LLTO membrane, production thereof and use thereof

Abstract

The invention relates to an LLTO membrane, to the production thereof and to the use thereof in the membrane electrodialysis of LION. It is an object of the present invention to specify a membrane that achieves a durably high current density in the electrodialysis of LION. This is surprisingly achieved by gold plating LLTO on the anode side. The gold layer brings about an increase in current density. It has been found that the gold plating of LLTO is particularly successful when the surface of the LLTO is subjected to acid treatment prior to gold plating. The gold layer adheres better to the acid-treated surface, thus maintaining the beneficial effect of the gold for longer.

Description

202500056 1LLTO membrane, production thereof and use thereofThe invention relates to a membrane containing a lithium lanthanum titanium oxide (LLTO). The invention further relates to a process for producing this membrane and to the use thereof.Production of lithium-ion batteries (LIB) necessarily requires lithium (Li). Due to its high reactivity, lithium occurs in nature not as the pure substance but rather always in bonded form. The employed starting material for production of LIB is generally lithium in the form of lithium hydroxide (LiOH) or lithium carbonate (U2CO3).In most natural deposits Li is present in the form of lithium oxide (Li2O) or salts such as lithium chloride (LiCI). Lithium oxide is a constituent of ores such as pegmatite, whereas lithium chloride is present in dissolved form in the brines of Li salt lakes. In the course of mining of lithium ores the lithium is initially converted into a respective soluble lithium compound, such as lithium sulfate or lithium chloride, by leaching with acid and then converted into lithium carbonate (Li2COs) by subsequent steps. In a further process step the lithium carbonate may be converted into lithium hydroxide by reaction with quicklime or calcium hydroxide. The extraction of Li and its transformation into LiOH is described in: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.Rich Li deposits are known but the production of LiOH from the Li compounds present therein is very energy intensive and generates large volumes of wastewater. There is also a strategic need to be independent of the owners of the deposits.One solution to this problem could be to carry out material recovery of end-of-life LIB so that the lithium present therein may be reused as a raw material for new batteries.Recycling processes for LIB have in the past already been developed to industrial maturity but usually with the aim of recovering the metals Fe, Ni, Mn, Co, Cu, Mg and Al present therein. The alkali metal Li was generally not recovered, since its high reactivity makes it difficult to separate from scrap batteries and it was available at low cost and in sufficient amounts from natural deposits. The extraction of Li from used LIB for a long time seemed simply uneconomic.However, there is now growing social and economic pressure to recover lithium from used LIB. The success of this proposal presupposes that recycled Li can be offered to producers of LIB in an acceptable quality such that the production processes for LIB from recycled Li do not differ from those from mined virgin Li. It goes without saying that there must be no adverse effect on battery quality. Recycled Li, especially in the form of LiOH, must consequently meet very stringent specifications as regards purity. In addition, the process for recovering Li from end-of-life batteries must be as energy-efficient as possible. The process should also use only little water.202500056 2Known processes for recovering lithium from old batteries are summarized in: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 104-121DOI: 10.1016 / j.mineng.2O17.04.008.A technology that is mentioned only in passing as “LISM” in the above review article is the electrolysis of Li-containing waters in the presence of so-called LiSICon membranes.LiSICon stands for lithium super ionic conductor. This is a class of inorganic material which is electrically insulating but at the same time has an intrinsic conductivity for Li ions. The constitution of LiSICon is described as ceramic or glass-like. The transport mechanism for Li derives from the crystal structure of the material. The Li ions are - in simple terms - “passed through” the crystals. Commercially available LiSICon materials include lithium aluminium titanium phosphate (LATP), lithium aluminium titanium silicon phosphate (LATSP), lithium aluminium germanium phosphate (LAGP) and lithium lanthanum titanium oxide (LLTO). These materials were originally developed as solid-state electrolytes for LIBs. An overview of the transport mechanisms of LiSICons, their crystal structure and production may be found in:Palakkathodi Kammampata et al.: Cruising in ceramics — discovering new structures for all-solid- state batteries — fundamentals, materials, and performances. Ionics 24, 639-660 (2018), DOI: 10.1007 / sl 1581-017-2372-7,Philippe Knauth: Inorganic solid Li ion conductors: An overview. Solid State Ionics, Volume 180, Issues 14-16, 2009, Pages 911-916, ISSN 0167-2738,https: / / doi.Org / 10.1016 / j.ssi.2009.03.022.Their selective conductivity for Li ions means that LiSICon materials can be used as a membrane for separating Li from Li-containing mixtures. The Li must be present in the mixture in ionic form, for instance as an Li salt dissolved in water. The driving force that is needed to push the Li ions through the LiSICon membrane is an electrical voltage. An electrochemical cell comprising two electrodes and a LiSICon membrane that divides the cell into two compartments is constructed for this purpose. Each compartment has an electrode in it. Each electrode is assigned a polarity: One electrode forms the anode, the other electrode the cathode. The polarity of the electrodes is determined by a voltage source. The compartments are referred to as anodic or cathodic depending on the polarity of the electrode present in the respective compartment. An electrical voltage is applied to the electrodes and the anodic compartment is filled with the Li-containing water as anolyte. The cathodic compartment is filled with aqueous catholyte. Due to the voltage between the anode and the cathode the cations are attracted by the cathode. The membrane transfers the Li cations to the cathodic compartment. The water in the cathodic compartment therefore becomes enriched with Li, while the water on the anodic side becomes depleted in Li. Such a process is referred to as membrane electrodialysis.202500056 3Above a certain cell voltage an electrolysis of the water present in the electrolytes occurs simultaneously with the membrane electrodialysis. This causes oxygen O2 to be formed at the anode and hydrogen H2 to be formed at the cathode. However, the yield of hydrogen and oxygen is not comparable to a pure electrolysis of water, since, depending on the composition of the anolyte and the salts present therein, other electrolysis products may also be formed.Processes for extracting lithium using LiSICon membranes have already been described in the prior art.For instance, Zhen Li et al. describe a process in which weakly lithium-containing water from the Red Sea is used as raw material:Zhen Li et al.: Continuous electrical pumping membrane process for seawater lithium mining.Energy Environ. Sci., 2021, 14, 3152 DOI: 10.1039 / d1ee00354b.The Zhen Li research group uses LLTO as the membrane. The separated target product is lithium phosphate (Li3PO4), which is suitable for production of lithium iron phosphate (LFP) batteries. LIB having a different cathode material such as for example nickel manganese cobalt oxide (NMC) or lithium manganese oxide (NMO) cannot be directly produced by this process. Over a period of about 20 h at a voltage of 3.25 V, Zhen Li et al. achieved a maximum current of 1.5 mA for a membrane having a diameter of 20 mm and a