The present invention relates to a method and an apparatus for the electro- chemical extraction of lithium from aqueous lithium sources and the recovery of the extracted lithium to high purity lithium solutions.

The advancement of electro-mobility and the exploitation of renewable energy sources have raised a high demand for lithium as a basic material for high-performance batteries. Currently, this demand is met by extracting lithium from natural resources, such as solid minerals or salt lake brines, or by recycling batteries and glasses. These methods, however, are time- consuming, complex and in many cases environmentally harmful. Moreover, the resources mentioned will not satisfy the increasing demand for lithium in the long run. Therefore, new sources and routes for the recovery of lithium must be accessed. Such sources may be, for example, seawater or saline waste waters obtained as by-products or waste-products in industrial processes.

Seawater is available in virtually unlimited supply and is thus a promising source of lithium despite its low concentration in lithium of approximately 0.17 ppm. Saline waste waters, on the other hand, offer the advantage of increased lithium concentration due to their prior processing. Brines of desalination or zero liquid discharge plants and also produced waters of gas or oil extraction wells are particularly attractive as sources of lithium since they are readily available and high in lithium compared to seawater. The recovery of pure lithium from aqueous lithium sources, however, is challenging since these sources may contain different ionic species and in some cases also biological microorganisms, small plants or algae.

In electrochemical lithium recovery processes, certain ions, such as chloride Cl , may trigger parasitic side-reactions. Parasitic side-reactions may compete with the lithium uptake or release and/or may produce undesired chemical species which may corrode or block the electrodes. Furthermore, biological organisms may cause the build-up of biofouling at the electrodes which may also block the electrodes. During the lithium recovery undesired chemical species may pollute high purity recovery solutions and may thus complicate the production of high purity lithium. Hence such lithium recovery processes are currently neither long term stable nor efficient.

The present invention is therefore based on the object to overcome these disadvantages by providing a method and an apparatus for an efficient and long term stable lithium extraction and recovery from aqueous lithium sources containing dissolved lithium.

This object is achieved according to the invention by a method for the extraction of lithium as claimed in claim 1 and a lithium-recovery unit as claimed in claim 28. Advantageous developments and embodiments are described in the dependent claims.

A method for extracting lithium from an aqueous lithium source comprises a lithium extraction step and a lithium recovery step. In the lithium extraction step, dissolved lithium ions Li ⁺ are electrochemically extracted from an aqueous lithium source in an electrochemical cell which comprises in a housing: a working electrode, a counter electrode and the aqueous lithium source as an electrolyte separating the working electrode and the counter electrode from each other. The aqueous lithium source comprises the dissolved lithium ions Li ⁺ and dissolved chloride ions Cl . The working elec trode of the electrochemical cell comprises a lithium storage material, a binder and an electrically conductive additive. In the extraction step, the dissolved lithium ions Li ⁺ are electrochemically intercalated, i.e. extracted by intercalation, from the aqueous lithium source into the lithium storage material of the working electrode at a working potential of the working electrode which is below a threshold potential for the electrochemical formation of chlorine at the counter electrode. In the recovery step, the extracted lithium ions Li ⁺ are electrochemically recovered from the lithium storage material of the working electrode to an aqueous recovery solution in the electrochemical cell which comprises in the housing: the working elec trode, the counter electrode and the aqueous recovery solution as an electrolyte separating the working electrode and the counter electrode from each other. The extracted lithium ions Li ⁺ are electrochemically deinterca- lated, i.e. extracted by deintercalation, from the lithium storage material of the working electrode into the aqueous recovery solution. The described method thus allows an efficient and selective extraction and recovery of Li ⁺ ions from aqueous lithium sources and may be used to produce high purity lithium solutions.

The method may be conducted in concentrated aqueous lithium sources with a high salinity of more than BO g/l, more than 100 g/l or even more than 200 g/l, in particular in aqueous lithium sources with a concentration of dissolved chloride ions Cl of 20 g/l to 120 g/l. In this application, salinity refers to the total amount of salts dissolved as ions in the aqueous lithium source. A saline solution is meant to be regarded as an aqueous lithium source if the saline solution has a concentration of dissolved lithium ions Li ⁺ of at least 0.3 mg/I, preferably at least 10 mg/I, most preferably at least 30 mg/I.

