WO2008001541A1

WO2008001541A1 - Negative electrode for non-aqueous electrolyte secondary battery

Info

Publication number: WO2008001541A1
Application number: PCT/JP2007/058416
Authority: WO
Inventors: Kayoko Shinzawa; Yoshiki Sakaguchi; Kiyotaka Yasuda
Original assignee: Mitsui Mining & Smelting Co., Ltd.
Priority date: 2006-06-30
Filing date: 2007-04-18
Publication date: 2008-01-03
Also published as: JP2008066279A

Abstract

A negative electrode (10) for use in a non-aqueous electrolyte secondary battery comprises an active material layer (12) containing a particle (12a) of an active material. At least a part of the surface of the particle (12a) is coated with a layer of a metal material (13). A void is formed between the particles (12a) that are coated with the metal material (13). The layer is composed of two or more different metal materials. Preferably, the layer has a multi-layered structure composed of different metal materials. Also preferably, each of the two or more different metal materials (13) contains at least copper.

Description

Specification

Anode for non-aqueous electrolyte secondary battery

Technical field

The present invention relates to a negative electrode for a non-aqueous electrolyte secondary battery such as a lithium secondary battery.

Background art

[0002] The applicant of the present invention first includes a pair of front and back surfaces that are in contact with an electrolytic solution and have conductivity, and an active material layer including active material particles between the surfaces, for a non-aqueous electrolyte secondary battery. A negative electrode was proposed (see Patent Document 1). The active material layer of the negative electrode is infiltrated with a metal material having a low ability to form a lithium compound, and active material particles are present in the infiltrated metal material. Since the active material layer has such a structure, in this negative electrode, even if fine particles are generated due to the expansion and contraction of the particles due to charge and discharge, it is difficult for the particles to fall off. As a result, the use of this negative electrode has the advantage of extending the cycle life of the battery.

[0003] If the negative electrode is left in the atmosphere for a long period of time, the metal material may be oxidized and a large amount of oxygen may be taken into the negative electrode. In particular, the tendency is remarkable when copper, which is a metal, is easily oxidized as the metal material. When the metal material is oxidized, the overvoltage of the first charge becomes high. An increase in overvoltage causes generation of lithium dendrites on the negative electrode surface and decomposition of the non-aqueous electrolyte. Further, when the metal material is oxidized, the cycle characteristics are liable to deteriorate.

[0004] Patent Document 1 discloses that the surface layer covering the surface of the active material layer may have a multilayer structure of two or more layers. However, even if the surface layer has such a multilayer structure, it is not easy to reliably prevent the oxidation of the metal material existing inside the active material layer. That is, in the manufacturing method specifically disclosed in the example of Patent Document 1, there may be too few voids in the active material layer, and the plating solution cannot sufficiently penetrate into the active material layer. It may not be possible to reliably prevent the oxidation of the metal material existing inside the layer.

[0005] Patent Document 1: US2006— 0121345A1

[0006] Accordingly, an object of the present invention is to provide a negative electrode for a non-aqueous electrolyte secondary battery, whose performance is further improved as compared with the above-described conventional negative electrode. Disclosure of the invention

[0007] The present invention includes an active material layer including particles of an active material, and at least a part of the surface of the particles is covered with a metal material layer, and the metal material layer covers the active material layer. A negative electrode for a non-aqueous electrolyte secondary battery in which voids are formed between particles,

The layer provides a negative electrode for a non-aqueous electrolyte secondary battery in which two or more different metal material forces are also configured.

Brief Description of Drawings

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

FIG. 2 is a schematic diagram showing an enlarged main part of the active material layer in the negative electrode shown in FIG.

FIG. 3 (a) to FIG. 3 (d) are process diagrams showing a method for manufacturing the negative electrode shown in FIG.

[FIG. 4] FIGS. 4 (a) and 4 (b) show the SEM image of the longitudinal section of the negative electrode active material layer obtained in Example 3 and the results of measuring the presence of Ni in the longitudinal section by EPMA. FIG.

FIG. 5 is a diagram showing the results of GDS measurement of the presence of each element in the longitudinal section of the negative electrode active material layer obtained in Example 3.

Detailed Description of the Invention

Hereinafter, the present invention will be described based on its preferred embodiments with reference to the drawings. FIG. 1 shows a schematic diagram of a cross-sectional structure of an embodiment of a negative electrode for a non-aqueous electrolyte secondary battery of the present invention. The negative electrode 10 of the present embodiment includes a current collector 11 and an active material layer 12 formed on at least one surface thereof. Note that FIG. 1 shows a state where the active material layer 12 is formed only on one surface of the current collector 11 for the sake of convenience! Although the active material layer is formed on both surfaces of the current collector, the active material layer 12 is formed on both surfaces of the current collector. It may be.

[0010] The active material layer 12 includes active material particles 12a. The active material layer 12 is formed, for example, by applying a slurry containing active material particles 12a. Examples of the active material include silicon materials, tin materials, aluminum materials, and germanium materials. Examples of soot-based materials include tin, conoret, carbon, nickel, and chromium. An alloy containing at least one is preferably used. In order to improve the capacity density per weight of the negative electrode, a silicon-based material is particularly preferable.

[0011] As the silicon-based material, a material capable of occluding lithium and containing silicon, for example, silicon alone, an alloy of silicon and metal, silicon oxide, or the like can be used. These materials can be used alone or in combination. 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. Of these metals, Cu, Ni, and Co are preferred, and Cu and Ni are preferably used because of their excellent electronic conductivity and low ability to form lithium compounds. In addition, lithium may be occluded in an active material having a silicon-based material force before or after the negative electrode is incorporated in the battery. A particularly preferable silicon-based material is silicon or silicon oxide having a high lithium storage capacity.

