JP2005310662A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery having a large initial capacity and a long charge/discharge cycle lifetime. <P>SOLUTION: In this nonaqueous electrolyte battery having a positive electrode, a negative electrode containing a mesophase pitch graphite material, and a nonaqueous electrolyte. The negative electrode satisfies the following formula 1 (0.001 ≤ A<SB>1</SB>/B<SB>1</SB>≤0.003) and has a density of 1.3-1.75 g/cm<SP>3</SP>. In this formula, A<SB>1</SB>represents a negative electrode density increased quantity (g/cm<SP>3</SP>) when the line pressure of a room temperature press using a roll with a diameter of 180 cm is increased from 20 to 180 Kg/cm step by step, while B<SB>1</SB>is 160 kg/cm as the line pressure change quantity of the room-temperature press. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

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

  In recent years, electronic devices such as mobile communication devices, notebook computers, palmtop computers, integrated video cameras, portable CD (MD) players, cordless telephones, etc. have been reduced in size and weight. As a power source, a battery having a small size and a large capacity is particularly demanded.

  Examples of batteries that are widely used as power sources for these electronic devices include primary batteries such as alkaline manganese batteries, and secondary batteries such as nickel cadmium batteries and lead storage batteries. Among them, the non-aqueous electrolyte secondary battery using a lithium composite oxide for the positive electrode and a carbonaceous material capable of occluding and releasing lithium ions for the negative electrode is small and light, has a high unit cell voltage, and can obtain a high energy density. Has been attracting attention. An example of the carbonaceous material is disclosed in Patent Document 1.

  In non-aqueous electrolyte secondary batteries, there is a great demand for small-sized mobile terminals, and an increase in capacity is required. 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 negative electrode.

However, in order to increase the packing density of the negative electrode, it is necessary to apply an excessive pressing pressure in the negative electrode manufacturing press process, which slows the penetration of the non-aqueous electrolyte into the negative electrode and makes the distribution of the non-aqueous electrolyte uneven. Thus, charging / discharging reaction spots are likely to occur, and the charge / discharge cycle maintenance rate deteriorates.
JP 2001-229924 A

  An object of the present invention is to provide a nonaqueous electrolyte secondary battery having a high initial capacity and a long charge / discharge cycle life.

A first nonaqueous electrolyte secondary battery according to the present invention is a nonaqueous electrolyte secondary battery comprising a positive electrode, a negative electrode containing a mesophase pitch-based graphite material, and a nonaqueous electrolyte,
The negative electrode satisfies the following formula (1) and has a density of 1.3 to 1.75 g / cm ³ .

0.001 ≦ (A ₁ / B ₁ ) ≦ 0.003 (1)
A ₁ is an increase in negative electrode density (g / cm ³ ) when the linear pressure of a room temperature press using a roll having a diameter of 180 cm is increased stepwise from 20 to 180 kg / cm, and B ₁ is that of the room temperature press. The amount of change in linear pressure is 160 kg / cm.

A second non-aqueous electrolyte secondary battery according to the present invention is a non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode containing a graphite material other than a mesophase pitch system, and a non-aqueous electrolyte,
The negative electrode satisfies the following formula (2) and has a density of 1.3 to 1.65 g / cm ³ .

0.0033 ≦ (A ₂ / B ₂ ) ≦ 0.007 (2)
A ₂ is the negative electrode density increase amount (g / cm ³ ) when the linear pressure of the room temperature press using a roll having a diameter of 180 cm is increased stepwise from 20 to 160 kg / cm, and B ₂ is the temperature of the room temperature press. The amount of change in linear pressure is 140 kg / cm.

  According to the present invention, a nonaqueous electrolyte secondary battery having a high initial capacity and a long charge / discharge cycle life can be provided.

  Embodiments of the present invention include a first nonaqueous electrolyte secondary battery using a mesophase pitch-based graphite material and a second nonaqueous electrolyte secondary battery using a graphite material other than a mesophase pitch-based material. .

First, the first nonaqueous electrolyte secondary battery will be described. When a mesophase pitch-based graphite material is used, the negative electrode density increase amount (A ₁ / B ₁ ) with respect to the linear pressure at the time of roll pressing is reduced as shown in the following formula (1) to reduce the negative electrode density to 1.3 to By setting it in the range of 1.75 g / cm ³ , it is possible to increase the density without applying an excessive pressure in the pressing process for producing the negative electrode, and therefore, the pores on the negative electrode surface are prevented from being blocked by the press. And the permeability of the nonaqueous electrolyte of the negative electrode can be improved. Moreover, the crack of the graphite material at the time of a press can be suppressed. As a result, a nonaqueous electrolyte secondary battery having a high initial capacity and a long charge / discharge cycle life can be realized.

  The positive electrode, negative electrode, and nonaqueous electrolyte of the first nonaqueous electrolyte secondary battery will be described.

1) Negative electrode The negative electrode includes a current collector and a negative electrode layer that is supported on one or both surfaces of the current collector and contains a mesophase pitch-based graphite material.

In this negative electrode, the negative electrode density increase amount (A ₁ / B ₁ ) with respect to the linear pressure at the time of roll press satisfies the following formula (1).

