JP6201294B2 - Negative electrode of non-aqueous electrolyte secondary battery - Google Patents

Description

  The present invention relates to a negative electrode for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery using the same.

  The electrode mixture layer of the negative electrode of a non-aqueous electrolyte secondary battery such as a lithium ion battery is formed by applying an electrode mixture paste to a current collector, drying, and then compressing to a predetermined packing density after drying Is done. The electrode mixture paste is made by adding and mixing a powdery active material with a binder (hereinafter sometimes referred to as a binder) and a conductive agent.

  Here, as the binder, a particulate polymer can be used in order to prevent separation of the active material during production and use. Further, as the particulate polymer, those having a particle size distribution can be used in order to achieve close contact between the active materials or between the active material and the current collector.

  Patent Document 1 discloses a binder composed of a particulate polymer having an average particle size of 0.01 to 10 μm as a particle size distribution and a negative electrode using the binder. In addition, the binder is not crushed even when pressed, and the amount of addition is also specified. It is known that the surface of the electrode mixture layer can be completely covered with the binder by using the binder.

JP 2011-198548 A

The negative electrode of Patent Document 1 is excellent in that peeling of the mixture is suppressed and charge / discharge characteristics are improved. However, depending on the relationship between the particle size of the negative electrode active material and the particle size of the binder, the peel strength decreases and the resistance increases.
The present invention reduces the reaction resistance of a non-aqueous electrolyte secondary battery while maintaining the peel strength of the negative electrode of the non-aqueous electrolyte secondary battery even when such a particulate polymer is used as a binder for the negative electrode active material. It is intended.

  The negative electrode of the nonaqueous electrolyte secondary battery of the present invention includes a negative electrode active material and a binder, and the negative electrode active material has a volume-based cumulative 50% particle size of 8.8 to 12.7 μm, The volume-based cumulative 50% particle size is 80 to 130 nm, and the volume-based cumulative 50% particle size ratio of the negative electrode active material and the binder is 67.7 to 195.8.

  The ratio of the cumulative 50% particle size is preferably 79.4 to 158.8, particularly preferably 97.7 to 133.7. It is preferable that a film containing boron is formed on the surface of the negative electrode.

  The negative electrode active material preferably has an oil absorption of 50 to 66 ml / 100 g when the oil absorption of linseed oil is measured. The negative electrode active material preferably has a volume-based cumulative 10% particle size of 3.4 to 6.3 μm and a volume-based cumulative 90% particle size of 25.8 μm or less.

  It is particularly preferable that the cumulative 90% particle size based on the volume is 21.0 μm or less. The negative electrode active material is preferably made of graphite having an amorphous carbon film. The binder is preferably made of a modified styrene-butadiene copolymer latex.

  The nonaqueous electrolyte secondary battery of the present invention includes a positive electrode having a positive electrode active material, the negative electrode, and an electrolyte. The positive electrode active material preferably has a volume-based cumulative 50% particle size of 3.8 to 5.2 μm, and an oil absorption of 31 to 50 ml / 100 g when the oil absorption of linseed oil is measured. The non-aqueous electrolyte secondary battery is preferably a lithium ion secondary battery.

  ADVANTAGE OF THE INVENTION According to this invention, the reaction resistance of a nonaqueous electrolyte secondary battery can be reduced, maintaining the peeling strength of the negative electrode of a nonaqueous electrolyte secondary battery.

It is a schematic diagram showing the relationship between an active material particle size and a binder particle size. It is a schematic diagram showing the relationship between oil absorption amount and a lithium ion path | route. It is a graph which shows the relationship between the particle size ratio of a negative electrode active material and a binder, and peeling strength. It is a graph which shows the relationship between the particle size ratio of a negative electrode active material and a binder, and reaction resistance. It is a graph which shows the relationship between the particle size of a binder, and the difference of the reaction resistance by the presence or absence of the boron of a SEI membrane | film | coat.

  The non-aqueous electrolyte secondary battery of the present embodiment includes a positive electrode, a negative electrode, and a non-aqueous electrolyte. The nonaqueous electrolyte secondary battery is preferably a lithium ion secondary battery. Hereinafter, although the structure in the case of manufacturing as a lithium ion secondary battery is shown below, this invention is not limited to this.

<Configuration of negative electrode>
The negative electrode of the nonaqueous electrolyte secondary battery of the present embodiment is obtained by binding a negative electrode active material with a binder (hereinafter also referred to as a binder) and laminating the negative electrode current collector. The negative electrode is preferably formed as an electrode plate by applying an electrode mixture paste (slurry) to the negative electrode current collector, drying it, and pressing it to a predetermined packing density after drying.

  The electrode mixture paste is preferably a mixture of a powdered negative electrode active material and a binder added to a dispersant or solvent. The property of the electrode mixture paste may be a slurry. The paste is preferably filtered through a filter in order to remove foreign substances or aggregates of particles.

<Material of negative electrode active material>
A carbon material is preferably used as the negative electrode active material because it can occlude and release lithium ions and the like. As the core material of the carbon material, it is preferable to use graphite, and it is more preferable to use natural graphite because the discharge potential is flat, the true density is high, and the filling property is good. Moreover, it is preferable to use particulate graphite as the shape of the graphite. The particulate graphite is preferably obtained by spheroidizing a flaky natural graphite into a roughly spherical shape.

