Method for producing a lithium-containing electrode, and electrochemical cell

Abstract

The invention relates to a method for producing a lithium-containing electrode (4) for a lithium-ion containing accumulator, comprising the following steps: - producing a metal foil (6); - coating a surface (26) of the metal foil (6) with a graphite layer (28); - introducing the metal foil (6) provided with a graphite layer (28) into an electrochemical cell (2) as a first electrode (8), which is used as a cathode (10); - introducing a second, lithium-containing electrode (12) into the cell (2), said electrode being used as an anode (14); - wherein lithium compounds (30) are introduced into the second, lithium-containing electrode (12), said lithium compounds originating from thermally decomposed electrode material of a lithium-ion accumulator; - introducing a non-aqueous electrolyte (16) into the electrochemical cell (2); - closing the electrochemical cell (2) under inert conditions; - applying an electrical current to the electrodes (8, 12) of the electrochemical cell (2) such that lithium ions (18) migrate from the lithium-containing anode (14) to the cathode (10) in the form of the metal foil (6) provided with the graphite layer (28), through the non-aqueous electrolyte (16); and - the surface (26) of the metal foil (6) provided with the graphite layer (28) is intercalated to form lithium ions.

Description

[0001] Description

[0002] Process for producing a lithium-containing electrode and electrochemical cell

[0003] The invention relates to a method for producing a lithium-containing electrode according to patent claim 1 and to an electrochemical cell according to patent claim 13.

[0004] The recovery of valuable metals such as nickel, cobalt, and lithium from lithium-ion batteries occurs after cell reprocessing—i.e., after an electrical safety discharge, deactivation of remaining lithium metal, and evaporation of the electrolytes through a pyrometallurgical and hydrometallurgical process. In addition to cobalt-nickel-copper alloys, the pyrometallurgical process also produces lithium-containing slag. The lithium contained in this lithium-containing slag is to be recycled.

[0005] According to the state of the art, this lithium is extracted from the slag using hydrometallurgical processes. The quenched slag is crushed using rod mills and then passed over a magnetic disk to remove ferromagnetic alloy components trapped in the slag, such as nickel and cobalt. The slag is then leached with sulfuric acid. The lithium ions go into solution and the remaining solids, which include gypsum and silicates, are separated by filtration. To prevent unwanted precipitation of lithium carbonate during subsequent purification steps, the filtrate is diluted below the solubility limit of lithium carbonate. The diluted filtrate is adjusted to a pH of 7 by adding calcium oxide to remove impurities such as aluminum, magnesium, iron, manganese, silicon and sulfate.At this pH value, aluminum, iron and silicon as well as some heavy metals precipitate as hydroxides. Some of the sulfate combines with the calcium oxide to form sparingly soluble calcium sulfate (gypsum). After a solid / liquid separation, the pH value is raised to 12 by adding more calcium oxide. At this pH value, magnesium and other metals that are present in the solution as hydroxides or sulfate residues are completely precipitated. After a further solid / liquid separation, excess calcium is precipitated as lime by adding sodium carbonate and separated from the solution by filtration. The solution is greatly concentrated by evaporating the water and the lithium is precipitated as lithium carbonate by adding sodium carbonate at 100 °C. The separated lithium carbonate is washed with ethanol because, unlike water, lithium carbonate is insoluble in ethanol. The purity of this crude carbonate is approx.98% and requires repeated refining for reuse in batteries. The total recovery of lithium, i.e. the yield from the slag, using this method according to the state of the art is approximately 65%. This low yield is due to the fact that part of the lithium is bound into the mixed metal oxides with aluminum, magnesium, iron, or manganese during the precipitation of the impurities with calcium oxide. This lithium is so firmly bound chemically that it would have to be separated from the starting solution using a solvent extraction process in order to significantly increase the recovery.

[0006] This described prior art process demonstrates the very high wet-chemical complexity, which is both economically and ecologically unprofitable. The object of the invention is therefore to provide a process and an electrochemical cell for recycling lithium, achieving a higher yield than in the prior art and simultaneously enabling the recycled lithium to be recycled in a new electrode.

