JP2002151155A - Lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery ensuring current shutoff when overcharged, without decreasing its discharging capacity characteristic. SOLUTION: The lithium ion secondary battery comprises a positive electrode wherein lithium/transition metal compound oxide with lithium carbonate of 0 to less than 0.5 wt.% absorbed thereto and lithium/transition metal compound oxide with lithium carbonate of 0.5 to 5 wt.% absorbed thereto are mixed in a weight ratio of 70-98 to 30-2; a negative electrode, a nonaqueous electrolyte, and a current shutoff means for the inside of the battery, the means being actuated upon buildup of the internal pressure of the battery. Preferably, the lithium/transition metal compound oxide of the positive electrode are lithium/ cobalt compound oxide and the negative electrode is made of a carbonaceous material enabling lithium ions to be released and inserted.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a lithium-transition metal composite oxide, particularly a lithium-cobalt composite oxide, comprising a means for interrupting current inside a battery by increasing the internal pressure of the battery.
Alternatively, the present invention relates to a lithium ion secondary battery using a lithium nickel composite oxide as a positive electrode active material, and more particularly, to a lithium ion secondary battery in which current interruption reliably operates during overcharge.

[0002]

2. Description of the Related Art Lithium ion secondary batteries use lithium ions, and thus can have a high capacity. Recently, lithium ion secondary batteries have been widely used as power sources for mobile phones and electronic terminals. In particular, for an electrode material that greatly contributes to the performance improvement of a lithium ion battery, for example, for a positive electrode, LiCoO ₂ or LiNi
For negative electrodes such as O ₂ and LiMnO ₂ , lithium metals, alloys thereof, and carbon materials have been developed and studied, and electric characteristics such as discharge capacity have been improved. By the way, a power generating element body of a lithium ion secondary battery generally includes a positive electrode, a negative electrode, an insulating separator interposed between the positive electrode and the negative electrode,
And an electrolyte for transferring lithium ions between the positive electrode and the negative electrode. As the electrolytic solution, a carbonate-based organic material is often used as a main component. For this reason, in order to prevent liquid leakage, the outer shell of the lithium ion secondary battery contains a power generating element body using a metal material such as stainless steel. It has a closed structure. Therefore, in the case of an overcharged state in which a predetermined current or more flows for some reason, the electrolytic solution in the lithium ion secondary battery is electrolyzed to gasify and the internal pressure of the battery increases, and the battery can May be damaged. For this reason, the lithium ion secondary battery has a mechanism for interrupting the current when the internal pressure increases, for example, a current interrupting mechanism for separating the positive electrode current collector and the positive electrode tab and interrupting conduction when the internal pressure increases. Have been. However, since this current cutoff mechanism is an operating mechanism on the premise that the internal pressure of the battery rises, if the temperature rises but the internal pressure does not rise so much, the current cutoff mechanism is It will not work.

For this reason, Japanese Patent Application Laid-Open No. Hei 4-3298278 discloses that lithium carbonate is added to a positive electrode made of a lithium-cobalt composite oxide in an amount of 0.5% by weight to 15% by weight. Lithium carbonate is converted into carbon dioxide gas, the internal pressure of the battery is further increased, and the current cutoff mechanism is operated. Also, Japanese Patent Application Laid-Open No. 4-32926
In JP-A-4-329927, the surface of a positive electrode made of a lithium-cobalt composite oxide is coated with lithium carbonate.
As in the technique of No. 8, lithium carbonate is converted into carbon dioxide gas to activate the current cutoff mechanism.

However, Japanese Unexamined Patent Publication No.
In the method disclosed in the above, although the current interrupting mechanism operates, it is difficult to uniformly disperse lithium carbonate in the positive electrode active material. Unless used in a large amount, a sufficient burst prevention effect in overcharging could not be obtained. Further, the method disclosed in JP-A-4-329268 has a problem that the conditions for synthesizing the active material are limited and the discharge capacity characteristics and the cycle characteristics are deteriorated.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide a lithium ion secondary battery in which current interruption is reliably performed during overcharge without lowering discharge capacity characteristics and cycle characteristics.

