Electrolyte and zinc / bromine flow battery

Abstract

A zinc / bromine flow battery comprises an electrolyte. The electrolyte comprises an active substance, a supporting electrolyte, and an additive. The additive is selected from at least one of 1-ethyl-3-methylimidazolium bromide, 1-ethyl-3-methylimidazolium chloride, 1-carboxymethyl-3-methylimidazolium bromide, and 1-carboxymethyl-3-methylimidazolium chloride. The use of the additive can regulate the deposition morphology of zinc and suppress zinc dendrites, and can also inhibit the diffusion of bromine on the positive electrode side, thereby improving the coulombic efficiency of the battery. Moreover, the complex product formed by the additive complexed with bromine does not produce solids during long-term cycling, whereas the complex product of conventional MEP in the later stage of battery cycling forms solids which are unevenly distributed on the electrode surface, reducing the voltage efficiency of the battery and shortening the service life of the battery.

Description

An electrolyte and zinc / bromine flow battery Technical Field

[0001] This application relates to an electrolyte and a zinc / bromine flow battery, belonging to the field of zinc / bromine flow battery technology. Background Technology

[0002] The zinc / bromine flow battery (ZBFB) was proposed by Lim et al. in 1977. Both the positive and negative electrodes use ZnBr2 as the active material, effectively solving the problem of cross-contamination of the electrolyte. During charging, the positive electrode Br2... - It is oxidized to Br2, and the negative electrode is Zn. 2+ It is reduced to metallic Zn, giving the battery a theoretical voltage of 1.85V. This is because ZnBr2 has high solubility in water (>7.5 mol L⁻¹). -1 This battery has a high theoretical energy density (>400Wh kg). -1 With its advantages of safety and low cost, zinc / bromine flow batteries have promising application prospects in the field of large-scale energy storage. After decades of development, zinc / bromine flow batteries have reached the application demonstration stage.

[0003] Although zinc / bromine flow batteries have made significant technological progress, their large-scale commercialization still faces many challenges. On the one hand, the reaction at the negative electrode involves the deposition and dissolution of zinc metal. During zinc deposition, high current or prolonged charging can lead to uneven zinc ion concentration distribution near the electrode, resulting in severe concentration polarization on the electrode surface. Furthermore, minute defects on the electrode surface can cause uneven electric field distribution, leading to inconsistent zinc growth rates at different nucleation sites and ultimately, zinc dendrite formation. During long-term cycling, zinc dendrite shedding reduces battery efficiency. The detached zinc cannot dissolve during discharge and accumulates in the electrolyte. Additionally, hydrogen evolution and water corrosion of zinc at the negative electrode further contribute to bromine accumulation at the positive electrode, reducing both coulombic and voltage efficiencies. Furthermore, the detached zinc can clog pipelines, directly causing battery failure, while dendrite punctures in the separator can directly lead to short-circuit failure. On the other hand, the Br2 products generated after charging on the positive electrode side have low solubility in water and volatilize to the top of the electrolyte. During discharge, this not only causes significant concentration polarization but also reduces the amount of active material in the battery, decreasing its capacity, efficiency, and lifespan. The strong corrosiveness of Br2 also increases the maintenance costs of battery components. Water-soluble Br2 easily diffuses through the separator to the negative electrode side and reacts with deposited zinc, reducing the battery's coulombic efficiency. The traditional solution is to add a bromine complexing agent, MEP. While MEP can inhibit bromine volatilization and diffusion to the negative electrode side, bromine gradually accumulates on the positive electrode side over time. The complex products formed by MEP and polybrominated compounds tend to solidify, resulting in uneven contact with the electrode surface and further reducing the battery's voltage efficiency and cycle life.

