JP2006190514A - CURRENT COLLECTOR FOR LITHIUM SECONDARY BATTERY ANODE FOR Si- AND Sn-BASED ACTIVE MATERIALS AND ITS MANUFACTURING METHOD AS WELL AS LITHIUM SECONDARY BATTERY USING THE CURRENT COLLECTOR - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a current collector for a lithium secondary battery anode with its adhesion with Si-system and Sn-based active materials well improved, and with excellent charge/discharge cycle characteristics. <P>SOLUTION: The collector for the lithium secondary battery anode for Si-based and Sn-based active materials has a copper electrocoating layer taking on a particulate shape, with its particles swollen toward the surface side, with a swollen part of a diameter of 0.5 to 5 μm, and with the number of particles distributed at a density of 500,000 to one million pieces per mm<SP>2</SP>. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a current collector for a negative electrode of a lithium secondary battery for Si-based and Sn-based active materials.

  As a negative electrode for a lithium secondary battery using a non-aqueous electrolyte, metal lithium, a lithium alloy, or a carbon material capable of inserting and extracting lithium has been conventionally known. A lithium negative electrode has a feature of a large charge / discharge capacity, but has a problem of short circuit due to the growth of dendrite, and a carbon material is generally used. However, a negative electrode using a carbon material does not have a short-circuit problem due to the growth of dendrites and is excellent in safety like a lithium negative electrode, but the usable current density is low and the charge / discharge capacity is not sufficient. In addition, the advent of negative electrode materials that can be used for a long time with a single charge is desired.

  As such a negative electrode material, a material using a simple substance such as Si or Sn or an intermetallic compound thereof has been proposed (for example, Patent Document 1 (Japanese Patent Laid-Open No. 2002-83594)). However, when an Sn or Si-based material is used as the active material of the negative electrode material, there is a problem that the active material is pulverized / dropped off due to volume expansion during charging, and the cycle life is short. For this problem, Patent Document 2 (Japanese Patent Application Laid-Open No. 2002-313319) proposes to roughen the current collector. In this proposal, the surface of the current collector is roughened, thereby forming a gap that becomes wider in the thickness direction of the silicon thin film provided by sputtering on the surface of the current collector toward the concave and convex valleys of the rough surface. A wide gap is maintained in the vicinity of the current collector surface, and this gap absorbs changes in volume expansion and contraction of the active material due to charge and discharge.

Moreover, in patent document 3 (Unexamined-Japanese-Patent No. 2002-319408), the copper foil surface was burnt-plated, granular copper was made to adhere, and also this granular copper was covered with the dense plating copper layer, and was plated. There has been proposed a method in which an active material is formed on the surface in a state of being separated into columns by using a current collector. According to this method, it is said that the stress due to expansion and contraction of the active material accompanying the charge / discharge cycle can be relieved by the gaps in the columnar portions.
JP 2002-83594 A JP 2002-313319 A JP 2002-319408 A

However, in the above-mentioned Patent Document 2, with respect to the roughened state, electrolytic copper plating is performed on the current collector, and the surface roughness Ra and Ry are merely disclosed, and more favorable performance with respect to battery characteristics and the like. It is not clear about the detailed roughening conditions for exhibiting.
Further, in the method proposed in Patent Document 3, in order to sufficiently obtain the effect of roughening, when the amount of electrodeposition is increased to increase the number of particles, the adhesion with the substrate becomes insufficient, and the copper powder is The problem of dropping out occurs, and satisfactory results cannot be obtained.
Under such circumstances, the present invention provides a current collector for a lithium secondary battery negative electrode having excellent charge / discharge cycle characteristics, in which the adhesion between the current collector and the Si-based and Sn-based active materials is sufficiently improved. It is for the purpose.

  As a result of intensive studies, the present inventors have provided a copper electrodeposition layer on the surface of the current collector, and formed the surface state so as to satisfy specific conditions. It has been found that the adhesion with the active material can be improved and charge / discharge characteristics can be improved, and the present invention has been achieved.

