WO2025047326A1 - Negative electrode and secondary battery - Google Patents

Abstract

The purpose of the present invention is to provide an electrode which is capable of improving the rate characteristics when applied to a battery.　The present invention provides a negative electrode which includes a current collector and an active material layer that is provided on the surface of the current collector and contains an active material, a conductive assistant, and a binder, wherein: the active material contains non-spherical particles; the active material layer is composed of granulated bodies; and in a volume-based particle size distribution of the granulated bodies as determined by a laser diffraction/scattering method, D50 is more than 70 μm but not more than 120 μm, and the difference between D10 and S90 is 50 μm or more.

Description

Anode and secondary battery

The present invention relates to a negative electrode and a secondary battery.

In electrodes used in lithium-ion batteries and the like, an active material layer is formed on a current collector. There are many methods for forming an active material layer on a current collector when making electrodes, but they can be broadly divided into wet methods and dry methods from the viewpoint of whether or not the electrode mixture containing the active material is made into a slurry. In the wet method, the active material, polymer, solvent, etc. are mixed to obtain a slurry, and the obtained slurry is applied to a current collector and dried to form an electrode. On the other hand, in the dry method, an electrode is formed without going through a process of applying a slurry of the solvent and active material. For example, an electrode is formed by forming a granule containing the active material in advance and pressing it onto a current collector.

Technology in which granules containing an active material are formed in advance and then used to form an active material layer is disclosed in Patent Documents 1 and 2.

International Publication No. 2015/029829 JP 2020-129448 A

In lithium ion secondary batteries, it is desirable to use electrodes that have excellent rate characteristics in order to meet the demand for rapid charging and discharging, that is, electrodes that can withstand charging and discharging at large currents. In Patent Document 1, a lithium ion secondary battery is produced using negative electrode granules with a D50 of 70 μm and positive electrode granules with a D50 of 40 μm, but it was found that this lithium ion secondary battery does not have sufficiently good rate characteristics. In Patent Document 2, a positive electrode using granules with a D50 of 44.6 μm is disclosed, but it was found that even when a battery is produced using such granules, a battery with sufficiently good rate characteristics cannot be obtained.

The inventors conducted research and found that in a negative electrode having an active material layer using a granule containing an active material containing non-spherical particles, by appropriately adjusting the particle size distribution of the active material, a negative electrode with less active material falling off from the current collector can be obtained, and a secondary battery equipped with such a negative electrode has good rate characteristics.

The present invention has been made in consideration of the above circumstances, and provides an electrode (particularly a negative electrode) having an electrode (particularly a negative electrode) active material layer that has high adhesion to a current collector, and a secondary battery including the same.

That is, according to the present invention, there are provided the following electrodes and batteries.
1. A current collector;
an active material layer provided on a surface of the current collector, the active material layer including an active material, a conductive additive, and a binder;
the active material comprises non-spherical particles;
The active material layer is composed of granules,
In a volume-based particle size distribution of the granules by a laser diffraction/scattering method, D50 is more than 70 μm and 120 μm or less, and the difference between D10 and D90 is 50 μm or more.

2. A secondary battery in which the negative electrode described in 1. above and the positive electrode are stacked in multiple layers with a separator between them, optionally rolled up, and housed in an outer container, with a positive electrode terminal electrically connected to the positive electrode and a negative electrode terminal electrically connected to the negative electrode being pulled out from the outer container.

The present invention makes it possible to provide a secondary battery with high rate characteristics and a negative electrode for use therein.

The following describes an embodiment of the present invention.

One embodiment of the present invention comprises:
The negative electrode includes a current collector and an active material layer provided on a surface of the current collector, the active material layer including an active material, a conductive additive, and a binder. The active material includes non-spherical particles,
The active material layer is composed of granules,
The granules are characterized in that, in a volume-based particle size distribution measured by a laser diffraction/scattering method, D50 is greater than 70 μm and not greater than 120 μm, and the difference between D10 and D90 is 50 μm or more.

