JP3068712B2 - 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 having improved characteristics so that the voltage between both electrodes is gradually reduced when the remaining capacity becomes small during the operation of the battery.

[0002]

2. Description of the Related Art A half-width value of an X-ray diffraction peak corresponding to the 002 plane distance is 1 ° or less, and the 002 plane distance d002.
A lithium secondary battery using a pulverulent carbonaceous material having a graphite structure with a size of 3.35 to 3.42 angstroms and a crystallite size (Lc) of 100 to 500 angstroms for a negative electrode is closed during discharge of the battery. Since the circuit voltage can maintain a high voltage for a long time, the energy density is higher and the performance is higher than when other carbonaceous materials are used for the negative electrode.

[0003] On the other hand, for example, an arbitrary organic polymer compound or a compound thereof is fired at a maximum temperature of 900 to 1800 ° C. in an inert gas or nitrogen gas atmosphere,
The spacing d002 between the 002 planes determined by x-ray diffraction, such as a carbon substance obtained by finely pulverizing pitch coke, is 3.43 to 3.9.
Angstrom, crystallite size Lc in the C-axis direction is 9
When a negative electrode of a lithium secondary battery having a graphite structure of up to 99 angstroms is used as a negative electrode, although the battery exhibits excellent life characteristics, a voltage change with respect to the depth of discharge is large in a charge / discharge curve, and the battery capacity is a carbon having the graphite structure. It depends more on the set value of the discharge cut-off voltage than when the material is used for the negative electrode.

As an example, LiClO ₄ is used as an electrolyte in natural graphite that has been subjected to a high-purification treatment in a dehumidified air atmosphere.
1M was dissolved in a mixed solvent of ethylene carbonate and 1,2-dimethoxyethane, and a non-aqueous electrolyte was used. The counter electrode was lithium metal. The current density was 0.5 mA / cm until the voltage became 0 V based on 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 had a purity of 99.99%, an average particle size of 7 μm,
The 002 plane spacing determined by X-ray wide-angle diffraction is 3.3
6 Å, Chinese flake natural graphite having a crystallite size Lc of 480 Å in the C-axis direction, which contains 5% by weight of PTFE as a binder.

[0005] Lithium this as is apparent from FIG. 1, natural graphite capable of releasing potential during lithium release occurs a gentle plateau in realm of 0～0.3V, to 3.0V in low voltage range It releases more than 85% of the amount. Due to such characteristics, when the graphite material is used as a negative electrode material of a battery, a high closed circuit voltage can be maintained for a long time during discharging of the battery, and the energy density is lower than when other carbonaceous materials are used for the negative electrode. High performance. Note that the length of the flat portion and the flatness thereof also vary depending on the place 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 in ethylene carbonate.
A solution prepared by dissolving in a mixed solvent of 2-dimethoxyethane as a non-aqueous electrolyte is used in combination with a polypropylene separator to produce a lithium secondary battery, which is operated to obtain a discharge curve shown in FIG. However, FIG.
It is a discharge curve after charging to 1V, and draws a flat voltage curve until the end of discharge.

[0007]

However, in the battery in which the negative electrode is made of the carbonaceous material having the graphite structure, the battery voltage drops immediately before the capacity is exhausted, that is, near the inflection point A in FIG. This makes it difficult for the battery user to notice a decrease in the capacity of the battery used, and in some cases, the operation using the battery-powered device is suddenly interrupted or the device is partially damaged due to the battery running out.

For example, when a personal computer or the like is operated on a battery, if the battery voltage suddenly drops while data is being stored on or read from the floppy disk, the data on the floppy disk may be lost. , The system may be destroyed.

The cause of this phenomenon is a sudden change in the electrode potential immediately before the discharge of all of the releasable lithium by the lithium occluded graphite which is the negative electrode material during discharge.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and has as its object to maintain a high discharge potential and to be flat, and to reduce the voltage between both electrodes when the remaining capacity becomes small during operation of the battery. An object of the present invention is to provide a non-aqueous electrolyte secondary battery in which a sudden decrease in voltage and a problem associated therewith are prevented by gradually decreasing 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 A mixture of natural or artificial graphite and 5 to 30% by volume of the graphite mixed with a carbonaceous material other than graphite capable of absorbing and releasing lithium (however, the volume% referred to here)
Is obtained by doping lithium into the carbonaceous material made of natural or artificial graphite at a certain current density using lithium metal as a counter electrode until the voltage between terminals becomes 0 V, and then releasing this lithium until the voltage becomes 1.0 V. Percentage of undoped capacity A and undoped capacity B obtained as a result of performing the same operation on the other carbonaceous material capable of inserting and extracting lithium, defined as a numerical value expressed by B / A × 100. Wherein the graphite structure of the natural or artificial graphite is:
The plane spacing d002 of the 002 plane determined by X-ray wide-angle diffraction is
3.35 to 3.42 angstroms;
The carbonaceous material has a 002 plane determined by X-ray wide-angle diffraction.
The spacing is between 3.43 and 3.9 angstroms.

