JPH1173944A - Negative electrode for lithium ion secondary battery and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high capacity battery electrode, and improve the discharge capacity of a battery by including a carbon material and basic tin chloride. SOLUTION: This negative electrode is manufactured by having tin dichloride dihydrate dispersed in water, followed by drying. More, specifically, natural flake graphite having an average particle size of about 10 μm as carbon material, for example and tin dichloride dihydrate are dispersed in a prescribed quantity of water, and then dried naturally. The resulting dispersion is successively dried in air of about 200 deg.C to form a negative electrode carbon material. This powdery negative electrode material is mixed with about 5 wt.% of PTFE and powder molded into a sheet form, which is then blanked in circle shape to form a negative electrode. Since water is used in this treatment, the particle form of the resulting negative electrode material powder is uneven. Therefore, when drying is performed after the tin dichloride dihydrate has been dissolved in ethanol, and the carbon material has been dispersed therein, a negative material having substantially the same particle form as graphite particle can be provided.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an improvement in the discharge capacity of a negative electrode in a lithium ion secondary battery using graphite or carbon for the negative electrode.

[0002]

2. Description of the Related Art All batteries using metallic lithium as a negative electrode active material are called lithium batteries. To be precise, it is called "manganese dioxide lithium battery" in combination with the positive electrode active material. Lithium batteries are batteries that have been put into practical use in recent years,
Since the voltage is twice as high as that of a normal dry battery, the capacity is large, and the storage life is 5 years or more, the battery is widely used although it is expensive. Especially in recent years, as a result of the rapid development of miniaturization technology in electronic devices,
The battery used as the power source also needs to be downsized,
Improvements in high energy density, large capacity, and high electromotive force have become essential, and research and development of new lithium ion secondary batteries has been activated.

However, conventional lithium secondary batteries often use metallic lithium foil or the like as a negative electrode active material, and therefore have the following problems to be solved. That is, metallic lithium is eluted into the electrolytic solution with discharging, so that lithium is reprecipitated during charging. At this time, lithium precipitates in a dendritic (dendritic) state or becomes fine particles. Dendrites cause short-circuits or drop off, resulting in a reduction in capacity, leading to a decrease in cycle characteristics and safety. Since dendrites are easily generated at high currents, rapid charging deteriorates cycle life.

Therefore, a method has been proposed in which a material capable of absorbing lithium, such as a lithium / aluminum alloy or a wood alloy, is used for the negative electrode. However, the processability as an electrode is reduced. The alloy has problems such as embrittlement and falling off.

[0005] Among the substances capable of storing lithium, the most probable negative electrode material is carbon. In recent years, studies on supporting carbon on various carbon materials such as graphite have been actively conducted. According to Japanese Patent Application Laid-Open No. 57-208079, when charging is performed using graphite as a negative electrode, lithium in the positive electrode is electrochemically intercalated (inserted) between layers of the negative electrode graphite, and lithium is replaced with graphite by discharging. The ions can be deintercalated into the electrolyte from the layers and returned to the positive electrode. During charge-discharge, lithium maintains high ionicity both when present in the positive electrode, in the negative electrode, and when present in the electrolytic solution. For this reason, a secondary battery utilizing these mechanisms has been generally called a lithium ion secondary battery.

[0006] The charge and discharge characteristics of graphite are similar to those of an alloy negative electrode when charged and discharged at a constant current using metallic lithium as a counter electrode.
It has a flat and stable portion near 0.2 V, which is convenient from the viewpoint of stable operation of the device. The theoretical discharge capacity of graphite is 372 mAh / g. However, for a demand for a high-capacity battery, a negative electrode material exceeding this theoretical value and having the highest possible capacity is desired.

One of the promising materials as a high capacity negative electrode material is non-graphitizable carbon. The non-graphitizable carbon is La
It is composed of a structure in which graphite layers of about 2 to 5 nm of graphite are stacked at random in an amount of 3 to 5 layers, and lithium ions are clustered and occluded in closed pores of about 10 angstroms generated in the firing step. It is believed that. The charge and discharge characteristics of the non-graphitizable carbon have a characteristic that when charged and discharged at a constant current using metallic lithium as a counter electrode, each voltage of 0 to 3 V does not show a flat portion and changes smoothly. This has the advantage that the remaining amount of the battery can be determined by the change in potential when used in a battery, but is disadvantageous from the viewpoint of stable operation of the device because the voltage is not constant. In addition, if the lithium storage mechanism is a cluster, the safety concerns cannot be eliminated as in the case of metallic lithium.

