JP3510175B2 - Method for producing negative electrode for secondary battery - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a negative electrode for a secondary battery, which uses a sintered body having silicon as an active material as an electrode material, a method for producing the same, and a non-aqueous secondary battery using the same.

[0002]

2. Description of the Related Art With the spread of mobile phones, notebook computers, and the like, high-capacity lithium secondary batteries containing a positive electrode active material and a negative electrode active material capable of inserting and releasing lithium ions have been attracting attention. In particular, there is a growing demand for space-saving thin rectangular batteries. In the current prismatic battery, positive and negative electrodes are used, in which a paint mixed with an electrode active material, a binder, a conductive material, etc. is applied on a strip-shaped metal foil in order to increase the efficiency of battery reaction by increasing the electrode area. After they are wound together with the separator, they are crushed and stored in a battery can.

The ratio of active material in this electrode is about 40.
20% to 30% by volume, the rest is binder, conductive material, metal foil, etc.
It is composed of 30% by volume and 30-40% by volume of pores.
Therefore, there is a problem that what does not originally contribute to the capacity of the electrode, such as the binder, the conductive material, and the metal foil, limits the battery capacity per volume. Further, when the wound electrode is housed in a rectangular battery can, the corner portion of the battery can cannot be filled and wasteful space is created, further reducing the capacity per unit volume.

Therefore, as one means for increasing the capacity per unit volume, an attempt has been made to form the electrode by a sintered body substantially made of an active material. When the electrode is made of a sintered body, it does not contain a binder, and since the conductive material can be unnecessary or reduced to a small amount, the packing density of the active material can be increased and the capacity per unit volume can be increased. it can. For example, Japanese Unexamined Patent Publication (Kokai) No. 5-299090 discloses a negative electrode obtained by pressure-bonding a copper foil onto a sintered body of petroleum pitch or a carbonaceous material, and Japanese Unexamined Patent Publication No. 8-180904 discloses a positive electrode formed of a sintered body of lithium composite oxide. Is disclosed.

Further, as a negative electrode active material, there is conventionally known a coke (for example, JP-A-62-122066 and JP-A-1-20).
No. 4361) and glassy carbon (Japanese Patent Laid-Open No. 2-6685).
No. 6, etc., amorphous carbon, natural (Japanese Patent Publication No. 62-234).
33) or man-made (JP-A-4-190555) graphite materials such as graphite have been proposed. However, the capacity per unit volume is insufficient regardless of whether an amorphous or crystalline carbon material is used, and further improvement in performance is desired.

Therefore, in order to increase the capacity per unit volume, attempts have been made to construct a negative electrode by using silicon or its compound as a negative electrode active material. For example, in Japanese Unexamined Patent Publication No. 7-29602, Li _x Si (0 ≦ x ≦ 5) is used as a negative electrode active material, graphite as a conductive material and a binder are added and molded into pellets, and a conductive adhesive is collected. A method for producing a negative electrode as an electric body is also disclosed in JP-A-5-744.
Japanese Patent Laid-Open No. 63-63 discloses a method of manufacturing a negative electrode by using a silicon single crystal as an active material and scissors with a nickel mesh.

[0007]

However, in order to increase the capacity per unit volume, even if the negative electrode using silicon as the active material is made of a sintered body, a large contact between the current collector and the sintered body is caused. The current situation is that the resistance increases the internal resistance of the battery and does not necessarily lead to a large capacity improvement. Also, considering the required capacity when used for mobile phones, etc., the bottom area of the electrode is 1 due to restrictions such as battery thickness.
It is desirable that it is cm ² or more. However, in the sintered body negative electrode containing silicon as a main component, those satisfying these at the same time could not be obtained by the conventionally known technique.

Therefore, an object of the present invention is to provide a method of manufacturing a negative electrode for a secondary battery, which can reduce the contact resistance between a current collector and a sintered body in a negative electrode containing silicon as an active material. .

