Electrode and nonaqueous electrolyte secondary battery

Abstract

The present invention provides a composite sheet having good moldability and stretchability. An electrode according to an exemplary embodiment of the present invention comprises a core material and a composite sheet bonded to a surface of the core material. The composite sheet contains a conductive material-coated active material, which is obtained by applying a conductive material on a surface of an active material, and a fibrous binder. The compression ratio of the conductive material-coated active material is 22% or less. The fibrous binder has a crystallite size of 30.5-35.6 nm, and contains polytetrafluoroethylene as a main component.

Description

Electrode and non-aqueous electrolyte secondary battery

[0001] The present disclosure relates to an electrode, particularly an electrode produced by a dry method, and a non-aqueous electrolyte secondary battery including the electrode.

[0002] Electrodes for nonaqueous electrolyte secondary batteries such as lithium ion batteries are generally produced by a wet method in which an electrode mixture slurry containing an active material, a binder, etc. is applied to the surface of a core material such as a metal foil, and the coating is dried and compressed. Recently, a dry method has been considered in which an electrode mixture sheet is produced by stretching and forming the electrode mixture into a sheet, and the sheet is then bonded to a core material to produce an electrode. Patent Document 1 discloses an electrode mixture sheet produced by mixing an active material, a fibrillizable binder such as polytetrafluoroethylene (PTFE), and a conductive material using a mill, and then treating the mixture under high pressure and large shear force for a long period of time to fibrillate the PTFE.

[0003] Special table 2019-512872 publication

[0004] However, in the above-mentioned dry method, if the binder is insufficiently or excessively fibrillated, the strength of the composite sheet may be weakened and it may not be possible to form it into a sheet. Furthermore, even if the formability is good, poor stretchability of the composite sheet may result in reduced electrode productivity or reduced strength in the electrode thickness direction. The physical properties of the composite sheet vary significantly depending on the state of the fibrillated fibrous binder, but until now, it has not been clear what specific state the fibrous binder should be in to achieve a high-quality composite sheet.

[0005] An object of the present disclosure is to provide a composite sheet having good formability and stretchability.

[0006] The electrode according to the present disclosure comprises a core material and a composite sheet bonded to the surface of the core material, the composite sheet including a conductive material-coated active material in which a conductive material is attached to the surface of the active material, and a fibrous binder, the conductive material-coated active material having a compression degree of 22% or less, the fibrous binder having a crystallite size of 30.5 nm or more and 35.6 nm or less, and containing polytetrafluoroethylene as a main component.

[0007] The non-aqueous electrolyte secondary battery according to the present disclosure includes the above-described electrode. The above-described electrode configuration is preferably applied to a positive electrode.

[0008] According to one aspect of the present disclosure, a composite sheet having both formability and extensibility can be obtained.

[0009] 1 is a cross-sectional view of an electrode according to an embodiment; FIG. 2 is a diagram illustrating a process of preparing an electrode mixture and a process of preparing a mixture sheet from the electrode mixture when manufacturing an electrode according to an embodiment; and FIG. 3 is a diagram illustrating a process of bonding a mixture sheet to a surface of a core material when manufacturing an electrode according to an embodiment.

[0010] As described above, in electrodes fabricated by a dry process, controlling the fibrous binder to an appropriate state is important for achieving a high-quality composite sheet (composite layer). The inventors' studies have revealed that simultaneously mixing an active material, binder, and conductive material results in much of the conductive material being captured by the binder, leading to insufficient fibrillation of the binder. While the details of this mechanism are unclear, it is believed that the surface of the binder that captures the conductive material becomes slippery, preventing shear stress and making it difficult for fibrillation to occur. The inventors have found that sufficient fibrillation can be achieved by precombining the active material and conductive material to form a conductive-material-coated active material in which the conductive material is attached to the surface of the active material, and then mixing the composite with the binder.

[0011] The present inventors have further studied and found that a composite sheet having good moldability and stretchability can be obtained by setting the compression rate of the conductive material-coated active material to 22% or less, using polytetrafluoroethylene as the main component of the binder, and setting the crystallite size of the binder to 30.5 nm or more and 35.6 nm or less. By setting the compression rate of the conductive material-coated active material to 22% or less, maintaining high fluidity of the conductive material-coated active material in the composite sheet, and setting the crystallite size of the polytetrafluoroethylene within the above range, the above-mentioned effects can be specifically obtained.

