JP2001176512A - Lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium secondary battery superior in high temperature shelf life. SOLUTION: This lithium secondary battery has a positive electrode and a negative electrode, respectively absorbing and desorbing lithium ion, and an organic electrolyte in which the electrolyte including lithium ion is dissolved, and the positive electrode and the negative electrode are mounted through a separator. The refractory graphitic carbon forming the negative electrode is heated in an inert gas atmosphere or in an atmosphere for the graphitic carbon to slightly oxidize to remove the decomposed gas, and heated under pressure to obtain the concentration (butanol method) of the refractory graphitic carbon of 1.6-1.8 (g/cc).

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 suitable for a portable device such as a portable telephone or a notebook personal computer, a driving power source for an electric vehicle, and a power storage power source.

[0002]

2. Description of the Related Art A lithium secondary battery using an amorphous carbon material for a negative electrode has been developed and widely used in notebook personal computers and mobile phones.

As a conventional technique using a carbon material for the negative electrode,
Japanese Patent No. 2,630,93 discloses that an n-doped carbon material having a density (ρ) of 1.7 <ρ <2.18 (g / cc) is used.
No. 9. JP-A-7-307164 discloses the use of a carbon material having a density (ρ) of 1.7 ≦ ρ ≦ 2.1 (g / cc) in a heat-treated product of mesophase pitch and mesophase pitch green coke. ing.

Furthermore, the density (ρ _B ) measured by the butanol method is ρ _B ≦ 1.7 (g / cc), and the ratio ρ _H / ρ to the density (ρ _H ) measured by the helium method is _B
The use of a carbon material satisfying ≧ 1.15 for the negative electrode is disclosed in JP-A-8-115723.

[0005]

As described above, lithium secondary batteries using an amorphous carbon material for the negative electrode have been widely used. The mounting of secondary batteries on electric vehicles is being studied.

However, in such automobiles, the temperature of the battery mounting portion is expected to rise to 50 to 60 ° C. in the summer, and the storability of the lithium secondary battery at such a high temperature is insufficient. Is a problem in practical use.

Regarding the cause of high temperature deterioration of a lithium secondary battery using an amorphous carbon material for a negative electrode,
The battery after being left in the charged state was disassembled and analyzed.

[0008] As a result of taking out the positive electrode and the negative electrode and separately evaluating the electrode characteristics, the positive electrode capacity after standing was hardly changed from the initial capacity. However, it was found that the negative electrode capacity was greatly reduced with respect to the initial capacity.

Therefore, a detailed analysis of the negative electrode revealed that deposits of lithium carbonate and the like, which are considered to have been formed by the reaction between the electrolyte and lithium stored in the negative electrode, were formed on the surface of the carbon material particles used for the negative electrode. It was found that a large amount was formed.

That is, it is considered that the reason why the capacity of the negative electrode is greatly reduced by leaving it at a high temperature is that deposits formed on the surface of the carbon material hinder the insertion and extraction of lithium ions.

As a prior art, Japanese Patent No. 2,630,93
9, JP-A-7-307164 or JP-A-8-1
Similarly, when the carbon material disclosed in No. 15723 is used as a negative electrode, if the carbon material is placed in a high-temperature environment for a long time as described above, a deposit is formed on the carbon surface and the carbon material is deteriorated. (Hereinafter referred to as high-temperature storage), it was found that the negative electrode characteristics could not be maintained.

An object of the present invention is to provide a highly reliable lithium secondary battery having excellent storage characteristics, particularly for portable equipment such as a cellular phone, and as a drive power source for an electric vehicle. .

[0013]

The gist of the present invention to achieve the above object is as follows.

[1] A lithium secondary battery including a positive electrode and a negative electrode that occlude and release lithium ions, and an organic electrolytic solution in which an electrolyte containing the lithium ions is dissolved, wherein the positive electrode and the negative electrode are arranged via a separator. In the above, the non-graphitizable carbon forming the negative electrode is subjected to a heat treatment in an inert atmosphere or an atmosphere in which the graphitized carbon is slightly oxidized to remove the decomposition gas, and then heat-treated under pressure to perform the heat treatment under pressure. Density of graphitized carbon (butanol method) is 1.6 to 1.8 g / cc
It is a lithium secondary battery characterized by the following.

