Negative electrode and secondary battery

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

Anodes and secondary batteries

[0001] The present invention relates to a negative electrode and a secondary battery.

[0002] Electrodes used in lithium-ion batteries and the like have an active material layer formed on a current collector. There are many methods for forming an active material layer on a current collector when producing an electrode. These methods can be broadly divided into wet methods and dry methods, depending on whether or not the electrode mixture containing the active material is slurried. In the wet method, an active material, a polymer, a solvent, and the like are mixed to obtain a slurry, and the resulting 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 undergoing a step of applying a slurry of a solvent and an active material. For example, an electrode is formed by forming granules containing the active material in advance and pressing them onto a current collector.

[0003] Patent Documents 1 and 2 disclose a technique in which granules containing an active material are formed in advance and then used to form an active material layer.

[0004] International Publication No. 2015 / 029829 Japanese Patent Application Laid-Open No. 2020-129448

[0005] In lithium-ion secondary batteries, it is desirable to use electrodes with excellent rate characteristics, i.e., electrodes that can withstand large current charging and discharging, in order to meet the demand for rapid charging and discharging. Patent Document 1 discloses a lithium-ion secondary battery fabricated 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. Patent Document 2 also discloses a positive electrode using granules with a D50 of 44.6 μm, but it was found that even when a battery was fabricated using such granules, it did not have sufficiently good rate characteristics.

[0006] As a result of investigations, the present inventors have found that in a negative electrode having an active material layer using granules containing an active material including non-spherical particles, by appropriately adjusting the particle size distribution of the active material, a negative electrode with little active material falling off from the current collector can be obtained, and a secondary battery including such a negative electrode has good rate characteristics.

[0007] The present invention has been made in view 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.

[0008] That is, the present invention provides the following electrode and battery: 1. A negative electrode comprising: a current collector; and an active material layer provided on the surface of the current collector, the active material layer including an active material, a conductive additive, and a binder, wherein the active material includes non-spherical particles, the active material layer is made of granules, and 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 more than 120 μm, and the difference between D10 and D90 is 50 μm or more.

[0009] 2. A secondary battery in which a plurality of negative electrodes and positive electrodes described in 1 above are stacked with separators interposed therebetween, optionally wound together, 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.

[0010] According to the present invention, it is possible to provide a secondary battery having high rate characteristics and a negative electrode used therein.

[0011] Hereinafter, an embodiment of the present invention will be described.

[0012] One embodiment of the present invention is a negative electrode including a current collector and an active material layer provided on a surface of the current collector, the active material including non-spherical particles, the active material layer being composed of granules, and the granules having a volume-based particle size distribution measured by a laser diffraction / scattering method, with D50 being greater than 70 μm and not greater than 120 μm, and the difference between D10 and D90 being 50 μm or greater.

[0013] In one embodiment, the negative electrode refers to, but is not particularly 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. The active material layer contains an active material, a conductive additive, and a binder, which are aggregated together. The current collector can be made of metal, such as copper, nickel, titanium, or stainless steel. The current collector is preferably made of a planar metal, particularly a metal foil. The current collector will be described in detail below.

[0014] Carbon-based active materials can be used as the active material. Examples of carbon-based active materials include natural graphite, artificial graphite, hard carbon, soft carbon, carbon black, and any mixtures thereof. Natural graphite includes natural graphite particles coated with amorphous carbon, while artificial graphite includes artificial graphite particles coated with amorphous carbon. These natural and artificial graphite particles can be used in the form of primary particles, particles formed by aggregation of primary particles to form secondary particles, or mixtures thereof. The negative electrode active material can also be a mixture of a carbon-based active material and a silicon-based active material. The negative electrode active material can also 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 below.

[0015] The conductive additive is a material for reducing the resistance of the electrode. Examples of the conductive additive 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 detail later.

