Energy storage element and energy storage device

Abstract

A power storage element according to an aspect of the present invention comprises a positive electrode, which has a positive electrode active material layer containing a positive electrode active material, and a non-aqueous electrolyte, wherein: the positive electrode active material contains a transition metal element and also contains a polyanion compound having a surface of which at least a part is covered with carbon; the ratio of a second BET specific surface area, which is the BET specific surface area of the carbon, to a first BET specific surface area, which is the BET specific surface area of the positive electrode active material layer, is greater than 10% and less than 35%; and the non-aqueous electrolyte contains a sulfur element-free electrolyte salt and a sulfur-based compound.

Description

Technical Field

[0001] The present invention relates to a power storage element and a power storage device.

Background Art

[0002] Non-aqueous electrolyte secondary batteries typified by lithium ion secondary batteries are widely used in electronic devices such as personal computers and communication terminals, and automobiles, etc. because of their high energy density. The above non-aqueous electrolyte secondary battery generally has a pair of electrodes electrically isolated by a separator, and a non-aqueous electrolyte interposed between the electrodes, and is configured to charge and discharge by transferring charge carrier ions between both electrodes. Further, as a power storage element other than the non-aqueous electrolyte secondary battery, capacitors such as lithium ion capacitors and electric double layer capacitors are also widely spread.

[0003] In recent years, polyanion compounds such as inexpensive and highly safe olivine-type positive electrode active materials have been attracting attention as the positive electrode active material used in the above power storage element. For example, in the case of an olivine-type positive electrode active material, since the electron conductivity is low, it has been difficult to obtain a discharge capacity close to the theoretical capacity, but a technique of coating the surface with carbon has been proposed in order to improve the electron conductivity (see Patent Document 1).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] When attempting to apply a power storage element to a starting battery for an automobile or the like, initial output characteristics in a low-temperature environment are required. Since the initial output characteristics in this low-temperature environment are also affected by factors other than the electron conductivity of the positive electrode active material, further improvement is required even when using a polyanion compound having a carbon coating on the surface.

[0006] The objective of the present invention is to provide an energy storage element and an energy storage device that have high initial output in low-temperature environments. [Means for solving the problem]

[0007] A power storage element according to one aspect of the present invention comprises a positive electrode having a positive electrode active material layer containing a positive electrode active material, and a non-aqueous electrolyte, wherein the positive electrode active material contains a polyanionic compound that includes a transition metal element and whose surface is coated with carbon at least a portion thereof, the ratio of the second BET specific surface area (BET specific surface area of ​​the carbon) to the first BET specific surface area (BET specific surface area of ​​the positive electrode active material layer) is greater than 10% and less than 35%, and the non-aqueous electrolyte contains an electrolyte salt that does not contain sulfur elements and a sulfur-based compound.

[0008] A power storage device according to another aspect of the present invention comprises two or more power storage elements, and one or more power storage elements according to the other aspect of the present invention. [Effects of the Invention]

[0009] One aspect of the present invention relates to an energy storage element that exhibits high initial output in low-temperature environments.

[0010] Another aspect of the present invention relates to a power storage device that has high initial output in low-temperature environments. [Brief explanation of the drawing]

[0011] [Figure 1] Figure 1 is a perspective view showing one embodiment of an energy storage element. [Figure 2] Figure 2 is a schematic diagram showing one embodiment of an energy storage device configured by assembling multiple energy storage elements. [Modes for carrying out the invention]

[0012] First, an overview of the energy storage elements disclosed herein will be provided.

[0013] A power storage element according to one aspect of the present invention comprises a positive electrode having a positive electrode active material layer containing a positive electrode active material, and a non-aqueous electrolyte, wherein the positive electrode active material contains a polyanionic compound that includes a transition metal element and whose surface is coated with carbon at least a portion thereof, the ratio of the second BET specific surface area (BET specific surface area of ​​the carbon) to the first BET specific surface area (BET specific surface area of ​​the positive electrode active material layer) is greater than 10% and less than 35%, and the non-aqueous electrolyte contains an electrolyte salt that does not contain sulfur elements and a sulfur-based compound.

[0014] The energy storage element exhibits high initial output in low-temperature environments. While the reason for this is not entirely clear, it can be inferred, for example, as follows: When the ratio of the second BET specific surface area to the first BET specific surface area is less than 35%, the amount of carbon coating is relatively reduced, improving the diffusion of charge carrier ions such as lithium ions. On the other hand, when the ratio of the second BET specific surface area to the first BET specific surface area is greater than 10%, the contact resistance between positive electrode active materials in the positive electrode active material layer can be reduced. Thus, by ensuring that the ratio of the second BET specific surface area to the first BET specific surface area satisfies the above range, the initial output of the energy storage element in low-temperature environments can be increased. In addition, because the non-aqueous electrolyte contains a sulfur-based compound, a relatively low-resistance film is formed on the surface of the negative electrode of the energy storage element, further increasing the initial output of the energy storage element in low-temperature environments. Therefore, it is presumed that the energy storage element has high initial output in low-temperature environments, due to the combination of the ratio of the second BET specific surface area to the first BET specific surface area satisfying the above range and the non-aqueous electrolyte containing the above sulfur-based compound.

[0015] The above-mentioned "BET specific surface area" is determined by immersing the sample in liquid nitrogen and supplying nitrogen gas, which causes nitrogen molecules to physically adsorb onto the particle surface. The pressure and amount of nitrogen adsorbed at that time are then measured. The BET specific surface area is measured using the following method: Using a specific surface area measuring device manufactured by Yuasa Ionics Corporation (product name: MONOSORB), the amount of nitrogen adsorbed onto the sample (m³) is measured. 2The BET specific surface area (m²) is calculated by dividing the obtained adsorption amount by the mass (g) of the sample. 2 Let's assume it's / g).

