JP2015050035A - Positive electrode for nonaqueous electrolyte secondary batteries, and nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery which has a flat wound electrode body, a high capacity, and good charge and discharge cycle characteristics, and enables the achievement of good productivity; and a positive electrode allows such a nonaqueous electrolyte secondary battery to be arranged.SOLUTION: A positive electrode for nonaqueous electrolyte secondary batteries is to be used for a nonaqueous electrolyte secondary battery having a wound electrode body arranged by superposing a positive electrode, a negative electrode and a separator on one another, and then winding them together spirally, followed by flattening to have a compressed form in section, and a nonaqueous electrolyte. The positive electrode comprises: a current collector made of metal; and a positive electrode mixture layer formed on each side of the current collector, and including a positive electrode active material, a conductive assistant and a binding agent. The current collector has a thickness of 11 μm or less, and a tensile strength of 2.5 N/mm or larger. The positive electrode mixture layer includes: vinylidene fluoride-chlorotrifluoroethylene copolymer as the binding agent; and graphite as the conductive assistant. A nonaqueous electrolyte secondary battery comprises a flat wound electrode body having the positive electrode.

Description

  The present invention includes a nonaqueous electrolyte secondary battery having a flat wound electrode body, high capacity, good charge / discharge cycle characteristics and good productivity, and a positive electrode capable of constituting the nonaqueous electrolyte secondary battery It is about.

  In recent years, with the development of portable electronic devices such as mobile phones and laptop computers, and the practical application of electric vehicles, small and light non-aqueous electrolyte secondary batteries have become necessary. .

  As such a non-aqueous electrolyte secondary battery that is reduced in size and weight, for example, a positive electrode and a negative electrode are overlapped with a separator interposed therebetween and wound into a spiral shape, and the cross section becomes flattened. The thing of the structure where the shape | molded flat wound electrode body was accommodated in the thin exterior body like the laminated film exterior body comprised with a square (square cylinder shape) exterior can and a metal laminate film is mentioned.

  However, in the flat wound electrode body as described above, in the curved portion (particularly, the innermost curved portion), cracks in the positive electrode mixture layer (mixture layer containing the positive electrode active material) or current collector As a result, there is a risk that the production efficiency of the battery may be reduced because a large number of manufactured batteries include those with low reliability due to the above-described cracking or breaking.

  Under such circumstances, a technique for suppressing cracking of the positive electrode mixture layer in the flat wound electrode body has been developed. In Patent Document 1, a fluorine atom-containing polymer material formed from a monomer such as vinylidene fluoride or chlorotrifluoroethylene is used as a binder for a mixture layer, and the elastic modulus of the mixture layer is set to a specific value. In addition, there has been proposed a technique for increasing the flexibility of the positive electrode by setting the tensile strength of the current collector to a specific value.

  In Patent Document 2, by adjusting the tensile elastic modulus of the binder contained in the positive electrode mixture layer and the volume ratio of the binder in the positive electrode mixture layer to have a specific relationship, It has been shown that a positive electrode capable of improving the reliability, productivity, and load characteristics of a non-aqueous electrolyte secondary battery while suppressing the occurrence of the cracks is shown.

JP 2005-56743 A JP 2012-28158 A

  However, the demand for higher capacity of non-aqueous electrolyte secondary batteries is expected to continue in the future. For example, there is a problem due to defects in the positive electrode in the flat wound electrode body, such as increasing the density of the positive electrode mixture layer. It is also predicted that it will respond to such a request in a manner that makes it easier to express. Along with this, development of a technique for suppressing the defects of the positive electrode that may occur in the flat wound electrode body to a higher degree is also required.

  The present invention has been made in view of the above circumstances, and the object thereof is a non-aqueous electrolyte secondary battery having a flat wound electrode body, high capacity, good charge / discharge cycle characteristics and good productivity. An object of the present invention is to provide a battery and a positive electrode that can constitute the nonaqueous electrolyte secondary battery.

  The positive electrode for a non-aqueous electrolyte secondary battery of the present invention that can achieve the above object is a spirally wound electrode body in which a positive electrode, a negative electrode, and a separator are overlapped and wound in a spiral shape, and a non-aqueous electrolyte. A positive electrode used for a non-aqueous electrolyte secondary battery having a metal current collector and a positive electrode active material formed on both surfaces of the current collector, a conductive additive and a binder The current collector has a thickness of 11 μm or less and a tensile strength of 2.5 N / mm or more, and the positive electrode mixture layer is used as the binder. It contains vinylidene fluoride-chlorotrifluoroethylene copolymer and graphite as the conductive additive.

  The nonaqueous electrolyte secondary battery of the present invention has a wound electrode body in which a positive electrode, a negative electrode, and a separator are overlapped and wound in a spiral shape, and the cross section is flattened, and a nonaqueous electrolyte. The positive electrode is a positive electrode for a non-aqueous electrolyte secondary battery according to the present invention.

  According to the present invention, a non-aqueous electrolyte secondary battery having a flat wound electrode body, having a high capacity, good charge / discharge cycle characteristics and good productivity, and the non-aqueous electrolyte secondary battery are configured. The positive electrode to be obtained can be provided.

It is a fragmentary longitudinal cross-sectional view which represents typically an example of the nonaqueous electrolyte secondary battery of this invention. FIG. 2 is a perspective view of FIG. 1.

  The positive electrode for a non-aqueous electrolyte secondary battery of the present invention (hereinafter sometimes simply referred to as “positive electrode”) has a positive electrode mixture layer containing a positive electrode active material, a conductive additive and a binder, and a metal current collector. It has a structure formed on both sides of the body.

