Method and apparatus for imaging electrochemical cell electrodes

Abstract

An electrochemical cell 300 includes a working electrode 306 having a channel 322 passing at least partially therethrough; a transparent window 320 providing optical access to the working electrode; a

Description

FIELD OF THE INVENTION The invention relates to a method and apparatus for imaging electrodes. In particular, but not exclusively, the invention relates to imaging electrochemical cell electrodes in an optically accessible electrochemical cell, using optical microscopy, during an electrochemical process. BACKGROUND In order to advance battery technologies, such as those based on lithium-ion batteries, it is helpful to understand how battery materials function on the nano- to mesoscale during real-time operation. For example, it has been demonstrated that optical measurements of electrochemical cell electrodes during electrochemical processes (such as charging and discharging) can provide useful mechanistic information (Merryweather et al., Nature, Vol. 594, 2021, pages 522 to 528). In contrast to previous methods for operando imaging of lithium-ion dynamics, based on sophisticated synchrotron X-ray or electron microscopy techniques, the use of an optical scattering microscope has been shown to resolve nanoscopic lithium-ion dynamics in battery materials in an elegant manner suitable for high-throughput analysis. However, whilst charging, discharging and degradation mechanisms relating to individual particles (e.g., particles of LixCoO2) within an electrode matrix have been investigated in such experiments, previous studies into such electrochemical processes have been limited to dilute electrodes. In this context, the term “dilute” is used as an umbrella term including porous, self-standing electrodes with a relatively low density of active material particles, such as, LixCoO2, compared with conventional electrodes having a relatively high density of active material particles. The relatively sparse distribution of active material particles within an inactive matrix means that changes to the active material particles during electrochemical processes, such as charge-discharge operations, can be monitored with reference to the inactive matrix. For example, a reference laser beam directed onto the inactive matrix, with a spot size that can be positioned between active material particles, can be used as a reference point to adjust the image focus of imaging apparatus, in order to account for any change in position of the electrode surface during electrochemical processes. For example, the use of a reference laser beam with a spot size of 8 to 15 microns at full-width-half-maximum means that the spot is much larger than the roughness of the inactive matrix in the region between active material particles, thus providing a clear reflection. In contrast, where the active material particles are densely packed, such as in conventional electrodes, the surface of the electrode is too rough to apply such a technique as the spacing between active material particles is less than the spot size of such a laser beam. Further, changes in the active particles during electrochemical processes means that the surface of such densely packed conventional electrodes does not provide a consistent reflection that can be used to aid the adjustment of image focus during the electrochemical processes. In order to perform optical measurements on dilute electrodes during their operation, a relatively complex operando cell apparatus can be used. Figure 1A shows an exploded-view arrangement of an exemplary prior art system 100 for analysing the electrochemical performance of a dilute electrochemical cell electrode 104 by scattering microscopy. The system 100 comprises an optical apparatus 120 that is used to interrogate a dilute electrochemical cell electrode 104 in an operando cell apparatus. The operando cell apparatus includes an optical window 102 within a lower casing 101, the optical window 102 providing optical access to the dilute electrode 104. The dilute electrode 104 is a free-standing electrode of Lithium Cobalt Oxide (LCO) powder distributed in a polymerbased matrix. The free-standing dilute electrode 104 is in electrical contact with an aluminium mesh 106, which is, in turn, in electrical contact with a probe 108 to forming the working electrode. The aluminium mesh 106 is separated, by a separator 110, from a lithium metal material 111 and further probe 112, which together form the counter electrode. An upper casing 103 cooperates with the lower casing 101 to seal the half-cell stack, which is wetted with a standard lithium hexafluorophosphate (LiPF6) and carbonate liquid electrolyte (LP30). The optical apparatus 120 comprises a laser diode as a light source 122 that is directed through the optical window 102 of the operando cell apparatus, as shown by an arrow 114. Prior to passing through the optical window 102, the light will have passed through a series of optical components, including a lens 124, polarizing beam splitter 126, quarter wave plate 128 and objective 130. The light source 122 is used to illuminate the dilute electrode 104 and the optical components are arranged such that incident light is focused on an active particle in the dilute electrode 104 and the light subsequently scattered by the active particle is collected via a further lens 132, such that the collected light is imaged at a complementary metal oxide semiconductor (CMOS) camera 134. Such a system enables the dilute electrochemical cell electrode to be imaged whilst the desired currentvoltage conditions are applied to the operando cell apparatus. The operando cell apparatus described above, whilst modular and re-usable, is unsuitable for routine battery testing because it is considered to be relatively expensive, difficult to assemble and to have unreliable cycling performance. In order to improve understanding of the processes during real-time operation of conventional electrochemical cells, such as coin cells, it would be beneficial optically to interrogate conventional electrochemical cells during electrochemical processes. In particular, it would be beneficial optically to interrogate dense, conventional electrochemical cell electrodes during electrochemical processes. Figure 1B shows a cross-sectional view of a conventional electrochemical coin cell 200 of the prior art. Within the coin cell 200 there is shown a stack of layers including a foil layer 204 upon which a working electrode layer 206 is positioned. The foil layer 204 is in electrical contact with a first part of the casing 202 of the coin cell 200. The working electrode layer 206 is separated from a counter electrode 210 by a separator 208. In between the counter electrode 210 and a second part of the cell casing 216 there is a spacer layer 212 and a spring 214. Conventional electrochemical coin cells are not optically accessible, not least since both the first part of the casing 202 and the foil layer 204 are optically opaque, thereby preventing optical access to one planar face of the working electrode layer 206. Additionally, the separator 208, counter electrode 210, spacer layer 212 and the second part of the casing 216 prevent optical access to the other planar face, opposite the planar face of the working electrode layer 206 that is adjacent to the foil layer 204. As described above, not only do such conventional electrochemical cells not provide optical access to the active material, even if they did, the rough surface of dense electrochemical cell electrodes due to the high density of active material particles is difficult to image in real time compared to the relatively low