JP7289413B1 - 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 relative to the total mass of the non-aqueous electrolyte, the separator has a structure having slit-like pores or a lamellar skeleton structure, the JIS Gurley is within 300 seconds/100 mL, and the negative electrode is , a negative electrode current collector, and a negative electrode mixture layer applied to the negative electrode current collector, wherein the negative electrode mixture layer contains graphite particles as a negative electrode active material, and the density of the negative electrode mixture layer is 1.5. It is 2 g/cm3 or more and 1.5 g/cm3 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 The negative electrode has a skeletal structure and a JIS Gurley of 300 seconds/100 mL or less, and the negative electrode has a negative electrode current collector and a negative electrode mixture layer applied to the negative electrode current collector, and the negative electrode mixture layer contains graphite particles as a negative electrode active material, and the negative electrode mixture layer has a density of 1.2 g/cm ³ or more and 1.5 g/cm ³ or less.

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, the JIS Gurley is within 300 seconds/100 mL, and the 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. It is characterized by having a density of 1.2 g/cm ³ or more and 1.5 g/cm ³ or less.

In addition, as a third aspect, the non-aqueous electrolyte secondary battery according to the present invention, in the above first and second aspects, has a specific surface area (m ² /g) of the graphite particles and a median of the graphite particles It is characterized in that the product with the diameter (μm) is 30 (×10 ⁻⁶ m ³ /g) or less.

Further, as a fourth aspect of the non-aqueous electrolyte secondary battery according to the present invention, in any one of the above first to third aspects, the positive electrode comprises a positive electrode current collector and the positive electrode current collector and a positive electrode material layer applied to the positive electrode material layer, wherein the positive electrode material layer includes a layered oxide and an olivine compound as a positive electrode active material.

Further, as a fifth aspect, in the non-aqueous electrolyte secondary battery according to the present invention, in the above fourth aspect, the density of the positive electrode mixture layer is 2.3 g/cm ³ or more and 3.5 g/cm ³ It is characterized by the following.

Further, as a sixth aspect of the non-aqueous electrolyte secondary battery according to the present invention, in any one of the above first to fifth 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.

As a seventh aspect, the nonaqueous electrolyte secondary battery according to the present invention is characterized in that, in the above sixth aspect, the lithium salt is LiPF ₆ .

Further, as an eighth aspect of the non-aqueous electrolyte secondary battery according to the present invention, in any one of the first to seventh 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).

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

Further, as a tenth aspect, the non-aqueous electrolyte secondary battery according to the present invention, in any one of the above first to ninth aspects, has an upper limit voltage of 4.2 V and a maximum voltage of 0.2 V in a -20° C. environment. 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 achieving both thermal stability and energy density, the positive electrode mixture layer 7b preferably contains both a layered oxide and an olivine compound as positive electrode active materials. Thereby, thermal stability can be improved. In particular, it is considered that thermal stability can be improved by surrounding the layered oxide with the olivine compound.

A layered oxide contributes to an improvement in energy density. Layered oxides include general formula LiMO ₂ such as LiCoO ₂ , LiNiO ₂ , LiMnO ₂ and LiCoNiO ₂ (M is commonly Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Ti, Cr, Pb, Sb, BZr, W, P, V, Ca, Sr, Ga, In, Si, Mo, Sn, Ag, Ce, Pr, Ge, Bi, Ba, Er, La, Sm, Yb and one or more elements selected from S), and those in which some elements of these positive electrode active materials are replaced with different elements, and Co, Ni and Mn. At least one type of active material particles selected from the group consisting of layered oxides, such as a ternary system containing, may be used in combination of two or more types. 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 replaced with Mn, and the like, and have the general formula LiMn _z M2 _b Fe _1-z-b PO ₄ (where 0<z≦ 0.9, 0≤b≤0.1, 0<z+b<1, M2 is Ni, Co, Ti, Cu, Zn, Mg, Zr, Ca, Y, Mo, Ba, Pb, Bi, La, Ce , Nd, Gd, Al, Ga and Sr). 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 weight of both the layered oxide and the olivine compound is preferably 50% by mass or less from the viewpoint of energy density, and 10% by mass from the viewpoint of sufficiently improving thermal stability. It is preferable that it is above. More preferably, it is 40% by mass or less and 20% by mass or more. In addition, the measurement of mass% may be performed in a state in which each film or additive is included. 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.