0.001 ≦ (A ₁ / B ₁ ) ≦ 0.003 (1)
A ₁ is an increase in negative electrode density (g / cm ³ ) when the linear pressure of a room temperature press using a roll having a diameter of 180 cm is increased stepwise from 20 to 180 kg / cm, and B ₁ is that of the room temperature press. The amount of change in linear pressure is 160 kg / cm.

  The reason why the diameter of the roll is 180 cm is that the pressure applied to the negative electrode is made uniform to obtain accurate negative electrode characteristics. Moreover, since a higher density tends to be obtained at a low linear pressure when hot pressing is performed, it is desirable to perform room temperature pressing in order to obtain accurate negative electrode characteristics.

When the density change amount (A ₁ / B ₁ ) is smaller than 0.001, it is necessary to apply an excessive press to make the negative electrode density 1.3 to 1.75 g / cm ^3. As a result, the permeability of the non-aqueous electrolyte decreases, and both the initial capacity and the charge / discharge cycle life are not improved. On the other hand, when the density change amount (A ₁ / B ₁ ) is greater than 0.003, cracks in the mesophase pitch-based graphite material increase when the negative electrode density is set to 1.3 to 1.75 g / cm ^3. Both the initial capacity and the charge / discharge cycle life are not improved. A more preferable range of the density change amount (A ₁ / B ₁ ) is 0.0015 to 0.0025.

  The mesophase pitch-based graphite material is obtained, for example, by subjecting mesophase pitch-based carbon, mesophase pitch-based carbon fiber, mesophase microspheres, etc. to heat treatment at 500 to 3000 ° C.

Plane spacing d ₀₀₂ of (002) plane of the mesophase pitch-based graphitic material, it is desirable to below 0.3365 nm. A more preferable range of the inter-surface distance d ₀₀₂ is 0.336 nm or less. 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.

The ratio of the peak height (I ₁₀₁ ) representing diffraction of the (101) plane to the peak height (I ₁₀₀ ) representing diffraction of the (100) plane by the X-ray diffraction method of the mesophase pitch-based graphite material is expressed as I _101. When / I ₁₀₀ is satisfied, it is desirable that 1.3 ≦ I ₁₀₁ / I ₁₀₀ ≦ 2.7. A more preferable range of the peak height ratio (I ₍₁₀₁₎ / I ₍₁₀₀₎ ) is 1.7 ≦ I ₁₀₁ / I ₁₀₀ ≦ 2.7.

  The average particle size of the mesophase pitch-based graphite material is desirably in the range of 10 to 35 μm. A more preferable range is 12 to 30 μm.

The specific surface area of the mesophase pitch-based graphite material by the BET method is desirably in the range of 0.5 to 3.5 m ² / g. A more preferable range is 0.9 to ³ m ² / g.

  The mesophase pitch-based graphite material may contain additive elements such as B, O, and Si. The type of additive element can be one type or two or more types.

The negative electrode is, for example, a kneaded mesophase pitch-based graphite material, a binder and, if necessary, a conductive agent in the presence of a solvent, and the resulting suspension is applied to a current collector and dried. It is produced by pressing once at a desired pressure or by multistage pressing 2-5 times. The density change amount (A ₁ / B ₁ ) can be adjusted, for example, by the particle size, specific surface area, or additive element of the mesophase pitch-based graphite material. Specifically, when the average particle size of the mesophase pitch-based graphite material is increased, the density change amount (A ₁ / B ₁ ) can be increased, and when boron is added to the mesophase pitch-based graphite material, the graphite is increased. Since the hardness of the quality material can be increased, the amount of density change (A ₁ / B ₁ ) can be reduced. Furthermore, when the boron-added mesophase pitch-based graphite material is oxidized, the density change amount (A ₁ / B ₁ ) can be increased. In addition, the press conditions at the time of negative electrode production may be the same as or different from the case of measuring the density variation (A ₁ / B ₁ ).

The reason why the negative electrode density is in the range of 1.3 to 1.75 g / cm ³ is for the reason described below. Since the density change amount (A ₁ / B ₁ ) is small as shown in the above-described formula (1), when the negative electrode density is less than 1.3 g / cm ³ , the binding property and conductivity between the graphite material particles are improved. descend. On the other hand, in order to make the negative electrode density larger than 1.75 g / cm ³ , it is necessary to perform an excessive press, so that the permeability of the nonaqueous electrolyte is reduced due to the blockage of the pores on the negative electrode surface, or the graphite There are many cracks in quality materials. A more preferable range of the negative electrode density is 1.35 to 1.7 g / cm ³ .

In order to sufficiently obtain the effects of the present invention, it is desirable that the basis weight of the negative electrode layer be in the range of 70 to 115 g / m ² (more preferably 85 to 110 g / m ² ).

  As the binder used for the negative electrode, for example, at least one water-soluble binder such as carboxymethyl cellulose (CMC) or styrene butadiene rubber (SBR) can be used.

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

  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, copper, stainless steel, or nickel.

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

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.

  The positive electrode layer may contain a conductive agent. Examples of the conductive agent include acetylene black, carbon black, and graphite.

  The positive electrode layer can contain a binder. 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, and drying to form a thin plate.

3) Nonaqueous electrolyte A nonaqueous electrolyte contains a nonaqueous solvent and an electrolyte (for example, lithium salt) dissolved in the nonaqueous solvent. The form of the non-aqueous electrolyte can be liquid, gel or solid.

