JP2013114882A - Lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery that has a large initial discharge capacity.SOLUTION: The lithium ion secondary battery includes a positive electrode including a positive electrode active material layer that is provided on a collector and contains a positive electrode active material capable of absorbing and desorbing lithium and containing manganese as a major constituent transition metal element; a negative electrode including a negative electrode active material layer that is provided on a collector and contains a negative electrode active material containing carbon or silicon as a major constituent; a nonaqueous electrolyte; and a separator. The lithium ion secondary battery includes a protective layer disposed between the negative electrode active material layer and the separator, and containing a material that undergoes an oxidation-reduction reaction at a potential higher than the negative electrode active material, and a polymer compound with a carboxyl group. The mass ratio of the protective layer to the negative electrode active material layer of the lithium ion secondary battery is higher than or equal to 0.03.

Description

  The present invention relates to a lithium ion secondary battery. More specifically, the present invention relates to a lithium ion secondary battery having an excellent initial discharge capacity.

  In recent years, in order to cope with air pollution and global warming, reduction of carbon dioxide emissions has been strongly 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 will be the key to the practical application of these technologies is thriving. Has been done.

  As a secondary battery for driving a motor, a lithium ion secondary battery having a high theoretical energy is attracting attention, and is currently being developed rapidly. A lithium ion secondary battery is generally formed by applying a slurry containing a positive electrode active material or the like on a positive electrode current collector and a slurry containing a negative electrode active material or the like on a negative electrode current collector. It has a configuration that includes a negative electrode and an electrolyte positioned therebetween and is housed in a battery case.

  In order to improve the capacity characteristics and output characteristics of the lithium ion secondary battery, selection of each active material is extremely important.

Conventionally, for the purpose of providing a lithium secondary battery with good load characteristics, it has been proposed to use a solid solution of LiNiO ₂ and Li ₂ MnO ₃ as a positive electrode active material (see Patent Document 1).

JP-A-9-55211

Further, as a result of further studies by the inventor, the solid solution described in Patent Document 1 has Li ₂ MnO ₃ as a parent structure, but the initial state occurs when manganese (Mn) is eluted from the solid solution. A new problem to be solved has been found that the discharge capacity decreases.

  The present invention has been made to solve such a new problem. An object of the present invention is to provide a lithium ion secondary battery having an excellent initial discharge capacity.

  The present inventor has intensively studied to achieve the above object. As a result, a positive electrode having a positive electrode active material layer containing a positive electrode active material containing manganese as a main constituent transition metal element and capable of inserting and extracting lithium on a current collector, and a negative electrode active material containing carbon or silicon as a main constituent element A negative electrode having a negative electrode active material layer on the current collector, a nonaqueous electrolyte, and a separator, and having a predetermined protective layer between the negative electrode active material layer and the separator The inventors have found that the above object can be achieved and have completed the present invention.

That is, the lithium ion secondary battery of the present invention includes a positive electrode having a positive electrode active material layer containing a positive electrode active material containing manganese as a main constituent transition metal element, which can occlude and release lithium, and carbon or silicon. The negative electrode which has the negative electrode active material layer containing the negative electrode active material which has as a main structural element on a collector, a non-aqueous electrolyte, and a separator are provided.
The lithium ion secondary battery of the present invention includes a material that is oxidized and reduced at a potential more noble than the negative electrode active material and a polymer compound having a carboxyl group between the negative electrode active material layer and the separator. Has a protective layer.
Moreover, in the lithium ion secondary battery of this invention, the ratio of the said protective layer with respect to the said negative electrode active material layer is 0.03 or more by mass ratio.

  According to the present invention, a positive electrode having a positive electrode active material layer containing a positive electrode active material containing manganese as a main constituent transition metal element capable of occluding and releasing lithium on a current collector, and carbon or silicon as a main constituent element. A negative electrode having a negative electrode active material layer including a negative electrode active material on a current collector, a nonaqueous electrolyte, and a separator, and redox at a potential nobler than the negative electrode active material between the negative electrode active material layer and the separator The protective layer includes a substance to be treated and a polymer compound having a carboxyl group, and the ratio of the protective layer to the negative electrode active material layer is 0.03 or more by mass ratio. Therefore, a lithium ion secondary battery having an excellent initial discharge capacity can be provided.

It is sectional drawing which shows the outline of an example of the lithium ion secondary battery which concerns on one Embodiment of this invention.

  Hereinafter, a lithium ion secondary battery according to an embodiment of the present invention will be described in detail with reference to the drawings. In addition, the dimension ratio of drawing quoted by the following embodiment is exaggerated on account of description, and may differ from an actual ratio.

  As shown in FIG. 1, the lithium ion secondary battery 1 of this embodiment has a configuration in which a battery element 10 to which a positive electrode tab 21 and a negative electrode tab 22 are attached is enclosed in an exterior body 30. In the present embodiment, the positive electrode tab 21 and the negative electrode tab 22 are led out in the opposite direction from the inside of the exterior body 30 to the outside. Although not shown, the positive electrode tab and the negative electrode tab may be led out in the same direction from the inside of the exterior body to the outside. Moreover, such a positive electrode tab and a negative electrode tab can be attached to the positive electrode collector and negative electrode collector which are mentioned later by ultrasonic welding, resistance welding, etc., for example.

[Positive electrode tab and negative electrode tab]
The positive electrode tab 21 and the negative electrode tab 22 are composed of materials such as aluminum (Al), copper (Cu), titanium (Ti), nickel (Ni), stainless steel (SUS), and alloys thereof. However, the material is not limited to these, and a conventionally known material used as a tab for a lithium ion secondary battery can be used.

  The positive electrode tab and the negative electrode tab may be made of the same material or different materials. Further, as in the present embodiment, a separately prepared tab may be connected to a positive electrode current collector and a negative electrode current collector described later, and each positive electrode current collector and each negative electrode current collector described later are extended. A tab may be formed. Although not shown, the positive electrode tab and the negative electrode tab of the part taken out from the exterior body do not affect products (for example, automobile parts, particularly electronic devices, etc.) due to contact with peripheral devices or wiring and leakage. Thus, it is preferable to coat with a heat-resistant insulating heat-shrinkable tube or the like.

