WO2024008857A1

WO2024008857A1 - A system for coating a slurry onto a substrate and a method of coating using said system

Info

Publication number: WO2024008857A1
Application number: PCT/EP2023/068688
Authority: WO
Inventors: Masahiro Nomura; Philip ÖZER; Jack BAHAR; Jairo Eduardo Noriega QUINTERO; Dongho Son
Original assignee: Northvolt Ab
Priority date: 2022-07-06
Filing date: 2023-07-06
Publication date: 2024-01-11

Abstract

A system for coating a slurry onto a substrate to form a battery electrode for a secondary battery. The electrode coating system comprises a slot-die (110) having an inlet (111) configured to receive slurry from a tank (120), and an ejection outlet (113) through which slurry is configured to be ejected onto the substrate (10), a feed valve (130) fluidly connecting the 5 tank (120) to the slot-die (110) at the inlet (111) of the slot-die (110), and a die recirculation valve (140), with a variable opening rate, fluidly connecting the slot-die (110) to the tank (120), and/or a feed recirculation valve (150), with a variable opening rate, fluidly connecting the feed valve (110) to the tank (120).

Description

A SYSTEM FOR COATING A SLURRY ONTO A SUBSTRATE AND A METHOD OF COATING USING SAID SYSTEM

TECHNICAL FIELD

The present disclosure generally pertains to a system for coating a slurry onto a substrate to form a battery electrode for a secondary battery, a method of coating a slurry using said system, a secondary battery electrode, a foil for such a battery electrode, and a secondary battery including the secondary battery electrode.

BACKGROUND

In the manufacture of rechargeable batteries, such as lithium-ion batteries, slurry coating on electrically conductive substrates is commonly performed using conventional slot-die technology. Normally, pressure is applied to the internal slot-die cavity by opening and closing a feed valve. A constant thickness of slurry is applied to the substrate when the feed valve is open. When the feed valve is closed and the pressure decreases inside the slot-die, the coating process is interrupted and no coating is performed.

A major drawback with current technology is that, in simply switching pressurizing valves on and off, it is difficult to control the thickness of the coating. The formation of ‘bumps’ at the leading edge zone and uneven ‘dripping’ at the trailing edge zone of the coated section is a result of unstable pressure control in the transition between coating and non-coating. The leading edge zone may be described as the foremost edge portion of a coated section and the trailing edge zone as the terminal edge portion of a coated section.

Another major drawback with current technology is that, since the pressure in the slot-die coating device is determined by opening or closing a feed valve, only the same thickness is achievable over the coated substrate. This poses a problem of load level balancing since, when rolling a subsection of a slurry coated electrode substrate into a roll, the curvature length of the respective anode and cathode substrates in the battery differs when moving radially outwards from the centre of the battery. As the radius of the roll increases, the curvature decreases. As a result, there is an imbalance between negative and positive capacity in the battery due to these curvature changes occurring, inevitably, as a result of increasing roll radius. When performing intermittent coating, inconsistent pressure in the transition between coated and uncoated sections leads to uneven thickness throughout the coated sections, in particular at the edges. Since this is an important quality factor in battery electrode manufacturing, and a higher demand is placed on the speed of production with thicker coatings and smaller uncoated areas, there is a need for improvements.

Furthermore, in cases where a continuous coating with varied thickness is desired, there is a need for an improved substrate coating technology to compensate for unbalanced loading levels.

SUMMARY

It is in view of the above considerations and others that the embodiments of the present invention have been made. The present disclosure aims at providing high quality secondary batteries that are efficient in manufacture.

According to a first aspect, a system for coating a slurry onto a substrate to form a battery electrode for a secondary battery is provided. The electrode coating system includes a slot-die having an inlet configured to receive slurry from a tank, and an ejection outlet through which slurry is configured to be ejected onto the substrate. Furthermore, the electrode coating system includes a feed valve which fluidly connects the tank to the slot-die at the inlet of the slot-die, and a die recirculation valve, with a variable opening rate, fluidly connecting the slot-die to the tank, and/or a feed recirculation valve, with a variable opening rate, fluidly connecting the feed valve to the tank.

By adapting the opening rate of the die recirculation valve and/or the feed recirculation valve, a precisely controlled pressure level may be regulated/maintained in the slot-die when the feed valve is open/closed, thereby achieving precise control of the thickness in the coated section.

Put differently, by controlling the opening degree/rate of the die recirculation valve and/or the feed recirculation valve, an excessive amount of slurry in the beginning of a coated section as well as the formation of drop-like shapes at the end of a coated section may be avoided.

In general, the technology defined in the appended claims aims at providing a way to improve control of pressure in the slot-die. Precise control of pressure inside the slot-die by regulating the opening rate(s) of the die recirculation valve and/or the feed recirculation valve provides a more precisely determined coating pattern.

