JP2003297354A - Lithium secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium secondary battery capable of suppressing gas generation inside the battery when it is stored at high temperatures by restricting the residual carbon amount in its active material and of reducing a swell of the battery. <P>SOLUTION: The lithium secondary battery is composed of an electrode active material capable of at least occluding and emitting lithium ions, a binder, a positive and a negative electrode having electricity collector, and an electrolytic solution, wherein the electrode active material contained in the positive electrode is lithium composite oxide expressed by the following formula, and the carbon content of the lithium composite oxide is no more than 6.0×10<SP>-4</SP>when expressed by the mass ratio of C/M. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium secondary battery used as a power source for portable equipment.

[0002]

2. Description of the Related Art Lithium-ion secondary batteries have been widely used as power sources for portable devices in terms of high-power batteries. The lithium ion secondary battery has been on the market for more than 10 years and its characteristics have been improved. With regard to lithium-ion secondary batteries, high capacity and high safety are emphasized as technical issues.

Materials capable of reversibly occluding and releasing lithium ions are usually used for the positive electrode and negative electrode materials of lithium ion secondary batteries. As the positive electrode active material,
Lithium-containing metal oxides are often used, particularly lithium cobalt oxide, lithium nickel oxide, lithium manganate, and those obtained by adding an appropriate metal element to these.

However, since lithium nickelate has poor thermal stability, it has not been put to practical use as a positive electrode of a lithium ion secondary battery. Lithium cobaltate also desorbs lithium ions from its structure. However, its thermal stability decreases, and it rapidly decreases from a potential of about 4.3 V or more with respect to lithium metal, leaving a problem in safety during overcharge. Further, although lithium manganate has high thermal stability, its theoretical capacity is about half that of lithium cobalt oxide and lithium nickel oxide, so the capacity when used in a battery also decreases, and the lithium ion secondary battery It is a material that is difficult to take advantage of its high energy density.

[0005] Positive electrode active lithium-containing nickel-manganese-cobalt oxide has attracted attention as one of materials of the lithium ion secondary battery _{(LiNi x Mn y Co z O} 2) is
It has the same layered rock salt type crystal structure as lithium cobalt oxide and lithium nickel oxide. This structure is maintained even after desorption of lithium ions, and in comparison with lithium cobalt oxide, which is currently most widely used as a positive electrode material for lithium ion secondary batteries, batteries using this oxide are It is a safer battery against overcharging.

[0006] Incidentally, the _{LiNi x Mn y Co z O 2} , structural stability to the lithium ion desorption are said to be correlated to the amount of manganese. In addition, LiNi _x Mn _y Co _z
Since O ₂ has a discharge capacity similar to that of lithium cobalt oxide, it is possible to produce a battery that takes advantage of the feature of high energy density as in lithium cobalt oxide. In particular, since the high safety is required for a cell having high energy density large capacity, use of LiNi _x Mn _y Co _z O ₂ has both the characteristics is essential. Also, the point of reducing the amount of cobalt is a rare metal element, is an advantage for the LiNi _x Mn _y Co _z O ₂ utilization.

_{However, LiNi x Mn y Co z O} 2 are that contain nickel in its structure, since the oxidation-reduction potential of the nickel ion is low as compared with the redox potential of the manganese ions, cobalt ions , It is considered that nickel ions are oxidized more preferentially than manganese ions and cobalt ions at the time of charging, and a phenomenon that the average voltage at the time of charging is lower than that of a battery using lithium cobalt oxide is observed. There is. Also, along with it,
It has been observed that the average voltage during discharge is lower than that of a battery using lithium cobalt oxide, which reflects the low reduction potential of nickel ions during discharge. Thus, when the LiNi _x Mn _y Co _z O ₂ at a high state of charge, the amount of oxidized metal ions in the active material as compared with the case of using the same charged state lithium cobaltate is increasingly It is considered that the environment is such that carbon traces and the like remaining in the electrolytic solution and the active material are easily oxidized and gas is easily generated.