thickness of 55 pm. This gives a current density (3.14 cm2membrane area) of 4.8 A / m2or a current density of 0.26 A / m2at a normalized material thickness of 1 mm.Some of the authors of the cited article are also named as inventors in WO2022157624A1. This paper is also concerned with the extraction of Li using an LLTO membrane in an electrochemical cell. The cathode of this cell is made of copper hollow fibres coated with platinum / ruthenium.WO 2019055730 A1 is likewise concerned with the separation of lithium using LiSICon membranes. LLTO, LAGP and LATP are specifically mentioned. Li ions are the separated target product.The different LiSICon types used as the membrane material for electrodialysis of Li differ in terms of their conductivity for lithium and their selectivity for and resistance to undesirable cations present as impurities in most lithium-containing waters. There are also considerable cost differences between the individual LiSICon types.An LiSICon type having a particularly high Li conductivity is LAGTP, as described for instance by the Sofia Saffirio research group:Sofia Saffirio etal.: Lii.4Alo.4Geo.4Tii.4(P04)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.2021.11.014.202500056 4However, LAGTP is very costly on account of its germanium content.A comparatively inexpensive LiSICon material is lithium lanthanum titanium oxide (LLTO) as previously described by KNAUTH.However, when using this material it must be taken into account that LLTO has a low specific conductivity for Li compared to other LiSICons. At a given material thickness a low intrinsic Li conductivity of the membrane material results in low permeance of the membrane. Permeance is a measure of the conductivity of the membrane while taking its thickness into account. The higher the permeance of the membrane, the lower the demand for electrical energy for the dialysis process. LiSICon materials having a low intrinsic Li conductivity therefore reduce the permeability of the membrane and increase the energy demand of the process. It is theoretically possible to reduce the material thickness of the membrane and thus increase its permeability again. However, the thickness of the membrane material must not be below a certain minimum thickness for stability reasons. Accordingly, the low specific conductivity of the LLTO cannot simply be compensated by a low membrane thickness. This applies all the more if the membrane is subjected to wear during operation.As a result an LLTO membrane always has a lower permeance than a membrane made of a LiSICon having a higher intrinsic Li conductivity. Accordingly, a LISM process with an LLTO membrane requires more energy than a corresponding process with an LAGTP membrane.Extracting lithium with an LLTO membrane thus appears attractive especially where inexpensive electrical energy - especially from renewable sources - is available.Due to the still insufficient availability of renewable electrical energy and the relatively high costs of germanium-based LiSICons having high Li conductivity there is an urgent need to increase the Li conductivity of inexpensive LLTO.EP4509635A1 describes how a layered article suitable as a membrane may be produced from LLTO and LAGP. The aim is to achieve a better compromise between permeance and material costs.However, even an LiSICon membrane optimized in terms of material costs and permeance still has the disadvantage of high susceptibility to foreign ions compared to organic membrane materials: If LiSICon comes into contact with ions other than lithium they settle on the free lattice sites of the crystal structure where they block the absorption of lithium. This causes the lithium conductivity to fall severely; the LiSICon is effectively poisoned. For this reason, it is only possible to use LiSICon membranes where the lithium-containing material streams contain hardly any foreign ions. Good pre-separation of the foreign ions is necessary particularly in the electrodialytic separation of lithium in the context of battery recycling, because such streams contain many such impurities. Sodium cations Na+have been found to be particularly damaging to LiSICon membranes. This is true especially of phosphate-based LiSICons such as LAGTP or LATSP. Pre-separation can be accomplished in particular with organic membranes or other202500056 5separation mechanisms. Particularly when pre-separation is carried out using thermal separation processes, the energy requirement of the overall process increases significantly.Besides susceptibility to poisoning and high material costs, there is a further phenomenon that impacts unfavourably on the economic viability of LISM processes: flow of electrical current through the LiSICon material.LiSICons, like other glasses or ceramics, in principle belong to the class of electrical non-conductors (insulators) since at a temperature of 23°C they have a specific conductivity for electrons y (electrical conductivity) of less than 10'7S / cm (10'9S / m). In most cases, the electrical conductivity of phosphate-based LiSICon material at 23°C is less than 10'12S / m or even less than 10'16S / m. The insulation property means that no electrical short circuit occurs between the anode and the cathode within the electrochemical cell, even if both electrodes are placed directly on the LiSICon membrane.However, this statement does not apply to oxidic LiSICons and in particular not to LLTO. LLTO has an electrical conductivity at a temperature of 50°C of about 31.62*10-4S / cm (cf. Kammampata et al.;Fig. 12a), which is why from an electrical standpoint it is a semiconductor rather than an insulator. Due to its electrical conductivity LLTO cannot be used as a separator in LIB, but at most as a solid-state electrolyte.Electrodialysis of lithium hydroxide may in principle employ LLTO but only with a low current efficiency of about 20% which is likewise attributable to the certain electrical conductivity of the LLTO. In addition, the electrical conductivity of LLTO in the electromembrane dialysis of lithium hydroxide actually increases over time: It is thought that this has to do with erosion of non-metallic fractions from the ceramic, with the result that metallic elements are over-represented in the eroded surface. In the case of LLTO, lithium and lanthanum are removed from the surface of the membrane to leave behind electrically conductive non-stoichiometric titanium oxide. In this way the titanium oxide results in a conductivity for electrons over the eroded surface of the membrane.From an electrical standpoint, leakage currents, which are no longer available for charge balancing, develop over time. As a result of this it is no longer the entirety of the electrical current I flowing through the electrical voltage source between the anode and the cathode that is utilized for transport of the lithium cations, but only a part thereof. The proportion of the electrical current actually utilized for material transport in electrodialysis is usually referred to in the scientific literature as current efficiency or FARADAY efficiency. This efficiency is a measure of the substance-specific energy demand of the process.In addition to current efficiency, the rate of lithium transport is also one of the most important measures for describing such a process. This transport rate may be expressed in [gmum / m2h] and at 100% current efficiency is proportional to the current density, which is commonly expressed in [A / m2]. Current density i is the quotient of the current flowing across the cell and the effective area of the membrane.202500056 6The linear current density is also used to more precisely describe the membrane / the membrane materials essential for material separation in the presently described process with few parameters. This normalizes the current density over the thickness of the membrane material and is reported in [A / m2] for a membrane of 1 mm in thickness or in units of [A mm / m2].In the patent literature WO2024128590A1 in paragraph