In the active material layer 12, at least a part of the surface of the particle 12 a is covered with the layer of the metal material 13. The metal material 13 is a material different from the constituent material of the particles 12a. Voids are formed between the particles 12 a covered with the layer of the metal material 13. That is, the layer of the metal material 13 covers the surface of the particle 12a in a state in which a gap is secured so that the non-aqueous electrolyte containing lithium ions can reach the particle 12a. In FIG. 1, the layer of the metal material 13 is conveniently represented as a thick line surrounding the periphery of the particle 12a. This figure is a schematic view of the active material layer 12 viewed two-dimensionally. In actuality, each particle is in direct contact with other particles or through the metal material 13.

[0013] The metal material 13 is preferably present on the surface of the active material particles 12a over the entire thickness direction of the active material layer 12. The active material particles 12 a are preferably present in the matrix of the metal material 13. As a result, even if the particles 12a expand and contract due to charge and discharge, even if they become fine powder, they are less likely to fall off. In addition, since the electronic conductivity of the entire active material layer 12 is ensured through the metal material 13, the electrically isolated active material particles 12 a are generated, particularly in the deep part of the active material layer 12. The formation of the active material particles 12a is effectively prevented. This is particularly true when a semiconductor is used as an active material and electron conductivity is poor, and a material such as a silicon-based material is used. It is profit. The presence of the metal material 13 on the surface of the active material particles 12a over the entire thickness direction of the active material layer 12 can be confirmed by electron microscope mapping using the material 13 as a measurement object.

[0014] The layer of the metal material 13 covers the surface of the particle 12a continuously or discontinuously. When the layer of the metal material 13 continuously covers the surfaces of the particles 12a, it is preferable to form a fine void in the layer of the metal material 13 that allows the non-aqueous electrolyte to flow. When the layer of the metal material 13 covers the surface of the particle 12a discontinuously, it is covered with the layer of the metal material 13 out of the surface of the particle 12a, and the particle 12a is non-watered through the site. Electrolyte is supplied. In order to form the layer of the metal material 13 having such a structure, the metal material 13 may be deposited on the surfaces of the particles 12a by, for example, electroplating according to the conditions described later.

A gap is formed between the particles 12 a covered with the layer of the metal material 13. This void serves as a distribution path for the non-aqueous electrolyte containing lithium ions. The non-aqueous electrolyte easily reaches the active material particles 12a due to the presence of the voids, and thus the overvoltage of the initial charge can be lowered. As a result, lithium dendrite is prevented from being generated on the negative electrode surface. The generation of dendrite causes a short circuit between the two poles. The ability to reduce the overvoltage is also advantageous in terms of preventing decomposition of the non-aqueous electrolyte. This is because the irreversible capacity increases when the non-aqueous electrolyte is decomposed. Furthermore, the ability to reduce the overvoltage is advantageous in that the positive electrode is damaged.

[0016] Furthermore, the voids formed between the particles 12a also serve as a space for relieving the stress caused by the volume change of the active material particles 12a due to charge and discharge. The increase in the volume of the active material particles 12a whose volume has been increased by charging is absorbed by the voids. As a result, it is difficult for the fine particles of the particles 12a to be generated, and significant deformation of the negative electrode 10 is effectively prevented.

As schematically shown in FIG. 2, the negative electrode 10 of the present embodiment has a multilayer structure in which the layers of the metal material 13 are configured with different metal material forces. Each layer may be composed of a single metal, or may be composed of an alloying force of two or more metals. Specifically, the layer of the metal material 13 has a two-layer structure including an inner layer 13a covering the surface of the active material particles 12a and an outer layer 13b formed on the inner layer 13a. Such a lot Due to the layer structure, for example, the inner layer 13a is easily oxidized as a constituent material, and even when a material is used, it is not easily oxidized and the outer layer 13b configured by the material cover is used. By covering the inner layer 13a, it is possible to effectively prevent acidification of the inner layer 13a. As a result, even when the negative electrode 10 is left in the atmosphere for a long period of time, oxygen is hardly taken into the negative electrode, and the overvoltage of the initial charge can be lowered. In addition, deterioration of cycle characteristics can be prevented. As will be described later, since the preferred thickness of the outer layer 13b is extremely thin, the outer layer 13b actually does not cover the surface of the inner layer 13a evenly, that is, it exists discontinuously. It is considered a thing. In this specification, “a multilayer structure composed of different metal materials” means that a multilayer structure is formed by different types of metal materials constituting adjacent layers. Therefore, when the layer of the metal material 13 also has, for example, a Sn layer, a Ni layer, and a Sn layer force, the layer has a three-layer structure composed of different metal materials.

[0018] Two or more different metal materials constituting the metal material layer 13 preferably include at least copper. This is because copper is excellent in electrical conductivity and also excellent in relaxation of stress generated due to expansion and contraction of the active material due to high ductility. Copper is known to be a metal that is easily oxidized. Therefore, in the present embodiment, while taking advantage of the above-described advantages of copper, the drawback of copper that is easily oxidized is compensated by using other metal materials.

[0019] From the viewpoint of more effectively preventing oxidation, the outer layer 13b, which is a layer exposed on the surface, preferably has a metal force that is not easily oxidized. It is also preferable that the outer layer 13b has a metal force with a low lithium compound forming ability. From these viewpoints, it is preferable that the metal composing the outer layer 13b is composed of nickel, conoretole, chromium, gold, or an alloy thereof, which is a material that has a low ability to form a lithium compound and is hardly oxidized. “Lithium compound forming ability is low” means that lithium does not form an intermetallic compound or solid solution, or even if lithium is formed, lithium is in a very small force or very unstable.