[0016] The specific surface area of ​​the first BET is measured by the following method. For measuring the first BET specific surface area described above, if the positive electrode before the energy storage element is available, the sample of the positive electrode active material layer taken from that positive electrode will be used directly for measurement. On the other hand, if the measurement sample is taken from the positive electrode extracted by disassembling the energy storage element, the sample of the positive electrode active material layer to be used for measuring the first BET specific surface area described above will be prepared by the following method: Discharge the energy storage element at a constant current of 0.05C to the lower limit voltage during normal use. Disassemble the energy storage element, take out the positive electrode and use it as the working electrode, and assemble a half-cell with metallic Li as the counter electrode. At a current value of 10mA per gram of positive electrode active material, the potential of the working electrode is 2.0V vs. Li / Li + Constant current discharge is performed until the desired result is reached. The half-cell is disassembled, the working electrode is removed, and thoroughly washed with dimethyl carbonate. After drying under reduced pressure at room temperature for 24 hours, the powder of the positive electrode active material layer taken from the positive electrode is used as the measurement sample for the positive electrode active material layer to be used for the measurement of the first BET specific surface area. Next, 1.00 g of the positive electrode active material layer powder is placed in a sample tube for measurement and dried under reduced pressure at 120°C for 12 hours to thoroughly remove moisture from the sample. Then, it is cooled with liquid nitrogen and evacuated, and the adsorption isotherm is measured by nitrogen gas adsorption method in the range of 0 to 1 for relative pressure P / P0 (P0 = approximately 770 mmHg). Five points are extracted from the region of P / P0 = 0.05 to 0.3 of the obtained adsorption isotherm, and a BET plot is performed. The first BET specific surface area is calculated from the y-intercept and slope of the line. The dismantling of the energy storage element and the sampling of the positive electrode active material layer powder are carried out in an argon atmosphere with a dew point of -60°C or lower.

[0017] The second BET specific surface area is measured by the following method. The sample of the positive electrode active material layer to be used for measuring the second BET specific surface area is prepared by the following method. First, the powder of the positive electrode active material layer, collected in the same manner as the sample of the positive electrode active material layer to be used for measuring the first BET specific surface area, is subjected to wind classification or the like to remove any arbitrary components such as conductive agents mixed in the powder of the positive electrode active material layer. Then, the powder of the positive electrode active material surface coated with carbon is collected and its BET specific surface area is determined by the method described above. Next, the positive electrode active material surface coated with carbon is heat-treated at 400°C for 2 hours in an air atmosphere to remove the carbon coating and obtain the powder of the positive electrode active material. Next, the obtained powder of the positive electrode active material is collected and its BET specific surface area is determined by the method described above. Furthermore, the second BET specific surface area, which is the BET specific surface area of ​​the carbon, is calculated by determining the difference between the BET specific surface area of ​​the positive electrode active material surface coated with carbon and the BET specific surface area of ​​the positive electrode active material. Then, calculate the ratio (%) of the calculated 2nd BET specific surface area to the measured 1st BET specific surface area.

[0018] In this energy storage element, the positive electrode active material layer may contain substantially no conductive agent. When the positive electrode active material layer does not contain a conductive agent, the power retention rate of the energy storage element after high-temperature storage is increased. The reason for this is not entirely clear, but it can be speculated as follows: Typically, the particle size of the conductive agent is smaller than the particle size of the carbon-coated polyanion compound, so the larger the content of the conductive agent, the larger the BET specific surface area (first BET specific surface area) of the entire positive electrode active material layer. When the BET specific surface area of ​​the entire positive electrode active material layer increases, the contact area (reaction area) between the non-aqueous electrolyte and the polyanion compound increases, and transition metal elements in the polyanion compound may dissolve into the non-aqueous electrolyte. When these transition metal elements dissolve into the non-aqueous electrolyte, an increase in resistance is likely to occur after storage in a high-temperature environment. However, if the positive electrode active material layer substantially does not contain the conductive agent, the BET specific surface area of ​​the entire positive electrode active material layer decreases, resulting in a smaller contact area (reaction area) between the non-aqueous electrolyte and the polyanionic compound. Consequently, the increase in resistance after storage in a high-temperature environment is reduced. This is presumed to improve the power retention rate of the energy storage element after high-temperature storage.

[0019] A power storage device according to another aspect of the present invention comprises two or more power storage elements, and one or more power storage elements according to the other aspect of the present invention.

[0020] Because this energy storage device is equipped with energy storage elements that have high initial output in low-temperature environments, the initial output in low-temperature environments is enhanced.

[0021] The configuration of the energy storage element, the configuration of the energy storage device, the method for manufacturing the energy storage element, and other embodiments related 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.

[0022] <Configuration of energy storage element> [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. The positive electrode active material layer contains a positive electrode active material. The positive electrode active material contains a transition metal element and a polyanionic compound whose surface is coated with carbon at least partially.

[0023] (Positive electrode substrate) 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 (1975), which is 10 7 The 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 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] (Cathode active material layer) The positive electrode active material layer contains a positive electrode active material. The positive electrode active material layer may contain optional components such as a binder, a thickener, a filler, etc. as required. On the other hand, the positive electrode active material layer may or may not contain a conductive agent. The positive electrode active material contains a polyanion compound containing a transition metal element and having at least a part of its surface coated with carbon.

[0027] The polyanion compound can occlude and release ions. The polyanion compound is a compound containing an oxoacid anion (PO4 3- , SO4 2- , SiO4 4- , BO3 3- , VO4 3- , etc.), a transition metal element, and an alkali metal element or an alkaline earth metal element. The oxoacid anion may be a condensed anion (P2O7 4- , P3O 10 5- , etc.). The polyanion compound may further contain other elements (such as halogen elements, etc.). As the oxoacid anion of the polyanion compound, a phosphate anion (PO4 3- ) is preferred. As the transition metal element of the polyanion compound, an iron element, a manganese element, a nickel element, and a cobalt element are preferred, and an iron element is more preferred. As the alkali metal element or alkaline earth metal element of the polyanion compound, a lithium element is preferred.

[0028] The polyanion compound is preferably a compound represented by the following formula 1. Li a M b (AO c ) d X e ···1 In Formula 1, M is at least one transition metal element. A is at least one selected from B, Al, Si, P, S, Cl, Ti, V, Cr, Mo, and W. X is at least one halogen element. a, b, c, d, and e are numbers satisfying 0 < a ≤ 3, 0 < b ≤ 2, 2 ≤ c ≤ 4, 1 ≤ d ≤ 3, and 0 ≤ e ≤ 1. a, b, c, d, and e may all be integers or may be decimals.