[0004] For aqueous zinc-based batteries, the composition of the electrolyte is particularly important for its electrochemical performance, as the physicochemical properties of the electrolyte are closely related to the reversibility of the zinc anode, the zinc deposition morphology, the reaction mechanism, and the ion transport characteristics. Introducing additives into the electrolyte is a simple and effective method to regulate the electrolyte composition, and these additives can also play a role in all stages of zinc deposition nucleation and growth. Suppressing Br2 diffusion on the positive electrode side also includes electrode material modification and the addition of additives. Since adding additives to the electrolyte is generally compatible with electrode structure optimization and electrode interface modification, additives can serve as an effective supplement to other solutions. Therefore, developing efficient and multifunctional additives to suppress zinc dendrite growth and mitigate side reactions during discharge is of great significance for the large-scale application of zinc / bromine flow batteries. Summary of the Invention

[0005] In view of the above-mentioned development status, this application provides a multifunctional additive that can be used on the negative electrode side to inhibit zinc dendrite growth and on the positive electrode side to inhibit bromine diffusion, thereby improving the coulombic efficiency of the battery. It is also inexpensive, and after long-term operation, the bromine complex at the positive electrode does not form a solid, and the polybrominates remain in the oil phase. When used in a zinc / bromine flow battery, a single cell achieves a coulombic efficiency of 40 mA cm⁻¹. -2 40mAh cm -2 Under suitable operating conditions, it can operate stably for over 1000 hours (up to 4000 hours), demonstrating promising application prospects. A zinc / bromine stack assembled with an electrolyte formulated using this additive operates at a 120mAh cm⁻¹. -2 Under high areal capacity conditions, it has been operating stably for more than 1900 hours, far exceeding the commercial additive N-methyl-N-ethylpyrrolidine bromide (MEP).

[0006] According to one aspect of this application, an electrolyte is provided, the electrolyte comprising an active substance, a supporting electrolyte, and additives;

[0007] The additive is selected from at least one of 1-ethyl-3-methyl imidazolium bromide, 1-ethyl-3-methyl imidazolium chloride, 1-carboxymethyl-3-methyl imidazolium bromide, and 1-carboxymethyl-3-methyl imidazolium chloride.

[0008] Optionally, the concentration of the additive is 0.25 to 0.55 mol / L.

[0009] Optionally, the concentration of the additive is independently selected from any value among 0.25 mol / L, 0.3 mol / L, 0.35 mol / L, 0.4 mol / L, 0.45 mol / L, 0.5 mol / L, and 0.55 mol / L, or a range between any two of the above.

[0010] Optionally, the concentration of the additive is 0.35 to 0.45 mol / L.

[0011] Optionally, the active material includes a positive electrode active material and a negative electrode active material;

[0012] The positive electrode active material is selected from at least one of zinc bromide, potassium bromide, and sodium bromide;

[0013] The negative electrode active material is selected from at least one of zinc bromide, zinc chloride, zinc sulfate, zinc nitrate, and zinc acetate.

[0014] Optionally, the concentration of the active substance is 0.5–5 mol / L.

[0015] Optionally, the concentration of the active substance is independently selected from any value among 0.5 mol / L, 1.0 mol / L, 1.5 mol / L, 2.0 mol / L, 2.5 mol / L, 3.0 mol / L, 3.5 mol / L, 4.0 mol / L, and 5.0 mol / L, or a range between any two of the above.

[0016] Optionally, the supporting electrolyte is selected from at least one of KCl, NaCl, KBr, and NaBr.

[0017] Optionally, the concentration of the supporting electrolyte is 0.5–4 mol / L.

[0018] Optionally, the concentration of the supporting electrolyte is independently selected from any value among 0.5 mol / L, 1.0 mol / L, 1.5 mol / L, 2.0 mol / L, 2.5 mol / L, 3.0 mol / L, 3.5 mol / L, and 4.0 mol / L, or a range between any two of the above.

[0019] According to another aspect of this application, a zinc / bromine flow battery is provided, the zinc / bromine flow battery comprising a positive electrode, a negative electrode, and an electrolyte;

[0020] The electrolyte is selected from the electrolytes described above.

[0021] The zinc / bromine flow battery of this application operates at a temperature of -10 to 60°C.

