JPH04319265A - Non-aqueous electrolyte secondary battery - Google Patents

Abstract

PURPOSE:To obtain a high discharging capacity and a long cycle life and to lower the self-discharging ratio as well by prescribing morphologic parameters of a carbon material used for an anode within a set range. CONSTITUTION:A plastic mixture consisting of resin hardening by light and polycarbonate is injection-molded to give a substrate. When ultraviolet-rays are irradiated and three-dimensional cross-links are formed among the polycarbonate molecules, the morphological parameters are limited within a prescribed range and the resulting carbon material has a plurality of heat generating peaks and at least one heat generating peak of >=700 deg.C by differential thermal analysis under the conditions of >=53.65Angstrom of the plane distance of (002) planes and <1.70g/cm<3> of true density. When the material is used as an anode, a non- aqueous electrolyte secondary battery can have high discharging capacity and a long cycle life and lower the self-discharging ratio.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to non-aqueous electrolyte secondary batteries, and particularly to improvements in the negative electrode thereof.

0002

BACKGROUND OF THE INVENTION The remarkable progress in electronic technology in recent years has led to successive miniaturization and weight reduction of electronic devices. Along with this, batteries as mobile power sources are becoming smaller and smaller.
Light weight and high energy density are required. Conventionally, aqueous solution secondary batteries such as lead batteries and nickel-cadmium batteries have been mainstream as secondary batteries. However, although these batteries have excellent cycle characteristics, they are not fully satisfactory in terms of battery weight, energy density, and self-discharge.

[0003]Recently, therefore, research and development of non-aqueous electrolyte secondary batteries using lithium or lithium alloys as negative electrodes has been actively conducted as a secondary battery to replace the above-mentioned nickel-cadmium batteries and the like. This non-aqueous electrolyte secondary battery has high energy density, little self-discharge, and is lightweight, making it suitable as a mobile power source.

However, in this non-aqueous electrolyte secondary battery, lithium crystals grow in a dendrite shape during charging due to repeated charge/discharge cycles, and this crystal-grown lithium reaches the positive electrode, causing an internal short circuit. Because of this potential, a sufficient cycle life cannot be obtained, which is a major obstacle to practical application.

[0005] Therefore, a non-aqueous electrolyte secondary battery using a carbon material for the negative electrode has been proposed as a non-aqueous electrolyte secondary battery with high energy density and long cycle life. This non-aqueous electrolyte secondary battery takes advantage of the fact that carbon intercalation compounds of lithium can be easily formed electrochemically, and when such a non-aqueous electrolyte secondary battery is charged, Lithium supported on a carbon material, lithium in the crystal structure of a positive electrode active material, lithium dissolved in an electrolyte, or the like is doped between the carbon layers of the negative electrode. and,
The carbon material doped with lithium functions as a lithium electrode, and lithium is released from between the carbon layers during discharge.

[0006] A non-aqueous electrolyte secondary battery using this carbon material as a negative electrode has a high energy density, and unlike the above-mentioned non-aqueous electrolyte secondary battery using lithium as a negative electrode, lithium crystal growth does not occur. , withstand repeated charge/discharge cycles. For this reason, development is progressing toward practical use, and for example, battery structures are being developed to serve as power sources for devices that consume relatively large currents, such as video cameras and portable personal computers (so-called laptop computers). For example, a spiral electrode structure has been proposed.

[0007]

[Problem to be Solved by the Invention] By the way, as a power supply for equipment used in the above-mentioned video cameras, etc.,
It is important to minimize capacity loss due to storage. However, non-aqueous electrolyte secondary batteries that use carbon materials as negative electrodes have greater self-discharge than non-aqueous electrolyte secondary batteries that use lithium or lithium alloys as negative electrodes, and are not fully satisfactory in terms of storage stability. It couldn't be called anything. Therefore, the present invention was proposed in view of the actual situation, and provides a non-aqueous electrolyte secondary battery that has a long cycle life, high energy density, high self-discharge rate, and excellent storage stability. The purpose is to provide.

[0008]

Means for Solving the Invention In order to achieve the above-mentioned problems, the nonaqueous electrolyte secondary battery of the present invention has a (002) plane spacing of 3.65 Å or more and a true density of 1.70 g/ cm3
The carbonaceous material is characterized in that the negative electrode is made of a carbonaceous material that has a temperature of less than 700° C., has a plurality of exothermic peaks in differential thermal analysis, and has at least one exothermic peak at 700° C. or higher.

