Electrode and method for manufacturing electrode

Abstract

An electrode 1 includes: a current collector 2; an active material layer 4 stacked on a surface of the current collector 2; and a composite particle group 6 containing a plurality of composite particles 8 in which a conductive material and a binder are made composite and being dispersed in the manner of islands at an interface between the current collector 2 and the active material layer 4.

Description

Electrode and Method for Manufacturing the Same

[0001] The present disclosure relates to an electrode and a method for manufacturing the electrode.

[0002] Patent Document 1 discloses an electrode for a lithium-ion battery having a structure in which binder particle groups are scattered at the interface between a current collector foil and an active material layer.

[0003] Japanese Unexamined Patent Application Publication No. 2023-82849

[0004] As a result of intensive studies on the above-described conventional electrodes, the present inventors have come to recognize that there is room for improvement in the conventional electrodes in terms of improving the performance of a power storage device including a lithium-ion battery.

[0005] The present disclosure has been made in view of such circumstances, and one of its objects is to provide a technology for improving the performance of a power storage device.

[0006] One aspect of the present disclosure is an electrode. This electrode includes a current collector, an active material layer laminated on the surface of the current collector, a plurality of composite particles in which a conductive material and a binder are composite, and a composite particle group that is scattered in an island shape at the interface between the current collector and the active material layer.

[0007] Another aspect of the present disclosure is a method for manufacturing an electrode. This manufacturing method includes scattering composite particles in which a conductive material and a binder are composite in an island shape on the surface of a current collector by a dry method, and laminating an active material layer on the surface of the current collector where the composite particles are scattered in an island shape.

[0008] Any combination of the above components, and those obtained by converting the expressions of the present disclosure among methods, apparatuses, systems, etc. are also effective as aspects of the present disclosure.

[0009] According to the present disclosure, the performance of a power storage device can be improved.

[0010] It is a schematic diagram of a cross-section of an electrode according to an embodiment. FIG. 2(A) is a SEM image of a composite particle group arranged on the surface of a current collector. FIG. 2(B) is a schematic diagram of a cross-section of a composite particle. It is a diagram showing the composition, binder ratio, interfacial resistance, peel strength, and charge-discharge cycle retention rate in each example and each comparative example.

[0011] The present disclosure will be described below with reference to the drawings, based on preferred embodiments. The embodiments are illustrative and not limiting, and not all features or combinations thereof described in the embodiments are necessarily essential to the present disclosure. The same or equivalent components, members, and processes shown in each drawing are denoted by the same reference numerals, and redundant descriptions are omitted where appropriate. The scale and shape of each part shown in each drawing are set for convenience to facilitate explanation and are not to be interpreted restrictively unless otherwise specified. Furthermore, where terms such as "first," "second," etc. are used in this specification or claims, unless otherwise specified, these terms do not indicate any order or importance, but are used to distinguish one configuration from another. In addition, some components that are not important for explaining the embodiments are omitted in each drawing.

[0012] Figure 1 is a schematic cross-sectional view of an electrode 1 according to an embodiment. The electrode 1 is incorporated into an energy storage device that includes a rechargeable secondary battery such as a lithium-ion battery, nickel-metal hydride battery, or nickel-cadmium battery, or a capacitor such as an electric double-layer capacitor. The electrode 1 may be used as either the positive electrode or the negative electrode of the energy storage device. The electrode 1 is, for example, in the form of a sheet. The electrode 1 comprises a current collector 2, an active material layer 4, and a composite particle group 6.

[0013] The current collector 2 is made of metal foil. In the case of a typical lithium-ion secondary battery, the current collector 2 is made of aluminum foil or the like if it is the positive electrode, and copper foil or the like if it is the negative electrode.

[0014] The active material layer 4 is laminated on the surface of the current collector 2. The active material layer 4 may be laminated on only one main surface of the current collector 2, or on both main surfaces. The active material layer 4 contains an electrode mixture. For example, the active material layer 4 is an electrode mixture sheet, in which the electrode mixture is formed into a sheet, which is then pressed onto the surface of the current collector 2.

