JPH10261406A - Carbon electrode and non-aqueous electrolyte secondary battery using it as negative electrode - Google Patents

Abstract

(57) [Summary] [Purpose] Good charge / discharge cycle performance under high load, high energy density is obtained even at high power discharge at low temperature, and energy density decreases even if left in a charged state for a long time. It is an object of the present invention to provide a non-aqueous electrolyte secondary battery having a small amount. [Constitution] In a negative electrode composed of carbon fiber and granular carbon,
A carbon electrode whose resistance Rn (unit: mΩ) at an arbitrary number of cycles is represented by the following equation: Rn = R ₁ + α × n (R ₁ is the resistance mΩ in the first cycle, α is 0 ≦ α ≦ 0.0
A coefficient in the range of 5 and n represents the number of cycles, respectively.) A negative electrode composed of the carbon electrode, a positive electrode composed of the lithium-containing composite oxide, and an electrolytic solution obtained by dissolving a lithium salt in an organic solvent. battery.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-aqueous electrolyte secondary battery, and more particularly to a lithium ion secondary battery, and more particularly, to a charge / discharge cycle performance under a high current and an output under a low temperature. The present invention relates to a non-aqueous electrolyte secondary battery that has excellent performance and storage characteristics when fully charged.

[0002]

2. Description of the Related Art A lithium secondary battery has attracted much attention as a secondary battery having a high energy density. However, the lithium secondary battery using metal lithium as the negative electrode has poor electrochemical reversibility in the electrolytic solution itself, and has a great problem in safety. For such reasons, in recent years, lithium ion secondary batteries using a carbon material for the negative electrode and a lithium-containing composite oxide for the positive electrode have been widely put into practical use in recent years.

The carbon material for the negative electrode of the lithium ion secondary battery is roughly classified into graphitizable carbon and non-graphitizable carbon. Graphitizable carbon is known as a material with excellent cycle characteristics and high load characteristics because heat treatment at a high temperature improves the crystallinity of graphite, thereby increasing the discharge capacity. I have. At present, graphite-based carbon is mainly used as a negative electrode material of a lithium ion secondary battery.

Examples of the graphite-based carbon include natural graphite,
Artificial graphite, mesocarbon microbeads (MCMB),
Pitch-based carbon fiber, vapor-grown carbon fiber and the like can be mentioned. Above all, the vapor-grown carbon fiber is heat-treated at a high temperature of 2800 ° C. or more, whereby graphite crystallites are sufficiently developed, so that a reversible capacity of 300 to 360 mAh / g can be obtained. Furthermore, since the aspect ratio and specific surface area of the fiber can be adjusted, by controlling those physical properties, there is an advantage that the filling property, cycle characteristics, and load characteristics are excellent.

[0005]

A lithium ion secondary battery operates by exchanging lithium ions between a positive electrode and a negative electrode via an electrolytic solution. Since lithium ions react with water, lead-acid batteries,
Aqueous electrolytes such as nickel-cadmium secondary batteries cannot be used. Instead, organic solvent-based electrolytes are used. Compared with aqueous electrolytes, organic solvent electrolytes have a poor ion transmission rate, and when charged and discharged with a high current, the resistance of the electrolyte is high. Had been significantly reduced. Further, when charge and discharge are repeated at a high current, the discharge capacity deteriorates remarkably, and the capacity deterioration up to 100 cycles is particularly severe. In addition, the average operating voltage of the battery (the average operating voltage of the lithium ion secondary battery is usually set to 3.6 V) rapidly decreases with each cycle, so that not only the capacity but also a high voltage is required. No satisfactory performance has been achieved for electrical or electronic equipment.

Further, since the ion transfer rate of the electrolytic solution is low, the capacity and the operating voltage are remarkably reduced at a low temperature such as 0 ° C. In particular, when discharging at a relatively high current at a low temperature, the decrease in energy density is quite remarkable compared to discharging at room temperature.

