JP2006004903A - Anode for nonaqueous electrolyte secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prolong a cycle life of a nonaqueous electrolyte secondary battery by heightening a current collecting property of an anode and preventing fall-off of an activator caused by storing/releasing of lithium ion. <P>SOLUTION: The anode 10 has a pair of current collecting surface layers 4a, 4b at a front surface and back surface respectively, and an activator layer 3 interposed between the surface layers. The surface layers 4a, 4b are composed of elements 6 with a low lithium compound forming function and elements 5 with a high lithium compound forming function, and not any thick conductor film for current collection is formed thereon. The elements 5 with the high lithium compound forming function contained in the surface layers 4a, 4b are turned into fine particles in accordance with charge and discharge, and consequently, cracks are generated on the surface layers, and many fine pores 7, extending in a thickness direction to reach the activator layer 3, through which, electrolyte liquid can permeate, are formed. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a negative electrode for a non-aqueous electrolyte secondary battery. More specifically, the negative electrode has a high current collecting property, prevents the active material from falling off due to absorption and desorption of lithium ions, improves the cycle life, and can provide a non-aqueous electrolyte secondary battery with higher energy density. About.

  In a negative electrode for a lithium ion secondary battery in which an active material layer made of a metal alloying with Li such as tin is provided on a current collector layer made of a metal not alloyed with Li such as copper, the active material layer On top of this, it has been proposed to provide a surface coating layer made of a metal not alloyed with Li, or a surface coating layer made of an alloy of a metal not alloyed with Li and a metal alloyed with Li (see Patent Document 1). ). According to this patent document, since the reaction between the surface of the active material layer and the electrolytic solution can be suppressed by this surface coating layer, deterioration of the surface of the active material layer can be suppressed, and charge / discharge cycle characteristics can be improved. It is said that it can be improved.

  However, since the surface coating layer in this electrode only covers the active material layer in a thin island shape, a considerable area of the active material layer is exposed on the surface of the electrode and is in direct contact with the electrolyte. Therefore, the active material is easily dropped from the electrode due to the expansion and contraction of the active material due to the absorption and desorption of Li accompanying charging and discharging.

JP 2002-289178 A

  Accordingly, an object of the present invention is to provide a negative electrode for a non-aqueous electrolyte secondary battery that can eliminate the various drawbacks of the above-described conventional technology.

  The present invention comprises a pair of front and back current collecting surface layers that are in contact with an electrolytic solution and have conductivity, and an active material layer that is disposed between the surface layers and includes active material particles having a high ability to form a lithium compound. And at least one of the surface layers includes an element having a low lithium compound forming ability and an element having a high lithium compound forming ability, and does not have a thick film conductor for collecting current. The above object is achieved by providing a negative electrode for a non-aqueous electrolyte secondary battery.

  According to the negative electrode for a non-aqueous electrolyte secondary battery of the present invention, fine voids are formed in the current collecting surface layer by charging and discharging. The fine voids are large enough to penetrate the electrolytic solution and prevent the active material particles from falling off. As a result, after the active material and the electrolytic solution are in sufficient contact, the surface layer prevents the active material from falling off, and the current collecting property of the active material is ensured even after repeated charge and discharge. Furthermore, the secondary battery using this negative electrode has a low deterioration rate even when charging and discharging are repeated, and the cycle life is greatly prolonged, and the charging and discharging efficiency is also increased. Furthermore, since the conductive metal foil layer as the core material, that is, the current collector used in the conventional negative electrode is not used, the ratio of the active material to the entire negative electrode can be made higher than that of the conventional negative electrode. As a result, a secondary battery having a high energy density per unit volume and unit weight can be obtained.

  The present invention will be described below based on preferred embodiments with reference to the drawings. FIG. 1 shows a schematic diagram of an embodiment of the negative electrode of the present invention. The negative electrode 10 of the present embodiment has a first surface 1a and a second surface 1b which are a pair of front and back surfaces in contact with the electrolytic solution. The electrode 10 includes an active material layer 3 including active material particles 2 between both surfaces. The active material layer 3 is continuously covered with a pair of current collecting surface layers 4 a and 4 b formed on each surface of the layer 3. Each surface layer 4a, 4b includes a first surface 1a and a second surface 1b, respectively. As is clear from FIG. 1, the electrode 10 has a current collecting thick film conductor (for example, a metal foil or expanded metal having a thickness of about 8 to 35 μm) called a current collector, which has been used for conventional electrodes. Not done.

  The current collecting surface layers 4a and 4b have a current collecting function in the negative electrode 10 of the present embodiment. The surface layers 4a and 4b are also used to prevent the active material contained in the active material layer 3 from falling off due to expansion and / or contraction due to an electrode reaction.

  Each of the surface layers 4a and 4b is thinner than the thick film conductor for current collection used in conventional electrodes. Specifically, a thin layer of about 0.3 to 20 μm, particularly about 0.3 to 10 μm, particularly about 0.5 to 5 μm is preferable. As a result, the active material layer 3 can be continuously coated almost uniformly with the minimum necessary thickness. As a result, dropping of the active material particles 2 can be prevented. In addition, by making such a thin layer and not having a thick film conductor for current collection, the proportion of the active material in the whole negative electrode becomes relatively high, and per unit volume and per unit weight. Energy density can be increased. In the conventional electrode, the ratio of the thick film conductor for current collection to the entire electrode is high, so there is a limit to increasing the energy density. The thin surface layers 4a and 4b in the above range are preferably formed by electrolytic plating as described later. The two surface layers 4a and 4b may have the same thickness or different thicknesses. The constituent materials of the two surface layers 4a and 4b may be the same or different, but at least one of the surface layers has a low lithium compound forming ability and a lithium compound forming ability, as will be described later. It is necessary that the composition contains a high element. When one surface layer includes an element having a low lithium compound forming ability and an element having a high lithium compound forming ability, the other surface layer may be composed of an element having a low lithium compound forming ability. preferable. Preferably, the two surface layers 4a and 4b include an element having a low lithium compound forming ability and an element having a high lithium compound forming ability.

