High-solubility sulfonation-improved aqueous iron-based flow battery electrolyte

Abstract

The invention relates to a high-solubility sulfonation-improved aqueous iron-based flow battery electrolyte, belongs to the technical field of flow battery energy storage, and aims to solve the problem of low solubility of the iron-based flow battery electrolyte, the novel high-solubility iron-based flow battery electrolyte is synthesized by adopting an asymmetric sulfonation ligand to chelate an iron element. And the concentration of the active substance at normal temperature can reach 1.3 M or above. The electrolyte is mainly composed of one or more iron-containing salts, and the iron-containing salts are coordinated with 3-trihydroxymethyl methylamino-2-hydroxy propanesulfonic acid (TAPSO), trihydroxymethyl methylamino propanesulfonic acid (TAPS), trihydroxymethyl methylamino ethanesulfonic acid (TES) or derivative chemicals thereof to form an aqueous solution of a coordination compound. The electrolyte is matched with ferrocyanide, can provide voltage of 1.3 V or above, shows excellent cycling stability while keeping high solubility, and creates new possibility for development and application of a flow battery technology.

Description

Technical Field

[0001] The present invention belongs to the technical field of liquid flow battery energy storage, and more specifically, relates to a method for preparing a high-solubility sulfonated improved aqueous iron-based liquid flow battery electrolyte, which can be used in electric vehicles, public utility backup power supplies, and large-scale energy storage fields, etc. Background Art

[0002] With the growth of the global population and economic development, human demand for energy continues to increase. However, the exploitation and use of traditional fossil energy such as coal, oil, and natural gas have caused serious environmental pollution and climate change. Therefore, in order to achieve the goal of sustainable development, it is imperative to replace traditional fossil energy with renewable energy. In the context of the "dual carbon" era of carbon peak and carbon neutrality, renewable energy technology has made great progress. Renewable energy represented by wind power and solar power generation is widely favored, the installed capacity and total power generation continue to grow, and the proportion of non-fossil energy consumption continues to increase. However, wind power and solar power generation are easily affected by environmental factors and have non-steady-state characteristics of randomness, intermittency, and volatility. In order to ensure the safe operation of the power grid, balance the supply and demand of energy, enhance the value of new energy, and improve the consumption of new energy, it is crucial to develop various energy storage technologies. The existing new energy storage technologies mainly include: liquid flow battery energy storage, lithium-ion battery energy storage, compressed air energy storage, flywheel energy storage and other new energy storage technologies. The above various energy storage technologies are constantly developing and improving, providing important technical support for the large-scale development of new energy.

[0003] Compared with other energy storage technologies, the advantages of liquid flow battery energy storage technology are: (1) decoupling of power and capacity: the energy storage active materials and electrodes of liquid flow batteries are independent of each other, and the power and capacity can be flexibly adjusted as needed. The specific operations are as follows: the power of the liquid flow battery can be increased by increasing the number of single cells and the electrode area, and the capacity of the liquid flow battery can be increased by increasing the concentration or volume of the electrolyte; (2) long cycle life: during the charging and discharging process of liquid flow batteries, there is no phase change involved, and deep discharge can be performed. The cycle life can reach 20,000 to 30,000 times, and the service life can reach more than 20 years; (3) high safety: the positive and negative active materials of the liquid flow battery are stored in the positive and negative electrolytic storage tanks respectively, and self-discharge will not occur. The heat generated by the battery can also be taken away by the flowing electrolyte, avoiding damage to the battery structure or even combustion due to battery heating.

[0004] The electrolyte contains active substances that undergo redox reactions and is one of the key materials for flow batteries. It should meet the following requirements: (1) high stability; (2) high solubility; (3) high reactivity; and (4) wide potential range. Iron is the second most widely distributed metal element on Earth and ranks fourth in the element distribution sequence, only after oxygen, silicon, and aluminum. It is easy to obtain and inexpensive. Although the standard electrode potential of iron is only 0.77 V, different ligands can form complexes with iron, causing the electrode potential of the electrode reaction to shift in the positive and negative directions, thereby improving the reversibility of the battery and being used to form an all-iron flow battery. The iron-based electrolytes prepared by current technology usually have low solubility and poor stability of the complexes, which leads to precipitation of active substances and ion crosstalk during battery operation, resulting in decreased battery performance. Therefore, it is of great significance to develop a new type of negative electrode electrolyte to improve the solubility and stability of flow batteries. Summary of the invention

[0005] In order to solve the problem of low solubility of iron-based flow battery electrolyte, the present invention uses asymmetric sulfonated ligands to chelate iron elements to synthesize a new type of high-solubility iron-based flow battery electrolyte, and the active material concentration can reach more than 1.3M at room temperature. The electrolyte is compatible with ferrocyanide and can provide a voltage of more than 1.3V. While maintaining high solubility, it shows excellent cycle stability, creating new possibilities for the development and application of flow battery technology.

