JPH0636760A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PURPOSE:To keep the discharge potential high and flat and prevent the sudden voltage reduction and a trouble caused by it by using a material mixed with another carbon material capable of storing or releasing a preset quantity of lithium into graphite for a carbon material. CONSTITUTION:A mixture of natural or artificial graphite and another carbon material capable of storing or releasing lithium at 5-30vol.% against graphite is used for a carbon material in a nonaqueous electrolyte secondary battery using the carbon material for a negative electrode. The vol.% is defined as B/AX100, where A is the undoped volume obtained when lithium is doped into the carbon material made of natural or artificial graphite at a certain current density with counter electrodes made of a lithium metal used until the inter-terminal voltage is made 0v then lithium is released until 1.0v is attained, and B is the undoped volume obtained after the similar action for another carbon material capable of storing and releasing lithium. The discharge voltage is kept high and flat, and the sudden voltage reduction can be prevented.

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 whose characteristics are improved so that the voltage between both electrodes can be gently reduced when the remaining capacity becomes small during the operation of the battery.

[0002]

2. Description of the Related Art The half-width value of the X-ray diffraction peak corresponding to the spacing between 002 planes is 1 ° or less, and the spacing between 002 planes is d002.
Is 3.35-3.42 angstroms and the crystallite size (Lc) is 100-500 angstroms. A lithium secondary battery using a pulverized carbonaceous material having a graphite structure as a negative electrode is closed during discharge of the battery. Since the circuit voltage can be maintained at a high voltage for a long time, it has a higher energy density and higher performance than those using other carbonaceous materials for the negative electrode.

On the other hand, for example, an arbitrary organic polymer compound or a composite thereof is fired at a maximum temperature of 900 to 1800 ° C. under an inert gas or nitrogen gas atmosphere,
A carbon substance obtained by finely pulverizing pitch coke or the like has a surface spacing d002 of 002 planes determined by x-ray diffraction of 3.43 to 3.9.
Angstrom, C-axis crystallite size Lc is 9
When a lithium secondary battery having a graphite structure of about 99 Å is used as the negative electrode of the lithium secondary battery, the battery has excellent life characteristics, but has a large voltage change with respect to the depth of discharge in the charge / discharge curve, and its battery capacity is the carbon having the graphite structure. It depends much more on the set value of the discharge end voltage than when the quality material is used for the negative electrode.

As an example, natural graphite that has been subjected to high-purification treatment in a dehumidified air atmosphere and LiClO ₄ as an electrolyte are used.
A solution obtained by dissolving 1 M in a mixed solvent of ethylene carbonate and 1,2-dimethoxyethane was used as a non-aqueous electrolytic solution and a counter electrode was lithium metal, and a current density of 0.5 mA / cm until it reached 0 V on the basis of Li / Li ⁺ potential. after inserting lithium in ^second constant current, shows the voltage curve when to release lithium at a constant current of current density of 0.5 mA / cm ² in Figure 1. In addition,
At this time, the natural graphite has a purity of 99.99%, an average particle size of 7 μm,
The spacing between the 002 planes determined by X-ray wide-angle diffraction is 3.3.
This is a flaky natural graphite produced in China having a crystallite size Lc of 6 angstroms in the C-axis direction of 480 angstroms, and this graphite contains 5% by weight of PTFE as a binder.

As is clear from FIG. 1, natural graphite produces a smooth flat portion in the voltage range of 0 to 0.3 V between terminals when releasing lithium, and 3.0 V in a low voltage range.
It releases over 85% of the amount of lithium that can be released. Due to such characteristics, when the graphite material is used as the negative electrode material of the battery, the closed circuit voltage can be maintained high during the discharge of the battery for a long time, and the energy density is higher than that when other carbonaceous materials are used for the negative electrode. High and high performance. The length and flatness of this flat portion also differ depending on the origin and type of natural graphite.

This natural graphite is used as a negative electrode, LiCoO _{2 in} excess of the lithium storage capacity of the negative electrode is used as a positive electrode, and LiClO ₄ 1M is used as an electrolyte for ethylene carbonate, 1,
A non-aqueous electrolyte prepared by dissolving in a mixed solvent of 2-dimethoxyethane is used in combination with a polypropylene separator to prepare a lithium secondary battery, and when operated, the discharge curve shown in FIG. 2 is obtained. However, in FIG.
It is a discharge curve after charging to 1 V, and a flat voltage curve is drawn until the end of discharge.

