JPH1173961A - Manufacture of lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the manufacturing method of a lithium secondary battery capable of enhancing discharge capacity, the initial charge/discharge efficiency, and prolonging the cycle life. SOLUTION: In a manufacturing method of a lithium secondary battery having a positive electrode, a negative electrode containing a carbonaceous material capable of absorbing/releasing lithium ions, and a nonaqueous electrolyte, the carbonaceous material is obtained by graphitizing a carbon precursor containing 1-50 wt.% at least one kind selected from among polymers having at least one of an amide group and a nitrile group and thermosetting resin, and mesophase pitch obtained through heating at 200-500 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing a carbonaceous material contained in a negative electrode of a lithium secondary battery.

[0002]

2. Description of the Related Art In recent years, non-aqueous electrolyte batteries using lithium as a negative electrode active material have attracted attention as high energy density batteries, and manganese dioxide (MnO ₂ ) has been used as a positive electrode active material.
Fluorocarbon [(CF ₂ ) _n ], thionyl chloride (SOCl
Primary batteries using ₂ ) and the like are already widely used as power supplies for calculators, watches, and as backup batteries for memories.

Further, in recent years, with the reduction in size and weight of various electronic devices such as VTRs and communication devices, the demand for secondary batteries having a high energy density as a power source for these devices has increased, and lithium secondary batteries using lithium as a negative electrode active material have been required. Battery research is being actively conducted.

A lithium secondary battery uses lithium as a negative electrode and propylene carbonate (PC), 1,2-dimethoxyethane (DME), γ-butyrolactone (γ-
BL), a non-aqueous electrolyte in which a lithium salt such as LiClO ₄ , LiBF ₄ , or LiAsF ₆ is dissolved in a non-aqueous solvent such as tetrahydrofuran (THF) or a lithium ion conductive solid electrolyte. TiS ₂ ,
The use of compounds that undergo a topochemical reaction with lithium, such as MoS ₂ , V ₂ O ₅ , V ₆ O ₁₃ , and MnO ₂ , has been studied.

[0005] However, the above-mentioned lithium secondary battery has not yet been put to practical use. The main reason for this is that the charge / discharge efficiency is low and the number of times that charge / discharge can be performed (cycle life) is short. It is considered that this is largely due to the deterioration of lithium due to the reaction between the lithium of the negative electrode and the non-aqueous electrolyte. That is, lithium dissolved in the non-aqueous electrolyte as lithium ions at the time of discharge reacts with the solvent at the time of deposition at the time of charging, and its surface is partially inactivated. Therefore, when charge and discharge are repeated, lithium precipitates in a dendritic (dendritic) or small spherical shape, and further, phenomena such as detachment of lithium from the current collector occur.

Further, the use of a lithium alloy having a composition formula of Li _X A (A is composed of a metal such as Al) as a negative electrode is being studied. This negative electrode has a large amount of lithium ion storage and release per unit volume, and has a high capacity.However, the lithium ion expands and contracts when it is stored and released. You. For this reason, the lithium secondary battery provided with the negative electrode has a problem that the charge / discharge cycle life is short.

[0007] For this reason, by using a carbonaceous substance that absorbs and releases lithium, such as coke, a resin fired body, carbon fiber, and pyrolysis gas phase carbon, as a negative electrode incorporated in a lithium secondary battery, lithium and lithium can be removed. It has been proposed to improve the reaction with a non-aqueous electrolyte and the deterioration of the negative electrode characteristics due to the precipitation of dendrites.

[0008] In the negative electrode containing the carbonaceous material, in the carbonaceous material having a structure in which hexagonal mesh layers mainly composed of carbon atoms are stacked (graphite structure), lithium ions are present in a portion between the layers. It is considered that charging and discharging are performed by going in and out. For this reason, it is necessary to use a carbonaceous material having a somewhat developed graphite structure for the negative electrode of the lithium secondary battery. However, when a carbonaceous material obtained by pulverizing a graphitized giant crystal is used as a negative electrode in a non-aqueous electrolyte, the non-aqueous electrolyte is decomposed, and as a result, the capacity and charge / discharge efficiency of the battery are reduced. In addition, there is a problem that the cycle life is shortened because the capacity decrease becomes larger as the charge / discharge cycle progresses.

Further, studies have been made to increase the capacity of the negative electrode containing the carbonaceous material. For these reasons, J.I. Elec
trochem. Soc. , Vol. 142, no. 1
1, November 1995, pages 3668-367.
Table II on page 7 discloses a coin-type battery including a cathode including a fired epoxy novolak resin having a void having a diameter of 7.4 to 8.8 angstroms by a small angle X-ray scattering method, and an anode made of lithium foil. Have been. By using such a resin fired body, a high capacity of 372 mAh / g or more in unit weight capacity is obtained.
However, the fired resin body has an X-ray diffraction of 0.37.
Since it has a peak corresponding to d ₀₀₂ larger than nm (d ₀₀₂ is the spacing between (002) planes), and even if the true density is high, it is at most about 1.7 g / cm ³ .
The amount of occluded and released lithium ions per unit volume is small. Therefore, the lithium secondary battery provided with the negative electrode including the resin fired body has a problem that the initial charge / discharge efficiency and the discharge capacity are low and the charge / discharge cycle life is short.

[0010]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method of manufacturing a lithium secondary battery capable of improving discharge capacity, initial charge / discharge efficiency and cycle life.