That is, the present invention
(1) A negative electrode material for a lithium secondary battery having a copper electrodeposition layer on a current collector, wherein the copper electrodeposition layer has a granular shape, and the particle has a bulge toward the surface side, and the bulge The diameter of the part is 0.5 to 5 μm, and the number of particles is distributed at a density of 500,000 to 1,000,000 particles / mm ^2. Current collector for secondary battery negative electrode,
(2) The lithium for Si-based and Sn-based active materials according to (1) above, wherein the copper electrodeposited layer has an average particle size of 1 μm or more and a standard deviation of the particle size of 0.05 to 0.8. A secondary battery negative electrode current collector,
(3) The Si-based and Sn-based active material according to (1) or (2) above, wherein the bulge portion has a diameter of 0.5 to 5 μm and an average distance between grains of 1.0 μm or more. Current collector for lithium secondary battery negative electrode,
(4) After forming granular copper with a metal foil as a cathode under the plating conditions, the plating conditions are changed to smooth plating conditions through an intermediate stage of plating while changing the plating conditions toward the smooth plating conditions continuously or stepwise. The manufacture of a current collector for a negative electrode of a lithium secondary battery for Si-based and Sn-based active materials according to any one of the above (1) to (3), wherein smooth plating is performed so as to prevent detachment of the material Method,
(5) The current collector for a negative electrode of a lithium secondary battery for Si-based and Sn-based active materials according to any one of (1) to (3) manufactured by the method according to (4),
(6) Lithium secondary using the current collector for the negative electrode of the lithium secondary battery for Si-based and Sn-based active materials according to any one of (1), (2), (3) and (5) battery,
About.

  By using the current collector of the present invention, a lithium secondary battery excellent in charge / discharge capacity and charge / discharge cycle life can be obtained.

In the present invention, the current collector has a copper electrodeposition layer, and the surface state is important. That is, the copper electrodeposition layer has a granular shape and has a bulge toward the surface side. The bulge diameter is 0.5 to 5 μm, preferably 1 to 3 μm, and the number of particles is 500,000 to 1, It is important that they are distributed at a density of 000,000 pieces / mm ² , preferably 700,000 to 900,000 pieces / mm ² .

When the diameter of the bulge is less than 0.5 μm, the electrodeposited particles are densely distributed, and when the active material layer is formed, the gap between the convex portions is insufficient. Can not absorb the change of expansion and contraction. On the other hand, if the thickness exceeds 5 μm, when the active material layer is formed, the convex portion itself is too large, and the entire convex portion falls off due to expansion / contraction during charging / discharging.
If the number of particles is less than 500,000, the adhesion of the active material layer is poor, and the active material layer falls off due to expansion / contraction during charge / discharge. When the number of particles exceeds 1,000,000, when the active material layer is formed, the gap between the convex portions is insufficient, so that the change in volume expansion / contraction of the active material during charge / discharge can be absorbed. Absent.

Furthermore, it is preferable that the average particle diameter of the copper electrodeposition layer is 1 μm or more and the standard deviation of the particle diameter is 0.05 to 0.8. A more preferable standard deviation of the particle diameter is 0.05 to 0.5. The average distance between grains is preferably 1.0 μm or more.
When the average particle size of the copper electrodeposition layer is less than 1 μm, the electrodeposited particles are densely distributed, and when the active material layer is formed, the gap between the convex portions is insufficient, so that the active material during charging and discharging It cannot absorb the change of volume expansion and contraction.
The standard deviation of the particle size has a certain degree of variation and cannot be less than 0.05, and if it exceeds 0.8 μm, coarse electrodeposited particles are formed. It will fall off every time.