[0029] As M in Formula 1, it is preferably to contain at least one of Fe, Mn, Ni, and Co, more preferably the total content ratio of Fe, Mn, Ni, and Co in M is 50 mol% or more, still more preferably the content ratio of at least one of Fe, Mn, Ni, and Co in M is 50 mol% or more, and even more preferably the content ratio of Fe in M is 50 mol% or more. Also, as M, it is preferably at least one of Fe, Mn, Ni, and Co, and preferably Fe. As A, P is preferred. As X, F is preferred. In one embodiment, a = 1, b = 1, c = 4, d = 1, and e = 0 may be preferred in some cases.

[0030] Specific examples of the polyanion compound include, for example, LiFePO4, LiCoPO4, LiFe 0.5 Co 0.5 PO4, LiMnPO4, LiNiPO4, LiMn 0.5 Fe 0.5 PO4, LiCrPO4, LiFeVO4, Li2FeSiO4, Li2Fe2(SO4)3, LiFeBO3, LiFePO 3.9 F 0.2 , Li3V2(PO4)3, Li2MnSiO4, Li2CoPO4F, etc. Among these, LiFePO4 (lithium iron phosphate) is preferred. Atoms or polyanions in the polyanion compounds exemplified herein may be partially substituted with other atoms or anion species. The polyanion compound may be used alone or in combination of two or more.

[0031] Polyanionic compounds have at least a portion of their surface coated with carbon. This carbon coating, in this context, refers to carbon as an inorganic substance. The carbon coating on at least a portion of the polyanionic compound's surface improves its electronic conductivity.

[0032] Polyanion compounds are particles (powder). The average particle size of the polyanion compound is preferably, for example, 0.1 μm or more and 20 μm or less. Setting the average particle size of the polyanion compound above the lower limit facilitates the manufacture and handling of the polyanion compound. Setting the average particle size of the polyanion compound below the upper limit improves ion diffusion in the positive electrode active material layer. When using a composite of the polyanion compound and other materials such as carbon, the average particle size of the composite is considered the average particle size of the positive electrode active material. "Average particle size" refers to the value at which the volume-based integrated distribution, calculated according to JIS-Z-8819-2 (2001), is 50%, based on the particle size distribution measured by laser diffraction / scattering on a diluted solution obtained by diluting particles with a solvent, in accordance with JIS-Z-8825 (2013). Hereinafter, "average particle size" is synonymous with "average particle size".

[0033] 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 organic solvents such as hexane, can also be used during grinding. For classification, sieves and wind classifiers are used as needed, both dry and wet.

[0034] Polyanion compounds can be produced, for example, according to the following procedure. First, an aqueous solution of one or more of the above-mentioned transition metal element oxoate anion salts is mixed with an aqueous solution of sodium hydroxide (NaOH) in the presence of a buffer to produce a precursor which is a hydroxide of the above-mentioned transition metal element. Next, the prepared precursor is solid-phase mixed with lithium oxoate anion salt and a carbon raw material such as sucrose. Then, by calcining the resulting mixture in an inert atmosphere, a polyanion compound can be produced in which at least a portion of the surface is coated with carbon. Furthermore, the amount of carbon coating can be increased or decreased by increasing or decreasing the amount of carbon raw material such as sucrose added.

[0035] For example, if the polyanionic compound is lithium iron phosphate (LiFePO4), first, an aqueous solution of FeSO4 is added dropwise to the reaction vessel at a constant rate, while simultaneously adding aqueous solutions of NaOH, NH3, and NH2NH2 to maintain a constant pH, thereby producing an Fe(OH)2 precursor. Next, the prepared Fe(OH)2 precursor is removed from the reaction vessel and mixed in solid phase with LiH2PO4 and sucrose powder. Then, the resulting mixture is calcined in a nitrogen atmosphere at a calcination temperature of 550°C to 750°C, thereby producing a polyanionic compound in which LiFePO4 particles, as the polyanionic compound, are coated with carbon.

[0036] The polyanion compound content 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 polyanion compound content within the above range, it is possible to achieve both high energy density and manufacturability in the positive electrode active material layer.

[0037] The positive electrode active material may further contain positive electrode active materials other than polyanion compounds (hereinafter also referred to as "other positive electrode active materials"). Such other positive electrode active materials can be appropriately selected from known positive electrode active materials for lithium-ion secondary batteries. However, the lower limit of the total content of carbon-coated polyanion compounds in the positive electrode active material is preferably 90% by mass, and more preferably 99% by mass. The upper limit of the total content of carbon-coated polyanion compounds in the positive electrode active material may be 100% by mass. By using substantially only polyanion compounds as the positive electrode active material in this way, the initial output in the energy storage element under low-temperature conditions can be more reliably increased.

[0038] As the positive electrode active material for the above-mentioned known lithium-ion secondary battery, a material capable of intercalating and releasing lithium ions is usually used. Other positive electrode 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, etc. As an example of a lithium transition metal composite oxide having an α-NaFeO2 type crystal structure, 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) Examples include ]O2 (0≦x<0.5, 0<γ, 0<β, 0.5<γ+β<1). As a lithium transition metal composite oxide having a spinel-type crystal structure, Li xMn2O4, Li x Ni γ Mn (2-γ) Examples include O4. Examples of chalcogen compounds include titanium disulfide, molybdenum disulfide, and molybdenum dioxide. Some atoms in these materials may be substituted with atoms of other elements. The surfaces of these materials may be coated with other materials. In the positive electrode active material layer, one of these materials may be used alone, or two or more may be used in mixture form.

[0039] 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.

[0040] 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 active material can be stably maintained.

[0041] 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.

[0042] The filler is not particularly limited. Examples of fillers include polyolefins such as polypropylene and polyethylene, inorganic oxides such as silicon dioxide, alumina, titanium dioxide, calcium oxide, strontium oxide, barium oxide, magnesium oxide, and 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.