[0016] 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 carboxymethyl cellulose (CMC), xanthan gum, guar gum, and pectin. In addition, the binder may be polyacrylic acid, polymethacrylic acid, lithium polyacrylate, sodium polyacrylate, potassium polyacrylate, sodium polymethacrylate, potassium polymethacrylate; polyethyl acrylate, polyethyl acrylate, polybutyl acrylate, polymethyl methacrylate, polyethyl methacrylate, polybutyl methacrylate, polyacrylamide, polyacrylonitrile, or any mixture thereof. Furthermore, the binder may be a cellulose derivative, such as carboxymethyl cellulose (hereinafter referred to as "CMC"), or a metal salt of carboxymethyl cellulose (e.g., sodium carboxymethyl cellulose, potassium carboxymethyl cellulose). Binders will be described in detail below.

[0017] In one embodiment, the active material layer is composed of granules. The granules include an active material, a conductive additive, and a binder, and are aggregates of these. In one embodiment, the granules have a volume-based particle size distribution measured by a laser diffraction / scattering method, in which D50 is greater than 70 μm and not greater than 120 μm, and the difference between D10 and D90 is 50 μm or greater. By appropriately adjusting the particle size distribution of the granules constituting the active material layer, the number and number of voids (paths) that connect the current collector, which serves as a path for lithium ions, to the surface of the active material layer are maintained, thereby improving the rate characteristics of a secondary battery using the negative electrode of one embodiment.

[0018] In order for the granules constituting the active material layer to have a structure that allows the above-described particle size distribution to be observed, it is preferable to first form granules in which the active material, conductive additive, and binder are aggregated, and then layer the granules on a current collector to form the active material layer. The granules are particles (granules) obtained by aggregating a mixture of the above-described active material, conductive additive, binder, and, optionally, a solvent. In one embodiment, the active material layer used in the negative electrode is formed by applying granules in which the active material, conductive additive, and binder are aggregated to the surface of the current collector using 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. During pressing, the granules gradually crumble, resulting in the formation of an active material layer without eliminating 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 larger or equal number of pore paths extending in the thickness direction of the active material layer. The length of the pore paths can also 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). Even in 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 orientation of the active material. This will deteriorate the rate characteristics of a secondary battery using a negative electrode containing such an active material layer.

[0019] 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 refers to the particle size at 50% of the integrated value (median size: D50) in the volume-based particle size distribution measured by a laser diffraction / scattering method. Furthermore, it is preferable that the difference between D10 and D90 in the volume-based particle size distribution of the granules measured by the laser diffraction / scattering method is 50 μm or more.

[0020] From the viewpoint of optimal adhesion of the conductive additive to the active material surface, 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. 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 refers to the particle size at 50% of the integrated value (median size: D50) in the volume-based particle size distribution measured by a laser diffraction scattering method.

[0021] 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-like crystals and is sometimes called graphite. Natural graphite and artificial graphite include natural graphite coated with amorphous carbon and artificial graphite coated with amorphous carbon.

[0022] Here, amorphous carbon refers to a carbon material that is amorphous as a whole and has a random network structure of microcrystals, which 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.

[0023] The crystalline structure of artificial graphite is generally thinner than that of natural graphite. When artificial graphite is used as a negative electrode active material for nonaqueous electrolyte secondary batteries, particularly lithium ion secondary batteries, it must have an interlayer distance that allows lithium ions to be inserted. The interlayer distance that allows lithium ions to 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. In addition, the negative electrode active material may include a silicon-based active material (A) containing SiOx (where x is a number that satisfies 0.5≦x≦1.6) and a carbon-based active material.

[0024] The aspect ratio of the negative electrode active material is preferably 1.1 or more and 5 or less. Creating granules primarily composed of a negative electrode active material with an aspect ratio within this range facilitates adjusting the orientation of the active material layer through the pressing conditions, resulting in stable electrical characteristics of a secondary battery using a negative electrode with an active material layer with such adjusted orientation. 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 thus excellent rate characteristics cannot be obtained in a secondary battery using such a negative electrode. On the other hand, when granules are formed using an active material with an aspect ratio greater than 5, adjusting the pressing conditions to achieve the desired density results in excessively long path lengths in the active material layer, and the rate characteristics of a secondary battery using such a negative electrode tend to deteriorate regardless of how much the orientation is adjusted. The orientation of the active material in the active material layer can be roughly estimated using the volume-based particle size distribution measured by laser diffraction / scattering. In the volumetric particle size distribution of the granules constituting the active material layer as determined by laser diffraction / scattering method, it is preferred that D50 is more than 70 μm and 120 μm or less, and the difference between D10 and D90 is 50 μm or more.

