JP5135712B2 - Secondary battery - Google Patents

Description

  The present invention relates to a secondary battery such as a lithium ion secondary battery, and more particularly, to a secondary battery with improved cycle characteristics.

  In recent years, mobile devices have become more sophisticated and multifunctional, and as a result, secondary batteries used as power sources for mobile devices are also required to be smaller, lighter, and thinner. It has been demanded.

  There is a lithium ion secondary battery as a secondary battery that can meet this requirement. The battery characteristics of the lithium ion secondary battery vary greatly depending on the electrode active material used. In typical lithium ion secondary batteries currently in practical use, lithium cobaltate is used as the positive electrode active material and graphite is used as the negative electrode active material. The lithium ion secondary battery configured in this way is used. The battery capacity of is approaching the theoretical capacity, and it is difficult to increase the capacity significantly with future improvements.

  In view of this, it has been studied to use a silicon or tin alloyed with lithium at the time of charging as a negative electrode active material to realize a significantly increased capacity of a lithium ion secondary battery. However, when silicon or tin is used as the negative electrode active material, the expansion and contraction associated with charging and discharging is large, and the active material is pulverized or dropped from the current collector due to expansion and contraction associated with charging and discharging. There is a problem that the cycle characteristics deteriorate.

  On the other hand, in recent years, a negative electrode formed by laminating a negative electrode active material layer such as silicon on a negative electrode current collector by a vapor phase method or the like has been proposed (for example, JP-A-8-50922, Japanese Patent No. 2948205). Gazette and JP-A-11-135115). If it does in this way, it is supposed that a negative electrode active material layer and a negative electrode electrical power collector are integrated, and it can suppress that an active material is subdivided by the expansion / contraction accompanying charging / discharging. Moreover, the effect which improves the electronic conductivity in a negative electrode is also acquired.

  However, even in the negative electrode in which the negative electrode active material layer and the negative electrode current collector are integrated as described above, when charging and discharging are repeated, stress is applied to the interface due to severe expansion and contraction of the negative electrode active material layer, and the negative electrode active material layer becomes Cycle characteristics deteriorate due to falling off of the negative electrode current collector.

  In order to solve such problems, a battery in which impurities are doped in silicon (Japanese Patent Laid-Open No. 10-199524), a lithium ion secondary battery using an alloy or intermetallic compound of silicon and other elements ( JP 2000-243389 A, JP 10-302770 A) and the like have been proposed.

  Further, in Patent Document 1 to be described later, the negative electrode active material layer is a thin film mainly composed of silicon, and at least on the surface thereof, periodic groups 4th period, 5th period and 6th period IIIa group, IVa group, A lithium secondary battery using a thin film containing at least one element of group Va, group VIa, group VIIa, group VIII, group Ib and group IIb (excluding copper (Cu)) has been proposed. ing.

  In the thin film, the element forms a solid solution with silicon, but since it is in a non-equilibrium state, the mixing ratio of the element and silicon can be changed within a relatively wide range. As the element, cobalt, chromium, zinc, iron, zirconium, nickel and the like having a high diffusion coefficient into silicon are preferable. Since these elements do not react with lithium, if these elements are contained at least in the thin film on the surface, the charge / discharge reaction between the thin film surface and the electrolyte is suppressed, and the amount of expansion / contraction of the thin film accompanying charge / discharge decreases. It is described that the cycle characteristics are improved because the stress acting on the current collector from the thin film during charge / discharge is reduced.

JP 2003-7295 A (pages 3, 4, 6-8, Table 3)

  However, in the lithium secondary battery of Patent Document 1, since silicon forms a solid solution with the above elements, the processing characteristics of the negative electrode active material layer change greatly, and the same handling as in the past can be performed in the production of an electrode winding body and the like. There is a concern that it will disappear.

  The present invention has been made in view of such problems, and an object of the present invention is to provide a secondary battery that can obtain a large energy capacity, suppress structural breakdown of the negative electrode, and improve cycle characteristics. is there.

  As a result of intensive studies, the inventor discovered that by adding molybdenum alone or a compound thereof to the active material layer, cycle characteristics such as capacity retention ratio can be improved, and the present invention is completed. Arrived.

That is, the present invention includes a positive electrode, an electrolyte, and a negative electrode, and in the negative electrode active material layer constituting the negative electrode, as a negative electrode active material, a simple substance of silicon and a compound thereof, and a simple substance of tin and one or more of the compounds thereof In secondary batteries that contain
The present invention relates to a secondary battery, wherein molybdenum alone or a compound thereof is attached to the surface of the negative electrode active material or interposed between the negative electrode active materials.

  As described above, the present inventor has discovered that cycle characteristics can be improved by adding the molybdenum alone or a compound thereof to the negative electrode active material layer, thereby completing the present invention. As for the mechanism of action of the simple substance of molybdenum or its compound, there are still unclear points, and it is currently being studied. However, since molybdenum disulfide is generally used as a lubricant, the negative electrode By adhering to the surface of the active material or interposing between the negative electrode active materials, the stress between the active material particles generated by the expansion and contraction of the negative electrode active material accompanying charge / discharge is dispersed, and the active material particles It is conceivable to prevent the collapse of. In addition, the molybdenum simple substance or a compound thereof adheres to the surface of the negative electrode active material, so that the supply of the electrode reactant in the vicinity thereof is inhibited, and as a result, the charge / discharge reaction is suppressed, and the thin film accompanying charge / discharge is suppressed. Since the amount of expansion / contraction is reduced and the stress acting between the negative electrode active material layer and the negative electrode current collector is reduced, the cycle characteristics may be improved. Furthermore, it is conceivable that the molybdenum compound acts catalytically and affects the film composition on the negative electrode surface.

