JP2003303586A - Electrode for secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrode for a secondary battery capable of inhibiting the generation of wrinkle on a collector upon charging and discharging, and having superior charging and discharging cycle characteristic and high volumetric energy density. <P>SOLUTION: In this electrode for the secondary battery having a thin film composed of an active material and accumulated on the collector 1, a plurality of pole-shaped projecting parts 3 of thick film thickness are formed on the periphery of the thin film, and a cross-sectional area in the film face direction of a bottom part of one of the pole-shaped projecting parts 3 is 2×10<SP>-7</SP>m<SP>2</SP>or less, preferably, 7×10<SP>-10</SP>m<SP>2</SP>or more. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an electrode for a secondary battery such as a lithium secondary battery.

[0002]

2. Description of the Related Art An electrode for a lithium secondary battery in which a silicon-based thin film is formed on a current collector by a CVD method or a sputtering method is known. It is known that in such an electrode, repeated charging and discharging causes wrinkles in the current collector and an increase in the apparent thickness of the electrode, which reduces the volume energy density.

The cause of wrinkling of the current collector due to charge / discharge cycles is considered as follows. That is, for example, when the active material is silicon, silicon reacts with lithium during charging to form a compound. When silicon reacts completely with lithium, its volume increases about four times as much as before charging. Due to this volume expansion, a large stress is generated at the interface between the current collector and the silicon thin film, and this stress forms wrinkles in the current collector.

[0004]

As a method of preventing wrinkles from occurring in such a current collector, a plurality of columnar protrusions are formed on a thin film, and voids are secured around the columnar protrusions. A method is conceivable in which volume expansion is accommodated in this void so that large stress is not applied to the current collector. However, in order to reduce the stress applied to the current collector and suppress the generation of wrinkles, the size of the columnar convex portion and the size of the void around the convex portion have not been specifically examined.

An object of the present invention is to provide an electrode for a secondary battery, which can suppress the generation of wrinkles of a current collector due to charge / discharge, has excellent charge / discharge cycle characteristics, and has a high volume energy density. .

[0006]

The present invention is an electrode for a secondary battery formed by depositing a thin film of an active material on a current collector, and a plurality of pillars having a film thickness thicker than the periphery of the thin film. The protrusion is formed, and the cross-sectional area in the film surface direction at the bottom of one of the columnar protrusions is 2 × 10 ⁻⁷ m ² or less.

In the present invention, the cross-sectional area in the film surface direction at the bottom of one columnar convex is preferably 7 × 10 ^-10 m ^2.
That is all. By setting the cross-sectional area in the film surface direction at the bottom of one of the columnar protrusions to the above range, it is possible to suppress the generation of wrinkles in the current collector due to charge / discharge, and to secure a large charge / discharge capacity, The volume energy density can be increased.

In the present invention, the area ratio S1 of the cross-sectional area in the film surface direction at the bottom of the columnar convex portion to the electrode area (total area of the region of the columnar convex portion and the region other than the columnar convex portion) is , 0.2 ≦ S1 ≦ 0.8 is preferable. By setting the content within such a range, it is possible to suppress the generation of wrinkles in the current collector due to charge and discharge, and it is possible to obtain a high volume energy density.

In the present invention, when the average film thickness of the thin film in the region of the columnar convex portion is β and the average film thickness of the thin film in the region other than the columnar convex portion is α, the ratio α / β is 0 ≦.
It is preferable that α / β ≦ 0.6. By setting the content within such a range, it is possible to suppress the generation of wrinkles in the current collector due to charge and discharge, and it is possible to obtain a high volume energy density.

The method of forming the columnar convex portions of the thin film in the present invention is not particularly limited. For example, the columnar protrusion may be formed by a cut formed in the thickness direction of the thin film due to the expansion and contraction of the thin film. Alternatively, a columnar convex portion may be formed by disposing a mesh having a predetermined pore size on the current collector and depositing an active material on the current collector through the mesh.

The columnar protrusions may be formed by a lift-off process when depositing a thin film. That is, a pattern that dissolves in an alkaline solution or the like is formed in a region other than the columnar convex portion, a thin film is formed on the pattern, and then the pattern is removed using an alkaline solution or the like to form the columnar convex portion. You may.

Alternatively, a thin film of the active material may be uniformly formed on the current collector in advance, and the columnar protrusions may be formed on the thin film. In the present invention, the method of depositing the thin film is not particularly limited, and for example, a CVD method, a sputtering method, a vacuum vapor deposition method or the like can be used.

