Electrolyte additive for inhibiting water migration of alkaline zinc-iron flow battery and screening method thereof

Abstract

The invention discloses an alkali metal salt additive for solving the problem of electrolyte migration in an alkaline zinc-iron flow battery. The ion concentration and the ion strength of the negative electrode electrolyte can be improved, and the number and the size of water molecule clusters in the electrolyte are reduced, so that the problem of electrolyte migration in the battery circulation process is relieved, the cycle life of the battery is remarkably prolonged, and the electrolyte maintenance cost of the battery in the actual use process is greatly reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of alkaline zinc-iron liquid flow batteries, and relates to an electrolyte additive for solving water migration in alkaline zinc-iron liquid flow batteries and a screening method thereof. Background Art

[0002] Liquid flow batteries have the advantages of high safety, long life, environmental friendliness, and capacity and power decoupling, and have broad application prospects in the field of large-scale long-term energy storage. Alkaline zinc-iron liquid flow batteries use Zn(OH)4 2- / Zn redox couple as the negative electrode active couple, Fe(CN)6 3- / Fe(CN)6 4- As the active electrode pair at the positive electrode, the redox pair has outstanding advantages such as low production cost, high energy density (high output voltage), and high safety, and has great potential for industry development. However, due to the different compositions of the positive and negative electrolytes, and the zinc deposition-dissolution reaction at the negative electrode, there is a significant difference in ion concentration and ion strength between the electrolytes on both sides. Under the further influence of the electric field, the alkaline zinc-iron flow battery faces a serious water migration problem, that is, the bound water carried by the charge-balancing ions migrates from the negative electrode to the positive electrode, resulting in an irreversible change in the electrolyte level difference on both sides, which greatly increases the maintenance cost of the battery or stack, and becomes one of the key technical bottlenecks for its practical application.

[0003] Patent CN 111200146 A discloses a negative electrode electrolyte for an alkaline zinc-based flow battery, and its preparation and application. Small molecule sugars, small molecule alcohols or a complex of the two are introduced into the negative electrode electrolyte to inhibit the migration of water molecules in the negative electrode electrolyte to the positive electrode. The organic molecules used in this patent cannot dissociate in the electrolyte, so there is still a large difference in ionic strength between the positive and negative electrode electrolytes. Patent CN 115911473 A discloses an electrolyte additive, namely a multivalent anionic organic compound, as a water migration inhibitor for alkaline zinc-iron flow batteries, including ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA) and corresponding sodium, potassium and lithium salts. The above compounds are all organic weak acids and corresponding weak acid salts, and have a strong chelating effect with metal ions, and can react with Zn 2+ Therefore, how to effectively alleviate the water migration problem through electrolyte design, thereby improving the long-term cycle stability of the electrolyte; and guiding the screening of additives from the mechanism, is of great significance for breaking through the key technical bottleneck of alkaline zinc-iron flow batteries and promoting their commercialization process. Summary of the invention

[0004] In order to solve the water migration problem of the alkaline zinc-iron flow battery, the present invention proposes an alkaline zinc-iron flow battery electrolyte alkali metal salt additive to increase the ion concentration and osmotic pressure of the negative electrode electrolyte, thereby reducing the ion concentration and strength difference between the positive and negative electrode electrolytes, which can effectively improve the electrolyte water migration problem and realize the development of long-life, low-cost alkaline zinc-iron flow batteries. The specific technical solution is as follows: The first object of the present invention is to provide: a negative electrode electrolyte for an alkaline zinc-iron liquid flow battery, wherein the negative electrode electrolyte comprises a zinc precursor, a soluble alkali metal salt additive and an inorganic base.

[0005] The anions in the soluble alkali metal salt additive are one or more of chloride ions, sulfate ions, nitrate ions, perchlorate ions, trifluoromethanesulfonate ions, and bis(trifluoromethanesulfonyl)imide ions, and the cations in the soluble alkali metal salt additive are one or more of lithium ions, sodium ions, or potassium ions.

[0006] The zinc precursor is one of zinc chloride, zinc bromide, zinc iodide, zinc nitrate, zinc sulfate, zinc carbonate and zinc oxide, or a mixture of several of them.