[0012] Hereinafter, embodiments of an electrode and an electrode manufacturing method according to the present disclosure will be described in detail. The embodiments described below are merely examples, and the present disclosure is not limited to the following embodiments. The drawings referred to in the description of the embodiments are schematic, and the dimensional ratios of the components depicted in the drawings should be determined in consideration of the following description. Furthermore, configurations formed by selectively combining multiple embodiments and modified examples described below are included in the present disclosure.

[0013] The electrode according to the present disclosure is suitable for non-aqueous electrolyte secondary batteries such as lithium ion batteries, but can also be applied to batteries containing aqueous electrolytes or power storage devices such as capacitors. The following description will be given using an electrode for a non-aqueous electrolyte secondary battery as an example. The configuration and manufacturing method of the electrode for a non-aqueous electrolyte secondary battery according to the present disclosure may be applied to a negative electrode, but is particularly preferably applied to a positive electrode.

[0014] FIG. 1 is a cross-sectional view of an electrode 10 according to an embodiment. As shown in FIG. 1, the electrode 10 includes a core material 11 and a composite sheet 12 bonded to the surface of the core material 11. The composite sheet 12 is preferably bonded to both sides of the core material 11. The composite sheet 12 is produced by forming an electrode composite 20 (see FIG. 2 ), which will be described later, into a sheet, and is bonded to the core material 11 to form an electrode composite layer. The electrode 10 may be a long electrode that constitutes a wound electrode body, or a rectangular electrode that constitutes a stacked electrode body.

[0015] The core material 11 can be a metal foil or a film with a metal layer formed on its surface. The thickness of the core material 11 is, for example, 5 μm or more and 20 μm or less. When the electrode 10 is a positive electrode, the core material 11 can be a metal foil such as aluminum or an aluminum alloy. When the electrode 10 is a negative electrode, the core material 11 can be a metal foil such as copper or a copper alloy.

[0016] The composite sheet 12 includes a conductive-material-coated active material having a conductive material attached to the surface of the active material, and a fibrous binder. The thickness of the composite sheet 12 is, for example, 50 μm or more and 150 μm or less, preferably 80 μm or more and 140 μm or less, and more preferably 100 μm or more and 130 μm or less.

[0017] The degree of compression of the conductive material-coated active material is 22% or less. This ensures that the fluidity of the conductive material-coated active material in the composite sheet is at least as high as desired. If the degree of compression of the conductive material-coated active material exceeds 22%, it is not possible to achieve both formability and stretchability even if the crystallite size of the polytetrafluoroethylene is within the predetermined range described below. The lower limit of the degree of compression of the conductive material-coated active material is not particularly limited, but is, for example, 10%.

[0018] The degree of compression of the conductive material coated active material is calculated by the following formula. An example of an apparatus used to measure the degree of compression is a powder tester PT-X manufactured by Hosokawa Micron Corporation. Compressibility (%) = (packed bulk density - loose bulk density) / packed bulk density x 100

[0019] The active material constituting the conductive material-coated active material may be either a positive electrode active material or a negative electrode active material. Examples of conductive materials constituting the conductive material-coated active material include carbon black (CB) such as acetylene black (AB) and ketjen black, graphite, carbon nanotubes (CNT), carbon nanofibers, graphene, and other carbon materials. The content of the conductive material in the composite sheet 12 is, for example, 0.1% by mass or more and 5% by mass or less, preferably 0.2% by mass or more and 2% by mass or less, and more preferably 0.3% by mass or more and 1% by mass or less, relative to 100 parts by mass of the active material.

[0020] Generally, a lithium transition metal composite oxide is used as the positive electrode active material. Examples of metal elements contained in the lithium transition metal composite oxide include Ni, Co, Mn, Al, B, Mg, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Zr, Nb, In, Sn, Ta, and W. Among these, it is preferable to contain at least one selected from Ni, Co, and Mn. The content of the positive electrode active material is preferably 85% by mass or more and 99% by mass or less, and more preferably 90% by mass or more and 99% by mass or less, relative to the mass of the composite sheet 12 (the preferred content of the negative electrode active material is the same as the content of the positive electrode active material).

[0021] The positive electrode active material is a secondary particle formed by aggregation of a plurality of primary particles. The volume-based median diameter (D50) of the positive electrode active material is preferably 3 μm or more and 30 μm or less. D50 refers to the particle size at which the cumulative frequency in the volume-based particle size distribution is 50% from the smallest particle size, and is also called the median diameter. The particle size distribution of the positive electrode active material can be measured using a laser diffraction particle size distribution analyzer (e.g., MT3000II manufactured by Microtrac-Bell) using water as a dispersion medium.