  In addition, a current collector plate may be used for the purpose of taking out current outside the battery. The current collector plate is electrically connected to the current collector and the lead, and is taken out of the laminate sheet that is a battery exterior material. The material which comprises a current collector plate is not specifically limited, The well-known highly electroconductive material conventionally used as a current collector plate for lithium ion secondary batteries can be used. As a constituent material of the current collector plate, for example, metal materials such as aluminum (Al), copper (Cu), titanium (Ti), nickel (Ni), stainless steel (SUS), and alloys thereof are preferable, and more preferable. Aluminum (Al), copper (Cu), and the like are preferable from the viewpoints of light weight, corrosion resistance, and high conductivity. Note that the same material may be used for the positive electrode current collector plate and the negative electrode current collector plate, or different materials may be used.

[Exterior body]
For example, the exterior body 30 is preferably formed of a film-shaped exterior material from the viewpoints of miniaturization and weight reduction, but is not limited thereto, and the exterior body for a lithium ion secondary battery is not limited thereto. Conventionally known materials used for the body can be used. That is, a metal can case can also be applied.

  In addition, from the viewpoint that it is excellent in high output and cooling performance, and can be suitably used for batteries for large equipment of electric vehicles and hybrid electric vehicles, for example, a polymer-metal composite laminate sheet having excellent thermal conductivity is used. Can be mentioned. More specifically, an exterior body formed of an exterior material such as a laminate film having a three-layer structure in which polypropylene, aluminum, and nylon are laminated in this order can be applied.

[Battery element]
As shown in FIG. 1, the battery element 10 is formed by a positive electrode 11 in which a positive electrode active material layer 11B is formed on both main surfaces of a positive electrode current collector 11A, and a non-aqueous electrolyte (not shown) included in a separator 14. A plurality of the nonaqueous electrolyte layer 13 and the negative electrode 12 in which the negative electrode active material layer 12B is formed on both main surfaces of the negative electrode current collector 12A are stacked. In addition, the battery element 10 includes a protective layer 15 between the negative electrode active material layer 12 </ b> B and the separator 14. In the present embodiment, the protective layer 15 is formed over the entire surface of the negative electrode active material layer. However, the protective layer 15 is formed only on a part of the surface (not shown), or for example, the negative electrode active material layer The case where a protective layer is formed on the surface of the granular negative electrode active material itself contained in (not shown) is also included in the scope of the present invention.

  At this time, one main surface of the positive electrode active material layer 11B formed on one main surface of the positive electrode current collector 11A of one positive electrode 11 and the negative electrode current collector 12A of the negative electrode 12 adjacent to the one positive electrode 11 The negative electrode active material layer 12 </ b> B formed above faces each other through a nonaqueous electrolyte layer 13 and a protective layer 15 formed by a nonaqueous electrolyte (not shown) included in the separator 14. In this manner, a plurality of positive electrodes 11, non-aqueous electrolyte layers 13, protective layers 15, and negative electrodes 12 are laminated in this order.

  As a result, the adjacent positive electrode active material layer 11B, the nonaqueous electrolyte layer 13 formed of a nonaqueous electrolyte (not shown) included in the separator 14, the protective layer 15, and the negative electrode active material layer 12B are combined into one unit cell layer 16. Configure. Therefore, the lithium ion secondary battery 1 of the present embodiment has a configuration in which a plurality of single battery layers 16 are stacked so that they are electrically connected in parallel. Note that the negative electrode current collector 12 a located in the outermost layer of the battery element 10 has the negative electrode active material layer 12 </ b> B and the protective layer 15 formed on only one side.

  Furthermore, in this battery element 10, the ratio of the dry mass of the protective layer 15 to the dry mass of the negative electrode active material layer 12B is 0.03 or more, preferably 0.03 to 0.20, more preferably. 0.05-0.15. When the ratio of the dry mass of the protective layer to the dry mass of the negative electrode active material layer is less than 0.03 in terms of mass ratio, it does not have an excellent initial discharge capacity. In addition, if the ratio of the dry weight of the protective layer to the dry weight of the negative electrode active material layer is 0.03 or more in mass ratio, it has excellent initial discharge capacity. While maintaining the density, it has an excellent initial discharge capacity.

  In addition, an insulating layer (not shown) may be provided on the outer periphery of the unit cell layer to insulate between the adjacent positive electrode current collector and negative electrode current collector. Such an insulating layer is preferably formed of a material that retains the electrolyte contained in the electrolyte layer and the like and prevents electrolyte leakage from the outer periphery of the single cell layer. Specifically, general-purpose plastics such as polypropylene (PP), polyethylene (PE), polyurethane (PUR), polyamide resin (PA), polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polystyrene (PS), etc. Or thermoplastic olefin rubber can be used. Silicone rubber can also be used.

[Positive electrode current collector and negative electrode current collector]
The positive electrode current collector 11A and the negative electrode current collector 12A are made of a conductive material. The size of the current collector can be determined according to the intended use of the battery. For example, if it is used for a large battery that requires a high energy density, a current collector having a large area is used. There is no particular limitation on the thickness of the current collector. The thickness of the current collector is usually about 1 to 100 μm. The shape of the current collector is not particularly limited. In the battery element 10 shown in FIG. 1, in addition to the current collector foil, a mesh shape (expanded grid or the like) can be used. In the case where the negative electrode active material is formed directly on the negative electrode current collector 12A by sputtering or the like, it is desirable to use a current collector foil.

  There is no particular limitation on the material constituting the current collector. For example, a metal or a resin in which a conductive filler is added to a conductive polymer material or a non-conductive polymer material can be employed.