In the non-aqueous electrolyte secondary battery of the present invention,
In order to obtain a long cycle life and reduce the self-discharge rate, the negative electrode has a (002) plane spacing of 3.65.
Å or more, preferably 3.70 Å or more, true specific gravity 1.70 g
/cm3, and there are multiple exothermic peaks in differential thermal analysis in an air stream, with at least one being less than 7 cm3.
A carbon material that exists at temperatures above 00°C is used.

[0010] Examples of carbon materials having such properties include carbonaceous materials obtained by carbonizing organic materials by a method such as firing. As the organic material serving as the starting material for carbonization, furan resin made of furfuryl alcohol or a homopolymer or copolymer of furfural is suitable. Specifically, polymers consisting of furfural + phenol, furfuryl alcohol + dimethylol urea, furfuryl alcohol, furfuryl alcohol + formaldehyde, furfuryl alcohol + furfural, furfural + ketones, etc. can be mentioned. When these furan resins are fired at an appropriate temperature, they have the above-mentioned properties ((002) plane spacing of 3.65 Å or more, true specific gravity of 1.70 g/cm3).
A carbonaceous material is obtained which has multiple exothermic peaks (at least one of which is present at temperatures of 700°C or higher) in differential thermal analysis in an air stream, and this obtained carbonaceous material has , exhibits very good properties as a negative electrode material for batteries.

Alternatively, the hydrogen/carbon atomic ratio is 0 as a raw material.
．． Using petroleum pitch of 6 to 0.8, oxygen-containing functional groups are introduced into it and so-called oxygen crosslinking is performed to reduce the oxygen content to 1.
A carbonaceous material obtained by firing the precursor in an amount of 0 to 20% by weight is also suitable. Furthermore, it is also possible to use a carbonaceous material in which the amount of lithium doped is increased by adding a phosphorus compound or a boron compound when carbonizing the furan resin, petroleum pitch, or the like.

On the other hand, as the positive electrode material used in the above-mentioned non-aqueous electrolyte secondary battery, it is preferable to use a material containing sufficient lithium, such as the general formula LiMO2 (where M is at least one of Co and Ni). ) and intercalation compounds containing lithium are used. In particular, LiCoO2, LiCo0.8
Ni0.2 O2 has high voltage, high energy density,
This is desirable in terms of obtaining good cycle characteristics.

[0013] As the electrolytic solution, for example, an electrolytic solution containing a lithium salt dissolved in an organic solvent is used. Here, the organic solvent is not particularly limited, but for example, propene carbonate, ethylene carbonate, diethyl carbonate, 1,2-
Dimethoxyethane, 1,2-diethoxyethane, γ-butyrolactone, tetrahydrofuran, 1,3-dioxolane, 4-methyl-1,3-dioxolane, diethyl ether, sulfolane, methylsulfolane, acetonitrile, propionitrile, etc. alone or A mixed solvent of two or more types can be used. Any conventionally known electrolytes can be used, including LiClO4, LiAsF6, L
iPF6, LiBF4, LiB(C6H5)4
, LiCl, LiBr, CH3 SO3 Li, CF
3 SO3 Li etc.

[0014]

[Function] In a non-aqueous electrolyte secondary battery using a carbonaceous material as a negative electrode, the interplanar spacing of the (002) plane is 3.65 Å or more, the true density is less than 1.70 g/cm3, and multiple When a carbonaceous material that satisfies the conditions of having an exothermic peak and at least one exothermic peak at 700° C. or higher is used, a large discharge capacity and a long cycle life can be obtained, and the self-discharge rate can be reduced.

[0015] This is presumably due to the following reasons. That is, when carbon doped with lithium has an interlayer distance (distance between (002) planes) d002 of 3.65 Å, the interlayer distance increases upon doping with lithium. Therefore, in a carbonaceous material with d002 <3.65 Å, it is considered that doping with lithium becomes difficult due to the need to widen the interlayer distance, and this is considered to reduce the amount of doping. On the other hand, the true density ρ has a close relationship with the interlayer distance, and ρ>1.7
When it becomes 0 g/cm3, it becomes difficult to secure the above-mentioned interlayer distance, and the doping amount also decreases. The carbon material used in the non-aqueous electrolyte secondary battery is (002
) The spacing between the surfaces is 3.65 Å or more, and the true density is 1.70 g/
It is said to be less than cm3. Therefore, a large amount of lithium is doped between the carbon layers, which is presumed to achieve a long cycle life and a large discharge capacity of the battery. Further, the carbon material has a plurality of exothermic peaks in differential thermal analysis, and at least one of these peaks is at 700° C. or higher. In non-aqueous electrolyte secondary batteries, the self-discharge rate is considered to be influenced by the structure of the carbon material that serves as the negative electrode. If a carbon material with a structure in which at least one exothermic peak in differential thermal analysis is at 700°C or higher is used, the self-discharge rate will decrease due to reasons such as the lithium doped in the carbon material becoming difficult to release. reduce,
It is presumed that high storage stability can be obtained.

