WO2008018208A1

WO2008018208A1 - Non-aqueous electrolyte secondary battery

Info

Publication number: WO2008018208A1
Application number: PCT/JP2007/058084
Authority: WO
Inventors: Hitohiko Ide; Yanko Marinov Todorov; Yoshiki Sakaguchi
Original assignee: Mitsui Mining & Smelting Co., Ltd.
Priority date: 2006-08-10
Filing date: 2007-04-12
Publication date: 2008-02-14
Also published as: JP2008047305A

Abstract

A non-aqueous electrolyte secondary battery comprises a negative electrode having a negative electrode active material layer and a positive electrode having a positive electrode active material layer, wherein the negative electrode active material layer contains Si and the positive electrode active material layer contains a lithium-transition metal composite oxide containing Li and Co as constituent elements. The weight-based ratio of the amount of Co in the positive electrode to the amount of Si in the negative electrode (i.e., a Co/Si ratio) falls within the range from 0.3 to 5.5. Preferably, the content of Co in the lithium-transition metal composite oxide in the positive electrode active material layer is 10 to 40% by weight relative to the total amount of the transition metal contained in the composite oxide.

Description

Specification

Nonaqueous electrolyte secondary battery

Technical field

The present invention relates to non-aqueous electrolyte secondary batteries such as lithium secondary batteries.

Background art

A lithium transition metal composite oxide is generally used as a positive electrode active material in a lithium secondary battery. On the other hand, carbon-based materials such as graphite are generally used as the negative electrode active material. In addition, use of silicon-based materials and tin-based materials, which are materials having larger capacities than carbon-based materials, has been attempted as an active material of the next generation. These positive electrode active materials and negative electrode active materials are known to expand and contract when lithium is absorbed and released by charge and discharge of the battery. The stress caused by the volume change due to the expansion and contraction contributes to the damage to the positive electrode and the negative electrode, which tends to deteriorate the cycle characteristics of the battery.

[0003] For the purpose of preventing volume change due to charge and discharge, as a positive electrode active material, an active material whose crystal structure expands at the time of discharge and whose crystal structure shrinks at the time of charge; It has been proposed to mix and use an active material which shrinks and the crystal structure expands at the time of charge (see Patent Document 1). The active material layer of the positive electrode is composed of two or more mixture layers, and as the active material contained in one of the adjacent mixture layers, a lithium-containing transition metal oxide which expands during discharge and condenses during charge is used. It has been proposed to use a lithium-containing transition metal oxide that contracts during discharge and expands during charge as the active material contained in one of the mixture layers (see Patent Document 2).

On the other hand, with regard to the negative electrode active material, as the active material disposed on one side of the negative electrode current collector, a material having a larger change in expansion and contraction during charge and discharge than the active material disposed on the other side is used. It has been proposed to use (see Patent Document 3). In this document, an active material which expands in the charging process and contracts in the discharging process is arranged on one side of the positive electrode current collector, and an active material which contracts in the charging process and expands in the discharging process is arranged on the other side. It is also suggested to do!

[0005] With respect to the volume change caused by charge and discharge, Patent Document 4 separately from each of the documents described above Uses a spinel-type lithium manganese composite oxide, which is a material that has a large crystal lattice contraction during lithium release (that is, during battery charging) as a positive electrode active material, and expansion when lithium is stored as a positive electrode active material. It is pointed out that the use of a carbon material, which is a material to be used, has the disadvantage that the carbon material on the negative electrode side is easily loosened during charging. Therefore, in the document, it has been proposed to suppress the above-mentioned loosening on the negative electrode side during charging by suppressing the contraction of the positive electrode active material at the time of releasing lithium.

In each of the above documents, the negative electrode active material used is a carbon-based material, but the material expands when lithium is absorbed, but the degree of the expansion is In comparison with silicon-based materials and tin-based materials that are considered as next-generation negative electrode active materials, they are not so large. Therefore, in the systems described in the above-mentioned respective documents, the same applies to the case where a material having a larger degree of expansion at the time of lithium storage is used as the negative electrode active material instead of the material based on carbon. It is unclear whether the effect will be achieved or not.

Patent Document 1: Japanese Patent Application Laid-Open No. 5-82131

Patent Document 2: US Patent No. 5677083

Patent Document 3: Japanese Patent Application Laid-Open No. 10-064515

Patent Document 4: Japanese Patent Application Laid-Open No. 2002-75361

An object of the present invention is to provide a non-aqueous electrolyte secondary battery in which the degree of expansion and contraction of the entire battery due to charge and discharge is reduced as compared with the above-described battery of the prior art.

Disclosure of the invention

The present invention comprises a negative electrode having a negative electrode active material layer containing Si, and a positive electrode having a positive electrode active material layer containing a lithium transition metal composite oxide containing Li and Co as constituent elements, A weight ratio (CoZSi) of Co in the positive electrode to Si in the positive electrode is in the range of 0.3 to 5.5, to provide a non-aqueous electrolyte secondary battery.

Brief description of the drawings

FIG. 1 is a schematic view showing a cross-sectional structure of an embodiment of a negative electrode used in the non-aqueous electrolyte secondary battery of the present invention.

