The present invention relates to a method and an apparatus for the extraction of lithium from aqueous lithium sources which contain dissolved lithium, carbonate, calcium and/or magnesium ions.

The advancement of electro-mobility and the exploitation of renewable energy sources have raised a high demand for lithium as a raw 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. Besides this, 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 have to be opened up. Such sources may be, for example, seawater or saline waste waters which are obtained as by-products or waste-products in industrial processes.

Seawater is available in virtually unlimited supply. It thus is 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 an increased lithium concentration due to their prior processing. Brines of desalination plants or zero liquid discharge plants or produced waters of gas or oil extrac tion 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 due to the presence of different anions and cations in these sources.

In electrochemical lithium recovery processes, certain anions and cations set off parasitic side-reactions or produce undesired chemical species which may degrade the electrodes. A major problem are anions and cations, such as sodium, calcium, potassium, magnesium, chloride, bromide, carbonate or sulphate, which form insoluble compounds at the surfaces of the electrodes. Such deposits act as diffusion barriers to lithium ions at the surface of the electrodes and impair the uptake and the release of lithium at the electrodes. Since the electrodes presently available are very fragile and less resistant to harsh chemical treatments, the removal of solid deposits from the electrodes is cumbersome, if at all possible, and frequently causes severe damage to the electrodes. Furthermore, the low chemical resistance of the electrodes also places limits on the salinity of aqueous lithium sources to be processed. Hence such lithium recovery processes are currently neither long term stable nor efficient.

In the prior art, various water treatment methods are known for removing dissolved calcium, magnesium and/or carbonate from saline water. With these pre-treatment methods a built-up of carbonate deposits at the elec trodes may be prevented. Such pre-treatment methods, however, involve time- and energy consuming, difficult or environmentally harmful separation steps which increase the complexity and costs of the lithium recovery.

Another problem is the low selectivity of these carbonate removal methods. Frequently, lithium is bound or removed from the saline water together with other ionic species, precipitates or additives. This severely reduces the amount of lithium ions available for the lithium recovery.

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 from aqueous lithium sources containing dissolved lithium, carbonate, calcium and/or magnesium.

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 20. Advantageous developments and embodiments are described in the dependent claims.

A method for extracting lithium from an aqueous lithium source comprises the step of pre-treating an aqueous lithium source containing dissolved lithium ions Li ⁺ , dissolved carbonate ions C0 ₃ ² , dissolved calcium ions Ca ²⁺ and/or dissolved magnesium ions Mg ²⁺ by mixing the aqueous lithium source with a scale inhibitor, and the step of electrochemically extracting the dissolved lithium ions Li ⁺ from the pre-treated aqueous lithium source in an electrochemical cell which comprises in a housing: a working electrode, a counter electrode and the pre-treated aqueous lithium source as an electro lyte. The working electrode of the electrochemical cell comprises a lithium storage material, a binder and an electrically conductive additive. The scale inhibitor is configured to inhibit the formation of solid carbonate compounds, in particular calcium carbonate, magnesium carbonate and lithium carbonate, in the aqueous lithium source. The dissolved lithium Li ⁺ ions of the pre-treated aqueous lithium source are intercalated, i.e. extracted by intercalation, into the lithium storage material of the working electrode. The described method thus allows a highly selective and efficient extraction of Li ⁺ ions from aqueous lithium sources despite a high concentration of other anions and cations in the aqueous lithium source. The method may be easily implemented in existing industrial sites and may be used to produce high purity lithium solutions, which may, for example, be used for the production of lithium battery components. The method may be conducted with aqueous lithium sources which exhibit a salinity of more than BO g/l, more than 100 g/l or more than 200 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 prefera bly at least 30 mg/I. Aqueous lithium sources may, for example, be provided from sea water or saline industrial waste waters, in particular from brines of desalination or zero liquid discharge plants or produced waters of gas or oil production plants.

In this application intercalation and deintercalation refer to the reversible insertion and removal of lithium ions Li ⁺ into and from the crystal structure of a lithium storage material. The lithium storage material may be selected from the group of lithium iron phosphates, e.g. LiFeP0 ₄ , lithium cobalt oxides, e.g. LiCo0 ₂ , 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. LiNi ₀ .85Co ₀ .iAlo.o50 ₂ .

