WO2021123731A1

WO2021123731A1 - Methods of making anode architectures

Info

Publication number: WO2021123731A1
Application number: PCT/GB2020/053116
Authority: WO
Inventors: Robert Gruar
Original assignee: Dyson Technology Ltd
Current assignee: Dyson Technology Ltd
Priority date: 2019-12-16
Filing date: 2020-12-04
Publication date: 2021-06-24
Anticipated expiration: 2022-06-16
Also published as: CN114867887A; GB2590398B; GB2590398A; GB201918476D0; CN114867887B

Abstract

A method of making an anode architecture comprising an alkali metal anode and a transition metal current collector layer for use in an alkali metal ion cell. The method comprising: providing a reactor having a substrate support and a transition metal ion source; depositing an alkali metal electrode on a substrate provided on the substrate support; applying a irst biasing potential to the source to create a source potential, thereby creating a transition metal ion flux from the source to the substrate; applying a second biasing potential that, when applied to the substrate support, will create a substrate potential that is less than the source potential and greater than a ground potential; and applying the second biasing potential to the substrate support to control a potential difference between the substrate potential and the source potential, thereby controlling an arrival energy of the transition metal ions at the alkali metal electrode.

Description

Methods of Making Anode Architectures

The present invention relates to a method of making an anode architecture comprising an alkali metal anode and a transition metal current collector layer, and to an alkali metal ion cell with an alkali metal anode.

Introduction

Cell architectures typically include current collectors associated with each electrode, which serve to collect current and conduct it to an appropriate point within the cell. It is often beneficial to include a current collector, rather than relying on the electrode itself, since a current collector may be of greater electrical conductivity than the electrode material, or may have other desirable properties. The current collector is usually made of a conducting material such as a transition metal, and is often provided as a foil that is placed over and in electrical contact with the electrode. Where the electrode is a cathode, and the cathode material is sufficiently robust to undergo vacuum deposition processes, the current collector layer may be deposited directly onto the cathode.

Figure 1 illustrates a typical known cathode-current collector architecture 2 of this sort, with a cathode 3 and a current collector Iayer4. The current collector 4 has a grain structure that comprises a network of grain boundaries 5. The network extends from a surface of the current collector 4 adjacent to the anode, to an opposite surface 6. Grain boundaries 5 provide fast pathways for diffusion of metal ions, and thus in the assembled cell, metal ions from the cathode 3 will tend to diffuse through the grain boundaries 5 to the surface 6 of the current collector 4, particularly at high loading of the cell. Unless the surface 6 is otherwise protected, the metal ions will react with the surrounding atmosphere, causing an overall loss in metal ions and hence a loss in cell capacity. This can be mitigated by encapsulating the architecture in a thick (~5 pm) polymer encapsulation layer 7, but such a layer adds significantly to the parasitic mass and volume of the cell, thereby reducing its volumetric and gravimetric energy density.

Although this grain boundary diffusion mechanism will occur for any electrode material, it is particularly problematic where the electrode material is particularly reactive.

For anode current collectors the reactive nature of lithium can make interactions with current collector layers problematic. As a result, in cell architectures where the anode is a lithium metal layer, a separate current collector layer is often avoided, and instead a single thick lithium layer typically acts as both the anode and the current collector. This structure must be encapsulated in the same way as the structure of Figure 1, to prevent lithium loss, thereby reducing the volumetric and gravimetric energy density of the cell in the same way.

It is against this background that the invention has been devised.

Statements of Invention

Against this background the invention resides in a method of making an anode architecture comprising an alkali metal anode and a transition metal current collector layer for use in an alkali metal ion cell. The method comprises: providing a reactor having a substrate support and a transition metal ion source; depositing an alkali metal electrode on a substrate provided on the substrate support; applying a first biasing potential to the source to create a source potential, thereby creating a transition metal ion flux from the source to the substrate; selecting a second biasing potential that, when applied to the substrate support, will create a substrate potential that is less than the source potential and greater than a ground potential; and applying the second biasing potential to the substrate support to control a potential difference between the substrate potential and the source potential, thereby controlling an arrival energy of the transition metal ions at the alkali metal electrode.

