Non-aqueous electrolyte energy storage element and method for manufacturing a non-aqueous electrolyte energy storage element

Abstract

To provide a nonaqueous electrolyte power storage element with excellent DC resistance reduction effect at low temperatures and a manufacturing method for the same.SOLUTION: The nonaqueous electrolyte power storage element includes a positive electrode containing a positive electrode active material, a negative electrode, and a nonaqueous electrolyte containing a nonaqueous solvent and a propionic acid alkyl ester. A boron element is present on the surface of the positive electrode active material. The content of the boron element on the surface of the positive electrode active material is greater than or equal to 0.05 mol% and less than or equal to 1.50 mol% of the content of the metal elements other than the lithium element contained in the positive electrode active material.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a non-aqueous electrolyte energy storage element and a method for manufacturing a non-aqueous electrolyte energy storage element. [Background technology]

[0002] Non-aqueous electrolyte secondary batteries, such as lithium-ion batteries, are widely used in electronic devices like personal computers and communication terminals, as well as in automobiles, due to their high energy density. Other non-aqueous electrolyte energy storage elements, such as lithium-ion capacitors and electric double-layer capacitors, are also widely used.

[0003] As a positive electrode active material used in such non-aqueous electrolyte energy storage elements, a positive electrode active material has been proposed in which the surface of the positive electrode active material is coated with a metal oxide to reduce internal resistance and improve cycle characteristics (see Patent Document 1). [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2009-076279 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] In recent years, non-aqueous electrolyte energy storage devices have been required to exhibit various performance characteristics depending on the operating environment and application. For example, when considering use in low-temperature environments, it is desirable for them to exhibit better input / output performance at low temperatures.

[0006] The object of the present invention is to provide a non-aqueous electrolyte energy storage element that exhibits excellent DC resistance reduction effects at low temperatures, and a method for manufacturing such a non-aqueous electrolyte energy storage element. [Means for solving the problem]

[0007] A non-aqueous electrolyte storage element according to one aspect of the present invention includes a positive electrode containing a positive electrode active material, a negative electrode, and a non-aqueous electrolyte containing a non-aqueous solvent and an alkyl propionate. Boron element is present on the surface of the positive electrode active material, and the content of the boron element on the surface of the positive electrode active material is 0.05 mol% or more and 1.50 mol% or less with respect to the content of metal elements other than lithium element contained in the positive electrode active material.

[0008] A method for manufacturing a non-aqueous electrolyte storage element according to one aspect of the present invention includes producing a positive electrode using a positive electrode active material having boron element on its surface, and preparing a non-aqueous electrolyte containing a non-aqueous solvent and an alkyl propionate.

Effects of the Invention

[0009] According to one aspect of the present invention, it is possible to provide a non-aqueous electrolyte storage element having an excellent effect of reducing DC resistance at low temperatures. According to another aspect of the present invention, it is possible to manufacture a non-aqueous electrolyte storage element having an excellent effect of reducing DC resistance at low temperatures.

Brief Description of the Drawings

[0010] [Figure 1] FIG. 1 is a perspective view showing an embodiment of a non-aqueous electrolyte storage element. [Figure 2] FIG. 2 is a schematic view showing an embodiment of a power storage device constituted by aggregating a plurality of non-aqueous electrolyte storage elements.

Embodiments for Carrying Out the Invention

[0011] First, an overview of the non-aqueous electrolyte storage element disclosed by this specification will be described.

[0012] (1) A non-aqueous electrolyte energy storage element according to one aspect of the present invention comprises a positive electrode containing a positive electrode active material, a negative electrode, and a non-aqueous electrolyte containing a non-aqueous solvent and an alkyl propionate ester, wherein boron is present on the surface of the positive electrode active material, and the content of boron on the surface of the positive electrode active material is 0.05 mol% or more and 1.50 mol% or less relative to the content of metal elements other than lithium contained in the positive electrode active material.

[0013] The non-aqueous electrolyte energy storage element described in (1) above exhibits excellent reduction of DC resistance at low temperatures. The reason for this is not entirely clear, but the following reasons are speculated. In the non-aqueous electrolyte energy storage element described in (1) above, the non-aqueous electrolyte contains alkyl propionate as an additive. By containing low-viscosity alkyl propionate, the viscosity of the non-aqueous electrolyte is reduced, improving its ionic conductivity. On the other hand, alkyl propionate is easily oxidized and decomposed, and when alkyl propionate is oxidized and decomposed, the effect of improving the ionic conductivity of the non-aqueous electrolyte decreases. In contrast, in the non-aqueous electrolyte energy storage element described in (1) above, boron is present on the surface of the positive electrode active material, and the boron content on the surface of the positive electrode active material is between 0.05 mol% and 1.50 mol% relative to the content of metal elements in the positive electrode active material. This suppresses the oxidative decomposition of alkyl propionate on the surface of the positive electrode active material, thus maintaining the effect of improving the ionic conductivity of the non-aqueous electrolyte. Therefore, it is presumed that the non-aqueous electrolyte energy storage element described in (1) above has an excellent effect in reducing DC resistance at low temperatures. Here, the "non-aqueous solvent" of the non-aqueous electrolyte in the present invention refers to a substance that is liquid at 40°C and 1 atmosphere and does not substantially contain water for dissolving the electrolyte salt in the non-aqueous electrolyte, and whose content in the non-aqueous electrolyte is 10% by mass or more.

[0014] (2) The non-aqueous electrolyte energy storage element described in (1) above preferably has a content of the alkyl propionate ester in the non-aqueous electrolyte of 0.5% by mass or more and 6.0% by mass or less. By having a content of the alkyl propionate ester of 0.5% by mass or more and 6.0% by mass or less, the non-aqueous electrolyte energy storage element described in (2) above can further reduce the DC resistance at low temperatures.

[0015] (3) In the non-aqueous electrolyte energy storage element described in (1) or (2) above, it is preferable that the alkyl propionate ester is methyl propionate. Since the viscosity reduction effect of the non-aqueous electrolyte is further enhanced when the alkyl propionate ester is methyl propionate, the non-aqueous electrolyte energy storage element described in (3) above can further reduce the DC resistance at low temperatures.

