JP2012146590A - Positive electrode for nonaqueous electrolyte secondary battery, method for producing positive electrode, and nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery excellent in cycle characteristics and high in capacity by increasing the density of positive electrode active material particles in a positive electrode mixture layer.SOLUTION: A positive electrode for a nonaqueous electrolyte secondary battery includes a positive electrode current collector and a positive electrode mixture layer formed on a surface of the positive electrode current collector. The positive electrode mixture layer includes a positive electrode active material and a conductive auxiliary agent, and has a density in a range of 3.5 to 4.0 g/cm. The positive electrode active material contains a first active material particles having an average particle size (D) of 18 to 25 μm and a second active material particles having an average particle size (D) of 3 to 7 μm at a blending ratio of 9:1 to 6:4, the conductive auxiliary agent contains carbon black and expanded graphite at a blending ratio of 7:3 to 3:7, and the expanded graphite has an average particle size in a rage of 1 to 5 μm.

Description

  The present invention relates to a positive electrode for a non-aqueous electrolyte secondary battery, and in particular, relates to an improvement of a densified positive electrode mixture layer.

  In recent years, with the reduction in size and weight of electronic devices such as mobile phones and notebook computers, it is required to increase the capacity of secondary batteries that are power sources of these devices. Due to such demands, non-aqueous electrolyte secondary batteries capable of increasing energy density are widely used. A non-aqueous electrolyte secondary battery includes a positive electrode, a negative electrode, a separator disposed so as to be interposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte. The positive electrode, the negative electrode, and the separator are wound to form an electrode group.

  A positive electrode for a non-aqueous electrolyte secondary battery (hereinafter also simply referred to as a positive electrode) has a positive electrode current collector and a positive electrode mixture layer formed on the surface of the positive electrode current collector. The positive electrode mixture layer includes positive electrode active material particles, a conductive additive, and a binder.

  For the purpose of further increasing the capacity of the nonaqueous electrolyte secondary battery, the positive electrode active material particles are filled in the positive electrode mixture layer at a high density to increase the density of the positive electrode mixture layer. Increasing the thickness is being studied.

  The following Patent Document 1 discloses a positive electrode mixture layer using a conductive additive in which expanded graphite and carbon black are mixed at a blending ratio of 1: 1 to 5: 1. And it is stated that according to the nonaqueous electrolyte secondary battery provided with such a positive electrode mixture layer, the capacity of the battery is increased and the charge / discharge cycle characteristics are improved.

  Patent Document 2 below has a high capacity, load characteristics and cycle characteristics, characterized in that the average particle diameter of the positive electrode active material is in the range of 5 to 20 μm, and expanded graphite is used as the conductive material. Discloses an excellent non-aqueous secondary battery.

  Moreover, the following patent document 3 is disclosing the positive electrode containing the conductive support agent containing the expanded graphite with an average particle diameter of 1-30 micrometers. And it is described that according to the nonaqueous electrolyte secondary battery provided with such a positive electrode, the utilization of the active material can be improved and the capacity can be increased by improving the conductivity of the positive electrode.

Japanese Patent No. 3115256 JP 2001-319658 A JP-A-10-188993

  Patent Documents 1 to 3 all disclose a positive electrode containing expanded graphite as a conductive additive. However, in the positive electrodes disclosed in Patent Documents 1 to 3, the conductivity between the positive electrode active material particles could not be sufficiently ensured unless a conductive assistant was blended at a certain high ratio. For example, in the Example of Patent Document 1, a positive electrode mixture containing 6% by weight of a conductive additive is disclosed. However, when 6% by weight of the conductive assistant is blended, there is a problem that it is difficult to increase the density of the positive electrode active material particles. That is, in order to increase the density of the positive electrode active material particles, if the positive electrode mixture layer is rolled with a high pressure at the time of rolling of the positive electrode mixture, the positive electrode active material particles are cracked and become conductive. There is a problem that the cycle characteristics deteriorate due to a decrease in the cycle time.

  An object of the present invention is to provide a high-capacity non-aqueous electrolyte secondary battery excellent in cycle characteristics by increasing the density of positive electrode active material particles in a positive electrode mixture layer.

