JPH10172573A - Lithium secondary battery negative electrode, its manufacture, and secondary battery using negative electrode - Google Patents

Abstract

PROBLEM TO BE SOLVED: To heighten the yield of lithium secondary batteries and enhance the battery characteristics by improving the characteristics of the carbon electrode of the lithium battery. SOLUTION: A negative electrode of a lithium secondary battery is formed by applying to electricity collector a mixture paste consisting of a binder and carbon powder capable of occluding and emitting lithium, wherein the binder consists of a polymer composite containing 55-80wt.% polyvinylidene fluoride (PVDF) and 20-45wt.% polymethyl methacrylate (PMMA), and the PVDF in the polymer composite is in amorphous condition not having a peak inherent to PVDF crystal at the diffracting angle around 17.7, 18.5, and 20.1deg. in X-ray diffraction or in the imperfect crystalline condition with the peak suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a negative electrode for a lithium secondary battery using a carbon material as a negative electrode, a method for producing the same, and a secondary battery using the same. More specifically, the present invention relates to a polyvinylidene fluoride as a binder for the negative electrode. Negative electrode with excellent cycle characteristics, optimized using two kinds of polymer compounds, polymethyl methacrylate and polymethyl methacrylate, by optimizing the compounding ratio and the manufacturing method, and having excellent cycle characteristics The present invention relates to a method and a secondary battery using the same.

[0002]

2. Description of the Related Art In recent years, secondary batteries with higher energy density have been demanded for miniaturization and portability of portable telephones, video cameras, notebook computers, and the like, or practical use of electric vehicles. In particular, a lithium secondary battery using a non-aqueous electrolytic solution having relatively high safety as a negative electrode and a carbon material capable of outputting 3 V or more is expected. As a carbon material for these secondary batteries, graphite produced naturally or an organic raw material is fired at a high temperature of 2000 ° C. or higher, and a graphite-based carbon material having a flat potential characteristic with a developed graphite structure, or an organic raw material is used. 1000
A coke-based carbon material or the like, which is fired at a relatively low temperature of not more than ℃ and is expected to have a larger charge / discharge capacity than graphite-based materials, is used.

In addition, a spinel structure compound such as LiMn ₂ O ₄ or a lithium transition metal composite oxide having an α-NaFeO ₂ structure generally represented by LiMO ₂ is used for the positive electrode of these lithium secondary batteries. it can. Here, M is a single metal element or two or more metal elements selected from Co, Ni, Al, Mn, Ti, Fe and the like. Moreover,
Metal oxides such as MnO ₂ and V ₂ O ₅ into which lithium can be inserted, metal sulfides such as TiS ₂ and ZnS ₂ , and π such as polyaniline and polypyrrole having electrochemical redox activity
It is also possible to use a conjugated polymer, a disulfide compound that forms a sulfur bond in the molecule, or utilizes cleavage of the sulfur bond.

As the non-aqueous electrolyte, a solution in which a lithium salt is dissolved in an organic solvent is used. As the lithium salt, LiClO ₄ , LiPF ₆ , LiBF ₄ , L
iCF ₃ SO _{3 and the} like are used, and as an organic solvent, ethylene carbonate, propylene carbonate, γ-butyrolactone, sulfolane, diethyl carbonate, dimethyl carbonate, dimethoxyethane, diethoxyethane, 2-methyl-tetrahydrofuran, various glymes, etc. are used alone. Alternatively, a mixture of two or more types is used. On the other hand, instead of the electrolytic solution, a solid electrolyte in which a lithium salt is dissolved in a polymer compound having a polyethylene oxide structure, a gel electrolyte in which a polymer such as polyacrylonitrile or polyvinylidene fluoride is plasticized with an organic solvent, or the like may be used. It is possible.

[0005] As a method of manufacturing electrodes (positive electrode, negative electrode),
A small amount of a high molecular compound such as polyvinylidene fluoride (hereinafter referred to as PVDF) serving as a binder is dissolved in an organic solvent to disperse various active materials and, as appropriate, a conductive aid made of fine powder of carbon or metal. A method of applying an electrode mixture in a paste form to an electrode core material and then removing an organic solvent is widely used. In addition, there is a method using a polymer latex or dispersion as a binder.

