JP7289415B1 - Non-aqueous electrolyte secondary battery - Google Patents

Abstract

A non-aqueous electrolyte that can be used in a wide temperature range without impairing its performance, with little decrease in capacity even after repeated charge-discharge cycles at high temperatures, and with little dendrite precipitation even after repeated charge-discharge cycles below freezing. Provide secondary batteries. A nonaqueous electrolyte secondary battery according to the present invention includes an electrode element comprising a positive electrode, a negative electrode, and a separator, and a nonaqueous electrolyte, the nonaqueous electrolyte comprising: The non-aqueous electrolyte contains 2% or less vinylene carbonate with respect to the total mass of the non-aqueous electrolyte, the separator has a structure having slit-like pores or a lamellar skeleton structure, and has a porosity of 45% or more. It has a negative electrode current collector and a negative electrode mixture layer applied to the negative electrode current collector, the negative electrode mixture layer contains graphite particles as a negative electrode active material, and the specific surface area (m / g) of the graphite particles , and the median diameter (μm) of the graphite particles is 30 (×10 −6 m 3 /g) or less. [Selection drawing] Fig. 1

Description

The present invention relates to non-aqueous electrolyte secondary batteries.

Non-aqueous electrolyte secondary batteries are widely used because of their high energy density, and are used as power sources for small portable devices such as mobile phones, digital cameras, and laptop computers. In addition, non-aqueous electrolyte secondary batteries are being used in large-scale industrial applications, such as hybrid and electric vehicles, and electric power storage using natural energy sources such as solar and wind power, from the perspective of depletion of energy resources and global warming. is being developed.

In recent years, non-aqueous electrolyte secondary batteries have also attracted attention as power sources for powering drones, robots, electric vehicles, hybrid electric vehicles, etc., and are expected to expand their applications. Such power sources are required to have higher density and longer life, and to be able to be used in a wide temperature range without impairing their performance. Specifically, it is desirable that the motive power source should be capable of being repeatedly charged and discharged even at sub-freezing temperatures, with little decrease in capacity even after repeated charge-discharge cycles at high temperatures.

Graphite particles are generally used as the negative electrode active material of non-aqueous electrolyte secondary batteries from the viewpoint of increasing the energy density. In a non-aqueous electrolyte secondary battery using graphite particles as a negative electrode active material, many documents disclose techniques for discharging below freezing, but there are few documents disclosing that repeated charging is possible at below freezing. In particular, there is no technology related to non-aqueous electrolyte secondary batteries that can be charged and discharged at extremely low temperatures such as -20 ° C. or lower and that use graphite particles as a negative electrode active material, which hinders practical use in environments with particularly large temperature differences. It's here.

For example, Patent Literature 1 describes that by using a non-aqueous electrolyte to which vinylene carbonate (VC) is added, a dense thin film is formed on the surface of the negative electrode, thereby improving cycle characteristics and the like.
However, when graphite particles are used as the negative electrode active material, the addition of VC to the non-aqueous electrolyte improves the cycle characteristics of the non-aqueous electrolyte secondary battery at room temperature and high temperature. The reaction resistance of the negative electrode increases at low temperatures, and dendrite deposition tends to occur during charging.

On the other hand, for example, in Patent Document 2, by using LiBF ₄ and fluoroethylene carbonate in addition to VC as additives for the non-aqueous electrolyte, the denseness of the film on the negative electrode surface is reduced, and during charging at low temperatures It is described that dendrite precipitation can be suppressed.
However, the additive described in Patent Document 2 has a high reduction potential and is likely to generate gas, so there is a problem that it leads to deterioration during high-temperature cycles. In this way, it is not possible to avoid a trade-off between low-temperature characteristics and high-temperature cycle characteristics simply by changing the composition of the electrolyte additive, and it is possible to use it in a wide temperature range without impairing its performance. It has been considered difficult to provide a non-aqueous electrolyte secondary battery.

Here, in order to make the dendrite deposition less likely to occur during the above-described charging at sub-zero temperatures, it is of course possible to consider changing the design specifications of the separator and graphite particles. For example, Patent Document 3 describes that rate characteristics and cycle performance are improved by using a separator having a low electrical resistance and a low Gurley number.
Further, it is widely known that input/output characteristics are improved by reducing the particle size of graphite particles and increasing the surface area thereof.

Japanese Patent No. 3066126 JP 2013-218967 A Japanese Patent Application Laid-Open No. 2021-64620

However, when the design specifications of the separator and graphite particles are changed as described above, the reaction between the electrolytic solution and the graphite particles is likely to occur at high temperature, and there is a problem that the deterioration of the cycle characteristics is increased. In addition, simply reducing the particle size or increasing the surface area of the graphite particles may not necessarily suppress dendrite precipitation during sub-zero charging. Therefore, a trade-off between low-temperature cycle characteristics and high-temperature cycle characteristics cannot be avoided simply by changing these design specifications. It has been considered difficult to provide a secondary battery.

The present invention has been made in view of the above, and has little capacity decrease even when charging and discharging cycles are repeated at high temperatures, and dendrite precipitation is less likely to occur even when charging and discharging are repeated below freezing. Performance in a wide temperature range. An object of the present invention is to provide a non-aqueous electrolyte secondary battery that can be used without damaging the

In order to solve the above-described problems and achieve the object, as a first aspect, the non-aqueous electrolyte secondary battery according to the present invention comprises an electrode element consisting of a positive electrode, a negative electrode and a separator, and a non-aqueous electrolyte. A non-aqueous electrolyte secondary battery, wherein the non-aqueous electrolyte contains vinylene carbonate in an amount of 2% or less with respect to the total mass of the non-aqueous electrolyte, and the separator has a structure or lamella having slit-like pores It has a skeletal structure and a porosity of 45% or more, and the negative electrode includes a negative electrode current collector and a negative electrode mixture layer applied to the negative electrode current collector, and the negative electrode mixture layer is , containing graphite particles as a negative electrode active material, wherein the product of the specific surface area (m ² /g) of the graphite particles and the median diameter (μm) of the graphite particles is 30 (×10 ⁻⁶ m ³ /g) or less. It is characterized by

In addition, as a second aspect, the non-aqueous electrolyte secondary battery according to the present invention is a non-aqueous electrolyte secondary battery comprising an electrode element consisting of a positive electrode, a negative electrode, and a separator, and a non-aqueous electrolyte, The aqueous electrolyte contains vinylene carbonate in an amount of 2% or less with respect to the total mass of the non-aqueous electrolyte, the separator is formed by a dry stretching method, has a porosity of 45% or more, and the negative electrode has a negative electrode current collector and a negative electrode mixture layer applied to the negative electrode current collector, the negative electrode mixture layer contains graphite particles as a negative electrode active material, and the specific surface area of the graphite particles ( m ² /g) and the median diameter (μm) of the graphite particles is 30 (×10 ⁻⁶ m ³ /g) or less.

