Online recovery method for electrolyte of alkaline total-iron flow battery

Abstract

The invention relates to an online recovery method for an electrolyte of an alkaline total-iron flow battery, and belongs to the technical field of flow batteries. Aiming at the problem of capacity fading caused by consumption of positive oxidation-state substances due to spontaneous reduction of positive ferricyanide into ferrocyanide and permeation of negative polyhydroxy organic ligands of an alkaline total-iron flow battery, a chemical and electrochemical coordinated regulation strategy is provided; and the concentration of the oxidation-state ferricyanide in the positive electrode electrolyte is dynamically monitored and accurately adjusted in the operation process of the battery. According to the method, dynamic balance regulation and control of oxidation state / reduction state substances in the electrolyte are realized through an in-situ chemical or electrochemical regeneration means on the basis of a positive and negative electrode capacity matching relationship, so that the positive electrode electrolyte is repaired on line, and quantitative and controllable capacity recovery of the alkaline all-iron flow battery is realized. According to the technology, the stable cycle life of the battery can be greatly prolonged, the problem of electrolyte failure caused by polyhydroxy ligand migration is solved, the maintenance cost is reduced, and a key technical support is provided for practicability of the alkaline all-iron flow battery in a large-scale energy storage system.

Description

TECHNICAL FIELD

[0001] The application relates to an online capacity recovery method for a positive electrolyte of an alkaline full-iron flow battery, and belongs to the technical field of flow batteries. BACKGROUND

[0002] The alkaline full-iron flow battery is a new type of flow battery technology using soluble iron-based complexes as positive and negative active materials. The electrolyte is mainly composed of iron salts and small-molecule organic chelating ligands. Compared with the all-vanadium flow battery, the alkaline full-iron flow battery has more abundant raw material resources, and thus has many advantages such as low cost, high safety and environmental friendliness, and is expected to become one of the most competitive technologies in the large-scale energy storage field.

[0003] At present, the key technical bottleneck restricting the alkaline full-iron flow battery is its capacity attenuation problem, which is also a common problem of various dissolved flow batteries including the all-vanadium flow battery and the iron-chromium flow battery. The capacity attenuation of the flow battery is generally related to factors such as cross contamination of positive and negative electrolytes, decomposition of active materials, occurrence of side reactions and the like. The positive active material of the alkaline full-iron flow battery is generally ferricyanide, and the negative active material is an iron-based organic complex (for example, a complex formed by iron ions and triethanolamine, triisopropanolamine, bis(2-hydroxyethyl)amino(trihydroxymethyl)methane and other polyhydroxy organic ligand molecules). The capacity attenuation mainly comes from two aspects: (1) the instability of the positive active material. Since a small part of the charged state (oxidation state) ferricyanide will spontaneously convert into the discharged state (reduction state) ferrocyanide under the action of the alkaline environment and maintain in a certain reversible equilibrium state, the positive electrolyte generally adopts a mixed formula of ferrocyanide / ferricyanide, and the oxidation state ferricyanide is used to supplement the partial capacity loss of the positive electrolyte during discharge; (2) the instability of the negative active material. Since the iron ions in the iron-based complex form a relatively weak coordination interaction with the polyhydroxy organic molecules, according to the strength of the coordination ability of the organic molecules, an excess of 20% to 200% of the polyhydroxy organic ligand needs to be added to the negative electrolyte to ensure that the iron-based complex does not decompose.

[0004] In view of the above two capacity attenuation mechanisms, although the capacity stability of the alkaline full-iron flow battery can be improved to a certain extent by implementing corresponding control strategies, during the long-term operation of the alkaline full-iron flow battery, the excess free polyhydroxy organic ligand in the negative electrolyte will gradually penetrate to the positive side, and the reducing organic ligand will accelerate the conversion reaction of the positive ferricyanide to ferrocyanide, so that the charged state substance in the positive electrolyte gradually decreases. When the initially added oxidation state ferricyanide is completely consumed, the battery starts to have capacity attenuation due to the mismatch of the positive and negative capacities during the discharge process. Therefore, in order to maintain the long-term capacity stability of the alkaline full-iron flow battery, it is urgent to develop a technology capable of recovering the capacity of the positive electrolyte. SUMMARY

[0005] The present application aims to provide an online capacity recovery method for the positive electrolyte of an alkaline all-iron flow battery, which uses chemical and electrochemical combination control means to accurately control the oxidation state concentration in the positive electrolyte during battery operation, to achieve continuous repair and replenishment of active substances in the electrolyte, thereby realizing the in-situ online recovery of the capacity of the alkaline all-iron flow battery.

[0006] An online capacity recovery method for the positive electrolyte of an alkaline all-iron flow battery, characterized in that it comprises the following steps: a. monitoring the capacity decay state of the alkaline all-iron flow battery; b. when the capacity decay is lower than a first preset threshold, adding a chemical oxidizing agent to the positive electrolyte to convert ferrocyanide in the positive electrolyte to ferricyanide; c. when the capacity decay reaches or exceeds the first preset threshold, or the application of the chemical oxidizing agent is limited, introducing the positive electrolyte into the anode chamber of an external electrolytic cell, adding an alkaline aqueous solution to the cathode chamber of the external electrolytic cell, converting the ferrocyanide in the positive electrolyte to ferricyanide through an electrochemical oxidation process, and returning the treated positive electrolyte to the positive electrode of the alkaline all-iron flow battery.

[0007] The chemical oxidizing agent is selected from at least one of persulfate, hypochlorite, permanganate, hydrogen peroxide, and ozone.

[0008] The amount of the chemical oxidizing agent is determined according to the capacity decay value and / or the initial composition of the positive electrolyte.

[0009] The alkaline aqueous solution added to the cathode chamber of the external electrolytic cell is an aqueous solution of an alkali metal hydroxide, with a concentration ranging from 1 mol / L to 6 mol / L.

[0010] The operating parameters of the electrochemical oxidation process include current density and charge cutoff conditions; the current density is a value ranging from 10 mA / cm 2 to 100 mA / cm 2 ; the charge cutoff conditions are determined according to the initial discharge capacity, the current discharge capacity, and / or the initial ferricyanide content of the positive electrolyte.

[0011] The positive electrolyte of the alkaline all-iron flow battery comprises ferrocyanide, ferricyanide and a first supporting electrolyte; the ferrocyanide is selected from at least one of alkali metal ferrocyanide or quaternary ammonium salt ferrocyanide, and the concentration thereof is in the range of 0.005 mol / L to 1.5 mol / L; the ferricyanide is selected from at least one of alkali metal ferricyanide or quaternary ammonium salt ferricyanide, and the concentration thereof is in the range of 0 mol / L to 1.0 mol / L; and the first supporting electrolyte is selected from at least one of alkali metal hydroxide or alkali metal carbonate, and the concentration thereof is in the range of 0.1 mol / L to 6 mol / L.

[0012] The negative electrolyte of the alkaline all-iron flow battery comprises a ferrous organic complex and a second supporting electrolyte; the ferrous organic complex is formed by a ferrous source and a polyhydroxy organic ligand, the ferrous source is selected from at least one of soluble iron salt, and the concentration of the ferrous organic complex is in the range of 0.001 mol / L to 2.0 mol / L; the molar ratio of iron to the organic ligand is in the range of 1:(0.5-2); and the second supporting electrolyte is selected from at least one of alkali metal hydroxide or alkali metal carbonate, and the concentration thereof is in the range of 1 mol / L to 10 mol / L.

