JP2011192543A - Lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery in which the liquid injection property of a non-aqueous electrolyte is improved and thereby the productivity is improved.
SOLUTION: A lithium ion secondary battery having a positive electrode, a negative electrode, a non-aqueous electrolyte, and a separator, wherein the positive electrode is formed of a lithium-containing composite oxide on at least one surface of an aluminum foil as a current collector. A lithium ion secondary battery having a positive electrode mixture layer containing as an active material, wherein the aluminum foil constituting the positive electrode current collector has a surface wettability of 40 mN / m or more, The problem is solved.
[Selection figure] None

Description

  The present invention relates to a lithium ion secondary battery with good liquid injection properties.

  In recent years, with the reduction in size, weight, and performance of portable devices, there is an increasing demand for higher performance and improved safety of lithium ion secondary batteries that serve as power sources. Lithium-ion secondary batteries are also expanding their use in large and medium sizes such as electric cars and electric bicycles.

  Thus, since the demand for lithium ion secondary batteries is increasing, the production volume thereof continues to increase year by year, and there is a great expectation for cost reduction. In order to meet the needs of both supply increase and cost reduction in lithium ion secondary batteries, it is required to improve production efficiency.

  There are a number of factors that increase the productivity of the lithium ion secondary battery. For example, increasing the liquid injection property of the non-aqueous electrolyte is the most important improvement item. The reason for this is that since a highly volatile organic solvent is used for the non-aqueous electrolyte, the injection can be completed in a short time so that the composition of the non-aqueous electrolyte does not fluctuate during the injection process. What is required is that the non-aqueous electrolyte is uniformly and sufficiently permeated into the electrode, which greatly affects the battery performance.

  In recent lithium ion secondary batteries, for example, an electrode in which an electrode mixture layer containing an active material is formed on one or both sides of a metal foil or the like as a current collector is used. The active material in the electrode mixture layer is highly filled (the electrode mixture layer has a higher density). In particular, in the positive electrode, since an oxide containing lithium as a reaction source is used as an active material, it is necessary to insert it into the battery with higher filling. When the density of the electrode mixture layer is increased, the porosity is inevitably reduced, and accordingly, the permeability of the nonaqueous electrolytic solution in the electrode mixture layer is lowered. If the permeability of the non-aqueous electrolyte into the electrode mixture layer decreases, a great amount of time and labor will be spent in the non-aqueous electrolyte injection process at the time of battery production, and the penetration of the non-aqueous electrolyte will not be possible. If sufficient, there will be variations in the characteristics between the batteries, resulting in a decrease in yield, and there is a problem in that the productivity of the batteries decreases.

  For these reasons, studies have been made to improve the liquid injection property of the non-aqueous electrolyte during battery production and increase the productivity (for example, Patent Documents 1 and 2).

JP 2007-12391 A JP 2008-27879 A

  An object of the present invention is to provide a lithium ion secondary battery in which the injection property of a non-aqueous electrolyte is improved by means different from the conventional one, thereby improving the productivity.

  The lithium ion secondary battery of the present invention that has achieved the above object has a positive electrode, a negative electrode, a non-aqueous electrolyte, and a separator, and the positive electrode is on at least one surface of an aluminum foil that is a current collector. And a positive electrode mixture layer containing a lithium-containing composite oxide as a positive electrode active material, and the aluminum foil constituting the positive electrode current collector has a surface wettability of 40 mN / m or more. To do.

  According to the present invention, it is possible to provide a lithium ion secondary battery in which the liquid-injecting property of the non-aqueous electrolyte is improved and thereby the productivity is improved.

It is a figure which shows typically an example of the lithium ion secondary battery of this invention, (a) is the top view, (b) is the fragmentary longitudinal cross-sectional view. It is a perspective view of the lithium ion secondary battery shown in FIG.

  The positive electrode according to the lithium ion secondary battery of the present invention has a positive electrode mixture layer using a lithium-containing composite oxide as a positive electrode active material on one or both sides of an aluminum foil as a current collector. The aluminum foil serving as the positive electrode current collector has a surface wettability of 40 mN / m or more.

  The aluminum foil used for the positive electrode current collector is usually manufactured through a rolling process, but the oil used at the time of rolling tends to remain on the surface, whereby the wettability of the surface is, for example, 31 to 33 mN / m. In the positive electrode configured using the aluminum foil having low surface wettability in this way, the non-aqueous electrolyte does not easily penetrate into the positive electrode mixture layer. Therefore, when a battery is manufactured using this, the non-aqueous electrolyte is injected. (Hereinafter referred to as “injection”) is difficult, and the process takes time, and the non-aqueous electrolyte does not penetrate uniformly and sufficiently into the positive electrode mixture layer. As a result, productivity may be impaired.

  Therefore, in the present invention, the wettability of the surface of the aluminum foil serving as the positive electrode current collector is set to 40 mN / m, thereby increasing the permeability of the non-aqueous electrolyte into the positive electrode mixture layer, thereby improving the productivity of the battery. Achieved improvement. That is, in the positive electrode having a current collector of aluminum foil having a surface wettability of 40 mN / m or more, the non-aqueous electrolyte efficiently and quickly wraps around the interface between the current collector and the positive electrode mixture layer. The penetration of the non-aqueous electrolyte into the positive electrode mixture layer is promoted, and the permeability is improved. Further, in the lithium ion secondary battery of the present invention, the non-aqueous electrolyte can penetrate uniformly and sufficiently into the positive electrode mixture layer, so that the charge / discharge characteristics of the battery itself are also good.

The wettability of the surface of the aluminum foil serving as the positive electrode current collector is preferably 60 mN / m or more, whereby the density of the positive electrode mixture layer is very high (for example, 3.8 g / cm ³ or more). However, it is possible to obtain a battery in which the non-aqueous electrolyte has good liquid injection properties.

  The wettability of the surface of the aluminum foil can be measured by a Wilhelmy method using a commercially available device (for example, “CBVP-Z” manufactured by Kyowa Interface Science Co., Ltd.).