Next, the manufacturing method of the current collector of the present invention will be described.
The roughening treatment of the metal foil such as copper foil for printed wiring boards is performed by forming fine copper particles on the metal foil using a copper sulfate solution under the plating conditions, and then covering the fine copper particles under the smooth plating conditions and plating. In general, the plating is performed in two stages of preventing the fine copper particles from falling off. Also in the present invention, it can be produced by such a method. However, it is important to control the plating operation so as to form the surface state of the specific copper electrodeposition layer so as to be compatible with the Si-based and Sn-based active materials.

However, although the granular copper plating obtained by such a method has no problem when the granular copper is small, the adhesion with the current collector may be insufficient when the granular copper is large. In order to further improve the adhesion, it is more preferable to perform plating as follows. That is,
In the present invention, the plating is first performed under the burn plating conditions, and then the plating is performed while changing the burn plating condition from the burn plating step to the smooth plating condition side continuously or gradually. By performing the step, a copper electrodeposition layer having a predetermined surface state can be obtained with sufficient adhesion.

In the present invention, as a plating bath for forming a predetermined granular copper plating on the current collector, a copper sulfate bath, a copper pyrophosphate bath and the like are generally used. It can be performed under the conditions of 30 g / l, sulfuric acid 50 to 200 g / l, liquid temperature 20 to 50 ° C., and current density 5 to 50 A / dm ² .
In addition, for smooth plating used for so-called covering plating for preventing dropping of the deposited granular copper, a copper sulfate bath, a copper pyrophosphate bath or the like is generally used as a plating bath. In the case of using a bath, it can be carried out by plating under higher copper concentration conditions, higher liquid temperature, and lower current density conditions than burn plating. For example, copper 20-50 g / l, sulfuric acid 50-200 g / l, liquid temperature 30-50. It can be performed under the conditions of ° C. and current density of ^{2 to} 30 A / dm ² .

The change of the plating condition from the burn plating condition to the smooth plating condition can be carried out by increasing the copper concentration or decreasing the current density as described above. Specifically, for example, the following method is possible. It is.
(1) The current is gradually reduced from the burnt plating stage.
(2) Prepare a plurality of plating tanks, perform burnt plating in the first plating tank, and then gradually increase the copper concentration (or plating solution temperature) in the plating solution. Continue plating in the plating tank.
(3) When the coiled substrate is continuously processed, the distance between the anode and the anode is gradually increased from the inlet side to the outlet side to decrease the current density.
The current collector of the present invention thus obtained is particularly suitable as a current collector for a lithium secondary battery negative electrode using Si-based and Sn-based active materials as active materials.
For example, Si or an Si alloy can be used as the Si-based active material.
As the Sn-based active material, for example, Sn or an Sn alloy can be used.

  The current collector (metal foil) provided with the copper electrodeposition layer used in the present invention is preferably selected from Cu, Cu alloys, Ni, Ti, etc., which are inert to electrode reactions and have high electrical conductivity. Among these, Cu or Cu alloy in which Cu in the base material and Sn in the plating layer are diffused by heat treatment to form an intermetallic compound is preferable. Among Cu or Cu alloys, it is a precipitation hardening type Cu alloy that is excellent in strength and heat resistance, among which Ni 2.0-4.0 mass%, Si 0.5-1.0 mass%, and further Mg, Zn, One or more selected from Sn, P, Fe, and Ag is optionally contained in an amount of 0.005 to 1.0% by mass, the remainder being Cu and an inevitable impurity Cu alloy, or Cr of 0.1 to 1.0% by mass. %, Zr 0.05-0.4% by mass, and further containing one or more selected from Fe, Ti, Ni, P, Sn, Zn as needed, 0.005-1.0% by mass, and the balance Cu And Cu alloy which is an inevitable impurity is more preferable.