The natural or artificial graphite has an X-ray diffraction peak having a half width of 1 ° or less corresponding to a 002 plane spacing ,
The size of the sintered crystallite (Lc) is finely divided carbonaceous material having a graphite structure 100 to 500 Angstroms.

Examples of the graphite material include carbon fiber (VGCF) obtained by vapor phase growth,
An artificial graphite obtained by heat-treating an AN-based or PITCH- based carbon fiber to 1800 to 3000 ° C. in an inert atmosphere to be graphitized, Nippon Graphite, artificial graphite manufactured by Lonza, natural graphite manufactured in China, Madagascar, etc. Which have been subjected to a high purification treatment.

The other carbonaceous material capable of inserting and extracting lithium, which is mixed with graphite, has a crystallite size L in the C- axis direction.
c has a structure of 9 to 99 angstroms, and the average particle size measured by a laser diffraction type particle size analyzer is 0.0
1 to 50 [mu] m, the nitrogen adsorption specific surface area determined by the BET method 10~300m ² / g, H / C atomic ratio of finely divided carbonaceous material is 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 storage capacity in the first cycle, but the subsequent (reversible) release capacity is extremely large. Is not practical. 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 its surface,
It is considered that as a result of deposition of the product, intercalation and deintercalation of lithium into the carbonaceous material hardly occur.

The above-mentioned carbonaceous materials are generally prepared by converting an organic polymer compound into an inert gas or nitrogen gas atmosphere at a maximum of 90%.
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 a polyacrylonitrile resin, a vinyl acetate resin, a polyvinyl alcohol resin, a polyvinylidene chloride resin, a furfuryl alcohol resin, a furan resin, a phenol resin, and a polyamide. Resin, petroleum pitch,
Coal pitch, mesophase pitch, condensable polycyclic polynuclear aromatic (COPNA resin) and the like are listed. In addition, carbon black also shows good characteristics as the obtained carbide.

Next, the reason why the mixing ratio of the two is limited to the range of 5 to 30% by volume will be described. As described above, lithium is doped into the carbonaceous material made of natural or artificial graphite at a certain current density using lithium metal as a counter electrode until the inter-terminal voltage becomes 0 V, and then released until the voltage becomes 1.0 V. Percentage of the undoped capacity (mAh / g) A obtained in this case and the undoped capacity B obtained by performing the same operation on the other carbonaceous material capable of inserting and extracting lithium, = B / A × 100 is expressed in order to determine the compounding ratio, in order to determine the electrochemical characteristics of the carbonaceous material having no graphite structure, that is, the amount of lithium that can be inserted and extracted and the electrode potential at that time. It may be set appropriately according to the method. However, if the compounding ratio of the carbonaceous material is less than 5% by volume, a sharp change in the negative electrode potential at the end of the discharge is not alleviated much, and the amount 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 when determining the values of A and B is
It should be determined according to the application after the battery is completed, and it is desirable that the value be obtained by dividing the electrode area from the current value required for the completed battery. For example, when a device 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. These are all caused by the fact that the utilization rate when carbonaceous materials occlude and release lithium varies depending on the current density, and it is generally known that the higher the current density, the lower the utilization rate. Have been.

The above mixture is mixed with a binder and kneaded and granulated to form a negative electrode. On the other hand, an oxide or sulfide capable of absorbing and releasing lithium is used as a positive electrode, and a separator,
When combined with a non-aqueous electrolyte, a lithium secondary battery is obtained. The battery form may be any of a flat type and a spiral type.

As the positive electrode material, any material may be used as long as it is used for this type of battery, but it is particularly preferable to use a material containing a sufficient amount of lithium. For example, it is represented by Li Mn ₂ O ₄ or a general formula Li MO ₂ (where M represents at least one or a mixture of Co and Ni, such as LiCoO ₂ and LiCo _0.8 Ni _0.2 O ₂ ). Intermetallic compounds containing lithium and complex metal oxides are preferred.

