Method for recovering aqueous positive electrode electrolyte of redox flow battery

Abstract

The present application relates to a method for recovering an aqueous positive electrode electrolyte of a redox flow battery to be recovered, said aqueous positive electrode electrolyte comprising at least one electroactive compound comprising at least one oxidized or reduced form of a reduction / oxidation pair and an aqueous solvent, the invention relates to a method for producing an electroactive compound, the reduced form of which is a water-soluble organometallic complex, characterized in that the method comprises:-a step (300) of precipitating the electroactive compound, thereby obtaining a suspension,-a step (400) of separating the suspension, thereby obtaining a solid residue (52) and an effluent (54), and-a step (600) of drying the solid residue (52), comprising heating the solid residue (52) to a temperature lower than or equal to 40 DEG C, preferably lower than or equal to 35 DEG C, more preferably lower than or equal to 30 DEG C, further preferably lower than or equal to 25 DEG C, thereby obtaining a recovered electroactive compound.

Description

[0001] The present disclosure relates to a method for recycling aqueous positive electrolyte (posolyte) for redox flow batteries. Background Art

[0002] A redox flow battery is a system that uses a liquid (called an electrolyte) to store energy. Redox flow batteries store and generate electricity via oxidation-reduction (redox) reactions. They typically have two compartments separated by an ion exchange membrane, in which the current collectors (electrodes) are typically immersed.

[0003] One problem with current battery storage technologies is that they typically use ores and metals that have a significant environmental impact. Furthermore, the complex design and use of composite materials hinder the simple, economical, and efficient recycling of key materials. While they claim to address the environmental impact of energy production by storing renewable energy (and therefore reducing CO2 emissions per kilowatt-hour of electricity generated), life cycle assessments of these technologies indicate very modest sustainability. They cause resource depletion and significant pollution due to the waste represented by end-of-life batteries.

[0004] As for the prior art, recovery methods have been published in the literature, for example:

[0005] - Lithium-ion batteries (EP1269554B1): Method for recovering and separating critical materials. The method is complex and expensive to implement.

[0006] -Lead-acid batteries (CA2986001A1): Closed-loop electrochemical process to recover lead for reuse;

[0007] - Vanadium redox flow batteries: A recycling method has been proposed by mixing two electrolytes, which are operated continuously during the cycle to combat the crossover of vanadium ions through the membrane (Zhang, Y., Liu, L., Xi, J., Wu, Z., & Qiu, X. (2017). The benefits and limitations of electrolyte mixing in vanadium flow batteries. Applied Energy, 204, 373-381).

[0008] -Zn-Br redox flow battery: bromine neutralization and recovery process (CN103236570B).

[0009] Currently, there are no recycling solutions for redox flow batteries based on organic or organometallic compounds dissolved in aqueous media, especially organic compounds using redox couples.

[0010] KEMIWATT uses organic and organometallic electrolytes dissolved in an aqueous medium to limit the environmental and resource consumption impact of this technology (use of critical metals / rare earths). However, to date, there has been no solution for recycling such batteries. Summary of the Invention

[0011] The present application addresses the technical problem of providing a method for recycling redox flow batteries using redox couples based on organic and / or organometallic compounds in aqueous solution.

[0012] A specific object of the present application is to solve the technical problem of providing a method for recycling aqueous cathode electrolytes of redox flow batteries.

[0013] A specific object of the present application is to solve the technical problem of providing a simple method for treating spent aqueous cathode electrolytes of redox flow batteries and to separate the electroactive compounds, purify the latter and in particular use them as raw materials for new cathode electrolytes.

[0014] In particular, the present application addresses the aforementioned technical problems by limiting the environmental impact and the consumption of natural resources, or by limiting the amount of organic and / or organometallic compounds used in the electrolyte, particularly in the cathode electrolyte. Finally, the present application addresses the technical problem of reducing the production cost of redox flow batteries. DETAILED DESCRIPTION

[0015] The present application allows solving one, and preferably all, of the technical problems presented herein.

[0016] In order to enhance the eco-compatibility and economic competitiveness of redox flow batteries using aqueous electrolytes containing organic and / or organometallic compounds, the inventors have discovered and developed a method and system for recycling electroactive compounds of the positive electrode electrolyte, in particular for their reuse in new redox flow batteries, and thereby establishing a circular economy around the redox flow batteries.