[0006] To achieve the above object, the present invention adopts the following technical solution:

[0007] A high-solubility sulfonated improved aqueous iron-based liquid flow battery electrolyte, characterized in that the main components of the electrolyte are iron salt, chelating agent, supporting electrolyte, and auxiliary electrolyte; the chelating agent is one of 3-tris(hydroxymethyl)methylamine-2-hydroxypropanesulfonic acid (TAPSO) or its derivative chemicals, tris(hydroxymethyl)methylaminopropanesulfonic acid (TAPS) or its derivative chemicals, tris(hydroxymethyl)methylaminoethanesulfonic acid (TES) or its derivative chemicals, or a mixture of multiple thereof.

[0008] The concentration of the iron salt in the electrolyte C 1 is 0.1 to 2.0 mol / L, preferably 0.5 to 1.5 mol / L; the concentration of the chelating agent C 2 0.5~4.0mol / L, preferably 0.5~2.0mol / L; wherein C 1 :C 2 =1:1~1:1.5. The supporting electrolyte concentration C 3 is 0.1 to 8.0 mol / L, preferably 1.0 to 6.0 mol / L; the auxiliary electrolyte concentration C 4It is 0 to 2.0 mol / L, preferably 0.2 to 1.0 mol / L.

[0009] The iron salt is a divalent iron salt or / and a trivalent iron salt, and further is one of ferrous sulfate / ferrous iron, ferrous nitrate / ferrous iron, ferric chloride / ferrous iron, ferric acetate / ferrous iron, or a mixture of multiple thereof.

[0010] The derivative chemicals of TAPSO are sodium 3-trihydroxymethylamine-2-hydroxypropanesulfonate and potassium 3-trihydroxymethylamine-2-hydroxypropanesulfonate; the derivative chemicals of TAPS are sodium tris(hydroxymethyl)methylaminopropanesulfonate and potassium tris(hydroxymethyl)methylaminopropanesulfonate; the derivative chemicals of TES are sodium tris(hydroxymethyl)methylaminoethanesulfonate and potassium tris(hydroxymethyl)methylaminoethanesulfonate.

[0011] The supporting electrolyte includes but is not limited to potassium carbonate, sodium carbonate, lithium carbonate, lithium hydroxide, sodium hydroxide, potassium hydroxide or a mixture thereof. The auxiliary electrolyte is a neutral salt compound, including but not limited to one or a mixture of ionic compounds such as ammonium salts, sodium salts, potassium salts, lithium salts, etc., including but not limited to one or a mixture of potassium chloride, potassium nitrate, sodium nitrate, sodium chloride, ammonium chloride, potassium sulfate, sodium sulfate, and ammonium sulfate. The pH value of the electrolyte is 10.0 to 13.9.

[0012] The negative electrode electrolyte configuration method comprises:

[0013] Add iron salt, chelating agent and supporting electrolyte to the reactor from which air has been exhausted, mix and add deionized water to fully dissolve; then dissolve an appropriate amount of auxiliary electrolyte; and finally adjust the volume to the target volume.

[0014] The high-solubility sulfonated improved aqueous iron-based liquid flow battery electrolyte is characterized in that the operating temperature of the iron-based liquid flow battery electrolyte is between 20 and 80°C.

[0015] A water-based iron-based liquid flow battery, wherein the negative electrode electrolyte adopts the above-mentioned high-solubility sulfonated improved water-based iron-based liquid flow battery electrolyte, and the positive electrode electrolyte includes ferrocyanide, such as potassium ferrocyanide, sodium ferrocyanide, etc.

