JPH10116619A - Lithium ion secondary battery and its negative electrode - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance the safety with a lithium ion secondary battery by forming its negative electrode form a graphite which is capable of storing and emitting lithium ions and whose volume resistivity lies below a specific value, and by preventing a steep rise of the battery temp. even in case the battery is crushed. SOLUTION: A negative electrode plate 2 uses an artificial graphite as active material, followed by adding a binder, and a black mixture prepared in paste form by suspending it in a carboxymethylcellulose water solution is applied to both surfaces of a core consisting of a copper foil and subjected to a drying process, and the obtained product is rolled so that the carbon filling density becomes 1.4g/cc and cut into specified dimensions. The graphite used therein should have a volume resistivity of 5.0×10<-3> Ω.cm or below and be able to store and emit lithium ions. This permits preventing a steep temp. rise of the battery by lessening heat generation due to Joule's heat even in case the battery is crushed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a carbon material used for a negative electrode of a lithium ion secondary battery, particularly to graphite.

[0002]

2. Description of the Related Art In recent years, portable and cordless electronic devices have been rapidly advancing, and there is a high demand for a small and lightweight secondary battery having a high energy density as a driving power source for these devices. In this respect, non-aqueous electrolyte secondary batteries, particularly lithium secondary batteries, are highly expected as batteries having high voltage and high energy density.

Conventionally, as a lithium secondary battery, a battery system using a transition metal oxide or sulfide such as manganese dioxide or molybdenum disulfide for a positive electrode and using metal lithium or a lithium alloy for a negative electrode has been proposed. Was. However, when metal lithium is used for the negative electrode, lithium precipitates in the form of needles or moss during charging, which penetrates through the separator and contacts the positive electrode, causing internal short-circuiting and a sudden increase in battery temperature, leading to safety problems. There was a problem.

[0004] Therefore, a battery using a carbon material capable of inserting and extracting lithium in the negative electrode has been proposed. In this case, lithium ions are inserted between layers of the carbon material during charging, so that lithium is deposited on the negative electrode. R & D is being actively conducted at present, since it does not precipitate, improves the safety of the battery, and also has excellent rapid charging characteristics.

At this time, LiCoO ₂ or L
A lithium-containing metal oxide such as iNiO ₂ is used.

[0006]

However, when an accident occurs in which the battery is crushed, an external pressure is applied to the side surface of the battery, so that the positive electrode and the negative electrode break through the separator in the battery and come into contact with each other, thereby causing an internal short circuit. There was something. At the time of this internal short-circuit, a large current is concentrated on a portion where the positive electrode and the negative electrode are in contact with each other, and the contact resistance is increased, so that heat is generated due to Joule heat, thereby causing a problem that the battery temperature rises rapidly.

An object of the present invention is to solve the above-mentioned problems, and an object of the present invention is to provide a highly safe lithium ion secondary battery in which the battery temperature does not rise rapidly even when the battery is crushed. I do.

[0008]

In order to achieve the above object, the present invention provides a negative electrode comprising graphite having a volume resistivity of 5.0 × 10 ⁻³ Ω · cm or less capable of inserting and extracting lithium ions. Since the graphite material has a low volume resistivity even when an internal short circuit occurs in the battery, the battery temperature does not rise sharply.

On the other hand, regarding the physical properties of the carbon material of the negative electrode for a lithium secondary battery, Japanese Unexamined Patent Publication (Kokai) No. 6-119925 discloses pyrolytic carbons, cokes, glassy carbons, fired bodies of organic polymer compounds, carbon fibers, and the like. The volume specific resistance of carbon materials such as activated carbon, fired mesocarbon microbeads, etc. is set to 0.
It is disclosed that the thickness is from 0.01 to 10 Ωcm, which indicates that a carbon material negative electrode having a large capacity can be provided.