[0016]

EXAMPLE A preferred example of the present invention will be described based on experimental results.

Example 1 In this example, a carbon material with a (002) plane spacing of 3.75 Å, a true specific gravity of 1.58 g/cm3, and exothermic peaks at 669°C and 705°C in differential thermal analysis was used as a negative electrode. This is an example of a non-aqueous electrolyte secondary battery.

First, in order to fabricate the non-aqueous electrolyte secondary battery shown in FIG. 1, the negative electrode 1 was fabricated as follows.

Petroleum pitch is used as a starting material, and after introducing 10 to 20% by weight of oxygen-containing functional groups (so-called oxygen crosslinking), it is calcined at 1200°C in an inert gas stream to produce a product close to glassy carbon. A carbonaceous material with specific properties was obtained. As a result of X-ray diffraction measurement of this material,
The spacing of the (002) plane is 3.75 Å, and as a result of measuring the true specific gravity using the pycnometer method, the true specific gravity is 1.58 g.
/cm3. Further, when differential thermal analysis was performed in an air stream, exothermic peaks were found at 669°C and 705°C. This carbon material was pulverized to obtain carbon material powder with an average particle size of 10 μm.

The thus obtained carbonaceous material powder was used as a negative electrode active material carrier, and 90 parts by weight of this was mixed with 10 parts by weight of polyvinylidene fluoride (PVDF) as a binder,
A negative electrode mixture was prepared. This negative electrode mixture is mixed with N-
It was dispersed in methyl-2-pyrrolidone to form a slurry (paste). The thickness of the negative electrode current collector is 10 μm.
Using a strip-shaped copper foil, a negative electrode mixture slurry was applied to both sides of the current collector, and after drying the solvent, compression molding was performed using a roller press machine to form a sheet with a width of 41.5 mm and a length of 700 mm.
A slit strip-shaped negative electrode 1 was prepared.

[0021] Next, a positive electrode 2 was prepared as shown below. 0.5 mol of lithium carbonate and 1 mol of cobalt carbonate were mixed and fired in air at 900° C. for 5 hours to obtain LiCoO2 as a positive electrode active material. Then, 91 parts by weight of this LiCoO2, 6 parts by weight of graphite as a conductive agent, and 3 parts by weight of polyvinylidene fluoride as a binder were mixed to prepare a positive electrode mixture. This positive electrode mixture was dispersed in N-methylpyrrolidone to form a slurry (paste).

Next, a strip-shaped aluminum foil with a thickness of 20 μm is used as a positive electrode current collector, and a positive electrode mixture slurry is uniformly applied to both sides of the current collector. After drying the solvent, compression molding is performed to obtain a strip-shaped positive electrode. 2 was produced. The width of the electrode is 39.5m.
It was set as m.

[0023] The strip-shaped negative electrode 1 produced as described above,
A strip-shaped positive electrode 2 and a separator 3 made of a microporous polypropylene film with a thickness of 25 μm and a width of 44 mm were laminated in the order of negative electrode, separator, positive electrode, and separator to a thickness of 0.4 μm.
It was made into a laminate of mm. This laminate is wound in a spiral shape many times, and the final end of the outermost separator is fixed with tape 20.
An electrode was created. Note that the inner diameter of the hollow portion at the center of this spiral electrode was 3.5 mm, and the outer diameter was 19.7 mm.

