JP2001085007A - Negative electrode material for lithium ion secondary battery and manufacture of negative electrode material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a carbon negative electrode material for a lithium ion secondary battery having a large charging/discharging capacity. SOLUTION: A negative electrode active material is formed from carbon material meeting the condition that the carbon content is 94 wt.% or more and oxygen content is 5 wt.% or less, the d002 value obtained through X-ray diffraction is 0.38-0.45 nm, and that Vpore according to the following expression is 5-30; Vpore=(1-ρa/ρHe)×100, where ρa is the density measured by using water or ethanol as medium, and ρHe is the density measured by using helium gas as medium. With this carbon material, a secondary battery having a large discharging capacity can be manufactured.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a negative electrode for a high capacity lithium ion secondary battery.

[0002]

2. Description of the Related Art In recent years, an organic electrolyte secondary battery using lithium has a high energy density and plays a major role in reducing the size and weight of equipment. However, there is a strong demand for further miniaturization and weight reduction of devices, and a higher capacity secondary battery is required.

[0003] Current lithium ion secondary batteries generally use a carbonaceous material for a negative electrode and a lithium-containing compound for a positive electrode, and perform charging and discharging by moving lithium ions between the positive electrode and the negative electrode. The characteristics of a lithium ion secondary battery greatly depend on the characteristics of a negative electrode material. If highly crystalline carbon such as graphite is used, the output voltage is almost constant and the battery has excellent charge / discharge reversibility. However, it is said that charging beyond the theoretical capacity of C ₆ Li is difficult.

On the other hand, if undeveloped carbon having a layered structure is used, charging exceeding the theoretical capacity of C ₆ Li is possible, but only a material having a small initial charge / discharge efficiency has been developed. Therefore, development of a carbon anode material using coal or petroleum pitch, thermosetting resin, carbon fiber, or the like as a raw material has been attempted.

For example, JP-A-3-245458 discloses a method of using a carbon material containing 0.1 to 0.2 wt% of boron as a negative electrode material.
JP-A-6-119923 discloses a method of injecting lithium ions into a carbon material having a graphite crystal part and an amorphous part to form a negative electrode material, and JP-A-6-333559 discloses a vapor-grown carbon fiber. Discloses a method of using a composition containing 1 to 30% by weight of carbon black as a negative electrode material. However,
In any case, a negative electrode material having a sufficient charge / discharge capacity has not been developed.

[0006]

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and has as its object to provide a carbon anode material for a lithium ion secondary battery having a large charge / discharge capacity.

[0007]

The present invention solves the above-mentioned problems by using carbon satisfying the following conditions as a negative electrode active material. (1) The carbon content is 94 wt% or more and the oxygen content is 5 wt% or less. (2) The value of d002 obtained by X-ray diffraction is 0.38 nm or more, 0.4
5 nm or less. (3) V _pore obtained by the following formula is 5 or more and 30 or less.

[0008]

(Equation 2)

Where ρ _a is the density measured using water or ethanol as a medium, and ρ _He is the density measured using helium gas as a medium.

[0010] The carbon may be a resin having a phenolic hydroxyl group in an inert atmosphere or in a vacuum at 800 ° C or higher, 140 ° C or higher.
It can be obtained by heat treatment at 0 ° C. or lower.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is characterized in that carbon satisfying the following conditions is used as a negative electrode active material. (1) The carbon content is 94 wt% or more and the oxygen content is 5 wt% or less. (2) The value of d002 obtained by X-ray diffraction is 0.38 nm or more.
0.45 nm or less. (3) V _pore obtained by the following formula is 5 or more and 30 or less.

[0012]

(Equation 3)

Where ρ _a is the density measured using water or ethanol as a medium, and ρ _He is the density measured using helium gas as a medium. Crystal structure, pore structure,
It is easy to understand if it is divided into surface structures. As a result of repeated experiments, the present inventors have grasped the above carbon structure, and have come to find the above three items as a structure most suitable for lithium occlusion.

The first item relates to the surface structure. It is known that an oxygen-containing group called a surface functional group exists on the carbon surface and the crystal face. This oxygen group is considered to cause the irreversible capacity and the discharge potential to shift higher than the equilibrium potential of lithium. In the present invention, the amount of oxygen derived from this functional group needs to be 5 wt% or less.

The crystal structure is desirable from the viewpoint that non-graphitizable carbon in which a fine graphite structure is randomly oriented tends to form pores. However, this is the second item. It is desirable that the value be 0.38 nm or more and 0.45 nm or less.