As the electrode core material, aluminum foil or film-like carbon is widely used for the positive electrode, and copper foil is widely used for the negative electrode.

[0007] For the characteristics of the electrode, not only the types and mixing ratios of the materials to be used, but also the preferable state of each component in the electrode, or a manufacturing method for realizing it is important. As such an example, see Japanese Patent Application Laid-Open No.
Is raised. In this publication, the influence of the crystalline state of PVDF associated with the X-ray diffraction peak on the cycle characteristics of the battery is described.

On the other hand, the adhesiveness between the electrode mixture and the electrode core material is as follows:
It is extremely important for electrode characteristics, especially cycle characteristics. That is, when the adhesiveness is poor, the electrode mixture is peeled off during repeated charging and discharging, leading to early deterioration. Also,
When the adhesiveness is improved, peeling of the electrode mixture from the electrode core material (current collector) in the battery manufacturing process is reduced, which leads to an increase in yield. JP-A-6-52861 is an example of a technique for improving the adhesion between an electrode mixture and an electrode core material. In this, a mixture of PVDF and polymethacrylate having good adhesion to metal, particularly polymethyl methacrylate (hereinafter referred to as PMMA) is used as a binder, and 10 to 90% by weight of PMMA (5% in Example) is used.
0% by weight) in the electrode mixture improves cycle characteristics. According to various studies by the present inventors on the above-mentioned mixture, PVD
There is a problem that the cycle characteristics are not always improved depending on the mixing ratio of F and PMMA and the production method.

In general, in order to improve the performance of a secondary battery, it is important to suppress a decrease in capacity due to an increase in battery capacity and a charge / discharge cycle as much as possible. Non-aqueous electrolyte secondary batteries typified by already commercially available lithium-ion batteries are no exception, and many companies and research institutes are currently studying them, and there is a need for even higher performance. In a lithium secondary battery using a carbon electrode as a negative electrode, the charge / discharge cycle characteristics of the carbon negative electrode are extremely important.However, the volume change of the carbon negative electrode accompanying charge / discharge is large, Degradation due to peeling from the current collector or cracking of the electrode becomes a problem.

[0010]

An object of the present invention is to provide a polymer composite using two types of polymer compounds, PVDF and PMMA, as a binder for a negative electrode for a lithium secondary battery using a carbon material as a negative electrode. By optimizing the compounding ratio of the two and the state of the binder (the crystalline state of the polymer composite), the adhesion between the electrode mixture and the current collector, the manufacturing process, cracking during use, etc. An object of the present invention is to find a negative electrode having excellent flexibility which does not occur, that is, a negative electrode which is excellent in battery cycle characteristics and achieves an increase in yield during battery production. Another object of the present invention is to find an optimal method for producing a negative electrode having the above-mentioned characteristics, and to provide a lithium secondary battery using such a negative electrode.

[0011]

According to a first aspect of the present invention, there is provided a current collector comprising a mixed paste of a carbon powder capable of inserting and extracting lithium and a binder. A negative electrode for a lithium secondary battery, wherein the binder comprises a polymer composite of 55 to 80% by weight of polyvinylidene fluoride (PVDF) and 20 to 45% by weight of polymethyl methacrylate (PMMA). P in things
When the VDF has a diffraction angle of about 17.7 degrees in X-ray diffraction,
A negative electrode for a lithium secondary battery, which is in an amorphous state having no peak unique to PVDF crystal at about 8.5 degrees or about 20.1 degrees or in an incompletely crystalline state with suppressed peaks. Yes,

According to a second aspect of the present invention, the binder is PVD.
The negative electrode for a lithium secondary battery according to claim 1, comprising a polymer composite of F55 to 65 wt% and PMMA 35 to 45 wt%.

According to a third aspect of the present invention, a mixed paste comprising the carbon powder according to the first aspect and a binder dissolved in a solvent is applied to a current collector, and then the solvent is dissipated. After that, this is heated to a temperature of 180 to 250 ° C. and rapidly cooled from the temperature in the above range to room temperature to form an electrode, which is a method for producing a negative electrode for a lithium secondary battery.

