DE102022104935A1 - Process for producing a porous lithium layer - Google Patents

Abstract

The invention relates to a method for producing a porous lithium layer, comprising the steps of a) providing a substrate comprising at least one surface to be coated with a porous lithium layer, b) treating the at least one surface to be coated with a porous lithium layer with a plasma generated by a plasma source Plasma at a pressure of 100 mbar or less,c) tempering the substrate to a temperature of less than 180 °C, andd) depositing a porous lithium layer on the at least one surface of the substrate treated with a plasma in step b) at a pressure of 100 mbar or smaller.

Description

technical field

The invention relates to the vapor deposition of metals, more specifically to a vapor phase process for preparing a porous layer of lithium.

Background and prior art

With lithium-ion batteries, the use of lithium metal anodes can achieve a storage density that is 3 to 5 times higher - in contrast to current anode concepts based on conventional materials such as graphite. Nevertheless, lithium metal anodes have not been used in commercial battery types to date because instabilities on the surface during the charging and discharging process can lead to the growth of lithium dendrites and these can cause short circuits between the cathode and anode. The instabilities on the Li metal anode have their origin in the high reactivity of the element lithium with the electrolyte and associated with structural changes of the anode surface with the formation of a solid-electrolyte interface (SEI). The SEI is heterogeneously conductive for the Li ions on a lateral level, which results in increased local Li incorporation and the growth of dendrites is promoted. The dendrites penetrate the SEI and promote local directional growth by providing a fresh Li surface. Dendrite formation can be reduced by charging at low current densities. Charging processes with small charging currents are not practicable because of the low charging speed. Instead, innovative anode concepts focus on providing the largest possible surface area in the contact area between the metallic lithium anode and the electrolyte, thereby minimizing the current density at practicable charging speeds. In the prior art, this is accomplished by a porous structure of the anode.

It is known from the prior art that the morphology of layers that are deposited using physical vapor deposition can be changed by the process parameters. The microstructure that forms in the condensed layer depends, among other things, on the substrate temperature during deposition and is generally well matched to the three-zone model of for many materials Movchan & Demchishin (Phys. Met. Metallogr., Vol. 28, pp. 653-660, 1969 ) and the extended model of Anders (Thin Solid Films, Vol. 518, Issue 15, pp. 4087-4090, 2010 ) described. To generalize for different materials, the substrate temperature T _sub is usually related to the melting temperature T* of the layer material and the layer structure that forms is considered for the dimensionless characteristic number of the homologous temperature T _hom =T _sub /T*. A scientific investigation into the applicability of these structural zone models to materials with a relatively low melting temperature, such as lithium, or even into the formation of a porous structure immediately after deposition without further processing steps, is not known.

Known from the prior art are also methods for producing porous metal layers by means of plasma deposition, such as in US2018/277849 AA or by thermal spraying as described in US Pat U.S. 9,481,922 BB revealed.

However, none of these documents relates to a method for the production of porous metallic lithium layers.

In addition, the production of a lithiated silicon electrode by plasma spraying a suspension of precursor compounds in the U.S. 9,806,326 BB revealed. However, the layer obtainable in this way is not porous.

It is also known from the prior art that a porous structuring of the anode can be produced by a matrix of a host material. In Qin et al. (Acta Physico - Chimica Sinica, Vol. 37, pp. 1-10, 2021 ) a method is described in which a porous material made of copper foam is used as the host structure, on which lithium is subsequently deposited. It is known that these host structures can be pre-coated to improve battery performance. In Lu et al. (Journal of Energy Chemistry, Vol. 41, pp. 87-92, 2020 ) another method is described in which aluminum zinc oxide is used as an intermediary layer between the copper foam and the lithium. At Qin et al. (ACS Applied Materials and Interfaces, Vol. 10, pp. 27764-27770, 2018 ) describes the use of intermediate layers made of zinc oxide. In addition, mediator layers made of gold can also be advantageous, as in Huang et al. (Journal of Physics: Conference Series, Vol. 2009, 2021 ) described. Other suitable host materials can be nickel foams, as in Zhang et al. (Chemical Engineering Journal, Volume 405, 2021 ) described or foams of carbon as in Liu et al. (Advanced Materials, Vol. 30, 2018 ) described. Free-standing porous lithium structures without a host structure can also be produced by etching compact lithium layers, as in Yang et al. (Energy Storage Materials Vol. 26, pp. 385-390, 2020 ) is described.

However, these known methods for producing porous lithium layers have the disadvantage, on the one hand, that a porous host layer must first be produced, and on the other hand, they require the use of chemical etching agents. Both known processes from the prior art therefore require additional expenditure of time and money and are therefore disadvantageous from the point of view of process economy. A further disadvantage of the known methods is that the production of a host structure or the use of etching agents introduce impurities into the lithium layers thus obtained, which can have a disadvantageous effect on the properties of electrodes comprising these lithium layers, for example.

The present invention aims to obviate these disadvantages of the prior art.

The object of the present invention is therefore to provide a method which makes it possible to obtain a porous lithium layer on a substrate without prior porous host structure or chemical etchants, and which consequently enables time and cost savings with a simplified procedure.

This object is solved by the subject matter of the present main claim. Advantageous configurations are listed in the dependent claims.