Aqueous lithium sources may, for example, be provided from sea water or saline industrial waste waters, in particular brines of desalination or zero liquid discharge plants as well as produced waters of gas or oil production plants. In contrast to leachates of ores or recycled electronics, these aqueous lithium sources are readily accessible. The method may therefore be imple mented in the wastewater processing of a desalination plant, a zero liquid discharge plant, a gas or oil production plant. The salinity and/or concentra tion of dissolved chloride ions Cl of the aqueous lithium source may be adjusted by diluting the aqueous lithium source with water prior to the lithium extraction. Waste products of zero liquid discharge plants which contain precipitates of water soluble salts, may, for example, be diluted with water to provide an aqueous lithium source.

In this application intercalation and deintercalation refer to the reversible insertion and removal of lithium ions Li ⁺ into and from the crystal structure of the lithium storage material. The lithium storage material may be chosen from the group of lithium iron phosphates, e.g. LiFeP0 ₄ , lithium cobalt oxides, e.g. UC0O2, lithium nickel oxides, e.g. LiNi0 ₂ , lithium nickel cobalt oxides, e.g. LiNii- _x Co _x 0 ₂ , lithium manganese oxides, e.g. LiMn ₂ 0 ₄ , lithium nickel manga nese cobalt oxides, e.g. LiNii/ ₃ Coi/3Mni/ ₃ 0 ₂ , and/or lithium nickel cobalt aluminium oxides, e.g. LiNio.85Coo.1Alo.05O2.

Binders are materials which enhance the cohesion between the lithium storage material and the electrically conductive additive in the working electrodes. Binders are typically made of materials which are chemically and electrochemically inert in the aqueous lithium source. The binder may for example comprise or consist of fibrils. It may preferably be a fibrillated binder in which a material has been formed into fibrils during the fabrication of the working electrode. Suitable materials for binders may for example be chosen from the group of fluoropolymers, e.g. polyvinylidene fluoride (PVDF) and/or polytetrafluoroethylene (PTFE), or from the group of rubbers, e.g. NBR and/or SBR. In this application fibril refers to an elongated filament with an aspect ratio of more than 10, preferably more than 50. The length of filaments may be in the range between 0.2 pm and 500 pm, while their diameter may be in a range between 2 nm and 1 pm.

In this application an electrically conductive additive is meant to be a material which exhibits an electrical conductivity of more than 10 ⁶ S/m at a tempera- ture of 20 °C. Typically the electrically conductive additive and/or the binder may be finely dispersed in the working electrode. The electrically conductive additive may be an electrochemically inactive carbon material, for example selected from carbon black, porous carbons, carbon nanotubes, graphene, graphite, carbon fibres or mixtures thereof. Carbon nanotubes may be single- walled or multi-walled carbon nanotubes (MWCNTs), e.g. with a length in the range from 0.1 pm to 1000 pm, and a diameter in the range from 0.1 nm to 100 nm.

Although working electrodes with a lithium storage material may enable a highly selective extraction of lithium from aqueous lithium sources, these electrodes often fall short of expectations in aqueous lithium sources contain ing dissolved chloride ions Cl . It has been found that the equilibrium potential for the electrochemical formation of chlorine may unexpectedly shift below the equilibrium potential for the electrochemical formation of oxygen in the extraction and recovery of lithium from aqueous lithium sources and recovery solutions containing dissolved chloride ions Cl . As a result, dissolved chloride ions Cl may react to chlorine and subsequently to hypochlorite CIO at the electrodes. Hypochlorite is a strong oxidizing agent which may damage the working electrode.

In the described method, the extracted lithium ions Li ⁺ are electrochemically intercalated at a working potential of the working electrode which is below the threshold potential for the electrochemical formation of chlorine at the working electrode. Thus, the formation of chlorine and subsequently hy pochlorite is reduced at the working electrode and the performance and life time of the working electrode are enhanced.