  The current collector according to the positive electrode of the present invention has a thickness of 11 μm or less, preferably 10 μm or less. The positive electrode of the present invention is provided with such a thin current collector, thereby reducing the proportion occupied by the positive electrode current collector as much as possible out of the internal volume of the nonaqueous electrolyte secondary battery. Yes. Therefore, in the nonaqueous electrolyte secondary battery (nonaqueous electrolyte secondary battery of the present invention) formed using the positive electrode of the present invention, the amount of nonaqueous electrolyte introduced into the interior can be increased.

  In nonaqueous electrolyte secondary batteries, for example, attempts have been made to increase the capacity by setting the upper limit voltage of charging to 4.3 V or higher. However, as a result, the potential of the positive electrode becomes very high when the non-aqueous electrolyte secondary battery is charged, so that oxidative decomposition of the non-aqueous electrolyte occurs and the electrolyte in the positive electrode becomes insufficient. Decomposition products accumulate on the surface layer of the positive electrode active material contained, ion conduction paths between particles decrease, and these cause deterioration of charge / discharge cycle characteristics of the battery.

  However, if the nonaqueous electrolyte secondary battery uses the positive electrode of the present invention and increases the amount of the nonaqueous electrolyte introduced therein, the occurrence of the above problems is suppressed, and the deterioration of the charge / discharge cycle characteristics is suppressed. be able to.

  However, if the current collector of the positive electrode is thinned as described above, the strength is reduced, so that when the flat wound electrode body is formed, the current collector is likely to be broken, producing a non-aqueous electrolyte secondary battery. Sex is reduced.

  Therefore, in the positive electrode of the present invention, vinylidene fluoride-chlorotrifluoroethylene copolymer (VDF-CTFE) is used as a binder for the positive electrode mixture layer, and graphite is used as a conductive additive for the positive electrode mixture layer.

  Polyvinylidene fluoride (PVDF) is often used as the binder of the positive electrode mixture layer relating to the positive electrode for the nonaqueous electrolyte secondary battery. This PVDF undergoes deHF reaction in the presence of alkali components (such as unreacted raw materials of the positive electrode active material and by-products during the synthesis of the positive electrode active material) contained in the positive electrode active material to form a crosslink. Therefore, the positive electrode mixture layer tends to be hard. When a flat wound electrode body is formed using a positive electrode having a hard positive electrode mixture layer, the stress applied to the current collector at the time of winding increases, so that the current collector is thin and has low strength as described above. When using, tears are likely to occur.

  However, in the case of VDF-CTFE, even if a deHF reaction occurs in the presence of an alkali component, the reaction is stopped by the action of a structural unit derived from chlorotrifluoroethylene. Therefore, the flexibility of the positive electrode mixture layer is improved by using VDF-CTFE as the binder.

  Furthermore, when graphite is used for the conductive additive of the positive electrode mixture layer, the positive electrode active material particles in the positive electrode mixture layer can easily move due to the sliding action of graphite. Therefore, when stress is concentrated on a specific part of the positive electrode mixture layer due to the formation of the flat wound electrode body, the graphite acts like a lubricant and the positive electrode active material particles move, thereby reducing the damage to the current collector. Is done.

  As described above, in the positive electrode of the present invention, the flexibility of the positive electrode mixture layer is improved by using VDF-CTFE as the binder of the positive electrode mixture layer, and graphite is used as the conductive additive of the positive electrode mixture layer. The synergistic function of the current collector's damage mitigating action is achieved, and even when a thin current collector is used as described above, the current collector at the time of forming the flat wound electrode body The non-aqueous electrolyte secondary battery can be increased in productivity by suppressing the breakage of the battery, and the deterioration of battery characteristics such as the capacity that can be caused by the breakage of the current collector can be suppressed. Battery reliability can also be increased.

  VDF-CTFE is known to contribute to improving the charge / discharge cycle characteristics of a non-aqueous electrolyte secondary battery, but a non-aqueous electrolyte secondary battery formed using the positive electrode of the present invention, that is, In the non-aqueous electrolyte secondary battery of the present invention, as described above, in addition to the effect of simply using VDF-CTFE as the binder of the positive electrode mixture layer, the non-aqueous electrolyte secondary battery can be made non-aqueous by using a thin current collector. The effect of increasing the electrolytic mass functions synergistically. Therefore, in the non-aqueous electrolyte secondary battery using the positive electrode of the present invention (non-aqueous electrolyte secondary battery of the present invention), even if the upper limit voltage for charging is set to 4.3 V or higher and the capacity is increased, it is good. Charge / discharge cycle characteristics can be ensured.

  Only VDF-CTFE may be used as the binder of the positive electrode mixture layer, and other binders may be used in combination with VDF-CTFE. Specific examples of the binder that can be used in combination with VDF-CTFE include, for example, acrylonitrile, acrylate esters (such as methyl acrylate, ethyl acrylate, butyl acrylate, and 2-ethylhexyl acrylate) and methacrylate esters (methyl methacrylate). , Ethyl methacrylate, butyl methacrylate, etc.) a copolymer formed by two or more monomers including at least one monomer selected from the group consisting of: hydrogenated nitrile rubber; PVDF; vinylidene fluoride-tetrafluoroethylene copolymer (VDF-TFE); vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymer (VDF-HFP-TFE); Even when PVDF easily forms a crosslinked structure in the presence of an alkali, when used in combination with VDF-CTFE, the formation of a crosslinked structure is suppressed by the action of the structural unit derived from chlorotrifluoroethylene in VDF-CTFE. Therefore, the flexibility of the positive electrode mixture layer can be maintained.

