Positive electrode active material for energy storage elements, positive electrode for energy storage elements, energy storage elements, and energy storage devices.

Abstract

A positive electrode active material for a power storage element according to one aspect of the present invention is a compound which is represented by formula 1, of which at least a part of the surface is covered with carbon, and of which the half-width ratio of the peak in the charged state with respect to the peak in the discharged state for peaks corresponding to the (131) plane as observed by a powder X-ray diffraction method using CuKα rays is 0.85 to 1.13, inclusive.　Formula 1: LiFexMn(1-x)PO4, where (0≤x≤1)

Description

[Technical Field]

[0001] The present invention relates to a positive electrode active material for an energy storage element, a positive electrode for an energy storage element, an energy storage element, and an energy storage device. [Background technology]

[0002] Non-aqueous electrolyte secondary batteries, such as lithium-ion batteries, are widely used in electronic devices like personal computers and communication terminals, as well as automobiles, due to their high energy density. Generally, these non-aqueous electrolyte secondary batteries consist of a pair of electrodes electrically isolated by a separator, and a non-aqueous electrolyte interposed between these electrodes. They are configured to charge and discharge by transferring ions between the two electrodes. In addition to non-aqueous electrolyte secondary batteries, capacitors such as lithium-ion capacitors and electric double-layer capacitors are also widely used as energy storage elements.

[0003] In recent years, olivine-type cathode active materials have attracted attention as a cathode active material used in the above-mentioned energy storage elements because they are inexpensive and highly safe. However, because these olivine-type cathode active materials have low electronic conductivity, it has been difficult to obtain a discharge capacity close to the theoretical capacity. To improve this, a technique of coating the surface with carbon has been proposed (see Patent Document 1).

[0004] Furthermore, a manufacturing method for cathode active materials that involves a precursor has been proposed, for example, for so-called tri-element lithium transition metal composite oxide cathode active materials (see Patent Document 2). [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2008-034306 [Patent Document 2] Japanese Patent Publication No. 2021-051909 [Overview of the project] [Problems that the invention aims to solve]

[0006] However, because such carbon-coated olivine-type cathode active materials have a large specific surface area, energy storage elements using such cathode active materials are prone to increased resistance after storage in high-temperature environments, which may lead to a decrease in output maintenance efficiency.

[0007] Furthermore, when carbon-coated olivine-type cathode active materials are obtained through a manufacturing method that involves a precursor, it is difficult to control the crystal growth direction of the cathode active material, making it difficult to obtain sufficient properties as a cathode active material.

[0008] The present invention was made based on the circumstances described above, and aims to provide a positive electrode active material for an energy storage element that can reduce the increase in resistance after storage in a high-temperature environment for energy storage elements and energy storage devices. [Means for solving the problem]

[0009] The positive electrode active material for an energy storage element according to one aspect of the present invention has at least a portion of its surface coated with carbon, and in the peak corresponding to the (131) plane observed by powder X-ray diffraction using CuKα rays, the ratio of the full width at half maximum of the charging state peak to the discharge state peak is 0.85 or more and 1.13 or less, and is a compound represented by the following formula 1. LiFe x Mn (1-x) PO4(0≦x≦1) ···1 [Effects of the Invention]

[0010] A positive electrode active material for an energy storage element according to one aspect of the present invention can reduce the increase in resistance after storage in a high-temperature environment for energy storage elements and energy storage devices. [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 positive electrode active material for energy storage elements, positive electrode for energy storage elements, energy storage elements, and energy storage devices disclosed herein will be provided.

