JP2022134546A - Power storage element - Google Patents

Abstract

To provide a power storage element that is excellent in the effect of suppressing an increase in resistance at low temperatures after charge/discharge cycles.SOLUTION: A power storage element includes: an electrode unit 100 having an electrode body 50 in which a negative electrode 53 and a positive electrode 51 are laminated via a separator, and a sheet member laminated on the outer surface of the electrode body 50; and a rectangular case that accommodates the electrode unit 100. Therein, when a ratio T2/T1 of a maximum thickness T2 of the electrode body to an average inner dimension T1 in the thickness direction of the square case in a thickness direction of the electrode body is defined to be A, and a ratio T3/T1 of a maximum thickness T3 of the electrode unit to T1 is define to be B, A is 0.89 or more and a difference between B and A is 0.01 or more.SELECTED DRAWING: Figure 2

Description

The present invention relates to an electric storage element.

2. Description of the Related Art Electricity storage devices typified by lithium ion non-aqueous electrolyte secondary batteries are widely used in electronic devices such as personal computers, communication terminals, and automobiles because of their high energy density. The energy storage element generally includes an electrode body having a pair of electrodes electrically isolated by a separator, and a non-aqueous electrolyte interposed between the electrodes. configured to charge and discharge; Capacitors such as lithium ion capacitors and electric double layer capacitors are also widely used as storage elements other than non-aqueous electrolyte secondary batteries.

As such a storage element, an electrode group in which a positive electrode, a negative electrode, and a first separator disposed between the positive electrode and the negative electrode are wound, and a second separator is wound around the outer periphery of the central portion. A secondary battery with excellent manageability has been proposed (see Patent Document 1).

WO2019/069356

However, the inventors of the present invention have found that the above-described conventional secondary battery is not sufficiently effective in suppressing an increase in resistance at low temperatures after charge-discharge cycles.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a power storage device that is excellent in suppressing an increase in resistance at low temperatures after charge-discharge cycles.

An electric storage element according to one aspect of the present invention includes an electrode unit having an electrode body in which a negative electrode and a positive electrode are laminated with a separator interposed therebetween, and a sheet-like member laminated on the outer surface of the electrode body, and containing the electrode unit. A ratio T2/T1 of the maximum thickness T2 of the electrode body to the average inner dimension T1 in the thickness direction of the square case in the thickness direction of the electrode body is defined as A, and the electrode to T1 When the ratio T3/T1 of the maximum thickness T3 of the unit is defined as B, A is 0.89 or more and the difference between B and A is 0.01 or more.

ADVANTAGE OF THE INVENTION According to this invention, the electrical storage element which is excellent in the effect of suppressing the resistance increase at the low temperature after a charging/discharging cycle can be provided.

FIG. 1 is a schematic exploded perspective view showing a power storage device according to one embodiment of the present invention. FIG. 2 is a schematic cross-sectional view schematically showing an electrode unit of an electric storage device according to one embodiment of the present invention. FIG. 3 is a schematic cross-sectional view schematically showing an electrode body of a storage element according to one embodiment of the present invention. FIG. 4 is a partial cross-sectional view schematically showing part of the electrode unit of the storage element according to one embodiment of the present invention. FIG. 5 is a schematic diagram showing an embodiment of a power storage device configured by assembling a plurality of non-aqueous electrolyte power storage elements.

An electric storage element according to one aspect of the present invention includes an electrode unit having an electrode body in which a negative electrode and a positive electrode are laminated with a separator interposed therebetween, and a sheet-like member laminated on the outer surface of the electrode body, and containing the electrode unit. A ratio T2/T1 of the maximum thickness T2 of the electrode body to the average inner dimension T1 in the thickness direction of the square case in the thickness direction of the electrode body is defined as A, and the electrode to T1 When the ratio T3/T1 of the maximum thickness T3 of the unit is defined as B, A is 0.89 or more and the difference between B and A is 0.01 or more.

According to the power storage device, the effect of suppressing an increase in resistance at low temperatures after charge-discharge cycles is excellent. Although the reason for this is not certain, the following reason is presumed. The ratio A of the maximum thickness T2 of the electrode body to the average inner dimension T1 in the thickness direction of the rectangular case in the thickness direction of the electrode body is 0.89 or more, so that the electrode body and the inner wall of the rectangular case The gap between the electrodes is reduced, the expansion and contraction of the electrode due to charging and discharging is suppressed, and the generation of wrinkles in the electrode can be suppressed. As a result, an increase in resistance at low temperatures after charge-discharge cycles can be suppressed. In addition, when the difference between the ratio B of the maximum thickness T3 of the electrode unit to the T1 and the above A is 0.01 or more, the effect of winding the electrode by the sheet-like member is enhanced, so that the The expansion and contraction of the electrode is suppressed, and the generation of wrinkles in the electrode can be further suppressed. As a result, the effect of suppressing an increase in resistance at low temperatures after charge-discharge cycles can be further improved. Therefore, in the electric storage element, when A is 0.89 or more and the difference between B and A is 0.01 or more, the effect of suppressing an increase in resistance at low temperatures after charge-discharge cycles is excellent.

"Average inner dimension T1 in the thickness direction of the prismatic case" refers to the inner dimension of the prismatic case, which is the direction in which the positive electrode, the negative electrode, and the separator are laminated. ) exists, it shall be "the average length of the inner dimension of the rectangular case in the thickness direction excluding the thickness of other members". If there are multiple areas with different thicknesses in the direction of the inner dimension of the rectangular case, such as when the rectangular case is provided with a concave portion, the thickness in the inner dimension direction is the smallest. The average thickness of the region is defined as "the average inner dimension T1 in the thickness direction of the rectangular case". The thickness of the electrode body, the thickness of the electrode unit, and the thickness of other members are the thickness in the direction in which the positive electrode, the negative electrode, and the separator are superimposed, and the thickness in the inner dimension direction of the rectangular case. The "average inner dimension T1 in the thickness direction of the prismatic case" is a value when the prismatic case is not pressed and the inside of the prismatic case is empty.
The "maximum thickness T2 of the electrode body" and the "maximum thickness T3 of the electrode unit" are values obtained by the following equations.
Maximum thickness T2 of the electrode body
= thickness of positive electrode other than positive electrode active material layer (A ₀ ) × number of layers of positive electrode + thickness of positive electrode active material layer in discharged state (A ₁ ) × number of layers of positive electrode having positive electrode active material layer + negative electrode active material layer Thickness of negative electrode other than (B ₀ ) × number of layers of negative electrode + thickness of negative electrode active material layer in discharged state (B ₁ ) × number of layers of negative electrode having negative electrode active material layer + thickness of separator (C ₀ ) × separator number of layers Maximum thickness of electrode unit T3
= thickness of positive electrode other than positive electrode active material layer (A ₀ ) × number of layers of positive electrode + thickness of positive electrode active material layer in discharged state (A ₁ ) × number of layers of positive electrode having positive electrode active material layer + negative electrode active material layer Thickness of negative electrode other than (B ₀ ) × number of layers of negative electrode + thickness of negative electrode active material layer in discharged state (B ₁ ) × number of layers of negative electrode having negative electrode active material layer + thickness of separator (C ₀ ) × separator lamination number + sheet-shaped member thickness (D ₀ ) × number of laminated sheet-shaped members That is, the maximum thickness T3 of the electrode unit is the maximum thickness T2 of the electrode body and the sheet-shaped member thickness (D ₀ ) It is the sum of the number of layers of the sheet-shaped member and the product thereof. Here, the number of layers is the number of sheets at the point where the total number of stacked positive electrodes, negative electrodes, and separators is the maximum in the direction in which the positive electrodes, negative electrodes, and separators are stacked (the inner dimension direction in the thickness direction of the rectangular case). is.
The thicknesses of the positive electrode active material layer and the negative electrode active material layer in the discharged state are values obtained by the following method. The electric storage element is discharged at a constant current of 0.2 C to an SOC of 0% to be in a discharged state. This electric storage element is disassembled, the positive electrode and the negative electrode are taken out, washed with dimethyl carbonate or the like, dried under reduced pressure at room temperature for 24 hours or more, and then the thickness of the positive electrode active material layer and the thickness of the negative electrode active material layer are measured. Let these be the positive electrode active material layer thickness (A ₁ ) in the discharged state and the negative electrode active material layer thickness (B ₁ ) in the discharged state, respectively. Note that the average inner dimension and the thickness of each active material layer are the average values of the values measured at arbitrary five points. When active material layers are provided on both sides of one positive electrode or negative electrode, the total thickness of the active material layers on both sides is the thickness of the active material layer.