  The content of the binder in the positive electrode mixture layer is such that the positive electrode active material and the conductive additive in the positive electrode mixture layer can be satisfactorily bound to prevent desorption from these positive electrode mixture layers. From the viewpoint of improving the reliability of the battery in which the positive electrode is used more preferably, it is preferably 1% by mass or more. However, when the amount of the binder in the positive electrode mixture layer is too large, the amount of the positive electrode active material and the amount of the conductive auxiliary agent are decreased, and the effect of increasing the capacity may be reduced. Therefore, the content of the binder in the positive electrode mixture layer is preferably 1.6% by mass or less.

  Moreover, when using together VDF-CTEF and another binder for the binder which concerns on a positive electrode, from a viewpoint of ensuring the said effect by use of VDF-CTFE more favorably, in binder total quantity. The ratio of VDF-CTFE is preferably 20% by mass or more, and more preferably 50% by mass or more. In addition, since only VDF-CTFE may be used for the binder of the positive electrode mixture layer, the preferable upper limit of the ratio of VDF-CTFE in the total amount of the binder is 100% by mass.

  As described above, graphite (natural graphite, artificial graphite, etc.) is used for the conductive additive according to the positive electrode of the present invention.

  The average particle diameter of graphite is preferably 1 μm or more, more preferably 3 μm or more, and preferably 30 μm or less, more preferably 20 μm or less. Also in the case of graphite of such a size, since the sliding action becomes better, a positive electrode that can further improve the productivity and reliability of the nonaqueous electrolyte secondary battery can be configured.

The average particle diameter of graphite and negative electrode active material described later in the present specification is a volume standard in the case of obtaining an integral volume from particles having a small particle size distribution measured using a microtrack particle size distribution measuring device “HRA9320” manufactured by Nikkiso Co., Ltd. The value of 50% diameter (d ₅₀ ) median diameter in the integrated fraction.

  Only the graphite may be used for the conductive auxiliary agent according to the positive electrode of the present invention, but other conductive auxiliary agents may be used together with the graphite. Examples of other conductive aids that can be used in combination with graphite include carbon materials such as carbon blacks such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; carbon fibers; It is done. Also, conductive fibers such as metal fibers; carbon fluoride; metal powders such as aluminum; zinc oxide; conductive whiskers such as potassium titanate; conductive metal oxides such as titanium oxide; organics such as polyphenylene derivatives Conductive aids such as conductive materials can also be used with graphite.

  The content of graphite in the positive electrode mixture layer is preferably 0.3% by mass or more, and more preferably 0.5% by mass or more, from the viewpoint of better exhibiting the above action. However, if the amount of graphite in the positive electrode mixture layer is too large, the active material ratio may decrease, and the density and capacity may decrease. Therefore, the graphite content in the positive electrode mixture layer is preferably 5% by mass or less, and more preferably 2.5% by mass or less.

  Moreover, it is preferable that the total content of the conductive support agent in a positive mix layer is 1-5 mass%.

As the positive electrode active material according to the positive electrode of the present invention, a conventionally known positive electrode active material for a non-aqueous electrolyte secondary battery, for example, an active material capable of occluding and releasing lithium ions is used. The As a specific example of such a positive electrode active material, for example, a layered structure represented by Li _{1 + x} MO ₂ (−0.1 <x <0.1, M: Co, Ni, Mn, Al, Mg, etc.) Lithium-containing transition metal oxide, LiMn ₂ O ₄ and spinel-structured lithium manganese oxide obtained by substituting some of its elements with other elements, LiMPO ₄ (M: Co, Ni, Mn, Fe, etc.) Type compounds. Specific examples of the lithium-containing transition metal oxide having the layered structure include LiCoO ₂ and other oxides including 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, in order to increase the stability of the positive electrode active material in a state of being charged at a high voltage, when the non-aqueous electrolyte secondary battery is charged with a higher final voltage than usual before its use. The various active materials exemplified above preferably further contain a stabilizing element. Examples of such stabilizing elements include Mg, Ak, Ti, Zr, Mo, and Sn.

  The content of the positive electrode active material in the positive electrode mixture layer is preferably 94 to 98% by mass.

  In producing the positive electrode, a paste in which the positive electrode mixture containing the positive electrode active material, the conductive auxiliary agent and the binder is uniformly dispersed using a solvent such as N-methyl-2-pyrrolidone (NMP). A composition in the form of a slurry or slurry (the binder may be dissolved in a solvent) is applied to the surface of the positive electrode current collector and dried. A method of adjusting the thickness and density of the agent layer can be employed. However, the method for producing the positive electrode of the present invention is not limited to the above method, and other methods may be adopted.

  As described above, the current collector according to the positive electrode of the present invention has a thickness of 11 μm or less, preferably 10 μm or less. In the present invention, by using VDF-CTFE as a binder for the positive electrode mixture layer and using graphite as a conductive additive for the positive electrode mixture layer, a positive electrode having a current collector with such a thickness is used. Even in such a case, it is possible to suppress breakage of the current collector when the flat wound electrode body is used. However, if the strength of the current collector of the positive electrode is too small, there is a possibility that the action of suppressing breakage due to the use of VDF-CTFE will be insufficient. Therefore, in the positive electrode of the present invention, in addition to using VDF-CTFE as the binder of the positive electrode mixture layer, the collector has a tensile strength of 2.5 N / mm or more, preferably 2.7 N. The current collector is favorably suppressed from breaking when a flat wound electrode body is used by using a material of at least / mm. The tensile strength of the positive electrode current collector is preferably 3.9 N / mm or less.