[0016] Compared with the prior art, the present invention has the following significant advantages and beneficial effects:

[0017] 1. Chelating agents TAPSO, TAPS, and TES, due to the asymmetry of the structure and the introduction of sulfonic acid groups, can form multiple hydrogen bonds and other interactions (such as van der Waals forces) with surrounding water molecules, thereby enhancing the interaction with water and thus improving solubility.

[0018] 2. The ligand introduced in the present invention has multiple coordination sites and can form a stable coordination structure with the iron element, thereby improving the stability of the electrolyte, keeping the solution in a uniform state without precipitation of iron hydroxide, and effectively improving the operating stability and cycle life of the battery.

[0019] 3. The iron and chelating agent used in the present invention are both relatively abundant and low-cost materials. Compared with the raw materials of all-vanadium liquid flow batteries, the raw materials of the liquid flow battery are easier to obtain, and the electrolyte configuration method is simple, thereby reducing production costs.

[0020] In summary, the iron-based liquid flow battery of the present invention has low cost, improves the energy storage density per unit volume of electrolyte, exhibits excellent cycle stability while maintaining high solubility, and creates new possibilities for the development and application of liquid flow battery technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is the cyclic voltammetry curve of the negative electrode electrolyte of Example 1.

[0022] Figure 2 This is the charge and discharge curve of the first cycle of 500 cycles of the battery assembled with the electrolyte of Example 1.

[0023] Figure 3 The charge and discharge capacity-cycle curve of Example 1 within 500 cycles.

[0024] Figure 4 This is the charge and discharge efficiency-cycle curve within 500 cycles of Example 1.

[0025] Figure 5 This is the relationship between the absorbance of the electrolyte of each concentration in Example 2 and the iron ion concentration.

[0026] Figure 6 This is the battery efficiency-cycle curve of the high concentration electrolyte test in Example 5.

[0027] Table 1 is a comparison of the efficiency of different electrolytes in various embodiments. DETAILED DESCRIPTION

[0028] The present invention is described in detail below through specific examples, but the use and purpose of these exemplary embodiments are only used to illustrate the present invention, and do not constitute any form of limitation on the actual protection scope of the present invention, nor limit the protection scope of the present invention to these.

[0029] Embodiment 1:

[0030] Negative electrolyte: This embodiment provides a method for preparing a high-solubility sulfonated improved aqueous iron-based flow battery electrolyte, wherein the solute of the negative electrolyte includes 0.5 mol / L ferric chloride, 0.6 mol / L TAPSO, 3.0 mol / L potassium hydroxide, and 0.5 mol / L potassium chloride, and the solvent is deionized water;

[0031] Positive electrode electrolyte: The solutes include potassium ferrocyanide and sodium ferrocyanide, which are configured in a molar ratio of 1:1. The total ferrocyanide concentration is 0.5 mol / L (which matches the concentration of iron ions at the negative electrode). Potassium hydroxide solution is used to adjust the pH to be consistent with the negative electrode, and sodium chloride is added as a supporting electrolyte.

[0032] The negative electrode electrolyte is prepared as follows: exhaust the air in the reactor in advance, weigh 0.25 mol ferric chloride, 0.3 mol TAPSO, 1.5 mol potassium hydroxide, and 0.25 mol potassium chloride and put them into the reactor, add 450 mL of deionized water, stir at room temperature for 12 h to make a solution, dilute to 500 mL and transfer to a volumetric flask for storage, and let it stand for 24 h before use.

[0033] The preparation method of the positive electrode electrolyte is as follows: exhaust the air in the reactor in advance, weigh 0.125 mol of potassium ferrocyanide, sodium ferrocyanide and sodium chloride respectively, add 450 mL of deionized water, stir at room temperature for 1 hour to make a solution, add potassium hydroxide solution to adjust the pH to be consistent with the negative electrode, and make the volume to 500 mL, let it stand for 24 hours before use.

[0034] The above negative electrode electrolyte was subjected to electrochemical tests, such as Figure 1 As shown. The results show that the cyclic voltammetry curves at different scan rates have good reversibility, the voltage generated by the negative electrode is ~-1.0V, and the total voltage difference with the positive electrode can be about >1.3V. And with the increase of the scan rate, the current density gradually increases.