  Specifically, examples of the metal include aluminum (Al), nickel (Ni), iron (Fe), stainless steel (SUS), titanium (Ti), copper (Cu), and the like. In addition to these, it is preferable to use a clad material of nickel (Ni) and aluminum (Al), a clad material of copper (Cu) and aluminum (Al), or a plating material of a combination of these metals. Moreover, the foil by which aluminum is coat | covered on the metal surface may be sufficient. Among these, aluminum, stainless steel, copper, and nickel are preferable from the viewpoints of electronic conductivity, battery operating potential, and adhesion of the negative electrode active material by sputtering to the current collector.

  Examples of the conductive polymer material include polyaniline, polypyrrole, polythiophene, polyacetylene, polyparaphenylene, polyphenylene vinylene, polyacrylonitrile, and polyoxadiazole. Since such a conductive polymer material has sufficient conductivity without adding a conductive filler, it is advantageous in terms of facilitating the manufacturing process or reducing the weight of the current collector.

  Non-conductive polymer materials include, for example, polyethylene (PE; high density polyethylene (HDPE), low density polyethylene (LDPE), etc.), polypropylene (PP), polyethylene terephthalate (PET), polyether nitrile (PEN), polyimide (PI), polyamideimide (PAI), polyamide (PA), polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), polyacrylonitrile (PAN), polymethyl acrylate (PMA), polymethyl methacrylate (PMMA) , Polyvinyl chloride (PVC), polyvinylidene fluoride (PVDF), polystyrene (PS), and the like. Such a non-conductive polymer material has excellent potential resistance or solvent resistance.

A conductive filler can be added to the conductive polymer material or the non-conductive polymer material as necessary. In particular, when the resin used as the base material of the current collector is made of only a non-conductive polymer, a conductive filler is inevitably necessary to impart conductivity to the resin. The conductive filler can be used without particular limitation as long as it is a substance having conductivity. For example, a metal, conductive carbon, etc. are mentioned as a material excellent in electroconductivity, electric potential resistance, or lithium ion interruption | blocking property. The metal is not particularly limited, but nickel (Ni), titanium (Ti), aluminum (Al), copper (Cu), platinum (Pt), iron (Fe), chromium (Cr), tin (Sn), It is preferable to include at least one metal selected from the group consisting of zinc (Zn), indium (In), antimony (Sb), and potassium (K), or an alloy or metal oxide containing these metals. Moreover, there is no restriction | limiting in particular as electroconductive carbon. Preferably, it contains at least one selected from the group consisting of acetylene black, vulcan, black pearl, carbon nanofiber, ketjen black, carbon nanotube, carbon nanohorn, carbon nanoballoon, and fullerene. The amount of the conductive filler added is not particularly limited as long as it is an amount capable of imparting sufficient conductivity to the current collector, and is generally about 5 to 35% by mass.
However, the material is not limited to these, and a conventionally known material used as a current collector for a lithium ion secondary battery can be used.

[Positive electrode active material layer]
The positive electrode active material layer 11B needs to contain at least a positive electrode active material having manganese as a main constituent transition metal element capable of occluding and releasing lithium, and optionally contains a binder and a conductive auxiliary agent. You may go out. The phrase “manganese is the main constituent transition metal element” means that the proportion of manganese in the constituent transition metal element is 50 mol% or more in terms of molar ratio. Needless to say, the scope of the present invention includes a case where the positive electrode active material is composed only of a positive electrode active material containing manganese as a main constituent transition metal element capable of inserting and extracting lithium.

  Examples of the positive electrode active material having manganese as a main constituent transition metal element capable of inserting and extracting lithium include a composite oxide containing lithium and manganese, and lithium and manganese from the viewpoint of capacity and output characteristics. Examples thereof include phosphoric acid compounds, sulfuric acid compounds containing lithium and manganese, and solid solutions containing lithium and manganese. From the viewpoint of obtaining higher capacity and output characteristics, lithium manganese composite oxide is particularly preferable.

Specific examples of the composite oxide containing lithium and manganese include lithium nickel manganese composite oxide (LiNi _0.5 Mn _1.5 O ₄ ), lithium nickel manganese cobalt composite oxide (Li (NiMnCo) O ₂ , Li (LiNiMnCo) O ₂ ), lithium manganese composite oxide having a spinel structure (LiMn ₂ O ₄ ), and the like. Further, specific examples of the phosphoric acid compound containing lithium and manganese include a lithium iron manganese phosphoric acid compound (LiFeMnPO ₄ ). These may be used alone or in combination of two or more.

  In addition, in the above-described composite oxide, for the purpose of stabilizing the structure, for example, a material in which a part of manganese is substituted with another element can be given as a suitable example. Specific examples of the positive electrode active material that is a solid solution containing lithium and manganese include those represented by the general formula (1).

Li _2-0.5x Mn _1-x M _1.5x O ₃ (1)
(In the formula (1), Li is lithium, Mn is manganese, M is Ni _α Co _β Mn _γ M ¹ _δ (Ni is nickel, Co is cobalt, Mn is manganese, M ¹ is aluminum (Al), iron (Fe ), Copper (Cu), magnesium (Mg) and titanium (Ti), at least one selected from the group consisting of α, β, γ and δ is 0 <α ≦ 0.5, 0 ≦ β ≦ 0 .33, 0 <γ ≦ 0.5, 0 ≦ δ ≦ 0.1, α + β + γ + δ = 1)), and x satisfies the relationship 0.1 ≦ x ≦ 0.5. )

  In general, nickel (Ni), cobalt (Co) and manganese (Mn) are used in terms of improving the purity and electronic conductivity of materials, aluminum (Al), iron (Fe), copper (Cu), magnesium (Mg). ) And titanium (Ti) are known to contribute to capacity and output characteristics from the viewpoint of improving the stability of the crystal structure.

