Field of the Invention

The present invention relates to energy storage devices, intermediate structures for manufacture of energy storage devices and methods of manufacturing an energy storage device.

Background

Lithium-ion secondary batteries are the leading battery technology currently powering devices ranging from small personal devices to electric vehicles. Lithium-ion batteries are favoured for their high energy density and long cycle life, among other benefits. Lithium-ion batteries contain a plurality of lithium-ion secondary cells, one example of an alkali metal ion secondary cell.

There is a constant drive to investigate new materials for use as anode materials in energy storage devices. Silicon and silicon-based materials are of great interest for use as anodes in lithium-ion batteries, owing to their high gravimetric and volumetric energy density. However, the practical application of these materials is limited, because they exhibit significant volume expansion and shrinkage during the charging/discharging process. These structural changes can lead to significant irreversible capacity loss and short cycle life. Similar problems are also observed in a number of other materials systems - for example, tin and tin-based materials also experience significant volume changes during cycling.

One attempted solution to this problem has been to provide electrode having a structured surface. For example, US7402829B2 describes an energy storage device comprising an anode comprising an array of sub-micron silicon structures supported on a silicon substrate. Providing a structured surface can allow facile lithium-ion diffusion into the structure, while allowing space for material expansion during charge and discharge.

However, such solutions raise their own further problems - the structures may become detached or disconnected from the anode body as a result of the structural changes during cycling. This results in effective active material loss and lithium-ion trapping.

The present invention has been devised in light of the above considerations.

Summary of the Invention

It has been realised that provision of a suitable conformal coating on an electrode having a structured surface may reduce or avoid some or all or the above problems. Accordingly, in a first aspect, the present invention provides an electrode for use in an alkali metal ion secondary cell, the electrode comprising: an active material layer having a structured surface portion provided on at least one side of the active material layer; and a conformal coating layer covering at least a part of the structured surface portion, wherein the conformal coating layer comprises carbon nanotubes.

In a second aspect, the present invention provides a method of preparing an electrode for use in an alkali metal ion secondary cell, the method comprising steps of: providing an active material layer having a structured surface portion provided on at least one side of the active material layer; and applying a carbon nanotube dispersion comprising carbon nanotubes and a dispersion medium to the active material layer to thereby provide a conformal coating layer comprising carbon nanotubes covering at least a part of the structured surface portion.

In a third aspect, the present invention provides an electrode for use in an alkali metal ion secondary cell, wherein the electrode is produced according to a method of the second aspect.

The presence of a conformal coating layer can mechanically reinforce the structured surface portion, reducing the risk of structures becoming disconnected from the electrode body as a result of structural changes during cycling. Furthermore, because the conformal coating layer contains carbon nanotubes, which are known to have excellent electrical conductivity, the carbon nanotubes can act to electrically ‘bridge’ between portions of the structures even if they do crack during cycling. This may result in improved electrode performance for a structured electrode comprising such a conformal coating layer, as a result of reduction in active material loss and alkali metal ion trapping.

The term “conformal coating layer” is used herein to describe a layer which at least partly conforms to the contours of the substate on which the conformal coated layer is disposed: in the present instance, as the conformal coating layer covers at least a part of the structured surface portion of the electrode, the layer should at least partly conform to the contours of the part of the structured surface portion on which it is disposed.

As discussed above, the conformal coating layer comprises carbon nanotubes. The carbon nanotubes may include single-walled carbon nanotubes (SWCNTs). The carbon nanotubes may include multi-walled carbon nanotubes (MWNTs). In some arrangements, the carbon nanotubes may include substantially only SWCNTs. In other arrangements, the carbon nanotubes may include substantially only MWCNTs. In yet other arrangements, the carbon nanotube may include a mixture of SWCNTs and MWCNTs. The carbon nanotubes may have any suitable aspect ratio.

