JPH0935752A - Lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium secondary battery with high capacity and less self discharge by having a positive electrode, a negative electrode containing a specific carbonaceous material, and a lithium ion conductive electrolyte. SOLUTION: A carbonaceous material capable of absorbing/releasing a lithium ion comprising meso-phase pitch carbon fibers having an average fiber diameter of 2-40μm, an average length of 10-100μm, and a specific surface area of 0.5-1.5m<2> is obtained in such a way that a raw material such as petroleum pitch, coal tar, heavy oil, organic resin, and a synthetic polymer material is spun, obtained fibers are carbonized in an inert gas atmosphere at 800-1000 deg.C, and if necessary graphitized in an inert gas atmosphere at 1000-3200 deg.C. The carbonaceous material and a binder are kneaded, molded in a pellet, then impregnated with a lithium ion to obtain a negative electrode 5. A pellet-like positive electrode 3 is housed in a positive can 1 through a positive current collector 2, the pellet-like negative electrode 5 is put on the positive electrode 3 through a separator 4, a negative current collector 6 is arranged on the positive electrode 3, then a negative can 8 is fit to the opening of the positive can 1 through an insulating gasket 8.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a lithium secondary battery having an improved negative electrode containing a carbonaceous material.

[0002]

2. Description of the Related Art In recent years, with the development of electronic equipment, there has been a demand for the development of a secondary battery that is small, lightweight, has a high energy density, and can be repeatedly charged and discharged. As such a secondary battery, a lithium secondary battery including a negative electrode using lithium or a lithium alloy as an active material and a positive electrode using an oxide, sulfide, or selenide such as molybdenum, vanadium, titanium, or niobium as an active material is used. Secondary batteries are known.

However, a secondary battery provided with a negative electrode using lithium or a lithium alloy as an active material has a problem of short charge / discharge cycle life because dendrite of lithium is generated in the negative electrode when the charge / discharge cycle is repeated. .

From the above, a lithium secondary battery using a carbonaceous material which absorbs and releases lithium ions such as coke, graphite, carbon fiber, a resin fired body and pyrolysis vapor phase carbon for the negative electrode is proposed. Has been done. Since the lithium secondary battery can improve the deterioration of the negative electrode characteristics due to the dendrite deposition, the battery life and safety can be improved. In particular, a positive electrode containing a lithium manganese composite oxide whose main raw material is a lithium salt and manganese dioxide,
The lithium secondary battery including the negative electrode containing the carbonaceous material has been attracting attention because it has a high operating voltage and can significantly improve the charge / discharge cycle life.

However, the lithium secondary battery having the negative electrode containing the carbonaceous material has a problem that self-discharge is more likely to occur than the secondary battery having the negative electrode using lithium or a lithium alloy as an active material. It was

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to provide a lithium secondary battery having a high capacity and suppressing self-discharge by improving the carbonaceous material of the negative electrode.

[0007]

The lithium secondary battery of the present invention comprises a positive electrode, a negative electrode containing a carbonaceous material that absorbs and releases lithium ions, and a lithium ion conductive electrolyte. In, the carbonaceous material has an average fiber diameter of 2 μm to 40 μm and an average length of 10 μm to 1
In 00μm, specific surface area of 0.5m ² /g~1.5m ² /
g of the mesophase pitch carbon fiber. The specific surface area of carbon fiber is
B. by nitrogen adsorption E. FIG. T. It is measured by the one-point method.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION A lithium secondary battery (for example, a coin type lithium secondary battery) according to the present invention will be described below with reference to FIG. For example, a positive electrode 3 in the form of a pellet is accommodated in a positive electrode can 1 made of stainless steel via a positive electrode current collector 2. The separator 4 is disposed on the positive electrode 3. The pellet-shaped negative electrode 5 is arranged on the separator 4. A negative electrode current collector 6 (for example, nickel expanded metal or the like) is arranged on the negative electrode 5. A negative electrode can 7 made of, for example, stainless steel is attached to the opening of the positive electrode can 1 via an insulating packing 8. The lithium ion conductive electrolytic solution is used for the positive electrode 3,
The negative electrode 5 and the separator 4 are impregnated.

Next, the positive electrode 3, the negative electrode 5, the separator 4, and the lithium ion conductive electrolytic solution will be described. 1) Pellet-shaped positive electrode 3 The positive electrode 3 is produced by, for example, kneading an active material, a conductive material, and a binder, and forming the mixture into a pellet by pressure molding.