Further, a fourth aspect of the present invention is a lithium secondary battery using the negative electrode according to the first aspect.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, each of the above inventions will be described in detail. (1) Regarding the first and second aspects of the invention The first aspect of the invention provides a negative electrode for a lithium secondary battery obtained by applying a mixed paste of a carbon powder capable of inserting and extracting lithium and a binder to a current collector. Wherein the binder is PVDF
55-80% by weight of PMMA and 20-45% by weight of polymer composite, and PVD in said polymer composite
F: near 17.7 degrees of diffraction angle in X-ray diffraction;
A negative electrode for a lithium secondary battery according to the gist of the invention, which is in an amorphous state having no peak unique to PVDF crystals at about 5 degrees or 20.1 degrees or in an incompletely crystalline state in which the peak is suppressed. is there.

In the present invention, PVD constituting the binder
The mixing ratio of F and PMMA is 55 to 80% by weight and 2 respectively.
0-45% by weight. 20-4 PMMA in the binder
The reason for limiting to 5% by weight is that if it exceeds 45% by weight, PV
This is because the composite of DF and PMMA becomes hard and loses flexibility as an electrode, and the cycle characteristics in charge and discharge (for example, discharge capacity retention ratio) are inferior. Also, when the content is less than 20% by weight, the cycle characteristics in charge and discharge (for example, the discharge capacity retention ratio) are also poor.

FIG. 3 is a graph showing the relationship between the ratio (% by weight) of PMMA in the binder and the discharge capacity retention rate (%) in the charge / discharge cycle test. As is clear from this, PMMA in the binder is 20 to 45% by weight.
(Claim 1) is preferred, and more preferably 35 to 45.
% By weight (claim 2).

Next, the binder according to the present invention comprises a polymer composite having the above-mentioned composition, and the PVDF in the polymer composite has a diffraction angle of 1 in X-ray diffraction.
P near 7.7 degrees, 18.5 degrees, and 20.1 degrees
It is characterized by being in an amorphous state having no peak unique to the VDF crystal or in an incomplete crystalline state in which the peak is suppressed.

Here, the polymer composite of the present invention will be described. T. Nishi et al., Macromo
recules (Vol. 8, No. 6, 1975)
Year, P909), PVDF and PM
MA is known as a polymer alloy that is compatible at a molecular level under certain conditions due to strong interaction despite being a polymer compound. For example, in a molten state at about 200 ° C., they are in a compatible state, but when solidified to 150 ° C. or lower, phase separation occurs between PVDF and PMMA. However, PVDF and PMM which are melted and in a compatible state
When the composite with A is quenched, the compatible state is frozen and the phase separation can be partially or completely suppressed.
This also makes it possible to control the crystal state of PVDF. In particular, when PMMA is increased to a certain ratio or more, crystallization of PVDF can be substantially completely suppressed. The present invention has been completed by applying and examining a polymer alloy composed of PVDF and PMMA to a binder for a carbon electrode (negative electrode) based on the above findings.

When the above composite of PVDF and PMMA is used as a binder for a carbon electrode (negative electrode), the diffraction angles unique to PVDF are around 17.7 °, 18.5 ° and 28.5 °.
There is no peak at around 0.1 degree, that is, the PVDF in the binder (polymer composite) needs to be amorphous. Alternatively, the peaks existing near the PVDF-specific diffraction angles of 17.7 degrees, 18.5 degrees, and 20.1 degrees are suppressed, that is, the binder (polymer composite)
It is necessary to make the PVDF therein incompletely crystalline. In other words, the present invention relates to a method for producing PV in a binder (polymer composite).
It is important that the DF is made amorphous or incompletely crystalline and not crystalline.

FIG. 1 shows electrodes A-1 and B- according to the present invention.
2 shows the results obtained by measuring the diffraction line intensity at a diffraction angle of 2θ by X-ray diffraction for the crystalline state of PVDF in the binder of No. 2. Also, for comparison, PVDF alone, which is crystalline, and electrodes C-2 and E-1 outside the scope of the present invention were similarly measured and shown. As is clear from FIG.
Crystalline PVDF has a diffraction angle of around 17.7 degrees, 18.5
It can be seen that there are peaks of diffraction line intensity unique to PVDF near the temperature and around 20.1 °. On the other hand, the electrode A-1 according to the present invention has diffraction angles of about 17.7 degrees, 18.
PVD that should be around 5 degrees and around 20.1 degrees
F-specific peak was not observed at all and PVDF in the binder
Is amorphous. In the electrode B-2 according to the present invention, the peak unique to PVDF has a diffraction angle of 17.
It is suppressed at around 7 degrees, around 18.5 degrees, and around 20.1 degrees, indicating that the PVDF in the binder is in an incomplete crystalline state. Further, for the electrodes C-2 and E-1 outside the range of the present invention, the diffraction line intensity of PVDF alone is the same as that of the crystalline one, and the diffraction angle is 17.
Since peaks specific to PVDF are observed at around 7 degrees, around 18.5 degrees, and around 20.1 degrees, it is understood that the PVDF in this binder is crystalline.

