JP2002260633A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery capable of improving safety, while realizing a high output, in particular, high output at low temperatures. SOLUTION: A positive electrode mix layer is formed, by evenly applying a positive electrode mix containing lithium manganate, having a spinel structure which has an average diameter of 5-20 μm, scale-like graphite and polyvinylidene fluoride to a collector. The vacancy volume in the positive electrode mix layer is adjusted to 25-35%, with respect to the volume of the positive electrode mix layer. A negative electrode mix layer is formed by evenly applying a negative electrode mix, containing amorphous carbon having an average particle diameter of 2-20 μm, acetylene black and polyvinylidene fluoride to a collector. The vacancy volume in the negative electrode mix layer is adjusted to 30-40%, with respect to the volume of the negative electrode mix layer.

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, and more particularly to a non-aqueous electrolyte secondary battery having high output, particularly high output at low temperatures and improved safety.

[0002]

2. Description of the Related Art In the field of rechargeable secondary batteries, aqueous batteries such as lead batteries, nickel-cadmium batteries, and nickel-hydrogen batteries have hitherto been the mainstream. However, due to global warming and exhausted fuel, electric vehicles (E
V) and hybrid electric vehicles (HEV), in which a part of the drive is assisted by an electric motor, have attracted attention, and higher capacity and higher output batteries have been required for the batteries used for these power supplies. As a power supply that meets such requirements,
A non-electrolyte secondary battery having a high voltage has attracted attention.

[0003] A lithium transition metal oxide is used as a positive electrode material of a non-aqueous electrolyte secondary battery. Among them, lithium cobalt oxide is used in view of balance of capacity and cycle characteristics. Since the amount of resources is small and the cost is high, lithium manganate is promising as a battery material for EVs and HEVs, and is being developed. On the other hand, a carbon material is generally used for the negative electrode material, and this carbon material is made of natural graphite, flake-like, artificial graphite such as lump, graphite material such as mesophase pitch graphite, and furfuryl alcohol. An amorphous carbon material obtained by firing a furan resin or the like is used.

[0004] In particular, for use in EVs and HEVs, a very high current density in charging and discharging, and a long life and high output characteristics are required. When amorphous carbon is used as the negative electrode active material, a nonaqueous electrolyte secondary battery having a capacity higher than the theoretical capacity of graphite and excellent cycle characteristics can be obtained. In addition, since the voltage at the time of charging and discharging has a gradient, it is possible to easily and accurately estimate the state of the battery only by measuring the voltage. However, there are drawbacks in that the irreversible capacity is large, it is difficult to increase the capacity in a battery, and the electron conductivity between active materials is inferior to that of a graphite-based material. On the other hand, when a graphite material is used, the irreversible capacity is small and the voltage characteristics are flat, so that a high capacity,
A high-output nonaqueous electrolyte secondary battery can be obtained. However, since there is a large change in volume during charging and discharging, there is a problem that electron conductivity between the active material particles cannot be maintained for a long period of time and the life is early reached. Furthermore, when a large battery such as an electric vehicle is assumed, there is a problem that charge acceptability at a large current density is inferior to amorphous carbon. Also,
A plurality of batteries are connected and used for EVs and HEVs, and since the variation in the characteristics of the cells greatly affects the life characteristics and safety, a control system is usually used in combination with each battery. Monitors cell voltage, current, temperature, etc.
Although control is used to suppress variations, it is extremely difficult to accurately monitor the state of the battery from the voltage because graphite-based materials have flat voltage characteristics, or a highly accurate control system is required. Become. Therefore, as a negative electrode material of a non-aqueous electrolyte secondary battery used for an application such as an electric vehicle, it is desirable to mainly use amorphous carbon, and it is promising to improve the output power.

Further, in order to improve the electron conductivity in the mixture layer for both the positive electrode and the negative electrode, a conductive agent is added, the mixture density is increased, and a network between particles is secured, for example. Of non-aqueous electrolyte secondary batteries has been improved.

[0006]

However, in order to increase the output of a non-aqueous electrolyte secondary battery, it is necessary to improve the electron conductivity in the mixture layer as described above while diffusing lithium ions. Need to be better. For that purpose, a space for holding the non-aqueous electrolyte is required in the mixture layer. In particular, when the non-aqueous electrolyte secondary battery is used at a low temperature, the movement of lithium ions in the non-aqueous electrolyte becomes extremely slow, and the output characteristics are increased due to the distribution of the non-aqueous electrolyte in the mixture layer. Changes significantly. Furthermore, uniform distribution of the non-aqueous electrolyte in the mixture layer is an essential condition for causing a uniform electrode reaction, and when the electrode reaction is biased, the non-aqueous electrolyte secondary battery becomes excessive. When charged, the safety is compromised until the battery explodes.

