EP4569558A1 - Method for recycling aqueous posolyte of a redox flow battery - Google Patents

Abstract

The present invention relates to a method for recycling an aqueous posolyte of a redox flow battery to be recycled, the aqueous posolyte comprising at least one electroactive compound and an aqueous solvent, the electroactive compound comprising at least one oxidized or reduced form of a reducing/oxidising couple, the reduced form of the reducing/oxidising couple being a water-soluble organometallic complex, characterised in that the method comprises: - a step of precipitating (300) the electroactive compound, whereby a suspension is obtained, - a step of separating (400) the suspension, whereby a solid residue (52) and an effluent (54) are obtained, and - a step of drying (600) the solid residue (52), comprising heating the solid residue (52) to a temperature of less than or equal to 40°C, preferably less than or equal to 35°C, preferably less than or equal to 30°C, more preferably less than or equal to 25°C, whereby a recycled electroactive compound is obtained.

Description

Process for recycling aqueous posolyte from flow redox battery

The present invention relates to a process for recycling an aqueous posolyte from a flow redox battery.

State of the art

A Redox Flow Battery is a system using liquids (called electrolytes) to store energy. Redox flow batteries store electricity and generate it by redox reaction. They generally have two compartments separated by an ion exchange membrane, where current collectors (electrodes) are generally immersed.

One of the problems with current battery storage technologies in general is their use of ores and metals whose extraction has a massive impact on the environment. In addition, their complex design and the use of composite materials prevent simple, economical and efficient recycling of critical materials. While they claim to resolve the environmental impact of energy production by storing renewable energy (and therefore reducing CO emissions/kWh electricity produced), these technologies have a very mixed life cycle analysis. They cause a depletion of resources and significant pollution due to the waste represented by batteries at the end of their life.

For existing technologies, recycling methods have been published in the literature, such as:

Li-ion battery (EP1269554B1): method of recycling and separation of critical materials. Such a method is complex to implement and expensive;

Lead-acid battery (CA2986001A1): electrochemical lead recovery process for closed-loop reuse;

Vanadium flow redox battery: recycling methods have already been proposed by mixing the two electrolytes, and must also be operated continuously during cycling to counteract the cross-over 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);

Zn-Br flow redox battery: recovery process by neutralization of bromine (CN103236570B). Current recycling solutions do not exist for redox flow batteries which use redox couples based on organic or organometallic compounds, in particular organometallic, solubilized in an aqueous medium.

KEMIWATT uses electrolytes based on organic and organometallic compounds, dissolved in an aqueous medium, to limit the impact of this technology on the environment and the depletion of resources (use of critical metals / rare earths). However, to date, there is no recycling solution for such batteries.

Aims of the invention

The present invention aims to solve the technical problem of providing a recycling process for a flow redox battery using redox couples based on organic and/or organometallic compounds in aqueous solution.

The present invention aims in particular to solve the technical problem of providing a process for recycling aqueous posolyte for a flow redox battery.

The present invention aims in particular to solve the technical problem consisting of providing a simple process for treating aqueous posolytes for end-of-life flow redox batteries in order to isolate the electroactive compound(s), to purify them, and in particular to use it as raw material for new posolytes.

In particular, the present invention aims to resolve the technical problems stated above by limiting the impact on the environment and the depletion of natural resources or by limiting the quantity of new organic/organometallic compounds used in electrolytes, particularly in the posolyte. Finally, the present invention aims to solve the technical problem of reducing the manufacturing costs of redox flow batteries.

Detailed description of the invention

The present invention makes it possible to solve one, and preferably all, of the technical problems posed by the present invention.

To strengthen the eco-compatibility and economic competitiveness of flow redox batteries using aqueous electrolytes comprising organic and/or organometallic compounds, the inventors have discovered and developed a process and a system for recycling the electroactive compounds of the posolyte, in particular to reuse them in new redox flow batteries and thus create a circular economy around such a redox flow battery. Advantageously, recycling according to the present invention comprises the isolation of the electroactive compound(s) included in the posolyte at the end of the life of the battery, to then recover it, either directly in marketing it(them) in another application, or preferably by reintroducing it(them) into a new redox flow battery in the form of a new posolyte. A redox flow battery can advantageously be recycled once it has lost at least 20% of its initial capacity.

