JPH0737618A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PURPOSE:To provide a secondary battery with high voltage, high capacity, and long life by constituting a negative electrode of graphite spherical particles having an optical aeolotropic single phase, and graphite fine powder smaller in particle size than the graphite spherical particles. CONSTITUTION:Graphite of a mean particle size of 6mum is obtained by heat treating mesocarbon microbeads (MCMB) produced in a carbonizing process of pitch, for example, at 2800 deg.C for graphitization. The process is controlled so that spacing in 202 planes of MCMB exists within the range of 3.36-3.39Angstrom . Artificial graphite fine powder having a spacing in 002 planes of 3.37Angstrom and a mean particle size of 3mum is mixed with MCMB in a mixing ratio of 3-15wt.% based on the weight of the MCMB together with a preferable binder, and the mixture is applied to both sides of a copper foil to constitute a negative electrode. Absorption/desorption reaction of lithium is enhanced.

Description

Detailed Description of the Invention

[0001]

The present invention relates to a non-aqueous electrolyte secondary battery,
More specifically, the present invention relates to a small, lightweight, and improved secondary battery negative electrode.

[0002]

2. Description of the Related Art In recent years, portable electronic devices for consumer use,
Cordless is advancing rapidly. Along with this, there is an increasing demand for a small-sized, lightweight secondary battery having a high energy density, which serves as a driving power source. From this point of view, non-aqueous secondary batteries, especially lithium secondary batteries, have great expectations as batteries having high voltage and high energy density, and their development is urgently needed.

Heretofore, manganese dioxide, vanadium pentoxide, titanium disulfide and the like have been used as positive electrode active materials for lithium secondary batteries. A battery was constituted by these positive electrodes, a lithium negative electrode and an organic electrolytic solution, and charging and discharging were repeated. However, in a secondary battery using lithium metal for the negative electrode, problems such as internal short circuit due to dendrite-like lithium generated during charging and discharging and side reactions between the active material and the electrolyte are major obstacles to the conversion to a secondary battery. Further, no one has been found to be satisfactory in high-rate charge / discharge characteristics and over-discharge characteristics.

In recent years, the safety of lithium batteries has been pointed out severely, and it is very difficult to ensure the safety in battery systems using lithium metal or lithium alloy for the negative electrode.

On the other hand, a new type of electrode active material utilizing the intercalation reaction of a layered compound has been attracting attention, and a graphite intercalation compound has been used as an electrode material for secondary batteries for a long time. In particular, ClO ₄ ⁻ , PF ₆ ⁻ , BF
Graphite intercalation compounds incorporating anions such as ₄ ^- ions are used as positive electrodes.

On the other hand, graphite intercalation compounds incorporating cations such as Li ⁺ and Na ⁺ are considered as negative electrodes. However, the graphite intercalation compound incorporating cations is extremely unstable, and when natural graphite or artificial graphite is used as the negative electrode, it lacks stability as a battery, has a low capacity, and further decomposes the electrolytic solution. , Could not substitute for the lithium negative electrode.

Recently, a cation dope of pseudo graphite material obtained by carbonizing various hydrocarbons or polymer materials is effective as a negative electrode and has a relatively high utilization rate and excellent stability as a battery. Was found. And small and lightweight secondary batteries using the same are being actively researched.

On the other hand, since a carbon material is used for the negative electrode, the positive electrode active material has a higher voltage and Li
It has been proposed to use LiCoO ₂ or LiMn ₂ O ₄ which is a compound containing, and further a composite oxide in which some of Co and Mn are replaced with other elements.

[0009]

When a pseudo graphite material having a certain degree of disordered layer structure as described above is used as the negative electrode material,
The storage and release amount of lithium was calculated to be 100 to 1
Only a capacity of 50 mAh / g carbon can be obtained, and the polarization of the carbon electrode increases with charge and discharge. Therefore, when combined with a positive electrode such as LiCoO _2, it is difficult to obtain a satisfactory capacity and voltage.