[0007] The solution to the problem consists in a method for producing a lithium-containing electrode having the features of patent claim 1 and in an electrochemical cell having the features of patent claim 13. The method for producing a lithium-containing electrode for a lithium-ion-containing accumulator according to patent claim 1 comprises the following steps:

[0008] Manufacturing a metal foil,

[0009] Coating a surface of the metal foil with a graphite layer, introducing the metal foil provided with the graphite layer into an electrochemical cell as a first electrode which serves as a cathode, introducing a second lithium-containing electrode into the cell which serves as anode, wherein lithium compounds originating from decomposed (in particular thermally decomposed) electrode material of a lithium-ion accumulator are introduced into the second lithium-containing electrode,

[0010] Introducing a non-aqueous electrolyte into the electrochemical cell, sealing the electrochemical cell under inert conditions,

[0011] Applying electric current to the electrodes of the electrochemical cell so that lithium ions migrate from the lithium-containing anode through the non-aqueous electrolyte to the cathode in the form of the metal foil with the graphite layer and

[0012] - the graphite layer of the metal foil is intercalated with lithium ions.

[0013] The invention described here differs fundamentally from the prior art in that the slag, which contains lithium compounds and is usually obtained from the thermal decomposition of rechargeable batteries, is not broken down using wet-chemical methods, but is instead reintroduced into an electrode and an electrochemical cell. The process differs from a conventional electrochemical cell and a conventional lithium-ion battery in that the lithium ions migrate from the aforementioned electrode with the lithium slag via an electrolyte to the cathode and are intercalated into a layer specifically designed for this purpose, namely a graphite layer.

[0014] Intercalation is the incorporation of molecules or ions (or atoms, if applicable) into chemical compounds, whereby their structure does not significantly change during the incorporation process. In this specific case, the lithium ions position themselves between the individual graphene layers of the graphite, since only the weak van der Waals forces must be overcome for incorporation (intercalation) between the graphene layers.

[0015] Furthermore, the described method also differs from the prior art in that the cathode in this electrochemical cell, or in this process, is designed in such a way that a new electrode is created during the process by means of an electrically conductive metal foil provided with the graphite-containing layer into which the lithium ions are intercalated. This new electrode is already designed, by its surface structuring and its surface coating, as an electrode for a new lithium-ion battery, in this case, for example, in the form of an anode (in an accumulator (battery), the positively charged electrode is referred to as the cathode during the discharge process, and the negatively charged electrode is referred to as the anode).

[0016] The described process not only offers a technically inexpensive and economical way of recycling the lithium-containing slag that is produced during the decomposition, particularly the thermal decomposition, of old batteries, but the described process also provides a manufacturing process for a completely new, directly reusable electrode for a lithium-containing battery.

[0017] In an advantageous embodiment of the invention, the metal foil is a copper foil. Copper has very high electrical conductivity and is therefore well suited as an electrode material. Furthermore, it is relatively inexpensive and technically easy to handle. In principle, other highly conductive metals such as silver, gold, or aluminum are also suitable as materials for the metal foil.

[0018] In particular, electrolytically purified copper is used to produce the copper foil, which in turn has a purity of at least 99.9%, particularly preferably 99.99%. The electrical conductivity of copper increases with its purity.

[0019] It is also advisable to use graphite that is as pure and flawless as possible in terms of its crystal structure. Therefore, synthetically produced graphite is preferred.

[0020] Furthermore, to achieve good coating properties, the graphite is provided with a binder, and this mixture is preferably applied wet-chemically to the metal foil 6. Polyacrylic acid (PAA), acrylic-based copolymers (ACM), styrene-butadiene rubber (SPR), and / or carboxymethyl cellulose are preferably used as binders. Furthermore, the mixture of graphite and binder is preferably provided with a conductive additive, in particular carbon black, to produce the graphite layer.

