JP2003168425A - NEGATIVE ELECTRODE MATERIAL FOR Li SECONDARY BATTERY - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a new negative electrode material for a Li secondary battery wherein the battery capacity can be enhanced compared with that from a graphite material and wherein deterioration of cycle life characteristic is not invited. <P>SOLUTION: This is a negative electrode material for a Li secondary battery to include a laminated structural body A<SB>0</SB>as the basic unit wherein an active layer 1 composed of metal or an alloy to stare/release Li ions and a collecting layer 2 composed of metal or an alloy not to store/release Li ions are alternately laminated. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a Li-ion battery,
The present invention relates to a novel negative electrode material for a Li secondary battery such as a gel electrolyte type Li polymer battery and an intrinsic electrolyte type Li polymer battery, and a negative electrode having a high capacity density using the same.

[0002]

2. Description of the Related Art Li secondary batteries are widely used as a power source for various electric and electronic devices such as laptop computers and mobile phones, but there is an increasing demand for downsizing and weight reduction of these electric and electronic devices. Accordingly, demands for smaller size and higher capacity are increasing for Li secondary batteries.

By the way, the negative electrode of the current Li secondary battery is
Most of them are manufactured by applying a paste formed by mixing a carbon material such as graphite with a fluorine-based binder to a metal current collector such as Cu foil, Ni foil, and stainless steel foil. It is a thing. When the carbon material used as the negative electrode material is, for example, graphite, the crystal structure thereof has a six-membered ring structure formed by covalent bonds of carbon atoms spread in a plane to form one plane layer, and this layer is a fan layer. -It has a layered structure that is laminated at a predetermined interval by the force of Der Waals. And the above-mentioned interlayer distance is theoretically 0.
It is 3354 nm.

A graphite having such a crystal structure is used as a Li secondary battery.
When used as a negative electrode material for
It is supplied from the disintegration and the positive electrode, and its ionic radius is about 0.1 nm
The degree of Li ions penetrates between the layers of the graphite crystal described above.
Put in there, for example LiC ₆It is stored in the form of. So
Then, at the time of discharging, the above-mentioned occlusion Li is Li ion.
And is released from the interlayer to the electrolyte. For charging and discharging
The supplied and emitted electrons are collected by the collector.

In the case of graphite, the interlayer distance in the layered structure is an appropriate size with respect to the radius of Li ions, and the interlayer attractive force is a relatively weak Van der Waals force. Good resistance to expansion and contraction of layered structure due to occlusion / release. for that reason,
Even if the charging / discharging cycle is repeated many times, the problem of collapse of the layered structure hardly occurs, and the function as the negative electrode material is maintained for a long period of time.

For these reasons, a carbon material typified by graphite is mostly used as the negative electrode material of the current Li secondary battery. However, on the other hand, the carbon material has a capacity density (LiC ₆ : theoretical capacity of 372 mAh /
It cannot be said that g) is always a high value, and there is a problem that the recent demand for higher capacity cannot be sufficiently met.

[0007] Therefore, research has been conventionally conducted to manufacture a negative electrode material from a metal material having a higher capacity density than a carbon material. For example, metal Li has a capacity density (L
i: The material with the highest theoretical capacity of 3860 mAh / g). However, in the case of this metallic Li, it becomes a dendrite during charging and is deposited on the negative electrode, but up to the present, problems such as suppression of dendrite deposition are still unsolved. Therefore, there is still no prospect of commercialization.

As a material having a high capacity density, Al is used.
(LiAl: theoretical capacity 993 mAh / g) and Sn (Li _4.4
Sn: theoretical capacity 994 mAh / g) and the like are known.
For example, Japanese Patent Laid-Open No. 2001-68094 discloses a Sn negative electrode material. Although these metal-based materials have a high capacity density of 2.6 times higher than that of carbon materials, their crystal structure is not a layered structure like carbon materials. Therefore, while the charge / discharge cycle is repeated many times, due to the expansion / contraction of the crystal structure accompanying the absorption / desorption of Li ions, the crystal structure gradually collapses in a relatively short time and becomes a granular material. Therefore, there is a problem in that it falls off from the current collector and disperses in the electrolyte, resulting in deterioration of the battery.