Brief description of the drawings

• 1 shows a schematic representation of a preferred embodiment of the method for the production of a porous lithium layer.
• 2 shows a schematic representation of an alternative preferred embodiment of the invention with vacuum-technically separated vacuum recipients.
• 3 shows a schematic representation of a further alternative preferred embodiment of the invention with two separate vacuum locks.
• 4 shows a schematic representation of a further alternative particularly preferred embodiment of the invention, in which the substrate is processed in the form of a strip.
• 5 FIG. 12 shows a flow chart of the method for manufacturing a porous lithium film.
• 6 shows schematically, as a comparative example, the morphology of a lithium layer that forms after application of a deposition method without prior treatment of the substrate with a plasma.
• 7 FIG. 12 shows schematically the morphology of a lithium layer that forms after application of a deposition process after prior treatment of the substrate with a plasma according to the present invention.
• 8th shows schematically the structure of a layer system in cross section, obtainable by means of the method according to the invention.
• 9 shows scanning electron micrographs of the surface of lithium layers obtainable by means of the method according to the invention ( 9a , 9b) and as a comparative example ( 9c , 9d ).

Summary

1. a) Bereitstellen eines Substrats umfassend mindestens eine mit einer porösen Lithiumschicht zu beschichtende Oberfläche,
2. b) Behandeln der mindestens einen mit einer porösen Lithiumschicht zu beschichtenden Oberfläche mit einem mittels einer Plasmaquelle erzeugten Plasma bei einem Druck von 100 mbar oder kleiner,
3. c) Temperieren des Substrats auf eine Temperatur von kleiner als 180 °C, und
4. d) Abscheiden einer porösen Lithiumschicht auf der mindestens einen in Schritt b) mit einem Plasma behandelten Oberfläche des Substrats bei einem Druck von 100 mbar oder kleiner.

The invention relates to a method for producing a porous lithium layer, comprising the steps

1. a) providing a substrate comprising at least one surface to be coated with a porous lithium layer,
2. b) treating the at least one surface to be coated with a porous lithium layer with a plasma generated by a plasma source at a pressure of 100 mbar or less,
3. c) Tempering the substrate to a temperature of less than 180° C., and
4. d) depositing a porous lithium layer on the at least one surface of the substrate treated with a plasma in step b) at a pressure of 100 mbar or less.

Description of the invention

1. a) Bereitstellen eines Substrats umfassend mindestens eine mit einer porösen Lithiumschicht zu beschichtende Oberfläche,
2. b) Behandeln der mindestens einen mit einer porösen Lithiumschicht zu beschichtenden Oberfläche mit einem mittels einer Plasmaquelle erzeugten Plasma bei einem Druck von 100 mbar oder kleiner,
3. c) Temperieren des Substrats auf eine Temperatur von kleiner als 180 °C, und
4. d) Abscheiden einer porösen Lithiumschicht auf der mindestens einen in Schritt b) mit einem Plasma behandelten Oberfläche des Substrats bei einem Druck von 100 mbar oder kleiner.

According to the invention, there is provided a method for producing a porous lithium layer, comprising the steps

1. a) providing a substrate comprising at least one surface to be coated with a porous lithium layer,
2. b) treating the at least one surface to be coated with a porous lithium layer with a plasma generated by a plasma source at a pressure of 100 mbar or less,
3. c) Tempering the substrate to a temperature of less than 180° C., and
4. d) depositing a porous lithium layer on the at least one surface of the substrate treated with a plasma in step b) at a pressure of 100 mbar or less.

First of all, the method according to the invention comprises the provision of a substrate comprising at least one surface to be coated with a porous lithium layer.

The substrate is not particularly limited, and a variety of substrates can be provided with a porous lithium layer by the method of the present invention.

According to an advantageous embodiment of the invention, the substrate is selected from the group consisting of metal elements or metal alloys, non-metal elements, plastics and ceramics, and combinations thereof. These materials offer the advantage of particular suitability as electrode backing material for energy conversion devices and energy storage devices.

Any metal element or metal alloy can serve as the substrate, preferably the metal element or metal alloy is selected from a transition metal, lanthanide, element of group 13 or group 14 of the periodic table, or combinations thereof, more preferably the metal element is Ti, Zr, V, Cr, Mo , W, Mn, Fe, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au, Zn or a combination thereof.

In a particularly preferred embodiment of the invention, the substrate is copper or a copper alloy. These offer the advantage that, due to the high electrical conductivity and low costs, they represent particularly suitable electrode materials for storage or conversion devices of electrical energy.

Non-metal elements can also be used as substrate, preferably based on carbon, with graphite being particularly preferred.

Furthermore, polymeric plastics can be provided as a substrate with a porous lithium layer. The type of polymeric plastic is not particularly limited, the plastic is preferably selected from polyolefins, polyethylenes, polyethers, polyesters, polyamides, fluoropolymers, polyimides, polyetheretherketone, polyalkanes, polyalkenes, polyvinyl chlorides, polystyrenes, polycarbonates, cellulose hydrates and combinations and copolymers thereof, further preferably acrylonitrile-butadiene-styrene copolymer and ethylene-tetrafluoroethylene copolymer and particularly preferably poly(acrylonitrile-butyl acrylate).

Known ceramics can also be used as the substrate. A ceramic substrate is preferably selected from oxides, nitrides or carbides, particularly preferably aluminum oxide, silicon oxide, silicon nitride or silicon carbide.

The dimensions of the substrate are not particularly limited. However, the substrate preferably has a width of from about 10 mm to about 1000 mm, more preferably from about 100 mm to about 300 mm, and particularly preferably from about 150 mm to about 200 mm. In addition, the substrate has a Depth of about 10 mm to about 100,000 m, more preferably from about 100 mm to about 500 mm, and particularly preferably from about 150 mm to about 200 mm. Furthermore, the substrate has a thickness of about 1 μm to about 10 mm, more preferably from about 0.005 mm to about 5.000 mm. In a particularly preferred embodiment of the invention, the substrate has a thickness of approximately 0.01 mm to approximately 0.05 mm. In this particularly preferred thickness range, the substrate is particularly suitable for use as an electrode substrate for conversion and storage devices for electrical energy.

