JP2024014602A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery, in which reduction in capacity is suppressed even if charge and discharge cycles are repeated under a high temperature and dendrite precipitation is less likely to occur even if charging and discharging are repeated below a freezing point, and which can be used over a wide temperature range without sacrificing performance.

SOLUTION: A nonaqueous electrolyte secondary battery according to the invention includes: an electrode element comprising a positive electrode, a negative electrode, and a separator; and a nonaqueous electrolytic solution. The nonaqueous electrolytic solution contains 2% or less of vinylene carbonate relative to the weight of the entire nonaqueous electrolytic solution. The separator has a structure with a slit or a lamellar skeletal structure and also has a JIS Gurley value of 300 s/100 mL or less. The negative electrode includes a negative electrode current collector and a negative electrode mixture layer coated on the negative electrode current collector. The negative electrode mixture layer contains graphite as a negative electrode active material, and the product of a specific surface area (m²/g) of the graphite and a median diameter (μm) of the graphite is 30 (×10^-6 m³/g) or less.

SELECTED DRAWING: Figure 1

COPYRIGHT: (C)2024,JPO&INPIT

Description

The present invention relates to a non-aqueous electrolyte secondary battery.

Nonaqueous electrolyte secondary batteries are widely used because of their high energy density, and are installed as power sources in small portable devices such as mobile phones, digital cameras, and notebook computers. In addition, non-aqueous electrolyte secondary batteries are being used for large-scale industrial applications such as hybrid cars, electric cars, and electricity storage using natural energy generation such as solar and wind power, from the perspective of energy resource depletion and global warming. development is underway.

In recent years, non-aqueous electrolyte secondary batteries have attracted attention as power sources for drones, robots, electric vehicles, hybrid electric vehicles, etc., and their use is expected to expand. In addition to higher density and longer life, such power sources are required to be able to be used in a wide temperature range without deteriorating their performance. Specifically, it is desirable that the power source for motive power exhibits little loss in capacity even after repeated charging and discharging cycles at high temperatures, and is capable of being repeatedly charged and discharged even at sub-zero temperatures.

From the viewpoint of increasing energy density, graphite is generally used as the negative electrode active material of non-aqueous electrolyte secondary batteries. In non-aqueous electrolyte secondary batteries using graphite as a negative electrode active material, many documents disclose techniques related to discharging at subzero temperatures, but there are few documents disclosing that repeated charging is possible at subzero temperatures. In particular, there is no technology for 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 as the negative electrode active material, which has hindered their practical application in environments with particularly large temperature differences. Ta.

For example, Patent Document 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 is used as the negative electrode active material, adding VC to the nonaqueous electrolyte improves the cycle characteristics of the nonaqueous electrolyte secondary battery at normal and high temperatures; The reaction resistance of the negative electrode at the bottom becomes high, making dendrite precipitation more likely to occur during charging.

On the other hand, for example, Patent Document 2 states that by using LiBF ₄ and fluoroethylene carbonate as additives in a non-aqueous electrolyte in addition to VC, the density of the film on the negative electrode surface decreases 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 tends 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 the trade-off between low-temperature characteristics and high-temperature cycle characteristics simply by changing the composition of electrolyte additives, and it is possible to use the product over a wide temperature range without compromising its performance. It has been considered difficult to provide non-aqueous electrolyte secondary batteries.

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

Patent No. 3066126 JP2013-218967A JP2021-64620A

However, when the design specifications of the separator and graphite are changed as described above, a reaction between the electrolytic solution and the graphite becomes more likely to occur at high temperatures, resulting in a problem that the cycle characteristics deteriorate significantly. Furthermore, simply reducing the particle size of graphite or increasing the surface area may not necessarily suppress dendrite precipitation during subzero charging. Therefore, it is not possible to avoid trade-offs between low-temperature cycle characteristics and high-temperature cycle characteristics simply by changing these design specifications. It has been considered difficult to provide secondary batteries.

The present invention has been made in view of the above, and has performance over a wide temperature range, with little capacity loss even after repeated charging and discharging cycles at high temperatures, and with little dendrite precipitation even after repeated charging and discharging at sub-zero temperatures. The purpose of the present invention is to provide a non-aqueous electrolyte secondary battery that can be used without damaging the battery.

