Positive electrode mixture for non-aqueous electrolyte storage element, positive electrode for non-aqueous electrolyte storage element, and non-aqueous electrolyte storage element

Abstract

To provide a positive electrode for a non-aqueous electrolyte storage element and a non-aqueous electrolyte storage element that have both low-temperature characteristics and storage characteristics.SOLUTION: A positive electrode mixture for a non-aqueous electrolyte storage element according to an embodiment contains a lithium transition metal compound having a carbon-coated polyanion structure as a positive electrode active material, and the ratio of the BET specific surface area ratio increased by the carbon coating of the lithium transition metal compound having a polyanion structure to the BET specific surface area of the positive electrode mixture is 12% or more and 30% or less, and the positive electrode mixture does not substantially contain a conductive agent.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a positive electrode mixture for a non-aqueous electrolyte storage element, a positive electrode for a non-aqueous electrolyte storage element, and a non-aqueous electrolyte storage element.

Background Art

[0002] In recent years, as a positive electrode active material used in non-aqueous electrolyte storage elements such as non-aqueous electrolyte secondary batteries, transition metal compounds having a polyanion structure have attracted attention. Since this transition metal compound having a polyanion structure forms a polyanion structure by oxygen covalently bonding with an element other than the transition metal, it does not release oxygen even at a relatively high temperature, and it is known that using it as a positive electrode active material can improve the thermal safety of the storage element. In addition, in order to improve the conductivity (electronic conductivity) of a transition metal compound having a polyanion structure used as a positive electrode active material, a technique of carbon coating (carbon coat) on its surface is known (for example, see Patent Documents 1 to 3).

[0003] Patent Document 1 describes that "a second positive electrode active material in which a compound having an olivine structure represented by LiMn 0.7 Fe 0.3 PO4 and having the physical properties shown in Table 1 below was prepared with carbon coating." (paragraph

[0091] ), and Table 1 (paragraph

[0092] ) shows that the BET specific surface area is 15.5 m 2 / g, 21.0 m 2 / g, 28.0 m 2The second positive electrode active material per g is shown. "85 parts by mass of the second positive electrode active material, 7.5 parts by mass of acetylene black as a conductive assistant, 7.5 parts by mass of polyvinylidene fluoride as a binder, and an appropriate amount of N-methyl-2-pyrrolidone were mixed to produce a slurry. An aluminum foil was prepared as a current collector. The above slurry was coated in a film form on the surface of the aluminum foil. By drying the aluminum foil coated with the slurry, N-methyl-2-pyrrolidone was removed. Then, the aluminum foil was pressed to obtain a conjugate. The obtained conjugate was heat-dried in a dryer to produce a positive electrode for measuring reaction resistance... A coin-type battery was obtained." (Paragraph

[0094] ) is described.

[0004] Patent Document 2 states that "The positive electrode was fabricated as follows. Lithium iron phosphate (with its surface coated with carbon): 97 parts by mass, acetylene black as a conductive assistant: 1.5 parts by mass, and PVDF as a binder: 1.5 parts by mass were dispersed in NMP to prepare a slurry, which was coated on one side of an Al foil with a thickness of 12 μm, dried, and subjected to a pressing process to form a positive electrode mixture layer with a mass of approximately 17 mg / cm 2 on one side of the Al foil current collector... A positive electrode having a positive electrode mixture layer with a size of 20 mm × 30 mm on one side of the current collector was fabricated... A non-aqueous electrolyte battery was fabricated." (Paragraphs

[0121] ,

[0122] ) is described.

[0005] Patent Document 3 states that "[Example 1] As metal sources, iron citrate (FeC6H5O7·nH2O), manganese acetate tetrahydrate (Mn(CH3COO)2·4H2O), lithium dihydrogen phosphate, and lithium acetate were added to a zirconia pot, and further, acetone was added and pulverized and mixed using a planetary ball mill device. The obtained solution was dried to obtain raw material powder. This raw material powder was calcined preliminarily using a box-type electric furnace... To this preliminarily calcined body, 7 parts by weight of sucrose was added as a carbon source and a particle size control agent per 100 parts by weight... Next, final firing was performed using a tubular furnace capable of controlling the atmosphere. The firing atmosphere was an Ar atmosphere, the firing temperature was 700 °C, and the firing time was 10 hours. Through the above steps, the olivine-type positive electrode active material LiFe0.2 Mn 0.8 PO4 was obtained. An electrode (positive electrode) was fabricated using the synthesized active material, and the characteristics of the electrode, namely the capacity and rate characteristics, were measured. The method for fabricating the electrode will be described below. A positive electrode active material, a conductive agent, a binder, and a solvent were kneaded on a mortar to prepare a slurry. Acetylene black... was used as the conductive material... The composition of the electrode was such that the weight ratio of the positive electrode active material, the conductive material, and the binder was 82.5:10:7.5. (From paragraphs

[0071] to

[0074] ) It is described.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0007] An object of the present invention is to provide a positive electrode for a non-aqueous electrolyte storage element and a non-aqueous electrolyte storage element that achieve both low-temperature characteristics and storage characteristics.

Means for Solving the Problems

[0008] The positive electrode mixture for a non-aqueous electrolyte storage element according to one aspect of the present invention contains, in the positive electrode mixture, as a positive electrode active material, a lithium transition metal compound having a carbon-coated polyanion structure, and the ratio of the BET specific surface area increased by the carbon coating of the lithium transition metal compound having the polyanion structure to the BET specific surface area of the positive electrode mixture is 12% or more and 30% or less, and the positive electrode mixture substantially does not contain a conductive agent. The positive electrode for a non-aqueous electrolyte storage element according to another aspect of the present invention has the above positive electrode mixture. The non-aqueous electrolyte storage element according to still another aspect of the present invention includes the positive electrode described above.

Advantages of the Invention

[0009] According to the positive electrode mixture for a non-aqueous electrolyte storage element according to one aspect of the present invention, it is possible to provide a positive electrode for a non-aqueous electrolyte storage element and a non-aqueous electrolyte storage element in which both low-temperature characteristics and storage characteristics are achieved.