Further, what is important in the present invention is that the third item, V _pore, which represents the pore structure, is 5 or more and 30 or less. Various methods for evaluating pores have been studied.For example, as shown in JP-A-9-25883, analysis using the Dubinin-Astakhov equation based on the adsorption amount of molecules such as oxygen, ethane, and isobutane There is a technique to do.
However, such a method involves difficulty in measuring the amount of adsorption,
The pores of carbon cannot be accurately grasped.

Therefore, in the present invention, pores related to lithium occlusion are grasped from the density using helium gas as a medium and the density using water or ethanol as a medium. A method for measuring the density using a gas such as helium as a medium is known as a constant volume expansion method, and is suitable for measuring the true density because gas molecules enter into fine pores of a sample that cannot be entered by liquid molecules.
In particular, when helium gas is used, it can penetrate into pores of 1 nm or less. On the other hand, a density measurement method using water or ethanol is known as a liquid immersion method. It is said that this technique penetrates into pores up to approximately 2 nm.

In the present invention, V is calculated from the density obtained by the two methods.
_{pore = (1-ρ a /} ρ He) X 100 by the determined Vpore is 5 or more, it is desirable that 30 or less. When it is smaller than 5, it indicates that there are few pores of 2 nm or less, so that the amount of occluded lithium is small. A larger value of Vpore indicates that there are more pores of 2 nm or less, but the upper limit of this value is about 35 when calculated from the density of graphite and glassy carbon,
For practical carbon, 30 is the limit.

When the carbon source contains nitrogen atoms, sufficient desorption does not occur if the heat treatment temperature is low. Since nitrogen easily reacts with lithium, carbon having a nitrogen content of 3 wt% or less is used in the present invention.

The carbon as described above is pitch, polyimide,
In general, any raw material, such as polyamide, from which carbon can be obtained by firing can be prepared.
As described, it is desirable that the resin is obtained by heat-treating a resin having a phenolic hydroxyl group.
The heat treatment at this time is desirably performed at 800 ° C. or more and 1400 ° C. or less in an inert atmosphere or vacuum as described in claim 4. When the temperature is lower than this temperature, carbonization does not proceed sufficiently, and elements other than carbon remain much. For this reason, the electric conductivity is insufficient, and the development of pores of about 1 nm is also insufficient. On the other hand, if the temperature is higher than 1400 ° C., rearrangement of carbon proceeds, and although pores are formed, the pores are closed.

When the carbon obtained as described above is used as an electrode, it is desirable to mix the carbon with an appropriate binder and apply it on a conductive foil. Various conductive foils can be used. Among them, a copper foil or a sheet-like graphite which is electrochemically inactive near the equilibrium potential of lithium is preferable. In particular, sheet graphite is particularly desirable because it can itself be used as an active material.

The electrode thus obtained is most suitable as a negative electrode of a lithium ion secondary battery.

[0023]

EXAMPLES The present invention will be described below with reference to specific embodiments, but the present invention is of course not limited thereto. In the following examples, the carbon material of the present invention which is originally used as a negative electrode is used as a positive electrode, and metallic lithium is used as a negative electrode. This is intended to simplify the insertion and desorption of lithium into the carbon material by using metallic lithium as a source of lithium ions, and to more clearly prove the properties of the carbon material of the present invention. It is apparent to those skilled in the art that the configuration of the embodiment of the present invention proves usefulness as a negative electrode in a lithium ion battery, which is the original purpose.

Various measurements in the examples were performed by the following methods.

(Charge / Discharge Characteristics) 3 g of carbon powder was mixed with 3 g of a binder obtained by dissolving 10% by weight of polyvinylidene fluoride in N-methylhydroxylidine, coated on a copper foil having a thickness of 20 μm, and dried to obtain an electrode plate. . In addition, 1 mol of LiPF ₆ was added to an organic solvent in which ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 1: 1.
/ l was dissolved to prepare an electrolytic solution. Using lithium metal as a carbon electrode and a counter electrode, insert a porous polypropylene impregnated electrolyte solution between the two, insert these into a 2016 type coin case, press seal and perform a coin for evaluation. A battery was manufactured.

With respect to the battery obtained in this way, 0.1%
Charge at a constant current of 2 mA until the potential becomes 0 V, and
The charging was terminated when the current reached 10 mA while maintaining the potential of V. Next, discharging was performed at a constant current of 0.2 mA until the potential became 1.5 V.

The density of the active material on the copper foil was determined as follows. The electrode plate was punched to a diameter of 12.5 mm, and the thickness and weight were measured. Next, 10 sheets of the copper foil itself were punched out, the weight and the thickness were measured, and the average value was obtained. The average value of the weight per copper foil was subtracted from the weight of the punched electrode plate to obtain the weight of the active material. Next, the volume of the active material was determined by subtracting the average of the thickness per copper foil from the thickness of the electrode plate. The active material density on the copper foil was determined from the weight and volume of the active material.