[0007] The inorganic base is one or a mixture of lithium hydroxide, sodium hydroxide and potassium hydroxide.

[0008] The zinc precursor is dissolved in the electrolyte to generate Zn(OH)4 2- Or the concentration of the complex formed by the anion in the soluble salt additive and the zinc precursor is 0.01~3.0 mol / L, preferably 0.1~1.0 mol / L; the concentration of the alkali metal salt additive is 0.05~3.0 mol / L, preferably 0.1~1.0 mol / L; the concentration of the inorganic base is 0.01~6.0 mol / L.

[0009] The second object of the present invention is to provide: a method for preparing a negative electrode electrolyte for an alkaline zinc-iron liquid flow battery, comprising the following steps: Step 1: After the zinc precursor and the strong base are fully mixed, deionized water is slowly added and stirred to obtain a clear and transparent uniform solution; Step 2: Add an alkali metal salt additive to the above solution, slowly add deionized water and continue stirring to obtain a negative electrode electrolyte.

[0010] In the step 1 and the step 2, the stirring temperature is 10-100°C, and the stirring time is 0.1-3 h.

[0011] The third object of the present invention is to provide a method for screening soluble alkali metal salt additives in negative electrode electrolytes for alkaline zinc-iron flow batteries, wherein the additives are used to alleviate water molecule clustering and avoid cross-membrane migration of charge-balancing ions, comprising the following steps: Step 1: By adding the single point energy and the free energy correction term, the Gibbs free energy of the anions of different soluble alkali metal salt additives before and after binding with water molecules is calculated respectively; Step 2: Calculate the Gibbs free energy change ∆G of the binding process, and select the one with higher ∆G as an additive to alleviate the migration of electrolyte during battery cycling.

[0012] In the step 2, the screening basis is the largest absolute value of ∆G, which is used to indicate that the binding force with water molecules is the weakest among various additives.

[0013] Beneficial Effects The invention scheme of the negative electrode electrolyte for alkaline zinc-iron flow batteries provided by the present invention can effectively improve the water migration problem of traditional alkaline zinc-iron flow batteries. Alkali metal salts generally have high solubility in water. On the one hand, the salts can be completely dissociated in water, increasing the ion concentration and ionic strength of the negative electrode liquid. On the other hand, the introduction of some anions will destroy the hydrogen bond network of the negative electrode liquid and reduce the size and content of water molecule clusters. In addition, the present invention provides a screening method for alkali metal salt additives for solving the water migration problem of alkaline zinc-iron flow batteries, evaluates the binding energy change between different additive anions and water molecules, and screens out alkali metal salt additives with potential application prospects. The preparation process of the present invention is simple and the effect is significant. It can effectively improve the cycle life of the battery system and reduce the system maintenance cost caused by electrolyte migration. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 : Effect of different LiTFSI addition concentrations on the performance of alkaline zinc-iron flow batteries.

[0015] Figure 2 : Electrolyte migration test of alkaline zinc-iron flow batteries in Comparative Example 1 and Examples 1-3 of the present invention.

[0016] Figure 3 : In Example 3, the alkaline zinc-iron flow battery at 80 mA cm -2 Cyclic stability test under current density conditions.

[0017] Figure 4 : Free energy change of the binding of TFSI anion and EDTA anion with water molecules.

[0018] Figure 5 : The free energy change of the binding of different anions to water molecules in Examples 1-6.

[0019] In the legend of the accompanying drawings, CE stands for Charge Efficiency, VE stands for Voltage Efficiency, and EE stands for Energy Efficiency. DETAILED DESCRIPTION

[0020] Various indicative implementation cases of the present invention are now described in detail. This detailed description should not be considered as a limitation to the invention, but should be understood as a detailed description of certain aspects, features, implementation cases and effects of the present invention.