[0070] for example formulates a parameter that corresponds with or is at least proportional to current efficiency. The object of that document is to increase the current efficiency of an electrodialysis of lithium hydroxide. However, the electrodialysis disclosed in WO2024128590A1 operates with a combination of an anion exchange membrane and a bipolar membrane, both made of organic material. By contrast, LiSICon membranes are inorganic cation exchange membranes. The problem of reduced current efficiency caused by material changes in LLTO membranes does not occur in organic membranes.The Kazuya Sasaki research group recently succeeded in increasing the FARADAY efficiency of an electrodialysis with an LLTO membrane by placing platinum-based electrodes directly onto the membrane. In addition, the employed electrochemical cell was fitted with a nickel-containing auxiliary electrode:Sasaki, K., Shin-mura, K., Honda, S. et al. A three-electrode dual-power-supply electrochemical pumping system for fast and energy efficient lithium extraction and recovery from solutions.Commun Eng 3, 29 (2024). https: / / doi.org / 10.1038 / s44172-024-00174-8.EP4134156A1 discloses an ion-conducting composite membrane. The composite membrane comprises a porous support structure provided with an ion-conductive coating. The ionic conductive coating may contain LLTO. The ionic conductive coating may be provided with catalytically active material point by point or over its whole area. The catalytically active material may be gold. The specific (comparative) examples 1 B employ a nickel catalyst arranged on an LLTO sintered body. A grid-shaped gold electrode is also applied by sputtering. A lattice-shaped platinum electrode is sputtered onto the other side. The composite membrane is used for electrodialysis of lithium. The alignment of the electrodes in the electrochemical cell is not disclosed.WO2024063872A1 describes that a LiSICon membrane is to be mechanically treated to obtain its unspecified performance in the electrodialysis of lithium. The aim of the treatment is to reduce the thickness of the membrane. The membrane may alternatively be etched. The etching process is not further specified.JP2024155152A describes an LLTO membrane for separation of lithium which is coated with platinum on both sides.CN118267965A describes a process for producing LLTO which comprises a plurality of sintering steps and an acid treatment. The obtained LLTO is used as an absorber for lithium.202500056 7Having regard to the prior art it is an object of the present invention to achieve a durably high linear current density in the membrane electrodialysis of lithium hydroxide.This is surprisingly achieved by gold plating the LLTO on the anode side once the LLTO has first been activated by means of an acid treatment. The gold layer brings about an increase in the linear current density and at the same time protects the surface activated by the acid. Both of these things lead to a durably lower specific energy demand of the LISM process. To ensure that this mode of action takes full effect, the gold plating should be carried out over the greatest possible area so that the LLTO is ideally completely covered by gold on the anode side. It is not necessary to cover parts of the LLTO that are not in contact with the anolyte. However, the regions of the membrane that come into contact with the anolyte, i.e. the active surface area of the membrane, should be gold plated to an extent of at least 80%, better still to an extent of 90% to 100%. Point-by-point or network gold plating is insufficient. There is ideally no direct contact between the LLTO and the anolyte but rather only indirect contact via the gold.The gold layer need not be particularly thick. Just a few layers of gold atoms will suffice. Thus only a small amount of gold is required. The material costs for the gold plating are relatively low, in particular a gold plated LLTO membrane is cheaper to produce than an LAGTP membrane which has a similar permeance to the gold plated LLTO membrane.It has been found that the gold plating of LLTO is particularly successful when the surface of the LLTO is subjected to acid treatment prior to gold plating. The acid treatment activates the surface. The gold layer adheres better to the acid-treated surface, thus maintaining the beneficial effect of the gold for longer.In this context the present application discloses a process for producing hydrogen and lithium hydroxide comprising the following non-chronological steps:a) providing at least one electrochemical cell, wherein the electrochemical cell has at least the following features:i. the electrochemical cell comprises a first compartment filled with an aqueous anolyte;ii. the electrochemical cell comprises a second compartment filled with an aqueous catholyte;Hi. the electrochemical cell comprises a two-sided membrane arranged between the first compartment and the second compartment which has an areal gold plating on its first side, is in contact there with the anolyte via a first active surface area and on its second side is in contact with the catholyte via a second active surface area, wherein the membrane contains a lithium lanthanum titanium oxide having the LLTO stoichiometry:Li3xLa<2 / 3)-xn(1 / 3)-2xTiO3 (LLTO)202500056 8wherein in LLTO: 0 < x < 0.16 and wherein the lithium lanthanum titanium oxide may optionally be doped with at least one doping element selected from the group consisting of the following doping elements: Ag, Al, Zr, Nd, Sr, Ta, Y, Ge, Nb; iv. the electrochemical cell comprises an anode contacted with the anolyte;v. the electrochemical cell comprises a cathode contacted with the catholyte; vi. the electrochemical cell comprises an electrical voltage source connected to the anode via a first electrical line and to the cathode via a second electrical line; b) providing dissolved lithium salt in the anolyte;c) applying to the electrochemical cell a first electrical voltage U acting between the anode and the cathode by means of theelectrical voltage source;d) synthesizing lithium hydroxide in the catholyte;e) synthesizing hydrogen in the catholyte.An essential feature of this electrodialytic process for producing lithium hydroxide and hydrogen is the use of the LLTO membrane having an areal gold plating on its anode side.The production of such a membrane is a first subject of the invention. The process according to the invention for producing the membrane comprises the following non-chronological steps:a) providing a two-sided flat solid body, wherein the solid body contains a lithium lanthanum titanium oxide having the LLTO stoichiometry:Li3xLa(2 / 3)-xn(i / 3).2xTiO3 (LLTO)wherein in LLTO: 0 x < 0.16 and wherein the lithium lanthanum titanium oxide may optionally be doped with at least one doping element selected from the group consisting of the following doping elements: Ag, Al, Zr, Nd, Sr, Ta, Y, Ge, Nb;b) providing an acid;c) providing gold;d) treating one side of the flat solid body with the acid;e) coating the acid-treated side of the solid body with the gold;f) obtaining a membrane comprising the flat solid body having an areal gold plating on its acid- treated side.The acid treatment before the gold plating is crucial.A second subject of the invention is the LLTO membrane provided with an areal gold plating after acid treatment. This membrane comprises a two-sided flat solid body, wherein the solid body contains a lithium lanthanum titanium oxide having the LLTO stoichiometry:202500056 9Li3xLa<2 / 3)-xn(1 / 3)-2xTiO3 (LLTO)wherein in LLTO: 0 x < 0.16 and wherein the lithium lanthanum titanium oxide may optionally be doped with at least one doping element selected from the group consisting of the following doping elements: Ag, Al, Zr, Nd, Sr, Ta, Y, Ge, Nb; wherein at least one side of the flat solid body is acid-treated and wherein the acid-treated side of the solid body has an areal gold plating.Such a membrane is obtainable for example by the process according to the invention for producing the membrane and is preferably used in the process disclosed herein for producing lithium hydroxide and hydrogen. The membrane according to the invention may also be used in a different process for producing lithium hydroxide and hydrogen.The use of the membrane according to the invention in a process for producing lithium hydroxide and hydrogen from lithium-containing waters is therefore a third subject of the invention.In the simplest case the use comprises performing an electrodialytic process for producing lithium hydroxide and hydrogen in the presence of the membrane according to the invention. The lithium-containing water is used as feed / anolyte.The membrane described here consists partially