In order to solve the above-mentioned problems and achieve the objects, a nonaqueous electrolyte secondary battery according to the present invention includes, as a first aspect, an electrode element consisting of a positive electrode, a negative electrode, and a separator, and a nonaqueous electrolyte. A non-aqueous electrolyte secondary battery, wherein the non-aqueous electrolyte contains 2% or less vinylene carbonate based on the total weight of the non-aqueous electrolyte, and the separator has a slit structure or a lamellar skeleton structure. None, JIS Gurley is within 300 seconds/100 mL, the negative electrode has a negative electrode current collector, and a negative electrode mixture layer coated on the negative electrode current collector, and the negative electrode mixture layer is a negative electrode It contains graphite as an active material, and the product of the specific surface area (m ² /g) of the graphite and the median diameter (μm) of the graphite is 30 (×10 ⁻⁶ m ³ /g) or less. Features.

Further, 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 non-aqueous electrolyte secondary battery comprising: The aqueous electrolyte contains 2% or less of vinylene carbonate based on the total weight of the non-aqueous electrolyte, the separator is formed by a dry stretching method, and has a JIS Gurley rating of 300 or less, and the negative electrode includes: It 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 as a negative electrode active material and has a specific surface area (m ² /g) of the graphite. ) and the median diameter (μm) of the graphite is 30 (×10 ⁻⁶ m ³ /g) or less.

Further, as a third aspect, the non-aqueous electrolyte secondary battery according to the present invention is characterized in that, in the first and second aspects described above, 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 It is characterized in that the salt concentration is 1.0 mol/L or more and 1.5 mol/L or less.

Further, 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 above, the lithium salt is LiPF ₆ . .

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 above, the positive electrode includes a current collector and a coating applied to the current collector. The positive electrode mixture layer has a positive electrode active material having the general formula LiMO ₂ (M is Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, It is characterized by containing a lithium transition metal oxide represented by one or more elements selected from Zn, Al, Ti, Cr, Pb, Sb and B).

Further, 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 above, the negative electrode contains only graphite as a negative electrode active material. Features.

Further, as an eighth aspect, the non-aqueous electrolyte secondary battery according to the present invention is provided with an upper limit voltage of 4.2 V, 0.0 V in a −20° C. environment in any one of the first to seventh aspects above. When CC-CV charging at 2C, cut-off current 0.05C and CC discharge at 1.0C with a lower limit voltage of 2.7V are repeated for 60 cycles, the upper limit voltage is 4.2V in a 25°C environment before and after that. The capacity retention rate was 95 when a capacity confirmation test was conducted by performing one cycle of CC-CV charging at 0.5C and a cut-off current of 0.05C and one cycle of CC discharging at a lower limit voltage of 2.7V and 0.5C. % or more.

According to the present invention, there is little capacity loss even after repeated charging and discharging cycles at high temperatures, dendrite precipitation does not easily occur even after repeated charging and discharging at below freezing temperatures, and it can be used in a wide temperature range without deteriorating performance. 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 an embodiment of the present invention. FIG. 2 is a diagram for explaining the configuration of an electrode group included in a nonaqueous electrolyte secondary battery according to an embodiment of the present invention. FIG. 3 is a diagram showing a part of the cross section taken along the line AA shown in FIG.

Embodiments of the present invention will be described below, but the present invention is not limited to the following description. Further, various changes or improvements can be made to this embodiment, and forms with such changes or improvements 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 an embodiment of the present invention. FIG. 2 is a diagram for explaining the configuration of an electrode group included in a nonaqueous electrolyte secondary battery according to an embodiment of the present invention. FIG. 3 is a diagram showing a part of the cross section taken along the line AA shown in FIG.

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

The laminate sheet 2 accommodates the electrode group 5 and constitutes an exterior body of the non-aqueous electrolyte secondary battery 1. The laminate sheet 2 has a rectangular outer edge. Further, the laminate sheet 2 is formed by pasting together 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. The peripheral edge of the laminate sheet 2 has a structure in which the sheet members are sealed together by heat welding, and the electrode group 5 is sealed by the laminate sheet 2. Further, the positive electrode terminal 3 and the negative electrode terminal 4 extend from a part of the peripheral edge of the laminate sheet 2, and 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, respectively. 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, the heat-fusible resin layers on the inner surfaces of the sealant 6 and the laminate sheet 2 are bonded together by heating, so that it is possible to prevent the non-aqueous electrolyte from penetrating and leaking.

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

Further, one end of the negative electrode terminal 4 is connected to the negative electrode 8 forming the electrode group 5 . Negative electrode terminal 4 and negative electrode 8 are electrically connected via negative electrode current collector lead 11 (see FIG. 3). Negative current collector lead 11 is connected to negative electrode terminal 4 via weld 12 .