density of active material particles in dilute electrodes, as changes to the high density of active material particles during operation cannot be tracked with reference to an inactive matrix, as is possible with dilute electrochemical cell electrodes. Further, if the arrangement of the mesh layer 106 and the dilute electrochemical electrode 104 described with reference to Figure 1A were replaced with a densely packed conventional electrochemical cell electrode cast on a foil layer, electrolyte within such an operando cell could not penetrate the conventional foil layer to reach the active material of the electrochemical cell electrode. This contrasts with the free-standing dilute electrochemical cell electrode of the prior art, which is porous (and does not include a foil layer), thereby enabling electrolyte access to the sparsely distributed active material particles with the free-standing matrix. SUMMARY OF INVENTION In order to mitigate for at least some of the above-described problems, there is provided an electrochemical cell comprising: a working electrode comprising a channel passing at least partially therethrough; a transparent window configured to provide optical access to the working electrode; and a reflection interface between a portion of the transparent window and a portion of the channel, wherein the reflection interface is configured to change in a manner indicative of a corresponding change in the working electrode. Advantageously, the electrochemical cell is optically accessible such that a portion of the working electrode comprising active material particles can be imaged in an improved manner during an electrochemical process. The electrochemical cell may comprise a foil layer on a first surface of the working electrode. The transparent window may be configured to provide optical access to a second surface of the working electrode positioned opposite to the first surface. Beneficially, the foil layer provides support for the working electrode without obscuring the portion of the working electrode layer that is imaged. The electrochemical cell may comprise a foil layer on a first surface of the working electrode, wherein the transparent window is configured to provide optical access to the first surface of the working electrode through one or more apertures in the foil layer. Beneficially, the foil layer provides support for the working electrode without obscuring the portion of the working electrode layer that is imaged. The foil layer may be a current collector and / or a mesh layer. Advantageously the current collector provides electrical conductivity across the portion of the working electrode comprising the active material particles. Beneficially, the use of a mesh layer provides channels for electrolyte access to the active material particles of the working electrode. The channel may extend through the working electrode and the foil layer. Beneficially, the channel provides the means for electrolyte to access the active material particles whilst simultaneously providing an interface between the transparent window and channel that can provide a reflection interface for improved imaging of the working electrode during an electrochemical process. The electrochemical may be a free-standing working electrode. Advantageously, the working electrode is self-supporting and therefore does not require additional layers. In use, the channel may comprise an electrolyte. Advantageously, the electrolyte is provided within the portion of the working electrode comprising the active material particles whilst also providing a material with a different refractive index that can be used to form an interface usable for tracking changes at the surface of the working electrode. The working electrode may be in mechanical communication with the transparent window. Beneficially, physical changes in the working electrode, such as changes in position due to swelling and contraction during electrochemical processes cause physical changes in the position of the transparent window and hence the reflection interface formed between the channel through the working electrode and the transparent window, which can be monitored such to provide improved imaging of the working electrode during operation of the electrochemical cell. The working electrode may comprise an active material wherein an active material loading of the working electrode may be at least 80%. The electrochemical cell may be a coin cell and / or mounted on a printed circuit board. Beneficially, the coin cell can be integrated into known systems for extensive performance testing, whilst also providing optical access to aid improvements in related battery technologies. Furthermore, according to the present invention there is also provided a method for imaging a working electrode during at least part of an electrochemical process, the method comprising: monitoring a reflection at an interface between a transparent window and a channel that extends at least partially through the working electrode; imaging a portion of the working electrode during at least part of an electrochemical process with an imaging apparatus; determining a change in the reflection at the interface; and adjusting the image focus of the imaging apparatus in response to determining the change. Advantageously, an improved system for stable imaging of working electrodes during their operation is provided. In use, the channel may comprise an electrolyte. Advantageously, the electrolyte is provided directly to the portion of the working electrode comprising the active material particles, whilst also functioning as part of a reflection interface that is usable as part of a feedback mechanism for stable imaging of a working electrode during its use. The step of monitoring the reflection may comprise directing a reference beam at the interface and detecting the position and / or intensity of the reflected reference beam. Beneficially, changes in the reflection of a reference beam indicate changes in the working electrode, thereby enabling adjustments to be made when imaging the working electrode. The step of imaging the portion of the working electrode may comprise imaging a first surface of the working electrode positioned opposite to a second surface of the working electrode, wherein the working electrode comprises a foil layer on the second surface. Advantageously, the foil provides support and uniform electrical conductivity without inhibiting optical inspection of the working electrode. The step of imaging the portion of the working electrode may comprise imaging a first surface of the working electrode, wherein the working electrode comprises a foil layer comprising one or more apertures on the first surface. Advantageously, the foil provides support without inhibiting optical inspection of the working electrode. The step of adjusting the image focus may comprise dynamically altering the focal plane of the imaging apparatus, thereby compensating for changes in the position of the portion of the working electrode in a direction substantially perpendicular to the focal plane. Beneficially, improved stability in image focus is provided during electrochemical processes. There is also provided a method of preparing an imaging system for imaging a working electrode during at least part of an electrochemical process in accordance with the method described herein. The method of preparing the imaging system may comprise the step of forming a channel at least partially through the working electrode. Advantageously, the provision of a channel enables electrolyte access to the active material particles of a working electrode whilst providing the means to form an interface to monitor