An electrolyte additive

Abstract

The present invention relates to a multivalent anion-based organic additive for solving the problem of electrolyte migration in an alkaline zinc-iron flow battery. Specifically, by adding a multivalent anion-based organic additive to the negative electrolyte of the alkaline zinc-iron flow battery, the ion concentration and ionic strength of the negative electrolyte are increased, and the difference in ion concentration and ionic strength from the positive electrolyte is reduced, thereby effectively suppressing the electrolyte migration problem caused by the inconsistent ion concentration and ionic strength between the positive and negative electrolytes of the alkaline zinc-iron flow battery, and greatly reducing the maintenance cost of the electrolyte during the actual use of the battery.

Description

Technical Field

[0001] The present invention belongs to the field of flow batteries, and particularly relates to a multivalent anion-based organic additive for solving the electrolyte migration problem of an alkaline zinc-iron flow battery. Background Art

[0002] The alkaline zinc-iron flow battery uses zinc and iron, which are rich in resources, as active substances, and has advantages such as low electrolyte cost (~$100 / kWh) and high open-circuit voltage (1.74V), and has good application prospects in the field of distributed energy storage. The alkaline zinc-iron flow battery was proposed by G-B. Adams et al. in the United States in 1979. Compared with the all-vanadium flow battery and zinc-bromine flow battery that are currently in the demonstration application stage, the alkaline zinc-iron flow battery has not yet made significant breakthrough progress. One of the key technical bottlenecks is that during the charge and discharge process of the alkaline zinc-iron flow battery or stack, under the combined action of the electric field gradient and concentration gradient, the bound water carried by the charge-balancing ions migrates from the negative electrode to the positive electrode of the battery or stack, and this process is irreversible, resulting in serious electrolyte migration, thereby causing the imbalance of the electrolyte volume of the battery or stack, and further leading to the reduction of the stability of the battery or stack, and greatly increasing the maintenance cost of the battery or stack.

[0003] Patent CN201611133465.X, WOCN17111224 disclose an alkaline zinc-iron flow battery, and the redox couple of the battery positive electrode is Fe(CN)6 3- / Fe(CN)6 4- and the redox couple of the negative electrode is Zn(OH)4 2- / Zn. During the charge and discharge process of this battery system, the positive and negative electrode electrochemical reactions are as follows:

[0004]

[0005]

[0006] It can be seen from the positive and negative electrode electrochemical reaction equations of the alkaline zinc-iron flow battery that during the battery charging process, the concentration of divalent Zn(OH)4 2- on the negative electrode side gradually decreases, and the concentration of Fe(CN)6 3- on the positive electrode side gradually increases; according to the solution ionic strength calculation formula:

[0007]

[0008] where: ci is the molality of ion i, and zi is the charge number of the ion.

[0009] According to the research in the literature (ACS Appl. Mater. Interfaces 2020, 12, 51573 - 51580), it is known that during the charge and discharge process of an alkaline zinc - iron flow battery, the total ion concentration in the positive electrolyte is always higher than that in the negative electrolyte. Moreover, due to the larger number of charges carried by the positive - electrode redox couple, the ionic strength in the positive electrolyte is much higher than that in the negative electrolyte. The combined effect of these two factors causes water in the negative electrolyte to gradually migrate to the positive electrode during the operation of the battery, resulting in electrolyte imbalance.

[0010] Regarding the problem of electrolyte migration in alkaline zinc - iron flow batteries, Patent 201710830958.7 discloses a symmetric alkaline zinc - iron flow battery. In this symmetric alkaline zinc - iron flow battery, the positive and negative electrolytes are both mixed aqueous solutions of ferrocyanide, zinc salt or / and zinc oxide and strong base. Since the same electrolyte is used for both the positive and negative electrodes, the concentration gradient and ionic strength of the positive and negative electrolytes are basically the same, avoiding the problem of water migration existing in the alkaline zinc - iron flow batteries disclosed in Patent CN201611133465.X and WOCN17111224. However, in this symmetric alkaline zinc - iron flow battery, because both the positive and negative electrolytes are mixed aqueous solutions of ferrocyanide, zinc salt or / and zinc oxide and strong base, there is a common - ion effect in the electrolyte, making the solubility of Fe(CN)6 3- / Fe(CN)6 4- relatively low (≤0.6 mol L -1 ), resulting in a low energy density of the battery, which is not conducive to practical applications.