As a third aspect, the non-aqueous electrolyte secondary battery according to the present invention is characterized in that, in the above-described first and second aspects, the separator does not contain a filler.

Further, as a fourth aspect of the non-aqueous electrolyte secondary battery according to the present invention, in any one of the first to third aspects, the non-aqueous electrolyte further contains a lithium salt, and the lithium The salt concentration is 1.0 mol/L or more and 1.5 mol/L or less.
The lithium salt concentration of 1.0 mol/L means that 1 L of the non-aqueous electrolyte contains 1.0 mol of the lithium salt.

In addition, as a fifth aspect of the non-aqueous electrolyte secondary battery according to the present invention, in any one of the first to fourth aspects, the lithium salt is _LiPF6 . .

Further, as a sixth aspect of the non-aqueous electrolyte secondary battery according to the present invention, in any one of the first to fifth aspects, the positive electrode comprises a positive electrode current collector and the positive electrode current collector a positive electrode mixture layer coated on a positive electrode active material, the positive electrode mixture layer having the general formula LiMO ₂ (M is Na, Mg, Sc, Y, Mn, Fe, Co, Ni, one or more elements selected from Cu, Zn, Al, Ti, Cr, Pb, Sb and B).

In addition, as a seventh aspect of the non-aqueous electrolyte secondary battery according to the present invention, in any one of the first to sixth aspects, the negative electrode contains only graphite particles as a negative electrode active material. characterized by

In addition, as an eighth aspect, the non-aqueous electrolyte secondary battery according to the present invention, in any one of the above first to seventh aspects, has an upper limit voltage of 4.2 V and a voltage of 0.2 V in an environment of -20°C. 2C, when CC-CV charging with a cutoff current of 0.05C and CC discharging with a lower limit voltage of 2.7V and 1.0C are repeated 60 cycles, the upper limit voltage is 4.2V in an environment of 25°C before and after that, The capacity retention rate is 95 when a capacity confirmation test is performed by performing one cycle of CC-CV charging at 0.5 C and a cutoff current of 0.05 C and CC discharging at a lower limit voltage of 2.7 V and 0.5 C. % or more.

According to the present invention, even if the charge-discharge cycle is repeated at high temperatures, there is little decrease in capacity, and even if the charge-discharge is repeated at sub-zero temperatures, dendrite precipitation does not occur easily. A non-aqueous electrolyte secondary battery can be obtained.

FIG. 1 is a diagram for explaining the configuration of a non-aqueous electrolyte secondary battery according to one embodiment of the present invention. FIG. 2 is a diagram for explaining the configuration of an electrode group included in a non-aqueous electrolyte secondary battery according to an embodiment of the invention. FIG. 3 is a diagram showing a part of the AA line cross section shown in FIG.

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below, but the present invention is not limited to the following description. In addition, various modifications or improvements can be added to the present embodiment, and forms to which such modifications or improvements are added can also be included in the present invention.

(Embodiment)
FIG. 1 is a diagram for explaining the configuration of a non-aqueous electrolyte secondary battery according to one embodiment of the present invention. FIG. 2 is a diagram for explaining the configuration of an electrode group included in a non-aqueous electrolyte secondary battery according to an embodiment of the invention. FIG. 3 is a diagram showing a part of the AA line cross section shown in FIG.

A non-aqueous electrolyte secondary battery 1 includes a laminate sheet 2 , a positive electrode terminal 3 , a negative electrode terminal 4 , an electrode group 5 and a sealant 6 . The electrode group 5 is composed of a positive electrode 7 , a negative electrode 8 and a separator 9 .

The laminate sheet 2 accommodates the electrode group 5 and constitutes the exterior body of the non-aqueous electrolyte secondary battery 1 . The laminate sheet 2 has a rectangular outer edge. The laminate sheet 2 is formed by bonding two sheet members with the electrode group 5 interposed therebetween. One side of the electrode group 5 is provided with a convex sheet member, and the other side of the electrode group 5 is provided with a concave sheet member. At the periphery of the laminate sheet 2 , the sheet members are sealed with each other by heat welding, and the electrode group 5 is sealed by the laminate sheet 2 . Moreover, the positive electrode terminal 3 and the negative electrode terminal 4 are extended from a part of the peripheral portion of the laminate sheet 2 and have a structure in which the laminate sheet 2 is thermally welded. Specifically, a sealant 6 is interposed as an adhesive layer between the positive electrode terminal 3 and the negative electrode terminal 4 and the laminate sheet 2 . That is, the portions of the surfaces of the positive electrode terminal 3 and the negative electrode terminal 4 that pass through the sealing portion of the laminate sheet 2 are coated with the sealant 6 . With such a configuration, since the heat-fusible resin layer on the inner surface of the sealant 6 and the laminate sheet 2 are joined by heating, it is possible to prevent the non-aqueous electrolyte from penetrating and leaking.

One end of the positive electrode terminal 3 is connected to a positive electrode 7 forming the electrode group 5 . The positive electrode terminal 3 and the positive electrode 7 are electrically connected via a positive current collecting lead 10 . The positive collector lead 10 is connected to the positive terminal 3 via the welded portion 12 .

One end of the negative electrode terminal 4 is connected to the negative electrode 8 forming the electrode group 5 . The negative electrode terminal 4 and the negative electrode 8 are electrically connected via a negative electrode current collecting lead 11 (see FIG. 3). The negative electrode current collecting lead 11 is connected to the negative electrode terminal 4 via the welded portion 12 .

The positive electrode 7 and the negative electrode 8 constitute an electrode group 5 by being laminated with a separator 9 interposed therebetween. The electrode group 5 is alternately laminated in the order of the separator 9, the negative electrode 8, the separator 9, the positive electrode 7, and the separator 9 from the outermost layer (see FIG. 3).

A non-aqueous electrolyte is held in the laminate sheet 2 . Although not shown, the non-aqueous electrolyte is prepared by dissolving a lithium salt in a non-aqueous solvent.

Although FIG. 1 shows a configuration example in the case of a laminated nonaqueous electrolyte secondary battery as an example of the embodiment, the shape of the nonaqueous electrolyte secondary battery in the present invention is not particularly limited. , a cylindrical shape, a rectangular shape, a coin shape, or the like. In addition, the exterior body of the non-aqueous electrolyte secondary battery is not particularly limited, and known materials such as laminated film, aluminum, aluminum alloy, and stainless steel can be used.
The non-aqueous electrolyte, positive electrode, negative electrode, separator, and package (laminate sheet) are described in detail below.

