JP2018206472A - Lithium ion battery - Google Patents

Abstract

To provide a lithium ion battery excellent in discharge rate characteristics.SOLUTION: A lithium ion battery comprises a positive electrode, a negative electrode, and an electrolyte. The positive electrode has a collector and a positive electrode mixture formed on the collector. The positive electrode mixture has a positive electrode active material and a conductive material. The conductive material contains acetylene black whose coarse particle ratio is 1 ppm or lower. Preferably, the positive electrode active material is a layered lithium-nickel-manganese-cobalt composite oxide. Also preferably, the average particle diameter of the acetylene black is 20 to 100 nm.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a lithium ion battery.

  Lithium ion batteries are secondary batteries with high energy density, and are used as power sources for portable devices such as notebook computers and mobile phones, taking advantage of their characteristics. In recent years, lithium ion secondary batteries are expected to be used not only for portable devices but also for electric vehicles, hybrid electric vehicles, and the like. Since electric vehicles and hybrid electric vehicles require a large current instantaneously at the time of engine start, for example, excellent output characteristics are required for lithium ion batteries.

  For example, Patent Document 1 discloses a lithium ion battery in which carbon nanofibers are added to a positive electrode conductive material for the purpose of improving output characteristics.

  Patent Document 2 discloses a battery in which activated carbon is added to a positive electrode for the purpose of reducing internal resistance of the battery and improving output characteristics at a large current.

JP 2004-220909 A JP 2007-317583 A

  However, according to the study of the present inventor, it has been clarified that not only the kind of the conductive material but also one characteristic of the conductive material affects the output characteristic.

  An object of the present invention has been made in view of the above problems, and is to provide a lithium ion battery having excellent output characteristics.

The inventors of the present invention are affected by not only the type of conductive material but also the coarse particle content of the conductive material, and there is a risk that the output characteristic will deteriorate when the coarse particle content is large. It was clarified that there was.
The lithium ion battery which concerns on one embodiment of this invention is a lithium ion battery provided with a positive electrode, a negative electrode, and electrolyte solution, Comprising: The said positive electrode is a positive electrode mixture formed in the collector and the said collector. The positive electrode mixture has a positive electrode active material and a conductive material, and the conductive material contains acetylene black having a coarse particle content of 1 ppm or less.
Here, the coarse particle content indicates the remaining amount of coarse particles that could not be removed when passed through a sieve (330 mesh) having an opening of 45 μm. 1 ppm is one millionth and is 1 mg / Kg.
In the lithium ion battery according to the present invention, the positive electrode active material is preferably a layered lithium / nickel / manganese / cobalt composite oxide (NMC).
In the lithium ion battery according to the present invention, the average particle diameter of acetylene black is preferably 20 to 100 nm.

  The lithium ion battery of the present invention can provide a lithium ion battery having excellent output characteristics.

It is a graph which shows the measurement result of the discharge rate and discharge capacity ratio of the lithium ion battery of an Example. It is a figure which shows the measurement result of the direct current | flow resistance (DCR) of the lithium ion battery of an Example. It is a cross-sectional perspective view which shows an example of a structure of a cylindrical lithium ion secondary battery.

  In the following embodiments, when ranges are shown as A to B, A or more and B or less are shown unless otherwise specified.

  First, an outline of a lithium ion battery (hereinafter also referred to as a lithium ion secondary battery) will be briefly described. The lithium ion secondary battery includes a battery container and a positive electrode, a negative electrode, a separator, and an electrolytic solution housed therein. A separator is disposed between the positive electrode and the negative electrode.

  When charging the lithium ion battery, a charger is connected between the positive electrode and the negative electrode. At the time of charging, lithium ions inserted into the positive electrode active material are desorbed and released into the electrolytic solution. The lithium ions released into the electrolytic solution move in the electrolytic solution, pass through a separator made of a microporous film, and reach the negative electrode. The lithium ions that have reached the negative electrode are inserted into the negative electrode active material constituting the negative electrode.

  When discharging, an external load is connected between the positive electrode and the negative electrode. At the time of discharging, the lithium ions inserted into the negative electrode active material are desorbed and released into the electrolytic solution. At this time, electrons are emitted from the negative electrode. Then, the lithium ions released into the electrolytic solution move in the electrolytic solution, pass through a separator made of a microporous film, and reach the positive electrode. The lithium ions reaching the positive electrode are inserted into the positive electrode active material constituting the positive electrode. At this time, when lithium ions are inserted into the positive electrode active material, electrons flow into the positive electrode. In this way, discharge is performed by the movement of electrons from the negative electrode to the positive electrode.

  Thus, charging / discharging can be performed by inserting and desorbing lithium ions between the positive electrode active material and the negative electrode active material. A configuration example of an actual lithium ion battery will be described later (see, for example, FIG. 3).

  Next, a positive electrode, a negative electrode, an electrolytic solution, a separator, and other constituent members that are constituent elements of the lithium ion secondary battery of the present embodiment will be sequentially described.

1. Positive electrode In this embodiment, the positive electrode shown below is included. The positive electrode (positive electrode plate) of the present embodiment is composed of a current collector and a positive electrode mixture formed on the surface thereof. The positive electrode mixture is a layer containing at least a positive electrode active material, and in the present embodiment, the positive electrode active material contains a layered lithium-nickel-manganese-cobalt composite oxide. This positive electrode mixture contains acetylene black as a conductive material in addition to the positive electrode active material, and is formed (coated) on, for example, both surfaces of the current collector.

(A) Positive electrode active material The positive electrode mixture is a layer containing at least a positive electrode active material, and in the present embodiment, a positive electrode active material contains a layered lithium-nickel-manganese-cobalt composite oxide (NMC). . By using this NMC, a lithium ion secondary battery having a high capacity and excellent safety can be obtained.

In addition to the layered lithium / nickel / manganese / cobalt composite oxide (hereinafter referred to as NMC), the positive electrode active material includes a lithium-containing composite metal oxide containing spinel type lithium / manganese oxide, an olivine type lithium oxide, It may contain a chalcogen compound, manganese dioxide or the like. Among these, it is preferable to use a lithium-containing composite metal oxide. The lithium-containing composite metal oxide is a metal oxide containing lithium and a transition metal or a metal oxide in which a part of the transition metal in the metal oxide is substituted with a different element. Here, examples of the different elements include Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B, and Mn, Al, Co, Ni, Mg or the like is preferable. One kind or two or more kinds of different elements may be used. The lithium-containing composite metal oxide, for _{example, Li X CoO 2, Li X} NiO 2, Li X Co Y Ni 1-Y O 2, Li X Co Y M 1-Y Oz, Li X Ni 1-Y M Y O _Z , LiMPO ₄ , Li ₂ MPO ₄ F (in the above formulas, M is Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, V, and B) And at least one element selected from the group consisting of X = 0 to 1.2, Y = 0 to 0.9, and Z = 2.0 to 2.3.
Here, the value of X indicating the molar ratio of lithium is increased or decreased by charging and discharging. Examples of the olivine type lithium oxide include LiFePO ₄ . Examples of the chalcogen compound include titanium disulfide and molybdenum disulfide. One positive electrode active material may be used alone, or two or more positive electrode active materials may be used in combination as described above.

