Method for producing a substrate, which is coated with an alkali metal, by means of a promoter layer, and a coated substrate

Abstract

The present invention relates to a method for producing a substrate (2) which is coated with an alkali metal (1), in which method a promoter layer (3) which is composed of a material which reacts with the alkali metal (1) by at least partial chemical reduction of the promoter layer (3) is applied to a surface of the substrate (2) and a surface of the promoter layer (3) is acted on by an alkali metal (1) and then the alkali metal (1) is converted into the solid phase and a coating containing the alkali metal is formed.

Claims

1. A method for producing a substrate formed from copper or nickel coated with a lithium layer, wherein a mediator layer comprising copper oxide on a surface of the substrate is chemically formed from copper or a mediator layer comprising nickel oxide on a surface of the substrate is chemically formed from nickel, a surface of the mediator layer is coated with liquid lithium and the mediator layer reacts with the liquid lithium by at least partial chemical reduction wherein the liquid lithium forms a solid phase lithium and a coating of the lithium is formed producing a lithium coated copper foil substrate or lithium coated nickel foil substrate.

2. The method as claimed in claim 1, wherein the mediator layer reacts when in contact with the lithium of the lithium coating to form a mediating interface or boundary layer.

3. The method as claimed in claim 1, wherein the mediator layer is formed by thermal oxidation, sputtering, chemical vapor deposition, wet-chemical coating or heat treatment.

4. The method as claimed in claim 1, wherein the lithium of the lithium layer is applied on the mediator layer by laser melting, melting using a heating apparatus, knife coating, application using a slot die, spraying, spreading, dip coating, thermal spraying or lamination.

5. The method as claimed in claim 1, wherein the mediator layer is formed with a thickness of between 0.1 nm and 1000 nm before coating with lithium.

Description

(1) Exemplary embodiments of the invention are represented in the drawings, and are explained below with reference to FIGS. 1 to 10.

(2) In the figures

(3) FIG. 1 shows a schematic side view of an alkali metal applied by heated nozzle;

(4) FIG. 2 shows a view corresponding to FIG. 1 of the alkali metal applied by heated reservoir;

(5) FIG. 3 shows a view corresponding to FIG. 1 of the alkali metal applied by laser melting;

(6) FIG. 4 shows a view corresponding to FIG. 1 of the alkali metal applied by knife coating;

(7) FIG. 5 shows a view corresponding to FIG. 1 of the alkali metal applied by spreading;

(8) FIG. 6 shows a view corresponding to FIG. 1 of the alkali metal applied by dip;

(9) FIG. 7 shows a view corresponding to FIG. 1 of the alkali metal applied by spray;

(10) FIG. 8 shows a view corresponding to FIG. 1 of the alkali metal applied by continuous nozzle;

(11) FIG. 9 shows a view corresponding to FIG. 1 of the alkali metal applied by pressure roll; and

(12) FIG. 10 shows a view corresponding to FIG. 1 of the alkali metal applied by glide.

(13) FIG. 1 in a schematic side view shows a method for producing a substrate 2 coated with lithium as alkali metal 1. The substrate 2 in the exemplary embodiment represented is a copper foil which by thermal oxidation has been provided with a copper oxide layer as mediator layer 3. The mediator layer 3 is therefore in direct, i.e., immediate, contact with the substrate. The lithium is melted by means of a heated nozzle 5. The lithium foil is passed through the electrically heated nozzle 5 and is melted in the process. The substrate 2 is conveyed past flush below the nozzle 5, and the mediator layer 3 is disposed in turn on the substrate. The melted lithium, then, impinges on a surface of the mediator layer 3 that faces the nozzle 5, where it cools and goes back into a solid aggregate state, with the formation of an impervious lithium layer which is in direct contact with the mediator layer 3.

(14) As shown in FIG. 2, in a view corresponding to FIG. 1, a preferably electrically heated reservoir 6 is also able to accommodate liquefied lithium and apply the lithium to the mediator layer 3 by a slot die of the reservoir 6, this stop die facing the substrate 2. The reservoir 6 in this case has the form of a funnel, to allow targeted exit flow through the slot die. Recurring features in this figure and the following figures are given identical reference symbols.