As discussed above, the conformal coating layer may be formed by applying a carbon nanotube dispersion comprising carbon nanotubes and a dispersion medium to the active material layer. The dispersion medium may be any suitable dispersion medium. For example, it may be an aqueous dispersion medium, or a non-aqueous dispersion medium. The dispersion medium may comprise a polymer. Non-aqueous dispersion mediums may be preferred over aqueous dispersion mediums, due to their generally lower reactivity with certain active electrode materials that may constitute part of the active material layer. Examples of suitable dispersion mediums include an aqueous CMC solution, or a NMP - PVDF solution. Where the dispersion medium comprises a polymer, preferably this is selected to be stable in the working voltage of the electrode. Where the dispersion medium comprises a polymer, preferably this polymer can plasticise with electrolyte. This can help to ensure that the coating layer does not block Li-ion transport.

In some arrangements, the surface tension of the CNT dispersion may be less than the surface tension of water. That is, it may be less than about 72 mN/m at 20 °C. In this regard, nonaqueous dispersions might be preferred over aqueous dispersions, as the surface tension of a non-aqueous dispersion may be lower than the surface tension of an aqueous dispersion, thereby allowing for improved ease in forming a conformal coating. In some arrangements, the contact angle of the dispersion on the active material layer is between 0-90 ° (i.e. so that it wets the surface). The contact angle may be 80 ° or less, 70 ° or less, 60 ° or less, 50 ° or less, 40 ° or less, 30 ⁰ or less, 20 ° or less, or 10 ° or less. It may be preferable to provide a dispersion which has a contact angle of as close to 0 ⁰ as possible. This can allow for improved wetting of the surface by the dispersion.

The carbon nanotubes may be present in the dispersion in an amount of from 0.1 wt% to 10 wt%, based on the total weight of components in the carbon nanotube dispersion. In some arrangements, the carbon nanotubes may be present in an amount of <5 wt%, or <1 wt%.

The dispersion medium may comprise, or consist of, a sacrificial solvent. In such cases, once the carbon nanotube dispersion comprising carbon nanotubes and a dispersion medium to the active material layer has been performed, a drying step may be performed to remove at least some of the sacrificial solvent. In this case, the step of applying and drying the carbon nanotube dispersion to form the conformal coating may be referred to as a ‘solvent casting’ process. Examples of suitable sacrificial solvents include NMP, acetone, ethanol, hexane, and/or water.

In some embodiments the coated active material layer is dried to remove at least 90 wt% of the sacrificial solvent, based on the total amount of dispersion medium in the original applied coating layer, for example to remove at least 95 wt%, or at least 99 wt% of the sacrificial solvent.

The drying step may include a step of drying the coated active material layer at a temperature lower than the boiling point of the sacrificial solvent. Alternatively or additionally, it may include a step of drying the coated active material layer at a temperature higher than the boiling point of the sacrificial solvent. In some methods, a two stage drying step may be performed, wherein drying is first performed at a temperature lower than the boiling point of the sacrificial solvent in the dispersion medium until the majority of the sacrificial solvent has been removed, for example until at least 95 wt%, at least 99 wt% or at least 99 wt% of the sacrificial solvent has been removed, following by drying under vacuum at a temperature above the boiling point of the sacrificial solvent. Such a method may allow for removal of substantially all sacrificial solvent from the conformal coating layer.

The drying step may be performed in any suitable manner, however in one preferred arrangement, the drying step may include a step of drying the coated active material layer under vacuum, for example in a vacuum oven.

The drying step may be performed for a time period selected based on the composition of the conformal coating layer, and/or the composition of the active material layer to be coated, and/or the thickness of the conformal coating layer, and/or the selected drying temperature. In some methods, the drying step may be performed for a time period in a range of from 1 hour to 48 hours, e.g. about 12 hours. Preferably drying is performed for a time of at least 1 hour, at least 2 hours, at least 3 hours, at least 4 hours or at least 5 hours.

After drying, the carbon nanotubes may be present in the conformal coating layer in an amount of at least 10 wt%, based on the total weight of components in the conformal coating layer. For example, the carbon nanotubes may be present in the conformal coating layer in an amount of at least 20 wt%, at least 30 wt%, at least 40 wt%, or at least 50 wt%. The carbon nanotubes may be present in an amount of up to 90 wt%, for example up to 80 wt%, up to 70 wt% or up to 60 wt%. In some methods, the carbon nanotubes may be present in the conformal coating layer in an amount of around 50 wt%, e.g. in a range of from 40 to 60 wt%. The remaining portion of the coating layer may comprise or consist of one or more binder materials, and, optionally, residual sacrificial solvent. The above amounts may help to ensure a suitable balance between good conductivity/porosity and good adhesive/cohesive strength of the conformal coating layer.

