JPH024990B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an electrolytic solution for a circulating electrolyte type zinc-bromine secondary battery, and more particularly to a composition of a negative electrode solution that suppresses the formation of zinc dendrites. Recently, zinc-bromine secondary batteries have been attracting attention and being developed from the viewpoint of effective energy utilization. For example, the first
An electrolyte circulation type zinc-bromine secondary battery as shown in the figure is used. Figure 1 shows its basic configuration, in which 1 is a single cell, 2 is a positive electrode chamber, 3 is a negative electrode chamber, 4 is a diaphragm (ion exchange membrane or porous thin film separator), 5 is a positive electrode, 6 is a negative electrode, 7 is a positive electrode liquid, 8 is a negative electrode liquid, 9 is a positive electrode liquid storage tank, 10 is a negative electrode liquid storage tank, and 11 and 12 are pumps. In terms of these electrolyte circulation type zinc-bromine secondary batteries, the zinc deposited on the negative electrode 6 surface shown in the figure during charging is caused by uneven electric field distribution near the negative electrode surface.
Due to disturbances in the flow of the negative electrode liquid, it is difficult to smooth the surface, and dendrites, which are dendrites, are often formed. Especially as the number of charge/discharge cycles increases, the following problem occurs. That is, since this dendrite-like precipitated zinc is very brittle, it easily falls off from the electrode, reducing the energy efficiency of the battery. Further, even if the dendrite does not fall off from the electrode, it continues to grow in a dendritic shape, penetrating and breaking the diaphragm 4, causing a short circuit with the positive electrode 5, and ultimately causing destruction of the battery. In order to prevent the above-mentioned drawbacks, it has been necessary to prevent the generation and length of dendrites by adding certain additives to the negative electrode liquid. Conventionally, nonionic surfactants, galvanizing brighteners, and the like have been used as dendrite inhibitors, but these have the problem of lack of stability when the number of charge/discharge cycles is large and over a long period of time. In particular, if an inexpensive porous thin film is used as the diaphragm, bromine molecules percolate and diffuse from the positive electrode chamber into the negative electrode chamber little by little, which tends to cause problems with bromine resistance to dendrite inhibitors, and it is difficult to charge and discharge many times over a long period of time. It was not possible to maintain stable performance over cycle use. Furthermore, even with bromine-resistant materials, as shown in FIG. 2, dendrites often occurred due to the formation of a non-uniform zinc oxide film on the surface of the zinc deposited on the negative electrode surface. FIG. 2 is a schematic diagram showing the mechanism by which dendrites are generated on an oxide film. FIG. 2-A shows the electrode surface at the end of charging during a short period of charging and discharging cycles, and metallic zinc Zn is deposited on the negative electrode 6. Second
- Figure 6 shows the electrode surface at the initial stage of discharge, and zinc ions Zn ⁺⁺ are eluted from the surface of the electrode 6. Second
-Figure C shows the situation at the end of discharge after many charge-discharge cycles, with a non-uniform oxide film on metallic zinc Zn.
Forms ZnO. Figure 2-D shows the situation when charging is restarted after the oxide film is formed, and metal zinc is selectively deposited on the non-uniform oxide film ZnO. Figure 2-E shows the situation in the middle stage of charging, and dendrite dendrites are generated on metallic zinc selectively deposited on the oxide film. In other words, when using bromine-resistant additives,
Even if dendrites are not formed while the number of charge/discharge cycles is small, as the number of cycles increases, dendrites are formed due to the oxide film generated during discharge, and it is necessary to prevent this. An object of the present invention is to provide an electrolyte composition for preventing the generation and growth of dendrites in an electrolyte circulation type zinc-bromine secondary battery as described above. The present inventors have investigated the factors behind the formation of dendrites in electrolyte circulation type zinc-bromine secondary batteries, and based on this, the above invention has been achieved by adding certain additives to the negative electrode electrolyte. This invention was discovered by discovering that this object could be achieved. The electrolytic solution of the zinc-bromine battery of the present invention contains divalent tin ions (Sn ⁺⁺ ) in a zinc bromine (ZnBr ₂ ) aqueous solution containing bromine and zinc as active materials, and also contains methyl, ethyl, and morpholinium. - An electrolytic solution characterized by containing 0.5 mol/bromide and methyl ethyl pyrrolidinium bromide each. Divalent tin ions contained in the electrolytic solution prevent the formation of a zinc oxide film formed during discharge, and suppress the generation of dendrites due to the formation. Furthermore, bromine complexing agents consisting of specific heterocyclic quaternary ammonium salts, namely methyl ethyl morpholinium and methyl ethyl pyrodinium bromide, which are contained in a specific amount in the electrolyte together with divalent tin ions, It works to capture bromine (generated in the positive electrode chamber during charging) diffused into the negative electrode chamber through the diaphragm (for example, a porous membrane) and to suppress the generation of dendrites. That is, in the present invention, the generation and growth of dendrites are prevented by using divalent tin ions and bromine-resistant bromine complexing agents methyl ethyl morpholinium and methyl ethyl pyrrolidinium bromide. There is. The negative electrode electrolyte composition of the zinc-bromine secondary battery is based on zinc bromide (ZnBr ₂ ), to which various divalent tin ions (Sn ⁺⁺ ) are added, and
Those with various added amounts of valent tin ions (Sn ⁺⁺ ), and A bromine complex forming agent with a large dendrite suppressing effect, methyl, ethyl, morpholinium,