[0017] Advantageously, recycling according to the present application involves separating the electroactive compounds contained in the cathode electrolyte of the spent battery, with the intention of subsequently upcycling it directly for use in another application, or preferably by reintroducing it into a new redox flow battery in the form of fresh cathode electrolyte. Redox flow batteries can advantageously be recycled once they have lost at least 20% of their initial capacity.

[0018] Thus, by means of the present application it is possible to limit the amount of newly introduced materials for the production of redox flow batteries and / or to limit the consumption of natural or synthetic raw materials.

[0019] The present application therefore relates to a method for recovering an aqueous cathode electrolyte of a redox flow battery to be recovered, the aqueous cathode electrolyte comprising at least one electroactive compound and an aqueous solvent, the electroactive compound comprising at least one oxidized or reduced form of a redox couple, the reduced form of the redox couple being a water-soluble organometallic complex, characterized in that the method comprises:

[0020] - a precipitation step of the electroactive compound, thereby obtaining a suspension,

[0021] - a separation step of the suspension, thereby obtaining a solid residue and an effluent, and

[0022] - a drying step of the solid residue, comprising heating the solid residue to a temperature lower than or equal to 40°C, preferably lower than or equal to 35°C, more preferably lower than or equal to 30°C, further preferably lower than or equal to 25°C, thereby obtaining recovered electroactive compounds.

[0023] By electroactive compound is meant an organic or organometallic compound which belongs to a redox couple and indiscriminately represents the oxidant (oxidized form) of the redox couple, the reductant (reduced form) of the redox couple, or a mixture of an oxidant and a reductant of the redox couple.

[0024] By aqueous electrolyte is meant the aqueous solution containing the electrochemical compounds and placed in the positive and negative electrode compartments of the redox flow battery.

[0025] In the case of catholyte, it is designated as the electrolyte in the positive compartment of a redox flow battery, and in the case of negolyte, it is designated as the electrolyte in the negative compartment of a redox flow battery.

[0026] By water-soluble organometallic complex is meant an organometallic complex whose solubility in water at 25° C. is higher than or equal to 0.1 mol / L, preferably higher than or equal to 0.3 mol / L, advantageously higher than or equal to 0.5 mol / L, i.e. this means that an aqueous solution comprising at least 0.1 mol / L, preferably at least 0.3 mol / L, advantageously at least 0.5 mol / L of such a complex does not exhibit a precipitate or an insoluble fraction at 25° C.

[0027] Preferably, the metal of the organometallic complex is selected from iron or copper. Preferably, the metal of the organometallic complex in its reduced form has an oxidation number between 0 and 2, preferably 0 if the metal is copper, or 2 if the metal is iron. More preferably, the reduced form of the redox couple is an iron organometallic complex having an oxidation number of 2 in its reduced form and is preferably selected from ferrocene and ferrocyanide ions, advantageously selected from ferrocyanide ions.

[0028] Preferably, the method comprises, in sequence:

[0029] - a collection step of an aqueous cathode electrolyte of a redox flow battery comprising at least one electroactive compound,

[0030] - a precipitation step of the electroactive compound, thereby obtaining a suspension,

[0031] - a separation step of the suspension, thereby obtaining a solid residue and an effluent, and

[0032] - optionally, a water washing and water grinding step of the solid residue obtained after the separation step, followed by a second separation step, thereby obtaining a washed solid residue, and

[0033] - a drying step of the solid residue or the washed solid residue to obtain a dry solid residue.

[0034] The steps of the method may be performed using any technique known to those skilled in the art.

[0035] The collecting step is preferably performed by pumping the cathode electrolyte from the redox flow battery to be recovered to a container, preferably directly at the battery's site of use. In one embodiment, the collecting step further comprises the step of transferring the electrolyte from the container to a reactor.

[0036] The collecting step is preferably performed after the step of fully discharging the redox flow battery. In other words, the cathode electrolyte collected in the collecting step is preferably a cathode electrolyte in which the electroactive compound is in its reduced form.

[0037] The aqueous cathode electrolyte collected from the redox flow battery is spent aqueous cathode electrolyte because it has undergone at least one charge and / or discharge cycle. Preferably, the spent aqueous cathode electrolyte is collected at the end of the cycle life of the battery.

[0038] The separation step is preferably performed via filtration, for example using a decanter centrifuge.

[0039] The solid residue obtained after said separation step comprises the precipitated electroactive compound.

[0040] The washing with water and water grinding steps allow an improvement in the purity of the solid residue and, in particular if the precipitant is hardly volatile, the removal of the precipitant. However, this increases the amount of effluent to be treated.

