Method for producing a substrate coated with a lithium layer by means of a mediator layer, and a coated substrate

Description

[0001] The present invention relates to a method for producing a substrate made of copper or nickel coated with a lithium layer by means of an intermediary layer made of copper oxide or nickel oxide, as well as to a coated substrate.

[0002] Metallic lithium anodes represent a key to increased energy densities for a variety of lithium-based battery technologies, for example, lithium-ion batteries, lithium-sulfur batteries, solid-state batteries, or lithium-air batteries. Since currently used production technologies such as extrusion and rolling for the required lithium foils with foil thicknesses of less than 50 µm and foil widths of greater than 100 mm are limited due to the mechanical properties of lithium, no processes are available for producing roll stock with these dimensions, which hinders the scaling up of battery technologies to larger cell formats.

[0003] To achieve thin material thicknesses despite the low mechanical stability of lithium, a thin lithium foil is laminated to a copper foil, as described, for example, in EP 2 472 648 A1, but this does not achieve the desired thicknesses. Deposition of metallic lithium on copper foil via an evaporation process is described, for example, in JP 2009 032597 A,

[0004] US 2005 / 0089757 A1 or WO 2005 / 001157 A2. The disadvantage of this method is that relatively low deposition rates result, and in addition to the copper foil, components near the substrate are also coated with lithium, which leads to considerable effort in servicing, maintenance, and cleaning the system.

[0005] Electrochemical deposition of lithium, such as the process described in US 2001 / 041294 A1, also suffers from similar disadvantages. Since direct deposition of lithium from the melt is also not readily possible due to poor wetting of copper foil or similar substrates caused by the high surface tension of molten metals, the desired layer thicknesses cannot be achieved using this method either.

[0006] In the document US 3634144 A an anode for lithium batteries is disclosed which comprises a porous metal substrate coated with liquid lithium.

[0007] The present invention is therefore based on the object of proposing a method which avoids the disadvantages mentioned, thus enabling the deposition of an alkali metal on large areas and small thicknesses.

[0008] This object is achieved according to the invention by a method according to claim 1 and a substrate according to claim 5. Advantageous embodiments and further developments are described in the dependent claims.

[0009] A method for producing a substrate coated with an alkali metal comprises a step in which a mediator layer is applied to a surface of the substrate. The mediator layer is made of a material that reacts with the alkali metal by at least partial chemical reduction. A surface of the mediator layer is exposed to an alkali metal, and the alkali metal is subsequently converted into the solid phase or solid state, and a coating with the alkali metal is formed.

[0010] By means of an intermediary layer which chemically reacts with the alkali metal by at least partial chemical reduction of the intermediary layer or forms an alloy, the wettability of the substrate with regard to the alkali metal is improved, so that larger areas can be coated and / or greater layer thicknesses can be achieved. The alkali metal can be gaseous, liquid or solid when applied or exposed. The intermediary layer is typically applied from the gas phase, the liquid phase or as a solid. According to the invention, the alkali metal is lithium and is liquid when applied. After the application of the liquid lithium, the invention provides for converting the lithium into the solid phase or the solid state of aggregation by reducing the temperature.The surface of the mediator layer is preferably wetted or brought into contact with the alkali metal at an elevated temperature, particularly preferably at a melting temperature of the alkali metal to be applied.

[0011] According to the invention, the intermediary layer may be formed from silicon, tin, antimony, aluminum, magnesium, bismuth, or alloys of the aforementioned chemical elements, or may comprise the aforementioned elements or alloys. These elements or alloys preferably form an alloy with an alkali metal such as lithium.

[0012] The mediator layer is formed from a material that reacts upon contact with the alkali metal to form a mediating interface or boundary layer, namely by reducing an oxide material. The oxide material is formed from copper oxide or nickel oxide. The mediator layer reacts with the lithium to form a mediating boundary layer by at least partially reducing the metal oxide.

[0013] It can be provided that the mediator layer is formed from a material that allows, as a chemical reaction, intercalation (for example, in graphite) or insertion (for example, in LiMeO2) of the alkali metal, alloy formation, or material transformation (conversion) by reduction. The material transformation by reduction preferably occurs as Cu2O + 2Li → Li2O + 2Cu or NiO + 2Li → Li2O + Ni.

[0014] Typically, the intermediary layer is formed from Cu2O or NiO, since these can be easily reduced by liquid lithium. Not according to the invention, the intermediary layer can be formed from lithium metal oxides (such as LiCoO2, LiNiO2, LiFePO4, LiMnO2, Li2Mn3NiO8, LiNiCoMnO2, LiNiCoAlO2, Li4Ti5O12 or their derivatives) or carbon in graphite form, since these materials enable both intercalation and insertion.

