JP4873281B2 - Method for producing lithium battery negative electrode - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a negative electrode used for a lithium secondary battery and a method for producing the same. In particular, the present invention relates to a lithium battery negative electrode capable of suppressing hydrolysis of an inorganic solid electrolyte thin film used for the negative electrode.
[0002]
[Prior art]
Practical use of lithium secondary batteries using organic electrolyte is progressing. The feature is that the energy output per unit volume or unit weight is higher compared to other batteries, and the development of practical use has been promoted as a power source for mobile communications, notebook computers and electric vehicles. Yes.
[0003]
In order to improve the performance of lithium secondary batteries, there are attempts to use lithium metal as the negative electrode, but dendritic lithium metal growth occurs on the negative electrode during charge and discharge, causing an internal short circuit with the positive electrode, and finally There is a risk of explosion. As a technique for suppressing this risk, as described in JP-A-2000-340257, formation of a sulfide-based inorganic solid electrolyte thin film on lithium metal has been studied.
[0004]
On the other hand, the inorganic solid electrolyte thin film has high hygroscopicity and hydrolyzability, and at the same time has a defect of deterioration due to moisture absorption (hydrolysis). In order to eliminate this drawback, it has been studied to form a protective thin film such as an oxide thin film on the surface of the inorganic solid electrolyte thin film.
[0005]
[Problems to be solved by the invention]
However, since the oxide thin film has low ionic conductivity, it must be removed when used in a battery. At that time, the oxide thin film is difficult to remove from the inorganic solid electrolyte thin film, and there is a problem that the productivity of the battery is lowered.
[0006]
Accordingly, a main object of the present invention is to provide a lithium battery negative electrode that can prevent deterioration of the inorganic solid electrolyte thin film due to moisture and does not hinder the workability in assembling the battery, and a method for manufacturing the same. .
[0007]
[Means for Solving the Problems]
The present invention achieves the above object by using a flexible protective sheet that can be easily peeled instead of the oxide thin film.
[0008]
That is, the lithium battery negative electrode of the present invention is characterized in that in the lithium battery negative electrode in which an inorganic solid electrolyte thin film is formed on a lithium-containing metal, the inorganic solid electrolyte thin film is covered with a flexible protective sheet.
[0009]
By closely covering the negative electrode surface with a flexible protective sheet, it is possible to prevent the negative electrode from being deteriorated by moisture before the lithium secondary battery is assembled. Moreover, when actually used for a lithium secondary battery, the protective sheet can be easily peeled off, which does not hinder the secondary battery assembly work.
[0010]
Hereinafter, the configuration of the present invention will be described in more detail.
[0011]
The material of the protective sheet is preferably one that satisfies the following conditions: (1) having flexibility, (2) not reacting with the inorganic solid electrolyte thin film, and (3) having high moisture resistance.
[0012]
Usually, since an inorganic solid electrolyte thin film is formed in a tape shape and rolled and handled, the flexibility is preferably such that it can be rolled in a state of covering a protective sheet.
[0013]
As the inorganic solid electrolyte, a sulfide system, an oxide system, a nitride system, and a mixed system thereof such as an oxynitride system and an oxysulfide system are conceivable. Examples of the sulfide herein include Li ₂ S and a compound of Li ₂ S and P ₂ S ₅ , SiS ₂ , GeS ₂ , and Ga ₂ S ₃ . Examples of the oxynitride include Li ₃ PO _{4 -X} N _{2X / 3} , Li ₄ SiO _{4 -X} N _{2X / 3} , Li ₄ GeO _{4 -X} N _{2X / 3} (0 <X <4), Li ₃ BO _3-X N _{2X / 3} (0 <X <3). By using a material that does not react with these for the protective sheet, deterioration of the negative electrode is suppressed.
[0014]
In terms of moisture resistance, it is important that the protective sheet itself hardly absorbs moisture as much as possible.
[0015]
As a material satisfying the above three conditions, plastic is suitable. More specifically, polyethylene and polypropylene are preferable.
[0016]
Further, the protective sheet is preferably a material that does not react with the lithium-containing metal. Usually, an inorganic solid electrolyte is formed on the lithium-containing metal and the protective sheet and the lithium-containing metal are not in direct contact with each other, but a material that does not react with the lithium-containing metal is preferable in consideration of the case where the inorganic solid electrolyte is peeled off. The lithium-containing metal includes lithium alloys as well as lithium metal itself. Specific examples of lithium alloys include alloys of lithium and In, Ti, Zn, Bi, Sn, and the like.
