Method for calculating amount-of-substance ratio of iron ions to chromium ions in electrolyte and application of method

Abstract

The invention provides a method for calculating the amount-of-substance ratio of iron ions to chromium ions in electrolyte and application thereof, the method for calculating the amount-of-substance ratio of iron ions to chromium ions in electrolyte comprises the following steps: determining the ion exchange capacity [IEC] of an ionic membrane used by an iron-chromium flow battery; setting the expected operating temperature T of the battery and the state of charge SOC of the electrolyte; calculating the ratio of the amount of substance of divalent iron ions to trivalent chromium ions according to the following formula: n (Fe < 2 + >) / n (Cr < 3 + >) = 0.1 alpha + 0.2 beta (T) + 0.5 gamma (SOC); alpha is a proportioning coefficient determined based on the ion exchange capacity of the ionic membrane; beta (T) is a temperature correction factor; gamma (SOC) is a correlation factor with the state of charge of the electrolyte. By adjusting the concentration ratio of main active substances in the initial electrolyte, the effects of controlling hydrogen evolution and inhibiting capacity fading of the iron-chromium flow battery can be achieved.

Description

Technical Field

[0001] This invention relates to iron-chromium redox flow battery technology, and more particularly to a method for calculating the molar ratio of iron ions to chromium ions in an electrolyte and its application. Background Technology

[0002] Flow batteries are a large-scale, high-efficiency electrochemical energy storage technology. The active materials are mainly found in the electrolyte, offering advantages such as the high safety of aqueous electrolytes, long battery cycle life, electrolyte recyclability, and environmental friendliness. Because stationary energy storage systems do not require high energy density, flow batteries are widely used in large-scale energy storage applications. Flow batteries convert chemical energy into electrical energy at the battery stack through redox reactions between different redox couples in the electrolyte.

[0003] The iron-chromium flow battery was the earliest proposed flow battery technology, utilizing Fe on the positive electrode side. 2+ / Fe 3+ The redox couple and the negative electrode side Cr 2+ / Cr 3+ The redox reaction of an electron couple converts chemical energy into electrical energy. Compared to vanadium redox flow batteries, iron-chromium flow batteries have a wider operating temperature range and are applicable to a wider range of environments; iron-chromium flow batteries do not release chlorine gas, thus offering better safety; the low-valence iron and chromium ions have relatively low toxicity; and the iron-chromium electrolyte is cheaper and can be recycled and reused. Therefore, iron-chromium flow batteries are considered the most promising energy storage technology for large-scale, long-term, high-temperature environments.

[0004] Iron-chromium electrolyte is the most critical component of iron-chromium flow batteries, and its performance directly affects the battery's performance. Currently, iron-chromium electrolytes suffer from problems such as excessively rapid chromium aging, severe ion migration, and electrolyte valence state shift. Furthermore, iron-chromium electrolytes typically operate at high temperatures, which exacerbates the hydrogen evolution reaction of divalent chromium at the negative electrode, preventing trivalent iron at the positive electrode from returning to divalent status. This leads to a decreasing rechargeable capacity, creating a vicious cycle where the battery experiences continuous capacity decay, and coulombic efficiency and energy efficiency decline steadily.

[0005] Existing technical solutions propose that the iron ion concentration range in the iron-chromium electrolyte be 1-1.3 mol / L, the chromium ion concentration range be 1-1.3 mol / L, and the hydrochloric acid concentration be 2.5-3 mol / L. Existing technical solutions have a relatively wide range of iron-chromium ion concentrations in the iron-chromium electrolyte and do not make strict specifications for the positive and negative electrode electrolytes of iron-chromium flow batteries under different test conditions.

[0006] In Fe-Cr electrolytes, when the iron and chromium ions in the solution are controlled at a 1:1 ratio to maintain charge balance in the redox reaction, due to the Cr... 2+ / Cr 3+The reaction kinetics of the redox couple are significantly slower than those of Fe. 2+ / Fe 3+ The negative electrode reaction is delayed during the reaction process, which leads to increased polarization, reduced energy efficiency, reduced discharge capacity, and reduced coulombic efficiency. This significantly increases the operating cost of the system, reduces system performance, and affects the long-term reliability of the battery.

