JP2002157999A - Method of manufacturing electrode for secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method of an electrode for a secondary battery capable of forming the secondary battery for controlling diffusion of a current collector material to an active material thin film, improving adhesion of the active material thin film to a current collector, with an excellent cycle characteristic. SOLUTION: In this method of manufacturing the electrode for the secondary battery by accumulating the active material thin film on the current collector forming a surface treatment layer, the active material thin film is accumulated on a current collector surface after removing at least a part of the surface treatment layer by cleaning the current collector surface with liquid when the diffusion of the current collector material to the active material thin film is to be enhanced to the low activity of the current collector surface, and the active material thin film is accumulated on the current collector surface without cleaning the current collector surface when the diffusion of the current collector material to the active material thin film is not needed to be enhanced since the activity of the current collector surface is high.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing an electrode for a secondary battery such as a lithium secondary battery,
In particular, the present invention relates to a method for manufacturing an electrode for a secondary battery, which is manufactured by depositing an active material thin film on a current collector.

[0002]

2. Description of the Related Art In recent years, a lithium secondary battery, which has been actively researched and developed, has a charge / discharge voltage,
Battery characteristics such as charge-discharge cycle life characteristics and storage characteristics are greatly affected. For this reason, the battery characteristics have been improved by improving the electrode active material.

[0003] When lithium metal is used as the negative electrode active material, a battery having a high energy density per weight and per volume can be formed. However, there is a problem that lithium precipitates in a dendrite shape during charging and causes an internal short circuit. Was.

[0004] On the other hand, there has been reported a lithium secondary battery using aluminum, silicon, tin, or the like which electrochemically alloys with lithium during charging as an electrode. Among these, silicon is particularly promising as a negative electrode for a battery having a large theoretical capacity and exhibiting a high capacity, and various secondary batteries using this as a negative electrode have been proposed (JP-A-10-25).
No. 5768). However, this type of alloy electrode does not have sufficient cycle characteristics because the alloy itself, which is an electrode active material, is pulverized by charging and discharging and the current collecting characteristics are deteriorated.

[0005]

SUMMARY OF THE INVENTION The present applicant has formed a microcrystalline silicon thin film or an amorphous silicon thin film on a copper foil by a CVD method or a sputtering method and used this as an electrode to obtain 4000 mAh / It has already been found that a lithium secondary battery having a capacity as high as about g and having good cycle characteristics enough to withstand practical use can be obtained (Japanese Patent Application No. 11-301646).

In this electrode, it is necessary that the adhesion of the silicon thin film to the copper foil serving as the current collector is good, and such adhesion requires that the copper element from the copper foil be applied to the silicon thin film. Have been found to result from diffusion and formation of a solid solution. In addition, it has been found that when the diffusion of the copper element into the silicon thin film increases and an intermetallic compound of silicon and copper is formed, the adhesion decreases and the capacity decreases. Therefore, in manufacturing such an electrode, it is important to control the diffusion of the current collector material into the active material thin film.

An object of the present invention is to provide a secondary battery in which the diffusion of a current collector material into an active material thin film is controlled, and the adhesion of the active material thin film to the current collector is good and the cycle characteristics are excellent. It is an object of the present invention to provide a method for manufacturing a secondary battery electrode which can be used.

[0008]

According to a first aspect of the present invention, there is provided a method of manufacturing an electrode for a secondary battery, comprising: depositing an active material thin film on a current collector having a surface treatment layer formed thereon; A method of manufacturing an electrode, in order to enhance the diffusion of the current collector material into the active material thin film, a step of cleaning the current collector surface with a liquid to remove at least a part of the surface treatment layer, and after the cleaning step. Depositing an active material thin film on the surface of the current collector.

According to the first aspect of the present invention, the diffusion of the current collector material into the active material thin film is suppressed by the presence of the surface treatment layer formed by rust prevention treatment or the like. When the adhesiveness of the material thin film is not good, the diffusion of the current collector material into the active material thin film can be enhanced to obtain good adhesiveness.

A method for manufacturing a secondary battery electrode according to a second aspect of the present invention is a method for manufacturing a secondary battery electrode by depositing an active material thin film on a current collector having a surface-treated layer formed thereon. Yes, when the activity of the current collector surface is low and it is necessary to increase the diffusion of the current collector material into the active material thin film, the current collector surface was washed with a liquid to remove at least a part of the surface treatment layer. rear,
If the active material thin film is deposited on the current collector surface and the current collector surface activity is high and it is not necessary to increase the diffusion of the current collector material into the active material thin film, the current collector surface should not be washed. The method is characterized in that an active material thin film is deposited on the current collector surface.

