Electrochemical cell and separator

Abstract

The invention pertains to an electrochemical cell having an anode, a cathode, and a separator between the anode and the cathode separating them from each other, wherein the anode is molten sodium, the separator is a sodium ion conducting inorganic membrane, the separator having a surface in contact with the molten sodium anode, the separator comprising metallic zinc at its surface facing the anode, wherein the metallic zinc is present in an amount in the range of 0.05-20 g / 1000cm2. It has been found that the presence of metallic zinc at the surface of the separator which faces a molten sodium anode leads to excellent wetting of the surface by the sodium, without high temperature activation or operating at higher temperatures being required. A zinc-containing separator and a method for its manufacture are also claimed.

Description

[0001] Electrochemical cell and separator

[0002] The present invention pertains to an electrochemical cell having a sodium-based anode. The invention also pertains to a separator suitable for use in such an electrochemical cell.

[0003] In particular, the invention pertains to an electrochemical cell having an anode, a cathode, and a separator between the anode and the cathode separating them from each other, wherein the anode is a molten sodium-based anode and the separator is a sodium ion conducting inorganic membrane.

[0004] Various cells of this type are known. They include cells having molten sodium as the anode with different types of cathode, e.g., a sodium sulphide / sulphur-based cathode, a ZEBRA type cathode, i.e. a sodium metal halide-based cathode, where the metal is, among others, nickel, iron, copper, manganese, chromium or zinc, or an alkali metal aluminium / aluminium halide cathode. The anode and the cathode are separated from each other by a separator which is a selective sodium ion conducting inorganic membrane.

[0005] For example, US20140186684 describes an electrochemical cell comprising a housing comprising a metallic material, wherein the housing defines a portion of an anode compartment containing an alkali metal, and a component disposed within the anode compartment, wherein the component comprises a sacrificial metal having an oxidation potential less than the oxidation potential of the housing material. The electrochemical cell may have a so-called ‘shim’ in the anode compartment adjacent to the separator surface. The term “shim” refers to a thin piece of a material, used to fill gaps, make something level, support something, or adjust something to fit properly. It is indicated that these shimstructures are present to provide a structure for electrical and thermal conduction within the cell and enhanced transport and utilization of the anodic material. It is indicated that the shim-structure is usually designed in a manner that renders the shims flexible enough to allow the deposition and transport of metallic anodic material (e.g., sodium) between the shim-structure and the surface of the separator.

[0006] For electrochemical cells having molten sodium as anode to function properly it is necessary that the molten sodium of the anode is in excellent contact with the separator. If this is not the case, the transport of sodium ions through the separator will be insufficient, and the battery will not function properly. Therefore, one of the challenges to be addressed is toensure proper wetting of the surface of the separator that is in contact with the molten sodium, which contact should be maintained during the lifetime of the electrochemical cell.

[0007] EP0032033A2 describes a beta-alumina solid electrolyte element which has had at least a part of its surface pre-treated with a metal such as lead or bismuth which will form an alloy with sodium. The purpose of this cation is to improve the wetting of the beta-alumina solid electrode by the sodium. Conveniently the electrolyte is coated with an aqueous solution of e.g. lead acetate, dried and the lead acetate decomposed by heating to leave a lead coating. However, the wetting is active only at elevated temperatures above -300 C thus limiting the applicability of this method.

[0008] GB2195329 describes a method of preparing a surface of a ceramic solid electrolyte conductor of alkali metal ions for wetting by said alkali metal in solid form. The method comprises doping the ceramic surface with an oxide of a transition metal. It is indicated that for cells with a sodium anode and a separator which is beta-alumina or Nasicon (Nai+xZr2SixP3-xOi2, 0 < x < 3) , doping may be by means of an oxide of a single transition metal selected from the group comprising iron, nickel, copper, manganese, cobalt, chromium and molybdenum. It is indicated that oxides of iron, nickel, copper, manganese and chromium are electrochemically inert in the cell anode environments of such cells, and are readily available at acceptable prices.

[0009] While the metals used in GB2195329 do result in good wetting of the separator by molten sodium, this is only the case after treatment at high temperature. In particular, GB2195329 tests wettability of the coated separators at 250°C. The electrochemical cells described in this reference are indicated as high temperature electrochemical power cells. In the examples results are given of cells operated at temperatures in the range of 262-280°C. GB2195329 indicates that this is much lower than the cells in which lead acetate was used to improve the wetting of the separator of a cell with a sodium anode. At 250°C separators provided with ferric chloride showed good wettability for liquid sodium, while separators provided with lead acetate did not. It is stated that separators having lead acetate to improve wetting need to be heated to a temperature of 350-400°C to promote sodium wetting.