In the present invention, the average film thickness β of the thin film in the region of the columnar convex portion is preferably about 1 to 20 μm. The active material used in the present invention is not particularly limited, and the present invention can be effectively used for an active material whose volume expands and contracts due to charge and discharge. In the case of a lithium secondary battery, a substance that occludes lithium by alloying with lithium is preferably used. Examples thereof include silicon, germanium, tin, lead, zinc, magnesium, sodium, aluminum, potassium and indium. Among these, silicon is particularly preferably used because of its high theoretical capacity. The silicon is preferably amorphous silicon or microcrystalline silicon.

In the present invention, a thin film made of an active material may be deposited on both surfaces of the current collector.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in more detail based on the following examples, but the invention is not intended to be limited to the following examples, and may be appropriately modified within the scope of the invention. It is possible to carry out.

Example 1 [Production of Electrode] Electrolytic copper foil (thickness: 26 μm) as a current collector
Was used. The pattern shown in FIG. 3 was printed on the current collector using an ink that was previously dissolved in an alkaline solution. In the pattern shown in FIG. 3, the ink does not exist in the circular portion, and the ink exists in the area other than the circular portion.
The radius of the circular portion is r, and the distance between the centers of the circular portions is a. As the pattern, eight kinds of patterns having r and a as shown in Table 1 were formed. The pattern thickness was 20 μm.

A silicon thin film, which is an active material, was formed on the current collector on which the pattern was formed as described above, as follows. Using 4-inch single crystal silicon as a target, an active material was deposited on the current collector by an RF magnetron sputtering device. The current collector was fixed to a rotary drum provided in the vacuum chamber, and the inside of the vacuum chamber was evacuated to 8 × 10 ⁻⁴ Pa or less.
Next, sputtering was started while introducing argon gas from the inlet at a flow rate of 50 sccm. RF power is 35
It was set to 0W. The thin film was formed to have a thickness of 10 μm.

After forming a silicon thin film on one surface of the current collector as described above, 1% by weight NaO heated to 50 ° C.
The pattern on the current collector was dissolved by immersing it in the H 2 aqueous solution for 5 minutes. As a result, the silicon thin film formed on the pattern was removed, and columnar protrusions having a columnar shape were formed on the current collector. It was confirmed by observing with an optical microscope that the columnar protrusions thus formed had a shape corresponding to a predetermined pattern.

The current collector on which the thin film having columnar convex portions was formed as described above was cut into a size of 2 cm × 2 cm, and electrodes 1 to 8 were obtained as shown in Table 1. As a comparative example, a current collector not printed with the above pattern was used, and a silicon thin film was formed to a thickness of 6 μm in the same manner as above, and an electrode of the comparative example was obtained in the same manner as above.

[Measurement of Charging / Discharging Characteristics] A test cell was prepared by using the electrodes 1 to 8 obtained above and the electrodes of the comparative examples as working electrodes. Lithium metal was used as the counter electrode and the reference electrode. In addition, as the electrolytic solution, LiP was added to an equal volume mixed solvent of ethylene carbonate and dimethyl carbonate.
An electrolytic solution in which F ₆ was dissolved at 1 mol / liter was used. In this unipolar test cell, the reduction of the working electrode is charged,
Oxidation is used as electric discharge.

A charge / discharge test was carried out at 25 ° C. for each of the above test cells. The electrodes 1 to 8 were charged with a constant current of 4 mA, and the electrodes of the comparative examples were charged with a constant current of 2 mA until the potential based on the reference electrode reached 0 V.
It discharged until it reached 2.0V. This was set as one cycle of charge / discharge, and 10 cycles of charge / discharge were performed. The discharge capacity at the 10th cycle and the electrode thickness at the 10th cycle were measured. The electrode thickness was measured with a micrometer.

Table 1 shows the sizes of r and a of the pattern used to manufacture the electrodes 1-8.

[0023]

[Table 1]

S1 shown in Table 1 indicates the ratio of the areas of the circular portions in the pattern. S1 can be obtained from the following formula. ^{S1 = πr 2 / (√3a 2} /2) N in S1 / N in Table 1 shows the number of the circular portion per 1 m ^2. N can be calculated by the following formula.

