JP2006059690A - Non-aqueous electrolyte secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a non-aqueous electrolyte secondary battery improved in charge and discharge cycle life. <P>SOLUTION: The non-aqueous electrolyte secondary battery comprises a positive electrode, a negative electrode including a negative electrode active material, and a non-aqueous electrolyte. The negative electrode has a pore volume of 0.15-0.35 cc/g at the pore diameter 10 μm or less, has a peak in the pore diameter 0.4-3.5 μm in the increased volume pore distribution, and the tilting at the accumulation 40-60% of the accumulation pore distribution curve in the range of pore diameter 0.001-10 μm is in the range 1.5-4.5. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a non-aqueous electrolyte secondary battery.

  In recent years, various electronic devices such as VTRs, mobile phones, personal computers, and cordless portable electronic devices have become smaller and lighter, and the demand for high energy density of these devices has increased, and metal lithium has been used as the negative electrode active material. Nonaqueous electrolyte secondary batteries such as lithium secondary batteries and lightweight secondary batteries using carbon for the negative electrode (for example, Patent Document 1) have been proposed.

As carbon used for the negative electrode, it has been proposed to use various carbonaceous materials such as coke, graphite, resin fired body, pyrolytic vapor phase carbon, etc., and using this negative electrode, LiCoO ₂ , LiNiO ₂ , LiMn are used for the positive electrode. A lithium ion secondary battery using a chalcogen compound such as ₂ O ₄ has been put into practical use.

  Lithium ion secondary batteries have various characteristics depending on the material of the carbonaceous material of the negative electrode. For example, as in Patent Document 2, a lithium ion secondary battery including carbon fibers having a lamellar structure in the cross-sectional direction of the fiber diameter as a negative electrode active material has excellent charge / discharge characteristics. Further, a lithium ion secondary battery containing graphite having a high degree of graphite as a negative electrode active material has high charging energy.

  The lithium ion secondary battery is higher in safety than a secondary battery using metallic lithium as a negative electrode, and is widely used as a power source for various portable terminals. In particular, there is an increasing demand for secondary batteries for small portable terminals, and there is a demand for increasing the capacity of secondary batteries. On the other hand, there is a demand for small size and light weight, and these demands are contradictory.

  In order to satisfy these requirements, it is desirable to increase the packing density of the electrodes.

However, when the packing density of the electrode is increased, the penetration of the nonaqueous electrolyte into the electrode is delayed, and the charge / discharge cycle maintenance rate is deteriorated due to the expansion / contraction of the electrode accompanying charge / discharge, particularly stress due to the expansion / contraction of the negative electrode. Cause problems. In particular, when a graphite-based carbonaceous material coated with amorphous coke as described in Patent Document 3 was used as a negative electrode active material, the cycle life was significantly reduced.
Japanese Unexamined Patent Publication No. Sho 63-121260 JP-A-5-89879 JP 2001-229924 A

  An object of the present invention is to provide a nonaqueous electrolyte secondary battery having an improved charge / discharge cycle life.

  A nonaqueous electrolyte secondary battery according to the present invention is a nonaqueous electrolyte secondary battery comprising a positive electrode, a negative electrode including a negative electrode active material, and a nonaqueous electrolyte, wherein the negative electrode has a pore diameter of 10 μm or less. A pore volume of 0.15 cc / g to 0.35 cc / g, a peak in a pore diameter of 0.4 to 3.5 μm in an increased volume pore distribution, and a pore diameter of 0.001 μm to 10 μm The slope of the cumulative pore distribution curve at 40 to 60% is in the range of 1.5 to 4.5.

  The nonaqueous electrolyte secondary battery according to the present invention is a nonaqueous electrolyte secondary battery comprising a positive electrode, a negative electrode including a negative electrode active material, and a nonaqueous electrolyte, wherein the negative electrode has a pore diameter of 10 μm. The following pore volume is 0.15 cc / g to 0.35 cc / g, the increased volume pore distribution curve has a peak in the range of the pore diameter of 0.4 to 3.5 μm, and the pore diameter of 0. The cumulative pore distribution curve in the range of 001 μm to 10 μm is characterized in that the pore diameters at the cumulative 80%, 70%, 60%, and 20% satisfy the following formulas (1) to (4). is there.

R ₈₀ ≧ 0.2 (1)
R ₇₀ ≧ 0.3 (2)
R ₆₀ ≧ 0.4 (3)
1.5 ≧ R ₂₀ ≧ 0.8 (4)
However, R ₈₀ is the pore diameter (μm) at the cumulative 80% of the cumulative pore distribution curve, R ₇₀ is the pore diameter (μm) at the cumulative 70% of the cumulative pore distribution curve, and R ₆₀ is the above-mentioned The pore diameter (μm) at a cumulative 60% of the cumulative pore distribution curve, and R ₂₀ is the pore diameter (μm) at a cumulative 20% of the cumulative pore distribution curve.

