Positive electrode for nonaqueous electrolyte secondary batteries, and nonaqueous electrolyte secondary battery

Abstract

Provided is a positive electrode for nonaqueous electrolyte secondary batteries that makes it possible to suppress deterioration of charge / discharge cycle characteristics even when a positive electrode mixture sheet has a low porosity. A positive electrode (11) is provided with a positive electrode core material (30) and a positive electrode mixture sheet (32) bonded to the surface of the positive electrode core material (30). The positive electrode mixture sheet (32) contains a fibrous binder and a conductive material-coated active material in which a conductive material is attached to the surface of the positive electrode active material. The porosity of the positive electrode mixture sheet (32) is 17.8-23.2%. The mode diameter of the positive electrode mixture sheet (32) obtained by a mercury intrusion method is more than 0.19 μm and less than 0.87 μm.

Description

Positive electrode for non-aqueous electrolyte secondary battery, and non-aqueous electrolyte secondary battery

[0001] The present disclosure relates to a positive electrode for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery.

[0002] Electrodes for nonaqueous electrolyte secondary batteries such as lithium ion secondary batteries are generally produced by a wet method in which an electrode composite slurry containing an active material, a binder, etc. is applied to the surface of a core material, which is a metal foil, to form a composite layer, and then the composite layer is dried and compressed. In recent years, a dry method has been considered in which an electrode composite containing an active material, a binder, etc. is stretched and formed into a sheet to produce a composite sheet, and the sheet is then bonded to a core material to produce an electrode (e.g., Patent Document 1).

[0003] JP-T-2019-512872 A JP-A-2015-138617 JP-A-2015-201442 A

[0004] However, voids that hold the non-aqueous electrolyte are formed in the composite layer or composite sheet, and the amount and properties (e.g., pore diameter) of these voids have a significant effect on battery characteristics. Specifically, by reducing the porosity of the positive electrode composite sheet (i.e., increasing the density of the positive electrode composite sheet), the battery capacity can be improved. However, this reduces the amount of non-aqueous electrolyte held in the positive electrode composite sheet, which can lead to a problem of easily degrading charge-discharge cycle characteristics.

[0005] Therefore, an object of the present disclosure is to provide a positive electrode for a non-aqueous electrolyte secondary battery that can suppress deterioration in charge-discharge cycle characteristics while providing a positive electrode mixture sheet with low porosity, and a non-aqueous electrolyte secondary battery using the positive electrode.

[0006] The positive electrode for a non-aqueous electrolyte secondary battery according to the present disclosure comprises a positive electrode core material and a positive electrode composite sheet joined to a surface of the positive electrode core material, the positive electrode composite sheet including a conductive material-coated active material in which a conductive material is adhered to a surface of a positive electrode active material, and a fibrous binder, the positive electrode composite sheet having a porosity of 17.8% or more and 23.2% or less, and a mode diameter of the positive electrode composite sheet obtained by mercury porosimetry exceeding 0.19 μm and less than 0.87 μm.

[0007] A non-aqueous electrolyte secondary battery according to the present disclosure is characterized by including the positive electrode for a non-aqueous electrolyte secondary battery.

[0008] According to one aspect of the present disclosure, it is possible to provide a positive electrode for a nonaqueous electrolyte secondary battery, in which a positive electrode mixture sheet has a low porosity and yet is capable of suppressing deterioration in charge-discharge cycle characteristics, and a nonaqueous electrolyte secondary battery using the positive electrode.

[0009] Fig. 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery according to an embodiment; Fig. 2 is a cross-sectional view of a positive electrode according to an embodiment; Fig. 3 is a diagram illustrating a process of preparing a positive electrode composite and a process of preparing a positive electrode composite sheet from the positive electrode composite when manufacturing a positive electrode according to an embodiment; Fig. 4 is a diagram illustrating a process of joining a positive electrode composite sheet to a surface of a positive electrode core material when manufacturing a positive electrode according to an embodiment.

[0010] Hereinafter, embodiments of a positive electrode for a nonaqueous electrolyte secondary battery and a nonaqueous electrolyte secondary battery including the positive electrode according to the present disclosure will be described. 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 with reference to the following description. Furthermore, configurations formed by selectively combining multiple embodiments and modified examples described below are included in the present disclosure.