It is preferable that the number of laminations of the sheet-shaped member is two or more. When the number of layers of the sheet-shaped member is 2 or more, the effect of seaming by the sheet-shaped member is enhanced, the effect of suppressing the generation of wrinkles in the electrode is enhanced, and the expansion and contraction of the electrode is suppressed. It is possible to reduce the gap between the distances between Therefore, it is possible to further improve the effect of reducing the rate of increase in resistance at low temperatures after charge-discharge cycles.

It is preferable to use the separator as the sheet member. By using the separator as the sheet member, productivity can be improved.

Preferably, the negative electrode has a negative electrode active material, and the negative electrode active material comprises graphite, silicon, silicon oxide, or a combination thereof. Graphite, silicon and silicon oxide have relatively large expansion and contraction during charging and discharging. When the negative electrode active material contains graphite, silicon, silicon oxide, or a combination thereof, it is possible to further enhance the effect of suppressing an increase in resistance at low temperatures after charge-discharge cycles.

A configuration of a power storage element, a configuration of a power storage device, a method for manufacturing the power storage element, and other embodiments according to an embodiment of the present invention will be described in detail. Note that the name of each component (each component) used in each embodiment may be different from the name of each component (each component) used in the background art.

<Storage element>
An electric storage element according to an embodiment of the present invention includes an electrode unit having an electrode body in which a negative electrode and a positive electrode are laminated with a separator interposed therebetween and a sheet-like member laminated on the outer surface of the electrode body; a non-aqueous electrolyte; A prismatic case containing the electrode unit and the non-aqueous electrolyte is provided. The electrode body is usually a laminated type in which a plurality of positive electrodes and a plurality of negative electrodes are laminated with separators interposed therebetween, or a wound type in which positive electrodes and negative electrodes are laminated with separators interposed and wound. The non-aqueous electrolyte exists in a state impregnated with the positive electrode, the negative electrode and the separator. A non-aqueous electrolyte secondary battery (hereinafter also simply referred to as a “secondary battery”) will be described as an example of the storage element.

FIG. 1 is a schematic exploded perspective view showing a non-aqueous electrolyte secondary battery as an example of the storage element 1. FIG. As shown in FIG. 1, the storage element 1 includes a flat rectangular parallelepiped rectangular case 3, an electrode unit 100 housed in the rectangular case 3, a negative electrode terminal 5 and a positive electrode terminal 4 provided on a case cover 3b. and The square case 3 has a case body 3a, which is a rectangular parallelepiped housing with an open top, and an elongated rectangular plate-like case cover 3b capable of closing the elongated rectangular opening of the case body 3a. ing. In FIG. 1, the X-axis direction indicates the height direction of the electrode unit 100 and the case main body 3a, the Y-axis direction indicates the thickness direction of the electrode unit 100 and the case main body 3a, and the Z-axis direction indicates the electrode unit 100 and the case main body 3a. The width direction of the case main body 3a is shown.

The storage element 1 includes an electrode unit 100 housed in a rectangular case 3, and a positive lead 14 and a negative lead 15 electrically connected to both ends of the electrode unit 100, respectively. In the electric storage element 1 , an electrode unit 100 having an electrode body having a positive electrode and a negative electrode wound with a separator sandwiched therebetween and a sheet-like member laminated on the outer surface of the electrode body is housed in a rectangular case 3 . The positive electrode is electrically connected to the positive terminal 4 via a positive lead 14 . The negative electrode is electrically connected to the negative terminal 5 via the negative lead 15 .

[Electrode unit]
FIG. 2 is a schematic cross-sectional view schematically showing the electrode unit 100 in the electric storage device 1. As shown in FIG. FIG. 3 is a schematic cross-sectional view schematically showing the electrode body 50 of the storage element according to one embodiment of the present invention. Moreover, FIG. 4 is a partial cross-sectional view schematically showing a part of the electrode unit 100 of the electric storage element 1. As shown in FIG. As shown in FIGS. 2 to 4, the electrode unit 100 includes an electrode assembly 50 in which a positive electrode 51 and a negative electrode 53 are stacked with a first separator 52 and a second separator 54 interposed therebetween, and an electrode assembly 50 laminated on the outer surface of the electrode body 50. It has a sheet-like member 60 that is

(electrode body)
As shown in FIG. 3, the electrode body 50 is a wound electrode body in which a negative electrode 53 and a positive electrode 51 are flatly wound with two separators consisting of a first separator 52 and a second separator 54 interposed therebetween. That is, in the electrode body 50 , a strip-shaped second separator 54 is laminated around the outer circumference of a strip-shaped negative electrode 53 , a strip-shaped positive electrode 51 is laminated around the outer periphery of the second separator 54 , and a strip-shaped first separator is laminated around the outer periphery of the positive electrode 51 . A separator 52 is laminated. More specifically, in the electrode body 50 configured as described above, the negative electrode 53 and the positive electrode 51 are wound with the first separator 52 and the second separator 54 interposed therebetween while being shifted from each other in the winding axis direction. .

In addition, as shown in FIG. 2 , the end portion (end point) of the outermost periphery of the wound negative electrode 53 is the boundary between the electrode body 50 and the sheet-like member 60 . That is, the electrode body 50 is formed by the positive electrode 51 , the negative electrode 53 , and the first separator 52 and the second separator 54 up to the position in contact with the terminal portion of the outermost periphery of the negative electrode 53 .

