ROLLING DEVICE FOR ELECTRODE SHEET

A rolling device is a rolling device that is arranged on a conveyance path on which an electrode sheet of an electricity storage device is conveyed and includes a pair of rolling rolls. Each of the rolling rolls includes a base material, a ground layer formed at least on a rolling area of an outer peripheral surface of the base material that rolls the electrode sheet, and a diamond-like carbon (DLC) coating film formed on an outer peripheral surface of the ground layer. The ground layer is harder than the base material.

CROSS REFERENCE OF RELATED APPLICATION

This application claims the benefit of priority to Japanese Patent Application No. 2023-095148 filed on Jun. 9, 2023. The entire contents of this application are hereby incorporated herein by reference.

BACKGROUND OF THE DISCLOSURE

The present disclosure relates to a rolling device for electrode sheet.

For example, Japanese Laid-open Patent Publication No. 2018-190681 discloses a method for manufacturing an electrode material including a step of pressing an electrode material obtained by coating a band-like metal foil with an active material slurry layer and a ceramic slurry layer in a thickness direction. The ceramic slurry layer includes ceramic particles. In the method described in Japanese Laid-open Patent Publication No. 2018-190681, pressing is performed using a press roll having an outer peripheral surface of a roll body coated with a wear resistance layer of diamond-like carbon (DLC) as conveying the electrode material. According to Japanese Laid-open Patent Publication No. 2018-190681, direct slide contact of the roll body with the ceramic particles is eliminated by the wear resistance layer, and wear of the roll body is suppressed.

SUMMARY

As described in Japanese Laid-open Patent Publication No. 2018-190681, by forming a DLC coating film having a high hardness on a surface of a rolling roll that rolls an electrode sheet of an electricity storage device, a wear resistance of the rolling roll can be increased. However, the DLC coating film has a low toughness, and the DLC coating film peels off from a base material of the rolling roll in some cases. When the DLC coating peels off, the base material rapidly wears. When the base material rapidly wears, there is a probability that fine particles of the base material generated by wear become contaminants and affect quality of the electrode sheet.

Therefore, herein, a rolling device for electrode sheet in which contaminants are less likely to be generated even when a DLC coating film of a rolling roll peels off is proposed.

A rolling device for electrode sheet disclosed herein is a rolling device that is arranged on a conveyance path on which an electrode sheet of an electricity storage device is conveyed and includes a pair of rolling rolls. Each of the rolling rolls includes a base material, a ground layer formed at least on a rolling area of an outer peripheral surface of the base material that rolls the electrode sheet, and a diamond-like carbon (DLC) coating film formed on an outer peripheral surface of the ground layer. The ground layer is harder than the base material.

According to the rolling device for electrode sheet, the ground layer that is harder than the base material is provided between the base material and the DLC coating film. With the ground layer, even when the DLC coating film of the rolling roll peels off, the base material does not contact the electrode sheet. Therefore, wear of the base material is prevented. Moreover, the ground layer is harder than the base material and is less likely to wear. As a result, contamination can be suppressed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of a rolling device for an electrode sheet of an electricity storage device will be described below. Note that, as a matter of course, the preferred embodiments described herein are not intended to be particularly limiting the present disclosure. The accompanying drawings are schematic and do not necessarily reflect an actual implemented product.

FIG.1is a schematic side view of a rolling device10for electrode sheet according to an embodiment.FIG.2is a schematic plan view of the rolling device10for electrode sheet. Herein, the rolling device10rolls an electrode sheet1of a lithium-ion secondary battery. However, the electrode sheet1is not limited to an electrode sheet of a lithium-ion secondary battery and may be an electrode sheet of any one of various known electricity storage devices. The term “electricity storage device” generally refers to devices from which electric energy can be taken out and encompasses so-called electricity storage batteries (chemical batteries), such as a lithium-ion secondary battery, a nickel hydrogen battery, or the like and capacitors (physical batteries), such as an electric double-layered capacitor or the like.

The electrode sheet1includes a band-like electrode foil2and an active material layer3formed on the electrode foil2. A positive electrode sheet is configured such that a positive electrode active material layer3including a positive electrode active material is formed on a surface of a band-like electrode foil2(for example, aluminum foil) having a width and a thickness that are determined in advance. In a lithium-ion secondary battery, the positive electrode active material can discharge lithium ions during charging and absorb lithium ions during discharging like a lithium transition metal composite material, for example. A negative electrode sheet is configured such that a negative electrode active material layer3including a negative electrode active material is formed on a surface of a band-like electrode foil2(for example, copper foil) having a width and a thickness that are determined in advance. In a lithium-ion secondary battery, the negative electrode active material can absorb lithium ions during charging and discharge lithium ions absorbed during charging during discharging like natural graphite, for example. As to the positive electrode active material and the negative electrode active material, various other materials than the materials described above have been proposed and there is no particular limitation.

