GB2543827A - Metal ion battery current collector - Google Patents

Abstract

A metal ion cell stack comprises a plurality of cell units, where each of the cell units comprises: a negative electrode comprising a layer of negative electrode active material disposed over an anode current collector, a positive electrode comprising a layer of positive electrode active material disposed over a cathode current collector, and a separator and electrolyte layer disposed between the negative electrode and the positive electrode. A first of the cell units comprises an electrode (2b) (for example an anode) with a thermal conductance higher than the thermal conductance of an electrode (2a) of the same polarity (in this example another anode) in a second of the cell units. In one example the high thermal conductance electrode (2b) may be an anode with a copper current collector while the lower thermal conductance electrode (2a) may be an anode with an aluminium current collector.

Description

Metal ion battery current collector TECHNICAL FIELD

This invention relates to a sodium ion or other metal-ion cell stack, in particular the utilisation of more than one type of current collector on the anode side, and the inclusion of these in mixed metal current collector cell stacks for sodium ion batteries.

BACKGROUND ART

Sodium-ion batteries are analogous in many ways to the lithium-ion batteries that are in common use today; they are both reusable secondary batteries that comprise an anode (negative electrode), a cathode (positive electrode) and an electrolyte material, both are capable of storing energy, and they both charge and discharge via a similar reaction mechanism. When a sodium-ion (or lithium-ion) cell is charging, Na+ (or Li+) ions de-intercalate from the cathode and insert into the anode. Meanwhile charge balancing electrons pass from the cathode through the external circuit containing the charger and into the anode of the cell. During discharge the same process occurs but in the opposite direction with the external circuit able to power a load.

Lithium-ion battery technology has enjoyed a lot of attention in recent years and provides the preferred portable battery for most electronic devices in use today; however lithium is not a cheap metal to source and is considered too expensive for use in large scale applications. By contrast sodium-ion battery technology is still in its relative infancy but is seen as advantageous; sodium is much more abundant than lithium and some researchers predict this will provide a cheaper and more durable way to store energy into the future, particularly for large scale applications such as storing energy on the electrical grid. Nevertheless a lot of work has yet to be done before sodium-ion batteries are a commercial reality.

During fabrication of the battery cells a slurry is made with the active cathode or anode material along with a solvent and additives, this slurry is coated onto a foil. This foil has multiple functions, it acts as a substrate for the electrode material but also as a current collector to distribute the current generated during operation of the battery cell.

One of the significant benefits of sodium ion batteries is that the negative electrode materials can be coated onto a low cost and low specific gravity aluminium substrate, whereas typically in lithium ion batteries copper substrates (current collectors) must be used for the anode. This is due to the alloying of the lithium with aluminium at potentials less than 0.33V vs Li/Li+, which does not occur with sodium ions. While a benefit of using a copper current collector over aluminium is its ductility, so that thin current collectors can be made therefore increasing the potential volumetric energy density of the cell, copper is an expensive element and in addition has a specific gravity of 8.94g/cm3 which significantly adds to the weight of the battery.

Typically a cell stack will contain a single substrate/current collector type for the positive electrode, and a single substrate/current collector type for the negative electrode (which may be the same type as, or a different type to, the substrate/current collector type for the positive electrode). Examples of this can be found for sodium ion batteries. US 2012 0021273A1 proposes the use of an aluminium or aluminium alloy current collector on both the positive and negative side of a sodium ion battery. WO 2014017968 A1 proposes the use of a pure iron or iron alloy with less than 10% alloying materials as a current collector in a lithium or sodium battery cell. The current collector may comprise a roll of an iron or iron alloy foil laminated with a foil of either copper or aluminium or copper or aluminium alloy. JP2015526859A also proposes the use of a combined iron and copper current collector. In all these cases there is a single current collector type for each polarity of current collector.

Lithium ion pouch cells and sodium ion pouch cells can suffer from degradation due to excessive or non-uniform heating of the electrode stack during operation of the cells. This is particularly true for higher capacity cells which may comprise many layers of stacked electrodes. When discharged at high rate the temperature at the centre of the cell may be significantly higher compared with the outer layers of the cell. It is desirable for the thermal energy transfer within the cell to be sufficient to avoid degradation of the cell.

