JPH09330717A - Lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery employing a low crystalline carbon as a negative active substance which is superior in safety, high charging capacity, and superior practicality. SOLUTION: A lithium ion secondary battery having a positive electrode made of lithium transition metal oxide, a negative electrode made of a carbonaceous material, and an organic electrolyte 3, wherein a negative electrode active substance is composed of carbon particles of 3.45Å or more in average inter- layer distance [d002], and the carbon particles 1 are coated with an ion conductive polymer solid electrolyte 2 with lithium ion conductivity at normal temperature is 5×10<-5> S/cm or more.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium ion secondary battery using a negative electrode made of a carbon material, and more particularly to improvement of a negative electrode active material.

[0002]

2. Description of the Related Art A lithium battery using lithium metal for a negative electrode having high energy density and high electromotive force is
A needle-like resinous product (dendrites) is formed on the surface of the lithium metal, which penetrates the separator and contacts the positive electrode, causing a short circuit, which causes serious safety problems such as heat generation and ignition. There is.

As an alternative to the lithium metal negative electrode, in Japanese Patent Publication Nos. 4-24831 and 5-17669, a charge / discharge reaction can be carried out by inserting and extracting lithium ions between carbon layers. Many carbon materials are being studied. However, the carbon material is
The influence on the battery characteristics differs depending on the starting material, the degree of graphitization, etc., and it is difficult to uniquely determine the optimum carbon material.

Currently, most of carbon materials used in commercialized lithium ion secondary batteries are graphitized carbon having a laminated structure or low crystalline carbon. Of these, graphitized carbon is limited in capacity per carbon weight to 372 mAh / g, which is the theoretical capacity of natural graphite, and the laminated structure grown in the three-dimensional direction increases the reactivity with the electrolytic solution. Since the internal pressure of the battery rises due to the generation of decomposed gas, there are problems of safety and limitation of combination with the electrolytic solution. In fact, "6th
In the 2nd Electrochemical Spring Conference Proceedings "(1995), it was reported that when graphitized carbon was combined with propylene carbonate, which is a typical organic solvent, it could not be charged. ing.

On the other hand, in a part of the low crystalline carbon, a large amount of lithium ions can be occluded not only in the part having the laminated structure but also in the non-crystalline part, so that it has a larger capacity than natural graphite. Moreover, since the low crystallinity carbon has an immature laminated structure, the restrictions on the combination of chargeable / dischargeable electrolytes can be relaxed, and it is therefore expected as a negative electrode active material for lithium ion secondary batteries. ing.

[0006]

However, the low crystalline carbon has a higher possibility that metallic lithium is deposited on the surface of the carbon as compared with the graphitized carbon, and the resinous product is It is liable to occur, and the deposition of metallic lithium causes problems that safety is impaired, smooth charge / discharge reaction is hindered, and charge capacity is deteriorated.

Therefore, the present invention solves the above problems when a low crystalline carbon is used for the negative electrode active material, and is a lithium ion which is excellent in safety and has a large charge capacity and excellent practicality. The purpose is to provide a secondary battery.

[0008]

[Means for Solving the Problems] To clarify the difference between the charge and discharge characteristics of graphitized carbon and low crystalline carbon, the present inventors firstly investigated mesocarbon microbeads. ，
Graphitized carbon (d002: 3.3) fired at 000 ° C
7Å) and low crystalline carbon (d00)
2: 3.61 A) was used to examine the negative electrode charge / discharge characteristics.

The results are shown in FIG. When a lithium ion secondary battery is charged, lithium ions are occluded in the carbon and the potential approaches 0 V which is the potential of metallic lithium. Here, as shown in the figure, the low crystalline carbon has a much higher capacity than the graphitized carbon, and the behavior of voltage change is not observed in the high potential region. Almost no liquid decomposition occurs.

However, as can be seen from the charging curve of the low crystalline carbon, in the low crystalline carbon, since most of the charging capacity is obtained near 0 V, there is a possibility that lithium metal may be deposited on the surface of the carbon. Becomes higher. As a result of studying the cause, it was found that the low crystalline carbon has a slow diffusion rate of lithium ions into the carbon and the charging potential caused by the diffusion overvoltage is close to the deposition potential of metallic lithium. .