[0025] ^{N = 1 / (√3a 2/} 2) × 10 -12 Accordingly, S1 / N can be calculated by the following equation. S1 / N = πr ² × 10 ^-12 As described above, the columnar convex portion formed on the current collector is formed corresponding to the circular portion of the pattern. Therefore, the radius of the columnar protrusions formed on the current collector is r, and the distance between the centers of the columnar protrusions is a. Therefore, the cross-sectional area in the film surface direction at the bottom of the columnar projection corresponds to the area of the circular portion of the pattern, and S1 is the cross-sectional area in the film surface direction at the bottom of the columnar projection with respect to the electrode area. It becomes the area ratio. Further, N is the number of columnar protrusions per 1 m ² . Therefore, S1 / N represents the cross-sectional area (unit: m ² ) in the film surface direction at the bottom of one columnar protrusion.

Table 2 shows the discharge capacity at the 10th cycle of the test cell. Table 2 shows the electrode thickness after 10 cycles, the electrode thickness increase rate, and the electrode thickness increase rate specified by the discharge capacity.

In Table 2, the electrode thickness after the charge / discharge cycle is the electrode thickness after 10 cycles. The electrode thickness increase rate is a value obtained by subtracting the electrode thickness before charging from the electrode thickness after 10 cycles. The electrode thickness of the electrodes 1 to 8 before charging is
All were 36 μm. The electrode thickness of the comparative electrode before charging was 32 μm.

The electrode thickness increase rate normalized by the discharge capacity is
It is a value obtained by dividing the above electrode thickness increase rate by the discharge capacity.

[0029]

[Table 2]

FIG. 4 is a diagram showing the relationship between S1 shown in Table 2 and the electrode thickness increase rate normalized by the discharge capacity. As is apparent from FIG. 4, the electrode thickness increase rate normalized by the discharge capacity is 100% / mAh when S1 is 0.2 ≦ S1.
This is when ≦ 0.8. Therefore, by setting S1 in such a range, it is possible to suppress the generation of wrinkles in the current collector due to charge and discharge, and to provide a secondary battery electrode having a high volume energy density.

As is clear from FIG. 4 and Table 1, S
1 / N, that is, by setting the cross-sectional area in the film surface direction at the bottom of one columnar protrusion to 2 × 10 ⁻⁷ m ² or less, the generation of wrinkles of the current collector due to charge / discharge is suppressed, and the volume energy density is reduced. Can be increased. Further, it can be seen that it is preferable to set the cross-sectional area in the film surface direction at the bottom of one columnar convex portion to 7 × 10 ⁻¹⁰ m ² or more.

Example 2 [Production of Electrode] A current collector similar to that in Example 1 was used, and a thickness of 3 μm was previously formed on the current collector under the same conditions as in Example 1.
A silicon thin film to be a base layer having a thickness of 5 μm, 5 μm, and 8 μm was formed. Then, the same pattern as the electrode 4 shown in Table 1 was printed on this silicon thin film in the same manner as in Example 1. Furthermore, a silicon thin film was formed on this under the same conditions as in Example 1. The total thickness of the silicon thin film is 10
It was formed to have a thickness of μm. Therefore, for a silicon thin film having a thickness of 3 μm formed as the underlayer, a silicon thin film having a thickness of 7 μm is formed and a thickness of 5 μm is used as the underlayer.
The thickness of the silicon thin film of m is 5μ
In the case where a silicon thin film having a thickness of 8 μm was formed as a base layer and a silicon thin film having a thickness of 8 μm was formed as a base layer, a silicon thin film having a thickness of 2 μm was formed.

Then, in the same manner as in Example 1, the pattern was removed by immersing in an alkaline aqueous solution to form columnar convex portions. In the same manner as in Example 1, the pieces were cut into a size of 2 cm × 2 cm to form electrodes 9 to 11.

FIG. 1 is a schematic sectional view showing an electrode formed as described above. A silicon thin film as a base layer 2 is formed on the current collector 1, and a silicon thin film to be the columnar convex portions 3 is formed on the base layer 2. α represents the average film thickness of the silicon thin film of the underlayer 2, and β represents the average film thickness of the silicon thin film in the region of the columnar convex portion. β is a value obtained by adding the average film thickness of the base layer 2 and the average film thickness of the columnar convex portions 3. 3 a indicates the bottom of the columnar protrusion 3. The cross-sectional area of the bottom portion 3a in the film direction is S1 / N described above.

