JPH04312764A - Separator for zinc-bromine battery - Google Patents

Abstract

PURPOSE:To suppress self-discharge to improve energy efficiency and durability of a battery by carrying fluorocarbon-based cation-exchange resin on hydrophilic particles-containing finely porous polymer substrate in highly dispersed condition. CONSTITUTION:Fluorocarbon-based cation-exchange resin is carried on a finely porous polymer substrate which is made of an acid-resistant and bromine- resistant material such as silica, etc., and contains hydrophilic particles having a shape able to be mixed. Here, as the ion-exchange resin, ion-exchangeable resin having a specified chemical formula is used and at the time of carrying the resin on the substrate, a solution using a volatile organic solvent is used. By using a separator 2 prepared in the way for a battery main body 1, transmission of bromide ion is effectively suppressed, so that while voltage efficiency of the battery is prevented from lowering, Coulomb's efficiency is increased and energy efficiency of the battery is improved and at the same time owing to the durability of the ion-exchange resin to bromine, the battery can have a long life.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a highly durable separator used in zinc-bromine batteries with high energy efficiency.

0002

[Prior art] Zinc-bromine batteries are metal batteries with electrolyte circulation.
It is a type of halogen battery that uses an aqueous zinc bromide solution as an electrolyte, and is a promising candidate battery for secondary batteries for power such as electric vehicle drive sources.

[0003] Since the reactants and constituent materials of such zinc-bromine batteries are cheap, they are expected to be low-cost. Furthermore, the generated voltage is high and the reversibility of the electrode reaction is good, so they are used not only in electric vehicles but also in other applications. It is about to be put into practical use in a wide range of fields.

[0004] Such a zinc-bromine battery was disclosed in Japanese Patent Application Laid-Open No. 1983
-122835, Japanese Unexamined Patent Publication No. 58-199167, and US Pat. No. 4,105,829.

The electrolyte used in this zinc-bromine battery includes, in addition to zinc bromide, which is an active material, a complexing agent that reacts with bromine to form a complex compound that is insoluble in the electrolyte and has a higher specific gravity than the electrolyte. An aqueous solution containing KCl or NH4 is used, and if necessary, KCl or NH4 is added to improve the conductivity of the electrolyte.
Electrolytes with added supporting electrolytes such as Cl may also be used. As the complexing agent, methyl ethylmorpholinium bromide, methyl ethylpyridinium bromide, etc. are used.

A diagram of the principle of a zinc-bromine battery is shown in FIG. The electrode reaction tank 1 is partitioned by a separator 2 into a positive electrode chamber 1a and a negative electrode chamber 1b. This separator prevents the positive electrode 3 and the negative electrode 4 from coming into direct contact and causing a short circuit, and bromine, which is the active material of the positive electrode, moves to the negative electrode side and causes a chemical reaction with zinc, which is the active material of the negative electrode, resulting in self-discharge. It is intended to suppress this. The electrode chambers 1a and 1b are connected to a positive electrode liquid storage tank 7 and a negative electrode liquid storage tank 8 by pipes 5 and 6, respectively.
are connected to form two electrolyte circulation paths. Each electrolytic solution is pumped into the electrode chambers 1a, 1b by pumps 9, 10 provided in the pipes 5, 6. Further, a bromine complex compound storage tank 11 is formed in the positive electrode liquid storage tank 7, and stores the bromine complex compound generated in the positive electrode chamber 1a, and supplies a part of it into the pipe 5 through a valve 12. do.

