A zinc-bromine flow battery positive electrode homogeneous electrolyte and application thereof

Abstract

The application discloses a zinc-bromine flow battery positive electrode homogeneous electrolyte, which comprises a main salt containing bromine ions and a complexing agent, and the complexing agent is selected from cyclodextrin derivatives, and is specifically selected from one or more of methylated cyclodextrin, hydroxyethyl cyclodextrin, hydroxypropyl cyclodextrin, carboxymethyl cyclodextrin, sulfate cyclodextrin, sulfonate cyclodextrin and phosphate cyclodextrin; in the zinc-bromine flow battery positive electrode homogeneous electrolyte, the mass fraction of the complexing agent is 12-30%. The zinc-bromine flow battery positive electrode homogeneous electrolyte disclosed in the application effectively identifies and complexes polybromide and bromine molecules by using a special complexing agent, thereby significantly improving the inhibition capacity on the shuttling effect; voltage polarization is small, voltage efficiency is high; under the condition of only using a low-cost microporous polymer diaphragm, a zinc-bromine flow battery with excellent CE, VE and EE is obtained, and the cost of the zinc-bromine flow battery is significantly reduced.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and more particularly to a homogeneous electrolyte for the positive electrode of a zinc-bromine flow battery and its application. Background Technology

[0002] Positive electrode active material based on Br2 / Br - Zinc-bromine flow batteries with redox pairs are currently a research hotspot in the field of novel energy storage batteries. They offer high energy density and efficiency, support fast charging and deep charge / discharge, are low-cost, highly safe, and have minimal environmental impact, making them suitable for applications such as grid peak shaving, wind power generation, and regional power supply. However, highly reactive elemental bromine rapidly permeates the separator to the zinc anode surface, reacting chemically with zinc and causing severe self-discharge. Furthermore, the volatility and corrosiveness of elemental bromine lead to irreversible capacity decay and shorten battery life. Therefore, introducing bromine complexing agents into the electrolyte to reduce free bromine is a highly effective strategy for mitigating self-discharge and improving the coulombic and energy efficiency of zinc-bromine flow batteries.

[0003] In zinc-bromine flow batteries, halogenated quaternary ammonium salts, such as N-ethyl bromide and methylpyrrolidine (MEP), are widely used as bromine complexing agents. These quaternary ammonium salts complex with polybrominates or bromine molecules to form an oil phase with a higher density than the bulk aqueous electrolyte. This oil phase deposits at the bottom of the positive electrode, affecting reaction kinetics and reducing the voltage efficiency of the zinc-bromine flow battery. Furthermore, the bromine complexes dissociate at 50°C, failing to effectively complex bromine and limiting the operating temperature range of the zinc-bromine flow battery.

[0004] Therefore, developing novel, highly efficient homogeneous bromine complexing agents is key to suppressing self-discharge and improving the energy efficiency of zinc-bromine flow batteries. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention discloses a homogeneous electrolyte for the positive electrode of a zinc-bromine flow battery. A special complexing agent is used to effectively identify and complex polybrominates and bromine molecules, significantly improving the ability to suppress the shuttle effect. Furthermore, it exhibits low voltage polarization and high voltage efficiency. By using only a low-cost microporous polymer membrane, a zinc-bromine flow battery with excellent CE, VE, and EE is obtained, significantly reducing the cost of zinc-bromine flow batteries.

[0006] The specific technical solution is as follows:

[0007] A homogeneous electrolyte for the positive electrode of a zinc-bromine flow battery includes a bromide-containing main salt and a complexing agent.

[0008] The complexing agent is selected from cyclodextrin derivatives;

[0009] The cyclodextrin derivative is selected from one or more of methylated cyclodextrin, hydroxyethyl cyclodextrin, hydroxypropyl cyclodextrin, carboxymethyl cyclodextrin, sulfate cyclodextrin, sulfonate cyclodextrin, and phosphate cyclodextrin.

[0010] In the homogeneous electrolyte of the zinc-bromine flow battery positive electrode, the mass fraction of the complexing agent is 12-30%.