[0025] 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, and even more preferably 98.5 parts by mass or less. The proportion of the active material involved in forming the granules is at least 95% or more, preferably 97% or more, and even more preferably 99% or more.

[0026] The conductive additive 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 additive preferably has an aggregate structure in which nanoparticle-sized carbons are linked together like beads. Examples of carbon black that can be used include acetylene black and ketjen black. Carbon nanotubes, which are formed from carbon atoms such as graphene into a cylindrical shape with a diameter of several nanometers and a length of several millimeters, can also be used. In particular, bundles of single-walled carbon nanotubes, each of which has a substantially single layer around the cylinder, or multi-walled carbon nanotubes, each of which has multiple layers around the cylinder, can be used.

[0027] In one embodiment, when the total amount of the granules is 100 parts by mass, the content of the conductive additive 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.

[0028] In one embodiment, the granules further contain a binder. Preferably, the conductive additive and the binder are present on the surface and inside of the granules. By doing so, when the granules are pressed, the desired density can be obtained without excessively crushing the pores between the active material particles or excessively orienting them.

[0029] The binder may include, 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.

[0030] In one embodiment, when the total amount of the granules is 100 parts by mass, the content of the binder is preferably 0.5 parts by mass or more, more preferably 0.7 parts by mass or more, even more preferably 1.0 part 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.

[0031] A method for producing a negative electrode according to one embodiment preferably includes the steps of spraying a dispersion of a conductive additive onto active material particles while mixing and stirring the active material particles to form granules, and placing the granules on a current collector and pressing them. The method for forming the granules is not particularly limited, and examples include spray granulation, stirring granulation, fluidized bed granulation, rolling granulation, extrusion granulation, and compression granulation. These granulation methods may be selected or combined as desired. Examples of spray granulation methods include spray-drying granulation. In the spray-drying granulation method, for example, a dispersion of a negative electrode active material and a conductive additive is sprayed into high-temperature gas in a spray dryer to form granules. In the stirring granulation method, for example, a dispersion of a conductive additive is sprayed onto the negative electrode active material particles while mixing and stirring the negative electrode active material. 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 a desired electrode density while maintaining high adhesion between the current collector and the active material layer and maintaining the desired pore paths between the active material particles.

[0032] By applying a dispersion containing a highly dispersed conductive additive and binder to active material particles without the addition of other organic solvents, an active material with a uniformly dispersed conductive additive on its surface can be prepared. The diameter of the granules formed varies depending on the equipment used to mix and stir these materials, but it can be controlled by adjusting the amount and timing of spraying the dispersion of conductive additive and binder, as well as the 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.

[0033] 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 preferred. When the current collector is a negative electrode current collector, a material selected from the group consisting of copper, stainless steel, nickel, titanium, or an alloy thereof is preferred, with copper being particularly preferred. 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.

[0034] When granules are placed on the surface of a current collector or a current collector and pressed, the pressing method is not particularly limited as long as it can form electrode granules containing an electrode active material on the current collector. In one embodiment, a roll-type pressure molding method is preferably used. 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 roll and then wound up. The granules are supplied onto the current collector before passing through the rolls and compressed as they pass between the rolls to form an electrode sheet. To adjust the amount of granules supplied, it is preferable to smooth the surface with a squeegee or the like before supplying the granules between the rolls to adjust the thickness. It is preferable to previously form a current collector surface layer containing a binder to enhance adhesion to the granules on the current collector foil. By adjusting the number of times, pressure, temperature, etc., of pressing the electrode sheet, the adhesion between the active material layer and the current collector and the length of the voids between the active material particles can be adjusted.

