WO2014208272A1

WO2014208272A1 - Positive electrode for nonaqueous electrolyte secondary batteries, and nonaqueous electrolyte secondary battery

Info

Publication number: WO2014208272A1
Application number: PCT/JP2014/064392
Authority: WO
Inventors: 裕志橋本; 大桃　義智; 阿部　浩史
Original assignee: 日立マクセル株式会社
Priority date: 2013-06-27
Filing date: 2014-05-30
Publication date: 2014-12-31
Also published as: KR20160024910A; CN105359301B; TWI622208B; JPWO2014208272A1; JP6345659B2; TW201517360A; CN105359301A; KR102211028B1

Abstract

Provided are: a nonaqueous electrolyte secondary battery which comprises a flattened wound electrode body and has high capacity, good charge/discharge cycle characteristics and good productivity; and a positive electrode which enables the configuration of this nonaqueous electrolyte secondary battery. A positive electrode for nonaqueous electrolyte secondary batteries, which is used for a nonaqueous electrolyte secondary battery that has a wound electrode body obtained by winding a laminate of a positive electrode, a negative electrode and a separator into a coil shape and flattening the wound electrode body to have a flattened cross-section and that has an upper limit for charging voltage set to 4.3 V or more, and which comprises a collector that is formed of a metal and positive electrode mixture layers that are formed on both surfaces of the collector and contain a positive electrode active material, a conductive assistant and a binder. The collector has a thickness of 11 μm or less and a tensile strength of 2.5 N/mm or more. The positive electrode mixture layers contain, as the binder, a vinylidene fluoride-chlorotrifluoroethylene copolymer. A nonaqueous electrolyte secondary battery which is provided with a nonaqueous electrolyte and a flattened wound electrode body having the above-described positive electrode, and which has an upper limit for charging voltage set to 4.3 V or more.

Description

Positive electrode for nonaqueous electrolyte secondary battery and nonaqueous electrolyte secondary battery

The present invention includes a non-aqueous 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 non-aqueous 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.

In Patent Document 3, a vinylidene fluoride-chlorotrifluoroethylene copolymer that can correspond to the above-described fluorine atom-containing polymer material is used as a binder for a positive electrode or a negative electrode. It has been shown that the ionic conductivity of the agent layer and the negative electrode mixture layer can be increased, thereby improving the charge / discharge cycle characteristics of the nonaqueous electrolyte secondary battery.

JP 2005-56743 A JP 2012-28158 A JP 2004-87325 A

By the way, in recent years, in response to the demand for higher capacity of non-aqueous electrolyte secondary batteries, studies are being made to respond to this by increasing the upper limit voltage at the time of charging than before. On the other hand, however, when the charging voltage of the nonaqueous electrolyte secondary battery is increased, there is a problem that the positive electrode active material is deteriorated and the charge / discharge cycle characteristics of the nonaqueous electrolyte secondary battery are lowered.

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 has achieved the above-mentioned object is a wound electrode body (hereinafter referred to as “flattened surface”) 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. A positive electrode used in a non-aqueous electrolyte secondary battery having a non-aqueous electrolyte and a charging upper limit voltage set to 4.3 V or higher, And a positive electrode mixture layer containing a positive electrode active material, a conductive additive and a binder formed on both surfaces of the current collector, and the current collector has a thickness Is 11 μm or less, and the tensile strength is 2.5 N / mm or more, and the positive electrode mixture layer contains a vinylidene fluoride-chlorotrifluoroethylene copolymer as the binder. To do.

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, and the upper limit voltage for charging is set to 4.3 V or higher.

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. It is explanatory drawing of the measuring method of the peeling strength of the positive mix layer and collector in the positive electrode used for the nonaqueous electrolyte secondary battery of an Example and a comparative example.

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 the nonaqueous electrolyte secondary battery formed using the positive electrode of the present invention, the capacity is increased 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.

As shown in Patent Document 3, VDF-CTFE is known to contribute to improving the charge / discharge cycle characteristics of a non-aqueous electrolyte secondary battery, but is formed using the positive electrode of the present invention. In the non-aqueous electrolyte secondary battery, that is, the non-aqueous electrolyte secondary battery of the present invention, in addition to the effect obtained by simply using VDF-CTFE as the binder of the positive electrode mixture layer, the non-aqueous electrolyte mass described above is used. Therefore, the charge / discharge cycle characteristics can be ensured while increasing the capacity by setting the upper limit voltage for charging to 4.3 V or higher.