[0022] The negative electrode active material may be, for example, a carbon-based active material such as natural graphite, such as flake graphite, lump graphite, or amorphous graphite, or artificial graphite, such as lump artificial graphite (MAG) or graphitized mesophase carbon microbeads (MCMB).The negative electrode active material may also be an active material containing Si, Sn, or the like, which alloys with lithium.

[0023] The fibrous binder can be obtained, for example, by applying a predetermined shear force to a particulate binder to fibrillate it. The fibrous binder becomes entangled with the active material particles and binds the active material particles together, thereby enabling the composite sheet 12 to be formed into a single sheet. The fibrous binder is preferably a material that can be fibrillated, adheres to the surfaces of the active material particles, and has chemical resistance to the electrolyte solution.

[0024] The fibrous binder is primarily composed of polytetrafluoroethylene (PTFE). Here, "primary component" refers to the component with the highest mass ratio, and the proportion of the primary component among all components is, for example, 90% or more. The fibrous binder may be substantially composed of PTFE alone. Note that, within the scope of the present disclosure, the composite sheet 12 may also contain a binder other than a fibrous binder, such as polyvinylidene fluoride (PVDF).

[0025] The crystallite size of the fibrous binder is 30.5 nm or more and 35.6 nm or less. The crystallite size is determined by X-ray diffraction measurement. Details of the X-ray diffraction measurement will be described later (see Examples). By keeping the crystallite size of the fibrous binder within this range and setting the compression rate of the conductive material-coated active material to 22% or less, a composite sheet that combines formability and extensibility can be obtained. If the crystallite size is less than 30.5 nm, for example, the fibers become too thin, reducing the extensibility of the composite sheet 12. If the crystallite size is more than 35.6 nm, the extensibility is significantly reduced, and in some cases not only the extensibility but also the formability may be reduced.

[0026] The fibrous binder (PTFE) preferably has a predetermined branched structure. A branched PTFE molecule is a molecule having one or more branching points within the molecule. On the other hand, a linear PTFE molecule is a molecule having no branching points within the molecule. The fibrous binder may be composed solely of branched PTFE molecules, or may be a mixture of branched PTFE molecules and linear PTFE molecules. When the fibrous binder is a mixture, each molecule does not need to satisfy the area ratio (Z) condition described below, as long as the mixture satisfies the area ratio (Z) condition.

[0027] In solid-state F-NMR measurement of the fibrous binder, the ratio of the area of ​​a second peak other than the first peak and its spinning side band, which exists between δ = -60 and -200 ppm, to the area of ​​the first peak derived from F in the linear portion detected at δ = -122 ppm (hereinafter sometimes referred to as "area ratio (Z)") satisfies the condition of 0.01% or more and 10% or less. The lower limit of the area ratio (Z) is preferably 0.02%, more preferably 0.05%. The upper limit of the area ratio (Z) is preferably 5%, more preferably 2%, and particularly preferably 1%.

[0028] The first peak is the linear portion of PTFE (-CF ２ -CF ２ -CF ２ The second peak is a peak derived from F in the branched portion of PTFE, and appears sharply at δ = -122 ppm. The second peak is a peak derived mainly from F in the branched portion of PTFE, but also contains a peak derived from F in the linear portion. The area ratio (Z) generally indicates the proportion of branched structures introduced into the molecules of the fibrous binder. The peak derived from F in the linear portion included in the second peak does not substantially affect the calculation of the proportion of branched structures.

[0029] The solid-state 19F-NMR spectrum of PTFE having a branched structure is disclosed, for example, in JP 2008-069280 A and Polymers Research Vol. 60, No. 4, 145-157, 2003. Fibrous binders have similar 19F-NMR spectra. In the structural formula below, peaks derived from F marked with b to d and g are detected only in the branched structure, while peaks derived from F marked with a, e and f are detected in both the linear and branched structures. The peak derived from F marked with a is the first peak detected at δ = -122 ppm.

[0030] By using a fibrous binder having a branched structure in which the area ratio (Z) of the solid-state 19F-NMR spectrum satisfies the above condition, the modification effect of the composite sheet 12 becomes more pronounced. The PTFE constituting the fibrous binder may have a branched structure, but at least some of the molecules may be crosslinked. By introducing a crosslinked structure, for example, it becomes easier to achieve both good productivity and high strength of the composite sheet 12.