The non-aqueous electrolyte is prepared by appropriately combining an inducing solvent and an electrolyte, and any of these organic solvents and electrolytes can be used as long as they are used in this type of battery. Examples thereof include propylene carbonate, ethylene carbonate, 1.2-dimethoxyethane, 1,2-diethoxyethane, γ-butyrolactone, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolan, and 4-methyl-1,3- There are dioxolan, diethyl ether and sulfolane. As the electrolyte, LiClO ₄ , LiAsF 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 released from the lithium occluded in graphite. This is because the electrode potential of lithium that is released occlusion-than other carbonaceous materials similar state of being mixed towards the graphite having lithium occluded is less noble.

When this electrochemical reaction proceeds and a certain amount of lithium is released from the graphite, the electrode potential of the entire negative electrode mixture shifts to a noble direction, and as a result, the other carbonaceous material in the lithium storage state becomes lithium. To reach the electrode potential for performing occlusion and release of lithium, and lithium is simultaneously released therefrom.

When the discharge proceeds and the electrode potential of the negative electrode mixture shifts in 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 emitted from graphite is low and flat, whereas the potential for lithium released from other carbonaceous materials is noble and less flat, and the electrochemical reaction proceeds during discharge. At the same time, it is for each characteristic to move in the noble direction faster.

[0025]

The present invention has the following effects. In other words, by configuring the battery with the negative electrode mixture composed of the above mixture, the flatness, which is an advantage of using graphite as the negative electrode material in a normal discharge state, that is, the battery voltage is stably maintained as a result of maintaining a more noble state stably. On the other hand, while the battery capacity can be maintained, the voltage drop becomes gradual just before the battery capacity is exhausted. Part damage can be prevented beforehand.

[0026]

Next, an embodiment of the present invention will be described. However, the present invention is not limited to only the following examples.

FIG. 12 shows the structure of an AA lithium secondary battery according to the present invention. Basically, this lithium secondary battery is spirally wound with a separator 3 made of a porous film made of polypropylene between the positive electrode 1 and the negative electrode 2 to form a winding element. The positive electrode lead plate 4 connected to the positive electrode 1 side is housed in the bottomed cylindrical case 6 via the PP insulating plate 6a with the negative electrode lead plate 5 connected to the negative electrode 2 side projected downward. 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. This is completed by pouring the liquid into the case 6, fitting the positive electrode terminal plate 7 into the opening of the case 6 via the sealing gasket 8, and caulking.

The positive electrode 1 is made of LiCo as a positive electrode active material.
O ₂ , a conductive powder of carbon powder and an aqueous dispersion of PTFE were mixed at a weight ratio of 100: 10: 10, and kneaded in a paste with water to form a current collector having a thickness of 30 μm. It is a strip which is dried, rolled and cut to a predetermined size after being coated on both sides of a stainless steel foil. A part of the mixture is scraped off at right angles to the longitudinal direction of the strip, and the positive electrode lead plate 4 is placed here. Spot welded.

LiCoO _{2 as the} positive electrode active material was obtained by mixing cobalt oxide (Co ₂ O) 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. Also, among the mixing ratios of the above materials, P
The proportion of the aqueous dispersion of TFE is the proportion of the solids therein. Further, the theoretical charge amount of the positive electrode 1 at this time is 500 mA.

The negative electrode 2 was formed by kneading with water an aqueous dispersion of carbonaceous powder and PTFE at a weight ratio of 100: 5 and kneading with nickel into an expanded metal made of nickel, followed by drying and cutting to form a strip. Further, a part thereof is scraped perpendicularly to the longitudinal direction, and a nickel negative electrode lead plate 5 is spot-welded to the scraped part. The ratio of the aqueous dispersion of PTFE was the same as the ratio 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
An electrolyte in which ClO ₄ ) was dissolved at a ratio of 1 mol / 1 in a mixed solvent of ethylene carbonate and 1,2-dimethoxyethane, and 2.5 ml of the electrolyte was injected and sealed. The size of the completed battery is AA size and is 14.5 mm x 50 mm in diameter.

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

[0033]

[Table 1] The negative electrode sheet made of each of the above materials 1 to 6 was made of lithium metal at the counter electrode, and lithium was occluded at a constant current of 0.5 mA / cm ² until the voltage became 0 V based on Li / Li ⁺ potential. 1 and 3 to 7 show voltage curves when lithium is released at a constant current of 0.5 mA / cm ² with a counter electrode of lithium metal at a current density of 0.5 mA / cm ² . In addition, FIG.
In the case of graphite-based NG and CF shown in FIG. 3, a flat and gentle curve is drawn, and in the case of carbonaceous materials CK, CB, PR and MP other than graphite shown in FIGS. It has become.