(Sheet-shaped member)
A sheet member 60 is laminated on the outer surface of the electrode body 50 . The sheet-like member 60 is composed of a belt-like sheet body. The sheet-shaped member 60 preferably has insulating properties, and is made of resin films such as polyolefins such as polyethylene and polypropylene, cellulose, polyester, polyvinyl alcohol, polyimide, polyamide, polyamide-imide, polytetrafluoroethylene and vinylon. , tapes, non-woven fabrics, and the like. A separator, which will be described later, may be used as the sheet member 60 . As the sheet member 60, it is preferable to use the separator. By using the separator as the sheet member 60, productivity can be improved.

The sheet-like member 60 of this embodiment is composed of two sheet bodies consisting of a first sheet body 61 and a second sheet body 62. As shown in FIG. 52 are continuously connected, and the second sheet body 62 and the second separator 54 are continuously connected. Note that the first sheet body 61 and the first separator 52 and the second sheet body 62 and the second separator 54 do not have to be connected continuously.

The lower limit of the number of layers of the sheet members 60 is preferably 2, more preferably 3. When the number of layers of the sheet-shaped member 60 is two or more, the effect of seaming by the sheet-shaped member becomes higher, the effect of suppressing the occurrence of wrinkles in the electrode is enhanced, and the expansion and contraction of the electrode is suppressed. The difference in distance between electrodes can be reduced. Therefore, it is possible to further improve the effect of reducing the rate of increase in resistance at low temperatures after charge-discharge cycles. Here, the number of layers of the sheet-like member 60 shown in FIG. Regarding the number of laminations, the part where the winding of each sheet member does not exceed one turn is not added to the number of laminations. The upper limit of the number of laminated sheets 60 is, for example, 10, preferably 6, and more preferably 4. By setting the upper limit of the number of layers to be within the above range, the proportion of the electrode bodies in the rectangular case is increased, and an electric storage element with high energy density can be obtained.

The average thickness of the first sheet body 61 and the second sheet body 62 forming the sheet-like member 60 is, for example, preferably 10 μm or more and 30 μm or less, more preferably 13 μm or more and 25 μm or less. Since the average thickness of the first sheet member 61 and the second sheet member 62 is equal to or greater than the above lower limit, the required length of the sheet member can be shortened, and the sheet member can sufficiently exhibit the effect of winding. can be done. On the other hand, when the average thickness of the first sheet body 61 and the second sheet body 62 is equal to or less than the upper limit, it is possible to easily secure the number of layers of 2 or more. The average thickness of the first sheet member 61 and the second sheet member 62 is the average value of the thicknesses measured at arbitrary 10 points.

When A is the ratio T2/T1 of the maximum thickness T2 of the electrode body 40 shown in FIG. is 0.89, preferably 0.90. When A is equal to or greater than the lower limit, the gap between the electrode assembly and the rectangular case is reduced, and the generation of wrinkles in the electrode can be suppressed. As a result, an increase in resistance at low temperatures after charge-discharge cycles can be suppressed. On the other hand, the upper limit of A is preferably 0.99. When A is equal to or less than the upper limit, it is possible to improve productivity when inserting the electrode unit 100 having the electrode body 50 and the sheet-like member 60 into the case 3 .

When the ratio T3/T1 of the maximum thickness T3 of the electrode unit 100 shown in FIG. 4 to T1 is B, the lower limit of B is preferably 0.91, more preferably 0.92. When B is equal to or higher than the lower limit, it is possible to suppress the generation of wrinkles in the electrodes and the increase in the distance between the electrodes. The occurrence of wrinkles in the electrodes tends to cause an increase in resistance at low temperatures after charge-discharge cycles, and the increase in the distance between the electrodes tends to cause an increase in resistance at low temperatures after charge-discharge cycles. Therefore, when B is equal to or higher than the lower limit, the effect of suppressing an increase in resistance at low temperatures after charge-discharge cycles can be further improved. On the other hand, the upper limit of B is preferably 1.00. When B is equal to or less than the upper limit, the productivity when inserting the electrode body unit 100 into the case 3 can be improved.

The lower limit of the difference between B and A is 0.01, more preferably 0.02. When the difference between B and A is equal to or greater than the lower limit, the effect of seaming the electrode by the sheet-shaped member is enhanced, so that the generation of wrinkles in the electrode can be further suppressed. As a result, the effect of suppressing an increase in resistance at low temperatures after charge-discharge cycles can be further improved. The upper limit of the difference between B and A is preferably 0.05, more preferably 0.04. When the difference between B and A is equal to or less than the upper limit, the proportion of the electrode body in the rectangular case is increased, and an electric storage element with high energy density can be obtained.

(separator)
Separators such as the first separator 52 and the second separator 54 can be appropriately selected from known separators. As the separator, for example, a separator consisting of only a substrate layer, a separator having a heat-resistant layer containing heat-resistant particles and a binder formed on one or both surfaces of a substrate layer, or the like can be used. Examples of the shape of the base layer of the separator include woven fabric, nonwoven fabric, and porous resin film. Among these shapes, a porous resin film is preferred from the viewpoint of strength, and a non-woven fabric is preferred from the viewpoint of non-aqueous electrolyte retention. As the material for the base layer of the separator, polyolefins such as polyethylene and polypropylene are preferable from the viewpoint of shutdown function, and polyimide, aramid, and the like are preferable from the viewpoint of oxidative decomposition resistance. A material obtained by combining these resins may be used as the base material layer of the separator.

The heat-resistant particles contained in the heat-resistant layer preferably have a mass loss of 5% or less when the temperature is raised from room temperature to 500 ° C. in an air atmosphere of 1 atm, and the mass loss when the temperature is raised from room temperature to 800 ° C. is more preferably 5% or less. An inorganic compound can be mentioned as a material whose mass reduction is less than or equal to a predetermined value. Examples of inorganic compounds include oxides such as iron oxide, silicon oxide, aluminum oxide, titanium oxide, zirconium oxide, calcium oxide, strontium oxide, barium oxide, magnesium oxide, and aluminosilicate; nitrides such as aluminum nitride and silicon nitride. carbonates such as calcium carbonate; sulfates such as barium sulfate; sparingly soluble ionic crystals such as calcium fluoride, barium fluoride, and barium titanate; covalent crystals such as silicon and diamond; Mineral resource-derived substances such as zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica, or artificial products thereof. As the inorganic compound, a single substance or a composite of these substances may be used alone, or two or more of them may be mixed and used. Among these inorganic compounds, silicon oxide, aluminum oxide, or aluminosilicate is preferable from the viewpoint of the safety of the electric storage device.

The porosity of the separator is preferably 80% by volume or less from the viewpoint of strength, and preferably 20% by volume or more from the viewpoint of discharge performance. Here, the "porosity" is a volume-based value and means a value measured with a mercury porosimeter.