Thus, the present invention makes it possible to limit the quantity of newly introduced material for the production of redox flow batteries and/or to limit the consumption of raw materials, natural or synthesized.

Thus, the present invention relates to a process for recycling an aqueous posolyte from a redox flow battery to be recycled, the aqueous posolyte comprising at least one electroactive compound and an aqueous solvent, the electroactive compound comprising at least one oxidized or reduced form of an oxidizer/reducing couple, the reduced form of the oxidizing/reducing couple being a water-soluble organometallic complex, characterized in that it comprises:

- a step of precipitation of the electroactive compound, by which a suspension is obtained,

- a step of separating the suspension, whereby a solid residue and an effluent are obtained, and

- a step of drying the solid residue, comprising heating the solid residue to a temperature less than or equal to 40°C, preferably less than or equal to 35°C, preferably less than or equal to 30°C, more preferably less than or equal to at 25°C, whereby a recycled electroactive compound is obtained.

By electroactive compound is meant an organic or organometallic compound forming part of an oxidoreduction couple, and designates indifferently either the oxidant (the oxidized form) of the oxidoreduction couple, or the reducer (the reduced form) of the oxidoreduction couple, or the mixture of the oxidant and the reducer of the oxidoreduction couple.

By aqueous electrolyte is meant the aqueous solutions comprising the electroactive compound(s) and arranged in the positive and negative compartments of a flow redox battery.

We designate by posolyte, the electrolyte of the positive compartment of the flow redox battery, and by negolyte the electrolyte of the negative compartment of the flow redox battery.

By water-soluble organometallic complex is meant an organometallic complex having a solubility in water at 25°C greater than or equal to 0.1 mol/L, preferably greater than or equal to 0.3 mol/L, advantageously greater than or equal to 0.5 mol/L, that is to say 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 this complex does not present any precipitate or insoluble parts at 25°C.

Preferably, the metal of the organometallic complex is chosen from iron or copper. Preferably, the metal of the organometallic complex has, in its reduced form, a degree of oxidation of between 0 and 2, preferably equal to 0 if the metal is copper or equal to 2 if the metal is iron. Preferably, the reduced form of the oxidizer/reducer couple is an organometallic iron complex having a degree of oxidation in its reduced form equal to 2, preferably is chosen from ferrocene and the ferrocyanide ion, advantageously is the ferrocyanide ion.

Preferably, the process successively comprises:

- a step of collecting an aqueous posolyte from a flow redox battery comprising at least one electroactive compound,

- a step of precipitation of the electroactive compound and obtaining a suspension,

- a step of separating the suspension and obtaining a solid residue and an effluent,

- optionally, a step of rinsing with water and crushing with water the solid residue obtained after the separation step, followed by a second separation step, and obtaining a rinsed solid residue, and

- a step of drying the solid residue or the rinsed solid residue to obtain a dried solid residue.

The steps of the process can be implemented by any technique known to those skilled in the art.

The collection step is preferably carried out by pumping the posolyte from the redox flow battery to be recycled to a container, preferably directly at the site of use of the battery. According to one embodiment, the collection step further comprises a step of transferring the posolyte from the container to a reactor.

The collection step is preferably carried out after a complete discharge step of the redox flow battery. In other words, the posolyte collected during the collection step is preferably a posolyte whose electroactive compound is in its reduced form.

The aqueous posolyte collected from the redox flow battery is a used aqueous posolyte since it has undergone at least one charge and/or discharge cycle. Preferably, the spent aqueous posolyte is collected at the end of the battery life cycle.

The separation step is preferably carried out by filtration, for example using a centrifugal decanter. The solid residue obtained at the end of the separation step comprises the precipitated electroactive compound(s).

The water rinsing and water trituration step makes it possible to improve the purity of the solid residue, and in particular makes it possible to eliminate the precipitating agent used if it is weakly volatile. However, it increases the quantity of effluent to be treated.