reflections from a reference beam, wherein the reflections are indicative of changes in the working electrode that can be compensated for when imaging. The step of forming the channel may be implemented by forming the working electrode on a foil mesh or woven wire mesh comprising one or more channels. Alternatively, the channel is formed by perforating a foil layer and forming the working electrode on the perforated foil layer. Alternatively, the channel is formed by forming the working electrode on a foil layer and subsequently perforating the working electrode. Beneficially, conventional working electrodes can be adapted for an optically accessible electrochemical cell, or commercially available meshes or adapted foils can be provided such that active material particles are subsequently cast upon them, providing channels with the benefits described herein. The step of perforating may comprise the use of at least one of: mechanical perforation; laser drilling; and laser ablation. Beneficially, mechanical perforation is an elegant and effective way to adapt existing materials. Advantageously, laser drilling and laser ablation provide fine control for creating uniform channels through working electrodes. Furthermore, according to the present invention there is also provided a system for imaging a working electrode during at least part of an electrochemical process, the system comprising: a light source; and an imaging apparatus, wherein the system is configured to perform the method described herein. Preferably, the system comprises an electrochemical cell described herein. The system may comprise an electrical connection for receiving an electrochemical cell, such as a battery. The light source may comprise at least one of: a laser; a light emitting diode; and a lamp, and / or wherein the imaging apparatus comprises an optical microscope. Further aspects of the invention will be apparent from the description and the appended claims. BRIEF DESCRIPTION OF THE FIGURES The invention is further and more particularly described, by way of example only, and with reference to the accompanying drawings, in which: Figure 1A shows a system for analysing the electrochemical performance of a dilute battery electrode by interferometric scattering, in accordance with the prior art; Figure 1B shows a cross-section of an electrochemical coin cell, in accordance with the prior art; Figure 2 shows a cross-section of an optically accessible electrochemical cell and optical imaging system; Figure 3 shows a process flow for imaging a working electrode during an electrochemical process; Figures 4A and 4B shows a sequence of cross-sectional images as part of a process for forming a working electrode; Figures 5A and 5B shows a sequence of cross-sectional images as part of a process for forming a working electrode; Figures 6A to 6C shows a sequence of cross-sectional images as part of a process for forming a working electrode; Figure 7A shows an optical apparatus to enable autofocussing; and Figure 7B shows monitored reflections from a reference beam. DETAILED DESCRIPTION OF FIGURES As described above, known methods for the optical imaging of electrochemical cell electrodes during real-time operation are not suitable for application to conventional electrochemical cell electrodes. Figure 2 shows a cross-sectional view of an optically accessible electrochemical cell 300 forming part of a system for imaging a working electrode layer 306 of the electrochemical cell 300 during an electrochemical process. Advantageously, the arrangement described with reference to Figure 2 enables the optical imaging of conventional electrochemical cell electrodes during real-time operation, thereby facilitating a greater understanding of the processes occurring during operation in such conventional electrodes, and hence issues that can be addressed in order to advance such technologies. The electrochemical cell 300 comprises a cell casing 302, which in the orientation shown is the lower cell casing 302 (with the electrochemical cell 300 being orientable in any convenient direction in use). The lower cell casing 302 has an aperture 303 enabling optical access into the electrochemical cell 300, in contrast with the opaque, apertureless, casing 202 of the prior art electrochemical coin cell 200 described with reference to Figure 1B. There is a transparent window 320, which seals the aperture 303 whilst providing optical access into the interior of the electrochemical cell 300. The transparent window 320 is a glass coverslip affixed to the inside of the cell casing 302 using thermoplastic to create a seal. The thickness of the glass coverslip is approximately 170 microns and has a diameter of approximately 12 mm. The aperture 303 is circular with a diameter of 6 mm. In other, non-illustrated embodiments, the aperture 303 has a different shape and size whilst providing optical access to the interior of the electrochemical cell 300. In further non-illustrated embodiments, the transparent window 320 is formed from a different material, with the same or different thickness and / or diameter, to provide optical access to the working electrode of the electrochemical cell 300, such that the material is optically transparent at the wavelengths of light used to image the working electrode. In further non-illustrated embodiments, alternatively, the transparent window is affixed to the outside of the cell casing 302, or at least partially within the thickness of the cell casing 302. Whilst the transparent window 320 is described as being affixed to the cell casing 302 using a thermoplastic, in further examples, additional and / or alternative materials and / or techniques are used to affix the transparent window 320 to the cell casing 302. Above the transparent window 320, there is shown a working electrode layer 306. The working electrode layer 306 is a conventional working electrode layer with a high density of active material particles cast upon a foil layer 304. Together, the working electrode layer 306 comprising active material particles and the foil layer 304 form a working electrode. The foil layer 304 is an aluminium layer and acts as a foil current collector. In further examples, additionally or alternatively, the foil layer 304 comprises a different conductive material, such as copper. The working electrode layer 306 comprises active material particles. In an example, the active material comprises Lithium Cobalt Oxide (LCO). In further examples, the active material comprises Nickel-Manganese-Cobalt Oxide (NMC). In yet further examples, the working electrode layer 306 comprises any appropriate electrochemical cell material. In examples, additionally, the working electrode layer 306 comprises additional material, such as conductive additive material and / or binder material. The working electrode layer 306 has a circular face with a diameter of 10 mm and is formed on a circular aluminium foil layer 304 with a thickness of 20 microns. In further examples, the working electrode is formed from a working electrode layer 306 and foil layer with different shapes and / or dimensions. The working electrode layer 306 with active material particles is positioned between the transparent window 320 and the foil layer 304 of the working electrode such that active material particles are viewable through the transparent window 320. The working electrode layer 306 comprising active material particles is pressed against the internal face of the transparent window 320 such that the working electrode layer 306 and the transparent window 320 are in mechanical communication. In further examples, there are one or more further layers between the working electrode layer 