[0021] The composition of the alkaline zinc-iron flow battery of the embodiment and the comparative example is as follows: The positive electrode electrolyte composition: 1 mol L -1 Fe(CN)6 4- + 2 mol L -1 KOH; the volume of positive and negative electrolytes is 20 mL each; the positive and negative electrodes are porous carbon felt electrodes, and the graphite plate is used as the current collector; the separator is a sulfonated polyetheretherketone (SPEEK) ion conductive membrane. Negative electrolyte composition: 0.5 mol L -1 Zn(OH)4 2- + 4 mol L -1 NaOH + soluble alkali metal salt additives of different types and concentrations.

[0022] Preparation method of negative electrode electrolyte: 0.5 mol L -1 ZnO is dissolved in 5 mol L -1 A clear and transparent uniform solution is formed in NaOH, and then alkali metal salt additives of different types and concentrations are added, and stirring is continued at room temperature for 0.1 to 3 hours to obtain the negative electrode electrolyte used in the following Examples 1-6.

[0023] Example and Comparative Example Single Cell Assembly Method: The single cell is assembled in the following order: positive terminal plate, graphite current collector, positive electrode 3×3.5 cm 2 Carbon felt, sulfonated polyetheretherketone ion conductive membrane, negative electrode 3×3.5 cm 2 Carbon felt, graphite current collector, negative terminal plate.

[0024] Example and Comparative Example Alkaline zinc-iron flow battery test conditions: electrode effective area: 10.5 cm 2 The battery was charged and discharged in constant current mode at 80 mA cm -2 The battery was charged to 70% state of charge at a current density of 1.50 mA cm-1 and discharged with voltage as the cutoff condition at 80 mA cm-1. -2 The battery was discharged to 0.1 V at a current density of 2.5 %.

[0025] Some selected examples and comparative examples of the present invention are described in detail below.

[0026] Bistrifluoromethylsulfonyl imide anion (TFSI) is often used in alkali metal salt additives. - ) or ethylenediaminetetraacetate anion (EDTA 4- ), as a chelating agent, because it can form stable water-soluble complexes with a variety of metal ions. By DFT calculation, TFSI - With EDTA 4- The free energy change of binding with water molecules. The results show that TFSI - The free energy change between TFSI and water molecules is very high, indicating that its binding force with water molecules is weak. By introducing TFSI, the original hydrogen bond network of the negative electrode electrolyte is destroyed, the size and content of water molecule clusters are reduced, and the transmembrane migration of a large number of water molecule clusters accompanied by charge-balancing ions is effectively alleviated. - EDTA 4- Compare the free energy changes of binding with water molecules respectively, such as Figure 4 Based on this, embodiments 1-3 of the present invention use TFSI - Lithium ions were used as alkali metal salt additives to compare the effects of the additives on electrolyte migration at different concentrations.

[0027] Comparative Example 1 The conventional alkaline zinc-iron flow battery electrolyte group is used as a comparison. The cathode electrolyte composition is 1 mol L -1 Fe(CN)6 4- + 2 mol L -1 KOH; the negative electrode electrolyte composition is 0.5 mol L -1 Zn(OH)4 2- + 4 mol L -1 NaOH, the test single cell assembly method and battery test conditions are as described above.

[0028] At regular intervals, the volume of the positive and negative electrolytes is read at the end of the battery discharge, and a graph of the number of cycles versus the volume of the positive and negative electrolytes is drawn, such as Figure 1 As shown, as the cycle progresses, due to the imbalance between the ion concentration and ion strength of the positive and negative electrolytes, the electrolyte migrates from the negative electrode to the positive electrode. After about 80 cycles, the liquid in the negative electrode electrolyte tank can no longer support the normal operation of the battery. 85% of the total electrolyte migrates, and the average migration rate per cycle is 1.1%. Therefore, in actual application, this will greatly increase the maintenance cost of the system, which is not conducive to its application in large-scale energy storage.

[0029] The mobility refers to the percentage of the changed volume of the electrolyte to the initial volume, and the average mobility per cycle refers to the mobility divided by the corresponding number of cycles.