[0010] Operating the cells at a temperature of 250°C and lower may lead to dewetting of the surface.

[0011] While heating to a temperature of 250°C to promote wetting is an improvement as compared to heating to a temperature of 350-400°C, there is still need for a method for improving the wettability of separators for use in electrochemical cells with alkali metal anodes which canbe operated at lower temperatures. There is also need in the art for separators which are suitable for electrochemical cells operated at lower temperatures which do not show a risk of decreased wettability for the molten sodium anode over time. There is also need for electrochemical cells with a sodium anode which can be operated stably over prolonged periods of time, without dewetting of the separator surface. The present invention solves these problems.

[0012] The present invention is directed to an electrochemical cell having an anode, a cathode, and a separator between the anode and the cathode separating them from each other, wherein the anode is a molten sodium,

[0013] the separator is a sodium ion conducting inorganic membrane,

[0014] the separator has a surface in contact with the molten sodium anode, with the separator comprising metallic zinc at its surface facing the anode, wherein the metallic zinc is present in an amount in the range of 0.05-20 g / 1000cm2.

[0015] The invention also pertains to a separator suitable for use in such an electrochemical cell having a molten sodium anode, the separator being a sodium ion conducting inorganic membrane which comprises metallic zinc at the surface that upon incorporation into the electrochemical cell will face a molten sodium anode, wherein the metallic zinc is present in an amount in the range of 0.05-20 g / 1000cm2.

[0016] Within the context of the present specification the term sodium when used in the context of the sodium anode refers to a composition comprising at least 90 at.% of sodium atoms, in particular at least 95 at.%, more in particular at least 98 at.%. Higher relative amounts of sodium are preferred for reasons of efficacy.

[0017] It has been found that the presence of metallic zinc at the surface of the separator which faces a molten sodium anode leads to excellent wetting of the surface by the sodium, without high temperature activation or operating at higher temperatures being required. In fact, good wetting properties can already be achieved at temperatures as low as 100 - 120°C, i.e. just above the melting temperature of sodium metal at 97.8°C. This leads to stable electrochemical cells which can be activated and operated in an energy-efficient manner. Further, zinc has a low solubility in sodium. In consequence, the zinc layer on the separator is durable, resulting in stable wetting properties and an improved lifetime of the electrochemical cell. Additionally, zinc is attractive for its availably and relatively low cost. It is further advantageous from an environmental and health and safety point of view as compared to metals such as cobalt and chromium.Further advantages of the present invention and its specific embodiments will become apparent from the further specification.

[0018] The invention will be discussed in more detail below.

[0019] As indicated above, it is a key feature of the present invention that the separator comprises metallic zinc at its surface facing the anode. The presence of metallic zinc improves the wettability of the separator for molten sodium. Apparently zinc metal has a sodiophilic surface as a result of which a separator surface provided with metallic zinc shows excellent sodium wetting just above the melting temperature of sodium.

[0020] The metallic zinc is present in an amount in the range of 0.05-20 g / 1000cm2, in particular in an amount of 0.2-10 g / 1000cm2, more in particular in an amount of 1-7 g / 1000cm2. If the amount of zinc is too low, its effect on the wettability of the separator may be insufficient. Not wishing to be bound by theory, the wetting process is hypothesized to be a Cassie-Baxter wetting process in which sodiophilic zinc islands promote a Wenzel transition in which wetting occurs between the zinc islands. Clearly, if the amount of zinc is too low, surface dewetting may occur between the zinc islands, leading to a non-continuous sodium metal film on the anode-side surface of the separator. On the other hand, if the amount of zinc is too high, its presence will not further contribute to the wettability of the substrate, while it may lead to a decrease of conductivity of the separator. In the present invention the zinc is present at the surface of the separator facing the anode, e.g., in the form of zinc islands. As the skilled person will appreciate, this is a more intimate contact than will be afforded by the use of a shim, which is present adjacent to the separator surface. The presence of zinc at the surface of the separator facing the anode in an amount in the range of 0.05-20 g / 1000cm2ensures improved wetting of the anode; the use of a shim does not.