[2] In the lithium secondary battery, the charge / discharge capacity of the negative electrode of 1.5 to 0.02 V with respect to the lithium reference electrode is 280 mAh / g or more.

[0016] [3] and measured density ([rho _B) by butanol method of the flame graphitized carbon, the ratio of the measurement by helium Method Density _{(ρ H) (ρ H /} ρ B) is 1.05 or less.

[4] The interlayer distance of the six-membered ring layer of the non-graphitizable carbon is larger than 0.36 nm and smaller than 0.41 nm, preferably larger than 0.37 nm and 0.39 nm.
Is less than.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION The structure of amorphous carbon is such that a portion (six-membered ring layer portion) in which six-membered ring surfaces (hexagonal mesh surfaces) of carbon are stacked by about several nm is randomly connected to form a spherical shell. It is reported that the structure is a porous structure. The above-mentioned Japanese Patent No. 2,630,939 and JP-A-7-307164.
It is considered that the amorphous carbon material disclosed in Japanese Unexamined Patent Application Publication No. 8-115723 is also divided into a portion in which six-membered rings are stacked and a hollow portion (micropore) of a spherical shell.

On the other hand, the storage sites of lithium ions into the above-mentioned amorphous carbon material have so far been in the six-membered ring layer portion and the micropore portion.
B09.

Further, lithium stored in the six-membered ring layer portion and the micropore portion can be distinguished by nuclear magnetic resonance (NMR), and two signals (night shift: around 20 ppm and around 120 ppm) ), The 35th Battery Symposium Abstracts 2B1
0 reported. Furthermore, this report reports that lithium is stored at high density in the form of clusters in the micropore portion.

Based on these reports, many studies have been made with the aim of efficiently utilizing micropores and increasing the charge / discharge capacity of carbon anodes. The technique disclosed in JP-A-8-115723 is also performed for the purpose of increasing the amount of lithium stored in micropores.

By the way, the present inventors have found that lithium stored in the six-membered ring layer due to the charging reaction is different from lithium stored in the micropore in the ease of occurrence of a side reaction with the electrolyte during high-temperature storage. Therefore, the reactivity was examined.

It has been reported in the 35th Battery Symposium Abstracts 2B10 that lithium ions are absorbed into amorphous carbon first in the six-membered ring layer and then in micropores.

Therefore, negative electrodes having different charge depths are prepared.
The storage characteristics at 60 ° C. were examined. As a result, it was shown that the capacity was remarkably reduced in the negative electrode having a large charge depth in which lithium was occluded in the micropore.

To investigate the cause, the surface of the carbon particles of the negative electrode whose capacity was remarkably reduced was analyzed. As a result, a large amount of deposits such as lithium carbonate which appeared to have been produced by the reaction between the electrolyte and the stored lithium were found. I understood that. From this, it is assumed that lithium absorbed in the micropores has high reactivity, and that a side reaction with the electrolyte easily occurs.

From the above results, in the present invention, based on a different idea of suppressing the lithium occlusion / release reaction to micropores during charging / discharging, the carbon material whose lithium ions are occluded / emitted mainly in the six-membered ring layer is considered. Developed. That is, the micropores were closed or eliminated so that lithium ions could not enter. As a specific method, a method of heat-treating the carbon material in a pressurized state was performed. The specific examples are shown below.

As a raw material, a thermosetting resin such as phenol, furan, melamine, or epoxy resin, or an isotropic pitch obtained from petroleum pitch or coal pitch is used.
In this embodiment, a carbon material is manufactured by a two-stage heat treatment.

In the first stage, the above-mentioned raw material is placed in an inert atmosphere or an atmosphere in which the raw material is slightly oxidized, and
Decomposition gas is removed by heating once at a temperature of 0 ° C. or less.

In the second stage, the first step is performed under pressure in an inert atmosphere.
Treat again at a high temperature of 000C to 2000C.

The above pressure condition is desirably 10 atm or more, preferably 100 atm or more. The carbon material of the present invention can be obtained by the above two-stage heat treatment.

The carbon material is roughly classified into two types, ie, non-graphitizable carbon and graphitizable carbon. The carbon material of the present invention belongs to non-graphitizable carbon, and can be made of graphite by the above-mentioned pressurization and high-temperature heat treatment. Does not change.