  According to the present invention, it is possible to provide a non-aqueous electrolyte secondary battery having an improved charge / discharge cycle life.

  The nonaqueous electrolyte secondary battery according to the present invention includes a positive electrode, a negative electrode including a negative electrode active material, and a nonaqueous electrolyte.

  Hereinafter, the negative electrode, the positive electrode, and the nonaqueous electrolyte will be described.

1) Negative electrode The negative electrode includes a current collector and a negative electrode material layer formed on the current collector and containing a negative electrode active material. The negative electrode has a pore volume of 0.15 cc / g to 0.35 cc / g with a pore diameter of 10 μm or less, and has a peak at a pore diameter of 0.4 to 3.5 μm in an increased volume pore distribution. Such a negative electrode has a cumulative pore distribution curve with a pore diameter of 0.001 μm to 10 μm and a slope at a cumulative 40% to 60% of 1.5 to 4.5, or the cumulative It is desirable that the pore diameters at the cumulative 80%, 70%, 60%, and 20% of the pore distribution curve satisfy the following expressions (1) to (4). The pore diameter of the negative electrode is measured by, for example, a mercury intrusion method.

R ₈₀ ≧ 0.2 (1)
R ₇₀ ≧ 0.3 (2)
R ₆₀ ≧ 0.4 (3)
1.5 ≧ R ₂₀ ≧ 0.8 (4)
However, R ₈₀ is the pore diameter (μm) at the cumulative 80% of the cumulative pore distribution curve, R ₇₀ is the pore diameter (μm) at the cumulative 70% of the cumulative pore distribution curve, and R ₆₀ is the above-mentioned The pore diameter (μm) at a cumulative 60% of the cumulative pore distribution curve, and R ₂₀ is the pore diameter (μm) at a cumulative 20% of the cumulative pore distribution curve.

  The reason for defining the pore volume having a pore diameter of 10 μm or less in the above range will be described. If the pore volume is less than 0.15 cc / g, the expansion and contraction of the negative electrode active material accompanying charge / discharge cannot be sufficiently absorbed in the negative electrode, and the negative electrode swells as the charge / discharge cycle progresses. Becomes shorter. On the other hand, when the pore volume exceeds 0.35 cc / g, a high negative electrode capacity cannot be obtained. A more preferable range of the pore volume is 0.18 to 0.35 cc / g.

  Those having a peak at a pore diameter of 0.4 to 3.5 μm in the increased volume pore distribution have a high proportion of pores having a diameter of 0.4 to 3.5 μm. In such a negative electrode, charging / discharging is performed by setting the slope at the cumulative 40% to 60% of the cumulative pore distribution curve in the pore diameter range of 0.001 μm to 10 μm to the range of 1.5 to 4.5. The expansion and contraction of the active material accompanying the negative electrode can be most effectively mitigated by the voids in the negative electrode, and the change in the structure consisting of the contact between the active material and the binder in the negative electrode accompanying the charge / discharge cycle can be minimized. it can. As a result, a new surface of the active material appears during the cycle and is prevented from coming into contact with the non-aqueous electrolyte, and the cycle efficiency can be improved. Further, since the reaction between the active material and the nonaqueous electrolyte is suppressed, the safety of the secondary battery is also improved. In addition, when the inclination is out of the above range, the charge / discharge cycle life is shortened. When the inclination is smaller than 1.5, the variation in pore diameter is large. It is speculated that the proportion of large pores is small, but details are not clear. In order to obtain a good charge / discharge cycle life, the slope is more preferably in the range of 2.0 to 4.5.

  Here, in the cumulative pore distribution curve in the pore diameter range of 0.001 μm to 10 μm, the pore diameter on the horizontal axis is 0.001 to 10 μm, and the cumulative pore distribution (% display) on the corresponding vertical axis is 0. Display at ~ 120%. It should be noted that the porosity is integrated from the pore diameter of 10 μm toward the smaller direction, and the accumulation reaches 50% when the porosity reaches 50%. The ratio between the horizontal axis and the vertical axis is horizontal axis: vertical axis = 2: 1. The slope of the cumulative pore distribution in the range of 40 to 60% is the amount of change about the X axis in the section from the point where the distribution curve intersects the cumulative 40% axis to the point where the cumulative curve intersects the 60% axis. And the amount of change about the Y-axis is measured, and the slope of the curve {Y-axis change amount / X-axis change amount} in the accumulated 40% to 60% is calculated.

Further, instead of defining the slope or together with the slope, the pore diameters R ₈₀ , R ₇₀ , R ₆₀ , R _{20 at} the cumulative 80%, 70%, 60%, 20% of the cumulative pore distribution curve are the above ( It is desirable to satisfy the formulas (1) to (4). Such a negative electrode can most effectively relieve the expansion and contraction of the active material associated with charging / discharging in the voids in the negative electrode, and has a structure comprising contact between the active material in the negative electrode associated with the charging / discharging cycle and the binder. Changes can be kept to a minimum. As a result, a new surface of the active material appears during the cycle and is prevented from coming into contact with the non-aqueous electrolyte, and the cycle efficiency can be improved. More preferable ranges of the pore diameters R ₈₀ , R ₇₀ , R ₆₀ , R ₂₀ are as shown in the following (1) ′ to (4) ′.

