JPH113699A - Negative electrode for lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a negative electrode for secondary battery having excellent adhesiveness of a collector and the carbon material by forming a coating layer, which contacts with a negative electrode collector, of two layers or more, and coating a lower layer with the carbon material having a small specific surface area. and forming the thickness in a specified range. SOLUTION: As a carbon material to be coated on a surface of a negative electrode collector 20 for lithium ion secondary battery, the carbon material having a low specific surface area at 2 m<2> /g or less, especially the fiber-like carbon having a specific surface area at 1-2 m<2> /g is used for a lower layer, and thickness of the coating layer is set at 20-100 μm. An upper layer is coated with the carbon material having a large specific surface area and a high discharging capacity. In comparison with one-layer coating of a collector surface, besides the binder already exists in the lower layer, since the coating layer of the lower surface has a larger contact area in comparison with the surface area of the collector, quantity of the binder to be required for coating of the upper layer is little, and quantity of the binder for all coating layers can be reduced than that of the one-layer coating, and quantity of the binder can be set at 10 wt.% or less in relation to the total weight of the all coating layer, and in this condition, adhesiveness and discharging capacity are improved.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a nonaqueous electrolyte lithium ion secondary battery using a carbon material capable of inserting and removing lithium ions for a negative electrode.

[0002]

2. Description of the Related Art Conventionally, a negative electrode plate for a lithium secondary battery is manufactured by applying a mixed paste of a carbon material, a binder, and a solvent on a surface of a current collector such as a copper foil. As this carbon material, a material using pyrolytic carbon is known to be excellent. For example, JP-A-3-95857 discloses that a negative electrode is made of a carbon layer (upper layer) containing no iron element and an iron layer. A carbon layer (lower layer) containing the element,
It describes that a negative electrode having high capacity and low self-discharge is formed by applying two layers of high-capacity carbon containing iron element and carbon not containing iron element having low self-discharge. Japanese Patent Application Laid-Open No. 6-163030 discloses that, in a negative electrode having a carbon particle layer composed of particles of a carbon material and a binder provided on a current collector, the average between the current collector and the carbon particle layer is reduced. 5 μm comprising a carbon fine particle layer having a particle size of 5 μm or less and a binder
By providing a thin carbon fine particle layer to fill the current collector surface with carbon fine particles, to prevent the phenomenon that Li is deposited as fine powder on the surface of the current collector made of Cu or Ni,
It describes that capacity deterioration due to charge / discharge cycles is improved.

[0003]

In the negative electrode of a lithium ion battery, conventionally, a mixed paste of a carbon material, a binder and a solvent is applied to the surface of a current collector such as a copper foil or a nickel foil using various carbon materials. Is manufactured by
When a negative electrode was manufactured by this one-layer coating, it was found that a high-capacity carbon material required a large amount of binder because of poor adhesion to the current collector, and the capacity density of the electrode was reduced. As a result of examining the cause of poor adhesion to the current collector, a high-capacity carbon material has a large specific surface area of 3 to 50 m 2 / g.
The present inventors have found that the binder is adsorbed on the surface of the carbon material, and the amount of the binder effective for adhesion between the carbon material and the current collector decreases in accordance with the degree of surface adsorption. On the other hand, a carbon material having a low specific surface area of 2 m 2 / g or less has been conventionally recognized as being unsuitable as a negative electrode material of a lithium ion secondary battery, although it has excellent adhesion to a current collector, but has a small discharge capacity. Was. The present invention has been made in order to solve the above problems when using a high-capacity negative electrode material as a negative electrode material, and has a high capacity lithium having excellent adhesion between a current collector and a carbon material. An object of the present invention is to provide a negative electrode for an ion secondary battery.

[0004]

SUMMARY OF THE INVENTION The present invention relates to a negative electrode for a lithium ion secondary battery having a coating layer made of a carbon material on the surface of a current collector, comprising two or more coating layers made of a carbon material. A negative electrode for a lithium ion secondary battery, wherein a carbon material having a specific surface area of 2 m <2> / g or less is used as a carbon material of a coating layer in contact with the body, and the thickness of the coating layer is in a range of 20 to 100 [mu] m. provide. If the thickness of the coating layer is less than 20 μm, the average particle size of the graphite falls below the average particle size of the graphite, making uniform coating difficult, and the binding property with the current collector is unfavorably reduced. No improvement effect is obtained.