<Amorphous coat>
Graphite is preferably coated with a carbon film, and the carbon film is an amorphous carbon film made of amorphous carbon (hereinafter sometimes referred to as an amorphous coating) in order to suppress reactivity with the electrolyte. Preferably there is. In the present invention, amorphous carbon refers to carbon other than graphite, and refers to carbon having no crystallinity or lower crystallinity than graphite.

The negative electrode active material is preferably amorphous coated with a predetermined amount of amorphous carbon coating (hereinafter sometimes referred to as coating amount). In the present specification, the coating amount is represented by the amount (%) occupied by the mass part of the coating with respect to 100 parts by mass of the negative electrode active material.
When the coating amount is too small, the effect of suppressing the reactivity with the electrolyte is not sufficiently exhibited. On the other hand, if the amount of coating is excessive, the coating becomes too thick and the initial resistance may increase. The predetermined coating amount is preferably 2 to 7 parts by mass with respect to 100 parts by mass of the negative electrode active material.

<Particle size ratio of negative electrode active material and binder>
In the negative electrode, when a particulate polymer having an average particle size of 0.01 to 10 μm as the particle size distribution is used as the binder, when the amount of the binder used is reduced for the purpose of reducing the resistance inside the negative electrode, The peel strength between the electrode mixture layer and the current collector is reduced.

  In order to achieve close contact between the negative electrode active materials or between the negative electrode active material and the current collector, the negative electrode active material and the binder are preferably in the form of particles having a predetermined particle size distribution. In particular, the average particle size of the negative electrode active material and the binder, that is, the ratio of 50% cumulative particle size based on volume (hereinafter sometimes referred to as particle size ratio) is preferably within a predetermined range from the viewpoint of maintaining peel strength. . The predetermined range of the particle size ratio is preferably 67.7 to 195.8, more preferably 79.4 to 158.8, and particularly preferably 97.7 to 133.7.

  FIG. 1A shows a case where the particle size of the binder 2 is appropriate with respect to the particle size of the negative electrode active material 1 and the negative electrode active materials are firmly bound to each other. On the other hand, in the case of FIG. 1B, since the particle size of the binder 2 is smaller than the appropriate particle size, the binding between the negative electrode active materials is weakened. In the case of FIG. 1C, since the particle size of the binder 2 is larger than an appropriate particle size, the binding between the negative electrode active materials is weakened.

  When the particle size of the negative electrode active material is arbitrarily selected in order to obtain preferable electric characteristics of the negative electrode, the peel strength may be lowered. Therefore, by adjusting the particle size of the binder so that the particle size ratio falls within the predetermined range, it is possible to prevent a decrease in peel strength.

<Particle size distribution of negative electrode active material>
In the negative electrode in which the particle size ratio between the negative electrode active material and the binder is in the predetermined range, the negative electrode active material preferably has a predetermined particle size distribution. Moreover, the negative electrode active material having a desired particle size can be evaluated and selected based on the particle size distribution. As an example of the particle size distribution measurement, it can be measured by a laser diffraction particle size distribution measuring apparatus.

  As the predetermined particle size distribution, the volume-based cumulative 50% particle size (hereinafter abbreviated as D50) of the negative electrode active material is preferably 8.6 to 12.7 μm. This is because the non-aqueous electrolyte secondary battery of the present embodiment retains the battery characteristics even after repeated use at room temperature because the particle size distribution is in this range.

  Furthermore, in order to reduce the reaction resistance of the negative electrode, D50 is preferably 8.8 to 12.7 μm as the predetermined particle size distribution. Further, from the viewpoint of securing peel strength, it is particularly preferable that D50 is 12.7 μm as the predetermined range. In the present embodiment, the peel strength represents an average load per unit width necessary for peeling between the electrode mixture layer and the current collector.

  As the predetermined particle size distribution, the volume-based cumulative 10% particle size (hereinafter abbreviated as D10) of the negative electrode active material is preferably 3.4 μm or more, and more preferably 4.2 μm or more. This is because when the particle size distribution is in such a range, it is easy to pass the paste through a filter, that is, filter permeability is sufficiently ensured. In order to reduce the electrical resistance of the electrode plate, D10 is preferably 6.3 μm or less, and more preferably 5.5 μm or less, as the predetermined particle size distribution.

  As the predetermined particle size distribution, the volume-based cumulative 90% particle size (hereinafter abbreviated as D90) of the negative electrode active material is preferably 25.8 μm or less, and particularly preferably 21.0 μm or less. preferable. Since the particle size distribution is in such a range, the non-aqueous electrolyte secondary battery of the present embodiment is unlikely to deteriorate in battery characteristics even after repeated use of SOC (State of Charge) 0-100% at room temperature. It is.

<Structural characteristics of negative electrode active material>
In the negative electrode in which the negative electrode active material has the predetermined particle size distribution, the negative electrode active material preferably has a predetermined structural characteristic. The degree of the predetermined structural feature is that when the oil absorption amount of linseed oil (hereinafter sometimes simply referred to as oil absorption amount) is measured, the oil absorption amount Ab of the negative electrode active material is 50 to 66 ml / 100 g. Is preferable, and it is especially preferable that it is 50-52 ml / 100g.

  FIG. 2A shows a case where the oil absorption amount of the negative electrode active material (active material 3) and the size of the space 4 in the active material 3 are appropriate, and an external space that can take the lithium ion path 5 is secured. Has been. On the other hand, the case of FIG. 2B shows a state where there are many spaces 4 in the active material 3, that is, a state where the oil absorption amount of the active material 3 is excessive. Space is under pressure.