Examples of the electrolyte include lithium perchlorate (LiClO ₄ ), lithium tetrafluoroborate (LiBF ₄ ), lithium hexafluorophosphate (LiPF ₆ ), lithium hexafluoroarsenate (LiAsF ₆ ), and trifluoromethanesulfonic acid. Lithium (LiCF ₃ SO ₃ ), LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ or the like can be used, and two or more kinds may be mixed and used.

  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.

  It is preferable to add a surfactant such as trioctyl phosphate (TOP) to the non-aqueous solvent. By adding such a surfactant, it becomes possible to improve the wettability of the nonaqueous electrolyte with respect to the separator.

  The concentration of the electrolyte in the non-aqueous solvent is preferably 0.5 mol / L or more.

  An electrode group can be manufactured using the positive electrode and the negative electrode described above. The electrode group is formed, for example, by interposing a separator or a solid electrolyte layer between the positive electrode and the negative electrode. Specific production methods include the following (i) to (v).

  (I) The positive electrode and the negative electrode are wound spirally with a separator interposed therebetween.

  (Ii) The positive electrode and the negative electrode are wound into a flat shape with a separator interposed therebetween.

  (Iii) The positive electrode and the negative electrode are wound spirally with a separator interposed therebetween, and then compressed in the radial direction.

  (Iv) The positive electrode and the negative electrode are bent one or more times with a separator interposed therebetween.

  (V) The positive electrode and the negative electrode are laminated with a separator interposed therebetween.

  The electrode group need not be pressed, but may be pressed to increase the integrated strength of the positive electrode, the negative electrode, and the separator. It is also possible to heat at the time of pressing.

  The electrode group can contain an adhesive polymer in order to increase the integrated strength of the positive electrode, the negative electrode, and the separator. Examples of the adhesive polymer include polyacrylonitrile (PAN), polyacrylate (PMMA), polyvinylidene fluoride (PVdF), polyvinyl chloride (PVC), and polyethylene oxide (PEO).

  As the 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 .

  The width of the separator is desirably wider than the width of the positive electrode and the negative electrode. By adopting such a configuration, it is possible to prevent the positive electrode and the negative electrode from coming into direct contact without a separator.

  The shape of the container in which the electrode group is accommodated can be, for example, a bottomed cylindrical shape, a bottomed rectangular tube shape, a bag shape, a cup shape, or the like.

  This container can be formed from, for example, a laminate film including a resin layer, a metal plate, a metal film, or the like.

  The resin layer included in the laminate film can be formed from, for example, polyolefin (for example, polyethylene, polypropylene), polyamide, or the like. The resin layer can be formed of one type or two or more types of resins. In particular, it is desirable to use a laminate film including a metal layer and a resin layer formed on at least a part of the metal layer. The metal layer is responsible for blocking moisture and maintaining the shape of the container. Examples of the metal layer include aluminum, stainless steel, iron, copper, and nickel. Among these, aluminum that is lightweight and has a high function of blocking moisture is preferable. The metal layer can be formed of one type or two or more types of metals.

  The metal plate and the metal film can be formed from, for example, iron, stainless steel, and aluminum.

Next, the second nonaqueous electrolyte secondary battery will be described. In this secondary battery, the same positive electrode, non-aqueous electrolyte, separator, and container as those described in the first non-aqueous electrolyte secondary battery can be used. In the second non-aqueous electrolyte secondary battery, a graphite material other than the mesophase pitch type is used, and the negative electrode density increase amount (A ₂ / B ₂ ) with respect to the linear pressure at the time of roll press is higher than that of the first secondary battery. By increasing the negative electrode density in the range of 1.3 to 1.65 g / cm ³ , it is possible to increase the density without applying excessive pressure in the negative electrode manufacturing press step. It is possible to prevent the pores from being clogged, and to improve the permeability of the nonaqueous electrolyte of the negative electrode. Further, the graphite material can be appropriately pulverized during pressing to increase the activity of the graphite material. As a result, a nonaqueous electrolyte secondary battery having a high initial capacity and a long charge / discharge cycle life can be realized.

  Hereinafter, the negative electrode will be described.

  The negative electrode includes a current collector and a negative electrode layer that is supported on one or both surfaces of the current collector and contains a graphite material other than a mesophase pitch system.

  Examples of the graphite material other than the mesophase pitch system include artificial graphite, natural graphite, and composite graphite material in which a carbon material layer having lower crystallinity than this particle is formed on at least a part of the surface of the graphite material particle. be able to. From the viewpoint of suppressing the decomposition reaction of a non-aqueous solvent such as PC, it is desirable to use a composite graphite material.

Plane spacing d ₀₀₂ of (002) plane of the graphitic material other than the mesophase pitch is preferably equal to or less than 0.3365 nm. A more preferable range of the inter-surface distance d ₀₀₂ is 0.336 nm or less. 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.

The ratio of the peak height (I ₁₀₁ ) representing diffraction on the (101) plane to the peak height (I ₁₀₀ ) representing diffraction on the (100) plane by X-ray diffraction of graphite materials other than mesophase pitch systems When I ₁₀₁ / I ₁₀₀ is set, it is desirable that 1.3 ≦ I ₁₀₁ / I ₁₀₀ ≦ 2.7. A more preferable range of the peak height ratio (I ₍₁₀₁₎ / I ₍₁₀₀₎ ) is 1.7 ≦ I ₁₀₁ / I ₁₀₀ ≦ 2.7.