The substrate support may be static, or the substrate support may allow for movement of the substrate, for example in a continuous manufacturing process. To this end, the substrate support may itself be moveable, for example the substrate support may comprise a plurality of rotatable reels.

The method may comprise varying the second biasing potential to vary the potential difference.

The method may comprising cooling the substrate, for example by cooling the substrate support.

The transition metal current collector layer may comprises first and second current collector layers, and the method may comprises: providing a first transition metal ion source and applying a first biasing potential to the source to create a first source potential; selecting a second biasing potential that, when applied to the substrate support, will create a substrate potential that is less than the first source potential and greater than a ground potential; and applying the second biasing potential to the substrate support; thereby depositing a layer of the first transition metal on the alkali metal electrode; and subsequently: providing a second transition metal ion source and applying a third biasing potential to the source to create a second source potential; selecting a fourth biasing potential that, when applied to the substrate support, will create a substrate potential that is less than the second source potential and greater than the ground potential; and applying the fourth biasing potential to the substrate support; thereby depositing a layer of the second transition metal on the layer of the first transition metal.

In this way, a current collector layer may be deposited on the electrode that comprises two transition metal layers.

In this case the second biasing potential may be different to the fourth biasing voltage.

The alkali metal electrode may comprise, and/or be provided on, an elongate strip. The substrate support may comprise a rotatable reel for use in a reel-to-reel process. In this case, the step of arranging an alkali metal electrode on the substrate support may comprise reeling the elongate strip onto the reel; and the step of applying the second and/or fourth biasing potential to the substrate support may comprise contacting the reel with a brush connector and applying the biasing potential to the reel through the brush connector.

Particularly where the alkali metal electrode is a lithium electrode, the method may comprise selecting the first biasing potential such that an arrival energy of transition metal ions at the alkali metal electrode is less than 16 eV.

The invention also extends to an alkali metal ion cell comprising an alkali metal anode plated with a transition metal current collector layer. The alkali metal anode may in particular be a lithium anode.

In either the method or the cell described above, the transition metal current collector layer may comprise a material selected from the group consisting of: platinum, copper, nickel, cobalt, titanium, silver, chromium, iridium, tantalum, tungsten and molybdenum, and preferably from the group consisting of: platinum, copper, nickel, cobalt and titanium. In particular, the transition metal may be platinum.

The transition metal current collector layer may have a thickness t_¥ of between approximately 20 nm and approximately 500 nm.

The alkali metal anode comprises a thin film of alkali metal. The alkali metal may be lithium. The invention extends still further to a current collector for use with an electrode in an alkali metal ion cell. The current collector comprises: a first current collector layer comprising a first current collector material and having a first grain structure defining a first set of grain boundaries; a second current collector layer comprising a second current collector material having a second grain structure defining a second set of grain boundaries; and an interface region separating the first and second current collector layers, such that the first and second sets of grain boundaries do not cross the interface region.

As a result of this architecture, the grain boundary network of the current collector as a whole is discontinuous across the interface region between the first and second current collector. When the first current collector layer is in contact with an electrode (for example a lithium metal electrode) electrode material (for example lithium ions) will tend to diffuse into the first set of grain boundaries, away from the electrode. However, when the electrode material reaches the interface region, the grain boundary network is disrupted because the sets of grain boundaries do not extend beyond the interface region. Thus, the electrode material cannot easily diffuse further into the second current collector layer, and is retained in the first current collector layer. This reduces the parasitic loss of the electrode material via grain boundary diffusion. By substantially preventing diffusion of the electrode material through the entire thickness of the current collector layer, the protective layer that would otherwise be required to protect the surface of the current collector layer and prevent loss of electrode material at that surface, can be dispensed with. This reduces the parasitic mass of the overall cell, thereby improving its volumetric and gravimetric performance.