[0009]

In order to achieve the above object, the present invention comprises baking a coating film containing silicon formed by using a peelable film and a base material made of a conductive metal foil or mesh. It was completed by finding that the above-mentioned problems can be solved by doing so. That is, the method for producing a negative electrode for a secondary battery of the present invention comprises a) a step of preparing a slurry by adding a binder and a solvent to a negative electrode material containing silicon, and b) applying the slurry onto a peelable film. A step of removing the solvent to prepare a coating film, and c) peeling the coating film from the above film, press-bonding the coating film on a base material made of a conductive metal foil or mesh, and firing the base material. It is characterized in that it includes a step of integrating and sintering. Further, in order to increase the density of the coating film, a step of pressing after the coating film may be added.

By firing the coating film containing silicon and the base material made of a conductive metal in a non-oxidizing atmosphere, the contact area of the interface between the sintered body and the current collector is increased and integrated. The contact resistance between the binder and the current collector can be reduced, and a thin-film negative electrode with improved conductivity can be provided. Furthermore, if a peelable film is used, it is possible to continuously dry the slurry-coated film, peel the coating film, and recover the coating film and film using a film winding device, etc. The effect that can be simplified is obtained.

Further, it is desirable that the above-mentioned negative electrode material contains a material or carbon material that is carbonized by heat treatment. In that case, in the presence of the material or carbon material that carbonizes silicon or its compound by heat treatment, 600 It is preferable to use a composite powder obtained by heat treatment in a temperature range of ˜1400 ° C. in a non-oxidizing atmosphere.

Further, when the above coating film is sintered, it is preferable to perform the sintering at a temperature not higher than the melting point of the conductive metal base material, whereby the base material is integrated with the sintered body without being thermally deformed. Can be done.

Further, by using any one metal selected from stainless steel, copper group and platinum group as the conductive metal, it is electrochemically stable and conductive even in the reduced state of the negative electrode. A high current collector can be obtained.

In the secondary battery negative electrode of the present invention, a negative electrode material sintered body is obtained by firing a coating film made of a negative electrode material containing silicon and a binder, and a base material made of a conductive metal foil or mesh. It is characterized by being integrated with the above-mentioned base material.

The thickness of the negative electrode material sintered body is 10
It is preferable that the bottom area is 1 cm ² or more in order to form a battery. If the bottom area is 1 cm ² or more and the thickness is smaller than 10 μm, the mechanical strength required for battery production is not sufficient, and if it is larger than 500 μm, sufficient capacity cannot be obtained when the current density during charging / discharging is large.

It is preferable that the sintered body contains 30 to 90% by weight of silicon and 10 to 70% by weight of a carbon material.

The non-aqueous secondary battery of the present invention is a sintered body which contains silicon as an active material and is integrated with a base material made of a conductive metal foil or mesh, and has a bottom area of 1c.
a negative electrode having a thickness of m ² or more and a thickness of 10 to 500 μm, and a positive electrode mainly composed of a lithium transition metal oxide,
An electrolyte solution in which a lithium compound is dissolved in an organic solvent, or a solid electrolyte containing a lithium ion conductive non-aqueous electrolyte in which a lithium compound is dissolved in a polymer or an organic solvent in which a lithium compound is dissolved is retained. It is a feature, and it is preferable to use a sintered body made of a lithium transition metal oxide also for the positive electrode.

Further, it is preferable that the non-aqueous secondary battery of the present invention is subjected to an electrochemical charge / discharge treatment. By carrying out this treatment, not only charging and discharging at high current density are possible, but also high capacity is obtained.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION As the silicon powder used in the present invention, either crystalline or amorphous silicon simple substance can be used, and a compound containing silicon may be used. Examples of the silicon compound include inorganic silicon compounds such as silicon oxide and materials such as silicone resins and silicon-containing polymer compounds that can be decomposed or reduced in a non-oxidizing atmosphere like an organic silicon compound to be converted into silicon. Of these, simple substance of silicon is particularly preferable. The purity of the silicon powder is not particularly limited, but in order to obtain a sufficient capacity, the silicon content is preferably 90% by weight or more, which is 9 from the economical point of view.
It is preferably 9.999% by weight or less. The particle size of the silicon powder is not particularly limited, but from the viewpoint of handling, raw material price, and uniformity of the negative electrode material, the average particle size is 0.01
Those having a size of not less than 100 μm and not more than 100 μm are preferably used.