  The average particle diameter of the graphite material other than the mesophase pitch system is desirably in the range of 10 to 31 μm. A more preferable range is 12 to 26 μm.

The specific surface area of the graphite material other than the mesophase pitch system by the BET method is desirably in the range of 1.5 to 3.2 m ² / g. A more preferable range is 1.8 to 2.9 m ² / g.

The amount of increase in negative electrode density (A ₂ / B ₂ ) with respect to the linear pressure during roll press satisfies the following formula (2).

0.0033 ≦ (A ₂ / B ₂ ) ≦ 0.007 (2)
A ₂ is the negative electrode density increase amount (g / cm ³ ) when the linear pressure of the room temperature press using a roll having a diameter of 180 cm is increased stepwise from 20 to 160 kg / cm, and B ₂ is the temperature of the room temperature press. The amount of change in linear pressure is 140 kg / cm.

  The reason why the roll diameter is 180 cm and the room temperature press is for the same reason as described in the first non-aqueous electrolyte secondary battery described above.

When the density change amount (A ₂ / B ₂ ) is less than 0.0033, the graphite material is difficult to crack at the time of pressing, and instead, the high density is obtained by closing the pores on the negative electrode surface. The initial capacity and the charge / discharge cycle life are lowered due to the decrease in permeability. On the other hand, when the density change amount (A ₂ / B ₂ ) exceeds 0.007, the amount of pulverization of the graphite material at the time of pressing increases, so that the conductivity decreases and the amount of reaction with the non-aqueous electrolyte increases. Both initial capacity and charge / discharge cycle life are not improved.

The negative electrode is, for example, kneaded with a graphite material other than the mesophase pitch system, a binder and, if necessary, a conductive agent in the presence of a solvent, and the resulting suspension is applied to a current collector and dried. Thereafter, it is produced by pressing once or a multistage pressing 2-5 times at a desired pressure. When, for example, a composite graphite material is used as a graphite material other than the mesophase pitch system, the density change amount (A ₂ / B ₂ ) can be adjusted by the coating amount of the low crystalline carbon layer. Specifically, when the coating amount of the low crystalline carbon layer is increased, the hardness of the composite graphite material increases, so that the density change amount (A ₂ / B ₂ ) can be reduced. The pressing conditions for producing the negative electrode may be the same as or different from the pressing conditions for measuring the density variation (A ₂ / B ₂ ).

The reason why the negative electrode density is in the range of 1.3 to 1.65 g / cm ³ is due to the reason described below. Even if the density change amount (A ₂ / B ₂ ) is large as shown in the above formula (2), if the negative electrode density is less than 1.3 g / cm ³ , the amount of pulverization of the graphite material at the time of pressing is insufficient. As a result, the activity of the graphite material decreases. On the other hand, if the negative electrode density is larger than 1.65 g / cm ³ , the amount of pulverized graphite material during pressing increases, leading to a decrease in conductivity and an increase in the amount of reaction with the nonaqueous electrolyte.

In order to sufficiently obtain the effects of the present invention, it is desirable that the basis weight of the negative electrode layer be in the range of 70 to 115 g / m ² (more preferably 85 to 110 g / m ² ).

  As the binder, the conductive agent, and the current collector used for the negative electrode, the same materials as those described for the first nonaqueous electrolyte secondary battery can be used.

  The present invention can be applied to various forms of nonaqueous electrolyte secondary batteries such as thin, rectangular, cylindrical, or coin-type. An example of the thin, rectangular nonaqueous electrolyte secondary battery will be described with reference to FIGS.

  First, a thin nonaqueous electrolyte secondary battery will be described.

  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. The lid 6 is integrated with the container body 1. The container body 1 and the cover plate 6 are each composed of a laminate film. This laminate film includes a resin layer 7, a thermoplastic resin layer 8, and a metal layer 9 disposed between the resin layer 7 and the thermoplastic resin layer 8. A lid 6 is fixed to the container main body 1 by heat sealing using a thermoplastic resin layer 8, whereby the electrode group 2 is sealed in the container. A positive electrode tab 10 is connected to the positive electrode 3, and a negative electrode tab 11 is connected to the negative electrode 4, and each is pulled out of the container and serves as a positive electrode terminal and a negative electrode terminal.

  Next, the prismatic nonaqueous electrolyte secondary battery will be described.

  As shown in FIG. 3, an electrode group 13 is accommodated in a bottomed rectangular cylindrical container 12 made of metal such as aluminum. In the electrode group 13, a positive electrode 14, a separator 15 and a negative electrode 16 are laminated in this order and wound in a flat shape. A spacer 17 having an opening near the center is disposed above the electrode group 13.

  The nonaqueous electrolyte is held in the electrode group 13. A sealing plate 18 b having a liquid injection port 18 a and having a circular hole opened near the center is laser welded to the opening of the container 12. The liquid injection port 18a is closed with a sealing lid (not shown) after the liquid nonaqueous electrolyte has been injected. The negative electrode terminal 19 is disposed in a circular hole of the sealing plate 18b via a hermetic seal. The negative electrode tab 20 drawn out from the negative electrode 16 is welded to the lower end of the negative electrode terminal 19. On the other hand, a positive electrode tab (not shown) is connected to a container 12 that also serves as a positive electrode terminal.