SUMMARY OF THE INVENTION In view of the above-mentioned problems, an object of the present invention is to provide a non-aqueous electrolyte secondary battery capable of improving safety while achieving high output, particularly at low temperature.

[0008]

Means for Solving the Problems In order to solve the above problems, a first aspect of the present invention is to provide a lithium manganese composite oxide having an average particle diameter of 5 to 20 μm and a conductive agent mainly composed of a graphite-based carbon material. In a non-aqueous electrolyte secondary battery in which a positive electrode mixture containing a binder and a positive electrode mixture is substantially uniformly applied to both surfaces of the belt-shaped current collector to form a positive electrode mixture layer, the empty space in the positive electrode mixture layer is formed. The pore volume is 25% or more and 35% or less with respect to the volume of the positive electrode mixture layer. According to this aspect, by setting the pore volume in the positive electrode mixture layer to be not less than 25% and not more than 35% with respect to the volume of the positive electrode mixture layer, the non-aqueous electrolyte can be substantially uniformly formed in the positive electrode mixture layer. The average particle size of the positive electrode active material that is distributed 5 to 5
Since the electrode reaction between the 20 μm lithium manganese composite oxide and the non-aqueous electrolyte is performed almost uniformly, good output characteristics can be obtained even in a low-temperature atmosphere, and the electrode reaction does not partially concentrate during overcharge. A non-aqueous electrolyte secondary battery excellent in safety can be obtained.

In a second aspect of the present invention, a negative electrode mixture containing amorphous carbon having an average particle diameter of 5 to 20 μm, a conductive agent, and a binder is substantially evenly distributed on both surfaces of the belt-shaped current collector. In the non-aqueous electrolyte secondary battery in which the negative electrode mixture layer is formed by coating on the negative electrode mixture layer, the volume of the pores in the negative electrode mixture layer is 30% or more and 40% or less with respect to the volume of the negative electrode mixture layer. There is a feature.
According to this aspect, the non-aqueous electrolyte is almost uniformly formed in the negative electrode mixture layer by setting the pore volume in the negative electrode mixture layer to 30% or more and 40% or less with respect to the volume of the negative electrode mixture layer. The electrode reaction between the non-aqueous electrolyte and the amorphous carbon having an average particle diameter of 5 to 20 μm, which is a distributed negative electrode active material, is performed almost uniformly, so that good output characteristics can be obtained even in a low-temperature atmosphere and overcharging can be performed. A non-aqueous electrolyte secondary battery with excellent safety can be obtained without partial concentration of electrode reactions at times.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment in which a nonaqueous electrolyte secondary battery according to the present invention is applied to a cylindrical lithium ion secondary battery for an electric vehicle will be described below.

(Positive Electrode) Lithium manganate (positive active material) as a lithium manganese composite oxide having an average particle diameter of 5 to 20 μm and having a spinel structure is added to a positive active material 10
10 parts by mass of flake graphite as a conductive agent and 0 parts by mass of polyvinylidene fluoride (PVDF) as a binder of 5 parts by mass are added to 100 parts by mass of the positive electrode active material, and dispersed therein. N-methylpyrrolidone was added as a solvent and kneaded to obtain a slurry. This slurry has a thickness of 20μ
m was applied to both sides of an aluminum foil as a belt-shaped current collector by roll-to-roll transfer, and dried to form a substantially uniform and uniform positive electrode mixture layer on the aluminum foil. Thereafter, the positive electrode mixture layer was pressed at a predetermined pressure by a roll press, so that the volume of pores in the positive electrode mixture layer was in the range of 25% to 35% with respect to the entire volume of the positive electrode mixture layer.
After pressing, it was cut into a width of 80 mm to obtain a belt-shaped positive electrode. The volume of pores in the positive electrode mixture layer can be adjusted by changing the pressing pressure.