In order to form the Si-based active material or the Sn-based active material layer on the current collector provided with the copper electrodeposition layer having the specific surface state of the present invention, there is a plating method, a sputtering method, or the like.
A Si-based or Sn-based active material layer including a minute convex portion including a curved surface can be formed by adhering the active material to the granular copper having a specific surface state by a plating method, a sputtering method, or the like.
Preferably, the convex portions are formed so as to have a gap therebetween.
Preferably, when the gap between the minute convex portions is expressed by a porosity described later, it is preferably 10 to 60%.
The porosity is measured from the cross-sectional observation of the total thickness (t0) calculated from the theoretical volume obtained by dividing the contents of Sn and Cu in the deposited Sn alloy plating film by the specific gravity of each pure metal. The actual plating layer thickness (t) was calculated by the following formula.
Porosity = (1− (t0 / t)) × 100 (%)
By providing such voids in the structure of the Si-based and Sn-based active material layers on the outermost surface of the current collector, it is possible to absorb and relax the expansion stress when lithium is occluded in the negative electrode during lithium secondary battery discharge. It is possible to prevent detachment of the convex portion in the active material layer and peeling between the active material layer and the roughened Cu plating layer.

  As an Sn alloy as the Sn-based active material, Cu, Ni, Co, and Fe can be used as an alloy component, but an alloy with Cu is particularly preferable, and an alloy containing Cu with 30 to 45 mass% of Sn is particularly preferable. preferable. Moreover, it is especially preferable to have a Cu6Sn5 phase.

  In addition, as the Si alloy as the Si-based active material, Cu, Ni, or the like can be used as an alloy component. Cu is alloyed with Si, and the adhesion between the current collector and the active material is improved, which is more preferable.

In this invention, it is preferable that the Sn alloy plating film which has a micro convex part including a curved surface at least has the particle size of the micro convex part of 1-20 micrometers.
If the particle size is smaller than 1 μm, the particle spacing is fine and the porosity is small, the expansion during charge / discharge cannot be relaxed, the Sn alloy is pulverized and dropped, and the maintenance rate after the charge / discharge cycle is reduced. On the other hand, if it is larger than 20 μm, the Sn alloy plating film is cracked, pulverized and dropped due to expansion / contraction during charge / discharge, and the maintenance rate after the charge / discharge cycle is lowered. Moreover, it is preferable that the space | interval of minute convex-shaped parts has 10 to 60% when expressed with the said porosity.
A plating film having a minute convex portion including such a curved surface can be formed by controlling the plating conditions. For example, after the rough plating with Cu, a commercially available organic acid Sn-1 to 3% by mass Cu plating bath is used, and the metal concentration is adjusted so that the metal concentration in the bath is 7: 3 Sn: Cu. The plating thickness is 4 μm in terms of pure Sn at a current density of 10 A / dm ² and a bath temperature of 25-30 ° C.
The pure Sn equivalent thickness was calculated by analyzing the Sn content by ICP (inductively coupled plasma method).

In the present invention, it is preferable that the plating film having the minute convex portion having the curved surface is further heat-treated. The conditions for the heat treatment are a temperature of 100 ° C. to 400 ° C., preferably 150 to 300 ° C., not exceeding the melting point of the Sn alloy, and 20 seconds to 10 hours, preferably 1 minute to 1 hour.
By this heat treatment, an Sn—Cu intermetallic compound is formed in the plating film having a minute convex portion including a curved surface. By forming this intermetallic compound, the negative electrode material for a lithium secondary battery of the present invention can further improve the charge / discharge characteristics.

Hereinafter, the present invention will be described in more detail with reference to examples.
Example 1
Fine copper particles were deposited on the copper alloy foil for 2 minutes under the burn plating conditions of copper 20 g / l, sulfuric acid 100 g / l, liquid temperature 38 ° C., and current density 5 A / dm ² . Thereafter, the coating was performed for 30 seconds under smooth plating conditions of copper 40 g / l, sulfuric acid 100 g / l, liquid temperature 40 ° C., and current density 5 A / dm ² . A photomicrograph of the obtained current collector surface is shown in FIG.

Example 2
Fine copper particles were deposited on the copper alloy foil for 2 minutes under the burn plating conditions of copper 20 g / l, sulfuric acid 100 g / l, liquid temperature 38 ° C., and current density 5 A / dm ² . Thereafter, the current density was continuously reduced to 2.5 A / dm ² over 1.5 minutes, and then plated for 1 minute. A photomicrograph of the obtained current collector surface is shown in FIG.