[0020] On the other hand, as a constituent material of the inner layer 13a, it is preferable to use a metal material having a high lithium compound forming ability or a metal material having a low lithium compound forming ability. Lithiation Examples of the metal material having high compound forming ability include zinc, tin, silver, and alloys thereof. An example of a metal material having a low ability to form a lithium compound is copper. From the viewpoint of maintaining the strength of the negative electrode after charge and discharge, it is preferable to use a metal material having a low lithium compound forming ability. In particular, the constituent material of the inner layer 13a is preferably a highly ductile material in which the surface coating of the particles 12a is not easily broken even when the active material particles 12a expand and contract. From this point of view, it is preferable to use copper, which is a highly ductile material, as the constituent material of the inner layer 13a.

[0021] The inner layer 13a has a thickness force of S40 to 1500 nm, particularly 100 to 240 nm. A force that can reliably hold the active material particles 12a in the active material layer 12 is preferable. On the other hand, the thickness of the outer layer 13b is preferably 10 to 500 nm, particularly 10 to 100 nm, from the viewpoint of reliably preventing the oxidation of the inner layer 13a. In each of the inner layer 13a and the outer layer 13b having a thickness in this range, fine voids that allow the non-aqueous electrolyte to flow are formed. If these layers are too thick, fine voids are formed. In addition, the layer of the metal material 13 including the inner layer 13a and the outer layer 13b preferably has an average thickness of 0.05 to 2 / ζ πι, particularly 0.1 to 0.25 m. In other words, the metal material 13 preferably covers the surface of the active material particles 12a with a minimum thickness. As a result, while the energy density is increased, the particles 12a are prevented from falling off due to expansion and contraction due to charge / discharge and fine particles. The layer of the metal material 13 has a multilayer structure, and the thickness of each layer can be confirmed and measured by, for example, a glow discharge emission spectrometer (GDS). When each layer is formed by electrolytic plating, it can be confirmed and measured by the amount of current applied when each layer is formed. Specific measurement methods will be described in detail in Examples. Here, the “average thickness” is a value calculated based on a portion of the surface of the active material particle 12a that is actually covered with the metal material 13. Therefore, the surface of the active material particle 12a is covered with the metal material 13, and the portion is not a basis for calculating the average value!

[0022] As described later, the active material layer 12 preferably has a predetermined plating bath applied to the coating film obtained by applying a slurry containing particles 12a and a binder onto a current collector and drying the slurry. It is formed by performing the electrolytic plating used and depositing the metal material 13 between the particles 12a.

[0023] In order to form necessary and sufficient voids in the active material layer through which the non-aqueous electrolyte can flow It is preferable to sufficiently penetrate the plating solution into the coating film. In addition to this, it is preferable that the conditions for depositing the metal material 13 by electrolytic plating using the plating solution are appropriate. The plating conditions include the composition of the fitting bath, the pH of the plating bath, and the current density of electrolysis. The pH of the plating bath is preferably adjusted to 7.1 to L 1 particularly when the inner layer 13a is formed. By keeping the pH within this range, the dissolution of the active material particles 12a is suppressed, the surface of the particles 12a is cleaned, and the adhesion to the particle surfaces is promoted. Moderate gaps are formed. The pH value was measured with respect to the temperature at the time of plating.

[0024] When copper is used as the constituent metal of the inner layer 13a, it is preferable to form the inner layer 13a by electrolytic plating using a copper pyrophosphate bath. It is preferable to use a copper pyrophosphate bath, even if the active material layer 12 is thick, because the voids can be easily formed over the entire thickness direction of the layer. Also, copper is deposited on the surface of the active material particles 12a, and it is difficult for copper to precipitate between the particles 12a, which is preferable in that the voids between the particles 12a are formed successfully. When using a copper pyrophosphate bath, the bath composition, electrolysis conditions and pH are preferably as follows.

'Copper pyrophosphate trihydrate: 85-120gZl

-Pig 13 gin Kazikum: 300-600g / l

'Potassium nitrate: 15-65gZl

'Bath temperature: 45-60 ° C

'Current density: l ~ 7AZdm ²

• pH: Add ammonia water and polyphosphoric acid to adjust the pH to 7.1 to 9.5.

[0025] Especially when using a copper pyrophosphate bath, the ratio of the weight of PO to Cu (P O ZCu

2 7 2 7

It is preferable to use one having a P ratio defined by If the P ratio is less than 5, the copper covering the active material particles 12a tends to be thick, and it may be difficult to form desired voids between the particles 12a. If a P ratio exceeding 12 is used, the current efficiency will be poor and gas generation will be likely to occur, which may reduce production stability. When a copper pyrophosphate bath having a P ratio of 6.5 to 10.5 is used as a more preferable copper pyrophosphate bath, the size and number of voids formed between the active material particles 12a are reduced in the active material layer 12. This is very advantageous for the flow of the water electrolyte.

[0026] When the inner layer 13a is an alloy of two or more metals, the inner layer 13a can be formed by alloy bonding. For example, when a copper nickel alloy is used as the inner layer 13a, electrolytic plating may be performed using a copper nickel alloy plating bath described below.

'Nickel sulfate hexahydrate 45gZl

'Copper pyrophosphate trihydrate 36gZl

• Potassium pyrophosphate 30gZl

PH 8.2

'Bath temperature 55 ° C

'Current density 3AZdm ²

By performing electrolytic plating under the above conditions, an alloy with a copper to nickel ratio (weight ratio) of approximately 8: 2 is formed.