[0006]

Means for Solving the Problems The above problems are (1) 0 to
Lithium transition metal composite oxide having less than 0.5% by weight of lithium carbonate adsorbed thereon and 0.5 to 5% by weight of lithium carbonate
7% by weight of the adsorbed lithium transition metal composite oxide
A lithium ion secondary battery comprising a positive electrode, a negative electrode, a non-aqueous electrolyte, and a current interrupting means inside the battery that is activated by an increase in the internal pressure of the battery, wherein the positive electrode is mixed at a ratio of 0 to 98/30 to 2; The lithium transition metal composite oxide of
The problem is solved by the lithium ion secondary battery according to (1), which is made of a lithium-cobalt composite oxide, and wherein the negative electrode is a carbon material capable of releasing and inserting lithium ions.

[0007]

DETAILED DESCRIPTION OF THE INVENTION In the present invention, a lithium transition metal composite oxide having 0 to less than 0.5% by weight of lithium carbonate adsorbed thereon and a lithium transition metal having 0.5 to 5% by weight of lithium carbonate adsorbed therein A positive electrode composed of a composite oxide is used. In addition, the concept of the lithium transition metal composite oxide in which lithium carbonate of 0 to less than 0.5% by weight is adsorbed also includes a lithium transition metal composite oxide in which lithium carbonate is not adsorbed. The lithium transition metal composite oxide having lithium carbonate adsorbed in an amount of 0 to less than 0.5% by weight and the lithium transition metal composite oxide having lithium carbonate adsorbed in an amount of 0.5 to 5% by weight have a weight ratio of 70 to 70%. 98 / 30-2. That is, according to the structure of the present invention, aggregation of lithium carbonate does not occur, unlike when lithium carbonate and a lithium transition metal composite oxide are simply mixed, and a small amount of lithium carbonate works effectively. In addition, unlike the case where a high concentration of lithium carbonate is adsorbed on a single active material, the entire surface of the active material is covered with lithium carbonate and the charge / discharge reaction is not hindered. In addition, an operation effect of current interruption can be obtained without lowering cycle characteristics. Unless a lithium transition metal composite oxide having lithium carbonate adsorbed in an amount of 0 to less than 0.5% by weight is used, the discharge capacity characteristics deteriorate. If the lithium transition metal composite oxide in which lithium carbonate is adsorbed in an amount of 0.5 to 5% by weight is not used, the operation effect of current interruption is reduced. A lithium transition metal composite oxide to which 0 to less than 0.5% by weight of lithium carbonate is adsorbed;
The mixing ratio with the lithium transition metal composite oxide adsorbed by 5 to 5% by weight is 70 to 98/30 to 2, preferably 75 to 95/25 to 5 by weight.

[0008] As such a lithium transition metal complex ^{_{oxide, LiCoO 2, LiNiO 2, LiMn}} 2 O 4 or the like can be used, as is LiCoO _2, can be a known, also, LiCo _X P _{( 1-X)}
A part of Co such as O ₂ (0 <X <1) is replaced with another element (P,
Al, Mn, Ni, etc.) can also be used. Similarly, a lithium nickel complex _{oxide, LiNiO2, LiNi X P (1} -X) O 2
(0 <X <1) and other parts of Mn to other elements (P, A
l, Mn, Co, etc.).