Mesophase pitch obtained from petroleum pitch, coal pitch disclosed in Japanese Patent No. 2,630,939 or JP-A-7-307164.
Alternatively, if petroleum or coal coke is used as a raw material,
It becomes graphitizable carbon and cannot be used as a raw material of the present invention because it is graphitized even when the pressure and heat treatment of the present invention is performed.

In order to use as a negative electrode of a lithium secondary battery, it is subjected to pressure and heat treatment, and then crushed and sieved (classified). As the negative electrode material, the average particle size is 10 to 25 μm.
m, particles having a particle size of 50 μm or less are 95% by volume fraction.
The above is desirable.

In the present invention, the obtained carbon material is characterized in that the density is increased as compared with the non-pressed one, because the micropores are closed by pressurization and heating and the ratio thereof is reduced.

In the case of a carbon material obtained by a heat treatment at 1,000 to 2,000 ° C. under no pressure, JIS R72
Density (ρ) measured by the butanol method based on
_B ) is about 1.5 (g / cc).

On the other hand, under the pressure of the present invention of 1,000
It intended to have been processed to 2,000 ° C., the density ([rho _B) increases with 1.6~1.8 (g / cc). Further, the density measured by another helium method shows almost the same value as that measured by the butanol method, and the ratio (ρ _H ) of the density (ρ _B ) obtained by the butanol method to the density (ρ _H ) obtained by the helium method is obtained.
/ Ρ _B ) is 1.05 or less.

The helium method is considered to have a higher density than the butanol method if there are many micropores into which helium can enter. The carbon material of the present invention is characterized in that since the micropores are closed by pressurization, there is no difference in density between the two by the above-described measurement method.

In the present invention, the following features are directly observed. This is because, in the first stage heat treatment, a decomposition gas is generated, so that a trace of bubbles is generated by gas ejection. In the second stage of high-temperature heat treatment at 1000 to 2000 ° C., pressure is applied, so that traces of bubbles due to gas ejection as described above disappear, and the carbon particle surface becomes smooth. In the absence of such pressurization, traces of bubbles can be seen even at high temperatures.

Using the carbon powder of the present invention obtained as described above, a negative electrode was prepared by the following method, and the charge / discharge characteristics and high-temperature storage characteristics were examined.

Polyvinylidene fluoride (PVdF) was used as a binder in an amount of 10% by weight to 90% by weight of the carbon powder of the present invention.
In addition, an appropriate amount of n-methyl-2-pyrrolidone (NMP) was further added as a solvent to form a paste. After this paste was applied to a copper foil as a current collector, NMP was dried and then pressure-formed to form a negative electrode.

A negative electrode 21 made of the carbon powder and a counter electrode 2
2 and the reference electrode 23 were made of lithium metal, and the charge / discharge characteristics of the negative electrode were examined using an electrochemical cell as shown in FIG. The result is shown by a curve 11 in FIG. 1, and for comparison, the result of the charge / discharge characteristics of the conventional carbon anode is shown by a curve 12 in FIG.

As can be seen from FIG. 1, the charge / discharge characteristics of the conventional carbon anode firstly show a region where the voltage gradually changes from 1.5 V to 0.02 V, and then a flat region below 0.02 V. Is roughly divided into two areas.

On the other hand, in the negative electrode of the present invention, 1.5 V
Only a gradually changing region in the range from 0.02V to 0.02V appears, and a flat region below 0.02V is hardly seen.

As described above, it is considered that lithium ions are occluded in the six-membered ring layer and then occluded by the micropores in the charging reaction, so that the lithium ions gradually change in the range of 1.5 V to 0.02 V. The area is the response of the former, and
It is considered that the reaction is caused by the latter in a flat region of 02 V or less.

In the negative electrode of the present invention, 1.5 V to 0.02 V
Because it is only a smoothly changing area in the range of,
It can be seen that the reaction in charge and discharge is mainly a reaction in which lithium ions are occluded in the six-membered ring layer, and almost no lithium ions are occluded in the micropores. This can be said to be a significant feature of the present invention.

