WO2016075465A1

WO2016075465A1 - Electrode structure and method of manufacture thereof

Info

Publication number: WO2016075465A1
Application number: PCT/GB2015/053427
Authority: WO
Inventors: Patrick Grant; Chun Huang
Original assignee: Isis Innovation Limited
Priority date: 2014-11-14
Filing date: 2015-11-11
Publication date: 2016-05-19
Also published as: GB201420309D0; GB2532275A; CN107112143B; CN107112143A; EP3218948A1

Abstract

An electrode structure comprises a conductive substrate or current collector and a first layer comprising a first electrochemically active material, the first active material being characterized by one or more first electrochemical properties. A second layer comprises a second electrochemically active material, the second active material being characterized by one or more second electrochemical properties, at least one of which is different from said one or more first electrochemical properties. The first and second layers are provided at predefined locations within the electrode structure based on the respective first and second electrochemical properties of the first and second electrochemically active materials to maximise the performance of each of the first and second layers. A method of fabricating such an electrode structure is also provided.

Description

Electrode Structure and Method of Manufacture Thereof

The present invention relates to the manufacture of electrode structures for electrochemical energy storage devices such as batteries, e.g. lithium ion batteries, and supercapacitors and fuel cells.

The invention also relates to the manufacture of electrochemical energy storage devices such as batteries, in particular lithium ion batteries, supercapacitors and fuel cells.

The invention also relates to electrode structures for electrochemical energy storage devices such as batteries, e.g. lithium ion batteries, and supercapacitors and fuel cells. In addition, the invention relates to electrochemical energy storage devices such as batteries, in particular lithium ion batteries, supercapacitors and fuel cells.

Batteries e.g. lithium ion (Li-ion) batteries are widely used as storage devices due to their low cost and useful high energy density (i.e . good capability for storing energy). However, batteries have a low power density meaning the energy they can provide per unit time is limited. Supercapacitors, by comparison, have a lower energy density and high power density, meaning they can be charged quickly and do not lose their storage capabilities over time, but the amount of energy per unit weight that they can store is relatively low. In addition, due to the amount of energy per unit weight that they can store, they are usually more costly to manufacture per unit energy. Supercapacitors and batteries can be used together in hybrid systems, complementing each other to meet peak power demands. However, the power density of solid-state superconductors is undermined due to the limited number of mobile ions and their low mobilities that lead to slow charging/discharging kinetics.

It is known to use slurry casting to make electrodes for storage devices such as batteries. Slurry casting is a well-established, mature, stable and automated manufacturing methodology for large volume manufacture. Electrodes manufactured by slurry casting are monolithic. They are intended to, and usually have, the same microstructure from point to point. Generally, they also have a uniform porosity - the electrodes must have some porosity to allow, in use, an electrolyte to permeate the electrode. The current slurry casting manufacturing route has been optimised for industrial application but offers very restricted scope to tailor or design the through thickness micro or meso structure of the electrode: it is essentially the same at all places. It is well known for electrodes in Li-ion batteries, other types of batteries and supercapacitors to be formed of a mixture of materials with a monolithic composition i.e. the local composition is the same at all locations in the component. For example, the negative electrode of Li-ion batteries usually uses a mixture of a lithium host metal oxide, a binder and an electrically conducting carbonaceous material. The fractions of these constituents, and the local porosity, are essentially constant everywhere in the component.

As a development on this, WO2014/147419 (the entire contents of which are hereby incorporated by reference) provides a method of manufacture of an electrode having a non-uniform microstructure or mesostructure comprising providing a first suspension containing a first electrode material and a second suspension containing a second electrode material, delivering the first and second suspension to a spraying means, operating the spraying means for a period of time to spray the first and second suspension on to a substrate located a distance from the spraying means, and controlling the mass flow rate of the first electrode material and/or the second electrode material to and/or through the spraying means as a function of time for at least a portion of the period of time. This enables an electrode having a non-uniform microstructure or mesostructure to be produced. Since the lithium ion diffusion rate through the electrode varies, the lithium-host metal oxide may not be utilised efficiently in some regions - especially furthest from the separator between the anode and the cathode. This is even more pronounced with thicker electrodes. It results in a relatively low power per weight or per unit volume of the electrode or device, and is a waste of costly materials. Strategies to enable a more homogeneous distribution of lithium ions would be beneficial.

Presently available energy storage devices such as batteries and supercapacitors thus suffer from several drawbacks and limitations. The present invention has been devised with the foregoing in mind. According to a first aspect of the present invention there is provided an electrode structure comprising a conductive substrate or current collector, a first layer comprising a first electrochemically active material, the first active material being characterized by one or more first electrochemical properties, and a second layer comprising a second electrochemically active material, the second active material being characterized by one or more second electrochemical properties, at least one of which is different from said one or more first electrochemical properties. The first and second layers are preferably provided at predefined locations within the electrode structure based on the respective first and second electrochemical properties of the first and second electrochemically active materials to maximise the performance of each of the first and second layers.

Advantageously, the invention provides for the design and ability to make a multifunctional electrode comprising discrete layers, where each layer of material is chosen according to a rational design and placed selectively within the electrode structure to maximise the inherent material benefits and thus improve overall performance . This may also enable the use of more expensive materials to be controlled, limited and/or minimised. The electrode structure may further comprise a proton conducting membrane or separator.

In an embodiment, the electrode structure further comprises a third layer of high electrically conducting material, which typically may comprise carbon. The third layer may be or comprise graphene. Preferably, the layer of high electrically conducting material, e.g. graphene, may have a thickness of approximately at least l Onm, or at least 50nm, or at least l OOnm, or at least 150nm, or at least 175 nm. Preferably, the layer of high electrically conducting material, e.g. graphene, may have a thickness of approximately up to 400nm, or up to 300nm, or up to 300nm, or up to 250nm, or up to 225nm. Optionally or preferably, the thickness of the layer of high electrically conducting material, e.g. graphene, may be approximately 200nm.

In an embodiment, the first layer is adjacent the substrate, the second layer is adjacent the first layer and the second layer is adjacent the proton conducting membrane or substrate. Where present, the third layer may be located between the substrate and the first layer. Alternatively, the conductive substrate may be a third layer of high electrically conducting carbon material or graphene located adjacent the first layer.

The electrode structure may comprise one or more additional layers each comprising an electrochemically active material that is characterized by one or more electrochemical properties, at least one of which is different from said one or more first and/or second electrochemical properties.

According to another aspect of the invention, there is provided a supercapacitor comprising the electrode structure described above. The thickness of the first layer may be greater than that of the second layer, or the thickness of the first layer may be equal to or smaller than that of the second layer. The thickness of the first layer may be approximately in the range of 200 nanometres to 200 micrometres, and is preferably approximately 32 micrometres. The thickness of the second layer may be approximately in the range 200 nanometres to 190 micrometres, and is preferably approximately 650 nanometres.