The process according to the invention preferably does not include a step of rinsing the solid residue.

The drying step can be carried out by heating and/or putting the solid residue under reduced pressure.

It was surprisingly discovered that the drying temperature of the solid residue has a great influence on the electrochemical properties of the recycled electroactive compound obtained at the end of the recycling process: a drying temperature of the solid residue greater than 40°C results in a significant drop in the performance of a battery comprising a recycled posolyte comprising such a recycled electroactive compound.

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

To improve the drying step, it may also be advantageous to carry it out under reduced pressure. Preferably, the drying step is carried out at an absolute pressure less than or equal to 1 bar, preferably less than or equal to 0.8 bar, advantageously less than 0.5 bar.

Preferably, the precipitation step comprises the addition of an anti-solvent of the electroactive compound and/or the addition of an acid or a base and/or the addition of a salt in the aqueous posolyte .

Preferably, the step of adding an anti-solvent is carried out in a reactor tank, with stirring.

By anti-solvent we mean an organic solvent in which the electroactive compound is less soluble than in water.

Preferably, the anti-solvent is chosen for its ability to lower the solubility of the electroactive compound of the initial aqueous medium, preferably chosen from solvents in which the electroactive compound is 5 times less soluble than in water, more preferably 10 times less soluble, advantageously 100 times less soluble. In other words, the ratio between the solubility of the electroactive compound in water to the solubility of the electroactive compound in the anti-solvent is preferably greater than or equal to 5, more preferably greater than or equal to 10, advantageously greater than or equal to 100. The solubility of the electroactive compound in water or the anti-solvent is the maximum concentration, in g/mol at 25°C, at which the electroactive compound can dissolve in water or the anti-solvent , respectively, by forming a homogeneous mixture, that is to say without formation of a precipitate.

Preferably the anti-solvent is chosen from polar aprotic and protic solvents miscible with water comprising an alcohol function, a nitrile function or a ketone function.

Preferably, the anti-solvent is an organic solvent, preferably chosen from the group of polar aprotic and protic solvents miscible with water, more preferably chosen from alcohols, preferably aliphatic alcohols, advantageously saturated aliphatic alcohols, such as methanol, ethanol, or 1-propanol and iso-propanol, and organic solvents comprising a nitrile function, such as acetonitrile, or a ketone function, such as acetone, or any of their mixtures. Using a mixture of two or more anti-solvents increases the amount of precipitated electroactive compound.

Preferably, the acid is a strong acid or a weak acid. The strong acid may be chosen from the group consisting of sulfuric acid, hydrochloric acid, nitric acid, hydroiodic acid, hydrobromic acid, perchloric acid, permanganic acid, acid manganic acid, chloric acid, phosphoric acid or any mixture thereof. The weak acid may comprise at least one carboxylic acid function, 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 makes it possible to increase the quantity of electroactive compound precipitated. Preferably, the acid is sulfuric acid or acetic acid, advantageously sulfuric acid.

Preferably, the quantity of acid added to the aqueous posolyte corresponds to the quantity of acid necessary to obtain a pH less than or equal to 10, preferably less than or equal to 8, preferably less than or equal to 7, plus preferably less than or equal to 6. More preferably, the quantity of acid added to the aqueous posolyte corresponds to the quantity of acid necessary to obtain a pH less than or equal to 10 and greater than or equal to 1, preferably less or equal to 8 and greater than or equal to 2, more preferably less than or equal to 6 and greater than or equal to 3.

Preferably, the acid is added with stirring.

Preferably, the base is an inorganic base. The base can be chosen from the group consisting of alkaline hydroxides, such as NaOH or KOH, and alkaline carbonates, such as NasCOs or K2CO3. Preferably, the quantity of base added to the aqueous posolyte corresponds to the quantity of base necessary to obtain a pH greater than or equal to 7, preferably greater than or equal to 8, preferably greater than or equal to 10. Preferably again, the quantity of base added to the aqueous posolyte corresponds to the quantity of base necessary to obtain a pH less than or equal to 14 and greater than or equal to 7, preferably less than or equal to 13 and greater than or equal to 10.