In one embodiment, the negative electrode refers to, but is not limited to, a negative electrode used in a non-aqueous secondary battery material. The negative electrode is composed of a current collector and an active material layer provided on the surface of the current collector, and the active material layer contains an active material, a conductive additive, and a binder, which are aggregated together. As the current collector, for example, a metal such as copper, nickel, titanium, or stainless steel can be used. As the current collector, it is preferable to use a flat metal, particularly a metal foil. The current collector will be described in detail later.

A carbon-based active material can be used as the active material. As the carbon-based active material, natural graphite, artificial graphite, hard carbon, soft carbon, carbon black, or any mixture of these can be selected. Natural graphite includes natural graphite whose particle surface is coated with amorphous carbon, and similarly, artificial graphite includes artificial graphite whose particle surface is coated with amorphous carbon. These natural graphites and artificial graphites can be used in the form of primary particles, particles in which the primary particles are aggregated to form secondary particles, or mixtures of these. In addition, a mixture of a carbon-based active material and a silicon-based active material can be used as the negative electrode active material. The negative electrode active material can include metal materials such as aluminum, lithium, silver, bismuth, calcium, cerium, indium, magnesium, tin, zinc, and nickel. The negative electrode active material will be described in detail later.

The conductive additive is a material for reducing the resistance of the electrode. Examples of conductive additives include carbon black such as acetylene black and ketjen black, activated carbon, graphite, mesoporous carbon, fullerenes, carbon nanotubes, carbon nanofibers, carbon nanobrushes, and other carbon fibers. The conductive additive will be described in more detail later.

Examples of binders include fluororesins such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), and polyvinyl fluoride (PVF); conductive polymers such as polyanilines, polythiophenes, polyacetylenes, and polypyrroles; synthetic rubbers such as styrene butadiene rubber (SBR), butadiene rubber (BR), chloroprene rubber (CR), isoprene rubber (IR), and acrylonitrile butadiene rubber (NBR); and polysaccharides such as carboxymethylcellulose (CMC), xanthan gum, guar gum, and pectin. Also, as the binder, polyacrylic acid, polymethacrylic acid, lithium polyacrylate, sodium polyacrylate, potassium polyacrylate, sodium polymethacrylate, potassium polymethacrylate; ethyl polyacrylate, ethyl polyacrylate, butyl polyacrylate, methyl polymethacrylate, ethyl polymethacrylate, butyl polymethacrylate, polyacrylamide, polyacrylonitrile, and any mixtures thereof can be used. Furthermore, as the binder, carboxymethylcellulose (hereinafter referred to as "CMC"), which is a derivative of cellulose, or a metal salt of carboxymethylcellulose (e.g., sodium carboxymethylcellulose, potassium carboxymethylcellulose) can also be used. The binder will be described in detail later.

In one embodiment, the active material layer is made of granules. The granules include an active material, a conductive additive, and a binder, and are aggregated together.
In one embodiment, the granules have a volume-based particle size distribution measured by a laser diffraction/scattering method, in which D50 is more than 70 μm and 120 μm or less, and the difference between D10 and D90 is 50 μm or more. By appropriately adjusting the particle size distribution of the granules constituting the active material layer, voids (paths) that connect from the current collector, which serves as a path for lithium ions, to the surface of the active material layer and the number of voids are maintained, thereby improving the rate characteristics of a secondary battery using the negative electrode of one embodiment.

In order for the granules constituting the active material layer to have a structure that allows the above-mentioned particle size distribution to be observed, it is preferable to first form granules in which the active material, conductive assistant, and binder are aggregated, and then stack the granules on the current collector to form the active material layer. The granules are particles (granulated material) obtained by aggregating a mixture of the above-mentioned active material, conductive assistant, and binder, and optionally a solvent, as raw materials. The active material layer used in the negative electrode of one embodiment is formed by applying granules in which the active material, conductive assistant, and binder are aggregated to the surface of the current collector by a dry coating method. Once the granules are formed, the active material layer can be formed by adjusting the pressing conditions to obtain the desired density. When the active material layer is pressed, the granules gradually collapse, forming the active material layer without eliminating the voids between the active material particles. Compared to an active material layer formed by a wet coating method using a slurry containing a mixture of an active material, a conductive additive, and a binder, the active material layer in one embodiment formed using granules has a greater or equal number of pore paths in the thickness direction of the active material layer. The length of the pore paths can be adjusted by changing the pressing conditions, such as the number of presses and pressure, while keeping the particle size of the active material and the amount of binder the same (i.e., using the same granules). Here, even in the case of an active material layer formed using granules, if the density is increased without adjusting the pressing conditions, the granules will be crushed back to the original active material particle state, resulting in the active material being oriented, and the rate characteristics of a secondary battery using a negative electrode containing such an active material layer will deteriorate.