According to the second aspect of the present invention, when the activity of the current collector surface is low and it is necessary to increase the diffusion of the current collector material into the active material thin film,
By cleaning the current collector surface with a liquid to remove at least a part of the surface treatment layer, diffusion of the current collector material into the active material thin film is enhanced. If the current collector surface has high activity and it is not necessary to increase the diffusion of the current collector material into the active material thin film, the active material thin film can be placed on the current collector surface without cleaning the current collector surface. Is deposited.

According to the second aspect of the present invention, the diffusion of the current collector material into the active material thin film can be controlled in accordance with the surface activity of the current collector used. Against, it is possible to improve the adhesion of the active material thin film,
An electrode for a secondary battery having excellent cycle characteristics can be obtained.

A current collector having a low surface activity is, for example, a metal foil finished by rolling. Examples of the metal foil finished by rolling include rolled copper foil. Since the surface of the rolled copper foil is finished in the rolling step, there are few irregularities and the surface area is small. Therefore, the surface of the rolled copper foil has low activity. In addition, the surface of the copper foil is usually subjected to a rust-preventive treatment using a known chromate or the like as disclosed in JP-A-11-158652, and a surface treatment layer is formed by the rust-preventive treatment. . For this reason, when a microcrystalline silicon thin film or an amorphous silicon thin film is formed on a rolled copper foil, diffusion of Cu in the copper foil into the silicon thin film becomes insufficient, and good adhesion may not be obtained. . In such a case, the diffusion of Cu in the copper foil to the silicon thin film can be enhanced by cleaning the surface of the copper foil with a liquid and removing at least a part of the surface treatment layer by the rust prevention treatment. Adhesion of the silicon thin film to the substrate can be improved.

A current collector having a high surface activity includes a metal foil having a surface with irregularities formed by electrolysis or etching. Electrolysis is a method of forming irregularities on the surface by depositing granular metal on the surface of a metal foil by an electrolytic plating method. Etching is a method of roughening the surface by dry etching or wet etching.

An example of the metal foil having an uneven surface formed by electrolysis is an electrolytic copper foil. Similarly to the rolled copper foil, the surface of the electrolytic copper foil is usually subjected to rust-prevention treatment by chromate, and a surface treatment layer by the rust-prevention treatment is present on the surface. By this surface treatment layer, similarly to the rolled copper foil, diffusion of Cu of the copper foil into the silicon thin film is suppressed. However, in the case of the electrolytic copper foil, such a surface treatment layer is present because the surface activity is high. However, Cu in the copper foil is sufficiently diffused into the silicon thin film, and a silicon thin film having good adhesion to the copper foil is formed.

In the case of electrolytic copper foil, if such a surface treatment layer is removed by washing with a liquid, Cu diffuses into the silicon thin film at a high concentration, and a crystalline intermetallic compound is formed. The strength of the body may be reduced, the battery capacity may be reduced, or the cycle characteristics may be reduced. Therefore, according to the second aspect of the present invention, when using such an electrolytic copper foil, it is preferable to deposit a silicon thin film on the copper foil surface without cleaning the copper foil surface.

Hereinafter, matters common to the first and second aspects of the present invention will be described as "the present invention". The liquid used for cleaning in the present invention is appropriately selected according to the surface treatment layer to be removed. For example, in the case of rust prevention treatment by chromate, a solution containing a surfactant such as a detergent, an organic solvent, and the like can be given. The detergent may be a neutral detergent or a weakly acidic or slightly alkaline detergent. Examples of the organic solvent include ethyl alcohol, acetone, and trichloroethylene. The cleaning is preferably performed by ultrasonic cleaning while applying ultrasonic waves.

The washing method is not particularly limited, and can be carried out by various methods. For example, the cleaning may be performed while the current collector is immersed in a cleaning tank containing the cleaning liquid and applying ultrasonic waves, or the cleaning liquid may be sprayed onto the current collector in a shower shape. Further, the cleaning may be performed while the cleaning liquid is caused to flow on the surface of the current collector. In these cases, it is preferable to perform cleaning while applying ultrasonic waves. Further, the cleaning may be performed while heating.