[0010] However, the above-mentioned publication does not disclose any purpose for improving the safety of the battery when the battery is crushed. In the present invention, an appropriate range of volume resistivity is set for the purpose of improving the safety of the battery, assuming the pressure applied to the graphite powder so as to obtain the density when the negative electrode is formed.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION According to the first aspect of the present invention, in a negative electrode for a lithium ion secondary battery, a volume resistivity at which lithium ions can be absorbed and released is 5.0.
Since graphite having a density of × 10 ⁻³ Ω · cm or less is used, even when the battery is crushed and an internal short circuit occurs, the generation of Joule heat can be suppressed by the low volume resistance of the graphite.

According to a second aspect of the present invention, there is provided a positive electrode plate using a lithium-containing transition metal composite oxide in a lithium ion secondary battery, and a negative electrode using graphite capable of inserting and extracting lithium ions. And a volume resistivity of graphite of 5.0 × 10 ⁻³ Ω · cm or less.

According to a third aspect of the present invention, there is provided the lithium ion secondary battery according to the second aspect, wherein the filling density of graphite in the negative electrode plate is 1.2 to 2.0 g / cc. .

Next, a description will be given of a lithium ion secondary battery accommodating an electrode plate group in which a positive electrode plate and a negative electrode plate are wound together with a separator in an embodiment of the present invention.

In the present invention, the volume resistivity of graphite is 5.0.
× desirably in a 10 ^-3 Ω · cm or less, graphite resistance is larger than this the battery is crushed broken separator, if the positive and negative electrode plates are in contact with the battery, a large current to the contact point Concentrates to generate large Joule heat, and the temperature of the battery may rise sharply locally.

The volume resistivity of an electrode plate using graphite varies depending on the density of graphite per electrode plate volume. Essentially, the energy density of a battery is determined by how much electrode material is packed into a limited volume in a battery container. Therefore, it is advantageous that the packing density is as large as possible. If the packing density is high, the volume resistivity is small, which is preferable.However, if the packing density is too large, the pores in the electrode plate are reduced, so that the electrolyte does not easily penetrate, and the migration resistance of lithium ions increases. The characteristics deteriorate.

Therefore, the packing density of graphite per negative electrode volume is desirably 2.0 g / cc or less. On the other hand, if the packing density is too low, the volume resistivity increases and the battery capacity decreases, so that the packing density is desirably 1.2 g / cc or more.

The non-aqueous electrolyte used in the lithium ion secondary battery of the present invention is prepared by dissolving an electrolyte in a non-aqueous solvent. As the non-aqueous solvent, an organic solvent generally used for a lithium ion secondary battery can be used alone or in combination of several kinds. For example, ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate ( B
Preferred are cyclic carbonates such as C), chain carbonates such as dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC), and aliphatic carboxylic acids such as methyl propionate and ethyl propionate. Particularly, a mixed system of a cyclic carbonate and a chain carbonate or a mixed system of a cyclic carbonate, a chain carbonate and an aliphatic carboxylic acid ester is more preferable.

Examples of the electrolyte include lithium perchlorate.
(LiClO_Four), Lithium hexafluorophosphate (LiP
F₆), Lithium borofluoride (LiBF_Four), Hexafluoride
Lithium (LiAsF)₆), Trifluoromethanesulfur
Lithium phonate (LiCF_ThreeSO _Three), Bistrifluoro
Lithium methylsulfonylimide [LiN (CF_ThreeS
O_Two)_Two] Alone or several kinds thereof
Can be used in combination, but especially hexafluorophosphoric acid
Lithium (LiPF₆) Are preferred.

The amount of the electrolyte dissolved in the non-aqueous solvent is 0.1.
2 mol / l to 2 mol / l, especially 0.5 mol / l
It is desirably 1.5 mol / l.

As the positive electrode active material used in the lithium ion secondary battery of the present invention, various lithium-containing transition metal oxides (for example, lithium manganese composite oxide such as LiMn ₂ O ₄ , lithium-containing nickel such as LiNiO ₂₎ Oxides, lithium-containing cobalt oxides such as LiCoO _{2 and the} like, manganese, nickel, those in which part of cobalt is replaced by other transition metals, or vanadium oxide containing lithium, etc., and chalcogen compounds (for example, Manganese dioxide, titanium disulfide, molybdenum disulfide, etc.). Among them, it is preferable to use a lithium-containing transition metal oxide.