The shape of the substrate is likewise not particularly restricted; the substrate to be coated using the method according to the invention can have any desired shape. However, these can preferably be present in the form of plates, foils, tapes or three-dimensional objects of any desired shape.

In a particularly preferred embodiment of the invention, the at least one surface of the substrate to be coated with a porous lithium layer is not designed as a porous structure. In this context, a porous structure is to be understood as meaning any open-pore or closed-pore, net-like structure provided with cavities and/or holes of any size and thus permeable structure.

In a preferred embodiment of the invention, after providing a substrate and before treating at least one surface of the substrate to be coated with a porous lithium layer with a plasma generated by a plasma source, the substrate can be wet-chemically cleaned with subsequent drying. This improves the adhesion of the porous lithium layer to be deposited on the substrate.

The methods of wet-chemical cleaning of the substrate are not particularly limited and also depend on the type of substrate, preferably the wet-chemical cleaning can be an ultrasonic treatment in conjunction with surfactants, rinsing with organic solvents such as ethanol or acetone, rinsing with deionized water, treatment with acidic and /or oxidizing agents such as sulfuric acid/hydrogen peroxide, acetic acid, citric acid, hydrochloric acid/hydrogen peroxide or hydrofluoric acid, and ammonium hydroxide, and combinations thereof. In a preferred embodiment, the wet-chemical cleaning can include alkaline solutions. This is associated with a good cleaning effect in connection with a surface-friendly treatment, especially in the case of metallic substrates such as copper or copper alloys.

Subsequent drying can be carried out using any desired method; drying can preferably include air drying, drying by means of an air stream, heat drying, microwave drying, vacuum drying and combinations thereof, drying by means of an air stream is particularly preferred. This represents a drying process that is technically particularly easy to implement.

In a particularly preferred embodiment of the invention, the optional wet-chemical cleaning of the substrate does not result in the formation of a porous structure as defined above on the at least one surface of the substrate to be coated with a porous lithium layer.

Subsequent to the provision of a substrate comprising at least one surface to be coated with a porous lithium layer with subsequent optional wet-chemical cleaning and drying of the substrate, the method according to the invention can include introducing the substrate into a vacuum recipient. Such a vacuum recipient can preferably serve to carry out the method according to the invention for producing a porous lithium layer and more preferably comprise a plasma source, means for tempering the substrate to a defined temperature and a device for depositing a lithium layer.

Smuggling into a vacuum recipient is not particularly restricted and can preferably take place by means of a vacuum lock with a lock valve and particularly preferably by means of a vacuum lock with a roller lock.

Furthermore, following the provision of a substrate with optional wet-chemical cleaning, drying and smuggling into a vacuum recipient, the method according to the invention comprises treating the at least one surface to be coated with a porous lithium layer with a plasma generated by a plasma source at a pressure of 100 mbar or less .

The specified pressure can preferably relate to the measured pressure inside a vacuum recipient.

For this purpose, the substrate with at least one surface to be coated with a porous lithium layer can be exposed to a plasma generated by a plasma source in a stationary manner or alternatively be guided past a plasma generated by a plasma source or over a plasma in a translatory manner by means of a transport device. However, the substrate can preferably be guided in a translatory manner by means of a transport device over a plasma generated by means of a plasma source.

If, in a preferred embodiment, the substrate is guided past a plasma generated by a plasma source in a translatory manner by means of a transport device, this results in a simplified procedure.

In the context of this embodiment, the translatory speed of the transport device is not particularly limited, but this is preferably from about 0.1 cm/s to about 100.0 cm/s, more preferably from about 1 cm/s to about 10 cm/s, and more preferably from about 3 cm/s to about 6 cm/s.

Furthermore, the distance from the plasma source to the substrate is also not particularly limited, but is preferably from about 1 mm to about 500 mm, more preferably from about 10 mm to about 200 mm, and particularly preferably from about 120 mm to about 160 mm.

In addition, the effective current density of the carriers to the substrate is not particularly limited, and may preferably be from about 10 mA/cm ² to about 80 mA/cm ² , more preferably from about 20 mA/cm ² to about 60 mA/cm ² , and particularly preferably from about 30 mA/cm ² to about 50 mA/cm ² .

The current density and in particular its lateral distribution in the experimental setup when the plasma source is operated is measured using sensors arranged in an array. Each of these sensors consists of a small metal plate of a defined size, to which voltages of 0 to 300 V are applied and the charge carriers are extracted from the plasma to the metal plate. As the voltage rises, the measured current increases, which saturates at around 200 V. The locally acting current density can be determined from the saturation current and the surface area of the metal plate.

The integrally acting charge per unit area applied to the substrate can preferably be from about 0.1 A*s/cm ² to about 300.0 A*s/cm ² , more preferably from about 0.5 A*s/cm ² to about 25.0 A*s/cm ² , and more preferably from about 1.0 A*s/cm ² to about 3.0 A*s/cm ² .

The integrally acting charge is calculated by integrating the current density distribution over the infinitesimal residence time of the substrate.

The plasma source is not particularly restricted; any known plasma source is suitable for treating the at least one surface of the substrate to be coated.

In a preferred embodiment of the invention, the plasma source is a hollow cathode arc discharge of a hollow cathode arc source or a magnetron.

A plasma source in the form of a hollow cathode arc discharge of a hollow cathode arc source according to a preferred embodiment of the invention is disclosed in the reference EP 2 087 503 B1 described. The disclosure of this reference is incorporated herein by reference in its entirety. As disclosed in this publication, such a device comprises a low-voltage arc discharge source arranged in a vacuum chamber for generating a plasma from which charge carriers can be extracted and with which the surface of a substrate can be applied. The low voltage arc discharge source further includes a cathode, an anode, and a power source for maintaining a discharge between the cathode and the anode. Furthermore, there is a voltage source for generating a BIAS voltage, which includes the acceleration of the charge carriers onto the substrate surface. This offers the advantage that the acceleration of charge carriers from the plasma in the direction of the substrate can be increased and the etching rate can thus be increased.