It is preferable that washing with pure water and washing with a cleaning agent are alternately repeated, and finally, washing with pure water and drying are performed. Drying is preferably performed in an atmosphere of an inert gas such as nitrogen when it is desired to prevent oxidation of the current collector surface.

In the present invention, the method for depositing the active material thin film on the current collector is not particularly limited.
A method of forming a thin film from a gas phase, such as a sputtering method, a sputtering method, a vacuum evaporation method, and a thermal spraying method, and a method of forming a thin film from a liquid phase, such as a plating method. After the formation of the thin film, if the diffusion of the current collector material into the active material thin film is insufficient, a heat treatment may be performed. Before the active material thin film is deposited, the surface of the current collector may be activated by irradiating the surface of the current collector with plasma or ions of an inert gas such as Ar or hydrogen or hydrogen.

When a silicon thin film is used as an active material thin film, a copper element (C
u). Since such Cu diffuses from the current collector surface into the silicon thin film, the entire current collector does not necessarily need to be formed from Cu or an alloy thereof, and at least the surface portion of the current collector is Cu or What is necessary is just to be formed from the alloy. For example, a current collector having a Cu layer formed on the surface of a metal foil such as a nickel foil may be used.

[0022]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in more detail with reference to specific embodiments. However, the present invention is not limited to the following embodiments, and the scope thereof is not changed. It can be implemented with appropriate changes.

[Preparation of Current Collector] As current collectors, rolled copper foil (thickness 18 μm) and electrolytic copper foil (thickness 17 μm) were prepared. Both the rolled copper foil and the electrolytic copper foil have been subjected to rust prevention treatment by chromate on the surface.

Further, as the etched rolled copper foil, one obtained by etching the surface of the rolled copper foil was prepared. The etching was performed by a spray process using a trade name "MECetchBOND CZ-8100" manufactured by Mec Co., Ltd. as an etchant at a processing temperature of 35 ° C., a spray pressure of 0.2 MPa, and an etching amount of 3 μm. Then wash with water and pickle (treatment temperature: room temperature, spray pressure:
(0.06 MPa, treatment time: 10 seconds), washed with water, and furthermore, as a rust preventive for preventing oxidation of the copper surface, trade name "MECetchBOND CL-83" manufactured by MEC Corporation.
Using a 20-fold dilution of "00", a rust-proof treatment was performed by dipping at a treatment temperature of room temperature and a treatment time of 10 seconds, followed by washing with water and drying.

[Cleaning of Current Collector Surface] The current collector was cleaned by the following steps (1) to (4). In each washing step, pure water or a neutral detergent was put in the washing tank, the current collector was immersed in the washing tank, and washing was performed by applying ultrasonic waves. As pure water,
The one having a conductivity of about 15 MΩ · cm was used. As the neutral detergent, a non-phosphorus soap solution (trade name “Pure Soft”, 5 times diluted by Iuchi Seieido) was used.

Ultrasonic cleaning with 25 ° C pure water for 10 minutes Ultrasonic cleaning with 25 ° C neutral detergent for 10 minutes Ultrasonic cleaning with 25 ° C pure water for 10 minutes Ultrasonic cleaning with 25 ° C neutral detergent for 10 minutes 25 ° C pure water Ultrasonic cleaning 10 minutes Ultrasonic cleaning with pure water at 25 ° C 10 minutes Drying in nitrogen atmosphere at 105 ° C 14 hours

[Formation of Silicon Thin Film] A microcrystalline silicon thin film was formed on the current collector after the above cleaning step. Before forming the silicon thin film, the surface of the current collector was irradiated with hydrogen plasma to perform a pretreatment. A current collector was placed on the heater in the reaction chamber, and the pressure in the reaction chamber was reduced to 40P by a vacuum exhaust device.
a (0.3 Torr). Next, a hydrogen (H ₂ ) gas was introduced into the reaction chamber so as to have a flow rate of 200 sccm, the current collector serving as a substrate was heated to 210 ° C., and hydrogen was applied under a condition of high frequency power of 20 W (high frequency power density of 62 mW / cm ² ). Plasma was generated, and this was irradiated on the current collector for 10 minutes.