Hence, by means of the technique defined in the appended claims, a more uniform coating thickness may be provided on substrates in battery electrode manufacturing. The concept of uniformity also includes shortening leading edge zone height and trailing edge zone length of a coated section.

Furthermore, where a patterned coating is desired, such as a coating having a sloped thickness, this is also achievable by means of the system described herein.

In any case, the coating speed may be increased, with a lower defect rate and a more even loading level throughout the coated section.

According to a second aspect, a method of coating a slurry onto a substrate to form a battery electrode for a secondary battery is provided The method includes the steps of: (a) providing a system according to the above, (b) supplying a slurry to the slot-die by opening the feed valve, thereby ejecting slurry from the slot-die onto the substrate to be coated, and (c) regulating a pressure level in the slot-die by actuating the die recirculation valve and/or the feed recirculation valve.

According to a third aspect, a secondary battery electrode obtainable by the above method is provided.

According to a fourth aspect, a secondary battery comprising the secondary battery electrode obtainable by the above method is provided.

For instance, the secondary battery is a jellyroll type battery.

According to a fifth aspect, a coated foil for a secondary battery electrode according to the above is provided. The coated foil has a coating with a periodic pattern. The periodic pattern is divided into subsections, each subsection having a gradually increasing and/or decreasing thickness extending between neighbouring subsections along the foil.

According to a sixth aspect, a coated foil for a secondary battery electrode according to the above is provided, having a uniform coating thickness extending along the foil. An idea behind the invention is to make use of pressure controlling valve(s) which regulate(s) the pressure inside the slot-die and thereby also the flow of slurry in/out from the slot-die. By this arrangement, the coating of slurry on the substrate forming a battery electrode may be controlled to have a certain thickness, such as uniform or tapered thickness.

The substrate may be coated periodically, such as intermittently with gaps between coated sections, or continuously with a uniform or periodic pattern. In the periodic pattern case, the periodic pattern is preferably divided into subsections, each subsection having a gradually increasing and/or decreasing thickness extending between neighbouring subsections along the substrate, or foil, to be coated.

The above-described battery electrodes/foils may be for, or comprised in, a vehicle battery for propelling a vehicle. The vehicle may for example be a fully electrically propelled vehicle or a hybrid vehicle.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments disclosed herein are illustrated by way of example, and by not by way of limitation, in the figures of the accompanying drawings. Like reference numerals refer to corresponding parts throughout the drawings, in which

Figure 1 schematically illustrates a system for coating a slurry onto a substrate to form a battery electrode for a secondary battery according to an embodiment,

Figure 2 illustrates a planar view and a cross-section view of a coated section on a substrate according to prior art,

Figure 3 illustrates a planar view and a cross-section view of a coated section on a substrate according to an embodiment,

Figure 4 shows a graph indicating the internal pressure of a slot-die as a function of time according to prior art,

Figure 5 shows a graph indicating the internal pressure of a slot-die as a function of time according to an embodiment,

Figure 6 illustrates a rolled up cylindrical secondary battery from a top view, Figures 7-9 illustrate different anode and cathode configurations according to different embodiments,

Figures 10-12 illustrate different anode configurations according to different embodiments, and

Figures 13-15 illustrate different cathode configurations according to different embodiments.

Figures 16-18 illustrate different anode and cathode configurations according to different embodiments,

DETAILED DESCRIPTION

Embodiments of the present disclosure will now be described in more detail. The invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those persons skilled in the art.

With reference to Figure 1, a system 100 for coating a slurry 20 onto a substrate 10 to form a battery electrode for a secondary battery is shown. The system 100 includes a slot-die 110 which is adapted to eject a slurry 20 onto the substrate 10. The substrate 10 is preferably electrically conductive and may be described as a foil, wound up on a roller 30 which rotates during manufacturing of the battery electrode. Rotation of the roller 30 is illustrated by the curved arrow in Figure 1. The substrate 10 may be an aluminium foil or a copper foil. Optionally, the slurry 20 to be deposited onto the substrate 10 is either an anode slurry or a cathode slurry. The slot-die 110 has an inlet 111 which is in fluid communication with a tank 120 where the slurry is held. Moreover, the slot-die 110 has an ejection outlet 113 through which the slurry, supplied from the tank 120, is ejected onto the substrate 10.

The system 100 further includes a feed valve 130 that fluidly connects the tank 120 to the slotdie 110 at the inlet 111 of the slot-die 110. The feed valve 130 may be a solenoid valve that is either fully opened or fully closed depending on a desired coating length L of slurry 20 on the substrate 10, see for instance Figure 2 (prior art) and Figure 3. When the feed valve 130 is open, the slurry goes into the slot-die 110 through the inlet 111, and the slot-die 110 thereby coats the substrate 10. On the other hand, when the feed valve 130 is closed, the slurry is redirected to a return piping, also referred to as a feed return line 170. In Figure 1, the feed return line 170 is arranged between the feed recirculation valve 150 and the tank 120. Other positions are possible for the feed return line 170 as long as it fulfils its purpose of guiding slurry back to the tank 120 from the feed valve 130, indirectly from the slot-die 110.