On the other hand, it has been reported that when a highly crystalline graphite material is used as the negative electrode material, gas is generated at the surface of the graphite electrode due to decomposition of the electrolytic solution during charging, and the intercalation reaction of lithium is difficult to proceed. . However, it has been found that a high temperature calcined product of coke, etc., gives a relatively high capacity (200 to 250 mAh / g), although some gas is generated. However, due to the large expansion and contraction of graphite in the C-axis direction with charge and discharge, the molded body swells, and the original shape can no longer be maintained, causing a decrease in conductivity between the negative electrode mixture particles and the like, which leads to deterioration of charge and discharge cycle characteristics Cause deterioration.

An object of the present invention is to provide a non-aqueous electrolyte secondary battery which solves the above-mentioned conventional problems, has a high voltage and a high capacity, and has excellent cycle characteristics.

[0012]

In order to solve these problems, the present invention provides a graphite material comprising spherical particles having an optically anisotropic and single phase in the negative electrode, and the average particle size thereof is By using a composite carbon material composed of other different graphite fine particles smaller than spherical particles, it is possible to prevent the decrease in conductivity between negative electrode mixture particles caused by swelling and collapse of the molded body due to charge and discharge. is there.

Generally, the amount of lithium that can be chemically intercalated between graphite layers is such that the first stage graphite intercalation compound C in which one atom of lithium is inserted for every six atoms of carbon is used.
₆ Li is reported to be the upper limit, in which case the active material will have a capacity of 372 mAh / g. When the pseudo graphite material as described above is used, the amount of lithium that can be intercalated is small because the layered structure of graphite is undeveloped, and the charge / discharge reaction is noble to metallic lithium.
Since it proceeds at a potential near 0 V, it was not suitable as a negative electrode material. When the high temperature calcined product of coke is used for the negative electrode, depending on the type of coke, the initial value is 200 to 25.
It was found to have a capacity of 0 mAh / g.

Therefore, as a result of studying the shape of the carbonaceous material particles, the present inventors have found that a graphite material composed of spherical particles is used as a negative electrode graphite material, particularly mesocarbon made from mesophase spherules produced in the carbonization process of pitch. It was found that a graphitized product obtained by subjecting microbeads to a heat treatment can be used. The degree of graphitization is an important factor in all of these, and the interplanar spacing (d002) of the 002 planes is preferably 3.36Å or more and 3.39Å or less. Generally 3.
In the pseudographitic state of 40 Å or more, the capacity is small and the polarization as the carbon electrode is large as in the case of other pseudographite materials.

However, when the above graphite material composed of spherical particles is used for the negative electrode, the initial value is 200 to 250 mAh.
/ G has a high capacity, but the swelling of the molded body with charge and discharge,
Collapse is seen, and the capacity deterioration with the cycle increases.

Therefore, the present inventors have prepared a composite carbon material by adding and mixing other graphite fine powder having an average particle size smaller than that of the graphite material composed of the above-mentioned spherical particles. The above-mentioned problems were solved by preventing a decrease in conductivity between the mixture particles due to swelling and collapse of the molded article.

For the purpose of filling the intergranular spaces of the graphite material composed of spherical particles and improving the conductivity, the average particle diameter of other different graphite fine powders added is different from that of the graphite material composed of spherical particles as the base material. Obviously, it must be smaller than the average particle size.

As the fine graphite powder used in the present invention, generally commercially available artificial graphite made from petroleum or coal, or finely pulverized products such as natural graphite can be used.

When fine powder of pseudo-graphite material such as coke is used in place of the fine graphite powder, the improvement of the conductivity is not sufficient, so that there is no significant effect. Cause decline. When carbon black such as acetylene black is added, an increase in specific surface area of the composite carbon material leads to an increase in gas generation during battery charging.

Further, in the present invention, the mixing ratio of the graphite material composed of spherical particles and the fine graphite powder is important, and the addition amount of the fine graphite powder in the above composite carbon material is relative to the graphite material composed of spherical particles. 20 wt% or less is preferable, and more preferably 3 wt% or more and 15 wt% or less. If it is less than 3% by weight, the effect of the graphite fine powder cannot be fully utilized and the cycle characteristics are deteriorated. 15% by weight, especially 2
If the amount exceeds 0% by weight, the packing density of the graphite material composed of spherical particles decreases and the capacity of the battery decreases.

