Method for applying a protective layer to a metal or metal alloy surface and articles including such a protective layer - Patents.com

Abstract

The present invention discloses a method of applying a protective layer on at least a portion of an exposed surface of a substrate, comprising the steps of activating a nitrogen-containing gas by atmospheric pressure plasma discharge in a plasma discharge chamber to obtain an activated gas, and contacting the exposed surface with an afterglow of the activated gas emitted from the plasma discharge chamber, wherein a protective layer is formed on at least a portion of the exposed surface, the surface comprising a metallic element or an alloy thereof, the metallic element being an alkali metal or an alkaline earth metal, and the surface and the plasma discharge chamber moving relative to one another while the exposed surface is in contact with the afterglow of the activated gas. The present invention further relates to an article comprising a substrate and a protective layer covering at least a portion of the substrate, the substrate and the protective layer sharing an interface comprising a metallic element or an alloy thereof, the protective layer comprising a plurality of spaced apart pillars protruding from the interface, the pillars being made of stacked layers of crystals of a nitride of the metallic element, the stacked layers of crystals comprising a polyhedral shape.

Description

[Technical field]

[0001] The present invention relates to a method for applying a protective layer to a surface of a substrate, the surface comprising a metallic element or an alloy thereof, in particular the metallic element being an alkali metal or an alkaline earth metal. The present invention further relates to an article comprising such a substrate and a protective layer disposed on or covering at least a portion of the substrate. The present invention further relates to an electrode, in particular an anode, comprising the article, and to a battery (cell) comprising the electrode. [Background technology]

[0002] In recent years, interest in high-density energy batteries and battery cells containing alkali metal anodes and alkaline earth metal anodes has been rapidly increasing. In particular, the alkali metals lithium (Li) and sodium (Na) and the alkaline earth metal magnesium (Mg) have attracted interest as anode electrode materials for Li-ion and lithium-sulfur (Li-S) batteries and battery cells, etc. The interest in lithium is related to the fact that lithium has the lowest reduction potential of the elements, allowing lithium-based batteries to have very high cell potentials. Lithium is the third lightest element and has one of the smallest ionic radii of singly charged ions. Furthermore, the use of lithium in the anodes can reduce or even minimize the presence of non-electroactive materials in the electrolyte of the battery or battery cell. These factors allow lithium-based batteries to have high weight and volume and power density compared to current batteries, e.g., batteries containing intercalation-based anodes such as carbon or graphite anodes.

[0003] Alkali metal anodes, such as lithium metal anodes, and alkaline earth metal anodes, such as magnesium metal anodes, when used in non-aqueous electrochemical cells (electrochemical cells, i.e., battery cells containing a non-aqueous electrolyte), develop a surface film due to reactions between the metal and the electrolyte and materials that migrate from the cathode to the electrolyte. This surface film is known in the battery art as a solid electrolyte interface (SEI) layer.

[0004] The SEI layer is typically conductive to the metal ions that make up the anode while mitigating reactions of the metal with the electrolyte and its components. However, the SEI layer can also reduce the discharge voltage and capacity of the cell, and a reduced effectiveness or efficiency of the SEI layer often leads to corrosion of the metal anode.

[0005] In the case of a Li-ion battery, the SEI layer comprises and consists essentially of the reduction products of a carbonate-based electrolyte.

[0006] In the case of Li-S batteries, the materials transferred from the cathode to the electrolyte generally include electrochemical reduction products from the sulfur cathode in the form of polysulfides. Polysulfides are highly reactive and often weaken or even ineffective the SEI layer in Li-S batteries and battery cells, thus making them unstable and resulting in the corrosion of lithium. Reactions of polysulfides can result in so-called interpolysulfide transport and electrolyte depletion. This leads in most cases to a decrease in the coulombic efficiency of the Li-S battery.

[0007] Furthermore, during charging and discharging, especially repeated charging and discharging, of a battery cell containing an anode based on an alkali or alkaline earth metal, the uneven dissolution and deposition of the metals on the electrode (especially the anode) surface leads to the formation of needle-like deposits, i.e., (needle-like) dendrites. When a battery separator is provided between the anode and cathode of the battery cell, these needle-like deposits can penetrate the battery separator material, thereby increasing the risk of short circuiting the battery cell and reducing the safety of the battery cell.

[0008] The above-mentioned problem of unstable SEI layer in Li-S batteries is particularly due to the presence of lithium nitrate (LiNO 3 It is known in the state of the art that the effects of nitrate, such as nitrate salts of alkali metals, can be significantly reduced by the presence of alkali metal nitrates such as nitrate salts of alkali metals.

[0009] Another solution is to provide a protective layer on the surface of the electrode. Known materials suitable for use as a protective layer include Li 3 PO 4 , carbon-based materials and Al 2 O 3 These include inorganic materials such as ethylene oxide (PEO) and organic materials, in particular polymers such as polyethylene oxide (PEO) and ionomers (polymers with ionic properties), such as sulfonated tetrafluoroethylene-based fluoropolymer-copolymers, e.g. Nafion™ (CAS31175-20-9). Although such materials can suppress the growth of lithium dendrites to a certain extent, they exhibit some drawbacks such as complicated manufacturing processes, low mechanical strength and / or low Li-ion conductivity. Furthermore, when organic materials are used, higher operating temperatures are also required to generate sufficient Li-ion conductivity. For example, PEO and Nafion™ require operating temperatures of about 60°C.

[0010] Lithium nitride (Li 3 N), sodium nitride (Na 3 N) and magnesium nitride (Mg 3 N 2 Alkali metal nitrides or alkaline earth metal nitrides such as ZnO, ZnSe, and ZnSe have high ionic conductivity (6*10 at room temperature) as solid lithium ion conductors. -3 It is known that it can provide mechanical strengths of up to 10 ...

[0011] “An ex-situ nitridation route to synthesize Li 3"N-modified Li anodes for lithium secondary batteries," Y. J. Zhang, W. Wang et al., J. Power Sources 277 (2015), pp. 304-311, describes the process of preheating a lithium substrate in a nitrogen atmosphere of at least 100 sccm (N 2 ) gas flow on the lithium substrate. 3 The drawback of this method is that 3 The N-layer is porous, allowing the electrolyte to pass through and lithium dendrites to form. A further drawback is that the process is time-consuming and difficult to control. Poor process control can result in polycrystalline Li-ion transport that exhibits non-uniform Li-ion transport. 3 N-layers can be created, which can bring very high power to specific areas, leading to dendrite growth.

[0012] WO2013 / 055573 discloses a lithium metal substrate including a ceramic protective layer on the exposed surface of the substrate. The protective layer is made of lithium nitride, is highly conductive to lithium ions, and protects the lithium metal surface from reaction with components in the electrolyte. The lithium nitride layer is obtained by exposing the lithium metal substrate to a plasma containing ions of a gas such as nitrogen.

[0013] "Flower-shaped lithium nitride as a protective layer via facile plasma activation for stable lithium metal anodes," Ke Chen, Rajesh Pathak et al., Energy Storage Materials 18 (2019), pp. 389-396, describes the rapid plasma activation of N 2 By plasma activation in an ambient environment, lithium nitride (LiN) was formed as a protective layer on the lithium metal surface, especially on the lithium metal electrode. 3In addition, a lithium metal substrate is placed in the discharge space between a pair of opposing electrodes and contacted with nitrogen gas activated in a low-pressure plasma discharge for several minutes. 2 The plasma is N 2 The gas was generated by introducing it into an evacuated quartz tube and subjecting it to an electric field. Under these conditions, the distinctive flower-shaped Li 3 A protective N layer is formed. The density of the flower-shaped layer increases with increasing exposure time to the nitrogen plasma. 3 N crystals were observed, which are hexagonal Li2+ bounded by Li2+ ions. 2 This structure provides open tunnels in the N-Li-N structure to ensure Li-ion conductivity. 3 Typical thicknesses of the N layer range up to 100 μm (4 min processing time). 3 The N layer has a high Young's modulus of 48 GPa and can mechanically block Li dendrites.

[0014] After cycling (charging / discharging or plating / stripping), the flower Li 3 The morphology of N changes to a hemispherical shape, which results in the formation of an interconnected interfacial protective layer. The drawback is that the Li metal surface coverage is highly uniform, limiting Li ion transport, which limits the battery capacity. A further drawback is that the pits and defects present on the lithium metal surface act as nuclei for non-uniform lithium deposition and dendrite formation, which limits the maximum life of the anode.