[FIG. 2] It is process drawing which shows the manufacturing method of the negative electrode shown in FIG. Detailed Description of the Invention

The present invention will be described below based on its preferred embodiments. The non-aqueous electrolyte secondary battery (hereinafter, also simply referred to as a secondary battery or battery) of the present invention has a positive electrode, a negative electrode, and a separator disposed therebetween as basic components. The space between the positive electrode and the negative electrode is filled with a non-aqueous electrolyte via a separator. The battery of the present invention may be in the form of a cylinder, a square, a coin or the like provided with these basic components. It is not limited to these forms.

The positive electrode used in the battery of the present invention is, for example, one obtained by forming a positive electrode active material layer on at least one surface of a current collector. The positive electrode active material layer contains an active material. What is used in the present invention as this active material is a lithium transition metal complex oxide containing Li and Co as constituent elements. Since this composite oxide contains Co as one of its constituent elements, it has the property of contracting when storing lithium and expanding when releasing lithium. The lithium-transition metal complex oxide used as a positive electrode active material of lithium secondary batteries includes, in addition to a material containing Co as one of its constituent elements, for example, Ni as a constituent element such as LiNiO.

2

And those containing Mn as a constituent element such as LiMn O are known.

twenty four

However, these materials have an expansion force at the time of lithium storage or a degree of contraction at the time of lithium storage that is smaller than that of the lithium-transition metal complex oxide containing Li and Co used in the present invention as constituent elements. It is a thing.

As the above-mentioned complex acid product, for example, one represented by the following formula (1) is used.

LiCo M O (1)

X 1-X 2

(Wherein, X represents a positive number less than 1 and M represents a metal element).

Examples of the metal element represented by M in the above formula include transition metal elements other than Co and typical metal elements other than Li. Examples of transition metal elements include Ni, Mn, Fe, V, Zr, Ti, Mo, W, Nb and the like. In particular, it is preferable to use Ni and Z or Mn as a transition metal element. On the other hand, examples of typical metal elements include Mg, Al and Ga. As described above, X in the complex acid product is a positive number less than 1, preferably 0.1 to 0.4, more preferably 0.1 to 0.30.

[0015] Further, the amount of Co contained in the above complex oxide is the transition metal contained in the complex oxide. The amount is preferably 10 to 40% by weight, more preferably 10 to 30% by weight, based on the total amount of the group elements. By setting the amount of Co in the above-mentioned complex acid product within this range, it is possible to make the degree of contraction when the complex acid occludes lithium within an appropriate range. .

As a specific example of the above-mentioned complex acid product, for example, Li (Co Mn Ni) 0 (wherein, a + b + c a b c 2

= 1, 0 <a <l, 0 ≤ b <l, 0 ≤ c <l, but b and c do not simultaneously become 0), Li (Co Fe a b

0 (wherein, a + b = l, 0 <a <l, 0 <b <l), Li (Co V) 0 (where in formula a + b = 1, 0 times a

2 a b 2

1, 0 <b <l), etc. These complex acids may be used alone or in combination of two or more. Among these complex acid products, it is preferable to use a material having a large volume change due to the absorbed and released lithium. The reason is that the material containing Si, which is a negative electrode active material used in combination with the composite acid oxide, is a material having a large volume change due to lithium absorption and release, and thus contains the composite acid oxide and Si. This is because the volume change of the entire battery is easily offset by the material. Preferred materials from this viewpoint are, for example, LiMn Co Ni O, LiMn Co Ni O, LiMn Co Ni

0.3 0.4 0.3 2 0.45 0.10 0.45 2 1/3 1/3 o etc. are mentioned. These materials are known methods such as lithium carbonate and transition metals

1/3 2

It is produced by firing the oxide with the atmosphere.

The positive electrode used in the present invention is prepared by suspending the above composite oxide in a suitable solvent together with a conductive agent such as acetylene black and a binder such as polyvinylidene fluoride to prepare a positive electrode mixture, After applying and drying on at least one surface of a current collector such as an aluminum foil, it can be obtained by roll rolling and pressing.

The negative electrode used in the battery of the present invention is, for example, one obtained by forming a negative electrode active material layer on at least one surface of a current collector. The negative electrode active material layer contains an active material. What is used in the present invention as this active substance is a substance containing Si. The negative electrode active material containing Si has a property of expanding when occluding lithium and contracting when releasing. The degree of expansion and contraction is extremely large as compared with the carbon-based material conventionally used as the negative electrode active material of lithium secondary batteries.

The negative electrode active material containing Si is capable of absorbing and releasing lithium ions. For example, silicon alone, an alloy of silicon and metal, silicon oxide, etc. can be used. Ru. These materials can be used alone or in combination of two or more. Examples of the metal include one or more elements selected from the group consisting of Cu, Ni, Co, Cr, Fe, Ti, Pt, W, Mo and Au. Among these metals, it is desirable to use Cu and Ni from the viewpoint that Cu, Ni and Co are preferred, in particular, the excellent electron conductivity and the low ability to form a lithium compound. In addition, before or after the negative electrode is incorporated into the battery, lithium may be absorbed into the active material which is also a silicon-based material. Particularly preferable silicon-based materials are point force silicon or silicon dioxide having a high lithium storage capacity.

The negative electrode active material layer may be, for example, a continuous thin film layer made of the above-mentioned negative electrode active material, a coating layer containing particles of the above-mentioned negative electrode active material, or a sintered layer containing particles of the above-mentioned negative electrode active material. possible . Also, it may be a layer having a structure shown in FIG. 1 described later.