[0011]

According to the present invention, there is provided a method of manufacturing a lithium secondary battery comprising a positive electrode, a negative electrode containing a carbonaceous substance that occludes and releases lithium ions, and a non-aqueous electrolyte. The carbonaceous material may be a polymer having at least one of an amide group and a nitrile group, and 1 to 50% by weight of at least one selected from thermosetting resins, and a mesophase pitch heated at 200 to 500 ° C. A method for manufacturing a lithium secondary battery is provided, which is manufactured by a method including a step of performing a graphitization treatment on a carbon precursor containing the same.

According to the present invention, there is provided a method for manufacturing a lithium secondary battery comprising a positive electrode, a negative electrode containing a carbonaceous material that occludes and releases lithium ions, and a non-aqueous electrolyte, wherein the carbonaceous material is an amide. 1 to 50% by weight of at least one selected from a polymer having at least one of a group and a nitrile group, and a thermosetting resin;
A step of spinning the carbon precursor containing the mesophase pitch heated at 0 ° C. at 200 to 500 ° C., a step of pulverizing the spun carbon precursor, and a step of subjecting the pulverized carbon precursor to a graphitization treatment; And a method for producing a lithium secondary battery characterized by being produced by a method comprising:

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a lithium secondary battery (for example, a cylindrical lithium secondary battery) provided with a negative electrode containing a carbonaceous material manufactured by the method according to the present invention will be described in detail with reference to FIG. .

For example, a cylindrical container 1 having a bottom made of stainless steel has an insulator 2 disposed at the bottom. Electrode group 3
Are stored in the container 1. The electrode group 3 includes:
A belt-like material in which a positive electrode 4, a separator 5 and a negative electrode 6 are laminated in this order is spirally wound so that the separator 5 is located outside.

The container 1 contains an electrolytic solution. The insulating paper 7 having a central portion opened is placed above the electrode group 3 in the container 1. The insulating sealing plate 8
The sealing plate 8 is disposed at the upper opening of the container 1 and caulked in the vicinity of the upper opening inward.
Is fixed to the container 1 in a liquid-tight manner. The positive terminal 9 is
The insulating sealing plate 8 is fitted at the center. Positive lead
One end of the cathode 10 is connected to the positive electrode 4, and the other end is connected to the positive electrode terminal 9.
Connected to each other. The negative electrode 6 is connected to the container 1 as a negative terminal via a negative lead (not shown).

Next, the positive electrode 4, the separator 5, the negative electrode 6, and the electrolytic solution will be described in detail.

1) Positive Electrode 4 The positive electrode 4 is prepared by suspending a conductive agent and a binder in a positive electrode active material in an appropriate solvent, applying the suspension to a current collector, and drying to form a thin plate. Is done.

As the positive electrode active material, various oxides,
For example, manganese dioxide, lithium manganese composite oxide,
Examples include lithium-containing nickel oxide, lithium-containing cobalt compound, lithium-containing nickel-cobalt oxide, lithium-containing iron oxide, lithium-containing vanadium oxide, and chalcogen compounds such as titanium disulfide and molybdenum disulfide. Above all, when lithium cobalt oxide {Li _x CoO ₂ (0.8 ≦ x ≦ 1)}, lithium nickel oxide (LiNiO ₂ ), and lithium manganese oxide (LiMn ₂ O ₄ or LiMnO ₂ ) are used, high voltage Is preferred because

Examples of the conductive agent include acetylene black, carbon black, graphite and the like.

Examples of the binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), ethylene-propylene-diene copolymer (EPDM), and styrene-butadiene rubber (SBR). it can.

The mixing ratio of the positive electrode active material, the conductive agent and the binder is 80 to 95% by weight of the positive electrode active material, 3 to 2% of the conductive agent.
It is preferable that the content be in the range of 0% by weight and 2 to 7% by weight of the binder.

As the current collector, for example, aluminum foil, stainless steel foil, nickel foil or the like can be used.

2) Separator 5 The separator 5 may be, for example, a synthetic resin nonwoven fabric,
A polyethylene porous film, a polypropylene porous film, or the like can be used.

3) Negative Electrode 6 The negative electrode 6 contains a carbonaceous material that stores and releases lithium ions. The carbonaceous material is produced, for example, by a method described below.

First, the mesophase pitch is subjected to a heat treatment at 200 to 500 ° C. in an atmosphere containing oxygen gas such as air. To this, 1 to 50% by weight of a polymer having at least one of an amide group and a nitrile group, a thermosetting resin, or both of the polymer and the thermosetting resin is added. After cooling the obtained carbon precursor, it is pulverized and subjected to a graphitization treatment to produce a carbonaceous material.

The carbonaceous material obtained by such a method has a multi-phase structure as shown in FIG. 2, for example, and is a graphite crystallite having a structure in which a plurality of hexagonal mesh layers 20 are laminated with a certain regularity. It includes a graphite structure region having C and an amorphous carbon structure region. In the amorphous carbon structure region, the arrangement of the hexagonal reticular layers 20 of graphite crystallites is not regular, and for example, voids (defects) such as region D exist. Most of the voids (50% or more of all voids) have a diameter of 0.1 mm.
5-5 nm. Since lithium ions can form a complex in the void having such a diameter,
The voids can be impregnated with lithium ions at high density. The diameter of the gap can be measured by a small-angle X-ray scattering method.