[0013] The positive electrode active material for an energy storage element according to one aspect of the present invention has at least a portion of its surface coated with carbon, and in the peak corresponding to the (131) plane observed by powder X-ray diffraction using CuKα rays, the ratio of the full width at half maximum of the charging state peak to the discharge state peak is 0.85 or more and 1.13 or less, and is a compound represented by the following formula 1. LiFe x Mn (1-x) PO4(0≦x≦1) ···1

[0014] The positive electrode active material for the energy storage element has at least a portion of its surface coated with carbon and is a compound represented by formula 1 above. In the peak corresponding to the (131) plane observed by powder X-ray diffraction using CuKα rays, the ratio of the full width at half maximum of the charging state peak to the discharge state peak has a specific range, thereby reducing the resistance increase after storage of the energy storage element in a high-temperature environment. The reason for this is presumed to be as follows: As mentioned above, since the carbon-coated olivine-type positive electrode active material has a large specific surface area, energy storage elements using such a positive electrode active material are prone to resistance increase after storage in a high-temperature environment. The inventors have found that in energy storage elements using a carbon-coated olivine-type positive electrode active material, in particular, if the anisotropic expansion that occurs when the positive electrode active material changes from a discharge state to a charging state is large, current collection degradation is likely to occur, which tends to cause a further increase in resistance. It was found that by suppressing this anisotropic expansion of the positive electrode active material, a suppressive effect on the resistance increase after storage of the energy storage element in a high-temperature environment can be obtained. Even if at least a portion of the surface of the positive electrode active material for the energy storage element is coated with carbon, by setting the half-width ratio of the charging state peak to the discharge state peak to 0.85 or more and 1.13 or less in the peak corresponding to the (131) plane observed by powder X-ray diffraction using CuKα rays, anisotropic expansion that occurs when transitioning from a discharge state to a charging state is suppressed. As a result, it is presumed that the current collection performance of the positive electrode can be maintained well. Therefore, the positive electrode active material for the energy storage element can reduce the increase in resistance after storage in a high-temperature environment of the energy storage element.

[0015] The positive electrode active material for the energy storage element preferably has a full width at half maximum (FWHM) ratio of 0.85 to 1.10. Having a FWHM ratio of 0.85 to 1.10 further reduces the resistance increase after storage in a high-temperature environment for the energy storage element.

[0016] A positive electrode for an energy storage element according to one aspect of the present invention comprises a positive electrode mixture layer containing the positive electrode active material. Since the positive electrode for an energy storage element comprises a positive electrode mixture layer containing the positive electrode active material, the increase in resistance after storage in a high-temperature environment of the energy storage element can be reduced.

[0017] The positive electrode for the energy storage element preferably has a peak differential pore volume of the positive electrode mixture layer of 5×10 -3 cm 3 / (g·nm) or more and 8×10 -3 cm 3 / (g·nm) or less. When the peak differential pore volume of the positive electrode mixture layer is 5×10 -3 cm 3 / (g·nm) or more and 8×10 -3 cm 3 / (g·nm) or less, the increase in resistance after storage of the energy storage element in a high-temperature environment can be further reduced.

[0018] The energy storage element according to one aspect of the present invention includes the positive electrode. Since the energy storage element includes a positive electrode containing the positive electrode active material, the increase in resistance after storage in a high-temperature environment can be reduced.

[0019] The energy storage device according to one aspect of the present invention includes two or more energy storage elements and one or more energy storage elements according to one aspect of the present invention. Since the energy storage device includes the energy storage element according to one aspect of the present invention, the increase in resistance after storage in a high-temperature environment can be reduced.

[0020] In this specification, the oxidation reaction in which ions (lithium ions in the case of a non-aqueous electrolyte lithium ion energy storage element) involved in the charge-discharge reaction are released from the positive electrode active material is referred to as "charging", and the reduction reaction in which ions involved in the charge-discharge reaction are occluded in the positive electrode active material is referred to as "discharging".

[0021] the constitution of the positive electrode active material for the energy storage element, the constitution of the positive electrode for the energy storage element, the constitution of the energy storage element, the constitution of the energy storage device, the manufacturing method of the positive electrode for the energy storage element and the manufacturing method of the energy storage element, and other embodiments will be described in detail. Note that the names of the constituent members (each constituent element) used in each embodiment may be different from the names of the constituent members (each constituent element) used in the background art.

