Method for modifying zinc-bromine battery positive electrode material by using ionic liquid bromine complexing agent

Abstract

The invention belongs to the field of electrochemistry, and relates to a method for modifying a zinc-bromine battery positive electrode material by an ionic liquid bromine complexing agent, which innovates the use mode of the ionic liquid bromine complexing agent as an electrolyte additive in a zinc-bromine battery and performs surface modification on an original carbon electrode. The method comprises the following steps: firstly, treating an electrode to improve the surface hydrophilicity and improve the internal pore structure type of the material; then, the ionic liquid is uniformly distributed on the surface of the electrode and in the internal pore diameter in a dipping-ultrasonic-spraying combined mode, uniform modification of an electrode material is realized, and finally, a total battery is assembled to evaluate the practicability of the experimental strategy. Ionic liquid which can have strong interaction with polybromine species is optimized, the defects that the ionic liquid serving as an electrolyte additive and bromine species generate out-of-phase complexation products to be dispersed in an electrolyte, the utilization rate of active substances is low, reaction kinetics is slow and the like are overcome, and by means of effective attachment of the ionic liquid on the surface of an electrode, the reaction efficiency is improved. Effective fixation of bromine species on the surface of the electrode is achieved, and battery capacity loss caused by the shuttle effect of the bromine species is relieved. The method comprises an original electrode pretreatment method, an electrode material modification method and ionic liquid selection. The zinc-bromine battery prepared by the invention has the advantages of high charge-discharge efficiency, low battery self-discharge rate, fast reaction kinetics and the like.

Description

Technical Field

[0001] The invention belongs to the technical field of liquid flow batteries, and in particular relates to a method for modifying a positive electrode material of a zinc-bromine battery with an ionic liquid bromine complexing agent. Background Art

[0002] Lithium-ion batteries dominate the electric vehicle and energy storage system industry with their high energy efficiency, acceptable lifespan, and high power performance. However, the recent frequent fire accidents in energy storage power stations using lithium-ion batteries have caused major concerns about safety. In order to enable large-scale batteries to meet the life requirements and safety guarantees of energy storage power stations, aqueous batteries have received increasing attention. Aqueous zinc-bromine batteries have attracted widespread attention due to their high energy density, long service life, and environmental friendliness. The redox reaction mechanisms that occur in aqueous zinc-bromine batteries include:

[0003] Br2 / Br in the positive electrode - Redox couple ( E0 = 1.065 V vs standard hydrogen electrode (SHE) and Zn in the negative electrode 2+ / Zn redox couple ( E0=-0.76Vvs SHE). During the charging process, Br - Losing electrons and being oxidized to Br2, it will continue to react with Br - Combine to form a highly soluble polybrominated species Br 2n+1 - (n=1、2、、); at the same time, Zn 2+ Electrons will be deposited on the electrode surface to form metallic zinc. However, the highly soluble polybrominated substances will diffuse through the battery separator uncontrollably to the negative electrode to react with metallic zinc, resulting in self-discharge and a low coulombic efficiency of the battery.

[0004] Adding ionic liquid bromine complexing agents to the electrolyte is considered to be a relatively effective strategy. The positive charge carried by the ionic liquid bromine complexing agent combines with the negatively charged polybromine anion to form a large-sized polybromine complex product, which is blocked on the positive electrode side due to the limited size of the battery separator. The complex products formed between the commonly used ionic liquids such as N-methyl-N-ethylpyrrole ammonium bromide, 1-ethylpyridinium-1-ammonium bromide and N-methyl-N-ethylmorpholinium ammonium bromide and polybromine species are dense oily substances, which will be deposited at the bottom of the battery due to gravity and form a phase separation with the electrolyte. For example, patent publication number CN115966740A discloses an alkaline zinc-bromine flow battery. The alkaline zinc-bromine flow battery mainly includes a positive electrode, a diaphragm, a negative electrode and an electrolyte. The positive and negative electrolytes include ZnBr2, an alkali and a complexing agent. The complexing agent is one or more of tetramethylammonium bromide, tetraethylammonium bromide, tetrapropylammonium bromide and tetrabutylammonium bromide. The positive electrode of the battery undergoes a redox reaction of bromine, and the negative electrode undergoes a deposition and dissolution reaction of metallic zinc. The complexing agent in the electrolyte can complex bromine into a solid complex, thereby inhibiting the disproportionation reaction of bromine in the alkaline zinc-bromine flow battery. However, the generation of phase separation leads to an uneven electrolyte, which is not conducive to the electrochemical reaction of the active component to gain and lose electrons when in contact with the electrode, resulting in problems such as low utilization of active substances and slow reaction kinetics. Summary of the invention

