JP2971451B1 - Lithium secondary battery - Google Patents

Abstract

An object of the present invention is to provide a lithium secondary battery having excellent charge / discharge cycle life characteristics. SOLUTION: The lithium composite metal oxide represented by a composition formula LiNi _1-x M _x O ₂ (wherein M is composed of one or more elements and x represents 0 <x ≦ 0.5) And a positive electrode 4 containing a conductive agent containing a carbonaceous material A having an average particle size of 100 nm or less and a carbonaceous material B having an average particle size of 1 μm or more, and a binder containing an acrylic rubbery copolymer. It is characterized by.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a lithium secondary battery, and more particularly to a lithium secondary battery having an improved positive electrode.

[0002]

2. Description of the Related Art In recent years, as electronic devices such as mobile phones and VTRs have been reduced in size and demand has increased, there has been a demand for higher capacity secondary batteries which are power sources for these electronic devices. In addition, air pollution due to exhaust gas from automobiles has become a social problem, and there is a demand for a lightweight and high-performance secondary battery as a power supply for electric vehicles.

[0003] As such a secondary battery, L is used as an active material.
A non-aqueous electrolyte secondary battery including a positive electrode containing a cobalt-based oxide such as iCoO ₂ and a negative electrode containing a carbonaceous material has been developed and is now widely used.

However, LiCoO ₂ , which is a positive electrode material of the secondary battery, is expensive because of containing Co and is limited in resources.
O ₂ or LiNi _1-x Co _{x in} which Ni is partially replaced by Co
Metal oxide compounds such as O ₂ and LiMn ₂ O ₄ have been proposed and are being actively researched.

[0005] In particular, LiNiO ₂ and LiNi _1-x Co
_x A positive electrode containing a nickel-based oxide such as O ₂ as an active material can have a higher energy density than a positive electrode containing a cobalt-based oxide as an active material, and can reduce the cost of a battery. In addition, the non-aqueous electrolyte secondary battery has a feature that the capacity characteristics are improved.

Incidentally, aside from the laboratory, industrially, the positive electrode for the secondary battery is a coating liquid obtained by adding an active material to a solution in which a binder is dispersed in an organic solvent, stirring and mixing. Is applied on a current collector, dried, and then rolled into a thin plate. Examples of the binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), ethylene-propylene diene copolymer (EPDM), and styrene butadiene rubber (S
BR) or the like can be used. Among these binders, polyvinylidene fluoride is excellent in dissolution resistance to an electrolytic solution and liquid retention, and is one of the materials suitable as a binder for the secondary battery electrode.

However, polyvinylidene fluoride,
A copolymer having a basic structure of vinylidene fluoride has a low alkali resistance, so that when an alkali salt is contained in the coating liquid, the coating liquid gels and hardens in a relatively short time. There is.

In general, in the case of lithium composite metal oxides represented by LiCoO ₂ and LiNiO ₂ , it is inevitable that surplus alkali is mixed during the production process. In particular, LiNiO
_{In the case of 2} , since an excessive amount of alkali is required at the time of synthesis, gelling of the coating liquid due to this excess alkali salt occurs remarkably. The cause of this gelation is considered as follows.
That is, the lithium salt (alkali component) contained in the synthesized active material causes a dehydrofluorination (HF) reaction of polyvinylidene fluoride to generate a carbon-carbon double bond. This double bond is extremely unstable and causes a cross-linking reaction between molecules or within a molecule, that is, gelation. If such a reaction occurs during the preparation of the coating liquid, it becomes difficult to apply the coating liquid to the current collector. Further, when the crosslinking reaction occurs, polyvinylidene fluoride functioning as a binder in the coating liquid decreases. Therefore, even if the coating liquid can be applied to the current collector to obtain a positive electrode, the adhesion between the positive electrode layer and the current collector is reduced, and the current is collected by the positive electrode layer as the charge / discharge cycle proceeds. There is a problem in that the charge / discharge cycle life is reduced due to peeling from the body.

As a countermeasure for preventing gelation, a method of removing excess alkali salts by sintering and washing the lithium composite metal oxide with water may be considered. However, washing with water makes the manufacturing operation complicated, which leads to an increase in cost. In addition, LiNiO ₂ is unstable with water, and thus has a problem in that the performance is greatly deteriorated due to washing with water.

In Japanese Patent Application Laid-Open No. 9-306502, a positive electrode active material composed of a composite metal oxide such as LiNi _0.8 Co _0.2 O ₂ is provided with a conductive auxiliary such as conductive carbon black and a vinylidene fluoride compound. It is disclosed that gelation of a slurry is prevented by adding a coalesced substance and an organic acid such as maleic acid and kneading the mixture in the presence of a solvent to prepare a slurry.

However, the adhesion between the slurry and the current collector is reduced by the salt generated by the neutralization reaction between the alkali salt and the organic acid contained in the composite metal oxide, and the charge-discharge cycle life is reduced. Is shortened.

In a non-aqueous electrolyte secondary battery using a cobalt-based oxide as a positive electrode active material, two types of carbonaceous materials having different average particle sizes, such as acetylene black and graphite, are used as a conductive agent for the positive electrode. That is being considered. When such a conductive agent is used, large current discharge characteristics and cycle life of the secondary battery are improved.

However, when a nickel-based oxide is used as a positive electrode active material and polyvinylidene fluoride or a copolymer having a basic structure of vinylidene fluoride is used as a binder, acetylene black is used as a conductive agent. When a carbonaceous substance having a small average particle size is added, there is a problem that the above-mentioned gelation of the coating liquid is accelerated. Although the cause is not clear, acetylene black has a high liquid absorbing property, so that the solvent in the coating liquid is absorbed by acetylene black, so that the solid content concentration of the coating liquid gradually increases, and the above-mentioned alkali-based polyvinylidene fluoride is dissolved. It is considered that the removal of the HF reaction and the subsequent crosslinking reaction are promoted.

[0014]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned conventional problems, and an object thereof is to provide a lithium secondary battery having excellent charge-discharge cycle life characteristics.

[0015]

The lithium secondary battery according to the present invention has a composition formula of LiNi _1-x M _x O ₂ (where M is
Is composed of one or more elements, and x represents 0 <x ≦ 0.5), a carbonaceous material A having an average particle diameter of 100 nm or less, and a carbonaceous material A having an average particle diameter of 1 μm or more. A conductive agent containing carbonaceous material B, and a binder made of an acrylic rubbery copolymer and a vinylidene fluoride-based fluororesin;
Having a structure in which a positive electrode mixture containing
, A negative electrode, and a non-aqueous electrolyte, and
Mixing ratio of vinylidene trifluoride resin is 45% by weight or less
It is characterized by being.

[0016]

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a lithium secondary battery (for example, a cylindrical lithium secondary battery) according to the present invention will be described with reference to FIG. For example, a cylindrical container 1 with a bottom made of stainless steel has an insulator 2 disposed at the bottom. Electrode group 3
Are stored in the container 1. The electrode group 3 includes:
It has a structure in which a belt-like material in which a positive electrode 4, a separator 5 and a negative electrode 6 are laminated in this order is spirally wound.

The container 1 contains a non-aqueous electrolyte. A PTC element 7 having an opening in the center,
A safety valve 8 disposed on the TC element 7 and a hat-shaped positive terminal 9 disposed on the safety valve 8 are caulked and fixed to an upper opening of the container 1 via an insulating gasket 10. The positive electrode terminal 9 has a vent hole (not shown). One end of the positive electrode lead 11 is connected to the positive electrode 4, and the other end is connected to the PTC element 7. The negative electrode 6 is connected to the container 1 as a negative terminal via a negative lead (not shown).

Next, the positive electrode 4, the separator 5, the negative electrode 6, and the non-aqueous electrolyte will be described in detail. 1) Positive electrode 4 This positive electrode 4 is made of lithium represented by a composition formula: LiNi _1-x M _x O ₂ (where M is one or more elements, and x represents 0 <x ≦ 0.5). Composite metal oxide (active material)
And a mixture containing a conductive agent containing carbonaceous material A having an average particle size of 100 nm or less and carbonaceous material B having an average particle size of 1 μm or more, and a binder containing an acrylic rubbery copolymer. It has a structure carried on the body.