[0043] In the energy storage element, the positive electrode active material layer may also contain substantially no conductive agent. The carbon coating at least a portion of the surface of the polyanion compound is not considered a conductive agent. "Substantially containing no conductive agent in the positive electrode active material layer" means that the content of conductive agent in the positive electrode active material layer that would adversely affect the improvement of initial output in low-temperature environments, which is a problem of this embodiment, is substantially 0% by mass. However, this does not exclude the possibility of containing trace amounts of conductive agent in the positive electrode active material layer within a range that does not hinder the improvement of initial output in low-temperature environments. Specifically, "substantially containing no conductive agent in the positive electrode active material layer" means that the upper limit of the conductive agent content in the positive electrode active material layer is 2% by mass, more preferably 1% by mass, even more preferably 0.5% by mass, and particularly preferably 0% by mass.

[0044] When the positive electrode active material layer contains a conductive agent, 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. Alternatively, these materials may 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.

[0045] The positive 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 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.

[0046] The ratio of the second BET specific surface area to the first BET specific surface area is more than 10% and less than 35%, preferably 11% to 34%, more preferably 12% to 33%, even more preferably 13% to 32%, and even more preferably 14% to 31%. By satisfying the above range for the ratio of the second BET specific surface area to the first BET specific surface area, the initial output of the energy storage element in a low-temperature environment is increased.

[0047] The ratio of the second BET specific surface area to the first BET specific surface area can be adjusted by increasing or decreasing the amount of carbon coating on the polyanionic compound. Specifically, this can be adjusted by controlling the amount of carbon raw material such as sucrose added to the solid phase when preparing a polyanionic compound in which at least a portion of the surface is coated with carbon, and by controlling the calcination temperature of the resulting mixture.

[0048] The first BET specific surface area of ​​the positive electrode active material layer is 2.0 m². 2 / g or more 10.0m 2 It is preferable that it be less than or equal to / g, and 0.5m 2 / g or more 8.0m 2 It is more preferable that the value be less than or equal to / g. The advantage of the above-mentioned first BET specific surface area satisfying the above range is that it is possible to achieve both output characteristics and lifespan characteristics.

[0049] [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.

[0050] (Negative electrode substrate) 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.

[0051] 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 energy storage element.

[0052] (Negative electrode active material layer) 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.

[0053] 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.

[0054] 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.

[0055] "Graphite" refers to the average lattice plane spacing (d) of the (002) plane, determined by X-ray diffraction before charging or discharging, or during the discharge state. 002 ) 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. Artificial graphite is preferred from the standpoint of obtaining materials with stable physical properties.

[0056] "Non-graphite carbon" refers to the average lattice plane spacing (d) of the (002) plane, which is determined by X-ray diffraction before charging or during the discharge 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.

[0057] Here, "discharge state" refers to a state in which sufficient lithium ions that can be absorbed and released during charging and discharging are released from the carbon material, which is the negative electrode active material. 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 metallic Li as the counter electrode, this is the state in which the open-circuit voltage is 0.7V or higher.

[0058] "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.

[0059] "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.

[0060] 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.

[0061] 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.

[0062] [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.

[0063] 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.

[0064] 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.

[0065] 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.

[0066] [Non-aqueous electrolytes] The non-aqueous electrolyte contains an electrolyte salt that does not contain sulfur and a sulfur-based compound. A non-aqueous electrolyte solution may be used as the non-aqueous electrolyte. The non-aqueous electrolyte solution contains a non-aqueous solvent, the above-mentioned sulfur-free electrolyte salt dissolved in the non-aqueous solvent, and the above-mentioned sulfur-based compound. The above-mentioned sulfur-based compound corresponds to an additive other than the non-aqueous solvent and the electrolyte salt. In addition to the non-aqueous solvent, the above-mentioned sulfur-free electrolyte salt, and the above-mentioned sulfur-based compound, the non-aqueous electrolyte solution may also contain other additives.

[0067] As the non-aqueous solvent, any known non-aqueous solvent can be appropriately selected. Examples of non-aqueous solvents include cyclic carbonates, linear carbonates, carboxylic acid esters, phosphate esters, ethers, amides, and nitriles. As the non-aqueous solvent, compounds in which some of the hydrogen atoms contained in these compounds are substituted with halogens may also be used.

[0068] Examples of cyclic carbonates include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinylene carbonate (VC), vinylethylene carbonate (VEC), chloroethylene carbonate, fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), styrene carbonate, 1-phenylvinylene carbonate, and 1,2-diphenylvinylene carbonate. Among these, EC is preferred.

[0069] 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.

[0070] 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.

[0071] The non-aqueous electrolyte of the energy storage element contains a sulfur-based compound as an additive. This sulfur-based compound is not present in the electrolyte salt. The presence of the sulfur-based compound in the non-aqueous electrolyte forms a relatively low-resistance film on the surface of the negative electrode of the energy storage element, thereby increasing the initial output of the energy storage element in low-temperature environments.

[0072] Examples of the above sulfur-based compounds include chain compounds containing the element sulfur (sulfur-based chain compounds) and cyclic compounds containing the element sulfur (sulfur-based cyclic compounds). Examples of sulfur-based chain compounds include imide salts such as lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and lithium bis(pentafluoroethanesulfonyl)imide (LiBETI), as well as dimethyl sulfite, methyl methanesulfonate, busulfan, methyl toluenesulfonate, dimethyl sulfate, diethyl sulfate, dipropyl sulfate, dibutyl sulfate, dimethyl sulfone, diethylsulfone, dimethyl sulfoxide, and diethyl sulfoxide.