[0022] The beneficial effects that this application can produce include:

[0023] Compared to widely used bromine complexing agents (N-methyl-N-ethylpyrrolidine bromide, MEP), the additive used in this application exhibits advantages such as longer battery life and lower cost because the complexed polybrominates remain in the oil phase at the positive electrode, unlike MEP which forms a solid. A single additive can effectively regulate zinc deposition morphology, suppress zinc dendrites, and inhibit bromine diffusion at the positive electrode, thereby improving battery coulombic efficiency. This method is simple and efficient, avoiding the problems of complex electrolyte composition and high costs associated with introducing multiple substances at both the positive and negative electrodes during electrolyte optimization. Furthermore, the additive used is a commonly used industrial substance, further reducing costs and facilitating industrialization. Attached Figure Description

[0024] Figure 1 shows the test example 1 of this application at 40mA cm -2 20mAh cm -2 Under the specified conditions, (a is without additives, scale bar is 30 μm; b is a magnified view without additives, scale bar is 5 μm; c is with additives, scale bar is 30 μm; d is a magnified view with additives, scale bar is 5 μm) are scanning electron microscope images of zinc deposited on the surface in solution.

[0025] Figure 2 shows the test example 1 of this application at 40mA cm -2 20mAh cm -2 Under the conditions, (a, b) depth microscopy and (b, d) surface profile images of zinc deposited in solutions without additives and (c, d) with additives (a, c; scale bar 80 μm) (b: difference between highest and lowest point is 27.52 μm; d: difference between highest and lowest point is 16.03 μm).

[0026] Figure 3 shows the test example 2 of this application at 2.5mA cm. -2 Under the conditions, (a, b) are in-situ optical microscopy images of zinc deposited in solutions without additives and (c, d) with additives. (a, c) are the original zinc metal electrode surfaces, and (b, d) are the respective metal electrode surfaces after 120 s of deposition (ad: scale bar is 75 μm).

[0027] Figure 4 shows the 5mA cm in Test Example 3 of this application. -2 5mA h cm -2 In-situ optical microscopy images of zinc deposition under certain conditions: (a) and (b) show the electrode surface state after the first deposition-dissolution cycle in electrolytes without additives and containing EMBr, respectively; (c) and (d) show the electrode surface state after two deposition-dissolution cycles in electrolytes without additives and containing EMBr, respectively (ad: scale bar is 75 μm).

[0028] Figure 5 shows a comparison of the XRD patterns of zinc deposited in two electrolytes in Test Example 4 of this application (a) and (b) schematic diagrams of the (101) and (002) crystal planes of the zinc lattice.

[0029] Figure 6 shows a photograph of the electrolyte tank on the positive electrode side at the end of charging of the zinc / bromine flow battery in Test Example 5 of this application: (a) without additives, (b) with EMBr added as an additive.

[0030] Figure 7 shows a comparison of (a) long-cycle charge-discharge curves and (b) energy efficiency of zinc / bromine flow batteries when EMBr and commercial MEP were used as additives, respectively, in Test Example 5 of this application.

[0031] Figure 8 shows (a) a photograph, (b) a long-cycle charge-discharge curve, and (c) a cycle efficiency of the zinc / bromine liquid flow stack when EMBr is used as an additive in Test Example 5 of this application.

[0032] Figure 9 shows the changes in bromine content and polybrominated state in different electrolyte systems: (a) 2M ZnBr2 + 3M KCl + 0.4M MEP, (b) 2M ZnBr2 + 3M KCl + 0.4M EMBr. Detailed Implementation

[0033] The present application is described in detail below with reference to the embodiments, but the present application is not limited to these embodiments.

[0034] Unless otherwise specified, all raw materials used in the embodiments of this application were purchased through commercial channels.

[0035] This application was characterized using scanning electron microscopy, depth microscopy, in-situ optical microscopy, and X-ray diffraction.

[0036] Assembly of a single-cell (zinc / bromine flow battery): The single-cell structure includes end plates, a graphite current collector, carbon felt measuring 6×8×0.5cm (length×width×height) as positive and negative electrodes, a Daramic 900μm polyethylene porous membrane as a separator, a flow frame, a silicone pad, positive and negative electrode electrolyte storage tanks, a pump, and piping. The following examples and comparative examples use a single cell assembled in this manner for testing.

[0037] The zinc / bromine flow batteries assembled in this application embodiment and comparative example differ from those in Example 1 only in their electrolyte composition.

[0038] Example 1

[0039] An electrolyte containing 2 mol / L ZnBr2, 3 mol / L KCl and 0.4 mol / L 1-ethyl-3-methylimidazolium bromide (EMBr) was prepared in water. The same electrolyte was used for both the positive and negative electrodes. Zinc / bromine flow batteries were assembled and tested. The specific test results are shown in Table 1.