[0009]

DISCLOSURE OF THE INVENTION The present invention aims to provide a novel negative electrode material and a negative electrode using the same, which can solve the above-mentioned problems associated with the conventionally known negative electrode materials in Li secondary batteries. And Specifically, since it is a metal or alloy, it has a higher capacity density than that of a carbon material, but it is a metal or alloy,
An object is to provide a negative electrode material having a charge / discharge cycle life equivalent to that of a carbon material and a negative electrode using the same.

[0010]

In order to achieve the above object, in the present invention, an active layer made of a metal or an alloy that absorbs and releases Li ions and a metal or an alloy that does not absorb and release Li ions are used. A negative electrode material for a Li secondary battery is provided, in which the formed current collecting layer includes a laminated structure that is alternately laminated as a basic unit.

Further, in the present invention, the mixture of the above-mentioned powder of the negative electrode material and the binder is applied to one side or both sides of the current collector, which is used for a Li secondary battery. A negative electrode is provided.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION First, the design concept of the material that enables the development of the negative electrode material of the present invention will be described. 1) In the layered structure of a carbon material, the place where Li ions are occluded / released electrochemically is between layers located between plane layers formed by covalent bonds of carbon atoms. Electrochemical occlusion / release of Li ions does not occur in the flat layer itself.

That is, the carbon material has a layered structure in which a field for occluding / desorbing Li ions and a field for not occluding / desorbing Li ions (planar layer) are alternately laminated. As a result, the expansion / contraction of the layered structure due to the absorption / desorption of Li ions is relieved, and this manifests itself as a good charge / discharge cycle life. However, on the other hand,
There is a problem that the capacity density of the carbon material is not so high.

2) By the way, when a high capacity density is to be obtained, like Li or Sn, it absorbs Li ions.
It is advantageous to use emissive metallic materials. However, in the case of a metallic material, it is generally practically impossible to make the crystal structure by itself into a layered structure having the above-mentioned function such as a carbon material. For example, a layered structure such as a carbon material, that is, a field for occluding and releasing Li ions and L
If a layered structure in which fields that do not occlude / desorb i ions are alternately laminated can be formed of a metal material, that material has a high capacity density and, in addition, a good charge / discharge cycle life characteristic like a carbon material. It is considered that it can function as a material.

3) By the way, as the metal material, there is a material which does not electrochemically store and release Li ions, that is, which does not alloy with Li. Therefore, electrochemically L
If a thin layer of a metal material that occludes / desorbs i ions and a thin layer of a metal material that does not electrochemically occlude / desorb Li ions are alternately laminated, the resulting laminated structure is the same as that of a carbon material. It is thought that they may exhibit similar functions. Moreover, in the case of the laminated structure, since the layer that absorbs and releases Li ions is made of a metal material, the capacity density should be higher than that of a carbon material.

Further, even when the metal material which absorbs and releases Li ions is pulverized due to expansion and contraction accompanying the absorption of Li ions, the thin layer absorbs and absorbs Li ions.
It is not released, that is, it is sandwiched by a thin layer of another metal material that does not cause pulverization due to expansion and contraction, so the situation in which the produced pulverized material is dispersed in the electrolyte and causes battery deterioration is suppressed. It is considered to be one.

Based on the above idea and consideration, the present inventor
The above-mentioned negative electrode material having the above-mentioned configuration and the negative electrode material having the above-mentioned configuration are combined with each other in combination with the types of the metal materials forming the above-mentioned thin layers, the thickness of both the thin layers, and the method for forming the laminated structure. We succeeded in developing the negative electrode used.
First, the negative electrode material of the present invention will be described in detail.

FIG. 1 shows an example A ₀ of a laminated structure which is a basic unit of the negative electrode material of the present invention. This laminated structure A ₀ is L
a layer 1 made of a metal material that absorbs and releases i ions, and L
It has a structure in which layers 2 made of a metal material that does not store and release i ions are alternately laminated. Each of the lowermost layer and the uppermost layer in FIG. 2 is composed of a layer 2 of a metal material that does not store or release Li ions.

The layer 1 means an active layer which enables the entire battery reaction by absorbing and releasing Li ions, and is hereinafter referred to as an active layer. The layer 2 also stores and releases Li ions. However, in the sense that it has conductivity and collects electrons generated in the active layer 1,
It is called a current collecting layer. When this laminated structure A ₀ is used as a negative electrode material, Li ions are occluded in each active layer 1 during charging. At the time of discharge, the Li ions stored therein are released from the active layer 1, and the electrons supplied to or generated in each active layer 1 at that time are not only electrically conductive in each active layer itself, but also each Current is collected by the current collecting layer 2 adjacent to the active layer 1.