To operate a hollow cathode plasma, the cathode of the low-voltage arc discharge source is designed as a hollow cathode. It is also necessary to admit a gas through the hollow cathode into the vacuum chamber. The gas can preferably include argon, nitrogen, hydrogen and a combination thereof, and the gas can particularly preferably be argon.

During operation, the substrate can be connected to the electrical ground of the vacuum chamber and the BIAS voltage can be connected between the substrate and the ground of the vacuum chamber on the one hand and an electrode close to the plasma potential in terms of voltage on the other. The electrode can be a cathode or an anode of the low-voltage arc discharge source. In a particularly preferred embodiment, however, no BIAS voltage is connected between the substrate and the vacuum chamber ground.

In a further preferred embodiment, the substrate can be placed at a defined electrical potential based on the plasma potential of the plasma source. Depending on this, charge carriers emitted by the plasma source are accelerated in the direction of the substrate. These charge carriers can preferably be electrons or ions of the gas provided by the hollow cathode. If these charge carriers are electrodes, this offers the advantage of electrochemical activation of the substrate surface. If these charge carriers are ions of the gas provided by the hollow cathode, material can be removed and the surface roughness of the substrate can thus be changed. In a particularly preferred embodiment, the charge carriers are argon ions.

A plasma source in the form of such a hollow cathode arc discharge of a hollow cathode arc source has the advantage that complex insulation measures between the substrate and the electrical ground of the device are not required as a result.

A plasma source in the form of a magnetron is from J.Faber et al. in Vacuum Vol. 64 (2002), pp. 55-63 been described. The disclosure of this reference is incorporated herein by reference in its entirety. As in particular with reference to Section 2 and 1 As can be seen from this journal article, a movable substrate serves as the cathode at ground potential of a magnetron discharge. The anode is spaced about 100 mm from the substrate and is connected to a high voltage source. A dark space shield surrounds the anode at a distance of about 2 mm. An arrangement of SmCo permanent magnets located directly below the substrate, which is customary for planar magnetrons, is used to generate a strong magnetic field. With this embodiment of the plasma source, a good decoupling of the local process pressure in the area affected by the plasma from the rest of the recipient can be achieved, whereby the process of treatment with a plasma and the deposition process can be operated in parallel with several substrates or in the form of strip-shaped substrates. In addition, components dusted from the substrate surface are primarily caught on the metallic surfaces of the dark room shield, which means that the deposition process is not affected by contamination and very clean coatings can be achieved.

According to the invention, the treatment with a plasma takes place at a pressure in the vacuum recipient of about 100 mbar or less, preferably from about 1*10 ^-5 mbar to about 1*10 ^-3 mbar, and particularly preferably from about 1*10 ^-4 mbar to about 8*10 ^-4 mbar.

A low pressure in the vacuum recipient is advantageous, for example and in particular, in order to enable parallel operation of treating a substrate with a plasma and depositing a lithium layer without mutual impairment. In addition, at a low process pressure within the preferred ranges, the scattering of the charge carriers on the process gas and residual gas is reduced, as a result of which the step of treating with a plasma can take place with greater efficiency. However, the coupling of higher plasma powers requires a higher intake of process gas for stable process conditions without flashovers in the form of cold cathode arcs. This must be weighed up and optimized for the respective parameter constellation.

Following the treatment of the at least one surface to be coated with a porous lithium layer with a plasma generated by a plasma source at a pressure of 100 mbar or less, the substrate is tempered to a temperature of less than 180° C. according to the invention. Preferably, the substrate can be tempered to a temperature in a range from about −50° C. to about 175° C., more preferably from about 10° C. to about 175° C., even more preferably from about 30° C. to about 175° C , particularly preferably from about 50 °C to about 175 °C and in an alternative particularly preferred embodiment from about 160 °C to about 170 °C.

If the substrate temperature is 180°C or higher, no porous lithium layer can be formed during the deposition step. If, on the other hand, the substrate temperature is in one of the preferred ranges, a homologous temperature of 0.5≦T _hom ≦1 is established for the substrate and zone 3 is thus completely covered in the structure zone model explained as follows.

Table 1 shows selected values of the homologous temperature T _hom at different substrate temperatures T _sub calculated as an example for the material lithium. Table 1: Selected values of the homologous temperature T _hom at different substrate temperatures T _Sub calculated as an example for the material lithium. T _Sub in °C -250 -200 -150 -100 -50 0 50 100 150 180 _{Thomas, Li} 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Zone 1 zone 2 zone 3

Zone 1 includes the forming structure at a low homologous temperature (T _hom < 0.25). Zone 2 is defined by the region 0.25 < T _hom < 0.45 where surface diffusion is dominant for growth. Finally, zone 3 includes the region T _hom > 0.45, in which the growth is determined by volume diffusion. Good agreement with the model was found, for example, for the materials titanium, nickel and tungsten for homologous temperatures T _hom < 0.6. In contrast, the validity of the three-zone model for high homologous temperatures T _hom > 0.6 has not been fully proven.

As a result, without wanting to be limited by theoretical considerations, the condensing lithium atoms or lithium atom clusters also have more and more energy with increasing temperature of the lower limit, in order to be able to arrange themselves more and more easily in a defined morphology with a porous structure on the substrate surface due to diffusion effects .

On the other hand, again without wishing to be limited by theoretical considerations, in the preferred ranges the energy of the condensing lithium atoms or lithium atom clusters is too low to form a liquid film from a lithium melt and to produce a compact, dense structure of a lithium layer. In the preferred areas, it is increasingly easier to form a porous structure while at the same time avoiding a compact structure for the deposited lithium layer.