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

Preferably, the inorganic salt is chosen from inorganic salts whose cation corresponds to the cation or to one of the cations included in the aqueous posolyte to be recycled.

The addition of an anti-solvent of the electroactive compound, the addition of an acid or a base and the addition of a salt in the aqueous posolyte can be combined in pairs or implemented all together to optimize the precipitation of the electroactive compound, depending on its solubility.

Preferably, the precipitation step comprises the addition to the aqueous posolyte of an anti-solvent of the electroactive compound, preferably of an anti-solvent of the reducer of the oxido/reducing couple included in the posolyte. The anti-solvent is as defined above.

Preferably, the volume of anti-solvent added representing between 1% and 70% of the volume of the aqueous posolyte to be treated, preferably between 20% and 40%, preferably between 25% and 35%. Preferably, the concentration of electroactive compound is greater than or equal to 0.1 M, preferably greater than or equal to 0.2 M, preferably between 0.1 M and 10 M. Preferably, the concentration of organometallic complex is greater than or equal to 0.1 M, preferably greater than or equal to 0.2 M, preferably between 0.1 M and 10 M.

Preferably, the anti-solvent added to the aqueous posolyte is at a temperature between 0°C and 15°C.

Preferably, the process according to the invention comprises a chemical reduction step before the precipitation step, comprising bringing the posolyte into contact with a reducing agent capable of reducing the oxidized form of the oxidizer/reducing couple. Preferably, the chemical reduction step is between the collection step and the precipitation step.

This step is preferably implemented when the posolyte has been collected from a flow redox battery which has not been completely discharged before the collection step.

Thus, preferably, the electroactive molecule to be precipitated is the reducer of the oxidizer/reducer couple included in the posolyte. By reducing agent capable of reducing the oxidant of the oxido/reducing couple, we mean any compound belonging to an oxido/reducing couple different from the oxido/reducing couple included in the posolyte and whose standard redox potential is strictly lower than the standard potential d redox of the oxido/reducer couple included in the posolyte.

Preferably, the step of bringing the posolyte into contact with a reducing agent capable of reducing the oxidant of the oxidant/reducing couple comprises the addition of a reducing agent to the aqueous electrolyte. Preferably, the addition of a reducing agent to the aqueous electrolyte is done by controlling the pH, which must preferably remain greater than or equal to 8.

Preferably, the reducing agent is chosen from the group consisting of H2O2, Na2SC>3, Na2S2C>4, Na2S2C>3, N ₂ H ₄ (hydrazine), l ₂ (iodine) and organic reducing agents such as ascorbic acid, citric acid, and glucose derivatives.

Preferably, during the precipitation step, the aqueous electrolyte is at a temperature between 5°C and 40°C, preferably between 10°C and 35°C, advantageously between 15°C and 30°C .

According to one embodiment, the method according to the invention further comprises a step of formulating the recycled electroactive compound comprising dissolving the recycled electroactive compound in an aqueous medium to obtain a recycled posolyte.

This formulation step may also include the addition of other constituents to the recycled posolyte, such as additives.

The choice of other constituents depends on the performance desired for the recycled posolyte.

The method according to the invention may further comprise a step of introducing the recycled posolyte obtained in the formulation step (700) into the positive compartment of a flow redox battery.

According to a variant, the method according to the invention further comprises a step of treating the effluent obtained at the end of the separation step to obtain a treated effluent. The treated effluent can be reused in the precipitation stage.

The method according to the invention may further comprise a step of verifying the purity of the solid residue, for example by chemical and/or electrochemical analysis. According to one embodiment of the process of the invention, the aqueous posolyte to be recycled may comprise at least one additive. In this embodiment, depending on the solubility of the additive, the additive is either recycled with the electroactive compound, and is therefore included in the solid residue, or included in the effluent obtained at the end of the process.

By additive we mean any compound likely to increase certain physicochemical properties of the posolyte.