As well as distributing current generated during operation of a cell, the current collector of a cell electrode can dissipate heat generated during operation of the cell. However, especially with larger format cells or higher power cells, good heat distribution and extraction are extremely important to prevent overheating. In addition to using current collectors to dissipate heat there are also several examples of the inclusion in the cell design of a feature specifically intended for thermal energy transfer. US7592097B2 proposes a cell design with multiple parallel connections to the cell casing to assist in heat dissipation from the cell. US5501916A proposes a through hole traversing a battery main body and opened to the outside wherein the hole contains heat dissipating fins. US8546009B2 proposes a thermally conductive core for thermal energy transfer. US20140205882A1 proposes a lithium ion pouch cell with a thermal conductive element, which may be a current collector, which extends beyond the electrode stack to make good thermal contact with the laminated aluminium pouch material and therefore provide good thermal energy transfer. In the examples described in US20140205882A1 the anode current collector is copper foil and the cathode current collector is aluminium foil, as is conventional for lithium ion cells and the aluminium cathode current collector and the copper anode current collector each extend beyond the electrode stack to provide improved thermal energy transfer to the pouch material. US6716552B2 also proposes the use of an electrode in thermal contact with the packaging material of the battery cell. JP2011192476A proposes reducing the temperature distribution within the electrode of a lithium ion secondary battery with the use of a shaft core of a wound electrode group formed by bonding an aluminium positive electrode shaft core part and a copper negative electrode shaft core part while insulating them from each other with an insulating material.

SUMMARY OF INVENTION A first aspect of the invention provides a metal ion cell stack comprising a plurality of cell units, wherein each of the cell units comprises: a negative electrode comprising a layer of negative electrode active material disposed over an anode current collector, a positive electrode comprising a layer of positive electrode active material disposed over a cathode current collector, and a separator and electrolyte layer disposed between the negative electrode and the positive electrode; wherein a first of the cell units comprises an electrode with a thermal conductance higher than the thermal conductance of an electrode of the same polarity in a second of the cell units.

It should be understood that the terms “first” and “second” in “a first [second] of the cell units” serve only to distinguish the two cell units from another, and do not imply anything about the position of the cell units in the cell stack.

As used herein, a “cell unit” consist of a single anode, a single cathode and a separator. A “cell stack” consists of multiple cell units stacked vertically and/or horizontally. Multiple cell units or multiple cell stacks may be used in conjunction to form a battery.

The present invention looks at mixed current collector types to improve the thermal and volumetric energy density properties compared to aluminium current collectors only, and also providing some of the benefits from the aluminium current collectors in terms of weight and cost.

Although it is known for anode current collectors and cathode current collectors in a cell to made of different materials, eg US20140205882A1 proposes a cell in which the anode current collector is copper foil and the cathode current collector is aluminium foil, there is no suggestion in the prior art of providing anode current collectors of two different materials, or providing cathode current collectors of two different materials, in a cell stack.

Sodium ion batteries can utilise a low cost aluminium or aluminium alloy current collector on both the anode and cathode side of the cell. This is not the case in lithium ion batteries where copper is typically used as a current collector. In lithium ion batteries aluminium can alloy with the lithium at voltages less than 0.3V vs Li, however this is not the case with sodium ions where aluminium and sodium do not alloy. Although aluminium offers a lower cost current collector the heat conduction and the electronic conduction is much reduced compared to copper, and heat transfer out of the cells may be an issue, in particular for high power or large format cells.

By incorporating at least one additional high heat conductance current collector into a cell stack for, for example, a sodium ion battery, the heat distribution and heat dissipation in the cell stack can be improved.

None of the prior art documents mentioned above which address the issue of thermal energy transfer within a lithium-ion cell suggest the use of more than one type of current collector on the anode electrodes.

Table 1 shows results from a simple 1D model dissipating heat from the centre of the electrode in an example 0.5Ah sodium ion pouch cell. The thermal conductivity for aluminium is 205W/mK and for copper is 385W/mK. In the example, the aluminium foil thickness is 20um, the results for the copper are shown for thicknesses of 20um and 12um.

Equation 1

Using Equation 1 and calculating the heat dissipation for a rate equivalent to 0.5W the inner temperatures are significantly higher for the aluminium current collector compared to the copper, even with the thinner copper foil thickness of 12pm.