(15) In a further embodiment of the invention, which is represented in FIG. 3, a laser radiation source 4 directs electromagnetic laser radiation onto an impingement point of the foil-form lithium onto the substrate 2 or mediator layer 3, and so at this point the foil-form lithium is liquefied and is disposed on the substrate 2 by the copper oxide mediator layer 3.

(16) In a further embodiment, which is reproduced in FIG. 4, the substrate 2 is heated and lithium as the alkali metal 1 lies as a melt on the substrate 2. By means of a coating knife 7, the lithium is scraped off with a defined thickness and applied to the mediator layer 3, which as before is facing a source of the alkali metal 1 to be applied.

(17) In the exemplary embodiment represented in FIG. 5, cold lithium foil is passed over a hot substrate 2. The substrate 2 in this case is passed through an electric oven and heated in the process. Spreading of the cold lithium foil over the substrate 2 causes the lithium foil to melt at the point of impingement on the mediator layer 3 applied to the substrate 2, and the melted lithium foil wets this mediator layer 3 superficially. After cooling, therefore, there is again an impervious lithium coating.

(18) FIG. 6 in turn, in a schematic side view, shows a heated tank 8 containing melted lithium. The substrate 2 with mediator layer 3 applied thereon is passed through the tank 8. On departure from the tank 8, it is possible to scrape off excess lithium at a scraper 9 and so to establish a layer of lithium on the substrate 2 that is identical in layer thickness on either side.

(19) FIG. 7 shows an exemplary embodiment wherein coating of the substrate 2 provided with the mediator layer 3 is accomplished by spray coating, where the alkali metal is deposited from a spraying nozzle 10 onto the substrate running past below the spraying nozzle 10.

(20) In the case of the exemplary embodiment reproduced in FIG. 8, the substrate 2 is coated on either side by the guiding of solid lithium to the substrate 2 from both sides. Because the substrate 2 is passed through a heated continuous nozzle 11 before the lithium impinges on the substrate 2 or on the mediator layer 3 disposed on either side of the substrate 2 and melts there, the substrate 2 is coated in such a way that an impervious lithium layer is formed.

(21) In a further exemplary embodiment, represented in FIG. 9, the substrate 2 is coated by a pressure roll method, by introduction of liquid lithium into surface indentations in a heated applicator roll and transport of the liquid lithium with the roll onto the substrate 2, where it impinges on the mediator layer 3 and is deposited.

(22) In a further exemplary embodiment, which is represented in FIG. 10, the substrate 2 is coated by the uniform guiding of the substrate over a lithium melt bath, the substrate 2 being just in contact with the lithium surface. In this surface contact drawing, as it is called, the adjustment of layer thickness is accomplished by factors including the travel speed of the substrate 2. The coating width is undertaken by the width of the lithium melt bath, and hence, when using a foil whose width is greater than the lithium melt bath, an uncoated margin is produced.

(23) The method described in various embodiments therefore enables extensive deposition of lithium on various substrates 2 by application of a lithiophilic mediator layer 3 which is thin—typically between 0.1 nm and 1000 nm in thickness. Examples of possible substrate materials used include copper foils, nickel foils, perforated metal foils, carbon fibers, especially carbon fiber mats, nonwoven webs made of carbon nanotubes (CNT nonwovens), woven metal wire fabrics, or polymeric substrates 2 such as polyimide films, woven polyimide fiber fabrics or laid polyimide fiber scrims.

