JP2002151157A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve high-rate discharging characteristic, output density, and charge acceptability of a nonaqueous electrolyte secondary battery. SOLUTION: A positive electrode is used, which has lithium cobaltate as positive electrode active material. The weight by which a positive electrode active material mix containing the positive electrode active material is applied to one side of a current collector is not less than 0.6 g/100 cm2 nor more than 1.1 g/100 cm2. A graphitization-resistant carbonaceous material is used as negative electrode active material such that a spacing d002 along the direction of C-axis, obtained through X-ray diffraction, assumes a value of not less than 0.36 nm nor more than 0.40 nm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a non-aqueous electrolyte secondary battery suitable for applications requiring high output.

[0002]

2. Description of the Related Art Since a lithium secondary battery is a lightweight and high-capacity density battery, its demand has been increasing as a battery for portable equipment such as mobile phones and personal computers. Also, taking advantage of the large capacity density,
The development of batteries for electric vehicles and batteries for UPS is also actively pursued.

[0003] In such a lithium secondary battery, a power generation element in which a positive electrode and a negative electrode in which an active material mixture is coated on a current collector such as a metal foil is wound through a separator is provided in a battery container. It has a sealed structure and is usually manufactured by applying as much active material mixture as possible on a given current collector area in order to maximize the capacity density.

[0004]

In recent years, a hybrid electric vehicle using a motor and an engine has attracted attention, and lithium secondary batteries for this purpose have been actively developed. I have.

However, when a lithium secondary battery is put to practical use as a battery for a hybrid electric vehicle,
At present, it is extremely difficult. This is because batteries for hybrid electric vehicles are required to have a high capacity density, in particular, in addition to having a high capacity density that has been required for lithium secondary batteries in conventional applications. This is because, in a lithium secondary battery using a nonaqueous electrolyte having essentially low conductivity, sufficient high-rate discharge characteristics and a large output were not obtained.

An object of the present invention is to solve such a problem and to provide a non-aqueous electrolyte secondary battery having excellent high-rate discharge characteristics and output characteristics.

[0007]

Means for Solving the Problems A lithium secondary battery is characterized by having a high capacity density.
Until now, development has been pursued with the aim of forming as much active material as possible on the current collector. For this reason,
In order to obtain a large output, a method of increasing the electrode thickness to increase the battery capacity and reducing the load applied to the active material per unit weight has been adopted. However, the inventors have found that by reducing the amount of active material formed on the current collector per unit area, the capacity density is reduced, but the output density is increased and the high-rate discharge characteristics can be improved. The inventors have found out that they can do so and have reached the present invention.

That is, in the present invention, lithium cobalt oxide is used as a positive electrode active material, and the coating weight of the positive electrode active material mixture containing the positive electrode active material on one surface of the current collector is 0.6 g / 100 c.
A nonaqueous electrolyte secondary battery comprising a positive electrode having a m ² or more and 1.1 g / 100 cm ² or less. By using such a positive electrode, the high-rate discharge characteristics of the nonaqueous electrolyte battery can be improved, and the output density can be increased. If the coating weight is too small, less than 0.6 g / 100 cm ² , the internal resistance of the battery increases, which is not preferable.

Further, in the present invention, as the negative electrode active material, a non-graphitizable carbon material having a plane spacing d ₀₀₂ value in the C-axis direction obtained by an X-ray diffraction method of not less than 0.36 nm and not more than 0.40 nm is used. It is preferable to use it, whereby the charge acceptability of the nonaqueous electrolyte battery of the present invention can be improved as compared with the case where another carbon material such as natural graphite or artificial graphite is used.

[0010]

DETAILED DESCRIPTION OF THE INVENTION For example, a battery of the present invention comprises a positive electrode using lithium cobalt oxide as a positive electrode active material, a separator, a negative electrode using a carbon material capable of inserting and extracting lithium ions as a negative electrode active material, and an organic electrolyte. It is an organic electrolyte secondary battery.

The lithium cobaltate in the present invention has a skeletal structure represented by LixCoO ₂ , and the value of x is particularly preferably 0.9 ≦ x ≦ 1.2.

In the case where a part of the Co component is substituted in the basic skeleton structure, it is preferable to substitute at least one element other than the alkali metal element.
It is replaced by a transition metal element such as Fe, Ti, Cr and Cu, an alkaline earth metal element such as Mg and Ca, and an element such as Al, In, Ga, B and Si.

The positive electrode may contain other active materials in addition to the above-mentioned lithium cobalt oxide as a positive electrode active material, or may be formed by adding an additive such as a transition metal compound. In the present invention, lithium cobalt oxide is preferably used as a main component.