[0041] The method of the present application preferably does not have a washing step for solid residues.

[0042] The drying step may be performed by heating the solid residue and / or placing the residue under reduced pressure.

[0043] It was surprisingly found that the drying temperature of the solid residue has a significant influence on the electrochemical properties of the recovered electroactive compound obtained after the recovery process: a drying temperature of the solid residue above 40°C leads to a significant reduction in the performance of batteries with recovered positive electrolytes containing said recovered electroactive compounds.

[0044] Preferably, the step of drying the solid residue comprises heating the solid residue to a temperature between 15°C and 35°C, more preferably between 20°C and 30°C, advantageously between 23°C and 27°C.

[0045] In order to improve the drying step, it is also advantageous to use reduced pressure.Preferably, the drying step is carried out at an absolute pressure lower than or equal to 1 bar, more preferably lower than or equal to 0.8 bar, advantageously lower than 0.5 bar.

[0046] Preferably, the precipitation step comprises adding an antisolvent for the electroactive compound and / or adding an acid or a base and / or adding a salt to the aqueous cathode electrolyte.

[0047] Preferably, the anti-solvent addition step is performed in the reactor vessel with stirring.

[0048] By antisolvent is meant an organic solvent in which the solubility of the electroactive compound is lower than its solubility in water.

[0049] Preferably, the antisolvent is selected for its ability to reduce the solubility of the electroactive compound in the initial aqueous medium and is preferably selected from solvents in which the electroactive compound is 5 times less soluble than in water, more preferably 10 times less soluble, and advantageously 100 times less soluble. In other words, the ratio of the solubility of the electroactive compound in water to the solubility of the electroactive compound in the antisolvent is preferably greater than or equal to 5, more preferably greater than or equal to 10, and advantageously greater than or equal to 100. The solubility of the electroactive compound in water or in the antisolvent is the maximum concentration, expressed in g / mol, at 25° C. at which the electroactive compound can dissolve in water or in the antisolvent, respectively, to form a homogeneous mixture, i.e., without forming a precipitate.

[0050] Preferably, the anti-solvent is selected from water-soluble aprotic and protic polar solvents, the solvent comprising an alcohol functional group, a nitrile functional group or a ketone functional group.

[0051] Preferably, the antisolvent is an organic solvent, preferably selected from the group consisting of water-soluble aprotic and protic polar solvents, more preferably selected from alcohols, preferably fatty alcohols, advantageously saturated fatty alcohols such as methanol, ethanol, or 1-propanol and isopropanol, and organic solvents containing a nitrile functional group such as acetonitrile, or a ketone functional group such as acetone, or any mixture thereof. Using a mixture of at least two antisolvents increases the amount of precipitated electroactive compound.

[0052] Preferably, the acid is a strong acid or a weak acid. The strong acid may be selected from the group consisting of sulfuric acid, hydrochloric acid, nitric acid, hydroiodic acid, hydrobromic acid, perchloric acid, permanganic acid, manganic acid, chloric acid, phosphoric acid, or any mixture thereof. The weak acid may comprise at least one carboxylic acid functional group such as formic acid, acetic acid, benzoic acid, citric acid, lactic acid, oxalic acid, or maleic acid. Preferably, the acid is a strong acid. The use of a strong acid allows for an increase in the amount of electroactive compound precipitated. More preferably, the acid is sulfuric acid or acetic acid, advantageously sulfuric acid.

[0053] Preferably, the amount of acid added to the aqueous positive electrode electrolyte corresponds to the amount of acid required to obtain a pH lower than or equal to 10, preferably lower than or equal to 8, more preferably lower than or equal to 7, and further preferably lower than or equal to 6. Even more preferably, the amount of acid added to the aqueous positive electrode electrolyte corresponds to the amount of acid required to obtain a pH lower than or equal to 10 and higher than or equal to 1, more preferably lower than or equal to 8 and higher than or equal to 2, further preferably lower than or equal to 6 and higher than or equal to 3.

[0054] Preferably, the acid is added with stirring.

[0055] Preferably, the base is an inorganic base. The base can be selected from the group consisting of alkaline hydroxides, such as NaOH or KOH, and alkaline carbonates, such as Na2CO3 or K2CO3.

[0056] Preferably, the amount of base added to the aqueous positive electrolyte corresponds to the amount of base required to obtain a pH higher than or equal to 7, preferably higher than or equal to 8, more preferably higher than or equal to 10. Further preferably, the amount of base added to the aqueous positive electrolyte corresponds to the amount of base required to obtain a pH lower than or equal to 14 and higher than or equal to 7, more preferably lower than or equal to 13 and higher than or equal to 10.

