Electrochemical method for production of lithium metal and products comprising lithium metal

Abstract

The present invention concerns an electrochemical method for the production of lithium metal and / or products comprising lithium metal. This objective is achieved by using a composition comprising an ionic liquid, a lithium salt and a precise aliquot of water within a single electrolytic cell.

Description

[0001] Electrochemical method for the production of lithium metal and products comprising lithium metal DESCRIPTION

[0002] TECHNICAL FIELD

[0003] The present invention relates to an electrochemical method for the production of lithium metal and products comprising lithium metal, such as lithium alloys or metal multilayers comprising lithium.

[0004] In particular, the method is based on the electrodeposition of lithium metal from mixtures of ionic liquids, water and lithium-ion salts.

[0005] STATE OF THE ART

[0006] Lithium metal is usually produced by electrolysis of a LiCI-KCI eutectic at high temperatures of up to about 600°C. A graphite electrode (cathode) is used where CI2 is produced while the lithium metal floats over the molten electrolyte. Once the lithium metal has been obtained, in order to have lithium films it is necessary to carry out a hot extrusion process with which it is possible to obtain films with a thickness of about 20 microns.

[0007] It is also possible to prepare thin films of lithium metal by passing a slightly oxidized copper foil to the surface in a bath of molten lithium metal (over 200°C). This turns out to be an expensive method due to the need to handle such a reactive chemical compound in a liquid state.

[0008] Another possible method of producing lithium metal involves thermal vaporization: thin films of lithium metal can be deposited on copper by vaporizing lithium at high temperatures. This is also a very expensive and energy-intensive process; moreover, only small surfaces (in the order of cm2) can be covered with lithium.

[0009] Other methodologies involve the use of sulfuric acid-based solutions, which however involve the development of gaseous CI2, or involve the use of fluorinated compounds. Still other methodologies, such as described in US 1 1 ,486,045, involve the use of anhydrous ionic liquids and avoid the use of lithium halides due to the possible concomitant development of gaseous halogens (in particular CI2). In these methodologies there is no presence of water in the electrolyte mixture, the water molecules are generated in-situ. The electrolyte is chosen from: bis(trihaloalkylsulfonyl)imides, bis(trihalosulfonyl)amides and bis(halosulfonyl)amides and corresponding acids in a non-aqueous environment such as acetonitrile.

[0010] The above technologies have numerous disadvantages because, apart from the high costs of the reagents, they mainly involve the use of fluorine-based ionic liquids, such as bis(trifluoroalkylsulfonyl)imide (TFSI), known to have high toxicity and to be difficult to handle. In addition, TFSI anion monoprotic acid (HTFSI) is a very strong acid characterized by high toxicity and corrosivity. Therefore, the use of TFSI and more generally of compounds containing halogens and in particular fluorine, such as those indicated, involves heavy chemical and structural limitations. In fact, it is necessary to divide the electrolytic cell into two separate compartments, an anode compartment and a cathode compartment, connected to each other for instance with a salt bridge, with consequent pressure drops between the two compartments.

[0011] The separation between the two compartments is necessary because the acid form HTFSI, coming into contact with the metallic Li formed during reduction, would corrode the Li product making the process of no interest. In addition, the presence of HTFSI acid severely limits the use of the cell, since HTFSI acid corrodes the cell material bringing unwanted species into solution and compromising the mechanical resistance of the cell itself.

[0012] It should be emphasized that not only the cell cannot be recovered, but also the electrolyte solution is substantially disposable, since, following the formation of HTFSI acid, it is very difficult and economically disadvantageous to reform the ionic liquid in its cation+1-

[0013] TFSh1form.

[0014] In addition, the use of lithium compounds such as LiCI, LiF, LiPFe and LiBF4 is explicitly prohibited in the technology described above as they form halogens under the reaction conditions used.

[0015] In addition, these technologies require two different sources of lithium ions, one to the cathode compartment as LiTFSI, which is reduced by providing metallic Li, and the other to the anode compartment, as a lithium compound (not containing halogens), which must necessarily be added intermittently between batches or fed continuously to the anode during the electrolysis process as said lithium compound is consumed by the reaction.

[0016] Finally, these systems show a low lithium reduction rate due to the low applicable electrical current (less than 1 mA / cm2): in fact, the two compartments of the cell are separated and connected via a salt bridge, resulting in higher lithium ion migration times than systems without a salt bridge, since the presence of the bridge inevitably involves a longer migration path. Higher electric currents that do not support the electrochemical process should also be avoided. In addition, such a cell configuration, which involves a large amount of electrolyte, causes high resistance, and this results in a low current density to promote the process.

[0017] There are also well-known documents describing the production of Li in a single electrolytic cell. For instance, an electrochemical process for the recovery of lithium metal from lithium salts in a medium consisting of an organic acid (such as HTFSI) and an ionic liquid, preferably with the same TFSI anion, is described. Electrolysis is conducted in a strictly anhydrous environment to allow the controlled deposition of lithium metal. The reactions at the anode are not explicated, probably the cation of the ionic liquid would be oxidized, but it is certainly not possible the O2 evolution, given the explicit elimination of water, which instead, in such conditions, would produce H2 instead of Lithium. The reason why the conditions must be strictly anhydrous depends on the fact that the presence of even traces of water would lead to deleterious effects: in particular, at the cathode the water would be preferentially reduced compared to lithium, with the consequent evolution of hydrogen gas and the formation of lithium hydroxides. This not only prevents the deposition of lithium metal but also leads to the formation of resistive layers (such as LiOH or Li2CO3) that hinder further electrochemical processes.