  As described above, the two surface layers 4a and 4b include the first surface 1a and the second surface 1b, respectively. When the negative electrode 10 of the present embodiment is incorporated in a battery, the first surface 1a and the second surface 1b are in contact with the electrolytic solution. In contrast, a thick film conductor for collecting current in a conventional electrode does not come into contact with the electrolyte solution when the active material layer is formed on both sides thereof, and the active material layer is not provided on one side. Even if it is formed, only one surface is in contact with the electrolyte. That is, the negative electrode 10 of the present embodiment does not have the current collecting thick film conductor used in the conventional electrode, and the layers positioned on the outermost surface of the electrode, that is, the surface layers 4a and 4b have the current collecting function. And the function of preventing the active material from falling off.

  Since each of the surface layers 4a and 4b each including the first surface 1a and the second surface 1b has a current collecting function, when the negative electrode 10 of this embodiment is incorporated in a battery, The surface layers 4a and 4b also have an advantage that a lead wire for extracting current can be connected.

  As described above, at least one of the surface layers 4a and 4b includes an element having a low lithium compound forming ability and an element having a high lithium compound forming ability. Since each surface layer 4a, 4b has such a configuration, the negative electrode 10 of this embodiment has the following advantages. This advantage will be described with reference to FIG. FIG. 2 is an enlarged schematic view showing a main part of the negative electrode 10 of the present embodiment.

  When the negative electrode 10 of the present embodiment is incorporated into a non-aqueous electrolyte secondary battery and charged and discharged, as shown in FIG. 2A, the ability to form lithium compounds contained in the surface layers 4a and 4b is high. Element 5 gradually becomes fine due to absorption and desorption of lithium. On the other hand, the element 6 having a low lithium compound forming ability contained in each of the surface layers 4a and 4b does not change even when charging / discharging is performed.

  When the atomization of the element 5 having a high ability to form a lithium compound proceeds, as shown in FIG. 2B, cracks are generated on the surfaces of the surface layers 4a and 4b. And the part which the crack produced becomes many fine space | gap 7. FIG.

  As this crack progresses, the fine void 7 further grows, and as shown in FIG. 2C, the fine void 7 extends in the thickness direction of at least one of the surface layers 4a and 4b and reaches the active material layer 3. . The fine gap 7 extends while bending. In FIG. 2 (c), all the formed fine voids 7 are drawn so as to reach the active material layer 3. However, this is exaggerated for easy understanding of the present invention. In fact, not all the fine voids 7 reach the active material layer 3.

  As a result of the formation of such fine voids 7, the electrolyte can sufficiently penetrate into the active material layer 3, and the reaction with the active material particles 2 occurs sufficiently. Since the nonaqueous electrolytic solution has a smaller surface tension than the aqueous electrolytic solution, the nonaqueous electrolytic solution can sufficiently penetrate into the active material layer 3 even if the width of the fine gap 7 is small. As a result of forming the fine voids 7 in this way, in the negative electrode 10 of the present embodiment, it is possible to prevent the particles 2 of the active material from falling off while ensuring the penetration of the electrolytic solution into the active material layer 3. .

  Since the reaction in the negative electrode 10 occurs centering on the surface facing the counter electrode, it is sufficient that the fine void 7 is formed in at least one of the pair of surface layers 4a and 4b. However, in practical batteries, a separator and a counter electrode are often disposed on both sides of the negative electrode. When the negative electrode 10 of this embodiment is applied to such a battery, it is preferable that the fine voids 7 are formed in both the pair of surface layers 4a and 4b. In the case of using the negative electrode 10 in which fine voids are formed only in one surface layer of the pair of surface layers 4a and 4b, a set of such negative electrodes 10 is prepared, and the fine voids in the negative electrodes 10 and 10 are formed. By using the surface layers on the side that is not facing each other in an overlapping manner, the same effect as that of the negative electrode 10 in which the fine voids 5 are formed in both of the pair of surface layers can be obtained. In particular, a negative electrode having a configuration in which the surface layers of the negative electrodes 10 and 10 on which the fine voids are not formed are opposed to each other, a conductive metal foil is sandwiched between the negative electrodes 10 and 10, and the three members are joined and integrated. It is preferable. This is because by adopting such a configuration, the strength of the whole negative electrode is further improved. This is advantageous from the viewpoint of ensuring the strength of the negative electrode against bending stress when it is necessary to bend the negative electrode when producing a battery. As the conductive metal foil, various materials made of a material having a low lithium forming ability such as an electrolytic copper foil, a rolled copper alloy foil, and a stainless steel foil can be used. From the balance between the effect of improving the strength and the energy density, the thickness is preferably about 5 to 35 μm, particularly 12 to 18 μm. In addition, as the conductive metal foil, a porous metal such as a punching metal or a metal foil having a lithium layer formed on the surfaces of the various foils exemplified above can be used.