The intercalation of lithium Li ⁺ may be conducted at a working potential of the counter electrode which is equal or higher than the threshold potential of the working electrode. Hence the dissolved chloride ions Cl react to chlorine at the counter electrode and their concentration is reduced at the working electrode. The intercalation may preferably be conducted at a working potential of the counter electrode at which dissolved chloride ions Cl of the aqueous lithium source react to chlorine at the counter electrode and subsequently to hypochlorite CIO at the counter electrode which may act as a disinfectant and/or anti-fouling agent at the counter electrode. Hence biofouling may be prevented at the counter electrode. Therefore both, the working electrode and the counter electrode, may be protected from damage and their efficiency and life-time may be improved.

In some implementations of the method, the electrochemical cell may comprise a control unit which is configured to keep the working potential of the working electrode below the threshold potential for the electrochemical formation of chlorine at the working electrode. The control unit may further be implemented to keep the working potential of the counter electrode during the lithium extraction from the aqueous lithium source above this threshold potential. Therefore any tedious monitoring of the chloride concentration and/or removal of chloride from the aqueous lithium source may be omitted.

Besides the formation of chlorine, the formation of hydrogen gas may have a negative impact on the intercalation and deintercalation of lithium into and from the lithium storage material. In order to avoid the formation of hydrogen gas, the lithium extraction and recovery may be conducted within the electrochemical stability window of the aqueous lithium source and the aqueous recovery solution, respectively, meaning that the working potential of the electrode may be higher than the electrochemical potential for the formation of hydrogen at the electrode. At a neutral pH and a temperature of 25 °C the electrochemical stability window ranges from 2.5 V to 3.8 V for pure water on a Li/Li ⁺ potential scale and may range from 2 V to 5 V in aqueous lithium sources or aqueous recovery solutions on a Li/Li ⁺ potential scale. In an electrochemical lithium extraction this relates to a range from -0.8 V to 0.5 V on a Ag/AgCI potential scale.

Subsequent to the lithium extraction step, in which the dissolved lithium ions Li ⁺ are extracted to the lithium storage material of the working electrode, the aqueous lithium source may be replaced by an aqueous recovery solution which is used as an electrolyte to recover the lithium ions Li ⁺ from the storage material of the working electrode. In this recovery step, the lithium ions Li ⁺ may be electrochemically deintercalated from the lithium storage material into an aqueous recovery solution in which the concentration of dissolved chloride ions Cl is below a threshold concentration for the formation of chlorine at the working electrode.

In an aqueous lithium recovery solution the threshold concentration of the chlorine formation may typically be BO mM if no other sources of chloride or other undesired ionic species are present in the electrochemical cell. For the lithium recovery it may therefore be sufficient to supply the electrochemical cell with an aqueous lithium recovery solution, e.g. a lithium chloride LiCI or lithium hydroxide LiOH solution, which exhibits a concentration of dissolved chloride ions Cl of 30 mM or less. Since the formation of chlorine is sup pressed with such aqueous recovery solutions, the electrochemical extraction of the lithium ions Li ⁺ from the lithium storage material may be conducted within the entire electrochemical stability window of the aqueous recovery solution up to the onset of the electrochemical formation of oxygen.

In the described method, the working electrode may be a porous electrode and/or a film electrode. Such working electrodes require only small amounts of electrode materials but offer large active surface areas for the extraction and release of lithium. The working electrodes may in particular exhibit porosities of at least 50 %, more preferably at least 55 % and/or film thickness in the range of 500 pm to 50 pm, more preferably in the range of 250 pm to 50 pm. High porosities and/or thin film thicknesses may be achieved with fibrillated binders, which may for example comprise or consisting of PTFE fibrils, and carbon nanotubes as electrically conductive additive in the working electrode. The fibrils and the carbon nanotubes act as a highly efficient binder. They enhance the chemical and mechanical stability of the working electrode. Hence even porous electrodes and/or thin film electrodes may be resistant to harsh chemical environments, e.g. concentrated aqueous lithium sources, and to wear from the flow of the aqueous lithium source, rinsing liquids or recovery solutions past the surfaces of the electrodes, as for example in a continuous feed or cyclic operation mode of the electrochemical cell. These aspects are of particular importance if the lithium extraction is to be up-scaled to an industrial scale demanding high throughputs of the aqueous lithium source. The working electrode may be a dry-film electrode. In this application, an electrode is meant to be regarded a dry-film electrode if the electrode is fabricated in a solvent-free dry-film process in which a film electrode is formed from particulate dry electrode materials without the use of solvents.