[0015] EP1739732 describes high melting point dielectrics, in particular metal nitrides such as sintered ceramics, e.g. AlN, Si 3 N 4This paper discloses a method for forming a nitride film of BN and BN. The method includes the steps of providing a solid dielectric on at least one of the opposing surfaces of a pair of opposing electrodes at about atmospheric pressure, introducing nitrogen gas into the space between the opposing electrodes, applying an electric field to the nitrogen gas in a pulsed manner, and contacting the resulting (pulsed) plasma gas with an object to be treated in a diffusion region outside the discharge space between the opposing electrodes to form a nitride film on the object surface. The plasma is composed of primarily neutral, active N 2 It contains N species and therefore reduces plasma damage. 2 When using, low activity nitrogen species and nitrogen ions N 2 + It is believed that species are generated from the plasma. In a remote-type example, a solid dielectric may be stretched to form a plasma-inducing nozzle that blows and guides the plasma gas toward a silicon wafer located outside the discharge space. The distance between the nozzle of the discharge electrode and the substrate is important to obtain a film of the desired composition. [Prior art documents] [Patent documents]

[0016] [Patent Document 1] WO2013 / 055573 [Patent Document 2] EP1739732 [Non-patent literature]

[0017] [Non-Patent Document 1] "An ex-situ nitridation route to synthesize Li3N-modified Li anodes for lithium secondary batteries", YJ Zhang, W. Wang et al., J. Power Sources 277 (2015), pp. 304-311 [Non-Patent Document 2] "Flower-shaped lithium nitride as a protective layer via facile plasma activation for stable lithium metal anodes", Ke Chen, Rajesh Pathak et al., Energy Storage Materials 18 (2019), pp. 389-396 Summary of the Invention [Problem to be solved by the invention]

[0018] The present invention aims to overcome one or more of the above mentioned drawbacks. It is an object of the present invention to provide an improved method for passivating a surface comprising a metallic element or an alloy of metallic elements, where the protective layer results in a more stable and / or improved electrode or battery performance. It is an object of the present invention to provide such a method which, among other advantages, is simpler, has a shorter processing time and / or results in better process control.

[0019] The present invention further aims to provide an article comprising a substrate having a surface comprising a metallic element or an alloy of metallic elements and a protective layer covering at least a portion of the surface, the protective layer providing a more stable and / or improved electrode or battery performance, the protective layer having, among other advantages, a particular structure or morphology compared to state-of-the-art protective layers and / or having improved properties, especially for use as an electrode. [Means for solving the problem]

[0020] Thus, according to a first aspect of the present invention, there is provided a method of applying a protective layer onto a surface of a substrate as claimed in the accompanying claims. The method described herein provides a substrate comprising an exposed surface comprising a metallic element and / or an alloy of metallic elements. The metallic element is an alkali metal or an alkaline earth metal. The method comprises: (i) activating a gas by atmospheric pressure plasma discharge to obtain an activated gas, the gas being nitrogen (N2 and (ii) contacting the exposed surface with an activation gas, whereby a protective layer is formed on at least a portion of the exposed surface.

[0021] The protective layer advantageously comprises at least 60 mol % of nitrides of metal elements, preferably at least 70 mol %, more preferably at least 80 mol %, most preferably at least 90 mol % of nitrides of metal elements, the mol % being capable of being determined by XPS.

[0022] According to the invention, the exposed surface is contacted with the activated gas in the afterglow resulting from the atmospheric pressure plasma discharge. The afterglow refers to the activated gas that is emitted from the atmospheric pressure plasma discharge, i.e., emitted from the plasma discharge chamber. In other words, the afterglow is located distal to the plasma discharge chamber. The atmospheric pressure plasma discharge induces the formation of reactive species in the activated gas. These reactive species have a limited lifetime but typically still exist in the afterglow, possibly at a lower concentration, less activity, and / or in a relaxed state.

[0023] Afterglow plasma treatment, or in other words plasma treatment at a location far from the plasma discharge, is considered a "milder" treatment compared to direct exposure to atmospheric pressure plasma discharge. This allows control of the reaction depth of the substrate with reactive species, which can be limited to the surface or surface area of ​​the substrate, and the modification of the bulk of the substrate can be reduced to a minimum or even avoided. With such a milder treatment, the risk of overheating and undesired modification of the substrate surface before or during the reaction with the plasma afterglow reactive species can also be better controlled.

[0024] Advantageously, the exposed surface of the substrate and the plasma discharge chamber (and hence the afterglow) are moved with respect to each other, i.e. relative to one another, while contacting the exposed surface with the activated gas in the afterglow.

[0025] By moving the exposed surface and the plasma discharge chamber relative to one another during plasma (afterglow) treatment, the presence of localized points of high concentration of activated gas emitted from the plasma discharge (chamber) and contacting the substrate surface can be better controlled or, if necessary, avoided. In addition, the temperature of the substrate surface can be better controlled and the risk of undesired overheating and / or melting at the substrate surface can be minimized. This is particularly important when the substrate surface comprises a low melting point material, since melting can locally alter the substrate properties. Also, relaxation of the substrate surface can advantageously occur between subsequent contacts with activated gas.

[0026] Advantageously, the surface of the substrate and the plasma discharge chamber, i.e. the afterglow, move relative to each other along (in the direction of) the exposed substrate surface, e.g. in a direction parallel to the exposed surface. Advantageously, the substrate moves relative to the plasma afterglow and the plasma discharge chamber. Alternatively or additionally, the plasma discharge chamber moves relative to the substrate surface. This relative movement of the surface and the afterglow allows the protective layer to be applied to the desired part of the substrate surface, and allows different parts of the substrate surface to be subjected to different intensities of the activation gas as a function of time. Alternatively, according to a further embodiment of the invention, the substrate moves towards and away from the plasma afterglow, e.g. in a direction perpendicular to the exposed surface. Advantageously, the surface is contacted with the activation (afterglow) gas in multiple passes.

[0027] The inventors have surprisingly discovered that by contacting the surface of the substrate with the afterglow of an atmospheric pressure plasma discharge, but without directly exposing the substrate to the plasma discharge (i.e. by placing the surface in the afterglow and thus far away from the plasma discharge), it is possible to obtain nitrides of metal elements that are highly crystalline and have a specific morphology, which is further described below. This morphology forms instantly and is maintained with increasing treatment times. Surprisingly, it has been observed that such protective layers, when used in electrodes for batteries (cells), result in a more stable interface between the metal and the electrolyte and an optimal electric field due to a low activation barrier for metal ion diffusion.

[0028] The gas activated by the atmospheric plasma discharge may include a carrier gas. Alternatively or additionally, the gas activated by the atmospheric plasma discharge may include one or more of a precursor, in particular a precursor gas, vapor, aerosol, or combinations thereof, which is introduced during the atmospheric plasma discharge or in the afterglow resulting from the atmospheric plasma discharge.

[0029] Advantageously, the gas comprises nitrogen in an amount of at least 90 vol%, such as at least 95 vol%, preferably at least 98 vol%, more preferably at least 99 vol%, most preferably at least 99.5 vol%, especially at least 99.95 vol%.

[0030] Advantageously, O in the gas 2 The concentration of the oxidizing gas is 0.5 vol% or less, for example, 0.25 vol% or less, 0.1 vol% or less, 0.075 vol% or less, preferably 0.05 vol% or less, 0.025 vol% or less, 0.01 vol% or less, 0.0075 vol% or less, and more preferably 0.005 vol% or less.

[0031] The inventors have demonstrated that technical quality N with a purity of only about 90 vol.% is sufficient for the method disclosed herein. 2 It has been discovered that even by using gases, a protective layer can still be obtained that consists mainly of nitrides of metal elements and is substantially free of impurities such as oxides, hydroxides or carbonates of metal elements. "Substantially free of impurities" means that the concentration of these impurities in the protective layer is below the detection limit of analytical techniques. Thus, the method described herein is surprisingly robust against contamination by impurities and allows the use of cheaper substrates.

[0032] Advantageously, the alkali metal is lithium (Li) or sodium (Na), and the corresponding alkali metal nitride is lithium nitride (Li 3 N) or sodium nitride (Na 3Advantageously, the alkali metal alloy comprises at least 5 wt% lithium or sodium, preferably at least 7.5 wt%, more preferably at least 10 wt% lithium or sodium, relative to the total mass of the alkali metal alloy.

[0033] Advantageously, the alkaline earth metal is magnesium (Mg) and the corresponding alkaline earth metal nitride is magnesium nitride (Mg 3 N 2 Advantageously, the alloy of alkaline earth metals comprises at least 5 wt% magnesium, preferably at least 7.5 wt%, more preferably at least 10 wt% magnesium relative to the total mass of the alkaline earth metal alloy.

[0034] Advantageously, the exposed surface is contacted with the activation gas at a temperature of not more than 700°C, preferably not more than 180°C, more preferably not more than 120°C, most preferably not more than 100°C, especially not more than 75°C.

[0035] Advantageously, the step of contacting the exposed surface with the activated gas in the afterglow comprises alternating periods of contact of the exposed surface with a higher concentration of reactive species and periods of contact of the exposed surface with a lower concentration of reactive species, which are advantageously low-frequency periods having a duration of, for example, at least 0.5 seconds, advantageously at least 1 second, and can be obtained by repeatedly locally treating the surface with the plasma afterglow in multiple passes or by repeatedly moving the exposed surface towards and away from the afterglow or vice versa.