[0011] FIG. 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery according to an embodiment. The nonaqueous electrolyte secondary battery 10 shown in FIG. 1 includes a wound electrode assembly 14 formed by winding a positive electrode 11 and a negative electrode 12 with a separator 13 interposed therebetween, a nonaqueous electrolyte, insulating plates 18 and 19 disposed above and below the electrode assembly 14, respectively, and a battery case 15 for accommodating the above components. The battery case 15 is composed of a cylindrical case body 16 with a bottom and a sealing body 17 that closes the opening of the case body 16. Note that, instead of the wound electrode assembly 14, other electrode bodies may be used, such as a laminated electrode body formed by alternately stacking positive and negative electrodes with separators interposed therebetween. Examples of the battery case 15 include cylindrical, prismatic, coin-shaped, or button-shaped metal cases, and resin cases (so-called pouch-shaped cases) formed by laminating resin sheets.

[0012] The non-aqueous electrolyte has, for example, ion conductivity (e.g., lithium ion conductivity). The non-aqueous electrolyte includes, for example, a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of the non-aqueous solvent include esters, ethers, nitriles, amides, and mixed solvents of two or more of these. Examples of the non-aqueous solvent include ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and mixed solvents of these. The non-aqueous solvent may contain a halogen-substituted compound (e.g., fluoroethylene carbonate) in which at least a portion of the hydrogen atoms of these solvents are substituted with halogen atoms such as fluorine. The electrolyte salt includes, for example, LiPF ６ Lithium salts such as

[0013] The case body 16 is, for example, a cylindrical metal container with a bottom. A gasket 28 is provided between the case body 16 and the sealing body 17 to ensure airtightness inside the battery. The case body 16 has, for example, a protruding portion 22, which is a portion of the side surface that protrudes inward and supports the sealing body 17. The protruding portion 22 is preferably formed in an annular shape along the circumferential direction of the case body 16, and supports the sealing body 17 on its upper surface.

[0014] The sealing body 17 has a structure in which a filter 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked in this order from the electrode body 14 side. Each component constituting the sealing body 17 has, for example, a disk or ring shape, and all components except for the insulating member 25 are electrically connected to each other. The lower valve body 24 and the upper valve body 26 are connected to each other at their respective centers, and the insulating member 25 is interposed between their respective peripheral edges. When the internal pressure of the nonaqueous electrolyte secondary battery 10 increases due to heat generation caused by an internal short circuit or the like, for example, the lower valve body 24 deforms and ruptures, pushing the upper valve body 26 toward the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. When the internal pressure further increases, the upper valve body 26 ruptures, and gas is discharged from the opening of the cap 27.

[0015] 1 , a positive electrode lead 20 attached to the positive electrode 11 passes through a through-hole in the insulating plate 18 and extends toward the sealing body 17, and a negative electrode lead 21 attached to the negative electrode 12 passes outside the insulating plate 19 and extends toward the bottom of the case body 16. The positive electrode lead 20 is connected by welding or the like to the underside of a filter 23, which is the bottom plate of the sealing body 17, and a cap 27, which is the top plate of the sealing body 17 and is electrically connected to the filter 23, serves as the positive electrode terminal. The negative electrode lead 21 is connected by welding or the like to the inner bottom surface of the case body 16, and the case body 16 serves as the negative electrode terminal.

[0016] The positive electrode 11, the negative electrode 12, and the separator 13 that constitute the nonaqueous electrolyte secondary battery 10 will be further described below.

[0017] Fig. 2 is a cross-sectional view of a positive electrode according to an embodiment. As shown in Fig. 2, the positive electrode 11 includes a positive electrode core material 30 and a positive electrode composite sheet 32 ​​bonded to the surface of the positive electrode core material 30. The positive electrode composite sheet 32 ​​may be bonded to only one surface of the positive electrode core material 30, or may be bonded to both surfaces of the positive electrode core material 30. The positive electrode composite sheet 32 ​​is produced, for example, by forming a positive electrode composite 40 (see Fig. 3), which will be described later, into a sheet, and is bonded to the positive electrode core material 30 to form a positive electrode composite layer.

[0018] The positive electrode core material 30 can be, for example, a foil of a metal such as aluminum or an aluminum alloy that is stable within the potential range of the positive electrode 11, or a film having such a metal disposed on its surface. The thickness of the positive electrode core material 30 is, for example, 5 μm or more and 20 μm or less.