[0073] Examples of sulfur-based cyclic compounds include ethylene sulfite, propylene sulfite, sulfolane, thioanisole, tetramethylene sulfoxide, diphenyl sulfide, diphenyl disulfide, dipyridinium disulfide, compounds having a sultone structure, and compounds having a cyclic sulfate structure. Examples of compounds having a sultone structure include propanesultone, propensultone, butanesultone, and butensultone. Examples of compounds having the above-mentioned cyclic sulfate structure include 4,4'-bis(2,2-dioxo-1,3,2-dioxathiolane), 4-methylsulfonylethyl-2,2-dioxo-1,3,2-dioxathiolane, ethylene sulfate, 4-fluoro-2,2-dioxo-1,3,2-dioxathiolane, 4,5-difluoro-2,2-dioxo-1,3,2-dioxathiolane, propylene glycol sulfate, butylene glycol sulfate, pentene glycol sulfate, 4-5,dimethyl-dioxo-1,3,2-dioxathiolane, 4-fluorosulfonyloxymethyl-2,2-dioxo-1,3,2-dioxathiolane, and 4-methylsulfonyloxymethyl-2,2-dioxo-1,3 Examples include 2-dioxathiolane, 4-ethylsulfonyloxymethyl-2,2-dioxo-1,3,2-dioxathiolane, 4-trifluoromethylsulfonyloxymethyl-2,2-dioxo-1,3,2-dioxathiolane, 4-methylsulfonyloxymethyl-5-fluoro-2,2-dioxo-1,3,2-dioxathiolane, 4-methylsulfonyloxymethyl-5-methyl-2,2-dioxo-1,3,2-dioxathiolane, 4,4'-bis(5-fluoro-2,2-dioxo-1,3,2-dioxathiolane), 4,4'-bis(5-methyl-2,2-dioxo-1,3,2-dioxathiolane), and 4,4'-bis(5-ethyl-2,2-dioxo-1,3,2-dioxathiolane). Among these, sulfur-based cyclic compounds are preferred, and compounds having a sultone structure and compounds having a cyclic sulfate structure are more preferred. These sulfur-based compounds may be used individually or in combination of two or more.

[0074] The lower limit of the content of the sulfur-based compound in the non-aqueous electrolyte is preferably 0.1% by mass, more preferably 0.2% by mass, and still more preferably 0.3% by mass. On the other hand, the upper limit of this content is preferably 9% by mass, more preferably 8% by mass or less, and still more preferably 5% by mass. By setting the content of the sulfur-based compound to be above the lower limit and below the upper limit, the initial output of the energy storage element in a low-temperature environment can be more reliably increased.

[0075] The non-aqueous electrolyte may contain additives other than the sulfur-based compounds mentioned above (hereinafter also referred to as "other additives"). However, the lower limit of the total content of the sulfur-based compounds in the total additives contained in the non-aqueous electrolyte is preferably 50% by mass, and more preferably 70% by mass. The upper limit of the total content of the sulfur-based compounds in the total additives may be 100% by mass. By using substantially only the sulfur-based compounds as additives in this way, the initial output of the energy storage element in a low-temperature environment can be more reliably increased.

[0076] Other additives include, for example, 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); aromatic compounds such as biphenyl, alkylbiphenyl, terphenyl, partially hydrogenated terphenyl, cyclohexylbenzene, t-butylbenzene, t-amylbenzene, diphenyl ether, and dibenzofuran; and 2-fluorobiphenyl, o-cyclohexylfluorobenzene, and p-cyclohexyl phosphate. Examples include partial halides of the aforementioned aromatic compounds such as ruolobenzene; halogenated anisole compounds such as 2,4-difluoroanisole, 2,5-difluoroanisole, 2,6-difluoroanisole, and 3,5-difluoroanisole; vinylene carbonate, methylvinylene carbonate, ethylvinylene carbonate, succinic anhydride, glutaric anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, cyclohexanedicarboxylic acid anhydride; perfluorooctane, tristrimethylsilyl borate, tristrimethylsilyl phosphate, tetrakithtrimethylsilyl titanate, lithium monofluorophosphate, and lithium difluorophosphate. These additives may be used individually or in combination of two or more.

[0077] The above-mentioned electrolyte salt that does not contain sulfur element 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.

[0078] Examples of lithium salts include inorganic lithium salts such as LiPF6, LiPO2F2, LiBF4, and LiClO4, and lithium oxalate salts such as lithium bis(oxalate)borate (LiBOB), lithium difluorooxalate borate (LiFOB), and lithium bis(oxalate)difluorophosphate (LiFOP). Among these, inorganic lithium salts are preferred, and LiPF6 is more preferred.

[0079] The sulfur-free 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 content of sulfur-free electrolyte salts within the above range, the ionic conductivity of the non-aqueous electrolyte can be increased.

[0080] The shape of the 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. Figure 1 shows an example of a rectangular battery, specifically an energy storage element 1. 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.

[0081] <Configuration of the energy storage device> The energy storage elements of this embodiment can be mounted as an energy storage unit (battery module) comprising multiple energy storage elements in power supplies for vehicles such as electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs), power supplies for electronic devices such as personal computers and communication terminals, or power supplies for power storage. In this case, it is sufficient that the technology of the present invention is applied to at least one of the energy storage elements included in the energy storage unit. An energy storage device according to one embodiment of the present invention comprises two or more energy storage elements and one or more energy storage elements according to the above embodiment of the present invention (hereinafter referred to as the "second embodiment"). The energy storage device according to the second embodiment only needs to have the technology according to one embodiment of the present invention applied to at least one energy storage element included in the energy storage device, and may comprise one energy storage element according to the above embodiment of the present invention and one or more energy storage elements not relating to the above embodiment of the present invention, or may comprise two or more energy storage elements according to the above embodiment of the present invention. Figure 2 shows an example of a second embodiment of an energy storage device 30, which is formed by further assembling energy storage units 20, each comprising two or more electrically connected energy storage elements 1. The energy storage device 30 may include busbars (not shown) that electrically connect two or more 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 energy storage elements.

[0082] <Manufacturing method for energy storage elements> The method for manufacturing the energy storage element of this embodiment can be appropriately selected from known methods. The manufacturing method includes, for example, preparing an electrode body, preparing a non-aqueous electrolyte, and housing 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 stacking or winding the positive electrode and the negative electrode via a separator.

[0083] 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.

