Electrode drying apparatus and method of use thereof

Abstract

The present disclosure relates to a method and corresponding apparatus for drying an electrode sheet which comprises three stages that sequentially remove solvent from a film layer, a first stage using irradiation, a controlled stage using selective irradiation. The controlled stage may preferably be carried out via a second stage using heat, and a final stage using irradiation. The method achieves improved mechanical properties of the resultant dried films by taking the film layer from a liquid state through a semi-solid state to a dried film layer in a controlled manner.

Description

[0001] ELECTRODE DRYING APPARATUS AND METHOD OF USE THEREOF

[0002] TECHNICAL FIELD

[0003] The present disclosure relates to an apparatus and a method for drying an electrode sheet. In particular, the disclosure provides an in-line drying apparatus, and method of use thereof, that enables reduced drying times, improved drying of a coated electrode, and real-time monitoring of the quality of the electrode sheet during and / or following the drying process.

[0004] BACKGROUND

[0005] The electrode manufacturing process typically involves a series of steps optimised in order to produce the electrodes in a low cost and efficient way. When the electrode is manufactured using a wet-coating process, these steps may consist of mixing, coating, drying, calendaring, post-drying, and cell assembly.

[0006] In order to achieve consistency of performance in electrodes, a homogeneous film coating free of defects is required. In particular, homogeneity in distribution of the microporous phase is desirable to provide improved mechanical stability and electrical contact. The microstructure of the electrode coating is especially sensitive to parameters in the drying process, as well as the interaction between the drying process and the preceding and subsequent steps of mixing and calendaring, respectively.

[0007] The drying process can be challenging to carry out at high speed, as this can impact the quality of the electrode produced. Conversely, low speed in-line processes can require long manufacturing lines with a resultant high CAPEX and OPEX. There remains a need to increase efficiency of processes for drying coated electrodes.

[0008] SUMMARY

[0009] In an aspect, there is provided a method of drying an electrode sheet coated with an active material slurry, the method comprising: in an first stage, drying a portion of the electrode sheet using a convection oven comprising an irradiation source for a duration of from 5 to 35% of the total drying time, wherein the irradiation source is applied for at least part of the first stage using a first stage maximum power intensity of at least 0.5 W / cm2, wherein said irradiation source comprises flood exposure; and in a controlled stage, drying at least part of the portion of the electrode sheet using a convection oven comprising a controllable irradiation source for a duration of from 65 to 95% of the total drying time, wherein the irradiation source is applied using a controlled stage maximum power intensity of at least 0.5 W / cm2, said controllable irradiation source being applied at different intensities across the area of irradiation .

[0010] Preferably, the controlled stage comprises a second stage and a final stage, wherein the second stage comprises drying the portion of the electrode sheet using a convection oven for a duration of from 5 to 45% of the total drying time, wherein an irradiation source is optionally applied at a second stage maximum power intensity of less than 2 W / cm2, said second stage maximum power intensity being lower than the first stage maximum power intensity; and wherein the final stage comprises drying at least part of the portion of the electrode sheet using a convection oven comprising a controllable irradiation source for a duration of from 20 to 90% of the total drying time, wherein the irradiation source is applied using a final stage maximum power intensity of at least 1 W / cm2, said controllable irradiation source being applied at different intensities across the area of irradiation.

[0011] Preferably, the method is an in-line method, and the controllable irradiation is applied during at least part of the controlled stage at differing intensities in the cross direction as the electrode sheet moves through the convection oven in a machine direction.

[0012] In another aspect, there is provided a method of drying an electrode sheet coated with an active material slurry, the method comprising: in an first stage, drying a portion of the electrode sheet using a convection oven comprising an irradiation source for a duration of from 5 to 35% of the total drying time, wherein the irradiation source is applied for at least part of the first stage using a first stage maximum power intensity of at least 0.5 W / cm2, wherein said irradiation source comprises flood exposure; and in a second stage, drying the portion of the electrode sheet using a convection oven for a duration of from 5 to 45% of the total drying time, wherein an irradiation source is optionally applied at a second stage maximum power intensity of less than 2 W / cm2, said second stage maximum power intensity being lower than the first stage maximum power intensity; and in a final stage, drying at least part of the portion of the electrode sheet using a convection oven comprising an irradiation source for a duration of from 20 to 90% of the total drying time, wherein the irradiation source is applied using a final stage maximum power intensity of at least 0.5 W / cm2.

[0013] Preferably, the method is an in-line method, and the electrode sheet moves through the convection oven in a machine direction.

[0014] BRIEF DESCRIPTION OF THE FIGURES

[0015] Examples of the present invention are described below with reference to the accompanying drawings, in which:

[0016] Figure 1 is a schematic illustration depicting a side view of an electrode sheet;

[0017] Figures 2a, 2b and 2c are schematic illustrations of the slurry composition during, respectively, the first, second and final stages; and

[0018] Figures 3a, 3b and 3c are a photographs of a sample electrode sheet during the first stage (3a), at the transition between the first and second stages (3b), and when in the final stage (3c);

[0019] DETAILED DESCRIPTION

[0020] Electrodes for electrochemical cells are typically manufactured from an active material slurry. In these processes, the slurry is deposited on a conductive foil, dried and then subsequently processed and incorporated into a cell. The subsequent processing steps typically include calendering, and shaping the electrode by mechanical processes to ensure it is a suitable size and shape for the cell which is being made.

[0021] The active material slurry may vary in composition, though it typically contains active material, binder, solvent and optional additives (such a conductive additives to improve conductivity of the resultant electrode).

[0022] The active material may be a cathode active material or an anode active material. These materials are well known in the art and described in detail elsewhere. Typically, the active material will comprise at least 80% of the solid components (i.e. the non- solvent components) in the slurry, such as at least 90% more typically at least 95% of the solid components.

[0023] The solvent is typically the only volatile liquid in the slurry.