  Moreover, it does not specifically limit as a binder, For example, the following materials are mentioned. Polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polyether nitrile (PEN), polyacrylonitrile (PAN), polyimide (PI), polyamide (PA), cellulose, carboxymethylcellulose (CMC), ethylene-acetic acid Vinyl copolymer, polyvinyl chloride (PVC), styrene / butadiene rubber (SBR), isoprene rubber, butadiene rubber, ethylene / propylene rubber, ethylene / propylene / diene copolymer, styrene / butadiene / styrene block copolymer, and Its hydrogenated product, thermoplastic polymer such as styrene / isoprene / styrene block copolymer and its hydrogenated product, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), tetrafluoro Tylene / hexafluoropropylene copolymer (FEP), tetrafluoroethylene / perfluoroalkyl vinyl ether copolymer (PFA), ethylene / tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene / Fluoropolymers such as chlorotrifluoroethylene copolymer (ECTFE) and polyvinyl fluoride (PVF), vinylidene fluoride-hexafluoropropylene fluororubber (VDF-HFP fluoropolymer), vinylidene fluoride-hexafluoropropylene-tetra Fluoroethylene fluoro rubber (VDF-HFP-TFE fluoro rubber), vinylidene fluoride-pentafluoropropylene fluoro rubber (VDF-PFP fluoro rubber), vinylidene fluoride- Interfluoropropylene-tetrafluoroethylene fluorine rubber (VDF-PFP-TFE fluorine rubber), vinylidene fluoride-perfluoromethyl vinyl ether-tetrafluoroethylene fluorine rubber (VDF-PFMVE-TFE fluorine rubber), vinylidene fluoride -Vinylidene fluoride-based fluororubber such as chlorotrifluoroethylene-based fluororubber (VDF-CTFE-based fluororubber), epoxy resin and the like. Among them, polyvinylidene fluoride (PVDF), polyimide (PI), styrene-butadiene rubber (SBR), carboxymethylcellulose (CMC), polypropylene (PP), polytetrafluoroethylene (PTFE), polyacrylonitrile (PAN), polyamide (PA) ) Is more preferable. These suitable binders are excellent in heat resistance, have a very wide potential window, are stable at both positive electrode potential and negative electrode potential, and can be used for a positive electrode (and negative electrode) active material layer.

  However, the material is not limited to these, and a known material conventionally used as a binder for a lithium ion secondary battery can be used. These binders may be used alone or in combination of two or more.

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

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

  However, the material is not limited to these, and a conventionally known material that is used as a conductive additive for a lithium ion secondary battery can be used. These conductive assistants may be used alone or in combination of two or more.

  In addition, the conductive binder having the functions of the conductive assistant and the binder may be used instead of the conductive assistant and the binder, or may be used in combination with one or both of the conductive assistant and the binder. . As the conductive binder, for example, commercially available TAB-2 (manufactured by Hosen Co., Ltd.) can be used.

[Negative electrode active material layer]
The negative electrode active material layer 12B needs to contain at least a negative electrode active material containing carbon or silicon as a main constituent element, and may contain a binder or a conductive aid as necessary. In addition, the binder and the conductive auxiliary agent described above can be used.

Examples of the negative electrode active material having carbon or silicon as the main constituent element include graphite (natural graphite, artificial graphite, etc.), which is highly crystalline carbon, low crystalline carbon (soft carbon, hard carbon), carbon black (ketchen) Carbon materials such as black, acetylene black, channel black, lamp black, oil furnace black, thermal black), fullerene, carbon nanotubes, carbon nanofibers, carbon nanohorns, carbon fibrils, etc .; silicon simple substance, alloy, oxide (monoxide) Examples thereof include silicon (SiO), SiO _x (0 <x <2), carbide (SiC), etc. The term “carbon or silicon as the main constituent element” means the ratio of carbon or silicon in the constituent elements. Is 5 by mass Refers to mass% or more. Negative electrode active material of these may be used alone individually or as a combination of two or more thereof.

  Further, the thickness of each active material layer (active material layer on one side of the current collector) 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 usually about 1 to 500 μm, preferably 2 to 100 μm in consideration of the intended use of the battery (emphasis on output, energy, etc.) and ion conductivity.

  Furthermore, when the optimum particle diameter is different for expressing the unique effect of each active material, the optimum particle diameters may be used in combination for expressing each unique effect. There is no need to make the particle size of the material uniform.

  For example, when an alloy in the form of particles is used as the negative electrode active material, the average particle size of the alloy is not particularly limited as long as it is approximately the same as the average particle size of the negative electrode active material contained in the existing negative electrode active material layer. From the viewpoint of increasing the output, it is preferably in the range of 1 to 20 μm. In the present specification, the “particle diameter” refers to the outline of active material particles (observation surface) observed using an observation means such as a scanning electron microscope (SEM) or a transmission electron microscope (TEM). It means the maximum distance among any two points. As the value of “average particle diameter”, the average particle diameter of particles observed in several to several tens of fields using an observation means such as a scanning electron microscope (SEM) or a transmission electron microscope (TEM). The calculated value shall be adopted. The particle diameters and average particle diameters of other components can be defined in the same manner.

  However, it is not limited to such a range at all, and it goes without saying that it may be outside this range as long as the effects of the present embodiment can be expressed effectively.

[Nonaqueous electrolyte layer]
As the nonaqueous electrolyte layer 13, for example, a separator 14 which will be described later may include a nonaqueous electrolyte to form a layer structure. Examples of the non-aqueous electrolyte include a non-aqueous electrolyte, a polymer gel electrolyte, and a solid polymer electrolyte.

  The non-aqueous electrolyte is preferably, for example, normally used in lithium ion secondary batteries, and specifically has a form in which a supporting salt (lithium salt) is dissolved in an organic solvent.

Examples of the lithium salt include lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ), lithium perchlorate (LiClO ₄ ), lithium hexafluoroarsenate (LiAsF ₆ ), six Inorganic acid anion salts such as lithium fluorotantalate (LiTaF ₆ ), lithium tetrachloroaluminate (LiAlCl ₄ ), lithium decachlorodecaborate (Li ₂ B ₁₀ Cl ₁₀ ), lithium trifluoromethanesulfonate (LiCF ₃₎ Organic acids such as SO ₃ ), lithium bis (trifluoromethanesulfonyl) imide (Li (CF ₃ SO ₂ ) ₂ N), lithium bis (pentafluoroethanesulfonyl) imide (Li (C ₂ F ₅ SO ₂ ) ₂ N) List at least one lithium salt selected from anionic salts Can.