[0040] Example 2

[0041] An electrolyte containing 2 mol / L ZnBr2, 3 mol / L KCl and 0.4 mol / L 1-ethyl-3-methylimidazolium chloride (EMCl) was prepared in water. The same electrolyte was used for both the positive and negative electrodes. Zinc / bromine flow batteries were assembled and tested. The specific test results are shown in Table 1.

[0042] Example 3

[0043] An electrolyte containing 2 mol / L ZnBr2, 3 mol / L KCl and 0.4 mol / L 1-carboxymethyl-3-methylimidazolium bromide (CMBr) was prepared in water. The same electrolyte was used for both the positive and negative electrodes. Zinc / bromine flow batteries were assembled and tested. The specific test results are shown in Table 1.

[0044] Example 4

[0045] An electrolyte containing 2 mol / L ZnBr2, 3 mol / L KCl and 0.4 mol / L 1-carboxymethyl-3-methylimidazolium chloride (CMCl) was prepared in water. The same electrolyte was used for both the positive and negative electrodes. Zinc / bromine flow batteries were assembled and tested. The specific test results are shown in Table 1.

[0046] Example 5

[0047] An electrolyte containing 2 mol / L ZnBr2, 3 mol / L KCl, and 0.25 mol / L 1-ethyl-3-methylimidazolium bromide (EMBr) was prepared in water. The same electrolyte was used for both the positive and negative electrodes. A zinc / bromine flow battery was assembled and tested. The battery showed no significant performance degradation after 600 cycles, with an average coulombic efficiency of 92.5%, a voltage efficiency of 87.1%, and an energy efficiency of 80.6%.

[0048] Example 6

[0049] An electrolyte containing 2 mol / L ZnBr2, 3 mol / L KCl, and 0.55 mol / L 1-ethyl-3-methylimidazolium bromide (EMBr) was prepared in water. The same electrolyte was used for both the positive and negative electrodes. A zinc / bromine flow battery was assembled and tested. The battery showed no significant performance degradation after 600 cycles, with an average coulombic efficiency of 96.9%, a voltage efficiency of 85.1%, and an energy efficiency of 82.5%.

[0050] Example 7

[0051] An electrolyte solution containing 2 mol / L ZnBr2, 3 mol / L KCl + 0.4 M 1-ethyl-3-methylimidazolium bromide (EMBr) as the negative electrode and 4 M NaBr + 3 M KCl + 0.4 M 1-ethyl-3-methylimidazolium bromide (EMBr) as the positive electrode was prepared in water. A zinc / bromine flow battery was then assembled and tested. The battery showed no significant performance degradation after 200 cycles, with an average coulombic efficiency of 93.4%, a voltage efficiency of 82.1%, and an energy efficiency of 76.7%.

[0052] Example 8

[0053] An electrolyte solution containing a negative electrode of 2 mol / L ZnSO4, 3 mol / L KCl + 0.4 M 1-ethyl-3-methylimidazolium bromide (EMBr) and a positive electrode of 2 mol / L ZnBr2, 3 mol / L KCl + 0.4 M 1-ethyl-3-methylimidazolium bromide (EMBr) was prepared in water. A zinc / bromine flow battery was then assembled and tested. The battery showed no significant performance degradation after 100 cycles, with an average coulombic efficiency of 91.8%, a voltage efficiency of 74.5%, and an energy efficiency of 69.4%.

[0054] Comparative Example 1

[0055] An electrolyte containing 2 mol / L ZnBr2, 3 mol / L KCl and 0.4 mol / L N-methyl-N-ethylpyrrolidine bromide (MEP) was prepared in water. The same electrolyte was used for both the positive and negative electrodes. Zinc / bromine flow batteries were assembled and tested. The specific test results are shown in Table 1.

[0056] Comparative Example 2

[0057] An electrolyte containing 2 mol / L ZnBr2, 3 mol / L KCl and 0.4 mol / L 1-hexyl-3-methylimidazolium bromide was prepared in water. The same electrolyte was used for both the positive and negative electrodes. Zinc / bromine flow batteries were assembled and tested. The specific test results are shown in Table 1.

