TECHNICAL FIELD

The present disclosure relates to electrodes. More particularly, but not necessarily exclusively, the disclosure relates to electrodes having improved electrochemical performance (e.g. specific capacity, on an electrode mass basis, charging rate and cycle life/capacity fade, e.g. as a result of cycling stresses) and/or having a more efficient route (e.g. solvent-free/dry) to manufacture, as compared with existing electrodes.

BACKGROUND

Electrodes are often formed by a slurry-cast process. Active material powder, binder, solvent and conductive additives are mixed together to form a slurry. The slurry is then cast onto a current collector foil and dried to remove the solvent. The dried and hardened slurry is then calendared to yield a sheet in the form of an agglomerated powder and conductive additives (bound by the binder) on the foil.

For a lithium-ion negative graphite electrode, the agglomerated graphitic powder is an active material, whereas the foil (e.g. copper: typically, for an illustrative double sided lithium-ion negative electrode, ~0.5 g by weight for every gram of active material, e.g. graphite), binder (-0.05 g), conductive additives (-0.01 g) and electrolyte (-0.2 g) are inactive and therefore do not positively contribute to properties associated with electrochemical performance (e.g. specific capacity). Taking graphite as an example for the active material powder, the theoretical specific capacity normalised to the mass of active material is 372 mA h g ^-1 . However, graphite accounts for only 57% of wet electrode mass, reducing the theoretical specific electrode capacity down from 372 to 212 mA h g ^-1 .

It would be desirable to provide an electrode that has improved or alternative electrochemical performance, and/or to obviate, mitigate and/or ameliorate one or more deficiencies in known electrodes, whether identified herein or otherwise. Alternatively or additionally, it would be desirable to provide an improved or alternative method of manufacture. SUMMARY

According to a first aspect of the present disclosure, there is provided an electrode, comprising a monolithic electrochemically active material which is patterned with a plurality of channels.

According to a second aspect of the present disclosure, there is provided an electrochemical cell, comprising the electrode according to the first aspect (e.g. as the negative electrode thereof), a further electrode (optionally a positive lithium electrode) and optionally a separator therebetween, optionally wherein the electrochemical cell further comprises electrolyte, and/or optionally wherein the electrochemical cell is a galvanic cell such as a coin cell, pouch cell, prismatic cell, or a battery of cells, and/or optionally wherein the electrochemical cell is lithium, potassium, sodium or aluminium electrochemical cell, and/or optionally wherein the electrochemical cell is a secondary battery such as a lithium ion secondary battery.

According to a third aspect of the present disclosure, there is provided a method of manufacturing an electrode, comprising:

(A) providing a monolithic electrochemically active material; and patterning the monolithic electrochemically active material with a plurality of channels; or

(B) providing a polymer (e.g. a film thereof); and patterning the polymer with a plurality of channels; and pyrolyzing the polymer (before or after patterning) to form a monolithic electrochemically active material.

DEFINITIONS

The following definitions apply for terms used herein. In the event that any term is not specifically defined here or otherwise, the standard meaning in the present technical field prevails. This standard meaning may bear in mind definitions provided in common general knowledge (e.g. standard textbooks) in the present technical field. Usefully, for example, chemical terms may be interpreted in accordance with the IUPAC Gold Book Version 3.0.1. The term “at least one” is synonymous with “one or more”, e.g. one, two, three, four, five, six, or more.

Only certain numerical ranges are explicitly stated herein. The lower limit of any range may be combined with the upper limit of any related range to form a range that is not explicitly described.

Unless otherwise indicated, the terms “around”, “about” or “approximately”, as applied to numerical values, generally encompass or refer to a range of values that one skilled in the art would consider equivalent to the recited values (e.g. having the same function or result and/or achieving the function/result substantially in the same way). Suitably, where one of the terms is used in relation to a numerical value, it can represent (in increasing order of preference) a 10%, 5%, 2%, 1% or 0% deviation from that value (e.g. “around 0.1” can represent 0.09 to 1.1 , 0.095 to 0.105, 0.098 to 0.102, 0.099 to 0.101 or 0.1 respectively).

Unless otherwise indicated, the terms “approximately” or “substantially”, as applied to characteristics, generally encompass slight deviations in that characteristic which the skilled person would nonetheless consider equivalent (e.g. having the same function or result and/or achieving the function/result substantially in the same way).

However, when the monolithic electrochemically active material is “substantially free of binder and/or conductive additives”, this means that there is less than 1 wt% of said binder and conductive additives (total). Generally speaking, less binder and conductive additive content is preferred, such as less than 0.5 wt%, preferably less than 0.25 wt%, preferably less than 0.1 wt% and preferably 0 wt%.