[0024] The solvent may be selected from water, / V-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), / V, / V-dimethylacetamide (DMA), l-butylpyrrolidin-2-one (NBP), dimethyl isosorbide (DMI), dimethylsulfoxide (DMSO), triethylphosphate, gamma-valerolactone (GVL), and an alkoxy- / V-substituted propanamide according to the formula (I), wherein R1- R3independently are Ci-6 alkyl; for example the compound of formula (I) is 3-methoxy- / V, / V-dimethylpropanamide, and combinations thereof

[0025] Preferably, the solvent is selected from / V-methyl-2-pyrrolidone (NMP), 1- butylpyrrolidin-2-one (NBP), water, or mixtures thereof.

[0026] In some embodiments, the solvent is water.

[0027] In some embodiments, the solvent is NMP or NBP.

[0028] Water is typically used for an anode active material, while NMP or NBP is typically used for a cathode active material.

[0029] The active material slurry will usually comprise from 25 to 35 wt% solvent, with the exact amount varying depending on the active material, binder and solvent which is used. Although the amount of solvent may vary, generally speaking the active material slurry will be formulated to have a given viscosity and rheology to make it suitable for coating onto a conductive foil.

[0030] Thus, referring to Figure 1, the electrode sheet 100 may comprise a conductive foil 102 and active material slurry 104 coated on one or both sides of the conductive foil.

[0031] The foil typically has a thickness of from 1 to 20 pm. The active material slurry layer typically has a thickness of from 50 to 500 pm, such as from 150 to 450 pm, preferably from 200 to 400 pm.

[0032] The exact thickness of the active material slurry layer may vary and is dependent on the end use (i.e. the cell design), as well as whether the layer is to form a cathode or an anode.

[0033] The active material slurry layer typically contains a relatively high amount of solvent when deposited, to ensure it has the suitable viscoelastic properties to allow deposition and effective coating. Typical solvent levels range from 25 to 35 wt%, such as from 27 to 30 wt%.

[0034] Upon deposition, the slurry formulation 104, if hypothetically viewed in the cross section, is represented schematically in Figure 2a. In this figure, it can be seen that the high solvent 105 levels allow relatively free movement of the active material 106 , binder and other components within the layer 104.

[0035] As the solvent 105 is removed, the slurry layer 104 resembles the schematic shown in Figure 2b, wherein the free movement of active material 106 becomes more restricted and the layer reduces in thickness. In this scenario, some components such as binder 108 are still able to move and can be influenced, for instance, by the flux of evaporating solvent 105.

[0036] However, when sufficient solvent 105 has been removed, the layer 104 resembles the schematic in Figure 3c. In this representation, it can be seen that the only solvent 105 present resides in the pores of an otherwise compacted layer 104. Removal of the solvent 104 does not significantly impact the thickness of the layer 104. Moreover, in this configuration, the binder 108 is unable to freely move, and it remains in place irrespective of the solvent flux.

[0037] The inventors have recognised that when the film is in the configuration represented in Figure 2b, the binder is particularly sensitive to solvent flux. If solvent is removed too quickly, the binder can migrate to the surface of the layer, leading to delamination of the electrode layer and cracks forming within the layer. In contrast, the binder is not particularly impacted by the solvent removal rate when the layer is configured in line with either Figure 2a or Figure 2c. After investigating this phenomenon, the inventor recognised that the configuration of the slurry layer may be determined by simple visual inspection, and / or measurement of the surface temperature. Thus, active material, the predominant solid material in the layer, is typically highly light absorbing and will determine the colour and visual properties of the slurry.

[0038] When in the configuration depicted in Figure 2a, the active material is fully wetted, and the layer displays a saturated, dark colouration that is very consistent across the layer. Such a layer is shown in a photograph in Figure 3a.

[0039] When in the configuration in Figure 2c, the active material is not wetted, and the layer consequently displays typically a lighter colouration which is consistent across the surface. Such a layer is shown in the photograph in Figure 3c. Notably, the lighter colour is likely an artefact of the high surface area of the active material that provides a dull, matt effect. This is contrasted with a wetted material which displays a much fuller, darker colour. While this is typical, the exact colouration of the layers may be reversed, with the wetted layer having a lighter colouration. What is significant is that there will be a change in colouration.

[0040] Thus, when in the configuration in Figure 2b, the wetting at the visible surface is changing from fully wetted to dried. This exposes non-wetted active material at the surface of the layer, leading to a change in colouration and often an inconsistency in the colouration across the layer. This is shown in Figure 3b.

[0041] These colour changes can be small, and so another characteristic of these transitions between different states has been found to be surface temperature of the electrode sheet.

[0042] Thus, while solvent is rapidly evaporating as is characteristic of the electrode sheet in Figure 2a, the surface temperature tends to be very even. Typically, the surface temperature will be around 85-90 °C for water evaporating in a normal atmosphere. However, the exact temperature will vary depending on the solvent and the conditions in the convection oven.

[0043] Once part of the surface transitions into the condition shown in Figure 2b, solvent evaporation no longer has the consistent cooling effect, and removal of the heat generated by the applied radiation is less rapid. This leads to a sharp temperature rise. Typically, for water as the solvent, the surface temperature will rise sharply to above 100 °C, i.e. above the boiling temperature of the water.

[0044] This sharp and sudden increase in temperature only occurs where the transition to the partially dried electrode sheet has occurred, and other parts of the electrode sheet may still be wetted. Thus, the electrode sheet shown in Figure 3b, if irradiated, would show temperature significant variations across its width (i.e. in the cross direction if moving in the machine direction).

[0045] The temperature variations can be significant, depending on the intensity of the irradiation. These significant increases in temperature, and the prolonged exposure of the electrode sheet to the high temperatures, can lead to components within the electrode sheet such as the binder becoming damaged.

[0046] The present disclosure provides a method of drying an electrode sheet that optimises the solvent removal as the electrode layer is being dried. The method of the disclosure provides reduced drying times without compromising the properties of the dried electrode.

[0047] The method of the disclosure comprises several drying stages, each being tailored to control the solvent removal rate to optimise the drying time.

[0048] For each drying stage, the electrode sheet is exposed to a convection oven. Typically, the method is an in-line method, and the electrode sheet will pass through a convection oven in a machine direction.

[0049] The temperature and conditions in the convection oven may be the same for each stage, or may vary. For instance, the temperature may remain the same, with the speed of the air passing over the electrode sheet varying. Alternatively, the temperature may vary, with the speed of the air passing over the electrode sheet remaining constant. Of course, both the temperature and the speed of air may vary between each stage, as well as within an individual stage.