The invention also relates to a system for recycling an aqueous posolyte from a flow redox battery comprising:

- a device for collecting an aqueous posolyte from a flow redox battery, the aqueous posolyte comprising at least one electroactive compound and an aqueous solvent,

- a device for precipitating the electroactive compound by adding an anti-solvent of the electroactive compound and/or adding an acid or a base and/or adding a salt, providing a suspension comprising a solid residue and a effluent, and

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

The collection device preferably comprises a collection tank for the collected aqueous posolyte and a device capable of transferring the posolyte from the redox flow battery to be recycled to the collection tank. The collection tank is for example in fluid connection with the positive tank of the redox flow battery to be recycled. The aqueous posolyte collected from the redox flow battery is a used aqueous posolyte since it has undergone at least one charge and/or discharge cycle. Preferably, the spent aqueous posolyte is collected at the end of the battery life cycle.

The device according to the invention may further comprise a first storage tank comprising an anti-solvent of the electroactive compound of the aqueous posolyte collected and/or an acid or a solution of a base and/or a solution of a salt such as defined in the description of the process according to the invention, preferably a storage tank for an anti-solvent of the electroactive compound of the aqueous posolyte collected. The first storage tank is in fluid connection with the precipitation device.

According to one embodiment, the recycling system according to the invention comprises a discharge device capable of reducing the oxidant of the oxidant/reducing couple of the posolyte. The discharge device is preferably in fluid connection with a second storage tank comprising a reducer capable of reducing the oxidant of the oxidizer/reducer couple as defined above. The discharge device is preferably in fluid connection with the collection tank and the precipitation device.

Preferably, the recycling system according to the invention further comprises a separation device making it possible to separate the suspension resulting from the precipitation device into a solid residue and an effluent. For example, the separation device may be a centrifugal decanter.

The solid residue obtained in the separation device comprises the precipitated electroactive compound(s).

The separation device is preferably in fluid connection with the precipitation device and the drying or formulation device.

According to one embodiment of the invention, the separation device is capable of partially or completely drying the solid residue, possibly rinsed. According to this embodiment, the drying device is included in the separation device.

Alternatively, the device according to the invention comprises a separate drying device for the solid residue by heating and/or by placing the solid residue under reduced pressure, optionally rinsed.

According to one embodiment, the device of the invention further comprises a device for treating the effluent from the separation device to obtain a treated effluent. The treatment device is in fluid connection with the storage tank and/or with the precipitation device.

Preferably, the recycling system according to the invention further comprises a formulation device formulating the solid residue in the form of a recycled posolyte.

The device according to the invention may further comprise a formulation reservoir comprising an aqueous solution possibly comprising one or more additives, as defined above. The formulation tank is in fluid connection with the formulation device.

At the outlet of the formulation device, the recycled posolyte can be introduced into the positive compartment of a new redox flow battery, preferably by a fluid connection.

Preferably the recycling system according to the invention is for the implementation of the method according to the invention.

Advantage of the invention

It is particularly surprising that such electroactive compounds can be recycled by precipitation. The recycling process according to the invention is particularly easy to implement and therefore particularly innovative. This process makes it possible to obtain very good recycling yields of electroactive compounds.

Quite surprisingly, the electroactive compounds recycled by the process of the invention are reusable for new cycling in a new redox flow battery with very satisfactory performance, particularly in terms of capacity and/or ohmic resistance (< 2 Q.cm ² ) and stability during the repetition of the operating cycles of the redox battery, which is substantially stable over several tens or hundreds of cycles. Such a performance was not expected by those skilled in the art.

Another advantage of the process of the invention is that a small quantity (in proportion to the volume treated) of reagents is used. Additionally, these reagents are readily available (and already used in many other applications) and inexpensive. For example, ethanol does not represent an environmental threat.

In addition, the precipitation is rapid, and the process according to the invention does not generate pollution from the recycled electroactive compounds: the purification of the solid residue only goes through an evaporation step. Advantageously, the yield of the process according to the invention and its low cost allow industrialization of the process and the system according to the invention.

Unless explicitly stated otherwise, the expressions “from X to Y” and “between X and Y” designate intervals whose limits X and Y are included.

Figures

[Fig 1] Figure 1 is a schematic block diagram of a process according to the invention.

