JP4188591B2 - Lithium secondary battery - Google Patents

Description

【００６２】
上記表１から実施例１〜４，比較例５〜９により、通常ガスを生じるような添加元素を加えた正極活物質であっても、γ−ブチロラクトンの使用によりガス発生を抑えることができ、薄い外装体を使用することによって、より小型の電池を作成できることがわかる。なお、厚みの変化量は０．２mm以内であれば許容範囲である。
【００６３】
実施例１〜１０、比較例１，１０から、添加元素により、低温特性が向上したことがわかる。なお、−２０℃での比容量は、１０％以上を許容値とした。
【００６４】
実施例１，５，６，７、比較例１，２，１０から、添加元素の原子比が２％を越えると、容量低下を引き起こし高容量の電池には向かないことがわかる。なお、１Ｃ容量は、本実施例では５５０mAh 以上のものを許容範囲とした。一方、添加元素の原子比が０．００１％未満では低温での比容量が低下し、低温動作に問題が生じることがわかる。
【００６５】
また、γ−ブチロラクトンの添加量は実施例１，９，１０、比較例３，４から６０〜９５体積％が適当であることがわかる。
【００６６】
このことから、本発明の二次電池は、高温で膨れることなく、また低温特性もより改善された電池であることがわかる。
【００６７】
【発明の効果】
以上のように、本発明によれば低温でも高い放電容量を示し、保存時にも膨れることのないリチウム二次電池を提供できる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a lithium secondary battery, and more particularly to improvement of an electrolytic solution using an electrode and a nonaqueous solvent.
[0002]
[Prior art]
In recent years, with the miniaturization of electronic devices, batteries are also required to be small and light. Under such circumstances, lithium secondary batteries are most actively researched and developed as one of the batteries that can satisfy these requirements. Recently, in order to achieve higher capacity, a battery using a flexible aluminum laminate film as an exterior body has also appeared.
[0003]
A problem with the aluminum laminate film is that the battery swells when a gas is generated from the inside of the battery after the battery is formed. This problem can be solved by using γ-butyrolactone as an electrolyte solution as disclosed in, for example, Japanese Patent Application Laid-Open No. 2000-236868.
[0004]
On the other hand, a problem with lithium secondary batteries is that capacity at low temperatures is insufficient. In response to this, various solutions have been proposed as disclosed in, for example, JP-A-6-290809 and JP-A-8-138738. However, these are mainly improvements in the composition of the electrolytic solution, and it has been more difficult to improve the low-temperature characteristics on the premise that γ-butyrolactone that suppresses swelling is used.
[0005]
[Problems to be solved by the invention]
An object of the present invention is to provide a thin lithium secondary battery having good low temperature characteristics and a low profile by suppressing gas generated from the battery electrode when stored at a high temperature in a battery in which an additive element is added to the positive electrode active material. It is.
[0006]
[Means for Solving the Problems]
That is, the above object is achieved by the following configuration of the present invention.
And the positive electrode to the armor body, a negative electrode, and an electrolyte is loaded,
As a positive electrode active material, it has a lithium-containing composite oxide represented by the formula LiCo _1-x Nb _x O ₂ (X = 0.00001 to 0.02) ,
Containing γ-butyrolactone in an amount of 60 to 95% by volume as an electrolyte solvent;
The lithium secondary battery whose thickness of the said exterior body is 0.3 mm or less .
[0007]
DETAILED DESCRIPTION OF THE INVENTION
In the lithium secondary battery of the present invention, a positive electrode, a negative electrode, and an electrolyte are loaded in an outer package. As a positive electrode active material, lithium cobaltate and 0.001 to 2 atomic% with respect to cobalt of the lithium cobaltate. And a lithium-containing composite oxide having an auxiliary component element M (transition metals and typical metal elements excluding Li and Co), 60 to 95% by volume of γ-butyrolactone as a solvent for the electrolyte, and a thickness of 0. It has an outer package of 3 mm or less.
[0008]
With such a configuration, a lithium secondary battery with good low-temperature characteristics and no gas generation even at high temperatures can be provided, and even when a thin film-like exterior body is used, the exterior body can be prevented from swelling.