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 may refer 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.

An electrically conductive additive is a material which exhibits an electrical conductivity of more than 10 ⁶ S/m at a temperature 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.

Unlike pre-precipitation methods, the pre-treatment of the aqueous lithium source with scale-inhibitors does not involve any complex, environmentally harmful, energy- and/or time consuming processing steps or equipment, such as evaporators, crystallization chambers, filters or pumps. It can be easily implemented in a batch, cyclic or a continuous feed stream mode of the aqueous lithium source and does not require any extreme temperatures or harsh pH-regimes. The intercalation of the lithium ions may thus be con ducted in a modest pH-range between 6 and 9 in which the lithium storage material exhibits a high efficiency, while the electrode is consistently pro tected against carbonate scaling. Besides this, the pre-treatment with scale- inhibitors may be highly selective to calcium and/or magnesium carbonate. The formation of undesired lithium compounds may be suppressed and as a result a high amount of dissolved lithium ions remains available for the extraction into the working electrode. This is of particular importance if lithium is to be extracted from aqueous lithium sources with a low content of lithium.

The scale inhibitor is preferably a water-soluble scale inhibitor chosen from the group of organophosphorus scale inhibitors, in particular water soluble polymers of phosphonic acid and derivatives thereof, and/or the group of organic polymer scale inhibitors, including water soluble polymers of maleic anhydride, maleic acid, acrylic acid, methacryclic acid and/or co-polymers thereof. In contrast to chelating scale inhibitors, which function in a sto- chiometric manner, these scale inhibitors act as nucleation inhibitors, crystal modifiers or dispersants and thus are very efficient even in small quantities. They are electrochemically stable and prevent the deposition of solid carbon ates at the surfaces of the electrodes without chemically degrading the electrodes or affecting the reversibility of the intercalation and deintercala- tion of lithium into and from the working electrode. Hence the life-time of the electrode is prolonged and the lithium extraction may be conducted at a stable working potential. Scale inhibitors of the group of organic polymers further offer the great advantage of a good environmental compatibility. If used appropriately, tedious post-treatments for the deposition of the delithiated aqueous lithium source may be avoided. For a synergistically enhanced inhibition efficiency, scale inhibitors of the group of organophos- phorus scale inhibitors and/or the group of organic polymer scale inhibitors may be blended together, preferably at ratios between 0.4 to 1.0 relative to each other.

In the described method, the working electrode may be a film electrode or a sheet electrode. Such electrodes require only small amounts of electrode material but offer large active surface areas for the extraction of lithium. The film thickness of the film electrode or sheet electrode may be in the range of 500 pm to 50 pm, preferably in the range of 250 pm to 50 pm. The working electrode may further comprise an electrically conducting current collector electrode, which may reinforce the working electrode. This current collector electrode is 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.

The described method may be conducted with a working electrode which exhibits a porosity of at least 50 %, more preferably a porosity of at least 55 %. A high porosity increases the active surface area of the electrode and thus significantly enhances the diffusion and uptake of lithium during the lithium extraction. Film electrodes and/or electrodes with porosities of 50 % or more may be achieved with fibrillated binders, which may for example comprise or consist of PTFE fibrils, and carbon nanotubes as electrically conductive additive in the working electrode. The fibrils and the carbon nanotubes act as highly efficient binder and may enhance both the chemical and the mechanical stability of the working electrode. Hence chemical degradation of the working electrode due to concentrated aqueous lithium sources, scale inhibitors or cleaning fluids may be prevented and the method may also be conducted in a continuous feed or cyclic mode since the elec- trades can withstand a permanent flow passing their surfaces. These aspects are of particular importance if the lithium extraction is to be up-scaled to an industrial scale demanding high throughputs of the pre-treated 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 method may be conducted with dry- film electrodes that are non-toxic and fabricated without any aggressive or environmentally harmful solvents, the environmental performance of the lithium extraction may be improved appreciably. This particularly holds, 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 the described method, the extraction of lithium may for example 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 electrochemical extraction of lithium ions within the electrochemical stability window of aqueous lithium sources and exhibits a good environmental compatibility. Hence the postprocessing of the delithiated aqueous lithium source may be reduced. The working electrode may further comprise polytetrafluoroethyl- ene (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 elec trodes display an excellent chemical and electrochemical stability in harsh chemical environments and may be operated in the electrochemical stability window of pre-treated aqueous lithium sources without irreversible structural changes. They may exhibit a prolonged-lifetime even if they are employed as film electrodes and/or highly porous electrodes in concentrated aqueous lithium sources.