  The tensile strength of the current collector referred to in this specification is a pre-treatment where the current collector is cut into a 15 mm × 250 mm rectangle to form a test piece. It is a value obtained by conducting a test at a crosshead speed of 10 mm / min using “SDT-52 type”).

  Examples of the current collector having the tensile strength as described above include the following.

  The material of the current collector for the positive electrode is preferably an aluminum alloy whose main component is aluminum. The aluminum alloy has an aluminum purity of 99.0% by mass or more, and as other additive components, for example, Si ≦ 0.6% by mass, Fe ≦ 0.7% by mass, Cu ≦ 0.25% by mass, Mn ≦ 1. It is desirable to contain 5% by mass, Mg ≦ 1.3% by mass, and Zn ≦ 0.25% by mass. A foil or film made of such a material can be used as a current collector.

  In addition, since it will become difficult to ensure the said tensile strength when a collector is too thin, it is preferable that the thickness is 6 micrometers or more.

  The thickness of the positive electrode mixture layer in the positive electrode is preferably 30 to 80 μm per side. In the positive electrode mixture layer, the filling rate is preferably 75% or more from the viewpoint of higher capacity. Note that when the filling rate of the positive electrode mixture layer is increased, the hardness of the positive electrode mixture layer is increased, so that the current collector is easily broken in the flat wound electrode body. By using VDF-CTFE as a binder for the agent layer and using graphite as a conductive additive for the positive electrode mixture layer, for example, the filling rate of the positive electrode mixture layer is 77% or more (more preferably 78% or more) ), It is possible to suppress the breaking of the thin current collector as described above.

  However, when the filling rate of the positive electrode mixture layer is too high, the number of pores in the positive electrode mixture layer becomes too small, and the permeability of the nonaqueous electrolyte (nonaqueous electrolyte solution) into the positive electrode mixture layer decreases. Since there exists a possibility, it is preferable that the filling rate is 83% or less.

The filling rate of the positive electrode mixture layer is determined by the following formula.
Filling rate (%) = 100 × (actual density of positive electrode mixture layer / theoretical density of positive electrode mixture layer)

The “theoretical density of the positive electrode mixture layer” in the above formula for calculating the filling rate of the positive electrode mixture layer is a density (positive electrode mixture) calculated from the density and content of each component of the positive electrode mixture layer. The density obtained by assuming that there are no vacancies in the layer), and the “actual density of the positive electrode mixture layer” is measured by the following method. First, it cuts a positive electrode to a size of 1 cm × 1 cm, a thickness micrometer _{(l 1),} measuring the mass _{(m 1)} a precision balance. Next, the positive electrode material mixture layer is scraped off, and only the current collector is taken out, and the thickness (l _c ) and mass (m _c ) of the current collector are measured in the same manner as the positive electrode. From the obtained thickness and mass, the actual density (d _ca ) of the positive electrode mixture layer is determined by the following formula (the unit of thickness is cm, and the unit of mass is g).
d _ca = (m ₁ −m _c ) / (l ₁ −l _c )

  The non-aqueous electrolyte secondary battery of the present invention only needs to include a flat wound electrode body having the positive electrode for a non-aqueous electrolyte secondary battery of the present invention and a non-aqueous electrolyte. There is no restriction | limiting, Each structure and structure employ | adopted as the nonaqueous electrolyte secondary battery known conventionally can be applied.

  As a negative electrode, what formed the negative mix layer containing a negative electrode active material in the single side | surface or both surfaces of a collector is mentioned, for example. The negative electrode mixture layer contains, in addition to the negative electrode active material, a binder and, if necessary, a conductive aid, and includes, for example, a negative electrode active material and a binder (further, a conductive aid). A negative electrode mixture-containing composition (slurry etc.) obtained by adding a suitable solvent to the mixture (negative electrode mixture) and kneading thoroughly is applied to the surface of the current collector and dried to obtain a desired thickness. Can be formed.

  Examples of the negative electrode active material include graphite materials such as natural graphite (flaky graphite), artificial graphite, and expanded graphite; graphitizable carbonaceous materials such as coke obtained by calcining pitch; furfuryl alcohol resin ( Non-graphitizable carbonaceous material such as amorphous carbon obtained by low-temperature firing of PFA), polyparaphenylene (PPP) and phenolic resin; amorphous carbon or resin is supported on the surface of graphite material Examples thereof include carbon materials such as surface-treated carbon materials. In addition to the carbon material, lithium or a lithium-containing compound can also be used as the negative electrode active material. Examples of the lithium-containing compound include lithium alloys such as Li—Al, and alloys containing elements that can be alloyed with lithium such as Si and Sn. Furthermore, oxide-based materials such as Sn oxide and Si oxide can also be used. The content of the negative electrode active material in the negative electrode mixture layer is preferably 97 to 99% by mass, for example.

  It is preferable to use a surface-treated carbon material as the negative electrode active material because it can prevent excessive reaction with the nonaqueous electrolyte.