[0050] The first stage comprises drying a portion of the electrode sheet using a convection oven comprising an irradiation source, wherein the irradiation source is applied for at least part of the first stage using a first stage maximum power intensity of at least 1 W / cm2. The irradiation source in the first stage is a flood exposure. The flood exposure ensures the layer is heated across its entire surface, and solvent removal is rapid and even.

[0051] During the first drying stage, the slurry layer is typically in the condition as shown in Figure 2a. When in this state, solvent removal may be rapid without risk of solvent flux causing binder migration to the surface. Therefore, the first stage includes an irradiation source to increase the drying power.

[0052] The irradiation source is applied at a first stage maximum power intensity of at least 0.5 W / cm2, preferably at least 1 W / cm2, typically from 2 to 23 W / cm2.

[0053] The power intensity will often depend on the solvent. Thus, solvents with a higher vapour pressure and lower boiling point will require a lower power intensity. If the power intensity is too high, there is a risk of too rapid evaporation of the solvent from the surface, which creates a top layer of dried electrode with a fully wetted electrode underneath. This mismatch leads to bubbling and other defects in the layer being formed.

[0054] For water, the first stage maximum power intensity is preferably from 2 to 13 W / cm2.

[0055] For NMP, the first stage maximum power intensity is preferably from 2 to 23 W / cm2.

[0056] The irradiation may be applied for the entire first stage, or only part of the first stage. Likewise, the power intensity of the irradiation may vary across the first stage, provided the maximum power meets the above criteria.

[0057] The controlled stage begins when the electrode layer resembles the material shown in Figure 2b. That is, when the binder has a high tendency to migrate with the solvent flux as it evaporates.

[0058] In this stage, the irradiation is applied using a controllable radiation source. This type of radiation source is has a controllable power across the area of irradiation, such that the irradiation applied may be at differing intensities over the area of irradiation. Particularly, for an in-line process, the irradiation applied may be at differing intensities in the cross direction as the electrode sheet moves through the convection oven in a machine direction. This controlled exposure allows for the different stages of solvent removal across the electrode sheet to be accommodated. For instance, in an electrode sheet as shown in Figure 3b, the irradiation may be selectively applied to areas containing more solvent to facilitate drying, and applied at reduced power or not applied in areas that are relatively dry after the first stage.

[0059] The inventors have found that electrode sheets in the condition shown in Figures 2b and 3b are particularly sensitive to solvent removal. Consequently, it can be advantageous to remove solvent slowly when the electrode sheet is in this condition.

[0060] The controlled stage in the method of the disclosure therefore preferably comprises a second stage and a final stage. The second stage is characterised by having low or no irradiation, so as to allow slow removal of the solvent primarily or only by convection drying. The final stage allows controlled exposure to irradiation over at least part of the layer.

[0061] Thus, in the second stage, any irradiation that may be applied is used at a reduced power compared to the first stage, and in any event at a maximum power of less than 2 W / cm2. Preferably, the second stage maximum power intensity is less than 1 W / cm2, more preferably less than 0.5 W / cm2.

[0062] Most preferably, the second stage does not use any irradiation.

[0063] The temperature and air flow of the convection oven of the second stage may be the same as the first stage. When configured in this way, the reduced power intensity of any radiation applied inevitably results in a lower rate of solvent removal than achieved in the first stage.

[0064] However, in the event that the oven conditions in the second stage are different from the oven conditions in the first stage, it is important to ensure lower solvent removal rates for the second stage than obtained during the first stage.

[0065] Preferably, the maximum rate of solvent removal during the second stage is less than 80% the maximum rate of solvent removal during the first stage, more preferably less than 70% such as less than 60% or even less than 50% the maximum rate of solvent removal during the first stage. Once enough solvent has been removed, the layer resembles the scenario in Figure 2c. When in this configuration, the method enters the final stage in which higher power irradiation is applied. This typically results in an increase in the rate of solvent removal, reducing the overall processing time for the method. The high rate of solvent removal does not impact the integrity of the dried film, since binder migration is minimal during this stage.

[0066] The maximum final irradiation power is therefore higher than the maximum second irradiation power. Suitable power levels for the final irradiation power match those for the first irradiation power.

[0067] Thus, the irradiation source is applied at a final stage maximum power intensity of at least 0.5 W / cm2, preferably at least 1 W / cm2, typically from 2 to 23 W / cm2.

[0068] For water, the final stage maximum power intensity is preferably from 2 to 13 W / cm2.

[0069] For NMP, the final stage maximum power intensity is preferably from 2 to 23 W / cm2.

[0070] An important aspect of the method is the transition points between the first and controllable stages, and specifically between the first, second and final stages. These may be characterised in a number of ways.

[0071] For instance, the transition points may be characterised by the appearance of the surface of the slurry layer. As discussed above, the lower levels of solvent result in a change in the colouration and increase in temperature of the surface, with a distinct transition occurring when the surface begins to show drier patches corresponding to the transition from the slurry in a mobile configuration (i.e. a wet slurry as shown in Figure 2a) to a more restricted configuration (i.e. a semi-solid slurry as shown in Figure 2b). The transition can also marked by an increase in surface temperature.

[0072] The change in colouration typically manifests in a lightening of the colour, which can be characterised by a change in the lightness in the HSL (Hue Saturation Lightness) scale.

[0073] Using an L value scaled to 100 (i.e. 100 is pure white and 0 is pure black), the transition from wet to semi-dry will typically correspond to a change in L value of at least 15, typically at least 20, such as at least 25 or even at least 30. The transition from the first stage to the controlled (or second) stage of the method may therefore occur when the slurry surface has changed L value, using an HSL colour scheme with L scaled to 100, by at least 15, typically at least 20, more typically at least 25, preferably at least 30.

[0074] As the transition is inevitably not consistent across the surface, the transition from the first stage to the controlled (or second) stage of the method may be characterised as when at least 10% of the cross direction (i.e. transverse to the machine direction) has an L value, using an HSL colour scheme with L scaled to 100, at least 15 higher, typically at least 20 higher, more typically at least 25 higher, preferably at least 30 higher, than the L value of the 10% of the cross direction having the lowest L value.