0.8 ≧ R ₈₀ ≧ 0.3 (1) '
1.0 ≧ R ₇₀ ≧ 0.4 (2) '
1.1 ≧ R ₆₀ ≧ 0.5 (3) '
1.2 ≧ R ₂₀ ≧ 0.9 (4) '
Examples of the negative electrode active material include composite graphite materials in which a carbon material layer having lower crystallinity than the particles is formed on at least a part of the surface of the graphite material particles, mesophase pitch carbon fibers, mesophase pitch microspheres, and the like. Can be mentioned. The composite graphite material is preferably used from the viewpoint of capacity.

  The negative electrode active material desirably has a particle size distribution satisfying the following formulas (5) to (8).

1.5 × D ₅₀ -3 ≦ D ₈₀ ≦ 1.5 × D ₅₀ +3 (5)
_{D 50 / 2-2 ≦ D 20 ≦} D 50/2 + 2 (6)
_{D 50 / 3-2 ≦ D 10 ≦} D 50/3 + 2 (7)
_{D 50 / 5-2 ≦ D 5 ≦} D 50/5 + 2 (8)
However, D ₈₀ is the negative electrode active material particle diameter (μm) at a cumulative 80%, D ₅₀ is a negative electrode active material particle diameter (μm) at a cumulative 50%, and D ₂₀ is a negative electrode active material particle diameter at a cumulative 20% (μm). μm), D ₁₀ is the negative electrode active material particle diameter (μm) at a cumulative 10%, and D ₅ is the negative electrode active material particle diameter (μm) at a cumulative 5%.

  According to the negative electrode active material having an appropriate width in the particle size distribution that satisfies the above expressions (5) to (8), the charge / discharge cycle life can be further improved.

It is desirable that the peak position (P) and the D ₅₀ in the increased volume pore distribution curve satisfy the following formula (9).

0.1 × D ₅₀ −1.5 ≦ P ≦ 0.1 × D ₅₀ +1.5 (9)
In the composite graphite material described above, the expansion volume per active material particle is about 10% of the diameter. Therefore, according to the pore having the diameter represented by (0.1 × D ₅₀ ± 1.5), the active material Can be sufficiently absorbed. Therefore, when the peak position (P) in the increased volume pore distribution curve satisfies the above-mentioned formula (9), the increase in the thickness of the negative electrode during the charge / discharge cycle can be further suppressed, and the charge / discharge cycle life is further improved. be able to.

The negative electrode active material particle diameter D ₅₀ at a cumulative 50% is desirably in the range of 10 to 25 μm, and more preferably 15 to 25 μm.

  The negative electrode active material desirably has a spherical shape with an aspect ratio in the range of 1 to 1.8. Thereby, the expansion of the active material due to charging can be brought closer to a three-dimensionally uniform direction, and the expansion relaxation effect by the pores can be further enhanced.

  The negative electrode active material desirably has an R value measured by Raman spectrum of 0.3 or more in intensity ratio and 1 or more in area ratio. As a result, reactions and side reactions that occur on the surface of the active material that occur during charging occur uniformly, and the expansion of the active material due to charging can be made closer to a three-dimensionally uniform direction. In addition, if the strength ratio is larger than 1.5 and the area ratio is larger than 4, the ratio of the low crystalline structure region in the active material is increased, so that a high capacity may not be obtained. It is desirable to set the upper limit of the ratio to 1.5 and the upper limit of the area ratio to 4. A more preferable range of the area ratio is 1 to 3.

The negative electrode active material has a specific surface area by the BET method in the range of 1.5 m ² / g or more and 5 m ² / g or less, and in the powder X-ray diffraction measurement, the interplanar spacing d ₀₀₂ is 0.336 nm or less, In the measurement using CuKα rays, it is preferable that peaks appear at diffraction angles 2θ of 42.8 ° to 44.0 ° and 45.5 ° to 46.6 °. Thereby, a high capacity can be achieved simultaneously with the cycle characteristics. The lower limit of the surface spacing d ₀₀₂ is spacing of (002) plane in complete graphite crystal d _002, i.e. it is preferable to 0.3354 nm.

In the negative electrode, the ratio of the peak intensity I ₀₀₄ representing diffraction on the (004) plane to the peak intensity I ₁₀₀ representing diffraction on the (100) plane obtained by wide-angle X-ray diffraction was (I ₀₀₄ / I ₁₀₀ ). In this case, it is desirable that (I ₀₀₄ / I ₁₀₀ ) ≦ 0.28. As a result, the expansion of the negative electrode due to charging can be brought closer to a three-dimensionally uniform direction, and the expansion relaxation efficiency by the pores can be increased.

