JP2008066279A - Negative electrode for nonaqueous electrolyte secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a negative electrode for nonaqueous electrolyte secondary batteries capable of reducing an excess voltage for initial charge. <P>SOLUTION: The negative electrode 10 for non-aqueous electrolyte secondary batteries 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 coated with the metal material 13. The layer is composed of not less than two different metal materials. Preferably, the layer has a multi-layered structure composed of different metal materials. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

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

  The present applicant has previously proposed a negative electrode for a non-aqueous electrolyte secondary battery that includes a pair of front and back surfaces that are in contact with the electrolyte and have conductivity, and an active material layer that includes active material particles between the surfaces. (See Patent Document 1). A metal material having a low lithium compound forming ability is infiltrated into the active material layer of the negative electrode, and active material particles are present in the infiltrated metal material. Since the active material layer has such a structure, even if the particles are pulverized due to expansion and contraction of the particles due to charge and discharge, the active material layer is unlikely to fall off. As a result, when this negative electrode is used, there is an advantage that the cycle life of the battery becomes long.

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

  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 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. In some cases, the oxidation of the metal material existing inside the metal cannot be reliably prevented.

International Publication No. 2005/055345 Pamphlet

  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 negative electrode of the prior art described above.

The present invention includes an active material layer containing particles of an active material, and at least a part of the surface of the particles is coated with a layer of a metal material, and between the particles coated with the layer of the metal material. A negative electrode for a non-aqueous electrolyte secondary battery in which a void is formed,
A non-aqueous electrolyte secondary battery negative electrode is provided in which the layer is composed of two or more different metal materials.

  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 is used as one of the constituent metals of the layer. Even so, the amount of the metal exposed on the surface of the layer is less than when the metal is used alone. Therefore, oxidation becomes difficult to occur. In particular, when the layer is composed of two or more kinds of metal materials different from each other, the oxidation of the innermost layer covering the surface of the particle is prevented by the layer located on the upper side. Thereby, the overvoltage of the initial charge can be lowered. As a result, lithium dendrite is prevented from being generated on the surface of the negative electrode. In addition, the nonaqueous electrolytic solution is hardly decomposed, and an increase in irreversible capacity is prevented. Furthermore, the positive electrode is less likely to be damaged. In addition, cycle characteristics are improved. In addition, even if the particles are pulverized due to expansion and contraction due to charging / discharging, the particles do not easily fall off.

  The present invention will be described below based on 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 this embodiment includes a current collector 11 and an active material layer 12 formed on at least one surface thereof. 1 shows a state in which the active material layer 12 is formed only on one side of the current collector 11 for convenience, the active material layer may be formed on both sides of the current collector. .

  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-based materials, tin-based materials, aluminum-based materials, and germanium-based materials. As the tin-based material, for example, an alloy containing tin, cobalt, carbon, and at least one of nickel and chromium is preferably used. In order to improve the capacity density per weight of the negative electrode, a silicon-based material is particularly preferable.

  As the silicon-based material, a material that can occlude lithium and contains 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. Among these metals, Cu, Ni, and Co are preferable, and Cu and Ni are preferably used from the viewpoint of excellent electronic conductivity and a low ability to form a lithium compound. Further, lithium may be occluded in an active material made of a silicon-based material before or after the negative electrode is incorporated in the battery. A particularly preferable silicon-based material is silicon or silicon oxide in view of the high occlusion amount of lithium.

  In the active material layer 12, at least a part of the surface of the particle 12 a is covered with a 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 provided 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 reality, each particle is in direct contact with other particles or through a metal material 13.

  The metal material 13 is preferably present on the surface of the active material particles 12 a 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. Thus, even if the particles 12a are pulverized due to expansion and contraction due to charge / discharge, the particles 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, and in particular, the electrically isolated active material is deep in the active material layer 12. Generation of the particles 12a of the substance is effectively prevented. This is particularly advantageous when a material that is a semiconductor and has poor electron conductivity, such as a silicon-based material, is used as the active material. 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 target.