Since the extraction method is highly selective to lithium, it may be used to produce high purity lithium solutions of at least 99.99 % purity, which may be metallised to high purity lithium metal. The high selectivity of the electro chemical extraction step results from the intercalation of lithium ions Li ⁺ into the lithium storage material in the working electrode. In order to ensure a reversible intercalation, the redox reactions at the working electrode may take place within the electrochemical stability window of the aqueous lithium source, avoiding the formation of hydrogen or oxygen gas. At a neutral pH and a temperature of 25 °C the 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 saline solutions on a Li/Li ⁺ potential scale. The dissolved lithium ions Li ⁺ may thus be extracted from the pre-treated aqueous lithium source at a working potential of the working electrode in a range between 2.5 V and 3.8 V on a Li/Li ⁺ potential scale, possibly in a range between 2 V and 5 V on a Li/Li ⁺ potential scale. After the extraction of lithium into the working electrode, the lithium ions may be recovered from the working electrode into an aqueous recovery solution, for example a lithium chloride or a lithium hydroxide solution, by reversing the electrochemical cell polarity, wherein the working potential of the working electrode remains within the stability window of water.

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 from a non-porous material, in order to avoid the co adsorption of lithium 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 do not absorb or discharge anions or cations of the aqueous lithium source and are chemically inert in the pre-treated aqueous lithium source. Hence they are very durable and show a long-term stable performance without any additional means of protection or requirements to the ionic composition of the electrolyte.

A lithium-recovery unit comprises an electrochemical cell and a mixing unit. The electrochemical cell of the lithium-recovery unit comprises in a housing: a working electrode, a counter electrode and an electrolyte; wherein the working electrode comprises a lithium storage material, binder and an electrically conductive additive. The mixing unit is configured to mix an aqueous lithium source comprising dissolved lithium ions Li ⁺ , dissolved carbonate ions C0 ₃ ² , dissolved calcium ions Ca ²⁺ and/or dissolved magnesium ions Mg ²⁺ with a scale inhibitor, wherein the scale inhibitor is configured to inhibit the formation of solid carbonate compounds, in particular calcium and/or magnesium carbonate, in the aqueous lithium source. The mixing unit is further configured to supply the aqueous lithium source mixed with the scale inhibitor as an electrolyte to the electrochemical cell. The electrochemi cal cell is configured to electrochemically intercalate the dissolved lithium ions Li ⁺ from the electrolyte into the lithium storage material of the working electrode.

Mixing refers to combining the aqueous lithium source and the scale inhibitor to a homogeneous liquid. The scale inhibitor is preferably added to the aqueous lithium source before or during the nucleation stage of the crystal growth of carbonates in the aqueous lithium source. The mixing unit may be separate from the lithium-recovery unit but may in some embodiments be integrated into the electrochemical cell. Mixing may be achieved by stirring and/or by diffusive mixing which may reduce the nucleation sites of calcite crystals in the aqueous lithium source, e.g. by releasing the scale inhibitor at multiple locations in the aqueous lithium source.

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. An exemplary embodiment of the invention is illustrated in the drawing and will be explained below with reference to figures 1 to 5. 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 an example of a method for extract ing lithium from an aqueous lithium source in a lithium-recovery unit,

Fig. 2 displays scanning electron microscopy images of a working elec trode of a of a lithium-recovery unit without (left image) and with (right image) scale inhibitors,

Fig. 3 displays the capacity retention of a working electrode of a lithium- recovery unit without and with scale inhibitors,

Fig. 4 displays the cyclovoltammogram of an electrochemical cell of a lithium-recovery unit with scale inhibitors, and

Fig. 5 displays discharging/charging voltage curves of an electrochemical cell of a lithium-recovery unit with scale inhibitors.