[0015] The electrode mixture contains an electrode active material and a conductive material. The electrode mixture may also contain a binder as needed. For example, the electrode mixture is a so-called dry mixture, with a solvent content of less than 10% by mass, less than 5% by mass, less than 0.1% by mass, or substantially 0% of the total mass of the electrode mixture. In other words, the solid content of the electrode mixture is 90% or more, 95% or more, 99.9% or more, or substantially 100% of the total mass of the electrode mixture.

[0016] In typical lithium-ion secondary batteries, the electrode active materials are lithium nickel-cobalt-manganese composite oxide (NCM), lithium nickel-cobalt-aluminum composite oxide (NCA), lithium iron phosphate (LFP), etc. for the positive electrode, and graphite, etc. for the negative electrode. Conductive materials include carbon black (CB), acetylene black (AB), Ketjenblack, carbon nanotubes (CNT), natural graphite, artificial graphite, etc. Binders include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), polyethylene (PE), fluororubber, ethylene propylene diene rubber, styrene-butadiene, cellulose resins, polyacrylic acid, etc.

[0017] The composite particle group 6 includes multiple composite particles 8 and is scattered in an island-like manner at the interface between the current collector 2 and the active material layer 4. Each island-like portion may consist of a single composite particle 8 or an aggregate of multiple composite particles 8. Figure 2(A) is a scanning electron microscope (SEM) image of the composite particle group 6 arranged on the surface of the current collector 2. The SEM image also shows that the composite particle group 6 is scattered in an island-like manner on the surface of the current collector 2. The island-like portions may be distributed regularly or irregularly. The surface of the current collector 2 is exposed in the gaps between the island-like portions, and the current collector 2 and the active material layer 4 are in direct contact. The gaps between the island-like portions may be, for example, one or more composite particles or one or more island-like portions.

[0018] Figure 2(B) is a schematic diagram of a cross-section of the composite particle 8. The composite particle 8 is a composite of the conductive material 10 and the binder 12. As an example, the composite particle 8 has a structure in which one or more binders 12 are attached to the surface of the conductive material 10. The composite particle 8 can be produced by mixing the conductive material 10 and the binder 12 using a known mixer.

[0019] Examples of conductive materials 10 include carbon black (CB), acetylene black (AB), Ketjenblack, carbon nanotubes (CNT), natural graphite, and artificial graphite. When the conductive material 10 contains at least one of natural graphite and artificial graphite, it is possible to easily form a state in which the binder 12 adheres to the surface of the conductive material 10.

[0020] Examples of binder 12 include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), polyethylene (PE), fluororubber, ethylene propylene diene rubber, styrene butadiene, cellulose resin, and polyacrylic acid.

[0021] When the weight of the binder 12 is Wb, the weight of the conductive material 10 is Wc, the true density of the conductive material 10 is TD, and the particle size of the conductive material 10 is PS, the composite particle 8 may satisfy the condition 0.3 ≤ Wb / [(Wc × 6) / (TD × PS)] ≤ 14.0. The above-mentioned "Wb / [(Wc × 6) / (TD × PS)]" means the ratio of the weight of the binder 12 to the surface area of ​​the conductive material 10, assuming that the conductive material 10 is spherical. Hereafter, this ratio will be appropriately referred to as the "binder ratio".

[0022] The weight Wb of the binder 12 and the weight Wc of the conductive material 10 can be measured, for example, with an electronic balance (TX323n: manufactured by Shimadzu Corporation). The true density TD of the conductive material 10 can be measured, for example, with a true density analyzer (Ultrapyc5000: manufactured by Anton Paar). The particle size PS of the conductive material 10 may be, for example, a volume-based median diameter D50. In this disclosure, the median diameter D50 means the particle size at which the cumulative frequency of the smallest particle size accounts for 50% in the volume-based particle size distribution. The particle size distribution can be measured, for example, with a particle size distribution / particle shape analyzer (SYNC: manufactured by Microtrac-Bell).

[0023] The basis weight of the composite particles 8 per unit area of ​​the current collector 2 is, for example, 0.10 g / m². ２ 3g / m or more ２ The following is the result. This makes it easier to adjust the peeling strength of the active material layer 4 from the current collector 2 and the interfacial resistance between the current collector 2 and the active material layer 4 to the values ​​required for the energy storage device.