[0035] A second embodiment of the present invention is a secondary battery in which a plurality of the above-described negative electrodes and positive electrodes are stacked with a separator interposed 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 are drawn out from the outer container. The secondary battery according to the second embodiment can be fabricated using the above-described negative electrode according to a known method. In the second embodiment, the positive electrode refers to, but is not particularly limited 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 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.

[0036] The positive electrode active material may be a composite oxide of lithium and a transition metal, 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, or lithium-nickel-cobalt-manganese-aluminum composite oxide; TiS, FeS, MoS ２ transition metal sulfides such as MnO, V ２ O ５ , V ６ O １３ , TiO ２and one or more transition metal oxides selected from the group consisting of olivine-type lithium phosphates. The olivine-type lithium phosphates contain, 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, as well as lithium, phosphorus, and oxygen. These compounds may have some elements partially substituted with other elements to improve their properties. Among these, the positive electrode active material is preferably a Li-based phosphate having an average particle size of 3 to 15 μm. 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, and b+c+d=1, and M is at least one element selected from the group consisting of Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr, and Cr) is preferably used. Examples of binders that form the positive electrode and material layer together with the positive electrode active material 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 carboxymethyl cellulose (CMC), xanthan gum, guar gum, and pectin. Examples of conductive additives 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, electrode additives commonly used for electrode formation, such as thickeners, dispersants, and stabilizers, can be used appropriately in the positive electrode active material layer.

[0037] 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, a film, a nonwoven fabric, or the like. Examples of the porous separator include polyolefin-based porous separators such as polypropylene-based and polyethylene-based separators; and porous separators formed from polyvinylidene fluoride, polyethylene oxide, polyacrylonitrile, polyvinylidene fluoride-hexafluoropropylene copolymer, or the like.

[0038] The separator may contain inorganic particles such as ceramic particles, or may further include a layer containing inorganic particles.

[0039] In the secondary battery of the second embodiment, a plurality of negative electrodes and positive electrodes are stacked with a separator interposed therebetween, optionally wound, and housed in an outer container. The secondary battery is formed by extending 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. The outer container can be, for example, a flexible film having a resin layer on both sides of a metal layer such as aluminum as a base material, or a metal can made of aluminum, iron, stainless steel, or the like. The positive electrode terminal can be made of aluminum or an aluminum alloy, and the negative electrode terminal can be made of copper, a copper alloy, or a nickel-plated copper or copper alloy.

[0040] The shape of the secondary battery of the second embodiment may be, for example, a square, cylindrical, coin, or button shape housed in an outer container such as an aluminum can, an iron can, a stainless steel can, etc. A pouch-shaped secondary battery housed in an outer container mainly made of an aluminum film is also preferred.

[0041] Although the embodiments of the present invention have been described above, these are merely examples of the present invention, and various other configurations may be adopted. Furthermore, the present invention is not limited to the above-described embodiments, and modifications and improvements within the scope of achieving the object of the present invention are included in the present invention.

[0042] EXAMPLES The present invention will be described below with reference to examples and comparative examples, but the present invention is not limited to these.

[0043] <Production of Negative Electrode> (Examples 1 to 4 and Comparative Examples 1 to 4) Separately from the formation of the conductive layer and adhesive layer, granulated particles for the negative electrode active material layer were prepared. A negative electrode dispersion was prepared by highly dispersing negative electrode active material particles, including artificial graphite with an average particle size of 12 μm and silicon monoxide with an average particle size of 5 μm, single-walled carbon nanotubes (SWCNT) as a conductive additive, and polyacrylic acid (PAA) and styrene-butadiene rubber (SBR) as binders, together with a solvent. The negative electrode dispersion prepared by the above procedure was then sprayed into a spray dryer by rotating an atomizer at a predetermined rotation speed, yielding granules. The mass ratio of artificial graphite / silicon monoxide / CNT / PAA / SBR was 93.45:4.0:0.05:0.5:2.0. The artificial graphite was available from Nippon Graphite Industries Co., Ltd., the PAA from Fujifilm Wako Pure Chemical Industries, Ltd., and the SWCNT from OCSiAl.