  The collapse of the active material particles destroys the electrode structure, causes peeling from the current collector, or causes a side reaction in which the electrolytic solution decomposes due to the appearance of a new surface, and therefore must be suppressed as much as possible. The addition of the molybdenum alone or a compound thereof to the negative electrode active material layer can improve cycle characteristics such as capacity retention rate by suppressing the collapse of the active material particles regardless of the mechanism of action. it can. As a result, it becomes possible to use one or more of the simple substance of silicon and a compound thereof, and the simple substance of tin and the compound thereof as the negative electrode active material, thereby realizing high capacity of the secondary battery. it can.

  In the present invention, the particle size of the molybdenum alone or its compound is preferably 4 μm or less. More preferably, the particle size of the molybdenum alone or a compound thereof is smaller than the average particle size of the negative electrode active material. Here, the average particle diameter is an average particle diameter of all particles including not only primary particles but also secondary particles formed by coalescence of primary particles.

  Further, it is preferable that the simple substance of molybdenum or a compound thereof adheres to the surface of the negative electrode active material and covers a part thereof.

  In addition, the content of the molybdenum alone or a compound thereof in the negative electrode active material layer is preferably 20% by mass or less.

  Further, molybdenum disulfide is preferably included as the molybdenum compound.

  In addition, oxygen may be contained as an element constituting the negative electrode active material layer. For example, the oxygen content in the negative electrode active material layer may be 3 atomic% or more and 45 atomic% or less.

  Further, it is preferable that a part or all of the simple substance of silicon or tin is alloyed with the current collector constituting the negative electrode.

  The surface of the current collector constituting the negative electrode on which the negative electrode active material layer is provided is preferably roughened. For example, the negative electrode current collector may have a surface roughness Ra value of 0.01 to 1 μm.

  The electrolyte solvent may include a compound in which part or all of the hydrogen atoms of the cyclic carbonate or chain carbonate are substituted with fluorine atoms.

  The compound is preferably difluoroethylene carbonate.

  Moreover, it is good that the lithium compound is contained in the positive electrode active material which comprises the said positive electrode.

  The electrolyte salt of the electrolyte may include a lithium compound having boron and fluorine as constituent elements.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  In the present embodiment, an example in which the secondary battery according to the present invention is configured as a lithium ion secondary battery will be described.

  FIG. 1 is a perspective view (a) and a sectional view (b) showing a configuration of a lithium ion secondary battery 10 based on the present embodiment. As shown in FIG. 1, the secondary battery 10 is a rectangular battery, the electrode winding body 6 is accommodated in the battery can 7, and the electrolytic solution is injected into the battery can 7. The opening of the battery can 7 is sealed with a battery lid 8. The electrode winding body 6 is formed by winding a strip-shaped negative electrode 1 and a strip-shaped positive electrode 2 facing each other with a separator (and an electrolyte layer) 3 therebetween, and winding them in the longitudinal direction. The negative electrode lead 4 drawn out from the negative electrode 1 is connected to a battery can 7, and the battery can 7 also serves as a negative electrode terminal. A positive electrode lead 5 drawn out from the positive electrode 2 is connected to a positive electrode terminal 9.

  As a material for the battery can 7 and the battery lid 8, iron, aluminum, or the like can be used. However, when the battery can 7 and the battery lid 8 made of aluminum are used, the positive electrode lead 5 is welded to the battery can 7 and the negative electrode lead 4 is connected to the terminal pin 9 in order to prevent reaction between lithium and aluminum. It is preferable that the structure be

  The negative electrode 1 includes, for example, a negative electrode current collector and a negative electrode active material layer provided on the negative electrode current collector.

  The negative electrode current collector is preferably formed of a metal material that does not form an intermetallic compound with lithium (Li). When the negative electrode current collector is a material that forms an intermetallic compound with lithium, the negative electrode current collector expands and contracts due to a reaction with lithium accompanying charge / discharge. As a result, the structure of the negative electrode current collector is broken and the current collecting property is lowered. In addition, the ability to hold the negative electrode active material layer is reduced, and the negative electrode active material layer is easily detached from the negative electrode current collector.

  Examples of the metal element that does not form an intermetallic compound with lithium include copper (Cu), nickel (Ni), titanium (Ti), iron (Fe), and chromium (Cr). Note that in this specification, the metal material includes not only a single metal element but also an alloy composed of two or more metal elements, or one or more metal elements and one or more metalloid elements.

  The negative electrode current collector is preferably made of a metal material containing a metal element that forms an alloy with the negative electrode active material layer. In this case, the adhesion improves the adhesion between the negative electrode active material layer and the negative electrode current collector, and the negative electrode active material is suppressed from being subdivided due to expansion and contraction associated with charge and discharge. This is because the negative electrode active material layer can be prevented from falling off. Moreover, the effect which improves the electronic conductivity in a negative electrode is also acquired.

  The negative electrode current collector may be a single layer or may be composed of a plurality of layers. In the case of a plurality of layers, the layer in contact with the negative electrode active material layer is preferably made of a metal material alloyed with silicon, and the other layers are preferably made of a metal material that does not form an intermetallic compound with lithium.