(Non-aqueous electrolyte)
The non-aqueous electrolyte contains a lithium salt, VC, a non-aqueous solvent, and other additives.

Lithium salts include, for example, one selected from LiPF ₆ , LiBF ₄ , LiClO ₄ , Li(FSO ₂ ) ₂ N, Li(CF ₃ SO ₂ ) ₂ N, or a mixture of two or more, It preferably contains at least _LiPF6 . LiPF ₆ is preferably contained as a lithium salt by comprehensively judging the solubility in non-aqueous solvents, the charge/discharge characteristics when used as a secondary battery, the output characteristics, the cycle characteristics, and the like. The concentration of the lithium salt is preferably 0.5 mol/L or more, and preferably 1.0 mol/L or more, in order to sufficiently contain the lithium salt-derived component when forming the film in the initial activation. is more preferred. Also, if the concentration of the lithium salt is too high, a film derived from VC will not be formed sufficiently and the high-temperature cycle characteristics will deteriorate. More preferred.

Non-aqueous solvents include, but are not limited to, dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), propylene carbonate (PC), One or a mixed solvent of two or more selected from methyl propionate, methyl acetate, methyl formate, methyl butyrate, dioxolane, 2-methyltetrahydrofuran, tetrahydrofuran, dimethoxyethane, γ-butyrolactone, acetonitrile, and benzonitrile. DMC, DEC, DPC, EMC, EC, and PC are particularly preferred as the non-aqueous solvent, and EC is particularly preferred from the viewpoint of good film formation on the negative electrode active material.

On the other hand, if the amount of VC added is too large, the film will be excessively formed, and dendrite precipitation will easily occur. Therefore, the amount of VC added is preferably 2% or less with respect to the total mass of the non-aqueous electrolyte. On the other hand, if the amount of VC added is less than 0.1% with respect to the total mass of the non-aqueous electrolyte, the formation of the film tends to be insufficient, which is not suitable.

In addition, as an additive other than the lithium salt, a substance other than VC may be included as necessary, such as fluoroethylene carbonate, 1,3,2-dioxathiolane 2,2-dioxide, 1,5,2,4- Dioxadithiane 2,2,4,4-tetraoxide, tris(trimethylsilyl)phosphite, 1-propene 1,3-sultone, Li ₂ PO ₂ F ₂ and other known additives. However, if these additives are added excessively, gas is likely to be generated during high-temperature cycles, causing a decrease in capacity. It is preferably 1% or less, more preferably 1% or less. Moreover, when these additives are used, they must be used together with VC from the viewpoint of suppressing gas generation.

(positive electrode)
The positive electrode 7 has a positive electrode current collector 7a and a positive electrode mixture layer 7b applied to the positive electrode current collector 7a.

The positive electrode current collector 7a is not particularly limited, but it is preferable to use a metal. Specific examples include aluminum, nickel, stainless steel, titanium, and other alloys. Among them, aluminum is preferable from the viewpoint of electronic conductivity and battery operating potential. Moreover, the thickness of the positive electrode current collector 1a is preferably 1 to 50 μm.

From the viewpoint of energy density, the positive electrode mixture layer 7b has a positive electrode active material of general formula LiMO ₂ such as LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiCoNiO ₂ (M is commonly Na, Mg, Sc, Y, one or more elements selected from Mn, Fe, Co, Ni, Cu, Zn, Al, Ti, Cr, Pb, Sb and B), and these positive electrode active materials At least one active material particle selected from the group consisting of layered oxides such as those in which a part of the element is replaced with a different element, and a ternary system containing Co, Ni and Mn, and two or more They may be used in combination. For the purpose of improving cycle characteristics and thermal stability, inorganic substances such as magnesium oxide, aluminum oxide, aluminum fluoride, niobium oxide, titanium oxide or tungsten oxide, or polyethylene glycol or polyethylene may be added to the surface of the layered oxide. A coating of ion-conducting polymers such as oxides, derivatives or salts thereof may be applied. The average particle diameter (D50: median diameter) of the layered oxide is preferably 1 μm or more and 100 μm or less, more preferably 1 μm or more and 20 μm or less.

In addition, the positive electrode mixture layer 7b may further contain an olivine compound as a positive electrode active material for the purpose of improving thermal stability. Examples of olivine compounds include LiFePO ₄ and LiFePO ₄ in which part of Fe is substituted with Mn. The surface of the olivine compound may be coated with a carbon material for the purpose of improving electrical conductivity. The mass ratio of the olivine compound to the total mass of the positive electrode active material is preferably 40% by mass or less from the viewpoint of energy density, and is preferably 10% by mass or more from the viewpoint of sufficiently improving thermal stability. preferable. The average particle size (D50) of the olivine compound is preferably smaller than the average particle size of the layered oxide from the viewpoint of filling property and ion conductivity.

From the viewpoint of ensuring conductive paths in the electrodes, the positive electrode mixture layer 7b preferably further contains a known conductive agent. Examples of known conductive agents include, but are not limited to, graphite, graphene, carbon black, activated carbon and carbon fiber. One type may be selected from these conductive agents and used, or two or more types may be used in combination.

A known binder may also be used for the positive electrode mixture layer 7b. Examples of known binders include, but are not limited to, polyvinylidene fluoride, polytetrafluoroethylene, polyvinylpyrrolidone, polyvinyl fluoride, polyethylene, polypropylene, nitrile resins, styrene-butadiene rubber, acrylic resins and copolymers thereof. mentioned. One type may be selected from these binders and used, or two or more types may be used in combination. In addition, other known additives such as dispersants, pH adjusters, and water scavengers may be included.

From the viewpoint of energy density, the density of the positive electrode mixture layer 7b is preferably 2.3 g/cm ³ or more, more preferably 2.5 g/cm ³ or more. Further, in order to secure appropriate voids inside the positive electrode mixture layer 7b to maintain good electrolyte impregnation and obtain good cycle characteristics, it is preferably 3.5 g/cm ³ or less, and 2.9 g/cm 3 or less. cm ³ or less is more preferable.

The positive electrode mixture layer 7b is applied to one side or both sides of the positive electrode current collector 7a, and the coating amount per one side is preferably 75 g/m ² or more and 200 g/m ² or less. By setting the coating amount of the positive electrode mixture layer 7b to the above value, it is possible to ensure a sufficient energy density and to maintain good output characteristics and cycle characteristics. Moreover, in order to form the positive electrode current collecting lead 10, it is preferable to leave a region where the positive electrode mixture layer 7b is not applied on the positive electrode current collector 7a.

(negative electrode)
The negative electrode 8 has a negative electrode current collector 8a and a negative electrode mixture layer 8b applied to the negative electrode current collector 8a.