The positive electrode 7 and the negative electrode 8 are laminated with a separator 9 in between to form an electrode group 5 . In the electrode group 5, separators 9, negative electrodes 8, separators 9, positive electrodes 7, and separators 9 are stacked alternately in this order from the outermost layer (see FIG. 3).

The non-aqueous electrolyte is held within the laminate sheet 2. Although not shown, the non-aqueous electrolyte is prepared by dissolving an electrolyte in a non-aqueous solvent.

Although FIG. 1 shows a configuration example of a laminated non-aqueous electrolyte secondary battery as an example of an embodiment, the shape of the non-aqueous electrolyte secondary battery in the present invention is not particularly limited, and may be a flat type or a flat type. , cylindrical shape, square shape, coin shape, etc. Moreover, the exterior body of the nonaqueous electrolyte secondary battery is not particularly limited, and in addition to a laminate film, publicly known exterior bodies such as aluminum, aluminum alloy, and stainless steel can be used.
The nonaqueous electrolyte, positive electrode, negative electrode, separator, and exterior body (laminate sheet) will be described in detail below.

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

Examples of the lithium salt include one or a mixture of two or more selected from LiPF ₆ , LiBF ₄ , LiClO ₄ , Li(FSO ₂ ) ₂ N, Li(CF ₃ SO ₂ ) ₂ N, etc. It is preferable that at least LiPF ₆ is included. It is preferable to include LiPF ₆ as a lithium salt after comprehensively evaluating the solubility in non-aqueous solvents, charge/discharge characteristics when used as a secondary battery, output characteristics, cycle characteristics, etc. The concentration of the lithium salt is preferably 0.5 mol/L or more, and 1.0 mol/L or more, in order to ensure that components derived from the lithium salt are sufficiently included during film formation during initial activation. is even more preferable. Furthermore, if the concentration of the lithium salt is too high, a film derived from VC will not be sufficiently formed and high-temperature cycle characteristics will deteriorate, so it is preferably 3.0 mol/L or less, and preferably 1.5 mol/L or less. More preferred.

Non-aqueous solvents are not particularly limited, but include, for example, dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), ethylmethyl carbonate (EMC), ethylene carbonate (EC), propylene carbonate (PC), Examples include one or more mixed solvents selected from methyl propionate, methyl acetate, methyl formate, methyl butyrate, dioxolane, 2-methyltetrahydrofuran, tetrahydrofuran, dimethoxyethane, γ-butyrolactone, acetonitrile, and benzonitrile. It is particularly preferable to use DMC, DEC, DPC, EMC, EC, and PC as the nonaqueous solvent, and it is particularly preferable to include EC from the viewpoint of forming a good film on the negative electrode active material.

Furthermore, if the amount of VC added is too large, the coating will be overformed and dendrite precipitation will likely occur. Therefore, the amount of VC added is preferably 2% or less based on the weight of the entire non-aqueous electrolyte. On the other hand, if the amount of VC added is less than 0.1% based on the weight of the entire non-aqueous electrolyte, the formation of a film tends to be insufficient, which is unsuitable.

In addition, additives other than lithium salts may include additives other than VC 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 may be mentioned. However, if excessive amounts of these additives are added, gas is likely to be generated during high-temperature cycles, leading to a decrease in capacity. It is preferably 1% or less, and more preferably 1% or less. Furthermore, when using these additives, they must be used in combination with VC from the viewpoint of suppressing gas generation.

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

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

From the viewpoint of energy density, _the positive electrode mixture layer 7b uses _the general formula _{LiMO 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. Contains at least one type of active material particles selected from the group consisting of layered oxides, such as those in which some elements are replaced with different elements, and ternary layered oxides containing Co, Ni, and Mn, and two or more types. May be used in combination. In addition, 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, polyethylene It may also be coated with a coating of ion conductive polymers such as oxides, derivatives or salts thereof. The average particle size (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.

Further, 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 the Fe is replaced with Mn. The surface of the olivine compound may be coated with a carbon material for the purpose of improving electrical conductivity. The weight ratio of the olivine compound to the total weight of the positive electrode active material is preferably 40% by weight or less from the viewpoint of energy density, and preferably 10% by weight 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 properties, ionic conductivity, and the like.

