JP3906342B2 - Negative electrode for non-aqueous electrolyte secondary battery and method for producing the same - Google Patents

Description

  The present invention relates to a negative electrode used for a non-aqueous electrolyte secondary battery such as a lithium ion secondary battery. The present invention also relates to a method for producing the negative electrode.

  It has been proposed that metallic lithium is affixed to the negative electrode of a non-aqueous electrolyte secondary battery to enhance the battery's overdischarge resistance. For example, in a non-aqueous electrolyte secondary battery in which a lithium-containing composite oxide of a transition metal is used for the positive electrode and a carbon material is used for the negative electrode, and the positive electrode plate and the negative electrode plate are wound together with the separator, the outermost periphery of the negative electrode plate It has been proposed to attach a metal lithium foil to a portion corresponding to the above and not facing the positive electrode plate (see Patent Documents 1 to 3).

  However, in the negative electrode, since the active material is exposed on the outermost surface that is in contact with the nonaqueous electrolytic solution, the active material is dropped due to expansion and contraction of the active material due to absorption and desorption of lithium ions. Cheap. As a result, the cycle life of the battery tends to decrease. In addition, since lithium metal is also exposed on the outermost surface, in some cases, lithium dendrite is generated, which falls off the negative electrode or penetrates the separator and contacts the positive electrode, causing internal short circuit and ignition. There is a risk.

  By the way, in a non-aqueous electrolyte secondary battery, a trace amount of water is often mixed in the manufacturing process. In the battery, moisture reacts with the non-aqueous electrolyte and decomposes it. Accordingly, it has been proposed to improve the charge / discharge cycle characteristics by reducing the moisture contained in the nonaqueous electrolyte secondary battery (see Patent Document 4). However, reducing the moisture to a satisfactory level is not economical because it takes excessive labor and time.

  Apart from moisture, current collectors and active materials inevitably contain trace amounts of oxygen. Oxygen forms a compound with lithium during charging and discharging. Since Li—O has a relatively strong binding force, the amount of lithium that can be used reversibly decreases due to the formation of the compound. That is, the irreversible capacity increases.

JP-A-5-144472 JP-A-5-144473 JP-A-7-94211 Japanese Patent Laid-Open No. 2001-2223030

  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 has an active material layer between a pair of current collecting surface layers made of a metal material having a low ability to form a lithium compound,
At least one of the current collecting surface layers is formed with a large number of fine voids which are open in the surface thereof and extend in the thickness direction and allow the nonaqueous electrolyte to penetrate,
The active material layer includes particles of an active material having a high lithium compound forming ability that occludes lithium before the start of charging , and the active material layer includes a metal material having a low lithium compound forming ability. Permeates throughout the thickness direction of
The object is achieved by providing a negative electrode for a non-aqueous electrolyte secondary battery that is not provided with a conductive metal foil layer as a core material.

Further, the present invention provides a preferred method for producing the negative electrode as
A thin layer covering made of a material different from the material constituting the current collecting surface layer is formed on one surface of the carrier foil , and the electron conductivity of the surface of the current collecting layer on the carrier foil is not uniform. After forming a state, a surface layer for current collection is formed by electrolytic plating,
A conductive slurry containing active material particles is applied to the current collecting surface layer to form an active material layer, and the current collecting surface layer and the active material layer are arranged in this order on the carrier foil. Forming the negative electrode precursor prepared in
The metal lithium foil is sandwiched between both negative electrode precursors so that the active material layers in each negative electrode precursor face each other, and the metal lithium foil and both negative electrode precursors are integrated by bonding,
Lithium is heated to a temperature sufficient for thermal diffusion to diffuse lithium from the metallic lithium foil into the active material layer, and then
The present invention provides a method for producing a negative electrode for a non-aqueous electrolyte secondary battery, wherein the carrier foil is separated from each negative electrode precursor.

Further, the present invention has an active material layer between a pair of current collecting surface layers made of a metal material having a low ability to form a lithium compound,
At least one of the current collecting surface layers is formed with a large number of fine voids which are open in the surface thereof and extend in the thickness direction and allow the nonaqueous electrolyte to penetrate,
The active material layer includes particles of an active material having a high lithium compound forming ability that occludes lithium before the start of charging , and the active material layer includes a metal material having a low lithium compound forming ability. Permeates throughout the thickness direction of
The present invention provides a negative electrode for a non-aqueous electrolyte secondary battery comprising a conductive metal foil layer as a core material.

  According to the negative electrode of the present invention, even when lithium is consumed during charging and discharging, lithium is dissolved and supplied from the metal lithium layer. Therefore, the problem of so-called “lithium depletion”, which is a concern during battery design in which the amount (capacity) of the positive electrode active material is reduced compared to the amount (capacity) of the negative electrode active material, is solved. As a result, the initial irreversible capacity can be reduced, and the charge / discharge efficiency (cycle characteristics) in each charge / discharge cycle is improved. Moreover, since the active material occludes lithium before the start of charging / discharging, volume increase resulting from occlusion of lithium during charging can be reduced. This greatly contributes to the improvement of cycle life.

  In addition, even when a minute amount of moisture or oxygen is contained in the battery components such as the negative electrode, the moisture and oxygen react with metal lithium and are consumed. Decrease. This also makes it possible to reduce the initial irreversible capacity and improve the charge / discharge efficiency (cycle characteristics) in each charge / discharge cycle.

  Furthermore, a space is generated in the metallic lithium layer after the lithium is dissolved, and the space relieves stress caused by expansion / contraction of the active material during charging / discharging, thereby suppressing pulverization of the active material. . In addition, even if the active material is finely powdered, the active material is not exposed on the surface of the electrode and is embedded in the electrode, so that the active material is prevented from falling off and is repeatedly charged and discharged. Even so, the current collector of the active material is secured. Moreover, since the metal lithium layer is not exposed on the surface of the negative electrode and is located inside, the lithium dendrite is prevented from being generated.

The present invention will be described below based on preferred embodiments with reference to the drawings. FIG. 1 schematically shows the structure of an embodiment of the negative electrode of the present invention. Note that the negative electrode of the present embodiment is outside the scope of the present invention because lithium is not occluded in the active material before the start of charging. This embodiment is an embodiment for the purpose of showing a basic structure which is a premise of the present invention. The negative electrode of the present embodiment falls within the scope of the present invention by occluding lithium in the active material before the start of charging. The negative electrode 1 of this embodiment has two negative electrode precursors 2 and a metal lithium layer 3 as its basic constituent members. The metal lithium layer 3 is sandwiched between the negative electrode precursors 2.