In putting a lithium-ion battery into practical use, various tests are required to test whether it can withstand various practical uses. Among them, gas generation inside the battery during high temperature storage is required. Suppressing leads to prevention of battery swelling and is one of the important matters. In particular, in a battery using an aluminum laminate film for the outer casing, the reduction of the bulge of the battery during high temperature storage is remarkable as compared with the can type due to the low strength of the outer casing, which is an important issue for practical use. Has become. As means for suppressing gas generation inside the battery during high temperature storage, (1) powder characteristics of positive electrode active material, (2) additive to positive electrode, (3)
It has been clarified and disclosed that the electrolytic solution composition and (4) the thermal stability of the electrolyte lithium salt contribute. These are measures primarily for blister reduced when using lithium cobalt oxide for the positive _{electrode, LiNi x Mn y Co z O} 2 as the positive electrode active material
When using, different measures are necessary.

[0009]

SUMMARY OF THE INVENTION An object of the present invention is to suppress gas generation inside the battery during high temperature storage in a high charge state, and in particular, it is essential for high energy density batteries. It is an object of the present invention to provide a lithium ion secondary battery in which gas generation is reduced when a lithium-containing nickel-manganese-cobalt composite oxide, which is a positive electrode active material having high capacity and high thermal stability, is used.

[0010]

That is, the above object is achieved by the following constitution of the present invention. (1) An electrode active material capable of occluding and releasing lithium ions, a positive electrode and a negative electrode each having a binder and a current collector, and an electrolytic solution, and the electrode active material contained in the positive electrode is as follows. A lithium secondary battery represented by the formula, wherein the amount of carbon contained in the lithium composite oxide is less than 6.0 × 10 ^{−4 in} a mass ratio of C / M. LiMO ₂ (M = Co, Mn, and Ni). (2) The specific surface area of the lithium composite oxide is 0.1 m ^2.
/ g or more and 0.8m ² / g or less and average diameter is 4μm or more 2
The lithium secondary battery according to (1) above, having a size of 5 μm or less. (3) The lithium secondary battery according to (1) or (2) above, wherein the lithium composite oxide is represented by the following formula. _{Li x Mn y Ni z Co 1} -yz O w (0.85 ≦ x ≦ 1.1,0 ≦ y ≦ 0.6,0 ≦ z ≦
1, 1 ≦ w ≦ 2. )

[0011]

[Function] During high temperature storage, particularly during high temperature storage in a highly charged state, the positive electrode active material is in a high oxidation state, and carbon traces remaining in the electrolytic solution and the active material are easily oxidized, Oxidation of these to hydrocarbon gas or carbon dioxide causes the battery to swell. Therefore, by regulating the amount of carbon remaining in the active material to a certain amount, it is possible to suppress gas generation inside the battery, and especially lithium-containing nickel that is a positive electrode active material having high capacity and high thermal stability.
It is effective when a manganese-cobalt composite oxide is used. This oxide is an essential material for the production of high energy density batteries.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The lithium secondary battery of the present invention comprises an electrode active material capable of occluding and releasing lithium ions, a binder, a positive electrode and a negative electrode having a current collector, and an electrolytic solution. However, the electrode active material contained in the positive electrode is a lithium composite oxide represented by the following formula, and the amount of carbon contained in the lithium composite oxide is 6.0 × 10 at a C / M mass ratio. ^{It is} less than ^-4 . LiMO ₂ (M = Co, Mn, and Ni).

In the present invention, the positive electrode active material is a ternary lithium composite oxide represented by the above formula.

By using such a lithium-containing nickel-manganese-cobalt composite oxide, it is possible to produce a battery that takes advantage of the features of high energy density and high safety. Further, the amount of cobalt, which is a rare metal element, can be reduced.