[0007] Substances with the same concentration of iron and chromium ions exhibit different reactivity, resulting in different reactions at the positive electrode (Fe). 3+ Excessive idleness or negative electrode Cr 2 + Insufficient reduction, Fe 3+ With Cr 2+ The difference in migration rates intensifies ion transmembrane migration, leading to reduced availability of active materials in the electrolyte and accelerated capacity decay. Furthermore, a higher iron-chromium ion concentration ratio results in excessively high chromium ion utilization at the negative electrode. A 1:1 ratio typically requires maintaining high acidity to improve conductivity, but high H... + The concentration further exacerbates hydrogen evolution.

[0008] High concentration of Cr in iron-chromium electrolyte at low temperature 3+ Iron-chromium ions readily crystallize (e.g., CrCl3·6H2O), making it difficult to adjust the Cl- concentration in a 1:1 system to adapt to electrolyte stability at different temperatures. Furthermore, at a 1:1 concentration, to avoid hydrogen evolution and crystallization, the iron-chromium ion concentration typically needs to be limited to 1.0-1.3 mol / L, which negatively impacts the improvement of the energy density of the iron-chromium electrolyte.

[0009] The ion exchange membrane is one of the most critical components in a flow battery. Different ion exchange membranes, even those of different thicknesses, have different effects on the ion exchange of the electrolyte. Therefore, a 1:1 iron-chromium electrolyte is not suitable for most ion exchange membranes, and using different ion exchange membranes may lead to completely different results.

[0010] Meanwhile, iron-chromium redox flow batteries, with their four core advantages of safety, long lifespan, low cost, and wide temperature range, have become the preferred technology for long-term energy storage of more than 4 hours. Although the electrolyte in a flow battery can be recycled multiple times, the strong side reactions of an initial 1:1 iron-chromium electrolyte cause the entire system to decay to negligible levels within just a hundred or even a few dozen cycles, greatly increasing the frequency of electrolyte recovery and, in turn, increasing the cost of iron-chromium electrolyte recovery. Summary of the Invention

[0011] The purpose of this invention is to address the problems of performance degradation and increased cost caused by ion migration, hydrogen evolution, and crystallization in traditional iron-chromium electrolytes. This invention proposes a method for calculating the molar ratio of divalent iron ions to trivalent chromium ions in the initial electrolyte of an iron-chromium flow battery. This method can control hydrogen evolution and suppress capacity decay of the iron-chromium flow battery by adjusting the concentration ratio of the main active substances in the initial electrolyte.

[0012] It should be noted that, in this invention, unless otherwise specified, the specific meaning of "comprising" in relation to composition and description includes both open-ended meanings such as "comprising," "including," etc., and closed-ended meanings such as "composed of," "consisting of," etc., and similar meanings.

[0013] To achieve the above objectives, the technical solution adopted by this invention is: a method for calculating the molar ratio of iron ions to chromium ions in an electrolyte, specifically a method for calculating the molar ratio of divalent iron ions to trivalent chromium ions in the initial electrolyte of an iron-chromium flow battery, comprising the following steps:

[0014] Step 1. Determine the ion exchange capacity [IEC] of the ion-exchange membrane used in the iron-chromium redox flow battery;

[0015] Step 2. Set the expected operating temperature T of the battery and the state of charge (SOC) of the electrolyte;

[0016] Step 3. According to the following formula n(Fe) 2+ ) / n(Cr 3+ )=0.1α+0.2β(T)+0.5γ(SOC) Calculate the molar ratio of divalent iron ions to trivalent chromium ions;

[0017] Wherein, n(Fe 2+ () represents the amount of ferrous ions in the initial electrolyte;

[0018] n(Cr 3+ () represents the amount of trivalent chromium ions in the initial electrolyte;

[0019] α is a ratio coefficient determined based on the ion exchange capacity of the ion exchange membrane, and α = 0.8 + 0.2 × [IEC], where [IEC] is the ion exchange capacity of the ion exchange membrane, in meq / g;

[0020] β(T) is the temperature correction factor, and β(T) = e -0.005(T-25) T is the operating temperature of the electrolyte, in °C.