[0007] Conversely, lithium ion secondary batteries have a lower self-discharge rate than batteries having aqueous electrolytes, such as lead-acid batteries and nickel-cadmium secondary batteries, due to the low ion conductivity of the electrolyte. The rates are quite low. Although this is an advantage of the organic electrolyte secondary battery, in the case of a lithium ion secondary battery, since the active material is lithium ion having a high chemical activity, it easily reacts with the solvent in the electrolyte. For example, if the battery is fully charged and left for one month or longer, the side reaction reduces the capacity by about 5 to 10% with respect to the capacity before leaving, and also reduces the average operating voltage of the battery. . This decrease in capacity is not high as compared with the above-mentioned secondary battery using an aqueous electrolyte. However, the aqueous secondary battery is restored to the capacity before being left by being recharged, whereas the capacity of the lithium ion secondary battery is not restored. That is, since the side reaction between lithium ions and the electrolyte is an irreversible reaction, even if the battery is recharged, the lithium ions that have once undergone a side reaction turn into side reaction products and do not contribute to the charge / discharge reaction of the battery. It is thought to be.

The present invention has been made to solve the above problems. That is, the capacity and energy density are prevented from deteriorating due to charging and discharging at a high current, the energy density is not reduced even when discharging at a high current at a low temperature, and the energy density is reduced in a long-term charging state. An object of the present invention is to provide a lithium ion secondary battery in which a decrease is suppressed.

[0009]

As a result of intensive studies, it has been found that the state of formation of the passive film formed during charging affects the above-mentioned problems. The passive film is formed mainly on the negative electrode side during the charge of the first cycle, particularly when the solvent in the electrolytic solution reacts with lithium ions. The formed film serves as a protective film for the electrolyte of the negative electrode, so that the decomposition reaction of the electrolyte is suppressed after the second cycle. However, it is not completely protected from the decomposition reaction,
The film gradually grows on the negative electrode side by repeated charge and discharge. Since the reaction for forming the coating is an irreversible reaction, it is caused by the deterioration of the discharge capacity in each charge / discharge cycle. In addition, since the resistance of the negative electrode side increases due to the growth of the coating, the resistance increases in addition to the resistance loss (also referred to as IR drop), which lowers the operating voltage of the battery or deteriorates the charge / discharge characteristics under a high load. . Various methods have been tried for the qualitative and quantification of the passive film, and among them, a method of indirectly estimating from the resistance on the negative electrode side obtained from a Cole-Cole plot obtained by impedance measurement is preferable. When the charge-discharge cycle is repeated, the resistance on the negative electrode side obtained by this method increases with each cycle, but it is found that the above problem can be improved if the rate of increase does not exceed a certain range. Reached.

In order to solve the above-mentioned problem, the invention according to claim 1 provides an electrochemical cell in which a counter electrode and a reference electrode are made of metallic lithium in a resistance Rn at an arbitrary number of charge / discharge cycles calculated by AC impedance measurement.
(Unit: mΩ) is a carbon electrode represented by the following formula: Rn = R ₁ + α × n (R ₁ is the resistance mΩ in the first cycle, α is 0 ≦ α ≦ 0.0
And n represents the number of cycles, respectively.) The invention according to claim 2 has a packing density of 1.3 to 1.8.
g / cm ³ , wherein the carbon electrode according to claim 1 contains 10 to 70% by weight of carbon fibers having an aspect ratio of 5 to 10.
The invention according to claim 4, wherein the negative electrode comprising the carbon electrode, the positive electrode comprising a lithium-containing composite oxide, and a solvent comprising a cyclic carbonate and a chain carbonate, LiPF ₆ is used. This is a non-aqueous electrolyte secondary battery having a dissolved electrolyte.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION

<< Negative Electrode >> The negative electrode of the present invention is formed of a carbon material. In the present invention, it is preferable that carbon fibers are contained in the negative electrode carbon material in an amount of 10 to 70% by weight. As the carbon fiber, conventionally known carbon fibers can be used without limitation, and among them, particularly preferred is a vapor grown carbon fiber produced by a substrate method or a fluidized gas phase method. Among them, those having an average diameter of 1 to 5 are preferable.
μm, the average fiber length is 5 to 30 μm, and the aspect ratio is 5 to 10. Since the fibers having such a shape have a moderate degree of entanglement of the fibers, it is advantageous to obtain high conductivity when forming a single aggregate of electrodes. If the aspect ratio is too low, it is difficult to obtain high conductivity, and if the aspect ratio is too high, a high packing density cannot be obtained.