As the separator in the battery of the present invention, a synthetic resin non-woven fabric, a polyolefin such as polyethylene or polypropylene, or a porous film of polytetrafluoroethylene is preferably used. From the viewpoint of suppressing the heat generation of the electrode generated at the time of overcharging of the battery, it is preferable to use a separator in which a thin film of a phenate derivative is formed on one side or both sides of the microporous polyolefin membrane. The separator preferably has a piercing strength of not less than 0.2 NZ m and not more than 0.49 wm, and preferably has a tensile strength in the winding axial direction of not less than 0 MPa and not more than 150 MPa. Even with the use of a negative electrode active material that greatly expands and contracts with charging and discharging, damage to the separator can be suppressed, and the occurrence of internal short circuit can be suppressed.

The non-aqueous electrolytic solution is a solution in which a lithium salt as a supporting electrolyte is dissolved in an organic solvent.

As a lithium salt, CF 2 SO 4 Li (CF 2 SO 4) NLi (C 4 F 2 SO 4) NLi ゝ LiCIO, LiAl

3 3 3 2 2 5 2 2 4

Examples thereof include CI, LiPF, LiAsF, LiSbF, LiCl, LiBr, Lil, LiC 3 F 2 SO 4 and the like. these

4 6 6 6 4 9 3

These can be used alone or in combination of two or more. Among these lithium salts, CF SO Li, (CF 2 SO 4) NLi, and (C 4 F 2 SO 4) NLi are preferred because of their excellent resistance to water degradation.

It is preferable to use 3 3 3 2 2 5 2 2. Examples of the organic solvent include ethylene carbonate, jetyl carbonate, dimethyl carbonate, propylene carbonate, butylene carbonate and the like. In particular, 0.5 to 5% by weight of vinylene carbonate with respect to the whole non-aqueous electrolyte And 0.1 wt% to 1 wt% of dibiylsulfone and 0.1 wt% to 1.5 wt% of 1,4-butanediol dimethanesulfonate are preferred from the viewpoint of further improving charge-discharge cycle characteristics. Although the details are not clear about the reason, the formation of a film on the positive electrode by step decomposition of 1,4 butanediol dimethan sulfonate and divinyl sulfone makes the film containing sulfur more dense. It is considered to be for. In particular, as non-aqueous electrolytes, halogen atoms such as 4-fluoro-1, 3-dioxolan-2, 4-chloro-1, 3-dioxolan-1, 4-trifluoromethyl-1, 3-dioxolane 2-, etc. It is also preferable to use a high dielectric constant solvent having a dielectric constant of 30 or more, such as a cyclic carbonate derivative having It is because reduction resistance is high and decomposition is difficult. In addition, it is also preferable to use an electrolyte prepared by mixing the above-mentioned high dielectric constant solvent with a low viscosity solvent having a viscosity of 1 mPa · s or less such as dimethyl carbonate, jetyl carbonate, or methyl ethyl carbonate. It is because higher ion conductivity can be obtained. Furthermore, it is also preferable that the content of fluorine ions in the electrolytic solution is in the range of 14 mass ppm or more and 1290 mass ppm or less. If the electrolytic solution contains an appropriate amount of fluorine ions, a film such as lithium fluoride derived from the fluorine ions is formed on the negative electrode, which is also considered to be capable of suppressing the decomposition reaction of the electrolytic solution on the negative electrode. . Furthermore, it is preferable that at least one additive in the acid anhydride and its derivative group be contained in an amount of 0.001% by weight to 10% by weight. As a result, a film is formed on the surface of the negative electrode, which is also a force capable of suppressing the decomposition reaction of the electrolytic solution. As this additive, a cyclic compound containing one c (= o) -0-c (= o) group in the ring is preferable. For example, succinic anhydride, dartalic anhydride, maleic anhydride, phthalic anhydride, Phthalic anhydride derivatives such as 2-sulfobenzoic acid anhydride, citraconic acid anhydride, itaconic acid anhydride, diglycolic acid anhydride, hexafluoroglutaric acid anhydride, 3-fluorophthalic acid anhydride, 4-fluorophthalic acid anhydride, or Anhydrous 3, 6-Epoxide 1, 2, 3, 6-Tetrahydrophthalic Acid, Anhydrous 1, 8 Naphthalic Acid, Anhydrous 2, 3 Naphthalene Carboxylic Acid, Anhydrous 1, 2-Cyclopentanedicarboxylic Acid, 1, 2- Cyclo Tetrahydrophthalic acid such as anhydrous 1,2 cycloalkanedicarboxylic acid such as hexanedicarboxylic acid or cis 1,2,3,6 tetrahydrophthalic anhydride or 3,4,5,6-tetrahydrophthalic anhydride Anhydride, or Hexahi Rofutaru anhydride (cis isomer, trans isomer And 3,4,5,6-tetrachlorophthalic anhydride, 1,2,4-benzenetricarboxylic anhydride, pyromellitic dianhydride, or derivatives thereof.