  On the other hand, if the amount of oil absorption of the negative electrode active material is too small, after the negative electrode active material repeatedly expands and contracts, the liquid retaining property cannot be ensured, and the durability of the battery decreases due to a decrease in the capacity retention rate of the battery. The higher the capacity retention rate, the smaller the decrease in the battery capacity after repeated charge / discharge. It also indicates that the battery characteristics after repeated use are maintained.

  When the particle size distribution and the oil absorption amount are in the above ranges, the non-aqueous electrolyte secondary battery of the present embodiment retains battery characteristics even after repeated use at room temperature. The evaluation method of the oil absorption amount Ab will be described later.

<Binder material>
In the negative electrode in which the particle size ratio of the negative electrode active material and the binder is in the predetermined range, and the negative electrode active material has the predetermined particle size distribution, the binder (hereinafter sometimes referred to as a binder) is in the form of particles. It is preferable to contain a polymer. As the polymer, styrene-butadiene rubber (hereinafter sometimes referred to as styrene-butadiene copolymer latex) or modified styrene-butadiene copolymer latex is preferable. Hereinafter, it may be collectively referred to as SBR.

<Binder particle size distribution>
Further, the particulate polymer of the binder preferably has a predetermined particle size distribution. Further, binder particles having a desired particle size can be evaluated and selected based on the particle size distribution. As an example of the particle size distribution measurement, it can be measured by a laser diffraction particle size distribution measuring apparatus.

  The predetermined particle size distribution preferably has an average particle size, that is, a volume-based cumulative 50% particle size (hereinafter abbreviated as d) of 80 to 130 nm. This is because the reaction resistance of the nonaqueous electrolyte secondary battery of the present embodiment is reduced when the particle size ratio and the particle size distribution of the negative electrode active material and the binder are in such ranges.

<Configuration of positive electrode>
The positive electrode of the nonaqueous electrolyte secondary battery of the present embodiment is obtained by binding a positive electrode active material with a binder and laminating the positive electrode current collector.

<Particle size distribution of positive electrode active material>
In the non-aqueous electrolyte secondary battery in which the negative electrode active material has the predetermined particle size distribution and structural characteristics, the positive electrode active material preferably has the predetermined particle size distribution. Moreover, the positive electrode active material having a desirable particle size can be evaluated and selected based on the particle size distribution. As an example of the particle size distribution measurement, it can be measured by a laser diffraction particle size distribution measuring apparatus.

  As the predetermined particle size distribution, the volume-based cumulative 50% particle size (hereinafter abbreviated as cD50) of the positive electrode active material is preferably 3.8 to 5.2 μm. The particle size distribution of each of the negative electrode and positive electrode active materials and the oil absorption amount Ab of the negative electrode active material are within such a range that the nonaqueous electrolyte secondary battery of the present embodiment can be used at low temperatures. This is because the battery characteristics are maintained.

<Structural characteristics of positive electrode active material>
In the non-aqueous electrolyte secondary battery in which the negative electrode active material has the predetermined particle size distribution and structural characteristics, it is preferable that the positive electrode active material has the predetermined structural characteristics. As the degree of the predetermined structural feature, the oil absorption amount cAb of the positive electrode active material is preferably 50 to 66 ml / 100 g. Since the particle size distribution and the oil absorption amount of each active material of the positive electrode and the negative electrode are within such ranges, the non-aqueous electrolyte secondary battery of the present embodiment retains battery characteristics even after repeated use at low temperatures. It is.

<Evaluation method of oil absorption>
The oil absorption amount of the positive electrode and negative electrode active materials in the nonaqueous electrolyte secondary battery of the present embodiment will be described in more detail. The positive electrode and negative electrode active materials for the nonaqueous electrolyte secondary battery of the present embodiment, which have desirable structural characteristics, can be evaluated and selected based on the oil absorption.
The amount of oil absorption varies greatly depending on the structural characteristics of the powder, but the influence of the size and shape of the particles is particularly large. It is considered big.

  As an example of oil absorption measurement, linseed oil is added to the powder little by little, and the state of the powder is confirmed while kneading. There is a thing which makes oil absorption amount. As a measuring method, it can carry out based on A method prescribed | regulated to JISK-6217 (test method of the basic performance of carbon black for rubber | gum), for example.

<Manufacture of negative electrode active material>
The negative electrode active material of the present embodiment is a carbon material such as pitch produced using oil such as heavy oil, polymer such as polyvinyl alcohol (PVA), coal or petroleum as a raw material on the surface of particulate graphite. And a coating material containing an additive such as a solvent, if necessary, can be produced by coating by a vapor phase method or a liquid phase method and firing in an inert atmosphere.

Examples of the vapor phase method include a CVD (Chemical Vapor Deposition) method. Examples of the inert atmosphere at the time of firing include an N ₂ atmosphere, a rare gas atmosphere such as Ar, and combinations thereof. Firing is preferably performed at 800 ° C. or lower. By such firing, a negative electrode active material having a desired amorphous coat can be obtained.

<Adjustment of coating amount of negative electrode active material>
In the present embodiment, the coating amount represents a part by mass of the coating film per 100 parts by mass of the negative electrode active material. A desired coating amount can be obtained by appropriately adjusting the amount of the coating material used when forming the coating.