Further, it is preferable that the carbon fibers have been subjected to a graphitization treatment. Those subjected to the graphitization treatment have a high capacity per unit weight and a high true density, so that a battery having a high capacity can be provided.
Further, the change in potential during the process of occluding lithium ions is a potential close to the potential of the lithium electrode (0 V
(In the vicinity), the operating voltage when the battery is combined with the positive electrode can be maintained at a high and flat potential, and a battery with high energy density can be provided. The graphitization treatment is usually performed in an inert gas atmosphere at 2800 ° C. or higher. The graphitized carbon fiber has a carbon network spacing distance determined by X-ray diffraction, that is, d ₀₀₂ is 0.3354 to 0.3375 nm, more preferably 0.3354 to 0.3365 nm, and the carbon network Lc, which is the size of the graphite crystallite in the vertical direction (c-axis direction), is 50 to 2500 nm, and more preferably 70 to 2500 nm. Note that d
₀₀₂ and Lc are determined by the Gakushin method.

[0013] The carbon fiber is subjected to a graphite finishing treatment and then a fiber cutting treatment to obtain the above-mentioned average fiber length of 5%.
-30 μm carbon fibers can be obtained. As a method of cutting fibers, ball mills, stamp mills, jet mills and other conventional methods can be used,
More preferably, there is a cutting method using a high impact force, for example, a method using a hybridizer, or a method using an isostatic press to compress fibers to compress the fibers.

The carbon constituting the negative electrode of the present invention also contains carbon other than the carbon fibers described above. The different carbon mentioned here is carbon having a shape different from that of the carbon fiber, such as natural graphite, artificial graphite, mesocarbon microbeads (MCMB), petroleum coke, and hard carbon. Among them, preferred are those having an average particle size of 5 to 40 μm.
Of carbon. When the particle size is within the above range, the gap between the carbon fiber aggregates can be efficiently filled, and a coal cable electrode having a high packing density can be obtained. The content of the coal cord is appropriately selected from 30 to 90% by weight according to the amount of the carbon fiber.

The carbon is preferably carbonized. The reason is the same as in the case of the carbon fiber. That is, d ₀₀₂ is 0.3354 to 0.3375
nm, more preferably 0.3354 to 0.336.
5 nm, and Lc is 50 to 2500 nm, more preferably 70 to 2500 nm.

[0016] The negative electrode of the present invention mainly comprises the carbon fiber and the carbon. The mixing ratio of carbon fiber and carbon is
Carbon fiber: carbon = 10-70: 90-30 as weight ratio
Are selected as appropriate. If the content of the carbon fiber is too low, the conductivity of the electrode becomes poor, and among the objects of the present invention, improvement in cycle performance particularly under high load cannot be achieved. If the content of the carbon fiber is too high, it becomes impossible to increase the packing density of the electrode, so that the formation of a passive film becomes excessive or the capacity of the battery cannot be improved.

The carbon fiber and the mixture of the carbon are:
Those whose specific surface area does not exceed 5 m ² / g are desirable,
More preferably, it does not exceed 3 m ² / g. If the specific surface area is too high, the contact area with the electrolytic solution becomes large, so that the charge / discharge efficiency representing the ratio between the charge capacity and the discharge capacity decreases. This means that the irreversible reactivity is high, resulting in a decrease in capacity per cycle. Furthermore, since the contact area with the electrolytic solution is large, a decomposition reaction of the electrolytic solution is easily caused at the time of an internal short circuit such as nailing of the battery. Gas is generated by the decomposition reaction and the internal pressure of the battery is rapidly increased, so that the battery is ruptured. The specific surface area can be measured by a BET method.

B of the mixture of the carbon fiber and the carbon
The specific surface area determined by ET method that does not exceed 5 m 2 ^/ g, the said carbon fibers, each of the specific surface area of the carbon may have shall not exceed 10 m 2 / ^g, more preferably 5 m ² / G. When mixed, the specific surface area and the mixing ratio of each of the carbon fiber and the carbon are adjusted so that the specific surface area of the mixture does not exceed 5 m ² / g. However, the mixing ratio is selected so that the carbon fiber is in the range of 10 to 70% by weight.