[0005] Aiming at the technical problems of low utilization rate of active substances and slow reaction kinetics of oily phase separation products generated by ionic liquid and bromine species, the present invention provides a method for modifying positive electrode materials of zinc-bromine batteries with ionic liquid bromine complexing agents.

[0006] In order to achieve the above object, the technical solution of the present invention is achieved as follows:

[0007] A method for modifying a positive electrode material of a zinc-bromine battery with an ionic liquid bromine complexing agent comprises the following steps:

[0008] (1) heat-treating the commercial electrode material, pore-forming and activating the electrode surface, washing it after natural cooling to remove impurities on the surface and in the internal cavity, and drying it to obtain the pretreated electrode material;

[0009] (2) dissolving the ionic liquid in the binder solution according to a specific loading amount to obtain a uniform solution, stirring the solution ultrasonically, and selecting an appropriate solvent to ensure that the two can be uniformly mixed therein according to the difference in properties between the ionic liquid and the binder;

[0010] (3) The pretreated electrode material is immersed in the above solution and ultrasonically treated so that the binder and ionic liquid molecules can be evenly attached to the electrode surface and internal pores. The immersed electrode is taken out and suspended above the solution to dry. The remaining solution is then sprayed on the front and back of the electrode multiple times using a spray gun. The modified electrode is obtained after drying.

[0011] The conductive current collector is a carbon electrode such as carbon felt, graphite felt, carbon cloth or carbon paper.

[0012] The conductive current collector needs to be heat treated and cleaned before infiltration; the heat treatment temperature is 300-900° C. and the time is 2-10 hours.

[0013] The infiltration method is: immersing the carbon electrode in the mixed solution for 30-60 minutes and ultrasonically treating for 5-10 minutes.

[0014] The ionic liquid is a quaternary ammonium salt ionic liquid.

[0015] The quaternary ammonium salt ionic liquid is tetramethylammonium bromide, tetraethylammonium bromide, tetrapropylammonium bromide, tetrabutylammonium bromide or hexadecyltrimethylammonium bromide.

[0016] The binder is a solution formed by mixing an adhesive polymer material with a solvent. The binder is a perfluorosulfonic acid resin solution (Nafion solution), a polytetrafluoroethylene emulsion or a polyvinylidene fluoride solution. The mass ratio of the ionic liquid to the binder is (3-9):1.

[0017] According to the different polarities of the binder molecules and the ionic liquid, a single or mixed solvent with good solubility for both is selected for testing, usually a mixture of one or more of ethanol, water, N-dimethylacetamide, N-methylpyrrolidone, etc., with a mass ratio of (100-1000):1 to the ionic liquid.

[0018] The ionic liquid loading on the zinc-bromine battery positive electrode material is 3-10 cm 2 g -1

[0019] Beneficial effects of the present invention: The present invention changes the traditional strategy of adding ionic liquids as bromine complexing agents to the electrolyte and adopts an ionic liquid bromine complexing agent electrode modification strategy. The ionic liquid bromine complexing agent can form a solid product with the polybromine species, and the solid bromine complex is directly attached to the electrode surface to achieve effective fixation of the bromine species. The binder molecules on the positive electrode side of the present invention are entangled with the ionic liquid molecules and firmly attached to the electrode surface, ensuring the bromine complexing ability of the ionic liquid while adhering to the electrode surface. The solid complexing product formed on the electrode surface is conducive to the participation of electrons in the chemical reaction and improves the reaction kinetics. The innovative use of the ionic liquid bromine complexing agent effectively alleviates the many problems caused by the production of phase separation complexing agent materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0021] Figure 1 This is a flow chart for the preparation of zinc-bromine battery positive electrode materials modified by ionic liquid bromine complexing agents.