[0031] The content of the fibrous binder is preferably 0.1 parts by mass or more and 5 parts by mass or less, more preferably 0.2 parts by mass or more and 2 parts by mass or less, and particularly preferably 0.4 parts by mass or more and 1 part by mass or less, relative to 100 parts by mass of the active material. If the content of the fibrous binder is within this range, the modifying effect of composite sheet 12 becomes more pronounced.

[0032] When composite sheet 12 is divided into three equal parts in the thickness direction, namely, a first region, a second region, and a third region from the core material 11 side, the content of the fibrous binder in the first region (a), the content of the fibrous binder in the second region (b), and the content of the fibrous binder in the third region (c) preferably satisfy (c-a) / (a+b+c)≦±10%, and more preferably satisfy (c-a) / (a+b+c)≦±5%. In electrode 10, the fibrous binder is not unevenly distributed in a part of composite sheet 12, but is present substantially uniformly throughout the entire composite sheet 12.

[0033] The density of the composite sheet 12 is not particularly limited. When the composite sheet 12 constitutes the positive electrode composite layer, the density is 2.5 g / cm ３ 4.5g / cm or more ３ Preferably, 3.0 g / cm or less ３ 4.2g / cm or more ３ More preferably, 3.0 g / cm or less ３ 4.0g / cm or more ３ The following is particularly preferred: Electrode 10 may be provided with an intermediate layer interposed between core material 11 and composite sheet 12. The intermediate layer contains, for example, a conductive material and a binder, and improves the bonding strength of composite sheet 12 to core material 11 and reduces the interfacial resistance.

[0034] An example of a method for manufacturing an electrode 10 will be described below with reference to Figures 2 and 3. While a method for manufacturing a positive electrode is exemplified below, this method can also be applied to the manufacture of a negative electrode. Figure 2 shows the steps of preparing an electrode composite 20 and preparing a composite sheet 12 from the electrode composite 20, and Figure 3 shows the step of joining the composite sheet 12 to a core material 11.

[0035] The electrode 10 is manufactured through the following steps: (1) a conductive material-coated active material preparation step in which an active material and a conductive material are combined, (2) an electrode mixture preparation step in which an electrode mixture containing a conductive material-coated active material and a fibrous binder and having a solids concentration of substantially 100% is prepared (see left side of FIG. 2), (3) a composite sheet preparation step in which the electrode mixture is formed into a sheet to prepare a composite sheet 12 (see right side of FIG. 2), and (4) a joining step in which the composite sheet 12 is joined to the surface of the core material 11 (see FIG. 3).

[0036] <Conductive Material-Coated Active Material Production Process> The conductive material-coated active material can be obtained, for example, by combining an active material and a conductive material using existing equipment. Specifically, the active material and the conductive material are charged into an existing device such as an agitation granulator or particle compositer and agitated, thereby adhering the conductive material to the surface of the active material and combining them to produce the conductive material-coated active material. Examples of devices used for combining include NOB300-Nobilta (registered trademark) and Mechanofusion (registered trademark) manufactured by Hosokawa Micron Corporation, a hybridization system manufactured by Nara Machinery Works, an FM mixer and COMPOSI manufactured by Nippon Coke & Engineering Co., Ltd., a high-speed mixer manufactured by EarthTechnica Corporation, an intensive mixer manufactured by Nippon Eirich Co., Ltd., a balance granulator manufactured by Freund Turbo Corporation, and a vertical granulator manufactured by Powrex Corporation.

[0037] In the conductive material-coated active material preparation step, the conductive material is composited so that the degree of compression of the conductive material-coated active material is 22% or less. The inventors have conducted a detailed investigation into the relationship between the composite treatment time (stirring treatment time) between the active material and the conductive material and the degree of compression of the conductive material-coated active material, and have found that the degree of compression decreases from a treatment time of 0 minutes (no composite treatment) until a predetermined treatment time, but increases once the predetermined treatment time is exceeded. The composite treatment time for achieving a degree of compression of the conductive material-coated active material of 22% or less depends on the composite conditions, such as the propeller rotation speed during stirring, but is, for example, 20 minutes to 40 minutes. Note that the degree of compression of the conductive material-coated active material remains substantially unchanged in the electrode composite preparation step, composite sheet preparation step, and bonding step described below. Setting the degree of compression to 22% or less in the conductive material-coated active material preparation step allows the degree of compression of the conductive material-coated active material contained in the electrode 10 to be 22% or less.