The carbon fiber and the mixture of carbon are:
Those subjected to a graphitization treatment are preferred. The reason is,
The same applies to the case of the carbon fiber and the carbon. That is, it is preferable that d ₀₀₂ is 0.3354 to 0.3375 nm and Lc is 50 to 2500 nm.

A binder may be added to the negative electrode of the present invention in addition to the carbon fiber and the carbon. The material of the binder is not particularly limited as long as it has a binding property between carbons and a binding property to a current collector.Preferably, the binder is a fluorinated resin such as polyvinylidene fluoride or polytetrafluoroethylene, polyethylene or the like. Examples thereof include polyolefin such as polypropylene, diene rubber, and polyimide such as Chiron 6.

The negative electrode of the present invention is formed by, for example, applying a slurry obtained by dispersing and mixing a mixed carbon comprising the carbon fiber and the carbon and the binder in a solvent to a current collector. The solvent is not particularly limited. For example, when polyvinylidene fluoride is used as a binder, N-methyl-2-pyrrolidone, dimethylformamide, cyclopentanone, and the like are preferable, and N-methyl-2-pyrrolidone is particularly preferable. . For the current collector, a metal foil, a metal plate, a metal mesh, a metal porous body is suitably used, and in the case of a negative electrode, those which are not easily reduced electrochemically are preferable, such as copper, titanium, and nickel. Generally, a copper foil is used.

After the slurry is applied to the current collector, the slurry is dried at, for example, 100 to 150 ° C. to evaporate the solvent, thereby obtaining a negative electrode. In order to suitably carry out the present invention, the packing density of the active material layer composed of the carbon fiber, the granular coal cable, and the binder can be increased by pressing the negative electrode as needed. The pressing method is not limited, and a conventionally known method can be used. Particularly preferred is a method using a roll press machine composed of two rolls having a fixed gap. The packing density of the active material layer after pressing is 1.3 to
It is desirably 1.8 g / cm ³ . If the packing density is less than 1.3 g / cm ³ , the resistance will increase remarkably each time charging and discharging are repeated, and will exceed the range of the α value in the mathematical formula described in claim 1. If the packing density is too low, the porosity in the active material layer of the negative electrode is high, and the permeability of the electrolyte is high, so that the generated passive film is likely to expand and increase the resistance of the negative electrode. If the packing density exceeds 1.8 g / cm ³ , the electrolyte does not easily penetrate into the active material layer of the negative electrode, so that ion transfer between the electrolyte and the negative electrode deteriorates.
After all, the resistance of the negative electrode is increased. Then, the range of the α value cannot be satisfied. In addition, even if the mixing ratio of the carbon fibers is kept within the range of 10 to 70% by weight, the α value can be kept within the predetermined range unless the packing density is 1.3 to 1.8 g / cm ^3. Sometimes it goes away. The thickness of the negative electrode is desirably as small as possible from the viewpoint of suppressing the resistance. However, if the thickness is too small, the ratio of the thickness of the current collector to the thickness of the active material layer increases, so that the capacity per volume is significantly reduced. Practically, it is selected in the range of 100 to 200 μm.

The resistance of the negative electrode in the present invention can be obtained from a Cole-Cole plot obtained by the AC impedance measurement. The Cole-Cole plot can be obtained by a conventionally known method if measured at a constant temperature. That is, it is measured by sweeping the frequency under a constant output. The present invention is characterized in that the resistance Rn (unit: mΩ) of the negative electrode at an arbitrary number of cycles is a function of the cycle according to the following equation. Rn = R ₁ + α × n where R ₁ is the resistance mΩ of the negative electrode in the first cycle, and α is 0
A coefficient in the range of ≦ α ≦ 0.05, and n represents the number of cycles. Here, when α is in the range of 0 to 0.05, the carbon electrode and the secondary battery which are excellent in high load cycle performance, output characteristics under low temperature, and long-term storage performance, which are the objects of the present invention, Can be provided. R ₁ ≦ Rn
Therefore, α becomes a numerical value higher than 0. α is 0.0
If it is higher than 5, the resistance of the negative electrode will increase too much and the object of the present invention cannot be achieved.