As the active material, various oxides (for example, lithium-manganese composite oxide, manganese dioxide, lithium-containing nickel oxide, lithium-containing cobalt oxide, lithium-containing nickel-cobalt oxide, and amorphous containing lithium) are used. Vanadium pentoxide), chalcogen compounds (eg, titanium disulfide, molybdenum disulfide, etc.) and the like. In particular, it is preferable to use a lithium manganese composite oxide. Among such lithium manganese composite oxides, the composition formula is Li _x MnO _y (however, x
And y are atomic ratios, 0.05 ≦ x ≦ 0.35, 1.8 ≦
It is preferable to use those represented by (y ≦ 2.0). The secondary battery including the positive electrode containing the lithium-manganese composite oxide having such a composition can improve the discharge capacity.

Examples of the conductive material include artificial graphite, carbon black (for example, acetylene black, etc.), nickel powder and the like. Examples of the binder include polytetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, polymethacrylic acid, polyacrylate, polymethacrylate, polyacrylate, polymethacrylate, and acrylic. Copolymer of either acid or methacrylic acid and either acrylic acid ester or methacrylic acid ester, copolymer of any one of acrylic acid ester or methacrylic acid ester and other monomer, etc. Can be mentioned. 2) Pelletized Negative Electrode 5 The negative electrode 5 is prepared by, for example, kneading a carbonaceous material that absorbs and releases lithium ions and a binder, pressurizing the mixture into pellets, and then electrolytically impregnating the lithium ions. It is produced by containing. The carbonaceous material has an average fiber diameter of 2 μm to 40 μm and an average length of 1
0 μm to 100 μm with a specific surface area of 0.5 m ² / g
It is composed of mesophase pitch carbon fiber of 1.5 m ² / g.

The mesophase pitch carbon fiber is a fiber produced from petroleum pitch, coal tar, heavy oil, etc. as a raw material and is fired in an inert gas stream or in the atmosphere,
It is a carbonaceous fiber produced by carbonization.
Such a carbon fiber can be produced, for example, by the methods shown in (1) and (2) below.

(1) Petroleum pitch, coal tar, heavy oil, organic resin, synthetic polymer material or the like is used as a raw material, and this is used in an inert gas such as nitrogen gas or argon gas.
After carbonization at a temperature of 00 ° C. to 1000 ° C. under normal pressure or under pressure, if necessary, 100 in an inert gas.
A carbon fiber is produced by graphitizing at a temperature of 0 ° C. to 3200 ° C. under normal pressure or under pressure.

(2) Melting a mesophase pitch-based raw material and then spinning the resulting fiber to infusibilize it, and 2,000 ° C. in an inert gas such as nitrogen gas or argon gas.
Below, more preferably at a temperature of 600 ℃ ~ 1500 ℃,
After carbonization under atmospheric pressure or pressure, if necessary, carbonization is carried out in an inert gas at a temperature of 1000 ° C to 3200 ° C, more preferably 2500 ° C to 3200 ° C, and graphitized under atmospheric pressure or pressure. Make fibers.

Among the methods (1) and (2), it is preferable to use the carbon fiber produced by the method (2). The reason for limiting the average fiber diameter of the carbon fibers to the above range is as follows. When the average fiber diameter is less than 2 μm, the carbon fibers easily pass through the pores of the separator, which causes a short circuit with the positive electrode. On the other hand, when the average fiber diameter exceeds 40 μm, the specific surface area of the carbon fibers is reduced and the amount of occlusion / release of lithium ions is reduced.
A more preferable average fiber diameter is 4 μm to 20 μm.

The reason why the average length of the carbon fiber is limited to the above range is as follows. When the average length is less than 10 μm, the carbon fibers easily pass through the pores of the separator, which causes a short circuit with the positive electrode. on the other hand,
If the average length exceeds 100 μm, the specific surface area of the carbon fibers is reduced, and the amount of lithium ions stored and released is reduced. A more preferable average length is 20 μm to 60 μm.

The specific surface area of the carbon fiber is limited to the above range for the following reason. When the specific surface area is less than 0.5 m ² / g, the amount of lithium ions stored and released decreases. On the other hand, the specific surface area is 1.5 m
^When it exceeds ² / g, a side reaction occurs on the surface of the negative electrode, which causes a significant decrease in discharge capacity due to self-discharge. More preferred specific surface area of 0.7m ² /g~1.2m ² / g.

The carbon fibers may have regularity or disorder in the orientation of the graphite crystals, or may have a regularity portion and a disordered portion.
As the orientation of the graphite crystal in the longitudinal section of the carbon fiber, for example,
Radial orientation, radial orientation in the vicinity of the surface and disordered orientation in the interior, strip orientation, lamella orientation and the like can be mentioned.