  Examples of the current collector include a copper plate and a copper foil.

  Examples of the binder contained in the negative electrode material layer include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), ethylene-propylene-diene copolymer (EPDM), styrene-butadiene rubber (SBR), Carboxymethyl cellulose (CMC) or the like can be used.

  The negative electrode material layer can contain a conductive agent. Examples of the conductive agent include flaky graphite, acetylene black, and carbon black.

  For example, the negative electrode is prepared by kneading a negative electrode active material, a binder, and a conductive agent in the presence of a solvent, applying the obtained slurry to a current collector, drying, and then pressing once at a desired pressure or The negative electrode pore distribution increases the electrode density before pressing by adjusting the particle size distribution of the negative electrode active material and the dispersion state of the slurry (slurry solid content). And it can set within the target range by making pore distribution uniform.

2) Positive electrode The positive electrode includes a current collector and a positive electrode material layer that is supported on one or both surfaces of the current collector and includes an active material.

  The positive electrode material layer includes a positive electrode active material, a binder, and a conductive agent.

Examples of the positive electrode active material include various oxides such as manganese dioxide, lithium manganese composite oxide, lithium-containing nickel oxide, lithium-containing cobalt oxide, lithium-containing nickel cobalt oxide, lithium-containing iron oxide, and lithium. Examples thereof include vanadium oxide and chalcogen compounds such as titanium disulfide and molybdenum disulfide. Among them, when a lithium-containing cobalt oxide (for example, LiCoO ₂ ), a lithium-containing nickel cobalt oxide (for example, LiNi _0.8 Co _0.2 O ₂ ), or a lithium manganese composite oxide (for example, LiMn ₂ O ₄ , LiMnO ₂ ) is used. This is preferable because a high voltage can be obtained. As the positive electrode active material, one kind of oxide may be used alone, or two or more kinds of oxides may be mixed and used.

  Examples of the conductive agent include acetylene black, carbon black, and graphite.

  Examples of the binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), polyethersulfone, ethylene-propylene-diene copolymer (EPDM), and styrene-butadiene rubber (SBR). be able to.

  The blending ratio of the positive electrode active material, the conductive agent and the binder is preferably in the range of 85 to 98% by weight of the positive electrode active material, 1 to 10% by weight of the conductive agent, and 1 to 5% by weight of the binder.

  As the current collector, a conductive substrate having a porous structure or a non-porous conductive substrate can be used. These conductive substrates can be formed from, for example, aluminum, stainless steel, or nickel.

  The positive electrode is produced, for example, by suspending a conductive agent and a binder in an appropriate solvent in a positive electrode active material, applying the suspension to a current collector, drying it, and applying a press.

  A separator or a gel or solid electrolyte layer can be disposed between the positive electrode and the negative electrode.

  As this separator, a microporous film, a woven fabric, a non-woven fabric, a laminate of the same material or different materials among these can be used. Examples of the material for forming the separator include polyethylene, polypropylene, ethylene-propylene copolymer, and ethylene-butene copolymer. As a material for forming the separator, one type or two or more types selected from the types described above can be used.

  The thickness of the separator is preferably 30 μm or less, and more preferably 25 μm or less. Moreover, it is preferable that the lower limit of thickness is 5 micrometers, and a more preferable lower limit is 8 micrometers.

  The separator preferably has a heat shrinkage rate of 20% or less at 120 ° C. for 1 hour. The heat shrinkage rate is more preferably 15% or less.

  The separator preferably has a porosity in the range of 30 to 60%. A more preferable range of the porosity is 35 to 50%.

The separator preferably has an air permeability of 600 seconds / 100 cm ³ or less. The air permeability means time (seconds) required for 100 cm ³ of air to pass through the separator. The upper limit value of the air permeability is more preferably 500 seconds / 100 cm ³ . The lower limit value of the air permeability is preferably 50 seconds / 100 cm ^3, and a more preferable lower limit value is 80 seconds / 100 cm ³ .

3) Nonaqueous electrolyte A nonaqueous electrolyte contains a nonaqueous solvent and an electrolyte such as a lithium salt dissolved in the nonaqueous solvent. A polymer material can be added to the nonaqueous electrolyte for the main purpose of gelation.

  Examples of the non-aqueous solvent include cyclic carbonate {ethylene carbonate (EC), propylene carbonate (PC), etc.}, chain carbonate {methyl ethyl carbonate (MEC), diethyl carbonate (DEC), dimethyl carbonate (DMC), etc.}, Sultone compounds (sultone compounds having at least one double bond in the ring, 1,3-propane sultone (PS), etc.), vinylene carbonate (VC), vinyl ethylene carbonate (VEC), phenyl ethylene carbonate (phEC), γ -Butyrolactone (GBL), γ-valerolactone (VL), methyl propionate (MP), ethyl propionate (EP), 2-methylfuran (2Me-F), furan (F), thiophene (TIOP), catechol carbonate (CA TC), ethylene sulfite (ES), 12-crown-4 (Crown), tetraethylene glycol dimethyl ether (Ether), cyclohexylbenzene (CHB), 2,4-difluoroanisole (DFA) and the like. The type of the non-aqueous solvent can be one type or two or more types.