  The surface of the negative electrode current collector on which the negative electrode active material layer is provided is preferably roughened. For example, the surface roughness Ra value of the negative electrode current collector is preferably 0.1 μm or more. This is because the adhesion between the negative electrode active material layer and the negative electrode current collector is improved. On the other hand, the Ra value is 3.5 μm or less, more preferably 3.0 μm or less. This is because if the surface roughness is too large, the negative electrode current collector tends to crack as the negative electrode active material layer expands. The surface roughness Ra value is an arithmetic average roughness Ra specified in JIS B0601. The surface roughness Ra of the area | region in which the negative electrode active material layer is provided among negative electrode collectors should just be in said range.

  The negative electrode active material layer contains one or more of a simple substance of silicon and a compound thereof, and a simple substance of tin and a compound thereof as a negative electrode active material. Of these, silicon is particularly preferable. Silicon is excellent in the ability to alloy lithium ions to be incorporated and to re-release the alloyed lithium as lithium ions. When a lithium ion secondary battery is constructed, a large energy density can be realized. Silicon may be contained as a simple substance, an alloy, a compound, or a mixture of two or more thereof.

  And the molybdenum simple substance or its compound (henceforth abbreviate | omitting a molybdenum material) adheres to the surface of the negative electrode active material, or intervenes between negative electrode active materials.

  The negative electrode active material layer may be a coating type having a thickness of about 70 to 80 μm or a thin film type having a thickness of about 5 to 6 μm.

  In the case of the coating type, the negative electrode active material layer includes a simple substance of silicon and a compound thereof, a negative electrode active material fine particle composed of one or more of a simple substance of tin and the compound thereof, and a fine particle composed of a molybdenum material. If necessary, it is formed on the negative electrode current collector with a conductive material such as a carbon material and a binder (binder) such as polyimide or polyvinylidene fluoride.

  At this time, the experiment described in Examples described later revealed that the effect is most effective when the particle size of the molybdenum material is 4 μm or less, and more preferably smaller than the average particle size of the negative electrode active material. . At this time, it is preferable that a simple substance of molybdenum or a compound thereof adheres to the surface of the negative electrode active material and covers a part thereof.

  Further, molybdenum disulfide is preferably included as a molybdenum compound. As will be described in the examples described later, it has become clear from experiments that molybdenum disulfide is most effective.

  In addition, the content of the molybdenum material in the negative electrode active material layer is preferably 20% by mass or less. Although the effect becomes higher as the content of the molybdenum material increases, an excessive amount of the molybdenum material is not preferable because the amount of the negative electrode active material decreases and the charge / discharge capacity decreases. From the experiments described in the examples described later, it has become clear that the content should be 20% by mass or less.

  In the case of a thin film type, the negative electrode active material layer is formed by forming a negative electrode active material layer composed of a single substance of silicon and a compound thereof and a simple substance of tin and one or more of the compounds on a negative electrode current collector. On top of that, a layer made of molybdenum material is laminated.

  At this time, a part or all of the simple substance of silicon or tin is preferably alloyed with the current collector constituting the negative electrode. This is because, as described above, the adhesion between the negative electrode active material layer and the negative electrode current collector can be improved. Specifically, it is preferable that the constituent elements of the negative electrode current collector are diffused in the negative electrode active material layer, the constituent elements of the negative electrode active material layer are diffused in the negative electrode current collector, or they are mutually diffused at the interface. This is because even if the negative electrode active material layer expands and contracts due to charge and discharge, dropping from the negative electrode current collector is suppressed. In the present application, the above-described element diffusion is included in one form of alloying.

  When the negative electrode active material layer contains a simple substance of tin, a cobalt layer is preferably laminated on the tin layer, and both are alloyed by heat treatment after lamination. If it does in this way, charging / discharging efficiency will become high and cycling characteristics will improve. Although the details of this cause are unknown, it is considered that the structural stability of the tin layer when the charge / discharge reaction is repeated is improved by containing cobalt that does not react with lithium.

  In the case where the negative electrode active material layer includes a simple substance of silicon, copper, nickel, and iron are given as metal elements that do not form an intermetallic compound with lithium and alloy with silicon in the negative electrode active material layer. Among these, copper is particularly preferable because a negative electrode current collector having sufficient strength and conductivity can be obtained.

  In addition, oxygen is preferably contained as an element constituting the negative electrode active material layer. This is because oxygen can suppress expansion and contraction of the negative electrode active material layer, and can suppress a decrease in discharge capacity and swelling. At least part of oxygen contained in the negative electrode active material layer is preferably bonded to silicon, and the bonding state may be silicon monoxide, silicon dioxide, or other metastable state.

  The oxygen content in the negative electrode active material layer is preferably in the range of 3 atomic% to 45 atomic%. If the oxygen content is less than 3 atomic%, a sufficient oxygen-containing effect cannot be obtained. In addition, if the oxygen content is higher than 45 atomic%, the energy capacity of the battery is reduced, the resistance value of the negative electrode active material layer is increased, and it is swollen by local lithium insertion, and the cycle characteristics are reduced. It is because it is thought that it will do. Note that a coating formed on the surface of the negative electrode active material layer by decomposition of the electrolyte solution or the like by charge / discharge is not included in the negative electrode active material layer. Therefore, the oxygen content in the negative electrode active material layer is a numerical value calculated without including this film.