In the present invention, the positive electrode active material mixture means a mixture of a positive electrode active material, a conductive auxiliary agent, a binder and the like at an appropriate ratio. When adjusting this mixture, the average diameter of the secondary particles of the positive electrode active material is desirably 15 μm or less from the viewpoint of coatability such as minimizing the roughness of the coated surface of the electrode plate.

As the conductive agent, carbon compounds, for example, carbon blacks such as natural graphite, artificial graphite, channel black, acetylene black, Ketjen black, furnace black, and carbon fibers can be used.
As the binder, polyvinylidene fluoride, polyimide resin, PTFE, styrene butadiene rubber, fluoro rubber,
Or the like, which is difficult to dissolve in the electrolytic solution.
When the mixture is used as a paste, for example, the composition of the coating solution is such that the conductive agent 1 is added to 100 parts by weight of the active material.
-10 parts by weight, 2-20 parts by weight of binder, and 30 parts of solvent
To 300 parts by weight.

Examples of the current collector include metal foils such as aluminum, copper, nickel, and stainless steel; conductive films such as inorganic oxides, organic polymer materials, and carbon; and metal-deposited films (for example, polyethylene terephthalate as a base film). , Polyimide, polyphenylene sulfide,
Examples of the metal to be deposited include gold, copper, and aluminum. ) Can be used. In addition, the form of such a conductive substrate can be various forms such as a continuous sheet, a perforated sheet, a mesh sheet, etc., but a continuous sheet is particularly preferable. Note that the active material is preferably applied to such a current collector on both surfaces.

The negative electrode may be manufactured such that an amount of the negative electrode active material capable of accepting the total amount of charge of the positive electrode is arranged on the opposite surface of the positive electrode. Form.

The negative electrode active material used in the present invention is particularly preferably a carbon material. Carbon materials are roughly classified into graphite materials with a uniform crystal structure and non-graphitized materials with a disordered crystal structure.The former includes natural graphite and artificial graphite, and the latter has a crystal structure. 2000, although disturbed
There are graphitizable carbon (soft carbon) that easily becomes graphite by heating at ３3000 ° C. and hardly graphitizable carbon (hard carbon) that hardly becomes graphite. Specifically, for example,
Graphite, coke (petroleum coke, pitch coke,
Needle coke, etc.), resin film fired carbon, fiber fired carbon, vapor grown carbon, and the like can be used.

Particularly, among carbon materials, in the case of the present invention,
It is preferable to use a non-graphite carbon material having a disordered crystal structure. By using a non-graphite material, the cycle life characteristics at high temperatures can be improved. Non-graphite-based carbon materials can be produced, for example, by heat-treating petroleum pitch, polyacene, polyparaphenylene, polyfurfuryl alcohol, polysiloxane, and the like. Or soft carbon. For example, a firing temperature of about 500 ° C. to 800 ° C. is suitable for producing hard carbon, and a firing temperature of 800 ° C. to 1000 ° C.
A moderate firing temperature is suitable for the production of soft carbon.

When a non-graphite carbon material having a disordered crystal structure is used as an active material, and a negative electrode active material mixture containing the same is applied on a current collector to form a negative electrode, The negative electrode active material mixture application weight on one side of the conductor was 0.25
g / 100 cm ² or more and 0.5 g / 100 cm ² or less. In this case, the average diameter of the secondary particles of the negative electrode material is preferably 15 μm or less from the viewpoint of coatability such as minimizing the roughness of the coated surface. The negative electrode active material mixture is usually mainly made of a carbon material and a binder, and the same binder as used in the positive electrode is used as the binder, and such a mixture is used as a paste. In this case, for example, 2 to 20 parts by weight of the binder and 30 to 300 parts by weight of the solvent are mixed with 100 parts by weight of the carbon material.

Further, among the non-graphite-based carbon materials, it is preferable to use a non-graphitizable carbon material, whereby the charge acceptability can be improved. When a non-graphitizable carbon material is used, it is preferable to use a carbon material having a d ₀₀₂ value obtained by an X-ray diffraction method of 0.36 nm or more and 0.40 nm or less. This is because if it is less than 0.36, the charge acceptability is remarkably reduced, and if it exceeds 0.40, the true density of the carbon material becomes too small, and the energy density of the battery becomes small.