[0021] γ(SOC) is a correlation factor with the state of charge of the electrolyte, and γ(SOC) = (SOC / 100) 0.5 The SOC refers to the state of charge of the electrolyte.

[0022] Furthermore, the range of [IEC] is 0.9-1.1 meq / g.

[0023] Furthermore, the range of T is 25℃-65℃.

[0024] Furthermore, the range of SOC is 50%-90%.

[0025] Furthermore, to effectively suppress the hydrogen evolution side reaction, the ratio of the total amount of ferrous ions and chromium ions to the amount of chloride ions in the initial electrolyte must satisfy the following condition: 0.3 ≤ [n(Fe 2+ )+n(Cr 3+ )] / n(Cl - )≤0.4;

[0026] Wherein, n(Fe 2+ () represents the amount of ferrous ions in the initial electrolyte;

[0027] n(Cr 3+ () represents the amount of trivalent chromium ions in the initial electrolyte;

[0028] n(Cl - () represents the amount of chloride ions in the initial electrolyte.

[0029] Furthermore, the n(Cl) - The selection range is 5-9 mol / L.

[0030] Another objective of this invention is to disclose an electrolyte for an iron-chromium redox flow battery, comprising a positive electrode electrolyte and a negative electrode electrolyte, wherein both the positive and negative electrode electrolytes contain chloride ions, ferrous ions, and chromium ions, and the initial molar ratio of ferrous ions to chromium ions, n(Fe), is... 2+ ) / n(Cr 3+ The electrolyte's properties are determined by three factors: the characteristics of the ion-exchange membrane, the operating temperature of the electrolyte, and the state of charge of the electrolyte, and satisfy the following relationship:

[0031] n(Fe 2+ ) / n(Cr 3+ )=0.1α+0.2β(T)+0.5γ(SOC);

[0032] Wherein, α is the ratio coefficient determined based on the ion exchange capacity of the ion exchange membrane, and α=0.8+0.2×[IEC], where [IEC] is the ion exchange capacity of the ion exchange membrane, in meq / g;

[0033] β(T) is a temperature correction factor, and β(T)=e^(-0.005*(T-25)), where T is the operating temperature of the electrolyte, in °C;

[0034] γ(SOC) is a correlation factor with the state of charge of the electrolyte, and γ(SOC) = (SOC / 100)^0.5, where SOC is the state of charge of the electrolyte.

[0035] Furthermore, in the initial electrolyte, the ratio of the total amount of ferrous ions and chromium ions to the amount of chloride ions satisfies: 0.3 ≤ [n(Fe 2+ )+n(Cr 3+ )] / n(Cl - )≤0.4.

[0036] Furthermore, the chloride ions are provided by hydrochloric acid, the concentration of which is in the range of 2.5-3 mol / L.

[0037] Furthermore, to prevent excessive initial volume difference between the positive and negative electrodes from causing electrolyte migration and thus affecting electrolyte capacity retention and efficiency, the volume V of the positive electrode electrolyte is controlled. 正 / V 负 The concentration range is 0.9-1.1, the concentration range of the divalent iron ions is 0.9-1.4 mol / L, and / or the concentration range of the trivalent chromium ions is 1.2-1.7 mol / L.

[0038] Another object of the present invention discloses a method for preparing an electrolyte for an iron-chromium flow battery, characterized by comprising the following steps:

[0039] First, calculate the ratio of the amount of divalent iron ions to trivalent chromium ions in the initial electrolyte of the iron-chromium flow battery.