[0036] According to a second aspect of the present invention, there is provided an article as claimed in the accompanying claims. The article described herein comprises a substrate and a protective layer covering at least a portion of the substrate. The protective layer and the substrate share an interface. The interface comprises a metallic element and / or an alloy of a metallic element. The metallic element is an alkali metal or an alkaline earth metal. Advantageously, the protective layer is conductive to ions of the corresponding metallic element. Advantageously, the protective layer comprises a plurality of pillars protruding from the interface. Advantageously, the pillars are made of stacked layers of crystals of a nitride of the metallic element. Advantageously, the pillars are spaced apart along the substrate surface / interface. Advantageously, the stacked layers of crystals have a substantially polyhedral shape or structure.

[0037] Advantageously, each of the plurality of pillars has a cross-section in a plane perpendicular to the projection direction of the pillar, for example a cross-section substantially parallel to the plane of the interface, having a polygonal shape, which may be a fern, a kite, a butterfly or in particular a star. Advantageously, each of the plurality of pillars comprises a tip forming a vertex or a side of a polyhedral shape or structure.

[0038] Preferably, the metal element is an alkali metal, more preferably, the metal element is lithium, Li 3 The stacked layers of N crystals comprise a substantially hexagonal bipyramidal structure.

[0039] Advantageously, the pillars have a height of between 5 nm and 500 μm, such as between 10 nm and 100 μm, preferably between 50 nm and 50 μm, such as between 250 nm and 25 μm, most preferably between 500 nm and 15 μm, in particular between 1 μm and 15 μm, such as between 5 μm and 15 μm. The height refers to the free height, for example the height protruding from the interface.

[0040] According to a third aspect of the invention, there is provided an article comprising a substrate and a protective layer covering at least a part of the substrate. The protective layer and the substrate share an interface. The interface comprises a metallic element and / or an alloy of a metallic element. The metallic element is an alkali metal or an alkaline earth metal. Advantageously, the protective layer comprises a nitride of a metallic element. Advantageously, the protective layer is made substantially of the alpha phase of a nitride of a metallic element and is substantially free of the beta phase of a nitride of a metallic element. Preferably, the protective layer is made of at least 90%, such as at least 95% (metal basis) of the alpha phase of a nitride of a metallic element. A high concentration of the alpha phase of the nitride presents the advantage that the Li-ion conductivity can be maintained at a high level.

[0041] Advantageously, the α phase of the nitride is oxidized during repeated plating / stripping cycles of the article, particularly at currents of 1 mA / cm 2 for at least 250 cycles, such as at least 300 cycles, at least 400 cycles, preferably at least 500 cycles, more preferably at least 600 cycles, and most preferably at least 700 cycles, such as at least 750 cycles.

[0042] According to a fourth aspect of the present invention there is provided an article combining the features and advantages of the second and third aspects above.

[0043] Advantageously, the protective layer comprises at least 60 mol % of nitrides of metal elements, preferably at least 70 mol %, at least 80 mol % or at least 90 mol % of nitrides of metal elements, the mol % being capable of being determined by XPS.

[0044] Advantageously, the articles of the second, third and fourth aspects of the invention are obtainable by the method of the first aspect of the invention.

[0045] According to a fifth aspect of the present invention there is provided an electrode comprising an article of the second, third or fourth aspect of the present invention. The electrode may be an anode.

[0046] According to a further aspect of the invention there is provided a battery cell comprising an electrode according to the fifth aspect, in particular as an anode.

[0047] The advantage of plasma-based methods compared to non-plasma-based methods for applying a protective layer is that the parameters of the deposition process can be more precisely controlled, thereby allowing better control of the composition of the resulting protective layer and / or reducing the formation of any unwanted by-products, and / or the process can be carried out at lower temperatures. A further advantage is that plasma-based methods are more efficient in the use of resources such as energy.

[0048] The inventors have further surprisingly discovered that the methods of the present invention result in protective layers having particular morphologies and / or particular crystallinity that, when used in electrode and battery applications, are highly stable and exhibit exceptional performance characteristics, especially upon repeated plating and stripping or charge and discharge cycles.

[0049] It has been found that protective layers according to embodiments of the present disclosure have one or more of the following advantages: - the protective layer is formed on the surface of the substrate and does not extend over most of the substrate; - improving the stability of the protective layer during repeated plating and stripping cycles or charge and discharge cycles; - Improved charge-discharge efficiency (Coulombic efficiency), especially over long-term use; - improved mechanical stability of the protective layer, - improving the flexibility of the protective layer, as well as - Improved interface between substrate and electrolyte.

[0050] Aspects of the present invention will now be described in more detail with reference to the accompanying drawings, in which like reference numerals illustrate like features, and in which: [Brief description of the drawings]

[0051] [Figure 1]FIG. 1 shows a schematic diagram of a plasma discharge device that can be used in the present invention. [Diagram 2] 1 is a schematic diagram of an article of the present invention; [Figure 3A] 2 is an SEM image of the surface of an article obtained by the method of the present invention. [Figure 3B] 1 is an SEM image of the surface of an article obtained by a method of the prior art. [Figure 4] FIG. 1 is a diagram illustrating a schematic configuration of a typical coin battery. [Figure 5A] 2 is an SEM image of the surface of an article obtained by the method of the present invention. [Figure 5B] 2 is an SEM image of the surface of an article obtained by the method of the present invention. [Figure 5C] 2 is an SEM image of the surface of an article obtained by the method of the present invention. [Figure 5D] 2 is an SEM image of the surface of an article obtained by the method of the present invention. [Figure 6] FIG. 1 shows a schematic diagram of a symmetric coin cell configuration. [Figure 7A] FIG. 13 is a graph showing EIS results for a symmetric coin cell including a lithium metal anode without a protective layer. [Figure 7B] 1 is a graph showing EIS results for a symmetric coin cell including a lithium metal anode with a protective layer. [Figure 8A] 1 is a graph showing the stability of lithium metal anodes with and without protective layers upon repeated plating and stripping of lithium. [Figure 8B] 1 is a graph showing a close up of the stability of a lithium metal anode having a protective layer. [Figure 9A] 1 is an SEM image of the surface of a lithium metal anode with a protective layer after repeated plating and stripping of lithium. [Figure 9B] 1 is a close-up SEM image of the protective layer. [Figure 9C] 1 is a close-up SEM image of the protective layer. [Figure 9D]FIG. 2 illustrates the geometric contours of morphological features in a protective layer. [Figure 10] FIG. 13 shows a 3D model of a protective layer on lithium metal after repeated plating and stripping of lithium. [Figure 11A] 1A-1D are SEM images of the surface of a lithium metal anode without a protective layer upon repeated lithium plating and stripping for 175 hours and 54 days, respectively. [Figure 11B] 1A-1D are SEM images of the surface of a lithium metal anode with a protective layer upon repeated lithium plating and stripping for 175 hours and 54 days, respectively. [Figure 12] FIG. 1 shows a schematic diagram of the hexagonal bipyramidal structure of stacked layers of Li3N. [Figure 13A] 2 is a SEM image of the surface of an article obtained by the method of the invention using nitrogen as precursor gas and argon as carrier gas for the plasma discharge. [Figure 13B] 1 is a graph showing the stability of lithium metal anodes with and without protective layers upon repeated lithium plating and stripping. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0052] According to an aspect of the invention, there is provided a method for applying a protective layer onto an exposed surface of a substrate. The surface comprises or consists of a metal element and / or an alloy of a metal element. The surface may comprise two or more metal elements and / or an alloy of two or more metal elements. The metal element is an alkali metal or an alkaline earth metal. The resulting protective layer comprises or consists of a nitride of the metal element.

[0053] The alkali metal may be any element in Group 1 (Ia) of the periodic system of elements (PSE), i.e. lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs) or francium (Fr). Preferably, the alkali metal is lithium (Li) and the corresponding alkali metal nitride is lithium nitride (Li 3 However, the alkali metal may also be sodium (Na), and the corresponding alkali metal nitride is sodium nitride (Na 3 N).

[0054] The surface may comprise an alloy of an alkali metal. Preferably, the alkali metal alloy comprises lithium (Li), e.g. at least 5 wt% lithium, preferably at least 10 wt% lithium, relative to the total mass of the alkali metal alloy, and / or sodium (Na), e.g. at least 5 wt% sodium, e.g. at least 10 wt%, at least 15 wt%, at least 20 wt%, at least 25 wt%, preferably at least 30 wt% sodium, relative to the total mass of the alkali metal alloy. The alkali metal alloy may further comprise one or more other elements, e.g. an element of group 2 of the PSE, e.g. magnesium (Mg), and / or an element of group 3 of the PSE, e.g. aluminum (Al), and / or an element of group 4 of the PSE, e.g. silicon (Si).