The present invention is characterized in that the weight ratio (CoZSi) of Co in the positive electrode to Si in the negative electrode is set in the range of 0.3 to 5.5. As described above, the positive electrode active material used in the present invention has the property of expanding when storing lithium and shrinking when releasing lithium, while the negative electrode active material expands when storing lithium and is released. It has the property of shrinking from time to time. That is, the positive electrode active material and the negative electrode active material used in the present invention exhibit opposite behavior of volume change with respect to lithium absorption and release. Therefore, when the battery of the present invention is charged, the negative electrode active material expands and the positive electrode active material contracts. Conversely, during discharge, the positive electrode active material expands and the negative electrode active material contracts. Thus, the volume of the expansion of the negative electrode active material is absorbed by the volume of the contraction of the positive electrode active material during charging, and conversely, the volume of the expansion of the positive electrode active material during discharge is the negative electrode active. It is absorbed by the volume of material contraction. This makes it possible to suppress the occurrence of stress due to the volume change of the entire battery during charging and discharging, and it is effective to damage the positive electrode, the negative electrode, the separator, etc. Is prevented. As a result, the cycle characteristics of the battery are improved.

Particularly, in the present invention, with regard to lithium absorption and release, the ratio of the positive electrode active material and the negative electrode active material, which exhibit the opposite behavior of the volume change, is not simply used, specifically the weight ratio of CoZSi. Is set within the above range, it is possible to more effectively suppress the occurrence of stress due to the volume change of the entire battery. Specifically, by setting the weight ratio of CoZSi to 0.3 or more, the deterioration of the rate characteristic caused by the positive electrode due to the reduction of the amount of Co is avoided, and the Si negative electrode has a high energy density (or high energy density) (or Capacity) will be obtained. Further, by setting the weight ratio of CoZSi to 5.5 or less, the balance between the contraction of the positive electrode active material and the expansion of the negative electrode active material at the time of charge becomes good, and the stress is sufficiently relieved. From the viewpoint of making these advantageous effects more remarkable, it is particularly preferable to set the weight ratio of CoZSi to 0.4 to 2.5, particularly 0.4 to 1.

FIG. 1 is a schematic view of the cross-sectional structure of a preferred embodiment of the negative electrode used in the present invention. The negative electrode 10 of the present embodiment has a current collector 11 and at least one surface thereof. The formed active material layer 12 is provided. In FIG. 1, although the state in which the active material layer 12 is formed only on one side of the current collector 11 is shown for convenience! / They, the active material layer is formed on both sides of the current collector. It is done.

In the active material layer 12, at least a part of the surface of the particle 12a of the active material containing Si is coated with a metal material having a low ability to form a lithium compound. The metal material 13 is a material different from the material of the particles 12a. An air gap is formed between the particles 12a coated with the metal material. That is, the metal material covers the surface of the particles 12 a in a state where a clearance is reached so that the non-aqueous electrolyte containing lithium ions can reach the particles 12 a. In FIG. 1, the metal material 13 is conveniently represented as a thick line surrounding the periphery of the particle 12a. The figure is a schematic view of the active material layer 12 two-dimensionally. In actuality, each particle is brought into contact with other particles directly through the metallic material 13. “Low ability to form lithium compound” means that it does not form an intermetallic compound or a solid solution with lithium, or if it is formed, lithium has a slight amount of force or is very unstable.

The metal material 13 has conductivity, and examples thereof include copper, nickel, iron, cobalt, alloys of these metals, and the like. In particular, the metal material 13 is preferably a highly ductile material in which the coating on the surface of the particles 12a is not easily broken even if the particles 12a of the active material expand and contract. It is preferable to use copper as such a material.

Preferably, the metal material 13 is present on the surface of the particles 12 a of the active material over the entire thickness of the active material layer 12. Preferably, particles 12 a of the active material are present in the matrix of the metal material 13. As a result, even if the fine particles are shredded due to expansion and contraction of the particles 12a due to charge and discharge, it is difficult for the particles to come off. In addition, since the electron conductivity of the entire active material layer 12 is ensured through the metal material 13, electrically isolated active material particles 12 a are formed, particularly in the deep part of the active material layer 12. The formation of particles 12a of the active material is effectively prevented. The presence of the metal material 13 on the surface of the particles 12a of the active material over the entire thickness of the active material layer 12 can be confirmed by electron microscopic mapping using the material 13 as a measurement target.

The metal material 13 covers the surface of the particles 12 a continuously or discontinuously. In the case where the metallic material 13 continuously coats the surface of the particle 12a, the coating of the metallic material 13 It is preferable to form a minute gap which allows the solution to flow. When the metallic material 13 covers the surface of the particle 12a discontinuously, the non-aqueous electrolytic solution is supplied to the particle 12a through the portion not covered with the metallic material 13 in the surface of the particle 12a. . In order to form a coating of the metal material 13 of such a structure, the metal material 13 may be deposited on the surface of the particle 12a by electrolytic plating according to the conditions described later, for example.

The metal material 13 covering the surface of the particles 12 a of the active material preferably has an average thickness of 0.5 to 2 / ζπι, and more preferably 0.1 to 0.25 / zm. And, it's too thin! That is, the metal material 13 covers the surface of the particles 12 a of the active material with a minimum thickness. As a result, while the energy density is increased, the particles 12 a are prevented from coming off due to expansion and contraction and pulverization due to charge and discharge. Here, the “average thickness” is a value calculated based on the portion of the surface of the particles 12 a of the active material that is actually covered by the metal material 13. Therefore, the part of the surface of the particles 12a of the active material which is not coated with the metal material 13 can not be used as a basis for calculating the average value.

