Operating an electrochemical cell in the context of processing lithium-containing water

Abstract

The invention relates to operating an electrochemical cell (0) for the purpose of processing lithium-containing water. The aim of the invention is to operate the cell (0) with an improved current yield, keep its permeance constant, and improve the purity of the target product (LIOH). An essential aspect of the method according to the invention is that it is carried out in a three-chamber cell comprising an anode separator (3) and a cathode separator (4). A basic concept of the method according to the invention is to use an inorganic cathode separator (4) which has an ion selectivity in favour of lithium. This means that fewer cationic impurities (Mem+) can be converted into undesirable by-products, which increases the current yield and improves the purity of the target product (LiOH). The anode separator (2) is anion-conductive. An additional essential aspect of the operation according to the invention is that two electrochemical methods are carried out simultaneously, namely on the one hand a membrane-supported electrodialysis of ions and on the other hand an electrolysis of water.

Description

[0001] Operation of an electrochemical cell in the context of the treatment of lithium-containing water

[0002] The invention relates to the operation of an electrochemical cell for the purpose of processing lithium-containing water.

[0003] Lithium-containing waters are mixtures containing predominantly water and dissolved lithium compounds. They may also contain other dissolved substances, such as sulfates, bisulfates, carbonates, bicarbonates, hydroxides, chlorides, or fluorides of the following elements: B, Na, Mg, Al, Si, K, Ca, Mn, Fe, Co, Ni, and Cu. Lithium-containing waters may also contain organic compounds.

[0004] Lithium-containing waters occur naturally, for example as lithium brine in salt lakes, as seawater, or as groundwater. Lithium-containing waters also occur in deep boreholes or as mine water. Finally, lithium-containing waters are also produced during the recycling of used lithium-ion batteries (LIBs) and in the production of new LIBs. The origins of lithium-containing waters vary greatly, as does their composition: Not only can the lithium concentration vary greatly, but also the quantity and type of other dissolved substances.

[0005] Lithium-containing waters serve as a starting material for the extraction of lithium-containing compounds, particularly lithium carbonate (Li2CO3) or lithium hydroxide (LiOH). Both are required for the production of lithium hydroxide (LIB). Due to the sharply increasing demand for new lithium hydroxide (LIB) and the increasing amount of used lithium hydroxide (LIB), numerous processes have been developed for the treatment of lithium-containing waters, most of which aim to extract lithium carbonate (Li2COa) or lithium hydroxide (LiOH) with the highest possible purity. The processes have been optimized for the desired target compound and the composition of the lithium-containing water used. An overview is provided below:

[0006] Wietelmann, U. and Steinbild, M. (2014). Lithium and lithium compounds. In Ullmann's Encyclopedia of Industrial Chemistry, (Ed.). DOI: 10.1002 / 14356007. a15_393.pub2.

[0007] Commercially predominant refining processes, especially in the extraction of primary lithium from salt lakes, are those that primarily operate thermally or with crystallization effects. This is very resource-intensive in many respects.

[0008] For this reason, newer processes for extracting lithium compounds from lithium-containing waters have been developed that use electrical energy. These are electrochemical processes, particularly electrolysis or electromembrane dialysis. The fundamental advantage of these electrochemical processes is that they are very resource-efficient when using green electricity. The disadvantage is the complex equipment technology; in particular, the electrochemical cells in which the processing takes place are very demanding from a materials science perspective.

[0009] A selection of electrochemical processes for separating lithium from water from the recycling of used batteries or seawater has been compiled:

[0010] Pankaj K. Choubey et al.: Advance review on the exploitation of the prominent energy storage element Lithium. Part II: From sea water and spent lithium ion batteries (LIBs), Minerals Engineering, Volume 110, 2017, Pages 104-121 , DOI: 10.1016 / j.mineng.2017.04.008.

[0011] In Section 2.2, Choubey et al. describe modern electrodialytic processes carried out in electrochemical cells, each equipped with two different ion-conducting membranes: an anion-conducting membrane and a cation-conducting membrane. The desired ionic conductivity of the membranes is achieved by impregnation with an ionic liquid.

[0012] CA 3077834 A1 describes a process for producing high-purity lithium hydroxide from highly contaminated sources, such as brine from salt lakes. A complex interconnection of different purification stages is disclosed, including electrodialysis with a three-chamber cell equipped with a bipolar membrane. A bipolar membrane is the combination of an anion exchange membrane and a cation exchange membrane. The chemical nature of these membranes is not disclosed in CA 3077834 A1.

[0013] EP 2 841 623 B1 discloses a process for producing lithium hydroxide using an electrochemical cell having three compartments and two separators (a so-called three-chamber cell). In the three-chamber process known from EP 2 841 623 B1, the central compartment of the three-chamber cell is exposed to an aqueous stream containing lithium sulfate. Lithium hydroxide is initially introduced into the cathodic compartment, while ammonia water is fed into the anodic compartment. An aqueous solution containing lithium hydroxide is withdrawn from the cathodic compartment of the cell, and ammonium sulfate is formed in the anodic compartment. In addition, oxygen is produced on the anodic side and hydrogen at the cathode. The three-chamber cell is operated under basic conditions.

[0014] The three-chamber cell known from EP 2 841 623 B1 inherently has two membranes that separate the central compartment of the anodic compartment and the cathodic compartment, respectively. Membrane materials considered include perfluorinated polymers, styrene, or divinylbenzene membranes. In particular, cation exchange membranes or PEEK-reinforced membranes are to be used. Examples include the commercial ion exchange membranes Asahi AAV, Fumatech FAB, Astrom Neosepta®, or Lanxess ionac®. The chemical nature of these ion exchange membranes is not disclosed in EP 2 841 623 B1, but it is highly likely that they are organic membrane materials.

[0015] A fundamental disadvantage of polymer membranes is their water permeability. This causes the anolyte to be diluted with water from the catholyte. Furthermore, organic ion exchange membranes not only allow Li + , but also Na+ happen, so that the purity of the target product is impaired as soon as Na is present in the starting material. In addition to the purity of the target product, the power efficiency of the process also suffers: The valuable electrical energy in electrolysis with organic membranes is also used to remove unwanted Na + into the second compartment. Once in the second compartment, the Na + Furthermore, they are converted into unwanted byproducts via unintended electrochemical processes. The energy efficiency of the process is limited in relation to the yield of the target product Li. Finally, these membranes are sensitive to the presence of divalent cations such as Mg. 2+ and Ca 2+These cations poison the membrane over time, reducing its conductivity for lithium. This manifests itself in a decrease in the membrane's permeance, i.e., its surface-related lithium conductivity relative to its thickness. This means that less lithium can be separated from the lithium-containing water. Due to reduced current efficiency, increasing contamination of the target product with foreign cations, and decreasing permeance, the operation of the electrochemical cell quickly becomes uneconomical.

[0016] In view of this state of the art, the present invention is based on the object of operating the electrochemical cell with better current efficiency, keeping the permeance constant over time and improving the purity of the target product.

[0017] This object is achieved in that the electrochemical cell is operated as follows: a) At least one electrochemical cell is provided which has at least the following features: i) the electrochemical cell comprises an anode and a cathode; ii) the electrochemical cell comprises a cathode separator and an anode separator; iii) the electrochemical cell comprises an anodic compartment, a central compartment and a cathodic compartment; iv) the cathode separator separates the central compartment from the cathodic compartment; v) the anode separator separates the central compartment from the anodic compartment;vi) the cathode separator contains an inorganic material which is electrically insulating and which has a conductivity for anions and a conductivity for cations, wherein the conductivity for cations is greater than the conductivity for anions and wherein the conductivity for Li cations (Li; + ) is greater than the conductivity for cationic impurities (Me m+ ); vii) the anode separator contains an organic material which has conductivity for anions (X n- , OH and a conductivity for cations, with the conductivity for anions (X nOH j is greater than the conductivity for cations; viii) the inorganic material and / or the organic material is electrically insulating; b) a catholyte is provided in the cathodic compartment, the catholyte containing at least: water (H2O), Li cations (Lrj, hydroxide ions (OH ; c) a central electrolyte is provided in the central compartment, the central electrolyte containing at least: water (H2O), Li cations (Lrj, anions (X n j and cationic impurities (Me m+ ); d) An anolyte is provided in the anodic compartment, the anolyte containing at least: water (H2O) and anions (X nj; e) At least one electrical voltage source is provided which can be connected to the anode via a first electrical line and to the cathode via a second electrical line; f) The electrochemical cell is subjected to an electrical voltage U obtained from the electrical voltage source in such a way that an electrical current / flows between the anode and the cathode.

[0018] A basic idea of ​​the inventive method is to use an inorganic cathode separator that exhibits ion selectivity in favor of lithium. This means that the material from which the cathode separator is made has a higher conductivity for Li+ cations than for other cations such as Na + or other cationic impurities Me m+This results in fewer cationic impurities entering the cathodic compartment and being converted into undesirable by-products, which increases the current yield and improves the purity of the target product.

