JP2002075451A - Lithium secondary cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium secondary cell with small self-discharging ratio, excellent cycle property, and high charge-discharge efficiency. SOLUTION: The lithium secondary cell comprises a positive pole containing a conductive polymer enabled to occlude and release lithium, or a dopant containing conductive polymer obtained by making above conductive polymer contain a dopant as a main material; a negative pole containing graphite of which, the length of crystal grain Lc in the direction of c axis at X-ray diffraction is 150 Å or more (provided that the grain with the length of crystal grain Lc in the direction of c axis at X-ray diffraction of less than 300 Å is excluded) as a maim material; a separator installed between those positive and negative poles; and an electrolyte liquid with an electrolyte solute dissolved in a solvent. A mixed solvent of ethylene carbonate and a solvent with low boiling point (provided that dimethoxyethane is excluded) is used as a solvent of the above material.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium secondary battery, and more particularly to an improvement in an electrolyte for a lithium secondary battery using a carbon material containing graphite as a single component or a main component as a negative electrode material.

[0002]

2. Description of the Related Art In recent years,
As a negative electrode material of a lithium secondary battery, a carbon material has been studied as a negative electrode material that replaces a conventional lithium alloy because it has excellent flexibility and there is no possibility of depositing mossy lithium.

[0003] By the way, the carbon material which has been mainly studied in the past is coke, and graphite has been almost excluded from the scope of the study. However, in coke, the amount of lithium inserted is not large enough, so that it is difficult to obtain a large-capacity battery. To the knowledge of the present inventors, US Pat.
Only 4,423,125 can be mentioned.

The above-mentioned US Patent Publication discloses a solution in which a carbon material capable of absorbing lithium as an active material is used as a negative electrode material, and LiAsF ₆ as an electrolyte solute is dissolved in 1,3-dioxolane as a solvent in an electrolytic solution. Has been proposed, and the publication reports that a secondary battery having excellent cycle characteristics can be obtained.

[0005] However, in the embodiment described later,
As shown by the characteristics of a conventional battery, the conventional secondary battery has not only cycle characteristics (cycle life), but also a capacity per unit weight of graphite (mAh / g), an initial charge / discharge efficiency (%), and a battery capacity. (%), Self-discharge rate (% / month),
In many respects, such as charge / discharge efficiency (%), the characteristics were poor, and the secondary battery was not sufficiently satisfactory for practical use.

This is presumed to be due to polymerization of 1,3-dioxolane on the negative electrode side (reduction side).

The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide a graphite having a large battery capacity, a small self-discharge rate, excellent cycle characteristics, and high charge / discharge efficiency. An object of the present invention is to provide a lithium secondary battery used as a negative electrode material.

[0008]

In order to achieve the above object, in a lithium secondary battery according to the present invention, a conductive polymer capable of inserting and extracting lithium or a dopant containing a dopant in the conductive polymer is used. A positive electrode mainly composed of a conductive polymer and graphite having a crystallite size Lc of 150 ° or more in the c-axis direction in X-ray diffraction (however,
The crystallite size Lc in the c-axis direction in X-ray diffraction is 30
A negative electrode having a main material of less than 0 °), a separator interposed between the positive and negative electrodes, and an electrolytic solution in which an electrolyte solute is dissolved in a solvent, and ethylene carbonate as the solvent, A mixed solvent with a low boiling point solvent (excluding dimethoxyethane) was used.

In the lithium secondary battery according to the present invention, the conductive polymer as the material of the positive electrode or the dopant-containing conductive polymer obtained by adding a dopant to the conductive polymer may be a conductive polymer such as polyaniline. Also, a perfluorocarbon sulfonic acid (manufactured by DuPont,
Dopant-containing conductive polymers doped with trade name “Nafion”, lithium paratoluenesulfonate, lithium polyvinylsulfonate, lithium persulfate, porphyrin and porphyrin can be used.

[0010]

Embedded image

The material of the positive electrode is kneaded with a conductive agent such as acetylene black and carbon black and a binder such as polytetrafluoroethylene (PTFE) and polyvinylidene difluoride (PVdF) to form a positive electrode mixture. Used as Among the above-mentioned conductive polymers and dopant-containing conductive polymers, those having excellent conductivity may be kneaded with a binder without blending a conductive agent to form a positive electrode mixture.

In the lithium secondary battery of the present invention, as a material of the negative electrode, graphite having a crystallite size Lc of 150 ° or more in the c-axis direction in X-ray diffraction as described above (however, A carbon material having a single component or a main component (excluding those having a crystallite size Lc of less than 300 ° in the c-axis direction) is used.

The above graphite has an average particle size of 1%.
Graphite in the range of ３０30 μm, the d value (d ₀₀₂ ) of the lattice plane (002) plane in X-ray diffraction is 3.35 to 3.40
で, a specific surface area of 0.5 to 50 m ² /
g, graphite having a true density of 1.9 to 2.3 g / c.
It is preferable to use graphite within the range of m ³ , and further, when the size La of the crystallite in the a-axis direction in X-ray diffraction is 150 ° or more and the value of the atomic ratio of H / C is 0.1 or less. Yes, G value in Raman analysis (1360 cm ^-1 / 1
More preferably, graphite having a 590 cm ^-1 ) of 0.05 or more is used.

[0014] The graphite as described above may be natural graphite, artificial graphite, or quiche graphite. By the way, quiche graphite is used in smelting furnaces at steelworks.
When iron is melted at a temperature of 00 ° C. or more, carbon contained in iron sublimates and adheres to the furnace wall and is recrystallized, and has a crystallinity higher than that of natural graphite. It is a high carbon material. If necessary, a mixture of two or more of these graphites may be used. The artificial graphite referred to here includes graphite-based substances such as expanded graphite obtained by further processing and denaturing graphite.