The positive electrode 4 is prepared, for example, by applying a paste obtained by dispersing the active material, the conductive agent and the binder in an appropriate solvent to a current collector, and drying the paste to form a thin plate. It is produced by cutting to a desired size.

When carboxymethylcellulose (CMC) is added as a thickener during the preparation of the paste,
This is preferable because the viscosity of the paste can be easily controlled. (1-1) Active Material As the element M in the composition formula of the lithium composite metal oxide, at least one selected from Co, B, Al, Mn, and Fe can be used. The reason why the value of x in the composition formula is set to 0.5 or less is as follows. When the value of x exceeds 0.5, although the gelation of the coating liquid is unlikely to occur, the improvement in capacity characteristics and the reduction in cost of the battery, which are features of a secondary battery including a positive electrode containing a nickel-based oxide, are achieved. It may not be possible. A more preferable range of x is 0.1 ≦ x ≦ 0.4. As such a lithium composite metal oxide, for example, LiNi _1-x Co _{x
O 2, LiNi 1-xy Co} x B y O 2, LiNi 1-xy
_{Co x Al y O 2, LiNi} 1-xy Co x Mn y O 2,
LiNi _1-xy Co _x Fe _y O _{2 and the} like can be mentioned. However, x and y in these composition formulas are 0 <x ≦
0.5, 0 ≦ y <0.5, 0 <x + y ≦ 0.5.

(1-2) Conductive agent Examples of the carbonaceous material A include acetylene black and Ketjen black. Among them, acetylene black is preferable.

Examples of the carbonaceous material B include artificial graphite and natural graphite. The carbonaceous material A
Is determined by observation with a transmission electron microscope (TEM). On the other hand, the average particle size of the carbonaceous material B is:
Laser diffraction particle size distribution analyzer measurements from (d ₅₀₎
Is determined. The average particle size of the carbonaceous material A is 100 n.
m or the average particle size of the carbonaceous material B is 1
When the thickness is less than μm, it is difficult to form a conductive network of the carbonaceous material densely and uniformly in the positive electrode. The average particle diameter of the carbonaceous material A is preferably 10 nm or more and 100 nm or less, and more preferably 30 to 60 nm. On the other hand, the average particle diameter of the carbonaceous material B is desirably 1 μm or more and 50 μm or less, and a more preferred range is 3 to 30 μm.

The particle shape of the carbonaceous material A and the carbonaceous material B can be, for example, various shapes such as a sphere and a scale. It is preferable that the sum of the contents of the carbonaceous material A and the carbonaceous material B be 10 parts by weight or less based on 100 parts by weight of the active material. If the content exceeds 10 parts by weight, the active material filling density of the positive electrode may decrease. In addition, the amount of the binder adsorbed on these carbonaceous materials A and B increases, and the function of the binder is reduced, and the adhesion between the current collector and the mixture may be reduced.

It is preferable that the content of the carbonaceous material A is not less than 20% by weight and not more than 80% by weight based on the sum of the contents of the carbonaceous material A and the carbonaceous material B. This is due to the following reasons. If the content is less than 20% by weight, it may be difficult to form a dense and uniform conductive network of carbonaceous materials A and B in the positive electrode. On the other hand, if the content exceeds 80% by weight, it may be difficult to form the above-mentioned conductive network densely and uniformly, and the effective amount of the binder not adsorbed to the carbonaceous material A And the adhesion between the current collector and the mixture may be reduced. The content of the carbonaceous material A is the same as the carbonaceous material A and the carbonaceous material B.
25% by weight or more and 50% by weight based on the total content of
It is more preferable to set the following.

(1-3) Binder The binder contains an acrylic rubbery copolymer. The acrylic rubbery copolymer is obtained by copolymerizing a rubbery copolymer with at least one or more raw material monomers selected from unsaturated carboxylic acids and unsaturated carboxylic acid esters.

Examples of the rubbery copolymer include an aromatic vinyl-conjugated diene copolymer and an ethylenic nitrile compound-conjugated diene copolymer. The aromatic vinyl-conjugated diene copolymer is obtained by copolymerizing an aromatic vinyl compound and a conjugated diene compound as raw material monomers.

Examples of the aromatic vinyl compound include styrene, α-methylstyrene, vinyltoluene,
pt-butyltoluene and the like can be mentioned. Examples of the conjugated diene compound include butadiene and piperylene.

The above-mentioned ethylenic nitrile compound-conjugated diene copolymer is obtained by copolymerizing a raw material monomer, an ethylenic nitrile compound and a conjugated diene compound.

The ethylenic nitrile compound includes:
For example, (meth) acrylonitrile, crotonnitrile, allylnitrile and the like can be mentioned. Examples of the conjugated diene-based compound include the same compounds as described above.

As the unsaturated carboxylic acid, for example,
Acrylic acid, methacrylic acid, itaconic acid, fumaric acid, maleic acid, crotonic acid, citraconic acid, mesaconic acid, glutaconic acid, monooctyl maleate, monobutyl maleate, monooctyl itanate, and the like can be given. Further, unsaturated carboxylic anhydrides such as acrylic acid, methacrylic anhydride, and maleic anhydride may be used.

The unsaturated carboxylic acid esters include:
For example, methyl acrylate, ethyl acrylate, propyl acrylate, isopropyl acrylate, butyl acrylate, octadecyl acrylate, hydroxyethyl acrylate, propylene glycol acrylate, acrylamide, glycidyl acrylate, methyl methacrylate,
Ethyl methacrylate, propyl methacrylate, isopropyl methacrylate, butyl methacrylate, cyclohexyl methacrylate, 2- (diethylamino) methacrylate
Examples thereof include ethyl, hydroxypropyl methacrylate, methacrylamide, glycidyl methacrylate, dimethylaminoethyl methacrylate, and tert-butylaminoethyl methacrylate.

Examples of the compound having a nitrile group include:
(Meth) acrylonitrile, crotonnitrile, allylnitrile and the like can be mentioned. The content of the acrylic rubbery copolymer is preferably 10 parts by weight or less, more preferably 5 parts by weight or less based on 100 parts by weight of the active material.

The binder may be composed of the acrylic rubbery copolymer and a vinylidene fluoride-based fluororesin. As such vinylidene fluoride-based fluororesin,
Examples thereof include polyvinylidene fluoride (PVdF), a raw material monomer in which at least one of hydrogen or fluorine of vinylidene fluoride is substituted with another substituent, and modified polyvinylidene fluoride obtained by polymerizing vinylidene fluoride. The ratio of the structural units derived from vinylidene fluoride in the vinylidene fluoride-based fluororesin is preferably 90% by weight or more.

The content of the vinylidene fluoride-based fluororesin in the binder is preferably not more than 50% by weight. When the content of the vinylidene fluoride-based fluororesin is 50,
If the content is more than 10% by weight, it may be difficult to suppress the gelation of the paste. A more preferable range of the content is 30 to 45% by weight.

(1-4) Current Collector Examples of the current collector include aluminum foil, stainless steel foil, and titanium foil having a thickness of 10 to 40 μm.

(1-5) Solvent As the organic solvent for dispersing the binder, N-
Polar solvents such as methyl-2-pyrrolidone (NMP) and dimethylformamide (DMF) are used. in this case,
It is preferable to use a polar rubbery copolymer obtained by copolymerizing a polymer having a nitrile group with the acrylic rubbery copolymer, since the polar rubbery copolymer is easily dispersed in the polar solvent.

2) Separator 5 As the separator 5, for example, a nonwoven fabric made of synthetic resin,
A polyethylene porous film, a polypropylene porous film, or the like can be used.

3) Negative Electrode 6 The negative electrode 6 has a structure in which a mixture containing a negative electrode active material is supported on a current collector. As the negative electrode active material, for example,
Examples include carbonaceous materials that occlude and release lithium ions, chalcogen compounds that occlude and release lithium ions, and light metals that occlude and release lithium ions. Above all, a negative electrode containing a carbonaceous substance or a chalcogen compound that occludes and releases lithium ions is preferable because battery characteristics such as cycle life of the secondary battery are improved.

Examples of the carbonaceous material that occludes / releases lithium ions include coke, carbon fiber, pyrolytic gas-phase carbon material, graphite, fired resin, fired mesophase pitch-based carbon fiber, and fired mesophase spherical carbon. be able to. Above all, mesophase pitch-based carbon fiber or mesophase spherical carbon which is graphitized at 2500 ° C. or higher is preferable because the electrode capacity is increased.