[0017]
The thickness of the protective sheet is desirably 5 μm or more and 100 μm or less. This is because, if the lower limit is not reached, the strength is weak and easily broken. On the other hand, if the upper limit is exceeded, the volume and weight of the entire negative electrode become too large, resulting in wasted material, and the size when rolled is increased. For example, when a protective sheet is wound 1000 times (about 1 km in length) on a core material having a diameter of 30 cm, the roll diameter is about 10 cm larger for a 100 μm sheet than for a 5 μm sheet.
[0018]
Usually, a laminate of a lithium-containing metal, an inorganic solid electrolyte thin film, and a protective sheet is processed into a tape shape. In that case, it is preferable that the width of the protective sheet is formed to be about 5 to 10 mm wider than the width required for assembling the battery. When the protective sheet is laminated on the surface of the inorganic solid electrolyte thin film, both side surfaces of the inorganic solid electrolyte thin film are not covered with the protective sheet, and it is considered that deterioration due to moisture absorption occurs from there. Since this deterioration is thought to stop at a depth of 1 mm or less from both sides of the protective sheet, it is necessary to cut both sides before assembling the battery by forming a laminate wider than the required width in advance. Can do.
[0019]
The inorganic solid electrolyte thin film and the protective sheet are preferably in close contact so that the absorption of moisture into the inorganic solid electrolyte thin film can be suppressed as much as possible. However, the two do not need to be bonded. This is because, when the battery is assembled, the protective sheet is removed, but if the two are adhered, it is difficult to remove the protective sheet. Therefore, it is sufficient that the protective sheet covers the inorganic solid electrolyte thin film to such an extent that the positional displacement between the inorganic solid electrolyte thin film and the protective sheet does not easily occur.
[0020]
The method for producing a lithium battery negative electrode of the present invention comprises a step of forming an inorganic solid electrolyte thin film on a lithium-containing metal, and a step of immediately covering the electrolyte thin film with a flexible protective sheet after forming the electrolyte thin film. It is characterized by that.
[0021]
By covering with a protective sheet immediately after forming the inorganic solid electrolyte thin film, deterioration of the inorganic solid electrolyte thin film due to hydrolysis can be suppressed.
[0022]
Here, the term “immediately after” refers to a short time that the inorganic solid electrolyte thin film absorbs water and does not deteriorate. Practically, a protective sheet coating step is performed following the inorganic solid electrolyte thin film forming step.
[0023]
Various known methods can be used for forming the inorganic solid electrolyte thin film on the lithium-containing metal. For example, a sputtering method, a laser ablation method, an ion plating method, or the like can be used.
[0024]
It is preferable to use a protective sheet that has been previously dried. By the drying process, the moisture content of the protective sheet is reduced as much as possible to minimize the deterioration of the inorganic solid electrolyte thin film. The drying step is preferably held at a temperature of 50 ° C. or higher for 30 minutes or longer. The lower limit of the drying temperature is that if it is less than 50 ° C., the drying time is long. The upper limit of the drying temperature may be determined in consideration of the melting point and softening point of the protective sheet. The upper limit temperature seems to be about 80 ° C for polyethylene and about 100 ° C for polypropylene. The drying time was selected so that the protective sheet could be sufficiently dried when the temperature was 50 ° C. or higher. Therefore, there is no particular upper limit on the drying time, but if it is too long, the production efficiency is lowered, so it is desirable to select a practical time.
[0025]
Further, this drying step is preferably performed in a vacuum of 133 Pa (1 Toor) or less. Moisture removal is further facilitated by drying in vacuum. This drying step is preferably performed in the same chamber as the step of forming the inorganic solid electrolyte thin film and the step of covering the inorganic solid electrolyte thin film with the protective sheet, but may be performed independently.
[0026]
The step of coating the protective sheet on the inorganic solid electrolyte thin film is preferably performed by winding it into a roll. That is, when winding a tape in which an inorganic solid electrolyte thin film is formed on a lithium-containing metal in a roll shape, it is preferable to wind up a protective sheet on the surface of the inorganic solid electrolyte thin film. By winding the protective sheet along with the roll of the inorganic solid electrolyte thin film, the time until the protective sheet is mounted can be shortened, and the deterioration of the inorganic solid electrolyte thin film can be reduced. Moreover, by winding up in roll shape, a protective sheet can be kept in close contact with the inorganic solid electrolyte thin film without using an adhesive or the like. Furthermore, by winding up in a roll shape, the protective sheet can serve as a buffer against mechanical impacts to prevent damage to the surface of the inorganic solid electrolyte thin film, and can also serve as a partition between adjacent turns.