[0016] (4) In the non-aqueous electrolyte energy storage element described in any one of (1) to (3) above, it is preferable that the non-aqueous solvent does not contain a fluorinated cyclic carbonate. In the non-aqueous electrolyte energy storage element described in (4) above, the amount of high-resistance layer derived from lithium fluoride due to the decomposition of the non-aqueous electrolyte is reduced, and the viscosity reduction effect of the non-aqueous electrolyte can be more effectively exhibited, thereby further reducing the DC resistance at low temperatures.

[0017] (5) The non-aqueous electrolyte energy storage element described in any one of (1) to (4) above preferably has an average particle size of 0.1 μm or more and 20 μm or less of the positive electrode active material. By having an average particle size of the positive electrode active material within the above range, side reactions on the surface of the positive electrode active material can be suppressed, and the packing density of the positive electrode active material layer can be increased, thereby improving the conductivity of charge transport ions such as lithium ions. Here, "average particle size" means the value at which the volume-based integrated distribution calculated in accordance with JIS-Z-8819-2 (2001), based on the particle size distribution measured by laser diffraction / scattering method on a dilution of particles diluted with a solvent in accordance with JIS-Z-8825 (2013), becomes 50%.

[0018] (6) A method for manufacturing a non-aqueous electrolyte energy storage element according to one aspect of the present invention comprises preparing a positive electrode using a positive electrode active material in which boron elements are present on the surface, and preparing a non-aqueous electrolyte containing a non-aqueous solvent and an alkyl propionate ester.

[0019] According to the method for manufacturing a non-aqueous electrolyte energy storage element described in (6) above, it is possible to manufacture a non-aqueous electrolyte energy storage element that has an excellent effect in reducing DC resistance at low temperatures.

[0020] The configuration of a non-aqueous electrolyte energy storage element, the configuration of an energy storage device, a method for manufacturing a non-aqueous electrolyte energy storage element, and other embodiments according to one embodiment of the present invention will be described in detail. Note that the names of the components (each element) used in each embodiment may differ from the names of the components (each element) used in the background art.

[0021] <Configuration of a non-aqueous electrolyte energy storage element> A non-aqueous electrolyte energy storage element (hereinafter also simply referred to as "energy storage element") according to one 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 container for housing the electrode body and the non-aqueous electrolyte. The electrode body is usually a stacked type in which a plurality of positive electrodes and a plurality of negative electrodes are stacked with a separator in between, or a wound type in which the positive electrode and negative electrode are wound together with a separator in between. The non-aqueous electrolyte exists in a state impregnated with the positive electrode, negative electrode, and separator. As an example of a non-aqueous electrolyte energy storage element, a non-aqueous electrolyte secondary battery (hereinafter also simply referred to as "secondary battery") will be described.

[0022] (positive electrode) The positive electrode comprises a positive electrode substrate and a positive electrode active material layer disposed directly on the positive electrode substrate or via an intermediate layer.

[0023] The positive electrode substrate is conductive. Whether or not it is conductive is determined by the volume resistivity measured in accordance with JIS-H-0505 (1970), which is 10 -2The determination is made using Ω·cm as the threshold. The positive electrode substrate material can be a metal such as aluminum, titanium, tantalum, or stainless steel, or an alloy thereof. Among these, aluminum or an aluminum alloy is preferred from the viewpoint of high potential resistance, high conductivity, and cost. Examples of positive electrode substrates include foil, vapor-deposited film, mesh, and porous material, with foil being preferred from the viewpoint of cost. Therefore, aluminum foil or aluminum alloy foil is preferred as the positive electrode substrate. Examples of aluminum or aluminum alloys include A1085, A3003, and A1N30 as specified in JIS-H-4000 (2014) or JIS-H-4160 (2006).

[0024] The average thickness of the positive electrode substrate is preferably 3 μm to 50 μm, more preferably 5 μm to 40 μm, even more preferably 8 μm to 30 μm, and particularly preferably 10 μm to 25 μm. By setting the average thickness of the positive electrode substrate within the above range, it is possible to increase the strength of the positive electrode substrate while increasing the energy density per unit volume of the non-aqueous electrolyte energy storage element.

[0025] The intermediate layer is a layer placed between the positive electrode substrate and the positive electrode active material 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 active material layer. The composition of the intermediate layer is not particularly limited and may include, for example, a binder and a conductive agent.

[0026] The positive electrode active material layer contains a positive electrode active material. The positive electrode active material layer may optionally contain conductive agents, binders, thickeners, fillers, and other optional components.

[0027] The positive electrode active material can be appropriately selected from known positive electrode active materials. For lithium-ion secondary batteries, materials capable of intercalating and releasing lithium ions are typically used as positive electrode active materials. Examples of positive electrode active materials include lithium transition metal composite oxides having an α-NaFeO2 crystal structure, lithium transition metal composite oxides having a spinel crystal structure, polyanionic compounds, chalcogen compounds, and sulfur. Examples of lithium transition metal composite oxides having an α-NaFeO2 crystal structure include Li[Lix Ni (1-x) O2(0 ≦ x < 0.5), Li[Li x Ni γ Co (1-x-γ) O2(0 ≦ x < 0.5, 0 < γ < 1, 0 < 1 - x - γ), Li[Li x Co (1-x) O2(0 ≦ x < 0.5), Li[Li x Ni γ Mn (1-x-γ) O2(0 ≦ x < 0.5, 0 < γ < 1, 0 < 1 - x - γ), Li[Li x Ni γ Mn β Co (1-x-γ-β) O2(0 ≦ x < 0.5, 0 < γ, 0 < β, 0.5 < γ + β < 1, 0 < 1 - x - γ - β), Li[Li x Ni γ Co β Al (1-x-γ-β) O2(0 ≦ x < 0.5, 0 < γ, 0 < β, 0.5 < γ + β < 1, 0 < 1 - x - γ - β), etc. can be mentioned. As the lithium transition metal composite oxide having a spinel crystal structure, Li x Mn2O4, Li x Ni γ Mn (2-γ) O4, etc. can be mentioned. As the polyanion compound, LiFePO4, LiMnPO4, LiNiPO4, LiCoPO4, Li3V2(PO4)3, Li2MnSiO4, Li2CoPO4F, etc. can be mentioned. As the chalcogen compound, titanium disulfide, molybdenum disulfide, molybdenum dioxide, etc. can be mentioned. Atoms or polyanions in these materials may be partially substituted with atoms or anion species composed of other elements. In the positive electrode active material layer, one of these materials may be used alone, or two or more of them may be mixed and used.