A sensor, such as a pressure gauge (not shown) may also be arranged in conjunction with the die recirculation valve and/or the feed recirculation valve. Monitoring of pressure can then be achieved in the return piping 160, 170. By means of the pressure gauge, active feedback of the return pipe opening can be done for better control of internal pressure in the slot-die 110. Put differently, fractuation of internal pressure can be controlled by controlling internal pressure in the return piping.

Typically, the feed valve 130 opens and closes at a fixed rate which depends on the desired length of the coated and uncoated sections on the substrate 10. For instance, a coated section may have a coating length L of 500-1000 mm. Between two coated sections, an uncoated 5-30 mm long gap may be provided. The fixed opening rate of the feed valve 130 enables intermittent coating on the substrate 10. Intermittent coating is suitable for the manufacture of so-called tabless battery cells where the coated substrate is rolled up (together with other layers) into a cylinder which forms the secondary battery.

A pump 180 is also provided in the system 100. The pump 180 is connected to the tank 120 and the feed valve 130, and is configured to pump slurry from the tank 120 to the slot-die 110 via the feed valve 130. In Figure 1, the pump 180 is arranged between the tank 120 and the feed valve 130. However, the pump 180 may be located elsewhere in the system 100, as long it pumps slurry to the slot-die 110.

Furthermore, the system 100 includes a die recirculation valve 140 which fluidly connects the slot-die 110 to the tank 120. As opposed to the feed valve 130, which has a fixed opening rate, the die recirculation valve 140 has a variable opening rate. For instance, the opening rate of the die recirculation valve 140 may vary between 0% and 100% depending on the opening rate of the feed valve 130. Preferably, the opening rate of the die recirculation valve 140 varies between 50% to 100% of the opening rate of the feed valve 130. The opening rate of the die recirculation valve 140 is regulated based on the pressure inside the slot-die 110. For instance, the opening rate of the die recirculation valve 140 is adjusted manually. However, the opening rate of the die recirculation valve 140 may also be adjusted automatically. Preferably, artificial intelligence is implemented for feedback control of the system 100 and its components. The opening of the die recirculation valve 140 and/or the feed recirculation valve 150 may also be regulated in relation to the feed valve 130.

When the system 100 is operating, slurry may exit the slot-die 110 at a backflow outlet 112 of the slot-die 110, pass through the die recirculation valve 140 and back into the tank 120 via a return piping, also referred to as a die return line 160. In Figure 1, the die return line 160 is arranged between the die recirculation valve 140 and the tank 120. Other positions are possible for the die return line 160 as long as it fulfils its purpose of guiding slurry back to the tank 120 from the slot-die 110. Preferably, the die recirculation valve 140 is an active valve in the sense that it may actively contribute in regulating the pressure inside the slot-die 110, such as in a pressurized space 114 of the slot-die 110.

The die recirculation valve 140, also referred to as an actuated valve, provides active control of the slot-die 110 pressure. Active control of the die recirculation valve 140 depends on the pressure in the pressurized space 114 of the slot-die 110. For instance, the die recirculation valve 140 may be a pneumatically controlled or electronically controlled solenoid valve or piezo valve. Digital control of the die recirculation valve 140 is preferred. In some cases, an electronic motor driven valve may be used as the die recirculation valve 140.

The system 100 as shown in Figure 1 also includes a feed recirculation valve 150. The feed recirculation valve 150 fluidly connects the feed valve 130 to the tank 120. Similarly as for the die recirculation valve 140, the feed recirculation valve 150 has a variable opening rate. For instance, the opening rate of the feed recirculation valve 150 may vary between 0% and 100% depending on the opening rate of the feed valve 130. Preferably, the opening rate of the feed recirculation valve 150 varies between 50% to 100% of the opening rate of the feed valve 130. The opening rate of the feed recirculation valve 150 is regulated based on the pressure inside the slot-die 110. For instance, the opening rate of the feed recirculation valve 140 is adjusted manually. However, the opening rate of the feed recirculation valve 140 may also be adjusted automatically. As mentioned above, artificial intelligence may also implemented for feedback control of the system 100.

