JP2000011991A - Organic electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve high rate discharge characteristic and cycle life characteristic by interposing a layer formed by a binder and a carbon material on the surface of a collector used in positive pole or negative pole, and providing an active material layer for positive pole or an active material layer for a negative pole on the layer. SOLUTION: An organic electrolyte secondary battery has a positive pole taking lithium composite oxide as an active material and a negative pole taking carbon material as an active material. It is desirable that a binder use polyvinylidene fluoride(PVdF) and the carbon material uses acethylene black or carbon powder. PVdF and acethylene black are mixed at a mixing weight ratio of 30:70, N-methyl-2 pyrolidone(NMP) as a dispersing solvent is added, and the materials are kneaded and dispersed to be slurry in form. The slurry is applied to both sides of an aluminum foil and both sides of a copper foil at 1 g/m2, and dried to make a collector for positive pole and a collector for negative pole. Active materials for the positive pole and the negative pole are applied to make an electrode and a battery.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to improvement of high-rate discharge characteristics and cycle life characteristics of an organic electrolyte secondary battery.

[0002]

2. Description of the Related Art Lithium secondary batteries, which can be used for a long time, are used as main power supplies or backup power supplies for video equipment such as portable telephones, cordless telephones, video cameras, office equipment such as personal computers, home electric appliances, electric vehicles, and the like. Highly required. As the positive electrode active material used in these lithium secondary batteries, lithium cobalt composite oxide, lithium nickel composite oxide, lithium manganese composite oxide, and the like are used. Attention has been paid to lithium manganese composite oxides using inexpensive manganese as a main raw material.

In general, a battery is assembled using a carbon material used as a negative electrode active material in a state where lithium is released, that is, in a discharged state. Accordingly, the positive electrode is also in a discharged active material, for example, LiMn ₂ O ₄ (lithium manganate), L
iCoO ₂ (lithium cobaltate) and LiNiO ₂ (lithium nickelate) are used. Then, a lithium secondary battery that can be charged and discharged by performing initial charging is obtained.

In a lithium secondary battery, a lithium composite metal oxide as a positive electrode active material, a carbon material powder and a binder as a negative electrode active material are dispersed in N-methyl-2-pyrrolidone (NMP) to form a slurry. Then, it is applied to both sides of a metal foil as a current collector, the solvent is dried, and then compression molded by a roller press to obtain a positive electrode plate and a negative electrode plate.
The binder is mainly polyvinylidene fluoride (P
VdF). However, when polyvinylidene fluoride is used as a binder, there is a problem that adhesion between the current collector and the positive electrode mixture layer or between the current collector and the negative electrode mixture layer is poor, or the positive electrode mixture layer or the negative electrode There is a problem that the binding force in the mixture layer is weak. When drying and removing NMP as a dispersant, a conductive agent and a binder having a small specific gravity float on the electrode surface as NMP is vaporized and evaporated.
In addition, a decrease in the concentration of the conductive agent present in the vicinity of the current collector is also one of the causes of a decrease in the adhesion strength between the current collector and the mixture layer. As a result, during a manufacturing process such as an electrode plate cutting process or a winding process, a part of the positive electrode material mixture layer or the negative electrode material mixture layer may peel off or fall off the current collector, causing a micro short circuit or a variation in battery capacity. Become. In addition, due to charge and discharge, the lithium composite metal oxide of the positive electrode and the carbon material of the negative electrode expand and contract, so that the mixture layer of the positive electrode and the negative electrode separates and falls off from the current collector, and the internal resistance of the battery increases. There were problems such as a decrease in high-rate discharge characteristics.