[0019] As an inorganic material, which has a higher conductivity for Li +than for other cations, so-called LiSICons are preferably used. LiSICon stands for Lithium Super Ionic Conductor. This is a class of inorganic, (glass) ceramic material that is electrically insulated but also has intrinsic conductivity for Li ions. The transport mechanism for Li is based on the crystal structure of the material. Simply put, the Li ions are "passed through" the crystals. Commercially available LiSICon materials include lithium aluminum titanium phosphate (LATP), lithium aluminum titanium silicon phosphate (LATSP), lithium aluminum germanium phosphate (LAGP), and lithium lanthanum titanium oxide (LLTO). These materials were originally developed as solid-state electrolytes for LiB. An overview of the transport mechanisms of LiSICons, their crystal structure, and production is provided by:

[0020] Palakkathodi Kammampata et al.: Cruising in ceramics — discovering new structures for allsolid-state batteries — fundamentals, materials, and performances. Ionics 24, 639-660 (2018) DOI: 10.1007 / S11581-017-2372-7.

[0021] Yedukondalu Meesala et al.: Recent Advancements in Li-Ion Conductors for All-Solid-State Li-Ion Batteries. ACS Energy Lett. 2017, 2, 12, 2734-2751 DOI: 10.1021 / acsenergylett.7b00849.

[0022] Spezielle LiSICon Stöchiometrien werden beschrieben von:

[0023] Sofia Saffirio et al.Lh 4AI04Geo4Tii 4(PO4)3 promising NASICON-structured glass-ceramic electrolyte for all-solid-state Li-based batteries: Unravelling the effect of diboron trioxide, Journal of the European Ceramic Society, Volume 42, Issue 3, 2022, Pages 1023-1032 DOI 10.1016 / j.jeurceramsoc.2021.1 1.014.

[0024] Eongyu Yi et al. Materials that can replace liquid electrolytes in Li batteries: Superionic conductivities in Li1.7AI03Ti1.7Si04P2.6O12. Processing combustion synthesized nanopowders to free standing thin films. Journal of Power Sources, Volume 269, 2014, Pages 577-588, DOI 10.1016 / j.jpowsour.2014.07.029.

[0025] Due to their selective conductivity for Li ions, LiSICon materials can be used as membranes to separate lithium from Li-containing mixtures. The lithium must be present in the mixture in ionic form, for example, as a Li salt dissolved in water.

[0026] It is generally known in the art to use LiSICon to separate lithium from aqueous streams:

[0027] WO 2019055730 A1 roughly describes the selective extraction of lithium on a LiSICon membrane.

[0028] In WO 2022157624 A1, a three-chamber LLTO membrane cell is used for the electrodialysis of seawater for the purpose of extracting lithium.

[0029] US 2012103826 A1 describes both a two-chamber cell with a LiSICon membrane and a three-chamber cell with compartments separated by an organic cation exchange membrane and an organic anion exchange membrane, respectively. These cells are used to produce lithium products.

[0030] A key aspect of the process according to the invention is that it is carried out in a three-chamber cell. While an electrochemical cell in its simplest design comprises only two compartments separated from each other by exactly one separator, a three-chamber cell has two separators that divide the cell into three compartments. To distinguish between the two separators, we refer to an anode separator and a cathode separator. The anode separator is arranged on the anode side, while the cathode separator is installed on the cathode side. The essential components of a three-chamber cell are therefore an anode, an anode separator, a cathode separator, and a cathode. The first, anodic compartment is formed between the anode and anodic separator. The second, cathodic compartment is formed between the cathodic separator and cathode.The third compartment is created centrally between the anodic separator and cathodic separator and is therefore called the central compartment.

[0031] In addition to the basic functional elements mentioned, the electrochemical cell according to the invention can also contain further components, such as catalysts to accelerate water electrolysis, porous transport layers (PTL), flow fields (FF) for transporting the electrolytes, or spacers. Furthermore, the individual functional elements can also be combined to form integrated components, such as membrane electrode assemblies (MEAs). It is also possible for several electrochemical cells to be interconnected to form an aggregate, for example, by series or parallel connection. In the case of a series connection, the anode of one cell can be in direct electrical contact with the cathode of the neighboring cell and vice versa. Directly contacted electrodes of different polarities can also be combined in a bipolar plate.

[0032] In addition to the solid functional elements listed above, the electrochemical cell also contains a liquid electrolyte, which is necessary for its operation. In this case, the electrolyte in the anodic compartment is called the anolyte, while the electrolyte in the cathodic compartment is called the catholyte. The anolyte is therefore in contact with the anode, while the catholyte is in contact with the cathode. The electrolyte in the central compartment is called the central electrolyte. It is separated from the two electrodes by the two separators.

[0033] According to the invention, the electrochemical cell operates as follows: The central electrolyte serves as the feed for the process; it contains the lithium cations that are part of the target product. The central electrolyte is fed from the lithium-containing water to be processed. The synthesis of the target product occurs at the cathode. Consequently, the target product is found in the catholyte. The anolyte serves as a sink for anionic contaminants, from which the cathode separator is protected according to the invention.

[0034] A further essential aspect of the method according to the invention is that two electrochemical processes are carried out simultaneously, namely, on the one hand, a membrane-supported electrodialysis of ions and, on the other hand, an electrolysis of water.

[0035] Electrodialysis serves, on the one hand, to convert the Li cations Li contained in the central electrolyte +in the catholyte. In addition, the anionic impurities contained in the feed X n " electrodialytically enriched in the anolyte to prevent the impurities from coming into contact with the cathode separator. In this way, the cathode separator is protected from being affected by the anionic impurities X n ~ poisoned. To make this possible, the anode separator is anion-conductive.

[0036] In parallel, the process according to the invention involves the electrochemical splitting (electrolysis) of water into hydrogen and oxygen. The hydroxide ions OH- formed as an intermediate during the water splitting combine with the Li+ cations enriched in the catholyte to form lithium hydroxide (LiOH) or its monohydrate (LiOH°H2O), the desired target product.

[0037] The particular advantage of the inventive combination of an anion exchange membrane as an anode separator with an inorganic ion-selective LiSICon membrane as a cathode separator is that the anion exchange membrane protects the LiSICon membrane from the harmful influence of the anions contained in the feed, such as sulfate, carbonate, hydroxide, chloride, and fluoride. These anions are transferred via the organic anion exchange membrane into the anodic compartment, preventing them from damaging the LiSICon membrane. The service life of the LiSICon material used as a cathode separator is thereby significantly improved, especially when the feed contains numerous anionic contaminants. This is especially the case when lithium-containing water originating from recycling or the production of lithium-ion batteries is used as the feed.

[0038] The process according to the invention is preferably carried out continuously. This means that at least the flowable components, namely the electrolytes and the electric current, are provided continuously. The supply of the non-flowable components, such as the cell and the voltage source, is always continuous, even if these components only need to be provided once. The individual process steps of the operation take place simultaneously in a continuous process.

[0039] Preferably, the process is partially run in a basic environment. Specifically, basic conditions should prevail in the central electrolyte and the catholyte. This means that the central electrolyte also contains hydroxide ions (OH-). The pH of the central electrolyte should be between 9 and 12, measured with a glass electrode at a temperature of 25°C. If the central electrolyte contains lithium, its pH must be adjusted prior to treatment. The basicity of the central electrolyte is important because the LiSICon membrane materials used exhibit the greatest stability under these conditions. Acidic conditions lead to material damage over extended periods of operation and must therefore be avoided.

[0040] The anolyte is preferably strongly acidic, or the pH of the anolyte is continuously reduced by at least one of the anode reactions, the formation of protons, and often reaches a pH value of less than 4, measured with a glass electrode at a temperature of 25°C.

[0041] The anions contained in the central electrolyte are specifically sulfate, bisulfate, carbonate, bicarbonate, hydroxide, chloride, or fluoride. These anions are regularly found in lithium-containing waters and thus also in the central electrolyte. There, these anions are more highly concentrated than the hydroxide ions.

[0042] The cationic contaminants also present in the central electrolyte are specifically cations of the following elements: B, Na, Mg, Al, Si, K, Ca, Mn, Fe, Co, Ni, Cu, and C. These are also frequently found in lithium-containing waters. The alkali and alkaline earth metals are particularly prevalent in lithium-containing brines of natural origin, while the metal cations are particularly prevalent in lithium-containing waters resulting from the reprocessing of spent LIBs or from production waste from LIB production. Such streams also often contain carbon compounds originating from adhesives, binders, the anode materials of the LIBs, carbon black, or the plastic packaging of the LIBs. These can then contain carbonaceous acids, or their cations, as well as uncharged organic decomposition products from the upstream steps of LIB recycling.

[0043] According to the invention, the cathode separator is lithium-selective, meaning it conducts Li cations better than other cations. The specific Li + -Conductivity G of the inorganic material used as cathode separator should be at least 1 * 10 -5 S / cm or better at least 5*10 5 S / cm or even better at least 10*10 -5 S / cm and maximum 100*10' 5 S / cm. The specific ion conductivity o is measured by impedance spectroscopy. The temperature-dependent value is measured at 23°C. Impedance spectroscopy is performed as follows:

[0044] The measurement setup consists of two cylindrical electrodes between which the sample is placed. To ensure optimal contact with the electrodes and reproducible contract pressure, a weight is placed on the sample.

[0045] A potentiostat (ZAHNER-elektrik I. Zahner-Schiller GmbH & Co. KG, Kronach-Gundelsdorf, Germany) is connected to the electrodes and controlled by Thales software (ZAHNER). Measurements are performed in a frequency range of 1 Hz to 4 MHz and an amplitude of 5 mV using polished samples onto which a thin, conductive gold layer has been sputtered.