[0024] The manufacturing method for the electrode 1 according to this embodiment includes a composite particle 8 attachment step and an active material layer 4 lamination step. In the composite particle 8 attachment step, the composite particles 8 are scattered in an island-like manner on the surface of the current collector 2 by a dry method. As an example, the composite particles 8 are prepared in advance by mixing the conductive material 10 and the binder 12. Then, the composite particles 8 are attached to the surface of the current collector 2 by a dry method such as electrostatic screen printing. By using a dry method for attaching the composite particles 8, the composite particles 8 can be scattered in an island-like manner on the surface of the current collector 2 more easily and reliably than when a wet method is used. In addition, the scattered state of the composite particles 8 can be maintained more stably.

[0025] In this disclosure, "dry method" means a method of applying or printing a paint containing composite particles 8 to the surface of a current collector 2, wherein the solid content of the paint is 90% or more. The solid content of the paint in the dry method may be 95% to 100%. In this disclosure, "wet method" means a method of applying or printing a paint to the surface of a current collector 2, wherein the solid content of the paint is less than 90%.

[0026] In the lamination process of the active material layer 4, the active material layer 4 is laminated onto the surface of the current collector 2, where the composite particles 8 are scattered in an island-like manner. As an example, the electrode mixture is first formed into a sheet using a stretching roller or the like to create the active material layer 4. Then, the active material layer 4 is pressed onto the surface of the current collector 2 using a press roller or the like. As a result, the active material layer 4 is laminated onto the surface of the current collector 2, and an electrode 1 is obtained in which the composite particle group 6 is scattered in an island-like manner at the interface between the current collector 2 and the active material layer 4. Prior to the lamination process of the active material layer 4, the current collector 2 on which the composite particles 8 are printed may be heated. This can melt the binder 12, and therefore the composite particles 8 can adhere more firmly to the current collector 2 and the active material layer 4. Alternatively, the lamination of the active material layer 4 onto the current collector 2 may be carried out by applying an electrode mixture slurry to the surface of the current collector 2, rather than by bonding the current collector 2 and the active material layer 4 together.

[0027] As described above, in the electrode 1 according to this embodiment, the composite particle group 6 is scattered in an island-like manner at the interface between the current collector 2 and the active material layer 4. This allows the conductive material to compensate for the decrease in conductivity between the current collector 2 and the active material layer 4 caused by the binder. Therefore, while increasing the peeling strength of the active material layer 4 from the current collector 2, similar to the conventional structure in which only the binder is scattered at the interface between the current collector 2 and the active material layer 4, the interfacial resistance between the current collector 2 and the active material layer 4 can be reduced compared to the conventional structure. In addition, the charge-discharge cycle maintenance rate of the energy storage device can be increased compared to the conventional structure. Thus, the performance of the energy storage device can be improved. The charge-discharge cycle maintenance rate can also be called the charge-discharge cycle characteristics or capacity maintenance rate.

[0028] Furthermore, if the binder ratio in the composite particles 8 is 0.3 to 14.0, the performance of the energy storage device can be further improved. In addition, the composite particles 8 of this embodiment have a structure in which the binder 12 is attached to the surface of the conductive material 10. This makes it easier to achieve the effects described above, such as increased peeling strength, decreased interfacial resistance, and improved charge-discharge cycle maintenance rate.

[0029] The embodiments of this disclosure have been described in detail above. The embodiments described above are merely examples of how to implement this disclosure. The content of the embodiments does not limit the technical scope of this disclosure, and many design changes, such as changes, additions, and deletions of components, are possible as long as they do not depart from the spirit of the invention as defined in the claims. A new embodiment with design changes will have the combined effects of both the embodiment and the variation. In the embodiments described above, the content in which such design changes are possible is emphasized with notations such as "of this embodiment" or "in this embodiment," but design changes are also permitted even if there are no such notations. Furthermore, any combination of components included in each embodiment is also valid as an embodiment of this disclosure. The hatching applied to the cross-section in the drawings does not limit the material of the object to which the hatching is applied.