This is a simple model which illustrates the issue of heat conductivity as a potential problem in switching to an aluminium anode current collector which may be solved with the inclusion of at least one higher thermal conductivity current collector in the stack. As either the cell stack capacity or the rate at which the cell stacks are operated is increased this difference in thermal dissipation between copper and aluminium may become very important in the cell stacks, high temperatures inside the cells may cause problems with thermal degradation of the materials leading to capacity loss and detrimental implications for the cell stack operation and safety.

Layers with increased thermal conductivity within the cell stack can also dissipate thermal energy away from the tabs where electrical connections to the electrodes pass through the laminated packaging material. These tabs are only part of the width of the pouch in order to pass through the pouch material and maintain a high quality seal to the surroundings. Therefore the current density in these regions is greater than in the rest of the cell and can lead to significant increases in temperature if the thermal dissipation is not sufficient.

Table 1

The electrode in the first cell unit may comprise a current collector layer with a higher thermal conductance than the current collector layer of the electrode of the same polarity in the second cell unit.

The current collector layer of the electrode in the first cell unit may comprise a material having a thermal conductivity higher than the thermal conductivity of the current collector layer of the electrode of the same polarity in the second cell unit.

The current collector layer of the electrode in the first cell unit may have a cross-sectional area greater than the cross-sectional area of the current collector layer of the electrode of the same polarity in the second cell unit. For example it may have a thickness greater than the thickness of the current collector layer of the electrode of the same polarity in the second cell unit.

The first unit cell may be disposed within an interior of the cell structure. As noted above the temperature at the centre of a cell may be significantly higher than the temperature of the outer layers of the cell, for example when a cell is discharged at a high rate. Providing the unit cell with the high thermal conductance electrode within the cell structure, for example at or near the centre of the cell structure provides effective cooling of the centre of the cell structure.

The first and second cell units may be laterally spaced, or they may be vertically stacked. A cell may comprise a plurality of first cell units and a plurality of second cell units, each of the first cell units comprising an electrode with a thermal conductance higher than the thermal conductance of an electrode of the same polarity in the second cell units; and, in the interior of the cell structure, the cell units may be arranged such that a first cell unit is disposed between a first cell unit and a second cell unit. Similarly, in the interior of the cell structure, the cell units may be arranged such that a second cell unit is disposed between a second cell unit and a first cell unit. The units cells are then arranged in the sequence Η H L L Η H etc. (where H is a unit cell with a high thermal conductance electrode (a first unit cell) and L is a unit cell without a high thermal conductance electrode (a second unit cell)). Such a sequence of unit cells may be readily implemented in a cell stack having double-sided anodes, that is where an anode active material is disposed on both sides of an anode current collector, by alternating between a high thermal conductivity anode collector and a lower thermal conductivity anode collector. (It will be understood that cell units at or near the boundaries of the cell stack may not be arranged in this sequence, either through constraints on the layer structure or through a choice to concentrate unit cells with a high thermal conductance electrode in the interior of the cell stack.) A cell stack may comprise a plurality of first cell units and a plurality of second cell units, each of the first cell units comprising an electrode with a thermal conductance higher than the thermal conductance of an electrode of the same polarity in the second cell units; and the cell units may arranged such that the first cell units are preferentially disposed within the interior of the cell stack and the second cell units are preferentially disposed near the upper and lower boundaries of the cell stack.

The first unit cell(s) may comprise a copper anode collector layer and the second unit cell(s) may comprise an aluminium anode collector layer.

The first unit cell(s) may comprise an aluminium anode collector layer and the second unit cell(s) may comprise a stainless steel aluminium anode collector layer.

The cell stack may be a sodium ion cell stack, although the invention is not limited to use with sodium ion cells.

The present invention may be applied to any type of cell, so that a cell of the invention may for example comprise a pouch cell or a wound cell. One or more cells of the invention may be incorporated into a battery.

BRIEF DESCRIPTION OF DRAWINGS

Figure 1 shows the layers contained in a typical metal-ion pouch cell stack including a sequence which can be repeated to increase the number of layers in the cell stack and therefore the capacity of the cell stack.

Figure 2 shows the layers contained in a cell stack according to an example of the present invention with two aluminium anode current collector layers (2a) towards the outside of the cell stack and one copper current collector layer (2b) towards to the centre of the cell stack.