For the positive electrode, artificial graphite, carbon black (eg, acetylene black) or nickel powder can be used as a conductive material.

On the other hand, as the graphite used for the negative electrode, those obtained by subjecting mesophase spherules obtained by firing mesophase pitch to high-temperature treatment and graphite materials such as artificial graphite and natural graphite are suitable.

[0024]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows the structure of a prototype cylindrical battery to demonstrate the effects of the present invention. The dimensions of this battery are 20 mm in diameter and 70 mm in total height.

Example 1 In FIG. 1, a positive electrode plate 1 was made of lithium carbonate (Li ₂ Co ₃ ) and tricobalt tetroxide (Co ₃
O ₄ ) and lithium cobalt oxide (LiCoO ₂ ) fired at 900 ° C. in air as an active material, and 3% by weight of acetylene black as a conductive agent, and then polytetrafluoride as a binder A mixture made into a paste by kneading a 7% by weight aqueous dispersion of ethylene resin is applied to both sides of a core material made of aluminum foil, dried and rolled, and then reduced to a size of 57 mm in width and 520 mm in length. It is cut out. The positive electrode lead piece 4 is spot-welded to the end.

The negative electrode plate 2 is made of artificial graphite (average particle size 25 μm).
m) as an active material, and 5% by weight of styrene butadiene rubber with respect to the active material as a binder, mixed, and then suspended in an aqueous solution of carboxymethylcellulose to form a paste-like mixture. Is coated and dried on both sides of the sample, and then rolled so that the packing density of carbon becomes 1.4 g / cc, and cut into a size of 0.2 mm in thickness, 59 mm in width and 550 mm in length. A negative electrode lead piece 5 is spot-welded to an end of the negative electrode plate 2.

The separator 3 was formed by cutting a porous film made of a polypropylene resin more widely than the positive electrode plate 1 and the negative electrode plate 2.

The whole of the positive electrode plate 1 and the negative electrode plate 2 was spirally wound with a separator interposed therebetween to form an electrode plate group.

Next, a lower insulating ring 6 is provided below the electrode plate group.
Battery case 7 with a diameter of 20 mm and a height of 70 mm
And the negative electrode lead piece 5 was spot-welded to the battery case 7. Further, an upper insulating ring 8 was mounted on the upper side of the electrode plate group, a groove was formed in the upper part of the battery case 7, and then a non-aqueous electrolyte was injected. Ethylene carbonate (EC) as electrolyte
And dimethyl carbonate (DMC) at a volume ratio of 1: 1 and 1 mol / l of lithium hexafluorophosphate (Li)
Was used to dissolve the PF _6). After the assembly sealing plate 9 in which the gasket was previously assembled and the positive electrode lead piece 4 were spot-welded, the assembly sealing plate 9 was mounted on the battery case 7 to obtain a battery A of the present example.

Example 2 A negative electrode material having an average particle size of 15
A battery was formed in the same manner as in (Example 1) except that artificial graphite having a thickness of μm was used, and was referred to as Battery B in Example 2.

Example 3 The density of the negative electrode material was 1.9 g / c.
A battery was formed in the same manner as in (Example 1) except that c was used, and battery C in Example 3 was obtained.

Example 4 The density of the negative electrode material was 1.2 g / c.
A battery was formed in the same manner as in (Example 1) except that c was used, and battery D in Example 4 was obtained.

(Comparative Example 1) A negative electrode material having an average particle size of 6 μm
A battery was constructed in the same manner as in Example 1 except that m artificial graphite was used.

(Comparative Example 2) A negative electrode material having an average particle diameter of 20
A battery was formed in the same manner as in (Example 1) except that natural graphite having a thickness of μm was used, and the battery was designated as Battery F in Comparative Example 2.

