JPH09320593A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte lithium secondary battery that has a high capacity and is excellent in battery characteristics such as the flatness of its discharge voltage arid a high charge-discharge life. SOLUTION: In a nonaqueous electrolyte secondary battery having a negative electrode 3c made from a carbonaceous material that stores and releases lithium ions, a nonaqueous electrolyte, and a positive electrode 3a made from a lithium- cohtaining oxide, the negative electrode 3c made form the carbonaceous material is formed from carbon powders in which a graphite structure has a spacing d002 of 0.335 to 0.337nm on its (002) face by X-ray diffraction method, with the peak intensity ratio (P101 /P100 ) of a (101) diffraction peak P101 to a (100) diffraction peak P100 being 1.0 to 1.5, and with the ratio (La/Lc) of the length La of a crystallite in the direction of the (a)-axis to the length Lc of the crystallite in the direction of the (c)-axis being less than 1.5, and which immediately settles when dispersed in ethanol (ethyl alcohol) and held stationary.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a non-aqueous electrolyte secondary battery, and more particularly to a lithium secondary battery having an improved negative electrode.

[0002]

_2. Description of the Related Art Recently, manganese dioxide (MnO ₂ ), fluorocarbon [(CF ₂ ) n], thionyl chloride (SOCl ₂ ) or the like has been used as a positive electrode active material, while lithium-free non-aqueous material has been used as a negative electrode active material. BACKGROUND ART Electrolyte batteries have attracted attention as high energy density batteries, and have been widely used as power sources for calculators and watches or backup batteries for memory devices.

Further, with the miniaturization and weight reduction of new portable electronic devices such as VTRs with built-in cameras, mobile phones, and laptop computers, the demand for high energy density secondary batteries as their power sources is increasing. As a result, lithium secondary batteries using lithium as a negative electrode active material have been actively researched.

By the way, when a non-aqueous electrolyte battery is used as a secondary battery, it is desired to have a higher energy density, that is, a higher capacity and a higher voltage than those of an aqueous electrolyte secondary battery such as a lead battery and a nickel-cadmium battery. . Then, as a positive electrode active material satisfying such high capacity and high voltage, LiC is used.
Examples thereof include oO ₂ and LiMn ₂ O ₄ type materials that exhibit a high voltage of 4V. On the other hand, as the negative electrode, lithium alloys such as metallic lithium and carbonaceous materials capable of absorbing and releasing lithium ions are being studied. However, the metallic lithium has a problem of a short circuit due to a dendritic product (dendrites) associated with charge / discharge, and the lithium alloy has a problem of electrode collapse caused by expansion / contraction associated with charge / discharge.

In order to solve the above problem, it has been attempted to incorporate a negative electrode made of a carbonaceous material which absorbs and releases lithium into a lithium secondary battery, such as coke, a resin fired body, carbon fiber, and pyrolytic vapor phase carbon. There is. That is, it has been proposed to use a carbonaceous negative electrode to improve the reaction between lithium and the non-aqueous electrolyte, and further, to improve the deterioration of the negative electrode characteristics due to dendrite deposition.

[0006]

The carbonaceous negative electrode has a structure (graphite structure) in which the hexagonal net-like layers mainly composed of carbon atoms are stacked among the carbonaceous materials. It is believed that lithium ions move in and out to perform the required charging and discharging. However, if a carbonaceous material obtained by powdering giant crystals that have advanced graphitization is used as a negative electrode in a non-aqueous electrolyte, the non-aqueous electrolyte decomposes, resulting in low battery capacity and charge / discharge efficiency. Further, as the charge / discharge cycle progresses, the crystal structure or the fine structure of the carbonaceous material collapses, and the lithium occlusion / desorption ability deteriorates,
There is a problem that the cycle life is reduced.

Further, in the graphitized material, since the powder is flaky, the surface in the c-axis direction of the graphite crystallite into which lithium ions enter is smaller than the area exposed to the electrolytic solution. There is a problem that the capacity rapidly decreases during the charge / discharge cycle. Then, the problem of the sudden decrease in capacity is improved by adding carbon black or the like, but on the other hand, there arises a problem that the packing density of the negative electrode decreases, resulting in a high capacity lithium secondary battery. It couldn't be realized. Further, even in carbon fibers with advanced graphitization, when powdered, the non-aqueous electrolytic solution decomposes, and as with the case of using giant crystal powder, there is a problem that the performance as a negative electrode is significantly reduced. Was.