  When a large amount of amorphous carbon is added to form a thick film, the amorphous carbon does not cover the graphite, and a lump is formed only with non-graphitic carbon. The battery characteristics may not be obtained. The gas phase method is preferred as the coating method because there is little risk of forming a lump consisting only of non-graphitic carbon and an amorphous coat can be formed uniformly.

<Manufacture of negative electrode>
The negative electrode of the present embodiment can be manufactured by applying the negative electrode active material for the nonaqueous electrolyte secondary battery to the negative electrode current collector. In the present specification, the negative electrode may refer to a negative electrode. When applying the negative electrode active material to the negative electrode current collector, it is preferable to apply a paste that can be obtained by adding the negative electrode active material and the binder to a dispersant (solvent) and mixing them. It is preferable to further mix a thickener with the paste. After coating, it is preferable to dry and press.

  As the negative electrode current collector, a metal foil is preferable, and a copper foil is more preferable. The dispersant is preferably water. As the binder, polyvinylidene fluoride (PVDF) is preferable. As the thickener, carboxymethyl cellulose Na salt (CMC) is preferable.

  A negative electrode active material other than the above negative electrode active material can be used in combination with the negative electrode of the present embodiment. There is no restriction | limiting in particular as a negative electrode active material which can be used together, What has a lithium occlusion ability to 2.0V or less on the basis of Li / Li + is used preferably. As the negative electrode active material to be used in combination, lithium metal, lithium alloy, transition metal oxide / transition metal nitride / transition metal sulfide capable of doping / dedoping lithium ions, and combinations thereof are preferable. .

<Production of positive electrode>
The positive electrode is preferably manufactured by applying a positive electrode active material to a positive electrode current collector. When applying the positive electrode active material to the positive electrode current collector, it is preferable to apply a paste that can be obtained by adding a positive electrode active material, a conductive material, and a binder to a dispersant (solvent) and mixing them. After coating, it is preferable to dry and press.

The positive electrode current collector is preferably a metal foil, and more preferably an aluminum foil. Examples of the positive electrode active material include LiCoO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , LiNiO ₂ , LiNi _x Co _(1-x) O ₂ , and LiNi _x Co _y Mn _(1-xy) O _2. A lithium-containing composite oxide is preferred. As the dispersant, N-methyl-2-pyrrolidone is preferable. Carbon powder is preferable as the conductive material. As the binder, polyvinylidene fluoride (PVDF) is preferable.

<Composition of non-aqueous electrolyte>
The non-aqueous electrolyte is preferably a liquid, gel or solid, and more preferably a non-aqueous electrolyte in which a lithium-containing electrolyte is dissolved in a mixed solvent of two or more carbonate solvents. The mixed solvent is preferably a mixture of a high dielectric constant carbonate solvent and a low viscosity carbonate solvent.

  As the high dielectric constant carbonate solvent, propylene carbonate, ethylene carbonate or a combination thereof is preferable. As the low viscosity carbonate solvent, diethyl carbonate, methyl ethyl carbonate, dimethyl carbonate or a combination thereof is preferable. As a combination of two or more carbonate solvents, ethylene carbonate (EC) / dimethyl carbonate (DMC) / ethyl methyl carbonate (EMC) is particularly preferable.

The lithium-containing electrolyte is preferably a lithium salt. Examples of the lithium salt include LiBOB (lithium bisoxalate borate), LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , Li ₂ SiF ₆ , LiOSO ₂ C _k F _{(2k + 1)} (k = 1 to 8), or LiPF _n {C _k F _{(2k + 1)} } _(6-n) (n = 1 to 5 integer, k = 1 to 8 integer), or a combination thereof is preferable.

LiBF ₄ or LiBOB, which is a salt containing boron, is preferable because the battery characteristics after a stable SEI (Solid Electrolyte Interface) film is formed and used repeatedly are retained. When the negative electrode according to the present embodiment is combined with LiBOB, which is an electrolyte, the decrease in battery output is reduced, so LiBOB is particularly preferable.

<Composition of separator>
As the separator, a porous polymer film is preferable because it electrically insulates the positive electrode and the negative electrode and allows lithium ions to pass therethrough. The film is preferably a polyolefin porous film, particularly preferably a PP (polypropylene) porous film, a PE (polyethylene) porous film, or a laminated porous film of PP (polypropylene) -PE (polyethylene). .

<Case>
As the case, it is preferable to use a case suitable for the type of the secondary battery. As the secondary battery type, a cylindrical type, a coin type, a square type, or a film type is preferable.
The nonaqueous electrolyte secondary battery of the present embodiment can be mounted on a transport machine such as an electric vehicle (EV) or a plug-in hybrid vehicle (PHV) and used as a drive power source. Note that the present invention is not limited to the above-described embodiment, and can be changed as appropriate without departing from the spirit of the present invention.

1. Production of negative electrode active material <graphite>
In each of the examples and comparative examples, natural graphite having an average particle diameter (nominal diameter) of 6 to 25 μm was used as the particulate graphite. The surface of the particulate graphite was coated by a CVD method using pitch as a coating material, and baked in an N ₂ atmosphere to produce a negative electrode active material. Selection of the negative electrode active material having a predetermined particle size distribution will be described later.

