Auxiliary-electrolyte-free high-specific-capacity and stable-circulation flow battery positive electrode electrolyte and preparation method and application of auxiliary-electrolyte-free high-specific-capacity and stable-circulation flow battery positive electrode electrolyte

Abstract

The invention discloses an auxiliary electrolyte-free high-specific-capacity and stable-circulation flow battery positive electrode electrolyte as well as a preparation method and application thereof, and relates to the field of flow batteries. The positive electrolyte is composed of iron cyanide with the molar concentration of 0.1-3 mol / L and the balance of water, the iron cyanide contains complex cations, and the complex cations are at least two of Li < + >, Na < + >, K < + > and NH4 < + >. No auxiliary electrolyte is added into the positive electrode electrolyte, the cyanide solubility of iron can be effectively improved through the hetero-ion effect, when the positive electrode electrolyte is subsequently applied to assembly of the iron-based flow battery, the specific capacity and the cycling stability of the flow battery are improved, and moreover, the electrolyte is neutral and has low requirements on acid and alkali corrosion resistance of corollary equipment, so that the positive electrode electrolyte can be applied to the assembly of the iron-based flow battery. The applied iron-based flow battery has low cost and practicability in an energy storage technology, and is beneficial to a large-scale commercialization process.

Description

Technical Field

[0001] The invention relates to the field of liquid flow batteries, and in particular to a positive electrode electrolyte for a liquid flow battery without auxiliary electrolyte, high specific capacity and stable circulation, and a preparation method thereof. Background Art

[0002] Energy storage technology is a key technology for achieving the effective utilization of intermittent clean energy such as wind energy and solar energy and building new power systems. Aqueous flow batteries have become one of the most competitive technologies in the field of large-scale long-term energy storage due to their inherent safety, independent adjustment of energy and output power, and other advantages. Currently, the most widely used all-vanadium liquid flow battery has a high initial installation cost and is unstable due to the limited resources of vanadium ore, the active material. At the same time, the electrolyte of this system is highly acidic and highly corrosive. These problems have hindered the large-scale commercialization of all-vanadium liquid flow batteries.

[0003] Iron-based flow batteries mainly include iron-chromium, iron-vanadium, zinc-iron, sulfur-iron and all-iron. The positive or negative electrode of the battery uses iron salt as the redox active substance, and the conversion of electrical energy and chemical energy is achieved through the change of the valence state of the iron element. Since iron compounds are abundant in the earth and the raw materials are easy to obtain, they have broad application prospects in the field of renewable energy storage. Therefore, iron-based flow batteries are low-cost and practical, and have become a large-scale energy storage technology with great potential.

[0004] The capacity of a flow battery is proportional to the concentration of active substances in the electrolyte. Increasing the concentration of active substances can increase the amount of energy stored per unit volume, i.e., the volumetric capacity. However, the molar concentration of the dissolved active substances in the cathode electrolyte of the current iron-based flow battery is low, and the volumetric capacity of the cathode electrolyte is low, which affects the energy density of the battery. Moreover, due to the low molar concentration of active substances in the cathode electrolyte, auxiliary electrolytes need to be added to promote ion transport, which will inevitably lead to raw material costs. Summary of the invention

[0005] The present invention provides a positive electrode electrolyte for a liquid flow battery with high specific capacity and stable cycle without auxiliary electrolyte, and a preparation method and application thereof, so as to solve the problem that the solubility of active substances in the positive electrode electrolyte of the current liquid flow battery is low and affects the energy density of the battery. The present application does not need to add auxiliary electrolytes, and increases the concentration of active substances in the positive electrode electrolyte through the heteroion effect, so as to realize a liquid flow battery with high specific capacity and stable cycle.