The carbonaceous material had a powder X-ray diffraction of 0.34.
It has a peak corresponding to d ₀₀₂ of 0 nm or less. It is almost difficult for carbonaceous materials without such peaks to use the voids as lithium ion storage sites. The carbonaceous material has a particle size of 0.1 in powder X-ray diffraction.
It may have only a peak corresponding to d ₀₀₂ of 340 nm or less, or may have a peak corresponding to d ₀₀₂ exceeding 0.340 nm. However, powder X
The carbonaceous material whose peak corresponding to d ₀₀₂ of 0.370 nm or more is detected by line diffraction has a true density of 1.8 g / c.
It may become smaller than m ^3.

The spacing d of the (002) plane of the carbonaceous material
₀₀₂ can be determined from the position of the peak and the half width of the diffraction pattern obtained by powder X-ray diffraction. As the calculation method, the half-width half-point method specified in the Gakushin method is used. In the powder X-ray diffraction measurement, CuKα was used as an X-ray source,
Use high-purity silicon as a standard. In addition, the half value width midpoint method is described in the literature of "Tanso (carbon)", 1963, p25.

The true density of the carbonaceous material is 1.8 g / cm
³ or more. The true density of the carbonaceous material is 1.8 g / cm
^If it is less than ³ , the graphite structure region in the carbonaceous material will be insufficient, and the degree of graphitization of the graphite structure region will decrease, and the volume specific capacity (mAh / cc) of the negative electrode will decrease. The true density of a carbonaceous material whose microstructure is composed only of a graphite structure region is 2.25 g / c.
m ³ , and the true density of the carbonaceous material obtained by the method of the present invention does not reach 2.25 g / cm ³ .

The above-mentioned heat treatment step will be described.

Examples of the mesophase pitch include a pitch obtained from petroleum or coal, a synthetic pitch, and the like.

By performing the heat treatment, the mesophase pitch can be melted, and the dispersibility of the polymer (solid) and the thermosetting resin (solid) in the mesophase pitch can be improved. As a result, the size of the void in the carbonaceous material is 0.5 to 5 nm in diameter.
It becomes easy to become. Further, by performing the heat treatment, the mesophase pitch can be subjected to infusibility treatment. If the temperature of the heat treatment is lower than 200 ° C., it becomes difficult to melt the mesophase pitch. On the other hand, when the temperature of the heat treatment exceeds 500 ° C.,
The mesophase pitch is easily oxidized and may burn. A more preferred range of the heat treatment temperature is in the range of 200 to 400 ° C.

Next, a polymer having at least one of the amide group and the nitrile group and a thermosetting resin will be described.

By adding the polymer and the thermosetting resin to the heat-treated mesophase pitch in an amount of 1 to 50% by weight, graphite crystals develop even when the graphitization treatment is performed at a high temperature of, for example, 2000 ° C. or more. It is possible to form a portion that becomes a gap without developing and a portion that does not develop. this is,
This is because graphite crystals do not develop even if the polymer or resin is subjected to heat treatment.

Examples of the polymer having an amide group include aramid having an aromatic (benzene ring) and an amide group.

Examples of the polymer having a nitrile group include:
For example, polyacrylonitrile and the like can be mentioned.

Examples of the thermosetting resin include a phenol resin and a furan resin.

Among the above-mentioned polymers and thermosetting resins, polyacrylonitrile is preferred.

The reason why the amounts of the polymer and the thermosetting resin in the carbon precursor are set in the above-described ranges is as follows. When the amount is less than 1% by weight,
The proportion of voids in the carbonaceous material decreases, and the fine structure of the carbonaceous material approaches a graphite structure (true density of 2.25 g / cm ³ ). On the other hand, if the amount exceeds 50% by weight, the proportion of voids having a diameter exceeding 5 nm in the voids in the carbonaceous material becomes high. A void having a diameter exceeding 5 nm cannot form a lithium complex, so that it is difficult to impregnate lithium ions with high density. If the amount exceeds 50% by weight, the true density may be less than 1.8 g / cm ³ . Accordingly, the lithium secondary battery including the negative electrode containing such a carbonaceous material has reduced discharge capacity and cycle life. A more preferable range of the compounding amount is 20 to 40% by weight.

The above-described graphitization process will be described.

The graphitization treatment can be performed, for example, by a heat treatment at 2000 ° C. or higher in an inert gas atmosphere such as an argon atmosphere. When the temperature of the graphitization treatment is less than 2000 ° C., the X
In some cases, a peak corresponding to d ₀₀₂ of 0.340 nm or less does not exist in a line diffraction peak. A more preferable range of the graphitization temperature is 2400 to 3000 ° C.

When raising the temperature of the carbon precursor to a temperature at which the carbonization treatment is performed, it is preferable to set the temperature raising rate to 100 ° C./hr or less. By increasing the temperature at such a rate, a gap having a diameter of 0.5 to 5 nm is easily obtained. If the heating rate is higher than 100 ° C./hr, the carbon precursor may be cracked during the graphitization treatment. It is more preferable that the heating rate be 50 ° C./hr or less. Further, from the viewpoint of improving the productivity of the carbonaceous material, it is preferable that the heating rate be 20 ° C./hr or more.

The carbonaceous material may be present in the negative electrode in the form of fibrous, spherical, or a mixture of fibrous and spherical. The fibrous carbonaceous material is produced, for example, by the method described below.