[0084] The negative electrode and separator can be prepared by known methods. The positive electrode can be prepared by known methods, except that a polyanionic compound containing the above-mentioned transition metal element and having at least a portion of its surface coated with carbon is used, and the conductive agent is handled as described above. The non-aqueous electrolyte can be prepared by known methods, except that the above-mentioned non-aqueous solvent, an electrolyte salt that does not contain sulfur elements, the above-mentioned sulfur-based compound as an additive, and any other additives are used.

[0085] <Other Embodiments> Furthermore, the energy storage element of the present invention is not limited to the embodiments described above, 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 with well-known technology. In addition, a part of the configuration of one embodiment may be deleted. Also, well-known technology may be added to the configuration of one embodiment.

[0086] In the above embodiment, the case in which the energy storage element is used as a rechargeable non-aqueous electrolyte secondary battery (e.g., a lithium-ion secondary battery) has been described, but the type, shape, dimensions, capacity, etc. of the 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.

[0087] In the above embodiment, an electrode body in which a positive electrode and a negative electrode are stacked with a separator in between has been described, but the electrode body does not need to have a separator. For example, the positive electrode and the negative electrode may be in direct contact with each other, with a non-conductive layer formed on the active material layer of either the positive electrode or the negative electrode. [Examples]

[0088] The present invention will be described in more detail below with reference to examples. The present invention is not limited to the following examples.

[0089] [Example 1] (Preparation of polyanion compounds) 750cm 3 2dm 3 Add 1 mol / dm³ to the reaction vessel. 3 While adding an aqueous FeSO4 solution dropwise at a constant rate, maintain a pH of 10.0 ± 0.1 during the process at a rate of 4 mol / dm³ 3 NaOH aqueous solution and 0.5 mol / dm 3 NH3 aqueous solution and 0.5 mol / dm 3 An aqueous solution of NH2NH2 was added dropwise to prepare an Fe(OH)2 precursor. The temperature of the reaction vessel was set to 50°C ± 2°C. Next, the prepared Fe(OH)2 precursor was removed from the reaction vessel, and 116 parts by mass of LiH2PO4 and 10 parts by mass of sucrose powder were solid-phase mixed with 100 parts by mass of the Fe(OH)2 precursor. The resulting mixture was then calcined at a calcination temperature of 650°C under a nitrogen atmosphere to prepare a polyanionic compound in which the polyanionic compound LiFePO4 is coated with carbon.

[0090] (Fabrication of the positive electrode) The polyanion compound obtained above, N-methylpyrrolidone (NMP) as the dispersion medium, and PVDF as the binder were used. The polyanion compound, binder, and dispersion medium were mixed. At that time, the solid content mass ratio of the positive electrode active material to the binder was set to 95:5, and an appropriate amount of dispersion medium was added to the mixture to adjust the viscosity and prepare a positive electrode active material layer paste. Next, the positive electrode active material layer paste was applied onto an aluminum foil as the positive electrode substrate, dried at 120°C, and roll-pressed to form a positive electrode active material layer on the positive electrode substrate. The amount of positive electrode active material layer paste applied was 10 mg / cm² in terms of solid content. 2 This is how the positive electrode was obtained.

[0091] (Measurement of BET specific surface area) The BET specific surface area of ​​the positive electrode active material layer obtained above was measured using the measurement method described above to obtain the first BET specific surface area. After measuring the BET specific surface area of ​​the carbon-coated polyanion compound obtained above using the measurement method described above, the coating carbon was removed by firing to obtain the polyanion compound. Next, the BET specific surface area of ​​the obtained polyanion compound was measured using the measurement method described above, and the difference between the BET specific surface area of ​​the carbon-coated polyanion compound and the BET specific surface area of ​​the polyanion compound from which the carbon was removed was calculated to obtain the second BET specific surface area. Then, the ratio (%) of the second BET specific surface area of ​​the carbon to the first BET specific surface area of ​​the obtained positive electrode active material layer was obtained. The results are shown in Table 1.

[0092] (Fabrication of the negative electrode) Graphite was used as the negative electrode active material, SBR as the binder, and CMC as the thickener. The negative electrode active material, binder, thickener, and water as the dispersion medium were mixed. The solid content mass ratio of negative electrode active material:binder:thickener was set to 97:2:1. An appropriate amount of water was added to the resulting mixture to adjust the viscosity and prepare a negative electrode mixture paste. This negative electrode mixture paste was applied to copper foil, which served as the negative electrode substrate, and dried to create a negative electrode active material layer on the substrate. Subsequently, a roll press was performed to produce the negative electrode.

[0093] (Preparation of non-aqueous electrolytes) A mixed solvent of EC and EMC in a volume ratio of 3:7 is mixed with 1 mol / dm³ of LiPF6, which is an electrolyte salt that does not contain sulfur. 3 The solution was dissolved at the specified concentration, and 4,4'-bis(2,2-dioxo-1,3,2-dioxathiolane), a sulfur-based compound, was dissolved at a concentration of 0.5% by mass as an additive to prepare a non-aqueous electrolyte.

[0094] (Fabrication of energy storage elements) Next, the positive electrode and the negative electrode were laminated together via a separator consisting of a polyethylene microporous membrane substrate and a heat-resistant layer formed on the polyethylene microporous membrane substrate to create an electrode body. The heat-resistant layer was positioned on the surface facing the positive electrode. This electrode body was placed in a rectangular aluminum container, and the positive electrode terminals and negative electrode terminals were attached. After the non-aqueous electrolyte was injected into the container, it was sealed to obtain the energy storage element of Example 1.

[0095] [Example 2] The energy storage element of Example 2 was fabricated in the same manner as in Example 1, except that the firing temperature for the polyanion compound was set to 675°C. For Example 2, the ratio (%) of the second BET specific surface area to the first BET specific surface area was calculated in the same manner as in Example 1. The results are shown in Table 1.

[0096] [Example 3] The energy storage element of Example 3 was fabricated in the same manner as in Example 2, except that 4-methylsulfonylethyl-2,2-dioxo-1,3,2-dioxathiolane was dissolved at a concentration of 0.5% by mass as a sulfur compound in the preparation of the non-aqueous electrolyte, instead of 4,4'-bis(2,2-dioxo-1,3,2-dioxathiolane) as an additive. In Example 3, the same positive electrode active material layer as in Example 2 was used, so as shown in Table 1, the ratio (%) of the second BET specific surface area to the first BET specific surface area in Example 3 is the same value as in Example 2.