  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 covers the surface of the particle 12a continuously, 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, the non-aqueous electrolyte is applied to the particle 12a through a portion of the surface of the particle 12a that is not covered with the layer of the metal material 13. 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, electrolytic plating in accordance with conditions described later.

  Gaps are formed between the particles 12 a covered with the layer of the metal material 13. This void serves as a flow path for the non-aqueous electrolyte containing lithium ions. Since the non-aqueous electrolyte easily reaches the active material particles 12a due to the presence of the voids, it is possible to reduce the overvoltage of the initial charge. As a result, lithium dendrite is prevented from being generated on the surface of the negative electrode. The generation of dendrite causes a short circuit between the two poles. The ability to reduce the overvoltage is also advantageous from the viewpoint 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 less susceptible to damage.

  Further, the voids formed between the particles 12a also have a function as a space for relieving 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, pulverization of the particles 12a is difficult to occur, and significant deformation of the negative electrode 10 is effectively prevented.

  In the negative electrode 10 of this embodiment, as schematically shown in FIG. 2, the layer of the metal material 13 has a multilayer structure composed of different metal materials. Each layer may be composed of a single metal or may be composed of an alloy 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. With such a multilayer structure, for example, even when a material that is easily oxidized is used as the constituent material of the inner layer 13a, the inner layer 13a is covered with the outer layer 13b that is made of a material that is not easily oxidized. Thus, the oxidation of the inner layer 13a can be effectively prevented. As a result, even when the negative electrode 10 is left in the atmosphere for a long period of time, oxygen is less likely to be taken into the negative electrode, and the overvoltage during initial charging can be reduced. 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 does not actually cover the surface of the inner layer 13a uniformly, and is mottled, ie, discontinuously. It is considered a thing. In the present 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 is composed of, for example, a Sn layer, a Ni layer, and a Sn layer, the layer has a three-layer structure composed of different metal materials.

  It is preferable that two or more different metal materials constituting the metal material layer 13 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. It is known that copper is 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.

  From the viewpoint of preventing oxidation more effectively, the outer layer 13b, which is a layer exposed on the surface, is preferably made of a metal that is not easily oxidized. The outer layer 13b is also preferably made of a metal having a low ability to form a lithium compound. From these viewpoints, the metal constituting the outer layer 13b is preferably made of nickel, cobalt, chromium, gold, or an alloy thereof, which is a material that has a low ability to form a lithium compound and is hardly oxidized. “Low lithium compound forming ability” means that lithium does not form an intermetallic compound or solid solution, or even if formed, lithium is in a very small amount or very unstable.

  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. Examples of the metal material having a high lithium 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 viewpoint, it is preferable to use copper, which is a highly ductile material, as the constituent material of the inner layer 13a.

  The inner layer 13a preferably has a thickness of 40 to 1500 nm, particularly 100 to 240 nm, from the viewpoint that the active material particles 12a can be reliably held in the active material layer 12. 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 that the oxidation of the inner layer 13a can be reliably prevented. 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 the thicknesses of these layers are too thick, it is difficult to form fine voids. Moreover, it is preferable that the layer of the metal material 13 including the inner layer 13a and the outer layer 13b has an average thickness of 0.05 to 2 μm, particularly 0.1 to 0.25 μm. That is, the metal material 13 preferably covers the surface of the active material particles 12a with a minimum thickness. This prevents the particles 12a from falling off due to expansion and contraction due to charge and discharge and pulverization while increasing the energy density. 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 optical 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 12 a that is actually covered with the metal material 13. Accordingly, the portion of the surface of the active material particle 12a that is not covered with the metal material 13 is not used as a basis for calculating the average value.

