JP2023117881A - Non-aqueous electrolyte storage element - Google Patents

Abstract

To provide a non-aqueous electrolyte storage element capable of suppressing decrease in capacity retention rate during charge-discharge cycling.SOLUTION: A non-aqueous electrolyte storage element according to one aspect of the invention has a flat wound electrode body wound by laminating a positive electrode and a negative electrode through a porous substrate, the flat wound electrode body comprises two opposing flat sections and two R sections connecting ends of the two flat sections, the negative electrode has a negative electrode binder layer, density of the negative electrode binder layer in at least one of three layers from an innermost layer to a third layer of the R section is lower than density of the negative-electrode binder layer in the layer of the flat section, and a difference between an open circuit voltage at 30% SOC and that at 70% SOC is less than or equal to 0.2 V.SELECTED DRAWING: Figure 1

Description

The present invention relates to a non-aqueous electrolyte storage element.

Non-aqueous electrolyte secondary batteries, typified by lithium-ion secondary batteries, are widely used in electronic devices such as personal computers, communication terminals, and automobiles because of their high energy density. The non-aqueous electrolyte secondary battery generally has an electrode body having a pair of electrodes and a separator, a non-aqueous electrolyte, and a container for containing the electrode body and the non-aqueous electrolyte. It is configured to charge and discharge by transferring charge-transport ions at . Capacitors such as lithium ion capacitors and electric double layer capacitors are also widely used as non-aqueous electrolyte storage elements other than non-aqueous electrolyte secondary batteries.

Conventionally, carbon materials such as graphite have been used as active materials contained in the negative electrodes of non-aqueous electrolyte storage elements. is in progress. As an example of a non-aqueous electrolyte storage element, Patent Document 1 discloses a non-aqueous electrolyte secondary battery using lithium iron phosphate as a positive electrode active material and graphite or the like as a negative electrode.

JP-A-2002-117908

A non-aqueous electrolyte secondary battery that uses lithium iron phosphate as the positive electrode active material and graphite as the negative electrode has the characteristic that the voltage change due to changes in the state of charge is flat over a wide range of state of charge (voltage flatness). Excellent for However, in a non-aqueous electrolyte secondary battery having voltage flatness, bias in charging/discharging reactions within the electrode body tends to occur during the charging/discharging process. If the charge-discharge reaction in the electrode body is biased, lithium metal or the like tends to deposit on the negative electrode surface during charge at the point where the charge-discharge reaction concentrates, so there is a risk that the capacity retention rate, etc., will decrease significantly with charge-discharge cycles. .

SUMMARY OF THE INVENTION It is an object of the present invention to provide a non-aqueous electrolyte storage element capable of suppressing a decrease in capacity retention rate during charge/discharge cycles.

A non-aqueous electrolyte storage element according to one aspect of the present invention comprises a flat wound electrode body formed by laminating and winding a positive electrode and a negative electrode via a porous base material, the flat wound electrode body is composed of two opposing flat portions and two R portions connecting the ends of the two flat portions, the negative electrode includes a negative electrode mixture layer, and the innermost layer to the third layer of the R portion The negative electrode mixture density in at least one negative electrode mixture layer is lower than the negative electrode mixture density in the negative electrode mixture layer in the flat portion, and the difference between the open circuit voltage at SOC 30% and the open circuit voltage at SOC 70% is 0. .2V or less.

A non-aqueous electrolyte storage element according to another aspect of the present invention comprises a flat wound electrode body formed by laminating and winding a positive electrode and a negative electrode with a porous substrate interposed therebetween, The electrode body is composed of two flat portions facing each other and two R portions connecting ends of the two flat portions. The negative electrode mixture density in the negative electrode mixture layer of at least one of the layers is lower than the negative electrode mixture density in the negative electrode mixture layer in the flat portion, and the positive electrode active material of the positive electrode is a lithium transition metal phosphate compound or spinel. type lithium-manganese-containing composite oxide, and the negative active material of the negative electrode comprises graphite or lithium titanate.

A non-aqueous electrolyte storage element according to one aspect of the present invention can suppress a decrease in capacity retention rate during charge-discharge cycles.

