JP2000058069A - Lithium ion secondary battery - Google Patents

Abstract

(57) [Problem] To provide a lightweight, high energy density lithium ion secondary battery used for various electronic devices, power supplies for electric vehicles, and the like. SOLUTION: By using a graphite sheet obtained by sintering a polymer film as a current collector of a battery, a lightweight, high energy density lithium ion secondary battery excellent in cycle characteristics can be provided.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a large, small, thin, and lightweight secondary battery used in the fields of electronic equipment, electric vehicles, and the like, and more particularly to a current collector. The present invention relates to a high-energy-density lithium-ion secondary battery characterized by comprising a flexible graphite sheet.

[0002]

2. Description of the Related Art In recent years, an organic electrolyte secondary battery using lithium has a high energy density and plays a major role in reducing the size and weight of equipment. However, there is an increasing demand for further downsizing and weight reduction of devices, and on the other hand, a demand for large, lightweight, high capacity secondary batteries such as power supplies for electric vehicles.

[0003] A conventional lithium secondary battery is disclosed in
No. 90863, JP-A-63-121260,
As disclosed in JP-A-3-49155, a transition metal composite oxide containing lithium and cobalt as main components is used as a positive electrode active material, and a carbon material is used as a negative electrode active material. The positive electrode active material is a metal current collector such as aluminum or stainless steel, and the negative electrode active material is a metal current collector made of a copper foil having a thickness of 10 to 20 μm. The active material is an aqueous binder or a non-aqueous binder. The adhesive is applied to one side or both sides of the current collector and held.

As described above, in the conventional lithium secondary battery, since a metal having a large specific gravity is used for the current collector, there is a problem that the energy density per unit weight of the battery does not increase. Further, such a current collector has poor adhesion to an active material, increases contact resistance, causes an increase in impedance, and causes cycle deterioration, which is also a problem.

On the other hand, in recent years, the development of high-capacity large-sized secondary batteries such as power supplies for electric vehicles has been actively pursued from the viewpoint of dealing with environmental problems such as air pollution and global warming. Electric vehicles are powered by nickel-hydrogen storage alloy batteries, lead-acid batteries,
Batteries such as nickel-cadmium batteries have been studied for practical use.

However, the total weight of the battery is 300 to 50.
Since it is very heavy at 0 kg and the energy density per unit weight is small, there is a problem that a traveling distance per charge is restricted, and there is an urgent need to develop a secondary battery having a high weight energy density.

[0007]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned conventional problems. A high-capacity lithium ion secondary battery having improved cycle current characteristics by reducing the weight of the battery by improving the current collector. The purpose is to provide.

[0008]

In order to achieve this object, a lithium ion secondary battery of the present invention is characterized in that the current collector is made of a specific graphite sheet. More specifically, the present invention is characterized in that a graphite sheet which is manufactured by baking a polymer film and has a curvature radius of 1 mm or less and an angle of 160 ° or more and has excellent flexibility is used as a current collector.

As a result, the battery of the present invention is lighter in weight, has improved adhesion to an active material, and has excellent cycle characteristics, as compared with a conventional battery using a metal current collector having a large specific gravity. A lithium ion secondary battery having a high energy density can be provided.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention according to claim 1 of the present invention is directed to a lithium ion secondary battery characterized by using a flexible graphite sheet as a current collector. Since the weight is lighter than the current collector made of metal such as copper or nickel used for the current collector, the battery can be reduced in weight by using the current collector of the present invention. It has the effect of increasing the energy density per weight. Further, since the current collector of the present invention has flexibility, the shape of the battery such as a square or a cylinder is not limited, and the current collector has an action capable of coping with batteries of various shapes such as a sheet, a square, and a cylinder. Have.

Further, by using the current collector of the present invention for a large power source such as an electric vehicle, the weight can be reduced and the energy density per unit weight can be increased. As a result, the traveling distance per charge can be reduced. It has the effect that it can be greatly extended.

Further, since the adhesion between the active material and the current collector of the present invention is very good, a decrease in the electric capacity due to a decrease in the adhesion between the current collector and the active material due to the charge / discharge cycle is prevented, and the cycle characteristics Excellent lithium ion secondary battery can be provided.