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 the binder of 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 nonaqueous electrolyte secondary batteries. 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, by using VDF-CTFE as the binder, the flexibility of the positive electrode mixture layer is improved. Therefore, even if a thin current collector is used as described above, formation of a flat wound electrode body is possible. It is possible to increase the productivity of the non-aqueous electrolyte secondary battery by suppressing current collector breakage at the time, and to suppress deterioration of battery characteristics such as capacity that can be caused by current collector breakage The reliability of the nonaqueous electrolyte secondary battery can also be improved.

Only the VDF-CTFE may be used as the binder of the positive electrode mixture layer, or other binders may be used in combination with the 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); and the like. Even when PVDF is easy to form a crosslinked structure in the presence of 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.

In addition, when VDF-CTEF and another binder are used in combination with the binder related to the positive electrode, from the viewpoint of better securing the above-described effect by using VDF-CTFE, The ratio of VDF-CTFE is preferably 20% by mass or more, and more preferably 50% by mass or more. Since only VDF-CTFE may be used as 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 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 ) and the like. 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, Al, Ti, Zr, Mo, and Sn.

Examples of the conductive additive according to the positive electrode of the present invention include graphites such as natural graphite (eg, flaky graphite) and artificial graphite; acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, and the like. It is preferable to use carbon materials such as carbon blacks; carbon fibers; and conductive fibers such as metal fibers; carbon fluorides; metal powders such as aluminum; zinc oxide; and conductive materials such as potassium titanate. Conductive whiskers; conductive metal oxides such as titanium oxide; organic conductive materials such as polyphenylene derivatives; and the like can also be used.

In producing the positive electrode, a paste in which the positive electrode mixture containing the positive electrode active material, the conductive auxiliary agent, the binder and the like 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 the binder of the positive electrode mixture layer, even when the positive electrode has a current collector having such a thickness, the current collector when a flat wound electrode body is obtained. This makes it possible to suppress body tears. 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 current 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. This is a value obtained by performing a test using a SDT-52 type ") at a crosshead speed of 10 mm / min.

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. 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 content of each component other than the binder in the positive electrode mixture layer is preferably 94 to 98% by mass for the positive electrode active material, and preferably 1 to 5% by mass for the conductive assistant.

The nonaqueous electrolyte secondary battery of the present invention comprises a flat wound electrode body having the positive electrode for a nonaqueous electrolyte secondary battery of the present invention and a nonaqueous electrolyte, and the upper limit voltage of charging is 4.3 V or more. Any other configuration and structure may be used as long as they are set, and each configuration and structure employed in a conventionally known non-aqueous electrolyte secondary battery can be applied.

Examples of the negative electrode include those in which a negative electrode mixture layer containing a negative electrode active material is formed on one side or both sides of a current collector. 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 And 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 a lithium alloy such as Li—Al, and an alloy containing an element 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 excessive reaction with the non-aqueous electrolyte can be prevented.

As the negative electrode active material, in particular, 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, the permeability of the nonaqueous electrolyte into the negative electrode mixture layer Is preferable. 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.

In addition, the average particle diameter of the carbon material referred to in the present specification is obtained by, for example, dissolving the carbon material using a laser scattering particle size distribution meter (for example, Microtrack particle size distribution measuring device “HRA9320” manufactured by Nikkiso Co., Ltd.) The value of 50% diameter (d ₅₀ ) median diameter in the volume-based integrated fraction when the integrated volume is obtained from particles having a small particle size distribution measured by dispersing the carbon material in a medium that does not swell.

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, styrene butadiene rubber (SBR), ethylene-acrylic acid copolymer, Na ⁺ ion crosslinked product of the copolymer, ethylene-methacrylic acid, 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 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 them, 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 side 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. The thickness of the current collector is not particularly limited, but is preferably 5 to 50 μm, for example.

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, di Chirueteru, can be an aprotic organic solvent such as 1,3-propane sultone as alone in a mixed solvent or a mixture of two or more.

The lithium 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] At least one selected from the above. 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, cyclohexylbenzene, biphenyl, fluorobenzene, and t-butylbenzene may be added as appropriate.