[0018] In addition, the development of hydrogen introduces safety concerns and drastically reduces the coulombic efficiency of the process.

[0019] It follows that the teaching of the art is to strictly avoid the presence of water, since even minimal quantities of it make the process inefficient, unstable and dangerous. It should also be noted that in the context of lithium-ion batteries (LiB), perfluoroalkyl and polyfluoroalkyl substances (PFAS) known as bis-perfluoroalkyl sulphonimides (bis-FASI) are considered highly polluting and internationally recognized as toxic

[0020] (httDs: / / www.nature.com / articles / s41467-024-49753-5; httDs: / / echa.europa.eu / it / - / echa-

[0021] -restriction- https: / / cen.acs.org / policv / chemical-requlation / battle-

[0022] Therefore, the need to have technologies that make the production of lithium metal possible is currently unresolved:

[0023] - without the release of harmful gases or vapours such as those based on gaseous halogens (in particular chlorine), without the use of fluorine or fluorinated compounds, which are dangerous and / or toxic to humans and the environment, at temperatures much lower than those reported in known art (e.g. <100°C), in film form without having to perform extrusion steps, in the form of a thin film with a thickness of the order of microns,

[0024] - without the use of highly corrosive and toxic acids,

[0025] - with a high deposition rate of Li,

[0026] - with the complete recovery or reuse of the reagents used in the reaction,

[0027] - with easy, fast and directly industrializable reactions.

[0028] Unless specifically excluded in the detailed description below, what is described in this chapter is to be considered as an integral part of the detailed description.

[0029] SUMMARY OF THE INVENTION

[0030] The above issues are resolved by the method claimed in claim 1 .

[0031] Therefore, it is an object of this invention an electrochemical method for the production of lithium metal, a product comprising lithium metal or lithium-based alloys or metal multilayers comprising lithium. The method involves the use of one or more lithium salts and one or more ionic liquids in a mixture, with the addition of a well-defined portion of water.

[0032] Another object of the invention is a composition, also called "electrolyte solution", which comprises one or more lithium salts, one or more ionic liquids having an anion in common with the lithium salt and a well-defined quantity of water, part of which can derive from the solvation of lithium salts.

[0033] Another object of the invention is an electrolytic cell to carry out the method of the invention, said cell housing both electrodes (anode and cathode) in a single compartment or container in the presence of the electrolyte solution.

[0034] Other objects of the invention are: lithium metal, products comprising lithium metal, lithium-based alloys, metal multilayers comprising lithium, such as electrodes comprising lithium metal having a micrometric thickness, with lithium deposited on a support, alone or in the form of an alloy or as a metal multilayer. These and other objects of the invention will be evident from the detailed description of the invention.

[0035] BRIEF DESCRIPTION OF THE FIGURES

[0036] The invention is described below on the basis of non-limiting examples illustrated by way of example in the following figures, which refer respectively to:

[0037] Figure 1 : schematic representation of an example of an electrolytic cell comprising a positive graphite electrode, a negative aluminium electrode and a mixture comprising an ionic liquid, i.e. BMIM-CI, water (>0-20% by volume with respect to the volume of the composition) and a lithium salt, i.e. LiCI;

[0038] Figure 2: schematic representation of the electrochemical reaction of the electrolytic cell;

[0039] Figure 3a, b: X-ray diffractogram of powders (a) and energy loss spectroscopy map (b) of the lithium aluminium alloy;

[0040] Figure 4a, b: photograph of the evolution of gas at the positive electrode (a) and graph representing the increase in oxygen concentration as a function of the time of use of the cell (b);

[0041] Figure 5a, b:1H NMR spectrum portion of the ionic anhydrous fluid, ionic liquid hydrate, and ionic liquid hydrate after use of the cell, i.e. following the formation of hydrochloric acid (a); photograph of lithium metal deposited on the negative copper electrode (b).

[0042] DETAILED DESCRIPTION OF THE INVENTION

[0043] The following definitions are given in the context of the present invention.

[0044] Electrolytic cell: electrochemical cell comprising a current generator, two electrodes and a liquid with a non-zero electrical conductivity. In the electrolytic cell, oxidation and reduction reactions are not spontaneous, and are promoted by electric current. The electrolytic cell converts electrical energy into chemical energy. Galvanic cell: electrochemical cell comprising two electrodes and a liquid with a nonzero electrical conductivity. In the galvanic cell, oxidation and reduction reactions are spontaneous. Said galvanic cell converts chemical energy into electrical energy.

[0045] Positive electrode or anode: inside the electrolytic cell is the electrode whereon the oxidation reaction takes place.

[0046] Negative electrode or cathode: inside the electrolytic cell is the electrode whereon the reduction reaction takes place.

