JP2002246027A - Lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium secondary battery showing high discharge capacity, even at a low temperatures, and without swelling during storage. SOLUTION: The lithium secondary battery uses a lithium contained composite oxide, consisting of lithium cobaltate and a 0001-2 atom.% auxiliary element M (a transition or typical metal element other than Li, Co), based on cobalt of the lithium cobaltate, as a positive electrode active material and contains 60-95 vol.% γ-butyrolactone as the solvent for an electrolyte.

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, and more particularly to an improvement in an electrolyte using an electrode and a non-aqueous solvent.

[0002]

2. Description of the Related Art In recent years, with the miniaturization of electronic devices, there has been a demand for smaller and lighter batteries.
Under these circumstances, lithium secondary batteries are being actively researched and developed as one of the batteries that can satisfy these requirements. In recent years, batteries using a flexible aluminum laminate film for an outer package have come to appear in order to achieve higher capacity.

[0003] One problem with the aluminum laminate film is that if gas is generated from inside the battery after the battery has been produced, the battery will swell. This problem can be solved by using γ-butyrolactone as the electrolyte, as shown in, for example, JP-A-2000-236868.

On the other hand, a problem with lithium secondary batteries is that the capacity at low temperatures is insufficient. On the other hand, for example, JP-A-6-290809,
Various solutions have been proposed as disclosed in Japanese Patent Application Laid-Open No. 8-13838. However, these are mainly for improving the composition of the electrolytic solution, and the γ-
On the premise of using butyrolactone, it was more difficult to improve low-temperature properties.

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a battery in which an additive element is added to a positive electrode active material, by suppressing the gas generated from the battery electrode when stored at a high temperature, thereby achieving a good low-temperature characteristic and a low profile. Is to provide a lithium secondary battery.

[0006]

That is, the above object is achieved by the following constitution of the present invention. (1) A positive electrode, a negative electrode, and an electrolyte are loaded in an outer package. As a positive electrode active material, lithium cobalt oxide and a subcomponent element M (0.001 to 2 atomic% with respect to cobalt of the lithium cobalt oxide) A transition metal and a typical metal element excluding Li and Co) and γ-butyrolactone as a solvent for the electrolyte.
A lithium secondary battery containing 5% by volume and having a thickness of the outer package of 0.3 mm or less. (2) The subcomponent elements are Ti, Nb, Sn and Mg
The lithium secondary battery according to the above (1), wherein the lithium secondary battery is any one or two or more types of

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION A lithium secondary battery according to the present invention is provided with a positive electrode, a negative electrode, and an electrolyte loaded in an outer package, and as a positive electrode active material, lithium cobaltate and cobalt of the lithium cobaltate are used. A lithium-containing composite oxide containing 0.001-2 atomic% of a sub-component element M (transition metal and typical metal element except Li and Co), and γ-butyrolactone as a solvent for the electrolyte in an amount of 60 to 95% by volume. It has an outer package having a thickness of 0.3 mm or less.

With such a configuration, it is possible to provide a lithium secondary battery having good low-temperature characteristics and no gas generation even at high temperatures, and it is possible to prevent swelling of the outer package even when a thin film-shaped package is used.

The positive electrode of the lithium secondary battery of the present invention is made of a mixture containing a positive electrode active material, a conductive auxiliary such as graphite, and a binder such as polyvinylidene fluoride.

[0010] As the positive electrode active material, a material obtained by adding some subcomponent elements to lithium cobalt oxide (LiCoO ₂ ) is used. The subcomponent element may be a typical metal or a transition metal, but is preferably one or more of Ti, Nb, Sn, and Mg, and furthermore, one or more of Ti and Nb, and has been confirmed to have improved temperature characteristics. It is.

The content of the subcomponent element M with respect to Co in the lithium cobalt oxide is 0.001-2 atomic% in total,
In particular, it is preferably 0.01 to 1 atomic%, more preferably 0.01 to 0.1 atomic%. When the content of the subcomponent exceeds the above range, the capacity is reduced. When the content is too small, the effect of improving the low-temperature characteristics is hardly obtained.

