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. Apart from that, 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 need to 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 or 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 due to the presence of different anions and cations in these sources.

In electrochemical lithium recovery processes, certain anions and cations trigger parasitic side-reactions which degrade the electrode materials or produce undesired chemical species. A major problem are anions and cations, such as sodium, calcium, potassium, magnesium, chloride, bromide, carbon ate or sulphate, which form insoluble compounds at the surfaces of the electrodes. Such deposits act as a diffusion barrier for lithium ions and impair the uptake and the release of lithium at the electrodes. As the electrodes presently available are very fragile and less resistant to harsh chemical treatments, the removal of solid deposits, in particular carbonates, from the electrodes is cumbersome, if at all possible, and frequently causes severe damage to the electrodes. Besides this, the low chemical resistance of the electrodes also places limits on the salinity of the aqueous lithium source to be processed. 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 from aqueous lithium sources containing dissolved lithium, carbonate, calcium and/or magnesium ions.

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

A method for extracting lithium from an aqueous lithium source comprises the step of removing dissolved calcium ions Ca ²⁺ and/or dissolved magnesium ions Mg ²⁺ by nanofiltration from 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⁺ , the step of removing water from the aqueous lithium source by reverse osmosis, and the step of electro- chemically extracting dissolved lithium ions Li ⁺ from the aqueous lithium source in an electrochemical cell. The electrochemical cell 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 working electrode of the electrochemical cell comprises a lithium storage material, a binder and an electrically conductive additive. The dissolved lithium Li ⁺ ions are electrochemically extracted from the aqueous lithium source by intercalating the dissolved lithium ions Li ⁺ 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 which may be used to produce high purity lithium solutions and may be easily implemented in water desalination or zero liquid discharge plants.

An aqueous lithium source may be an aqueous saline solution which exhibits 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 be provided for example from sea water or saline industrial waste waters. They may exhibit a salinity of more than 30 g/l, more than 100 g/l or even more than 200 g/l. In this application, salinity refers to the total amount of salts dissolved as ions in the 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 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 _0.85 Co ₀ .iAlo.o ₅ 0 ₂ . Working electrodes with these lithium storage materials enable the production of high purity lithium solution since the intercalation and deintercalation of lithium to the lithium storage material is highly selective to lithium ions. The lithium extraction and recov ery, however, may be suppressed by the formation of solid carbonates at the surfaces of the electrodes. The described extraction method prevents such deposits without harsh chemical treatments or high losses in the lithium content of the aqueous lithium source. The nanofiltration is very effective in selectively removing polyvalent ions, including dissolved Ca ²⁺ and Mg ²⁺ , while the reverse osmosis also retains monovalent ions, including dissolved ions Li ⁺ . Hence high levels of dissolved lithium ions Li ⁺ remain in the aqueous lithium source while the counter-ions for the deposition of carbonates, e.g. Ca ²⁺ and Mg ²⁺ , are removed. Therefore the formation of calcium carbonate and/or magnesium carbonate is prevented and high lithium extraction and recovery rates are maintained. In addition to this, purified desalinated water may be supplied by the method. The method may therefore be implemented in a desalination plant or share processing steps, e.g. the reverse osmosis step, with desalination plants saving resources.

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 within this application meant to be 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.

In the nanofiltration step prior to the reverse osmosis step, dissolved calcium ions Ca ²⁺ and/or dissolved magnesium ions Mg ²⁺ are removed from the aqueous lithium source. Nanofiltration is a pressure-driven membrane separation process that forms the transition between ultrafiltration and reverse osmosis. It may be typically conducted in a cross-flow mode in which the aqueous lithium source is fed in a tangential flow across a nanofiltration membrane at pressures in the range from 10 bar to 50 bar. The tangential flow may wash any residues off the surface of the nanofiltration membrane and may thus enable a stable continuous feed stream mode. The nanofiltra tion membrane may exhibit a pore size of 1 nm to 20 nm in diameter. It may be configured to retain also organic matter and polyvalent cations, including S0 ₄ ² and C0 ₃ ² , to aid the production of soft drinking water. The nanofiltra tion may, for example, be conducted with a composite membrane comprising a polyamide layer deposited on a polyethersulfone or polysulfone porous layer. These nanofiltration membranes may be especially permeable for dissolved, i.e. hydrated, lithium ions Li ⁺ and may thus enable a high lithium yield in the lithium extraction step.