[Measurement of Charge / Discharge Characteristics] Using the electrodes 9 to 11 as working electrodes, a test cell was prepared in the same manner as in Example 1 and a charge / discharge test was conducted. In the same manner as in Example 1, the electrode thickness increase rate standardized by the discharge capacity was obtained for each electrode and is shown in Table 3. It should be noted that Table 3 also shows the values of the electrode 4 in Example 1 and the comparative example.

[0036]

[Table 3]

FIG. 5 is a diagram showing the relationship between α / β shown in Table 3 and the electrode thickness increase rate normalized by the discharge capacity. Figure 5
As is clear from the above, when α / β is in the range of 0 ≦ α / β ≦ 0.6, it is possible to suppress the generation of wrinkles of the current collector due to charge / discharge and to increase the volume energy density. You can Further, it is more preferable that α / β is within the range of 0 ≦ α / β ≦ 0.5.

In the above embodiment, the columnar convex portions are formed by the lift-off process, but the present invention is not limited to this. Further, the shape of the columnar protrusion is not particularly limited, and may be, for example, a truncated cone-shaped columnar protrusion 3 as shown in FIG. In such a case, it suffices that the cross-sectional area of the bottom portion 3a of the truncated cone-shaped columnar protrusion 3 in the film surface direction be within the range of the present invention.

Further, in the above embodiment, the silicon thin film is exemplified as the thin film of the active material, but the present invention is not limited to this, and an active material thin film whose volume expands and contracts by charging and discharging is used. On the other hand, the present invention can be applied.

Further, in the above embodiment, an example of a lithium secondary battery is shown, but the present invention can be applied to an electrode which occludes and releases an alkali metal and an alkaline earth metal other than lithium. It is a thing.

[0041]

EFFECTS OF THE INVENTION According to the present invention, it is possible to suppress the generation of wrinkles in the current collector due to charging / discharging, to provide a secondary battery electrode having excellent charge / discharge cycle characteristics and high volume energy density. it can.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view showing an electrode for a secondary battery of an example according to the present invention.

FIG. 2 is a schematic cross-sectional view showing an electrode for a secondary battery of another embodiment according to the present invention.

FIG. 3 is a plan view showing a pattern for forming a columnar convex portion used in an example of the present invention.

FIG. 4 is a diagram showing a relationship between S1 and an electrode thickness increase rate normalized by a discharge capacity in an example of the present invention.

FIG. 5 is a diagram showing a relationship between α / β and an electrode thickness increase rate normalized by a discharge capacity in an example of the present invention.

[Explanation of symbols]

1 ... Current collector 2 ... Thin film of active material to be the underlying layer 3 ... Columnar convex part 3a ... Bottom of columnar protrusion α: Average thickness of underlayer β: average thickness of thin film in the region of columnar protrusion

   ─────────────────────────────────────────────────── ─── Continued front page    F-term (reference) 5H029 AJ03 AJ05 AL11 AM03 AM05                       AM07 CJ11 CJ24 CJ25 CJ28                       DJ14 DJ18 HJ04 HJ07                 5H050 AA07 AA08 BA17 CB11 FA10                       FA15 FA20 GA24 GA25 GA27                       HA04 HA07

Claims (6)

[Claims] 1. An electrode for a secondary battery formed by depositing a thin film of an active material on a current collector, wherein the thin film has a plurality of columnar protrusions having a thickness thicker than its surroundings. An electrode for a secondary battery, wherein the cross-sectional area in the film surface direction at the bottom of one of the columnar protrusions is 2 × 10 ⁻⁷ m ² or less. 2. The electrode for a secondary battery according to claim 1, wherein a cross-sectional area in the film surface direction at the bottom of one of the columnar protrusions is 7 × 10 ⁻¹⁰ m ² or more. 3. The area ratio S1 of the cross-sectional area in the film surface direction at the bottom of the columnar protrusion to the electrode area is 0.
The electrode for a secondary battery according to claim 1 or 2, wherein the range is 2 ≦ S1 ≦ 0.8. 4. The ratio α / β when the average film thickness of the thin film in the region of the columnar convex portion is β and the average film thickness of the thin film in the region other than the columnar convex portion is α, 0 ≦ α / β ≤0.6
The secondary battery electrode according to any one of claims 1 to 3, wherein the electrode is for a secondary battery. 5. The electrode for a secondary battery according to claim 1, wherein the columnar convex portion is formed by a lift-off process when depositing the thin film. 6. The secondary battery electrode according to claim 1, wherein the active material contains silicon as a main component.