The second current collector material may be different to the first current collector material.

The first and second current collector materials may be metallic materials, which may be transition metals or alloys thereof. For example, each of the first and second current collector materials may be selected from the group consisting of: platinum, copper, nickel, cobalt, titanium, silver, chromium, iridium, tantalum, tungsten and molybdenum, and preferably from the group consisting of: platinum, copper, nickel, cobalt and titanium.

The first current collector material may be copper and/or the second current collector material may be platinum.

The first and second current collector layers may be in direct contact at the interface region. Alternatively, the current collector may comprise a third current collector layer between the first and second current collector layers comprising a third current collector material. In this case, the third current collector layer may define the interface region.

Where the first and second current collector materials are metallic materials, the third current collector material may comprise an intermetallic material comprising a mixture of the first and second current collector materials.

The current collector may have a thickness t_cc of between approximately 20 nm and approximately 500 nm.

The current collector may have a thickness t_c and the first current collector layer may have a thickness T_fcc that is at least 10% of the thickness t_¥ of the current collector, preferably 10 to 90% of the thickness t_cc of the current collector.

The invention also extends to a multi-layer electrode architecture for use in an alkali metal ion cell. The multi-layer electrode architecture comprises an electrode layer, and the current collector of any preceding claim, wherein the first current collector layer is in contact with the electrode layer.

The invention extends further to a method of making a multi-layer electrode architecture for use in an alkali metal ion cell. The method comprises: providing an electrode having an electrode surface; on the electrode surface, depositing a first current collector layer of a first current collector material, the first current collector layer having a first grain structure defining a first set of grain boundaries; and on the first current collector layer, depositing a second current collector layer of a second current collector material, the second current collector layer having a second grain structure defining a second set of grain boundaries, thereby defining a boundary between the first and second current collector layers, such that the first and second sets of grain boundaries do not cross the boundary.

The method may further comprise allowing the first and second current collector materials to interdiffuse at the boundary to form a third current collector layer between the first and second current collector layers, the third current collector layer defining an interface region, such that the first and second sets of grain boundaries do not cross the interface region.

The electrode may be a negative electrode or an anode. The anode may comprise lithium or UPON, in which case the alkali metal ion cell may be a lithium cell. Preferred and optional features of one aspect or embodiment may be used alone, or in appropriate combination, with other aspects and embodiments also.

Brief description of the drawings

Figure 1 has already been described above by way of background to the invention. In order that the present invention may be more readily understood, embodiments of the invention will now be described, by way of example, with reference to the remainder of the accompanying Figures, in which:

Figure 2 is a schematic cross-sectional view of a multilayer electrode architecture for use in an alkali metal ion cell, comprising an electrode and a current collector architecture;

Figure 3 is a schematic cross-sectional view of another multilayer electrode architecture for use in an alkali metal ion cell;

Figure 4 is a schematic view of apparatus for depositing a transition metal layer on an alkali metal surface, the apparatus supporting a substrate comprising the alkali metal surface and a target providing a source for transition metal ions;

Figures 5A to 5C are schematic diagrams illustrating the effect of varying a potential Vs_ub applied to the substrate of the apparatus of Figure 4 while a fixed potential Vs_ource is applied to the target of the apparatus of Figure 4;

Figures 6A to 6E are schematic illustrations of steps in a method of making the multilayer electrode architectures o Figures 2 and 3; and

Figure 7 is a schematic view of another apparatus for depositing a current collector layer on an anode.

Detailed description of embodiments of the invention

Figure 2 illustrates a multilayer electrode architecture 10, comprising an electrode in the form of an electrode layer 12 and a current collector in the form of a current collector layer 14. In use, the electrode architecture 10 is integrated into a cell (not shown).