It is preferable that the positive electrode mixture layer 7b further contains a known conductive material from the viewpoint of ensuring a conductive path in the electrode. Known conductive materials include, but are not particularly limited to, graphite, graphene, carbon black, activated carbon, and carbon fiber. One type of these conductive materials may be selected and used, or two or more types may be used in combination.

The positive electrode mixture layer 7b may further include a known binding material. Known binders include, but are not particularly limited to, polyvinylidene fluoride, polytetrafluoroethylene, polyvinylpyrrolidone, polyvinyl fluoride, polyethylene, polypropylene, nitrile resins, styrene-butadiene rubber, acrylic resins, and copolymers thereof. Can be mentioned. One type of these binders may be selected and used, or two or more types may be used in combination. In addition to these, other known additives such as a dispersant, a pH adjuster, and a moisture scavenger may also 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, and more preferably 2.5 g/cm ³ or more. In addition, in order to ensure appropriate voids inside the positive electrode mixture layer 7b to maintain good electrolyte impregnation properties and obtain good cycle characteristics, it is preferably 3.5 g/cm ³ or less, and 2.9 g/cm 3 or less. It is more preferable that it is below ^cm3 .

The positive electrode mixture layer 7b is applied to one or both sides of the current collector 7a, and the amount applied 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 sufficient energy density and maintain good output characteristics and cycle characteristics. Further, in order to form the positive electrode current collector lead 10, it is preferable to leave an area on the current collector 7a where the positive electrode mixture layer 7b is not applied.

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

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

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

Note that the negative electrode mixture layer 8b may contain a thickener as necessary. Examples of thickeners include carboxymethylcellulose and polyvinyl alcohol. In addition to this, other additives such as a dispersant may also be included.

The negative electrode mixture layer 8b is applied to one or both sides of the 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 ensure sufficient energy density and maintain good output characteristics and cycle characteristics. In addition, from the viewpoint of suppressing lithium metal dendrite precipitation during charging, the coating amount of the negative electrode mixture layer 8b is preferably determined so that the positive and negative electrode capacity ratio is 1.01 or more, and is preferably 1.1 or more. It is even more preferable to specify.
Here, the positive and negative electrode capacity ratio is calculated by the following formula (1).
(Positive and negative electrode capacity ratio) = (Single-sided negative electrode capacity per area)
/(Single-sided positive electrode capacity per area)...(1)
However, if the value of the positive and 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, so the value of the positive and negative electrode capacity ratio should be 1.3 or less. It is preferably 1.2 or less, and more preferably 1.2 or less.

The density of the negative electrode mixture layer 8b is preferably 1.0 g/cm ³ or more from the viewpoint of energy density, and 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 electrolyte impregnation property, the density of the negative electrode mixture layer 8b is 1.7 g/cm ³ It is preferably at most 1.5 g/cm ³ , more preferably at most 1.5 g/cm 3 .

(Separator)
Separator 9 is formed by a dry stretching method. In general, methods for producing separators are roughly divided into two types: dry stretching method and wet method. However, separator 9 produced by dry stretching method has highly crystalline crystals, unlike separator produced by wet method. It has a structure in which a non-crystalline part and an amorphous part with low crystallinity coexist. For example, the separator 9 produced by a dry stretching method has a structure in which a plurality of slit-like (flat plate-shaped) pores are formed and a lamellar skeleton structure in which the polymer is crystallized in a partially folded state. . On the other hand, a separator produced by a wet method has a mesh-like shape, for example, a spider's web shape (see, for example, Japanese Patent Application Laid-Open No. 2020-198316). Therefore, those skilled in the art can easily distinguish between a separator produced by a dry stretching method and a separator produced by a wet method by observing an electron microscope image or the like.

Furthermore, from the viewpoint of making it difficult for vinylene carbonate, which is necessary to form a film on the surface of the negative electrode during initial activation, to become diffusion rate-limiting, the JIS Gurley of the separator 9 needs to be within 300 seconds/100 mL. Here, JIS Gurley is the value of air permeation resistance (sec/100 mL) determined by the measurement method defined in JISP8117:2009. When the JIS Gurley value is larger than 300 seconds/100 mL, diffusion of substances in the electrolyte solution including vinylene carbonate tends to become rate-limiting, making it difficult to form a thin and uniform film. Moreover, since the diffusion of lithium ions tends to become rate-limiting during charging at low temperatures, dendrite precipitation is even more likely to occur.