Examples of the electrolyte include lithium perchlorate (LiClO ₄ ), lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ), lithium hexafluoroarsenide (LiAsF ₆ ), and trifluorometa. Examples include lithium salts such as lithium sulfonate (LiCF ₃ SO ₃ ), lithium bistrifluoromethylsulfonylimide [(LiN (CF ₃ SO ₂ ) ₂ ], LiN (C ₂ F ₅ SO ₂ ) ₂ . The type of electrolyte can be one type or two or more types.

  The amount of the electrolyte dissolved in the non-aqueous solvent is preferably 0.5 to 2.5 mol / L. A more preferable range is 1 to 2.5 mol / L.

  In order to improve the wettability with the separator, the nonaqueous electrolyte preferably contains a surfactant such as trioctyl phosphate (TOP). The addition amount of the surfactant is preferably 3% or less, and more preferably in the range of 0.1 to 1%.

  The amount of non-aqueous electrolyte is preferably 0.2 to 0.6 g per 100 mAh of battery unit capacity. A more preferable range of the nonaqueous electrolytic mass is 0.25 to 0.55 g / 100 mAh.

  The form of the non-aqueous electrolyte secondary battery according to the present invention is not particularly limited, and can be various forms such as thin, square, cylindrical, coin-type, etc., but flat electrodes such as thin and square It is suitable for a secondary battery using a group.

  Hereinafter, an example of a thin nonaqueous electrolyte secondary battery is shown in FIGS.

  As shown in FIG. 1, an electrode group 2 is accommodated in a container body 1 having a rectangular cup shape. The electrode group 2 has a structure in which a laminate including a positive electrode 3, a negative electrode 4, and a separator 5 disposed between the positive electrode 3 and the negative electrode 4 is wound into a flat shape. The nonaqueous electrolyte is held in the electrode group 2. A part of the edge of the container body 1 is wide and functions as the lid plate 6. The container body 1 and the cover plate 6 are each composed of a laminate film. The laminate film includes an external protective layer 7, an internal protective layer 8 containing a thermoplastic resin, and a metal layer 9 disposed between the external protective layer 7 and the internal protective layer 8. A lid 6 is fixed to the container body 1 by heat sealing using a thermoplastic resin of the inner protective layer 8, whereby the electrode group 2 is sealed in the container. A positive electrode tab 10 is electrically connected to the positive electrode 3, and a negative electrode tab 11 is electrically connected to the negative electrode 4, and each is drawn out of the container and serves as a positive electrode terminal and a negative electrode terminal.

  In the thin non-aqueous electrolyte secondary battery illustrated in FIGS. 1 and 2, an example using a cup-shaped container has been described, but the shape of the container is not particularly limited, and can be, for example, a bag shape. .

[Example]
Hereinafter, embodiments of the present invention will be described in detail.

Example 1
<Negative electrode active material>
A composite graphite material obtained by surface-coating a carbon material having a grain size adjusted and spheroidized on a carbon material having lower crystallinity than graphite was used as a main negative electrode active material. This active material had an aspect ratio of 1.3 and an average particle diameter D ₅₀ of 19 μm. Particle size distribution of the negative electrode active material, D ₈₀ is 30 [mu] m, D ₅₀ is 19 .mu.m, D ₂₀ is 10 [mu] m, D ₁₀ is 6 [mu] m, D ₅ was 4.5 [mu] m. An outline of the relative frequency distribution of the particle diameter is shown in FIG. The horizontal axis in FIG. 3 is the particle diameter (μm), and the vertical axis is the relative particle amount (%) for each particle diameter. When the average particle diameter D ₅₀ is 19 μm, the range of D ₈₀ represented by the above-mentioned formula (5) is 25.5 to 31.5 μm, and the range of D ₂₀ represented by the above-described formula (6) is 7.5. The range of D ₁₀ represented by the above-mentioned formula (7) is 4.3 to 8.3 μm and the range of D ₅ represented by the above-described formula (8) is 1.8 to 5.8 μm. Therefore, the particle size distribution of the negative electrode active material used in Example 1 satisfies the expressions (5) to (8) described above.

Further, the negative electrode active material has a specific surface area of 3 m ² / g by the BET method, and in powder X-ray diffraction measurement, the interplanar spacing d ₀₀₂ is 0.336 nm or less, and the diffraction angle 2θ is 42.8 ° to Peaks appeared at 44.0 ° and 45.5 ° to 46.6 °. In addition, the space _| interval d002 of (002) plane is the value calculated | required by the half-value-width midpoint method from the powder X-ray diffraction spectrum. At this time, scattering correction such as Lorentz scattering was not performed.