FIG. 1 is a graph showing an example of charge/discharge curves of a non-aqueous electrolyte storage element for explaining voltage flatness. FIG. 2 is a see-through perspective view showing one embodiment of the non-aqueous electrolyte storage element. FIG. 3 is a schematic diagram showing an embodiment of a power storage device configured by assembling a plurality of non-aqueous electrolyte power storage elements. FIG. 4 is an enlarged view of a main portion of the non-aqueous electrolyte storage element.

First, an outline of the non-aqueous electrolyte storage element disclosed by this specification will be described.

A non-aqueous electrolyte storage element according to one aspect of the present invention comprises a flat wound electrode body formed by laminating and winding a positive electrode and a negative electrode via a porous base material, the flat wound electrode body is composed of two opposing flat portions and two R portions connecting the ends of the two flat portions, the negative electrode includes a negative electrode mixture layer, and the innermost layer to the third layer of the R portion The negative electrode mixture density in at least one of the negative electrode mixture layers (hereinafter also referred to as “the negative electrode mixture density of the innermost layer to the third layer of the R section”) is the negative electrode mixture density in the negative electrode mixture layer in the flat portion. density (hereinafter also referred to as “flat portion negative electrode mixture density”), and the difference between the open circuit voltage at SOC 30% and the open circuit voltage at SOC 70% is 0.2 V or less.

The non-aqueous electrolyte storage element can suppress a decrease in capacity retention rate during charge-discharge cycles. Although the reason for this is not clear, the following is presumed.

In the non-aqueous electrolyte storage element, the difference between the open circuit voltage at SOC 30% and the open circuit voltage at SOC 70% is 0.2 V or less. have Here, in order to explain "voltage flatness", FIG. 1 shows an example of a charge/discharge curve of a non-aqueous electrolyte storage element with SOC (%) on the horizontal axis and battery voltage (V) on the vertical axis. In FIG. 1, LFP/Gr is a lithium ion secondary battery using lithium iron phosphate (LFP) as the positive electrode and graphite (Gr) as the negative electrode, and NCM/Gr is lithium containing Ni, Co and Mn. It is a lithium ion secondary battery using a transition metal composite oxide (NCM) as a positive electrode and graphite (Gr) as a negative electrode. In FIG. 1, the NCM/Gr battery voltage monotonously increases from SOC 30% to SOC 70%, whereas the LFP/Gr battery voltage is substantially constant from SOC 30% to SOC 70%. That is, LFP/Gr in FIG. 1 has voltage flatness.

On the other hand, if the conventional non-aqueous electrolyte storage device has the above-described voltage flatness, charge-transporting ions such as lithium ions in the positive electrode mixture layer and the negative electrode mixture layer are not distributed due to the uneven charge/discharge reaction in the electrode body. If the distribution is uneven, it is presumed that the potential difference between the positive electrode mixture layer and the negative electrode mixture layer makes it difficult for the force to eliminate the unevenness to work. In particular, in the R portion of the flat wound electrode body, current concentration is more likely than in the flat portion, and the inner peripheral portion from the innermost layer to the third layer is also a portion where the non-aqueous electrolyte is difficult to permeate. Therefore, the charge/discharge reaction tends to be biased. As a result, lithium metal or the like tends to deposit on the negative electrode surface during charging in the innermost layer to the inner peripheral portion of the third layer from the innermost layer of the flat wound electrode body, and the capacity retention rate decreases with charge-discharge cycles. There is a risk of

In contrast, in the non-aqueous electrolyte storage element according to one aspect of the present invention, the negative electrode mixture density in at least one of the negative electrode mixture layers from the innermost layer to the third layer of the R portion is the same as that of the flat portion. Since the density of the negative electrode mixture is lower than that of the negative electrode mixture layer, the charge acceptability of the negative electrode can be improved in the inner peripheral portion from the innermost layer to the third layer of the R section. As a result, it is presumed that the charge/discharge reaction in the electrode body is less likely to be biased, and as a result, a decrease in the capacity retention rate during charge/discharge cycles can be suppressed.

Here, the negative electrode mixture density in at least one negative electrode mixture layer from the innermost layer to the third layer of the R portion is 0.80 times or more the negative electrode mixture density in the negative electrode mixture layer in the flat portion. 0.99 times or less.