Here, examples of the natural graphite include graphite from Sri Lanka, graphite from Madagascar, flake graphite from Korea, soil graphite from Korea, and graphite from China. Examples of artificial graphite include coke graphite. The lattice plane (002) of these natural graphite and artificial graphite in X-ray diffraction.
Table 1 shows the d value (d ₀₀₂ ) of the plane and the crystallite size Lc in the c-axis direction in X-ray diffraction.

[0016]

[Table 1]

The commercial products of the above natural graphite include:
"NG-2", "NG-2L", "N
G-4 "," NG-4L "," NG-7 "," NG-7 "
L "," NG-10 "," NG-10L "," NG-1 "
2 "," NG-12L "," NG-14 "," NG-1 "
4L "," NG-100 "," NG-100L "(high purity graphite having a purity of 99% or more);" CX-3000 "," FBF "," BF "," CB "manufactured by Chuetsu Graphite Co., Ltd.
R "," SSC-3000 "," SSC-600 ",
"SSC-3", "SSC", "CX-600", "C
PF-8 "," CPF-3 "," CPB-6S "," C
PB "," 96E "," 96L "," 96L-3 ",
"90L-3", "CPC", "S-87", "K-
3 "," CF-80 "," CF-48 "," CF-3 "
2 "," CP-150 "," CP-100 "," C
P "," HF-80 "," HF-48 "," HF-3 "
2 "," SC-120 "," SC-80 "," SC-6 "
0 "," SC-32 "(above, scaly graphite)," APF-
3000 "," APF "," AX-600 "," S-
3 "," AP-6 "," AP-3 "," 300F ",
"150F" (above, ground graphite); "CSSP", "CSPE", "CSP", "Special C" manufactured by Nippon Graphite Industry Co., Ltd.
P "," CP "," CP-B "," CB-150 ",
“CB-100”, “F $ 1”, “F $ 2”, “F $”
3 "," SF-A "," SF-B "(above, scaly graphite)," AOP "," AUP "," ASSP "," AS "
P "," AP "," Blue P "," APB "," PD ",
"CA.C", "P @ 1" (above, earth graphite), "AC
P-1000 "," ACP "," ACCB-150 ",
"SP-5", "SP-5L", "SP-10", "S
P-10L "," SP-20 "," SP-20L ",
"SCB + 100", "SP-300", "HOP"
(High purity graphite having a purity of 97.5% or more).

[0018] Commercially available products of the above artificial graphite include:
"RA-3000", "RA-15", manufactured by Chuetsu Graphite Co., Ltd.
"RA-44", "GX-600", "G-6S",
"G-3", "G-150", "G-100", "G-
48 "," G-30 "," G-50 ";" HAG-150 "," HAG-15 "," HAG "manufactured by Nippon Graphite Industry Co., Ltd.
-5 "," PAG-15 "," PAG-5 "," PAG
-80 "," PAG-60 "," SGS-100 ",
“SGS-50”, “SGS-25”, “SGS-1”
5 "," SGS-5 "," SGS-1 "," SGP-1 "
00 "," SGP-50 "," SGP-25 "," SG
P-15 "," SGP-5 "," SGP-1 "," SG
O-100 "," SGO-50 "," SGO-25 ",
"SGO-15", "SGO-5", "SGO-1",
"SGX-100", "SGX-50", "SGX-2
5 "," SGX-15 "," SGX-5 "," SGX-
In addition to "1", "QP-2", "QP-5", "QP-10",
"QP-20" is exemplified.

As a commercial product of artificial graphite obtained by further processing and denaturing natural graphite, natural graphite powder is used for pitch,
AOP-AOP manufactured by Nippon Graphite Industries Co., Ltd.
Pi5 "," AOP-B5 "," AOP-A5 "," A
OP-T1 "is exemplified.

As a commercial product of expanded graphite obtained by expanding the layers of natural graphite by acid treatment, “S” manufactured by Chuetsu Graphite Co., Ltd.
SLF "," SSMF "," SSFF "," SLF ",
"SMF", "SFF", "EMK", "ELF",
“EMF”, “EFF”, “CMF”; “EXP-SPM”, “EXP-12M”, “EX” manufactured by Nippon Graphite Industries, Ltd.
P-80M "and" EXP-SM ".

Examples of the graphite other than natural graphite and artificial graphite include the above-described quiche graphite commercially available from Kansai Thermochemical Co., Ltd., and may be used as the carbon material in the present invention.

The carbon material used in the present invention may be composed of only the above-described graphite, or may be composed of graphite as a main component and other carbon materials.

Examples of the carbon material containing graphite as a main component include a mixture of graphite and a carbon material such as carbon black having a crystallite size Lc of 8 ° or less in the c-axis direction in X-ray diffraction. , A mixture of graphite and pitch (both petroleum pitch and coal pitch can be used) having a Lc of 150 ° or more. In the latter production, in order to obtain a fired product having an Lc of 150 ° or more, it is necessary to fire at a temperature of 1000 ° C. or more, and usually fire at a temperature of 1000 to 3000 ° C.

By adding carbon black or pitch to graphite, the adhesion of graphite to a core (conductive substrate) can be improved. The preferred addition ratio of carbon black or pitch to graphite is in the range of 2 to 10 parts by weight of carbon black or pitch based on 100 parts by weight of graphite. This is because when the addition ratio of carbon black or pitch is less than 2 parts by weight, the above-mentioned effect of improving the adhesiveness is not sufficiently exhibited, while when the addition ratio exceeds 10 parts by weight, the energy density decreases. In addition, this leads to a reduction in the battery capacity, and neither is preferable.

