JP2006216518A - Negative electrode material for lithium secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a negative electrode material for lithium secondary battery which can prevent separation of the coating containing Sn-Cu or Sn-Cu alloy intermetallic compound from the current collector by absorbing and relaxing stress generated at expansion and shrinking of the negative electrode at charge and discharge and which is improved in charge and discharge cycle characteristics. <P>SOLUTION: This is a negative electrode material for lithium secondary battery having a coating layer at the current collector, and the coating layer is a granular rough plated film of Cu formed on the current collector and an Sn alloy plated film having a minute convex shape part including at least a curved face portion formed on top of it. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a negative electrode material for a lithium secondary battery.

  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.

  Proposals of negative electrode materials that meet these requirements have also been made. For example, by using a negative electrode for a lithium secondary battery in which a tin film laminated on the surface of the current collector by electroplating is used, the lithium secondary battery having high current density and energy density and improved charge / discharge cycle characteristics A battery has been proposed (Patent Document 1 (Japanese Patent Laid-Open No. 2001-68094)). In addition, a copper foil that has been electroplated or electrolessly plated with an Sn alloy or the like, or a copper foil that has been heat-treated after plating and made into an intermetallic compound, is used as a negative electrode material for a non-aqueous electrolyte secondary battery. It has been proposed to enable charging / discharging (Patent Documents 2 and 3 (JP 2001-256967 A, JP 2001-256968 A)).

In the above proposal, it is said that the action of suppressing the generation of lithium dendrite at the time of charging / discharging can be promoted by setting the tin film crystal formed on the current collector to 1 μm or less. ing. In Patent Document 4 (Japanese Patent Laid-Open No. 2002-198091), the Sn coating of the current collector is alloyed to disperse the alloy components in the Sn film, and the Sn particles fall off due to the volume change accompanying the charge / discharge reaction. It is supposed to prevent.
Patent Document 5 (Japanese Patent Laid-Open No. 2003-7305) describes a negative electrode material in which a current collector surface is roughened and an active material thin film is formed thereon. Adhesion can be improved by this pretreatment, and the active material thin film is separated into columns by separating the active material thin film into columns by causing cracks in the thickness direction at the valleys of the rough surface unevenness due to expansion and contraction of the active material thin film during charging and discharging. It is described that the stress relaxation action of this void is utilized.
And in patent document 6 (Unexamined-Japanese-Patent No. 2002-313319), after roughening plating of Cu, the gap | interval which becomes wide as it goes to the trough part of the unevenness | corrugation of the collector surface is formed by CVD of Si. A negative electrode characterized by is described.

JP 2001-68094 A JP 2001-256967 A JP 2001-256968 A JP2002-198091 JP2003-7305 JP 2002-313319 A

  However, as these negative electrode materials also repeat the charge / discharge cycle, the Sn or Sn alloy layer is pulverized due to expansion and contraction accompanying lithium occlusion / release at that time, and falls off the current collector. In addition, this phenomenon cannot be sufficiently suppressed even if the plating particles are simply miniaturized, and the charge / discharge cycle characteristics are sufficiently high due to the roughening of the current collector and the formation of voids due to cracks in the valleys. It is not satisfactory. Further, in the proposal of the above-mentioned JP-A-2002-313319, since a gap is formed which becomes wider as it goes to the concave and convex valleys on the current collector surface, the volume expansion of the active material at the time of charging / discharging in that part, The stress of the current collector due to shrinkage can be absorbed, but since there are almost no voids on the active material surface, when the active material expands during charging, the active material surface cannot absorb the stress, and the active material However, the charge / discharge cycle life is still not fully satisfactory.

  The present invention further enhances the ability to absorb and relax the stress generated during the expansion / contraction of the negative electrode during charge / discharge, thereby removing the coating containing the Sn—Cu alloy or Sn—intermetallic compound from the current collector. It is an object of the present invention to provide a negative electrode material for a lithium secondary battery that can prevent the above and improve the charge / discharge cycle characteristics.