2. Battery manufacturing <Negative electrode>
In Examples and Comparative Examples, water was used as a dispersant, and the negative electrode active material, a binder, and a thickener were mixed at 98/1/1 (mass ratio) to obtain a negative electrode mixture paste. . Modified styrene-butadiene copolymer latex (SBR) particles were used as the binder. Selection of the binder having a predetermined particle size distribution will be described later.

The paste was applied to both sides of a copper foil as a current collector by a doctor blade method, dried at 150 ° C. for 30 minutes, and pressed using a press machine to form an electrode layer. A negative electrode was obtained as described above. The negative electrode layer had a basis weight of 18 mg / cm ² and a density of 1.4 g / cm ³ per side.

<Positive electrode>
As the positive electrode active material, particles of LiNi _1/3 Co _1/3 Mn _1/3 O ₂ were used. Selection of the positive electrode active material having a predetermined particle size distribution will be described later. Using N-methyl-2-pyrrolidone as a dispersant, mixing the positive electrode active material, acetylene black as a conductive agent, and PVDF as a binder at 93/4/3 (mass ratio), A positive electrode mixture paste was obtained.

The positive electrode mixture paste was applied to both surfaces of an aluminum foil as a current collector by a doctor blade method, dried at 150 ° C. for 30 minutes, and pressed using a press machine to form an electrode layer. As described above, a positive electrode was obtained. The positive electrode layer had a basis weight of 30 mg / cm ² and a density of 2.8 g / cm ³ per side.

<Nonaqueous electrolyte>
A mixed solution of ethylene carbonate (EC) / dimethyl carbonate (DMC) / ethyl methyl carbonate (EMC) = 3/4/3 (volume ratio) was used as a solvent, and the following lithium salt was used as an electrolyte at a concentration of 1.1 mol / L. It melt | dissolved and the nonaqueous electric field liquid was prepared. The lithium salts of Examples 1-12, 17-37, 97-100, 106-113, and 137-152 and Comparative Examples 1-87, 95, 96, 101-105, 114-136, and 153-175 are LiPF ₆ did. The lithium salts of Examples 13 to 16 and Comparative Examples 88 to 94 were LiBOB.

<Package>
The separator composed of the positive electrode, the negative electrode, the nonaqueous electrolyte, and the PE (polyethylene) porous film was packaged in a cylindrical case to obtain a lithium ion secondary battery.

3. Battery member and battery characteristic evaluation <particle size of negative electrode active material>
About the negative electrode active material of each Example and a comparative example, the particle size distribution was evaluated and selected. Using a laser diffraction particle size distribution analyzer SALD-2300 as an apparatus, D50, D10, and D90 were determined.
D50 of Examples 1-12 and Comparative Examples 1-87 was 6.8-24.3 micrometers, and it showed to Tables 1-3 mentioned later. D50 of Examples 13-16 is the same as that of Examples 9-12. Moreover, D50 of Comparative Examples 88-94 is the same as that of Comparative Examples 26-32.

The D50 of Examples 17 to 22 and 97 to 100 and Comparative Examples 95, 96 and 101 to 105 were 6.8 to 15.6 μm, and are shown in Table 6 described later. In Examples 23 to 26 and 106 to 109 , D10 was 1.7 to 7.5 μm, and D50 was 8.9 to 12.5 μm, which are shown in Table 7 described later. In Examples 27 to 30 and 110 to 113 , D90 was set to 16.7 to 48.9 μm, and D50 was set to 9.2 to 12.3 μm, which are shown in Table 8 described later.

<Particle size of positive electrode active material>
About the positive electrode active material of each Example and the comparative example, the particle size distribution was evaluated and selected. CD50 was calculated | required using laser diffraction type particle size distribution measuring device SALD-2300 as an apparatus. The cD50 of Examples 1 to 30, 97 to 100, 106 to 113 and Comparative Examples 1 to 96 and 101 to 105 was 6 μm. The cD50 of Examples 31 to 37 and 137 to 152 and Comparative Examples 114 to 136 and 153 to 175 were 2.6 to 8.3 μm, and are shown in Table 9 described later.

<Binder particle size>
The SBR particles, which are binders of the examples and comparative examples, were selected by evaluating the particle size distribution. A laser diffraction particle size distribution analyzer SALD-2300 was used as an apparatus, and the SBR particle size (binder average particle size) d was determined. D of Examples 1-12 and Comparative Examples 1-87 was 0.050-0.270 micrometers, and it showed to Tables 1-3 mentioned later. D of Examples 13-16 is the same as that of Examples 1-4. Moreover, d of Comparative Examples 88-94 is the same as that of Comparative Examples 26-32. Examples 17 to 37, 97 to 100, 106 to 113, and 137 to 152 and Comparative Examples 95 , 96, 101 to 105, 114 to 136, and 153 to 175 d were set to 0.095 μm.

<Oil absorption amount of negative electrode active material>
The oil absorption amount Ab is measured using linseed oil with an oil absorption measuring device S410 manufactured by Asahi Soken Co., Ltd., in accordance with Method A defined in JIS K-6217 (Testing method for basic performance of carbon black for rubber). did. About the negative electrode active material of each Example and a comparative example, oil absorption amount Ab was evaluated and selected.

Oil absorption amount Ab of Examples 1-16 , 23-30, 106-113, and Comparative Examples 1-94 was 50-62 ml / 100g. The oil absorption amounts Ab of Examples 17 to 22 and 97 to 100 and Comparative Examples 95, 96 and 101 to 105 were 43.0 to 78.0 ml / 100 g, and are shown in Table 6 described later. The oil absorption amounts Ab of Examples 31 to 37 and 137 to 152 and Comparative Examples 114 to 136 and 153 to 175 were 56.0 to 58.0 ml / 100 g, and are shown in Table 9 described later.