The chalcogen compounds that occlude and release lithium ions include titanium disulfide (TiS ₂ ), molybdenum disulfide (MoS ₂ ), and niobium selenide (NbS).
e ₂ ). When such a chalcogen compound is used for the negative electrode, the capacity of the negative electrode increases although the voltage of the secondary battery drops, and the capacity of the secondary battery is improved. Further, since the negative electrode has a high diffusion rate of lithium ions, the rapid charge / discharge performance of the secondary battery is improved.

Examples of the light metal include aluminum, aluminum alloy, magnesium alloy, lithium metal, lithium alloy and the like. As the current collector, for example, a copper foil, a nickel foil, or the like can be used, but a copper foil is most preferable in consideration of electrochemical stability, flexibility at the time of winding, and the like. The thickness of the foil at this time is preferably 8 μm or more and 40 μm or less.

For the negative electrode (for example, a negative electrode using a carbonaceous material as a negative electrode active material), for example, a paste obtained by dispersing the carbonaceous material and a binder in an appropriate solvent is applied to a current collector and dried. And made into a thin plate.

Examples of the binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), ethylene-propylene-diene copolymer (EPDM), and styrene-butadiene rubber (SBR). it can. The mixing ratio of the negative electrode material and the binder is preferably in the range of 80 to 98% by weight of the negative electrode material and 2 to 20% by weight of the binder. In particular, the carbon material was coated in an amount of 10
It is preferable to set it in the range of 0 to 200 g / m ² .

4) Non-aqueous electrolyte The non-aqueous electrolyte has a composition in which an electrolyte is dissolved in a non-aqueous solvent. Examples of the non-aqueous solvent include cyclic carbonates such as propylene carbonate (PC) and ethylene carbonate (EC), for example, dimethyl carbonate (DM)
C), linear carbonates such as methyl ethyl carbonate (MEC) and diethyl carbonate (DEC);
Chain ethers such as 2-dimethoxyethane (DME) and diethoxyethane (DEE), tetrahydrofuran (THF) and 2-methyltetrahydrofuran (2-Me
Cyclic ethers such as THF) and crown ethers;
At least one selected from fatty acid esters such as butyrolactone (γ-BL), nitrogen compounds such as acetonitrile (AN), and sulfur compounds such as sulfolane (SL) and dimethylsulfoxide (DMSO) can be used.

Among them, those composed of at least one selected from EC, PC and γ-BL, EC, PC and γ-B
At least one selected from L and DMC, MEC, DE
It is desirable to use a mixed solvent comprising at least one selected from C, DME, DEE, THF, 2-MeTHF, and AN. Further, when using a negative electrode containing a carbonaceous material that occludes and releases lithium ions, from the viewpoint of improving the cycle life of a secondary battery including the negative electrode, EC, PC and γ-BL, EC and PC And MEC, E
C and PC and DEC, EC and PC and DEE, EC and AN,
EC and MEC, PC and DMC, PC and DEC, or E
It is desirable to use a mixed solvent consisting of C and DEC.

Examples of the electrolyte include lithium perchlorate (LiClO ₄ ) and lithium hexafluorophosphate (Li
PF ₆ ), lithium borofluoride (LiBF ₄ ), lithium arsenic hexafluoride (LiAsF ₆ ), lithium trifluoromethasulfonate (LiCF ₃ SO ₃ ), lithium aluminum tetrachloride (LiAlCl ₄ ), lithium bistrifluoromethylsulfonylimide [LiN (CF ₃ SO
₂ ) ₂ ] and the like. Among them, LiPF ₆ , LiBF ₄ , LiN (CF ₃ SO ₂ ) ₂
The use of is preferred because conductivity and safety are improved.

The amount of the electrolyte dissolved in the non-aqueous solvent is preferably in the range of 0.5 mol / L to 2.0 mol / L. The lithium secondary battery according to the present invention described in detail above includes a lithium composite metal oxide represented by the above composition formula LiNi _1-x M _x O ₂ , a carbonaceous material A having an average particle size of 100 nm or less, and an average particle size. Is provided with a positive electrode containing a conductive agent containing a carbonaceous substance B of 1 μm or more and a binder containing an acrylic rubbery copolymer. Since the binder containing the acrylic rubbery copolymer has excellent alkali resistance, it is possible to suppress the gelation of the paste containing the alkali salt derived from the lithium composite metal oxide. As a result, since it is possible to use a conductive agent containing two types of carbonaceous materials having significantly different average particle diameters as described above, a micro conductive network of the carbonaceous material A and a macroscopic network of the carbonaceous material B And a conductive network can be formed, and the conductivity of the positive electrode can be improved. Further, the binder containing the acrylic rubbery copolymer hardly swells even when immersed in an electrolytic solution for a long time,
And it has excellent adhesion to the current collector. Therefore, by providing a positive electrode including the lithium composite metal oxide, the conductive agent, and the binder, a lithium secondary battery having excellent cycle life can be realized.

Further, another lithium secondary battery according to the present invention comprises a lithium composite metal oxide represented by the above composition formula LiNi ₁ -xM _x O ₂ , a carbonaceous material A having an average particle diameter of 100 nm or less, and A positive electrode containing a conductive agent containing a carbonaceous substance B having an average particle diameter of 1 μm or more and a binder containing an acrylic rubbery copolymer and a vinylidene fluoride-based fluororesin is provided. As described above, the acrylic rubbery copolymer is excellent in alkali resistance and adhesion to a current collector, but is slightly inferior in liquid retention of an electrolytic solution. If the liquid retaining property of the binder is insufficient, the electrolyte between the positive electrode and the negative electrode runs short with the progress of the charge / discharge cycle, and the discharge capacity of the battery gradually decreases before the positive electrode or the negative electrode is deteriorated. That is, the liquid wiping phenomenon is apt to occur. On the other hand, the vinylidene fluoride-based fluororesin has the problem of gelation of the paste as described above, but is excellent in the liquid retaining property of the electrolytic solution. By using both the copolymer and the fluororesin as the binder, the gelation of the paste containing the lithium composite metal oxide and the conductive agent can be suppressed, and the adhesion between the current collector and the paste can be reduced. The lithium secondary battery can improve not only the charge / discharge cycle life but also the large current discharge characteristics of the lithium secondary battery.

[0050]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below in detail with reference to the drawings. Example 1 <Preparation of Positive Electrode> Nickel hydroxide powder [Ni _0.8 Co _0.2 (OH) ₂ ] in which a part of Ni was replaced with Co was mixed with lithium hydroxide monohydrate (LiOH · H ₂ O). By mixing and firing, a LiNi _0.8 Co _0.2 O ₂ powder was obtained. From the X-ray diffraction pattern, it was confirmed that the obtained powder had a LiNiO ₂ structure.

Subsequently, the LiNi _0.8 Co _0.2 O ₂
100 parts by weight of powder, 2.5 parts by weight (50% by weight) of acetylene black having an average particle diameter of 50 nm, and 1 part by weight
2.5 parts by weight (50% by weight) of μm flake graphite (artificial graphite) was mixed with a mixer. The mixture was dispersed in N-methyl-2-pyrrolidone in which 3 parts by weight of an acrylic rubbery copolymer obtained by copolymerizing methyl methacrylate, itaconic acid and acrylonitrile in styrene-butadiene rubber (SBR) was dissolved. Further, carboxymethylcellulose (CMC) as a thickener was added and stirred to prepare a paste (coating liquid) of 5000 mPa · s.

The coating solution was left at room temperature (25 ° C.) for 20 hours, and the viscosity was measured using a B-type viscometer.
The viscosity after time was about 5000 mPa · s, which was almost the same as immediately after preparation.

Thus, this coating solution was applied to both sides of an aluminum foil as a current collector, dried, and rolled to produce a positive electrode. <Production of Negative Electrode> A carbonaceous material was produced by graphitizing mesophase pitch carbon fibers using mesophase pitch as a raw material.

Subsequently, a mixture of 5 parts by weight of graphite and 5 parts by weight of polyvinylidene fluoride with respect to 100 parts by weight of the carbonaceous material was dispersed in N-methylpyrrolidone to form a paste, which was then used as a current collector substrate. A negative electrode was prepared by applying the coating on both sides of the copper foil, drying it, and performing roll pressing.