[0013] The first preset threshold is a certain percentage of the capacity attenuation of the battery relative to its initial capacity, and the percentage is in the range of 10% to 30%.

[0014] The separator used in the alkaline all-iron flow battery is selected from at least one of sulfonated polyether ether ketone ion exchange membrane, perfluorosulfonic acid ion exchange membrane, polyether sulfone porous ion conductive membrane, polybenzimidazole porous ion conductive membrane and polyolefin porous ion conductive membrane.

[0015] The method is applied to an alkaline all-iron flow single battery, an alkaline all-iron flow stack module or an alkaline all-iron flow battery system.

[0016] The chemical oxidant is selected from one or more of persulfate, hypochlorite, permanganate, hydrogen peroxide, ozone and a combination thereof.

[0017] The addition amount m of the chemical oxidant o The calculation is obtained according to the following formula or its deformation form:

[0018]

[0019] C0 is the initial discharge capacity, C i is the current cycle discharge capacity, 26.8 is the capacity constant corresponding to 1 mol of electron transfer, N Fe is the proportionality factor of the initial amount of substance of ferricyanide in the positive electrolyte to the concentration and volume of oxidized ferricyanide, M On is the molar mass of the oxidant. e The number of redox electrons that the oxidant can provide.

[0020] During the recovery process, the capacity recovery amount is calculated as follows:

[0021] a) Determine the theoretical target recovery amount: based on the battery's initial capacity (C0) and the current capacity before the recovery operation (C... i ), calculate the theoretical number of moles of ferrocyanide (Fe(II)CN) required to fully restore capacity, as the theoretical target recovery amount (n target_Fe(II)CN );

[0022] b) Determine the theoretical oxidation supply: Based on the recovery method adopted, calculate the maximum theoretical oxidation capacity that can be provided, as the theoretical oxidation supply (n). oxide_Fe(II)CN ),in:

[0023] i. If recovery is achieved by adding a chemical oxidant, the theoretical oxidation supply is based on the mass (m) of the added chemical oxidant. ox ), molar mass (M) ox ) and unit electron transfer number (n e )calculate;

[0024] ii. If recovery is performed using an external electrolytic cell with electrochemical oxidation, the theoretical oxidation supply is based on the set charging cutoff capacity (C). c )calculate;

[0025] c) Calculate the predicted number of recovered moles:

[0026] i. If a chemical oxidant is added, first calculate a value related to the battery's historical operating cycle count (N). cyc ) and the initial free ligand concentration in the negative electrode electrolyte (C L,0 The relevant ligand influence factor (LIF) is used, and this LIF and a preset ligand influence correction coefficient (k) are employed. LIF ), for the theoretical oxidation supply (n) determined in step b), oxide_Fe(II)CN After correction, an effective oxidation supply (n) is obtained. oxide,eff Then compare the effective oxidation supply with the theoretical target recovery amount in step a), and take the smaller of the two as the predicted number of moles recovered (n). recov_corr );

[0027] ii. If an external electrolytic cell is used for electrochemical oxidation, the correction for ligand influence factors is not considered. The theoretical oxidation supply in step b) is directly compared with the theoretical target recovery in step a), and the smaller of the two is taken as the predicted number of moles recovered (n).recov_corr );

[0028] d) Calculate the predicted capacity recovery: based on the predicted number of recovered moles (n) obtained in step c). recov_corr The final predicted capacity recovery (ΔC) is calculated. pred_corr ).

[0029] The theoretical target recovery amount (n) in step a) target_Fe(II)CN n is determined by the following formula: target_Fe(II)CN =(C0-C i ) / F e Where C0 is the initial capacity, C i For the current capacity, F e This represents the Faraday electrochemical equivalent of a single electron transfer.

[0030] When a chemical oxidant is added, the theoretical oxidation supply (n_oxide_Fe(II)CN) is determined by the following formula:

[0031] n oxide_Fe(II)CN =(m ox / M ox )*n e ; where m ox For the mass of the oxidant, M ox n is the molar mass of the oxidant. e This represents the number of electrons transferred per unit of the oxidizing agent.

[0032] When a chemical oxidant is added, the effective oxidation supply (n_oxide,eff) is determined by the following formula:

[0033] n oxide,eff =n oxide_Fe(II)CN *(1-k LIF *LIF); where the ligand influence factor LIF = N cyc *C L,o ;k LIF The correction factor for ligand influence related to the properties of the negative electrode ligand molecule itself; N_cyc is the number of cycles before recovery; C_L,0 is the initial molar concentration of free ligands in the negative electrode electrolyte;

[0034] When using an external electrolytic cell for electrochemical oxidation, the theoretical oxidation supply (n) oxide_Fe(II)CN n is determined by the following formula: oxide_Fe(II)CN =C c / F e Among them, C c F represents the charging cutoff capacity of the external electrolytic cell. e This represents the Faraday electrochemical equivalent of a single electron transfer.

[0035] The beneficial effects of this invention are:

[0036] (1) The online capacity recovery method for the positive electrode electrolyte of alkaline all-iron flow battery proposed in this invention addresses the core decay mechanism caused by the accumulation of ferrocyanide in the positive electrode electrolyte, which leads to capacity mismatch between the positive and negative electrodes. By combining chemical and electrochemical parallel control methods, the amount of recovery agent added or the setting of electrochemical recovery parameters can be directly determined based on the capacity decay value. This eliminates the need for complicated steps such as sampling, detection, and analysis, and achieves online capacity recovery of the battery in a simple, flexible, precise and controllable manner. This method is of great significance for the large-scale application of alkaline all-iron flow batteries.

[0037] (2) This invention solves the capacity decay problem during battery cycling without replacing all internal components of the battery. It can directly restore the battery capacity retention rate from 0-80% to 90-100% in one go. The capacity recovery speed is fast and the effect is significant. (3) This invention effectively improves the long-term stable operation time of alkaline all-iron flow batteries, significantly reduces the maintenance cost during battery use, and further reduces the energy storage cost per kilowatt-hour of the battery throughout its entire life cycle. Attached Figure Description

[0038] Figure 1 This is a graph showing the change in capacity recovery rate. Detailed Implementation

[0039] A method for online capacity recovery of the positive electrode electrolyte in an alkaline all-iron flow battery involves the following steps: When the battery capacity decays below a certain percentage (e.g., 10%, 15%, 20%, 25%, 30%), a precise amount of oxidant is added based on the capacity decay value to induce a directional chemical reaction between the accumulated ferrocyanide and ferricyanide in the positive electrode electrolyte, thereby adjusting the ferricyanide concentration in the positive electrode electrolyte in real time. When the battery capacity decays above a certain percentage (e.g., 10%, 15%, 20%, 25%, 30%) or when the solubility of the oxidant is limited, after the battery charging is completed, the positive electrode electrolyte is introduced into the anode of an external electrolytic cell device, an alkaline aqueous solution is added to the cathode of the electrolytic cell, the operating parameters of the electrolytic cell device are set, and the remaining ferrocyanide in the positive electrode electrolyte is converted into ferricyanide through an electrochemical charging process. Then, the positive electrode electrolyte is reintroduced into the positive electrode of the alkaline all-iron flow battery to achieve online recovery of the battery discharge capacity.

