder and an electroconductive carbon powder were 
uniformly mixed so that the latter constituted 30 weight %. The mixture 
was placed in an even thickness on the bottom of a metallic mold, of which 
the temperature was set by 10.degree. C. higher than the softening point 
of the resin, and then the mixture was heat-pressed to produce a 
carbon-plastic plate of a 10 cm square with a thickness of 1.0 mm. This 
plate was placed as the positive electrode in one compartment of a 
flow-type electrolytic cell, of which the separator was a cation-exchanges 
membrane. In the other compartment, a rolled zinc plate of a purity of 
99.99% was placed as the negative electrode. Though this negative 
compartment, a constant quantity of a negative aqueous electrolyte 
containing zinc bromide, 3.0 mol/l in concentration, and potassium 
chloride, 4.0 mol/l in concentration, was circulated. In the positive 
compartment, a positive electrolyte solution containing zinc bromide and 
potassium chloride, both in the same concentration as in the negative 
electrolyte, and additionally containing 3.0 mol/l bromine, was 
circulated. Then, at a current density of 40 mA/cm.sup.2 discharge test 
was carried out at room temperature. The single electrode potential of the 
positive electrode was observed with a silver-silver chloride reference 
electrode having a Luggin capillary. And also the bromine concentration in 
the positive electrolyte was observed. The results are shown in Table 1. 
The results when using a platinum plate as the positive electrode are also 
shown in Table 1. 
TABLE 1 
______________________________________ 
Positive Positive electrode potential (V) 
electrode Br.sub.2 2.0-3.0 M/l 
Br.sub.2 0.4-0.8 M/l 
______________________________________ 
Carbon- 0.512 0.121 
plastic 
(comparative 
example 1) 
Platinum 0.792 0.731 
(reference 
example) 
______________________________________ 
The value at bromine concentration 2.0-3.0 M/l corresponds to the initial 
and middle stage of discharge, and the value at 0.4-0.8 M/l corresponds to 
the last stage of discharge. It is seen that in the carbon-plastic 
electrode there is a large potential drop at the last stage of discharge. 
EXAMPLES 1-4 AND COMATIVE EXAMPLES 2-3 
We prepared several kinds of twill fabrics having different weights per 
area, using spun yarns of different yarn count numbers, composed of 2.0 d 
regenerated cellulosic fibers. Also, using spun yarns of different yarn 
count numbers, composed of 2.0 d regenerated cellulosic fibers, we 
prepared several kinds of interlock stitch knit fabrics having different 
weights per area. These woven or knit fabrics were immersed in an aqueous 
solution of ammonium secondary phosphate, then squeezed and dried so that 
the fabrics would contain 10% of ammonium secondary phosphate based on the 
fiber weight. Thereafter, the fabrics were heated in an inert gas current 
at 270.degree. C. for 30 minutes, and subsequently heated from 270.degree. 
C. to 850.degree. C., spending about 90 minutes. The fabrics were further 
treated for 30 minutes in a gas current which contained 40% by volume of 
steam, to produce porous carbon fiber woven fabrics (A) and knit fabrics 
(M) each having a weight per area of 45-50 g/m.sup.2. The fabrics 
subjected to activation treatment in steam for 60 minutes and given the 
same order of weight per area as (A) and (M) were named (B) and (N) 
respectively for woven and knit fabrics. Further, the starting material 
fabrics, from which (B) and (N) were obtained, were subjected to the same 
treatment as that gave (A) and (M). The thus-obtained porous carbon fiber 
fabrics were immersed in an aqueous solution of ferric chloride so that 
the fabrics would contain ferric chloride corresponding to 4.3 and 4.7 
weight % Fe, respectively. After drying, the fabrics were heated from 
100.degree. C. to 850.degree. C. in a nitrogen gas current containing 40 
volume % of steam. After being maintained in this condition for 15 
minutes, the fabrics were cooled in an inert gas. After being washed with 
a 1N HCl solution, the fabrics were rinsed with water and dried. The 
thus-obtained porous carbon fiber woven fabrics and knit fabrics, each 
having a weight per area of 45-50 g/m.sup.2, were named (C) and (P), 
respectively. The various porous carbon fiber fabrics thus obtained were 
each placed on the bottom of the metallic mold mentioned in Comparative 
Example 1, and the carbon-plastic powder mixture used in Comparative 
Example 1 was placed in an even thickness on the fabric, and heat-pressed 
to produce an electrode (positive pole) of a carbon-plastic plate, 10 cm 
square, 1 mm in thickness, on which surface the porous carbon fiber fabric 
was joined. 
Discharge experiments of the zinc-bromine battery which used the electrode 
of the present invention as the positive pole were carried out in the same 
way as in Comparative Example 1, and the results shown in Table 2 were 
obtained. 