[0011] This invention discloses, for the first time, the use of cyclodextrin in the preparation of a homogeneous electrolyte for the positive electrode of a zinc-bromine flow battery. Experiments revealed that cyclodextrin molecules can form inclusion compounds with dissolved bromine in the electrolyte. However, directly using α-cyclodextrin or β-cyclodextrin did not significantly improve the ability to suppress the shuttle effect. Extensive testing revealed that the poor improvement might be due to issues with solubility and the cyclodextrin's structure. Using structurally modified cyclodextrin derivatives increased its solubility, thus significantly improving the ability to suppress the shuttle effect. However, the experiments also showed that this conclusion only applied to β-cyclodextrin derivatives. When using α-cyclodextrin derivatives, even with increased solubility, the ability to suppress the shuttle effect was comparable to that of α-cyclodextrin, showing no significant improvement.

[0012] The experiment also revealed that the state of the electrolyte changed depending on the mass fraction of the complexing agent in the electrolyte. When the mass fraction was less than 12%, the positive electrode electrolyte underwent phase separation, with a dark reddish-brown bromine phase appearing at the bottom. When the mass fraction was greater than 30%, the positive electrode electrolyte underwent phase separation, with the bromine phase unevenly distributed and concentrated near the positive electrode. When the mass fraction was 12–15%, the electrolyte remained homogeneous, but the color turned yellow; when the mass fraction was 25–30%, the electrolyte was metastable, with uneven color distribution, and the color was darker near the positive electrode.

[0013] Preferably, the mass fraction of the complexing agent in the homogeneous electrolyte of the zinc-bromine flow battery positive electrode is 15-25%. When the mass fraction of the cyclodextrin derivative is between 15-25%, the cyclodextrin derivative can combine with free bromine in the electrolyte and present a homogeneous overall phase without obvious phase separation.

[0014] Further preferably, the mass fraction of the complexing agent is 20-25%, more preferably 20%.

[0015] With continuous optimization of the above parameters, the ability of the complexing agent to suppress the shuttle effect gradually increases, and the final zinc-bromine flow battery has higher CE, VE and EE.

[0016] Experiments have shown that changes in the types of substituents in cyclodextrin derivatives can affect not only their own solubility but also their binding affinity to dissolved bromine.

[0017] Preferably, the complexing agent is selected from one or more of 2-methyl-β-cyclodextrin, 2-hydroxyethyl-β-cyclodextrin, 2-hydroxypropyl-β-cyclodextrin, 2,3-hydroxypropyl-β-cyclodextrin, 3-carboxymethyl-β-cyclodextrin, sodium β-cyclodextrin sulfate, sodium β-cyclodextrin sulfonate, and sodium β-cyclodextrin phosphate;

[0018] Further preferably, it is one or more of 2-hydroxyethyl-β-cyclodextrin, 2-hydroxypropyl-β-cyclodextrin, 2,3-hydroxypropyl-β-cyclodextrin, and sodium β-cyclodextrin sulfonate; more preferably, it is 2-hydroxypropyl-β-cyclodextrin and / or 2,3-hydroxypropyl-β-cyclodextrin.

[0019] With continuous optimization of the types of complexing agents, the ability of complexing agents to suppress the shuttle effect has gradually increased, resulting in higher CE, VE, and EE of the final zinc-bromine flow battery.

[0020] The homogeneous electrolyte for the positive electrode of a zinc-bromine flow battery disclosed in this invention:

[0021] The bromide-containing main salt is selected from one or more of zinc bromide, potassium bromide, and sodium bromide.

[0022] The concentration of bromide ion main salt in the homogeneous electrolyte of the zinc-bromine flow battery positive electrode is 0.1–5.0 mol / L; preferably 1–3 mol / L.

[0023] The homogeneous electrolyte at the positive electrode of the zinc-bromine flow battery also includes a conductive support agent.

[0024] The conductive support agent is selected from one or more of potassium chloride, ammonium chloride, sodium chloride, and zinc chloride;

[0025] The concentration of the conductive support in the homogeneous electrolyte of the positive electrode of the zinc-bromine flow battery is 0.5–5.0 mol / L; preferably 2–3 mol / L.