[0040] Preferably, the alkaline all-iron flow battery includes a positive electrode electrolyte, a negative electrode electrolyte, a separator, and a current collector;

[0041] Preferably, the positive electrode electrolyte includes ferrocyanide, ferricyanide, and a supporting electrolyte, wherein the ferrocyanide is selected from one or more of sodium ferrocyanide, potassium ferrocyanide, and ammonium ferrocyanide, and the concentration is between 0.005-1.5 mol / L, preferably between 0.3-0.8 mol / L; the ferricyanide is selected from potassium ferricyanide, etc., and the concentration of potassium ferricyanide is between 0-1.0 mol / L, preferably between 0.1-0.5 mol / L; the supporting electrolyte is selected from one or more of potassium hydroxide, sodium hydroxide, lithium hydroxide, potassium carbonate, sodium carbonate, and lithium carbonate, and the concentration is between 0.1-6 mol / L, preferably between 1-3 mol / L.

[0042] Preferably, the negative electrode electrolyte comprises an iron-based organic complex and a supporting electrolyte, wherein the iron source is selected from one or more of ferric sulfate, ferric chloride, ferric acetate, and ferric nitrate, and the organic ligand is selected from triethanolamine, triisopropanolamine, 3-[NN-bis(2-hydroxyethyl)amino]-2-hydroxypropanesulfonic acid, bis(2-hydroxyethyl)amino(tris(hydroxymethyl)methane, N,N,N'N'-tetra(2-hydroxyethyl)ethylenediamine, N,N,N',N'-tetra(2-hydroxypropyl)ethylenediamine, N,N,N',N'-penta(2-hydroxypropyl)ethylenediamine, and N,N,N',N',N'-penta(2-hydroxypropyl)ethylenediamine. The iron-based organic complex is selected from one or more of polyhydroxy organic ligands containing 1-3 amino groups and 3-5 hydroxyl groups, such as diethylenetriamine. The concentration of the iron-based organic complex is between 0.001-2.0 mol / L, preferably between 0.5-0.8 mol / L. The molar ratio of iron to organic ligand is 1:(0.5-2). The supporting electrolyte is selected from one or more of potassium hydroxide, sodium hydroxide, lithium hydroxide, potassium carbonate, sodium carbonate, and lithium carbonate, with a concentration between 1-10 mol / L, preferably between 3-5 mol / L.

[0043] Preferably, the diaphragm is one of sulfonated polyether ether ketone ion exchange membrane, perfluorosulfonic acid ion exchange membrane, polyethersulfone porous ion conduction membrane, polybenzimidazole porous ion conduction membrane, and polyolefin porous ion conduction membrane, with sulfonated polyether ether ketone ion exchange membrane being the most preferred.

[0044] Preferably, the current collector is graphite or carbon felt.

[0045] Preferably, the battery is an alkaline all-ferrous flow single cell, an alkaline all-ferrous flow stack module, or an alkaline all-ferrous flow battery system.

[0046] Preferably, the oxidant is one or more of the following: ammonium persulfate, potassium persulfate, sodium persulfate, sodium hypochlorite, calcium hypochlorite, potassium permanganate, hydrogen peroxide, and ozone.

[0047] Preferably, the optimal amount of oxidant added is m o (Unit: g) can be calculated using the following formula:

[0048]

[0049] Where C0 is the initial discharge capacity (unit: Ah), C i The discharge capacity for the i-th cycle (unit: Ah) is given by N, where the constant 26.8 represents the theoretical capacity corresponding to 1 mol of electron transfer. Fe M represents the initial amount of potassium ferricyanide in the positive electrode electrolyte (in mol). O n is the molar mass of the oxidant (unit: g / mol). e The number of redox electrons that the oxidant can provide.

[0050] Preferably, the alkaline aqueous solution added to the cathode of the external electrolytic cell is one or more of potassium hydroxide, sodium hydroxide, and lithium hydroxide, with a concentration between 1-6 mol / L, and preferably between 2-3 mol / L;

[0051] Preferably, the operating parameters of the external electrolytic cell device include: current density and charging cutoff conditions;

[0052] Preferably, the current density is 10-100 mA / cm². 2 Preferred 40-80 mA / cm 2 ;

[0053] Preferably, the charging cutoff condition adopts capacity cutoff, and the optimal charging capacity C c It can be calculated using the following formula:

[0054] C c =C0-C i +26.8×N Fe

[0055] Preferably, the online capacity recovery method can employ one or both of the following: oxidant chemical method or external electrolytic cell electrochemical method.

[0056] Example 1

[0057] An alkaline all-iron flow battery was assembled with a positive electrode electrolyte consisting of 0.5 mol / L sodium ferrocyanide + 0.1 mol / L potassium ferricyanide + 1 mol / L sodium hydroxide, and a negative electrode electrolyte consisting of 0.5 mol / L ferric chloride + 0.75 mol / L bis(2-hydroxyethyl)amino(tris(hydroxymethyl)methane) + 4 mol / L potassium hydroxide. Long-term cycling tests were conducted using this electrolyte formulation. The initial discharge capacity was 23.11 Ah. After 127 cycles, the capacity decayed to 18.49 Ah (20% capacity decay). Adding 10.00 g of sodium persulfate to the positive electrode electrolyte restored the discharge capacity to 20.64 Ah after 130 cycles, recovering 89.31% of the capacity. Subsequently, the capacity slowly decayed.

[0058] Example 2

[0059] An alkaline all-iron flow battery was assembled with a positive electrode electrolyte consisting of 0.5 mol / L sodium ferrocyanide + 0.1 mol / L potassium ferricyanide + 1 mol / L sodium hydroxide, and a negative electrode electrolyte consisting of 0.5 mol / L ferric chloride + 0.75 mol / L bis(2-hydroxyethyl)amino(tris(hydroxymethyl)methane) + 4 mol / L potassium hydroxide. Long-term cycling tests were conducted using this electrolyte formulation. The initial discharge capacity was 23.08 Ah. After 122 cycles, the capacity decayed to 18.46 Ah (20% capacity decay). Adding 20.00 g of sodium persulfate to the positive electrode electrolyte caused the discharge capacity to stabilize at 22.81 Ah after 125 cycles, with a capacity retention of 98.83%. Subsequently, the capacity slowly decayed.

[0060] Example 3

[0061] An alkaline all-iron flow battery was assembled with a positive electrode electrolyte consisting of 0.5 mol / L sodium ferrocyanide + 0.1 mol / L potassium ferricyanide + 1 mol / L sodium hydroxide, and a negative electrode electrolyte consisting of 0.5 mol / L ferric chloride + 0.75 mol / L bis(2-hydroxyethyl)amino(tris(hydroxymethyl)methane) + 4 mol / L potassium hydroxide. Long-term cycling tests were conducted using this electrolyte formulation. The initial discharge capacity was 23.10 Ah. After 128 cycles, the capacity decayed to 18.48 Ah (20% capacity decay). Adding 40.00 g of sodium persulfate to the positive electrode electrolyte restored the discharge capacity to 23.08 Ah after 130 cycles, with a capacity retention of 99.91%. The battery then stabilized for up to 200 cycles.