The current collector layer 14 comprises a first current collector layer 16 adjacent to the electrode, and a second current collector layer 18 that overlies the first current collector layer 16. In this way, the first current collector layer 16 is sandwiched between the electrode layer 12 and the second current collector layer 18. An interface region 20, in this case a substantially two-dimensional interfacial plane, is defined between the first and second current collector layers 16, 18, while an electrode interface 22 is defined between the first current collector layer 16 and the electrode layer 12. An outer surface 24 of the second current collector layer 18 is a free surface that, in use, may be exposed to the atmosphere in the cell.

The first current collector layer 16 comprises a first current collector material and has a first grain structure that comprises a first set of grain boundaries defining a first grain boundary network 26. The first grain boundary network 26 extends between the electrode interface 22 and the interface region 20. The second current collector layer 18 comprises a second current collector material and has a second grain structure that comprises a second set of grain boundaries defining a second grain boundary network 28. The second grain boundary network 28 extends between the interface region 20 and the outer surface 24.

Importantly, the first and second grain boundary networks 26, 28 do not cross the interface region 20. Said another way, the first and second grain boundary networks 26, 28 each truncate at the interface region 20, such that the grain boundary networks 26, 28 are occluded at the interface region 20. Viewing the combined first and second grain boundary networks 26, 28 as a collective grain boundary network for the entire current collector layer 14, it could be said that the collective grain boundary network is discontinuous across the interface region 20, and hence discontinuous between the first and second current collector layers 16, 18.

As a result of this discontinuity, if metal ions from the electrode 12 diffuse into the first set of grain boundaries 26 of the first current collector layer at the electrode interface 22, grain boundary diffusion will be halted at the interface region 20 because the grain boundary network is disrupted. Thus, the metal ions will not easily diffuse further into the second current collector layer 18, and are retained in the first current collector layer 16. This reduces the parasitic loss of the electrode material via grain boundary diffusion, and avoids the need for an encapsulation layer.

Considering the various layers of the architecture in more detail, the electrode layer 12 comprises an electrode material. In this example, the electrode 12 is an anode comprising an anode material that acts as a source for alkali metal ions. The anode material may for example be an alkali metal, such as lithium, which may be provided as a lithium thin film deposited on a substrate. In other examples, the anode material may comprise an alkali-metal-ion- containing compound, such as lithium phosphorus oxy-nitride (“UPON”), which may similarly be provided as a thin film deposited on a substrate. In the example of Figure 2, the current collector layer 14 is deposited directly onto the electrode layer 12 to create a multi-layer thin film architecture, as will be described in more detail below.

Considering first and second current collector layers 16, 18 in more detail, each of the first and second current collector materials are conducting materials, and are preferably transition metals such as platinum, copper, nickel, cobalt, titanium, silver, chromium, iridium, tantalum, tungsten or molybdenum. Particularly preferred transition metals are platinum, copper, nickel, cobalt and titanium, and in a particularly preferred embodiment, the first current collector material is copper and the second current collector material is a transition metal such as platinum.

In the example of Figure 2, the first and second current collector materials are different materials. Forming the first and second current collector layers 16, 18 of different current collector materials is a simple and effective way to ensure that the first and second grain boundary networks 26, 28 are distinct, and do not extend across the interface region 20. This is particularly beneficial when the second current collector layer 18 is a deposited layer that is deposited directly onto the first current collector layer 16, since it allows the second current collector layer 18 to be deposited directly over the first current collector layer 16, thereby providing a simple fabrication process, whilst maintaining separate grain boundary networks across the interface region 20. However, in other embodiments, the first and second current collector materials may be the same material, so long as the first and second grain boundary networks 26, 28 do not extend across the interface region 20.

The current collector layer 14 has a total thickness t_cc that is between approximately 20 nm and approximately 500 nm. In other words, the combined thicknesses of the first and second current collector layers 16, 18 is between approximately 20 nm and approximately 500 nm. The total thickness t_cc of the current collector layer 14 is substantially the same as the total thickness of the current collector layer 4 of Figure 1 , and thus there is no need to provide additional thickness as a result of the multi-layer architecture of the current collector 14 of Figure 2.