The positive electrode sheet further includes a protective layer4that protects the active material layer3and prevents reaction with an electrolyte solution. The protective layer4herein includes a passive state of a metal oxide (ceramic). The protective layer4includes, for example, an aluminum oxide (alumina). However, as to the protective layer4, various other materials than the materials described above have been proposed and there is no particular limitation. The protective layer4is formed to overlap an upper layer of the positive electrode active material layer3. InFIG.1andFIG.2, as the electrode sheet1, a positive electrode sheet including the protective layer4is illustrated. In the following description, it is assumed that the electrode sheet1includes the protective layer4, unless specifically stated otherwise.

A raw material of the active material layer3is made into a slurry state and is applied to the electrode foil2. The raw material of the active material layer3may be applied to one surface of the electrode foil2and may be applied to both surfaces thereof. For a positive electrode sheet, a material of the protective layer4is made into a slurry state and is applied to coat over the active material layer3. The electrode sheet1is formed to include a portion on which the active material layer3and the protective layer4are applied and a non-coated portion on which neither the active material layer3nor the protective layer4is formed. The raw materials of the active material layer3and the protective layer4in a slurry state that have been applied are dried in a drying step. Thereafter, the rolling device10continuously presses the electrode sheet1as conveying the electrode sheet1in a longitudinal direction, to adjust each of respective thicknesses of the active material layer3and the protective layer4to a thickness in a predetermined range and densify an active material in the active material layer3.

As illustrated inFIG.1, the rolling device10is arranged on a conveyance path on which the electrode sheet1of the electricity storage device is conveyed. As illustrated inFIG.1, the rolling device10itself may include a mechanism that conveys the electrode sheet1. In this preferred embodiment, the rolling device10includes an unwinding device20, a pressing device30, and a winding device40. The electrode sheet1with the active material layer3and the protective layer4dried thereon is supplied to the unwinding device20by a roll. The winding device40winds the electrode sheet1unwound from the unwinding device20and rolled by the pressing device30. However, the rolling device10may be provided on another conveyance path of the electrode sheet1on which the electrode sheet1is conveyed by some other conveyance device.

The rolling device10(specifically, the pressing device30) includes a pair of rolling rolls31U and31D. The rolling roll31U at an upper side and the rolling roll31D at a lower side face each other in an up-down direction with the conveyance path of the electrode sheet1interposed therebetween. The rolling roll31U at the upper side is lowered by an unillustrated driving device when rolling the electrode sheet1and presses the electrode sheet1with the rolling roll31D at the lower side. The thickness of the active material layer3and the protective layer4after being rolled are measured by a film thickness sensor35. A position of the rolling roll31U at the upper side in the up-down direction or a pressing pressure thereof is feedback controlled in accordance with the thickness of the active material layer3and the protective layer4measured by the film thickness sensor35. Note that, in the following description, when the rolling roll31U at the upper side and the rolling roll31D at the lower side are not distinguished from each other, the reference symbol31is used to express them as the rolling rolls31, the pair of rolling rolls31, or the like as appropriate.

Although there is no particular limitation on a pressing force of the rolling device10, the pressing force can reach a linear pressure of 40000 N/cm in some cases. Lengths of the rolling rolls31U and31D in a width direction of the electrode sheet1can be 100 cm in some cases. As to conveyance speed of the electrode sheet1during rolling, there is no particular limitation, but the conveyance speed can reach 100 m/min in some cases. The linear pressure of the rolling device10is high, the conveyance speed of the electrode sheet1is high, and the protective layer4is hard, so that a large friction force is applied to the pair of rolling rolls31U and31D. A Vickers' hardness of a ceramic portion of the protective layer4is, for example, 1400 HV to 1800 HV. As the conveyance speed of the electrode sheet1is high, a wear amount of the rolling rolls31per unit hour is increased.

FIG.3is a schematic cross-sectional view of the rolling roll31. As illustrated inFIG.3, the rolling roll31includes a base material32, a ground layer33, and a diamond-like carbon (DLC) coating film34. The base material32is a base body of the rolling roll31formed into a cylindrical shape. The base material32is formed of, for example, a high carbon content bearing steel material (SUJ material), and is preferably formed of, for example, SUJ2having high versatility. However, a material of the base material32is not limited to the SUJ material. The base material32may be formed of, for example, chrome steel, chromium molybdenum steel, stainless steel, or the like. As a material of the base material32, a material having a strength to bear the pressing pressure of the rolling roll31and being excellent in terms in availability, workability, cost, or the like is preferable. In this preferred embodiment, a hardness of the base material32does not need to be very high. A Vickers' hardness of a preferable material as the base material32is, for example, 600 HV or more and 900 HV or less.