According to a second aspect of the present invention, the graphite sheet is produced by firing an aromatic polyimide film having a thickness of 300 μm or less in an inert gas at a maximum temperature of 2500 ° C. or more. The lithium ion secondary battery according to claim 1, wherein a graphite sheet having good quality and excellent flexibility can be manufactured, and by using this as a current collector, it is light in weight and has a low energy density per unit weight. Has an effect that a lithium ion secondary battery having a large value can be provided.

According to a third aspect of the present invention, the graphite sheet has an electric conductivity of 2500 S / cm or more and 5500
The range of S / cm or less, The claim 1 characterized by the above-mentioned.
A lithium ion secondary battery according to item 2,
This has the effect of providing a high-capacity lithium ion secondary battery that is lightweight and has excellent cycle characteristics.

According to a fourth aspect of the present invention, there is provided the lithium ion secondary battery according to the first to third aspects, wherein the graphite sheet density is in a range of 0.4 g / cc to 1.5 g / cc. The secondary battery has a function of providing a high-capacity lithium ion secondary battery that is lightweight and has excellent cycle characteristics.

According to a fifth aspect of the present invention, in the structure of the graphite sheet, the (002) plane spacing of the graphite is in a range of 0.3354 nm to 0.3375 nm. It is a lithium ion secondary battery according to 3, 4, and by using a graphite sheet having this structure as a current collector,
This has the effect of reducing the weight of the battery and increasing the energy density per unit weight of the battery. Furthermore, since the current collector of the present invention has flexibility, the shape of the battery is not limited, and the current collector has an action capable of coping with batteries having various shapes such as a sheet shape, a square shape, and a cylindrical shape.

Further, by using the current collector of the present invention for a large power source such as an electric vehicle, the weight is reduced, the energy density per unit weight is increased, and as a result, the mileage per charge is reduced. It has the effect that it can be greatly extended.

Also, since the adhesion between the active material and the current collector of the present invention is very good, a decrease in the electric capacity due to a decrease in the adhesion between the current collector and the active material due to charge / discharge cycles is prevented, and the cycle characteristics are improved. Excellent lithium ion secondary battery can be provided.

According to a sixth aspect of the present invention, there is provided the negative electrode active material, wherein any one of amorphous carbon and graphite or a mixture thereof is provided on a graphite sheet. It is a lithium ion secondary battery described in 3, 4, and 5, which is lighter in weight than a current collector made of a metal such as copper or nickel used for a conventional current collector. The use of the electric conductor can reduce the weight of the battery, and has the effect of increasing the energy density per unit weight of the battery.

Further, the current collector of the present invention has a function of absorbing and releasing lithium in addition to a current collecting function, and thus has a high energy density lithium ion secondary battery in which the amount of active material is substantially increased. Has the effect of providing. Similarly, by using the current collector of the present invention for a large power source such as an electric vehicle, the weight is reduced, the energy density per unit weight is increased, and as a result, the mileage per charge is significantly increased. It has the effect that it can be extended.

In addition, since the adhesion between the active material such as amorphous carbon and graphite carbon and the current collector of the present invention is very good, the charge due to the decrease in the adhesion between the current collector and the active material due to the charge / discharge cycle. This has the effect of preventing a decrease in discharge capacity and providing a lithium ion secondary battery having excellent cycle characteristics. Furthermore, since the current collector of the present invention has flexibility, the shape of the battery is not limited, and the current collector has an action capable of coping with batteries having various shapes such as a sheet shape, a square shape, and a cylindrical shape.

According to a seventh aspect of the present invention, the negative electrode active material is provided after at least one surface of the graphite sheet is subjected to a porous treatment by a physical or mechanical method in advance. According to the surface treatment, the current collector of the present invention itself absorbs and releases lithium in addition to the current collecting function. The function as a negative electrode active material is more improved than that of an untreated current collector.

Therefore, by providing the negative electrode active material after the surface treatment of the present invention, it is possible to provide a lightweight and high capacity lithium ion secondary battery.

