JPH10228896A - Non-aqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To accomplish increase in energy density and improvement of charging-discharging efficiency by using a composite carbon material consisting of a graphite material, which is prepared by heat treating and graphitizing a meso-carbon micro bead made of a mesophase micro sphere, and a slightly graphitizable carbon material, which is prepared by carbonizing a resin selected from a group consisting of phenol resin, furan resin and the like, as a negative electrode. SOLUTION: As a graphite material, a graphitized carbon material, which is prepared by heat treating a meso-carbon micro bead made of petroleum pitch, is used. As a slightly graphitizable carbon material, a carbon material, which is prepared by heat treating and carbonizing phenol resin, is used. A composite carbon material consisting of these carbon materials is used as a negative electrode. In this case, a mixing ratio of the slightly graphitizable carbon material in the composite carbon material ranges from 10% to 50% on a weight base. In this way, in which the slightly graphitizable carbon is mixed with the graphite material consisting of mesophase group carbon material, fracture of the carbon granule due to pressurization can be prevented, while high capacity and excellent charging-discharging efficiency can be accomplished.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-aqueous electrolyte secondary battery, and more particularly to an improvement in a negative electrode thereof.

[0002]

2. Description of the Related Art In recent years, mobile phones, PHSs, and camera-integrated Vs
The market for portable electronic devices such as TRs is growing rapidly. Along with this, various demands for batteries as drive power supplies are intensifying, and high energy density, long life,
A battery having high safety and the like is expected.

[0003] From such a viewpoint, non-aqueous secondary batteries, especially lithium secondary batteries, are highly expected as batteries having high voltage and high energy density. In particular, recently, a battery system using a lithium-containing transition metal oxide as a positive electrode active material and a carbonaceous material for a negative electrode has attracted attention as a lithium secondary battery having a high energy density.

[0004] Since both the positive and negative electrodes of the battery utilize the intercalation and deintercalation reactions of the active material, short-circuiting due to precipitation of dendritic Li does not occur, and the battery is excellent in safety.

[0005] Proposals for using a carbon material for the negative electrode material include:
For example, Japanese Patent Application Laid-Open No.
-208079 and 58-102464. However, graphite has crystallites that develop, which causes the destruction of the crystal structure during the insertion and extraction of lithium, resulting in poor storage and cycling characteristics and high reactivity. This causes a drawback that the initial charge / discharge efficiency is reduced.

[0006] In order to solve such disadvantages, it has been proposed to use a carbon material having a low degree of graphitization and a crystallite that has not developed much. Specifically, it has been proposed to define the degree of graphitization by the sintering temperature. A method of using an organic sinter obtained at a sintering temperature of 1500 ° C. or less as a negative electrode material is disclosed in JP-A-58-93176 and JP-A-58-93176. -2
No. 35372 is disclosed in each publication.

However, a carbon material having a low degree of graphitization has a large irreversible capacity and a small density as compared with a carbon material having a high degree of graphitization, so that the energy density of the obtained battery is low and the battery capacity is insufficient. Was something.

On the other hand, there has been proposed an attempt to improve the initial charge / discharge efficiency by regulating the specific surface area of the carbon material to suppress side reactions such as decomposition of the electrolytic solution occurring at the carbon-electrolyte interface, and to improve the initial charge / discharge efficiency. .

For example, Japanese Patent Application Laid-Open No. 5-242890 discloses
Is 0.5 m of the specific surface area of the carbonaceous material ^Two/ G or more, 10m
^Two/ G or less are disclosed.

JP-A-6-295725 discloses a powder having a specific surface area of 1 to 10 m ² / g, an average particle size of 10 to 30 μm and a particle size of 10 μm or less. It has been proposed to use graphite powder having at least one of a content of powder having a particle size of 30 μm or more of 10% or less as a negative electrode material.

Particularly, in the case of a graphite-based material, the primary cause of the initial irreversible capacity is a decomposition reaction of the electrolytic solution occurring at the carbon-electrolyte interface, so that reducing the surface area of the carbon material improves the initial charge / discharge efficiency. Important for.

[0012]

However, when a carbon material is actually used as a negative electrode in a battery, the carbon material as a powder is slurried and applied to a current collector, and then the carbon material is added to increase the packing density. It is common to use after pressure forming. At first glance, it seems that the surface area decreases because the porosity decreases when the negative electrode is formed under pressure, but surprisingly, the specific surface area of the negative electrode significantly increases when the negative electrode is compressed under pressure. Was found by the present inventors.