Brief Description of the Drawings

[0010]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0011] First, an overview of the positive electrode mixture for a non-aqueous electrolyte storage element, the positive electrode for a non-aqueous electrolyte storage element, and the non-aqueous electrolyte storage element disclosed in this specification will be described.

[0012] The positive electrode mixture for a non-aqueous electrolyte storage element according to one aspect of the present invention contains, in the positive electrode mixture, as a positive electrode active material, a lithium transition metal compound having a carbon-coated polyanion structure, and the ratio of the BET specific surface area increased by the carbon coating of the lithium transition metal compound having the polyanion structure to the BET specific surface area of the positive electrode mixture is 12% or more and 30% or less, and the positive electrode mixture substantially does not contain a conductive agent.

[0013] The positive electrode for a non-aqueous electrolyte storage element according to another aspect of the present invention has the positive electrode mixture, and the non-aqueous electrolyte storage element according to still another aspect of the present invention includes the positive electrode.

[0014] According to the above positive electrode mixture for non-aqueous electrolyte storage elements, it is possible to provide a positive electrode for non-aqueous electrolyte storage elements and a non-aqueous electrolyte storage element that achieve both low-temperature characteristics and storage characteristics. By increasing the ratio of the above BET specific surface area, the low-temperature characteristics can be improved, and by decreasing it, the storage characteristics can be improved. Therefore, by setting the ratio of the above BET specific surface area to 12% or more and 30% or less, both low-temperature characteristics and storage characteristics can be achieved. The BET specific surface area can be calculated by the one-point method using nitrogen gas adsorption for a powder dried in advance at 120 °C for 10 minutes.

[0015] The ratio of the above BET specific surface area is determined by the method described below. Disassemble the non-aqueous electrolyte storage element in a discharged state and take out the positive electrode. Take out the positive electrode mixture from the positive electrode, Measure its BET specific surface area and designate it as (C). The positive electrode active material Disperse it in a solvent such as N-methylpyrrolidone (NMP) to remove the binder. For the mixed powder of the dried positive electrode active material and the conductive agent, the conductive agent is removed by air classification treatment. Measure the BET specific surface area of the obtained positive electrode active material (lithium transition metal compound having a carbon-coated polyanion structure) powder and designate it as (A). Thereafter, the lithium transition metal compound having a polyanion structure with the carbon coating removed is prepared by heat-treating the above positive electrode active material powder at 400 °C in an air atmosphere, and its BET specific surface area is measured and designated as (B). (A - B) / C The value calculated by the formula of ×100 is defined as "the ratio of the BET specific surface area increased by the carbon coating of the lithium transition metal compound having a polyanion structure to the BET specific surface area of the above positive electrode mixture".

[0016] In this specification, "carbon coating" means carbon coated on the surface of a lithium transition metal compound having a polyanion structure to enhance its conductivity, and "conductive agent" means a conductive material other than the above coated carbon contained in the positive electrode mixture to enhance the conductivity of the positive electrode.

[0017] In addition, "substantially free of a conductive agent in the positive electrode active material" means that the content of the conductive agent in the positive electrode active material for a non-aqueous electrolyte storage element, which is an issue of the positive electrode active material for a non-aqueous electrolyte storage element according to one aspect of the present invention, is substantially 0% by mass, which has an adverse effect on the compatibility between the low-temperature characteristics and the storage characteristics of the non-aqueous electrolyte storage element. However, it does not exclude the inclusion of a trace amount of the conductive agent in the positive electrode active material as long as it does not inhibit the compatibility between the above-mentioned low-temperature characteristics and storage characteristics. Therefore, "substantially free of a conductive agent in the positive electrode active material" means that the content of the conductive agent is less than 1% by mass of the positive electrode active material, preferably 0.5% by mass or less, and more preferably 0% by mass. The content of the conductive agent can be confirmed by calculating it by the following method. Disassemble the non-aqueous electrolyte storage element in a discharged state and take out the positive electrode. Take out the positive electrode active material from the positive electrode, disperse it in a solvent such as N-methylpyrrolidone (NMP) to remove the binder. Perform air classification on the mixed powder of the dried positive electrode active material and the conductive agent. From the masses of the conductive agent and the positive electrode active material taken out in this way, the content of the conductive agent is calculated.

[0018] By substantially not containing a commonly used conductive agent such as acetylene black (for example, refer to the conductive aids and conductive materials in Patent Documents 1 to 3 above) in the positive electrode active material, it is possible to suppress the BET specific surface area of the entire positive electrode active material from becoming too large. Therefore, side reactions on the positive electrode surface during storage at high temperatures and the like can be suppressed, leading to an improvement in storage characteristics. In addition, when a positive electrode is manufactured using such a positive electrode active material, there is an advantage that a more uniform state can be produced. On one hand, since the retention property of the positive electrode mixture decreases due to substantially not containing a common conductive agent in the positive electrode mixture, in order to suppress the decrease in the retention property of the positive electrode mixture, it is preferable that the positive electrode active material has a certain degree of large surface roughness. Therefore, by setting the ratio of the BET specific surface area increased by the carbon coating of the lithium transition metal compound having the polyanion structure to the BET specific surface area of the positive electrode mixture to 12% or more, the surface roughness of the carbon coating surface of the lithium transition metal compound having the polyanion structure becomes appropriately large, and the retention property of the positive electrode mixture is compensated. As a result, lithium ions can be sufficiently supplied to the lithium transition metal compound having the polyanion structure, and the low-temperature characteristics can be improved. Further, by setting the ratio of the BET specific surface area to 30% or less, side reactions on the carbon coating surface of the lithium transition metal compound having the polyanion structure can be suppressed, and the storage characteristics can be improved.

[0019] The configuration of the non-aqueous electrolyte storage element, the configuration of the power storage device, the manufacturing method of the non-aqueous electrolyte storage element, and other embodiments according to an embodiment of the present invention (hereinafter referred to as "this embodiment") will be described in detail. Note that the names of the respective constituent members (each constituent element) used in each embodiment may be different from the names of the respective constituent members (each constituent element) used in the background art.