[0046] The measurement results are presented in Nyquist diagrams and evaluated using the Analysis software (ZAHNER). The electrical resistance is read from the maximum of the Nyquist diagram curve. The specific ionic conductivity o [mS / cm] is then calculated using the formula o=(h-10 4 ) / (R-TT / 4-d 2 ), where h is the height of the sample in mm, R is the measured electrical resistance in Q and d is the diameter of the sample in mm.

[0047] This Li selectivity and conductivity is achieved by most LiSICon materials. Therefore, the cathode separator is preferably manufactured using LiSICon material, contains a LiSICon, or even consists entirely of a LiSICon.

[0048] Specifically, the following LiSICons come into consideration:

[0049] • LATP with the following stoichiometry: Lii+ x Al x Ti2- x (PO4)3 where 0.1^x<0.3, preferably x=0.3.

[0050] • LATSP with the following stoichiometry: Lii+ x +yAl x Ti2-xSi y P3-yOi2 where: 0.1 <x<0.3 und 0.2<y<0.4.

[0051] • LAGTSP with the following stoichiometry: Lii+x+yAl x Ti2-xSi y P3- y Oi2 * nGeÜ2 where: 0 <x<1 und 0<y<1 und 0<n<1

[0052] • LAGTP with the following stoichiometry: Lii 4AI04(Gei- x Ti x )i .6 (PO4)3 where: 0 <x<1.

[0053] • LAGP with the following stoichiometry: Lii+ x AlxGe2-x (PO4)3 where x=0 or x=0.2 or x=0.4.

[0054] • LLTO with the following stoichiometry: Li3xLa(2 / 3)- x n(i / 3)-2xTiO3 where: 0 <x<0.16.

[0055] • Doped LLZO with the following stoichiometry: Lie 4La3Zr1 4M06O12 where M is selected from the group consisting of the following elements: Ta, Sb, Nb.

[0056] • Undoped LLZO with the following stoichiometry: Li?La3Zr20i2

[0057] LiSICons are commercially available, for example the LAGP Ampcera™ from MSE Suplies®, Tuscon, USA.

[0058] The material from which the anode separator is made or what it contains is an anion-conductive organic material. Such materials are also often referred to as ionomers. This is preferably a polymer having a backbone to which at least one cationic functional group is bonded. The latter is preferably a quaternized trialkyl ammonium salt. Polystyrene, polysulfone, poly(ether sulfone) or poly(phenylene oxide), polyvinylidene fluoride, or polytetrafluoroethylene is preferably used as the backbone. Very particularly preferably, the quaternized trialkyl ammonium salt is bonded via a benzyl(methyl) group to a backbone made of polystyrene, polysulfone, poly(ether sulfone) or poly(phenylene oxide). The anion-conductive polymer thus obtained is used in the production of the anode separator, is contained in the anode separator, or the anode separator consists entirely of this polymer.

[0059] Such polymers are commercially available. Examples include Fumasep FAPQ from Fumatech, Neosepta membranes from ASTOM, Selemion membranes from AGC, and AHA membranes from Eurodia Industrie SAS.

[0060] A preferred embodiment of the invention provides for the use of an electrochemical cell with an auxiliary cathode. The auxiliary cathode is contacted with the central electrolyte and connected to the electrical voltage source via a third electrical line.

[0061] The electrochemical cell equipped with the auxiliary cathode therefore has the following features: ix) the electrochemical cell comprises an auxiliary cathode; x) the auxiliary cathode is in contact with the central electrolyte; xi) the auxiliary cathode can be connected to the electrical voltage source via a third electrical line.

[0062] The auxiliary cathode opens up the possibility of operating the cell in particularly preferred operating modes, which are characterized by different operating states.

[0063] In a first variant, the operation has at least two operating states, namely: p) a production state in which the anode and the cathode are connected to the electrical voltage source via the first and the second electrical lines, respectively, and in which the anode and cathode are subjected to the electrical voltage U, so that the electrical current / flows between the anode and the cathode; r) a regeneration state in which the anode and the auxiliary cathode are connected to the electrical voltage source via the first and the third electrical lines, respectively, and in which the anode and the auxiliary cathode are subjected to the electrical voltage U, so that the electrical current / flows between the anode and the auxiliary cathode.

[0064] In the production state, the Li is separated from the central electrolyte and LiOH is formed. In the regeneration state, the cathode separator is regenerated. This process flushes the cathode separator of surface-concentrated metal ions that cannot pass through it, since the electrical voltage no longer impedes their removal. This cleanses the membrane surface of impurities, largely restoring the original permeance and permeability.

[0065] According to a preferred embodiment of the first operating variant, the two operating states (production / regeneration) alternate. The regeneration phase is significantly shorter than the production phase. Specifically, the duration of the production state tp should be more than 10 or even more than 100 times as long as the duration of the regeneration state tp. Therefore, tp > f* tp applies, with f greater than 1, with f greater than 10, or with f greater than 100.

[0066] Instead of only temporarily switching on the auxiliary cathode, it is also possible to leave it switched on permanently, thus operating in a combined production-regeneration mode. In this second operating mode variant, the auxiliary cathode is designed to be smaller in area than the actual cathode. Specifically, the auxiliary cathode area AAK should be selected such that AK > f* AAK with f greater than 1, f greater than 10, or f greater than 100. The area K is the area of ​​the cathode. The area factor f here corresponds to the time factor f of the alternating operating mode.

[0067] In the combined production and regeneration state, the anode is connected to the electrical voltage source via the first electrical line and the cathode and the auxiliary cathode via the second electrical line, and the anode, cathode and auxiliary cathode are subjected to the electrical voltage U, so that the electrical current I flows between the anode, cathode and auxiliary cathode.

[0068] The advantage of the combined production and regeneration mode is that the means for switching between the production modes (switching relays) are eliminated. The disadvantage is that the ratio f cannot be changed as easily.

[0069] A third operating mode with two operating states is also advantageous: a combined production and regeneration state and a pure production state. This involves alternating between the production state (p) and the combined production and regeneration state (k), with a single production state (p) being carried out for a time period of tp and a single combined production and regeneration state (k) being carried out for a time period of t«, where tp > g* tK with g greater than 50 or with g greater than 500 or with g greater than 5000. The factor g is significantly higher here than in the other two operating modes (factor f). This results in a particularly long production period. The productivity of the electrochemical cell is thus increased in the long term.

[0070] The auxiliary cathode is preferably located outside the central compartment. This way, it doesn't take up any space within the cell and doesn't interfere with the ion exchange. It is sufficient for the auxiliary cathode to be in contact with the central electrolyte. Surprisingly, it is sufficient to place the auxiliary cathode in the central electrolyte reservoir or in the central electrolyte supply line.

[0071] The auxiliary electrode is preferably made of a textile material. These include linear textile structures such as threads, yarns, wires, or fibers, as well as flat textile structures such as woven, knitted, warp-knitted, nonwoven fabrics, scrims, felt, or fleece. The textile material must be electrically conductive and cathodically active. This can be achieved, for example, with nickel-containing materials. In the simplest case, a nickel-containing stainless steel is used as the material for the auxiliary cathode. Pure nickel can also be used. The material is usually used as a wire or wire mesh. Of course, pure titanium or higher-quality electrode materials such as Ti, Pt, Nb, or similar can also be used.

[0072] When the electrochemical cell is operated according to the invention, electrolysis of water and electrodialysis of anions occur according to the electrochemical model. This is noticeable by the formation of oxygen at the anode and hydrogen at the cathode, as well as by the depletion of anions in the central electrolyte and their enrichment in the anolyte. Furthermore, operation according to the invention leads to the electrochemical synthesis of lithium hydroxide and / or lithium hydroxide monohydrate at the cathode. These target products can be separated from the catholyte or even precipitate spontaneously within it. All of this occurs simultaneously.

[0073] The operation of an electrochemical cell described here preferably comprises electrolysis of water (H2O) and electrodialysis of anions (X n ). Particularly preferably, the operation also includes a synthesis of lithium hydroxide and / or lithium hydroxide monohydrate (LiOH°H2O).

[0074] The method according to the invention will now be explained in more detail using exemplary embodiments. For this purpose, the following are shown:

[0075] Fig. 1 : Basic structure of three-chamber cell;

[0076] Fig. 2: Plant with three-chamber cell in continuous operation;

[0077] Fig. 3: Membrane dialysis in operation;

[0078] Fig. 4: Water electrolysis in operation;

[0079] Fig. 5p: Three-chamber cell with full-surface auxiliary cathode in production mode; Fig. 5r: Three-chamber cell with full-surface auxiliary cathode in regeneration mode;

[0080] Fig. 6: Three-chamber cell with reduced auxiliary cathode in combined operation;

[0081] Fig. 7: Three-chamber cell with perforated auxiliary cathode in combined operation;

[0082] Fig. 8: Three-chamber cell with upstream auxiliary cathode in combined operation.

[0083] Figure 1 shows the basic structure of an electrochemical three-chamber cell as operated in the process according to the invention.