[0027]
Moreover, it is preferable to perform the formation process of an inorganic solid electrolyte thin film and the coating process of a protective sheet in a continuous vacuum chamber. Inorganic solid electrolyte thin films are often deposited in a vacuum. In that case, if the protective sheet coating process is performed in a continuous vacuum chamber, the time required to attach the protective sheet can be shortened and the productivity is reduced. Can be made more efficient. Usually, in the negative electrode, a lithium-containing metal layer is formed on a metal foil serving as a current collector, an inorganic solid electrolyte thin film is formed thereon, and a protective sheet is further coated thereon. In that case, the lithium-containing metal layer is preferably formed in the same vacuum chamber.
[0028]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below.
<Configuration of negative electrode>
As shown in FIG. 1, this negative electrode has a lithium metal thin film 2 formed on a copper foil 1 serving as a current collector, an inorganic solid electrolyte thin film 3 formed thereon, and a protective sheet 4 further formed thereon. It is a covered configuration. The copper foil 1, the lithium metal thin film 2, and the inorganic solid electrolyte thin film 3 are integrated, but the protective sheet 4 is only held in contact with the inorganic solid electrolyte thin film 3, and can be easily removed. it can.
[0029]
<Method for producing negative electrode>
Said negative electrode can be manufactured with the manufacturing line shown in FIG.
[0030]
This line includes a supply chamber 10 in which a supply reel 5 for the copper foil 1 is disposed, a lithium metal thin film deposition chamber 11, an ion beam processing chamber 12, a solid electrolyte deposition chamber 13, and a winding chamber 14. Each chamber 10-14 is a vacuum chamber comprised continuously.
[0031]
The copper foil 1 drawn out from the supply reel 5 is taken up on the take-up reel 6 in the take-up chamber 14 through the lithium metal thin film forming chamber 11, the ion beam processing chamber 12, and the solid electrolyte film forming chamber 13 in order. Meanwhile, a lithium metal thin film is formed on the copper foil 1 in the lithium metal thin film deposition chamber 11. In the ion beam processing chamber 12, the lithium metal thin film is subjected to ion beam processing. In the solid electrolyte film forming chamber 13, a solid electrolyte thin film is formed on the lithium metal thin film. In addition to the take-up reel 6, the take-up chamber 14 is provided with a supply reel 7 for the protective sheet 4. When the copper foil on which the lithium metal thin film and the inorganic solid electrolyte thin film are formed is taken up by the take-up reel 6, the protective sheet 4 drawn out from the supply reel 7 is also taken up by the take-up reel 6.
[0032]
Through the above steps, the negative electrode having the laminated structure shown in FIG. 1 can be obtained in a state of being wound on a take-up reel.
[0033]
<Example>
The copper foil 1 having a thickness of 10 μm and a width of 50 mm is pulled out from the supply reel by the line shown in FIG. On the copper foil 1, a lithium metal thin film having a thickness of 10 μm was formed in a lithium metal thin film deposition chamber 11 by a vacuum deposition method.
[0034]
The lithium metal thin film formed on the copper foil was subjected to ion beam processing in a vacuum ion beam processing chamber 12 before forming the inorganic solid electrolyte thin film. Pressure ^{2 × 10 - 4 Torr (267} × 10 - 4 Pa) in flowing mixed gas of argon and nitrogen (Ar 75 vol%, 25 vol% nitrogen), the ion gun, 20 mA, an ion beam at 500V to the sample surface did.
[0035]
A sulfide inorganic solid electrolyte thin film having a thickness of 0.5 μm was formed on the lithium metal thin film in a solid electrolyte film forming chamber 13 by a vacuum deposition method.
[0036]
The composition of the sulfide inorganic solid electrolyte is as follows.
No.1 Li ₂ S: P ₂ S ₅ = 80: 20 (molar ratio)
No.2 Li ₂ S: P ₂ S ₅ : Li ₃ PO ₄ = 79: 20.5: 0.5 (molar ratio)
[0037]
As a method for forming the lithium metal thin film and the inorganic solid electrolyte thin film, a sputtering method, a laser ablation method, and an ion plating method are also possible.
[0038]
The copper foil on which the lithium metal thin film and the inorganic solid electrolyte thin film are formed is wound on the take-up reel 6. At that time, the protective sheet 4 pulled out from the protective sheet supply reel 7 is also wound up. As the protective sheet 4, a polypropylene sheet having a thickness of 50 μm was used. Polyethylene may be used instead of polypropylene. The polypropylene sheet was previously dried. Drying conditions were such that the rotary pump was evacuated to a degree of vacuum of 0.1 Torr (13.3 Pa), a temperature of 60 ° C., and a holding time of 5 hours.