[0027] In consideration of the penetration of the bath of the outer layer 13b performed in the next step, it is preferable that the ratio of the voids in the active material layer 12 when the inner layer 13a is formed is 20% by volume or more. . As a method of increasing the void ratio in the active material layer 12 by reducing the thickness of the inner layer 13a, a method of pressing the coating film 15 after forming the coating film 15 and before forming the inner layer 13a can be mentioned. The pressure for the pressing process is preferably 50 to 500 MPa, particularly 100 to 500 MPa.

[0028] By subjecting the coating film 15 to a press treatment before the penetration plating, the progress of plating can be accelerated, and a thin and uniform coating of the metal material 13 is formed on the surface of the active material particles 12a. As a result of the study by the present inventors, it has been found that this is possible. The reason for this is that the degree of contact between the particles 12a is increased by the press treatment, whereby the electron conductivity in the coating film 15 is improved, and the region where plating nuclei are generated is in the thickness direction of the coating film 15. This is thought to be due to the increase. From this point of view, the pressure of the press treatment is preferably 50 to 500 MPa, particularly 100 to 5 OOMPa! /.

[0029] On the other hand, when nickel is used as the constituent metal of the outer layer 13b, it is preferable to form the outer layer 13b by electrolytic plating using a watt bath. When using a Watt bath, the bath composition and electrolysis conditions are preferably as follows. NiSO 6Η O 150 ~ 300gZl

4 2

NiCl-6H O 30-60gZl

• H

'Bath temperature: 40 ~ 70 ° C

'Current density: 0.5-20AZdm ²

[0030] By adding various additives used in the electrolytic solution for producing copper foil such as protein, active sulfur compound, cellulose, etc. to the various baths described above, the characteristics of the metal materials constituting the inner layer 13a and the outer layer 13b are improved. It is also possible to adjust appropriately.

[0031] The ratio of voids in the active material layer 12 formed by the above method, that is, the void ratio is

15 to 45% by volume, particularly 20 to 40% by volume is preferable. By setting the porosity within this range, it is possible to form necessary and sufficient voids in the active material layer 12 through which the non-aqueous electrolyte can flow. The porosity is measured by the following procedures (1) to (7).

(1) The weight per unit area of the coating film formed by applying the slurry is measured, and the weight of the particles 12a and the weight of the binder are calculated from the blending ratio of the slurry.

(2) From the weight change per unit area after electroplating, calculate the weight of the plated metal species. This operation is performed separately for each of the inner layer 13a and the outer layer 13b.

(3) After electrolytic plating, the thickness of the active material layer 12 is obtained by SEM observation of the cross section of the negative electrode

(4) The volume of the active material layer 12 per unit area is calculated from the thickness of the active material layer 12.

(5) The respective volumes are calculated from the weight of the particles 12a, the weight of the binder, the weight of the plating metal species, and the respective mixing ratios.

(6) The void volume is calculated by subtracting the volume of the particles 12a, the volume of the binder, and the volume of the metal species from the volume of the active material layer 12 per unit area.

(7) Divide the void volume calculated in this way by the volume of the active material layer 12 per unit area, and multiply the result by 100 to obtain the void ratio (%).

[0032] The porosity can also be controlled by appropriately selecting the particle size of the active material particles 12a. From this viewpoint, the maximum particle size of the particles 12a is preferably 30 m or less, more preferably 10 m or less. In addition, when the particle size is expressed in terms of D value, 0 1 to 8 μm, particularly 0.3 to 4 μm is preferable. The particle size of the particles is measured by laser diffraction / scattering particle size distribution measurement and electron microscope observation (SEM observation).

[0033] If the amount of the active material relative to the whole negative electrode is too small, it is difficult to sufficiently increase the energy density of the battery. On the other hand, if the amount is too large, the strength decreases and the active material tends to fall off. Considering these, the thickness of the active material layer 12 is preferably 10 to 40 / ζ πι, more preferably 15 to 30 μm, and still more preferably 18 to 25 μm.

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. Further, 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.

[0035] When the negative electrode 10 is thin and has a surface layer or has the surface layer, a secondary battery is assembled using the negative electrode 10, and the battery is initially charged. The overvoltage can be reduced. This means that lithium can be prevented from being reduced on the surface of the negative electrode 10 when the secondary battery is charged. The reduction of lithium leads to the generation of dendrites that cause short circuits between the two electrodes.

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 continuously covers the surface of the active material layer 12, the surface layer has a large number of fine voids (not shown) that are open to the surface and communicate with the active material layer 12. Have, prefer to have. It is preferable that the fine voids exist in the surface layer so as to extend in the thickness direction of the surface layer. The fine voids allow the non-aqueous electrolyte to flow. The role of the fine voids is to supply a non-aqueous electrolyte into the active material layer 12. When the surface of the negative electrode 10 is viewed in plan by an electron microscope, the fine voids are the ratio of the area covered with the metal material 13, that is, the coverage is 95% or less, particularly 80% or less, particularly 60% or less. Such a size is preferable.

[0037] The surface layer is composed of a metal compound having a low lithium compound forming ability. This metal material can be the same as the constituent material of the outer layer 13b described above. Alternatively, it may be different from the constituent material 13 of the outer layer 13b. For example, when the inner layer 13a is composed of a metal material having a low lithium compound forming ability, the surface layer is a constituent material of the inner layer 13a. It may be formed of a material. In this case, the metal material constituting the surface layer and the constituent material of the outer layer 13b are different. Considering the ease of manufacturing the negative electrode 10, it is preferable that the constituent material of the outer layer 13b and the metal material constituting the surface layer are the same type.