  The negative electrode precursor 2 includes a current collecting surface layer 4 and an active material layer 5 disposed on one surface of the current collecting surface layer 4. As shown in FIG. 1, the metal lithium layer 3 is sandwiched between the negative electrode precursors 2 so that the active material layers 5 of the negative electrode precursors 2 face each other and the current collecting surface layer 4 faces outward. ing. As is clear from FIG. 1, the negative electrode 1 has a current collector thick film conductor (for example, a metal foil or expanded metal having a thickness of about 8 to 35 μm) that has been used for a conventional negative electrode. Not done.

  The current collecting surface layer 4 has a current collecting function. The current collecting surface layer 4 is also used to prevent the active material contained in the active material layer 5 from falling off due to expansion and contraction of the active material due to absorption and desorption of lithium ions. Yes. The current collecting surface layer 4 is preferably made of a metal that can be a current collector of a non-aqueous electrolyte secondary battery. In particular, it is preferably made of a metal that can be a current collector of a lithium secondary battery. An example of such a metal is a metal material having a low lithium compound forming ability. Specific examples include copper, nickel, iron, cobalt, and alloys of these metals. Of these metals, it is particularly preferable to use copper and nickel or an alloy thereof. From the viewpoint of increasing the strength of the negative electrode 1, nickel is preferably used. The two current collecting surface layers 4 may be made of the same or different materials. “Lithium compound forming ability is low” 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.

  Each surface layer 4 for current collection is thinner than the thick film conductor for current collection used in conventional electrodes. Specifically, a thin layer of about 0.3 to 10 μm, particularly about 1 to 5 μm is preferable. As a result, the active material layer 5 can be continuously and uniformly coated with the minimum necessary thickness. As a result, it is possible to prevent the active material pulverized due to charge / discharge from falling off. The thin current collecting surface layer 4 in the above range is preferably formed by electrolytic plating as will be described later. The two current collecting surface layers 4 may have the same thickness or different thicknesses.

  Each current collecting surface layer 4 has a large number of fine voids 6 that are open in the surface and communicate with the active material layer 5. The fine void 6 exists in the current collecting surface layer 4 so as to extend in the thickness direction of the current collecting surface layer 4. By forming the fine voids 6, the non-aqueous electrolyte can sufficiently penetrate into the active material layer 5, and the reaction with the active material occurs sufficiently. The fine gap 6 is a fine one having a width of about 0.1 μm to about 10 μm when the current collecting surface layer 4 is observed in cross section. Although fine, the fine gap 6 has a width that allows the non-aqueous electrolyte to penetrate. However, since the nonaqueous electrolytic solution has a smaller surface tension than the aqueous electrolytic solution, it can sufficiently penetrate even if the width of the fine gap 6 is small. The fine voids 6 are preferably formed at the same time when the current collecting surface layer 4 is formed by electrolytic plating. In the present embodiment, the fine voids 6 are formed in each current collecting surface layer 4. However, if the fine voids 6 are formed in at least one current collecting surface layer 4, a desired effect can be obtained. Is done.

When viewed in plan by electron microscopic observation of the surface of the current collecting surface layer 4, the average open area of the micropores 6 is 0.1 to 50 [mu] m ^2, preferably 0.1 to 20 [mu] m ^2, more preferably 0 About ⁵ to 10 μm ² . By setting the opening area within this range, it is possible to effectively prevent the active material from falling off while ensuring sufficient permeation of the non-aqueous electrolyte. Further, the charge / discharge capacity can be increased from the initial stage of charge / discharge. When the active material is particles, the average pore area is 0.1 to 50%, particularly 0.1 to 0.1% of the maximum cross-sectional area of the particles of the active material from the viewpoint of more effectively preventing the particles from falling off. It is preferably ˜20%. The maximum cross-sectional area of the particles of the active material means the maximum cross-sectional area when the particle diameter (D ₅₀ value) of the particles of the active material is measured and the particles are regarded as a sphere having a diameter of D ₅₀ value.

  When the surface of the current collecting surface layer 4 is viewed in plan by electron microscope observation, the ratio of the total opening area of the fine voids 6 to the area of the observation field (this ratio is referred to as the opening ratio) is 0.1 to 0.1. 20%, preferably 0.5 to 10%. This reason is the same reason as setting the aperture area of the fine gap 6 within the above range. Furthermore, for the same reason, when the surface of the current collecting surface layer 4 is viewed in plan by electron microscope observation, no matter what observation field is taken, 1 to 20,000 in a 100 μm × 100 μm square field range, In particular, it is preferable that 10 to 1,000, especially 50 to 500 fine voids 6 exist.

  The active material layer 5 located immediately inside the current collecting surface layer 4 contains an active material having a high ability to form a lithium compound. Examples of the active material include silicon-based materials, tin-based materials, aluminum-based materials, and germanium-based materials. Since each active material layer 5 is covered with each current collecting surface layer 4, the active material is effectively prevented from falling off due to absorption and desorption of lithium ions. Since the active material can be in contact with the electrolytic solution through the fine gap 6, the electrode reaction is not hindered.

  For example, as shown in FIG. 1, the active material layer 5 is formed by applying a conductive slurry containing active material particles 7. Instead of applying the slurry, a gas deposition method may be used. In the gas deposition method, active material particle powder (Si, etc.) is mixed with a carrier gas (nitrogen, argon, etc.) in a reduced pressure space, and aerosolized in a state of being aerosolized, so that the substrate (current collector foil) This is a technique of forming a film on the surface by pressure bonding. Since a coating film can be formed at room temperature, the composition change is small even when a multi-component active material powder is used as compared with the following thin film forming means. Further, an active material film layer having a large number of voids can be formed by adjusting the injection conditions (active material particle diameter, gas pressure, etc.) of the same method. The active material layer 5 may be formed of a thin layer of active material formed by various thin film forming means. Examples of the thin film forming means include chemical vapor deposition (CVD), physical vapor deposition (PVD), sputtering, and electrolytic plating.

  When the active material layer 5 includes active material particles 7, the particles 7 include, for example, a) silicon simple substance or tin simple particles, b) at least silicon or a mixture of tin and carbon, c A) mixed particles of silicon or tin and metal, d) compound particles of silicon or tin and metal, e) mixed particles of compound particles of silicon or tin and metal, and metal particles, f) silicon alone or tin Examples include particles in which the surface of a single particle is coated with a metal, and g) particles containing these oxides. When using particles b), c), d), e) and f), the silicon-based material fine powder resulting from the absorption and desorption of lithium compared to the case of b) using the silicon or tin particles There is an advantage that the conversion is further suppressed. Further, there is an advantage that electron conductivity can be imparted to silicon which is a semiconductor and has poor electron conductivity.