306 and the transparent window 320 such that the functionality described herein is maintained. Both the working electrode layer 306 and the foil layer 304 are shown with a plurality of channels 322 passing through both the working electrode layer 306 and the foil layer 304. The channels 322 enable electrolyte within the electrochemical cell 300 to access the working electrode layer 306. This is in contrast to conventional electrodes, where a foil layer without channels would prevent electrolyte accessing the working electrode layer comprising active material particles when pressed against the transparent window 320. Whilst the channels 322 shown in Figure 2 are shown to extend through the entirety of the height of the working electrode layer 306 and the foil layer 304 of the working electrode, in further non-illustrated examples, one or more of the channels 322 passes partially through the height of the working electrode layer 306 such that the functionality described herein is preserved. The interfaces between the transparent window 320 and the channels 322 provide clean and flat surfaces with a difference in refractive indices that can be used as a reference reflection interface for image focus stabilisation, as described herein. For example, the difference in refractive indices for the glass of the transparent window 320 and electrolyte within the channels 322 provides an interface location for reflecting a reference beam, as described herein. Further, as the working electrode layer 306 is mechanically coupled to the transparent window, changes in the position of the reflection interface between the transparent window 320 and the channel 322 enable the corresponding changes in the surface of the working electrode layer 306 in the vicinity of the reflection interface to be tracked, thereby providing an input for the imaging focus of the working electrode layer 306 to be adjusted in the event of any changes in the position of the working electrode layer 306 (e.g., due to swelling / contraction during operation of the electrochemical cell 300). Similarly, during operation, changes in the refractive index of a material, such as an electrolyte, within the channel, change the properties of the interface and hence the reflection of a reference beam directed at the interface. Such a change is indicative of behaviour in the working electrode layer 306 and may be used to inform an adjustment of an image focus of a portion of the working electrode layer 306 during an electrochemical process. The plurality of channels 322 form circularly cross-sectioned holes through the working electrode, with diameters of 100 microns and a relative linear separation of 300 microns, arranged in a regular square pattern of holes. In further non-illustrated examples, holes of any cross-sectional size and shape form the plurality of channels 322, in any appropriate regular or irregular formation, or pattern across the working electrode. A conductive mesh 318 is shown above the foil layer 304 and is in electrical communication with the lower cell casing 302. The conductive mesh 318 is an aluminium mesh with open holes of 1 mm diameter and narrow ribbons of aluminium metal of 140 microns in width. Whilst a conductive mesh 318 is shown in Figure 2, in further non-illustrated examples, a conductive mesh 318 is not included. For example, the working electrode layer 306 may be in direct electrical contact with the cell casing 302. Alternatively and / or additionally, the transparent window 320 may be at least partially electrical conductive, thereby to provide electrical communication between the working electrode layer 306 and the cell casing 302. In further non-illustrated examples, additional and / or alternative material is used to provide electrical connection to the cell casing 302. Above the mesh 318 there is shown in Figure 2 a separator 308, a counter electrode 310, a spacer 312 and a spring 314, corresponding to the separator 208, counter electrode 210, spacer 212 and spring 214 described with respect to the conventional electrochemical coin cell 200 of the prior art shown in Figures 1A and 1B. The separator 308 is a glass fibre separator and the spacer 312 is a stainless steel spacer. In further non-illustrated examples, additional and / or alternative layers and / or components are implemented. The stack of layers within the electrochemical cell 300 is wetted with an electrolyte. In an example, 200pl of LP57 electrolyte is used. In further examples, a different amount and / or type of electrolyte is used. The layers within the electrochemical cell 300 are encased with the upper cell casing 316, thereby sealing the electrolyte within the electrochemical cell 300. The working electrode layer 306 is optically accessible through the transparent window 320 created within the aperture 303 in the lower cell casing 302. Whilst the electrochemical cell 300 is described with respect to particular layers formed in a particular order, in further examples, additional, fewer and / or alternative layers and / or components are used to provide an optically accessible electrochemical cell in accordance with the claimed invention. For example, in further examples, the working electrode is a free-standing electrode layer that does not require a foil layer 304. In further examples, the working electrode comprises a foil layer 304 on the surface of the working electrode layer 306 positioned between the transparent window 320 and the working electrode layer 306. In such a configuration, the foil layer 304 comprises one or more apertures, which expose corresponding portions of the working electrode layer 306, thereby to enable optical access to the working electrode layer 306 through the foil layer 304, such that the working electrode layer 306 is imaged in accordance with the methods described herein. The cell casing 302, 316 forms a coin cell casing, such as a modified CR2032 coin cell casing. In further examples, the electrochemical cell 300 has the form of a different coin cell. In further examples, the electrochemical cell 300 is a different type of cell, such as a pouch cell or a Swagelok (RTM) type cell, and the internal components are arranged analogously whilst providing the benefits described herein. Electrical connections made to the upper cell casing 316 and the lower cell casing 302 facilitate the application of current-voltage conditions, such as charging and discharging conditions, which cause electrochemical processes to occur within the electrochemical cell 300. Such electrochemical processes can be imaged by imaging a portion of the working electrode layer 306 that is optically accessible through the transparent window 320. Advantageously, the arrangement of the entities in the electrochemical cell 300 is such that electrolyte can access the active material of the working electrode layer 306 via the channels 322, whilst facilitating improved imaging, as described herein. Further, the electrochemical cell 300 is relatively cost effective, easy to assemble, can be integrated into route battery testing and provide much improved cycling performance compared with known cells. The working electrode layer 306 is imaged using an imaging apparatus 328. The imaging apparatus 328 comprises a light source 330 providing light for illuminating / imaging in a direction, as indicated by an arrow 326. The light source is a 532 nm light emitting diode (LED). In further examples, alternative and / or additional light sources, such as laser diodes, lamps and / or LEDs with different peak wavelengths are used for illumination / imaging of the working electrode layer 306. In order to image the working electrode layer 306, the electrochemical