[Formula] and bromide B methyl, ethyl, pyrodinium,

[Formula] We prepared a negative electrode electrolyte containing 0.5 mole of each bromide, performed charging and charge/discharge tests using these electrolytes, and then performed a cycle test to evaluate the electrode condition. As a result, Sn ⁺⁺ Regarding the point of suppressing the generation of dendrites during initial charging with a small number of charging/discharging cycles when only 100% is added to the negative electrolyte,
Sn ⁺⁺ concentration 1×10 ^-5 mol/~5×10 ^-4 mol/
Although it has some effect at (nearly saturation amount),
There was no effect at 5×10 ^-6 mol/. 5 more
Addition of Sn ⁺⁺ of 10 ^-6 mol/~1×10 ^-5 mol/,
When the above-mentioned methyl, ethyl, morpholinium, bromide and methyl, ethyl, pyrrolidinium, bromide complex forming agents are mixed in a fixed amount of 0.5 mol each, they are more effective than when Sn ⁺⁺ ion or the complex forming agent is used alone. It was seen. Furthermore, Sn ⁺⁺
ion and methyl, ethyl, morpholinium, 1 mole of bromide, or Sn ⁺⁺ ion and methyl,
A negative electrode electrolyte solution containing 1 mole of ethyl, pyrrolidinium, and bromide also gave similar or slightly inferior results to those described above. Next, we performed a charge-discharge-charge cycle test and found that compared to the electrode situation using a negative electrode electrolyte containing only a complexing agent, the
The electrode condition using a negative electrode electrolyte containing a complex forming agent mixed with 10 ^-6 mol/~5×10 ^-4 mol/Sn ⁺⁺ and 0.5 mol/each was much better, and the initial charge The current electrodeposition condition could be maintained during subsequent charging, demonstrating that divalent tin ions are effective. The amount added is 1×10 ^-5 mol/
~ Saturation amount is desirable, SnCl ₂ as Sn ⁺⁺ ,
SnBr ₂ , SnSO ₄ , etc. can be used. In addition, each of the above-mentioned A and B complex forming agents was added at 0.5 mol/
Even after repeating 20 charge/discharge cycles (6 hours) using an electrolytic solution with a Sn ⁺⁺ concentration of 1 × 10 ^-5 mol/~saturation amount, the same electrodeposition situation as in the initial stage was observed, indicating that divalent tin The effectiveness of adding the ion and bromide complexing agents methyl, ethyl, morpholinium, and methyl, ethyl, pyrrolidinium, bromide was demonstrated. Next, examples of the present invention will be described below. Example 1 A liquid circulating zinc-bromine secondary battery as shown in Fig. 1 was used, and the negative electrode electrolyte was based on 3 mol of zinc bromide and 5 x 10 ^-6 mol/ to 5 of Sn ⁺⁺ as an oxidation inhibitor. ×10 ^-4 mol/, and add methyl, ethyl, morpholinium, bromide, and predetermined amounts (0.5 mol/each) of methyl, ethyl, morpholinium, and bromide as bromine complex forming agents with a large dendrite suppressing effect. Using the prepared material, it was charged at a current density of 40 mA/cm ² for 6 hours to electrodeposit zinc on a carbon plastic electrode, and the condition of the electrodeposited surface was evaluated according to the following criteria. Evaluation Criteria A: Excellent smooth surface. B: A small amount of unevenness is present, C: A large amount of dendrites are present. Note that for A and B, A is optimal for practical use. The results are shown in Table 1.