After cycling 100 of a flow redox battery, an aqueous posolyte of the flow redox battery is collected during a collection step 200. The electroactive compound(s) included in the aqueous posolyte is (are) optionally brought into contact with a reducer in order to be discharged during a chemical reduction step (250). The electroactive compound(s) included in the aqueous posolyte is(are) precipitated during a precipitation step 300, preferably by addition of an anti-solvent and/or the addition of an acid or a base and/or the addition of a salt in the aqueous electrolyte of the electroactive compound in the aqueous posolyte. The suspension obtained at the end of the precipitation step 300 is then separated into a solid residue and an effluent during a separation step 400. A solid residue comprising the electroactive compound(s) and an effluent are obtained. The solid residue can be rinsed with water and triturated during a step 500 of rinsing with water and water crushing. The solid residue, rinsed or not, is then dried during a drying step 600 in order to reduce the quantity of water and/or solvent present in the solid residue. The drying temperature must not exceed 40°C. The recycled electroactive compound obtained at the end of the drying step can then be formulated during a formulation step 700 in order to obtain a recycled posolyte. The recycled posolyte can be used in a new flow redox battery, alone or mixed with a posolyte comprising native electroactive compound(s), i.e. never having been used in a charge and/or discharge cycle of a flow redox battery. In parallel, the effluent obtained at the end of the separation step can be treated during a treatment step 800, in order to obtain a treated effluent capable of being reused in the precipitation step 300 during a future implementation of the method according to the invention.

[Fig 2] Figure 2 is a schematic representation of a recycling system 1 of an aqueous posolyte of a redox flow battery 10 according to the invention.

A posolyte 20 from a flow redox battery 10 is collected in a collection device 30, possibly conveyed to a discharge device 35, then conveyed to a precipitation device 40. An anti-solvent and/or an acid or a base and/or a solution of a salt of the electroactive compound of the aqueous posolyte collected from a storage tank 45 is added to the posolyte 20 in the precipitation device 40 in order to precipitate the electroactive compound(s) ) of the posolyte 20. The suspension obtained is separated, preferably by filtration, in a separation device 50. A solid residue 52 comprising the electroactive compound(s) of the posolyte 20 and an effluent 54 are obtained. The effluent 54 is collected in an effluent collection device 80, and can be treated then redirected to the storage tank 45. Optionally, the solid residue 52 is rinsed with water and triturated with water, then separated a second time to remove the wash water. The residue 52 (rinsed or not) is then dried or partially dried, either directly in the separation device, or after having been transferred to a drying device 60. The drying device allows heating under controlled temperature and/or putting under reduced pressure the residue 52 and thus reduce the quantity of water and solvents present in the solid residue 52. The solid residue 52 is then conveyed to a formulation device 70. An aqueous solution optionally comprising additives is also introduced into the formulation device 70 from a formulation tank 75 in order to prepare a recycled posolyte 78. The recycled posolyte 78 can then be introduced into the positive compartment of a new redox flow battery 90. [Fig 3] Figure 3 is a graph representing accessible capacity in battery cycling (TRL 4) (as a percentage of the theoretical capacity of the electrolytes) of a battery comprising electrolytes with native electroactive compound and two batteries A and B comprising a negolyte with native electroactive compound and a posolyte with recycled electroactive compound, the recycled electroactive compound having been dried under different conditions for batteries A and B.

[Fig 4] Figure 4 is a graph representing the battery resistance (TRL 4) measured by polarization curve of a battery comprising electrolytes with native electroactive compound and two batteries A and B comprising a negolyte with native electroactive compound and a posolyte with recycled electroactive compound, the recycled electroactive compound having been dried under different conditions.

[Fig 5] Figure 5 is a graph representing the capacity accessible in battery cycling (TRL 4) (as a percentage of the theoretical capacity of the electrolytes) of a battery comprising electrolytes with native electroactive compound and a battery comprising electrolytes with compound electroactive recycled on the posolyte side and on the negolyte side.

[Fig 6] Figure 6 is a graph representing the battery resistance (TRL 4) measured by polarization curve of a battery comprising electrolytes with native electroactive compound and a battery comprising ones with recycled electroactive compound.