The positive electrode active material in the present invention is preferably represented by the following formula. _{Li x Mn y Ni z Co 1} -yz O w 0.85 ≦ x ≦ 1.1,0 ≦ y ≦ 0.6,0 ≦ z ≦ 1,
1 ≦ w ≦ 2. Further, preferably, 0 ≦ y ≦ 0.
5, 0.2 <z <0.8.

By using a composite oxide having the above composition range and performing an annealing treatment at a high temperature, while maintaining high characteristics,
Swelling can be prevented during storage at high temperature.

When the manganese content is 0.5 or less and the nickel content is 0.2 or more and 0.8 or less as a practical composition range, the performance of the active material can be exhibited and swelling is suppressed. When nickel deviates from this range, the effect is recognized but it is not put to practical use. On the other hand, as the amount of nickel decreases, the capacity of the active material decreases, which affects battery characteristics, so it cannot be said to be practical.

In the present invention, the amount of carbon contained in the positive electrode active material having the above composition is regulated. In this case, when the three elements nickel, manganese, and cobalt are M and carbon is C, the amount of carbon contained is C / M in mass ratio.
Is less than 6.0 × 10 ⁻⁴ , preferably 5.5 × 10 ⁻⁴ or less. Although the lower limit is not particularly limited, it is usually about 1.5 × 10 ⁻⁴ . The above values are values obtained by a so-called combustion method in which a carbon dioxide spectrum generated when a sample is burned is measured to obtain the amount of carbon.

On the other hand, inductively coupled plasma atomic emission spectrography
Rometry: hereinafter referred to as ICP analysis), when analyzing the element (carbon amount) in the sample, C / Li in terms of mass ratio is preferably less than 5.0 × 10 ⁻³ , more preferably 4.5.
It is × 10 ⁻³ or less.

In order to control the residual carbon amount as described above, since the carbon source is mainly derived from the fact that lithium existing in excess in the active material absorbs carbon dioxide existing in the air, The carbon dioxide concentration may be controlled during the substance synthesis and / or during storage.

The structure of the lithium secondary battery is not particularly limited, but is usually composed of a positive electrode, a negative electrode and a separator,
It is applied to stacked type batteries and cylindrical type batteries. Such a positive electrode, a separator and a negative electrode are laminated in this order and pressure-bonded to form an electrode group.

The electrode is preferably an electrode active material and a binder,
A composition with a conductive additive is used if necessary. In particular, when a single metal foil is used for the electrode, a metal crystal such as lithium dendrite, which does not contribute to the battery characteristics, is deposited on the surface, and the battery characteristics deteriorate with the lapse of use time.

In the present invention, the positive electrode active material capable of inserting and extracting lithium ions is the composite metal oxide represented by the above formula. The average particle diameter of the oxide powder is preferably 5 to 20 .mu.m, particularly 7 to 15 .mu.m. If the particle size is too small, the workability of the electrode will deteriorate, and the stability of the electrode will deteriorate.On the other hand, if it is too large, it will take time for the ions to diffuse into the particles, preventing uniform charge / discharge. However, problems such as deterioration of rate characteristics will occur.

The BET specific surface area is 0.1 m ² /
g to 1.0 m ² / g or less, particularly 0.1~0.8m about ² / g are preferred. If the specific surface area is small, the same problem occurs as when the average particle size is large, and if it is too large, the same problem occurs as when the average particle size is small.

In the present invention, as the negative electrode active material capable of inserting and extracting lithium ions, carbon materials, metallic lithium,
Examples include lithium alloys and oxides.

Examples of the carbon materials include natural graphite, mesophase carbon microbeads (MCMB), mesophase carbon fibers (MCF), cokes, glassy carbon, and organic polymer compound fired bodies. Also,
Examples of lithium alloys include Li-Al, LiSi, and LiSn. Examples of the oxide include Nb ₂ O ₅ and SnO. These are usually used as powder.