[0022] Figure 2 It is an optical photograph comparison after adding the same mass of different ionic liquids into the bromine standard solution.

[0023] Figure 3 This is a SEM image comparing the surface morphology of the electrode material before and after treatment. DETAILED DESCRIPTION

[0024] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0025] Example 1

[0026] A method for modifying a positive electrode material of a zinc-bromine battery with an ionic liquid bromine complexing agent, such as Figure 1 As shown, the following steps are included:

[0027] 20 mg of tetramethylammonium bromide ionic liquid was used as a bromine complexing agent to modify the electrode surface, and 5 mg of PVD F was used as a binder. The two were uniformly mixed and dissolved in 5 g of N-methylpyrrolidone to prepare a mixed slurry; a commercial carbon felt with a thickness of 5 mm was heat-treated at 500 ° C for 5 h in an air atmosphere, and after natural cooling, it was cleaned with ethanol, acetone, and deionized water to remove surface impurities and dried in an oven as a pretreated carbon felt for use; the pretreated carbon felt was immersed in the above mixed slurry and ultrasonically treated for 30 min, then it was fished out and placed on the top of the mixed slurry for preliminary drying; then the remaining slurry was sprayed on both sides of the electrode to reach 5 mg cm -2 The loading amount was placed in a vacuum oven for drying.

[0028] Example 2

[0029] A method for modifying a positive electrode material of a zinc-bromine battery with an ionic liquid bromine complexing agent comprises the following steps:

[0030] 20 mg of tetraethylammonium bromide ionic liquid was used as a bromine complexing agent to modify the electrode surface, and 5 mg of PVD F was used as a binder. The two were uniformly mixed and dissolved in 5 g of N-methylpyrrolidone to prepare a mixed slurry; a commercial carbon felt with a thickness of 5 mm was heat-treated at 500 ° C for 5 h in an air atmosphere, and after natural cooling, it was cleaned with ethanol, acetone, and deionized water to remove surface impurities and dried in an oven as a pretreated carbon felt for use; the pretreated carbon felt was immersed in the above mixed slurry and ultrasonically treated for 30 min, then it was fished out and placed on the top of the mixed slurry for preliminary drying; then the remaining slurry was sprayed on both sides of the electrode to reach 5 mg cm -2 The loading amount was placed in a vacuum oven for drying.

[0031] Example 3

[0032] A method for modifying a positive electrode material of a zinc-bromine battery with an ionic liquid bromine complexing agent comprises the following steps:

[0033] 20 mg of tetrapropylammonium bromide ionic liquid was used as a bromine complexing agent to modify the electrode surface, and 5 mg of PVD F was used as a binder. The two were uniformly mixed and dissolved in 5 g of N-methylpyrrolidone to prepare a mixed slurry; a commercial carbon felt with a thickness of 5 mm was heat-treated at 500 ° C for 5 h in an air atmosphere, and after natural cooling, it was cleaned with ethanol, acetone, and deionized water to remove surface impurities and dried in an oven as a pretreated carbon felt for use; the pretreated carbon felt was immersed in the above mixed slurry and ultrasonically treated for 30 min, then it was fished out and placed on the top of the mixed slurry for preliminary drying; then the remaining slurry was sprayed on both sides of the electrode to reach 5 mg cm -2 The loading amount was placed in a vacuum oven for drying.