According to another aspect of the present invention, there is provided a battery comprising the electrode structure described above. The thickness of the first layer may be equal to, or greater or smaller than that of the second layer. Two or more layers may be fabricated for the anode or the cathode . The thickness of the first or second or any additional layer may be equal to, greater than or smaller than the other layers. The range of thickness for each layer is in the range from approximately 10 micrometres to 200 micrometres.

According to a further aspect of the present invention there is provided a method of fabricating an electrode structure, the method comprising the steps of providing a conductive substrate or current collector, providing a first layer comprising a first electrochemically active material, the first active material being characterized by one or more first electrochemical properties, at a predefined location within the electrode structure based on the first electrochemical properties to maximise the performance of the first layer, and providing a second layer comprising a second electrochemically active material, the second active material being characterized by one or more second electrochemical properties, at least one of which is different from said one or more first electrochemical properties, at a predefined location within the electrode structure based on the second electrochemical properties to maximise the performance of the second layer.

Fabrication preferably may be achieved by a layer-by-layer processing technique, e.g. a layer-by-layer processing technique comprising spraying. As such, aspects of the invention provide a method of fabricating discrete layered structured components that improve electrical storage performance for example in batteries, electrochemical supercapacitors and fuel cells. Aspects and embodiments of the invention provide for multiple layers to be built using a large variety of material combinations to make multi-functional components to achieve best performances for different applications.

Advantageously, the processing technique may provide for building electrodes layer- by-layer rather than casting monolithic electrodes. Furthermore, by using multiple sprayheads, the materials for individual layers are flexible to be changed.

The method may further comprise providing an ionic conducting membrane or a separator such as a proton or lithium conducting membrane. It may also comprise providing a third layer of high electrically conducting carbon material or graphene. The third layer may have a thickness of approximately at least l Onm, or at least 50nm, or at least l OOnm, or at least 150nm, or at least 175nm. The third layer may have a thickness of approximately up to 400 nanometres, or up to 300nm, or up to 300nm, or up to 250nm, or up to 225nm. Optionally or preferably, the thickness of the third layer may be approximately 200nm. In an embodiment, the first layer is provided adjacent the substrate, the second layer is provided adjacent the first layer and the second layer is provided adjacent the ionic conducting membrane or separator and, when present, the third layer is provided between the substrate and the first layer. Alternatively, the conductive substrate is a third layer of high electrically conducting carbon material or graphene provided adjacent the first layer.

One or more additional layers may be provided each comprising an electrochemically active material that is characterized by one or more electrochemical properties, at least one of which is different from said one or more first and/or second electrochemical properties. According to a further aspect of the invention, there is provided a method of forming a supercapacitor comprising the steps described above . The thickness of the first layer may be greater than that of the second layer, or the thickness of the first layer may be equal to or smaller than that of the second layer. The thickness of the first layer may be approximately in the range from 200 nanometres to 200 micrometres and is optionally or preferably 32 micrometres. The thickness of the second layer may be approximately in the range from 200 nanometres to 190 micrometres and is optionally or preferably 650 nanometres. According to another aspect of the present invention, there is provided a method of forming a battery comprising the steps described above. The thickness of the first layer may be equal to, or greater or smaller than that of the second layer. Preferably, two or more layers are fabricated for the anode or the cathode. The thickness of the first or second or any additional layer may be equal to, greater than or smaller than the other layers. The thickness for each layer may be from approximately 10 micrometres and/or up to 200 micrometres.

In embodiments for the electrode structure, the supercapacitor, the battery or the method(s) described above, one of the first and second layers may comprise a material having a first density and/or porosity and the other of the first and second layers may comprise a material having a second density and/or porosity that is greater than the first density and/or porosity. The said one of the first and second layers may be a porous or mesoporous material and the said other of said first and second layers may be a non-porous or substantially non-porous material. The said one of the first and second layers may be or comprise a composite having a high surface area and/or an open structure and the said other of the first and second layers may be or comprise a composite having a significantly lower surface area and/or a closed or substantially closed structure. The said one of the first and second layers may be or comprise a composite of high weight percent electrochemically active material and said other of the first and second layers may be or comprise a composite of low or equal weight percent electrochemically active material. The first and/or second layers may be or comprise a metal oxide. The said one of the first and second layers may be or comprise mesoporous Ti0₂ and said other of the first and second layers may be or comprise Ti0₂. One or both of said first and second layers may also comprise a high aspect ratio material, e.g. a one-dimensional material such as micro- or nano-wires or micro- or nano-tubes or carbon nanotubes. The density and/or porosity of the first layer may be greater than the density and/or porosity of the second layer, or the density and/or porosity of the first layer may be less than the density and/or porosity of the second layer.

Aspects and embodiments of the invention may provide for tailoring the material porosity for a specific purpose, and grading with layers. This further provides for heterogeneity within the layers and/or electrode structure, chosen and tailored depending on the required end product.

In order that aspects and embodiments of the invention may be well understood, it will now be described by way of example only with reference to the accompanying drawings, in which: Figure 1 shows schematically one half of a cell of a supercapacitor according to an embodiment;

Figure 2 shows schematically one half of a cell of a supercapacitor according to another embodiment;

Figure 3 shows schematically one half of a cell of a lithium ion battery according to another embodiment;

Figure 4 shows schematically an exploded view of one half of a cell of a supercapacitor according to the embodiment of Figure 2;

Figure 5 shows an apparatus for carrying out a method according to an embodiment of the invention;

Figure 6a shows cyclic voltammogram curves of a solid-state supercapacitor according to an embodiment;

Figure 6b shows cyclic voltammogram curves of a solid-state supercapacitor according to another embodiment;

Figure 6c shows comparison cyclic voltammogram curves of a known solid-state supercapacitor with monolithic composite electrodes of p-TiC and c-TiC + MWNTs + ionomer;

Figure 6d shows the normalised cyclic voltammetry peak current of a solid-state supercapacitor according to a known monolithic electrode;

Figure 6e shows the normalised cyclic voltammetry peak current of a solid-state supercapacitor according to the three-layer electrode embodiment of Figure 2; Figure 7a shows a Nyquist plot of a solid-state supercapacitor according to an embodiment;

Figure 7b shows a Nyquist plot of a solid-state supercapacitor according to another embodiment;

Figure 8a shows a cyclic voltammogram curve of a solid-sate supercapacitor according to an embodiment;

Figure 8b shows cyclic voltammogram curves of a solid-sate supercapacitor according to another embodiment;

Figure 9a shows galvanostatic charge/discharge curves of a solid-state supercapacitor according to an embodiment;