[0047] Ionic liquid: compound in the liquid state comprising only positive and negative ions and their combination, i.e. their salts. An ionic liquid is a chemical compound consisting entirely of ions (anions and cations) that is in a liquid state at or near room temperature (typically below 100°C). Unlike common salts, which are found in a solid state at room temperature (such as sodium chloride), ionic liquids have very low melting points, often due to the asymmetrical nature or large volume of the ions that compose them, which prevents the formation of a rigid crystal structure. Ionic liquids have very low vapor pressures, which means they evaporate little or nothing, making them useful in applications wherein vapor emission needs to be minimized. They also have high ionic conductivity and can remain stable over a wide range of electrical potentials, making them useful for electrochemical applications such as batteries and supercapacitors.

[0048] In this document, the terms "approximately" or "about" as used herein when referring to a measurable value such as a quantity, a time duration, and the like, are intended to encompass variations of ±20%, ±10%, ±5%, ±1 %, or ±0.1 % from the specified value, wherein such variations are appropriate to perform the methods described.

[0049] In this document, the terms "electrochemical composition", "electrolyte solution", "electrolyte mixture" and "electrochemical mixture" as used herein are to be considered synonymous, unless otherwise indicated. The present invention concerns the production of lithium metal by electrolytic methods. Lithium metal can also be in the form of a metal alloy or a metal multilayer, preferably metal alloys are lithium-aluminium, lithium-zinc, lithium-nickel, and lithiummagnesium alloys.

[0050] The invention method is a low-temperature method carried out in an electrolysis cell wherein the anode and cathode are housed in a single container and immersed in a single electrolyte solution containing a lithium salt and an ionic liquid having the same anion in common with each other; the composition also provides for the presence of water in a certain concentration range; the electrolytic reaction takes place at low temperature, wherein the low temperature is a temperature typically below the melting point of lithium (180°C). Preferably the temperature is in the range 0-120°C, preferably 0-100°C, more preferably 15-90°C

[0051] The composition of the electrolyte solution is as follows: ionic liquid, lithium salt, water in concentration >0% by volume relative to the volume of the composition (v / v), preferably between 0.1% and 20% v / v, preferably between 1 % and 10% v / v, measured by nuclear magnetic resonance (NMR). In particular, the NMR spectra were obtained using a Bruker Avance Neo 400 MHz NMR spectrometer, operating at 400.13 MHz for1H. The spectra of1H NMR were obtained at 363°K with a 5 mm probe for liquids and a standard acquisition / pulsation sequence without a lock on deuterium. Water can also be supplied as solvation water. Water, which forms the source of oxygen that evolves at the anode as O2, can be added intermittently between batches or added continuously to the electrolytic cell during the electrolysis process as it is consumed by the reaction.

[0052] With reference to the presence of water in the electrolyte mixture, this is essential for the performance of the reaction, but, in principle, it is not necessary for it to be present in specific and determined quantities because: - its lower limit is related to the amount of lithium that is deposited at the cathode; as the amount of water decreases, the amount of lithium deposited decreases (in theory, a single molecule of water could be sufficient for the deposition of lithium, but then, once that is consumed, the process would stop, unless other molecules are added);

[0053] - the upper limit is linked to the competition between the formation of Li(S) and that of H2(g), which would prevail for higher quantities of water, as explained by the competitive reaction (2) indicated below.

[0054] In the context of micro batteries, for which powers of the order of pA are required, very small quantities of water may be acceptable.

[0055] The lithium salt can be chosen from one or more of the following compounds: LiX, with X=halide ion, carbonate ion, sulphate ion, borate ion, preferably chloride ion, bromide ion, iodide ion;

[0056] The ionic liquid can be chosen from one or more of the following compounds:

[0057] 1 -R-3-methylimidazolium-X

[0058] 1 -R-pyridinium-X

[0059] 1 -R-piperidinium-X

[0060] 1 - R-pyrrolidi n- 1 -ium-X

[0061] Tetra-R-ammonium-X

[0062] Tri-R-sulphonium-X

[0063] Tetra-R-phosphonium-X wherein

[0064] X=halide ion, carbonate ion, sulphate ion, borate ion, preferably chloride ion, bromide ion, iodide ion;

[0065] R = linear, branched or cyclic alkyl chain C1-C10; with the condition that X is the same anion for the lithium salt and for the ionic liquid. Lithium salt is dissolved in the electrolyte composition, and electrical energy is applied to the anode and cathode to form lithium metal on the cathode of the electrolytic cell. Lithium metal produced on the surface of the cathode has a purity of more than 97% by weight or even more than 99% by weight and can be used without further purification.

[0066] The power supply applied to the cell has a current density greater than approximately 0.1 mA / cm2, preferably greater than approximately 0.5 mA / cm2, preferably greater than approximately 1 mA / cm2, preferably greater than approximately 2 mA / cm2, preferably greater than approximately 3 mA / cm2, preferably greater than approximately 4 mA / cm2, preferably greater than approximately 5 mA / cm2, preferably greater than approximately 6 mA / cm2, preferably greater than approximately 10 mA / cm2, preferably greater than approximately 15 mA / cm2, preferably greater than approximately 20 mA / cm2, the upper limit of current density being only a function of the cell size. In the cell, the material the cathode is made of can be chosen from aluminium, copper, nickel, magnesium, zinc, iron, steel such as stainless steel.

[0067] In the cell, the material the anode is made of can be chosen from graphite, platinum.