The above-mentioned carbon material is kneaded with a binder such as polytetrafluoroethylene or polyvinylidene difluoride by a conventional method, and used as a negative electrode mixture.

In the present invention, as the graphite, the above-mentioned expanded graphite or the like is used in place of the powdered graphite, and sheet graphite obtained by heat-treating and pressure-forming the expanded graphite is used as the carbon material. Good.

In the lithium secondary battery according to the present invention, a mixed solvent of ethylene carbonate and a low boiling point solvent (excluding dimethoxyethane) is used as a solvent in the electrolytic solution as described above. . In addition, in this specification, a low boiling point solvent refers to a solvent having a boiling point of 150 ° C. or lower.

Here, the low boiling point solvent is 1,
Low-boiling ether solvents such as 2-diethoxyethane (DEE) and ethoxymethoxyethane (EME), dimethyl carbonate (DMC), and diethyl carbonate (DE
Ester-based low-boiling solvents such as C) can be used,
In particular, it is preferable to use an ester-based low boiling point solvent such as dimethyl carbonate (DMC) and diethyl carbonate (DEC).

When dimethyl carbonate is used as the low-boiling solvent, the conductivity of dimethyl carbonate is high, so that the high-rate discharge characteristics are particularly improved. When diethyl carbonate is used, the viscosity of diethyl carbonate at low temperatures is low, Because of the excellent conductivity, the low-temperature discharge characteristics are particularly improved.

When mixing the above-mentioned low-boiling solvent with ethylene carbonate, ethylene carbonate is mixed with ethylene carbonate.
It is preferable that the content is contained at a ratio of 0 to 80% by volume because the battery capacity at the time of high-rate discharge increases.

In the lithium secondary battery of the present invention, examples of the electrolyte solute dissolved in the above-mentioned mixed solvent include LiPF ₆ , LiBF ₄ , and LiClO 2.
₄ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiN (CF ₃
SO ₂ ) ₂ and LiAsF ₆ can be used. When dissolving such an electrolyte solute in the above-described solvent, the ratio of the electrolyte solute is preferably set to 0.1 to 3 mol / liter, more preferably,
0.5-1.5 mol / l.

Here, the charge / discharge cycle characteristics of a lithium secondary battery using graphite satisfying the conditions of the present invention as a negative electrode material and a lithium secondary battery using coke not satisfying the conditions of the present invention as a negative electrode material are described. Investigation and the results are shown in FIG. Here, FIG. 1 shows the potential (V) of the negative electrode with respect to the Li / Li + unipolar potential on the vertical axis and the capacity (mAh / g) per 1 g of the carbon material (graphite or coke) on the horizontal axis. It is a graph, and shows the charge and discharge cycle characteristics of a lithium secondary battery using graphite satisfying the conditions of the present invention as a negative electrode material by a solid line, and charging and discharging of a lithium secondary battery using coke not satisfying the conditions of the present invention as a negative electrode material. The cycle characteristics are shown by broken lines. The direction of the arrow in the figure indicates the direction of the rise and fall of the negative electrode potential during charging and discharging. Note that the illustrated charge / discharge cycle characteristics are data for both batteries when a mixed solvent of ethylene carbonate and dimethyl carbonate at a volume ratio of 1: 1 is used as an electrolyte solvent.

First, a charge / discharge cycle of a lithium secondary battery using coke which does not satisfy the conditions of the present invention as a negative electrode material will be described with reference to FIG. 3 (V) before initial charging
The potential (point a) of the negative electrode, which was about
As Li is occluded in the coke, the potential approaches the Li / Li ⁺ unipolar potential (the potential of the negative electrode on the vertical axis indicates this potential as a reference (0 V)). When charging is completed, point b (negative electrode potential: 0 V, capacity) : About 300 mAh / g). In addition, the coke at the point b exhibits brown to red. Then
When the first discharge is performed, the potential of the negative electrode increases as the discharge proceeds, and reaches a point c (capacity: 50 to 100 mAh / g) indicating the discharge termination potential (about 1 V). At the time of the first discharge, the point corresponding to the capacity indicated by P in the figure is that Li is captured by coke because it reaches the point c in a hysteresis without returning to the route followed during the initial charging. This is because in the subsequent electrode reaction in charge and discharge, only the amount of Li corresponding to the capacity indicated by Q in the figure can participate in the reaction. By repeating the subsequent charge / discharge cycle, the potential of the negative electrode fluctuates in a cycle such as c → b → c → b.

Next, a charge / discharge cycle in a lithium secondary battery using graphite satisfying the conditions of the present invention as a negative electrode material will be described. Before the initial charging, the potential (point a) of the negative electrode was about 3 (V), as in the lithium secondary battery using coke as the negative electrode material. / Li + unipolar potential, and at the completion of charging, reaches a point d (capacity: 375 mAh / g) where the potential with respect to the Li / Li + monopolar potential is 0 V. Note that the graphite at the point d has a golden color, which confirms that C ₆ Li was generated from X-ray diffraction. Next, when the first discharge is performed, the potential of the negative electrode increases as the discharge proceeds, and reaches a point e (capacity: 25 mAh / g) indicating a discharge end potential (about 1 V). By repeating the subsequent charge / discharge cycle, the potential of the negative electrode fluctuates in a cycle such as e → d → e → d.