A constant potential AC impedance method is available for measuring the resistance generated when the reactive species move. The average inter-layer distance [d002] is one of the parameters for determining the graphitization degree using this method. And the diffusion coefficient of lithium ions in the negative electrode near the metallic lithium potential were investigated. From FIG. 3 showing this result, when d002 becomes 3.45Å or more, the diffusion rate of lithium remarkably decreases. As can be seen from this result, d002 is 3.45.
With a low crystalline carbon of Å or higher, the diffusion overvoltage is large,
Lithium metal easily deposits on the carbon surface.

The present inventors next examined a method for preventing the deposition of metallic lithium on the surface of such a low crystalline carbon. As a result, it has been found that the above problem can be solved by coating the surface of the low crystalline carbon with a layer having lithium ion conductivity and high electric conductivity. As shown in FIG. 1, when the carbon particles 1 are covered with the solid polymer electrolyte membrane 2, the solid polymer electrolyte membrane 2
Due to the shielding effect, the direct contact of the organic electrolytic solution 3 with the carbon surface is suppressed.

Here, when the solid polymer electrolyte membrane is provided,
Due to the shielding effect, lithium ions cannot penetrate into the inside of the carbon, which makes it difficult to store lithium ions. Therefore, it is necessary to limit the conductivity. That is, when the solid polymer electrolyte membrane has low conductivity and high lithium ion conductivity as compared with the low crystalline carbon that is the active material, the overvoltage for deposition of lithium metal is The interface between the membrane and the organic electrolyte is larger than the interface with the membrane, and the formation of a resinous product of lithium on the carbon electrode surface is prevented.

In order to exhibit such a shielding effect and obtain sufficient battery characteristics in the charge / discharge reaction, the solid polymer electrolyte membrane has a lithium ion conductivity of 5 × 1 at room temperature.
It is necessary to be 0 ^-5 S / cm or more, and if it is lower than this, the battery characteristics deteriorate. On the other hand, such a polymer solid electrolyte membrane also has a shielding effect on the growth of resinous products from the inside when lithium is deposited on the surface of carbon particles, and thus short-circuiting due to the penetration of the separator. This brings about an effect of providing a lithium-ion secondary battery that is hard to cause and has high safety.

The present invention has been completed based on the above findings, and in a lithium ion secondary battery having a positive electrode made of a lithium transition metal oxide, a negative electrode made of a carbon material, and an organic electrolyte, a negative electrode active material is obtained. The substance consists of carbon particles having an average inter-layer distance [d002] of 3.45 Å or more, and the carbon particles have a high ionic conductivity of lithium ion conductivity of 5 × 10 ^-5 S / cm or more at room temperature. The present invention relates to a lithium ion secondary battery characterized by being coated with a molecular solid electrolyte.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION The carbon material used in the present invention is
As the low crystalline carbon, carbon particles having an average interlayer distance [d002] of 3.45Å or more (usually up to about 4.0Å) may be used, and the raw material and the manufacturing method thereof are not particularly limited. The crystal lattice size [Lc] in the c-axis direction is 100.
When it is not more than Å, this is a carbon having a more undeveloped laminated structure, so that a negative electrode having a large capacity can be obtained, which is particularly preferable.

In the present invention, while such carbon particles are used as a negative electrode active material, the carbon particles have a lithium ion conductivity of 5 × 10 ⁻⁵ S / cm at room temperature.
Above, it is preferably coated with an ion conductive polymer solid electrolyte having a concentration of 1 × 10 ⁻⁴ S / cm or more. As the polymer solid electrolyte having lithium ion conductivity as described above, polyethylene glycol, polyethylene oxide, polypropylene oxide, a copolymer of ethylene oxide and propylene oxide, a mixture thereof, or a crosslinked isocyanate thereof. And so on.

