JP3435731B2 - Non-aqueous electrolyte secondary battery - Google Patents

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 using a lithium transition metal composite oxide as a positive electrode active material,
Particularly, it relates to improvement of load characteristics.

[0002]

2. Description of the Related Art Recent remarkable advances in electronic technology have made electronic devices smaller and lighter one after another. Along with this, batteries, which are used as mobile power sources, are required to be smaller and lighter and have high energy density.

Conventionally, an aqueous solution type secondary battery such as a lead battery or a nickel-cadmium battery has been mainly used as a secondary battery for general use. However, although these aqueous secondary batteries have excellent cycle characteristics, they cannot be said to be sufficiently satisfactory in terms of battery weight and energy density.

Therefore, recently, substances capable of doping and dedoping lithium ions such as lithium and lithium alloys and carbon materials have been used as the negative electrode, and lithium composite oxides such as lithium cobalt composite oxides have been used for the positive electrode. Research and development of non-aqueous electrolyte secondary batteries to be used are actively conducted. This battery has a high battery voltage, a high energy density, and excellent cycle characteristics.

[0005]

By the way, in the above non-aqueous electrolyte secondary battery, the energy density,
Due to its excellent charge / discharge cycle characteristics, it is expected to be used as a power supply for portable equipment with relatively large power consumption, and improvement of load characteristics is one of the important issues.

Generally, in order to improve the load characteristics of a battery, the electron conductivity in the electrode is improved to facilitate the movement of electrons, and the porosity in the electrode is increased to increase the porosity in the electrode. A method that facilitates the permeation of the electrolytic solution and the movement of ions is effective.

Here, in the above non-aqueous electrolyte secondary battery, the lithium-transition metal composite oxide used as the positive electrode active material is itself a material having a relatively large electric resistance. Therefore, the electron conductivity is insufficient when the positive electrode is composed of only this lithium-transition metal composite oxide.
Therefore, electron conductivity is improved by adding a conductive agent such as highly conductive graphite powder to a positive electrode using a lithium-transition metal composite oxide as a positive electrode active material.

That is, a positive electrode mixture prepared by mixing a lithium composite oxide powder serving as a positive electrode active material, a graphite powder serving as a conductive agent, and a binder is applied to a positive electrode current collector. Is low, and the adhesion between the lithium-transition metal composite oxide powder and the graphite powder is insufficient, so that the action of the conductive agent cannot be effectively obtained. Therefore, the positive electrode mixture layer is thinned by compression. Is used as.

However, in such a positive electrode in which the positive electrode mixture layer is compressed, although the electrode filling property and the adhesion between the powders in the positive electrode mixture layer are improved, the voids are crushed by the compression and the voids are crushed. The rate is low. Therefore, ions do not move smoothly, and especially when the thickness of the positive electrode mixture layer is large, the low porosity significantly reduces the contact area with the electrolytic solution, making it difficult to diffuse the ions. As a result, sufficient load characteristics cannot be obtained.

Therefore, the present invention has been proposed in view of such conventional circumstances, and has a positive electrode excellent in electron conductivity, ion mobility, and electrode filling property, and has good load characteristics. An object is to provide a non-aqueous electrolyte secondary battery.

[0011]

In order to achieve the above-mentioned object, the inventors of the present invention have conducted extensive studies, and as a result, as a conductive agent, graphite powder having high conductivity and amorphous carbon powder having low bulk density were used as positive electrodes. We have found that a positive electrode excellent in both electron conductivity and ion mobility can be obtained by adding it to the mixture.

The present invention has been completed based on such findings, and a negative electrode using a negative electrode active material of a metal material mainly containing lithium or a carbonaceous material capable of doping / dedoping lithium. In a non-aqueous electrolyte secondary battery comprising a positive electrode using a lithium transition metal composite oxide as a positive electrode active material and a non-aqueous electrolyte solution, the positive electrode is a lithium transition metal composite oxide powder serving as a positive electrode active material. A positive electrode mixture obtained by mixing a carbon powder serving as a conductive agent and a binder is applied to a positive electrode current collector, and is a compression-molded strip-shaped electrode. The carbon powder serving as the conductive agent does not include graphite powder. It is a mixed carbon powder with a regular carbon powder, and 10 to 30% by weight of amorphous carbon powder is mixed in the mixed powder, and the porosity of the positive electrode mixture is 26% or more and 39% or less . , The above graphite is scale Graphite, and the amorphous carbon is acetylene black.