The subcomponent may be substituted for Co. Preferably, the positive electrode active material is represented by the following composition formula. LiCo _1-x M _x O ₂ (x = 0.00001 to 0.02, M: transition metal element excluding Li and Co, or typical metal element)

[0013] As the substitution element M, in particular, Ti, Nb, S
n, Mg, and more preferably Ti, Nb. These may be used alone or two or more of them may be substituted. When two or more types are used, the combination is free and the total amount of replacement may be the above value.

As the conductive auxiliary, graphite, carbon black, carbon fiber, nickel, aluminum,
Examples thereof include metals such as copper and silver, and graphite and carbon black are particularly preferable.

Examples of the binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF),
Ethylene-propylene-gen copolymer (EPDM), styrene-butadiene rubber (SBR) and the like can be used.

The negative electrode usually has a carbonaceous material, a conductive additive and a binder.

As the carbonaceous material, for example, artificial graphite, natural graphite, pyrolytic carbon, coke, resin fired body, mesophase sphere, mesophase pitch, etc. can be used.

As the conductive additive, for example, acetylene black, carbon black, or the like can be used.

Examples of the binder include styrene-butadiene latex (SBR), carboxymethyl cellulose (CMC), polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), and ethylene-propylene-gen copolymer (EPDM). ), Nitrile-butadiene rubber (NB
R), vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, polytrifluoroethylene (PTrFE), vinylidene fluoride-trifluoroethylene copolymer , Vinylidene fluoride-tetrafluoroethylene copolymer and the like can be used.

In manufacturing an electrode, first, an active material and, if necessary, a conductive auxiliary are dispersed in a binder solution to prepare a coating solution.

Then, the electrode coating solution is applied to a current collector. The means for applying is not particularly limited, and may be determined as appropriate according to 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.
Thereafter, if necessary, a rolling treatment is performed by a flat plate press, a calender roll, or the like.

The current collector may be appropriately selected from ordinary current collectors according to the shape of the device using the battery and the method of arranging the current collector in the case. Generally, aluminum or the like is used for the positive electrode, and copper, nickel, or the like is used for the negative electrode.
Note that a metal foil, a metal mesh, or the like is generally used as the current collector. Although the metal mesh has lower contact resistance with the electrode than the metal foil, a sufficiently low contact resistance can be obtained even with the metal foil.

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

The non-aqueous electrolytic solution according to the present invention contains 60 to 95% by volume of γ-butyrolactone (γ-BL) in the solvent component,
A composition containing preferably 70 to 90% by volume, particularly 75 to 85% by volume as a main component, and an electrolyte dissolved in a non-aqueous solvent comprising a mixed solvent with one or more solvents such as a chain carbonate, a cyclic carbonate, and a chain ester. Has. If the composition ratio of γ-butyrolactone deviates from 60 to 95% by volume in the solvent to be mixed, formation of a film on the surface of the carbon material constituting the electrode at the time of the first charge becomes insufficient, and the battery capacity is reduced.

The separator is impregnated with a non-aqueous electrolyte in which an electrolyte is dissolved in an organic solvent. Further, the separator may be replaced with a solid electrolyte.

The separator sheet forming the separator may be made of one or more of polyolefins such as polyethylene and polypropylene (in the case of two or more, there are two or more films bonded together).
Examples include polyesters such as polyethylene terephthalate, thermoplastic fluororesins such as ethylene-tetrafluoroethylene copolymer, and celluloses. The form of the sheet is such that the air permeability measured by the method specified in JIS-P8117 is about 5 to 2000 seconds / 100 cc, and the thickness is 5 to 100.
There is a microporous membrane film of about μm, woven fabric, non-woven fabric, etc.

Further, a gel type polymer may be used. For example, (1) polyethylene oxide (PEO), polyalkylene oxide such as polypropylene oxide,
(2) a copolymer of ethylene oxide and acrylate,
(3) a copolymer of ethylene oxide and glycyl ether, (4) a copolymer of ethylene oxide, glycyl ether and allyl glycyl ether, (5) polyacrylate (6) polyacrylonitrile (PAN) (7) polyvinylidene fluoride , Vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-ethylene chloride trifluoride copolymer, vinylidene fluoride-hexafluoropropylene fluororubber, vinylidene fluoride "tetrafluoroethylene-hexafluoropropylene fluororubber, etc. And the like.