By the way, when this battery is charged, oxidation of bromine ions occurs at the positive electrode, reduction of zinc ions occurs at the negative electrode, production of bromine occurs at the positive electrode, and electrodeposition of zinc occurs at the negative electrode. Bromine generated at the positive electrode reacts with the complexing agent in the electrolytic solution to form a complex compound 13, which is separated and stored in the bromine complex compound storage tank 11. However, this complexing agent does not capture all of the generated bromine, and due to dissociation equilibrium, it usually
．． 05-0.2 mol/l of free bromine remains in the catholyte. This free bromine diffuses to the negative electrode liquid side through the separator 2 and reacts directly with zinc, which is the negative electrode active material. This becomes self-discharge, and the amount of charged electricity is lost. Therefore, as the amount of bromine passing through the separator increases, the ratio of the amount of discharged electricity to the amount of charged electricity, that is, the coulombic efficiency of charging and discharging, decreases. As described above, the bromine permeability of the separator 2 greatly affects the coulombic efficiency of the battery.

Free bromine Br2 passing through the separator in the electrolytic solution mostly forms complex ions with bromine ions Br-, and behaves as negatively charged complex ions like Br3-. Therefore, the permeation of free bromine can be suppressed by using a cation exchange membrane as a separator, which easily passes unionized or positively charged cations but does not allow anions to pass through (Journ
al of Power Sources 7, p3
13 (1979)).

However, in cation exchange membranes, the passage of anions is excessively suppressed and the flow of ions is not smooth, so that the internal resistance of the battery increases compared to ordinary separators such as microporous membranes. This in turn caused a problem in that the voltage efficiency decreased and eventually the energy efficiency of the battery deteriorated. Additionally, cation exchange membranes have the disadvantage of being expensive.

Furthermore, if the thickness of the cation exchange membrane is made thinner, the membrane resistance in the electrolyte solution can be reduced, although the actual cation exchange performance will be lowered. However, in this case, the mechanical strength of the membrane decreases, so that it cannot be used practically.

[0011] As described above, separators for zinc-bromine batteries have had the contradictory problem that if the permeability of bromide ions is suppressed, the internal resistance increases, and if the resistance of the membrane is reduced, the amount of permeation of bromide ions increases. .

[0012] Therefore, an attempt was made to obtain a low-cost membrane that suppresses bromine permeability without significantly increasing electrical resistance by imparting some cation exchange properties to a silica-polyolefin microporous membrane that has low membrane resistance. An attempt was made. For example, the U.S. Department of Energy introduced SO3 - groups into a separator membrane by impregnating a microporous membrane with sulfonated polysulfone (hereinafter abbreviated as SPS), giving it cation exchange membrane properties (Report No. SAND-89). -1914C).

However, since this SPS is concentrated and deposited on the surface of the microporous membrane, it tends to block the pores of the microporous membrane, and when impregnated in an amount that provides sufficient cation exchangeability, it causes an increase in electrical resistance. Moreover, free bromine oxidizes SPS itself and makes it brittle, so it could not be used as a separator for zinc-bromine batteries.

[0014]

[Problems to be Solved by the Invention] Therefore, although the present invention can effectively suppress the permeation of free bromine, the increase in membrane resistance in the electrolytic solution is small, and moreover, it is possible to effectively suppress the permeation of free bromine. The purpose of the present invention is to provide a separator for zinc-bromine batteries that is stable even in liquid.

[0015]

[Means for Solving the Problems] The separator for a zinc-bromine battery of the present invention has a substrate made of a polymer having pores and containing hydrophilic particles, and a separator supported on the substrate in a highly dispersed state. fluorocarbon ion exchange resin.

[0016]

[Action] Most of the free bromine Br2 in the electrolyte of a zinc-bromine battery forms a complex ion with the bromide ion Br-.
They have negative charges such as r3 − and Br5 − and are dissolved in the electrolyte. Therefore, using a cation exchange separator will suppress the permeation of these negatively charged bromine complex ions.

On the other hand, in order to charge and discharge a zinc-bromine battery, ions as charge carriers must pass through the separator, and if this ion permeation is suppressed, the internal resistance increases.

Therefore, if a complete cation exchange membrane is used as a separator, the permeation of bromine can be sufficiently suppressed, but the increase in electrical resistance will also be large, making it impractical. Practically speaking, it is necessary to have appropriate cation exchange membrane properties in order to suppress the permeation of free bromine without significantly increasing electrical resistance.