[0006] In order to solve the above technical problems, one of the purposes of the present invention is to provide a positive electrode electrolyte for a liquid flow battery with high specific capacity and stable circulation without auxiliary electrolyte, wherein the positive electrode electrolyte is composed of iron cyanide with a molar concentration of 0.1-3 mol / L and the balance of water, wherein the iron cyanide contains a coordinated cation, and the coordinated cation is Li+ 、Na + , K + and NH4 + At least two of the .

[0007] By adopting the above scheme, no auxiliary electrolyte is needed to be added to the positive electrolyte of the present application, and the added iron cyanide contains at least two coordinating cations, which effectively improve the solubility of the active substance through the heteroion effect. As the concentration of the active substance in the positive electrolyte increases, the amount of active substances that can participate in the electrochemical reaction in the positive electrolyte of the flow battery per unit volume is increased, and the increase in the total concentration of the corresponding coordinating cations is sufficient to support ion transmission. When it is subsequently applied to assembling iron-based flow batteries, it helps to improve the specific capacity and cycle stability of the flow battery and saves raw material costs. The positive electrolyte is neutral, and in terms of material cost and supporting equipment, the acid and alkali corrosion resistance requirements of the pipeline and the storage tank are not high; in terms of stability, compared with the conventionally used alkaline iron cyanide electrolyte, the water decomposition potential of the neutral electrolyte is higher, so the side reaction of the neutral electrolyte is less, so that the battery using the neutral electrolyte is easier to obtain excellent cycle stability performance.

[0008] As a preferred embodiment, the molar concentration of iron cyanide in the positive electrode electrolyte is 1-1.6 mol / L.

[0009] As a preferred embodiment, the iron cyanide is ferrocyanide or ferrocyanide.

[0010] By adopting the above scheme, during the charging and discharging process of the positive electrode electrolyte assembled flow battery of the present application, Fe(CN)6 4- and Fe(CN)6 3- The conversion of Fe 2+ and Fe 3+ During charging, electrical energy is converted into chemical energy, which can be stored in the electrolyte; during discharging, chemical energy is converted into electrical energy, which can be used by the load.

[0011] As a preferred embodiment, the ferrocyanide is at least one of lithium ferrocyanide, sodium ferrocyanide, potassium ferrocyanide and ammonium ferrocyanide.

[0012] As a preferred embodiment, the ferrocyanide is at least one of lithium ferrocyanide, sodium ferrocyanide, potassium ferrocyanide and ammonium ferrocyanide.

[0013] As a preferred embodiment, the coordinating cation is Li + 、Na + , K + and NH4 + At least two of the .

[0014] As a preferred embodiment, the iron cyanide is potassium ferrocyanide and sodium ferrocyanide in a molar ratio of (1-3):(1-3).

[0015] As a preferred embodiment, the iron cyanide is potassium ferrocyanide and sodium ferrocyanide in a molar ratio of 1:1.

[0016] In order to solve the above technical problems, the second purpose of the present invention provides a method for preparing a positive electrode electrolyte for a liquid flow battery with high specific capacity and cyclic stability without an auxiliary electrolyte, comprising the following steps: dissolving iron cyanide in water, stirring at room temperature until completely dissolved, and calibrating with water to a volume corresponding to the required molar concentration.

[0017] In order to solve the above technical problems, the third purpose of the present invention provides a high specific capacity and cyclically stable liquid flow battery positive electrode electrolyte without auxiliary electrolyte for use in the field of preparing iron-based liquid flow batteries. Since iron compounds are abundant in the earth and the raw materials are easily available and low in cost, iron-based liquid flow batteries have low cost and practicality in energy storage technology, which is conducive to large-scale commercialization process.

[0018] As a preferred embodiment, the iron-based flow battery includes but is not limited to iron-chromium flow battery, iron-vanadium flow battery, sulfur-iron flow battery, zinc-iron flow battery, all-iron flow battery and organic-iron flow battery.

[0019] In order to solve the above technical problems, the fourth object of the present invention provides an iron-based liquid flow battery, which adopts the above-mentioned high specific capacity and cyclically stable liquid flow battery positive electrode electrolyte without auxiliary electrolyte, including a positive electrode electrolyte, a negative electrode electrolyte and a diaphragm.