[Table] 3-1 and 2 are comparative examples. As shown in the table above, Sn ⁺⁺ alone cannot be expected to be very effective in suppressing dendrites.
In addition, a small amount, that is, 5 × 10 ^-6 mol/(Experiment number 1-
The addition of 5) resulted in the generation of a large amount of dendrites as in the comparative example (experiment number 3-1). Example 2 Continuing from Example 1, after initial charging for 3 hours at a current density of 40 mA/cm ² using an electrolytic solution with the same composition,
A 3-cycle test of discharging and charging, in which the battery was discharged for 2 hours and then charged for 2 hours, was conducted, and the condition of the electrodeposited surface was observed and evaluated based on evaluation criteria. Table 2 shows the results.

[Table] As shown in the above table, 0.5 mol/each of complex forming agents A and B were added, and further Sn ⁺⁺ concentration was 1×10 ^-5 to 5×10 ^-4
It can be seen that Sn ⁺⁺ has the ability to maintain the initial electrodeposition state when using a zinc bromide electrolyte containing mol. Example 3 Using the electrolyte compositions that obtained A and B ratings in Example 2 (experiment numbers 1-1c to 4c and 2-1c to 4c), the batteries were charged for 6 hours at a current density of 30 mA/cm ² and then discharged for 6 hours. As a result of a cycle test, the electrodeposited surface with the above composition maintained the same electrodeposited surface even after 20 cycles. In the present invention, by containing divalent tin ions and bromine complexing agents methyl ethyl morpholinium and methyl ethyl pyrrolidinium bromide in the electrolyte solution of the zinc-bromine battery, during discharge, It is possible to prevent the formation of a zinc oxide film on the negative electrode, and almost completely prevent the generation and growth of dendrites in zinc-bromine batteries. By using the electrolyte of the present invention, it is possible to suppress the generation and growth of dendrites even when repeated charge/discharge cycles.
The charge/discharge life of a bromine secondary battery can be extended.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram showing the basic structure of an electrolyte circulation type zinc-bromine secondary battery, and FIG. 2 is a schematic diagram showing the mechanism by which dendrites are generated on an oxide film. 1: Single cell, 2: Positive electrode chamber, 3: Negative electrode chamber, 4:
Diaphragm, 5: positive electrode, 6: negative electrode, 7: positive electrode electrolyte,
8: negative electrode electrolyte, 9: positive electrode liquid storage tank, 10: negative electrode liquid storage tank, 11, 12: pump, Zn: precipitated zinc,
ZnO: oxide film, ZnD: dendrite.

Claims (1)

[Claims] 1 The electrolyte circulated to the negative electrode side of a circulating electrolyte zinc-bromine battery contains divalent tin ions and contains methyl, ethyl, morpholinium,
An electrolytic solution for a zinc-bromine battery, characterized in that it contains 0.5 mol/each of bromide and methyl ethyl pyrrolidinium bromide.