[0057] Preferably, the salt is an inorganic salt, preferably KCl or NaCl; or an organic salt, preferably sodium acetate or ammonium carbonate.

[0058] Preferably, the inorganic salt is selected from inorganic salts having a cation corresponding to the cation or one of the cations contained in the aqueous cathode electrolyte to be recovered.

[0059] Addition of an antisolvent for the electroactive compound, addition of an acid or base, and addition of a salt to the aqueous positive electrolyte can be combined in pairs or can all be added together to optimize electroactive compound precipitation, depending on its solubility.

[0060] Preferably, the precipitation step comprises adding an antisolvent for the electroactive compound, preferably an antisolvent for the reducing agent of the redox couple contained in the cathode electrolyte, to the aqueous cathode electrolyte. The antisolvent is as defined above.

[0061] Preferably, the volume of the added antisolvent accounts for 1% to 70% of the volume of the aqueous cathode electrolyte to be treated, preferably 20% to 40%, more preferably 25% to 35%. Preferably, the concentration of the electroactive compound is greater than or equal to 0.1M, preferably greater than or equal to 0.2M, more preferably 0.1M to 10M. Preferably, the concentration of the organometallic complex is greater than or equal to 0.1M, preferably greater than or equal to 0.2M, more preferably 0.1M to 10M.

[0062] Preferably, the temperature of the antisolvent added to the aqueous cathode electrolyte is 0°C to 15°C.

[0063] Preferably, the method of the present application comprises a chemical reduction step before the precipitation step, comprising contacting the cathode electrolyte with a reducing agent capable of reducing the oxidized form of the redox couple. Preferably, the chemical reduction step is between the collection step and the precipitation step.

[0064] This step is preferably performed when the cathode electrolyte is collected from a redox flow battery, which has not been fully discharged prior to the collection step.

[0065] Thus, preferably, the electroactive compound to be precipitated is the reducing agent of the redox couple contained in the positive electrode electrolyte.

[0066] By reducing agent capable of reducing the oxidant of a redox couple is meant any compound which belongs to a redox couple different from the redox couple contained in the positive electrode electrolyte and which has a standard redox potential strictly lower than that of the redox couple contained in the positive electrode electrolyte.

[0067] Preferably, the step of contacting the positive electrolyte with a reducing agent capable of reducing the oxidant of the redox couple comprises adding the reducing agent to the aqueous electrolyte. Preferably, the reducing agent is added to the aqueous electrolyte while monitoring the pH, which must preferably be maintained above or equal to 8.

[0068] Preferably, the reducing agent is selected from the group consisting of: H2O 2、 NaSO 3、 Na2S2O 4、 Na2S2O3, N2H4 (hydrazine), I2 (iodine) and organic reducing agents such as ascorbic acid, citric acid and glucose derivatives.

[0069] Preferably, during the precipitation step, the temperature of the aqueous electrolyte is between 5°C and 40°C, preferably between 10°C and 35°C, advantageously between 15°C and 30°C.

[0070] In one embodiment, the method of the present application further comprises a step of formulating the recovered solid residue, comprising dissolving the recovered solid residue in an aqueous medium to obtain a recovered cathode electrolyte.

[0071] The formulating step may also include adding other ingredients, such as additives, to the recycled cathode electrolyte.

[0072] The choice of other components depends on the expected performance of the recycled cathode electrolyte.

[0073] The method of the present application may further comprise the step of inputting the recovered cathode electrolyte obtained after the preparation step (700) into the cathode compartment of the redox flow battery.

[0074] In one variant, the method of the present application further comprises a step of treating the effluent obtained after the separation step to obtain a treated effluent. The treated effluent can be reused in the precipitation step.

[0075] The method of the present application may further comprise the step of verifying the purity of the solid residue, for example by chemical and / or electrochemical analysis.

[0076] In one embodiment of the method of the present application, the aqueous cathode electrolyte to be recovered may contain at least one additive. In this embodiment, the additive is recovered together with the electroactive compound, in which case it is contained in the solid residue; or it is contained in the effluent obtained after the completion of the method, depending on the solubility of the additive.

[0077] By additive is meant any compound that can enhance some physicochemical properties of the positive electrode electrolyte.