The first current collector layer 16 has a thickness t_fCC that is at least 10% of the thickness t_cc of the current collector layer 14, and is preferably 10 to 90% of the thickness t_c of the current collector layer 14. Correspondingly, the second current collector layer 18 has a thickness f_scc that is at most 90% of the thickness t_c of the current collector layer 14, and is preferably 10 to 90% of the thickness t_c of the current collector layer 14. In particular the relative thickness of the two layers may be chosen so as to minimise the amount of the relatively more expensive material, and maximise the amount of the relatively less expensive material, so as to reduce the overall cost of the current collector layer. Figure 3 shows an alternative multilayer electrode architecture 110, also comprising an electrode in the form of an electrode layer 112 and a current collector in the form of a current collector layer 114. This example is substantially the same as the example of Figure 2, except that, unlike in the example of Figure 2 where the first and second current collector layers 16, 18 are in direct contact at the interface region 20, in the example of Figure 3, a third current collector layer 130 is located between the first and second current collector layers 116, 118, such that the third current collector layer 130 defines the interface region 120, and the first and second current collector layers 116, 118 are not in direct contact.

In this example, the first and second current collector materials are different metallic materials, and the third current collector material is an intermetallic compound of the first and second current collector materials. This intermetallic compound, and hence the third current collector layer 130, is formed by interdiffusion of the first and second current collector materials at the interface region 120. The first and second current collector materials may be specifically selected to provide a desirable intermetallic compound. For example, the materials may be cooper and tin, providing a copper-tin intermetallic, or copper and zinc providing a brass (copper-zinc) intermetallic. The intermetallic may have a thickness of approximately 5 to 30 nm, for example 10 nm.

The intermetallic material and hence the third current collector layer 130 that defines the interface region 120 is substantially free from grain boundaries. As a result, the third current collector layer 130 performs the same function of disrupting the overall grain boundary network across the interface region 120, such that the first and second grain boundary networks 126, 128 of the respective first and second current collector materials do not cross the interface region 120.

The multilayer electrode architectures 10, 110 described above may be fabricated by any suitable method. In one exemplary fabrication method, an electrode 12, 112 is provided, and the first current collector layer 16, 116 is first deposited on the electrode, for example by sputtering. Next, the second current collector layer 18, 118 is deposited on the first current collector layer 16, 116, also by sputtering. In examples where a third current collector layer 130 is present, the first and second current collector layer 116, 118 are allowed to interdiffuse to form the intermetallic compound of the third current collector layer 130. Examples are also envisaged in which the various layers are provided as separate foils that are layered together.

However, one particularly beneficial method of providing current collector layer architectures on alkali metal anodes, and particularly for depositing transition metal current collector layers directly onto lithium anodes, will now be described below. It is emphasised that while such a method may be used to deposit the multi-layered current collector layers of Figures 2 and 3 directly onto lithium anodes, and is described below for in use for this particular application, the method and apparatus described may also be used to deposit any other metallic layer directly onto an alkali metal surface such as a lithium surface: for example it may be used to deposit a single transition metal layer onto lithium, which may serve a purpose other than a current collector.

Figure 4 illustrates apparatus 40 for depositing a transition metal layer, such as the current collector layer 14 described above, onto an alkali metal surface, such as an anode surface of a lithium anode.

The apparatus 40 is an improved version of a conventional sputtering reactor, and to this end is a reactor that comprises a substrate support 42 for supporting a substrate 44, and a holder for receiving a transition metal ion source, exemplified here as a sputtering target 46. Although not shown, the substrate support 42 and holder maybe enclosed in a sealable chamber, such that the atmosphere in the chamber can be controlled during deposition. The apparatus 40 may also comprises cooling means for cooling the substrate: in this case, the cooling means is integrated into the substrate support 42.