  The negative electrode active material is a non-aqueous electrolyte (non-aqueous electrolyte) negative electrode mixture, especially when a carbon material having amorphous carbon supported on the surface of a graphite material and having an average particle size of 8 to 18 μm and relatively small particles is used. This is preferable because the permeability into the layer is improved. The reason is not clear, but if the carbon material is relatively small particles, the pores formed in the negative electrode mixture layer are uniformed when the negative electrode is pressed. It is thought that the electrolyte solution easily penetrates. In addition, this type of graphite has a high lithium ion acceptability (ratio of constant current charge capacity to the total charge capacity). By using this graphite as a negative electrode active material, a non-aqueous electrolyte having excellent charge / discharge cycle characteristics. A secondary battery can be provided.

  The conductive aid is not particularly limited as long as it is an electron conductive material, and may not be used. Specific examples of conductive aids include acetylene black; ketjen black; carbon blacks such as channel black, furnace black, lamp black, and thermal black; carbon materials such as carbon fibers; and conductive fibers such as metal fibers. Carbon fluoride, metal powders such as copper and nickel, organic conductive materials such as polyphenylene derivatives, and the like. These may be used alone or in combination of two or more. . Among these, acetylene black, ketjen black and carbon fiber are particularly preferable. However, when a conductive additive is used for the negative electrode, the content of the conductive additive in the negative electrode mixture layer is preferably 10% by mass or less in order to increase the capacity.

As a binder concerning a negative mix layer, any of a thermoplastic resin and a thermosetting resin may be sufficient. Specifically, for example, the same material as the binder according to the positive electrode of the present invention, a styrene butadiene rubber (SBR), an ethylene-acrylic acid copolymer, or a Na ⁺ ion crosslinked product of the copolymer, ethylene-methacrylic Acid copolymer or Na ⁺ ion crosslinked product of the copolymer, ethylene-methyl acrylate copolymer, Na ⁺ ion crosslinked product of the copolymer, ethylene-methyl methacrylate copolymer or the copolymer Na ⁺ ion crosslinked body etc. can be used, These materials may be used individually by 1 type, and may use 2 or more types together.

Among these, PVDF, SBR, ethylene-acrylic acid copolymer or Na ⁺ ion crosslinked product of the copolymer, ethylene-methacrylic acid copolymer or Na ⁺ ion crosslinked product of the copolymer, ethylene-acrylic acid A methyl copolymer or a Na ⁺ ion crosslinked product of the copolymer, an ethylene-methyl methacrylate copolymer or a Na ⁺ ion crosslinked product of the copolymer is particularly preferable. The content of the binder in the negative electrode mixture layer is preferably 1 to 5% by mass, for example.

  The thickness of the negative electrode mixture layer (when the negative electrode mixture layer is formed on both sides of the current collector, the thickness per one surface thereof) is preferably 30 to 80 μm.

  The current collector used for the negative electrode is not particularly limited as long as it is an electron conductor that is substantially chemically stable in the nonaqueous electrolyte secondary battery. Examples of the material constituting the current collector include stainless steel, nickel or an alloy thereof, copper or an alloy thereof, titanium or an alloy thereof, carbon, conductive resin, carbon, or the like on the surface of copper or stainless steel. A material obtained by treating titanium is used. Among these, copper and copper alloys are particularly preferable. These materials can also be used after oxidizing the surface. Moreover, it is preferable to give an unevenness | corrugation to the collector surface by surface treatment. Examples of the shape of the current collector include films, sheets, nets, punched materials, lath bodies, porous bodies, foamed bodies, and molded bodies of fiber groups, in addition to foils. Although the thickness of a collector is not specifically limited, For example, it is preferable that it is 5-50 micrometers.

  As the non-aqueous electrolyte, for example, a solution (non-aqueous electrolyte) prepared by dissolving a lithium salt in the following non-aqueous solvent can be used.

  Examples of the solvent include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), γ-butyrolactone (γ-BL ), 1,2-dimethoxyethane (DME), tetrahydrofuran (THF), 2-methyltetrahydrofuran, dimethyl sulfoxide (DMSO), 1,3-dioxolane, formamide, dimethylformamide (DMF), dioxolane, acetonitrile, nitromethane, Methyl formate, methyl acetate, phosphoric acid triester, trimethoxymethane, dioxolane derivative, sulfolane, 3-methyl-2-oxazolidinone, propylene carbonate derivative, tetrahydrofuran derivative, diethyl Aprotic organic solvents such as ruether and 1,3-propane sultone can be used singly or as a mixed solvent in which two or more are mixed.

The inorganic ion salt according to the non-aqueous electrolyte solution, for _{example, LiClO 4, LiPF 6, LiBF} 4, LiAsF 6, LiSbF 6, 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] There may be mentioned at least one selected. The concentration of these lithium salts in the non-aqueous electrolyte is preferably 0.6 to 1.8 mol / l, and more preferably 0.9 to 1.6 mol / l.

  Non-aqueous electrolytes used in non-aqueous electrolyte secondary batteries include vinylene carbonate, vinyl ethylene carbonate, anhydrous water for the purpose of further improving charge / discharge cycle characteristics and improving safety such as high-temperature storage and prevention of overcharge. Additives (including these derivatives) such as acid, sulfonic acid ester, dinitrile, 1,3-propane sultone, diphenyl disulfide, cyclohexyl benzene, biphenyl, fluorobenzene, and t-butyl benzene may be added as appropriate.

  Further, as the non-aqueous electrolyte of the non-aqueous electrolyte secondary battery, a gel (gel electrolyte) obtained by adding a known gelling agent such as a polymer to the non-aqueous electrolyte can be used.

  In the non-aqueous electrolyte secondary battery of the present invention, a separator containing the non-aqueous electrolyte is disposed between the positive electrode and the negative electrode. As the separator, an insulating microporous thin film having a large ion permeability and a predetermined mechanical strength is used. Moreover, what has a function which a hole is obstruct | occluded by fusion | melting of a structural material above fixed temperature (for example, 100-140 degreeC), and raises resistance (namely, what has a shutdown function) is preferable.