First, the mesophase pitch is subjected to a heat treatment at 200 to 500 ° C. in an atmosphere containing oxygen gas such as air. To this, 1 to 50% by weight of a polymer having at least one of an amide group and a nitrile group, a thermosetting resin, or both of the polymer and the thermosetting resin is added. The obtained carbon precursor was converted to 20
A fibrous carbonaceous material (carbon fiber) can be produced by spinning at 0 to 500 ° C., pulverizing, and subjecting to graphitization.

The spinning step will be described.

The spinning step is performed in an atmosphere containing oxygen gas such as air. By performing the spinning step in such an atmosphere, the carbon precursor can be infusibilized together. The reason why the spinning temperature is set in the above range is as follows. Spinning temperature 20
When the temperature is lower than 0 ° C., it is difficult to perform spinning because the carbon precursor does not melt. On the other hand, if the spinning temperature exceeds 500 ° C., the carbon precursor may burn.

From the viewpoint of further increasing the hardness of the carbon precursor,
After spinning, heat treatment (for example, 350 to 500 ° C.) may be performed.

The pulverizing step will be described.

In the pulverization, the average fiber diameter is 1 to 20 μm,
Further, it is preferable to carry out the treatment so that the fiber has an average fiber length of 5 to 60 μm. In particular, it is preferable to perform the pulverization so that the aspect ratio (fiber length / fiber diameter) of the pulverized carbon precursor is 2 to 10. In particular, it is more preferable that the pulverization is performed so that the fiber has the above-described average fiber diameter, average fiber length, and aspect ratio, and has a specific surface area of 0.1 to ⁵ m ² / g.

The carbon fibers obtained by the above method preferably have a graphite crystallite orientation of a radial type. here,
The radial orientation of the graphite crystallite means that the hexagonal mesh plane layer of the graphite crystallite in at least the side peripheral surface region of the carbon fiber is exposed to the side peripheral surface of the carbon fiber. The radial orientation includes orientations belonging to a lamella type or a Brooks-Taylor type. FIG. 3 shows an example in which among the carbon fibers 21 having the radial orientation, the hexagonal mesh plane layers of all the graphite crystallites 22 included in the carbon fibers 21 face the side peripheral surface of the carbon fibers 21. The carbon fibers 21 have voids 23 in the amorphous carbon structure region. Among carbon fibers in which the orientation of graphite crystallites belongs to the radial type, those having an appropriate disturbance in the orientation are preferable. The carbon fibers having an appropriate disorder in the orientation have high strength and little structural deterioration due to Li ion occlusion / release reactions, so that the life characteristics are improved. In particular, it is preferable that the orientation of graphite crystallites existing inside the carbon fiber is disordered. Such a carbon fiber is easy to occlude and release Li ions from the side peripheral surface, so that not only life characteristics are improved but also rapid charge / discharge performance is improved. However, if the orientation of the graphite crystal of the carbon fiber is made coaxial and tubular (onion type), there is a possibility that the internal diffusion of lithium ions may be hindered.

The negative electrode 6 is prepared, for example, by mixing a binder dispersed in an appropriate solvent (for example, an organic solvent) and the carbonaceous material obtained by the above-mentioned method, and forming the resulting suspension into a current collector. It can be produced by applying, drying and pressing. In the pressing step, two to five times of multi-stage pressing may be performed.

Examples of the binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), ethylene-propylene-diene copolymer (EPDM), styrene-butadiene rubber (SBR),
Carboxymethyl cellulose (CMC) or the like can be used.

The mixing ratio of the carbonaceous material and the binder is as follows:
It is preferable that the carbon material is in the range of 90 to 98% by weight and the binder is in the range of 2 to 10% by weight. In particular, the negative electrode 6 preferably has a carbonaceous material content in the range of 5 to ²⁰ mg / cm ² .

As the current collector, for example, a copper foil, a stainless steel foil, a nickel foil or the like can be used.

4) Electrolyte The non-aqueous electrolyte is prepared by dissolving an electrolyte in a non-aqueous solvent.

As the non-aqueous solvent, a known non-aqueous solvent as a solvent for a lithium secondary battery can be used, and is not particularly limited. Ethylene carbonate (EC) has a melting point lower than that of ethylene carbonate and a donor. It is preferable to use a non-aqueous solvent mainly composed of a mixed solvent with one or more non-aqueous solvents having a number of 18 or less (hereinafter, referred to as a second solvent). Such a non-aqueous solvent is advantageous in that it is stable with respect to the carbonaceous material having a graphite structure that constitutes the negative electrode, hardly causes reductive decomposition or oxidative decomposition of the electrolytic solution, and has high conductivity.

The non-aqueous electrolyte containing ethylene carbonate alone has the advantage of being hardly reductively decomposed to the graphitized carbonaceous material, but has a high melting point (39 ° C. to 4 ° C.).
0 ° C.) Because of its high viscosity, the conductivity is low and it is not suitable for a secondary battery operated at room temperature. The second solvent mixed with ethylene carbonate has a lower viscosity than the ethylene carbonate in the mixed solvent to improve conductivity. Further, by using the second solvent having a donor number of 18 or less (provided that the number of donors of ethylene carbonate is 16.4), the ethylene carbonate is not easily solvated selectively with lithium ions, and the graphite structure is developed. It is considered that the reduction reaction of the second solvent with respect to the carbonaceous material is suppressed. Further, by setting the number of donors of the second solvent to 18 or less, the oxidative decomposition potential easily becomes 4 V or more with respect to the lithium electrode, and there is an advantage that a high-voltage lithium secondary battery can be realized.