Here, the average particle size of the granules is preferably more than 70 μm and not more than 120 μm. In one embodiment, the average particle size of the granules means the particle size at 50% of the cumulative value in the volumetric particle size distribution measured by the laser diffraction/scattering method (median size: D50). Furthermore, in the volumetric particle size distribution of the granules measured by the laser diffraction/scattering method, it is preferable that the difference between D10 and D90 is 50 μm or more.

The average particle size of the active material particles is preferably 1.0 μm or more, more preferably 1.5 μm or more, and even more preferably 2.0 μm or more, from the viewpoint of optimal adhesion of the conductive assistant to the active material surface. It is also preferably 20.0 μm or less, more preferably 10.0 μm or less, and even more preferably 5.0 μm or less. In the present invention, the average particle size of the active material particles means the particle size at an integrated value of 50% (median size: D50) in the volume-based particle size distribution measured by a laser diffraction scattering method.

The negative electrode active material includes a carbon-based active material. The carbon-based active material is preferably natural graphite, artificial graphite, hard carbon, soft carbon, or any mixture thereof. Here, graphite is a carbon material with hexagonal plate-shaped crystals in a hexagonal system, and is sometimes called graphite or graphite. Natural graphite and artificial graphite include natural graphite coated with amorphous carbon, and artificial graphite coated with amorphous carbon.

Here, amorphous carbon refers to a carbon material that is amorphous overall, with a random network structure of microcrystals that may partially have a structure similar to graphite. Examples of amorphous carbon include carbon black, coke, activated carbon, carbon fiber, carbon nanotubes, hard carbon, soft carbon, and mesoporous carbon. When using artificial graphite, it is preferable that the interlayer distance d value (d002) is 0.33 nm or more.

The crystal structure of artificial graphite is generally thinner than that of natural graphite. When artificial graphite is used as a negative electrode active material for non-aqueous electrolyte secondary batteries, particularly lithium ion secondary batteries, it is necessary that the interlayer distance allows lithium ions to be inserted. The interlayer distance at which lithium ions can be inserted and removed can be estimated by the d value (d002), and if the d value is 0.33 nm or more, lithium ions can be inserted and removed without any problems. The negative electrode active material may also include a silicon-based active material (A) containing SiOx (wherein x is a number that satisfies 0.5≦x≦1.6) and a carbon-based active material.

The aspect ratio of the negative electrode active material is preferably 1.1 or more and 5 or less. When a granule is prepared that is mainly composed of a negative electrode active material having an aspect ratio in this range, the orientation of the active material layer can be easily adjusted by the pressing conditions, and the electrical characteristics of a secondary battery using a negative electrode having an active material layer with an adjusted orientation are stabilized. When the aspect ratio of the negative electrode active material is less than 1.1, the active material particles are packed too densely, making it impossible to adjust the orientation of the active material layer, and excellent rate characteristics cannot be obtained in a secondary battery using such a negative electrode. On the other hand, when a granule is formed using an active material with an aspect ratio of more than 5, if the pressing conditions are adjusted to obtain a desired density, the path length formed in the active material layer becomes too long, and the rate characteristics of a secondary battery using such a negative electrode tend to decrease no matter how much the orientation is adjusted.
The orientation of the active material in the active material layer can be roughly estimated from the volumetric particle size distribution measured by a laser diffraction/scattering method. In the volumetric particle size distribution measured by a laser diffraction/scattering method of the granules constituting the active material layer, it is preferable that D50 is more than 70 μm and 120 μm or less, and the difference between D10 and D90 is 50 μm or more.

In one embodiment, when the total amount of the granules is 100 parts by mass, the content of the active material is preferably 95.0 parts by mass or more, more preferably 96 parts by mass or more, even more preferably 97 parts by mass or more, and preferably 99.5 parts by mass or less, more preferably 99.0 parts by mass or less, even more preferably 98.5 parts by mass or less. The proportion of the active material involved in the formation of the granules is at least 95% or more, preferably 97% or more, and even more preferably 99% or more.