Although the negative electrode current collector 8a is not particularly limited, it is preferable to use a metal. Specific examples include aluminum, nickel, stainless steel, titanium, and other alloys. Among them, copper is preferable from the viewpoint of electronic conductivity and battery operating potential. Moreover, the thickness of the negative electrode current collector 8a is preferably 1 μm or more and 50 μm or less. In order to form the negative electrode current collecting lead 11, it is preferable to leave a region where the negative electrode mixture layer 8b is not applied on the negative electrode current collector 8a.

From the viewpoint of increasing the density and operating voltage, the negative electrode mixture layer 8b preferably contains only graphite particles as the negative electrode active material. The graphite particles have a product of specific surface area S (m ² /g) and median diameter (D50) (μm) of 30 (×10 ⁻⁶ m ³ /g) or less. It is preferably 5 (×10 ⁻⁶ m ³ /g) or more from the viewpoint of output characteristics, suppression of increase in initial irreversible capacity, and high density of electrodes. Here, the specific surface area is preferably 1 m ² /g or more from the viewpoint of output characteristics, and preferably 10 m ² /g or less to suppress an increase in initial irreversible capacity due to excessive film formation. Also, the median diameter is preferably 5 μm or more from the viewpoint of high density, and preferably 20 μm or less to keep good output characteristics.
Other negative electrode active materials include silicon-based and lithium-titanium oxide-based materials, but silicon-based particles may collapse due to cycles, and lithium-titanium oxide-based materials may have a lower energy density per mass. Therefore, it is preferable to use only graphite particles.

In addition, the negative electrode mixture layer 8b may contain a thickener as needed. Examples of thickening agents include carboxymethyl cellulose and polyvinyl alcohol. In addition, other additives such as dispersants may be included.

The negative electrode mixture layer 8b is applied to one side or both sides of the negative electrode current collector 8a. The coating amount per side is preferably 40 g/m ² or more and 120 g/m ² or less. By setting the coating amount of the negative electrode mixture layer 8b to the above value, it is possible to secure a sufficient energy density and to maintain good output characteristics and cycle characteristics. In addition, from the viewpoint of suppressing deposition of lithium metal dendrites during charging, the coating amount of the negative electrode mixture layer 8b is preferably determined so that the positive/negative electrode capacity ratio is 1.01 or more, and is set to 1.1 or more. It is more preferable to define
Here, the positive/negative electrode capacity ratio is calculated by the following formula (1).
(positive and negative electrode capacity ratio) = (negative electrode capacity of one side per area)
/ (capacity of positive electrode on one side per area) (1)
However, if the value of the positive/negative electrode capacity ratio is too large, the amount of the negative electrode mixture layer 8b that does not contribute to the charge/discharge reaction will increase and the energy density will decrease. It is preferably 1.2 or less, more preferably 1.2 or less.

From the viewpoint of energy density, the density of the negative electrode mixture layer 8b is preferably 1.0 g/cm ³ or more, more preferably 1.2 g/cm ³ or more. In addition, in order to obtain good cycle characteristics by ensuring appropriate voids inside the negative electrode mixture layer 8b and maintaining good impregnation with the electrolyte, the density of the negative electrode mixture layer 8b is set to 1.7 g/cm ^{3 .} It is preferably 1.5 g/cm 3 or less, more preferably 1.5 g/cm ³ or less.

(separator)
The separator 9 is formed by a dry drawing method. In general, the method for producing a separator is roughly divided into two types: a dry drawing method and a wet method. It has a structure in which a portion and an amorphous portion with low crystallinity are mixed. For example, the separator 9 produced by the dry stretching method has a structure in which a plurality of slit-shaped (flat plate-shaped) pores are formed (slit-shaped pore structure), and crystals in which the polymer is partially folded. form a lamellar skeletal structure. On the other hand, a separator produced by a wet method has a mesh shape, for example, a spider web shape (see, for example, Japanese Patent Application Laid-Open No. 2020-198316). Therefore, a person skilled in the art can easily distinguish between separators produced by the dry stretching method and separators produced by the wet method by observation of electron microscope images or the like.

Moreover, the porosity of the separator 9 needs to be 45% or more from the viewpoint that the vinylene carbonate required for forming a film on the surface of the negative electrode in the initial activation is less likely to be diffusion rate-determining. Here, the porosity is calculated from the actual volume measured by a mercury intrusion porosimeter and the theoretical volume calculated from the electrode constituent material. If the porosity of the separator 9 is less than 45%, diffusion of substances in the electrolytic solution, including vinylene carbonate, tends to be rate-determining, making it difficult to form a thin and uniform film. In addition, diffusion of lithium ions during charging tends to be rate-determining at low temperatures, so dendrite precipitation is even more likely to occur. Moreover, the porosity of the separator 9 is desirably 65% or less. If the porosity of the separator 9 is more than 65%, the strength will be low, which may cause problems in workability. Also, those with a percentage of 65% or less are more readily available than those with a percentage of more than 65%.

The JIS Gurley of the separator 9 is desirably within 300 seconds/100 mL. Here, JIS Gurley is the value of air resistance (seconds/100 mL) obtained by the measurement method specified in JIS P8117:2009.

At least polyethylene (PE) is preferably used for the separator 9 from the viewpoint of ensuring safety against short circuits and overcharging by the shutdown function. The separator 9 may be composed only of PE, or may have a layer structure in which PE is combined with polypropylene (PP), polyimide, aramid, or the like from the viewpoint of heat resistance and oxidation resistance. For example, a separator having a three-layer structure of PP/PE/PP in which PP is arranged on both sides of a base layer made of PE may be used. In addition, the separator 9 preferably does not contain a filler made of inorganic particles such as alumina, boehmite, magnesium oxide, etc., which promotes the decomposition of the electrolytic solution during high-temperature cycles. good too.

The shape of the electrode element is not particularly limited. good too.

(Exterior body)
The laminate sheet 2 used for the exterior body is a multilayer film consisting of a metal layer and a resin layer covering the metal layer. For the metal layer, it is desirable to use aluminum or an aluminum alloy containing aluminum as a main component for weight reduction. The laminate sheet 2 uses an aluminum sheet (sheet member) having a thickness of 138 to 168 μm as a base material. The aluminum sheet is provided with a heat-fusible resin layer (for example, PE, PP, etc.) having a thickness of 40 to 100 μm on one side of the base layer, and an insulating thick layer on the other side. It has a protective layer (eg, polyethylene terephthalate, nylon, etc.) with a thickness of 10 to 20 μm, and is laminated in a three- to five-layer structure.

The reason why the non-aqueous electrolyte secondary battery 1 having the configuration described above can achieve both excellent high-temperature cycle characteristics and an effect of suppressing dendrite deposition during charging below freezing is as follows.