  The negative electrode active material layer is preferably formed by alternately laminating a first layer having a low oxygen content and a second layer having a higher oxygen content than the first layer. One or more layers are preferably present between the first layers. In this case, it is because the expansion | swelling and shrinkage | contraction accompanying charging / discharging can be suppressed more effectively. For example, the silicon content in the first layer is preferably 90 atomic% or more, and oxygen may or may not be contained, but the oxygen content is preferably as low as possible, More preferably, it does not contain oxygen and its content is zero. This is because a higher discharge capacity can be obtained in this case. On the other hand, the silicon content in the second layer is preferably 90 atomic% or less, and the oxygen content is preferably 10 atomic% or more. This is because structural destruction due to expansion and contraction can be more effectively suppressed. The first layer and the second layer may be laminated in the order of the first layer and the second layer from the negative electrode current collector side, but are laminated in the order of the second layer and the first layer. The surface may be the first layer or the second layer. Moreover, it is preferable that the oxygen content changes stepwise or continuously between the first layer and the second layer. This is because, if the oxygen content changes abruptly, the diffusibility of lithium ions decreases and the resistance may increase.

  Note that the negative electrode active material layer may contain one or more constituent elements other than silicon and oxygen. Examples of other elements include cobalt (Co), iron (Fe), tin (Sn), nickel (Ni), copper (Cu), manganese (Mn), zinc (Zn), indium (In), silver ( Ag), titanium (Ti), germanium (Ge), bismuth (Bi), antimony (Sb), or chromium (Cr).

  The positive electrode 2 includes, for example, a positive electrode current collector and a positive electrode active material layer provided on the positive electrode current collector.

  The positive electrode current collector is preferably made of a metal material such as aluminum, nickel, or stainless steel.

  The positive electrode active material layer includes, for example, one or more materials capable of releasing lithium ions during charging and re-occluding lithium ions during discharging, as a positive electrode active material. A conductive material such as a carbon material and a binder (binder) such as polyvinylidene fluoride are preferably included.

As a material capable of releasing and re-occluding lithium ions, for example, a lithium transition metal composite oxide composed of lithium and a transition metal element M represented by a general formula Li _x MO ₂ is preferable. This is because the lithium transition metal composite oxide can generate a high electromotive force and has a high density when a lithium ion secondary battery is configured, thereby realizing further increase in capacity of the secondary battery. Because you can. Note that M is one or more transition metal elements, and is preferably at least one of cobalt and nickel, for example. x varies depending on the state of charge (discharge state) of the battery, and is usually a value in the range of 0.05 ≦ x ≦ 1.10. Specific examples of such a lithium transition metal composite oxide include LiCoO _{2 and} LiNiO ₂ .

  When a particulate lithium transition metal composite oxide is used as the positive electrode active material, the powder may be used as it is. However, at least part of the particulate lithium transition metal composite oxide contains this lithium transition metal. A surface layer containing at least one member selected from the group consisting of oxides, halides, phosphates, and sulfates having a composition different from that of the metal composite oxide may be provided. This is because the stability can be improved and the decrease in discharge capacity can be further suppressed. In this case, the constituent element of the surface layer and the constituent element of the lithium transition metal composite oxide may diffuse to each other.

  Moreover, it is preferable that a positive electrode active material layer contains at least 1 sort (s) of the group which consists of a simple substance and a compound of a 2nd group element, a 3rd group element, or a 4th group element in a long period type periodic table. This is because the stability can be improved and the decrease in discharge capacity can be further suppressed. Examples of the Group 2 element include magnesium (Mg), calcium (Ca), and strontium (Sr), among which magnesium is preferable. Examples of the Group 3 element include scandium (Sc) and yttrium (Y). Among them, yttrium is preferable. Examples of the group 4 element include titanium and zirconium (Zr), and zirconium is particularly preferable. These elements may be dissolved in the positive electrode active material, or may exist as a simple substance or a compound at the grain boundary of the positive electrode active material.

  The separator 3 separates the negative electrode 1 and the positive electrode 2 and allows lithium ions to pass through while preventing a short circuit of current due to contact between both electrodes. As a material of the separator 3, for example, a thin film such as microporous polyethylene or polypropylene in which a large number of minute through holes are formed is preferable.

  The electrolytic solution is composed of, for example, a solvent and an electrolyte salt dissolved in the solvent, and may contain an additive as necessary.

  Examples of the solvent for the electrolytic solution include cyclic carbonates such as 1,3-dioxolane-2-one (ethylene carbonate; EC) and 4-methyl-1,3-dioxolan-2-one (propylene carbonate; PC). And nonaqueous solvents such as chain carbonates such as dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC). Any one of the solvents may be used alone, or two or more of them may be mixed and used. For example, by using a mixture of a high dielectric constant solvent such as ethylene carbonate or propylene carbonate and a low viscosity solvent such as dimethyl carbonate, diethyl carbonate, or ethyl methyl carbonate, it has high solubility in electrolyte salts and high ionic conductivity. And can be realized.

  The solvent may contain sultone. This is because the stability of the electrolytic solution is improved, and swelling of the battery due to a decomposition reaction or the like can be suppressed. As the sultone, those having an unsaturated bond in the ring are preferable, and 1,3-propene sultone shown in Chemical Formula 1 is particularly preferable. This is because a higher effect can be obtained.

  In addition, a cyclic carbonate having an unsaturated bond such as 1,3-dioxol-2-one (vinylene carbonate; VC) or 4-vinyl-1,3-dioxolan-2-one (VEC) is used as a solvent. It is preferable to use a mixture. This is because a decrease in discharge capacity can be further suppressed. In particular, it is preferable to use both VC and VEC because higher effects can be obtained.