cell 300 is held in place in an appropriate location, such as a part of a sample stage. In an example, the imaging apparatus 328 comprises a fixed location for receiving the electrochemical cell 300 and for making electrical connections to the electrodes of the electrochemical cell 300, such that appropriate current-voltage conditions can be applied whilst imaging the working electrode layer 306. The light source 330 is arranged in order to direct light through an optical microscope comprising a lens system 332, polarising beam splitter 334 or beam splitter, quarter wave plate 336 and objective 338, to an visually exposed portion of the working electrode 306, such that the portion of the working electrode 306 is imaged. Light scattered by the portion of the working electrode layer 306 is then directed through the objective 338, quarter wave plate 336 and by the beam splitter 334, through the lens system 340 to the camera 342, where images of the portion of the working electrode layer 306 are captured. Whilst the optical microscope comprises a particular arrangement for imaging the working electrode layer 306, in further examples, alternative and / or additional optical components are implemented in order to provide imaging focussed on a portion of the working electrode layer 306 during an electrochemical process. The imaging apparatus 328 is in communication with a computing device 352 via a communication path 346 between an interface of the imaging apparatus 328 and an interface 358 of the computing device 352. The computing device 352 comprises a processor 354 and a memory 356. The computing device 352 is in communication with a network 360 via a communication path 350 between the computing device 352 and the network 360. The imaging apparatus 328 is also in communication with the network 360 via a communication path 348 and the interface 344 of the imaging apparatus 328. The computing device 352 is in communication with an autofocussing apparatus 800, which is described in further detail with reference to Figure 7. The autofocussing apparatus 800 is used to direct and measure a reflection of a reference beam 324 from a reflection interface formed in the optically accessible electrochemical cell 300. Whilst the imaging apparatus 328 and autofocussing apparatus 800 are shown in communication with a computing device 352 and a network 360, in further non-illustrated embodiments, the imaging apparatus 328 and autofocussing apparatus 800 are implemented and controlled in any suitable manner in accordance with the techniques described herein. In an example, the computing device 352 is integrated into the imaging apparatus 328 and / or autofocussing apparatus 800. During imaging of a portion of the working electrode layer 306, it is important to maintain the focus of the surface of the portion of the working electrode layer 306 that is being imaged. The computing device 352 is configured to communicate with the imaging apparatus 328 and autofocussing apparatus 800 such that the optical components of the imaging apparatus 328 and / or the relative position of the imaging apparatus 328 with respect to the electrochemical cell 300 are dynamically controlled. In order to maintain the focus of the surface of the portion of the working electrode layer 306 that is being imaged, the focal plane of the optical apparatus is adjusted in a direction substantially perpendicular to the surface of the portion of the working electrode layer 306 (which may be substantially perpendicular to the planar surface of the working electrode layer 306 adjacent to the transparent window 320). In Cartesian coordinates, where the planar surface of the working electrode layer 306 lies in a plane substantially defined by the x-axis and the y-axis, the focal plane of the optical apparatus is adjusted along the z-axis in order to provide a focussed image of a portion of the working electrode layer 306. Whilst a portion of the working electrode layer 306 initially may be brought into focus by changing the relative position of the focal plane in a direction perpendicular to the z-axis such that the portion coincides with the focal plane of the optical apparatus 328, during real-time operation of the electrochemical cell 300 by the application of current-voltage conditions, the position of the portion of the working electrode layer 306 may change, such that it no longer coincides with the focal plane. Such changes can result from the working electrode layer 306 swelling or contracting during operation, changes in the refractive index of electrolyte within the electrochemical cell 300 during operation, and changes due to drift in the sample stage supporting the electrochemical cell during operation. As described above, the high density of active material in conventional electrochemical cell electrodes means that the surface is rough and not suitable for using as a reference point for maintain focus stabilisation, as with dilute electrochemical cell electrodes, where a reference light beam with a suitable spot size is positioned on the relatively smooth and inactive surface between active particles such that changes in the z-position of the dilute electrode can be tracked. Advantageously, the channels 322 of the optically accessible electrochemical cell 300 not only provide paths for electrolyte to reach the active material particles of the working electrode layer 306, but they also provide a reference reflection interface between a portion of the transparent window 320 and a portion of the channel 322 that can be used to monitor changes in the working electrode layer 306 that result in changes with respect to the relative difference in position between the portion of the working electrode layer 306 that is being imaged and the focal plane of the optical apparatus 328 that is being used to image the portion of the working electrode layer 306. Figure 3 shows a process flow S400 for imaging a portion of the working electrode layer 306 of the optically accessible electrochemical cell 300 during an electrochemical process. The process flow S400 is performed using the imaging apparatus 328 described with reference to Figure 2 and / or in other embodiments referenced in the description that provide similar or identical functionality, in order to image a portion of the working electrode layer 306 of the optically accessible electrochemical cell 300. The process flow S400 is controlled using a computing device 352 described with reference to Figure 2. The computing device 352 is used to control the imaging process directly through control of the imaging apparatus 328 and / or indirectly via a computing network 360. The process is initiated at a first step S402 and moves to step S404, where a portion of the working electrode layer 306 is imaged. The process may be initiated by a user identifying a portion of the working electrode layer 306 to image and initiating an experiment to perform an electrochemical process at the electrochemical cell 300 whilst simultaneously imaging a portion of the working electrode layer 306 and automatically maintaining focus of the portion of the working electrode layer 306 that is to be imaged. In order to image a portion of the working electrode layer 306, the imaging apparatus 328 is arranged such that the portion of the working electrode layer 306 is illuminated with light, as shown by the arrow 326 in Figure 2. The image focus of the imaging apparatus 328 is adjusted such that the focal plane of the imaging apparatus 328 coincides with the portion of the working electrode layer 306 that is being imaged. An electrochemical process is initiated by applying the appropriate current-voltage conditions to the