The voids formed between the particles 12 a coated with the metal material 13 serve as a flow path of the non-aqueous electrolyte containing lithium ions. Since the non-aqueous electrolyte flows smoothly in the thickness direction of the active material layer 12 due to the presence of the voids, the cycle characteristics can be improved. Furthermore, the voids formed between the particles 12a also function as spaces for relieving stress caused by volume change of the particles 12a of the active material during charge and discharge. An increase in the volume of the particles of the active material 12a whose volume has been increased by charging is absorbed in this space. As a result, fine particles of the particles 12 a are less likely to occur, and significant deformation of the negative electrode 10 is effectively prevented.

The active material layer 12 is preferably prepared by applying a slurry containing the particles 12 a and a binder onto a current collector and drying it, as described later. It is formed by performing the electrolytic plating used and depositing the metal material 13 between the particles 12a.

In order to form necessary and sufficient voids in the active material layer 12 in which the non-aqueous electrolytic solution can flow, it is preferable to allow the plating solution to sufficiently penetrate into the coating film. In addition to this, it is preferable to set appropriate conditions for depositing the metal material 13 by electrolytic plating using the plating solution. Plating conditions include plating bath composition, plating bath pH, There is the current density of the solution. With regard to the pH of the plating bath, it is preferable to adjust it to 7. 1 to: L 1. By setting the pH to this range, the dissolution of the particles 12a of the active material is suppressed, the surface of the particles 12a is cleaned, and the plating on the particle surface is promoted, and at the same time, between the particles 12a. An adequate void is formed. The pH value is measured at the temperature at plating.

When copper is used as the metal material 13 for plating, it is preferable to use a copper pyrophosphate bath. When nickel is used as the metal material, it is preferable to use, for example, an alkaline nickel bath. In particular, it is preferable to use a copper pyrophosphate bath, even if the active material layer 12 is thickened, since the above-mentioned voids can be easily formed over the entire thickness of the layer. In addition, the metal material 13 is deposited on the surface of the particles 12a of the active material, and the precipitation of the metal material 13 is less likely to occur between the particles 12a, so that the gaps between the particles 12a are well formed. The point is also preferable. When a copper pyrophosphate bath is used, its bath composition, electrolytic conditions and pH are preferably as follows.

'Pyrophosphate copper trihydrate: 85-120 g Zl

-Pi 13 Jin Kajikumu: 300-600 g / l

'Potassium nitrate: 15-65g Zl

'Bath temperature: 45-60 ° C

'Current density: 1 to 7 AZdm ²

• pH: Add ammonia water and polyphosphoric acid and adjust to pH 7. 9-9. 5.

[0036] In particular, when a copper pyrophosphate bath is used, the ratio of the weight of P 2 O to the weight of Cu (P 0 ZCu

2 7 2 7

It is preferable to use one having a P ratio of 5 to 12 as defined in the above. When the P ratio is less than 5, the metal material 13 covering the particles 12a of the active material tends to be thick, and it may be difficult to form desired voids between the particles 12a. In addition, when the P ratio exceeds 12, the current efficiency may be deteriorated, and gas generation may easily occur, which may lower the production stability. Furthermore, using a bath having a P ratio of 6.5 to 50.5 as a preferable pyrophosphate copper bath, the size and number of the void formed between the particles 12a of the active material non-aqueous in the active material layer 12 It becomes very advantageous to the distribution of electrolyte solution.

When an alkaline nickel bath is used, the bath composition, electrolytic conditions and pH are as follows: It is preferable that

'Nickel sulfate: 100 to 250 g Zl

Ammonium chloride: 15 to 30 g Zl

'Boric acid: 15-45 g Zl

'Bath temperature: 45-60 ° C

'Current density: 1 to 7 AZdm ²

• pH: 25 weight ^_0/0 aqueous ammonia: 100～300GZl adjusted to be Roita8～ 11 in the range of.

When this alkaline nickel bath is compared with the above-mentioned copper pyrophosphate bath, an appropriate gap tends to be formed in the active material layer 12 when the copper pyrophosphate bath is used, and the life of the negative electrode is increased. Easy to plan, so preferred.

The properties of the metal material 13 can be appropriately adjusted by adding various additives used in an electrolytic solution for producing a copper foil, such as protein, active sulfur compound, and cellulose, to the various plating baths described above. It is.

The proportion of voids in the entire active material layer formed by the various methods described above, that is, the void ratio is preferably about 15 to 45% by volume, and particularly preferably about 20 to 40% by volume. By setting the porosity within this range, it is possible to form a void capable of circulating the non-aqueous electrolyte in the active material layer 12 as necessary and sufficiently. The void volume of the active material layer 12 is measured by mercury intrusion method CFIS R 1655). Mercury porosimetry is a method for obtaining information on the physical shape of a solid by measuring the size and volume of pores in the solid. The principle of the mercury intrusion method is to apply pressure to mercury and inject it into the pores of the object to be measured, and to measure the relationship between the pressure at that time and the volume of mercury that has been pressed (entered). is there. In this case, the water silver also intrudes into the large air gap forces present in the active material layer 12 in sequence. In the present invention, the void volume measured at a pressure of 90 MPa is regarded as the total void volume. The porosity (%) of the active material layer 12 is obtained by dividing the amount of voids per unit area measured by the above method by the apparent volume of the active material layer 12 per unit area and multiplying it by 100. Ask.