[0034] Example 4

[0035] A method for modifying a positive electrode material of a zinc-bromine battery with an ionic liquid bromine complexing agent comprises the following steps:

[0036] 20 mg of tetrabutylammonium bromide ionic liquid was used as a bromine complexing agent to modify the electrode surface, and 5 mg of PVD F was used as a binder. The two were uniformly mixed and dissolved in 5 g of N-methylpyrrolidone to prepare a mixed slurry; a commercial carbon felt with a thickness of 5 mm was heat-treated at 500 ° C for 5 h in an air atmosphere, and after natural cooling, it was cleaned with ethanol, acetone, and deionized water to remove surface impurities and dried in an oven as a pretreated carbon felt for use; the pretreated carbon felt was immersed in the above mixed slurry and ultrasonically treated for 30 min, then it was fished out and placed on the top of the mixed slurry for preliminary drying; then the remaining slurry was sprayed on both sides of the electrode to reach 5 mg cm -2The loading amount was placed in a vacuum oven for drying.

[0037] Example 5

[0038] A method for modifying a positive electrode material of a zinc-bromine battery with an ionic liquid bromine complexing agent comprises the following steps:

[0039] 20 mg of tetrabutylammonium bromide ionic liquid was used as a bromine complexing agent to modify the electrode surface, and 5 g of a 5% Nafion solution was used as a binder. The two were evenly mixed and distributed in a mixed solvent of water and isopropanol to prepare a mixed slurry; a commercial carbon felt with a thickness of 5 mm was heat-treated at 500°C for 5 h in an air atmosphere, and after natural cooling, it was cleaned with ethanol, acetone, and deionized water to remove surface impurities, and dried in an oven as a pretreated carbon felt for use; the pretreated carbon felt was immersed in the above mixed slurry and ultrasonically treated for 30 min, and then it was fished out and placed on the top of the mixed slurry for preliminary drying; and then the remaining slurry was sprayed on both sides of the electrode to reach 5 mg cm -2 The loading amount was placed in a vacuum oven for drying.

[0040] Comparative Example 1

[0041] A zinc-bromine battery positive electrode material, the preparation method comprising the following steps:

[0042] Two pieces of original carbon felt were washed with ethanol, acetone and deionized water to remove surface impurities, and dried in an oven for later use without any subsequent treatment.

[0043] Comparative Example 2

[0044] A zinc-bromine battery positive electrode material, the preparation method comprising the following steps:

[0045] Take 20 mg of N-methyl-N-ethylpyrrolidone bromide ionic liquid as a bromine complexing agent to modify the electrode surface, and use 5 mg of PVDF as a binder. The two are evenly mixed and dissolved in N-methylpyrrolidone to prepare a mixed slurry; a commercial carbon felt with a thickness of 5 mm is heat-treated at 500°C for 5 h in an air atmosphere, and after natural cooling, it is cleaned with ethanol, acetone, and deionized water to remove surface impurities, and placed in an oven to dry as a pretreated carbon felt; the pretreated carbon felt is immersed in the above mixed slurry and ultrasonically treated for 30 minutes, then it is fished out and placed on the top of the mixed slurry for preliminary drying; then the remaining slurry is applied to both sides of the electrode by spraying, and placed in a vacuum oven for drying.

[0046] Test Case

[0047] The electrodes prepared in Examples 1-5 and Comparative Examples 1 and 2 were tested respectively. If there is no special introduction, the positive electrode is the prepared electrode, the commercial carbon felt is used as the negative electrode, and the positive and negative electrolytes are 50 ml 2M ZnBr2 + 3M KCl as electrolytes to assemble zinc-bromine symmetric flow batteries for electrochemical performance testing. During the test, the charging capacity is 10 mA h cm -2 , the current density is 80 mA cm -1 , the discharge cut-off voltage is 0.8V, and the battery reaction active area is 2cm x 2cm.

[0048] Figure 2 The same mass of different ionic liquids were added to the bromine standard solution, and the optical photos were compared. Figure 2 It can be seen that after adding the same mass of ionic liquid to the bromine standard solution, different states of bromine complex products are formed due to the difference in binding ability to bromine species. N-methyl-N-ethylpyrrolidine bromide forms an oily product, while quaternary ammonium salts form solid particulate complexes, but the formation amount is significantly different, which directly affects the fixation ability of bromine species in the subsequent electrode modification process. Figure 3 The surface morphology of the commercial carbon felt before and after heat treatment in Example 1 is changed. Figure 3 It can be seen that after heat treatment, the surface pores of the original carbon felt increase and become more obvious, providing opportunities for the subsequent infiltration of the ionic liquid mixed solution and providing space for the fixation of the bromine complex product.