<< Positive Electrode >> As the positive electrode active material, a conventionally known lithium-containing composite oxide is used. For example, LiCo
_{O 2, LiNiO 2, LiMnO 2} , LiFeO 2 and,
LiM _1-x N _x O ₂ (where M is Fe, Co, Ni
Wherein N is a transition metal and X is 0 ≦ X
≦ 1), LiMn ₂ O ₄ , LiMn ₂ -YN _Y
O ₄ (where N is a transition metal and Y is 0 ≦ Y ≦ 2
), LiV ₂ O ₅ and the like.

Since the positive electrode active material generally has poor conductivity, it is desirable to add a conductive auxiliary material to increase the conductivity. The conductive auxiliary material is not particularly limited as long as it has conductivity, and among them, a carbonaceous material is preferable.For example, various carbon blacks such as acetylene black, natural graphite, artificial graphite, and amorphous materials such as petroleum coke can be used. Examples include carbon, non-graphitizable carbon, and carbon fiber.

A binder may be added to the positive electrode of the present invention in addition to the lithium-containing composite oxide and the conductive auxiliary material. The binder may be the same as that used for the negative electrode.

In the method for producing a positive electrode according to the present invention, a slurry obtained by dispersing and mixing the lithium-containing composite oxide, the conductive auxiliary material and the binder in a solvent is applied to a current collector in the same manner as in the case of the negative electrode. It will dry. As the current collector, a metal foil is suitable as in the case of the negative electrode. However, a material which is not easily oxidized electrochemically is preferable for the positive electrode, for example, aluminum, titanium, and the like. Generally, an aluminum foil is used. After application and drying, if necessary, a press treatment is performed in the same manner as the negative electrode. The packing density of the active material layer of the positive electrode after pressing is 2.5 to 3.5 g / cm ^3.
It is desirable that If the packing density is too low, the amount of the lithium-containing composite oxide, which is a source of lithium ions, decreases, so that it is difficult to increase the battery capacity. If the packing density is too high, as in the case of the negative electrode, the permeability of the electrolyte is poor and sufficient battery characteristics cannot be obtained. The thickness of the positive electrode is appropriately selected in the range of 100 to 200 μm, as in the case of the negative electrode.

<< Electrolyte >> The secondary battery of the present invention uses an organic solvent-based electrolyte. The electrolytic solution of the present invention is obtained by dissolving a lithium salt in an organic solvent. A conventionally known lithium salt is suitably used. For example, LiPF ₆ , Li
ClO ₄ , LiBF ₄ , LiAsF ₆ , Li (CF ₃ S
O ₂ ) ₂ N and the like. The organic solvent is preferably one composed of a carbonate, and a mixed solvent of a cyclic carbonate and a chain carbonate is particularly preferable. Examples of the cyclic carbonate include ethylene carbonate, propylene carbonate, and butylene carbonate, and these may be used alone or in combination. Examples of the chain carbonate include dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate and the like, and these may be used alone or in combination.

<< Secondary Battery >> The secondary battery of the present invention is manufactured as follows. That is, the negative electrode and the positive electrode are wound through a separator to form a spiral electrode. It is manufactured by placing the spiral electrode in a battery can, injecting the electrolytic solution, attaching a cap and sealing. There is no particular limitation on the shape of the secondary battery of the present invention manufactured as described above. For example, the shape is a cylindrical shape, a square shape, a coin shape, a button shape, and the like, and the size is not limited.

[0030]

EXAMPLES The present invention will be described below with reference to examples, but the present invention is not limited by these examples. Example 1 A vapor-grown carbon fiber having an average diameter of 2 μm and an average length of 80 μm was immersed in an argon gas atmosphere at 2800 ° C. for 30 minutes.
Graphitized for minutes. Then, the obtained graphitized vapor-grown carbon fiber was used as a hybridizer (Nara Machinery Co., Ltd., NHS
-1) and subjected to a high impact treatment at 4000 rpm for 2 minutes. The resulting _{d 002} of carbon fiber 0.33
60 nm, Lc is 100 nm, and the specific surface area is 1.4 m ² /
g, the average diameter was 2 μm, the average length was 20 μm, and the aspect ratio was 10. This carbon fiber has an average particle size of 20 μm,
Specific surface area is 0.7 m ² / g, d ₀₀₂ is 0.3360 n
Spherical carbon (mesocarbon microbeads MCMB, manufactured by Osaka Gas Co., Ltd.) having m and Lc of 100 nm was mixed at a weight ratio of 1: 1. The specific surface area of the obtained mixed carbon was 1.1 m ² / g. To 90 parts by weight of this mixed carbon, 10 parts by weight of polyvinylidene fluoride were added, and N was added.
The mixture was sufficiently mixed and dispersed in -methyl-2-pyrrolidone to obtain a slurry. This slurry was applied to both sides of a copper foil having a thickness of 10 μm, dried at 100 ° C., and roll-pressed to obtain a negative electrode. The packing density is 1.6 g / cm ³ and the thickness is 1
It was 30 μm.