[0044] Next, a predetermined amount of the obtained granules was supplied onto a current collector traveling from the unwinding side to the winding side of an electrode coater while smoothing the surface with a squeegee. An adhesive auxiliary layer containing carbon black and binders (CMC and SBR) had been previously formed on the surface of the current collector. After supplying the granules onto the current collector, the amount of granules supplied to the press roll was adjusted by the squeegee roll before the granules were compressed by the press roll. The granules were then passed between the press rolls to form an active material layer of the desired density. The volume-based particle size distribution of the granules constituting the active material layer, measured by a laser diffraction scattering method, is shown in Table 1.

[0045] <Battery manufacturing> As the positive electrode active material, LiNi with an average particle size of 4.5 μm ０．９２ Mn ０．０３ Co ０．０５ O ２ The 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 a density of 3.5 g / cm ３The eight layers of the positive electrode and nine layers of the negative electrodes of the previously obtained Examples or Comparative Examples were stacked with a polyolefin-based porous separator interposed therebetween, 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, a mixed solvent of ethylene carbonate and diethyl carbonate (ethylene carbonate:diethyl carbonate=3:7 (volume ratio)) was added with LiPF as an electrolyte. ６ The electrolyte solution in which the above was dissolved 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 container was sealed with the positive electrode terminal and the negative electrode terminal extended to the outside, thereby obtaining a secondary battery.

[0046] <Rate characteristics> The prepared battery was charged at a constant current of 0.2 C up to 4.2 V, and after reaching 4.2 V, the charge was switched to a constant voltage and charged down to 0.015 C, and discharged at a constant current of 0.2 C and 3 C down to 2.5 V, and the rate characteristics were determined as 3 C / 0.2 C, which is the ratio of the capacity at 2.5 V between the 3 C discharge and the 0.2 C discharge. Generally, the capacity tends to decrease more easily as the current value increases, so the closer the current value is to 100%, the better the rate characteristics are.

[0047] <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 laser diffraction scattering method using a dry particle size distribution measurement 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.

[0048]

[0049] As shown in Table 1, secondary batteries using negative electrodes with active material layers having a granule particle size distribution D50 of 80 to 120 μm exhibit good rate characteristics. On the other hand, secondary batteries using negative electrodes with active material layers having a granule particle size distribution D50 of 70 μm or less exhibit poor rate characteristics. It is believed that negative electrodes with active material layers having a granule particle size distribution D50 of 70 μm or less, as in Comparative Example 1, tend to clog the voids that serve as ion paths, resulting in poor rate characteristics for secondary batteries using such negative electrodes. Even when using negative electrodes with active material layers having granule particle sizes that are too large, as in Comparative Example 2, secondary batteries with excellent rate characteristics could not be obtained. It is presumed that this is because large granules hinder increased press density, resulting in insufficient contact of the conductive additive between the active material particles and making it impossible to keep up with rapid charging and discharging. Furthermore, the granules constituting the negative electrode active material layer used in the secondary batteries of Comparative Examples 3 and 4 had a small difference between D10 and D90 in the particle size distribution, i.e., a narrow particle size distribution width, which is thought to have made it difficult to obtain a desired active material layer density and to adjust the paths of voids present in the active material layer, resulting in a deterioration in the rate characteristics of the secondary batteries.

[0050] The electrode for a lithium ion secondary battery and the lithium ion secondary battery 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

1. A negative electrode comprising: a current collector; and an active material layer provided on a surface of the current collector, the active material comprising an active material, a conductive additive and a binder, wherein the active material comprises non-spherical particles, the active material layer is composed of granules, and 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.

2. The negative electrode according to claim 1, wherein the granules are an aggregate of at least an active material, a conductive additive, and a binder.

3. 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.

4. A secondary battery in which the negative electrode according to any one of claims 1 to 3 and a 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.

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