[0009]
The positive electrode of the lithium secondary battery of the present invention is produced from a mixture containing a positive electrode active material, a conductive additive 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 sub-component element may be either a typical metal or a transition metal, but is preferably Ti, Nb, Sn, and Mg, or any one or more of Ti and Nb, and an element that has been confirmed to improve temperature characteristics. It is.
[0011]
The content of the subcomponent element M with respect to Co in the lithium cobalt oxide is preferably 0.001 to 2 atomic%, particularly 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 decreases, and when the content is too small, it is difficult to obtain the effect of improving the low temperature characteristics.
[0012]
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, Co, or typical metal element)
[0013]
As the substitution element M, Ti, Nb, Sn, Mg, and further Ti and Nb are particularly preferable. 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 substitution only needs to be the above value.
[0014]
Preferred examples of the conductive assistant include graphite, carbon black, carbon fiber, and metals such as nickel, aluminum, copper, and silver, and graphite and carbon black are particularly preferable.
[0015]
As the binder, for example, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), ethylene-propylene-gen copolymer (EPDM), styrene-butadiene rubber (SBR), or the like can be used.
[0016]
The negative electrode usually has a carbonaceous material, a conductive aid and a binder.
[0017]
As the carbonaceous material, for example, artificial graphite, natural graphite, pyrolytic carbon, coke, resin fired body, mesophase microsphere, mesophase pitch, and the like can be used.
[0018]
As the conductive assistant, for example, acetylene black, carbon black, or the like can be used.
[0019]
Examples of the binder include styrene / butadiene latex (SBR), carboxymethylcellulose (CMC), polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), ethylene-propylene-gen copolymer (EPDM), and nitrile. -Butadiene rubber (NBR), vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, polytrifluoroethylene (PTrFE), vinylidene fluoride-trifluoro An ethylene copolymer, a vinylidene fluoride-tetrafluoroethylene copolymer, or the like can be used.
[0020]
In producing the electrode, first, an active material and, if necessary, a conductive additive are dispersed in a binder solution to prepare a coating solution.
[0021]
And this electrode coating liquid is apply | coated to a collector. The means for applying is not particularly limited, and may be appropriately determined according to the material and shape of the current collector. In general, 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, a rolling process is performed using a flat plate press, a calendar roll, or the like.
[0022]
The current collector may be appropriately selected from ordinary current collectors according to the shape of the device using 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. In addition, a metal foil, a metal mesh, etc. are normally used for a collector. The metal mesh has a smaller contact resistance with the electrode than the metal foil, but a sufficiently small contact resistance can be obtained even with the metal foil.
[0023]
Then, the solvent is evaporated to produce an electrode. The coating thickness is preferably about 50 to 400 μm.
[0024]
The non-aqueous electrolyte in the present invention is composed of 60 to 95% by volume, preferably 70 to 90% by volume, particularly 75 to 85% by volume of γ-butyrolactone (γ-BL) in the solvent component, It has a composition in which the electrolyte is dissolved in a non-aqueous solvent composed of a mixed solvent with one or more solvents such as cyclic carbonate and chain ester. When the composition ratio of γ-butyrolactone deviates from 60 to 95% by volume, the solvent to be mixed results in insufficient film formation on the surface of the carbon material constituting the electrode during the first charge, and the battery capacity decreases.
[0025]
The separator is impregnated and held with a nonaqueous electrolytic solution in which an electrolyte is dissolved in an organic solvent. The separator may be replaced with a solid electrolyte.
[0026]
The separator sheet forming the separator is composed of one or more of polyolefins such as polyethylene and polypropylene (in the case of two or more, there is a laminate of two or more films), polyethylene terephthalate Such polyesters, thermoplastic fluororesins such as ethylene-tetrafluoroethylene copolymer, and celluloses. The form of the sheet includes a microporous membrane film, a woven fabric, a non-woven fabric, etc. having an air permeability measured by the method specified in JIS-P8117 of about 5 to 2000 seconds / 100 cc and a thickness of about 5 to 100 μm.
[0027]
Further, a gel type polymer may be used. For example,
(1) Polyalkylene oxides such as polyethylene oxide (PEO) and 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-trichloroethylene copolymer, vinylidene fluoride-hexafluoropropylene fluororubber, vinylidene fluoride “tetrafluoroethylene- Fluoropolymers such as hexafluoropropylene fluororubber can be used.