  The present inventors blended two types of positive electrode active material particles having an average particle diameter in a specific range as a positive electrode active material, and further added carbon black and expanded graphite having an average particle diameter in a specific range: According to the positive electrode mixture compounded at a mixing ratio of 3 to 3: 7, the occurrence of particle cracking of the positive electrode active material particles is suppressed during rolling of the positive electrode mixture layer, and the density of the positive electrode mixture layer is easily increased. I found out that In addition, by mixing the positive electrode active material particles having such an average particle diameter, carbon black, and expanded graphite, the positive electrode active material particles are sufficiently brought into contact with each other. However, it has been found that sufficient conductivity can be obtained, and the present invention has been completed.

That is, one aspect of the present invention includes a positive electrode current collector and a positive electrode mixture layer formed on the surface of the positive electrode current collector, the positive electrode mixture layer includes a positive electrode active material and a conductive additive, and has a density. in the range of 3.5~4.0g / cm ^3, average particle diameter (D ₅₀₎ the positive electrode active material average particle diameter (D ₅₀₎ and the first active material particles of 18~25μm second active of 3~7μm The material particles are contained in a blending ratio of 9: 1 to 6: 4, the conductive assistant contains carbon black and expanded graphite in a blending ratio of 7: 3 to 3: 7, and the average particle diameter of the expanded graphite is It is a positive electrode for nonaqueous electrolyte secondary batteries in the range of 1 to 5 μm. In such a positive electrode, the first active material particles and the second active material particles each having predetermined average particles are blended at a predetermined blending ratio, so that the positive electrode active material particles can be easily filled at a high density. In addition, by blending carbon black and expanded graphite having an average particle diameter within a predetermined range at a predetermined blending ratio, the conductivity imparting effect by the conductive auxiliary agent can be sufficiently maintained, and at the time of rolling Since expanded graphite becomes a lubricant, it is easy to increase the density.

Another aspect of the present invention is a mixture layer forming step for forming a positive electrode mixture layer by applying a positive electrode mixture slurry to the surface of a positive electrode current collector, and a rolling step for rolling the positive electrode mixture layer. When provided with a positive electrode mixture slurry as a positive electrode active material, the average particle diameter (D ₅₀₎ a first active material particles 18~25μm average particle diameter (D ₅₀₎ and a second active material particles 3~7μm In a blending ratio of 9: 1 to 6: 4, and further, carbon black as a conductive additive and expanded graphite having an average particle diameter in the range of 1 to 5 μm in a blending ratio of 7: 3 to 3: 7. It is a manufacturing method of the positive electrode for nonaqueous electrolyte secondary batteries which contains and is rolled so that it may become a range of density 3.5-4.0 g / cm < ³ > in a rolling process. The positive electrode mixture slurry having such a composition is rolled to a density in the range of 3.5 to 4.0 g / cm ³ in the rolling process, thereby suppressing cracks and the like of the positive electrode active material and increasing the density. Can be rolled.

  Another aspect of the present invention is a nonaqueous electrolyte secondary battery including an electrode group obtained by winding the positive electrode and the negative electrode with a separator interposed therebetween, and a nonaqueous electrolyte.

  ADVANTAGE OF THE INVENTION According to this invention, the high capacity | capacitance nonaqueous electrolyte secondary battery excellent in cycling characteristics can be provided.

It is a longitudinal cross-sectional schematic diagram which shows roughly the structure of the square-shaped nonaqueous electrolyte secondary battery which concerns on one Embodiment of this invention.

First, the positive electrode for a nonaqueous electrolyte secondary battery according to this embodiment will be described in detail.
The positive electrode for a non-aqueous electrolyte secondary battery of the present embodiment has a positive electrode current collector and a positive electrode mixture layer formed on the surface of the positive electrode current collector. The positive electrode mixture layer includes positive electrode active material particles, a conductive auxiliary agent, and a binder.

  As the positive electrode active material particles, a lithium-containing composite oxide is usually used. Any lithium-containing composite oxide can be used without particular limitation as long as it is conventionally used as a positive electrode active material for a non-aqueous electrolyte secondary battery. In particular, for example, a lithium-containing composite oxide having a layered or hexagonal crystal structure or a spinel structure containing at least one element selected from the group consisting of Ni, Co, and Mn is preferable. In particular, lithium-containing composite oxides containing Ni have a high capacity, but the filling properties tend to decrease. According to the present invention, even when such lithium-containing composite oxides are used, the filling properties are reduced. Since it can be improved, an extremely high capacity can be realized.