The size (hole diameter, length) of the fine gap 7 varies depending on the number of times the battery is charged and discharged. Therefore, the size of the fine gap 7 cannot be uniquely determined. However, as a result of the study by the present inventors, when the surface layers 4a and 4b are viewed in plan by electron microscope observation, the average pore area of the fine voids 7 is 0.1 to 100 μm ² , particularly 1 to 30 μm ^2. It was found that the active material particles 2 could be effectively prevented from falling off while ensuring sufficient permeation of the electrolyte solution. In addition, when the surface layers 4a and 4b are viewed in plan by electron microscope observation, no matter what observation field is taken, 1 to 20,000, particularly 10 to 1000, especially 30 in the square field of view of 1 cm × 1 cm. It has been found that it is preferable that ˜500 fine voids 7 exist (this value is referred to as distribution rate). Furthermore, when the surface layers 4a and 4b are viewed in plan by electron microscope observation, the ratio of the total opening area of the fine voids 7 to the area of the observation field (this ratio is referred to as the opening ratio) is 0.1 to 10%. In particular, it has been found that 1 to 5% is preferable. The fine gap 7 has a width of about 0.1 to 100 μm when the surface layers 4 a and 4 b are observed in cross section, but in order to more effectively suppress the falling off of the active material, 0.1 to 50 μm, In particular, it is preferable to use a fine one of about 0.1 to 10 μm.

  In order to form a desired fine void, the ratio of the element 5 having a high lithium compound forming ability and the element 6 having a low lithium compound forming ability contained in each of the surface layers 4a and 4b should be within an appropriate range. As a result of the examination by the present inventors, it has been found that it is good. Specifically, in each of the surface layers 4a and 4b, the element 6 having a low lithium compound forming ability preferably occupies 50% by weight or more. In particular, each of the surface layers 4a and 4b preferably includes 50 to 99.9% by weight of an element having a low lithium compound forming ability and 0.1 to 50% by weight of an element having a high lithium compound forming ability. In particular, it is preferable that the composition contains 70 to 99% by weight of an element having a low ability to form a lithium compound and 1 to 30% by weight of an element having a high ability to form a lithium compound.

  As the element 6 having a low lithium compound forming ability, nickel, copper, iron, cobalt, or an alloy thereof is preferably used. In particular, it is preferable to use a nickel-tungsten alloy because the surface layer 4 can have high strength. On the other hand, tin, zinc, aluminum, indium or the like is preferably used as the element 5 having a high lithium compound forming ability. These elements can be used alone or in combination of two or more.

  Returning to FIG. 1 again, the active material layer 3 positioned between the two surface layers 4a and 4b includes active material particles 2 having a high lithium compound-forming ability. Examples of the active material include silicon-based materials, tin-based materials, aluminum-based materials, and germanium-based materials. In particular, a silicon-based material is preferable. Since the active material layer 3 is covered with the two surface layers 4a and 4b, the active material is effectively prevented from falling off due to absorption and desorption of lithium ions. Moreover, since the active material particles 2 can be in contact with the electrolytic solution for the reason described later, the electrode reaction is not hindered.

The active material particles 2 have a maximum particle size of preferably 50 μm or less, more preferably 20 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 1 to 5 μm. If the maximum particle size is more than 50 μm, the particles are likely to fall off, and the life of the electrode may be shortened. There is no particular limitation on the lower limit of the particle size, and the smaller the better. In view of the method for producing the particles, the lower limit is about 0.01 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 active material tends to fall off. Considering these, the amount of the active material is preferably 5 to 80% by weight, more preferably 10 to 50% by weight, and still more preferably 20 to 50% by weight with respect to the whole negative electrode.

  The thickness of the active material layer 3 can be appropriately adjusted according to the ratio of the amount of the active material to the whole negative electrode and the particle size of the active material, and is not particularly critical in this embodiment. Generally, it is about 1 to 100 μm, particularly about 3 to 40 μm. The active material layer is preferably formed by applying a conductive slurry containing particles of the active material, as will be described later.

  In the active material layer 3, it is preferable that a metal material having a low ability to form a lithium compound penetrates between particles contained in the layer. It is preferable that the metal material penetrates over the entire thickness direction of the active material layer 3. The active material particles 2 are preferably present in the infiltrated metal material. That is, it is preferable that the active material particles 2 are not exposed on the surface of the negative electrode 10 and are embedded in the surface layers 4a and 4b. As a result, the adhesion between the active material layer 3 and the surface layers 4a and 4b becomes strong, and the dropping of the active material particles 2 is further prevented. In addition, since electronic conductivity is ensured between the surface layers 4a and 4b and the active material particles 2 through the material penetrated into the active material layer 3, it is possible to generate an electrically isolated active material, Generation of an electrically isolated active material in the deep part of the material layer 3 is effectively prevented, and the current collecting function is maintained. As a result, functional degradation as a negative electrode is suppressed. In addition, the life of the negative electrode can be extended. 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 metal material having a low lithium compound forming ability penetrating into the active material layer 3 may be the same as or different from the metal material having a low lithium compound forming ability contained in at least one surface layer. May be. For example, (a) when each surface layer includes a metal material having a low lithium compound forming ability, the metal material may be the same as the metal material having a low lithium compound forming ability penetrating into the active material layer 3. Good. In this case, since each material is the same, there exists an advantage that the manufacturing method mentioned later does not become complicated. Alternatively, (b) each surface layer includes a metal material having a low lithium compound forming ability, and one of the metal materials is different from the metal material having a low lithium compound forming ability penetrating into the active material layer 3. May be. Furthermore, (c) each surface layer includes a metal material having a low lithium compound forming ability, and both of the metal materials are different from the metal material having a low lithium compound forming ability penetrating into the active material layer 3. It may be.

  In the case of (c), the metal material having a low ability to form a lithium compound contained in each surface layer may be the same or different. That is, (i) a metal material having a low lithium compound forming ability contained in each surface layer is the same, and the metal material has a low lithium compound forming ability penetrating into the active material layer 3; And (ii) different metal materials with low lithium compound-forming ability contained in each surface layer, and all of the metal materials have low ability to form lithium compounds penetrating into the active material layer 3 May differ from metal material.