[0011] On the premise of ensuring a relatively high solubility of Fe(CN)6 3- / Fe(CN)6 4- in the positive electrolyte, Patent CN201811372457.X and the literature (ACS Appl. Mater. Interfaces 2020, 12, 51573 - 51580) disclose a negative electrolyte for an alkaline zinc - based flow battery, its preparation and application. The negative electrolyte is a supramolecular complex solution formed by mixing a zinc precursor, an organic additive and a strong base. Among them, the organic additive is small - molecule sugars, small - molecule alcohols or a complex of both. When applied to an alkaline zinc - based flow battery, it effectively inhibits the electrolyte migration problem caused by the inconsistent osmotic pressure of the positive and negative electrolytes in the alkaline zinc - based flow battery. The organic additives used in this patent and the literature are small - molecule sugars, small - molecule alcohols or a complex of both, and there are no dissociable groups in the additives, resulting in a relatively large difference in ionic strength between the positive and negative electrolytes.

[0012] Therefore, how to alleviate or solve the problem of water migration in an alkaline zinc-iron flow battery or stack during charging and improve the cycle stability of the battery or stack is of great significance for breaking through the key technical bottlenecks in the application of alkaline zinc-iron flow batteries and promoting their practicalization process. SUMMARY OF THE INVENTION

[0013] To solve the problem of water migration existing in traditional alkaline zinc-iron flow batteries, the present invention proposes a multivalent anion-type organic additive for solving the electrolyte migration problem of alkaline zinc-iron flow batteries. The proposed multivalent anion-type organic additive has a relatively large molecular size and cannot reach the positive electrolyte from the negative electrolyte through the membrane material, thereby endowing the negative electrolyte with a high ion concentration and osmotic pressure. In addition, this multivalent anion-type organic additive can be completely dissociated into anions in the alkaline zinc-iron flow battery electrolyte, reducing the difference in ionic strength from the positive electrolyte. The synergistic effect of the two can significantly improve the water migration problem existing in traditional alkaline zinc-iron flow batteries. To achieve the above effects, the specific technical solutions are as follows:

[0014] On the one hand, the present invention provides a negative electrode liquid for an alkaline zinc-iron flow battery, and the negative electrolyte includes a mixed solution formed by a zinc precursor, a multivalent anion-type organic additive, and a strong base;

[0015] The multivalent anion-type organic additive is one or more of a multivalent anion-type organic compound, a sodium salt, a potassium salt, or a lithium salt corresponding to the multivalent anion-type organic compound; the multivalent anion-type organic compound includes one or more of ethylenediamine o-dihydroxyphenylacetic acid (EDDHA), 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid (DO3A), ethylenediaminetetraacetic acid (EDTA), 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA), diethylenetriaminepentaacetic acid (DTPA), ethylenediaminetetramethylenephosphonic acid (EDTMPA), and hydroxyethylidene diphosphonic acid (EA). The specific structures are as follows:

[0016]

[0017] Based on the above scheme, preferably, the molar concentration of Zn(OH)4 2- (or the complex formed by the multivalent anion-type organic additive and the zinc precursor) generated after the zinc precursor is dissolved in the strong base is 0.01 - 2 mol / L -1 , the concentration of the multivalent anion-type organic additive is 0.05 - 1.0 mol / L -1 , and is further preferably 0.2 - 0.7 mol / L -1 ; the molar concentration of the strong base is 0.01 - 6 mol / L -1 .

[0018] On the other hand, the present invention provides a method for preparing the negative electrode electrolyte for the above-mentioned alkaline zinc-iron flow battery. The negative electrode electrolyte can be prepared by the following two methods:

[0019] Method 1: Mix a zinc precursor with a strong base and slowly add deionized water, and stir well at 20-100 °C for 0.1-5 hours to form a uniform solution. Then add a polyvalent anion organic additive, and slowly add deionized water and stir well at 20-100 °C for 0.1-5 hours to obtain the negative electrode electrolyte.