[0062] Example 4

[0063] An alkaline all-iron flow battery was assembled with a positive electrode electrolyte consisting of 0.5 mol / L sodium ferrocyanide + 0.1 mol / L potassium ferricyanide + 1 mol / L sodium hydroxide, and a negative electrode electrolyte consisting of 0.5 mol / L ferric chloride + 0.6 mol / L bis(2-hydroxyethyl)amino(tris(hydroxymethyl)methane) + 4 mol / L potassium hydroxide. Long-term cycling tests were conducted using this electrolyte formulation. The initial discharge capacity was 23.12 Ah. After 120 cycles, the capacity decayed to 18.50 Ah (20% capacity decay). Adding 20.00 g of sodium persulfate to the positive electrode electrolyte caused the discharge capacity to stabilize at 22.93 Ah after 123 cycles, with a capacity retention of 99.16%. Subsequently, the capacity slowly decayed.

[0064] Example 5

[0065] An alkaline all-iron flow battery was assembled with a positive electrode electrolyte consisting of 0.5 mol / L sodium ferrocyanide + 0.1 mol / L potassium ferricyanide + 1 mol / L sodium hydroxide, and a negative electrode electrolyte consisting of 0.5 mol / L ferric chloride + 1.0 mol / L bis(2-hydroxyethyl)amino(tris(hydroxymethyl)methane) + 4 mol / L potassium hydroxide. Long-term cycling tests were conducted using this electrolyte formulation. The initial discharge capacity was 23.09 Ah. After 122 cycles, the capacity decayed to 18.50 Ah (20% capacity decay). Adding 20.00 g of sodium persulfate to the positive electrode electrolyte caused the discharge capacity to stabilize at 22.65 Ah after 125 cycles, with a capacity retention of 98.10%. Subsequently, the capacity slowly decayed.

[0066] Example 6

[0067] An alkaline all-iron flow battery was assembled with a positive electrode electrolyte consisting of 0.5 mol / L sodium ferrocyanide + 0.2 mol / L potassium ferricyanide + 1 mol / L sodium hydroxide, and a negative electrode electrolyte consisting of 0.5 mol / L ferric chloride + 0.75 mol / L bis(2-hydroxyethyl)amino(tris(hydroxymethyl)methane) + 4 mol / L potassium hydroxide. Long-term cycle testing was conducted using this electrolyte formulation. The initial discharge capacity was 23.23 Ah. After 210 cycles, the capacity decayed to 18.58 Ah (20% capacity decay). Adding 20.00 g of sodium persulfate to the positive electrode electrolyte caused the discharge capacity to stabilize at 22.83 Ah after 212 cycles, with a capacity retention of 98.30%. Subsequently, the capacity decayed slowly.

[0068] Example 7

[0069] An alkaline all-iron flow battery was assembled with a positive electrode electrolyte consisting of 0.5 mol / L sodium ferrocyanide + 0.3 mol / L potassium ferricyanide + 1 mol / L sodium hydroxide, and a negative electrode electrolyte consisting of 0.5 mol / L ferric chloride + 0.75 mol / L bis(2-hydroxyethyl)amino(tris(hydroxymethyl)methane) + 4 mol / L potassium hydroxide. Long-term cycling tests were conducted using this electrolyte formulation. The initial discharge capacity was 23.45 Ah. After 280 cycles, the capacity decayed to 18.76 Ah (20% capacity decay). Adding 20.00 g of sodium persulfate to the positive electrode electrolyte caused the discharge capacity to stabilize at 22.91 Ah after 283 cycles, with a capacity retention of 97.70%. Subsequently, the capacity slowly decayed.

[0070] Example 8

[0071] An alkaline all-iron flow battery was assembled. The positive electrode electrolyte consisted of 0.5 mol / L sodium ferrocyanide + 0.1 mol / L potassium ferricyanide + 1 mol / L sodium hydroxide, and the negative electrode electrolyte consisted of 0.5 mol / L ferric chloride + 0.75 mol / L N,N,N',N',N'-penta(2-hydroxypropyl)diethylenetriamine + 4 mol / L potassium hydroxide. Long-term cycling tests were conducted using this electrolyte formulation. The initial discharge capacity was 23.14 Ah. After 315 cycles, the capacity decayed to 18.51 Ah (20% capacity decay). Adding 10.00 g of sodium persulfate to the positive electrode electrolyte restored the discharge capacity to 20.72 Ah after 317 cycles, with a capacity retention of 89.55%. Subsequently, the capacity slowly decayed.

[0072] Example 9

[0073] An alkaline all-iron flow battery was assembled. The positive electrode electrolyte consisted of 0.5 mol / L sodium ferrocyanide + 0.1 mol / L potassium ferricyanide + 1 mol / L sodium hydroxide, and the negative electrode electrolyte consisted of 0.5 mol / L ferric chloride + 0.75 mol / L N,N,N',N',N'-penta(2-hydroxypropyl)diethylenetriamine + 4 mol / L potassium hydroxide. Long-term cycling tests were conducted using this electrolyte formulation. The initial discharge capacity was 23.15 Ah. After 326 cycles, the capacity decayed to 18.52 Ah (20% capacity decay). Adding 20.00 g of sodium persulfate to the positive electrode electrolyte restored the discharge capacity to 22.95 Ah after 328 cycles, recovering 99.14% of the capacity. Subsequently, the capacity slowly decayed.

[0074] Example 10

[0075] An alkaline all-iron flow battery was assembled. The positive electrode electrolyte consisted of 0.5 mol / L sodium ferrocyanide + 0.1 mol / L potassium ferricyanide + 1 mol / L sodium hydroxide, and the negative electrode electrolyte consisted of 0.5 mol / L ferric chloride + 0.75 mol / L N,N,N',N',N'-penta(2-hydroxypropyl)diethylenetriamine + 4 mol / L potassium hydroxide. Long-term cycling tests were conducted using this electrolyte formulation. The initial discharge capacity was 23.10 Ah. After 308 cycles, the capacity decayed to 18.48 Ah (20% capacity decay). Adding 40.00 g of sodium persulfate to the positive electrode electrolyte restored the discharge capacity to 23.09 Ah after 310 cycles, with a capacity retention of 99.96%. The battery then stabilized for 600 cycles.

[0076] Example 11

[0077] An alkaline all-iron flow battery was assembled. The positive electrode electrolyte consisted of 0.5 mol / L sodium ferrocyanide + 0.3 mol / L potassium ferricyanide + 1 mol / L sodium hydroxide, and the negative electrode electrolyte consisted of 0.5 mol / L ferric chloride + 0.75 mol / L N,N,N',N',N'-penta(2-hydroxypropyl)diethylenetriamine + 4 mol / L potassium hydroxide. Long-term cycle testing was conducted using this electrolyte formulation. The initial discharge capacity was 24.06 Ah. After 448 cycles, the capacity decayed to 19.25 Ah (20% capacity decay). Adding 20.00 g of sodium persulfate to the positive electrode electrolyte restored the discharge capacity to 23.65 Ah after 450 cycles, with a capacity retention of 98.29%. Subsequently, the capacity slowly decayed.