Next, the temperature of the current collector as a substrate is set to 210 ° C.
Under the pressure of 40 Pa (0.3 Torr),
High frequency power 555W (high frequency power density 1713mW / c
m ² ) and a silane (SiH ₄ ) gas flow rate of 10 scc
m, the hydrogen gas flow rate was 200 sccm, and the plasma C
A microcrystalline silicon thin film was formed on the current collector by the VD method. By forming for about 180 minutes, the thickness is about 8 to 10
A microcrystalline silicon thin film of μm was formed. The substrate temperature finally rose to 260 ° C.

[Evaluation of Cu Diffusion in Electrode] With respect to each electrode obtained as described above, the appearance after forming a thin film and the state of diffusion of Cu were evaluated. Regarding the appearance after the formation of the thin film, it was evaluated whether the active material thin film was peeled off from the current collector, and color unevenness and discoloration were evaluated. When Cu diffused into the silicon thin film forms an intermetallic compound with Si, the gray of the silicon thin film becomes grayish white. Cu
Was determined from the results of analysis by SIMS and XPS described later in addition to whether or not the silicon thin film turned grayish white. Table 1 shows the results.

[0030]

[Table 1]

As is clear from Table 1, when the rolled copper foil is used as the current collector, if the silicon thin film is formed without performing the cleaning step, the adhesion between the silicon thin film and the current collector becomes insufficient. The silicon thin film has peeled off. In contrast,
When the cleaning step was performed, such peeling was not observed. Therefore, when the rolled copper foil is used as the current collector, Cu from the current collector is easily diffused into the silicon thin film by removing the surface treatment layer by the rust prevention treatment on the surface of the rolled copper foil by washing. It is considered that the adhesion was improved.

When an electrolytic copper foil is used as a current collector, its surface is active.
It seems that u has sufficiently diffused into the silicon thin film. When cleaning is performed to remove at least a part of the surface treatment layer, it is considered that Cu excessively diffuses into the silicon thin film and an intermetallic compound is formed.

Also, when the rolled copper foil thus etched is used as a current collector, it is considered that excessive Cu diffuses into the silicon thin film by washing as in the case of the electrolytic copper foil. Therefore, it is understood that it is preferable to form a silicon thin film without performing cleaning on the electrolytic copper foil or the etched rolled copper foil used in the above experiment.

[Analysis by SIMS and XPS] The electrodes a1 and a4 shown in Table 1 were analyzed by SIMS.
Using O ₂ ⁺ as a sputtering source, the concentration distribution of copper element (Cu ⁺ ) in the depth direction of the silicon thin film was measured. The thickness of the silicon thin film at the electrode a1 is about 8 μm, and the thickness of the silicon thin film at the electrode a4 is about 10 μm.

FIG. 1 shows a SIMS profile for the electrode a1, and FIG. 2 shows a SIMS profile for the electrode a4. As is clear from FIG. 1, in the electrode a1 used as a current collector after washing the rolled copper foil, a diffusion region having a thickness of about 1 to 2 μm exists at the interface between the silicon thin film and the current collector. I have. Further, in the portion above the diffusion region (surface-side portion), almost no copper element was detected, indicating that excessive diffusion was not occurring.

Further, in the electrode a4 used as a current collector without washing the electrolytic copper foil, as shown in FIG. 2, the interface portion between the silicon thin film and the current collector has a thickness of about 3 to 4 μm. It can be seen that the diffusion region A exists. Also, the diffusion region A
It can be seen that almost no copper element was detected in the upper portion, and no excessive diffusion was observed. In addition, in the region B near the current collector, since the surface of the electrolytic copper foil has irregularities, it can be seen that a high concentration of copper is detected.

Next, in order to measure the concentration distribution near the interface between the silicon thin film and the current collector by XPS, an electrode having a thin silicon thin film was manufactured. Specifically, the same electrolytic copper foil as described above was used as a current collector, and two types of current collectors were prepared by washing the electrolytic copper foil and by not cleaning the electrolytic copper foil. Under the same conditions, a microcrystalline silicon thin film having a thickness of about 2 μm was formed. About the obtained electrode, X
By PS, silicon (Si), copper (C
The concentrations (existence ratio) of u), carbon (C), and (O) were measured.