As the second solvent, for example, chain carbon is preferable, and among them, dimethyl carbonate (DM
C), methyl ethyl carbonate (MEC), diethyl carbonate (DEC), ethyl propionate, methyl propionate, or propylene carbonate (P
C), γ-butyrolactone (γ-BL), acetonitrile (AN), ethyl acetate (EA), propyl formate (P
F), methyl formate (MF), toluene, xylene or methyl acetate (MA). These second solvents can be used alone or in the form of a mixture of two or more. In particular, the second type solvent more preferably has a donor number of 16.5 or less.

The viscosity of the second solvent is 2 at 25 ° C.
It is preferably 8 cp or less. The blending amount of the ethylene carbonate in the mixed solvent is 10 to 10 by volume.
Preferably it is 80%. If you deviate from this range,
There is a possibility that charge and discharge efficiency may decrease due to a decrease in conductivity or decomposition of the solvent. A more preferable blending amount of the ethylene carbonate is 20 to 75% by volume ratio. By increasing the amount of ethylene carbonate in the non-aqueous solvent to 20% by volume or more, solvation of ethylene carbonate with lithium ions is facilitated, so that the effect of suppressing the decomposition of the solvent can be improved.

A more preferred composition of the mixed solvent is EC
And MEC, EC and PC and MEC, EC and MEC and DE
C, EC, MEC, DMC, EC, MEC, PC, DE
In the mixed solvent of C, the volume ratio of MEC is preferably 30 to 80%. Thus, the volume ratio of MEC is set to 30.
By setting the content to 80%, more preferably 40% to 70%, the conductivity can be improved. On the other hand, from the viewpoint of suppressing the reductive decomposition reaction of the solvent, using an electrolytic solution in which carbon dioxide (CO ₂ ) is dissolved is effective in improving the capacity and cycle life.

The main impurities present in the mixed solvent (non-aqueous solvent) include water and organic peroxides (eg, glycols, alcohols, carboxylic acids). It is considered that each of the impurities forms an insulating film on the surface of the graphitized material and increases the interfacial resistance of the electrode. Therefore, there is a possibility that the cycle life and capacity may be reduced. In addition, self-discharge during high-temperature (60 ° C. or higher) storage may increase. For this reason, it is preferable that the impurities be reduced as much as possible in the electrolytic solution containing the non-aqueous solvent. Specifically, the water content is 50 pp
m or less, and the amount of the organic peroxide is preferably 1000 ppm or less.

As the electrolyte contained in the non-aqueous electrolyte, for example, lithium perchlorate (LiClO ₄ ), lithium hexafluorophosphate (LiPF ₆ ), lithium borofluoride (LiBF ₄ ), lithium arsenide hexafluoride (LiBF ₄ ) LiAs
F ₆ ), lithium trifluorometasulfonate (LiC
F ₃ SO ₃ ) and lithium salts (electrolytes) such as lithium bistrifluoromethylsulfonylimide [LiN (CF ₃ SO ₂ ) ₂ ]. Among them, LiPF ₆ , Li
It is preferable to use BF ₄ or LiN (CF ₃ SO ₂ ) ₂ .

The amount of the electrolyte dissolved in the non-aqueous solvent is desirably 0.5 to 2.0 mol / 1.

In FIG. 1 described above, an example in which the present invention is applied to a cylindrical lithium secondary battery has been described, but the present invention can be similarly applied to a prismatic lithium secondary battery. Further, the electrode group housed in the battery container is not limited to the spiral shape, but may be a form in which a plurality of positive electrodes, separators, and negative electrodes are stacked in this order.

As described in detail above, the manufacturing method according to the present invention is a method for manufacturing a lithium secondary battery including a positive electrode, a negative electrode containing a carbonaceous substance that occludes and releases lithium ions, and a non-aqueous electrolyte. The carbonaceous material may include at least one selected from a polymer having at least one of an amide group and a nitrile group and a thermosetting resin by 1 to 1.
It is produced by a method including a step of subjecting a carbon precursor containing 50% by weight and a mesophase pitch heated at 200 to 500 ° C. to a graphitization treatment. According to such a method, it is possible to obtain a carbonaceous material (graphitic material) in which voids having a diameter of mostly 0.5 to 5 nm are formed between graphite crystals. The void having such a size can be impregnated with lithium ions at a high density. In addition, the carbonaceous material has an X-ray diffraction of 0.340.
Since it has a peak corresponding to _d002 of nm or less, it can be used as a storage site for lithium at a high ratio in addition to the graphite storage layer between the graphite crystal layers.
Furthermore, since the carbonaceous material has a true density of 1.8 g / cm ³ or more, the ratio between the region having the graphite crystal structure and the voids can be made appropriate. as a result,
The negative electrode containing the carbonaceous material has a capacity equal to the theoretical capacity of graphite (3
72 mAh / g), so that the discharge capacity and cycle life of the lithium secondary battery can be improved. In addition, since the carbonaceous material has a higher ratio of graphite crystallites having improved crystallinity as compared with the conventional carbonaceous material, the initial charge / discharge efficiency of the lithium secondary battery can be improved.