(X-ray diffraction) d002 was obtained as follows. First, the measurement was performed using a normal X-ray diffractometer (Rotorflex RU-200B manufactured by Rigaku Denki). Next, the background was removed from the obtained diffractogram by the method of Somneverlt-Visser's. D002 was obtained from the (002) peak of the diffraction pattern obtained in this manner using Bragg's equation.

(Elemental analysis) As for the ratio of the elements in the carbon material, the ratios of carbon, hydrogen and nitrogen were determined by a CHN analyzer by a combustion method, and the rest was regarded as the ratio of oxygen. The detection limit by this method is 0.1 wt%.

(Density) The density was measured with a He gas by a dry automatic densitometer (Acupic 1330, manufactured by Shimadzu Corporation). On the other hand, the density by the liquid immersion method was determined using ion-exchanged water in which 0.1 wt% of a surfactant (Tween 20) was mixed. The effect of the surfactant on the density was 0.001 g / cm ³ .

Example 1 A resol-type phenol resin solution was cured by raising the temperature to 180 ° C. in air and holding for 1 hour. The obtained polymer resin was pulverized by a planetary ball mill so that the average particle diameter became 10 μm.

The resin powder thus obtained was subjected to a heat treatment in argon at a temperature from 600 ° C. to 1600 ° C. for 1 hour. The measurement results are shown in (Table 1). As shown in the table, the carbon according to the present invention has a higher lithium storage capacity than graphite.

[0033]

[Table 1]

(Example 2) A carbon powder obtained by a method similar to that of Example 1 was mixed with a polymer powder obtained by a method similar to that of Example 1,
The heat treatment was performed at 0 ° C. for 0, 1, 10, 30, and 100 hours, respectively, and the characteristics were measured. The results are shown in (Table 2). As shown in the table, the carbon according to the present invention has a higher lithium storage capacity than graphite.

[0035]

[Table 2]

Example 3 A polyimide resin (UPILEX, Ube Industries) was treated in argon at 500 ° C. for 1 hour, and then pulverized by a planetary ball mill so as to have an average particle diameter of 10 μm.

The thus obtained powder was subjected to a heat treatment in argon at a temperature of 600 ° C. to 1600 ° C. for 1 hour. The measurement results are shown in (Table 3). As shown in the table, when nitrogen is present at 3 wt% or more, the discharge capacity becomes small.

[0038]

[Table 3]

[0039]

According to the present invention, carbon satisfying the following conditions is used as a negative electrode active material. (1) The carbon content is 94 wt% or more and the oxygen content is 5 wt% or less. (2) The value of d002 obtained by X-ray diffraction is 0.38 nm or more, 0.4
5 nm or less. (3) V _pore obtained by the following formula is 5 or more and 30 or less.

[0040]

(Equation 4)

Where ρa is the density measured using water or ethanol as a medium, and ρHe is the density measured using helium gas as a medium.

Such a carbon material can be obtained by using a resin having a phenolic hydroxyl group as a raw material and performing heat treatment in an inert atmosphere or in a vacuum at a temperature of 800 ° C. or more and 1400 ° C. or less. By using the carbon obtained according to the present invention, a secondary battery having a large discharge capacity can be manufactured.

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Claims (6)

[Claims] 1. A negative electrode material for a lithium ion secondary battery, wherein carbon satisfying the following conditions is used as a negative electrode active material. (1) The carbon content is 94 wt% or more and the oxygen content is 5 wt% or less. (2) The value of d002 obtained by X-ray diffraction is 0.38 nm or more and
0.45 nm or less. (3) V _pore obtained by the following formula is 5 or more and 30 or less. (Equation 1) Where ρ _a is the density measured using water or ethanol as a medium, and ρ _He is the density measured using helium gas as a medium. 2. The negative electrode material for a lithium ion secondary battery according to claim 1, wherein the nitrogen content of carbon is 3 wt% or less. 3. The method for producing a negative electrode material for a lithium ion secondary battery according to claim 1, further comprising a step of heat treating the resin having a phenolic hydroxyl group. 4. The method for producing a negative electrode material for a lithium ion secondary battery according to claim 3, wherein the heat treatment is performed at 800 ° C. or higher and 1400 ° C. or lower in an inert atmosphere or in a vacuum. 5. A negative electrode for a lithium ion secondary battery, wherein the negative electrode material for a lithium ion secondary battery according to claim 1 is applied on a conductive foil. 6. A lithium ion secondary battery, wherein the negative electrode for a lithium ion secondary battery according to claim 5 is used as a negative electrode.