The conductive assistant preferably contains at least one selected from the group consisting of carbon black, carbon nanofibers, and carbon nanotubes.
The carbon black used as the conductive assistant preferably has an aggregate structure in which carbon nanoparticles are connected in a beaded shape. As the carbon black, acetylene black, ketjen black, etc. can be used. Carbon nanotubes formed of a substance composed of carbon atoms such as graphene into a cylindrical shape with a diameter of several nm and a length of several mm can also be used. In particular, single-walled carbon nanotubes formed in a substantially single layer around the cylinder or multi-walled carbon nanotubes formed in a multiple layer around the cylinder can be bundled.

In one embodiment, when the total amount of the granules is 100 parts by mass, the content of the conductive assistant is preferably 0.01 parts by mass or more, preferably 0.05 parts by mass or more, more preferably 0.1 parts by mass or more, even more preferably 3.0 parts by mass or more, and preferably 2.0 parts by mass or less, more preferably 1.5 parts by mass or less, even more preferably 0.5 parts by mass or less.

In one embodiment, the granules further contain a binder. It is preferable that the conductive assistant and the binder are present on the surface and inside of the granules. In this way, when the granules are pressed, the desired density can be obtained without over-compressing the pores between the active material particles or over-orienting them.

The binder may contain, for example, polyvinylidene fluoride (PVDF), polyvinylpyrrolidone (PVP), polytetrafluoroethylene, polyimide, polyamideimide, polyamide, styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), polyvinyl alcohol, polyvinyl butyral, or two or more of these, but is not limited to these, and can be changed appropriately depending on the solvent.

In one embodiment, when the total amount of the granules is 100 parts by mass, the binder content is preferably 0.5 parts by mass or more, more preferably 0.7 parts by mass or more, even more preferably 1.0 parts by mass or more, and preferably 5.0 parts by mass or less, more preferably 3.0 parts by mass or less, even more preferably 2.0 parts by mass or less.

A method for manufacturing a negative electrode according to one embodiment preferably includes a step of forming granules by spraying and adding a dispersion liquid in which a conductive assistant is dispersed to the active material particles while mixing and stirring the active material particles, and a step of placing the granules on a current collector and pressing the granules. The method for forming the granules is not particularly limited, and examples of the method include a spray granulation method, an agitation granulation method, a fluidized bed granulation method, a rolling granulation method, an extrusion granulation method, and a compression granulation method. These granulation methods may be selected or combined as desired. An example of a spray granulation method is a spray drying granulation method. In the spray drying granulation method, for example, a dispersion liquid in which a negative electrode active material and a conductive assistant are dispersed is sprayed into high-temperature gas in a spray dryer to form granules. In the agitation granulation method, for example, a dispersion liquid in which a conductive assistant is dispersed is sprayed and added to the negative electrode active material particles while mixing and stirring the negative electrode active material to form granules. By adjusting the number of presses and the pressure applied to the active material layer containing the granules thus formed, it is possible to obtain high adhesion between the current collector and the active material layer, maintain the desired pore paths between the active material particles, and obtain the desired electrode density.

By attaching a dispersion liquid containing a highly dispersed conductive additive and binder to active material particles without containing any other organic solvents, it is possible to prepare an active material with a uniformly formed conductive additive on its surface. The diameter of the granules formed will vary depending on the equipment used to mix and stir these materials, but it is possible to control this by changing the amount and timing of spraying the dispersion liquid of the conductive additive and binder, as well as mixing and stirring times. In addition, by classifying the formed granules through a sieve, it is possible to obtain granules with any particle size distribution.

In one embodiment, the material of the current collector is not particularly limited, but when the current collector is a positive electrode current collector, aluminum foil is preferable. When the current collector is a negative electrode current collector, the material is selected from the group consisting of copper, stainless steel, nickel, titanium, or an alloy thereof, with copper being particularly preferable. The shape of the current collector layer is not particularly limited, and may be foil, flat, or mesh-like. For example, the thickness is 0.001 mm or more and 0.5 mm or less.