The spiral electrode described above was housed in a nickel-plated iron battery can 5. An insulating plate 4 is provided on both the upper and lower surfaces of the spiral electrode, and an aluminum positive electrode lead 12 is provided.
was led out from the positive electrode current collector and welded to the battery lid 7, and a nickel negative electrode lead 11 was led out from the negative electrode current collector and welded to the battery can 5. Inside this battery can 5 are propylene carbonate and 1,
LiPF in an equal volume mixed solvent with 2-dimethoxyethane
An electrolytic solution in which 6 was dissolved at a ratio of 1 mol/l was injected. Finally, by caulking the battery can 5 through the insulating sealing gasket whose surface is coated with asphalt, the battery lid 7
A cylindrical non-aqueous electrolyte secondary battery (Example Battery 1) with a diameter of 20 mm and a height of 50 mm was used.
)It was created.

Example 2 In this example, a carbon material having a (002) plane spacing of 3.73 Å, a true specific gravity of 1.58 g/cm3, and exothermic peaks at 680°C and 726°C in differential thermal analysis was used as a negative electrode. This is an example of a non-aqueous electrolyte secondary battery.

After crosslinking petroleum pitch with oxygen in the same manner as in Example 1, it was fired at 1300° C. in an inert gas stream to obtain a carbon material having properties similar to glassy carbon. X of this material
As a result of line diffraction measurement, the spacing of the (002) plane was 3.
73 Å, and the true specific gravity by pycnometer method is 1.
It was 58g/cm3. Further, in differential thermal analysis in an air stream, exothermic peaks existed at 680°C and 726°C. This carbonaceous material was pulverized to have an average particle size of 10 μm. The procedure was the same as in Example 1 except that the carbon material powder obtained in this way was used as a negative electrode active material carrier.
A cylindrical non-aqueous electrolyte secondary battery (Example Battery 2) with a diameter of 0 mm and a height of 50 mm was created.

Example 3 In this example, a carbon material having a (002) plane spacing of 3.71 Å, a true specific gravity of 1.58 g/cm3, and exothermic peaks at 691°C and 737°C in differential thermal analysis was used as a negative electrode. This is an example of a non-aqueous electrolyte secondary battery.

After crosslinking petroleum pitch with oxygen in the same manner as in Example 1, it was fired at 1400° C. in an inert gas stream to obtain a carbon material having properties similar to glassy carbon. X of this material
As a result of line diffraction measurement, the spacing of the (002) plane is 3
．． 71 Å, and the true specific gravity according to the pycnometer method is 1
．． It was 58g/cm3. In addition, in differential thermal analysis in an air stream, the exothermic peaks were 691°C and 737°C.
existed in This carbonaceous material is crushed and the average particle size is 10μ.
It was set as m. In the same manner as in Example 1, except that the carbonaceous material powder obtained in this way was used as a negative electrode active material carrier,
A cylindrical non-aqueous electrolyte secondary battery (Example Battery 3) with a diameter of 20 mm and a height of 50 mm was created.

Example 4 In this example, a carbon material having a (002) plane spacing of 3.67 Å, a true specific gravity of 1.59 g/cm3, and exothermic peaks at 704°C and 758°C in differential thermal analysis was used as a negative electrode. This is an example of a non-aqueous electrolyte secondary battery.

After crosslinking petroleum pitch with oxygen in the same manner as in Example 1, it was fired at 1600° C. in an inert gas stream to obtain a carbon material having properties similar to glassy carbon. X of this material
As a result of line diffraction measurement, the spacing of the (002) plane is 3
．． 67 Å, and the true specific gravity according to the pycnometer method is 1
．． It was 59g/cm3. Further, in differential thermal analysis in an air stream, exothermic peaks were found at 704°C and 758°C. This carbonaceous material is crushed and the average particle size is 10 μm.
And so. A cylindrical nonaqueous electrolyte secondary battery with a diameter of 20 mm and a height of 50 mm (
Example battery 4) was created.

Comparative Example 1 This comparative example uses a carbon material as a negative electrode, which has a spacing of (002) planes of 3.79 Å, a true specific gravity of 1.58 g/cm3, and an exothermic peak at 639°C in differential thermal analysis. This is an example of a water electrolyte secondary battery.

After crosslinking petroleum pitch with oxygen in the same manner as in Example 1, it was fired at 1000° C. in an inert gas stream to obtain a carbon material having properties similar to glassy carbon. X of this material
As a result of line diffraction measurement, the spacing of the (002) plane is 3
．． 79 Å, and the true specific gravity according to the pycnometer method is 1
．． It was 58g/cm3. Further, in differential thermal analysis in an air stream, an exothermic peak existed at 639°C. This carbonaceous material powder was pulverized to have an average particle size of 10 μm. A diameter of 20 mm, a diameter of 20 mm,
A cylindrical non-aqueous electrolyte secondary battery (Comparative Example Battery 1) with a height of 50 mm was created.