[0019] Polymer substrate having pores used in the present invention (
Microporous polymers have improved wettability with electrolyte by containing hydrophilic particles, and therefore have low membrane resistance in the liquid. The separator membrane of the present invention has the following effects because the cation exchange resin is supported on the substrate in a highly dispersed state.

The fluorocarbon ion exchange resin used in the present invention has sufficient durability against bromine, which is a halogen element. Furthermore, the hydrophilic particles in the polymer base of the present invention impart hydrophilicity to the separator membrane. The ion exchange resin of the present invention has an affinity for the exposed hydrophobic parts of the polymer fabric, mainly in the gaps between the hydrophilic particles embedded in the polymer base, and is in a highly dispersed state where it appears to be adsorbed and supported. . Therefore, the supported ion exchange resin is not locally unevenly distributed and does not block the pores of the microporous polymer substrate. Therefore, ions such as Zn2+, K+, etc. in the electrolytic solution can pass through these pores smoothly, and the membrane resistance of the separator in the electrolytic solution does not become very large.

[0021] Furthermore, since the ion exchange membrane supported in a highly dispersed state imparts ion exchange properties to the entire separator, bromine (Br2) is complex-ionized Br3.
-, Br5-, etc. are suppressed, and the permeation of bromine, which can lead to self-discharge, is also reduced.

[0022]

Effects of the Invention The separator of the present invention has a microporous membrane with excellent bromine resistance and low membrane resistance in an electrolytic solution, which contains hydrophilic particles in the base polymer, and is highly dispersed throughout the membrane. The separator is made of a fluorine-based cation exchange resin with good bromine resistance supported on the separator, and the electrical resistance of the separator increases only slightly, effectively suppressing the permeation of bromine (bromine anion complex). Therefore, by using the separator of the present invention, it is possible to increase the coulombic efficiency while suppressing a decrease in the voltage efficiency of the battery as much as possible, and the energy efficiency of the battery is improved.

[0023] Furthermore, since the fluorine-based cation exchange resin has good durability against bromine, the above effects are maintained for a long period of time. Therefore, it becomes possible to realize a long-life zinc-bromine battery.

[0024]

[Example] The present invention will be explained in more detail using specific examples and examples.

(Specific Example 1) The hydrophilic particles to be present in the substrate are particles or powder made of acid-resistant and bromine-resistant substances such as oxide particles such as silica and clay substances, and organic particles such as polyvinyl alcohol. , and other forms that can be mixed with the polymer substrate can be used. Particularly preferred are fine particles of silica.

(Specific Example 2) As the fluorocarbonaceous ion exchange resin used in the present invention, an ion exchange resin represented by the general formula (1) is suitable.

[0027]

[Chemical formula 1]

[0028] l is 5 to 13.5, m is 0, 1, 2, etc.
..., n is 1, 2, ..., x is about 1000. A typical example of such a resin is Nafion (a registered trademark of Du Pont).

(Specific Example 3) A separator for a zinc-bromine battery in which a fluorocarbon ion exchange resin is supported in a highly dispersed state on a substrate made of a polymer having pores and containing hydrophilic particles. The manufacturing method includes a step of containing a fluorocarbonaceous ion exchange resin solution in a substrate formed of a polymer having pores and containing hydrophilic particles, and fixing the contained ion exchange resin to the substrate. It consists of a process.

In the process of impregnating a fluorocarbon ion exchange resin solution into a substrate made of a polymer having pores, since the fluorocarbon ion exchange resin is in solution, it is easy to coat the surface or inside of the polymer substrate. The fluorocarbon ion exchange resin can be supported not only on the surface of the polymer substrate but also on the walls of the pores that are intricately present inside the substrate.

As the ion exchange resin solution, one using a volatile organic solvent is suitable. Further, a mixed solvent with water may be used. Examples of suitable organic solvents include alcohols. Its concentration is not particularly limited.