  As described later, the active material layer 12 is preferably electrolyzed using a predetermined plating bath on a coating film obtained by applying a slurry containing particles 12a and a binder onto a current collector and drying the slurry. It forms by plating and depositing the metal material 13 between the particle | grains 12a.

  In order to form necessary and sufficient voids in the active material layer through which the non-aqueous electrolyte can be circulated, it is preferable to sufficiently infiltrate the plating solution into the coating film. In addition to this, it is preferable to make conditions suitable for depositing the metal material 13 by electrolytic plating using the plating solution. The plating conditions include the composition of the plating 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 11 particularly when the inner layer 13a is formed. By controlling the pH within this range, the dissolution of the active material particles 12a is suppressed, the surface of the particles 12a is cleaned, and plating on the particle surface is promoted. Is formed. The value of pH is measured at the temperature at the time of plating.

When copper is used as the constituent metal of the inner layer 13a, the inner layer 13a is preferably formed 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. Further, copper is deposited on the surface of the active material particles 12a, and copper is less likely to be deposited between the particles 12a, which is preferable in that the voids between the particles 12a are successfully formed. When using a copper pyrophosphate bath, the bath composition, electrolysis conditions and pH are preferably as follows.
Copper pyrophosphate trihydrate: 85-120 g / l
-Potassium pyrophosphate: 300-600 g / l
-Potassium nitrate: 15-65 g / l
-Bath temperature: 45-60 ° C
・ Current density: 1 to 7 A / dm ²
PH: Ammonia water and polyphosphoric acid are added to adjust the pH to 7.1 to 9.5.

In particular, when a copper pyrophosphate bath is used, it is preferable to use one having a P ratio defined by a ratio of P ₂ O ₇ weight to Cu weight (P ₂ O ₇ / Cu) of 5 to 12. . 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. Further, when a P ratio exceeding 12 is used, current efficiency is deteriorated, and gas generation is likely to occur, so that production stability may be lowered. 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 may be reduced. This is very advantageous for the flow of the water electrolyte.

When the inner layer 13a is an alloy of two or more kinds of metals, the inner layer 13a can be formed by alloy plating. 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 45g / l
・ Copper pyrophosphate trihydrate 36g / l
-Potassium pyrophosphate 30g / l
・ PH 8.2
・ Bath temperature 55 ℃
・ Current density 3A / dm ²
By performing electrolytic plating under the above conditions, an alloy having a copper to nickel ratio (weight ratio) of approximately 8: 2 is formed.

  In consideration of the penetration of the plating bath of the outer layer 13b performed in the next step, the proportion of voids in the active material layer 12 at the time when the inner layer 13a is formed is preferably 20% by volume or more. As a method of reducing the thickness of the inner layer 13a and increasing the void ratio in the active material layer 12, 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 of the press treatment is preferably 50 to 500 MPa, particularly 100 to 500 MPa.

  By subjecting the coating film 15 to press treatment prior to osmotic plating, the progress of plating can be accelerated, and a thin and uniform coating of the metal material 13 can be formed on the surface of the active material particles 12a. It became clear as a result of examination of inventors. 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 area where plating nuclei are generated increases in the thickness direction of the coating film 15. This is thought to be caused by From this viewpoint, the pressure of the press treatment is preferably 50 to 500 MPa, particularly 100 to 500 MPa.

On the other hand, when nickel, for example, is used as the constituent metal of the outer layer 13b, the outer layer 13b is preferably formed by electrolytic plating using a watt bath. When using a Watt bath, the bath composition and electrolysis conditions are preferably as follows.
・ NiSO ₄・ 6H ₂ O 150 ~ 300g / l
・ NiCl ₂ .6H ₂ O 30-60 g / l
・ H ₃ BO ₃ 30-40 g / l
-Bath temperature: 40-70 ° C
Current density: 0.5 to 20 A / dm ²

  By appropriately adding the various additives used in the electrolytic solution for producing copper foil such as protein, active sulfur compound, and cellulose to the various plating baths, the characteristics of the metal materials constituting the inner layer 13a and the outer layer 13b are appropriately adjusted. Is also possible.