[0038] The current collector 11 in the negative electrode 10 may be the same as that conventionally used as the current collector of the negative electrode for a non-aqueous electrolyte secondary battery. It is preferable that the current collector 11 is composed of a metal material having a low lithium compound forming ability as described above. Examples of such metal materials are as already described. In particular, it is preferably made of copper, nickel, stainless steel or the like. Also, it is possible to use a copper alloy foil represented by Corson alloy foil. Further, as the current collector, a metal foil having a normal tensile strength (JIS C 2318) of preferably 500 MPa or more, for example, a copper film layer formed on at least one surface of the aforementioned Corson alloy foil can be used. Furthermore, it is also preferable to use a current collector having a normal elongation CFIS C 2318) of 4% or more. This is because, when the tensile strength is low, stress is generated due to the stress when the active material expands, and when the elongation is low, the current collector may crack. The thickness of the current collector 11 is not critical in this embodiment. Considering the balance between maintaining the strength of the negative electrode 10 and improving the energy density, it is preferably 9 to 35 / ζ πι. In addition, when using a copper foil as the current collector 11, it is preferable to perform a chromate treatment or an antifungal treatment using an organic compound such as a triazole compound and an imidazole compound.

Next, a preferred method for producing 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 active material particles and a binder, and then electrolytic plating is repeatedly performed on the coating film using different types of plating baths. Do.

First, a current collector 11 is prepared as shown in FIG. Then, a slurry containing active material particles 12 a is applied onto the current collector 11 to form a coating film 15. In addition to the active material particles, the slurry contains a binder and a diluent solvent. The slurry may contain a small amount of conductive carbon material particles such as acetylene black graphite. In particular, when the active material particles 12a also have a silicon-based material force, it is preferable that the conductive carbon material is contained in an amount of 1 to 3% by weight with respect to the weight of the active material particles 12a. Conductive carbon material When the content of is less than 1% by weight, the viscosity of the slurry decreases and the settling of the active material particles 12a is promoted, so that it is difficult to form a good coating film 15 and uniform voids. If the content of the conductive carbon material exceeds 3% by weight, plating nuclei concentrate on the surface of the conductive carbon material, and it becomes difficult to form a good coating.

[0041] As the binder, styrene butadiene rubber (SBR), polyvinylidene fluoride (PVDF), polyethylene (PE), ethylene propylene monomer (EPDM), or the like is used. As a diluting solvent, N-methylpyrrolidone, cyclohexane or the like is used. The amount of the active material particles 12a 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. A dilute solvent is added to these to form a slurry.

[0042] The formed coating film 15 has a large number of minute spaces between the particles 12a. The current collector 11 on which the coating film 15 is formed is immersed in the first plating bath. The first bath is low in the ability to form a lithium compound, contains a metal material, or contains a metal material that has a high ability to form a lithium compound. By immersing in the first plating bath, the plating solution enters the minute space in the coating film 15 and reaches the interface between the coating film 15 and the current collector 11. Under this condition, electrolytic plating is performed to deposit plated metal species on the surface of the particles 12a (hereinafter, this plating is also referred to as penetration plating). The penetration is performed by using the current collector 11 as a force sword, immersing the counter electrode as the anode in the plating bath, and connecting both electrodes to the power source.

[0043] It is preferable that the deposition of the metal material by the penetration adhesion proceeds by applying one side force of the coating film 15 to the other side. Specifically, as shown in FIGS. 3 (b) to (d), the interfacial force between the coating film 15 and the current collector 11 is electrolysis so that the deposition of the plating metal species proceeds toward the surface of the coating film. Make a mess. By precipitating the plating metal species in this way, the surface of the active material particles 12a can be successfully coated with the metal species, and voids can be successfully formed between the particles 12a coated with the plating metal species. Can be formed. In addition, the void ratio of the voids can be easily set to the preferred range described above.

[0044] As described above, the conditions of the penetration plating for depositing the plating metal species include the composition of the plating bath, the pH of the plating bath, and the current density of electrolysis. Such conditions are as described above. [0045] As shown in FIGS. 3 (b) to (d), the deposition of the plating metal species proceeds from the interface between the coating film 15 and the current collector 11 toward the surface of the coating film. When electrolytic plating is performed, in the forefront portion of the precipitation reaction, fine particles 14 having a substantially constant thickness and serving as a plating nucleus of the plating metal species are present in layers. As the precipitation of the plated metal species proceeds, adjacent fine particles 14 combine to form larger particles, and when the deposition proceeds further, the particles combine to continuously coat the surface of the active material particles 12a. Will come to do.

[0046] Electrolytic plating using the first plating bath is terminated when the plating metal species is deposited in the entire thickness direction of the coating film 15. In this manner, a negative electrode precursor is obtained as shown in FIG. Next, instead of this bathing bath, use a second bathing bath, and perform electrolytic bathing according to the same procedure as described above. The second bath has a low ability to form lithium compounds and contains a metal material.

[0047] By performing electroplating with the second plating bath in the same procedure as the electrolytic plating with the first plating bath, this is deposited on the inner layer that also has the plating metal seed strength. An outer layer is formed which also has a different kind of plating metal species than the metal species. The electrolytic plating with the second plating bath is terminated when the plating metal species is deposited in the entire thickness direction of the coating film 15. In this case, a surface layer (not shown) can be formed on the upper surface of the active material layer 12 by adjusting the end point of electroplating.