It is understood from Table 2 that, when the positive electrode of the 
present invention was used, not only in the early stage of discharge 
(Br.sub.2 2.0-3.0 Mol/l concentration) but also in the last stage of 
discharge (Br.sub.2 0.4-0.8 Mol/l concentration), there was only small 
drop in the positive potential and the energy efficiency was maintained 
stably. 
TABLE 2 
______________________________________ 
Name of fabric Positive electrode 
joined to the Fiber potential (V) 
positive elec- 
TPV .sub.30.sup.1000 
density Br.sub.2 2.0- 
Br.sub.2 0.4- 
Re- 
trode (cc/g) (g/cc) 3.0 M/l 
0.8 M/l 
marks 
______________________________________ 
A 0.04 0.235 0.632 0.450 Comp. 
Ex. 2 
B 0.10 0.171 0.785 0.715 Ex. 1 
C 0.35 0.181 0.806 0.731 Ex. 2 
M 0.05 0.205 0.695 0.545 Comp. 
Ex. 3 
N 0.11 0.176 0.737 0.630 Ex. 3 
P 0.41 0.165 0.781 0.676 Ex. 4 
______________________________________ 
Weight per area of fabric: 45-50 g/m.sup.2 
EXAMPLES 5-6 AND COMATIVE EXAMPLE 4 
Regenerated cellulosic cut fibers (mono-filament denier: 2.mu., length: 3 
mm) were immersed into an aqueous ammonium secondary phosphate, 
centrifuged and dried so that the ammonium salt would be contained in the 
fibers in an amount of 9.5 weight % based on the fiber weight. The fibers 
were then heated in an inert gas current at 270.degree. C. for 30 minutes, 
and subsequently heated from 270.degree. C. to 850.degree. C., spending 
about 90 minutes. The fibers were further treated in a nitrogen gas 
current containing 40 volume % steam, for 30 minutes or 60 minutes to 
obtain porous carbon fibers A and B, respectively. A part of porous carbon 
fiber A were immersed in an aqueous solution of magnesium acetate so that 
fiber A would contain 2.1 weight % magnesium acetate so that fiber A would 
contain 2.1 weight % magnesium acetate based on the fiber weight. 
Thereafter, fiber A was dried at 90.degree. C. and heated from 100.degree. 
C. to 850.degree. C. in a nitrogen gas current containing 40 volume % 
steam. After being maintained in this condition for 25 minutes, fiber A 
was cooled in an inert gas current, washed with a 0.1N HCl solution, 
rinsed with water and dried to obtain porous carbon fiber C. 70 weight % 
respectively of fibers A, B and C thus obtained, 30 weight % of 
polypropylene single fibers, and small quantities of a viscosity 
increasing agent and a binder were mixed to prepare a paper-making 
solution. After paper making and drying, the mixture was heat-pressed to 
obtain papers AP, BP and CP (of which the weight per area was 50 
g/m.sup.2) containing fibers A, B and C, respectively. 
The three kinds of porous carbon fiber papers were placed on the bottom of 
the metallic mold used in Comparative Example 1, and on this the 
carbon-plastic powder mixture used in Comparative Example 1 was placed in 
an even thickness. The whole body was heat-pressed to obtain a compound 
electrode (positive electrode) composed of a carbon-plastic plate (10 mm 
square, 1 mm thick), on which surface the porous carbon fiber paper was 
joined. 
Discharge experiment of the zinc-bromine battery in which the electrode of 
the present invention was used as the positive electrode, was carried out 
in the same manner as in Comparative Example 1, and the results in Table 3 
were obtained. 
TABLE 3 
______________________________________ 
Porous 
carbon 
TPV .sub.30.sup.1000 of 
fiber 
Paper joined 
porous density Positive electrode 
to the carbon in the potential (V) 
positive fiber paper Br.sub.2 2.0- 
Br.sub.2 0.4- 
Re- 
electrode 
(cc/g) (g/cc) 3.0 M/l 
0.8 M/l 
marks 
______________________________________ 
AP 0.04 0.22 0.60 0.43 Comp. 
Ex. 4 
BP 0.10 0.21 0.75 0.69 Comp. 
Ex. 5 
CP 0.35 0.18 0.79 0.72 Comp. 
Ex. 6 
______________________________________ 
It is understood from Table 3 that, when the positive electrode of the 
present invention was used, not only in the early stage of discharge 
(Br.sub.2 2.0-3.0 Mol/l concentration) but also in the last stage of 
discharge (Br.sub.2 0.4-0.8 Mol/l concentration), there was small 
potential drop in the positive electrode and the energy efficiency was 
stably maintained.