A three-electrode beaker cell was assembled using the produced negative electrode as a working electrode, lithium metal as a counter electrode and a reference electrode. In the electrolyte, a mixed solvent of ethylene carbonate (EC), propylene carbonate (PC), and dimethyl carbonate (DMC) (volume mixing ratio: EC: PC) was used so that LiPF ₆ had a concentration of 1.2 mol / liter. : DMC = 3: 3: 4). The battery was charged at a current value of 100 mA until the voltage reached 0 V with respect to the reference electrode (meaning a reaction for absorbing lithium ions into the working electrode).
Assuming that discharging to V is one cycle, impedance measurement (apparatus; Solartron 12) is performed for each cycle.
86 & 1250). The sweep frequency was 10 kHz to 50 mHz, and the measurement temperature was 20 ° C. The α value was calculated from the resistance for each cycle obtained from the impedance measurement. Table 1 shows the results.

90 parts by weight of LiCoO _{2 having an} average particle diameter of 10 μm, 5 parts by weight of acetylene plaque, and 5 parts by weight of polyvinylidene fluoride were sufficiently mixed and dispersed in N-methyl-2-pyrrolidone to obtain a slurry. Thick this slurry 2
It was applied to both sides of a 0 μm aluminum foil, dried at 100 ° C., and roll-pressed to obtain a positive electrode. The packing density was 3.3 g / cm ³ and the thickness was 130 μm.

The thus obtained negative electrode and positive electrode were wound up through a polypropylene separator to obtain a spiral electrode. This electrode was placed in a cylindrical battery can having a diameter of 17 mm and a height of 500 mm. In this battery can, LiPF ₆
To a concentration of 1.2 mol / liter with ethylene carbonate (EC) and propylene carbonate (P
C) and dimethyl carbonate (DMC), and an electrolytic solution obtained by dissolving in a mixed solvent (volume mixing ratio: EC: PC: DMC = 3: 3: 4) was injected. Then, the positive electrode cap was swaged and sealed to obtain a secondary battery.

The obtained secondary battery was charged at a constant current of 3 A and a constant current of a charging upper limit voltage of 4.1 V and a constant voltage (a charging time of 2.5 hours) in a thermostat at 20 ° C. 3A,
Constant current discharge at a discharge lower limit voltage of 2.5 V was repeated 300 times. 1 cycle, 100 cycles, 200 cycles, 3
Table 2 shows the discharge capacity at the 00th cycle.

The obtained secondary battery was charged at a constant current of 1 A and a constant current of a charging upper limit voltage of 4.1 V and a constant voltage (a charging time of 2.5 hours) in a thermostat at 0 ° C., and the output was 50 W. /
A constant output discharge was performed at kg, 100 W / kg, and 200 W / kg, respectively. The lower discharge limit voltage was 2.5V. Table 2 shows the energy density at each output.

The obtained secondary battery was charged in a constant temperature bath at 20 ° C. with a constant current of 1 A and a constant current of a charging upper limit voltage of 4.1 V and constant voltage charging (charging time: 2.5 hours), and the current value was increased. 1A,
The constant current discharge at a discharge lower limit voltage of 2.5 V was repeated 30 times. After charging again, it was transferred to a 45 ° C. constant temperature bath and left for one month. After leaving, the current value is 1A, the discharge lower limit voltage is 2.
The battery was discharged at a constant current of 5 V to obtain a discharge capacity after standing. The ratio of the energy density obtained from the discharge capacity and the average operating voltage at the 30th cycle before leaving, and the energy density obtained from the discharge capacity and the average operating voltage after leaving was obtained. Table 3 shows the results.