The active material particles 7 preferably have a maximum particle size of 50 μm or less, and more preferably 20 μm or less. Moreover, when the particle size of the particle 7 is expressed by a D ₅₀ value, it is preferably 0.1 to 8 μm, particularly preferably 1 to 5 μm. When the maximum particle size is more than 50 μm, the particles 7 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 7, the lower limit is about 0.01 μm. The particle size of the particles 7 is measured by a laser diffraction / scattering particle size distribution measuring device or an electron microscope.

  If the amount of the active material relative to the whole negative electrode 1 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 entire negative electrode 1. The thickness of the active material layer 5 can be appropriately adjusted according to the ratio of the amount of the active material to the whole negative electrode, and is not particularly critical in the present embodiment. The thickness in the case where the active material layer 5 includes the active material particles 7 is generally 1 to 100 μm, particularly about 3 to 40 μm.

  When the active material layer 5 includes the active material particles 7, it is preferable that a metal material having a low lithium compound forming ability permeates throughout the thickness direction of the active material layer 5. The active material particles 7 are preferably present in the permeated material. That is, it is preferable that the active material particles 7 are not substantially exposed on the surface of the negative electrode 1 and are embedded in the current collecting surface layer 4. As a result, the adhesion between the active material layer 5 and the current collecting surface layer 4 becomes strong, and the active material is further prevented from falling off. In addition, since electronic conductivity is ensured between the current collecting surface layer 4 and the active material through the material that has penetrated into the active material layer 5, it is possible to generate an electrically isolated active material, particularly the active material layer. The generation of an electrically isolated active material in the deep part of 5 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 ability to form a lithium compound penetrating over the entire thickness direction of the active material layer 5 is preferably the same material as the metal material constituting the current collecting surface layer 4. However, if the lithium compound forming ability is low, a material different from the metal material constituting the current collecting surface layer 4 may be used.

  It is preferable that the metal material with low lithium compound forming ability penetrates the active material layer 5 in the thickness direction. As a result, the two current collecting surface layers 4 are electrically conducted through the metal material and the metal lithium layer 3, and the electron conductivity of the negative electrode 1 as a whole is further increased. That is, the negative electrode 1 of the present embodiment as a whole has a current collecting function. The permeation of the metal material having a low lithium compound forming ability throughout the thickness direction of the active material layer 5 can be determined by electron microscope mapping using the material as a measurement object. A preferred method for allowing a metal material having a low ability to form a lithium compound to penetrate into the active material layer 5 will be described later.

  The active material layer 5 preferably contains conductive carbon material or conductive metal material particles 8 in addition to the active material particles 7. Thereby, the negative electrode 1 is further provided with electronic conductivity. From this viewpoint, the amount of the conductive carbon material or conductive metal material particles 8 contained in the active material layer 5 is 0.1 to 20% by weight, particularly 1 to 10% by weight, based on the active material layer 5. It is preferable. 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.

  The metallic lithium layer 3 interposed between the two active material layers 5 constitutes a local battery with the active material (negative electrode active material) in the presence of the non-aqueous electrolyte. As a result, metal lithium is electrochemically intercalated from the metal lithium layer 3 into the adjacent active material. Alternatively, lithium intercalates into the active material due to a lithium concentration gradient. Thus, the metal lithium layer 3 functions as a lithium supply source. As a result, even if lithium is consumed by charging and discharging, lithium is supplied from the metal lithium layer 3, so that the problem of lithium depletion is solved. Thereby, the life of the negative electrode 1 is extended. Further, even if a minute amount of moisture is present in the constituent materials of the battery including the negative electrode 1 (for example, the positive electrode, the electrolyte solution, etc.), the moisture reacts with metallic lithium and is consumed. Decrease. Furthermore, a trace amount of oxygen inevitably contained in the current collector and the active material is captured by metallic lithium. As a result, the initial irreversible capacity can be reduced, and the charge / discharge efficiency (cycle characteristics) in each charge / discharge cycle is improved. In addition, the metal lithium layer 3 is not exposed on the surface of the negative electrode 1 and is located inside the negative electrode 1, and lithium intercalates into the active material, causing internal short circuit and ignition. There is little risk of the formation of a lithium dendrite. A space is generated in the metal lithium layer 3 after the lithium is dissolved, and the space relieves stress caused by expansion / contraction of the active material at the time of charging / discharging, thereby suppressing pulverization of the active material. There are also advantages.

The amount of metallic lithium is preferably 0.1% or more and 50% or less with respect to the initial charge theoretical capacity of the negative electrode active material because the capacity recovery characteristics are improved. A more preferred range is 5 to 50%, a more preferred range is 10 to 40%, and a particularly preferred range is 20 to 40%. When silicon is used as the negative electrode active material, for example, silicon occludes lithium up to the state represented by the composition formula SiLi _4.4. Therefore, the occlusion amount of lithium is 100% of the initial charge theoretical capacity of silicon. “Present” means that lithium is occluded by silicon to the state represented by the composition formula SiLi _4.4 .

  The total thickness of the negative electrode 1 having the above configuration is preferably about 2 to 50 μm, particularly about 10 to 50 μm in consideration of maintaining the strength of the negative electrode 1 and increasing the energy density.

  Next, the preferable manufacturing method of the negative electrode 1 shown in FIG. 1 is demonstrated, referring FIG. First, the negative electrode precursor 2 is manufactured. For the production of the negative electrode precursor 2, a carrier foil 11 is prepared as shown in FIG. There are no particular restrictions on the material of the carrier foil 11. The carrier foil 11 is preferably conductive. In this case, the carrier foil 11 may not be made of metal as long as it has conductivity. However, the use of the metal carrier foil 11 has an advantage that the carrier foil 11 can be melted, made into a foil and recycled after the production of the negative electrode precursor 2. Considering the ease of recycling, the material of the carrier foil 11 is preferably the same as the material of the current collecting surface layer 4 formed by electrolytic plating described later. Since the carrier foil 11 is used as a support for manufacturing the negative electrode precursor 2, 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.

  The carrier foil 11 can be manufactured by, for example, electrolysis or rolling. By manufacturing by rolling, carrier foil 11 with low surface roughness can be obtained. On the other hand, by producing the carrier foil 11 by electrolysis, the production from the carrier foil 11 to the production of the negative electrode precursor 2 can be performed in-line. Performing in-line is advantageous in terms of stable production of the negative electrode precursor 2 and reduction of production costs. When the carrier foil 11 is manufactured by electrolysis, the rotating drum is used as a cathode, and electrolysis is performed in an electrolytic bath containing metal ions such as copper and nickel 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. 2B, a thin layer covering 12 made of a material different from the material constituting the current collecting surface layer 4 is formed on one surface of the carrier foil 11. Thereafter, the current collecting surface layer 4 is formed by electrolytic plating. By this operation, the number of fine voids 6 formed in the current collecting surface layer 4 and the opening area can be easily controlled.