[0020] Method 2: Dissolve a zinc precursor and a polyvalent anion organic additive in water to form a uniform solution (the zinc precursor and the polyvalent anion organic additive form a complex), then slowly add a strong base, and stir well at 20-100 °C for 0.1-5 hours to obtain the negative electrode electrolyte.

[0021] Based on the above scheme, preferably, the strong base is at least one of NaOH, KOH, and LiOH; the zinc precursor is at least one of zinc chloride, zinc bromide, zinc iodide, zinc sulfate, zinc nitrate, zinc carbonate, and zinc oxide.

[0022] Advantages of the present invention

[0023] 1. Effectively improve the water migration problem existing in the alkaline zinc-iron flow battery: The polyvalent anion organic additive proposed by the present invention effectively improves the problem that water in the electrolyte migrates from the negative electrode to the positive electrode due to the inconsistent concentration and ionic strength of the positive and negative electrode electrolytes during the operation of the traditional alkaline zinc-iron flow battery, effectively reducing the system maintenance cost caused by electrolyte migration and showing good application prospects.

[0024] 2. The polyvalent anion organic additive can be dissociated into ionic form in the solution, improving the solution conductivity and having no significant impact on the battery performance. Compared with additives such as small molecule sugars or small molecule alcohols, the polyvalent anion organic additive provided by the present invention further reduces the difference in ionic strength from the positive electrode electrolyte, improving the stability of the alkaline zinc-iron flow battery.

[0025] 3. The introduction of the polyvalent anion organic additive does not reduce the solubility of the active substance in the negative electrode electrolyte, enabling the battery to have a high energy density.

[0026] 4. The added polyvalent anion organic additive has a low cost: The listed polyvalent anion organic additives are widely sourced and easily available in the market, capable of maintaining the low-cost advantage of the alkaline zinc-iron flow battery and having good application prospects in the energy storage field.

[0027] 5. The added polyvalent anionic organic additives are safe and non-toxic: The listed polyvalent anionic organic additives are all safe and non-toxic, ensuring the high safety and high stability of the alkaline zinc-iron flow battery. Brief Description of the Drawings

[0028] Figure 1 Effect of additives with different EDTA concentrations on the performance of the alkaline zinc-iron flow battery.

[0029] Figure 2 Electrolyte migration test of the alkaline zinc-iron flow battery without using polyvalent anionic organic additives and the alkaline zinc-iron flow battery in Examples 1-3 of the present invention.

[0030] Figure 3 In Example 3, the alkaline zinc-iron flow battery at 80 mA cm -2 Cyclic stability test under the working current density condition. Detailed Description of the Invention

[0031] The following is a further description in combination with specific examples, so that the advantages and features of the present invention can be more easily understood by those skilled in the art. The raw materials used in the following examples are all commercially available conventional products.

[0032] Composition of the alkaline zinc-iron flow battery in the following examples and comparative examples: Composition of the positive electrolyte: 0.8 mol L -1 Fe(CN)6 4- + 3 mol L -1 KOH; the volume of each of the positive and negative electrolytes is 80 mL; the positive and negative electrodes are both porous carbon felt electrodes, and the graphite plate is used as the current collector; the ion conductive membrane is a polybenzimidazole ion conductive membrane. Composition of the negative electrolyte in the following examples: 0.4 mol L -1 Zn(OH)4 2- + 3 mol L -1 NaOH + polyvalent anionic organic additives of different types and concentrations. Preparation method of the negative electrolyte in the following examples:

[0033] Method 1: Dissolve 0.4 mol L -1 ZnO in 3.8 mol L -1 NaOH, add polyvalent anionic organic additives of different types and concentrations after forming a homogeneous solution, and stir well at 25 °C for 0.1 - 5 hours to obtain the negative electrolytes used in Examples 1-5 below.

[0034] Method 2: Mix 0.4 mol L -1 ZnBr with the sodium salts corresponding to polyvalent anionic organic compounds of different types and concentrations (at 0.4 mol L -1Taking sodium ethylenediaminetetraacetate (EDTA-4Na) as an example), after mixing and adding deionized water, stirring at room temperature for 10 minutes to form a homogeneous solution, 0.5 mol / L -1 NaOH was added to obtain the negative electrode electrolyte used in Examples 6-10 below.