As NMC, those represented by the following composition formula (Formula 1) are preferably used.
Li _{(1 + δ)} Mn _X Ni _Y Co _(1-XYZ) M _Z O ₂ (Chemical Formula 1)
In the above composition formula (Chemical formula 1), (1 + δ) is a composition ratio of Li (lithium), X is a composition ratio of Mn (manganese), Y is a composition ratio of Ni (nickel), (1-XYZ) Indicates the composition ratio of Co (cobalt). Z represents the composition ratio of the element M. The composition ratio of O (oxygen) is 2.

  The element M includes Ti (titanium), Zr (zirconium), Nb (niobium), Mo (molybdenum), W (tungsten), Al (aluminum), Si (silicon), Ga (gallium), Ge (germanium), and Sn. It is at least one element selected from the group consisting of (tin).

  The composition ratios are −0.15 <δ <0.15, 0.1 <X ≦ 0.5, 0.6 <X + Y + Z ≦ 1.0, and 0 ≦ Z ≦ 0.1.

  Hereinafter, the positive electrode mixture and the current collector will be described in detail. The positive electrode mixture contains a positive electrode active material, a binder, and the like, and is formed on the current collector. Although there is no restriction | limiting in the formation method, it forms as follows, for example. (A) positive electrode active material, (b) conductive material, (c) binder, (d) thickener used as required, and (e) other additives are mixed into a sheet, This is crimped to a current collector (dry method). Further, a positive electrode active material, a conductive material, a binder, a thickener used as necessary, and other additives are dissolved or dispersed in a dispersion solvent to form a slurry, which is applied to a current collector, Dry (wet method).

  (A) As the particles of the positive electrode active material, those in the form of a lump, polyhedron, sphere, ellipsoid, plate, needle, column or the like are used. Among them, it is preferable that the primary particles are aggregated to form secondary particles, and the shape of the secondary particles is spherical or elliptical.

In an electrochemical element such as a battery, the active material in the electrode expands and contracts as it is charged / discharged, so that the active material is easily damaged by the stress or the conductive path is broken.
For this reason, it is preferable to use particles in which primary particles are aggregated to form secondary particles, rather than using single particles of only primary particles, because the stress of expansion and contraction can be relieved and the above deterioration can be prevented. In addition, it is preferable to use spherical or oval spherical particles rather than plate-like particles having axial orientation, since the orientation in the electrode is reduced, so that the expansion and contraction of the electrode during charge / discharge is reduced. Further, it is preferable because other materials such as a conductive material are easily mixed uniformly when forming the electrode.

  The range of the average particle diameter d50 of the NMC particles applied to the positive electrode active material (the average particle diameter d50 of the secondary particles when primary particles are aggregated to form secondary particles) is as follows. . The lower limit of the range is 0.1 μm or more, preferably 0.5 μm or more, more preferably 1 μm or more, further preferably 3 μm or more, and the upper limit is 20 μm or less, preferably 18 μm or less, more preferably 16 μm or less, and even more preferably. Is 15 μm or less. If it is less than the above lower limit, the tap density (fillability) may be lowered, and a desired tap density may not be obtained. If the upper limit is exceeded, it takes time to diffuse lithium ions in the particles, so that the battery performance is lowered. There is a risk of inviting. On the other hand, when the above upper limit is exceeded, there is a possibility that the mixing property with other materials such as a binder and a conductive material may be lowered during the formation of the electrode. Therefore, when this mixture is slurried and applied, it may not be applied uniformly, which may cause problems such as streaking. Here, the tap density (fillability) may be improved by mixing two or more positive electrode active materials having different average particle diameters d50. The average particle diameter d50 can be obtained from the particle size distribution obtained by the laser diffraction / scattering method.

  The range of the average particle diameter of the primary particles in the case where the primary particles are aggregated to form secondary particles is as follows. The lower limit of the range is 0.01 μm or more, preferably 0.05 μm or more, more preferably 0.08 μm or more, further preferably 0.1 μm or more, and the upper limit is 3 μm or less, preferably 2 μm or less, more preferably 1 μm. Hereinafter, it is more preferably 0.6 μm or less. When the above upper limit is exceeded, it becomes difficult to form spherical secondary particles, and battery performance such as output characteristics may be reduced due to a decrease in tap density (fillability) and a decrease in specific surface area. Moreover, if it is less than the said minimum, there exists a possibility of producing problems, such as the reversibility of charging / discharging degrading by crystallinity fall.

The range of the BET specific surface area of the positive electrode active material particles such as NMC is as follows. The lower limit of the range is 0.1 m ² / g or more, preferably 0.3 m ² / g or more, more preferably 0.4 m ² / g or more, and the upper limit is 4.0 m ² / g or less, preferably 2 .5m ² / g, more preferably at most 1.5 m ² / g. If it is less than the said minimum, there exists a possibility that battery performance may fall. When the above upper limit is exceeded, the tap density is difficult to increase, and the miscibility with other materials such as a binder and a conductive material may be reduced. Therefore, there is a possibility that applicability at the time of slurrying and applying this mixture is deteriorated. The BET specific surface area is a specific surface area (area per unit g) determined by the BET method.

(B) Conductive Material In the present invention, the positive electrode conductive material contains acetylene black having a coarse particle content of 1 ppm or less.

  Here, the coarse particle content indicates the remaining amount of coarse particles that could not be removed when passed through a sieve (330 mesh) having an opening of 45 μm. 1 ppm is one millionth and is 1 mg / Kg. It can be measured in accordance with JIS K6218-3 (carbon black for rubber-incidental characteristics-part 3: how to determine sieve residue).