[0030] The embodiments may be specified by the following items: [Item 1] An electrode (1) comprising a current collector (2), an active material layer (4) laminated on the surface of the current collector (2), and a group of composite particles (6) scattered in an island-like manner at the interface between the current collector (2) and the active material layer (4), each containing a plurality of composite particles (8) formed by the compounding of a conductive material (10) and a binder (12). [Item 2] The electrode (1) of Item 1, wherein when the weight of the binder (12) is Wb, the weight of the conductive material (10) is Wc, the true density of the conductive material (10) is TD, and the particle size of the conductive material (10) is PS, the composite particles (8) satisfy the condition 0.3 ≤ Wb / [(Wc × 6) / (TD × PS)] ≤ 14.0. [Item 3] The electrode (1) according to Item 1 or Item 2, wherein the composite particle (8) has a structure in which a binder (12) is attached to the surface of a conductive material (10). [Item 4] The electrode (1) according to Item 3, wherein the conductive material (10) includes at least one of natural graphite and artificial graphite. [Item 5] A method for manufacturing the electrode (1), comprising scattering composite particles (8), in which the conductive material (10) and binder (12) are compounded, in an island-like manner on the surface of a current collector (2) by a dry method, and laminating an active material layer (4) on the surface of the current collector (2) where the composite particles (8) are scattered in an island-like manner.

[0031] The following describes embodiments of the present invention, but these embodiments are merely illustrative examples for suitably illustrating the present invention and do not limit the present invention in any way.

[0032] (Example 1) <Preparation of positive electrode active material layer> A lithium transition metal composite oxide as the positive electrode active material and AB as the conductive material were put into a pulverizer (NOB300 - Novilta (registered trademark): manufactured by Hosokawa Micron Corporation) and mixed for 20 minutes to obtain a positive electrode composite material. The weight ratio of lithium transition metal composite oxide to AB was 99.1:0.9. The obtained positive electrode composite material and PTFE as a binder were put into a pulverizer (Wonder Crusher WC-3: manufactured by Osaka Chemical Co., Ltd.) and kneaded at a rotation speed of 8400 rpm for 2 minutes to obtain a positive electrode mixture. The weight ratio of positive electrode composite material to PTFE was 100:1.0. The obtained positive electrode mixture was supplied between a pair of stretching rolls, and the pair of rolls were rotated under conditions of a roll peripheral speed ratio of 1:1 and a linear pressure of 0.1 t / cm to obtain a sheet-like positive electrode active material layer.

[0033] <Preparation of Composite Particles> Natural graphite as a conductive material and PVdF as a binder were put into a pulverizer (Wonder Crusher WC-3: manufactured by Osaka Chemical Co., Ltd.) and kneaded at a rotation speed of 8400 rpm for 2 minutes to obtain composite particles. The weight of the natural graphite added was 2.5 g (Wc) and the weight of the PVdF added was 47.5 g (Wb). Each weight was measured using an electronic balance (TX323n: manufactured by Shimadzu Corporation).

[0034] The true density (TD) of the natural graphite used was measured using a true density analyzer (Ultrapyc 5000: manufactured by Anton Paar) at a temperature of 25°C and a cell volume of 3.5 cm³. ３ Measurements were taken under the specified conditions. In addition, the particle size distribution of natural graphite was measured using a particle size distribution / particle shape analyzer (SYNC: Microtrac-Bell) with a measurement time of 10 s. From the obtained particle size distribution, the D50 of natural graphite was calculated twice, and the average value was taken as the particle size PS of natural graphite. Then, the binder ratio in the composite particles was calculated using the weight Wb of PVdF, the weight Wc of natural graphite, the true density TD of natural graphite, and the particle size PS of natural graphite.

[0035] <Fabrication of the positive electrode> Using an electrostatic screen printing device (Type T-1: manufactured by Berg Industries Co., Ltd.), composite particles were printed onto the surface of aluminum foil, which was to be used as a current collector, at a distance of 4 mm and a voltage of 4 kV to obtain a current collector with composite particles. Then, the current collector with composite particles and the positive electrode active material layer were passed between a pair of rolling rolls, and the pair of rolls were rotated at a roll peripheral speed ratio of 1:1 and a linear pressure of 1 t / cm to obtain the positive electrode.