[0030] In the embodiment, the alkaline zinc-iron flow battery device and the test conditions are the same as those in Comparative Example 1, except for the composition of the negative electrode electrolyte, which is specifically shown in Table 1: Table 1 Composition of negative electrode electrolytes in Examples 1-6 Example Negative electrolyte composition Example 1 <![CDATA[0.5 mol L -1 Zn(OH)4 2- + 4 mol L -1 NaOH + 0.1 mol L -1 LiTFSI]]> Example 2 <![CDATA[0.5 mol L -1 Zn(OH)4 2- + 4 mol L -1 NaOH + 0.2 mol L -1 LiTFSI]]> Example 3 <![CDATA[0.5 mol L -1 Zn(OH)4 2- + 4 mol L -1 NaOH + 0.5 mol L -1 LiTFSI]]> Example 4 <![CDATA[0.5 mol L -1 Zn(OH)4 2- + 4 mol L -1 NaOH + 0.2 mol L -1 LiCl]]> Example 5 <![CDATA[0.5 mol L -1 Zn(OH)4 2- + 4 mol L -1 NaOH + 0.2 mol L -1 LiClO4]]> Example 6 <![CDATA[0.5 mol L -1 Zn(OH)4 2- + 4 mol L -1 NaOH + 0.2 mol L -1 LiCF3SO3]]> Implementation results analysis Example 1

[0031] For alkaline zinc-iron flow batteries with alkali metal salt additives, the negative electrode electrolyte composition is 0.5 mol L -1 Zn(OH)4 2- + 4 mol L -1 NaOH + 0.1 mol L -1 When LiTFSI is used, the battery performance is not significantly affected ( Figure 2 ), the battery water migration phenomenon is significantly improved. After about 80 cycles, 43% of the total electrolyte migrates, and the average migration rate per cycle is 0.54%. The electrolyte migration phenomenon is improved to a certain extent.

[0032] Example 2

[0033] When the negative electrode electrolyte composition is 0.5 mol L -1 Zn(OH)4 2- + 4 mol L -1 NaOH + 0.2 mol L -1 When LiTFSI is used, the battery performance is not significantly affected ( Figure 2 ), after about 80 cycles, 31% of the total electrolyte migrated, and the average migration rate per cycle was 0.39%. The electrolyte migration phenomenon was significantly improved, and the battery operated continuously and stably for more than 310 cycles.

[0035] When the negative electrode electrolyte composition is 0.5 mol L -1 Zn(OH)4 2- + 4 mol L -1 NaOH + 0.5 mol L -1 When LiTFSI is used, the battery performance is not significantly affected ( Figure 2 ), after about 80 cycles, 15% of the total electrolyte migrated, with an average migration rate of 0.19% per cycle. The migration of the electrolyte was significantly suppressed, and the battery operated stably for more than 500 cycles ( Figure 3 ).

[0036] Example 4

[0037] When the negative electrode electrolyte composition is 0.5 mol L -1 Zn(OH)4 2- + 4 mol L -1 NaOH + 0.2 mol L -1 When LiCl was used, the battery performance was not significantly affected. After about 80 cycles, 89% of the total electrolyte migrated, and the average migration rate per cycle was 1.11%. The electrolyte migration phenomenon was improved to a certain extent.

[0038] Example 5

[0039] When the negative electrode electrolyte composition is 0.5 mol L -1 Zn(OH)4 2- + 4 mol L -1 NaOH + 0.2 mol L -1 When LiClO4 was used, the battery performance was not significantly affected. After about 80 cycles, 55% of the total electrolyte migrated, and the average migration rate per cycle was 0.69%. The electrolyte migration phenomenon was significantly improved.

[0040] Example 6

[0041] When the negative electrode electrolyte composition is 0.5 mol L -1 Zn(OH)4 2- + 4 mol L -1 NaOH + 0.2 mol L -1 When LiCF3SO3 was used, the battery performance was not significantly affected. After about 80 cycles, 39% of the total electrolyte migrated, and the average migration rate per cycle was 0.49%. The electrolyte migration phenomenon was improved to a certain extent.

[0042] The free energy changes of the binding of different anions to water molecules in Examples 2 and 4-6.