In the process of repeating the charge / discharge cycle described above, the expansion and contraction of the active layer 1 causes the metal material constituting the active layer to be pulverized. However, in the laminated structure A ₀ , the active layer 1 is sandwiched by the current collecting layers 2 laminated on both sides thereof, and the current collecting layer 2 is not powdered during the charging / discharging cycle and is made of a metal thin film. Since the layer remains as it is, even if the active layer 1 is granulated, the situation in which the granular material is dispersed in the electrolyte is suppressed by the current collecting layer 2 serving as one kind of protective wall. .

In the laminated structure A ₀ , the metal material forming the active layer 1 may be any material as long as it absorbs and releases Li ions, and is not particularly limited. For example, Sn , Zn, Ag, Al and the like, Cu—Sn alloys, and Cu—Zn alloys having a Zn content of 40 mass% or more. Further, the metal material forming the current collecting layer 2 may be one that does not occlude and release Li ions, and for example, Cu, Ni, Fe, stainless steel, and Cu-containing less than 40 mass% of Cu-
A Zn alloy etc. can be mentioned.

The thickness of the active layer 1 is set to be equal to or larger than the ionic radius of Li ions (about 0.1 nm). However, if the thickness exceeds 100 μm, pulverization tends to occur in the process of repeated absorption and desorption of Li ions, so the upper limit of the thickness is preferably set to 100 μm or less. 1
It is practical that the thickness is 0 nm to 10 μm. On the other hand, the thickness of the current collecting layer 2 is not particularly limited, but a thicker one is more effective in securing the strength as the negative electrode material. However, from the viewpoint of capacity density, it is advantageous to be as thin as possible. The thickness is appropriately set depending on the required characteristics of the battery and the type of metal material used, but is generally 0.1 nm to 100 nm.
It may be about μm. Preferably 10 nm to 100 μ
m.

The number of active layers 1 and current collecting layers 2 in the laminated structure A ₀ is not particularly limited. However, as is clear from the above description, one active layer and two current collecting layers laminated on both surfaces thereof must be the minimum unit. This laminated structure A ₀ is, for example,
It can be manufactured by applying a vacuum deposition method, a rolling foil laminating method, an electroplating method, or the like.

For example, in the case of the vacuum deposition method, a thin Cu foil is prepared as a current collector for the whole, and an Sn layer (active layer) and a Cu layer (current collector layer) are formed on the Cu foil by PVD method or CVD method. The desired laminated structure A ₀ can be manufactured by alternately laminating a desired number of layers, and finally laminating a Cu layer which is a current collecting layer. Even when applying the laminated method of rolled foil, C
It can be manufactured by alternately stacking u foils and Sn foils, finally stacking a Cu layer, and then press-molding the whole and bringing them into close contact.

However, the above-mentioned method requires a great number of steps when the thickness of each layer is reduced and the number of layers to be laminated is increased. From that point of view, the electroplating method can form a laminated structure with a small number of steps, but since the types of metal materials of the active layer and the current collecting layer are different, a large number of steps are required to repeatedly form each layer. Becomes From this point of view, it is preferable to apply the pulse plating method when manufacturing the laminated structure A ₀ . This is because the active layers and the current collecting layers can be alternately and continuously laminated with one type of electrolytic solution.

For example, when the active layer is formed of a Cu--Sn alloy and the current collecting layer is formed of a Cu layer, an aqueous solution containing sulfuric acid, copper sulfate and tin sulfate as the main components is used as the working electrode (cathode). ) Is made of stainless steel, Ti, Cu, etc., and the counter electrode (anode) is made of lead, and by switching the energizing current and energizing potential at regular intervals, the Cu plating layer and the Cu-Sn alloy plating layer alternate on the cathode. Can be laminated on. In that case, the thickness of the plating layer can be arbitrarily changed by appropriately selecting the respective plating times.

The electrolytic drum (Ti, stainless steel, etc.) used in the production of the electrolytic copper foil is used as the cathode,
By combining an insoluble electrode such as Pb as an anode and applying the above-described pulse plating method, the laminated structure of the present invention can be continuously manufactured. By the way, in the case of the laminated structure A ₀ shown in FIG. 1, both the lowermost layer and the uppermost layer are current collecting layers that do not store Li ions. Therefore,
Even if the electrolyte is present around the laminated structure A ₀ , Li ions are not occluded in the active layer from the thickness direction of the laminated structure A ₀ and therefore are not released from the active layer.