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, from the viewpoint of energy density and cycle characteristics. Although the mechanism is not clear, it is speculated that if the density of the positive electrode mixture layer is less than 2.3 g/cm ³ , voids in the positive electrode may cause the paths between the positive electrode active material and the conductive aid to not connect at some points. Therefore, due to the volume expansion and contraction of the positive electrode that occurs when the cycle is repeated, many electrically isolated active materials are produced with each cycle, and the number of effective active materials in the positive electrode decreases. It is considered that the active material alone is used for charging and discharging, and is abused, resulting in deterioration of cycle characteristics.

Here, the density of the positive electrode mixture layer 7b is preferably 3.5 g/cm ³ or less, more preferably 2.9 g/cm ³ or less, from the viewpoint of cycle characteristics and prevention of dendrite precipitation. Although the mechanism is not clear, it is presumed that if the density of the positive electrode mixture layer is higher than 3.5 g/cm ³ , appropriate voids cannot be secured in the positive electrode and the electrolyte does not sufficiently permeate. As a result, there are places where the supply of ions from the electrolyte to the active material is insufficient during charge/discharge, and only the specific active material to which sufficient ions have been supplied is subjected to overuse during charge/discharge, resulting in poor cycle characteristics. It seems to do. In addition, by securing the appropriate voids as described above, unevenness in the reaction distribution of the positive electrode does not occur during charging, so ions are uniformly supplied from the positive electrode to the negative electrode through the separator, so that the ions on the negative electrode It is presumed that the unevenness of the dendritic grains is less likely to occur, and the dendrite precipitation is suppressed.

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 preferably 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.

The density (negative electrode density) of the negative electrode mixture layer 8b is 1.2 g/cm ³ or more. Also, the negative electrode density is 1.5 g/cm ³ or less. When the negative electrode density exceeds 1.5 g/cm ³ , the contact area between the negative electrode active material and the electrolytic solution is relatively decreased due to the increased number of contact points between the negative electrode particles. As a result, the area where VC reacts with the negative electrode becomes smaller, and an excessive amount of VC locally reacts, so that a non-uniform and thick film is formed. If the negative electrode density is less than 1.2 g/cm ³ , there will be places where the paths of the conductive aid in the negative electrode are not connected, resulting in uneven electronic conductivity of the negative electrode and a film formed by VC. is assumed to be non-uniform. As described above, when the negative electrode density is 1.2 g/cm ³ or more and 1.5 g/cm ³ or less, there is no unevenness in the film around each negative electrode particle, and a uniform and thin film containing a sufficient amount of lithium salt is formed. It is considered possible.

(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, and a lamellar skeleton structure in which the polymer is partially folded and crystallized. . 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.

In addition, from the viewpoint of making it difficult for vinylene carbonate, which is necessary for forming a film on the surface of the negative electrode in the initial activation, to become diffusion rate-determining, the JIS Gurley of the separator 9 must be 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. If the JIS Gurley value is greater than 300 seconds/100 mL, the diffusion of substances in the electrolyte, including vinylene carbonate, tends to be rate-determining, making it difficult to form a thin and uniform coating. In addition, diffusion of lithium ions during charging at low temperatures tends to be rate-determining, so dendrite precipitation is even more likely to occur.

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 decrease in the concentration of the lithium salt is suppressed. As a result, when the film is formed, the lithium salt-derived component is sufficiently contained, and dendrite precipitation hardly occurs 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, for example, a wet method, is used, it is difficult to eliminate the uneven lithium salt concentration inside the separator during the film formation, and the lithium salt is formed on the surface of the negative electrode 8. Film formation is likely to occur in a deficient state. 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. Regarding the pore structure of the separator 9, no clear tendency of influence on high-rate charging/discharging or direct current resistance after the coating is completely formed has been observed. has not been 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 slit-like pores produced by a dry stretching method is an effective way to obtain a non-aqueous electrolyte secondary battery having desired characteristics. It turned out to be one of the essential requirements.