The positive electrode, the separator made of a porous film made of polyethylene and the negative electrode were laminated in this order, and then spirally wound to form an electrode group.
As a non-aqueous electrolyte, lithium hexafluorophosphate (LiPF) was mixed in a mixed solvent of ethylene carbonate (EC) and methyl ethyl carbonate (MEC) (mixing volume ratio 1: 2).
₆ ) is dissolved in 1M, and the electrode group and the non-aqueous electrolyte are respectively accommodated in a stainless steel bottomed cylindrical container, and a cylindrical lithium ion secondary battery (18650 size) having a designed rated capacity of 1900 mAh is used. ) Assembled.

Example 2 100 parts by weight of LiNi _0.8 Co _0.2 O ₂ powder similar to that described in Example 1, 2 parts by weight (40% by weight) of acetylene black having an average particle diameter of 50 nm, and 1 μm flaky graphite (artificial graphite)
3 parts by weight (60% by weight) were mixed by a mixer, and the obtained mixture was mixed with 3 parts by weight of an acrylic rubbery copolymer of the same type as described in Example 1 and dissolved in N-methyl- It was dispersed in 2-pyrrolidone, carboxymethylcellulose (CMC) was further added as a thickener, and the mixture was stirred to prepare a paste (coating liquid) of 5000 mPa · s.

This coating solution was left at room temperature for 20 hours, and the viscosity was measured using a B-type viscometer. The viscosity after 20 hours was about 5000 mPa · s, which was almost the same as immediately after the preparation.

The obtained coating solution was applied to both sides of an aluminum foil as a current collector, dried, and rolled to produce a positive electrode. Design rated capacity 1 from the positive electrode in the same manner as in Example 1.
900 mAh cylindrical lithium ion secondary battery (186
50 size).

Example 3 100 parts by weight of a LiNi _0.8 Co _0.2 O ₂ powder as described in Example 1, 3 parts by weight (60% by weight) of acetylene black having an average particle diameter of 50 nm and an average particle diameter of 50% 1 μm flaky graphite (artificial graphite)
2 parts by weight (40% by weight) were mixed by a mixer, and the obtained mixture was mixed with 3 parts by weight of an acrylic rubbery copolymer of the same type as described in Example 1 to dissolve N-methyl- It was dispersed in 2-pyrrolidone, carboxymethylcellulose (CMC) was further added as a thickener, and the mixture was stirred to prepare a paste (coating liquid) of 5000 mPa · s.

This coating solution was left at room temperature for 20 hours, and the viscosity was measured using a B-type viscometer. The viscosity after 20 hours was approximately 5000 mPa · s, which was almost the same as immediately after the preparation.

The obtained coating solution was applied to both sides of an aluminum foil as a current collector, dried and rolled to produce a positive electrode. Design rated capacity 1 from the positive electrode in the same manner as in Example 1.
900 mAh cylindrical lithium ion secondary battery (186
50 size).

Example 4 100 parts by weight of a LiNi _0.8 Co _0.2 O ₂ powder as described in Example 1 and 2.5 parts by weight of acetylene black having an average particle size of 50 nm (50 parts by weight)
% By weight) and 2.5 parts by weight (50% by weight) of flaky graphite (artificial graphite) having an average particle diameter of 20 μm by a mixer, and the obtained mixture was mixed with the same kind as described in Example 1. Is dispersed in N-methyl-2-pyrrolidone in which 3 parts by weight of an acrylic rubbery copolymer is dissolved, carboxymethylcellulose (CMC) is added as a thickener, and the mixture is stirred to obtain a paste (coating) of 5000 mPa · s. Liquid).

The coating solution was left at room temperature for 20 hours, and the viscosity was measured using a B-type viscometer. The viscosity after 20 hours was approximately 5000 mPa · s, which was almost the same as immediately after the preparation.

The obtained coating solution was applied to both sides of an aluminum foil as a current collector, dried, and rolled to produce a positive electrode. Design rated capacity 1 from the positive electrode in the same manner as in Example 1.
900 mAh cylindrical lithium ion secondary battery (186
50 size).

Example 5 100 parts by weight of a LiNi _0.8 Co _0.2 O ₂ powder as described in Example 1 and 2.5 parts by weight of acetylene black having an average particle diameter of 50 nm (50 parts by weight)
% By weight) and 2.5 parts by weight (50% by weight) of flaky graphite (artificial graphite) having an average particle diameter of 20 μm by a mixer, and the obtained mixture was mixed with the same kind as described in Example 1. Is dispersed in N-methyl-2-pyrrolidone in which 5 parts by weight of an acrylic rubbery copolymer is dissolved, carboxymethylcellulose (CMC) is added as a thickener, and the mixture is stirred to obtain a 5000 mPa · s paste (coating). Liquid).

This coating solution was left at room temperature for 20 hours, and the viscosity was measured using a B-type viscometer. The viscosity after 20 hours was approximately 5000 mPa · s, which was almost the same as immediately after the preparation.

The obtained coating solution was applied to both sides of an aluminum foil as a current collector, dried, and rolled to produce a positive electrode. Design rated capacity 1 from the positive electrode in the same manner as in Example 1.
900 mAh cylindrical lithium ion secondary battery (186
50 size).

Example 6 100 parts by weight of a LiNi _0.8 Co _0.2 O ₂ powder as described in Example 1 and 3.5 parts by weight of acetylene black having an average particle diameter of 50 nm (50 parts by weight)
% By weight) and 3.5 parts by weight (50% by weight) of flaky graphite (artificial graphite) having an average particle size of 1 μm were mixed by a mixer, and the obtained mixture was mixed with the same kind as described in Example 1. Is dispersed in N-methyl-2-pyrrolidone in which 10 parts by weight of an acrylic rubbery copolymer is dissolved, and carboxymethylcellulose (CMC) is added as a thickening agent, followed by stirring to obtain a paste having a viscosity of 5000 mPa · s. (Coating liquid) was prepared.

This coating solution was left at room temperature for 20 hours, and the viscosity was measured using a B-type viscometer. The viscosity after 20 hours was about 5000 mPa · s, which was almost the same as immediately after the preparation.

The obtained coating solution was applied to both sides of an aluminum foil as a current collector, dried, and rolled to produce a positive electrode. Design rated capacity 1 from the positive electrode in the same manner as in Example 1.
900 mAh cylindrical lithium ion secondary battery (186
50 size).

Example 7 100 parts by weight of a LiNi _0.8 Co _0.2 O ₂ powder similar to that described in Example 1, 5 parts by weight (50% by weight) of acetylene black having an average particle diameter of 50 nm, and an average particle diameter of 50% 5 parts by weight (50% by weight) of 20 μm flaky graphite (artificial graphite) are mixed by a mixer, and the obtained mixture is mixed with 5 parts by weight of an acrylic rubbery copolymer of the same type as described in Example 1. The resulting mixture was dispersed in N-methyl-2-pyrrolidone in which a portion was dissolved, carboxymethylcellulose (CMC) was added as a thickener, and the mixture was stirred to prepare a paste (coating liquid) having a viscosity of 5000 mPa · s.

This coating solution was left at room temperature for 20 hours, and the viscosity was measured using a B-type viscometer. The viscosity after 20 hours was approximately 5000 mPa · s, which was almost the same as immediately after the preparation.

The obtained coating solution was applied to both sides of an aluminum foil as a current collector, dried, and rolled to produce a positive electrode. Design rated capacity 1 from the positive electrode in the same manner as in Example 1.
900 mAh cylindrical lithium ion secondary battery (186
50 size).

Example 8 100 parts by weight of the same LiNi _0.8 Co _0.2 O ₂ powder as described in Example 1 and 3.5 parts by weight of acetylene black having an average particle diameter of 50 nm (50 parts by weight)
% By weight) and 3.5 parts by weight (50% by weight) of flaky graphite (artificial graphite) having an average particle size of 1 μm were mixed by a mixer, and the obtained mixture was mixed with the same kind as described in Example 1. Is dispersed in N-methyl-2-pyrrolidone in which 12 parts by weight of an acrylic rubbery copolymer is dissolved, and carboxymethylcellulose (CMC) is further added as a thickener and stirred to obtain a paste (coating material) of 5000 mPa · s. Liquid).