[0068] With reference to Fig. 1 , which represents a non-limiting embodiment of the present invention, the production device can advantageously be an electrolytic cell (A) comprising an anode (1 ), i.e. a positive electrode, graphite and a cathode (2), i.e. a negative electrode, in the form of a copper or aluminium metal sheet.

[0069] The device consists of a single container (3) containing, as an electrolyte, a mixture of an ionic liquid, a lithium salt and a quantity of water not exceeding 10% v / v and a current generator (4) connected to the cathode (2) and the anode (1 ) to provide a current density suitable for conducting the method of the invention. Water acts as a carrier of lithium ions and at the same time is a source of electrons producing oxygen gas at the graphite anode as an oxidation product. An acid corresponding to the counterion of lithium salt is produced in the electrolyte. The cell comprises the negative electrode, the positive electrode, and the mixture comprising: one or more ionic liquids, water (1 -10% v / v), and a lithium salt. Each component will then be described in detail.

[0070] In a non-limiting embodiment, the electrolytic cell comprises a current generator connected to the two electrodes, wherein the positive electrode, i.e. the anode, is graphite, and the negative electrode, i.e. the cathode, is made of copper or aluminium. In addition, the electrolytic cell comprises a mixture wherein the electrodes are immersed comprising LiCI as a lithium salt, butyl-methyl-imidazolium chloride (BMIM-CI) as an ionic liquid, and a well-defined aliquot of water with a maximum value of 10% v / v. According to this example, the electrodes and electrochemical mixture are contained in a single glass container.

[0071] Oxidation and reduction reactions take place within the same electrolytic cell, in particular oxidation, i.e. transfer of one or more electrons, takes place at the positive electrode, i.e. the anode, while reduction, i.e. acquisition of one or more electrons, takes place at the negative electrode, i.e. the cathode.

[0072] In the present invention, the material to be reduced is lithium cation, which passes from the oxidation state +1 to the oxidation state 0, corresponding to lithium metal (Li++ e- - Li).

[0073] That is, through the method of the present invention it is possible to form lithium metal starting from a lithium salt, i.e. starting from a lithium salt solution it is possible to obtain solid lithium metal. Since this is a reduction process, the lithium cation acquires an electron, supplied by the negative electrode, so the reduction of the lithium ion takes place at the surface of the negative electrode, and it is therefore at the surface of the latter that the lithium metal is formed.

[0074] To summarize, lithium enters in the solution as a salt with a +1 oxidation state, then migrates to the negative electrode, where it receives an electron reducing itself to lithium metal and remaining sticking to the surface of the negative electrode. It should be emphasized that the deposit of lithium on the surface of the electrode does not hinder the deposition of other lithium metal; in fact, since lithium is a metal, the electrons of the negative electrode can pass through the deposited lithium and reach new lithium in the +1 oxidation state at the surface, reducing it and thus thickening the deposit of lithium on the negative electrode. Therefore, the lithium metal layer can be of any thickness you want, preferably with a thickness between 1 and 10 pm, preferably about 5 mm. It should be emphasized that the upper value of the range is not limiting and can be higher; the upper value of 10 mm is indicated only because it is of commercial interest to produce materials with low thickness.

[0075] The flow of electrons to the negative electrode results from the oxidation process that occurs simultaneously on the positive electrode. According to the present invention, the material that oxidizes is the oxygen atom provided by the water molecules present in the mixture, wherein oxygen passes from an oxidation state of -2 to an oxidation state of 0, evolving into gaseous oxygen.

[0076] This lithium production method therefore has the advantage of being a green method since during oxidation no hazardous or waste gases or gases harmful to humans and the environment are generated, but only oxygen is produced.

[0077] In addition, the ionic liquids used also do not contain fluorine, although this halogen can be used as an ion with the same functionality as chlorine, bromine and iodine, which are considered preferred in the context of the present invention. It should therefore be emphasized that the present invention can use fluorine, but this choice, even if possible, is not preferable.

[0078] The process of forming lithium metal is therefore a cycle that involves the reduction of lithium through electrons obtained from the oxidation of water. In conclusion, the present invention receives a lithium salt and water as input and produces lithium metal and oxygen gas. During this process, the ionic liquid is not consumed, a significant advantage over state-of-the-art technologies wherein the degradation products of the ionic liquid are difficult to reuse. In more detail, the conversion of the degradation products of the ionic liquid back into ionic liquid is not easy and presents considerable difficulties.

[0079] According to an embodiment, the present invention can be represented by the following equation:

[0080] 2l_iCI + H2O + 2AI --- -> 2Li Al +1 / 2O2+ 2HCI (1 )

[0081] The competitive reaction is that of water electrolysis by oxidation of oxygen atoms and reduction of hydrogen ions, according to the reaction

[0082] 2H2O — > 2H2+ O2 (2)

[0083] This reaction becomes more and more favored as the amount of water in the electrochemical mixture increases; this quantity is therefore kept in a very specific range, so as to disadvantage the reaction (2).

[0084] Figure 2 shows a schematic representation of equation (1 ).