  Examples of the organic solvent include cyclic carbonates such as propylene carbonate (PC) and ethylene carbonate (EC); chain carbonates such as dimethyl carbonate (DMC), methyl ethyl carbonate (EMC), and diethyl carbonate (DEC). Ethers such as tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, 1,2-dibutoxyethane; lactones such as γ-butyrolactone; nitriles such as acetonitrile; methyl propionate; Esters such as amides; Amides such as dimethylformamide; One using at least one selected from methyl acetate and methyl formate, or a mixture using an organic solvent such as an aprotic solvent can be used. .

  Examples of the polymer gel electrolyte include those containing a polymer constituting the polymer gel electrolyte and an electrolytic solution in a conventionally known ratio. For example, from the viewpoint of ionic conductivity and the like, it is desirable to set it to about several mass% to 98 mass%. The polymer gel electrolyte is a solid polymer electrolyte having ion conductivity containing the above-described electrolytic solution usually used in a lithium ion secondary battery. However, the present invention is not limited to this, and includes a structure in which a similar electrolyte solution is held in a polymer skeleton having no lithium ion conductivity. Examples of the polymer having no lithium ion conductivity used in the polymer gel electrolyte include polyvinylidene fluoride (PVDF), polyvinyl chloride (PVC), polyacrylonitrile (PAN), and polymethyl methacrylate (PMMA). Can be used. However, it is not necessarily limited to these. In addition, since polyacrylonitrile (PAN), polymethyl methacrylate (PMMA), and the like are in a class that has almost no ionic conductivity, it can be a polymer having the ionic conductivity. Here, the polymer used for the polymer gel electrolyte is exemplified as a polymer having no lithium ion conductivity.

  Examples of the solid polymer electrolyte include a structure in which the lithium salt is dissolved in polyethylene oxide (PEO), polypropylene oxide (PPO), and the like, and does not contain an organic solvent. Therefore, when the non-aqueous electrolyte layer is composed of a solid polymer electrolyte, there is no fear of liquid leakage from the battery, and the reliability of the battery can be improved.

  The thickness of the nonaqueous electrolyte layer is preferably thinner from the viewpoint of reducing internal resistance. The thickness of the electrolyte layer can be adjusted by, for example, the thickness of the separator described later in detail, and is usually 1 to 100 μm, preferably 5 to 50 μm. A polymer gel electrolyte or a solid polymer electrolyte matrix polymer can exhibit excellent mechanical strength by forming a cross-linked structure. In order to form a crosslinked structure, a suitable polymerization initiator is used to polymerize a polymer for forming a polymer electrolyte (for example, polyethylene oxide (PEO) or polypropylene oxide (PPO)) by thermal polymerization, ultraviolet polymerization, A polymerization treatment such as radiation polymerization or electron beam polymerization may be performed.

[Separator]
The separator 14 is not particularly limited as long as it includes the above-described non-aqueous electrolyte and can form a non-aqueous electrolyte layer. For example, a microporous film or a porous flat plate made of an inorganic material such as polyolefin such as polyethylene (PE) or polypropylene (PP) or insulating ceramics, or a non-woven fabric can be used.

[Protective layer]
The protective layer 15 satisfies at least a predetermined relationship with the above-described negative electrode active material layer, and further includes at least a material that is oxidized and reduced at a higher potential than the above-described negative electrode active material, and a polymer compound having a carboxyl group. It is necessary to be out. However, other binders described above may be included.

The substance that can be oxidized and reduced at a more noble potential than the negative electrode active material is not particularly limited as long as it can be oxidized and reduced at a relatively noble potential with respect to the applied negative electrode active material. A preferable example is lithium titanate (Li ₄ Ti ₅ O ₁₂ ).

  The polymer compound having a carboxyl group is not particularly limited as long as it has at least a function of binding the above-described substances. Preferable examples include carboxymethyl cellulose (CMC) and polyacrylic acid (PAA). These may be used alone or in combination of two or more.

  The amount of the polymer compound having a carboxyl group contained in the protective layer is preferably about the same as, for example, a general binder amount. Specifically, the amount is 0.5 to 15% by mass with respect to the protective layer. Yes, more preferably 1 to 10% by mass.

Next, an example of the manufacturing method of the lithium ion secondary battery of this embodiment mentioned above is demonstrated.
First, a positive electrode is produced. For example, when the granular positive electrode active material represented by the general formula (1) is used, the positive electrode active material and a conductive aid such as acetylene black, if necessary, a binder such as polyvinylidene fluoride (PVDF), and A positive electrode slurry is prepared by mixing with a viscosity adjusting solvent such as N-methylpyrrolidone.
Next, this positive electrode slurry is applied to a positive electrode current collector, dried, and compression molded to form a positive electrode active material layer.

  Moreover, a negative electrode is produced. For example, when a negative electrode active material such as granular graphite is used, a viscosity adjustment such as a negative electrode active material and a conductive aid such as acetylene black, a binder such as polyvinylidene fluoride (PVDF), and N-methylpyrrolidone as necessary. A negative electrode slurry is prepared by mixing with a solvent. Thereafter, this negative electrode slurry is applied to a negative electrode current collector, dried and compression molded to form a negative electrode active material layer.

Furthermore, a protective layer is provided on the negative electrode active material. For example, lithium titanate (Li ₄ Ti ₅ O ₁₂ ) as a granular material that is oxidized and reduced at a noble potential from the negative electrode active material, carboxymethyl cellulose (CMC) as a polymer compound having a carboxyl group, distilled water, and the like The protective layer slurry is prepared by mixing with a viscosity adjusting solvent. Thereafter, this protective layer slurry is applied to the negative electrode active material layer, dried, and compression molded to form a protective layer.