It is preferable to use at least polyethylene (PE) for the separator 9 from the viewpoint of ensuring safety against short circuits and overcharging through a shutdown function. The separator 9 may be made of only PE, or may have a layered structure in which PE is combined with polypropylene (PP), polyimide, aramid, etc. 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 material layer made of PE may be used. Further, it is preferable that the separator 9 does not contain a filler made of inorganic particles such as alumina, boehmite, or magnesium oxide, which promotes decomposition of the electrolyte during high-temperature cycles. Good too.

The shape of the electrode element is not particularly limited, and the positive electrode 7, negative electrode 8, and separator 9 may be wound, laminated in a single layer manner, or folded in a zigzag manner. 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. In order to reduce the weight of the metal layer, it is desirable to use aluminum or an aluminum alloy containing aluminum as a main component. The laminate sheet 2 uses an aluminum sheet (sheet member) with a thickness of 138 to 168 μm as a base material. The aluminum sheet has a heat-fusible resin layer (for example, PE, PP, etc.) with a thickness of 40 to 100 μm on one side of the base layer, and a thick insulating layer on the other side. It has a protective layer (for example, 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 above configuration allows both excellent high-temperature cycle characteristics and the effect of suppressing dendrite precipitation during sub-zero charging 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 negative electrode surface during initial activation, resulting in excellent high-temperature cycle characteristics. can do. Here, if there is too much coating component derived from VC, the denseness of the coating may become high, leading to an increase in resistance at low temperatures. It is necessary to maintain appropriate density of the film.

In order to ensure that the components derived from the lithium salt are sufficiently included during film formation, the lithium salt in the non-aqueous electrolyte needs to be supplied without stagnation through the separator 9. When the lithium salt is consumed due to the formation of a film on the surface of the negative electrode 8, the concentration of the lithium salt tends to temporarily decrease at the interface between the separator 9 and the negative electrode 8. In order to immediately eliminate this problem, it is necessary to supply the lithium salt from the positive electrode side to the negative electrode side inside the separator 9 without any delay. Therefore, by using a separator 9 made by a dry stretching method, the lithium salt of the non-aqueous electrolyte is supplied without any stagnation due to its lamellar skeleton structure having a straight slit-like pore structure. A decrease in the concentration of lithium salt at the interface with the negative electrode 8 is suppressed. As a result, a sufficient amount of components derived from lithium salts are included during film formation, and dendrite precipitation is less likely to occur when charging at low temperatures. On the other hand, if a separator with a non-straight pore structure, such as one made by a wet method, is used, it is difficult to eliminate the imbalance in the lithium salt concentration inside the separator during film formation, and the lithium salt is not concentrated on the surface of the negative electrode 8. Film formation is likely to occur in a deficient state. As a result, a film derived from VC is produced excessively and the density becomes too high, so dendrite precipitation tends to occur at low temperatures. As for the pore structure of the separator 9, there is no clear tendency for it to affect high rate charging/discharging or DC resistance after complete film formation, so it may be related to the suppression of dendrite precipitation at low temperatures. has not been discussed. However, the inventors of the present application have earnestly investigated and found that it is possible to obtain a non-aqueous electrolyte secondary battery having desired characteristics by using a separator 9 having a lamellar skeleton structure and having slit-like pores produced by a dry drawing method. It turned out to be one of the essential requirements.

Further, since the JIS Gurley of the separator 9 is below the above value, the permeability of the non-aqueous electrolyte is improved, and therefore lithium salt is naturally easily supplied. However, JIS Gurley only indicates the air permeation resistance in an environment with a pressure difference, and does not uniquely determine the ease of diffusion of lithium salt in the non-aqueous electrolyte. Merely having Gurley below the above-mentioned value does not mean that the supply of lithium salt is sufficiently prevented. In other words, use a separator 9 that has both a JIS Gurley value of not more than the above value and a lamellar skeleton structure with a straight slit-like pore structure that is produced when produced by the dry stretching method. For the first time, it is possible to ensure that sufficient components derived from lithium salts are included during film formation.

Furthermore, since the pore structure of the separator 9 becomes straight, the contact area with the non-aqueous electrolyte can be reduced, so that deterioration due to side reactions during high-temperature cycles can be suppressed.