[0007]

However, in a battery in which the negative electrode is made of the carbonaceous material having the above graphite structure, the battery voltage suddenly drops just before the capacity is exhausted, that is, near the inflection point A in FIG. For this reason, it is difficult for the battery user to notice the decrease in the capacity of the battery being used, and it is inevitable that the device using the battery-using device may be suddenly interrupted or part of the device may be damaged due to battery exhaustion.

For example, when a personal computer or the like is operated by a battery, if the battery voltage suddenly drops while storing data in the floppy disk or reading the data, the data of the floppy disk , The system may be destroyed.

The cause of this phenomenon is a rapid change in the electrode potential immediately before the lithium-occluding graphite, which is the negative electrode material, discharges all of the releasable amount of lithium during discharge.

The present invention is intended to solve the above problems, and an object thereof is to maintain the discharge potential high and flat, and to reduce the voltage between both electrodes when the remaining capacity becomes small during battery operation. The purpose of the present invention is to provide a non-aqueous electrolyte secondary battery that prevents a sudden voltage drop and the problems associated with it by gently reducing the voltage.

[0011]

In order to achieve the above object, a non-aqueous electrolyte secondary battery of the present invention is a non-aqueous electrolyte secondary battery using a carbonaceous material for a negative electrode, wherein the carbonaceous material is It is characterized by being a mixture of natural or artificial graphite and another carbonaceous material capable of occluding and releasing lithium from 5 to 30% by volume based on the graphite. However, the capacity% used here is defined as follows. That is, it is obtained by doping lithium into the carbonaceous material composed of natural or artificial graphite at a certain current density with lithium metal as the counter electrode until the inter-terminal voltage becomes 0 v, and then releasing this lithium to 1.0 v. De-doping capacity (mAh / g) A
And the dedoping capacity B obtained as a result of performing the same operation on the other carbonaceous material capable of absorbing and desorbing lithium = B / A × 100.

The natural or artificial graphite has an X-ray diffraction peak having a half-width value of 1 ° or less, which corresponds to the spacing between 002 planes,
The interplanar spacing d002 of the 002 plane is 3.35 to 3.42 angstrom, and the crystallite size (Lc) is 100 to 500.
It is a finely powdered carbonaceous material having an angstrom graphite structure.

Examples of this graphite material include carbon fiber (VGCF) obtained by a vapor phase growth method, P
Artificial graphite obtained by heat-treating AN-based and PICH-based carbon fibers at 1800 to 3000 ° C in an inert atmosphere, Nippon Graphite Co., Ltd., artificial graphite manufactured by Lonza, natural graphite manufactured in China, Madagascar, etc. The high-purified product is listed.

Other carbonaceous materials capable of absorbing and desorbing lithium, which are mixed with the above graphite, are 0 obtained by X-ray wide angle diffraction.
The spacing of 02 planes is 3.43 to 3.9 angstroms,
It has a structure in which the crystallite size Lc in the C-axis direction is 9 to 99 angstroms, the average particle size measured by a laser diffraction particle size analyzer is 0.01 to 50 μm, and the nitrogen adsorption specific surface area determined by the BET method is 10 to 300 m ² / g,
It is a fine powder carbonaceous material having an H / C atomic ratio of 0.15 or less.

When the particle size is smaller than the above value and the specific surface area is larger than the above value, the carbonaceous material has a large lithium occlusion amount in the first cycle, but the subsequent (reversible) emission amount is extremely large. It is not practical because it becomes too small. Although the details of this phenomenon are unknown, a carbonaceous material having a small particle size and a large specific surface area is likely to cause a side reaction such as decomposition of an electrolytic solution or a competitive reaction on the surface thereof.
It is thought that this is because as a result of the deposition of the product, intercalation and deintercalation of lithium into the carbonaceous material are less likely to occur.

The above-mentioned carbonaceous materials are generally organic polymer compounds at a maximum of 90 in an inert gas or nitrogen gas atmosphere.
It is obtained by heating at 0 to 1600 ° C. and carbonizing. Various materials can be used as the organic polymer compound as the starting material, and examples thereof include polyacrylonitrile resin, vinyl acetate resin, polyvinyl alcohol resin, polyvinylidene chloride resin, furfuryl alcohol resin, furan resin, phenol resin, and polyamide. Resin, petroleum pitch,
Examples include coal-based pitch, mesophase pitch, condensed polycyclic polynuclear aromatic (COPNA resin), and the like. Further, carbon black as the obtained carbide also shows good characteristics.