When the granules are placed on the surface of the current collector or the surface of the current collector and pressed, the pressing method is not particularly limited as long as it can form the electrode granules containing the electrode active material on the current collector. In one embodiment, it is preferable to use a roll-type pressure molding method. In the roll-type pressure molding method, a long current collector is arranged so that the current collector passes between a pair of rolls while the current collector is unwound from a rolled roll and wound up. The granules are supplied onto the current collector before passing through the rolls, and are compressed when passing between the rolls to form an electrode sheet. In order to adjust the amount of granules supplied, it is preferable to supply the thickness while smoothing the surface with a squeegee or the like before supplying it between the rolls. It is preferable to previously form a current collector surface layer containing a binder to enhance the binding property with the granules on the current collector foil. By adjusting the number of times of pressing the electrode sheet, the pressing pressure, the temperature, etc., it is possible to adjust the adhesion between the active material layer and the current collector and the length of the path of the gap between the active material particles.

A second embodiment of the present invention is a secondary battery in which the above-mentioned negative electrode and positive electrode are stacked in multiple layers with a separator interposed therebetween, optionally rolled up, and housed in an outer container, and a positive electrode terminal electrically connected to the positive electrode and a negative electrode terminal electrically connected to the negative electrode are drawn out from the outer container. The secondary battery according to the second embodiment can be produced using the above-mentioned negative electrode in accordance with a known method.
In the second embodiment, the positive electrode is not particularly limited, but refers to a positive electrode used in a non-aqueous secondary battery material. For example, the positive electrode is composed of a positive electrode current collector and a positive electrode active material layer provided on the surface of the positive electrode current collector, and the active material layer includes a positive electrode active material, a conductive additive, and a binder. The positive electrode active material layer including the positive electrode active material, the conductive additive, and the binder can be formed on at least one surface of the positive electrode current collector.
The positive electrode current collector can be made of stainless steel, aluminum, nickel, titanium, or a positive electrode current collector in which the surface of aluminum or stainless steel is surface-treated with carbon, nickel, titanium, or silver.

The positive electrode active material includes one or more selected from the group consisting of composite oxides of lithium and transition metals, such as lithium-nickel composite oxide, lithium-cobalt composite oxide, lithium-manganese composite oxide, lithium-nickel-manganese composite oxide, lithium-nickel-cobalt composite oxide, lithium-nickel-aluminum composite oxide, lithium-nickel-cobalt-aluminum composite oxide, lithium-nickel-manganese-cobalt composite oxide, lithium-nickel-manganese-aluminum composite oxide, and lithium-nickel-cobalt-manganese-aluminum composite oxide; transition metal sulfides, such as TiS ₂ , FeS, and MoS ₂ ; transition metal oxides, such as MnO, V ₂ O ₅ , V ₆ O ₁₃ , and TiO ₂ ; and olivine-type lithium phosphate. The olivine-type lithium phosphate contains, for example, one or more elements selected from the group consisting of Mn, Cr, Co, Cu, Ni, V, Mo, Ti, Zn, Al, Ga, Mg, B, Nb, and Fe, lithium, phosphorus, and oxygen. These compounds may have some elements partially substituted with other elements in order to improve their properties. Among these, as the positive electrode active material, it is preferable to use lithium nickel manganese cobalt oxide having an average particle size of 3 to 15 μm and represented by Li _a Ni _b Mn _c Co _d M _x O ₂ (wherein a, b, c, d and x satisfy 0.9≦a≦1.2, 0<b<1, 0<c≦0.5, 0<d≦0.5, 0≦x≦0.3, b+c+d=1, and M is at least one selected from the group consisting of Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr and Cr).
Examples of the binder that forms the positive electrode and the material layer together with the positive electrode active material include fluororesins such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinyl fluoride (PVF), etc.; conductive polymers such as polyanilines, polythiophenes, polyacetylenes, polypyrroles, etc.; synthetic rubbers such as styrene butadiene rubber (SBR), butadiene rubber (BR), chloroprene rubber (CR), isoprene rubber (IR), acrylonitrile butadiene rubber (NBR), etc.; and polysaccharides such as carboxymethyl cellulose (CMC), xanthan gum, guar gum, pectin, etc.
Examples of the conductive assistant include carbon black such as acetylene black and ketjen black, activated carbon, graphite, mesoporous carbon, fullerenes, carbon fibers such as carbon nanofibers, carbon nanotubes, and carbon nanobrushes. In addition, the positive electrode active material layer may appropriately contain electrode additives that are generally used for forming electrodes, such as thickeners, dispersants, and stabilizers.