  The acetylene black is preferably particles having an average particle size of 20 nm or more and 100 nm or less, and is not particularly limited as long as it is in this particle size range. Here, examples of the particles include granular, flake-like, spherical, columnar, and irregular shapes. The “granular” is not an irregular shape but a shape having almost equal dimensions (see JIS Z2500: 2000). The “flake shape (strip shape)” is a plate-like shape (see JIS Z2500: 2000) and is also referred to as a scale-like shape because it is a thin plate shape like a scale. Here, in this specification, it analyzes from the result of SEM observation, and makes the range whose aspect-ratio (particle diameter a / average thickness t) 2-100 is flake shape (piece shape). The particle diameter a here is defined as the square root of the area S when the flaky particles are viewed in plan, and this is defined as the particle diameter in this specification. The “spherical shape” is a shape that is almost a sphere (see JIS Z2500: 2000). Further, the shape does not necessarily need to be spherical, and the ratio of the major axis (DL) to the minor axis (DS) of the particles (DL) / (DS) (sometimes referred to as spherical coefficient or sphericity) is 1. In this case, the particle diameter refers to the long diameter (DL). Examples of the “columnar shape” include a substantially columnar shape and a substantially polygonal column, and the particle diameter refers to the height of the column.

  When the average particle diameter of acetylene black contained in the conductive material exceeds 100 nm, the number of contact points with the positive electrode active material is reduced, the conductive network between the active materials is hindered, and the input / output characteristics of the battery tend to deteriorate. On the other hand, when the average particle size is less than 20 nm, the dispersibility in the positive electrode mixture is deteriorated, and the battery performance is significantly deteriorated due to an adverse effect such as segregation of acetylene black. As described above, the average particle diameter of acetylene black is preferably 20 nm or more and 100 nm or less, more preferably 30 nm or more and 80 nm or less, and further preferably 40 nm or more and 60 nm or less.

  Note that the average particle diameter of the conductive material is an arithmetic average particle diameter obtained by photographing with a scanning electron microscope photographed at a magnification of 200,000 and measuring all the diameters of the in-image particle images.

  The content of the conductive material is preferably 0.2% by mass or more, more preferably 0.5% by mass or more, and further preferably 1% by mass or more, based on the total amount of the positive electrode mixture. The upper limit of the content of the conductive material Is preferably 30% by mass or less, more preferably 20% by mass or less, and still more preferably 10% by mass or less. Within the above range, the battery capacity and input / output characteristics are excellent.

  Furthermore, the content of acetylene black contained in the conductive material is preferably 0.1% by mass or more and 15% by mass or less, and preferably 1% by mass with respect to the total amount of the positive electrode mixture, from the viewpoint of conductivity and high capacity. As mentioned above, 10 mass% or less is more preferable, and 2 mass% or more and 5 mass% or less are more preferable. Within the above range, the battery capacity and input / output characteristics are excellent.

(C) Binder (c) The binder for the positive electrode is not particularly limited. When the positive electrode mixture is formed by a coating method, a material having good solubility and dispersibility in the dispersion solvent is selected. Is done. Specific examples include resin polymers such as polyethylene, polypropylene, polyethylene terephthalate, polymethyl methacrylate, polyimide, aromatic polyamide, cellulose, and nitrocellulose; SBR (styrene-butadiene rubber), NBR (acrylonitrile-butadiene rubber), fluorine Rubbery polymers such as rubber, isoprene rubber, butadiene rubber, ethylene-propylene rubber; styrene / butadiene / styrene block copolymer or hydrogenated product thereof, EPDM (ethylene / propylene / diene terpolymer), styrene / Thermoplastic elastomeric polymers such as ethylene / butadiene / ethylene copolymers, styrene / isoprene / styrene block copolymers or hydrogenated products thereof; syndiotactic-1,2-polybutadiene, polyacetic acid Soft resinous polymers such as Nyl, ethylene / vinyl acetate copolymer, propylene / α-olefin copolymer; polyvinylidene fluoride (PVDF), polytetrafluoroethylene, fluorinated polyvinylidene fluoride, polytetrafluoroethylene / ethylene Fluoropolymers such as copolymers and polytetrafluoroethylene / vinylidene fluoride copolymers; polymer compositions having ion conductivity of alkali metal ions (particularly lithium ions), and the like. Of these, one type may be used alone, or two or more types may be used in combination. From the viewpoint of the stability of the positive electrode, it is preferable to use a fluorine-based polymer such as polyvinylidene fluoride (PVDF) or a polytetrafluoroethylene / vinylidene fluoride copolymer.

  The content of the binder with respect to the total amount of the positive electrode mixture is preferably 0.1% by mass or more, more preferably 1% by mass or more, and further preferably 3% by mass or more. The upper limit is preferably 50% by mass or less, more preferably 40% by mass or less, further preferably 30% by mass or less, and particularly preferably 10% by mass or less. By setting it as the said range, battery performance, such as cycling characteristics, can be made more favorable.

(D) Thickener (d) The thickener is used to adjust the viscosity of the slurry. When an aqueous solvent is used as the dispersion solvent, it is preferable to use a thickener, but when an organic solvent is used, it may not be used.

  In order to improve the packing density of the positive electrode active material, the layer formed on the current collector using the wet method or the dry method is preferably consolidated by a hand press, a roller press, or the like.

(Current collector)
There is no restriction | limiting in particular as a material of the electrical power collector for positive electrodes, As a specific example, metal materials, such as aluminum and stainless steel, are mentioned. Of these, aluminum is preferable.

  There is no restriction | limiting in particular as a shape of an electrical power collector, The material processed into various shapes can be used. Specific examples of the metal material (for example, Al) include a metal foil, a metal thin film, and an expanded metal. Among these, it is preferable to use a metal thin film. In addition, you may form a thin film suitably in mesh shape. Although the thickness of a metal thin film is arbitrary, the range is as follows. The lower limit of the range is 1 μm or more, preferably 3 μm or more, more preferably 5 μm or more, and the upper limit is 1 mm or less, preferably 100 μm or less, more preferably 50 μm or less. If it is less than the said minimum, intensity | strength required as a collector may be insufficient. Moreover, when the said upper limit is exceeded, flexibility will fall and workability may deteriorate.

(Positive electrode mixture density)
The range of the density of the positive electrode mixture is preferably 2.4 g / cm ³ or more and 3.3 g / cm ³ or less from the viewpoint of input / output characteristics. It is preferably 80 g / m ² or more and 200 g / m ² or less, and more preferably 90 g / m ² or more and 150 g / m ² or less.

2. Negative electrode In this embodiment, the negative electrode shown below is included. The negative electrode (negative electrode plate) of the present embodiment is composed of a current collector and a negative electrode mixture formed on the surface thereof. The negative electrode mixture contains a negative electrode active material that can electrochemically occlude and release lithium ions.