  Further, a carbonate ester derivative having a halogen atom may be mixed and used as the solvent. This is because a decrease in discharge capacity can be suppressed. In this case, it is more preferable to use a mixture with a cyclic carbonate having an unsaturated bond. This is because a higher effect can be obtained. The carbonic acid ester derivative having a halogen atom may be either a cyclic compound or a chain compound, but the cyclic compound is preferable because a higher effect can be obtained. Such cyclic compounds include 4-fluoro-1,3-dioxolan-2-one (FEC), 4-chloro-1,3-dioxolan-2-one, 4-bromo-1,3-dioxolane- 2-one, 4,5-difluoro-1,3-dioxolane-2-one (DFEC), and the like can be mentioned. Among them, DFEC and FEC having a fluorine atom, particularly DFEC are preferable. This is because a higher effect can be obtained.

Examples of the electrolyte salt of the electrolytic solution include lithium salts such as lithium hexafluorophosphate (LiPF ₆ ) and lithium tetrafluoroborate (LiBF ₄ ). Any one electrolyte salt may be used alone, or two or more electrolyte salts may be mixed and used.

  The electrolytic solution may be used as it is, or may be retained in a polymer compound to form a so-called gel electrolyte. In that case, the electrolyte may be impregnated in the separator 3, or may exist in a layer between the separator 3 and the negative electrode 1 or the positive electrode 2. As the polymer material, for example, a polymer containing vinylidene fluoride is preferable. This is because the redox stability is high. Moreover, as a high molecular compound, what was formed by superposing | polymerizing a polymeric compound is also preferable. Examples of the polymerizable compound include monofunctional acrylates such as acrylic acid esters, monofunctional methacrylates such as methacrylic acid esters, polyfunctional acrylates such as diacrylic acid esters and triacrylic acid esters, dimethacrylic acid esters and trimethacrylic acid esters. There are polyfunctional methacrylates such as acrylonitrile, methacrylonitrile, and the like. Among them, esters having an acrylate group or a methacrylate group are preferable. This is because the polymerization proceeds easily and the reaction rate of the polymerizable compound is high.

  The lithium ion secondary battery 10 can be manufactured as follows, for example.

  First, the negative electrode active material layer is formed on the negative electrode current collector, and the negative electrode 1 is manufactured.

  In the case of forming a coating type negative electrode active material layer, for example, first, a simple substance of silicon and a compound thereof, and a negative electrode active material containing at least one of a simple substance of tin and a compound thereof are pulverized into fine particles. And this and the fine particle which consists of molybdenum materials are mixed with a electrically conductive material and a binder (binder) as needed, and a mixture is prepared. Next, this mixture is dispersed in a dispersion medium such as N-methylpyrrolidone (NMP) to form a slurry, and after applying this mixture slurry to the negative electrode current collector, the dispersion medium is evaporated and compression molded. Thus, the negative electrode 1 is produced.

  When forming a thin film type negative electrode active material layer, first, a simple substance of silicon and its compound, and a simple substance of tin are applied to the negative electrode current collector by, for example, a vapor phase method, a thermal spraying method, a firing method or a liquid phase method. And a negative electrode active material layer containing at least one of the compounds. Examples of the vapor phase method include a physical deposition method and a chemical deposition method. Specifically, a vacuum deposition method, a sputtering method, an ion plating method, a laser ablation method, a CVD method (Chemical Vapor Deposition; chemical vapor phase). Either a growth method) or a thermal spraying method may be used. Examples of the liquid phase method include plating. In addition, the negative electrode active material layer may be formed by combining two or more of these methods, and other methods.

  Next, a layer made of a molybdenum material is laminated thereon by a vacuum deposition method or an RF sputtering method.

  When oxygen is included in the negative electrode active material layer, the oxygen content is, for example, that oxygen is included in the atmosphere when forming the negative electrode active material layer, or oxygen is included in the atmosphere during firing or heat treatment. Or by adjusting the oxygen concentration of the negative electrode active material particles to be used.

  In addition, as described above, when the negative electrode active material layer is formed by alternately stacking the first layer having a low oxygen content and the second layer having a higher oxygen content than the first layer, You may make it adjust by changing the oxygen concentration in atmosphere, and after forming a 1st layer, you may make it form a 2nd layer by oxidizing the surface.

  Note that after the negative electrode active material layer is formed, heat treatment may be performed in a vacuum atmosphere or a non-oxidizing atmosphere to further alloy the interface between the negative electrode current collector and the negative electrode active material layer.

  Next, a positive electrode active material layer is formed on the positive electrode current collector. For example, a positive electrode active material and, if necessary, a conductive material and a binder (binder) are mixed to prepare a mixture, which is dispersed in a dispersion medium such as NMP to form a slurry. After applying the slurry to the positive electrode current collector, the positive electrode 2 is formed by compression molding.

  Next, the electrode winding body 6 is formed by making the negative electrode 1 and the positive electrode 2 face each other with the separator 3 interposed therebetween and winding in the longitudinal direction. At this time, the negative electrode 1 and the positive electrode 2 are disposed so that the negative electrode active material layer and the positive electrode active material layer face each other. Next, the electrode winding body 6 is inserted into a rectangular battery can 7, and the battery lid 8 is welded to the opening of the battery can 7. Next, after injecting an electrolyte from the electrolyte inlet formed in the battery lid 8, the inlet is sealed. As described above, the rectangular lithium ion secondary battery 10 is assembled.