[0028]
The gel polymer may be mixed with an electrolytic solution, or may be applied to a separator or an electrode. Furthermore, the gel polymer may be cross-linked by ultraviolet rays, EB, heat or the like by adding an initiator.
[0029]
For the battery outer body in the present invention, a sheet-like film having a thickness of 0.3 mm or less, particularly 0.15 mm or less is used. A positive electrode, a negative electrode, and a separator are accommodated and placed in the exterior body. The lower limit of the thickness of the exterior body is not particularly limited, but is usually about 0.03 mm. The battery is sealed and vacuum sealed.
[0030]
The exterior body in the present invention is laid on a flexible film. By using a flexible film and evacuating the inside of the battery, the film is densely attached to the battery electrode, and a thinner and smaller battery can be created. The structure of the film is not particularly defined, but an aluminum laminate film having a resin layer sandwiched between the upper and lower sides is preferable.
[0031]
The exterior bag is composed of a laminated 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 bag is formed in a bag shape in which two laminated films are bonded in advance to each other by thermally bonding the heat-adhesive resin layers on the end surfaces of the three sides to form a first seal portion. Alternatively, a single laminate film may be folded and the end faces of both sides may be thermally bonded to form a seal portion to form a bag.
[0032]
As a laminate film, a laminate film having a laminated structure of a heat-adhesive resin layer / polyester resin layer / metal foil / polyester resin layer from the interior side is used to ensure insulation between the metal foil constituting the laminate film and the lead-out terminal. It is preferable to use it. By using such a laminate film, the polyester resin layer having a high melting point remains undissolved at the time of thermal bonding, so that a separation distance between the lead-out terminal and the metal foil of the outer bag can be secured and insulation can be secured. Therefore, the thickness of the polyester resin layer of the laminate film is preferably about 5 to 100 μm.
[0033]
Here, the exterior body using a flexible film can reduce the thickness of the battery, which is advantageous for downsizing of the battery. However, since it is soft, the battery expands even if a small amount of gas is generated from the battery. There are drawbacks.
[0034]
The positive electrode of the present invention is superior in low-temperature characteristics compared to ordinary lithium cobalt oxide, but on the other hand, the electrode surface has high activity, and when the battery is fully charged and stored at high temperature, it reacts with the electrolyte solution to react with the gas. There was a problem that caused.
[0035]
For this reason, in the case of a small and thin battery, there was a drawback that a higher performance active material could not be used. The γ-butyrolactone used in the present invention is less susceptible to oxidation than dimethyl carbonate (DMC), methyl ethyl carbonate (MEC), and diethyl carbonate (DEC), which are often used in lithium secondary batteries that normally use an outer can. It is difficult to generate gas during high-temperature storage of charging. For this reason, it is possible to use a high-performance active material even in a battery using a thin and soft film for the outer casing, compared to an outer can that is relatively hard and difficult to deform, and it is possible to create a small high-performance battery. Become.
[0036]
【Example】
Specific examples of the present invention are shown below.
<Example 1>
Polymer substance: PVDF [Kynar 761A (manufactured by Elf Atchem)], electrolyte: ethylene carbonate: γ-butyrolactone = 2: 8 (volume ratio), a solution obtained by dissolving LiBF4 at a concentration of 2M, solvent Acetone was mixed in a weight ratio such that polymer substance: electrolyte solution: solvent = 3: 7: 20 to prepare a first solution.
[0037]
In this first solution, the positive electrode active material: LiCo _0.999 Nb _0.001 O ₂ , conductive auxiliary agent: acetylene black in a weight ratio, the first solution: active material: conductive auxiliary agent = 2: 7.5: 1.2 Then, a positive electrode slurry was obtained.
[0038]
In addition, a second solution prepared in the same manner as the first solution except that the polymer material: electrolyte solution: solvent = 3: 7: 5 was used, and graphite was added as a negative electrode active material in a weight ratio. A solution: active material = 2: 1 was added and dispersed to obtain a negative electrode slurry.