A non-aqueous electrolyte storage element according to another aspect of the present invention comprises a flat wound electrode body formed by laminating and winding a positive electrode and a negative electrode with a porous substrate interposed therebetween, The electrode body is composed of two flat portions facing each other and two R portions connecting ends of the two flat portions. The negative electrode mixture density in the negative electrode mixture layer of at least one of the layers is lower than the negative electrode mixture density in the negative electrode mixture layer in the flat portion, and the positive electrode active material of the positive electrode is a lithium transition metal phosphate compound or spinel. type lithium-manganese-containing composite oxide, and the negative active material of the negative electrode comprises graphite or lithium titanate. By combining the positive electrode active material and the negative electrode active material, the difference between the open circuit voltage at SOC 30% and the open circuit voltage at SOC 70% can be 0.2 V or less. It has a voltage flatness in which a voltage change due to a change in the state of charge is flat, but the negative electrode mixture density in at least one of the negative electrode mixture layers from the innermost layer to the third layer of the R portion is the same as that of the flat portion. Since the density of the negative electrode mixture is lower than that of the negative electrode mixture layer, the charge-discharge reaction in the electrode body is less likely to be biased, and as a result, a decrease in the capacity retention rate during charge-discharge cycles can be suppressed.

In the present invention, the “R portion” is the portion where the positive electrode and the negative electrode are bent. The positive and negative electrodes of the R section are bent in a semicircular or semielliptical shape, and as shown in FIG. , and a straight line perpendicular to a straight line passing through the set centers and connecting both centers of the two R portions is defined as a boundary line between the R portion and the flat portion. A “flat portion” is a portion where the positive electrode and the negative electrode are substantially flat. "SOC" is an abbreviation for State Of Charge, and represents the state of charge of the non-aqueous electrolyte storage element as a ratio of the remaining capacity at that time to the capacity at full charge. A fully discharged state is expressed as SOC 0%. Here, the "fully charged state" refers to a state in which the non-aqueous electrolyte storage element is charged to the upper limit voltage under the recommended or specified charging conditions. is a state in which the non-aqueous electrolyte storage element is discharged to the lower limit voltage by adopting charging conditions recommended or specified for the non-aqueous electrolyte storage element.

The negative electrode mixture density (g/cm ³ ) can be calculated from the area, thickness and mass of the negative electrode mixture layer.

The open circuit voltage was obtained by charging the non-aqueous electrolyte storage element at a charging current of 0.1 C from a completely discharged state to each SOC at a constant current, and then measuring the voltage after 30 minutes with no current applied. can ask.

A configuration of a non-aqueous electrolyte storage element, a configuration of a storage device, and other embodiments according to one 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.

<Structure of non-aqueous electrolyte storage element>
A non-aqueous electrolyte storage element according to an embodiment of the present invention comprises an electrode body having a positive electrode, a negative electrode and a separator, a non-aqueous electrolyte, and a sealable container for containing the electrode body and the non-aqueous electrolyte. And prepare. The electrode body is a wound type in which a positive electrode and a negative electrode are laminated with a separator interposed therebetween and wound. The non-aqueous electrolyte exists in a state contained in 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 a non-aqueous electrolyte storage element.

In the non-aqueous electrolyte storage element of one embodiment of the present invention, the upper limit of the difference between the open circuit voltage at SOC 30% and the open circuit voltage at SOC 70% is 0.2V, preferably 0.15V, and 0.2V. 10V is more preferred. By setting the difference between the open-circuit voltage at SOC 30% and the open-circuit voltage at SOC 70% to be equal to or less than the upper limit, it is possible to relatively widen the range of the state of charge in which the change in voltage due to the change in the state of charge becomes flat (voltage flatness). enhanced). On the other hand, the lower limit of the difference between the open circuit voltage at SOC 30% and the open circuit voltage at SOC 70% is not particularly limited, but may be 0.05V or 0.10V, for example. The difference between the open circuit voltage at SOC 30% and the open circuit voltage at SOC 70% may be equal to or greater than any of the lower limits and equal to or less than any of the upper limits.

(positive electrode)
The positive electrode has a positive electrode base material and a positive electrode material mixture layer arranged directly on the positive electrode base material or via an intermediate layer.