Based on the charge / discharge cycle characteristics shown in FIG.
Comparing the battery characteristics of a lithium secondary battery using graphite as a negative electrode material and a lithium secondary battery using coke as a negative electrode material, the charge per gram of graphite at the time of initial charging in a lithium secondary battery using graphite as a negative electrode material Capacity is 375mA
The charging capacity per gram of coke at the time of initial charging in a lithium secondary battery using coke as a negative electrode material is as small as about 300 mAh / g (point b), whereas it is as large as about h / g (point d). A lithium secondary battery using graphite as a negative electrode material has a large capacity per gram of graphite up to a discharge termination potential of 1 V of about 350 mAh / g (de), whereas a lithium secondary battery using coke as a negative electrode material has a large capacity. The capacity per g of coke up to the discharge end voltage 1V is 20
It is as small as about 0 to 250 mAh / g (bc).

This means that the charge / discharge efficiency of a lithium secondary battery using graphite satisfying the conditions of the present invention as a negative electrode material is higher than that of a lithium secondary battery using coke not satisfying the conditions of the present invention as a negative electrode material. Means that.

The charge / discharge curve of a lithium secondary battery using graphite as a negative electrode material is as follows during discharge from point d to point e.
While the negative electrode potential rises sharply when approaching point e almost flat, the charge / discharge curve of the lithium secondary battery using coke as the negative electrode material gradually increases from point b to point c. I have.

This means that the lithium secondary battery using graphite satisfying the conditions of the present invention as a negative electrode material has a flatter discharge voltage than the lithium secondary battery using coke not satisfying the conditions of the present invention as a negative electrode material. It means that it is also excellent in terms of sex.

The charge / discharge efficiency of the lithium secondary battery using graphite satisfying the conditions of the present invention as a negative electrode material is lower than that of the lithium secondary battery using coke not satisfying the conditions of the present invention as a negative electrode material as described above. Is high, and that the discharge voltage is flat means that the discharge capacity of a lithium secondary battery using graphite as a negative electrode material satisfying the conditions of the present invention is a coke that does not satisfy the conditions of the present invention as a negative electrode material. This means that it is larger than a lithium secondary battery.

[0040]

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to the following Examples, and may be carried out by appropriately changing the scope of the present invention. Is possible.

(Reference Example 1) [Preparation of Positive Electrode] Cobalt carbonate and lithium carbonate were mixed with Co:
After mixing at an atomic ratio of 1: 1 Li, 900 ° C in air
For 20 hours to obtain LiCoO ₂ .

LiC as the cathode material thus obtained was used.
oO ₂ was mixed with acetylene black as a conductive agent and a fluororesin dispersion as a binder in a weight ratio of 9;
The mixture was mixed at a ratio of 0: 6: 4 to obtain a positive electrode mixture. And
This positive electrode mixture was rolled into an aluminum foil as a current collector, and heat-treated under vacuum at 250 ° C. for 2 hours to produce a positive electrode.

[Preparation of Negative Electrode] A fluororesin dispersion as a binder was added to each of natural graphite, artificial graphite and Lonza graphite produced in China as negative electrode materials of 400 mesh pass in a weight ratio of 95: 5. The mixture was mixed to obtain a negative electrode mixture. Then, these negative electrode mixtures were rolled into aluminum foils as current collectors, respectively, and heat-treated under vacuum at 250 ° C. for 2 hours to prepare negative electrodes mainly composed of carbon materials. Here, the particle size of graphite used for the negative electrode is 40
A 0 mesh pass is preferable, and 2 to 14 μm is preferable.

The size Lc of the crystallite in the c-axis direction in the X-ray diffraction of the natural graphite, artificial graphite and Lonza graphite produced in China was 300 ° or more.

Here, X-ray diffraction was carried out under the following measurement conditions (the same X-ray diffraction was used under the same measurement conditions). Source: CuKα Slit: Divergent slit 1 °, Scattering slit 1 °, Receiving slit 0.3mm Goniometer radius: 180mm Graphite curved crystal monochromator

[Preparation of Electrolyte Solution] A mixed solvent of ethylene carbonate and dimethyl carbonate having a volume ratio of 1: 1 was added to
LiPF ₆ was dissolved at 1 mol / liter to prepare an electrolytic solution. The mixing ratio is preferably in the range of 0.001: 1-1: 0.01.

[Preparation of Reference Batteries BA1 to BA3] A cylindrical nonaqueous electrolyte secondary battery was prepared using the positive and negative electrodes and the electrolyte described above. Reference battery BA1 using natural graphite as a carbon material, reference battery BA using artificial graphite
2. Use Lonza graphite as reference battery BA3
Expressed by In addition, as the separator, ion-permeable polypropylene (trade name “Duraguard” manufactured by Daicel Corporation) was used.

FIG. 2 is a cross-sectional view of the prepared reference batteries BA1 to BA3. The reference batteries BA1 to BA3 have a positive electrode 1 and a negative electrode 2, a separator 3 separating these electrodes, a positive electrode lead 4, and a negative electrode lead 5. , A positive electrode external terminal 6, a negative electrode can 7, and the like. The positive electrode 1 and the negative electrode 2 are housed in a negative electrode can 7 while being spirally wound through a separator 3 into which an electrolytic solution has been injected. The positive electrode 1 is connected to a positive electrode external terminal 6 through a positive electrode lead 4. The negative electrode 2 is connected to a negative electrode can 7 via a negative electrode lead 5, so that chemical energy generated inside the battery BA1 can be taken out as electric energy.

Comparative Example 1 As a negative electrode material, Lc was 26.
A comparative battery BC1 was produced in the same manner as in Reference Example 1 except that coke was used.