[0038] <Electrode Mixture Preparation Process> The electrode mixture 20 shown in FIG. 2 is a powder obtained by stirring and mixing raw materials such as a conductive material-coated active material and a particulate binder in a mixer 30. The electrode mixture 20 has a solids concentration of substantially 100%. The particulate binder in the raw materials is fibrillated into a fibrous binder by being stirred and mixed in the mixer 30. Fibrillation is performed so that the crystallite size of the fibrous binder is 30.5 nm or more and 35.6 nm or less. The content of the fibrous binder (i.e., the content of the particulate binder) is as described above.

[0039] The particulate binder used in the electrode composite preparation process is primarily composed of polytetrafluoroethylene (PTFE). The particulate binder may contain components other than PTFE, such as polyvinylidene fluoride (PVdF), as long as the objectives of the present disclosure are not impaired. However, it may essentially contain only PTFE. The average particle size of the particulate binder is not particularly limited, but is preferably 100 μm or more and 500 μm or less, and more preferably 200 μm or more and 400 μm or less. The average particle size of the particulate binder can be determined by observing the PTFE particles with a scanning electron microscope (SEM). Specifically, 100 randomly selected particles are extracted, and the diameter of the circumscribed circle of each of the 100 particles is measured and the measured values ​​are averaged to determine the average particle size.

[0040] A conventionally known mechanical stirring mixer capable of applying mechanical shear force can be used as the mixer 30. Specific examples of the mixer 30 include a cutter mill (such as the Wonder Crusher manufactured by Osaka Chemical Co., Ltd.), a pin mill, a bead mill, a kneader (such as a kneader or a Banbury mixer), a planetary mixer, and a fine particle compounding device (a device in which shear force is generated between a specially shaped rotor that rotates at high speed inside a tank and an impact plate).

[0041] The mixing time by the mixer 30 (the time for applying shear force to the composite raw material) varies depending on the type of the mixer 30, but is preferably within a few minutes, for example, 0.5 to 10 minutes. The rotation speed of the propeller of the mixer 30 is, for example, 1,000 to 10,000 rpm.

[0042] By stirring and mixing the raw materials using a mixer 30, an electrode mixture 20 is obtained, which includes a conductive-coated active material having a compressibility of 22% or less and a fibrous binder primarily composed of polytetrafluoroethylene having a crystallite size of 30.5 nm to 35.6 nm, and has a solids concentration of substantially 100%. The electrode mixture 20 obtained by the mixer 30 may be used to prepare a composite sheet 12, or the electrode mixture 20 obtained by the mixer 30 may be fed to a pair of rolls (not shown) to form a sheet and promote fibrillation of the PTFE. Specifically, the electrode mixture 20 is fed into a gap formed between the pair of rolls and rolled by the pair of rolls to form a sheet. By feeding the electrode mixture 20 to the pair of rolls to form a sheet, the non-fibrillated particulate binder in the electrode mixture can be fibrillated. The obtained electrode mixture sheet is pulverized, for example, by a pulverizer or the like and returned to the electrode mixture 20. A conventionally known pulverizer is used.

[0043] <Composite Sheet Manufacturing Step> In the composite sheet manufacturing step, the electrode composite 20 obtained in the electrode composite manufacturing step is used to manufacture the composite sheet 12 by a dry method.

[0044] In the composite sheet preparation process, as shown in FIG. 2 , the electrode composite 20 is supplied to a pair of rolls 31 and rolled through a gap formed between the pair of rolls 31 to prepare a composite sheet 12. Note that the rotation axes of the rolls shown in FIG. 2 are parallel, and the arrows drawn on each roll indicate the rotation direction of each roll. The pair of rolls 31 have, for example, the same diameter. The composite sheet 12 may be passed through the gap between the pair of rolls 31 multiple times.

[0045] The thickness of the composite sheet 12 can be controlled by, for example, the gap between the rolls, the circumferential speed of the rolls, the number of times the composite sheet 12 is passed through the rolls, etc. In the composite sheet production process, the electrode composite 20 may be formed into a sheet using rolls with a circumferential speed ratio that differs by two or more times. By varying the circumferential speed ratio of the rolls, it becomes easier to make the composite sheet 12 thinner, improving productivity. Furthermore, in the composite sheet production process, the composite sheet 12 may be compressed at the end. The linear pressure at that time is, for example, 1 t / cm or more and 3 t / cm or less.