The average particle size of the lithium cobalt composite oxide or lithium nickel composite oxide, which is a typical example of the lithium transition metal composite oxide used in the present invention, is 1 to 3.
It may be about 0 μm. As a preferred embodiment, 0 to
The average particle size of the lithium cobalt composite oxide to which less than 0.5% by weight of lithium carbonate is adsorbed is 10 to 25 μm. If the average particle size is smaller than 10 μm, there is a tendency for the problem of reduced thermal stability to occur, and if it is larger than 25 μm, there tends to be a problem of reduced capacity in large current discharge. The average particle size of the lithium cobalt composite oxide or lithium nickel composite oxide in which lithium carbonate is adsorbed at 0.5 to 5% by weight is preferably less than 1 to 10 μm.
When the average particle size is smaller than 1 μm, there is a tendency that a problem of a decrease in capacity per volume (due to an increase in bulk density) occurs,
When it is 10 μm or more, there is a tendency that a problem of a decrease in the carbon dioxide gas generation rate (due to a decrease in specific surface area) occurs. For these reasons, particularly preferred embodiments of the positive electrode active material include 0 to
The lithium cobalt composite oxide or lithium nickel composite oxide to which less than 0.5% by weight of lithium carbonate is adsorbed, preferably the lithium cobalt composite oxide has an average particle size of 10 to 25 μm and the lithium carbonate is 0.1 to 0.1 μm. 5
The average particle size of the lithium cobalt composite oxide or lithium nickel composite oxide, preferably lithium cobalt composite oxide, adsorbed by ５5% by weight is less than 1 to 10 μm.

[0010] The average particle size of the lithium transition metal composite oxide can be measured by the following method. First, a granular substance such as a lithium-cobalt composite oxide to be measured is charged into an organic liquid such as water or ethanol, and is charged at 35k.
A dispersion process is performed for about 2 minutes by applying ultrasonic waves of about Hz to 40 kHz. The amount of the particulate matter to be measured is such that the laser transmittance (the ratio of the output light amount to the incident light amount) of the dispersion liquid after the dispersion treatment is 70% to 90%. next,
This dispersion is applied to a Microtrac particle size analyzer, and the particle size (D1, D2, D
3 ...) and the number of particles present for each particle size (N1, N2,
N3 ...) is measured. Note that the Microtrac particle size analyzer calculates the particle size distribution of the group of particles having the theoretical intensity closest to the observed scattering intensity distribution. That is, the particle is assumed to be a sphere having a sectional circle having the same area as the projected image obtained by the irradiation of the m laser beam, and the diameter (equivalent sphere diameter) of this sectional circle is measured as the particle diameter.

The average particle size (μm) is calculated from the particle size (D) of the individual particles obtained above and the number (N) of each particle size by using the following equation. Average particle size (μm) = (ΔND ³ / ΔN) ^1/3

Further, a conductive material can be added to the lithium transition metal composite oxide used in the present invention. As the conductive material, known materials, for example, artificial graphite, natural graphite, acetylene black, oil furnace black,
Ketjen black, mesophase carbon microphase and the like are exemplified. In particular, it is preferable to use granular flaky graphite, spherical graphite and mesophase carbon microphase having a particle diameter of 3 μm or more, and flaky graphite is particularly preferable in terms of cycle characteristics. In the present invention, the term “granular” includes scaly, spherical, pseudo-spherical, massive, and whisker-like shapes.

As a method of obtaining a lithium transition metal composite oxide having lithium carbonate adsorbed thereon, for example, a method of mixing and firing a required amount of lithium carbonate at the time of synthesizing the lithium transition metal composite oxide, and a method of mixing lithium hydroxide And baking and then reacting by blowing carbon dioxide gas, and a method of adding an aqueous solution of lithium carbonate to the synthesized lithium transition metal composite oxide, kneading and drying, and the like.

The lithium transition metal composite oxide having lithium carbonate adsorbed thereon is laminated on a metal current collector such as an aluminum foil together with a binder such as polyvinylidene fluoride and a conductive material to form a positive electrode active material. Form a layer. The method for forming the positive electrode active material layer is not particularly limited.
An O-based (M is Co or Ni) composite oxide, a binder, and a conductive agent are dispersed in a solvent such as N-methyl-2-pyrrolidone to form a paste, and the paste is formed on both surfaces of a metal current collector. And then drying it to evaporate the solvent, and then rolling it with a roller press or the like to form an active material layer.