The carbon nanotube dispersion used to form the conformal coating layer, and accordingly, the conformal coating layer formed from said carbon nanotube dispersion, may comprise one or more binder materials. The binder materials may be selected from one or more of: polyvinylidene fluoride (PVDF), Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), carboxymethyl cellulose (CMC), polyethylene oxide (PEO), polyacrylic acid (PAA), Polyacrylonitrile (PAN), and Polyvinylpyrrolidone (PVP). PVDF may be particularly preferred as this can partially swell in some electrolytes (when the electrode is use as part of a cell comprising an electrolyte), maintaining the possibility for ionic conductivity of the electrolyte, while also maintaining structural stability of the conformal coating layer.

The dynamic viscosity of the carbon nanotube dispersion may be selected to be in a range of 100 to 2000 cP, as determined by use of a suitable rheometer, such as a cone and plate rheometer, at a shear rate of 100s’ ¹ . Selection of an appropriate viscosity can help to ensure conformal coating of the carbon nanotube dispersion on the active material layer. The viscosity may depend on the concentration of carbon nanotubes in the dispersion, as well as the composition of the dispersion medium.

The step of applying the carbon nanotube dispersion to the active material layer may be performed by any suitable method. In some methods, the step of applying the carbon nanotube dispersion to the active material layer is performed using a method selected from: blade coating, slot-die coating, spin coating, dip coating, comma bar coating, and gravure coating.

The conformal coating layer may cover some, or all, of the structured surface portion of the active material layer. For example, in some embodiments, the conformal coating lay may cover at least 50%, at least 60%, at least 70%, at least 80%, at least 90%, at least 95% or substantially 100% of the structured surface portion of the active material layer. Preferably, the conformal coating layer covers all of the structured surface portion. In some arrangements, the conformal coating layer may cover substantially all of the side of the active material layer on which the structured surface portion is formed.

The conformal coating layer may have a thickness of 1 pm or less, more preferably 500 nm or less, more preferably 100 nm or less, e.g. 90 nm or less, 80 nm or less, 70 nm or less, 60 nm or less or 50 nm or less.

The thickness of the conformal coating layer may be greater than or equal to the diameter of the CNTs forming a part of the conformal coating layer. Typical dimensions of CNT diameters vary, but may be in a range of from about 0.4 to about 40 nm. Accordingly, in some embodiments, the conforming coating layer may have a thickness of at least 1 nm, at least 2 nm, at least 5 nm, or at least 10 nm thick. Providing a conformal coating layer with a thickness in this range can help to ensure that the conformal coating layer provides suitable mechanical support for the structured surface portion of the active material layer.

The thickness of the conformal coating layer may be measured as the thickness of the layer in a direction perpendicular to the surface on which the coating layer is disposed. In some arrangements, the thickness of the conformal coating layer may be substantially equal across the extent of the coating layer. In other arrangements, the thickness of the conformal coating layer may vary across the extent of the conformal coating layer. For example, the conformal coating layer may be thicker at locations between structures forming a part of the structured surface portion as compared with locations on said structures.

The conformal coating layer may be a substantially continuous layer. Alternatively, the conforming coating layer may be non-continuous.

Where the conformal coating layer is formed by applying a carbon nanotube dispersion comprising carbon nanotubes and a dispersion medium comprising a sacrificial solvent to the active material layer, and performing a drying stage to remove at least some of the sacrificial solvent, the thickness of the conformal coating layer (ranges for which are specified above) may be the thickness after drying has been performed. The active material layer may be a layer which undergoes significant volume change during a charge/discharge cycle when used in an alkali metal ion secondary cell, e.g. as a result of incorporation of the alkali metal ions into the active material layer. For example, the active material layer may undergo volume changes of 100% or more, 200% or more, or 300% or more when the active material alkali metal ions are incorporated into the active material layer to saturation, e.g. when fully lithiated, for a lithium-ion cell.