[0024] According to the present invention, at least one surface of the graphite sheet is preliminarily made porous by laser irradiation, and then any one of amorphous carbon and graphite carbon or a mixture thereof is used as a negative electrode active material. The lithium ion secondary battery according to claim 7, wherein the current collector according to the present invention is provided by:
In addition to the current collecting function, the function of the negative electrode active material itself for absorbing and releasing lithium is further improved as compared with the untreated current collector.

Therefore, by providing the negative electrode active material after the surface treatment of the present invention, it is possible to provide a lightweight and high capacity lithium ion secondary battery.

According to a ninth aspect of the present invention, a structure in which either graphite carbon or amorphous carbon or a mixture thereof is provided on a graphite sheet is used as a negative electrode active material. A lithium ion secondary battery according to any one of Items 6, 7, and 8, which has an effect of providing a lightweight, high-capacity lithium ion secondary battery having excellent cycle characteristics.

[0027] According to a tenth aspect of the present invention, a phenol resin is coated on a graphite sheet at a temperature of 700 ° C to 1500 ° C.
10. The lithium ion secondary battery according to claim 6, wherein an amorphous carbon synthesized by a temperature treatment in between is provided as an active material layer. It has the function of providing a high capacity lithium ion secondary battery.

[0028] In the eleventh aspect of the present invention, spherical graphite, acicular graphite, or flaky graphite or a mixture thereof is provided on a graphite sheet. The lithium ion secondary battery according to any one of claims 1 to 3, which has an effect of providing a lightweight, high capacity lithium ion secondary battery.

According to a twelfth aspect of the present invention, the carbon powder provided on the graphite sheet has an average particle size of 15 μm or less, and the carbon powder layer has a thickness of 0.05 mm to 0.3 mm. The bulk density is in the range of 0.7 g / cc to 1.5 g / cc.
To 11. The lithium ion secondary battery according to any one of Items 1 to 11, which has a function of providing a lightweight, high capacity lithium ion secondary battery.

According to a thirteenth aspect of the present invention, there is provided a lithium ion secondary battery according to any one of the first to twelfth aspects, wherein a powdery active material is provided from a paste on a graphite sheet by a printing method. This has the effect of providing a lightweight, high-capacity lithium ion secondary battery with excellent productivity.

According to the present invention, it is preferable that any one of lithium cobaltate, lithium nickelate, lithium manganate, or a composite thereof is provided as a positive electrode active material on a graphite sheet. It is a characteristic lithium ion secondary battery,
This has the effect of providing a lithium ion secondary battery that is lightweight and has an increased energy density per unit weight.

The present invention according to claim 15 is characterized in that one or both of the components provided with a positive electrode active material or a negative electrode active material on a graphite sheet are used as electrodes of a secondary battery for automobiles. The lithium ion secondary battery according to any one of claims 1 to 12, which is lighter in weight than a current collector made of a metal such as copper or nickel used for a conventional current collector. By using the current collector, the weight of the battery can be reduced, and the energy density per unit weight of the battery can be increased. This has the effect of reducing the weight and increasing the energy density per unit weight, and as a result, the traveling distance per charge can be greatly extended.

Hereinafter, the battery to which the present invention is applied will be described with reference to the drawings, but the present invention is not limited thereto.

FIG. 1 is a sectional view of an example of a battery to which the present invention is applied. The positive electrode active material 1 is held on the positive electrode current collector,
Further, the negative electrode active material 3 is held on a negative electrode current collector, and these two electrodes are separated by a porous separator 5 impregnated with a non-aqueous solvent electrolyte obtained by dissolving a lithium salt. 5 shows a coin-type battery manufactured by caulking the storage case 7.

In the battery of the present invention, various materials used in conventional lithium ion secondary batteries can be used in combination, and there is no particular limitation.

For example, as the negative electrode active material of the battery of the present invention, a carbonaceous material that absorbs and releases lithium can be used. Examples of such a material include graphite, a high molecular compound (a phenol resin or a furan resin obtained by firing and carbonizing), glassy carbons, carbon fibers, activated carbon, and the like.

As the positive electrode active material, a chargeable / dischargeable lithium-containing compound can be used. For example, the general formula L
ixMOy (M represents a transition metal element such as Co, Ni, Mn, and Fe, x is represented by 0 ≦ x ≦ 2, y is 1 ≦ y ≦ 5), and specific examples are LiCoO2, LiNiO2, and LiMnO.
2, LiMn2O2 and the like.