The ratio of voids in the active material layer 12 formed by the above-described method, that is, the void ratio is preferably about 15 to 45% by volume, particularly about 20 to 40% by volume. By setting the porosity within this range, it is possible to form necessary and sufficient voids in the active material layer 12 that allow the nonaqueous electrolyte to 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 electrolytic plating, the weight of the plated metal species is calculated. 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 determined 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 compounding ratios.
(6) From the volume of the active material layer 12 per unit area, the volume of the voids is calculated by subtracting the volume of the particles 12a, the volume of the binder, and the volume of the plating metal species.
(7) The void volume calculated in this way is divided by the volume of the active material layer 12 per unit area, and a value obtained by multiplying by 100 is defined as a void ratio (%).

The porosity can also be controlled by appropriately selecting the particle size of the active material particles 12a. From this viewpoint, the particle 12a has a maximum particle size of preferably 30 μm or less, more preferably 10 μm or less. Moreover, when the particle diameter of the particle is expressed by a D ₅₀ value, it is preferably 0.1 to 8 μm, particularly preferably 0.3 to 4 μm. The particle size of the particles is measured by laser diffraction / scattering particle size distribution measurement and electron microscope observation (SEM observation).

  If the amount of the active material relative to the whole negative electrode is too small, it is difficult to sufficiently improve the energy density of the battery. Conversely, 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 μm, 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 0.25 μm or less, preferably 0.1 μm or less. There is no restriction | limiting in the lower limit of the thickness of a surface layer.

  When the negative electrode 10 has the thin surface layer or does not have the surface layer, a secondary battery is assembled using the negative electrode 10 to reduce the overvoltage when the battery is initially charged. Can do. 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 a short circuit 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 microscopic voids (not shown) that are open at the surface and communicate with the active material layer 12. It is preferable. The fine voids are preferably present 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 electron microscope observation, the fine voids are such that the ratio of the area covered with the metal material 13, that is, the coverage is 95% or less, particularly 80% or less, especially 60% or less. It is preferable that the size is large.

  The surface layer is made of a metal material having a low lithium compound forming ability. This metal material can be the same kind 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 made of a metal material having a low ability to form a lithium compound, the surface layer may be made of a constituent material of the inner layer 13a. 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 manufacture of the negative electrode 10, the constituent material of the outer layer 13b and the metal material constituting the surface layer are preferably the same type.

  As the current collector 11 in the negative electrode 10, the same one as conventionally used as the current collector of the negative electrode for the non-aqueous electrolyte secondary battery can be used. The current collector 11 is preferably made of a metal material having a low lithium compound forming ability as described above. Examples of such metallic 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 a 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 preferable to use a current collector having a normal elongation (JIS C 2318) of 4% or more. This is because when the tensile strength is low, wrinkles are generated due to 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 μm. In addition, when using copper foil as the electrical power collector 11, it is preferable to give the rust prevention process using organic compounds, such as a chromate process and a triazole type compound and an imidazole type compound.

  Next, the preferable manufacturing method of the negative electrode 10 of this embodiment is demonstrated, referring FIG. In this production 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. The slurry contains a binder and a diluting solvent in addition to the active material particles. The slurry may contain a small amount of conductive carbon material particles such as acetylene black and graphite. In particular, when the active material particles 12a are made of a silicon-based material, the conductive carbon material is preferably contained in an amount of 1 to 3% by weight based on the weight of the active material particles 12a. When the content of the conductive carbon material is less than 1% by weight, the viscosity of the slurry is lowered and the sedimentation of the active material particles 12a is promoted, so that it is difficult to form a good coating film 15 and uniform voids. . On the other hand, 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.

  As the binder, styrene butadiene rubber (SBR), polyvinylidene fluoride (PVDF), polyethylene (PE), ethylene propylene diene 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 dilution solvent is added to these to form a slurry.