[0036] The interfacial resistance between the current collector and the active material layer in the obtained positive electrode was measured using an interfacial resistance meter (XF057: HIOKI E.E. CORPORATION) under the conditions of a measurement current of 100 μA, a voltage range of 0.5 V, and a measurement speed of Normal. Furthermore, the peel strength of the active material layer relative to the current collector in the obtained positive electrode was measured using a Tensilon universal material tester (RTC-1350A: A&D Corporation) under the conditions of a test speed of 20 mm / min and a load range of 1 N / 1%. The interfacial resistance was found to be 0.8 Ωm. ２ A double circle (◎) indicates less than 0.8 Ωm. ２ The above is 1.0 Ωm ２ Use a circle (○) for values ​​less than 1.0 Ωm. ２ The above were evaluated as "X" (×). Regarding peel strength, 1.0 N / m or more was evaluated as "◎", 0.8 N / m or more but less than 1.0 N / m was evaluated as "○", and less than 0.8 N / m was evaluated as "×". In both the evaluation of interfacial resistance and peel strength, evaluations of "◎" and "○" mean that the electrodes meet the performance requirements when used in energy storage devices, while evaluation "×" means that the performance requirements are not met.

[0037] <Preparation of Test Cell> A negative electrode active material mixture electrode was prepared as the negative electrode. A polyolefin separator was sandwiched between the positive and negative electrodes to create an electrode body. This electrode body and a non-aqueous electrolyte were housed in an outer casing to obtain the test cell according to Example 1. The non-aqueous electrolyte was prepared by mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 1:3 as a non-aqueous solvent, and LiPF as the electrolyte salt. ６ A solution prepared by dissolving the substance in 1.0 mol / L was used.

[0038] The charge-discharge cycle retention rate in the test cell of Example 1 was measured under the conditions of a charge-discharge current of 0.5 C and a charge amount of 450 mAh / g using a charge-discharge evaluation apparatus (TOSCAT 3000, manufactured by Toyo System Co., Ltd.). In this measurement, after charging at a constant current of 0.5 C until the cell voltage reached 4.2 V, it was discharged at a constant current of 0.5 C until the cell voltage reached 2.5 V. The discharge capacity at this time was taken as the initial discharge capacity. Also, while repeating this charge-discharge process, the discharge capacity was measured every predetermined cycle. Further, the ratio of the discharge capacity after each cycle to the initial discharge capacity was calculated as the charge-discharge cycle retention rate. And for the charge-discharge cycle retention rate, 85% or more was evaluated as ◎, 80% or more and less than 85% was evaluated as ○, and 80% or less was evaluated as ×. In the evaluation of the charge-discharge cycle retention rate, evaluations of ◎ and ○ mean that the performance required for the electrode when used in the energy storage device is satisfied, and evaluation of × means that the performance is not satisfied.

[0039] (Example 2) A test cell was fabricated in the same manner as in Example 1, except that the weight of natural graphite was 25 g and the weight of PVdF was 25 g in the fabrication of the composite particles. The binder ratio, interfacial resistance, peel strength, and charge-discharge cycle retention rate were calculated and evaluated.

[0040] (Example 3) Artificial graphite was used as the conductive material of the composite particles. A test cell was fabricated in the same manner as in Example 1, except that the weight of artificial graphite was 25 g and the weight of PVdF was 25 g in the fabrication of the composite particles. The binder ratio, interfacial resistance, peel strength, and charge-discharge cycle retention rate were calculated and evaluated. The particle size PS of the artificial graphite is about 4.5 times that of the natural graphite.

[0041] (Example 4) Natural graphite and artificial graphite were used as the conductive materials of the composite particles. A test cell was fabricated in the same manner as in Example 1, except that the total weight of natural graphite and artificial graphite was 37.5 g and the weight of PVdF was 12.5 g in the fabrication of the composite particles. The weight ratio of natural graphite to artificial graphite was 1:2.