Furthermore, 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. Further, those having a function of increasing the resistance by clogging the holes by melting the constituent materials at a certain temperature (eg, 100 to 140 ° C.) (that is, having a shutdown function) are 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 positive and negative electrode active materials, the conductive auxiliary agent, the binder, and the like detached from the positive and negative electrodes do not pass through, for example, 0.01 to 1 μm. 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.

And in the battery of this invention, since a flat winding electrode body is used, the rectangular (square tube-shaped) exterior can which can make a battery thin can be used for an exterior 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 non-aqueous electrolyte secondary battery of the present invention is used with an upper limit voltage of charging of 4.3 V or higher, and thus increasing the capacity by setting the upper limit voltage of charging higher than usual. Even when used repeatedly over a long period of time, it is possible to stably exhibit excellent characteristics. In addition, it is preferable that the upper limit voltage of charge of a nonaqueous electrolyte secondary battery is 4.7V or less.

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 _0.98 Al _0.008 Mg _0.008 Ti _0.004 O _{2 as} positive electrode active material: 96.9 parts by mass, acetylene black as conductive auxiliary agent: 1.5 parts by mass, and VDF as a binder -CTFE: Mix 1.6 parts by mass to make a positive electrode mixture, add NMP as a solvent to this positive electrode mixture, and use "Claremix CLM0.8 (trade name)" manufactured by M Technique The mixture was processed at a rotational speed of 10,000 min ⁻¹ for 30 minutes to obtain a paste-like mixture. To this mixture, NMP as a solvent was further added, and the mixture was treated at a rotational speed of 10,000 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 3.75 g / cm ³ and the filling rate was 76%.

<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 electrolyte is injected from the inlet provided on the cover plate, and is allowed to stand for 1 hour. The inlet is then sealed, and the structure shown in FIG. An electrolyte 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 a battery outer package, and 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.

The cover plate 9 is inserted into the opening of the battery case 4, and the joint of the two is welded, whereby the opening of the battery case 4 is sealed 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.

Example 2
LiCo _0.98 Al _0.008 Mg _0.008 Ti _0.004 O _{2 as} a positive electrode active material: 97.1 parts by mass, acetylene black as a conductive auxiliary agent: 1.5 parts by mass, and a binder A positive electrode mixture-containing composition was prepared in the same manner as in Example 1 except that 1.0 part by mass of VDF-CTFE and 0.4 part by mass of PVDF were mixed to form a positive electrode mixture, and this positive electrode mixture was used. did

Then, a positive electrode was produced in the same manner as in Example 1 except that the positive electrode mixture-containing composition was used, and a rectangular nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that this positive electrode was used. did.

Example 3
LiCo _0.98 Al _0.008 Mg _0.008 Ti _0.004 O _{2 as} a positive electrode active material: 97.3 parts by weight, acetylene black as a conductive auxiliary agent: 1.5 parts by weight, and a binder VDF-CTFE: 0.6 parts by mass and PVDF: 0.6 parts by mass were mixed to prepare a positive electrode mixture, and a positive electrode mixture-containing composition was prepared in the same manner as in Example 1 except that this positive electrode mixture was used. did.

Example 4
LiCo _0.98 Al _0.008 Mg _0.008 Ti _0.004 O _{2 as} positive electrode active material: 97.5 parts by mass, acetylene black as conductive auxiliary agent: 1.5 parts by mass, and binder VDF-CTFE: 0.2 parts by mass and PVDF: 0.8 parts by mass were mixed to prepare a positive electrode mixture, and a positive electrode mixture-containing composition was prepared in the same manner as in Example 1 except that this positive electrode mixture was used. did.

Example 5
LiCo _0.98 Al _0.008 Mg _0.008 Ti _0.004 O _{2 as} a positive electrode active material: 96.9 parts by mass, acetylene black as a conductive auxiliary agent: 1.5 parts by mass, and a binder A positive electrode mixture-containing composition was prepared in the same manner as in Example 1 except that 0.4 part by mass of VDF-CTFE and 1.2 parts by mass of PVDF were mixed to form a positive electrode mixture, and this positive electrode mixture was used. did.

Example 6
LiCo _0.98 Al _0.008 Mg _0.008 Ti _0.004 O _{2 as} a positive electrode active material: 97.3 parts by mass, acetylene black as a conductive auxiliary agent: 1.5 parts by mass, and VDF as a binder -CTFE: A positive electrode mixture-containing composition was prepared in the same manner as in Example 1 except that 1.2 parts by mass were mixed to obtain a positive electrode mixture, and this positive electrode mixture was used.