  The wettability of the surface of the aluminum foil is, for example, an organic solvent (for example, an organic solvent used in a non-aqueous electrolyte described later, specifically, cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate; dimethyl carbonate Chain carbonates such as methyl carbonate, diethyl carbonate and methyl ethyl carbonate; chain esters such as methyl propionate; cyclic esters such as γ-butyrolactone; dimethoxyethane, diethyl ether, 1,3-dioxolane, diglyme, triglyme, tetraglyme Chain ethers; Cyclic ethers such as dioxane, tetrahydrofuran and 2-methyltetrahydrofuran; Nitriles such as acetonitrile, propionitrile and methoxypropionitrile; Ethylene Sulfites such as glycol sulfite; etc.) cleaning and by annealing treatment of aluminum foil, it can be adjusted to a value of the by surface treatment of the aluminum foil (such as corona discharge treatment).

  The shape of the aluminum foil may be the same as that used for a positive electrode current collector in a conventionally known lithium ion secondary battery. For example, the thickness is preferably 10 to 30 μm. Moreover, the width | variety of aluminum foil can be 30-100 cm etc. according to the specification of the coating device which apply | coats a positive mix containing composition (after-mentioned), for example. In addition, the aluminum foil which concerns on this invention may be comprised with the aluminum (aluminum alloy) containing metal elements other than aluminum in the range which does not impair the effect of this invention.

  The positive electrode mixture layer formed on the surface of the current collector made of the aluminum foil is composed of a positive electrode mixture containing a binder and a conductive additive in addition to the positive electrode active material.

As the positive electrode active material, those conventionally used for lithium ion secondary batteries, that is, lithium-containing composite oxides capable of occluding and releasing Li (lithium) ions are used. Specifically, for example, Li _{1 + x} MO ₂ (−0.1 <x <0.1, M: Co, Ni, Mn, Al, Mg, etc. Note that the element M is a metal element other than Li and is 10 Lithium-containing transition metal oxide having a layered structure represented by the following formula: LiMn ₂ O ₄ or a part of its element substituted with another element, Limanganese oxide having a spinel structure, LiMPO ₄ (M: Co, Ni, Mn, Fe, etc.) represented by an olivine type compound can be used. Specific examples of the lithium-containing transition metal oxide having a layered structure include LiCoO ₂ and LiNi _1-x Co _xy Al _y O ₂ (0.1 ≦ x ≦ 0.3, 0.01 ≦ y ≦ 0. 2) and other oxides containing at least Co, Ni and Mn (LiMn _1/3 Ni _1/3 Co _1/3 O ₂ , LiMn _5/12 Ni _5/12 Co _1/6 O ₂ , LiNi _{3 / 5} Mn _1/5 Co _1/5 O ₂ etc.). In particular, an active material containing 40% or more of Ni is preferable because the battery has a high capacity, and O (oxygen atom) may be substituted with 1 atom% of fluorine or sulfur atom.

  As the binder related to the positive electrode mixture layer, for example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC) and the like are preferably used. Moreover, as a conductive support agent which concerns on a positive mix layer, natural or artificial graphite, carbon black, carbon fiber, a carbon nanotube etc. are used suitably, for example.

  For the positive electrode, for example, a paste-like or slurry-like positive electrode mixture-containing composition in which a positive electrode active material, a binder, and a conductive additive are dispersed in a solvent such as N-methyl-2-pyrrolidone (NMP) is prepared. The binder may be dissolved in a solvent), and this is applied to one or both sides of the current collector made of the aluminum foil, dried, and then subjected to a calendering process as necessary. . However, the manufacturing method of the positive electrode is not limited to the above method, and may be manufactured by other manufacturing methods.

  The thickness of the positive electrode mixture layer is preferably, for example, 10 to 100 μm per one side of the current collector. Moreover, as a composition of a positive mix layer, it is preferable that the quantity of a positive electrode active material is 60-95 mass%, for example, it is preferable that the quantity of a binder is 1-15 mass%, and the quantity of a conductive support agent. Is preferably 3 to 20% by mass.

  As the negative electrode according to the lithium ion secondary battery of the present invention, one in which a negative electrode mixture layer containing a negative electrode active material or a binder is formed on one side or both sides of a current collector can be used. The negative electrode active material is not particularly limited as long as it is a conventionally used lithium ion secondary battery, that is, an active material capable of occluding and releasing Li (lithium) ions. Specifically, for example, natural graphite, pyrolytic carbons, cokes, glassy carbons, fired bodies of organic polymer compounds, mesocarbon microbeads (MCMB), carbon materials such as carbon fibers, etc. Graphite materials (natural graphite, pyrolytic carbons, MCMB, carbon fiber and other graphitized carbon obtained by graphitizing at 2800 ° C. or higher) are particularly preferable.

  Moreover, the binder which concerns on a negative mix layer can use the same thing as the various binders illustrated previously as a binder for positive mix layers. Furthermore, you may add various carbon blacks, such as acetylene black, a carbon nanotube, etc. to a negative mix layer as a conductive support agent.

  In the lithium ion secondary battery of the present invention, it is preferable to use a negative electrode having an arithmetic average roughness (Ra) of 0.7 μm or more on the surface of the negative electrode mixture layer. When such a negative electrode is used, the permeability of the non-aqueous electrolyte into the negative electrode mixture layer is also improved by the embossing effect on the negative electrode surface.

  However, when an electrode with a large surface roughness is used, the convex portion of the electrode is difficult to separate in the battery, the distance between the electrode and the opposite electrode is reduced, and the capacity decreases when the battery is repeatedly charged and discharged. In extreme cases, a short circuit is expected. In addition, when an electrode with a large surface roughness is used in combination with a separator having a low puncture strength, for example, the protruding portion of the electrode serves as a trigger and the separator tears when an impact such as dropping or vibration is applied to the battery. In some cases, the electrodes may be short-circuited. Therefore, it is preferable that the surface roughness of the negative electrode mixture layer is suppressed to some extent to suppress a decrease in battery reliability. Specifically, the arithmetic average roughness (Ra) of the negative electrode mixture layer surface is preferably 1.2 μm or less. In this case, for example, the porous layer (I) and the porous layer (II) described later If a laminated separator having the above is used, a decrease in battery reliability due to the above-described reason can be sufficiently suppressed.