[0015] The intermediary layer can be formed by thermal oxidation, vapor deposition processes, preferably sputtering and / or chemical vapor deposition (CVD), wet chemical coating, and / or annealing. Thermal oxidation of a copper or nickel foil, in particular, enables rapid and efficient formation of the intermediary layer.

[0016] The alkali metal is typically applied by laser melting, melting using a heating device, doctor blade coating, application using a slot die, brushing, spraying alkali metal melts, dip coating, gas-phase processes, thermal spraying, pressure rolling, and / or lamination. Using the described processes, the alkali metal can be liquefied or converted into a gaseous state, as well as applied to the surface of the intermediary layer with defined dimensions. However, the intermediary layer can also be used for deposition of the alkali metal from the gas phase using physical vapor deposition (PVD). According to the invention, the alkali metal is lithium and is liquid upon application.

[0017] Typically, the intermediary layer is formed with a thickness between 0.1 nm and 1000 nm, preferably between 1 nm and 500 nm, particularly preferably between 10 nm and 200 nm, in order to achieve sufficient coverage of the substrate for the subsequent application of the alkali metal.

[0018] The alkali metal is lithium, as lithium is mainly used for battery applications where thin layers are important.

[0019] The substrate is made of copper or nickel or comprises copper or nickel and the intermediary layer is made of copper oxide in the case of a copper substrate and of nickel oxide in the case of a nickel substrate.

[0020] The substrate is a solid, non-porous solid. According to the invention, the substrate is a planar film with a thickness of less than 20 µm, preferably with a thickness of less than 10 µm.

[0021] The thermal oxidation may be carried out over a period of not more than 2 minutes, preferably less than 1.5 minutes, particularly preferably less than 1 minute, in order to generate a sufficiently thick oxide layer as quickly and efficiently as possible.

[0022] A coated substrate produced using the described method thus has a layer sequence of substrate-mediating layer-alkali metal layer. Before the alkali metal layer is applied, the substrate has the mediating layer, which is made of a material that chemically reacts with the alkali metal. The coated substrate can therefore have the properties already described and, in particular, can be produced using the described method. The described substrate is typically used as an anode in a battery cell.

[0023] Embodiments of the invention and non-inventive examples are shown in the drawings and are described below with reference to Figures 1 to 10 explained.

[0024] They show: Fig. 1 a schematic side view of an alkali metal application using a heated nozzle; Fig. 2 a Figure 1 corresponding view of the alkali metal deposition using a heated reservoir; Fig. 3a Figure 1 corresponding view of the alkali metal deposition by laser melting; Fig. 4a Figure 1 corresponding view of the alkali metal application by doctor blades; Fig. 5a Figure 1 corresponding view of the alkali metal application by brushing; Fig. 6a Figure 1 corresponding view of the alkali metal application by means of a dip coating; Fig. 7a Figure 1 corresponding view of the alkali metal application by means of a spray coating; Fig. 8a Figure 1corresponding view of the alkali metal application using a flow nozzle; Fig. 9a Figure 1 corresponding view of the alkali metal application by means of a pressure roller and Fig. 10a Figure 1 corresponding view of the alkali metal application using a sliding coating.

[0025] Figure 1shows, in a schematic side view, a method for producing a substrate 2 coated with lithium as the alkali metal 1. In the illustrated embodiment, the substrate 2 is a copper foil which has been provided with a copper oxide layer as a mediator layer 3 by thermal oxidation. The mediator layer 3 is thus in direct, i.e. immediately touching, contact with the substrate. The lithium is melted by a heated nozzle 5. The lithium foil is guided through the electrically heated nozzle 5 and melted in the process. The substrate 2, on which the mediator layer 3 is arranged, is passed in alignment below the nozzle 5. The molten lithium then strikes a surface of the mediator layer 3 facing the nozzle 5, cools there and returns to a solid state, so that a closed lithium layer is formed which is in direct contact with the mediator layer 3.

[0026] As in Figure 2 in one Figure 1 As shown in the corresponding view, a preferably electrically heated reservoir 6 can also hold liquefied lithium and apply the lithium to the mediator layer 3 through a slotted nozzle of the reservoir 6 facing the substrate 2. The reservoir 6 is funnel-shaped to enable targeted outflow through the slotted nozzle. Recurring features are provided with identical reference numerals in this figure and the following figures.

[0027] In a further embodiment of the invention, which is Figure 3 As shown, electromagnetic laser radiation is directed by a laser radiation source 4 onto an impact point of the foil-shaped lithium on the substrate 2 or the mediator layer 3, so that the foil-shaped lithium is liquefied at this point and is arranged on the substrate 2 by the mediator layer 3 made of copper oxide.

[0028] In a further embodiment, which is Figure 4 As shown, the substrate 2 is heated and lithium as the alkali metal 1 lies as a melt on the substrate 2. By means of a doctor blade 7, the lithium is stripped off with a defined thickness and applied to the mediator layer 3, which, as before, faces a source of the alkali metal 1 to be applied.