In the reverse osmosis step, water is removed from the aqueous lithium source. While hydrated monovalent or polyvalent ions, larger molecules and/or particles may be retained in the aqueous lithium source at an osmosis membrane, water may move through the osmosis membrane if a pressure is applied to the aqueous lithium source which is higher than the osmotic pressure across the osmosis membrane. The applied pressure may be typically in the range from 50 bar to 100 bar, depending on the mechanical durability of the osmosis membrane. The osmosis membrane may be a water- permeable hollow fibre membrane comprising cellulose acetate and/or cellulose triacetate or a water-permeable composite membrane comprising a polyamide layer deposited on a polyethersulfone or polysulfone porous layer. The pore size of the osmosis membrane is typically in the range of 0.2 nm to 0.6 nm, thus water molecules which exhibit a diameter of approximately 0.1 nm may pass through the osmosis membrane. Like the nanofiltration step, the reverse osmosis step may also be conducted in a continuous feed stream mode, e.g. in a single-pass operation or multiple-pass recirculation.

In the lithium extraction step, a reversible intercalation of lithium may be enabled if the redox reactions at the electrodes take place within the electro chemical stability window of the aqueous lithium source in order to avoid the formation of hydrogen or oxygen gas. 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 saline solutions on a Li/Li ⁺ potential scale. The dissolved lithium ions Li ⁺ may thus be extracted from the aqueous lithium source at a working potentials of the working electrode in a range between 2.5 V to 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.

The working electrode of the electrochemical cell may be preferably 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 thin film thicknesses may be achieved with fibrillated binders, which may for example comprise or consist of PTFE fibrils, and carbon nanotubes in the working electrode. The fibrils and the carbon nanotubes are chemically inert and 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 as produced in the reverse osmosis step of the described method, and may also be resistant to wear from the flow of the aqueous lithium source, rinsing liquids or recovery solutions past the surfaces of the electrodes, 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 a highly efficient binder in the dry-film electrode. Since the lithium extraction 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 desalination of sea water or post-processing of waste water from industrial desalination 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-% polytetrafluorethylene fibrils and 3 weight-% to 10 weight-% multi-walled carbon nanotubes. These working electrodes may be operated with a large lithium capacity within the electrochemical stability window of aqueous lithium sources 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. This electrode is typically designed as a planar current collector. Preferably the 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 be laminated onto the 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 from 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 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 any additional means of protection or specific requirements to the ionic composition of the electrolyte in the electrochemi cal cell.

Subsequent to the lithium extraction, the aqueous lithium source may be replaced with an aqueous recovery solution in the electrochemical cell, for example an aqueous lithium chloride LiCI or lithium hydroxide LiOH solution. The extracted lithium ions Li ⁺ may be electrochemically recovered from the working electrode to this aqueous recovery solution by reversing the electro chemical cell polarity. In order to avoid the formation of hydrogen gas or oxygen gas, the extracted lithium ions Li ⁺ may be electrochemically recovered at a working potential of the working electrode in the range between 2.5 V and 3.8 V on a Li/Li ⁺ potential.

A lithium-recovery unit comprises a nanofiltration unit, a reverse osmosis unit and an electrochemical cell. The nanofiltration unit is configured to remove dissolved calcium ions Ca ²⁺ and/or dissolved magnesium ions Mg ²⁺ from 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⁺ . The reverse osmosis unit is configured to remove water from the aqueous lithium source and the electrochemical cell is configured to electro chemically extract dissolved lithium ions Li ⁺ from the aqueous lithium source. The electrochemical cell comprises in a housing: a working electrode, an inert counter electrode and an electrolyte, wherein the working electrode com prises a lithium storage material, a binder and an electrically conductive additive. The electrochemical cell is configured to electrochemically extract the dissolved lithium ions Li ⁺ , wherein the dissolved lithium ions Li ⁺ are intercalated into the storage material of the working electrode using the aqueous lithium source as the electrolyte in the electrochemical cell.

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 drawings and will be explained below with reference to figures 1 to 4. 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 schematically an example of a method for extracting lithium from an aqueous lithium source in a lithium-recovery unit, Fig. 2 illustrates schematically an example of an electrochemical cell of a lithium-recovery unit for extracting lithium from an aqueous lithium source,

Fig. 3 displays the cyclovoltammogram of an electrochemical cell of a lithium-recovery unit, and

Fig. 4 displays discharging/charging voltage curves of an electrochemical cell of a lithium-recovery unit.

Figure 1 illustrates schematically an example of a method for extracting lithium from an aqueous lithium source in a lithium-recovery unit. For the lithium extraction dissolved calcium ions Ca ²⁺ and/or dissolved magnesium ions Mg ²⁺ are removed by nanofiltration SI from 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⁺ . Subsequently, water is removed from an aqueous lithium source by reverse osmosis S2 and dissolved lithium ions Li ⁺ are electrochemically extracted S3 from the aqueous lithium source in an electrochemical cell. The electrochemical cell may be configured as illustrated and described in the example of figure 2. It comprises in a housing 1: a working electrode 3, a counter electrode 4 and the aqueous lithium source as an electrolyte 5 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 Li ⁺ ions are electrochemically extracted from the aqueous lithium source 5 by intercalating the dissolved lithium ions Li ⁺ into the lithium storage material of the working electrode 3.