[0020] As a preferred embodiment, the negative electrode electrolyte is an alkaline aqueous solution containing 0.1-3 mol / L polysulfide.

[0021] As a preferred embodiment, the polysulfide includes but is not limited to Li2S x 、Na2S x 、K2S x 、(NH4)2S x At least one of .

[0022] As a preferred solution, the auxiliary electrolyte includes but is not limited to at least one of KOH, NaOH, LiOH, and NH4OH.

[0023] As a preferred embodiment, the diaphragm is a cation exchange membrane.

[0024] As a preferred embodiment, the diaphragm is a potassium-type perfluorosulfonic acid cation exchange membrane.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] 1. The positive electrode electrolyte of the present application does not need to add auxiliary electrolytes. The electrolyte contains at least two coordinating cations. Through the heteroion effect, the solubility of the active substance of iron cyanide is effectively improved, and the increase in the total concentration of the corresponding coordinating cations is sufficient to support ion transport. When it is subsequently used to assemble iron-based liquid flow batteries, it helps to improve the specific capacity of the liquid flow battery, achieve higher energy density and cycle stability, and save raw material costs.

[0027] 2. The positive electrode electrolyte of the present application is neutral, and the requirements for acid and alkali corrosion resistance of the supporting equipment are not high. In terms of stability, compared with the commonly used alkaline iron cyanide electrolyte, the water decomposition potential of the neutral electrolyte is higher, so the neutral electrolyte has fewer side reactions, making it easier for batteries using neutral electrolytes to obtain excellent cycle stability performance.

[0028] 3. Since iron compounds are abundant in the earth and the raw materials are easily available and low-cost, the iron-based liquid flow battery used in this application is low-cost and practical in energy storage technology, with high battery specific capacity and long charge and discharge cycle life, which is conducive to large-scale commercialization. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 : The solution properties of a positive electrode electrolyte of a flow battery with high specific capacity and stable circulation without auxiliary electrolyte in Example 1 of the present invention;

[0030] Figure 2 : The solution properties of a positive electrode electrolyte of a flow battery with high specific capacity and stable circulation without auxiliary electrolyte in Example 2 of the present invention;

[0031] Figure 3 : The solution properties of the positive electrode electrolyte obtained in step (2) of the method for preparing a positive electrode electrolyte for a liquid flow battery with high specific capacity and stable circulation without auxiliary electrolyte in Example 3 of the present invention;

[0032] Figure 4 : The solution properties of the positive electrode electrolyte obtained in step (2) of the method for preparing a positive electrode electrolyte for a liquid flow battery in Comparative Example 1 of the present invention;

[0033] Figure 5 : The solution properties of the positive electrode electrolyte obtained in step (2) of the method for preparing a positive electrode electrolyte for a liquid flow battery in Comparative Example 2 of the present invention;

[0034] Figure 6 : The solution properties of the positive electrode electrolyte obtained in step (2) of the method for preparing a positive electrode electrolyte for a liquid flow battery in Comparative Example 3 of the present invention;

[0035] Figure 7: The solution properties of the positive electrode electrolyte obtained in step (2) of the method for preparing a positive electrode electrolyte for a liquid flow battery in Comparative Example 4 of the present invention;

[0036] Figure 8 : is a curve showing the change of specific capacity with voltage during the first discharge test of the iron-sulfur flow battery assembled with the positive electrode electrolyte of Example 1 of the present invention;

[0037] Fig. 9 : The discharge specific capacity and cycle performance of the iron-sulfur flow battery assembled with the positive electrode electrolyte of Example 1 of the present invention during the constant current charge and discharge test process;

[0038] Fig.10 : is a curve showing the change of specific capacity with voltage during the first discharge test of the iron-sulfur flow battery assembled with the positive electrode electrolyte of Example 2 of the present invention;