[0035] The positive and negative electrolytes mentioned above are pumped into the corresponding liquid storage tanks respectively. The remaining components of the flow battery include: battery stack, control system, peristaltic pump, anti-corrosion pipeline, test system, etc. The battery stack includes: end plates, gaskets, bipolar plates, proton exchange membranes, and graphite felt electrodes. The graphite felt electrode is 2.5cm×3.0cm, and the proton exchange membrane is Nafion 211. Before the test, an inert gas is circulated in the battery to avoid inactivation of active substances. Then the battery is connected to the battery test system at 80mA / cm 2 Start testing.

[0036] Figure 2This is the first cycle charge and discharge curve of the battery assembled with the electrolyte of Example 1 after 500 cycles. It can be seen that its coulombic efficiency (CE) is close to 100%, and its energy efficiency (EE) is greater than 81%, which has very excellent performance.

[0037] Figure 3 , 4 The charge and discharge capacity and efficiency-cycle curves of the battery in Example 1 within 500 cycles are shown. According to the results, the capacity remains basically unchanged within 500 cycles, the overall EE is stable at around 80%, and the CE is always close to 100%.

[0038] Embodiment 2:

[0039] Negative electrolyte: This embodiment provides a method for preparing a sulfonated modified aqueous iron-based flow battery electrolyte with high solubility at different concentrations. The solute of the negative electrolyte includes ferric chloride with a concentration of 0.05 to 1.5 mol / L, TAPSO with a concentration of 0.05 to 1.5 mol / L, a ratio of ferric chloride to TAPSO of 1:1, each concentration gradient is 0.1 mol / L, potassium hydroxide with a concentration of 3.0 mol / L, and the solvent is deionized water;

[0040] The negative electrode electrolyte is prepared as follows: exhaust the air in the reactor in advance, weigh 0.05-1.5 mol ferric chloride, 0.05-1.5 mol TAPSO, and 1.5 mol potassium hydroxide in turn and put them into the reactor, add 450 mL of deionized water, stir at room temperature for 12 hours to make a solution, make the volume to 500 mL and transfer to a volumetric flask for storage, prepare 8 electrolyte solutions with a concentration gradient of 0.1-0.8 mol / L, and use them after standing for 24 hours.

[0041] The above electrolytes were tested for UV absorbance in turn, and a linear relationship was established with the iron ion concentration. The results are as follows: Figure 5 The results show that the maximum solubility of the electrolyte is 1.33 mol / L (when the molar ratio of ferric chloride to TAPSO is 1:1, the saturation concentration of ferric chloride is 1.33 mol / L), which is far beyond the current technical level.

[0042] Embodiment 3:

[0043] Negative electrolyte: This embodiment provides a method for preparing a high-solubility sulfonated improved aqueous iron-based flow battery electrolyte, wherein the solute of the negative electrolyte includes 0.5 mol / L ferric chloride, 0.6 mol / L TAPS, 3.0 mol / L potassium hydroxide, and 0.5 mol / L potassium chloride, and the solvent is deionized water;

[0044] Positive electrode electrolyte: The solutes include potassium ferrocyanide and sodium ferrocyanide, which are configured in a 1:1 molar ratio. The total ferrocyanide concentration is 0.5 mol / L to match the negative electrode. Potassium hydroxide solution is used to adjust the pH to be consistent with the negative electrode, and sodium chloride is added as a supporting electrolyte.

[0045] The negative electrode electrolyte is prepared as follows: exhaust the air in the reactor in advance, weigh 0.25 mol ferric chloride, 0.3 mol TAPS, 1.5 mol potassium hydroxide, and 0.25 mol potassium chloride and put them into the reactor, add 450 mL of deionized water, stir at room temperature for 12 h to make a solution, dilute to 500 mL and transfer to a volumetric flask for storage, and let it stand for 24 h before use.

[0046] The preparation method of the positive electrode electrolyte is as follows: exhaust the air in the reactor in advance, weigh 0.125 mol of potassium ferrocyanide, sodium ferrocyanide and sodium chloride respectively, add 450 mL of deionized water, stir at room temperature for 1 hour to make a solution, add potassium hydroxide solution to adjust the pH to be consistent with the negative electrode, and make the volume to 500 mL, let it stand for 24 hours before use.

[0047] After the battery system is assembled, other reference conditions are the same as those in Example 1. To prevent the active material from being deactivated, wait until the inert gas fills the entire battery system. Connect to the battery test system and start testing.