[0048] The negative electrode 10 thus obtained is suitably used as a negative electrode for a non-aqueous electrolyte secondary battery such as a lithium secondary battery. In this case, the positive electrode of the battery is prepared by suspending a positive electrode active material and, if necessary, a conductive agent and a binder in an appropriate solvent to produce a positive electrode mixture, applying this to a current collector, drying it, and then rolling it. It is obtained by pressing, cutting and punching. As the positive electrode active material, conventionally known positive electrode active materials such as lithium-containing metal composite oxides such as lithium nickel composite oxide, lithium manganese composite oxide, and lithium cobalt composite oxide are used. In addition, as a positive electrode active material, at least LiCoO

2

Lithium transition metal composite oxide containing both Zr and Mg and a mixture of lithium transition metal composite oxide having a layered structure and containing at least both Mn and Ni are also preferably used. Can do. The use of a positive active material can be expected to increase the end-of-charge voltage without deteriorating charge / discharge cycle characteristics and thermal stability. Cathode active material The average primary particle size is 5 μm or more and 10 μm or less, and the weight average molecular weight of the binder used for the positive electrode is preferably 350,000 or more because of the balance between packing density and reaction area. , 000, 000 or less of polyvinylidene fluoride is preferable. This is because it can be expected to improve the discharge characteristics in a low temperature environment.

[0049] As the battery separator, a synthetic resin nonwoven 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 that occurs when the battery is overcharged, it is preferable to use a separator in which a polyolefin film is formed on one or both surfaces of the polyolefin microporous membrane. The separator preferably has a puncture strength of 0.2 NZ w m or more and 0.49 N / μm or less and a tensile strength in the winding axis direction of 40 MPa or more and 150 MPa or less. This is because even when a negative electrode active material that expands and contracts greatly with charge and discharge is used, damage to the separator can be suppressed, and occurrence of internal short circuits can be suppressed.

[0050] The nonaqueous electrolytic solution is a solution in which a lithium salt as a supporting electrolyte is dissolved in an organic solvent.

Lithium salts include LiCIO, LiAlCl, LiPF, LiAsF, LiSbF, LiBF, LiSCN,

4 4 6 6 6 4

Examples include LiCl, LiBr, Lil, LiCF SO, LiC F SO and the like. Examples of organic solvents include

3 3 4 9 3

Examples include ethylene carbonate, jetino carbonate, dimethylol carbonate, propylene carbonate, butylene carbonate, and the like. Especially for the whole non-aqueous electrolyte

0.5 to 5% by weight of bi -.. Ren carbonate and 0.1 to 1 wt% of divinyl sulfone, 0 1 to 1 5 weight ^_0/0 1, 4 also contain a butanedioldimethanesulfonate is charged The viewpoint power for further improving the discharge cycle characteristics is preferable. The reason for this is not clear, but the 1,4-butanediol dimethanesulfonate and divinylsulfone gradually decompose to form a film on the positive electrode, resulting in a denser film containing sulfur. It is thought that it is to become.

[0051] Especially for non-aqueous electrolytes, 4 fluoro 1,3 dioxolane 1 2-on, 4 black mouth

-Use a high dielectric constant solvent with a relative dielectric constant of 30 or more, such as cyclic carbonic acid ester derivatives having a halogen atom such as 1,3 dioxolan-2-one or 4-trifluoromethyl-1,3-dioxolan-2-one. It is also preferable. This is because it is difficult to be decomposed due to its high resistance to reduction. In addition, the above-mentioned high dielectric constant solvent, dimethyl carbonate, jetyl carbon Also preferred is an electrolyte mixed with a low-viscosity solvent having a viscosity of 1 mPa · s or less, such as a sodium salt or methylethyl carbonate. This is because higher ion conductivity can be obtained. Furthermore, it is also preferable that the content of fluorine ions in the electrolytic solution is within the range of 14 mass ppm or more and 1290 mass ppm or less. When the electrolyte solution contains an appropriate amount of fluorine ions, a coating film such as lithium fluoride derived from the fluorine ions is formed on the negative electrode, which can suppress the decomposition reaction of the electrolyte solution in the negative electrode. . Furthermore, it is preferable that 0.001% by mass to 10% by mass of an acid anhydride and at least one additive in the group consisting of derivatives thereof are contained. As a result, a film is formed on the surface of the negative electrode, which can suppress 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 preferred, for example, succinic anhydride, dartaric anhydride, maleic anhydride, phthalic anhydride, Phthalic anhydride derivatives such as 2-sulfobenzoic anhydride, citraconic anhydride, itaconic anhydride, diglycolic anhydride, hexafluoroglutaric anhydride, 3-fluorophthalic anhydride, 4-fluorophthalic anhydride, or 3,6-epoxy anhydride 1,2,3,6-tetrahydrophthalic acid, 1,8 naphthalic anhydride, 2,3 naphthalene carboxylic anhydride, 1,2-cyclopentanedicarboxylic anhydride, 1,2-cyclo 1,2-cycloalkanedicarboxylic anhydrides such as hexanedicarboxylic acid, or tetrahydrophthalates such as cis 1,2,3,6-tetrahydrophthalic anhydride or 3,4,5,6-tetrahydrophthalic anhydride Acid anhydride or hexahi Lophthalic anhydride (cis isomer, trans isomer), 3, 4, 5, 6-tetrachlorophthalic anhydride, 1, 2, 4 benzenetricarboxylic anhydride, dianhydropyromellitic acid, or their derivatives Etc.

