JPH09213306A - Secondary battery with non-aqueous electrolyte - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent slipping-off of a carbon material from a negative electrode caused by repetitive cycle of charging and discharging operations by specifying the component composition of a binder for negative electrode. SOLUTION: A binder consists of a mixture of polyvinyl acetate(PVAc) and polyvinyl pyrolidone(PVP) in a proportion by weight of 3:7 to 8:2. Accordingly the carbon material is not likely slipping off from the negative electrode when the charging and discharging cycle is repeated, so that the resultant battery is equipped with an excellent charge/discharge cyclic characteristic. The reason therefor is as follows; PVAc of soft nature is suitable for binding together of particles of carbon material and also binding of the carbon material with a negative electrode electricity collector but is likely to go in structural destruction with application of an external force at the time of rolling, etc., to lead to drop of the binding strength, while the PVP of rigid nature is not likely to make structural destruction even with application of external force but has a small binding force. If the two sorts of polymers are used in combination in the specified mix proportion, they complement each other, so that a bond strength greater than a conventional binder can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-aqueous electrolyte secondary battery using a carbon material as a negative electrode material, and more specifically to a binder used for a negative electrode for the purpose of improving its charge / discharge cycle characteristics. Regarding improvement of agents.

[0002]

2. Description of the Related Art In recent years,
A non-aqueous electrolyte secondary battery typified by a lithium secondary battery,
It is attracting attention because it can achieve higher voltage and higher energy density than an alkaline secondary battery. As a negative electrode material for a non-aqueous electrolyte secondary battery, a metallic material (such as metallic lithium) was initially considered.

However, when a metal material is used as a negative electrode material, repeated charging and discharging may cause dendritic electrodeposits (dendrites) to grow and cause an internal short circuit. For this reason, in recent years, carbon materials such as coke and graphite that have no risk of causing an internal short circuit have been proposed as negative electrode materials for non-aqueous electrolyte secondary batteries that replace metal materials.

For a negative electrode using a carbon material as a negative electrode material, carbon powder is mixed with a binder solution to prepare a negative electrode mixture slurry,
This negative electrode mixture slurry is applied on a negative electrode current collector and dried. Thus, conventionally, a fluororesin such as polyvinylidene fluoride (PVdF) has been mainly used as a binder for the negative electrode of the non-aqueous electrolyte secondary battery.

However, when a fluororesin is used as the binder, the binding property between the carbon material and the negative electrode current collector is poor, and the carbon material easily falls off from the negative electrode after repeated charging and discharging. There is a problem that the discharge cycle characteristics are generally poor.

The present invention has been made to solve this problem, and an object thereof is to provide a good binding property between a carbon material and a negative electrode current collector and to have excellent charge / discharge cycle characteristics. A non-aqueous electrolyte secondary battery is provided.

[0007]

The non-aqueous electrolyte secondary battery (the battery of the present invention) according to the present invention for achieving the above object is
A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode formed by forming a negative electrode mixture layer containing a carbon material and a binder on a negative electrode current collector, and a non-aqueous electrolyte secondary battery comprising: The binder is polyvinyl acetate (PVAc) and polyvinylpyrrolidone (PV
P) in a weight ratio of 3: 7 to 8: 2.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION Examples of the carbon material include coke, graphite, and a fired organic material. For higher capacity,
The d value (d ₀₀₂ ) on the lattice plane (002) plane is 3.35.
~ 3.38Å, and the crystallite size in the c-axis direction (Lc)
Is preferably a carbon material having a value of 150Å or more.