[0084] The electrochemical cell 0 comprises two electrodes, namely an anode 1 and a cathode 2. Two separators are arranged between anode 1 and cathode 2, namely an anode separator 3 and a cathode separator 4. The anode separator 3 is closer to the anode 1 than the cathode separator 4, while the cathode separator 4 is closer to the cathode 2 than the anode separator 3. Between the electrodes 1, 2 and separators 3, 4 there are three compartments 5, 6, 7 within the electrochemical cell 0, which is why it is also called a three-chamber cell. The first compartment 5 extends between anode 1 and anode separator 3 and is therefore called the anodic compartment 5. The second compartment 6 extends analogously between cathode separator 4 and cathode 2 and is therefore called cathodic compartment 6.In the center of the electrochemical cell 0 is the third compartment 7, which is accordingly referred to as the central compartment 7. The central compartment 7 is bordered on one side by the anode separator 3 and on the other side by the cathode separator 4.

[0085] The materiality of the separators 3, 4 is important: The anode separator 3 must have conductivity for anions. If the anode separator 3 also has conductivity for cations, the conductivity for cations should be lower than that for anions. This is the case for most anion-conducting materials. Suitable anion-conducting materials are organic in nature. Examples are polymers that have a backbone to which at least one cationic functional group is bound. The cationic functional group enables intrinsic transport of anions, in particular hydroxide ions (OH'), through the anode separator 3, while cations such as protons (H +) can hardly overcome the anode separator 3. The cathode separator 4 is configured in the opposite way: It has a higher conductivity for cations than for anions. Furthermore, the cation conductivity of the cathode separator 4 is selective: The conductivity for Li cations (Li+) must be greater than for other cations such as Ca+ or Na+. This means that the cathode separator 4 preferentially allows Li cations to pass through, while other cations are transported at least more slowly or not at all. The cathode separator 4 is practically impermeable to anions. Suitable materials that exhibit these properties are inorganic LiSICons, a special type of (glass) ceramic with intrinsic conductivity for Li ions. At least one of the two separators 3, 4 must be electrically insulating. This is the case with inorganic LiSICons.If an inorganic LiSiCon is used as the cathode separator, the electrical conductivity of the organic anode separator 1 is of secondary importance. However, an organic anode separator that also has low electrical conductivity is preferred.

[0086] Specifically, the specific conductivity for electrons y (electrical conductivity) of the electrically insulating separator at a temperature of 23°C should be less than 10~ 7 S / cm (10~ 9 S / m) or less than 10 -12 S / m or less than 10 -16S / m. Such values ​​are typical for inorganic materials; from an electron-conducting perspective, they qualify as non-conductors. The LiSiCons described exhibit specific conductivities in this range and are therefore considered electrical insulators. Ionomers used as anode separators do not always achieve these specific conductivities. Therefore, the cathode separator is preferably used as an electrical insulator. The specific conductivity for electrons y is measured using impedance spectroscopy, as described above for ionic conductivity.

[0087] The electrical conductivity of at least one of the two separators 3, 4 is important because the two electrodes 1, 2 must be electrically insulated from one another. This prevents an electrical short circuit from occurring within the electrochemical cell 0 when an electrical voltage U is applied to the electrodes 1, 2. To apply such a voltage U, an electrical voltage source 8 is provided, which can be electrically connected to the anode 1 or the cathode 2 via a first electrical line 9 and a second electrical line 10. The circuit is selected such that the positive pole (+) of the electrical voltage source 8 is connected to the anode 1, while the negative pole (-) is connected to the cathode 2. Consequently, when the voltage source 8 is switched on, the cathode 2 is negatively charged, while the anode 1 is positively charged. The switching of the voltage source 8 and the level of the voltage U will be explained in more detail later.

[0088] In addition to the solid components of the electrochemical cell 0 described so far, it also comprises flowable elements in the form of three electrolytes 11, 12, 13. The first electrolyte 11 is located in the first, anodic compartment 5 and is referred to as anolyte 11. The second electrolyte 12 is located in the second, cathodic compartment 6 and is analogously referred to as catholyte 12. The central electrolyte 13 is arranged in the central compartment 7. All electrolytes 11, 12, 13 are flowable but not necessarily completely liquid. They can be multiphase mixtures with a liquid, a solid, and a gaseous phase. However, all three electrolytes contain at least liquid water. The exact composition of the individual electrolytes 11, 12, 13 will be explained later.It is important to understand that the composition of anolyte, catholyte and central electrolyte constantly changes during operation of the electrochemical cell 0.

[0089] Furthermore, the three electrolytes 11, 12, 13 can preferably be replaced continuously, so that despite the changing composition, a steady flow process through the three compartments 5, 6, 7 of the electrochemical cell 0 is established. This is illustrated in Figure 2 using a schematic setup of a system 14.

[0090] Within the system 14, the electrochemical cell 0 is divided into two circuits 15, 16, namely an anodic circuit 15 and a cathodic circuit 16. In the anodic circuit 15, the anolyte 11 circulates through the anodic compartment 5, while the catholyte 12 circulates through the cathodic compartment 6. The central electrolyte 13 does not circulate but flows through the central compartment 7 only once in a straight passage 17.

[0091] Unlike the configuration shown in Figure 2, it is also possible to circulate the central electrolyte 13 through the central compartment 7. This requires a large buffer tank for the central electrolyte, from which the central compartment 7 is supplied with central electrolyte. The central electrolyte withdrawn from the central compartment 7 is recycled to the buffer tank. During operation, the lithium concentration in this central circuit would continue to fall until a "limit concentration" is reached. After that, the entire contents of this buffer tank would be replaced, and the next batch would be processed and the lithium separated.

[0092] With regard to the circulation of anolyte 11 and catholyte 12 shown in Figure 2, it should be noted that these two electrolytes 11, 12 are provided continuously, namely in their respective circuits 15, 16. The central electrolyte 13 is also provided continuously in the system 14 shown in Figure 2, but in a straight pass 17. Alternatively, it would be conceivable not to allow all three electrolytes 11, 12, 13 to flow and accordingly to provide them only once in their respective compartments 5, 6, 7. This would be a batch process, which is not preferred. A mixed continuous / batch operation is also conceivable, in which, for example, anolyte 11 and catholyte 12 circulate continuously, while the central electrolyte is only presented once as a batch in the central compartment 7. It is also conceivable to provide only the catholyte 12 as a batch and to continuously replace the anolyte 11 and central electrolyte 13.

[0093] Therefore, when reference is made here to providing the electrolytes, this includes both the one-time provision (batch) and the continuous provision in the circuit 15, 16 or in a straight pass 17. The operation of the electrochemical cell according to the invention also includes mixed forms of one-time and continuous provision (continuous / batch operation) of the electrolytes 11, 12, 13.

[0094] The operation of the electrochemical cell 0 according to the invention serves to process lithium-containing water. Consequently, the system 14 shown in Figure 2 is a system for processing lithium-containing water. The lithium-containing water is fed into the central compartment 7 as fresh water 18. From a process engineering perspective, the fresh water 18 is to be regarded as a feed. Within the electrochemical cell 0, when the voltage U is switched on, various electrochemical processes take place, which will be explained in more detail later. These lead to the lithium contained in the fresh water 18, or more precisely, the lithium cations Li dissolved therein +, are depleted from the fresh water 18 and enriched in the catholyte 12. As the lithium is depleted from the fresh water 18, it becomes wastewater 19, which is withdrawn from the central compartment 7. The lithium concentration in the wastewater 19 is therefore lower than the lithium concentration in the fresh water 18. The wastewater 19 is therefore rather low in lithium. From an electrochemical perspective, both the fresh water 18 and the wastewater 19 can be regarded as the central electrolyte 13. The transition of the fresh water 18 into the wastewater 19 due to the depletion of lithium is to be understood and understood as the previously discussed change in the composition of the electrolyte during cell operation. The anolyte 11 and the catholyte 12 also change their composition as a result of the electrochemical processes:

[0095] In the catholyte 12, Li is enriched so that the concentration of lithium in the catholyte 12 +, which is withdrawn from the cathodic compartment 6, is greater than in the catholyte 12 which is fed into the cathodic compartment 6. The withdrawn catholyte 12 + is lithium-rich, while the supplied catholyte 12' is low in lithium. To enable the recirculation of the catholyte 12, a primary separation device 20 is incorporated in the cathodic circuit 16, which separates the desired lithium compound, namely lithium hydroxide LiOH or lithium hydroxide monohydrate (LiOH^2 H 2 O), from the lithium-rich catholyte 12. + separated, leaving the low-lithium catholyte 12' behind and recycled into the cathodic compartment 6. From a process engineering perspective, the separated lithium hydroxide (monohydrate) represents the target product of the process.

[0096] Analogously, a secondary separation device 21 is incorporated in the anolytic circuit 15, which serves to separate unwanted anions X contained in the fresh water 18. n' from the process. The unwanted anions X n ' These are, for example, sulfates or chlorides or other monovalent or multivalent negatively charged ions. These ions originate from salts dissolved in the fresh water 18. To ensure that the charge balance remains constant and to prevent anions from concentrating in the central electrolyte or even unwanted anode reactions such as chlorine gas (Cl2), these anions are depleted from the central electrolyte 13 and enriched in the anolyte 11. The concentration of these anions X n ' therefore increases in the anolyte 11, so that the concentration of anions X n ' in the anolyte 11 withdrawn from the anodic compartment + is greater than in the supplied anolyte 1 T. The withdrawn anolyte 1 1 +is therefore rich in these anions, while the supplied anolyte 1 T is anion-poor. The exact process by which the anions are enriched in the anolyte will be explained later. However, it is important to note that the anions X n ' in the anolyte 11 can combine to form other substances, especially acids, salts, or molecular gases, depending on the composition of the anolyte. The anions can therefore also be separated by the secondary separation apparatus 21 as part of such a compound. If these compounds are harmless, the removed anolyte 11 can be + also be disposed of as secondary wastewater. In this case, the anolytic circuit 15 is eliminated, and the secondary separation apparatus 21 would be dispensable.