  Next, the positive electrode tab is attached to the positive electrode, and the negative electrode tab is attached to the negative electrode, and then the positive electrode, a separator such as polyolefin, and the negative electrode provided with a protective layer are laminated. Further, the laminated product is sandwiched between polymer-metal composite laminate sheets, and the outer peripheral edge except one side is heat-sealed to form a bag-like exterior body.

  After that, a non-aqueous electrolyte containing a lithium salt such as lithium hexafluorophosphate and an organic solvent such as ethylene carbonate (EC) is prepared, and injected into the inside from the opening of the exterior body. Is heat-sealed and sealed. Thereby, a laminate-type lithium ion secondary battery is completed.

  EXAMPLES Hereinafter, although an Example and a comparative example demonstrate this invention further in detail, this invention is not limited to these Examples.

Example 1
<Preparation of positive electrode>
Li _1.85 Ni _0.18 Co _0.10 Mn _0.87 O ₃ (in the formula (1), α = 0.40, β = 0.22, γ = 0.38, δ = 0) X = 0.3) 90 parts by mass, 5 parts by mass of acetylene black as a conductive additive, and 5 parts by mass of polyvinylidene fluoride as a binder are mixed and dispersed in N-methylpyrrolidone to form a positive electrode I got a slurry. Next, the obtained positive electrode slurry was uniformly applied to both surfaces of a positive electrode current collector made of an aluminum foil so that the thickness of each positive electrode active material layer was 30 μm and dried to obtain a positive electrode.

<Production of negative electrode>
90 parts by mass of graphite powder as a negative electrode active material, 5 parts by mass of acetylene black as a conductive additive, and 5 parts by mass of polyvinylidene fluoride as a binder are mixed and dispersed in N-methylpyrrolidone to obtain a negative electrode slurry. It was. Next, the obtained negative electrode slurry was uniformly applied to both surfaces of a negative electrode current collector made of copper foil so that the thickness of each negative electrode active material layer was 30 μm, and dried in a vacuum for 24 hours to obtain a negative electrode. Obtained.

<Formation of protective layer>
Mixing 97 parts by mass of lithium titanate (Li ₄ Ti ₅ O ₁₂ ) as a substance that is oxidized and reduced at a noble potential from the negative electrode active material and 3 parts by mass of carboxymethyl cellulose (CMC) as a polymer compound having a carboxyl group Furthermore, it was dispersed in distilled water so as to have a solid content of 10% by mass to obtain a protective layer slurry. Next, the obtained protective layer slurry was dropped so that the ratio of the protective layer to the negative electrode active material layer of the negative electrode was 0.03 by mass ratio, and then dried at 80 ° C. Furthermore, it was pressed at a surface pressure of 100 MP and dried at 80 ° C. in a vacuum to obtain a negative electrode provided with a protective layer.

<Production of battery>
The produced positive electrode and the negative electrode provided with a protective layer were opposed to each other, and a separator (material: polyolefin, thickness: 20 μm) was disposed therebetween.
Next, this negative electrode, separator, and positive electrode laminate was placed on the bottom side of a coin cell (CR2032, material: stainless steel (SUS316)).
In addition, a gasket for maintaining insulation between the positive electrode and the negative electrode is attached, the following electrolyte is injected using a syringe, a spring and a spacer are stacked, the upper side of the coin cell is overlapped, and sealed by caulking. Thus, a lithium ion secondary battery of this example was obtained.
As an electrolytic solution, lithium hexafluorophosphate as a supporting salt is mixed with an organic solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) are mixed at a ratio of EC: DEC = 1: 2 (volume ratio). (LiPF ₆₎ the concentration was used dissolved at a 1 mol / L.

(Example 2)
In forming the protective layer, the same operation as in Example 1 was repeated except that the protective layer slurry was dropped so that the ratio of the protective layer to the negative electrode active material layer of the negative electrode was 0.05 in terms of mass ratio. An example lithium ion secondary battery was obtained.

(Example 3)
In forming the protective layer, the same operation as in Example 1 was repeated except that the protective layer slurry was dropped so that the ratio of the protective layer to the negative electrode active material layer of the negative electrode was 0.13 by mass ratio. An example lithium ion secondary battery was obtained.

Example 4
In forming the protective layer, the same operation as in Example 1 was repeated except that the protective layer slurry was dropped so that the ratio of the protective layer to the negative electrode active material layer of the negative electrode was 0.20 in mass ratio. An example lithium ion secondary battery was obtained.

(Example 5)
In forming the protective layer, 97 parts by mass of lithium titanate (Li ₄ Ti ₅ O ₁₂ ) as a substance that is oxidized and reduced at a noble potential from the negative electrode active material, and 1.5 parts by mass of styrene butadiene rubber (SBR) as a binder And 1.5 parts by mass of carboxymethyl cellulose (CMC) as a polymer compound having a carboxyl group, and further dispersed in distilled water to a solid content of 10% by mass to obtain a protective layer slurry, The obtained protective layer slurry was dropped so that the ratio of the protective layer to the negative electrode active material layer of the negative electrode was 0.13 by mass ratio, dried at 80 ° C., further pressed at a surface pressure of 100 MP, and vacuum The lithium ion secondary battery of this example was obtained by repeating the same operation as in Example 1 except that it was dried at 80 ° C. to obtain a negative electrode provided with a protective layer.

(Example 6)
In forming the protective layer, 97 parts by mass of lithium titanate (Li ₄ Ti ₅ O ₁₂ ) as a substance that is oxidized and reduced at a noble potential from the negative electrode active material, and polyacrylic acid (PAA) as a polymer compound having a carboxyl group ) 3 parts by mass is mixed and further dispersed in N-methylpyrrolidone so as to have a solid content of 10% by mass to obtain a protective layer slurry, and then the obtained protective layer slurry is protected against the negative electrode active material layer of the negative electrode After dropping dropwise so that the ratio of the layer is 0.13 by mass ratio, the film is dried at 80 ° C., further pressed at a surface pressure of 100 MP, and dried at 80 ° C. in a vacuum to provide a protective layer. The lithium ion secondary battery of this example was obtained by repeating the same operation as in Example 1 except that.