For the purpose of improving the adhesion strength between the current collector and the mixture layer, a copolymer of a monomer containing vinylidene fluoride as a main component and an unsaturated dibasic monoester as a binder has been proposed. I have. However, when this binder is used, the adhesion strength between the positive electrode and negative electrode mixture layers and the current collector is improved, but the abnormal temperature rise under a high voltage causes the binder to decompose and generate hydrogen fluoride. However, the battery reacts with a lithium intercalation compound (GIC) on the surface of the negative electrode plate or precipitated metallic lithium, generates abnormal heat, and may explode or explode the battery.
Examples of the binder other than the fluororesin include a synthetic rubber such as acrylic rubber and styrene / butadiene rubber, and a thermosetting resin such as an epoxy resin. However, since these resins are dissolved or swelled in the electrolytic solution, there is a problem that the adhesion between the current collector and the mixture layer and the inside of the mixture layer cannot be maintained for a long time. When a thermosetting resin such as an epoxy resin is used, 180.degree.
It is necessary to heat to a temperature of not less than ° C. If the heating is insufficient, the curing will be insufficient, and the resistance to the electrolytic solution will be significantly reduced. Diene-based synthetic rubbers such as styrene-butadiene rubber have electrolytic solution resistance, but it is very difficult to uniformly disperse them with active material particles, so it is necessary to add cellulose or a surfactant. There is a problem that it dissolves in the liquid and lowers the charge / discharge efficiency of the battery.

[0006] Further, if the adhesion between the current collector and the mixture layer is insufficient, a part of the current collector and the mixture layer is peeled off, and as a result, the current collector and the mixture layer are separated. The current concentrates on the contact surface of. In the case of the negative electrode, metallic lithium is easily deposited on the peeled portion. When the electrodes are in such a state, there is a problem that the thermal stability of the battery is lost, and in some cases, the battery does not only lose its function but also explodes or explodes.

[0007] In a battery for an electric vehicle or a large-sized power storage, a very large number of batteries (cells) are connected in series or in parallel, so that each battery is required to have high reliability. Note that among the lithium composite metal oxides, LiM
Since n ₂ O ₄ is very thermally stable, it is attracting attention as a raw material for these large lithium secondary batteries. In addition, manganese has abundant resources and is inexpensive. However, the capacity of the active material is as low as about 70% of LiCoO ₂ and about 50% of LiNiO ₂ , and there is a problem that charge / discharge cycle characteristics are poor. Furthermore, since the adhesive strength between the current collector and the active material mixture layer is small, it cannot follow the expansion and contraction of the active material mixture layer during charge and discharge, and peeling occurs at this interface to reduce electron conductivity, There is also a problem that the charge-discharge cycle life characteristic is significantly reduced.

[0008]

SUMMARY OF THE INVENTION It is an object of the present invention to provide an organic electrolyte secondary battery which uses a lithium composite oxide as a positive electrode active material, has good high-rate discharge characteristics, and has an excellent cycle life. Is what you do.

[0009]

In order to solve the above-mentioned problems, the present invention provides an organic electrolyte secondary battery having a positive electrode using a lithium composite oxide as an active material and a negative electrode using a carbon material as an active material. On the surface of the current collector used for the positive electrode or the negative electrode, there is a layer made of a binder and a carbon material, and a positive electrode active material layer or a negative electrode active material layer is provided on this layer. It is characterized by having.

[0010]

Embodiments of the present invention will be described below. 1. In positive present embodiment, a Li _{1. 107} Mn _1. Of composition: ₈₉₃ O ₄ lithium-manganese composite oxide. The lithium manganese composite oxide having an average particle diameter of 15 μm, graphite (average particle diameter of about 0.5 μm), and carbon fibers (average fiber diameter of 0.5 μm)
m, fiber length 10-20 μm; vapor grown carbon fiber (VGCF) manufactured by Showa Denko KK) and PVdF at 80: 6: 4: 1.
The mixture is sufficiently mixed with a mixing ratio of 0, and an appropriate amount of NMP serving as a dispersion solvent is added thereto, sufficiently kneaded and dispersed to form a slurry.
This slurry is applied to both sides of a 20 μm thick aluminum foil. It should be noted that a battery was prepared by using an aluminum foil as it is as described below as a current collector, and a battery was prepared by using two types of aluminum foil whose surfaces were previously coated with a mixture of carbon powder and PVdF, and compared. Electrodes obtained by applying a slurry-like active material to these current collectors, after drying the solvent, were rolled with a roller press, cut to a length of 50 mm and cut to a length of 450 mm to form a short thin positive electrode. Produced. The positive electrode active material layer has a thickness of about 250 g / side on one side of the current collector.
m ^{2 is} applied, and the density is 2.6 to 2.7 g / cm ³ .