Specific examples of the lithium-containing composite oxide containing at least one element selected from the group consisting of Ni, Co and Mn include, for example, lithium nickelate (LiNiO ₂ ), a modified lithium nickelate, lithium cobaltate ( LiCoO ₂ ), lithium cobaltate modifications, lithium manganate (LiMn ₂ O ₄ ), lithium manganate modifications, some of these oxides Co, Ni or Mn, other transition metal elements, aluminum And the like substituted with a typical metal element such as magnesium or an alkaline earth metal element such as magnesium.

More preferably, for example, the general _{formula: Li x M y Me 1-} y O z (M is, Ni, at least one element selected from the group consisting of Co and Mn, Me is Al, Mg, Fe , Ca, Ti, Zr, and P, at least one element selected from the group consisting of 0.9 ≦ x ≦ 1.1, 0.9 ≦ y ≦ 2.0, and 1.9 ≦ z ≦ 4. A lithium-containing composite oxide represented by 0).

Positive electrode active material of the present embodiment, the average particle diameter (D ₅₀₎ mean particle diameter and the first active material particles of 18~25μm (D ₅₀₎ and a second active material particles of 3 to 7 [mu] m 9: 1 to 6: 4 in a mixing ratio. Such a positive electrode active material usually has at least two peaks, a peak in the particle size distribution of the first active material particles and a peak in the particle size distribution of the second active material particles. Specifically, it has one peak between 18 and 25 μm and one peak between 3 and 7 μm. Densifying the positive electrode mixture layer obtained by rolling by using a positive electrode active material in which the first active material particles and the second active material particles are mixed at a mixing ratio of 9: 1 to 6: 4. Can do. The average particle diameter (D ₅₀ ) of the positive electrode active material particles means the median diameter of the volume-based particle size distribution as measured with a particle size distribution meter.

The positive electrode active material particles are composed of first active material particles having an average particle diameter (D ₅₀ ) of 20 to 23 μm and second active material particles having an average particle diameter (D ₅₀ ) of 3 to 5 μm of 8: 2 to 7: 3. It is preferable to contain by the compounding ratio of. By containing such a positive electrode active material, more excellent filling properties can be obtained, so that the positive electrode active material particles in the positive electrode mixture layer can be densified.

When the positive electrode active material particles having an average particle diameter (D ₅₀ ) exceeding 25 μm are contained, the electron conductivity and ionic conductivity at the time of charge / discharge are lowered. In addition, when the positive electrode active material particles having an average particle diameter (D ₅₀ ) of less than 3 μm are contained, the positive electrode active material particles of less than 3 μm generate a sliding resistance during rolling, thereby sufficiently increasing the density. It becomes difficult.

Further, the average particle diameter of the entire positive electrode active material particles (D _50), 10 to 25 [mu] m, further particularly 12～18Myuemu.

The conductive additive contains carbon black and expanded graphite having an average particle diameter in the range of 1 to 5 μm in a mixing ratio of 7: 3 to 3: 7, preferably in a mixing ratio of 6: 4 to 4: 6. . When the compounding ratio of carbon black is too larger than 7: 3, the bulk density of the conductive auxiliary agent increases and the filling property of the positive electrode active material particles decreases. Moreover, when the compounding ratio of the expanded graphite whose average particle diameter is in the range of 1 to 5 μm is more than 3: 7, the capacity and cycle characteristics are lowered due to the decrease in conductivity. The average particle diameter (D ₅₀ ) of expanded graphite means the median diameter of the volume-based particle size distribution measured with a particle size distribution meter.

  The type of carbon black used in the present embodiment is not particularly limited, and any carbon black can be used without particular limitation as long as it is conventionally used as a conductive additive for a positive electrode of a nonaqueous electrolyte secondary battery. Specific examples of carbon black include acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, and the like. The average particle diameter of carbon black is not particularly limited, and is not particularly limited as long as it is in the range of, for example, 10 to 300 nm, which has been conventionally used as a conductive additive.