  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 plating bath includes a metal material having a low lithium compound forming ability or a metal material having a high lithium compound forming ability. 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 plating metal species on the surfaces of the particles 12a (hereinafter, this plating is also referred to as permeation plating). The osmotic plating is performed by using the current collector 11 as a cathode, immersing a counter electrode as an anode in a plating bath, and connecting both electrodes to a power source.

  The deposition of the metal material by the osmotic plating is preferably progressed from one side of the coating film 15 to the other side. Specifically, as shown in FIGS. 3B to 3D, electrolytic plating is performed so that 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. I do. By precipitating the plating metal species in this way, the surface of the active material particles 12a can be successfully coated with the plating metal species, and a void is successfully formed between the particles 12a coated with the plating metal species. be able to. In addition, it becomes easy to set the void ratio of the voids to the above-described preferable range.

  As described above, the conditions of the infiltration 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 already described.

  As shown in FIGS. 3B to 3D, electrolytic plating is performed so that 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. And, in the forefront portion of the precipitation reaction, fine particles 14 made of plating nuclei of plating metal species are present in layers with a substantially constant thickness. As the deposition of the plating metal species proceeds, adjacent microparticles 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. It becomes like this.

  The 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 plating bath, a second plating bath is used, and electrolytic plating is performed by the same procedure as described above. The second plating bath contains a metal material having a low lithium compound forming ability.

  By performing electroplating with the second plating bath in the same procedure as the electroplating with the first plating bath, a different type of plating from the metal species deposited on the inner layer made of the plating metal species deposited previously. An outer layer made of a metal species is formed. The electrolytic plating by 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 the electrolytic plating.

The negative electrode 10 thus obtained is suitably used as a negative electrode for a nonaqueous 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 a suitable solvent to prepare a positive electrode mixture, applying this to a current collector, drying, roll rolling, It is obtained by pressing, further 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. Further, as the positive electrode active material, a lithium transition metal composite oxide in which LiCoO ₂ contains at least both Zr and Mg, a lithium transition metal composite oxide having a layered structure and containing at least both Mn and Ni, A mixture thereof can also be preferably used. The use of such a positive electrode active material can be expected to increase the end-of-charge voltage without deteriorating charge / discharge cycle characteristics and thermal stability. The average value of the primary particle diameter of the positive electrode active material is preferably 5 μm or more and 10 μm or less in view of the packing density and the reaction area, and the weight average molecular weight of the binder used for the positive electrode is 350,000 or more and 2,000. It is preferable that the polyvinylidene fluoride is 1,000 or less. This is because it can be expected to improve discharge characteristics in a low temperature environment.

  As the battery separator, a synthetic resin nonwoven fabric, a polyolefin such as polyethylene or polypropylene, or a polytetrafluoroethylene porous film 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 thin film of a ferrocene derivative is formed on one side or both sides of a polyolefin microporous membrane. The separator preferably has a puncture strength of 0.2 N / μm thickness or more and 0.49 N / μm thickness 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 greatly expands and contracts with charge and discharge is used, damage to the separator can be suppressed, and occurrence of internal short circuits can be suppressed.

The nonaqueous electrolytic solution is a solution in which a lithium salt as a supporting electrolyte is dissolved in an organic solvent. Examples of the lithium salt include LiClO ₄ , LiA1Cl ₄ , LiPF ₆ , LiAsF ₆ , LiSbF ₆ , LiBF ₄ , LiSCN, LiCl, LiBr, LiI, LiCF ₃ SO ₃ , LiC ₄ F ₉ SO _{3 and the} like. Examples of the organic solvent include ethylene carbonate, diethyl carbonate, dimethyl carbonate, propylene carbonate, butylene carbonate, and the like. In particular, 0.5 to 5% by weight of vinylene carbonate and 0.1 to 1% by weight of divinyl sulfone and 0.1 to 1.5% by weight of 1,4-butanediol dimethanesulfonate with respect to the whole non-aqueous electrolyte. It is preferable from the viewpoint of further improving the charge / discharge cycle characteristics. The details are not clear, but 1,4-butanediol dimethanesulfonate and divinylsulfone are decomposed stepwise to form a film on the positive electrode, so that the film containing sulfur becomes denser. It is thought that it is to become.