The negative electrode of the present invention comprises PVDF and PMMA.
By making the composition of the binder composed of the composite of the above as described above and making the PVDF in the binder (polymer composite) amorphous or incompletely crystalline, the electrode has high flexibility. And cycle characteristics in charge and discharge (for example,
Discharge capacity retention ratio) can be improved.

It should be noted that between the degree of crystallization of PVDF in the binder and the discharge capacity retention rate as cycle characteristics,
There is a correlation, and this will be described below. FIG. 3 is a graph showing the relationship between the ratio of PMMA in the electrode and the discharge capacity retention ratio for electrodes having the same electrode manufacturing method.

As can be seen from FIG. 3, PMM in the binder
In the range of 20 to 45% by weight of A in the present invention (claim 1), if the ratio is 45% by weight or less, the discharge capacity retention ratio increases with an increase in the mixing ratio of PMMA. This is considered to be due to suppression of the development of the peak unique to PVDF in the X-ray diffraction described above. That is, in the region where the PMMA in the binder is 45% by weight or less, the more the peak is suppressed, the more the deterioration of the electrode due to the cycle maintenance is suppressed, but the PMMA ratio is as follows:
The discharge capacity retention ratio is preferably 20% by weight or more exceeding 80%. Further, the peak of the PVDF is completely suppressed (no peak), and the discharge capacity retention ratio exceeds 90%.
Obviously, weight% or more is very preferable (PM
MA 35 to 45% by weight, claim 2).

However, when the proportion of PMMA exceeds 45% by weight, the characteristics are sharply reduced. this is,
As described above, it is presumed that the apparent glass transition temperature of the composite of PVDF and PMMA increases, the electrode becomes hard, and adversely affects the charge / discharge reaction of the carbon negative electrode accompanied by a large volume change. Further, in this case, as described above, the electrode is easily cracked in the battery manufacturing process and the like,
It leads to a decrease in yield.

(2) Regarding the Invention of Claim 3 The invention of claim 3 is a method for manufacturing a negative electrode for a lithium secondary battery according to the invention of claim 1. That is, a mixed paste consisting of the carbon powder according to claim 1 and a binder dissolved in a solvent is applied to a current collector, and then the solvent is dissipated. The electrode is heated to a temperature and rapidly cooled from the temperature in the above range to room temperature to form an electrode.

In the production of the electrode according to the present invention, P
From the temperature of 180 ° C or higher (180-250 ° C) at which the composite of VDF and PMMA melts, quenching treatment to room temperature (for example, the time required to reach 20 ° C room temperature is within 1 minute)
There is a need to. The reason for limiting the heating temperature in this way is that
If the temperature is lower than 180 ° C., even when rapidly cooled to room temperature, the PVDF in the binder (polymer composite) does not become amorphous or incompletely crystalline, so that the cycle characteristics in charge and discharge are inferior. If the temperature exceeds 250 ° C., the polymer component may be decomposed, which is not preferable. Also, even if the heating temperature is in the range of 180 to 250 ° C., it is gradually cooled (for example, the time required to reach room temperature of 20 ° C. is 30 minutes).
If so, the binder (polymer composite) as described above
The PVDF inside does not become amorphous or incompletely crystalline, and the cycle characteristics in charge and discharge are inferior.

As will be apparent from the examples described later, even when the ratio of PMMA in the binder is 40% by weight and the same, the difference in the method of manufacturing the electrodes and the difference in the state of the binder resulting therefrom. Since a large difference occurs in the cycle characteristics, it is extremely important to maintain the crystalline state of PVDF in the binder (polymer composite) as described above by employing the above-described electrode manufacturing method.