[0039] Fig.11 : is a curve showing the change of specific capacity with voltage during the first discharge test of the iron-sulfur flow battery assembled with the positive electrode electrolyte of Comparative Example 1 of the present invention;

[0040] Fig.12 : is a curve showing the change of specific capacity with voltage during the first discharge test of the iron-sulfur flow battery assembled with the positive electrode electrolyte of Comparative Example 2 of the present invention;

[0041] Fig.13 : is a curve showing the change of specific capacity with voltage during the first discharge test of the positive electrode electrolyte assembled sulfur-iron flow battery of Comparative Example 3 of the present invention;

[0042] Fig.14 : This is a curve showing the change of specific capacity with voltage during the first discharge test of the sulfide-iron flow battery assembled with the positive electrode electrolyte of Comparative Example 4 of the present invention. DETAILED DESCRIPTION

[0043] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0044] Preparation Example 1

[0045] A potassium-type perfluorosulfonic acid cation exchange membrane, the preparation method comprising the following steps:

[0046] (1) placing the cut perfluorosulfonic acid cation exchange membrane in a 5 wt% H2O2 aqueous solution and keeping it at 80°C for 1 hour;

[0047] (2) The membrane was taken out and placed in a 5 wt% H2SO4 aqueous solution at 80°C for 1 h;

[0048] (3) The membrane was then placed in a 1 mol / L KOH aqueous solution and kept warm at 60°C for 3 h. Finally, the membrane was soaked in deionized water for later use.

[0049] Example 1

[0050] A positive electrode electrolyte for a flow battery with high specific capacity and stable circulation without auxiliary electrolyte, the preparation method of which comprises the following steps:

[0051] (1) Dissolve K4Fe(CN)6 and Na4Fe(CN)6 active substances in deionized water, stir at room temperature for 2 h, and add deionized water to the solution in small amounts and multiple times while maintaining mechanical stirring;

[0052] (2) After the active material is completely dissolved, the solution is calibrated with deionized water to a volume corresponding to the required molar concentration. The positive electrode electrolyte contains 0.5 mol / L of K4Fe(CN)6 and 0.5 mol / L of Na4Fe(CN)6 active materials. The resulting positive electrode electrolyte is as follows: Figure 1 As shown, the solution is a clear aqueous solution without precipitation, indicating that the active substances K4Fe(CN)6 and Na4Fe(CN)6 in the positive electrode electrolyte are fully dissolved. This is because the heteroionic effect increases the solubility of the active substances in the electrolyte.

[0053] Example 2

[0054] A positive electrode electrolyte for a flow battery with high specific capacity and stable circulation without auxiliary electrolyte, the preparation method of which comprises the following steps:

[0055] (1) Dissolve K4Fe(CN)6 and Na4Fe(CN)6 active substances in deionized water, stir at room temperature for 2 h, and add deionized water to the solution in small amounts and multiple times while maintaining mechanical stirring;

[0056] (2) After the active material is completely dissolved, the solution is calibrated with deionized water to a volume corresponding to the required molar concentration. The positive electrode electrolyte contains 0.8 mol / L of K4Fe(CN)6 and 0.8 mol / L of Na4Fe(CN)6 active materials. The resulting positive electrode electrolyte is as follows: Figure 2 As shown, the solution is a clear aqueous solution without precipitation, and the K4Fe(CN)6 and Na4Fe(CN)6 active substances in the positive electrode electrolyte are fully dissolved, indicating that the concentration of active substances in the positive electrode electrolyte of Example 2 is higher than that of Example 1.