Next, the reason why the compounding ratio of both is limited to the range of 5 to 30% by volume will be described. As described above, the capacity% is obtained by doping lithium into the carbonaceous material composed of natural or artificial graphite at a certain current density using lithium metal as the counter electrode as described above until the terminal voltage becomes 0 v, and then releases this lithium until 1.0 v is reached. The ratio of the dedoping capacity (mAh / g) A obtained in the case of the above and the dedoping capacity B obtained as a result of performing the same operation on the other carbonaceous material capable of absorbing and desorbing lithium = B / It is expressed by A × 100, and to determine the blending ratio, the electrochemical characteristics of the carbonaceous material having no graphite structure, that is, the amount of lithium that can be absorbed and released and the electrode potential at that time are determined. It may be properly set depending on the method, but when the blending ratio of the carbonaceous material is less than 5% by volume, the abrupt change of the negative electrode potential at the end of discharge is not alleviated so much and exceeds 30% by volume. Does not Ikase properties to keep the battery voltage during discharge and Gill to a high voltage, it is limited to the range of the.

The current density for determining the values of A and B is
It should be decided according to the application after the battery is completed, and it is desirable that the electrode area is divided by the current value required for the completed battery. For example, when the equipment requiring high load discharge is used, the current densities of A and B must be high, and in the case of low load discharge, it may be low. All of these are due to the fact that the utilization factor when the carbonaceous material occludes and releases lithium changes depending on the current density. Generally, the higher the current density, the lower the utilization factor. Has been.

A binder is mixed with the above mixture and kneaded and granulated to form a negative electrode. On the other hand, an oxide or sulfide capable of inserting and extracting lithium is used as a positive electrode, and a separator,
A lithium secondary battery can be obtained by combining it with a non-aqueous electrolyte or the like. The battery can be either flat or spiral.

As the positive electrode material, any material can be used as long as it is used in this type of battery, but it is particularly preferable to use a material containing a sufficient amount of lithium. For example, a composite metal oxide represented by Li Mn ₂ O ₄ or a general formula Li MO ₂ (where M represents at least one kind or a mixture of Co and Ni, such as LiCoO ₂ and LiCo _0.8 Ni _0.2 O 2). An intercalation compound containing a metal or lithium is suitable.

The non-aqueous electrolytic solution is prepared by appropriately combining an inducing solvent and an electrolyte, and any organic solvent and electrolyte can be used as long as they are used in this type of battery. Examples of propylene carbonate, ethylene carbonate, 1.2-dimethoxyethane, 1,2-diethoxyethane, γ-butyrolactone, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, 4-methyl-1,3- There are dioxolane, diethyl ether and sulfolane. As the electrolyte, LiClO ₄ , LiAs F ₆ , LiB
F ₆ , LiPF ₆ , LiCF ₃ SO ₃ , LiCl and the like are listed.

[0022]

When lithium is electrochemically released from the negative electrode having the above structure, lithium is first occluded in graphite. This is because the lithium-occluded graphite absorbs lithium more than other carbonaceous materials in a similar state in which graphite is mixed. This is because the electrode potential emitted is base.

When this electrochemical reaction proceeds and a certain amount of lithium is released from graphite, the electrode potential of the entire negative electrode mixture shifts to a noble direction. As a result, the other carbonaceous material in the lithium occlusion state is lithium. The electrode potential for occluding and releasing is reached, and lithium is also released from here.

When the discharge progresses and the electrode potential of the negative electrode mixture shifts to a more noble direction, the amount of lithium released from other carbonaceous materials increases and the amount of lithium released from graphite decreases. This is because the potential released from graphite is base and has flatness, whereas the potential from lithium released from other carbonaceous materials is noble and not flat, and the progress of electrochemical reaction during discharge. Along with that, it is due to the characteristics of each of them, which will move to the noble direction sooner.

[0025]

The present invention has the following effects. That is, by constructing a battery with a negative electrode mixture composed of the above mixture, flatness, which is an advantage of using graphite as a negative electrode material in a normal discharge state, that is, a more base state is stably maintained, resulting in a high and stable battery voltage. While the battery capacity can be maintained, the voltage drop becomes gentle immediately before the battery capacity is exhausted, so it is easy to recognize that the battery capacity is low, and it is possible to use the equipment that accompanies sudden battery power interruptions or battery exhaustion. Part damage can be prevented in advance.

[0026]

Embodiments of the present invention will be described below. However, the present invention is not limited to the following examples.