[0020] <Configuration of Non-Aqueous Electrolyte Storage Element> The non-aqueous electrolyte storage element (hereinafter also simply referred to as "storage element") according to this embodiment includes an electrode body having a positive electrode, a negative electrode, and a separator, a non-aqueous electrolyte, and a container that houses the electrode body and the non-aqueous electrolyte. The electrode body is usually a laminated type in which a plurality of positive electrodes and a plurality of negative electrodes are laminated via a separator, or a wound type in which the positive electrode and the negative electrode are wound in a state of being laminated via a separator. The non-aqueous electrolyte exists in a state contained in the positive electrode, the negative electrode, and the separator. As an example of the non-aqueous electrolyte storage element, a non-aqueous electrolyte secondary battery (hereinafter also simply referred to as "secondary battery") will be described.

[0021] (Positive Electrode) The positive electrode has a positive electrode substrate and a positive electrode mixture layer disposed directly or via an intermediate layer on the positive electrode substrate.

[0022] The positive electrode substrate has conductivity. Whether it has "conductivity" is determined based on the volume resistivity measured in accordance with JIS-H-0505 (1975) with 10 7 Ω·cm as the threshold value. As the material of the positive electrode substrate, metals such as aluminum, titanium, tantalum, stainless steel, or alloys thereof are used. Among these, aluminum or an aluminum alloy is preferable from the viewpoints of potential resistance, high conductivity, and cost. Examples of the positive electrode substrate include foil, vapor deposition film, mesh, porous material, etc., and foil is preferable from the viewpoint of cost. Therefore, an aluminum foil or an aluminum alloy foil is preferable as the positive electrode substrate. Examples of aluminum or an aluminum alloy include A1085, A3003, A1N30, etc. defined in JIS-H-4000 (2014) or JIS-H4160 (2006).

[0023] 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, still 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, it is possible to increase the strength of the positive electrode substrate while increasing the energy density per unit volume of the secondary battery.

[0024] The intermediate layer is a layer disposed between the positive electrode substrate and the positive electrode 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 configuration of the intermediate layer is not particularly limited. For example, it contains a binder and a conductive agent.

[0025] The positive electrode mixture layer contains a positive electrode active material. The positive electrode mixture layer contains optional components such as a conductive agent, a binder, a thickener, and a filler as required. The positive electrode mixture layer is composed of a positive electrode mixture containing a positive electrode active material and the above optional components.

[0026] In this embodiment, the positive electrode active material contains a lithium transition metal compound having a carbon-coated polyanion structure. As the lithium transition metal compound having a polyanion structure, the general formula Li α M β (PO4) α (where M is one or more transition metals, and α and β are integers that satisfy the stoichiometric composition according to the valence of M). For example, when M is specifically one transition metal selected from Fe, Mn, Ni, Co, and V, there are LiFePO4, LiMnPO4, LiNiPO4, LiCoPO4, Li3V2(PO4)3, etc. When M is two or more transition metals, there are LiFe x Mn 1-x PO4(0 < x < 1), LiFe y M 1-y PO4 (M is one or more transition metals, 0.5 ≤ y < 1), etc. In addition, there are Li2MnSiO4, Li2CoPO4F, etc.

[0027] The lithium transition metal compound having a carbon-coated polyanion structure is preferably prepared by mixing raw materials of transition metal compounds containing Fe, Mn, Ni, Co, V, etc. with Li phosphates such as LiH2PO4 and carbon raw materials such as sucrose powder, and then firing. By controlling the type of carbon raw material, the mixing amount of the carbon raw material, the firing temperature, the firing time, etc. during the above firing, the ratio of the BET specific surface area increased by the carbon coating of the lithium transition metal compound having a polyanion structure to the BET specific surface area of the positive electrode mixture can be made 12% or more and 30% or less. For example, when preparing LiFePO4, the firing temperature is preferably from 650 °C to 700 °C, and the firing time is preferably from 2 to 12 hours.

[0028] In this embodiment, the positive electrode mixture layer may contain other positive electrode active materials other than the lithium transition metal compound having a polyanion structure, as long as it does not impair the compatibility between the low-temperature characteristics and the storage characteristics of the non-aqueous electrolyte storage element, which is an issue of the positive electrode mixture for non-aqueous electrolyte storage elements according to one aspect of the present invention. Examples of other cathode active materials include, for example, lithium transition metal composite oxides having an α-NaFeO2-type crystal structure, lithium transition metal composite oxides having a spinel-type crystal structure, chalcogen compounds, sulfur, and the like. Examples of the lithium transition metal composite oxide having an α-NaFeO2-type crystal structure include, for example, Li[Li x 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), 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), 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), 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), and the like. Examples of the lithium transition metal composite oxide having a spinel-type crystal structure include Li x Mn2O4, Li x Ni γ Mn (2-γ) O4, and the like. Examples of the chalcogen compound include titanium disulfide, molybdenum disulfide, molybdenum dioxide, and the like. Atoms or polyanions in these materials may be partially substituted with atoms or anion species composed of other elements. These materials may have their surfaces coated with other materials.

[0029] The positive electrode active material is usually in the form of 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 setting the average particle size of the positive electrode active material to be not less than the above lower limit, the production or handling of the positive electrode active material becomes easier. By setting the average particle size of the positive electrode active material to be not more than the above upper limit, the electron conductivity of the positive electrode mixture layer is improved. When a composite of the positive electrode active material and other materials is used, the average particle size of the composite is taken as the average particle size of the positive electrode active material. The "average particle size" means a value at which the volume-based cumulative distribution calculated in accordance with JIS-Z-8819-2 (2001) based on the particle size distribution measured by the laser diffraction / scattering method for a diluted solution in which particles are diluted with a solvent is 50%, in accordance with JIS-Z-8825 (2013).

[0030] To obtain powder with a predetermined particle size, a pulverizer, a classifier, or the like is used. Examples of the pulverization method include methods using a mortar, a ball mill, a sand mill, a vibration ball mill, a planetary ball mill, a jet mill, a counter jet mill, a swirling air flow type jet mill, or a sieve. During pulverization, wet pulverization in which water or an organic solvent such as hexane coexists can also be used. As the classification method, a sieve, an air classifier, or the like is used as needed for both dry and wet processes.