[0026] The zinc-bromine flow battery positive electrode electrolyte disclosed in this invention needs to contain both bromide ions and zinc ions. When the bromide-containing main salt is selected from zinc bromide, the added conductive support can be selected from types that do not contain bromine, such as potassium chloride and sodium chloride. When the bromide-containing main salt does not contain zinc ions, the added conductive support needs to be selected from types that contain zinc ions, such as zinc chloride.

[0027] The present invention also discloses a zinc-bromine flow battery, comprising a positive electrode, a negative electrode, an electrolyte, and a separator, wherein the positive electrode electrolyte is selected from the above-mentioned homogeneous electrolyte for the positive electrode of the zinc-bromine flow battery.

[0028] Preferably, the diaphragm is selected from microporous polymer diaphragms.

[0029] When using the aforementioned special homogeneous positive electrode electrolyte, this invention can suppress the shuttle effect by using only inexpensive microporous polymers as membranes, thereby significantly reducing production costs and making large-scale industrial production possible.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] This invention discloses a homogeneous electrolyte for the positive electrode of a zinc-bromine flow battery. For the first time, cyclodextrin derivatives are used as complexing agents, which can effectively recognize and complex polybrominates and bromine molecules, significantly improving the ability to suppress the shuttle effect. Furthermore, it exhibits low voltage polarization and high voltage efficiency. By using only a low-cost microporous polymer membrane, a zinc-bromine flow battery with excellent CE, VE, and EE is obtained, significantly reducing the cost of zinc-bromine flow batteries. Detailed Implementation

[0032] The present invention will be described in further detail below with reference to embodiments and comparative examples, but the implementation of the present invention is not limited thereto.

[0033] Example 1

[0034] A homogeneous positive electrode electrolyte was prepared by mixing the complexing agent 2-hydroxypropyl-β-cyclodextrin with zinc bromide and potassium chloride. The concentration of zinc bromide in the electrolyte was 2.5 mol / L, the concentration of potassium chloride was 3 mol / L, and the mass fraction of the complexing agent was 20%.

[0035] Example 2

[0036] The preparation process is basically the same as in Example 1, with the only difference being:

[0037] Replace the mass fraction of the complexing agent in the electrolyte with 15%.

[0038] Example 3

[0039] The preparation process is basically the same as in Example 1, with the only difference being:

[0040] Replace the mass fraction of the complexing agent in the electrolyte with 25%.

[0041] Example 4

[0042] The preparation process is basically the same as in Example 1, with the only difference being:

[0043] The complexing agent in the electrolyte was replaced with an equal mass of 2,3-hydroxypropyl-β-cyclodextrin.

[0044] Examples 5-10

[0045] The preparation process is basically the same as in Example 1, with the only difference being:

[0046] The complexing agent in the electrolyte was replaced sequentially with equal masses of 2-methyl-β-cyclodextrin, 2-hydroxyethyl-β-cyclodextrin, 3-carboxymethyl-β-cyclodextrin, sodium β-cyclodextrin sulfate, sodium β-cyclodextrin sulfonate, and sodium β-cyclodextrin phosphate.

[0047] Comparative Example 1

[0048] An electrolyte was prepared by mixing zinc bromide (2.5 mol / L), potassium chloride (3 mol / L), and water.

[0049] Comparative Example 2

[0050] A positive electrode electrolyte was prepared by mixing the complexing agent methylpyrrolidine with zinc bromide and potassium chloride. The concentration of zinc bromide in the electrolyte was 2.5 mol / L, the concentration of potassium chloride was 3 mol / L, and the concentration of the complexing agent was 0.8 mol / L.

[0051] Comparative Example 3

[0052] The positive electrode electrolyte was prepared by mixing the complexing agent α-cyclodextrin with zinc bromide and potassium chloride. The concentration of zinc bromide in the electrolyte was 2.5 mol / L, the concentration of potassium chloride was 3 mol / L, and the concentration of the complexing agent was the saturation concentration, i.e., the mass concentration was 14.5%.