The same procedure as in Example 1 was conducted except that the positive electrode mixture-containing composition was used, and 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. Then, a square nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that this positive electrode was used.

Example 7
The same natural graphite as that used as the negative electrode active material in Example 1 and a surface-treated carbon material having an average particle diameter of 10 μm in which amorphous carbon is supported on the surface of natural graphite are mixed at a mass ratio of 1: 1. To obtain a mixture. This mixture (negative electrode active material): 97.5% by mass, SBR: 1.5% by mass, and carboxymethylcellulose (thickener): 1% by mass are mixed with water to form a slurry-like negative electrode mixture A composition was prepared. 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: 86 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 the above-mentioned negative electrode.

Comparative Example 1
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 2
LiCo _0.98 Al _0.008 Mg _0.008 Ti _0.004 O _{2 as} a positive electrode active material: 96.9 parts by mass, acetylene black as a conductive auxiliary agent: 1.5 parts by mass, and PVDF as a binder : 1.6 parts by mass were mixed to obtain a positive electrode mixture, and a positive electrode mixture-containing composition was prepared in the same manner as in Example 1 except that this positive electrode mixture was used.

A positive electrode was prepared in the same manner as in Comparative Example 1 except that the positive electrode mixture-containing composition was used, and a rectangular nonaqueous electrolyte secondary battery was prepared in the same manner as in Example 1 except that this positive electrode was used. did.

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 (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.

Comparative Example 4
A positive electrode was produced in the same manner as in Example 1 except that the same positive electrode mixture-containing composition as that prepared in Comparative Example 2 was used, and a rectangular nonaqueous electrolyte was obtained in the same manner as in Example 1 except that this positive electrode was used. A secondary battery was produced.

The composition of the binder in the positive electrode used in the nonaqueous electrolyte secondary batteries of Examples and Comparative Examples, the density of the positive electrode mixture layer (actual density) and the filling rate are shown in Table 1, the composition of the current collector, and Table 2 shows the amount of the non-aqueous electrolyte injected into these non-aqueous electrolyte secondary batteries. 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.

The following evaluations were performed on the nonaqueous electrolyte secondary batteries of Examples and Comparative Examples and the positive electrodes used in 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 evenly 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.), and crosshead 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.

<Peel strength between positive electrode mixture layer and current collector in positive electrode (peel strength)>
A sample obtained by cutting the double-sided coated part of the positive electrode into 10 cm in the longitudinal direction and 1 cm in the width direction was adhered to one side of a double-sided tape (“Nystack NW-15” manufactured by Nichiban) and the other side of the double-sided tape As shown in FIG. 3, this surface was adhered to the sample mounting surface 100 of a 90 ° peel tester (“TE-3001” manufactured by Tester Sangyo Co., Ltd.). The end of the sample (positive electrode 1) opposite to the side adhered to the sample setting surface 100 is sandwiched between jigs 101 of a 90 ° peeling tester and peeled at an angle of 90 ° with respect to the sample setting surface 100. The sample 1 was pulled in the longitudinal direction (the direction of the arrow in the figure) at a speed of 50 mm / min to peel off the positive electrode mixture layer and the current collector, and the strength at that time was measured. It can be evaluated that a higher-reliability battery can be formed because the higher the peel strength, the better the suppression of the positive electrode active material and the conductive additive from the positive electrode mixture layer.

<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 3.3 V, and the discharge capacity at that time was determined. For the battery of Example 1, the discharge capacity was measured under the same conditions as described above except that the upper limit voltage during charging was 4.2 V.

<Charge / discharge cycle characteristics evaluation of non-aqueous electrolyte secondary battery>
For each battery of the example and the comparative example, a series of operations of constant current-constant voltage charging and constant current discharging performed under the same conditions as in the above-described discharge capacity measurement except that the environmental temperature was set to 45 ° C. was set as one cycle. The number of cycles in which the discharge capacity was 60% or more of the discharge capacity in the first cycle was determined. For the battery of Example 1, the number of cycles was also measured under the same conditions as described above except that the upper limit voltage during charging was 4.2 V.