As mentioned above, although this invention was demonstrated based on the preferable embodiment, this invention is not restrict | limited to the said embodiment. For example, the layer of the metal material 13 in the above embodiment has a high lithium compound forming ability, a low metal material or lithium compound forming ability, an inner layer composed of the metal material, and a low lithium compound forming ability. Force that was a two-layer structure of an outer layer made of a metal material. Alternatively, another layer of one or more layers is provided between the inner layer and the outer layer, or a layer of a multilayer structure of three or more layers may be used. Another layer of one or more layers may be further provided outside the layer, and a layer having a multilayer structure of three or more layers may be provided. Furthermore, one or more other layers may be provided between the inner layer and the outer layer, and one or more other layers may be further provided outside the outer layer. [0053] The above embodiment is an example in which the layer of the metal material 13 has a multilayer structure. However, as long as the metal material force of more than one kind is also configured, the layer is a single layer. There may be. In this case, the layer of the metal material 13 is a layer having an alloying force of two or more kinds of metals. Examples of the combination of two or more metals include a combination of one or more metals that are easily oxidized and have a low or high lithium forming ability and one or more metals that are not easily oxidized and have a low lithium forming ability. It is done. An example of a metal that is easily oxidized and has a low lithium forming ability is copper. Examples of the metal that is oxidized and has a low lithium forming ability include nickel, conoretole, chromium, and gold. In order to form such a layer having alloy strength, the penetration plating shown in Fig. 3 may be performed by the alloy plating method. The ratio (molar ratio) of the metal layer 13 to the metal layer 13 that is easily oxidized and has a low or high lithium forming ability, and a metal that is not easily oxidized and has a low lithium forming ability (molar ratio). : It is preferable that it is 3-9: 1.

Example

[0054] 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 such embodiments.

[Example 1]

A current collector having an electrolytic copper foil strength of 18 m in thickness was acid-washed at room temperature for 30 seconds. After the treatment, it was washed with pure water for 15 seconds. A slurry containing Si particles was applied on the current collector to a 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). The average particle size D of Si particles is 2

50 m. The average particle size D is measured by the Nikkiso Co., Ltd.

50

No. 9320—X100).

[0056] The current collector on which the coating film was formed was passed through a roll press, and the coating film was pressed.

The press pressure was 300 MPa. A current collector having a coating film subjected to press working is immersed in a copper pyrophosphate bath having the following bath composition, and copper is infiltrated into the coating film by electrolysis. An inner layer having copper strength was formed. The electrolysis conditions were as follows. DSE was used for the anode. A DC power source was used as the power source. The thickness of the formed inner layer was 200 nm as a result of cross-sectional SEM observation of a sample product produced in the same manner. 'Copper pyrophosphate trihydrate: 105gZl

• Potassium pyrophosphate: 450g / l

'Potassium nitrate: 30gZl

• P ratio: 7.7

'Bath temperature: 50 ° C

• Current density: 3AZdm ²

• pH: Ammonia water and polyphosphoric acid were added to adjust to pH 8.2.

[0057] The penetration staking was terminated when copper was deposited over the entire thickness direction of the coating film.

Next, instead of the copper pyrophosphate bath, a second osmotic plating was performed using a Watt bath having the following composition. As a result, an outer layer made of nickel was formed on the inner layer made of copper. The electrolysis conditions were as follows. A nickel electrode was used as the anode. A DC power source was used as the power source. The intended negative electrode was obtained by the above operation.

'Bath temperature 50 ° C

'Current density 0.5 A / dm ²

• pH: 5

[0058] The average thickness of the inner layer and the outer layer was determined by the following method. Determine the ratio of the amount of electricity when copper penetration is applied to the amount of electricity when nickel penetration is applied. Separately from this, it is confirmed by GDS that copper and nickel are present uniformly in the thickness direction of the active material layer. When copper and nickel are present uniformly, the thickness ratio of the inner layer made of copper and the outer layer made of nickel is determined by the amount of current that is applied when copper penetrates and the nickel penetrates. It can be regarded as equivalent to the ratio of the energization amount at the time. Therefore, the thicknesses of the inner layer and the outer layer can be calculated from the total value of the thicknesses of the inner layer and the outer layer obtained by cross-sectional SEM observation of the active material layer and the ratio of the energization amount. Energization amount when subjected to copper infiltration plated that put the present embodiment 2321CZdm ^2, energization amount of time was one row penetration plated nickel was 18CZdm ^2. The inner layer thickness thus determined is 20 Onm, the thickness of the outer layer was 5 nm.

[Example 2]

A negative electrode was obtained in the same manner as in Example 1 except that the amount of current with which nickel penetrated was 183 CZdm ² . The thickness of the outer layer obtained in the same manner as in Example 1 was 50 nm.

[Example 3]

The film thickness of the slurry containing the active material was 20 m. The amount of electricity when copper penetration was applied was 2321 CZdm ² , and the amount of electricity when nickel penetration was found was 73 CZdm ² . A negative electrode was obtained in the same manner as Example 1 except for these. The thickness of the inner layer obtained in the same manner as in Example 1 was 200 nm, and the thickness of the outer layer was 20 nm.

[Example 4]

In place of the Watt bath, a second bathing penetration was performed using a cobalt bath having the following composition. At this time, the energization amount was 73 CZdm ² . A negative electrode was obtained in the same manner as Example 1 except for these. The outer layer thickness determined in the same manner as in Example 1 was 20 nm.

'Bath temperature 40 ° C

'Current density 0.5 A / dm ²

PH: 5

[Example 5]

Instead of the Watt bath, a second bathing penetration was performed using a chromium bath having the following composition. Current amount at this time was 62CZdm ^2. Except for these, a negative electrode was obtained in the same manner as in Example 1. The thickness of the outer layer obtained in the same manner as in Example 1 was 15 nm.

'Bath temperature 40 ° C

'Current density 0.5 A / dm ²

[0063] [Comparative Example 1]

Negative in the same manner as in Example 1 except that the 2nd penetration with a watt bath is not performed. Got the pole.