The above-mentioned lithium ion conductivity means that a solid polymer electrolyte and LiPF ₆ which is a lithium electrolyte are mixed at a weight ratio of 5: 1 using a solvent and dried to form a thin film using stainless steel. Create a sandwiched cell and use F at room temperature.
AC impedance (frequency 100 mHz to 100 kHz) using an FT (frequency Fourier transform) analyzer
z) The value of the bulk impedance (resistance) is measured. In the battery, the solid polymer electrolyte coats carbon particles, so the direct lithium ion conductivity of the electrolyte cannot be measured.However, the measurement conditions are considered to be almost the same as the internal conditions of the battery. The change is considered to be small.

If the thickness of such a polymer solid electrolyte membrane is too thin, the surface of the carbon particles cannot be completely covered, and the shielding effect due to the provision of this membrane decreases. Usually 0.05 μm or more, preferably 0.1 μm
The above average film thickness is preferable. Further, if the film thickness is too thick, the lithium ion transfer resistance increases and the battery characteristics deteriorate, so the thickness is preferably 1 μm or less.

The formation of the polymer solid electrolyte membrane is generally performed by
The solid polymer electrolyte may be dissolved in a solvent such as toluene, xylene or N-methylpyrrolidone to prepare a resin solution, and carbon powder may be dipped in the resin solution, followed by evaporation of the solvent. After forming a solid polymer electrolyte membrane on the surface of carbon particles in this way, this carbon powder is kneaded with a binder, a solvent, etc. to prepare a negative electrode mixture, which is used as a current collector material such as copper foil. And dried to obtain a sheet-shaped negative electrode molded body.

In consideration of productivity, a method in which a negative electrode mixture containing carbon powder, a binder and a solvent is applied to a current collecting material, and then a resin solution of a polymer solid electrolyte is applied and dried. May be adopted. In this case, if a large amount of voids that are not covered with the solid polymer electrolyte remain in the negative electrode coating film, the conduction of lithium ions will be hindered and the battery capacity will decrease, so the resin liquid should be sufficiently filled in the negative electrode coating film. Is preferred.

In FIG. 1, a negative electrode coating film containing carbon particles 1 coated with a polymer solid electrolyte membrane 2 as described above is formed on a current collecting material 4 to form a sheet-shaped negative electrode molded body 5, Is schematically shown in a state of being loaded in the battery. The shielding effect of the ion conductive polymer solid electrolyte membrane 2 prevents the organic electrolyte solution 3 from contacting the surface of the carbon particles 1. As a result, the deposition of a resinous product of lithium metal on the surface can be avoided, and a lithium ion secondary battery having excellent safety and battery characteristics can be manufactured.

In the present invention, the positive electrode used in combination with the negative electrode is not particularly limited as long as it is a lithium transition metal oxide, and examples thereof include lithium nickel oxide such as LiNiO ₂ , lithium cobalt oxide such as LiCoO ₂ , and LiMnO 2. A metal oxide such as lithium manganese oxide such as _{4 is used} as a positive electrode active material, and a mixture of this and a conductive additive or a binder such as polytetrafluoroethylene is added to a current collecting material such as a stainless steel net. As the material, a finished product is used.

Further, as the electrolyte of the organic electrolytic solution, Li
ClO ₄ , LiPF ₆ , LiBF ₄ , LiAsF ₆ , L
iSbF ₆ , LiCF ₃ SO ₃ , LiCF ₃ CO _2, etc., and others, Li ₂ C ₂ F ₄ (SO ₃ ) ₂ , LiN (C
_{F 3 SO 2) 2, LiC} (CF 3 SO 2) 3, LiC n
F _{2n + 1} SO ₃ (n> = 2) and the like are used alone or in combination of two or more. Among these, LiPF ₆ and LiC _n F _{2n + 1} SO ₃ (n> = 2) are preferably used because they have good charge and discharge characteristics. The concentration of these electrolytes in the electrolytic solution is not particularly limited, but it is usually 0.1 to 2 mol / liter, preferably 0.4 to 1 mol / liter.

The organic solvent used in the organic electrolytic solution includes diethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, propylene carbonate, ethylene carbonate, butylene carbonate, esters such as γ-butyrolactan, 1,2-dimethoxyethane,
Examples thereof include ethers such as 1,2-dimethoxymethane, and carbonic acid esters are preferable also from the viewpoint of high voltage resistance.

[0026]

EXAMPLES Next, the examples of the present invention will be described in more detail, but the present invention is not limited to these examples. Hereinafter, "parts" means "parts by weight".