In such a process, dry and solvent-free electrode materials, i.e. the lithium storage material, the binder and the electrically conductive additive, may be mixed and homogenised in a mortar to form a dry and solvent-free powder mixture. The mixture may be heated and rolled out to a dry and solvent-free film forming the dry-film electrode. In the processing of the mixture the binder, e.g. PTFE, becomes fibrillated, meaning that fibrils are formed in the fabrication of the electrode. These fibrils are an efficient binder in the dry-film electrode. Since the lithium extraction and release may be conducted with dry-film electrodes that are non-toxic and fabricated without any aggressive or environmentally harmful solvents, their environmental performance may be improved appreciably. This aspect becomes even more important when multiple working electrodes are used to up-scale the method to an industrial scale, e.g. for the post-processing of waste water from desalination plants, zero liquid discharge plants, gas production plants or oil production plants.

In one implementation of the method, the extraction and recovery of lithium may be performed with a working electrode that comprises spinel-type lithium manganese (III, IV) oxide LiMn ₂ 0 ₄ as lithium storage material. Spinel- type lithium manganese (III, IV) oxide LiMn ₂ 0 ₄ is a very promising lithium storage material since it allows a highly selective and reversible electrochemi cal extraction of lithium ions within the electrochemical stability window of aqueous lithium sources and exhibits a good environmental compatibility. For working electrodes comprising lithium (III, IV) manganese oxide LiMn ₂ 0 ₄ as a lithium storage material, a suitable threshold potential may be 0.95 V on a Ag/AgCI potential scale in the presence of the aqueous lithium source. The intercalation of lithium Li ⁺ may be conducted at a working potential of the working electrode which is below this threshold potential, while the working potential of the counter electrode is maintained above this threshold poten tial. Thus both, the working electrode and the counter electrode, may be protected from the damaging effects described. If the electrochemical formation of hydrogen is to be avoided, the electrochemical extraction of lithium may be conducted within the electrochemical stability window of the aqueous lithium source, i.e. at a working potential of the working electrode in a range between 0.3 V up to a working potential below the threshold poten tial of 0.95 V on a Ag/AgCI potential scale in the presence of the aqueous lithium source.

In the lithium recovery step, lithium ions Li ⁺ may be electrochemically deintercalated from the lithium storage material into an aqueous recovery solution in which the concentration of dissolved chloride ions Cl is below a threshold concentration for the formation of chlorine at the working elec trode. Since the formation of chlorine is suppressed in such aqueous recovery solutions, the electrochemical extraction of the lithium ions Li ⁺ from the lithium storage material may be conducted at higher working potentials, i.e. in a range between 0.3 V and 1.2 V on a Ag/AgCI potential scale.

A working electrode comprising lithium (III, IV) manganese oxide LiMn ₂ 0 ₄ as a lithium storage material may further comprise polytetrafluoroethylene (PTFE) fibrils as binder and multi-walled carbon nanotubes (MWCNTs) as electrically conductive additive. The working electrode may for example comprise 85 weight-% to 95 weight-% LiMn ₂ 0 ₄ , 2 weight-% to 5 weight-% PTFE fibrils and 3 weight-% to 10 weight-% MWCNTs. These working electrodes may be operated with a large lithium capacity within the electrochemical stability window of aqueous lithium sources and aqueous recovery solutions, respec tively, and may be configured as porous electrodes and/or dry-film electrodes which exhibit a prolonged lifetime even in concentrated aqueous lithium sources.

The working electrode may further comprise an electrically conducting current collector electrode, which may reinforce the working electrode. The current collector electrode is preferably chemically and electrochemically inert to the aqueous lithium source and typically exhibits a higher electrical conductivity than the working electrode. The working electrode may, for example, be laminated onto a planar current collector electrode, facing the counter electrode.