The apparatus 40 also comprises a first biasing means 48 arranged to apply a first biasing potential to the target 46, thereby creating a source potential Vs_ource at the target. When the first biasing potential is applied to create the source potential, a transition metal ion flux is created between the source and the substrate 44 supported by the substrate holder 42.

A second biasing means 49 is arranged to apply a second biasing potential to the substrate holder 42, thereby creating a substrate potential V_Sub at the substrate holder 42. The second biasing means 49 is configured such that the second biasing potential can be varied, and specifically can be varied to apply a potential that is variable between a maximum potential that is equal to the source potential Vs_ource applied to the target 46, and a minimum potential that is equal to a ground potential V_Ground.

The value of the second biasing voltage can be selected to create a particular potential difference between the target 46 and the substrate holder 42. The higher the potential difference, the greater the degree of attraction of the target material (i.e. the transition metal ions) towards the substrate 44, and thus the higher the arrival energy of the transition metal ions at the substrate 44. In this way, the potential applied to the substrate holder 42 can be used to tune the arrival energy of the transition metal ions at the substrate 44.

Figures 5A to 5C show examples of the effect of the potential V_Sub applied to the substrate 44, when a fixed potential Vs_ource is applied to the target 46. In Figure 5A, the potential Vs_ub applied to the substrate 44 is equal to the ground potential Vcr_oun_d: the lowest boundary for the substrate potential Vs_ub. In this situation, the potential difference between the potential Vs_our_ce applied to the target 46 and the potential Vs_ub = Vcr_oun_d applied to the substrate 44 is at its greatest, and thus the transition metal ions have the highest arrival energy that can be achieved using this apparatus.

In Figure 5B, the potential Vs_ub applied to the substrate 44 is equal to the source potential Vs_our_ce: the highest boundary for the substrate potential VSub. In this situation, the potential difference between the potential Vs_our_ce applied to the target 46 and the potential Vs_ub = Vs_our_ce applied to the substrate 44 is substantially zero, and thus the transition metal ions have the lowest arrival energy that can be achieved using this apparatus.

In Figure 5C, the potential Vs_ub applied to the substrate 44 has been specifically chosen to be at an optimum level, and is selected to be lower than the source potential Vs_our_ce and greater than the ground potential Vcround, i.e. V _Groun_d < Vs_Ub< Vsource- In this situation, the potential difference between the potential applied to the target 46 and the potential applied to the substrate 44 is at a specifically chosen intermediate level selected to optimise the arrival energy of the transition metal ions at the substrate.

It has been found that the above apparatus and method is particularly beneficial in depositing transition metal layers on lithium, and in fact allows the deposition of transition metal layers directly onto lithium metal surfaces, which to the inventors’ knowledge has not previously been achieved. Deposition of transition metals directly onto a lithium metal surface has previously been found to be impossible, since the potential that must be applied to a transition metal target to provide the required flux of transition metal ions results in an arrival energy of the transition metal ions which, while acceptable for deposition on many substrate surfaces, is problematic for deposition on a lithium surface, since a lithium surface is particularly prone to over-energising or destruction.

By applying a carefully selected potential to the substrate holder 42 and hence to the substrate 44, sufficient source potential can still be applied to the target 46 to provide the required flux of transition metal ions, but the arrival energy at the substrate 44 can be tuned to a level that is appropriate for a lithium surface. In particular, tuning the arrival energy to less than 16 eV has been found to permit deposition of the current collector material onto the lithium surface.

A method of making the multilayer electrode architectures 10 of Figures 2 and 3 will now be described with reference to Figures 6A to 6E and Figure 4. First, the user first provides a substrate 50 as shown in Figure 6A. Next, as shown in Figure 6B a lithium anode layer 12, 112 is deposited on the substrate 50, for example by sputtering. Sputtering may take place using the apparatus of Figure 4, or using any other suitable sputtering apparatus. If using the apparatus of Figure 4, the lithium anode layer 12 may be deposited on the substrate 50 immediately prior to the subsequent steps now described, which may be advantageous to preserve a pristine lithium anode surface prior to the subsequent steps.