The evaluation results are shown in Table 3. In Table 3, the discharge capacity of each non-aqueous electrolyte secondary battery and the number of cycles 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. Moreover, in Table 3, about the nonaqueous electrolyte secondary battery of Example 1 and the comparative example 1, the discharge capacity calculated | required as the upper limit voltage at the time of charge being 4.2V, and the cycle number at the time of charge / discharge cycle characteristic evaluation were respectively referred Examples are shown as Example 1 and Reference Example 2.

As shown in Table 3, the nonaqueous electrolyte secondary batteries of Examples 1 to 7 having a positive electrode in which the current collector has an appropriate thickness and tensile strength and uses VDF-CTFE as the binder of the positive electrode mixture layer Compared to the case of the reference example in which the upper limit voltage of charging is 4.2 V, the discharge capacity is large and high capacity can be achieved. Further, the positive electrodes used in the batteries of Examples 1 to 7 have high bending strength and peel strength, and it can be said that the batteries of Examples 1 to 7 using these positive electrodes have good productivity and reliability.

In addition, since the nonaqueous electrolyte secondary batteries of Examples 1 to 7 have an upper limit voltage of charging of 4.35 V, the charge / discharge cycle is compared to the cases of Reference Examples 1 and 2 where the upper limit voltage is 4.2 V. Although the number of cycles at the time of characteristic evaluation is small, the number of cycles is larger than the batteries of Comparative Examples 1 to 4 charged with the same upper limit voltage as the batteries of Examples 1 to 7, and better charge / discharge cycle characteristics can be secured. Yes. In particular, the battery of Example 7 using a carbon material having a small particle diameter supporting amorphous carbon on the surface of graphite as the negative electrode active material has excellent charge / discharge cycle characteristics as compared with the batteries of other examples. ing.

That is, the battery of Comparative Example 1 in which the current collector has a thick positive electrode and the battery of Comparative Example 2 in which the current collector is thick and has a positive electrode that does not use VDF-CTFE as the binder of the positive electrode mixture layer. Compared with the battery of an Example, there are few cycles at the time of charging / discharging cycling characteristics evaluation, and charging / discharging cycling characteristics are inferior.

The battery of Comparative Example 3 having a positive electrode with a low tensile strength of the current collector and the battery of Comparative Example 4 having a positive electrode that did not use VDF-CTFE as the binder of the positive electrode mixture layer Compared to a battery, the discharge capacity is small, the number of cycles during evaluation of charge / discharge cycle characteristics is small, and charge / discharge cycle characteristics are inferior. Furthermore, since the positive electrodes used in the batteries of Comparative Examples 3 and 4 have low bending strength, it can be said that these batteries have poor productivity.

The present invention can be implemented in other forms as long as it does not depart from the spirit of the present invention. The embodiments disclosed in the present application are examples, and the present invention is not limited to these embodiments. The scope of the present invention is construed in preference to the description of the appended claims rather than the description of the above specification, and all modifications within the scope equivalent to the claims are construed in the scope of the claims. included.

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.

1 Positive electrode 2 Negative electrode 3 Separator

Claims

A non-aqueous battery having a spirally wound electrode body in which a positive electrode, a negative electrode, and a separator are wound in a spiral shape and having a flat cross section and a non-aqueous electrolyte, and an upper limit voltage for charging is set to 4.3 V or higher A positive electrode used in an electrolyte secondary battery,
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, the positive electrode for a non-aqueous electrolyte secondary battery.
The positive electrode for a nonaqueous electrolyte secondary battery according to claim 1, wherein the current collector has a thickness of 6 μm or more.
The content of the binder in the positive electrode mixture layer is 1 to 1.6% by mass, and the proportion of the vinylidene fluoride-chlorotrifluoroethylene copolymer in the total amount of the binder is 20% by mass or more. The positive electrode for nonaqueous electrolyte secondary batteries according to claim 1 or 2.
The positive electrode for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 3, 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 positive electrode is a positive electrode for a nonaqueous electrolyte secondary battery according to any one of claims 1 to 4,
A non-aqueous electrolyte secondary battery, wherein an upper limit voltage for charging is set to 4.3 V or more.
6. The non-aqueous electrolyte secondary battery according to claim 5, wherein the negative electrode contains a carbon material having an average particle diameter of 8 to 18 μm carrying amorphous carbon on the surface of graphite as a negative electrode active material.
The nonaqueous electrolyte secondary battery according to claim 5 or 6, wherein the nonaqueous electrolyte secondary battery has a rectangular tube-shaped outer can.