or completely of conventional lithium lanthanum titanium oxide whose LLTO stoichiometry is disclosed inter alia by KAMMAMPATA and KNAUTH. The crystal structure of LLTO was likewise described by these authors. LLTO is commercially available in powder form or in the form of flat discs that are directly employable as a membrane.According to the invention the LLTO membrane is provided with an areal gold plating on one side. The membrane is to be installed in the electrochemical cell in such a way that the gold layer is aligned in the direction of the anode. The cathodic side of the membrane need not be gold plated.It is important that on the anode side the membrane is provided with a gold layer over the greatest possible area - point-by-point or network gold plating is insufficient. Preferably at least 80%, better still 90% to 100%, of the anode side of the membrane should have a gold plating. The reference value here is the area Aaa of the first active surface area of the membrane facing the anode. The first active surface area of the membrane is the part of the membrane coming into contact with the anolyte. The electrochemically unusable sub-areas of the membrane, occupied for instance by seals, are not included.The other side of the membrane facing the cathode need not have a gold plating. However a small degree of gold plating cannot be ruled out as a consequence of production. Preferably at most 10%, better still 0% to 1%, of the area Aak of the second active surface area should have a gold plating. The second active surface area is the part of the membrane coming into contact with the catholyte.It is surprising that the anode need not have electrical contact with the gold layer to achieve the desired effect. This is advantageous firstly because it is easier to assemble the cell without electrode / membrane202500056 10contact. Experimental studies further show that electrical contact between the membrane and the electrode leads to more severe ageing of the membrane. The anode is thus preferably spaced apart from the membrane. The cathode may also be spaced apart from the membrane. In this respect the proposed cell design differs from those where noble metal electrodes are placed directly on the membrane. Spacing of the electrodes may be effected by installation of spacers between the electrode and the membrane.The gold coating allows continuous electrodialysis of lithium-containing waters as anolyte which contain not only lithium but also impurities in the form of cations of B, Na, Mg, Al, Si, K, Ca, Mn, Co, Ni, Cu. These cationic impurities are typical of lithium-containing waters derived from the workup of used lithium-ion batteries.The resistance of the gold-coated LLTO membrane to sodium is particularly noteworthy. The proportion of Na+cations in the anolyte may be between 0 mol% and 20 mol% without significantly limiting the lithium conductivity of the membrane. The molar proportions are based on the lithium content in the anolyte.In addition to the cationic impurities, the anolyte can also contain anions other than hydroxide, such as sulfate, hydrogen sulfate, carbonate, hydrogen carbonate, chloride or fluoride. The concentration of these foreign anions should be at most 1 mol to 1 mol of the hydroxide ions contained in the anolyte, otherwise the current density of the process will decrease sharply and the ageing of the membrane will increase.The electrodialysis may be operated such that it forms not only lithium hydroxide and hydrogen at the cathode but also oxygen O2 at the anode. The process accordingly includes the step of “synthesis of oxygen in the anolyte”. The gaseous oxygen O2 is formed by the water electrolysis running simultaneously with the lithium dialysis. Other oxidation products or compounds may also be formed at the anode, depending on the presence of further elements in the anolyte.The electrodialysis should be performed at a current density / of between 10 A / m2and 500 A / m2. The current density / is calculated as the quotient of the electric current I flowing through the electrical voltage source and the area Aaaof the first active surface area on the first side of the membrane: / = / / Aaa.The linear current density is obtained by dividing the current density by the thickness of the membrane. The electrical voltage U should be in the range from 3 V to 12 V. Preference is given to a voltage range of 4 V to 8 V. The electrical voltage is optimally about 8 V.The temperature at which the electrochemical cell is operated is ideally between 30°C and 60°C. This is because experimental data show that the efficiency of electrodialysis increases with temperature.However, a higher temperature should be avoided because ageing of the membrane increases from about 60°C.The stoichiometry of lithium lanthanum titanium oxide is in the range for x specified in the formula for LLTO of 0 to 0.16 inclusive.202500056 11Li3xLa<2 / 3)-xn(1 / 3)-2xTiO3 (LLTO)The proportions of lithium and lanthanum can therefore vary. The square n in the formula represents the vacancies on the A-sides; cf. article by KNAUTH in section 2.1 and KAMMAMPATA et al. on page 654, top right with additional evidence. The LLTO may additionally be doped with at least one doping element. Suitable doping elements are: Ag, Al, Zr, Nd, Sr, Ta, Y, Ge, Nb. Specific examples of LLTO are lithium lanthanum titanium oxides having the stoichiometry Lio saLao.ssTiOs or Lio.29Lao.57Ti03.In the course of the electrodialysis lithium hydroxide / lithium hydroxide monohydrate is formed at the cathode. Depending on its concentration in the catholyte the substance formed is in dissolved form. It is separated from the catholyte in a corresponding separation step. This allows the catholyte to be reused. It is preferable when the catholyte is circulated through the second compartment and the lithium hydroxide / lithium hydroxide monohydrate is separated from the catholyte outside the cathodic compartment.The membrane employed in the electrodialysis process according to the invention is made by singlesided gold plating of a flat solid body made of LLTO. The gold plating may be carried out by conventional coating processes classified as physical vapour deposition (PVD), such as for instance sputtering.Chemical or electrochemical gold plating may alternatively be undertaken.As mentioned above, the LLTO solid is subjected to an acid treatment prior to the coating with gold, to increase the durability of the gold layer. The acid treatment is effected by contacting the LLTO with the acid. The acid is preferably hydrogen chloride (HCI). It is alternatively possible to use phosphoric acid or nitric acid or sulfuric acid. The hydrogen chloride is preferably contacted with the LLTO in liquid form, preferably at a temperature of 20°C to 80°C over a period of 20 h to 80 h. Liquid hydrogen chloride is in the simplest case an aqueous solution of hydrogen chloride, i.e. hydrochloric acid. However, anhydrous hydrogen chloride may also be used.It has proven advantageous to mask the side of the LLTO solid membrane not to be gold plated during the acid treatment, thus preventing the acid from unnecessarily attacking the other side. The acid treatment is then substantially one-sided. Masking may in the simplest case be achieved by masking the side of the solid body not to be gold plated with an acid-resistant adhesive film. The side of the solid body not to be treated may alternatively be sealed off from the acid during the acid treatment. This may be done for example by installation into an electrochemical cell: The acid treatment is then carried out in the electrochemical cell (in situ) by charging the first compartment with the acid. An electrical voltage need not be applied to the cell during in-situ treatment with acid.In a particularly preferred development of the process for producing the membrane, the solid body is sintered prior to the acid treatment. This improves the crystal structure of the solid body. The sintering is caried out at an end temperature in the range between 900°C and 1200°C which is maintained for a period in the range from 1 h to 16 h.202500056 12The invention and its advantageous characteristics shall now be more particularly elucidated with reference to experiments. In the figures:Figure 1 : shows a schematic diagram of a first measurement cell;Figure 2: shows a schematic diagram of a second measurement cell;Figure 3: shows a schematic diagram of a third measurement cell;Figure 4: shows a schematic diagram of a fourth measurement cell;Figure 5: shows an SEM image of surface of LLTO solid body before sintering;Figure 6: shows an SEM image of surface of LLTO solid body after sintering;Figure 7: shows an SEM image of surface of LLTO solid