The gel polymer may be mixed with an electrolytic solution, or may be applied to a separator or an electrode. Further, by adding an initiator, the gel polymer may be cross-linked by ultraviolet rays, EB, heat or the like.

The outer package of the battery according to the present invention has a thickness of 0.3 mm.
A sheet film having a thickness of not more than 0.15 mm is used. A positive electrode, a negative electrode, and a separator are housed, arranged, and loaded in the outer package. The lower limit of the thickness of the exterior body is not particularly limited, but is usually 0.03.
mm. The battery is sealed and vacuum sealed.

The outer package of the present invention is made of a flexible film. By using a flexible film and evacuating the inside of the battery, the film closely adheres to the battery electrode,
A thinner and smaller battery can be created. Although the structure of the film is not particularly limited, an aluminum laminate film having a resin layer sandwiched between upper and lower portions is preferable.

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

The laminated film has a laminated structure of a heat-adhesive resin layer / polyester resin layer / metal foil / polyester resin layer from the interior side in order to ensure insulation between the metal foil constituting the laminated film and the lead terminals. It is preferable to use a laminate film. By using such a laminated film, the high melting point polyester resin layer remains without melting at the time of thermal bonding, so that a separation distance between the lead terminal and the metal foil of the outer package can be ensured, and insulation can be ensured. Therefore, the thickness of the polyester resin layer of the laminate film is preferably about 5 to 100 μm.

Here, an outer package using a flexible film can reduce the thickness of the battery, which is advantageous for miniaturization of the battery. However, since the battery is soft, the battery expands even if a small amount of gas is generated from the battery. There is a disadvantage that it will.

Although the positive electrode of the present invention is excellent in low-temperature characteristics as compared with ordinary lithium cobalt oxide, it has a high activity on the surface of the electrode, and reacts with the electrolytic solution when the battery is fully charged and stored at a high temperature. Gas is generated.

Therefore, in the case of a small and thin battery, there is a drawback that a higher-performance active material cannot be used. The γ-butyrolactone used in the present invention is dimethyl carbonate (DMC), methyl ethyl carbonate (MEC), which is commonly used in lithium secondary batteries that usually use an outer can.
Compared with diethyl carbonate (DEC), it is hardly oxidized and hardly generates gas during high temperature storage at full charge. For this reason, compared to an outer can that is relatively hard and hard to deform, even a battery that uses a thin and soft film for the outer body can use a higher-performance active material, making it possible to create a small high-performance battery. Become.

[0036]

EXAMPLES Specific examples of the present invention will be described below. Example 1 Polymer substance: PVDF [Kynar 761A (manufactured by Elf Atochem)], electrolyte solution: ethylene carbonate: γ-butyrolactone = 2: 8 (volume ratio) LiBF 4 in a mixed solvent
Was dissolved at a concentration of 2M, and a solvent: acetone was mixed at a weight ratio of polymer substance: electrolyte solution: solvent = 3: 7: 20 to prepare a first solution.

In the first solution, a positive electrode active material: LiCo
_0.999 Nb _0.001 O ₂ , conductive auxiliary agent: acetylene black in a weight ratio of the first solution: active material: conductive auxiliary agent = 2: 7.
5: 1.2, and dispersed to obtain a positive electrode slurry.

Further, polymer substance: electrolyte solution: solvent = 3:
A graphite was added as a negative electrode active material to the second solution prepared in the same manner as the first solution except that the ratio was 7: 5, so that the second solution: active material = 2: 1. And dispersed to obtain a negative electrode slurry.

Using the first solution, the slurry for the positive electrode, and the slurry for the negative electrode, an electrode group consisting of a positive electrode, a gelled solid electrolyte, a negative electrode, a gelled solid electrolyte, a positive electrode, and a laminate was prepared. Was placed in a sheet-like exterior body (aluminum laminate pack, thickness: 100 μm) and sealed with a sealer. The size of the electrode was 30 mm × 40 mm.