[0031] 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, both high energy density and manufacturability of the positive electrode mixture layer can be achieved.

[0032] As described above, in this embodiment, the positive electrode mixture substantially does not contain a conductive agent, but it does not exclude the inclusion of a trace amount of the conductive agent in the positive electrode mixture within a range that does not impede the compatibility of the low-temperature characteristics and storage characteristics of the non-aqueous electrolyte storage element. The conductive agent is not particularly limited as long as it is a material having conductivity. Examples of such conductive agents include carbonaceous materials, metals, conductive ceramics, etc. Examples of carbonaceous materials include graphite, non-graphitic carbon, graphene-based carbon, etc. Examples of non-graphitic carbon include carbon nanofibers, pitch-based carbon fibers, carbon black, etc. Examples of carbon black include furnace black, acetylene black, ketjen black, etc. Examples of graphene-based carbon include graphene, carbon nanotubes (CNT), fullerenes, etc. Examples of the shape of the conductive agent include powder form, fibrous form, etc. As the conductive agent, one of these materials may be contained alone, or two or more of them may be mixed and contained. Further, these materials may be contained in a composite form. For example, a material in which carbon black and CNT are composite may be contained.

[0033] The content of the conductive agent in the positive electrode mixture layer is less than 1% by mass, preferably 0.5% by mass or less, and more preferably 0% by mass. By setting the content of the conductive agent below the above upper limit, it is possible to achieve both the low-temperature characteristics and the storage characteristics of the non-aqueous electrolyte storage element.

[0034] Examples of the binder include thermoplastic resins such as fluororesins (polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), etc.), polyethylene, polypropylene, polyacrylic, polyimide, etc.; elastomers such as ethylene-propylene-diene rubber (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, etc.; polysaccharide polymers, etc.

[0035] The content of the binder in the positive electrode mixture 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 binder within the above range, the active material can be stably held.

[0036] Examples of the thickener include polysaccharide polymers such as carboxymethyl cellulose (CMC) and methyl cellulose. When the thickener has a functional group that reacts with lithium or the like, this functional group may be deactivated in advance by methylation or the like.

[0037] The filler is not particularly limited. Examples of the filler include polyolefins such as polypropylene and polyethylene, inorganic oxides such as silicon dioxide, alumina, titanium dioxide, calcium oxide, strontium oxide, barium oxide, and magnesium oxide, 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, and mineral resource-derived substances such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica, or artificial products thereof.

[0038] The positive electrode mixture layer may contain typical non-metal elements such as B, N, P, F, Cl, Br, and I, typical metal 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, and Nb as components other than the positive electrode active material, the conductive agent, the binder, the thickener, and the filler.

[0039] (Negative electrode) The negative electrode has a negative electrode substrate and a negative electrode mixture layer disposed directly or via an intermediate layer on the negative electrode substrate. The configuration of the intermediate layer is not particularly limited, and can be selected, for example, from the configurations exemplified for the positive electrode above.

[0040] The negative electrode substrate has conductivity. As the material of the negative electrode substrate, metals such as copper, nickel, stainless steel, nickel-plated steel, aluminum, or alloys thereof, carbonaceous materials, etc. are used. Among these, copper or a copper alloy is preferred. Examples of the negative electrode substrate include foil, vapor deposition film, mesh, porous material, etc., and foil is preferred from the perspective of cost. Therefore, a copper foil or a copper alloy foil is preferred as the negative electrode substrate. Examples of the copper foil include rolled copper foil, electrolytic copper foil, etc.

[0041] 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, still 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, it is possible to increase the strength of the negative electrode substrate while increasing the energy density per unit volume of the secondary battery.

[0042] The negative electrode binder layer contains a negative electrode active material. The negative electrode binder layer may contain optional components such as a conductive agent, a binder, a thickening agent, a filler, etc. as necessary. The optional components such as the conductive agent, the binder, the thickening agent, the filler, etc. can be selected from the materials exemplified in the above positive electrode.

[0043] The negative electrode binder layer may contain typical non-metal elements such as B, N, P, F, Cl, Br, I, etc., typical metal elements such as Li, Na, Mg, Al, K, Ca, Zn, Ga, Ge, Sn, Sr, Ba, etc., and transition metal elements such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Zr, Ta, Hf, Nb, W, etc. as components other than the negative electrode active material, the conductive agent, the binder, and the thickening agent.

[0044] The negative electrode active material can be appropriately selected from known negative electrode active materials. As the negative electrode active material for a lithium-ion secondary battery, a material that can usually occlude and release lithium ions is generally used. Examples of the negative electrode active material include metallic Li; metals or semi-metals such as Si and Sn; metal oxides or semi-metal oxides such as Si oxide, Ti oxide, and Sn oxide; Li4Ti5O 12 、LiTiO 2、Titanium-containing oxides such as TiNb2O7; polyphosphoric acid compounds; silicon carbide; carbon materials such as graphite (graphite), non-graphitic carbon (easily graphitizable carbon or hardly graphitizable carbon), etc. Among these materials, graphite and non-graphitic carbon are preferred. In the negative electrode binder layer, one of these materials may be used alone, or two or more of them may be used in combination.

[0045] "Graphite" refers to a carbon material having an average lattice plane spacing (d 002 ) of 0.33 nm or more and less than 0.34 nm as determined by X-ray diffraction before charge-discharge or in the discharged state. Examples of graphite include natural graphite and artificial graphite. From the viewpoint of obtaining a material with stable physical properties, artificial graphite is preferred.

[0046] "Non-graphitic carbon" refers to a carbon material having an average lattice plane spacing (d 002 ) of 0.34 nm or more and 0.42 nm or less as determined by X-ray diffraction before charge-discharge or in the discharged state. Examples of non-graphitic carbon include hardly graphitizable carbon and easily graphitizable carbon. Examples of non-graphitic carbon include, for example, resin-derived materials, petroleum pitch or petroleum pitch-derived materials, petroleum coke or petroleum coke-derived materials, plant-derived materials, alcohol-derived materials, etc.