The discharge capacity of non-graphitizable carbon is 500-80.
It is also called 0 mAh / g.

As another high capacity negative electrode material, there is a tin composite oxide. When the tin composite oxide is charged and discharged with a constant current using lithium metal as a counter electrode, 500 to 800 mAh /
It is known that a high discharge capacity of g can be obtained,
It is known that the irreversible capacity is large, and that the capacity is extremely reduced when deep charge / discharge is repeated, and there is still room for improvement.

[0010] First, although the lithium occlusion mechanism of the tin composite oxide has not been clarified, the charge and discharge characteristics thereof are similar to those of the non-graphitizable carbon, and the voltage is not constant. From the standpoint of stable operation, it can be disadvantageous as with non-graphitizable carbon.

The present inventors have investigated the charge / discharge characteristics of various metal oxides, sulfides, sulfates, etc. in order to elucidate the charge / discharge mechanism of the above-mentioned tin composite oxide. As a result, the inventors formed an alloy with lithium. It has been found that the resulting metals, in particular, compounds of tin, lead, indium, molybdenum, tungsten, aluminum, manganese, iron and zinc exhibit charge and discharge characteristics similar to those of the tin composite oxide.

Further, the following research has revealed the following. When graphite is treated with tin tetrachloride and water, SnCl (OH) (basic tin chloride) and / or SnO are generated by the tin tetrachloride and a large amount of water and adhere to the graphite surface. Then 2
The tin compound is fixed by drying in the air at 00 ° C. The amount of the attached tin compound is extremely small, and the charge / discharge capacity is only slightly increased. However, although the reason is not clear, the cycle characteristics of graphite itself can be greatly improved. When graphite is treated with tin tetrachloride, sulfuric acid and water, SnCl (OH) and SnSO ₄ are mixed with tin tetrachloride, sulfuric acid and a large amount of water.
Is formed and adheres to the graphite surface. Then 200 ° C
Although the tin compound is fixed by drying in air, the amount of the tin compound attached is relatively large, so that not only cycle characteristics are improved, but also the charge / discharge capacity is increased.

[0013]

SUMMARY OF THE INVENTION An object of the present invention is to provide a high capacity negative electrode carbon material for a lithium ion secondary battery, and to further improve the discharge capacity of the lithium ion secondary battery.

[0014]

The products from tin tetrachloride shown in the above are basic tin chloride SnCl (O
H), the possibility of tin dichloride was investigated. Tin tetrachloride is volatile and generates toxic gases, whereas tin dichloride is commercially available as a dihydrate and does not generate toxic gases due to volatilization. Therefore, handling is easy.

Since tin dichloride dihydrate swells and becomes semi-molten in the electrolyte solution EC + DEC as it is,
Although it is not suitable as a negative electrode material, it is considered that by dispersing in water, a basic hydroxyl group is introduced to form basic tin chloride, which becomes stable to an electrolytic solution. Graphite and tin dichloride dihydrate are weighed at a weight ratio of 1: 1, dispersed in water, and dried. Furthermore, it was dried in air at 200 ° C. to obtain a negative electrode material.
This had a discharge capacity of 540 mAh / g, which was a very high capacity.

Next, the negative electrode material obtained by the treatment with water has a non-uniform powder particle shape and is merely a mixture of graphite and a basic chloride. Therefore, when tin dichloride dihydrate is dissolved in ethanol, graphite is dispersed, and dried and treated, the particle shape is almost the same as graphite particles, that is, tin dichloride dihydrate is converted to graphite film. It will be in the state covered in the shape. It is also conceivable to treat the ethanol-treated graphite with water to introduce a basic group.

As described above, according to the present invention, there is provided a negative electrode for a lithium ion secondary battery containing a carbon material and basic tin chloride. According to the present invention, there is also provided a method for producing a negative electrode for a lithium ion secondary battery, in which a carbon material and tin dichloride dihydrate are dispersed in water and then dried. According to the present invention, there is also provided a method for producing a negative electrode for a lithium ion secondary battery, in which tin dichloride dihydrate is dissolved in ethanol, a carbon material is dispersed, and then dried.