[0058] Comparative Example 3

[0059] An electrolyte containing 2 mol / L ZnBr2, 3 mol / L KCl and 0.4 mol / L 1-hexadecyl-3-methylimidazolium bromide was prepared in water. The same electrolyte was used for both the positive and negative electrodes. Zinc / bromine flow batteries were assembled and tested. The specific test results are shown in Table 1.

[0060] Comparative Example 4

[0061] An electrolyte containing 2 mol / L ZnBr2, 3 mol / L KCl and 0.4 mol / L 1-propyl-3-methylimidazolium bromide was prepared in water. The same electrolyte was used for both the positive and negative electrodes. Zinc / bromine flow batteries were assembled and tested. The specific test results are shown in Table 1.

[0062] Comparative Example 5

[0063] An electrolyte containing 2 mol / L ZnBr2, 3 mol / L KCl and 0.4 mol / L 1-butyl-3-methylimidazolium bromide was prepared in water. The same electrolyte was used for both the positive and negative electrodes. Zinc / bromine flow batteries were assembled and tested. The specific test results are shown in Table 1.

[0064] Comparative Example 6

[0065] An electrolyte containing 2 mol / L ZnBr2, 3 mol / L KCl and 0.4 mol / L 1-ethyl-3-methylpyridine bromide was prepared in water. The same electrolyte was used for both the positive and negative electrodes. Zinc / bromine flow batteries were assembled and tested. The specific test results are shown in Table 1.

[0066] Comparative Example 7

[0067] An electrolyte containing 2 mol / L ZnBr2, 3 mol / L KCl and 0.2 mol / L 1-ethyl-3-methylimidazolium bromide (EMBr) was prepared in water. The same electrolyte was used for both the positive and negative electrodes. Zinc / bromine flow batteries were assembled and tested. The specific test results are shown in Table 1.

[0068] Comparative Example 8

[0069] An electrolyte containing 2 mol / L ZnBr2, 3 mol / L KCl and 0.6 mol / L 1-ethyl-3-methylimidazolium bromide (EMBr) was prepared in water. The same electrolyte was used for both the positive and negative electrodes. Zinc / bromine flow batteries were assembled and tested. The specific test results are shown in Table 1.

[0070] Comparative Example 9

[0071] An electrolyte containing 2 mol / L ZnBr2, 3 mol / L KCl and 0.8 mol / L 1-ethyl-3-methylimidazolium bromide (EMBr) was prepared in water. The same electrolyte was used for both the positive and negative electrodes. Zinc / bromine flow batteries were assembled and tested. The specific test results are shown in Table 1.

[0072] Comparative Example 10

[0073] An electrolyte containing 2 mol / L ZnBr2 and 3 mol / L KCl was prepared in water. The same electrolyte was used for both the positive and negative electrodes. A zinc / bromine flow battery was assembled and tested. The specific test results are shown in Table 1.

[0074] The effect of adding EMBr on zinc deposition was verified by tests 1-5, using an electrolyte containing EMBr (the electrolyte prepared in Example 1) and an electrolyte without EMBr (the electrolyte differed from the electrolyte prepared in Example 1 in that it did not contain EMBr, but the rest of the composition was the same as the electrolyte prepared in Example 1):

[0075] Zn|| graphite symmetric cells were assembled using different electrolytes. Their structure is similar to that of the aforementioned Zn / Br flow cell. Specifically, the structure of a single cell includes an end plate, a graphite current collector, a carbon felt measuring 6×8×0.5cm (length×width×height) as the positive electrode, and no carbon felt at the negative electrode. When the electrolyte flows through the negative electrode cavity, zinc is directly deposited on the graphite current collector. A Daramic 900μm polyethylene porous membrane is used as the separator, along with a flow frame, a silicone pad, positive and negative electrolyte storage tanks, a pump, and pipelines.

[0076] Test Example 1: Effect of EMBr additive on zinc deposition morphology and structure

[0077] To verify the effect of EMBr additive on zinc deposition morphology, Zn||graphite symmetric cells were assembled using different electrolytes. The compositions of the two electrolytes were (2M ZnBr2+3M KCl, 2M ZnBr2+3M KCl+0.4M EMBr).