According to another method of manufacturing a lithium secondary battery according to the present invention, the carbonaceous material is selected from a polymer having at least one of an amide group and a nitrile group and a thermosetting resin. 1 to 5 at least one
A step of spinning a carbon precursor containing 0% by weight and a mesophase pitch heated at 200 to 500 ° C at 200 to 500 ° C, a step of pulverizing the spun carbon precursor, and a step of pulverizing the pulverized carbon precursor. And performing a graphitization process. According to such a method, most of the voids having a diameter of 0.5 to 5 nm are formed between the graphite crystals, and the X-ray diffraction has a peak corresponding to d ₀₀₂ of 0.340 nm or less, and is true. A carbon fiber powder having a density of 1.8 g / cm ³ or more can be obtained. As a result, a lithium secondary battery with significantly improved initial charge / discharge efficiency, discharge capacity, and cycle life can be provided.

Further, according to such a method, carbon fibers having a graphite crystallite orientation in a radial type can be easily obtained. Since the carbon fibers having such an orientation can improve the lithium ion occlusion / release speed from the side peripheral surface as compared with the carbon fibers having no orientation, the initial charge / discharge efficiency and discharge of the secondary battery can be improved. The capacity and cycle life can be further improved.

[0068]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below in detail with reference to the drawings.

Example 1 First, lithium cobalt oxide (LiCoO ₂ ) powder 9
1% by weight of acetylene black 3.5% by weight, graphite 3.5% by weight, and ethylene propylene diene monomer powder 2% by weight and toluene were added and mixed together, applied to an aluminum foil (30 μm) current collector, and then pressed. This produced a positive electrode.

The mesophase pitch obtained from petroleum pitch was heated to 300 ° C. in air, and 30% by weight of polyacrylonitrile was added.
After spinning at 0 ° C., the mixture was infusibilized, and then carbonized at 600 ° C. in an argon gas atmosphere. The average particle diameter was 11 μm, the particle diameter was 1 to 80 μm, and 90% by volume was present. Less particles of 0.5 μm or less (5
% Or less). This was heated at 80 ° C./hr in an inert gas atmosphere and graphitized at 3000 ° C. to produce a fibrous carbon powder.

The obtained fibrous carbon powder has an average fiber diameter of 7 μm, an average fiber length of 40 μm, and an average particle diameter of 20 μm.
μm. In the particle size distribution, 90% by volume or more was present in the range of 1 to 80 µm, and the particle size distribution of the particles having a particle size of 0.5 µm or less was 0% by volume. The specific surface area by the N ₂ gas adsorption BET method was 3 m ² / g. True density is 2.1g
/ Cm ³ . From a (002) lattice image obtained by X-ray diffraction, it was confirmed that the fine structure of the fibrous carbon powder was a mixture of a graphite structure and an amorphous structure. Also,
It was confirmed by the small-angle X-ray scattering method described below that a large number of fine voids having a diameter of 1 to 2 nm exist in the amorphous structure portion. In the powder X-ray diffraction of the carbonaceous material, 0.3
Peak corresponding to d ₀₀₂ of 36nm was present. This 0.336 nm peak is attributable to the graphite structure of the carbonaceous material. Furthermore, the orientation of graphite crystallites in the fiber cross section observed by SEM belonged to the radial type. However, there were no missing portions in the fibers because the orientation was slightly disordered.

The measurement by the small-angle X-ray scattering method was performed by the method described below.

That is, Siemens
The measurement was performed in a transmission type arrangement using a powder X-ray apparatus (trade name: D5000, manufactured by the company) and Cu · Kα as a tube. The measurement conditions are as follows. As the sample holder, a vertical and horizontal size chamber having a size of 13 mm, a height of 9 mm, and a thickness of 1.5 mm was used. The sample holder was arranged in the device so as to be orthogonal to the primary light beam. 25 thickness as window material
A μm polymer film (Kapton foil) was used.
The amount of carbonaceous material contained in the sample holder is 150 mg
It was in the range of ２００200 mg. The incident angle and the scattering angle were each set to 0.1 °. The width of the receiving slit is 0.1
mm.

The scattering angle was raised from 0.4 ° to 10 ° in increments of 0.05 °, and the small-angle scattering intensity at each scattering angle was measured. The average value of the obtained scattering intensities is defined as I (q),
Shown in the following Equation 1 from Equation (1) it was determined rotation radius R _g of the gap.

[0075]

(Equation 1)

Here, q is a wave vector, N is the number of voids in the carbonaceous material, and V is the total volume of the voids.

The obtained radius of gyration R _g is substituted into the following equation (2) to obtain the diameter R _s of the voids of the carbonaceous material by the small-angle X-ray scattering method.

R _g = (3/5) ^1/2 × R _s (2) Next, the mesophase pitch-based fibrous carbon powder 9
6.7% by weight was mixed with 2.2% by weight of styrene-butadiene rubber and 1.1% by weight of carboxymethylcellulose, applied to a copper foil as a current collector, dried, and pressed to produce a negative electrode. .

After the positive electrode, the separator made of a porous film made of polyethylene, and the negative electrode were laminated in this order, the electrode group was formed by spirally winding the negative electrode so as to be positioned outside.

Further, lithium hexafluorophosphate (LiP)
F ₆ ) in a mixed solvent of ethylene carbonate (EC) and methyl ethyl carbonate (MEC) (mixing volume ratio 5
0:50) to prepare a non-aqueous electrolyte.

The above-mentioned electrode group and the above-mentioned electrolytic solution were housed in a stainless steel bottomed cylindrical container, respectively, to assemble the above-mentioned cylindrical lithium secondary battery shown in FIG.

Example 2 Example 1 was repeated except that the carbonaceous material described below was used.
The above-described cylindrical lithium secondary battery shown in FIG.