[0048] Embodiment 4:

[0049] Negative electrolyte: This embodiment provides a method for preparing a high-solubility sulfonated improved aqueous iron-based flow battery electrolyte, wherein the solute of the negative electrolyte includes 0.5 mol / L ferric chloride, 0.6 mol / L TES, 3.0 mol / L potassium hydroxide, and 0.5 mol / L potassium chloride, and the solvent is deionized water;

[0050] Positive electrode electrolyte: The solutes include potassium ferrocyanide and sodium ferrocyanide, which are configured in a 1:1 molar ratio. The total ferrocyanide concentration is 0.5 mol / L to match the negative electrode. Potassium hydroxide solution is used to adjust the pH to be consistent with the negative electrode, and sodium chloride is added as a supporting electrolyte.

[0051] The negative electrode electrolyte is prepared as follows: exhaust the air in the reactor in advance, weigh 0.25 mol ferric chloride, 0.3 mol TES, 1.5 mol potassium hydroxide, and 0.25 mol potassium chloride and put them into the reactor, add 450 mL of deionized water, stir at room temperature for 12 h to make a solution, dilute to 500 mL and transfer to a volumetric flask for storage, and let it stand for 24 h before use.

[0052] The preparation method of the positive electrode electrolyte is as follows: exhaust the air in the reactor in advance, weigh 0.125 mol of potassium ferrocyanide, sodium ferrocyanide and sodium chloride respectively, add 450 mL of deionized water, stir at room temperature for 1 hour to make a solution, add potassium hydroxide solution to adjust the pH to be consistent with the negative electrode, and make the volume to 500 mL, let it stand for 24 hours before use.

[0053] After the battery system is assembled, other reference conditions are the same as those in Example 1. To prevent the active material from being deactivated, wait until the inert gas fills the entire battery system. Connect to the battery test system and start testing.

[0054] Embodiment 5:

[0055] Negative electrolyte: This embodiment provides a method for preparing a high-solubility sulfonated modified aqueous iron-based flow battery electrolyte, which is intended to examine the battery performance when the active material concentration is close to saturation. The solutes of the negative electrolyte include 1.2 mol / L ferric chloride, 1.2 mol / L TAPSO, 4.0 mol / L potassium hydroxide, 0.5 mol / L potassium chloride, and the solvent is deionized water;

[0056] Positive electrode electrolyte: The solute includes potassium ferrocyanide and sodium ferrocyanide, which are configured in a molar ratio of 1:1, and the total ferrocyanide concentration is 0.8 mol / L. Potassium hydroxide solution is used to adjust the pH to be consistent with the negative electrode, and sodium chloride is added as a supporting electrolyte. Note that the volume of the positive electrode electrolyte used in this embodiment is 1.5 times that of the negative electrode to keep the total amount of active substances matched.

[0057] The negative electrode electrolyte is prepared as follows: exhaust the air in the reactor in advance, weigh 0.6 mol ferric chloride, 0.6 mol TAPSO, 2 mol potassium hydroxide, and 0.25 mol potassium chloride and put them into the reactor, add 480 mL of deionized water, stir at room temperature for 12 h to make a solution, dilute to 500 mL and transfer to a volumetric flask for storage, and let it stand for 24 h before use.

[0058] The preparation method of the positive electrode electrolyte is as follows: exhaust the air in the reactor in advance, weigh 0.4 mol of potassium ferrocyanide, sodium ferrocyanide and sodium chloride respectively, add 480 mL of deionized water, stir at room temperature for 1 hour to make a solution, add potassium hydroxide solution to adjust the pH to be consistent with the negative electrode, and make the volume to 500 mL, let it stand for 24 hours before use.

[0059] After the battery system is assembled, other reference conditions are the same as those in Example 1. To prevent the active material from being deactivated, wait until the inert gas fills the entire battery system. Connect to the battery test system and start testing.

[0060] Battery test results are as follows Figure 6As shown, the results show that the battery still maintains a very stable efficiency of about 75% within 200 cycles. Although the efficiency is lower than that of Example 1, the energy storage capacity is 2.4 times that of Example 1.

[0061] Table 1 is a comparison of the efficiency of different electrolytes in various embodiments (500 cycles).