Figure 9b shows galvanostatic charge/discharge curves of a solid-state supercapacitor according to another embodiment;

Figure 10 shows a Ragone plot of power density against energy density showing the performance of a solid-state supercapacitor according to an embodiment compared with known examples;

Figures l l i-iii and iv-vi show galvanostatic discharge/charge plots of potential against capacity for three discharge/charge cycles of a lithium ion battery electrode manufactured in accordance with an embodiment of the invention at 0. 1 C;

Figures 12i-iii and iv-vi show volumetric discharge/charge plots of potential against capacity for three discharge/charge cycles of a lithium ion battery electrode manufactured in accordance with an embodiment of the invention at 0. 1 C;

Figures 13 i-ii i and iv-vi show galvanostatic discharge/charge plots of potential against capacity for three discharge/charge cycles of a lithium ion battery electrode manufactured in accordance with an embodiment of the invention at 1 C; and

Figures 14i-iii and iv-vi show volumetric discharge/charge plots of potential against capacity for three discharge/charge cycles of a lithium ion battery electrode manufactured in accordance with an embodiment of the invention at 1 C.

Figure 1 shows schematically one half 10 of a supercapacitor cell. The full cell comprises a similar or identical second half of a supercapacitor cell (not shown) but which would be provided in a mirror-image fashion on the opposite side of the line or plane of symmetry 12. The cell 10 comprises a first conducting current collector 14, which carries an electrode 16. The electrode 16 comprises two layers. A first layer 18 adjacent the current collector 14 is a layer comprising a composite of a high weight percent (wt%) electrically conducting carbonaceous material and a low wt% electrochemically active metal oxide to provide a high double layer charge storage. A second layer 20 adjacent the first layer 18 comprises a composite of a high wt% high surface area porous electrochemically active metal oxide and a low wt% electrically conducting carbonaceous material. This layer 20 is for catalysing redox reactions between the electrode 16 and electrolyte 42. A proton (H⁺) conducting membrane 22 is provided adjacent the second layer 20. The mirror-image set of layers forming the other half of the cell (not shown) comprises a similar electrode and terminates in a second conducting current collector. The electrode 16 shown in Figure 1 may be an anode or a cathode, with the electrode in the other half of the cell (not shown) being the respective cathode/anode.

Figure 2 shows schematically one half 30 of a supercapacitor cell. As for Figure 1 , the full cell comprises a similar or identical second half of a supercapacitor cell (not shown) but which would be provided in a mirror-image fashion on the opposite side of the line or plane of symmetry 32. Also similarly to Figure 1 , the cell 30 comprises a first conducting current collector 34, which carries an electrode 36. The electrode 36 comprises two layers. A first layer 38 adjacent the current collector 34 is a layer comprises a composite of a high wt% electrically conducting carbonaceous material and a low wt% electrochemically active metal oxide to provide a high double layer charge storage. A second layer 40 adjacent the first layer 38 comprises a composite of a high wt% high surface area electrochemically active metal oxide and a low wt% electrically conducting carbonaceous material. This layer 40 is for catalysing redox reactions between the electrode 36 and electrolyte 42. A proton (H⁺) conducting membrane 42 is provided adjacent the second layer 40. An additional (third) layer 44 comprising a material of high electrically conductive material is provided between the current collector 34 and the first electrode layer 38. Layer 44 can either be included as an additional layer in electrode 36, or layer 44 can be considered as a surface treatment on the current collector 34. The mirror-image set of layers forming the other half of the cell (not shown) comprises a similar triple-layer electrode and terminates in a second conducting current collector. The electrode 36 shown in Figure 1 may be an anode or a cathode, with the electrode in the other half of the cell (not shown) being the respective cathode/anode. As will be discussed in greater detail below, the resulting structure, as shown in Figure 2, couples both high energy and power densities by designing a suitable material composition in the right layer at the right place within the structure. Figure 3 shows schematically one half 50 of a cell of a lithium ion battery. The full cell comprises a similar second half of a battery cell (not shown) but which would be provided in a mirror-image fashion on the opposite side of the line or plane of symmetry 52. The half battery cell 50 comprises a first conducting current collector 54, which carries an anode 56. The anode 56 comprises two layers. A first layer 58 adjacent the current collector 54 is a composite of a high wt% high power density electrochemically active metal oxide and a low wt% electrically conducting carbonaceous material. A second layer 60 adjacent the first layer 58 comprises a composite of a high wt% high energy density electrochemically active metal oxide and a low wt% electrically conducting carbonaceous material. A separator 62 comprising lithium (Li⁺) ions is provided adjacent the second layer 60. The mirror-image set of layers forming the other half of the cell (not shown) comprises a similar electrode and terminates in a second conducting current collector. The electrode 56 shown in Figure 3 may be an anode or a cathode, with the electrode in the other half of the cell (not shown) being the respective cathode/anode .

Although the example shown in Figure 3 is for a Li-ion battery, it will be appreciated that the present invention may be employed in other batteries such as Li-air, Na-air, Na-S batteries etc.

For the supercapacitor embodiments of Figures 1 and 2, the first electrode layer 18, 38 comprises commercially available, non-porous titanium dioxide or titania (c-Ti0₂) and carbon nanotubes (CNTs), preferably multi-walled nanotubes (MWNTs). The c-Ti0₂ may typically be or comprise nanoparticles approximately 30nm in size. However, the nanoparticle size may range from approximately 5 nanometres to 200 nanometres. The first layer 18, 38 might typically have a thickness of 32 micrometres, although other thicknesses are possible, for example from 200 nanometres to 200 micrometres. Such a layer 18, 38 of c-Ti0₂ and MWNTs is known to have a high electric double layer capacitance (EDLC), and is suitable for the bulk of the electrode. The thickness of this layer 18, 38 is thus greater in comparison to the second layer 20, 40 (described more below).

The second electrode layer 20, 40 comprises porous Ti0₂ (p-Ti0₂) and carbon nanotubes (CNTs), preferably multi-walled nanotubes (MWNTs). The p-Ti0₂ is or comprises laboratory manufactured crystals typically approximately 220nm in size with ~50nm diameter internal channels. However, the crystal size may range from approximately 150 nanometres to 500 nanometres with a range of diameter for the internal channels from approximately 10 nanometres to 80 nanometres. The use of p- Ti0₂ enables exploitation of the high surface area to enhance the hydroxyl group pseudo-capacitive reactions at the electrode/membrane interface . However, since p- Ti0₂ is synthesised with low availability and hence a high cost, the use of p-Ti0₂ needs to be minimised to be focussed in the locations where pseudo-capacitive reactions occur most efficiently. For this reason, the layer is preferably thin e.g. approximately 650nm although again other thicknesses are possible . For example, the range of thicknesses may span from approximately 200 nanometres to 190 micrometres.