On the other hand, the positive electrode is a compound containing lithium ions.
LiCoO that is a product₂And LiMn₂O _Four, And even both
Part of Co or Mn may be replaced with other elements such as Co, M
A composite oxide substituted with n, Fe, Ni or the like can be used.
The composite oxide is, for example, a carbonate of lithium or cobalt.
Alternatively, using an oxide as a raw material, depending on the target composition, these
Can be easily obtained by mixing and baking
It Of course, when other raw materials are used
Wear. Usually the firing temperature is between 650 ° C and 1200 ° C.
Is set.

The electrolytic solution and the separator are not particularly limited, and any conventionally known one can be used.

[0023]

The graphite material composed of spherical particles according to the present invention is suitable for the intercalation / deintercalation reaction of lithium because the graphite material has a spherical shape. Side reactions are less likely to occur. However, swelling of the negative electrode molded body is observed with charge / discharge cycles, contact between particles is deteriorated, and conductivity is lowered, resulting in significant capacity deterioration.

Therefore, by mixing and dispersing other graphite fine powder having an average particle size smaller than that into the graphite material composed of the above-mentioned spherical particles, good contact between the mixture particles can be maintained, and conductivity between the spherical particles can be maintained. It was possible to prevent the capacity deterioration due to the cycle by preventing the deterioration of the performance.

Therefore, by combining this negative electrode with a positive electrode made of a lithium-containing composite oxide, it becomes possible to obtain a secondary battery having a high voltage and a high capacity and excellent in cycle characteristics.

[0026]

The present invention will be described in detail with reference to the following examples. FIG. 1 shows a vertical cross-sectional view of the cylindrical battery used in this example. In the figure, 1 is a battery case formed by processing an organic electrolyte resistant stainless steel plate, 2 is a sealing plate provided with a safety valve, and 3 is an insulating packing. Reference numeral 4 denotes an electrode plate group, in which the positive electrode and the negative electrode are spirally wound a plurality of times with a separator interposed between them.
It is stored inside. A positive electrode lead 5 is drawn out from the positive electrode and connected to the sealing plate 2, and a negative electrode lead 6 is drawn out from the negative electrode and connected to the bottom of the battery case 1. Insulating rings 7 are provided above and below the electrode plate group 4, respectively.

The positive and negative electrode plates, electrolytic solution and the like will be described in detail below. The positive electrode was LiCoO ₂ synthesized by mixing Li ₂ Co ₃ and CoCO ₃ and firing at 900 ° C. for 10 hours.
3 parts by weight of acetylene black,
4 parts by weight of graphite and 7 parts by weight of a fluororesin binder were mixed and suspended in an aqueous solution of carboxymethyl cellulose to form a paste. This paste has a thickness of 0.03 mm
Applied to both sides of the aluminum foil, dried and rolled to a thickness of 0.1
The electrode plate was 8 mm, 40 mm wide, and 260 mm long.

The negative electrode was mesocarbon microbeads (d002 = 3.37Å; average particle size 6 μm) heat-treated at 2800 ° C. (hereinafter, abbreviated as MCMB) and artificial graphite fine powder (manufactured by SEC, artificial graphite SGP-). 3; d002 =
3.37Å; average particle size 3 μm) in a mixing ratio as shown in Table 1, 100 parts by weight of this carbon material and 100 parts by weight of a fluororesin-based binder, and suspended in a carboxymethylcellulose aqueous solution. Made into a paste. Then, this paste is applied to both sides of a copper foil having a thickness of 0.02 mm, dried and rolled to a thickness of 0.19 mm, a width of 40 mm, and a length of 28.
The electrode plate was 0 mm. Then, a lead was attached to each of the positive and negative plates, and the thickness was 0.025 mm, the width was 46 mm, and the length was 7 mm.
It was spirally wound through a separator made of polypropylene of 00 mm and was housed in a battery case having a diameter of 13.8 mm and a height of 50 mm. As the electrolytic solution, a solution prepared by dissolving lithium perchlorate in a mixed solvent of equal volume of propylene carbonate and ethylene carbonate at a rate of 1 mol / liter was used. This electrolytic solution was injected under reduced pressure and sealed to obtain a test battery. Then, these test batteries were charged / discharged at a current of 100 mAh and cut off at a voltage of 4.
A constant current charge / discharge test was conducted under the conditions of 1 V and a discharge end voltage of 3.0 V. The comparison of the cycle characteristics is shown in FIG.