Although the charge / discharge capacity of the carbon anode of the present invention is reduced because lithium ions are not occluded in the micropores, the charge / discharge capacity in the region where the potential changes gradually from 1.5 V to 0.02 V is obtained. When comparing the conventional carbon anode, the charge and discharge capacity of 280 mAh / g or more can be obtained with the carbon anode of the present invention, while the conventional carbon anode is about 200 mAh / g to 240 mAh / g. This is presumably because the micropores were reduced and the ratio of the six-membered ring layer was increased by performing the pressure and heat treatment.

Further, with respect to the negative electrode of the present invention and the conventional negative electrode in which lithium ions were occluded to 0 V on the basis of the lithium reference electrode, the lithium N based on LiCl (0 ppm) was used.
MR ⁽⁷ Li-NMR) was analyzed.

In the negative electrode of the present invention, one signal was confirmed at about 20 ppm, whereas in the conventional negative electrode, 20 pp
Two signals were observed around m and 120 ppm.

The signal of lithium metal based on LiCl as a reference (0 ppm) is about 270 ppm, and the signal of 120 ppm confirmed in the conventional negative electrode indicates lithium occluded in micropores in a cluster state with reduced ionicity. it is conceivable that.

Since no signal of 120 ppm appears in the carbon powder of the present invention, it is analyzed that there is almost no occlusion of lithium ions in the micropores.

Further, the negative electrode of the present invention and the conventional negative electrode in which lithium ions were occluded up to 0 V were taken out, left at 60 ° C. for 20 days in a state of being immersed in an electrolytic solution, and measured for discharge capacity after leaving the storage. Was examined. As a result, it was found that the capacity was reduced by about 50% in the conventional negative electrode, but the storage property was improved in the negative electrode of the present invention when the capacity was reduced to 20% or less.

The cluster-like lithium occluded in the micropores easily reacts with the electrolytic solution because of its low ionicity and high activity, and the reaction product accumulates and hinders the charge / discharge reaction, resulting in poor storage characteristics. It is considered to be. However, lithium stored in the six-membered ring layer is considered to have relatively low reactivity because of its small night shift and high ionicity. In the negative electrode of the present invention, the occlusion of lithium into the micropores is suppressed and mainly occluded in the six-membered ring, so that the high-temperature storage characteristics are considered to be improved.

Further, the optimum conditions of the carbon material of the present invention having excellent storage characteristics were examined by preparing various materials while changing the second-stage pressurizing condition and the heat treatment temperature. As a result, the density (ρ _B ) measured by the butanol method is 1.6 to 1.7.
In the case where the distance is less than 0.36 nm, the interlayer distance of the six-membered ring layer measured by the X-ray diffraction method is larger than 0.36 nm and 0.41 nm.
The decrease in the capacity of the negative electrode during high-temperature storage was small.

The density (ρ _B ) measured by the butanol method is 1.7 to 1.8, and the distance between the six-membered ring layers is 0.3.
It was larger than 7 nm and smaller than 0.39 nm, and the decrease in negative electrode capacity during high-temperature storage was small.

As the positive electrode material of the lithium secondary battery of the present invention, a lithium-containing transition metal oxide is desirable.
Chemical formula LiCoO ₂ , LiM _x Co _1-x O ₂ , LiMn
_{2 O 4, Li 1 + x} Mn 2-x O 4, Li 1 + x M y Mn 2-xy O
₄ (M is Fe, Ni, Cr, Mn, Al, B, Si, T
By using a compound represented by at least one of i, x> 0, y> 0), a lithium secondary battery having excellent storability can be realized.

As a solvent for the organic electrolyte solution used in the present invention, a mixed solvent of propylene carbonate and ethylene carbonate or a single solvent thereof may be used. , Methyl propionate, ethyl propionate,
It is a mixed solvent to which at least one of dimethoxyethane and 2-methyltetrahydrofuran is added, and the volume fraction of a mixed solvent of propylene carbonate and ethylene carbonate or a single solvent is preferably from 0.3 to 0.6.

On the other hand, in the present invention, LiP is used as the lithium salt.
F ₆ , LiBF ₄ , LiClO ₄ , (C ₂ F ₅ SO ₂ ) ₂ NL
It is desirable to use at least one kind of i, (CF ₃ SO ₂ ) ₂ NLi, and its concentration is in a range of 0.5 to 1.5 mol / l.