[0057] Example 3

[0058] A positive electrode electrolyte for a flow battery with high specific capacity and stable circulation without auxiliary electrolyte, the preparation method of which comprises the following steps:

[0059] (1) Dissolve K4Fe(CN)6 and Na4Fe(CN)6 active substances in deionized water, stir at room temperature for 2 h, and add deionized water to the solution in small amounts and multiple times while maintaining mechanical stirring;

[0060] (2) After the active material is fully dissolved, the solution is calibrated with deionized water to a volume corresponding to the required molar concentration. The positive electrode electrolyte contains 0.9 mol / L of K4Fe(CN)6 and 0.9 mol / L of Na4Fe(CN)6 active materials. The resulting positive electrode electrolyte is as follows: Figure 3 As shown, there is a large amount of white precipitate in the positive electrode electrolyte, and the active substances Na4Fe(CN)6 and K4Fe(CN)6 cannot be completely dissolved, indicating that in the positive electrode electrolyte system containing Na4Fe(CN)6 and K4Fe(CN)6, the total molar concentration of Na4Fe(CN)6 and K4Fe(CN)6 that can be completely dissolved should be below 1.8 mol / L;

[0061] (3) Filter the positive electrode electrolyte to remove the precipitate so that it can be used for the subsequent assembly of the iron-based liquid flow battery.

[0062] Comparative Example 1

[0063] A positive electrolyte for a liquid flow battery, the preparation method of which comprises the following steps:

[0064] (1) Dissolve the K4Fe(CN)6 active material in deionized water, stir at room temperature for 2 h, and add deionized water to the solution in small amounts and multiple times while maintaining mechanical stirring;

[0065] (2) After the active material is fully dissolved, the solution is calibrated with deionized water to a volume corresponding to the required molar concentration. The positive electrode electrolyte contains 1 mol / L of K4Fe(CN)6 active material. The obtained positive electrode electrolyte is as follows Figure 4 As shown, there is a small amount of precipitation in the positive electrode electrolyte, and the active material K4Fe(CN)6 cannot be completely dissolved, indicating that the molar concentration of the active material in the positive electrode electrolyte of Comparative Example 1 should be below 1 mol / L, which is significantly lower than that of Example 1;

[0066] (3) Filter the positive electrode electrolyte to remove the precipitate so that it can be used for the subsequent assembly of the iron-based liquid flow battery.

[0067] Comparative Example 2

[0068] A positive electrolyte for a liquid flow battery, the preparation method of which comprises the following steps:

[0069] (1) Dissolve the Na4Fe(CN)6 active material in deionized water, stir at room temperature for 2 h, and add deionized water to the solution in small amounts and multiple times while maintaining mechanical stirring;

[0070] (2) After the active material is fully dissolved, the solution is calibrated with deionized water to a volume corresponding to the required molar concentration. The positive electrode electrolyte contains 1 mol / L of Na4Fe(CN)6 active material. The obtained positive electrode electrolyte is as follows Figure 5 As shown, a large amount of white precipitate is precipitated in the positive electrode electrolyte, and the active material Na4Fe(CN)6 cannot be completely dissolved, indicating that the molar concentration of the active material in the positive electrode electrolyte of Comparative Example 2 should be below 1 mol / L, which is significantly lower than that of Example 1;

[0071] (3) Filter the positive electrode electrolyte to remove the precipitate so that it can be used for the subsequent assembly of the iron-based liquid flow battery.

[0072] Comparative Example 3

[0073] A positive electrolyte for a liquid flow battery, the preparation method of which comprises the following steps:

[0074] (1) Dissolve K4Fe(CN)6 active material and KCl auxiliary electrolyte in deionized water, stir at room temperature for 2 h, and add deionized water to the solution in small amounts and multiple times while maintaining mechanical stirring;

[0075] (2) After the active material is fully dissolved, the solution is calibrated with deionized water to a volume corresponding to the required molar concentration. The positive electrode electrolyte contains 1 mol / L K4Fe(CN)6 active material and 0.1 mol / L KCl auxiliary electrolyte. The resulting positive electrode electrolyte is as follows Figure 6 As shown, there is a large amount of precipitation in the positive electrode electrolyte;

[0076] (3) Filter the positive electrode electrolyte to remove the precipitate so that it can be used for the subsequent assembly of the iron-based liquid flow battery.