[0021] The non-aqueous electrolyte preferably comprises ethylene carbonate, propylene carbonate or dimethyl carbonate, acetonitrile or an ionic liquid. Propylene carbonate is particularly preferred because it is inexpensive to produce and can be operated without problems. Ionic liquids are salts whose melting point is in particular less than 100 °C. Like all salts, they comprise anions and cations. By varying these, the physicochemical properties of an ionic liquid can be varied within wide limits and optimized to meet technical requirements. Typical cations can be imidazolium or pyridinium, ammonium and / or phosphonium. Possible anions are halides and weakly coordinating ions such as tetrafluoroborates or hexafluorophosphates, but also trifluoroacetates, triflates and tosylates.

[0022] In an advantageous embodiment of the invention, the lithium of the lithium compound in the second electrode comprises lithium calcium silicates and / or lithium magnesium silicates and / or lithium manganese oxides and / or lithium cobalt oxides and / or lithium nickel oxides. These are typically compounds that are present in the slag or ash produced during the thermal decomposition of lithium-ion batteries.

[0023] The process and the electrochemical cell operate under inert conditions. Argon is a particularly suitable inert gas. Nitrogen would react with the lithium ions in the non-aqueous electrolyte. A vacuum atmosphere is also inefficient due to the high vapor pressure of the materials used.

[0024] A further component of the invention is an electrochemical cell for producing an electrode for a lithium accumulator. This comprises an anode containing a lithium compound derived from a preferably thermal decomposition of used lithium accumulators and a cathode comprising a metal foil provided with a graphite layer on its surface, wherein the anode and the cathode are separated from one another by a non-aqueous electrolyte.

[0025] The advantages of this electrochemical cell over the prior art have already been explained with reference to the process according to the invention. These include, firstly, the technically inexpensive, direct recycling of lithium-containing compounds from the preferably thermal decomposition of old accumulators, while simultaneously producing a new cathode for a lithium accumulator.

[0026] This electrochemical cell is particularly advantageous when it features multiple pairs of anodes and cathodes connected in parallel. This allows for upscaling of anode production and the industrial recycling of lithium slag from recycled batteries.

[0027] To produce the second electrode, the anode of the electrochemical cell, it is advantageous to mix the lithium compound, i.e., the slag from the thermal decomposition of old accumulators, with a conductive material, particularly carbon particles, and then compress it. Porous carbon particles are particularly preferred. Such a compressed electrode made of lithium compounds and carbon particles is particularly well suited for use in the electrochemical cell described.

[0028] Further embodiments and features are explained in more detail with reference to the following figures. Features with the same designation but in different embodiments are provided with the same reference symbol. The illustrations in the following figures are purely schematic representations and do not represent a limitation of the scope of protection.

[0029] Showing:

[0030] Figure 1 shows an electrochemical cell for producing a lithium-containing electrode and a corresponding method therefor,

[0031] Figure 2 shows an upscaling version of the electrochemical cell according to Figure 1, Figure 3 shows a schematic representation of the electrodes from Figure 1 to explain the microstructure.

[0032] Figure 4 enlarged section IV from Figure 3 to illustrate the microstructure.

[0033] Figure 1 schematically describes an electrochemical cell 2 and a method for operating this cell 2. The electrochemical cell 2 comprises a first electrode 8, which is designed as a cathode 10. On the other hand, the cell 2 comprises a second, lithium-containing electrode 12, which is designed as an anode 14. Both electrodes 8, 12 are immersed in a non-aqueous, liquid electrolyte 16, wherein upon application of an electric current via contacts 46, lithium ions 18, which are formed by the Li +migrate from the anode 14 to the cathode 10 in the cell 2. Furthermore, the electrochemical cell 2 is operated under an inert gas 44, in this case argon, the inert gas 44 being led out of the cell 2 and passed into an inert gas circuit 44, where it is processed in a gas cleaning system 36 and then fed back into the cell 2. Furthermore, the electrolyte 16 is also processed, for which purpose an electrolyte circulation system 38 is used, which contains a pump 40 and a filter 42. In this case, propylene carbonate is used as the electrolyte 16.