[0040] Then, based on the calculated ratio of the target substances of ferrous ions to chromium ions and the concentration range, raw materials containing ferrous ions, chromium ions and chloride ions are weighed and provided to prepare the electrolyte.

[0041] Another objective of this invention is to disclose the application of an iron-chromium flow battery electrolyte in the field of iron-chromium flow batteries.

[0042] Another objective of this invention is to disclose an iron-chromium redox flow battery, comprising a battery stack, a positive electrolyte storage tank, a negative electrolyte storage tank, and a circulation pipeline, wherein the positive electrolyte storage tank and the negative electrolyte storage tank are filled with the aforementioned iron-chromium redox flow battery electrolyte.

[0043] The method for calculating the molar ratio of iron ions to chromium ions in an electrolyte and its application, as described in this invention, have the following advantages compared to existing technologies:

[0044] 1) This invention calculates the initial iron-chromium ion concentration ratio by considering the migration characteristics of the ion exchange membrane, electrolyte temperature, and electrolyte state of charge (SOC). The ion exchange membrane characteristic ratio coefficient α (corresponding to ion exchange capacity [IEC]) adapts to the differences in ion migration among different ion exchange membranes, the temperature correction factor β(T) balances the fluctuations in ion reactivity over a wide temperature range, and the SOC correlation factor γ(SOC) matches the battery's different charge and discharge states. This achieves the following objectives: first, to promote migration equilibrium between the positive and negative electrodes, blocking or reducing the migration of active materials; second, to improve the utilization rate of iron-chromium active materials and avoid the loss of Fe2 at the positive electrode. 3+ Excessive idleness and negative electrode Cr 2+ The problem of insufficient reduction; thirdly, to suppress discharge capacity decay and control the negative electrode SOC level, and reduce side reactions such as hydrogen evolution, support the efficient and stable operation of iron-chromium redox flow batteries under different operating conditions.

[0045] 2) The Fe in the positive and negative electrolytes of the iron-chromium redox flow battery of the present invention 2+ Cr 3+ Cl - Satisfying n(Fe) 2 + ) / n(Cr 3+ )=0.1α+0.2β(T)+0.5γ(SOC), and 0.3≤[n(Fe 2+ )+n(Cr 3+ )] / n(Cl - When the concentration is ≤0.4, the following beneficial effects can be achieved: this concentration relationship synergistically promotes the migration balance of positive and negative electrode ions, effectively reduces battery polarization, and effectively controls side reactions such as hydrogen evolution. As a result, at 200 mA·cm⁻¹, 2 Under the test conditions, the energy efficiency (stable above 68%) and coulombic efficiency (stable above 98%) of the iron-chromium redox flow battery were improved simultaneously, and the cycle stability was significantly enhanced (under optimal conditions, the capacity retention rate was above 90% after 100 charge-discharge cycles), thereby reducing the consumption and recovery costs of electrolyte.

[0046] In summary, this invention addresses the issues of hydrogen evolution control and capacity decay in iron-chromium redox flow batteries by adjusting the concentration ratio of the main active materials in the initial electrolyte. This method has promising application prospects and large-scale promotion potential in the field of iron-chromium redox flow batteries. Attached Figure Description

[0047] Figure 1 This is a comparison chart of coulombic efficiency between the examples and the comparative examples;

[0048] Figure 2 This is a comparison chart of the energy efficiency of the examples and comparative examples;

[0049] Figure 3 This is a comparison chart of capacity retention rates between the examples and the comparative examples;

[0050] Figure 4 This is a comparison chart of energy density between the examples and the comparative examples. Detailed Implementation

[0051] The present invention will be further described below with reference to embodiments. The description of the technical features described below is based on representative embodiments and specific examples of the present invention, but the present invention is not limited to these embodiments and specific examples. It should be noted that:

[0052] Unless otherwise stated, all units used in this specification are international standard units, and all numerical values ​​and ranges appearing in this invention should be understood to include systematic errors that are unavoidable in industrial production.