Preferably, the feed recirculation valve 150 is an active valve in the sense that it may actively contribute in regulating the pressure inside the slot-die 110, such as in the pressurized space 114 of the slot-die 110. The feed recirculation valve 150, also referred to as an actuated valve, provides active control of the slot-die 110 pressure. Active control of the feed recirculation valve 150 depends on the pressure in the pressurized space 114 ofthe slot-die 110. For instance, the feed recirculation valve 150 may be a pneumatically controlled or electronically controlled solenoid valve or piezo valve. Digital control of the feed recirculation valve 150 is preferred. In some cases, an electronic motor driven valve may be used as the feed recirculation valve 150.

The internal pressure of the slot-die 110 may also be described as being controlled by the opening times of the different valves. Typically, the valves 130-150 are opened at a micro second order.

The internal pressure of the slot-die 110 may be controlled in any, or a combination of, embodiments for pressure control above, so as to control the internal pressure of the slot-die 110, such as to between 10 kPa and 350 kPa, or preferably between 25 kPa and 200 kPa, and where the pressure level in the slot-die (110) may be kept within ±10 %, or preferably within ±5 %, or even more preferably within ±2.5 %.

Table 1 shows an example of intermittent coating according to an embodiment, hereby referred to as Example 1, with pressure regulation of the die recirculation valve 140 and the feed recirculation valve 150 in relation to the feed valve 130. In accordance with Example 1, the feed valve 130 is open, the die recirculation valve 140 is partially open, such as 0% to 100%, preferably between 50% and 100% and the feed recirculation valve 150 is closed. In this case, the die recirculation valve 140, together with the closed feed recirculation valve 150, actively controls the internal pressure of the slot-die and provides a clean cut with a uniform height (with respect to the remaining coated section) at the beginning of the coated section, see area A in Figure 3 as opposed to Figure 2 which instead shows a prior art example where the feed valve is simply opened and closed with no pressure regulation thereby creating a high so called ‘leading edge’ in area/zone A. The high leading edge zone in the prior art example has a bumplike shape as seen from a cross-section view (the bottom drawing in Figure 2), and is a result of excess flow of slurry due to uncontrolled pressure in the pressurized space 114 of the slotdie 110 prior to opening of the feed valve 130. This problem is mitigated by the technique described herein, and as shown in for instance Examples 1 and 2 below.

According to Example 1, when the feed valve 130 is closed, the die recirculation valve 140 continues to be partially open, such as 0% to 100%, preferably between 50% and 100% and the feed recirculation valve 150 remains closed. Also in this case, the die recirculation valve 140, together with the closed feed recirculation valve 150, actively controls the internal pressure of the slot-die 110 by redirecting slurry to the tank 120 via the die return line 160 and adapting the pressure inside the slot-die 110. When the feed valve 130 is closed, the internal pressure of the slot-die 110 is maintained at a certain level. Thanks to this, a clear end of the coated section is provided on the substrate, see area B in Figure 3 as opposed to Figure 2 showing the prior art example where the feed valve is simply opened and closed with no pressure regulation thereby creating a so called ‘trailing edge’ in area/zone B. The trailing edge zone in the prior art example of Figure 2 is extended a result of an uncontrolled decrease in pressure inside the slot-die 110.

Table 1. Timing table of valves describing pressure regulation according to an embodiment.

Table 2 shows another example of intermittent coating according to an embodiment, hereby referred to as Example 2, with pressure regulation of the die recirculation valve 140 and the feed recirculation valve 150 in relation to the feed valve 130. In accordance with Example 2, when the feed valve 130 is open, the opening of the die recirculation valve 140 is adjusted manually, and the feed recirculation valve 150 is partially open, such as 0% to 100%, preferably between 50% and 100%. In this case, the die recirculation valve 140 and/or the feed recirculation valve 150 actively controls internal pressure of the slot-die by providing a clean cut at the beginning of the coated section, see area A in Figure 3.

According to Example 2, when the feed valve 130 is closed, the opening of the die recirculation valve 140 is adjusted manually, and the feed recirculation valve 150 is partially open, such as 0% to 100%, preferably between 50% and 100%. Also in this case, the die recirculation valve 140 and/or the feed recirculation valve 150 actively control the internal pressure of the slot-die by redirecting slurry to the tank via the die return line 160 and/or the feed return line 170 and adapting the pressure inside the slot-die 110. Put differently, when the feed valve 130 is closed, the internal pressure of the slot-die 110 is maintained at a certain pressure level. Thanks to this, a clear end of the coated section is provided on the substrate, see area B in Figure 3 as opposed to Figure 2 showing a prior art example where the feed valve is simply opened and closed with no pressure regulation.

Table 2. Timing table of valves describing pressure regulation according to another embodiment.