[0022] <Positive electrode active material for energy storage element> The positive electrode active material for the energy storage element (hereinafter also simply referred to as the positive electrode active material) has at least a part of its surface coated with carbon.

[0023] The positive electrode active material is a compound represented by the following formula 1. LiFe x Mn (1-x) PO4(0≦x≦1) ···1 The compound represented by the above formula 1 is a phosphate compound containing iron, manganese or a combination thereof and lithium. The compound represented by the above formula 1 has an olivine-type crystal structure. A compound having an olivine-type crystal structure has a crystal structure that can be attributed to the space group Pnma. The crystal structure attributable to the space group Pnma means having peaks attributable to the space group Pnma in the X-ray diffraction pattern. Since the compound represented by the above formula 1 is a polyanion salt in which the oxygen desorption reaction from the crystal lattice does not easily proceed, it has high safety and is also inexpensive.

[0024] Examples of the positive electrode active material include lithium iron phosphate (LiFePO4), lithium manganese phosphate (LiMnPO4), lithium iron manganese phosphate (LiFe x Mn (1-x) PO4, 0<x<1) or a combination thereof. By including iron, manganese or a combination thereof as a transition metal element in the positive electrode active material, the charge and discharge capacity can be further increased.

[0025] The compound represented by the above formula 1 may contain transition metal elements other than iron and manganese and typical elements such as aluminum. However, it is preferable that the compound represented by the above formula 1 is substantially composed of iron, manganese or a combination thereof, lithium, phosphorus and oxygen.

[0026] In the above formula 1, the upper limit of x is 1, and 0.95 is preferable. Also, the lower limit of x is 0, and 0.25 is preferable. By the value of x being below the above upper limit or above the above lower limit, it has more excellent life characteristics. Incidentally, x may be substantially 1.

[0027] The average particle size of the primary particles of the positive electrode active material is preferably, for example, 0.01 μm or more and 0.20 μm or less, and more preferably 0.02 μm or more and 0.10 μm or less. By setting the average particle size of the primary particles of the compound represented by Formula 1 within the above range, the diffusivity of lithium ions in the pores of the positive electrode mixture layer is improved.

[0028] The average particle size of the secondary particles of the positive electrode active material is preferably 3 μm to 20 μm, and more preferably 5 μm to 15 μm. By setting the average particle size of the secondary particles of the compound represented by Formula 1 within the above range, manufacturing and handling become easier, and the diffusivity of lithium ions in the pores of the positive electrode mixture layer is improved.

[0029] The "average particle size of primary particles" is a value measured by observation using a scanning electron microscope. The "average particle size of secondary particles" is a value that corresponds to 50% of the volume-based integrated distribution calculated according to JIS-Z-8819-2 (2001), based on the particle size distribution measured by laser diffraction / scattering on a dilution of particles diluted with a solvent, in accordance with JIS-Z-8825 (2013).

[0030] 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.

[0031] The positive electrode active material can have its electronic conductivity improved by having at least a portion of its surface coated with carbon. The carbon content in the positive electrode active material is preferably 0.5% by mass or more and 5% by mass or less. By setting the carbon content within this range, electrical conductivity can be increased, as can the density of the positive electrode mixture layer and, consequently, the capacity of the energy storage element.

[0032] In the positive electrode active material, the lower limit of the full width at half maximum (FWHM) ratio of the charging state peak to the discharge state peak, observed by powder X-ray diffraction using CuKα rays, is 0.85, with 0.88 being preferred. On the other hand, the upper limit of the FWHM ratio of the charging state peak to the discharge state peak is 1.13, with 1.10 being preferred. By setting the FWHM ratio of the charging state peak to the discharge state peak to be above the lower limit or below the upper limit, anisotropic expansion that occurs when the positive electrode active material changes from a discharge state to a charge state is suppressed. As a result, it is presumed that the current collection performance of the positive electrode can be maintained well. Therefore, the positive electrode active material for the energy storage element can reduce the increase in resistance after storage in a high-temperature environment of the energy storage element.