  Hereinafter, the negative electrode mixture and the current collector will be described in detail. The negative electrode mixture contains a negative electrode active material and a binder, and is formed on the current collector. Although there is no restriction | limiting in the formation method, it forms as follows, for example. (A) a negative electrode active material, (b) a conductive material, (c) a binder, (d) a thickener used as necessary, and other additives are mixed to form a sheet, which is collected. Crimp to electrical body (dry method). Moreover, a positive electrode active material, a conductive material, a binder, a thickener used as necessary, and other additives are dissolved or dispersed in a dispersion solvent to form a slurry, which is applied to the surface of the current collector. And dry (wet method). This negative electrode mixture may be formed (coated) on both surfaces of the current collector, for example.

(A) Negative electrode active material (a) As the negative electrode active material, for example, graphitizable carbon is used. Graphitizable carbon has graphitization properties that graphitize by heat treatment at 800 ° C. or higher. On the other hand, non-graphitizable carbon has non-graphitization property that hardly graphitizes even by heat treatment at 2800 ° C. or higher. This is because graphitizable carbon has an atomic arrangement structure that easily forms a layered structure, and has a property of easily changing to a graphite structure by heat treatment at a relatively low temperature as compared with non-graphitizable carbon.

  The graphitizable carbon has a mass of 550 ° C. of 75% or more with respect to the mass of 25 ° C. in the air stream determined by thermogravimetry (TG), and the mass of 650 ° C. is 20% of the mass of 25 ° C. % Or less.

From the viewpoint of battery performance, it is more preferable that the mass at 550 ° C in the air stream has 85% or more with respect to the mass at 25 ° C, and the mass at 650 ° C is 10% or less with respect to the mass at 25 ° C. .
Furthermore, it is preferable that the mass of 550 ° C. in the air stream is 95% or more with respect to the mass of 25 ° C., and the mass of 650 ° C. is less than 5% with respect to the mass of 25 ° C.

  When the mass at 550 ° C. in the air stream is less than 75% with respect to the mass at 25 ° C., the input / output characteristics are degraded, and when the mass at 650 ° C. exceeds 20% with respect to the mass at 25 ° C., the life characteristics are degraded. To do. The thermogravimetric measurement here can be measured using a TG analyzer (for example, TG / DTA6200, manufactured by SII Nano Technology Co., Ltd.). For example, a sample of 10 mg is taken, and the mass is measured at each temperature at a temperature rising rate of 1 ° C./min with alumina as a reference under a flow of dry air of 300 mL / min.

  As a method for forming graphitizable carbon, for example, a material exhibiting graphitizable properties is fired in an inert atmosphere at 800 ° C. or higher, and then this is known in a jet mill, a vibration mill, a pin mill, a hammer mill or the like. The graphitizable carbon can be obtained by pulverizing by the above method and adjusting the average particle diameter so as to be in the range of 5 to 30 μm.

  The material exhibiting graphitizability is not particularly limited, and examples thereof include thermoplastic resins, naphthalene, anthracene, phenanthrolen, coal tar, tar pitch, etc., preferably coal-based coal tar or petroleum-based tar. Is used.

  Here, the graphitizable carbon is defined as having a surface spacing d002 value in the C-axis direction obtained by an X-ray wide angle diffraction method of 0.34 nm or more and 0.40 nm or less.

The graphitizable carbon preferably has a surface spacing d002 value in the C-axis direction obtained by an X-ray wide angle diffraction method of 0.34 nm or more and less than 0.36 nm, and is 0.341 nm or more and 0.355 nm or less. More preferably, it is 0.342 nm or more and 0.35 nm or less.
Although the graphitizable carbon can be used as it is as a negative electrode active material for a lithium ion battery, a specific surface area is expected to be large depending on pulverization conditions, and desired characteristics may not be exhibited. Therefore, it is preferable to adjust the physical properties shown in the following (1) to (3) by forming a carbon layer or the like on the surface of the graphitizable carbon.

  (1) About the average particle diameter (d50), the range is as follows. It is preferably 5 to 30 μm, more preferably 10 to 25 μm, and still more preferably 12 to 23 μm.

  If it is below the above upper limit, unevenness on the electrode surface is unlikely to occur, the short circuit of the battery can be suppressed, and the Li diffusion distance from the particle surface to the inside becomes relatively short, so the input / output characteristics of the lithium ion battery are improved. Tend. Moreover, if it is more than the said minimum, a specific surface area can be made into an appropriate range, and while it is excellent in the initial charge-and-discharge efficiency of a lithium ion battery, there exists a tendency for the contact of particle | grains to be good and input / output characteristics to be excellent.

  The particle size distribution can be measured with a laser diffraction particle size distribution measuring device (SALD-3000J, manufactured by Shimadzu Corporation) by dispersing the sample in purified water containing a surfactant, and the average particle size is 50% d. Calculate as

(2) About the BET specific surface area (measurement temperature: 77K) calculated | required from nitrogen adsorption measurement, the range is as follows. Preferably 1.0~5.0m is ² / g, more preferably 1.3~4.0m ² / g, more preferably from 1.5~3.0m ² / g. If it is at least the above lower limit, the input / output characteristics are excellent.

  In addition, the specific surface area by nitrogen adsorption can be calculated | required using BET method from the adsorption isotherm obtained by the nitrogen adsorption measurement at 77K.

(3) Regarding the carbon dioxide adsorption amount (measurement temperature: 273 K) up to a relative pressure of 0.03, the ranges are as follows. Preferably 0.01~4.0cm ³ / is g, more preferably 0.05~1.5cm ³ / g, further preferably 0.1~1.2cm ³ / g.

  In addition, the specific surface area by carbon dioxide adsorption can be calculated | required using a BET method from the adsorption isotherm obtained by the carbon dioxide adsorption measurement by 273K. If it is above the lower limit, the input characteristics are excellent, and if it is below the upper limit, the loss of the initial additional reverse capacity is small and the life characteristics are excellent.

  Thus, graphitizable carbon has a mass of 550 ° C. of 75% or more with respect to a mass of 25 ° C. in an air stream determined by thermogravimetry (TG), and a mass of 650 ° C. of 25 ° C. Alternatively, a carbon layer may be formed on the surface of graphitizable carbon serving as a nucleus of 20% or less.