[0028] The positive electrode active material is usually in the form of particles (powder). The lower limit of the average particle size of the positive electrode active material is preferably 0.1 μm, more preferably 1 μm, and even more preferably 2 μm. By setting the average particle size of the positive electrode active material to be above the above lower limit, side reactions at the positive electrode can be suppressed and the conductivity of charge transport ions such as lithium ions can be improved. On the other hand, the upper limit of the average particle size of the positive electrode active material is preferably 20 μm, more preferably 9 μm, and even more preferably 4 μm. By setting the average particle size of the positive electrode active material to be below the above upper limit, the packing density of the positive electrode active material layer can be increased, so that the conductive paths between the positive electrode active materials can be maintained well.

[0029] To obtain powder with a predetermined particle size, grinders and classifiers are used. Examples of grinding methods include using mortars, ball mills, sand mills, vibrating ball mills, planetary ball mills, jet mills, counter-jet mills, swirling airflow jet mills, or sieves. Wet grinding, which involves the coexistence of water or a non-aqueous solvent such as hexane, can also be used during grinding. For classification, sieves and air classifiers are used as needed, both dry and wet.

[0030] In this non-aqueous electrolyte energy storage element, the presence of boron on the surface of the positive electrode active material suppresses the oxidative decomposition of the alkyl propionate ester on the positive electrode active material surface, which has an effect of improving ionic conductivity due to its low viscosity. As a result, the effect of improving the ionic conductivity of the non-aqueous electrolyte can be maintained. Therefore, this non-aqueous electrolyte energy storage element exhibits excellent reduction of DC resistance at low temperatures.

[0031] It is even more preferable that the boron element is present at the grain boundaries of the primary particles of the positive electrode active material. The presence of the boron element at the grain boundaries of the primary particles can further suppress the oxidative decomposition of the alkyl propionate ester on the surface of the positive electrode active material, thereby further reducing the DC resistance of the non-aqueous electrolyte energy storage element at low temperatures. The boron element may be present in the form of a layer on the surface of the positive electrode active material or as scattered particles.

[0032] The content of boron on the surface of the positive electrode active material is 0.05 mol% to 1.50 mol%, preferably 0.10 mol% to 1.20 mol%, and more preferably 0.20 mol% to 1.00 mol%, relative to the content of metal elements other than lithium contained in the positive electrode active material. By having a boron content that is above the lower limit and below the upper limit relative to the content of metal elements other than lithium contained in the positive electrode active material, the non-aqueous electrolyte energy storage element can further reduce its DC resistance at low temperatures.

[0033] In the present invention, the content of boron and metal elements contained in the positive electrode active material can be determined by inductively coupled plasma atomic emission spectroscopy (ICP). The measurement of the content of boron and metal elements contained in the positive electrode active material is performed by the following procedure. First, the non-aqueous electrolyte energy storage element is discharged at a current value of 0.05C to the lower limit voltage for normal use. Here, "normal use" refers to the case in which the non-aqueous electrolyte energy storage element is used by adopting the discharge conditions recommended or specified for the non-aqueous electrolyte energy storage element. Next, the non-aqueous electrolyte energy storage element is disassembled, and the positive electrode active material is collected from the positive electrode, which has been completely discharged. Then, the collected positive electrode active material is dissolved in an acid capable of dissolving the metal elements and boron elements contained in the positive electrode active material by microwave decomposition. Next, this solution is diluted to a certain volume with pure water to be used as the measurement solution. Then, using a multi-type ICP emission spectrometer ICPE-9820 (manufactured by Shimadzu Corporation), the concentrations of boron and metal elements contained in the positive electrode active material are measured by ICP emission spectroscopy. From the obtained concentrations of boron and metal elements contained in the positive electrode active material, the element content in the positive electrode active material is quantified. In calculating the concentrations of boron and metal elements contained in the positive electrode active material, a calibration curve method can be used, for example, by creating a calibration curve from solutions of known concentrations of boron and metal elements contained in the positive electrode active material, and then determining the concentrations of boron and metal elements contained in the positive electrode active material. The presence of boron on the surface of the positive electrode active material can be confirmed, for example, by observing the surface of the positive electrode active material with a scanning electron microscope-energy dispersive X-ray analyzer (SEM-EDX) or electron probe microanalyzer (EPMA).

[0034] The content of the positive electrode active material in the positive electrode active material layer is preferably 50% to 99% by mass, more preferably 70% to 98% by mass, and even more preferably 80% to 95% by mass. 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 in the positive electrode active material layer.

[0035] 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. Examples of carbonaceous materials include graphite, non-graphitic carbon, and graphene-based carbon. Examples of non-graphitic carbon include carbon nanofibers, pitch-based carbon fibers, and carbon black. Examples of carbon black include furnace black, acetylene black, and Ketjen black. Examples of graphene-based carbon include graphene, carbon nanotubes (CNTs), and fullerenes. The conductive agent can take the form of powder or fiber. One of these materials may be used alone as the conductive agent, or two or more may be used in mixture form. These materials may also be used in composite form. For example, a composite material of carbon black and CNTs may be used. Among these, carbon black is preferred from the viewpoint of electronic conductivity and coating properties, and acetylene black is particularly preferred.