In the present invention, graphite preferably has a purity of 98% or more as fixed carbon.

In the non-aqueous electrolyte secondary battery of the present invention, the non-aqueous electrolyte, the negative electrode active material constituting the negative electrode, and the positive electrode active material constituting the positive electrode are usually used in this type of non-aqueous electrolyte secondary battery. Any of the above can be used.

As the electrolytic solution, for example, a lithium salt is electrolyzed.
Quality, and use an electrolyte that is a solution of this in an organic solvent.
Be done. Here, the organic solvent is not particularly limited.
However, for example, propylene carbonate, ethylene
Carbonate, 1,2-dimethoxyethane, 1,2-die
Toxyethane, γ-butyrolactone, tetrahydrofura
1,3-dioxolane, 4-methyl-1,3-dio
Xoxolane, diethyl ether, sulfolane, methyl sulphate
Horan, acetonitrile, propionitrile etc. can be used alone
Or two or more kinds of mixed solvents can be used. Electrolyte also
Any known one can be used, LiClO_Four,
LiAsF₆, LiPF₆, LiBF_Four, LiB (C₆
H_Five)_Four, LiCl, LiBr, CH₃SO₃Li, C
F₃SO ₃Li or the like is used.

A material that can be doped with lithium and dedoped can be used for the negative electrode. Examples of such a material include a conductive polymer such as lithium metal, a lithium alloy, polyacetylene, and the like, or a carbonaceous material such as coke and the like.

Of these, carbonaceous materials are preferable because they provide excellent cycle life. Such a carbonaceous material is obtained by carbonizing an organic material by a method such as firing, and a furan resin composed of furfuryl alcohol or furfural homopolymer or copolymer is suitable as a starting material for carbonization. Specifically, furfural + phenol, furfuryl alcohol + dimethylol urea, furfuryl alcohol, furfuryl alcohol +
Examples of the polymer include formaldehyde, furfuryl alcohol + furfural, and furfural + ketones.

Alternatively, a petroleum pitch having a hydrogen / carbon atomic ratio of 0.6 to 0.8 is used as a starting material, a functional group containing oxygen is introduced into the petroleum pitch, and so-called oxygen cross-linking is performed to obtain an oxygen content of 10 to 20% by weight. A carbonaceous material obtained by firing the precursor after being used as the precursor is also suitable. Further, it is also possible to use a carbonaceous material having a large doping amount with respect to lithium by adding a phosphorus compound or a boron compound when carbonizing the furan resin or petroleum pitch.

As the positive electrode active material, a material capable of doping and dedoping lithium is used, and it is preferable to use a material containing a sufficient amount of lithium. For example,
A lithium-cobalt composite oxide represented by the general formula LiMO ₂ (M is at least one of Co and Ni),
Lithium nickel composite oxide, lithium cobalt
A nickel composite oxide is preferable, and a lithium / manganese composite oxide or a lithium / vanadium composite oxide may be used.

Here, the positive electrode using the above-mentioned lithium-transition metal composite oxide specifically comprises a transition metal composite oxide powder serving as a positive electrode active material, a carbonaceous material powder serving as a conductive agent, and a binder. The mixed positive electrode mixture is applied to the positive electrode current collector, and the positive electrode mixture layer is further compressed to reduce the thickness. At this time, the addition of the conductive agent and the compression of the positive electrode mixture layer are both performed to enhance the conductivity of the positive electrode and improve the load characteristics of the battery.

That is, the lithium composite oxide powder is a substance having a large electric resistance, and when the positive electrode is composed of this alone, the electron conductivity is insufficient. The conductive agent kneaded in the positive electrode mixture together with the lithium composite oxide powder is used to supplement the conductivity of such a lithium transition metal composite oxide.