[0039]
Using the first solution, the positive electrode slurry, and the negative electrode slurry, a positive electrode-gelled solid electrolyte-negative electrode-gelled solid electrolyte-positive electrode: an electrode group composed of a laminate is produced, and this is formed into a sheet. The outer package (aluminum laminate pack, thickness 100 μm) was sealed with a sealer. The size of the electrode was 30 mm × 40 mm.
[0040]
The battery prepared by this method was charged and discharged at 25 ° C. at a cutoff of 4.2 to 3.0 V and 1.0 C, and the capacity was measured. Then, the specific capacity at −20 ° C. was measured. Moreover, it was set as the 4.2V full charge state, and it injected | thrown-in to 90 degreeC open, and measured the change of battery thickness.
[0041]
< Reference Example 2>
The battery was assembled and charged / discharged in the same manner as in Example 1 except that the positive electrode active material was LiCo _0.999 Ti _0.001 O _2. After charging and discharging, the battery was fully charged and placed in an oven to change the battery thickness. It was measured.
[0042]
< Reference Example 3>
The battery was assembled in the same manner as in Example 1 except that the positive electrode active material was LiCo _0.999 Sn _0.001 O ₂ , charged and discharged, then fully charged, and put into an oven to change the battery thickness. It was measured.
[0043]
< Reference Example 4>
The battery was assembled and charged / discharged in the same manner as in Example 1 except that the positive electrode active material was LiCo _0.999 Mg _0.001 O _2. After charging and discharging, the battery was fully charged and placed in an oven to change the battery thickness. It was measured.
[0044]
<Example 5>
A battery was assembled and charged and discharged in the same manner as in Example 1 _except that the positive electrode active material was LiCo _0.99999 Nb _0.00001 O _2. After charging and discharging, the battery was fully charged and placed in an oven to measure changes in battery thickness.
[0045]
<Example 6>
A battery was assembled and charged and discharged in the same manner as in Example 1 _except that the positive electrode active material was LiCo _0.9999 Nb _0.0001 O _2. After charging and discharging, the battery was fully charged and placed in an oven to measure changes in battery thickness.
[0046]
<Example 7>
A battery was assembled and charged / discharged in the same manner as in Example 1 except that the positive electrode active material was LiCo _0.99 Nb _0.01 O _2, and then fully charged, put into an oven, and the change in battery thickness was measured.
[0047]
<Example 8>
A battery was assembled and charged and discharged in the same manner as in Example 1 except that the positive electrode active material was LiCo _0.98 Nb _0.02 O _2. After charging and discharging, the battery was fully charged and placed in an oven to measure changes in battery thickness.
[0048]
<Example 9>
The battery was assembled in the same manner as in Example 1 except that the composition of the electrolytic solution was ethylene carbonate (EC): γ-butyrolactone = 4: 6 (volume ratio), charged and discharged, and then fully charged. The change in battery thickness was measured.
[0049]
<Example 10>
A battery was assembled in the same manner as in Example 1 except that the composition of the electrolytic solution was ethylene carbonate (EC): γ-butyrolactone = 5: 95 (volume ratio), charged and discharged, and then fully charged. The change in battery thickness was measured.
[0050]
<Comparative Example 1>
The battery was assembled and charged / discharged in the same manner as in Example 1 _except that the positive electrode active material was LiCo _0.999999 Nb _0.000001 O _2. After charging and discharging, the battery was fully charged and placed in an oven to change the battery thickness. It was measured.
[0051]
<Comparative example 2>
A battery was assembled and charged / discharged in the same manner as in Example 1 except that the positive electrode active material was LiCo _0.9 Nb _0.1 O _{2. The} battery was fully charged, put in an oven, and the change in battery thickness was measured.
[0052]
<Comparative Example 3>
The battery was assembled in the same manner as in Example 1 except that the composition of the electrolytic solution was ethylene carbonate (EC): γ-butyrolactone = 5: 5 (volume ratio). After charging and discharging, the battery was fully charged. The change in battery thickness was measured.
[0053]
<Comparative example 4>
The battery was assembled in the same manner as in Example 1 except that the composition of the electrolyte was γ-butyrolactone = 100 (volume ratio). After charging and discharging, the battery was fully charged and placed in an oven to change the battery thickness. Was measured.