This coating solution was left at room temperature for 20 hours, and the viscosity was measured using a B-type viscometer. The viscosity after 20 hours was approximately 5000 mPa · s, which was almost the same as immediately after the preparation.

The obtained coating solution was applied to both sides of an aluminum foil as a current collector, dried, and rolled to produce a positive electrode. Design rated capacity 1 from the positive electrode in the same manner as in Example 1.
900 mAh cylindrical lithium ion secondary battery (186
50 size).

(Example 9) Nickel hydroxide powder [Ni _0.8 Co _0.17 Al
_0.03 (OH) ₂ ] and lithium hydroxide monohydrate (LiOH
· H ₂ O) were mixed, LiNi by firing
_0.8 Co _0.17 Al _0.03 O ₂ powder was obtained. From the X-ray diffraction pattern, it was confirmed that the obtained powder had a LiNiO ₂ structure.

The obtained LiNi _0.8 Co _0.17 Al _0.03 O
Example 1 except that powder ₂ was used as the positive electrode active material
A cylindrical lithium ion secondary battery (18650 size) having a design rated capacity of 1900 mAh was assembled in the same manner as described above.

The coating liquid for the positive electrode was left at room temperature for 20 hours, and the viscosity was measured using a B-type viscometer.
The viscosity after time was about 5000 mPa · s, which was almost the same as immediately after preparation.

Example 10 Nickel hydroxide powder [Ni _0.8 Co _0.17 (OH) ₂ ] in which a part of Ni was replaced by Co
, Lithium borate (Li ₂ B ₄ O ₇ ) and lithium hydroxide monohydrate (LiOH · H ₂ O) were mixed and calcined to obtain LiNi _0.8 Co _0.17 B _0.03 O ₂ powder. . From the X-ray diffraction pattern, it was confirmed that the obtained powder had a LiNiO ₂ structure.

The obtained LiNi _0.8 Co _0.17 B _0.03 O ₂
A cylindrical lithium ion secondary battery (18650 size) having a designed rated capacity of 1900 mAh was assembled in the same manner as in Example 1 except that the powder was used as the positive electrode active material.

The coating solution for the positive electrode was left at room temperature for 20 hours, and the viscosity was measured using a B-type viscometer.
The viscosity after time was about 5000 mPa · s, which was almost the same as immediately after preparation.

Example 11 Styrene was used as a binder.
Except for using an acrylic rubbery copolymer obtained by copolymerizing butadiene rubber (SBR) with methyl methacrylate, itaconic acid, fumaric acid and acrylonitrile,
A cylindrical lithium ion secondary battery (18650 size) having a designed rated capacity of 1900 mAh was assembled in the same manner as in Example 1.

(Example 12) As a binder, styrene-
Except for using an acrylic rubbery copolymer obtained by copolymerizing methyl methacrylate, acrylic acid and itaconic acid with butadiene rubber (SBR), a cylindrical lithium having a design rated capacity of 1900 mAh was used in the same manner as in Example 1. An ion secondary battery (18650 size) was assembled.

Comparative Example 1 100 parts by weight of LiNi _0.8 Co _0.2 O ₂ powder as described in Example 1 and 5 parts by weight of acetylene black having an average particle diameter of 50 nm were mixed by a mixer, and the resulting mixture was obtained. Is dispersed in N-methyl-2-pyrrolidone in which 3 parts by weight of polyvinylidene fluoride is dissolved, and a paste (coating liquid) having a viscosity of 5000 mPa · s
Was prepared. The obtained coating liquid lacked fluidity and was not suitable for application to a current collector.

When this coating solution was left at room temperature for 20 hours, the coating solution was completely cured and could not be applied to the current collector. Therefore, a coating liquid was prepared again in the same manner, and immediately applied to the current collector to produce a positive electrode. A cylindrical lithium ion secondary battery (18650 size) having a designed rated capacity of 1900 mAh was assembled from the positive electrode in the same manner as in Example 1.

Comparative Example 2 100 parts by weight of a LiNi _0.8 Co _0.2 O ₂ powder as described in Example 1, 2.5 parts by weight of acetylene black having an average particle diameter of 50 nm, and scale having an average particle diameter of 1 μm 2.5 parts by weight of flake graphite are mixed by a mixer, and the obtained mixture is dispersed in N-methyl-2-pyrrolidone in which 3 parts by weight of polyvinylidene fluoride is dissolved, and a paste (coating liquid) having a viscosity of 5000 mPa · s Was prepared. The coating liquid immediately after preparation had fluidity suitable for coating on a current collector. When this coating liquid was left at room temperature for 20 hours, the coating liquid did not reach curing, but gelation was observed, and application to the current collector could not be performed. Therefore,
A coating liquid was prepared again, and immediately applied to the current collector to prepare a positive electrode. A cylindrical lithium ion secondary battery (18650 size) having a designed rated capacity of 1900 mAh was assembled from the positive electrode in the same manner as in Example 1.

Comparative Example 3 100 parts by weight of LiNi _0.8 Co _0.2 O ₂ powder similar to that described in Example 1, 2.5 parts by weight of flaky graphite having an average particle diameter of 1 μm, and an average particle diameter of 2
2.5 parts by weight of 0-μm flaky graphite was mixed by a mixer, and the obtained mixture was dissolved in N— containing 3 parts by weight of an acrylic rubbery copolymer of the same type as described in Example 1. It was dispersed in methyl-2-pyrrolidone, carboxymethylcellulose (CMC) was further added as a thickener, and the mixture was stirred to prepare a paste (coating liquid) having a viscosity of 5000 mPa · s.

This coating solution was left at room temperature for 20 hours, and the viscosity was measured using a B-type viscometer. The viscosity after 20 hours was approximately 5000 mPa · s, which was almost the same as immediately after the preparation.

Thus, this coating solution was applied to both sides of an aluminum foil as a current collector, dried, and rolled to produce a positive electrode. From the positive electrode, a cylindrical lithium ion secondary battery (1
8650 size).

Comparative Example 4 100 parts by weight of the same LiNi _0.8 Co _0.2 O ₂ powder as described in Example 1 and 5 parts by weight of acetylene black having an average particle diameter of 50 nm were mixed by a mixer, and the resulting mixture was obtained. Is dispersed in N-methyl-2-pyrrolidone in which 3 parts by weight of an acrylic rubbery copolymer of the same type as described in Example 1 is dissolved, and carboxymethyl cellulose (CMC) is further used as a thickener.
The viscosity is 5000 mPa · s by adding and stirring.
(Paste solution) was prepared. The obtained coating liquid lacked fluidity and was not suitable for application to a current collector.

This coating solution was left at room temperature for 20 hours, and the viscosity was measured using a B-type viscometer. The viscosity after 20 hours was approximately 5000 mPa · s, which was almost the same as immediately after the preparation. However, it lacked fluidity and was not suitable for application to a current collector.

Then, this coating solution was applied to both sides of an aluminum foil as a current collector, dried, and rolled to produce a positive electrode. From the positive electrode, a cylindrical lithium ion secondary battery (1
8650 size).

Comparative Example 5 100 parts by weight of the same LiNi _0.8 Co _0.2 O ₂ powder as described in Example 1 and 5 parts by weight of acetylene black having an average particle diameter of 50 nm were mixed by a mixer, and the resulting mixture was obtained. Is dissolved in N in which 3 parts by weight of polyvinylidene fluoride and 0.1 part by weight of maleic acid are dissolved.
-Dispersed in methyl-2-pyrrolidone, having a viscosity of 5000
An mPa · s paste (coating liquid) was prepared. The obtained coating liquid lacked fluidity and was not suitable for application to a current collector.

When this coating solution was allowed to stand at room temperature (25 ° C.) for 20 hours, it showed almost the same viscosity after 20 hours, and no gelation was observed. However, the coating solution lacked fluidity and was applied to a current collector. It was not suitable.

The obtained coating liquid was applied to both sides of an aluminum foil as a current collector, dried, and rolled to produce a positive electrode. The obtained positive electrode had a large variation in thickness. Design rated capacity 1900 from the positive electrode in the same manner as in Example 1.
A mAh cylindrical lithium ion secondary battery (18650 size) was assembled.