  It is preferable that the metal material having a low ability of forming a lithium compound penetrating into the active material layer 3 penetrates the active material layer 3 in the thickness direction and is connected to both surface layers 4a and 4b. As a result, the two surface layers 4a and 4b are electrically connected through the material, and the electron conductivity of the whole negative electrode is further increased. That is, the negative electrode 10 of the present embodiment as a whole has a current collecting function. The fact that a metal material having a low ability to form a lithium compound penetrates over the entire thickness direction of the active material layer 3 and the surface layers 4a and 4b are connected is confirmed by electron microscope mapping using the material as a measurement target. can do. A preferred method for allowing a metal material having a low lithium compound forming ability to penetrate into the active material layer 3 will be described later.

  The space between the active material particles 2 in the active material layer 3 is preferably not completely filled with a metal material having a low lithium compound forming ability, but preferably has voids 8 between the particles 2 ( It should be noted that this void is different from the fine void 7 formed in the current collecting surface layers 4a and 4b). The presence of the voids 8 relieves stress caused by the active material particles 2 absorbing and desorbing lithium and expanding and contracting. From this viewpoint, the ratio of the voids 8 in the active material layer 3 is preferably about 5 to 30% by volume, particularly about 5 to 9% by volume. The ratio of the gap 8 can be obtained by electron microscope mapping. As described later, since the active material layer 3 is formed by applying and drying a conductive slurry containing the active material particles 2, the active material layer 3 naturally has voids 8. Therefore, in order to set the ratio of the voids 8 within the above range, for example, the particle diameter of the active material particles 2, the composition of the conductive slurry, and the application conditions of the slurry may be appropriately selected. Alternatively, after the slurry is applied and dried to form the active material layer 3, the ratio of the voids 8 may be adjusted by pressing under appropriate conditions.

  The active material layer 3 preferably contains a conductive carbon material in addition to the active material particles 2. This further imparts electronic conductivity to the negative electrode 10. From this viewpoint, the amount of the conductive carbon material contained in the active material layer 3 is preferably 0.1 to 20% by weight, particularly 1 to 10% by weight. For example, particles such as acetylene black and graphite are used as the conductive carbon material. The particle diameter of these particles is preferably 40 μm or less, and particularly preferably 20 μm or less from the viewpoint of further imparting electron conductivity. The lower limit of the particle size of the particles is not particularly limited and is preferably as small as possible. In view of the method for producing the particles, the lower limit is about 0.01 μm.

  Next, the preferable manufacturing method of the negative electrode 10 of this embodiment is demonstrated, referring FIG. First, a carrier foil 11 is prepared as shown in FIG. There are no particular restrictions on the material of the carrier foil 11. For example, copper foil can be used. Since the carrier foil 11 is used as a support for manufacturing the negative electrode 10, it is preferable that the carrier foil 11 has such a strength that no twist or the like occurs in the manufacturing process. Therefore, the carrier foil 11 preferably has a thickness of about 10 to 50 μm. As described above, an important role of the carrier foil 11 is a support for manufacturing the negative electrode 10. Therefore, when the strength of the surface layer 4 is sufficient, it is not always necessary to manufacture the negative electrode 10 using the carrier foil.

  The carrier foil 11 is preferably manufactured by electrolysis. Specifically, using a rotating drum as a cathode, electrolysis is performed in an electrolytic bath containing metal ions such as copper to deposit metal on the drum peripheral surface. The carrier foil 11 is obtained by peeling the deposited metal from the drum peripheral surface.

  Next, as shown in FIG. 3B, a thin release layer 12 is formed on one surface of the carrier foil 11. When the carrier foil 11 is manufactured by electrolysis, the carrier foil 11 has a smooth glossy surface on one side and a matte surface with unevenness on the other side. The glossy surface is the surface facing the drum peripheral surface, and the matte surface is the precipitation surface. In this production method, the release layer 12 may be formed on either the glossy surface or the matte surface.

  The release layer 12 is preferably formed by, for example, chromium plating, nickel plating, lead plating, chromate treatment, or the like. The reason for this is that an oxide or acid salt layer is formed on the surface of the release layer 12 by these treatments, and this layer lowers the adhesion between the carrier foil 11 and the later-described electrolytic plating layer and improves the peelability. It is because it has a function to make it. An organic compound can also be used as the release agent. In particular, it is preferable to use a nitrogen-containing compound or a sulfur-containing compound. Examples of nitrogen-containing compounds include benzotriazole (BTA), carboxybenzotriazole (CBTA), tolyltriazole (TTA), N ′, N′-bis (benzotriazolylmethyl) urea (BTD-U), and 3-amino. Triazole compounds such as -1H-1,2,4-triazole (ATA) are preferably used. Examples of the sulfur-containing compound include mercaptobenzothiazole (MBT), thiocyanuric acid (TCA), 2-benzimidazolethiol (BIT), and the like. These organic compounds are used by being dissolved in alcohol, water, acidic solvent, alkaline solvent or the like. For example, when CBTA is used, the concentration is preferably 2 to 5 g / 1. For the formation of the release layer 12 made of an organic compound, a dipping method can be employed in addition to the coating method. The thickness of the release layer 12 is preferably 0.05 to 3 μm because it can be successfully peeled off. The surface roughness Ra of the release layer 12 after the release layer 12 is formed is 0.01 to 3 μm, particularly 0.01 to 1 μm, as in the case where the active material layer 3 is directly formed on the carrier foil 11. In particular, the thickness is preferably 0.01 to 0.2 μm.

  After the release layer 12 is formed, as shown in FIG. 3C, a surface layer 4b containing an element having a low lithium compound forming ability and an element having a high lithium compound forming ability is formed on the release layer 12. There is no restriction | limiting in particular in the formation method of the surface layer 4b, A well-known various thin film formation method is employable. For example, methods such as electrolytic plating, sputtering, chemical vapor deposition, and physical vapor deposition can be used. A particularly preferable forming method is electrolytic plating.