  In addition, the arithmetic mean roughness (Ra) of the negative electrode mixture layer surface referred to in the present specification is the arithmetic mean roughness defined in JIS B 0601, and specifically, a confocal laser microscope (manufactured by Lasertec Corporation). “Real-time scanning laser microscope 1LM-21D”) is used to measure a visual field of 1 mm × 1 mm with 512 × 512 pixels, and the numerical value is obtained by arithmetically averaging the absolute values from the average line of each point.

Examples of the negative electrode having an arithmetic average roughness (Ra) on the surface of the negative electrode mixture layer of 0.7 μm or more and 1.2 μm or less include a negative electrode using graphite whose surface is coated with a low crystalline carbon material as a negative electrode active material and the like, and more specifically, R value is the peak intensity ratio of 1360 cm ^-1 to the peak intensity of 1580 cm ^-1 in the argon ion laser Raman spectrum _(I 1360 _/ I ₁₅₈₀₎ of 0.1 to 0.5 And a negative electrode containing, as a negative electrode active material, graphitic carbon having a ₀₀₂ plane spacing d ₀₀₂ of 0.338 nm or less. This type of graphitic carbon is particularly preferably used as a negative electrode active material because it has excellent load characteristics, and particularly has excellent characteristics of charging at a low temperature of 0 ° C. or lower.

  The R value is obtained from a Raman spectrum obtained using an argon laser having a wavelength of 514.5 nm [for example, “T-5400” (Laser power: 1 mW) manufactured by Ramanaor Inc.].

The graphitic carbon whose R value and d ₀₀₂ satisfy the above values is, for example, natural graphite having d ₀₀₂ of 0.338 nm or less or graphite obtained by spherically shaping artificial graphite, and the surface thereof is an organic compound. And calcining at 800-1500 ° C. and then pulverizing and sizing through a sieve. The organic compound covering the base material includes aromatic hydrocarbons; tars or pitches obtained by polycondensation of aromatic hydrocarbons under heat and pressure; tars mainly composed of a mixture of aromatic hydrocarbons. , Pitch or asphalt; In order to coat the base material with the organic compound, a method of impregnating and kneading the base material into the organic compound can be employed. Also, the R value and d ₀₀₂ satisfy the above values by a vapor phase method in which hydrocarbon gas such as propane or acetylene is carbonized by pyrolysis and deposited on the surface of graphite having d ₀₀₂ of 0.338 nm or less. Graphite carbon can be produced.

The negative electrode may use only the graphitic carbon having the R value of 0.1 or more and 0.5 or less and d ₀₀₂ of 0.338 nm or less as the negative electrode active material. For example, the arithmetic average of the surface of the negative electrode mixture layer In order to adjust the roughness (Ra) to the above value, it may be used in combination with the other carbon materials exemplified above. However, the R value is 0.1 or more and 0.5 or _less, the following graphitic carbon _{d 002} is 0.338nm, the street, has good charging characteristics at low temperature (e.g., 0 ℃ less) Therefore, when using this graphitic carbon, it is preferable to use it in a proportion of 30% by mass or more in the total amount of the negative electrode active material from the viewpoint of ensuring such effects better. , More preferably 80% by mass or more.

  The negative electrode is prepared, for example, by preparing a negative electrode mixture-containing composition in which a negative electrode active material and a binder and, if necessary, a conductive additive are dispersed in a solvent such as N-methyl-2-pyrrolidone (NMP) or water. (However, the binder may be dissolved in a solvent), which is applied to one or both sides of the current collector, dried, and then subjected to a calendering process as necessary. However, the manufacturing method of the negative electrode is not limited to the above method, and may be manufactured by other manufacturing methods.

  The thickness of the negative electrode mixture layer is preferably 10 to 100 μm per side of the current collector per side of the current collector. Moreover, as a composition of a negative mix layer, it is preferable that the quantity of a negative electrode active material is 80-95 mass%, for example, it is preferable that the quantity of a binder is 1-20 mass%, and uses a conductive support agent. When it does, it is preferable that the quantity is 1-10 mass%.

  As the current collector for the negative electrode, a foil made of copper or nickel, a punching metal, a net, an expanded metal, or the like can be used, but a copper foil is usually used. In the negative electrode current collector, when the thickness of the entire negative electrode is reduced in order to obtain a battery having a high energy density, the upper limit of the thickness is preferably 30 μm, and the lower limit is preferably 5 μm.

  As the separator according to the lithium ion secondary battery of the present invention, a separator used in a normal lithium ion secondary battery, for example, a microporous membrane made of polyolefin such as polyethylene (PE) or polypropylene (PP) is used. Can do. The microporous film constituting the separator may be, for example, one using only PE or one using PP, or a laminate of a PE microporous film and a PP microporous film. There may be.

  The separator according to the battery of the present invention includes a porous layer (I) composed of a microporous film mainly composed of a thermoplastic resin, and a porous layer (II) mainly composed of an inorganic filler having a heat resistant temperature of 150 ° C. or higher. Is preferably used.

  The porous layer (I) relating to the laminated separator is mainly for ensuring a shutdown function, and a thermoplastic resin in which a lithium ion secondary battery is a main component of the porous layer (I) When the temperature exceeds the melting point, the thermoplastic resin related to the porous layer (I) melts and closes the pores of the separator, thereby causing a shutdown that suppresses the progress of the electrochemical reaction.