  As the expanded graphite, a conventionally known method, specifically, for example, highly crystallized graphite such as natural graphite, quiche graphite, and pyrolytic graphite is mixed acid of concentrated sulfuric acid and nitric acid, concentrated sulfuric acid and permanganese. Oxidation treatment such as chemical treatment, electrolytic treatment, etc., soaking in a strong oxidizing solution such as mixed acid of potassium acid, concentrated sulfuric acid and hydrogen peroxide water, etc. to produce graphite-sulfuric acid intercalation compound, water washing and drying It is obtained by compressing expanded graphite obtained by rapid heating and expanding in the C-axis direction of the graphite crystal into a sheet, and then pulverizing with a known pulverizer such as a jet mill and classifying as necessary. The expanded graphite particles having an average particle diameter in the range of 1 to 5 μm are used without particular limitation.

  The average particle diameter of the expanded graphite used in the present embodiment is in the range of 1 to 5 μm, and preferably in the range of 3 to 5 μm. When the particle size of the expanded graphite is less than 1 μm, the fine expanded graphite generates slip resistance during rolling, thereby reducing the filling property of the positive electrode mixture during rolling. Further, when the expanded graphite particle size exceeds 5 μm, the expanded graphite size is too large, which makes it difficult to fill the voids formed between the positive electrode active material particles. For this reason, the mixture density is lowered and the conductivity is also lowered.

  The blending ratio of the conductive additive to 100 parts by mass of the positive electrode active material is preferably 0.5 to 3 parts by mass, and more preferably 1 to 2 parts by mass. When there are too many conductive assistants, it tends to be difficult to increase the density of the positive electrode active material particles. When the amount of the conductive auxiliary is too small, it tends to be difficult to ensure sufficient conductivity in the positive electrode.

  In addition, the blending ratio of the expanded graphite having an average particle diameter of 1 to 5 μm with respect to 100 parts by mass of the positive electrode active material is preferably 0.2 to 2 parts by mass, and more preferably 0.4 to 1.5 parts by mass. When the proportion of expanded graphite is too large, it tends to be difficult to ensure sufficient conductivity in the positive electrode. Moreover, when the compounding ratio of expanded graphite is too low, there exists a tendency for the densification of a positive electrode active material particle to become difficult because the slip property of a positive electrode mixture falls at the time of rolling.

The binder used in the present embodiment is not particularly limited as long as it is a binder that has been conventionally used for binding positive electrode active material particles of a nonaqueous electrolyte secondary battery. Specific examples of the binder include, for example, polyethylene, polypropylene, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), and vinylidene fluoride-hexafluoro. A propylene copolymer or the like is used. The content ratio of the binder is 100 parts by mass of the positive electrode active material.
It is preferably 0.5 to 3 parts by mass, and more preferably 0.7 to 1.5 parts by mass.

The positive electrode is produced, for example, by the following method. First, the positive electrode mixture slurry is prepared by mixing the positive electrode active material particles, the conductive additive, the binder, and the dispersion medium as described above. And the obtained positive mix slurry is apply | coated on both surfaces of a positive electrode electrical power collector, and it is made to dry as needed, and forms a positive mix layer. Then, the positive electrode mixture layer formed on the surface of the positive electrode current collector is rolled with a roller or the like so as to be in a density range of 3.5 to 4.0 g / cm ³ .

  As the positive electrode current collector, a sheet or a metal foil containing a metal material conventionally used as a positive electrode current collector of a nonaqueous electrolyte secondary battery can be used without any particular limitation. Specific examples of the metal material include stainless steel, aluminum, titanium, and the like. The thickness of the positive electrode current collector is not particularly limited, but is preferably 5 to 20 μm.

The positive electrode mixture slurry as the positive electrode active material, the average particle diameter (D ₅₀₎ mean particle diameter and the first active material particles of 18~25μm (D ₅₀₎ and a second active material particles of 3 to 7 [mu] m 9:. 1 to 6: 4 is blended at a blending ratio, and carbon black and expanded graphite having an average particle diameter in the range of 1 to 5 μm are blended at a blending ratio of 7: 3 to 3: 7 as a conductive assistant, It is prepared by mixing a binder and a known dispersion medium such as N-methyl-2-pyrrolidone (NMP). The blending ratio of the conductive additive and the binder in the positive electrode mixture slurry is as described above.

The positive electrode mixture slurry thus prepared is applied to the surface of the positive electrode current collector and then dried to form a positive electrode mixture layer. Then, the positive electrode mixture layer formed on the surface of the positive electrode current collector is rolled with a roller or the like so that the density is in the range of 3.5 to 4.0 g / cm ^3. can get. The linear pressure during rolling is preferably about 1.0 × 10 ^{3 to} 2.0 × 10 ³ kgf / cm, for example. Moreover, rolling may be performed only once or multiple times.