In addition, in the step of fixing the ion exchange resin to the substrate, the fluorocarbon polymer portion of the fluorocarbon ion exchange resin is removed from the fluorocarbon polymer portion of the polymer substrate containing hydrophilic particles without the hydrophilic particles. The material is selectively deposited on exposed areas of the fabric and crystals grow there. The selective precipitation is thought to be due to the affinity between the hydrophobic parts. Therefore, the fluorocarbon ion exchange resin is firmly supported in a highly dispersed state on the polymer base forming the separator.

According to this manufacturing method, a substrate formed of a polymer containing hydrophilic particles and having pores, and a fluorine supported in a highly dispersed state on the surface and/or in the pores of the substrate are used. Zinc consisting of carbonaceous ion exchange resin
Separators for bromine batteries can be easily produced.

(Specific Example 4) In the step of impregnating the substrate formed of the polymer of Specific Example 3 with a fluorocarbonaceous ion exchange resin solution, the impregnation method is not particularly limited, but the impregnation method may be a dipping method,
A brush coating method, a spray method, etc. are used. After impregnation,
In order to remove the solvent, treatment such as reduced pressure treatment may be performed.

(Specific Example 5) In the step of fixing the ion exchange resin to the substrate in Specific Example 3, the ion exchange resin is crystallized and fixed to the polymer substrate by heat treatment. The heat treatment temperature is 1
The temperature is preferably 50°C or lower. If the temperature exceeds 150°C, the hydrophilic side chains of the ion exchange resin will be damaged and the ion exchange properties will be impaired.

(Example 1) The chemical structural formula of the cation exchange resin solution used in this example is

[0037]

[Chemical formula 1]

[0038] where l is 5 to 13.5, m is 1 to 3,
It was obtained by dissolving a fluorocarbonaceous cation exchange resin in which n is about 1 and x is about 1000 in 5% by weight ethyl alcohol. This solution was further diluted with ethyl alcohol to prepare three types: a 2.5% solution and a 1% solution. A silica-polyolefin microporous membrane having a polymer base was immersed in each solution, placed in a vacuum desiccator and degassed for 2 hours, and then the microporous membrane was pulled out of the solution and dried in air. Thereafter, it was dried for 2 hours in a constant temperature bath at 80° C. to obtain the separator of this example.

Furthermore, as a comparative example, a separator carrying SPS (sulfonated polysulfone) was prepared. Dissolve SPS in dimethylformamide to 10% by weight
A solution was prepared. A silica-polyolefin microporous membrane was immersed in this SPS solution for 30 minutes in a vacuum desiccator.
After degassing for 2 hours, the microporous membrane was taken out of the solution and dried in air, and then further dried for 2 hours in a constant temperature bath at 80° C. to obtain a separator of a comparative example.

The dispersion state of the ion exchange resin in the separator of this example and the separator of the comparative example was compared by examining the sulfur distribution of SO3 in the ion exchange resin using EPMA. The results are shown in FIGS. 2 and 3. In the separator of this example, sulfur is uniformly distributed from the front surface to the back surface. In contrast, the SPS of the comparative example was concentrated near the surface. For this reason, the micropores on the surface of the microporous membrane that is the separator base are blocked, and the path for ions to pass is narrowed, making it difficult for not only anions but also cations to pass through, resulting in an unnecessarily increased resistance.

It was also found that SPS has insufficient bromine resistance compared to the ion exchange resin of this example. In other words, a membrane made only of SPS was prepared, and free bromine was added to 0.2 m
When immersed for 144 hours in a solution containing OL/L, it turned yellow and became extremely brittle and easily broken. This could also be confirmed from the IR spectrum.

Evaluation of electric resistance and bromine permeability coefficient of the separator of the present example, the SPS-impregnated separator as a comparative example, and the separator that was simply dried in an 80° C. thermostat for 2 hours without impregnating it with anything. Charge/discharge evaluation was performed on an 8-cell battery.