[0021] The metallic zinc is present to provide improved wetting of the surface of the separator facing the anode. To achieve this effect, it is preferred that zinc is distributed over at least 50% of the anode-side separator surface which when the battery is in use is in contact with sodium. In other words, preferably the metallic zinc is homogeneously distributed over the surface of the separator facing the anode which when the battery is in use is in contact with sodium. Within the context of the present specification a homogeneous distribution means that of the anode-side surface of the separator that when the battery is in use is in contact with sodium, the amount of zinc present at the square cm that has the lowest zinc content is at least 10%of the average amount of zinc per square cm of the anode side separator surface that is in contact with sodium. Preferably, the amount of zinc present at the square cm that has the lowest zinc content is at least 25% of the average amount of zinc per square cm of the anode side separator surface that is in contact with sodium when the battery is used, more in particular at least 50%.

[0022] Within the context of the present specification the terms zinc or metallic zinc refer to a metal composition comprising at least 70 at.% of zinc atoms, in particular at least 80 at.%, more in particular at least 90 at.%, still more in particular at least 95 at.%, even more in particular at least 98 at.% zinc. Higher relative amounts of zinc are preferred for reasons of efficacy.

[0023] In one embodiment, the metallic zinc is embedded in a glassy layer bonded to the membrane, in particular a glassy layer derived from sodium silicate or sodium polyphosphate.

[0024] The invention also pertains to a method for manufacturing the separator of the present invention. In the method according to the invention, a sodium ion conducting inorganic membrane is subjected to a zinc application processes which results in the deposition of a zinc-containing layer on the surface of the sodium conducting inorganic membrane. Various zinc application methods may be considered.

[0025] In one embodiment, the zinc application process comprises the step of contacting the ion conducting inorganic membrane with a liquid in which zinc is dispersed or dissolved in a liquid medium, followed by removal of the liquid medium The zinc may be provided in the form of metallic particles, but also in the form of an organic or inorganic Zn-salt or in the form of zinc sulphide particles. Examples of suitable organic salts include salts of organic acids, such as acetate or citrate. Examples of inorganic salts include chlorides, sulfates, nitrates, carbonate, and hydroxides. Combinations of various forms of zinc may also be used.

[0026] After contacting the separator with the liquid comprising zinc, the liquid medium can be removed, e.g., through evaporation. It is within the scope of the skilled person to select a suitable liquid medium. If removal of the medium through evaporation is desired, a relatively low boiling point is preferred, e.g. at most 120°C at atmospheric pressure. A liquid medium comprising water may be considered preferred.

[0027] Where the zinc is not provided in metallic form, a reduction step is carried out to convert zinc from its ionic form into its metallic form. This also applies when part of the zinc is present in the form of oxide or hydroxide as may occur if the separator is stored under conditions under which zinc may be converted into oxides or hydroxides. A separate reduction step may be carried out, but otherwise reduction of the zinc ions to form metallic zinc will take placeduring start-up of the battery by reaction with sodium. For reasons of process control it may be preferred to carry out a separate reduction step. Methods for effecting reduction of the zinc are known to the skilled person.

[0028] In one embodiment, the liquid medium comprising zinc further comprises at least one compound which improves bonding of the zinc to the separator. Examples of suitable compounds include sodium silicate and sodium polyphosphate. For example use may be made of an aqueous solution comprising zinc particles and sodium silicate or sodium polyphosphate, e.g., sodium polyhexametaphosphate. Upon heating, the silicate or polyphosphate will be converted to a glassy layer which will entrap the zinc particles.

[0029] An additional advantage of the use of a dispersion of zinc in a solution of a sodium silicate or sodium polyphosphate is that this dispersion is surprisingly stable over time, despite the relatively high specific density of metallic zinc particles compared to the aqueous solution. In consequence, the dispersion can be made and used over a prolonged time while its properties remain constant. The use of sodium polyphosphate is considered preferred in this respect.

[0030] As will be clear to the skilled person, other binders may also be used, as long as they result in a glassy layer which is permeable to sodium ions, which bonds to the separator, which is solid at the conditions prevailing during startup and use of the power storage cell, and which otherwise does not interfere with operating of the cell.

[0031] In one embodiment, the zinc is provided in the form of metallic zinc particles with a particle size in the range of 5-100 micron (D50), in particular in the range of 10-50 micron (D50). It is preferred for the particles to extend beyond the surface of any coating layer. The maximum size will be governed by practical considerations relating to cell design. It is within the scope of the skilled person to select a suitable particle size.