  In addition, when the electrolytic solution is held in the polymer compound, the electrolyte is gelled by injecting the polymerizable compound together with the electrolytic solution into a container made of a packaging material such as a laminate film and polymerizing the polymerizable compound in the container. Turn into. Further, a metal can may be used as the container in order to cope with the large expansion and contraction of the electrode. Further, before winding the negative electrode 1 and the positive electrode 2, a gel electrolyte is applied to the negative electrode 1 or the positive electrode 2 by a coating method or the like, and then the negative electrode 1 and the positive electrode 2 are wound with the separator 3 interposed therebetween. You may make it turn.

  After the assembly, when the lithium ion secondary battery 10 is charged, lithium ions are released from the positive electrode 2, move to the negative electrode 1 side through the electrolytic solution, and are reduced in the negative electrode 1, and the generated lithium forms a negative electrode active material and an alloy. Formed and taken into the negative electrode 1. When discharge is performed, lithium taken into the negative electrode 1 is re-released as lithium ions, moves to the positive electrode 2 side through the electrolytic solution, and is occluded by the positive electrode 2 again.

  In this case, in the lithium ion secondary battery 10, since the negative electrode active material layer contains a simple substance of silicon and a compound thereof, and a simple substance of tin and one or more of the compounds as a negative electrode active material. The capacity of the battery can be increased. In addition, since the molybdenum material adheres to the surface of the negative electrode active material or is interposed between the negative electrode active materials, cycle characteristics such as capacity retention ratio are improved.

  Hereinafter, specific examples of the present invention will be described in detail. In the following description of the examples, the symbols and symbols used in the embodiments are used correspondingly.

Examples 1-1 to 1-12
As a negative electrode active material, a simple substance of silicon in the form of fine particles having an average particle diameter of 1 μm (silicon; Si) is used to form a coating type negative electrode active material layer. A secondary battery 10 was produced.

<Formation of negative electrode 1>
First, silicon crushed into chips (purity 99%) was pulverized to an average particle size of 1 μm using a jet mill. This silicon powder and a fine particle powder (molybdenum material) made of molybdenum alone or a compound thereof are dispersed in an NMP solution of 3% by mass of polyamic acid (a precursor of polyimide which is a binder). The slurry was applied, and this slurry was applied to a negative electrode current collector made of an electrolytic copper foil. After the solvent was evaporated and dried, roll pressing was performed and compression molding was performed. Thereafter, heat treatment was performed in vacuum at 400 ° C. for 3 hours to form a negative electrode active material layer. Then, the negative electrode lead 4 was attached and the negative electrode 1 was formed.

  In Examples 1-1 to 1-12, the slurry was prepared so that the mass ratio of silicon, polyimide (PI), and molybdenum material in the negative electrode active material after the heat treatment became the ratio described in the column of the negative electrode composition in Table 1. The composition was adjusted.

<Preparation of lithium ion secondary battery 10>
Next, lithium cobaltate (LiCoO ₂ ) as a positive electrode active material, carbon black as a conductive material, and polyvinylidene fluoride (PVdF) as a binder are mixed to prepare a mixture, and this mixture Is dispersed in NMP as a dispersion medium to form a slurry, and this mixture slurry is applied to a positive electrode current collector made of an aluminum foil, and after the dispersion medium is evaporated and dried, the positive electrode active material is formed by compression molding. A layer was formed. Then, the positive electrode lead 5 was attached and the positive electrode 2 was formed.

  Next, the negative electrode 1 and the positive electrode 2 were opposed to each other with a separator 3 made of a microporous polyethylene film having a thickness of 23 μm interposed therebetween, and wound to prepare an electrode winding body 6. Next, the electrode winding body 6 is inserted into a rectangular battery can 7, and the battery lid 8 is welded to the opening of the battery can 7. Next, after injecting an electrolyte from the electrolyte inlet formed in the battery lid 8, the inlet was sealed, and the lithium ion secondary battery 10 was assembled.

The electrolyte was LiPF ₆ as an electrolyte salt in a mixed solvent in which ethylene carbonate (EC), vinylene carbonate (VC), and diethyl carbonate (DEC) were mixed at a mass ratio of EC: VC: DEC = 20: 10: 70. Was dissolved as a standard electrolyte solution at a concentration of 1 mol / dm ³ (1M).

Comparative Example 1-1 and Comparative Examples 1-1-11 to 1-15
As a comparative example for Examples 1-1 to 1-12, in Comparative Example 1-1, a negative electrode active material layer not containing a molybdenum material was formed. A negative electrode active material layer was formed using 6 μm artificial graphite powder, and secondary batteries were fabricated in the same manner as described above.

Examples 1-21 to 1-25, and Comparative Examples 1-2 and 1-3
In order to investigate the influence of the particle size of the molybdenum material, the negative electrode active material layer was formed by changing the particle size of molybdenum disulfide in various ways, and a secondary battery was fabricated in the same manner as described above.