[0019] The positive electrode mixture sheet 32 ​​includes a conductive material-coated active material in which a conductive material is attached to the surface of the positive electrode active material, and a fibrous binder. The thickness of the positive electrode mixture sheet 32 ​​is, for example, 50 μm to 150 μm, preferably 60 μm to 140 μm, and more preferably 70 μm to 130 μm.

[0020] The porosity of the positive electrode mixture sheet 32 ​​is 17.8% or more and 23.2% or less, preferably 18.0% or more and 22.0% or less. The mode diameter of the positive electrode mixture sheet 32 ​​obtained by mercury porosimetry is greater than 0.19 μm and less than 0.87 μm, preferably 0.2 μm or more and 0.8 μm or less, and more preferably 0.4 μm or more and 0.6 μm or less. The above-mentioned range of the porosity of the positive electrode mixture sheet 32 ​​falls within a low porosity range (i.e., a high-density range) compared to a typical positive electrode mixture layer. However, it is presumed that the mode diameter of the positive electrode mixture sheet 32 ​​satisfying the above-mentioned range ensures that the positive electrode mixture sheet 32 ​​has adequate pores for retaining the non-aqueous electrolyte. As a result, even when the non-aqueous electrolyte secondary battery is repeatedly charged and discharged, the amount of non-aqueous electrolyte retained in the positive electrode mixture sheet 32 ​​is prevented from decreasing, thereby suppressing deterioration in the charge-discharge cycle characteristics. It is easier to set the porosity and mode diameter within the above ranges when manufacturing a positive electrode by a dry method using a positive electrode mixture sheet than when manufacturing a positive electrode by a wet method using an electrode mixture slurry.

[0021] The porosity of the positive electrode composite sheet 32 ​​is expressed as a percentage of the volume of the portion excluding all materials constituting the positive electrode composite sheet 32, where the apparent volume (volume including voids) of the positive electrode composite sheet 32 ​​is taken as 100%. Specifically, the weight of the positive electrode 11 is first measured, and the weight of the positive electrode composite sheet 32 ​​is calculated by subtracting the weight of the positive electrode core material 30 from the measured weight. This weight is then divided by the volume of the positive electrode composite sheet 32, which is calculated by multiplying the thickness of the positive electrode composite sheet 32, which is calculated by subtracting the thickness of the positive electrode core material 30 from the average thickness of the positive electrode 11, by the area of ​​the positive electrode composite sheet 32, to obtain the density Dc of the positive electrode composite sheet 32. Next, the positive electrode composite true density Mc is calculated from the true densities and mass ratios of all materials contained in the positive electrode composite sheet 32. The porosity P is then calculated using the density Dc of the positive electrode composite sheet 32 ​​and the positive electrode composite true density Mc according to the following formula: P={1-(Dc / Mc)}×100

[0022] The mode diameter refers to the pore diameter of the pore that appears most frequently in the pore diameter distribution (differential pore volume distribution) of the positive electrode composite sheet 32 ​​measured by mercury intrusion porosimetry. It is also called the maximum frequency pore diameter. The differential pore volume distribution is a distribution curve in which the pore volume corresponding to the pore diameter (μm) is plotted against the pore diameter, and the pore diameter that takes the maximum value (peak value) in the range of 0.003 μm to 2 μm in this distribution curve is calculated as the "mode diameter."

[0023] Measurement of the pore size distribution by mercury intrusion porosimetry can be performed using an apparatus such as the Autopore IV9500 series manufactured by Micromeltix. During measurement, the measurement sample is sealed in a sample container under an inert atmosphere, mercury is injected into the sample container, and pressure is applied to the mercury. Here, the pressure applied to the mercury is appropriately adjusted depending on the pore size that the measurement sample may have, and is not particularly limited. For example, it is preferable to measure the pressure by changing the pressure from 0.5 psi (3.4 kPa) to 60,000 psi (413,400 kPa) in order to be able to measure the pore size over a wide range. The measurement sample may be a positive electrode 11 that has not been charged or discharged.

[0024] The density of the positive electrode mixture sheet 32 ​​is set to 3.55 g / cm in order to suppress the deterioration of the charge-discharge cycle characteristics. ３ Above, 3.8g / cm ３ Preferably, 3.6 g / cm or less ３ Above, 3.8g / cm ３ More preferably, 3.65 g / cm ３ 3.8g / cm or more ３ The following is more preferable: The density of the positive electrode mixture sheet 32 ​​is calculated according to the method for calculating the density Dc of the positive electrode mixture sheet 32 ​​described above.