[0075] The L value of the cross direction may be determined by measuring the HSL value of a 1 cm strip of the surface of the electrode sheet in the cross direction.

[0076] The transition from the second stage to the final stage may be characterised as when the 10% of the cross direction (i.e. transverse to the machine direction) having the highest L value, using an HSL colour scheme with L scaled to 100, is no more than 15 higher, preferably no more than 10 higher, more preferably no more than 5 higher, than the L value of the 10% of the cross direction having the lowest L value.

[0077] Thus, the transition to the final stage is characterised by an evenness in colouration across the entirety of the surface of the electrode sheet. This evenness is indicative that the solvent has been removed from the surface, and only remains in the pores of the layer between the solid materials. When in this state, the rate of solvent removal may be increased, as the tendency for binder migration caused by solvent flux is minimal.

[0078] As an alternative, or in addition to, the above characterisations of the transition points between the first stage, second stage and final stage, the solvent content of the layer may be used.

[0079] For instance: the first stage may be characterised by a solvent content, relative to the solvent content of the active material slurry, of from 65% to 100%; the second stage may be characterised by a solvent content, relative to the solvent content of the active material slurry, of from 25% to 75%; and the final stage may be characterised by a solvent content, relative to the solvent content of the active material slurry, of from 0% to 35%.

[0080] The exact transition between the stages will depend on the binder system used, and specifically the propensity of the binder to interact with the solvent which is influenced by its solubility in the solvent amongst other things, as well as the solvent content of the original slurry at the start of the method.

[0081] Preferably: the first stage may be characterised by a solvent content, relative to the solvent content of the active material slurry, of from 67% to 100%; the second stage may be characterised by a solvent content, relative to the solvent content of the active material slurry, of from 33% up to 67%; and the final stage may be characterised by a solvent content, relative to the solvent content of the active material slurry, of from 0% up to 33%

[0082] Typically, these values correspond to the second stage being characterised by a solvent content of the active material slurry of from 10 to 20 wt%. Thus, the controlled stage will typically begin when the solvent content of the active material slurry in the electrode sheet reaches 20 wt%.

[0083] The transition from the second to the final stage may also be characterised by the lack of change in the thickness of the layer upon solvent removal. Thus, broadly speaking the final stage is characterised by the layer being compacted such that solvent resides only in the pores of the layer. The final stage may therefore be characterised by the lack of reduction in thickness of the layer while solvent is being removed.

[0084] Even though the transition point between the stages broadly corresponds to about a third of the solvent being removed in each stage (or a third of the solvent being removed to transition from the first to the controlled stage), the time of each stage may vary depending on the conditions used in the method. In particular, since the solvent removal may be rapid in the first stage, this is typically short in comparison to the other stages and characterised by having rapid rates of solvent removal.

[0085] Temperature may also be used as a marker for the transition from the first stage to the second stage. Thus, as solvent is no longer present at the surface, the cooling effect of evaporation is reduced and surface temperature under irradiation increases rapidly. The end of the first stage may therefore be characterised when there is a difference in the surface temperature of 10 °C or more across the width of the surface (i.e. in the cross direction), preferably 5 °C or more.

[0086] Thus, the second stage may be characterised as beginning when at least 10% of the cross direction (i.e. transverse to the machine direction) shows a temperature of at least 10 °C, preferably at least 5 °C, higher than the 10% of the cross direction having the lowest temperature.

[0087] This may lead to transition into the second stage, wherein no or only low power irradiation is applied until the final stage.

[0088] While solvent removal may not lead to binder migration in the final stage (or the latter part of the controlled stage), some care is needed to ensure the integrity of the electrode layer is maintained. For instance, too much irradiation may lead to overheating of areas of the layer, risking damage to the components within the layer. Likewise, solvent removal can be slow, as it involves removing solvent down to residual levels where it may be held within pores or adhered to surfaces by strong interactions. Consequently, the time for the final stage (and by extension the controlled stage) may be long, at least in comparison to the first stage.

[0089] The length of the second stage (or the initial part of the controlled stage) is quite variable and depends on the system. It may be relatively short compared to the final stage, or it may be longer than the final stage, since solvent removal rate is reduced to protect the integrity of the layer.

[0090] The method may preferably therefore be characterised by: a first stage comprising from 5 to 35% of the total drying time, preferably from 10 to 25% of the total drying time, such as from 10 to 20% of the total drying time; a second stage comprising from 5 to 45% of the total drying time, preferably from 15 to 40% of the total drying time, such as from 25 to 40% of the total drying time; and a final stage comprising from 20 to 90% of the total drying time, preferably from 30 to 80% of the total drying time, such as from 30 to 60% of the total drying time. By "total drying time" is meant the total time that the electrode sheet is resident in the convection oven during the drying process. Typically, this corresponds to the total combined time for the first, second and final stages (or more broadly the first and controlled stages).

[0091] Preferably, the second stage is longer than the first stage, for instance at least 20% longer than the first stage, preferably at least 40% longer than the first stage.

[0092] The method of the disclosure may also comprise actively monitoring the electrode sheet, and adjusting the conditions based on that monitoring.

[0093] For instance, the method may comprise monitoring the colour properties of the electrode sheet at various points as it moves in the machine direction. Once certain changes in the colour properties are detected, the method may move from the first stage to the second stage, or from the second stage to the final stage.

[0094] This configuration of the method is particularly useful where there may be some variability in the slurry or processing conditions. In these scenarios, a convection oven having controllable irradiating elements along the length of the oven may be used. The irradiating elements may be reduced in power or turned off when the second stage is detected, and increased in power or turned on when the final stage is detected. The exact point of transition from the first stage to the second stage, and from the second stage to the final stage, may vary, but can still be precisely controlled by actively monitoring the colour properties of the electrode sheet as it transitions through the oven. This configuration allows the same oven to be used for multiple different types of electrode sheet (for instance having different types of or thicknesses of active material slurry layers) without significant reconfiguration of the convection oven.

[0095] The active monitoring may also be monitoring of the surface temperature of the electrode sheet.