  The carbon layer can be formed, for example, by attaching an organic compound (carbon precursor) that remains carbonaceous by heat treatment to the surface of the graphitizable carbon, and then firing. The method for attaching the organic compound to the surface of the graphitizable carbon is not particularly limited. For example, after the graphitizable carbon serving as a nucleus is dispersed and mixed in a mixed solution obtained by dissolving or dispersing the organic compound in a solvent. Examples include a wet method for removing a solvent, a dry method in which graphitizable carbon and an organic compound are mixed together as solids, and mechanical energy is applied to the mixture to adhere, and a vapor phase method such as a CVD method. Among these, the wet method is preferable from the viewpoint of being uniform and easy to control the reaction system and maintaining the shape of graphitizable carbon.

The organic compound may be a polymer compound such as a thermoplastic resin or a thermosetting resin, and is not particularly limited, but a thermoplastic polymer compound is preferably used.
The thermoplastic polymer compound is carbonized via the liquid phase to generate carbon having a small specific surface area. Therefore, when this coats the graphitizable carbon surface, the specific surface area of the negative electrode active material itself becomes small. This is preferable because the initial irreversible capacity of the lithium ion battery can be reduced as a result. The thermoplastic polymer compound is not particularly limited. For example, ethylene heavy end pitch, crude oil pitch, coal tar pitch, asphalt cracking pitch, pitch generated by thermally decomposing polyvinyl chloride, naphthalene, etc. A synthetic pitch produced by polymerization in the presence of superacidity can be used. Further, thermoplastic synthetic resins such as polyvinyl chloride, polyvinyl alcohol, polyvinyl acetate, and polyvinyl butyral, or natural products such as starch and cellulose can also be used. These organic compounds may be used alone or in combination of two or more.

  The solvent for dissolving and dispersing the organic compound is not particularly limited. For example, when the organic compound is pitches, tetrahydrofuran, toluene, xylene, benzene, quinoline and the like can be used. In addition, a suitable solvent may be used depending on the type of the organic compound.

  The solvent can be removed by heating in a normal pressure or reduced pressure atmosphere. The temperature at the time of solvent removal is preferably 200 ° C. or lower when the atmosphere is air. When the removal temperature exceeds 200 ° C., oxygen in the atmosphere reacts with the organic compound and the solvent (especially when creosote oil is used), the amount of carbon produced by the calcination fluctuates, and the porosity increases, so In some cases, the physical properties of the substance may be deviated and desired characteristics cannot be expressed.

  The firing conditions for coating with the carbon layer may be appropriately determined in consideration of the carbonization rate of the organic compound, and are not particularly limited, but are preferably 700 to 1400 ° C. and 800 to 1300 in a non-oxidizing atmosphere. More preferably, it is in the range of ° C. When the firing temperature is less than 700 ° C., when used as a negative electrode active material, the initial irreversible capacity of the lithium ion battery tends to increase. It only causes an increase. Examples of the non-oxidizing atmosphere include an inert gas atmosphere such as nitrogen, argon, and helium, and a vacuum atmosphere.

  The firing time is appropriately selected depending on the type of organic compound used and the amount of the organic compound used, and is not particularly limited. The baking apparatus to be used is not particularly limited as long as it is a reaction apparatus having a heating mechanism, and examples thereof include a baking apparatus capable of processing by a continuous method, a batch method, or the like.

  Here, the graphitizable carbon obtained by the firing treatment is preferably subjected to pulverization because individual particles may be aggregated, and further when adjustment to a desired average particle size is necessary. A pulverization process may be performed.

  Further, the thermogravimetric measurement result (TG) of graphitizable carbon (coated) having a carbon layer formed by the above method becomes a nucleus due to the influence of the formed carbon layer (crystallinity, coating amount, etc.). May differ from thermogravimetric measurement results of graphitizable carbon (before coating). In order to obtain the desired characteristics, even in the thermogravimetric measurement result of graphitizable carbon (coated) with a carbon layer formed, the mass at 550 ° C. in the air stream is 75% relative to the mass at 25 ° C. It is preferable that the mass at 650 ° C. is 20% or less with respect to the mass at 25 ° C.

  In addition, the crystallinity of the surface carbon layer is preferably lower than the graphitizable carbon that is the nucleus. By making the crystallinity of the surface carbon layer lower than the graphitizable carbon as the core, the familiarity between the negative electrode active material for lithium ion batteries and the electrolytic solution is improved, and as a result, the life characteristics tend to be improved. When such graphitizable carbon is used as a negative electrode active material, it is excellent in safety, input / output characteristics, and life characteristics.

  Furthermore, a carbonaceous material having high conductivity such as graphite and activated carbon may be mixed and used as the negative electrode active material.

  In the case of mixing, the mixing ratio (mass ratio) of the graphite material is preferably graphitizable carbon / graphite = 100/0 to 10/90, more preferably 100/0 to 50/50, and 100 / 0-80 / 20 is more preferable. By using the graphite under such conditions as the negative electrode active material, battery performance such as higher energy density and higher output can be improved. Also, graphitizable carbon and non-graphitizable carbon may be used in combination. The mixing ratio (mass ratio) of the non-graphitizable carbon is preferably graphitizable carbon / non-graphitizable carbon = 100/0 to 10/90, more preferably 100/0 to 50/50, and 100/0 to 70. / 30 is more preferable.

  The content (addition amount, ratio, amount) of the negative electrode active material with respect to the mass of the negative electrode mixture is preferably 20% by mass or more, more preferably 50% by mass or more, and more than 70% by mass with respect to the total amount of the negative electrode active material. Is more preferable.

(B) Conductive material (b) In addition, a carbonaceous material having properties different from those of graphitizable carbon may be added as a conductive material. The above properties include one or more characteristics of X-ray diffraction parameters, average particle diameter, aspect ratio, BET specific surface area, orientation ratio, Raman R value, tap density, true density, pore distribution, circularity, and ash content. Show.

  As the conductive material, natural graphite, graphite such as artificial graphite (graphite), carbon black such as acetylene black, and amorphous carbon such as needle coke can be used. These may be used alone or in combination of two or more. Thus, by adding a conductive material, there are effects such as reducing the resistance of the electrode.

  The range of the content of the conductive material relative to the mass of the negative electrode mixture is preferably in the range of 1 to 45% by mass, and preferably 2 to 42% by mass from the viewpoint of improving conductivity and reducing the initial irreversible capacity. More preferably, it is more preferably 3 to 40% by mass.

(C) Binder (c) The binder for the negative electrode active material is not particularly limited as long as it is a material that is stable to the non-aqueous electrolyte and the dispersion solvent used in forming the electrode. A binder similar to that used as the binder can be used. Specific examples include polyethylene, polypropylene, polyethylene terephthalate, polymethyl methacrylate, SBR (styrene-butadiene rubber), NBR (acrylonitrile-butadiene rubber), polyvinylidene fluoride, polytetrafluoroethylene, and fluorinated polyvinylidene fluoride. . These may be used alone or in combination of two or more.