In addition, since the JIS Gurley of the separator 9 is equal to or less than the above value, the permeability of the non-aqueous electrolyte is improved, so that the lithium salt is naturally easily supplied. However, JIS Gurley is a value that indicates air resistance in an environment with a pressure difference, and does not uniquely determine the ease of diffusion of lithium salt in a non-aqueous electrolyte. Just because the Gurley is equal to or less than the above value does not mean that the supply of lithium salt is not sufficiently stopped. In other words, a separator 9 having both a JIS Gurley 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 is used. Only then can the component derived from the lithium salt be sufficiently contained at the time of film formation.

Further, 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. It is better to have one, that is, the smaller the surface roughness. 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. be able to. 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>
72 mass % of LiNi _0.5 Co _0.2 Mn _0.3 O ₂ (NCM) as a positive electrode active material, 18 mass % of LiMn _0.7 Fe _0.3 PO ₄ (LMFP), and 3 graphite particles as a first conductive agent. % by mass, 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. At this time, the mass ratio of NCM and LMFP is NCM:LMFP=80:20.

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 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-hole pore structure because it is made by a dry drawing method. The JIS Gurley of the prepared separator is 250 seconds/100 mL. 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>
In the −20° C. cycle test, if the capacity retention rate is less than 95%, the possibility of dendrite precipitation is extremely high. In addition, even if the capacity retention rate is less than 75% in the 45 ° C. cycle test, the overall judgment is "x" because the cycle characteristics at high temperatures are not sufficient, and if the capacity retention rate is 75% or more and less than 80%, it is "○". , and when the capacity retention rate was 80% or more, it was indicated as "⊚". In other words, only when the capacity retention rate of the −20° C. cycle test is 95% or more and the capacity retention rate of the 45° C. cycle test is 80% or more, the overall judgment is “⊚”.
Table 1 shows the physical property values in Example 1.

(Example 2)
In Example 2, a non-aqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that the density of the negative electrode mixture layer was set to 1.20 g/cm ³ . Table 1 shows the physical property values in Example 2.

(Example 3)
In Example 3, a non-aqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that the density of the negative electrode mixture layer was set to 1.50 g/cm ³ . Table 1 shows the physical property values 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 the physical property values 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 the physical property values 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 the physical property values 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 the physical property values 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 the physical property values 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 property values 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 the physical property values 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 the physical property values 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 the density of the negative electrode mixture layer was 1.3 g/cm ³ . Table 1 shows the physical property values 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 the density of the positive electrode mixture layer was 2.3 g/cm ³ . Table 1 shows the physical property values in Example 13.

(Example 14)
In Example 14, a non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1, except that a positive electrode having a positive electrode mixture layer with a density of 3.0 g/cm ³ was used. Table 1 shows the physical property values 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 a positive electrode having a positive electrode mixture layer with a density of 3.2 g/cm ³ was used. Table 1 shows the physical property values in Example 15.

(Example 16)
In Example 16, 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 5.1 m ² /g and a median diameter of 1.9 μm were used as the negative electrode active material. bottom. Table 1 shows the physical property values in Example 16.

(Example 17)
In Example 17, 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 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 the physical property values in Example 17.

(Example 18)
In Example 18, a non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1, except that a JIS Gurley separator of 195 seconds/100 mL was used. Table 1 shows the physical property values in Example 18.

(Example 19)
In Example 19, 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 the physical property values in Example 19.

(Example 20)
In Example 20, 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 the physical property values in Example 20.

(Example 21)
In Example 21, a non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1, except that only NCM was used as the positive electrode active material without using LMFP. Table 1 shows the physical property values in Example 21.

(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 the density of the negative electrode mixture layer was set to 1.10 g/cm ³ . Table 2 shows the physical property values in Comparative Example 1.

(Comparative example 2)
In Comparative Example 2, a non-aqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that the density of the negative electrode mixture layer was set to 1.60 g/cm ³ . Table 2 shows the physical property values 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 JIS Gurley of 260 seconds/100 mL made of PE produced by a wet stretching method was used as the separator. . 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 the physical property values 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 PE/PP/PE three-layer structure produced by a dry stretching method and a JIS Gurley rating of 530 seconds/100 mL was used. A non-aqueous electrolyte secondary battery was produced. Table 2 shows the physical property values 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 the physical property values 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 the physical property values 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 the physical property values in Comparative Example 7.