A positive 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 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 alloy 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 intermediate layer is a layer arranged between the positive electrode substrate and the positive electrode material mixture layer. The intermediate layer contains a conductive agent such as carbon particles to reduce the contact resistance between the positive electrode substrate and the positive electrode mixture layer. The composition of the intermediate layer is not particularly limited, and includes, for example, a binder and a conductive agent.

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

In one embodiment of the present invention, the positive electrode active material is appropriately selected from positive electrode active materials that can make the difference between the open circuit voltage at SOC 30% and the open circuit voltage at SOC 70% of the non-aqueous electrolyte storage element 0.2 V or less. Among them, a lithium transition metal phosphate compound or a spinel-type lithium-manganese-containing composite oxide is preferable. Examples of the lithium transition metal phosphate compound include those represented by Formula 1 below.
LiFe _x Mn _(1−x) PO ₄ (0≦x≦1) . . . 1
Examples of the spinel-type lithium-manganese-containing composite oxide include lithium manganate (LiMn ₂ O ₄ ) and lithium nickel manganate (Li _x Ni _γ Mn _(2-γ) O ₄ (0<x<2)). be done. The lithium transition metal phosphate compound or spinel-type lithium-manganese-containing composite oxide may be partially substituted with atoms or anion species of other elements, or may be coated with other materials.

The lower limit of the total content of the lithium transition metal phosphate compound or the spinel-type lithium-manganese-containing composite oxide with respect to all positive electrode active materials is preferably 70% by mass, more preferably 80% by mass, and even more preferably 90% by mass. When the total content is equal to or higher than the lower limit, the difference between the open circuit voltage at SOC 30% and the open circuit voltage at SOC 70% can be reduced.

The positive electrode active material is preferably lithium iron phosphate (LiFePO ₄ ) in the case of the lithium transition metal phosphate compound, and lithium manganate (LiMn ₂ O ₄ ) in the case of the spinel-type lithium-manganese-containing composite oxide. The lower limit of the total content of lithium iron phosphate or lithium manganate in all positive electrode active materials is preferably 70% by mass, more preferably 80% by mass, and even more preferably 90% by mass. When the total content of lithium iron phosphate or lithium manganate in all positive electrode active materials is equal to or higher than the lower limit, the difference between the open circuit voltage at SOC 30% and the open circuit voltage at SOC 70% can be made smaller. . On the other hand, the total content of lithium iron phosphate or lithium manganate with respect to all positive electrode active materials may be 100% by mass. Moreover, it is preferable that the positive electrode active material consists essentially of lithium iron phosphate or lithium manganate.

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 lower limit, the production or handling of the positive electrode active material becomes easy. By making the average particle size of the positive electrode active material equal to or less than the above upper limit, the electron conductivity of the positive electrode mixture 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. "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. 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.

The content of the positive electrode active material in the positive electrode mixture 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 of the positive electrode mixture layer and manufacturability.

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 positive electrode mixture 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 positive electrode mixture 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 positive electrode 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.

The positive electrode mixture 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

(negative electrode)
The negative electrode has a negative electrode base material and a negative electrode mixture layer disposed on the negative electrode base material directly 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 positive electrode.

In the negative electrode, the negative electrode mixture density in at least one negative electrode mixture layer from the innermost layer to the third layer of the R portion is lower than the negative electrode mixture density in the negative electrode mixture layer in the flat portion. The upper limit of the ratio of the negative electrode mixture density in at least one negative electrode mixture layer from the innermost layer to the third layer of the R section to the negative electrode mixture density in the flat portion of the negative electrode mixture layer is preferably 0.99 times. 0.97 times is more preferable, 0.95 times is more preferable, and 0.94 times is particularly preferable. The lower limit is preferably 0.80 times, more preferably 0.85 times, and even more preferably 0.90 times. The control of the negative electrode mixture density in at least one layer from the innermost layer to the third layer in the R portion and the negative electrode mixture layer in the flat portion can be performed, for example, by intermittent pressing of the negative electrode mixture layer, but a method other than the intermittent pressing. can be controlled by

A negative electrode base material has electroconductivity. 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 negative electrode substrates include foils, deposited films, meshes, porous materials, and the like, and foils are preferred 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 unit volume of the secondary battery can be increased while increasing the strength of the negative electrode substrate.