[0036] The content of the conductive agent in the positive electrode active material layer is preferably 1% by mass or more and 10% by mass or less, and 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 non-aqueous electrolyte energy storage element can be increased.

[0037] Examples of binders include thermoplastic resins such as fluororesins (polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), etc.), polyethylene, polypropylene, polyacrylic, and polyimide; elastomers such as ethylene-propylene-diene rubber (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), and fluororubber; and polysaccharide polymers.

[0038] The binder content in the positive electrode active material layer is preferably 1% by mass or more and 10% by mass or less, and more preferably 3% by mass or more and 9% by mass or less. By keeping the binder content within the above range, the positive electrode active material can be stably maintained.

[0039] Examples of thickening agents include polysaccharide polymers such as carboxymethylcellulose (CMC) and methylcellulose. If the thickening agent has a functional group that reacts with lithium or the like, this functional group may be deactivated beforehand by methylation or the like.

[0040] The filler is not particularly limited. Examples of fillers include polyolefins such as polypropylene and polyethylene; inorganic oxides such as silicon dioxide, aluminum oxide, titanium dioxide, calcium oxide, strontium oxide, barium oxide, magnesium oxide, and aluminosilicates; hydroxides such as magnesium hydroxide, calcium hydroxide, and aluminum hydroxide; 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; mineral resource-derived materials such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica, or artificial products thereof.

[0041] The positive electrode active material layer may contain typical nonmetallic elements such as N, P, F, Cl, Br, and I, typical metallic elements such as Li, Na, Mg, Al, K, Ca, Zn, Ga, Ge, Sn, Sr, and Ba, and transition metal elements such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Zr, Nb, and W as components other than the positive electrode active material, conductive agent, binder, thickener, and filler.

[0042] (Negative electrode) The negative electrode comprises a negative electrode substrate and a negative electrode active material layer disposed directly on the negative electrode substrate or via an intermediate layer. The configuration of the intermediate layer is not particularly limited and can be selected from, for example, the configurations exemplified in the positive electrode.

[0043] The negative electrode substrate is electrically conductive. Suitable materials for the negative electrode substrate include metals such as copper, nickel, stainless steel, nickel-plated steel, and aluminum, or alloys thereof, as well as carbonaceous materials. Among these, copper or copper alloys are preferred. Examples of negative electrode substrates include foil, vapor-deposited film, mesh, and porous materials, with foil being preferred from a cost perspective. Therefore, copper foil or copper alloy foil is preferred as the negative electrode substrate. Examples of copper foil include rolled copper foil and electrolytic copper foil.

[0044] The average thickness of the negative electrode substrate is preferably 2 μm to 35 μm, more preferably 3 μm to 30 μm, even more preferably 4 μm to 25 μm, and particularly preferably 5 μm to 20 μm. By setting the average thickness of the negative electrode substrate within the above range, it is possible to increase the strength of the negative electrode substrate while increasing the energy density per unit volume of the non-aqueous electrolyte energy storage element.

[0045] The negative electrode active material layer contains a negative electrode active material. The negative electrode active material layer optionally contains conductive agents, binders, thickeners, fillers, and other optional components. These optional components can be selected from the materials exemplified above for the positive electrode.

[0046] The negative electrode active material layer may contain typical nonmetallic elements such as B, N, P, F, Cl, Br, and I, typical metallic elements such as Li, Na, Mg, Al, K, Ca, Zn, Ga, Ge, Sn, Sr, and Ba, and transition metallic elements such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Zr, Ta, Hf, Nb, and W as components other than the negative electrode active material, conductive agent, binder, thickener, and filler.

[0047] The negative electrode active material can be appropriately selected from known negative electrode active materials. For lithium-ion secondary batteries, materials capable of intercalating and releasing lithium ions are typically used as negative electrode active materials. Examples of negative electrode active materials include: metallic Li; metals or metalloids such as Si and Sn; metal oxides or metalloid oxides such as Si oxide, Ti oxide, and Sn oxide; and Li4Ti5O 12 LiTiO 2、Examples of materials include titanium-containing oxides such as TiNb2O7; polyphosphate compounds; silicon carbide; and carbon materials such as graphite and non-graphitizable carbon (easily graphitizable carbon or poorly graphitizable carbon). Among these materials, graphite and non-graphitizable carbon are preferred. In the negative electrode active material layer, one of these materials may be used alone, or two or more may be used in mixture form.

[0048] "Graphite" refers to the average lattice plane spacing (d) of the (002) plane, determined by X-ray diffraction before charging or discharging, or after discharge. 002 This refers to carbon materials with a n-scale between 0.33 nm and less than 0.34 nm. Examples of graphite include natural graphite and artificial graphite. Natural graphite is preferred from the standpoint of superior input / output characteristics.

[0049] "Non-graphite carbon" refers to the average lattice plane spacing (d) of the (002) plane, as determined by X-ray diffraction before charging / discharging or in the discharged state. 002 This refers to carbon materials with a nautical radius of 0.34 nm or more and 0.42 nm or less. Non-graphitized carbons include poorly graphitizable carbons and easily graphitizable carbons. Examples of non-graphitized carbons include resin-derived materials, petroleum pitch or materials derived from petroleum pitch, petroleum coke or materials derived from petroleum coke, plant-derived materials, and alcohol-derived materials.

[0050] Here, the "discharged state" of a carbon material such as graphite refers to a state in which the carbon material, which is the negative electrode active material, is discharged to the extent that charge transport ions such as lithium ions that can be absorbed and released during charging and discharging are sufficiently released. For example, in a half-cell using a negative electrode containing a carbon material as the negative electrode active material as the working electrode and lithium metal (Li) as the counter electrode, this is the state in which the open-circuit voltage is 0.7V or higher.

[0051] "Non-graphitizable carbon" refers to the above d 002 This refers to carbon materials with a wavelength between 0.36 nm and 0.42 nm.

[0052] "Easily graphitizable carbon" refers to the above d 002 This refers to carbon materials with a wavelength of 0.34 nm or more and less than 0.36 nm.