FIG. 3 is an XPS profile of an electrode used without cleaning the electrolytic copper foil, and FIG. 4 is an XPS profile of an electrode used after cleaning the electrolytic copper foil. 3 and 4, the horizontal axis indicates the time of sputtering the surface of the silicon thin film, and indicates the depth of the silicon thin film. The vertical axis indicates the abundance ratio of each constituent element.

As is apparent from FIG. 3, in the electrode used as the current collector without cleaning the electrolytic copper foil, a diffusion region exists at the interface between the silicon thin film and the current collector. In addition, copper (Cu) is not detected in the portion above the diffusion region, and it can be seen that excessive diffusion has not occurred.

On the other hand, as shown in FIG. 4, in the electrode used as a current collector by cleaning the electrolytic copper foil, high concentration copper (Cu) was detected in a wide area, and in the surface area, Also, copper (Cu) is detected. Therefore, copper (C
It can be seen that u) is excessively diffused.

A lithium secondary battery was manufactured using the electrode a1 shown in Table 1, and the charge and discharge cycle characteristics were evaluated. As a result, good cycle characteristics were obtained. For the electrodes a2 to a6, a lithium secondary battery could not be manufactured because the silicon thin film was peeled off or became brittle as an electrode.

[0042]

According to the present invention, since the diffusion of the current collector material into the active material thin film can be controlled, the adhesion of the active material thin film to the current collector is good, and the cycle characteristics are excellent. It can be used as an electrode for a secondary battery.

[Brief description of the drawings]

FIG. 1 is a SIM showing a concentration distribution of a copper element in a silicon thin film in an electrode used as a current collector after washing a rolled copper foil.
S profile.

FIG. 2 is an SI showing a concentration distribution of copper element in a silicon thin film in an electrode used as a current collector without cleaning an electrolytic copper foil.
MS profile.

FIG. 3 is an XPS profile showing a concentration distribution of each constituent element in a silicon thin film in an electrode used as a current collector without cleaning an electrolytic copper foil.

FIG. 4 is a graph showing the concentration distribution of each constituent element in a silicon thin film in an electrode used as a current collector after washing an electrolytic copper foil.
PS profile.

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 5H017 AA03 AS02 BB16 CC01 DD01 EE01 5H050 AA07 AA14 BA16 CB11 DA03 DA07 FA15 FA19 FA20 GA12 GA14 GA17 GA18 GA24 GA25 GA27

Claims (9)

[Claims] 1. A method of manufacturing an electrode for a secondary battery by depositing an active material thin film on a current collector having a surface treatment layer formed thereon, comprising: diffusing the current collector material into the active material thin film. Cleaning the current collector surface with a liquid to remove at least a part of the surface treatment layer, and depositing the active material thin film on the current collector surface after the cleaning step. A method for producing an electrode for a secondary battery comprising: 2. A method of manufacturing an electrode for a secondary battery by depositing an active material thin film on a current collector having a surface treatment layer formed thereon, wherein the current collector has low surface activity. When it is necessary to increase the diffusion of the material into the active material thin film, the surface of the current collector is washed with a liquid to remove at least a part of the surface treatment layer, and then the surface of the current collector is removed. When an active material thin film is deposited, the activity of the current collector surface is high, and it is not necessary to increase the diffusion of the current collector material into the active material thin film, without cleaning the current collector surface, A method for manufacturing an electrode for a secondary battery, comprising depositing the active material thin film on a current collector surface. 3. The method for producing an electrode for a secondary battery according to claim 2, wherein the current collector having a low surface activity is a metal foil finished by rolling. 4. The method for producing an electrode for a secondary battery according to claim 2, wherein the current collector having a high surface activity is a metal foil having a surface having irregularities formed by electrolysis or etching. 5. The diffusion of the current collector material into the active material thin film as long as no intermetallic compound of the active material material and the current collector material is formed. The method for producing an electrode for a secondary battery according to any one of the above. 6. The electrode for a secondary battery according to claim 1, wherein the surface of the current collector is irradiated with plasma or ions before depositing the active material thin film. Manufacturing method. 7. The method for producing an electrode for a secondary battery according to claim 1, wherein the surface treatment layer is a surface treatment layer obtained by rust prevention treatment. 8. The method according to claim 1, wherein the active material thin film is a thin film containing Si as a main component. 9. At least a surface portion of the current collector is Cu
Or the alloy thereof, and the current collector material diffused into the active material thin film is Cu.
9. The method for producing an electrode for a secondary battery according to any one of the above items 8.