The electrical resistance measurement cell used for electrical resistance measurement is shown in FIG. The measurement cell includes a separator 14 and an inner plate 17.
, 18, and carbon electrodes 22, 23 are provided between the inner plates 17, 18 and the outer plates 19, 20. Further, electrolyte chambers 15 and 16 are provided on both sides of the separator 14 between the carbon electrodes 22 and 23. Note that the reference numeral 21 is a measurement meter (LCR meter). The resistance value of the separator is determined by measuring the resistance between the carbon electrodes with alternating current when the separator is sandwiched in the center of the measurement cell and when it is not sandwiched, and calculating from the difference. In addition, the measurement electrolyte contains 2 mol/
1 of zinc bromide aqueous solution was used.

FIG. 5 shows the measurement cell used for measuring the bromine permeability coefficient.
Shown below. This cell is partitioned into two rooms by a separator 14 in the center. Note that the reference numeral 27 is a stirrer. One concentration chamber 25 contains 2 mol/l ZnBr2
, 0.2 mol/l Br2 and the other dilution chamber 26 contains a bromine concentrate containing 2 mol/l ZnBr2, 0.2 mol/l ZnBr2.
The same amount of 02 mol/l Br2 was added to each chamber, and the bromine concentration in both chambers was measured over time. Bromine is in the concentrated chamber 2
5 to the thinner dilute chamber 26, the rich chamber 25
The bromine concentration in the dilution chamber 26 decreases over time, and the bromine concentration in the dilution chamber 26 increases over time. The change at this time is expressed by the following equation based on Fick's first law.

-1/2Ln|(2C-C0)/C0
|=APt/V (2)

P: Permeability coefficient (cm/sec) A
: Separator area (cm2) V : Electrolyte volume (cm3) C : Bromine concentration at time t (mol/l) C0
: Initial bromine concentration (mol/l) t : Time (sec)

For each of the above two chambers, the above formula (2)
Plotting the left side of with respect to time yields a straight line,
The bromine permeability coefficient is calculated from the slope.

Table 1 shows the measurement results of electrical resistance and bromine permeability coefficient together with the impregnated amount (weight increase). Also,
Figures 6 and 7 show the relationship between the amount of impregnation, the area resistance, and the bromine permeability coefficient.

[0049]

[Table 1]

As is clear from Table 1 and FIGS. 6 and 7, the fluorocarbon-impregnated separator of this example and the SPS of the comparative example
It can be seen that in both impregnated separators, as the amount of impregnation increases, the electrical resistance gradually increases, and the bromine permeability coefficient tends to decrease significantly. However, when comparing the separator of the present example with the separator of the comparative example, the decrease in the bromine permeability coefficient is greater in the present example, although the increase in electrical resistance is smaller. In other words, in this example, the bromine permeation is less than 1/5 with an increase in resistance of about 30% (comparing samples No. R1 and No. 3), whereas in the comparative example, the electrical resistance is about twice as high. Even so, the bromine permeability coefficient is not 1/5 (comparing sample Nos. R1 and R4). As described above, the separator of this example was an excellent separator for zinc-bromine batteries with low electrical resistance and a large effect of suppressing bromine permeation.

Furthermore, the above separator was used in an 8-cell stack battery with an electrode area of 900 cm2, and the current density was set to 2.
Table 2 shows the results of battery efficiency when charging and discharging at 0 mA/cm2. As is clear from the table, in the case of the SPS impregnated separator of the comparative example, there is almost no effect when the amount of impregnation is small, and when the amount of impregnation is increased (sample No. R4), the voltage efficiency decreases significantly, so the energy efficiency is only about 1%. only increases. On the other hand, in the case of the fluorocarbon-impregnated separators of this example (sample Nos. 1, 2, and 3), the voltage efficiency hardly changes and the coulomb efficiency improves, so the energy efficiency improves by up to 3%. .