[0054]
<Comparative Example 5>
The battery was assembled in the same manner as in Example 1 except that the composition of the electrolytic solution was ethylene carbonate (EC): diethyl carbonate (DEC) = 2: 8 (volume ratio). After charging and discharging, the battery was fully charged. Then, it was put into an oven and the change in battery thickness was measured.
[0055]
<Comparative Example 6>
The battery was assembled and charged and discharged in the same manner as in Example 1 except that the composition of the electrolyte was ethylene carbonate (EC): methyl ethyl carbonate (MEC) = 2: 8 (volume ratio). And put into an oven to measure the change in battery thickness.
[0056]
<Comparative Example 7>
A battery was assembled and charged in the same manner as in Example 1 except that the positive electrode active material was LiCo _0.999 Ti _0.001 O ₂ and the electrolyte composition was ethylene carbonate (EC): methyl ethyl carbonate = 2: 8 (volume ratio). After discharging, the battery was fully charged, put into an oven, and the change in battery thickness was measured.
[0057]
<Comparative Example 8>
A battery was assembled and charged in the same manner as in Example 1 except that the positive electrode active material was LiCo _0.999 Sn _0.001 O ₂ and the electrolyte composition was ethylene carbonate (EC): methyl ethyl carbonate = 2: 8 (volume ratio). After discharging, the battery was fully charged, put into an oven, and the change in battery thickness was measured.
[0058]
<Comparative Example 9>
A battery was assembled and charged in the same manner as in Example 1 except that the positive electrode active material was LiCo _0.999 Mg _0.001 O ₂ and the electrolyte composition was ethylene carbonate (EC): methyl ethyl carbonate = 2: 8 (volume ratio). After discharging, the battery was fully charged, put into an oven, and the change in battery thickness was measured.
[0059]
<Comparative Example 10>
A battery was assembled and charged and discharged in the same manner as in Example 1 except that the positive electrode active material was LiCoO ₂ and the electrolyte composition was ethylene carbonate (EC): γ-butyrolactone = 2: 8 (volume ratio). Thereafter, the battery was fully charged, put in an oven, and the change in battery thickness was measured.
[0060]
The results are shown in Table 1. In the table, EC: ethylene carbonate, γBL: γ-butyrolactone, MEC: methyl ethyl carbonate, DEC: diethyl carbonate, respectively.
[0061]
[Table 1]

[0062]
According to Examples 1 to 4 and Comparative Examples 5 to 9 from Table 1 above, even a positive electrode active material to which an additive element that normally generates gas can be added, gas generation can be suppressed by using γ-butyrolactone, It can be seen that a smaller battery can be produced by using a thin outer package. In addition, if the amount of change in thickness is within 0.2 mm, it is acceptable.
[0063]
It can be seen from Examples 1 to 10 and Comparative Examples 1 and 10 that the low temperature characteristics were improved by the additive element. The specific capacity at −20 ° C. was allowed to be 10% or more.
[0064]
From Examples 1, 5, 6, and 7 and Comparative Examples 1, 2, and 10 it can be seen that if the atomic ratio of the additive element exceeds 2%, the capacity is reduced and it is not suitable for a high-capacity battery. In this embodiment, the 1C capacity is 550 mAh or more as the allowable range. On the other hand, it can be seen that when the atomic ratio of the additive element is less than 0.001%, the specific capacity at a low temperature is lowered, causing a problem in low temperature operation.
[0065]
Moreover, it turns out that 60-95 volume% is suitable for the addition amount of (gamma) -butyrolactone from Example 1,9,10 and Comparative Examples 3,4.
[0066]
From this, it can be seen that the secondary battery of the present invention is a battery that does not swell at high temperatures and has improved low-temperature characteristics.
[0067]
【The invention's effect】
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.

Claims (1)

The outer body is loaded with a positive electrode, a negative electrode, and an electrolyte,
As a positive electrode active material, it has a lithium-containing composite oxide represented by the formula LiCo _1-x Nb _x O ₂ (X = 0.00001 to 0.02) ,
Containing γ-butyrolactone in an amount of 60 to 95% by volume as an electrolyte solvent;
The lithium secondary battery whose thickness of the said exterior body is 0.3 mm or less.