<Evaluation of lithium ion secondary battery>
For the fabricated secondary batteries of Examples 1-1 to 1-12 and Comparative Example 1-1, a cycle test was performed to measure the capacity retention rate. In one cycle of this cycle test, the battery is first charged with a constant current of 500 mA until the battery voltage reaches 4.2 V, then charged with a constant voltage of 4.2 V until the current value reaches 50 mA, and then 500 mA. The battery was discharged until the battery voltage became 2.5 V at a constant current of. This charge / discharge cycle is performed 100 times at room temperature, and the capacity retention rate (%) of the 100th cycle of the following formula
= (Discharge capacity at 100th cycle / Discharge capacity at 1st cycle) × 100 (%)
The capacity retention rate (%) at the 100th cycle, defined by The results are shown in Table 1.

  As shown in Table 1, in Examples 1-1 to 1-12 to which the molybdenum material was added, the capacity retention rate was improved as compared with Comparative Example 1-1 to which no additive was added. Among the molybdenum materials, the effect of improving the capacity retention rate by adding molybdenum disulfide and metal molybdenum is high.

  FIG. 2 is a graph showing the relationship between the mass fraction of molybdenum material in Examples 1-5 to 1-8, the capacity retention rate, and the discharge capacity at the 100th cycle. Although the capacity retention rate increases as the content of molybdenum material increases, the degree of improvement decreases at 5% or more. On the other hand, when the molybdenum material is added, the amount of the active material is reduced by that amount, so that the initial discharge capacity in the first cycle is reduced. As a result of examining how much the initial capacity is reduced by the addition of the molybdenum material, when the initial capacity of the additive-free Comparative Example 1-1 is set to 100%, 99% in Example 1-5 and Example 1-6 97%, 94% in Example 1-7 and 90% in Example 1-8. When the discharge capacity at the 100th cycle is compared in consideration of the decrease in the initial capacity, when the initial capacity of Comparative Example 1-1 is set to 100%, 62% in Comparative Example 1-1 and 68 in Example 1-1. %, 74% in Example 1-2, 72% in Example 1-3, and 72% in Example 1-4. As shown in FIG. 2 illustrating this, when the content of the molybdenum material exceeds 10% by mass, the capacity retention rate is improved, but the initial capacity is reduced, so that the discharge capacity at the 100th cycle is almost unchanged. do not do. When the content of the molybdenum material exceeds 20% by mass, the influence of the decrease in the initial capacity becomes significant. Therefore, the content of the molybdenum material in the negative electrode active material layer is preferably 20% by mass or less.

  About Comparative Examples 1-11 to 1-15, the cycle test was done similarly to the above and the result of having measured the capacity | capacitance maintenance factor is shown in Table 2. As shown in Table 2, when the negative electrode active material is artificial graphite powder, it can be seen that the effect of improving the capacity retention rate due to the addition of the molybdenum material is not observed.

  In addition, with respect to Examples 1-21 to 1-24, Example 1-7, and Comparative Examples 1-2 to 1-4, a cycle test was performed in the same manner as described above, and the results of measuring the capacity retention rate were shown in Table 3 and As shown in FIG.

  As shown in Table 3 and FIG. 3, when the particle size of molybdenum disulfide is 4.1 μm or less, the volume retention rate is improved as compared with Comparative Example 1-1 without addition, and 3.8 μm or less. It has been found that the effect is large when it is, and more preferably, the effect is remarkable when it is smaller than the substantial average particle diameter (1 μm) of the negative electrode active material. Considering the initial reduction in discharge capacity due to the addition of molybdenum material, the particle size of molybdenum disulfide is preferably 4 μm or less.

  Further, when the negative electrode active material layer to which the molybdenum compound was added was subjected to elemental analysis by SEM (Scanning Electron Microscope) -EDX (Energy Dispersive X-ray Fluorescence Spectrometer), it was found on the surface of the active material particles. It was confirmed that the molybdenum compound particles were present in the form of a film. On the other hand, as in Comparative Examples 1-2 to 1-4, when the particle size of the molybdenum material particles is much larger than the particle size of the negative electrode active material and there is no effect due to the addition of the molybdenum material, the molybdenum material particles are the negative electrode. It was confirmed that the existence state was scattered in the active material layer. From the above experiment, it was found that the effect is high when the particle diameter of the molybdenum material fine particles is smaller than the particle diameter of the negative electrode active material and exists on the surface in the form of a film.

Examples 2-1 and 2-2, and Comparative Example 2
First, a 6 μm-thick tin layer and a 2 μm-thick cobalt layer were laminated on the electrolytic copper foil whose surface was roughened by vacuum deposition. As described above, when a cobalt layer is laminated on a tin layer and alloyed by heat treatment after the lamination, charge / discharge efficiency is increased and cycle characteristics are improved. On top of that, a molybdenum disulfide layer was formed by RF sputtering so that the thickness was 0.2 μm or 0.5 μm in terms of a crystal oscillator thickness meter. This was heat-treated in an argon atmosphere for 12 hours at 280 ° C. to form the negative electrode 1.

  Next, a secondary battery was produced in the same manner as in Example 1, and the capacity retention rate after 100 cycles was measured. The results are shown in Table 4.

  From Table 4, it can be seen that in Examples 2-1 and 2-2 in which molybdenum disulfide layers are stacked, the capacity retention ratio is improved. In addition, as for the tin vapor deposition negative electrode with favorable cycling characteristics, as a result of carrying out the line analysis of the electrode cross section by Auger electron spectroscopy, it confirmed that the collector and the active material layer interface were alloyed.