[0096] In some variants, the method of the disclosure may comprise monitoring the surface temperature of the electrode sheet through the drying process; and transitioning to the second stage when the surface temperature of the electrode sheet reaches a threshold temperature. The above description of the controlled stage comprising the second and final stages is one possibility to achieve the controlled removal of the solvent with minimal risk of damaging the electrode. However, an alternative embodiment of the method is to selectively apply irradiation throughout the controlled stage, effectively forgoing the second stage. For instance, this may be achieved by selectively applying irradiation that varies in the cross direction as the electrode sheet moves in the machine direction.

[0097] For instance, the method of the disclosure may comprise monitoring the surface temperature of the electrode sheet through the drying process, the drying process comprising a first stage and a controlled stage; the first stage comprising drying a portion of the electrode sheet using a convection oven comprising an irradiation source for a duration of from 5 to 35% of the total drying time, wherein the irradiation source is applied for at least part of the first stage using a first stage maximum power intensity of at least 0.5 W / cm2, wherein said irradiation source comprises flood exposure; and the controlled stage comprising drying the electrode sheet using a convection oven comprising a controllable irradiation source for a duration of from 65 to 95% of the total drying time, wherein the irradiation source is applied using a final stage maximum power intensity of at least 0.5 W / cm2, and wherein the irradiation source is applied at reduced power or not applied to areas of the surface of the electrode sheet that reach a threshold temperature.

[0098] The "threshold temperature" will be specific to the solvent used and dependent on the drying conditions in the convection oven, as well as the thermal stability of the components in the electrode active layer.

[0099] For water as the solvent, the temperature of the electrode sheet during the first stage is typically around 85 to 90 °C. This can be characterised as first stage stead state temperature, or the temperature at which solvent is removed under irradiation during the first stage. The threshold temperature is always above this temperature, and typically at least 5-10 °C above this temperature, or about 95-100 °C for water as the solvent.

[0100] Thus, the start of the controlled stage is characterised when at least 10% of the cross direction (i.e. transverse to the machine direction) shows a temperature of at least 10 °C, preferably at least 5 °C, higher than the 10% of the cross direction having the lowest temperature. Once enough solvent has been removed, the temperature will rise and the risk of damage to the electrode increases significantly. The threshold temperature will be dependent on the components in the electrode, and may be 5-10 °C above the first stage steady state temperature. It could however be 20-50 °C higher than the first stage steady state temperature for thermally stable systems.

[0101] The controlled stage is characterised by stopping the flood exposure, such that irradiation only occurs on areas of the surface that have a temperature below the threshold temperature. Areas of the surface that are at or above the threshold temperature have a reduced intensity of irradiation, or no irradiation.

[0102] This controlled exposure may continue for the entirety of the controlled phase, such that the surface is selectively exposed to irradiation to ensure it is maintained at a suitable temperature, for instance between 50 °C below the threshold temperature and up to the threshold temperature, preferably between 25 °C below the threshold temperature and up to the threshold temperature.

[0103] For water as the solvent, this corresponds to temperature ranges of, for instance, from 80 to 130 °C, or from 80 to 105 °C, or from 85 to 100 °C as typical examples.

[0104] The active layer of the electrode sheet is typically applied by screen printing or similar deposition techniques. Using such methodologies, there is usually a sloped edge to the sheet, which has reduced thickness. Active monitoring and selective irradiation based on surface temperature can ensure that the edge of the electrode sheet is not over exposed.

[0105] It can however be advantageous to control the application of irradiation to reduce the likelihood of overexposure at the edges of the electrode sheet.

[0106] Thus, it is preferable that the flood exposure in the first stage is not applied to any sloped edge of the electrode sheet. The flood exposure is preferably applied during the first stage to the area of electrode sheet having consistent thickness.

[0107] In a variation of the method, the method comprises a second stage and a third stage, and the second stage is characterised by stopping (or reducing) the flood exposure, so that the rate of solvent removal is reduced. This ensures that the surface remains and is maintained below the threshold temperature. In this variation, the transition to the third stage may be characterised by a consistency in the surface temperature being achieved once the solvent has been evenly and sufficiently removed across the entire surface. Thus, once an even surface temperature has been reached, for instance no greater than 5 °C across the entire surface, the method transitions into the third stage.

[0108] The disclosure also relates to an apparatus for drying an electrode sheet transitioning through the apparatus in a machine direction for a total drying length, said apparatus comprising: a first section configurable for convection drying a substrate transitioning through the section in the machine direction, said section comprising from 5 to 35% the total drying length and having irradiation elements configurable to irradiate a substrate moving in the machine direction with a power intensity of at least 0.5 W / cm2, wherein said irradiation source comprises flood exposure; and a controllable section configurable for convection drying a substrate transitioning through the section in a machine direction, said section comprising from 65 to 95% of the total drying length and having irradiation elements configurable to irradiate a substrate moving in the machine direction with a power intensity of at least 0.5 W / cm2, wherein said controllable irradiation source being configurable to be applied at different intensities across the area of irradiation.

[0109] Preferably, the apparatus is an in-line apparatus and the controllable irradiation source is configurable to be applied at differing intensities in the cross direction as the electrode sheet moves through the convection oven in a machine direction.

[0110] By "in-line" is meant that the apparatus is set up to dry an electrode sheet moving through the apparatus in a machine direction, for instance in a roll-to-roll drying process.

[0111] In some embodiments, the apparatus comprises a first section configurable for convection drying a substrate transitioning through the section in the machine direction, said section comprising from 5 to 35% the total drying length and having irradiation elements configurable to irradiate a substrate moving in the machine direction with a power intensity of at least 0.5 W / cm2, wherein said irradiation source comprises flood exposure; a second section configurable for convection drying a substrate transitioning through the section in a machine direction, said section comprising from 5 to 45% the total drying length; and a final section configurable for convection drying a substrate transitioning through the section in a machine direction, said section comprising from 20 to 90% of the total drying length and having irradiation elements configurable to irradiate a substrate moving in the machine direction with a power intensity of at least 0.5 W / cm2; and wherein the second and final sections comprise a controllable irradiation source configurable to be applied at different intensities across the area of irradiation

[0112] In some embodiments, the apparatus comprises a first section configurable for convection drying a substrate transitioning through the section in the machine direction, said section comprising from 5 to 35% the total drying length and having irradiation elements configurable to irradiate a substrate moving in the machine direction with a power intensity of at least 0.5 W / cm2, wherein said irradiation source comprises flood exposure, and a temperature sensor configurable to monitor the surface temperature of an electrode sheet being exposed to irradiation from the irradiation elements; and a controllable section configurable for convection drying a substrate transitioning through the section in a machine direction, said section comprising from 65 to 95% of the total drying length and having irradiation elements configurable to irradiate a substrate moving in the machine direction with a power intensity of at least 0.5 W / cm2, and a temperature sensor configurable to monitor the surface temperature of an electrode sheet being exposed to irradiation from the irradiation elements, said irradiation elements being configurable to selectively apply irradiation at varying intensities in the cross direction, preferably wherein said irradiation elements are in communication with the temperature sensors and configurable to adjust the irradiation intensity based on the surface temperature of an electrode sheet moving in the machine direction.