The term “consists essentially of” is used herein to denote that a given component consists of only designated materials and optionally other materials that do not materially negatively affect the characteristic(s) of the product. In the context of the monolithic electrochemically active material, for example, this term may be understood to denote that the monolithic electrochemically active material consists of only the designated material (e.g. the pyrolytic graphite) and optional other materials which do not negatively affect the electrochemical performance of the monolithic electrochemically active material. Suitably, component which consists essentially of a designated material (or materials) comprises greater than or equal to about 95 wt%, more suitably greater than or equal to about 99 wt%, more suitably greater than or equal to about 99.5 wt% of the designated material(s); more suitably greater than or equal to about 99.9 wt%, based on the total weight of the component. Pyrolytic graphite is a graphitic (layered) material that is produced by pyrolysis of a hydrocarbon polymer. Pyrolysis involves heating a material to above its decomposition temperature, thereby causing the polymer to break down and convert into a graphitic crystalline material. Pyrolytic graphite is similar, structurally, to graphite. Pyrolytic graphite can be produced from various hydrocarbon polymers (such as polyimide, pitch and the like) and for present purposes is preferably produced from a film of such materials.

Typical pyrolytic graphite is dense (0.85 to 2.13 g/crrr ³ ), highly electrically conductive (10,000-20,000 S cm ^-1 ; in-plane conductivity) and thermally conductive (700-1950 W rrr ¹ K ^-1 ) material with a highly ordered non-porous microstructure. Previous studies have used XRD to show that typical pyrolytic graphite sheet has an interplanar spacing of 0.33584 nm, mosaic angle spread of 8.2° and an in-plane lattice parameter of 0.2463 nm. Raman also revealed an IG/IC’ ratio of 3.1, typical of multi-layered graphite, and found that a D band was not detected within experimental error, indicative of high crystallinity and minimal defects. Properties of such materials are described in Nakamura, S., Miyafuji, D., Fujii, T., Matsui, T.

& Fukuyama, H. Low temperature transport properties of pyrolytic graphite sheet. Cryogenics 86, 118-122, doi: 10.1016/j. cryogenics.2017.08.004 (2017), the entire contents of which are incorporated herein by reference.

Pyrolytic graphite is available from commercial suppliers, such as Panasonic "PGS" Graphite Sheets (e.g. EYGS091203, Panasonic). See also PGS 31 December 2021 datasheet https://industrial.panasonic.eom/cdbs/www-data/pdf/AYA0000/A YA0000C59.pdf; accessible July 2022, the entire contents of which are hereby incorporated by reference.

Reference is also made to Yuan, G. and Cui, Z., 2021 , Polyimide-Derived Graphite Films with High Thermal Conductivity, In Polyimide, IntechOpen. https://www.intechopen.com/online-first/78817, as accessible July 2022 (the entire contents of which are hereby incorporated by reference). This describes polyimide-derived materials, including their properties and methods of manufacture.

The term “monomer” is a standard term in the art. For the avoidance of any doubt, monomers are molecules that can be bonded to other molecules to form a polymer or a copolymer comprising units of the monomer.

The term “polymer” is a standard term in the art. For the avoidance of any doubt, a polymer refers to a molecule comprising two or more (such as three or more, four or more, five or more, six or more, seven or more, eight or more, nine or more, or ten or more) monomer units. A polymer may comprise many monomer units, such as 100 or more monomer units.

The term “monolithic” as used herein is intended to distinguish the electrode active materials of the present disclosure from slurry-cast, powdered and/or agglomerated (e.g. bound by binder) materials in the art. Monolithic refers to a substantially continuous and/or integral structure. Such materials may (preferably) be substantially crystalline (i.e. not amorphous).

The electrodes of the present disclosure may comprise a single crystal of electrode active materials, though it is also intended to that the “monolithic electrochemically active materials” could comprise multiple crystals provided they were held together in a way which the skilled person would understand to be a polycrystalline material (e.g. one in a macroscopic sheet). Optionally, the material has a high degree of texture and/or wherein individual grains comprising the material are oriented relative to the sheet. For the purposes of this disclosure, the material must be electrochemically active.

The terms “monolithic electrochemically active material” and “monolithic electrode active material” are used interchangeably throughout and refer to the active material usable in an electrode for ion/charge exchange/intercalation of active electrochemical species from electrolyte (e.g. lithium ions in a lithium ion battery).

The term “strength” is well understood. The strength of a material may be considered in terms of the flexural, tensile, etc. properties of the material.

Where the amount of a particular ingredient is expressed as a “wt%, this may be calculated as follows: weiqht of particular inqredient in a material wt% = - - - ^J - ^J — _J - — - f- - — - x 100% total weight of the material

The electrodes of the present disclosure comprise a monolithic electrochemically active material which is patterned with a plurality of channels. Optionally, these channels are patterned in an array or a semi-regular array (e.g. a substantially repeating pattern of channels). In this context, the repeating pattern may not be completely perfect and minor deviations (e.g. where some channels are out-of-place or missing, defects) from perfect repetition is intended to be encompassed by the present disclosure. The array design may be informed by Penrose filling/tiling. The repeating pattern may be different in different regions of the monolith electrochemical electrochemically active material. Various properties of these channels are defined with reference to exemplary/reference Figs. 1 and 2. These depict 2D arrays (extending in the plane of a sheet of monolithic active electrode material) comprising a plurality of channels 3 in an ordered hexagonal array (schematic of an individual channel 3 in Fig. 1, array in Fig. 2). Other array designs are possible, as discussed below, and the hexagonal array depicted in these figures is merely an example. In general terms, the design/shaping/layout of channels and the array constituting a plurality of said channels will dependent on the specific transport and mechanical requirements of the electrode.