The method for depositing a lithium layer is not particularly limited, but any known method can be used here. A lithium layer can preferably be deposited by means of thermal vapor deposition, electron beam vapor deposition or magnetron sputtering; a lithium layer can particularly preferably be deposited by thermal vapor deposition.

If, in a particularly preferred embodiment of the invention, a lithium layer is deposited by means of thermal vapor deposition, this results in the technical advantage of a lower outlay in terms of apparatus and simpler process control, which results in cost advantages.

In thermal evaporation, metallic lithium is provided in a crucible. This crucible is heated to a defined temperature by means of a device designed to supply heat in the form of a resistance heater or induction heater. This causes the metallic lithium to go into the gas phase and a vapor cloud is generated above the crucible. In a preferred embodiment, the substrate can be transported across this vapor cloud.

The crucible temperature is not particularly restricted here; the crucible temperature is preferably in a range from about 500° C. to about 1400° C., more preferably from about 550° C. to about 800° C. and particularly preferably from about 590° C. to about 650° C

If the crucible temperature is below a temperature of about 500° C., the observed coating rate is too low for a process-economical deposition of metallic lithium. If the crucible temperature is above about 1400 °C, reactions with the crucible material are to be expected, which means that no purely metallic lithium layers can be deposited. In addition, no stable process conditions and the formation of spattering from the melt are to be expected during evaporation with a crucible temperature above about 1400 °C.

If the crucible temperature is more preferably from about 550° C. to about 800° C., this results in the technical advantage that the evaporation rate can be regulated very precisely via the crucible temperature. The evaporation rate shows an exponential dependence on the crucible temperature, so that at a crucible temperature above about 800 °C, the smallest temperature differences of about 1 K cause very strong changes in the evaporation rate. In the temperature range above about 800 °C, external influences due to changes in heat dissipation as a result of thermal conduction and thermal radiation also become very significant, so that precise regulation of the crucible temperature is associated with a great deal of technical effort. If the crucible temperature is particularly preferably in the range from about 590° C. to about 650° C., the functional dependency of the evaporation rate on the crucible temperature can be approximated by a linear function, which means that the evaporation rate can be adjusted very precisely using suitable electronic control.

The crucible material can be any material suitable for this temperature range; the crucible material can preferably be stainless steel.

According to the invention, a lithium layer is deposited at a pressure of 100 mbar or less, preferably from about 1*10 ^-7 mbar to about 5*10 ^-4 mbar, and particularly preferably from about 1*10 ^-6 mbar to about 1*10 ^{- 5} mbar. These specified ranges can be achieved in particular in all of the embodiments described below.

The pressure range during the treatment of the at least one surface of the substrate to be coated with a porous lithium layer with a plasma generated by a plasma source can be identical to or different from the pressure range for the deposition of a lithium layer.

In one embodiment, the substrate is treated with a plasma and a lithium layer is deposited in parallel. However, this has the disadvantage that when the steps for treating the substrate with a plasma and for depositing a lithium layer are carried out in parallel, the two processes can influence each other and both processes cannot be carried out with sufficient quality.

An alternative embodiment represents the sequential sequence of the steps of treating the substrate with a plasma and depositing a lithium layer, in which the respective other process station is deactivated.

In a further alternative preferred embodiment, the two steps of treating with a plasma and depositing a lithium layer are each carried out in a separate vacuum recipient. The process pressures in the two vacuum recipients for the step of treating the substrate with a plasma and for the step of depositing a lithium layer can preferably be decoupled with a separate lock valve. However, the embodiment with two separate vacuum recipients is associated with increased outlay in terms of equipment, since a separate system of vacuum pumps has to be installed and operated for each vacuum recipient.

The deposition rate of metallic lithium depends on the temperature of the crucible and the pressure inside the vacuum recipient. Preferably, the static lithium metal deposition rate is from about 1 nm/sec to about 500 nm/sec, and more preferably from about 20 nm/sec to about 100 nm/sec. The deposition rate R can be determined ex situ by measuring the deposition time and by determining the weight of the substrate before and after deposition. For this purpose, the surface A to be coated and the initial mass m ₁ of the substrate before the deposition of a layer, the deposition time t and the mass m ₂ of the substrate after the deposition of a lithium layer are determined. From the mass density p of the deposited layer, the thickness D of the deposited layer and finally the deposition rate R can be determined according to Equation 1. $$R=\frac{D}{t}=\frac{m_2-m_1}{t\cdot A\cdot \rho }$$

In addition, the deposition rate can be determined in situ by using oscillating crystals and used as a controlled variable for setting the crucible temperature. In many cases, the geometric conditions of the deposition device require that the apparatus for measuring the deposition rate by means of oscillating crystals must be installed at a different position than the location at which the layer is deposited on the substrate. As a result, a different deposition rate than that measured on the substrate is measured with the apparatus for measuring the deposition rate by means of oscillating crystals. As a rule, the deviation between the two measurement methods can be explained by a linear functional relationship to be discribed. Therefore, both measuring methods for determining the deposition rate usually have to be calibrated to one another.

During the deposition of a metallic lithium layer on at least one surface of the substrate, a porous lithium layer is formed, more specifically an open-porous lithium layer, ie one provided with interconnected cavities. Without wishing to be limited by purely theoretical considerations, it is assumed that the plasma treatment of the substrate surface roughens it on the one hand, and surface activation takes place through plasma action on the other. As a result, the condensation process of the metallic lithium is influenced by surface effects at the interface of the substrate so that the formation of a porous layered structure takes place.

