JP3126030B2 - Lithium secondary battery - Google Patents

Description

The present invention relates to a lithium secondary battery, and more particularly, to an output using a spherical carbon material, particularly a spherical carbon material having optical anisotropy, as a carrier for a negative electrode active material. The present invention relates to a lithium secondary battery having high density and discharge capacity and excellent cycle characteristics.

Conventional technology Lithium as the negative electrode active material, metal chalcogenide, metal oxide and conjugated polymer material as the positive electrode active material,
A so-called lithium secondary battery using a solution in which various salts are dissolved in an aprotic organic solvent as an electrolytic solution has attracted attention as a kind of high energy density secondary battery, and has been actively studied.

However, in a conventional lithium secondary battery, lithium as a negative electrode active material is often used in the form of a single foil or a foil containing a small amount of another alkali metal,
As charge and discharge are repeated, dendritic lithium precipitates and both electrodes are short-circuited, so that the charge and discharge cycle life is short. Further, since an alkali metal having high reactivity with water is used, there are some problems in safety such as ignition and heat generation.

In view of this, a method has been proposed in which a fusible alloy containing aluminum, lead, cadmium, and indium is used to deposit lithium as an alloy during charging and dissolve lithium from the alloy during discharging [US Patent No. 4002492 ( 1977)]. However, in such a method, the precipitation of dendritic lithium can be suppressed, but the energy density decreases.

Similarly, various attempts have been made to carry lithium on a carbon material for the purpose of eliminating the precipitation of dendritic lithium and solving the problem of a decrease in the energy density of the alloy type. . For example,
Attempts have been made to use a fibrous or powdered carbon material for the carrier [see, for example, JP-A-62-1039.
No. 91, JP-A-63-114056 and JP-A-62-2258]. However, in this type of conventional lithium secondary battery, the output density, the discharge capacity, and the cycle characteristics have not been sufficient yet.

Accordingly, an object of the present invention is to improve the output density, discharge capacity, and cycle characteristics of a lithium secondary battery using a carbon material as a lithium carrier.

Means for Solving the Problems In view of the circumstances described above, the present inventors have conducted intensive studies, and as a result, the spherical carbon material, particularly the crystal group constituting the unit particle has a specific structure, and thus has an optically anisotropic property. By using a spherical carbon material having properties as a lithium carrier,
Surprisingly, they have found that the above problem can be solved, and have completed the present invention.

That is, the present invention provides a negative electrode body for a lithium secondary battery, wherein the negative electrode active material of lithium metal or lithium ion is supported on a spherical carbon material, particularly a spherical carbon material having optical anisotropy. It is intended to provide a lithium secondary battery using the negative electrode body.

As described above, there has been a report using a powdered carbon material as a lithium carrier before the present application, but a spherical carbon material as in the present invention, particularly a powdered carbon material composed of spherical particles having a special crystal structure, has been used. It is clearly distinguished from the present invention.

First, in the present invention, a spherical carbon material is used as a carbon material serving as a lithium carrier.

Among the spherical carbon materials, carbon materials having optical anisotropy are particularly preferable.

This optical anisotropy results from the layered crystal structure of the unit particles. That is, the unit particles of the carbon material used in the present invention consist of crystal groups arranged in layers in a direction perpendicular to the diameter direction of the spherical particles, as in the case of mesocarbon microbeads described later. On the other hand, in ordinary crystalline carbon particles, crystal groups are arranged in a spherical shell shape due to thermodynamic restrictions.
In other words, they tend to be in the form of onions,
No layered structure as in the present invention is employed.

By having such a layered crystal structure, many end faces of the so-called six-carbon ring network plane constituting each crystal group are present on the spherical surface, and lithium can freely enter and leave between the network planes. A space between the mesh planes is a place where a battery reaction occurs, and the presence of many entrances and exits means that the reaction area becomes large, and therefore, the power density is excellent. In the case of crystalline carbon particles that have been conventionally used, such entrances and exits cannot be present on the particle surface due to the spherical shell arrangement, which is disadvantageous in terms of a reaction area.

Preferred specific examples of such a layered crystal structure carbon material include mesocarbon microbeads or carbides or graphitized products thereof.

Mesocarbon microbeads include coal tar, coal tar pitch, petroleum heavy oil (eg asphalt)
Or ethylene bottom oil as a starting material, and heat-treat them under normal pressure to 20 kg / cm ² · G at a temperature of 350 to 450 ° C to separate and purify spherulites from the reaction solution. Is obtained by Further, the obtained mesocarbon microbeads, if necessary, in an inert atmosphere as a powder,
The carbonized or graphitized mesocarbon microbeads are obtained by carbonization or further graphitization. In particular, in that the layered structure develops and has a crystal structure closer to graphite,
Coal-based raw materials such as coal tar or coal tar pitch are used as starting materials, and mesocarbon microbeads obtained by heat-treating them and hydrogenating the coal tar pitch are subjected to heat treatment to form mesophase pitch, which is used as an inert atmosphere. Mesocarbon microbeads obtained by spraying into the inside or spheroidizing treatment in a liquid such as silicone or a carbide or graphitized product thereof are preferable.