(Comparative Example 8)
In Comparative Example 8, a non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1, except that 3% by mass of VC was dissolved as an additive in the non-aqueous electrolyte. Table 2 shows the physical property values 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 property values in Comparative Example 9.

(Comparative Example 10)
In Comparative Example 10, a non-aqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that a positive electrode having a positive electrode mixture layer with a density of 2.2 g/cm ³ was used. Table 2 shows the physical property values in Comparative Example 10.

(Comparative Example 11)
In Comparative Example 11, a non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1, except that a positive electrode having a positive electrode mixture layer with a density of 3.6 g/cm ³ was used. Table 2 shows the physical property values in Comparative Example 11.

Table 3 shows the test results of Examples 1 to 21 and Comparative Examples 1 to 11.

From the results shown in Table 3, the non-aqueous electrolyte secondary batteries of Examples 1 to 21, which satisfy all the constituent requirements of the present invention, have little capacity decrease even after repeated charge-discharge cycles at high temperatures, and have a temperature of -20°C. It was found that dendrite precipitation hardly occurs even after repeated charging and discharging at extremely low temperatures. Among them, Examples 1 to 3, 7, 10 to 21 in which LiPF ₆ was used as a lithium salt, its 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 Example 1, there are places where the paths of the conductive aid in the negative electrode are not connected, so the electronic conductivity of the negative electrode becomes uneven, and it is thought that the film formed by VC becomes uneven. be done. As in Comparative Example 2, the number of contact points between the negative electrode particles is increased, so that the contact area of the electrolyte solution of the negative electrode active material is relatively decreased. As a result, the area where VC reacts with the negative electrode becomes small, and an excessive amount of VC locally reacts, so that a non-uniform and thick film is formed. As in Comparative Example 10, when the density of the positive electrode is less than 2.3 g/cm ³ , there are places where the path of the conductive aid in the positive electrode is not connected, so the reaction distribution of the positive electrode becomes uneven and reaches the negative electrode. It is believed that the ions generated by the VC are non-uniform and the film produced by the VC is non-uniform. As in Comparative Example 11, when the density of the positive electrode is greater than 3.5 g/cm ³ , the electrolytic solution does not sufficiently permeate the positive electrode, resulting in uneven reaction distribution of the positive electrode and non-uniform ions reaching the negative electrode. , VC are believed to produce non-uniform coatings. As a result, the area where VC reacts with the negative electrode becomes small, and an excessive amount of VC locally reacts, so that a non-uniform and thick film is formed. In addition, 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. It is considered that a good film could not be formed. As in Comparative Example 4, when the JIS Gurley is high, the lithium salt supply is stagnant and the lithium salt concentration at the interface between the separator and the negative electrode decreases, as in Comparative Example 3, so that a uniform negative electrode coating is formed. presumably could not be formed. If the additive was LiPO ₂ F ₂ or FEC, or if VC was not added, as in Comparative Examples 5 to 7, 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. The non-aqueous electrolyte secondary batteries of Comparative Examples 1 to 11, in which any one of the negative electrode, the separator, and the electrolyte additive did not satisfy the constituent requirements, exhibited the desired characteristics in at least one of the −20° C. cycle test and the 45° C. cycle test. It was an unsatisfactory result.

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 (9)

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 JIS Gurley of 300 seconds/100 mL or less,
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 density of the negative electrode mixture layer is 1.2 g/cm ³ or more and 1.5 g/cm ³ or less.
A non-aqueous electrolyte secondary battery characterized by: 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.
The non-aqueous electrolyte secondary battery according to claim 1 , 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 layered oxide and an olivine compound as positive electrode active materials,
The non-aqueous electrolyte secondary battery according to claim 1 , characterized in that: The density of the positive electrode mixture layer is 2.3 g/cm ³ or more and 3.5 g/cm ³ or less,
4. The non-aqueous electrolyte secondary battery according to claim 3 , 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 ;
6. The non-aqueous electrolyte secondary battery according to claim 5 , 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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