Further, the compression of the positive electrode material mixture layer reduces the thickness of the positive electrode material mixture layer to improve the electrode filling property and enhances the adhesion between the lithium-transition metal composite oxide powder and the electrically conductive agent to function as the electrically conductive agent. This is a process that is performed in order to acquire it more effectively. FIG. 1 shows a coin-type non-aqueous electrolyte secondary battery (sample battery 1: positive electrode volume density 2.27 g / c) in which a cylindrical positive electrode having a positive electrode mixture layer compressed and adjusted to various volume densities is housed.
m ³ , sample battery 2: positive electrode volume density of 2.83 g / c
m ³ , sample battery 3: positive electrode volume density 3.09 g / c
m ³ , sample battery 4: positive electrode volume density 3.35 g / c
m ³ ), the relationship between the discharge current and the discharge capacity when charging with a charging current of 1 mA and discharging with various discharge currents is shown.
The diameter of the positive electrode used was 15.5 mm. As described above, the non-aqueous electrolyte secondary battery in which the positive electrode having a small compression amount and a small volume density is housed has a short discharge capacity, and the non-aqueous electrolyte secondary battery containing a positive electrode having a large compression amount and a large volume density is housed. The larger the liquid secondary battery, the larger the discharge capacity can be obtained.

However, the compression of the positive electrode mixture layer is essential for enhancing the electronic conductivity and the electrode filling property, but it crushes the pores of the positive electrode mixture layer to reduce the ion mobility, resulting in a non-aqueous electrolyte solution. It also reduces the contact area with. For this reason,
As shown in FIG. 1, even when the compression is sufficiently performed (sample battery 3 and sample battery 4), the discharge capacity becomes small in the large discharge current range and the load characteristics are insufficient.

Therefore, in the present invention, even if compression is performed,
In order to obtain a positive electrode that retains the pores of the positive electrode mixture layer and is excellent in both electron conductivity and ion mobility, a mixed carbon powder of graphite powder and amorphous carbon powder is used as the carbonaceous material as the conductive agent. I will.

That is, the above graphite powder is a carbonaceous material having an extremely high electric conductivity. The graphite powder having high electric conductivity imparts sufficient electron conductivity to the positive electrode mixture layer. On the other hand, the amorphous carbon powder is a carbonaceous material having a very small bulk density, though it has a lower conductivity than graphite powder. When such an amorphous carbon powder is added to the positive electrode mixture layer, when compressed, the amorphous carbon powder having a low bulk density acts so as to maintain pores between the powders, and the sufficient amount is sufficient. Porosity is maintained. Therefore, it is possible to realize a positive electrode that is excellent in both electron conductivity and ion mobility, and that has a good contact property with the non-aqueous electrolyte even if the thickness of the positive electrode material mixture layer is designed to be excellent. Will be done.

Here, it is desirable that the ratio of the amorphous carbon powder to the whole conductive agent is 10 to 30% by weight. When the proportion of the amorphous carbon powder in the whole conductive agent is less than 10% by weight, the effect of maintaining pores by the amorphous carbon powder cannot be sufficiently obtained. On the other hand, when the proportion of the amorphous carbon powder in the whole conductive agent exceeds 30% by weight, the positive electrode material mixture layer cannot be sufficiently thinned even if compressed this time, and the electrode packing density becomes small. Battery capacity is insufficient.

Note that graphite is one of the allotropes of carbon, and is produced naturally as well as artificially produced by heat treating amorphous carbon (calcined carbon) at about 2400 to 3000 ° C. It The graphite powder used in the present invention may be either natural graphite powder or artificial graphite powder. For example,
As natural graphite, flake graphite is a highly pure graphite crystal structure that is well developed among natural graphite, but among artificial graphite, for example, 3000 ° C.
It may be scaled by heat treatment at a higher temperature. Naturally, natural graphite is preferable.

Among natural graphites, earthy graphite is not preferable because it has poor crystallinity and contains many impurities. Further, it is considered that ordinary artificial graphite is not preferable because it has an incomplete crystal structure and is easily oxidized. Moreover, the purity of the flake graphite is preferably 98% or more as fixed carbon.

On the other hand, as the amorphous carbon powder, a material having a low bulk density such as acetylene black is selected and used.

The negative electrode and the positive electrode described above are made into desired electrode shapes and then housed together with the non-aqueous electrolytic solution in the battery can to function as the negative electrode and positive electrode of the non-aqueous electrolytic solution secondary battery. . Here, the configuration and shape of the electrode and the battery can are
The electrodes may be spiral electrodes, laminated electrodes, or battery cans of button type, cylindrical type, etc., in accordance with ordinary non-aqueous electrolyte secondary batteries.