  In the step of rolling the positive electrode current collector, the positive electrode current collector on which the positive electrode mixture layer is formed is preferably rolled while being heated to 20 to 100 ° C. Thereby, the density of a positive mix layer can be made higher.

The density of the positive electrode mixture layer after rolling is 3.5 to 4.0 g / cm ³ , and preferably 3.7 to 4.0 g / cm ³ . When the density of the positive electrode mixture layer after rolling is less than 3.5 g / cm ³ , a sufficiently high capacity battery cannot be obtained, and when the density exceeds 4.0 g / cm ³ , the positive electrode active material There is a possibility that the particles are compressed too much to be easily rolled and cracks may occur. The density of the positive electrode mixture layer can be calculated from the mass of the components of the positive electrode mixture contained in the positive electrode mixture layer per unit volume (generally, the total of the positive electrode active material, the conductive material and the binder). it can.

  The porosity of the positive electrode mixture layer after rolling thus obtained is preferably 20% or less, more preferably 18% or less. Moreover, as thickness of the positive mix layer after rolling, it is preferable that it is the range of 30-90 micrometers, Furthermore, 40-60 micrometers.

  Next, the nonaqueous electrolyte secondary battery of this embodiment will be described. The non-aqueous electrolyte secondary battery of this embodiment includes an electrode group and a non-aqueous electrolyte obtained by winding the positive electrode and the negative electrode as described above with a separator interposed therebetween.

If a negative electrode is conventionally used as a negative electrode of a nonaqueous electrolyte secondary battery, it may be used without limitation. Such a negative electrode is obtained, for example, by applying a negative electrode mixture slurry containing a negative electrode active material and a binder to a negative electrode current collector, drying, and rolling. Examples of the negative electrode active material include carbon materials, metals, alloys, metal oxides, and metal nitrides. As specific examples of the carbon material, for example, natural graphite and artificial graphite are preferably used. As the metal or alloy, lithium alone, lithium alloy, silicon alone, silicon alloy, tin alone, tin alloy and the like are preferable. Specific examples of the metal oxide include SiO _x (0 <x <2, preferably 0.1 ≦ x ≦ 1.2). Examples of the negative electrode current collector include sheets and foils containing materials such as copper, nickel, and stainless steel.

As the nonaqueous electrolyte, any nonaqueous electrolyte that has been conventionally used as a nonaqueous electrolyte for a nonaqueous electrolyte secondary battery can be used without any particular limitation. For example, a liquid electrolyte composed of a nonaqueous solvent and a solute dissolved therein is preferred. As a specific example of the nonaqueous solvent, for example, a mixed solvent of a cyclic carbonate such as ethylene carbonate or propylene carbonate and a chain carbonate such as dimethyl carbonate, diethyl carbonate, or ethyl methyl carbonate is preferably used. Further, γ-butyrolactone, dimethoxyethane and the like are also used. Examples of lithium salts include inorganic lithium fluorides and lithium imide compounds. Examples of the inorganic lithium fluoride include LiPF ₆ and LiBF ₄ , and examples of the lithium imide compound include LiN (CF ₃ SO ₂ ) ₂ .

  Any separator can be used without particular limitation as long as it is conventionally used as a separator for non-aqueous electrolyte secondary batteries. Specific examples include microporous films made of polyethylene, polypropylene, and the like. Although the thickness of a separator is not specifically limited, For example, it is preferable that it is 10-30 micrometers.

  The present invention is applicable to non-aqueous electrolyte secondary batteries having various shapes such as a cylindrical shape and a rectangular shape.

  Next, the present invention will be specifically described based on examples and comparative examples. The scope of the present invention is not construed as being limited in any way by the examples.