In addition, in order to ensure that the components derived from VC and the components derived from lithium salt are uniformly contained in the non-aqueous electrolyte during film formation, the surface of the graphite that is the negative electrode mixture layer 8b must be smooth. In other words, the surface roughness must be small. The surface roughness of graphite can be compared by the value of (specific surface area) x (median diameter). For example, if the roughness calculation target is assumed to be a perfect sphere with uniform mass distribution, the specific surface area is proportional to the reciprocal of the radius, so by multiplying by the radius, a feature quantity that depends only on the surface shape can be calculated. be able to. Applying this idea to solid objects, not limited to spheres, (specific surface area) x (median diameter) can be a feature representing surface roughness, so the smaller this value is, the smaller the surface roughness is, that is, the smoother the surface. It can be assumed that it has. By using smooth graphite with a value of (specific surface area) x (median diameter) of 30 (x10 ^-6 m ³ /g) or less as the negative electrode active material, more uniform film formation is likely to occur, and as a result, The proportion of components derived from VC is less likely to increase locally, and dendrite precipitation can be suppressed due to the above-mentioned effects. On the other hand, in the case of graphite with a value of (specific surface area) × (median diameter) larger than 30 (×10 ⁻⁶ m ³ /g), the surface roughness of graphite is large, so that the VC-derived components of the surface coating and The ratio with the component derived from lithium salt becomes non-uniform, and there are places where the ratio of the component derived from VC increases locally. At that location, the reaction resistance at low temperatures becomes locally high and dendrite precipitation is likely to occur.
Here, the value of (specific surface area) x (median diameter) is 5 (x10 ^-6 m ³ /g) or more from the viewpoint of output characteristics, suppressing increase in initial irreversible capacity, and increasing electrode density. It is preferable. This lower limit value is calculated based on the specific surface area and median diameter of graphite.

Furthermore, graphite with a smooth surface, where the value of (specific surface area) x (median diameter) is less than the above upper limit, can suppress excessive contact with the electrolyte during cycling, so it is possible to reduce the reductive decomposition of the electrolyte. This has the effect that deterioration due to reactions is less likely to occur.

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

With the above configuration, a suitable amount of VC-derived components and lithium salt-derived components are included during film formation during initial activation, and a film with appropriate density can be formed on the negative electrode surface. It is possible to cause a unique phenomenon that cannot be obtained by a single component or a random combination of components. Therefore, dendrite precipitation due to increased resistance can be suppressed even during charging at sub-zero temperatures. In addition, there is no excessive contact between the electrolyte and other components, which prevents deterioration during high-temperature cycles. It becomes possible to provide batteries.

EXAMPLES Hereinafter, the present invention will be explained in more detail by way of examples, but the present invention is not limited to the following embodiments.

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

The obtained positive electrode active material slurry was applied to both sides of a 20 μm thick aluminum foil serving 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 subjected to press processing, and the density of the positive electrode mixture layer was set to 2.5 g/cm ³ . Thereafter, it was cut so that the uncoated portion of the rectangular side of the portion coated with the positive electrode mixture layer protruded into a rectangular shape as a positive electrode current collector lead.

<Preparation of negative electrode>
96.7% by weight of graphite with a specific surface area of 2.1 m ² /g and a median diameter (D50) of 6.4 μm as the negative electrode active material, 0.3% by weight of acetylene black as the conductive material, and styrene-butadiene as the binder. A negative electrode active material slurry was prepared by mixing 1.5% by weight of rubber, 1.5% by weight of carboxymethyl cellulose as a thickener, and an appropriate amount of ion-exchanged water as a viscosity adjusting solvent.

The obtained negative electrode active material slurry was applied to both sides of a 10 μm thick copper foil serving as 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 subjected to press processing, and the density of the negative electrode mixture layer was set to 1.45 g/cm ³ . Thereafter, it was cut so that the uncoated portion of the rectangular side of the portion coated with the negative electrode mixture layer protruded into a rectangular shape as a negative electrode current collector lead.

<Preparation of electrode element>
Next, an electrode element was produced by alternately stacking a positive electrode and a negative electrode with separators connected in a meandering manner. The separator used had a three-layer structure of PE/PP/PE produced by a dry stretching method. Since this separator is manufactured by a dry stretching method, it has a slit hole structure. Moreover, the JIS Gurley of the prepared separator is 250 seconds/100 mL. Subsequently, the positive electrode current collector lead and the negative electrode current collector lead were bundled together and connected to the positive electrode current collector tab and the negative electrode current collector tab by ultrasonic welding.