[0005] As the carbon material, pyrolytic carbons, cokes, graphites, glassy carbons, organic polymer compound fired bodies, activated carbon, and the like are generally used as negative electrode materials. There are scaly, massive, spherical, fibrous, etc. In general, a correlation is found between the shape of graphite and the specific surface area, and the specific surface area of general flaky graphite is 5 to 5.
100 m 2 / g, the specific surface area of general massive graphite is 2.5
５5 m 2 / g, specific surface area of general fibrous graphite is 1-2
m 2 / g, the specific surface area of general spheroidal graphite is 0.5 to 2 m
It is known to be 2 / g. The present invention provides a spherical carbon or a fibrous carbon having a small specific surface area of 2 m 2 / g or less,
In particular, it has been found that the use of fibrous graphite as the lower layer results in a significant improvement in discharge capacity.

Further, the present invention uses the carbon material having a small specific surface area to reduce the total amount of binder necessary for forming two or more coating layers by using the carbon material of the coating layer.
The amount can be reduced to 10 wt% or less, which is smaller than the required amount in the related art, based on the total weight of the binder, and the effect of improving the discharge capacity can be obtained.

[0007]

According to the present invention, the amount of the binder adsorbed on the surface of the carbon material can be suppressed to a minimum by using a carbon material having a low specific surface area of 2 m 2 / g or less for the lower layer.
Therefore, since a binder effective for bonding the carbon material and the current collector is sufficiently present, a negative electrode plate having excellent adhesion between the current collector and the carbon material can be provided. Then, a carbon material having a high discharge capacity is applied to the upper layer. In this case, the binder is already present in the lower layer, compared to coating the current collector surface with a single layer, and the contact area of the lower coating layer is larger than the surface area of the current collector. It is also possible to reduce the amount of binder required for coating the upper layer. In addition, the lower-layer carbon material with a low specific surface area has an improved discharge capacity compared to a high-discharge capacity negative electrode manufactured by coating even though the discharge capacity itself is smaller than the high-discharge capacity carbon material. I do.
Although the mechanism that causes the improvement in the discharge capacity is not clear, it is estimated that the reduction in the amount of binder improves the electron conductivity in the electrode plate and facilitates the insertion and removal of lithium ions. Is done. As the thickness of the lower coating layer becomes thinner, the ratio of the high-capacity carbon material used for the upper layer increases, so that the total capacity of the negative electrode material increases. However, if the thickness is less than 20 μm, the binding property with the current collector becomes poor. The optimum value is in the range of 20 to 100 μm. Furthermore, as a result of examining carbon materials of various shapes as the carbon material used for the lower layer, it was found that the fibrous carbon material had the best adhesion to the current collector. The cause is that the spherical and massive carbon material is in point contact with the current collector and the binder, whereas the fibrous carbon material is in linear contact, so the contact area between the carbon material and the binder is large. This is considered to be because bonding between the carbon material and the binder and bonding between the current collector and the binder are strengthened.

[0008]

EXAMPLES For comparative experiments, carbon materials having different particle shapes and specific surface areas, polyvinylidene fluoride as a binder, and N-methyl-2-pyrrolidone as a solvent were mixed, and the mixture was mixed on a copper foil having a thickness of 20 μm. , A solvent was dried, and compression molding was performed by a press machine to produce a negative electrode plate. The thickness of the coating layer is such that the thickness of the coating layer after press molding is 1
The electrode was prepared so as to have a thickness of 20 μm, and the amount of the binder was a minimum value at which no peeling occurred when the electrode after press molding was wound around a φ2 mm cylinder. The electrode was press-formed to have a porosity of 30%, and then a coin cell shown in FIG. 1 was assembled using lithium metal as a counter electrode. In the coin-type cell shown in FIG. 1, a separator 5 containing an electrolytic solution is provided between a negative electrode material 3 and a positive electrode material 6, and is sealed by a positive electrode can 1 and a negative electrode current collector 2 via a gasket 4. .
As the electrolytic solution, a solution prepared by dissolving 1 mol / liter of lithium hexafluorophosphate in a mixed solvent of ethylene carbonate and diethyl carbonate was used. After discharging the cell to 0 V at a constant current corresponding to the current density of the negative electrode plate of 0.5 mA / cm 2, the cell is charged to 1.2 V at the same current, and the carbon material including the carbon material per unit weight and the binder weight is included. Was examined for discharge capacity per unit weight. Table 1 shows the results.

[0009] Spheroidal graphite, for example, has a particle size of 5
の も の 20 μm can be used. Usually, the smallest specific surface area of the obtained spheroidal graphite is about 0.5 m 2 / g, but these can be used in the present invention. Also,
Examples of the fibrous graphite include, for example, an average fiber diameter of 1 to 10 μm.
m, an average fiber length of about 10 to 100 μm can be used. Usually, the smallest specific surface area of the obtained fibrous graphite is about 1.0 m2 / g, but these can be used in the present invention.

[0010]

[Table 1]

As is apparent from Table 1, although the capacity per unit weight of the carbon material increases as the specific surface area increases, a large amount of binder is required to maintain the adhesion to the current collector. I understand. Comparing the discharge capacity per unit weight of carbon material including binder weight,
It can be seen that the difference in discharge capacity between the carbon material types becomes smaller.