[0052]

[Table 2]

(Example 2) Sol as a fluorocarbon solution
5% Na manufactured by Ution Technology
A separator was prepared by impregnating a silica-polyolefin microporous membrane with Nafion in the same manner as in Example 1 except that a Nafion (Nafion is a registered trademark of Du Pont) solution was used. However, in this example, Na
The fion solution was used as it is without being diluted, and no deaeration was performed during immersion, and the immersion was merely immersed for 2 hours. Note that, after immersion, the membrane taken out was once dried in air as in Example 1, and then dried in a constant temperature bath at 80° C. for 1 hour. The measurement results of the electrical resistance and bromine permeability coefficient of the membrane obtained in this example are shown in Table 3 (Sample No. 4). The film of this example has an electrical resistance of 13%.
However, the bromine permeability coefficient was less than half, indicating that the Coulombic efficiency could be improved without reducing the voltage efficiency too much.

Furthermore, the distribution of sulfur in the separator of this example was examined by EPMA in the same manner as in Example 1. As a result, it was found that sulfur was uniformly distributed throughout the thickness of the separator, and Nafion was distributed throughout the microporous membrane. Furthermore, when the bromine resistance was evaluated in the same manner as in Example 1, no signs of deterioration were observed either with the naked eye or on an IR chart.

[0055]

[Table 3]

(Example 3) A 2.5% Nafion solution was applied with a brush to both sides of a silica-polyolefin microporous membrane, and after drying in air, the same solution was applied again with a brush. After repeating this operation five times, it was dried for 1 hour in a constant temperature bath at 80° C. to obtain the separator of this example. The measurement results of the electrical resistance and bromine permeability coefficient of the separator of this example are shown in Table 3 (Sample No. 5). In this example, the electrical resistance is approximately 40%.
However, the bromine permeability coefficient was reduced to one-fourth or less, and the Coulombic efficiency could be improved without significantly lowering the voltage efficiency.

Example 4 A 2.5% Nafion solution was applied to both sides of a silica-polyolefin microporous membrane using an atomizer spray. After drying in air, the above solution was applied again by spraying. After repeating this operation five times, it was dried for 1 hour in a constant temperature bath at 80° C. to obtain the separator of this example. The measurement results of the electrical resistance and bromine permeability coefficient of the membrane obtained in this example are shown in Table 3 (Sample No. 6). Although the electrical resistance of the membrane of this example increased by about 30%, the bromine permeability coefficient was less than one-fourth, and the Coulombic efficiency could be improved without significantly lowering the voltage efficiency.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of a zinc-bromine battery.

FIG. 2 is an EPMA output diagram showing the sulfur distribution state of the separator of this example.

FIG. 3 is an EPMA output diagram showing the sulfur distribution state of a separator impregnated with SPS of a comparative example.

FIG. 4 is a schematic diagram of a cell for measuring electrical resistance of a separator used in this example.

FIG. 5 is a schematic diagram of a cell for measuring the bromine permeability coefficient of the separator used in this example.

FIG. 6 is a diagram showing the relationship between the amount of cation exchange resin impregnated and the electrical resistance of the separator.

FIG. 7 is a diagram showing the relationship between the amount of cation exchange resin impregnated and the bromine permeability coefficient of the separator.

[Explanation of symbols]

1 Battery body 1a Positive electrode side reaction tank 1b Negative electrode side reaction tank 2, 14 Separator 3 Positive electrode 4 Negative electrode 5　Positive electrode liquid piping 6 Negative electrode liquid piping 7 Positive electrode liquid storage tank 8 Negative electrode liquid storage tank 9, 10 Electrolyte pump 11 Bromine complex compound storage tank 12 Valve 15, 16 Electrolyte chamber 17,18 Inner plate 19,20 Outer panel 21 LCR meter 22, 23 Carbon electrode 25 Concentrated liquid chamber 26 Dilute liquid chamber 27 Starla

Claims (1)

[Claims] Claim 1: Zinc-bromine comprising a substrate formed of a polymer having pores and containing hydrophilic particles, and a fluorocarbon ion exchange resin supported on the substrate in a highly dispersed state. Battery separator.