First, in the non-aqueous electrolyte secondary battery 1, since the non-aqueous electrolyte contains VC, it becomes easier to form an appropriately dense film on the surface of the negative electrode during initial activation, thereby realizing excellent high-temperature cycle characteristics. can do. Here, if the coating component derived from VC is too large, the density of the coating increases, which may lead to an increase in resistance at low temperatures. It is necessary to keep the denseness of the film moderately.

The lithium salt in the non-aqueous electrolyte must be supplied through the separator 9 without stagnation in order to ensure that the lithium salt-derived component is sufficiently contained during film formation. When the lithium salt is consumed by forming a film on the surface of the negative electrode 8, the lithium salt concentration at the interface between the separator 9 and the negative electrode 8 tends to temporarily decrease. In order to solve this immediately, it is necessary to supply the lithium salt from the positive electrode side to the negative electrode side inside the separator 9 without delay. Therefore, by using a separator made by a dry stretching method as the separator 9, the lithium salt of the non-aqueous electrolyte is supplied without delay due to the lamellar skeleton structure having a straight slit-like pore structure. and the negative electrode 8, the lithium salt concentration decrease is suppressed. As a result, when the film is formed, the lithium salt-derived component is sufficiently contained, and dendrite precipitation is less likely to occur when the battery is charged at a low temperature. On the other hand, when a separator having a non-straight pore structure, which is produced by a wet method or the like, is used, it is difficult to eliminate the uneven lithium salt concentration inside the separator during the film formation, and the lithium salt concentration on the surface of the negative electrode 8 is difficult to eliminate. Film formation is likely to occur in a state in which is deficient. As a result, the film derived from VC is excessively formed and the density becomes too high, so dendrite precipitation easily occurs at low temperatures. With regard to the pore structure of the separator 9, no clear tendency of influence on high-rate charging/discharging, direct current resistance, etc. after complete film formation has been observed. Relationships were never discussed. However, as a result of extensive research by the inventors of the present application, the use of a separator 9 having a lamellar skeleton structure having a slit-like pore structure produced by a dry stretching method is necessary to obtain a non-aqueous electrolyte secondary battery having desired characteristics. was found to be one of the essential requirements for

Further, when the porosity of the separator 9 is equal to or higher than the above value, the permeability of the non-aqueous electrolyte is improved, so that the lithium salt is naturally easily supplied. However, the porosity is only a value that indicates the structure of the separator, and does not uniquely determine the ease of diffusion of the lithium salt in the non-aqueous electrolyte. However, it is not enough to stop the supply of lithium salt. That is, the separator 9 has both a porosity value equal to or less than the above value and a lamellar skeleton structure having a straight slit-like pore structure generated when produced by a dry stretching method. Only by using it can the components derived from the lithium salt be sufficiently contained at the time of film formation.

In addition, since the straight pore structure of the separator 9 can reduce the contact area with the non-aqueous electrolyte, deterioration due to side reactions during high-temperature cycles can be suppressed.

In order to uniformly contain the VC-derived component and the lithium salt-derived component in the non-aqueous electrolyte when forming the film, the surface of the graphite particles constituting the negative electrode mixture layer 8b must be smooth. There must be one, that is, the surface roughness must be small. The surface roughness of graphite particles can be compared by the value of (specific surface area)×(median diameter). For example, if the roughness calculation target is assumed to be a perfect sphere with a uniform mass distribution, the specific surface area is proportional to the reciprocal of the radius. can be done. If this idea is applied to solids that are not limited to spheres, (specific surface area) x (median diameter) can be a feature value that expresses surface roughness. can be inferred to have By using smooth graphite particles with a value of (specific surface area)×(median diameter) of 30 (×10 ⁻⁶ m ³ /g) or less as the negative electrode active material, it becomes easier to form a more uniform film, resulting in , the proportion of VC-derived components is less likely to increase locally, and dendrite precipitation can be suppressed by the above effect. On the other hand, in the case of graphite particles having a value of (specific surface area)×(median diameter) larger than 30 (×10 ⁻⁶ m ³ /g), the surface roughness of the graphite particles is large. The ratio of the component and the lithium salt-derived component becomes non-uniform, and there are places where the ratio of the VC-derived component is locally high. At that location, the reaction resistance at low temperatures locally increases, and dendrite precipitation tends to occur.
Here, the value of (specific surface area)×(median diameter) is 5 (×10 ⁻⁶ m ³ /g) or more from the viewpoint of output characteristics, suppression of increase in initial irreversible capacity, and high density of electrodes. is preferred. This lower limit is calculated based on the specific surface area and median diameter of the graphite particles.

Furthermore, graphite particles with a smooth surface such that the value of (specific surface area) × (median diameter) is equal to or less than the above upper limit can suppress excessive contact with the electrolyte during cycling, so the reduction of the electrolyte The effect is that deterioration due to a decomposition reaction is less likely to occur.

In addition, as described above, the concentration of the lithium salt in the non-aqueous electrolyte is preferably 1.0 mol/L or more in order to ensure that the lithium salt is sufficiently supplied when the film is formed on the negative electrode surface. . However, if the lithium salt is too excessive, the film derived from vinylene carbonate will not be formed sufficiently and the high-temperature cycle characteristics will deteriorate, so the concentration of the lithium salt is preferably 1.5 mol/L or less. In order to form a film derived from a lithium salt of good quality, the lithium salt is preferably LiPF ₆ .

By adopting the above-described structure, it is possible to form a coating on the surface of the negative electrode that contains an appropriate amount of the VC-derived component and the lithium salt-derived component when the coating is formed in the initial activation, and that has an appropriate density. It is possible to cause a peculiar phenomenon that cannot be obtained by a single component or a random combination. Therefore, dendrite deposition due to increased resistance can be suppressed even during charging below freezing. In addition, since there is no excessive contact between the electrolyte and other components, it is possible to prevent deterioration during high-temperature cycles. It becomes possible to provide batteries.

EXAMPLES The present invention will be described in more detail below with reference to Examples, but the present invention is not limited to the following embodiments.

(Example 1)
<Preparation of positive electrode>
90% by mass of LiNi _0.5 Co _0.2 Mn _0.3 O ₂ (NCM) as a positive electrode active material, 3% by mass of graphite particles as a first conductive agent, and 3% by mass of acetylene black as a second conductive agent %, 4% by mass of polyvinylidene fluoride (PVDF) as a binder, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) as a viscosity adjusting solvent to prepare a positive electrode active material slurry.