  As a result of intensive studies, the present inventors have roughened the surface of the negative electrode current collector with Cu granular plating, and provided each of the Sn alloy plating films having a minute convex portion including a curved surface closely adhered thereon. It has been found that the formation of a void structure between the convex portions acts very effectively on the absorption and relaxation of stress due to expansion / contraction during the charge / discharge cycle, and the present invention has been achieved.

That is, the present invention
(1) A negative electrode material for a lithium secondary battery having a coating layer on a current collector, wherein the coating layer is a granular rough plating film of Cu formed on the current collector, and further formed on at least A negative electrode material for a lithium secondary battery, which has a fine convex portion including a curved surface portion, and the fine convex particles are Sn alloy plating films having voids therebetween.
(2) A negative electrode material for a lithium secondary battery having a coating layer on a current collector, wherein the coating layer is a granular rough plating film of Cu formed on the current collector, and further a curved surface formed thereon A negative electrode material for a lithium secondary battery, characterized in that the negative convex material is a Sn alloy plating film having a minute convex portion composed of a portion, and wherein the minute convex particles have voids therebetween.
(3) The negative electrode material for a lithium secondary battery according to (1) or (2), wherein the fine convex particle diameter of the Sn alloy plating is 1 to 20 μm.
(4) The negative electrode material for a lithium secondary battery according to any one of (1) to (3), wherein the Sn alloy is a Sn—Cu alloy.
(5) The negative electrode material for a lithium secondary battery according to (4), wherein the Sn alloy is an alloy of Sn and 30 to 45% by mass of Cu.
(6) The negative electrode material for a lithium secondary battery according to (4), wherein the Sn alloy is an alloy having a Cu6Sn5 phase.
(7) The negative electrode material for a lithium secondary battery according to any one of (1) to (6), which is further heat-treated.
(8) The surface roughness of the Sn or Sn alloy plating film surface is Ra 0.5 to 5.0 μm, Ry 3.0 to 30 μm, according to any one of (1) to (7) above Negative electrode material for lithium secondary battery.
(9) The negative electrode material for a lithium secondary battery according to any one of (1) to (8), wherein the current collector is a Cu-Cr-Zr-based or Cu-Ni-Si-based copper alloy foil.
(10) The present invention relates to a battery using the negative electrode material for a lithium secondary battery according to any one of (1) to (9).

  Unlike the active material form described in JP-A-2002-313319, the void structure formed in the covering layer on the current collector surface of the negative electrode material of the present invention is not only in the vicinity of the interface between the active material and the rough plating, but also in the active material form. Since voids are also formed on the surface of the material, not only the stress generated in the active material and current collector during charging / discharging is relieved, but the active material expands during charging and the stress generated between the active materials. The charge / discharge cycle life can be extended. In this way, it can act very effectively to absorb stress due to expansion / contraction during charging / discharging, thus preventing the coating layer from being pulverized and peeling off, and improving the charge / discharge characteristics of the lithium secondary battery. Can do.

  The current collector used in the present invention is preferably selected from Cu, Cu alloy, 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.

The rough plating film on the current collector surface of the present invention is formed by plating granular copper on the current collector surface.
The formation of the granular Cu plating film is an indispensable film in order to form a minute convex portion having a curved surface thereon. It is impossible to form the curved convex portion directly from the current collector surface with a good adhesive force.

The grained granular Cu plating of the present invention has a particle size of 0.5 to 5.0 μm, preferably 1.5 to 2.5 μm. Plating is performed so that a film of 30 μm, preferably 0.10 to 0.25 μm, and Ry of 0.5 to 3.0 μm, preferably 1.0 to 2.5 μm, is formed.
When the particle size of Cu is smaller than 0.5 μm, when the surface roughness Ra is smaller than 0.05, or when Ry is smaller than 0.5 μm, the shape having a fine convex portion having a curved surface I can't. When the particle size of Cu is larger than 5.0 μm, when the surface roughness Ra is larger than 0.30 μm, or when Ry is larger than 3.0 μm, the adhesion with the material is deteriorated.