[0097] [Example 4] In preparing the positive electrode, the energy storage element of Example 4 was prepared in the same manner as in Example 1, except that acetylene black was added as a conductive agent to the positive electrode active material layer paste. Specifically, the preparation of the positive electrode active material layer paste in Example 4 was carried out as follows: The above-mentioned polyanion compound, N-methylpyrrolidone (NMP) as a dispersion medium, acetylene black as a conductive agent, and PVDF as a binder were used. The above-mentioned polyanion compound, conductive agent, binder, and dispersion medium were mixed. At that time, the solid content mass ratio of the polyanion compound:conductive agent:binder was set to 90:5:5, and an appropriate amount of dispersion medium was added to the mixture to adjust the viscosity and prepare the positive electrode active material layer paste. For Example 4, the ratio (%) of the second BET specific surface area to the first BET specific surface area was calculated in the same manner as in Example 1. The results are shown in Table 1.

[0098] [Example 5] The energy storage element of Example 5 was fabricated in the same manner as in Example 1, except that the firing temperature for the polyanion compound was set to 690°C. For Example 5, the ratio (%) of the second BET specific surface area to the first BET specific surface area was calculated in the same manner as in Example 1. The results are shown in Table 1.

[0099] [Example 6] An energy storage element of Example 6 was fabricated in the same manner as in Example 1, except that the firing temperature for the polyanion compound was set to 700°C. For Example 6, the ratio (%) of the second BET specific surface area to the first BET specific surface area was calculated in the same manner as in Example 1. The results are shown in Table 1.

[0100] [Comparative Example 1] A storage element for Comparative Example 1 was fabricated in the same manner as in Example 1, except that the firing temperature for the polyanion compound was set to 630°C. For Comparative Example 1, the ratio (%) of the second BET specific surface area to the first BET specific surface area was calculated in the same manner as in Example 1. The results are shown in Table 1.

[0101] [Comparative Example 2] A storage element for Comparative Example 2 was fabricated in the same manner as in Example 1, except that the firing temperature for the polyanion compound was set to 720°C. For Comparative Example 2, the ratio (%) of the second BET specific surface area to the first BET specific surface area was calculated in the same manner as in Example 1. The results are shown in Table 1.

[0102] [Comparative Example 3] A power storage element for Comparative Example 3 was fabricated in the same manner as in Example 2, except that LiPO2F2 was dissolved at a concentration of 1% by mass instead of 4,4'-bis(2,2-dioxo-1,3,2-dioxathiolane) as an additive in the preparation of the non-aqueous electrolyte. In Comparative Example 3, the same positive electrode active material layer as in Example 2 was used, so as shown in Table 1, the ratio (%) of the second BET specific surface area to the first BET specific surface area in Comparative Example 3 is the same as in Example 2.

[0103] [Comparative Example 4] A storage element for Comparative Example 4 was fabricated in the same manner as in Example 2, except that lithium bis(oxalate) difluorophosphate (LiFOP) was dissolved at a concentration of 1% by mass as an additive in the preparation of the non-aqueous electrolyte. In Comparative Example 4, the same positive electrode active material layer as in Example 2 was used, so as shown in Table 1, the ratio (%) of the second BET specific surface area to the first BET specific surface area in Comparative Example 4 is the same as in Example 2.

[0104] [Comparative Example 5] A storage element for Comparative Example 5 was fabricated in the same manner as in Example 2, except that lithium bis(oxalate) borate (LiBOB) was dissolved at a concentration of 1% by mass instead of 4,4'-bis(2,2-dioxo-1,3,2-dioxathiolane) as an additive in the preparation of the non-aqueous electrolyte. In Comparative Example 5, the same positive electrode active material layer as in Example 2 was used, so as shown in Table 1, the ratio (%) of the second BET specific surface area to the first BET specific surface area in Comparative Example 5 is the same as in Example 2.

[0105] [Comparative Example 6, Comparative Example 7] In the preparation of the polyanion compounds, the energy storage elements of Comparative Examples 6 and 7 were fabricated in the same manner as in Example 1, except that the amount of sucrose powder mixed was 15 parts by mass and the firing temperature was 680°C and 750°C, respectively. For Comparative Examples 6 and 7, the ratio (%) of the second BET specific surface area to the first BET specific surface area was calculated in the same manner as in Example 1. The results are shown in Table 1.

[0106] [evaluation] (Capacity verification test) Each of the above energy storage elements was charged with a constant current of 0.1C to 3.6V in a 25°C environment, and then charged with a constant voltage at 3.6V. The charging was terminated when the charging current dropped to 0.02C. After a 10-minute rest period following charging, the elements were discharged with a constant current of 0.1C to 2.0V in a 25°C environment. A 10-minute rest period was followed after discharge. The above cycle was repeated twice.

[0107] (Initial power output performance test in a low-temperature environment) The initial output performance in a low-temperature environment was evaluated using the following procedure. Each of the above energy storage elements was charged with a constant current of 0.1C to 3.6V in a 25°C environment, and then charged with a constant voltage at 3.6V. The charging termination condition was when the charging current reached 0.02C. After a 10-minute rest period following charging, a constant current discharge was performed with a discharge current of 0.1C to 2.0V in a 25°C environment, and the "0.1C discharge capacity in a 25°C environment" was measured. Next, half of this "0.1C discharge capacity in a 25°C environment" was set as the State of Charge (SOC) 50%, and constant current charging was performed with a charging current of 0.1C in a 25°C environment from a fully discharged state until the SOC reached 50%. After that, the elements were stored in a -10°C environment for 3 hours, discharged with a discharge current of 0.1C for 30 seconds, a 10-minute rest period was observed, and then supplemental charging was performed with a charging current of 0.1C for 30 seconds until the SOC reached 50%. Similarly, the discharge current was adjusted to 0.3C and 0.5C, and each discharge was performed for 30 seconds. After a 10-minute rest period, supplemental charging was performed with a charging current of 0.1C until the State of Charge (SOC) reached 50%. The VI characteristic was plotted from the current and the voltage at 10 seconds after the start of discharge for each discharge. After linear approximation of the VI characteristic using the least squares method, the maximum output current value corresponding to the discharge termination voltage was calculated. Furthermore, the initial output in a low-temperature environment (-10°C) (shown as "Initial Output (-10°C)" in Table 1) was calculated by multiplying the above maximum output current value by the above discharge termination voltage. The above discharge termination voltage was set to 2.0V. The results of the initial output performance test in a low-temperature environment (-10°C) are shown in Table 1.