[0085] According to this embodiment, the electrolytic cell is a single container that comprises a positive electrode, i.e. a commercial graphite electrode, a negative electrode, i.e. an aluminium foil, and a mixture comprising an ionic liquid, such as butyl-methyl-imidazolium chloride (BMIM-CI) as an ionic liquid, water (>0-20% v / v) and lithium chloride as a lithium salt.

[0086] The electrolytic cell is connected to an external circuit, which supplies an electric current to the cell to promote reduction and oxidation reactions. The value of the applied current preferably has a range of 0.5-10 mA / cm2, more preferably in use an electric current of about 2 mA / cm2is applied.

[0087] Advantageously, using a current above 1 -2 mA / cm2allows for rapid lithium reduction. In particular, there is a direct relationship between the current value and the lithium reduction rate, so that as the electric current increases, the lithium reduction rate increases. The increased rate allows to get a product in a shorter time, increasing the productivity of the system.

[0088] In the liquid mixture, the lithium salt in solution separates into anion, i.e. Cl’, and cation, i.e. Li+. The cation migrates in the mixture to the surface of the negative electrode, i.e. an aluminium foil, where it undergoes a reduction process forming lithium metal on the surface of the foil.

[0089] According to an embodiment of the invention, if metallic aluminium is used as an anode, the metallic lithium will not remain deposited as an outer layer on the aluminium foil but will form an alloy on the surface of the aluminium (LiAl in reference to equation (1 )). The alloy formed belongs to the cubic crystal system and is assigned to the known alloy LiAl (ICSD 24019). The assignment is made by comparing X-ray diffractograms (XRDs) from powders (Figure 3a). The coexistence of Li and Al in the same portion of material is also confirmed by an energy loss spectroscopy (EELS) map. This spectroscopy, performed with a ThermoFisher Scientific Talos 200S microscope, equipped with a post-column spectrometer (Gatan Inc.) is based on irradiation of the sample with a beam of electrons with a predetermined kinetic energy and the measurement of the latter after impact with the sample. The variation in kinetic energy is related to the elemental composition of the sample, since different elements modify the kinetic energy of the electrons in different and identifying ways. Figure 3b shows a cross-section of the product of the reduction reaction, i.e. the lithium-aluminium alloy, and the respective EELS map of this section, wherein it is possible to see how lithium is distributed in the aluminium matrix.

[0090] Simultaneously with the process just described, at the positive electrode water is oxidized to form gaseous oxygen, as shown in Figures 4a and 4b. Figure 4a shows a photo of the graphite electrode whereon gas bubbles evolve during the application of the external current. The identification of this gas was made by means of a mass spectrometer and in particular the oxygen signal is related to the reaction time, i.e. the amount of oxygen is measured as a function of the time in use of the cell. It is immediately visible how the amount of gaseous oxygen increases over time, indicating that at the positive electrode water is oxidized by evolving gaseous oxygen.

[0091] The oxidation of water to oxygen gas also produces H+ions which, in the case of lithium chloride, form hydrochloric acid. The formation of hydrochloric acid is confirmed by1H NMR spectroscopy, which shows the characteristic signal of an acidic proton at 15 ppm (Figure 5a). Therefore, according to the present invention: the electrochemical mixture, before the reduction and oxidation reactions has a first pH and after the reactions has a second pH, the second pH being lower than the first.

[0092] It is crucial to emphasize the role of water in the process. In fact, in a control experiment, a device consisting of a single container containing, as an electrolyte, a mixture of an ionic liquid, a lithium salt and no amount of water and a current generator connected to the cathode and anode was used to provide a current density. When the same amount of current as in the previously described embodiment was supplied to the device, i.e. 2 mA / cm2, no lithium deposit was obtained on the negative electrode. This is because in the control example the only agent capable of oxidizing is the chloride ion; however, in the current regime used according to the present invention, the electrical energy supplied is not sufficient to convert the chloride ion into chlorine gas, i.e. 2CI’ + 2e~ — > Cl2.

[0093] Given its key role, it is important to define the amount of water within the electrochemical mixture. It must be >0% v / v, is variable and preferably within a concentration range between 0.1 % and 20% by volume with respect to the volume of the composition (v / v). However, it should be emphasized that the lower limit is higher than 0% v / v, while the upper limit is defined as the amount of water that prevents the formation of lithium metal. The lower limit is due to the fact that water is the medium through which electrons are supplied to reduce the lithium cation; therefore, if the electrolyte mixture had a quantity of water below the indicated threshold value, the deposition of lithium would not take place or would take place with difficulty because there would be too little oxygen atoms to oxidize to supply electrons to the lithium cation and the process would be exhausted quickly. The upper limit is due to the competition between lithium and hydrogen cations in the reduction reaction. In fact, after the oxidation of the oxygen of the water molecule, hydrogen cations are released into solution. As previously described, these cations form hydrochloric acid with chloride anions by acidifying the solution. However, the formation of hydrochloric acid is not the only possible reaction for hydrogen cations; in contact with the negative electrode, they undergo a reduction process by acquiring an electron and forming molecular hydrogen, i.e. 2H++ 2e_— H2. This reduction process competes with the reduction of lithium cations, so if the amount of water in the electrochemical mixture exceeds a threshold value, i.e. 20% v / v, the reduction of lithium will be inhibited and at the negative electrode the preferential evolution of hydrogen gas will be noted. It should also be noted that the reduction of H+occurs even when the amount of water is below the maximum threshold, i.e. 20% v / v; however, being in smaller quantities than lithium, molecular hydrogen is a minor product and its production does not affect the production of lithium metal (with reference to equation (1 ), both HCI and H2 are among the products).