electrodes 306, 310 of the electrochemical cell 300. An example of an electrochemical process is charging and discharging the electrochemical cell 300 using CC CV-CC CV cycling protocols at a constant-current (CC) rate of C / 5 (nominal 5 hour (dis)charging time) with a constant voltage (CV) of 1 hour. In further examples, different electrochemical processes are instigated. The process then moves to step S406, where it is determined whether the imaging is complete. The decision to determine whether the imaging is complete or not is optionally based on user intervention, or alternatively and / or additionally, by control of the imaging apparatus 328 based on computer implemented instructions executed by the computing device 352, for example, based on computer implemented instructions stored in the memory 356 of the computing device 352. If the imaging has not finished, the process moves to step S408, where a reference reflection is monitored. The reference reflection is monitored by the computing device 352 in combination with the imaging apparatus 328. The computing device 352 is integrated into the imaging apparatus 328 in an example. The computing device 352 is configured to implement a hardware or software feedback protocol, for instance a proportional integral derivative (PID) control system such that inputs based on the monitored reflection of a reference beam are processed by the processor 354 in order to determine focussing control of the imaging apparatus 328. Imaging and focus adjustment can occur in series or in parallel. For example, the imaging apparatus 328 can continue to take images of the portion of the working electrode layer 306 whilst the autofocussing apparatus 800 simultaneously monitors the reference reflection, such that the computing device 352 coordinates the imaging and focus adjustment in any suitable manner. The reference reflection is monitored by directing focussed radiation at a reference reflection interface comprising a portion of the transparent window 320 and a portion a channel 322. Advantageously, the reflection interface provides the means for obtaining a clean and consistent reflection that is inextricably linked to the working electrode layer 306 such that a change in a reflection at the reflection interface is indicative of a change in the working electrode layer 306. This means, for example, that if the working electrode layer 306 changes during operation such that the portion being imaged moves out of the focal plane of the imaging apparatus 328, the change in reflection at the reflection interface is used as an input to compensate for the change by adjusting the focal plane of the imaging apparatus 328 in the z-direction substantially perpendicular to the planar surface of the portion of the working electrode layer 306 that is being imaged, thereby maintaining a stable focus. The reference reflection is monitored using the optical apparatus 800 described with reference to Figures 7A and 7B, in combination with the computing device 352 and / or imaging apparatus 328. Figure 7A shows an optical apparatus 800A to enable autofocussing of the imaging apparatus 328, as controlled by the computing device 352, thereby to illustrate the principle of using a reference beam reflected at a reflection interface to determine changes in reflections and to compensate for them accordingly. There is shown a reference beam 802 from a light source that is directed through a focusing lens 816 and via a mirror 818 through an objective 820 such that it is focussed at a reflection interface. The light source is a collimated low power laser beam with a wavelength of 785 nm. In further examples, any suitable wavelength and power of light source is used to provide a reference beam. As illustrated with the arrow 324 in Figure 2, the reference beam is directed at an interface between the transparent window 320 and the channel 322 passing through the working electrode layer 306. Advantageously, the channel 322 may contain electrolyte, thereby simultaneously providing electrolyte to the active material particles of the working electrode layer 306 whilst also providing a reference point to determine any changes that may alter the image focus of the working electrode layer 306. In the example of Figure 7A, there is shown the transparent window 320 such that there is a reflection interface between the transparent window 320 and the channel 322 is at a first position 810. The reference beam 802 is focussed at the interface between the transparent window 320 in the first position 810 and the portion of the channel 322. The reference beam 802 is focussed such that it provides a focus spot at the reflection interface with a Gaussian full width half maximum of 5 to 15 microns. In further examples, a different spot size is used in order to monitor a change at the reflection interface. A first reflected beam 804 from the reflection interface between the transparent window 320 in the first position 810 and the channel 322 is directed through the objective 820 and a cylindrical lens 822 to a camera 824, where the first reflected beam 804 provides a reflection line at a first position 804’, as shown in the projection 800B of the x-y plane of a reference beam image captured by the camera 824 at Figure 7B. When the transparent window 320 moves in the z-direction 812 such that the reflection interface between the transparent window 320 and the channel 322 is at a second position 810’, there is a corresponding change in the reflection of the reference beam 802 such that a second reflected beam 806 from the reflection interface formed by the transparent window 320 and the channel 322 in the second position 810’ provides a reflection line at a second position 806’, as shown in the projection 800B of a reference beam image captured by the camera 824 at Figure 7B. The z-direction 812 corresponds to a direction substantially perpendicular to the planar surface of the working electrode layer 306. Such a change in position of the working electrode layer 306 occurs, for example, due to electrode swelling or contraction under operating conditions. When the reflection interface between the transparent window 320 and the channel moves from the second position 810’ to a third position 810”, there is a corresponding change in the reflection of the reference beam 802 such that a third reflected beam 808 from the reflection interface formed by the transparent window 320 and the channel 322 in the third position 810” provides a reflection line at a third position 808’, as shown in the projection 800B of a reference beam image captured by the camera 824 at Figure 7B. As shown at Figure 7A the lateral position of the reflected beams 804, 806, 808 changes in a lateral direction 814 in the x-y plane, perpendicular to the movement of the interface between the transparent window 320 and the channel 322 in the z-direction, as a function of the position of the reflection interface between the transparent window 320 and the channel 322. Accordingly, by monitoring the reflection of the reference beam, changes in the reflection at the reflection interface can be determined and corresponding changes in the working electrode layer 306 can be taken into account. Whilst the arrangement of the autofocussing optical apparatus 800A, 800B described with reference to Figures 7A and 7B is such that the monitored reflections manifest as lines at the camera 824, in further examples, additional and / or alternative detections of monitored reflections are used to determine changes in the position of the interface between the transparent window 320 and the channel 322, for example based on changes in position and intensity of detected