[0053] In this specification, the range of values ​​referred to as "value A to value B" refers to the range including the endpoint values ​​A and B.

[0054] In this specification, the numerical range indicated by "above" or "below" refers to the numerical range that includes the stated number.

[0055] In this specification, the word "may" has two meanings: to perform a certain process and not to perform a certain process.

[0056] In this specification, the terms "optional" or "optional" are used to indicate the use or omission of certain substances, components, procedures, application conditions, etc.

[0057] In this instruction manual, when "room temperature" or "room temperature" is used, the temperature can be 15-25℃.

[0058] Unless otherwise specified, all reagents or instruments used in this instruction manual are commercially available products.

[0059] Example 1

[0060] This embodiment discloses an electrolyte for an iron-chromium redox flow battery, comprising a positive electrode electrolyte and a negative electrode electrolyte. Both the positive and negative electrode electrolytes contain chloride ions, ferrous ions, and chromium ions. The initial molar ratio of ferrous ions to chromium ions in the electrolyte, n(Fe2), is... 2+ ) / n(Cr 3+ The result is calculated using the following formula:

[0061] n(Fe 2+ ) / n(Cr 3+ = 0.1α + 0.2β(T) + 0.5γ(SOC)

[0062] And control [n(Fe 2+ )+n(Cr 3+ )] / n(Cl - ) = 0.36;

[0063] The positive and negative electrode electrolytes were prepared with a volume ratio of 1:1, using an initial iron-chromium electrolyte. The specific design conditions for Example 1 were: an ambient temperature of 25°C, an ion exchange capacity of 1.1 meq / g for the ion-exchange membrane, and a preset SOC value of 50% for the battery. At this time, the ratio of the amount and concentration of iron-chromium ions in the initial electrolyte was 0.675, with Fe... 2+ Cr 3+ Cl - The ion concentrations were 1.05 mol / L, 1.55 mol / L, and 7.2 mol / L, respectively.

[0064] Example 2

[0065] This embodiment discloses an electrolyte for an iron-chromium redox flow battery, which is basically the same as that in Embodiment 1, except that the specific design conditions are different. The test environment temperature is 25°C, the ion exchange capacity of the ion-exchange membrane used is 1 meq / g, and the battery is preset to reach a SOC value of 70%. At this time, the initial ratio of iron-chromium ion content and concentration in the electrolyte is 0.758, and Fe... 2+ Cr 3+ Cl - The ion concentrations were 1.05 mol / L, 1.39 mol / L, and 6.8 mol / L, respectively.

[0066] Example 3

[0067] This embodiment discloses an electrolyte for an iron-chromium redox flow battery, which is basically the same as that in Embodiment 1, except that the specific design conditions are different. The test environment temperature is 25°C, the ion exchange capacity of the ion-exchange membrane used is 0.9 meq / g, and the battery is preset to reach a SOC value of 90%. At this time, the initial ratio of iron-chromium ion content and concentration in the electrolyte is 0.854, and Fe... 2+ Cr 3 + Cl - The ion concentrations were 1.05 mol / L, 1.23 mol / L, and 6.3 mol / L, respectively.

[0068] Example 4

[0069] This embodiment discloses an electrolyte for an iron-chromium redox flow battery, which is basically the same as that in Embodiment 1, except that the specific design conditions are different. The test environment temperature is 45℃, the ion exchange capacity of the ion-exchange membrane used is 1.1 meq / g, and the battery is preset to reach a SOC value of 70%. At this time, the initial ratio of iron-chromium ion content and concentration in the electrolyte is 0.731, and Fe... 2+ Cr 3 + Cl - The ion concentrations were 1.05 mol / L, 1.44 mol / L, and 6.9 mol / L, respectively.