[0047] Here, the "discharged state" means a state in which lithium ions that can be occluded and released during charge-discharge are sufficiently released from the carbon material that is the negative electrode active material. For example, in a half-cell using a negative electrode containing a carbon material as the working electrode and metallic Li as the counter electrode, it is a state where the open circuit voltage is 0.7 V or more.

[0048] "Hardly graphitizable carbon" refers to a carbon material having the above d 002 of 0.36 nm or more and 0.42 nm or less.

[0049] "Easily graphitizable carbon" refers to a carbon material having the above d 002 of 0.34 nm or more and less than 0.36 nm.

[0050] 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, 1 nm or more and 100 μm or less. When the negative electrode active material is a carbon material, a titanium-containing oxide, or a polyphosphoric acid compound, its average particle size may be 1 μm or more and 100 μm or less. When the negative electrode active material is Si, Sn, a Si oxide, or a Sn oxide, etc., its average particle size may be 1 nm or more and 1 μm or less. By setting the average particle size of the negative electrode active material to be equal to or greater than the above lower limit, the production or handling of the negative electrode active material becomes easier. By setting the average particle size of the negative electrode active material to be equal to or less than the above upper limit, the electron conductivity of the negative electrode binder layer is improved. To obtain a powder with a predetermined particle size, a pulverizer, a classifier, etc. are used. The pulverization method and the classification method can be selected, for example, from the methods exemplified for the positive electrode above. When the negative electrode active material is a metal such as metallic Li, the negative electrode active material may be in the form of a foil.

[0051] The content of the negative electrode active material in the negative electrode binder 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, both high energy density and manufacturability of the negative electrode binder layer can be achieved.

[0052] (Separator) The separator can be appropriately selected from known separators. As the separator, for example, a separator composed only of a base material layer, a separator in which a heat-resistant layer containing heat-resistant particles and a binder is formed on one or both surfaces of the base material layer, etc. can be used. Examples of the shape of the base material layer of the separator include woven fabric, non-woven fabric, porous resin film, etc. Among these shapes, a porous resin film is preferable from the viewpoint of strength, and a non-woven fabric is preferable from the viewpoint of the liquid retention property of the non-aqueous electrolyte. As the material of the base material layer of the separator, polyolefins such as polyethylene and polypropylene are preferable from the viewpoint of the shut-down function, and polyimides, aramids, etc. are preferable from the viewpoint of oxidation decomposition resistance. As the base material layer of the separator, a material in which these resins are compounded may also be used.

[0053] 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 at 1 atm, and more preferably have a mass loss of 5% or less when heated from room temperature to 800 °C. Examples of materials with a mass loss of less than or equal to a predetermined value include inorganic compounds. 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; and substances derived from mineral resources such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica, or artificial products thereof. As the inorganic compound, these substances may be used alone or in combination, or two or more of them may be mixed and used. Among these inorganic compounds, from the viewpoint of the safety of the energy storage element, silicon oxide, aluminum oxide, or aluminosilicate is preferable.

[0054] From the viewpoint of strength, the porosity of the separator is preferably 80% by volume or less, and from the viewpoint of discharge performance, it is preferably 20% by volume or more. Here, the "porosity" is a value based on volume and means the measured value by a mercury porosimeter.

[0055] As the separator, a polymer gel composed of a polymer and a non-aqueous electrolyte may be used. Examples of the polymer include polyacrylonitrile, polyethylene oxide, polypropylene oxide, polymethyl methacrylate, polyvinyl acetate, polyvinyl pyrrolidone, polyvinylidene fluoride, and the like. Using a polymer gel has the effect of suppressing liquid leakage. As the separator, a porous resin film or non-woven fabric as described above and a polymer gel may be used in combination.

[0056] (Non-aqueous electrolyte) The non-aqueous electrolyte can be appropriately selected from known non-aqueous electrolytes. A non-aqueous electrolyte solution may be used as the non-aqueous electrolyte. The non-aqueous electrolyte solution contains a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent.

[0057] The non-aqueous solvent can be appropriately selected from known non-aqueous solvents. Examples of the non-aqueous solvent include cyclic carbonates, chain carbonates, carboxylic acid esters, phosphate esters, sulfonic acid esters, ethers, amides, nitriles, etc. As the non-aqueous solvent, those in which some of the hydrogen atoms contained in these compounds are substituted with halogens may be used.

[0058] Examples of the cyclic carbonate include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinylene carbonate (VC), vinyl ethylene carbonate (VEC), chloroethylene carbonate, fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), styrene carbonate, 1-phenylvinylene carbonate, 1,2-diphenylvinylene carbonate, etc. Among these, EC is preferred.

[0059] Examples of the chain carbonate include diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diphenyl carbonate, trifluoroethyl methyl carbonate, bis(trifluoroethyl) carbonate, etc. Among these, EMC is preferred.

[0060] 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 cyclic carbonate and a chain carbonate in combination. By using a cyclic carbonate, dissociation of the electrolyte salt can be promoted and the ionic conductivity of the non-aqueous electrolyte can be improved. 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 in combination, 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.

[0061] The electrolyte salt can be appropriately selected from known electrolyte salts. Examples of the electrolyte salt include lithium salts, sodium salts, potassium salts, magnesium salts, onium salts, etc. Among these, lithium salts are preferable.

[0062] Examples of the lithium salt include inorganic lithium salts such as LiPF6, LiPO2F2, LiBF4, LiClO4, LiN(SO2F)2, lithium oxalate salts such as lithium bis(oxalate)borate (LiBOB), lithium difluorooxalate borate (LiFOB), lithium bis(oxalate)difluorophosphate (LiFOP), and lithium salts having a halogenated hydrocarbon group such as LiSO3CF3, LiN(SO2CF3)2, LiN(SO2C2F5)2, LiN(SO2CF3)(SO2C4F9), LiC(SO2CF3)3, LiC(SO2C2F5)3. Among these, inorganic lithium salts are preferable, and LiPF6 is more preferable.