As briefly mentioned in relation Example 1 and Example 2 above, Figure 2 shows a prior art example of an intermittently coated section. Area A is referred to as a leading edge zone and area B is referred to as a trailing edge zone. The above drawing shows the coated section from a top view, with slurry 20 coated onto a substrate 10. The drawing just below illustrates the same coated section from a side view. The length L of the coated section is not well defined as indicated by the dotted part at the trailing edge zone B. As is clear from the prior art coating in Figure 2, an initial bump is provided at the leading edge zone A. This is an effect of lack of pressure control inside the slot-die. Hence, when the slot-die begins coating after a period of non-coating, a built up pressure inside the slot-die ejects an excess flow of slurry onto the substrate, thereby creating a high leading edge zone, which is not uniform in thickness, as compared to the majority of the coated section. A similar problem occurs at the trailing edge zone B of the coated section. When the slot-die stops coating, the lack of pressure control inside the slot-die leads to an additional ejection of slurry onto the substrate, thereby creating a drip like structure on the substrate at the trailing edge zone B. See also Figure 4 which illustrates how the internal pressure inside the slot-die in a conventional setup increases locally after the feed valve has closed. When the valve reopens, a high leading edge is observed.

Figure 3 schematically illustrates an embodiment according to the invention, which makes use of active pressure control within the slot-die 110 by actively controlling/varying the opening rate of the die recirculation valve 140 and/or the feed recirculation valve 150 who are synchronized with the operation of the feed valve 130. By keeping the pressure inside the slotdie 110 at a steady level throughout the coating process (i.e. during the whole process of intermittent coating), both the leading edge zone A and the trailing edge zone B display a clean cut with no excess slurry 20 ejected onto the substrate 10. Hence, by maintaining the pressure inside the slot-die 110 at a certain level, precise control of the pressure inside the slot-die provides a more uniform thickness throughout the coated section, along the length L. This is further exemplified in Figure 5.

The shape of the curve in Figure 5 resembles a periodic step-function where each step corresponds to a coating step. In the first coating step, the feed valve 130 is completely open and the die recirculation valve 140 and/or the feed recirculation valve 150 has a relatively smaller opening compared to the feed valve 130, such as 0-50 % of the feed valve opening, or preferably 0-30 %, or even more preferably 20-30 %. The pressure inside the slot-die 110 is constant during coating, such as a pressure in the range 10-350 kPa, or preferably 25-200 kPa. When the feed valve 130 closes at the end of the first coating step, the die recirculation valve 140 and/or the feed recirculation valve 150 has a relatively larger opening compared to the feed valve 130 which is closed, where the die recirculation valve 140 and/or the feed recirculation valve are at least partly open, such as 1-100 %, or preferably 50-100 %, in relation to their respective fully open position. This leads to an instantly decreased pressure in the slot-die 110. When the feed valve 130 reopens to coat the substrate 10, the pressure has been maintained at a certain level in the slot-die 110 so that no overflow of slurry is ejected at the beginning, thereby achieving a more “clean cut” edge that what is known from prior art (see reference to Figures 2 and 3 above). A similar clean cut is achieved when closing the feed valve 130, due to the pressure control regulated by the die recirculation valve 140 and/or the feed recirculation valve 150.

The system 100 according to what has been described above may also be used to perform continuous coating on the substrate 10, where the coating has a predetermined average thickness, such as 50-200 pm. The coating may have a uniform thickness, where the coating thickness variation is controlled, such as a maximum coating thickness deviation from the average coating thickness of below 5 pm, or preferably below 2 pm, or even more preferably below 1 pm, or, as is sometimes preferred, a varying thickness along the length of the coated substrate 10. This will be described in relation to Figures 6-15 below. For instance, instead of performing intermitted coating where the feed valve 130 alternates between being opened and closed to provide areas on the substrate 10 which are either coated or non-coated with slurry 20, the feed valve 130 may be open over a whole coating session thus leading to continuous coating. As described above, the opening rate of the die recirculation valve 140 and/or the feed recirculation valve 150 is adaptable to precisely regulate the pressure inside the slot-die 110 and thereby also the structural shape and thickness of the coating.

After coating, and before continued process steps such as calendaring, slitting etc., the coated substrate 10 may be referred to as a coated foil. The coated foil may have different thickness profiles depending on the application. For instance, the continuously coated foil may have a coating with a periodic pattern of anode/cathode material (i.e. slurry) over the length of the foil, such as a toothed pattern, a zigzag pattern, a shark-tooth pattern or the like. The periodic pattern is divided into subsections, each subsection having a gradually increasing and/or decreasing thickness extending between neighbouring subsections along the foil. A subsection may be described as having a tapered shape or a slope-like shape. At the beginning of each new periodic pattern, or subsection, the coated foil is then to be cut into smaller electrode parts, or sheets. The electrode sheets may also be referred to as secondary battery electrodes. For instance, the electrodes may be stacked alternately and rolled to form a cylindrical secondary battery. The secondary battery may be described as an electrode assembly.