[Example]
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings described above.

  First, the amount of change in negative electrode density with respect to linear pressure was measured for six types of graphite materials described below.

<Negative electrode A using composite graphite material A>
After spheroidizing the natural graphite, chemical vapor deposition is performed at 1000 ° C. in a benzene / N ₂ stream to form a carbon material layer having low crystallinity on the surface in the amount shown in Table 1 below, thereby obtaining a composite graphite material A. It was.

The average particle diameter (D50) of the obtained composite graphite material A, the specific surface area by BET method, the (002) plane spacing (d ₀₀₂ ) by the powder X-ray diffraction, the peak height ratio (I ₍₁₀₁₎ / I _{( 100)} ) is shown in Table 1 below.

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. Further, the peak height ratio (I ₍₁₀₁₎ / I ₍₁₀₀₎ ) was determined after subtracting the baseline from the spectrum similarly measured using CuKα rays.

  The specific surface area was measured by the BET method by the method described below.

  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.

Carboxymethylcellulose is added in the blending amount shown in Table 1 below with respect to 90 parts by weight of the obtained composite graphite material A powder and 10 parts of the scaly graphite powder, and the SBR emulsion contains 100 parts by weight of the graphite material powder. The mixture was added in an amount of 1.5 parts by weight, and these were kneaded in the presence of water to prepare a slurry. The slurry was applied to both sides of a current collector made of a copper foil having a thickness of 12 μm and dried. The negative electrode density after drying was measured, and the results are shown in FIG. In FIG. 4, the horizontal axis represents the linear pressure (kg / cm) during roller pressing, and the vertical axis represents the negative electrode density (g / cm ³ ).

  After the room temperature press was performed once at a linear pressure of 20 kg / cm with a roller having a diameter of 180 cm, the negative electrode density was measured, and then the room temperature press with a higher linear pressure was performed once, and the density measurement was performed again. It was. In this way, the density measurement with the linear pressure increased stepwise was performed until the press linear pressure reached 160 kg / cm. A part of the result is shown in FIG.

From this measurement result, (A ₂ / B ₂ ) of the above-described equation ( ₂ ) was calculated, and the result is shown in Table 2 below.

  The negative electrode density was measured by the method described below.

  A negative electrode having an active material coated on both sides was cut into a size of 5 cm × 5 cm, and the total weight and thickness of the electrode were measured. Next, the negative electrode layer was peeled off from both surfaces of the electrode using acetone, and the weight and thickness of the current collector were measured. The density of the electrode was calculated by (total electrode weight−current collector weight) / ((electrode thickness−current collector thickness) × area).

<Negative electrode B using composite graphite material B>
After spheroidizing natural graphite, chemical vapor deposition is performed at a temperature shown in Table 1 below in a benzene / N ₂ stream to form a carbon material layer having low crystallinity on the surface in the amount shown in Table 1 below. A graphite material was obtained.

The average particle diameter (D50) of the obtained composite graphite material B, the specific surface area by the BET method, the (002) plane spacing (d ₀₀₂ ) by the powder X-ray diffraction, the peak height ratio (I ₍₁₀₁₎ / I _{( 100)} ) is shown in Table 1 below.

  Carboxymethylcellulose is added in the blending amounts shown in Table 1 below with respect to 90 parts by weight of the obtained composite graphite material B powder and 10 parts of the scaly graphite powder, and the SBR emulsion contains 100 parts by weight of the graphite material powder. The mixture was added in an amount of 1.5 parts by weight, and these were kneaded in the presence of water to prepare a slurry. The slurry was applied to both sides of a current collector made of a copper foil having a thickness of 12 μm and dried. The negative electrode density after drying was measured, and the results are also shown in FIG.

The change in negative electrode density when subjected to room temperature pressing with a roller having a diameter of 180 cm was measured in the same manner as described above, and a part of the result is also shown in FIG. Further, (A ₂ / B ₂ ) of the above-described equation ( ₂ ) is calculated from the measurement result, and the result is shown in Table 2 below.

<Negative electrode C using artificial graphite>
The average particle size (D50), specific surface area by BET method, interplanar spacing (d ₀₀₂ ) by powder X-ray diffraction (d ₀₀₂ ), and peak height ratio (I ₍₁₀₁₎ / I ₍₁₀₀₎ ) are shown in Table 1 below. An artificial graphite powder having the value shown in FIG.

  Carboxymethylcellulose is added to the obtained artificial graphite powder 90 parts by weight and flaky graphite powder 10 weights in the blending amount shown in Table 1 below, and the SBR emulsion is added to 100 weight parts of the graphite material powder. It added so that it might become 1.5 weight part, these were knead | mixed in presence of water, and the slurry was prepared. The slurry was applied to both sides of a current collector made of a copper foil having a thickness of 12 μm and dried. The negative electrode density after drying was measured, and the results are also shown in FIG.

The change in negative electrode density when subjected to room temperature pressing with a roller having a diameter of 180 cm was measured in the same manner as described above, and a part of the result is also shown in FIG. From this measurement result, (A ₂ / B ₂ ) of the above-described equation ( ₂ ) was calculated, and the result is shown in Table 2 below.