[0078] Example 12

[0079] An alkaline all-iron flow battery was assembled. The positive electrode electrolyte consisted of 0.5 mol / L sodium ferrocyanide + 0.3 mol / L potassium ferricyanide + 1 mol / L sodium hydroxide, and the negative electrode electrolyte consisted of 0.5 mol / L ferric chloride + 0.75 mol / L N,N,N',N',N'-penta(2-hydroxypropyl)diethylenetriamine + 4 mol / L potassium hydroxide. Long-term cycling tests were conducted using this electrolyte formulation. The initial discharge capacity was 24.09 Ah. After 455 cycles, the capacity decayed to 19.27 Ah (20% capacity decay). Adding 40.00 g of sodium persulfate to the positive electrode electrolyte restored the discharge capacity to 24.08 Ah after 458 cycles, recovering 99.95% of the capacity. The battery then stabilized for 730 cycles.

[0080] Example 13

[0081] An alkaline all-ferric flow battery was assembled. The positive electrode electrolyte consisted of 0.5 mol / L sodium ferrocyanide + 0.1 mol / L potassium ferricyanide + 1 mol / L sodium hydroxide, and the negative electrode electrolyte consisted of 0.5 mol / L ferric chloride + 0.75 mol / L bis(2-hydroxyethyl)amino(tris(hydroxymethyl)methane) + 4 mol / L potassium hydroxide. Long-cycle testing was conducted using this electrolyte formulation. The initial discharge capacity was 23.15 Ah, and after 170 cycles, the capacity decayed to 9.26 Ah (60% capacity decay). After the 171st charge cycle, the positive electrode electrolyte was introduced into the anode of an external electrolytic cell, and a 2 mol / L sodium hydroxide aqueous solution was added to the cathode. The current density of the electrolytic cell was set to 60 mA / cm². 2 The charging capacity cutoff condition is 10Ah. After charging is completed, the positive electrolyte is reintroduced into the positive electrode of the alkaline all-iron flow battery. After the 172nd cycle of discharge, the capacity recovers to 13.89Ah, and the capacity retention rate recovers to 82.98%. Subsequently, the capacity slowly decays.

[0082] Example 14

[0083] An alkaline all-ferric flow battery was assembled. The positive electrode electrolyte consisted of 0.5 mol / L sodium ferrocyanide + 0.1 mol / L potassium ferricyanide + 1 mol / L sodium hydroxide, and the negative electrode electrolyte consisted of 0.5 mol / L ferric chloride + 0.75 mol / L bis(2-hydroxyethyl)amino(tris(hydroxymethyl)methane) + 4 mol / L potassium hydroxide. Long-cycle testing was conducted using this electrolyte formulation. The initial discharge capacity was 23.11 Ah, and after 175 cycles, the capacity decayed to 9.24 Ah (60% capacity decay). After the 176th charge cycle, the positive electrode electrolyte was introduced into the anode of an external electrolytic cell, and a 2 mol / L sodium hydroxide aqueous solution was added to the cathode. The current density of the electrolytic cell was set to 60 mA / cm². 2 The charging capacity cutoff condition is 20Ah. After charging is completed, the positive electrolyte is reintroduced into the positive electrode of the alkaline all-iron flow battery. After the 177th cycle, the discharge capacity is restored to 23.07Ah, and the capacity retention rate is restored to 99.97%. It then stabilizes for 260 cycles.

[0084] Comparative Example 1

[0085] An alkaline all-iron flow battery was assembled with a positive electrode electrolyte of 0.5 mol / L sodium ferrocyanide + 1 mol / L sodium hydroxide and a negative electrode electrolyte of 0.5 mol / L ferric chloride + 0.75 mol / L bis(2-hydroxyethyl)amino(tris(hydroxymethyl)methane) + 4 mol / L potassium hydroxide. Long-term cycling tests were conducted using this electrolyte formulation. The initial discharge capacity was 22.92 Ah. After 51 cycles, the capacity decayed to 18.34 Ah. Adding 10.00 g of sodium persulfate to the positive electrode electrolyte restored the discharge capacity to 20.36 Ah after 54 cycles, recovering 89.64% of the capacity. Subsequently, the capacity slowly decayed.

[0086] Comparative Example 2

[0087] An alkaline all-iron flow battery was assembled with a positive electrode electrolyte of 0.5 mol / L sodium ferrocyanide + 1 mol / L sodium hydroxide and a negative electrode electrolyte of 0.5 mol / L ferric chloride + 0.75 mol / L bis(2-hydroxyethyl)amino(tris(hydroxymethyl)methane) + 4 mol / L potassium hydroxide. Long-term cycling tests were conducted using this electrolyte formulation. The initial discharge capacity was 22.94 Ah. After 54 cycles, the capacity decayed to 18.35 Ah. Adding 40.00 g of sodium persulfate to the positive electrode electrolyte restored the discharge capacity to 22.93 Ah after 57 cycles, achieving a capacity retention of 99.97%. Subsequently, the capacity slowly decayed.

[0088] Comparative examples show that when the battery capacity decays by approximately 20%, capacity recovery using chemical oxidation is affected by several factors, including the amount of oxidant (Examples 1-3, 8-10), the ligand concentration in the initial negative electrode electrolyte (Examples 4-5), the molecular type (Examples 2, 9), and the potassium ferricyanide concentration in the initial positive electrode electrolyte (Examples 2, 6, 7). When the positive electrode electrolyte does not contain potassium ferricyanide (Comparative Examples 1-2), the capacity of the alkaline all-iron flow battery exhibits a linear decay trend with increasing cycle count. However, as the initial potassium ferricyanide concentration in the positive electrode electrolyte increases (Examples 2, 6, 7), the number of stable cycle counts for battery capacity significantly increases. This indicates that battery capacity decay is closely related to the potassium ferricyanide content in the electrolyte. Figure 1 The figures show the capacity retention changes of alkaline all-iron flow batteries before and after recovery in Examples 3, 12, and Comparative Example 2. It is clear from the figures that, in the examples, when the battery capacity decayed to about 80%, adding a certain amount of recovery agent to the positive electrode electrolyte resulted in the battery discharge capacity recovering to over 99.5% of its initial capacity and remaining stable after 2-3 charge-discharge cycles. The number of stable cycles was related to the initial potassium ferricyanide concentration in the electrolyte. In contrast, Comparative Example 2 still showed continuous capacity decay after recovery, indicating that the presence of initial potassium ferricyanide was crucial for maintaining stable battery operation. When the battery capacity decayed to about 60%, electrochemical capacity recovery was performed using an external electrolyzer. Under appropriate recovery conditions, the battery discharge capacity could also be recovered to over 99.8% of its initial capacity and remained stable (Example 14).

[0089] In summary, the online capacity recovery method for the positive electrode electrolyte of the alkaline all-iron flow battery proposed in this invention is applicable to different negative electrode electrolyte systems and different degrees of capacity decay, with significant battery capacity recovery effects. The capacity recovery rate fluctuates within the range of 80% to 100%, which may depend on factors such as the initial electrolyte formulation, the number of battery cycles, the degree of capacity decay, and the capacity recovery method. By optimizing the recovery conditions, the capacity recovery rate can reach 99.5%.

[0090] To better apply the capacity recovery method to practical processes, this invention further proposes to establish a capacity recovery rate prediction model based on the recovery method, providing theoretical guidance for the actual operation of the capacity recovery method.