[0025] The degree of compression of the conductive material-coated active material is preferably, for example, 22% or less, which ensures a predetermined level of fluidity of the conductive material-coated active material in the positive electrode mixture sheet 32. The lower limit of the degree of compression of the conductive material-coated active material is not particularly limited, but is, for example, 10%.

[0026] 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

[0027] Examples of the conductive material 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 positive electrode mixture sheet 32 ​​is, for example, preferably 0.1% by mass or more and 5% by mass or less, more preferably 0.2% by mass or more and 2% by mass or less, and even more preferably 0.3% by mass or more and 1% by mass or less, relative to 100 parts by mass of the positive electrode active material, in order to increase the electronic conductivity of the conductive material-coated active material and to increase the pore diameter in the positive electrode mixture sheet 32 ​​(i.e., to increase the mode diameter measured by mercury intrusion porosimetry).

[0028] 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 element 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 positive electrode composite sheet 32.

[0029] The positive electrode active material is, for example, 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.

[0030] 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 particles of the conductive material-coated active material and binds the particles together. This allows the positive electrode mixture sheet 32 ​​to be formed into a single sheet. The fibrous binder is preferably a material that can be fibrillated, adheres to the particle surfaces of the conductive material-coated active material, and has chemical resistance to the electrolyte solution.

[0031] The fibrous binder is mainly composed of, for example, polytetrafluoroethylene (PTFE). Here, the term "main component" refers to the component with the highest mass ratio. The proportion of the main component in all components of the fibrous binder is preferably, 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 positive electrode mixture sheet 32 ​​may contain a binder that does not become fibrous, such as polyvinylidene fluoride (PVDF).

[0032] The fibrous binder preferably has a branched structure. A molecule having a branched structure is a molecule having one or more branch points in the molecule. On the other hand, a molecule having a linear structure means a molecule having no branch points in the molecule. The fibrous binder may be composed of, for example, only PTFE molecules having a branched structure, or may be a mixture of PTFE molecules having a branched structure and PTFE molecules having a linear structure.

[0033] The content of the fibrous binder is, for example, 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 more preferably 0.4 parts by mass or more and 1 part by mass or less, relative to 100 parts by mass of the positive electrode active material, in order to improve the formability and extensibility of the positive electrode composite sheet 32.

[0034] When the positive electrode mixture sheet 32 ​​is divided into three equal parts in the thickness direction, namely, a first region, a second region, and a third region from the positive electrode core material 30 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 the positive electrode 11, by satisfying the above formula, the fibrous binder can be present substantially uniformly throughout the entire positive electrode mixture sheet 32.

[0035] Positive electrode 11 may be provided with an intermediate layer interposed between positive electrode core material 30 and positive electrode mixture sheet 32. The intermediate layer contains, for example, a conductive material and a binder, and improves the bonding strength between positive electrode core material 30 and positive electrode mixture sheet 32 ​​and reduces the interfacial resistance.

[0036] An example of a method for manufacturing the positive electrode 11 will be described below with reference to Fig. 3 and Fig. 4. Fig. 3 shows a process for preparing a positive electrode composite and a process for preparing a positive electrode composite sheet from the positive electrode composite, and Fig. 4 shows a process for joining the positive electrode composite sheet to a positive electrode core material.

[0037] The positive electrode 11 is manufactured through the following steps: (1) a conductive material-coated active material preparation step in which a positive electrode active material and a conductive material are combined; (2) a positive electrode composite preparation step in which a positive electrode composite 40 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. 3 ); (3) a positive electrode composite sheet preparation step in which the positive electrode composite 40 is formed into a sheet to prepare a positive electrode composite sheet 32 ​​(see right side of FIG. 3 ); and (4) a joining step in which the positive electrode composite sheet 32 ​​is joined to the surface of the positive electrode core material 30 (see FIG. 4 ).

[0038] <Conductive Material-Coated Active Material Preparation Process> The conductive material-coated active material can be obtained, for example, by combining a positive electrode active material and a conductive material using an existing device. Specifically, the positive electrode 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 positive electrode active material and combining them to produce a 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.

[0039] In the conductive material-coated active material preparation step, the time for compounding the positive electrode active material and the conductive material (stirring time) is preferably 20 minutes or more and 40 minutes or less, although this depends on compounding conditions such as the rotation speed of the propeller during stirring. Within this range, a conductive material-coated active material with a compressibility of 22% or less can be easily obtained.