The current collector 14, 34 may be formed of copper, or other materials such as indium tin oxide coated glass. The proton conducting membrane 22, 42 may be or comprise Nafion, or other types of proton conducting materials of a gel polymer electrolyte comprising polyvinyl alcohol (PVA) mixed with an acid such as sulfuric acid H₂S0₄ or phosphorus acid H₃P0₄, or a gel polymer electrolyte comprising polyvinylidene fluoride (PVDF)/lithium trifluoromethane sulfonate (LiTFS) with polyethylene oxide (PEO), or other types of proton conducting solid-state electrolytes such as lithium sulfide Li₂S - phosphorus pentasulfide P₂S₅ glass ceramic.

In the embodiment of Figure 2, the additional or third layer 44 is preferably graphene . The layer 44 may be fabricated by providing few-layer graphene sheets (400 S cm^"1) made by shear exfoliation in an aqueous suspension sprayed on top of the first layer. The layer 44 may also be fabricated by providing mono-layer, bi-layer or few-layer graphene sheets made by chemical vapour deposition either directly on the current collector 34 or sprayed on top of the first layer 38. The thickness of the graphene may typically be approximately 200nm. The thicknesses may however range from 10 nanometres to 400 nanometres. The reason for providing the graphene is to decrease the electric transfer resistance from the electrode to the current collector.

To show this in greater detail, Figure 4 shows schematically an exploded view of one half 30 of a cell of a supercapacitor according to the embodiment of Figure 2. The layer 44 can be seen to comprise graphene sheets. The first layer 38 of the electrode 36 comprises ionomer coated commercially available, non-porous titanium dioxide or titania Ti0₂ nanoparticles 38a and ionomer coated MWNTs 38b. The second layer 40 of the electrode 36 comprises ionomer coated, high surface porous Ti0₂ 40a and ionomer coated MWNTs 40b. Co-spraying with ionomer helps to ensure H⁺ mobility throughout the electrode .

Thus each of the embodiments of Figures 1 -4 comprises a layer of dense or non- porous material (e.g. c-Ti0₂) and a layer of less dense, more porous material (e.g. p- Ti0₂). In the embodiment of Figure 3, for use in a lithium ion battery, the constituents of the first and second layers 58, 60 of the anode 56 are effectively reversed as compared with the first and second layers 18, 20 of the supercapacitor embodiment of Figure 1. That is to say, for a battery, the first layer 58 comprises p- Ti0₂ and the second layer 60 comprises c-Ti0₂. By comparison, for a supercapacitor, the equivalent first layer 18, 38 comprises c-Ti0₂ and the equivalent second layer 20, 40 comprises p-Ti0₂. These arrangements are chosen due to the electrochemical properties and principles upon which energy storage devices such as batteries and supercapacitors operate . Importantly, however, layers of the same electrochemical properties can be utilised in each type of device, but placed differently and strategically to achieve optimal performance. Although the embodiments shown in Figures 1 -4 relate to two- or three-layered arrangements, it will be appreciated that one or more additional layers each comprising an electrochemically active material that is characterized by one or more electrochemical properties may also be present. Again, at least one of the layers may have an electrochemical property (or properties) which differ(s) from the electrochemical properties of the other layers, the layers being located specifically and strategically with respect to each other and the other components on the storage device to achieve optimal performance .

Referring to Figure 3, the separator 62 may be a polyolefin separator such as a polyethylene (PE) and polypropylene (PP), or glass microfibre separator. The component 62 may also be a lithium conducting membrane comprising a gel polymer electrolyte such as polyvinylidene fluoride (PVDF)-hexafluoropropylene (HFP) copolymer. The component 62 may also be a lithium conducting gel polymer electrolyte mixed with ceramic particles such as silicon dioxide or silica Si0₂ particles. The component 62 may also be separator coated by a gel polymer electrolyte such as PVDF-co-HFP coated PE. The component 62 may also be or comprise other types of solid-state ceramic electrolytes such as Li₂0-Al₂0₃-Si0₂-P₂0₅-Ti0₂-Ge0₂. Figure 5 shows an apparatus 70 for spray forming of electrodes for electrochemical energy storage devices as described above. The system 70 comprises one or more syringe pumps 72, which are each connected to one or more spray heads 74 by a tube 76. The spray heads 74 can spray materials either alternately or simultaneously. A distance below the spray head 74, there is a heated stage 78. Both the spray head 74 and the heated stage 78 are movable in the x- and y- directions. Either the spray head 74 or the heated stage 78 moves during the spray processing . On the stage 78, there is a substrate 80, which can be metal or alloy-based such as aluminium, copper foil, thin sheet of steel, or glass-based such as indium tin oxide (ITO) coated glass, or polymer- based such as Nafion membrane. A mask 82 is located between the spray head 74 and the substrate 80 for applications to spray patterns.

In use, the syringe pumps 72 contain a suspension containing the intended electrode material(s), e.g. p-Ti0₂, c-Ti0₂, MWNTs or equivalents as discussed above, and preferably an aqueous suspension, but may also be alcohol-based suspension or N- methyl-2-pyrrolidone (NMP)-based suspension or the application of spraying lithium ion batteries. The syringe pump 72 is operable to deliver the suspension to the spray head 74 via the tube 76. The spray head 74 sprays the suspension towards the substrate 80. The material being sprayed forms a spray cone 84. By moving the stage 78 in the x- and/or y- directions, or by moving the spray head 74 in the x- and/or y- directions, coverage of larger areas of the substrate 80 can be achieved. The stage 78 is heated, which helps to dry the deposited suspension such that the electrode material(s) are left in place . The mask 82 acts to allow the suspension to be sprayed on to certain regions of the substrate 80, but not on to others.

In order to form the solid-state supercapacitor structure of Figure 2, for example, the multiple sprayheads 72 are used consecutively to spray the various layers in position. A composite comprising a high wt% electrochemically high surface area active metal oxide and a low wt% electrically conducting carbonaceous material is sprayed onto a proton conducting membrane 42 to form the second layer 40. The spray heads 72 are then used to form layer 38, being a composite comprising one or more electrochemical active materials. A third head consecutively sprays a thin layer 44 of a high electrically conducting carbonaceous material e.g. graphene . It was found that spraying the graphene was effectively achieved by spraying an aqueous, exfoliated graphene . Typically, the thickness of the layer of graphene may be approximately 200nm, but other thicknesses are possible . For example, the range of thicknesses may be from approximately 10 nanometres to 400 nanometres. The supercapacitor structure of Figure 1 can be formed according to the same process, omitting spraying the layer of graphene 44.