The negative electrode active material is not particularly limited.
From the viewpoint of discharge capacity characteristics, a carbon material capable of inserting and extracting lithium ions can be preferably used. The carbon material capable of occluding and releasing lithium ions will be described in detail. In the present invention, various natural and artificial carbon materials can be applied, for example, coke such as pitch coke and petroleum coke, graphite, pyrolytic carbon, carbon fiber, and activated carbon. And the like, and the shape may be an appropriate shape such as a fiber shape, a scale shape, or a spherical shape. In the negative electrode active material of the present invention, in particular, fibrous graphite is suitably used in view of safety, high capacity, and cycle characteristics.

The carbon material is laminated on a metal current collector such as a copper foil with a binder such as polyvinylidene fluoride to form a negative electrode active material layer. The method for forming the negative electrode active material layer is not particularly limited. For example, the carbon material and the binder are dispersed in a solvent such as N-methyl-2-pyrrolidone to form a paste, and the paste is formed of a metal. After applying a uniform thickness to both surfaces of the current collector and drying it to evaporate the solvent, the active material layer is formed by rolling this with a roller press or the like.

The non-aqueous electrolyte used in the present invention is not particularly limited as long as it is a commonly used non-aqueous electrolyte. The non-aqueous electrolyte of the lithium secondary battery is generally composed of an organic solvent composed of a mixed solvent obtained by mixing a high dielectric constant solvent and a low viscosity solvent, and a lithium salt. Ethylene carbonate, propylene carbonate, dimethyl sulfoxide, γ-butyl lactone and the like, and a low viscosity solvent such as dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, dioxolan, tetrahydrofuran, 1,2-dimethoxyethane and the like are appropriately mixed and mixed. A non-aqueous electrolyte may be obtained by mixing a lithium salt such as LiPF ₆ or LiBF ₄ with the mixed solvent.

Preferred compositions of the non-aqueous electrolyte include ethylene carbonate and / or propylene carbonate in an amount of from 20 to 50% by weight, and dimethyl carbonate, diethyl carbonate and / or ethyl methyl carbonate in an amount of from 50% by weight or more. 80% by weight as a mixed solvent, and LiPF ₆ , L
a lithium salt such as iBF ₄ may be blended.

As the separator, a known separator can be used. For example, a separator composed of a polyethylene film, a separator composed of a polypropylene film, and a separator composed of a three-layer structure of a polypropylene / polyethylene / polypropylene film can be exemplified.

In the present invention, there is provided a current interrupting means inside the battery which is activated by an increase in the internal pressure of the battery. As the current interrupting means, a known means can be adopted, and there is no particular limitation as long as the current inside the battery is interrupted by an increase in battery internal pressure.

Hereinafter, embodiments of the present invention will be described. Example A lithium-cobalt composite oxide (LiCoO ₂ ) on which lithium carbonate was adsorbed was prepared by the following method to prepare a positive electrode. [Preparation of lithium cobalt composite oxide with 0.2% lithium carbonate adsorption] Stoichiometric amount of cobalt oxide (average particle size of 17
μm) and 1.01 times the stoichiometric amount of lithium carbonate in dry air at 900 ° C. for 9 hours, followed by 500 ° C.
For 12 hours to obtain a lithium-cobalt composite oxide having an average particle size of 20 μm. The lithium cobalt composite oxide contained 0.2% of lithium carbonate.
Had been adsorbed. [Preparation of lithium-cobalt composite oxide adsorbing 2.0% of lithium carbonate] Stoichiometric amount of cobalt oxide (average particle size 4μ)
m) and 1.06 times the stoichiometric amount of lithium carbonate were calcined in dry air at 900 ° C. for 9 hours to obtain lithium cobaltate powder having an average particle size of 5 μm. In addition, 2.0% of lithium carbonate was adsorbed in the lithium-cobalt composite oxide. Next, 90 parts by weight of the lithium-cobalt composite oxide obtained by mixing the above two kinds of lithium-cobalt composite oxides in the ratio shown in Table 1 below, 7 parts by weight of flaky graphite, and 3 parts by weight of polyvinylidene fluoride were mixed with N-
It was uniformly dispersed in methyl 2-pyrrolidone to obtain a slurry.
This slurry is made into an aluminum foil (thickness: 20) serving as a current collector.
μm) and dried to obtain a positive electrode.