[0063] The content of the electrolyte salt in the non-aqueous electrolyte is preferably 0.1 mol / dm 3 or more and 2.5 mol / dm 3 or less at 20 °C and 1 atm, more preferably 0.3 mol / dm 3 or more and 2.0 mol / dm 3 or less, even more preferably 0.5 mol / dm 3 or more and 1.7 mol / dm 3 or less, and still more preferably 0.7 mol / dm 31.5 mol / dm or less is particularly preferred. By setting the content of the electrolyte salt within the above range, the ionic conductivity of the non-aqueous electrolyte can be increased. 3 It is particularly preferable that it is as follows. By setting the content of the electrolyte salt within the above range, the ionic conductivity of the non-aqueous electrolyte can be increased.

[0064] The non-aqueous electrolyte may contain additives in addition to the non-aqueous solvent and the electrolyte salt. Examples of the additives include halogenated carbonates such as fluoroethylene carbonate (FEC) and difluoroethylene carbonate (DFEC); 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, alkyl biphenyl, terphenyl, partially hydrogenated terphenyl, cyclohexylbenzene, t-butylbenzene, t-amylbenzene, diphenyl ether, and dibenzofuran; partially halogenated compounds of the 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, methyl vinylene carbonate, ethyl vinylene carbonate, succinic anhydride, glutaric anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, cyclohexanedicarboxylic anhydride; ethylene sulfite, propylene sulfite, dimethyl sulfite, propane sultone, propene sultone, butane sultone, methyl methanesulfonate, busulfan, methyl toluenesulfonate, dimethyl sulfate, ethylene sulfate, sulfolane, dimethyl sulfone, diethyl sulfone, dimethyl sulfoxide, diethyl sulfoxide, tetramethylene sulfoxide, diphenyl sulfide, 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-propene sultone, 1,3-propane sultone, 1,4-butane sultone, 1,4-butene sultone, perfluorooctane, tris(trimethylsilyl) borate, tris(trimethylsilyl) phosphate, tetrakis(trimethylsilyl) titanate, lithium monofluorophosphate, and lithium difluorophosphate, etc.These additives may be used alone or in combination of two or more.

[0065] The content of the additive contained in the non-aqueous electrolyte is preferably 0.01% by mass or more and 10% by mass or less, more preferably 0.1% by mass or more and 7% by mass or less, still more preferably 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 with respect to the mass of the entire non-aqueous electrolyte. By setting the content of the additive within the above range, the capacity retention performance or cycle performance after high-temperature storage can be improved, or the safety can be further improved.

[0066] A solid electrolyte may be used for the non-aqueous electrolyte, or a non-aqueous electrolyte and a solid electrolyte may be used in combination.

[0067] As the solid electrolyte, any material having ion conductivity such as lithium, sodium, calcium, etc. and being solid at normal temperature (for example, from 15°C to 25°C) can be selected. Examples of the solid electrolyte include sulfide solid electrolytes, oxide solid electrolytes, oxynitride solid electrolytes, polymer solid electrolytes, and the like.

[0068] Examples of the sulfide solid electrolyte in the case of a lithium-ion secondary battery include, for example, Li2S-P2S5, LiI-Li2S-P2S5, Li 10 Ge-P2S 12 and the like.

[0069] The shape of the non-aqueous electrolyte storage element of this embodiment is not particularly limited, and examples include cylindrical batteries, prismatic batteries, flat batteries, coin-type batteries, button-type batteries, and the like. FIG. 1 shows a non-aqueous electrolyte storage element 1 as an example of a prismatic battery. Note that the figure is a perspective view of the inside of the container. An electrode body 2 having a positive electrode and a negative electrode wound with a separator interposed therebetween is housed in a prismatic container 3. The positive electrode is electrically connected to a positive electrode terminal 4 via a positive electrode lead 41. The negative electrode is electrically connected to a negative electrode terminal 5 via a negative electrode lead 51.

[0070] <Configuration of the energy storage device> The non-aqueous electrolyte energy storage element of the present embodiment can be mounted as an energy storage unit (battery module) configured by aggregating a plurality of non-aqueous electrolyte energy storage elements 1 in a power source for automobiles such as electric vehicles (EVs), hybrid vehicles (HEVs), plug-in hybrid vehicles (PHEVs), a power source for electronic devices such as personal computers and communication terminals, or a power source for power storage. In this case, the technology of the present invention may be applied to at least one non-aqueous electrolyte energy storage element included in the energy storage unit. FIG. 2 shows an example of an energy storage device 30 in which energy storage units 20 in which two or more non-aqueous electrolyte energy storage elements 1 electrically connected are further aggregated. The energy storage device 30 may include a bus bar (not shown) for electrically connecting two or more non-aqueous electrolyte energy storage elements 1, a bus bar (not shown) for electrically connecting two or more energy storage units 20, and the like. The energy storage unit 20 or the energy storage device 30 may include a state monitoring device (not shown) for monitoring the state of one or more non-aqueous electrolyte energy storage elements.

[0071] <Method for manufacturing a non-aqueous electrolyte energy storage element> The method for manufacturing the non-aqueous electrolyte energy 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 accommodating 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 the electrode body by laminating or winding the positive electrode and the negative electrode via a separator.

[0072] The method of accommodating the non-aqueous electrolyte in the container can be appropriately selected from known methods. For example, when a non-aqueous electrolyte solution is used as the non-aqueous electrolyte, the non-aqueous electrolyte solution may be injected from an injection port formed in the container and then the injection port may be sealed.

[0073] <Other embodiments> Furthermore, the non-aqueous electrolyte storage element of the present invention is not limited to the above-described embodiments, and various modifications may be made without departing from the gist of the present invention. For example, the configuration of 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 well-known techniques. Further, a part of the configuration of one embodiment may be deleted. In addition, well-known techniques may be added to the configuration of one embodiment.