In the present invention, in order to form granular Cu plating on the current collector, copper sulfate is used as a plating bath, and plating is performed by setting conditions such as concentration, plating temperature, stirring, and current density. can do.
In addition, it is important that the Sn alloy plating of the present invention is formed on the granular Cu plating in a minute convex shape having at least a curved surface. It is more preferable that the entire minute convex portion is substantially curved.
It is also important that the minute convex portions maintain a gap between each other. In particular, when this gap is expressed by a porosity described later, it is preferably 5 to 70%, and more preferably 10 to 60%.
By providing such voids in the structure of the Sn alloy coating layer 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 discharge of the lithium secondary battery. Detachment of the convex portion in the coating layer and peeling between the Sn alloy coating layer and the roughened Cu plating layer can be prevented.

As the Sn alloy, 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. Moreover, it is especially preferable to have a Cu6Sn5 phase.
Even when an intermetallic compound layer is formed by heat treatment after Sn-M eutectoid plating is applied to the current collector, the plating layer is cracked, and the expansion stress is mitigated by the voids. Although it can suppress that Sn-M coating layer pulverizes and peels, it is still inadequate and cannot extend to the extent which can satisfy cycle life.

  Further, it is known that after Cu roughening treatment, Sn plating is performed, cracks are formed on the surface, and heat treatment is further performed. However, the surface caused by such simple cracks does not have a convex part having curved surfaces with a gap between them, and the stress relaxation action at the time of charging / discharging is still insufficient. The cycle life cannot be improved sufficiently.

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 roughening plating with Cu, using a commercially available organic acid Sn-1 to 3% by mass Cu plating bath, the metal concentration is adjusted to Sn20 to 50 g / l, Cu20 to 30 g / l, and the target precipitation composition is It is formed by adjusting the plating conditions to a current density of 5 to 20 A / dm ² , a bath temperature of 20 to 30 ° C., etc. so that the shape after electrodeposition is a fine convex part having a curved surface at Sn 40 to 80% by mass. be able to.

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.
Cu-2.51 mass% Ni-0.45 mass% Si-0.15 mass% Mg alloy having a composition of 18 μm in thickness is subjected to alkaline electrolytic degreasing and sulfuric acid pickling, and then thickened using a copper sulfate bath. A current collector was obtained by roughing plating of Cu with a thickness of about 3 μm, and this current collector was subjected to Sn—Cu alloy plating. Sn-Cu alloy plating uses an organic acid Sn-1 to 3 mass% Cu commercial bath as a plating bath, and the metal concentration in the bath is Sn20 so that the electrodeposition composition is Sn-20 to 60 mass% Cu. It adjusted in the range of -50 g / L and Cu20-30 g / L. The amount of electrodeposition was adjusted so that the amount of Sn in the film was the same in consideration of battery performance. That is, the plating thickness was adjusted according to the Sn—Cu composition so that Sn equivalent to 4 μm was electrodeposited in terms of pure Sn.

The curved shape of the Sn alloy plating film was controlled by adjusting the plating conditions at a current density of 5 to 20 A / dm ² , a bath temperature of 28 ° C., and a stirring of the plating bath of 1 m / s or less.
1 to 6 are photographs of the surface and cross section of roughened Cu plating and Sn alloy plating. It can be seen that the Sn alloy is electrodeposited into a shape having a minute convex portion including a curved surface portion on the granular rough plating.
Thus, about the created negative electrode, Sn-Cu composition of a film | membrane, Sn adhesion amount, identification of the formed compound, film | membrane shape, porosity, and charging / discharging cycling characteristics were evaluated.
The Sn—Cu coating composition was analyzed by ICP (inductively coupled plasma method).
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 (%)

Identification of the plating film by X-ray diffraction was measured using a Co tube from the negative electrode surface after the heat treatment.
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.