As shown in Fig. 1, double-headed arrow 7 refers to the channel diameter and doubleheaded arrow 9 refers to the channel pitch. In the event a channel is non-uniform/circular, then channel diameter is to be measured as the minor dimension (i.e. narrowest point on the channel opening). The channel pitch is measured as the distance from the centre of a channel to the centre of an adjacent channel.

This depicts a high symmetry arrangement. In general, 2D repeating arrays may be defined by two lattice parameters (pitches) and/or the angle between them. For circular channels, a symmetric arrangement is generally preferable. As a result, the hexagonal array has a single pitch (the two independent lattice parameters are identical by symmetry). For non-circular channels, two different pitches are preferred. In the event that two different pitches are relevant, then references to pitch herein refers to the short pitch.

It is also generally preferred that the width of material will be the same or similar (for example, if channels are wider along one axis, the pitch will be greater, such that the width of monolith is similar).

Only a small number of channels is shown in Fig. 2, but it will be appreciated that a monolithic electrochemically active material may comprise a great number of channels. For example, the channels may cover substantially an entire surface (e.g. the main/major surface, e.g. the largest surface of a plate) of a monolithic electrochemically active material with the spacing of channels in the array being determined by the channel pitch. In the event there is 100% coverage of a major surface, it will be appreciated that channel pitch dictates channel density (number of channels in a given surface area). It will be appreciated that a large number of laterally disconnected channels allows the monolithic electrode to remain as one connected piece of material. Suitably, the openings of the plurality of channels covers at least around 70% of a monolithic electrochemically active material surface (e.g. the main/major surface, e.g. the largest surface of a plate), optionally at least around 80%, optionally at least around 90%, optionally at least around 95%, such as around 100% - such ranges may be combined with any other features of the electrodes described and claimed herein.

These ranges may be understood as being equivalent to porosity (attributable to channels), which depends closely on the hole pitch. Generally speaking, a higher pitch will lead to a lower porosity (and vice versa). For example, a pitch of around 200 pm may give rise to a porosity (due to channels) of around 23%, a pitch of around 160 pm may give rise to a porosity (due to channels) of around 35% and a pitch of around 120 pm may give rise to a porosity (due to channels) of around 63%.

In the event that less than 100% of the surface is covered with channels, then channel pitch should be calculated based only on the area which is covered with channels. In other words, if a first area is covered with channels and second area is covered with channels but spaced away from the first area, then the distance between the two areas should not be included in a calculation of pitch.

Sizing and other channel characteristics/dimensions can be confirmed using microscopy techniques described and exemplified herein. For example, a representative photograph (optical microscopy or SEM) can be taken to capture an image of a portion of the array of channels on a monolithic electrochemically active material surface (e.g. a 5x5 mm square thereof). Example images are shown in Fig. 2B.

Using visual inspection (or computer aided analysis), the characteristics (e.g. pitch) can be determined for each channel and the results plotted on a histogram. The pitch may therefore be understood to be an average. If the plurality of channels has a pitch of around 150 to 200 pm, then the mean average pitch between channels must be around 150 to 200 pm and the majority (greater than around 50%) of channels should have a pitch around 150 to 200 pm.

A channel may fully or partially penetrate the monolithic electrochemically active material (e.g. sheet thereof). The channels may therefore be understood as a through bore or blind bore, respectively. Preferably, the channels fully penetrate (through bore). This enables electrolyte to access the channels from both sides (e.g. major surfaces of the monolithic electrochemically active material sheet). The channel diameter may remain substantially constant through the monolithic electrochemically active material (straight sides) or it may vary (e.g. adopting an hour-glass shape).

The monolith material density (active material/intrinsic density) is defined by its production process, for example around 1.9 g/cc for pyrolytic graphite. The average density of the patterned electrode is given by the final mass of the patterned electrode divided by the volume of a flat sheet of equivalent thickness and area. This value is directly related to the patterned porosity (not the intrinsic porosity of the sheet).

Intrinsic porosity of the monolith material is given by its intrinsic density, as produced, divided by the theoretical maximum density for a perfect crystalline structure. For example, pyrolytic graphite has a density around 1.9 g/cc compared to the ideal density of graphite of 2.2 g/cc, hence an intrinsic material porosity of 16%.

The patterned porosity or channel porosity is given by the average density of the patterned region of the electrode divided by the intrinsic porosity of the material.

For example, pyrolytic graphite sheet may have a density (active material/intrinsic density) of 1.9 g/cnv ³ (datasheet), which is equivalent to 16% porosity compared to graphite crystal (2.26 g/cm- ³ ).

“Tortuosity” may be calculated as follows:

Actual flow path length Tortuosity = -

Straight distance between the ends of the flow path

BRIEF DESCRIPTION OF THE FIGURES

Fig. 1 : schematic showing a plurality of channels in a hexagonal array.