With respect to the secondary batteries of Examples 1 to 12 and Comparative Examples 1 to 5, types and mixing ratios of the positive electrode active material, the conductive agent and the binder are shown in Tables 1 to 4 below. Examples 1 to 1 obtained
For the positive electrodes of the secondary batteries of Comparative Example 2 and Comparative Examples 1 to 5, a peel strength test was performed with a tensile tester in order to examine the adhesion to the current collector. Each positive electrode was cut into a width of 2 cm and a length of 5 cm, the paste-coated surface was adhered to a glass surface with a double-sided tape, and one end was peeled off at a constant speed while maintaining an angle of 170 to 180 ° in the long side direction. The strength was measured as the adhesion strength, and the results are shown in Table 5 below.

Further, cycle life tests were performed on the secondary batteries of Examples 1 to 12 and Comparative Examples 1 to 5. The charging was performed at a charging current of 900 mA up to 4.2 V at 20 ° C., and then maintained at a constant voltage of 4.2 V for a total of 8 hours. The discharge was performed at a constant current of 900 mA, and the discharge end voltage was set to 2.7 V. The rest time after charging and discharging was 30 minutes each. Such charge / discharge is repeated, and the discharge capacity is measured for each cycle. The number of cycles at which the discharge capacity reaches 80% of the capacity in the first cycle is defined as the battery life. The results are shown in Table 5 below.

[0099]

[Table 1]

[0100]

[Table 2]

[0101]

[Table 3]

[0102]

[Table 4]

[0103]

[Table 5]

As is apparent from Tables 1 to 5, a conductive agent containing carbonaceous material A having an average particle size of 100 nm or less and a carbonaceous material B having an average particle size of 1 μm or more, and a binder containing an acrylic rubbery copolymer. Positive electrodes of Examples 1 to 12 containing a binder,
The positive electrode of Comparative Example 1 containing acetylene black as a conductive agent and polyvinylidene fluoride as a binder, and the positive electrode of Comparative Example 2 using polyvinylidene fluoride instead of an acrylic rubbery copolymer as a binder, Carbonaceous material B
The positive electrode of Comparative Example 3 consisting of only the carbonaceous material A
Comparative Example 5 consisting of only the positive electrode of Comparative Example 4 and acetylene black as the conductive agent, polyvinylidene fluoride as the binder, and neutralization of the positive electrode active material
It can be seen that the adhesion strength between the current collector and the coating liquid is higher than that of the positive electrode. In addition, it can be seen that the secondary batteries of Examples 1 to 12 have a longer cycle life than the secondary batteries of Comparative Examples 1 to 5.

Example 13 <Production of Positive Electrode> LiNi similar to that described in Example 1 was used.
_0.8 Co _0.2 O ₂ powder 100 parts by weight, average particle size 50n
m acetylene black 2.5 parts by weight (50% by weight) and flaky graphite (artificial graphite) 2.5 having an average particle size of 1 μm
Parts by weight (50% by weight) are mixed by a mixer, and the resulting mixture is mixed with 1 part by weight of polyvinylidene fluoride (33% by weight).
N- in which 2 parts by weight (67% by weight) of an acrylic rubbery copolymer of the same type as described in Example 1 were dissolved.
It was dispersed in methyl-2-pyrrolidone, carboxymethylcellulose (CMC) was further added as a thickener, and the mixture was stirred to prepare a paste (coating liquid) having a viscosity of 5000 mPa · s.

This coating solution was left at room temperature for 20 hours, and the viscosity was measured using a B-type viscometer. The viscosity after 20 hours was approximately 5000 mPa · s, which was almost the same as immediately after the preparation.

Therefore, this coating solution was applied to both sides of an aluminum foil as a current collector, dried, and rolled to produce a positive electrode. From the positive electrode, a cylindrical lithium ion secondary battery (1
8650 size).

Example 14 100 parts by weight of a LiNi _0.8 Co _0.2 O ₂ powder as described in Example 1 and 2.5 parts by weight of acetylene black having an average particle size of 50 nm (5 parts by weight)
0% by weight) and 2.5 parts by weight (50% by weight) of flaky graphite (artificial graphite) having an average particle diameter of 1 μm are mixed by a mixer, and the obtained mixture is mixed with 1.5 parts by weight of polyvinylidene fluoride (50% by weight). %) And 1.5 parts by weight (50% by weight) of an acrylic rubbery copolymer of the same type as described in Example 1 were dispersed in N-methyl-2-pyrrolidone dissolved therein, and the viscosity was further increased. Carboxymethylcellulose (C
MC) and stirred to obtain a viscosity of 5000 mP.
An a · s paste (coating solution) was prepared.

This coating solution was left at room temperature for 20 hours, and the viscosity was measured using a B-type viscometer. The viscosity after 20 hours was approximately 5000 mPa · s, which was almost the same as immediately after the preparation.

The obtained coating liquid was applied to both sides of an aluminum foil as a current collector, dried, and rolled to produce a positive electrode. Design rated capacity 1 from the positive electrode in the same manner as in Example 1.
900 mAh cylindrical lithium ion secondary battery (186
50 size).

Example 15 100 parts by weight of a LiNi _0.8 Co _0.2 O ₂ powder similar to that described in Example 1 and 2.5 parts by weight of acetylene black having an average particle diameter of 50 nm (5 parts by weight)
0% by weight) and 2.5 parts by weight (50% by weight) of flaky graphite (artificial graphite) having an average particle diameter of 20 μm are mixed by a mixer, and the obtained mixture is 1 part by weight of polyvinylidene fluoride (33% by weight). And 2 parts by weight (67% by weight) of an acrylic rubbery copolymer of the same type as described in Example 1 was dispersed in N-methyl-2-pyrrolidone dissolved therein, and carboxymethyl cellulose was further used as a thickener. (CMC)
The viscosity is 5000 mPa · s by adding and stirring.
(Paste solution) was prepared.

The coating liquid was left at room temperature for 20 hours, and the viscosity was measured using a B-type viscometer. The viscosity after 20 hours was about 5000 mPa · s, which was almost the same as immediately after the preparation.

The obtained coating liquid was applied to both sides of an aluminum foil as a current collector, dried, and rolled to produce a positive electrode. Design rated capacity 1 from the positive electrode in the same manner as in Example 1.
900 mAh cylindrical lithium ion secondary battery (186
50 size).

Example 16 100 parts by weight of a LiNi _0.8 Co _0.2 O ₂ powder as described in Example 1 and 2.5 parts by weight of acetylene black having an average particle diameter of 50 nm (5 parts by weight)
0% by weight) and 2.5 parts by weight (50% by weight) of flaky graphite (artificial graphite) having an average particle size of 20 μm are mixed by a mixer, and the obtained mixture is mixed with 1.5 parts by weight of polyvinylidene fluoride (50% by weight). %) And 1.5 parts by weight (50% by weight) of an acrylic rubbery copolymer of the same type as described in Example 1 were dispersed in N-methyl-2-pyrrolidone dissolved therein, and the viscosity was further increased. By adding carboxymethylcellulose (CMC) as an agent and stirring, the viscosity becomes 5,000.
An mPa · s paste (coating liquid) was prepared.

This coating solution was left at room temperature for 20 hours, and the viscosity was measured using a B-type viscometer. The viscosity after 20 hours was approximately 5000 mPa · s, which was almost the same as immediately after the preparation.

The obtained coating solution was applied to both sides of an aluminum foil as a current collector, dried, and rolled to produce a positive electrode. Design rated capacity 1 from the positive electrode in the same manner as in Example 1.
900 mAh cylindrical lithium ion secondary battery (186
50 size).

Example 17 100 parts by weight of a LiNi _0.8 Co _0.2 O ₂ powder similar to that described in Example 1 and 2.5 parts by weight of acetylene black having an average particle diameter of 50 nm (2
5% by weight) and 7.5 parts by weight (75% by weight) of flaky graphite (artificial graphite) having an average particle diameter of 20 μm are mixed by a mixer, and the resulting mixture is mixed with 2 parts by weight of polyvinylidene fluoride (40% by weight). And 3 parts by weight (60% by weight) of an acrylic rubbery copolymer of the same type as described in Example 1 are dispersed in N-methyl-2-pyrrolidone dissolved therein, and carboxymethyl cellulose is further used as a thickener. (CMC)
The viscosity is 5000 mPa · s by adding and stirring.
(Paste solution) was prepared.