Examples of the separator include a microporous film (for example, polyethylene or polypropylene) separator, an organic polymer electrolyte (for example, a complex of a polyether such as PEO and an alkali metal salt, polyvinylidene fluoride,
A gel-like material in which an electrolyte solution is contained in an organic polymer such as PAN), an inorganic solid electrolyte, or the like. In addition to a sheet-like material, a form directly formed on the surface of a positive electrode or a negative electrode sheet And the like can be used.

Examples of the non-aqueous electrolyte include propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, γ-butyrolactone,
Various lithium salts, for example, LiClO ₄ , LiBF ₆ , LiP, are added to at least one or more aprotic organic solvents such as 1,2-dimethoxyethane and tetrahydroxyfuran.
A solution in which F ₆ , LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , LiAsF ₆ or the like is dissolved can be used.

The battery of the present invention comprises the above-mentioned components. For example, the positive electrode and the negative electrode are formed into a sheet shape, and these electrodes are wound into a roll shape, for example, through a separator as a separator. It is preferably configured by adopting the spiral structure.

The sheet-shaped electrode plate is produced, for example, by applying a paste-like electrode mixture onto a metal foil current collector sheet made of copper, aluminum or the like by a reverse roll method, a doctor blade method, or the like. The sheet electrode coated with the mixture is hot-air-dried or vacuum-dried, and then uniformly pressed and compressed by a roll press, so that the electrode porosity is uniformly adjusted to a range of about 25 to 50%. Then, the sheet-shaped electrode plate manufactured by these methods is cut into a length of one battery such as a cylinder, a long cylinder, and a square, and an electrode group in which a positive electrode sheet, a separator, and a negative electrode sheet are sequentially laminated is formed. It is wound in a roll around the core material and stored in a battery container.

For example, the non-aqueous electrolyte secondary battery of the present invention manufactured as described above can be used for applications requiring a large output and good high-rate discharge characteristics, for example, for power tools, vacuum cleaners, HEVs that can be suitably used for assistive bicycles and HEVs, and in particular, those that use the non-graphitizable carbon material having the above-mentioned specific surface spacing to improve charge acceptability,
It can be suitably used for applications in which charge receiving performance is important such as use.

[0027]

EXAMPLES Five types of long cylindrical batteries having the same battery size and different active material coating weights and negative electrode materials were produced.

First, a positive electrode sheet was prepared. 89 parts by weight of LiCoO ₂ having an average particle size of 13 μm as an active material and 4 parts by weight of acetylene black as a conductive agent were mixed, and polyvinylidene fluoride as a binder was added at a ratio of 7 parts by weight. , N-methyl-2-pyrrolidone was added and kneaded to prepare a slurry-type positive electrode mixture coating solution. Then, apply this positive electrode mixture coating solution to a thickness of 15 μm.
A single-sided coating was applied to both sides of a μm aluminum foil so as to have the same application weight. In addition, in order to compare the differences depending on the application weight, four types of positive electrode sheets having different application weights as shown in Table 1 below were prepared.

Next, a negative electrode sheet was prepared. 94 parts by weight of hard carbon having an average particle size of 11 μm as a negative electrode active material, 6 parts by weight of polyvinylidene fluoride as a binder, N-methyl-2-pyrrolidone as a solvent, kneading, and slurrying A positive electrode mixture coating solution was prepared. Next, this negative electrode mixture coating solution was applied to both sides of a 10-μm-thick copper foil so as to have the same application weight, one surface at a time. In addition, according to the capacity of the positive electrode, four types of negative electrode sheets having different application weights were prepared as shown in Table 1 below so that the utilization rate of the negative electrode was the same in each battery. Further, one type of negative electrode sheet was produced in the same procedure as above, except that the negative electrode active material was changed to artificial graphite having an average particle size of 11 μm.

The positive and negative electrode sheets were dried with hot air and compressed by a roll press. The porosity was 37% for the positive electrode and 35% for the negative electrode.

Next, the positive and negative electrode sheets were wound into a roll through a 30 μm-thick polyethylene separator to produce a long cylindrical power generating element, which was 95 m in height.
m, width 60 mm, thickness 26 mm, and wall thickness 0.5 mm were stored in a stainless steel long cylindrical container, and then the electrolyte was sealed therein. Thus, batteries A, B, C, D, and E were produced. The electrode area of the facing part of each battery was 47 A for Battery A.
19 cm2, 5564 cm2 for battery B, 6760 cm2 for batteries C and E, and 7800 cm2 for battery D. The electrolyte used was a solution obtained by dissolving LiPF ₆ at a concentration of 1 M in a solvent in which EC and DEC were mixed at a ratio of 1: 1.