Figure 1 illustrates in a schematic diagram an example of a method for extracting lithium from an aqueous lithium source in a lithium-recovery unit. For the extraction of lithium an aqueous lithium source 5 which contains dissolved lithium ions Li ⁺ , dissolved carbonate ions C0 ₃ ^2~ , dissolved calcium ions Ca ²⁺ and/or dissolved magnesium ions Mg ²⁺ is pre-treated SI by mixing the aqueous lithium source 5 with a scale inhibitor 5a. The scale inhibitor 5a is configured to inhibit the formation of solid carbonate compounds, in particu lar calcium carbonate, magnesium carbonate and lithium carbonate, in the aqueous lithium source 5. The dissolved lithium ions Li ⁺ are electrochemically extracted S2 from the pre-treated aqueous lithium source 5, 5a in an electro chemical cell which comprises in a housing 1: a working electrode 3, a counter electrode 4 and the pre-treated aqueous lithium source 5, 5a as an electrolyte separating 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 additive. The dissolved lithium ions Li ⁺ are electrochemically extracted S2 from the pre-treated aqueous lithium source 5, 5a in the electrochemical cell by intercalating the dissolved lithium Li ⁺ ions into the lithium storage material of the working electrode 3 (cathode).

Intercalation and deintercalation refers to the reversible insertion and removal of lithium ions Li ⁺ into 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, 5a via an electron conductor 2, a power source 2a and the pre-treated aqueous lithium source 5, 5a, which represents an ionic conductor.

In the example of figure 1 the aqueous lithium source 5 is a waste water brine of a desalination plant with a concentration of dissolved lithium ions Li ⁺ of 0.3 mg/I. In contrast to leachates of ores or recycled electronics, brines from desalination plants or produced waters of gas or oil production plants are readily accessible 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 30 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 preferably at least 30 mg/I.

In the example of figure 1, the aqueous lithium source 5 and the scale inhibitor 5a are supplied to the mixing unit 6 and to the electrochemical cell in a continuous feed stream mode, indicated by the arrows in figure 1. The method may, however, also be conducted in a batch mode or a cyclic mode in which the mixture of the aqueous lithium source 5 and the scale inhibitor 5a is cycled through the electrochemical cell in a loop. Mixing may be achieved with stirrers or agitators as indicated in figure 1 and/or by diffusive mixing, e.g. by injecting/releasing solutions of the scale inhibitor 5a at multiple, evenly spaced locations into the aqueous lithium source 5. The mixing unit 6 may be a separate unit or may be integrated into the electrochemical cell; stirrers, agitators or injection nozzles may for example be located in the electrolyte of the electrochemical cell.

The scale inhibitor 5a may be chosen from the groups of organophosphorus scale inhibitors, in particular from the group of water soluble polymers of phosphonic acid, e.g. aminotrimethylene phosphonic acid (ATMP), ethyl- enediamine-tetramethylene phosphonic acid (EDTMP), diethylenetriamine- pentamethylene phosphonic acid (DETPMP), bis(hexamethylene) triamine- pentamethylene phosphonic acid) (BHMT), pentaethylenehexamine octakis- methylene phosphonic acid (PEHOMP), l-hydroxyethylidene-1,1- diphosphonic acid (HEDP) or phosphino-carboxylic acids (PCA), and/or the group of organic polymer scale inhibitors, including water soluble polymers of maleic anhydride, maleic acid, acrylic acid, methacryclic acid and/or copoly mers thereof, e.g. poly(acrylic acid- co-maleic acid) or poly (acrylic amide-co- maleic acid). Commercially available examples include Albrivap ^® DSB (based on a polyphosphonic acid) from Rhodia, Belgard ^® EV2000 (based on a terpolymer of maleic acid), Belgard ^® EVN (based on a maleic acid homopoly mer) or Belgard ^® EV2030 (based on polymaleic acid copolymers) from BWA Water Additives or Sokalan ^® PM10 and PM15 (based on acrylic acid copoly mers) from BASF.