Since the surface area of the negative electrode affects the actual battery characteristics, no matter how the specific surface area of the negative electrode active material powder is specified, the increase in the surface area of the negative electrode in this pressing step is not controlled. As long as the initial charge / discharge efficiency of the actual battery is not improved.

The present inventors have conducted intensive studies to solve the above-mentioned problems, and as a result, have found that the rate of increase of the specific surface area due to the pressurization of the negative electrode greatly differs depending on the carbon material.

The present invention has been made based on such findings, and it is an object of the present invention to provide a non-aqueous electrolyte secondary battery having higher energy density and superior charge / discharge efficiency as compared with conventional batteries. Is to do.

[0016]

The above objects can be attained by the present invention described below.

That is, the present invention provides a non-aqueous electrolyte comprising a positive electrode using a lithium-containing composite oxide as an active material, a non-aqueous electrolyte, and a negative electrode using a carbon material capable of occluding and releasing lithium as an active material. In a secondary battery, a nonaqueous electrolyte secondary battery is disclosed in which the negative electrode is a composite carbon material composed of a graphite material A and a non-graphitizable carbon material B.

In order to achieve the above object, the present invention provides a non-aqueous liquid by controlling a change in electrode surface area due to pressure by using a composite carbon material of graphite material A and non-graphitizable carbon B for the negative electrode. The purpose is to increase the charge / discharge efficiency of the electrolyte secondary battery.

Generally, in a negative electrode produced by slurrying a negative electrode carbon material and applying the slurry to a current collector, when pressure is applied, carbon particles are crushed and the surface area increases. The rate of the increase in the surface area is relatively small in a carbon material obtained by calcining and carbonizing glassy carbon, for example, a thermosetting resin such as a phenol resin or a furan resin.

However, graphitized mesocarbon microbeads, bulk mesophases, and mesophase-based carbon fibers made from mesophase obtained by heat-treating petroleum pitch or coal tar have an increased surface area due to pressure. Is very large.

That is, these mesophase-based carbon materials have lower mechanical strength than glassy carbon materials and the like, and the surface of the mesophase-based carbon material is coated with non-graphitizable carbon. The crushing of the carbon particles destroys the non-graphitizable carbon coating,
This is because more carbon net surfaces are exposed.

Therefore, the present inventors use a composite carbon material composed of a graphite material A and a non-graphitizable carbon material B as a negative electrode active material, thereby alleviating an increase in the specific surface area of the electrode sheet due to pressurization. By doing so, the above problem was solved.

In the present invention, the compounding ratio of the graphite material A and the non-graphitizable carbon material B is important, and the amount of the non-graphitizable carbon material B in the composite carbon material is 1 by weight.
It is preferably from 0 to 50%, more preferably from 15 to 35%.
% Is more preferred.

If it is less than 10%, the effect of the non-graphitizable carbon material B cannot be sufficiently utilized, and the rate of increase in the specific surface area due to the pressurization increases, and as a result, the initial charge / discharge efficiency decreases. On the other hand, if it exceeds 50%, the packing density of the negative electrode material decreases, and the capacity as a battery decreases.

On the other hand, as a cathode material, for example, LiCo
O ₂ , LiNiO ₂ , LiMnO _{2 and the} like are mentioned as preferable ones. Further, Co, Ni, M of the above compounds
A composite oxide in which a part of n is substituted with another element, for example, Co, Ni, Mn, Fe, or the like can be used. The composite oxide is, for example, a carbonate or oxide of lithium or cobalt as a raw material, and blending them according to a desired composition.
It can be easily obtained by firing.

As described above, the battery of the present invention has the greatest feature in using a composite carbon material composed of a graphite material A and a non-graphitizable carbon material B as a negative electrode active material. As other members constituting the battery such as a non-aqueous electrolyte and a separator, it is possible to use various materials conventionally used or proposed as non-aqueous electrolyte secondary batteries.

Since the surface of the mesophase-based carbon material according to the present invention is covered with the non-graphitizable carbon film, side reactions such as decomposition of the electrolytic solution are relatively unlikely to occur as compared with other graphite materials. However, this coating is crushed by pressurization of the negative electrode sheet to expose the active carbon netting surface, thereby increasing the surface area of the negative electrode, thereby lowering the initial charge / discharge efficiency and deteriorating the capacity accompanying the cycle.