[0033] In the peak corresponding to the (131) plane observed by powder X-ray diffraction using CuKα rays of the positive electrode active material, the upper limit of the full width at half maximum (FWHM) of the discharge state peak is preferably 0.170. The lower limit of the FWHM of the discharge state peak is preferably 0.110. By having the FWHM of the discharge state peak be below the upper limit and above the lower limit, the initial output characteristics of the energy storage element can be improved.

[0034] In the peak corresponding to the (131) plane observed by powder X-ray diffraction using CuKα rays of the positive electrode active material, the upper limit of the full width at half maximum (FWHM) of the charged state peak is preferably 0.185. The lower limit of the FWHM of the charged state peak is preferably 0.120. By having the FWHM of the charged state peak be below the upper limit and above the lower limit, the low-temperature characteristics of the energy storage element can be improved.

[0035] The positive electrode active material is assigned to the orthorhombic space group Pnma in the powder X-ray diffraction pattern using CuKα rays. The full width at half maximum of the peak corresponding to the (131) plane observed by powder X-ray diffraction using CuKα rays can be obtained from the diffraction peak located at 2θ = 35.6 ± 0.5° in the X-ray diffraction pattern using CuKα rays.

[0036] The full width at half maximum (FWHM) of the diffraction peak of the positive electrode active material is measured using an X-ray diffractometer (Rigaku, model: MiniFlex II). Specifically, the measurement is performed according to the following conditions and procedures: The radiation source is CuKα, and the acceleration voltage and current are 30kV and 15mA, respectively. The sampling width is 0.01deg, the scanning time is 14 minutes (scan speed is 5.0), the diverging slit width is 0.625deg, the receiving slit width is open, and the scattering slit is 8.0mm. The obtained X-ray diffraction data is automatically analyzed using "PDXL," the software included with the X-ray diffractometer, and the FWHM output is used to determine the FWHM. When analyzing the X-ray diffraction data, peaks originating from Kα2 are not removed.

[0037] To measure the full width at half maximum of the diffraction peak in the discharge state of the positive electrode active material, the powder of the positive electrode active material before charging and discharging, before the positive electrode is manufactured, can be used directly for measurement. When taking a sample from the positive electrode extracted after disassembling an energy storage element, before disassembling the element, discharge it at a constant current value (0.1C) that is one-tenth of the current value that would produce the same amount of electricity as the nominal capacity of the energy storage element when a constant current is applied to the element for one hour at 25°C, until the voltage reaches the lower limit of the specified voltage. Disassemble the energy storage element, extract the positive electrode, assemble a battery with a metallic lithium electrode as the counter electrode, and at 25°C, discharge at a current value of 10mA per gram of positive electrode mixture until the terminal voltage of the positive electrode reaches 2.0V (vs.Li / Li + Constant current discharge is performed until the device reaches a state of complete discharge. The device is then disassembled again, and the positive electrode is removed. The removed positive electrode is thoroughly washed to remove the electrolyte adhering to it using dimethyl carbonate, dried at room temperature for 24 hours, and then the positive electrode mixture on the positive electrode substrate is collected and subjected to measurement. On the other hand, to measure the full width at half maximum of the diffraction peak in the charged state, a constant current discharge is performed at a current value of 0.05C down to the lower limit voltage during normal use, and the positive electrode is adjusted to a completely discharged state. The energy storage element is disassembled, the positive electrode is removed, and a battery is assembled with a metallic lithium electrode as the counter electrode. At a current value of 0.1C in a 25°C environment, the terminal voltage of the positive electrode is 4.1V (vs. Li / Li +After constant current charging until it reaches 4.1V (vs.Li / Li), + The positive electrode is charged at a constant voltage using the terminal voltage of the positive electrode. The charging is terminated when the current value reaches 0.02C. In this charged state, the device is disassembled again and the positive electrode is removed. The removed positive electrode is thoroughly washed to remove the electrolyte adhering to it using dimethyl carbonate, dried at room temperature for 24 hours, and then the positive electrode mixture on the positive electrode substrate is collected and subjected to measurement. The dismantling and subsequent re-dismantling of the energy storage element, as well as the cleaning and drying of the positive electrode, shall be carried out in an argon atmosphere with a dew point of -60°C or lower.