  R value by the Raman spectrum measurement of the negative electrode active material was 0.39 in intensity ratio, and 1.58 in area ratio.

  The particle diameter, aspect ratio, specific surface area, and R value of the negative electrode active material were measured by the methods described below.

(Particle size of negative electrode active material)
The particle size of the negative electrode active material is a value obtained from the particle size distribution measured by the microtrack method. For example, D ₅₀ indicates the particle size of particles that have reached 50% by integrating their volume from particles having a small particle size. And Actual measurement was performed using a laser light scattering particle size distribution meter. This is a measurement that utilizes the light scattering phenomenon that occurs when a laser beam is applied to a particle. The intensity and scattering angle of the scattered light depend largely on the size of the particle. The particle size distribution of the powder can be obtained by measuring the angle with an optical detector and computerizing it.

(Aspect ratio of negative electrode active material)
As the aspect ratio of the negative electrode active material, a value obtained by measuring the major axis / minor axis ratio of the particle shape (ellipse) observed with a scanning electron microscope (SEM) and taking the average value thereof was used.

(Measurement of specific surface area by BET method)
As a measuring device, a product name manufactured by Yuasa Ionics used Kantasorb. The sample amount was set to around 0.5 g, and the sample was deaerated at 120 ° C. for 15 minutes as a pretreatment.

(Measurement of R value)
For the Raman spectrum of the negative electrode active material, peak separation was performed, and D (A1g): peak derived from disorder of the structure of graphite near 1360 cm ⁻¹ , D ′ (A1g): disorder of the structure of graphite near 1620 cm ⁻¹ Derived peak, D: Peak derived from disorder of graphite structure of amorphous carbon, G (E2g): Peak derived from graphite structure in the vicinity of 1580 cm ⁻¹ , G: Peak derived from graphite structure of amorphous carbon were obtained.

Calculating the intensity of each peak, intensity and I _D the sum of the intensity of a peak derived from the D-band, the ratio of I _G the sum of the intensity of a peak derived from the G band (I _D / I _G) Ratio. Also, the area of each peak is calculated, and the ratio of the sum of the peak area values derived from the _D band to the sum of the peak area values derived from the G band, S _G (S _D / S _G ) was defined as the area ratio.

<Production of negative electrode>
A carboxy having a etherification degree distribution of 0.6 to 0.8 and a weight average molecular weight distribution of 200,000 to 250,000 with respect to a total of 100 parts by weight of 90 parts by weight of the negative electrode active material and 10 parts by weight of flake graphite. 1.8 parts by weight of methylcellulose (CMC) and 1.5 parts by weight of styrene butadiene rubber (SBR) were added and kneaded in the presence of water to prepare a paste having a solid content of 50% by weight. The obtained paste was applied to both sides of a negative electrode current collector made of a copper foil having a thickness of 12 μm, dried and pressed to obtain an electrode density of 1.45 g / cm ³ on both sides of the negative electrode current collector. A negative electrode having a structure in which a negative electrode material layer was supported was produced.

  The pore diameter distribution of the obtained negative electrode was measured by the method described below.

(Measurement of pore diameter distribution)
The pore diameter was measured by mercury porosimetry using a measuring device manufactured by Cantachrome and having a model number of AUTOSCAN-33. The pore volume (cc / g) with a pore diameter of 10 μm or less is shown in Table 2 below. The increased volume pore distribution is obtained from the measurement results and is shown in FIG. Increased volume pore distribution indicates the amount of change in the cumulative pore volume for each diameter with respect to the pore diameter. The horizontal axis is the pore diameter (μm), and the vertical axis is the cumulative pore volume at each pore diameter. (Cc / g). As shown in FIG. 4, when the pore diameter was 100 μm or less, a sharp peak appeared when the pore diameter was 0.5 to 3.0 μm. Since the peak top position (P) is 0.7 μm, the relationship of the above-mentioned formula (9) (P = 0.1 × D ₅₀ ± 1.5) is established with D _{50 of} the negative electrode active material. It was.

  Further, from the measurement results, the cumulative pore distribution at pore diameters of 0.001 to 10 μm was calculated and shown in FIG. The pore diameter on the horizontal axis was 0.001 to 10 μm, and the cumulative pore distribution (% display) on the corresponding vertical axis was displayed as 0 to 120%. In addition, the porosity was integrated from a pore diameter of 10 μm toward a smaller direction, and the place where the porosity reached 50% was regarded as 50% cumulative. The ratio of the horizontal axis and the vertical axis was horizontal axis: vertical axis = 2: 1. It was 2.5 when the inclination in the range whose cumulative pore distribution is 40 to 60% was calculated. Note that the slope is the amount of change on the X axis and the change on the Y axis in mm in the section from the point A where the distribution curve intersects the cumulative 40% axis to the point B where the cumulative curve intersects the 60% axis. The slope of the curve in the cumulative 40% to 60% {Y-axis change amount (mm) / X-axis change amount (mm)} was calculated.