[0097] If a suitable acid such as sulfuric acid H2SO4 or hydrochloric acid HCl is formed in an anode reaction, the withdrawn anolyte 1 1 +can also be used to treat black mass: Black mass is a mixture that results from the disintegration of used LIBs. In addition to lithium, it also contains the electrode materials typically found in LIBs: Ni, Mn, Co, Al, Fe, and graphite, as well as copper as a conductor material. Using these acids, the metals contained in the black mass can be dissolved. An acidic, extracted anolyte 11 + could thus be used in upstream process stages of LIB processing.

[0098] As already mentioned, during the operation of the electrochemical cell 0 according to the invention, various electrochemical processes take place simultaneously. Which processes occur precisely depends on the precise composition of the electrolytes 11, 12, 13. In any case, however, membrane-supported electrodialysis of the ions present and electrolysis of water take place simultaneously. For better understanding, both processes will now be explained separately: electrodialysis using Figure 3 and electrolysis using Figure 4.

[0099] The electrodialysis shown in Figure 3 serves on the one hand to convert the Li cations Li contained in the central electrolyte 13 + in the catholyte 12. In addition, the unwanted anions X contained in the central electrolyte 13, originating from the fresh water 18, are n-electrodialytically enriched in the anolyte 11 to prevent these anions from coming into contact with the cathode separator 4. In this way, the cathode separator 4 is protected from being affected by the anions X n ~ and poisoned by products of the anode reaction. In addition, the depletion of anions X n- from the central electrolyte 13, the formation of undesirable compounds of these anions with the free Li cations. This increases the yield of the target compound lithium hydroxide LiOH.

[0100] The electrodialysis shown in Figure 3 and described here is enabled by the special ionic conductivity of the separator materials used and driven by the applied electrical voltage U. The polarity of the electrical voltage source 8 is selected such that the cathode 2 is negatively charged, while the anode 1 is positively charged. According to generally accepted models of electrochemistry, the negatively charged anions migrate toward the anode 1, whereas the positively charged cations move in the opposite direction toward the cathode 2. To balance the charge, an electric current / flows along the electrical lines 9, 10 from the anode 1 to the cathode 2.

[0101] The anions X n~ and OH' can pass from the central electrolyte 13 into the anolyte 11 because the anode separator 3 located on the path has an intrinsic anion conductivity. Since the hydroxide ions OH' are usually less concentrated in the central electrolyte 13 than other anions, primarily non-hydroxide ions migrate through the anode separator 3. Vice versa, the Li cations Li + due to the intrinsic lithium ion conductivity of the cathode separator 4 from the central compartment 7 into the catholyte 12. Since the cathode separator 4 is made of a material that has a higher conductivity for Li cations than for other cations, other monovalent or polyvalent metal cations Me m+, which are contained as impurities in the fresh water 18, hardly leave the central electrolyte 13 in the direction of the catholyte 12, although they also strive towards the cathode 2 due to their positive charge. Only a small part of the cationic impurities Me m+ succeeds in overcoming the cathode separator 4 and forming undesirable by-products in the catholyte 12 (not shown). The majority of the cationic impurities Me m+ remains in the central electrolyte 13 and is discharged from the process with the wastewater 19. Thanks to the special cation selectivity of the cathode separator 4 in favor of lithium, the purity of the target product LiOH / LiOH°H2O is increased and the power efficiency of the process is improved, since hardly any electrical energy is wasted on removing the cationic impurities Me m+ from the central compartment 7 to the cathodic compartment 6.

[0102] In parallel to the electrodialysis shown in Figure 3, the electrochemical splitting (electrolysis) of water H2O into hydrogen H2 and oxygen O2 shown in Figure 4 takes place in the process according to the invention. More precisely, two water splitting processes are practiced in parallel, namely a basic water splitting according to equation (1) in the catholyte and an acidic water splitting according to equation (2) in the anolyte.

[0103] 2 H2O + 2 e ' — >• H2 + 2 OH' (1) Reduction / Cathode reaction

[0104] H2O — > 1 O2 + 2H + + 2 e' (2) Oxidation / anode reaction

[0105] In the present process, the electrochemical cell 0 is filled with a basic, water-based electrolyte, and a voltage is applied between anode 1 and cathode 2. In this case, the basic electrolytes are the central electrolyte 13 and the catholyte 12. Consequently, it must be ensured that the central electrolyte 13 and the catholyte 12 are provided in the alkaline range. Since the starting materials used as electrolytes can be acidic depending on their origin, these electrolytes must be made alkaline. The corresponding alkaline, lithium-containing water is accordingly fed into the central compartment 7 as fresh water 18. The alkalinity of the catholyte 12 is preferably ensured by a minimum amount of lithium hydroxide dissolved in the catholyte 12. The hydroxide ions OH' originating from the LiOH determine the alkalinity of the catholyte 12.The minimum amount of LiOH is ensured by operating the primary separation device 20 in such a way that the LiOH is not completely removed from the cathodic circuit 17. To start the process, a starting dose of LiOH is added to the catholyte 12. On the cathode side of the three-chamber cell, i.e., in the central electrolyte 13 and the catholyte 12, the water H2O is decomposed into hydrogen H2 and hydroxide ions OH- (Equation 1). The anode separator 3 primarily transports anions X. n ' into the anodic compartment 5, where they can be oxidized to corresponding compounds such as chlorine in the case of chloride ions. Furthermore, water H2O is oxidized at the anode 1 and protons H +and oxygen O2 (Equation 2). In this way, oxygen O2 is formed on the anode side, while hydrogen H2 is formed on the cathode side. At the anode, the protons also form the corresponding acids of the anions, such as hydrochloric acid or sulfuric acid, each in a dissociated form.

[0106] In addition, the presence of the hydroxide ions OH' in the catholyte 12 causes them to react with the Li cations Li + to form lithium hydroxide LiOH (equation 3).

[0107] Li+ + OH- ->■ LiOH (3) Synthesis of LiOH

[0108] The LiOH is initially dissolved, i.e. as Li + Cations and hydroxide ions OH'. As soon as the concentration of these ions reaches the saturation limit, solid lithium hydroxide (LiOH) or lithium hydroxide monohydrate (LiOH°H2O) precipitates, the target product of the process.

[0109] The target product is obtained from the catholyte 12 via the primary separation device 20. Hydrogen H2 and oxygen O2 are byproducts. Due to their gaseous form, the two byproducts easily escape from the catholyte 12 and the anolyte 11, respectively, and can be collected and utilized accordingly. A separation device for the byproduct hydrogen H2 or oxygen O2 is not absolutely necessary, but is conceivable.

[0110] To understand the overall process, it is important to recognize that the Li cations Li + , which form LiOH in the catholyte 12, only entered the catholyte 12 by means of membrane-supported electrodialysis (Figure 3). The hydroxide ions OH', which in the catholyte 12 react with the migrated Li cations Li according to equation 3, +are formed in the catholyte 12, namely by the cathode reaction of water splitting according to equation 1 (Figure 4). The inventive operation of the electrochemical cell 0 is therefore based on the simultaneous implementation of water electrolysis and membrane-supported electrodialysis.

[0111] A particular embodiment of the operation according to the invention provides that the electrochemical cell is operated with an auxiliary cathode. Figures 5p, 5r, 6, 7, and 8 each show a configuration of a three-chamber cell with an auxiliary cathode. Figures 5p and 5r show a full-surface auxiliary cathode 221, while Figures 6, 7, and 8 each show a reduced-size auxiliary cathode 222. In all cases, the auxiliary cathode 221 / 222 is in contact with the central electrolyte 13. The auxiliary cathode 221 shown in Figures 5p, 5r has the same surface area as the other two electrodes 1, 2. The auxiliary cathode 221 can be connected to the negative pole of the voltage source 8 via a third electrical line 23 (Figure 5r). The connection between the negative pole of the voltage source 8 and the cathode 2 via the second electrical line 10 is then interrupted, so that the current I flows between the anode and the auxiliary cathode 221.In this operating state, shown in Figure 5r, the electrochemical cell 0 is in regeneration mode. During regeneration, hydroxide ions OH' are formed at the auxiliary cathode 221 according to equation (1). These ions capture protons H. + by forming a water molecule (H2O). In this way, the pH value in the central electrolyte 13 is increased to operate in the desired alkaline range of pH 9 to 10.5. Regeneration mode is always interrupted when the pH value has dropped to an undesirable, acidic value below 8. Adjusting the pH value via the auxiliary electrode 221 is very quick: Regeneration mode only needs to take about 1 / 100 of the time of production operation. Adding alkaline compounds to the central electrolyte 13 is therefore unnecessary.