The separator used in the present invention is preferably a polyethylene microporous membrane having a thickness of 20 to 50 μm.

The lithium secondary battery of the present invention has excellent storage characteristics at a high temperature, and is particularly suitable for a mobile phone, a portable information terminal device, a personal computer, a portable audio device, a portable device, and especially a drive power source and an electric power source for an electric vehicle. Used as a power supply for storage.

Hereinafter, the present invention will be specifically described with reference to the drawings. FIG. 2 shows an electrochemical cell used for performing an electrochemical evaluation of the negative electrode in the present invention. Negative electrode 21, Li metal counter electrode 2
2. Li metal reference electrode 23, electrolytic solution 24, glass container 25
It consists of. Using a three-electrode electrochemical cell using Li metal as a reference potential, the occlusion / release reaction of lithium ions to / from the negative electrode 21 was examined.

The electrolyte 24 contains propylene carbonate (PC) and diethyl carbonate (D
A solution in which 1 mol / l of LiPF ₆ was dissolved in the mixed solvent of EC) was used.

Example 1 Phenol resin, furan resin, melamine resin, epoxy resin, isotropic coal pitch,
The carbon material of the present invention was prepared using isotropic petroleum pitch as a raw material.

As a first stage heat treatment using each of the raw materials, the N ₂ gas was passed through the electric furnace at normal pressure and 8 H.
Heat at 00 ° C. for 2 hours. The generated gas was exhausted together with N ₂ .

Next, as a second-stage heat treatment, the carbon material obtained by the first-stage heat treatment was heated at 100 atm and 1200 ° C. for 2 hours by an autoclave having a heating section therein. During heating, the autoclave was purged with N ₂ gas and maintained at 100 atm.

The density of the obtained carbon material was measured by a butanol method and a helium method. The distance between the six-membered ring layers was measured by X-ray diffraction. Table 1 shows the results.

[0066]

[Table 1]

Using the respective carbons of the present invention prepared by the above method, a negative electrode was prepared by the following method, and a charge / discharge test was performed at room temperature using the electrochemical cell shown in FIG.

90% by weight of each carbon powder of the present invention
And polyvinylidene fluoride (PVdF) as a binder
Was added as a solvent, and n-methyl-2- as a solvent was further added.
An appropriate amount of pyrrolidone (NMP) was added to form a paste. After applying this mixture paste to a copper foil as a current collector, NM
After drying P, pressure molding was performed to form a negative electrode.

The charging (lithium occlusion) method was a constant current / constant voltage charging method (0.5 mA / cm ² constant current charging, 0 V constant voltage charging), a charging time was selected as a termination condition, and the condition was 24 hours. .

The charge / discharge curve of the carbon negative electrode of the present invention when a furan resin is used as a raw material at room temperature is shown as a curve 11 in FIG. The potential change of the carbon anode according to the present invention due to the insertion and extraction of lithium ions appears only in a gradually changing region in the range of 1.5 V to 0.02 V, and almost no flat region of 0.02 V or less is observed.

In the present embodiment, all the carbon materials are denoted by 1 in FIG.
1. The charge-discharge curve is almost the same as that of No. 1 and is 1.5 to 0.02
Table 1 shows the charge / discharge capacities in the region where the voltage gradually changes in the range of V.
Put it all together.

Further, all the carbon materials of this example were subjected to ⁷ Li-NMR measurement by the following method.
The negative electrode was taken out in a charged state, washed sufficiently with dimethyl carbonate (DMC), vacuum-dried, and the carbon powder mixture was scraped off the copper foil to obtain a sample for NMR measurement.

The NMR measurement was carried out at room temperature at an observation frequency of 155.
37Hz, MASGNN mode, sample rotation speed 4000H
z, the number of times of integration was 100 times. Note that LiCl was used as an external standard sample. Table 1 shows the shift values of all the confirmed signals.

Further, all the carbon materials of the present embodiment were taken out in a charged state, and sealed while being immersed in the electrolyte so that oxygen and moisture in the atmosphere were not mixed.
After storing for 0 days, the electrochemical cell shown in FIG. 2 was constructed again and the capacity retention was examined. Table 1 shows these results.