Comparative Example 1
On the copper alloy foil, fine copper particles were deposited for 1 minute under the condition of burnt plating with copper 20 g / l, sulfuric acid 100 g / l, liquid temperature 38 ° C., and current density 5 A / dm 2. Thereafter, the coating was performed for 30 seconds under smooth plating conditions of copper 40 g / l, sulfuric acid 100 g / l, liquid temperature 40 ° C., and current density 5 A / dm ² . A photomicrograph of the resulting current collector surface is shown in FIG.

Comparative Example 2
Fine copper particles were deposited on the copper alloy foil for 3 minutes under the condition of burnt plating with copper 20 g / l, sulfuric acid 100 g / l, liquid temperature 38 ° C., and current density 5 A / dm ² . Thereafter, plating was performed for 30 seconds under smooth plating conditions of copper 40 g / l, sulfuric acid 100 g / l, liquid temperature 40 ° C., and current density 5 A / dm ² .
In addition to the surface characteristics of the current collector obtained by electrodepositing the copper fine copper particles obtained in the above examples and comparative examples, a lithium secondary electrode having a negative electrode formed by applying 4 μm of tin-copper alloy plating to this current collector The charge / discharge cycle characteristics of the battery are shown in the table.
In Comparative Example 2, the granular copper dropped out after the roughening (with powder falling), so the charge / discharge cycle characteristics and the like were not evaluated.
The charge / discharge cycle characteristics were evaluated under the following conditions. A bipolar beaker cell was used in the glove box, and metallic lithium having a thickness of 0.3 mm was used as a counter electrode. The electrolyte was made 1 mol / L by dissolving LiPF ₆ in a solvent of ethylene carbonate / dimethyl carbonate (1: 1 (vol)) solution. The charge was 0.25 mA / cm ² (up to 0 V vs Li / Li +), the discharge was 1.0 mA / cm ² (up to 2.0 V vs Li / Li +), and a 20-cycle charge / discharge cycle test was performed.

2 is a micrograph of the current collector surface obtained in Example 1. FIG. 4 is a micrograph of the current collector surface obtained in Example 2. FIG. 4 is a photomicrograph of the current collector obtained in Comparative Example 1.

Claims (6)

A current collector for a negative electrode of a lithium secondary battery having a copper electrodeposition layer on a current collector, wherein the copper electrodeposition layer is granular, and the grains have a bulge toward the surface side, and the bulge portion The lithium secondary material for Si-based and Sn-based active materials has a diameter of 0.5 to 5 μm and a number of particles distributed at a density of 500,000 to 1,000,000 particles / mm ^2. Current collector for battery negative electrode.   The negative electrode for lithium secondary battery for Si-based and Sn-based active materials according to claim 1, wherein the average particle size of the copper electrodeposition layer is 1 µm or more and the standard deviation of the particle size is 0.05 to 0.8. Current collector.   3. The negative electrode for a lithium secondary battery for Si-based and Sn-based active materials according to claim 1 or 2, wherein the diameter of the bulge portion is 0.5 to 5 [mu] m and the average distance between grains is 1.0 [mu] m or more. Current collector.   After forming granular copper with the metal foil as the cathode under the plating condition, the plating condition is changed to the smooth plating condition side continuously or step by step to the smooth plating condition to remove the granular copper. The method for producing a current collector for a negative electrode of a lithium secondary battery for Si-based and Sn-based active materials according to claim 1, wherein smooth plating is performed so as to prevent.   The current collector for a lithium secondary battery negative electrode for a Si-based and Sn-based active material according to any one of claims 1 to 3, produced by the method according to claim 4.   A lithium secondary battery using the current collector for a negative electrode of a lithium secondary battery for Si-based and Sn-based active materials according to any one of claims 1, 2, 3, and 5.