The negative electrode contains a negative electrode current collector, a binder which is formed on the negative electrode current collector, and which comprises a carbon material and a mixture of PVAc and PVP in a weight ratio of 3: 7 to 8: 2. And a negative electrode mixture layer. The negative electrode mixture layer is a carbon material (powder)
Is mixed with a binder solution (aqueous solution or non-aqueous solution) to prepare a negative electrode mixture slurry, and the negative electrode mixture slurry is applied onto a negative electrode current collector and dried. Examples of the negative electrode current collector include copper foil and nickel foil. PVA
The reason that the weight ratio of c to PVP is limited to 3: 7 to 8: 2 is that if it is out of this range, excellent charge / discharge cycle characteristics cannot be obtained. The negative electrode mixture layer may contain a thickener. A typical example of the thickener is CMC (carboxymethyl cellulose).

Mixture of PVAc and PVP (binder)
Is preferably contained in the negative electrode mixture layer in an amount of 0.75 to 10% by weight. If the content of the binder in the negative electrode mixture layer deviates from this range, the initial capacity decreases. This is because when the binder content is less than 0.75% by weight, the binding between the particles of the carbon material and the binding between the carbon material and the current collector become insufficient. If the content exceeds 10% by weight,
It is considered that the decrease in the contact area between the carbon material and the non-aqueous electrolytic solution reduces the liquid content of the negative electrode and makes the electrode reaction non-uniform.

The non-aqueous electrolyte is not particularly limited, and any conventionally known one can be used. Specific examples of the solvent include ethylene carbonate, propylene carbonate,
Butylene carbonate, vinylene carbonate, cyclopentanone, sulfolane, 3-methylsulfolane, 2,
4-dimethylsulfolane, 3-methyl-1,3-oxazolidin-2-one, γ-butyrolactone, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, butyl methyl carbonate, ethyl propyl carbonate, butyl ethyl carbonate, di Propyl carbonate, 1,2-
Dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, methyl acetate, ethyl acetate and a mixed solvent of two or more of these, and specific examples of the solute include LiPF ₆ , LiBF ₄ , and L.
iClO ₄ , LiCF ₃ SO ₃ , LiAsF ₆ , LiN
(CF ₃ SO ₂ ) ₂ , LiOSO ₂ (CF ₂ ) ₃ CF ₃
, Respectively.

The positive electrode material is not particularly limited, and conventionally known materials can be used. As a specific example, LiC
oO ₂ , LiNiO ₂ , LiMnO ₂ , LiVO ₂ , L
iNbO ₂ .

INDUSTRIAL APPLICABILITY The present invention can be applied not only to lithium secondary batteries but also to non-aqueous electrolyte secondary batteries having a carbon material as a negative electrode material.

In the battery of the present invention, P is used as the binder.
Since VAc and PVP are used in a predetermined ratio, it is difficult for the carbon material to drop from the negative electrode when charging and discharging are repeated. For this reason, the battery of the present invention is excellent in charge / discharge cycle characteristics. This is for the following reason. That is, flexible PVA
c is suitable for binding the particles of the carbon material to each other and binding the carbon material to the negative electrode current collector, but if external force is applied during rolling, structural destruction occurs and the binding strength decreases. However, by itself, practically sufficient binding strength cannot be obtained. On the other hand, a rigid PVP cannot easily be structurally destroyed even when an external force is applied, but it cannot be used alone as a binder because it has a small binding force. However, when PVAc and PVP are used together in a predetermined ratio, the above-mentioned drawbacks of one polymer are complemented by the advantages of the other polymer, and a binding strength much higher than that of a conventional fluororesin is exhibited. is there.

[0015]

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to the following examples, and the present invention may be practiced by appropriately changing the gist of the invention. Is possible.

(Experiment 1) [Preparation of Positive Electrode] 95 parts by weight of a mixture of LiCoO ₂ powder (active material) and artificial graphite powder (conductive agent) in a weight ratio of 9: 1, and 5 parts by weight of PVdF in 5% by weight N-- A methyl-2-pyrrolidone solution is kneaded to prepare a positive electrode mixture slurry, and this positive electrode mixture slurry is applied to both surfaces of an aluminum foil as a positive electrode current collector by a doctor blade method, and then the temperature is 150 °.
It was vacuum dried at C for 2 hours to prepare a positive electrode in which a positive electrode mixture layer having a thickness of 50 μm was formed on each surface of the aluminum foil.