(Charge / Discharge Characteristics of Each Battery) FIG. 3 shows the 250 mA of the reference batteries BA1 to BA3 and the comparative battery BC1.
FIG. 4 is a graph showing the charge / discharge characteristics after the second cycle in (constant current discharge), with the vertical axis representing voltage (V) and the horizontal axis representing time (h). The charge and discharge characteristics of the battery BA1 and the reference battery BA2 were compared with those of the comparative battery B.
It is a comparison with the charge / discharge characteristics of C1.
5 is a graph showing the potential (V) of the negative electrode with respect to the Li + monopolar potential, with the charge and discharge capacity (mAh / g) on the horizontal axis.

From these figures, it can be seen that the reference batteries BA1 to BA3 using Chinese natural graphite, artificial graphite and Lonza graphite as the negative electrode material had better charge / discharge than the comparative battery BC1 using coke as the negative electrode material. It is understood to have properties.

FIG. 6 shows the above reference batteries BA1, BA
2 is a graph showing the cycle characteristics of Comparative Battery No. 2 and Comparative Battery BC1, the discharge capacity (mAh / g) on the vertical axis, and the number of cycles on the horizontal axis. From the figure, reference battery cells BA1 and BA2 using Chinese natural graphite and artificial graphite as the negative electrode material
Compared to the comparative battery BC1 using coke as the negative electrode material,
It can be seen that excellent cycle characteristics are exhibited.

The above reference batteries BA1 to BA3 and comparative battery BC1 were each charged, stored at room temperature for one month, and measured for storage characteristics. As a result, the self-discharge rate is 2 to 5% / in the above reference batteries BA1 to BA3.
And 15% / month for the comparative battery BC1.

(Reference Example 2) As a negative electrode material, a carbon material obtained by calcining a mixture obtained by adding 5 parts by weight of pitch to 100 parts by weight of natural graphite produced in China at a temperature of 1000 ° C. was used. Otherwise, the procedure of Example 1 was repeated to fabricate a reference battery BA4. Here, the carbon material obtained as described above had a crystallite size Lc of 150 ° in the c-axis direction in X-ray diffraction.

FIG. 7 is a graph showing the cycle characteristics of the reference battery BA4, the vertical axis representing the discharge capacity (mAh / g) of the battery, and the horizontal axis representing the number of cycles. FIG.
For comparison, the cycle characteristics of the reference battery BA1 using only the above-described natural graphite as a negative electrode material and the comparative battery BC1 using coke are also shown for comparison.

As can be seen from the figure, the reference battery BA4 exhibits superior cycle characteristics because the carbon material is less likely to fall off from the electrode than the comparative battery BC1 using coke and the reference battery BA1 using only natural graphite. You can see that

(Reference Example 3) As a negative electrode material, 5 parts by weight of carbon black having a crystallite size Lc of 8 ° in the c-axis direction in X-ray diffraction was added to 100 parts by weight of the above natural graphite produced in China. A reference battery BA5 was produced in the same manner as in Reference Example 1 except that the mixture was used.

FIG. 8 is a graph showing the cycle characteristics of the reference battery BA5, the vertical axis representing the discharge capacity (mAh / g) of the battery, and the horizontal axis representing the number of cycles. FIG.
For comparison, the cycle characteristics of the reference battery BA1 using only the above-described natural graphite as a negative electrode material and the comparative battery BC1 using coke are also shown for comparison.

As can be seen from the figure, the reference battery BA5 exhibits superior cycle characteristics because the carbon material is less likely to fall off the electrode, compared to the reference battery BC1 using coke and the reference battery BA1 using only natural graphite. You can see that

Reference Example 4 As a solvent for the electrolytic solution, ethylene carbonate (EC) and dimethyl carbonate (DM)
Reference battery B instead of the mixed solvent having a volume ratio of 1: 1 with C)
In A6, a mixed solvent of ethylene carbonate and diethyl carbonate (DEC) at a volume ratio of 1: 1 was mixed with a comparative battery B.
In C2, a mixed solvent of ethylene carbonate and dipropyl carbonate (DPC) having a volume ratio of 1: 1 was used, and in a conventional battery, 1,3-dioxolane (1,3-DOL) was used.
Other than that, the reference battery BA
6. A comparative battery BC2 and a conventional battery were produced.

FIG. 9 is a graph showing the charge / discharge characteristics of these batteries, with the vertical axis representing the negative electrode potential (V) and the horizontal axis representing the charge / discharge capacity.

It can be seen from the figure that the reference battery BA6 exhibits superior charge / discharge characteristics as compared with the comparative battery BC2 and the conventional battery, similarly to the above-described reference battery BA1.

Reference Example 5 The lattice plane (0
Negative electrodes were manufactured using 13 types of carbon materials having different d values (d ₀₀₂ ) on the 02) plane, and 13 types of batteries were manufactured in the same manner as in Reference Example 1 except for the above.

FIG. 10 is a graph showing the relationship between the d ₀₀₂ value of the carbon material and the discharge capacity of the battery. The vertical axis indicates the discharge capacity (mAh / g) of the battery, and the d _{002 of} the carbon material used.
₀₀₂ value (Å) is shown on the horizontal axis.

From the figure, d ₀₀₂ is 3.35 to 3.40 °.
It can be seen that a battery using graphite as a carbon material has a large discharge capacity.

Reference Example 6 A negative electrode was manufactured using 12 kinds of carbon materials having different true densities.
In the same manner as described above, 12 types of batteries were produced.

FIG. 11 is a graph showing the relationship between the true density of the carbon material and the discharge capacity of the battery.
Ah / g) is shown on the vertical axis, and the true density (g / cm ³ ) of the carbon material used is shown on the horizontal axis.

From the figure, it can be seen that the true density is 1.9 to 2.3 g / c.
battery using graphite as m ³ as the carbon material is found to have a large discharge capacity.

(Reference Example 7) A negative electrode was manufactured using nine kinds of carbon materials having different average particle diameters.
In the same manner as in the above, nine types of batteries were produced.