[0055] The alkaline earth metal may be any element of group 2 (IIa) of the Periodic Table of the Elements (PSE), in particular beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr) and barium (Ba). Preferably, the alkaline earth metal is magnesium (Mg) and the corresponding alkaline earth metal nitride is magnesium nitride (Mg). 2 N 3 ).

[0056] The surface may comprise an alloy of an alkaline earth metal. Preferably, the alkaline earth metal alloy comprises magnesium (Mg), for example at least 5 wt% magnesium, for example at least 10 wt%, at least 15 wt%, at least 17 wt%, at least 20 wt%, at least 25 wt%, at least 30 wt%, preferably at least 34 wt%, for example at least 35 wt% magnesium, based on the total mass of the alkaline earth metal alloy. The alkaline earth metal alloy may further comprise one or more other elements, for example an element of group 1 of the PSE, for example lithium (Li) or sodium (Na), and / or an element of group 3 of the PSE, for example aluminum (Al), and / or an element of group 4 of the PSE, for example silicon (Si).

[0057] According to the invention, a protective layer is applied onto a surface of a substrate exposed to the afterglow of an atmospheric plasma discharge. A gas is activated by the atmospheric plasma discharge. The activated gas is released from the plasma discharge chamber forming a so-called afterglow in which reactive species are present. The afterglow is located away from the atmospheric plasma discharge, i.e. distal to the plasma discharge. The surface of the substrate is brought into contact with the afterglow. The reactive species react with metal elements on the exposed surface, thereby producing a protective layer.

[0058] Atmospheric pressure plasma discharge can be obtained by direct current (DC) excitation (DC plasma discharge) or alternating current (AC) excitation (AC plasma discharge), by inductively coupled plasma excitation, by excitation by radio waves or microwaves (radio frequency or microwave plasma discharge), or by other excitation means known in the art. The atmospheric pressure plasma discharge by DC excitation can be, but is not limited to, an electric arc discharge (arc plasma discharge). The atmospheric pressure plasma discharge by AC excitation can be, but is not limited to, a corona discharge, a dielectric barrier discharge (DBD), a piezoelectric direct discharge, or a plasma jet.

[0059] With reference to FIG. 1, an atmospheric plasma jet device (or plasma torch) 1 can be utilized to carry out an exemplary embodiment of the method of the invention. The device 1 makes it possible to obtain an atmospheric plasma discharge by dielectric barrier discharge (DBD). The device 1 advantageously comprises a first electrode 2 and a second electrode 3. The second electrode 3 can be arranged coaxially with the first electrode 2. As an example, the first electrode 2 can be arranged in the center and the second electrode 3 can be arranged around the first electrode 2 and coaxially with the first electrode 2. An electrical insulator 4 is arranged coaxially between the first central electrode 2 and the second outer electrode 3. A discharge lumen 5, in which the plasma discharge occurs, is placed between the electrical insulator 4 and the first electrode 2. In this case, the second electrode 3 acts as a high-voltage electrode and the first electrode 2 can be grounded. Alternatively, the discharge lumen 5 can be placed between the electrical insulator 4 and the second electrode, with the first electrode 2 being the high-voltage electrode. High voltage (HV) electrode refers to an electrode connected to a radio frequency power source 6 as known in the art. It will be understood that the first and second electrodes may alternatively have a planar configuration or any other suitable configuration, such as an elliptical configuration, with an electrical insulator interposed therebetween and spaced from one electrode to define a discharge lumen.

[0060] The electrical insulator 4 is Al. 2 O 3 or the like. Advantageously, the spacing between the outer surface of the first electrode 2 (or possibly the second electrode 3) and the inner surface of the electrical insulator 4 defining the discharge lumen 5 is between 0.1 mm and 10 mm, for example 1-5 mm, preferably about 1.5 mm. The distance can be controlled by a ceramic spacer 7. The discharge lumen 5 extends between a distal end 8 and a proximal end 9, forming an outlet.

[0061] A feed port arranged at the distal end 8 of the discharge lumen 5 allows the supply of a carrier gas 11 to the discharge lumen 5. The carrier gas is advantageously an inert gas such as nitrogen, helium or argon, or a combination of two or more of these. Preferably, the carrier gas comprises nitrogen. More preferably, the carrier gas consists essentially of nitrogen. When the power supply 6 is operated, a plasma discharge is generated in the discharge lumen 5, which excites the carrier gas 11 in the discharge lumen. The plasma-excited carrier gas leaves the discharge lumen 5 at the proximal end 9. An afterglow zone 12 is formed in an adjacent but distal region, i.e. at a distance from and / or downstream of the proximal end 9. The substrate is introduced into the afterglow zone 12.

[0062] The precursor 13, in particular in the form of a gas, vapor or aerosol, can be applied either directly into the plasma discharge chamber (discharge lumen 5) or into the afterglow zone 12 through a feed inlet 14. By way of example, the central first electrode 2 is hollow with an internal lumen. The precursor 13 is introduced through the internal lumen of the ground electrode 2 and is released in the afterglow zone 12 where it can be activated by a plasma-excited carrier gas. The activated precursor subsequently reacts with the exposed surface of the substrate to be treated. In this method, the precursor is advantageously not introduced in the plasma discharge or in the afterglow.

[0063] A slit opening 15 can optionally be provided between the proximal end 9 and the plasma afterglow zone 12. The slit opening 15 allows for control of the supply of precursor 13. The slit opening 15 may have a width between 0.1 mm and 5 mm, for example between 0.2 mm and 2.5 mm, between 0.25 mm and 1 mm, preferably about 0.5 mm in the method of the present invention.

[0064] A substrate having a surface containing metal elements can be placed in the afterglow zone 12 by a substrate holder, for example a plate or grid or tray, on or in which the substrate is placed. The substrate holder can be moved relative to the plasma jet device 1 to subject the entire surface to plasma treatment. According to a preferred embodiment, the substrate holder and the plasma jet device 1 can be moved relative to each other, in particular the substrate holder and the afterglow can be moved relative to each other. Preferably, the exposed surface is oriented towards the proximal end 9 of the discharge lumen 5 to ensure optimal contact with the activation gas in the afterglow zone 12. Advantageously, the substrate holder can be moved relative to the plasma jet afterglow in a direction extending along the substrate surface, for example substantially parallel to the surface. Alternatively or additionally and advantageously, the substrate holder can be moved towards and away from the plasma jet afterglow, i.e. the substrate surface can be moved towards and away from the plasma jet afterglow.

[0065] The carrier gas is advantageously nitrogen (N 2 ) or nitrogen (N 2 According to one embodiment, the precursor gas, if used, comprises or consists essentially of nitrogen. In addition to nitrogen, the carrier gas and / or precursor gas may consist essentially of or include one or more further inert gases, such as helium (He) or argon (Ar).

[0066] Preferably, the carrier gas and / or precursor gas contains N in an amount of at least 90 vol%, such as at least 92 vol%, preferably at least 95 vol%, such as at least 97.5 vol%, more preferably at least 98 vol%, most preferably at least 99 vol%, such as at least 99.5 vol%, at least 99.75 vol%, at least 99.9 vol%, or at least 99.95 vol%. 2 The inventors have used technical quality N with a purity of about 90 vol.%. 2It has been observed that even when gases are used, it is possible to obtain a protective layer consisting essentially of nitrides of metal elements, with impurities such as corresponding metal oxides, hydroxides, carbonates, etc., being absent or present only at levels that are not detectable by available analytical methods.

[0067] Advantageously, the protective layer obtained by the method of the present disclosure comprises a nitride of a metal element in an amount of at least 60 mol%, at least 70 mol%, at least 75 mol%, at least 80 mol%, at least 85 mol%, at least 90 mol%, or at least 95 mol% of the nitride of a metal element, the Mol% being expressed in relation to the composition of the protective layer and determined by XPS.

[0068] Advantageously, an oxidizing gas, in particular O, in the carrier gas and / or precursor gas is 2 The concentration of is 5 vol% or less, for example, 1 vol% or less, 0.75 vol% or less, preferably 0.5 vol% or less, for example, 0.25 vol% or less, 0.1 vol% or less, 0.075 vol% or less, more preferably 0.05 vol% or less, for example, 0.025 vol% or less, 0.01 vol% or less, 0.0075 vol% or less, and most preferably 0.005 vol% or less.

[0069] The power supply 6 is preferably arranged to supply an AC or DC voltage of between 0.5 kV and 50 kV, for example between 1 kV and 10 kV. The voltage may be applied by the power supply to either or both of the first and second electrodes as a continuous wave, i.e. the plasma discharge may be a continuous wave discharge. Alternatively, the voltage may be applied by the power supply to either or both of the first and second electrodes as a pulsed wave, i.e. the plasma discharge may be a pulsed plasma discharge.

[0070] The frequency of the voltage applied by the power supply may be from kHz to GHz, for example 18 kHz or 13.56 MHz.