  Specific examples of such a separator include a sheet (porous sheet), a nonwoven fabric or a woven fabric composed of a material such as polyethylene solvent, hydrophobic polymer such as polyethylene, polypropylene, or glass fiber having organic solvent resistance and hydrophobicity; And a porous material in which fine particles of the exemplified polyolefin polymer are fixed with an adhesive.

  The pore diameter of the separator is preferably such that the active material of the positive and negative electrodes, the conductive auxiliary agent, the binder and the like detached from the positive and negative electrodes do not pass through, and is preferably 0.01 to 1 μm, for example. The thickness of the separator is generally 8-30 μm, but is preferably 10-20 μm in the present invention. Further, the porosity of the separator is determined according to the constituent material and thickness, but is generally 30 to 80%.

  In the battery of the present invention, as described above, the positive electrode of the present invention and the negative electrode are overlapped via the separator, wound into a spiral shape, and flattened in a cross-sectional shape by flattening. A wound electrode body is used.

  In the battery of the present invention, since a flat wound electrode body is used, a rectangular (rectangular cylindrical) outer can that can make the battery thinner can be used for the outer body. Moreover, the battery of this invention can also use the exterior body which consists of a laminate film which formed the resin layer in the single side | surface or both surfaces of a metal layer.

  The nonaqueous electrolyte secondary battery of the present invention can exhibit good battery characteristics even when the upper limit voltage for charging is used at about 4.2 V, as in the case of ordinary nonaqueous electrolyte secondary batteries. Even if the upper limit voltage of charging is increased to 4.3 V or more, the amount of nonaqueous electrolyte in the battery can be increased by using a thin current collector for the positive electrode, so that excellent charge / discharge cycle characteristics can be secured. . Therefore, the nonaqueous electrolyte secondary battery of the present invention exhibits stable and excellent characteristics even when repeatedly used over a long period of time while increasing the capacity by setting the upper limit voltage of charging higher than usual. Is possible.

  In addition, it is preferable that the upper limit voltage of charge of a nonaqueous electrolyte secondary battery is 4.7V or less.

  The non-aqueous electrolyte secondary battery of the present invention can be applied to the same applications as conventionally known non-aqueous electrolyte secondary batteries.

  Hereinafter, the present invention will be described in detail based on examples. However, the following examples do not limit the present invention.

Example 1
<Preparation of positive electrode>
LiCo _0.97 Al _0.02 Zr _0.01 O _{2 as} positive electrode active material: 96.9 parts by mass, acetylene black as a conductive auxiliary agent: 1.2 parts by mass and graphite (average particle size: 3 μm): 0 .3 parts by mass and VDF-CTFE as a binder: 1.6 parts by mass were mixed to make a positive electrode mixture. To this positive electrode mixture, NMP as a solvent was added, and “Clare” manufactured by M Technique Co., Ltd. Using a “mix CLM0.8 (trade name)”, a treatment was performed at a rotational speed of 10000 min ⁻¹ for 30 minutes to obtain a paste-like mixture. NMP which is a solvent was further added to this mixture, and the mixture was treated at a rotational speed of 10000 min ⁻¹ for 15 minutes to prepare a positive electrode mixture-containing composition.

The positive electrode mixture-containing composition is applied to both sides of an aluminum alloy foil (1100, thickness: 10.0 μm, tensile strength: 2.5 N / mm) as a current collector, and vacuum-dried at 120 ° C. for 12 hours. Then, a press treatment was further performed to produce a positive electrode having a positive electrode mixture layer having a thickness of 61 μm on both sides of the current collector. The density (actual density) of the positive electrode mixture layer after the press treatment determined by the above method was 3.85 g / cm ³ and the filling rate was 78%.

<Production of negative electrode>
Natural graphite: 97.5% by mass (average particle size: 19.3 μm), SBR: 1.5% by mass, and carboxymethyl cellulose (thickener): 1% by mass are mixed with water to form a slurry. A negative electrode mixture-containing composition was prepared. This negative electrode mixture-containing composition was applied to both sides of a copper foil (thickness: 6 μm) as a current collector, vacuum-dried at 120 ° C. for 12 hours, and further subjected to a press treatment to form both sides of the current collector. A negative electrode having a negative electrode mixture layer with a thickness of 73 μm was prepared.

<Production of electrode body>
The positive electrode and the negative electrode are overlapped via a separator (a polyethylene porous film having a thickness of 14 μm and an air permeability of 300 seconds / 100 cm ³ ), wound in a spiral shape, and then the cross section becomes flat. In this way, a flat wound electrode body was produced.

<Preparation of non-aqueous electrolyte>
LiPF ₆ was dissolved at a concentration of 1.2 mol / l in a mixed solvent of methyl ethyl carbonate, diethyl carbonate and ethylene carbonate (volume ratio 2: 1: 3), and vinylene carbonate: 2% by mass, vinyl ethylene carbonate. 1% by mass was added to prepare a non-aqueous electrolyte (non-aqueous electrolyte).

<Battery assembly>
The electrode body is inserted into a prismatic battery case made of aluminum alloy having an outer dimension of 4.0 mm in thickness, 34 mm in width, and 50 mm in height, the lead body is welded, and the lid plate made of aluminum alloy is attached to the battery case. Welded to the open end of the. Thereafter, the non-aqueous electrolysis is injected from the inlet provided in the cover plate, and after standing for 1 hour, the inlet is sealed. The rectangular non-aqueous electrolyte having the structure shown in FIG. 1 and the appearance shown in FIG. A secondary battery was produced.