  Subsequently, as shown in FIG. 3 (d), an active material layer 3 is formed on the surface layer 4 b by applying a conductive slurry containing active material particles. In this case, when the surface layer 4b is formed by electrolytic plating, the surface of the surface layer 4b to which the conductive slurry is applied is a precipitation surface, that is, a mat surface, and the surface roughness is high. By applying the conductive slurry to the surface of the surface layer 4b in such a surface state, there is an advantage that the adhesion between the active material particles and the surface layer 4b is improved.

  The slurry contains active material particles, conductive carbon material particles, a binder, a diluting solvent, and the like. Among these components, the particles of the active material and the particles of the conductive carbon material are as described above. 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 in the slurry is preferably about 14 to 40% by weight. The amount of the conductive carbon material particles is preferably about 0.4 to 4% by weight. The amount of the binder is preferably about 0.4 to 4% by weight. Moreover, it is preferable that the quantity of a dilution solvent shall be about 60 to 85 weight%.

The active material layer 3 formed by drying the slurry has a large number of minute spaces between the particles. After the active material layer 3 is formed, a surface layer 4a containing an element having a low lithium compound forming ability and an element having a high lithium compound forming ability is formed on the active material layer 3 as shown in FIG. Form. The method for forming the surface layer 4a can be the same as the method for forming the surface layer 4b described above. In particular, in order for the material constituting the surface layer 4a to permeate throughout the thickness direction of the active material layer 3 so that the surface layers 4a and 4b are electrically connected, the surface layer 4a is formed by electrolytic plating. It is advantageous to form. That is, it is advantageous to form at least one surface layer by electrolytic plating (hereinafter, this electrolytic plating is also referred to as permeation plating). By immersing the carrier foil 11 on which the active material layer 3 is formed in the plating bath, the plating solution enters the minute space in the active material layer 3 and enters the interface between the active material layer 3 and the surface layer 4b. In this state, electrolytic plating is performed. The conditions of the osmotic plating are important in order to deposit a metal material having a low lithium compound forming ability in the active material layer 3. For example, when the surface layer 4a is formed by electrolytic plating, the surface layer 4a is made of, for example, copper (an element having a low lithium compound forming ability) and tin (an element having a high lithium compound forming ability). The electrolysis conditions are as follows. As the plating bath, one having a composition described in ordinary literature, generally a cyan bath, a pyrophosphate bath, a borofluoride bath, or the like can be used. Among these, an example of the bath composition and electrolysis conditions of the cyanide bath is shown below.
CuCN: 35 g / l
・ Na ₂ SnO ₃ .2H ₂ O: 60 g / l
・ NaCN: 25 g / l
・ Current density: 5 A / dm ²
・ Bath temperature: 65 ° C
When this plating bath is used, copper (an element having a low ability to form a lithium compound) and tin (an element having a high ability to form a lithium compound) are deposited between particles of the active material in the active material layer 3 by osmotic plating. Furthermore, a surface layer 4 a having copper and tin as constituent materials is formed on the active material layer 3.

  Further, when the surface layer is made of, for example, nickel (an element having a low lithium compound forming ability) and silicon (an element having a high lithium compound forming ability), silicon oxide particles are suspended from an electrolytic plating solution containing nickel. A desired surface layer can be obtained by turbidity, performing electrolytic plating while stirring the plating solution, and incorporating silicon into the surface layer. In this case, only nickel (an element having a low ability to form a lithium compound) is deposited between the active material particles in the active material layer 3 by osmotic plating.

  Finally, the carrier foil 11 is peeled and separated from the surface layer 4b as shown in FIG. Thereby, the negative electrode 10 is obtained. Note that the release layer 12 may remain on the carrier foil 11 side or may remain on the surface layer 4b side depending on the thickness and type of the release treatment agent. Or it may remain in both of them. In any case, since the thickness of the release layer 12 is extremely thin, there is no influence on the performance of the obtained negative electrode 10.

  In the above-described method, both surface layers 4a and 4b are configured to include an element having a low lithium compound forming ability and an element having a high lithium compound forming ability. Instead, one surface layer is composed of an element having a low lithium compound forming ability and an element having a high lithium compound forming ability, and the other surface layer is made of an element having a low lithium compound forming ability. You can also In this case, electrolysis is performed using two types of plating baths: a plating bath containing an element having a low lithium compound forming ability and an element having a high lithium compound forming ability and a plating bath containing an element having a low lithium compound forming ability. Plating may be performed.

  Moreover, in the above-mentioned method, two operations, ie, an osmotic plating on the active material layer 3 and an operation of forming a surface layer on the surface of the active material layer 3 were performed simultaneously. In this case, the metal material having a low ability to form a lithium compound deposited in the active material layer 3 is the same as the metal material having a low ability to form a lithium compound contained in the surface layer. Instead of these operations, the two operations may be performed separately. That is, after performing the osmotic plating operation on the active material layer 3, the carrier foil 11 on which the active material layer 3 is formed is immersed in another plating bath, and the surface layer is formed on the active material layer 3 by electrolytic plating. It may be formed. By carrying out this operation, the metal material having a low ability to form a lithium compound contained in each surface layer and the metal material having a low ability to form a lithium compound deposited in the active material layer 3 can be made different. it can.

  According to this manufacturing method, the negative electrode which can use both surfaces of a negative electrode for an electrode reaction is obtained only by forming the active material layer once. In the conventional negative electrode, in order to use both surfaces of the electrode for the electrode reaction, it is necessary to form active material layers on both surfaces of the current collector. In other words, it was necessary to perform the operation of forming the active material layer twice. Therefore, according to this production method, the production efficiency of the negative electrode is greatly improved. Further, according to the present manufacturing method, the carrier foil 11 is not peeled until the negative electrode 10 is incorporated into the battery, and the carrier foil 11 is peeled off just before the incorporation, so that the negative electrode 10 that is thin and easily wrinkled is difficult to roll. There is also an advantage that it can be transported with good handling properties.