The obtained positive electrode active material slurry was applied to both surfaces of a 20 μm thick aluminum foil as a positive electrode current collector and dried to form a positive electrode mixture layer. At this time, the coating amount per one side of the positive electrode mixture layer was 96 g/m ² . Subsequently, the positive electrode was press-worked to set the density of the positive electrode mixture layer to 2.5 g/cm ³ . After that, it was cut so that the non-coated portion protruded in a rectangular shape as a positive electrode current collecting lead from one side of the rectangle where the positive electrode mixture layer was coated.

<Production of negative electrode>
96.7% by mass of graphite particles having a specific surface area of 2.1 m ² /g and a median diameter (D50) of 6.4 μm as a negative electrode active material, 0.3% by mass of acetylene black as a conductive agent, and styrene as a binder 1.5% by mass of butadiene rubber, 1.5% by mass of carboxymethyl cellulose as a thickener, and an appropriate amount of ion-exchanged water as a viscosity adjusting solvent were mixed to prepare a negative electrode active material slurry.

The obtained negative electrode active material slurry was applied to both surfaces of a copper foil having a thickness of 10 μm, which was a negative electrode current collector, and dried to form a negative electrode mixture layer. At this time, the coating amount per one side of the negative electrode mixture layer was 58 g/m ² . Subsequently, the positive electrode was press-worked to set the density of the negative electrode mixture layer to 1.45 g/cm ³ . After that, it was cut so that the non-coated portion protruded in a rectangular shape as a negative electrode current collecting lead from one side of the rectangle where the negative electrode mixture layer was coated.

<Production of electrode element>
Next, an electrode element was produced by alternately laminating the positive electrode and the negative electrode with the separators connected in a zigzag shape. The separator used had a three-layer structure of PE/PP/PE produced by a dry stretching method. This separator has a slit-like pore structure because it is produced by a dry drawing method. The JIS Gurley of the prepared separator is 250 seconds/100 mL, and the porosity is 48.7%. Subsequently, the positive electrode current collecting lead and the negative electrode current collecting lead were bundled and connected to the positive electrode current collecting tab and the negative electrode current collecting tab by ultrasonic welding.

<Adjustment of non-aqueous electrolyte>
A mixed solvent of ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate in a volume ratio of 2:5:3 was mixed with 1.3 mol/L of LiPF ₆ as a lithium salt and 1.0% by mass of VC as an additive. was used as a non-aqueous electrolyte.

<Production of non-aqueous electrolyte secondary battery>
As the exterior body, prepare two rectangular laminated films having a structure in which a heat-sealable resin layer made of polyolefin, a metal layer made of aluminum foil, and a protective layer made of nylon resin and polyester resin are laminated in this order. bottom. The heat-sealable resin layers of the two laminate films were arranged so as to face each other, and the laminate films were superimposed by mirror alignment so that the electrode element was housed in the two housing recesses. The electrode elements were arranged such that the portions of the terminals (tabs) where the heat-sealable resin portions were formed passed between the peripheral edges of the two laminate films, and a portion of each terminal was exposed to the outside. In this state, the heat-sealing resin layers of the peripheral edges of the laminate films were heat-sealed on three sides including the side from which each terminal of the laminate films extended. Subsequently, the non-aqueous electrolytic solution prepared above was injected from one side of the exterior body that was not heat-sealed. Next, under a reduced pressure environment, the remaining one side of the exterior body was heat-sealed to fabricate a non-aqueous electrolyte secondary battery.

<Evaluation by -20°C cycle test>
Evaluation by a −20° C. cycle test was performed on the produced non-aqueous electrolyte secondary battery. In an environment of −20° C., 60 cycles of CC-CV charging with an upper limit voltage of 4.2 V, 0.2 C and a cutoff current of 0.05 C and CC discharge with a lower limit voltage of 2.7 V and 1.0 C were repeated. Before and after that, CC-CV charging with an upper limit voltage of 4.2 V, 0.5 C and a cutoff current of 0.05 C and CC discharging with a lower limit voltage of 2.7 V and 0.5 C were performed one cycle each in an environment of 25 ° C. Conduct a capacity confirmation test, and assume that the discharge capacity in the capacity confirmation test before the charge/discharge cycle in a -20°C environment is 100%. The discharge capacity at -20°C was taken as the capacity retention rate [%] after the cycle.

<Evaluation by 45°C cycle test>
Evaluation by a 45° C. cycle test was performed on the produced non-aqueous electrolyte secondary battery. In an environment of 45° C., 500 cycles of CC-CV charging with an upper limit voltage of 4.2 V, 1.0 C and a cutoff current of 0.05 C and CC discharge with a lower limit voltage of 2.7 V and 1.0 C were repeated. Before and after that, CC-CV charging with an upper limit voltage of 4.2 V, 0.5 C and a cutoff current of 0.05 C and CC discharging with a lower limit voltage of 2.7 V and 0.5 C were performed one cycle each in an environment of 25 ° C. Discharge in the capacity confirmation test after the charge-discharge cycle in a 45 ° C. environment when the discharge capacity in the capacity confirmation test before the charge-discharge cycle in the 45 ° C. environment is considered to be 100%. The capacity was taken as the capacity retention rate [%] after the 45° C. cycle.

<Comprehensive evaluation of cycle test results>
If the capacity retention rate is less than 95% in the −20° C. cycle test, it is very likely that dendrite precipitation has occurred. Also, when the capacity retention ratio is less than 75% in the 45° C. cycle test, the cycle characteristics at high temperatures are not sufficient. In the comprehensive judgment, all the cases where the capacity retention rate of the −20° C. cycle test was less than 95% or the capacity retention rate of the 45° C. cycle test was less than 75% were evaluated as “×”. Only when the capacity retention rate of the −20° C. cycle test was 95% or more and the capacity retention rate of the 45° C. cycle test was 80% or more, the comprehensive evaluation was given as “⊚”. Those that were neither "×" nor "⊚" were overall judged as "○".
Table 1 shows physical properties and test results in Example 1.

(Example 2)
In Example 2, a non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1, except that graphite particles having a specific surface area of 3.3 m ² /g and a median diameter of 7.6 μm were used as the negative electrode active material. bottom. Table 1 shows physical properties and test results in Example 2.

(Example 3)
In Example 3, a non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1, except that graphite particles having a specific surface area of 1.3 m ² /g and a median diameter of 18.5 μm were used as the negative electrode active material. bottom. Table 1 shows physical properties and test results in Example 3.

(Example 4)
In Example 4, a non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1, except that a solution obtained by dissolving 0.9 mol/L of LiPF ₆ as a lithium salt was used as the non-aqueous electrolyte. Table 1 shows physical properties and test results in Example 4.

(Example 5)
In Example 5, a nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that a solution obtained by dissolving 1.6 mol/L of LiPF ₆ as a lithium salt was used as the nonaqueous electrolyte. Table 1 shows physical properties and test results in Example 5.