[Example 1]
(I) Preparation average particle diameter of the positive electrode (D ₅₀₎ 20 [mu] m and LiNi _0.80 Co _0.15 Al _0.05 O ₂ having an average particle diameter (D ₅₀₎ 5 [mu] m and LiNi _0.80 Co _0.15 Al _0.05 O ₂ of 8: 2 in weight ratio For 100 parts by mass of the mixed positive electrode active material particles, 0.9 parts by mass of the binder (PVDF, weight average molecular weight 1 million), 0.75 parts by mass of acetylene black, 0.75 parts by mass of expanded graphite, and a dispersion medium A positive electrode mixture slurry was prepared by mixing an appropriate amount of N-methyl-2-pyrrolidone (NMP). The expanded graphite was prepared as follows. A graphite intercalation compound was obtained by adding 50 g of natural graphite to 500 g of concentrated sulfuric acid of 98% by mass and adding an appropriate amount of 25% by mass of hydrogen peroxide while stirring. And the obtained graphite intercalation compound was washed with water and dried, and expanded graphite was obtained by heating at 800 ° C. The obtained expanded graphite was compression molded into a sheet with a roll and then pulverized with a jet mill to obtain expanded graphite particles having an average particle diameter (D ₅₀ ) of 3 μm.

And after apply | coating the obtained positive mix slurry on both surfaces of the positive electrode electrical power collector which consists of 15-micrometer-thick aluminum foil (Sumi Light Aluminum Foil Co., Ltd. aluminum alloy foil, Vespa FS13), at 80 degreeC A positive electrode mixture layer was formed by drying for 10 minutes. The density of the positive electrode mixture layer before rolling was 2.2 g / cm ³ . And the positive electrode sheet was produced by rolling a coating film with a roller so that the density of a positive mix layer might be 3.7 g / cm < ³ >, and the porosity may be 17%. The linear pressure of the roller was 2.0 × 10 ³ kgf / cm. The thickness of the positive electrode mixture layer after rolling was 50 μm, and the thickness of the entire sheet was 115 μm. A positive electrode was produced by cutting the positive electrode sheet into a width of 35 mm and a length of 450 mm.

(Ii) Production of negative electrode 100 parts by mass of artificial graphite as a negative electrode active material, 1.0 part by mass of a styrene-butadiene copolymer (BM-400B manufactured by Nippon Zeon Co., Ltd.) as a binder, and a thickener A negative electrode mixture slurry was prepared by mixing 1.0 part by mass of a certain carboxymethyl cellulose (CMC) and an appropriate amount of water as a dispersion medium.

  After apply | coating the negative mix slurry obtained on both surfaces of the negative electrode collector which consists of copper foil with a thickness of 10 micrometers, the negative mix layer was formed by making it dry at 80 degreeC for 20 minutes. And the negative electrode sheet was produced by rolling a coating film with a roller so that the thickness of a negative mix layer might be set to 140 micrometers. And the negative electrode was produced by cut | disconnecting a negative electrode sheet into width 37mm x length 460mm.

(Iii) Production of Electrode Group A positive electrode lead made of aluminum was attached to the end of the positive electrode current collector of the positive electrode, and a negative electrode lead made of nickel was attached to the end of the negative electrode current collector of the negative electrode. Then, a positive electrode and a negative electrode were wound through a separator made of a polyethylene microporous film having a thickness of 15 μm between them to produce an electrode group of type 504042 having a substantially elliptical cross section. .

(Iv) Production of Battery A square lithium ion secondary battery as shown in FIG. 1 was produced as follows.
The electrode group (electrode group 21) obtained as described above was accommodated in the rectangular battery case 20. The battery case 20 has a bottom part and a side wall, the top part is opened, and the shape is substantially rectangular. The thickness of the main flat part of the side wall was 80 μm. Thereafter, an insulator 24 for preventing a short circuit between the battery case 20 and the positive electrode lead 22 was disposed on the electrode group 21. Next, a rectangular sealing plate 25 having a negative electrode terminal 27 surrounded by an insulating gasket 26 in the center was disposed in the opening of the battery case 20. The negative electrode lead 22 was connected to the negative electrode terminal 27. The positive electrode lead 23 was connected to the lower surface of the sealing plate 25. The end of the opening and the sealing plate 25 were welded by laser to seal the opening of the battery case 20. Thereafter, a nonaqueous electrolyte was injected into the battery case 20 from the injection hole of the sealing plate 25. As the non-aqueous electrolyte, a solution obtained by dissolving LiPF ₆ at a concentration of 1.4 mol / L in a mixed solvent containing ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate in a volume ratio of 3: 3: 4 was used. Finally, the liquid injection hole was closed by welding with a sealing plug 29 to produce a prismatic lithium ion secondary battery having a height of 42 mm, a width of 40 mm, a thickness of about 5.0 mm, and a design capacity of 1000 mAh.