On the other hand, in a negative electrode made of carbonized material such as coke or carbon fiber having a low degree of graphitization, decomposition of the non-aqueous electrolyte can be suppressed to some extent, but the capacity and charge / discharge efficiency are low,
In addition, there are problems that the overvoltage during charging and discharging is large, the flatness of the discharge voltage of the battery is low, and the cycle life is short.

Regarding the carbonaceous negative electrode, it has been proposed to control the degree of graphitization of various carbonized materials and graphitized materials, and to provide optimum parameters for the graphite structure (for example, Japanese Patent Laid-Open No. 62-268058). , JP-A-2-82466, JP-A-4-61747
No. 4-115458, No. 4-184862, No. 4-19
No. 0557), a negative electrode having sufficient characteristics has not been obtained. A carbonaceous negative electrode using a powdered material of carbon fiber is also known (see, for example, JP-A-4-79170).
No. 4, JP-A-4-82172, etc.), the performance is not satisfactory. That is, the carbonaceous negative electrodes of conventionally known non-aqueous electrolyte lithium secondary batteries all have cycle characteristics,
Not all the characteristics of the battery such as high rate characteristics were satisfactory.

The present invention has been made in consideration of the above circumstances, and provides a non-aqueous electrolyte lithium secondary battery having high capacity, excellent charge / discharge efficiency, flatness of discharge voltage, and excellent charge / discharge life. For the purpose of provision.

[0011]

A non-aqueous electrolyte secondary battery according to the present invention comprises a carbonaceous negative electrode which absorbs and releases lithium ions, a non-aqueous electrolyte, and a positive electrode comprising a lithium-containing oxide. In the non-aqueous electrolyte secondary battery provided with, the carbonaceous negative electrode has an interplanar spacing d ₀₀₂ of 0.335 to 0.337 nm in the (002) plane of the graphite structure by the X-ray diffraction method, and a (101) diffraction peak P _101. And the peak intensity ratio (P ₁₀₁ / P ₁₀₀ ) of the (100) diffraction peak P ₁₀₀ is 1.0 to 1.5, and the ratio of the crystallite length La in the a-axis direction to the crystallite length Lc in the c-axis direction (La / It is characterized in that Lc) is less than 1.5, and that it is formed of a material mainly composed of carbon powder that immediately precipitates when dispersed and allowed to stand in ethanol (ethyl alcohol).

In the present invention, the positive electrode is prepared by suspending a positive electrode active material, a conductive material and an adhesive material in a suitable solvent, applying the suspension to a current collector and drying it to form a thin plate. Here, examples of the positive electrode active material include manganese dioxide, lithium-manganese composite oxide, lithium-containing nickel oxide, lithium-containing cobalt compound, lithium-containing nickel-cobalt oxide, lithium-containing vanadium oxide, titanium disulfide, and titanium disulfide. Examples thereof include chalcogen compounds such as molybdenum sulfide. Among them, lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), and lithium manganese oxide (LiMn ₂ O ₄ , LiMnO ₂ ) are preferably used because a high voltage can be obtained.

Examples of the conductive material include acetylene black, carbon black, graphite, and the like, and examples of the binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), ethylene-. Propylene-diene copolymer (EPDM), styrene-butadiene rubber (SBR), etc. can be used.

The mixing ratio of the positive electrode active material, the conductive material and the binder may be 80 to 95 mass% of the positive electrode active material, 3 to 20 mass% of the conductive material, and 2 to 7 mass% of the binder. preferable.
As the current collector, for example, aluminum foil, stainless steel foil, nickel foil, titanium foil or the like can be used.

In the present invention, as the separator that electrically insulates between the positive electrode and the negative electrode, for example, a synthetic resin non-woven fabric, a polyethylene porous film, or a polypropylene porous film can be used.

In the present invention, the carbonaceous material forming the negative electrode can be produced, for example, as follows. First, by using a mesophase pitch system as a main raw material, a short fiber having a fiber length of 0.5 to 1 m is spun by a melt blow method, and then carbonized to an extent that it can be infusibilized and pulverized. Here, the heat treatment for carbonization is 600 to 2000 ° C., preferably 600 to 2000 ° C.
It is desirable to carry out at 800 ° C. Then, after the carbonization,
The mesophase pitch carbon fiber described above is produced by graphitizing the crushed carbon fiber at 2000 ° C. or higher, more preferably 2500 to 3200 ° C. At this time, the crushing and firing steps are extremely important, and the carbon fibers are less likely to be longitudinally cracked by using a ball mill or a jet mill during crushing, and by uniformly crushing, the average fiber length is 10 to
It is desirable that the average fiber length is 100 μm, more preferably 15 to 50 μm, and the average fiber diameter is 4 to 15 μm, and more preferably 7 to 10 μm.