As the organic solvent of the non-aqueous electrolyte,
Propylene carbonate, ethylene carbonate, 1,
One or a mixture of two or more of 2-butylene carbonate, 1,2-dimethoxyethane, γ-butyrolactone, tetrahydrofuran, 2-methyltetrahydrofuran, dioxane, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, dipropyl carbonate, etc. May be used.

The supporting electrolyte is LiPF6, LiC
l04, LiAsF6, LiSbF6, LiBF4, Li
I, LiBr, LiCl, LiSO3CH3, LiSO3C
F3 and the like.

The non-aqueous electrolyte used in the battery of the present invention is not limited to a liquid, but may be a gel or a solid.

The thickness, shape, aperture ratio and the like of the current collector of the present invention are optimally determined according to the type of active material of the positive electrode and the negative electrode of the battery, the type of electrolyte and electrolyte, and the purpose of use of the battery. Can be

In the following examples, in order to clarify the characteristics of the negative electrode, a negative electrode structure (a negative electrode active material, a negative electrode current collector, etc.) originally used as a negative electrode was used as a positive electrode, and metallic lithium was used as a negative electrode. Using. The reason is that the use of metallic lithium as a source of lithium ions simplifies the insertion and extraction of lithium into and from the negative electrode structure, and proves the characteristics of the secondary battery using the current collector of the present invention. Intended.

(Embodiment 1) As a carbon material used as a negative electrode active material, a novolak-type phenol resin is used as a raw material, which is heat-treated at 180 ° C. in an inert gas atmosphere. 5 ° C / in an active gas atmosphere
Heat treatment was performed at 500 ° C. for 1 hour at a heating rate of 1 minute, and then the obtained heat-treated product was ground using a planetary ball mill so as to have an average particle size of 10 μm.

Next, the powder of the heat-treated product was heat-treated at 1000 ° C. for 1 hour at a rate of 5 ° C./min in an inert gas atmosphere to obtain amorphous carbon. This amorphous carbon powder is
g was mixed with 3 g of a binder obtained by dissolving polyvinylidene fluoride in N-methylpyrrolidone at 10% by weight, and the mixture was mixed with a flexible graphite sheet of the present invention having a thickness of 150 μm, a weight of 10 mg, and a density of 1.0 g / cc. Was applied on a current collector composed of and dried to obtain an electrode plate.

As a result of X-ray diffraction measurement of this carbon powder, the d (002) plane was 0.375 nm. Also,
The specific surface area by the BET method was 200 m 2 or less. The active material on the current collector was 10.4 mg.

Also, LiPF6 was added to an organic solvent obtained by mixing ethylene carbonate and diethyl carbonate at a ratio of 1: 1 with 1 m
ol / l was dissolved to prepare an electrolytic solution. Using lithium metal as a counter electrode of the carbon electrode, sandwiching between the two electrodes a porous polypropylene impregnated with an electrolytic solution, placing them in a 2016 type coin case, and pressing and sealing the coin battery for evaluation Was prepared.

Regarding the battery manufactured as described above,
Charging was performed at a constant current of 0.2 mA until the potential became 0 V, and the charging was terminated while maintaining the potential of 0 V for 20 hours. Next, discharging was performed at a constant current of 0.2 mA until the potential became 1.5 V. FIG. 2 shows a discharge curve. The solid line indicates battery characteristics using the graphite sheet of the present invention as a current collector, and the dotted line indicates characteristics of a conventional battery (using a copper foil as a current collector) described in Comparative Example 1 below. The result was 650 mAh / g by using the current collector of the present invention.
, And exhibited the characteristics of a high capacity secondary battery that has never been achieved before.

The graphite sheet of the present invention is obtained by coating a 75 μm-thick polyimide film under an inert atmosphere.
The graphite sheet obtained by sintering at 00 ° C. can produce a highly flexible graphite sheet that can be bent at a curvature radius of 1 mm or less and an angle of 160 ° or more.

With the graphite sheet having such flexibility, the lightweight and high-capacity lithium secondary battery of the present invention can be realized. Note that conventional sheet-like graphite which is generally known does not have the above-described bending and flexibility, and has different raw materials, manufacturing steps, and physical properties of the sheet.