The present invention will now be described using non-limiting examples.

Examples:

Example 1: Recycling of a posolyte comprising the ferrocyanide/ferricyanide redox couple - Effect of drying temperature

The recycling process was implemented on a posolyte used in a battery (> 500 cycles and 4 months of cycling).

At the end of cycling, the electroactive compound(s) of the ferrocyanide/ferricyanide redox couple of the posolyte is(are) first chemically reduced, for example by addition of H ₂ C>2 , by controlling the pH (which should preferably remain greater than 8) and with stirring, in order to obtain an electrolyte comprising 100% ferrocyanide. The concentration of electroactive compounds in the negolyte is 0.2 M and the concentration of electroactive compounds in the posolyte is 0.7 M.

The precipitation is then carried out by adding a volume of 96% ethanol (in liquid form) to the electrolyte with magnetic stirring, the volume of ethanol corresponding to 30% of the volume of the posolyte; the quantity of ethanol must be controlled, because if it exceeds a certain volume, the effect is counterproductive and the ferrocyanide redissolves in the solvent mixture.

The solution is then filtered (for example on filter paper (5-10 μm)), then the solid residue obtained is dried by evaporation of the residual traces of solvent (water + ethanol) at two temperatures: 20 ° C and 45 ° C.

The nature and quantity of solvent used in each case, as well as the yields and purities obtained, are presented in Table 1.

[Table 1]

When the filtered powder is heated on a hot plate at 45°C, it gradually releases moisture, forming a paste again; it must then be re-filtered to obtain a powder, which is then re-dried in the open air without any special heating.

UV analysis of the 2 samples indicates satisfactory purities (greater than 90%), and an unchanged signature compared to the initial powder.

Each recycled electroactive compound A and B was then resolubilized in an aqueous medium to obtain two recycled posolytes A and B. Each posolyte was tested in a battery, in association with a non-recycled negolyte comprising (M3CH) as electroactive compound:

[Chem 1]

Figures 3 and 4 present the performances obtained with a “native” battery comprising electrolytes with only native electroactive compounds (M3CH compound with the negolyte and ferrocyanide ion with the posolyte), and with the two batteries A and B comprising the same negolyte but the recycled posolyte A or B, respectively.

We observe that the capacity (Figure 3) and the battery resistance (Figure 4) are identical for the native battery and battery A (the visible difference between the two curves is included in the reproducibility error), but that the performances of battery B, which includes the recycled posolyte obtained after heating the ferrocyanide to 45 °C, are significantly lower than those of the native batteries and A, in terms of accessible capacity.

An excess of negolyte was added at the time of cycle 13 of battery B to ensure that the limitation was indeed due to the posolyte.

We therefore observe that, unexpectedly, the drying temperature of recycled ferrocyanide has a deleterious effect on its redox properties. This was completely unpredictable as no degradation is visible by UV characterization, which is the characterization method indicated for the ferrocyanide compound.

Example 2: Recycling and battery testing of the ferrocyanide/ferricyanide redox couple (posolyte) and anthraquinone (M3CH) (negolyte)

The recycling process was implemented on electrolytes used in batteries (>350 cycles and 6 months of cycling). The results present both the characteristics of the recycling process and the performance of batteries including the recycled electrolytes. Concerning the negolyte, the electroactive molecule (M3CH) of the negolyte in reduced form is discharged (ie is oxidized) automatically in the air by the action of dioxygen in the air. Then, the precipitation of the electroactive molecule is carried out by acidification of the negolyte solution, up to a pH value less than or equal to 6. The process was tested with several types of acid (strong acid eg sulfuric acid, weak acid eg acetic acid), leading to equivalent results. The quantity of acid to add depends solely on the volume of negolyte to be reprocessed and its initial pH. It is added with stirring. As soon as the pH value is less than or equal to 6, precipitation is instantaneous. Filtration of the effluent can be carried out on a filter with a large pore size, because the cake obtained is very compact and forms a block. The precipitate must then be rinsed with water to remove traces of acid, then spread to facilitate the drying stage, and eliminate residual traces of solvent.