It is reported that the obtained mesocarbon microbeads have a lamellar structure as shown in FIG. 1 of the attached drawings [see Brooks and Taylor; Carbon, 3, 185 (1965)], and each lamellar is a condensed polycyclic aromatic compound. Zimmer and White; “Discl
ination structure in carbonaceous Mesophase and Gr
aphite "Aerespace Report (1976)". The end face of the plane of the fused polycyclic network is particularly an active site.

In the present invention, the spherical carbon material as described above is used as a lithium carrier, but the particle size and specific surface area of such a carbon material are not particularly limited. The particle size is usually in the range of 0.1 to 150 μm, preferably 0.5 to 80 μm, and the specific surface area is usually 50 m ² / g or less, preferably 5 m ² / g or less.

By using the spherical carbon material as described above,
Various advantages are obtained.

First, battery characteristics such as output density are improved. This is because, as described above, the active area involved in the battery reaction increases on the surface of the sphere. For example, in the case where the conventional fibrous carbon material was used as a carrier, the output density was generally about 9 W / kg, but in the lithium secondary battery of the present invention, it was 20 W / kg.
Increase to about kg.

Also, the discharge capacity per unit volume (weight) increases. This is because the discharge capacity per unit particle increases and the particles can be packed at a high density. Further, by adjusting the particle size distribution by classification or the like, close-packing becomes possible, and the discharge capacity per unit volume (weight) further increases. Regarding the discharge capacity, for example, in a lithium secondary battery using a conventional fibrous carrier, generally, it was about 70 Ah / (190 Ah / kg), but in the lithium secondary battery of the present invention, it was 100 Ah / (270 Ah / / kg).

Further, since the spherical carbon material has high physical stability, the active material has sufficient durability even when the active material frequently enters and exits the carbon layer, thereby significantly improving the cycle characteristics of the secondary battery. Conventional lithium secondary batteries that do not use carbon materials had a life of about 200 cycles,
In the lithium secondary battery of the present invention, it is extended to about 500 cycles.

The spherical carbon material giving such advantageous battery characteristics uses a binder, and carbon: binder = 99 to 7
The negative electrode body can be formed by molding at a weight ratio of 0: 1 to 30. Since molding can be performed by various shaping techniques such as extrusion molding, injection molding, and compression molding, productivity is excellent.

Even when the spherical carbon material used in the present invention is a spherical carbon material having crystal anisotropy, selectivity in orientation is lost by molding, and as a result, the whole becomes isotropic.
As a result, the anisotropy of the electrode disappears, the electrode is stabilized, and the battery life is further improved.

As described above, the lithium secondary battery of the present invention has a feature in the negative electrode, and a conventionally used positive electrode or electrolytic solution can be used.

That is, as the positive electrode active material, for example, TiS ₂ , MoS ₃ , Nb
_{Se 3, FeS, VS 2,} VSe metal chalcogenide having a layered structure of _{two such, CoO 2, Cr 3 O 5} , TiO 2, CuO, V 3 O 6, Mo 3 O, V 2 O 5
Metal oxides such as (· P ₂ O ₅ ) and MnO ₂ (· Li ₂ O), and conductive conjugated polymer substances such as polyacetylene, polyaniline, polyparaphenylene, polythiophene, and polypyrrole can be used. . Preferably, V ₂ O ₅ , MnO ₂
Is used.

Examples of the electrolyte include propylene carbonate, ethylene carbonate, γ-butyrolactone, tetrahydrofuran, 2-methyltetrahydrofuran, dioxolan, 4-methyldioxolan, sulfolane,
Aprotic solvents such as 2-dimethoxyethane, dimethylsulfoxide, acetonitrile, N, N-dimethylformamide, diethyleneglycol-dimethylether, preferably tetrahydrofuran, 2-methyltetrahydrofuran, dioxolan, and stable even in strong reducing atmospheres such as 4-methyldioxolane LiBF ₄ , LiClF ₄ , L
_{iAsF 6, LiSbF 6, LiAlO 4} , LiAlCl 4, LiPF 6, LiCl, can be used to dissolve the salts formed solvated hard anions such as LiI.

In addition, a separator such as a generally used porous polypropylene nonwoven fabric including a polyolefin-based porous membrane, a current collector, a gasket, a sealing plate, a battery component such as a case and the carbon anode of the present invention as described above. It can be used to assemble a lithium secondary battery in the form of a cylinder, a square, a button or the like according to a conventional method.

The lithium secondary battery of the present invention thus obtained has lithium on the negative electrode carrier during charging, which is formed as an intercalation compound, and lithium is released from the carrier during discharging, and is used as a power source for portable electronic devices and the like, or various memories and the like. It can be suitably used for solar backup and the like.

Examples Hereinafter, the present invention will be described in more detail with reference to Examples.