The active material layer may comprise an electrochemically active material comprising one or more of graphite, Si, SiOx and Sn. Such materials are commonly commercially available.

The active material may be provided as a film having a thickness in a range of from about 1 pm to about 20 pm. Accordingly, in some embodiments, the active material layer may have a thickness of at least 1 pm, at least 2 pm, at least 5 pm, or at least 10 pm thick. However, if the active material layer is too thick, ion transport kinetics can become worse. Accordingly, in some embodiments, the active material layer may be 20 pm or less, 18 pm or less, 15 pm or less, or 10 pm. In some embodiments, the active material lay may be a film having a thickness of about 10 pm. .

The structured surface portion of the active material layer may be a 3D structured portion comprising a plurality of 3D microstructures or nanostructures. The microstructures or nanostructures may be three-dimensional features disposed on, or integral with, a surface of the active material layer.

The term “microstructures” is used herein to generally refer to microscale structures, i.e. structures having at least one dimension smaller than 1 mm, as measurable using standard techniques well known in the art, for example scanning electron microscopy (SEM) or transmission electron microscopy (TEM). Microstructures may have at least one dimension, two dimensions, or all three dimensions having a size in a range of from 1 p to 1000 pm. In some cases, at least one dimension, two dimensions, or all three dimensions may have a size of 100 pm or less, or 10 pm or less.

The term “nanostructures” is used herein to generally refer to nanoscale structures, i.e. structures having at least one dimension smaller than 1 pm, as measurable using standard techniques well known in the art, for example scanning electron microscopy (SEM) or transmission electron microscopy (TEM). Nanostructures may have at least one dimension, two dimensions, or all three dimensions having a size in a range of from 1 nm to 1000 nm. In some cases, at least one dimension, two dimensions, or all three dimensions may have a size of 100 nm or less, or 10 nm or less.

The specific size and shape of the 3D microstructures or nanostructures is not particularly limited, however in some arrangements, the 3D microstructures or nanostructures may comprise pillar formations (‘pillars’). In other arrangements, the 3D microstructures or nanostructures may comprise cavities, holes or pores formed in the active material layer. For example, the active material layer may comprise a microporous film layer. In some embodiments, the active material lay may comprise an active material layer as disclosed in EP4014266A1 , the contents of which are hereby incorporated by reference. That is, it may comprise a nanostructured amorphous anode layer, said layer comprising silicon and being nanostructured with a plurality of columns, nanoparticle aggregates, and interfaces there between.

Where the 3D microstructures or nanostructures comprise pillars, the pillars may have an aspect ratio in a range of from 1 :1 to 100:1, e.g. in a range of from 5:1 to 10:1. For example, in one suitable arrangement, the pillars may have a height of about 9 pm and a diameter of about 2 pm.

The 3D microstructures or nanostructures may be provided in an array. The array may be a regular or an irregular array. The 3D microstructures or nanostructures may be generally aligned with one another. In one preferred arrangement, the 3D microstructures or nanostructures comprises an array of vertically-aligned pillars.

The structured surface portion may be defined by its total surface area, or by the surface area enhancement provided by the structuring. The total surface area of the structured surface portion may be defined as the surface area of each 3D micro/nanostructure times the number of 3D micro/nanostructures (assuming that each 3D micro/nanostructure is geometrically similar). The Surface Area Enhancement may be defined as the Total Surface Area of the 3D micro/nanostructure divided by the footprint area (also referred to as the planar area) of the structured surface portion of active material layer.

The active material layer having a structured surface portion may be formed in any suitable manner. In some embodiments, the active material layer having a structured surface portion may be formed using OVD techniques, or using PVD techniques. In some cases, the active material layer having a structured surface portion may be obtained from a third party supplier before the conformal coating layer is applied.

As discussed above, the electrode is for use in an alkali metal ion secondary cell. Accordingly, in a fourth aspect, the present invention provides an electrochemical secondary cell comprising an electrode according to the first aspect.