A polymer gel composed of a polymer and a non-aqueous electrolyte may be used as the separator. Examples of polymers include polyacrylonitrile, polyethylene oxide, polypropylene oxide, polymethyl methacrylate, polyvinyl acetate, polyvinylpyrrolidone, polyvinylidene fluoride, and the like. The use of polymer gel has the effect of suppressing liquid leakage. As the separator, a polymer gel may be used in combination with the porous resin film or non-woven fabric as described above.

The average thickness of the separator is, for example, preferably 10 μm or more and 30 μm or less, more preferably 13 μm or more and 25 μm or less. When the average thickness of the separator is equal to or greater than the above lower limit, the desired strength of the separator can be maintained. On the other hand, when the average thickness of the separator is equal to or less than the above upper limit, the capacity of the cell can be improved. In addition, let the average thickness of a separator be the average value of the thickness measured in ten arbitrary places.

(negative electrode)
The negative electrode 53 has a negative electrode base material and a negative electrode active material layer disposed directly on the negative electrode base material or via an intermediate layer.

A negative electrode base material has electroconductivity. Whether or not a material has "conductivity" is determined using a volume resistivity of 10 ⁷ Ω·cm as a threshold measured according to JIS-H-0505 (1975). As materials for the negative electrode substrate, metals such as copper, nickel, stainless steel, nickel-plated steel, aluminum, alloys thereof, carbonaceous materials, and the like are used. Among these, copper or a copper alloy is preferred. Examples of the negative electrode substrate include foil, deposited film, mesh, porous material, and the like, and foil is preferable from the viewpoint of cost. Therefore, copper foil or copper alloy foil is preferable as the negative electrode substrate. Examples of copper foil include rolled copper foil and electrolytic copper foil.

The average thickness of the negative electrode substrate is preferably 2 μm or more and 35 μm or less, more preferably 3 μm or more and 30 μm or less, even more preferably 4 μm or more and 25 μm or less, and particularly preferably 5 μm or more and 20 μm or less. By setting the average thickness of the negative electrode substrate within the above range, the energy density per volume of the secondary battery can be increased while increasing the strength of the negative electrode substrate.

The intermediate layer is a layer arranged between the negative electrode substrate and the negative electrode active material layer. The intermediate layer reduces the contact resistance between the negative electrode substrate and the negative electrode active material layer by containing a conductive agent such as carbon particles. The composition of the intermediate layer is not particularly limited, and includes, for example, a binder and a conductive agent.

The negative electrode active material layer contains a negative electrode active material. The negative electrode active material layer contains arbitrary components such as a conductive agent, a binder, a thickener, a filler, etc., as required.

The negative electrode active material layer contains typical nonmetallic elements such as B, N, P, F, Cl, Br, and I, Li, Na, Mg, Al, K, Ca, Zn, Ga, Ge, Sn, Sr, Ba, and the like. and transition metal elements such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Zr, Ta, Hf, Nb, and W are used as negative electrode active materials, conductive agents, binders, and thickeners. You may contain as a component other than a sticky agent and a filler.

The negative electrode active material can be appropriately selected from known negative electrode active materials. Materials capable of intercalating and deintercalating lithium ions are usually used as negative electrode active materials for lithium ion secondary batteries. Examples of negative electrode active materials include metallic lithium; metals or semimetals such as silicon and tin; metal oxides and semimetal oxides such as silicon oxide, titanium oxide and tin oxide; Li ₄ Ti ₅ O ₁₂ ; Titanium-containing oxides such as LiTiO _{2 and} TiNb ₂ O ₇ ; polyphosphate compounds; silicon carbide; carbon materials such as graphite and non-graphitizable carbon (easily graphitizable carbon or non-graphitizable carbon) be done. Among these materials, graphite and non-graphitic carbon are preferred. In the negative electrode active material layer, one type of these materials may be used alone, or two or more types may be mixed and used.

Preferably, the negative electrode active material contains graphite, silicon, silicon oxide, or a combination thereof. Graphite, silicon and silicon oxide have relatively large expansion and contraction during charging and discharging. When the negative electrode active material contains graphite, silicon, silicon oxide, or a combination thereof, it is possible to further enhance the effect of suppressing an increase in resistance at low temperatures after charge-discharge cycles.

“Graphite” refers to a carbon material having an average lattice spacing (d ₀₀₂ ) of the (002) plane of 0.33 nm or more and less than 0.34 nm as determined by X-ray diffraction before charging/discharging or in a discharged state. Graphite includes natural graphite and artificial graphite. Artificial graphite is preferable from the viewpoint that a material with stable physical properties can be obtained.

“Non-graphitic carbon” refers to a carbon material having an average lattice spacing (d ₀₀₂ ) of the (002) plane of 0.34 nm or more and 0.42 nm or less as determined by X-ray diffraction before charging/discharging or in a discharged state. . Non-graphitizable carbon includes non-graphitizable carbon and graphitizable carbon. Examples of non-graphitic carbon include resin-derived materials, petroleum pitch or petroleum pitch-derived materials, petroleum coke or petroleum coke-derived materials, plant-derived materials, and alcohol-derived materials.

Here, the term “discharged state” means a state in which the carbon material, which is the negative electrode active material, is discharged such that lithium ions that can be intercalated and deintercalated are sufficiently released during charging and discharging. For example, in a single electrode battery using a negative electrode containing a carbon material as a negative electrode active material as a working electrode and metal Li as a counter electrode, the open circuit voltage is 0.7 V or higher.

The term “non-graphitizable carbon” refers to a carbon material having a d ₀₀₂ of 0.36 nm or more and 0.42 nm or less.

“Graphitizable carbon” refers to a carbon material having a d ₀₀₂ of 0.34 nm or more and less than 0.36 nm.

The negative electrode active material is usually particles (powder). The average particle size of the negative electrode active material can be, for example, 1 nm or more and 100 μm or less. When the negative electrode active material is a carbon material, a titanium-containing oxide or a polyphosphate compound, the average particle size may be 1 μm or more and 100 μm or less. When the negative electrode active material is silicon, tin, silicon oxide, tin oxide, or the like, the average particle size may be 1 nm or more and 1 μm or less. By making the average particle size of the negative electrode active material equal to or greater than the above lower limit, the production or handling of the negative electrode active material is facilitated. By setting the average particle diameter of the negative electrode active material to the above upper limit or less, the electron conductivity of the negative electrode active material layer is improved. "Average particle size" is based on JIS-Z-8825 (2013), based on the particle size distribution measured by a laser diffraction / scattering method for a diluted solution in which particles are diluted with a solvent, JIS-Z-8819 -2 (2001) means a value at which the volume-based integrated distribution calculated according to 50%. A pulverizer, a classifier, or the like is used to obtain powder having a predetermined particle size. A pulverizer, a classifier, or the like is used to obtain powder having a predetermined particle size. Pulverization methods include, for example, methods using a mortar, ball mill, sand mill, vibrating ball mill, planetary ball mill, jet mill, counter jet mill, whirling jet mill, or sieve. At the time of pulverization, wet pulverization in which water or an organic solvent such as hexane is allowed to coexist can also be used. As a classification method, a sieve, an air classifier, or the like is used as necessary, both dry and wet. When the negative electrode active material is metal such as metal Li, the negative electrode active material may be foil-shaped.