If the average fiber length is less than 10 μm, the carbon fibers are liable to be longitudinally cracked by crushing, while if the average fiber length exceeds 100 μm, it is not preferable because the current collector cannot be coated. When the average fiber diameter is less than 4 μm, the strength of the fiber becomes brittle, while the average carbon fiber diameter is 15
If it exceeds μm, coating on the current collector cannot be performed, which is not preferable.

In the present invention, the solute which constitutes the nonaqueous electrolytic solution as one component is, for example, lithium perchlorate (LiCl).
Examples thereof include lithium salts (electrolytes) such as O ₄ ), lithium hexafluorophosphate (LiPF ₆ ), lithium borofluoride (LiBF ₄ ), and lithium trifluorometasulfonate (LiCF ₃ SO ₃ ). Of these, LiPF ₆ and LiBF ₄ are more preferable in consideration of safety and battery performance.
The concentration dissolved in these electrolytes is 0.7 to 1.7.
The mol / l range is preferred. If it is out of the above range,
The conductivity of the electrolytic solution may be insufficient, and the charge / discharge efficiency may deteriorate.

The negative electrode provided in the non-aqueous electrolyte secondary battery according to the present invention has a graphite structure in which the interplanar spacing and surface properties are selected so that lithium ions can easily enter, and the negative electrode is dispersed in ethanol (ethyl alcohol). It is formed from a carbonaceous powder that settles immediately (generally, at room temperature within seconds) when left to stand. In other words, because the negative electrode is made of carbonaceous powder that does not contain submicron-order ultrafine powder (excluding fine pine) that floats in ethanol, the decomposition of the electrolyte can be easily suppressed and prevented. As a result, it functions as a good negative electrode without causing a decrease in battery capacity and charge / discharge efficiency.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIG.

This embodiment is a case of a cylindrical lithium secondary battery, and 1 is a bottomed cylindrical container made of, for example, stainless steel, and an insulator 2 is arranged at the bottom thereof. Reference numeral 3 denotes an electrode group, which is housed in the container 1. Here, the electrode group 3 is formed of a positive electrode 3a, a separator 3b, and a negative electrode 3c, and a strip-shaped material in which the positive electrode 3a, the separator 3b, and the negative electrode 3c are stacked in this order is spirally formed so that the negative electrode 3c is located outside. It has a structure wound around.

In the cylindrical container 1, a non-aqueous electrolyte is stored together with the electrode group 3, and further, an insulating paper 4 having a central opening above the electrode group 3 in the cylindrical container 1, And the insulating sealing plate 5 is attached and arranged. In addition, the insulating sealing plate 5 mounted and arranged in the upper opening of the cylindrical container 1
Is fixed to the cylindrical container 1 in a liquid-tight manner by caulking in the vicinity of the upper opening. The positive electrode terminal 6 is fitted in the center of the insulating sealing plate 5 and is connected to the positive electrode 3 a via the positive electrode lead 7. On the other hand, the negative electrode 3c is connected to the cylindrical container 1 forming the negative electrode terminal via a negative electrode lead (not shown).

In the above structure, the positive electrode 3a is manufactured as follows. First, 91% by mass of lithium cobalt oxide (Li _x CoO ₂ [0.8 ≦ x ≦ 1]) powder was added to 3.5% by mass of acetylene black and 3.5% by mass of graphite.
And ethylene propylene diene monomer powder 2% by mass
Toluene was added to and mixed with each other, and the mixture was coated on an aluminum foil (30 μm) current collector, and then pressed.

Further, the negative electrode 3c has a 2
Mixture of carbonaceous A (90% by mass) and B (10% by mass) 96
%, Styrene-butadiene rubber 2.5% by mass, and carboxymethylcellulose 1.5% by mass were mixed together, and the mixture was applied to a copper foil as a current collector and dried to prepare a negative electrode.

Carbonaceous material A; carbonized at 650 ° C., pulverized, and then 3000
Firing at ℃, average fiber length 20μm, average fiber diameter 8μm,
The specific surface area by the N ₂ gas adsorption BET method is 1.0 m ² / g, and the interplanar spacing d ₀₀₂ of the (002) plane of the graphite structure by the X-ray diffraction method is 0.
3364 nm mesophase pitch carbon fiber powder, and 1 g of this carbon powder was sampled in a test tube, 10 ml of ethanol was put in this test tube, and the mixture was shaken with a rubber stopper and allowed to stand, A powder that has been confirmed to settle quickly.

Carbonaceous B Artificial graphite having a block-like shape having a particle size of 15 μm or less, 92.2% by volume, d ₀₀₂ of 0.3365 nm, and a specific surface area of 8.6 m ² / g.