[0078] At a current density of 40 mA cm -2 Surface capacity is 20mAh cm -2 After zinc was deposited under harsh operating conditions, the surface deposition morphology of the zinc anode was characterized using SEM.

[0079] As shown in Figure 1, when using an electrolyte without EMBr, a large amount of loose, irregular moss-like deposits were generated on the zinc deposition surface. These deposits easily become "dead zinc" during subsequent cycles, thus affecting the coulombic efficiency of the battery (Figure 1a). Upon magnification (Figure 1b), it was found that these zinc flakes, perpendicular to the substrate or at a large angle, were dispersed within the deposits, potentially puncturing the separator and affecting the battery's cycle stability. In contrast, a dense, uniform, and smooth zinc deposition structure could be achieved in an electrolyte containing EMBr (Figures 1c and 1d), demonstrating its effective improvement in the deposition morphology of the zinc anode.

[0080] The surface undulations of zinc deposits were studied using ultra-deep surface profilometry. As shown in Figures 2a, b, c, and d, compared with electrolytes without EMBr, the difference between the highest and lowest points of the zinc surface deposited in electrolytes containing EMBr was smaller, indicating a smoother surface. This further verifies the beneficial effects of EMBr, suggesting that it may improve the reversibility and cycle stability of the zinc anode.

[0081] Test Example 2: In-situ optical microscopy observation of the effect of EMBr additive on zinc deposition nucleation.

[0082] The macroscopic morphology of zinc deposition is closely related to the nucleation mechanism. To more accurately study the influence of additives on the nucleation process during zinc deposition, an in-situ optical microscope cell suitable for observing the electrode surface was designed to track and observe the surface changes during zinc deposition in real time. Figure 3 shows the changes in zinc nucleation and growth on the zinc metal surface observed in a planar in-situ cell, with a charge-discharge current density of 5 mA cm⁻¹. -2 In the absence of additives, as shown in Figure 3a, zinc preferentially nucleates at defects on the zinc metal surface, and subsequently grows preferentially at the nucleated sites. When the deposition time reaches 120s, the number of zinc nuclei is small and the nucleation sites are sparse. During deposition, zinc preferentially grows at the nucleated sites, and the original zinc foil surface is still exposed (Figure 3b). Under the same conditions, the system containing additives (Figure 3d) has a large number of nuclei, and the nuclei are small and dense. During deposition, the sites easily grow and merge together. When the growth process ends, the original zinc foil surface (Figure 3c) is basically uniformly covered. This indicates that the addition of additives can induce uniform zinc nucleation, which is beneficial to the uniform deposition of zinc in the future.

[0083] Test Example 3: In-situ optical microscopy observation of the effect of EMBr additive on zinc deposition growth.

[0084] The addition of additives significantly affected the nucleation process, resulting in marked differences in the morphology of the subsequently grown zinc. At a charge / discharge current density of 5 mA cm⁻¹ -2 The surface capacity is 2.5 mA h cm -2As shown in Figure 4a, an optical microscope image of the zinc electrode surface after the initial deposition and dissolution reveals that without additives (Figure 4a), after a discharge process with the same charging time, a large amount of zinc remains on the electrode surface, with numerous protrusions at the edges. However, with the addition of EMBr additives (Figure 4b), the zinc residue is relatively small, and the edges are more even, indicating that the introduction of additives improves the reversibility of cycling. The protrusions in Figure 4a, due to the tip effect, have a high charge density and are prone to becoming preferential growth sites for the next deposition, leading to uneven zinc deposition. During dissolution, insufficient discharge induces dendrite formation (Figure 4c), a process observed in situ using an optical microscope. With the addition of EMBr additives, dense zinc deposition is induced during cycling (Figure 4d).

[0085] Test Example 4: Effect of EMBr additive on zinc deposition morphology

[0086] 40mA cm -2 20mA h cm -2 Under the same conditions, the zinc deposited in Zn||graphite symmetric cells in two different electrolytes was analyzed by XRD (Figure 5). Without additives, the deposited zinc was predominantly deposited on the 101 crystal plane, while with additives, the 002 crystal plane was predominant. The 002 crystal plane is parallel to the substrate; zinc deposited with this plane as the dominant crystal plane is less prone to dendrite formation, which is beneficial for the long-term stable operation of the cell.