[0032] In a preferred embodiment the zinc application process comprises the step of contacting the ion conducting inorganic membrane with a liquid comprising dispersed metallic zinc particles, in particular zinc particles with a particle size stipulated above, and silicate or sodium polyphosphate solution in a liquid medium, followed by removal of the liquid medium.

[0033] Other methods which may be used to apply zinc onto the separator include contacting the separator with zinc particles in solid form followed by pressing, (reactive) CVD (chemical vapour deposition) in which an organic zinc-containing compound, e.g., diethylzinc is decomposed at the separator to form a thin film of zinc metal, PVD (physical vapour deposition), e.g., in the form of a sputtering or evaporation process via which a thin film ofmetallic Zn is applied onto the separator, and electrochemical deposition from a zinc salt based solution. Metal atomic layer deposition or metal organic molecular layer deposition may also be used. In all application methods, but in particular for the layer deposition methods, care should be taken to monitor that the amount of zinc is not so high that the sodium conductivity of the separator is affected.

[0034] It has been found that using metallic zinc particles, in particular metallic zinc particles with the particle size stipulated above, in combination with the use of silicate or polyphosphate results in zinc-containing coatings with good properties. If silicate or polyphosphate are used, the thickness of the silicate or polyphosphate layer is generally made to be in the range of 0.1-100 micron, in particular in the range of 1-50 micron, more in particular in the range of 5-20 micron. The coating can be applied in one or more layers.

[0035] The electrochemical cell according to the invention has an anode, a cathode, and a separator between the anode and the cathode separating them from each other. The anode is a molten sodium anode.

[0036] The cathode may be any cathode suitable for use in combination with an alkali metal containing anode. Examples of suitable cathodes comprise cathodes having a composition selected from the group of sulphide-containing cathode compositions, metal halide-containing cathode compositions, such as nickel and aluminium halide containing cathode compositions. In one embodiment the cathode has a composition selected from the group of nickel-halide containing cathode compositions and aluminium halide containing cathode compositions, the halide preferably being chloride.

[0037] In a preferred embodiment of the present invention, the cathode composition is an aluminium halide-containing cathode composition, in particular an aluminium chloride containing cathode composition. This is because electrochemical cells having an aluminium halide-containing cathode composition can be operated at relatively low operating temperatures ranging from the melting point of sodium as lower limit to, e.g., 300°C as upper limit (although higher temperatures of e.g. up to 350°C or 400°C are also possible). A more preferred range may be 100-250°C, in particular 130-210°C. If the operating temperature of the electrochemical cell is in this range it is of particular advantage that high-temperature pretreatment to improve wetting can be avoided. Additionally, higher operating temperatures place additional requirements on the cell structure and the materials used therein, in particular on the sealings applied. The possibility of using lower operating temperatures leads to increased design flexibility.A suitable aluminium chloride-containing cathode composition generally comprises metallic aluminium, sodium chloride, and NaAICI4. When charging the electrochemical cell Na+ moves from the anode to the cathode through the separator, resulting in the formation of Na metal in the anode and the reaction of Al with NaCI to form NaAICI4 in the cathode compartment, followed by the reaction of NaAICI4 with further Al to form NaAI2CI7. Upon discharging of the electrochemical cell, Na+ moves from the cathode to the anode through the separator, and the reactions in the cathode compartment are reversed.

[0038] The cathode compartment may contain a current collector if so desired. As will be evident to the skilled person, conventional conductive materials be used as current collector.

[0039] The separator used in the present invention is an alkali metal ion conducting inorganic membrane. A key feature of the separator used in the present invention is that it is permeable to ions of the alkali metal making up the anode, but not to the alkali metal itself, or to other components of the cathode or the anode. A further key feature of the separator is that it is stable under reaction conditions. That is, it should not physically or chemically degrade during use of the electrochemical cell. Suitable separators include those selected from the group of beta-alumina and Nasicon. Both materials are well known for use as separators in electrochemical cells and require no further elucidation here.

[0040] In general, a suitable sodium sulphide containing cathode composition comprises liquid sulphur and sodium sulphides with a carbon current collector as e.g. is described in ‘The sodium-sulphur battery’ edited by Sudworth and Tilley (1985).