When rolling rectangular-shaped battery electrodes into a cylindrical battery structure, also referred to as a jellyroll, a mismatch occurs between the curvature lengths of the different electrodes. Due to the wound structure of the jellyroll, only considering planar interfacial interactions, such as in two-dimensional (2D) pouched cell configurations, is not enough to properly account for loading level balancing, i.e. balancing of negative/positive capacity based on curvature. As the radius of the jellyroll increases, the curvature decreases (arc length increases). In other words, towards the centre of the roll, the curvature of the rolled up electrode is higher than at the outermost radius. The curvature difference between the anodes/cathodes in the rolled up battery gives rise to the mismatch in loading level balancing.

In Figure 6, a schematic illustration of a jellyroll is presented without winding. In reality, the different electrodes are wound several times. The circles with a dashed pattern are anodes 41, 42 coated on both sides of a current collector 40 shown as white circles. Preferably, the anode current collector 40 is made of copper. The black circles are cathodes 51, 52 coated on both sides of a cathode current collector 50 shown as circles with a dotted pattern. Preferably, the cathode current collector 50 is made of aluminium. Between the anodes 41, 42 and cathodes 51, 52, a separator 60 is arranged shown as circles with a checked pattern.

To avoid the risk of overcharging and developing short circuits in the battery, it is desirable to have a larger capacity per area on the anode. At the outermost portion of the jellyroll, the arc length of the cathode is substantially larger than that of the anode. Hence, a way of achieving a balance between the negative/positive capacity is to provide an electrode design which takes into account the effects of the curvature mismatch between the different electrodes in the jellyroll.

A way of solving the above mismatch problem is to provide a relatively thicker anode at the centre of the jellyroll, and phase it out to be thinner as the radius increases. Preferably, the cathode material is relatively thinner as compared to the anode at the centre. Hence, the thickness ratio between the anode and cathode at the centre of the jellyroll is larger than the thickness ratio between the anode and cathode at the periphery of the jellyroll, i.e. towards the outer wall, or radial end of, the jellyroll.

The length L of the electrodes may be 200-2000 mm, preferably 500-1500 mm. The ratio of maximum to minimum thickness at the respective edge portions of the electrode is 1.01-1.50, preferably 1.03-1.20, even more preferably 1.04-1.10. The thickness of the coating ranges between 30-300 pm, preferably 50-200 pm, even more preferably 75-150 pm at the thinnest part of the anodes, which corresponds with the thickest part of the cathodes and vice versa. See also the explanatory description in relation to Figures 7-15 below.

For the electrode, where the thickness increases in a direction towards the centre when rolled up into a jellyroll, the tapered or slope-like shape may be described by the general formula y = -a*ln(x) + b, where y is a highlow thickness ratio and x is the electrode distance from center of the jellyroll. For instance, ‘a’ may be within the range of 0.001-0.040, such as between 0.004-0.020, or 0.008-0.016 and ‘b’ may be within the range of 1.010-1.040, such as between 1.015-1.035, or 1.020-1.030. For the electrode, where the thickness decreases in a direction towards the centre when rolled up into a jellyroll, the tapered or slope-like shape may be described by the general formula y = c*ln(x) + d. ‘c’ may be within the range of 0.001-0.020, such as between 0.004-0.016, or 0.008-0.012 and ‘d’ may be within the range of 0.940-0.990, such as between 0.95-0.985, or 0.965-0.980. Examples of logarithmic relations for the electrode, such as the cathode and/or the anode, with a thickness increase in a direction towards the centre when rolled up into a jellyroll where the balancing mismatch is reduced to a minimum are y = -0.011n(x) + 1.0225, y = -0.0041n(x) + 1.0117 and y = -0.0151n(x) + 1.0379. Examples of logarithmic relations for the electrode, such as the cathode and/or the anode, with a thickness decrease in a direction towards the centre when rolled up into a jellyroll where the balancing mismatch is reduced to a minimum are: y = 0.0091n(x) + 0.9782, y = 0.00421n(x) + 0.9885 and y = 0.01411n(x) + 0.9643.

Different electrode configurations obtained using the system 100 as described above are illustrated in Figures 7-15. The arrow R indicates the direction out from the center and towards an outer wall of the jellyroll. The arrow R may also be referred to as a radius direction. The double coated electrode configurations in Figures 7-15 may also be seen as subsections of a coated foil.

In the examples below, the anode side 42 is facing outwards from the centre of the roll and the cathode side 51 is facing inwards. Put differently, the curvature mismatch described briefly above primarily needs to be considered when the cathode coating is on the outer side (towards the wall) and the anode coating is on the inner side of the separator, i.e. closer to the centre of the roll. Only in this case will the mismatch lead to overcharging of the anode since the cathode is has a relatively longer arc length, or curvature length, than the anode. However, when the anode side 42 is facing inwards from the centre of the roll and the cathode side 51 is facing outwards, it may be beneficial to compensate for the curvature mismatch as well as the amount of redundant coated material can be reduced, as described in relation to Figures 16-18.