[0049] As can be seen from Table 1, the electrodes were modified and assembled into full batteries for electrochemical performance testing. The efficiency of the batteries assembled with carbon felt modified with ionic liquid was higher than that of the batteries composed of original carbon felt. As the binding ability between ionic liquid and bromine species increased, the energy, voltage and coulomb efficiency of the symmetrical battery increased. It is worth noting that compared with PVDF binder, Nafion as a binder slightly improved the battery performance.

[0050] Table 1 Comparison of battery performance with different electrodes

[0051] name Energy efficiency / % Voltage efficiency / % Coulomb efficiency / % Example 1 80.96 95.40 84.86 Example 2 81.89 97.05 84.38 Example 3 82.30 95.68 86.06 Example 4 83.10 95.58 86.94 Example 5 83.21 97.84 88.11 Comparative Example 1 80.30 95.2 84.28 Comparative Example 2 80.37 95.96 83.75

[0052] Example 6

[0053] A method for modifying a positive electrode material of a zinc-bromine battery with an ionic liquid bromine complexing agent comprises the following steps:

[0054] 30 mg of tetramethylammonium bromide ionic liquid was used as a bromine complexing agent to modify the electrode surface, and 10 mg of PVDF was used as a binder. The two were uniformly mixed and dissolved in 30 g of N-methylpyrrolidone to prepare a mixed slurry; a commercial carbon felt with a thickness of 5 mm was heat-treated at 500 ° C for 5 h in an air atmosphere, and after natural cooling, it was cleaned with ethanol, acetone, and deionized water to remove surface impurities and dried in an oven as a pretreated carbon felt for use; the pretreated carbon felt was immersed in the above mixed slurry and ultrasonically treated for 30 min, then it was fished out and placed on the top of the mixed slurry for preliminary drying; then the remaining slurry was sprayed on both sides of the electrode to reach 5 mg cm -2 The loading amount was placed in a vacuum oven for drying.

[0055] Example 7

[0056] A method for modifying a positive electrode material of a zinc-bromine battery with an ionic liquid bromine complexing agent comprises the following steps:

[0057] 90 mg of tetramethylammonium bromide ionic liquid was used as a bromine complexing agent to modify the electrode surface, and 10 mg of PVDF was used as a binder. The two were uniformly mixed and dissolved in 9 g of N-methylpyrrolidone to prepare a mixed slurry; a commercial carbon felt with a thickness of 5 mm was heat-treated at 500 ° C for 5 h in an air atmosphere, and after natural cooling, it was cleaned with ethanol, acetone, and deionized water to remove surface impurities and dried in an oven as a pretreated carbon felt; the pretreated carbon felt was immersed in the above mixed slurry and ultrasonically treated for 30 min, then it was fished out and placed on the top of the mixed slurry for preliminary drying; then the remaining slurry was sprayed on both sides of the electrode to reach 10 mg cm -2 The loading amount was placed in a vacuum oven for drying.

[0058] Example 8

[0059] A method for modifying a positive electrode material of a zinc-bromine battery with an ionic liquid bromine complexing agent comprises the following steps:

[0060] 20 mg of hexadecyltrimethylammonium bromide ionic liquid was used as a bromine complexing agent to modify the electrode surface, and 5 g of a 5% Nafion solution was used as a binder. The two were evenly mixed and distributed in a mixed solvent of water and isopropanol to prepare a mixed slurry; a commercial carbon felt with a thickness of 5 mm was heat-treated at 300°C for 10 h in an air atmosphere, and after natural cooling, it was washed with ethanol, acetone, and deionized water to remove surface impurities, and dried in an oven as a pretreated carbon felt for use; the pretreated carbon felt was immersed in the above mixed slurry and ultrasonically treated for 30 min, and then it was fished out and placed on the top of the mixed slurry for preliminary drying; and then the remaining slurry was sprayed on both sides of the electrode to reach 5 mg cm-2 The loading amount was placed in a vacuum oven for drying.