Further, it is desirable to use a composite powder containing a carbon material as the negative electrode material used in the present invention. The composite powder has a range in which sufficient sintering occurs in the range where silicon is not melted in the presence of a carbon material or a material that carbonizes by heat treatment in a non-oxidizing atmosphere, that is, 6
It is manufactured by heat treatment at 00 to 1400 ° C, preferably 800 to 1200 ° C. Examples of the carbon material used here include coke, glassy carbon, graphite, carbide of pitch, and a mixture thereof.

As the material carbonized by heat treatment, a thermosetting resin such as a phenol resin, an epoxy resin, an unsaturated polyester resin, a furan resin, a urea resin, a melamine resin, an alkyd resin or a xylene resin, naphthalene or acenaphthylene. , Phenanthrene, anthracene, triphenylene, pyrene, chrysene, naphthacene, picene, perylene, pentaphene, pentacene and the like condensed polycyclic hydrocarbon compounds or derivatives thereof, or a pitch containing a mixture of the above compounds as a main component, Pitch is preferred.

As the conductive metal used for the substrate, any one metal selected from stainless steel, copper group and platinum group can be used, but it is easily reduced, has high conductivity, and is inexpensive. Copper is preferred. The conductive metal may be either foil or mesh, but the thickness is preferably 3 to 100 μm.

The releasable film may be any film having a smooth surface and a releasable coating film. For example, polymer films such as polyethylene, polypropylene, polyethylene terephthalate and polyethylene naphthalate can be used. . It is preferable that these films are peeled off. And the thickness is 3 ~ 100μ
m is desirable.

For coating the negative electrode material on the conductive substrate or film, a known binder dissolved in a suitable solvent such as water or N-methyl-2-pyrrolidone can be used. The solvent may be either aqueous or non-aqueous. As the binder, any conventionally known material such as polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyvinyl alcohol and polyvinylpyrrolidone can be used.

The temperature at which the negative electrode coating film is sintered is preferably not higher than the melting point of the conductive metal used. For example, when copper is used, the melting point is 1083 ° C. or lower, preferably 500 to
It is 900 ° C. When firing this negative electrode coating film, it can also serve as firing for producing the composite powder containing the above-mentioned silicon and carbon material.

The thickness of the sintered negative electrode material is preferably 10 μm or more from the viewpoint of strength, and 500 μm or less from the viewpoint of performance at high current density.

Further, in order to facilitate the handling during the construction of the battery, the negative electrode material sintered body preferably has a bottom area of 1 cm ² or more.

Further, the sintered body of the present invention is preferably a porous body having a porosity of 15 to 60% so that the electrolytic solution sufficiently contacts the active material.

As the positive electrode material used as the positive electrode active material of the present invention, any conventionally known material can be used.
i _x CoO ₂ , Li _x NiO ₂ , MnO ₂ , Li _x MnO ₂ ,
_{Li x Mn 2 O 4, Li} x Mn 2-y O 4, α-V 2 O 5, TiS
₂ etc.

The non-aqueous electrolyte used in the present invention is a non-aqueous electrolytic solution prepared by dissolving a lithium compound such as LiPF ₆ as an electrolyte in an organic solvent such as ethylene carbonate or dimethyl carbonate, or a solid solution of the lithium compound in a polymer. Alternatively, a polymer solid electrolyte holding an organic solvent in which a lithium compound is dissolved can be used.