[0071] According to an embodiment of the invention, the exposed surface comprising the metal element and / or its alloy is contacted after activating the gas by atmospheric pressure plasma discharge in the so-called indirect, remote or afterglow plasma treatment, thereby keeping the surface to be treated remote from the discharge lumen 5. The inventors have surprisingly found that it is possible to obtain nitrides of metal elements that are highly crystalline and have a specific morphology, as further described below, by contacting the surface of the substrate with the afterglow of an atmospheric pressure plasma discharge, without exposing the surface directly to the plasma discharge. This morphology is formed immediately, within one minute after contacting the exposed surface with the afterglow of the activated gas. Moreover, this morphology is maintained with increasing treatment time. In particular, the protective layer may comprise multiple columns of nitrides of metal elements and / or may be substantially monocrystalline. By monocrystalline it is meant that the nitride is present in a crystal comprising a substantially single type of crystal lattice. In the present invention, the single type of crystal lattice is in particular a substantially α-phase type crystal lattice. Surprisingly, it has been observed that such protective layers, when used in electrodes for batteries (cells), result in a more stable interface between the metal and the electrolyte and an optimal electric field with a low activation barrier to metal ion diffusion.

[0072] To minimize the risk of melting of the metallic element and / or its alloy, the temperature of the surface of the metal and / or metal alloy exposed in step (ii) is advantageously kept below the melting temperature of the metallic element and / or its alloy. This can be achieved by exposing the substrate surface to a plasma afterglow treatment and by avoiding direct exposure to the plasma discharge (plasma discharge chamber). Additionally or alternatively, this can be achieved by moving the substrate surface relative to the plasma discharge chamber and thus relative to the afterglow as described above. The skilled person will be able to select the appropriate temperature taking into account the nature of the metal or metal alloy. Preferably, the temperature of the exposed surface in step (ii), i.e. during the plasma afterglow treatment, is 700° C. or less, preferably 500° C. or less, more preferably 400° C. or less, most preferably 250° C. or less, in particular 200° C. or less, more particularly 180° C. or less, preferably 120° C. or less, more preferably 100° C. or less, in particular 75° C. or less.

[0073] The plasma jet apparatus 1 can be mounted on an XY table and the apparatus 1 moved over the exposed surface to be treated. Alternatively, the substrate can be placed on an XY table and moved under the plasma jet apparatus 1. In this way, larger surfaces can be treated and / or multi-pass treatment can be performed.

[0074] Advantageously, step (ii) comprises contacting the surface of the substrate with the activation (afterglow) gas in multiple passes. This can be performed by moving the outlet (proximal end 9) of the discharge lumen and the surface of the substrate relative to each other in repeated passes. By doing so, it is achieved that the same point on the surface of the substrate is alternately exposed to a higher concentration of activation gas, for example if the point is closer to the outlet of the discharge lumen, and to a lower concentration of activation gas, for example if the point is further away from the outlet of the discharge lumen. The number of passes may depend on a number of factors such as the applied power, the distance between the surface and the outlet of the discharge lumen, and the metal element, and is advantageously between 1 and 10, advantageously between 1 and 8, advantageously between 1 and 5.

[0075] Each unit portion of the exposed surface is advantageously contacted with the activation gas in the afterglow for a total treatment time of between 0.1 seconds and 5 minutes, between 1 second and 4 minutes, such as between 2 seconds and 4.5 minutes, between 5 seconds and 4 minutes, between 10 seconds and 3.5 minutes, between 15 seconds and 3 minutes, preferably between 20 seconds and 2.5 minutes, such as between 25 seconds and 2 minutes, more preferably between 30 seconds and 90 seconds, such as about 60 seconds. Total treatment time may refer to the time of a single pass or multi-pass treatment in which a unit surface area is contacted by the activation gas.

[0076] The treatment time depends on, but is not limited to, the thickness of the resulting protective layer, the gas composition, the gas flow, the configuration of the plasma discharge equipment (such as the distance between electrodes, if present), and the type of plasma discharge (AC or DC or other, pulsed or continuous wave, etc.), the plasma discharge power, or the temperature of the exposed surface containing the metal element or its alloy. The treatment time is advantageously selected such that reaction with the active species present in the plasma afterglow remains limited to the substrate surface such that a protective layer of the desired thickness can be formed on the substrate surface and reaction of the bulk of the substrate can be reduced to a minimum or even avoided.

[0077] Pretreatment may be performed on the substrate, particularly the surface exposed to the activated gas. Pretreatment may include one or more reduction pretreatments and cleaning. Pretreatment may be performed by plasma discharge or by another method, such as a method using one or more liquids. However, it will be understood that the method described herein does not require pretreatment of the exposed surface before application of the protective layer.

[0078] Post-treatments, especially thermal post-treatments such as drying steps, may be carried out on the protective layer.

[0079] The plasma device may be mounted in a closed environment (not shown) that is advantageously filled with an inert gas such as nitrogen, helium or argon or a mixture of two or more of these. 2 The closed environment makes it possible to reduce the presence of unwanted impurities in the afterglow zone.

[0080] According to a second aspect of the present invention, there is provided an article comprising a substrate and a protective layer disposed on at least a portion of the substrate. The protective layer is advantageously obtained by a method as described herein. The protective layer and the substrate share an interface. The interface comprises a metallic element and / or an alloy of a metallic element. The metallic element is an alkali metal or an alkaline earth metal. The metallic element and the substrate are as defined above in connection with the method of the present invention. The protective coating advantageously comprises a nitride of the metallic element.

[0081] 2, an article 20 according to an embodiment described herein includes a substrate 21 and a protective layer 22 disposed thereon. Bulk substrate 21 and protective layer 22 share an interface 23. Thus, the protective layer is advantageously disposed directly on substrate 21 at interface 23, without any other intervening layers therebetween.

[0082] Advantageously, the protective layer is conductive to ions of the metallic element, for example, if the metallic element is lithium (Li), the protective layer is advantageously conductive to lithium ions.

[0083] Preferably, the protective layer is impermeable to electrons. Preferably, when the article is used as an electrode, preferably as an anode, in a battery or battery cell, the protective layer is passivated against reaction with the electrolyte. In other words, the protective layer advantageously minimizes reaction between the substrate and the electrolyte.

[0084] Advantageously, the protective layer comprises a plurality of pillars protruding from the interface, where protruding from the interface means that the pillars grow and are therefore present on a surface that is in a different direction to the plane of the interface. Advantageously, the plurality of pillars are spaced apart.

[0085] The pillars may be branched, for example comprising side branches. Alternatively, the pillars may be substantially straight. The pillars may comprise a pyramidal tip at the top of the pillar and / or at the side branches of the pillar. Advantageously, each of the pillars comprises a cross section in a plane perpendicular to the projection (i.e. extension) direction of the pillar or of the branches, having a polygonal shape. The polygonal shape may be fern-shaped, kite-shaped, butterfly-shaped or especially star-shaped.

[0086] Advantageously, the pillars may have a height between 10 nm and 100 μm, such as between 25 nm and 75 μm, preferably between 50 nm and 50 μm, such as between 75 nm and 45 μm, between 100 nm and 40 μm, between 150 nm and 35 μm, between 200 nm and 30 μm, more preferably between 250 nm and 25 μm, such as between 350 nm and 20 μm, most preferably between 500 nm and 15 μm, in particular between 1 μm and 15 μm, or between 5 μm and 15 μm. The height of the pillars can be increased by exposing the substrate to the plasma afterglow for an increasing contact time. The thickness and height can be measured by scanning electron microscopy (SEM) of a cross-section of the article. For example, an FEI NovaSEM 450 scanning electron microscope may be used. The cross-section can be made by slicing the article with a ceramic knife. The sliced ​​samples can then be placed on an SEM sample holder placed in a transfer module (trademark: Kammrath Weiss Gmbh) to avoid any contamination by atmospheric moisture and oxygen during transfer to the scanning electron microscope.

[0087] Advantageously, the pillars are made of multiple stacked layers of crystals of nitrides of metallic elements. Advantageously, all stacked layers have the same or identical crystal structure. Each of these stacked layers may have a polyhedral shape. A polyhedral shape refers to a three-dimensional shape with flat polygonal faces, straight edges and sharp corners or vertices. This shape typically forms on the substrate surface when the substrate comes into contact with the plasma afterglow and is maintained with increasing exposure time to the plasma afterglow.

[0088] Preferably, adjacent layers of crystals within a pillar have substantially the same orientation. Without wishing to be bound by any theory, the first crystals of the nitride of the metal element can act as nuclei or seeds for the growth of further crystals, thereby obtaining adjacent layers with substantially the same orientation. The orientation of the layers can be determined by the long axis of the geometric shape. The layers of crystals further include a width and a length.