The battery produced by this method is
After charging / discharging was performed at a cutoff of 4.2 to 3.0 V and 1.0 C at ℃ and the capacity was measured, the specific capacity at -20 ℃ was measured. Further, the battery was put into a 4.2 V fully charged state, and the battery was put in an open state at 90 ° C., and the change in the battery thickness was measured.

Example 2 The positive electrode active material was LiCo _0.999
A battery was assembled and charged and discharged in the same manner as in Example 1, _{except that} Ti _0.001 O ₂ was used.

Example 3 The positive electrode active material was LiCo _0.999
A battery was assembled and charged / discharged in the same manner as in Example 1 _{except that} Sn _0.001 O ₂ was used. Then, the battery was charged to a full charge, put into an oven, and a change in battery thickness was measured.

Example 4 The positive electrode active material was LiCo _0.999
A battery was assembled and charged and discharged in the same manner as in Example 1 _{except that} Mg _0.001 O ₂ was used. Then, the battery was fully charged, put into an oven, and a change in battery thickness was measured.

Example 5 The positive electrode active material was LiCo
_A battery was assembled and charged and discharged in the same manner as in Example 1 _{except that 0.99999} Nb _0.00001 O ₂ was used.
It was put into an oven and the change in battery thickness was measured.

Example 6 The positive electrode active material was LiCo _0.9999
A battery was assembled and charged and discharged in the same manner as in Example 1 _{except that} Nb was set to _0.0001 O ₂ , charged to a fully charged state, put into an oven, and measured for changes in battery thickness.

Example 7 The positive electrode active material was LiCo _0.99 N
A battery was assembled and charged and discharged in the same manner as in Example 1 _{except that} b was changed to _0.01 O ₂ , charged to a fully charged state, put into an oven, and measured for changes in battery thickness.

Example 8 The positive electrode active material was LiCo _0.98 N
A battery was assembled and charged / discharged in the same manner as in Example 1 _{except that} b was changed to _0.02 O _{2. The} battery was charged to a fully charged state, put into an oven, and a change in battery thickness was measured.

Example 9 A battery was assembled and charged and discharged in the same manner as in Example 1 except that the composition of the electrolytic solution was changed to ethylene carbonate (EC): γ-butyrolactone = 4: 6 (volume ratio). Thereafter, the battery was fully charged, put into an oven, and the change in battery thickness was measured.

Example 10 The composition of the electrolytic solution was changed to ethylene carbonate (EC): γ-butyrolactone = 5: 95.
A battery was assembled and charged and discharged in the same manner as in Example 1, except that the volume ratio was changed.

Comparative Example 1 The positive electrode active material was LiCo _0.9999
A battery was assembled and charged and discharged in the same manner as in Example 1 _{except that} Nb was set to _0.0001 O ₂ , charged to a fully charged state, put into an oven, and measured for changes in battery thickness.

Comparative Example 2 The positive electrode active material was LiCo _0.9 N
A battery was assembled and charged / discharged in the same manner as in Example 1 _{except that} b _0.1 O ₂ was used.

Comparative Example 3 A battery was assembled and charged and discharged in the same manner as in Example 1 except that the composition of the electrolytic solution was changed to ethylene carbonate (EC): γ-butyrolactone = 5: 5 (volume ratio). Thereafter, the battery was fully charged, put into an oven, and the change in battery thickness was measured.

Comparative Example 4 A battery was assembled and charged and discharged in the same manner as in Example 1 except that the composition of the electrolytic solution was changed to γ-butyrolactone = 100 (volume ratio). And the change in battery thickness was measured.

<Comparative Example 5> The composition of the electrolyte was changed to ethylene carbonate (EC): diethyl carbonate (DEC).
= 2: 8 (volume ratio), except that the battery was assembled and charged / discharged in the same manner as in Example 1. The battery was charged to a full charge, placed in an oven, and the change in battery thickness was measured.