The negative electrode mixture layer contains a negative electrode active material. The negative electrode mixture layer contains arbitrary components such as a conductive agent, a binder, a thickener, a filler, etc., if necessary. Optional components such as conductive agents, binders, thickeners, and fillers can be selected from the materials exemplified for the positive electrode.

The negative electrode mixture 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.

In one embodiment of the present invention, the negative electrode active material is appropriately selected from negative electrode active materials that can make the difference between the open circuit voltage at SOC 30% and the open circuit voltage at SOC 70% of the non-aqueous electrolyte storage element 0.2 V or less. Among them, graphite or lithium titanate is preferable. Here, “graphite” is a carbon material having an average lattice spacing (d ₀₀₂ ) of the (002) plane determined by X-ray diffraction before charging/discharging or in a discharged state of 0.33 nm or more and less than 0.34 nm. Say. Graphite includes natural graphite and artificial graphite. Artificial graphite is preferable from the viewpoint that a material with stable physical properties can be obtained. In addition, the “discharged state” of the negative electrode means a state in which the negative electrode active material is discharged such that lithium ions that can be intercalated and deintercalated are sufficiently released from the negative electrode active material during charging and discharging. For example, in a single electrode battery using a negative electrode containing graphite 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 lower limit of the total content of graphite or lithium titanate for all negative electrode active materials is preferably 70% by mass, more preferably 80% by mass, and even more preferably 90% by mass. When the total content of graphite or lithium titanate is at least the lower limit, the difference between the open circuit voltage at SOC 30% and the open circuit voltage at SOC 70% can be reduced. On the other hand, the total content of graphite or lithium titanate in all negative electrode active materials may be 100% by mass. Moreover, it is preferable that the negative electrode active material consists essentially of graphite or lithium titanate.

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. The average particle size of the negative electrode active material may be 1 μm or more and 100 μm or less. By setting the average particle size of the negative electrode active material to the above lower limit or more, 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 mixture layer is improved. 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 positive electrode.

The content of the negative electrode active material in the negative electrode mixture 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 of the negative electrode mixture layer and manufacturability.

(separator)
The separator has a porous substrate. The separator can be appropriately selected from known separators. As the separator, for example, a separator consisting of only a porous substrate layer, a separator having a heat-resistant layer containing heat-resistant particles and a binder formed on one or both surfaces of the porous substrate layer, or the like can be used. . Examples of the shape of the porous substrate 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 retention of a non-aqueous electrolyte. As the material for the porous substrate 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 oxidation resistance to decomposition. A material obtained by combining these resins may be used as the porous substrate 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 non-aqueous electrolyte 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.

(Non-aqueous electrolyte)
The non-aqueous electrolyte can be appropriately selected from known non-aqueous electrolytes. 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, methyl methanesulfonate, busulfan, methyl toluenesulfonate, dimethyl sulfate, ethylene sulfate, sulfolane, dimethylsulfone, diethyl Sulfone, dimethylsulfoxide, diethylsulfoxide, tetramethylenesulfoxide, diphenylsulfide, 4,4'-bis(2,2-dioxo-1,3,2-dioxathiolane), 4-methylsulfonyloxymethyl-2,2-dioxo- 1,3,2-dioxathiolane, thioanisole, diphenyl disulfide, dipyridinium disulfide, 1,3-propenesultone, 1,3-propanesultone, 1,4-butanesultone, 1,4-butenesultone, perfluorooctane, boric acid Tristrimethylsilyl, 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 nonaqueous 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 nonaqueous 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, and to further improve safety.

FIG. 2 shows a non-aqueous electrolyte storage element 1 (non-aqueous electrolyte secondary battery) as an example of a rectangular battery. In addition, the same figure is taken as the figure which saw through the inside of a container. An electrode body 2 having a positive electrode and a negative electrode wound with a separator sandwiched therebetween is housed in a rectangular container 3 . Further, the container 3 is sealed while the non-aqueous electrolyte is added. As the container 3, a known metal container, resin container, or the like, which is usually used as a container for a non-aqueous electrolyte storage element, can be used.