Comparative Example 1 In Example 1, the second heat treatment was performed under the conditions of 100 atm and 1200 ° C., but in Comparative Example 1, the second heat treatment was performed at normal pressure without applying pressure. A conventional carbon material was prepared. In Comparative Example 1, all the conditions such as the raw material and the heat treatment temperature were the same as in Example 1 except that the second stage heat treatment was at normal pressure.

Using the carbon material of this comparative example, all the same evaluations as in Example 1 were performed. Density, 6-member ring interlayer distance, 1.5
V to 0.02 V charge / discharge capacity, MNR shift value, 60
Table 1 shows the measurement results of the capacity retention rate after storage at 200C for 20 days.

Comparative Example 2 A carbon powder was prepared in the same manner as in Example 1 except that the carbon material was changed and mesophase pitch was used. Using the carbon material of this comparative example, similarly to Example 1, the density, the distance between the six-membered ring layers, and 1.5 V to 0.0
Table 1 shows the measurement results of the charge / discharge capacity at 2 V, the MNR shift value, and the capacity retention rate after storage at 60 ° C. for 20 days.

From the comparison of the measurement results of Example 1 and Comparative Examples 1 and 2, the carbon anode of the present invention had only one kind of occlusion state of lithium at the time of charging, whereas Comparative Examples 1 and 2 had a lithium occlusion state of 2 types. It can be seen from the NMR results that it is a species.

As described above, in the carbon negative electrode of the comparative example, 20 p, which indicates a state where lithium ions are occluded in the six-membered ring layer.
Both an NMR signal near pm and an NMR signal near 120 ppm indicating the state of occlusion in the micropores are observed. However, in the carbon anode of Example 1, 20 p
Only NMR signals near pm were confirmed. This indicates that the reaction in charge and discharge is mainly a reaction in which lithium ions are occluded in the six-membered ring layer, and almost no lithium ions are occluded in the micropores.

When comparing the charge and discharge capacities in a region where the potential changes gradually from 1.5 V to 0.02 V, the carbon anode of the comparative example is about 200 to 240 mAh / g, With the carbon negative electrode, a charge / discharge capacity of 280 mAh / g or more was obtained. The results show that the application of pressure and heat treatment reduced the number of micropores and increased the ratio of the six-membered ring layer.

Further, in the carbon material of Example 1, since the micropores were closed by pressurization, there was almost no difference due to the difference in density measurement method, and the density (ρB) measured by the butanol method and the helium method The ratio (ρH / ρB) of the density (ρH) measured according to
This range has been found to be desirable.

Example 2 Using a furan resin as a raw material,
The heat treatment was performed by changing the pressure condition of the second stage in the range of 10 to 120 atm, and a carbon powder was prepared in the same manner as in Example 1.

Using the carbon material of this example, the density, the distance between the six-membered ring layers, the charge / discharge capacity of 1.5 V to 0.02 V, and the capacity retention rate after storage at 60 ° C. for 20 days, as in Example 1. Was measured. Table 2 shows the results.

[0084]

[Table 2]

[Example 3] Isotropic petroleum pitch was used as a raw material, and the heat treatment temperature in the second stage was 1000 ° C to 2000 ° C.
The carbon powder was prepared in the same manner as in Example 1 except that the temperature was changed in the range of ° C.

Using the carbon material of this example, in the same manner as in Example 1, the density, the distance between the six-membered ring layers, the charge / discharge capacity of 1.5 V to 0.02 V, and the capacity retention rate after storage at 60 ° C. for 20 days Was measured. Table 3 shows the results.

[0087]

[Table 3]

From the results of Examples 2 and 3, when the density (ρB) measured by the butanol method is in the range of 1.6 to less than 1.7, the interlayer distance of the six-membered ring layer is larger than 0.36 nm, and A carbon material that satisfies the condition of less than .41 nm has a large capacity retention rate at high temperature storage. When the density (ρB) measured by the butanol method is in the range of 1.7 to 1.8, the carbon material satisfying the condition that the interlayer distance between the six-membered ring layers is larger than 0.37 nm and smaller than 0.39 nm. However, the capacity retention rate at high temperature storage is large, which is preferable.