[0108] (Percentage of output retention after storage in a high-temperature environment) (1) Output performance tests at SOC 15% were conducted on the energy storage elements of Example 2 and Example 4 according to the following procedure. Constant current charging was performed with a charging current of 0.1C to 3.6V in a 25°C environment, and then constant voltage charging was performed at 3.6V. The charging termination condition was when the charging current became 0.02C. After a 10-minute rest period after charging, constant current discharge was performed with a discharge current of 0.1C to 2.0V in a 25°C environment, and the "0.1C discharge capacity in a 25°C environment" was measured. Next, the amount of electricity was set to "initial SOC 15%" at 15 / 100 of this "0.1C discharge capacity in a 25°C environment". Next, constant current charging was performed with a charging current of 0.1C from the fully discharged state until the amount of electricity reached the initial SOC 15%. Subsequently, the battery was discharged for 30 seconds at a discharge current of 0.1C, followed by a 10-minute rest period, and then supplemental charging for 30 seconds at a charging current of 0.1C until the SOC reached 15%. Similarly, the discharge and supplemental charging were performed in the same manner, except that the discharge current was changed to 0.3C and 0.5C, and the supplemental charging time was adjusted to reach 15% SOC. The VI characteristic was plotted from the current and voltage at 10 seconds after the start of discharge for each discharge. After linear approximation of the VI characteristic using the least squares method, the maximum output current value corresponding to the discharge termination voltage was calculated, and then the "output at the initial SOC of 15%" was obtained by multiplying the above maximum output current value by the above discharge termination voltage. The above discharge termination voltage was set to 2.0V. (2) Next, the energy storage elements of Example 2 and Example 4 were charged with a constant current of 0.1C to 3.6V in a 25°C environment, and then charged with a constant voltage at 3.6V. The charging termination condition was when the charging current became 0.02C. After charging to 100% SOC in this manner, they were stored in a constant temperature bath at 85°C for 10 days. After 10 days, the energy storage elements of Example 2 and Example 4 were stored in a 25°C environment for 3 hours, and then discharged with a constant current of 0.1C to 2.0V. After that, they were charged with a constant current of 0.1C to 3.6V, and then charged with a constant voltage at 3.6V. The charging termination condition was when the charging current became 0.02C. After charging, a 10-minute rest period was observed, followed by constant current discharge at a discharge current of 0.1C to 2.0V in a 25°C environment to measure the "0.1C discharge capacity at 25°C after storage in a high-temperature environment". Next, 15 / 100 of this "0.1C discharge capacity at 25°C after storage in a high-temperature environment" was set as the "15% SOC after storage in a high-temperature environment", and constant current charging was performed at a charging current of 0.1C until the charge reached 15% SOC after storage in a high-temperature environment. Subsequently, the VI characteristics were plotted using the same method as in (1) above, and the "output at 15% SOC after storage in a high-temperature environment" was determined. (3) The output retention rate after storage in a high-temperature environment (labeled "Output retention rate after high-temperature storage" in Table 2) was calculated by dividing the output at SOC 15% after storage in the high-temperature environment by the initial output at SOC 15% and multiplying by 100. The results are shown in Table 2.

[0109] [Table 1]

[0110] [Table 2]

[0111] As shown in Table 1 above, Examples 1 to 6, in which the ratio of the specific surface area of ​​the second BET to the specific surface area of ​​the first BET was greater than 10% but less than 35%, and the non-aqueous electrolyte contained a sulfur-based compound, showed high initial output in low-temperature environments. On the other hand, Comparative Examples 1, 2, 6, and 7, in which the ratio of the specific surface area of ​​the second BET to the specific surface area of ​​the first BET was 10% or less or 35% or more, and Comparative Examples 3 to 5, in which the non-aqueous electrolyte did not contain a sulfur-based compound, showed lower initial output in low-temperature environments compared to Examples 1 to 6.

[0112] Furthermore, as shown in Table 2 above, when comparing Example 2 and Example 4, which have the same ratio of the specific surface area of ​​the second BET to the specific surface area of ​​the first BET and the same sulfur compound, Example 2, in which the positive electrode active material layer does not contain a conductive agent, showed a higher power retention rate after storage in a high-temperature environment than Example 4, in which the positive electrode active material layer contains a conductive agent. Note that while Examples 2 and 4 show the power retention rate after storage in a high-temperature environment at relatively low SOC, it is believed that similar effects can be obtained at relatively high SOC as well.

[0113] The results above demonstrate that the energy storage element exhibits high initial output in low-temperature environments. [Explanation of Symbols]

[0114] 1. 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 having a positive electrode active material layer containing positive electrode active material, Non-aqueous electrolytes and Equipped with, The above positive electrode active material contains a polyanionic compound that includes a transition metal element and whose surface is coated with carbon at least partially. The ratio of the second BET specific surface area to the first BET specific surface area, which is the BET specific surface area of ​​the positive electrode active material layer, is greater than 10% and less than 35%. The above-mentioned second BET specific surface area was calculated by subtracting the BET specific surface area of ​​the polyanion compound measured with the carbon removed from the surface from the BET specific surface area of ​​the polyanion compound measured with the carbon coated on it. The above non-aqueous electrolyte is an energy storage element containing an electrolyte salt that does not contain sulfur elements and a sulfur-based compound.

2. The energy storage element according to claim 1, wherein the content of the conductive agent in the positive electrode active material layer is 2% by mass or less.

3. A power storage device comprising two or more energy storage elements, and one or more energy storage elements as described in claim 1 or claim 2.