(Negative Electrode) The average particle diameter of the negative electrode active material is 5
To 20 μm of amorphous carbon powder, 5 parts by mass of acetylene black as a conductive agent with respect to 100 parts by mass of the negative electrode active material, and 10 parts by mass of polyfluoro as a binder with respect to 100 parts by mass of the negative electrode active material. And vinylidene chloride, and N-methylpyrrolidone as a dispersing solvent was added thereto to obtain a kneaded slurry. This slurry is applied to both sides of a rolled copper foil as a 10 μm-thick strip-shaped current collector by transfer using a roll-to-roll method, and dried to form a roughly uniform and uniform negative electrode mixture layer on the copper foil. did. Thereafter, by pressing the negative electrode mixture layer at a predetermined pressure with a roll press machine, the volume of pores in the negative electrode mixture layer is reduced to 30% of the total volume of the positive electrode mixture layer.
４０40%. After pressing, it was cut into a width of 85 mm to obtain a strip-shaped negative electrode. As in the case of the positive electrode, the volume of pores in the negative electrode mixture layer can be adjusted by changing the pressing pressure.

(Preparation of Battery) The prepared positive and negative electrodes were
An electrode group was prepared by winding the electrode group together with a 0 μm polyethylene separator, and this electrode group was inserted into a cylindrical battery container.
After injecting a predetermined amount of the non-aqueous electrolyte, the upper lid was swaged to obtain a cylindrical lithium ion secondary battery. For the non-aqueous electrolyte, lithium hexafluorophosphate (LiPF ₆ ) is used in a mixed solution of ethylene carbonate and dimethyl carbonate.
Was dissolved at 1 mol / liter.

[0014]

Next, according to the present embodiment, in the positive electrode mixture layer,
An example of a battery manufactured by changing the volume of pores in the negative electrode mixture layer will be described. The battery of the comparative example produced for comparison is also described.

Example 1 As shown in Table 1 below, in Example 1, the ratio of the volume of pores in the positive electrode mixture layer to the total volume of the positive electrode mixture layer (hereinafter, referred to as positive electrode porosity). , And the ratio of the volume of pores in the negative electrode mixture layer to the entire volume of the negative electrode mixture layer (hereinafter, referred to as negative electrode porosity) was 35% by volume, to produce a battery. Positive electrode porosity and negative electrode porosity are obtained by peeling the mixture layer from the current collectors of the positive and negative electrodes,
It was measured by the mercury intrusion method (the same applies to the following Examples and Comparative Examples). The pore volume calculated from the values of the true specific gravity, mass blending amount, and thickness of the electrode plate after pressing of the various materials present in the mixture layer was very close to the measurement result by the mercury intrusion method. Was done.

[0016]

[Table 1]

(Examples 2 and 3, Comparative Examples 1 to 4) As shown in Table 1, in Example 2, the porosity of the positive electrode was 30% by volume, and the porosity of the negative electrode was 35% by volume. Positive electrode porosity of 3
A battery was manufactured with 5% by volume and a negative electrode porosity of 35% by volume. In Comparative Examples 1 to 4, the positive electrode porosity and the negative electrode porosity were respectively set to 24% by volume, 35% by volume, and 36% by volume.
% By volume and 35% by volume, 24% by volume and 28% by volume, 3
Batteries were prepared at 6% by volume and 41% by volume.

(Examples 4 to 6, Comparative Examples 5 and 6) As shown in Table 1, in Example 4, the porosity of the positive electrode was 30% by volume, and the porosity of the negative electrode was 30% by volume. Positive electrode porosity of 3
A battery was manufactured with 0% by volume and negative electrode porosity of 35% by volume. In Example 6, the positive electrode porosity was 30% by volume and the negative electrode porosity was 40% by volume. In Comparative Examples 5 and 6, batteries were manufactured with positive electrode porosity and negative electrode porosity of 30% by volume and 28% by volume, 30% by volume and 41% by volume, respectively.

(Test) Next, the following tests were carried out for the batteries of the examples and the comparative examples manufactured as described above.

First, constant-current constant-voltage charging (set voltage 4.1) at an atmosphere of 25 ° C. at a rate of 3 hours (0.33 C).
After performing V) for 5 hours, the battery was discharged at an hourly rate (1C) until the discharge end voltage reached 2.7 V, and charged again under the same conditions.
Next, in accordance with Japan Storage Battery Manufacturers Association Standard SBA8503, discharge was performed at respective current values of discharge currents 1, 3, and 6 A, the voltage at the 5th second was measured, and the initial output was obtained from the current-voltage characteristics.