[0053] The negative electrode active material is usually in the form of particles (powder). The average particle size of the negative electrode active material can be, for example, between 1 nm and 100 μm. If the negative electrode active material is a carbon material, titanium-containing oxide, or polyphosphate compound, its average particle size may be between 1 μm and 100 μm. If the negative electrode active material is Si, Sn, Si oxide, or Sn oxide, its average particle size may be between 1 nm and 1 μm. Setting the average particle size of the negative electrode active material above the lower limit makes it easier to manufacture or handle. Setting the average particle size of the negative electrode active material below the upper limit improves the electronic conductivity of the active material layer. To obtain powder with a predetermined particle size, a pulverizer or classifier is used. The pulverizing method and classification method can be selected from, for example, the methods exemplified above for the positive electrode. If the negative electrode active material is a metal such as metallic Li, the negative electrode active material may be in the form of foil.

[0054] 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, and 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 in the negative electrode active material layer.

[0055] (Separator) The separator can be appropriately selected from known separators. Examples of separators include a separator consisting only of a base layer, or a separator in which a heat-resistant layer containing heat-resistant particles and a binder is formed on one or both sides of the base layer. Examples of the base layer shape of the separator include woven fabric, nonwoven fabric, and porous resin film. Among these shapes, porous resin film is preferred from the viewpoint of strength, and nonwoven fabric is preferred from the viewpoint of liquid retention of non-aqueous electrolytes. As for the material of the base layer of the separator, polyolefins such as polyethylene and polypropylene are preferred from the viewpoint of shutdown function, and polyimide and aramid are preferred from the viewpoint of oxidative degradation resistance. A composite material of these resins may also be used as the base layer of the separator.

[0056] The heat-resistant particles contained in the heat-resistant layer preferably have a mass loss of 5% or less when heated from room temperature to 500°C in an air atmosphere of 1 atmosphere, and more preferably have a mass loss of 5% or less when heated from room temperature to 800°C. Inorganic compounds are examples of materials with a mass loss of less than the specified amount. 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 aluminosilicates; 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 materials such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica, or artificial products thereof. These inorganic compounds may be used individually or in combination, or two or more may be used as a mixture. Among these inorganic compounds, silicon dioxide, aluminum oxide, or aluminosilicates are preferred from the viewpoint of safety for energy storage elements.

[0057] The porosity of the separator is preferably 80 volume% or less from the viewpoint of strength, and preferably 20 volume% or more from the viewpoint of discharge performance. Here, "porosity" refers to a volume-based value and means the measurement value obtained using a mercury porosimeter.

[0058] A polymer gel composed of a polymer and a non-aqueous electrolyte may be used as a separator. Examples of polymers include polyacrylonitrile, polyethylene oxide, polypropylene oxide, polymethyl methacrylate, polyvinyl acetate, polyvinylpyrrolidone, and polyvinylidene fluoride. Using a polymer gel has the effect of suppressing leakage. A polymer gel may also be used in combination with a porous resin film or nonwoven fabric as described above as a separator.

[0059] (Non-aqueous electrolytes) A non-aqueous electrolyte contains a non-aqueous solvent, an electrolyte salt dissolved in this non-aqueous solvent, and an alkyl propionate ester as an additive. A non-aqueous electrolyte solution may also be used as the non-aqueous electrolyte.

[0060] Examples of non-aqueous solvents include cyclic carbonates, linear carbonates, phosphate esters, sulfonic acid esters, ethers, amides, and nitriles. Compounds in which some of the hydrogen atoms are substituted with halogens may also be used as non-aqueous solvents.

[0061] Preferably, the non-aqueous solvent does not contain fluorinated cyclic carbonates. By not containing fluorinated cyclic carbonates, the amount of high-resistance layer derived from lithium fluoride (LiF) due to the decomposition of the non-aqueous electrolyte is reduced, and the viscosity reduction effect of the non-aqueous electrolyte can be more effectively exhibited. Therefore, the non-aqueous electrolyte energy storage element can further reduce its DC resistance at low temperatures.

[0062] Examples of cyclic carbonates include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinylethylene carbonate (VEC), chloroethylene carbonate, styrene carbonate, 1-phenylvinylene carbonate, and 1,2-diphenylvinylene carbonate. Among these, EC is preferred.

[0063] Examples of linear carbonates include diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diphenyl carbonate, trifluoroethyl methyl carbonate, and bis(trifluoroethyl) carbonate. Among these, EMC is preferred.

[0064] It is preferable to use a cyclic carbonate or a linear carbonate as the non-aqueous solvent, and it is more preferable to use a cyclic carbonate and a linear carbonate in combination. Using a cyclic carbonate can promote the dissociation of the electrolyte salt and improve the ionic conductivity of the non-aqueous electrolyte. Using a linear carbonate can keep the viscosity of the non-aqueous electrolyte low. When using a cyclic carbonate and a linear carbonate in combination, the volume ratio of the cyclic carbonate to the linear carbonate (cyclic carbonate:linear carbonate) is preferably in the range of 5:95 to 50:50.

[0065] The electrolyte salt can be appropriately selected from known electrolyte salts. Examples of electrolyte salts include lithium salts, sodium salts, potassium salts, magnesium salts, and onium salts. Among these, lithium salts are preferred.

[0066] Examples of lithium salts include inorganic lithium salts such as LiPF6, LiPO2F2, LiBF4, LiClO4, and LiN(SO2F)2, and lithium salts having halogenated hydrocarbon groups such as LiSO3CF3, LiN(SO2CF3)2, LiN(SO2C2F5)2, LiN(SO2CF3)(SO2C4F9), LiC(SO2CF3)3, and LiC(SO2C2F5)3. Among these, inorganic lithium salts are preferred, and LiPF6 is more preferred.