[0034] Figure 2 shows an embodiment analogous to the electrochemical cell 2 from Figure 1, but a large number of pairs of first electrons 8 and second electrons 12, i.e. pairs of cathodes 10 and anodes 14, are connected in parallel in this cell 2. The contact 46 as well as the inert gas circuit 34 and the electrolyte circulation system 38 are designed analogously to cell 2 in Figure 1. This embodiment according to Figure 2 serves in particular to achieve upscaling for the processing of lithium-containing compounds and the production of an electrode 4 (cf. Figures 3 and 4). The electrochemical process and the preparatory steps for producing the anode 14 and the cathode 10 for the electrochemical cell 2 are discussed in more detail below.

[0035] Since lithium has the lowest standard potential of -3.04 V in the periodic table and is therefore the least noble of all elements, electrolytes that dissociate HsCh ions, i.e. mainly aqueous electrolytes, cannot be used for lithium-containing cells. Otherwise, during electrochemical deposition, the hydrogen would be deposited rather than the lithium. The electrochemical window of water is 1.2 V. In this electrochemical potential of the electrode, the electrolyte is neither oxidized nor reduced. This range results from the difference between oxidation potential (anodic limit) and reduction potential (cathodic limit). Outside this range, the electrolyte reacts at the electrode surface. Water is electrolyzed in the process.

[0036] Instead of water, aprotic, polar solvents such as ethylene carbonate, propylene carbonate or dimethyl carbonate, acetonitrile or ionic liquids are used for the deposition of lithium. The organic carbonates and ionic liquids have a larger electrochemical window. For propylene carbonate, the electrochemical window is 4 V, and ionic liquids show a value of 3 V to 6 V. Since the electrochemical window is reduced from 4 V to 2 V by small amounts of water (approx. 3 wt. %), it is necessary to work in anhydrous form. In addition, lithium reacts with nitrogen to form lithium nitride and with oxygen to form lithium oxide even at room temperature. For this reason, the electrochemical cell 2 is purged with inert gas 44, in particular argon or sulfur dioxide.The gas purging of cell 2 takes place in the inert gas circuit 34, wherein the inert gas 44 is constantly circulated between cell 2 and the gas purification system 36 and continuously processed. The gas purification system 36 removes oxygen and moisture from the inert gas 44 using a copper catalyst and a molecular sieve. This achieves purity levels of up to < 1 ppm O2 and < 1 ppm H2O. In addition, the electrochemical cell 2 must be sealed off from the environment to prevent air humidity, oxygen, or nitrogen from entering cell 2. For processing, the electrolyte 16 is also constantly pumped through filter systems 42 by means of a pump 40 and returned to the sealed electrochemical cell 2.

[0037] To produce the second electrode 12, i.e. the anode 14, a lithium slag is used which is produced during the thermal decomposition of old, used lithium-ion batteries. This second electrode 12 therefore comprises lithium compounds 30, as is shown, for example, in Figure 3. These lithium compounds 30, which can also be referred to as lithium slag because they originate from the thermally decomposed residues of lithium accumulators, contain lithium calcium silicates, lithium magnesium silicates, lithium fluoride and / or lithium aluminite. During the decomposition of certain NMC batteries which contain manganese, lithium manganese oxide (Li2Mn2O3 or the associated spinel type LiM^Cg) is also present in the slag, depending on the manganese content. This lithium slag must first be processed mechanically by breaking the slag and grinding it into particles in a vibrating disc mill or a ball mill.The powder obtained from the slag is then thoroughly mixed with electrically conductive carbon particles 31 (e.g. electrically conductive soot, porous conductive carbon powder or graphite) in a plowshare mixer for battery masses. The mixing drum has a ceramic lining. Mixing elements, mixer shaft and measuring heads are preferably provided with a thin, solid ceramic coating (e.g. aluminum oxide or tungsten carbide) so that foreign ions can be avoided. Homogeneous mixing is achieved in the plowshare mixer in a short time. Porous conductive carbon powder in the form of carbon particles 31, available for example under the trade name Porocarb, is particularly well suited as an additive for electrode formulation because local areas of high porosity are present after electrode compaction.