The ground layer33is a layer harder than the base material32and is formed on a surface of the base material32. The ground layer33is formed at least on a rolling area32a(seeFIG.2) of an outer peripheral surface of the base material32that rolls the electrode sheet1. As illustrated inFIG.2, the rolling area32ais an area that contacts the electrode sheet1during rolling. A width of the rolling area32ain a width direction of the electrode sheet1is equal to a width of the electrode sheet1. The ground layer33may be formed on a portion of the outer peripheral surface of the base material32that includes the rolling area32aand is broader than the rolling area32a. For example, the ground layer33may be formed on the entire outer peripheral surface of the base material32.

In this preferred embodiment, the ground layer33is formed of cemented carbide. More specifically, the ground layer33is formed of a thermal-sprayed film of cemented carbide. The cemented carbide is a material obtained by sintering tungsten carbide with metal, such as cobalt, nickel, or the like. In the cemented carbide, the metal that bonds fine particles of tungsten carbide is not limited to cobalt or nickel. The thermal-sprayed film is a coating film formed by spraying a material in a molten state or a state close to a molten state to a base material. The thermal-sprayed film of the cemented carbide is formed, for example, by a high-speed flame spraying method. However, the ground layer33may be a thermal-sprayed film of some other material than cemented carbide, for example, a material (so-called cermet) including a metal oxide (ceramic) and metal. As another option, the ground layer33may be formed by some other method than thermal spraying. For example, the ground layer33may be formed of hard chrome plating.

The ground layer33is formed of a material harder than at least the base material32. A Vickers' hardness of the ground layer33is preferably 900 HV or more. The Vickers' hardness of the ground layer33is more preferably 1200 HV or more. A Vickers' hardness of cemented carbide is about 1100 HV to 1500 HV. A Vickers' hardness of hard chrome plating is about 750 HV to 1000 HV.

The ground layer33is preferably formed thick. As a method for forming the ground layer33to a large thickness, thermal spraying is suitable. The ground layer33is preferably formed to have an average thickness of 50 μm or more. More preferably, the ground layer33is formed to have an average thickness of 100 μm or more.

As illustrated inFIG.3, the DLC coating film34is formed on an outer peripheral surface of the ground layer33. The DLC coating film34is a carbon coating film having a carbon bond structure of both of diamond and graphite. The DLC coating film34is formed, for example, by plasma chemical vapor deposition (CVD) method in which hydrogen carbonate gas is converted into plasma and is vapor deposited or sputtering using graphite as a raw material. A Vickers' hardness of the DLC coating film34is, for example, 1600 HV or more and 2500 HV or less. The Vickers' hardness of the DLC coating film34is larger than the Vickers' hardness of the ground layer33. An average thickness of the DLC coating film34is, for example, 2 μm to 3 μm. A realizable thickness of the DLC coating film34is several μm or less. An average thickness of the ground layer33is larger than the average thickness of the DLC coating film34. The average thickness of the ground layer33is preferably ten times the average thickness of the DLC coating film34or more.

Effects of Preferred Embodiment

Effects that can be achieved by the rolling device10for the electrode sheet1according to this preferred embodiment will be described below.

The rolling device10for the electrode sheet1according to this preferred embodiment is arranged on the conveyance path on which the electrode sheet1of the electricity storage device is conveyed and incudes the pair of rolling rolls31. Each of the rolling rolls31includes the base material32, the ground layer33formed at least on the rolling area32aof the outer peripheral surface of the base material32that rolls the electrode sheet1, and the DLC coating film34formed on the outer peripheral surface of the ground layer33. The ground layer33is harder than the base material32.

The DLC coating film34has a low toughness and can peel off from the base material32in some cases. For example, when the rolling rolls31bite a foreign matter or the like, the DLC coating film34tends to peel off. According to a known technology, each of rolling rolls that roll electrode sheet1of the electricity storage device is configured such that a DLC coating film is formed on an outer peripheral surface of a base material. Therefore, in the known rolling rolls, when the DLC coating film peels off, the base material rapidly wears. The electrode sheet1, specifically, the protective layer4is very hard, the pressing pressure is large, and the conveyance speed of the electrode sheet1is high, and therefore, the base material rapidly wears. When the base material rapidly wears, there is a probability that fine particles of the base material generated by wear become contaminants and affect quality of the electrode sheet1.