  The covering 12 is used for forming a large number of fine voids in the current collecting surface layer 4 by making the electron conductivity of the surface on which the current collecting surface layer 4 is formed nonuniform. The covering 12 is preferably formed so that the thickness thereof is 0.001 to 1 μm, particularly 0.002 to 0.5 μm, particularly 0.005 to 0.2 μm. This is because the coating body 12 coats the surface of the carrier foil discontinuously, for example, in an island shape, by making it thin to this extent.

  The covering 12 is made of a material different from the constituent material of the current collecting surface layer 4. Thus, the current collecting surface layer 4 can be successfully peeled from the carrier foil 11 in the peeling step. In particular, the covering 12 is a material different from the constituent material of the current collecting surface layer 4, and Cu, Ni, Co, Mn, Fe, Cr, Sn, Zn, In, Ag, Au, C, Al, It is preferable that at least one element of Si, Ti, and Pd is included.

  There is no restriction | limiting in particular in the formation method of the covering body 12. FIG. For example, the formation method of the covering 12 can be selected in relation to the formation method of the current collecting surface layer 4. Specifically, when the current collecting surface layer 4 is formed by electrolytic plating, it is preferable from the viewpoint of manufacturing efficiency and the like that the covering 12 is also formed by electrolytic plating. However, the covering 12 can be formed by other methods such as electroless plating, sputtering, physical vapor deposition, chemical vapor deposition, sol-gel, or ion plating.

When the covering 12 is formed by electrolytic plating, an appropriate plating bath and plating conditions are selected according to the constituent material of the covering 12. For example, when the covering 12 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 12 is used to make the electron conductivity of the surface on which the current collecting surface layer 4 is formed nonuniform. Therefore, if the electron conductivity of the constituent material of the covering 12 is significantly different from the electron conductivity of the carrier foil 11, the electron conductivity of the surface on which the current collecting surface layer 4 is formed is immediately non-uniform by forming the covering 12. It becomes a state. For example, carbon is used as the constituent material of the covering 12. 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 12, depending on the formation of the covering body 12, The electronic conductivity of the surface on which the electric surface layer 4 is formed does not immediately become uneven. Therefore, when the covering 12 is made of such a material, the carrier foil 11 on which the covering 12 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 12 (and the exposed surface of the carrier foil 11) is oxidized (see FIG. 2C). By this operation, the electronic conductivity of the surface on which the current collecting surface layer 4 is formed becomes non-uniform. When electrolytic plating described later is performed in this state, a difference in electrodeposition rate occurs between the surface of the covering 12 and the exposed surface of the carrier foil 11, and the fine voids 6 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 12 formed in the atmosphere for about 10 to 30 minutes. However, it is not hindered to forcibly oxidize the carrier foil 11 on which the covering 12 is formed.

  The reason why the carrier foil 11 on which the covering 12 is formed is dried when it is exposed to an oxygen-containing atmosphere is to efficiently oxidize. For example, when the covering 12 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 12, a drying operation is not necessary. After the covering body 12 is formed, the covering body 12 may be left in the air as it is.

  After oxidizing the covering 12, as shown in FIG. 2 (d), a release agent 13 is applied thereon. The stripping agent 13 is used for successfully stripping the negative electrode precursor 2 from the carrier foil 11 in the stripping step described later. As the release agent 13, it is preferable to use an organic compound, and it is particularly 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. The step of applying the release agent 13 is only performed in order to successfully release the negative electrode precursor 2 from the carrier foil 11 in the release step described later. Therefore, even if this step is omitted, the current collecting surface layer 4 having a large number of fine voids 6 can be formed.

Next, as shown in FIG. 2 (e), after the release agent 13 is applied, the constituent material of the current collecting surface layer 4 is electrodeposited by electrolytic plating to form the current collecting surface layer 4. In the formed current collecting surface layer 4, a large number of the fine voids 6 having the diameter described above are formed at the existence density described above. In FIG. 2 (e), the fine gap 6 is drawn at the position of the apex of the covering 12, but this is for convenience. The fine gap 6 is not necessarily formed at the apex position. The plating bath and plating conditions are appropriately selected according to the constituent material of the current collecting surface layer 4. For example, when the current collecting surface layer 4 is made of Ni, a Watt bath or a sulfamic acid bath having the following composition can be used as a plating bath. When these plating baths are used, the bath temperature is preferably about 40 to 70 ° C., and the current density is preferably about 0.5 to 20 A / dm ² .
・ 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

  Next, as shown in FIG. 2 (f), an active material layer 5 is formed on the current collecting surface layer 4 by applying a conductive slurry containing particles of the active material. The slurry contains active material particles, conductive carbon material or conductive metal material particles, a binder, a diluting solvent, and the like. Among these components, polyvinylidene fluoride (PVDF), polyethylene (PE), ethylene propylene diene monomer (EPDM) and the like are used as the binder. As a diluting solvent, N-methylpyrrolidone, cyclohexane or the like is used. The amount of the active material in the slurry is preferably about 14 to 40% by weight. The amount of the conductive carbon material or conductive metal 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. A dilute solvent is added to these components to prepare a slurry. Note that the active material layer may be formed by using a gas deposition method, as described above, instead of the method of applying the slurry. According to this method, since an active material layer can be formed without using a diluting solvent, there is an advantage that a subsequent drying step is not required. The drying step affects the oxidation of the active material particles and the binder and slurry dilution solvent. Therefore, the gas deposition method that does not require a drying step is a preferable method because the influence on the negative electrode can be minimized.

  After the coating film of the slurry is dried and the active material layer 5 is formed, the carrier foil 11 on which the active material layer 5 is formed is immersed in a plating bath containing a metal material having a low lithium compound forming ability. Perform electrolytic plating. By immersion in the plating bath, the plating solution enters the active material layer 5 and reaches the interface between the active material layer 5 and the current collecting surface layer 4, and electrolytic plating is performed in this state. As a result, (a) a metal having a low lithium compound forming ability on the inside of the active material layer 5 and (b) the inner surface side of the active material layer 5 (that is, the surface side facing the current collecting surface layer 4). The material precipitates and permeates throughout the thickness direction of the active material layer 5. In this way, the negative electrode precursor 2 is formed on the carrier foil 11.