The absorption and release of Li ions into and from the active layer, in other words, the exchange reaction of Li ions between the electrolyte and the active layer, occurs at the point where the active layer is exposed, that is, in FIG. Only proceed on four sides of the laminated structure A ₀ . Therefore, the active layer is not effectively utilized with the laminated structure A ₀ shown in FIG. Therefore, in order to put this laminated structure A ₀ into practical use as a negative electrode material, it is necessary to widen the contact area between the active layer and the electrolyte to promote the Li ion transfer reaction between them.

Therefore, in the present invention, the laminated structure A ₀ is actually used as a negative electrode material after the following treatment. The first measure is a measure in the thickness direction of the laminated structure A ₀ to form a large number of minute cracks reaching the inside of the laminate by taking measures such as needle punching, expanding, and embossing. By performing such a treatment, the electrolyte penetrates from the minute clutch to the inside of the laminated structure A ₀ , and the contact area of the active layer with the electrolyte is effectively increased.

The second measure is a process of crushing the laminated structure A ₀ into powder. By this treatment, as in the case of the current carbon materials, the exposed active layer area is increased and the contact area with the electrolyte is significantly increased. Of these treatments, the latter pulverization treatment is effective in that the work is simple and the contact area between the active layer and the electrolyte is significantly increased.

The negative electrode of the present invention is manufactured by mixing the above-mentioned negative electrode material and a binder of, for example, a fluororesin at a predetermined ratio, and coating the mixture on a current collector such as a Cu foil. In that case, since the metal material of the above-mentioned negative electrode material (laminated structure) is easily oxidizable, in order to prevent its surface oxidation, it is immersed in, for example, a chromic acid solution and then washed with water to prevent the surface from being oxidized. It is preferable to form a rust film.

[0032]

EXAMPLES 1) Production of Negative Electrode Material Pulse plating was performed under the following conditions. Electrolyte: H ₂ SO ₄ 1 mol / L, SnSO ₄ 1 mol /
L, CuSO ₄ 0.1 mol / L, liquid temperature 40 ° C., working electrode (cathode): vertical 22 cm, width 35 cm, thickness 1 mm Ti plate (effective area 500 cm ² ), counter electrode (anode): vertical 22 cm, width 35 cm , P with a thickness of 5 mm
b plate (insoluble electrode), mode of pulse energization: pulse energization time is T ₁ (1 second), T ₂
(5 seconds), the current density at the pulse time T ₁ is 1 A / dm ² , and the current density at the pulse time T ₂ is 5 A.
Set to / dm ² .

Then, the pulse energization is repeated 120 times.
After performing the above-mentioned pulse plating, the deposit was peeled off from the Ti plate. A sheet having an area of about 500 cm ² and a thickness of 10 μm (average value) was obtained. When the cross section of this sheet was observed with a scanning electron microscope, the sheet had a layered structure,
The layers under pulsed current T ₁ had a thickness of about 3 nm, and the layers under pulsed current T ₂ had a thickness of about 80 nm.

Further, when a sample was taken from the sheet and EPMA analysis was performed from the surface thereof, pulse energization T ₁
The layer at the time was almost pure copper, the layer at the time of pulse current T ₂ was Cu—Sn alloy, and the Sn content was about 60 mass%. Then, the sheet was roughly pulverized by a cutter mill and further pulverized by a vibration mill to obtain a powder having an average particle size of 3 μm. Then, this powder was immersed in a chromic acid solution having a concentration of 5% for 1 minute, washed with water and dried to obtain a negative electrode material of the present invention.

2) Assembly of Li-ion secondary battery The above powder and a fluorine-based binder were mixed in a weight ratio of 9: 1, and the mixture was formed on both sides of a rolled copper foil having a width of 60 mm and a length of 600 mm, respectively. The negative electrode of the present invention was manufactured by coating at 40 μm. The Sn content in the coating film was about 50% by mass.

On the other hand, lithium cobalt oxide, a fluorine-based binder, and graphite powder were mixed in a weight ratio of 8: 1: 1, and the mixture was applied to both sides of an aluminum foil having a width of 60 mm, a length of 600 mm and a thickness of 20 μm. A positive electrode was manufactured by coating with a thickness of 160 μm. Then, a polypropylene separator is sandwiched between the negative electrode and the positive electrode, and the whole is spirally wound to have a diameter of 18
A cylindrical electrode plate having a height of 65 mm and a height of 65 mm was formed.