In the above materials, combinations of carbonaceous materials of the negative electrode are NG / CK, NG / CB, CF / MP, C
F / PR, each having a mixing ratio shown in Table 2. In this case, the volume% of the mixing ratio is such that lithium is doped into the carbonaceous material made of natural or artificial graphite at a constant current of 0.5 mA / cm ^{2 with} the counter electrode being lithium metal until the terminal voltage becomes 0 V. Undoped capacity (mAh / mA) obtained when this lithium is discharged at a constant current of 0.5 mA / cm ² until the voltage becomes 1.0 V.
g) Percentage ratio of A to the undoped capacity B obtained as a result of performing the same operation on the other carbonaceous material capable of inserting and extracting lithium, expressed as a percentage = B / A × 100. The mixing ratio is determined with reference to the characteristics of the individual materials shown in FIGS.

[0035]

[Table 2] Next, the batteries of each composition were subjected to an upper limit voltage of 4.1 V and a lower limit voltage of 2.0.
When the first cycle of charging and discharging was performed at a constant current of V, the discharge curves of FIGS. 8 to 11 were obtained. In the figure, A indicates an inflection point at which the voltage at the end of discharge suddenly changes.

As shown in FIGS. 8 to 11, when the carbonaceous material capable of occluding and releasing lithium other than graphite in the combination of a and b is 100, 50 (% by volume), the discharge curve has no flatness. Obviously, 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%), a sharp drop in potential from the inflection point A at the end of discharge is recognized as in the conventional case.

On the other hand, as can be seen from the figures, in order to maintain the high voltage and the flatness and reduce the voltage drop at the end of the discharge to a detectable state, the composition of c to e is 5 to 5. It is appropriate to mix in the range of 30 (% by volume), the voltage drop is gradual from the inflection point A until the capacity is completely lost, and there is a considerable time. When the user notices the sluggish state, the user can obtain a sufficient time for measures such as stopping the device, charging, and replacing the battery.

[Brief description of the 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 a negative electrode is made of natural graphite.

FIG. 3 is a graph showing a voltage curve when lithium is released 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 phenolic carbide.

7 is a graph showing the voltage curve in the case where releasing lithium from meso Phase pitch carbon.

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

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

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

FIG. 11 is a graph showing a battery voltage curve using a negative electrode by 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.

Continued on the front page (72) Inventor Yoshiro Harada 5-36-11 Shimbashi, Minato-ku, Tokyo Inside Fuji Electric Chemical Co., Ltd. (72) Inventor Hidenori Nakura 5-36-11 Shimbashi, Minato-ku, Tokyo Fuji Electric (56) References JP-A-5-290844 (JP, A) JP-A-1-3111565 (JP, A) JP-A-3-129664 (JP, A) (58) Fields investigated (Int) .Cl. ⁷ , DB name) H01M 4/02-4/04 H01M 4/58 H01M 10/40

Claims (2)

(57) [Claims] 1. A non-aqueous electrolyte secondary battery using a carbonaceous material for a negative electrode, wherein the carbonaceous material is natural or artificial graphite and 5 to 30% by volume of graphite capable of inserting and extracting lithium from the graphite. A mixture obtained by kneading a carbonaceous material other than the above (however, the capacity% referred to here is a lithium metal as a counter electrode at a certain current density and a terminal is connected to the carbonaceous material made of the natural or artificial graphite). The same operation is performed on the undoped capacity A obtained when lithium is doped until the voltage becomes 0 V and thereafter the lithium is released until the voltage becomes 1.0 V, and on the other carbonaceous material capable of inserting and extracting lithium. % Of the undoped capacity B obtained as a result of the above-mentioned process = B / A × 100
An in defined as numerical values revealed), wherein the natural or
The graphite structure of artificial graphite was determined by X-ray wide-angle diffraction.
The distance d002 between the 002 surfaces is 3.35 to 3.42 angstroms.
And the other carbonaceous material is an X-ray wide-angle
The spacing between the 002 planes obtained by folding is 3.43 to 3.9 inches.
Nonaqueous secondary battery, which is a Ngusutoromu. 2. The crystal structure of the natural or artificial graphite,
Half-width value of the X-ray diffraction peak corresponding to a spacing of 002 surface thereof at 1 ° or less, the sintered crystallite size Lc is 100 to 500 Å, the crystal structure of the other carbonaceous materials, the C-axis direction The crystallite size Lc is 9 to 50 angstroms, and the average particle size measured by a laser diffraction type particle size analyzer is 0.01.
～50Myuemu, non-aqueous electrolyte secondary battery according to claim 1, wherein the nitrogen adsorption specific surface area determined by the BET method 10~300m ² / g, H / C atomic ratio is 0.15 or less.