[0038] The positive electrode active material for energy storage elements can reduce the increase in resistance after storage in a high-temperature environment.

[0039] [Method for manufacturing the positive electrode active material for the energy storage element] The above-mentioned positive electrode active material can be manufactured, for example, according to the following procedure. First, in a reaction vessel containing deionized water, a mixed aqueous solution of FeSO4 and MnSO4 in any ratio is added dropwise at a constant rate, while simultaneously adding aqueous NaOH solution, aqueous NH3 solution, and aqueous NH2NH2 solution dropwise to maintain a constant pH value. x Mn (1-x) (OH)2 precursor is prepared. Next, the prepared Fe x Mn (1-x) The (OH)2 precursor is removed from the reaction vessel and mixed in solid phase with LiH2PO4 and sucrose powder. The resulting mixture is then calcined in a nitrogen atmosphere at a calcination temperature of 550°C to 750°C to produce a positive electrode active material having an olivine-type crystal structure and in which at least a portion of the surface of the positive electrode active material represented by the following formula 1 is coated with carbon. LiFe x Mn (1-x) PO4(0≦x≦1) ···1

[0040] The full width at half maximum (FWHM) of the discharge and charge states at the peak corresponding to the (131) plane, observed by powder X-ray diffraction using CuKα rays of the positive electrode active material, can be adjusted in the manufacturing method of the positive electrode active material by controlling the concentrations of the NaOH aqueous solution and the NH3 aqueous solution. The concentration range of the NH3 aqueous solution is 0.25 mol / dm³. 3 More than 1mol / dm 3 The following is preferable: The concentration of the above NH3 aqueous solution is 1 mol / dm³ 3 If the concentration exceeds this, the precursor may not be analyzed according to the desired composition. On the other hand, if the concentration of the NH3 aqueous solution is 0.25 mol / dm³ 3 If the value is less than [value missing], a uniform elemental distribution within a single precursor particle may not be achieved. This may result in insufficient control of the crystal growth direction, making it difficult to adjust the full width at half maximum (FWHM) of the peaks within a favorable range. Furthermore, using NH2NH2 as an antioxidant for the precursor allows for adjustment of the FWHM of the peaks in the cathode active material within a favorable range.

[0041] In other words, the positive electrode active material is manufactured via a precursor, Fe x Mn (1-x) By controlling the pH and NH2NH2 concentration during (OH)2 precursor preparation to a suitable range, the full width at half maximum of the above peak can be adjusted to a good range.

[0042] <Positive electrode for energy storage element> The positive electrode for the energy storage element (hereinafter also simply referred to as the positive electrode) includes the positive electrode active material. The positive electrode comprises a positive electrode substrate and a positive electrode mixture layer disposed directly on the positive electrode substrate or via an intermediate layer.

[0043] [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 7The 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-H4160 (2006). 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.

[0044] The intermediate layer is a layer placed 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 composition of the intermediate layer is not particularly limited and may include, for example, a binder and a conductive agent.

[0045] [Positive electrode mixture layer] The positive electrode mixture layer contains the positive electrode active material described above. Since the positive electrode is equipped with a positive electrode mixture layer containing the positive electrode active material, the resistance increase after storage in a high-temperature environment of the energy storage element can be reduced. The positive electrode mixture layer may optionally contain optional components such as conductive agents, binders, thickeners, and fillers.