  Further, the porous layer (II) according to the multilayer separator has a function of preventing a short circuit due to direct contact between the positive electrode and the negative electrode even when the internal temperature of the lithium ion secondary battery is increased. Yes, its function is secured by an inorganic filler having a heat resistant temperature of 150 ° C. or higher. That is, when the battery becomes hot, even if the porous layer (I) shrinks, the porous layer (II) that does not easily shrink can cause the positive and negative electrodes directly when the separator is thermally contracted. It is possible to prevent a short circuit due to the contact of. Moreover, since this heat-resistant porous layer (II) acts as a skeleton of the separator, the thermal contraction of the porous layer (I), that is, the thermal contraction of the entire separator itself can be suppressed.

  Further, the porous layer (II) of the laminated separator is more porous than the microporous membrane made of polyolefin used as a separator of a conventional lithium ion secondary battery, and penetrates the non-aqueous electrolyte. Good properties. Therefore, by arranging the separator so that the porous layer (II) faces the positive electrode, combined with the action of the aluminum foil used for the positive electrode current collector, non-aqueous electrolysis to the positive electrode mixture layer The permeability of the liquid can be further improved, and the liquid injection property of the battery can be further improved.

  The thermoplastic resin as the main component of the porous layer (I) of the laminated separator has an electrical insulation property, is electrochemically stable, and has a non-aqueous electrolysis provided in the battery described in detail later. There is no particular limitation as long as it is a thermoplastic resin that is stable to a liquid or a solvent (details will be described later) used in producing a laminated separator, but polyolefins such as PE, PP, ethylene-propylene copolymer; Polyesters such as polyethylene terephthalate and copolyesters are preferred.

  In addition, it is preferable that a separator has the property (namely, shutdown function) which the hole obstruct | occludes in 80 degreeC or more (more preferably 100 degreeC or more) 170 degrees C or less (more preferably 150 degrees C or less). Therefore, the porous membrane (I) has a melting point, that is, a melting temperature measured using a differential scanning calorimeter (DSC) of 80 ° C. or higher (more preferably 100 ° C. or higher) in accordance with JIS K 7121. ) More preferably, a thermoplastic resin having a temperature of 170 ° C. or lower (more preferably 150 ° C. or lower) is used as its constituent component, which is a single-layer microporous film mainly composed of PE, or 2 of PE and PP. It is preferably a laminated microporous membrane having 5 layers laminated.

  For example, when the porous layer (I) is composed of a thermoplastic resin having a melting point of 80 ° C. or more and 150 ° C. or less such as PE and a thermoplastic resin having a melting point exceeding 150 ° C. such as PP. For example, a microporous film formed by mixing PE and a resin having a higher melting point than PE such as PP is used as a porous layer (I), or has a higher melting point than PE such as a PE layer and a PP layer. When the laminated microporous film constituted by laminating the layers made of the resin is used as the porous layer (I), the melting point of the thermoplastic resin constituting the porous layer (I) is 80 The resin (for example, PE) having a temperature of from 150 ° C. to 150 ° C. is preferably 30% by mass or more, and more preferably 50% by mass or more.

  As the microporous membrane as described above, for example, a microporous membrane composed of the above-mentioned exemplified thermoplastic resin used in a conventionally known lithium ion secondary battery or the like, that is, a solvent extraction method, a dry type Alternatively, an ion-permeable microporous film manufactured by a wet stretching method or the like can be used.

  Further, the porous layer (I) may contain a filler or the like in order to improve the strength and the like within a range that does not impair the action of imparting the shutdown function to the separator. Examples of the filler that can be used for the porous layer (I) include the same fillers that can be used for the porous layer (II) described later (an inorganic filler having a heat resistant temperature of 150 ° C. or higher).

  The particle diameter of the filler is an average particle diameter, for example, preferably 0.01 μm or more, more preferably 0.1 μm or more, preferably 10 μm or less, more preferably 1 μm or less. The average particle diameter of the filler referred to in the present specification is measured by, for example, using a laser scattering particle size distribution meter (for example, “LA-920” manufactured by HORIBA), dispersing these fine particles in a medium in which the filler is not dissolved. The same applies to the inorganic filler according to the porous layer (II) described later. ].

  By providing the porous layer (I) having the above-described configuration, it becomes easy to provide a shutdown function to the separator, and it is possible to easily ensure safety when the internal temperature of the battery rises.

  The content of the thermoplastic resin in the porous layer (I) is preferably as follows, for example, in order to make it easier to obtain the shutdown effect. Since the thermoplastic resin is the main component of the porous layer (I), it is 50% by volume or more, more preferably 70% by volume or more, in the total volume of all the components of the porous layer (I), It may be 100% by volume. Furthermore, the porosity of the porous layer (II) obtained by the method described later is 20 to 60%, and the volume of the thermoplastic resin is 50% or more of the pore volume of the porous layer (II). Preferably there is.

  The inorganic filler related to the porous layer (II) has a heat-resistant temperature of 150 ° C. or higher, is stable to the non-aqueous electrolyte of the battery, and is electrochemically stable to be hardly oxidized or reduced in the battery operating voltage range. However, fine particles are preferable from the viewpoint of dispersion, and alumina, silica, and boehmite are preferable. Alumina, silica, and boehmite have high oxidation resistance, and the particle size and shape can be adjusted to the desired numerical values, making it easy to accurately control the porosity of the porous layer (II). It becomes. In addition, as for the inorganic filler whose heat-resistant temperature is 150 degreeC or more, the thing of the said illustration may be used individually by 1 type, and may use 2 or more types together, for example.

  The shape of the inorganic filler having a heat resistant temperature of 150 ° C. or higher related to the porous layer (II) is not particularly limited, and is substantially spherical (including true spherical), substantially elliptical (including elliptical), plate-like, etc. Various shapes can be used.

  In addition, if the average particle size of the inorganic filler having a heat resistance temperature of 150 ° C. or higher (the average particle size of the plate-like filler and other shape fillers; the same applies hereinafter) of the porous layer (II) is too small, the ion permeability is high. Since it falls, it is preferable that it is 0.3 micrometer or more, and it is more preferable that it is 0.5 micrometer or more. In addition, if the inorganic filler having a heat resistant temperature of 150 ° C. or higher is too large, the electrical characteristics are likely to be deteriorated. Therefore, the average particle diameter is preferably 5 μm or less, and more preferably 2 μm or less.