[Embodiment 4] FIG. 3 is a schematic sectional view showing an embodiment of a lithium secondary battery of the present invention. Positive electrode 3
0, a separator 31, a negative electrode 32, and a separator 31 in this order, wound, and housed in a battery can 33. Positive electrode 3
The positive electrode tab 34 is attached to the negative electrode 32, the negative electrode tab 35 is connected to the negative electrode 32, and the negative electrode tab 35 is connected to the battery can 33.

Further, a safety valve (current cutoff valve) 37 is connected to the battery inner lid 36, and the safety valve (current cutoff valve) 37 is deformed by an internal pressure rise of 10 atm or more, and the electrical connection between the two is cut off. Hereinafter, a method for manufacturing the lithium secondary battery of this example will be described.

For a positive electrode active material represented by LiCoO ₂ , polyvinylidene fluoride (PVdF) was used as a binder, and graphite powder was used as a conductive additive.
%, 7%, and 5%, and N-methyl-2-pyrrolidone (NMP) was added as a solvent to prepare a positive electrode mixture paste.

Using an aluminum foil having a thickness of 20 μm, intermittent coating was performed on one surface of the aluminum foil, in which an applied portion and an uncoated portion were provided at regular intervals. Thereafter, the NMP in the applied positive electrode mixture paste was dried to form a positive electrode mixture film.

Further, a positive electrode mixture film was similarly formed on the other surface of the Al foil to obtain a coated electrode. At this time, the coated portion and the uncoated portion were exactly overlapped on both sides of the Al foil. Thereafter, the above-mentioned coated electrode was pressure-formed by a roll press to produce a positive electrode sheet.

Further, the positive-molded positive electrode sheet was cut into strips with the uncoated portion on one side, that is, the portion where the Al foil was exposed, and the positive electrode tab was used as a current collector.
1. Spot welding was applied to the foil portion.

On the other hand, Table 1 of Example 1 was used as a negative electrode active material.
The non-graphitizable carbon of the present invention produced using the isotropic coal pitch of the raw material shown in FIG. The non-graphitizable carbon and polyvinylidene fluoride (PVdF) are each 90% by weight,
10%, and N-methyl-
2-Pyrrolidone (NMP) was added to prepare a negative electrode mixture.

This negative electrode mixture was coated on an 18 μm Cu foil in the same manner as the above-mentioned positive electrode, provided with an applied portion and an uncoated portion, and a coated electrode was prepared and pressed to obtain a negative electrode sheet. Further, a strip was cut out while leaving an uncoated portion on one side, that is, a portion of the Cu foil, and a negative electrode tab was attached as a current collector to the Cu foil portion on one side by spot welding.

Using the above-described positive electrode and negative electrode and a polyethylene microporous membrane separator, a positive electrode, a separator, a negative electrode,
The electrodes were stacked in the order of the separators, which were spirally wound to form an electrode group. The positive electrode tab and the negative electrode tab were configured to be above and below the wound group. This electrode group was placed in a battery can, and the positive electrode tab was connected to the battery inner lid, and the negative electrode tab was connected to the battery can by spot welding.

As an electrolytic solution, a solution was prepared by dissolving 1 mol / l of LiPF ₆ in a mixed solvent of 1: 1 propylene carbonate (PC) and diethyl carbonate (DEC), and these electrolytic solutions were injected. Thereafter, the battery lid was attached to the battery can to produce a cylindrical battery.

Comparative Example 3 A conventional non-graphitizable carbon produced using the raw materials shown in Table 1 of Comparative Example 1 and isotropic coal pitch was used as the negative electrode active material in the same manner as in Example 4. A lithium secondary battery was manufactured.

Using five lithium secondary batteries prepared in Example 4 and Comparative Example 3, 10 cycles were performed at 25 ° C. as aging charge / discharge until the battery capacity was stabilized.

First, the battery was charged at a constant current of 0.5 A, and when the battery voltage reached 4.2 V, the battery was then charged at a constant voltage of 4.2 V. After 5 hours from the start of charging, the battery was charged. The charging was terminated (hereinafter, abbreviated as 0.5 A, 4.2 V, 5 hour end constant current constant voltage charging).

The discharge is a constant current discharge at a current of 0.5 A.
The operation was terminated when the battery voltage reached 2.8 V (hereinafter, referred to as “0.8 V”).
5A, abbreviated as 2.8V termination constant current discharge).