[0031]

The positive electrode mixture obtained by kneading the lithium-transition metal composite oxide powder serving as the positive electrode active material, the carbonaceous material powder serving as the conductive agent, and the binder is applied to the positive electrode current collector, and the positive electrode mixture is further mixed. In the strip-shaped positive electrode formed by compression-molding the agent layer, a mixed carbon powder of graphite powder and amorphous carbon powder is used as the conductive agent, and the ratio of the amorphous carbon powder in the mixed carbon powder to be the conductive agent is When it is 10 to 30% by weight and the porosity of the positive electrode material mixture layer is 26% or more and 39% or less , both the electron conductivity and the ion mobility are excellent, and the load characteristics of the battery are improved. . This is for the following reason.

That is, the graphite powder is a carbon powder having a high electric conductivity. The positive electrode mixture layer is provided with sufficient electron conductivity by adding the graphite powder having high conductivity. On the other hand, the amorphous carbon powder is a carbon material powder having a very low bulk density, though it has a lower conductivity than graphite powder. When this amorphous carbon powder having a low bulk density is added to the positive electrode mixture layer, when the positive electrode mixture layer is compressed, the amorphous carbon powder acts so as to hold pores between the powders and has sufficient pores. The rate is maintained.

Therefore, both the electron conductivity and the ion mobility are excellent, and even when the thickness of the positive electrode mixture layer is designed to be large, the load characteristics that the contact area with the non-aqueous electrolyte is sufficiently secured are excellent. The positive electrode will be realized.

[0034]

EXAMPLES Preferred examples of the present invention will be described below based on experimental results.

Example 1 FIG. 1 shows a coin type non-aqueous electrolyte secondary battery prepared in this example. This non-aqueous electrolyte secondary battery was manufactured as follows.

First, a negative electrode can was produced by pressure-bonding metallic lithium 2 as a negative electrode active material to the anode cup 1.

Next, the positive electrode 3 was produced as follows. 91 parts by weight of LiCoO _{2 as a} positive electrode active material, 6 parts by weight of mixed carbon powder of graphite powder and amorphous carbon powder as a conductive material, polyvinylidene fluoride (PV) as a binder.
3 parts by weight of DF) was kneaded to prepare a positive electrode mixture. In addition,
Here, artificial graphite having a purity of 99.9% was used as the graphite powder, and acetylene black was used as the amorphous carbon powder. Further, the proportion of the conductive agent in the positive electrode mixture is 6% by weight,
The ratio of acetylene black in the conductive agent was set to 10% by weight.

The thus prepared positive electrode mixture was made into a slurry by dispersing N-methylpyrrolidone as a solvent, and this positive electrode mixture slurry was formed into a positive electrode current collector having a thickness of 2
A positive electrode material mixture layer was formed by applying it on a 0 μm band-shaped Al foil and drying it. Next, this positive electrode mixture layer was compression-molded by a roller press machine into a band shape, and further cut by using a cylindrical cutter to manufacture a cylindrical positive electrode having a diameter of 15.5 mm.

The cylindrical positive electrode 3 thus prepared was fitted into a cathode can 4, a separator 5 was placed thereon, and a nonaqueous electrolytic solution was injected. Then, in a further step, the anode cup on which the metallic lithium was pressure-bonded was placed on the separator so that the exposed side of the metallic lithium was the separator side,
A coin type non-aqueous electrolyte secondary battery having a diameter of 20 mm and a height of 1.6 mm was produced by caulking the anode cup 1 and the cathode can 4 with an insulating sealing gasket 6 interposed therebetween.

Example 2 A coin type non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that the proportion of acetylene black in the conductive agent was 30% by weight.

Comparative Example 1 A coin type non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that acetylene black was not added to the positive electrode mixture.

Regarding the non-aqueous electrolyte secondary battery thus produced, first, the dry volume density of the positive electrode, the volume density after impregnation with the electrolyte, the porosity, and the DC resistance were examined. The porosity was determined by measuring the impregnated amount of the electrolytic solution when the positive electrode was impregnated with the electrolytic solution and determining the impregnated amount of the electrolytic solution as the pore volume. The DC resistance was measured in a fully charged state.

[0043]

[Table 1]

As can be seen from Table 1, the positive electrodes of Examples 1 and 2 using the mixed carbon powder of graphite powder and acetylene black powder as the conductive agent were the same as those of Comparative Example 1 using only the graphite powder as the conductive agent. It has a higher porosity and lower electrical resistance than the positive electrode. This tendency becomes more remarkable as the proportion of acetylene black in the conductive agent increases, as can be seen by comparing the positive electrodes of Examples 1 and 2.