Figure 3 shows the layers contained in a cell stack according to an example of the present invention with a repeating unit for increasing the number of layers and therefore the capacity of the cell stack. Within the repeating unit there is one aluminium anode current collector (2a) and one copper anode current collector (2b).

Figure 4 shows the layers contained in a cell stack according to an example of the present invention with a repeating unit for increasing the number of layers and therefore the capacity of the cell stack. This demonstrates the use of another possible combination of current collector materials for use with the present invention. Within the repeating unit there is one aluminium anode current collector (2b) and one stainless steel anode current collector (2a).

Figure 5 shows a cell stack incorporating both an aluminium anode current collector (2a) and a copper anode current collector (2b) which may be wound to produce a jelly-roll or prismatic cell.

Figure 6 shows an example of a wound electrode stack.

Figure 7 shows a stack of layers incorporating both an aluminium anode current collector (2a) and a copper anode current collector (2b) which may be wound to produce a jelly-roll or prismatic cell. In this case the aluminium anode current collector layer (2a) and the copper anode current collector layer (2b) are adjacent to one another laterally forming a single layer within the stack.

Figure 8 shows the layers contained in a cell stack according to an example of the present invention with a repeating unit for increasing the number of layers and therefore the capacity of the cell stack. This demonstrates the use of another possible combination of current collector materials for use with the present invention. Within the repeating unit there are two different thickness of anode current collector with the anode current collector layers (2b) having a thickness greater than the anode current collector layers (2a).

DETAILED DESCRIPTION OF INVENTION

Figure 1 shows the basic structure of an example of a sodium ion cell stack to which the present invention may be applied. The labelled repeating unit of additional double sided coated cathode electrode (layers (5) and layer (6)), separator (4) and double sided coated anode electrode (layers (3) and layer (2)) may be repeated multiple times to build up a higher capacity cell stack.

Example 1

In a first embodiment of the present invention the cathode coatings (5) were formed by mixing nickel based sodium layered oxide material with small quantities of a carbon black conductive additive and a PVDF/CTFE copolymer binder with NMP solvent. This slurry was cast onto carbon coated aluminium which forms the cathode current collector (6) and dried before the slurry was also cast on the opposite side. These electrodes were then vacuum dried, cut and calendered (rolled) before use in the cell stack. The conductive carbon additive was C65 from TimCal. The ratio of the components was 87% active material, 6% binder and 5% conductive additive.

The anode coatings (3) were formed by mixing a disordered carbon with NMP, PvDF binder and a carbon black conductive additive, and casting this slurry onto both a carbon coated aluminium anode current collector (2a) and a copper foil anode current collector (2b). The ratio of the components was 90% active material, 5% binder and 5% conductive additive. For the double sided coatings, the coating was dried and the casting repeated on the opposite side, the coatings were then cut and vacuum dried before being assembled into cells.

The cells stacks were formed by z-folding a polypropylene separator material (4) between the layers. The cathode current collector layers (6) were welded together ultrasonically with tabbing material, the anode current collector layers (2) were similarly welded together ultrasonically with tabbing material. This stack was placed in a formed pouch of laminated aluminium (1). An electrolyte consisting of a 1M solution of NaPF6 in an organic solvent mix of EC:PC:DEC (ethylene carbonate, propylene carbonate and diethyl carbonate) was added to the cell stack which was subsequently vacuum sealed.

The cells stack consists of the sequence as shown in Figure 2. Layers 2a which denote the outer anode current collectors are formed from aluminium but the central anode current collector layer 2b is formed from copper. Use of copper for at least one anode current collector layer 2b provides a high thermal conductivity anode current collector that in turn provides improved dissipation of heat generated within the cell stack during operation of the cell stack compared to a cell stack having every anode current collector layer made of aluminium, while the use of aluminium for other anode current collector layer 2a reduces the cost and weight of the cell compared to a cell stack having every anode current collector layer made of copper.

Example 2

Figure 3 shows a cell stack manufactured in the same process as described in the first example but shows a repeating unit including the aluminium anode current collector layer 2a and the copper current collector layer 2b. In this example the metal composition of the current collector for the anode electrodes alternates between copper and aluminium. It is not the intention of the present invention to limit the scope of the pattern in which the different current collectors are deployed. Other patterns with more infrequent use of the higher thermal conductivity current collectors are included in the invention as is the use of a pattern in which there is not a simple repeating unit through the stack. For example it may be advantageous to cluster the high thermal conductivity current collector layers close to the centre of the cell stack.