Fig. 2: (A) Optical microscopy of electrodes with 200 pm (upper left), 160 pm (lower left), 120 pm (lower right) pitch electrolyte channels and the control pyrolytic graphite sheet electrode (upper right) without electrolyte channels; (B) Images used for hole diameter analysis of pyrolytic graphite sheet electrodes: 200 pm pitch, 160 pm pitch and 120 pm pitch.

Fig. 3: Histogram of hole diameters extracted from micrographs. Fig. 4: SEM images of electrolyte channels drilled in pyrolytic graphite sheet. Hole diameter was 100 pm, as indicated in (B), and hole pitch for this sample was 160 pm.

Fig. 5 : Galvanostatic cycling between 5 mV and 1.5 V at rates of 0.1 C to 2 C (raw data), where 1 C = 372 mA g-1 (current normalised to dry electrode mass).

Fig. 6 : Galvanostatic cycling between 5 mV and 1.5 V at indicated charge rates between 0.1 C and 2 C, where 1 C = 372 mA g-1 (current normalised to dry electrode mass).

Fig. 7: Voltage-Capacity profile for (upper) first and (lower) second cycle (at 0.1 C).

Fig. 8. Schematic drawing depicting modification of cells and jelly rolls (Example 2).

DETAILED DESCRIPTION

According to a first aspect of the present disclosure, there is provided an electrode, comprising a monolithic electrochemically active material which is patterned with a plurality of channels.

The electrodes of the present disclosure, comprising monolithic electrochemically active material, do not rely on slurry-cast processes for manufacturing. As a result, the monolithic electrochemically active material does not need to include inactive binders and/or conductive additives.

Additionally, the monolithic electrochemically active materials of the present disclosure may be understood to be more robust/stronger (less brittle) than the slurry-cast equivalents. It will be appreciated that agglomerated powders (bound by a binder) have poor electrical conductivity and are relatively weak/brittle. Agglomerated powders must be supported by a current collector foil to maintain structural integrity. In contrast, the more robust/stronger monolithic electrochemically active materials of the present disclosure do not need a foil current collector to maintain structural integrity (although a foil may nonetheless be useful if electrical conductivity is low). In other words, the monolithic electrochemically active materials of the present disclosure are free-standing. Numerous other advantages flow from this, such as lower cost, less weight, avoiding electrochemical degradation reactions associated with the metal, avoid high contact resistance with the metal, etc. Collectively, this means that a greater proportion (e.g. on an electrode mass basis) of the electrode is formed from active material (the monolithic electrochemically active material), rather than inactive materials such as binders, conductive additives and foil current collectors. It follows that the electrochemical performance of the electrodes of the present disclosure can be improved as compared to slurry-cast equivalents. For example, as discussed and exemplified below, electrodes of the present disclosure may have improved or comparable specific capacity (on an electrode mass basis), as compared to slurry-cast equivalents.

Moreover, it will be appreciated that avoiding slurry-cast processes for manufacture also avoids the need for binders, conductive additives, current collectors and solvents. This may lead to a more efficient manufacturing routes for the electrodes of the present disclosure. Alternatively or additionally, this may lead to safer manufacturing routes and/or reduced costs.

The monolithic electrochemically active material may have: at least around 1 channel per mm ² , optionally at least around 10 channels per mm ² , optionally at least around 20 channels per mm ² , optionally at least around 25 channels per mm ² ; and/or at most around 100,000,000 channels per mm ² , optionally at most around 1,000,000 channels per mm ² , optionally at most around 10,000 channels per mm ² , optionally at most around 1 ,000 channels per mm ² , optionally at most around 100 channels per mm ² .

The monolithic electrochemically active material may be a layered material, e.g. in terms of its crystal structure. Here, the layers may be understood to be arranged substantially parallel to the plane (of an electrode). The electrochemically active material may be a sheet. The monolithic electrochemically active material may be a carbonaceous material (such as graphite or hard carbon/non-graphitizing carbon).

The monolithic electrochemically active material may be a crystalline material (e.g. a polycrystalline material), preferably wherein the monolithic electrochemically active material is pyrolytic graphite (e.g. a sheet thereof) or nickel manganese cobalt oxide (e.g. a sheet thereof).

The monolithic electrochemically active material may comprise pyrolytic graphite (e.g. a sheet thereof). Pyrolytic graphite is a preferred implementation throughout the present disclosure. The monolithic electrochemically active material preferably consists or consists essentially of said layered material and/or crystalline material and/or pyrolytic graphite material (e.g. a sheet thereof).

Agglomerated powders (such as graphitic powder) may be understood to have a highly disordered microstructure, with particles in the powder essentially randomly oriented and dispersed throughout. Such agglomerated powders forming a complex, stoichiometric network of surrounding porosity. Typical charge-discharge cycles using such electrodes involve mass transport/shuttling of electroactive species (e.g. lithium ions) into and-out of the active material. However, in doing so, the electroactive species must navigate around the highly disordered agglomerated graphitic powder microstructure. This means that the electroactive species follow a high-diffusion path during charge-discharge cycles, reducing performance.