[0046] <Bonding Process> For example, a pair of rolls 32 shown in Fig. 3 is used to bond the composite sheet 12 and the core material 11. By passing the composite sheet 12 and the core material 11 between the pair of rolls 32, an electrode 10 is obtained in which the composite sheet 12 is bonded to the surface of the core material 11. Although Fig. 3 shows the composite sheet 12 bonded to only one surface of the core material 11, it may be bonded to both surfaces of the core material 11. In this case, two composite sheets 12 may be bonded to both surfaces of the core material 11 simultaneously, or a first composite sheet 12 may be bonded to one surface of the core material 11, and then a second composite sheet 12 may be bonded to the other surface of the core material 11.

[0047] At least one of the pair of rolls 32 may be heated. The temperature of these rolls is preferably from room temperature to 300°C or less, more preferably from room temperature to 200°C or less. The linear pressure between the pair of rolls 32 is, for example, from 0.1 t / cm to 2 t / cm or less. An intermediate layer such as an adhesive layer may be formed on both sides of the core material 11 to be subjected to this bonding step.

[0048] The electrode 10 obtained in this manner may be subjected to a compression step, if necessary. Specifically, the electrode 10 may be conveyed to a predetermined gap provided between a pair of opposing rolls and compressed by the gap. The linear pressure between the pair of rolls compressing the electrode 10 is, for example, 1 t / cm or more and 3 t / cm or less.

[0049] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0050] Example 1 Preparation of a Positive Electrode Composite Sheet Using a NOB300-Nobilta (registered trademark) manufactured by Hosokawa Micron Corporation, 100 parts by mass of a lithium transition metal composite oxide as a positive electrode active material and 0.8 parts by mass of acetylene black as a conductive material were mixed for 20 minutes to perform a composite treatment, thereby preparing a conductive-material-coated positive electrode active material in which acetylene black was adhered to the particle surfaces of the positive electrode active material. The degree of compression of the conductive-material-coated positive electrode active material was 18.6%.

[0051] Next, this conductive material-coated positive electrode active material and polytetrafluoroethylene (PTFE) were charged into a Wonder Crusher manufactured by Osaka Chemical Co., Ltd. in a mass ratio of 100:0.6 and mixed for 2 minutes at room temperature and a rotation speed of 8,400 rpm. TE5448 manufactured by Mitsui Chemours was used as the PTFE. Solid-state 19F-NMR measurement of TE5448 revealed that the ratio of the area of ​​the second peak to the area of ​​the first peak was 0.051%. The mixing process fibrillated the PTFE particles to form fibrous PTFE, resulting in a positive electrode composite in which the carbon-coated positive electrode active material and fibrous PTFE were uniformly dispersed. The resulting positive electrode composite had a solids concentration of 100%.

[0052] The above-mentioned positive electrode composite was rolled between a pair of rolls to produce a positive electrode composite sheet. At this time, the peripheral speed ratio of the pair of rolls was set to 1:1, and the linear pressure between the pair of rolls was set to 0.1 t / cm. Using the positive electrode composite sheet obtained by the first roll rolling as described above, the formability was evaluated as follows. The evaluation result of formability was ○.

[0053] [Evaluation of formability] The positive electrode composite sheet was visually observed and evaluated according to the following criteria: ∘: Can be formed into a single sheet to form a positive electrode composite layer; ×: Cannot be formed into a sheet, or even if formed into a sheet, cannot hold a single sheet and breaks into small pieces.

[0054] Furthermore, the positive electrode composite sheet subjected to the first roll rolling was stretched using a pair of rolls to produce a stretched positive electrode composite sheet. At this time, the peripheral speed ratio of the pair of rolls was set to 1:2, the linear pressure between the pair of rolls 31 was set to 0.5 t / cm, and the gap between the pair of rolls was set to half the film thickness of the positive electrode composite sheet before stretching. When stretching the positive electrode composite sheet subjected to the first roll rolling as described above, the stretchability was evaluated as follows. The stretchability evaluation result was ○. Note that the positive electrode composite sheet for which the evaluation result of formability was × was unable to maintain its shape and therefore could not be evaluated for stretchability, and therefore the evaluation result of stretchability was set to ×.

[0055] [Evaluation of Stretchability] The positive electrode composite sheet was visually observed after the first rolled pass during stretching as described above, and the stretchability was evaluated according to the following criteria: ◯: The sheet did not break while the positive electrode composite powder was being formed into a sheet. ×: The sheet broke while the positive electrode composite powder was being formed into a sheet.