Comparative Example 1 A coin battery was manufactured in the same manner as in the first embodiment except that the current collector was a commonly used copper foil. The copper foil used at this time had a thickness of 20 μm and a weight of 16 mg. For this battery, the potential was 0 at a constant current of 0.2 mA.
The battery was charged until the voltage reached V and the potential was kept at 0 V for 20 hours to complete the charging. Next, discharging was performed at a constant current of 0.2 mA until the potential became 1.5 V. The result is shown in FIG.
Is indicated by a dotted line. The discharge capacity was 500 mAh / g.

As described above, by using the current collector of the present invention for a lithium ion secondary battery, a high-capacity lithium ion secondary battery having a reduced battery weight and an increased energy density per unit can be realized.

(Embodiment 2) In Embodiment 1,
The carbon material provided on the graphite sheet is 7.3 graphite.
The battery characteristics were evaluated in the same manner as in Embodiment 1 except that mg was used. As a result, the discharge capacity was 700 mAh / g.
372 mAh /, which is the theoretical capacity of graphite.
g. This is because the current collector of the present invention used as a current collector also has a function as an active material for absorbing and releasing lithium in addition to a current collecting function. As described above, the lithium ion secondary battery of the present invention has a light current collector, so that the battery weight can be reduced, and a high-capacity lithium ion secondary battery, which has never existed before, can be realized.

The cycle characteristics are such that the capacity increases each time charge / discharge is repeated, and the battery characteristics do not deteriorate after several tens of charge / discharge cycles, and a high-capacity lithium ion secondary battery with excellent cycle characteristics is provided. can do.

In order to explain the above high capacity and excellent cycle characteristics, in the battery configuration described in the first embodiment, only the current collector of the present invention was used without providing the negative electrode active material on the negative electrode current collector. When the battery characteristics were evaluated in the same manner as in Embodiment 1, the initial discharge capacity was 200 m as shown in FIG.
Ah / g, and the capacity increased after the second time. This proves that the current collector of the present invention has a function of inserting and extracting lithium ions in addition to the function of collecting current, and at the same time, it is accompanied by an increase in discharge capacity in cycle characteristics. It is clear in form.

Therefore, by using the graphite sheet of the present invention as a current collector, the energy density per unit weight is increased, and a lithium ion secondary battery having excellent cycle characteristics can be realized.

(Embodiment 3) In Embodiment 1,
Embodiment 1 A carbon material provided on a graphite sheet is used in Embodiment 1.
The mixture of amorphous carbon and graphite described in
The battery characteristics were evaluated in the same manner as in Embodiment 1 except for using mg, and the same effect as in Embodiment 1 was obtained.

(Embodiment 4) In Embodiment 1,
The battery characteristics were evaluated in the same manner as in Embodiment 1, except that the current collector of the present invention in which one side of the graphite sheet had been made porous with a needle having a needle diameter of 0.3 mm was used. As a result, the discharge capacity was 700 mAh / g.

By subjecting the current collector of the present invention to a porous treatment, lithium ions can be easily absorbed and the discharge capacity can be increased. As described above, the lithium ion secondary battery of the present invention can reduce the weight and provide a high-capacity secondary battery.

Further, graphite and a mixture of graphite and amorphous carbon were respectively provided on the porous graphite sheet, and the battery characteristics were similarly evaluated. As a result, the discharge capacity was increased.

Incidentally, the battery characteristics using the current collector of the present invention which had been made porous by a laser also resulted in an increase in discharge capacity.

In the above embodiment, the case where the graphite sheet current collector of the present invention is applied to a coin-type lithium ion secondary battery has been described. However, the current collector of the present invention has flexibility. A spirally wound battery can be applied to a rectangular, cylindrical, or other shape lithium ion secondary battery.

[0062]

As described above, the present invention can be applied to batteries of various shapes, and does not use a conventional metal current collector. it can. Further, since the current collector of the present invention has a function of acting as an active material itself in addition to the current collecting function, high capacity can be achieved.

As described above, the secondary battery using the current collector of the present invention can provide a novel secondary battery having a light weight and a high energy density. It can be expected that the effect is particularly large.