monitored reflections. Whilst a particular arrangement of optical components for monitoring a reference beam reflection is described with respect to Figures 7A and 7B, in further examples additional and / or alternative optical components are used in order to provide the functionality described herein. The process then moves to step S410, where it is determined if the reference reflection has changed. If the reference reflection has not changed, the process moves to step S404 the portion of the working electrode layer 306 is imaged again by the imaging apparatus 328. If it is determined at step S406 that the imaging has finished, the process ends at step S414. If it is determined at step S410 that the reference reflection has changed, the process moves to step S412, where the image focus of the portion of the working electrode is adjusted, thereby to compensate for changes during the electrochemical process. As the change in the reflection of the reference beam at the reflection interface between the transparent window 320 and the channel 322 varies as a function of a corresponding change in the working electrode layer 306, the focal plane of the imaging apparatus 328 is adjusted based on the change in the reflection at the reflection interface, in order to compensate for differences between the position of the focal plane and the portion of the working electrode layer 306 that is being imaged. The imaging and monitoring steps of the process flow S400 are repeated for the desired duration of the operation of the electrochemical cell 300 that is being investigated. Whilst particular steps are shown in the process flow S400, the skilled person understands that in further non-illustrated embodiments, additional and / or alternative steps are implemented, whilst providing the functionality described herein. In further embodiments, the steps of process flow S400 are performed in any manner, simultaneously or sequentially, in order to provide the functionality of adjusting the imaging of a working electrode in response to determining a change in a reference beam reflected at a reflection reference interface described herein. Advantageously, the arrangement of the electrochemical cell 300 described with reference to Figure 2 is such that a portion of the working electrode can be monitored in real time in an improved manner. The rate at which imaging and adjusting image focus occurs is configurable in order to determine desired data. In order to prepare a working electrode layer 306 and foil layer 304 with a density of active material particles and channels 322 that simultaneously facilitates operation within an optically accessible electrochemical cell 300 whilst enabling improved, stable, imaging, processing steps as described with reference to Figures 4 to 6 are undertaken. Cross-sectional examples of the working electrodes are described showing a number of regular channels passing through the working electrodes. In further examples, the working electrode comprises any number of channels formed in any appropriate pattern and / or with any suitable frequency, density and regularity / irregularity. Typically, the working electrodes have a circular planar face for implementation in a coin cell apparatus, however, in further examples, working electrodes comprising channels are formed in any suitable shape and size. Figures 4A and 4B shows a sequence of cross-sectional images as part of a process for forming a perforated working electrode, such as the working electrode layer 306 with a foil layer 304 and channels 322 described herein. A foil layer 504 is provided with a working electrode layer 506 formed on the foil layer, as shown at Figure 4A. The foil layer 504 and the working electrode layer 506 are mechanically perforated using a stamp 502 with a plurality of needles 503 to form a plurality of channels 508 through both the foil layer 504 and the working electrode layer 506, as shown at Figure 4B. Alternatively and / or additionally, the foil layer 504 and the working electrode layer 506 are perforated using laser drilling or laser ablation to introduce channels 508. In further examples, any suitable technique for perforating the foil layer 504 and / or the working electrode layer 506 is used in order to provide a perforated working electrode. Whilst the mechanical perforation using the needles 503 of the stamp 502 are shown to be directed first through the foil layer 504 and then the working electrode layer 504 comprising the active material particles, in further examples the needles 503 of the stamp 502 are directed first through the working electrode layer 506 and then the foil layer 504 in order to preserve a flatter surface of the working electrode layer 506. Figures 5A and 5B shows a sequence of cross-sectional images as part of a process for forming a working electrode, such as the working electrode layer 306 with a foil layer 304 and channels 322 described herein. A foil or woven wire layer with one or more holes passing through its thickness is provided, such as the mesh foil layer 604, with a plurality of channels 608 through the mesh foil layer 604, as shown at Figure 5A. Subsequently, a working electrode layer 606 is cast on the mesh foil layer 604, such that a working electrode layer 606 with a plurality of channels 608 is formed, as shown at Figure 5B. Figures 6A to 6C shows a sequence of cross-sectional images as part of a process for forming a working electrode, such as the working electrode layer 306 with a foil layer 304 and channels 322 described herein. A foil layer 704 is provided, along with a stamp 702 comprising a plurality of needles 703, as shown at Figure 6A. The foil layer 704 is mechanically perforated using the stamp 702 to form a foil layer 704 with a plurality of channels 708, as shown at Figure 6B. Subsequently, a working electrode is cast on the perforated foil layer 704 to form a working electrode layer 706 with a plurality of channels 708, as shown at Figure 6C. Alternatively and / or additionally, the foil layer 704 is perforated using laser drilling or laser ablation to introduce channels 708. In further examples, any suitable technique for perforating the foil layer 704 is used in order to provide a mesh upon which a working electrode layer 706 is formed in order to provide a perforated working electrode. Beneficially, the use of a working electrode with a plurality of channels passing through them means that portions of the working electrode layer that are being imaged can be selected to be a consistent distance away from a point of access of electrolyte (e.g., always a predetermined distance from a hole passing through the working electrode). Various further aspects and embodiments of the present invention will be apparent to those skilled in the art in view of the present disclosure. “and / or” where used herein is to be taken as specific disclosure of each of the two specified features or components with or without the other. For example “A and / or B” is to be taken as specific disclosure of each of (i) A, (ii) B and (iii) A and B, just as if each is set out individually herein. Unless context dictates otherwise, the descriptions and definitions of the features set out above are not limited to any particular aspect or embodiment of the invention and apply equally to all aspects and embodiments which are described. It will further be appreciated by those skilled in the art that although the invention has been described by way of example with reference to several embodiments. It is not limited to the disclosed embodiments and that alternative embodiments could be constructed without departing from the scope of the invention as defined in the appended claims.