[0094] In another embodiment, the negative electrode is copper, while the remaining working conditions are the same as those of the previous embodiment. That is, an electrolytic cell is provided, comprising a positive electrode, preferably graphite, a negative electrode, i.e. copper, an electrochemical mixture comprising an ionic liquid and a lithium salt, preferably BMIM-CI and LiCI, and the concentration of water is between 1 % and 20% v / v. This electrolytic cell is connected to an external current system. In use, current is supplied to the electrolytic cell via the electrodes and promotes reduction reactions, i.e. Li++ e’ Li, and oxidation reactions, i.e. 2 O2-^ O2+ 4e_, forming lithium metal at the interface with the negative electrode and evolving oxygen gas at the interface with the positive electrode.

[0095] The product obtained according to this embodiment is not a lithium-copper alloy, but a bi-layer wherein lithium is deposited on the surface of copper, as shown in Figure 5b. Advantageously, the thickness of the lithium foil can be controlled by the operator according to the use time of the electrochemical cell. It is therefore possible to make a metal bi-layer wherein one layer is copper, and one layer is lithium and wherein the thickness of the lithium layer does not exceed 1 pm.

[0096] It is possible to associate the electrolytic cell with a direct or indirect monitoring system of the amount of water, for instance by monitoring the pH of the solution, since hydrochloric acid is developed following the oxidation of oxygen, or by monitoring the amount of oxygen gas produced at the anode. In this way, it is possible to configure the cell for the restoration of water and lithium salt, for instance via a manually operated dripper funnel or automatically via an actuator designed to open / close the funnel opening, when the amount of water consumed falls below the preset threshold value, preferably 1 %.

[0097] In this configuration, the electrolytic cell can operate continuously.

[0098] It should be noted that, unlike the known art, the method allows not to chemically alter the ionic liquid, but only consumes water and lithium salt which, once restored, keep the cell operating continuously. In this case, it is possible to add a base to counteract the increase in acidity of the electrochemical liquid.

[0099] In another embodiment, the negative electrode is magnesium, while the remaining working conditions are the same as those of the previous embodiment. That is, an electrolytic cell is supplied, comprising a positive electrode, preferably graphite, a negative electrode, i.e. magnesium, an electrochemical mixture comprising an ionic liquid and a lithium salt, preferably BMIM-CI and LiCI, and the concentration of water is between 1 % and 20% v / v. This electrolytic cell is connected to an external current system. In use, current is supplied to the electrolytic cell through electrodes and promotes reduction, i.e.

[0100] Li++ e- — > Li, and oxidation reactions, i.e. 2 O2’ -^ 02 + 4e_, forming lithium metal at the interface with the negative electrode and evolving oxygen gas at the interface with the positive electrode.

[0101] The product obtained according to this embodiment is an alloy of lithium and magnesium.

[0102] In another embodiment, the negative electrode is zinc, while the remaining working conditions are the same as those of the previous embodiment. That is, an electrolytic cell is supplied, comprising a positive electrode, preferably graphite, a negative electrode, i.e. zinc, an electrochemical mixture comprising an ionic liquid and a lithium salt, preferably BMIM-CI and LiCI, and the concentration of water is between 1 % and 20% v / v. This electrolytic cell is connected to an external current system. In use, current is supplied to the electrolytic cell through electrodes and promotes reduction, i.e. Li++ e’ -> Li, and oxidation reactions, i.e. 2 O2’ -^ 02 + 4e_, forming lithium metal at the interface with the negative electrode and evolving oxygen gas at the interface with the positive electrode.

[0103] The product obtained according to this embodiment is an alloy of lithium and zinc.

[0104] In another embodiment, the negative electrode is iron, while the remaining working conditions are the same as those of the previous embodiment. That is, an electrolytic cell is supplied, comprising a positive electrode, preferably graphite, a negative electrode, i.e. iron, an electrochemical mixture comprising an ionic liquid and a lithium salt, preferably BMIM-CI and LiCI, and the concentration of water is between 1 % and 20% v / v. This electrolytic cell is connected to an external current system. In use, current is supplied to the electrolytic cell through electrodes and promotes reduction, i.e. Li++ e’ -> Li, and oxidation reactions, i.e. 2 O2’ -^ 02 + 4e_, forming lithium metal at the interface with the negative electrode and evolving oxygen gas at the interface with the positive electrode.

[0105] The product obtained according to this embodiment is a bilayer of lithium and iron. In another embodiment, the negative electrode is stainless steel, while the remaining working conditions are the same as those of the previous embodiment. That is, an electrolytic cell is provided, comprising a positive electrode, preferably graphite, a negative electrode, i.e. stainless steel, an electrochemical mixture comprising an ionic liquid and a lithium salt, preferably BMIM-CI and LiCI, and the concentration of water is between 1 % and 20% v / v. This electrolytic cell is connected to an external current system. In use, current is supplied to the electrolytic cell through electrodes and promotes reduction, i.e. Li++ e- — > Li, and oxidation reactions, i.e. 2 O2’ -^ 02 + 4e_, forming lithium metal at the interface with the negative electrode and evolving oxygen gas at the interface with the positive electrode.