[0040] <Positive Electrode Composite Preparation Process> The positive electrode composite 40 shown in Fig. 3 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 50. The positive electrode composite 40 has a solid content 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 50. The content of the fibrous binder (i.e., the content of the particulate binder) is as described above.

[0041] The particulate binder used in the positive electrode composite preparation process is primarily composed of, for example, polytetrafluoroethylene (PTFE). The particulate binder may contain a non-fibrous binder, such as polyvinylidene fluoride (PVDF), as long as the object of the present disclosure is not impaired. 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 observation 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.

[0042] A conventionally known mechanical stirring mixer capable of applying mechanical shear force can be used as the mixer 50. Specific examples of the mixer 50 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 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).

[0043] The mixing time by the mixer 50 (the time for applying shear force to the composite raw material) varies depending on the type of the mixer 50, but is preferably within a few minutes, for example, 0.5 to 10 minutes. The rotation speed of the propeller of the mixer 50 is, for example, 1,000 to 10,000 rpm.

[0044] By stirring and mixing the raw materials in the mixer 50, a cathode mixture 40 containing a conductive material-coated active material and a fibrous binder and having a solids concentration of substantially 100% is obtained. The cathode mixture 40 obtained by the mixer 50 may be used to prepare a cathode mixture sheet 32, or the cathode mixture 40 obtained by the mixer 50 may be fed to a pair of rolls (not shown) to form a sheet and promote fibrillation of the binder. Specifically, the cathode mixture 40 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 cathode mixture 40 to the pair of rolls to form a sheet, the non-fibrillated particulate binder in the electrode mixture can be fibrillated. The obtained cathode mixture sheet is pulverized, for example, by a pulverizer or the like and returned to the cathode mixture 40. A conventionally known pulverizer may be used.

[0045] <Positive Electrode Composite Sheet Fabrication Step> In the positive electrode composite sheet fabrication step, positive electrode composite sheet 32 ​​is fabricated by a dry method using positive electrode composite 40 obtained in the positive electrode composite fabrication step.

[0046] In the positive electrode mixture sheet preparation process, as shown in FIG. 3 , the positive electrode mixture 40 is supplied to a pair of rolls 51 and rolled through a gap formed between the pair of rolls 51 to prepare a positive electrode mixture sheet 32. Note that the rotation axes of the rolls shown in FIG. 3 are parallel, and the arrows drawn on each roll indicate the rotation direction of each roll. The pair of rolls 51 have, for example, the same diameter. The positive electrode mixture sheet 32 ​​may be passed through the gap between the pair of rolls 51 multiple times.

[0047] The thickness of the positive electrode mixture sheet 32 ​​can be controlled by, for example, the gap between the rolls, the circumferential speed of the rolls, the number of times the positive electrode mixture sheet 32 ​​passes through the rolls, etc. In the positive electrode mixture sheet production step, the positive electrode mixture 40 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 positive electrode mixture sheet 32 ​​thinner, improving productivity. Furthermore, in the positive electrode mixture sheet production step, the positive electrode mixture sheet 32 ​​may be compressed at the end. The linear pressure at this time is, for example, 1 t / cm or more and 3 t / cm or less.

[0048] <Bonding Step> A pair of rolls 52 shown in FIG. 4 is used to bond the positive electrode mixture sheet 32 ​​and the positive electrode core material 30, for example. The positive electrode mixture sheet 32 ​​and the positive electrode core material 30 pass between the pair of rolls 52, thereby obtaining a positive electrode 11 in which the positive electrode mixture sheet 32 ​​is bonded to the surface of the positive electrode core material 30. In FIG. 4, the positive electrode mixture sheet 32 ​​is bonded to only one surface of the positive electrode core material 30, but it may also be bonded to both surfaces of the positive electrode core material 30. In this case, two positive electrode mixture sheets 32 may be bonded to both surfaces of the positive electrode core material 30 simultaneously, or one positive electrode mixture sheet 32 ​​may be bonded to one surface of the positive electrode core material 30, and then a second positive electrode mixture sheet 32 ​​may be bonded to the other surface of the positive electrode core material 30.

[0049] At least one of the pair of rolls 52 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 52 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 surfaces of the positive electrode core material 30 to be subjected to this bonding step.

[0050] The positive electrode 11 obtained in this manner may be subjected to a compression step as necessary. Specifically, the positive electrode 11 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 positive electrode 11 is, for example, 1 t / cm or more and 3 t / cm or less.