[0056] Finally, the stretched positive electrode mixture sheet was passed between two rolls at room temperature and repeatedly stretched until a desired film thickness was reached, thereby obtaining a positive electrode mixture sheet to be used for the positive electrode.

[0057] [Fabrication of Positive Electrode (Bonding Step)] The positive electrode composite sheet was placed on a core material made of aluminum foil, and the laminate of the positive electrode composite sheet and the core material was pressed (linear pressure: 1 t / cm) using two rolls at room temperature to obtain a positive electrode in which the positive electrode composite sheet was bonded to both sides of the core material. The density of the positive electrode active material in the positive electrode composite sheet was 3.6 g / cm ３ When the positive electrode composite sheet (positive electrode composite layer) was divided into three equal parts in the thickness direction, which were defined as a first region, a second region, and a third region from the core side, the content of fibrous PTFE in the first region (a), the content of fibrous PTFE in the second region (b), and the content of fibrous PTFE in the third region (c) were (c−a)×100 / (a+b+c)=1%.

[0058] A sample (PTFE) was extracted from the positive electrode by the following method, and then subjected to X-ray diffraction measurement to evaluate the crystallite size of the fibrous PTFE. The crystallite size of the fibrous PTFE in the positive electrode was evaluated by the following method. The crystallite size was 31.4 nm.

[0059] [Crystallite Size Measurement] (Acid Dissolution) The positive electrode composite sheet peeled from the positive electrode was placed in a glass beaker, and 30 mL of aqua regia (36% hydrochloric acid: 70% nitric acid = 3:1 (volume ratio)) was added and heated to dissolve. The solution was allowed to cool and diluted with pure water, and then suction filtered using a membrane filter (pore size 0.1 μm). The residue on the filter was washed with pure water and then heated and dried in a dryer at 50 ° C. for 6 hours to obtain a sample (PTFE). (X-ray diffraction measurement) Apparatus: SmartLab (manufactured by Rigaku Corporation) Stage: standard stage Sample holder: glass sample plate Irradiated X-ray: Cu-Kα Output: 40 kV-30 mA Optical system: CBO-α optical system (focusing method) Scanning axis: 2θ / θ Scanning speed: 1° / min (narrow scan) Step: 0.01° Scanning range: 15° or more and 21° or less (PTFE peak portion) Detector: SC detector Slit conditions: 1 / 4° (DS), 1 / 4° (SS), 0.3 mm (RS) From the half width of the peak obtained in the above measurement, the crystallite size of PTFE was calculated using the Scherrer formula.

[0060] Example 2 A positive electrode was prepared and evaluated in the same manner as in Example 1, except that the composite treatment time in the preparation of the positive electrode composite sheet was changed to 30 minutes. The degree of compression of the conductive material-coated positive electrode active material was 16.5%, and the crystallite size of the PTFE was 32.1 nm. The evaluation results for formability and stretchability were both good.

[0061] <Example 3> In the preparation of the positive electrode composite sheet, the composite treatment time was changed to 30 minutes, the mixing device was changed from a Wonder Crusher to an intensive mixer manufactured by Nippon Eirich Co., Ltd., and the mixing conditions were changed to a rotation speed of 6300 rpm and 3 minutes. A positive electrode was prepared and evaluated in the same manner as in Example 1. The crystallite size of PTFE was 31.8 nm. The evaluation results of moldability and stretchability were both ○.

[0062] Example 4 A positive electrode was prepared and evaluated in the same manner as in Example 1, except that the composite treatment time in the preparation of the positive electrode composite sheet was changed to 40 minutes. The degree of compression of the conductive material-coated positive electrode active material was 20.7%, and the crystallite size of the PTFE was 32.4 nm. The evaluation results for formability and stretchability were both good.

[0063] Comparative Example 1 A positive electrode was produced and evaluated in the same manner as in Example 1, except that in the production of a positive electrode composite sheet, no composite treatment was performed, and a lithium transition metal composite oxide, acetylene black, and PTFE were mixed together. The degree of compression of the lithium transition metal composite oxide not coated with a conductive material was 33.7%, and the crystallite size of the PTFE was 37.5 nm. The evaluation results for formability and stretchability were both x.

[0064] Comparative Example 2 A positive electrode was prepared and evaluated in the same manner as in Example 1, except that the composite treatment time in the preparation of the positive electrode composite sheet was changed to 10 minutes. The degree of compression of the conductive material-coated positive electrode active material was 23.6%, and the crystallite size of the PTFE was 35.9 nm. The evaluation results for formability and stretchability were both x.