The content of the negative electrode active material in the negative electrode active material layer is preferably 60% by mass or more and 99% by mass or less, more preferably 90% by mass or more and 98% by mass or less. By setting the content of the negative electrode active material within the above range, it is possible to achieve both high energy density and manufacturability of the negative electrode active material layer.

The conductive agent is not particularly limited as long as it is a conductive material. Examples of such conductive agents include carbonaceous materials, metals, and conductive ceramics. Carbonaceous materials include graphite, non-graphitic carbon, graphene-based carbon, and the like. Examples of non-graphitic carbon include carbon nanofiber, pitch-based carbon fiber, and carbon black. Examples of carbon black include furnace black, acetylene black, and ketjen black. Graphene-based carbon includes graphene, carbon nanotube (CNT), fullerene, and the like. The shape of the conductive agent may be powdery, fibrous, or the like. As the conductive agent, one type of these materials may be used alone, or two or more types may be mixed and used. Also, these materials may be combined for use. For example, a composite material of carbon black and CNT may be used. Among these, carbon black is preferable from the viewpoint of electron conductivity and coatability, and acetylene black is particularly preferable.

The content of the conductive agent in the negative electrode active material layer is preferably 1% by mass or more and 10% by mass or less, more preferably 3% by mass or more and 9% by mass or less. By setting the content of the conductive agent within the above range, the energy density of the secondary battery can be increased.

Binders include, for example, fluorine resins (polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), etc.), thermoplastic resins such as polyethylene, polypropylene, polyacryl, and polyimide; ethylene-propylene-diene rubber (EPDM), sulfone Elastomers such as modified EPDM, styrene-butadiene rubber (SBR) and fluororubber; polysaccharide polymers and the like.

The content of the binder in the negative electrode active material layer is preferably 1% by mass or more and 10% by mass or less, more preferably 3% by mass or more and 9% by mass or less. By setting the content of the binder within the above range, the active material can be stably retained.

Examples of thickeners include polysaccharide polymers such as carboxymethylcellulose (CMC) and methylcellulose. When the thickener has a functional group that reacts with lithium or the like, the functional group may be previously deactivated by methylation or the like.

A filler is not specifically limited. Fillers include polyolefins such as polypropylene and polyethylene, inorganic oxides such as silicon dioxide, alumina, titanium dioxide, calcium oxide, strontium oxide, barium oxide, magnesium oxide and aluminosilicate, magnesium hydroxide, calcium hydroxide, hydroxide Hydroxides such as aluminum, carbonates such as calcium carbonate, sparingly soluble ionic crystals such as calcium fluoride, barium fluoride, and barium sulfate, nitrides such as aluminum nitride and silicon nitride, talc, montmorillonite, boehmite, zeolite, Mineral resource-derived substances such as apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica, or artificial products thereof may be used.

(positive electrode)
The positive electrode 51 has a positive electrode base material and a positive electrode active material layer disposed directly on the positive electrode base material or via an intermediate layer. The structure of the intermediate layer is not particularly limited, and can be selected from, for example, the structures exemplified for the negative electrode.

A positive electrode base material has electroconductivity. As the material for the positive electrode substrate, metals such as aluminum, titanium, tantalum and stainless steel, or alloys thereof are used. Among these, aluminum or an aluminum alloy is preferable from the viewpoint of potential resistance, high conductivity, and cost. Examples of the positive electrode substrate include foil, deposited film, mesh, porous material, and the like, and foil is preferable from the viewpoint of cost. Therefore, aluminum foil or aluminum alloy foil is preferable as the positive electrode substrate. Examples of aluminum or aluminum alloys include A1085, A3003, A1N30, etc. defined in JIS-H-4000 (2014) or JIS-H4160 (2006).

The average thickness of the positive electrode substrate is preferably 3 μm or more and 50 μm or less, more preferably 5 μm or more and 40 μm or less, even more preferably 8 μm or more and 30 μm or less, and particularly preferably 10 μm or more and 25 μm or less. By setting the average thickness of the positive electrode substrate within the above range, the energy density per volume of the secondary battery can be increased while increasing the strength of the positive electrode substrate.

The positive electrode active material layer contains a positive electrode active material. The positive electrode active material layer contains optional components such as a conductive agent, a binder, a thickener, a filler, etc., as required. Optional components such as a conductive agent, a binder, a thickener, and a filler can be selected from the materials exemplified for the negative electrode.

The positive electrode active material can be appropriately selected from known positive electrode active materials. As a positive electrode active material for lithium ion secondary batteries, a material capable of intercalating and deintercalating lithium ions is usually used. Examples of positive electrode active materials include lithium-transition metal composite oxides having an α-NaFeO ₂ type crystal structure, lithium-transition metal composite oxides having a spinel-type crystal structure, polyanion compounds, chalcogen compounds, and sulfur. Examples of lithium transition metal composite oxides having an α-NaFeO ₂ type crystal structure include Li[Li _x Ni _(1-x) ]O ₂ (0≦x<0.5), Li[Li _x Ni _γ Co _{( 1-x-γ)} ]O ₂ (0≦x<0.5, 0<γ<1), Li[Li _x Co _(1-x) ]O ₂ (0≦x<0.5), Li[ Li _x Ni _γ Mn _(1-x-γ) ]O ₂ (0≦x<0.5, 0<γ<1), Li[Li _x Ni _γ Mn _β Co _(1-x-γ-β) ] O ₂ (0≦x<0.5, 0<γ, 0<β, 0.5<γ+β<1), Li[Li _x Ni _γ Co _β Al _(1-x-γ-β) ]O ₂ ( 0≦x<0.5, 0<γ, 0<β, 0.5<γ+β<1) and the like. Examples of lithium transition metal composite oxides having a spinel crystal structure include Li _x Mn ₂ O ₄ and Li _x Ni _γ Mn _(2-γ) O ₄ . Examples of _polyanion compounds include _LiFePO4 , _{LiMnPO4 , LiNiPO4} , _{LiCoPO4, Li3V2(PO4)3 , Li2MnSiO4 , Li2CoPO4F and the} like. Examples of chalcogen compounds include titanium disulfide, molybdenum disulfide, and molybdenum dioxide. The atoms or polyanions in these materials may be partially substituted with atoms or anionic species of other elements. These materials may be coated with other materials on their surfaces. In the positive electrode active material layer, one kind of these materials may be used alone, or two or more kinds may be mixed and used.

The positive electrode active material is usually particles (powder). The average particle size of the positive electrode active material is preferably, for example, 0.1 μm or more and 20 μm or less. By making the average particle size of the positive electrode active material equal to or more than the above lower limit, manufacturing or handling of the positive electrode active material becomes easy. By setting the average particle size of the positive electrode active material to the above upper limit or less, the electron conductivity of the positive electrode active material layer is improved. Note that when a composite of a positive electrode active material and another material is used, the average particle size of the composite is taken as the average particle size of the positive electrode active material.