  Since the inorganic filler having a heat resistant temperature of 150 ° C. or higher in the porous layer (II) is mainly contained in the porous layer (II), the amount in the porous layer (II) Is 50% by volume or more, preferably 70% by volume or more, more preferably 80% by volume or more, and still more preferably 90% by volume or more. By making the inorganic filler in the porous layer (II) high as described above, the thermal contraction of the entire separator can be satisfactorily suppressed even when the lithium ion secondary battery becomes high temperature. The occurrence of a short circuit due to direct contact between the positive electrode and the negative electrode can be better suppressed.

  In addition, the porous layer (II) binds inorganic fillers having a heat-resistant temperature of 150 ° C. or higher, or binds the porous layer (I) and the porous layer (II) as necessary. In view of this, it is preferable to contain an organic binder. From such a viewpoint, the preferred upper limit of the amount of the inorganic filler having a heat resistant temperature of 150 ° C. or higher in the porous layer (II) is, for example, that of the porous layer (II) It is 99.5 volume% in the whole volume of a structural component. If the amount of the inorganic filler having a heat resistant temperature of 150 ° C. or higher in the porous layer (II) is less than 70% by volume, for example, it is necessary to increase the amount of the organic binder in the porous layer (II). In some cases, the pores of the porous layer (II) are easily filled with an organic binder, and the function as a separator may be lowered. In addition, when the porous layer is made porous by using a pore-opening agent, the filler There is a concern that the effect of suppressing heat shrinkage may be reduced due to an excessive increase in the distance between each other.

  The porous layer (II) preferably contains an organic binder in order to ensure the shape stability of the separator and to integrate the porous layer (II) and the porous layer (I). Examples of organic binders include ethylene-vinyl acetate copolymers (EVA, those having a structural unit derived from vinyl acetate of 20 to 35 mol%), ethylene-acrylic acid copolymers such as ethylene-ethyl acrylate copolymers, and fluorine-based binders. Rubber, styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), hydroxyethyl cellulose (HEC), polyvinyl alcohol (PVA), polyvinyl butyral (PVB), polyvinyl pyrrolidone (PVP), cross-linked acrylic resin, polyurethane, epoxy resin, etc. In particular, a heat-resistant binder having a heat-resistant temperature of 150 ° C. or higher is preferably used. As the organic binder, those exemplified above may be used singly or in combination of two or more.

  Among the organic binders exemplified above, highly flexible binders such as EVA, ethylene-acrylic acid copolymer, fluorine-based rubber, and SBR are preferable. Specific examples of such highly flexible organic binders include Mitsui DuPont Polychemical's “Evaflex Series (EVA)”, Nihon Unicar's EVA, Mitsui DuPont Polychemical's “Evaflex-EAA Series (Ethylene). -Acrylic acid copolymer) ", Nippon Unicar EEA, Daikin Industries" DAI-EL Latex Series (Fluororubber) ", JSR" TRD-2001 (SBR) ", Nippon Zeon" BM-400B " (SBR) ".

  When the organic binder is used for the porous layer (II), it can be used in the form of an emulsion dissolved or dispersed in the solvent for the composition for forming the porous layer (II) described later. Good.

  For example, the laminated separator includes a porous layer (II) -forming composition (such as a liquid composition such as a slurry) containing an inorganic filler having a heat resistant temperature of 150 ° C. or higher, and a porous layer (I). It can be manufactured by coating the surface of a microporous membrane for construction and drying to a predetermined temperature to form a porous layer (II).

  The composition for forming the porous layer (II) contains an inorganic binder having a heat-resistant temperature of 150 ° C. or higher and, if necessary, an organic binder and the like, and these are dispersed in a solvent (including a dispersion medium, the same applies hereinafter). It has been made. The organic binder can be dissolved in a solvent. The solvent used in the composition for forming the porous layer (II) is not particularly limited as long as it can uniformly disperse the inorganic filler and can uniformly dissolve or disperse the organic binder. Common organic solvents such as hydrocarbons, furans such as tetrahydrofuran, and ketones such as methyl ethyl ketone and methyl isobutyl ketone are preferably used. In addition, for the purpose of controlling the interfacial tension, alcohols (ethylene glycol, propylene glycol, etc.) or various propylene oxide glycol ethers such as monomethyl acetate may be appropriately added to these solvents. In addition, when the organic binder is water-soluble or used as an emulsion, water may be used as a solvent. In this case, alcohols (methyl alcohol, ethyl alcohol, isopropyl alcohol, ethylene glycol, etc.) are appropriately added. It is also possible to control the interfacial tension.

  The composition for forming the porous layer (II) preferably has a solid content containing, for example, an inorganic filler having a heat resistant temperature of 150 ° C. or higher and an organic binder, for example, 10 to 80% by mass.

  In addition, the porous layer (II) forming composition is applied onto a substrate such as a film or a metal foil, dried at a predetermined temperature, and then peeled off from the substrate as necessary to form the porous layer (II). A laminated separator can also be produced by forming a porous film and laminating and integrating the porous film and the microporous film for forming the porous layer (I). In this case, in order to integrate the microporous membrane for forming the porous layer (I) and the porous membrane to be the porous layer (II), for example, the porous layer (I) and the porous layer ( It is possible to use a method of superimposing II) and pasting them together using a roll press or the like.

  In the laminated separator, the porous layer (I) and the porous layer (II) do not have to be one each, and a plurality of layers may be present in the separator. For example, a configuration in which the porous layer (I) is disposed on both sides of the porous layer (II) or a configuration in which the porous layer (II) is disposed on both sides of the porous layer (I) may be employed. However, increasing the number of layers may increase the thickness of the separator and increase the internal resistance of the battery or decrease the energy density. Therefore, it is not preferable to increase the number of layers. The total number of the porous layers (I) and (II) is preferably 5 or less.