<Negative electrode D using composite graphite material C>
After spheroidizing natural graphite, chemical vapor deposition is performed at a temperature shown in Table 1 below in a benzene / N ₂ stream to form a carbon material layer having low crystallinity on the surface in the amount shown in Table 1 below. A graphite material C was obtained.

The average particle diameter (D50) of the obtained composite graphite material C, the specific surface area by the BET method, the (002) plane spacing (d ₀₀₂ ) by the powder X-ray diffraction, the peak height ratio (I ₍₁₀₁₎ / I _{( 100)} ) is shown in Table 1 below.

  Carboxymethylcellulose was added at a blending ratio shown in Table 1 below with respect to 90 parts by weight of the obtained composite graphite material C powder and 10 parts of the scaly graphite powder, and the SBR emulsion contained 100 parts by weight of the graphite material powder. The mixture was added in an amount of 1.5 parts by weight, and these were kneaded in the presence of water to prepare a slurry. The slurry was applied to both sides of a current collector made of a copper foil having a thickness of 12 μm and dried.

The change in negative electrode density when subjected to room temperature pressing with a roller having a diameter of 180 cm was measured in the same manner as described above. From the measurement results, (A ₂ / B ₂ ) in the above-described equation ( ₂ ) is calculated, and the results are shown in Table 2 below.

<Negative electrode E using mesophase microsphere A>
The mesophase pitch spherules were heat-treated at the temperatures shown in Table 1 below to obtain a graphite material. The average particle diameter (D50) of the obtained graphite material, the specific surface area by the BET method, the (002) plane spacing (d ₀₀₂ ) by the powder X-ray diffraction, the peak height ratio (I ₍₁₀₁₎ / I _{(100 )} ) Is shown in Table 1 below.

  Carboxymethylcellulose is added in the blending amount shown in Table 1 below with respect to 90 parts by weight of the obtained mesophase spherule A powder and 10 parts of flaky graphite powder, and the SBR emulsion contains 100 parts by weight of graphite material powder. The mixture was added in an amount of 1.5 parts by weight, and these were kneaded in the presence of water to prepare a slurry. The slurry was applied to both sides of a current collector made of copper foil having a thickness of 12 μm and dried. The negative electrode density after drying was measured, and the results are also shown in FIG.

In the same manner as described above, the change in negative electrode density when a room temperature press is performed with a roller having a diameter of 180 cm is measured up to a press linear pressure of 180 kg / cm, and a part of the result is also shown in FIG. From this measurement result, (A ₁ / B ₁ ) of the above-described equation ( ₁ ) was calculated, and the result is shown in Table 3 below.

<Negative electrode F using mesophase microsphere B>
The mesophase pitch spherules were heat-treated at the temperatures shown in Table 1 below to obtain a graphite material. The average particle diameter (D50) of the obtained graphite material, the specific surface area by the BET method, the (002) plane spacing (d ₀₀₂ ) by the powder X-ray diffraction, the peak height ratio (I ₍₁₀₁₎ / I _{(100 )} ) Is shown in Table 1 below.

  Carboxymethylcellulose was added at a blending ratio shown in Table 1 below with respect to 90 parts by weight of the obtained mesophase spherule B powder and 10 parts of the scaly graphite powder, and the SBR emulsion contained 100 parts by weight of the graphite material powder. The mixture was added in an amount of 1.5 parts by weight, and these were kneaded in the presence of water to prepare a slurry. The slurry was applied to both sides of a current collector made of a copper foil having a thickness of 12 μm and dried. The negative electrode density after drying was measured, and the results are also shown in FIG.

In the same manner as described above, the change in negative electrode density when a room temperature press is performed with a roller having a diameter of 180 cm is measured up to a press linear pressure of 180 kg / cm, and a part of the result is also shown in FIG. From this measurement result, (A ₁ / B ₁ ) of the above-described equation ( ₁ ) was calculated, and the result is shown in Table 3 below.

<Mesophase microsphere B + negative electrode G of scaly graphite powder 20 parts>
1.2 parts by weight of carboxymethylcellulose is added to 80 parts by weight of the mesophase spherule B powder and 20 parts by weight of the scaly graphite powder, and the SBR emulsion is 1.5 parts by weight with respect to 100 parts by weight of the graphite material powder. They were added so as to be parts by weight, and these were kneaded in the presence of water to prepare a slurry. The slurry was applied to both sides of a current collector made of a copper foil having a thickness of 12 μm and dried.

The change in negative electrode density when a room temperature press was performed with a roller having a diameter of 180 cm was measured up to a press linear pressure of 180 kg / cm in the same manner as described above. From this measurement result, (A ₁ / B ₁ ) of the above-described equation ( ₁ ) was calculated, and the result is shown in Table 3 below.

<Negative electrode H containing mesophase carbon fiber A>
The mesophase pitch carbon fiber was heat-treated at the temperature shown in Table 1 below to obtain a graphite material. The average particle diameter (D50) of the obtained graphite material, the specific surface area by the BET method, the (002) plane spacing (d ₀₀₂ ) by the powder X-ray diffraction, the peak height ratio (I ₍₁₀₁₎ / I _{(100 )} ) Is shown in Table 1 below.