[0113] Other examples relate to an apparatus for drying an electrode sheet transitioning through the apparatus in a machine direction for a total drying length, said apparatus comprising: a first section configurable for convection drying a substrate transitioning through the section in the machine direction, said section comprising from 5 to 35% the total drying length and having irradiation elements configurable to irradiate a substrate moving in the machine direction with a power intensity of at least 0.5 W / cm2, wherein said irradiation source comprises flood exposure; a second section configurable for convection drying a substrate transitioning through the section in a machine direction, said section comprising from 5 to 45% the total drying length; and a final section configurable for convection drying a substrate transitioning through the section in a machine direction, said section comprising from 20 to 90% of the total drying length and having irradiation elements configurable to irradiate a substrate moving in the machine direction with a power intensity of at least 0.5 W / cm2..

[0114] Any of the apparatus of the disclosure may be configured to carry out the method of the disclosure.

[0115] That is, the apparatus of the disclosure may be configured such that the second section comprises an irradiation element, wherein the irradiation element of the second section is configured to irradiate the substrate at a lower power intensity that the irradiation elements of the first and / or final sections.

[0116] The irradiation elements of the apparatus of the disclosure may comprise, or consist of, a flood lamp such a Carbon or Medium wave Infrared Lamp, a xenon lamp, LED module, and / or laser source such as a diode laser or a VCSEL laser

[0117] For instance, the irradiation elements may comprise or consist of an IR lamp or a vertical-cavity surface-emitting laser (VCSEL).

[0118] Preferably, the controllable light source is a laser. This allows a higher level of control over the selective irradiation. This may also be achieved using lamps configured with suitable blocking elements that reduce or block the irradiation.

[0119] The apparatus of the disclosure may comprise one or more monitoring elements capable of measuring the colouration or surface temperature of a substrate moving in the machine direction, preferably over a portion of the substrate running in the cross direction.

[0120] The one or more monitoring elements may be present in the first, second, final and / or controllable sectons, preferably all sections of the apparatus.

[0121] In particular, the colouration being measured may be capable of determining the lightness of the substrate (i.e. the amount of black / white luminescence).

[0122] The one or more monitoring elements may be coupled to a control element that can, based on the signal from the monitoring elements, selectively control the irradiation elements. Such irradiation elements may be referred to as controllable irradiation elements.

[0123] An embodiment of the disclosure relates to an apparatus wherein the irradiation element comprises a controllable irradiation element, preferably a selectively controllable irradiation element. For instance, preferably the apparatus comprises a selectively controllable laser such as a VCSEL.

[0124] Preferably, the controllable irradiation element comprises an array of irradiation sources.

[0125] For instance, when the irradiation element is a VCSEL laser, the laser may comprise a plurality of diodes spaced in the machine and / or cross direction. A width of each diode of the plurality of diodes may be between about 5 mm and 45 mm, optionally between about 5 mm and 32 mm.

[0126] Based on a signal from the monitoring element, the control element may alter the irradiation pattern, irradiation power intensity, irradiation wavelength and / or duration of irradiation of the controllable irradiation element.

[0127] This means that the same apparatus can be used for different substrates (e.g. a different slurry composition or thickness) without the need to manually adjust and optimize the settings. This reduces CAPEX due to the need for fewer apparatus.

[0128] An apparatus with monitoring and control elements is capable of dynamic feedback. This means that the drying process is optimized in real time based on a signal from the monitoring element. An apparatus capable of dynamic feedback ensures that a suitable drying process is provided regardless of run-to-run variation and ensures that the irradiation power intensity and thus drying profile is tailored to the substrate being processed.

[0129] In some embodiments, the monitoring element is configured to monitor the colour variation across the substrate and the control element is configured to selectively adjust the irradiation power intensity.

[0130] For instance, in such an apparatus, if the monitoring element detects that the slurry at the edges of the substrate are drying faster than the slurry at the centre, the control element can reduce the power intensity of the irradiation beam at said edges. Alternatively, if the monitoring element detects that the colour across the substrate is patchy due to the formation of dry spots, it can adjust the irradiation element such that a patterned power intensity is provided. This means that the apparatus is configured to automatically ensure that 'dry spots' are irradiated at a lower power intensity than wet areas thereby reducing the risk of over-drying, delamination and / or cracking. This provides an improved method and reduces OPEX due to the automatic dynamic feedback system.

[0131] The monitoring element may also be configured to measure the surface temperature of a substrate, particularly the surface temperature across the cross direction. The monitoring element may be configured to control the irradiation elements when a threshold temperature is reached.

[0132] In a preferred embodiment, the monitoring element is configured to monitor the substrate in the second section. In the second section, it is critical that fast drying is avoided in order to prevent cracking and / or delamination. Accordingly, monitoring this section provides a benefit. In such embodiments, preferably the control element is also configured to adjust the irradiation power intensity in the second section.

[0133] For instance, the control element may be configured to reduce the irradiation intensity in the first section upon receiving a signal that at least 10% of the cross direction (i.e. transverse to the machine direction) has an L value, using an HSL colour scheme with L scaled to 100, at least 15 higher, typically at least 20 higher, typically at least 25 higher, preferably at least 30 higher, than the L value of the 10% of the cross direction having the lowest L value. That is, when the substrate has reached the end of stage 1 of the method of the disclosure, the control system is configured to reduce or switch off the irradiation source in order to protect the substrate from over drying.