[Evaluation of square battery]
The cycle characteristics of the obtained prismatic lithium ion secondary battery were evaluated by the following methods.
(1) Evaluation of discharge capacity density In a 20 degreeC environment, it charged / discharged on the following conditions and calculated | required the initial capacity.
Based on the initial capacity, the discharge capacity density was determined from the following formula.
Discharge capacity density (mAh / cm ³ ) = initial discharge capacity (mAh) / volume of mixture layer (cm ³ )
<Charging / discharging conditions>
Constant current charging: Charging current value 700mA / end-of-charge voltage 4.2V
Constant voltage charging: Charging voltage value 4.2V / end-of-charge current 100mA
Constant current discharge: discharge current value 1000 mA / discharge end voltage 2.5 V
(2) Evaluation of capacity maintenance rate Under 20 degreeC environment, it charged / discharged on the following conditions and calculated | required the initial capacity.
Thereafter, charging and discharging were repeated 500 cycles under the following conditions in a 20 ° C. environment, and the discharge capacity at the 500th cycle was determined. And the cycle capacity maintenance rate was calculated | required by the following formula.
Cycle capacity retention rate (%) = 500th cycle discharge capacity / initial discharge capacity × 100

<Charging / discharging conditions>
Constant current charging: Charging current value 700mA / end-of-charge voltage 4.2V
Constant voltage charging: Charging voltage value 4.2V / end-of-charge current 100mA
Constant current discharge: discharge current value 1000 mA / discharge end voltage 2.5 V

(3) Evaluation of increase rate of battery thickness The charge / discharge cycle was repeated under the same conditions as in (1), and the thickness of the battery after 500 cycles was measured. And the increase rate of the battery thickness after 500 cycles with respect to the initial battery thickness measured beforehand was calculated | required by the following formula.
Rate of increase in battery thickness (%) = battery thickness after 500 cycles / initial battery thickness × 100

  The results are shown in Table 1.

[Examples 2-3 and Comparative Examples 1-2]
A positive electrode mixture slurry was prepared in the same manner as in Example 1 except that the mixing ratio of acetylene black and expanded graphite to 100 parts by mass of the positive electrode active material particles in the positive electrode slurry was changed to the ratio shown in Table 1. Type lithium ion secondary batteries were fabricated and evaluated. The results are shown in Table 1.

As in Examples 1 to 3 in Table 1, according to the positive electrode having a mixture layer density of 3.69 g / cm ³ or more using a positive electrode containing acetylene black and expanded graphite as a conductive additive, the capacity after 500 cycles It was found that the maintenance rate was high and the battery thickness increase rate after 500 cycles was low. On the other hand, in Comparative Example 1 using only acetylene black, it was difficult to increase the density of the positive electrode. Further, in Comparative Example 2 using expanded graphite, the positive electrode could be densified, but it was found that the capacity retention rate after 500 cycles was low and the battery thickness increase rate after 500 cycles was high. It was found that the positive electrode containing carbon black and expanded graphite of Examples 1 to 3 according to the present invention was capable of increasing the density of the mixture layer to 3.69 g / cm ³ or more and excellent in cycle characteristics. .

[Examples 4-7 and Comparative Examples 3-4]
A positive electrode mixture slurry was prepared in the same manner as in Example 1 except that the composition of the positive electrode active material in the positive electrode slurry and the average particle size of the positive electrode active material were changed as shown in Table 2. A secondary battery was fabricated and evaluated. The results are shown in Table 2.

As in Examples 4 and 5 in Table 2, when the mixing ratio of the positive electrode active material is in the range of 6: 4 to 9: 1, the mixture density is 3.61 g / cm ³ or more and the capacity is maintained after 500 cycles. It has been found that the rate is improved and the battery thickness increase rate is reduced. On the other hand, when the ratio was 5: 5, the mixture density was as low as 3.48, and it was found that the characteristics of the capacity retention rate and the battery thickness increase rate were reduced.
When the case where the large average particle diameter of the positive electrode active material is in the range of 25 to 18 μm and the small average particle diameter is in the range of 3 to 7 μm is combined as in Examples 6 and 7 in Table 2, the mixture density is It was found to be 3.59 g / cm ³ or more, the capacity retention rate after 500 cycles was high, and the battery thickness increase rate was low. On the other hand, in the case of Comparative Example 4 using only one kind of positive electrode active material particles having an average particle diameter, the mixture density is 3.40 g / cm ³ , the capacity retention rate is low, and the battery thickness increase rate is Was found to be expensive.