  As another method of this manufacturing method, the active material layer 3 is formed directly on the carrier foil 11 on which the release layer 12 is formed without forming the surface layer 4b, and then the entire active material layer 3 is electroplated. Accordingly, the surface layer 4 b can be formed between the active material layer 3 and the carrier foil 11, and the surface layer 4 a can be formed on the active material layer 3. The electrolytic plating conditions in this case can be the same as those described above. According to this alternative method, each surface layer can be formed by a single operation of electrolytic plating, so that the production of the negative electrode 10 can be made efficient.

The negative electrode 10 thus obtained is used with a known positive electrode, separator, and non-aqueous electrolyte solution to form a non-aqueous electrolyte secondary battery. The positive electrode is prepared by suspending a positive electrode active material and, if necessary, a conductive agent and a binder in an appropriate solvent to prepare a positive electrode mixture, applying this to a current collector, drying it, then rolling and pressing, and further cutting. It is obtained by punching. As the positive electrode active material, conventionally known positive electrode active materials such as lithium nickel composite oxide, lithium manganese composite oxide, and lithium cobalt composite oxide are used. As the separator, a synthetic resin nonwoven fabric, polyethylene, polypropylene porous film, or the like is preferably used. In the case of a lithium secondary battery, the nonaqueous electrolytic solution is a solution in which a lithium salt that is a supporting electrolyte is dissolved in an organic solvent. Examples of the lithium _{salt, LiBF 4, LiClO 4, LiAlCl} 4, LiPF 6, LiAsF 6, LiSbF 6, LiSCN, LiCl, LiBr, LiI, etc. _{LiCF 3 SO 3, LiC 4 F} 9 SO 3 are exemplified.

  As another embodiment of the negative electrode of the present invention, as shown in FIG. 4, there is a negative electrode 10 ′ in which fine voids 7 are already formed in the surface layers 4 a and 4 b when the surface layers 4 a and 4 b are formed. In the negative electrode 10 ', the fine voids 7 are already formed in the surface layers 4a and 4b before charging and discharging, and the fine voids 7 are further grown by charging and discharging. A method of manufacturing the negative electrode 10 'shown in FIG. 4 will be described with reference to FIG.

  First, a carrier foil 11 is prepared as shown in FIG. As the carrier foil 11, the same one as shown in FIG. 3A can be used. Next, as shown in FIG. 5B, a thin layer covering 13 made of a material different from the material constituting the surface layer 4 b is formed on one surface of the carrier foil 11. Thereafter, the surface layer 4b is formed by electrolytic plating. By this operation, the fine voids 7 can be formed in the surface layer 4b, and the number of fine voids 7 and the opening area can be easily controlled.

  The covering 13 is used to form a large number of fine voids 7 in the surface layer 4b by making the electron conductivity of the surface on which the surface layer 4b is formed nonuniform. The covering 13 is preferably formed to have a thickness of 0.001 to 1 μm, particularly 0.002 to 0.5 μm, especially 0.005 to 0.2 μm. This is because the covering 13 covers the surface of the carrier foil 11 in a discontinuous manner, for example, in an island shape.

  The covering 13 is made of a material different from the constituent material of the surface layer 4b. Thus, the surface layer 4b can be successfully peeled from the carrier foil 11 in the peeling step. In particular, the covering 13 is a material different from the constituent material of the surface layer 4b, and Cu, Ni, Co, Mn, Fe, Cr, Sn, Zn, In, Ag, Au, C, Al, Si, Ti And at least one element selected from Pd.

  There is no restriction | limiting in particular in the formation method of the covering body 13. FIG. For example, the formation method of the covering 13 can be selected in relation to the formation method of the surface layer 4b. Specifically, when the surface layer 4b is formed by electrolytic plating, it is preferable from the viewpoint of manufacturing efficiency and the like that the covering 13 is also formed by electrolytic plating. However, the covering 13 can also be formed by other methods such as electroless plating, sputtering, physical vapor deposition, chemical vapor deposition, sol-gel, or ion plating.

When forming the covering 13 by electrolytic plating, an appropriate plating bath and plating conditions are selected according to the constituent material of the covering 13. For example, when the covering 13 is made of tin, a plating bath having the following composition or a tin fluoride bath can be used. When this plating bath is used, the bath temperature is preferably about 15 to 30 ° C., and the current density is preferably about 0.5 to 10 A / dm ² .
・ SnSO ₄ 30-70 g / l
・ H ₂ SO ₄ 60-150 g / l
・ Cresol sulfonic acid 70-100g / l

  As described above, the covering 13 is used to make the electron conductivity of the surface on which the surface layer 4b is formed nonuniform. Therefore, if the electron conductivity of the constituent material of the covering 13 is significantly different from the electron conductivity of the carrier foil 11, the formation of the covering 13 immediately causes the surface of the surface layer 4b to have a non-uniform electronic conductivity. Become. For example, carbon is used as a constituent material of the covering body 13. On the other hand, when a material having the same degree of electronic conductivity as that of the carrier foil 11, for example, various metal materials such as tin, is used as the constituent material of the covering body 13, The electronic conductivity of the surface on which the layer 4b is formed does not immediately become uneven. Therefore, when the covering 13 is made of such a material, the carrier foil 11 on which the covering 13 is formed is preferably exposed to an oxygen-containing atmosphere, for example, air, in a dry state. As a result, the surface of the covering 13 (and the exposed surface of the carrier foil 11) is oxidized (see FIG. 5C). By this operation, the electronic conductivity of the surface on which the surface layer 4b is formed becomes nonuniform. When electrolytic plating described later is performed in this state, a difference in electrodeposition rate occurs between the surface of the covering 13 and the exposed surface of the carrier foil 11, and the fine voids 7 can be easily formed. The degree of oxidation is not critical in the present invention. For example, the inventors have found that it is sufficient to leave the carrier foil 11 with the covering 13 formed in the atmosphere for about 10 to 30 minutes. However, forcibly oxidizing the carrier foil 11 on which the covering 13 is formed is not hindered.