[0018]

FIG. 1 shows the configuration of a test cell for a lithium ion secondary battery. An excessive amount of metallic lithium was used for the counter electrode 10, and filter paper was used three times for the separator 12.
The counter electrode 10 and the working electrode 14 were pressed by a stainless steel bar 16 urged by a spring 15. The stainless steel rod 16 was placed in a glass tube 18 and the cell was sealed by an O-ring 20. 22 is a Marloind film, 2
4 is a conducting wire. As the electrolytic solution 26, a solution obtained by adding lithium perchlorate to ethylene carbonate (EC) and diethyl carbonate (DEC) at a weight ratio of 1: 1 was used. (Example 1) 0.5 g of natural flaky graphite (BF-10A made by Chuetsu graphite from Brazil) having an average particle diameter of 10 µm and 0.5 g of tin dichloride dihydrate (Wako Pure Chemicals reagent special grade) were weighed. , Dispersed in water, air dried, then 200 ° C
The resultant was dried in the air to obtain a carbon material for a negative electrode.

Next, this powdery carbon material for a negative electrode was mixed with about 5% by weight of PTFE, powder-formed and formed into a sheet, which was punched into a circular shape having a diameter of 6.0 mm to obtain a negative electrode. The prepared electrode has an actual electrode weight of 2-3 mg.
Before and after. It was dried in a vacuum at 120 ° C. for one day and night to make it completely dry. The produced negative electrode was assembled in a test cell shown in FIG. 1 and a charge / discharge test was performed by the following method.

The initial charging (charging is the direction of the current in which lithium ions enter the carbon electrode) is performed by changing the current density from 0.1 V to 0.2 V from 3 V, which is the potential difference of lithium, until it reaches 0 V. , 0.5 and 1.0 mA / cm ² , and the discharge was performed up to 3.0 V at the same current density. The charge / discharge after the second time was measured between 0 V and 3.0 V with the same current density. FIG. 2 shows the obtained charge / discharge curve, and FIG. 3 shows the rate characteristics and cycle characteristics of the discharge capacity.
Shown in (Example 2) 0.5 g of natural flaky graphite (BF-10A made by Chuetsu graphite from Brazil) having an average particle size of 10 µm and 0.5 g of tin dichloride dihydrate (Wako Pure Chemical Reagent Special Grade) were weighed. Then, ethanol was added to dissolve tin dichloride dihydrate, and the ethanol was dried in a vacuum dryer at 50 ° C. Next, it was dried in air at 200 ° C. to obtain a carbon material for a negative electrode.

FIG. 3 shows the rate characteristics and cycle characteristics of the discharge capacity of the obtained carbon material in the charge / discharge test conducted in the same manner as in Example 1. The charge and discharge curves were almost the same as those in Example 1. (Comparative Example 1) FIG. 3 shows the cycle characteristics of the discharge capacity in the charge / discharge test obtained in the same manner as in Example 1 by using the graphite of Examples 1 and 2 untreated.

Comparing Examples 1 and 2 with Comparative Example 1, it can be seen that although the cycle characteristics are insufficient due to the tin carrying treatment, the discharge capacity is clearly large at the same current density. The current density showing a discharge capacity comparable to that of graphite alone at a current density of 0.1 mA / cm ² was 0.4 mA / cm 2.
^It is considered to be about ² and this is the same for the charging capacity, so not only can the charging time be shortened,
This indicates that a large current discharge is also possible. Furthermore, since this test result is converted by the capacity per weight excluding only PTFE used as a binder, it can be said that the discharge capacity is comparable to that of an oxide negative electrode requiring a large amount of a conductive material.

[0023]

According to the present invention, the capacity of a carbon material for a negative electrode of a lithium ion secondary battery can be increased by an extremely easy method, and a negative electrode material having excellent rate characteristics can be provided. By using this negative electrode material, a large-capacity lithium-ion secondary battery having excellent large-current charge / discharge characteristics can be manufactured.

[Brief description of the drawings]

FIG. 1 is a structural explanatory view of a test cell used for a charge / discharge test.

FIG. 2 is a charge / discharge curve diagram in a test cell of a negative electrode material for a lithium ion secondary battery of Example 1 of the present invention.

FIG. 3 is a graph showing cycle characteristics of discharge capacity in Examples 1 and 2 of the present invention and Comparative Example 1.

Claims (3)

[Claims] 1. A negative electrode for a lithium ion secondary battery comprising a carbon material and basic tin chloride. 2. A method for producing a negative electrode for a lithium ion secondary battery, comprising dispersing a carbon material and tin dichloride dihydrate in water and drying the dispersion. 3. A method for producing a negative electrode for a lithium ion secondary battery, comprising dissolving tin dichloride dihydrate in ethanol, dispersing a carbon material, and drying.