<Oil absorption amount of positive electrode active material>
About the positive electrode active material of each Example and the comparative example, oil absorption amount cAb was evaluated similarly to the negative electrode active material, and was selected. The oil absorption amount cAb of Examples 1 to 30 , 97 to 100 and 106 to 113 and Comparative Examples 1 to 96 and 101 to 105 were set to 36 to 40 ml / 100 g. The oil absorption amounts cAb of Examples 31 to 37 and 137 to 152 and Comparative Examples 114 to 136 and 153 to 175 were 18.0 to 54.0 ml / 100 g, and are shown in Table 9 described later.

<Filter permeability>
For Examples 23 to 26 and 106 to 109 , filter permeation tests were performed after preparing pastes containing a negative electrode active material. In the test, the filter permeability of each paste was measured under the following measurement conditions.
300 ml of the paste was filtered with a filter having a pore size of 50 μm, and it was determined whether or not the entire amount could pass through the filter. As shown in the filter permeability of Table 7 which will be described later, the results are shown as ◯ for those that permeate and as x for those that do not.

<Peel strength>
The peel strength was measured using a tensile compression tester SV-201NA-50SL manufactured by Imada Manufacturing Co., Ltd. The relationship between the particle size ratio (D50 / d) of the negative electrode active material and the binder and the peel strength of the negative electrode in Examples 1 to 12 and Comparative Examples 1 to 87 is shown in FIG.

  As shown in FIG. 3 and Tables 1 to 3, the lithium ion secondary batteries of Examples 1 to 12 in which D50 / d is included in the range of 67.7 to 195.8 are comparative examples 12 when D50 is the same. There was a tendency for the peel strength to be higher than -32. The same tendency was seen also in Comparative Examples 1-11 and 33-87.

  Since the negative electrodes of Examples 13 to 16 and Comparative Examples 88 to 94 are the same as the negative electrodes of Examples 9 to 12 and Comparative Examples 26 to 32, the peel strength is as shown in Table 4 described later. From the above, it was found that when the particle size distribution of the negative electrode active material and the binder is in such a range, the peel strength of the negative electrode is increased and the durability of the battery is improved.

<Measurement of low-temperature reaction resistance>
The low temperature reaction resistance was measured as follows. First, the lithium secondary battery after the conditioning treatment was adjusted to a SOC (State of Charge) 60% charge state. Thereafter, the electrical resistance was measured by an AC impedance method at a frequency of 10 mHz to 1 MHz under a temperature condition of −30 ° C.

(1) Evaluation of Influence of Particle Size Distribution of Negative Electrode Active Material and Binder FIG. 4 shows the relationship between D50 / d and −30 ° C. reaction resistance of lithium ion secondary batteries in Examples 1 to 12 and Comparative Examples 1 to 87. And shown in Tables 1-3. In the lithium ion secondary batteries of Examples 1 to 12, D50 / d is included in the range of 67.7 to 195.8, and the average particle diameter D50 of the negative electrode active material is in the range of 8.8 to 12.7 μm. And the average particle diameter d of the binder is included in the range of 0.080 to 0.130 μm.

  As shown in FIG. 4 and Tables 1 to 3, the lithium ion secondary batteries of Examples 1 to 12 showed a tendency for the reaction resistance to be smaller than those of Comparative Examples 1 to 87, which was less than 500 mΩ. From this, it was found that when the particle size distribution of the negative electrode active material and the binder is in such a range, the reaction resistance of the nonaqueous electrolyte secondary battery including the negative electrode is reduced, and the battery performance is improved.

(2) Evaluation of influence of boron in SEI film In Examples 9 to 16 and Comparative Examples 26 to 32 and 88 to 94, the relationship between presence / absence of boron in the SEI film, -30 ° C reaction resistance and capacity retention rate It shows in FIG. Details of the capacity maintenance rate measurement will be described later. In FIG. 5, the amount of increase in resistance according to Examples 13 to 16 is surrounded by a broken line.

  As shown in FIG. 5 and Table 4, the lithium ion secondary batteries of Examples 13 to 16 and Comparative Examples 88 to 94 containing boron in the SEI film were more reactive than Examples 9 to 12 and Comparative Examples 1 to 87. While the resistance increases, the capacity retention rate is improved. This indicates that the use of LiBOB as the electrolyte resulted in boron being contained in the SEI film, so that the negative electrode was firmly covered.

  It will be described below that an increase in reaction resistance of the SEI film due to boron is suppressed by the negative electrode active material and the binder having a predetermined particle size distribution. As shown in FIG. 5 and Table 4, in the lithium ion secondary batteries of Examples 13 to 16, D50 / d is included in the range of 67.7 to 195.8, D50 is about 12.7 μm, and d is included in the range of 0.080 to 0.130 μm.

  In each of the lithium ion secondary batteries of Examples 13 to 16 in which boron is contained in the SEI film, an increase in reaction resistance with respect to Examples 9 to 12 is suppressed, and the increase in resistance is 50 mΩ or less. On the other hand, each of the lithium ion secondary batteries of Comparative Examples 88 to 94 has an increase in reaction resistance with respect to Comparative Examples 26 to 32 of 60 mΩ or more.