In contrast, the layered material and/or crystalline material and/or pyrolytic graphite material (e.g. a sheet thereof) of the present disclosure present short diffusion paths (e.g. in regions of low diffusivity, electrolyte domain within electrode, active material) and thereby improve performance as compared to powder counterparts. The patterned sheets may preferably be designed to have no low diffusivity (electrolyte domain, within the electrode), thereby minimising diffusion path length (tortuosity = ~1).

Moreover, in use/charge-discharge cycling, slurry-agglomerate-based/slurry-cast electrodes generate a complex stress state and relatively weak connections between particles making up the agglomerate can lead to failure. Here, individual particles (or regions thereof) are disconnected leading to “dead” zones and lost capacity (fade).

The patterned monolithic electrochemically active material of the present disclosure manages the diffusion/intercalation profile in a predictable way (e.g. so that it substantially does not exceeds the critical stress, which profile is moreover greater due to the monolithic nature of the material). The plurality of channels can be designed with both mechanical stress and electrochemical processes in mind. The result is that that cycle life can be improved. In the case that the material is layered/ textured, the intercalation expansion can be specifically managed to be out of plane, and therefore further reduce damaging in plane stresses. The pyrolytic graphite may be derived from pyrolysis of a hydrocarbon polymer (optionally polyimide or pitch), preferably a film, optionally a consol idated/com pressed hydrocarbon polymer film.

The monolithic electrochemically active material has a high electrode active material content (e.g. carbon/graphite/pyrolytic graphite content), optionally of at least around 95%, optionally at least around 99%, optionally at least around 99.5%, optionally at least around 99.9% electrode active material, on a weight basis.

The monolithic electrochemically active material may have an in-plane conductivity of: at least around 1x1 O' ⁹ S-cnv ¹ , optionally at least around 1x1 O' ⁵ S-cm’ ¹ , optionally at least around 1 S-crm ¹ , optionally at least around 10,000 S-crm ¹ ; and/or at most around 50,000 S-cnr ¹ , optionally at most around 30,000 S-crn’ ¹ ; optionally at most around 20,000 S-crn’ ¹ ; and/or around 1x10’ ⁹ to 50,000 S-crn’ ¹ , optionally around 1 to 30,000 S-crn’ ¹ .

The monolithic electrochemically active material may have a porosity (attributable to the channels) of: at most around 75%, optionally at most around 65%; and/or at least around 0%, optionally at least around 1%, optionally at least around 10%, optionally at least around 20%; and/or around 1 to 75%, optionally around 1 to 65%, optionally around 10 to 65%.

The monolithic electrochemically active material (including channels) may have a density (sheet density) of: at least around 0.568 g/cm ³ ; optionally at least around 1 g/cm ³ , optionally at least around 1.58 g/cm ³ , optionally at least around 1.75 g/cm ³ , optionally at least around 1.9 g/cm ³ ; and/or at most around 2.266 g/cm ³ , optionally at most around 2 g/cm ³ ; and/or around 0.568 g/cm ³ to around 2.266 g/cm ³ , optionally 1 g/cm ³ to around 2 g/cm ³ .

The monolithic electrochemically active material may be understood to be not a powder or agglomerated powder.

The monolithic electrochemically active material may be understood to be not slurry-cast. The monolithic electrochemically active material may be substantially free of binder and/or conductive additives.

The ordered structure of layered and/or crystalline materials leads to faster diffusion (desirable) parallel to the layers, in the monolithic electrochemically active material microstructure. With appropriate patterning, the diffusion process is consistent through the entire patterned electrode, yielding substantially uniform potential and intercalation response. For example, pyrolytic graphite has a substantially flat microstructure made up of a series of stacked crystalline layers generally lining up (face-to-face) throughout the material bulk, orientation parallel to the macroscopic sheet. Overall, this improves capacity retention at elevated current densities. In contrast, powdered materials are highly disordered (particles are more randomly oriented, leading to longer electrolyte flow path lengths) and this equates to a high-diffusion path microstructure.

The monolithic electrochemically active material may be a sheet of material having a thickness of: at least around 25 pm, optionally at least around 20 pm; and/or at most around 5 mm, optionally at most around 2.5 mm, optionally at most around 1 mm, at most around 500 pm, at most around 250 pm, optionally at most around 50 pm, optionally at most around 30 pm; and/or around 25 to 5 mm, optionally around 25 to 2.5 mm, optionally around 25 to 1 mm, optionally around 25 to 500 pm, optionally around 10 to 250 pm, optionally around 10 to 100 pm, optionally around 10 to 50 pm, optionally around 20 to 30 pm, optionally around 25 pm.

The channels in the plurality of channels may have a channel diameter of: at most around 100 pm; and/or at least around 0.1 pm, at least around 1 pm, optionally at least around 10 pm; and/or around 0.1 to 100 pm, optionally around 10 to 100 pm, optionally around 100 pm.