The positive electrode is electrically connected to the positive electrode terminal 4 via a positive electrode lead 41 . The negative electrode is electrically connected to the negative terminal 5 via a negative lead 51 . In FIG. 2, the winding axis direction of the electrode body 2 in the non-aqueous electrolyte storage element 1 is the X direction, the thickness direction of the non-aqueous electrolyte storage element 1 is the Y direction, and the axial direction (X direction) is perpendicular to the direction of the axis (X direction). A direction perpendicular to the thickness direction (Y direction) is referred to as a Z direction. The Z direction is parallel to the flat portion (that is, the surface of the flat portion) of the electrode body 2 in the non-aqueous electrolyte storage element 1 and coincides with the winding direction of the electrode body 2 at the flat portion. Here, the thickness direction of the non-aqueous electrolyte storage element 1 coincides with the thickness direction of the electrode body 2 . The thickness direction of the electrode body 2 corresponds to the stacking direction of the positive electrode, the negative electrode and the separator, and also corresponds to the direction perpendicular to the surfaces of the positive electrode, the negative electrode and the separator.

<Configuration of power storage device>
The non-aqueous electrolyte storage element 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 It can be installed in a power source for electric power storage or the like as an electric storage unit (battery module) configured by collecting a plurality of non-aqueous electrolyte electric storage elements 1 . In this case, the technology of the present invention may be applied to at least one non-aqueous electrolyte storage element included in the storage unit.
FIG. 3 shows an example of a power storage device 30 in which a power storage unit 20 in which two or more electrically connected non-aqueous electrolyte power storage elements 1 are assembled is further assembled. The power storage device 30 includes a bus bar (not shown) that electrically connects two or more non-aqueous electrolyte power storage elements 1, a bus bar (not shown) that electrically connects two or more power storage units 20, and the like. may The power storage unit 20 or the power storage device 30 may include a state monitoring device (not shown) that monitors the state of one or more non-aqueous electrolyte power storage elements.

<Method for manufacturing non-aqueous electrolyte storage element>
A method for manufacturing the non-aqueous electrolyte storage element of the present embodiment can be appropriately selected from known methods. The manufacturing method includes, for example, preparing an electrode body, preparing a non-aqueous electrolyte, and storing the electrode body and the non-aqueous electrolyte in a container. Preparing the electrode body includes preparing a positive electrode and a negative electrode, and forming an electrode body by stacking and winding the positive electrode and the negative electrode with a separator interposed therebetween.

A suitable method for containing the non-aqueous electrolyte in the container can be selected from known methods. For example, after injecting the non-aqueous electrolyte from an injection port formed in the container, the injection port may be sealed.

<Other embodiments>
It should be noted that the non-aqueous electrolyte storage element of the present invention is not limited to the above 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 non-aqueous electrolyte storage element is used as a chargeable/dischargeable non-aqueous electrolyte secondary battery (for example, a lithium ion secondary battery). , size, capacity, etc. are arbitrary. The present invention can also be applied to capacitors such as various secondary batteries, electric double layer capacitors, and lithium ion capacitors.

In the above embodiments, the electrode body in which the positive electrode and the negative electrode are laminated with the separator interposed therebetween has been described, but the electrode body may not include the separator. For example, the positive electrode and the negative electrode may be in direct contact with each other in a state in which a non-conductive porous base material is formed on the active material layer of the positive electrode or the negative electrode.

In the above embodiment, the positive electrode active material includes the lithium transition metal phosphate compound represented by Formula 1 above, or a spinel-type lithium manganese-containing composite oxide of either lithium manganate or lithium nickel manganate, and the negative electrode Although the case where the active material contains graphite or lithium titanate has been described, the configurations of the positive electrode active material and the negative electrode active material are not limited to the above embodiments. For example, a positive electrode active material other than the lithium transition metal phosphate compound represented by the above formula 1 or a spinel-type lithium manganese-containing composite oxide of either lithium manganate or lithium nickel manganate is selected, and the negative electrode active material is A material other than graphite or lithium titanate may be selected as the material, and the difference between the open circuit voltage at SOC 30% and the open circuit voltage at SOC 70% may be 0.2 V or less.

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

[Example 1]
(Preparation of positive electrode)
Lithium iron phosphate (LiFePO ₄ ) was used as a positive electrode active material. Using N-methylpyrrolidone (NMP) as a dispersion medium, the positive electrode active material, acetylene black (AB) as a conductive agent, and polyvinylidene fluoride (PVDF) as a binder at a mass ratio of 90:5:5 in terms of solid content. A positive electrode mixture paste containing was prepared. The positive electrode material mixture paste was applied to both surfaces of an aluminum foil serving as the positive electrode base material 42a, dried, and then pressed. As a result, a positive electrode 42 was obtained in which the positive electrode mixture layers 42b were laminated on both sides of the positive electrode substrate 42a.