Further, the battery assembled from the above members is characterized by undergoing a process of charging / discharging at a low current (aging process), and thereafter exhibiting charge / discharge at a high current density and a high capacity function as a battery. It is what In addition,
The low charge / discharge efficiency at high current densities of batteries that do not undergo this process is presumed to be because reactions involving a structural change in which lithium is inserted into crystalline silicon to amorphize cannot follow high current. It

The present invention will be described in detail below with reference to examples. Example 1. Equal weight of a commercially available crystalline silicon powder having a purity of 99.9% and an average particle diameter of 1 μm and a phenol resin were mixed and stirred, and cured at 80 ° C. for 3 days. The phenolic resin used here is cresol (m-cresol content rate 38%) 1
To 50 parts by weight, 135 parts by weight of 30% formaldehyde aqueous solution and 7.5 parts by weight of 25% ammonia water are mixed, and the mixture is heated to 85 ° C.
After heating at 105 ° C. for 105 minutes, water removed by vacuum distillation was used. The obtained silicon-containing phenol resin cured product was calcined in a nitrogen atmosphere at 1100 ° C. for 3 hours, and was dry pulverized to obtain a silicon / carbon composite powder. The obtained silicon / carbon composite powder is made into a slurry using an N-methyl-2-pyrrolidone solution of polyvinylidene fluoride which is a binder, coated on a PET (polyethylene terephthalate) film and dried to form a coating film. 20m after peeling from
It was cut out into m × 20 mm, placed on a 22 mm × 20 mm copper foil, and pressure-bonded by a press machine. This copper foil-containing coating film was baked at 800 ° C. for 3 hours in a nitrogen atmosphere to give a negative electrode. The thickness of the negative electrode material sintered body was 220 μm.

The positive electrode was manufactured as follows. Lithium carbonate Li ₂ CO ₃ and cobalt carbonate CoCO ₃ are in a molar ratio of 1:
Weigh 2 and wet-mix with a ball mill using isopropyl alcohol, evaporate the solvent and calcinate at 800 ° C. for 1 hour. After recalculating the calcined powder with a vibration mill, it is 20 mm x 20 mm at a molding pressure of 127 MPa and a thickness of 0.5 m.
After being pressure-molded into a pellet of m, it was baked at 800 ° C. for 10 hours to obtain a positive electrode.

The electrolyte used was a solution of 1 mol / L of LiPF ₆ dissolved in a mixed solvent of ethylene carbonate and dimethyl carbonate at a volume ratio of 1: 1.

The prismatic battery thus produced was left at room temperature for one day and then subjected to the charge / discharge test described later. This battery was charged with a constant current of 1.5 mA, and its discharge capacity was examined. The charging / discharging cycle started from charging. The results are shown in Table 1.

Comparative Example 1. A rectangular battery was prepared and a charge / discharge test was conducted in the same manner as in Example 1 except that a commercially available silicon wafer having a thickness of 200 μm was used as it was for the negative electrode. In this case, it was found that all batteries having a charge amount of more than 20 mAh were short-circuited during the test. Therefore, since it is necessary to limit the amount of lithium inserted into the negative electrode, it is not possible to obtain a high discharge capacity with a commercially available silicon wafer. The results are shown in Table 1.

Comparative Example 2. The silicon / carbon composite powder obtained by the method according to Example 1 was slurried using polyvinylidene fluoride as a binder and N-methyl-2-pyrrolidone as a solvent, and applied to a copper foil at 140 ° C. It was dried, cut out to a size of 20 mm × 20 mm, and pressure-bonded with a flat plate press to obtain a negative electrode. The coating thickness of this negative electrode is 210 μm
Met. Below, a prismatic battery was prepared in the same manner as in Example 1, and a charge / discharge test was conducted. The results are shown in Table 1.

[0038]

[Table 1]

As is clear from Table 1, in Example 1,
Although a high capacity of 78 mAh was obtained, Comparative Examples 1 and 2
However, only a low capacity of 5 mAh was obtained.

Example 2. A negative electrode was obtained in the same manner as in Example 1, except that the coating film was cut into a size of 20 mm × 40 mm and placed on a 20 mm × 45 mm copper foil. Except that the size of the positive electrode was 20 mm x 40 mm,
The same operation as in Example 1 was carried out to obtain a positive electrode. As the electrolytic solution, a solution obtained by dissolving LiPF ₆ at 1 mol / L in a mixed solvent of ethylene carbonate and dimethyl carbonate at a volume ratio of 1: 1 was used.