FIG. 12 is a graph showing the relationship between the average particle size of the carbon material and the discharge capacity of the battery. The vertical axis indicates the discharge capacity (mAh / g) of the battery, and the average particle size of the carbon material used. (Μm) is shown on the horizontal axis.

From the figure, it can be seen that a battery using graphite having an average particle size of 1 to 30 μm as a carbon material has a large discharge capacity.

Reference Example 8 Negative electrodes were produced using 13 kinds of carbon materials having different specific surface areas, and 13 kinds of batteries were produced in the same manner as in Reference Example 1 except for the above.

FIG. 13 is a graph showing the relationship between the specific surface area of the carbon material and the discharge capacity of the battery. The vertical axis indicates the discharge capacity (mAh / g) of the battery, and the specific surface area (m ² / g) on the horizontal axis.

As shown in the figure, the specific surface area is 0.5 to 50 m ² /
It can be seen that a battery using graphite as a carbon material has a large discharge capacity.

Reference Example 9 A negative electrode was manufactured using eleven kinds of carbon materials having different crystallite sizes Lc in the c-axis direction in X-ray diffraction. Eleven types of batteries were produced.

FIG. 14 is a graph showing the relationship between Lc of the carbon material and the discharge capacity of the battery, and shows the discharge capacity (mA) of the battery.
h / g) on the vertical axis, and Lc (Å) of the carbon material used.
Is a graph showing the horizontal axis.

From the figure, it can be seen that Lc is 150 ° or more, especially 30 °.
It can be seen that a battery using graphite of 0 ° or more as a negative electrode material has a large discharge capacity.

Reference Example 10 As the electrolyte, an electrolyte in which LiPF ₆ was dissolved at a ratio of 1 mol / liter in a mixed solvent shown in Tables 4 and 5 was used. In the same manner as described above, 21 types of batteries were produced.

Then, each battery in Reference Example 10 was discharged at 100 mA to obtain graphite characteristics [capacity per unit weight (mAh / g) and initial charge / discharge efficiency (%)] and battery characteristics [battery capacity (mAh)]. , Self-discharge rate (% / month), cycle life (times), charge / discharge efficiency (%)], and the results are shown in Tables 2 and 3.

[0080]

[Table 2]

[0081]

[Table 3]

(Reference Example 11) As the electrolytic solution, an electrolytic solution in which LiPF ₆ was dissolved at a ratio of 1 mol / liter in the mixed solvents shown in Tables 4 and 5 was used. In the same manner as described above, 21 types of batteries were produced.
The batteries of Reference Example 11 were also discharged at 100 mA as described above, and the same items as those of Reference Example 10 were measured. The results are shown in Tables 4 and 5.

[0083]

[Table 4]

[0084]

[Table 5]

Here, as is clear from the results of Tables 2 to 5, when a mixed solvent of a low boiling point solvent composed of dimethyl carbonate or diethyl carbonate and ethylene carbonate is used, the above graphite characteristics and Battery characteristics were improved.

REFERENCE EXAMPLE 12 As shown in Tables 6 and 7, as an electrolyte, dimethyl carbonate, diethyl carbonate, 1,2-diethoxyethane, ethoxymethoxyethane, which are solvents having a low boiling point, relative to ethylene carbonate were used. Are mixed with each other in a volume ratio of 1: 1 to use an electrolyte solution in which LiPF ₆ is dissolved at a ratio of 1 mol / liter, and otherwise the same as in Reference Example 1 above. Seed batteries were made. And, as for each battery in Reference Example 12, 100
After discharging at mA, the same items as those in Reference Example 10 were measured, and the results are shown in Tables 6 and 7.

[0087]

[Table 6]

[0088]

[Table 7]

As a result, when a mixed solvent in which dimethyl carbonate, diethyl carbonate, 1,2-diethoxyethane and ethoxymethoxyethane, which are low-boiling solvents, were mixed with ethylene carbonate was used, the above graphite characteristics and Battery characteristics were high.

(Reference Example 13) As shown in Tables 8 and 9, as the electrolytic solution, a mixed solvent of ethylene carbonate and dimethyl carbonate, which is a low boiling point solvent, mixed at a volume ratio of 1: 1 was used. Six kinds of batteries were produced in the same manner as in Reference Example 1 except that the electrolyte solution in which each of the electrolyte solution solutes shown was dissolved at a rate of 1 mol / liter was used. And also about each battery in this reference example 13, it discharged at 100 mA as mentioned above and measured about the same item as the above-mentioned reference example 10, and the result was shown in Table 8 and Table 9.

[0091]

[Table 8]

[0092]

[Table 9]

As a result, it was found that each electrolyte obtained by dissolving each electrolyte solute shown in the same table in a mixed solvent obtained by mixing ethylene carbonate with dimethyl carbonate as a low boiling point solvent at a volume ratio of 1: 1 was used. Also, the graphite characteristics and battery characteristics described above were high.

Reference Example 14 Ethylene carbonate (EC) and dimethyl carbonate (D
MC) is 100: 0, 90:10, 8
0:20, 70:30, 60:40, 50:50, 4
0:60, 30:70, 20:80, 10:90, 0:
100 kinds of 11 solvents, an electrolytic solution in which LiPF ₆ was dissolved in each solvent at a rate of 1 mol / liter, LiNiO ₂ was used as a positive electrode material, and other than the above reference examples In the same manner as in Example 1, 11 types of batteries were produced.

For each battery in Reference Example 14, the battery was discharged at 1 A to determine the battery capacity (mAh), and the volume mixing ratio (volume%) of ethylene carbonate (EC) and dimethyl carbonate (DMC) and the battery capacity (M
Ah) was examined, and the results are shown in the graph of FIG.