Example 1 As a polymer solid electrolyte, 5 parts of polyethylene glycol (lithium ion conductivity: 1.0 × 10 ⁻⁴ S / cm) was used, and this and 1 part of LiPF ₆ were mixed with cyclohexanone / toluene ( A resin liquid was prepared by dissolving in a mixed solvent of 45 parts by weight ratio of 1: 1). Carbon powder (d002: 3.50Å with an average particle size of 10 μm) obtained by crushing bulk carbon obtained by heat treating carbon micro beads extracted from petroleum pitch at 900 ° C. to 1 part of this resin liquid, Lc: 18Å) 15 parts were mixed and sufficiently stirred, and then the solvent was removed to obtain carbon powder coated with the polymer solid electrolyte membrane. The thickness of the film was 0.3 μm as observed with a scanning line electron microscope.

Next, 9 parts of the above carbon powder and 1 part of polyvinylidene fluoride as a binder were mixed using 8 parts of N-methylpyrrolidone as a solvent to prepare a negative electrode mixture. This was applied onto a copper foil having a thickness of 18 μm and dried, and then a sheet negative electrode having a total thickness of 60 μm was produced using a roll press. This sheet negative electrode is used as a working electrode, and a lithium electrode is used as a counter electrode and a reference electrode.
F ₆ for ethylene carbonate and ethyl methyl carbonate
A cell was prepared by using a solution prepared by dissolving 1 mol / liter of a solution in a mixed solvent having a weight ratio of 1: 1 to 1 mol / liter as an organic electrolytic solution.

Example 2 As carbon powder, naphthalene pitch was 1,000 ° C.
Average particle size obtained by crushing bulk carbon heat-treated at 1
0 μm carbon powder (d002: 3.80Å, Lc:
A cell was prepared in the same manner as in Example 1 except that 12Å) 15 parts were used. The thickness of the polymer solid electrolyte membrane was 0.3 μm.

Example 3 As a solid polymer electrolyte, a copolymer of ethylene oxide and propylene oxide (lithium ion conductivity: 8.
A cell was prepared in the same manner as in Example 1 except that 5 parts of 0 × 10 ⁻⁵ S / cm) was used. The thickness of the polymer solid electrolyte membrane was 0.3 μm.

Example 4 As a resin liquid, 5 parts of polyethylene glycol (lithium ion conductivity: 1.0 × 10 ⁻⁴ S / cm) and LiPF ₆ 1 were used.
A cell was prepared in the same manner as in Example 1 except that a mixed solution of 30 parts of cyclohexanone / toluene (weight ratio 1: 1) mixed solvent was used. The thickness of the polymer solid electrolyte membrane was 0.5 μm.

Example 5 90% carbon microbeads extracted from petroleum pitch
Carbon powder having an average particle size of 10 μm obtained by crushing bulk carbon heat-treated at 0 ° C. (d002: 3.50Å, L
c: 18Å) 9 parts and polyvinylidene fluoride 1 part as a binder were mixed using 8 parts of N-methylpyrrolidone as a solvent to prepare a negative electrode mixture. This has a thickness of 1
After being applied on a copper foil of 8 μm and dried, a sheet negative electrode having a total thickness of 60 μm was produced using a roll press.
Next, a resin solution prepared by dissolving 1 part of polyethylene glycol in 9 parts of a mixed solvent of cyclohexanone / toluene (weight ratio 1: 1) was dried on the sheet negative electrode so that the average film thickness after drying was 0.3 μm. Topped to dryness and dried. Using this sheet negative electrode, a cell was prepared in the same manner as in Example 1 below.

Comparative Example 1 A cell was produced in the same manner as in Example 1 except that the treatment with the resin liquid was not carried out in the production of the sheet negative electrode.

Comparative Example 2 A cell was produced in the same manner as in Example 2 except that the treatment with the resin liquid was not carried out in the production of the sheet negative electrode.

Comparative Example 3 As carbon powder, an average particle diameter of 10 μm obtained by crushing bulk carbon obtained by heat-treating petroleum pitch at 2,000 ° C.
Carbon powder (d002: 3.43Å, Lc: 265
A cell was prepared in the same manner as in Example 1 except that Å) was used. The thickness of the polymer solid electrolyte membrane is 0.3 μm.
It was.