This coating solution was left at room temperature for 20 hours, and the viscosity was measured using a B-type viscometer. The viscosity after 20 hours was about 5000 mPa · s, which was almost the same as immediately after the preparation.

The obtained coating liquid was applied to both sides of an aluminum foil as a current collector, dried, and rolled to produce a positive electrode. Design rated capacity 1 from the positive electrode in the same manner as in Example 1.
900 mAh cylindrical lithium ion secondary battery (186
50 size).

Example 18 100 parts by weight of LiNi _0.8 Co _0.2 O ₂ powder similar to that described in Example 1 and 2.5 parts by weight of acetylene black having an average particle diameter of 50 nm (2
5% by weight) and 7.5 parts by weight (75% by weight) of flaky graphite (artificial graphite) having an average particle size of 20 μm are mixed by a mixer, and the obtained mixture is mixed with 3 parts by weight of polyvinylidene fluoride (60% by weight). And 2 parts by weight (40% by weight) of an acrylic rubbery copolymer of the same type as described in Example 1 was dispersed in N-methyl-2-pyrrolidone dissolved therein, and carboxymethyl cellulose was further used as a thickener. (CMC)
The viscosity is 5000 mPa · s by adding and stirring.
(Paste solution) was prepared. The obtained coating liquid had fluidity suitable for coating on a current collector. The coating solution was left at room temperature for 20 hours, and the viscosity was measured using a B-type viscometer. The viscosity after 20 hours was 6500 mP.
a · s.

Subsequently, a coating solution was applied to both sides of an aluminum foil as a current collector, dried, and rolled to produce a positive electrode. From the positive electrode, a cylindrical lithium ion secondary battery (1
8650 size).

(Example 19) 100 parts by weight of a LiNi _0.8 Co _0.2 O ₂ powder similar to that described in Example 1 and 2.5 parts by weight of acetylene black having an average particle diameter of 50 nm (2
5% by weight) and 7.5 parts by weight (75% by weight) of flaky graphite (artificial graphite) having an average particle diameter of 20 μm are mixed with a mixer, and the obtained mixture is mixed with 2.5 parts by weight of polyvinylidene fluoride (50% by weight). %) And 2.5 parts by weight (50% by weight) of an acrylic rubbery copolymer of the same type as described in Example 1 were dispersed in N-methyl-2-pyrrolidone dissolved therein, and the viscosity was further increased. By adding carboxymethylcellulose (CMC) as an agent and stirring, the viscosity becomes 5,000.
An mPa · s paste (coating liquid) was prepared.

This coating solution was left at room temperature for 20 hours, and the viscosity was measured using a B-type viscometer. The viscosity after 20 hours was approximately 5000 mPa · s, which was almost the same as immediately after the preparation.

The obtained coating solution was applied to both sides of an aluminum foil as a current collector, dried, and rolled to produce a positive electrode. Design rated capacity 1 from the positive electrode in the same manner as in Example 1.
900 mAh cylindrical lithium ion secondary battery (186
50 size).

Example 20 100 parts by weight of a LiNi _0.8 Co _0.2 O ₂ powder as described in Example 1 and 3.5 parts by weight of acetylene black having an average particle diameter of 50 nm (5 parts by weight)
0% by weight) and 3.5 parts by weight (50% by weight) of flaky graphite (artificial graphite) having an average particle size of 20 μm are mixed by a mixer, and the obtained mixture is mixed with 3.5 parts by weight of polyvinylidene fluoride (35 parts by weight). %) And 6.5 parts by weight (65% by weight) of an acrylic rubbery copolymer of the same type as described in Example 1 were dispersed in N-methyl-2-pyrrolidone dissolved therein, and the viscosity was further increased. By adding carboxymethylcellulose (CMC) as an agent and stirring, the viscosity becomes 5,000.
An mPa · s paste (coating liquid) was prepared.

This coating solution was left at room temperature for 20 hours, and the viscosity was measured using a B-type viscometer. The viscosity after 20 hours was approximately 5000 mPa · s, which was almost the same as immediately after preparation.

The obtained coating solution was applied to both sides of an aluminum foil as a current collector, dried, and rolled to produce a positive electrode. Design rated capacity 1 from the positive electrode in the same manner as in Example 1.
900 mAh cylindrical lithium ion secondary battery (186
50 size).

Example 21 100 parts by weight of the same LiNi _0.8 Co _0.2 O ₂ powder as described in Example 1 and 3.5 parts by weight of acetylene black having an average particle size of 50 nm (5 parts by weight)
0% by weight) and 3.5 parts by weight (50% by weight) of flaky graphite (artificial graphite) having an average particle size of 20 μm are mixed by a mixer, and the obtained mixture is mixed with 3.5 parts by weight of polyvinylidene fluoride (29 parts by weight). %) And 8.5 parts by weight (71% by weight) of an acrylic rubbery copolymer of the same type as described in Example 1 were dispersed in N-methyl-2-pyrrolidone dissolved therein, and the viscosity was further increased. By adding carboxymethylcellulose (CMC) as an agent and stirring, the viscosity becomes 5,000.
An mPa · s paste (coating liquid) was prepared.

This coating solution was left at room temperature for 20 hours, and the viscosity was measured using a B-type viscometer. The viscosity after 20 hours was approximately 5000 mPa · s, which was almost the same as immediately after the preparation.

The obtained coating liquid was applied to both sides of an aluminum foil as a current collector, dried, and rolled to produce a positive electrode. Design rated capacity 1 from the positive electrode in the same manner as in Example 1.
900 mAh cylindrical lithium ion secondary battery (186
50 size).

Example 22 A part of Ni was converted to Co and Al.
Nickel hydroxide powder [Ni _0.8 Co _0.17Al
0.03 (OH) ₂ ] and lithium hydroxide monohydrate (LiOH
· H ₂ O) were mixed, LiNi by firing
_0.8 Co _0.17 Al _0.03 O ₂ powder was obtained. From the X-ray diffraction pattern, it was confirmed that the obtained powder had a LiNiO ₂ structure.

The obtained LiNi _0.8 Co _0.17 Al _0.03 O
Example 1 except that powder ₂ was used as the positive electrode active material
A cylindrical lithium ion secondary battery (18650 size) having a design rated capacity of 1900 mAh was assembled in the same manner as in Example 3.

The coating solution for the positive electrode was left at room temperature for 20 hours, and the viscosity was measured using a B-type viscometer.
The viscosity after time was about 5000 mPa · s, which was almost the same as immediately after preparation.

(Example 23) Nickel hydroxide powder [Ni _0.8 Co _0.17 (OH) ₂ ] in which a part of Ni was replaced by Co
, Lithium borate (Li ₂ B ₄ O ₇ ) and lithium hydroxide monohydrate (LiOH · H ₂ O) were mixed and calcined to obtain LiNi _0.8 Co _0.17 B _0.03 O ₂ powder. . From the X-ray diffraction pattern, it was confirmed that the obtained powder had a LiNiO ₂ structure.

The resulting LiNi _0.8 Co _0.17 B _0.03 O ₂
Example 13 except that powder was used as the positive electrode active material
A cylindrical lithium ion secondary battery (18650 size) having a design rated capacity of 1900 mAh was assembled in the same manner as described above.

The coating solution for the positive electrode was left at room temperature for 20 hours, and the viscosity was measured using a B-type viscometer.
The viscosity after time was about 5000 mPa · s, which was almost the same as immediately after preparation.

(Example 24) <Preparation of positive electrode> LiNi similar to that described in Example 1 was used.
_0.8 Co _0.2 O ₂ powder 100 parts by weight, average particle size 50n
m acetylene black 2.5 parts by weight (50% by weight) and flaky graphite (artificial graphite) 2.5 having an average particle size of 1 μm
Parts by weight (50% by weight) were mixed with a mixer, and the resulting mixture was treated with 1% by weight of a monomer obtained as an anhydride by replacing two fluorines of a modified polyvinylidene fluoride (vinylidene fluoride with a carboxy group), and vinylidene fluoride. And 2 parts by weight (67% by weight) of an acrylic rubbery copolymer of the same type as described in Example 1. Dispersed in N-methyl-2-pyrrolidone,
Furthermore, carboxymethylcellulose (CM
The viscosity is 5000 mPa by adding C) and stirring.
・ A paste (coating liquid) was prepared.