Of these, artificial graphite having a lattice spacing between (002) planes of 0.335 to 0.380 nm is particularly preferable. The surface spacing between the (002) planes can be calculated by X-ray diffraction. Since natural graphite contains impurities, when the compound represented by the formula (1) forms a film during the initial charging, the quality of the film may be deteriorated. By using artificial graphite, the influence of impurities can be avoided, so that a film having good ion permeability can be formed.

When these are used as powders, the average particle size is preferably 1 to 30 μm, particularly 5 to 25 μm. If the average particle size is too small, the charge / discharge cycle life tends to be short, and the capacity variation (individual difference) tends to increase. If the average particle size is too large, the variation in capacity becomes extremely large and the average capacity becomes small. It is considered that the reason why the capacity varies when the average particle diameter is large is that the contact between the negative electrode active material such as graphite and the current collector and the contact between the negative electrode active materials vary.

A conductive aid is added to the electrode, if necessary. The conductive aid is preferably graphite, carbon black, acetylene black, carbon fiber, a metal such as nickel, aluminum, copper or silver, and particularly preferably graphite or carbon black.

The electrode composition of the positive electrode is preferably in the range of active material: conductive assistant: binder = 80 to 94: 2 to 8: 2 to 18 by weight ratio, and in the negative electrode, the active material: conductive assistant is in weight ratio. Agent: Binder = The range of 70 to 97:00 to 25: 3 to 10 is preferable.

To manufacture the electrode, first, an active material, a binder, and optionally a conductive auxiliary agent are dispersed in a binder solution to prepare a coating solution.

As the binder, an elastomer such as styrene-butadiene rubber (SBR) or a resin material such as polyvinylidene fluoride (PVDF) can be used. Moreover, you may add additives, such as carboxymethyl cellulose (CMC), as needed.

Then, the electrode coating solution is applied to the current collector. The means for applying is not particularly limited and may be appropriately determined depending on the material and shape of the current collector. Generally, a metal mask printing method, an electrostatic coating method, a dip coating method, a spray coating method, a roll coating method, a doctor blade method, a gravure coating method, a screen printing method and the like are used.
Then, if necessary, rolling treatment is performed by a flat plate press, a calendar roll, or the like.

The current collector may be appropriately selected from ordinary current collectors depending on the shape of the device used by the battery, the method of arranging the current collector in the case, and the like. Generally, aluminum or the like is used for the positive electrode and copper, nickel or the like is used for the negative electrode.
A metal foil, a metal mesh, or the like is usually used as the current collector. Although the metal mesh has a smaller contact resistance with the electrode than the metal foil, the metal foil can also obtain a sufficiently small contact resistance.

Then, the solvent is evaporated to produce an electrode. The coating thickness is preferably about 50 to 400 μm.

In the present invention, as the lithium ion conductive material, either a non-aqueous electrolytic solution in which a lithium salt is dissolved or a gel polymer can be used.

The solvent of the electrolytic solution is not particularly limited as long as it has good compatibility with the solid polymer electrolyte, the electrolyte salt and the like, but a polar organic solvent which does not decompose even at a high operating voltage in a lithium battery or the like. , For example, ethylene carbonate (EC), propylene carbonate (PC),
Butylene carbonate, dimethyl carbonate (DM
C), carbonates such as diethyl carbonate (DEC) and ethyl methyl carbonate, cyclic ethers such as tetrahydrofuran (THF) and 2-methyltetrahydrofuran, cyclic ethers such as 1,3-dioxolane and 4-methyldioxolane, γ- Lactones such as butyrolactone and sulfolane are preferably used. 3-Methylsulfolane, dimethoxyethane, diethoxyethane, ethoxymethoxyethane, ethyl diglyme and the like may be used.

Examples of the supporting salt containing lithium ions include LiClO ₄ , LiPF ₆ , LiBF ₄ , and LiAs.
F ₆ , LiCF ₃ SO ₃ , LiCF ₃ CF ₂ SO ₃ , Li
C (CF ₃ SO ₂ ) ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN
_{(CF 3 CF 2 SO 2)} 2, LiN (CF 3 SO 2) (C 4 F
₉ SO ₂ ) and salts such as LiN (CF ₃ CF ₂ CO) ₂ or mixtures thereof.