The battery whose initial output was measured was allowed to stand in a constant temperature bath at -25 ° C. for 24 hours to cool the battery to -25 ° C., and the battery was cooled at room temperature (25 ° C.) as described above. Under the same conditions as the measurement of the initial output, the output characteristics at a low temperature (−25 ° C.) were measured.

Next, in order to confirm the output deterioration due to the charge / discharge cycle, the battery was allowed to stand at room temperature for 24 hours and then charged at a constant current and a constant voltage (at a set voltage of 0.33 C) in an atmosphere of 25 ° C. at a rate of 0.33 C. 4.1 V) for 5 hours and then 1 hour rate (1
In C), the discharge was repeated until the discharge end voltage reached 2.7 V, and after 100 cycles, the output of the battery was measured in the same manner as the measurement of the initial output.

The battery was continuously charged at room temperature with a constant current of 10 A to make it overcharged. At that time, the maximum temperature of the battery surface was measured and the behavior was observed (overcharge test).

Table 2 shows the test results of these series of tests.

[0025]

[Table 2]

As shown in Tables 1 and 2, in the batteries of Examples 1 to 3 having a positive electrode porosity of 25% by volume or more and 35% by volume or less, an output at a low temperature of 280 W or more was ensured, and charging and discharging were performed. Even after repeating the cycle 100 times, the initial output 83
% Output is maintained.

On the other hand, in the batteries of Comparative Examples 1 and 3 in which the porosity of the positive electrode was less than 25% by volume, the output at a low temperature was remarkably reduced. Significantly damaged. This is because the phenomenon in which the battery of Comparative Example 1 is the same as the batteries of Examples 1 to 3 has the same negative electrode porosity, and thus the nonaqueous electrolyte distribution in the positive electrode mixture layer is uneven. As a result, it is considered that the electrode reaction was concentrated on the portion where the non-aqueous electrolyte was present, and that portion was overcharged from an early stage. Conversely, in the batteries of Comparative Examples 2 and 4 in which the positive electrode porosity exceeds 35% by volume, since the initial output is low, the electron conductivity in the positive electrode mixture layer is reduced and the electrode reaction becomes uneven. It is thought that there is. However, these batteries did not reach a rupture state due to poor output characteristics from the beginning.

On the other hand, in the batteries of Examples 4 to 6 in which the porosity of the negative electrode is 30% by volume or more and 40% by volume or less, an output at a low temperature of 280 W or more is secured as in the case of the positive electrode. The output of 80% or more of the initial output is maintained even after the number of repetitions.

On the other hand, in the battery of Comparative Example 5 in which the porosity of the negative electrode was less than 30% by volume, the output at a low temperature was significantly reduced.
Further, in the overcharge test, the battery ruptured and safety was significantly impaired. This is because, similarly to the case of the positive electrode, the distribution of the non-aqueous electrolyte in the negative electrode mixture layer is non-uniform, and as a result, the electrode reaction concentrates on the portion where the non-aqueous electrolyte is present, and that portion is early in excess. It is considered that the state of charge has been reached. If the electrode reaction of the negative electrode is partially biased, lithium ions cannot be inserted between the active materials, and metallic lithium precipitates on the surface of the negative electrode mixture layer. As a result, not only is the output significantly reduced, but also safety is impaired such as rupture. Conversely, the negative electrode porosity is 4
Even in the battery of Comparative Example 6 exceeding 0% by volume, the initial output was significantly reduced.

As described above, the vacancy volume in the electrode mixture layer has a range in which both electron conductivity and lithium ion diffusivity can be maintained well. % Or more and 35% by volume or less, and the negative electrode porosity of 30% by volume or more and 40% by volume or less is present in the mixture layer, so that good output characteristics can be obtained even at low temperatures, and at the time of overcharging. In addition, a battery whose safety is ensured can be obtained.

In the present embodiment, lithium manganate is used as the positive electrode active material, flake graphite is used as the positive electrode conductive agent, polyvinylidene fluoride is used as the binder, acetylene black is used as the negative electrode conductive agent, and ethylene carbonate and dimethyl are used as the nonaqueous electrolyte. Although a solution in which lithium hexafluorophosphate is dissolved in a mixed solution with carbonate is exemplified, as described in detail below, any of those commonly used in the claims described above can be used. It is.