FIG. 12 shows the structure of an AA lithium secondary battery according to the present invention. This lithium secondary battery is basically wound in the same manner as in the past with a separator 3 made of a polypropylene porous film sandwiched between a positive electrode 1 and a negative electrode 2 to be spirally wound to form a winding element. The positive electrode lead plate 4 connected to the positive electrode 1 side, and the negative electrode lead plate 5 connected to the negative electrode 2 side projected to the lower part in the bottomed cylindrical case 6 via the PP insulating plate 6a. Then, the negative electrode lead plate 5 is connected to the center of the inner bottom surface of the bottomed cylindrical case 6 by spot welding, and the positive electrode lead plate 4 is spot welded to the bottom of the positive electrode terminal plate 7 with a safety valve. It is completed by pouring liquid into the case 6, fitting the positive electrode terminal plate 7 into the opening of the case 6 through the sealing gasket 8, and caulking.

The positive electrode 1 is made of LiCo which is a positive electrode active material.
O ₂ and carbon powder as a conductive material and an aqueous dispersion of PTFE were mixed at a weight ratio of 100: 10: 10, and the mixture was kneaded into a paste with water to form a current collector having a thickness of 30 μm. A strip-shaped product obtained by coating on both sides of stainless steel foil, drying, rolling, and cutting to a predetermined size, and scraping a part of the mixture material in a direction orthogonal to the longitudinal direction of the strip-shaped product. Spot welded.

LiCoO _{2 as the} positive electrode active material was obtained by mixing cobalt oxide (CoO) and lithium carbonate (Li ₂ CO ₃ ) at a molar ratio of 2: 1 and heating the mixture in air at 900 ° C. for 48 hours. Using. Of the mixing ratios of the above materials, P
The proportion of the aqueous dispersion of TFE is the proportion of solids therein. Further, the theoretical charging electricity amount of the positive electrode 1 at this time is 500 mA.

The negative electrode 2 is formed into a strip shape by press-fitting an aqueous dispersion of carbonaceous powder and PTFE in a weight ratio of 100: 5 and kneading with water into a nickel expanded metal, drying and cutting. Further, a portion of the negative electrode lead plate 5 made of nickel is spot-welded and spot-welded at a portion orthogonal to the longitudinal direction. The ratio of the aqueous dispersion of PTFE was the same as that of the solid content as described above, and the weight of the carbon powder in the negative electrode was 1.5 g, which was prepared by combining the materials described below.

The non-aqueous electrolyte is lithium perchlorate (Li
Cl O ₄₎ ethylene carbonate, 1,2-dimethoxyethane and an electrolyte dissolved in a proportion of 1 mol / 1 in a mixed solvent of ethane, was injected after sealing the 2.5ml therein. The size of the completed battery is AA, with a diameter of 14.5 mm x 50 mm.

The above-mentioned carbonaceous powder of the negative electrode is a combination of graphite or another carbonaceous material capable of occluding and releasing lithium, and their types and physical properties are as shown in Table 1 below.

[0033]

[Table 1] In addition, with respect to the negative electrode sheet made of each of the above materials 1 to 6, the counter electrode was made of lithium metal, and lithium was occluded at a constant current of 0.5 mA / cm ^{2 at} a current density of 0 V on the basis of Li / Li ⁺ potential. 1 and 3 to 7 show the voltage curves when lithium is released at a constant current with a current density of 0.5 mA / cm ² using lithium metal as the counter electrode. Note that FIG.
In the case of the graphite-based NG and CF shown in FIG. 3, a flat and gentle curve is drawn, and in the case of the carbonaceous materials CK, CB, PR and MP other than the graphite shown in FIGS. 4 to 7, there is no flatness. Has become.

Among the above materials, the combination of the carbonaceous materials for the negative electrode is NG / CK, NG / CB, CF / MP, C.
F / PR, and the mixing ratios are shown in Table 2, respectively. In this case, the volume ratio of the mixing ratio is such that lithium is doped into the carbonaceous material made of the natural or artificial graphite with a constant current of 0.5 mA / cm ² using lithium metal as the counter electrode until the terminal voltage becomes 0 v. Then, the de-doping capacity (mAh / mAh) obtained when the lithium was discharged at a constant current with a current density of 0.5 mA / cm ² until it reached 1.0 V
g) A and the de-doping capacity B obtained as a result of performing the same operation on the other carbonaceous material capable of absorbing and desorbing lithium, expressed as a percentage = B / A × 100, and The mixing ratio is determined with reference to the characteristics of the individual materials shown in FIGS.