[0106] The product obtained according to this embodiment is a bilayer of lithium and stainless steel.

[0107] In another embodiment, the negative electrode is nickel, while the remaining working conditions are the same as those of the previous embodiment. That is, an electrolytic cell is supplied, comprising a positive electrode, preferably graphite, a negative electrode, i.e. nickel, an electrochemical mixture comprising an ionic liquid and a lithium salt, preferably BMIM-CI and LiCI, and the concentration of water is between 1 % and 20% v / v. This electrolytic cell is connected to an external current system. In use, current is supplied to the electrolytic cell through electrodes and promotes reduction, i.e. Li++ e’ -> Li, and oxidation reactions, i.e. 2 O2’ -^ 02 + 4e_, forming lithium metal at the interface with the negative electrode and evolving oxygen gas at the interface with the positive electrode.

[0108] The product obtained according to this embodiment is an alloy of lithium and nickel.

[0109] In the previous examples, the electrochemical mixture comprises BMIM-CI as an ionic liquid and lithium chloride as a lithium salt. However, this combination is not limiting. In fact, it is possible to extend the present invention to other combinations of ionic liquids and lithium salts or mixtures of a lithium salt and a magnesium salt or aluminium or zinc as long as the anion is common to both. In particular, the anion is a halide, preferably chloride, bromide and iodide. In addition, the ability to vary the mixture by changing the halide is limited by the halogen reduction potential.

[0110] Therefore, the electrochemical mixture according to the present invention may comprise all ionic liquids and lithium salts wherein the anion is a halide or a carbonate or a sulphate or a borate, preferably it is a halide provided that the following conditions are satisfied:

[0111] • the anion of the ionic liquid must be able to form a lithium salt, so that the ionic liquid and lithium salt have the same anion; can be used an ion among chloride, bromide, iodide and carbonate.

[0112] • this anion must not undergo oxidation when an electric current up to a value of 10 mA / m2or greater is supplied to the cell.

[0113] The main advantages of the present invention are summarized below:

[0114] - advantageously, lithium metal alloys or lithium metal multilayers are produced with other metals at low temperature by the process according to the present invention. It is known that the production of lithium from lithium salts requires high temperatures, i.e. about 600°C, to break the ionic bond between the lithium cation and the anion. According to the present invention, the temperature is significantly reduced, since the process by which lithium is separated from its anion is not based on thermal energy but rather on the solubility of the lithium salt in the ionic liquid, i.e. lithium is dissolved in the electrochemical mixture. The lower limit to the temperature usable in the process according to the present invention is variable and is determined by the melting point of the ionic liquid. In fact, the ionic liquid must be liquid in order to dissolve the lithium salt. In a realisation example wherein the ionic liquid is BMIM-CI, the minimum operating temperature is below 100°C, even below 66°C. At the same time, it is possible to produce molecular oxygen and hydrogen that can be recovered and stored separately.

[0115] - advantageously, the present invention allows the production of lithium metal and / or products comprising lithium metal with a fluorine-free process;

[0116] - advantageously, the present invention allows the production of lithium metal and / or products comprising lithium metal at a high rate by virtue of the electric current applied to the two electrodes (up to 10 mA / cm2or higher) and the reduced migration path, given the possibility of working in a single reactor;

[0117] - advantageously, the present invention demonstrates for the first time the possibility of using a lithium ion-based electrolyte within ionic liquids comprising halides, preferably chloride and / or bromide, allowing to work with water as the only reducing agent; this makes the ionic liquid reusable ad- libitum since it is not involved in the reaction; in addition, it is sufficient to bring the water concentration back to values between 1 % and 20% v / v and restore lithium salt to allow the method according to the present invention to operate continuously.

[0118] The process described above is easily scalable to provide industrial-scale production of relatively pure lithium metal, which does not require a subsequent high-temperature purification step.

[0119] The following examples are provided to illustrate the invention and are not to be considered limiting its scope.

[0120] EXAMPLES

[0121] A sample of BMIM-CI / hydrated LiCI was prepared in a glove box by mixing 3.48 g of BMIM- Cl and 0.4 g of hydrated LiCI in a 16 mL glass container used as the electrolytic cell, using graphite as the positive electrode and a 100 microns thick aluminium foil as the negative electrode. An external current generator, connected to the two electrodes, provided a current of 2 mA / cm2. After the transfer of electrical energy, the positive electrode showed an evolution of bubbles and the formation of a darker layer occurred on the surface of the aluminium foil, i.e. the electrical energy was converted into chemical energy. X-ray diffraction analysis on the aluminium foil showed the presence of Li Al on the surface of Al. In addition to chloride, the invention can be extended to other counter-ions such as bromide (BMIM-Br / LiBr), iodide (BMIM-I / Lil), carbonate ((BMIM)2-CO3 / Li2CO3).