The porosity can be controlled by appropriately selecting the particle size of the particles 12 a of the active material. In this respect, the particles 12a preferably have a maximum particle size of 30. It is m or less, more preferably 10 / z m or less. The particle diameter of the particles is preferably 0.1 to 8 μm, particularly preferably 0.3 to 4 μm, in terms of D50 value. The particle size of the particles is measured by laser diffraction / scattering particle size distribution measurement, electron microscopic observation (SEM observation).

If the amount of the active material relative to the entire negative electrode is too small to sufficiently improve the energy density of the battery, on the other hand, if the amount is too large, the strength tends to decrease and the active material tends to fall off easily. Taking these into consideration, the thickness of the active material layer is 10 to 40 111, preferably 15 to 30 m, and more preferably 18 to 25 m.

In the negative electrode 10 of the present embodiment, a thin surface layer (not shown) may be formed on the surface of the active material layer 12. The negative electrode 10 may not have such a surface layer. The thickness of the surface layer is as thin as 0.25 μm or less, preferably 0.1 μm or less. There is no limit to the lower limit of the thickness of the surface layer. By forming the surface layer, dropping of the particles 12a of the micronized active material can be further prevented. However, in the present embodiment, by setting the porosity of the active material layer 12 within the above-described range, it is possible to sufficiently prevent the particles 12 a of the finely divided active material from falling off without using the surface layer. It is possible.

[0043] When the negative electrode 10 has a thickness as described above, has a surface layer, or has a surface layer, the secondary battery is assembled using the negative electrode 10, and the battery is initially charged. The over-voltage can be reduced. This means that reduction of lithium on the surface of the negative electrode 10 can be prevented during charging of the secondary battery. The reduction of lithium leads to the generation of dendrite which causes a short circuit between the two poles.

When the negative electrode 10 has a surface layer, the surface layer covers the surface of the active material layer 12 continuously or discontinuously. When the surface layer covers the surface of the active material layer 12 continuously, the surface layer has a large number of microvoids (not shown) open on its surface and communicating with the active material layer 12. It is preferable to have it. The microvoids are preferably present in the surface layer so as to extend in the thickness direction of the surface layer. The microvoids allow the non-aqueous electrolyte to flow. The role of the microvoids is to supply the non-aqueous electrolyte into the active material layer 12. When the surface of the negative electrode 10 is viewed in plan by electron microscope observation, the percentage of the area covered with the metal material 13, that is, the coverage is 95% or less, particularly 80% or less, and particularly 60% or less It is preferable that it is such a magnitude | size. When the coverage exceeds 95%, high viscosity The non-aqueous electrolyte may not easily enter, and the range of choices of the non-aqueous electrolyte may be narrowed.

[0045] The surface layer has a low ability to form a lithium compound, and the metal material force is configured. This metal material may be the same as or different from the metal material 13 present in the active material layer 12. The surface layer may have a structure of two or more layers composed of two or more different metal materials. In consideration of the ease of manufacture of the negative electrode 10, the metal material 13 present in the active material layer 12 and the metal material constituting the surface layer are preferably the same type.

In the negative electrode 10 of the present embodiment, the porosity in the active material layer 12 is a high value, so that the resistance to bending is high. Specifically, the MIT folding resistance measured in accordance with JIS C 6471 preferably has a high folding resistance of 30 times or more, more preferably 50 times or more. The high folding resistance is extremely advantageous because the negative electrode 10 is broken when the negative electrode 10 is folded or wound and housed in the battery case. As a MIT folding apparatus, for example, a film-clad film bending fatigue tester (No. 549) manufactured by Toyo Seiki Seisaku-sho, Ltd. is used, and a bending radius of 0.8 mm, a load of 0.5 kgf, and a sample size of 15 X 150 mm can do.

As the current collector 11 of the negative electrode 10, the same one as conventionally used as a current collector of a negative electrode for a non-aqueous electrolyte secondary battery can be used. The current collector 11 is preferably composed of a metal material having a low ability to form a lithium compound as described above. Examples of such metal materials are as described above. In particular, it is preferably made of copper, nickel, stainless steel or the like. In addition, copper alloy foils represented by Corson alloy foils can also be used. Furthermore, as the current collector, a metal foil having a normal tensile strength (JIS C 2318) of preferably 500 MPa or more, for example, one obtained by forming a copper coating layer on at least one surface of the aforementioned Corson alloy foil can be used. Furthermore, it is also preferable to use one having a normal elongation of CFIS C 2318) of 4% or more as a current collector. If the tensile strength is low and the stress is generated when the active material expands, a crack may occur in the current collector due to the stress if the elongation is low. By using these current collectors, it is possible to further improve the bending resistance of the negative electrode 10 described above. The thickness of the current collector 11 is preferably 9 to 35 / ζπι in consideration of the balance between maintaining the strength of the negative electrode 10 and improving the energy density. When copper foil is used as the current collector 11, chromate treatment, triazole type compound and imidazole type compound It is preferable to use any organic compound to protect against mildew.