[0078] The present application also relates to a system for recycling an aqueous cathode electrolyte of a redox flow battery, comprising:

[0079] - a collection device for an aqueous cathode electrolyte from a redox flow battery, said aqueous cathode electrolyte comprising at least one electroactive compound and an aqueous solvent,

[0080] - a precipitation device for the electroactive compound by adding an antisolvent for the electroactive compound and / or adding an acid or base and / or adding a salt to provide a suspension comprising a solid residue and an effluent, and

[0081] - a drying device allowing drying of the solid residue at a temperature lower than or equal to 40°C, preferably lower than or equal to 35°C, more preferably lower than or equal to 30°C, further preferably lower than or equal to 25°C.

[0082] The collection device preferably includes a collection container for the collected aqueous cathode electrolyte; and a device capable of transferring the cathode electrolyte from the redox flow battery to the collection container. The collection container is, for example, in fluid communication with the cathode compartment of the redox flow battery to be recycled. The aqueous cathode electrolyte collected from the redox flow battery is spent aqueous cathode electrolyte, as it has undergone at least one charge and / or discharge cycle. Preferably, the spent aqueous cathode electrolyte is collected at the end of the battery's cycle life.

[0083] The apparatus of the present application may further comprise a first storage container comprising an antisolvent for the collected electroactive compound of the aqueous positive electrolyte and / or an acid or base solution and / or a salt solution, as defined in the description of the method of the present application, more preferably a container for storing the antisolvent for the collected electroactive compound of the aqueous positive electrolyte. The first storage container is in fluid communication with the precipitation device.

[0084] In one embodiment, the recovery system of the present application includes a discharge device capable of reducing the oxidant of the redox pair in the positive electrode electrolyte. The discharge device is preferably in fluid communication with a second storage container containing a reducing agent capable of reducing the oxidant of the redox pair, as defined above. The discharge device is preferably in fluid communication with a collection container and a precipitation device.

[0085] Preferably, the recovery system of the present application further comprises a separation device to separate the suspension from the sedimentation device into a solid residue and an effluent. For example, the separation device may be a decanter centrifuge.

[0086] The solid residue obtained in the separation device comprises the precipitated electroactive compound.

[0087] The separation device is preferably in fluid communication with the precipitation device and the drying or formulation device.

[0088] In one embodiment of the present application, the separation device is capable of partially or completely drying the optionally washed solid residue. In this embodiment, the drying device is included in the separation device.

[0089] Alternatively, the system of the present application comprises a separation device for drying the solid residue by heating and / or subjecting the optionally washed solid residue to reduced pressure.

[0090] In one embodiment, the system of the present application further comprises a device for treating the effluent from the separation device to obtain treated effluent. The treatment device is in fluid communication with the storage container and / or the precipitation device.

[0091] Preferably, the recycling system of the present application further comprises a preparation device to prepare the solid residue into the form of recycled positive electrode electrolyte.

[0092] The system of the present application may further comprise a preparation container comprising an aqueous solution, the aqueous solution optionally comprising one or more additives as defined above. The preparation container is in fluid communication with the preparation device.

[0093] Upon leaving the dispensing device, the recovered cathode electrolyte may be introduced into the cathode compartment of a new redox flow battery, preferably via fluid communication.

[0094] Preferably, the recovery system of the present application is used to implement the method of the present application.

[0095] Beneficial effects

[0096] Particularly surprising is that the electroactive compound can be recovered by precipitation. The recovery method of the present application is particularly easy to implement and therefore particularly innovative. Using this method, it is possible to obtain very good recovery rates of the electroactive compound.

[0097] Most surprisingly, the electroactive compounds recovered by the method of the present application can be reused for new cycles in new redox flow batteries, especially in terms of capacity and / or ohmic resistance (<2Ω.cm 2 ) and has very satisfactory performance in repeated working cycles of redox batteries, with the battery remaining substantially stable over dozens or hundreds of cycles. Such performance is unexpected for those skilled in the art.

[0098] Another advantage of the present method is that it uses small amounts of reagents (proportional to the volume processed). In addition, these reagents are readily available (and already used in many other applications) and are inexpensive. For example, ethanol poses no threat to the environment.

[0099] Furthermore, precipitation is rapid and the method of the present invention does not produce any contamination of the recovered electroactive compound: purification of the solid residue requires only an evaporation step. Advantageously, the yield of the method of the present invention and its reduced costs allow the industrialization of the method and system of the present invention.