In the described method, the counter electrode may be an activated or inert carbon electrode, a graphite electrode, an Ag/AgCI electrode, an electrode based on the Prussian blue structure or a noble metal electrode. The counter electrode may preferably be a non-porous electrode, i.e. an electrode which is manufactured of a non-porous material, in order to avoid the co-adsorption of lithium or other ionic species at the counter electrode. Most preferably, the counter electrode may be an inert redox electrode, which merely acts as an electron donor and acceptor in an electrochemical water splitting reaction, such as electrodes comprising glassy carbon, platinum or titanium. These electrodes are chemically inert to aqueous lithium sources containing hypochlorite and do not absorb or discharge anions or cations of the aqueous lithium source. Hence they are very durable and show a long-term stable performance without specific requirements to the ionic composition of the electrolytes.

As described previously, the aqueous lithium source may be replaced by an aqueous recovery solution to recover the lithium ions Li ⁺ from the lithium storage material of the working electrode. If chloride and/or hypochlorites of the aqueous lithium source remain in the electrochemical cell, they may represent undesired sources of chloride during the lithium recovery step. The removal of chloride and hypochlorites, such as potassium hypochlorite and/or sodium hypochlorite, from the working electrode may be particularly cumber some if the working electrode is a fragile thin film electrode and/or a porous electrode. Critical concentrations of potassium hypochlorite and/or sodium hypochlorite may be reached at approximately 20 g/l of dissolved chloride Cl and 10 mg/I of dissolved potassium K ⁺ and/or dissolved sodium Na ⁺ in the aqueous lithium source. In some implementations of the method, the electrochemical cell may comprise a cation exchange membrane which is configured to inhibit the diffusion of dissolved chloride ions Cl from the aqueous lithium source to the working electrode.

Preferably, the cation exchange membrane may be placed in front of a surface of the working electrode which is in contact with the aqueous lithium source or may cover a surface of the working electrode which is in contact with the aqueous lithium source. The cation exchange membrane is permeable to lithium cations and water but protects the working electrode from adsorbing high concentrations of chloride and/or hypochlorite from the aqueous lithium source which may suppress the lithium deintercalation in the recovery step. The cation exchange membrane may also protect the working electrode from adsorbing other anions, which may be released to the recovery solution in the recovery step. The cation exchange membrane may thus notably improve the production of high purity lithium solutions in the recovery step.

The cation exchange membrane may be removed from the electrochemical cell before the lithium recovery step to allow an undisturbed diffusion of the extracted lithium ions Li ⁺ from the working electrode into the recovery solution. However, in some cases, if for example the chloride concentration of the recovery solution may not be reliably adjustable below the threshold concentration of the chlorine formation at the working electrode, the cation exchange membrane may remain in the electrochemical cell to keep the chloride concentration at the surface of the working electrode below the threshold concentration for the formation of chlorine.

In order to minimise the effects of the cation exchange membrane on the diffusion of the dissolved lithium ions Li ⁺ , the cation exchange membrane may exhibit a thickness of less than 125 pm. Preferably, the cation exchange membrane comprises a sulfonated tetra-fluoroethylene based fluoropolymer- copolymer. These cation exchange membranes are chemically inert and highly permeable to lithium cations and water.

A lithium-recovery unit comprises an electrochemical cell which comprises in a housing: a working electrode, a counter electrode and an electrolyte separating the working electrode and the counter electrode from each other. The working electrode comprises a lithium storage material, a binder and an electrically conductive additive. The electrochemical cell is configured to electrochemically extract dissolved lithium ions Li ⁺ from an aqueous lithium source which comprises dissolved lithium ions Li ⁺ and dissolved chloride ions Cl using the aqueous lithium source as an electrolyte in the electrochemical cell wherein the dissolved lithium ions Li ⁺ are intercalated into the lithium storage material of the working electrode at a working potential of the working electrode which is below a threshold potential for the electrochemi cal formation of chlorine at the working electrode. The electrochemical cell is further configured to electrochemically deintercalate the extracted lithium ions Li ⁺ from the lithium storage material of the working electrode into an aqueous recovery solution using the aqueous recovery solution as an electro lyte in the electrochemical cell. The electrochemical cell may be configured to electrochemically extract dissolved lithium ions Li ⁺ from an aqueous lithium source at a working potential of the counter electrode which is equal or higher than the threshold potential of the working electrode, i.e. a working potential at which dissolved chloride ions Cl react to chlorine at the counter electrode and subsequently to hypochlorite CIO at the counter electrode. The hypochlorite CIO may acts as a disinfectant and/or anti-fouling agent at the counter electrode. It may thus protect the counter electrode from biofouling while the working electrode is being protected from damaging hypochlorite.