That is, as the positive electrode active material that can be used in other than this example, a material into which lithium can be inserted and desorbed, and a lithium manganese composite oxide into which a sufficient amount of lithium has been inserted in advance is preferable. Lithium manganate having a manganese or a material in which a part of manganese or lithium in a crystal is substituted or doped with another element can be suitably used. In order to ensure high capacity, high output, and safety, use lithium manganate, a lithium-manganese composite oxide, instead of lithium-cobalt composite oxide and lithium-nickel composite oxide for the positive electrode active material. Is preferred.

Examples of the binder that can be used in other than this example include Teflon (registered trademark), polyethylene, polystyrene, polybutadiene, butyl rubber, nitrile rubber, styrene / butadiene rubber, polysulfide rubber, nitrocellulose, and cyanoethylcellulose. , Various latex,
Acrylonitrile, vinyl fluoride, vinylidene fluoride,
There are polymers such as propylene fluoride and chloroprene fluoride, and mixtures thereof.

Further, there is no particular limitation on the positive electrode conductive material that can be used in other than this example as long as it is a graphite-based carbon material. For example, natural graphite, various artificial graphite materials, carbonaceous materials such as coke and the like may be used.
It is not particularly limited, such as a sphere, a fiber, and a lump.
Also, there is no particular limitation on the negative electrode conductive agent that can be used in other than this example. For example, amorphous carbon such as Ketjen black may be used.

As the non-aqueous electrolyte, an electrolyte obtained by dissolving a general lithium salt in an organic solvent as an electrolyte can be used. Further, the lithium salt and the organic solvent used are not particularly limited. For example, in addition to this example, LiClO ₄ , LiAsF ₆ , LiBF ₄ ,
LiB (C ₆ H ₅ ) ₄ , CH ₃ SO ₃ Li, CF ₃ SO
₃ Li or a mixture thereof can be used. Further, non-aqueous electrolyte organic solvents other than this example include propylene carbonate, diethyl carbonate, 1,2-dimethoxyethane, 1,2-diethoxyethane, γ-butyrolactone, tetrahydrofuran, 1,3-dioxolan, 4- Methyl-1,3-dioxolan, diethyl ether, sulfolane, methylsulfolane, acetonitrile, propionitonyl and the like, or a mixed solvent of two or more thereof are used. The mixing ratio is not limited.

[0036]

As described above, according to the present invention,
The volume of pores in the positive electrode mixture layer is 2 times the volume of the positive electrode mixture layer.
The nonaqueous electrolyte is substantially uniform in the mixture layer by setting the pore volume in the negative electrode mixture layer to 30% to 40% with respect to the volume of the negative electrode mixture layer. And the electrode reaction between the active material and the non-aqueous electrolyte is performed almost uniformly, so that good output characteristics can be obtained even in a low-temperature atmosphere, and safety can be achieved without partial concentration of electrode reactions during overcharge. In addition, an effect that a non-aqueous electrolyte secondary battery having excellent characteristics can be obtained can be obtained.

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Kensuke Hironaka 2-7-7 Nihonbashi Honcho, Chuo-ku, Tokyo F-term in Shin-Kobe Electric Co., Ltd. 5H029 AJ02 AJ06 AJ12 AK03 AL07 AM03 AM05 AM07 BJ02 BJ14 CJ22 DJ08 EJ04 EJ12 HJ05 HJ09 5H050 AA02 AA03 AA06 AA15 AA19 BA17 CA09 CB08 DA02 DA03 DA10 DA11 EA08 EA24 FA05 FA20 GA22 HA05 HA09

Claims (2)

[Claims] 1. A lithium-manganese composite oxide having an average particle diameter of 5 to 20 μm, a conductive agent mainly containing a graphite-based carbon material,
In a non-aqueous electrolyte secondary battery in which a positive electrode mixture containing a binder and a positive electrode mixture layer are formed on the both sides of a belt-shaped current collector substantially uniformly, a pore volume in the positive electrode mixture layer Is 25% or more and 35% or less based on the volume of the positive electrode mixture layer. 2. A negative electrode mixture containing amorphous carbon having an average particle diameter of 5 to 20 μm, a conductive agent, and a binder is substantially uniformly coated on both surfaces of a belt-shaped current collector, and a negative electrode mixture layer is formed. In the formed non-aqueous electrolyte secondary battery, the volume of pores in the negative electrode mixture layer is 30% to 40% of the volume of the negative electrode mixture layer. Liquid secondary battery.