Therefore, by blending the non-graphitizable carbon with the graphite material composed of the above-mentioned mesophase-based carbon material, it is possible to prevent the carbon particles from being broken by pressurization and to prevent a decrease in the initial charge / discharge efficiency. Therefore, by combining this negative electrode with a positive electrode made of a lithium-containing composite oxide, a secondary battery having high capacity and excellent charge / discharge efficiency can be obtained.

[0029]

Embodiments of the present invention will be described below in detail with reference to the drawings.

[0030]

FIG. 1 is a longitudinal sectional view of a cylindrical battery used in this example and a comparative example. In FIG. 1, reference numeral 1 denotes a stainless steel battery case, and reference numerals 2, 3, and 4 denote positive and negative electrode sheets and separators, respectively. The positive and negative electrodes are spirally wound through the separator and housed in the case 1. ing. 5 is PTC, 6 is safety valve,
Reference numeral 7 denotes an insulating cap.

[Examples 1 to 5] (Preparation of negative electrode sheet) As graphite material A, 28 mesocarbon microbeads produced using stone extraction pitch as a raw material were used.
A carbon material heat-treated at 00 ° C. and graphitized is used. As a non-graphitizable carbon material B, a phenol resin is used at 1500 ° C.
A carbon material carbonized by heat treatment in the above was used, and these carbon materials were blended at a blending ratio as shown in Table 1 to obtain a negative electrode active material. Polyvinylidene fluoride (PVD)
F) 10% by weight and 2% by weight of acetylene black are mixed to prepare a negative electrode mixture, which is dispersed in N-methyl-2-pyrrolidone to form a slurry. The slurry is applied to both surfaces of a copper foil, dried, and then roll-pressed. By compression molding with a machine, the negative electrode packing density was adjusted to a range of 1.4 to 1.6 g / cc to obtain a negative electrode sheet.

Table 1 shows the packing density (g / cc) of the negative electrodes after the formation of each negative electrode under pressure. The densities of the negative electrodes of Examples 1 to 4 after forming under pressure could be about 1.5 (g / cc). However, the phenol resin fired body in the negative electrode active material of Example 5 was 75% by weight. The negative electrode has a packing density of 1.4 (g / c
c).

(Measurement of Specific Surface Area of Negative Electrode Sheet) The above negative electrode sheet was cut into 20 cm ² , and nitrogen was used as an adsorbed gas. E. FIG. T. The specific surface area was measured by the one-point method. Table 1 shows the measurement results. The specific surface area decreases as the weight percentage of the phenol resin fired body increases.

[0034]

[Table 1]

(Preparation of Positive Electrode Sheet) The positive electrode was prepared by mixing lithium carbonate and manganese dioxide, and baking at 750 ° C. for 12 hours. 3.5% by weight of polyvinylidene fluoride (PVDF) was blended to prepare a positive electrode mixture, which was dispersed in N-methyl-2-pyrrolidone to form a slurry, and this slurry was used as a positive electrode current collector in a 10 μm-thick strip. Was coated on both sides of an aluminum foil, dried, and then compression-molded with a roll press to obtain a positive electrode sheet.

(Assembly of Battery) A tab was attached to each of the positive and negative electrodes, spirally wound through a polypropylene separator, and housed in a battery can. In this battery can,
Ethylene carbonate and diethyl carbonate:
An electrolyte obtained by dissolving lithium hexafluorophosphate at a ratio of 1 mol / liter in a solvent mixed at a volume ratio of 7 was injected.
Then, the battery can was caulked via a green sealed gasket coated with tar, and the battery lid was fixed to produce a cylindrical nonaqueous electrolyte battery.

[Examples 6 to 10] Except that the graphite material A as the negative electrode active material in Examples 1 to 5 was subjected to a heat treatment at 2800 ° C. with a carbon fiber produced from petroleum pitch as a graphitized carbon material. Produced batteries in the same manner as in Examples 1 to 5. Table 2 shows the compounding ratio of the negative electrode active materials of Examples 6 to 10, the negative electrode packing density (g / cc) after pressure formation, and the negative electrode specific surface area (m ² / g) after pressure formation.