[0067] The electrolyte salt content in non-aqueous electrolytes is 0.1 mol / dm³ at 20°C and 1 atm. 3 More than 2.5mol / dm 3 Preferably, it is 0.3 mol / dm³ 3 More than 2.0mol / dm 3 It is more preferable that it be less than or equal to 0.5 mol / dm 3 More than 1.7mol / dm 3 It is even more preferable that the following is the case: 0.7 mol / dm 3 More than 1.5mol / dm 3 The following is particularly preferable. By setting the electrolyte salt content within the above range, the ionic conductivity of the non-aqueous electrolyte can be increased.

[0068] The non-aqueous electrolyte contains an alkyl propionate ester as an additive, in addition to the non-aqueous solvent and electrolyte salt. The inclusion of a low-viscosity alkyl propionate ester in the non-aqueous electrolyte reduces its viscosity and improves its ionic conductivity. Examples of alkyl propionate esters include methyl propionate and ethyl propionate. Methyl propionate is preferred as the alkyl propionate ester. The use of methyl propionate as the alkyl propionate ester further enhances the viscosity reduction effect of the non-aqueous electrolyte, thereby allowing the non-aqueous electrolyte energy storage element to further reduce its DC resistance at low temperatures.

[0069] The content of the alkyl propionate ester in the non-aqueous electrolyte is preferably 0.5% by mass or more and 6.0% by mass or less, more preferably 1.0% by mass or more and 5.0% by mass or less, and even more preferably 2.0% by mass or more and 3.0% by mass or less. By having the content of the alkyl propionate ester between the lower limit and the upper limit, the non-aqueous electrolyte energy storage element can further reduce its DC resistance at low temperatures and further improve its safety.

[0070] Non-aqueous electrolytes may contain additives other than alkyl propionate esters. Other additives include, for example, oxalates such as lithium bis(oxalate)borate (LiBOB), lithium difluorooxalate borate (LiFOB), and lithium bis(oxalate)difluorophosphate (LiFOP); imide salts such as lithium bis(fluorosulfonyl)imide (LiFSI); aromatic compounds such as biphenyl, alkylbiphenyl, terphenyl, partially hydrogenated terphenyl, cyclohexylbenzene, t-butylbenzene, t-amylbenzene, diphenyl ether, and dibenzofuran; partially halides of the above aromatic compounds such as 2-fluorobiphenyl, o-cyclohexylfluorobenzene, and p-cyclohexylfluorobenzene; halogenated anisole compounds such as 2,4-difluoroanisole, 2,5-difluoroanisole, 2,6-difluoroanisole, and 3,5-difluoroanisole; vinylene carbonate (VC), methylvinylene carbonate, ethylvinylene carbonate, lithium monofluorophosphate, and difluoro Lithium phosphate (LiDFP), succinic anhydride, glutaric anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, cyclohexanedicarboxylic acid anhydride; ethylene sulfite, propylene sulfite, dimethyl sulfite, methyl methanesulfonate, busulfan, methyl toluenesulfonate, dimethyl sulfate, ethylene sulfate, sulfolane, dimethyl sulfone, diethyl sulfone, dimethyl sulfoxide, diethyl sulfoxide, tetramethylene sulfoxide, diphenyl sulfide, 4 Examples include ,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-propensultone, 1,3-propanesultone, 1,4-butanesultone, 1,4-butensultone, perfluorooctane, tristrimethylsilyl borate, tristrimethylsilyl phosphate, tetrakithrimethylsilyl titanate, and the like. These other additives may be used individually or in combination of two or more.

[0071] The content of other additives in the non-aqueous electrolyte is preferably 0.3% to 9.0% by mass, more preferably 0.4% to 5.0% by mass, and particularly preferably 0.5% to 6.0% by mass, relative to the total amount of other additives. By setting the content of other additives within the above range, it is possible to improve the capacity retention performance or charge / discharge cycle performance after high-temperature storage, or to further improve safety.

[0072] The boron content in the non-aqueous electrolyte is preferably 0.028% by mass or less, and more preferably 0.012% by mass or less. By keeping the boron content in the non-aqueous electrolyte below the above upper limit, the oxidative decomposition of the alkyl propionate can be effectively suppressed.

[0073] The shape of the non-aqueous electrolyte energy storage element in this embodiment is not particularly limited, and examples include cylindrical batteries, prismatic batteries, flat batteries, coin-type batteries, button-type batteries, and the like.

[0074] Figure 1 shows a non-aqueous electrolyte energy storage element 1 as an example of a rectangular battery. Note that the figure is a transparent view of the inside of the container. An electrode body 2 having a positive electrode and a negative electrode wound around a separator is housed in a rectangular container 3. 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 electrode terminal 5 via a negative electrode lead 51.

[0075] <Configuration of the energy storage device> The non-aqueous electrolyte energy storage element of this embodiment can be installed in an energy storage device that comprises an energy storage unit (battery module) composed of multiple non-aqueous electrolyte energy storage elements, such as an automotive power supply for electric vehicles (EVs), hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), an electronic device power supply for personal computers, communication terminals, or a power storage power supply. In this case, it is sufficient that the technology of the present invention is applied to at least one of the non-aqueous electrolyte energy storage elements included in the energy storage device. Figure 2 shows an example of an energy storage device 30 which is formed by further assembling energy storage units 20, each of which is an assembly of two or more electrically connected non-aqueous electrolyte energy storage elements 1. The energy storage device 30 may include busbars (not shown) that electrically connect two or more non-aqueous electrolyte energy storage elements 1, busbars (not shown) that electrically connect two or more energy storage units 20, etc. The energy storage unit 20 or the energy storage device 30 may include a condition monitoring device (not shown) that monitors the state of one or more non-aqueous electrolyte energy storage elements 1.

[0076] <Method for manufacturing a non-aqueous electrolyte energy storage element> The method for manufacturing a non-aqueous electrolyte energy storage element of this embodiment comprises preparing a positive electrode using a positive electrode active material having boron elements on its surface, and preparing a non-aqueous electrolyte containing a non-aqueous solvent and an alkyl propionate ester. According to this method for manufacturing a non-aqueous electrolyte energy storage element, it is possible to manufacture a non-aqueous electrolyte energy storage element that exhibits excellent DC resistance reduction effects at low temperatures.