The prismatic battery thus manufactured was allowed to stand at room temperature for a whole day and night, and then the current to the negative electrode was 5.3 mA.
After performing a process of inserting and releasing lithium at (40 mA / g), a charge / discharge test was performed at a current value of 20 mA (150 mA / g). The results are shown in Table 2.

Example 3. The prismatic battery described in Example 1 was left at room temperature for a whole day and night, and then immediately subjected to a charge / discharge test at a current value of 20 mA. The results are shown in Table 2.

[0043]

[Table 2]

As is clear from Table 2, the effect of greatly increasing the capacity was obtained by subjecting the assembled battery to the electrochemical charge / discharge treatment.

[0045]

As is apparent from the above description, the method for producing a negative electrode for a secondary battery of the present invention comprises forming a coating film containing silicon on a peelable film, and then coating the coating film from the film. Since the coating film was peeled off, the coating film was pressure-bonded onto a base material made of a conductive metal foil or mesh and baked, and the coating film was integrated with the base material and sintered, so that a sintered body and a current collector were formed. The contact area of the interface with and can be increased, the contact resistance between the sintered body and the current collector can be reduced, and a negative electrode with improved conductivity can be provided.
Further, since the peelable film is used, it is possible to continuously perform the production of the coating film and the recovery of the film using the film winding device or the like, which can simplify the manufacturing process and reduce the manufacturing cost. .

The negative electrode for a secondary battery of the present invention is a sintered body which contains silicon as an active material and is integrated with a base material made of a conductive metal foil or mesh, and has a bottom area of 1 cm ² or more. Since the thickness is 10 to 500 μm, the internal resistance can be reduced and the battery capacity can be improved.

Since the non-aqueous secondary battery of the present invention uses the above-mentioned secondary battery negative electrode as the negative electrode, it is possible to provide a battery which can be made thinner while ensuring a predetermined capacity.

Since the non-aqueous secondary battery of the present invention is subjected to the electrochemical charge / discharge treatment, it is possible to provide a battery having a high capacity even at a high current density.

─────────────────────────────────────────────────── --- Continuation of the front page (72) Inventor Tadashi Hirabayashi 1334 Minato Minato, Wakayama City, Wakayama Prefecture, Kao Corporation Research Institute (72) Inventor Jun Jun, 1334 Minato Minato, Wakayama City, Wakayama Prefecture (56) References JP 8-222273 (JP, A) JP 8-180904 (JP, A) JP 9-249407 (JP, A) JP 8-236104 (JP, A) JP 5-144474 (JP, A) JP-A 5-129018 (JP, A) International Publication 98/024135 (WO, A1) International Publication 98/028804 (WO, A1) (58) Fields investigated (Int.Cl. ⁷ , DB) Name) H01M 4/04 H01M 4/02 H01M 4/38 H01M 4/64 H01M 10/40

Claims (4)

(57) [Claims] 1. A step of preparing a slurry by adding a binder and a solvent to a negative electrode material containing silicon, and b) applying the slurry on a peelable film,
A step of producing a coating film by removing the solvent, and c) peeling the coating film from the above film, press-bonding the coating film on a base material made of a conductive metal foil or mesh, and baking it to integrate it with the base material. A method for producing a negative electrode for a secondary battery, which comprises the steps of oxidization and sintering. 2. A sintered body containing silicon as an active material and integrated with a base material composed of a conductive metal foil or mesh, having a bottom area of 1 cm ² or more and a thickness of 10 to 50.
A negative electrode for a secondary battery having a size of 0 μm. 3. The negative electrode according to claim 2, a positive electrode mainly composed of a lithium transition metal oxide, an electrolytic solution in which a lithium compound is dissolved in an organic solvent, or a solid solution or a lithium compound dissolved in a polymer. A non-aqueous secondary battery comprising a solid electrolyte containing a lithium ion conductive non-aqueous electrolyte that holds the organic solvent. 4. A non-aqueous secondary battery obtained by subjecting the battery according to claim 3 to electrochemical charge / discharge treatment.