The lithium-recovery unit described is set up to carry out the described method, that is the described method can be carried out in particular with the lithium-recovery unit described.

Exemplary embodiments of the invention are illustrated in the drawing and will be explained below with reference to figures 1 to 3. The singular forms "a", "an" and "the" include plural referents unless the context indicates otherwise. The terms "a" (or "an"), as well as the terms "one or more," and "at least one" can be used interchangeably.

In the figures:

Fig. 1 illustrates in a schematic diagram a lithium-recovery unit for the extraction and recovery of lithium from aqueous lithium sources,

Fig. 2 displays a cyclovoltammogram of an electrochemical cell of a lithium- recovery unit in which the working potential of the working electrode is increased beyond the threshold potential for the electrochemical formation of chlorine, and

Fig. 3 displays discharging/charging voltage curves of an electrochemical cell of a lithium-recovery unit in which the working potential of the work ing electrode is kept below the threshold potential for the electro chemical formation of chlorine. Figure 1 illustrates in a schematic diagram a lithium-recovery unit for the extraction and recovery of lithium from aqueous lithium sources. The lithium- recovery unit comprises an electrochemical cell which comprises in a hous ing 1: a working electrode 3, a counter electrode 4 and an electrolyte 5, 6 which separates the working electrode 3 and the counter electrode 4 from each other. The working electrode 3 comprises a lithium storage material, a binder and an electrically conductive agent. The electrochemical cell is configured to electrochemically intercalate dissolved lithium ions Li ⁺ from an aqueous lithium source 5 which comprises dissolved lithium ions Li ⁺ and is used as an electrolyte in the electrochemical cell into the lithium storage material of the working electrode 3 at a working potential of the working electrode 3 which is below a threshold potential for the electrochemical formation of chlorine at the working electrode 3. The electrochemical cell is further configured to electrochemically deintercalate the extracted lithium ions Li ⁺ from the lithium storage material of the working electrode 3 into an aqueous recovery solution 6 using the aqueous recovery solution 6 as an electrolyte in the electrochemical cell. Moreover, the electrochemical cell may be configured to electrochemically extract the dissolved lithium ions Li ⁺ from an aqueous lithium source at a working potential of the counter electrode 4 which is equal or higher than the threshold potential of the working elec trode 3, i.e. a working potential at which dissolved chloride ions Cl react to chlorine at the counter electrode 4 and subsequently to hypochlorite CIO at the counter electrode 4. The hypochlorite CIO may acts as a disinfectant and/or anti-fouling agent at the counter electrode 4.

Intercalation and deintercalation refer to the reversible insertion and removal of lithium ions Li ⁺ into and from the crystal structure of the lithium storage material of the working electrode 3. Electroneutrality is preserved by provid ing or removing electrons to or from the working electrode 3 through an electrochemical reaction at the counter electrode 4 which is connected to the working electrode 3 in a closed electrochemical circuit 2, 2a, 3, 3a, 4, 5 via an electron conductor 2, a power source 2a and the aqueous lithium source 5, which represents an ionic conductor. The lithium-recovery unit may further comprise a control unit (not shown) which is configured to keep the working potential at the working electrode 3 below the threshold potential for the electrochemical formation of chlorine at the working electrode 3, and if applicable, the working potential of the counter electrode 4 equal to or above this threshold potential during the lithium extraction.