[0041] In general, a suitable metal-halide-containing cathode composition comprises, e.g., nickel and salt particles immersed in liquid sodium metal halide electrolyte as e.g. described by J. Sudworth in J. Power Sources, 51 (1994), p. 105-114 and in J. Power Sources 100 (2001),p149-163. Cells of this type are often described as ZEBRA cells.

[0042] The electrochemical cell may be manufactured by combining the cathode, anode, and separator in methods known in the art. These require no further elucidation here.

[0043] As will be evident to the skilled person, different embodiments of the present invention can be combined unless they are mutually exclusive. For example, preferences expressed in the context of the separator in the battery also apply to the separator itself and vice versa, unless they are mutually exclusive. For a further example, preferences expressed in the context ofthe manufacture of the separator also apply to the separator, and to the battery and its manufacture, and vice versa.

[0044] All percentages used herein are weight percentages, unless specified otherwise.

[0045] When amounts, concentrations, dimensions and other parameters are expressed in the form of a range, a preferable range, an upper limit value, a lower limit value or preferable upper and limit values, it should be understood that any ranges obtainable by combining any upper limit or preferable value with any lower limit or preferable value are also specifically disclosed, irrespective of whether the obtained ranges are clearly mentioned in the context.

[0046] The invention is elucidated by the examples, without being limited thereto or thereby.

[0047]

[0048] of a metal surfaces by molten sodium

[0049] The wetting of a zinc surface by molten sodium was investigated as follows: A piece of sodium metal was positioned next to a flat piece of zinc metal in an inert atmosphere in a hermetically closed glass ampoule. At the start, some residual oxidation of the sodium was observed due to oxygen remnants in the ampoule. After the oxidation had stopped and a completely inert gas atmosphere had been formed in the ampoule, the temperature in the ampoule was increased to 120°C. At this temperature the sodium had molten. By shaking the ampoule the molten sodium was brought into contact with the flat surface of the zinc piece. The liquid sodium immediately spread over the zinc surface, demonstrating good wetting behaviour and low contact angle already at a temperature of 120°C.

[0050] The experiment repeated except that a flat piece of stainless steel was used instead of a flat piece of zinc. In this experiment the molten sodium formed droplets which did not adhere to the steel surface but rolled off the surface. No wetting was observed.

[0051] This experiment shows that even at the low temperature of 120°C metallic zinc is well-wetted by molten sodium. The wetting behaviour of steel under these conditions was poor.

[0052] 2: NaCI / AI cell with zinc-

[0053]

[0054] to the invention) or zinc-free

[0055]

[0056] tubular membrane

[0057] Example 2a: NaCI / AI cell with zinc-containing tubular membrane (according to the invention)Zinc powder was dispersed in an aqueous sodium hexametaphosphate solution to form a homogeneous dispersion. Specifically, 10 grams of metallic zinc powder with a particle size of 37 micron was stirred into 4 ml of an aqueous solution of sodium hexametaphospate (24 g / 100 ml water). A stable grey dispersion was obtained, which did not wet the (borate) glass surface if the vessel in which it was prepared.

[0058] The dispersion was applied with a sponge applicator to one side of a tubular beta-alumina membrane with a 16 cm2active area. The metallic zinc was present on the membrane in an amount of 2.9 g / 1000cm2. The tubular membrane was closed to form an anode compartment. Before starting cell operation the anode compartment was evacuated to dehydrate the hexametaphosphate and let it form a glass-like coating with the zinc particles evenly distributed over the entire active anode-side beta-alumina surface. The dehydration took place at a temperature of 150°C and a pressure of 0,1 bar for 15 minutes. The anode compartment was then filled with inert gas.

[0059] The cathode compartment was filled with a mixture of NaCI (vacuum dried), aluminium particles, and sodium tetrachloroaluminate (NaAICI4, STCA). During cell operation STCA which is liquid above 154°C, serves as electrolyte with a high conductivity for sodium ions. After start up the cell was charged to a level of 4.5 Ah after which cycling was started. The cell temperature was 190°C. The internal Ohmic resistance of the cell was monitored. The minimum Ohmic resistance that was measured during cycling was 0.2 Ohm. The theoretical Ohmic resistance of the beta alumina separator (~90 mS / cm) at 190°C can be calculated as 0.1 Ohm (R = rho * d / A = d / sigma * A). With a sodium conductivity of 200 - 400 mS / cm of the catholyte the cathode contribution is estimated at R =~0.07 - 0.15 Ohm.