[0046] The positive electrode mixture layer may further contain positive electrode active materials other than the positive electrode active material having an olivine-type crystal structure. Such other positive electrode active materials can be appropriately selected from known positive electrode active materials commonly used in lithium-ion secondary batteries and the like. However, the lower limit of the total content of the positive electrode active material having an olivine-type crystal structure in the total positive electrode active material contained in the positive electrode mixture layer is preferably 90% by mass, and more preferably 99% by mass. The upper limit of the total content of the positive electrode active material in the total positive electrode active material may be 100% by mass. By using only the positive electrode active material having an olivine-type crystal structure as substantially the positive electrode active material in this way, the effects of the present invention can be further enhanced.

[0047] Commonly known positive electrode active materials for lithium-ion secondary batteries are materials that can intercept and release lithium ions. Examples of positive electrode active materials include lithium transition metal composite oxides having an α-NaFeO2 type crystal structure, lithium transition metal composite oxides having a spinel type crystal structure, polyanionic compounds, chalcogen compounds, sulfur, etc. Examples of lithium transition metal composite oxides having an α-NaFeO2 type crystal structure include 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, Lix Mn2O4, Li x Ni γ Mn (2-γ) Examples include O4. Examples of polyanion compounds include LiNiPO4, LiCoPO4, Li3V2(PO4)3, Li2MnSiO4, Li2CoPO4F, etc. Examples of chalcogen compounds include titanium disulfide, molybdenum disulfide, molybdenum dioxide, etc. Some atoms or polyanions in these materials may be substituted with atoms or anions of other elements. The surfaces of these materials may be coated with other materials. In the positive electrode mixture layer, one of these materials may be used alone, or two or more may be used in mixture form.

[0048] The content of total positive electrode active material in the positive electrode mixture layer is preferably 50% to 99% by mass, more preferably 70% to 98% by mass, and even more preferably 80% to 95% by mass. By setting the content of total positive electrode active material within the above range, it is possible to achieve both high energy density and manufacturability in the positive electrode active material layer.

[0049] The conductive agent is not particularly limited as long as it is a conductive material. Examples of such conductive agents include carbonaceous materials, metals, and conductive ceramics. Examples of carbonaceous materials include graphite, non-graphitic carbon, and graphene-based carbon. Examples of non-graphitic carbon include carbon nanofibers, pitch-based carbon fibers, and carbon black. Examples of carbon black include furnace black, acetylene black, and Ketjen black. Examples of graphene-based carbon include graphene, carbon nanotubes (CNTs), and fullerenes. The conductive agent can take the form of powder or fiber. One of these materials may be used alone as the conductive agent, or two or more may be used in mixture form. These materials may also be used in composite form. For example, a composite material of carbon black and CNTs may be used. Among these, carbon black is preferred from the viewpoint of electronic conductivity and coating properties, and acetylene black is particularly preferred.

[0050] The content of the conductive agent in the positive electrode mixture layer is preferably 0.5% by mass or more and 15% by mass or less, more preferably 1% by mass or more and 10% by mass or less, and even more preferably 2% by mass or more and 5% by mass or less. By setting the content of the conductive agent within the above range, the energy density of the energy storage element can be increased.

[0051] 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.

[0052] The binder content 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 keeping the binder content within the above range, the active material can be stably retained.

[0053] 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.

[0054] 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.

[0055] The positive electrode mixture 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.

[0056] The peak differential pore volume of the above cathode mixture layer is 5 × 10 -3 cm 3 / (g·nm) or more 10×10 -3 cm 3 It is preferable that the value is less than or equal to / (g·nm), and 5 × 10 -3 cm 3 / (g·nm) or more 8×10 -3 cm 3 It is more preferable that the peak differential pore volume of the positive electrode mixture layer is within the above range, which suppresses side reactions between the positive electrode active material and the electrolyte, thus providing a good suppression effect against resistance increase after storage of the energy storage element in a high-temperature environment.