(Preparation of negative electrode)
Graphite (Gr) was used as a negative electrode active material. A negative electrode mixture containing water as a dispersion medium, the negative electrode active material, styrene-butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener in a mass ratio of 96:2:2 in terms of solid content. An agent paste was prepared. A negative electrode mixture paste was applied to both surfaces of the copper foil, which is the negative electrode substrate 52a, and dried. After that, a roll press was performed. As a result, a negative electrode 52 was obtained in which the negative electrode mixture layers 52b were laminated on both sides of the negative electrode substrate 52a. Furthermore, intermittent pressing was used to control the negative electrode mixture density of the negative electrode mixture layer in the portion corresponding to the innermost layer to the third layer and the flat portion of the R portion when a flat-type wound electrode body described later was produced. The negative electrode mixture density in the negative electrode mixture layer from the innermost layer to the third layer of the R portion was 1.20 g/cm ³ , and the negative electrode mixture density in the negative electrode mixture layer in the flat portion was 1.27 g/cm ³ .

(Preparation of non-aqueous electrolyte)
A solution was prepared by dissolving LiPF ₆ as an electrolyte salt at a concentration of 1.0 mol/dm ³ in a non-aqueous solvent in which EC and EMC were mixed at a volume ratio of 30:70. The solution was obtained as a non-aqueous electrolyte.

(Fabrication of non-aqueous electrolyte storage element)
A polyolefin microporous film was used as a separator. The positive electrode 42 and the negative electrode 51 were layered and wound with the separator interposed therebetween to prepare a flat wound electrode assembly. This flat-type wound electrode body was placed in a container, and the non-aqueous electrolyte was injected thereinto to obtain a non-aqueous electrolyte storage element of Example 1.

[Example 2 and Comparative Examples 1 to 4]
Except that the positive electrode active material, the negative electrode mixture density in the negative electrode mixture layer from the innermost layer to the third layer of the R section, and the negative electrode mixture density in the negative electrode mixture layer in the flat portion were changed as shown in Table 1. In the same manner as in Example 1, non-aqueous electrolyte storage elements of Example 2 and Comparative Examples 1 to 4 were obtained.

(initial charge/discharge)
Initial charge/discharge was performed at 25° C. in the following manner for each of the obtained non-aqueous electrolyte storage elements. Constant-current and constant-voltage charging was performed with a charging current of 0.1C and a charge termination voltage of 3.6V. The charging termination condition was until the charging current reached 0.02C. A rest period of 10 minutes was then provided. After that, constant current discharge was performed with a discharge current of 0.1C and a discharge final voltage of 2.0V.

(Initial capacity confirmation test)
Next, for each non-aqueous electrolyte storage element, an initial capacity confirmation test was performed at 25° C. in the following manner.
Constant-current and constant-voltage charging was performed with a charging current of 0.1C and a charge termination voltage of 3.6V. The charging termination condition was until the charging current reached 0.02C. A rest period of 10 minutes was then provided. After that, constant current discharge was performed with a discharge current of 0.1C and a discharge final voltage of 2.0V. The discharge capacity at this time was defined as "initial discharge capacity". After that, each non-aqueous electrolyte storage element was subjected to constant current charging from a fully discharged state to an SOC of 30% at a charging current of 0.1 C. After 30 minutes with no current applied, the open circuit voltage and Further, constant current charging was performed at a charging current of 0.1 C to an SOC of 70%, and then the open circuit voltage was measured after 30 minutes with no current applied. As a result, in each of the non-aqueous electrolyte storage elements of Examples 1 and 2 and Comparative Examples 1 and 2, the difference in open circuit voltage was 0.2 V or less. On the other hand, in each of the nonaqueous electrolyte storage elements of Comparative Examples 3 and 4, the difference in the open circuit voltage exceeded 0.2V. Here, the state after the initial capacity confirmation test is assumed to be a completely discharged state, denoted as SOC 0%, and constant current and constant voltage charging is performed with the same amount of electricity as the initial discharge capacity at the charge end voltage of 3.6 V. The fully charged state was defined as a state of SOC 100%.