Claims

1. An electrochemical cell comprising:a working electrode comprising a channel passing at least partially therethrough;a transparent window configured to provide optical access to the working electrode; anda reflection interface between a portion of the transparent window and a portion of the channel, wherein the reflection interface is configured to change in a manner indicative of a corresponding change in the working electrode.

2. The electrochemical cell according to claim 1, comprising:a foil layer on a first surface of the working electrode, wherein the transparent window is configured to provide optical access to a second surface of the working electrode positioned opposite to the first surface.

3. The electrochemical cell according to claim 1, comprising:a foil layer on a first surface of the working electrode, wherein the transparent window is configured to provide optical access to the first surface of the working electrode through one or more apertures in the foil layer.

4. The electrochemical cell according to claim 2 or 3, wherein the foil layer is a current collector and / or a mesh layer.

5. The electrochemical cell according to any preceding claim, wherein the channel extends through the working electrode and the foil layer.

6. The electrochemical cell according to claim 1, wherein the working electrode is a free-standing working electrode.

7. The electrochemical cell according to any preceding claim, wherein the channel comprises an electrolyte.

8. The electrochemical cell according to any preceding claim, wherein the working electrode is in mechanical communication with the transparent window.

9. The electrochemical cell according to any preceding claim, wherein the working electrode comprises an active material, preferably wherein an active material loading of the working electrode is at least 80%.

10. The electrochemical cell according to any preceding claim, wherein the electrochemical cell is a coin cell and / or mounted on a printed circuit board.

11. A method for imaging a working electrode during at least part of an electrochemical process, the method comprising:monitoring a reflection at an interface between a transparent window and a channel that extends at least partially through the working electrode;imaging a portion of the working electrode during at least part of an electrochemical process with an imaging apparatus;determining a change in the reflection at the interface; and adjusting the image focus of the imaging apparatus in response to determining the change.

12. The method according to claim 11, wherein the channel comprises an electrolyte.

13. The method according to claim 11 or 12, wherein monitoring the reflection comprises directing a reference beam at the interface and detecting the position and / or intensity of the reflected reference beam.

14. The method according to any of claim 11 to 13, wherein imaging the portion of the working electrode comprises imaging a first surface of the working electrode positioned opposite to a second surface of the working electrode, wherein the working electrode comprises a foil layer on the second surface.

15. The method according to any of claim 11 to 14, wherein adjusting the image focus comprises dynamically altering the focal plane of the imaging apparatus, thereby compensating for changes in the position of the portion of the working electrode in a direction substantially perpendicular to the focal plane.

16. A method of preparing an imaging system for imaging a working electrode during at least part of an electrochemical process in accordance with the method of claims 11 to 15.

17. The method according to claim 16, comprising forming a channel at least partially through the working electrode.

18. The method according to claim 17, wherein the channel is formed by forming the working electrode on a foil mesh comprising one or more channels.

19. The method according to claim 17, wherein the channel is formed by perforating a foil layer and forming the working electrode on the perforated foil layer.

20. The method according to claim 17, wherein the channel is formed by forming the working electrode on a foil layer and subsequently perforating the working electrode.

21. The method according to claim 19 or 20, wherein perforating comprises the use of at least one of: mechanical perforation; laser drilling; and laser ablation.

22. A system for imaging a working electrode during at least part of an electrochemical process, the system comprising:an imaging apparatus comprising a light source, wherein the system is configured to perform the method of any one of claims 11 to 15.

23. The system according to claim 22, wherein the system comprises an electrochemical cell according to any of claims 1 to 10.

24. The system according to any of claim 22 and claim 23, wherein the system comprises an electrical connection for receiving a battery electrode.

25. The system according to any of claims 22 to 24, wherein the light source comprises at least one of: a laser; a light emitting diode; and a lamp, and / or wherein the imaging apparatus comprises an optical microscope.

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