The above positive electrode is wound together with a porous separator having a three-layer structure made of polypropylene, polyethylene and polypropylene and a negative electrode having graphitized carbon fibers as a negative electrode active material, and is wound into a cylindrical can having a height of 65 mm and an outer diameter of 18 mm. And a lithium ion secondary battery (discharge capacity 1500
mAh). The electrolyte was LiPF ₆ with respect to a mixture of 40 g of ethylene carbonate, 60 g of ethyl methyl carbonate and 40 g of dimethyl carbonate.
Was added at 1 mol / L to prepare and use a non-aqueous electrolyte.
In addition, the current interruption mechanism welds the positive electrode tab drawn from the positive electrode current collector to the current interruption thin film sealing the power generating element body, and when the internal pressure of the battery increases, the current interruption thin film A means was used to cut off the welded portion by being pushed up in the direction of the external pressure and to cut off conduction inside the battery.

[Discharge capacity test] The battery was charged at a constant current of 1.5 A until the charging voltage reached 4.2 V, subsequently charged at a constant voltage of 4.2 V until the total charging time reached 2.5 hours, and then the terminal Discharge is performed at 0.75 A until the inter-voltage becomes 3 V. At this time, the discharge capacity at the discharge at 0.75 A is obtained.

[Charge / Discharge Cycle Characteristic Test] The battery was charged at a constant current of 1.5 A until the charging voltage reached 4.2 V, and subsequently, the total charging time was 2.5 at a constant voltage of 4.2 V.
The four steps of charging until the time is reached, performing a 1-hour pause after the charging, discharging at 1.5 A until the terminal voltage becomes 3 V, and performing a 1-hour pause after the discharge are defined as one cycle. Next, 500 cycles of these four steps are performed at room temperature (20 ° C.), and the discharge capacity (mAh) in each cycle is measured. The ratio of the charge / discharge capacity in each cycle to the initial discharge capacity is defined as a discharge capacity change rate (%). Table 1 shows the discharge capacity change rate (%) at the first time and at the 500th cycle.

[Damage Resistance Test of Battery Can] The battery was visually observed as a state of overcharge by charging at 1.5 A until the charging voltage reached 10 V or 3 hours had passed.

The test results are summarized in Table 1.

[0027]

[Table 1]

[0028]

According to the present invention, it is possible to provide a lithium ion secondary battery in which current interruption is reliably performed during overcharge without deteriorating discharge capacity characteristics and cycle characteristics. In particular, as the positive electrode active material, 0 to 0.5% by weight
The average particle size of the lithium-cobalt composite oxide on which less than less than lithium carbonate is adsorbed is 10 to 25 μm, and the average particle size of the lithium-cobalt composite oxide on which lithium carbonate is adsorbed by 0.5 to 5% by weight is 1 When a mixture having a size of less than 10 μm is used, the above-mentioned effect becomes remarkable, and a higher performance and higher safety lithium ion secondary battery can be provided.

Continued on the front page F-term (reference) 5H022 AA09 CC01 KK01 5H029 AJ12 AK03 AL06 AM03 AM04 AM05 AM07 DJ08 HJ01 5H050 AA04 AA15 BA17 CA08 CB07 DA09 EA01 HA01

Claims (2)

[Claims] 1. A weight ratio of a lithium transition metal composite oxide in which 0 to less than 0.5% by weight of lithium carbonate is adsorbed to a lithium transition metal composite oxide in which lithium carbonate is adsorbed by 0.5 to 5% by weight. A lithium ion secondary battery comprising: a positive electrode, a negative electrode, a non-aqueous electrolyte, and a current interrupting means inside the battery that is activated by an increase in the internal pressure of the battery. 2. The lithium transition metal composite oxide of the positive electrode,
The negative electrode is made of a lithium-cobalt composite oxide, and the negative electrode is a carbon material capable of releasing and inserting lithium ions.
4. The lithium ion secondary battery according to 1.