The alkali metal ion secondary cell may be, for example, a lithium ion cell or a sodium ion cell. Preferably the alkali metal ion secondary cell is a lithium ion secondary cell.

The electrode may further comprise a current collector arranged to be in electrical communication with the active material layer. The current collector may be disposed on a first side of the active material layer, with the structured surface portion provided on a second side of the active material layer, opposite to the first side, although other arrangements are contemplated. The current collector may have any suitable form and/or composition as is known for an alkali metal ion secondary cell. For example, the current collector may comprise a foil layer.

The electrode may be an anode or may be a cathode. Preferably the electrode is an anode.

The alkali metal ion secondary cell may comprise a further electrode. The further electrode may also be an electrode according to the present invention, or may be a conventional electrode.

The alkali metal ion secondary cell may comprise an electrolyte. The choice of electrolyte is not particularly limited, and any suitable electrolyte may be used.

The alkali metal ion secondary cell may comprise a separator. The choice of separator is not particularly limited, and any suitable separator may be used.

The alkali metal ion secondary cell may be incorporated in an electrochemical energy storage device. Accordingly, in a fifth aspect, the present invention provides an electrochemical energy storage device comprising an electrochemical secondary cell according to the fourth aspect.

The invention includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided.

Summary of the Figures

Embodiments and experiments illustrating the principles of the invention will now be discussed with reference to the accompanying figures in which:

Figure 1 is a schematic figure illustrating an electrode according to the present invention.

Figure 2 is a schematic figure illustrating another electrode according to the present invention.

Detailed Description of the Invention

Aspects and embodiments of the present invention will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference.

Fig. 1 and 2 are both schematic figures illustrating electrodes 100, 100’ according to the present invention. As can be seen, the electrodes are generally similar, except for the form of the conformal coating layer. Accordingly, these figures will be discussed together.

Here, the electrode 100, 100’ comprises an active material layer 1 having a structured surface portion 3. The active material layer here conveniently comprises silicon (Si). The electrode comprises a current collector layer 5 disposed on an opposite side of the active material layer to the structured surface portion 3. The structured surface portion 3 comprises a plurality of three-dimensional pillar structures 7, having a height Xi of around 9 pm, and a diameter X ₂ of about 1.6 pm. The pillar structures are integral with the active material layer and are accordingly also formed of silicon.

A conformal coating layer 9, 9’ is provided which covers the structured surface portion 3. The conformal coating layer comprises single-walled carbon nanotubes (SWCNTs) and a PVDF binder, which acts to bind the SWCNTs together, and to the structured surface portion. Provision of this conformal coating layer can mechanically reinforce the structured surface portion, reducing the risk of parts of the pillar structures 7 becoming disconnected from the electrode body as a result of structural changes during cycling. Furthermore, because the conformal coating layer contains carbon nanotubes, which are known to have excellent electrical conductivity, the carbon nanotubes can act to electrically ‘bridge’ between portions of the structures even if they do crack during cycling. The electrode may therefore show improved performance as compared with a similarly structured electrode which does not comprise a coating layer.

In Fig. 1, the conformal coating layer 9 covers the three-dimensional structures of the structured surface portion, to provide a non-continuous costing layer. In Fig. 2, the conformal coating layer 9’ also extends in the spaces between the pillar structures 5 to provide a continuous layer extending across the entire structured surface portion. The arrangement shown in Fig. 2 may be preferred, as it may be more effective in preventing active material loss and alkali metal ion trapping than the arrangement shown in Fig. 1 .

The thickness of the conformal coating layer 9, 9’ is around 100 nm. For both of these arrangements, it can be seen that the thickness of the conformal coating layer is selected so that a space remain between adjacent pillars, even after coating. That is, the thickness of the coating layer at least on sidewall surfaces of the pillars is less than half of the total spacing between the pillars. This ensure maintenance of a high surface area for charge transfer, even with the coating layer.

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The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations.

Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.

Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise” and “include”, and variations such as “comprises”, “comprising”, and “including” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and/or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about,” it will be understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means for example +/- 10%.