Further, the cooling temperature in the quenching process may be at room temperature or lower. As a cooling method, a method using cold air, a method of immersing in a refrigerant such as liquid nitrogen or cold water, or the like may be used. In addition, further heating after the quenching treatment is not preferable because it causes phase separation of PVDF and PMMA and partial crystallization of PVDF, but heating at a relatively low temperature of 120 ° C. or less is acceptable.

(3) Regarding the Invention of Claim 4 The invention of claim 4 is a lithium secondary battery using the negative electrode according to claim 1. In the lithium secondary battery of the present invention, the negative electrode uses the above-described negative electrode of the present invention as a matter of course. As the positive electrode and the non-aqueous electrolyte, the above-mentioned known ones can be used.

In the carbon negative electrode of the present invention and the lithium secondary battery used for the negative electrode, the mixing ratio of PVDF and PMMA used as a binder for the negative electrode is limited, and the method of manufacturing the negative electrode is specialized. By improving the adhesiveness between the electrode core material and the electrode mixture and controlling the crystalline state of the binder in the negative electrode, the charge / discharge cycle characteristics are improved and the battery is manufactured using a flexible electrode during battery production. An improvement in yield can be achieved.

[0032]

Next, examples of the present invention (examples of the present invention) will be described in detail. In addition, comparative examples for further clarifying the effects of the present invention are also shown. Example 1 As shown in Table 1, 10 parts by weight of a binder having various compositions composed of PVDF and PMMA were dissolved in 1-methyl-2-pyrrolidone in a solution having an average particle size of 29 μm.
An electrode mixture paste was prepared by adding and kneading 90 parts by weight of an amorphous carbon material powder having an average interlayer distance of 0.343 nm. The above paste was applied on a copper foil having a thickness of 35 μm so that the carbon powder was about 0.01 g / cm ² , and heated at 200 ° C. to obtain 1-methyl-2-
The pyrrolidone was dissipated and dried. At this time, the electrode was rapidly cooled by leaving it in a room temperature (20 ° C.) atmosphere immediately from 200 ° C. The time required to reach room temperature is 2
5 seconds. Thereafter, an electrode was formed by press molding at a pressure of 1 t / cm ² .

[0033]

[Table 1]

As shown in Table 1, in the present invention, the composition of the binder was within the range of the invention (Test Nos. 1 to 6), and the electrode names were A-1, A-2 and A-, respectively. 3, B-1, B-
2, B-3. In Comparative Examples, the composition of the binder was out of the range of the invention (Test Nos. 7 to 10), and the electrode names were C-1, C-2, D-1, and D-2, respectively. For the electrode groups A to D prepared under the above-described conditions, in order to examine the crystal state of the binder, the diffraction line intensity at a diffraction angle 2θ was measured by X-ray diffraction. Rig-2400 manufactured by Rigaku Denki
Using an X-ray X-ray measuring apparatus, the measurement conditions of the optical system were as follows. Optical system measurement conditions: X-ray source is Cu target, wavelength 1.5405４０ (CuKα ray) X-ray intensity is 60 kV × 200 mA, divergence slit is 1 °, light receiving slit is 0.30 mm, scattering slit is 1 °, scan speed is 2 .0 ° / min using curved monochromator

FIG. 1 shows the results of X-ray diffraction measurement of the electrodes A-1, B-2 and C-2. For comparison, PV
The diffraction line intensity at a diffraction angle of 2θ by X-ray diffraction for crystalline DF alone is also shown. As is clear from FIG. 1, the crystalline PVDF has a diffraction angle around 17.7 degrees,
It can be seen that there are peaks of diffraction line intensity unique to PVDF around 18.5 degrees and around 20.1 degrees. on the other hand,
In the electrode A-1 according to the present invention, no peak unique to PVDF, which should exist near the diffraction angles of about 17.7 degrees, about 18.5 degrees, and about 20.1 degrees, was not observed at all. That is, it means that the PVDF in the binder according to the present invention is amorphous. Further, the electrode B-2 according to the present invention comprises:
The peak unique to PVDF has a diffraction angle of about 17.7 degrees, 1
It can be seen that it is suppressed around 8.5 degrees and around 20.1 degrees. That is, this is the PVDF in the binder
Means incomplete crystal state.