In an embodiment the stage 78 is heated e.g. maintained at 100°C. This causes the fugitive water in the suspension to evaporate continuously during spraying such that the meso-porous electrodes are formed and dried simultaneously. The required layers can be sprayed onto a separator 22 or membrane 42, 62 to form the half-cell structure as shown in Figures 1 -3. The resulting structure can then be flipped and sprayed using a similar process on the other side using the same procedure to directly form a battery/supercapacitor cell with no need for subsequent re- immersion into any liquid electrolyte.

The average mass loading of each electrode was 18 mg cm^"2 with a mass deviation of 3%. The range of mass loadings is from 1 mg cm^"2 to 100 mg cm^"2. Additionally, no binders were needed for the supercapacitors and batteries during processing, allowing for easy access of ions to the high-loading electrode surface area.

Embodiments of the invention also enable more than one electrochemically active material to be added to an electrode at different places in the electrode, so as to best utilise their differing electrochemical properties to produce a device with optimum performance characteristics. For example, it is known in the art to provide a mixture of Si and C in current anodes in lithium ion batteries in a blended mixture, but it is not known in the prior art to arrange these materials at specific places in the electrode . In embodiments of the invention, the layers within the fabricated electrode component have different material compositions to maximise electrochemical utilisation of different materials, and/or improve physical properties such as ion diffusion rate of the components. The multi-functional coatings or electrodes are made where each layer can be adjusted by spray duration to assist material utilisation efficiency, and its position within the multilayer through thickness direction controlled at will.

For the lithium ion battery structure of Figure 3, again multiple sprayheads 72 can be used to spray a composite comprising a high power lithium host metal oxide (such as p-Ti0₂) and a carbonaceous material (e.g. CNTs) as layer 58 directly onto the current collector 54 (typically Cu), and a composite comprising a high energy lithium host metal oxide (such as c-Ti0₂) and a carbonaceous material (e.g. CNTs) as layer 60. The range of thickness for each layer is from approximately 10 micrometres to 200 micrometres. The design of the layer order is critical, requiring an understanding of charge storage processes within the entire component and the function of individual layers.

Furthermore, for solid state lithium ion batteries, the main problem is their slow ion mobility. Here a solid material is used to form the separator, between the anode and cathode, which must allow conduction of ions but not electrons across the separator. In most lithium ion batteries, the ion conduction takes place in a liquid electrolyte that floods the polymer separator and fills pores in the electrode . Despite the slower ion mobilities in the solid separator rather than the liquid electrolyte, solid-state batteries are attractive because of their inherent safety (no flammable electrolytes) and the design flexibility they allow. The multiple sprayheads 72 can spray a layer of ceramic solid-state electrolyte membrane (relating to 62 in Figure 3). Then another sprayhead 72 consecutively sprays a composite comprising a high wt% gel polymer electrolyte and a low wt% an electrode material e.g. graphite (relating to layer 60 in Figure 3), before the graphite electrode is sprayed on layer 60 (relating to layer 58 in Figure 3) . This single process improves ionic conduction from the solid state electrolyte to the electrode and improves adhesion at the solid state electrolyte/electrode interface .

Figure 6b shows cyclic voltammetry (CV) curves of a solid-state supercapacitor according to the embodiment of Figure 1. For comparison, Figure 6a shows the CV curves of a solid-state supercapacitor without the layer of p-Ti0₂ and CNTs. Taking the exemplary measurement at 5 mVs^"1, the results show that the capacitance increased by 28%. This is because the highly reactive layer 20 of p-Ti0₂ and CNTs contributed high pseudo-capacitance. This layer 20 of p-Ti0₂ and CNTs was designed to be placed at the strategic location of the electrode 16/electrolyte 22 interface because this is where pseudo-capacitive reaction takes place most efficiently in a solid-state supercapacitor. The results at other scan rates show similar increases in capacitance when a layer 20 of p-Ti0₂ and CNTs is included between the proton conducting membrane 22 and the first electrode layer 18. I.e . by choosing to concentrate the laboratory synthesised p-Ti0₂ at the electrode 16/membrane 22 interface only, where there was more residual H and H₂0 molecules at the membrane, and the larger surface area p-Ti0₂ is likely to have more surface -OH^" groups, a larger contribution from the pseudo-capacitance was achieved, whereas the same fraction of p-Ti02 randomly mixed in the electrode was much less effective.

The results show more prominent redox reaction peaks than those obtained from an otherwise identical arrangement using randomly mixed p-Ti0₂ and c-Ti0₂ and MWNTs in monolithic electrodes in the same fractions. To demonstrate the difference, Figure 6c shows CV curves of a solid-state supercapacitor with monolithic electrodes of p-TiC and c-TiC + MWNTs + ionomer. The elongated tail at fast scan rates above 50 mV s ¹ indicates side reactions of the monolithic electrodes above 1 .3 V. The cathodic and anodic peaks in the CV curves of Figures 6a and 6b over a range of scan rates were due to the redox activity of H⁺ and H₂0 molecules from the membrane 22 or 42 and -OH^" on the Ti0₂ surface according to:

H₂0 <→ H⁺ + OH^"

OH^" <→ H⁺ + O^2".

Figure 6e shows a linear relationship between the normalised peak current from the CV curves and the square root of scan rate of Figure 6b, and that the slope of the graph was 5 1 times higher and the intercept was 12 times higher for the embodiment of Figure 2 or Figure 1 than an otherwise identical arrangement using randomly mixed p-Ti0₂ and c-Ti0₂ and MWNTs in monolithic electrodes in the same fractions. The normalised cyclic voltammetry peak current of a solid-state supercapacitor according to a monolithic electrode is shown in Figure 6d. The slope and intercept of the graph of Figure 6e are used to assess the pseudo-capacitive contribution of an electrode material, and show that the pseudo-capacitance for the embodiment of Figure 2 or Figure 1 was higher than that for the monolithic electrode. Figure 7b shows a Nyquist plot obtained from electrochemical impedance spectroscopy of a solid-state supercapacitor according to the embodiment of Figure 2. For comparison, Figure 7a shows a Nyquist plot of a solid-state supercapacitor without the layer of graphene 44. With each plot there is also shown a magnified view of the area of intersection on the real axis. The plots show that when the graphene layer 44 was included, the equivalent series resistance or ESR (intersection on the real axis at high frequency) decreased from 28 Ω to 3.5 Ω, and the charge transfer resistance (the semicircle diameter) decreased from 42 Ω to 8.5 Ω. The graphene layer 44 was designed to be placed at the strategic position of the electrode 36, 38/current collector 34 interface to transfer electrons from the electrode 36 to the current collector 34 most efficiently. Furthermore, the ESR of solid-state supercapacitors with the three-layer electrode structure and without a current collector 34 (i.e. by connecting directly to the graphene layer 44) was -7Ω . This suggests that, if made more robust, the graphene layer 44 may directly act as a current collector. A separate current collector 34 would then not be required.