The channels in the plurality of channels may have a pitch of: at most around 5000 pm, optionally at most around 2500 pm, optionally at most around 1000 pm, optionally at most around 500 pm, optionally at most around 200 pm, optionally at most around 180 pm; and/or at least around 0.11 pm, optionally at least around 1 pm, optionally at least around 10 pm, optionally at least around 50 pm, optionally at least around 120 pm, optionally at least around 140 pm; and/or around 0.11 to 5000 m, optionally around 10 to 2500 pm, optionally around 100 to 500 pm, optionally around 100 to 250 pm, optionally around 160 pm.

The channels in the plurality of channels may have a pitch to diameter ratio of: at most around 2, optionally at most around 1.6; and/or at least around 1.1; and/or around 1.1 to 2, optionally around 1.2, 1.6 or 2.0.

The channels in the plurality of channels may have a substantially circular, hexagonal or triangular, ellipse-shaped or slotted opening.

The channels in the plurality of channels may be substantially ordered in an array, such as a hexagonal array. Hexagonal arrays allow high packing density and minimise radial variation in distance. In general, the ordering in the array should preferably be configured to give consistent diffusion distances and/or to mitigate system stresses.

Properties of the channels, including their diameter, pitch, shape and array ordering, can be confirmed using microscopy, such as using optical microscopy or SEM and plotting the results on a histogram, as described and exemplified herein.

In an implementation, the monolithic electrochemically active material may have: an in-plane conductivity of around 1 to 30,000 S-cm ^-1 ; and a porosity (attributable to the channels) of around 1 to 40%; and a density (active material/intrinsic density) of at least around 1.58 g/cnv ³ .

In an implementation (the features of which may optionally be combined with the features in the preceding implementation), the channels in the plurality of channels: have a channel diameter of around 10 to 100 pm; and have a pitch of around 150 to 200 pm; and are substantially ordered in a hexagonal array.

The monolithic electrochemically active material in either implementation, or their combination, is preferably pyrolytic graphite (e.g. a sheet thereof).

According to a second aspect of the present disclosure, there is provided an electrochemical cell, comprising the electrode according to the first aspect (e.g. as the negative electrode thereof), a further electrode (optionally a positive lithium electrode) and optionally a separator therebetween, optionally wherein the electrochemical cell further comprises electrolyte, and/or optionally wherein the electrochemical cell is a galvanic cell such as a coin cell, pouch cell, prismatic cell or a battery of cells, and/or optionally wherein the electrochemical cell is lithium, potassium, sodium or aluminium electrochemical cell, and/or optionally wherein the electrochemical cell is a secondary battery such as a lithium ion secondary battery.

Suitable electrolytes depend on the nature of the desired electrochemical reason and are well-understood by the skilled person. Suitable electrolytes for a lithium-ion battery, for example, include lithium-based salts (e.g. LiPFe) in an organic solvent (e.g. ethylene carbonate, ethyl methyl carbonate, dimethyl carbonate, etc.) and/or a mixture thereof.

According to a third aspect of the present disclosure, there is provided a method of manufacturing an electrode, comprising:

(A) providing a monolithic electrochemically active material; and patterning the monolithic electrochemically active material with a plurality of channels; or

(B) providing a polymer (e.g. a film thereof); and patterning the polymer with a plurality of channels; and pyrolyzing the polymer (before or after patterning) to form a monolithic electrochemically active material.

Patterning may be by machining, drilling, laser cutting/ablation, (photo)lithography, reactive ion etching, ion ablation, oxygen plasma etching, embossing or additive manufacturing, or the like. Laser cutting offers good resolution combined with reasonable processing speeds and design flexibility for prototyping. Laser cutting may offer the advantage of generating a heat-affected zone around the cut, for example expanding the graphite, increasing surface area and ion access. Embossing or additive manufacturing may be used in the polymer precursors state (e.g. before pyrolysis).

Various implementations disclosed herein are particularly suited to lithium-ion electrochemical cells/batteries, which generally operate by means of intercalation-insertion of lithium ions into the electrochemically active material lattice. It will be appreciated that similar concepts apply to other battery/cell types, notably those which have sufficient electrical conductivity, similar solid state diffusion/stress limitations, etc. By way of example, graphitic electrodes can be used in potassium- or sodium-ion cells (potassium or sodium intercalation), e.g. those based around ternary solvents (such as glycol ethers/glymes). The same comments apply to other electrolyte types, such as aluminium-ion batteries (AICIT intercalation).

EXAMPLES

Starting materials

Pyrolytic graphite sheet (EYGS091203, Panasonic) and a commercial graphite anode sheet on copper foil (bc-cf-241-ss-005, MTI) were both used as received.

Laser cutting of electrodes and electrolyte channels

A compact laser micromachining system (A series, Oxford Lasers) was used for cutting of electrodes from the starting material sheets and introduction of electrolyte channels. A G- Code script was developed to allow fast manufacture of electrodes with different hole diameter, hole pitch, and electrode diameter values.

Specifications for the laser micromachining system used for electrode manufacture are presented in Table 1.

Table 1 Specifications of the Oxford Lasers A series micromachining system used for electrode manufacture.

Five different electrode types were prepared, as detailed in Table 2. Table 2 Five different electrode types were manufactured.