[0035] The following is the single cell assembly method for examples and comparative examples: The single cell was assembled in the following order: positive electrode end plate, graphite current collector, positive electrode 6×8 cm 2 carbon felt, polybenzimidazole ion conductive membrane, negative electrode 6×8 cm 2 carbon felt, graphite current collector, negative electrode end plate.

[0036] The following are the test conditions for the alkaline zinc-iron flow battery in examples and comparative examples: Electrode effective area: 48 cm 2 ; The battery adopts a constant current charge-discharge mode, charging for 18 min under a current density condition of 80 mA / cm -2 , and then discharging to 0.1 V under a current density condition of 80 mA / cm -2 .

[0037] Comparative Example 1

[0038] Taking the electrolyte composition of a traditional alkaline zinc-iron flow battery as a comparison, the positive electrode electrolyte composition is 0.8 mol / L -1 Fe(CN)6 4- + 3 mol / L -1 KOH; The negative electrode electrolyte composition is 0.4 mol / L -1 Zn(OH)4 2- + 3 mol / L -1 NaOH, and the test conditions are as described above.

[0039] At the end of discharge, the volumes of the positive and negative electrode electrolytes were measured. From Figure 2 it can be seen that in this traditional alkaline zinc-iron flow battery, after about 50 cycles, due to the imbalance of the ion concentration and ionic strength of the positive and negative electrode electrolytes, the electrolyte migrates from the negative electrode to the positive electrode. This electrolyte migration will increase the maintenance cost of the system in practical applications and is not conducive to large-scale applications.

[0040] Comparative Example 2

[0041] When the negative electrode electrolyte composition is 0.4 mol / L -1 Zn(OH)4 2- + 3 mol / L -1 NaOH + 0.5 mol / L -1When using glucose, the test conditions are as described above. Glucose will adsorb on the electrode, increasing electrode polarization, and the battery voltage efficiency will decrease from ~89% to 83%. After about 120 cycles of the battery, the volume of the negative electrolyte is 67 mL, the volume of the positive electrolyte is 93 mL, and the electrolyte migration amount has certain improvement compared with the blank electrolyte.

[0042] Examples 1 - 10

[0043] Same test conditions for the alkaline zinc - iron flow battery: Effective area of the electrode: 48 cm 2 ; The battery adopts a constant - current charge - discharge mode. It is charged for 18 min under a current density condition of 80 mA cm -2 , and then discharged until 0.1 V under the condition of a voltage cut - off, at a current density condition of 80 mA cm -2 . Composition of the positive electrolyte: 0.8 mol L -1 Fe(CN)6 4- + 3 mol L -1 KOH; The volume of both the positive and negative electrolytes is 80 mL each; Both the positive and negative electrodes are porous carbon felt electrodes, and the graphite plate is used as the current collector plate; The ion - conducting membrane is a polybenzimidazole ion - conducting membrane.

[0044] The difference lies in the composition of the negative electrolyte, and the specific situation is as follows in the table:

[0045] Example Negative electrode electrolyte composition Example 1 <![CDATA[0.4 mol L -1 Zn(OH)4 2- + 3 mol L -1 NaOH + 0.1 mol L -1 EDTA]]> Example 2 <![CDATA[0.4 mol L -1 Zn(OH)4 2- +3 mol L -1 NaOH + 0.3 mol L -1 EDTA]]> Example 3 <![CDATA[0.4 mol L -1 Zn(OH)4 2- +3 mol L -1 NaOH + 0.7 mol L -1 EDTA]]> Example 4 <![CDATA[0.4 mol L -1 Zn(OH)4 2- +3 mol L -1 NaOH + 0.8 mol L -1 EDTA]]> Example 5 <![CDATA[0.4 mol L -1 Zn(OH)4 2- +3 mol L -1 NaOH + 0.3 mol L -1 DTPA]]> Example 6 <![CDATA[0.4 mol L -1 ZnBr2 + 0.4 mol L -1 EDTA-4Na + 0.5 mol L -1 NaOH]]> Example 7 <![CDATA[0.6 mol L -1 ZnBr2 + 0.6 mol L -1 EDTA-4Na + 0.5 mol L -1 NaOH]]> Example 8 <![CDATA[0.4mol L -1 Zn(OH)4 2- +3mol L -1 NaOH+0.3mol L -1 EDDHA]]> Example 9 <![CDATA[0.4 mol L -1 Zn(OH)4 2- + 3 mol L -1 NaOH + 0.3 mol L -1 DOTA]]> Example 10 <![CDATA[0.4 mol L -1 Zn(OH)4 2- +3 mol L -1 NaOH + 0.3 mol L -1 EDTMPA]]>