Examples and Comparative Examples Negative electrode plates were prepared by using carbon materials of various shapes having a specific surface area of 2 m 2 / g or less for the lower layer and carbon material B (bulky graphite) showing the highest capacity in the comparative experiment for the upper layer. Using the same spherical graphite C and fibrous graphite D as the lower layer carbon material species as in the comparative experiment, and varying the thickness of the lower coating layer and the amount of binder to produce a coin-shaped cell in the same manner as the comparative experiment Then, the discharge capacity was examined. The thickness of the upper coating layer was adjusted so that the thickness of the coating layer after press molding was 120 μm. The amount of the binder was set to the minimum necessary value at which no peeling occurred as in the comparative experiment. The amount of binder in the lower layer was 6 wt%, and the amount of binder in the upper layer was 12 wt%. Table 2 shows the results.
The lower layer thickness in Table 2 is the thickness after press molding.

[0013]

[Table 2]

As is apparent from Table 2, when the discharge capacity per unit weight of the carbon material including the binder weight is compared, when spherical graphite C is used as the carbon material used for the lower layer, the thickness of the lower coating layer is determined. In the range of 20 μm to 80 μm (Examples Nos. 1 to 4), 304A of carbon material B (bulk graphite) showing the highest discharge capacity in the comparative experiment
A discharge capacity of 306 to 310 Ah / kg exceeding h / kg was obtained. Further, when fibrous graphite D is used as the carbon material species used for the lower layer, the thickness of the lower coating layer is 20 to
In the range of 80 μm (Example Nos. 7 to 10),
The discharge capacity was 312 to 314 Ah / kg, and more excellent characteristics were obtained as compared with 306 to 310 Ah / kg when spherical graphite C was used. Although the cause of this difference in properties is unknown, the fibrous graphite D is closely bonded to the current collector, which improves the electron conductivity in the electrode plate and facilitates the insertion and desorption of lithium ions. It is presumed to have been done. When carbon D is used, a discharge capacity equivalent to 304 Ah / kg of carbon B can be obtained even when the thickness of the lower coating layer is 100 μm. In the case where the thickness of the lower coating layer exceeds 100 μm (Comparative Examples Nos. 5, 6, and 12), characteristics exceeding the discharge capacity in the case of single-layer coating of the carbon material B (bulk graphite) cannot be obtained.

In the present invention, various shapes of carbon materials having a specific surface area of more than 2 m 2 / g may be mixed with the carbon material having a specific surface area used for the lower layer. In this case, the adhesion with the current collector decreases with an increase in the amount of addition.
The addition amount is preferably 50 wt% or less. In addition, specific surface area 2
It is not necessary to appropriately form a three-layer structure by providing an intermediate layer in which both materials are mixed between a lower layer made of only a carbon material having a capacity of m2 / g or less and an upper layer made of a high-capacity carbon material. obtain. In addition, the binder is not limited to polyvinylidene fluoride, but a polyimide resin, polystyrene rubber, polytetrafluoroethylene, or the like can be used as appropriate. Further, the cell structure is not limited to the coin type, but may be any other flat type,
It can be applied regardless of the shape of the electrode such as a cylindrical type.

[0016]

According to the present invention, a negative electrode for a lithium ion secondary battery having a coating layer made of a carbon material on the surface of a current collector has a specific surface area of 2 m 2 / g or less as a carbon material of the coating layer in contact with the current collector. By using the carbon material of the above, and by setting the thickness of the layer in the range of 20 to 100 μm, a battery having a higher capacity can be obtained as compared with a battery using only a high-capacity negative electrode material manufactured by one-layer coating. It has a remarkable effect.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a coin cell of the present invention.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 positive electrode can 2 negative electrode current collector 3 negative electrode material 4 gasket 5 separator + electrolyte 6 positive electrode material

Claims (3)

[Claims] 1. A negative electrode for a lithium ion secondary battery having a coating layer made of a carbon material on the surface of a current collector, wherein the coating layer made of a carbon material has two or more layers, and the carbon material of the coating layer in contact with the current collector is provided. Negative electrode for a lithium ion secondary battery, wherein a carbon material having a specific surface area of 2 m @ 2 / g or less is used and the thickness of the layer is in the range of 20 to 100 .mu.m. 2. The negative electrode for a lithium ion secondary battery according to claim 1, wherein fibrous carbon is used as the carbon material having a specific surface area of 2 m 2 / g or less. 3. The negative electrode for a lithium ion secondary battery according to claim 1, wherein the amount of the binder in the coating layer is 10 wt% or less based on the total weight of the coating layer.