After manufacture, optical microscopy, as described elsewhere herein, was used to verify the electrode microstructures. Hole diameters were extracted from the micrographs using an open-source computer vision library.

The micrographs show cleanly cut electrolyte channels (Fig. 2; with 200 pm (upper left), 160 pm (lower left), 120 pm (lower right) pitch electrolyte channels and the control pyrolytic graphite sheet electrode (upper right) without electrolyte channels) with a narrow diameter distribution centered at 98 and 96 pm respectively for the 200 and 160 pm pitch arrays (Fig. 3).

The holes were less-cleanly cut for the 120 pm pitch array and median hole diameter reduced to 90 pm. The reduced cut consistency is reflected by a broader distribution of hole diameters (Fig. 3). Scanning electron microscopy confirmed the optical microscopy results (Fig. 4).

Coin cell assembly

Prior to assembly, all cell components and electrodes were washed in ethanol and dried under dynamic vacuum at 80°C for 12 hours before transfer to an argon glove box (M-Braun) with O2 <0.5 ppm and H2O <0.5 ppm. Three cells were prepared for each of the five electrode types in Table 2 to ensure statistical significance of results.

The electrodes were assembled into coin cells with a 0.12 mm thickness lithium metal counter electrode (LI00-FL-000105, Goodfellow) and a 16 mm diameter Whatman GF/A glass fibre separator (WHA1820-021, Merck, 0.26 mm thickness, 1.6 pm pore size). The as- purchased lithium foil was punched into 10 mm diameter circles for use as an electrode.

Two 75 pl quantities of lithium hexafluorophosphate solution in ethylene carbonate and dimethyl carbonate (746711-100ML, Merck) were added as electrolyte before and after the separator. The full cell stack with component details is shown below (with product codes and suppliers indicated). Two 0.5 mm thick spacers were used in each cell. A hydraulic crimper (MSK-110, MTI) was used to seal the coin cells at a pressure of 7 bar before removal from glove box. All cells were rested for 10 hours prior to electrochemical characterisation to facilitate electrode wetting.

Coin cell top (EQ-CR2032-CASE, PI-KEM) Spring (EQ-CR20WS-Spring316, PI-KEM) Spacer (EQ-CR20-Spacer316, PI-KEM) Lithium metal foil (L100-FL-D00105, Goodfellow)

Electrolyte (746711 -100ML, Merck) Glass fibre separator (WHA1820-021, Merck) Electrolyte (746711 - 10OM L, Merck) Sample electrode

Spacer (EQ-CR20-Spacer316, PI-KEM) Coin cell botom (EQ-CR2032-CASE, PI-KEM)

Example 1: Rate capability test

To determine the rate capacity of the cells, galvanostatic electrochemical cycling was conducted between 5.0 mV and 1.5 V at different C rates (5 cycles at 0.1 C, 0.2 C and 0.5 C, followed by 10 cycles at 1 C and 2 C, and 5 cycles at 0.1 C). Both charging and discharging were performed using a constant current procedure. The C rate was calculated based on a theoretical capacity of 372 mA h g ^-1 . All measurements were performed at room temperature (25°C). Prior to each change in cycling rate, the cells were rested for 1 hour at OCV to allow for electrode equilibration.

Electrodes were assembled into coin cells with a lithium metal counter electrode for electrochemical characterisation. Galvanostatic cycling was conducted at increasing cycling rates to assess the capacity of the pyrolytic graphite sheet electrodes relative to a commercial slurry cast graphite electrode. Reversible capacity at 0.1 C for the commercial slurry cast electrode was in line with the theoretical specific capacity for intercalation of lithium into graphite to form LiCe

(372 mA h g ^-1 , see Fig. 5 - Galvanostatic cycling between 5 mV and 1.5 V at rates of 0.1 C to 2 C (raw data). Specific discharge capacity is normalised to electrode mass, including active material, binder, conductive additive, and current collector) when normalised to the active material mass.

For consistency with the pyrolytic graphite sheet electrodes, the slurry cast electrode capacity was re-plotted normalised to the dry electrode mass, including binder, conductive additive, and current collector mass (Fig. 6: Galvanostatic cycling between 5 mV and 1.5 V at indicated charge rates between 0.1 C and 2 C).

The capacity of the slurry cast electrode normalised to the electrode mass was -240 mA h g ^-1 to facilitate direct comparison (including active material, binder, conductive additive, and current collector where applicable), highlighting the severe penalty of inactive materials accounting for just 64% of dry electrode mass.

Cells which showed unstable cycling were not plotted in Fig. 6, full data is shown in Fig. 5. Line indicates mean capacity for each electrode type.

The capacity of the pyrolytic graphite sheet without electrolyte channels was low

(< 10 mA h g ^-1 ) at all tested current densities, even 0.1 C. Low capacity of the unmodified pyrolytic graphite sheet was anticipated given the low electrolyte-accessible surface area of these electrodes. Electrolyte channels were introduced to increase specific surface area, thereby reducing areal current density and the path length for lithium ions within the graphite following intercalation. Introducing electrolyte channels in a hexagonal array with 200 pm pitch increased the capacity of the pyrolytic graphite sheet electrodes 10-fold to more than 100 mA h g- ¹ at 0.1 C (Fig. 6).