(Charge-discharge cycle test)
After the initial capacity confirmation test, each nonaqueous electrolyte storage element was subjected to a charge/discharge cycle test at 25° C. in the following manner. Constant-current and constant-voltage charging was performed with a charging current of 1.0C and a charging end voltage of 3.6V. The charging termination condition was until the charging current reached 0.05C. Thereafter, constant current discharge was performed with a discharge current of 1.0C and a discharge final voltage of 2.0V. After charging and after discharging, a rest time of 10 minutes was provided. This charging/discharging was performed 50 cycles.
After the charge-discharge cycle test, a capacity confirmation test was performed in the same manner as the "initial capacity confirmation test", and the discharge capacity at this time was defined as the "discharge capacity after the charge-discharge cycle test". The discharge capacity after the charge-discharge cycle test was divided by the initial discharge capacity to obtain the capacity retention rate (%). Table 1 shows the results.

As shown in Table 1, in the non-aqueous electrolyte storage element having voltage flatness, the negative electrode mixture density in the negative electrode mixture layer from the innermost layer to the third layer of the R section is flat. In Examples 1 and 2, the negative electrode mixture density in the innermost layer to the third layer of the negative electrode mixture layer in the R section is the same as the negative electrode mixture density in the negative electrode mixture layer in the flat portion. Compared with Comparative Examples 1 and 2, the capacity retention rate is high. In addition, in Comparative Examples 3 and 4, which are non-aqueous electrolyte storage elements that do not have voltage flatness, the capacity The retention rate is the same, and the capacity retention rates of Examples 1 and 2 are higher than those of Comparative Examples 3 and 4.
Based on these facts, the present invention solves the problem peculiar to a non-aqueous electrolyte storage element having voltage flatness. It is presumed that by controlling the density of the negative electrode mixture in the material layer, the charge-discharge reaction in the electrode body is less likely to be biased, and the problem has been solved.

INDUSTRIAL APPLICABILITY The present invention can be applied to electronic devices such as personal computers and communication terminals, non-aqueous electrolyte storage elements used as power sources for automobiles and the like.

1 Non-aqueous electrolyte storage element 2 Electrode body 3 Container 4 Positive electrode terminal 41 Positive electrode lead 42 Positive electrode 42a Positive electrode base material 42b Positive electrode mixture layer 5 Negative electrode terminal 51 Negative electrode lead 52 Negative electrode 52a Negative electrode base material 52b Negative electrode mixture layer 20 Electricity storage unit 30 power storage device

Claims (3)

A flat wound electrode body formed by laminating and winding a positive electrode and a negative electrode with a porous substrate interposed therebetween,
The flat wound electrode body is composed of two flat portions facing each other and two R portions connecting ends of the two flat portions,
The negative electrode comprises a negative electrode mixture layer,
the negative electrode mixture density in at least one negative electrode mixture layer from the innermost layer to the third layer of the R portion is lower than the negative electrode mixture density in the negative electrode mixture layer in the flat portion;
A non-aqueous electrolyte storage element in which the difference between the open circuit voltage at SOC 30% and the open circuit voltage at SOC 70% is 0.2 V or less. The negative electrode mixture density in at least one negative electrode mixture layer from the innermost layer to the third layer of the R portion is 0.80 to 0.99 times the negative electrode mixture density in the negative electrode mixture layer in the flat portion. The non-aqueous electrolyte storage element according to claim 1, wherein: A flat wound electrode body formed by laminating and winding a positive electrode and a negative electrode with a porous substrate interposed therebetween,
The flat wound electrode body is composed of two flat portions facing each other and two R portions connecting ends of the two flat portions,
The negative electrode comprises a negative electrode mixture layer,
the negative electrode mixture density in at least one negative electrode mixture layer from the innermost layer to the third layer of the R portion is lower than the negative electrode mixture density in the negative electrode mixture layer in the flat portion;
The positive electrode active material of the positive electrode includes a lithium transition metal phosphate compound or a spinel-type lithium manganese-containing composite oxide,
The negative electrode active material of the negative electrode contains graphite or lithium titanate.

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