  In particular, as the non-aqueous electrolyte, 4-fluoro-1,3-dioxolan-2-one, 4-chloro-1,3-dioxolan-2-one or 4-trifluoromethyl-1,3-dioxolane-2- It is also preferable to use a high dielectric constant solvent having a relative dielectric constant of 30 or more, such as a cyclic carbonate derivative having a halogen atom such as ON. This is because it has high resistance to reduction and is not easily decomposed. Further, an electrolytic solution obtained by mixing the high dielectric constant solvent and a low viscosity solvent having a viscosity of 1 mPa · s or less such as dimethyl carbonate, diethyl carbonate, or methyl ethyl carbonate is also preferable. This is because higher ionic 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 to 1290 mass ppm. This is because, when an appropriate amount of fluorine ions is contained in the electrolytic solution, a coating film such as lithium fluoride derived from fluorine ions is formed on the negative electrode, and it is considered that the decomposition reaction of the electrolytic solution in the negative electrode can be suppressed. . Furthermore, it is preferable that 0.001 mass%-10 mass% of at least 1 sort (s) of additives from the group which consists of an acid anhydride and its derivative (s) are contained. This is because a film is formed on the surface of the negative electrode, and the decomposition reaction of the electrolytic solution can be suppressed. As this additive, a cyclic compound containing a —C (═O) —O—C (═O) — group in the ring is preferable. For example, succinic anhydride, glutaric anhydride, maleic anhydride, phthalic anhydride, anhydrous Phthalic anhydride derivatives such as 2-sulfobenzoic acid, citraconic anhydride, itaconic anhydride, diglycolic anhydride, hexafluoroglutaric anhydride, 3-fluorophthalic anhydride, 4-fluorophthalic anhydride, or anhydrous 3,6 -Epoxy-1,2,3,6-tetrahydrophthalic acid, 1,8-naphthalic anhydride, 2,3-naphthalene carboxylic acid anhydride, 1,2-cyclopentane dicarboxylic acid anhydride, 1,2-cyclohexane dicarboxylic acid, etc. 1,2-cycloalkanedicarboxylic anhydride, or cis-1,2,3,6-tetrahydrophthalic anhydride or 3,4,5,6-tetrahydrophthal Tetrahydrophthalic anhydride such as acid anhydride, or hexahydrophthalic anhydride (cis isomer, trans isomer), 3,4,5,6-tetrachlorophthalic anhydride, 1,2,4-benzene Examples thereof include tricarboxylic acid anhydride, dianhydropyromellitic acid, and derivatives thereof.

  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 is composed of an inner layer made of a metal material having a high lithium compound forming ability or a metal material having a low lithium compound forming ability and a metal material having a low lithium compound forming ability. However, instead of this, another layer of one or more layers may be provided between the inner layer and the outer layer to form a multilayer structure of three or more layers, or further on the outside of the outer layer. One or more other layers may be provided to form a multilayer structure of three or more layers. Further, 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.

  Moreover, although the said embodiment is an example in case the layer of the metal material 13 is a multilayer structure, as long as this layer is comprised from 2 or more types of metal materials, even if this layer is a thing of a single layer, Good. In this case, the layer of the metal material 13 is a layer made of an alloy 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. An example of the metal that is easily oxidized and has a low lithium forming ability is copper. Examples of the metal that is not easily oxidized and has a low lithium forming ability include nickel, cobalt, chromium, and gold. In order to form a layer made of such an alloy, the permeation plating shown in FIG. 3 may be performed by an alloy plating method. The ratio (molar ratio) between the metal that is easily oxidized and has a low or high lithium forming ability and the metal that is not easily oxidized and has a low lithium forming ability in the layer of the metal material 13 is the former: the latter = 7: 3 to 9: 1 is preferable.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the scope of the present invention is not limited to such examples.