[0061] Example 9

[0062] A method for modifying a positive electrode material of a zinc-bromine battery with an ionic liquid bromine complexing agent comprises the following steps:

[0063] 20 mg of hexadecyltrimethylammonium bromide ionic liquid was used as a bromine complexing agent to modify the electrode surface, and 5 g of a 60% polytetrafluoroethylene emulsion as a binder were uniformly mixed and distributed in a mixed solvent of water and isopropanol to prepare a mixed slurry; a commercial carbon felt with a thickness of 5 mm was heat treated at 900 ° C for 2 h in an air atmosphere, and after natural cooling, it was cleaned with ethanol, acetone, and deionized water to remove surface impurities, and dried in an oven as a pretreated carbon felt; the pretreated carbon felt was immersed in the above mixed slurry and ultrasonically treated for 30 min, and then it was fished out and placed on the top of the mixed slurry for preliminary drying; and then the remaining slurry was sprayed on both sides of the electrode to reach 5 mg cm -2 The loading amount was placed in a vacuum oven for drying.

[0064] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for modifying a positive electrode material of a zinc-bromine battery with an ionic liquid bromine complexing agent, characterized in that: The following steps are involved: (1) mixing an ionic liquid and a binder to prepare a mixed solution; (2) soaking the electrode material in the mixed solution and subjecting it to ultrasonic treatment; The soaked electrode is taken out and dried, and then the mixed solution is sprayed on the front and back sides of the electrode again to obtain a modified electrode.

2. The method for modifying the positive electrode material of zinc-bromine battery with an ionic liquid bromine complexing agent according to claim 1, characterized in that: The electrode material needs to be heat treated and cleaned before infiltration in step (2); the heat treatment temperature is 300-900°C and the time is 2-10h.

3. The method for modifying the positive electrode material of zinc-bromine battery with an ionic liquid bromine complexing agent according to claim 2, characterized in that: The electrode material is a carbon electrode; the carbon electrode is carbon felt, graphite felt, carbon cloth or carbon paper.

4. The method for modifying the positive electrode material of zinc-bromine battery with an ionic liquid bromine complexing agent according to claim 1, characterized in that: The method of the immersion-ultrasonic treatment is: immersing the electrode material in the mixed solution for every 30-60 minutes, and ultrasonically treating for 5-10 minutes.

5. The method for modifying the positive electrode material of zinc-bromine battery with an ionic liquid bromine complexing agent according to claim 1, characterized in that: The ionic liquid is a quaternary ammonium salt ionic liquid.

6. The method for modifying the positive electrode material of zinc-bromine battery with an ionic liquid bromine complexing agent according to claim 5, characterized in that: The quaternary ammonium salt ionic liquid is tetramethylammonium bromide, tetraethylammonium bromide, tetrapropylammonium bromide, tetrabutylammonium bromide or hexadecyltrimethylammonium bromide.

7. The method for modifying the positive electrode material of zinc-bromine battery with an ionic liquid bromine complexing agent according to claim 1, characterized in that: The binder is a perfluorosulfonic acid resin solution, a polytetrafluoroethylene emulsion or a polyvinylidene fluoride solution, and the mass ratio of the ionic liquid to the binder solute is (3-9):

1.

8. The method for modifying the positive electrode material of zinc-bromine battery with an ionic liquid bromine complexing agent according to claim 7, characterized in that: The mixed solution is further added with a solvent, and the mass ratio of the solvent to the ionic liquid is (100-1000):

1.

9. The method for modifying the positive electrode material of zinc-bromine battery with an ionic liquid bromine complexing agent according to claim 8, characterized in that: The solvent is one or more of ethanol, water, N-dimethylacetamide and N-methylpyrrolidone.

10. The zinc-bromine battery positive electrode material prepared by the method according to any one of claims 1 to 9, characterized in that: The ionic liquid loading on the positive electrode material of zinc-bromine battery is 3-10cm 2 g -1 .

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