  Carboxymethylcellulose is added at a blending ratio shown in the following Table 1 with respect to 90 parts by weight of the obtained mesophase carbon fiber A powder and 10 parts of the scaly graphite powder, and the SBR emulsion contains 100 parts by weight of the graphite material powder. The mixture was added in an amount of 1.5 parts by weight, and these were kneaded in the presence of water to prepare a slurry. The slurry was applied to both sides of a current collector made of a copper foil having a thickness of 12 μm and dried. The negative electrode density after drying was measured, and the results are also shown in FIG.

In the same manner as described above, the change in negative electrode density when a room temperature press is performed with a roller having a diameter of 180 cm is measured up to a press linear pressure of 180 kg / cm, and a part of the result is also shown in FIG. From this measurement result, (A ₁ / B ₁ ) of the above-described equation ( ₁ ) was calculated, and the result is shown in Table 3 below.

<Negative electrode I containing mesophase carbon fiber B>
The mesophase pitch carbon fiber B was heat-treated at the temperature shown in Table 1 below to obtain a graphite material. The average particle diameter (D50) of the obtained graphite material, the specific surface area by the BET method, the (002) plane spacing (d ₀₀₂ ) by the powder X-ray diffraction, the peak height ratio (I ₍₁₀₁₎ / I _{(100 )} ) Is shown in Table 1 below.

  Carboxymethylcellulose was added at a blending ratio shown in Table 1 below with respect to 90 parts by weight of the obtained mesophase carbon fiber B powder and 10 parts of the scaly graphite powder, and the SBR emulsion contained 100 parts by weight of the graphite material powder. The mixture was added in an amount of 1.5 parts by weight, and these were kneaded in the presence of water to prepare a slurry. The slurry was applied to both sides of a current collector made of a copper foil having a thickness of 12 μm and dried.

The change in negative electrode density when a room temperature press was performed with a roller having a diameter of 180 cm was measured up to a press linear pressure of 180 kg / cm in the same manner as described above. From this measurement result, (A ₁ / B ₁ ) of the above-described equation ( ₁ ) was calculated, and the result is shown in Table 3 below.

  A square non-aqueous electrolyte secondary battery was manufactured by the method described below using the above nine types of negative electrodes.

(Examples 1-4 and Comparative Example 1)
<Production of negative electrode>
Among the negative electrodes A using the composite graphite material A described above, five types having densities shown in Table 2 below were prepared. Table 2 below shows the thickness (μm) per side of the negative electrode layer and the basis weight (g / m ² ) per side of the negative electrode layer.

  A square nonaqueous electrolyte secondary battery was manufactured using each negative electrode by the method described below.

<Preparation of positive electrode>
First, 90% by weight of lithium cobalt oxide (Li _x CoO ₂ ; X is 0 <X ≦ 1) powder, 5% by weight of acetylene black, and 5% by weight of polyvinylidene fluoride (PVdF) dimethylformamide (DMF) solution was added and mixed to prepare a slurry. The slurry was applied to both sides of a current collector made of an aluminum foil having a thickness of 15 μm, then dried and pressed to produce a positive electrode having a structure in which the positive electrode layer was supported on both sides of the current collector.

<Separator>
A separator made of a microporous polyethylene film having a thickness of 25 μm was prepared.

<Preparation of non-aqueous electrolyte>
A non-aqueous solvent was prepared by mixing ethylene carbonate (EC) and methyl ethyl carbonate (MEC) so that the volume ratio (EC: MEC) was 1: 2. Lithium hexafluorophosphate (LiPF ₆ ) was dissolved in the obtained non-aqueous solvent so that the concentration thereof was 1 mol / L to prepare a non-aqueous electrolyte (liquid non-aqueous electrolyte).

<Production of electrode group>
After ultrasonically welding a positive electrode lead made of a strip-shaped aluminum foil to the positive electrode current collector and ultrasonically welding a negative electrode lead made of a band-shaped nickel foil to the negative electrode current collector, the positive electrode and the negative electrode are interposed between the positive electrode and the negative electrode. After winding in a spiral shape through a separator, it was shaped into a flat shape to produce an electrode group.

  After housing the electrode group in an aluminum square can, the electrode group in the metal can was vacuum dried at 80 ° C. for 12 hours to remove moisture contained in the electrode group and the metal can.

  Subsequently, the liquid nonaqueous electrolyte is injected into the electrode group in the metal can so that the amount per battery capacity 1Ah is 4.8 g, and the injection hole is sealed by welding, whereby the structure shown in FIG. A rectangular nonaqueous electrolyte secondary battery having a thickness of 4.8 mm, a width of 30 mm, and a height of 48 mm was assembled.

(Examples 5 to 8 and Comparative Example 2)
Among the negative electrodes B using the composite graphite material B described above, five types having densities shown in Table 2 below were prepared. A square nonaqueous electrolyte secondary battery having the same configuration as that described in Example 1 was manufactured except that each negative electrode was used.

(Examples 9 to 12 and Comparative Example 3)
Among the negative electrodes C using artificial graphite described above, five types having densities shown in Table 2 below were prepared. A square nonaqueous electrolyte secondary battery having the same configuration as that described in Example 1 was manufactured except that each negative electrode was used.

(Examples 13 to 16)
Among the negative electrodes E using the mesophase microspheres A described above, four types having densities shown in Table 3 below were prepared. A square nonaqueous electrolyte secondary battery having the same configuration as that described in Example 1 was manufactured except that each negative electrode was used.