Next, among the batteries using acetylene black, the batteries of Example 2 and the battery of Comparative Example 1 were measured for discharge capacities when charged and discharged at various currents. The result is shown in FIG. The charging / discharging conditions are as follows. Charging conditions Charging current: 2 mA Charging time: 9 hours Upper limit voltage: 4.2 V Discharging conditions Discharging method: Constant current Discharge end voltage: 3.3 V Area density: 55.1 mg / cm ²

In FIG. 3, the battery of Example 2 and Comparative Example 1
Comparing the batteries No. 1 and No. 2, no difference was observed in the discharge capacities of both batteries when the discharge current was small. However, when the discharge current was increased, the battery of Comparative Example 1 had a large decrease in discharge capacity with an increase in discharge current, whereas the battery of Example 2 had a discharge current increased to 9 mA. Discharge current 1m
The discharge capacity is maintained at the same level as in A, and the load characteristics are excellent.

From the above results, a mixed carbon powder of graphite powder and acetylene black powder was used as the conductive agent for the positive electrode,
The ratio of the amorphous carbon powder in the mixed carbon powder as the conductive agent is 10 to 30% by weight, and the porosity is 26% or more and 39% or less.
It is lower, it was found to be effective in obtaining an excellent non-aqueous electrolyte secondary battery to the load characteristics.

[0048]

As is apparent from the above description, in the non-aqueous electrolyte secondary battery of the present invention, the positive electrode contains lithium transition metal composite oxide particles, graphite powder as a conductive agent, and amorphous carbon powder. The mixture of the mixed carbon powder and the binder is applied to the positive electrode current collector, the mixture is formed by compression molding, and the ratio of the amorphous carbon powder in the mixed carbon powder to be the conductive agent is Since it is 10 to 30% by weight and the porosity of the positive electrode mixture layer is 26% or more and 39% or less , the positive electrode is excellent in electron conductivity, ion mobility, and electrode filling property, and good load characteristics can be obtained. Obtainable.

Therefore, according to the present invention, it is possible to obtain a non-aqueous electrolyte secondary battery which is suitable as a power supply for portable equipment with large power consumption.

[Brief description of drawings]

FIG. 1 is a characteristic diagram showing a relationship between a discharge current and a discharge capacity of a non-aqueous electrolyte secondary battery in which a positive electrode compressed by various compression amounts is housed.

FIG. 2 is a schematic cross-sectional view showing one structural example of a non-aqueous electrolyte secondary battery to which the present invention is applied.

FIG. 3 shows the relationship between discharge current and discharge capacity of a non-aqueous electrolyte secondary battery in which acetylene black is added to the positive electrode mixture and a non-aqueous electrolyte secondary battery in which acetylene black is not added to the positive electrode mixture. It is a characteristic diagram.

[Explanation of symbols]

1 ... Anode cup 2 ... Negative electrode 3 ... Positive electrode 4 ... Cathode can 5 ... Separator 6 ... Sealing gasket

Claims (2)

(57) [Claims] 1. A negative electrode having a negative electrode active material of a metal material mainly containing lithium or a carbonaceous material capable of doping / dedoping lithium, a positive electrode having a lithium transition metal composite oxide as a positive electrode active material, and In a non-aqueous electrolyte secondary battery comprising an aqueous electrolyte, the positive electrode is a positive electrode obtained by mixing a lithium transition metal composite oxide powder serving as a positive electrode active material, a carbon powder serving as a conductive agent, and a binder. The mixture is applied to a positive electrode current collector, which is a compression-molded strip-shaped electrode, the carbon powder serving as the conductive agent is a mixed carbon powder of graphite powder and amorphous carbon powder, and in the mixed powder. Amorphous carbon powder is mixed in an amount of 10 to 30% by weight, the porosity of the positive electrode mixture is 26% or more and 39% or less , the graphite is flake graphite, and the amorphous carbon is acetylene black. Is Non-aqueous electrolyte secondary battery, characterized and. 2. The graphite is 9 as fixed carbon.
The non-aqueous electrolyte secondary battery according to claim 1, which has a purity of 8% or more.