[0134] Similarly, the monitoring and control elements may be configured to monitor the end of stage 2 of the disclosure. For instance, the control element may be configured to increase the irradiation intensity in the second section upon receiving a signal that the 10% of the cross direction (i.e. transverse to the machine direction) having the highest L value, using an HSL colour scheme with L scaled to 100, is no more than 15 higher, preferably no more than 10 higher, more preferably no more than 5 higher, than the L value of the 10% of the cross direction having the lowest L value. That is, when the substrate has reached the end of stage 2 of the method of the disclosure, the control system is configured to increase or switch on the irradiation source in order to protect the substrate from over drying.

[0135] Likewise, the monitoring element may be configured to monitor the surface temperature, particularly the surface temperature in the cross direction, of an electrode sheet.

[0136] For instance, the control element may be configured to reduce the irradiation intensity in the first section upon receiving a signal that a threshold temperature has been reached. Thereafter, the irradiation elements will be configured to selectively apply irradiation so as to maintain the surface temperature in a range at or slightly below, for instance up to 50 °C below or up to 25 °C below or even up to 10 °C below, the threshold temperature. This can be done by dynamic feedback, selectively applying irradiation periodically so as to raise the temperature to the threshold temperature, then reduce or remove the irradiation thus allowing the surface temperature to fall, and repeating this pattern until the electrode sheet is dried.

[0137] In another variant, the control element may be configured to reduce the irradiation intensity in the first section upon receiving a signal that a threshold temperature has been reached. Thereafter, the irradiation elements will be configured to apply irradiation at a reduced intensity (or no intensity) through the second section.

[0138] When the apparatus of the disclosure is used in the method of the disclosure, the control element responding to the signal of the monitoring element may be responsible for the transition from one method stage to the next. That is, a monitoring element configured to monitor the colour variation or surface temperature in the first section of the apparatus and thereby triggering the control element to reduce the irradiation power intensity may be the transition from stage 1 to stage 2. In such embodiments, the first and second sections may be the same portion of the apparatus. That is, the first section and the second section of the apparatus are the same portion of the apparatus in the machine direction, with the difference being that the irradiation element is configured to irradiate at a higher power intensity when the portion is a first section compared to when the portion is a second section.

[0139] The same may also be true for the second and / or final sections of the apparatus if the control system increasing the irradiation power intensity is the transition between the second stage and the final stage. Further examples will now be provided.

[0140] Example 1 : A method of drying an electrode sheet coated with an active material slurry, the method comprising: in a first stage, drying a portion of the electrode sheet using a convection oven comprising an irradiation source for a duration of from 5 to 35% of the total drying time, wherein the irradiation source is applied for at least part of the first stage using a first stage maximum power intensity of at least 0.5 W / cm2, wherein said irradiation source comprises flood exposure; and in a second stage, drying the portion of the electrode sheet using a convection oven for a duration of from 5 to 45% of the total drying time, wherein an irradiation source is optionally applied at a second stage maximum power intensity of less than 2 W / cm2, said second stage maximum power intensity being lower than the first stage maximum power intensity; and in a final stage, drying at least part of the portion of the electrode sheet using a convection oven comprising an irradiation source for a duration of from 20 to 90% of the total drying time, wherein the irradiation source is applied using a final stage maximum power intensity of at least 0.5 W / cm2.

[0141] Example 2. The method of example 1, wherein the method is an in-line method.

[0142] Example 3. The method of example 1 or example 2, wherein the first stage maximum power intensity is from 2 to 13 W / cm2, the second stage maximum power intensity is less than 1 W / cm2, and wherein the final stage maximum power intensity is from 2 to 13 W / cm2.

[0143] Example 4. The method of any of examples 1-3, wherein the solvent content of the active material slurry during the second stage is from 10 to 20 wt%.

[0144] Example 5. The method of any preceding example, comprising transitioning the electrode sheet from the first stage to the second stage when at least 10% of the cross direction (i.e. transverse to the machine direction) has an L value, using an HSL colour scheme with L scaled to 100, at least 20 higher than the L value of the 10% of the cross direction having the lowest L value.

[0145] Example 6. The method of any preceding example, comprising transitioning the electrode sheet from the second stage to the final stage when the 10% of the cross direction (i.e. transverse to the machine direction) having the highest L value, using an HSL colour scheme with L scaled to 100, is no more than 15 higher than the L value of the 10% of the cross direction having the lowest L value.

[0146] Example 7. The method of any preceding example, comprising transitioning the electrode sheet from the first stage to the second stage when at least 10% of the cross direction (i.e. transverse to the machine direction) shows a temperature of at least 10 °C, preferably at least 5 °C, higher than the 10% of the cross direction having the lowest temperature.

[0147] Example 8. The method of any preceding example dependent on example 3, wherein the maximum rate of solvent removal during the second stage is less than 60% of the maximum rate of solvent removal during the first stage.

[0148] Example 9. The method according to any preceding example, wherein the irradiation source is selected from laser, infra-red lamp, a LED module or Xenon Lamp, or a combination thereof.

[0149] Example 10. An apparatus for drying an electrode sheet transitioning through the apparatus in a machine direction for a total drying length, said apparatus comprising: a first section configurable for convection drying a substrate transitioning through the section in the machine direction, said section comprising from 5 to 35% the total drying length and having irradiation elements configurable to irradiate a substrate moving in the machine direction with a power intensity of at least 0.5 W / cm2, wherein said irradiation source comprises flood exposure using a source selected from laser, infra-red lamp, or a combination thereof; a second section configurable for convection drying a substrate transitioning through the section in a machine direction, said section comprising from 5 to 45% the total drying length; and a final section configurable for convection drying a substrate transitioning through the section in a machine direction, said section comprising from 20 to 90% of the total drying length and having irradiation elements configurable to irradiate a substrate moving in the machine direction with a power intensity of at least 0.5 W / cm2.

[0150] Example 11. The apparatus of example 10, comprising one or more monitoring elements capable of measuring the colouration or surface temperature of a substrate moving in the machine direction, preferably over a portion of the substrate running in the cross direction. Example 12. The apparatus of example 11, wherein the one or more monitoring elements are coupled to a control element that can, based on the signal from the monitoring elements, selectively control the irradiation elements. Example 13. The apparatus of any one of examples 10 to 12, wherein the apparatus is a continuous, in-line convection oven.