The concentration of the lithium salt in the electrolytic solution is 0.5 to
It is preferably 2.5 mol / liter, more preferably 0.
It is 8 to 1.5 mol / liter. If the concentration of lithium salt is higher than this range, the viscosity of the electrolyte will be high, and the discharge capacity at high rate or low temperature will drop, and if it is low, the lithium ion supply will not be in time, and the discharge capacity at high rate or The discharge capacity at low temperature decreases.

Examples of the gel polymer include those obtained by swelling a non-aqueous electrolyte solution in which the lithium salt is dissolved in polyacrylonitrile, polyethylene glycol, polyvinylidene fluoride (PVdF) or the like. If it is necessary to prevent a short circuit between the positive electrode and the negative electrode, a polymer porous film, for example, a polyolefin uniaxially or biaxially stretched film, a polyolefin woven fabric or the like may be used as a separator or a base material of a lithium ion conductive polymer. .

The film thickness of the gel polymer is 5 to 100 μm.
Further, it is preferably 5 to 60 μm, and particularly preferably 10 to 40 μm.

Other separator constituent materials include one or more kinds of polyolefins such as polyethylene and polypropylene (in the case of two or more kinds, a laminated product of two or more layers of film), polyethylene terephthalate, etc. Examples include polyesters, thermoplastic fluororesins such as ethylene-tetrafluoroethylene copolymer, and celluloses. Sheet form is JIS-P81
The air permeability measured by the method specified in 17 is 5 to 2000 seconds /
There are microporous film, woven cloth, non-woven cloth, etc. having a thickness of about 100 cc and a thickness of about 5 to 100 μm.

The outer package is composed of a laminate film in which a polyolefin resin layer such as polypropylene or polyethylene as a heat-adhesive resin layer or a heat-resistant polyester resin layer is laminated on both surfaces of a metal layer such as aluminum. . The exterior body is formed in a bag shape in which one side is opened by previously heat-bonding two laminated films to each other by thermally adhering the thermoadhesive resin layers on the end faces of the three sides. Alternatively, a single laminated film may be folded back and the end faces of both sides may be heat-bonded to form a seal portion to form a bag shape.

As the laminate film, in order to secure the insulation between the metal foil forming the laminate film and the lead-out terminal, for example, a laminated structure of a thermoadhesive resin layer / polyester resin layer / metal foil / polyester resin layer from the interior side is used. It is preferable to use a laminate film having the same. By using such a laminate film, the high melting point polyester resin layer remains without melting during heat bonding,
Insulation can be secured by ensuring a distance between the lead-out terminal and the metal foil of the outer bag. Therefore, the thickness of the polyester resin layer of the laminated film is preferably about 5 to 100 μm.

The present invention relates to a lithium ion battery using an electrolytic solution, but is not limited thereto,
It can also be applied when the electrolyte is a solid electrolyte.

[0046]

EXAMPLES The preparation of Example 1 a positive _{electrode, Li x Mn y Ni z Co} 1-yz O w as the positive electrode active material, at, x = 1, y = 0.33 , z = 0.33, w =
90% by mass of the composite metal oxide defined as 2 was used, 6% by mass of acetylene black was used as a conductive aid, and 4% by mass of polyvinylidene fluoride was used as a binder.

Artificial graphite as negative electrode active material: 92% by mass
8% by mass of polyvinylidene fluoride was used as a binder. As the electrolytic solution, a nonaqueous electrolytic solution was used in which a mixed solution having a volume ratio of EC / DEC = 3/7 was used as a solvent and LiPF ₆ was a solute at a ratio of 1 mol / L.