[0032]

[Table 1]

In order to compare the output characteristics of the batteries manufactured as described above, in a fully charged state (open terminal voltage is 4.1 V), at 25 ° C., 10 seconds at 5 A, then 10 seconds at 20 A. Further, by discharging the battery at 50 A for 10 seconds, the output value of each battery was obtained by extrapolating the current value at which the battery voltage became 2.5 V. FIG. 1 is a diagram showing the relationship between the output value of each battery thus obtained and the applied weight of the active material mixture of the positive electrode (per one side).

As shown in Table 1, as the weight of the applied battery increases, the capacity of the battery increases, and the energy density per volume of the battery increases. However, as can be seen from FIG. 1, the output of the battery increases as the application weight decreases. From this, to increase the output,
It can be seen that it is better to reduce the coating weight per unit area of the current collector, and that it is possible to obtain a larger output than before by setting it to 1.1 g / 100 cm ² or less per side.

Further, in order to compare the Wh capacity retention rate at the time of high-rate discharge, continuous discharge at 1,5,10,16,20C was performed. It can be seen that the weight has the same correlation as the output with respect to the weight, and the coating weight is 0.6 g / 100 cm ² or more and 1.1 g per side.
/ 100 cm ² or less was found to be good. By setting such a range, for example, the discharge capacity of 80% of the 1C discharge at the time of the 5C discharge cannot be ensured.
% Of discharge capacity can be secured.

FIG. 2 is a diagram showing a Cole-Cole plot of the battery C obtained by measuring the AC impedance in the fully charged state. Table 2 below shows the electrode facing area of each battery, and the AC internal resistance and DC internal resistance at 1 kHz. Note that SOC indicates the state of charge,
0% is a state where the open terminal voltage is 2.75V, and 100% is a state where the open terminal voltage is 4.1V. DC resistance is 5,
It is calculated from the voltage at the 10th second when discharging at a current of 20, 50 A. The measurement conditions of the AC impedance were an applied voltage of 10 mV and a frequency of 0.001 to 1000 H.
z, ambient temperature 25 ° C.

[0037]

[Table 2]

Although the Cole-Cole plot shows only the battery C, from the measurement of each battery, the value of the intercept with respect to the real axis becomes smaller as the weight of the applied battery becomes smaller, and the diameter of the semicircle becomes larger. I found that. This corresponds to the change in the AC resistance and the DC resistance in Table 2. The smaller the coating weight, the smaller the fixed resistance component such as the solution resistance, while the smaller the reaction area of the electrode. This indicates that an increase has occurred. From this, it can be seen that the output is increased by reducing the coating weight, and the high-rate discharge characteristics are improved. On the other hand, when the coating weight is reduced too much, the decrease in the fixed resistance component decreases the interface resistance. It can be seen that there is a boundary line at which the reduction of the coating weight can exert an effect because it is canceled by the increase.

Further, in order to compare the input characteristics indicating the charge acceptability of the battery, the battery was completely discharged (the open terminal voltage was 2.75).
V) at 25 ° C. and 5 A for 10 seconds, then 10
By charging for 10 seconds at A and further for 10 seconds at 30 A, the input value of each battery was determined by extrapolating the current value at which the battery voltage was 4.2 V. Table 3 shows the input values of the battery C and the battery E thus obtained.

[0040]

[Table 3]

As can be seen from Table 3, in battery E using artificial graphite as the negative electrode material, the input value was smaller than that in battery C using hard carbon as the negative electrode material with the same electrode facing area. I understand. This is because hard carbon having a disordered crystal structure has more sites for storing lithium ions than artificial graphite having a uniform crystal structure, so even when charged with a large current, lithium ions are quickly absorbed. This is due to occlusion in the crystal structure of carbon.

[0042]

According to the present invention, a battery having a large output, excellent high-rate discharge characteristics, and excellent input characteristics can be put to practical use.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the output value of a battery and the applied weight of a positive electrode active material mixture.

FIG. 2 is a view showing a Cole-Cole plot of a battery C;

Claims (2)

[Claims] Claims: 1. A lithium cobalt oxide is used as a positive electrode active material,
The coating weight of the positive electrode active material mixture containing the positive electrode active material on one surface of the current collector is 0.6 g / 100 cm ² or more and 1.1 g.
An organic electrolyte secondary battery comprising a positive electrode having a capacity of / 100 cm ² or less. 2. A non-graphitizable carbon material having a plane spacing d ₀₀₂ value in the C-axis direction obtained by X-ray diffraction of 0.36 nm or more and 0.40 nm or less is used as the negative electrode active material. The non-aqueous electrolyte secondary battery according to claim 1.