[0070] Example 5

[0071] This embodiment discloses an electrolyte for an iron-chromium redox flow battery, which is basically the same as that in Embodiment 1, except that the specific design conditions are different. The test environment temperature is 45℃, the ion exchange capacity of the ion-exchange membrane used is 1 meq / g, and the battery is preset to reach a SOC value of 90%. At this time, the initial ratio of iron-chromium ion content and concentration in the electrolyte is 0.827, and Fe... 2+ Cr 3+ Cl - The ion concentrations were 1.05 mol / L, 1.27 mol / L, and 6.4 mol / L, respectively.

[0072] Example 6

[0073] This embodiment discloses an electrolyte for an iron-chromium redox flow battery, which is basically the same as that in Embodiment 1, except that the specific design conditions are different. The test environment temperature is 45℃, the ion exchange capacity of the ion-exchange membrane used is 0.9 meq / g, and the battery is preset to reach a SOC value of 50%. At this time, the initial ratio of iron-chromium ion content and concentration in the electrolyte is 0.655, and Fe... 2+ Cr 3 + Cl - The ion concentrations were 1.05 mol / L, 1.6 mol / L, and 7.4 mol / L, respectively.

[0074] Example 7

[0075] This embodiment discloses an electrolyte for an iron-chromium redox flow battery, which is basically the same as that in Embodiment 1, except that the specific design conditions are different. The test environment temperature is 65℃, the ion exchange capacity of the ion-exchange membrane used is 1.1 meq / g, and the battery is preset to reach a SOC value of 90%. At this time, the initial ratio of iron-chromium ion content and concentration in the electrolyte is 0.803, and Fe... 2+ Cr 3 + Cl -The ion concentrations were 1.05 mol / L, 1.3 mol / L, and 6.5 mol / L, respectively.

[0076] Example 8

[0077] This embodiment discloses an electrolyte for an iron-chromium redox flow battery, which is basically the same as that in Embodiment 1, except that the specific design conditions are different. The test environment temperature is 65℃, the ion exchange capacity of the ion-exchange membrane used is 1 meq / g, and the battery is preset to reach a SOC value of 50%. At this time, the initial ratio of iron-chromium ion content and concentration in the electrolyte is 0.627, and Fe... 2+ Cr 3+ Cl - The ion concentrations were 1.05 mol / L, 1.67 mol / L, and 7.6 mol / L, respectively.

[0078] Example 9

[0079] This embodiment discloses an electrolyte for an iron-chromium redox flow battery, which is basically the same as that in Embodiment 1, except that the specific design conditions are different. The test environment temperature is 65℃, the ion exchange capacity of the ion-exchange membrane used is 0.9 meq / g, and the battery is preset to reach a SOC value of 70%. At this time, the initial ratio of iron-chromium ion content and concentration in the electrolyte is 0.689, and Fe... 2+ Cr 3 + Cl - The ion concentrations were 1.05 mol / L, 1.52 mol / L, and 7.1 mol / L, respectively.

[0080] Comparative Example

[0081] Similarly, control [n(Fe] in the comparative example 2+ )+n(Cr 3+ )] / n(Cl - ) = 0.36;

[0082] The positive and negative electrode electrolytes were prepared with a volume ratio of 1:1, using initial iron-chromium electrolytes. The specific design conditions for the comparative example were: an ambient temperature of 45℃, an ion exchange capacity of 1 meq / g for the ion-exchange membrane, and a preset SOC value of 70% for the battery.

[0083] The comparative example is an initial iron-chromium electrolyte with a molar ratio of 1:1, Fe 2+ Cr 3+ Cl - The ion concentrations were 1.05 mol / L, 1.05 mol / L, and 5.8 mol / L, respectively.

[0084] Specifically, the parameters and calculation results of Examples 1-9 and the comparative examples are shown in Table 1.

[0085] Table 1. Parameters and calculation results for Examples 1-9 and Comparative Examples

[0086]

[0087]

[0088] Single-cell performance tests were conducted on the electrolytes of Examples 1-9 and the comparative examples. Specific test data are shown in Table 2, and single-cell efficiency is shown in [reference needed]. Figure 1-4 .