When the particle size of the binder is smaller than an appropriate particle size with respect to the particle size of the negative electrode active material, the binder is too uniformly applied to the active material, and further, an SEI film containing boron is formed thereon. It is thought that the resistance increased greatly.
When the particle size of the binder is larger than the appropriate particle size with respect to the particle size of the negative electrode active material, since the thickness of the film formed by the binder is large, an SEI film further containing boron is originally formed in a place where the electric resistance is large. It is considered that the resistance is greatly increased.

  From the above, when the particle size distribution of the negative electrode active material and the binder is in such a range, even if an SEI film containing boron is formed on the negative electrode, the increase in reaction resistance of the nonaqueous electrolyte secondary battery is reduced. It was found that the output and durability of the battery can be improved at the same time.

<Measurement 1 of capacity maintenance ratio>
After the battery was manufactured, the battery was charged at a constant current up to 4.1 V at a charge rate of 1 C, and then subjected to a conditioning process by charging at a constant voltage for 2 hours. Thereafter, the battery was discharged to 3 V at a discharge rate of 0.3 C, and the discharge capacity at this time was defined as the initial battery capacity.

  About Example 9-16 of Table 4, and Comparative Examples 26-32 and 88-94, after charging to SOC80%, it stored at 60 degreeC for 60 days. After the storage period, the battery was charged at a constant current of up to 4.1 V at a charge rate of 1/3 C, and then discharged to 3 V at a discharge rate of 1/3 C. The discharge capacity at this time was defined as the post-test battery capacity.

For Examples 17-22, 27-37 , 97-100 , 110-113 and 137-152 and Comparative Examples 95, 96 , 101-105 , 114-136 and 153-175 listed in Tables 6, 8 and 9 After performing a charge / discharge test on each sample, the battery capacity after the test was determined. Table 5 shows the number, temperature, cycle (pulse), and number of cycles executed for each charge / discharge test. After the test, the battery capacity was measured at room temperature. Each battery was set to SOC = 100%, then discharged to 3 V at a discharge rate of 0.3 C, and the discharge capacity at this time was determined.

The capacity retention rate (%) was determined using the following formula.

Capacity maintenance rate (%) = (battery capacity after test / initial battery capacity) × 100

In general, if the capacity retention rate exceeds 98%, there is no visible Li deposition, and therefore it is estimated that the battery characteristics are maintained even after repeated use.

(1) Evaluation of influence of average particle size and oil absorption of negative electrode active material
About the lithium ion secondary battery of Examples 17-22 and 97-100 and Comparative Examples 95, 96, and 101-105, after repeating charging / discharging by the cycle of the test number 1, the capacity | capacitance maintenance factor was measured. Table 6 shows the relationship between D50 of the negative electrode active material and the oil absorption amount Ab, and the capacity retention rate of the lithium ion secondary battery.

As shown in Table 6, the negative electrode active material D50 was included in the range of 8.6 to 12.7 μm, and the oil absorption amount Ab was included in the range of 50 to 66 ml / 100 g. The secondary batteries had a higher capacity retention rate than Examples 97 to 100 and Comparative Examples 95, 96, and 101 to 105 , and were 98% or more.

  Unlike the low-temperature test in which the reactivity of the active material surface is rate-limiting, in the 25 ° C. pulse test according to test number 1, the diffusion resistance of lithium ions is the rate-limiting rate of the battery reaction. In other words, there is a problem of how to ensure a path through which lithium ions pass and smoothly penetrate into the electrode plate.

  The average particle size is considered to be related to the diffusion distance of lithium ions. When the particle size is larger than an appropriate size, the diffusion distance becomes long. For this reason, unevenness occurs in the concentration distribution of lithium ions, leading to a decrease in capacity retention rate. Further, when the particle size is smaller than an appropriate size, the spheroidization treatment is insufficient, and it is considered that the scale-like negative electrode active material is increasing. In this case, it is considered that the diffusion distance is longer because the lithium ion path is more complicated than the spherical particle.

  The oil absorption amount Ab is considered to be related to the volume of the pores in the active material. When the oil absorption amount Ab is larger than the appropriate range, the pores in the active material are large, and as a compensation, the diffusion path outside the active material is restricted, the diffusion distance becomes long, and the capacity retention rate is reduced. Is as described above. Further, when the oil absorption amount Ab is smaller than the appropriate range, the liquid retention of the electrode plate after the active material expands and contracts decreases, so that the capacity retention rate is lowered as described above.

  From the above, a battery in which the particle size distribution and the oil absorption amount of the negative electrode active material are in such a range can maintain the battery characteristics even after repeated use, and a high level required for a battery mounted as a driving power source for automobiles and the like. It was found that durability could be secured. The test results of test numbers 2 and 3 will be described later.

<Plate resistance and filter permeability>
The negative electrodes of Examples 23 to 26 and 106 to 109 were sandwiched between test copper plates, and the penetration resistance between the test copper plates was measured with a DC milliohm meter to determine the electrode plate resistance. Further, the filter permeability was evaluated as described above. Table 7 shows the relationship between D10 and D50 of the negative electrode active material, the filter permeability of the negative electrode mixture paste, and the electrode plate resistance of the negative electrode.