The layer thickness of the lithium layer is not particularly restricted; lithium layers of customary thicknesses can be obtained using the method according to the invention. Preferably, however, the layer thickness is from about 0.1 μm to about 50.0 μm, and more preferably from about 1 μm to about 10 μm. The layer thickness can be determined ex situ using stylus methods on a prepared step from the layer to the bare substrate in accordance with DIN EN 1071-1:2003 or by microscopic examinations of cross-sections of the layer in accordance with DIN EN 1071-10:2009.

Another way of measuring the layer thickness is to evaluate the in situ measured variable for the coating rate when using oscillating crystals with reference to the deposition time. In practice, the process is set up once using the above measurement methods and the measurement signal from the quartz crystals is calibrated to it. For production, the process parameters are usually controlled based on the measurement signal from the oscillating crystals, so that a defined layer thickness is set.

The structure of the deposited lithium layers - in particular the porosity - can be characterized with imaging microscopy methods. An image of the layer surface or the layer cross-section is created and the size of individual pores is measured using the magnification scale. The pore size distribution can be evaluated using static methods. According to the present invention, the pore size is not particularly limited, but may preferably range from about 0.5 μm to about 5.0 μm, and more preferably from about 1.0 μm to about 2.0 μm.

Following the step of depositing a porous lithium layer, the substrate can preferably be cooled in vacuo. This can be achieved by storing the substrate in a temperature-controlled area in the vacuum recipient for a defined period of time, with the thermal energy of the substrate being given off to the environment by radiant heat.

Following the deposition of a porous lithium layer and the optional cooling of the substrate in a vacuum, the substrate can preferably be removed from the vacuum. This is not particularly restricted; it can be carried out using a first vacuum lock with a lock valve, which is identical to the vacuum lock, by means of which the optional transfer into a vacuum recipient takes place.

In a preferred embodiment, the substrate can be removed from the vacuum by means of a second vacuum lock, which is different from a first vacuum lock, by means of which the optional transfer into a vacuum recipient takes place. In this way, an uninterrupted process sequence can be made possible and the effectiveness of a production can be increased. However, this embodiment with two separate vacuum locks is associated with increased outlay in terms of equipment.

In a particularly preferred embodiment, the substrate can be discharged from a first vacuum recipient by means of a first vacuum lock with roller valve and the substrate can be discharged from a second vacuum recipient by means of a second vacuum lock with roller valve. Here, the substrate is processed in the form of a strip. Continuous process operation can thereby be made possible and the effectiveness for production can be further increased.

The invention further relates to an electrode, preferably an anode, comprising a porous lithium layer, which can be obtained by the method described above. According to a particularly preferred embodiment, such an electrode does not comprise a porous substrate structure.

Furthermore, an energy storage or energy conversion device comprising an electrode, preferably an anode, as described above, is the subject matter of the present invention. The energy storage device is not particularly restricted; it can preferably be a capacitor, an accumulator or a battery. The energy conversion device can preferably be a fuel cell.

Detailed description

1 shows an exemplary experimental setup for carrying out the method for producing a porous lithium layer according to a preferred embodiment of the invention.

• Eine Vakuumschleuse (2a) mit Schleusenventil (2b) dient für den Transfer von Substraten (3c) in und aus dem Vakuum. Eine Substrattransportvorrichtung (3a) mit Substrathalterung (3b) bewirkt mittels einer translatorischen Bewegung den Transport des Substrats (3c) über Prozesseinrichtungen, insbesondere die Plasmaquelle (4a) und die Dampfwolke (5e).

The experimental setup includes a vacuum recipient (1) in which the following components are installed:

• A vacuum lock (2a) with a lock valve (2b) is used to transfer substrates (3c) into and out of the vacuum. A substrate transport device (3a) with a substrate holder (3b) uses a translational movement to transport the substrate (3c) over process devices, in particular the plasma source (4a) and the vapor cloud (5e).

A plasma source (4a) comprising a cathode (4b) and anode (4c), with which a low-pressure plasma (4d) is generated, is used to treat the at least one surface of the substrate (3c) to be coated with a porous lithium layer. The temperature of the substrate is set by a substrate heater (3d) optionally mounted on the substrate holder (3b). This can be done in a vacuum using a radiant heater.

A vapor source (5a) with a crucible (5b) containing the evaporating material (5c), metallic lithium, causes a lithium layer to be deposited. The crucible (5b) is thermally heated to a defined temperature with a heating device (5d), consisting of a resistance heater, whereby the evaporating material (5c) changes into the gas phase and a vapor cloud (5e) is generated above the vapor source (5a).

2 shows an example of an alternative embodiment in which a separate vacuum recipient (1a) is used for the process of treatment with a plasma and another separate vacuum recipient (1b) is used for the process of depositing lithium. The process pressures in the two vacuum recipients are decoupled by another sluice valve (2c).

3 shows an example of a further alternative preferred embodiment, in which the substrate vacuum lock (2a) with lock valve (2b) is transferred into the vacuum. After the steps for treating with a plasma and depositing a lithium layer have been carried out, the substrate is transferred out of the vacuum via a further lock valve (2d) and via a further vacuum lock (2h).

4 shows an example of a further alternative, particularly preferred embodiment, in which the substrate is rolled up in the form of a strip on an unwinding reel (3f) and is transferred into a vacuum lock (2a) with a roller lock (2e) in the vacuum. After the steps for treating with a plasma and depositing a lithium layer have been carried out, with the process pressures in the two vacuum recipients being decoupled by a further roller lock (2f), the substrate is guided over a further roller lock (2g) into a further vacuum lock (2h) transferred, rolled up on a take-up reel (3h) and transferred from the vacuum.

5 shows schematically a flow chart of the method according to a preferred embodiment of the invention. First, a substrate (101) to be coated is provided. This is then subjected to wet-chemical cleaning and drying (102) before the substrate is introduced into a glove box with a pure argon atmosphere (103). In the next step, the substrate is introduced into the vacuum recipient (1), in which the substrate surface to be coated is treated with a plasma (105). When the substrate is tempered to the target temperature (106), a lithium layer is subsequently deposited (107). The cooling of the substrate coated with a lithium layer in a vacuum (108) is followed by the discharge of the substrate into a glove box with a pure argon atmosphere (109).