[0038] The macroporous carbon particles 31 are used to improve the ionic conductivity in the electrode 12. As the anode degradation progresses, the proportion of macropores reduces capacity loss. Furthermore, the porosity of the particles 31 increases the mechanical stability of the electrode 12 after compaction. The mixed powder (mixture of lithium compound 30 and carbon particles 31) is filled into a mold and compacted in a pressing process. This can be uniaxial pressing using an upper punch or by isostatic pressing at approximately 300 bar in an oil bath. In this way, a preferably cylindrical electrode is formed. This second electrode 12 produced in this way is introduced into the cell 2 as the anode 14.

[0039] The corresponding cathode 10 of the electrochemical cell is constructed in advance in such a way that it is converted into an electrode 4 of a lithium accumulator during the process in the electrochemical cell 2. This means that the cathode 10 in the cell is modified during cell operation in such a way that at the end of the process it appears as a new, independent, lithium-containing electrode 4 (cf. Figure 4). This positive electrode of a lithium battery, i.e. the electrode 4, consists of a current collector, a metal foil 6 and an energy storage layer. This layer is the graphite layer 28 with the lithium ions intercalated therein.

[0040] Synthetically produced graphite is preferably used to manufacture the first electrode 8. Synthetic graphite is distinguished from natural graphite by its higher purity, better quality, and reproducible, tailored properties. This leads to greater cycle stability between charging and discharging processes and to better charging performance in general. The synthetic graphite is applied to a copper foil (metal foil 6) with a binder (e.g. polyacrylic acid (PAA), acrylic-based copolymers (ACM), styrene-butadiene rubber (SPR), carboxymethyl cellulose, which are soluble in water or ethanol) and a conductive additive (e.g. carbon black). Immediately after the wet-chemical application of an electrode slurry, drying takes place in an impingement jet dryer to remove the solvent used (e.g. ethanol or water).In the impact jet dryer, the slurry film containing the copper foil is suspended and dried as it passes through air jets that impinge at high speed through a nozzle onto the web to be dried, thereby causing high heat and mass transfer. This is followed by a final drying step in a vacuum oven to reduce the residual moisture content to a few ppm.

[0041] Copper foil is electrolytically purified copper that is drawn into a foil. The copper purified in this way has a very high purity of > 99.99%.

[0042] Figure 3 shows a very schematic microstructure of the respective electrodes 12, 8 in the cell 2. On the left-hand side, the anode 14 is shown in the form of the second electrode 12. This comprises, as already described, the lithium-containing compound 30, i.e. the processed lithium slag with the aforementioned lithium compounds. In addition, the structure is provided with conductive carbon particles 31 and compacted to form an electrode 12. The cathode 10 is shown on the right-hand side of Figure 3. This is the metal foil 6, in this case a copper foil, which is provided with the graphite layer 28, in which the lithium ions 18 are incorporated during operation of the cell 2.

[0043] Figure 4 shows the schematic microstructure of the resulting electrode 4, which is an enlarged representation of section IV of Figure 3. This electrode 4 is thus provided with the graphite layer 28, with the lithium ions 18 being embedded between the individual graphene layers 29 of the graphite layer 28. In principle, this electrode 4 can be used, as shown in Figure 4, in a new lithium accumulator, for example, as an anode.

[0044] In the described method for producing this electrode 4, recycled lithium-containing material is used to directly produce a new, ready-to-use electrode 4 for a new accumulator.