[0091] The capacity decay of alkaline all-iron flow batteries is mainly due to the positive electrode active material. (simplified to Fe(III)CN) transformed into (Simplified to Fe(II)CN), partly due to its own instability, but more importantly, due to the excessive reduction of organic ligands at the negative electrode permeating to the positive electrode and accelerating this conversion, or reacting directly with Fe(III)CN. This can be achieved by adding chemical oxidants (such as...) to the positive electrode electrolyte of the alkaline all-iron flow battery. Alternatively, it can be introduced into an external electrochemical oxidation recovery device, with the aim of oxidizing Fe(II)CN to Fe(III)CN, thereby achieving capacity recovery.

[0092] During battery capacity recovery, cross-penetration of the negative electrode ligands still occurs, which can interfere with the actual capacity recovery rate. Assume there exists an equivalent concentration C of the permeated ligands. L,eff Two parameters related to ligand permeation concentration were set, and the number of charge-discharge cycles (N) was also set. cyc ) and the original concentration of the negative electrode ligand (C L,0 This indicates that the more cycles, the greater the cumulative amount of ligand permeating to the positive electrode; the higher the initial ligand concentration, the greater the driving force for permeation, and the potentially greater the permeation amount per unit time. A ligand impact factor (LIF) is defined as: LIF = N cyc ·C L,0 Where N cyc C is the number of loops before capacity restoration. L,0 The initial molar concentration (mol / L) of free ligands in the negative electrode electrolyte is LIF. The unit of LIF is cyc·mol / L. A higher LIF value indicates a greater potential negative impact of ligand permeation on capacity recovery. This impact occurs partly by consuming some of the externally added oxidizing capacity and partly by directly reacting with newly generated Fe(III)CN, thus reducing the recovery effect. Assuming the electrolyte volume remains constant throughout the process, the data parameters before and after capacity recovery in the examples and comparative examples are statistically analyzed (Table 1).

[0093] Table 1. Summary of data before and after capacity recovery in Examples 1-14 and Comparative Examples 1-2.

[0094]

[0095] When ligand permeation correction is not considered, two core basic parameters are calculated: first, the number of moles of Fe(II)CN that the battery theoretically needs to oxidize to recover from its current decay state to its initial capacity; and second, the maximum number of moles of Fe(II)CN that can theoretically be oxidized based on the actual capacity recovery strategy adopted (the amount of oxidant added or the electrochemical recovery parameters).

[0096] Target recovered Fe(II)CN moles (theoretical maximum):

[0097] Where, ntarget\_Fe(II)CN :Make the battery capacity increase from the current value C i The amount of Fe(I)CN that needs to be oxidized to restore the initial value C0 is expressed in moles (mol), representing the "required amount" for capacity restoration.

[0098] C0: The initial capacity of the battery, i.e., the capacity before significant degradation, is measured in ampere-hours (Ah).

[0099] C i The actual capacity of the battery at the i-th cycle, i.e., the capacity after degradation, is expressed in ampere-hours (Ah).

[0100] (C0-C i ): The actual capacity degradation of the battery, expressed in ampere-hours (Ah).

[0101] F e Faraday's constant is applied to single-electron redox reactions (Fe(II)CN→e) - The electrochemical equivalent of Fe(III)CN represents the amount of charge corresponding to each mole of electron transfer, and is usually taken as 26.8 Ah / mol (i.e. 96485 C / mol / 3600 s / h).

[0102] The theoretical maximum number of moles of Fe(II)CN that can be oxidized by a chemical oxidant:

[0103] Electrochemical recovery of the number of moles of oxidizable Fe(II)CN (theoretical maximum):

[0104] Where, n oxidable\_Fe(II)CN The amount of Fe(II)CN that can be oxidized theoretically, based on the amount of oxidant added, is expressed in moles (mol) and represents the "supply" for capacity recovery.

[0105] m ox The mass of oxidant added to the electrolyte, expressed in grams (g).

[0106] M ox The molar mass of the oxidizing agent used is expressed in grams per mole (g / mol). Sodium persulfate (Na₂S₂O₈) is used as an example here. It is approximately 238.1 g / mol.

[0107] The amount of oxidant added is expressed in moles (mol).

[0108] n e The number of electrons transferred per unit amount of oxidizing agent, for example, sodium persulfate (Na2S2O8)n e =2.

[0109] C c : Charging cut-off capacity of external electrochemical recovery device, in ampere-hours (Ah).

[0110] F e (As defined above) represents the amount of charge corresponding to each mole of electron transfer, usually taken as 26.8 Ah / mol.

[0111] Base Model (M1)

[0112] First, an idealized capacity recovery prediction model (M1) is established, where capacity recovery is unaffected by other factors (such as side reactions caused by ligand permeation or the instability of Fe(III)CN). The actual amount of Fe(II)CN recovered depends on the two fundamental parameters mentioned above: one is the "demand" (n). target\_Fe(II)CN The second is "supply" (n oxidable\_Fe(II)CN If capacity recovery conditions are sufficient (n) oxidable\_Fe(II)CN >n target\_Fe(II)CN If n ), then the capacity recovery rate is limited by n. target\_Fe(II)CN If capacity recovery conditions are insufficient (n oxidable\_Fe(II)CN ≤n target\_Fe(II)CN If the capacity recovery rate is limited, then n oxidable\_Fe(II)CN Then we get:

[0113] Predicted number of Fe(II)CN moles recovered: n recov\_no\_corr =min(n oxidable\_Fe(II)CN ,n target\_Fe(II)CN )

[0114] n recov\_no\_corr The amount of Fe(II)CN that can actually be recovered (i.e. successfully oxidized) according to model M1, without considering the influence of ligands, is expressed in moles (mol).

[0115] n target\_Fe(II)CN (As defined above) Theoretically, the number of Fe(II)CN moles that need to be recovered.

[0116] n oxidable\_Fe(II)CN (As defined above) The maximum number of Fe(II)CN moles that the capacity recovery strategy can oxidize.

[0117] min(A,B): Takes the smaller of the two values ​​A and B.

[0118] Converting the predicted actual number of moles of Fe(II)CN oxidized into an equivalent capacity increase yields the predicted capacity recovery: ΔC pred\_no\_corr =n recov\_no\_corr ·F e .

[0119] ΔC pred\_no\_corr : Battery capacity recovery predicted by the basic model M1, in ampere-hours (Ah).

[0120] F e (As defined above) represents the amount of charge corresponding to each mole of electron transfer, usually taken as 26.8 Ah / mol.

[0121] The capacity recovery prediction results and relative error values ​​of the basic model (M1) are shown in Table 2. As can be seen from Table 2, for all embodiments and comparative examples, the error rate between the predicted and actual capacity recovery values ​​given by the M1 model is less than 10%. For capacity recovery strategies using chemical oxidants, when the "supply" (n...) oxidable\_Fe(II)CN ) lower than "demand" (n target\_Fe(II)CN When the capacity recovery error rate predicted by the M1 model is between 1.63% and 8.49%, i.e., Examples 1-2, 4-9, 11 and Comparative Example 1, the error rate is between 1.63% and 8.49%; when the "supply" (n) oxidable\_Fe(II)CN ) is higher than "demand" (n target\_Fe(II)CN When the recovery conditions were met (i.e., Examples 3, 10, 12, and Comparative Example 2), the capacity recovery error rate predicted by the model ranged from 0.17% to 0.43%. This indicates that when recovery conditions were sufficient, the M1 model was more accurate in predicting the capacity recovery rate. However, when recovery conditions were insufficient, the M1 model's predictions showed some bias, which was related to ligand permeation. When the molecular size of ligand 1 was small (Examples 1-2, 4-7, and Comparative Example 1), the ligand transmembrane permeation coefficient was large, with an average error rate of 4.83%. When the molecular size of ligand 2 was large (Examples 8-9 and 11), the ligand transmembrane permeation coefficient was small, with an average error rate of 1.94%. For the capacity recovery strategy using electrochemical oxidation (Examples 13-14), the M1 model was more accurate in predicting the capacity recovery rate (error rate < 0.50%). This was mainly because during electrochemical recovery, the electrolyte was introduced into an external capacity recovery device, and the influence of ligand transmembrane permeation during the recovery process was relatively small.