  0.1-20 mass% is preferable, as for the range of content of the binder with respect to the mass of a negative electrode mixture, 0.5-15 mass% is more preferable, and 0.6-10 mass% is further more preferable.

  When the content of the binder is 0.1% by mass or more, the negative electrode active material can be sufficiently bound, and sufficient mechanical strength of the negative electrode active material can be obtained. Sufficient battery capacity and electroconductivity are obtained as it is 20 mass% or less.

  As the binder, the range of the content of the binder with respect to the mass of the negative electrode mixture when a fluorine-based polymer typified by polyvinylidene fluoride is used as a main component is preferably 1 to 15% by mass, It is more preferable that it is 2-10 mass%, and it is further more preferable that it is 3-8 mass%.

(solvent)
As the dispersion solvent for forming the slurry, any type of solvent can be used as long as it can dissolve or disperse the negative electrode active material, the binder, and the conductive material and thickener used as necessary. There is no restriction, and either an aqueous solvent or an organic solvent may be used. Examples of the aqueous solvent include water, a mixed solvent of alcohol and water, and examples of the organic solvent include N-methylpyrrolidone (NMP), cyclohexanone, and methyl acetate. In particular, when an aqueous solvent is used, it is preferable to use a thickener. In addition to this thickener, a dispersant or the like is added and slurried using a latex such as SBR. In addition, the said dispersion solvent may be used individually by 1 type, or may be used in combination of 2 or more type.

(D) Thickener (d) The thickener is used to adjust the viscosity of the slurry. The thickener is not particularly limited, and specific examples include carboxymethyl cellulose and methyl cellulose. These may be used alone or in combination of two or more.

  When using a thickener, the range of the content of the thickener with respect to the mass of the negative electrode mixture is preferably 0.1 to 5 mass%, more preferably 0.5 to 3 mass%, More preferably, it is 0.6-2 mass%.

  The layer formed on the current collector using the above wet method or dry method is preferably consolidated by a hand press, a roller press or the like in order to improve the packing density of the negative electrode active material.

(Current collector)
The material and shape of the current collector for the negative electrode are not particularly limited, and copper foil is preferred from the viewpoint of ease of processing and cost. The copper foil includes a rolled copper foil formed by a rolling method and an electrolytic copper foil formed by an electrolytic method, both of which are suitable for use as a current collector.

The single-sided coating amount of the negative electrode mixture on the current collector is preferably 50 to 120 g / m ² and more preferably 60 to 100 g / m ² from the viewpoint of energy density and input / output characteristics.

Although there is no particular limitation to the configuration of the negative electrode mixture was formed by using the anode active material, preferably in the range of the negative electrode mixture density is ^{0.7~2g / cm 3, 0.8~1.9g / cm} 3 More preferably, it is 0.9-1.8 g / cm < ³ >.

When it is 0.7 g / cm ³ or more, the conductivity between the negative electrode active materials is improved, an increase in battery resistance can be suppressed, and the capacity per unit volume can be improved. When it is 2 g / cm ³ or less, there is less possibility of incurring deterioration in discharge characteristics due to an increase in initial additional reverse capacity and a decrease in permeability to the electrolyte near the interface between the current collector and the negative electrode active material.

3. Electrolytic Solution The electrolytic solution of the present embodiment is composed of a lithium salt (electrolyte) and a non-aqueous solvent that dissolves the lithium salt. Such an electrolytic solution may be referred to as a non-aqueous electrolytic solution. You may add an additive to this non-aqueous electrolyte solution as needed.

  The lithium salt is not particularly limited as long as it is a lithium salt that can be used as an electrolyte of a non-aqueous electrolyte solution for a lithium ion battery, and examples thereof include inorganic lithium salts, fluorine-containing organic lithium salts, and oxalatoborate salts. It is done.

Examples of the inorganic lithium _{salt, LiPF 6, LiBF 4, LiAsF} 6, LiSbF inorganic fluoride salts and the like _6, LiClO _4, Libro 4, LiIO and perhalogenate such as _4, an inorganic chloride salts such as LiAlCl _4, etc. Is mentioned. A fluorine-containing organic lithium salt, a fluoroalkyl fluorophosphate, or the like may be used. Examples of the oxalatoborate salt include lithium bis (oxalato) borate and lithium difluorooxalatoborate.

These lithium salts may be used alone or in combination of two or more. Among these, lithium hexafluorophosphate (LiPF ₆ ) is preferable when comprehensively judging the solubility in a solvent, charge / discharge characteristics in the case of a battery, output characteristics, cycle characteristics, and the like.

  The concentration of the electrolyte in the electrolytic solution is not particularly limited, but the concentration range of the electrolyte is preferably 0.5 to 2 mol / L, more preferably 0.6 to 1.8 mol / L, and More preferably, it is 7-1.8 mol / L.

  When the concentration is 0.5 mol / L or more, sufficient electric conductivity of the electrolytic solution can be obtained. Moreover, since a viscosity does not become high too much that a density | concentration is 2 mol / L or less, the fall of electrical conductivity can be suppressed.

  The non-aqueous solvent is not particularly limited as long as it is a non-aqueous solvent that can be used as an electrolyte solvent for lithium ion batteries. For example, the following cyclic carbonates, chain carbonates, chain esters, cyclic ethers and chain ethers are used. Etc.

  These include ethylene carbonate, propylene carbonate, butylene carbonate, dialkyl carbonate, dimethyl carbonate, diethyl carbonate, di-n-propyl carbonate, ethyl methyl carbonate, methyl n-propyl carbonate, ethyl n-propyl carbonate, methyl acetate, tetrahydrofuran , Dimethoxyethane, dimethoxymethane and the like.

  These may be used alone or in combination of two or more, but it is preferable to use a mixed solvent in which two or more compounds are used in combination, a high dielectric constant solvent of cyclic carbonates, chain carbonates, It is preferable to use in combination with a low-viscosity solvent such as a chain ester. One of the preferable combinations is a combination mainly composed of cyclic carbonates and chain carbonates. Among them, the total of the cyclic carbonates and the chain carbonates in the non-aqueous solvent is preferably 80% by volume or more, more preferably 85% by volume or more, and further preferably 90% by volume or more. preferable. And the thing whose capacity | capacitance of the cyclic carbonate with respect to the sum total of cyclic carbonate and chain carbonate is the following range is preferable.