Figure 8b shows the CV curves of a solid-sate supercapacitor according to the embodiment of Figure 2. For comparison, Figure 8a shows a CV curve of a solid-sate supercapacitor without the layer of graphene 44. The results shown were obtained at extremely fast scan rates. The CV curve for the solid-state supercapacitor without the graphene layer 44 (Figure 8a) was already elongated at 500 mVs^"1, as is expected for electrodes comprising CNTs and metal oxides. Remarkably, the CV curves for the device with the graphene layer (Figure 8b) maintained the basic parallelogram shapes of EDLC even up to an extremely fast scan rate of 1000 mVs^"1. The capacitance for the device without graphene (Figure 8a) was 52 Fg^"1 (3 1 mFcm^"2) at 500 mVs^"1, compared with 94 Fg^"1 (58 mFcm^"2) with a layer of graphene 44 at the same scan rate . As shown in Table 8 (discussed below), compared with a more than 86% capacitance decrease when the scan rate increased from 500 to 1000 mV s^"1, the capacitance decreased by only 21 % from 94 F g^"1 to 74 F g^"1 with the graphene layer. At such high scan rates, redox reactions do not have time to occur. Carbon nanotube-based electrodes normally show a sharp decrease in capacitance above 200 mV s^"1 and metal oxides such as Ti0₂ tend to have to operate at even slower scan rates due to a more restricted electric conduction. As expected the CV curve without the interfacial graphene layer shows an elongated, more resistive shape and slow current changes at potential reversals at 500 mV s^"1 (Figure 8a).

Figure 9b shows a galvanostatic charge/discharge curve of a solid-state supercapacitor according to the embodiment of Figure 2 (the three layer or "3L" arrangement). For comparison, Figure 9a shows a galvanostatic charge/discharge curve of a solid-state supercapacitor without the graphene layer 44 (the two layer or "2L" arrangement). It was found that the IR drop of the solid-state supercapacitor was reduced from 0.1 V to 0.05 V after adding the graphene layer 44, showing a significant reduction in the internal resistance of the device . The non-linearity of the curves without graphene indicated a large electrode internal resistance, and the decrease in slope above 1. 15 V further showed an increase in resistance and a decrease in Columbic efficiency. The curves with the graphene layer show typical "saw-tooth" shapes, a more linear response and reasonable symmetry. This charge/discharge shape is more likely to be observed in aqueous-based supercapacitors with high ion mobility and has rarely been seen in solid-state supercapacitors because normally the ionic mobility in solid-state supercapacitors is one to three orders slower than that of liquid electrolyte-based supercapacitors.

Figure 10 shows a Ragone plot of power density against energy density showing the performance of a solid-state supercapacitor according to the embodiment of Figure 2 compared with known examples representing some of the highest performing solid- state supercapacitors. Such a plot enables the performance of various energy-storing devices to be compared. The performance of various known commercial supercapacitors is represented by the band 90. An exemplary monolithic electrode containing a random mixture of exactly the same materials in the same fractions (i.e., the fractions of these constituents, and the local porosity, are essentially constant everywhere in the monolithic electrode) is represented by point 92 lying within this band 90. It can be seen at point 94 that, by adding a layer of p-Ti0₂ and CNTs 40, there is an increase in both the energy density and power density. By further adding a graphene layer 44 (point 96), the energy density of the 3L electrode increased by 4-5 times, and the power density increased by 30-40 times over a range of scan rates, compared with a monolithic electrode containing a random mixture of exactly the same materials in the same fractions (i.e., the fractions of these constituents, and the local porosity, are essentially constant everywhere in the monolithic electrode). The 3L electrode thus yielded the highest and most well-balanced energy density of 83 Wh kg^"1 due to its highly reactive pseudo-capacitive behavior at the electrode/proton conducting membrane interface, and a maximum power density of 1 1 kW kg^"1 at 1 A g^"1 due to the highly electrical conducting layer and material added.

Figures 1 1 to 14 show the comparison of lithium ion battery performances among four types of anodes according to or similar to the embodiment of Figure 3. On the plots "a" represents a lithium ion battery anode comprising a layer 60 of c-Ti0₂ and CNTs only (i.e. without layer 58); "b" represents a lithium ion battery anode comprising a layer 58 of p-Ti0₂ and CNTs only (i.e. without layer 60); "c" represents a double layer ("2L") anode comprising both layers 58 and 60 as illustrated in Figure 3 ; and "d" represents an inverted 2L structure where layers 58 and 60 are reversed (i.e., the p-Ti0₂ and CNTs layer 58 was instead nearest to the separator 62, and the c-Ti0₂ and CNTs layer 60 was nearest to the current collector 54). The total amount of materials sprayed for each anode was kept constant.

Figures l l (i)-(iii) and (iv)-(vi) show galvanostatic discharge/charge plots of potential against capacity for three discharge and charge cycles respectively at 0. 1 C of a lithium ion battery electrode at the second cycle when the capacity became steady, manufactured according to a to d as discussed above. (It is more common in the literature to quote the capacity at the second cycle and onwards instead of the first cycle, as there are likely to be side reactions taking place during the first cycle.) The results are summarised in Table 1 below.

Table 1

These results show how the double layered anode 58, 60 of Figure 3 provides the highest gravimetric capacity for both charge and discharge cycles at a relatively slow charging and discharging rate of 0. 1 C. Embodiments with a single layer 60 of c-Ti0₂ and CNTs only, without a layer 58 of p-Ti0₂, provided the next best results.

Figures 12(i)-(iii) and (iv)-(vi) show volumetric discharge/charge plots of potential against capacity for three discharge and charge cycles respectively at 0. 1 C of a lithium ion battery electrode at the second cycle, manufactured according to a to d as discussed above. The results are summarised in Table 2 below. Volumetric capacity at "a": "b": "c": "d": first cycle (mAh cm^"3) c-Ti0₂ p-Ti0₂ 2L Inverted 2L

Discharge 115.6 52.9 649.2 138.8

Charge 113.5 35.6 309.9 135.6

Table 2

These results show how the double layered anode 58, 60 of Figure 3 provides the highest volumetric capacity for both charge and discharge cycles at a relatively slow charging and discharging rate of 0. 1 C. Embodiments with the inverted 2L structure provided the next best results overall.

Figures 13 (i)-(iii) and (iv)-(vi) show galvanostatic discharge/charge plots of potential against capacity for three discharge and charge cycles respectively at 1 C of a lithium ion battery electrode at the second cycle, manufactured according to a to d as discussed above. The results are summarised in Table 3 below.