[0064] [Evaluation]

In order to make it easier to understand the effects of the present invention, the negative electrodes obtained in Examples and Comparative Examples were left in the atmosphere for 3 weeks, and then a lithium secondary battery was produced using the negative electrode. LiCo Ni Mn O was used as the positive electrode. As electrolyte, ethylene carbonate

1/3 1/3 1/3 2

And the geminal chill carbonate 1: solution obtained by dissolving LiPF of I mol / 1 to 1 volume ^_0/0 mixed solvent

6

In contrast, 2% by volume of behylene carbonate was externally added. As the separator, a 20 m thick polypropylene porous film was used. The obtained secondary battery was charged for the first time, and the voltage when the capacity was 0. ImAh was measured. Charging was performed in a constant current / constant voltage mode.

[0065] Separately from this measurement, a 100-cycle capacity retention rate was measured. The capacity retention rate is a value calculated by measuring the discharge capacity after 100 cycles, dividing that value by the discharge capacity at the 13th cycle, and multiplying by 100. The discharge cut-off voltage was 2.7V. Charging conditions were 0.5C and 4.2V, constant current and constant voltage. The discharge conditions were 0.5C and 2.7V, and a constant current. However, charge / discharge in the first cycle is 0.05C, charge / discharge in the second to fourth cycles is 0.1C, charge / discharge in the fifth to seventh cycles is 0.5C, and charge / discharge in the eighth to tenth cycles is 1C. These results are shown in Table 1.

[0066] Further, the longitudinal section of the active material layer of the negative electrode obtained in Example 3 was observed with a scanning electron microscope (SEM), and the presence of Ni in the longitudinal section was measured by EPMA. The results are shown in Figs. 4 (a) and (b). The presence of each element was measured by GDS for the longitudinal section of the active material layer. The results are shown in Fig. 5. In Fig. 5, the sensitivity of the vertical axis is changed depending on the type of element, so relative comparison between elements is not possible.

[0067] [Table 1] Inner layer Outer layer First charge 100 cycles after

Voltage capacity maintenance rate (%)

(@ 0.imAh, V)

Example 1 Cu (200 nm) Ni (5 nm) 2. 80 82 Example 2 Cu (200 nm) i "(50 nm) 2. 75 87 Example 3 Cu (200 nm) i (20 nm) 2. 73 91 Example 4 Cu (200nm) Co (20nm) 2. 60 89 Example 5 Cu (200nm) Cr (15mn) 2. 70 87 Comparative Example 1 Cu (200nm) None 3. 10 77

[0068] As is apparent from the results shown in Table 1, the battery using the negative electrode of each example had a lower voltage at the first charge than the battery using the negative electrode of Comparative Example 1, that is, the overvoltage was low. It turns out that it is low. It can also be seen that the battery using the negative electrode of each example has better cycle characteristics than the battery using the negative electrode of Comparative Example 1. The negative electrode of Comparative Example 1 has been confirmed to have good characteristics if it is used immediately after fabrication. For this evaluation, the characteristics deteriorated due to the influence of acid. It is presumed that

Further, as is apparent from the results shown in FIGS. 4 and 5, the inner layer copper and the outer layer nickel covering the surface of the Si particles are uniformly present in the thickness direction of the active material layer. This was confirmed.

Industrial applicability

[0070] According to the present invention, since the surface of the active material particles is covered with a layer composed of two or more different metal materials, a metal that is easily oxidized as one of the constituent metals of the layer. Even if is used, the amount of the metal exposed on the surface of the layer is less than when the metal is used alone. Therefore, it becomes difficult to cause acid soot. In particular, when the two or more metal material forces different from each other are formed, oxidation of the innermost layer covering the surface of the particle is prevented by the layer located above the layer. As a result, the overvoltage of the initial charge can be lowered. As a result, lithium dendrite is prevented from being generated on the negative electrode surface. In addition, the non-aqueous electrolyte is not easily decomposed, and an increase in irreversible capacity is prevented. Furthermore, the positive electrode is damaged. Also, the site characteristics are improved. In addition, the particles are expanded and contracted by charging and discharging. Even if it is fine, it is difficult for the dropout to occur.

Claims

The scope of the claims

[1] An active material layer including particles of an active material is provided, and at least a part of the surface of the particles is covered with a layer of a metal material, and between the particles covered with the layer of the metal material A negative electrode for a non-aqueous electrolyte secondary battery in which a gap is formed,

A negative electrode for a non-aqueous electrolyte secondary battery in which the layers are also composed of two or more different metal material forces.

2. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the layer has a multilayer structure of two or more layers in which different metal material forces are also configured.

[3] The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the two or more kinds of metal materials different from each other contain at least copper.

[4] The layer has a structure of two or more layers including at least an inner layer covering the surface of the particles and an outer layer formed on the inner layer,

The inner layer is made of a metal material having a high lithium compound forming ability or a metal material having a low lithium compound forming ability;

3. The negative electrode for a nonaqueous electrolyte secondary battery according to claim 2, wherein the outer layer is composed of a metal material having a low lithium compound forming ability.

[5] The inner layer is formed from zinc, tin or silver which is a metal material having a high lithium compound forming ability, and is formed from copper which is a metal material having a low strength or a lithium compound forming ability.

5. The negative electrode for a nonaqueous electrolyte secondary battery according to claim 4, wherein the outer layer is composed of nickel, cobalt, chromium, or gold, which is a metal material having a low ability to form a lithium compound.

6. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 4 or 5, wherein the inner layer is present on the surface of the active material particles over the entire thickness direction of the active material layer.

[7] The negative electrode for a nonaqueous electrolyte secondary battery according to [1], wherein the average thickness of the layer is 0.05 to 2 μm on average.

[8] A nonaqueous electrolyte secondary battery comprising the negative electrode for a nonaqueous electrolyte secondary battery according to claim 1.