The positive electrode 3a, the separator 3b made of a polyethylene porous film, and the negative electrode 3c are laminated in this order, and then spirally wound so that the negative electrode 3c is located outside to form the electrode group 3. did.

Further, in the above-mentioned constitution, the non-aqueous electrolyte is lithium hexafluorophosphate (LiPF ₆ ), ethyl carbonate (EC), propylene carbonate (PC),
Mixed solvent of methyl ethyl carbonate (MEC) and diethyl carbenate (DEC) (mixed volume ratio 40:10:
It was prepared by dissolving it in 40:10) at a ratio of 1.0 mol / l.

Comparative Example In the case of the above-mentioned Examples, except that the carbonaceous powder was used in which the ultrafine powder did not float and did not settle after the carbonaceous A was dispersed and allowed to stand in ethanol. A cylindrical lithium secondary battery similar to the above was assembled.

Cylindrical lithium secondary battery of these examples,
And about the cylindrical lithium secondary battery of Comparative Example, 20
The cycle characteristics were evaluated under the charge and discharge conditions of ℃, 500mA for 3 hours charging to 4.2V, and 500mA discharging to 2.7V.
As a result, in the case of the cylindrical lithium secondary battery of the example, as shown in FIG. 2 and FIG. 3, respectively, the capacity retention rate was high and the charge / discharge efficiency was also good, whereas the cylindrical lithium secondary battery of the comparative example. As shown in comparison with FIG. 2 and FIG. 3, the lithium secondary battery was low in capacity retention and inferior in charge / discharge efficiency.

Although the configuration example of the cylindrical lithium secondary battery has been described above, the present invention is not limited to the above examples, and various modifications can be made without departing from the spirit of the invention. . For example, the form / structure of the lithium secondary battery may be a disc type, a button type or a square type.

[0032]

As can be seen from the description of the above embodiment,
The negative electrode that occludes and releases lithium ions has a graphite structure (002) plane spacing d ₀₀₂ of 0.335 to
0.337 nm, peak intensity ratio of (101) diffraction peak P ₁₀₁ and (100) diffraction peak P ₁₀₀ (P ₁₀₁ / P ₁₀₀ ) 1.0 to
1.5, a ratio (La / Lc) of the crystallite length La in the a-axis direction to the crystallite length Lc in the c-axis direction (La / Lc) less than 1.5, and immediately settles when dispersed and allowed to stand in ethanol. According to the lithium secondary battery of the present invention formed by using the carbon powder as a raw material, it functions as a lithium secondary battery having a high capacity, excellent high rate characteristics, and excellent cycle characteristics. That is,
It is possible to provide a high-energy-density lithium secondary battery that is compatible with downsizing and weight reduction of camera-integrated VTRs, mobile phones, laptop computers, and the like.

[Brief description of drawings]

FIG. 1 is a partial cross-sectional view showing the configuration of a cylindrical lithium secondary battery of an example.

FIG. 2 is a characteristic diagram showing the capacity retention rate of the cylindrical lithium secondary battery of the embodiment in comparison with the capacity retention rate of the cylindrical lithium secondary battery other than the present invention.

FIG. 3 is a characteristic diagram showing the charging / discharging efficiency of the cylindrical lithium secondary battery of the embodiment in comparison with the charging / discharging efficiency of the cylindrical lithium secondary battery outside the present invention.

[Explanation of symbols]

 1 ………… Cylindrical container 2 ……… Bottom insulator 3 ……… Electrode group 3a ……… Positive electrode 3b ……… Separator 3c ……… Negative electrode 4 ……… Insulating paper 5 ……… Insulating sealing plate 6… …… Positive electrode terminal 7 ………… Positive electrode lead

Claims (1)

[Claims] 1. A non-aqueous electrolyte secondary battery comprising a carbonaceous negative electrode that occludes and releases lithium ions, a non-aqueous electrolyte, and a positive electrode comprising a lithium-containing oxide. The negative electrode has an interplanar spacing d ₀₀₂ of 0.335 to 0.337 nm in the (002) plane of the graphite structure by X-ray diffraction,
(101) diffraction peak P ₁₀₁ and (100) diffraction peak P
The peak intensity ratio of ₁₀₀ (P ₁₀₁ / P ₁₀₀ ) is 1.0 to 1.5,
Carbon powder having a ratio (La / Lc) of the crystallite length La in the a-axis direction and the crystallite length Lc in the c-axis direction of less than 1.5, and immediately settling when dispersed / stood in ethanol. A non-aqueous electrolyte secondary battery, which is formed of a material mainly containing.