[0062] sample Electrolyte CE / % VE / % EE / % Example 1 Fe-TAPSO 99.8 81.5 81.3 Example 3 Fe-TAPS 98.5 79.1 77.9 Example 4 Fe-TES 98.9 76.9 76.1 .

Claims

1. A high-solubility sulfonated improved aqueous iron-based flow battery electrolyte, characterized in that: The main components of the electrolyte are iron salt, chelating agent, supporting electrolyte and auxiliary electrolyte; the chelating agent is one of 3-tris(hydroxymethyl)methylamine-2-hydroxypropanesulfonic acid (TAPSO) or its derivative chemicals, tris(hydroxymethyl)methylaminopropanesulfonic acid (TAPS) or its derivative chemicals, tris(hydroxymethyl)methylaminoethanesulfonic acid (TES) or its derivative chemicals or a mixture of multiple thereof.

2. A high-solubility sulfonated improved aqueous iron-based flow battery electrolyte according to claim 1, characterized in that: The concentration C1 of the iron salt in the electrolyte is 0.1 to 2.0 mol / L, preferably 0.5 to 1.5 mol / L; the concentration C2 of the chelating agent is 0.5 to 4.0 mol / L, preferably 0.5 to 2.0 mol / L; wherein C1:C2=1:1-1:1.

5. The supporting electrolyte concentration C3 is 0.1-8.0 mol / L, preferably 1.0-6.0 mol / L; the auxiliary electrolyte concentration C4 is 0-2.0 mol / L, preferably 0.2-1.0 mol / L.

3. A high-solubility sulfonated improved aqueous iron-based flow battery electrolyte according to claim 1, characterized in that: The iron salt is a divalent iron salt or / and a trivalent iron salt, and further is one of ferrous sulfate / ferrous iron, ferrous nitrate / ferrous iron, ferric chloride / ferrous iron, ferric acetate / ferrous iron, or a mixture of multiple thereof.

4. A high-solubility sulfonated improved aqueous iron-based flow battery electrolyte according to claim 1, characterized in that: The derivative chemicals of TAPSO are sodium 3-trihydroxymethylamine-2-hydroxypropanesulfonate and potassium 3-trihydroxymethylamine-2-hydroxypropanesulfonate; the derivative chemicals of TAPS are sodium tris(hydroxymethyl)methylaminopropanesulfonate and potassium tris(hydroxymethyl)methylaminopropanesulfonate; the derivative chemicals of TES are sodium tris(hydroxymethyl)methylaminoethanesulfonate and potassium tris(hydroxymethyl)methylaminoethanesulfonate.

5. A high-solubility sulfonated improved aqueous iron-based flow battery electrolyte according to claim 1, characterized in that: The supporting electrolyte includes but is not limited to one or a mixture of potassium carbonate, sodium carbonate, lithium carbonate, lithium hydroxide, sodium hydroxide, potassium hydroxide; the auxiliary electrolyte is a neutral salt compound, including but not limited to one or a mixture of ionic compounds such as ammonium salt, sodium salt, potassium salt, lithium salt, etc.

6. A high-solubility sulfonated improved aqueous iron-based flow battery electrolyte according to claim 1, characterized in that: The pH value of the electrolyte is 10.0-13.

9.

7. A method for preparing a high-solubility sulfonated improved aqueous iron-based liquid flow battery electrolyte according to any one of claims 1 to 6, characterized in that: The following steps are involved: Add iron salt, chelating agent and supporting electrolyte to the reactor from which air has been exhausted, mix and add deionized water to fully dissolve; then dissolve an appropriate amount of auxiliary electrolyte; and finally adjust the volume to the target volume.

8. Use of a high-solubility sulfonated improved aqueous iron-based liquid flow battery electrolyte as described in any one of claims 1-6 as a negative electrode electrolyte for an iron-based liquid flow battery, wherein the operating temperature of the iron-based liquid flow battery electrolyte is 20 to 80°C.

9. An aqueous iron-based flow battery, characterized in that: The electrolyte is a high-solubility sulfonated improved aqueous iron-based liquid flow battery electrolyte as described in any one of claims 1 to 6.

10. The aqueous iron-based flow battery according to claim 9, characterized in that: The negative electrode electrolyte is a high-solubility sulfonated improved aqueous iron-based liquid flow battery electrolyte as described in any one of claims 1 to 6, and the positive electrode electrolyte includes ferrocyanide.

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