In the secondary battery of the second embodiment, the separator may be, for example, a porous separator. The separator may be in the form of a membrane, film, nonwoven fabric, or the like. Examples of the porous separator include polyolefin-based porous separators such as polypropylene-based and polyethylene-based; and porous separators formed from polyvinylidene fluoride, polyethylene oxide, polyacrylonitrile, polyvinylidene fluoride hexafluoropropylene copolymer, or the like.

The separator may contain inorganic particles such as ceramic particles, or may further contain a layer containing inorganic particles.

In the secondary battery of the second embodiment, negative electrodes and positive electrodes are stacked in multiple layers with separators between them, and may be wound and housed in an outer container. The secondary battery is formed by drawing out a positive electrode terminal electrically connected to the positive electrode and a negative electrode terminal electrically connected to the negative electrode from the outer container. For the outer container, for example, a flexible film having a resin layer on the front and back sides of a metal layer such as aluminum as a base material, or a metal can such as aluminum, iron, or stainless steel, can be used. The positive electrode terminal can be made of aluminum or an aluminum alloy, and the negative electrode terminal can be made of copper or a copper alloy or copper or copper alloys plated with nickel.

The shape of the secondary battery of the second embodiment can be, for example, a square, cylindrical, coin, or button type housed in an outer container made of an aluminum can, iron can, stainless steel can, or the like. Also preferred is a pouch-shaped secondary battery housed in an outer container made mainly of aluminum film.

The above describes the embodiments of the present invention, but these are merely examples of the present invention, and various configurations other than those described above can also be adopted. Furthermore, the present invention is not limited to the above-described embodiments, and modifications and improvements within the scope of the present invention are included in the present invention.

The present invention will be explained below with reference to examples and comparative examples, but the present invention is not limited to these.

<Production of negative electrode>
(Examples 1 to 4 and Comparative Examples 1 to 4)
Separately from the formation of the conductive layer and the adhesive layer, granulated particles of the negative electrode active material layer were prepared. A negative electrode dispersion liquid was prepared in which artificial graphite with an average particle size of 12 μm and silicon monoxide with an average particle size of 5 μm, which are negative electrode active material particles, single-wall carbon nanotubes (SWCNT) as a conductive assistant, and polyacrylic acid (PAA) and styrene butadiene rubber (SBR) as binders were highly dispersed together with a solvent. Next, the negative electrode dispersion liquid prepared by the above procedure was rotated at a predetermined rotation speed and sprayed into a spray dryer to obtain a granule. The mass ratio of artificial graphite/silicon monoxide/CNT/PAA/SBR was 93.45:4.0:0.05:0.5:2.0.
Artificial graphite from Nippon Graphite Industries Co., Ltd., PAA from Fujifilm Wako Pure Chemical Industries, Ltd., and SWCNT from OCSiAl Corporation can be used.

Then, a predetermined amount of the obtained granules was supplied onto a current collector traveling from the unwinding side to the winding side of the electrode coater while smoothing the surface with a squeegee. An adhesive auxiliary layer containing carbon black and binders (CMC and SBR) had been formed in advance on the surface of the current collector. After the granules were supplied onto the current collector, the amount of granules supplied to the press roll by the squeegee roll was adjusted before the granules were compressed by the press roll, and the granules were then passed between the press rolls to form an active material layer of the desired density. The volumetric particle size distribution of the granules constituting the active material layer, measured by the laser diffraction scattering method, is shown in Table 1.