body after acid treatment according to example 2;Figure 8: shows an SEM image of surface of LLTO solid body after acid treatment according to example 6;Figure 9: shows a photograph of gold plated surface after 60 h of electrodialysis in example 19 (noninventive);Figure 10: shows a photograph of gold plated surface after 600 h of electrodialysis in example 2;Figure 11 : shows a U / l diagram for example 28;Figure 12: shows a U / l diagram for example 29;Figure 13: shows a U / l diagram for example 31 ;Figure 14: shows a graphical representation of current increase over temperature for example 33;Figure 15: shows a graphical representation of current increase over voltage between 4 V and 8 V for example 34;Figure 16: shows a graphical representation of current increase over voltage between 4 V and 12 V for example 34.202500056 130. Experimental setupsFigure 1 is a schematic diagram cf the first experimental setup cf an electrochemical cell 0.The electrochemical cell 0 ccmprises a first ccmpartment 1 and a seccnd ccmpartment 2. The twc ccmpartments 1 and 2 are separated from cne ancther by a membrane 3. The membrane 3 is here designed as a circular flat membrane having a diameter cf 20 mm (this is not visible in the sectional representation of the figure). Other membrane shapes are possible. The materiality of membrane 3 will be more particularly elucidated later.A first electrode, namely an anode 4, is arranged within the first compartment 1 ; a second electrode, namely a cathode 5, is arranged in the second compartment 2.The anode 4 and cathode 5 employed were in each case a round disc having a diameter of 19.5 mm and a thickness of 1 mm. The material was in each case a titanium expanded metal sheet, coated on both sides with IrTi mixed oxide, 12 g lr / m2, 1 AF D 1.5 mm from Metakem GmbH, 61250 Usingen, Germany.The first compartment 1 is filled with an anolyte 7. The anolyte 7 is a liquid containing water and lithium salts dissolved therein. This means the anolyte 7 contains freely mobile lithium cations Li+and freely mobile anions of the dissolved salts, for example chloride Cl' or sulfate SO42'. Since these ions are mobile in the aqueous liquid, the anolyte 7 is a liquid electrolyte. In addition to lithium, the anolyte 7 may also contain other cations, such as Na+.The second compartment 2 is filled with a catholyte 8. Said compartment also contains water and salt dissolved therein, i.e. freely mobile anions and cations. The salt present in the catholyte 8 need not be a lithium salt: sodium, potassium, beryllium, magnesium or calcium salts may also be dissolved in the catholyte. It is however preferable that the catholyte 8 contains lithium cations Li+, specifically ideally more Li+ions than other cations. If the catholyte 8 contains too many cations other than Li+, the target product LiOH is obtained in low purity. In addition to the cations, the catholyte also contains the anions of the salts dissolved therein. The catholyte is thus likewise to be understood as a liquid electrolyte. The exact composition of the electrolytes is described in the following examples. Mixing of the two electrolytes 7 and 8 is precluded since the membrane 3 is liquid-tight.The two electrolytes 7 and 8 are each in contact with the corresponding electrodes: The anolyte 7 is in contact with the anode 4 and the catholyte 8 is in contact with the cathode 5. Since the anode 4 and the cathode 5 are designed as porous expanded metals, the electrolyte penetrates into the respective electrode.In the measurement setup shown in figure 1 the two electrodes 4 and 5 are arranged spaced apart from the membrane 3. This means there is no electrical contact either between the membrane 3 and the anode 4 or between the membrane 3 and the cathode 5. There is however contact between the membrane 3202500056 14and the anolyte 7 and between the membrane 3 and the catholyte 8: The contact between the membrane 3 and the anolyte 4 occurs via a first active surface area having an area of 7a. The contact between the membrane 3 and the catholyte 8 occurs via a second active surface area having an area of / k. The two active surface areas are parallel to one another and each extend perpendicularly to the plane of the drawing in figure 1. Thus, in each case only one dimension of the two active surface areas is illustrated and provided with measurements. The two active surface areas are the same size in the measurement setup shown: They are determined from the area of the membrane 3 minus the regions of the membrane not in contact with the electrolytes. These are typically the areas of the membrane in contact with a sealing element. To simplify the calculation of the active surface area, the membrane seal is disregarded, though in practice there is a seal via an O-ring not shown here. The area T k = Aaaof the two active surface areas is therefore calculated from the circle diameter of the membrane of 20 mm to about 314 mm2in each case.The membrane 3 has a layer structure: The basis is a solid body 31 in the form of a circular disc of lithium lanthanum titanium oxide having the stoichiometry Lio.aaLao.ssTiOa. The solid body 31 was commercially obtained from Toshima Manufacturing Co., Ltd., Higashimatsuyama, Japan. In the applicant's laboratories, the solid body 31 was first sintered and then treated with acid on one side to obtain an acid-treated surface 32. The acid-treated area 32 was then provided with a whole-area gold layer 33 in the applicant's laboratories. The sintering, acid treatment and gold plating of the solid body 31 shall now be more particularly elucidated in the following experimental description. For the experimental setup it is important that the membrane 3 is installed in the cell 0 in such a way that the gold layer 33 is arranged on the side of the anode 4. This means that the anolyte 7 contacts the membrane 3 via its gold layer 33. The area of the gold layer 33 corresponds to the area of the first active surface area -Aaa.The electrochemical cell 0 is provided with an electrical voltage source 9. The electrical voltage source 9 is connected to the anode 4 via a first electrical line 10 and to the cathode 5 via a second electrical line 11. The polarity is selected such that the positive pole of the electrical voltage source 9 is connected to the anode 4 and the negative pole is connected to the cathode 5.To start up the electrochemical cell 0, the electrical voltage source 9 is switched on, giving rise to an electrical voltage U generated by the electrical voltage source 9 between anode 4 and cathode 5. In the simplest case, the magnitude of the voltage U and its polarity are kept constant as DC voltage.The two electrolytes 7, 8 may in principle be maintained in two operating modes: batch operation or continuous operation. In batch operation a certain portion of the electrolyte 7, 8 remains in the respective compartment 1, 2 for the duration of the experiment. In continuous operation a fresh portion of the respective electrolyte is continuously added to the corresponding compartment, the same volume being withdrawn from the compartment. The fill level of the compartment accordingly remains constant in continuous operation though the electrolyte is continuously renewed. Hybrid forms of batch operation and continuous operation are also possible, for instance partially replacing the electrolyte at longer intervals during the experiment (semi-batch).202500056 15Irrespective of electrolyte maintenance, two electrochemical processes occur simultaneously in the electrochemical cell 0 when the electrical voltage source 9 is switched on: an electromembrane dialysis and a water electrolysis.In the course of the electromembrane dialysis, Li+cations move from the first compartment 1 across the membrane 3 into the second compartment 2 to the cathode 5. The driving force is the voltage U. The transport via liquid-tight membrane 3 is made possible by the intrinsic conductivity for Li+cations of the LLTO from which the solid body 31 of the membrane 3 is made. The gold layer 33 on the solid body 31 has no adverse effect on the conductivity of the membrane 3 for lithium cations. Since LLTO has a higher conductivity for lithium cations than for other cations (cation selectivity), the lithium cations Li+migrate preferentially through the membrane 3 towards the cathode 5, whereas sodium cations Na+tend to remain in the first compartment 1. The cation selectivity of the membrane 3 accordingly results in separation of Li+and Na+. The Cl" and SO42' anions present in the anolyte 7 in any case remain in the first compartment 1 at the anode 4 and may form gas. More specifically, the presence of chloride results in the formation at the anode 4 of chlorine gas which