[Preparation of Negative Electrode] 98 parts by weight of graphite powder [crystal size in the c-axis direction (Lc)>1000Å; d value (d ₀₀₂ ): 3.35Å] on the lattice plane (002) plane,
The binder is dispersed in an aqueous solution of 1 part by weight, and further 1 part by weight of carboxymethyl cellulose (CMC) is added and mixed,
A negative electrode mixture slurry was prepared. As a binder, PVP
Or PVAc alone, or PVAc and PVP
And the weight ratio of 1: 9, 2: 8, 3: 7, 4: 6, 5: 5,
Used at 6: 4, 7: 3, 8: 2, 9: 1. Then
Each negative electrode mixture slurry is applied to both surfaces of a copper foil as a negative electrode current collector by the doctor blade method, and dried to form a negative electrode formed by forming a negative electrode mixture layer having a thickness of 50 μm on each surface of the copper foil. did.

[Preparation of Non-Aqueous Electrolyte] LiPF ₆ (lithium hexafluorophosphate) was added to a mixed solvent of ethylene carbonate and diethyl carbonate at a volume ratio of 2: 3.
Mol / liter was dissolved to prepare a non-aqueous electrolytic solution.

[Battery Assembly] A cylindrical (AA size) non-aqueous electrolyte secondary battery (invention battery) A1 (PVA) having a positive electrode-dominated capacity using the above positive electrode, negative electrode and non-aqueous electrolytic solution.
c: PVP = 3: 7), A2 (PVAc: PVP = 4:
6), A3 (PVAc: PVP = 5: 5), A4 (PV
Ac: PVP = 6: 4), A5 (PVAc: PVP =
7: 3), A6 (PVAc: PVP = 8: 2) and non-aqueous electrolyte secondary battery (comparative battery) B1 (one PVP alone),
B2 (PVAc: PVP = 1: 9), B3 (PVAc:
PVP = 2: 8), B4 (PVAc: PVP = 9:
1) and B5 (single PVAc alone) were assembled. In addition,
A microporous film made of polypropylene was used as the separator.

[Relationship between PVAc / PVP Weight Ratio and Charge / Discharge Cycle Characteristic] For each battery, at 20 ° C., 20
After charging to 4.1V at 0mA, 2.7mA at 200mA
A charge cycle test was conducted with one cycle consisting of a step of discharging up to 5 V, and the PVAc of the binder was determined by the number of charge / discharge cycles until the discharge capacity decreased to 75% or less of the initial capacity (discharge capacity in the first cycle). The relationship between the PVP weight ratio and the charge / discharge cycle characteristics was investigated. The results are shown in FIG. FIG.
Is the relationship between the weight ratio of PVAc and PVP of the binder and the charge / discharge cycle characteristics, the vertical axis represents the number of charge / discharge cycles until the initial capacity is reduced to 75% or less, and the horizontal axis represents the PV of the binder.
It is the graph which took and showed the weight ratio of Ac and PVP.

As shown in FIG. 1, the batteries A1 to A6 are far superior to the batteries B1 to B5 in charge / discharge cycle characteristics. From this fact, in order to obtain a non-aqueous electrolyte secondary battery excellent in charge / discharge cycle characteristics, it is necessary to regulate the weight ratio of PVAc and PVP of the binder to 3: 7 to 8: 2. I understand.

(Experiment 2) 95 parts by weight of graphite powder (the same as that used in Experiment 1) and 5 parts by weight of PVdF in N were added.
It was dispersed in an MP solution to prepare a negative electrode mixture slurry. Next, a non-aqueous electrolyte secondary battery (conventional battery) C was assembled in the same manner as in Experiment 1 except that a negative electrode was produced using this negative electrode mixture slurry.