  The thickness of the separator according to the battery of the present invention (a separator made of a microporous membrane made of polyolefin or the laminated separator) is preferably 10 to 30 μm, for example.

  In the laminated separator, the thickness of the porous layer (II) [when the separator has a plurality of porous layers (II), the total thickness] is determined by each of the functions of the porous layer (II). From the viewpoint of exhibiting more effectively, it is preferably 3 μm or more. However, if the porous layer (II) is too thick, the energy density of the battery may be lowered. Therefore, the thickness of the porous layer (II) is preferably 8 μm or less.

  Further, in the laminated separator, the thickness of the porous layer (I) [when the separator has a plurality of porous layers (I), the total thickness thereof. same as below. ] Is preferably 6 μm or more, and more preferably 10 μm or more, from the viewpoint of more effectively exerting the above-described action (particularly shutdown action) by using the porous layer (I). However, if the porous layer (I) is too thick, there is a possibility that the energy density of the battery may be lowered. In addition, the force that the porous layer (I) tends to shrink is increased, and the heat of the entire separator is increased. There is a possibility that the action of suppressing the shrinkage becomes small. Therefore, the thickness of the porous layer (I) is preferably 25 μm or less, more preferably 20 μm or less, and further preferably 14 μm or less.

The porosity of the separator as a whole is preferably 30% or more in a dried state in order to secure the amount of electrolyte solution retained and to improve ion permeability. On the other hand, from the viewpoint of securing separator strength and preventing internal short circuit, the separator porosity is preferably 70% or less in a dry state. The porosity of the separator: P (%) can be calculated by obtaining the sum of each component i from the thickness of the separator, the mass per area, and the density of the constituent components using the following equation (1).
P = 100− (Σa _i / ρ _i ) × (m / t) (1)
Here, in the above formula, a _i : ratio of component i expressed by mass%, ρ _i : density of component i (g / cm ³ ), m: mass per unit area of separator (g / cm ² ), t: The thickness (cm) of the separator.

In the case of the multilayer separator, in the formula (1), m is the mass per unit area (g / cm ² ) of the porous layer (I), and t is the thickness of the porous layer (I) ( cm), the porosity: P (%) of the porous layer (I) can also be obtained using the formula (1). The porosity of the porous layer (I) determined by this method is preferably 30 to 70%.

Further, in the case of the multilayer separator, in the formula (1), m is the mass per unit area (g / cm ² ) of the porous layer (II), and t is the thickness of the porous layer (II) ( cm), the porosity: P (%) of the porous layer (II) can also be obtained using the formula (1). The porosity of the porous layer (II) obtained by this method is preferably 20 to 60%.

  The positive electrode, the negative electrode, and the separator are formed in the form of a laminated electrode body in which a separator is interposed between the positive electrode and the negative electrode, or a wound electrode body in which the separator is wound in a spiral shape. It can be used for the battery of the invention.

  In the multilayer electrode body and the wound electrode body, when the multilayer separator is used, as described above, the permeability of the non-aqueous electrolyte into the positive electrode mixture layer is further improved, and the battery injection property is improved. Therefore, it is preferable that the porous layer (II) is disposed so as to face at least the positive electrode. In this case, the porous layer (II), which mainly contains an inorganic filler having a heat-resistant temperature of 150 ° C. or more, and more excellent in oxidation resistance, faces the positive electrode, so that the oxidation of the separator by the positive electrode can be suppressed more favorably. Therefore, the storage characteristics and charge / discharge cycle characteristics of the battery at a high temperature can be improved.

  In addition, from the viewpoint of enhancing the permeability of the nonaqueous electrolyte solution to the negative electrode mixture layer, a separator having a porous layer (II) on both sides of the porous layer (I) is used, and the porous layer (II) It is preferable to face not only the positive electrode but also the negative electrode.

  On the other hand, when one surface of the multi-layer separator is the porous layer (I), it is preferable that the porous layer (I) faces the negative electrode. The thermoplastic resin melted from the porous layer (I) is suppressed from being absorbed into the electrode mixture layer, and can be efficiently used to close the pores of the separator.

As the non-aqueous electrolyte solution according to the lithium ion secondary battery of the present invention, a solution in which a lithium salt is dissolved in an organic solvent is used. The lithium salt is not particularly limited as long as it dissociates in a solvent to form Li ⁺ ions and hardly causes side reactions such as decomposition in a voltage range used as a battery. For example, LiClO ₄ , LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiSbF ₆ and other inorganic lithium salts, LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , Li ₂ C ₂ F ₄ (SO ₃ ) ₂ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , LiC _n F _{2n + 1} SO ₃ (n ≧ 2), LiN (RfOSO ₂ ) ₂ [where Rf is a fluoroalkyl group] and the like can be used. .

  The organic solvent used for the non-aqueous electrolyte is not particularly limited as long as it dissolves the lithium salt and does not cause a side reaction such as decomposition in a voltage range used as a battery. For example, cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate; chain carbonates such as dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate; chain esters such as methyl propionate; cyclic esters such as γ-butyrolactone; Chain ethers such as dimethoxyethane, diethyl ether, 1,3-dioxolane, diglyme, triglyme and tetraglyme; cyclic ethers such as dioxane, tetrahydrofuran and 2-methyltetrahydrofuran; nitriles such as acetonitrile, propionitrile and methoxypropionitrile Sulfites such as ethylene glycol sulfite, etc., and these should be used as a mixture of two or more. It can be. In order to obtain a battery with better characteristics, it is desirable to use a combination that can obtain high conductivity, such as a mixed solvent of ethylene carbonate and chain carbonate. In addition, for the purpose of improving safety such as improvement of charge / discharge cycle characteristics, high-temperature storage properties and prevention of overcharge, these non-aqueous electrolytes may be used in acid anhydrides, sulfonic acid esters, dinitriles, vinylene carbonates, 1,3- Additives (including these derivatives) such as propane sultone, diphenyl disulfide, cyclohexylbenzene, biphenyl, fluorobenzene, and t-butylbenzene may be added as appropriate.