In contrast, in the rolling device10according to this preferred embodiment, the ground layer33that is harder than the base material32is provided between the base material32and the DLC coating film34. Even when the DLC coating film34of the rolling rolls31peels off, with the ground layer33provided, the base material32does not contact the electrode sheet1. Therefore, wear of the base material32is prevented. Moreover, the ground layer33is harder than the base material32and is less likely to wear. As a result, contamination can be suppressed.

In the known rolling rolls, when the DLC coating film wears off through long-term use and a portion or an entire portion of the DLC coating film is eliminated, the base material rapidly wears. Therefore, during a period from when the DLC coating film has worn and peeled off to when wear or peeling off of the DLC coating film is found and the rolling rolls are changed, the wear of the base material progresses. Thus, a life of the rolling rolls is shortened. While the base material can be used, the DLC coating film is re-formed on the base material for reuse.

In contrast, in the rolling device10according to this preferred embodiment, it is the ground layer33that, after the DLC coating film34wears or peels off, contacts the electrode sheet1and wears. The ground layer33is harder than the base material32, and a wear amount thereof is smaller than that of the base material32. Therefore, the rolling rolls31can be maintained in a reusable state for a long period of time, and the DLC coating film34can be re-formed more times than that in the known technology. As a result, the life of the rolling rolls31can be extended longer than that in the known technology.

In this preferred embodiment, the ground layer33is formed of cemented carbide. According to the configuration described above, the hardness of the ground layer33formed of cemented carbide is high, and therefore, the wear amount of the rolling rolls31can be further reduced.

In this preferred embodiment, the average thickness of the ground layer33is larger than the average thickness of the DLC coating film34. According to the configuration described above, the ground layer33has a large thickness, and therefore, the life of the rolling rolls31can be further extended.

In this preferred embodiment, the average thickness of the ground layer33is 50 μm or more. According to the configuration described above, the ground layer33has a large thickness, and therefore, the life of the rolling rolls31can be further extended. According to a trial product manufactured by the inventors of the present application, the life of the rolling rolls31configured such that the average thickness of the ground layer33is 50 μm or more is several times that of the known rolling rolls (the material of the base material is SUJ2) in which the DLC coating film is formed on a surface of the base material or more.

In this preferred embodiment, the ground layer33is formed of a thermal-sprayed film. According to the configuration described above, a high degree of freedom for the material of the ground layer33is achieved. Thermal spraying has fewer restrictions on materials that can be used for forming a film. Moreover, the ground layer33can be easily formed to a large thickness by thermal spraying.

Other Preferred Embodiments

One preferred embodiment of the rolling device for the electrode sheet proposed herein has been described above. However, the preferred embodiment described above is merely an example, and a rolling device for an electrode sheet according to the present disclosure can be implemented in various other embodiments. For example, in the preferred embodiment described above, each of the rolling rolls31has a three-layered structure of the base material32, the ground layer33, and the DLC coating film34, but not limited thereto. The rolling roll may have a multilayer structure including four or more layers in which a base material is arranged in an innermost side in a radial direction, a DLC coating film is arranged in an outermost side in the radial direction, and a ground layer is arranged between the base material and the DLC coating film.

Furthermore, the preferred embodiment described above shall not limit the present disclosure, unless specifically stated otherwise. Various changes can be made to a technology described herein, and each of components and processes described herein can be omitted as appropriate or can be combined as appropriate, unless a particular problem occurs.

The present specification includes disclosure set force in the following items.

Item 1: A rolling device for electrode sheet that is arranged on a conveyance path on which an electrode sheet of an electricity storage device is conveyed, the rolling device comprising:a pair of rolling rolls,whereineach of the rolling rolls includesa base material,a ground layer formed at least on a rolling area of an outer peripheral surface of the base material that rolls the electrode sheet, anda diamond-like carbon (DLC) coating film formed on an outer peripheral surface of the ground layer, andthe ground layer is harder than the base material.

Item 2: The rolling device for electrode sheet according to Item 1, whereinthe ground layer is formed of cemented carbide.

Item 3: The rolling device for electrode sheet according to Item 1 or 2, whereinan average thickness of the ground layer is larger than an average thickness of the DLC coating film.

Item 4: The rolling device for electrode sheet according to any one of Items 1 to 3, whereinthe average thickness of the ground layer is 50 μm or more.

Item 5: The rolling device for electrode sheet according to any one of Items 1 to 4, whereinthe ground layer is formed of a thermal-sprayed film.