2. Negative electrode Average particle size of 20μ for inserting and removing lithium ions
m of amorphous carbon powder and PVDF are mixed at a weight ratio of 90:10, NMP as a dispersion solvent is added in an appropriate amount, and the mixture is sufficiently kneaded and dispersed to form a slurry. This slurry is applied to both sides of a copper foil having a thickness of 10 μm by a roll-to-roll transfer. Two types of current collectors were used, one using the above-mentioned aluminum foil as it was, and one having a mixture of carbon powder and PVdF previously applied to the surface of the aluminum foil. After drying the solvent, the electrode coated with the slurry-like active material was rolled with a roller press, and cut into 50 mm width and 500 mm length to produce a strip-shaped negative electrode. Note that the density of the negative electrode active material layer is about 1.0 g / cm ³ .

3. Battery The positive electrode and the negative electrode produced by the method described above were 25 μm thick,
It is wound through a separator made of a microporous polyethylene film having a width of 58 mm to form a spiral wound group. The wound group is inserted into a battery can, and a nickel tab terminal previously welded to the copper foil of the negative electrode current collector is welded to the bottom of the battery can.
The sum of the cross-sectional areas of the positive electrode active material layer, the negative electrode active material layer, and the separator was 2.00 to 2.10 cm ² . If it exceeds 2.10 cm ² , the diameter of the wound body becomes larger than the inner diameter of the negative electrode can, and the wound body cannot be inserted into the negative electrode can. On the other hand, 2.00cm
If it is less than ² , on the contrary, the diameter of the wound body becomes smaller than the inner diameter of the negative electrode can 6, and a sufficient capacity as a battery cannot be obtained. The cross-sectional area was adjusted by fixing the amount of the positive electrode active material and adjusting the respective lengths of the negative electrode and the separator to keep the cross-sectional area constant. At this time, the length of the negative electrode is 5c longer than the length of the positive electrode.
m and the length of the separator was further increased by 5 cm.

A mixed solvent of ethylene carbonate, dimethyl carbonate and diethyl carbonate is used, and the mixing ratio is 30: 5 by volume.
0:20. 1 mol of LiPF _{6 in} this mixed solvent
5 ml of the electrolytic solution dissolved at a concentration of / l was injected into the battery.
One of the positive electrode tab terminals is previously welded to the positive electrode current collector,
The other is welded to the positive electrode cap. The positive electrode cap was placed on the upper part of the negative electrode can, and the upper part of the negative electrode can was caulked via a gasket to seal the battery, thereby producing a cylindrical battery having a diameter of 18 mm and a height of 65 mm. In the positive electrode cap, a current cutoff mechanism (pressure switch) that operates in accordance with an increase in battery internal pressure and a valve mechanism that opens in response to a pressure higher than the pressure at which the current cutoff mechanism operates are incorporated. In this embodiment, the operating pressure is 9 kgf / cm ² , and the operating pressure is 20 kgf / cm ^2.
A valve mechanism of kgf / cm ² was used.

4. Initial charge / discharge test and cycle test After the produced battery was left at 25 ° C. for 24 hours, an initial charge / discharge test was performed to confirm a discharge capacity. That is, after charging for 4 hours at a charging voltage of 4.2 V (however, a limiting current of 430 mA), the battery was discharged under a condition of a discharging current of 280 mA (however, a discharging end voltage of 2.7 V).