[Examples 8 to 9 and Comparative Examples 5 to 6]
A positive electrode mixture slurry was prepared in the same manner as in Example 1 except that the particle size of the expanded graphite in the positive electrode slurry was changed as shown in Table 3, and a square lithium ion secondary battery was prepared and evaluated. The results are shown in Table 3.

  As in Examples 8 and 9 in Table 3, when the average particle diameter of the expanded graphite is in the range of 1 to 5 μm, the mixture density is 3.69 g / cc or more, and the capacity retention rate after 500 cycles is high. The rate of increase in thickness was low. On the other hand, in Comparative Example 5 in which the average particle diameter of the expanded graphite was 0.5 μm, since the particle size was too small, it was not possible to sufficiently increase the density due to friction caused by fine particles during rolling. Moreover, when the average particle diameter of the expanded graphite was 7 μm, the expanded graphite did not fit into the gaps between the active material particles, so that the mixture density decreased. As a result, in Comparative Example 5 and Comparative Example 6, the capacity retention rate was low and the battery thickness increase rate was high.

  A battery using the positive electrode for a nonaqueous electrolyte secondary battery of the present invention is useful as a power source suitable for reducing the size and weight of electronic devices such as mobile phones and laptop computers.

20 Battery Case 21 Electrode Group 22 Negative Electrode Lead 23 Positive Electrode Lead 24 Insulator 25 Sealing Plate 26 Insulating Gasket 27 Negative Electrode Terminal 29 Sealing

Claims (7)

A positive electrode current collector and a positive electrode mixture layer formed on the surface of the positive electrode current collector;
The positive electrode mixture layer includes a positive electrode active material and a conductive additive, and has a density in the range of 3.5 to 4.0 g / cm ³ .
The positive electrode active material average particle diameter (D ₅₀₎ a first active material particles with an average particle diameter of 18~25μm (D ₅₀₎ 3~7μm the second active material and particles 9: 1 to 6: 4 mixing ratio Contained in
The conductive assistant contains carbon black and expanded graphite in a blending ratio of 7: 3 to 3: 7,
The positive electrode for nonaqueous electrolyte secondary batteries, wherein the expanded graphite has an average particle diameter in the range of 1 to 5 μm.   The positive electrode for a nonaqueous electrolyte secondary battery according to claim 1, wherein the conductive auxiliary agent is contained in an amount of 0.5 to 3 parts by mass with respect to 100 parts by mass of the positive electrode active material.   The positive electrode for a nonaqueous electrolyte secondary battery according to claim 1, wherein 0.2 to 2 parts by mass of the expanded graphite is contained with respect to 100 parts by mass of the positive electrode active material.   4. The lithium-containing composite oxide containing at least one element selected from the group consisting of Ni, Co, and Mn, at least one of the first active material particles and the second active material particles. The positive electrode for nonaqueous electrolyte secondary batteries of Claim 1. A mixture layer forming step of forming a positive electrode mixture layer by applying a positive electrode mixture slurry to the surface of the positive electrode current collector, and a rolling step of rolling the positive electrode mixture layer,
The positive electrode mixture slurry contains 9: 1 first active material particles having an average particle diameter (D ₅₀ ) of 18 to 25 μm and second active material particles having an average particle diameter (D ₅₀ ) of 3 to 7 μm as a positive electrode active material. -6: 4, and further contains carbon black as a conductive additive and expanded graphite having an average particle size in the range of 1 to 5 μm in a ratio of 7: 3 to 3: 7.
A method for producing a positive electrode for a non-aqueous electrolyte secondary battery, wherein the rolling is performed so that the density is in a range of 3.5 to 4.0 g / cm ^{3 in} the rolling step.   The manufacturing method of the positive electrode for nonaqueous electrolyte secondary batteries of Claim 5 which contains the said conductive support agent 0.5-3 mass parts with respect to 100 mass parts of said positive electrode active materials.   A nonaqueous electrolyte secondary battery comprising an electrode group obtained by winding the positive electrode and the negative electrode according to any one of claims 1 to 4 with a separator interposed therebetween, and a nonaqueous electrolyte.

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