Examples 3-1 and 3-2 and Comparative Example 3
First, on the electrolytic copper foil whose surface is roughened, the thickness of the silicon oxide (purity 99%) used in Experiment 1 is reduced by electron beam evaporation while flowing oxygen gas diluted with argon into the chamber. A 4 μm partially oxidized amorphous silicon layer was formed. As in Experiment 2, a molybdenum disulfide layer was formed thereon by RF sputtering so that the thickness was 0.2 μm or 0.4 μm in terms of a quartz oscillator thickness meter. This was heat-treated in an argon atmosphere for 12 hours at 280 ° C. to form the negative electrode 1.

  Next, a secondary battery was produced in the same manner as in Example 1, and the capacity retention rate after 100 cycles was measured. The results are shown in Table 5.

  From Table 5, it can be seen that in Examples 3-1 and 3-2 in which molybdenum disulfide layers are stacked, the capacity retention ratio is improved. In addition, as for the silicon vapor deposition negative electrode with favorable cycling characteristics, as a result of carrying out the line analysis of the electrode cross section by Auger electron spectroscopy, it confirmed that the collector and the active material layer interface were alloyed.

Examples 4-1 to 4-3
Using the negative electrode produced in Experiment 3, the composition of the electrolytic solution was changed as shown in Table 6 to produce a secondary battery, and the capacity retention rate after 100 cycles was measured. The results are shown in Table 6.

Table 6 shows that as the solvent constituting the electrolyte, 4,5-difluoro-1,3-dioxolan-2-one (DFEC) and 4-fluoro-1,3-dioxolan-2-one (FEC) having fluorine atoms are used. In particular, it can be seen that when DFEC is added, the effect of improving the capacity retention rate by adding the molybdenum material is improved. It can also be seen that when LiBF ₄ is added as an electrolyte salt constituting the electrolytic solution, the effect of improving the capacity retention rate due to the addition of the molybdenum material is improved.

  The present invention has been described with reference to the embodiments and examples. However, the present invention is not limited to the above embodiments and examples, and can be variously modified.

  For example, in the above embodiments and examples, the case where a square can is used as the exterior member has been described, but the present invention can also be applied to the case where a film-like exterior material or the like is used as the exterior member. The shape may be any shape such as a cylindrical shape, a square shape, a coin shape, a button shape, a thin shape, or a large size.

  Moreover, although the case where the electrode winding body 6 which wound the negative electrode 1 and the positive electrode 2 was provided was demonstrated, this invention applies similarly also to the laminated type which laminated | stacked the negative electrode and the positive electrode 1 layer or multiple layers. be able to.

  The secondary battery according to the present invention uses a simple substance of silicon and tin as a negative electrode active material to achieve a large energy capacity and good cycle characteristics, thereby reducing the size, weight and thickness of mobile electronic devices. Contribute and improve its convenience.

It is the perspective view (a) and sectional drawing (b) which show the structure of the lithium ion secondary battery based on embodiment of this invention. It is a graph which shows the relationship between the mass fraction of molybdenum material based on Example 1 of this invention, a capacity | capacitance maintenance factor, and the discharge capacity after 100 cycles. It is a graph which shows the relationship between the particle size of molybdenum disulfide based on Example 1 of this invention, and a capacity | capacitance maintenance factor.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Negative electrode, 2 ... Positive electrode, 3 ... Separator, 4 ... Negative electrode lead, 5 ... Positive electrode lead,
6 ... electrode winding body, 7 ... battery can, 8 ... battery lid, 9 ... positive electrode terminal,
10 ... Lithium ion secondary battery

Claims (12)

A thin-film negative electrode active material layer comprising a positive electrode, an electrolyte, and a negative electrode, and the negative electrode active material includes a simple substance of silicon and a compound thereof, and a simple substance of tin and one or more of the compounds as a negative electrode active material It has been,
In the negative electrode, a secondary battery in which the negative electrode active material layer and molybdenum alone or a compound layer thereof are laminated in this order on a negative electrode current collector. The secondary battery according to claim 1, wherein the negative electrode active material layer is formed by a vacuum deposition method, and the molybdenum alone or a compound layer thereof is formed by an RF sputtering method . The negative active material layer is formed by alloying by heat treatment after lamination cobalt layer on the tin layer, the secondary battery according to claim 1. 2. The negative electrode active material layer according to claim 1, wherein the negative electrode active material layer is formed by alloying silicon with a metal element made of copper, nickel, or iron that does not form an intermetallic compound with lithium and forms an alloy with silicon . Secondary battery.   The secondary battery according to claim 1, wherein the molybdenum compound includes molybdenum disulfide. The silicon negative electrode active material is included, this and silicon, the negative active material has at least partially bond of oxygen contained in the layer, the coupling state silicon monoxide, silicon dioxide or quasi these non Ru stable state that is secondary battery in claim 1.   The secondary battery according to claim 1, wherein a part or all of the simple substance of silicon or tin is alloyed with the negative electrode current collector.   The secondary battery according to claim 1, wherein a surface of the negative electrode current collector on which the negative electrode active material layer is provided is roughened.   The secondary battery according to claim 1, wherein the electrolyte solvent includes a compound in which part or all of the hydrogen atoms of the cyclic carbonate or chain carbonate are substituted with fluorine atoms.   The secondary battery according to claim 9, wherein the compound is difluoroethylene carbonate.   The secondary battery according to claim 1, wherein the positive electrode active material constituting the positive electrode is made of a lithium compound.   The secondary battery according to claim 11, wherein the electrolyte salt of the electrolyte includes a lithium compound having boron and fluorine as constituent elements.

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