[0077] The positive electrode can be manufactured, for example, by applying a positive electrode mixture paste directly to a positive electrode substrate or via an intermediate layer, and then drying it. After drying, pressing or other processes may be performed as needed. The positive electrode mixture paste contains a positive electrode active material with boron elements present on its surface, and optional components such as a conductive agent and a binder, which constitute the positive electrode active material layer. The positive electrode mixture paste usually also contains a dispersion medium. The specific and preferred forms of the positive electrode prepared in this manufacturing method are the same as those of the positive electrode provided in the non-aqueous electrolyte energy storage element described above. As a method for presenting boron elements on the surface of the positive electrode active material, for example, a method can be used in which the positive electrode active material is mixed with an aqueous solution of boric acid (H3BO3) to prepare a suspension solution, and then the suspension solution is dried and heat-treated. The heat treatment temperature is preferably, for example, 200°C to 500°C.

[0078] Preparing a non-aqueous electrolyte containing a non-aqueous solvent and an alkyl propionate ester may be equivalent to preparing a non-aqueous electrolyte containing a non-aqueous solvent and an alkyl propionate ester. Preparing a non-aqueous electrolyte can be done by mixing the components constituting the non-aqueous electrolyte. That is, for example, a non-aqueous electrolyte can be obtained by dissolving components such as an alkyl propionate ester and an electrolyte salt in a non-aqueous solvent. Note that the non-aqueous electrolyte may also be prepared by methods other than preparation. The specific and preferred forms of the non-aqueous electrolyte prepared in this manufacturing method are the same as those of the non-aqueous electrolyte provided in the non-aqueous electrolyte energy storage element described above.

[0079] The manufacturing method typically further comprises preparing a negative electrode. The manufacturing method may further comprise, for example, forming an electrode body by stacking or winding a positive electrode and a negative electrode via a separator, and housing the electrode body and a non-aqueous electrolyte in a container.

[0080] The method for housing the non-aqueous electrolyte in a container can be appropriately selected from known methods. For example, when using a non-aqueous electrolyte solution, the non-aqueous electrolyte solution can be injected through an inlet formed in the container, and then the inlet can be sealed.

[0081] <Other Embodiments> Furthermore, the non-aqueous electrolyte energy storage element of the present invention is not limited to the above embodiments, and various modifications may be made without departing from the spirit of the present invention. For example, the configuration of one embodiment may be added to the configuration of another embodiment, and a part of the configuration of one embodiment may be replaced with the configuration of another embodiment or known technology. In addition, a part of the configuration of one embodiment may be deleted. Also, known technology may be added to the configuration of one embodiment.

[0082] In the above embodiment, the case in which the non-aqueous electrolyte energy storage element is used as a rechargeable non-aqueous electrolyte secondary battery (e.g., a lithium-ion secondary battery) was described, but the type, shape, dimensions, capacity, etc. of the non-aqueous electrolyte energy storage element are arbitrary. The present invention can also be applied to various secondary batteries, electric double-layer capacitors, or capacitors such as lithium-ion capacitors. [Examples]

[0083] The present invention will be described in more detail below with reference to examples. The present invention is not limited to the following examples.

[0084] [Example 1] (Fabrication of the positive electrode) As the positive electrode active material, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 was prepared. The average particle size of the positive electrode active material was 3 μm. Boron was coated onto the surface of the positive electrode active material by mixing the positive electrode active material with an aqueous solution of boric acid (H3BO3), preparing a suspension solution, drying the suspension solution, and then heat-treating it. The boron content on the surface of the positive electrode active material was 0.50 mol% relative to the content of all metal elements other than lithium contained in the positive electrode active material. Next, a positive electrode mixture paste was prepared using a positive electrode active material, acetylene black (AB) as a conductive agent, polyvinylidene fluoride (PVDF) as a binder, and N-methylpyrrolidone (NMP) as a dispersion medium. The mass ratio of the positive electrode active material, conductive agent, and binder was 93.0:4.5:2.5 (on a solid content basis). The positive electrode mixture paste was applied to both sides of an aluminum foil substrate and dried. After that, a roll press was performed to obtain the positive electrode. The mass per unit area of ​​the positive electrode active material layer was 8.9 mg / cm². 2 The mass per unit area of ​​the positive electrode active material layer is the sum of the two layers provided on both sides of the positive electrode substrate, and the same applies to the negative electrode active material layer.

[0085] (Fabrication of the negative electrode) Graphite was prepared as the negative electrode active material. A negative electrode mixture paste was prepared using the above negative electrode active material, styrene-butadiene rubber (SBR) as a binder, carboxymethylcellulose (CMC) as a thickener, and water as a dispersion medium. The mass ratio of the negative electrode active material, binder, and thickener was 98.0:1.0:1.0 (on a solid content basis). The negative electrode mixture paste was applied to both sides of a copper foil, which served as the negative electrode substrate, and dried. After that, a roll press was performed to obtain the negative electrode. The mass per unit area of ​​the negative electrode active material layer was 5.8 mg / cm². 2 That was the case.

[0086] (Non-aqueous electrolytes) A non-aqueous solvent prepared by mixing ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 30:35:35, to which 1.2 mol / dm³ of electrolyte salt was added. 3 A non-aqueous solution was prepared by dissolving LiPF6 at the specified concentration. Furthermore, a non-aqueous electrolyte was prepared by adding methyl propionate as an alkyl propionate additive at a concentration of 3.0% by mass, vinylene carbonate at 0.5% by mass, lithium difluorophosphate at 1.0% by mass, and lithium bis(oxalate)borate at 0.2% by mass, respectively, as other additives.

[0087] (Separator) A microporous membrane made of polyolefin was used as the separator.

[0088] (Assembly of non-aqueous electrolyte energy storage elements) A wound electrode body was obtained using the positive electrode, negative electrode, and separator described above. The electrode body was placed in a container, a non-aqueous electrolyte was injected, and the container was sealed to obtain the non-aqueous electrolyte energy storage element of Example 1.