[0045] Reference symbol list

[0046] 2 electrochemical cell

[0047] 4 lithium-containing electrode

[0048] 6 metal foil

[0049] 8 first electrode

[0050] 10 Cathode

[0051] 12 second electrode

[0052] 14 Anode

[0053] 16 non-aqueous electrolyte

[0054] 18 lithium ions

[0055] 26 Surface metal foil

[0056] 28 Graf it layer

[0057] 29 graphene layers

[0058] 30 lithium compounds

[0059] 31 carbon particles

[0060] 34 Inert gas circuit

[0061] 36 Gas cleaning system

[0062] 38 Electrolyte circulation system

[0063] 40 Pump

[0064] 42 filters

[0065] 44 Inert gas

[0066] 46 Contacting

Claims

Patent claims 1. A method for producing a lithium-containing electrode (4) for a lithium-ion battery, comprising the following steps: - producing a metal foil (6) , - coating a surface (26) of the metal foil (6) with a graphite layer (28), - introducing the metal foil (6) provided with the graphite layer (28) into an electrochemical cell (2) as the first electrode (8) which serves as the cathode (10), - introducing a second, lithium-containing electrode (12) into the cell (2) which serves as an anode (14), - wherein lithium compounds (30) originating from decomposed electrode material of a lithium-ion accumulator are introduced into the second lithium-containing electrode (12), - introducing a non-aqueous electrolyte (16) into the electrochemical cell (2), - closing the electrochemical cell (2) under inert conditions, - applying electric current to the electrodes (8) , (12) of the electrochemical cell (2) , so that lithium ions (18) migrate from the lithium-containing anode (14) through the non-aqueous electrolyte (16) to the cathode (10) in the form of the metal foil (6) provided with the graphite layer (28) and - the surface provided with the graphite layer (28) (26) of the metal foil (6) is intercalated to form lithium ions.

2. Method according to claim 1, characterized in that the metal foil (6) is a copper foil.

3. A method according to claim 2, characterized in that electrolytically purified copper is used to produce the copper foil.

4. Process according to claim 3, characterized in that the copper has a purity of at least 99.9%, preferably 99.99%.

5. Method according to one of the preceding claims, characterized in that the graphite is a synthetically produced graphite.

6. Method according to one of the preceding claims, characterized in that the graphite is provided with a binding agent and is applied to the metal foil (6).

7. The method according to claim 6, characterized in that the binder comprises polyacrylic acid (PAA), acrylic-based copolymers (ACM), styrene-butadiene rubber (SPR) and / or carboxymethyl cellulose.

8. Method according to one of claims 6 or 7, characterized in that the mixture of graphite and the binder for producing the graphite layer (28) is provided with a conductive additive, in particular with carbon black.

9. Method according to one of the preceding claims, characterized in that lithium compounds (30) are prepared from thermally decomposed electrode material of a lithium ion accumulator.

10. Method according to one of the preceding claims, characterized in that the non-aqueous electrolyte (16) comprises ethylene, propylene or dimethyl carbonate, acetonitrile or an ionic liquid.

11. Method according to one of the preceding claims, characterized in that the lithium compound (30) comprises lithium calcium silicates and / or lithium magnesium silicates and / or lithium manganese oxides and / or lithium cobalt oxides and / or lithium nickel oxides.

12. Method according to one of the preceding claims, characterized in that the inert conditions are provided by argon as a protective gas.

13. Electrochemical cell for producing an electrode of a lithium accumulator comprising an anode (14) with lithium compounds (30) which originate from the decomposition of used lithium accumulators and a cathode (10) comprising a metal foil (6) provided with a graphite layer (28) on its surface (26), and wherein the anode (14) and the cathode (10) are separated from one another by a non-aqueous electrolyte (16).

14. Electrochemical cell according to claim 13, characterized in that in the electrochemical cell (2) there are several pairs of anodes (14) and cathodes (10) connected in parallel.

15. Electrochemical cell according to claim 13 or 14, characterized in that the lithium compounds (30) are mixed with carbon particles (32), preferably with porous carbon particles (32).

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