[Comparison of Charging / Discharging Cycle Characteristics of Battery of Present Invention and Conventional Battery] Conventional battery C was 2 ° C. at 25 ° C.
After charging to 4.1V at 00mA, 2. Charge at 200mA.
A charging / discharging cycle test in which the step of discharging up to 75 V was defined as one cycle was conducted to examine the charging / discharging cycle characteristics. FIG.
Is the charging / discharging cycle characteristics of the battery A3 of the invention and the conventional battery C, and the vertical axis shows the discharge capacity (mAh) in each cycle.
Is a graph showing the number of charge / discharge cycles on the horizontal axis.

From FIG. 2, the battery A3 of the present invention is the conventional battery C3.
It is understood that the charging / discharging cycle characteristics are remarkably excellent as compared with.

(Experiment 3) Graphite powder (the same as that used in Experiment 1) 99-X parts by weight (X = 0.5, 0.7)
5,1,3,5,7.5,10,12,15,17.5
Or 20) was dispersed in an aqueous solution of X part by weight of a binder, and 1 part by weight of carboxymethyl cellulose (CMC) was further added and mixed to prepare a negative electrode mixture slurry. As a binder, PVAc and PVP were used at a weight ratio of 5: 5. Then, in the same manner as in Experiment 1 except that each negative electrode mixture slurry was used to manufacture a negative electrode, nonaqueous electrolyte secondary batteries (cells of the present invention) A7 (X = 0.75), A8 (X = X
1), A9 (X = 3), A10 (X = 5), A11 (X
= 7.5), A12 (X = 10) and non-aqueous electrolyte secondary battery (comparative battery) B6 (X = 0.5), B7 (X = 1)
2), B8 (X = 15), B9 (X = 17.5), B1
0 (X = 20) was assembled. X corresponds to the binder content (% by weight) of the negative electrode mixture layer.

[Relationship between Binder Amount X and Initial Capacity] Each battery was charged at 25 ° C. to 200 V at 4.1 V and then discharged at 200 mA to 2.75 V to form a negative electrode mixture layer. The relationship between the binder amount X and the initial capacity was examined. The results are shown in FIG. FIG. 3 is a graph showing the relationship between the binder amount X and the initial capacity, with the vertical axis representing the initial capacity (mAh) and the horizontal axis representing the binder amount X.

As shown in FIG. 3, the batteries A7 to A12 are
The initial capacity is much larger than the batteries B6 to B10. From this fact, in order to obtain a non-aqueous electrolyte secondary battery having a large initial capacity, the binder content of the negative electrode mixture layer is 0.75 to 1
It can be seen that it is preferable to regulate to 0% by weight.

[0028]

The battery of the present invention uses a mixture of polyvinyl acetate and polyvinylpyrrolidone in a predetermined ratio as a binder for the negative electrode, so that the carbon material is unlikely to fall off from the negative electrode even after repeated charging and discharging. Excellent charge / discharge cycle characteristics.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the weight ratio of PVAc and PVP and the charge / discharge cycle characteristics.

FIG. 2 is a graph showing charge / discharge cycle characteristics of a battery of the present invention and a conventional battery.

FIG. 3 is a graph showing the relationship between the binder content (% by weight) of the negative electrode mixture layer and the initial capacity.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Koji Nishio 2-5-5 Keihanhondori, Moriguchi-shi, Osaka Sanyo Electric Co., Ltd.

Claims (2)

[Claims] 1. A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode formed by forming a negative electrode mixture layer containing a carbon material and a binder on a negative electrode current collector, and a non-aqueous electrolytic solution. A non-aqueous electrolyte secondary battery, wherein the binder comprises a mixture of polyvinyl acetate and polyvinylpyrrolidone in a weight ratio of 3: 7 to 8: 2. 2. The negative electrode material mixture layer comprises 0.75 to 5% of the mixture.
The non-aqueous electrolyte secondary battery according to claim 1, containing 10% by weight.