[0087] Test Example 5: Application of EMBr additive in zinc / bromine flow batteries

[0088] Based on the previously demonstrated effect of EMBr in smoothing zinc deposition morphology, and to verify the feasibility of EMBr in zinc-based energy storage devices, the zinc anode was combined with a common Br2 / ... - The redox couple was matched and assembled into a zinc / bromine flow cell at a current density of 40 mA cm⁻¹. -2 The surface capacity is 40mAh cm -2 The additive was subjected to constant current charge and discharge with a discharge cutoff voltage of 0.1V to further study its role.

[0089] Figure 6 compares the bromine distribution photographs in the positive electrode electrolyte tank at the end of battery charging. It can be seen that for the battery without additives (Figure 6a), the electrolyte is dark reddish-brown, and a large amount of reddish-brown bromine vapor is observed above the solution. In contrast, in the system containing EMBr (Figure 6b), there is no bromine vapor above the solution, and the electrolyte is grayish-orange. This indicates that the additive has a significant binding ability with polybrominated anions, which can effectively reduce the vapor pressure of bromine, reduce bromine loss, inhibit bromine diffusion, and thus improve the coulombic efficiency of the battery.

[0090] As shown in Figure 7a, the battery with EMBr as the additive showed no fluctuations in its charge-discharge curve and no significant decrease in energy efficiency after about 4500 hours of operation (Figure 7b). However, when commercial MEP was used as the additive, the charge-discharge curve showed increased polarization after about 400 hours of operation, and its energy efficiency gradually decreased after about 100 cycles, further confirming the practicality of the developed EMBr additive.

[0091] Figure 8 shows the assembly of a zinc / bromine battery stack using an electrolyte formulated with EMBr as an additive. It can be seen that at a 120 mAh cm⁻¹… -2 With high surface capacity, the fuel cell stack can operate stably for more than 1,900 hours without significant performance degradation.

[0092] Figure 9 shows the changes in bromine content and polybrominate state in different electrolyte systems: (a) 2M ZnBr2 + 3M KCl + 0.4M MEP, (b) 2M ZnBr2 + 3M KCl + 0.4M EMBr. It can be seen that as the bromine content in the electrolyte increases from 1.2M to 4.0M, the MEP system gradually forms a solid precipitate, while the EMBr system remains in the oil phase. Under the same conditions, 1-ethyl-3-methylimidazolium chloride, 1-carboxymethyl-3-methylimidazolium bromide, and 1-carboxymethyl-3-methylimidazolium chloride all remain in the oil phase.

[0093] Table 1 Battery performance of different embodiments and comparative examples

[0094] Note: The efficiencies (coulomb efficiency, voltage efficiency, energy efficiency) mentioned in the table are average values ​​under a certain number of cycles. "Greater than" means that the highest number of cycles has not yet been updated during stable operation.

[0095] Constant current charging and discharging, current density 40mA cm-2, surface capacity 40mAh cm-2, discharge cutoff voltage 0.1V.

[0096] In the above examples and comparative examples, the active electrolyte material was 2 mol / L ZnBr2, and 3 mol / L KCl was added as a supporting electrolyte. Battery data from Examples 1-2 show that the introduction of 1-ethyl-3-methylimidazolium bromide (EMBr) or 1-ethyl-3-methylimidazolium chloride (EMCl) allows the battery to operate stably for over 700 cycles with an energy efficiency of at least 80%. Comparative Example 1 demonstrates the battery performance of a commercially available MEP, showing that its stability is significantly lower than that of EMBr or EMCl. This proves the effectiveness of EMBr or EMCl in improving the performance of zinc-bromine batteries. Battery data from Examples 3-4 show that the introduction of 1-carboxymethyl-3-methylimidazolium bromide (CMBr) or 1-carboxymethyl-3-methylimidazolium chloride (CMCl) allows the battery to operate stably for over 700 cycles with an energy efficiency of at least 75%. Comparative Example 1 shows that while the battery of a commercially available MEP has higher efficiency in the initial stage of operation, its stability is significantly lower than that of CMBr or CMCl.