A pulverizer, a classifier, or the like is used to obtain powder having a predetermined particle size. The pulverization method and the powder class method can be selected from, for example, the methods exemplified for the negative electrode.

The content of the positive electrode active material in the positive electrode active material layer is preferably 50% by mass or more and 99% by mass or less, more preferably 70% by mass or more and 98% by mass or less, and even more preferably 80% by mass or more and 95% by mass or less. By setting the content of the positive electrode active material within the above range, it is possible to achieve both high energy density and manufacturability of the positive electrode active material layer.

The positive electrode active material layer contains typical nonmetallic elements such as B, N, P, F, Cl, Br, and I, Li, Na, Mg, Al, K, Ca, Zn, Ga, Ge, Sn, Sr, Ba, and the like. typical metal elements, transition metal elements such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Zr, Nb, W are used as positive electrode active materials, conductive agents, binders, thickeners, fillers It may be contained as a component other than

(Non-aqueous electrolyte)
The non-aqueous electrolyte can be appropriately selected from known non-aqueous electrolytes. A non-aqueous electrolyte may be used as the non-aqueous electrolyte. The non-aqueous electrolyte contains a non-aqueous solvent and an electrolyte salt dissolved in this non-aqueous solvent.

The non-aqueous solvent can be appropriately selected from known non-aqueous solvents. Non-aqueous solvents include cyclic carbonates, chain carbonates, carboxylic acid esters, phosphoric acid esters, sulfonic acid esters, ethers, amides, nitriles and the like. As the non-aqueous solvent, those in which some of the hydrogen atoms contained in these compounds are substituted with halogens may be used.

Cyclic carbonates include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinylene carbonate (VC), vinylethylene carbonate (VEC), chloroethylene carbonate, fluoroethylene carbonate (FEC), and difluoroethylene carbonate. (DFEC), styrene carbonate, 1-phenylvinylene carbonate, 1,2-diphenylvinylene carbonate and the like. Among these, EC is preferred.

Chain carbonates include diethyl carbonate (DEC), dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), diphenyl carbonate, trifluoroethylmethyl carbonate, bis(trifluoroethyl) carbonate and the like. Among these, EMC is preferred.

As the non-aqueous solvent, it is preferable to use a cyclic carbonate or a chain carbonate, and it is more preferable to use a combination of a cyclic carbonate and a chain carbonate. By using a cyclic carbonate, it is possible to promote the dissociation of the electrolyte salt and improve the ionic conductivity of the non-aqueous electrolyte. By using a chain carbonate, the viscosity of the non-aqueous electrolyte can be kept low. When a cyclic carbonate and a chain carbonate are used together, the volume ratio of the cyclic carbonate to the chain carbonate (cyclic carbonate:chain carbonate) is preferably in the range of, for example, 5:95 to 50:50.

The electrolyte salt can be appropriately selected from known electrolyte salts. Examples of electrolyte salts include lithium salts, sodium salts, potassium salts, magnesium salts, onium salts and the like. Among these, lithium salts are preferred.

Lithium salts include inorganic lithium salts such as LiPF ₆ , LiPO ₂ F ₂ , LiBF ₄ , LiClO ₄ and LiN(SO ₂ F) ₂ , lithium bis(oxalate) borate (LiBOB), lithium difluorooxalate borate (LiFOB). , lithium oxalate salts such as lithium bis(oxalate) difluorophosphate ( _LiFOP ), _LiSO3CF3 , LiN _{( SO2CF3} ) ₂ , LiN _{( SO2C2F5} ) _{2 ,} LiN _{( SO2CF3} ) (SO ₂ C ₄ F ₉ ), LiC(SO ₂ CF ₃ ) ₃ , LiC(SO ₂ C ₂ F ₅ ) ₃ and other lithium salts having a halogenated hydrocarbon group. Among these, inorganic lithium salts are preferred, and _LiPF6 is more preferred.

The content of the electrolyte salt in the non-aqueous electrolyte is preferably 0.1 mol/dm3 or more and 2.5 mol/dm3 or less ^, and 0.3 mol/dm3 or more and 2.0 mol/dm3 or less at ²⁰ °C and ¹ atm. It is more preferably ³ or less, more preferably 0.5 mol/dm ³ or more and 1.7 mol/dm ³ or less, and particularly preferably 0.7 mol/dm ³ or more and 1.5 mol/dm ³ or less. By setting the content of the electrolyte salt within the above range, the ionic conductivity of the non-aqueous electrolyte can be increased.

The non-aqueous electrolyte may contain additives in addition to the non-aqueous solvent and electrolyte salt. Examples of additives include halogenated carbonates such as fluoroethylene carbonate (FEC) and difluoroethylene carbonate (DFEC); lithium bis(oxalate)borate (LiBOB), lithium difluorooxalateborate (LiFOB), lithium bis(oxalate ) oxalates such as difluorophosphate (LiFOP); imide salts such as lithium bis(fluorosulfonyl)imide (LiFSI); biphenyl, alkylbiphenyl, terphenyl, partially hydrogenated terphenyl, cyclohexylbenzene, t-butylbenzene , t-amylbenzene, diphenyl ether, dibenzofuran and other aromatic compounds; 2-fluorobiphenyl, o-cyclohexylfluorobenzene, p-cyclohexylfluorobenzene and other partial halides of the above aromatic compounds; 2,4-difluoroanisole, 2 Halogenated anisole compounds such as ,5-difluoroanisole, 2,6-difluoroanisole, 3,5-difluoroanisole; vinylene carbonate, methyl vinylene carbonate, ethyl vinylene carbonate, succinic anhydride, glutaric anhydride, maleic anhydride, anhydride Citraconic acid, glutaconic anhydride, itaconic anhydride, cyclohexanedicarboxylic anhydride; ethylene sulfite, propylene sulfite, dimethyl sulfite, propanesultone, propenesultone, butanesultone, methyl methanesulfonate, busulfan, methyl toluenesulfonate, dimethyl sulfate, sulfuric acid ethylene, sulfolane, dimethylsulfone, diethylsulfone, dimethylsulfoxide, diethylsulfoxide, tetramethylenesulfoxide, diphenylsulfide, 4,4'-bis(2,2-dioxo-1,3,2-dioxathiolane), 4-methylsulfonyloxy methyl-2,2-dioxo-1,3,2-dioxathiolane, thioanisole, diphenyl disulfide, dipyridinium disulfide, 1,3-propenesultone, 1,3-propanesultone, 1,4-butanesultone, 1,4- butene sultone, perfluorooctane, tristrimethylsilyl borate, tristrimethylsilyl phosphate, tetrakistrimethylsilyl titanate, lithium monofluorophosphate, lithium difluorophosphate and the like. These additives may be used singly or in combination of two or more.