[0100] Unless otherwise specified, the expressions “from X to Y” and “X to Y” represent a range including the limits X and Y. BRIEF DESCRIPTION OF THE DRAWINGS

[0101] [ Figure 1 ] Figure 1 It is a schematic block diagram of the method of the present application.

[0102] After the cycle 100 of the redox flow battery, the aqueous cathode electrolyte of the redox flow battery is collected in a collection step 200. The electroactive compounds contained in the aqueous cathode electrolyte are optionally contacted with an oxidizing agent of a reducing agent to be discharged in a chemical reduction step (250). In a precipitation step 300, the electroactive compounds contained in the aqueous cathode electrolyte are precipitated, preferably by adding an antisolvent and / or adding an acid or base and / or a salt to the aqueous electrolyte of the electroactive compounds in the aqueous cathode electrolyte. In a separation step 400, the suspension obtained after the precipitation step 300 is separated into a solid residue and an effluent. A solid residue and an effluent containing the electroactive compounds are obtained. The solid residue can be washed with water and ground in a water washing and water grinding step 500. The solid residue, whether washed or not, is then dried in a drying step 600 to reduce the amount of water and / or solvent in the solid residue. The drying temperature must not exceed 40°C. The recovered electroactive compound obtained after the drying step can be prepared in a preparation step 700 to obtain a recovered cathode electrolyte. The recovered cathode electrolyte can be used alone in a new redox flow battery or mixed with a cathode electrolyte containing one or more native electroactive compounds (i.e., these compounds have never been used in the charge and / or discharge cycle of the redox flow battery). At the same time, the effluent obtained after the separation step can be treated in a treatment step 800 to obtain a treated effluent that can be reused in the precipitation step 300 when the method of the present application is next implemented.

[0103] [ Figure 2 ] Figure 2 The recovery system 1 of the aqueous cathode electrolyte of the redox flow battery 10 of the present application is schematically illustrated.

[0104] The positive electrolyte 20 of the redox flow battery 10 is collected in a collection vessel 30 and optionally conveyed to a discharge device 35 and then to a precipitation device 40. The collected aqueous positive electrolyte electroactive compound acid or base and / or salt solution and / or antisolvent, fed from a storage vessel 45, is added to the positive electrolyte 20 in the precipitation device 40 to precipitate the electroactive compound of the positive electrolyte 20. The resulting suspension is separated, preferably by filtration, in a separation device 50. A solid residue 52 containing the electroactive compound of the positive electrolyte 20 and an effluent 54 are obtained. The effluent 54 is received in an effluent collector device 80 and can be processed and conveyed back to the storage vessel 45. Optionally, the solid residue 52 is rinsed and ground with water, and then separated a second time to remove the wash water. The residue 52 (whether rinsed or not) is dried or partially dried directly in the separation device or after being transferred to a drying device 60. The drying device allows the residue 52 to be heated at a controlled temperature and / or placed under reduced pressure, thereby reducing the amount of water and any solvent remaining in the solid residue 52. The resulting solid residue 52 is conveyed to the preparation device 70. An aqueous solution, optionally containing additives, is also conveyed from the preparation vessel 75 to the preparation device 70 to prepare a recovered cathode electrolyte 78. The recovered cathode electrolyte 78 can be fed into the anode compartment of a new redox flow battery 90.

[0105] [ Figure 3 ] Figure 3 Graph showing the usable battery cycle capacity (TRL 4) (as a percentage of the theoretical capacity of the electrolyte) for a battery comprising an electrolyte having a native electroactive compound, and two batteries A and B comprising a negative electrolyte having a native electroactive compound and a positive electrolyte having a recovered electroactive compound, the recovered electroactive compound of batteries A and B being dried under different conditions.

[0106] [ Figure 4 ] Figure 4 is a graph showing the cell resistance (TRL 4) measured on the polarization curves of a cell comprising an electrolyte having a naturally occurring electroactive compound, and two cells comprising a negative electrolyte having a naturally occurring electroactive compound and a positive electrolyte having a recovered electroactive compound, the recovered electroactive compound being dried under different conditions.

[0107] [ Figure 5 ] Figure 5 is a graph showing the usable battery cycle capacity (TRL4) (as a percentage of the theoretical capacity of the electrolyte) for batteries containing an electrolyte with a native electroactive compound and batteries containing an electrolyte with a recycled electroactive compound on the positive electrolyte side and the negative electrolyte side.

[0108] [ Figure 6 ] Figure 6 is a graph showing cell resistance (TRL 4) measured on polarization curves of a cell comprising an electrolyte with a native electroactive compound and a cell comprising an electrolyte with a recovered electroactive compound.