As a result, in the case where a mixed solvent of ethylene carbonate and dimethyl carbonate was used, the battery capacity when discharging at 1 A as described above was a mixed capacity containing ethylene carbonate in the range of 20 to 80% by volume. It was higher in batteries using solvents.

Reference Example 15 Ethylene carbonate (EC) and diethyl carbonate (D
EC) is 100: 0, 90:10, 8
0:20, 70:30, 60:40, 50:50, 4
0:60, 30:70, 20:80, 10:90, 0:
100 kinds of 11 solvents, an electrolytic solution in which LiPF ₆ was dissolved in each solvent at a rate of 1 mol / liter, LiNiO ₂ was used as a positive electrode material, and other than the above reference examples In the same manner as in Example 1, 11 types of batteries were produced.

For each battery in Reference Example 15, the battery was discharged at 1 A to obtain the battery capacity (mAh).
Volume mixing ratio (vol%) of ethylene carbonate (EC) and diethyl carbonate (DEC) and battery capacity (m
Ah) was examined, and the results are shown in the graph of FIG.

As a result, when a mixed solvent of ethylene carbonate and diethyl carbonate was used, the battery capacity in the case of discharging at 1 A as described above was a mixed capacity containing ethylene carbonate in the range of 20 to 80% by volume. It was higher in batteries using solvents.

Reference Example 16 Ethylene carbonate (EC) and dimethyl carbonate (D
MC) and diethyl carbonate (DEC) in a volume mixing ratio of 100: 0: 0, 90: 5: 5, 80: 10: 1.
0, 70:15:15, 60:20:20, 50: 2
5:25, 40:30:30, 30:35:35, 2
Using 11 kinds of solvents of 0:40:40, 10:45:45 and 0:50:50, LiPF was used for each solvent.
Eleven batteries were produced in the same manner as in Reference Example 1 except that an electrolytic solution in which ₆ was dissolved at a rate of 1 mol / liter was used, and LiNiO ₂ was used as a positive electrode material.

For each battery in Reference Example 16, the battery was discharged at 1 A to obtain the battery capacity (mAh).
The relationship between the volume mixing ratio (vol%) of ethylene carbonate (EC), dimethyl carbonate (DMC) and diethyl carbonate (DEC) and the battery capacity (mAh) was examined, and the results are shown in the graph of FIG.

As a result, when a mixed solvent of ethylene carbonate, dimethyl carbonate and diethyl carbonate was used, when the battery was discharged at 1 A as described above, the battery capacity of ethylene carbonate was 20 to 8
The value was higher in the battery using the mixed solvent contained in the range of 0% by volume.

Example 1 In order to obtain a conductive polymer used for the positive electrode, 0.5 mol of each monomer was dissolved in 1 liter of a 1N aqueous solution of borofluoric acid, and 0.5 mol of ammonium persulfate was added thereto. A solution dissolved in 1 liter of a prescribed aqueous solution of borofluoric acid was dropped and precipitated. Then, a 1N aqueous solution of sodium hydroxide was added thereto to remove anions, and this was thoroughly washed with water, washed with methanol, and dried in vacuo at 80 ° C. Five kinds of conductive polymer powders of polyaniline, polypyrrole, polythiophene, polycarbazole and polyiminodibenzyl shown in Table 12 were obtained.

Next, each of the above conductive polymer powders was molded under pressure, and the molded body was used as a positive electrode. On the other hand, lithium metal was used as a negative electrode, and Nafion (trade name), lithium polyvinyl sulfonate, lithium persulfate, para The battery was charged in a non-aqueous solvent in which a dopant selected from lithium toluene sulfonate was dissolved, and the dopant was doped into each conductive polymer powder used for the positive electrode.
-10 kinds of dopant-containing conductive polymers shown in Table 12 were obtained.

Next, in an electrolytic solution obtained by dissolving LiPF ₆ at a ratio of 1 mol / liter in a mixed solvent obtained by mixing ethylene carbonate and dimethyl carbonate at a volume ratio of 1: 1, the above conductive polymer and The dopant-containing conductive polymer was subjected to cathodic reduction at a constant current of 3 mA / cm ² to insert lithium into each of the conductive polymers and the dopant-containing conductive polymer.

Next, the 15 kinds of conductive polymers obtained as described above were used as a positive electrode material, and the other conditions were the same as in the case of Reference Example 1 except that the 15 kinds of Was prepared.

Then, each of the batteries according to this example was discharged at 100 mA to obtain a positive electrode capacity (mAh / g), a battery capacity (mAh), a self-discharge rate (% / month), a cycle life (times), and a charge. The discharge efficiency (%) was measured, and the results are shown in Tables 10 to 12.

[0108]

[Table 10]

[0109]

[Table 11]

[0110]

[Table 12]

As a result, in each of the batteries using the above-described conductive polymers and the dopant-containing conductive polymer for the positive electrode, the characteristics of the battery capacity, the self-discharge rate, and the cycle life were improved.

The characteristics shown in each of the above reference examples can be applied to the examples using the conductive polymer and the dopant-containing conductive polymer for the positive electrode as described above, and show the same characteristics. is there.

In the above embodiments, specific examples in which the present invention is applied to a cylindrical battery have been described. However, the shape of the battery is not particularly limited, and the present invention is applicable to lithium batteries having various shapes such as a rectangular shape and a flat shape. This is applicable to a secondary battery.

[0114]

The lithium secondary battery according to the present invention has excellent specific effects such as a large battery capacity and a high charge / discharge efficiency.