[0112] After regeneration is complete (Figure 5r), the system switches back to production mode (Figure 5p). In this mode, the negative pole of the electrical voltage source 8 is connected to the cathode 2 via the second electrical line 10. Current I flows between anode 1 and cathode 2. The auxiliary cathode 221 is de-energized.

[0113] Instead of alternating between production and regeneration, it is also possible to continuously raise the pH value using a reduced-size auxiliary electrode 222, which is also contacted with the central electrolyte 13 (Figure 6). The area of ​​the auxiliary electrode 222 is smaller than that of the other two electrodes 1, 2, for example, only approximately 1 / 100. The auxiliary electrode 222 is permanently connected to the negative pole of the electrical voltage source 8 via the second electrical line 10; the second electrical line 10 is branched accordingly. Thus, the current I flows continuously between the anode 2 and the auxiliary cathode 222 and the cathode 2. Due to the smaller area of ​​the auxiliary cathode 222, only a small amount of hydroxide ions OH' are formed in the central electrolyte 13, enough to keep the pH value constant in the desired basic range of 9 to 10.5. The addition of basic substances is then not necessary.Continuous operation with the reduced auxiliary cathode 222 is to be regarded as a combined regeneration-production state.

[0114] A particular advantage of the reduced-size auxiliary cathode 222 is that it can also be placed outside the electrochemical cell 0, for example, in the supply line of the central electrolyte 13. This saves space in the central compartment 7. A corresponding embodiment is shown in Figure 8. It is also possible to alternate the reduced-size auxiliary cathode 222 between a pure production state and a combined regeneration-production state. This is a mixture of the operating states shown in Figures 5p, 5r, and 6. This allows for greater flexibility in dimensioning the area of ​​the reduced-size auxiliary cathode 222 and the timing. This mixed operation is not shown.

[0115] Figure 7 shows another embodiment of an electrochemical cell 0 with a reduced-size auxiliary cathode 222. The reduction in size of the auxiliary cathode 222 is achieved by using a porous textile as the auxiliary cathode, such as a mesh fabric. Due to its porosity, the auxiliary cathode 222 has a surface area AK that is smaller than the surface area AK of the cathode 2. This is not directly apparent in Figure 7 because the textile auxiliary cathode 222 extends through the entire central compartment 7. The advantage of using a textile auxiliary cathode is that it has better permeability for the ions than a full-surface auxiliary cathode and therefore poses less of an obstacle to ion exchange.

[0116] Figure 8 shows yet another embodiment of an electrochemical cell 0 with a reduced-size auxiliary cathode 222. Here, the auxiliary cathode 222 is arranged outside the central compartment 7, more precisely, in a supply line 24 for the central electrolyte 13. The central electrolyte 13 flows in a straight passage 17 through the supply line 24 into the central compartment 7 and leaves it again (not shown). The advantage of arranging the auxiliary cathode 222 in the supply line 24 is that the ions in the central compartment 7 can move unhindered by the auxiliary cathode 222. The contact of the auxiliary cathode 222 with the central electrolyte 13 within the supply line 24 is sufficient.

[0117] Examples:

[0118] The advantages achieved by the process according to the invention will now be demonstrated with the help of experimental data:

[0119] Experimental setup

[0120] The electrochemical cell 0 used to conduct the experiments is shown in Figure 8. It comprises three compartments 5, 6, and 7. Compartments 7 and 6 are separated by an ion exchange membrane, the so-called cathode separator 4. Compartments 5 and 7 are separated from each other by a membrane, the so-called anode separator 3. An anode 1 is arranged in the first compartment 5. In the central compartment 7, an auxiliary cathode 222 is located in the feed line for the central electrolyte 13. A cathode 2 is arranged in the third compartment 6. The first compartment 5 can also be referred to as the anodic compartment, while the third compartment e is referred to as the cathodic compartment. The compartment 7 in the middle can also be referred to as the central compartment 7.

[0121] A first electrical line 9 connects the anode 1 to a voltage source 8. A second electrical line 10 connects the cathode 2 to the voltage source 8. A branch of the second electrical line 10 connects the voltage source 8 to the auxiliary cathode 222. The polarity of the voltage source 8 is selected such that the positive pole of the voltage source 8 is connected to the anode 1, while the negative pole of the voltage source 8 is connected to the cathode 2 and the auxiliary cathode 222.

[0122] An electric current I flows through the electrical lines 9 and 10 as well as via the electrical voltage source 8. Since the ion exchange membrane 4 is electrically insulated, there is no electrical short circuit between the two electrodes 1 and 2 via the ion exchange membrane 4.

[0123] The ion exchange membrane 4 is a flat membrane made entirely of LISiCon material. The other membrane 3 is also a flat membrane, made of an anion-conductive polymer, such as an AHA membrane from Eurodia Industrie SAS or a Neosepta membrane from ASTOM.

[0124] The auxiliary cathode 222 is a wire containing titanium, platinum, or stainless steel. The cathode 2 is also a flat metal sheet containing titanium or nickel. In the simplest case, stainless steel sheet is used as the cathode. Anode 1, cathode 2, and the two membranes 3, 4 have the same shape; they can be rectangular or circular. The auxiliary cathode 222 is a wire whose area, which projects into the supply line 24 for the central electrolyte 13, accounts for 2% of the active area A of the cathode 2. This is not shown to scale in the side view of Figure 8. Instead of sheets, expanded metal, grids, or meshes made of the specified materials can also be used as the electrode.

[0125] The electrochemical cell 0 has an active area A, which corresponds to the surface area of ​​the two membranes 3, 4, the anode 1, and the cathode 2. As mentioned, the auxiliary cathode 222 is smaller, its active surface being only 2 / 100*A.

[0126] During operation, compartments 5 and 7 are supplied with a feed 13. Compartments 5 and 7 can contain the same feed 13, or one of the compartments can contain a solution 11 different from 13. Feed 13 is an aqueous solution containing Li+ cations. From an electrochemical perspective, feed 13 can be considered an anolyte.

[0127] Feed 13 can be a Li liquor from a natural deposit or a material stream resulting from the processing of spent LIB. The concentration of Li+ cations in feed 13 should be at least 200 wt. ppm, based on the total mass of the feed. Seawater has a lower Li concentration and would therefore need to be concentrated before being used in the process. Feed 13 also contains anions such as sulfate or chloride. Feed 13 also contains impurities. Anions and impurities are not shown in Figure 1. The main component of feed 13 is water. The cathodic compartment 6 is supplied with a lean working medium 12'. The lean working medium 12' is water with a low concentration of Li+ cations. The concentration is at least 50 wt. ppm, based on the total mass of the lean working medium 12'. From an electrochemical point of view, the poor working medium 12 can be regarded as a catholyte.

[0128] The electrochemical cell 0 is also subjected to an electrical voltage U supplied from the voltage source 8. This causes the following:

[0129] First, water electrolysis occurs, in which water is electrochemically separated into hydrogen and oxygen. OH- and hydrogen are formed at the cathode 2 and the auxiliary cathode 222. However, the OH- anions cannot overcome the LiSiCon membrane 4 and combine with the Li present in the cathodic compartment 6. + Cations to lithium hydroxide. At the anode 1, oxygen and H + formed. In the central compartment 7 (the one with the auxiliary cathode 222), OH- anions are formed, which keep the pH value in compartment 7 stable.

[0130] The formation of LiOH in the cathodic compartment 6 is maintained by the migration of Li+ cations from the feed 13 toward the cathode 2, driven by the voltage U. They overcome the LiSiCon membrane 4 due to its conductivity for Li ions and accumulate in the working medium 12 (membrane electrolysis). This creates a rich working medium 12. + , which is withdrawn from the cathodic compartment 6. The concentration of Li+ ions in the rich working medium 12 + is larger than in the poor working medium 12'.

[0131] In the electrochemical cell 0, water electrolysis, membrane electrolysis of Li + and a synthesis of LiOH.

[0132] During the simultaneous operation of Li+ membrane electrolysis and water electrolysis in the electrochemical cell, lithium hydroxide LiOH and molecular hydrogen H2 are directly produced. The hydrogen is at least partially dissolved; it can also be present in gas bubbles. Depending on the temperature and the presence of crystallization nuclei, the LiOH precipitates in the cathodic compartment e or immediately after the rich working medium is withdrawn. + out of.

[0133] The feed 13 is depleted of Li+ by membrane electrolysis, producing wastewater 19.

[0134] Experimental procedure

[0135] To carry out the electrolysis, the electrolytes are first poured into the two anolyte and catholyte containers, with the inlet hoses clamped off. The electrolyte container, which supplies the part of the cell directly adjacent to the membrane, is referred to as the central compartment. The next step is to assemble the electrolysis cell. During assembly, the process should be carried out quickly to prevent the membranes from drying out. Once the electrolysis cell is assembled, it is connected to the anolyte and catholyte containers, as well as the central chamber. Care must be taken to ensure that the inlet and return lines are connected to the same side. The inlet lines to the cell can now be carefully opened, with the inlet hoses for the catholyte and central chamber being opened at the same time.

[0136] A round disc with a diameter of 19.5 mm and a thickness of 1 mm was used as the anode and cathode. The material in each case was a titanium expanded sheet coated on both sides with IrTi mixed oxide, 12 g lr / m 2 , 1 AF D1.5 mm from Metakem GmbH, 61250 Usingen, Germany.