dissolves in the aqueous solutions; otherwise the electrolysis forms especially oxygen O2.In the course of the water electrolysis water H2O present in the electrolyte 8 is reduced at the cathode, thus forming hydroxide ions OH" and hydrogen H2 at the cathode. The hydroxide ions OH" formed in the catholyte are attracted by the anode 4, but have little ability to cross the membrane 3, since LLTO does not have any appreciable anion conductivity. The OH' anions trapped in the second compartment form lithium hydroxide LiOH with the Li+cations newly arriving there. This is initially present in the form of dissolved ions and also in solid form once the solubility limit has been exceeded.During ongoing batch operation the concentration of Li+cations in the two electrolytes changes constantly: The concentration of lithium cations ci(f) in the anolyte 7 in principle decreases while the Li+concentration in the catholyte 02(f) in principle increases. The process comes to a halt when no more lithium is available in the anolyte 7. In continuous operation a steady-state flow process is operated, in which the concentrations c-i(t) and 02(f) are kept as constant as possible. This is achieved by continuously adding fresh lithium salt to the anolyte and by continuously removing lithium hydroxide from the catholyte. It is preferable when anolyte and catholyte are continuously withdrawn from the two compartments and recycled after the addition of the fresh Li salt and the removal of the lithium hydroxide. For the sake of simplicity this is not shown in the figures.Figure 2 is a schematic diagram of the second experimental setup of an electrochemical cell 0. The second experimental setup corresponds to the first experimental setup shown in figure 1 with the difference that the anode 4 is arranged directly on the anode-side gold layer 33 of the membrane 3. This brings about electrical contact between the anode 4 and the gold layer 33. The gold layer 33 is accordingly given a positive polarity by the electrical voltage source 9. From an electrochemical standpoint the gold layer 33 thus becomes part of the anode of the electrochemical cell 0 shown in Figure 2. In the second experimental setup the cathode 5 remains spaced apart from the membrane 3 in202500056 16the second compartment 2 without electrical contact to the membrane 3. The two respective electrolytes 7, 8 present in the respective compartments 1 , 2 still have contact with the respective electrodes 4, 5.Figure 3 is a schematic diagram of the third experimental setup of the electrochemical cell 0. The third experimental setup uses a membrane 3 that is coated on both sides with gold. In addition to the first gold layer 33 on the side of the anode 4 the membrane 3 has a second gold layer 34 on the side of the cathode 5. The second gold layer 34 is applied directly to the sintered LLTO solid body 31 without prior acid treatment. Apart from this, the third experimental setup corresponds to the second experimental setup shown in figure 2 with the exception that the cathode 5 is arranged directly on the second gold layer 34 of the membrane 3. This results in there being electrical contact between the cathode 5 and the second gold layer 34. The second gold layer 34 accordingly becomes a formal constituent of the cathode 5.Figure 4 is a schematic diagram of the fourth experimental setup of the electrochemical cell 0. The fourth experimental setup corresponds to the third experimental setup shown in figure 3 with the exception that the electrochemical cell 0 is additionally fitted with an auxiliary electrode 6. The auxiliary electrode 6 is arranged in the second compartment 2 where it is contacted with the catholyte 8. The auxiliary electrode 6 may also be arranged outside the second compartment 2. All that matters is that it is in contact with the catholyte 8. The auxiliary electrode 6 is a nickel wire. The auxiliary electrode 6 is connected via a third electrical line 12 to the negative pole of a second electrical voltage source 13. The positive pole of the second electrical voltage source 13 is connected to the negative pole of the first electrical voltage source 9 via a fourth electrical line 14. The two electrical voltage sources 9, 13 are accordingly connected in series. A second electrical voltage W may be applied between cathode 5 and auxiliary electrode 6 via the second electrical voltage source 13.Example 1 : Sintering of an LLTO solid bodyCircular, flat 0.8 mm thick discs of lithium lanthanum titanium oxide (Toshima Manufacturing Co., Ltd., Higashimatsuyama, Japan) having the stoichiometry Lio.saLao.ssTiOs (according to manufacturer's data) were sintered for 10 hours at 1100°C in a furnace. The furnace was filled with ambient air and was operated at standard pressure.Figure 5 shows an SEM image of the surface of a disc before sintering. Figure 6 shows an SEM image of the surface of a disc after sintering.Examples 2 to 9: Acid treatment of the surface of the sintered solid bodyThe discs sintered according to example 1 were treated with various mineral acids at various temperatures over various periods. The acids were 98% sulfuric acid (H2SO4), 65% phosphoric acid (H3PO5), 30% nitric acid (HNO3) and 37% hydrochloric acid (HCI). All acids were present in the form of an aqueous solution at the specified concentration. The variations in temperature, time and acid are reported202500056 17in table 1. Each disc was subjected to precisely one treatment variant. The treatment is carried out by immersing in the acid (ex situ) or is installed, masked on one side, in an electrochemical cell without applied voltage (in situ). To this end the first, anodic compartment was filled with the acid.T able 1 : Overview of acid treatmentExample 2 3 4 5 6 7 8 9 Acid H2SO4 H3PO5 HNO3 HCI HCI HCI HCI HCI Duration 16 h 16 h 16 h 76 h 24 h 76 h 76 h 76 h Temperature 150°C 150°C 60°C 60°C 60°C 25°C 60°C 60°C Location ex situ ex situ ex situ ex situ ex situ ex situ ex situ in situFigure 7 shows an SEM image of the acid-treated surface according to example 2.Figure 8 shows an SEM image of an acid-treated surface according to example 6.Example 10: Gold plating of the surface of the sintered solid body (noninventive)A disc sintered according to example 1 which had not been subjected to acid treatment was subjected to whole-area sputtering with gold on one side. The following process parameters were established on a Quorum model Q 150R ES plus: sputter current: 20 mA; max. for a sputter time of 12 min.; thickness 25 nm; tooling factor 2.3. This provided the sintered solid body with a gold layer. The layer thickness was about 25 nm. The layer thickness was determined by weighing.Examples 11 to 18: Gold platina of the surface of acid-treated solid bodiesIn each case one of the discs acid-treated according to examples 2 to 9 was subjected to whole-area sputtering with gold on one side. The following process parameters were established on a Quorum model Q 150R ES plus: sputter current: 20 mA; max. for a sputter time of 10 to 20 min.; thickness 25 nm; tooling factor 2.3. This provided the sintered acid-treated solid body with a gold layer. Varying the sputter time made it possible to establish layer thicknesses of between 10 nm and 40 nm.The discs used as substrate in the sputtering experiments and the achieved layer thicknesses are reported in table 2.Table 2: Overview of gold plating of acid-treated substratesExample 11 12 13 14 15 16 1Z 18Substrate from example 2 3 4 5 6 7 8 9Gold plating, thickness [nml 40 40 30 25 10 25 25 25202500056 18Example 19: Electrodialysis with membrane from example 10 (noninventive)The solid body obtained in example 10 that had been gold plated without prior acid treatment was installed as a membrane in an electrochemical cell.The anolyte employed was a 1 M LiOH solution in water. The catholyte was an aqueous solution of 5 mM LiOH (0.005 mol / L). The temperature was 20°C. The voltage U was 4 V. The linear current density was 0.25 mA / cm2at 1 mm membrane thickness.After a run time of 60 hours, electrodialysis was interrupted and the membrane removed. The gold coating had become detached from the LLTO solid body over large areas; cf. figure 9.Example 19a: Electrodialysis with untreated membrane gold-plated on both sides (noninventive)A disc sintered according to example 1 that had not been subjected to acid treatment was subjected to whole-area sputtering with gold on both sides. The following process parameters were established on a Quorum model Q 150R ES plus: sputter current: 20 mA; max. for a sputter time of 16 min.; thickness 40 nm; tooling factor 2.3. This provided the sintered solid body with a respective gold layer on each of its two sides. The layer thickness was in each