[0074] In the above embodiment, the case where the non-aqueous electrolyte storage element is used as a non-aqueous electrolyte secondary battery (for example, a lithium ion secondary battery) capable of charge and discharge has been described. However, the type, shape, dimensions, capacity, etc. of the non-aqueous electrolyte 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.

[0075] In the above embodiment, the electrode body in which the positive electrode and the negative electrode are laminated via a separator has been described. However, the electrode body may not include a separator. For example, the positive electrode and the negative electrode may be in direct contact with each other in a state where a layer having no conductivity is formed on the mixture layer of the positive electrode or the negative electrode.

Examples

[0076] Hereinafter, the present invention will be described more specifically by way of examples. The present invention is not limited to the following examples.

[0077] In the following examples and comparative examples, a non-aqueous electrolyte secondary battery was fabricated as the non-aqueous electrolyte storage element, and its low-temperature characteristics and storage characteristics were evaluated.

[0078] [Example 1] (Fabrication of positive electrode active material) (Fabrication of LiFePO4) First, an FeSO4 aqueous solution of 1 mol / dm 3 was dropped into a reaction vessel containing ion-exchanged water at a constant rate, and a 4 mol / dm 3 NaOH aqueous solution and a 0.5 mol / dm 3An aqueous solution of NH3 and an aqueous solution of NH2NH2 at 0.5 mol / dm 3 were dropped to prepare a Fe(OH)2 precursor. Next, the prepared Fe(OH)2 precursor was taken out of the reaction vessel and solid-phase mixed with LiH2PO4 and sucrose powder. Then, it was calcined at 650 °C for 5 hours under a nitrogen atmosphere to prepare LiFePO4 (LFP) having a carbon-coated polyanion structure.

[0079] (Preparation of the positive electrode) The above-prepared LiFePO4 was used as the positive electrode active material, N-methylpyrrolidone (NMP) was used as the dispersion medium, and PVDF was used as the binder. The above positive electrode active material, binder, and dispersion medium were mixed. At that time, they were mixed so that the solid content mass ratio of the active material:binder was 95:5. An appropriate amount of the dispersion medium was added to these mixtures to adjust the viscosity, and a positive electrode mixture paste was prepared. Next, the above positive electrode mixture paste was applied to both sides of an aluminum foil, which is the positive electrode substrate, leaving an uncoated portion (a portion where the positive electrode mixture layer is not formed), dried at 120 °C, and roll-pressed to form a positive electrode mixture layer on the positive electrode substrate. The coating amount of the positive electrode mixture paste was 10 mg / cm in terms of solid content 2 and thus the positive electrode according to Example 1 was prepared.

[0080] (Preparation of the negative electrode) Graphite was used as the negative electrode active material, SBR was used as the binder, and CMC was used as the thickener. The negative electrode active material, binder, thickener, and water as the dispersant were mixed. At that time, they were mixed so that the solid content mass ratio of the active material:binder:thickener was 97:2:1. An appropriate amount of water was added to these mixtures to adjust the viscosity, and a negative electrode mixture paste was prepared. This negative electrode mixture paste was applied to both sides of a copper foil, which is the negative electrode substrate, leaving an uncoated portion (a portion where the negative electrode mixture layer is not formed), dried, and roll-pressed to prepare a negative electrode mixture layer on the negative electrode substrate. Thus, the negative electrode was prepared.

[0081] (Preparation of the non-aqueous electrolyte) LiPF6 was added to a mixed solvent in which EC and EMC were mixed at a volume ratio of 3:7 at 1 mol / dm3 It was dissolved at the concentration of

[0082] (Fabrication of non-aqueous electrolyte secondary battery) The positive electrode and the negative electrode were laminated and opposed to each other through a separator composed of a microporous membrane substrate made of polyethylene and a heat-resistant layer formed on the microporous membrane substrate made of polyethylene, thereby fabricating an electrode body. Note that the heat-resistant layer was disposed on the surface facing the positive electrode. This electrode body was housed in a square container made of aluminum, and a positive electrode terminal and a negative electrode terminal were attached. After injecting the above non-aqueous electrolyte into the square container, it was sealed to fabricate a non-aqueous electrolyte secondary battery according to Example 1.

[0083] (Capacity retention rate after storage) Regarding the fabricated non-aqueous electrolyte secondary battery, after constant current charging up to 3.6 V at a current of 0.1 C in an environment of 25°C, constant voltage charging was performed at 3.6 V. Then, it was discharged at a constant current up to 2.0 V at a current of 0.1 C. The discharge capacity obtained at that time was defined as the "discharge capacity before storage". Thereafter, this non-aqueous electrolyte secondary battery was charged at a constant current up to 3.6 V at a current of 0.1 C in an environment of 25°C, then constant voltage charging was performed at 3.6 V, and it was stored for 10 days in an environment of 85°C. Then, it was discharged at a constant current up to 2.0 V at a current of 0.1 C in an environment of 25°C, further charged at a constant current up to 3.6 V at a current of 0.1 C, and then constant voltage charging was performed at 3.6 V. Then, it was discharged at a constant current up to 2.0 V at a current of 0.1 C. The discharge capacity at that time was defined as the "discharge capacity after storage". The percentage of the "discharge capacity after storage" with respect to the "discharge capacity before storage" was calculated by the formula of "discharge capacity after storage" / "discharge capacity before storage"×100, and defined as the "capacity retention rate after storage".

[0084] (0°C / 25°C 1C capacity ratio) Regarding the fabricated non-aqueous electrolyte secondary battery described above, after constant current charging at a current of 0.1C up to 3.6V under a 25°C environment, constant voltage charging was performed at 3.6V. Subsequently, constant current discharging was performed at a current of 1C up to 2.0V under a 0°C environment and a 25°C environment. The percentage of the discharge capacity under the 0°C environment with respect to the discharge capacity under the 25°C environment obtained at that time was calculated and defined as the "0°C / 25°C 1C capacity ratio".

[0085] [Examples 2 to 6] Except for changing the firing temperature when fabricating LiFePO4 from 650°C to 660°C, 670°C, 680°C, 690°C, and 700°C respectively to fabricate LiFePO4 with different BET specific surface areas increased by carbon coating, non-aqueous electrolyte secondary batteries according to Examples 2 to 6 were fabricated in the same manner as in Example 1.