[0137] The auxiliary cathode was a 2 mm thick wire that protruded approximately 3 mm into the inlet line of the central compartment. The material was a titanium wire coated with IrTi mixed oxide (12 g Ir / m2) from Metakem GmbH, 61250 Usingen, Germany.

[0138] The anode and cathode are connected to the voltage source, a Keithley 2400 potentiostat from Tektronix UK Ltd., Berkshire, UK. The auxiliary cathode is connected to the cathode.

[0139] The sampled membranes were also circular discs with a diameter of approximately 25 mm. The membrane thickness was approximately 1 mm. The material of the sampled LiSiCon membranes was Ampcera™ LISICON LAGP from MSE Suplies®, Tucson, USA. The material of the sampled organic anion exchange membranes was an AHA membrane from Eurodia Industrie SAS.

[0140] The electrolysis and the corresponding storage vessels are blanketed with nitrogen throughout the entire experiment to prevent the formation of lithium carbonate. Each cell has separate anolyte and catholyte vessels, as well as a separate middle chamber. Each vessel is filled with approximately 1 kg of liquid; the exact mass is determined by backweighing. In all experiments, the catholyte at the beginning of the experiments was always a 5 mmol / L LiOH solution (equivalent to 120 wt. ppm LiOH). The anolyte and the electrolyte in the middle chamber are lithium salt solutions in various concentrations and with different lithium salts. The anolyte vessel and the middle chamber can contain different solutions. The starting concentrations and the exact concentrations vary over the course of the experiment and are therefore also listed in the diagram of the experiments.

[0141] The experiment is started by switching on the pumps and the desired voltage. During the experiment, the voltage is cycled, controlled by the appropriately programmed Siemens LOGO! 230 RC control unit from Siemens. The maximum flow rate is 900 mL / minute and is determined using a SONOFLOW CO.55 / 060 V2.0 ultrasonic flow sensor from Sonotec. Sampling takes place every half hour or at longer intervals as agreed. A 3 mL pre-flow sample is drawn and discarded. With each sample, the current is recorded, and the pH and conductivity of the sample are determined. The samples are then returned to the appropriate containers to keep the volume nearly constant.

[0142] After the experiment, the containers are emptied, and all lines and the membrane are rinsed with deionized water. The cell is disassembled, the membrane is photographed, and SEM images of the catholyte and anolyte sides are taken to document any damage or changes to the membranes. The ion exchange membrane is examined microscopically for any changes. The side facing the middle chamber and the anolyte side are examined.

[0143] The membrane performance is determined by the permeability parameters (gLi*mm / m 2 *h) and permeance (g Li / m 2*h) measured. Permeance indicates how much mass of lithium is transported through the membrane per unit area and time. Permeability also takes membrane thickness into account, thus allowing for comparison between different membrane types with varying thicknesses. Both parameters are necessary for a comprehensive description of performance, as extremely thin membranes would enable extremely high permeance. However, if concentration polarization effects occur in the membrane cell, the permeabilities would be incorrectly represented. Considering membrane thickness is then no longer useful, as transport is not limited by the membrane.

[0144] All measured values ​​shown in the examples are subject to a measurement error of approximately + / - 10%, which can be attributed to inaccuracies in the positioning of the electrodes relative to each other, the determination of the thickness of the membrane samples and the concentration determination via conductivity measurements.

[0145] The concentration was determined inline using a conductivity measurement. The calibration curve shown in Figure 9 translates the conductivity into a concentration in the test results.

[0146] Figure 9: Conductivity as a function of the concentration of a LiOH solution (25°C)

[0147] Consequently, at concentrations above approximately 10% LiOH, it is hardly possible to accurately monitor the actual concentration using conductivity measurements.

[0148] The variations within the experiments are shown in Table 1. Table 1: Experimental overview

[0149] Apart from that, the following constant experimental conditions prevailed:

[0150] Timing: Power on for 50s - Power off for 10s

[0151] Catholyte concentration: 5 mmol / l LiOH - LiOH*H2O, 98%, Thermo Fisher Scientific Voltage: 6 V (between anode and cathode, as well as between auxiliary cathode and anode) Membrane: Ampcera® LISCON LAGP

[0152] Table 1 Experiment 1a:

[0153] The experimental setup was as described above (experimental procedure and detailed design as described in Table 1).

[0154] In the central compartment, an auxiliary cathode in the form of a stainless steel wire mesh (quality 1.4401) with a mesh size of 200|jm was inserted and provided with a wire which enables electrical contact from the outside (see Figure 7).

[0155] When the pumps for the three circuits were started up, a pulsed DC voltage of 6 V was applied between the anode and cathode. During the course of the experiment, the decrease in the pH value of the middle chamber could be monitored through regular sampling. It dropped from pH 9 to 7.3 after approximately 24 hours. The pH of the anolyte was 2.5 after 24 hours, and the current was approximately 30 mA.

[0156] After these 24 hours, the auxiliary cathode was operated exclusively at a voltage of 6 V for a total of 5 minutes. This resulted in a current of up to 200 mA during this period. The pH value of the middle chamber rose to more than 10.

[0157] Experiment 1 b:

[0158] When the cell is operated further under the same settings as in Experiment 1a (Experiment 1b), now again without voltage applied to the auxiliary cathode, the current through the cell increases to more than 70 mA, which is also associated with a correspondingly higher permeance.

[0159] The course of the measuring points from experiments 1 a and 1 b is shown in the diagram in Figure 10.

[0160] Figure 10: Diagram of experiments 1a and 1b

[0161] For a simplified overview, all results / values ​​are summarized in Table 2.

[0162] Table 2: Measured values ​​from the tests

[0163] Experiment 1c:

[0164] Using the same measurement setup, with the only difference being that the auxiliary cathode was now designed not as a wire mesh, but as a wire in the inlet line to the central chamber with a surface area of ​​2% of the electrode and membrane area (Figure 8). The results are also summarized in Table 2. Experiments 2 to 6:

[0165] These experiments were conducted in a setup similar to that used in Experiment 1c. The voltage between the anode and cathode was 6 V, and the voltage between the auxiliary cathode and anode was also 6 V. Point-by-point measurements of the current at the auxiliary cathode yielded values ​​between 1 and 20% of the current flowing between the anode and cathode. All results after 50 hours of cell operation are listed in Table 2.

[0166] Table 2:

[0167] Comparison tests:

[0168] For comparison, cells were constructed without an anion exchange membrane. Table 3 provides an overview.

[0169] Table 3: Comparative test overview

[0170] Table 3: The measured values ​​of the comparative tests are shown in Table 4.

[0171] Table 4: Measured values ​​from comparative tests

[0172] Table 4:

[0173] Attempt 7:

[0174] For comparison purposes, the setup used in Experiments 1 to 6 was modified by omitting the anion exchange membrane between the central compartment and the anodic compartment, so that the cell has only two separate compartments: one anodic compartment and one cathodic compartment. An auxiliary cathode was not used.

[0175] Since the pH value in the anodic compartment decreases over the course of the test due to proton formation at the anode, damage to the ceramic LiSiCon membrane occurs continuously at pH values ​​below 7. As a result, the results presented in Table 4 are not constant values, but rather continue to decrease over the test duration, especially after 5 to 50 hours. As an example, the data were presented after approximately 24 hours.

[0176] Attempt 8:

[0177] The same applies here as in Experiment 7, with the only difference being that a lithium sulfate solution was used for the experiments. Conclusion:

[0178] The experiments demonstrate that the inventive operation of a three-chamber cell with an anion exchange membrane as the anode separator (Experiments 1a to 7) maintains higher permeance and permeability than a two-chamber cell (Experiments 8 and 9) after the same operating time. This means that the same cathode separator wears more rapidly when treated with lithium-containing water if it is installed in a two-chamber cell than in a three-chamber cell. Consequently, higher efficiency can be expected over a long service life when operating the three-chamber cell according to the invention.

[0179] In all experiments, the levels of foreign cations in the catholyte were below the detection limit. The purity of the target product was not affected by cathodic impurities, such as sodium, contained in the central electrolyte.

[0180] Furthermore, a higher current flows during cell operation according to the invention (tests 1a to 7) than during non-inventive operation (tests 8 and 9). Consequently, productivity per unit area is better. This means that a system operated in this way can be smaller while maintaining the same production output.

[0181] A comparison of operation with an auxiliary cathode constructed as a wire mesh (Test 1) with those in which the auxiliary cathode was located in the feed line (Test 2 ff.) shows that the auxiliary cathode in the feed line leads to very good permeance in continuous operation, without the need for regular switching between the different operating modes. This simplification in the cell design, as well as in its implementation, demonstrates that arranging the auxiliary cathode in the feed line is the particularly preferred embodiment.