  The reason why the carrier foil 11 on which the covering 13 is formed is dried when exposed to an oxygen-containing atmosphere is to efficiently oxidize. For example, when the covering 13 is formed by electrolytic plating, the carrier foil 11 is lifted from the plating bath, dried using a dryer or the like, and then left in the atmosphere for a predetermined time. When a dry method such as a sputtering method or various vapor deposition methods is used as a method for forming the covering body 13, a drying operation is not necessary, and the covering body 13 may be left as it is in the air after the formation.

  After oxidizing the covering 13, a release layer 12 is formed thereon as shown in FIG. 5 (d). As the release agent, the same one as shown in FIG. 3B can be used. The step of forming the release layer 12 is merely performed in order to successfully release the negative electrode 10 ′ from the carrier foil 11 in the later-described release step (FIG. 5 (i)). Therefore, even if this step is omitted, fine voids can be formed in the surface layer 4b. Next, as shown in FIG.5 (e), after providing the release agent 12, the constituent material of the surface layer 4b is electrodeposited by electrolytic plating, and the surface layer 4b is formed. A large number of fine voids 7 are formed in the formed surface layer 4b. In FIG. 5 (e), the fine void 7 is drawn at the position of the apex of the covering 13, but this is for convenience. The minute gap 7 is not necessarily formed at the position of the apex.

  Next, as shown in FIG. 5 (f), an active material layer 3 is formed on the surface layer 4 b by applying a conductive slurry containing active material particles. Regarding the composition of the conductive slurry and the formation of the active material layer 3, the description regarding FIG.

  When the active material layer 3 is formed, as shown in FIG. 5G, a coating liquid containing a conductive polymer is applied thereon and dried to form a coating film 14. The coating liquid is obtained by dissolving a conductive polymer in a volatile organic solvent.

  There is no restriction | limiting in particular as a conductive polymer, A conventionally well-known thing can be used. Examples thereof include poly (vinylidene fluoride) (PVDF), polyethylene oxide (PEO), polyacrylonitrile (PAN), and polymethyl methacrylate (PMMA). In particular, it is preferable to use a lithium ion conductive polymer. The conductive polymer is preferably a fluorine-containing conductive polymer. This is because the fluorine-containing polymer has high thermal and chemical stability and excellent mechanical strength. Considering these, it is particularly preferable to use poly (vinylidene fluoride), which is a fluorine-containing polymer having lithium ion conductivity.

  As the organic solvent for dissolving the conductive polymer, for example, when polyvinylidene fluoride is used as the conductive polymer, N-methylpyrrolidone or the like can be used.

  As described above, the coating liquid is applied onto the active material layer 3. Since the active material layer 3 is a layer containing particles of the active material, the surface thereof is uneven. Accordingly, the coating liquid tends to accumulate in the recesses on the surface of the active material layer 3. When the solvent volatilizes in this state, the thickness of the coating film 14 becomes non-uniform. That is, the thickness of the coating film corresponding to the concave portion on the surface of the active material layer 3 is large, and the thickness of the coating film corresponding to the convex portion is small. In the present manufacturing method, a large number of fine voids are formed in the surface layer 4a by utilizing the thickness non-uniformity of the coating film 14.

  In the present manufacturing method, the mechanism by which many fine voids are formed in the surface layer 4a is considered as follows. The carrier foil 11 on which the coating film 12 is formed is subjected to an electrolytic plating process, and the carrier foil 11 on which the coating film 14 is formed is subjected to an electrolytic plating process, as shown in FIG. A surface layer 4a is formed thereon. This state is shown in FIG. 6 which is an enlarged view of the main part of FIG. Although the conductive polymer which comprises the coating film 14 is not like a metal, it has electronic conductivity. Accordingly, the coating film 14 has different electron conductivity depending on its thickness. As a result, when a metal is deposited by electrolytic plating on the coating film 14 containing a conductive polymer, a difference occurs in the deposition rate according to the electron conductivity, and the surface layer 4a has a fine void due to the difference in the deposition rate. 7 is formed. That is, a portion where the electrodeposition rate is low, in other words, a portion where the coating film 14 is thick is likely to become the fine void 7.

  The pore size and density of the fine voids 7 can be controlled by the concentration of the conductive polymer contained in the coating liquid. For example, when the concentration of the conductive polymer is low, the pore diameter tends to be small and the existence density tends to be small. Conversely, when the concentration of the conductive polymer is high, the pore diameter tends to increase. From this viewpoint, the concentration of the conductive polymer in the coating solution is preferably 0.05 to 5% by weight, particularly 1 to 3% by weight. The conductive polymer can be applied on the active material layer 3 by a dipping method in addition to a coating method.

  In this way, the negative electrode 10 'is obtained. Finally, as shown in FIG. 5 (i), the negative electrode 10 ′ is peeled and separated from the carrier foil 11.

  The present invention is not limited to the embodiment. For example, in order to increase the reaction active point between the active material particles and the electrolytic solution, a hole that opens on at least one surface of the negative electrode using a laser, a punch, a needle, or the like and reaches at least a part of the active material layer, Or you may form the through-hole extended in the thickness direction of a negative electrode.