  The concentration of the lithium salt in the electrolytic solution is preferably 0.5 to 1.5 mol / l, and more preferably 0.9 to 1.25 mol / l.

  Examples of the form of the lithium ion secondary battery of the present invention include a cylindrical shape (such as a rectangular tube shape or a cylindrical shape) using a steel can or an aluminum can as an outer can. Moreover, it can also be set as the soft package battery which used the laminated film which vapor-deposited the metal as an exterior body.

  The lithium ion secondary battery of this invention can be used for the same use as the various uses to which the conventionally known lithium ion secondary battery is applied.

  Hereinafter, the present invention will be described in detail based on examples. However, the following examples do not limit the present invention. In addition, the wettability of the surface of the aluminum foil in a present Example is the value measured using Kyowa Interface Science company "CBVP-Z" by the Wilhelmy method.

Example 1
<Preparation of positive electrode>
LiCoO _{2 as a} positive electrode active material: 70 parts by mass and LiNi _0.8 Co _0.2 O ₂ : 15 parts by mass, acetylene black as a conductive auxiliary agent: 10 parts by mass, and PVDF as a binder: 5 parts by mass, NMP was mixed uniformly as a solvent to prepare a positive electrode mixture-containing paste. The positive electrode mixture-containing paste is intermittently applied to both sides of an aluminum foil (thickness: 15 μm) having a surface wettability of 40 mN / m, dried, and then subjected to a calendering process so that the total thickness is 150 μm. The thickness of the agent layer was adjusted and cut to a width of 43 mm to produce a positive electrode. Further, a tab was welded to the exposed portion of the aluminum foil of the positive electrode to form a lead portion.

<Production of negative electrode>
Graphite (graphite A) having an average particle diameter D _50% of 16 μm, d ₀₀₂ of 0.3360 nm and an R value of 0.05, an average particle diameter D of _50% of 18 μm, d ₀₀₂ of 0.3380 nm, an R value Is a mixture of graphite carbon (graphite B) having a specific surface area of 3.2 m ² / g at a mass ratio of 30:70: 98 parts by mass, and a viscosity of 1500 to 5000 mPa · s. CMC aqueous solution having a concentration of 1% by mass adjusted to the range of: 1.0 part by mass and SBR: 1.0 part by mass of ion-exchanged water having a specific conductivity of 2.0 × 10 ⁵ Ω / cm or more. Mixing as a solvent, an aqueous negative electrode mixture-containing paste was prepared.

  The negative electrode mixture-containing paste is intermittently applied to both sides of a 10 μm-thick current collector made of copper foil, dried, and then calendered to give a total thickness of 142 μm. Was adjusted to a width of 45 mm to produce a negative electrode. Further, a tab was welded to the exposed portion of the copper foil of the negative electrode to form a lead portion.

  In addition, the arithmetic mean roughness (Ra) of the negative electrode mixture layer surface of the negative electrode measured using a confocal laser microscope was 1.2 μm.

<Preparation of separator>
Add 5 kg of ion-exchanged water and 0.5 kg of a dispersant (aqueous polycarboxylic acid ammonium salt, solid content concentration 40 mass%) to 5 kg of boehmite with an average particle diameter D of _50% of 1 μm. Dispersion was prepared by crushing for 10 hours with a ball mill at times / minute. The treated dispersion was vacuum-dried at 120 ° C. and observed with a scanning electron microscope (SEM).

  To 500 g of the above dispersion, 0.5 g of xanthan gum as a thickener and 17 g of a resin binder dispersion (modified polybutyl acrylate, solid content 45% by mass) as a binder are added and stirred with a three-one motor for 3 hours to form a uniform slurry. [Slurry for forming porous layer (II), solid content ratio 50 mass%] was prepared.

PE microporous separator for lithium ion secondary battery [Porous layer (I): thickness 16 μm, porosity 40%, average pore size 0.08 μm, PE melting point 135 ° C.] on one side corona discharge treatment (discharge amount) 40 W · min / m ² ), and a porous layer (II) forming slurry is applied to the treated surface by a micro gravure coater and dried to form a porous layer (II) having a thickness of 4 μm. A mold separator was obtained. The mass per unit area of the porous layer (II) in this separator was 5.5 g / m ² , the boehmite volume content was 95% by volume, and the porosity was 45%.

<Battery assembly>
The positive electrode and the negative electrode obtained as described above were stacked with the separator porous layer (II) facing the positive electrode and wound in a spiral shape to produce a wound electrode body. The obtained wound electrode body was crushed into a flat shape, put into an aluminum outer can having a thickness of 6 mm, a height of 50 mm, and a width of 34 mm, and a non-aqueous electrolyte (ethylene carbonate and ethyl methyl carbonate in a volume ratio of 1: 2). LiPF ₆ was dissolved at a concentration of 1.2 mol / l in the solvent mixed in the above and further 4% by mass of cyclohexylbenzene was added). Note that the amount of the non-aqueous electrolyte injected into the outer can was 3.2 g. A series of operations to inject a portion of the non-aqueous electrolyte into the outer can in an amount that does not overflow, decompress the inside of the outer can, and release the pressure when the injected non-aqueous electrolyte penetrates into the wound electrode body. Was repeated as one cycle, and the time from the start of injection of the non-aqueous electrolyte to the end of injection of the entire amount (hereinafter referred to as “injection time”) was measured.

  After injecting the non-aqueous electrolyte, the outer can was sealed to produce a lithium ion secondary battery having the structure shown in FIG. 1 and the appearance shown in FIG. This battery includes a cleavage vent for lowering the pressure when the internal pressure rises at the top of the can.