(Example 6)
In Example 6, the separator was a separator having a three-layer structure of PE/PP/PE produced by a dry stretching method and containing a filler on the positive electrode side. A water electrolyte secondary battery was produced. Table 1 shows physical properties and test results in Example 6.

(Example 7)
In Example 7, a non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1, except that a non-aqueous electrolyte in which 2.0% by mass of VC was dissolved as an additive was used. . Table 1 shows physical properties and test results in Example 7.

(Example 8)
In Example 8, VC was dissolved at a rate of 2.0% by mass as an additive, and LiBF ₄ was dissolved as a lithium salt at 1.3 mol / L as a non-aqueous electrolyte. A non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1. Table 1 shows physical properties and test results in Example 8.

(Example 9)
In Example 9, VC was dissolved at a rate of 2.0% by mass as an additive, and LiClO ₄ dissolved as a lithium salt at 1.3 mol/L was used as the non-aqueous electrolyte. A non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1. Table 1 shows the physical properties and test results in Example 9.

(Example 10)
In Example 10, a non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1, except that a non-aqueous electrolyte in which 0.5% by mass of VC was dissolved as an additive was used. . Table 1 shows physical properties and test results in Example 10.

(Example 11)
In Example 11, a non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1, except that a solution in which VC was dissolved at a rate of 0.1% by mass as an additive was used as the non-aqueous electrolyte. . Table 1 shows physical properties and test results in Example 11.

(Example 12)
In Example 12, a non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1, except that a solution obtained by dissolving 1.0 mol/L of LiPF ₆ as a lithium salt was used as the non-aqueous electrolyte. Table 1 shows physical properties and test results in Example 12.

(Example 13)
In Example 13, a non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1, except that a solution obtained by dissolving 1.5 mol/L of LiPF ₆ as a lithium salt was used as the non-aqueous electrolyte. Table 1 shows physical properties and test results in Example 13.

(Example 14)
In Example 14, a nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1, except that graphite particles having a specific surface area of 5.1 m ² /g and a median diameter of 1.9 μm were used as the negative electrode active material. bottom. Table 1 shows physical properties and test results in Example 14.

(Example 15)
In Example 15, a non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1, except that graphite particles having a specific surface area of 2.5 m ² /g and a median diameter of 12.0 μm were used as the negative electrode active material. bottom. Table 1 shows physical properties and test results in Example 15.

(Example 16)
In Example 16, a nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1, except that a separator with a porosity of 55.0% was used. Table 1 shows physical properties and test results in Example 16.

(Example 17)
In Example 17, a non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1, except that a separator with a porosity of 65.0% was used. Table 1 shows the physical properties and test results in Example 17.

(Example 18)
In Example 18, a nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1, except that a separator with a porosity of 45.0% was used. Table 1 shows physical properties and test results in Example 18.

(Example 19)
In Example 19, graphite particles having a specific surface area of 5.1 m ² /g and a median diameter of 1.9 μm as the negative electrode active material, a separator having a porosity of 65.0%, and 0.5% by mass of VC as an additive were used. A non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1, except that the non-aqueous electrolyte was used as the non-aqueous electrolyte. Table 1 shows physical properties and test results in Example 19.

(Example 20)
In Example 20, graphite particles having a specific surface area of 2.5 m ² /g and a median diameter of 12.0 μm were used as the negative electrode active material, and a separator having a porosity of 65.0% was used. A non-aqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that a non-aqueous electrolyte solution containing 0.5% by mass of the electrolyte was used. Table 1 shows physical properties and test results in Example 20.

(Example 21)
In Example 21, graphite particles having a specific surface area of 5.1 m ² /g and a median diameter of 1.9 μm as the negative electrode active material, a separator having a porosity of 65.0%, and 2.0% by mass of VC as an additive. A non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1, except that the non-aqueous electrolyte was used as the non-aqueous electrolyte. Table 1 shows physical properties and test results in Example 21.

(Example 22)
In Example 22, graphite particles having a specific surface area of 2.5 m ² /g and a median diameter of 12.0 μm as the negative electrode active material, a separator having a porosity of 65.0%, and 2.0% by mass of VC as an additive. A non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1, except that the non-aqueous electrolyte was used as the non-aqueous electrolyte. Table 1 shows physical properties and test results in Example 22.

(Example 23)
In Example 23, graphite particles having a specific surface area of 5.1 m ² /g and a median diameter of 1.9 μm as the negative electrode active material, a separator having a porosity of 45.0%, and 0.5% by mass of VC as an additive were used. A non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1, except that the non-aqueous electrolyte was used as the non-aqueous electrolyte. Table 1 shows the physical properties and test results in Example 23.

(Example 24)
In Example 24, graphite particles having a specific surface area of 2.5 m ² /g and a median diameter of 12.0 μm were used as the negative electrode active material, a separator having a porosity of 45.0% was used, and VC was 0 as an additive. A non-aqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that a non-aqueous electrolyte solution containing 0.5% by mass of the electrolyte was used. Table 1 shows physical properties and test results in Example 24.

(Example 25)
In Example 25, as the negative electrode active material, graphite particles having a specific surface area of 5.1 m ² /g and a median diameter of 1.9 μm, a separator having a porosity of 45.0%, and 2.0% by mass of VC as an additive. A non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1, except that the non-aqueous electrolyte was used as the non-aqueous electrolyte. Table 1 shows physical properties and test results in Example 25.

(Example 26)
In Example 26, graphite particles having a specific surface area of 2.5 m ² /g and a median diameter of 12.0 μm were used as the negative electrode active material, a separator having a porosity of 45.0% was used, and VC was used as an additive at 2 A non-aqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that a non-aqueous electrolyte solution containing 0.0% by mass of the electrolyte was used. Table 1 shows physical properties and test results in Example 26.

(Comparative example 1)
In Comparative Example 1, a non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1, except that graphite particles having a specific surface area of 2.8 m ² /g and a median diameter of 12.0 μm were used as the negative electrode active material. bottom. Table 2 shows physical properties and test results in Comparative Example 1.

(Comparative example 2)
In Comparative Example 2, a non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1, except that graphite particles having a specific surface area of 5.2 m ² /g and a median diameter of 10.9 μm were used as the negative electrode active material. bottom. Table 2 shows physical properties and test results in Comparative Example 2.

(Comparative Example 3)
In Comparative Example 3, a non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1, except that a separator made of PE produced by a wet stretching method and having a porosity of 47.0% was used. bottom. The separator according to Comparative Example 3 does not have a slit-like pore structure because it is produced by a wet drawing method. Table 2 shows physical properties and test results in Comparative Example 3.