The cylindrical electrode plate was placed in a bottomed cylindrical container made of stainless steel, and an electrolyte solution of LiBF ₄ was used as a solvent, and a solvent was a nonaqueous mixed solution of ethylene carbonate and dimethyl carbonate. The negative electrode terminal was taken out and then sealed to assemble a cylindrical Li-ion secondary battery with a battery capacity of 2000 mAh. For comparison, a cylindrical Li-ion secondary battery having the same specifications as in the example except that the negative electrode material was graphite powder, the negative electrode had a coating thickness of 100 μm, and the positive electrode had a coating thickness of 100 μm. Assembled. This battery is referred to as Comparative Example 1.

Further, the negative electrode material is Cu-having an average particle size of 3 μm.
A cylindrical Li-ion secondary battery was assembled with the same specifications as in the example except that the Sn alloy powder (Sn content was about 55 mass%) was used. This battery is referred to as Comparative Example 2. 3) Characteristics For these 3 types of batteries, the current is 2.0A (1C) and 1
After constant time constant current charging, battery voltage 4.2V constant voltage charging for 1.5 hours, discharge 0.4A (0.2
The charging / discharging cycle was repeated in which the discharge was stopped when the terminal voltage dropped to 2.5V.

Then, the battery capacity after 5 cycles was measured, and the number of cycles until the battery capacity became less than 70% of the initial capacity was measured, and the value was taken as the number of cycles until battery deterioration. When the number of cycles was more than 800, the battery performance was considered to be good and the charge / discharge cycle test was stopped. The above results are shown in Table 1.

[0040]

[Table 1]

As is clear from Table 1, the battery of the example using the negative electrode material of the present invention has a battery capacity of 50% as compared with the battery of the comparative example 1 using the current graphite material as the negative electrode material.
The cycle life has increased and the cycle life is almost the same. Further, Comparative Example 2 in which a Cu—Sn alloy is used as the negative electrode material
The battery of No. 1 has a battery capacity of 75 compared to the battery of Comparative Example 1.
%, The cycle life is extremely deteriorated.

[0042]

As is apparent from the above description, the negative electrode material of the present invention is compared with the case where the current graphite material is used.
It enables a significant increase in battery capacity and at the same time guarantees approximately the same cycle life characteristics. Therefore, high capacity,
It is useful as a negative electrode material for Li secondary batteries, which are strongly demanded to be miniaturized, and its industrial value is extremely large.

[Brief description of drawings]

FIG. 1 is a schematic view showing an example of a negative electrode material of the present invention.

[Explanation of symbols]

A ₀ Laminated structure 1 Layer of metal material that absorbs and releases Li ions (active layer) 2 Layer of metal material that does not absorb and release Li ions (collection layer)

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Claims (8)

[Claims] 1. A laminated structure, in which an active layer made of a metal or alloy that absorbs and releases Li ions and a current collecting layer that is made of a metal or alloy that does not absorb and release Li ions are alternately laminated. A negative electrode material for a Li secondary battery, characterized in that it is included as a unit. 2. The negative electrode material for a Li secondary battery according to claim 1, wherein the outermost layer of the laminated structure is the current collecting layer. 3. The active layer has a thickness of 0.1 nm to 100 μm.
m, and the thickness of the current collecting layer is 0.1 nm to 100 μm, and the negative electrode material for a Li secondary battery according to claim 1. 4. The metal or alloy forming the active layer is Sn, Zn, Ag, Al, Cu—Sn alloy or Z.
The negative electrode material for a Li secondary battery according to claim 1, which is a Cu—Zn alloy having an n content of 40 mass% or more. 5. The metal or alloy forming the current collecting layer is Cu, Ni, Fe, stainless steel, or a Cu—Zn alloy having a Zn content of less than 40% by mass. A negative electrode material for such a Li secondary battery. 6. The negative electrode material for a Li secondary battery according to claim 1, wherein the laminated structure is manufactured by a pulse plating method. 7. The L according to claim 1, wherein a plurality of cracks are formed in a thickness direction of the laminated structure.
i Negative electrode material for secondary battery. 8. A Li secondary battery, wherein the mixture of the powder of the negative electrode material according to any one of claims 1 to 6 and a binder is coated on one side or both sides of a current collector. Negative electrode for.