6 shows, as a comparative example, the morphology of a lithium layer (6a, 6b, 6c) that is formed after application of a deposition process in which the substrate (3c) is first heated to a defined substrate temperature T _sub and then by means of thermal vapor deposition a lithium layer is deposited. The substrate temperatures T _Sub to be used can have the following values, for example: T ₁ =30° C., T ₂ =165° C., T*=180° C. At a low substrate temperature T _{1 ,} a columnar structure (6a) forms, which can lead to so-called nodular defects and as a result of which a bump-like layer surface is produced. At a higher substrate temperature T ₂ <T', a dense structure (6b) with larger crystallites is formed, which has only a small number of defects and shows a largely flat layer surface. At substrate temperatures T* greater than or equal to the melting temperature of the layer material, a dense structure (6c) with very large crystallites and a flat layer surface is formed.

7 shows schematically the morphology of a lithium layer (7a, 7b, 7c) that forms after the application of a deposition process in which a treatment with a plasma as described according to the invention is first carried out on the substrate surface (3e) and then the substrate (3c) is brought to a defined substrate temperature T _Sub is heated and then a lithium layer is deposited by means of thermal vapor deposition. The substrate temperatures T _sub to be used can have the following values, for example: T ₁ =30° C., T ₂ =165° C., T*=180° C. At a low substrate temperature T ₁ , a porous structure is formed, which has a reduced mass density compared to the bulk value of the layer material. At a higher substrate temperature T ₂ < T*, a porous structure is formed with, in comparison to T ₁ , larger pores and a further reduced mass density. At substrate temperatures T* greater than or equal to the melting temperature of the layer material, a dense structure forms with very large crystallites, with a largely flat layer surface and with a mass density comparable to the bulk value.

8th shows schematically the structure of the layer system in cross section, with a substrate surface (3e) being located on a substrate (3c) and having been treated by means of a plasma. This also represents the interface between the substrate (3c) and a porous lithium layer (7a, 7b) deposited on it.

During the deposition of the lithium layer, the substrate temperature T _Sub was set to values between T ₁ ≦T _Sub ≦T ₂ <T*.

9 shows scanning electron micrographs of the surface of lithium layers deposited under different process conditions. The 9a and 9b show examples of the layer morphology produced, obtainable by means of the method according to the invention by treating the substrate with a plasma and then depositing a lithium layer at a substrate temperature of 50° C. or 150° C. According to the invention, the layer has a structure which is interspersed with cavities of different sizes. The 9c and 9d show, on the other hand, comparative examples of the layer morphology produced, which arise outside the process area according to the invention. At the shift in 9c the substrate was treated with a plasma and then a lithium layer was deposited at a substrate temperature above 180 °C. At 9d a sole deposition of a lithium layer took place at a substrate temperature of 35 °C without carrying out a prior treatment of the substrate with a plasma. The layers in the 9c and 9d show a dense compact structure without pores.

exemplary embodiments

The present invention is explained in more detail below using exemplary embodiments.

Example a

For the exemplary embodiment a according to the invention, an experimental setup comprising a vacuum recipient (1) is used to carry out the method for producing a porous lithium layer, as is shown in 1 is shown as an example. In the first process step, the substrate (3c) is provided, which in this example is made of copper, has a width and length of 20 mm and a thickness of 1 mm. A K-type thermocouple is fixed to the back of the substrate by spot welding. The substrate surface to be coated with lithium is then first sprayed with acetone and wiped with a cotton cloth and then sprayed with ethanol and wiped with a cotton cloth and then stored in the atmosphere for a period of 10 minutes to dry. The substrate is then transferred into a glove box with a dry argon atmosphere, the weight of the substrate is determined and then placed in a substrate holder (3b) located in the vacuum lock (2a), followed by evacuation of the vacuum lock (2a), followed by the opening of the sluice valve (2b) and followed by the transfer of the substrate holder (3b) together with the substrate (3c) into the vacuum recipient (1). A plasma source (4a) is installed in the vacuum recipient (1), which was previously evacuated to an initial pressure of <1*10 ^-5 mbar, at a distance of 145 mm from the substrate transport device (3a). The plasma source (4a) is activated with the parameters listed in Table 2 and the substrate is moved completely over the effective area of the low-pressure plasma (4d) at a translatory speed of 4.6 mm/s. As a result, a substrate surface (3e) which has been treated by means of a plasma is produced on the substrate (3c). The plasma source (4a) is then deactivated and the substrate is heated to a temperature of 50° C. using a substrate heater (3d) mounted on the substrate holder. At the same time, the heating device (5d) on the vapor source (5a) is activated so that the crucible (5b), which contains the evaporating material (5c), metallic lithium, is heated to a temperature of around 597°C to 605°C . As a result, the evaporating material (5c) is converted into the gas phase and a vapor cloud (5e) is generated above the vapor source (5a) and a process pressure of about 6.2*10 ^-5 mbar to 8.2*10 ^-5 mbar is created in the vacuum recipient a. As a result, the evaporant evaporates and deposits on surfaces at the level of the substrate transport device (3a) with a static deposition rate of about 19.2 nm/s to 23.1 nm/s, which corresponds to a dynamic deposition rate of about 0.57 µm*m/ min to 0.66 µm*m/min. The substrate is then moved completely over the vapor source (5a) at a constant translatory speed of about 2.6 mm/s to 3.5 mm/s and thereby a metallic lithium layer with a mass application of about 150 μg/cm ² to 219 μg / cm ² deposited on the substrate surface, which would correspond to a hypothetical assumed mass density of the lithium layer of 0.534 g / cm ³ , a layer thickness of about 2.8 microns to 4.1 microns. The vapor source (5a) is then deactivated and the substrate is stored in a vacuum at a pressure of <1*10 ^-5 mbar for more than 120 minutes until the temperature of the substrate has cooled to values of <30°C. The substrate is then transferred via the vacuum lock (2a) into a glove box with a dry argon atmosphere and the weight of the substrate is again determined. In the glove box with a dry argon atmosphere, the coated substrate is examined visually and the appearance is documented. The coating has a greyish color and low reflectivity, which is reflected in a matte appearance. The coated substrate is then placed in a transport box with a dry argon atmosphere and transferred to a scanning electron microscope under inert conditions. As by inclusion in 9a is shown, the deposited lithium layer has a structure with pore-like cavities with a lateral size of about 1 μm.