(Comparative Example 4)
In Comparative Example 4, the same procedure as in Example 1 was performed except that a separator having a three-layer structure of PE/PP/PE produced by a dry stretching method and having a porosity of 39.0% was used. A non-aqueous electrolyte secondary battery was produced by Table 2 shows physical properties and test results in Comparative Example 4.

(Comparative Example 5)
In Comparative Example 5, a non-aqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that LiPO ₂ F ₂ was dissolved at a rate of 1% by mass as an additive in the non-aqueous electrolyte. made. Table 2 shows physical properties and test results in Comparative Example 5.

(Comparative Example 6)
In Comparative Example 6, a non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1, except that a non-aqueous electrolyte containing no additive was used. Table 2 shows physical properties and test results in Comparative Example 6.

(Comparative Example 7)
In Comparative Example 7, a non-aqueous electrolyte secondary was prepared in the same manner as in Example 1, except that 3% by mass of fluoroethylene carbonate (FEC) was dissolved as an additive in the non-aqueous electrolyte. A battery was produced. Table 2 shows physical properties and test results in Comparative Example 7.

(Comparative Example 8)
In Comparative Example 8, a nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1, except that 3.0% by mass of VC was dissolved as an additive in the nonaqueous electrolyte. bottom. Table 2 shows physical properties and test results in Comparative Example 8.

(Comparative Example 9)
In Comparative Example 9, a non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1, except that 2.5% by mass of VC was dissolved as an additive in the non-aqueous electrolyte. bottom. Table 2 shows the physical properties and test results in Comparative Example 9.

From the results shown in Tables 1 and 2, the non-aqueous electrolyte secondary batteries of Examples 1 to 26, which satisfy all the constituent requirements of the present invention, show little decrease in capacity even after repeated charge-discharge cycles at high temperatures, and -20 It was found that dendrite precipitation does not easily occur even after repeated charging and discharging at an extremely low temperature of °C. Among them, Examples 1 to 3, 7, 10 to 26 in which LiPF ₆ was used as the lithium salt, the concentration was 1.0 mol/L or more and 1.5 mol/L or less, and the separator did not contain an inorganic filler. showed particularly excellent characteristics.

On the other hand, as in Comparative Examples 1 and 2, when the value of (specific surface area)×(median diameter) of the negative electrode is greater than 30 (×10 ⁻⁶ m ³ /g), the surface roughness is large and the negative electrode is uniform. It is considered that a good film could not be formed. As in Comparative Example 3, a wet separator does not have a lamellar skeleton structure, so the supply of lithium salt is delayed, and the lithium salt concentration at the interface between the separator and the negative electrode decreases, resulting in a uniform negative electrode coating. could not be formed. When the porosity is low as in Comparative Example 4, the lithium salt supply is stagnant and the lithium salt concentration at the interface between the separator and the negative electrode is reduced, as in Comparative Example 3. Therefore, a uniform negative electrode is obtained. It is considered that the film could not be formed. Also, as in Comparative Examples 5 to 7, when the additive was LiPO ₂ F ₂ or FEC, or when VC was not added, it is considered that a uniform negative electrode film derived from VC could not be formed. It is believed that when a large amount of VC was added as in Comparative Examples 8 and 9, the denseness of the film increased, leading to an increase in resistance at low temperatures. As described above, the non-aqueous electrolyte secondary batteries of Comparative Examples 1 to 9, in which any one of the negative electrode, the separator, and the non-aqueous electrolyte additive did not satisfy the constituent requirements, were at least On the other hand, the result was that the desired characteristics (capacity retention rate) could not be obtained.

As described above, according to the present invention, it is possible to provide a non-aqueous electrolyte secondary battery that can be used in a wide temperature range without impairing performance.

Although several embodiments have been specifically described, these are merely examples, and the present invention is not limited to these embodiments and examples, and various modifications based on the technical idea of the present invention can be changed.

INDUSTRIAL APPLICABILITY The present invention provides a non-aqueous electrolyte secondary battery that can be used in a wide temperature range without impairing its performance and exhibits excellent characteristics. have availability.

REFERENCE SIGNS LIST 1 non-aqueous electrolyte secondary battery 2 laminate sheet 3 positive electrode terminal 4 negative electrode terminal 5 electrode group 6 sealant 7 positive electrode 8 negative electrode 9 separator 10 positive electrode current collecting lead 11 negative electrode current collecting lead 12 welded portion

Claims (7)

A non-aqueous electrolyte secondary battery comprising an electrode element consisting of a positive electrode, a negative electrode and a separator, and a non-aqueous electrolyte,
The non-aqueous electrolyte contains 2% or less vinylene carbonate with respect to the total mass of the non-aqueous electrolyte,
The separator has a structure having slit-like pores or a lamellar skeleton structure, and has a porosity of 45% or more,
The negative electrode is
a negative electrode current collector;
a negative electrode mixture layer applied to the negative electrode current collector;
has
The negative electrode mixture layer contains graphite particles as a negative electrode active material,
The product of the specific surface area (m ² /g) of the graphite particles and the median diameter (μm) of the graphite particles is 30 (×10 ⁻⁶ m ³ /g) or less.
A non-aqueous electrolyte secondary battery characterized by: The separator is filler-free,
The non-aqueous electrolyte secondary battery according to claim 1 , characterized in that: The non-aqueous electrolyte further contains a lithium salt,
The concentration of the lithium salt is 1.0 mol/L or more and 1.5 mol/L or less.
The non-aqueous electrolyte secondary battery according to claim 1 , characterized in that: the lithium salt is _LiPF6 ;
4. The non-aqueous electrolyte secondary battery according to claim 3 , characterized in that: The positive electrode is
a positive electrode current collector;
a positive electrode mixture layer applied to the positive electrode current collector;
has
The positive electrode mixture layer contains a positive electrode active material of the general formula LiMO ₂ (M is Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Ti, Cr, Pb, Sb and one or more elements selected from B) containing a lithium transition metal oxide represented by
The non-aqueous electrolyte secondary battery according to claim 1 , characterized in that: The negative electrode contains only graphite particles as a negative electrode active material,
The non-aqueous electrolyte secondary battery according to claim 1 , characterized in that: In an environment of -20°C, when CC-CV charging with an upper limit voltage of 4.2 V, 0.2 C and a cutoff current of 0.05 C and CC discharging with a lower limit voltage of 2.7 V and 1.0 C are repeated for 60 cycles , CC-CV charging with an upper limit voltage of 4.2 V, 0.5 C, and a cutoff current of 0.05 C, and CC discharge with a lower limit voltage of 2.7 V, 0.5 C are performed one cycle each before and after that in an environment of 25 ° C. 2. The non-aqueous electrolyte secondary battery according to claim 1, wherein a capacity retention rate is 95% or more when a capacity confirmation test is performed.

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