The lithium-recovery unit of the example of figure 1 may be used to conduct a lithium extraction method comprising a lithium extraction step and a lithium recovery step. In the lithium extraction step, the electrochemical cell of the example of figure 1 is operated with an aqueous lithium source 5 as an electrolyte from which dissolved lithium ions Li ⁺ are electrochemically extracted into the working electrode 3 by intercalating the lithium ions Li ⁺ into the lithium storage material of the working electrode 3. Subsequently, the aqueous lithium source 5 is replaced by an aqueous recovery solution 6 and the extracted lithium ions Li ⁺ are electrochemically recovered from the lithium storage material of the working electrode 3 to the aqueous recover solution 6 by deintercalation. Alternatively, the method may also be conducted by installing the working electrode 3 subsequently in two separate electrochemi cal cells, in which the first electrochemical cell is operated with the aqueous lithium source 5 to conduct the lithium extraction step, while the second electrochemical cell is operated with the aqueous recovery solution 6 to conduct the lithium recovery step.

In the example of figure 1 the aqueous recovery solution 6 exhibits a concen tration of dissolved chloride ions Cl below a threshold concentration for the electrochemical formation of chlorine at the working electrode 3. The aqueous recovery solution 6 is an aqueous lithium chloride solution LiCI and/or lithium hydroxide LiOH. In these solutions the threshold concentration of the chlorine formation is typically 30 mM if no other sources of chloride or other ionic species are present in the electrochemical cell. The concentration of dissolved chloride ions Cl is therefore kept below 30 mM in the aqueous lithium recovery solution 6 in the example of figure 1 so that the electro chemical cell contains an aqueous lithium chloride solution of e.g. lithium chloride LiCI with a concentration of chloride Cl of 30 mM or less during the lithium recovery step.

The aqueous lithium source 5 of the example in figure 1 is a waste water brine of a desalination plant with a concentration of dissolved lithium ions Li ⁺ of 0.3 mg/I and a chloride concentration of 20 g/l. In contrast to leachates of ores or recycled electronics, brines from desalination or zero liquid discharge plants or produced waters of gas or oil production plants are readily accessi ble since they are mere waste products. The lithium extraction method, however, may also be conducted with other aqueous lithium sources 5, in particular aqueous lithium sources 5 with a salinity of more than BO g/l, more than 100 g/l or even more than 200 g/l and a concentration of dissolved lithium ions Li ⁺ of at least 0.3 mg/I, preferably at least 10 mg/I, most prefera bly at least 30 mg/I.

In the example of figure 1, the working electrode 3 comprises spinel-type lithium manganese (III, IV) oxide LiMn ₂ 0 ₄ as lithium storage material, polytetrafluorethylene (PTFE) fibrils as binder and multi-walled carbon nanotubes (MWCNTs) as electrically conductive additive, for example 85 % to 95 % LiMn ₂ 0 ₄ , 2 % to 5 % PTFE fibrils and 3 % to 10 % MWCNTs by weight. Alternatively, the working electrode 3 may comprise a lithium storage material chosen from the group of lithium iron phosphates, lithium cobalt oxides, lithium nickel oxides, lithium nickel cobalt oxides, lithium nickel manganese cobalt oxides and/or lithium nickel cobalt aluminium oxides. The binder may be selected from the group of fluoropolymers, such as PTFE and/or polyvinylidene fluoride (PVDF), or from the group of rubbers, e.g. NBR and/or SBR. It may for example also comprise or consist of PVDF fibrils. The electrically active additive may alternatively or additionally comprise electro- chemically inactive carbon materials such as carbon black, porous carbons, single-walled carbon nanotubes, graphene, graphite, carbon fibres or mixtures thereof.

Spinel-type lithium manganese (III, IV) oxide LiMn ₂ 0 ₄ exhibits a good thermal stability and is chemically and electrochemically stable within the electro chemical stability window of the aqueous lithium source 5 and the aqueous recovery solution 6. In the example of figure 1, the dissolved lithium ions Li ⁺ are electrochemically extracted from the aqueous lithium source 5 to the lithium storage material LiMn ₂ 0 ₄ at a working potential of the working electrode 3 (discharging) which is kept below a threshold potential of 0.95 V on a Ag/AgCI scale for the electrochemical formation of chloride at the working electrode 3, while the working potential of the counter electrode 4 is kept equal to or above this threshold. For an efficient lithium extraction the dissolved lithium ions Li ⁺ are extracted from the aqueous lithium source in a range between 0.3 V up to a potential below the threshold potential of 0.95 V on a Ag/AgCI on a potential scale, and are recovered from the lithium storage material in a range between 0.3 V to 1.2 V on a Ag/AgCI potential scale. The control unit may be configured to adjust the working potentials of the electrodes 3, 4 within these ranges.