[0029]

[Table 1]

In the battery 1 containing no graphite fine powder, the capacity was significantly deteriorated with the cycle. On the other hand, 3 to 15
It can be seen that in batteries 2 to 5 using the composite material mixed by weight%, the capacity deterioration accompanying the cycle is extremely small while maintaining the high capacity. The battery 6 containing 25% by weight of graphite fine powder has relatively good cycle characteristics, but the capacity becomes extremely small. This is because the graphite fine powder became dominant and the filling amount of the mixture was reduced. The average discharge voltage was about 3.7 V in each case.

Comparative Example 1 In place of the fine graphite powder used in Example, commercially available artificial graphite (d002 = 3.42Å)
A battery of Comparative Example 1 was prepared under the same conditions as in Example 1 except that the composite carbon material containing 5% by weight of was used for the negative electrode.

(Comparative Example 2) A composite carbon material was used under the same conditions as in the Example, except that a composite carbon material prepared by mixing 5% by weight of commercially available acetylene black was used instead of the fine graphite powder in the Example. The battery of Example 2 was used.

The batteries of Comparative Examples 1 and 2 were subjected to a charge / discharge test under the same conditions as in Examples, and their cycle characteristics are shown in FIG.

It can be seen that the battery of Comparative Example 1 has a smaller initial capacity than that of the battery 3 of Example, and has a large capacity deterioration due to cycling. This is because the added coke fine powder has a smaller lithium storage capacity than the graphite fine powder, and further, the effect of improving the conductivity between the graphite base material particles composed of spherical particles is inferior to the graphite fine powder. Probably the cause.

In the battery of Comparative Example 2, the initial capacity was remarkably reduced. This is because the addition of acetylene black with a large specific surface area promotes the decomposition of the electrolytic solution on the surface of the negative electrode mixture,
This is probably because the lithium intercalation reaction was inhibited.

[0036]

As is apparent from the above description, according to the present invention, the negative electrode is composed of a graphite material composed of spherical particles and another fine graphite powder having an average particle size smaller than the spherical particles. By using such a composite carbon material, it is possible to provide a secondary battery having high voltage and high capacity and excellent in cycle characteristics.

[Brief description of drawings]

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

FIG. 2 is a diagram showing a comparison of cycle characteristics of examples.

FIG. 3 is a diagram showing a comparison of cycle characteristics between an example and a comparative example.

[Explanation of symbols]

 1 Battery case 2 Sealing plate 3 Insulating packing 4 Electrode plate 5 Positive electrode lead 6 Negative electrode lead 7 Insulating ring

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Toru Takai 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd.

Claims (6)

[Claims] 1. A non-aqueous electrolyte secondary battery comprising a positive electrode made of a lithium-containing composite oxide, a non-aqueous electrolyte, and a rechargeable negative electrode, wherein the negative electrode is optically anisotropic and unitary. A non-aqueous electrolyte secondary battery, which is a composite carbon material composed of a graphite material composed of spherical particles having one phase and another different graphite fine powder having a smaller average particle diameter. 2. The graphite material comprising spherical particles is graphitized by subjecting mesocarbon microbeads made of mesophase microspheres, which are generated in the carbonization process of pitch, as a raw material to heat treatment. Water electrolyte secondary battery. 3. The graphite material composed of the spherical particles has a lamellar structure, and the graphite material has a lamella structure of 0.
The surface spacing (d002) of 02 surfaces is 3.36 Å or more 3.39
The non-aqueous electrolyte secondary battery according to claim 1 or 2, which is Å or less. 4. The non-aqueous electrolyte secondary battery according to claim 1, wherein the mixing ratio of the graphite fine powder in the composite carbon material is 20% by weight or less with respect to the spherical material. . 5. The nonaqueous electrolyte secondary battery according to claim 4, wherein the mixing ratio of the fine graphite powder in the composite carbon material is 3 to 15% by weight with respect to the spherical material. 6. The non-aqueous electrolyte secondary according to claim 1, wherein the fine graphite powder in the composite carbon material is either artificial graphite made from petroleum or coal pitch or natural graphite. battery.