[0109] The present application will now be described with reference to non-limiting examples.

[0110] Example:

[0111] Example 1: Recovery of cathode electrolyte containing ferrocyanide / ferrocyanide redox couple - Temperature effect on dry The impact of dryness

[0112] A recycling method for used cathode electrolytes in batteries (>500 cycles and 4 months of cycle time).

[0113] At the end of the cycle time, the electroactive compound of the ferrocyanide / ferricyanide redox couple in the positive electrolyte is first chemically reduced, for example by adding H2O2, while monitoring the pH value (which must preferably be kept above 8) and stirring, to obtain an electrolyte containing 100% ferrocyanide. The concentration of the electroactive compound in the negative electrolyte is 0.2M, and the concentration of the electroactive compound in the positive electrolyte is 0.7M.

[0114] The precipitation is then carried out by adding 96% ethanol (in liquid form) by volume to the electrolyte under magnetic stirring; the volume of ethanol corresponds to 30% of the volume of the positive electrode electrolyte; the amount of ethanol must be controlled because if a certain volume is exceeded, the effect will be counterproductive and the ferrocyanide will redissolve in the solvent mixture.

[0115] The solution is subsequently filtered (for example on (5-10 μm) filter paper) and the solid residue obtained is dried by evaporation at two temperatures: 20° C. and 45° C. to remove remaining traces of solvent (water+ethanol).

[0116] Table 1 gives the type and amount of solvent used in each case, as well as the yield and purity obtained.

[0117] Table 1

[0118]

[0119]

[0120] When the filtered powder is heated on a hot plate at 45°C, it gradually releases its water content and forms a paste again; it then has to be re-filtered to obtain a powder that is re-dried in the open air without any special heating.

[0121] UV analysis of both samples gave satisfactory purity (greater than 90%) with no change in characteristics compared to the initial powder.

[0122] Each of the recovered electroactive compounds A and B was redissolved in an aqueous medium to obtain two recovered cathode electrolytes A and B. Each cathode electrolyte was tested in a cell together with a non-recycled anode electrolyte comprising (M3CH) as the electroactive compound:

[0123] [Chemical structure 1]

[0124]

[0125] Figure 3 and Figure 4 The performance levels obtained using a "natural" battery comprising an electrolyte with only natural electroactive compounds (compound M3CH in the negative electrolyte and ferrocyanide ions in the positive electrolyte) and two batteries A and B comprising the same negative electrolyte but containing recycled positive electrolyte A or B, respectively.

[0126] It can be observed that the capacity of the natural battery and battery A ( Figure 3 ) and internal battery resistance ( Figure 4 ) are identical (the visible difference between the two curves is due to the reproducibility error), but the performance of cell B containing the recovered positive electrolyte obtained after heating ferrocyanide at 45 °C is significantly lower than that of the native cell and cell A in terms of available capacity.

[0127] Excess anode electrolyte was added at cycle 13 of cell B to verify that the limitation was indeed due to the cathode electrolyte.

[0128] Thus, it was unexpectedly observed that the drying temperature of the recovered ferrocyanide has a detrimental effect on its redox properties. This was completely unexpected since no degradation was observed under UV characterization, which is an indicative characterization method for ferrocyanide compounds.

[0129] Example 2: Redox coupling of ferrocyanide / ferrocyanide (positive electrolyte) and anthraquinone (M3CH) (negative electrolyte) Recovery of electrolytes and battery testing

[0130] The recycling method was performed on used electrolytes in cells (>350 cycles and 6 months of cycle time). The results present the characteristics of the recycling method and the performance of cells containing the recycled electrolytes.

[0131] Regarding the negative electrode electrolyte, the electroactive molecules (M3CH) in the negative electrode electrolyte in a reduced state will automatically discharge (i.e., be oxidized) by the action of dioxygen in the air. The precipitation of the electroactive molecules is caused by the acidification of the negative electrode electrolyte solution to a pH value lower than or equal to 6. The method was tested with several types of acids (strong acids such as sulfuric acid; weak acids such as acetic acid) and the same results were obtained. The amount of acid to be added depends only on the volume of the negative electrode electrolyte to be treated and its initial pH value. It is added under stirring. Once the pH value is lower than or equal to 6, the precipitation is instantaneous. The effluent can be filtered on a large-pore filter because the filter cake obtained is very compact and forms lumps. The precipitate must be rinsed with water to remove traces of acid and then spread out to facilitate the drying step and remove residual traces of the solvent.