[0039]
A lithium secondary battery composed of the obtained negative electrode, a separator (porous polymer film), a positive electrode, an organic electrolyte, and the like was produced. The battery production procedure and battery evaluation results are described below.
[0040]
Heats a mixed solution of ethylene carbonate (EC) and propylene carbonate (PC), cools a polyacrylonitrile (PAN) dissolved at a high concentration, and contains a large amount of EC and PC in which LiPF ₆ is dissolved PAN to make. In this PAN, LiCoO ₂ particles serving as an active material and carbon particles imparting electron conductivity were mixed and coated on a 20 μm thick aluminum foil (positive electrode current collector) to a thickness of 300 μm to form a positive electrode.
[0041]
The battery was manufactured in an argon gas atmosphere with a dew point of −60 ° C. or lower. The protective sheet was peeled off from the negative electrode on which the inorganic solid electrolyte thin film was formed, and placed in a stainless steel sealed container together with the separator (porous polymer film) and the positive electrode. The protective sheet could be easily removed without any problem. Further, a transverse electrolyte solution in which 1 mol% of LiPF ₆ was dissolved as an electrolytic salt was dropped into a mixed solution of ethylene carbonate and propylene carbonate to produce a lithium secondary battery.
[0042]
The charge / discharge characteristics of the produced battery were evaluated. As a result, both the negative electrode samples No. 1 and No. 2 had a charging voltage of 4.2 V, and the capacity until the voltage decreased to 3.0 V by 100 mA discharge was 0.5 Ah (ampere hour). The energy density was 500 Wh (kilowatt hour) / 1 (liter). Furthermore, cycle charge / discharge was performed under the same conditions, and the cycle was stable for 300 cycles or more.
[0043]
【Effect of the invention】
As described above, according to the lithium battery negative electrode of the present invention, it is possible to prevent the inorganic electrolyte thin film from absorbing and deteriorating by covering the inorganic electrolyte thin film with the protective sheet that can be easily peeled off.
[0044]
Moreover, the lithium battery negative electrode negative electrode of this invention can shorten time until a protective sheet is mounted | worn with an inorganic electrolyte thin film, and can reduce deterioration of an inorganic solid electrolyte thin film.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view of a lithium battery negative electrode of the present invention.
FIG. 2 is an explanatory view showing a method for producing a lithium battery negative electrode of the present invention.
[Explanation of symbols]
1 Copper foil
2 Lithium metal thin film
3 Inorganic solid electrolyte thin film
4 Protective sheet
5 Supply reel
6 Take-up reel
7 Supply reel
10 Supply room
11 Lithium metal thin film deposition chamber
12 Ion beam processing room
13 Solid electrolyte deposition chamber
14 Winding room

Claims (6)

Forming an inorganic solid electrolyte thin film on the lithium-containing metal;
Immediately after forming the electrolyte thin film, comprising covering the electrolyte thin film with a flexible protective sheet ,
A method for producing a lithium battery negative electrode, characterized in that an inorganic solid electrolyte thin film forming step and a protective sheet coating step are performed in a continuous vacuum chamber . The method for producing a lithium battery negative electrode according to claim 1 , wherein the protective sheet is subjected to a drying step. The method for producing a lithium battery negative electrode according to claim 1 or 2 , wherein the drying step is held at a temperature of 50 ° C or higher for 30 minutes or longer. Comprising a step of winding a tape in which an inorganic solid electrolyte thin film is formed on a lithium-containing metal into a roll,
The method for producing a lithium battery negative electrode according to any one of claims 1 to 3 , wherein a protective sheet is also wound on the surface of the inorganic solid electrolyte thin film during the winding step. Before forming the inorganic solid electrolyte thin film, comprising a step of forming a lithium-containing metal thin film on the metal foil serving as a current collector,
The lithium battery negative electrode according to any one of claims 1 to 4, wherein the lithium-containing metal thin film forming step, the inorganic solid electrolyte thin film forming step, and the protective sheet coating step are performed in a continuous vacuum chamber. Production method. After forming the lithium-containing metal thin film and before forming the inorganic solid electrolyte thin film, the lithium-containing metal thin film is subjected to an ion beam treatment,
6. The method for producing a lithium battery negative electrode according to claim 5, wherein the forming step of the lithium-containing metal thin film, the ion beam treatment step, the forming step of the inorganic solid electrolyte thin film and the covering step of the protective sheet are performed in a continuous vacuum chamber. .

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