The content of the additive contained in the non-aqueous electrolyte is preferably 0.01% by mass or more and 10% by mass or less, and 0.1% by mass or more and 7% by mass or less with respect to the total mass of the non-aqueous electrolyte. More preferably, it is 0.2% by mass or more and 5% by mass or less, and particularly preferably 0.3% by mass or more and 3% by mass or less. By setting the content of the additive within the above range, it is possible to improve capacity retention performance or cycle performance after high-temperature storage, or to further improve safety.

A solid electrolyte may be used as the non-aqueous electrolyte, or a non-aqueous electrolyte and a solid electrolyte may be used in combination.

The solid electrolyte can be selected from arbitrary materials such as lithium, sodium, calcium, etc., which have ion conductivity and are solid at room temperature (for example, 15° C. to 25° C.). Examples of solid electrolytes include sulfide solid electrolytes, oxide solid electrolytes, oxynitride solid electrolytes, and polymer solid electrolytes.

Examples of sulfide solid electrolytes include Li ₂ SP ₂ S ₅ , LiI—Li ₂ SP ₂ S ₅ , Li ₁₀ Ge—P ₂ S ₁₂ and the like.

<Power storage device>
The power storage device of the present embodiment is a power source for automobiles such as electric vehicles (EV), hybrid vehicles (HEV), and plug-in hybrid vehicles (PHEV), power sources for electronic devices such as personal computers and communication terminals, or power sources for power storage. For example, it can be mounted as a power storage device configured by collecting a plurality of power storage elements 1 . In this case, the technology of the present invention may be applied to at least one power storage element included in the power storage device.
FIG. 5 shows an example of a power storage device 30 in which power storage units 20 each including two or more electrically connected power storage elements 1 are assembled. The power storage device 30 may include a bus bar (not shown) that electrically connects two or more power storage elements 1, a bus bar (not shown) that electrically connects two or more power storage units 20, and the like. The power storage unit 20 or power storage device 30 may include a state monitoring device (not shown) that monitors the state of one or more power storage elements.

<Method for manufacturing power storage element>
A method for manufacturing the electric storage device of the present embodiment can be appropriately selected from known methods. The manufacturing method includes, for example, preparing an electrode unit, preparing a non-aqueous electrolyte, and housing the electrode unit and the non-aqueous electrolyte in a rectangular case. Preparing the electrode unit includes preparing a positive electrode and a negative electrode, forming an electrode body by laminating or winding the positive electrode and the negative electrode with a separator interposed therebetween, and forming a sheet-like member on the outer surface of the electrode body. and laminating.

Enclosing the non-aqueous electrolyte in the rectangular case can be appropriately selected from known methods. For example, when a non-aqueous electrolyte is used as the non-aqueous electrolyte, the non-aqueous electrolyte may be injected from an inlet formed in the rectangular case, and then the inlet may be sealed.

<Other embodiments>
It should be noted that the electric storage device of the present invention is not limited to the above-described embodiments, and various modifications may be made without departing from the gist of the present invention. For example, the configuration of another embodiment can be added to the configuration of one embodiment, and part of the configuration of one embodiment can be replaced with the configuration of another embodiment or a known technique. Furthermore, some of the configurations of certain embodiments can be deleted. Also, well-known techniques can be added to the configuration of a certain embodiment.

In the above embodiment, the storage element is used as a chargeable/dischargeable non-aqueous electrolyte secondary battery (for example, a lithium ion secondary battery), but the type, shape, size, capacity, etc. of the storage element are arbitrary. . The present invention can also be applied to various secondary batteries.

In addition, although the wound electrode body is used in the above embodiment, a laminated electrode body formed by stacking a sheet-like positive electrode, a negative electrode, and a separator may be provided. In the case of such a laminated electrode body, the sheet member is arranged so as to wind around the outside of the laminated electrode body.

Further, in the above embodiment, the electrode body has a hollow area in the central portion, but an electrode body that does not have a hollow area in the central portion may be provided.

In the above embodiment, the electrode unit uses two separators, the first separator and the second separator, but the first separator and the second separator may be connected at the center. That is, one sheet of separator may be folded back at the central portion.

In the above embodiment, the sheet-shaped member has the first sheet body and the second sheet body continuously connected to the first separator and the second separator, but the length of the first separator and the second separator is extended to The region extending from the outermost peripheral end portion of the negative electrode may be the first sheet body and the second sheet body, and separate first sheet bodies and second sheet bodies may be formed at the ends of the first separator and the second separator. It may be fixed and used. Moreover, it is preferable that the edge of the outermost periphery of the sheet member is fixed by means such as tape or heat sealing.

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

[Example 1, Example 2 and Comparative Examples 1 to 5]
(Preparation of negative electrode)
A negative electrode mixture paste containing graphite as a negative electrode active material, SBR as a binder, CMC as a thickener, and water as a dispersion medium was prepared. The mass ratio of the negative electrode active material, the binder, and the thickener was 97:2:1 in terms of solid content. This negative electrode mixture paste was applied to both sides of the copper foil as the negative electrode substrate so that the negative electrode active material layer non-laminated portion was formed on one edge of the copper foil as the negative electrode substrate. Next, a negative electrode active material layer was produced by drying. After drying, the negative electrode active material layer was roll-pressed so as to obtain a predetermined packing density, thereby obtaining a negative electrode.

(Preparation of positive electrode)
LiNi _0.6 Mn _0.2 Co _0.2 O ₂ powder as a positive electrode active material, acetylene black as a conductive agent, PVDF as a binder, and N-methylpyrrolidone (NMP) as a non-aqueous dispersion medium were used to form a positive electrode. A mixture paste was prepared. The mass ratio of the positive electrode active material, the binder and the conductive agent was 90:5:5 in terms of solid content. This positive electrode material mixture paste was applied to both surfaces of an aluminum foil serving as a positive electrode base material so that a negative electrode active material layer non-laminated portion was formed on one edge of the aluminum foil. Next, by drying, a positive electrode active material layer was produced. After drying, the positive electrode active material layer was roll-pressed so as to obtain a predetermined packing density, thereby obtaining a positive electrode.

(Non-aqueous electrolyte)
The non-aqueous electrolyte was prepared by dissolving LiPF ₆ at a salt concentration of 1.2 mol/dm ³ in a solvent in which EC and EMC were mixed at a volume ratio of 30:70.

(separator)
A polyethylene microporous film having a thickness of 20 μm was used as the separator.

(Sheet-shaped member)
The above separator was used as the sheet member. The number of laminations of the sheet-shaped member was as shown in Table 1.

(Production of power storage element)
The positive electrode, the negative electrode, and the separator were laminated and wound to produce an electrode body. Using the above separator as a sheet-like member, the separator was continuously wound so as to obtain the number of layers shown in Table 1, thereby producing an electrode unit. The separator used as the sheet-like member is continuously connected to the separator used when the electrode body was produced. Thereafter, the non-laminated portion of the positive electrode active material layer of the positive electrode substrate and the non-laminated portion of the negative electrode active material layer of the negative substrate were welded to the positive electrode lead and the negative electrode lead, respectively, and enclosed in the case body of the square case. Next, after welding the case main body and the case cover, the above non-aqueous electrolyte was injected and sealed. Thus, the electric storage elements of Examples 1 and 2 and Comparative Examples 1 to 5 were obtained.