The coating liquid was left at room temperature for 20 hours, and the viscosity was measured using a B-type viscometer. The viscosity after 20 hours was about 5000 mPa · s, which was almost the same as immediately after the preparation.

Thus, this coating solution was applied to both sides of an aluminum foil as a current collector, dried, and rolled to produce a positive electrode. From the positive electrode, a cylindrical lithium ion secondary battery (1
8650 size).

Comparative Example 6 100 parts by weight of LiNi _0.8 Co _0.2 O ₂ powder similar to that described in Example 1, 2.5 parts by weight of acetylene black having an average particle diameter of 50 nm, and scale having an average particle diameter of 20 μm 2.5 parts by weight of glassy graphite is mixed with a mixer, and the resulting mixture is mixed with polyvinylidene fluoride 3
A part by weight was dispersed in the dissolved N-methyl-2-pyrrolidone to prepare a paste (coating liquid) having a viscosity of 5000 mPa · s. The coating liquid immediately after preparation had fluidity suitable for coating on a current collector. When this coating liquid was left for 20 hours, the coating liquid did not reach curing, but gelation was observed, and application to the current collector could not be performed. Therefore, a coating liquid was prepared again, and immediately applied to the current collector to prepare a positive electrode. A cylindrical lithium ion secondary battery (18650 size) having a designed rated capacity of 1900 mAh was assembled from the positive electrode in the same manner as in Example 1.

Comparative Example 7 100 parts by weight of the same LiNi _0.8 Co _0.2 O ₂ powder as described in Example 1 and 5 parts by weight of acetylene black having an average particle diameter of 50 nm were mixed by a mixer, and the resulting mixture was obtained. Is dispersed in N-methyl-2-pyrrolidone in which 1 part by weight of polyvinylidene fluoride and 2 parts by weight of an acrylic rubbery copolymer of the same type as described in Example 1 are dissolved, and further as a thickener A paste (coating liquid) having a viscosity of 5000 mPa · s was prepared by adding carboxymethyl cellulose (CMC) and stirring. The obtained coating liquid lacked fluidity and was not suitable for application to a current collector.

When this coating solution was left at room temperature for 20 hours and the viscosity was measured using a B-type viscometer, the viscosity after 20 hours was about 5000 mPa · s, which was almost the same as immediately after the preparation. However, it lacked fluidity and was not suitable for application to a current collector.

The obtained coating solution was applied to both sides of an aluminum foil as a current collector, dried, and rolled to produce a positive electrode. Design rated capacity 1 from the positive electrode in the same manner as in Example 1.
900 mAh cylindrical lithium ion secondary battery (186
50 size).

Comparative Example 8 100 parts by weight of the same LiNi _0.8 Co _0.2 O ₂ powder as described in Example 1 and 5 parts by weight of acetylene black having an average particle diameter of 50 nm were mixed by a mixer, and the resulting mixture was obtained. Was dissolved in 3 parts by weight of a modified polyvinylidene fluoride (obtained by substituting the two fluorines of vinylidene fluoride with carboxyl groups and polymerizing 1% by weight of an anhydride monomer and vinylidene fluoride). -Methyl-2-
It was dispersed in pyrrolidone to prepare a paste (coating liquid) having a viscosity of 5000 mPa · s. The obtained coating liquid lacked fluidity and was not suitable for application to a current collector.

When this coating solution was left at room temperature for 20 hours, the coating solution was completely cured and could not be applied to the current collector. Therefore, a coating liquid was prepared again in the same manner, and immediately applied to the current collector to produce a positive electrode. A cylindrical lithium ion secondary battery (18650 size) having a designed rated capacity of 1900 mAh was assembled from the positive electrode in the same manner as in Example 1.

For the secondary batteries of Examples 13 to 24 and Comparative Examples 6 to 8, the types and mixing ratios of the positive electrode active material, the conductive agent and the binder are shown in Tables 6 to 9 below. Example 13 obtained
The positive electrodes of the secondary batteries of Comparative Examples 6 to 8 and Comparative Examples 6 to 8 were subjected to a peel strength test using a tensile tester in the same manner as described above, and the adhesion strength was measured.
Shown in Table 10 also shows the results of Example 1.

Examples 13 to 24 and Comparative Examples 6 to 8
Was subjected to a cycle life test in the same manner as described above, and the results are shown in Table 10 below. In addition,
Table 10 also shows the results of Example 1.

Further, the above-described Embodiments 1 and 13 to
24 and the secondary batteries of Comparative Examples 6 to 8 were measured for large-current discharge characteristics. This large current discharge characteristic was defined by a method of defining the ratio of each discharge capacity obtained when discharging at two current values. That is, 1 which is the nominal capacity of the battery
When the current of 1900 mAh for discharging 900 mAh in 1 hour is 1 C, the discharge capacity when discharging at 0.2 C (0.2 C) and the discharging capacity when discharging at 3 C (3 C)
Is measured, and the value of the discharge capacity (3C) / discharge capacity (0.2C), which is the ratio of the two discharge capacities, is defined as the large current discharge capacity ratio. The results are shown in Table 10 below.

[0149]

[Table 6]

[0150]

[Table 7]

[0151]

[Table 8]

[0152]

[Table 9]

[0153]

[Table 10]

As is clear from Tables 6 to 10, a conductive agent containing carbonaceous material A having an average particle size of 100 nm or less and carbonaceous material B having an average particle size of 1 μm or more, a vinylidene fluoride fluororesin and an acrylic rubber The positive electrodes of Examples 1 and 13 to 24 containing a binder containing a porous copolymer were used in Comparative Examples 6 and 8 in which the binder consisted only of a vinylidene fluoride-based fluororesin.
It can be seen that the adhesion strength between the current collector and the coating liquid is higher than that of the positive electrode of Comparative Example 7 and the positive electrode of Comparative Example 7 in which the conductive agent is only the carbonaceous material A. In addition, it can be seen that the secondary batteries of Examples 1 and 13 to 24 are superior to the secondary batteries of Comparative Examples 6 to 8 in both cycle life and large current discharge characteristics.

In the above-described embodiment, an example in which the present invention is applied to a cylindrical lithium ion secondary battery has been described. However, a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte are accommodated in a bottomed rectangular cylindrical container. The same can be applied to a prismatic lithium ion secondary battery having a different structure.

[0156]

As described above in detail, according to the lithium secondary battery of the present invention, it is possible to suppress the gelation of the coating liquid on the positive electrode, and to improve the adhesion between the coating liquid and the current collector. And a remarkable effect such as improvement in the charge / discharge cycle life can be obtained.

[Brief description of the drawings]

FIG. 1 is a partial cross-sectional view showing a lithium secondary battery (for example, a cylindrical lithium secondary battery) according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... container, 3 ... electrode group, 4 ... positive electrode, 5 ... separator, 6 ... negative electrode, 8 ... insulating sealing plate.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-4-162357 (JP, A) JP-A-8-222206 (JP, A) JP-A-62-160656 (JP, A) JP-A-9-96 199132 (JP, A) JP-A-9-213337 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) H01M 4/36-4/62 H01M 4/02-4/04 H01M 10/40

Claims (3)

(57) [Claims] 1. A lithium composite metal oxide represented by a composition formula: LiNi _1-x M _x O ₂ (where M is one or more elements and x represents 0 <x ≦ 0.5) And a conductive agent containing a carbonaceous material A having an average particle size of 100 nm or less and a carbonaceous material B having an average particle size of 1 μm or more, and a binder made of an acrylic rubbery copolymer and a vinylidene fluoride-based fluororesin.
Having a structure in which a positive electrode mixture containing
, A negative electrode, and a non-aqueous electrolyte, and a mixing ratio of a vinylidene fluoride-based fluororesin in the binder.
Is 45% by weight or less . 2. The vinylidene fluoride-based fluorine in the binder.
The mixing ratio of the resin must be within the range of 30 to 45% by weight.
The lithium secondary battery according to claim 1, wherein: 3. The non-aqueous electrolyte comprises EC, PC and γ-B.
L, EC and PC and MEC, EC and PC and DEC, EC and
PC and DEE, EC and AN, EC and MEC, PC and DM
C or a non-aqueous solvent comprising PC and DEC;
And an electrolyte dissolved in a solvent.
Item 3. The lithium secondary battery according to any one of Items 1 to 2.