With reference to Figure 7, the anode material is uniformly coated on both sides of the anode current collector 40. On the other hand, the cathode material is coated on one side of the cathode current collector 50 in a way such that the thickness decreases in a direction towards the centre when rolled up into a jellyroll. Hence, in the region where the anode 42 and cathode 51 meet (separated by a separator, not shown), the anode 42 is relatively thicker compared to the cathode 51 in the region of the electrodes facing the centre of the jellyroll. As will be clear from Figures 8-9, this principle is applicable to the remaining embodiments as well.

In Figure 8, the cathode material is uniformly coated on both sides of the cathode current collector 50. The anode 42 on one side of the anode current collector 40 facing the cathode 51, has a relatively thicker cross-section closer to the center of the jellyroll than the cathode 51. Figure 9 shows a situation where the anode 42 on one side of the anode current collector 40 has a sloped thickness that matches a sloped thickness of the cathode 51 on one side of the cathode current collector 50 facing the anode 42.

Figures 10-12 illustrate different anode profile thickness shapes. In Figure 10, the thickness of the anode 42 on one side of the anode current collector 40 decreases gradually over the length L of the anode 42 along the increasing radius direction R. The thickness may also be described as tapering along the radius direction R. In Figure 11, the thickness of the anode 42 decreases over a part of the length L of the anode 42 along the radius direction R. In Figure 12, the thickness of the anode 42 decreases logarithmically over the length L of the anode 42 along the radius direction R. Matching cathode structures are shown in Figures 13-15. In Figure 13, the thickness of the cathode 51 on one side of the cathode current collector 50 increases gradually over the length L of the cathode 51 along the radius direction R. In Figure 14, the thickness of the cathode 51 increases over a part of the length L of the cathode 51 along the radius direction R. In Figure 15, the thickness of the cathode 51 increases with a curved, or bent, shape over the length L of the cathode 51 along the radius direction R.

With reference to Figure 16, the anode material is uniformly coated on both sides of the anode current collector 40. On the other hand, the cathode material 51 is coated on one side of the cathode current collector 50 in a way such that the thickness decreases in a direction towards the centre when rolled up into a jellyroll on one side of the cathode current collector 50. Furthermore, the cathode material 52 is coated on the other side of the cathode current collector 50 in a way such that the thickness increases in a direction towards the centre when rolled up into a jellyroll on one side of the cathode current collector 50. Hence, in the region where the anode 42 and cathode 51 meet (separated by a separator, not shown), the anode 42 is relatively thicker compared to the cathode 51 in the region of the electrodes facing the centre of the jellyroll on one side, and on the other side the cathode 51 is relatively thicker compared to the anode 42 in the region of the electrodes facing the centre of the jellyroll. Such electrodes will compensate for the imbalance between negative and positive capacity in the battery due to the curvature changes previously described. Furthermore, all anode and cathode profile thickness shapes, illustrated in Figures 10-15, are applicable to this embodiment as well. As will be clear from Figures 17-18, these principles are applicable to the remaining embodiments as well.

In Figure 17, the cathode material is uniformly coated on both sides of the cathode current collector 50. The anode 42 on one side of the anode current collector 40 facing the cathode 51, has a relatively thicker cross-section closer to the center of the jellyroll than the cathode 51, where the other side of the anode collector 40, facing a second cathode (not shown), has a relatively thinner cross-section closer to the center of the jellyroll than the cathode (not shown). Figure 18 shows a situation where the anode 42 on one side of the anode current collector 40 has a sloped thickness that matches a sloped thickness of the cathode 51 on both sides of the cathode current collector 50 facing the anodes 42.

After coating the substrate with the slurry, the coated substrate is typically calendered. After calendering, the cross sectional shape of the coating as schematically illustrated in Figures 6- 18 will still be tapered in either one of the radius direction R. In the thicker part of the coating, there will be a higher solids content relative to the thinner part. The density will be the same even though the thickness is tapered/sloped.

The technique described in relation to Figure 6-18 may be used for cylindrical batteries in general. However, it has been shown that it may be particularly suitable for so-called Targe format batteries’ having a diameter of at least 40 mm. As mentioned, the slurry is coated on both sides of the substrate forming the electrode. Moreover, the anode should be made thicker than the cathode. Both the anode and the cathode have a coating which increases in (relative) thickness the further away from the centre of the battery towards the radial end, also referred to as the periphery.