[0042] (Example 5) A test cell was fabricated in the same manner as in Example 1, except that artificial graphite was used as the conductive material for the composite particles, and the weight of artificial graphite was 47.5 g and the weight of PVdF was 2.5 g in the preparation of the composite particles. The binder ratio, interfacial resistance, peel strength, and charge-discharge cycle retention rate were calculated and evaluated.

[0043] (Example 6) A test cell was fabricated in the same manner as in Example 1, except that the weight of natural graphite was 47.5 g and the weight of PVdF was 2.5 g in the preparation of the composite particles. The binder ratio, interfacial resistance, peel strength, and charge-discharge cycle retention rate were calculated and evaluated.

[0044] (Example 7) A test cell was fabricated in the same manner as in Example 1, except that artificial graphite was used as the conductive material for the composite particles. The binder ratio, interfacial resistance, peel strength, and charge-discharge cycle retention rate were calculated and evaluated.

[0045] (Comparative Example 1) A test cell was fabricated in the same manner as in Example 1, except that 50 g of PVdF was used instead of the composite particles. The binder ratio, interfacial resistance, peel strength, and charge-discharge cycle retention rate were calculated and evaluated.

[0046] (Comparative Example 2) A test cell was fabricated in the same manner as in Example 1, except that 50 g of artificial graphite was used instead of the composite particles. The binder ratio, interfacial resistance, peel strength, and charge-discharge cycle retention rate were calculated and evaluated.

[0047] Figure 3 shows the composition, binder ratio, interfacial resistance, peel strength, and charge / discharge cycle maintenance rate for each example and comparative example. In Figure 3, the weight of graphite and the weight of PVdF are listed as a weight ratio. As shown in Figure 3, Examples 1 to 7, in which composite particles were scattered at the interface between the current collector and the active material layer, all showed good interfacial resistance, peel strength, and charge / discharge cycle maintenance rate. On the other hand, in Comparative Example 1, in which only binder was scattered at the interface, the peel strength was good, but the interfacial resistance and charge / discharge cycle maintenance rate were poor. Furthermore, in Comparative Example 2, in which only conductive material was scattered at the interface, all of the peel strength, interfacial resistance, and charge / discharge cycle maintenance rate were poor. From this, it was confirmed that the performance of the energy storage device can be improved by scattering composite particles in an island-like manner at the interface between the current collector and the active material layer.

[0048] Furthermore, Examples 1 to 5, where the binder ratio was between 0.3 and 14.0, showed better interfacial resistance, peel strength, and charge / discharge cycle maintenance rate than Example 6, where the binder ratio was less than 0.3. In addition, Examples 1 to 5 showed better interfacial resistance and charge / discharge cycle maintenance rate than Example 7, where the binder ratio was greater than 14.0. From this, it was confirmed that the performance of the energy storage device can be further improved by setting the binder ratio to between 0.3 and 14.0.

[0049] This disclosure can be used for electrodes and methods for manufacturing electrodes.

[0050] 1 Electrode, 2 Current collector, 4 Active material layer, 6 Composite particle group, 8 Composite particles, 10 Conductive material, 12 Binder.

Claims

1. An electrode comprising: a current collector; an active material layer laminated on the surface of the current collector; and a group of composite particles containing a plurality of composite particles in which a conductive material and a binder are combined, and which are scattered in an island-like manner at the interface between the current collector and the active material layer.

2. The electrode according to claim 1, wherein, when the weight of the binder is Wb, the weight of the conductive material is Wc, the true density of the conductive material is TD, and the particle size of the conductive material is PS, the composite particles satisfy the condition 0.3 ≤ Wb / [(Wc × 6) / (TD × PS)] ≤ 14.

0.

3. The electrode according to claim 1 or 2, wherein the composite particles have a structure in which the binder is attached to the surface of the conductive material.

4. The electrode according to claim 3, wherein the conductive material comprises at least one of natural graphite and artificial graphite.

5. A method for manufacturing an electrode, comprising scattering composite particles, which are a composite of a conductive material and a binder, in an island-like manner on the surface of a current collector by a dry method, and laminating an active material layer on the surface of the current collector on which the composite particles are scattered in an island-like manner.

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