Example 1 [Preparation of mesocarbon microbeads] Using dehydrated coal tar as a starting material, 3 kg / cm ³
The reaction was carried out at 385 ° C. for 14 hours under the pressure of G, and the generated spherulites were separated from the reaction tar by a high-temperature centrifuge, washed with toluene, and dried at 150 ° C. for 3 hours under a nitrogen atmosphere.

The obtained mesocarbon microbeads were carbonized at 1000 ° C. for 1 hour in a reducing atmosphere, and subsequently, in a nitrogen atmosphere,
It was graphitized at 2800 ° C. for 1 hour.

Table 1 shows the properties of the obtained graphitized mesocarbon microbeads.

[Preparation of Negative Electrode Body] The graphitized mesocarbon microbeads and polyethylene powder as a binder were mixed at a weight ratio of 90:10, and 30 mg of the mixture was pressure-formed to form a pellet.

The pellet is used as a cathode, and in propylene carbonate in which LiClO ₄ is dissolved at a concentration of 1 mol /, the anode is selected as lithium metal, and the cathode is reduced under the conditions of a current density of 0.5 mA / cm ² and an electrolysis time of 13 hours. A negative electrode body was prepared by supporting lithium.

[Preparation of Battery] In addition to the negative electrode body obtained above, electrolytic manganese dioxide was used as the positive electrode body, and LiClO ₄ was used as the electrolyte at a concentration of 1 mol /.
Was dissolved in propylene carbonate and other ordinary battery components were used to prepare a lithium secondary battery. The sectional view is shown in FIG. In FIG. 2, 1 is a negative electrode body, 2 is a positive electrode body, 3 is a separator, 4 is a current collecting layer, 5
Denotes a negative electrode can, 6 denotes a positive electrode can, and 7 denotes an insulating packing. This battery exhibited an average operating voltage of 3.7V.

[Measurement of Battery Characteristics] The output density, discharge capacity and cycle characteristics of the lithium secondary battery obtained in this example were measured.

The measurement was usually performed under a constant charge / discharge of 0.636 mA (0.5 mA / cm ³ , 0.9 cmφ). The discharge capacity was evaluated until the voltage decreased to 2.0 V, and the cycle characteristics were evaluated based on the number of cycles until the initial discharge capacity decreased to 90%. The output density was determined from a current-voltage curve obtained by changing the discharge current density.

As a control, a conventional lithium secondary battery using a fibrous carbon material as a lithium carrier was also measured under the same conditions.

 The results are shown in Table 2.

As is clear from Table 2, the lithium secondary battery of the present invention is superior in power density, discharge capacity and cycle characteristics as compared with the conventional lithium secondary battery.

FIG. 3 shows a graph in which the discharge capacity and the number of cycles are plotted for the lithium secondary battery of the present invention and a conventional lithium secondary battery using a fibrous carbon material. The solid line indicates the lithium secondary battery of the present invention, and the dashed line indicates the conventional lithium secondary battery.

According to the present invention, a lithium secondary battery having a high output density, an increased capacity per unit volume (weight), and improved cycle characteristics, and a negative electrode body thereof are provided.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing the structure of mesocarbon microbeads, which is a preferred carbon material used in the present invention. FIG. 2 is a sectional view of the lithium secondary battery prepared in Example 1. FIG. 3 is a graph comparing the cycle characteristics of the lithium secondary battery prepared in Example 1 and a conventional lithium secondary battery. The symbols in the drawings have the following meanings. 1: negative electrode body, 2: positive electrode body, 3: separator, 4: current collecting layer, 5: negative electrode can, 6: positive electrode can, 7: insulating packing

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-4-115458 (JP, A) JP-A-4-188559 (JP, A) JP-A-4-115457 (JP, A) JP-A-61- 205611 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01M 4/58 H01M 4/02 C01B 31/00-31/04

Claims (5)

(57) [Claims] 1. A lithium metal or lithium ion negative electrode active material having a carbonized or graphitized particle size of 0.1 μm or more and 150 μm or more.
A negative electrode body for a lithium secondary battery, wherein the negative electrode body is supported on mesocarbon microbeads having a specific surface area of 5 m ² / g or less, which is not more than μm. 2. The mesocarbon microbeads having a particle size of 0.5
The negative electrode body for a lithium secondary battery according to claim 1, wherein the negative electrode body has a thickness of at least 80 µm. 3. The mesocarbon microbeads use a binder, and the mesocarbon microbeads: binder =
3. The negative electrode body for a lithium secondary battery according to claim 1, wherein the negative electrode body is formed by molding at a weight ratio of 99 to 70: 1 to 30. 4. Mesocarbon microbeads are obtained by subjecting a starting material to heat treatment under a pressure of normal pressure to 20 kg / cm ² · G at a temperature of 350 to 400 ° C. to separate and purify spherulites from a reaction solution. The negative electrode body for a lithium secondary battery according to any one of claims 1 to 3, wherein the negative electrode body is obtained by: 5. The negative electrode body for a lithium secondary battery according to claim 4, wherein the spherulites subjected to high-temperature centrifugal separation are washed and dried.