[0046] Analysis of experimental results

[0047] Example 1

[0048] For the alkaline zinc - iron flow battery with a multivalent anionic organic additive added, the composition of the negative electrolyte is 0.4 mol L -1 Zn(OH)4 2- + 3 mol L -1 NaOH + 0.1 mol L -1 EDTA; Compared with the performance of the blank battery, adding 0.1 molL -1 EDTA to the negative electrolyte has no obvious effect on the battery performance ( Figure 1 ), after about 100 cycles of the battery, the volume of the negative electrolyte is 46 mL, the volume of the positive electrolyte is 114 mL, and the electrolyte migration amount has certain improvement.

[0049] Example 2

[0050] When the composition of the negative electrolyte is 0.4 mol L -1 Zn(OH)4 2- + 3 mol L -1NaOH + 0.3 mol L -1 EDTA, the battery performance did not decrease significantly ( Figure 1 ). After about 600 cycles, the volume of the negative electrolyte was 54 mL, and the volume of the positive electrolyte was 106 mL. The electrolyte migration was significantly improved.

[0051] Example 3

[0052] When the negative electrolyte composition was 0.4 mol L -1 Zn(OH)4 2- + 3 mol L -1 NaOH + 0.7 mol L -1 EDTA, the battery performance did not decrease significantly ( Figure 1 ). After about 550 cycles, the volume of the negative electrolyte was 75 mL, and the volume of the positive electrolyte was 85 mL. The electrolyte migration was significantly improved, and the battery operated stably for more than 1600 cycles with stable performance ( Figure 3 ).

[0053] Example 4

[0054] When the negative electrolyte composition was 0.4 mol L -1 Zn(OH)4 2- + 3 mol L -1 NaOH + 0.8 mol L -1 EDTA, after 650 cycles, the volume of the negative electrolyte was 77 mL, and the volume of the positive electrolyte was 83 mL. The electrolyte migration was small, and the battery efficiency did not decrease significantly.

[0055] Example 5

[0056] When the negative electrolyte composition was 0.4 mol L -1 Zn(OH)4 2- + 3 mol L -1 NaOH + 0.3 mol L -1 DTPA, since the carboxylate ions with 5 negative charges in DTPA, compared with EDTA (carboxylate ions with 4 negative charges) at the same concentration, can make the negative electrolyte have a higher ionic strength. After 622 cycles, the volume of the negative electrolyte was 61 mL, and the volume of the positive electrolyte was 99 mL. The electrolyte migration was smaller than that when EDTA was used as an additive, and the battery efficiency did not decrease significantly.

[0057] Example 6

[0058] When the negative electrolyte composition was 0.4 mol L -1ZnBr2 + 0.4 mol L -1 EDTA - 4Na + 0.5 mol L -1 When the electrolyte of the negative electrode is composed of 0.4 mol L

[0059] Example 7

[0060] When the electrolyte of the negative electrode is composed of 0.6 mol L -1 ZnBr2 + 0.6 mol L -1 EDTA - 4Na + 0.5 mol L -1 NaOH, after the battery goes through 587 cycles, the volume of the electrolyte of the negative electrode is 59 mL, and the volume of the electrolyte of the positive electrode is 101 mL. The migration amount of the electrolyte is smaller than that when using the blank electrolyte, and the battery efficiency has no obvious attenuation.