[evaluation]
(Capacity retention rate after charge-discharge cycle test)
(1) Initial charge/discharge test Initial charge/discharge was performed on each of the obtained electric storage elements under the following conditions. After constant current charging with a charging current of 0.2C to 4.25V at 25°C, constant voltage charging was performed at 4.25V. The charging termination condition was until the charging current reached 0.01C. After a rest period of 10 minutes was provided after charging, the battery was discharged at a constant current of 0.2 C to 2.75 V at 25°C. A rest period of 10 minutes was provided after discharge.
Next, the battery was charged at a constant current of 1.0C to 4.25V at 25°C, and then charged at a constant voltage of 4.25V. The charging termination condition was until the charging current reached 0.01C. After a rest period of 10 minutes was provided after charging, the battery was discharged at a constant current of 1.0 C to 2.75 V at 25°C. The discharge capacity at a discharge current of 1.0 C obtained in this test was defined as "initial discharge capacity". In addition, the thickness of the positive electrode active material layer and the thickness of the negative electrode active material layer in the discharged state were determined by the method described above separately, and the maximum thickness T2 of the electrode body and the maximum thickness T3 of the electrode unit were determined by the above formula. . The ratio A of the maximum thickness T2 of the electrode body to the average inner dimension T1 in the thickness direction of the rectangular cases of Examples 1 and 2 and Comparative Examples 1 to 5, and the ratio A of the maximum thickness T3 of the electrode unit to T1 Table 1 shows the ratio B and the difference between B and A above.
(2) Capacity retention rate after charge-discharge cycle test For each storage element after measuring the “initial discharge capacity”, in a constant temperature chamber at 60 ° C., after constant current charging at a charging current of 1.0 C to 4.25 V, 4 It was constant voltage charged at 0.25V. The charging termination condition was until the charging current reached 0.01C. A rest period of 10 minutes was then provided. Thereafter, constant current discharge was performed at a discharge current of 1.0 C to 2.75 V, followed by a rest period of 10 minutes. This charging and discharging was regarded as one cycle, and 1000 cycles were carried out. After 1000 cycles, the discharge capacity was measured at a discharge current of 1.0 C under the same conditions as the "initial discharge capacity" measurement test, and the discharge capacity at this time was defined as "discharge capacity after 1000 cycles". The value expressed as a percentage of the "discharge capacity after 1000 cycles" to the "initial discharge capacity" was defined as the "capacity retention rate (%)" after the charge-discharge cycle test.

(Rate of increase in low-temperature DC resistance after charge-discharge cycle test)
The rate of increase in low-temperature direct current resistance (DCR) after the charge-discharge cycle test was evaluated for each storage element under the following conditions. After measuring the initial discharge capacity and after the charge/discharge cycle test of 1000 cycles, each storage element was placed in a constant temperature oven at 25° C. and charged at a constant current of 0.1 C with an amount of electricity corresponding to 50% of the initial discharge capacity. did. Under these conditions, the SOC (State of Charge) of each storage element was set to 50%. Next, each storage device was stored in a constant temperature bath at -10°C for 3 hours, and then discharged at currents of 0.1C, 0.2C and 0.3C for 30 seconds. After completion of each discharge, constant current charging was performed at a current of 0.05 C to make the SOC 50%. The low-temperature DC resistance, which is a value corresponding to the gradient, was obtained from a current-voltage performance graph obtained by plotting the voltage 10 seconds after the start of discharge on the vertical axis and the discharge current on the horizontal axis. Then, the value expressed as a percentage of the increase rate of the "low-temperature DC resistance after the charge-discharge cycle test" with respect to the "low-temperature DC resistance after the initial discharge capacity measurement" is the "low-temperature DC resistance increase rate (%) after the charge-discharge cycle test. ” was obtained by the following formula.
Low-temperature DC resistance increase rate = (low-temperature DC resistance after charge-discharge cycle test) / (low-temperature DC resistance after initial discharge capacity measurement) × 100-100
The results are shown in Table 1 below.

As shown in Table 1 above, in Examples 1 and 2 in which A is 0.89 or more and the difference between B and A is 0.01 or more, the low-temperature DC resistance increase rate after the charge-discharge cycle test is low. was confirmed. On the other hand, in Comparative Examples 1 to 5 in which A is 0.89 or more and the difference between B and A is 0.01 or more, the low-temperature DC resistance increase rate after the charge-discharge cycle test was very high. . In addition, Example 1, Example 2, and Comparative Example 1 in which A is 0.95 have better capacity retention rates after the charge-discharge cycle test than Comparative Examples 2 to 5 in which A is 0.88. However, as described above, Comparative Example 1 in which the difference between B and A is less than 0.01 has a high low-temperature DC resistance increase rate after the charge-discharge cycle test, and the requirement that A is 0.89 or more is insufficient for the present invention. No effect was obtained. Furthermore, from the comparison between Example 2 and Comparative Example 2, and Comparative Example 1 and Comparative Example 3, in which B is the same, the effect of the ratio of the thickness T3 of the electrode unit to the average inner dimension T1 of the case on the effect of the present invention is It also turned out to be small. That is, it was found that the resistance increase at low temperature after charge-discharge cycles was suppressed by satisfying both requirements that A was 0.89 or more and the difference between B and A was 0.01 or more.

As a result, it was shown that the power storage device has an excellent effect of suppressing an increase in resistance at low temperatures after charge-discharge cycles.

The storage device is suitable for use as a power source for electronic devices such as personal computers, communication terminals, and automobiles.

1 Storage Element 3 Rectangular Case 3a Case Body 3b Case Cover 4 Positive Electrode Terminal 5 Negative Electrode Terminal 14 Positive Electrode Lead 15 Negative Lead 20 Energy Storage Unit 30 Power Storage Device 50 Electrode Body 51 Positive Electrode 52 First Separator 53 Negative Electrode 54 Second Separator 60 Sheet-shaped Member 61 first sheet body 62 second sheet body 100 electrode unit

Claims (4)

an electrode unit having an electrode body in which a negative electrode and a positive electrode are laminated via a separator, and a sheet-like member laminated on the outer surface of the electrode body;
and a rectangular case that houses the electrode unit,
Let A be the ratio T2/T1 of the maximum thickness T2 of the electrode body to the average inner dimension T1 in the thickness direction of the rectangular case in the thickness direction of the electrode body, and the ratio T3 of the maximum thickness T3 of the electrode unit to T1. A power storage element wherein A is 0.89 or more and the difference between B and A is 0.01 or more, where B is a ratio T3/T1. 2. The electric storage device according to claim 1, wherein the number of layers of the sheet-shaped member is two or more. 3. The electric storage element according to claim 1, wherein the separator is used as the sheet member. The negative electrode has a negative electrode active material,
4. The storage element of claim 1, 2 or 3, wherein the negative electrode active material comprises graphite, silicon, silicon oxide, or a combination thereof.

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