  In the above-described embodiment, the surface layers 4a and 4b have a single layer structure, but instead, at least one surface layer may have a multilayer structure of two or more layers. For example, at least one surface layer has a lower layer made of nickel (an element having a low lithium compound forming ability) and an element having a high lithium compound forming ability, and a copper (an element having a low lithium compound forming ability) and a lithium compound forming ability. By adopting an upper two-layer structure composed of high elements, significant deformation of the negative electrode due to the volume change of the active material can be more effectively prevented. When the surface layer has a multi-layer structure, at least one metal material having a low lithium compound forming ability contained in the surface layer is different from the metal material having a low lithium compound forming ability penetrating into the active material layer 3. It can be. Alternatively, all of the metal material having a low lithium compound forming ability contained in each surface layer may be a different material from the metal material having a low lithium compound forming ability penetrating into the active material layer 3.

  In addition, when the constituent material of the surface layer is different from the material penetrating into the active material layer 3, the material penetrating into the active material layer 3 is the boundary portion between the active material layer 3 and the surface layer. May exist. Alternatively, the material penetrating into the active material layer 3 may constitute a part of the surface layer beyond the boundary portion. Conversely, the constituent material of the surface layer may exist in the active material layer 3 beyond the boundary.

  Moreover, the metal material deposited in the active material layer 4 is obtained by performing the operation of precipitating a material having a low ability to form a lithium compound in the active material layer 4 using two or more different plating baths. Two or more different multilayer structures can be used.

  Further, in the manufacturing method shown in FIG. 5, the covering 13 is not formed when the surface layer 4b is formed, and instead, the conductive polymer described above is included on the carrier foil 11 (or the release layer 12). Many fine voids can be formed in the surface layer 4b by applying the coating liquid and forming the surface layer 4b thereon. In that case, the surface roughness Ra (JIS B 0601) of the carrier foil 11 is 0.05 to 5 μm, particularly 0.2 to 0.8 μm, so that a fine void having a desired diameter and density can be easily formed. It is preferable from the point which can do.

It is a schematic diagram which shows the cross-section of one Embodiment of the negative electrode of this invention. It is a schematic diagram which expands and shows the principal part of the negative electrode shown in FIG. It is process drawing which shows an example of the manufacturing method of the negative electrode shown in FIG. It is a schematic diagram which shows the cross-section of another embodiment of the negative electrode of this invention. It is process drawing which shows an example of the manufacturing method of the negative electrode shown in FIG. It is a schematic diagram which shows the state in which a surface layer and a fine space | gap are formed.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1a 1st surface 1b 2nd surface 2 Active material particle | grains 3 Active material layer 4a, 4b Current collecting surface layer 5 Element with high lithium compound formation ability 6 Element with low lithium compound formation ability 7 Fine void 8 Void 10, 10 'Negative electrode 11 Carrier foil 12 Release layer 13 Cover 14 Conductive polymer coating

Claims (13)

  A pair of current-collecting surface layers that are in contact with the electrolyte and have conductivity, and an active material layer that is disposed between the surface layers and contains active material particles having a high lithium compound-forming ability, and at least one of them The surface layer of the non-aqueous composition is configured to include an element having a low ability to form a lithium compound and an element having a high ability to form a lithium compound, and does not have a thick film conductor for current collection. Negative electrode for electrolyte secondary battery.   2. The non-aqueous solution according to claim 1, wherein the surface layer comprises 50 to 99.9 wt% of an element having a low lithium compound forming ability and 0.1 to 50 wt% of an element having a high lithium compound forming ability. Negative electrode for electrolyte secondary battery.   The non-aqueous electrolyte secondary solution according to claim 1 or 2, wherein the element having a low ability to form a lithium compound is nickel, copper, iron or cobalt, and the element having a high ability to form a lithium compound is tin, zinc, aluminum or indium. Battery negative electrode.   4. The non-flank according to claim 1, wherein at least one of the surface layers is formed with a large number of fine voids extending in the thickness direction so as to reach the active material layer and allow the electrolyte solution to penetrate. 5. Negative electrode for water electrolyte secondary battery.   The fine voids are formed at the time of forming the surface layer, or an element having a high lithium compound forming ability contained in the surface layer is pulverized due to charge / discharge, whereby the surface layer is formed. The negative electrode for a nonaqueous electrolyte secondary battery according to claim 4, wherein the negative electrode is formed by causing cracks in the battery.   The negative electrode for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 5, wherein the active material having a high ability to form a lithium compound is a silicon-based material.   In the active material layer, a metal material having a low lithium compound forming ability permeates between particles of the active material so that both surface layers are electrically connected, and the whole has a current collecting function as a whole. Item 7. The negative electrode for a nonaqueous electrolyte secondary battery according to any one of Items 1 to 6.   The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein each of the surface layers has a thickness of 0.3 to 20 μm.   The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the active material layer is formed by applying a conductive slurry containing particles of the active material.   The negative electrode for a nonaqueous electrolyte secondary battery according to any one of claims 1 to 9, wherein at least one of the surface layers is formed by electrolytic plating.   11. The metal material having a low ability to form a lithium compound contained in the surface layer and the metal material having a low ability to form a lithium compound penetrating into the active material layer are different from each other. The electrode for nonaqueous electrolyte secondary batteries in any one of.   11. The metal material having a low ability to form a lithium compound contained in the surface layer and the metal material having a low ability to form a lithium compound penetrating into the active material layer are the same type. The electrode for nonaqueous electrolyte secondary batteries in any one of. At least one of the surface layers has a multilayer structure of two or more layers, and at least one of the constituent materials of each layer in the surface layer of the multilayer structure has a low ability to form a lithium compound penetrating into the active material layer The electrode for a nonaqueous electrolyte secondary battery according to any one of claims 1 to 10, wherein the electrode is a material different from the material.