The working electrode 3 in the example of figure 1 is a dry-film electrode with a film thickness of 231 pm, a density of 1.9 g/cm ³ and a porosity of 55 %. It has been laminated onto a current collector electrode 3a, e.g. a platinum chip, using an electrically conductive adhesion promoter. In the example of figure 1, the electrolytes 5, 6 are fed or cycled through the electrochemical cell in a continuous feed stream, as indicated by the arrows in figure 1. Since the working electrode 3 is mechanically stable it can withstand a continuous feed stream for a prolonged time. In a continuous feed stream mode, the concentration of hypochlorites at the working electrode 3 may be reduced by dilution, e.g. by a continuous delivery of unconsumed, hypochlorite-free aqueous lithium source 5 to the working electrode 3; a continuous feed stream may, however, also reduce the concentration of hypochlorite at the counter electrode 4. Alternatively, the extraction and/or recovery of lithium maybe conducted in a batch mode.

The counter electrode 4 may be an activated or inert carbon electrode, a graphite electrode, an Ag/AgCI electrode, an electrode based on the Prussian Blue structure or a noble metal electrode. In the example of figure 1 a mesh of non-porous platinized titanium is used as an inert redox counter electrode. Inert redox counter electrodes provide and remove electrons in an electro chemical water splitting reaction without absorbing or discharging anions or cations of the aqueous lithium source 5. They may therefore be operated independently of a specific composition or specific ionic species in the aqueous electrolyte of the electrochemical cell.

The electrochemical cell in the example of figure 1 further comprises a cation exchange membrane 3b which is configured to inhibit the diffusion of dissolved chloride ions Cl from the aqueous lithium source 5 to and into the working electrode 3. In the example of figure 1 the surface of the working electrode 3 facing the counter electrode 4 is covered by the cation exchange membrane 3b while the reverse of the working electrode 3 is protected by the non-porous current collector electrode 3a. The cation exchange membrane 3b exhibits a thickness of 75 pm and comprises a sulfonated tetrafluoroethylene based fluoropolymer-copolymer. It protects the working electrode 3 from adsorbing chloride Cl in the lithium extraction step which may compromise the lithium deintercalation in the lithium recovery step. Therefore, tedious cleaning procedures or means for adjusting the chloride concentration below the threshold for the formation of chlorine in the recovery step may be omitted.

Figure 2 demonstrates the negative effect of the formation of chlorine and subsequently hypochlorite in the lithium extraction. The figure displays a cyclovoltammogram of an electrochemical cell in which the working potential of the working electrode 3 is increased above the threshold potential for the electrochemical formation of chlorine at the working electrode 3. The electrochemical cell and the aqueous lithium source 5 are configured as described in the example of figure 1. When the working potential of the working electrode 3 is increased beyond a potential threshold for the formation of chlorine of 0.95 V on a Ag/AgCI potential scale, a steep increase in the cell current is observed due to the electrochemical formation of chlorine. The cyclovoltammogram further reveals the gradual decay of the working electrode 3. It has been found that this degradation occurs due to the formation of hypochlorite in the aqueous lithium source 5. The decay of the working electrode 3 causes the overall cell current to decrease gradually with each potential cycle.

Figure 3 displays discharging/charging voltage curves of an electrochemical cell in which the working potential of the working electrode 3 is kept below the threshold potential for the electrochemical formation of chlorine at the working electrode 3. The charge and discharge current density is 0.3 mA/cm ² and the maximum working potential of the working electrode 3 is kept below the threshold potential of 0.95 V on a Ag/AgCI potential scale. Therefore, the formation of chlorine and subsequently hypochlorite is suppressed and the performance and the life-time of the working electrode 3 are extended significantly. Features of the different embodiments which are merely disclosed in the exemplary embodiments of course can be combined with one another and can also be claimed individually.