[0089] [Example 2] A non-aqueous electrolyte energy storage element of Example 2 was obtained in the same manner as in Example 1, except that the content of boron element coated on the surface of the positive electrode active material was changed as shown in Table 1.

[0090] [Comparative Example 1] LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 A non-aqueous electrolyte energy storage element of Comparative Example 1 was obtained in the same manner as in Example 1, except that O2 was used as is without coating its surface with boron.

[0091] [Preparation of reference samples for DC resistance evaluation at low temperatures] For the DC resistance evaluation at low temperatures described below, non-aqueous electrolyte energy storage elements (samples without methyl propionate) were prepared in the same manner as in Example 1, Example 2, and Comparative Example 1, except that the non-aqueous electrolyte did not contain methyl propionate.

[0092] [evaluation] (Initial charge / discharge) Each obtained non-aqueous electrolyte energy storage element was subjected to constant current charging at 25°C with a charging current of 1.0C up to 4.10V, followed by constant voltage charging at 4.10V. The charging termination condition was set to 3 hours from the start of charging. After a 10-minute rest period, constant current discharge was performed with a discharge current of 1.0C up to 2.50V, followed by another 10-minute rest period. These charging and discharging cycles constituted one cycle, and two initial charge-discharge cycles were performed.

[0093] (DC resistance at low temperatures) For each non-aqueous electrolyte energy storage element, the State of Charge (SOC) was adjusted to 85% at 25°C with a charging current of 1.0C, and then stored in a constant temperature bath at 65°C for 180 days under high-temperature conditions. Next, for each non-aqueous electrolyte energy storage element after storage in a high-temperature environment, it was stored in a 25°C constant temperature bath for 3 hours, then discharged at 25°C with a discharge current of 1.0C to 2.50V, followed by a 10-minute rest period, and then charged with a constant current of 1.0C to adjust the SOC to 50%. After storage in a -10°C constant temperature bath for 3 hours, it was discharged at -10°C with a constant current of 0.2C, 0.5C, or 1.0C for 30 seconds each. After each discharge, it was charged with a constant current of 0.2C to bring the SOC to 50%. The relationship between the current in each discharge and the voltage at 10 seconds after the start of discharge was plotted, and the DC resistance [mΩ] at low temperatures after storage in a high-temperature environment for each non-aqueous electrolyte energy storage element was determined from the slope of the straight line obtained from the three plots. Furthermore, the DC resistance at low temperatures was determined for each of the methyl propionate-free samples from Example 1, Example 2, and Comparative Example 1 using the same procedure. Then, the relative ratio of the DC resistance of each of the above non-aqueous electrolyte energy storage elements at low temperatures was determined, with the methyl propionate-free sample set to 100. The results are shown in Table 1.

[0094] [Table 1]

[0095] As shown in Table 1, in Examples 1 and 2, where the non-aqueous electrolyte contained alkyl propionate, boron was present on the surface of the positive electrode active material, and the boron content on the surface of the positive electrode active material was 0.05 mol% to 1.50 mol% relative to the content of metal elements other than lithium in the positive electrode active material, the DC resistance at low temperatures was significantly reduced compared to the case where the non-aqueous electrolyte did not contain methyl propionate.

[0096] On the other hand, in Comparative Example 1, where the surface of the positive electrode active material was not coated with boron, the DC resistance at low temperatures increased when the non-aqueous electrolyte contained methyl propionate compared to when the non-aqueous electrolyte did not contain methyl propionate. Furthermore, in Comparative Example 1, the boron content in the positive electrode active material was measured by ICP emission spectroscopy after initial charging and discharging, and it was found to be less than the detection limit of 0.02 mol%. From this, it can be concluded that when the non-aqueous electrolyte contains 0.2 mass% lithium bis(oxalate)borate as other additives, there is virtually no boron element derived from lithium bis(oxalate)borate on the surface of the positive electrode active material. [Industrial applicability]

[0097] This invention can be applied to non-aqueous electrolyte energy storage elements used as power sources for electronic devices such as personal computers and communication terminals, as well as automobiles and the like. [Explanation of Symbols]

[0098] 1. Non-aqueous electrolyte energy storage element 2 Electrode body 3 containers 4 Positive terminal 41 Positive lead 5 Negative terminal 51 Negative lead 20 Energy storage units 30 Energy storage devices

Claims

1. A positive electrode containing a positive electrode active material, The negative electrode and, A non-aqueous electrolyte containing a non-aqueous solvent and an alkyl propionate ester. It is equipped with, The boron element is present on the surface of the above positive electrode active material. A non-aqueous electrolyte energy storage element in which the content of the boron element on the surface of the positive electrode active material is 0.05 mol% or more and 1.50 mol% or less relative to the content of metal elements other than lithium element contained in the positive electrode active material.

2. The non-aqueous electrolyte energy storage element according to claim 1, wherein the content of the alkyl propionate ester in the non-aqueous electrolyte is 0.5% by mass or more and 6.0% by mass or less.

3. The non-aqueous electrolyte energy storage element according to claim 1 or claim 2, wherein the alkyl propionate ester is methyl propionate.

4. The non-aqueous electrolyte energy storage element according to claim 1 or claim 2, wherein the non-aqueous solvent does not contain a fluorinated cyclic carbonate.

5. The non-aqueous electrolyte energy storage element according to claim 1 or claim 2, wherein the average particle size of the positive electrode active material is 0.1 μm or more and 20 μm or less.

6. The process involves creating a positive electrode using a positive electrode active material that has boron elements present on its surface, To prepare a non-aqueous electrolyte containing a non-aqueous solvent and an alkyl propionate ester, Equipped with, A method for manufacturing a non-aqueous electrolyte energy storage element, wherein the content of the boron element on the surface of the positive electrode active material is 0.05 mol% or more and 1.50 mol% or less relative to the content of metal elements other than lithium element contained in the positive electrode active material.

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