[0122] Table 2. Prediction results of the basic model (M1)

[0123]

[0124]

[0125] Modified model (M2)

[0126] Based on model M1, a modified model (M2) is established that considers the cumulative effect of ligand penetration, making it closer to the actual capacity recovery process. It assumes that the penetration of negative electrode ligands affects the actual capacity recovery of the positive electrode, and this effect is related to the number of battery cycles and the initial ligand concentration at the negative electrode. This effect is quantified by the "ligand influence factor" (LIF).

[0127] LIF=N cyc ·C L,0

[0128] N cyc Number of charge / discharge cycles before capacity recovery operation.

[0129] C L,0 : Initial molar concentration (mol / L) of free ligands in the negative electrode electrolyte.

[0130] Since the cumulative permeation effect of ligands reduces chemical or electrochemical recovery capacity, a larger LIF indicates a greater cumulative negative impact (1-k). LIF The smaller the LIF factor, the lower the effective number of oxidizable moles n. oxidable\_Fe(II)CN,eff Below the ideal value n oxidable\_Fe(II)CN .

[0131] Effective number of moles that can be oxidized and recovered: n oxidable\_Fe(II)CN,eff =n oxidable\_Fe(II)CN ·(1-k LIF ·LIF)

[0132] n oxidable\_Fe(II)CN,eff The actual effective amount of Fe(II)CN oxidatively restored after considering the effect of ligand permeation is expressed in moles (mol).

[0133] n oxidable\_Fe(II)CN (As defined above) The maximum number of moles that a capacity recovery strategy can oxidize under ideal conditions (without ligand influence).

[0134] k LIF : Ligand Influence Correction Coefficient. This is an empirical fitting parameter, whose physical meaning is the degree to which a unit LIF value weakens the theoretical recovery ability. This coefficient is related to factors such as the ligand molecule's own structure and its transmembrane permeability coefficient. Its unit is the reciprocal of the LIF unit, i.e., (cyc·mol / L). -1 .

[0135] LIF: Ligand Influence Factor, which comprehensively reflects the cumulative effect of cycle number and initial ligand concentration.

[0136] (1-k LIF•LIF: A correction term, a dimensionless scaling factor (typically between 0 and 1) representing the percentage of the actual recoverable capacity relative to the ideal recoverable capacity due to the negative effects of the ligand. The effect of the ligand is relatively small when the chemical oxidant is sufficient. LIF ≈0, this factor is 1, the model degenerates to the uncorrected case; when using electrolytic recovery in an electrolytic cell, the correction term (1-k) LIF ·LIF) is set to 1, meaning that the penetration of ligands into the battery is not considered at this time.

[0137] The actual amount of Fe(II)CN recovered still depends on the "demand" (n target\_Fe(II)CN ) and "corrected supply" (n oxidable\_Fe(II)CN,eff The smaller of the two.

[0138] Predicted recovery of Fe(II)CN moles (corrected): n recov\_corr =min(n target\_Fe(II)CN ,n oxidable\_Fe(II)CN,eff )

[0139] n recov\_corr The amount of Fe(II)CN that can actually be recovered (i.e. successfully oxidized) according to the M2 model, taking into account the influence of ligands, is expressed in moles (mol).

[0140] n target\_Fe(II)CN (As defined above) The target is to restore the number of Fe(II)CN moles.

[0141] n oxidable\_Fe(II)CN,eff (From the previous formula of the M2 model) After considering the influence of ligands, the maximum number of Fe(II)CN that the capacity recovery strategy can actually oxidize.

[0142] The number of Fe(II)CN moles predicted by the M2 model and actually recovered after considering the ligand effect was converted back to the equivalent capacity increase.

[0143] Predicted capacity recovery: ΔC pred\_corr =n recov\_corr ·F e

[0144] Based on the basic parameters and prediction results in the M1 model, the k in the M2 model... LIF Perform empirical fitting, especially in cases of insufficient chemical oxidants (n oxidable\_Fe(II)CN,eff <n target\_Fe(II)CN In the case of n... oxidable\_Fe(II)CN,eff =n oxidable\_Fe(II)CN ·(1-k LIF ·LIF)=ΔC pred\_no\_corr / F e ·(1-k LIF·LIF) more closely approximates the actual number of moles recovered, n actual =ΔC actual / F e hour, Calculate k for each data point LIF Theoretical value:

[0145] Example 1: k LIF1 = (1 - 2.15 / 2.25) / 31.75 ≈ 0.001415

[0146] Example 2: k LIF2 =(1-4.35 / 4.50) / 30.50≈0.001109

[0147] Example 4: k LIF3 =(1-4.43 / 4.50) / 12.00≈0.001338

[0148] Example 5: k LIF4 =(1-4.18 / 4.50) / 61.00≈0.001147

[0149] Example 6: k LIF6 = (1 - 4.25 / 4.50) / 52.50 ≈ 0.001067

[0150] Example 7: k LIF7 = (1 - 4.15 / 4.50) / 70.00 ≈ 0.001118

[0151] Comparative Example 1:k LIF1′ = (1 - 2.21 / 2.25) / 12.75 ≈ 0.001434

[0152] ligand 1's k LIF The values ​​are quite close; the average value is taken as 0.001237 (cyc·mol / L). -1 .

[0153] Example 8: k LIF8 = (1 - 2.21 / 2.25) / 78.75 ≈ 0.000232

[0154] Example 9: k LIF9 = (1 - 4.43 / 4.50) / 81.50 ≈ 0.000197

[0155] Example 11: k LIF11 =(1-4.40 / 4.50) / 112.00≈0.000203

[0156] ligand 2's k LIFThe values ​​are quite close; the average value is taken as 0.000211 (cyc·mol / L). -1 .

[0157] Based on LIF and k in the M2 model LIF The parameter fitting values ​​were used to quantify and empirically correct the complex effects of ligand permeation, aiming to provide a more realistic capacity recovery prediction (Table 3). As shown in Table 3, for all examples and comparative examples, the absolute error rate between the predicted and actual capacity recovery values ​​given by the M2 model was less than 1%. For electrolytes containing ligand 1 and ligand 2, the prediction accuracy (mean absolute error rates of 0.52% and 0.13%, respectively) was significantly better than that of the M1 model (mean absolute error rates of 4.83% and 1.94%, respectively). This demonstrates the importance of quantifying the cumulative effect of ligand permeation and incorporating it into the recovery model, which not only conforms to electrochemical principles but also improves the accuracy of predictions in practice.