case about 40 nm.The anolyte employed was a 1 M LiOH solution in water. The catholyte was an aqueous solution of 5 mM LiOH (0.005 mol / L). The temperature was 20°C. The voltage U was 4 V. The linear current density was 0.25 mA / cm2at 1 mm membrane thickness.Examples 20 to 27: Electrodialysis with membrane from examples 11 to 18The acid-treated and gold-plated solid bodies obtained in examples 11 to 18 were each installed as a membrane in an electrochemical cell according to experimental setup 1.The anolyte employed was a 1 M LiOH solution in water. The catholyte was an aqueous solution of 5 mM LiOH (0.005 mol / L). The temperature was 20°C. The voltage U was 4 V. The linear current densities achieved are reported in table 3.Table 3: Overview of electrodialysis with gold-plated, acid-treated membranesElectrodialysis example 20 21 22 23 24 25 26 27 Membrane from example 11 12 13 14 15 16 17 18 Current density at 1 mm membrane 0,013 0.11 0.49 0.92 0.99 1.21 1.31 2.17 thickness[mA / cm2]202500056 19After a maximum run time of 600 hours, the electrodialyses were terminated and the membrane removed in each case. The gold coating was undamaged; cf. figure 10, which shows a photograph of the membrane from example 2.Conclusion from electrodialysis experimentsThe acid treatment improves the durability of the gold layer over a longer period. The acid treatment with HCI results in a higher current density in the results normalized to 1 mm membrane thickness. Best results were achieved with in-situ treatment (example 27).Examples 28 to 32: Investigation of influence of contactingTo investigate the effects of the contacting of the electrodes on the membrane, the membrane obtained from example 18 was installed in various embodiments of an electrochemical cell. An electrodialysis was then performed with the respective measurement setup. The anolyte used was 1 M LiOH in water and the catholyte was 5 mM LiOH in water. The voltage U of the first electrical voltage source was 4 V in all experiments with the exception of example 32. In the latter it was 6 V. The voltage of the second electrical voltage source W in examples 31 and 32 was 3 V.The course of the voltage and the current is apparent for example from the graphs shown in figures 11 to 13. These show the course of the measured data over time and also give a pointer to what is meant by the description “pronounced ageing” or “minimal ageing” of a membrane in use. Table 4 provides an overview of these examples.202500056 20Table 4: Overview of contacting experimentsContacting example 28 29 30 31 32 Embodiment of cell 1 2 3 4 4 Voltage U 4 V 4 V 4 V 4 V 6 V Voltage W 3 V 3 V Diagram Fig. 11 Fig. 12 Fig. 13Ageing behaviour Minimal Minimal Minimal ageing Very Very ageing over ageing over over 90 h, pronounced pronounced 400 h 75 h, then then ageing ageing measurement pronounced very within 22 h. within 24 h. time ageing up to pronouncedapprox. ageing and400 h experimentmeasurement ended bytime membranefailure.Maximum linear Approx. 4 to Approx. Approx. 4 to Approx. 3.2 mA / current density of a 5 mA / cm25 mA / cm25 mA / cm25.8 mA / cm2cm21 mm thickmembraneConclusion from contacting experimentsThe arrangement of the electrodes directly on the membrane proposed by SASAKI et al (examples 29, 30, 31 , 32) shows the highest current densities at a specified voltage of 4 V at the start of the electrodialyses, but in this arrangement the membranes showed pronounced ageing. The relatively rapid decline in current density at constant voltage over time is regarded as ageing. The more rapidly it falls, the more pronounced the ageing of the membrane. Once the current density has fallen below a threshold defined as 50% of the initial current density, the electrodialyses are stopped and the membrane must be replaced. The embodiments without contacting (example 28) do not result in any significant ageing over a period of weeks, with only slight ageing observed even at elevated temperatures of 50°C.The comparison with example 19a shows that gold-plating on both sides with the direct contacting proposed by SASAKI et. al. does not achieve a better result.202500056 21Example 33: Investigation of influence of temperatureTo investigate the influence of temperature, the membrane obtained from example 18 was installed in an electrochemical cell of the first embodiment. An electrodialysis was then carried out at increasing temperatures. The anolyte used was 1 M LiOH aqueous solution and the catholyte was 5 mM LiOH. The voltage was 4 V.The course of the increase in current IT / IO is shown in figure 14. In the figure, IT describes the current at the respective temperature and Io the current at room temperature.Conclusion from investigation of temperatureIncreasing the temperature brings about an increase in current to almost 200% of the current measured at room temperature. In the operation of an electrochemical cell, the electrolytes often undergo heating as a result of the applied voltage and through energy losses of the circulating pumps. Since this increase in temperature has a positive effect on the performance of the membrane and thus of the electrochemical cell, no cooling measures are taken to significantly reduce the electrolyte temperatures in use.Example 34: Investigation of influence of voltageTo investigate the influence of the electrical voltage, the membrane obtained from example 18 was installed in an electrochemical cell of the first embodiment. An electrodialysis was then carried out at increasing voltages. The anolyte used was 1 M LiOH aqueous solution and the catholyte was 5 mM LiOH.The course of the increase in current lu / lo is shown in figures 15 and 16. Therein, lu describes the current at the respective voltage and Io the current at a voltage of 4 V. Measured points from several measurement series (figure 15: several experiments with variation of voltage between 4 V and 8 V and figure 16: an experiment with variation of voltage between 4 V and 12 V) with different membranes in the voltage range from 2 to 12 V that differ only negligibly from one another are shown. At about 8 V and above the current is associated with more pronounced ageing of the membrane overtime, with the result that a divergence of various experimental results, which varies according to the duration of the measurement, is measured here.Extrapolation of the linear relationship to 0% improved current (intercept with the x axis) gives the cumulative value for the voltage required for anodic and cathodic water splitting at the electrodes occurring simultaneously in the cells, elevated by the voltage drop due to the internal resistance of the cells.202500056 22Conclusion from investigation of voltageIncreasing the voltage results in a near-linear increase in current at low voltages. Much higher voltages result in divergences from the behaviour described here, which is then also associated with more pronounced ageing. Therefore, high voltages of over 12 V, ideally of over 8 V, should be avoided in use.Summary of the experimental results:It is apparent from the experiments that the performance of the membrane can be markedly increased by inventive treatment of the LLTO membrane surface. A current density of about 20 A / m2is easily attainable via the experimental setup (about 2 V voltage drop over the membrane) for a 1 mm thick membrane at 4 V voltage. Taking into account the possible increase in current density by increasing the voltage (+100% per 2 V) and increasing the temperature (+100% at a temperature of about 50°C) it is easy to estimate that current densities of about 200 A / m2are possible at only 12 V and 50°C. A technically easily realizable reduction of the membrane thickness to 0.5 mm would allow a further 100% increase to about 400 A / m2.The maximum achievable current density has not hitherto been described for LLTO membranes over such long periods of time.202500056 23List of reference symbols0 Electrochemical cell1 First compartment2 Second compartment3 Membrane31 Solid body32 Acid-treated surface33 (First) gold layer (anode side)34 Second gold layer (cathode side)4 Anode5 Cathode6 Auxiliary electrode7 Anolyte8 Catholyte9 (First) electrical voltage source10 First electrical line11 Second electrical line12 Third electrical line13 Second electrical voltage source14 Fourth electrical lineAaa Area of first active surface area of the membrane Aak Area of second active surface area of the membrane U (First) electrical voltageW (Second) electrical voltageci(f) Concentration of Li+cations in the anolyte02(f) Concentration of Li+cations in the catholyteH2O WaterH2 HydrogenO2 OxygenH+ProtonsLi+Lithium cationsOH" Hydroxide anionsNa+Lithium cationsCl' Chloride anionsSO ' Sulfate anionsLiOH Lithium hydroxide