(Examples 17 to 19)
Among the negative electrodes F using the mesophase microspheres B described above, three types having densities shown in Table 3 below were prepared. A square nonaqueous electrolyte secondary battery having the same configuration as that described in Example 1 was manufactured except that each negative electrode was used. When a negative electrode having a press linear pressure of 174 kg / cm, an electrode density of 1.52 g / cm ³ and a negative electrode layer thickness of 66 μm per side was produced, both the active material and the copper foil were produced due to the high linear pressure. There was a lot of damage to the cell, so I didn't want to make it into a cell. In particular, the active material was broken.

(Examples 20 to 22)
Among the negative electrodes H using the above-described mesophase carbon fibers A, three types having densities shown in Table 3 below were prepared. A square nonaqueous electrolyte secondary battery having the same configuration as that described in Example 1 was manufactured except that each negative electrode was used. A negative electrode having a press linear pressure of 120 kg / cm, an electrode density of 1.52 g / cm ³ and a negative electrode layer thickness of 66 μm per side was produced. There was a lot of damage to the cell, so I didn't want to make it into a cell. In particular, the active material was broken.

(Examples 23 to 26)
Among the negative electrode G of 20 parts of mesophase microspheres B + flaky graphite, four types having densities shown in Table 3 below were prepared. A square nonaqueous electrolyte secondary battery having the same configuration as that described in Example 1 was manufactured except that each negative electrode was used.

(Comparative Example 4)
Among the negative electrodes D using the composite graphite material C described above, those having densities shown in Table 2 below were prepared. A square nonaqueous electrolyte secondary battery having the same configuration as that described in Example 1 was manufactured except that this negative electrode was used.

(Comparative Example 5)
Among the negative electrodes I using the mesophase carbon fibers B described above, those having densities shown in Table 3 below were prepared. A square nonaqueous electrolyte secondary battery having the same configuration as that described in Example 1 was manufactured except that this negative electrode was used.

  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, a charge / discharge cycle in which a constant current and constant voltage charge at a charge rate of 1.0 C and a charge end voltage of 4.2 V is performed for 3 hours and then discharged to 1.0 V at 1.0 C is repeated at 20 ° C. The discharge capacity at the cycle was measured, the capacity maintenance rate at 500 cycles was calculated with the discharge capacity at the first cycle as 100%, and the results and the discharge capacity at the first cycle are shown in Tables 1 and 2 below.

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

As is clear from Tables 1 and 2, when using a graphite material other than the mesophase pitch system, (A ₂ / B ₂ ) is 0.0033 to 0.007 and the density is 1.3 to 1.65 g. It turns out that the secondary battery of Examples 1-12 provided with the negative electrode of / cm < ³ > is excellent in both the initial stage capacity | capacitance and the capacity | capacitance maintenance factor at the time of 500 cycles.

On the other hand, in the secondary batteries of Comparative Examples 1 to 3 in which the negative electrode density deviated from an appropriate value, the capacity retention rate at 500 cycles was lower than in Examples 1 to 12. Further, in the secondary battery of Comparative Example 4 in which (A ₂ / B ₂ ) is smaller than 0.0033, the initial capacity and the capacity retention rate at 500 cycles were low for the electrode density.

On the other hand, from the results in Table 3, when using a mesophase pitch-based graphite material, (A ₁ / B ₁ ) is 0.001 to 0.003 and the density is 1.3 to 1.75 g / cm ³ . It can be seen that the secondary batteries of Examples 13 to 26 including the negative electrode are excellent in both the initial capacity and the capacity retention rate at 500 cycles.

  On the other hand, in the secondary battery of Comparative Example 5 having a low negative electrode density, the initial capacity was lower than that of Examples 13 to 26.

  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. The partial notch perspective view which shows the square nonaqueous electrolyte secondary battery which is an example of the nonaqueous electrolyte secondary battery which concerns on this invention. The characteristic view which shows the relationship between the press linear pressure and electrode density about the graphite material used for the nonaqueous electrolyte secondary battery of an Example.

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 (2)

A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode containing a mesophase pitch-based graphite material, and a non-aqueous electrolyte,
The said negative electrode satisfy | fills following (1) Formula, and a density is 1.3-1.75g / cm < ³ >, The nonaqueous electrolyte secondary battery characterized by the above-mentioned.
0.001 ≦ (A ₁ / B ₁ ) ≦ 0.003 (1)
A ₁ is an increase in negative electrode density (g / cm ³ ) when the linear pressure of a room temperature press using a roll having a diameter of 180 cm is increased stepwise from 20 to 180 kg / cm, and B ₁ is that of the room temperature press. The amount of change in linear pressure is 160 kg / cm. A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode containing a graphite material other than a mesophase pitch system, and a non-aqueous electrolyte,
The said negative electrode satisfy | fills following (2) Formula, and a density is 1.3-1.65 g / cm < ³ >, The nonaqueous electrolyte secondary battery characterized by the above-mentioned.
0.0033 ≦ (A ₂ / B ₂ ) ≦ 0.007 (2)
A ₂ is the negative electrode density increase amount (g / cm ³ ) when the linear pressure of the room temperature press using a roll having a diameter of 180 cm is increased stepwise from 20 to 160 kg / cm, and B ₂ is the temperature of the room temperature press. The amount of change in linear pressure is 140 kg / cm.

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