[0029] In the Figure 5 In the illustrated embodiment, cold lithium foil is passed over a hot substrate 2. The substrate 2 is passed through an electric furnace and heated in the process. By passing the cold lithium foil over the substrate 2, the lithium foil melts at the point of impact on the intermediary layer 3 applied to the substrate 2 and wets this intermediary layer 3 over the entire surface. After cooling, a continuous lithium coating is thus formed.

[0030] Figure 6, again shows a schematic side view of a heated basin 8 containing molten lithium. The substrate 2 with the intermediary layer 3 applied thereto is guided through the basin 8. Upon leaving the basin 8, it is possible to strip off excess lithium using a stripper 9, thereby achieving an identical layer thickness of the lithium layer on both sides of the substrate 2.

[0031] In Figure 7 an embodiment is shown in which the coating of the substrate 2 provided with the mediator layer 3 is carried out by means of a spray coating, in which the alkali metal is deposited from a spray nozzle 10 onto the substrate passing below the spray nozzle 10.

[0032] In the Figure 8In the exemplary embodiment shown, the substrate 2 is coated on both sides by supplying lithium as a solid to the substrate 2 from both sides. Since the substrate 2 is passed through a heated flow nozzle 11, before the lithium impinges on the substrate 2 or the mediator layer 3 arranged on both sides of the substrate 2, it melts there and coats the substrate 2 in such a way that a continuous lithium layer is formed.

[0033] In another, in Figure 9 In the embodiment shown, the coating of the substrate 2 is carried out by a pressure roller process in which liquid lithium is introduced into surface depressions of a heated application roller and transported by the roller onto the substrate 2, where it strikes the mediator layer 3 and is deposited.

[0034] In another embodiment, which is shown in Figure 10As shown, the coating of the substrate 2 is achieved by uniformly sliding the substrate over a lithium melt bath, with the substrate 2 just in contact with the lithium surface. During this so-called surface contact drawing, the layer thickness is adjusted, among other things, by the travel speed of the substrate 2. The coating width is determined by the width of the lithium melt bath, so that using a foil with a greater width than the lithium melt bath creates an uncoated edge.

[0035] With the method described in various embodiments or non-inventive examples, large-area deposition of lithium on different substrates 2 can be enabled by applying a thin, typically between 0.1 nm and 1000 nm thick, lithiophilic mediator layer 3. Examples of substrate materials that can be used include copper foils, nickel foils, perforated metal foils, carbon fibers, in particular carbon fiber mats, nonwovens made of carbon nanotubes (CNT nonwovens), metal wire mesh, or polymeric substrates 2 such as polyimide films or polyimide fiber fabrics or scrims.

[0036] According to the invention, the mediator layer 3 is not made of a material such as silicon, tin, antimony, aluminum, magnesium, bismuth or an alloy of the aforementioned chemical elements such as CuSn, which forms an alloy with lithium. Alternatively, the mediator layer 3 can also be made of a material that reacts in contact with liquid lithium to form a mediating interface, e.g. by reducing an oxidic material to a material that forms alloys with lithium, for example aluminum oxide. This can also be realized, for example, using materials that allow intercalation or insertion of lithium. These can be materials that are used as active materials in lithium-ion batteries, e.g. LiCoO 2 , LiNiO 2 , LiFePO 4 , LiMnO 2 , Li 2 Mn 3 NiO 8 , LiNiCoMnO 2 , LiNiCoAlO 2 , Li 4 Ti 5 O 12 or carbon in graphite form.Alternatively, however, materials can also react in contact with liquid lithium to form compounds that allow intercalation or insertion or other chemical reactions. For example, conversion materials in which no intercalation or insertion occurs, but instead a metal oxide is directly (reversibly) converted, such as vanadium oxide, manganese oxide, iron oxide, copper oxide, sulfur, or sulfides. Instead of lithium, sodium can also be used in an analogous manner to form homologous layers from the melt. An example reaction of a conversion material could be: MeO + 2 Li → Me + Li 2 O.

[0037] In general, an oxide intermediary layer 3 can be used, which is at least partially (but also completely) reduced by lithium or sodium. This oxide intermediary layer can therefore comprise a metal oxide, with oxides or oxide compounds of the following elements being considered as the metal oxide: magnesium, aluminum, silicon, calcium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, zirconium, molybdenum, indium, tin, antimony, lead, and bismuth.

[0038] According to the invention, the mediator layer is made of copper oxide or nickel oxide.