(Comparative Example 3) The average particle size of the negative electrode material was 7 μm.
A battery was constructed in the same manner as in (Example 1) except that m natural graphite was used.

(Comparative Example 4) The density of the negative electrode material was 2.1 g / c.
A battery was formed in the same manner as in (Example 1) except that c was used, and the battery was designated as Battery H in Comparative Example 4.

(Comparative Example 5) The density of the negative electrode material was set to 1.0 g / c.
A negative electrode plate was prepared in the same manner as in (Example 1) except that c was used. However, the amount of graphite was reduced in this negative electrode plate to reduce the density of graphite, and a sufficient initial capacity was obtained. Did not.

The graphites of Examples 1 to 4 and Comparative Examples 1 to 5 were measured for the volume resistivity using a four-terminal powder resistance measuring apparatus (MCP-PD41, Mitsubishi Chemical Corporation) according to JIS-K7.
194 was measured. The pressure applied to the powder at the time of measuring the resistance of the powder was adjusted so that each negative electrode material had a density constituting a negative electrode plate.

Batteries A, B, C, and D according to the present invention and batteries E, F, G, and H as comparative examples were prepared by 50 cells each.
The battery was subjected to constant voltage and constant current charging at 20 ° C. with a charging voltage of 4.2 V, a charging time of 2 hours, and a limiting current of 800 mA, and a crush test of the battery was performed.

In the crush test of the battery, a cylindrical round bar having a diameter of 6 mm was used, and the round bar was pressed against the center of the battery from a direction perpendicular to the direction in which the outer dimension of the battery became the longest. Was crushed until the thickness became half. The high rate characteristics were evaluated by taking the ratio of the discharge capacity at a low current (200 mA) to the discharge capacity at a high current (2000 mA).
(Table 1) shows the firing rate of the battery by the crush test, the volume resistivity of graphite, the density of the negative electrode carbon material, and the capacity ratio at low current and high current (capacity at high current / capacity at low current). Is shown for each experimental example.

[0041]

[Table 1]

According to Table 1, the batteries A, B, and
When the firing rates of C and D are compared with those of the batteries E, F, G and H of the comparative example, the effect of the battery according to the present invention is clear. Batteries A, B, C, and D using graphite having a volume resistivity of 5 × 10 ⁻³ Ω · cm or less did not show a sharp rise in battery temperature even when a battery crush test was performed. On the other hand, the battery H of Comparative Example 4 did not have a sharp rise in temperature, but had a graphite density of 2.0 or more, so the high-rate characteristics of the battery were poor, and it was difficult to withstand practical use.

[0043]

As described above, the lithium ion secondary battery of the present invention is formed using graphite having a volume resistivity of 5 × 10 ⁻³ Ω · cm or less. However, heat generation due to Joule heat is small, and a rapid temperature rise of the battery can be prevented.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a lithium ion secondary battery according to one embodiment of the present invention.

[Explanation of symbols]

 Reference Signs List 1 positive electrode plate 2 negative electrode plate 3 separator 4 positive electrode lead piece 5 negative electrode lead piece 6 lower insulating ring 7 battery case 8 upper insulating ring 9 assembly sealing plate

 ──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Hajime Nishino 1006 Kazuma Kadoma, Osaka Prefecture Inside Matsushita Electric Industrial Co., Ltd.

Claims (3)

[Claims] 1. A negative electrode for a lithium ion secondary battery containing graphite whose volume resistivity capable of inserting and extracting lithium ions is 5.0 × 10 ⁻³ Ω · cm or less. 2. A positive electrode plate using a lithium-containing transition metal composite oxide, and a negative electrode plate using graphite capable of inserting and extracting lithium ions, wherein the graphite has a volume resistivity of 5. A lithium ion secondary battery having a resistance of 0 × 10 ⁻³ Ω · cm or less. 3. The graphite packing density of the negative electrode plate is 1.2.
The lithium ion secondary battery according to claim 2, wherein the weight is 2.0 g / cc to 2.0 g / cc.