[0065] <Comparative Example 3> In the preparation of the positive electrode composite sheet, the composite treatment time was changed to 30 minutes, and the mixing device was changed from a Wonder Crusher to an intensive mixer manufactured by Nippon Eirich Co., Ltd., and the mixing conditions were changed to a rotation speed of 6300 rpm and 4 minutes. A positive electrode was prepared and evaluated in the same manner as in Example 1. The crystallite size of PTFE was 30.4 nm. The evaluation result of moldability was ○, but the evaluation result of stretchability was ×.

[0066] Comparative Example 4 A positive electrode was prepared and evaluated in the same manner as in Example 1, except that the composite treatment time in the preparation of the positive electrode composite sheet was changed to 50 minutes. The degree of compression of the conductive material-coated positive electrode active material was 23.1%, and the crystallite size of the PTFE was 35.7 nm. The evaluation result for formability was ○, but the evaluation result for stretchability was ×.

[0067] Comparative Example 5 A positive electrode was produced and evaluated in the same manner as in Example 1, except that the composite treatment time in the production of the positive electrode composite sheet was changed to 60 minutes. The degree of compression of the conductive material-coated positive electrode active material was 25.9%, and the crystallite size of the PTFE was 36.6 nm. The evaluation result for formability was ○, but the evaluation result for stretchability was ×.

[0068]

[0069] As shown in Table 1, in the examples, positive electrode composite sheets with good formability and stretchability were obtained. On the other hand, the positive electrode composite sheets obtained in the comparative examples had poor formability and could not be evaluated for stretchability, or the evaluation result for formability was ○ but the stretchability was insufficient. This shows that by containing a conductive material-coated active material with a compression rate of 22% or less and a fibrous binder with a crystallite size of 30.5 nm or more and 35.6 nm or less, a composite sheet with good formability and stretchability can be obtained.

[0070] The present disclosure is further described by the following embodiments. Aspect 1: An electrode comprising a core material and a composite sheet bonded to a surface of the core material, wherein the composite sheet includes a conductive-material-coated active material in which a conductive material is attached to a surface of the active material, and a fibrous binder, wherein the conductive-material-coated active material has a compressibility of 22% or less, and the fibrous binder has a crystallite size of 30.5 nm to 35.6 nm and is primarily composed of polytetrafluoroethylene. Aspect 2: The electrode according to Aspect 1, wherein, in solid-state 19F-NMR measurement of the fibrous binder, the ratio of the area of ​​a second peak other than the first peak and its spinning side band, which is present between δ = -60 and -200 ppm, to the area of ​​a first peak derived from F in a linear portion detected at δ = -122 ppm, is 0.01% or more and 10% or less. Configuration 3: The electrode according to Configuration 1 or 2, wherein the content of the fibrous binder is 0.1 parts by mass or more and 5 parts by mass or less per 100 parts by mass of the active material. Configuration 4: The electrode according to any one of Configurations 1 to 3, wherein the electrode is a positive electrode. Configuration 5: A nonaqueous electrolyte secondary battery comprising the electrode according to any one of Configurations 1 to 4.

[0071] 10 Electrode, 11 Core material, 12 Composite sheet, 20 Electrode composite, 30 Mixer, 31, 32 Roll

Claims

1. An electrode comprising a core material and a composite sheet bonded to the surface of the core material, the composite sheet including a conductive-material-coated active material having a conductive material attached to the surface of the active material, and a fibrous binder, the conductive-material-coated active material having a degree of compression of 22% or less, the fibrous binder having a crystallite size of 30.5 nm or more and 35.6 nm or less, and primarily composed of polytetrafluoroethylene.

2. The electrode according to claim 1, wherein in solid-state 19F-NMR measurement of the fibrous binder, the ratio of the area of ​​a second peak other than the first peak and its spinning side band, which exists between δ = -60 and -200 ppm, to the area of ​​a first peak derived from F in a linear portion detected at δ = -122 ppm, is 0.01% or more and 10% or less.

3. The electrode according to claim 1, wherein the content of the fibrous binder is 0.1 parts by mass or more and 5 parts by mass or less per 100 parts by mass of the active material.

4. The electrode of claim 1, wherein the electrode is a positive electrode.

5. A non-aqueous electrolyte secondary battery comprising the electrode according to any one of claims 1 to 4.

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