[0043] Gaussian 16 calculation software was used, and the M06-2X function was used to calculate the Gibbs free energy before and after the anions of different alkali metal salt additives combined with water molecules by adding the single point energy and the free energy correction term. The Gibbs free energy change of the binding process was calculated by the following formula:

[0044] By calculating the free energy changes of the four anions binding to water molecules, such as Figure 5As shown in Table 2, the binding force between chloride ions and water molecules is the strongest, with a free energy change value of −0.067 eV. The binding energy between perchlorate ions, trifluoromethanesulfonate ions, and bis(trifluoromethanesulfonyl)imide ions and water increases in turn, which are 0.11 eV, 0.15 eV, and 0.24 eV, respectively. The results show that the binding force between bis(trifluoromethanesulfonyl)imide ions and water molecules is the weakest, because its introduction as an additive has the strongest destructive power on the original hydrogen bond network of the negative electrode liquid, and reduces the size and content of water molecule clusters the most, so it has the strongest ability to alleviate water molecule clusters and avoid cross-membrane migration of charge-balancing ions.

[0045] Table 2 Free energy change of examples and comparative examples Example Compound Free energy change ∆G (eV) Example 2 <![CDATA[Cl − -H2O]]> −0.067 Example 4 <![CDATA[ClO4 − -H2O]]> 0.11 Example 5 <![CDATA[OTf − -H2O]]> 0.15 Example 6 <![CDATA[TFSI − -H2O]]> 0.24

Claims

1. A negative electrode electrolyte for an alkaline zinc-iron flow battery, characterized in that: The negative electrode electrolyte includes a zinc precursor, a soluble alkali metal salt additive and an inorganic base; the anions in the soluble alkali metal salt additive are one or more of chloride ions, sulfate ions, nitrate ions, perchlorate ions, trifluoromethanesulfonate ions, and bistrifluoromethanesulfonyl imide ions, and the cations in the soluble alkali metal salt additive are one or more of lithium ions, sodium ions or potassium ions.

2. The negative electrode electrolyte for alkaline zinc-iron flow battery according to claim 1, characterized in that: The zinc precursor is one of zinc chloride, zinc bromide, zinc iodide, zinc nitrate, zinc sulfate, zinc carbonate and zinc oxide, or a mixture of several of them.

3. The negative electrode electrolyte for alkaline zinc-iron flow battery according to claim 1, characterized in that: The inorganic base is one or a mixture of lithium hydroxide, sodium hydroxide and potassium hydroxide.

4. The negative electrode electrolyte for alkaline zinc-iron flow battery according to claim 1, characterized in that: The zinc precursor is dissolved in the electrolyte to generate Zn(OH)4 2- Or the concentration of the complex formed by the anion in the salt additive and the zinc precursor is 0.01~3.0 mol / L; the concentration of the soluble alkali metal salt additive is 0.05~3.0 mol / L; the concentration of the inorganic base is 0.01~6.0 mol / L.

5. The method for preparing the negative electrode electrolyte for alkaline zinc-iron flow battery according to any one of claims 1 to 4, characterized in that: The following steps are involved: Step 1: After the zinc precursor and the inorganic base are fully mixed, deionized water is slowly added and stirred to obtain a clear and transparent uniform solution; Step 2: Add an alkali metal salt additive to the above solution, slowly add deionized water and continue stirring to obtain a negative electrode electrolyte.

6. The preparation method according to claim 5, characterized in that: In the steps 1 and 2, the stirring temperature is 10-100°C and the stirring time is 0.1-3 h.

7. The method for screening soluble alkali metal salt additives in the negative electrode electrolyte for alkaline zinc-iron flow battery according to claim 1, wherein the additive is used to alleviate water molecule clustering and avoid cross-membrane migration of charge-balancing ions, characterized in that: The following steps are involved: Step 1: By adding the single point energy and the free energy correction term, the Gibbs free energy of the anions of different soluble alkali metal salt additives before and after binding with water molecules is calculated; Step 2: Then calculate the Gibbs free energy change value ∆G of the binding process, and select those with higher ∆G as additives.

8. The method for screening additives according to claim 7, characterized in that: The screening basis described in step 2 is that the absolute value of ∆G is the largest, which is used to indicate that the binding force with water molecules is the weakest among various additives.

Images