Further, for the electrode C-2 outside the range of the present invention, peaks unique to PVDF are observed at diffraction angles of about 17.7 degrees, about 18.5 degrees, and about 20.1 degrees. This means that if the composition ratio of PVDF and PMMA of the binder is out of the range of the present invention, the PVDF will be in a crystalline state. Table 1 shows each diffraction angle (about 17.7 degrees, 1
Around 8.5 degrees and around 20.1 degrees). In the table, “No” has no unique peak,
Suppression means that a unique peak is suppressed, and presence means that there is a unique peak.

For each of these electrodes, the presence or absence of cracks during molding (here, press molding) was observed, and the results are shown in Table 1.

For each of these electrodes, the discharge capacity retention as a cycle characteristic was measured. This measurement is
Lithium metal is used for the counter electrode and the reference electrode, and ethylene carbonate and dimethyl carbonate are used as an electrolyte in a volume ratio of 1:
Using a non-aqueous electrolyte obtained by dissolving LiPF ₆ at a rate of 1 mol / liter in a solution mixed at a rate of 1, a triode-type test cell as shown in FIG. 2 was assembled. At a current density of 0.5 mA / cm ² , a charge lower limit voltage was set to 0 V and a discharge upper limit voltage was set to 3 V with respect to a metallic lithium reference electrode, and a charge / discharge test was performed. However, here,
The reaction in which lithium was stored in the carbon electrode was defined as charging, and the reaction in which lithium was released from the carbon electrode was defined as discharging. This cycle characteristic is shown by the discharge capacity retention rate of the 100th cycle of each electrode when the discharge capacity of three cycles is 100%. These results are shown in Table 1.

As is clear from Table 1, the composition of the binder is within the range of the present invention (Examples Nos. 1 to 6 of the present invention, electrode A group)
In the case of group B), PVDF in the binder was in an amorphous or imperfect crystalline state, no cracks were observed in molding, and the discharge capacity retention rate as cycle characteristics was high. . The composition of the binder is out of the range of the present invention (Comparative Example N
o. Nos. 7 to 10), No. 7 (electrode C-
1), No. 8 (electrode C-2) did not show any cracks, but PVDF became crystalline, and the discharge capacity retention rate as cycle characteristics was low. In addition, No. 9 (electrode D-1),
No. 10 (electrode D-2) is cracked by molding (about 10 to 10).
Cracks were observed at a rate of 25%, indicating that the discharge capacity retention rate was low although it was amorphous.

Example 2 In Example 1, the test No.
1 having the same composition as that of No. 1 (PVDF 60% by weight and PMMA
In the same manner as in Example 1, an electrode mixture paste was produced, and the paste was applied to a current collector and dried to produce an electrode. In the manufacture of this electrode, as shown in Table 2, after the mixed paste was applied to the current collector to dissipate the solvent, the mixture was reheated to various temperatures as shown in Table 2, and rapidly cooled to room temperature. Were manufactured under various conditions.

[0041]

[Table 2]

A sample which was gradually cooled from a heating temperature of 200 ° C. to room temperature (the time required for the room temperature to reach 20 ° C., 30 minutes) was prepared in Comparative Example No. 16 (electrode E-1, reference). In addition, the example of the present invention is a material that is rapidly cooled after heating to 180 to 250 ° C.
No. Nos. 11 to 13 (electrodes E-1 to 3). 14 to 16 (electrodes E-1-4, E-1-5, E-1).

Each of these electrodes was measured by X-ray diffraction in the same manner as in Example 1. For PVDF in the binder, the diffraction angles were around 17.7 °, 18.5 °, and 2 °.
Measure the degree of PVDF specific peak around 0.1 degree,
The results are shown in Table 2. Further, for each of these electrodes, during molding (here, press molding), as in Example 1.
The presence or absence of cracks was observed, and the results are shown in Table 2. Further, for each electrode, a test cell was assembled in the same manner as in Example 1, a charge / discharge test was performed under the same test conditions, and a discharge capacity retention ratio as a cycle characteristic was measured.
It was also described in.

Inventive Example No. 11 to 13 (electrode E-1-
The results of the measurement by X-ray diffraction of 1) to 3) are as follows.
Similar to the group, no diffraction peak specific to PVDF is observed at all (amorphous), and the discharge capacity retention ratio is high.