In comparison to other pre-precipitation methods, a pre-treatment with the scale inhibitors 5a may easily be implemented without negative effects on the lithium intercalation and deintercalation at the working electrode 3. Further more, the scale inhibitors 5a are very efficient and may be applied in small quantities of typically less than 10 mg/I of the aqueous lithium source 5. In the example of figure 1, a dosage of 8 mg/I aqueous lithium source 5 of a non toxic hydrolyzed poly(maleic anhydride) has been used as a scale inhibitor 5a to stabilize the performance of the working electrode 3. Since scale inhibi tors 5a of the group of organophosphorus scale inhibitors and the group of organic polymer scale inhibitors may exhibit different inhibition mechanisms, they may be blended together for a synergistically enhanced inhibition efficiency. Satisfactory results may for example be attained with the blend of scale inhibitors 5a summarized in table 1. These scale inhibitors 5a are environmentally sustainable and may be applied even at elevated tempera tures.

Table 1: Scale inhibitors for the extraction of lithium from aqueous lithium sources 5 in the temperature range from 15 °C to 60 °C

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, polytetrafluoroethylene (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. LiMn ₂ 0 ₄ exhibits a good thermal stability compared to other lithium storage materials. It is chemically and electrochemically stable within the electro chemical stability window of aqueous lithium sources 5 and environmental compatible. Together with the PTFE fibrils and the MWCNTs it forms a thermally, mechanically and chemically durable working electrode 3 which may be configured as porous and/or thin film electrode for aqueous lithium sources 5 which may be pre-treated with scale inhibitors 5a without any damage to the working electrode. Alternatively the lithium storage material may be selected 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 including polyvi- nylidene fluoride (PVDF) or from the group of rubbers, preferably NBR and or SBR, and the electrically active additive may be selected of the group of electrochemically inactive carbon materials including carbon black, porous carbons, carbon nanotubes, graphene, graphite, carbon fibres or mixtures thereof.

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. 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 utilized as an inert redox counter electrode. Inert redox counter electrodes provide and remove electrons in an electrochemical water splitting reaction without absorbing or discharging anions or cations of the aqueous lithium source. They may therefore be operated independently of a specific composition or specific ionic species in the electrolyte of the electro chemical cell, i.e. in the pre-treated aqueous lithium source.

Subsequent to the lithium extraction the pre-treated aqueous lithium source 5, 5a in the electrochemical cell may be replaced by an aqueous recovery solution, e.g. an aqueous lithium chloride or lithium hydroxide solution, and the extracted lithium ions may be electrochemically recovered from the working electrode 3 to the recovery solution by reversing the electrochemical cell polarity. For a reversible deintercalation of the lithium ions from the working electrode 3 into the recovery solution (charging), the working potential of the working electrode 3, may, like in the case of the intercalation (discharging), remain within the electrochemical stability window of the aqueous recovery solution, in the example of figure 1 in the range between 2.5 V and 3.8 V on a Li/Li ⁺ potential scale. With the present method, the coloumbic efficiency (ratio of discharge capacity to charge capacity) of the working electrode 3 may thus be stabilised above 90 % for more than 50 extraction and recovery cycles.

Figures 2 and 3 display scanning electron microscopy images and capacity retention results of the working electrode 3 of a lithium recovery unit as described in the example of figure 1. Without scale inhibitors 5a, the surface of the working electrode 3 is blocked by micrometer-sized crystals, as can be seen in the left image of figure 2. Energy-dispersive X-ray spectroscopy revealed that the crystals are mainly based on calcium and magnesium. The crystals act as a diffusion barrier to lithium ions and lower the capacity retention of the working electrode 3 shown in figure 3 (full square symbols). With the scale inhibitors 5a the growth and/or deposition of such crystals may be suppressed, as seen in the right image of figure 2, and a long term stable capacity retention may be achieved as shown in figure 3 (open triangular symbols). Figures 4 and 5 display a cyclovoltammogram and discharging/charging voltage curves of an electrochemical cell of a lithium recovery unit described in the example of figure 1. The cyclovoltammogram in figure 4 and the discharging/charging voltage curves of figure 5 reveal that the scale inhibi tors 5a enable reversible two-fold phase changes in the lithium storage material during the intercalation and deintercalation of lithium without any parasitic electrochemical reactions of the scale inhibitors 5a.

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.