Claims

CLAIMS1. Electrochemical method for producing lithium metal and products comprising lithium metal comprising the steps of:• preparing an electrolytic cell equipped with a positive electrode and a negative electrode housed in a single container;• preparing an electrolyte composition comprising: o an aliquot of an ionic liquid o an aliquot of a lithium salt o water in the range >0-?20% v / v via1H NMR nuclear magnetic resonance using a Bruker Avance Neo 400 MHz NMR spectrometer, operating at 400.13 MHz for 1 h, at a temperature of 363°K with a 5 mm probe for liquids and standard acquisition / pulsation sequence without a lock on deuterium o with the condition that the ionic liquid and the lithium salt have the same anion;• feeding said cell with the electrolytic composition and immersing the positive electrode and the negative electrode in it;• providing an electric current to the electrodes electrically connected to each other to generate gaseous oxygen to the positive electrode and lithium metal to the negative electrode.

2. The method according to claim 1 , wherein the percentage by volume of water in the electrolyte composition is in the range 0.1 -10% v / v, preferably between 1 % v / v and 10% v / v, more preferably between 3% v / v and 10% v / v.

3. The method according to anyone of claims 1 -2, wherein the lithium salt is chosen from lithium halide, lithium carbonate, lithium sulphate, lithium borate and mixtures thereof, preferably lithium chloride, lithium bromide, lithium iodide.

4. The method according to anyone of claims 1 -3, wherein the ionic liquid is one or more, chosen from:1 -R-3-methylimidazolium-X1 -R-pyridinium-X1 -R-piperidinium-X1 - R-pyrrolidi n- 1 -ium-XTetra-R-ammonium-XTri-R-sulphonium-XTetra-R-phosphonium-X wherein:X = halide ion, carbonate ion, sulphate ion, borate ion, preferably chloride, bromide, iodide;R = linear, branched or cyclic alkyl chain C1-C10.

5. The method according to anyone of claims 1 -4, wherein the product comprising lithium metal is in the form of metal foil, alloy comprising lithium metal, multilayer metal comprising lithium.

6. The method according to anyone of claims 1 -5, wherein the temperature at which the method is conducted is below the melting point of lithium metal, preferably in the range of 0-120°C, preferably 0-100°C, more preferably 15-90°C.

7. The method according to anyone of claims 1 -6, wherein the current density of the electric current is greater than approximately 0.1 mA / cm2, preferably greater than approximately 0.5 mA / cm2, preferably greater than approximately 1 mA / cm2, preferably greater than approximately 2 mA / cm2, preferably greater than approximately 3 mA / cm2, preferably greater than approximately 4 mA / cm2, preferably greater than approximately 5 mA / cm2, preferably greater than approximately 6 mA / cm2, preferably greater than approximately 10mA / cm2, preferably greater than approximately 15 mA / cm2, preferably greater than approximately 20 mA / cm2.

8. The method according to anyone of claims 1 -7, wherein the material the positive electrode is made of is chosen from: graphite, platinum.

9. The method according to anyone of claims 1 -8, wherein the material the negative electrode is made of is chosen from: copper, aluminium, magnesium, zinc, iron, nickel, steel, preferably stainless steel.

10. The method according to anyone of claims 1 -9, wherein the lithium generated at the negative electrode has a purity of approximately 97% by weight, preferably 99% by weight or greater.

11. Lithium metal and products comprising lithium metal obtained directly by the method according to anyone of claims 1 -10.

12. An electrolytic cell for the production of lithium metal and products comprising lithium metal according to the method of anyone of claims 1 -10, comprising: a negative electrode (2), a positive electrode (1 ) placed in a single container (3) containing, as an electrolyte, a mixture of an ionic liquid, a lithium salt and a quantity of water not exceeding 20% v / v and a current generator (4) connected to the negative electrode (2) and the positive electrode (1 ) to generate gaseous oxygen on the positive electrode and lithium metal on the negative electrode; with the condition that the ionic liquid and the lithium salt have the same anion.

13. An electrolyte composition comprising:- an aliquot of an ionic liquid- an aliquot of a lithium salt- water in the range >0-?20% v / v, preferably 0.1 -10% v / v, preferably 1 -10% v / v, more preferably between 3% v / v and 10% v / v via nuclear magnetic resonance1H NMR, measured via nuclear magnetic resonance (NMR) usinga Broker NMR Avarice Neo 400 MHz spectrometer, operating at 400.13 MHz for 1 h, at a temperature of 363°K with a 5 mm probe for liquids and standard acquisition / pulsation sequence without a lock on deuterium. with the condition that the ionic liquid and the lithium salt have the same anion.

14. The composition according to the above claim wherein the lithium salt is chosen from lithium halide, lithium carbonate, lithium sulphate, lithium borate and mixtures thereof, preferably lithium chloride, lithium bromide, lithium iodide.

15. The composition according to anyone of claims 13-14, wherein the ionic liquid is one or more, chosen from:1 -R-3-methylimidazolium-X1 -R-pyridinium-X1 -R-piperidinium-X1 - R-pyrrolidi n- 1 -ium-XTetra-R-ammonium-XTri-R-sulphonium-XTetra-R-phosphonium-X wherein:X = halide ion, carbonate ion, sulphate ion, borate ion, preferably chloride ion, bromide ion, iodide ion;R = linear, branched or cyclic alkyl chain C1-C10.

16. Use of the composition according to anyone of claims 13-15 as an electrolyte in electrochemical cells for the deposition of lithium metal and products comprising lithium metal.

Images