Table 1 shows the pore diameters R ₈₀ , R ₇₀ , R ₆₀ , and R ₂₀ at the cumulative 80%, 70%, 60%, and 20% of the cumulative pore distribution curve.

<Preparation of positive electrode>
First, 41.7 parts by weight of a 12% strength by weight polyvinylidene fluoride resin (PVdF) in N-methylpyrrolidone solution, 100 parts by weight of LiCoO ₂ powder as an active material, graphite powder as a conductive filler (trade name, manufactured by Lonza); KS4) 5 parts by weight were mixed and kneaded. Subsequently, 15 parts by weight of N-methylpyrrolidone was further added to the mixture, and the solid matter was dispersed using a bead mill to prepare a positive electrode coating slurry.

Next, the positive electrode coating slurry is applied to both sides of an aluminum foil (current collector) having a thickness of 15 μm so as to be 194 g / m ² , dried, and then subjected to pressing and slitting to obtain a thickness. A belt-like positive electrode having a thickness of 130 μm and a width of 49.5 mm was produced.

  Next, lead tabs were joined to the positive and negative electrode current collectors, respectively, and a flat electrode group was produced by winding up and spiraling through two polyethylene porous membranes using an automatic winding machine. .

Next, a stretched nylon film having a thickness of 25 μm, an aluminum sheet having a thickness of 40 μm, and a linear low density polyethylene (LLDPE) film having a thickness of 30 μm are laminated and bonded in this order via a urethane-based adhesive. .6mm film material for exterior material is folded in half, the cup part is drawn on one side, an electrode group is inserted into this cup part, and the other side is filled with a liquid injection port around the surface having the cup part. After injecting a non-aqueous electrolyte, the liquid injection port is heat sealed and sealed, thereby having the structure shown in FIGS. 1 and 2 and having a thickness of 3.6 mm. A thin lithium ion secondary battery having a width of 35 mm and a height of 62 mm was assembled. The non-aqueous electrolyte has a composition in which LiPF ₆ is dissolved at a concentration of 1.2 mol / L in a mixed non-aqueous solvent in which ethylene carbonate and ethyl methyl carbonate are mixed at a weight ratio of 1: 2.

(Examples 2 to 10 and Comparative Examples 1 to 3)
Except that the particle size distribution, aspect ratio, specific surface area, negative electrode paste solid content, strength ratio of R value and area ratio of the negative electrode active material are set as shown in Table 1 below, it was explained in Example 1 described above. Similarly, a thin non-aqueous electrolyte secondary battery was produced. In addition, as shown in Table 2, the particle size distribution of the negative electrode active material used in Examples 2 to 10 satisfies the relationship of the above-described formulas (5) to (8), but is used in Comparative Examples 1 to 3. The particle size distribution of the negative electrode active material did not satisfy the relationships of the expressions (5) to (8). In addition, the aspect ratio “undefined” in Table 2 means that the aspect ratio varies depending on the particle and cannot be specified as a numerical value.

For the negative electrodes of the secondary batteries of Examples 2 to 10 and Comparative Examples 1 to 3, the pore volume, peak position, curve slope, and R _{80 to} R ₂₀ were measured in the same manner as described in Example 1 above. The results are shown in Table 2 below.

  Further, for the secondary batteries of Examples 2 to 10 and Comparative Examples 1 to 3, the swelling and capacity retention rate of the battery container after 50 cycles and after 500 cycles were measured by the method described below. It is shown in Table 2.

  Each secondary battery was subjected to constant current / constant voltage charging to 0.2 V at 0.2 C at room temperature for 10 hours as an initial charge / discharge process, and then discharged to 3.0 V at 0.2 C at room temperature.

Next, constant current / constant voltage charging was performed for 3 hours at 1.0 C to 4.2 V at room temperature, and then discharged to 3.0 V at 1.0 C at room temperature. Thereafter, the thickness t _{0 of the} battery container was measured.

Next, after charging and discharging at the charge / discharge rate of 1C, the end-of-charge voltage of 4.2V, and the end-of-discharge voltage of 3.0V are repeated 500 times at room temperature, the battery container is kept discharged in the fifth and 500th cycles. The thickness t ₁ was measured, and the rate of change in thickness of the battery container after cycling was determined from the following formula (A). Further, the discharge capacities at 50 cycles and 500 cycles were measured, the discharge capacity at the first cycle was taken as 100%, and the capacity retention rates at 50 cycles and 500 cycles were calculated, and the results are shown in Tables 1 and 2 below. Show.

((T ₁ −t ₀ ) / t ₀ ) × 100 (%) (A)
However, t ₀ is the thickness of the battery container before the cycle test, and t ₁ is the thickness of the battery container after 50 cycles or 500 cycles.

Here, 1C is a current value necessary for discharging the nominal capacity (Ah) in one hour.