[0089] The tests include:

[0090] Battery efficiency test: A 5W single-cell battery was used, employing constant current charging + constant current discharging mode. The constant current charging current density was 180mA / cm. 2 The cutoff voltage is 1.2V, and then at 180mA / cm 2 The current is constant and discharged to the cutoff voltage of 0.6V for a total of 100 cycles.

[0091] EE = Discharge Wh / Charge Wh × 100%; CE = Discharge Ah / Charge Ah × 100%; ED = Discharge Wh / Active Material Volume L; Capacity Retention = Discharge Ah / Maximum Discharge Ah.

[0092] Table 2 Test Results

[0093]

[0094]

[0095] Figure 1 The diagram shows the coulombic efficiency of the electrolyte batteries in Examples 1-9 and the comparative examples. The iron-chromium batteries under these conditions exhibit high coulombic efficiency (>98%). However, in Examples 7-9, the higher operating temperature lowered the barrier to the hydrogen evolution side reaction at the negative electrode, resulting in a decrease in coulombic efficiency.

[0096] Figure 2 The diagram shows the energy efficiency of the electrolyte batteries in Examples 1-9 and the comparative examples. When the battery operating temperature is too high (65°C) or the expected state of charge (SOC) is too high (90%), the battery often exhibits poor efficiency. This is because the proportion of divalent chromium ions at the negative electrode is too high, and the high-temperature environment intensifies the hydrogen evolution reaction, leading to a decrease in efficiency, which is consistent with the expected test results. Conversely, when the battery's state of charge is too low, the reactive materials cannot react sufficiently, and the efficiency cannot reach its optimal level.

[0097] Figure 3The graph shows the capacity retention rates of the electrolyte batteries in Examples 1-9 and the comparative examples. It can be seen that State of Charge (SOC) has the greatest impact on electrolyte capacity retention, followed by temperature, while the type of ion exchange membrane used has the least impact. This is because SOC directly reflects the final state of the electrolyte after charging; excessively high ion utilization will lead to greater battery side reactions, while temperature affects the extent of these side reactions. A larger ion exchange capacity membrane allows for more ion migrations within the same time frame when the battery has the same state of charge, which also affects the battery capacity retention rate to some extent. The low capacity retention rate of Example 7 is attributed to high SOC, high temperature, and the largest ion exchange capacity of the ion exchange membrane used.

[0098] Figure 4 Energy density graphs of electrolyte batteries in Examples 1-9 and the comparative examples. Energy density reflects the ion concentration participating in the discharge process. Electrolytes with relatively high SOC have a larger initial energy density, but the decay of capacity retention will reduce the average energy density.

[0099] When the battery's state of charge (SOC) is too high, the utilization rate of Cr in the negative electrode increases significantly, leading to Cr... 2+ Increased content leads to a large number of hydrogen evolution negative reactions, resulting in rapid capacity decay. When the temperature is too high, not only does the activity of the active substances in the electrolyte increase, but the conditions for the generation of side reactions also decrease, which also leads to capacity decay. For different types of ion exchange membranes, a relatively high ion exchange capacity can accelerate electron migration and improve conductivity, but at the same time, it can cause problems such as swelling.

[0100] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for calculating the molar ratio of iron ions to chromium ions in an electrolyte, characterized in that, Includes the following steps: Step 1. Determine the ion exchange capacity [IEC] of the ion-exchange membrane used in the iron-chromium redox flow battery; Step 2. Set the expected operating temperature T of the battery and the state of charge (SOC) of the electrolyte; Step 3. According to the following formula: n(Fe 2+ ) / n(Cr 3+ )=0.1α+0.2β(T)+0.5γ(SOC) Calculate the molar ratio of divalent iron ions to trivalent chromium ions; Wherein, n(Fe 2+ () represents the amount of ferrous ions in the initial electrolyte; n(Cr 3+ () represents the amount of trivalent chromium ions in the initial electrolyte; α is a ratio coefficient determined based on the ion exchange capacity of the ion exchange membrane, and α = 0.8 + 0.2 × [IEC], where [IEC] is the ion exchange capacity of the ion exchange membrane, in meq / g; β(T) is the temperature correction factor, and β(T) = e -0.005(T-25) T is the operating temperature of the electrolyte, in °C. γ(SOC) is a correlation factor with the state of charge of the electrolyte, and γ(SOC) = (SOC / 100) 0.5 The SOC refers to the state of charge of the electrolyte.