Example 3

Although the above examples have used copper and aluminium as the current collectors, the present invention is not limited to these metals. It is intended that any suitable material may be used for the current collectors, for example one material that has a high thermal conductivity and one material that is cheap and preferably has a low density so as to minimise the cell stack weight. The materials used for the current collectors may also be alloys. As an alternative to being a pure metal or an alloy the current collectors may also have their own structure, for example an aluminium foil may have a carbon coating or more than one material may have been used to make a bimetallic foil; other possible layered or three dimensional featured structures are also incorporated into the present invention.

Moreover, the invention does not require that the high thermal conductivity current collector is made of a different material to other conductivity current collector, and current collectors of different thermal conductivities may be obtained with current collectors having the same material composition but different structures. A simple example of a structural difference between the two types of current collector for one of the electrode types may be a difference in thickness of the current collector (2). Figure 8 shows a schematic in which one anode current collector layer (2b) has greater thickness, and hence a greater cross-sectional area and a greater thermal conductance, than another anode current collector layer (2a). Typically the thicker current collector layer (2b) may have a thickness of between twice and five times the thickness of the thinner current collector layer (2a), and therefore have a thermal conductance of between two and five times the thermal conductance of the thinner current collector layer (2a). The key feature being that at least one of the cathode or anode electrodes has more than one type of current collector, that is has at least one higher thermal conductance current collector (2b) and at least one lower current collector (2a). This allows the optimization of cost, energy density, thermal distribution and other factors for the cell stack according to the needs of the application.

Where current collectors of two different materials are used, the invention is not limited to the use of copper and aluminium and other materials may be used. For example figure 4 shows an example where the anode current collectors are a mixture of aluminium and stainless steel. Stainless steel has the advantage of being low cost. In this case the aluminium is the higher thermal conductivity layer (2b) and the stainless steel is the low conductivity layer (2a).

Example 4

The present invention is not intended to be limited to sodium ion batteries. It is applicable to other metal-ion cell stack systems for example but not limited to lithium-ion, aluminium-ion and magnesium-ion cell stacks. (It should be noted that the embodiment of fig 3 could not be applied to a Li-ion cell stack, since an Al anode current collector is not suitable for use in a lithium ion cell as explained above. However, other embodiments of the invention may be applied in a lithium cell stack, for example by using a thicker copper foil for one of the anode current collector layers in a lithium-ion cell stack and using a thinner copper foil for another of the anode current collector layers in the cell stack.)

Example 5

The present invention is not limited to the pouch cell format as described in the previous examples. It can also be extended to other formats. For example a wound format for a jelly roll or prismatic cell. Figure 5 shows an example of stack which may be wound as demonstrated in Figure 6.

Furthermore, it is possible to change the type of current collector, and hence change the thermal conductance of the current collector, laterally along the sheet within a layer as shown in Figure 7 as well or instead of changing the type of current collector used in different layers. (It should be noted that Figure 7 does not show the complete layer structure, and only shows layers in the vicinity of the current collector layer having a change in type/composition.) The lateral connection between the different current collectors may be formed by any suitable method. For example, during the reel-to-reel electroplating manufacture of the current collector foil (2) the composition of the foil may be compositionally modulated as described in US6344123B1. This modulation can occur at the interval required to produce the length needed to form the final rolled battery. This change may be gradual or abrupt. This results in a layer (2) as in Figure 7 with either a gradual or abrupt change between a high conductivity composition and a lower conductivity composition. This foil may then be coated with an appropriate electrode material as described in example 1. This embodiment may be particularly applicable to a wound design of cell where the high thermal conductivity current collector can then be used in the centre of the roll.

Other cell designs incorporating the present invention but not stated here are also included within the scope of the invention. It is also not intended to limit the scope of the components of the cell stack other than the current collectors to those given in the above examples. The electrolyte may be in the form of a liquid, polymer or gel. The anode material may be a form of carbon, such as hard carbon (amorphous carbon), graphite, graphene, or other forms or mixtures of carbon. The anode may also comprise a metal or metal containing for example lithium, sodium, aluminium, magnesium, tin, antimony, germanium or silicon. The anode may be a composite or nanostructured material. The cathode material may comprise layered oxides, silicates, sulfates, fluorophosphates or phosphates. The cathode may be a composite or nanostructured material.