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

<Production of non-aqueous electrolyte secondary battery>
As the exterior body, two rectangular laminate films are prepared that have 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. did. The heat-sealing resin layers of the two laminate films were arranged to face each other, and the laminate films were overlapped in mirror alignment so that the electrode elements were housed in the two housing recesses. The electrode element was arranged so that a portion of each terminal (tab) on which a heat-sealing resin portion was 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-sealable resin layers at the peripheral edges of the laminate films were heat-sealed on three sides including the side where each terminal of the laminate film extended. Subsequently, the non-aqueous electrolyte prepared above was injected from one side of the exterior body that was not heat-sealed. Next, the remaining one side of the exterior body was heat-sealed in a reduced pressure environment to produce a non-aqueous electrolyte secondary battery.

<Evaluation by -20℃ cycle test>
The fabricated non-aqueous electrolyte secondary battery was evaluated by a -20°C cycle test. CC-CV charging with an upper limit voltage of 4.2 V, 0.2 C, and cut-off current of 0.05 C and CC discharging with a lower limit voltage of 2.7 V and 1.0 C were repeated for 60 cycles in a −20° C. environment. In addition, before and after that, CC-CV charging with an upper limit voltage of 4.2 V, 0.5 C, and cut-off current of 0.05 C and CC discharging with a lower limit voltage of 2.7 V, 0.5 C were performed in a 25°C environment for one cycle each. A capacity confirmation test is conducted after a charge/discharge cycle in a -20℃ environment, assuming that the discharge capacity in the capacity confirmation test before a charge/discharge cycle in a -20℃ environment is 100%. The discharge capacity in was defined as the capacity retention rate [%].

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

<Comprehensive judgment of cycle test results>
If the capacity retention rate is less than 95% in the -20°C cycle test, there is a very high possibility that dendrite precipitation has occurred, so the overall evaluation was given as "x", and when it was 95% or more, it was given "◎". Also, if the capacity retention rate is less than 75% in the 45°C cycle test, the cycle characteristics at high temperatures are not sufficient, so the overall judgment is "×", and if the capacity retention rate is 75% or more and less than 80%, it is "○". If the capacity maintenance rate was 80% or more, it was marked as "◎". That is, only when the capacity retention rate in the -20°C cycle test was 95% or more and the capacity retention rate in the 45°C cycle test was 80% or more, the overall evaluation was given as "◎".
Table 1 shows the physical property values and test results in Example 1.

(Example 2)
In Example 2, a nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1, except that graphite with a specific surface area of 3.3 m ² /g and a median diameter of 7.6 μm was used as the negative electrode active material. . Table 1 shows the physical property values 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 with a specific surface area of 1.3 m ² /g and a median diameter of 18.5 μm was used as the negative electrode active material. . Table 1 shows the physical property values 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 0.9 mol/L of LiPF ₆ dissolved as a lithium salt was used as the non-aqueous electrolyte. Table 1 shows the physical property values and test results in Example 4.

(Example 5)
In Example 5, a non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1, except that 1.6 mol/L of LiPF ₆ dissolved as a lithium salt was used as the non-aqueous electrolyte. Table 1 shows the physical property values and test results in Example 5.

(Example 6)
In Example 6, a non-containing material was prepared in the same manner as in Example 1, except that 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 was used. A water electrolyte secondary battery was fabricated. Table 1 shows the physical property values 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 solution containing 2.0% by weight of VC as an additive was used as the non-aqueous electrolyte. . Table 1 shows the physical property values and test results in Example 7.

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

(Example 9)
In Example 9, VC was dissolved at a ratio of 2.0% by weight as an additive, and 1.3 mol/L of LiClO ₄ was dissolved as a lithium salt and the non-aqueous electrolyte was used. A non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1. Table 1 shows the physical property values 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 solution containing 0.5% by weight of VC as an additive was used as the non-aqueous electrolyte. . Table 1 shows the physical property values 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 containing 0.1% by weight of VC as an additive was used as the non-aqueous electrolyte. . Table 1 shows the physical property values and test results in Example 11.

(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 with a specific surface area of 2.8 m ² /g and a median diameter of 12.0 μm was used as the negative electrode active material. . Table 1 shows the physical property values 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 with a specific surface area of 5.2 m ² /g and a median diameter of 10.9 μm was used as the negative electrode active material. . Table 1 shows the physical property values and test results in Comparative Example 2.

(Comparative example 3)
In Comparative Example 3, a nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1, except that a JIS Gurley made of PE produced by a wet stretching method at 260 seconds/100 mL was used as the separator. . The separator according to Comparative Example 3 was produced by a wet stretching method, and therefore did not have a slit hole structure. Table 1 shows the physical property values and test results in Comparative Example 3.