[0151] Example 14. The apparatus of any one of examples 10 to 13, wherein the apparatus is configured such that the second section comprises an irradiation element, wherein the irradiation element of the second section is configured to irradiate the substrate at a lower power intensity that the irradiation elements of the first and / or final sections.

[0152] Example 15. The apparatus of any one of examples 10 to 14, configured to carry out the method of any one of claims 1 to 9.

Claims

26CLAIMS1. A method of drying an electrode sheet coated with an active material slurry, the method comprising: in a first stage, drying a portion of the electrode sheet using a convection oven comprising an irradiation source for a duration of from 5 to 35% of the total drying time, wherein the irradiation source is applied for at least part of the first stage using a first stage maximum power intensity of at least 0.5 W / cm2, wherein said irradiation source comprises flood exposure; and in a controlled stage, drying at least part of the portion of the electrode sheet using a convection oven comprising a controllable irradiation source for a duration of from 65 to 95% of the total drying time, wherein the irradiation source is applied using a final stage maximum power intensity of at least 0.5 W / cm2, said controllable irradiation source being applied at different intensities across the area of irradiation.

2. The method of claim 1, wherein the method is an in-line method, and the controllable irradiation is applied during at least part of the controlled stage at differing intensities in the cross direction as the electrode sheet moves through the convection oven in a machine direction.

3. The method of claim 1 or claim 2, wherein the controlled stage comprises a second stage and a final stage, wherein the second stage comprises drying the portion of the electrode sheet using a convection oven for a duration of from 5 to 45% of the total drying time; and the final stage comprises drying at least part of the portion of the electrode sheet using a convection oven comprising a controllable irradiation source for a duration of from 20 to 90% of the total drying time, wherein the irradiation source is applied using a final stage maximum power intensity of at least 0.5 W / cm2, said controllable irradiation source being applied at different intensities across the area of irradiation.

4. The method of claim 3, wherein the first stage maximum power intensity is from 2 to 13 W / cm2, the second stage maximum power intensity is less than 1 W / cm2, and wherein the final stage maximum power intensity is from 2 to 13 W / cm2.

5. The method of claim 3 or claim 4, wherein the solvent content of the active material slurry during the second stage is from 10 to 20 wt%.

6. The method of any claim dependent on claim 3, comprising transitioning the electrode sheet from the second stage to the final stage when the 10% of the cross direction (i.e. transverse to the machine direction) having the highest L value, using an HSL colour scheme with L scaled to 100, is no more than 15 higher than the L value of the 10% of the cross direction having the lowest L value.

7. The method of any claim dependent on claim 3, wherein the maximum rate of solvent removal during the second stage is less than 60% of the maximum rate of solvent removal during the first stage.

8. The method of claim 1 or claim 2, wherein the method comprises monitoring the surface temperature of the electrode sheet through the drying process, the drying process comprising a first stage and a controlled stage; the first stage comprising drying a portion of the electrode sheet using a convection oven comprising an irradiation source for a duration of from 5 to 35% of the total drying time, wherein the irradiation source is applied for at least part of the first stage using a first stage maximum power intensity of at least 1 W / cm2, wherein said irradiation source comprises flood exposure; and the controlled stage comprising drying the electrode sheet using a convection oven comprising a controllable irradiation source for a duration of from 65 to 95% of the total drying time, wherein the irradiation source is applied using a controlled stage maximum power intensity of at least 1 W / cm2, and wherein the irradiation source is applied at reduced power or not applied to areas of the surface of the electrode sheet that reach a threshold temperature.

9. The method of claim 8, wherein the surface temperature is maintained at a temperature of between 25 °C below the threshold temperature and up to the threshold temperature through the controlled phase, and wherein the threshold temperature is preferably 100 °C.

10. The method of any preceding claim, comprising transitioning the electrode sheet from the first stage to the controlled stage when at least 10% of the cross direction (i.e. transverse to the machine direction) has an L value, using an HSL colour scheme with L scaled to 100, at least 20 higher than the L value of the 10% of the cross direction having the lowest L value.

11. The method of any preceding claim, comprising transitioning the electrode sheet from the first stage to the controlled stage when at least 10% of the cross direction shows a temperature of at least 10 °C higher than the 10% of the cross direction having the lowest temperature.

12. An apparatus for drying an electrode sheet transitioning through the apparatus in a machine direction for a total drying length, said apparatus comprising: a first section configurable for convection drying a substrate transitioning through the section in the machine direction, said section comprising from 5 to 35% the total drying length and having irradiation elements configurable to irradiate a substrate moving in the machine direction with a power intensity of at least 1 W / cm2, wherein said irradiation source comprises flood exposure; and a controllable section configurable for convection drying a substrate transitioning through the section in a machine direction, said section comprising from 65 to 95% of the total drying length and having irradiation elements configurable to irradiate a substrate moving in the machine direction with a power intensity of at least 1 W / cm2, wherein said controllable irradiation source being configurable to be applied at different intensities across the area of irradiation.

13. The apparatus of claim 12, wherein the controllable section comprises a second section and a final section, the second section configurable for convection drying a substrate transitioning through the section in a machine direction, said section comprising from 5 to 45% the total drying length; and a final section configurable for convection drying a substrate transitioning through the section in a machine direction, said section comprising from 20 to 90% of the total drying length and having controllable irradiation elements configurable to irradiate a substrate moving in the machine direction with a power intensity of at least 1 W / cm2, said controllable irradiation elements being configurable to be applied at differing intensities in the cross direction as the electrode sheet moves through the convection oven in a machine direction.

14. The apparatus of any of claims 12 to 13, comprising one or more monitoring elements capable of measuring the colouration or surface temperature of a substrate moving in the machine direction, preferably over a portion of the substrate running in the cross direction.2915. The apparatus of claim 14, wherein the one or more monitoring elements are coupled to a control element that can, based on the signal from the monitoring elements, selectively control the irradiation elements, and where in the apparatus is preferably configured to carry out the method of any one of claims 1 to 11.

Images