[0051] As described above, by employing a dry method in which a positive electrode composite sheet is prepared using a conductive material-coated active material and the positive electrode composite sheet is bonded to the surface of a positive electrode core material to prepare a positive electrode, it is easy to adjust the porosity and mode diameter to fall within the above ranges. The porosity and mode diameter can be adjusted to fall within the above ranges, for example, by adjusting the linear pressure when compressing the above-described positive electrode composite sheet 32 ​​or positive electrode 11, or by adjusting the content of the conductive material.

[0052] The negative electrode 12 includes, for example, a negative electrode core material and a negative electrode composite layer provided on the surface of the negative electrode core material. The negative electrode composite layer may be provided on only one surface of the negative electrode core material, or on both surfaces of the negative electrode core material. The negative electrode core material may be, for example, a foil of a metal stable within the potential range of the negative electrode 12, such as copper or a copper alloy, or a film with such a metal disposed on the surface layer. The negative electrode composite layer includes, for example, a negative electrode active material and a binder. The negative electrode 12 can be produced, for example, by applying a negative electrode composite slurry containing a negative electrode active material, a binder, etc. to the surface of a negative electrode current collector, drying the coating, and then rolling the coating using a roller or the like. The above manufacturing method is an example of a wet method, but a dry method may also be used, as with the positive electrode 11.

[0053] The negative electrode active material is not particularly limited as long as it can reversibly absorb and release lithium ions, and generally, a carbon material such as graphite is used. The graphite may be any of natural graphite such as flake graphite, lump graphite, and amorphous graphite, or artificial graphite such as lump artificial graphite and graphitized mesophase carbon microbeads. Furthermore, metals that alloy with Li, such as Si and Sn, metal compounds containing Si and Sn, and lithium-titanium composite oxides may also be used as the negative electrode active material. For example, SiO ｘ (0.5≦x≦1.6) or Li ２ｙ SiO （２＋ｙ） A Si-containing compound in which fine particles of Si are dispersed in a lithium silicate phase represented by (0<y<2) may be used in combination with graphite.

[0054] Examples of binders include fluorine-based resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide-based resins, acrylic-based resins, polyolefin-based resins, styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC) or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), and polyethylene oxide (PEO).

[0055] The separator 13 may be, for example, a porous sheet having ion permeability and insulating properties. Specific examples of porous sheets include a microporous thin film, a woven fabric, and a nonwoven fabric. Suitable materials for the separator include olefin-based resins such as polyethylene and polypropylene, and cellulose. The separator 13 may be a laminate having a cellulose fiber layer and a thermoplastic resin fiber layer such as an olefin-based resin. Alternatively, the separator 13 may be a multilayer separator including a polyethylene layer and a polypropylene layer, and the surface of the separator 13 may be coated with a material such as an aramid-based resin or ceramic.

[0056] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0057] Example 1 Preparation of Positive Electrode Composite Sheet Using a NOB300-Nobilta (registered trademark) manufactured by Hosokawa Micron Corporation, 99.1 parts by mass of a lithium transition metal composite oxide as the positive electrode active material and 0.9 parts by mass of acetylene black as the conductive material were mixed for 20 minutes to perform a composite treatment, thereby preparing a conductive-material-coated 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 active material was 18.6%.

[0058] Next, this conductive material-coated active material and polytetrafluoroethylene (PTFE) were added to a Wonder Crusher manufactured by Osaka Chemical Co., Ltd. in a mass ratio of 100:1.0 and mixed for 2 minutes at room temperature and 8,400 rpm. TE5448 manufactured by Mitsui Chemours was used as the PTFE. Through the mixing process, the PTFE particles were fibrillated to form fibrous PTFE, resulting in a positive electrode composite in which the carbon-coated active material and fibrous PTFE were uniformly dispersed. The resulting positive electrode composite had a solids concentration of 100%.

[0059] The positive electrode composite sheet was produced by rolling the positive electrode composite between a pair of rolls, with the peripheral speed ratio of the rolls set to 1:1 and the linear pressure between the rolls set to 0.1 t / cm.

[0060] The positive electrode composite sheet from the first roll-rolling was then stretched with a pair of rolls to produce a stretched positive electrode composite sheet, with the peripheral speed ratio of the pair of rolls set to 1:2, the linear pressure between the pair of rolls set to 0.5 t / cm, and the gap between the pair of rolls set to half the film thickness of the positive electrode composite sheet before stretching.

[0061] 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.