  FIG. 1 is a partial cross-sectional view thereof. A positive electrode 1 and a negative electrode 2 are wound in a spiral shape via a separator 3 and then pressed so as to be flattened to form a flat wound electrode body 6 having a rectangular shape ( A rectangular tube-shaped outer can 4 is housed together with a non-aqueous electrolyte. However, in FIG. 1, in order to avoid complication, a metal foil, a non-aqueous electrolyte, or the like as a current collector used for manufacturing the positive electrode 1 and the negative electrode 2 is not illustrated.

  The battery case 4 is made of an aluminum alloy and constitutes an outer package of the battery. The outer can 4 also serves as a positive electrode terminal. An insulator 5 made of a polyethylene sheet is disposed at the bottom of the battery case 4, and is connected to one end of each of the positive electrode 1 and the negative electrode 2 from the flat wound electrode body 6 made 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 battery case 4 via a polypropylene 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 battery case 4, and the opening part of the battery case 4 is sealed by welding the joint part of both, and the inside of the 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. As a result, the battery is sealed by welding. 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 Example 1, the battery case 4 and the cover plate 9 function as positive terminals by directly welding the positive electrode lead body 7 to the cover 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 battery case 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 body is not cross-sectional.

Examples 2-4
A positive electrode was produced in the same manner as in Example 1 except that the amounts of acetylene black and graphite as conductive aids were changed as shown in Table 1. And the square nonaqueous electrolyte secondary battery was produced like Example 1 except having used this positive electrode.

Examples 5-7
A positive electrode was produced in the same manner as in Example 1 except that the conductive auxiliary agent was changed to graphite having an average particle size shown in Table 1. And the square nonaqueous electrolyte secondary battery was produced like Example 1 except having used this positive electrode.

Example 8
A positive electrode was produced in the same manner as in Example 1 except that an aluminum alloy foil (3003) having a thickness of 8.0 μm and a tensile strength of 2.5 N / mm was used as the current collector. And the square nonaqueous electrolyte secondary battery was produced like Example 1 except having used this positive electrode.

Example 9
Mixed graphite obtained by mixing the same natural graphite as used in Example 1 and a surface-treated carbon material having an average particle diameter of 10 μm carrying amorphous carbon on the surface of natural graphite at a mass ratio of 1: 1: 97.5% by mass, SBR: 1.5% by mass, and carboxymethylcellulose (thickening agent): 1% by mass were mixed with water to prepare a slurry-like negative electrode mixture-containing composition. In the same manner as in Example 1, this negative electrode mixture-containing composition was applied on both sides of a current collector copper foil (thickness: 6 μm), vacuum-dried at 120 ° C. for 12 hours, and further subjected to press treatment. A negative electrode having a negative electrode mixture layer having a thickness of 73 μm on both surfaces of the current collector was prepared.

  And the square nonaqueous electrolyte secondary battery was produced like Example 1 except having used this negative electrode.

Comparative Example 1
A positive electrode was produced in the same manner as in Example 1 except that graphite was not used as the conductive additive and the amount of acetylene black was changed as shown in Table 1. And the square nonaqueous electrolyte secondary battery was produced like Example 1 except having used this positive electrode.

Comparative Example 2
A positive electrode was produced in the same manner as in Example 1 except that PVDF was used as the binder instead of VDF-CTFE. And the square nonaqueous electrolyte secondary battery was produced like Example 1 except having used this positive electrode.

Comparative Example 3
A positive electrode was produced in the same manner as in Example 1 except that the positive electrode current collector was changed to an aluminum alloy foil (1100) having a thickness of 15.0 μm and a tensile strength of 3.8 N / mm. A rectangular nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that.

Comparative Example 4
A positive electrode was produced in the same manner as in Example 1 except that the positive electrode current collector was changed to an aluminum alloy foil (A1N30) having a thickness of 10.0 μm and a tensile strength of 2.2 N / mm. A rectangular nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that.

  The following evaluation was performed about the nonaqueous electrolyte secondary battery of an Example and a comparative example, and the positive electrode used for these batteries.

<Bending strength of positive electrode>
The double-sided coated part of the positive electrode (the part where the positive electrode mixture layer is formed on both sides of the current collector) is cut into 5 cm in the long direction and 4 cm in the width direction to make a test piece, and the position 15 mm from the end on the long side of the test piece Was folded in the same direction as the folding direction when the wound electrode body was produced. After applying a load of 200 gf uniformly to the bent part of the test piece, set both ends of the test piece opened with a jig of a tensile tester (“SDT-52 type” manufactured by Imada Seisakusho Co., Ltd.). A tensile test was performed at a speed of 50 mm / min, and the strength when the bent portion of the test piece was broken was defined as the bending strength. It can be evaluated that the higher the bending strength, the better the productivity of the non-aqueous electrolyte secondary battery can be suppressed since the breakage of the positive electrode current collector can be better suppressed when the flat wound electrode body is formed.

<Measurement of discharge capacity of non-aqueous electrolyte secondary battery>
About each battery of an Example and a comparative example, after charging with a constant current of 0.2C to 4.35V at room temperature, the battery was charged at a constant voltage until the total charging time was 8 hours, and then the battery voltage at 0.2C at room temperature. Was discharged at a constant current up to 2.75 V, and the discharge capacity at that time was determined.