[0086] [Comparative Examples 1 to 3] Except for changing the firing temperature when fabricating LiFePO4 from 650°C to 630°C, 610°C, and 720°C respectively to fabricate LiFePO4 with different BET specific surface areas increased by carbon coating, non-aqueous electrolyte secondary batteries according to Comparative Examples 1 to 3 were fabricated in the same manner as in Example 1.

[0087] [Comparative Examples 4 and 5] Except for changing the firing temperature when fabricating LiFePO4 from 650°C to 655°C and 600°C respectively, LiFePO4 having a carbon-coated polyanion structure of Comparative Examples 4 and 5 was fabricated in the same manner as in Example 1. As the positive electrode active material, the prepared LiFePO4 above was used. As the dispersion medium, N-methylpyrrolidone (NMP) was used. As the conductive agent, acetylene black was used. And as the binder, PVDF was used. The above positive electrode active material, conductive agent, binder and dispersion medium were mixed. At that time, the solid content mass ratio of the active material:conductive agent:binder was mixed to be 90:5:5 (the content of the conductive agent was 5% by mass based on the positive electrode mixture). An appropriate amount of the dispersion medium was added to these mixtures to adjust the viscosity, and a positive electrode mixture paste was prepared. Next, the above positive electrode mixture paste was applied to both sides of the aluminum foil which was the positive electrode substrate, leaving an uncoated part (a part where the positive electrode mixture layer was not formed), dried at 120 °C, and roll-pressed to form a positive electrode mixture layer on the positive electrode substrate. The coating amount of the positive electrode mixture paste was 10 mg / cm 2 in terms of solid content. In this way, the positive electrodes according to Comparative Example 4 and Comparative Example 5 were prepared. Except for the above, in the same manner as in Example 1, non-aqueous electrolyte secondary batteries according to Comparative Example 4 and Comparative Example 5 were prepared.

[0088] Regarding the non-aqueous electrolyte secondary batteries according to Example 1 to Example 6, Comparative Example 1 to Comparative Example 5, the capacity retention rate after storage and the 0 °C / 25 °C 1C capacity ratio were determined by the above-described method. Also, by the above-described method, the ratio of the BET specific surface area increased by the carbon coating of LiFePO4 having a polyanion structure to the BET specific surface area of the positive electrode mixture (BET increase by the carbon coating of LFP / BET ratio of the whole mixture) was determined. The results are shown in Table 1.

[0089]

Table 1

[0090] From Table 1, it can be seen that for the positive electrode mixture, the ratio of the BET specific surface area increased by the carbon coating of LiFePO4, which is a lithium transition metal compound having a polyanion structure, to the BET specific surface area of the positive electrode mixture (denoted as "BET increase by carbon coating of LFP / BET ratio of the entire mixture" in Table 1) is 12% or more and 30% or less, and for Examples 1 to 6 of non-aqueous electrolyte secondary batteries using a positive electrode having a positive electrode mixture not containing acetylene black (AB) (denoted as "AB in the mixture" in Table 1), it is found that both the 0°C / 25°C 1C capacity ratio and the capacity retention rate after storage are high, and the low-temperature characteristics and storage characteristics are compatible. On the other hand, for Comparative Examples 1 and 2 of non-aqueous electrolyte secondary batteries using a positive electrode having a positive electrode mixture with the above BET specific surface area ratio of less than 12% and not containing AB, the capacity retention rate after storage is high, but the 0°C / 25°C 1C capacity ratio is low, and the low-temperature characteristics and storage characteristics cannot be made compatible. Also, for the non-aqueous electrolyte secondary battery according to Comparative Example 3 using a positive electrode having a positive electrode mixture with the above BET specific surface area ratio of more than 30% and not containing AB, the 0°C / 25°C 1C capacity ratio is high, but the capacity retention rate after storage is low, and the low-temperature characteristics and storage characteristics cannot be made compatible. On the other hand, for Comparative Examples 4 and 5 of non-aqueous electrolyte secondary batteries using a positive electrode having a positive electrode mixture containing 5% of acetylene black (AB) as a conductive agent, even when the above BET specific surface area ratio is in the range of 12% or more and 30% or less, or less than 12%, the 0°C / 25°C 1C capacity ratio is high, but the capacity retention rate after storage is low, and the low-temperature characteristics and storage characteristics cannot be made compatible. Therefore, from the present examples, it can be said that by setting the ratio of the BET specific surface area increased by the carbon coating of the lithium transition metal compound having a polyanion structure to the BET specific surface area of the positive electrode mixture to 12% or more and 30% or less, and substantially not containing a conductive agent in the positive electrode mixture, the low-temperature characteristics and storage characteristics can be made compatible.

Industrial Applicability

[0091] By using the positive electrode active material according to one aspect of the present invention, a non-aqueous electrolyte storage element having both low-temperature characteristics and storage characteristics can be provided. Therefore, this non-aqueous electrolyte storage element is useful as a non-aqueous electrolyte storage element for hybrid vehicles, plug-in hybrid vehicles, and electric vehicles.

Description of Symbols

[0092] 1 Non-aqueous electrolyte storage element 2 Electrode body 3 Container 4 Positive electrode terminal 41 Positive electrode lead 5 Negative electrode terminal 51 Negative electrode lead 20 Power storage unit 30 Power storage device

Claims

Claim 1 The positive electrode mixture contains, as a positive electrode active material, a lithium transition metal compound having a carbon-coated polyanion structure, wherein the ratio of the BET specific surface area increased by the carbon coating of the lithium transition metal compound having the polyanion structure to the BET specific surface area of the positive electrode mixture is 12% or more and 25% or less, and the positive electrode mixture for a non-aqueous electrolyte storage element substantially does not contain a conductive agent. Claim 2 A positive electrode for a non-aqueous electrolyte storage element having the positive electrode mixture according to Claim 1. Claim 3 A non-aqueous electrolyte storage element including the positive electrode according to Claim 2.

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