As the carbon fiber, the average value of the interplanar spacing d ₀₀₂ of the (002) plane of the graphite structure obtained by the X-ray diffraction method is 0.336 nm to 0.380 nm, and the carbon fiber is obtained by the X-ray diffraction method. It is preferable that the average value of the crystallite length Lc in the c-axis direction is 1 nm to 70 nm. The negative electrode containing the carbon fiber having the average value of the interplanar spacing d ₀₀₂ and the crystallite length Lc outside the above range has a decrease in the amount of lithium ions stored and released, deterioration of the graphite structure, and a gas caused by the reductive decomposition of the non-aqueous electrolyte. This may cause the generation of the secondary battery and reduce the capacity of the secondary battery.

As the binder, the same binder as the above-mentioned positive electrode 3 can be used. 3) Separator 4 Examples of the separator 4 include a polyolefin fiber nonwoven fabric and a polyolefin fiber porous membrane. Examples of the polyolefin fiber include a polypropylene fiber and a polyethylene fiber. 4) Lithium ion conductive electrolytic solution This electrolytic solution is prepared by dissolving an electrolyte in a non-aqueous solvent.

As the non-aqueous solvent, ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, γ-butyrolactone, sulfolane, acetonitrile, 1,2-dimethoxymethane,
There may be mentioned one or more solvents selected from 1,3-dimethoxypropane, dimethyl ether, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl carbonate, diethyl carbonate and ethylmethyl carbonate. In particular, it is preferable to use a non-aqueous solvent prepared by adding propylene carbonate to a mixed solvent of ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, and ethylene carbonate. A secondary battery provided with a non-aqueous electrolytic solution containing such a non-aqueous solvent is excellent in high discharge capacity during continuous charging in a high temperature environment, during use in a low temperature environment, and even during storage in a high temperature environment. It is possible to maintain excellent battery characteristics.

Examples of the electrolyte include lithium perchlorate (LiClO ₄ ), lithium hexafluorophosphate (L
iPF _6), lithium borofluoride (LiBF _4), lithium hexafluoroarsenate (LiAsF _6), lithium salts such as lithium trifluoromethane sulfonate (LiCF ₃ SO ₃₎ may be mentioned.

The amount of the electrolyte dissolved in the non-aqueous solvent is preferably 0.5 mol / l to 1.5 mol / l. Instead of using the lithium ion conductive electrolytic solution and the separator 4 in the secondary battery, a lithium ion conductive solid electrolyte that also serves as a separator can be used. As such a solid electrolyte, for example,
Examples thereof include a polymer solid electrolyte made of a polymer compound in which a lithium salt is complexed.

According to the lithium secondary battery of the present invention, the average fiber diameter is 2 μm to 40 μm and the average length is 10 μm to 1.
In 00μm, specific surface area of 0.5m ² /g~1.5m ² /
and a negative electrode containing mesophase pitch-based carbon fiber of g. The secondary battery including such a negative electrode can suppress the side reaction between the negative electrode and the lithium ion conductive electrolytic solution, so that self-discharge can be suppressed and the discharge capacity can be improved.

[0025]

Embodiments of the present invention will now be described in detail with reference to the drawings. Example 1 Lithium hydroxide and manganese dioxide were mixed so that the molar ratio of Li and Mn was 1: 3, and this mixture was heated at 400 ° C. for 20 hours to give Li _0.30 MnO.
A lithium manganese composite oxide represented by _1.89 was produced. The lithium manganese composite oxide, artificial graphite as a conductive material, and polytetrafluoroethylene as a binder are used so that the weight ratio of the active material, the conductive material and the binder is 90: 10: 5. After mixing and kneading, the mixture was molded into a pellet with a pressure press at a pressure of ² ton / cm ² to prepare a positive electrode having a diameter of 15 mm and a thickness of 0.80 mm.

On the other hand, a fiber produced by melting a mesophase pitch-based raw material and then spinning it was made infusible and carbonized in an inert gas at 650 ° C to 950 ° C under normal pressure. The obtained carbon fibers are finely pulverized and graphitized under an inert gas atmosphere at 2800 ° C. under normal pressure to have an average fiber diameter of 8 μm to 10 μm and an average fiber length of 20 μm.
.About.40 μm, B. E. FIG. T. A mesophase pitch carbon fiber having a specific surface area of 0.5 m ² / g measured by the one-point method was obtained. The obtained carbon fiber has an interplanar spacing d ₀₀₂ of the (002) plane of the graphite structure obtained by the X-ray diffraction method.
Was 0.338 nm. In addition, the average value of the crystallite length Lc in the c-axis direction obtained by the X-ray diffraction method was 30 nm. 95 parts by weight of such carbon fiber was mixed with 5 parts by weight of styrene-butadiene rubber as a binder and kneaded, and this was kneaded with a press machine at 3 ton / cm ^2.
The pressure was used to form a pellet having a diameter of 15 mm and a thickness of 0.96 mm.