[0077] Comparative Example 4

[0078] A positive electrolyte for a liquid flow battery, the preparation method of which comprises the following steps:

[0079] (1) Dissolve K4Fe(CN)6, Na4Fe(CN)6 active substances and KCl in deionized water, stir at room temperature for 2 h, and add deionized water to the solution in small amounts and multiple times while maintaining mechanical stirring;

[0080] (2) After the active material is completely dissolved, the solution is calibrated with deionized water to a volume corresponding to the required molar concentration. The positive electrode electrolyte contains 0.9 mol / L of K4Fe(CN)6 and 0.9 mol / L of Na4Fe(CN)6 active materials and 0.1 mol / L of KCl. The resulting positive electrode electrolyte is as follows: Figure 7 As shown, there is a large amount of white precipitate in the positive electrode electrolyte;

[0081] (3) Filter the positive electrode electrolyte to remove the precipitate so that it can be used for the subsequent assembly of the iron-based liquid flow battery.

[0082] Performance testing

[0083] The positive electrode electrolyte obtained after filtration in the embodiment or comparative example is used to assemble a sulfur iron liquid flow battery. The negative electrode electrolyte of the sulfur iron liquid flow battery is an aqueous solution containing 1 mol / L of K2S4 active material and 1 mol / L of KOH auxiliary electrolyte, and the diaphragm is the potassium-type perfluorosulfonic acid cation exchange membrane of Preparation Example 1. The constant current charge and discharge performance test of the sulfur iron liquid flow battery is carried out. The flow rate of the positive and negative electrolytes is 50 mL / min, the cut-off voltage of the charging process is 1.4 V, the cut-off voltage of the discharging process is 0.2 V, and the current density is 20 mA / cm 2 The test results are shown in Table 1 and Figure 8-14 shown.

[0084] Table 1 - Constant current charge and discharge test results of positive electrode electrolytes of the present application examples or comparative examples and their assembled sulfur-iron liquid flow batteries

[0085]

[0086] like Figure 8 As shown, the discharge capacity of the flow battery in Example 1 is 26.8Ah / L posolyte , the active material Fe(CN)6 in the positive electrolyte 4- The concentration of Fe(CN)6 in the positive electrode electrolyte is positively correlated with the specific capacity of the assembled sulfur-iron flow battery. This is because the concentration of the active substance directly determines the amount of substances that can participate in the electrochemical reaction inside the battery, which in turn affects the energy density of the battery. 4- As the concentration increases, the specific capacity of the sulfur-iron flow battery will also increase accordingly. Fig. 9 As shown, the iron-sulfur liquid flow battery assembled with the positive electrolyte of Example 1 can achieve at least 50 stable charge and discharge cycles and has excellent cycle stability.

[0087] By comparing Comparative Example 1 with Comparative Example 3 or Example 3 with Comparative Example 4, it can be found that the active substance of the positive electrode electrolyte in Comparative Example 1 is 1 mol / L K4Fe(CN)6. Fig.11As shown, the first discharge capacity of its flow battery is 22.8Ah / L posolyte In Comparative Example 3, the active substance of the positive electrode electrolyte is 1 mol / L K4Fe(CN)6, and 0.1 mol / L KCl auxiliary electrolyte is added. Fig.13 As shown, the first discharge capacity of its flow battery is 22.1Ah / L posolyte Compared with Comparative Example 1, the battery specific capacity of Comparative Example 3 is reduced; the active materials of the positive electrode electrolyte in Example 1 and Comparative Example 4 are both 0.9 mol / L K4Fe(CN)6 and 0.9 mol / L Na4Fe(CN)6, and Comparative Example 4 is additionally added with 0.1 mol / L KCl auxiliary electrolyte. Finally, the first discharge specific capacity of Example 1 is higher than that of Comparative Example 4. This shows that the addition of auxiliary electrolyte in Comparative Examples 3 and 4 reduces the discharge specific capacity, which is speculated to be because the addition of auxiliary electrolyte of the same ion reduces the solubility of the active material in the positive electrode electrolyte.