[0089] The distance between adjacent columns may be between 100 nm and 5 μm, for example between 200 nm and 4 μm, between 250 nm and 3 μm, preferably between 500 nm and 2 μm, in particular 1 μm. If the contact time of the plasma discharge increases, the distance between adjacent columns may decrease. If the contact time of the substrate with the plasma discharge increases beyond a certain value, the reaction of the active species with the substrate surface is no longer possible, and a large part of the substrate begins to be altered. This is usually undesirable, as it reduces the energy efficiency of the electrode containing such materials.

[0090] The inventors have surprisingly discovered that when the protective layer includes multiple pillars, diffusion channels for ions of metal elements are created between adjacent pillars, and such ion diffusion channels can contribute to an increase in ionic conductivity.

[0091] As an example, when the interface surface comprises Li or a Li alloy, it has been observed that the pillars and / or at least some of the deposited layers at the top of the pillars have the morphology of a hexagonal bipyramid, as shown in Figure 12, with eight vertices V1-V8, twelve faces F1-F12 and eighteen edges E1-E18. Geometrically, a hexagonal bipyramid consists of twelve triangular faces arranged in six pairs.

[0092] Advantageously, the hexagonal bipyramidal structure or morphology comprises an average angle between each side of each base apex of between 45° and 85°, preferably between 50° and 75°, for example between 50° and 60°, more preferably between 55° and 60°.

[0093] Advantageously, the hexagonal bipyramidal morphology comprises an average angle of each apex of the apex between 40° and 75°, preferably between 45° and 70°, for example between 50° and 65°, more preferably between 55° and 60°.

[0094] Li 3 The longest dimension of the layer of N crystals may be between 0.5 μm and 5 μm, preferably between 1 μm and 4 μm, and more preferably between 1.5 μm and 2.5 μm. 3 The shortest dimension of the layer of N crystals may be between 0.5 μm and 5 μm, preferably between 1 μm and 4 μm, more preferably between 1.5 μm and 2.5 μm.

[0095] Li 3 The hexagonal bipyramidal structure of the N crystals may have an aspect ratio between 0.75 and 1.25, for example between 0.8 and 1.2, preferably between 0.9 and 1.1, for example 1, the aspect ratio being the ratio of the width of the crystal to the height of the crystal. When the aspect ratio is between 0.9 and 1.1, the crystal can be considered to have a substantially symmetrical morphology or structure.

[0096] Advantageously, the protective layer is made substantially of the α-phase of the nitride of the metallic element, in particular the concentration of the α-phase of the nitride of the metallic element is at least 90%, for example at least 92.5%, at least 95%, at least 98% or at least 99% on a metallic basis. Advantageously, the protective layer is substantially free of the β-phase of the nitride of the metallic element. By "substantially free" it is meant that the concentration of the β-phase of the nitride of the metallic element is below the detection limit of analytical methods, in particular X-ray diffraction (XRD).

[0097] For example, if the interface surface contains Li or a Li alloy, the protective layer may be primarily crystalline α-Li. 3 N(α-phase Li 3 N). Advantageously, α-Li 3 The N crystalline phases are arranged in columns that protrude from the Li or Li alloy surface (interface) and are separated from each other (at least over a portion of their height) to create Li ion diffusion channels between them. 3N(β-phase Li 3 N), which is consistent with the absence of detectable amounts of β-Li 3 It means that it does not contain N. Advantageously, it is α-phase Li 3 N crystals have a hexagonal structure that includes edge-sharing layers of planar lithium hexagons centered on nitrogen in the ab plane. Each of these hexagons is connected by a lithium ion above and one below, both of which lie along the c plane. Furthermore, each nitrogen atom is coordinated by a total of eight lithium atoms in a hexagonal bipyramidal geometry.

[0098] The protective layers described herein have been found to be mechanically stable when subjected to varying electric fields in the form of repeated plating / stripping cycles, and when used in battery electrodes in the form of repeated charge / discharge cycles. In particular, the multiple columns and / or crystalline morphology remains stable during repeated plating / stripping cycles of the article, particularly at currents of up to 1 mA / cm. 2 is maintained for at least 250 cycles, such as at least 300 cycles, at least 400 cycles, preferably at least 500 cycles, more preferably at least 600 cycles, and most preferably at least 700 cycles, such as at least 750 cycles, at a current density of at least 250 cycles, such as at least 300 cycles, at least 400 cycles,

[0099] Advantageously, when the article is used in an electrode in a battery, the alpha phase of the nitride remains constant during repeated charge / discharge cycles of the article, particularly at 1 mA / cm 2 is maintained for at least 250 cycles, such as at least 300 cycles, at least 400 cycles, preferably at least 500 cycles, more preferably at least 600 cycles, and most preferably at least 700 cycles, such as at least 750 cycles, at a current density of at least 250 cycles, such as at least 300 cycles, at least 400 cycles,

[0100] Without wishing to be bound by any theory, the protective layer of the present invention contains highly conductive pathways (high ionic conductivity) through which excellent metal ion transport can occur. Such highly conductive pathways are advantageously provided by the columnar morphology and / or high concentration of alpha crystals in the protective coating.

[0101] The protective layer also provides improved resistance to volume changes of the substrate during charging and discharging of the battery compared to prior art protective layers having different crystallinity and / or morphology, leading to reduced damage of the protective layer over time.

[0102] Advantageously, the protective layer comprises at least 60 mol %, such as at least 70 mol %, at least 75 mol %, at least 80 mol %, at least 90 mol %, or at least 95 mol % of a nitride of a metal element.

[0103] Advantageously, the protective layer comprises up to 40 mol % of an oxide of a metal element, such as up to 20 mol %, up to 15 mol %, preferably up to 10 mol %, more preferably up to 5 mol %, most preferably up to 3 mol % of an oxide of a metal element.

[0104] Advantageously, the protective layer obtained as described herein has a Young's modulus of less than or equal to 45 GPa and exhibits a certain degree of flexibility, which makes it resistant to cracking of the layer due to repeated stripping / plating of the article. As a result of the certain degree of flexibility, the passivated substrate, i.e. the substrate comprising the protective layer of the invention, advantageously has a malleability comparable to that of a bare substrate.

[0105] The article of the invention is advantageously used as (part of) an electrode. The electrode is advantageously an anode. The electrode can be used in a battery or battery cell. Preferably, the electrode is used in a battery cell as an anode. Particularly preferred are lithium-ion, solid-state lithium-ion, Li-S (lithium-sulfur) or lithium-air batteries or battery cells comprising a lithium anode having a protective layer as described herein.

[0106] FIG. 4 shows an exemplary embodiment of a battery cell 40. The battery cell 40 has a coin cell configuration known in the art as a CR2032 type configuration. The battery cell 40 includes an anode 41 and a cathode 42. The anode 41 is an electrode according to the present invention. The battery cell 40 further includes a liquid electrolyte (not shown). The battery cell advantageously includes a battery separator membrane 43 between the anode 41 and the cathode 42. The cathode may be, for example, a sulfur-based cathode, such as a Li-S battery cell. The battery cell 40 further includes a coin cell lid 44, a coin cell base 45, a spacer 46, and a spring 47. The spacer 46 and the spring 47 provide good contact between the other components 41, 42, 43, 44, 45 of the battery cell 40.

[0107] The electrolyte may be a solid electrolyte. The solid electrolyte may be a solid polymer or a solid inorganic glass. For example, the solid electrolyte may be poly(ethylene oxide) (PEO) with a lithium salt dispersed in a polymer matrix of PEO.

[0108] Alternatively, the electrolyte may be a liquid electrolyte. The electrolyte may be an ionic liquid, optionally including an organic component, a salt-solvent mixture, preferably a supersaturated salt-solvent mixture. For example, the liquid electrolyte may be an ionic liquid with a lithium salt dissolved therein or a mixture of an ionic liquid with a lithium salt dissolved therein and an organic liquid. Examples of liquids that may be used include organic solvents such as polyethylene glycol dimethyl ether (PEG DME) or dioxolane mixed with dimethyl ether. The liquid electrolyte may include a compound of tetraethylene glycol dimethyl ether (PEGDME) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). A useful ionic liquid is methylbutylpyridinium trifluorosulfonyl imide (PYR14TFSI). In one example, the electrolyte has a 1:1 mass ratio of PYR14TFSI and PEGDME (1 mol / kg LiTFSI). In another example, the electrolyte comprises 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in dimethoxyethane (DME):1,3-dioxolane (DOL) (2:1 mass ratio).

[0109] Alternatively, the electrolyte may be a gel electrolyte. The gel electrolyte may be a polymer gelling organic medium. For example, the gel electrolyte may be a mixture of poly(methyl methacrylate) (PMMA), a lithium salt, and a small amount of liquid.