[Example 1]
A current collector made of an electrolytic copper foil having a thickness of 18 μm 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 μm 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 diameter D ₅₀ of the Si particles was 2 μm. The average particle diameter D ₅₀ was measured using a Microtrac particle size distribution measuring device (No. 9320-X100) manufactured by Nikkiso Co., Ltd.

The current collector on which the coating film was formed was passed through a roll press machine, 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, copper is osmotically plated on the coating film, and copper is coated on the surface of the Si particles. An inner layer consisting of 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: 105 g / l
-Potassium pyrophosphate: 450 g / l
・ Potassium nitrate: 30 g / l
-P ratio: 7.7
・ Bath temperature: 50 ° C
・ Current density: 3 A / dm ²
-PH: Ammonia water and polyphosphoric acid were added to adjust to pH 8.2.

The permeation plating 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.
・ NiSO ₄・ 6H ₂ O 250g / l
・ NiCl ₂・ 6H ₂ O 45g / l
・ H ₃ BO ₄ 30g / l
・ Bath temperature 50 ℃
・ Current density 0.5A / dm ²
・ PH: 5

The average thickness of the inner layer and the outer layer was determined by the following method. The ratio of the amount of energization when performing copper infiltration plating to the amount of energization when performing nickel infiltration plating is determined. 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 when copper is infiltrated and when the nickel is infiltrated. It can be regarded as equivalent to the ratio of the energization amount. 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 the cross-sectional SEM observation of the active material layer and the ratio of the energization amount. Energization amount when subjected to penetration of copper in this example 2321C / dm ^2, current amount when performing the penetration plating nickel was 18C / dm ^2. The inner layer thickness thus determined was 200 nm, and the outer layer thickness was 5 nm.

[Example 2]
A negative electrode was obtained in the same manner as in Example 1 except that the amount of current applied to the nickel infiltration plating was 183 C / dm ² . The outer layer thickness determined 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 μm. The energization amount when copper permeation plating was performed was 2321 C / dm ² , and the energization amount when nickel permeation plating was performed was 73 C / dm ² . A negative electrode was obtained in the same manner as Example 1 except for these. The thickness of the inner layer determined in the same manner as in Example 1 was 200 nm, and the thickness of the outer layer was 20 nm.

Example 4
Instead of the Watt bath, a second infiltration plating was performed using a cobalt bath having the following composition. The energization amount at this time was 73 C / dm ² . A negative electrode was obtained in the same manner as Example 1 except for these. The outer layer thickness obtained in the same manner as in Example 1 was 20 nm.
・ CoSO ₄・ 7H ₂ O 180g / l
・ H ₃ BO ₃ 30g / l
・ Bath temperature 40 ℃
・ Current density 0.5A / dm ²
・ PH: 5

Example 5
Instead of the Watt bath, a second infiltration plating was performed using a chromium bath having the following composition. The energization amount at this time was 62 C / dm ² . A negative electrode was obtained in the same manner as Example 1 except for these. The outer layer thickness obtained in the same manner as in Example 1 was 15 nm.
・ CrO ₃ 250g / l
・ H ₂ SO ₄ 2.5 g / l
・ Bath temperature 40 ℃
・ Current density 0.5A / dm ²

[Comparative Example 1]
A negative electrode was obtained in the same manner as in Example 1 except that the second permeation plating using a Watt bath was not performed.