Modifications and other variants of the described embodiments will come to mind to ones skilled in the art having benefit of the teachings presented in the foregoing description and associated drawings. Therefore, it is to be understood that the embodiments are not limited to the specific example embodiments described in this disclosure and that modifications and other variants are intended to be included within the scope of this disclosure.

Furthermore, although specific terms may be employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation. Therefore, persons skilled in the art would recognize numerous variations to the described embodiments that would still fall within the scope of the appended claims. As used herein, the terms “comprise/comprises” or “include/includes” do not exclude the presence of other elements or steps. Furthermore, although individual features may be included in different claims (or embodiments), these may possibly advantageously be combined, and the inclusion of different claims (or embodiments) does not imply that a certain combination of features is not feasible and/or advantageous. In addition, singular references do not exclude a plurality. Finally, reference numerals in the claims are provided merely as a clarifying example and should not be construed as limiting the scope of the claims in any way.

Claims

1. A system for coating a slurry onto a substrate to form a battery electrode for a secondary battery, said electrode coating system comprising

- a slot-die (110) having an inlet (111) configured to receive slurry from a tank (120), and an ejection outlet (113) through which slurry is configured to be ejected onto the substrate (10),

- a feed valve (130) fluidly connecting the tank (120) to the slot-die (110) at the inlet (111) of the slot-die (110), and

- a die recirculation valve (140), with a variable opening rate, fluidly connecting the slot-die (110) to the tank (120), and/or

- a feed recirculation valve (150), with a variable opening rate, fluidly connecting the feed valve (110) to the tank (120).

2. The system for coating a slurry onto a substrate according to claim 1, wherein the feed valve (130) has a fixed opening rate.

3. The system for coating a slurry onto a substrate according to claim 1 or 2, wherein the opening rate of the die recirculation valve (140) is dependent on the opening rate of the feed valve (130).

4. The system for coating a slurry onto a substrate according to any one of the preceding claims, wherein the opening rate of the feed recirculation valve (150) is dependent on the opening rate of the feed valve (130).

5. The system for coating a slurry onto a substrate according to any one of the preceding claims, wherein the die recirculation valve (140) and the feed recirculation valve (150) are actuated valves.

6. The system for coating a slurry onto a substrate according to any one of the preceding claims, wherein the die recirculation valve (140) and the feed recirculation valve (150) are solenoid valves or piezo valves.

7. The system for coating a slurry onto a substrate according to any one of the preceding claims, wherein a die return line (160) is arranged between the die recirculation valve (140) and the tank (120).

8. The system for coating a slurry onto a substrate according to any one of the preceding claims, wherein a feed return line (170) is arranged between the feed recirculation valve (150) and the tank (120).

9. The system for coating a slurry onto a substrate according to any one of the preceding claims, further comprising a pump (180) connected to the tank (120) and the feed valve (130), wherein the pump is configured to pump the slurry from the tank (120) to the slot-die (110), via the feed valve (130).

10. The system for coating a slurry onto a substrate according to any one of the preceding claims, wherein the substrate (10) is an electrically conductive substrate, preferably a foil, such as an aluminium foil or a copper foil.

11. A method of coating a slurry onto a substrate to form a battery electrode for a secondary battery, comprising the steps of:

(a) providing a system (100) according to any one of claims 1-10,

(b) supplying a slurry to the slot-die (110) by opening the feed valve (130), thereby ejecting slurry from the slot-die (110) onto the substrate (10) to be coated, and

(c) regulating a pressure level in the slot-die (110) by actuating the die recirculation valve (140) and/or the feed recirculation valve (150).

12. The method according to claim 11, wherein a predetermined pressure level is maintained in the slot-die (110) by regulating the opening rate of the die recirculation valve (140) and/or the feed recirculation valve (150).

13. The method according to claim 11 or 12, wherein the pressure is maintained at a predetermined level, such as between 10 kPa and 350 kPa, or preferably between 25 kPa and 200 kPa, and where optionally the pressure level in the slot-die (110) is kept within ±10 %, or preferably within ±5 %, or even more preferably within ±2.5 %.

14. A secondary battery electrode obtainable by the method claimed in any one of claims 11-13.

15. A secondary battery comprising a secondary battery electrode according to claim 14.

16. A secondary battery according to claim 15, wherein the secondary battery is a jellyroll type battery.

17. A coated foil for a secondary battery electrode according to claim 14, having a coating with a periodic pattern, wherein the periodic pattern is divided into subsections, wherein at least one subsection has a gradually increasing or decreasing thickness extending between neighbouring subsections along the foil.

18. A coated foil for a secondary battery electrode according to claim 14, having a uniform coating thickness extending along the foil.

19. A secondary battery comprising at least one subsection of the foil according to claim claim 17 and/or claim 18. 0. The secondary battery according to claim 19, wherein the thickness of said at least one subsection increases or decreases towards a centre of the secondary battery.