[0061] Example 8

[0062] When the electrolyte of the negative electrode is composed of 0.4 mol L -1 Zn(OH)4 2- + 3 mol L -1 NaOH + 0.3 mol L -1 EDDHA, in addition to the carboxylate ions with 4 negative charges in EDDHA, there are also -OH and nitrogen-containing functional groups that can form hydrogen bonds with water. Compared with EDTA (carboxylate ions with 4 negative charges) at the same concentration, the electrolyte of the negative electrode can not only have the same ionic strength but also have a richer hydrogen bond network, which can effectively lock the water molecules at the negative electrode. After the battery goes through 712 cycles, the volume of the electrolyte of the negative electrode is 66 mL, and the volume of the electrolyte of the positive electrode is 94 mL. The migration amount of the electrolyte is smaller than that when using EDTA as an additive, and the battery efficiency has no obvious attenuation.

[0063] Example 9

[0064] When the electrolyte of the negative electrode is composed of 0.4 mol L -1 Zn(OH)4 2- + 3 mol L -1 NaOH + 0.3 mol L -1When using DOTA, since in DOTA, in addition to the carboxylate ions with 4 negative charges, there are also nitrogen-containing functional groups (4) that can form hydrogen bonds with water. Compared with EDTA (carboxylate ions with 4 negative charges) at the same concentration, it can make the negative electrode electrolyte have a higher ionic strength. After 657 cycles of the battery, the volume of the negative electrode electrolyte is 63 mL, and the volume of the positive electrode electrolyte is 97 mL. The electrolyte migration amount is smaller than that when using EDTA as an additive, and the battery efficiency has no obvious attenuation.

[0065] Example 10

[0066] When the composition of the negative electrode electrolyte is 0.4 mol L -1 Zn(OH)4 2- +3 mol L -1 NaOH + 0.3 mol L -1 EDTMPA, since there are phosphate ions with 4 divalent negative charges in EDTMPA, compared with EDTA (carboxylate ions with 4 monovalent negative charges) at the same concentration, it can make the negative electrode electrolyte have a higher ionic strength. After 883 cycles of the battery, the volume of the negative electrode electrolyte is 72 mL, and the volume of the positive electrode electrolyte is 88 mL. The electrolyte migration amount is smaller than that when using EDTA as an additive, and the battery efficiency has no obvious attenuation.

Claims

1. A negative electrolyte for an alkaline zinc-iron flow battery, characterized in that: The negative electrode electrolyte comprises a mixed solution formed by a zinc precursor, a multivalent anionic organic additive, and a strong base; The multivalent anionic organic additive is one or more of a multivalent anionic organic compound, a sodium salt, a potassium salt, and a lithium salt corresponding to the multivalent anionic organic compound; the multivalent anionic organic compound comprises one or more of ethylenediamine o-dihydroxyphenylacetic acid, 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid, ethylenediaminetetraacetic acid, 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid, diethylenetriaminepentaacetic acid, ethylenediaminetetramethylenephosphonic acid, and hydroxyethylidene diphosphonic acid; The negative electrode electrolyte is prepared by the following method: Mix the zinc precursor and the strong base, add deionized water, and stir well at 20-100 °C for 0.1-5 hours to form a uniform solution. Then add the multivalent anionic organic additive, add deionized water, and stir well at 20-100 °C for 0.1-5 hours to obtain the negative electrode electrolyte; The molar concentration of Zn(OH)4 formed after the zinc precursor in the negative electrode electrolyte is dissolved in a strong base 2- is 0.01 - 2 mol / L -1 , the concentration of the multivalent anion organic additive is 0.05 - 1.0 mol / L -1 , and the molar concentration of the strong base is 3 - 6 mol / L -1 ; The strong base is at least one of NaOH, KOH, and LiOH.

2. The negative electrode electrolyte for an alkaline zinc-iron flow battery according to claim 1, characterized in that The concentration of the multivalent anionic organic additive in the negative electrode electrolyte is 0.2 - 0.7 mol L -1 .

3. The negative electrode electrolyte for an alkaline zinc-iron flow battery according to claim 1, characterized in that, The zinc precursor is at least one of zinc chloride, zinc bromide, zinc iodide, zinc sulfate, zinc nitrate, zinc carbonate, and zinc oxide, or two or more thereof.

4. An alkaline zinc-iron flow battery, characterized in that, The alkaline zinc-iron flow battery comprises the negative electrode electrolyte according to any one of claims 1 to 3.

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