No. 1 is an example in which roughening treatment and heat treatment were not performed. Since the roughening treatment was not performed, the minute convex portion including the curved surface portion was not formed.
No. Since 2-8 performed the roughening process, the micro convex part containing a curved-surface part was formed.
No. Nos. 2 and 3 are No. It has a discharge capacity and cycle maintenance rate better than 1. No. No. 2 with a larger particle size and a rougher surface roughness than No. 2. 3 is No.3. It can be seen that it has a discharge capacity and cycle maintenance ratio better than 2.
No. No. 4 is an example in which the Sn content is low in the Sn alloy composition, and the Cu3Sn phase was generated by the heat treatment, but the film peeled off and the cycle retention rate was inferior.
No. No. 5 is an example in which the Sn content is high and the porosity is low in the Sn alloy composition. The Sn alloy coating was pulverized and the cycle retention rate was inferior.

No. No. 6 is an example in which the heat treatment temperature is increased. Compared with FIGS. 2 and 3, there was no pure Sn phase and a Cu3Sn phase was produced. It was equivalent to 3.
No. No. 7 is an example in which no heat treatment was performed, but despite the fact that no heat treatment was performed, a Cu6Sn5 phase was generated and the battery performance was No. 7. Although it was inferior to 3, it was favorable.
No. No. 8 is No.8. No. 3 is an example in which the heat treatment time is increased, but the discharge capacity and the cycle retention rate were both equal to No. 3.

No. 2 is a micrograph of the surface of Cu roughening plating (magnification × 3500). No. 2 is a micrograph of the Sn—Cu plating surface (magnification × 3500). No. 3 is a micrograph (magnification × 3500) of the surface of the roughened Cu plating. No. 3 is a micrograph of the Sn-Cu plating surface (magnification × 3500). No. 2 is a micrograph (magnification × 1000) showing the cross-sectional shape of Sn—Cu plating. No. 3 is a photomicrograph (magnification × 1000) showing the cross-sectional shape of Sn-Cu plating 3.

Claims (10)

  A negative electrode material for a lithium secondary battery having a coating layer on a current collector, wherein the coating layer has a granular rough plating film of Cu formed on the current collector, and at least a curved surface portion formed thereon A negative electrode material for a lithium secondary battery, characterized in that the negative electrode material is a Sn alloy plating film having a minute convex portion including the fine convex particles and having voids therebetween.   A negative electrode material for a lithium secondary battery having a coating layer on a current collector, the coating layer comprising a granular rough plating film of Cu formed on the current collector and a curved surface portion formed thereon A negative electrode material for a lithium secondary battery, characterized in that it has a fine convex portion, and the fine convex particles are Sn alloy plating films having voids therebetween.   3. The negative electrode material for a lithium secondary battery according to claim 1, wherein the fine particle diameter of the Sn alloy plating is 1 to 20 μm.   The negative electrode material for a lithium secondary battery according to any one of claims 1 to 3, wherein the Sn alloy is a Sn-Cu alloy.   The negative electrode material for a lithium secondary battery according to claim 4, wherein the Sn alloy is an alloy of Sn and 30 to 45% by mass of Cu.   The negative electrode material for a lithium secondary battery according to claim 4, wherein the Sn alloy is an alloy having a Cu6Sn5 phase.   Furthermore, the negative electrode material for lithium secondary batteries of any one of Claims 1-6 heat-processed.   The negative electrode material for a lithium secondary battery according to any one of claims 1 to 7, wherein the surface roughness of the Sn alloy plating film surface is Ra 0.5 to 5.0 µm, Ry 3.0 to 30 µm. .   The negative electrode material for a lithium secondary battery according to any one of claims 1 to 8, wherein the current collector is a Cu-Cr-Zr-based or Cu-Ni-Si-based copper alloy foil.   The battery which uses the negative electrode material for lithium secondary batteries of any one of Claims 1-9.