example b

An equivalent production procedure is used for exemplary embodiment b according to the invention, with the substrate only being heated to a temperature of 150° C. before the lithium layer is deposited. The coating is also greyish in color with a matte appearance. As shown by the scanning electron micrograph in 9b is shown, the deposited lithium layer also has a structure with pore-like cavities, but with an increased lateral size of about 2 μm.

example c

An equivalent production procedure is used for comparative example c, the substrate being merely heated to a temperature of 180° C. before the lithium layer is deposited. The coating, on the other hand, has a dark greyish color with a matt appearance. As shown by the scanning electron micrograph in 9c As shown, the deposited lithium layer has a structure with a predominantly compact structure without voids and with a rough surface.

example d

An equivalent production procedure is used for comparative example d, but the surface of the substrate (3c) to be coated was not treated with a plasma and the substrate is heated to a temperature of 30° C. before the lithium layer is deposited. The coating, on the other hand, has a shiny metallic appearance. As shown by the scanning electron micrograph in 9d As shown, the deposited lithium layer has a structure with a predominantly compact structure without voids.

Industrial Applicability

The possible uses of the porous lithium layer obtainable by means of the method according to the invention are in the areas of energy conversion methods such as, for example, as electrodes of fuel cells, energy storage such as electrodes in capacitors or as electrodes in batteries as well as in the areas of catalytic converters, filters and sensors.

List of References

1

vacuum recipient

1a

Vacuum recipient for the plasma treatment step

1b

Vacuum recipient for the step of coating at least one surface of a substrate with lithium

2a

Vacuum lock for transferring the substrate into the vacuum

2b, 2c, 2d

sluice valve

2e, 2f, 2g

roller lock

2h

Vacuum lock for transferring the substrate from the vacuum

3a

substrate transport device

3b

substrate holder

3c

Substrate in the form of a plate or a three-dimensional object of any shape

3d

substrate heater

3e

Substrate surface after treatment with a plasma

3f

decoiler

3g

Substrate in the form of a tape

3 hours

take-up reel

4a

plasma source

4b

cathode

4c

anode

4d

low pressure plasma

5a

steam source

5b

crucible

5c

evaporation material

5d

heating device

5e

steam cloud

6a, 6b, 6c

developed morphology of the lithium layer without treating a substrate surface with a plasma

7a, 7b, 7c

developed morphology of the lithium layer after treatment of a substrate surface with a plasma

QUOTES INCLUDED IN DESCRIPTION

This list of the documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• US2018277849 [0004]
• US 9481922 [0004]
• US9806326 [0006]
• EP 2087503 B1 [0043]

Non-patent Literature Cited

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• Anders (Thin Solid Films, Vol. 518, Issue 15, pp. 4087-4090, 2010 [0003]
• Qin et al. (Acta Physico - Chimica Sinica, Vol. 37, pp. 1-10, 2021 [0007]
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Claims (14)

A method of making a porous lithium film, comprising the steps of a) providing a substrate comprising at least one surface to be coated with a porous lithium layer, b) treating the at least one surface to be coated with a porous lithium layer with a plasma generated by a plasma source at a pressure of 100 mbar or less, c) Tempering the substrate to a temperature of less than 180° C., and d) depositing a porous lithium layer on the at least one surface of the substrate treated with a plasma in step b) at a pressure of 100 mbar or less. The procedure according to claim 1 , wherein the substrate is selected from the group consisting of metal elements or metal alloys, non-metal elements, and plastics. The procedure according to claim 2 , wherein the substrate is copper or a copper alloy. The method of any one of the preceding claims, wherein the substrate is tempered to a temperature of about 50°C to about 175°C. The method according to any one of the preceding claims, wherein the plasma source is a hollow cathode arc discharge of a hollow cathode arc source or a magnetron. The method according to any one of the preceding claims, wherein the lithium layer is deposited by means of thermal evaporation. The method according to any one of the preceding claims, wherein steps b) and d) are carried out in separate vacuum recipients. The procedure according to claim 7 , further comprising the inward transfer of the substrate from a first vacuum recipient by means of a first vacuum lock with roller valve and the removal of the substrate from a second vacuum recipient by means of a second vacuum lock with roller valve. The method according to any one of the preceding claims, wherein the pressure in step b) is from about 1*10 ^-4 mbar to about 8*10 ^-4 mbar. The method according to any one of the preceding claims, wherein the pressure in step d) is from about 1*10 ^-6 mbar to about 1*10 ^-5 mbar. The method according to any one of the preceding claims, wherein after step a) a wet-chemical cleaning of the substrate with subsequent drying takes place. The procedure according to claim 11 , whereby the wet-chemical cleaning is carried out using alkaline solutions. An electrode comprising a porous layer of lithium obtainable according to any one of the preceding claims. An energy storage or energy conversion device comprising an electrode according to Claim 13 .

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