[0132] Table 2 gives the type and amount of solvent used for each electrolyte, as well as the yield and purity obtained. The amount of solvent required was 10% and 30% by volume for the negative and positive electrolytes, respectively. This addition tended to reduce the positive electrolyte as the concentration of the electroactive compound increased. The yield was above 65% and is expected to increase with optimized industrial processes. The yield was determined by quantitative proton NMR ( 1 The purity of the recovered electroactive compounds obtained after simple drying was estimated by H-qNMR. The purities were 92% and 93%, respectively, demonstrating the ease of removal of the solvent used for precipitation. In comparison, the purity of these same naturally occurring electroactive compounds was approximately 97% for anthraquinone and approximately 96% for the ferrocyanide salt.

[0133] Quantitative NMR: 1 H NMR spectra were recorded on a BRUKER AC 300P spectrometer (300 MHz). Maleic acid (Acros Organics) was used as an internal standard to assess the purity of the compounds.

[0134] [Table 2]

[0135]

[0136]

[0137] Figure 5 and Figure 6 The performance levels obtained using cells comprising electrolytes containing natural electroactive compounds and using recycled cells (ie, comprising negative and positive electrolytes formulated with electroactive compounds recovered under the conditions of Table 2 above) are given.

[0138] For both batteries, the available capacity ( Figure 5) are identical (the visible difference between the two curves is due to the reproducibility error), which surprisingly demonstrates that recovery via precipitation of the electroactive compound has no effect on its electrochemical activity. The capacity over-cycling trend is stable.

[0139] For both batteries, the battery internal resistance ( Figure 6 ) are also equal and remain constant during the cycle. This result surprisingly confirms that the solvent used for precipitation has no effect on the performance of the system.

[0140] Comparison between the two battery tests highlights the fact that the active materials of aqueous organic redox flow batteries can be recovered by precipitation and reused in new storage systems without degradation of performance.

Claims

1. A method for recycling an aqueous cathode electrolyte of a redox flow battery to be recycled, wherein the aqueous cathode electrolyte comprises at least one electroactive compound and an aqueous solvent, wherein the electroactive compound comprises at least one oxidized or reduced form of a redox couple, wherein the reduced form of the redox couple is a water-soluble organometallic complex, The method comprises: - a precipitation step (300) of said electroactive compound, thereby obtaining a suspension, - a separation step (400) of the suspension, thereby obtaining a solid residue (52) and an effluent (54), and - a drying step (600) of the solid residue (52), comprising heating the solid residue (52) to a temperature lower than or equal to 40°C, preferably lower than or equal to 35°C, more preferably lower than or equal to 30°C, further preferably lower than or equal to 25°C, thereby obtaining recovered electroactive compounds.

2. The method according to claim 1, wherein the precipitation step (300) comprises adding an antisolvent and / or an acid or a base and / or a salt of the electroactive compound to the aqueous cathode electrolyte.

3. The method of claim 1 or 2, wherein the precipitation step (400) comprises adding an antisolvent for the electroactive compound to the aqueous cathode electrolyte.

4. The method according to claim 3, wherein the volume of the added anti-solvent accounts for 1% to 70% of the volume of the aqueous cathode electrolyte to be treated, preferably 20% to 40%, more preferably 25% to 35%.

5. The method according to claim 4, wherein the concentration of the organometallic complex is higher than or equal to 0.1 M, preferably higher than or equal to 0.2 M.

6. The method according to any one of claims 3 to 5, wherein the temperature of the anti-solvent added to the aqueous cathode electrolyte is 0°C to 15°C.

7. The process according to any one of claims 3 to 6, wherein the antisolvent is selected from water-soluble aprotic and protic polar solvents comprising an alcohol function, a nitrile function or a ketone function.

8. The method according to any one of the preceding claims, further comprising a chemical reduction step (250) prior to the precipitation step (300), the chemical reduction step comprising contacting the cathode electrolyte with a reducing agent capable of reducing the oxidized form of a redox couple.

9. The method according to any one of the preceding claims, wherein the reduced form of the redox couple is an iron organometallic complex with an oxidation number of 2 and is preferably selected from ferrocene and ferrocyanide ions, advantageously the ferrocyanide ion.

10. The method according to any one of the preceding claims, characterized in that It also includes a step of formulating the recovered electroactive compound (700), which includes dissolving the recovered electroactive compound in an aqueous medium to obtain a recovered cathode electrolyte.

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