[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 _1/3 Ni _1/3 Mn _1/3 O ₂ was used as the positive electrode. As the electrolytic solution, a solution obtained by adding 2% by volume of vinylene carbonate to a solution of 1 mol / l LiPF ₆ dissolved in a 1: 1 volume% mixed solvent of ethylene carbonate and diethyl carbonate was used. As the separator, a 20 μm thick polypropylene porous film was used. The obtained secondary battery was charged for the first time, and the voltage when the capacity was 0.1 mAh was measured. Charging was performed in a constant current / constant voltage mode.

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

  Furthermore, while observing the longitudinal cross section of the active material layer of the negative electrode obtained in Example 3 with a scanning electron microscope (SEM), the presence state of Ni was measured by EPMA on the longitudinal section. The results are shown in FIGS. 4 (a) and 4 (b). Further, regarding the longitudinal section of the active material layer, the existence state of each element was measured by GDS. The result is shown in FIG. In FIG. 5, the sensitivity of the vertical axis is changed in range according to the type of element, so that a relative comparison between elements is not possible.

  As is clear from the results shown in Table 1, it can be seen that the battery using the negative electrode of each example has a lower voltage at the first charge, that is, a lower overvoltage than the battery using the negative electrode of Comparative Example 1. . In addition, it can 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. Although it has been confirmed that the negative electrode of Comparative Example 1 can obtain good characteristics if used immediately after production, it is assumed that the characteristics deteriorated due to the influence of oxidation in this evaluation. .

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

It is a schematic diagram which shows the cross-sectional structure of one Embodiment of the negative electrode for nonaqueous electrolyte secondary batteries of this invention. FIG. 2 is a schematic diagram illustrating an enlarged main part of an active material layer in the negative electrode illustrated in FIG. 1. It is process drawing which shows the manufacturing method of the negative electrode shown in FIG. It is a figure which shows the result of having measured the SEM image of the longitudinal cross section of the active material layer of the negative electrode obtained in Example 3, and the presence state of Ni in a longitudinal cross section by EPMA. It is a figure which shows the result of having measured the presence state of each element in the longitudinal cross-section of the active material layer of the negative electrode obtained in Example 3 by GDS.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Negative electrode for nonaqueous electrolyte secondary batteries 11 Current collector 12 Active material layer 12a Active material particles 13 Metal material 13a Inner layer 13b Outer layer 15 Coating film

Claims (8)

An active material layer containing active material particles is provided, and at least a part of the surface of the particles is covered with a metal material layer, and voids are formed between the particles covered with the metal material layer. A negative electrode for a non-aqueous electrolyte secondary battery,
A negative electrode for a nonaqueous electrolyte secondary battery, wherein the layer is composed of two or more different metal materials.   The negative electrode for a nonaqueous electrolyte secondary battery according to claim 1, wherein the layer has a multilayer structure of two or more layers made of different metal materials.   3. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the two or more kinds of metal materials different from each other contain at least copper. 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 composed of a metal material having a high ability to form a lithium compound or a metal material having a low ability to form a lithium compound;
The negative electrode for a nonaqueous electrolyte secondary battery according to claim 2, wherein the outer layer is made of a metal material having a low lithium compound forming ability. The inner layer is formed from zinc, tin or silver which is a metal material having a high ability to form a lithium compound, or is formed from copper which is a metal material having a low ability to form a lithium compound,
The negative electrode for a nonaqueous electrolyte secondary battery according to claim 4, wherein the outer layer is made of nickel, cobalt, chromium, or gold, which is a metal material having a low ability to form a lithium compound.   The negative electrode for a non-aqueous electrolyte secondary battery according to claim 4, wherein the inner layer is present on the surface of the active material particles over the entire thickness direction of the active material layer.   The negative electrode for a nonaqueous electrolyte secondary battery according to any one of claims 1 to 6, wherein the layer has an average thickness of 0.05 to 2 µm on average.   The nonaqueous electrolyte secondary battery provided with the negative electrode for nonaqueous electrolyte secondary batteries in any one of Claims 1 thru | or 7.