  As apparent from Tables 1 and 2, the pore volume is 0.15 cc / g to 0.35 cc / g, the peak position is in the range of 0.4 to 3.5 μm, and the slope is 1.5. -4.5, and the pore diameters at cumulative 80%, 70%, 60%, and 20% of the cumulative pore distribution curve satisfy Examples (1) to (4) described above. It can be understood that the secondary battery has a smaller battery swelling at 500 cycles than those of Comparative Examples 1 to 3, and a capacity retention rate at 50 cycles and 500 cycles is higher than those of Comparative Examples 1 to 3.

  Note that the present invention is not limited to the above-described embodiment as it is, and can be embodied by modifying the constituent elements without departing from the scope of the invention in the implementation stage. In addition, various inventions can be formed by appropriately combining a plurality of components disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, constituent elements over different embodiments may be appropriately combined.

The perspective view which shows the thin nonaqueous electrolyte secondary battery which is an example of the nonaqueous electrolyte secondary battery concerning this invention. The fragmentary sectional view which cut | disconnected the thin nonaqueous electrolyte secondary battery of FIG. 1 along the II-II line. FIG. 3 is a characteristic diagram illustrating an outline of a particle size distribution of a negative electrode active material used in the nonaqueous electrolyte secondary battery of Example 1; The characteristic view which shows the increase volume pore distribution of the negative electrode of the nonaqueous electrolyte secondary battery of Examples 1-4 and Comparative Examples 1-3. The characteristic view which shows the cumulative pore distribution in the pore diameter about 0.001-10 micrometers about the negative electrode of the nonaqueous electrolyte secondary battery of Examples 1-4 and Comparative Examples 1-3.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Container body, 2 ... Electrode group, 3 ... Positive electrode, 4 ... Negative electrode, 5 ... Separator, 6 ... Cover plate, 7 ... External protective layer, 8 ... Internal protective layer, 9 ... Metal layer, 10 ... Positive electrode tab, 11 ... negative electrode tab.

Claims (4)

  A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode containing a negative electrode active material, and a non-aqueous electrolyte, wherein the negative electrode has a pore volume of 0.15 cc / g to. 35 cc / g, with an increased volume pore distribution having a peak at a pore diameter of 0.4 to 3.5 μm, and a cumulative pore distribution curve in the range of pore diameters of 0.001 μm to 10 μm A non-aqueous electrolyte secondary battery having a slope at 60% of 1.5 to 4.5. A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode containing a negative electrode active material, and a non-aqueous electrolyte, wherein the negative electrode has a pore volume of 0.15 cc / g to. Cumulative pore distribution curve at 35 cc / g having a peak in the pore diameter range of 0.4 to 3.5 μm in the increased volume pore distribution curve and the pore diameter range of 0.001 μm to 10 μm A non-aqueous electrolyte secondary battery wherein the pore diameters at 80%, 70%, 60%, and 20% satisfy the following formulas (1) to (4):
R ₈₀ ≧ 0.2 (1)
R ₇₀ ≧ 0.3 (2)
R ₆₀ ≧ 0.4 (3)
1.5 ≧ R ₂₀ ≧ 0.8 (4)
However, R ₈₀ is the pore diameter (μm) at the cumulative 80% of the cumulative pore distribution curve, R ₇₀ is the pore diameter (μm) at the cumulative 70% of the cumulative pore distribution curve, and R ₆₀ is the above-mentioned The pore diameter (μm) at a cumulative 60% of the cumulative pore distribution curve, and R ₂₀ is the pore diameter (μm) at a cumulative 20% of the cumulative pore distribution curve. The non-aqueous electrolyte secondary battery according to claim 1, wherein the negative electrode active material has a particle size distribution satisfying the following formulas (5) to (8).
1.5 × D ₅₀ -3 ≦ D ₈₀ ≦ 1.5 × D ₅₀ +3 (5)
_{D 50 / 2-2 ≦ D 20 ≦} D 50/2 + 2 (6)
_{D 50 / 3-2 ≦ D 10 ≦} D 50/3 + 2 (7)
_{D 50 / 5-2 ≦ D 5 ≦} D 50/5 + 2 (8)
However, D ₈₀ is the negative electrode active material particle diameter (μm) at a cumulative 80%, D ₅₀ is a negative electrode active material particle diameter (μm) at a cumulative 50%, and D ₂₀ is a negative electrode active material particle diameter at a cumulative 20% (μm). μm), D ₁₀ is the negative electrode active material particle diameter (μm) at a cumulative 10%, and D ₅ is the negative electrode active material particle diameter (μm) at a cumulative 5%. The peak position (P) in the increased volume pore distribution curve and the negative electrode active material particle diameter (D ₅₀ ) at a cumulative 50% of the negative electrode active material satisfy the following formula (9): The nonaqueous electrolyte secondary battery according to any one of to 3.
0.1 × D ₅₀ −1.5 ≦ P ≦ 0.1 × D ₅₀ +1.5 (9)

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