Comparative Example 4 Hydroxypropyl cellulose was used as the polymer solid electrolyte.
A cell was prepared in the same manner as in Example 1, except that lithium (ion conductivity: 1.0 × 10 ⁻⁹ S / cm) was used and tetrahydrofuran was used as the solvent of the resin liquid. The thickness of the polymer solid electrolyte membrane was 0.3 μm.

Comparative Example 5 As carbon powder, an average particle size of 10 μm obtained by crushing bulk carbon obtained by heat treating petroleum pits at 2,000 ° C.
Carbon powder (d002: 3.43Å, Lc: 265
A cell was prepared in the same manner as in Example 1 except that Å) was used and the treatment with the resin liquid was not performed.

The cells of Examples 1 to 5 and Comparative Examples 1 to 5 were charged at ² mA / cm ² , and the charge potential at which lithium metal was deposited and the charge capacity at that time were measured. The results are shown in Table 1.

[0039]

From Table 1 above, in the cells of Examples 1 to 5 in which the polymer solid electrolyte membrane was formed on the carbon particle surface, the polymer solid electrolyte membrane was formed even when low crystalline carbon was used. Compared with the cells of Comparative Examples 1 and 2 which were not formed, the lithium metal deposition potential can be lowered, the overvoltage is increased, the safety is improved, and even if a solid polymer electrolyte membrane is provided. It can be seen that excellent charge capacity is obtained.

On the other hand, even when the solid polymer electrolyte membrane was formed on the surface of the particles of low crystalline carbon, the lithium ion conductivity of the membrane was less than 5 × 10 ^-5 S / cm in Comparative Example 4. In the cell (2), the effect of lowering the deposition potential due to the formation of the film is small, and the charge capacity is greatly reduced. Furthermore, d
In the cells of Comparative Examples 3 and 5 using the graphitized carbon having 002 of less than 3.45 Å, the charge capacity was low, and the effect of forming a polymer solid electrolyte membrane was hardly seen compared with both cells.

The cell of Comparative Example 3 using the graphitized carbon coated with the polymer solid electrolyte membrane is the cell of Comparative Example 5 using the graphitized carbon not coated with the above membrane. The reason for the lower charging capacity is that graphitized carbon originally has a low lithium metal deposition potential and metal deposition is less likely to occur. Is expected to decrease.

[0043]

As described above, according to the present invention, the negative electrode active material uses carbon particles having an average interlayer distance [d002] of 3.45 Å or more, and the carbon particles have lithium ion conductivity at room temperature. By coating with an ion conductive polymer solid electrolyte having a concentration of 5 × 10 ⁻⁵ S / cm or more, the deposition potential of lithium metal is lowered, and the safety is excellent and
A lithium ion secondary battery having a high charge capacity can be provided.

[Brief description of drawings]

FIG. 1 is a schematic view of a carbon negative electrode showing a state in which carbon particles used in the present invention are coated with an ion conductive polymer solid electrolyte.

FIG. 2 is a characteristic diagram showing charge / discharge characteristics of a negative electrode using a graphitized carbon and a low crystalline carbon.

FIG. 3 is a characteristic diagram showing the relationship between the average interlayer distance [d002] of carbon particles and the diffusion coefficient of lithium ions in the negative electrode near the metal lithium potential.

[Explanation of symbols]

 1 Carbon Particles 2 Ion Conducting Polymer Solid Electrolyte Membrane 3 Organic Electrolyte 4 Current Collecting Material 5 Sheet-like Negative Molded Body

Claims (1)

[Claims] 1. A positive electrode comprising a lithium transition metal oxide,
In a lithium ion secondary battery having a negative electrode made of a carbon material and an organic electrolytic solution, the negative electrode active material comprises carbon particles having an average interlayer distance [d002] of 3.45 Å or more, and the carbon particles are A lithium ion secondary battery, which is coated with an ion conductive polymer solid electrolyte having a lithium ion conductivity of 5 × 10 ⁻⁵ S / cm or more at room temperature.