As shown in Table 7, the negative electrodes of Examples 23 to 26 and 106 to 108 in which D10 of the negative electrode active material is included in the range of 6.3 μm or less and D50 is included in the range of 8.6 to 12.7 μm. The electrode plate resistance was lower than that of Example 109 , which was 3.0 mΩ or less.

A negative electrode mixture for producing negative electrodes of Examples 23 to 26 and 109 in which D10 of the negative electrode active material is included in the range of 3.4 μm or more and D50 is included in the range of 8.6 to 12.7 μm. The paste had preferable filter permeability as compared with the negative electrode mixture pastes of Examples 106 to 108 .

  When D10 is smaller than the appropriate range, the effect of dilatancy cannot be ignored and the filter permeability is considered to have deteriorated. Moreover, when D10 is larger than an appropriate range, it is considered that the contact between the active materials and between the active material and the current collector decreases, and the electrical resistance increases.

  From the above, it was found that a battery having a negative electrode in which the particle size distribution of the negative electrode active material falls within such a range has a low resistance of the negative electrode plate and can secure a high battery output. Moreover, since the paste with the particle size distribution of the negative electrode active material in the above range has preferable filter permeability, it was found that it can be uniformly applied to the negative electrode current collector, thereby contributing to improvement of battery characteristics.

<Measurement of capacity maintenance rate 2>
The method for measuring the capacity retention rate is as described above.
(2) Evaluation of influence of particle size distribution D90 and D50 of negative electrode active material
About the lithium ion secondary battery of Examples 27-30 and 110-113 , after repeating charging / discharging by the cycle of the test number 2, the capacity | capacitance maintenance factor was measured. Table 8 shows the relationship between D90 and D50 of the negative electrode active material and the capacity retention rate of the lithium ion secondary battery.

As shown in Table 8, the negative electrode of Examples 27 to 30 in which D90 of the negative electrode active material is included in the range of 25.8 μm or less and D50 is included in the range of 8.6 to 12.7 μm is Example 110. The capacity retention rate was higher than -113 , 83% or more.

When D90 is larger than the appropriate range, expansion / contraction per active material particle is large when lithium ions are inserted and desorbed during charge / discharge. For this reason, it is considered that by repeating charge and discharge, the active materials are separated from each other and isolated, and the capacity retention rate is reduced.
From the above, it was found that the battery characteristics within the range in which the particle size distribution of the negative electrode active material is within the range of battery characteristics even after repeated use.

(3) Evaluation of influence of particle size distribution and oil absorption of positive electrode active material
About the lithium ion secondary battery of Examples 31-37 and 137-152 and Comparative Examples 114-136 and 153-175 , after repeating charging / discharging by the cycle of the test number 3, the capacity | capacitance maintenance factor was measured. Table 9 shows the relationship between D50 and the oil absorption amount Ab of the negative electrode active material, cD50 and the oil absorption amount cAb of the positive electrode active material, and the capacity retention rate of the lithium ion secondary battery.

In the batteries of Examples 31 to 37, the negative electrode active material D50 was included in the range of 8.6 to 12.7 μm, the oil absorption amount Ab was included in the range of 50 to 66 ml / 100 g, and the positive electrode active material cD50. Is included in the range of 3.8 to 5.2 μm, and the oil absorption is included in the range of 31 to 50 ml / 100 g. As shown in Table 9, the batteries of Examples 31 to 37 had a higher capacity retention rate than Examples 137 to 152 and Comparative Examples 114 to 136 and 153 to 175, and were 98% or more.

  In the moderate low temperature region according to Test No. 3, securing the lithium ion path in the positive electrode and the negative electrode affects the durability of the battery. From the above, a battery in which the particle size distribution and the oil absorption amount of the positive electrode and the negative electrode active material are in a range is required for a battery mounted as a driving power source for automobiles, etc., even after repeated use at a low temperature. It was found that a high level of durability can be secured.

  Note that the present invention is not limited to the above-described embodiments, and can be modified as appropriate without departing from the spirit of the present invention.

DESCRIPTION OF SYMBOLS 1 Negative electrode active material 2 Binder 3 Active material 4 Space (in active material 3) 5 Lithium ion pathway

Claims (6)

A negative electrode active material, a binder, and a current collector made of copper foil coated with these,
The negative electrode active material has a volume-based cumulative 50% particle size of 8.8 to 12.7 μm,
The binder has a volume-based cumulative 50% particle size of 80 to 115 nm,
The ratio of the volume-based cumulative 50% particle size of the negative electrode active material and the binder is 79.4 to 158.8,
The binder is a negative electrode of a non-aqueous electrolyte secondary battery made of a modified styrene-butadiene copolymer latex.   The negative electrode of the nonaqueous electrolyte secondary battery according to claim 1, wherein a ratio of the cumulative 50% particle size is 97.7 to 133.7.   The negative electrode of the nonaqueous electrolyte secondary battery according to claim 1, wherein a coating film containing boron is formed on the surface. The negative electrode active material is
The nonaqueous electrolyte 2 according to any one of claims 1 to 3, wherein the volume-based cumulative 10% particle size is 3.4 to 6.3 µm, and the volume-based cumulative 90% particle size is 25.8 µm or less. Secondary battery negative electrode.   The negative electrode of the nonaqueous electrolyte secondary battery according to claim 4, wherein the cumulative 90% particle size based on volume is 21.0 μm or less. The negative electrode of the nonaqueous electrolyte secondary battery according to claim 1, wherein the negative electrode active material is made of graphite having an amorphous carbon film .

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