Table 3

These results show how the double layered anode 58, 60 of Figure 3 provides the highest gravimetric capacity for both charge and discharge cycles at a relatively fast charging and discharging rate of 1 C. Embodiments with a single layer 60 of c-Ti0₂ and CNTs only, without a layer 58 of p-Ti0₂, provided the next best results overall.

Figures 14(i)-(iii) and (iv)-(vi) show volumetric discharge/charge plots of potential against capacity for three discharge and charge cycles respectively at 1 C of a lithium ion battery electrode at the second cycle, manufactured according to a to d as discussed above. The results are summarised in Table 4 below.

Table 4 These results show how the double layered anode 58, 60 of Figure 3 provides the highest volumetric capacity for both charge and discharge cycles at a relatively fast charging and discharging rate of 1 C. Embodiments with the inverted 2L structure 5 provided the next best results overall.

Table 5 below shows gravimetric and volumetric discharge and charge values over cycle 2 as above and also for cycles 3-4 at 0. 1 C.

0 Table 5

These results show how the double layered anode 58, 60 of Figure 3 provides the highest gravimetric and volumetric capacities for both charge and discharge over three cycles at a relatively fast charging and discharging rate of 0. 1 C.

Table 6 below shows gravimetric and volumetric discharge and charge values over cycle 2 as above and also for cycles 3-4 at 1 C.

Table 6

20 These results show how the double layered anode 58, 60 of Figure 3 provides the highest gravimetric and volumetric capacities for both charge and discharge over three cycles at a relatively fast charging and discharging rate of 1 C. By comparison, Table 7 below provides data for known anodes and for a graphite anode.

Table 7

The results show that capacity increased 2-3 times for a double layer (2L) anode compared with a monolithic anode for lithium ion batteries at different charging rates. For example, the experimental results show that the gravimetric and volumetric capacities of the 2L layered anode were 1 -9 times higher than that of the inverted 2L anode at different charging and discharging rates using the same materials in the same fraction.

Table 8 below summarises the capacitance of types of electrodes fabricated by an identical spraying procedure in which the thicknesses, fractions and total weight of Ti0₂, MWNTs and ionomer in monolithic, two-layer and three-layer electrodes were kept the same. All electrode variants were characterised in the same solid-state supercapacitor configuration under the same electrochemical conditions. The 2L and 3L configurations produced the highest capacitance .

Table 8

The low capacitance of the monolithic electrodes was due to a relatively low surface area and porosity when p-Ti0₂ and c-Pi0₂ were randomly mixed with the MWNTs.

In summary, aspects and embodiments of the invention are thus based on understanding the electrochemical reactions during each stage of the charge/discharge process for energy storage devices, and then designing the structure of electrodes and solid-state electrolyte membranes. Of particular interest is the interface between the solid-state electrolyte 22, 42, 62 and electrode layer 20, 40, 60, between the electrode layer 18, 38, 58 and the current collector 14, 34, 54, and between the layers within the electrodes 18, 20; 38, 40, 44; 58, 60. This enables maximisation of the efficiency of individual layer function, achieving improvements in properties such as electrochemical reactivity, ion movements, electrical conductivity, permittivity and stability for multi-functional components.

The use of Ti0₂ in the electrode layers is advantageous because it can be made in a wide range of sizes and morphologies (e.g. compared to graphite). Ti0₂ is also low- cost, non-toxic and has good environmental compatibility. In the embodiments of Figures 1 -3, each electrode 16, 36, 56 has a double layer arrangement respectively comprising layers 18, 20; 38, 40; 58; 60. In the case of supercapacitors, it is advantageous for one layer to comprise a material with a high surface area, e .g. porous Ti0₂ (p-Ti0₂) for a higher pseudo-capacitive contribution. It is advantageous if the other layer to be a commercial material, as the bulk of the electrode. This is because mesoporous materials are expensive. In the case of lithium ion batteries, it is advantageous for one layer to comprise a material with an open structure and a high surface area, e.g. porous Ti0₂ (p-Ti0₂), for a high power density. It is advantageous for the other layer to be a commercial, smaller and non-porous material, e.g. commercial Ti0₂ (c-Ti0₂). This is because mesoporous materials are expensive and the smaller non-porous material provides a higher energy density. As such, in both cases, it is advantageous to utilise a double layer electrode structure, with one layer comprising a mesoporous material and the other a denser material such as a commercially available composite . In an embodiment, each layer may be or comprise Ti0₂, but it will be appreciated that other compounds may also be used. For example, but not limited to, silicon (Si), silicon dioxide (Si0₂), tin dioxide (Sn0₂) or stabilised lithium metal particle (SLMP).

Furthermore, the addition of a layer of graphene 44 in embodiments of the invention provides additional surprising effects. Experiments have shown that graphene wetted to the current collector 14, 34, 54 (which may typically be or comprise copper) provides improved performance . The graphene was found to be an effective interface and wetting agent. Thus, in particular, the combination of a layer of graphene, a layer of mesoporous material such as titania and a layer of denser material such as titania was found to provide superior performance (as evidenced by the results from impedance spectroscopy e.g. as shown in Figure 7b). The use of graphene is advantageous since it provides good electrical conductivity, but it will be appreciated that other types of carbon and/or two-dimensional conductive materials may alternatively be used. It is also possible, in an embodiment, to use the graphene layer 44 as the current collector, without including the current collector 14, 34, 54 shown in Figures 1 -3.

Aspects and embodiments of the invention thus provide for a layered electrode structure preferably fabricated by LbL spray deposition, where each layer was chosen and placed according to its function, to achieve one of the best balanced performances compared with randomly mixed monolithic composite electrodes of the same materials in the same fractions. The best results were achieved from the described 3L structure. The LbL approach maximised the function of the more costly and harder to obtain materials, and this approach may find applications in other applications such as fuel cells. Since no liquid electrolyte is utilised, the solid-state supercapacitor can be fabricated to be flexible, and showed excellent cycle behaviour, with capacitance maintained at 86% after 5000 cycles during tests even when continuously exposed to ambient air and moisture with no packaging, offering encouraging potential for future wearable sensors and medical devices.

Claims

CLAIMS:

1. An electrode structure comprising:

a conductive substrate or current collector;

a first layer comprising a first electrochemically active material, the first active material being characterized by one or more first electrochemical properties; and

a second layer comprising a second electrochemically active material, the second active material being characterized by one or more second electrochemical properties, at least one of which is different from said one or more first electrochemical properties; wherein

the first and second layers are provided at predefined locations within the electrode structure based on the respective first and second electrochemical properties of the first and second electrochemically active materials to maximise the performance of each of the first and second layers.