[0110] The battery separator membrane may be a porous separator membrane. Polymer battery separator membranes known in the art may be used, such as porous polypropylene (PP) membranes or porous polyethylene (PE) membranes. For example, a polypropylene membrane with a thickness of 25 μm and a porosity of 50% may be used. PP and PE are preferred materials due to their chemical inertness properties. However, it is preferred that the porous separators can absorb liquid electrolyte, but they are not easily wetted. To this end, the hydrophobic PP and PE may be treated with a surface treatment or coating, such as spray coating, dip coating, or plasma coating (atmospheric or low pressure plasma). Alternatively, the battery separator membrane may be a ceramic material. EXAMPLES

[0111] Example 1 A lithium metal substrate was treated with the method of the present invention. The plasma jet apparatus of FIG. 1 was used, where the lithium metal substrate was mounted on a substrate holder and placed in the afterglow zone 12 of an atmospheric pressure dielectric barrier discharge. The substrate holder was moved by an XY translation table in a plane parallel to the outlet of the discharge lumen of the plasma jet apparatus so as to expose the entire surface to the afterglow zone. The plasma jet apparatus was mounted in the closed environment described above. Nitrogen was added to the closed environment to limit the presence of unwanted impurities, especially oxygen and water vapor. The gases used for the plasma discharge were N as precursor and carrier gas. 2 It was. 2 The total nominal gas flow of carrier and precursor gases was 20 slm. The plasma power was 300 W and the frequency was 17 kHz. The exposed surface and the activated N 2 The duration of contact with the gas was 1 min (1 pass). The temperature of the exposed surface was about 80 °C. The distance between the proximal end of the discharge lumen and the exposed lithium metal surface of the substrate was 3 mm. The closed environment contained less than 50 ppm O 2 It included:

[0112] FIG. 3A shows an SEM image of the lithium nitride layer obtained after three passes. Three passes means that the exposed surface of the substrate was contacted with the activation gas three times, each of the three passes being carried out as described above. The protective layer clearly shows a morphology or structure comprising a number of pillars 30 protruding from the substrate. The pillars have pyramidal tips. The lithium nitride crystals are stacked in a direction away from the lithium metal surface of the substrate. In comparison, as shown in FIG. 3B, when the surface of the same substrate was exposed to a nitrogen flow at room temperature for several hours without any plasma discharge, the structure comprising the pillars was not observed.

[0113] Figures 5A, 5B, 5C and 5D show the morphology of the lithium nitride layer obtained after 2, 3, 4 and 5 passes, respectively, each of which was carried out as described above. The morphology of the protective layer after 2 and 3 passes (total treatment times of 1 min and 1.5 min for each part of the exposed surface) shows that the lithium nitride layer comprises a number of columns, whereas after 4 and 5 passes this is less clear. This can be explained by the fact that with each pass lithium nitride crystals can be obtained not only at the tops of the columns, but also at the sides, resulting in columns with a less "straight" or "linear" structure or morphology.

[0114] The inventors have discovered that for optimal performance of the protective layer, there are advantageously more "linear" or "straight" shaped columns, and therefore it may be beneficial to limit the number of passes or total processing time to obtain this particular and particularly beneficial morphology.

[0115] Example 2 Li-ion battery cells were fabricated to evaluate the charge / discharge characteristics and cycle life characteristics of the anodes including the protective layers. To reduce the risk of contamination during fabrication, the battery cells were fabricated in a glove box filled with argon and oxygen and with a moisture level of less than 1 ppm.

[0116] A battery cell of the type shown in FIG. 4 was prepared, in which the anode was the electrode obtained according to Example 1. A reference battery cell was prepared, in which the anode was lithium metal without any protective layer. For both battery cells, the cathode was self-prepared by coating a slurry containing appropriate amounts of lithium cobalt oxide (LCO), carbon and binder onto an aluminum foil. A polymer separator was placed between the anode and the cathode. The battery cells were charged with 1M lithium hexafluorophosphate (LiPF 6 The battery cell was filled with 90 μL of electrolyte containing 100 μL of 10 ...

[0117] Example 3 Li-S battery cells were fabricated to evaluate the charge / discharge characteristics and cycle life characteristics of the anodes including the protective layers. To reduce the risk of contamination during fabrication, the battery cells were fabricated in a glove box filled with argon and oxygen and with a moisture level of less than 1 ppm.

[0118] A battery cell of the type shown in FIG. 4 was prepared, in which the anode was the electrode obtained according to Example 1. A reference battery cell was prepared, in which the anode was lithium metal without any protective layer. For both battery cells, the cathode was self-prepared by coating a slurry containing appropriate amounts of sulfur, carbon and binder on aluminum foil. A polymer separator was placed between the anode and the cathode. The battery cells were then filled with a 2:1 mass ratio of dimethoxyethane (DME) / 1,3-dioxolane (DOL) (LiNO 3 The battery cells were filled with 90 μL of electrolyte containing 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in 0.5% CO (without additives). The battery cells were sealed with an air press.

[0119] Example 4 To evaluate how the internal resistance of the anode with and without the protective layer evolves as a function of time, Li-S symmetric battery cells were fabricated as shown in Figure 6. To reduce any contamination risk during fabrication, the battery cells were fabricated in a glove box filled with argon and oxygen and with a moisture level below 1 ppm.

[0120] The Li-S symmetric battery cell 60 is similar to the battery cell 40 of FIG. 4, except that both electrodes 61, 62 are identical. In the reference battery cell, the electrodes were lithium metal substrates without any protective layer. For the battery cell 60 according to the invention, the electrodes 61, 62 were obtained through Example 1. A polymer separator 63 was placed between the electrodes 61 and 62. The battery cell 60 was filled with 90 μL of electrolyte containing 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in dimethoxyethane (DME) / 1,3-dioxolane (DOL) (without LiNO3 additive) in a mass ratio of 2:1. Furthermore, a coin cell lid 64 and a coin cell base 65 as well as a spacer 66 and a spring 67 were provided to ensure good contact between the components of the cell 60. The battery cell 60 was sealed with an air press.

[0121] Electrochemical impedance spectroscopy (EIS) was performed on the reference symmetric battery cell and the symmetric battery cell according to the invention. An AC potential of 10 mV was applied to the cell during a frequency sweep. The frequency sweep was performed from 100 kHz to 0.01 Hz.

[0122] 7A and 7B show Nyquist plots of impedance data for a reference symmetric battery cell and a symmetric battery cell according to the invention, respectively. In the Nyquist plots, impedance data from each frequency point is plotted. The abscissa (x-axis) shows the real part of the impedance data, and the ordinate (y-axis) shows the imaginary part of the impedance data. 7A and 7B show results in 6 hour intervals from the start of the experiment (time 0) to 120 hours of testing (results at time 0, 6 hours, 12 hours, 18 hours, 24 hours, followed by 120 hours).

[0123] The impedance spectra of both the reference cell and the battery cell according to the invention consist of a semicircle in the high-mid frequency region, which corresponds to the surface layer resistance. When comparing the impedance spectra acquired from 0 to 120 hours of testing, it is clear from Figures 7A and 7B that the surface layer resistance formed in the symmetric cell of Figure 6 made with the electrode including the protective layer of Example 1 remains stable over time, indicating that the protective layer continues to provide excellent protection.

[0124] Example 5 Using the symmetric battery cell configuration 60 of Example 4, 2 The lithium plating / stripping behavior of the electrodes was evaluated by monitoring the evolution of overpotential over time when a constant current of 1000 V was applied. These measurements were performed using an Ametek PARSTAT PMC-200 multichannel potentiostat.

[0125] FIG. 8A shows potential values ​​measured for a duration of about 500 hours for a reference battery cell (A, black) and potential values ​​measured for a duration of about 1400 hours for a battery cell of the invention (B, grey). Up to a test duration of about 175 hours, the potential values ​​indicate degradation of the lithium metal. Higher absolute potential values, i.e., larger potential value increases or decreases, indicate greater degradation. It is therefore clear that the electrode comprising a lithium metal substrate without a protective layer shows greater degradation earlier (starting from a test duration of 175 hours) than the electrode of the invention. The electrode of the invention shows no detectable signs of degradation up to at least a test duration of about 1300 hours (longer times were not tested).

[0126] 8B shows a close-up view of the plating / stripping overpotential of an electrode of the invention after a test duration of about 1200 hours. The potential-time profile indicates excellent efficiency and stability of the protective layer and thus passivated lithium metal.

[0127] Figure 9A shows an SEM image of one of the electrodes in a symmetrical battery cell according to the invention at the end of a plating / stripping test (approximately 1400 hours of testing). An SEI layer 90 has formed on the electrode and some protective layer 91 is visible. Figures 9B and 9C show a detailed view of the morphology of protective layer 91. The pillar morphology as well as pyramidal tops 92 are still clearly visible, thus indicating that the layer morphology has not changed significantly. Figure 9D shows a geometrical schematic of the morphological features of the pyramidal tops superimposed on the individual crystalline layers.