Further, in Comparative Example No. FIG. 1 also shows the result of measurement by X-ray diffraction of Sample No. 16 (electrode E-1). As is clear from FIG. 1 and Table 2, the diffraction angle is around 17.7 degrees, and 18.
The peaks around 5 degrees and around 20.1 degrees developed to the same extent as in the case of PVDF alone. Therefore, No. 1
PVDF in the binder of No. 6 (electrode E-1) is crystalline and has a low discharge capacity retention ratio.

Also, in Comparative Example No. 14 (electrode E-1-
4), no. 15 (electrode E-1-5) has a lower heating temperature, but the measurement result by X-ray diffraction shows that the electrode E-1
As in the case of (1), it has a diffraction peak unique to PVDF (PVDF in the binder is crystalline), and the capacity retention is low.

In the present invention, the solvent used for dissolving the binder is not limited to 1-methyl-2-pyrrolidone, but has a solubility in both polymer components of PVDF and PMMA. I just need. As such an example,
N, N-dimethylformamide, dimethylsulfoxide and the like can be mentioned. Further, the polymer component does not necessarily need to be dissolved in an organic solvent, and PVDF and PMMA fine powder may be used in a state where both or one of them is simply dispersed. Further, in the present invention, the type of the carbon material is not limited to the type thereof, and can be applied to either graphite or coke.

[0048]

As described above in detail, the present invention relates to a negative electrode for a lithium secondary battery using a carbon material as a negative electrode, wherein polyvinylidene fluoride (PVDF) is used as a binder for the negative electrode.
Optimizing the state of the binder as a polymer composite by optimizing the compounding ratio and the manufacturing method using two types of polymer compounds, polymethyl methacrylate (PMMA) and non-crystalline PVDF Or incomplete crystallization)
In addition, a negative electrode and a secondary battery having extremely excellent cycle characteristics can be provided. In addition, the electrode is less likely to crack during manufacturing or use of the electrode, and has the effect of enabling cost reduction and improvement in characteristics with improvement in yield.

[Brief description of the drawings]

FIG. 1 shows a negative electrode, crystalline PVD of inventive examples (A-1, B-2) and comparative examples (C-2, E-1) by X-ray diffraction.
It is a typical diffraction pattern of F alone.

FIG. 2 is a schematic diagram of a three-electrode cell used for a charge / discharge cycle test in the present invention examples and comparative examples.

FIG. 3 is a graph showing a relationship between a ratio of PMMA in a binder for electrodes A to D and a discharge capacity retention rate in a charge / discharge cycle test.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Carbon test electrode 2 Copper foil 3 Stainless steel lead wire 4 Polypropylene nonwoven fabric 5 Polyethylene separator 6 Metal lithium reference electrode 7 Platinum lead wire 8 Metal lithium counter electrode 9 Stainless steel lead wire 10 Electrolyte 11 Electrolyte tank 12 Teflon stopper

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification symbol FI H01M 10/40 H01M 10/40 Z // (C09D 127/16 133: 12)

Claims (4)

[Claims] 1. A negative electrode for a lithium secondary battery obtained by applying a mixed paste of a carbon powder capable of inserting and extracting lithium and a binder to a current collector, wherein the binder is polyvinylidene fluoride (PVD). (PVDF) 55-80% by weight and polymethyl methacrylate (PMMA) 20-45% by weight of a polymer composite, and PVDF in said polymer composite
Has a diffraction angle of about 17.7 degrees in X-ray diffraction, and 18.5.
A negative electrode for a lithium secondary battery, wherein the negative electrode is in an amorphous state having no peak unique to PVDF crystals at about 20.1 degrees or in an incompletely crystalline state with suppressed peaks. 2. The negative electrode for a lithium secondary battery according to claim 1, wherein the binder comprises a polymer composite of 55 to 65% by weight of PVDF and 35 to 45% by weight of PMMA. 3. A current collector is coated with a mixed paste comprising the carbon powder according to claim 1 and a binder dissolved in a solvent,
Next, after dissipating the solvent,
A method for producing a negative electrode for a lithium secondary battery, wherein the electrode is heated to a temperature of ° C and rapidly cooled from a temperature in the above range to room temperature to form an electrode. 4. A lithium secondary battery using the negative electrode according to claim 1.