[0053] Comparative Example 4

[0054] A homogeneous positive electrode electrolyte was prepared by mixing the complexing agent 2-hydroxypropyl-α-cyclodextrin with zinc bromide and potassium chloride. The concentration of zinc bromide in the electrolyte was 2.5 mol / L, the concentration of potassium chloride was 3 mol / L, and the mass fraction of the complexing agent was 20%.

[0055] Comparative Example 5

[0056] The positive electrode electrolyte was prepared by mixing the complexing agent β-cyclodextrin with zinc bromide and potassium chloride. The concentration of zinc bromide in the electrolyte was 2.5 mol / L, the concentration of potassium chloride was 3 mol / L, and the concentration of the complexing agent was the saturation concentration, i.e., the mass concentration was 2%. A white precipitate appeared when the complexing agent was added further.

[0057] Performance testing:

[0058] 1. The positive electrode electrolytes prepared according to each embodiment or comparative example were applied to zinc-bromine flow batteries to test their performance in suppressing the shuttle effect. The experimental conditions were as follows:

[0059] The negative electrode electrolyte is a mixed aqueous solution of 2.5 mol / L zinc bromide and 3 mol / L potassium chloride. The electrode is a carbon felt with an electrode area of ​​9 cm². 2 The diaphragm is a microporous polyolefin diaphragm with a charge / discharge current density of 20 mA / cm². 2The charging time is 1 hour, and the discharge cutoff voltage is 0.6V. The average coulombic efficiency (CE), voltage efficiency (VE), and energy efficiency (EE) of the battery after 20 cycles are shown in Table 1 below.

[0060] Table 1

[0061] Example 1 95.1 84.9 80.7 Example 2 91.3 84.2 76.9 Example 3 93.5 82.7 77.3 Example 4 95.0 84.7 80.5 Example 5 92.8 81.8 75.9 Example 6 94.2 82.7 77.9 Example 7 93.8 82.6 77.5 Example 8 93.5 82.5 77.1 Example 9 94.1 83.2 78.3 Example 10 93.6 82.1 76.8 Comparative Example 1 75.4 80.2 60.5 Comparative Example 2 82.7 83.4 69.0 Comparative Example 3 85.1 83.5 71.1 Comparative Example 4 85.5 83.5 71.4 Comparative Example 5 86.7 83.7 72.6

[0062] 2. The electrolytes prepared according to each embodiment or comparative example were applied to zinc-bromine flow batteries to test the battery polarization performance. The experimental conditions were as follows:

[0063] The negative electrode electrolyte is a mixed aqueous solution of 2.5 mol / L zinc bromide and 3 mol / L potassium chloride. The electrode is a carbon felt with an electrode area of ​​9 cm². 2 The diaphragm is a microporous polyolefin diaphragm with a charge / discharge current density of 20 mA / cm². 2 The charging time was 1 hour, and the discharge cutoff voltage was 0.6V. The data are listed in Table 2 below.

[0064] Table 2

[0065] Example 1 1917 1628 289 Example 2 1920 1617 303 Example 3 1923 1590 333 Example 4 1919 1625 294 Example 5 1932 1580 352 Example 6 1925 1592 333 Example 7 1926 1591 335 Example 8 1924 1587 337 Example 9 1921 1598 323 Example 10 1927 1582 345 Comparative Example 1 1940 1556 384 Comparative Example 2 1929 1609 320 Comparative Example 3 1924 1607 317 Comparative Example 4 1922 1605 317 Comparative Example 5 1931 1616 315

[0066] The above-described embodiments are preferred embodiments, but the scope of protection of the present invention is not limited thereto. Those skilled in the art can easily understand the spirit of the present invention based on the above embodiments and make different extensions and variations, but as long as they do not depart from the spirit of the present invention, they are all within the scope of protection of the present invention.