[0128] The present invention 3 A 3D surface plot made by ImageJ software from the SEM image of the N layer clearly shows a protective layer containing a plurality of pillars (FIG. 10), the stacking of the lithium nitride crystal layers extending substantially along a plane away from the lithium metal surface, the plane being oriented substantially perpendicular to the surface. The average (measurable) height of the pillars is 5 μm, with a standard deviation of 0.9 μm. Such stacking appears to lead to anisotropy of the crystal lattice, which can impart beneficial properties such as low activation barriers to lithium ion migration along certain directions and high ionic conductivity in the subsequent material.

[0129] FIG. 11A shows the formation of lithium dendrites in the reference battery cell (without protective layer) for test durations of 175 hours or more, while the battery cell of the present invention does not show any dendrite formation after 54 days (1300 hours) of testing (FIG. 11B). Such lithium dendrites eventually lead to shorting of the cell. It is evident from FIG. 9A that the absolute overpotential values ​​are higher and that the overpotential signal of the reference battery cell is more unstable compared to the battery cell of the present invention. Peak values ​​are visible and become more frequent for test durations of more than 350 hours, indicating non-uniform plating / stripping of lithium due to the formation of a non-uniform surface film that increases in resistance over time. This is consistent with the increase in surface film resistance measured by impedance spectroscopy and discussed in Example 4, the results of which are shown in FIG. 7A and FIG. 7B. Thus, the protective layer provides clear protection against lithium dendrite formation by allowing a smooth and stable surface film while minimizing the increase in resistance.

[0130] Example 6 The lithium metal substrate was heated to 1000° C. The gas used for plasma discharge was N as a precursor gas in Ar as a carrier gas. 2 The process was carried out as described in Example 1, except that Ar carrier gas and N 2 The total nominal gas flow of the precursor gases was 20 slm. The plasma power was 300 W and the frequency was 17 kHz. The exposed surface and the activated N 2 The duration of contact with the gas was 2 min (3 passes). The temperature of the exposed surface was approximately 80 °C. The distance between the proximal end of the discharge lumen and the exposed lithium metal surface of the substrate was 3 mm. The closed environment contained less than 50 ppm O 2 It included:

[0131] 13A shows an SEM image of the obtained lithium nitride layer. The morphology of the protective layer after three passes (e.g., the protective layer of Example 1) is the same as that obtained when nitrogen is used as the precursor and carrier gas.

[0132] Example 7 To evaluate how the internal resistance of the anode with and without the protective layer of Example 6 evolves as a function of time, a Li-S symmetric battery cell was made as shown in FIG. 6. The battery cells were manufactured as described above. In the reference battery cell, the electrodes were lithium metal substrates without any protective layer. In the battery cell 60 according to the invention, the electrodes 61, 62 were obtained through Example 6. The other components of the battery cell were as described in Example 5.

[0133] 1mA / cm using a symmetrical battery cell configuration 60 2 The lithium plating / stripping behavior of the electrodes was evaluated by monitoring the evolution of overpotential over time when a constant current of 1000 V was applied. These measurements were performed using an Ametek PARSTAT PMC-200 multichannel potentiostat.

[0134] FIG. 13B shows potential values ​​measured for a duration of about 500 hours for a reference battery cell (C, black) and for a duration of about 700 hours for a battery cell of the invention (D, grey). Up to a test duration of about 175 hours, the potential values ​​indicate degradation of the lithium metal. Higher absolute potential values, i.e., larger potential value increases or decreases, indicate greater degradation. Thus, it is clear that the electrode comprising a lithium metal substrate without a protective layer shows greater degradation earlier (starting from a test duration of 175 hours) than the electrode of the invention. The electrode of the invention shows no detectable signs of degradation up to at least a test duration of about 700 hours (longer times were not tested). Furthermore, compared to the battery cell tested in Example 5 and shown in FIG. 8A, the stability of the battery cell of Example 7 is (surprisingly) even superior to that of the battery cell having electrodes 61, 62 obtained through Example 4 (i.e., only nitrogen gas was used as precursor and carrier gas), as indicated by the lower overpotential after 700 hours (54 mV compared to 83 mV observed in FIG. 8A). [Explanation of symbols]

[0135] 1. Atmospheric pressure plasma jet device 2 First electrode 3 Second electrode 4. Electrical insulators 5 Discharge lumen 6 Power supply 7 Ceramic Spacer 8 Distal end 9 Proximal end 10 Carrier gas supply port 11 Carrier gas 12 Afterglow Zone 13 Precursors 14 Precursor supply port 15 Slit mouth 20 Goods 21 Substrate 22 Protective layer 23 Interface 30 Pillars 40 Battery Cells 41 Anode 42 Cathode 43 Battery separator membrane 44 Coin battery cover 45 Coin Cell Base 46 Spacer 47 Spring 60 Li-S symmetrical battery cells 61 Electrode 62 electrodes 63 Polymer Separator 64 Coin battery cover 65 Coin Cell Base 66 Spacer 67 Spring 90 SEI layer 91 Protective layer 92 Top 110 Lithium Dendrites

Claims

1. A method for applying a protective layer (22) onto at least a portion of an exposed surface (23) of a substrate (21), comprising the steps of: (i) A process for activating a gas in a plasma discharge chamber by atmospheric pressure plasma discharge to obtain an activated gas, in which the gas is nitrogen (N ２ ) (ii) contacting the exposed surface (23) with an afterglow of an activated gas emitted from the plasma discharge chamber to form a protective layer on at least a portion of the exposed surface; wherein the surface comprises a metallic element and / or an alloy of metallic elements, the metallic element being an alkali metal or an alkaline earth metal, and the protective layer comprises a nitride of the metallic element, and wherein the surface and the plasma discharge chamber are moved relative to one another during contacting the exposed surface with the afterglow of an activation gas.

2. The gas contains at least 90 vol. % N ２ The method of claim 1 , comprising:

3. The method of claim 2, wherein the gas comprises N 2 in an amount of at least 95 vol %.

4. O in gas ２ The method according to claim 1 or 2, wherein the concentration of the oxidizing gas is 0.5 vol % or less.

5. The metal element is sodium (Na) or lithium (Li), and the nitride is sodium nitride (Na ３ N) or lithium nitride (Li ３ N).

6. The metal element is magnesium (Mg) and the nitride is magnesium nitride (Mg ３ N ２ 2. The method of claim 1 , wherein

7. The method of claim 1 , wherein the exposed surface remains remote from the atmospheric pressure plasma discharge.

8. The method of claim 1 , wherein the exposed surface is contacted with an activation gas at a temperature of 700° C. or less.

9. The method of claim 8, wherein the exposed surface is contacted with an activation gas at a temperature of not more than 180°C.

10. 2. The method of claim 1 , wherein contacting the exposed surface with an activation gas comprises alternating periods of contacting the exposed surface with an activation gas having a higher concentration of reactive species and periods of contacting the exposed surface with an activation gas having a lower concentration of reactive species.

11. 2. The method of claim 1, wherein the exposed surface is moved relative to the afterglow of activated gas exiting the plasma discharge chamber in a plane parallel to an exit of the plasma discharge chamber.

12. The method of claim 1 , wherein the surface is contacted with the afterglow of the activation gas in multiple passes.

13. 2. An article (20) obtainable by the method of claim 1, characterized in that the article comprises a substrate (21) and a protective layer (22) covering at least a portion of the substrate (21), the protective layer (22) and the substrate (21) sharing an interface (23), the interface (23) comprising a metallic element and / or an alloy of a metallic element, the metallic element being an alkali metal or an alkaline earth metal; the protective layer (22) comprising a plurality of spaced apart pillars (30) protruding from the interface (23), the pillars (30) being made of stacked layers of crystals of a nitride of the metallic element, the stacked layers of crystals comprising a polyhedral shape.

14. 14. The article of claim 13, wherein each of the plurality of posts includes a tip, the tips forming vertices or edges of a polyhedral shape.

15. The metallic element is lithium, Li ３ 15. The article of claim 13 or 14, wherein the stacked layers of N-crystals comprise a hexagonal bipyramidal structure.

16. 14. The article of claim 13, wherein the pillars have a free height between 10 nm and 100 μm.

17. 14. An article according to claim 13, wherein the protective layer is made substantially of the alpha phase of the nitride of the metallic element, preferably being substantially free of the beta phase of the nitride.

18. 18. The article of claim 17, wherein the protective layer is made of the alpha phase of a nitride of at least 90% of the metallic element on a metallic basis.

19. The α phase of the nitride is prone to damage during repeated plating / stripping cycles of the article, especially at currents of 1 mA / cm ２ 19. The article of claim 17 or 18, wherein the article is maintained at a current density of at least 250 cycles.

20. The article of claim 13, wherein the protective layer (22) comprises at least 60 mol % of a nitride of a metallic element.

21. An electrode (41, 61, 62) comprising the article (20) of claim 13.

22. The electrode (41) of claim 21, wherein the electrode is an anode.

23. A battery cell (40) comprising the electrode (41) of claim 21 as an anode.

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