  The cyclic carbonates are preferably 5 to 50% by volume, more preferably 10 to 35% by volume, and still more preferably 15 to 30% by volume. By using such a combination of non-aqueous solvents, battery cycle characteristics and high-temperature storage characteristics (particularly, remaining capacity and high-load discharge capacity after high-temperature storage) are improved.

  The additive is not particularly limited as long as it is an additive for an electrolyte solution of a lithium ion battery. For example, a heterocyclic compound containing sulfur or nitrogen, a cyclic carboxylic acid ester, a fluorine-containing cyclic carbonate, and other intramolecular molecules And compounds having an unsaturated bond. From the viewpoint of extending the life of the battery, fluorine-containing cyclic carbonates and other compounds having an unsaturated bond in the molecule are preferred.

  In addition to the above additives, other additives such as an overcharge inhibitor, a negative electrode film forming agent, a positive electrode protective agent, and a high input / output agent may be used depending on the required function.

  By using the other additives, it is possible to suppress a rapid electrode reaction at the time of abnormality due to overcharge, to improve capacity maintenance characteristics after storage at high temperature, to improve cycle characteristics, and to improve input / output characteristics.

4). Separator The separator is not particularly limited as long as it has ion permeability while electronically insulating the positive electrode and the negative electrode, and has resistance to oxidation on the positive electrode side and reducibility on the negative electrode side. As a material (material) of the separator satisfying such characteristics, a resin, an inorganic material, glass fiber, or the like is used.

  As the resin, an olefin polymer, a fluorine polymer, a cellulose polymer, polyimide, nylon (polyamide), or the like is used. It is preferable to select from materials that are stable with respect to the electrolytic solution and have excellent liquid retention properties, and it is preferable to use a porous sheet or a nonwoven fabric made of a polyolefin such as polyethylene or polypropylene.

  As the inorganic substance, oxides such as alumina and silicon dioxide, nitrides such as aluminum nitride and silicon nitride, sulfates such as barium sulfate, and the like are used. For example, what made the said inorganic substance of fiber shape or particle shape adhere to thin film-shaped base materials, such as a nonwoven fabric, a woven fabric, and a microporous film, can be used as a separator. As a thin film-shaped substrate, a substrate having a pore diameter of 0.01 to 1 μm and a thickness of 5 to 50 μm is preferably used.

5. Other components The positive electrode, the negative electrode, the separator, and the electrolytic solution are accommodated inside the battery outer package. A cleavage valve may be provided in this battery outer body (battery container, outer film, etc.). By opening the cleavage valve, it is possible to suppress an increase in pressure inside the battery and to improve safety.

  Moreover, you may provide the structure part which discharge | releases inert gas (for example, carbon dioxide etc.) with a temperature rise inside a battery container. By providing such a component, when the temperature inside the battery rises, the cleavage valve can be opened quickly due to the generation of inert gas, and safety can be improved. Examples of the material used for the component include lithium carbonate, polyethylene carbonate, and polypropylene carbonate.

(Lithium ion secondary battery)
Next, a configuration example of the lithium ion secondary battery will be described.

  Here, a cylindrical lithium ion secondary battery (18650 type) will be described. FIG. 3 is a cross-sectional perspective view showing an example of the configuration of a cylindrical lithium ion secondary battery.

  As shown in FIG. 3, a columnar lithium ion secondary battery 100 includes a bottomed cylindrical battery container 106 made of steel plated with nickel. The battery case 106 accommodates an electrode group in which a strip-like positive electrode plate 101 and a negative electrode plate 103 are wound in a spiral shape with a separator 105 interposed therebetween. In the electrode group, a positive electrode plate 101 and a negative electrode plate 103 are wound in a spiral shape in cross section via a separator 105 made of a polyethylene porous sheet. For example, the separator 105 is set to have a width of 58 mm and a thickness of 30 μm. A ribbon-like positive electrode tab terminal (positive electrode current collecting tab) 102 made of aluminum and having one end fixed to the positive electrode plate 101 is led out to the upper end surface of the electrode group. The other end of the positive electrode tab terminal 102 is ultrasonically welded to the lower surface of a disk-shaped battery lid that is disposed on the upper side of the electrode group and serves as a positive electrode external terminal. Meanwhile, a ribbon-like negative electrode tab terminal (negative electrode current collecting tab) 104 made of copper and having one end fixed to the negative electrode plate 103 is led out to the lower end surface of the electrode group. The other end portion of the negative electrode tab terminal 104 is joined to the inner bottom portion of the battery container 106 by resistance welding. Accordingly, the positive electrode tab terminal 102 and the negative electrode tab terminal 104 are led out to opposite sides of both end faces of the electrode group. In addition, the insulation coating which abbreviate | omitted illustration is given to the outer peripheral surface whole periphery of an electrode group. The battery lid is caulked and fixed to the upper part of the battery container 106 via an insulating resin gasket. For this reason, the inside of the lithium ion secondary battery 100 is sealed. In addition, a non-aqueous electrolyte (not shown) is injected into the battery container 106.

  Hereinafter, the present embodiment will be described in more detail based on examples. The present invention is not limited to the following examples.

[Production of positive electrode plate]
The positive electrode plate was produced as follows. Li (Ni _1/3 Mn _1/3 Co _1/3 ) O ₂ (MX6, manufactured by Umicore Japan Co., Ltd., average particle size (d50) 6.5 μm) as a positive electrode active material, and acetylene black A as a conductive material (Denka Co., Ltd., coarse particle content 10 ppm, average particle size 48 nm) or acetylene black B (Li-400, Denka Co., Ltd., coarse particle content 0.1 ppm, average particle size 48 nm) and polyfluoride as a binder A mixture of positive electrode materials was obtained by sequentially adding and mixing vinylidene (PVDF).

  In addition, content of a positive electrode active material, a electrically conductive material, and a binder was a positive electrode active material: conductive material: binder = 90: 4.5: 5.5 by mass ratio.

Further, N-methyl-2-pyrrolidone (NMP) as a dispersion solvent was added to the mixture and kneaded to form a slurry. This slurry was applied substantially evenly and uniformly to both surfaces of a 20 μm thick aluminum foil as a positive electrode current collector. Then, the drying process was performed and it pressed so that positive electrode mixture density might be 2.55 g / cm < ³ >. The coating amount on one side of the positive electrode mixture was 115 g / m ² .