(24) The mediator layer 3 is formed of a material such as silicon, tin, antimony, aluminum, magnesium, bismuth or an alloy of the stated chemical elements such as CuSn that forms an alloy with lithium. Alternatively, the mediator layer 3 may also be made of a material which reacts in contact with liquid lithium to form a mediating interface—for example, which reacts by reduction of an oxidic material to form a material that forms alloys with lithium, such as aluminum oxide, for example. This may also be realized, for example, through materials which permit intercalation or insertion of lithium. These may in particular be materials which are employed as active materials in lithium-ion batteries, e.g., LiCoO.sub.2, LiNiO.sub.2, LiFePO.sub.4, LiMnO.sub.2, Li.sub.2Mn.sub.3NiO.sub.8, LiNiCoMnO.sub.2, LiNiCoAlO.sub.2, Li.sub.4Ti.sub.5O.sub.12 or carbon in graphite form. Alternatively, however, there may also be materials which in contact with liquid lithium react to give compounds which permit intercalation or insertion or other chemical reactions, examples being conversion materials, with which there is no intercalation or insertion but instead, for example, a metal oxide is converted directly (reversibly), vanadium oxide, manganese oxide, iron oxide, copper oxide, sulfur or sulfides. Instead of lithium, it is also possible analogously to utilize sodium for the formation of homologous layers from the melt. An example reaction of a conversion material may be as follows: MeO+2 Li.fwdarw.Me+Li.sub.2O.

(25) Generally speaking, an oxidic mediator layer 3 which is at least partially (but also completely) reduced by lithium or sodium can be used. This oxidic mediator layer may therefore comprise a metal oxide, with oxides or oxide compounds of the following elements being suitable as the metal oxide: magnesium, aluminum, silicon, calcium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, zirconium, molybdenum, indium, tin, antimony, lead, bismuth.

(26) As for the application of the alkali metal 1, there are various possible procedures that can be employed to form the mediator layer 3. For example, a copper foil 12 μm in thickness can be surface-oxidized at 300° C. to 500° C. in a preheated muffle furnace (or in a high-temperature continuous oven for continuous substrates) to generate a thin oxide layer having a thickness of between 0.1 nm and 1000 nm, which serves as the mediator layer 3. In the same way, by oxidation of a nickel foil 20 μm in thickness, a nickel oxide mediator layer can be formed at 600° C. as well. The formation of Cu.sub.2O can be demonstrated by X-ray diffraction (XRD). Subsequently, in an Ar environment, solid lithium can be brought into contact with the oxidized cooper foil heated to 200° C., leading to the melting of the lithium. Then, as shown in FIG. 4, using the coating knife 7, a lithium layer with a thickness of 20 μm to 160 μm can be generated.

(27) In the case of the exemplary embodiment described above, parameters of the method can also be adapted to the intended use. The copper foil is preferably treated at 300° C. for one minute, forming a Cu.sub.2O layer of approximately 10 nm in thickness that can be wetted very effectively by liquefied lithium. At 400-500° C., the treatment times are less than one minute, but different, less favorable copper oxides such as CuO are also formed.

(28) In a variation of this exemplary embodiment, the mediator layer 3 is produced only in certain regions on the substrate 2. This enables the lithium layer to be imposed on the substrate 2 only in these regions, as well. Possible accordingly is the realization of patterned lithium layers and/or geometric shapes, or the generation of an uncoated marginal region for current collector tabs or the like.

(29) In a further exemplary embodiment, a carbon fiber nonwoven (Freudenberg H14) can be impregnated for 30 s in an ethanolic SnCl.sub.2 solution or SbCl.sub.3 solution (0.15 M), dried for 5 minutes, and then heat-treated in air in a preheated muffle furnace at 300° C. for 5 minutes. After that, reduction is carried out in the absence of oxygen at 700° C. for 60 minutes. The substrate 2 produced in this way is subsequently dipped in a lithium melt.

(30) In other exemplary embodiments, a woven copper fabric (wire thickness 0.05 mm, mesh size 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, called an ED foil, bearing dendritic copper structures in the low single-digit μm range, through electrodeposition, converted in turn by thermal oxidation into a wettable state.

(31) Whereas, for example, lithium beads are unsuitable for wetting on an untreated surface of a copper foil, the lithium beads can wet the foil surface in the case of a surface treated as described, on heating in argon to approximately 200° C. Accordingly it is possible to produce 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.

(32) Features of the various embodiments that are disclosed only in the exemplary embodiments can be combined with one another and claimed individually.