Next, as shown in Figures 6C and 6D, the current collector layer 14 is deposited on the lithium anode surface 22, 122 using the apparatus of Figure 4. In particular, the substrate 50 and associated lithium anode layer 12, 112 and lithium anode surface 22, 122, which together define the substrate 44 of Figure 4, are arranged on the substrate support 42 the reactor. A first target 46 of the first current collector material (in this case an appropriate transition metal such as copper) is also arranged in place in the reactor.

A substrate potential Vs_ub is selected and applied to the substrate, and a source potential Vs_ource is applied to the first target 46. The two potentials act to generate a flux of the first transition metal ions towards the lithium anode surface 22, 122, with the arrival energy of the first transition metal ions tuned so as to be a desired arrival energy. In this way, as shown in Figure 6C, the first current collector layer 16, 116 is deposited directly onto the lithium anode surface 22, 122 using the apparatus of Figure 4. During this step, the substrate 44 may additionally be cooled by the cooling means: this can be particularly beneficial in preventing adatom implantation or migration on condensation at the lithium anode surface.

When the first current collector layer 16, 116 has been deposited to the required thickness, the first target is switched for a second target 46 of the second current collector material (in this case another appropriate transition metal such as platinum). A source potential Vs_ource is applied to the second target to generate a flux of the second transition metal ions towards the first current collector layer 16, 116. Optionally, a different substrate potential Vs_ub may be selected and applied to the substrate 44, to control the arrival energy of the second transition metal ions. In this way, the second current collector layer 18, 118 is deposited directly onto the first current collector layer 16, 116 as shown in Figure 6D.

In an optional additional step shown in Figure E, which may be included to make the multi layer architecture of Figure 3, the first and second current collector materials of the first and second current collector layers 116, 118 are allowed to interdiffuse to form an intermetallic compound defining the third current collector layer 130. The apparatus and method described therefore allow for a transition metal layer such as a current collector layer 14 to be successfully deposited on a lithium surface, such as a lithium anode surface, without damage to the underlying lithium metal, and in a manner that provides a uniform transition metal layer without implantation of the transition metal in the lithium. If desired, multiple layers of different transition metals can be deposited using this technique, to provide current collector architectures of the type shown in Figures 2 and 3 above.

Figure 7 illustrates an alternative apparatus 140 which is suitable for use in a continuous reel- to-reel process. The apparatus comprises a substrate support 142 in the form of a rotatable reel, and a target 146. A first biasing means 148 is arranged to bias the target 146 in the same way as the first biasing means 48 described above in relation to Figure 4.

A second biasing means 149 is also provided for biasing the substrate support 142. The second biasing means 149 is substantially identical to the second biasing means 49 described above in relation to Figure 4, except that it is adapted to allow for connection to the reel defining the substrate support 142, while still allowing the reel to rotate. To this end, the apparatus of Figure 4 comprises a brush connector 152 to provide an electrical connection between the second biasing means 149 and the reel defining the substrate support 142.

In the examples described above in relation to Figures 2, 3 and 6, the current collector layer comprises two or three current collector layers. However, other numbers of current collector layers are also envisaged, and there is no limit to the number of layers that can be provided according to the above architectures and methods.