[0039] To form the intermediary layer 3, as well as to apply the alkali metal 1, various procedures can be used. For example, a 12 µm thick copper foil can be superficially oxidized in a preheated muffle furnace (or in a high-temperature continuous furnace for continuous substrates) at 300 °C to 500 °C to create a thin oxide layer with a thickness between 0.1 nm and 1000 nm, which serves as the intermediary layer 3. In the same way, a nickel oxide intermediary layer can be formed by oxidizing a 20 µm thick nickel foil at 600 °C. The formation of Cu2O can be detected by X-ray diffraction (XRD). Solid lithium can then be brought into contact with the oxidized copper foil heated to 200 °C in an Ar environment, which leads to the melting of the lithium. Using the squeegee 7, as shown in Figure 4 shown a 20 µm to 160 µm thick lithium layer can be generated.

[0040] In the previously described embodiment, process parameters can also be adapted to the intended application. Preferably, the copper foil is treated for one minute at 300 °C, forming an approximately 10 nm thick Cu2O layer that is very well wetted by liquefied lithium. At 400-500 °C, treatment times are less than one minute, but other, less favorable copper oxides such as CuO are also formed.

[0041] In a variation of this embodiment, the intermediary layer 3 is produced only in specific areas on the substrate 2. In this way, it is possible to ensure that the lithium layer can be applied to the substrate 2 only in these areas. This makes it possible to create structured lithium layers or geometric shapes, or to create an uncoated edge area for current collector flags or the like.

[0042] In another example not according to the invention, a carbon fiber fleece (Freudenberg H14) can be soaked in an ethanoic SnCl 2 solution or SbCl 3 solution (0.15 M) for 30 s, dried for 5 minutes, and then annealed in air in a preheated muffle furnace at 300 °C for 5 minutes. Subsequently, a reduction is carried out under oxygen exclusion at 700 °C for 60 minutes. The substrate 2 produced in this way is then immersed in a lithium melt.

[0043] In further examples not according to the invention, a copper mesh (wire thickness 0.05 mm, mesh width 0.2 mm) can also be treated under the same conditions as the copper foil and coated with lithium, or a rough copper foil is used, a so-called ED foil, which has dendritic copper structures in the low single-digit µm range by electrodeposition, which in turn is converted into a wettable state by thermal oxidation.

[0044] For example, while lithium spheres on an untreated surface of a copper foil are not suitable for wetting, when heated in argon to approximately 200 °C on a surface treated as described, the lithium sphere can wet the foil surface. This allows the production of electrodes for secondary batteries such as lithium-ion batteries, lithium-air batteries, lithium-sulfur batteries, or solid-state batteries, or electrodes for primary batteries such as lithium thionyl chloride batteries, lithium manganese oxide batteries, lithium sulfur oxide batteries, lithium carbon monofluoride batteries, lithium iodine batteries, or lithium iron sulfide batteries.

[0045] Only features of the various embodiments disclosed in the exemplary embodiments can be combined with one another.

Claims

1. A method for producing a copper or nickel substrate (2) coated with a lithium layer (1), in which a mediator layer (3) of copper oxide is applied to a surface of the substrate (2) of copper or a mediator layer (3) of nickel oxide is applied to a surface of a substrate (2) of nickel, and liquid lithium is applied to a surface of the mediator layer (3) and the mediator layer (3) reacts with the liquid lithium by at least partial chemical reduction, the mediator layer (3) reacting in contact with the lithium of the lithium layer (1) to form a mediating interface or boundary layer, and subsequently the liquid lithium is converted into the solid phase and a coating (1) is formed from the lithium, characterized in that the substrate (2) is a non-porous planar copper foil having a thickness of less than 20 µm or a non-porous planar nickel foil with a thickness of less than 20 µm.

2. The method according to claim 1, characterized in that the mediator layer (3) is formed by thermal oxidation, gas-phase coating processes, preferably sputtering and / or chemical vapour deposition, a wet-chemical coating and / or annealing.

3. The method according to one of the preceding claims, characterized in that the lithium of the lithium layer (1) is applied to the mediator layer (3) by laser melting, melting by means of a heating device, doctoring, application by means of a slot nozzle, spraying, brushing, dip coating, thermal spraying and / or laminating.

4. The method according to one of the preceding claims, characterized in that the mediator layer (3) is formed with a thickness of between 0.1 nm and 1000 nm, preferably between 1 nm and 500 nm, particularly preferably between 10 nm and 200 nm.

5. A substrate (2) having a mediator layer (3) and a lithium layer (1), wherein the substrate (2) is formed of copper as a non-porous copper foil having a thickness of less than 20 µm and the mediator layer (3) is formed of copper oxide, or the substrate (2) is formed of nickel as a non-porous nickel foil having a thickness of less than 20 µm and the mediator layer (3) is formed of nickel oxide, the mediator layer (3) between the substrate (2) and the lithium layer (1) being formed by an at least partial chemical reduction of the mediator layer (3) with the lithium to form a mediating interface or boundary layer.

6. A battery cell comprising an anode formed from the substrate according to claim 5.