Although the invention has been shown and described with respect to a certain embodiment or embodiments, equivalent alterations and modifications may occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described elements (components, assemblies, devices, compositions, etc.), the terms (including a reference to a "means") used to describe such elements are intended to correspond, unless otherwise indicated, to any element which performs the specified function of the described element (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein exemplary embodiment or embodiments of the invention. In addition, while a particular feature of the invention may have been described above with respect to only one or more of several embodiments, such feature may be combined with one or more other features of the other embodiments, as may be desired and advantageous for any given or particular application.

For example, the invention has been described with reference to embodiments in which the thermal conductance of the anode is different between one cell unit and another cell unit. However, the invention is not limited to this, and may additionally or alternatively be implemented by making the thermal conductance of the cathode different between one cell unit and another cell unit.

INDUSTRIAL APPLICABILITY

The invention relates to an improvement in sodium ion battery technology and may be applied for use in many different applications such as energy storage devices, rechargeable batteries and electrochemical devices. Advantageously the cell stacks according to the invention allow for a compromise between low cost aluminium current collectors and high thermal conductivity copper current collectors, allowing the balance between different factors to be optimised for applications as required depending on the cell stack size and rate at which the cell stacks need to be operated.

Claims (12)

1. A metal ion cell stack comprising a plurality of cell units, wherein each of the cell units comprises: a negative electrode comprising a layer of negative electrode active material disposed over an anode current collector, a positive electrode comprising a layer of positive electrode active material disposed over a cathode current collector, and a separator and electrolyte layer disposed between the negative electrode and the positive electrode; wherein a first of the cell units comprises an electrode with a thermal conductance higher than the thermal conductance of an electrode of the same polarity in a second of the cell units.

2. A metal ion cell stack as claimed in claim 1 wherein the electrode in the first cell unit comprises a current collector layer with a higher thermal conductance than the current collector layer of the electrode of the same polarity in the second cell unit.

3. A metal ion cell stack as claimed in claim 2 wherein the current collector layer of the electrode in the first cell unit comprises a material having a thermal conductivity higher than the thermal conductivity of the current collector layer of the electrode of the same polarity in the second cell unit.

4. A metal ion cell stack as claimed in claim 1, 2 or 3 wherein the current collector layer of the electrode in the first cell unit has a cross-sectional area greater than the cross-sectional area of the current collector layer of the electrode of the same polarity in the second cell unit.

5. A metal ion cell stack as claimed in claim 4 wherein the current collector layer of the electrode in the first cell unit has a thickness greater than the thickness of the current collector layer of the electrode of the same polarity in the second cell unit.

6. A metal ion cell stack as claimed in any preceding claim wherein the first unit cell is disposed within an interior of the cell structure.

7. A metal ion cell stack as claimed in claim 1 - 6 wherein the first and second cell units are laterally spaced.

8. A metal ion cell stack as claimed in claim 1 - 6 wherein the first and second cell units are vertically stacked.

9. A metal ion cell stack as claimed in claim 8 and comprising a plurality of first cell units and a plurality of second cell units, each of the first cell units comprising an electrode with a thermal conductance higher than the thermal conductance of an electrode of the same polarity in the second cell units; wherein, in the interior of the cell structure, the cell units are arranged such that a first cell unit is disposed between a first cell unit and a second cell unit.

10. A metal ion cell stack as claimed in claim 8 and comprising a plurality of first cell units and a plurality of second cell units, each of the first cell units comprising an electrode with a thermal conductance higher than the thermal conductance of an electrode of the same polarity in the second cell units; wherein the cell units are arranged such that the first cell units are preferentially disposed within the interior of the cell structure and the second cell units are preferentially disposed near the upper and lower boundaries of the cell structure.

11. A metal ion cell stack as claimed in any preceding claim wherein the first unit cell(s) comprise a copper anode collector layer and the second unit cell(s) comprise an aluminium anode collector layer.

12. A metal ion cell stack as claimed in any one of claims 1 to 10 wherein the first unit cell(s) comprise an aluminium anode collector layer and the second unit cell(s) comprise a stainless steel aluminium anode collector layer.