1 volume% of propylene carbonate was added to a solvent in which ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate and ethylene carbonate were mixed at a volume ratio of 40: 30: 10: 20, and the non-aqueous solvent was used as an electrolyte. The concentration of lithium perchlorate is 1
A lithium ion conductive electrolytic solution was prepared by dissolving so as to have a mol / l.

The positive electrode was housed in a positive electrode can whose inner surface was coated with colloidal carbon as a positive electrode current collector. On the other hand, a current collector made of expanded metal made of nickel was welded to the inner surface of the negative electrode can to integrate the container and the insulating packing. A metal lithium plate having a diameter of 15 mm and a thickness of 0.19 mm was pressure-bonded to the current collector in the negative electrode can, and the prepared negative electrode pellet was placed thereon. A separator made of a polypropylene fiber non-woven fabric impregnated with the electrolytic solution was placed on the negative electrode pellet. By fitting the negative electrode can and the positive electrode can through the insulating packing, the coin-type lithium secondary battery having an outer diameter of 20 mm and a thickness of 2.5 mm shown in FIG. 1 was assembled.
The metallic lithium is stored in the negative electrode pellets as lithium ions by storage aging performed after assembly. Example 2 B. by nitrogen adsorption of mesophase pitch carbon fiber
E. FIG. T. The specific surface area measured by the one-point method is 1.0 m ² / g
1 having the same configuration as that of the first embodiment except that
A coin-type lithium secondary battery having the structure shown in was assembled. Example 3 B. by nitrogen adsorption of mesophase pitch carbon fiber
E. FIG. T. The specific surface area measured by the one-point method is 1.5 m ² / g
1 having the same configuration as that of the first embodiment except that
A coin-type lithium secondary battery having the structure shown in was assembled. Comparative Example 1 B. by nitrogen adsorption of mesophase pitch carbon fiber.
E. FIG. T. The specific surface area measured by the one-point method is 0.4 m ² / g
1 having the same configuration as that of the first embodiment except that
A coin-type lithium secondary battery having the structure shown in was assembled. Comparative Example 2 B. Deposition by Nitrogen Adsorption of Mesophase Pitch-Based Carbon Fiber
E. FIG. T. The specific surface area measured by the one-point method is 2.0 m ² / g
1 having the same configuration as that of the first embodiment except that
A coin-type lithium secondary battery having the structure shown in was assembled.

The secondary batteries of Examples 1 to 3 and Comparative Examples 1 and 2 thus obtained were subjected to storage aging at room temperature for 7 to 14 days to occlude lithium ions in the carbon fiber of the negative electrode. The battery open circuit voltage after aging was 3.2V.

The discharge capacities of the secondary batteries of Examples 1 to 3 and Comparative Examples 1 and 2 which had been subjected to storage aging when discharged at a constant current of 250 μA were measured, and the results are shown in Table 1 below. . After these secondary batteries were stored at a temperature of 60 ° C. for 20 days, they were discharged at a constant current of 250 μA to 2.0 V and the residual capacity was measured. The results are also shown in Table 1.

[0031]

[Table 1]

As is clear from Table 1, the secondary batteries of Examples 1 to 3 showed less decrease in discharge capacity after high temperature storage than the secondary batteries of Comparative Examples 1 and 2, and suppressed self-discharge. I know that I can do it.

In addition, although the example applied to the coin type lithium secondary battery has been described in the above embodiment, the present invention can also be applied to flat type, cylindrical type and prismatic type lithium secondary batteries. In addition to the pellet-shaped positive electrode and negative electrode described above, as the positive electrode, an active material, a conductive material and a binder are suspended in an appropriate solvent, and the suspension is applied to a current collector and dried. , Those produced by pressing can be used,
As the negative electrode, one prepared by suspending the carbon fiber and the binder in an appropriate solvent, applying the suspension to a current collector, drying and then pressing can be used.

[0034]

As described above in detail, according to the present invention, by using carbon fiber having a fiber diameter, a fiber length and a specific surface area within the above ranges for a negative electrode, a lithium battery having a high capacity and a small self-discharge can be obtained. A secondary battery can be provided.

[Brief description of drawings]

FIG. 1 is a vertical cross-sectional view showing a lithium secondary battery (for example, a coin-type lithium secondary battery) according to the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Positive electrode can, 3 ... Positive electrode pellet, 4 ... Separator, 5 ...
Negative electrode pellet, 7 ... Negative electrode can, 8 ... Insulating packing.

Claims (1)

[Claims] 1. A lithium secondary battery comprising a positive electrode, a negative electrode containing a carbonaceous material that absorbs and releases lithium ions, and a lithium ion conductive electrolyte, wherein the carbonaceous material has an average fiber diameter of 2 μm to 40 μm with an average length of 10
μm to 100 μm and specific surface area of 0.5 m ² / g to 1.
A lithium secondary battery comprising a mesophase pitch carbon fiber of 5 m ² / g.