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 BO g/l, more than 100 g/l or even more than 200 g/l and a concen tration 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 nanofiltration step SI, dissolved calcium ions Ca ²⁺ and/or dissolved magnesium ions Mg ²⁺ are removed from the aqueous lithium source 5. In the example of figure 2, dissolved calcium ions Ca ²⁺ and/or dissolved magnesium ions Mg ²⁺ are removed in a continuous cross-flow nanofiltration using a composite nanofiltration membrane comprising a polyamide layer deposited on a polyethersulfone or polysulfone nanoporous layer with a pore size of 1 nm to 20 nm.

In the reverse osmosis step S2, water is removed from the aqueous lithium source 5 at a pressure in the range from 50 bar to 100 bar using a water- permeable composite reverse osmosis membrane comprising a cross-linked aromatic polyamide layer deposited on a polyethersulfone or polysulfone porous layer which exhibits a pore diameter in the range from 0.2 nm to 0.6 nm. These membranes offer a very high salt rejection at a high flux and are thus suitable to remove water of drinking water quality from the aqueous lithium source 5 without high losses in lithium. Hence the described method may be implemented into a seawater desalination or zero liquid discharge plant for the production of drinking water.

In the lithium extraction step S3, dissolved lithium ions Li ⁺ are electrochemi- cally extracted from the aqueous lithium source 5 to the lithium storage material at a working potential of the working electrode 3 (discharging) in the range between 2.5 V and 3.8 V on a Li/Li ⁺ potential scale. Subsequent to the lithium extraction, the aqueous lithium source 5 may be replaced with an aqueous recovery solution, for example an aqueous lithium chloride LiCI solution or an aqueous lithium hydroxide LiOH solution. The extracted lithium ions Li ⁺ may be electrochemically recovered from the working electrode 3 by reversing the electrochemical cell polarity and applying a working potential in range between 2.5 V and 3.8 V on a Li/Li ⁺ potential scale at the working electrode 3 (charging) to deintercalate the extracted lithium ions Li ⁺ from the lithium storage material to the aqueous recovery solution. Thus both drinking water and high purity lithium solutions with a purity of at least 99.99 % may be obtained with the described method. Figure 2 illustrates schematically an example of an electrochemical cell of a lithium-recovery unit for extracting lithium from an aqueous lithium source 5. The electrochemical cell comprises in a housing 1: a working electrode 3, a counter electrode 4 and an electrolyte separating the working electrode 3 and the counter electrode 4 from each other; wherein the working electrode 3 comprises a lithium storage material, binder and an electrically conductive additive. The electrochemical cell is configured to electrochemically extract dissolved lithium ions Li ⁺ from the aqueous lithium source 5 using the aqueous lithium source 5 as the electrolyte, wherein the dissolved lithium ions Li ⁺ are intercalated into the storage material of the working electrode 3.

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, 3, 3a, 4, 5 via an electron conductor 2, a power source 2a and the aqueous lithium source 5 or the aqueous recovery solution, which represent ionic conductors.

In the example of figure 2, 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 and is chemically and electrochemically stable within the electro chemical stability window of the aqueous lithium source 5 in the range between 2.5 V and 3.8 V on a Li/Li ⁺ potential scale. Together with the PTFE fibrils and the MWCNTs it thus forms a thermally, mechanically and chemically durable working electrode 3 which may be configured as porous and/or thin film electrode, which offers a large active surface area for the extraction of lithium. 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 polyvinylidene fluoride (PVDF) or from the group of rubbers, e.g. 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 2 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 2 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 electrochemical water splitting reaction without absorbing or discharging anions or cations from the aqueous lithium source. They may therefore be operated independently of specific ionic species in the electrolyte of the electrochemical cell.

Figures 4 and 5 display a cyclovoltammogram and discharging/charging voltage curves of an electrochemical cell of a lithium recovery unit as de scribed in the example of figures 1 and 2. Both, the cyclovoltammogram in figure 4 and the discharging/charging voltage curves of figure 5 display the typical voltage plateaus of the reversible two-fold phase changes in the lithium storage material during the intercalation and deintercalation of lithium without any parasitic side-reactions.

Features of the different embodiments which are merely disclosed in the exemplary embodiments as a matter of course can be combined with one another and can also be claimed individually.