As conditions for electrolytic plating, for example, when copper is used as a metal material having a low ability to form a lithium compound, when using a copper sulfate solution, the concentration of copper is 30 to 100 g / l, and the concentration of sulfuric acid is 50 to 200 g / l. The chlorine concentration may be 30 ppm or less, the liquid temperature may be 30 to 80 ° C., and the current density may be 1 to 100 A / dm ² . When using a copper pyrophosphate solution, the concentration of copper is 2 to 50 g / l, the concentration of potassium pyrophosphate is 100 to 700 g / l, the liquid temperature is 30 to 60 ° C., the pH is 8 to 12, and the current density is 1. ~10A / dm ² and may be set. By appropriately adjusting these electrolysis conditions, the metal material penetrates throughout the thickness direction of the active material layer 5.

  Thus, the negative electrode precursor 2 provided with the current collecting surface layer 4 and the active material layer 5 in this order on the carrier foil 11 is formed. Using this pair, as shown in FIG. 2G, the metal lithium foil 14 is sandwiched between the negative electrode precursors 2 so that the active material layers 5 in the negative electrode precursors 2 face each other. Thereby, the metallic lithium foil 14 and the two negative electrode precursors 2 are integrated by bonding. In this case, these three members can be bonded together by simply superimposing the metal lithium foil 14 and the negative electrode precursors 2 on each other and pressing them. In order to strengthen the bonding, these three members may be bonded using a conductive adhesive material such as a conductive paste.

  Finally, as shown in FIG. 2 (h), the negative electrode precursor 2 is peeled and separated from the carrier foil 11 at the interface between the current collecting surface layer 4 and the carrier foil 11. Thereby, the intended negative electrode 1 is obtained.

The negative electrode of the present embodiment 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. The lithium salt, for _{example, LiC1O 4, LiA1Cl 4, LiPF} 6, LiAsF 6, LiSbF 6, LiSCN, LiC1, LiBr, LiI, etc. _{LiCF 3 SO 3, LiC 4 F} 9 SO 3 are exemplified.

Next, second to fifth embodiments of the present invention will be described with reference to FIGS. With respect to these embodiments, the explanation in detail regarding the negative electrode 1 shown in FIG. Of these embodiments, the second embodiment shown in FIG. 3 is within the scope of the present invention, and the other embodiments have no lithium occluded in the active material before the start of charging. It is outside the scope of the invention. The negative electrode of the other embodiment falls within the scope of the present invention by occluding lithium in the active material before the start of charging.

  The negative electrode 1 shown in FIG. 3 is bonded after the bonding step shown in FIG. 2 (g) in the process diagrams shown in FIGS. 2 (a) to 2 (h) described with reference to the manufacturing method of the negative electrode 1 shown in FIG. It is obtained by thermally diffusing lithium from the metal lithium foil 14 into the active material particles by heating the body to a predetermined temperature. The heating temperature may be a temperature sufficient for thermal diffusion of lithium, and specifically 30 to 160 ° C., particularly preferably 60 to 150 ° C.

  In the negative electrode 1 thus obtained, the active material particles have sufficiently occluded lithium before the start of charging. Therefore, the volume increase resulting from occlusion of lithium during charging can be further reduced as compared with the negative electrode 1 shown in FIG. Further, due to the occlusion of lithium by the active material particles, a space larger than that of the negative electrode 1 shown in FIG. 1 is formed in the portion of the metal lithium foil 14 (see FIG. 2G). As a result, the generation of stress due to the expansion and contraction of the active material particles is further alleviated.

  In FIG. 3, the metal lithium foil 14 (see FIG. 2G) does not remain on the negative electrode 1 and is drawn as if it was occluded by the active material particles 7, but this is for convenience. The metal lithium foil may remain in the negative electrode 1, and even in that case, a negative electrode having desired performance can be obtained. Whether or not the metal lithium foil remains in the negative electrode 1 depends on the relative quantity relationship between the metal lithium foil to be used and the active material, the temperature and time of thermal diffusion. Even if a part of the metallic lithium foil remains in the negative electrode 1 before charging, lithium is consumed while charging and discharging are repeated, and the amount thereof gradually decreases.

  In the negative electrode 1 shown in FIG. 4, one active material layer 5 and one metal lithium layer 3 are disposed between a pair of current collecting surface layers 4. The negative electrode 1 of the present embodiment corresponds to the negative electrode 1 of the embodiment shown in FIG. 1 in which the metal lithium layer 3 is interposed between the two active material layers 5 and using only one active material layer 5. .

  A negative electrode 1 shown in FIG. 5 is obtained by forming a large number of holes 9 in the negative electrode 1 of the embodiment shown in FIG. The holes 9 are opened in each surface of the negative electrode 1 and extend in the thickness direction of the active material layer 5 and each current collecting surface layer 4. In the active material layer 5, the active material layer 5 is exposed on the wall surface of the hole 9. The role of the hole 9 is roughly divided into the following two.

  One is the role of supplying the electrolytic solution into the active material layer through the active material layer 5 exposed on the wall surface of the hole 9. Therefore, when the hole 9 is formed, the fine gap 6 described above may not be formed in the current collecting surface layer 4. The active material layer 5 is exposed at the wall surface of the hole 9, but a metal material having a low lithium compound forming ability penetrates between the active material particles 7 in the active material layer. Is prevented from falling off.

  The other is the role of relieving the stress caused by the volume change when the volume of the active material particles 7 in the active material layer changes due to charge / discharge. The stress is generated mainly in the plane direction of the negative electrode 1. Therefore, even if the volume of the active material particles 7 increases due to charging and stress occurs, the stress is absorbed by the holes 9 which are spaces. As a result, significant deformation of the negative electrode 1 is effectively prevented.

Another role of the hole 9 is that the gas generated in the negative electrode can be released to the outside. Specifically, gas such as H ₂ , CO, and CO ₂ may be generated due to moisture contained in a minute amount in the negative electrode. When these gases accumulate in the negative electrode, the polarization increases and causes charge / discharge loss. By forming the hole 9, the gas is released to the outside of the negative electrode through the hole 9, so that the polarization due to the gas can be reduced. Furthermore, as another role of the hole 9, there is a role of heat dissipation of the negative electrode. Specifically, since the specific surface area of the negative electrode is increased by forming the holes 9, the heat generated with the occlusion of lithium is efficiently released to the outside of the negative electrode. In addition, when stress is generated due to the volume change of the active material particles, heat may be generated due to the stress. Since the stress is relieved by forming the holes 9, the heat generation itself is suppressed.

  From the viewpoint of sufficiently supplying the electrolytic solution into the active material layer and from the viewpoint of effectively relieving the stress caused by the volume change of the particles of the active material, the hole area ratio of the holes 9 opened on the surface of the negative electrode 1 That is, the value obtained by dividing the total area of the holes 9 by the apparent area of the surface of the negative electrode 1 and multiplying by 100 is preferably 0.3 to 30%, particularly 2 to 15%. For the same reason, the hole diameter of the hole 9 opened on the surface of the negative electrode 1 is preferably 5 to 500 μm, particularly 20 to 100 μm. Also, by setting the pitch of the holes 9 to preferably 20 to 600 μm, more preferably 45 to 400 μm, the electrolyte can be sufficiently supplied into the active material layer, and the stress due to the volume change of the active material particles 7 can be increased. Can be effectively mitigated. Further, when attention is paid to an arbitrary portion on the surface of the negative electrode 1, an average of 100 to 250,000, particularly 1000 to 40000, and especially 5000 to 20000 holes 9 are opened in a 1 cm × 1 cm square observation field. It is preferable.

  The hole 9 may penetrate in the thickness direction of the negative electrode 1. However, in view of the role of the hole 9 to sufficiently supply the electrolyte into the active material layer and relieve the stress caused by the volume change of the particles of the active material, the hole 9 penetrates in the thickness direction of the negative electrode 1. It is not necessary to form holes on the surface of the negative electrode 1 and to reach at least the active material layer 5.

  The negative electrode 1 of this embodiment can be manufactured according to the manufacturing method shown in FIG. In detail, the process shown to Fig.2 (a)-FIG.2 (f) is performed, and a negative electrode precursor is obtained. Next, holes 9 are formed in the negative electrode precursor by a predetermined drilling process. Thereafter, the steps shown in FIGS. 2G and 2H are performed. There is no restriction | limiting in particular in the formation method of the hole 9. FIG. For example, the holes 9 can be formed by laser processing. Alternatively, drilling can be performed mechanically with a needle or punch. When both are compared, it is easier to obtain a negative electrode with good cycle characteristics and charge / discharge efficiency when laser processing is used. This is because, in the case of laser processing, the metal material dissolved and re-solidified by processing covers the surface of the active material particles present on the wall surface of the hole 9, so that the active material is prevented from being directly exposed, thereby This is because it is prevented from falling off from the wall surface of the hole 9. In the case of using laser processing, for example, after the step shown in FIG. 2F, the active material layer 5 may be irradiated with a laser. In addition, as another formation means of the hole 9, the hole 9 can also be formed using a sandblast process or using a photoresist technique. It is preferable that the holes 9 are formed so as to exist at substantially equal intervals. This is because the entire electrode can react uniformly.

  As a modification of the negative electrode shown in FIG. 5, the negative electrode 1 shown in FIG. In the negative electrode shown in FIG. 6, the metal lithium layer 3 formed on each surface of the conductive foil 10 such as a metal foil is sandwiched between the pair of negative electrode precursors 2. According to the negative electrode 1 of the present embodiment, the same effect as that of the negative electrode shown in FIG. 5 is achieved, and in addition, the conductive foil 10 provides an effect that higher strength is imparted to the negative electrode 1.

  The present invention is not limited to the embodiment. For example, a hole similar to the hole 9 formed in the negative electrode of the embodiment shown in FIGS. 5 and 6 may be formed in the negative electrode of the embodiment shown in FIGS. 3 and 4. Moreover, you may use the negative electrode shown in FIG. 6 without forming the hole 9 in this.

  Moreover, the negative electrode of each of the embodiments described above can be used alone, or a plurality of the negative electrodes can be used in an overlapping manner. In the latter case, a conductive foil (for example, a metal foil) serving as a core material can be interposed between adjacent negative electrodes.

  In each of the embodiments described above, the current collecting surface layer 4 has a single layer structure. However, instead of this, 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 current collecting surface layer has a multilayer structure, the ability to form a lithium compound that penetrates the active material layer 5 with at least one metal material having a low ability to form a lithium compound contained in the current collecting surface layer is low. The material can be different from the metal material. Alternatively, all of the metal material having a low lithium compound forming ability contained in each current collecting surface layer may be a different material from the metal material having a low lithium compound forming ability penetrating into the active material layer 5.

  When the constituent material of the current collecting surface layer is different from the material penetrating into the active material layer 5, the metal material penetrating into the active material layer 5 is separated from the active material layer 5 and the current collector. It may exist up to the boundary with the surface layer for use. Alternatively, the metal material penetrating into the active material layer 5 may constitute a part of the current collecting surface layer beyond the boundary portion. Conversely, the constituent material of the current collecting surface layer may be present in the active material layer 5 beyond the boundary.

  Moreover, the metal material deposited in the active material layer 5 is obtained by performing the operation of depositing a metal material having a low lithium compound forming ability in the active material layer 5 using two or more different plating baths. Two or more different multilayer structures can be used.

  In the manufacturing method shown in FIG. 2, the active material layer 5 is formed from a conductive slurry containing particles of the active material. Instead, a chemical vapor deposition method, a physical vapor deposition method, a sputtering method, and an electrolytic plating method are used. The active material layer 5 composed of a thin layer of the active material may be formed using a thin film forming means such as.

Hereinafter, the present invention will be described in more detail with reference to examples . The following reference examples serve as references for the present invention and are outside the scope of the present invention.

[ Reference Example 1]
The negative electrode shown in FIG. 5 was manufactured. A copper carrier foil (thickness 35 μm) obtained by electrolysis was acid-washed at room temperature for 30 seconds. Subsequently, it was washed with pure water at room temperature for 30 seconds. Next, a carrier foil was immersed in a tin plating bath having the following bath composition, and electrolytic plating was performed to form a coating made of tin. The current density was 2 A / dm ² and the bath temperature was 30 ° C. A tin electrode was used as the anode. A DC power source was used as the power source. The covering was formed unevenly to a thickness of 20 nm. After lifting from the plating bath, it was washed with pure water for 30 seconds, dried in the atmosphere, and allowed to stand for 15 minutes to oxidize the coating.
・ SnSO ₄ 50g / l
・ H ₂ SO ₄ 100 g / l
・ Cresol sulfonic acid 100g / l

  The carrier foil on which the covering was formed was immersed in a 3 g / l CBTA solution maintained at 40 ° C. for 30 seconds. Thus, a release layer forming process was performed. After the release layer forming treatment, the film was pulled up from the solution and washed with pure water for 15 seconds.

Next, the carrier foil was immersed in an H ₂ SO ₄ / CuSO ₄ plating bath to perform electroplating. Thus, a current collecting surface layer made of copper was formed on the surface of the carrier foil on which the covering was formed. The composition of the plating bath was 250 g / l for CuSO _{4 and} 70 g / l for H ₂ SO ₄ . The current density was 5 A / dm ² . The surface layer for current collection was formed to a thickness of 5 μm. After lifting from the plating bath, it was washed with pure water for 30 seconds and dried in the air.

Next, a slurry containing negative electrode active material particles was applied on the current collecting surface layer to a thickness of 20 μm to form an active material layer. The active material particles were made of Si, and the average particle size was D ₅₀ = 2 μm. The composition of the slurry was active material: acetylene black: styrene butadiene rubber = 98: 2: 1.7.

After the active material layer was formed, the carrier foil was immersed in a Watt bath having the following bath composition, and nickel was infiltrated into the active material layer by electrolysis. The current density was 5 A / dm ² , the bath temperature was 50 ° C., and the pH was 5. A nickel electrode was used as the anode. A DC power source was used as the power source. After lifting from the plating bath, it was washed with pure water for 30 seconds and dried in the air. Thus, a negative electrode precursor supported on the carrier foil was obtained. As a result of observing an electron microscope image, it was confirmed that a large number of fine voids were formed in the current collecting surface layer of the negative electrode precursor.
・ NiSO ₄・ 6H ₂ 0 250g / l
・ NiCl ₂・ 6H ₂ 0 45g / l
・ H ₃ BO ₄ 30g / l

The negative electrode precursor was irradiated with a YAG laser to regularly form holes penetrating the negative electrode precursor. The hole diameter was 24 μm, the pitch was 100 μm (10000 holes / cm ² ), and the hole area ratio was 4.5%.

  Next, a 30 μm thick metal lithium foil prepared separately from the negative electrode precursor was sandwiched between a pair of negative electrode precursors. The sandwiching was performed so that the active material layers in each negative electrode precursor face each other. Thus, each negative electrode precursor and metallic lithium were bonded and integrated. Finally, the carrier foil and the current collecting surface layer were peeled off to obtain a target negative electrode. The amount of metallic lithium in the negative electrode was 30% with respect to the initial charge theoretical capacity of the active material contained in the negative electrode.

  Using the obtained negative electrode, a nonaqueous electrolyte secondary battery was produced by the following method. The discharge capacity, negative electrode thickness change rate, and capacity retention rate after 100 cycles of this battery were measured and calculated by the following methods. These results are shown in Table 1 below.

[Production of non-aqueous electrolyte secondary battery]
The negative electrodes obtained in Reference Examples and Comparative Examples were used as working electrodes, and LiCoO ₂ was used as a counter electrode. The counter electrode was manufactured by coating LiCoO ₂ on an Al foil having a thickness of 20 μm so as to be 4 mAh / cm ² . The ratio of the positive electrode capacity to the negative electrode capacity (the former: the latter) was 1: 2. Both electrodes were opposed to each other through a separator. A non-aqueous electrolyte secondary battery was produced by a conventional method using a mixed solution (1: 1 volume ratio) of LiPF ₆ / ethylene carbonate and dimethyl carbonate as the non-aqueous electrolyte.

[Discharge capacity after one cycle]
The discharge capacity per unit area of the negative electrode was measured.

[Negative electrode thickness change rate]
Using an HS displacement cell manufactured by Hosen Co., Ltd., the thickness change of the negative electrode accompanying charging in one cycle was measured. In this displacement cell, the total thickness change of negative electrode + separator + positive electrode LiCoO ₂ is measured. However, since the positive electrode hardly expands due to charge / discharge and the contribution rate of the change in thickness of the negative electrode is large, the thickness change being measured can be regarded substantially as a change in thickness of the negative electrode.

[Capacity maintenance rate after 100 cycles]
The discharge capacity after 100 cycles was measured, and the value was divided by the maximum negative electrode discharge capacity and multiplied by 100.

[Comparative Example 1]
A negative electrode was obtained in the same manner as in Reference Example 1 except that a pair of negative electrode precursors were superposed so that their active material layers were opposed to each other without using a metal lithium foil. The obtained negative electrode was evaluated in the same manner as in Reference Example 1. The results are shown in Table 1.

As is clear from the results shown in Table 1, the battery using the negative electrode of Reference Example 1 has a high discharge capacity and capacity retention rate. Furthermore, it can be seen that the rate of change in the thickness of the negative electrode is small. On the other hand, it can be seen that the battery using the negative electrode of Comparative Example 1 that does not use lithium foil has a large change rate of the negative electrode thickness and a low capacity retention rate.

[ Reference Examples 2 to 4 and Comparative Example 2]
In this reference example and comparative example, the difference in the performance of the negative electrode due to the difference in the amount of metallic lithium contained in the negative electrode was evaluated. A negative electrode was produced in the same manner as in Reference Example 1 except that the negative electrode precursor was not subjected to drilling using a YAG laser and the amount of metallic lithium was changed to the values shown in Table 2. The obtained negative electrode had the structure shown in FIG. 1 (except for Comparative Example 1). As a result of observing an electron microscope image, it was confirmed that many fine voids were formed in the surface layer for current collection.

  For the obtained negative electrode, the charge capacity and discharge capacity after one cycle were measured. The results are shown in Table 2. Table 2 also describes the capacity reversibility after one cycle. The capacity reversibility after one cycle is a value obtained by dividing the discharge capacity after one cycle by the charge capacity after one cycle and multiplying it by 100. Note that metallic lithium was used for the counter electrode. Accordingly, the irreversible capacity derived from the negative electrode can be mainly estimated without including the irreversible capacity derived from the counter electrode. In addition, since the capacity is not regulated by the counter electrode, the entire capacity of the negative electrode is charged and discharged.

As is clear from the results shown in Table 2, it can be seen that the negative electrode of each reference example has a large capacity reversibility after one cycle as compared with the negative electrode of the comparative example. The reason for this is considered that the metal lithium contained in the negative electrode of each reference example removed moisture that adversely affects the performance of the negative electrode and trapped oxygen. In Reference Examples 3 and 4, the reason why the capacity reversibility after one cycle exceeds 100 is that the lithium metal added in advance contributes to the charge / discharge reaction.

[ Reference Examples 5 and 6 and Comparative Examples 3 and 4]
In this reference example and comparative example, the difference in the performance of the negative electrode due to the difference in moisture content contained in the negative electrode was evaluated. A negative electrode was prepared in the same manner as in Reference Example 1 except that the negative electrode precursor was not subjected to drilling using a YAG laser and the amount of metallic lithium was 40% of the initial charge theoretical capacity of silicon. The obtained negative electrode had the structure shown in FIG. As a result of observing an electron microscope image, it was confirmed that many fine voids were formed in the surface layer for current collection. The obtained negative electrode was dried in a vacuum chamber for 1 week under the condition of 160 ° C., and the water content was adjusted to 390 ppm ( Reference Example 5). Further, the obtained negative electrode was dried in a vacuum chamber for 3 hours under the condition of 160 ° C., and the moisture content was adjusted to 870 ppm ( Reference Example 6).

Separately from Reference Examples 5 and 6, the negative electrode obtained in Comparative Example 2 was dried in a vacuum chamber for 1 week under the condition of 160 ° C. to obtain a moisture content of 390 ppm (Comparative Example 3). Moreover, the obtained negative electrode was dried in a vacuum chamber for 3 hours under the condition of 160 ° C., and the moisture content was adjusted to 870 ppm (Comparative Example 4).

A battery was prepared using the obtained negative electrode, and cycle characteristics were measured. The results are shown in FIGS. In Reference Example 5, measurement was performed up to 100 cycles. The others were measured up to 50 cycles. As a counter electrode in the battery, LiCoO ₂ coated on an Al foil having a thickness of 20 μm was used. As the non-aqueous electrolyte, a mixed solution of LiPF ₆ / ethylene carbonate and dimethyl carbonate (1: 1 volume ratio) was used. The charge / discharge conditions are as follows.
Initial charge: constant current / constant voltage mode, current density 0.4 mA / cm ² , cutoff 4.15 V, 0.04 mA / cm ²
-Charging after the second cycle: constant current mode, current density 1.0 mA / cm ² , cutoff 3.95 V
Initial discharge: constant current / constant voltage mode, current density 0.4 mA / cm ² , cutoff 2.7 V
-Discharge after the second cycle: constant current mode, current density 1.0 mA / cm ² , cutoff 2.7 V

As is apparent from the results shown in FIGS. 7 and 8, it can be seen that the negative electrode of each reference example does not observe a decrease in capacity even after repeated charge and discharge. On the other hand, it can be seen that the capacity of the negative electrode of each comparative example gradually decreases with repeated charge and discharge.

[ Reference Example 7 and Comparative Example 5]
In this reference example and comparative example, the difference in the performance of the negative electrode due to the difference in the amount of oxygen contained in the negative electrode was evaluated. The negative electrode (moisture content of 390 ppm) obtained in Reference Example 5 was oxidized by a heat treatment in the atmosphere to obtain an oxygen concentration of 4000 ppm ( Reference Example 7). Further, the negative electrode obtained in Comparative Example 4 was further dried to obtain a moisture content of 390 ppm, and then oxidized by heat treatment in the atmosphere to obtain an oxygen concentration of 4000 ppm (Comparative Example 5).

The charge / discharge characteristics of the negative electrodes obtained in Reference Example 7 and Comparative Example 5 were evaluated. The result is shown in FIG. FIG. 9B is an enlarged view of the rising portion of the charge / discharge curve in FIG. As is clear from the results shown in FIGS. 9A and 9B, particularly the results shown in FIG. 9B, a shoulder portion peculiar to the oxidized negative electrode is observed in the negative electrode of Comparative Example 5. On the other hand, in the negative electrode of Reference Example 7, it can be seen that the shoulder portion has disappeared. The reason for this is considered that, in the negative electrode of Reference Example 7, the lithium metal contained therein captured oxygen.

FIG. 1 is a schematic view showing the structure of one embodiment of the negative electrode of the present invention. FIG. 2A to FIG. 2H are process diagrams showing a method for manufacturing the negative electrode shown in FIG. FIG. 3 is a schematic diagram showing the structure of the second embodiment of the negative electrode of the present invention. FIG. 4 is a schematic view showing the structure of the third embodiment of the negative electrode of the present invention. FIG. 5 is a schematic view showing the structure of the fourth embodiment of the negative electrode of the present invention. FIG. 6 is a schematic view showing the structure of the fifth embodiment of the negative electrode of the present invention. FIG. 7 is a graph showing the charge / discharge characteristics of batteries using the negative electrodes obtained in Reference Examples 5 and 6. FIG. 8 is a graph showing the charge / discharge characteristics of the batteries using the negative electrodes obtained in Comparative Examples 3 and 4. FIG. 9 is a graph showing charge / discharge characteristics of batteries using the negative electrodes obtained in Reference Example 7 and Comparative Example 5.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Negative electrode for non-aqueous electrolyte secondary batteries 2 Negative electrode precursor 3 Metal lithium layer 4 Current collecting surface layer 5 Active material layer 6 Fine void 7 Active material particles 9 Hole

Claims (5)

An active material layer between a pair of current collecting surface layers made of a metal material having a low ability to form a lithium compound;
At least one of the current collecting surface layers is formed with a large number of fine voids which are open in the surface thereof and extend in the thickness direction and allow the nonaqueous electrolyte to penetrate,
The active material layer includes particles of an active material having a high lithium compound forming ability that occludes lithium before the start of charging , and the active material layer includes a metal material having a low lithium compound forming ability. Permeates throughout the thickness direction of
A negative electrode for a non-aqueous electrolyte secondary battery, characterized by not having a conductive metal foil layer as a core material. ^{2. The} negative electrode for a nonaqueous electrolyte secondary battery according to claim 1, wherein the fine pores have an average pore area of 0.1 to 50 μm ² and a pore ratio of 0.1 to 20%.   The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the current collecting surface layer is formed by electrolytic plating. A method for producing a negative electrode for a non-aqueous electrolyte secondary battery according to claim 2,
A thin layer covering made of a material different from the material constituting the current collecting surface layer is formed on one surface of the carrier foil , and the electronic conductivity of the surface of the current collecting layer on the carrier foil is uneven. After forming a state, a surface layer for current collection is formed by electrolytic plating,
A conductive slurry containing active material particles is applied to the current collecting surface layer to form an active material layer, and the current collecting surface layer and the active material layer are arranged in this order on the carrier foil. Forming the negative electrode precursor prepared in
The metal lithium foil is sandwiched between both negative electrode precursors so that the active material layers in each negative electrode precursor face each other, and the metal lithium foil and both negative electrode precursors are integrated by bonding,
Lithium is heated to a temperature sufficient for thermal diffusion to diffuse lithium from the metallic lithium foil to the active material layer, and then
A method for producing a negative electrode for a non-aqueous electrolyte secondary battery, wherein the carrier foil is peeled and separated from each negative electrode precursor. An active material layer between a pair of current collecting surface layers made of a metal material having a low ability to form a lithium compound;
At least one of the current collecting surface layers is formed with a large number of fine voids which are open in the surface thereof and extend in the thickness direction and allow the nonaqueous electrolyte to penetrate,
The active material layer includes particles of an active material having a high lithium compound forming ability that occludes lithium before the start of charging , and the active material layer includes a metal material having a low lithium compound forming ability. Permeates throughout the thickness direction of
A negative electrode for a non-aqueous electrolyte secondary battery, comprising a conductive metal foil layer as a core material.

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