[Production of negative electrode plate]
The negative electrode plate was produced as follows. Easily graphitized carbon (hereinafter also referred to as soft carbon) was used as the negative electrode active material. Here, soft carbon having an average particle diameter (d50) of 20 μm, a specific surface area of 2.0 m ² / g, and a carbon coating amount of 1.0 mass% was used. Polyvinylidene fluoride was added as a binder to this soft carbon. These mass ratios were set to active material: binder = 92.4: 7.6. A dispersion solvent N-methyl-2-pyrrolidone (NMP) was added thereto and kneaded to form a slurry. A predetermined amount of this slurry was applied to both surfaces of a rolled copper foil having a thickness of 10 μm, which is a negative electrode current collector, substantially uniformly and uniformly. Then, the drying process was performed and it pressed so that negative electrode mixture density might be 1.15 g / cm < ³ >. The coating amount on one side of the negative electrode mixture was 72 g / m ² .

[Production of lithium-ion batteries]
A cylindrical lithium ion battery was produced as follows. First, the positive electrode plate and the negative electrode plate were cut into rectangular shapes of 445 mm × 54 mm and 480 mm × 56 mm, respectively. 45 mm and 20 mm portions of the double-sided mixture layer are removed from the ends on one side of the long side of the positive electrode plate and the negative electrode plate, respectively, and the ribbon-like positive electrode tab terminal and nickel made on the current collectors that appear after removal. The ribbon-shaped negative electrode tab terminals were joined using ultrasonic welding. Through a separator having a width of 58 mm and a thickness of 30 μm, the tabs of the positive electrode plate and the negative electrode plate were placed upside down and wound into a spiral cross section to produce a wound group. The wound group was housed in the battery container so that the negative electrode tab terminal hits the bottom side of the battery container. A rod-shaped welding electrode was passed through the central hollow portion of the wound group, and the battery bottom surface and the negative electrode tab terminal were joined by resistance welding. An insulating resin gasket was fitted into the battery container opening, and then the positive electrode tab terminal and the battery lid were joined by resistance welding. This was dried at 60 ° C. for 12 hours using a vacuum drier, and then non-aqueous electrolyte (hexafluoride into a mixed solution of ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate in a volume ratio of 2: 2: 3). 6 mL of lithium phosphate (LiPF ₆ ) dissolved to 1.2 mol / L (Mitsubishi Chemical Corporation) was added, and the battery container and the battery lid were caulked through an insulating resin gasket. Fixed. The inside of the battery is sealed by this caulking. A cylindrical lithium ion battery was produced as described above.

[Evaluation of output characteristics]
As an evaluation of the output characteristics of the lithium ion battery, the discharge rate characteristics were evaluated by the following method.

  The above lithium ion battery was first initialized / charged using a charging / discharging device (BATTERY TEST UNIT, manufactured by IEM Co., Ltd.) in a voltage range of 4.1 to 2.7 V in an environment of 25 ° C., and 0 The charge / discharge cycle with a current value of 2 C was repeated twice. The current value of 1C was determined with the discharge capacity at the second cycle at this time as the battery capacity. C means “current value (A) / battery capacity (Ah)”. Since the average value of the battery capacity of 10 batteries (5 batteries using acetylene black A and 5 batteries using acetylene black B (Li-400)) was 520 mAh, the current value of 1C was defined as 520 mA. The discharge rate characteristics shown below were measured.

First, the discharge capacity at a current value of 0.2 C and the discharge capacity at a current value of 5 C were measured.
After measuring the battery capacity, the battery is charged to 4.1 V with a current value of 0.2 C, and a constant current discharge with a final voltage of 2.7 V is performed with a current value of 0.2 C, and the capacity at the time of discharge is defined as the current value. The discharge capacity was 0.2 C. Next, the battery was charged to 4.1 V with a current value of 0.5 C, and a constant current discharge with a final voltage of 2.7 V was performed at each current value of 0.5 to 5 C. Based on the discharge capacity of 0.2 C, the discharge capacity ratio at each current value was determined. An average of five batteries was taken to determine the discharge rate characteristics of the batteries.

  The method for calculating the discharge capacity ratio is shown below.

<Calculation method of discharge capacity ratio>
Discharge capacity ratio = (discharge capacity at each current value / discharge capacity at 0.2 C) × 100

  In this test, the magnitudes of the discharge capacity ratios at 5C were compared, and the superiority or inferiority of the discharge rate characteristics of each battery was determined.

  Moreover, the direct current resistance (DCR) of the battery was obtained from the voltage drop 10 seconds after the start of discharge in the discharge of 0.2 to 5C, and the average of five batteries was taken and compared with each battery.

<DCR calculation method>
DCR = (battery voltage before the start of discharge−battery voltage 10 seconds after the start of discharge) / discharge current value

(Discharge rate characteristics, Example 1, Comparative Example 1)
FIG. 1 shows the discharge rate characteristics. In Example 1 in which 0.1 ppm of acetylene black B (Li-400) having a coarse particle content of 1 ppm or less was used as the conductive material, compared with Comparative Example 1 in which acetylene black A having a coarse particle content of 10 ppm was used, It was found that the discharge capacity ratio was improved by 1.5%.

(DCR, Example 1, Comparative Example 1)
FIG. 2 shows the DCR. In Example 1 using acetylene black B (Li-400) as the conductive material, it was found that DCR was reduced as compared with Comparative Example 1 using acetylene black A.

  From the above results, in Example 1 in which 0.1 ppm of acetylene black B (Li-400) having a coarse particle content of 1 ppm or less was used as the conductive material, Comparative Example 1 in which acetylene black A having a coarse particle content of 10 ppm was used. In comparison, it is considered that the discharge rate characteristics are improved by reducing the DCR. In addition, it is thought that the factor by which DCR was reduced in Example 1 using acetylene black B (Li-400) was because the conductive network in the electrode was improved by reducing the coarse particle content to 0.1 ppm. .

DESCRIPTION OF SYMBOLS 100 ... Cylindrical lithium ion secondary battery 101 ... Positive electrode plate 102 ... Positive electrode tab terminal 103 ... Negative electrode plate 104 ... Negative electrode tab terminal 105 ... Separator 106 ... Battery container

Claims (3)

A lithium ion battery comprising a positive electrode, a negative electrode, and an electrolyte solution,
The positive electrode has a current collector and a positive electrode mixture formed on the current collector,
The positive electrode mixture has a positive electrode active material and a conductive material,
The conductive material is a lithium ion battery containing acetylene black having a coarse particle content of 1 ppm or less.   The lithium ion battery according to claim 1, wherein the positive electrode active material is a layered lithium / nickel / manganese / cobalt composite oxide.   The lithium ion battery according to claim 1 or 2, wherein the average particle diameter of acetylene black is 20 to 100 nm.