Claims

1. A homogeneous electrolyte for the positive electrode of a zinc-bromine flow battery, comprising a bromide-containing main salt and a complexing agent, characterized in that: The complexing agent is selected from cyclodextrin derivatives; The cyclodextrin derivative is selected from one or more of methylated cyclodextrin, hydroxyethyl cyclodextrin, hydroxypropyl cyclodextrin, carboxymethyl cyclodextrin, sulfate cyclodextrin, sulfonate cyclodextrin, and phosphate cyclodextrin. In the homogeneous electrolyte of the zinc-bromine flow battery positive electrode, the mass fraction of the complexing agent is 12-30%. The bromide-containing main salt is selected from one or more of zinc bromide, potassium bromide, and sodium bromide. The homogeneous electrolyte at the positive electrode of the zinc-bromine flow battery also includes a conductive support agent. When the bromide-containing main salt is selected from zinc bromide, the conductive support agent is selected from one or more of potassium chloride, ammonium chloride, sodium chloride, and zinc chloride; When the bromide-containing main salt is selected from potassium bromide and / or sodium bromide, the conductive support is selected from zinc chloride.

2. The homogeneous electrolyte for the positive electrode of a zinc-bromine flow battery according to claim 1, characterized in that: In the homogeneous electrolyte of the zinc-bromine flow battery positive electrode, the mass fraction of the complexing agent is 15-25%.

3. The homogeneous electrolyte for the positive electrode of a zinc-bromine flow battery according to claim 1, characterized in that: The complexing agent is selected from one or more of 2-methyl-β-cyclodextrin, 2-hydroxyethyl-β-cyclodextrin, 2-hydroxypropyl-β-cyclodextrin, 2,3-hydroxypropyl-β-cyclodextrin, 3-carboxymethyl-β-cyclodextrin, sodium β-cyclodextrin sulfate, sodium β-cyclodextrin sulfonate, and sodium β-cyclodextrin phosphate; In the homogeneous electrolyte of the zinc-bromine flow battery positive electrode, the mass fraction of the complexing agent is 20-25%.

4. The homogeneous electrolyte for the positive electrode of a zinc-bromine flow battery according to claim 3, characterized in that: The complexing agent is selected from one or more of 2-hydroxyethyl-β-cyclodextrin, 2-hydroxypropyl-β-cyclodextrin, 2,3-hydroxypropyl-β-cyclodextrin, 3-carboxymethyl-β-cyclodextrin, sodium β-cyclodextrin sulfate, sodium β-cyclodextrin sulfonate, and sodium β-cyclodextrin phosphate.

5. The homogeneous electrolyte for the positive electrode of a zinc-bromine flow battery according to claim 4, characterized in that: The complexing agent is selected from one or more of 2-hydroxyethyl-β-cyclodextrin, 2-hydroxypropyl-β-cyclodextrin, 2,3-hydroxypropyl-β-cyclodextrin, and sodium β-cyclodextrin sulfonate.

6. The homogeneous electrolyte for the positive electrode of a zinc-bromine flow battery according to claim 5, characterized in that: The complexing agent is selected from 2-hydroxypropyl-β-cyclodextrin and / or 2,3-hydroxypropyl-β-cyclodextrin.

7. The homogeneous electrolyte for the positive electrode of a zinc-bromine flow battery according to claim 6, characterized in that: The homogeneous electrolyte at the positive electrode of the zinc-bromine flow battery contains a complexing agent with a mass fraction of 20%.

8. The homogeneous electrolyte for the positive electrode of a zinc-bromine flow battery according to claim 1, characterized in that: The concentration of bromide ion main salt in the homogeneous electrolyte of the positive electrode of the zinc-bromine flow battery is 0.1~5.0 mol / L; The concentration of the conductive support in the homogeneous electrolyte of the positive electrode of the zinc-bromine flow battery is 0.5~5.0 mol / L.

9. A zinc-bromine flow battery, comprising a positive electrode, a negative electrode, an electrolyte, and a separator, characterized in that, The positive electrode electrolyte is selected from the homogeneous positive electrode electrolyte of the zinc-bromine flow battery as described in any one of claims 1 to 8.

10. The zinc-bromine flow battery according to claim 9, wherein the separator is selected from microporous polymer separators.