Example 2 Of the mixed carbon, the weight mixing ratio between the carbon fiber and the granular coal cable was determined as follows: coal fiber: granular carbon = 70: 3.
0, the packing density of the negative electrode was 1.3 g / cm ³ , and the thickness was 1
A negative electrode and a secondary battery were produced in the same manner as in Example 1 except that the thickness was 90 μm, the packing density of the positive electrode was 3.5 g / cm ³ , and the thickness was 180 μm. In the same manner as in Example 1, the measurement of the negative electrode resistance, the calculation of the α value, the high load cycle test, the high output discharge test at a low temperature, and the standing test were performed. The results are shown in Tables 1 to 4.

Example 3 Of the mixed carbon, the weight mixing ratio between carbon fiber and granular carbon was determined as follows: carbon fiber: granular carbon = 10: 9.
0, the packing density of the negative electrode was 1.8 g / cm ³ , and the thickness was 1
A negative electrode and a secondary battery were produced in the same manner as in Example 1 except that the thickness was 10 μm, the positive electrode active material was LiMn ₂ O ₄ , the packing density of the positive electrode was 2.5 g / cm ³ , and the thickness was 200 μm. In the same manner as in Example 1, the measurement of the negative electrode resistance, the calculation of the α value, the high load cycle test, the high output discharge test at a low temperature, and the standing test were performed. Table 1 shows the results.
It is shown in FIG.

<< Comparative Example 1 >> Among the mixed carbon, the weight mixing ratio between carbon fiber and granular carbon was determined as follows: carbon fiber: granular carbon = 5: 95
The packing density of the negative electrode was 1.1 g / cm ³ and the thickness was 21.
A negative electrode and a secondary battery were produced in the same manner as in Example 1, except that the thickness was 0 μm, the packing density of the positive electrode was 3.8 g / cm ³ , and the thickness was 150 μm. In the same manner as in Example 1, the measurement of the negative electrode resistance, the calculation of the α value, the high load cycle test, the high output discharge test at a low temperature, and the standing test were performed. The results are shown in Tables 1 to 4.

Comparative Example 2 The weight mixing ratio of carbon fiber and granular carbon in the mixed carbon was determined as follows: carbon fiber: granular carbon = 80: 2
0, the packing density of the negative electrode was 1.9 g / cm ³ , and the thickness was 9
A negative electrode and a secondary battery were produced in the same manner as in Example 1, except that the thickness was 0 μm, the packing density of the positive electrode was 3.8 g / cm ³ , and the thickness was 150 μm. In the same manner as in Example 1, the measurement of the negative electrode resistance, the calculation of the α value, the high load cycle test, the high output discharge test at a low temperature, and the standing test were performed. The results are shown in Tables 1 to 4.

[0041]

[Table 1]

[0042]

[Table 2]

[0043]

[Table 3]

[0044]

[Table 4]

[0045]

According to the present invention, the charge / discharge cycle performance under a high load is good, a high energy density can be obtained even at a high output discharge at a low temperature, and even if the battery is left in a charged state for a long period of time, it has a significant effect. It is possible to provide a secondary battery that does not cause a significant capacity deterioration and a small decrease in energy density.

Claims (4)

[Claims] In an electrochemical cell using lithium metal as a counter electrode and a reference electrode, a resistance R at an arbitrary number of charge / discharge cycles calculated by AC impedance measurement.
A carbon electrode in which n (unit: mΩ) is represented by the following formula. Rn = R ₁ + α × n (R ₁ is the resistance mΩ in the first cycle, α is 0 ≦ α ≦ 0.0
A coefficient in the range of 5 and n represents the number of cycles, respectively) 2. The carbon electrode according to claim 1, wherein the packing density is 1.3 to 1.8 g / cm ³ . 3. The carbon electrode according to claim 1, comprising 10 to 70% by weight of carbon fibers having an aspect ratio of 5 to 10. 4. A non-aqueous solution comprising a negative electrode comprising the carbon electrode, a positive electrode comprising a lithium-containing composite oxide, and an electrolytic solution obtained by dissolving LiPF ₆ in a solvent comprising a cyclic carbonate and a chain carbonate. Electrolyte secondary battery.