[0060] Comparing the actual value of 0.2 Ohm with the theoretical values presented above clearly indicate that the entire anode-side surface in the cell takes part in the cell operation. This supports that the wetting of the separator surface in contact with the sodium anode is good.

[0061] Example 2b: NaCI / AI cell with zinc-free tubular membrane (comparative)

[0062] Example 2a was repeated, except that no zinc coating was applied to the separator surface to be in contact with the liquid sodium anode. The cell immediately showed very high internal cell resistance. While the resistance of the cell according to the invention of example 2a dropped to a value of less than 0.5 Ohm within 24 hrs (with the minimum Ohmic resistance measured during cycling being 0.2 Ohm), the resistance of the comparative cell of the present example remained above 5 Ohm for more than two weeks. Assuming that the cell according to the invention of Example 2a had a 100% wetted and working anode surface, it may be inferred that in the comparative cell of Example 2b, wetting of the separator surface at the side of the sodium anode was so incomplete that less than 5% of the membranesurface at the anode side participated in the charging and discharging processes. This clearly shows that the provision of zinc on the separator surface facing the molten sodium anode leads to a substantial improvement of battery performance, also over prolonged periods of time.

Claims

CLAIMS1. Electrochemical cell having an anode, a cathode, and a separator between the anode and the cathode separating them from each other, whereinthe anode is a molten sodium anode,the separator is a sodium ion conducting inorganic membrane,the separator having a surface in contact with the molten sodium anode,the separator comprising metallic zinc at its surface facing the anode, wherein the metallic zinc is present in an amount in the range of 0.05-20 g / 1000cm2.

2. Electrochemical cell according to any one of the preceding claims, wherein the metallic zinc is present in an amount in the range of 0.2-10 g / 1000cm2, in particular in an amount of 1-7 g / 1000cm2.

3. Electrochemical cell according to any one of the preceding claims, wherein the separator is an inorganic membrane selected from the group of beta-alumina and Nasicon.

4. Electrochemical cell according to any one of the preceding claims, wherein the cathode has a composition selected from the group of sulphur-containing cathode compositions, nickel-halide containing cathode compositions, and aluminium halide containing cathode compositions.

5. Electrochemical cell according to claim 4, wherein any one of the preceding claims where the cathode has a composition selected from the group of nickel-halide containing cathode compositions and aluminium halide containing cathode compositions, the halide preferably being chloride.

6. Electrochemical cell according to claim 5, wherein the cathode is an aluminium halide containing cathode.

7. Separator suitable for use in an electrochemical cell according to any one of the preceding claims, the separator being a sodium ion conducting inorganic membrane, the separator comprising metallic zinc at its surface that upon incorporation into the electrochemical cell will face an alkali metal anode, wherein the metallic zinc is present in an amount in the range of 0.05-20 g / 1000cm2.

8. Separator according to claim 7, wherein the metallic zinc is embedded in a glassy layer bonded to the membrane, in particular a glassy layer derived from sodium silicate or sodium polyphosphate.

9. Method for manufacturing a separator according to claim 7, wherein a sodium ion conducting inorganic membrane is subjected to a zinc application processes which results in the deposition of a zinc-containing layer on the surface of the alkali metal ion conducting inorganic membrane.

10. Method according to claim 9, wherein the zinc application process comprises the step of contacting the sodium ion conducting inorganic membrane with a liquid in which zinc is dispersed or dissolved in a liquid medium, followed by removal of the liquid medium.

11. Method according to claim 9 or 10, wherein the zinc application process comprises the step of contacting the sodium ion conducting inorganic membrane with a liquid medium comprising metallic zinc particles dispersed in a solution comprising silicate or polyphosphate, followed by removal of the liquid medium.

12. Method for manufacturing a separator according to claim 7, wherein the zinc application process is selected from:- contacting the separator with zinc particles in solid form followed by pressing,- (reactive) CVD (chemical vapour deposition) in which an organic zinc-containing compound, e.g., diethylzinc is decomposed at the separator to form a thin film of zinc metal, - PVD (physical vapour deposition), e.g., in the form of a sputtering or evaporation process, via which a thin film of metallic Zn is applied onto the separator, and- electrochemical deposition from a zinc salt based solution.

13. Method according to claim 7, wherein if the zinc is not provided in metallic form, a reduction step is applied to convert the zinc into its metallic form.

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