[0097] Comparative Examples 2-6 show that slightly altering the functional groups in EMBr significantly reduces performance. Using 1-hexyl-3-methylimidazolium bromide as an additive results in low voltage efficiency, leading to low energy efficiency. When using 1-hexadecyl-3-methylimidazolium bromide as an additive in Comparative Example 3, the complexation product between the EMBr and the polybrominated compounds on the positive electrode side after charging is solid, causing blockage of the flow battery's circuitry and preventing cycling. Using 1-propyl-3-methylimidazolium bromide or 1-butyl-3-methylimidazolium bromide in Comparative Examples 4-5, their strong binding affinity to the polybrominated compounds on the positive electrode side makes bromine dissociation difficult during discharge, significantly reducing the battery's voltage efficiency. Using 1-ethyl-3-methylpyridine bromide in Comparative Example 6 results in weaker binding affinity to the polybrominated compounds on the positive electrode, leading to lower coulombic efficiency. Similarly, while similar quaternary ammonium salts with modified functional groups all possess bromine complexation capabilities, they are not entirely suitable for stable cycling in zinc / bromine batteries, and some additives affect electrolyte solubility, thus reducing battery energy density. For example, 1-hexylpyridine bromide and 1-butyl-2-methylpyridine bromide are both quaternary ammonium salts with similar structures to EMBr, but they are not miscible with zinc bromide electrolyte and cannot be used to obtain zinc bromide electrolyte.

[0098] Comparative Examples 7-9 demonstrate the battery performance at different EMBr concentrations. The results show that when the EMBr concentration is below 0.2 mol / L, the coulombic efficiency is low due to the poor complexation ability of polybrominated compounds at such low concentrations. When the EMBr concentration is above 0.6 mol / L, the addition of a large amount of EMBr easily adsorbs onto the electrode surface, hindering the contact between the active material and the electrode, resulting in a decrease in voltage efficiency. In addition, higher concentrations also lead to increased costs, which is not conducive to commercial applications. Therefore, the preferred concentration in this application is 0.4 mol / L.

[0099] The above description is merely a few embodiments of this application and is not intended to limit this application in any way. Although this application discloses preferred embodiments as described above, it is not intended to limit this application. Any changes or modifications made by those skilled in the art without departing from the scope of the technical solution of this application using the disclosed technical content are equivalent to equivalent implementation cases and fall within the scope of the technical solution.

Claims

1. An electrolyte, characterized by, The electrolyte comprises an active substance, a supporting electrolyte and an additive; The additive is at least one selected from 1-ethyl-3-methyl imidazole bromide, 1-ethyl-3-methyl imidazole chloride, 1-carboxymethyl-3-methyl imidazole bromide and 1-carboxymethyl-3-methyl imidazole chloride.

2. The electrolyte according to claim 1, characterized in that, The electrolyte uses water as a solvent, and the concentration of the additive in the electrolyte is 0.25-0.55 mol / L.

3. The electrolyte of claim 1, wherein, The electrolyte uses water as a solvent, and the concentration of the additive in the electrolyte is 0.35-0.45 mol / L.

4. The electrolyte of claim 1, wherein, The active substance comprises a positive electrode active substance and a negative electrode active substance. The positive electrode active substance is bromine ion, preferably at least one selected from zinc bromide, potassium bromide and sodium bromide. The negative electrode active substance is zinc ion, preferably at least one selected from zinc bromide, zinc chloride, zinc sulfate, zinc nitrate and zinc acetate.

5. The electrolyte according to claim 1 or 4, characterized in that, The electrolyte uses water as a solvent, and the concentration of the active substance in the electrolyte is 0.5-5 mol / L.

6. The electrolyte of claim 1, wherein The supporting electrolyte is at least one selected from KCl, NaCl, KBr and NaBr.

7. The electrolyte according to claim 1 or 6, characterized in that, The electrolyte uses water as a solvent, and the concentration of the supporting electrolyte in the electrolyte is 0.5-4 mol / L.

8. A zinc / bromine flow battery characterized in that, The zinc / bromine flow battery comprises a positive electrode, a negative electrode and an electrolyte. The electrolyte is selected from the electrolyte according to any one of claims 1-7.

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