[0038]

[Table 2]

(Charge / Discharge Test) Each of the batteries obtained as described above was charged at a constant current at a charge current of 0.6 A and a charge end voltage of 4.2 V, and then was charged at 4.2 V for 4 hours at a constant voltage. Thereafter, a discharge current of 1.0 A and a discharge end voltage of 3.0 V
Under the following conditions. FIG. 2 shows the initial charge / discharge efficiency as a result.

Comparative Example 1 A battery was manufactured in the same manner as in the example except that the mesophase carbon microbeads were used as the negative electrode active material in the example.

Comparative Example 2 A battery was manufactured in the same manner as in Example except that only the mesophase-based carbon fiber was used as the negative electrode active material in Example.

Comparative Example 3 A battery was fabricated in the same manner as in the example except that only the phenol resin fired body was used as the negative electrode active material in the example.

Table 3 shows the negative electrode packing density (g / cc) of the negative electrodes of Comparative Examples 1 to 3 after pressure formation, and the negative electrode specific surface area (m ² / g) after pressure formation.

[0044]

[Table 3]

FIG. 2 shows the initial charge / discharge efficiency as a result of conducting a charge / discharge test on the batteries of Comparative Examples 1 to 3 under the same conditions as in the example.

(Characteristics of each battery) In Examples 1 to 10 and Comparative Examples 1 to 3, the charge / discharge efficiency was determined when the blending ratio of the phenol resin fired body to the mesophase graphite material in the negative electrode active material was about 25% by weight. It is almost maximal at about 90%.

When the blending ratio of the phenol resin fired body is in the range of 0 to 25% by weight, the increase in the specific surface area of the negative electrode due to the pressurization is reduced by adding the phenol resin fired body. It is considered that the decomposition reaction of the electrolyte solution occurring at the liquid interface was suppressed. On the other hand, when the blending ratio of the phenol resin fired body exceeds 50% by weight, the charge / discharge efficiency decreases with the addition of the phenol resin fired body. This is probably because the amount of lithium ions occluded at such sites increased.

In this example, when the blending ratio of the phenol resin fired body was within the range of 10 to 50% by weight, the initial charge / discharge efficiency was maintained while maintaining the negative electrode packing density at 1.5 (g / cc) or more. The capacity of the battery was able to be improved by improving the capacity.

[0049]

As is apparent from the above description, by using the composite carbon material composed of the graphite material A and the non-graphitizable carbon material B for the negative electrode according to the present invention, a high capacity and a high charge-discharge efficiency can be obtained. It is possible to provide a non-aqueous electrolyte secondary battery that is also excellent.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view showing a cylindrical battery according to an embodiment of the present invention.

FIG. 2 is a graph showing a comparison of initial charge / discharge efficiencies of each example and each comparative example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Battery (can) case 2 Positive electrode (sheet) 3 Negative electrode (sheet) 4 Separator 5 PTC 6 Safety valve (rupture disk) 7 (Insulation) cap

Claims (5)

[Claims] 1. A non-aqueous electrolyte secondary battery including a positive electrode using a lithium-containing composite oxide as an active material, a non-aqueous electrolyte, and a negative electrode using a carbon material capable of occluding and releasing lithium as an active material. A non-aqueous electrolyte secondary battery, wherein the negative electrode is a composite carbon material composed of a graphite material A and a non-graphitizable carbon material B. 2. A graphite material A in the composite carbon material
Is a graphitized by heat-treating mesocarbon microbeads composed of mesophase spherules obtained by carbonization of pitch, or graphitized by heat-treating carbon fibers composed of the mesophase spherules. 2. The non-aqueous electrolyte secondary battery according to 1. 3. The non-graphitizable carbon material B in the composite carbon material is a carbon material obtained by carbonizing a resin selected from the group consisting of a phenol resin, a furan resin and a cellulose resin. The non-aqueous electrolyte secondary battery according to the above. 4. The non-aqueous electrolyte solution according to claim 1, wherein the compounding ratio of the non-graphitizable carbon material B in the composite carbon material is within a range of 10 to 50% by weight. Next battery. 5. A negative electrode obtained by forming a negative electrode mixture comprising the composite carbon material and a binder in a layer on a current collector, wherein the packing density of the negative electrode mixture layer is 1.3 g / cc or more. And B.I. of the negative electrode mixture layer. E. FIG. T. Non-aqueous electrolyte secondary battery according to any one of claims 1 to 4 which is the surface area by one-point method is 1.5 m ² / g or less.