Comparative Example 2 For comparison, non-aqueous electrolysis using a carbon material as a negative electrode with a (002) plane spacing of 3.62 Å, true specific gravity of 1.60 g/cm3, and an exothermic peak of 798°C in differential thermal analysis. This is an example of a liquid secondary battery.

After crosslinking petroleum pitch with oxygen in the same manner as in Example 1, it was fired at 1800° C. in an inert gas stream to obtain a carbon material having properties similar to glassy carbon. X of this material
As a result of line diffraction measurement, the spacing of the (002) plane is 3
．． 62 Å, and the true specific gravity according to the pycnometer method is 1
．． It was 60g/cm3. Further, in differential thermal analysis in an air stream, an exothermic peak existed at 798°C. This carbonaceous material powder was pulverized to have an average particle size of 10 μm. A diameter of 20 mm was prepared in the same manner as in Example 1, except that the carbon material powder thus obtained was used as a negative electrode active material carrier.
A cylindrical non-aqueous electrolyte secondary battery (Comparative Example Battery 2) with a height of 50 mm was prepared.

Example batteries 1 to 4 prepared as described above
For Comparative Example Batteries 1 and 2, the capacity retention rate and self-discharge rate after 100 cycles of charging and discharging were examined. The capacity maintenance rate is determined by setting the charging upper limit voltage to 4.
After 2.5 hours of constant voltage and constant current charging with a charging current of 1 V and a charging current of 1 A, repeat the charge/discharge cycle of discharging to a final voltage of 2.75 V with a constant load of 6 Ω. The discharge capacity at the 100th cycle was recorded and calculated from these values. Also, the self-discharge rate is 1
In the 01st cycle, the charging upper limit voltage was set to 4.1 V and the charging current was set to 1 A, and after 2.5 hours of constant voltage constant current charging, 22
℃ for 720 hours, and then discharged to a final voltage of 2.75V under a constant load of 6Ω.
It was calculated by comparing it with the discharge capacity at the 0th cycle.

Table 1 shows the measurement results of the capacity retention rate and self-discharge rate of each battery.

[Table 1]

As shown in Table 1, Examples Batteries 1 to 4 have lower self-discharge rates than Comparative Examples Batteries 1 and 2, and the decrease in discharge capacity due to storage is very small. Furthermore, the initial capacity is large and the capacity retention rate is high. Therefore, from these results, the interplanar spacing of the (002) plane is 3.65 Å or more, the true specific gravity is less than 1.70 g/cm3, and multiple exothermic peaks are present in differential thermal analysis in an air stream. It has been shown that large capacity, long cycle life, and low self-discharge rate can be achieved by using a carbon material that satisfies the condition that at least one carbon material exists at a temperature of 700° C. or higher. In addition, since the capacity retention rate was superior in the order of Example Batteries 1, 2, 3, and 4, it was found that when a carbon material with a (002) plane spacing of 3.70 Å or more was used, a non-aqueous battery with particularly excellent discharge capacity It was found that an electrolyte secondary battery could be obtained.

For example, when a carbon material whose exothermic peaks in differential thermal analysis are all below 700°C is used (Comparative Example Battery 1), the self-discharge rate becomes large and the storage stability is insufficient. On the other hand, it was found that when a carbon material with a (002) plane spacing of less than 3.65 Å was used (Comparative Example Battery 2), the capacity and capacity retention rate were insufficient.

[0039]

Effects of the Invention As is clear from the above explanation, the non-aqueous electrolyte secondary battery of the present invention has the morphological parameters of the carbon material used for the negative electrode within a predetermined range. It is possible to obtain high capacity and long cycle life, and also to reduce the self-discharge rate. Therefore, it can supply sufficient power even to devices with relatively high power consumption such as laptops, computers, video cameras, etc. Moreover, it has excellent cycle life and storage stability, so it is also practical. Makes an excellent power source for equipment.

[0040]

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view showing a configuration example of a cylindrical non-aqueous electrolyte secondary battery.

Claims (1)

[Claims] Claim 1: The (002) plane has a spacing of 3.65 Å or more, a true density of less than 1.70 g/cm3, and has multiple exothermic peaks in differential thermal analysis.
A non-aqueous electrolyte secondary battery characterized in that a carbonaceous material having at least one exothermic peak at 00° C. or higher is used as a negative electrode.