Claims

202500056 24Claims1. Process for producing a membrane comprising the following non-chronological steps:a) providing a two-sided flat solid body, wherein the solid body contains a lithium lanthanum titanium oxide having the LLTO stoichiometry:Li3xLa(2 / 3)-xn(i / 3)-2xTiO3 (LLTO)wherein in LLTO: 0 x< 0.16 and wherein the lithium lanthanum titanium oxide may optionally be doped with at least one doping element selected from the group consisting of the following doping elements: Ag, Al, Zr, Nd, Sr, Ta, Y, Ge, Nb;b) providing an acid;c) providing gold;d) treating one side of the flat solid body with the acid;e) coating the acid-treated side of the solid body with the gold;f) obtaining a membrane comprising the flat solid body having an areal gold plating on its acid- treated side.

2. Process according to Claim 1 , characterized in that the acid is hydrogen chloride (HCI).

3. Process according to Claim 2, characterized in that the treatment with liquid hydrogen chloride is carried out at a temperature of 20°C to 80°C over a period of 20 h to 80 h.

4. Process according to any of Claims 1 to 3, characterized in that one side of the flat solid body is masked during the acid treatment.

5. Process according to any of Claims 1 to 4, characterized in that the acid treatment is carried out in an electrochemical cell.

6. Process according to any of Claims 1 to 5, characterized in that the flat solid body is sintered prior to the acid treatment.

7. Process according to Claim 6, characterized in that the sintering is carried out at an end temperature of 900°C to 1200°C over a period of 1 h to 16 h.

8. Membrane comprising a two-sided flat solid body, wherein the solid body contains a lithium lanthanum titanium oxide having the LLTO stoichiometry:Li3xLa<2 / 3)-xn(1 / 3)-2xTiO3 (LLTO)202500056 25wherein in LLTO: 0 < x < 0.16 and wherein the lithium lanthanum titanium oxide may optionally be doped with at least one doping element selected from the group consisting of the following doping elements: Ag, Al, Zr, Nd, Sr, Ta, Y, Ge, Nb;wherein at least one side of the flat solid body is acid-treatedand wherein the acid-treated side of the solid body has an areal gold plating.

9. Membrane according to Claim 8, wherein the flat solid body on its acid-treated, gold-plated side has an area Aaa, characterizedin that at least 80% of the area Aaa is gold-plated;orin that 90% to 100% of the area Aaa is gold-plated.

10. Membrane according to Claim 8 or 9, characterized in that the lithium lanthanum titanium oxide has the following stoichiometry:Lio.33Lao.5sTi03orLio.29Lao.57Ti0311. Membrane according to Claim 8 or 9 or 10 obtainable by a process according to any of Claims 1 to 7.

12. Use of a membrane according to any of Claims 8 to 11 in a process for producing lithium hydroxide and hydrogen from lithium-containing waters.

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