[0062] [Fabrication of Positive Electrode] The positive electrode composite sheet was placed on a positive electrode core material made of aluminum foil, and the laminate of the positive electrode composite sheet and the positive electrode core material was pressed (linear pressure: 1 t / cm) using a pair of rolls to obtain a positive electrode in which the positive electrode composite sheet was bonded to both sides of the core material. The porosity (density) and mode diameter by mercury porosimetry of the obtained positive electrode were measured. As a result, the porosity of the positive electrode composite sheet was 21.0% (density was 3.65 g / cm ３ ) and the mode diameter of the positive electrode composite sheet obtained by mercury porosimetry was 0.43 μm.

[0063] [Fabrication of Negative Electrode] Graphite, sodium salt of carboxymethyl cellulose, and styrene-butadiene rubber were mixed in a mass ratio of 98:1:1, and the mixture was kneaded with water to prepare a negative electrode composite slurry. This negative electrode composite slurry was applied to both sides of a copper foil with a thickness of 8 μm, and after drying the coating, the foil was rolled with a rolling roller to prepare a negative electrode.

[0064] [Preparation of non-aqueous electrolyte] Five parts by mass of vinylene carbonate (VC) was added to a non-aqueous solvent prepared by mixing ethylene carbonate (EC) and dimethyl carbonate in a volume ratio of 1:3, and LiPF ６ was dissolved in the solution at a concentration of 1.2 mol / L. This was used as a non-aqueous electrolyte.

[0065] [Fabrication of Non-Aqueous Electrolyte Secondary Battery] (1) A positive electrode lead was attached to the positive electrode, and a negative electrode lead was attached to the negative electrode. A separator made of a polyethylene microporous film was then interposed between the positive and negative electrodes, and the resulting structure was wound to produce a wound electrode assembly. (2) Insulating plates were placed above and below the electrode assembly, and the negative electrode lead was welded to the case body and the positive electrode lead was welded to a sealing member, and the electrode assembly was housed within the case body. (3) A non-aqueous electrolyte was injected into the case body under reduced pressure, and the opening of the case body was then sealed with a sealing member via a gasket. This resulted in a non-aqueous electrolyte secondary battery.

[0066] Example 2 A positive electrode was produced in the same manner as in Example 1, except that in the preparation of the positive electrode composite sheet, 99.5 parts by mass of a lithium transition metal composite oxide and 0.5 parts by mass of acetylene black were mixed for 20 minutes to perform a composite treatment. The porosity (density) and mode diameter of the positive electrode composite sheet were measured using mercury porosimetry. As a result, the porosity of the positive electrode composite sheet was 21.3% (density was 3.64 g / cm ３ The mode diameter of the positive electrode composite sheet obtained by mercury porosimetry was 0.52 μm. A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1 using the positive electrode.

[0067] Comparative Example 1: A positive electrode composite slurry was prepared by mixing 99.1 parts by mass of a lithium transition metal composite oxide, 0.9 parts by mass of acetylene black, and 1.0 part by mass of polyvinylidene fluoride (PVDF) as a binder, and then adding an appropriate amount of N-methyl-2-pyrrolidone (NMP). The positive electrode composite slurry was applied to both sides of a positive electrode core material made of aluminum foil (thickness 15 μm). The coating film (positive electrode composite layer) was dried, and then rolled (linear pressure: 1 t / cm) using a pair of rolls to produce a positive electrode.

[0068] The porosity (density) of the positive electrode mixture layer and the mode diameter by mercury porosimetry were measured for the obtained positive electrode. As a result, the porosity of the positive electrode mixture layer was 21.2% (density was 3.63 g / cm ３ The mode diameter of the positive electrode mixture layer obtained by mercury porosimetry was 0.19 μm. A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1 using the positive electrode.

[0069] Comparative Example 2 A positive electrode was produced in the same manner as in Example 1, except that in the preparation of the positive electrode composite sheet, 99.7 parts by mass of lithium transition metal composite oxide and 0.3 parts by mass of acetylene black were mixed for 20 minutes to perform a composite treatment. The porosity (density) and mode diameter by mercury porosimetry of the obtained positive electrode composite sheet were measured. As a result, the porosity of the positive electrode composite sheet was 22.8% (density was 3.57 g / cm ３ The mode diameter of the positive electrode composite sheet obtained by mercury porosimetry was 0.87 μm. A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1 using the positive electrode.