Next, a preferred method of manufacturing the negative electrode 10 of the present embodiment will be described with reference to FIG. In this manufacturing method, a coating film is formed on the current collector 11 using a slurry containing particles of an active material and a binder, and then electrolytic plating is performed on the coating film.

First, as shown in FIG. 2 (a), the current collector 11 is prepared. Then, a slurry containing particles 12 a of the active material is applied onto the current collector 11 to form a coating film 15. The surface roughness of the coating film-forming surface of the current collector 11 is preferably 0.5 to 4 / ζπι at the maximum height of the contour curve. When the maximum height is more than 4 m, the formation accuracy of the coating film 15 is lowered, and the current concentration with the penetration of the projections is likely to occur. If the maximum height is less than 0, the adhesion of the active material layer 12 is likely to be reduced. As the particles 12a of the active material, those having the above-mentioned particle size distribution and average particle diameter are preferably used.

The slurry contains, in addition to the particles of the active material, a binder, a dilution solvent, etc. The slurry may also contain a small amount of particles of conductive carbon material such as acetylene black or graphite. In particular, the conductive carbon material is preferably contained in an amount of 1 to 3% by weight based on the weight of the particles 12a of the active material. If the content of the conductive carbon material is less than 1% by weight, the viscosity of the slurry decreases and the sedimentation of the particles 12a of the active material is promoted, so that a good coating 15 and uniform voids can be formed. "Become. When the content of the conductive carbon material exceeds 3% by weight, plating nuclei are concentrated on the surface of the conductive carbon material, and it becomes difficult to form a good coating.

As the binder, styrene butadiene rubber (SBR), polybiphenyl difluoride (PVDF), polyethylene (PE), ethylene propylene diene monomer (EPDM), etc. are used. As a dilution solvent, N-methyl pyrrolidone, cyclohexane or the like is used. The amount of particles 12a of the active material in the slurry is preferably about 30 to 70% by weight. The amount of the binder is preferably about 0.4 to 4% by weight. Add a dilution solvent to these to make a slurry.

The formed coating film 15 has a large number of microspaces between the particles 12a. The current collector 11 on which the coating film 15 is formed is immersed in a plating bath containing a base metal material having a low lithium compound-forming ability. By immersion in the plating bath, the plating solution intrudes into the minute space in the coating film 15. To reach the interface between the coating 15 and the current collector 11. In this state, electrolytic plating is performed to deposit a plated metal species on the surface of the particles 12a (hereinafter, this plating is also called penetration plating). The penetration plating is performed by using the current collector 11 as a force sword, immersing a counter electrode as an anode in a plating bath, and connecting both electrodes to a power supply.

It is preferable that the deposition of the metallic material by penetration plating proceeds toward one side of the coating film 15 and the other side. Specifically, as shown in FIGS. 2 (b) to 2 (d), the interfacial force between the coating 15 and the current collector 11 is also such that the deposition of the metal material 13 proceeds toward the surface of the coating. Do the game. By depositing the metallic material 13 in this manner, the surface of the particles 12a of the active material can be successfully coated with the metallic material 13, and a void is successfully formed between the particles 12a coated with the metallic material 13. can do.

As described above, conditions of penetration for depositing the metal material 13 include the composition of the plating bath, the pH of the plating bath, the current density of electrolysis, and the like. Such conditions are as described above.

As shown in FIG. 2 (b) to (d), the deposition of the metallic material 13 proceeds from the interface between the coating 15 and the current collector 11 toward the surface of the coating. When plating is performed, in the foremost part of the precipitation reaction, fine particles 13a having a substantially constant thickness and also having a plating nuclear force of the metal material 13 are present in layers. When precipitation of the metal material 13 proceeds, adjacent fine particles 13a combine to form larger particles, and when precipitation proceeds further, the particles combine to cover the surface of the particles 12a of the active material continuously. It will be.

The penetration plating is terminated when the metallic material 13 is deposited over the entire area of the coating film 15 in the thickness direction. By adjusting the end point of plating, a surface layer (not shown) can be formed on the upper surface of the active material layer 12. Thus, as shown in FIG. 2 (d), the target negative electrode is obtained.

After penetration, it is also preferable to treat the negative electrode 10 for protection. As the protection treatment, for example, organic protection using triazole compounds such as benzotriazole, carboxybenzotriazole, tolyltriazole and the like or imidazole and the like, and inorganic protection using cobalt, nickel, chromate and the like can be adopted.

Example Hereinafter, the present invention will be described in more detail by way of examples. However, the scope of the present invention is not limited to the example embodiment.

Example 1

The current collector, which is also an electrolytic copper foil having a thickness of 18 m, was acid washed at room temperature for 30 seconds. After treatment, it was washed with pure water for 15 seconds. A slurry containing Si particles was applied onto the current collector to a film thickness of 15 m to form a coating film. The composition of the slurry was particles: styrene butadiene rubber (binder): acetylene black = 100: 1. 7: 2 (weight ratio). Average particle diameter D of Si particles is 2

50

It was 5 m. The average particle diameter D is determined by Microtrac particle size distribution measurement equipment manufactured by Nikkiso Co., Ltd.

50

Measurement was carried out using a measuring instrument (No. 9320-X100).

A current collector on which a coating is formed is immersed in a copper pyrophosphate bath having the following bath composition, and electrolytic penetration causes copper to penetrate the coating, thereby forming an active material layer. did. The conditions of electrolysis were as follows. DSE was used for the anode. The power supply used DC power supply.