<Comparative Example 6> The composition of the electrolyte was changed to ethylene carbonate (EC): methyl ethyl carbonate (ME
A battery was assembled and charged and discharged in the same manner as in Example 1 except that C) = 2: 8 (volume ratio), and then the battery was fully charged.
It was put into an oven and the change in battery thickness was measured.

Comparative Example 7 The positive electrode active material was LiCo _0.999
Ti _0.001 O _2, and the composition of the electrolytic solution was ethylene carbonate (EC): methyl ethyl carbonate = 2: 8
A battery was assembled and charged and discharged in the same manner as in Example 1 except that the volume ratio was changed.

Comparative Example 8 The positive electrode active material was LiCo _0.999
The composition of the electrolytic solution was set to Sn _0.001 O ₂ and ethylene carbonate (EC): methyl ethyl carbonate = 2: 8
A battery was assembled and charged and discharged in the same manner as in Example 1 except that the volume ratio was changed.

Comparative Example 9 The positive electrode active material was LiCo _0.999
Mg _0.001 O _2, and the composition of the electrolytic solution was ethylene carbonate (EC): methyl ethyl carbonate = 2: 8
A battery was assembled and charged and discharged in the same manner as in Example 1 except that the volume ratio was changed.

Comparative Example 10 The positive electrode active material was LiCoO ₂
And the composition of the electrolytic solution is ethylene carbonate (E
C): A battery was assembled and charged and discharged in the same manner as in Example 1 except that γ-butyrolactone was set to 2: 8 (volume ratio). It was measured.

Table 1 shows the above results. In the table, EC: ethylene carbonate, γBL: γ-butyrolactone, ME
C: represents methyl ethyl carbonate, DEC: represents diethyl carbonate.

[0061]

[Table 1]

From Table 1 above, Examples 1-4 and Comparative Examples 5-9
By using γ-butyrolactone, gas generation can be suppressed even with a positive electrode active material to which an additive element that normally generates a gas is added, and a smaller battery is created by using a thin exterior body. We can see that we can do it.
The change in thickness is within an allowable range if it is within 0.2 mm.

From Examples 1 to 10 and Comparative Examples 1 and 10, it can be seen that the low-temperature characteristics were improved by the added elements. The specific capacity at −20 ° C. was 10% or more as an allowable value.

Examples 1, 5, 6, 7 and Comparative Examples 1, 2, 1
From 0, it can be seen that when the atomic ratio of the added element exceeds 2%, the capacity is reduced and the battery is not suitable for a high capacity battery.
In this example, the allowable capacity of the 1C capacity was 550 mAh or more. On the other hand, if the atomic ratio of the additional element is 0.00
If it is less than 1%, the specific capacity at a low temperature is reduced, and a problem occurs at a low temperature operation.

The addition amount of γ-butyrolactone according to Examples 1, 9, and 10 and Comparative Examples 3 and 4 shows that 60 to 95% by volume is appropriate.

This indicates that the secondary battery of the present invention does not swell at high temperatures and has improved low-temperature characteristics.

[0067]

As described above, according to the present invention, it is possible to provide a lithium secondary battery that exhibits a high discharge capacity even at a low temperature and does not swell during storage.

Continued on the front page F term (reference) 5H011 CC10 KK01 5H029 AJ02 AJ07 AK03 AL06 AL07 AM00 AM03 AM16 BJ04 DJ02 DJ08 EJ01 EJ12 HJ01 5H050 AA06 AA13 BA17 CA08 CB07 CB08 DA02 DA09 EA02 HA01

Claims (2)

[Claims] A positive electrode, a negative electrode, and an electrolyte are loaded in an exterior body, and lithium cobalt oxide is used as a positive electrode active material, and 0.001 to 2 atomic% based on the cobalt of the lithium cobalt oxide.
A lithium-containing composite oxide having a sub-component element M (transition metal and typical metal element except for Li and Co), containing 60 to 95% by volume of γ-butyrolactone as a solvent for the electrolyte. A lithium secondary battery having a thickness of 0.3 mm or less. 2. The lithium secondary battery according to claim 1, wherein the subcomponent element is one or more of Ti, Nb, Sn and Mg.