<Battery manufacturing>
Lithium nickel manganese cobalt oxide with an average particle size of 4.5 μm and expressed as LiNi _0.92 Mn _0.03 Co _0.05 O ₂ was used as the positive electrode active material, and a slurry containing carbon black as a conductive additive and polyvinylidene fluoride (PVDF) as a binder was applied to an aluminum foil as a positive electrode current collector, and then pressed to obtain a positive electrode with a density of 3.5 g/cm ³ .
The eight positive electrode layers and the nine negative electrode layers of each of the previously obtained examples or comparative examples were laminated via a polyolefin-based porous separator, and a negative electrode terminal electrically connected to each negative electrode and a positive electrode terminal electrically connected to each positive electrode were provided to obtain a laminate. Next, an electrolyte solution in which LiPF ₆ was dissolved in a mixed solvent of ethylene carbonate and diethyl carbonate (ethylene carbonate:diethyl carbonate=3:7 (volume ratio) to a concentration of 1.0 mol/L was placed together with the obtained laminate in a container with a flexible film as a case, and the positive electrode terminal and the negative electrode terminal were pulled out to the outside and sealed to obtain a secondary battery.

<Rate characteristics>
The prepared battery was charged at a constant current of 0.2 C up to 4.2 V, switched to a constant voltage after reaching 4.2 V and charged down to 0.015 C, and discharged at constant currents of 0.2 C and 3 C up to 2.5 V, and the rate characteristic was determined as 3C/0.2C, which is the ratio of the capacity of the 3C discharge to the capacity of the 0.2C discharge at 2.5 V. Generally, the capacity tends to decrease more easily as the current value increases, so the closer to 100%, the better the rate characteristic.

<Measurement of particle size distribution by laser diffraction scattering method>
The volumetric particle size distribution of the granules constituting the active material layer was measured by a laser diffraction scattering method using a dry particle size distribution measuring unit (LA-960 LY-9505, Horiba, Ltd.). The D50 (median diameter), D10, and D90 were determined from the particle size distribution curve (cumulative distribution curve) of the granules constituting each active material layer.

As shown in Table 1, secondary batteries using negative electrodes with active material layers in which the particle size distribution of the granules has a D50 of 80 to 120 μm show good rate characteristics. On the other hand, secondary batteries using negative electrodes with active material layers in which the particle size distribution of the granules has a D50 of 70 μm or less have poor rate characteristics. As in Comparative Example 1, negative electrodes with active material layers in which the particle size distribution of the granules has a D50 of 70 μm or less tend to have clogged voids that serve as the ion path, which is thought to have reduced the rate characteristics of secondary batteries using such negative electrodes. Even when negative electrodes with active material layers in which the particle size of the granules is too large, as in Comparative Example 2, were used, secondary batteries with excellent rate characteristics could not be obtained. It can be inferred that this is because the large granules prevent the press density from being increased, which results in insufficient contact of the conductive assistant between the active material particles and makes it impossible to keep up with rapid charging and discharging. In addition, the granules constituting the negative electrode active material layer used in the secondary batteries of Comparative Example 3 and Comparative Example 4 have a small difference between D10 and D90 in the particle size distribution, i.e., the width of the particle size distribution is narrow, making it difficult to obtain the desired active material layer density and difficult to adjust the paths of the voids present in the active material layer, which is thought to have reduced the rate characteristics of the secondary battery.

The electrodes for lithium ion secondary batteries and the lithium ion secondary batteries of the present invention have been described in detail above. However, the present invention is not limited to the above-described embodiments and examples, and various improvements and modifications may be made without departing from the spirit and scope of the present invention.

Claims (4)

A current collector;
an active material layer provided on a surface of the current collector, the active material layer including an active material, a conductive additive, and a binder;
A negative electrode comprising:
the active material comprises non-spherical particles;
The active material layer is composed of granules,
In a volume-based particle size distribution of the granules measured by a laser diffraction/scattering method, D50 is greater than 70 μm and not greater than 120 μm, and the difference between D10 and D90 is 50 μm or more. The negative electrode according to claim 1, wherein the granules are aggregates of at least an active material, a conductive additive, and a binder. The negative electrode according to claim 1 or 2, wherein the aspect ratio of the active material is 1.1 or more and 5 or less. A secondary battery comprising a negative electrode according to any one of claims 1 to 3 and a positive electrode stacked together with a separator therebetween, optionally wound, and housed in an outer container, and a positive electrode terminal electrically connected to the positive electrode and a negative electrode terminal electrically connected to the negative electrode being pulled out from the outer container.