'Cyrophosphoric acid trihydrate: 105 g Zl

• Potassium pyrophosphate: 450 g / l

'Potassium nitrate: 30g Zl

• P ratio: 7.7

'Bath temperature: 50 ° C

• Current density: 3AZdm ²

• pH: Adjusted to pH 8.2 by adding ammonia water and polyphosphoric acid.

The penetration plating was terminated when copper was deposited all over the thickness direction of the coating film, washed with water, and treated with benzotriazole (BTA) to give a target negative electrode. The amount of Si in the negative electrode was as shown in Table 1.

A positive electrode was manufactured separately from the manufacturing of the negative electrode. What was shown in Table 1 was used as a positive electrode active material.

This was suspended in polyvinyl pyrrolidone which is a solvent for both acetylene black and poly (fluorinated fluoride) to obtain a positive electrode mixture. The positive electrode mixture was applied to a current collector made of an aluminum foil and dried, and then rolled and pressed to obtain a positive electrode. The amount of Co at the positive electrode is as shown in Table 1.

As an electrolytic solution, a 1: 1 volume% mixture of ethylene carbonate and jetyl carbonate is used. 2 volumes of vinylene carbonate to a solution of ImolZl LiPF in a solvent

6

% Externally added was used. As a separator, a polypropylene porous film with a thickness of 20 μm was used. The lithium secondary battery was produced using these members. The weight ratio of CoZSi in this battery is as shown in Table 1.

[Example 2 to 4 and Comparative Example 1]

A lithium secondary battery was produced in the same manner as in Example 1 except that the positive electrode active material shown in Table 1 was used and the amount of Co was set to the values shown in the same table to produce a positive electrode. The weight ratio of CoZSi in the obtained battery was as shown in Table 1.

[Evaluation 1]

Using the positive electrode and the negative electrode obtained in Examples and Comparative Examples, and the above-mentioned separator, the expansion coefficients of the entire members during the first charge were measured. For the measurement, an HS displacement cell manufactured by Takasen Co., Ltd. was used. In this displacement cell, the expansion coefficient of the total thickness of the negative electrode + separator + positive electrode is measured. The expansion rate is calculated by the following equation. Initial charge conditions were: 0. 05 C, final voltage 4.2 V, and constant current 'constant voltage'. The results are shown in Table 1.

Expansion coefficient (%) = [(Thickness after first charge) (Thickness before charge)] Z thickness before charge X 100 [0066] [Evaluation 2]

The capacity retention of the 50th cycle was measured for the lithium secondary batteries obtained in Examples and Comparative Examples. The capacity retention rate was calculated by measuring the discharge capacity at the 50th cycle, dividing the value by the discharge capacity at the 13th eye and multiplying by 100. The results are shown in Table 1. The charge condition was 0.5 C, 4.2 V, and the constant current was constant voltage. Discharge conditions were 0.5 C and 2.7 V, and constant current was applied. However, charge and discharge in the first cycle is set to 0.05 C, charge and discharge in the second to fourth cycles is 0.1 C, charge and discharge in the fifth to seventh cycles is 0.5 C, and charge and discharge in the eighth to tenth cycles is 1 C And

[0067] [Table 1]

As is clear from the results shown in Table 1, the secondary battery of each example has a low degree of expansion due to charging due to the weight ratio of CoZSi being set within a specific range. It can be seen that the capacity retention rate is increased by this. On the other hand, in the secondary battery of the comparative example, it can be seen that the capacity retention rate is lowered due to the fact that the degree of expansion due to charging increases. Industrial applicability

According to the present invention, by setting the amount of Si in the negative electrode active material and the amount of Co in the positive electrode active material in the battery within a specific range, the expansion and contraction of the entire battery due to charge and discharge. The degree of contraction is reduced, thereby reducing the stress generated by the expansion and contraction and reducing the damage to the positive electrode and the negative electrode. As a result, the cycle characteristics of the battery are improved.

Claims

The scope of the claims

[1] A negative electrode having a negative electrode active material layer containing Si, and a positive electrode having a positive electrode active material layer containing a lithium transition metal composite oxide containing Li and Co as constituent elements,

The weight ratio (CoZSi) of Co in a positive electrode with respect to Si is in the range of 0.3-5. The non-aqueous-electrolyte secondary battery characterized by these.

[2] The amount of Co contained in the lithium transition metal complex oxide in the positive electrode active material layer is 10 to 40% by weight based on the total amount of transition metals contained in the complex oxide. The non-aqueous electrolyte secondary battery according to claim 1.

[3] The non-aqueous electrolyte solution according to claim 1 or 2, wherein the transition metal other than Co contained in the lithium transition metal complex oxide in the positive electrode active material layer is Mn and Z or Ni. Next battery.

[4] The negative electrode has a negative electrode active material layer containing particles of an active material made of a Si-based material, and in the negative electrode, in the active material layer, at least a part of the surface of the particles has low ability to form a lithium compound. The non-aqueous electrolyte secondary battery according to claim 1, wherein a base metal material is coated and a void is formed between the particles coated with the metal material.

[5] The non-aqueous electrolyte secondary battery according to Claim 4, wherein the metal material is present on the surface of the particles over the entire area in the thickness direction of the negative electrode active material layer.