The nature and quantity of solvent used for each electrolyte, as well as the yields and purities obtained, are presented in Table 2. The necessary quantities of solvent are 10 and 30% by volume respectively for the negolyte and the posolyte. This addition tends to decrease for the posolyte when the concentration of electroactive compound increases. Yields are greater than 65%, with an expected improvement through the implementation of an optimized industrial process. The purity of the recycled electroactive compound obtained after simple drying is estimated by quantitative proton NMR (qNMR ¹ H) with the presence of an internal standard. This purity is 92 and 93% respectively, which proves the ease of elimination of the solvent used for precipitation. By comparison, the purity of these same native electroactive compounds are approximately 97% for anthraquinone and 96% for the ferrocyanide salt.

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

[Table 2]

Figures 5 and 6 present the performances obtained with a battery comprising electrolytes with native electroactive compound(s) and with a recycled battery, i.e. comprising a negolyte and a posolyte formulated from the compound(s) recycled electroactive(s) according to the conditions in Table 2 above.

The accessible capacity (Figure 5) is identical for the two batteries (the visible difference between the two curves is included in the reproducibility error), which surprisingly proves that recycling by precipitation of electroactive compounds has no impact on their electrochemical activity. The evolution of this capacity during cycling is stable.

The resistance measured in the battery (Figure 6) is also equivalent for the two batteries, and remains constant depending on the cycling. This result surprisingly confirms that the solvents used for precipitation have no impact on the performance of the system. The comparison of the two battery tests highlights that the active materials of an aqueous organic flow redox battery can be recycled by precipitation and reused in a new storage system without degradation of performance.

Claims

1. Process for recycling an aqueous posolyte from a redox flow battery to be recycled, the aqueous posolyte comprising at least one electroactive compound and an aqueous solvent, the electroactive compound comprising at least one oxidized or reduced form of an oxidizer/reducing couple , the reduced form of the oxidizer/reducer couple being a water-soluble organometallic complex, characterized in that it comprises: - a precipitation step (300) of the electroactive compound, whereby a suspension is obtained, - a step of separation (400) of the suspension, whereby a solid residue (52) and an effluent (54) are obtained, and - a step of drying (600) of the solid residue (52), comprising heating the solid residue (52) to a temperature less than or equal to 40°C, preferably less than or equal to 35°C, preferably less than or equal to 30°C, more preferably less than or equal to 25°C, whereby a recycled electroactive compound is obtained. 2. Method according to claim 1, in which the precipitation step (300) comprises the addition of an anti-solvent of the electroactive compound and/or the addition of an acid or a base and/or the adding a salt to the aqueous posolyte. 3. Method according to claim 1 or 2, wherein the precipitation step (400) comprises the addition to the aqueous posolyte of an anti-solvent of the electroactive compound. 4. Method according to claim 3, in which the volume of added anti-solvent representing between 1% and 70% of the volume of the aqueous posolyte to be treated, preferably between 20% and 40%, preferably between 25% and 35%. 5. Method according to claim 4, in which the concentration of organometallic complex is greater than or equal to 0.1 M, preferably greater than or equal to 0.2 M. 6. Method according to any one of claims 3 to 5, in which the anti-solvent added to the aqueous posolyte is at a temperature between 0 ° C and 7. Method according to any one of claims 3 to 6, in which the anti-solvent is chosen from polar aprotic and protic solvents miscible with water comprising an alcohol function, a nitrile function or a ketone function. 8. Method according to any one of the preceding claims, further comprising a chemical reduction step (250) before the precipitation step (300), comprising bringing the posolyte into contact with a reducing agent capable of reducing the oxidized form of the oxidizing/reducing couple. 9. Method according to any one of the preceding claims, in which the reduced form of the oxidizing/reducing couple is an organometallic iron complex having a degree of oxidation equal to 2, preferably is chosen from ferrocene and ferrocyanide ion , advantageously is the ferrocyanide ion. 10. Method according to any one of the preceding claims, characterized in that it further comprises a step of formulation (700) of the recycled electroactive compound comprising the dissolution of the recycled electroactive compound in an aqueous medium to obtain a recycled posolyte.