  Here, the battery shown in FIGS. 1 and 2 will be described. FIG. 1A is a plan view, and FIG. 1B is a partial cross-sectional view thereof. As shown in FIG. 2 is spirally wound through the separator 3 as described above, and then pressed so as to be flattened and accommodated in a rectangular tube-shaped outer can 4 together with the electrolyte as a flat wound electrode body 6 Has been. However, in FIG. 1, in order to avoid complication, a metal foil, an electrolytic solution, and the like as a current collector used for manufacturing the positive electrode 1 and the negative electrode 2 are not illustrated. Also, the separator layers are not shown separately.

  The outer can 6 is made of an aluminum alloy and constitutes an outer casing of the battery. The outer can 4 also serves as a positive electrode terminal. And the insulator 5 which consists of PE sheets is arrange | positioned at the bottom part of the armored can 4, and it connects to each one end of the positive electrode 1 and the negative electrode 2 from the flat wound electrode body 6 which consists of the positive electrode 1, the negative electrode 2, and the separator 3. The positive electrode lead body 7 and the negative electrode lead body 8 thus drawn are drawn out. A stainless steel terminal 11 is attached to a sealing lid plate 9 made of aluminum alloy for sealing the opening of the outer can 4 through a PP insulating packing 10, and an insulator 12 is attached to the terminal 11. A stainless steel lead plate 13 is attached.

  And this cover plate 9 is inserted in the opening part of the armored can 4, and the opening part of the armored can 4 is sealed by welding the junction part of both, and the inside of a battery is sealed. Further, in the battery of FIG. 1, a non-aqueous electrolyte inlet 14 is provided in the cover plate 9, and a sealing member is inserted into the non-aqueous electrolyte inlet 14, for example, laser welding or the like. (See FIG. 1 and FIG. 2, in practice, the non-aqueous electrolyte inlet 14 is actually sealed with the non-aqueous electrolyte inlet.) Although it is a member, for ease of explanation, it is shown as a non-aqueous electrolyte inlet 14). Further, the lid plate 9 is provided with a cleavage vent 15 as a mechanism for discharging the internal gas to the outside when the temperature of the battery rises.

  In the battery of this Example 1, the outer can 5 and the cover plate 9 function as a positive electrode terminal by directly welding the positive electrode lead body 7 to the lid plate 9, and the negative electrode lead body 8 is welded to the lead plate 13, The terminal 11 functions as a negative electrode terminal by conducting the negative electrode lead body 8 and the terminal 11 through the lead plate 13, but depending on the material of the outer can 4, the sign may be reversed. There is also.

  FIG. 2 is a perspective view schematically showing the external appearance of the battery shown in FIG. 1. FIG. 2 is shown for the purpose of showing that the battery is a square battery. FIG. 1 schematically shows a battery, and only specific members of the battery are shown. Also in FIG. 1, the inner peripheral portion of the electrode group is not cross-sectional.

Examples 2-8
The aluminum foil used for the current collector of the positive electrode has the surface wettability as shown in Table 1, and the mixing ratio (mass ratio) of graphite A and graphite B used for the negative electrode active material is shown in Table 1. A lithium ion secondary battery was produced in the same manner as in Example 1 except for the above.

Example 9
In the same manner as in Example 1, except that the same microporous membrane separator made of PE as that used for the laminated separator in Example 1 was used as the separator without forming the porous layer (II). A secondary battery was produced.

Comparative Example 1
The aluminum foil used for the current collector of the positive electrode has the surface wettability shown in Table 1, and the separator is the same as that used in Example 1 for the porous layer (I) of the multilayer separator. A lithium ion secondary battery was produced in the same manner as in Example 1 except that the microporous film made of PE was used.

<Measurement of charge current at -5 ° C and 10% charge>
The lithium ion secondary batteries of Examples 1 to 9 and Comparative Example 1 were allowed to stand in a thermostatic bath at −5 ° C. for 5 hours, and then each battery was subjected to a constant current of 1200 mA (1.0 C) up to 4.2 V. After reaching 4.2V, constant voltage charging was performed at 4.2V, and the current value was measured when the charging depth (the ratio of the actually charged capacity to the standard capacity) reached 10%. These results are also shown in Table 1.

  Table 1 also shows the arithmetic average roughness (Ra) of the surface of the negative electrode mixture layer measured for the negative electrode of each battery in the same manner as the negative electrode for the battery of Example 1.

  As shown in Table 1, the lithium ion secondary batteries of Examples 1 to 9 configured by using an aluminum foil having a surface wettability at a suitable value as the positive electrode current collector were prepared using an aluminum foil having a low surface wettability. Compared to the battery of Comparative Example 1 used for the positive electrode current collector, it can be seen that the time for injecting the non-aqueous electrolyte at the time of manufacture is very short, and the productivity is high.

1 Positive electrode 2 Negative electrode 3 Separator

Claims (5)

A lithium ion secondary battery having a positive electrode, a negative electrode, a non-aqueous electrolyte, and a separator,
The positive electrode has a positive electrode mixture layer containing a lithium-containing composite oxide as a positive electrode active material on at least one surface of an aluminum foil as a current collector,
The aluminum foil constituting the positive electrode current collector has a surface wettability of 40 mN / m or more, and is a lithium ion secondary battery.   2. A separator having a porous layer (I) composed of a microporous film mainly composed of a thermoplastic resin and a porous layer (II) mainly composed of an inorganic filler having a heat resistant temperature of 150 ° C. or higher. The lithium ion secondary battery described in 1.   The lithium ion secondary battery according to claim 2, wherein the porous layer (II) faces at least the positive electrode.   The negative electrode has a negative electrode mixture layer on at least one surface of a current collector, and the arithmetic average roughness (Ra) of the surface of the negative electrode mixture layer is 0.7 to 1.2 µm. The lithium ion ion secondary battery in any one of -3. The negative electrode mixture layer is, with R-values 0.1 to 0.5 is the peak intensity ratio of 1360 cm ^-1 to the peak intensity of 1580 cm ^-1 in the argon ion laser Raman spectrum, the surface spacing _{d 002} of the 002 plane is 0. The lithium ion secondary battery in any one of Claims 1-4 which contains the graphite which is 338 nm or less as a negative electrode active material.

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