[0035]

[Table 2] Next, the battery of each composition was tested with an upper limit voltage of 4.1 V and a lower limit voltage of 2.0.
When the charge and discharge of the first cycle was performed at a constant current as V, the discharge curves of FIGS. In the figure, A indicates an inflection point at which the voltage changes rapidly at the end of discharge.

As shown in FIGS. 8 to 11, when the carbonaceous material capable of occluding and releasing lithium other than graphite in the composition of a and b is 100, 50 (volume%), there is no flatness of the discharge curve, It is clear that the high voltage cannot be maintained.
On the other hand, graphite in the composition of f and g is 97-100
In the case of (capacity%), as in the conventional case, a sharp decrease in potential is recognized from the inflection point A at the end of discharge.

On the other hand, as is clear from each figure, in order to maintain the high voltage and the flatness and to alleviate the degree of the voltage drop at the end of discharge to a detectable state, the combination of c to e is 5-. It is appropriate to mix in the range of 30 (capacity%), and there is a gradual voltage drop from each inflection point A until the capacity is completely exhausted, and there is a considerable time. When the user notices the slow state, the user can obtain sufficient time for measures such as equipment stoppage, charging, and battery replacement.

[Brief description of drawings]

FIG. 1 is a graph showing a voltage curve when lithium is released from natural graphite.

FIG. 2 is a graph showing a battery voltage curve in which the negative electrode is made of natural graphite.

FIG. 3 is a graph showing a voltage curve when lithium is emitted from a coal-based carbon fiber.

FIG. 4 is a graph showing a voltage curve when lithium is released from coal coke.

FIG. 5 is a graph showing a voltage curve when lithium is released from carbon black.

FIG. 6 is a graph showing a voltage curve when lithium is released from a phenol resin carbide.

FIG. 7 is a graph showing a voltage curve when lithium is released from meso-fuse pitch carbon.

FIG. 8 is a graph showing a battery voltage curve using a negative electrode with a combination of NG / CK.

FIG. 9 is a graph showing a battery voltage curve using a negative electrode with a combination of NG / CB.

FIG. 10 is a graph showing a battery voltage curve using a negative electrode according to a combination of CF / MP.

FIG. 11 is a graph showing a battery voltage curve using a negative electrode according to a combination of CF / PR.

FIG. 12 is a cross-sectional view showing a structural example of a spiral type lithium secondary battery to which the present invention is applied.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yoshiro Harada 5-311, Shimbashi, Minato-ku, Tokyo Fuji Electric Chemical Co., Ltd. (72) Hidenori Nagura 5-36-11 Shinbashi, Minato-ku, Tokyo Fuji Electrochemical Co., Ltd.

Claims (2)

[Claims] 1. A non-aqueous electrolyte secondary battery using a carbonaceous material as a negative electrode, wherein the carbonaceous material is natural or artificial graphite, and 5 to 30% by volume of lithium can be absorbed and released with respect to the graphite. Is a mixture with a carbonaceous material (provided that the% by volume is obtained by doping lithium into the carbonaceous material composed of the natural or artificial graphite at a certain current density with the counter electrode being lithium metal until the terminal voltage becomes 0 v, Then 1.0v
To the dedoping capacity A obtained when this lithium is discharged until the following condition and the dedoping capacity B obtained as a result of performing the same operation on the other carbonaceous material capable of absorbing and desorbing lithium = B / A × 100) is defined as a numerical value expressed by B / A × 100). 2. The graphite structure of the natural or artificial graphite is 0.
The half-width value of the X-ray diffraction peak corresponding to the surface spacing of the 02 plane is 1
The crystal spacing of the 002 plane obtained by X-ray wide-angle diffraction is 3.35 to 3.42 angstroms, the crystallite size Lc is 100 to 500 angstroms, and the crystal structure of the other carbonaceous material And the characteristics are that the 002 plane spacing determined by wide-angle X-ray diffraction is 3.43 to 3.9 angstroms, and the crystallite size Lc in the C-axis direction is 9 to.
99 angstrom, the average particle size measured by a laser diffraction type particle size measuring device is 0.01 to 50 μm, BE
Nitrogen adsorption specific surface area determined by T method is 10 to 300 m ²
/ G, H / C atomic ratio is 0.15 or less, non-aqueous electrolyte secondary battery according to claim 1, characterized in that.