2. The electrode structure of claim 1 , further comprising a proton conducting membrane or separator.

3. The electrode structure of claim 1 or 2, further comprising a third layer of high electrically conducting carbon material.

4. The electrode structure of claim 3, wherein the third layer is or comprises graphene.

5. The electrode structure of claim 4, wherein the layer of graphene has a thickness of approximately in the range of 10 to 400nm, or 50 to 300nm, or 100 to 300nm, or 150 to 250nm, or 175 to 225nm and is optionally or preferably approximately 200nm.

6. The electrode structure of any of claims 2 to 5, wherein the first layer is adjacent the substrate, the second layer is adjacent the first layer and the second layer is adjacent the proton conducting membrane or substrate and, when dependent on claim 3, 4, or 5, the third layer is located between the substrate and the first layer.

7. The electrode structure of claim 1 or 2, wherein the conductive substrate is a third layer of high electrically conducting carbon material or graphene located adjacent the first layer.

8. The electrode structure of any preceding claim, comprising one or more additional layers each comprising an electrochemically active material that is characterized by one or more electrochemical properties, at least one of which is different from said one or more first and/or second electrochemical properties.

9. A supercapacitor comprising the electrode structure of any of claims 1 to 8.

10. The supercapacitor of any of claims 2 to 9, wherein the thickness of the first layer is greater than that of the second layer, or where the thickness of the first layer is equal to or smaller than that of the second layer.

1 1. The supercapacitor of claim 10, wherein the thickness of the first layer is approximately in the range of 200 nanometres to 200 micrometres, and is preferably approximately 32 micrometres.

12. The supercapacitor of claim 10 or 1 1 , wherein the thickness of the second layer is approximately in the range 200 nanometres to 190 micrometres, and is preferably approximately 650 nanometres.

13. A battery comprising the electrode structure of any of claims 1 to 8.

14. The battery of claim 13, wherein the thickness of the first layer is equal, or greater or smaller than that of the second layer.

15. The battery of claim 13 or 14, wherein two or more layers are fabricated for the anode or the cathode.

16. The battery of claim 15, wherein the thickness of the first or second or any additional layer is equal to, greater than or smaller than the other layers.

17. The battery of claim 16, wherein the range of thickness for each layer is in the range from approximately 10 micrometres to 200 micrometres.

18. A method of fabricating an electrode structure, the method comprising the steps of:

providing a conductive substrate or current collector;

providing a first layer comprising a first electrochemically active material, the first active material being characterized by one or more first electrochemical properties, at a predefined location within the electrode structure based on the first electrochemical properties to maximise the performance of the first layer; and

providing a second layer comprising a second electrochemically active material, the second active material being characterized by one or more second electrochemical properties, at least one of which is different from said one or more first electrochemical properties, at a predefined location within the electrode structure based on the second electrochemical properties to maximise the performance of the second layer.

19. The method of claim 18, further comprising providing an ionic conducting membrane or a separator such as a proton or lithium conducting membrane.

20. The method of claim 18 or 19, further comprising providing a third layer of high electrically conducting carbon material or graphene .

21. The method of claim 20, wherein the third layer has a thickness of approximately in the range of 10 nanometres to 400 nanometres, or 50 to 300nm, or

100 to 300nm, or 150 to 250nm, or 175 to 225nm and is optionally or preferably approximately 200nm.

22. The method of any of claims 18 to 21 , wherein the first layer is provided adjacent the substrate, the second layer is provided adjacent the first layer and the second layer is provided adjacent the ionic conducting membrane or separator and, when dependent on claim 14 or 15, the third layer is provided between the substrate and the first layer.

23. The method of claim 22, wherein the conductive substrate is a third layer of high electrically conducting carbon material or graphene provided adjacent the first layer.

24. The method of any of claims 18 to 23, comprising providing one or more additional layers each comprising an electrochemically active material that is characterized by one or more electrochemical properties, at least one of which is different from said one or more first and/or second electrochemical properties.

25. A method of forming a supercapacitor comprising the steps of any of claims 18 to 24, and wherein the thickness of the first layer is greater than that of the second layer.

26. The method of claim 25, wherein the thickness of the first layer is approximately in the range from 200 nanometres to 200 micrometres and is optionally or preferably 32 micrometres.

27. The method of claim 25 or 26, wherein the thickness of the second layer is approximately in the range from 200 nanometres to 190 micrometres and is optionally or preferably 650 nanometres.

28. A method of forming a battery comprising the method of any of claims 18 to 24.

29. The method of claim 28, wherein the thickness of the first layer is equal, or greater or smaller than that of the second layer.

30. The battery of claim 28 or 29, wherein two or more layers are fabricated for the anode or the cathode.

3 1. The battery of claim 30, wherein the thickness of the first or second or any additional layer is equal to, greater than or smaller than the other layers.

32. The battery of claim 3 1 , wherein the range of thickness for each layer is in the range from approximately 10 micrometres to 200 micrometres.

33. The electrode structure of any of claims 1 to 8, the supercapacitor of any of claims 9 to 12, the battery of any of claims 13 to 17 or the method of any of claims 18 to 32, wherein one of said first and second layers comprises a material having a first density and the other of said first and second layers comprises a material having a second density that is greater than the first density.

34. The electrode structure, supercapacitor, battery or method of claim 33 wherein said one of said first and second layers is a porous or mesoporous material and said other of said first and second layers is a non-porous or substantially non-porous material.

35. The electrode structure, supercapacitor, battery or method of claim 33 or 34, wherein said one of said first and second layers is or comprises a composite having a high surface area and/or an open structure and said other of said first and second layers is or comprises a composite having a significantly lower surface area and/or a closed or substantially closed structure .

36. The electrode structure, supercapacitor, battery or method of claim 33, 34 or 35, wherein said one of said first and second layers is or comprises a composite of high weight percent electrochemically active material and said other of said first and second layers is or comprises a composite of high, equal or low weight percent electrochemically active material.

37. The electrode structure, supercapacitor, battery or method of any of claims 33 to 36, wherein said first and/or second layers is/are or comprise a metal oxide.

38. The electrode structure, supercapacitor, battery or method of claim 37, wherein said one of said first and second layers comprises mesoporous Ti0₂ and said other of said first and second layers comprises Ti0₂.

39. The electrode structure, supercapacitor, battery or method of any of claims 33 to 38 wherein one or both of said first and second layers also comprise a high aspect ratio material, e.g. a one-dimensional material such as micro- or nano-wires or micro- or nano-tubes or carbon nanotubes.

40. The supercapacitor of any of claims 33 to 39, wherein the density of the first layer is greater than the density of the second layer.

41. The battery of any of claims 33 to 39, wherein the density of the first layer is greater, equal or less than the density of the second layer.