At the eleventh cycle, the sample was again subjected to 0.5 at 25 ° C.
A, 4.2 V, constant-current constant-voltage charging for 5 hours, and the rechargeable battery was stored in an atmosphere at 60 ° C. for 20 days. After saving,
The battery temperature was cooled to room temperature, and 0.5 A, 2.8 at 25 ° C.
V termination constant current discharge was performed to examine changes in battery capacity before and after storage.

The charge / discharge capacity at the 10th cycle before storage at 60 ° C., the charge / discharge capacity at the 11th cycle after storage, and the charge capacity at the 11th cycle of the five batteries prepared in Example 4 and Comparative Example 3, respectively. Table 4 shows the retention ratio of the discharge capacity with respect to.

[0105]

[Table 4]

Since the capacity retention of the lithium secondary battery of the present invention after storage at 60 ° C. is 80% or more, and the capacity retention of the comparative example is about 50%, the secondary battery of the present invention is excellent. It can be seen that it has high temperature storage characteristics.

[0107]

According to the carbon material used in the present invention, lithium ions are mainly absorbed and released in the six-membered layer by the charge and discharge reaction.
Since the lithium ions occluded in the six-membered ring layer have low reactivity with the electrolytic solution, a decrease in capacity due to a side reaction of the negative electrode can be reduced, and a lithium secondary battery having excellent high-temperature storage characteristics can be realized.

[Brief description of the drawings]

FIG. 1 is a graph showing a change in potential caused by insertion and extraction of lithium ions from a carbon material.

FIG. 2 is a schematic configuration diagram of an electrochemical cell used for measuring lithium ion occlusion and release characteristics of a negative electrode.

FIG. 3 is a schematic sectional view of a lithium secondary battery of the present invention.

[Explanation of symbols]

Reference numeral 21: negative electrode, 22: Li metal counter electrode, 23: Li metal reference electrode, 24: electrolytic solution, 25: glass container, 30: positive electrode, 3
DESCRIPTION OF SYMBOLS 1 ... Separator, 32 ... Negative electrode, 33 ... Battery can, 34 ... Positive electrode tab, 35 ... Negative electrode tab, 36 ... Battery inner lid, 37 ... Safety valve (current cutoff valve), 38 ... Gasket, 39 ... Insulating plate,
40 ... Battery outer lid.

 ────────────────────────────────────────────────── ─── Continuing from the front page (72) Inventor Yasushi Muranaka 7-1-1, Omika-cho, Hitachi City, Ibaraki Prefecture F-term in Hitachi Research Laboratory, Hitachi Ltd. 5H003 AA03 BA01 BB01 BC06 BD00 BD03 BD05 5H029 AJ04 AK03 AL07 AM03 AM05 AM07 BJ02 BJ14 CJ02 CJ12 CJ28 HJ08 HJ13 HJ18 HJ19

Claims (5)

[Claims] 1. A lithium secondary battery comprising a positive electrode and a negative electrode that occlude and release lithium ions, an organic electrolyte in which an electrolyte containing the lithium ions is dissolved, and the positive electrode and the negative electrode are arranged via a separator. The non-graphitizable carbon forming the negative electrode is subjected to a heat treatment in an inert atmosphere or an atmosphere in which the graphitized carbon is slightly oxidized to remove the decomposition gas, and then heat-treated under pressure to obtain the non-graphite. Density of carbonized carbon (butanol method) is 1.6 to 1.8 g
/ Cc. 2. 1.5 to 0.02 V based on a lithium reference electrode
The lithium secondary battery according to claim 1, wherein the charge / discharge capacity of the negative electrode is 280 mAh / g or more. 3. The measured density (ρ _B ) of the non-graphitizable carbon by a butanol method and the measured density (ρ _H ) of a helium method.
The lithium secondary battery according to claim 1, wherein a ratio (ρ _H / ρ _B ) of the lithium secondary battery is 1.05 or less. 4. The lithium secondary battery according to claim 1, wherein an interlayer distance of the six-membered ring layer of the non-graphitizable carbon is larger than 0.36 nm and smaller than 0.41 nm. 5. The lithium secondary battery according to claim 1, wherein an interlayer distance of the six-membered ring layer of the non-graphitizable carbon is larger than 0.37 nm and smaller than 0.39 nm.