2. The method for calculating the molar ratio of iron ions to chromium ions in an electrolyte according to claim 1, characterized in that, To effectively suppress the hydrogen evolution side reaction, the ratio of the total amount of ferrous ions and chromium ions to the amount of chloride ions in the initial electrolyte must satisfy the following condition: 0.3 ≤ [n(Fe 2+ )+n(Cr 3+ )] / n(Cl - )≤0.4; Wherein, n(Fe 2+ () represents the amount of ferrous ions in the initial electrolyte; n(Cr 3+ () represents the amount of trivalent chromium ions in the initial electrolyte; n(Cl - () represents the amount of chloride ions in the initial electrolyte.

3. An electrolyte for an iron-chromium redox flow battery, comprising a positive electrode electrolyte and a negative electrode electrolyte, wherein both the positive and negative electrode electrolytes contain chloride ions, ferrous ions, and chromium ions, characterized in that... The molar ratio of divalent ferric ions to trivalent chromium ions in the initial electrolyte satisfies the following relationship: n(Fe 2+ ) / n(Cr 3+ )=0.1α+0.2β(T)+0.5γ(SOC); Wherein, α is the ratio coefficient determined based on the ion exchange capacity of the ion exchange membrane, and α=0.8+0.2×[IEC], where [IEC] is the ion exchange capacity of the ion exchange membrane, in meq / g; β(T) is a temperature correction factor, and β(T)=e^(-0.005*(T-25)), where T is the operating temperature of the electrolyte, in °C; γ(SOC) is a correlation factor with the state of charge of the electrolyte, and γ(SOC) = (SOC / 100)^0.5, where SOC is the state of charge of the electrolyte.

4. The iron-chromium redox flow battery electrolyte according to claim 3, characterized in that, In the initial electrolyte, the ratio of the total amount of ferrous ions and chromium ions to the amount of chloride ions satisfies: 0.3 ≤ [n(Fe 2+ )+n(Cr 3+ )] / n(Cl - )≤0.

4.

5. The iron-chromium redox flow battery electrolyte according to claim 3, characterized in that, Controlling the volume V of the positive electrode electrolyte 正 / V 负 The concentration range is 0.9-1.1, the concentration range of the divalent iron ions is 0.9-1.4 mol / L, and / or the concentration range of the trivalent chromium ions is 1.2-1.7 mol / L.

6. The iron-chromium redox flow battery electrolyte according to claim 3, characterized in that, The chloride ions are provided by hydrochloric acid, the concentration of which is in the range of 2.5-3 mol / L.

7. A method for preparing an electrolyte for an iron-chromium flow battery, characterized in that, Includes the following steps: First, calculate the ratio of the amount of divalent iron ions to trivalent chromium ions in the initial electrolyte of the iron-chromium flow battery. Then, based on the calculated ratio of the target substances of ferrous ions to chromium ions and the concentration range, raw materials containing ferrous ions, chromium ions and chloride ions are weighed and provided to prepare the electrolyte.

8. The application of the iron-chromium redox flow battery electrolyte according to any one of claims 3-6 in the field of iron-chromium redox flow batteries.

9. A flow battery made of iron-chromium redox fluid, comprising a battery stack, a positive electrolyte storage tank, a negative electrolyte storage tank, and a circulation pipeline, characterized in that, The positive electrode electrolyte storage tank and the negative electrode electrolyte storage tank are filled with the iron-chromium redox flow battery electrolyte according to any one of claims 3-6.

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