With the above structure, the positive electrode and the negative electrode were laminated with a separator interposed therebetween to form a cell, which was housed in an aluminum laminate film outer casing having a thickness of 0.2 mm, and then an electrolytic solution was injected to complete the process. In addition, as the exterior body, one formed in a so-called deep-drawing type was used.

The battery thus prepared was subjected to a 0.2 C charge / discharge cycle, charged to 4.2 V, and held at 90 ° C. for 4 hours, and the battery thickness was measured. The gas generation was confirmed.

The amount of carbon remaining in the positive electrode active material is determined from the carbon dioxide gas spectrum when burned.
It was 195 ppm as determined by the so-called combustion method.

Example 2 — In the positive electrode active material, y =
A battery was prepared and evaluated in the same manner as in Example 1 except that 0.42 and z = 0.42 were used. The amount of carbon remaining in this positive electrode active material was 304 ppm as determined by the combustion method.

Comparative Example 1 In the positive electrode active material, y =
A battery was prepared and evaluated in the same manner as in Example 1 except that the one having 0.55 and z = 0.30 was used. The amount of carbon remaining in the positive electrode active material was 581 ppm as determined by the combustion method.

Comparative Example 2 In the positive electrode active material, y =
A battery was prepared and evaluated in the same manner as in Example 1 except that 0.70 and z = 0.10. The amount of carbon remaining in this positive electrode active material was 857 ppm as determined by the combustion method.

For the above Examples 1 and 2 and Comparative Examples 1 and 2, the mass ratios of the cell thickness change rate before and after the high temperature storage test at 90 ° C. for 4 hours and the residual carbon amount by the combustion method to each element are shown in Table 1. Shown in.

[0055]

[Table 1]

In Examples 1 and 2 using the active material in which the amount of residual carbon was regulated, the rate of change in thickness was greatly reduced as compared with Comparative Examples 1 and 2. By regulating the residual carbon amount, it is possible to provide a lithium ion secondary battery that suppresses gas generation during high temperature storage.

[0057]

As described above, according to the present invention, by controlling the amount of carbon remaining in the active material, it is possible to suppress the gas generation inside the battery during high temperature storage and to prevent the battery from bulging. Could be reduced.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Tsuyoshi Iijima             1-13-1, Nihonbashi, Chuo-ku, Tokyo             -In DC Inc. (72) Inventor Cho Suzuki             1-13-1, Nihonbashi, Chuo-ku, Tokyo             -In DC Inc. (72) Inventor Akinori Nishizawa             1-13-1, Nihonbashi, Chuo-ku, Tokyo             -In DC Inc. F term (reference) 5H029 AJ03 AJ12 AK03 AL02 AL06                       AL12 AM03 AM04 AM07 BJ04                       BJ12 DJ16 HJ01 HJ02 HJ05                       HJ07                 5H050 AA08 AA10 AA15 CA08 CA19                       CB02 CB08 CB12 FA17 HA01                       HA02 HA05 HA07

Claims (3)

[Claims] 1. An electrode active material contained in the positive electrode, the positive electrode and the negative electrode having at least a lithium ion occluding and releasing electrode active material, a binder, and a current collector, and an electrolytic solution. Is a lithium composite oxide represented by the following formula, and the amount of carbon contained in the lithium composite oxide is less than 6.0 × 10 ^{−4 in} a mass ratio of C / M. LiMO ₂ (M = Co, Mn, and Ni). 2. The lithium complex oxide has a specific surface area of 0.1 m ² / g or more and 0.8 m ² / g or less and an average diameter of 4 μm.
The lithium secondary battery according to claim 1, having a size of m or more and 25 μm or less. 3. The lithium secondary battery according to claim 1, wherein the lithium composite oxide is represented by the following formula. _{Li x Mn y Ni z Co 1} -yz O w (0.85 ≦ x ≦ 1.1,0 ≦ y ≦ 0.6,0 ≦ z ≦
1, 1 ≦ w ≦ 2. )