Some of the batteries that were subjected to the initial charge / discharge test were subjected to a cycle life test. That is, after charging at an ambient temperature of 50 ° C. at a charging voltage of 4.2 V (however, a limiting current of 430 mA) for 4 hours, charging and discharging were repeated under a condition of a discharging current of 280 mA (discharge end voltage of 2.7 V). The life was judged when it reached less than 70% of the initial discharge capacity.

[0016] The relationship between the discharge current value and the discharge capacity of some of the batteries that were subjected to the initial charge / discharge test was measured. That is, after charging at an ambient temperature of 50 ° C. at a charging voltage of 4.2 V (however, a limiting current of 430 mA) for 4 hours, a discharging current of 280 m
The discharge capacity was measured at A to 4300 mA.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to working examples.

(Example) Polyvinylidene fluoride (PVd)
F) and acetylene black are sufficiently mixed at a compounding weight of 30:70, an appropriate amount of NMP as a dispersion solvent is added thereto, and the mixture is sufficiently kneaded and dispersed to form a slurry. This slurry was applied to both sides of a 20 μm-thick aluminum foil using a gravure roll at 1 g / m ² and dried to prepare a positive electrode current collector. Polyvinylidene fluoride (PVdF) and acetylene black are sufficiently mixed at a compounding weight of 30:70, an appropriate amount of NMP as a dispersion solvent is added thereto, and the mixture is sufficiently kneaded and dispersed to form a slurry. This slurry was applied to both surfaces of a copper foil having a thickness of 10 μm at 1 g / m ² using a gravure roll and dried to prepare a negative electrode current collector. The present invention, on the surface of the current collector for the positive electrode and the negative electrode, after applying a mixture of polyvinylidene fluoride and acetylene black, and applying the active material for the positive electrode and the negative electrode by the above-described means, the electrode and A battery was produced.

Comparative Example In order to confirm the effects of the present invention, a positive electrode and a negative electrode were manufactured using an aluminum foil or a copper foil as a current collector. Other conditions for producing the electrode and the battery are as described above.

FIG. 1 shows the relationship between the discharge current and the capacity, and FIG. 2 shows the relationship between the cycle life characteristics of the two types of batteries. As shown in FIGS. 1 and 2, by providing a layer of a conductive agent and a binder on the surface of the current collector, the high-rate discharge characteristics are improved, and the cycle life characteristics at 50 ° C. are also significantly improved. ing. It is considered that the reason for this is that the present invention has improved current collecting properties. The layer of the conductive agent and the binder is 1 g / m this time.
^Although it was set to ² , it was found that preferably 0.5 to 5 g / m ² was good.

Although lithium manganate was used as the positive electrode active material in the examples, any transition metal oxide capable of reversibly inserting and releasing lithium ions may be used, and a lithium-containing transition metal oxide is particularly desirable. As the transition metal, Ti, V, Cr, Mn, Fe, Co, Ni, M
At least one selected from o, W, and Cu can be used. On the other hand, although an amorphous carbon powder was used as the negative electrode active material, pitch coke, petroleum coke, graphite, carbon fiber, activated carbon, and the like, or a mixture thereof can obtain the same effect.
Further, the adhesive strength between the current collector and the mixture layer can be further improved by adding a crosslinking agent of the binder to the dispersion solution of the conductive agent and the binder and performing thermosetting.

[0022]

According to the present invention, there is provided an organic electrolyte secondary battery having improved high-rate discharge characteristics and cycle life characteristics due to the presence of a layer made of a carbon material on the surface of a current collector used for a positive electrode or a negative electrode. Obtainable.

[Brief description of the drawings]

FIG. 1 Relationship between discharge current and capacity

FIG. 2 shows a relationship between cycle life characteristics.

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Claims (1)

[Claims] In an organic electrolyte secondary battery having a positive electrode using a lithium composite oxide as an active material and a negative electrode using a carbon material as an active material, a surface of a current collector used for the positive electrode or the negative electrode is provided. An organic electrolyte secondary battery comprising a layer comprising a binder and a carbon material, and a positive electrode active material layer or a negative electrode active material layer provided on this layer.