[Brief description of the drawings]

FIG. 1 is a graph showing charge / discharge cycle characteristics of reference batteries BA1 to BA3 and a comparative battery BC1.

FIG. 2 is a sectional view of a cylindrical battery.

FIG. 3 is a graph showing charge / discharge characteristics of reference batteries BA1 to BA3 and a comparative battery BC1.

FIG. 4 is a graph showing charge / discharge characteristics of a reference battery BA1.

FIG. 5 is a graph showing charge / discharge characteristics of a reference battery BA2.

FIG. 6 is a graph showing cycle characteristics of reference batteries BA1, BA2 and comparative battery BC1.

FIG. 7 is a graph showing charge / discharge characteristics of a reference battery BA4.

FIG. 8 is a graph showing cycle characteristics of a reference battery BA5.

FIG. 9 is a graph showing cycle characteristics of a reference battery BA6, a comparative battery BC2, and a conventional battery.

FIG. 10 is a graph showing the relationship between the d ₀₀₂ value of a carbon material and the discharge capacity of a battery.

FIG. 11 is a graph showing the relationship between the true density of a carbon material and the discharge capacity of a battery.

FIG. 12 is a graph showing the relationship between the average particle size of a carbon material and the discharge capacity of a battery.

FIG. 13 is a graph showing the relationship between the specific surface area of a carbon material and the discharge capacity of a battery.

FIG. 14 is a graph showing a relationship between a crystallite size Lc in the c-axis direction in X-ray diffraction of a carbon material and a discharge capacity of a battery.

FIG. 15 is a view showing a relationship between a volume mixing ratio (volume%) of ethylene carbonate (EC) and dimethyl carbonate (DMC) and a battery capacity (mAh) in Reference Example 14.

FIG. 16 is a diagram showing a relationship between a volume mixing ratio (vol%) of ethylene carbonate (EC) and diethyl carbonate (DEC) and a battery capacity (mAh) in Reference Example 15.

FIG. 17 is a diagram showing a relationship between a volume mixing ratio (vol%) of ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC) and a battery capacity (mAh) in Reference Example 16. .

[Explanation of symbols]

 BA1 Battery 1 Positive electrode 2 Negative electrode 3 Separator 4 Positive electrode lead 5 Negative electrode lead 6 Positive external terminal 7 Negative electrode can

Continuation of the front page (72) Koji Ueno 2-5-5 Keihanhondori, Moriguchi-shi, Osaka Sanyo Electric Co., Ltd. (72) Inventor Nobuhiro Furukawa 2-5-5 Keihanhondori, Moriguchi-shi, Osaka Inside Sanyo Electric Co., Ltd. (72) Inventor Toshiyuki Noma 2-5-5 Keihanhondori, Moriguchi-shi, Osaka Prefecture Inside Sanyo Electric Co., Ltd. (72) Inventor Masatoshi Takahashi 2-5-2 Keihanhondori, Moriguchi-shi, Osaka No. 5 Sanyo Electric Co., Ltd. F term (reference) 5H029 AJ02 AJ03 AK03 AK16 AL07 AM03 AM04 AM05 AM07 BJ02 BJ14 DJ09 HJ02 HJ04 HJ05 HJ07 HJ08 HJ13 5H050 AA02 AA08 BA17 CA08 CA20 CB08 DA13 EA04 HA07 HA02 HA07

Claims (12)

[Claims] 1. A cathode mainly composed of a conductive polymer capable of inserting and extracting lithium or a conductive polymer containing a dopant in which the conductive polymer is doped, and a crystallite size in the c-axis direction in X-ray diffraction. Lc is 150Å
A negative electrode mainly composed of the above graphite (excluding a crystallite size Lc of less than 300 ° in the c-axis direction in X-ray diffraction), a separator interposed between the positive and negative electrodes, and a solvent A lithium secondary battery comprising: an electrolytic solution in which an electrolyte solute is dissolved; and a mixed solvent of ethylene carbonate and a low boiling point solvent (excluding dimethoxyethane) as the solvent. 2. The lithium secondary battery according to claim 1, wherein the low-boiling solvent is an ester-based low-boiling solvent. 3. The method according to claim 1, wherein the low-boiling solvent is at least one solvent selected from dimethyl carbonate, diethyl carbonate, 1,2-diethoxyethane, and ethoxymethoxyethane. Lithium secondary battery. 4. The lithium secondary battery according to claim 1, wherein the mixed solvent comprises ethylene carbonate, dimethyl carbonate, and diethyl carbonate. 5. The lithium secondary battery according to claim 1, wherein the mixed solvent contains 20 to 80% by volume of ethylene carbonate. 6. The method according to claim 1, wherein the dopant is at least one selected from perfluorocarbonsulfonic acid, lithium paratoluenesulfonate, lithium polyvinylsulfonate, lithium persulfate, and porphyrin. The lithium secondary battery according to claim 1. 7. The X-ray diffraction of graphite (00)
2) The lithium secondary battery according to any one of claims 1 to 6, wherein the d value of the surface is 3.35 to 3.40 °. 8. The X-ray diffraction of graphite (00)
2) The lithium secondary battery according to any one of claims 1 to 7, wherein the d value of the surface is 3.554 to 3.365 °. 9. The lithium secondary battery according to claim 1, wherein the average particle size of the graphite is 1 to 30 μm. 10. A graphite having a specific surface area of 0.5 to 50.
The lithium secondary battery according to any one of claims 1-9, characterized in that the m ² / g. 11. The true density of the graphite is 1.9 to 2.3 g.
/ Cm < ³ >, The lithium secondary battery according to any one of claims 1 to 10, wherein 12. The lithium secondary battery according to claim 1, wherein LiPF ₆ is dissolved in the electrolyte as a solute.