(Comparative Example 1)
The same procedure as in Example 1 was repeated except that the negative electrode before forming the protective layer obtained in Example 1 was used instead of the negative electrode provided with the protective layer, and the lithium ion secondary battery of this example was manufactured. Obtained.

(Comparative Example 2)
In the formation of the protective layer, oxidation that is not a substance that is oxidized or reduced at a more noble potential than the anode active material used in place of lithium titanate (Li ₄ Ti ₅ O ₁₂ ) as a substance that is oxidized or reduced at a more noble potential than the anode active material 97 parts by mass of titanium (TiO ₂ ) and 3 parts by mass of polyvinylidene fluoride (PVDF) used instead of polyacrylic acid (PAA) as a polymer compound having a carboxyl group were mixed, and the solid content was further 10% by mass. Then, a protective layer slurry was obtained by dispersing in N-methylpyrrolidone so that the ratio of the protective layer to the negative electrode active material layer of the negative electrode was 0.13 by mass ratio. After that, it was dried at 80 ° C., further pressed at a surface pressure of 100 MP, and dried at 80 ° C. in a vacuum to obtain a negative electrode provided with a protective layer. Repeat work, to obtain a lithium ion secondary battery of the present embodiment.

(Comparative Example 4)
In forming the protective layer, 97 parts by mass of lithium titanate (Li ₄ Ti ₅ O ₁₂ ) as a substance that is oxidized and reduced at a noble potential from the negative electrode active material, and polyacrylic acid (PAA) as a polymer compound having a carboxyl group ) Was mixed with 3 parts by weight of polyvinylidene fluoride (PVDF) used instead, and further dispersed in N-methylpyrrolidone so as to have a solid content of 10% by weight to obtain a protective layer slurry, and then the obtained protection The layer slurry was dropped so that the ratio of the protective layer to the negative electrode active material layer of the negative electrode was 0.13 by mass ratio, then dried at 80 ° C., further pressed at a surface pressure of 100 MP, and in vacuum at 80 ° C. The lithium ion secondary battery of this example was obtained by repeating the same operation as in Example 1 except that the negative electrode provided with the protective layer was obtained by drying.

(Comparative Example 4)
In the formation of the protective layer, oxidation that is not a substance that is oxidized or reduced at a more noble potential than the anode active material used in place of lithium titanate (Li ₄ Ti ₅ O ₁₂ ) as a substance that is oxidized or reduced at a more noble potential than the anode active material 97 parts by mass of titanium (TiO ₂ ) and 3 parts by mass of carboxymethyl cellulose (CMC) as a polymer compound having a carboxyl group are mixed and further dispersed in N-methylpyrrolidone so as to have a solid content of 10% by mass. A protective layer slurry was obtained, and then the obtained protective layer slurry was dropped so that the ratio of the protective layer to the negative electrode active material layer of the negative electrode was 0.13 by mass ratio, followed by drying at 80 ° C. The same procedure as in Example 1 was repeated except that the negative electrode provided with a protective layer was obtained by pressing at a pressure of 100 MP and drying at 80 ° C. in a vacuum. To obtain a down secondary battery.

(Comparative Example 5)
In forming the protective layer, the same operation as in Example 1 was repeated except that the protective layer slurry was dropped so that the ratio of the protective layer to the negative electrode active material layer of the negative electrode was 0.01. An example lithium ion secondary battery was obtained.

[Performance evaluation]
The charge / discharge cycle test was performed on the lithium ion secondary batteries of the above examples, and the initial discharge capacity was measured. Specifically, in an atmosphere of 30 ° C., the battery is charged to 4.6 V by a constant current method (CC current: 0.1 C), paused for 10 minutes, and then 2 V by a constant current method (CC current: 0.1 C). Discharged until. Tables 1 and 2 show the results obtained and some of the specifications of each example.

  From Table 1, it can be seen that Examples 1 to 4 belonging to the scope of the present invention have an excellent initial discharge capacity as compared with Comparative Examples 1 to 5 outside the present invention. At the present time, it can be seen that Example 3 has the most excellent initial discharge capacity.

For example, in the same manner as in Example 3 having the most excellent initial discharge capacity, even when the ratio of the protective layer mass / the protective layer mass to the negative electrode active material layer mass is set to 0.13, the redox is performed at a higher potential than the negative electrode active material. Comparative Example 2 to Comparative Example 4 that do not have either or both of the substance to be used and the polymer compound having a carboxyl group are comparative examples in which the initial discharge capacity does not reach each of the examples and does not have a protective layer. It turns out that it is inferior to 1. Note that TiO ₂ is not a substance that oxidizes and reduces at a more noble potential than the negative electrode active material.

  For example, as in Comparative Example 5, when the ratio of the protective layer mass / the protective layer mass to the negative electrode active material layer mass is 0.01 which is less than 0.03, Even if it has both a substance that oxidizes and reduces at a more noble potential than the active material and a polymer compound having a carboxyl group, the initial discharge capacity does not reach each example, and from Comparative Example 1 that does not have a protective layer It turns out that it is inferior.

  That is, a positive electrode having a positive electrode active material layer containing a positive electrode active material that can occlude and release lithium on the current collector and that has manganese as a main constituent transition metal element; and a main constituent element of carbon or silicon on the current collector; In a lithium ion secondary battery including a negative electrode having a negative electrode active material layer containing a negative electrode active material, a non-aqueous electrolyte, and a separator, a potential higher than that of the negative electrode active material is provided between the negative electrode active material layer and the separator. And having a protective layer containing a substance that oxidizes and reduces with a polymer compound having a carboxyl group, and the ratio of the protective layer to the negative electrode active material layer is 0.03 or more in mass ratio. It can be seen that it has an initial discharge capacity.

  The present inventor presumes that the reason why such an effect is obtained is due to the following mechanism. However, the following mechanism is based solely on speculation. Therefore, it goes without saying that even if the above-described effect is obtained by a mechanism other than the following mechanism, it is included in the scope of the present invention.