[0158] Table 3. Prediction results of the modified model (M2)

[0159]

Claims

1. A method for online recovery of electrolyte in an alkaline all-iron flow battery, characterized in that, Includes the following steps: a. Monitor the capacity decay status of the alkaline all-iron flow battery; b. When the capacity decay is lower than a first preset threshold, a chemical oxidant is added to the positive electrode electrolyte to promote the conversion of ferrocyanide in the positive electrode electrolyte into ferricyanide; c. When the capacity decay reaches or exceeds the first preset threshold, or the application of the chemical oxidant is limited, the positive electrode electrolyte is introduced into the anode chamber of an external electrolytic cell, and an alkaline aqueous solution is added to the cathode chamber of the external electrolytic cell. The ferrocyanide in the positive electrode electrolyte is converted into ferricyanide through an electrochemical oxidation process, and the treated positive electrode electrolyte is returned to the positive electrode of the alkaline all-iron flow battery.

2. The method according to claim 1, characterized in that, The chemical oxidant is selected from at least one of persulfate, hypochlorite, permanganate, hydrogen peroxide, and ozone.

3. The method according to claim 1 or 2, characterized in that, The amount of chemical oxidant added is determined based on the capacity decay value and / or the initial composition of the positive electrode electrolyte.

4. The method according to claim 1, characterized in that, The alkaline aqueous solution added to the cathode chamber of the external electrolytic cell is an alkali metal hydroxide aqueous solution with a concentration in the range of 1 mol / L to 6 mol / L.

5. The method according to claim 1 or 4, characterized in that, The operating parameters of the electrochemical oxidation process include current density and charging cutoff conditions; the current density is 10 mA / cm². 2 Up to 100mA / cm 2 The value is within the specified range; the charging cutoff condition is determined based on the initial discharge capacity, the current discharge capacity, and / or the initial ferricyanide content of the positive electrode electrolyte.

6. The method according to claim 1, characterized in that, The positive electrode electrolyte of the alkaline all-iron flow battery includes ferrocyanide, ferricyanide, and a first supporting electrolyte; the ferrocyanide is selected from at least one of alkali metal ferrocyanide or quaternary ammonium salt ferrocyanide, and its concentration is in the range of 0.005 mol / L to 1.5 mol / L; the ferricyanide is selected from at least one of alkali metal ferricyanide or quaternary ammonium salt ferricyanide, and its concentration is in the range of 0 mol / L to 1.0 mol / L; the first supporting electrolyte is selected from at least one of alkali metal hydroxide or alkali metal carbonate, and its concentration is in the range of 0.1 mol / L to 6 mol / L.

7. The method according to claim 1, characterized in that, The negative electrode electrolyte of the alkaline all-iron flow battery includes an iron-based organic complex and a second supporting electrolyte; the iron-based organic complex is formed by an iron source and a polyhydroxy organic ligand, wherein the iron source is selected from at least one of soluble iron salts, and the concentration of the iron-based organic complex is in the range of 0.001 mol / L to 2.0 mol / L; the molar ratio of iron to the organic ligand is in the range of 1:(0.5-2); the second supporting electrolyte is selected from at least one of alkali metal hydroxides or alkali metal carbonates, and its concentration is in the range of 1 mol / L to 10 mol / L.

8. The method according to claim 1, characterized in that, The first preset threshold is a percentage of the battery capacity decaying to its initial capacity, and the percentage is in the range of 10% to 30%; the separator used in the alkaline all-iron flow battery is selected from at least one of sulfonated polyether ether ketone ion exchange membrane, perfluorosulfonic acid ion exchange membrane, polyether sulfone porous ion conduction membrane, polybenzimidazole porous ion conduction membrane, and polyolefin porous ion conduction membrane; the method is applied to alkaline all-iron flow single cell, alkaline all-iron flow stack module, or alkaline all-iron flow battery system.

9. The method according to claim 1, characterized in that, During the recovery process, the capacity recovery amount is calculated as follows: a) Determine the theoretical target recovery amount: based on the battery's initial capacity (C0) and the current capacity before the recovery operation (C... i ), calculate the theoretical number of moles of ferrocyanide (Fe(II)CN) required to fully restore capacity, as the theoretical target recovery amount (n target_Fe(II)CN ); b) Determine the theoretical oxidation supply: Based on the recovery method adopted, calculate the maximum theoretical oxidation capacity that can be provided, as the theoretical oxidation supply (n). oxide_Fe(II)CN ),in: i. If recovery is achieved by adding a chemical oxidant, the theoretical oxidation supply is based on the mass (m) of the added chemical oxidant. ox ), molar mass (M) ox ) and unit electron transfer number (n e )calculate; ii. If recovery is performed using an external electrolytic cell with electrochemical oxidation, the theoretical oxidation supply is based on the set charging cutoff capacity (C). c )calculate; c) Calculate the predicted number of recovered moles: i. If a chemical oxidant is added, first calculate a value related to the battery's historical operating cycle count (N). cyc ) and the initial free ligand concentration in the negative electrode electrolyte (C L,o The relevant ligand influence factor (LIF) is used, and this LIF and a preset ligand influence correction coefficient (k) are employed. LIF ), for the theoretical oxidation supply (n) determined in step b), oxide_Fe(II)CN After correction, an effective oxidation supply (n) is obtained. oxide,eff Then compare the effective oxidation supply with the theoretical target recovery amount in step a), and take the smaller of the two as the predicted number of moles recovered (n). recov_corr ); ii. If an external electrolytic cell is used for electrochemical oxidation, the correction for ligand influence factors is not considered. The theoretical oxidation supply in step b) is directly compared with the theoretical target recovery in step a), and the smaller of the two is taken as the predicted number of moles recovered (n). recov_corr ); d) Calculate the predicted capacity recovery: based on the predicted number of recovered moles (n) obtained in step c). recov_corr The final predicted capacity recovery (ΔC) is calculated. pred_corr ).

10. The method according to claim 9, characterized in that, The theoretical target recovery amount (n) in step a) target_Fe(II)CN n is determined by the following formula: target_Fe(II)CN =(C0-C i ) / F e ; Where C0 is the initial capacity, C i For the current capacity, F e This represents the Faraday electrochemical equivalent of a single electron transfer. When a chemical oxidant is added, the theoretical oxidation supply (n_oxide_Fe(II)CN) is determined by the following formula: n oxide_Fe(II)CN =(m ox / M ox )*n e ; where m ox For the mass of the oxidant, M ox n is the molar mass of the oxidant. e This represents the number of electrons transferred per unit of the oxidizing agent. When a chemical oxidant is added, the effective oxidation supply (n_oxide,eff) is determined by the following formula: n oxide,eff =n oxide_Fe(II)CN *(1-k LIF *LIF); where the ligand influence factor LIF = N cyc *C L,o ;k LIF The correction factor for ligand influence related to the properties of the negative electrode ligand molecule itself; N_cyc is the number of cycles before recovery; C_L,0 is the initial molar concentration of free ligands in the negative electrode electrolyte; When using an external electrolytic cell for electrochemical oxidation, the theoretical oxidation supply (n) oxide_Fe(II)CN n is determined by the following formula: oxide_Fe(II)CN =C c / F e Among them, C c F represents the charging cutoff capacity of the external electrolytic cell. e This represents the Faraday electrochemical equivalent of a single electron transfer.

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