First, mesophase pitch obtained from petroleum pitch was heated to 350 ° C. in air, and 30% by weight of aramid was added thereto. The obtained carbon precursor was infusibilized by spinning at 350 ° C. in air, and then 600 ° C. under an argon gas atmosphere.
And carbonized at 9 μm with an average particle size of 11 μm and a particle size of 1 to 80 μm.
Pulverize appropriately so that 0% by volume is present and particles having a particle size of 0.5 μm or less are reduced (5% or less). This was heated at 80 ° C./hr in an inert gas atmosphere and graphitized at 3000 ° C. to produce a fibrous carbon powder.

The obtained fibrous carbon powder had an average fiber diameter of 7 μm, an average fiber length of 40 μm, and an average particle diameter of 20 μm.
μm. In the particle size distribution, 90% by volume or more was present in the range of 1 to 80 µm, and the particle size distribution of the particles having a particle size of 0.5 µm or less was 0% by volume. The specific surface area by the N ₂ gas adsorption BET method was 3 m ² / g. The true density is 2.2g
/ Cm ³ . From a (002) lattice image obtained by X-ray diffraction, it was confirmed that the fine structure of the fibrous carbon powder was a mixture of a graphite structure and an amorphous structure. Also,
It was confirmed by the small-angle X-ray scattering method described above that there were many fine voids having a diameter of 1 to 3 nm in the amorphous structure portion. 0.3358 for powder X-ray diffraction of the carbonaceous material
There was a peak corresponding to _d002 in nm. The peak at 0.3358 nm is attributable to the graphite structure of the carbonaceous material. Furthermore, the orientation of graphite crystallites in the fiber cross section observed by SEM belonged to the radial type. However, there were no missing portions in the fibers because the orientation was slightly disordered.

Example 3 Example 1 was repeated except that the carbonaceous material described below was used.
The above-described cylindrical lithium secondary battery shown in FIG.

First, a mesophase pitch obtained from a petroleum pitch was heated to 400 ° C. in the air, and 10% by weight of a phenol resin was added. After spinning the obtained carbon precursor at 400 ° C. in the air to make it infusibilized, it was heated to 600 ° C. in an argon gas atmosphere.
And carbonized at 9 μm with an average particle size of 11 μm and a particle size of 1 to 80 μm.
Pulverize appropriately so that 0% by volume is present and particles having a particle size of 0.5 μm or less are reduced (5% or less). This was heated at 80 ° C./hr in an inert gas atmosphere and graphitized at 3000 ° C. to produce a fibrous carbon powder.

The obtained fibrous carbon powder had an average fiber diameter of 7 μm, an average fiber length of 40 μm, and an average particle diameter of 20 μm.
μm. In the particle size distribution, 90% by volume or more was present in the range of 1 to 80 µm, and the particle size distribution of the particles having a particle size of 0.5 µm or less was 0% by volume. The specific surface area by the N ₂ gas adsorption BET method was 3 m ² / g. The true density is 2.15
g / cm ³ . From a (002) lattice image obtained by X-ray diffraction, it was confirmed that the fine structure of the fibrous carbon powder was a mixture of a graphite structure and an amorphous structure. Further, it was confirmed by the small-angle X-ray scattering method described above that a large number of fine voids having a diameter of 0.5 to 3 nm exist in the amorphous structure portion. In the powder X-ray diffraction of the carbonaceous material, a value of 0.
Peak corresponding to d ₀₀₂ of 3360nm was present. The peak at 0.3360 nm is attributable to the graphite structure of the carbonaceous material. Furthermore, the orientation of graphite crystallites in the fiber cross section observed by SEM belonged to the radial type. However, there were no missing portions in the fibers because the orientation was slightly disordered.

Comparative Example 1 The microstructure was composed of only a graphite structure, and d ₀₀₂ was 0.3
At 354 nm, Lc is 100 nm or more, and the true density is 2.
The cylindrical lithium secondary battery shown in FIG. 1 described above was assembled in the same manner as in Example 1 except that artificial graphite of 25 g / cm ³ was used as the carbonaceous material of the negative electrode.

Comparative Example 2 The cylindrical lithium secondary battery shown in FIG. 1 was assembled in the same manner as in Example 1 except that the negative electrode carbonaceous material described below was used.

The carbonaceous material is obtained by mixing novolak resin with 110
It was produced by a carbonization treatment at 0 ° C. The true density of the carbonaceous material was 1.55 g / cm ³ . From the (002) lattice image by X-ray diffraction, it was confirmed that the fine structure of the carbonaceous material had a graphite structure and an amorphous structure. Further, it was confirmed by the small-angle X-ray scattering method described above that there were many voids having a diameter of 1 nm in the amorphous structure portion. Further, d ₀₀₂ of the carbonaceous material is a peak of 0.380nm was obtained. The crystallite length Lc in the C-axis direction is 1 nm
Met.

Reference Example Example 1 was repeated except that the carbonaceous material described below was used.
The above-described cylindrical lithium secondary battery shown in FIG.

First, 20% by weight of a phenol resin was added to the mesophase pitch obtained from petroleum pitch, and this was spun in air at 500 ° C. to make it infusibilized.
C. and appropriately pulverized so that 90 volume% exists with an average particle size of 11 μm and a particle size of 1 to 80 μm, and particles having a particle size of 0.5 μm or less are reduced (5% or less). Then, it was graphitized at 2700 ° C. in an inert gas atmosphere to produce a fibrous carbon powder.