(Comparative example 4)
Comparative Example 4 was carried out in the same manner as in Example 1, except that a JIS Gurley of 530 seconds/100 mL, which had a three-layer structure of PE/PP/PE produced by a dry stretching method, was used as a separator. A non-aqueous electrolyte secondary battery was fabricated. Table 1 shows the physical property values and test results in Comparative Example 4.

(Comparative example 5)
In Comparative Example 5, a non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1, except that LiPO ₂ F ₂ dissolved at a ratio of 1% by weight was used as an additive in the non-aqueous electrolyte. Created. Table 1 shows the physical property values 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 additives was used. Table 1 shows the physical property values 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 weight of fluoroethylene carbonate (FEC) was dissolved as an additive in the non-aqueous electrolyte. A battery was created. Table 1 shows the physical property values and test results 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 weight of VC was dissolved as an additive in the non-aqueous electrolyte. Table 1 shows the physical property values 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 weight of VC was dissolved as an additive in the non-aqueous electrolyte. did. Table 1 shows the physical property values and test results in Comparative Example 9.

From the results shown in Table 1, the non-aqueous electrolyte secondary batteries of Examples 1 to 11, which satisfy all the constituent requirements of the present invention, show little capacity loss even after repeated charge/discharge cycles at high temperatures, and have a temperature of -20°C. It was found that dendrite precipitation does not easily occur even after repeated charging and discharging at extremely low temperatures. Among them, the non-aqueous electrolytes of Examples 1 to 3 and 7 in which LiPF ₆ is used as the lithium salt, the concentration thereof is 1.0 mol/L or more and 1.5 mol/L or less, and the separator does not contain an inorganic filler. The secondary battery showed particularly excellent characteristics.

On the other hand, the nonaqueous electrolyte secondary batteries of Comparative Examples 1 to 9 in which any one of the negative electrode, separator, and electrolyte additive did not meet the structural requirements met the desired results in at least one of the -20°C cycle test and the 45°C cycle test. As a result, the following characteristics 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 degrading performance.

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

The present invention provides a non-aqueous electrolyte secondary battery that can be used in a wide temperature range without degrading performance and exhibits excellent characteristics, and thus contributes to the manufacture and sale of non-aqueous electrolyte secondary batteries. Has availability.

1 Nonaqueous 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 collector lead 11 Negative electrode current collector lead 12, 13 Welded part

Claims (8)

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 based on the total weight of the non-aqueous electrolyte,
The separator has a structure with slits or a lamellar skeleton structure, and has a JIS Gurley speed 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 as a negative electrode active material,
The product of the specific surface area (m ² /g) of the graphite and the median diameter (μm) of the graphite is 30 (×10 ⁻⁶ m ³ /g) or less,
A non-aqueous electrolyte secondary battery characterized by: 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 based on the total weight of the non-aqueous electrolyte,
The separator is formed by a dry stretching method, 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 as a negative electrode active material,
The product of the specific surface area (m ² /g) of the graphite and the median diameter (μm) of the graphite is 30 (×10 ⁻⁶ m ³ /g) or less,
A non-aqueous electrolyte secondary battery characterized by: The separator does not contain filler,
The non-aqueous electrolyte secondary battery according to claim 1 or 2, characterized in that: The non-aqueous electrolyte further includes 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 or 2, characterized in that: the lithium salt is LiPF ₆ ;
The non-aqueous electrolyte secondary battery according to claim 4, characterized in that: The positive electrode is
A current collector;
a positive electrode mixture layer applied to the current collector;
has
The positive electrode mixture layer has the general formula _LiMO2 (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);
The non-aqueous electrolyte secondary battery according to claim 1 or 2, characterized in that: The negative electrode contains only graphite as a negative electrode active material.
The non-aqueous electrolyte secondary battery according to claim 1 or 2, characterized in that: When CC-CV charging with an upper limit voltage of 4.2V, 0.2C and cut-off current of 0.05C and CC discharge with a lower limit voltage of 2.7V and 1.0C are repeated for 60 cycles in a -20°C environment. Before and after that, one cycle of CC-CV charging with an upper limit voltage of 4.2 V, 0.5 C, and a cut-off current of 0.05 C and a CC discharge with a lower limit voltage of 2.7 V and 0.5 C were performed in a 25°C environment. 3. The non-aqueous electrolyte secondary battery according to claim 1, wherein the non-aqueous electrolyte secondary battery has a capacity retention rate of 95% or more when a capacity confirmation test is conducted.

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