  In other words, if there is a protective layer containing a substance that is oxidized and reduced at a more noble potential than the negative electrode active material between the negative electrode active material layer and the separator, the redox potential of manganese ions eluted from the positive electrode is higher than that of lithium ions. Since it is noble, in order to reach the negative electrode active material, it can be trapped by a substance that is oxidized and reduced at a noble potential more than the negative electrode active material contained in the protective layer. At this time, it is necessary for the protective layer to contain a polymer compound having a carboxyl group that is considered to assist the trapping. Furthermore, since the desired effect cannot be obtained when the amount of the protective layer is small, it is considered that the ratio of the protective layer to the negative electrode active material layer needs to be 0.03 or more by mass ratio.

  In addition, in such a lithium ion secondary battery, the ratio of the protective layer to the negative electrode active material layer is 0.03 to 0.2 in terms of mass ratio, so that an excellent initial stage can be maintained while maintaining a high energy density. It can be seen that the battery has a discharge capacity. Furthermore, by setting it as the structure whose ratio of the protective layer with respect to a negative electrode active material layer is 0.05-0.15 by mass ratio, it may have a more excellent initial discharge capacity, maintaining a high energy density. I understand.

  From Table 2, it can be seen that Examples 3, 5, and 6 belonging to the scope of the present invention have excellent initial discharge capacities as compared with Comparative Examples 2 to 4 outside the present invention.

For example, it can be seen that when lithium titanate (Li ₄ Ti ₅ O ₁₂ ) is used as a material that is oxidized and reduced at a more noble potential than the negative electrode active material, a lithium ion secondary battery having an excellent initial discharge capacity can be obtained. In addition, as the polymer compound having a carboxyl group, carboxymethyl cellulose (CMC) or polyacrylic acid (PAA), among which carboxymethyl cellulose (CMC) is used, a lithium ion secondary battery having excellent initial discharge capacity is obtained. I understand that.

  Further, the polymer compound having a carboxyl group may be contained in at least a part as in Example 5, and excellent initial discharge capacity even when used in combination with another binder such as styrene butadiene rubber (SBR). It turns out that it becomes a lithium ion secondary battery which has.

From Table 1 and Table 2, even when Li _1.85 Ni _0.18 Co _0.10 Mn _0.87 O ₃ which is an example of the positive electrode active material represented by the general formula (1) described above is used, it is excellent. It can be seen that the lithium ion secondary battery has an initial discharge capacity.

  As mentioned above, although this invention was demonstrated with some embodiment and an Example, this invention is not limited to these, A various deformation | transformation is possible within the range of the summary of this invention.

  That is, in the above-described embodiments and examples, as the lithium ion secondary battery, a laminate type battery or a button type battery has been described as an example. However, a conventionally known form / structure such as a cylindrical or square can type battery can be used. Can also be applied.

  Further, for example, the present invention can be applied not only to the above-described stacked type (flat type) battery but also to conventionally known forms and structures such as a wound type (cylindrical type) battery.

  Furthermore, for example, the present invention is not only the above-described normal (internal parallel connection type) battery but also a bipolar type (internal series connection type) when viewed in terms of an electrical connection form (electrode structure) in a lithium ion secondary battery. ) Conventionally known forms and structures such as batteries can also be applied.

  A battery element in a bipolar battery generally includes a bipolar electrode in which a negative electrode active material layer is formed on one surface of a current collector and a positive electrode active material layer is formed on the other surface, an electrolyte layer, A plurality of layers.

DESCRIPTION OF SYMBOLS 1 Lithium ion secondary battery 10 Battery element 11 Positive electrode 11A Positive electrode collector 11B Positive electrode active material layer 12 Negative electrode 12A Negative electrode collector 12B Negative electrode active material layer 13 Nonaqueous electrolyte layer 14 Separator 15 Protective layer 16 Single battery layer 21 Positive electrode tab 22 Negative electrode tab 30 Exterior body

Claims (5)

A positive electrode having a positive electrode active material layer on the current collector, the positive electrode active material containing manganese as a main constituent transition metal element capable of occluding and releasing lithium;
A negative electrode having a negative electrode active material layer containing a negative electrode active material containing carbon or silicon as a main constituent element on a current collector;
A non-aqueous electrolyte,
A separator,
Between the negative electrode active material layer and the separator, a protective layer containing a substance that is oxidized and reduced at a more noble potential than the negative electrode active material, and a polymer compound having a carboxyl group,
The ratio of the said protective layer with respect to the said negative electrode active material layer is 0.03 or more by mass ratio, The lithium ion secondary battery characterized by the above-mentioned.   2. The lithium ion secondary battery according to claim 1, wherein the ratio of the protective layer to the negative electrode active material layer is 0.03 to 0.20 in mass ratio. A positive electrode active material containing manganese as a main constituent transition metal element capable of occluding and releasing lithium is represented by the general formula (1).
Li _2-0.5x Mn _1-x M _1.5x O ₃ (1)
(In the formula (1), Li is lithium, Mn is manganese, M is Ni _α Co _β Mn _γ M ¹ _δ (Ni is nickel, Co is cobalt, Mn is manganese, M ¹ is aluminum (Al), iron (Fe ), Copper (Cu), magnesium (Mg) and titanium (Ti), at least one selected from the group consisting of α, β, γ and δ is 0 <α ≦ 0.5, 0 ≦ β ≦ 0 .33, 0 <γ ≦ 0.5, 0 ≦ δ ≦ 0.1, α + β + γ + δ = 1)), and x satisfies the relationship 0.1 ≦ x ≦ 0.5. The lithium ion secondary battery according to claim 1, wherein the lithium ion secondary battery is a positive electrode active material represented by:   The lithium ion secondary battery according to any one of claims 1 to 3, wherein the substance that is oxidized and reduced at a more noble potential than the negative electrode active material is lithium titanate.   The lithium ion secondary battery according to any one of claims 1 to 4, wherein the polymer compound having a carboxyl group is at least one of carboxymethyl cellulose and polyacrylic acid.

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