It is emphasised that architectures of the type shown in Figures 2 and 3 may be produced by any suitable method, and need not necessarily be produced using the apparatus of Figures 4 or 7, and/or using the method of Figure 6. It is also emphasised that the method of Figure 6 and the apparatus of Figures 4 or 7 may be used to deposit any transition metal layer on any alkali metal surface, particularly a lithium surface, and their use is not limited to the architectures of Figures 1 and 3. However, particular advantages do arise when the apparatus of Figures 4 or 7, and/or the method of Figure 6, are used to produce the transition metal current collector architecture of Figures 2 or 3 on a lithium anode, since the method or apparatus allows a transition metal layer to be deposited directly on to the lithium metal surface, so that a separate current collector layer can act to protect the lithium anode, while the multi-layer architecture of the current collector layer guards against lithium loss through grain boundary diffusion. As a result, it is possible to fabricate a lithium anode and current collector in a single multi-layer thin film architecture, such that encapsulation layers can be dispensed with altogether, thereby providing a lithium cell of improved volumetric and gravimetric energy density. Other variations and modifications will be apparent to the skilled person without departing from the scope of the appended claims.

Claims

1. A method of making an anode architecture comprising an alkali metal anode and a transition metal current collector layer for use in an alkali metal ion cell, the method comprising: providing a reactor having a substrate support and a transition metal ion source; depositing an alkali metal electrode on a substrate provided on the substrate support; applying a first biasing potential to the source to create a source potential, thereby creating a transition metal ion flux from the source to the substrate; selecting a second biasing potential that, when applied to the substrate support, will create a substrate potential that is less than the source potential and greater than a ground potential; and applying the second biasing potential to the substrate support to control a potential difference between the substrate potential and the source potential, thereby controlling an arrival energy of the transition metal ions at the alkali metal electrode.

2. The method of Claim 1 , comprising varying the second biasing potential to vary the potential difference.

3. The method of any preceding claim, comprising cooling the substrate.

4. The method of any preceding claim, wherein the transition metal current collector layer comprises first and second current collector layers, and the method comprises: providing a first transition metal ion source and applying a first biasing potential to the source to create a first source potential; selecting a second biasing potential that, when applied to the substrate support, will create a substrate potential that is less than the first source potential and greater than a ground potential; and applying the second biasing potential to the substrate support; thereby depositing a layer of the first transition metal on the alkali metal electrode; and subsequently: providing a second transition metal ion source and applying a third biasing potential to the source to create a second source potential; selecting a fourth biasing potential that, when applied to the substrate support, will create a substrate potential that is less than the second source potential and greater than the ground potential; and applying the fourth biasing potential to the substrate support; thereby depositing a layer of the second transition metal on the layer of the first transition metal.

5. The method of Claim 4, wherein the second biasing potential is different to the fourth biasing voltage.

6. The method of Claim 1 , wherein: the substrate comprises and elongate strip; the substrate support comprises a rotatable reel for use in a reel-to-reel process; the step of arranging an alkali metal electrode on the substrate support comprises reeling the elongate strip onto the reel; and the step of applying the second and/or fourth biasing potential to the substrate support comprises contacting the reel with a brush connector and applying the biasing potential to the reel through the brush connector.

7. The method of any preceding claim, comprising selecting the first biasing potential such that an arrival energy of transition metal ions at the alkali metal electrode is less than 16 eV.

8. An alkali metal ion cell comprising an alkali metal anode plated with a transition metal current collector layer.

9. The method of any of Claims 1 to 7, or the alkali metal ion cell of Claim 8, wherein the transition metal current collector layer comprises a material selected from the group consisting of: platinum, copper, nickel, cobalt, titanium, silver, chromium, iridium, tantalum, tungsten and molybdenum, and preferably from the group consisting of: platinum, copper, nickel, cobalt and titanium.

10. The method of any of Claims 1 to 7 or Claim 9, or the alkali metal ion cell of Claim 8 or Claim 9, wherein the alkali metal is lithium and/or wherein the transition metal is platinum.

11. The method of any of Claims 1 to 7 or Claims 9 or 10, or the alkali metal ion cell of any of Claim 8 or Claims 9 or 10, wherein the transition metal current collector layer has a thickness between approximately 20 nm and approximately 500 nm.

12. The method of any of Claims 1 to 7 or Claims 9 to 11 , or the alkali metal ion cell of any of Claim 8 or Claims 9 to 11, wherein the alkali metal anode comprises a thin film of alkali metal.