[0088] The active substances of the positive electrolyte in Example 1 are 0.5 mol / L K4Fe(CN)6 and 0.5 mol / L Na4Fe(CN)6. Figure 8 As shown, the first discharge capacity of its flow battery is 26.8Ah / L posolyte , Example 1 and Comparative Example 2 have the same molar concentration of active substances. Compared with Comparative Example 2, Example 1 uses two coordinating cations, which effectively improve the solubility of the active substances and the specific capacity of the battery through the heteroionic effect. The increase in the total concentration of the corresponding coordinating cations is sufficient to support ion transport, and there is no need to add auxiliary electrolytes, which reduces the cost of raw materials. However, the solubility of the active substances in Comparative Example 2 is low, and a large amount of white precipitates are precipitated, resulting in the specific capacity of the flow battery in Comparative Example 2 being 17.9Ah / L posolyte , which is significantly lower than that of Examples 1 and 2.

[0089] The specific embodiments described above further illustrate the purpose, technical solutions and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. It is particularly pointed out that for those skilled in the art, any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention should be included in the scope of protection of the present invention.

Claims

1. A positive electrode electrolyte for a flow battery with high specific capacity and stable circulation without auxiliary electrolyte, characterized in that: The positive electrode electrolyte is composed of iron cyanide with a molar concentration of 0.1-3 mol / L and the balance of water, wherein the iron cyanide contains a complex cation, and the complex cation is Li + 、Na + , K + and NH4 + At least two of the .

2. A positive electrode electrolyte for a flow battery with high specific capacity and stable circulation without auxiliary electrolyte as claimed in claim 1, characterized in that: The iron cyanide is ferrocyanide and / or ferrocyanide.

3. A positive electrode electrolyte for a flow battery with high specific capacity and stable circulation without auxiliary electrolyte as claimed in claim 2, characterized in that: The ferrocyanide is at least one of lithium ferrocyanide, sodium ferrocyanide, potassium ferrocyanide and ammonium ferrocyanide; the ferrocyanide is at least one of lithium ferrocyanide, sodium ferrocyanide, potassium ferrocyanide and ammonium ferrocyanide.

4. A positive electrode electrolyte for a flow battery with high specific capacity and stable circulation without auxiliary electrolyte as claimed in claim 2, characterized in that: The coordinating cation is Li + 、Na + , K + and NH4 + At least two of the .

5. A positive electrode electrolyte for a flow battery with high specific capacity and stable circulation without auxiliary electrolyte as claimed in claim 1, characterized in that: The iron cyanide is potassium ferrocyanide and sodium ferrocyanide in a molar ratio of (1-3):(1-3).

6. A method for preparing a positive electrode electrolyte for a flow battery with high specific capacity and stable circulation without auxiliary electrolyte as claimed in any one of claims 1 to 5, characterized in that: The method comprises the following steps: dissolving iron cyanide in water, stirring at room temperature until completely dissolved, and then calibrating with water to a volume corresponding to a required molar concentration.

7. Use of a high specific capacity and cyclically stable positive electrode electrolyte for a flow battery without auxiliary electrolyte as claimed in any one of claims 1 to 6 in the preparation of an iron-based flow battery.

8. An iron-based liquid flow battery, characterized in that: A positive electrode electrolyte for a liquid flow battery with high specific capacity and stable circulation without auxiliary electrolyte as claimed in any one of claims 1 to 6, comprising a positive electrode electrolyte, a negative electrode electrolyte and a separator.

9. An iron-based liquid flow battery as claimed in claim 8, wherein the negative electrode electrolyte is an alkaline aqueous solution containing 0.1-3 mol / L polysulfide.

10. The iron-based liquid flow battery according to claim 8, wherein the diaphragm is a cation exchange membrane.

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