The obtained fibrous carbon powder has an average fiber diameter of 7 μm, an average fiber length of 40 μm, and an average particle diameter of 20 μm.
μm. In the particle size distribution, 90% by volume or more was present in the range of 1 to 80 µm, and the particle size distribution of the particles having a particle size of 0.5 µm or less was 0% by volume. The specific surface area by the N ₂ gas adsorption BET method was 3 m ² / g. True density is 2.0g
/ Cm ³ . From a (002) lattice image obtained by X-ray diffraction, it was confirmed that the fine structure of the fibrous carbon powder was a mixture of a graphite structure and an amorphous structure. Also,
It was confirmed by the small-angle X-ray scattering method described above that a large number of fine voids having a diameter of 0.5 to 2 nm existed in the amorphous structure portion. 0.33 powder X-ray diffraction of the carbonaceous material
There was a peak corresponding to _d002 at 60 nm.
The peak at 0.3360 nm is attributable to the graphite structure of the carbonaceous material. Furthermore, the orientation of graphite crystallites in the fiber cross section observed by SEM belonged to the radial type. However, there were no missing portions in the fibers because the orientation was slightly disordered.

The obtained secondary batteries of Examples 1 and 2, Comparative Examples 1 and 2, and Reference Example were charged at a charging current of 1 A to 4.2 V for 2.5 hours, and then discharged at a current of 2.7 V at 1 A. A charge / discharge cycle test was performed. From the results, the initial charge / discharge efficiency, the first cycle discharge capacity, and the capacity retention ratio at 300 cycles (relative to the first cycle discharge capacity) were measured for each secondary battery.
Shown in

[0095]

[Table 1]

As apparent from Table 1, a carbon precursor containing a polymer having an amide group or a nitrile group, or a thermosetting resin in an amount of 1 to 50% by weight and a mesophase pitch heated at 200 to 500 ° C. The secondary batteries of Examples 1 to 3 provided with the negative electrode containing the carbonaceous material produced by the method including the step of performing the graphitization treatment on the first charge-discharge efficiency, the discharge capacity, and the capacity retention rate at 300 cycles. It turns out that it is high.

On the other hand, the secondary battery of Comparative Example 1 provided with a negative electrode containing a carbonaceous material having a fine structure consisting only of a graphite structure and having a peak corresponding to d ₀₀₂ of 0.3354 nm has a lower initial charge / discharge efficiency. Although excellent, the discharge capacity and the capacity retention ratio are lower than those of Examples 1 to 3. on the other hand,
The microstructure is composed of a graphite structure and an amorphous structure,
It has a peak corresponding to d ₀₀₂ at 0 nm and a true density of 1.
It can be seen that the secondary battery of Comparative Example 2 including the negative electrode containing a carbonaceous material as small as 55 g / cm ³ has lower initial charge / discharge efficiency and discharge capacity than Examples 1 to 3. Further, a secondary battery of a reference example including a negative electrode containing a carbonaceous material obtained by a method of not performing a heat treatment on a mesophase pitch of a carbon precursor,
It can be seen that the discharge capacity and the capacity retention ratio are lower than those of Examples 1 to 3. If heat treatment is not applied to the mesophase pitch, the reaction between the mesophase pitch and the polymer or resin becomes a solid-phase reaction, and the dispersibility of the polymer or resin in the mesophase pitch decreases, and most voids formed in the carbonaceous material are reduced. Is larger than 5 nm.

[0098]

As described in detail above, according to the present invention, it is possible to provide a method of manufacturing a lithium secondary battery having a high capacity, an excellent initial charge / discharge efficiency, and an excellent cycle life.

[Brief description of the drawings]

FIG. 1 is a partial cross-sectional view showing an example of a lithium secondary battery (cylindrical lithium secondary battery) manufactured by a method according to the present invention.

FIG. 2 is a schematic view showing an example of a microstructure of a carbonaceous material produced by the method according to the present invention.

FIG. 3 is a schematic view showing an example of a carbon fiber produced by the method according to the present invention.

[Description of Signs] 1 ... Container, 3 ... Electrode group, 4 ... Positive electrode, 6 ... Negative electrode, 8 ... Sealing plate.

Claims (2)

[Claims] 1. A method for manufacturing a lithium secondary battery comprising a positive electrode, a negative electrode containing a carbonaceous substance that occludes and releases lithium ions, and a non-aqueous electrolyte, wherein the carbonaceous substance comprises an amide group and a nitrile. 1 to 50% by weight of at least one selected from a polymer having at least one of the groups and a thermosetting resin;
A method for producing a lithium secondary battery, characterized by being produced by a method comprising a step of subjecting a carbon precursor containing a mesophase pitch heated at 500 ° C. to a graphitization treatment. 2. A method for manufacturing a lithium secondary battery comprising a positive electrode, a negative electrode including a carbonaceous material that occludes and releases lithium ions, and a non-aqueous electrolyte, wherein the carbonaceous material includes an amide group and a nitrile. 1 to 50% by weight of at least one selected from a polymer having at least one of the groups and a thermosetting resin;
Spinning a carbon precursor containing a mesophase pitch heated at 500 ° C. at 200 to 500 ° C., pulverizing the spun carbon precursor, and subjecting the pulverized carbon precursor to a graphitization process. A method for producing a lithium secondary battery, characterized by being produced by a method comprising: