JP2012009209A - Negative electrode for lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion compensation method for effectively performing compensation of irreversible capacity and suppressing formation of an excess solid electrolyte interface (SEI) coat on a negative electrode in a lithium ion secondary battery.SOLUTION: A negative electrode 1 of a lithium ion secondary battery has a structure in which a lithium layer 4 including metal lithium is arranged between a collector 2 and a negative electrode active material layer 3. Thereby, supply of electrolyte which reacts with the lithium in a pre-doping reaction is restricted and thus, formation of an SEI coat generated by a reductive decomposition reaction of the electrolyte is suppressed.

Description

  The present invention relates to a negative electrode for a lithium ion secondary battery.

  In recent years, in order to cope with global warming, reduction of the amount of carbon dioxide is eagerly desired. In the automobile industry, there is a great expectation for reducing carbon dioxide emissions by introducing electric vehicles (EV) and hybrid electric vehicles (HEV), and the development of secondary batteries for motor drive that holds the key to commercialization of these is thriving. Has been done.

  As a secondary battery for driving a motor, it is required to have extremely high output characteristics and high energy as compared with a consumer lithium ion secondary battery used in a mobile phone, a notebook personal computer or the like. Therefore, lithium ion secondary batteries having a relatively high theoretical energy among all the batteries are attracting attention, and are currently being developed rapidly.

  Generally, a lithium ion secondary battery includes a positive electrode in which a positive electrode active material or the like is applied to the surface of the current collector using a binder, and a negative electrode in which a negative electrode active material or the like is applied to the surface of the current collector using a binder. It is connected via an electrolyte layer and is configured to be stored in a battery case.

Conventionally, carbon that is advantageous in terms of charge / discharge cycle life and cost, particularly graphite-based materials, has been used for the negative electrode of a lithium ion secondary battery. Recently, materials capable of being alloyed with lithium have been studied as high-capacity negative electrode active materials. For example, Si material occludes and releases 4.4 mol of lithium ions per mol during charge / discharge, and Li ₂₂ Si ₅ has a theoretical capacity of about 4200 mAh / g. Thus, since the material which can be alloyed with lithium can increase the energy density of an electrode, it is anticipated as a negative electrode material in a vehicle use.

  However, many of the lithium ion secondary batteries using such a carbon material having a large capacity or a material alloyed with lithium as a negative electrode active material have a large irreversible capacity during initial charge / discharge. For this reason, there exists a problem that the capacity utilization factor of the filled positive electrode falls and the energy density of a battery falls. Here, the irreversible capacity refers to the amount of lithium remaining in the negative electrode even after the discharge in the lithium ion secondary battery, in which all of the lithium occluded in the negative electrode during the initial charge cannot be released by the discharge. means. This problem of irreversible capacity has become a major development issue in the practical application to vehicle applications that require high capacity, and attempts to suppress the irreversible capacity have been actively made.

  As a technique for supplementing lithium corresponding to such irreversible capacity, for example, a technique disclosed in Patent Document 1 can be cited. In this technique, a lithium metal foil is superposed on the negative electrode surface, and lithium is pre-doped into the negative electrode.

JP-A-9-283179

  However, in the technique described in Patent Document 1, since the potential difference between the lithium layer and the negative electrode active material layer is base during the pre-doping reaction, the reductive decomposition of the electrolytic solution is promoted, and an excessive solid electrolyte coating (SEI coating) is formed. There was a problem of being formed.

  Therefore, an object of the present invention is to provide a means for suppressing the formation of an excessive SEI film.

  The inventors of the present invention have made extensive studies in view of the above problems. As a result, it has been found that the above problem can be solved by a negative electrode for a lithium ion secondary battery having a lithium layer containing metallic lithium in the negative electrode active material layer.

  Since the supply of the electrolytic solution that reacts with lithium during the pre-doping reaction is suppressed, the amount of the SEI film forming substance generated by the reductive decomposition reaction of the electrolytic solution can be suppressed.

1 is a schematic cross-sectional view showing the structure of a stacked lithium ion secondary battery. 1 is a schematic cross-sectional view showing a negative electrode for a lithium ion secondary battery according to a first embodiment. 1 is a schematic plan view showing a negative electrode for a lithium ion secondary battery according to a first embodiment. It is the cross-sectional schematic which shows the negative electrode for lithium ion secondary batteries of 2nd Embodiment. It is a cross-sectional schematic diagram which shows the negative electrode for lithium ion secondary batteries of 3rd Embodiment. It is a perspective view showing the appearance of a lithium ion secondary battery.

  First, although the nonaqueous electrolyte lithium ion secondary battery which is preferable embodiment is demonstrated, it is not restrict | limited only to the following embodiment. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted. In addition, the dimensional ratios in the drawings are exaggerated for convenience of explanation, and may be different from the actual ratios.

  When distinguished by the structure and form of the lithium ion secondary battery, it is not particularly limited, such as a stacked (flat) battery or a wound (cylindrical) battery, and can be applied to any conventionally known structure.

  Similarly, there is no particular limitation when distinguished by the form of electrolyte. For example, the present invention can be applied to any of a liquid electrolyte type battery in which a separator is impregnated with a nonaqueous electrolytic solution, a polymer gel electrolyte type battery also called a polymer battery, and a solid polymer electrolyte (all solid electrolyte) type battery. With respect to the polymer gel electrolyte and the solid polymer electrolyte, these can be used alone, or the polymer gel electrolyte or the solid polymer electrolyte can be used by impregnating the separator.

[Battery overall structure]
FIG. 1 is a schematic diagram showing the basic configuration of an embodiment of a flat (stacked) nonaqueous electrolyte lithium ion secondary battery (hereinafter also simply referred to as “stacked battery”). As shown in FIG. 1, the stacked battery 10 of the present embodiment has a structure in which a substantially rectangular power generation element 21 in which a charge / discharge reaction actually proceeds is sealed inside a battery exterior material 29 that is an exterior body. Have. Here, in the power generation element 21, the positive electrode in which the positive electrode active material layer 13 is disposed on both surfaces of the positive electrode current collector 11, the electrolyte layer 17, and the negative electrode active material layer 15 is disposed on both surfaces of the negative electrode current collector 12. It has a configuration in which a negative electrode is laminated. Specifically, the negative electrode, the electrolyte layer, and the positive electrode are laminated in this order so that one positive electrode active material layer 13 and the negative electrode active material layer 15 adjacent thereto face each other with the electrolyte layer 17 therebetween. .

  Thereby, the adjacent positive electrode, electrolyte layer, and negative electrode constitute one unit cell layer 19. Therefore, it can be said that the stacked battery 10 of the present embodiment has a configuration in which a plurality of the single battery layers 19 are stacked and electrically connected in parallel. In addition, although the positive electrode active material layer 13 is arrange | positioned only at one side in the outermost layer positive electrode collector located in both outermost layers of the electric power generation element 21, an active material layer may be provided in both surfaces. That is, instead of using a current collector dedicated to the outermost layer provided with an active material layer only on one side, a current collector having an active material layer on both sides may be used as it is as an outermost current collector. In addition, the arrangement of the positive electrode and the negative electrode is reversed from that in FIG. 1, so that the outermost layer negative electrode current collector is positioned on both outermost layers of the power generation element 21, and the outermost layer negative electrode current collector is disposed on one or both surfaces. A negative electrode active material layer may be disposed.

  The positive electrode current collector 11 and the negative electrode current collector 12 are respectively attached with a positive electrode current collector plate 25 and a negative electrode current collector plate 27 that are electrically connected to the respective electrodes (positive electrode and negative electrode), and are sandwiched between end portions of the battery exterior material 29. Thus, it has a structure led out of the battery exterior material 29. The positive electrode current collector plate 25 and the negative electrode current collector plate 27 are ultrasonically welded to the positive electrode current collector 11 and the negative electrode current collector 12 of each electrode, respectively, via a positive electrode lead and a negative electrode lead (not shown) as necessary. Or resistance welding or the like.

  FIG. 2 is a schematic cross-sectional view showing a negative electrode (first embodiment) used in the lithium ion secondary battery 10. FIG. 3 is a schematic plan view showing the negative electrode of the first embodiment. A negative electrode 1 according to the first embodiment shown in FIG. 2 includes a current collector 2, a negative electrode active material layer 3 formed on one surface of the current collector 2, and metallic lithium disposed in the negative electrode active material layer 3. A lithium layer 4.

  In a lithium ion secondary battery to which pre-doping is not applied, lithium ions move from the positive electrode active material layer to the negative electrode active material layer during the first charge. At that time, in relation to the potential window of the electrolytic solution, reductive decomposition of the electrolytic solution proceeds near the surface of the negative electrode active material layer at an initial stage or a base potential, and the decomposition product causes a solid electrolyte coating (SEI coating) to be formed. It is formed. After the SEI coating is formed, the SEI coating prevents direct contact between the electrolyte layer and the negative electrode active material layer, so that the reductive decomposition of the electrolyte does not proceed any further, and stable charge / discharge behavior is achieved. Will show. In addition, in order to smoothly perform the electron transfer reaction between the lithium ion and the positive electrode / negative electrode, it is necessary to form an SEI film having high reversibility. On the other hand, the pre-dope reaction proceeds at a lower potential even when compared with normal charge / discharge, and therefore, in the configuration in which the lithium layer is arranged on the negative electrode active material layer, the reductive decomposition of the electrolyte also proceeds. Prominent and excessive production of SEI film forming material.

  However, the negative electrode of the present embodiment has a lithium layer containing metallic lithium in the negative electrode active material layer. With such a configuration, the reaction field where the pre-doping reaction proceeds is in the lithium layer formed inside the negative electrode active material layer, the negative electrode active material layer in contact with the lithium layer, and the vacancies in the negative electrode active material layer It is under the coexistence of the electrolyte solution to be impregnated. As a result, the supply of the electrolytic solution to the reaction field is suppressed, and as a result, the amount of the product resulting from the reductive decomposition reaction of the electrolytic solution, that is, the SEI film forming substance is also suppressed. In other words, the generation of excessive electrolyte decomposition products can be suppressed both inside and outside the negative electrode active material layer, and as a result, while suppressing the phenomenon that cell resistance increases, lithium corresponding to irreversible capacity is compensated. It becomes possible to do.

  Hereinafter, each component of the negative electrode of the present embodiment will be described in detail.

[Current collector]
The current collector 2 is made of a conductive material. The material which comprises the electrical power collector 2 will not be restrict | limited especially if it has electroconductivity, For example, a metal and a conductive polymer may be employ | adopted. Specifically, metals such as iron, chromium, nickel, manganese, titanium, molybdenum, vanadium, niobium, aluminum, copper, silver, gold, and platinum; polyaniline, polypyrrole, polythiophene, polyacetylene, polyparaphenylene, polyphenylene vinylene, poly Examples thereof include conductive polymers such as acrylonitrile and polyoxadiazole. A so-called “resin current collector” having a configuration in which a conductive filler is dispersed in a base material made of a non-conductive polymer can also be adopted as one form of the current collector. Although the thickness of the electrical power collector 2 is not specifically limited, Usually, it is about 1-100 micrometers.

[Negative electrode active material layer]
The negative electrode active material layer 3 contains a negative electrode active material, and if necessary, a conductive additive, binder, electrolyte (polymer matrix, ion conductive polymer, electrolytic solution, etc.) for increasing electrical conductivity, and ion conductivity. The electrolyte support salt (lithium salt) may be further included.

  The compounding ratio of the components contained in the negative electrode active material layer 3 is not particularly limited, and can be selected by appropriately referring to known knowledge about the lithium ion secondary battery. Moreover, there is no restriction | limiting in particular also about the thickness of an active material layer, The conventionally well-known knowledge about a lithium ion secondary battery can be referred suitably. For example, the thickness of the negative electrode active material layer 3 is about 2 to 100 μm.

(Negative electrode active material)
The negative electrode active material is not particularly limited as long as it can reversibly occlude and release lithium, but preferably contains an element that can be alloyed with lithium. Examples of the form containing an element that can be alloyed with lithium include simple elements that can be alloyed with lithium, oxides and carbides containing these elements, and the like.

  By using an element that can be alloyed with lithium, a high-capacity battery having a higher energy density than that of a conventional carbon material can be obtained.

Elements that can be alloyed with lithium are not limited to the following, but specifically, Si, Ge, Sn, Pb, Al, In, Zn, H, Ca, Sr, Ba, Ru, Rh, Ir, Pd, Pt, Ag, Au, Cd, Hg, Ga, Tl, C, N, Sb, Bi, O, S, Se, Te, Cl and the like can be mentioned. Among these, from the viewpoint that a battery excellent in capacity and energy density can be configured, the negative electrode active material contains at least one element selected from the group consisting of Si, Ge, Sn, Pb, Al, In, and Zn. It is preferable to include. It is more preferable that an element of Si or Sn is included, and it is particularly preferable that Si is included. As the oxide, silicon monoxide (SiO), tin dioxide (SnO ₂ ), tin monoxide (SnO), or the like can be used. These may be used individually by 1 type and may use 2 or more types together.

In addition, carbon materials such as graphite, soft carbon, and hard carbon, metal materials such as lithium metal, lithium-transition metal composite oxides such as lithium-titanium composite oxide (lithium titanate: Li ₄ Ti ₅ O ₁₂ ), In addition, other conventionally known negative electrode active materials can be used. In some cases, two or more of these negative electrode active materials may be used in combination.

  However, in order to improve the capacity, it is preferable to include a large amount of a negative electrode active material containing an element that can be alloyed with lithium in the active material. In a more preferred form, specifically, in the negative electrode active material, the active material containing an element capable of alloying with lithium is 60% by mass or more, more preferably 80% by mass or more, still more preferably 90% by mass or more, particularly preferably. 100% by mass is contained.

(Conductive aid)
A conductive assistant means the additive mix | blended in order to improve electroconductivity. The conductive auxiliary agent that can be used in the present embodiment is not particularly limited, and conventionally known forms can be appropriately referred to. Examples thereof include carbon materials such as carbon black such as acetylene black, graphite, and carbon fiber. When the active material layer contains a conductive additive, an electronic network inside the active material layer is effectively formed, which can contribute to improvement of the output characteristics of the battery.

(Binder)
Although it does not specifically limit as a binder used for an active material layer, For example, the following materials are mentioned. Polyethylene, polypropylene, polyethylene terephthalate (PET), polyether nitrile, polyacrylonitrile, polyimide, polyamide, cellulose, carboxymethyl cellulose (CMC), ethylene-vinyl acetate copolymer, polyvinyl chloride, styrene-butadiene rubber (SBR), isoprene Rubber, butadiene rubber, ethylene / propylene rubber, ethylene / propylene / diene copolymer, styrene / butadiene / styrene block copolymer and its hydrogenated product, styrene / isoprene / styrene block copolymer and its hydrogenated product, etc. Thermoplastic polymer, polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), Tet Fluoroethylene / perfluoroalkyl vinyl ether copolymer (PFA), ethylene / tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene / chlorotrifluoroethylene copolymer (ECTFE), polyfluorinated Fluorine resin such as vinyl (PVF), vinylidene fluoride-hexafluoropropylene-based fluororubber (VDF-HFP-based fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-HFP-TFE-based fluorine) Rubber), vinylidene fluoride-pentafluoropropylene-based fluoro rubber (VDF-PFP-based fluoro rubber), vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene-based fluoro rubber (VDF-PFP-TFE fluoro rubber), vinylidene fluoride-perfluoromethyl vinyl ether-tetrafluoroethylene fluoro rubber (VDF-PFMVE-TFE fluoro rubber), vinylidene fluoride-chlorotrifluoroethylene fluoro rubber (VDF) Vinylidene fluoride fluororubbers such as (-CTFE fluororubber) and epoxy resins. Among these, polyvinylidene fluoride, polyimide, styrene / butadiene rubber, carboxymethyl cellulose, polypropylene, polytetrafluoroethylene, polyacrylonitrile, and polyamide are more preferable. These suitable binders are excellent in heat resistance, have a very wide potential window, are stable at both the positive electrode potential and the negative electrode potential, and can be used for the active material layer. These binders may be used individually by 1 type, and may use 2 or more types together.

  The amount of the binder contained in the negative electrode active material layer is not particularly limited as long as it is an amount capable of binding the active material, but preferably 0.5 to 15% by mass with respect to the active material layer. More preferably, it is 1-10 mass%.

(Electrolyte / Supporting salt)
Examples of the electrolyte include, but are not limited to, ion conductive polymers (solid polymer electrolytes) including lithium salts such as polyethylene oxide (PEO), polypropylene oxide (PPO), and copolymers thereof. There is nothing.

The supporting salt (lithium salt), but are not limited _{to, LiPF 6, LiBF 4, LiClO} 4, LiAsF 6, LiTaF 6, LiAlCl 4, inorganic acid anion salts such as _{Li 2 B 10 Cl 10; LiCF} 3 SO ₃ , organic acid anion salts such as Li (CF ₃ SO ₂ ) ₂ N and Li (C ₂ F ₅ SO ₂ ) ₂ N. These supporting salts may be used alone or in combination of two or more.

[Lithium layer]
As described above, the lithium layer includes metallic lithium. The metallic lithium may be in the form of a lithium alloy as long as the negative electrode active material can be doped. As the lithium alloy, lithium alloys such as simple elements which can be alloyed with lithium described in the section of (a) negative electrode active material, and oxides and carbides containing these elements can be preferably used. The form of the lithium layer is not particularly limited, and a lithium metal foil may be used as it is, or an aggregate of lithium particles or lithium alloy particles may be used. The lithium particles mean lithium powder in which metallic lithium is finely pulverized. The shape of the lithium particles is not particularly limited, and may take any structure such as a spherical shape, a rod shape, a needle shape, a plate shape, a column shape, an indefinite shape, a flake shape, and a spindle shape.

  When lithium particles are used, the average particle diameter is not particularly limited, but is preferably 1 to 60 μm, more preferably 2 to 55 μm, and particularly preferably 3 to 50 μm. If the average particle diameter of the lithium particles is within the above range, it is preferable because the handling is easy.

Furthermore, the specific surface area of the lithium particles measured by the BET single point method is preferably 0.1 to 10 m ² / g, more preferably 0.5 to 8 m ² / g.

  The thickness of the lithium layer is not particularly limited, but is preferably 1 to 100 μm, more preferably 2 to 95 μm. If the thickness of the lithium layer is less than 1 μm, the effect of improving the irreversible capacity by pre-doping may be insufficient. On the other hand, when it exceeds 100 μm, the thickness of the negative electrode active material layer may be larger.

  When lithium particles are used, the lithium layer may contain other components. Other components include a negative electrode active material, a conductive aid for increasing electrical conductivity, a binder, an electrolyte (polymer matrix, ion conductive polymer, electrolyte, etc.), and an electrolyte supporting salt (lithium for increasing ionic conductivity). Salt). The compounding ratio of the components in the lithium layer is not particularly limited. However, when importance is attached to the function of lithium pre-doping from the lithium layer to the negative electrode active material layer, the abundance ratio of lithium in the lithium layer is preferably 3% by mass or more, more preferably 5% by mass or more, and further preferably 8% by mass. % Or more. The lithium layer may have a form in which only one layer is present in the negative electrode active material layer as in the first embodiment shown in FIG. 2, or the negative electrode active material as in the embodiment shown in FIGS. There may be a form in which a plurality exist in the layer.

  Further, the arrangement of the lithium layers in the stacking direction is not particularly limited as long as the lithium layer is arranged inside the negative electrode active material layer, but it is preferably in contact with the interface between the current collector and the negative electrode active material layer. By adopting such an arrangement, the supply of the electrolytic solution to the reaction field is further suppressed, and the amount of the product resulting from the reductive decomposition reaction of the electrolytic solution, that is, the SEI film forming substance can be further suppressed.

  The ratio of the area in which the negative electrode active material layer and the lithium component in the lithium layer are in direct contact to the total area in which the negative electrode active material layer and the lithium layer are in direct contact with the current collector is 20 to 80%. Preferably there is. When the ratio exceeds 80%, the lithium component in the lithium layer disappears due to the lithium pre-doping reaction, which may increase the possibility of peeling of the negative electrode active material layer and the current collector. If the ratio is less than 20%, the time required for movement and diffusion of lithium ions in the negative electrode active material in the surface direction may be increased.

  In addition, the calculation method of the said ratio is based on the following method. That is, the negative electrode active material layer formed on the current collector is cross-sectional processed, and the vicinity of the current collector-negative electrode active material layer interface is observed with an optical microscope. Thereby, the ratio of the area where the negative electrode active material layer and the lithium component in the lithium layer are in direct contact with the sum of the areas where the negative electrode active material layer and the lithium layer are in direct contact with the current collector can be obtained. . This operation is performed at least 5 times, and the average of the contact ratio is taken.

  In this embodiment, after lithium pre-doping from the lithium layer to the active material layer, the lithium component in the lithium layer disposed between the current collector and the negative electrode active material layer disappears. Although SEI is formed at the boundary surface between the disappeared portion and the negative electrode active material layer, a conductive path in the negative electrode active material layer is sufficiently secured. Therefore, since the adverse effect of the electron conductivity from the negative electrode active material layer to the current collector can be minimized, the battery resistance value is hardly affected. Furthermore, in addition to the vacancies in the normal negative electrode active material, the ionic conductivity also contributes to the location where the lithium component disappears after the pre-dope reaction, and is further improved.

  The method for producing the negative electrode for a lithium ion secondary battery of the present embodiment is not particularly limited, and can be produced by applying a conventionally known method. Details will be described later together with a method of manufacturing a lithium ion secondary battery.

  The negative electrode for a lithium ion secondary battery according to the first embodiment described above has the following effects.

  The negative electrode for a lithium ion secondary battery of the first embodiment has a lithium layer containing metallic lithium in the negative electrode active material layer. Thereby, since the supply of the electrolytic solution that reacts with lithium during the pre-doping reaction is suppressed, the amount of the SEI film forming substance generated by the reductive decomposition reaction of the electrolytic solution can be suppressed.

  FIG. 4 is a schematic cross-sectional view showing the negative electrode for a lithium ion secondary battery of the second embodiment, and FIG. 5 is a schematic cross-sectional view showing the negative electrode for a lithium ion secondary battery of the third embodiment. In the embodiment shown in FIGS. 4 and 5, the lithium layer 3 is formed on both surfaces of the current collector. With such a configuration, the above effect can be obtained more easily. The shape of the lithium layer formed on one surface of the current collector and the shape / number of layers of the lithium layer formed on the other surface of the current collector may be the same as shown in FIG. It may be different as shown in FIG.

  The shape of the lithium layer in the area direction is not particularly limited, and may be any shape such as a circular shape, an elliptical shape, a quadrangular shape, a structure arranged in an array shape, or a comb shape.

  The lithium ion secondary battery described above is characterized by a negative electrode. Hereinafter, other main components will be described.

[Active material layer]
(Positive electrode (positive electrode active material layer))
The positive electrode active material layer 13 includes a positive electrode active material, and further includes other additives as necessary. As the positive electrode active material, for example, LiMn ₂ O ₄ , LiCoO ₂ , LiNiO ₂ , Li (Ni—Co—Mn) O _2, and lithium-such as those in which a part of these transition metals are substituted with other elements Examples include transition metal composite oxides, lithium-transition metal phosphate compounds, and lithium-transition metal sulfate compounds. In some cases, two or more positive electrode active materials may be used in combination. Preferably, a lithium-transition metal composite oxide is used as the positive electrode active material from the viewpoint of capacity and output characteristics. Of course, positive electrode active materials other than those described above may be used.

  The average particle diameter of each active material contained in the positive electrode active material layer 13 is not particularly limited, but is preferably 1 to 20 μm from the viewpoint of increasing the output.

  The positive electrode active material layer 13 includes a binder.

  Although it does not specifically limit as a binder used for an active material layer, For example, the following materials are mentioned. Polyethylene, polypropylene, polyethylene terephthalate (PET), polyether nitrile, polyacrylonitrile, polyimide, polyamide, cellulose, carboxymethyl cellulose (CMC), ethylene-vinyl acetate copolymer, polyvinyl chloride, styrene-butadiene rubber (SBR), isoprene Rubber, butadiene rubber, ethylene / propylene rubber, ethylene / propylene / diene copolymer, styrene / butadiene / styrene block copolymer and its hydrogenated product, styrene / isoprene / styrene block copolymer and its hydrogenated product, etc. Thermoplastic polymer, polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), Tet Fluoroethylene / perfluoroalkyl vinyl ether copolymer (PFA), ethylene / tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene / chlorotrifluoroethylene copolymer (ECTFE), polyfluorinated Fluorine resin such as vinyl (PVF), vinylidene fluoride-hexafluoropropylene-based fluororubber (VDF-HFP-based fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-HFP-TFE-based fluorine) Rubber), vinylidene fluoride-pentafluoropropylene-based fluoro rubber (VDF-PFP-based fluoro rubber), vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene-based fluoro rubber (VDF-PFP-TFE fluorine rubber), vinylidene fluoride-perfluoromethyl vinyl ether-tetrafluoroethylene fluorine rubber (VDF-PFMVE-TFE fluorine rubber), vinylidene fluoride-chlorotrifluoroethylene fluorine rubber (VDF) Vinylidene fluoride fluororubbers such as (-CTFE fluororubber) and epoxy resins. Among these, polyvinylidene fluoride, polyimide, styrene / butadiene rubber, carboxymethyl cellulose, polypropylene, polytetrafluoroethylene, polyacrylonitrile, and polyamide are more preferable. These suitable binders are excellent in heat resistance, have a very wide potential window, are stable at both the positive electrode potential and the negative electrode potential, and can be used for the active material layer. These binders may be used independently and may use 2 or more types together.

  The amount of the binder contained in the positive electrode active material layer is not particularly limited as long as it is an amount capable of binding the active material, but is preferably 0.5 to 15% by mass with respect to the active material layer. More preferably, it is 1-10 mass%.

  Examples of other additives that can be included in the positive electrode active material layer include a conductive additive, an electrolyte salt (lithium salt), and an ion conductive polymer.

  A conductive support agent means the additive mix | blended in order to improve the electroconductivity of a positive electrode active material layer. Examples of the conductive assistant include carbon materials such as carbon black such as acetylene black, graphite, and vapor grown carbon fiber. When the active material layer contains a conductive additive, an electronic network inside the active material layer is effectively formed, which can contribute to improvement of the output characteristics of the battery.

Examples of the electrolyte salt (lithium salt) include Li (C ₂ F ₅ SO ₂ ) ₂ N, LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiCF ₃ SO _{3 and the} like.

  Examples of the ion conductive polymer include polyethylene oxide (PEO) -based and polypropylene oxide (PPO) -based polymers.

  The compounding ratio of the components contained in the positive electrode active material layer is not particularly limited. The blending ratio can be selected by appropriately referring to known knowledge about the non-aqueous solvent secondary battery. The thickness of the active material layer is not particularly limited, and conventionally known knowledge about the battery can be referred to as appropriate. For example, the thickness of each active material layer is about 2 to 100 μm.

[Electrolyte layer]
A liquid electrolyte or a polymer electrolyte can be used as the electrolyte constituting the electrolyte layer 17.

  The liquid electrolyte has a form in which a lithium salt as a supporting salt is dissolved in an organic solvent as a plasticizer. Examples of the organic solvent that can be used as the plasticizer include carbonates such as ethylene carbonate (EC) and propylene carbonate (PC). Further, as the supporting salt (lithium salt), a compound that can be added to the active material layer of the electrode, such as LiBETI, can be similarly employed.

  On the other hand, the polymer electrolyte is classified into a gel electrolyte containing an electrolytic solution and an intrinsic polymer electrolyte containing no electrolytic solution.

  The gel electrolyte has a configuration in which the above liquid electrolyte is injected into a matrix polymer made of an ion conductive polymer. Examples of the ion conductive polymer used as the matrix polymer include polyethylene oxide (PEO), polypropylene oxide (PPO), and copolymers thereof. In such polyalkylene oxide polymers, electrolyte salts such as lithium salts can be well dissolved.

  In addition, when an electrolyte layer is comprised from a liquid electrolyte or a gel electrolyte, you may use a separator for an electrolyte layer. Specific examples of the separator include a microporous film made of polyolefin such as polyethylene or polypropylene.

  The intrinsic polymer electrolyte has a structure in which a supporting salt (lithium salt) is dissolved in the matrix polymer, and does not include an organic solvent that is a plasticizer. Therefore, when the electrolyte layer is composed of an intrinsic polymer electrolyte, there is no fear of liquid leakage from the battery, and the reliability of the battery can be improved.

  The matrix polymer of the gel electrolyte or the intrinsic polymer electrolyte can exhibit excellent mechanical strength by forming a crosslinked structure. In order to form a crosslinked structure, thermal polymerization, ultraviolet polymerization, radiation polymerization, electron beam polymerization, etc. are performed on a polymerizable polymer (for example, PEO or PPO) for forming a polymer electrolyte using an appropriate polymerization initiator. A polymerization treatment may be performed.

[Tab and Lead]
A tab may be used for the purpose of extracting the current outside the battery. The tab is electrically connected to the outermost layer current collector or current collector plate, and is taken out of the laminate sheet which is a battery exterior material.

  The material which comprises a tab in particular is not restrict | limited, The well-known highly electroconductive material conventionally used as a tab for lithium ion secondary batteries can be used. As a constituent material of the tab, for example, metal materials such as aluminum, copper, titanium, nickel, stainless steel (SUS), and alloys thereof are preferable, and aluminum, copper, and the like are more preferable from the viewpoint of light weight, corrosion resistance, and high conductivity. Is preferred. Note that the same material may be used for the positive electrode tab and the negative electrode tab, or different materials may be used.

  The positive terminal lead and the negative terminal lead are also used as necessary. As the material of the positive terminal lead and the negative terminal lead, a terminal lead used in a known lithium ion secondary battery can be used. It should be noted that the part taken out from the battery outer packaging material 29 has a heat insulating property so as not to affect the product (for example, automobile parts, particularly electronic devices) by contacting with peripheral devices or wiring and causing leakage. It is preferable to coat with a heat shrinkable tube or the like.

[Battery exterior materials]
As the battery exterior material 29, a known metal can case can be used, and a bag-like case using a laminate film containing aluminum that can cover the power generation element can be used. For example, a laminate film having a three-layer structure in which PP, aluminum, and nylon are laminated in this order can be used as the laminate film, but the laminate film is not limited thereto. A laminate film is desirable from the viewpoint that it is excellent in high output and cooling performance, and can be suitably used for a battery for large equipment for EV and HEV.

[Method for producing negative electrode / method for producing lithium ion secondary battery]
Hereinafter, although the manufacturing method of the negative electrode for lithium ion secondary batteries and a lithium ion secondary battery is demonstrated, it is not limited only to the following form.

  The method for producing the negative electrode is not particularly limited. However, a step of forming a lithium layer containing metallic lithium on the surface of the current collector (lithium layer forming step), and a step of forming a negative electrode active material layer on the surface of the current collector on which the lithium layer is disposed ( Negative electrode active material layer forming step).

(1) Lithium layer forming step In this step, first, a lithium layer is formed on the current collector surface. There is no restriction | limiting in particular about the formation method of a lithium layer, For example, the method of apply | coating the dispersion liquid (slurry) which disperse | distributed metallic lithium to the appropriate solvent on the surface of an electrical power collector, and drying can be illustrated. In addition, as other methods, a method of sticking or press-bonding a lithium foil to the surface of the current collector or the negative electrode active material layer, a method of forming a lithium layer on the surface of the current collector by vacuum deposition, or the like can be employed. Vacuum deposition is a preferred method in that it can form a thin film lithium layer that is difficult to manufacture industrially with lithium foil. A method in which a slurry containing a negative electrode active material, a conductive additive, a binder, and lithium metal is applied to the surface of the current collector and dried in the same manner as in the negative electrode active material layer forming step described later is also a preferred embodiment. Examples of the solvent used in this slurry include γ-butyrolactone (GBL), N-methyl-2-pyrrolidone (NMP), N, N-dimethylformamide, N, N-dimethylacetamide, N-methylformamide and the like. It is done.

  In addition, in the method of affixing or press-bonding a lithium foil on the surface of a collector or a negative electrode active material layer, the lithium foil may be affixed or pressure-bonded during the later-described (2) negative electrode active material layer forming step. .

(2) Negative electrode active material layer forming step In this step, first, a negative electrode active material and, if necessary, an electrode material containing a conductive additive, a binder, an electrolyte, and the like are dispersed in an appropriate slurry viscosity adjusting solvent to form a negative electrode. An active material slurry is prepared.

  The slurry viscosity adjusting solvent is not particularly limited, and examples thereof include N-methyl-2-pyrrolidone (NMP), N, N-dimethylformamide, N, N-dimethylacetamide, N-methylformamide and the like. The slurry is converted into ink from the solvent and the solid content using a homogenizer or a kneader.

  Next, the negative electrode active material slurry prepared above is applied to the surface of the current collector on which the lithium layer has been formed. The application means for applying the slurry to the current collector is not particularly limited, and generally used means such as a self-propelled coater, a doctor blade method, and a spray method can be employed.

  Subsequently, the coating film formed on the surface of the current collector on which the lithium layer has been formed is dried. Thereby, the solvent in a coating film is removed. The drying means for drying the coating film is not particularly limited, and conventionally known knowledge about electrode production can be appropriately referred to. For example, heat treatment is exemplified. Drying conditions (drying time, drying temperature, etc.) can be appropriately set according to the coating amount of the slurry and the volatilization rate of the slurry viscosity adjusting solvent. However, since the melting point of metallic lithium is 180 ° C, the drying temperature is preferably less than 180 ° C. The dried product obtained is pressed to adjust the electrode density, porosity, and thickness. This press treatment may be performed before drying. Thereby, the negative electrode for lithium ion secondary batteries of this embodiment is formed. In addition, when manufacturing the negative electrode by which a lithium layer is arrange | positioned on both surfaces of a collector like the form shown to FIG. 3 and 4, what is necessary is just to perform said process with respect to both surfaces of a collector.

  Through the above process, a negative electrode precursor in which a lithium layer containing metallic lithium is formed inside the negative electrode active material layer is obtained.

  The manufacturing method of the lithium ion secondary battery using the negative electrode precursor is not particularly limited. For example, a step of forming a power generation element using the negative electrode precursor (power generation element formation step) and a step of converting the negative electrode precursor to a negative electrode by doping metallic lithium into the negative electrode active material layer (lithium doping step) By performing the above, a lithium ion secondary battery can be manufactured. Hereinafter, it demonstrates in order of a process.

(3) Electric power generation element production process The negative electrode precursor and positive electrode obtained above are laminated via an electrolyte layer to produce an electric power generation element.

  First, a positive electrode is produced. The method for producing the positive electrode is not particularly limited, and a known method can be preferably used for the lithium ion secondary battery. Specifically, in the same manner as in the formation of the negative electrode active material layer, a positive electrode active material and, if necessary, an electrode material containing a binder, a conductive additive, an electrolyte, and the like are dispersed in a slurry viscosity adjusting solvent to obtain a positive electrode active material slurry. To prepare. Then, in the same manner as in the formation of the negative electrode active material layer, the positive electrode active material slurry is applied on the current collector, dried, and then pressed, thereby pressing the positive electrode active material layer on the surface (one side or both sides) of the current collector. Thus, a positive electrode in which is formed is obtained.

  Next, the power generation element can be manufactured by laminating the positive electrode and the negative electrode precursor so that the positive electrode active material layer and the negative electrode active material layer face each other via a separator (corresponding to an electrolyte layer). At this time, the stacking of the separator and the electrode is repeated until the number of single battery layers reaches a desired number.

  Then, a current collector plate and / or a lead is connected to each of the positive electrode and the negative electrode precursor, and the laminate is accommodated in an aluminum laminate film bag so that the current collector plate or the lead is led out. Thereafter, the electrolytic solution is injected by a liquid injector, and the end portion is sealed under reduced pressure to obtain a power generation element (battery).

  The power generation element (battery) in the case where the electrolyte is a liquid electrolyte has been described above as an example. The production of a power generation element (battery) using a gel electrolyte or an intrinsic polymer electrolyte can also be performed with reference to a known technique, and thus detailed description thereof is omitted here.

(4) Lithium doping step Subsequently, the negative electrode precursor is converted into a negative electrode by doping the lithium metal in the lithium layer into the negative electrode active material. Specifically, lithium in the lithium layer is doped into the negative electrode active material by injection of the electrolytic solution. When the lithium layer is composed of metallic lithium, that is, when the lithium layer is composed only of lithium element, the lithium layer disappears after doping with lithium, and the negative electrode precursor is converted into a negative electrode. . On the other hand, when the lithium layer is configured to include a material other than lithium element, such as a lithium alloy, the lithium layer (for example, an element that forms an alloy with lithium in the case of a lithium alloy) is present even after doping with lithium. Remains. Thereby, a lithium ion secondary battery having a negative electrode active material layer including a negative electrode active material doped with lithium ions is obtained.

  After the injection of the electrolytic solution, the lithium ion secondary battery is preferably aged for a predetermined time. The process may be performed only once or a plurality of times. By performing the aging process, the amount of lithium per unit area in the active material layer can be made uniform, and a battery with improved reliability can be obtained.

  The aging temperature is preferably 20 to 80 ° C., more preferably 40 to 60 ° C. in terms of shortening the time (aging time) required for making the amount of lithium uniform. The aging time is usually about 24 to 240 hours, although it varies depending on the doping amount of lithium.

  In the above description, the laminated battery in the case where the electrolyte is a liquid electrolyte has been described as an example, but the case where a gel electrolyte or an intrinsic polymer electrolyte is used can be implemented with reference to a known technique, and is omitted here. To do.

  The aging may be performed after battery assembly or preliminary charging. The conditions for the preliminary charging are not particularly limited, and for example, a method of charging at 20 to 60 ° C. for 10 minutes by a constant current method (current: 0.5 C) may be used.

<Appearance structure of lithium ion secondary battery>
FIG. 6 is a perspective view showing the appearance of a flat lithium ion secondary battery which is a typical embodiment of the secondary battery.

  As shown in FIG. 6, the flat lithium ion secondary battery 50 has a rectangular flat shape, and a positive electrode tab 58 and a negative electrode tab 59 for taking out electric power are drawn out from both sides thereof. Yes. The power generation element 57 is encased by the battery outer packaging material 52 of the lithium ion secondary battery 50, and the periphery thereof is heat-sealed. The power generation element 57 is sealed with the positive electrode tab 58 and the negative electrode tab 59 pulled out to the outside. Has been. Here, the power generation element 57 corresponds to the power generation element 21 of the lithium ion secondary battery 10 shown in FIG. 1 described above. The power generation element 57 is formed by laminating a plurality of single battery layers (single cells) 19 including a positive electrode (positive electrode active material layer) 13, an electrolyte layer 17, and a negative electrode (negative electrode active material layer) 15.

  The lithium ion secondary battery is not limited to a stacked flat shape. The wound lithium ion secondary battery may have a cylindrical shape, or may have a shape that is a flattened rectangular shape by deforming such a cylindrical shape. There is no particular limitation. In the said cylindrical shape thing, a laminate film may be used for the exterior material, and the conventional cylindrical can (metal can) may be used, for example, It does not restrict | limit. Preferably, the power generation element is covered with an aluminum laminate film. With this configuration, weight reduction can be achieved.

  Also, the removal of the tabs 58 and 59 shown in FIG. 6 is not particularly limited. The positive electrode tab 58 and the negative electrode tab 59 may be drawn out from the same side, or the positive electrode tab 58 and the negative electrode tab 59 may be divided into a plurality of parts and taken out from each side, as shown in FIG. It is not limited to. Further, in a wound type lithium ion battery, instead of a tab, for example, a terminal may be formed using a cylindrical can (metal can).

  The lithium ion secondary battery is used as a power source for driving a vehicle or an auxiliary power source that requires a high volume energy density and a high volume output density as a large capacity power source for an electric vehicle, a hybrid electric vehicle, a fuel cell vehicle, and a hybrid fuel cell vehicle. It can be suitably used.

  Hereinafter, although the said negative electrode for lithium ion secondary batteries and a lithium ion secondary battery using the same are demonstrated in detail through an Example, it is not limited to the following Example at all.

Example 1
A test cell having a negative electrode having the structure shown in FIG. 2 was produced by the following method.

(1) Lithium layer forming step A copper foil having a thickness of 15 μm was prepared as a negative electrode current collector. Next, one side of the copper foil was masked to form a 34 mm × 22 mm lithium layer on the copper foil as a substrate by vacuum deposition using lithium metal as a target. The lithium layer was silver white and the thickness was 4 μm.

(2) Negative electrode active material layer formation process Hard carbon (80 mass parts) was prepared as a negative electrode active material, acetylene black (10 mass parts) as a conductive support agent, and PVdF (10 mass parts) was prepared as a binder. These were dispersed in an appropriate amount of N-methyl-2-pyrrolidone (NMP), which is a slurry viscosity adjusting solvent, to prepare a negative electrode active material slurry.

The negative electrode active material slurry prepared above is applied to one side of the copper foil carrying the lithium layer prepared in (1) above, dried at 60 ° C., pressed, and subjected to a negative electrode active material layer (single side Thickness = 85 μm, negative electrode active material density = 9 mg / cm ² ) to form a negative electrode precursor. A nickel current extraction tab was joined to the obtained negative electrode by ultrasonic welding, and punched out so that the lithium layer was located in the center and the electrode part size was 36 mm × 26 mm. The lithium layer was in a position completely covered with the negative electrode active material layer and could not be observed even after punching. The ratio of the area where the negative electrode active material layer and the lithium component in the lithium layer were in direct contact to the total area where the negative electrode active material layer and the lithium layer were in direct contact with the current collector was 80%.

(3) Power generation element preparation step Separately, LiMn ₂ O ₄ (84 parts by mass) as a positive electrode active material, acetylene black (6 parts by mass) as a conductive auxiliary, and PVdF (10 parts by mass) as a binder, slurry viscosity adjusting solvent Was dispersed in an appropriate amount of NMP to prepare a positive electrode active material slurry.

On the other hand, an aluminum foil having a thickness of 20 μm was prepared as a positive electrode current collector. The positive electrode active material slurry prepared above was applied to one side of the current collector. Thereafter, drying was performed at 80 ° C., and press treatment was performed to form a positive electrode active material layer (thickness = 102 μm, positive electrode active material density = 25 mg / cm ² ), thereby completing a positive electrode. An aluminum current extraction tab was joined to the obtained positive electrode by ultrasonic welding, and punched out so that the electrode size was 34 mm × 24 mm.

A polyethylene microporous membrane (thickness = 25 μm) was prepared as a separator. Further, as an electrolytic solution, a solution in which LiPF _{6 as a} lithium salt was dissolved at a concentration of 1M in an equal volume mixed solution of ethylene carbonate (EC) and dimethyl carbonate (DMC) was prepared.

  The negative electrode precursor (including the lithium layer), separator, and positive electrode prepared / prepared above were laminated to produce a power generation element.

(4) Lithium dope process The obtained electric power generation element was mounted in the bag made from the aluminum laminate sheet which is an exterior, and the electrolyte solution prepared above was injected. Under vacuum conditions, the opening of the aluminum laminate sheet bag was sealed so that the current extraction tabs connected to both electrodes were led out to complete the test cell. By the impregnation with the electrolytic solution, the negative electrode active material layer is doped with lithium, and the negative electrode precursor is converted into a negative electrode.

  The test cell produced above was subjected to aging treatment at 55 ° C. for 120 hours. Thereby, the lithium contained in the lithium layer described above made the amount of lithium per unit area in the negative electrode active material layer uniform. The cell voltage after aging was 3.8V.

(Example 2)
In the (1) lithium layer forming step of Example 1, a test cell was prepared in the same manner as in Example 1 except that the area and thickness of the lithium layer were 26 mm × 18 mm and 6 μm, respectively. An aging process was performed. The cell voltage after aging was 3.8V. The ratio of the area where the negative electrode active material layer and the lithium component in the lithium layer were in direct contact to the total area where the negative electrode active material layer and the lithium layer were in direct contact with the current collector was 50%.

(Example 3)
In the (1) lithium layer forming step of Example 1, a test cell was prepared in the same manner as in Example 1 except that the area and thickness of the lithium layer were 18 mm × 13 mm and 13 μm, respectively. An aging process was performed. The cell voltage after aging was 3.8V. The ratio of the area in which the negative electrode active material layer and the lithium component in the lithium layer were in direct contact to the total area in which the negative electrode active material layer and the lithium layer were in direct contact with the current collector was 25%.

(Comparative Example 1)
Example 1 (1) The lithium layer forming step was not performed, and (2) after the negative electrode active material layer forming step, a 20 μm thick lithium rolled foil (17 mm × 4 mm) was pasted on the negative electrode active material layer Produced a test cell by the same method as in Example 1 described above. Further, an aging process was performed. The ratio of the area in which the negative electrode active material layer and the lithium component in the lithium layer were in direct contact to the total area in which the negative electrode active material layer and the lithium layer were in direct contact with the current collector was 7%.

(Comparative Example 2)
A test cell was prepared in the same manner as in Example 1 except that (1) the lithium layer formation step in Example 1 was not performed, and an aging step was further performed.

[Evaluation of test cell]
Each test cell subjected to the above aging treatment was charged for 3 hours in the atmosphere at 25 ° C. by a constant current constant voltage method (CCCV, current: 0.5 C, voltage: 4.2 V). Next, after a 30-minute pause, discharge treatment was performed to 2.5 V by a constant current method (CC, current: 0.5 C). At this time, initial discharge capacity, charge / discharge efficiency (initial discharge capacity / initial charge capacity), and resistance value were calculated. As for the resistance value, after charging and discharging, the cell voltage changes when charging to SOC 50% by constant current method (CC, current: 0.5C) and then discharging at current: 1C, 2C, 3C for 20 seconds. The average value was calculated from (ΔV) and the current value. The evaluation results are shown in Table 1.

  From the evaluation results shown in Table 1, it can be seen that in Examples 1 to 3, higher charge / discharge efficiency than that of Comparative Example 2 is ensured. This is considered to be due to the fact that the lithium pre-doping reaction from the lithium layer to the negative electrode active material proceeded without a shortage in the lithium pre-doping step.

Example 4
A test cell having a negative electrode having the form shown in FIG. 2 was produced by the method described below.

(1) Lithium layer forming step A copper foil having a thickness of 15 μm was prepared as a negative electrode current collector. Next, one side of the copper foil was masked to form a 34 mm × 22 mm lithium layer on the copper foil as a substrate by vacuum deposition using lithium metal as a target. The lithium layer was silver white and had a thickness of 8 μm.

(2) Negative electrode active material layer formation process Hard carbon (80 mass parts) was prepared as a negative electrode active material, acetylene black (10 mass parts) as a conductive support agent, and PVdF (10 mass parts) was prepared as a binder. These were dispersed in an appropriate amount of N-methyl-2-pyrrolidone (NMP), which is a slurry viscosity adjusting solvent, to prepare a negative electrode active material slurry.

The negative electrode active material slurry prepared above is applied to one side of the copper foil carrying the lithium layer prepared in (1) above, dried at 60 ° C., pressed, and subjected to a negative electrode active material layer (single side Thickness = 172 μm, negative electrode active material density = 18 mg / cm ² ). A nickel current extraction tab was joined to the negative electrode precursor thus obtained by ultrasonic welding, and punched out so that the electrode part size was 36 mm × 26 mm. The lithium layer was in a position completely covered with the negative electrode active material layer and could not be observed even after punching. The ratio of the area where the negative electrode active material layer and the lithium component in the lithium layer were in direct contact to the total area where the negative electrode active material layer and the lithium layer were in direct contact with the current collector was 80%.

(3) Power generation element manufacturing step Separately, LiNi _0.80 Co _0.15 Al _0.05 O ₂ (84 parts by mass) as a positive electrode active material, acetylene black (6 parts by mass) as a conductive additive, and PVdF (binder as a binder) 10 parts by mass) was prepared. These were dispersed in an appropriate amount of NMP, which is a slurry viscosity adjusting solvent, to prepare a positive electrode active material slurry.

On the other hand, an aluminum foil having a thickness of 20 μm was prepared as a positive electrode current collector. The positive electrode active material slurry prepared above was applied to one side of the current collector, dried at 80 ° C., and pressed, and the positive electrode active material layer (thickness = 92 μm, positive electrode active material density = 25 mg / cm ² ) was formed to complete the positive electrode. An aluminum current extraction tab was joined to the obtained positive electrode by ultrasonic welding, and punched out so that the electrode size was 34 mm × 24 mm.

A polyethylene microporous membrane (thickness = 25 μm) was prepared as a separator. Further, as an electrolytic solution, a solution in which LiPF _{6 as a} lithium salt was dissolved at a concentration of 1M in an equal volume mixed solution of ethylene carbonate (EC) and dimethyl carbonate (DMC) was prepared.

  The negative electrode precursor (including the lithium layer), separator, and positive electrode prepared / prepared above were laminated to produce a power generation element.

(4) Lithium dope process The obtained electric power generation element was mounted in the bag made from the aluminum laminate sheet which is an exterior, and the electrolyte solution prepared above was injected. Under vacuum conditions, the opening of the aluminum laminate sheet bag was sealed so that the current extraction tabs connected to both electrodes were led out to complete the test cell. By the impregnation with the electrolytic solution, the negative electrode active material layer is doped with lithium, and the negative electrode precursor is converted into a negative electrode.

  The test cell produced above was subjected to aging treatment at 55 ° C. for 120 hours. Thereby, the lithium contained in the lithium layer described above made the amount of lithium per unit area in the negative electrode active material layer uniform. The cell voltage after aging was 3.5V.

(Example 5)
A test cell was prepared in the same manner as in Example 5 except that the area and thickness of the lithium layer were 26 mm × 18 mm and 13 μm, respectively, in the (1) lithium layer formation step of Example 5, An aging process was performed. The cell voltage after aging was 3.5V. The ratio of the area where the negative electrode active material layer and the lithium component in the lithium layer were in direct contact to the total area where the negative electrode active material layer and the lithium layer were in direct contact with the current collector was 50%.

(Example 6)
In Example 5 (1) Lithium layer forming step, except that the area and thickness of the lithium layer were 18 mm × 13 mm and 26 μm, respectively, and the lithium rolled foil was directly attached to the copper foil instead of vacuum deposition. A test cell was prepared in the same manner as in No. 5. Further, an aging process was performed. The cell voltage after aging was 3.5V. The ratio of the area in which the negative electrode active material layer and the lithium component in the lithium layer were in direct contact to the total area in which the negative electrode active material layer and the lithium layer were in direct contact with the current collector was 25%.

(Comparative Example 3)
Example 5 (1) Lithium layer forming step was not performed, and (2) 95 μm thick lithium rolled foil (17 mm × 4 mm) was pasted on the negative electrode active material layer after the negative electrode active material layer forming step. A test cell was produced in the same manner as in Example 5 except for the above. Further, an aging process was performed. The cell voltage after aging was 3.65V. The ratio of the area in which the negative electrode active material layer and the lithium component in the lithium layer were in direct contact to the total area in which the negative electrode active material layer and the lithium layer were in direct contact with the current collector was 7%.

(Comparative Example 4)
A test cell was produced in the same manner as in Example 5 except that (1) the lithium layer formation step in Example 5 was not performed, and an aging step was further performed. The cell voltage after aging was 3.65V.

(Example 7)
A battery having a negative electrode having the structure shown in FIG. 2 was produced by the method described below.

(1) Lithium layer forming step A copper foil having a thickness of 15 μm was prepared as a negative electrode current collector. Hard carbon (73 parts by mass) as a negative electrode active material, acetylene black (9 parts by mass) as a conductive auxiliary agent, SBR (9 parts by mass) as a binder, and lithium powder (average particle size: 50 μm, 9 parts by weight) as a lithium source Prepared. These were dispersed in an appropriate amount of γ-butyrolactone (GBL), which is a slurry viscosity adjusting solvent, to prepare a negative electrode active material slurry containing a lithium component.

  Next, the negative electrode active material slurry containing the lithium component prepared above was applied to one side of the copper foil and dried at 130 ° C. to form a lithium layer.

(2) Negative electrode active material layer forming step On the other hand, hard carbon (80 parts by mass) was prepared as the negative electrode active material, acetylene black (10 parts by weight) as the conductive auxiliary agent, and PVdF (10 parts by mass) as the binder. These were dispersed in an appropriate amount of N-methyl-2-pyrrolidone (NMP), which is a slurry viscosity adjusting solvent, to prepare a negative electrode active material slurry.

The negative electrode active material slurry prepared above was applied to one side of the copper foil carrying the lithium layer prepared in (1) above, dried at 60 ° C., and pressed to form a negative electrode active material layer. . Taking into account both the lithium layer and the negative electrode active material layer, the thickness of one surface = 179 μm and the negative electrode active material density = 18 mg / cm ² . A nickel current extraction tab was joined to the obtained negative electrode by ultrasonic welding, and punched out so that the electrode size was 36 mm × 26 mm. The thickness ratio between the lithium layer and the negative electrode active material layer was 1: 4. The ratio of the area in which the negative electrode active material layer and the lithium component in the lithium layer were in direct contact to the total area in which the negative electrode active material layer and the lithium layer were in direct contact with the current collector was 23%. In addition, the ratio of the area where the negative electrode active material layer and the lithium component in the lithium layer are in direct contact depends on the following method. That is, the negative electrode active material layer formed on the current collector is cross-sectional processed, and the vicinity of the current collector-negative electrode active material layer interface is observed with an optical microscope. Thereby, the ratio of the area where the negative electrode active material layer and the lithium component in the lithium layer were in direct contact with respect to the total area where the negative electrode active material layer and the lithium layer were in direct contact with the current collector was determined. This operation was performed at least 5 times, and the average of the contact ratio was taken.

(3) Power generation element manufacturing step Separately, LiNi _0.80 Co _0.15 Al _0.05 O ₂ (84 parts by mass) as a positive electrode active material, acetylene black (6 parts by mass) as a conductive additive, and PVdF (binder as a binder) 10 parts by mass) was dispersed in an appropriate amount of NMP, which is a slurry viscosity adjusting solvent, to prepare a positive electrode active material slurry.

On the other hand, an aluminum foil having a thickness of 20 μm was prepared as a positive electrode current collector. The positive electrode active material slurry prepared above was applied to one side of the current collector, dried at 80 ° C., and pressed, and the positive electrode active material layer (thickness = 92 μm, positive electrode active material density = 25 mg / cm ² ) was formed to complete the positive electrode. An aluminum current extraction tab was joined to the obtained positive electrode by ultrasonic welding, and punched out so that the electrode size was 34 mm × 24 mm.

A polyethylene microporous membrane (thickness = 25 μm) was prepared as a separator. Further, as an electrolytic solution, a solution in which LiPF _{6 as a} lithium salt was dissolved at a concentration of 1M in an equal volume mixed solution of ethylene carbonate (EC) and dimethyl carbonate (DMC) was prepared.

  The negative electrode precursor (including the lithium layer), separator, and positive electrode prepared / prepared above were laminated to produce a power generation element.

(4) Lithium dope process The obtained electric power generation element was mounted in the bag made from the aluminum laminate sheet which is an exterior, and the electrolyte solution prepared above was injected. Under vacuum conditions, the opening of the aluminum laminate sheet bag was sealed so that the current extraction tabs connected to both electrodes were led out to complete the test cell. By the impregnation with the electrolytic solution, the negative electrode active material layer is doped with lithium, and the negative electrode precursor is converted into a negative electrode.

  The test cell produced above was subjected to aging treatment at 55 ° C. for 48 hours. The cell voltage after aging was 3.5V.

  Table 2 shows the results of evaluating the test cells of Examples 4 to 7 and Comparative Examples 3 to 4.

  From the evaluation results shown in Table 2, it can be seen that in Examples 4 to 7, higher charge / discharge efficiency than that of Comparative Example 4 is ensured. This is considered to be due to the fact that the lithium pre-doping reaction from the lithium layer to the negative electrode active material proceeded without a shortage in the lithium pre-doping step.

According to the structure of Example 7, since the specific surface area of the lithium particle in a lithium layer is large, a pre dope reaction rate becomes quick and is preferable. The specific surface area of the lithium particles in the lithium layer used in Example 7, measured by the BET single point method, was 0.8 m ² / g.

1 negative electrode,
2 current collector,
3, 15 negative electrode active material layer,
4 Lithium layer,
10, 50 lithium ion secondary battery,
11 positive electrode current collector,
12 negative electrode current collector,
13 positive electrode active material layer,
17 electrolyte layer,
19 cell layer,
21, 57 power generation element,
25 positive current collector,
27 negative current collector,
29, 52 Battery exterior material,
58 positive electrode tab,
59 Negative electrode tab.

Claims (7)

  A negative electrode for a lithium ion secondary battery, having a lithium layer containing metallic lithium inside the negative electrode active material layer.   The negative electrode for a lithium ion secondary battery according to claim 1, wherein the lithium layer is in contact with an interface between the current collector and the negative electrode active material layer.   The ratio of the area in which the negative electrode active material layer and the lithium component in the lithium layer are in direct contact to the total area in which the negative electrode active material layer and the lithium layer are in direct contact with the current collector is 20 to 80%. The negative electrode for lithium ion secondary batteries of Claim 1 or 2.   The negative electrode for a lithium ion secondary battery according to any one of claims 1 to 3, wherein the lithium layer has a thickness of 1 to 100 µm.   The lithium ion secondary battery using the negative electrode for lithium ion secondary batteries of any one of Claims 1-4. Disposing a lithium layer containing metallic lithium on the surface of the current collector;
Forming a negative electrode active material layer on the surface of the current collector on which the lithium layer is disposed;
The manufacturing method of the negative electrode for lithium ion secondary batteries containing. Forming a power generation element using the precursor of the negative electrode for a lithium ion secondary battery according to any one of claims 1 to 4,
Converting the precursor of the negative electrode for a lithium ion secondary battery into a negative electrode by doping metallic lithium into the negative electrode active material layer;
A method for producing a lithium ion secondary battery, comprising:

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