The set of electrodes with a narrow array pitch (160 pm) achieved an even higher capacity of 225 mA h g ^-1 at 0.1 C. The electrodes with 120 pm pitch showed highly variable capacity (see Fig. 5), most likely due to inconsistent wetting, and are not included in Fig. 6 for clarity. The pyrolytic graphite sheet electrodes exhibited good capacity retention with increasing charge rate. At the highest charge rates of 1 C and 2 C the capacity of the pyrolytic graphite sheet with 160 pm electrolyte channels is higher than the slurry cast electrode. These data shows that a pyrolytic graphite sheet electrode with electrolyte channels can achieve a specific capacity, normalised to electrode mass, that exceeds conventional slurry cast electrodes.

The staged intercalation of lithium into graphite is visible as a series of plateaus in the voltage-capacity profile. All electrodes show irreversible sloping capacity in the first cycle, associated with SEI formation, that is not seen in subsequent cycles (Fig. 7). Irreversible first cycle capacity is highest for the pyrolytic graphite sheet electrode with 160 pm, most likely due to an increased surface area compared to the other pyrolytic graphite sheet electrodes. Cycling is stable from cycle 2 onwards for all electrodes and little difference is observed at 0.1 C before and after the charge rate ramp (Fig. 6)

In contrast to the conventional slurry cast process, this work demonstrates a single, solvent- free production step for the transformation of commercial pyrolytic graphite sheet into high- density, free-standing, and low diffusion-path graphite electrodes. By introducing appropriately spaced electrolyte channels, the capacity of pyrolytic graphite sheet electrodes is shown to exceed commercial slurry cast graphite electrodes at elevated current densities.

In contrast to the slurry cast process, electrodes manufactured from pyrolytic graphite sheet do not require the use of conductive additive, binder, or a current collector foil. Omitting these inactive materials enables higher specific capacities on an electrode mass basis. Moreover, in combination with electrolyte channels, the uniform aligned sheets of pyrolytic graphite sheet create a low-diffusion path architecture with substantially uniform potential and intercalation response. Overall, this improves capacity retention at elevated current densities. Finally, both manufacturing process and electrode structure are simple and controlled. These attributes could prove invaluable in swiftly achieving gigafactory scale production.

Example 2: Evaluating potential commercial cell performance

To evaluate the potential commercial cell performance, calculations were performed to assess the impact of replacing the slurry cast graphite electrode of a commercial LGM50 cell with a patterned pyrolytic graphite sheet according to the present disclosure (the patterned pyrolytic graphite sheet having equivalent areal capacity). First, parametrization data (table below) for the LGM50 cell is used to establish the jelly roll areal capacity, thickness, wet mass loading and volume.

Next, the slurry cast negative electrode is swapped for a patterned pyrolytic graphite sheet electrode of equivalent areal capacity. The thickness and wet mass loading of the modified jelly roll are calculated. The area and wet mass of this modified jelly roll that will occupy the same volume as the original jelly roll is then calculated. Finally, the capacity, mass and gravimetric energy density of the modified LGM50 cell are calculated. Calculations shown in the table below.

The example considers a patterned pyrolytic graphite sheet electrode with a hexagonal array of 5 pm diameter, 20 pm pitch channels (6% porosity due to channels) and a gravimetric active material capacity of 450 mA h g ^-1 .

The voltage (cells); and areal capacity & volume (jelly rolls) parameters are assumed constant.

The additional calculations show that, without changing the cell chemistry, a pyrolytic graphite sheet electrode with a hexagonal array of 5 pm diameter, 20 pm pitch channels could increase the gravimetric energy density of a commercial LGM50 cell by 22% to 322 W h kg ^-1 . These calculations demonstrate that with appropriate design, these techniques could significantly improve the performance of commercial cells.

Fig. 8 depicts the modification with a schematic drawing of the cells and jelly rolls used herein. As labelled:

(A) Commercial LGM50 cell

(B) Commercial LGM50 jelly roll

(C) Modified LGM50 jelly roll

(D) Modified LGM50 cell

(1) Remove original jelly roll

(2) Swap negative electrode

(3) Replace jelly roll Parametrization data:

Those skilled in the art will recognise or be able to ascertain using no more than routine experimentation many equivalents to the specific embodiments described herein. The scope of the present disclosure herein is not intended to be limited to the above description, but rather is as set forth in the appended claims. Those of ordinary skill in the art will appreciate that various changes and modifications to this description may be made without departing from the spirit or scope of the present disclosure.

Any listing or discussion of an apparently prior-published document in this specification should not necessarily be taken as an acknowledgement that the document is part of the state of the art or common general knowledge. All references disclosed herein are to be considered to be incorporated herein by reference. All features discussed herein in respect of any products or methods relate to all other products or methods mutatis mutandis. For example, the features discussed herein in relation to electrodes could be adopted in the methods for manufacture of electrodes.