[0182] List of reference symbols

[0183] 0 electrochemical cell (three-chamber cell)

[0184] 1 anode

[0185] 2 Cathode

[0186] 3 Anode separator

[0187] 4 Cathode separator

[0188] 5 anodic compartment

[0189] 6 cathodic compartment

[0190] 7 central compartment

[0191] 8 electrical voltage source

[0192] 9 first electrical line

[0193] 10 second electrical line

[0194] 11 Anolyte

[0195] 11- Anolyte that is fed in (low in anions)

[0196] 11 + Anolyte, which is removed (rich in anions)

[0197] 12 Catholyte

[0198] 12" catholyte that is fed in (low lithium)

[0199] 12 + Catholyte that is removed (lithium-rich)

[0200] 13 Central electrolyte

[0201] 14 Appendix

[0202] 15 anodic circuit

[0203] 16 cathodic circuit

[0204] 17 straight passage

[0205] 18 Fresh water (containing lithium)

[0206] 19 Wastewater (low lithium)

[0207] 20 primary separation apparatus

[0208] 21 secondary separation apparatus

[0209] 221 full-surface auxiliary cathode

[0210] 222 reduced auxiliary cathode

[0211] 23 third electrical line

[0212] 24 Central electrolyte supply line

[0213] X n- Anions

[0214] Me m+ cationic impurities

[0215] OH' hydroxide ions

[0216] H + Protons

[0217] Li + Lithium cations

[0218] LiOH lithium hydroxide

[0219] H2 hydrogen

[0220] O2 oxygen

[0221] H2O water

Claims

Patent claims 1. A method for operating an electrochemical cell, comprising the following non-chronological steps: a) providing at least the electrochemical cell, which has at least the following features: i) the electrochemical cell comprises an anode and a cathode; ii) the electrochemical cell comprises a cathode separator and an anode separator; iii) the electrochemical cell comprises an anodic compartment, a central compartment and a cathodic compartment; iv) the cathode separator separates the central compartment from the cathodic compartment; v) the anode separator separates the central compartment from the anodic compartment; vi) the cathode separator contains an inorganic material which has a conductivity for anions and a conductivity for cations, wherein the conductivity for cations is greater than the conductivity for anions and wherein the conductivity for Li cations (Li +) is greater than the conductivity for cationic impurities (Me m+ ); vii) the anode separator contains an organic material which has conductivity for anions (X n OH j and a conductivity for cations, with the conductivity for anions (X n OH ) is greater than the conductivity for cations; viii) the inorganic material and / or the organic material is electrically insulating; b) providing a catholyte in the cathodic compartment, wherein the catholyte contains at least: water (H2O), Li cations (Li + ), hydroxide ions (OH'); c) providing a central electrolyte in the central compartment, wherein the central electrolyte contains at least: water (H2O), Li cations (Li + ), anions (X n ') and cationic impurities (Me m+ ); d) providing an anolyte in the anodic compartment, the anolyte containing at least: water (H2O) and anions (X n); e) providing at least one electrical voltage source which can be connected to the anode via a first electrical line and to the cathode via a second electrical line; f) Applying an electrical voltage U obtained from the electrical voltage source to the electrochemical cell in such a way that an electrical current / flows between the anode and the cathode.

2. Method according to claim 1, characterized in that at least some of the steps take place simultaneously and continuously.

3. A method according to claim 1 or 2, characterized in that the central electrolyte additionally contains hydroxide ions (OH ) and that the pH of the central electrolyte is between 9 and 12, measured with a glass electrode at a temperature of 25°C.

4. Process according to one of claims 1 to 3, characterized in that the anions (X nj are selected from the group consisting of sulfate, hydrogen sulfate, carbonate, hydrogen carbonate, hydroxide, chloride, fluoride.

5. A process according to claim 4, characterized in that the anions selected from said group (X n ) are more concentrated in the central electrolyte than hydroxide ions (OH ).

6. Process according to one of claims 1 to 5, characterized in that the cationic impurities (Me m+ ) are cations of elements selected from the group consisting of B, Na, Mg, Al, Si, K, Ca, Mn, Fe, Co, Ni, Cu, C.

7. Method according to one of the preceding claims, wherein the inorganic material contained in the cathode separator has a specific conductivity for Li cations o measured according to the method described here “impedance spectroscopy”, which at a temperature of 23°C is at least 1*10 5 S / cm or at least 5*10' 5S / cm or at least 10*10' 5 S / cm and maximum 100*10 -5 S / cm.

8. Process according to claim 7, characterized in that the inorganic material is a compound of the following stoichiometry (LATP): Lii + xAlxTi2-x(PO4)3 where 0.1^x<0.3, preferably x=0.

3.

9. Process according to claim 7, characterized in that the inorganic material is a compound of the following stoichiometry (LATSP): Lil+x+yAl x Ti2-xSiyP3-yOl2 where: 0.1 <x<0.3 und 0.2<y<0.

4.

10. The method according to claim 7, characterized in that the inorganic material is a compound of the following stoichiometry (LAGTSP): Lii+x+yAlxTi2-xSiyPs-yOi2 * nGeO2 where: 0 <x<1 und 0<y<1 und 0<n<1.

11. Process according to claim 7, characterized in that the inorganic material is a compound of the following stoichiometry (LAGTP): Lil 4Alo.4(Gei-xTix)l .6 (PÜ4)3 where: 0^x<1.

12. The method according to claim 7, characterized in that the inorganic material is a compound of the following stoichiometry (LAGP): Lii+xAlxGe2-x (PÜ4)3 where: x=0 or x=0.2 or x=0.

4.

13. Process according to claim 7, characterized in that the inorganic material is a compound of the following stoichiometry (LLTO): Li3xLa ( 2 / 3)-xn ( 1 / 3)-2xTiO3 where: 0^x<0.

16.

14. The method according to claim 7, characterized in that the inorganic material is a compound of the following stoichiometry (dot. LLZO): Li64La3Zr1.4M06O12 where M is selected from the group consisting of the following elements: Ta, Sb, Nb.

15. The method according to claim 7, characterized in that the inorganic material is a compound of the following stoichiometry (LLZO): Li?La3Zr20i2 16. The method according to any one of claims 1 to 15, characterized in that the organic material contained in the anode separator is a polymer having a backbone to which at least one cationic functional group is bonded.

17. The method according to claim 16, characterized in that the cationic functional group is a quaternized trialkyl ammonium salt.

18. The method according to claim 16 or 17, characterized in that the backbone is selected from the group consisting of polystyrene, polysulfone, poly(ethersulfone) or poly(phenylene oxide), polyvinylidene fluoride, or polytetrafluoroethylene.

19. The method according to claim 16, characterized in that the cationic functional group is a quaternized trialkyl ammonium salt, that the backbone is selected from the group consisting of polystyrene, polysulfone, poly(ether sulfone) or poly(phenylene oxide), and that the quaternized trialkyl ammonium salt is attached to the backbone via a benzyl(methyl) group.

20. A method according to any one of the preceding claims 1 to 19, characterized in that an electrochemical cell is provided which additionally has the following features: ix) the electrochemical cell comprises an auxiliary cathode; x) the auxiliary cathode is in contact with the central electrolyte; xi) the auxiliary cathode is connectable to the electrical voltage source via a third electrical line.

21. Method according to claim 20, characterized in that the method has two operating states, namely: p) a production state in which the anode and the cathode are connected to the electrical voltage source via the first and second electrical lines, respectively, and in which the anode and cathode are subjected to the electrical voltage U, such that the electrical current / flows between the anode and the cathode; r) a regeneration state in which the anode and the auxiliary cathode are connected to the electrical voltage source via the first and third electrical lines, respectively, and in which the anode and the auxiliary cathode are subjected to the electrical voltage U, such that the electrical current / flows between the anode and the auxiliary cathode.

22. Method according to claim 21, characterized by an alternating change between production state (p) and regeneration state (r), wherein a single production state is carried out over a time duration of tp and wherein a single regeneration state is carried out over a time duration of / R, wherein tp > f* / R with f greater than 1 or with f greater than 10 or with f greater than 100.

23. The method according to claim 20, characterized in that an electrochemical cell is provided which additionally has the following features: xii) the cathode has a cathode surface AK; xiii) the auxiliary cathode has an auxiliary cathode surface AK; and that the method has an operating state, namely: k) a combined production and regeneration state, in which the anode is connected to the electrical voltage source via the first electrical line and the cathode and the auxiliary cathode are connected to the electrical voltage source via the second electrical line, and in which the anode, cathode, and auxiliary cathode are subjected to the electrical voltage U, so that the electrical current / flows between the anode, cathode, and auxiliary cathode; wherein the cathode surface K and the auxiliary cathode surface AAK are selected such that AK > f* AAK with f greater than 1 or with f greater than 10 or with f greater than 100.

24. The method according to claim 23, characterized in that the method has two operating states, namely: k) the combined production and regeneration state, p) a production state in which the anode and the cathode are connected to the electrical voltage source via the first and the second electrical lines, respectively, and in which the anode and cathode are supplied with the electrical voltage U, so that the electrical current / flows between the anode and cathode; alternating between the production state (p) and the combined production and regeneration state (k), a single production state (p) being carried out for a time duration of tp and a single combined production and regeneration state (k) being carried out for a time duration of / K, where tp > g* tK with g greater than 50 or with g greater than 500 or with g greater than 5000.

25. Method according to one of the preceding claims 20 to 24, characterized in that the auxiliary electrode is arranged outside the central compartment and / or that the auxiliary electrode consists of a textile material.

26. A method according to any one of the preceding claims 1 to 25, characterized in that the operation comprises an electrolysis of water (H2O) and an electrodialysis of anions (X n j includes.

27. A process according to claim 26, characterized in that the operation comprises the synthesis of lithium hydroxide and / or lithium hydroxide monohydrate (LiOH ^O).

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