Further, since the adhesion between the active material and the current collector of the present invention is very good, a decrease in the battery capacity due to a decrease in the adhesion between the current collector and the active material due to charge / discharge cycles is prevented, and the cycle characteristics are improved. Excellent battery can be provided.

[Brief description of the drawings]

FIG. 1 is a sectional view of an example of a battery to which the present invention is applied.

2A is a diagram showing discharge characteristics using a current collector of the present invention. FIG. 2B is a diagram showing discharge characteristics using a conventional metal current collector.

FIG. 3 is a diagram showing the discharge characteristics of the graphite sheet of the present invention.

[Explanation of symbols]

 Reference Signs List 1 positive electrode active material 2 positive electrode current collector 3 negative electrode active material 4 negative electrode current collector 5 separator 6 insulating gasket 7 storage case

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Tomonari Nanai 3-1-1, Higashi-Mita, Tama-ku, Kawasaki-shi, Kanagawa Prefecture Inside Matsushita Giken Co., Ltd. 3-10-1 Mita Matsushita Giken Co., Ltd. (72) Inventor Souji Tsuchiya Inside Matsushita Giken Co., Ltd.

Claims (15)

[Claims] 1. A lithium ion secondary battery using a flexible graphite sheet as a current collector. 2. A lithium sheet according to claim 1, wherein the graphite sheet is produced by firing an aromatic polyimide film having a thickness of 300 μm or less in an inert gas at a maximum temperature of 2500 ° C. or more. Ion secondary battery. 3. The graphite sheet having an electric conductivity of 25.
3. The lithium ion secondary battery according to claim 1, wherein the lithium ion secondary battery has a range of not less than 00 S / cm and not more than 5500 S / cm. 4. A graphite sheet density of 0.4 g / c.
4. The lithium ion secondary battery according to claim 1, wherein the range is c to 1.5 g / cc. 5. The graphite sheet has a structure in which the (002) plane spacing of graphite is 0.3354 nm.
The lithium ion secondary battery according to any one of claims 1, 2, 3, and 4, wherein the lithium ion secondary battery has a wavelength in the range of 0.3 to 0.3375 nm. 6. A method according to claim 1, wherein one of amorphous carbon and graphite or a mixture thereof is provided as a negative electrode active material on a graphite sheet.
4. The lithium-ion secondary battery according to 4, 5. 7. A negative electrode active material is provided after at least one surface of a graphite sheet is subjected to a porous treatment by a physical or mechanical method in advance.
The lithium ion secondary battery according to 2, 3, 4, 5, or 6. 8. A graphite sheet, wherein at least one surface thereof has been made porous by laser irradiation in advance, and any one of amorphous carbon and graphite carbon or a mixture thereof is provided as a negative electrode active material. The lithium ion secondary battery according to claim 7. 9. The lithium according to claim 6, wherein a composition in which amorphous carbon or graphite or a mixture thereof is provided on a graphite sheet is used as a negative electrode active material. Ion secondary battery. 10. An amorphous carbon synthesized by a temperature treatment between 700 ° C. and 1500 ° C. on a graphite sheet as a phenolic resin is provided as an active material layer. The lithium ion secondary battery according to the above. 11. A spherical, needle-like,
11. The lithium ion secondary battery according to claim 6, wherein any one of flaky graphite or a mixture thereof is provided. 12. The carbon powder provided on the graphite sheet has an average particle diameter of 15 μm or less, a thickness of the carbon powder layer in a range of 0.05 mm to 0.3 mm, and a bulk density of 0.7 g / 12. The lithium ion secondary battery according to claim 1, wherein the range is from cc to 1.5 g / cc. 13. The lithium ion secondary battery according to claim 1, wherein the powdery active material is provided from a paste form on a graphite sheet by a printing method. 14. A lithium ion secondary battery wherein any one of lithium cobaltate, lithium nickelate, lithium manganate, or a composite thereof is provided as a positive electrode active material on a graphite sheet. . 15. A positive electrode active material on a graphite sheet,
13. The lithium ion secondary battery according to claim 1, wherein one or both of the components provided with the negative electrode active material are used as an electrode of an automobile secondary battery.