[0070] [Measurement of Capacity Retention Rate in Charge-Discharge Cycles] At an ambient temperature of 25°C, the nonaqueous electrolyte secondary batteries of each Example and Comparative Example were charged at a constant current of 0.5 C to 4.2 V, and then charged at a constant voltage of 1 / 20 C at 4.2 V. Subsequently, the batteries were discharged at a constant current of 0.5 C to 2.5 V. This cycle of charge and discharge constituted one cycle, and 200 cycles were performed. The capacity retention rate in charge-discharge cycles of the nonaqueous electrolyte secondary batteries of each Example and Comparative Example was calculated using the following formula, and the results are summarized in Table 1. Capacity retention rate = (discharge capacity at 200th cycle / discharge capacity at 1st cycle) × 100

[0071]

[0072] As shown in Table 1, the capacity retention rates during charge-discharge cycles were higher in Examples 1 and 2 than in Comparative Examples 1 and 2. Therefore, by using a positive electrode that includes a positive electrode core material and a positive electrode composite sheet bonded to the surface of the positive electrode core material, where the positive electrode composite sheet includes a conductive-material-coated active material in which a conductive material is attached to the surface of the positive electrode active material, and a fibrous binder, and where the porosity of the positive electrode composite sheet is 17.8% or more and 23.2% or less, and where the mode diameter of the positive electrode composite sheet obtained by mercury porosimetry is more than 0.19 μm and less than 0.87 μm, it can be said that, despite the low porosity of the positive electrode composite sheet, it is possible to suppress deterioration in charge-discharge cycle characteristics.

[0073] The present disclosure is further described by the following embodiments. Configuration 1: A positive electrode for a non-aqueous electrolyte secondary battery, comprising: a positive electrode core material; and a positive electrode composite sheet bonded to a surface of the positive electrode core material, wherein the positive electrode composite sheet includes a conductive material-coated active material in which a conductive material is attached to a surface of the positive electrode active material; and a fibrous binder, wherein the porosity of the positive electrode composite sheet is 17.8% or more and 23.2% or less, and wherein the mode diameter of the positive electrode composite sheet obtained by mercury porosimetry is more than 0.19 μm and less than 0.87 μm. Configuration 2: The density of the positive electrode composite sheet is 3.55 g / cm ３ Above, 3.8g / cm ３ The positive electrode for a non-aqueous electrolyte secondary battery according to Configuration 1, wherein the content of the fibrous binder is 0.1 parts by mass or more and 5.0 parts by mass or less per 100 parts by mass of the positive electrode active material. Configuration 3: The positive electrode for a non-aqueous electrolyte secondary battery according to Configuration 1 or 2, wherein the content of the fibrous binder is 0.1 parts by mass or more and 5.0 parts by mass or less per 100 parts by mass of the positive electrode active material. Configuration 4: A non-aqueous electrolyte secondary battery comprising the positive electrode for a non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 3.

[0074] REFERENCE SIGNS LIST 10 Non-aqueous electrolyte secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Separator, 14 Electrode body, 15 Battery case, 16 Case body, 17 Sealing body, 18, 19 Insulating plate, 20 Positive electrode lead, 21 Negative electrode lead, 22 Protruding portion, 23 Filter, 24 Lower valve body, 25 Insulating member, 26 Upper valve body, 27 Cap, 28 Gasket, 30 Positive electrode core material, 32 Positive electrode composite sheet, 40 Positive electrode composite, 50 Mixer, 51, 52 Pair of rolls.

Claims

1. A positive electrode for a non-aqueous electrolyte secondary battery comprising: a positive electrode core material; and a positive electrode composite sheet bonded to a surface of the positive electrode core material, the positive electrode composite sheet including a conductive-material-coated active material in which a conductive material is attached to a surface of a positive electrode active material; and a fibrous binder, the positive electrode composite sheet having a porosity of 17.8% or more and 23.2% or less, and a mode diameter of the positive electrode composite sheet obtained by mercury porosimetry of more than 0.19 μm and less than 0.87 μm.

2. The density of the positive electrode mixture sheet is 3.55 g / cm ３ Above, 3.8g / cm ３ 2. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein:

3. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the content of the fibrous binder is 0.1 parts by mass or more and 5.0 parts by mass or less per 100 parts by mass of the positive electrode active material.

4. A non-aqueous electrolyte secondary battery comprising the positive electrode for a non-aqueous electrolyte secondary battery according to claim 1 or 2.

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