<Charge / discharge cycle characteristics evaluation of non-aqueous electrolyte secondary battery>
About each battery of an Example and a comparative example, after charging with a constant current of 1C to 4.35V at room temperature, the battery was charged at a constant voltage until the total charging time was 2.5 hours, and then the battery voltage was 2 at 1C at room temperature. A series of operations for performing constant-current discharge up to .75 V was taken as one cycle, and these were repeated many times, and the discharge capacity at 500th cycle was divided by the discharge capacity at 1st cycle of Comparative Example 1 to obtain the capacity retention rate.

  The composition of the binder in the positive electrode used in the nonaqueous electrolyte secondary batteries of Examples and Comparative Examples, the density (actual density) and filling rate of the positive electrode mixture layer are shown in Table 1, and the composition of the current collector and the nonaqueous Table 2 shows the injection amount of the nonaqueous electrolyte in the electrolyte secondary battery, and Table 3 shows the evaluation results. In Table 1, “AB” means acetylene black. In Table 2, the amount of the non-aqueous electrolyte is shown as a relative value (mass basis) when the amount of the battery of Comparative Example 1 is 100. In Table 3, the discharge capacity of each non-aqueous electrolyte secondary battery and the capacity retention rate at the time of charge / discharge cycle characteristics evaluation are shown as relative values when the result of the battery of Comparative Example 1 is 100.

  As shown in Table 2, the thickness and tensile strength of the current collector are appropriate, and VDF-CTFE is used as the binder for the positive electrode mixture layer and graphite is used as the conductive additive for the positive electrode mixture layer. The positive electrodes according to the nonaqueous electrolyte secondary batteries of Examples 1 to 9 had high bending strength. Therefore, since the nonaqueous electrolyte secondary batteries of Examples 1 to 9 can satisfactorily suppress the occurrence of breakage of the positive electrode current collector in the flat wound electrode body, the productivity and reliability are good. It can be said.

  On the other hand, the positive electrode according to the battery of Comparative Example 1 in which graphite is not used as the conductive additive of the positive electrode mixture layer, and the Comparative Example 2 in which VDF-CTFE is not used as the binder of the positive electrode mixture layer. The positive electrode according to the battery had lower bending strength than the positive electrode according to the battery of each example. Therefore, it can be said that the batteries of Comparative Examples 1 and 2 are easily broken in the positive electrode current collector in the flat wound electrode body, and are inferior in productivity and reliability to the batteries of the examples.

  In addition, the nonaqueous electrolyte secondary batteries of Examples 1 to 9 have a higher capacity retention rate at the time of charge / discharge cycle characteristic evaluation than the batteries of Comparative Examples 1 to 4, and can ensure better charge / discharge cycle characteristics. It was.

  That is, the positive electrode according to the battery of Comparative Example 1 that does not use graphite as the conductive additive of the positive electrode mixture layer, the Comparative Example 2 that does not use VDF-CTFE as the binder of the positive electrode mixture layer, and the current collector The battery of Comparative Example 4 having a positive electrode with a low tensile strength of the body had a smaller discharge capacity, a lower capacity retention rate in charge / discharge cycle characteristic evaluation, and inferior charge / discharge cycle characteristics compared to the battery of the example. . Further, the battery of Comparative Example 3 having a positive electrode with a thick current collector has a small amount of non-aqueous electrolyte injected, and therefore has a low capacity retention rate at the time of charge / discharge cycle characteristic evaluation, and has poor charge / discharge cycle characteristics. It was.

1 Positive electrode 2 Negative electrode 3 Separator

Claims (8)

A positive electrode used in a non-aqueous electrolyte secondary battery having a non-aqueous electrolyte and a spirally wound electrode body having a transverse cross-section that is wound in a spiral shape by stacking a positive electrode, a negative electrode, and a separator,
A metal current collector, and a positive electrode mixture layer formed on both surfaces of the current collector, containing a positive electrode active material, a conductive additive, and a binder;
The current collector has a thickness of 11 μm or less and a tensile strength of 2.5 N / mm or more,
The positive electrode mixture layer contains a vinylidene fluoride-chlorotrifluoroethylene copolymer as the binder and graphite as the conductive additive, and is a positive electrode for a non-aqueous electrolyte secondary battery .   2. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the positive electrode mixture layer contains the graphite having an average particle diameter of 1 to 30 μm.   The positive electrode for a nonaqueous electrolyte secondary battery according to claim 1 or 2, wherein the content of the graphite in the positive electrode mixture layer is 0.3 mass% or more.   The positive electrode for a nonaqueous electrolyte secondary battery according to claim 1, wherein a filling rate of the positive electrode mixture layer is 75% or more. A non-aqueous electrolyte secondary battery having a non-aqueous electrolyte and a spirally wound electrode body in which a positive electrode, a negative electrode, and a separator are overlapped and wound in a spiral shape, and the cross section is flattened,
The said positive electrode is a positive electrode for nonaqueous electrolyte secondary batteries in any one of Claims 1-4, The nonaqueous electrolyte secondary battery characterized by the above-mentioned.   The non-aqueous electrolyte secondary battery according to claim 5, wherein the negative electrode includes a carbon material having an average particle diameter of 8 to 18 μm and supporting amorphous carbon on the surface of graphite as a negative electrode active material.   The nonaqueous electrolyte secondary battery according to claim 5 or 6, comprising a rectangular tube-shaped outer can.   The nonaqueous electrolyte secondary battery according to any one of claims 5 to 7, wherein an upper limit voltage of charging is set to 4.3 V or more.

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