[0057] The peak differential pore volume of the positive electrode mixture layer described above is measured by the following method. 1.00 g of the sample powder is placed in a sample tube and dried under reduced pressure at 120°C for 6 hours, and then at 180°C for another 6 hours to thoroughly remove moisture from the sample. Next, using the nitrogen gas adsorption method with liquid nitrogen, the isotherms of the adsorption and desorption sides are measured within the range of 0 to 1 for the relative pressure P / P0 (P0 = approximately 770 mmHg). Then, the cumulative pore volume curve is obtained by calculating using the BJH method with the isotherm of the desorption side. From this curve, the total pore volume (cm³) is calculated. 3 The value of ( / g) is obtained. Next, by linearly differentiating the above cumulative pore volume curve, the differential pore volume (cm³) is obtained, with the horizontal axis being pore diameter (nm) and the vertical axis being the differential pore volume (cm³). 3 A differential pore volume curve is obtained, denoted as / (g·nm). In this specification, "peak differential pore volume" refers to the differential pore volume that is the maximum value in the differential pore volume curve described above. A large "peak differential pore volume" means that there are many pores of a certain pore diameter.

[0058] The peak differential pore volume of the above positive electrode mixture layer is the Fe in the method for producing the positive electrode active material. x Mn (1-x) The pH can be adjusted by controlling the pH during the preparation of the (OH)2 precursor. A pH range of 8 to 10 is preferred at 50°C. Furthermore, by using NH3 as a complexing agent and NH2NH2 as an antioxidant in the method for producing the positive electrode active material, the peak differential pore volume of the positive electrode mixture layer can be brought within a favorable range.

[0059] <Energy storage element> An energy storage element according to one embodiment of the present invention comprises an electrode body having a positive electrode, a negative electrode, and a separator, a non-aqueous electrolyte, and a container for housing the electrode body and the non-aqueous electrolyte. The electrode body is usually a stacked type in which a plurality of positive electrodes and a plurality of negative electrodes are stacked with a separator in between, or a wound type in which the positive electrode and negative electrode are wound in a stacked state with a separator in between. The non-aqueous electrolyte exists in a state impregnated with the positive electrode, negative electrode, and separator. As an example of an energy storage element, a non-aqueous electrolyte secondary battery (hereinafter also simply referred to as a "secondary battery") will be described.

[0060] [Positive electrode] The positive electrode of the energy storage element is as described above. Since the energy storage element has a positive electrode containing the positive electrode active material, the increase in resistance after storage in a high-temperature environment can be reduced.

[0061] [Negative electrode] The negative electrode comprises a negative electrode substrate and a negative electrode mixture 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.

[0062] 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.

[0063] 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 secondary battery.

[0064] The negative electrode mixture layer contains a negative electrode active material. The negative electrode mixture layer optionally contains conductive agents, binders, thickeners, fillers, and other optional components. These optional components can be selected from the materials exemplified in the positive electrode above.

[0065] The negative electrode mixture 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.

[0066] 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 mixture layer, one of these materials may be used alone, or two or more may be used in mixture form.

[0067] "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.

[0068] "Non-graphite carbon" refers to the average lattice plane spacing (d) of the (002) plane, which is determined by X-ray diffraction before charging / discharging 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.

[0069] Here, "discharge state" refers to a state in which sufficient lithium ions capable of being absorbed and released during charging and discharging are released from the carbon material, which is the negative electrode active material. For example, in a monoelectrode battery 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.

[0070] "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.

[0071] "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.

[0072] 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 to 100 μm. If the negative electrode active material is a carbon material, titanium-containing oxide, or polyphosphate compound, its average particle size may be 1 μm to 100 μm. If the negative electrode active material is Si, Sn, Si oxide, or Sn oxide, its average particle size may be 1 nm to 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 negative electrode mixture layer. To obtain powder with a predetermined particle size, a pulverizer or classifier is used. The pulverizing method and powder grading 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.

[0073] The content of the negative electrode active material in the negative electrode mixture layer is preferably 60% by mass or more and 99% by mass or less, 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 mixture layer.

[0074] [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.

[0075] 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.

[0076] 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.

[0077] 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.

[0078] [Non-aqueous electrolytes] As the non-aqueous electrolyte, it 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 this non-aqueous solvent.

[0079] As the non-aqueous solvent, it can be appropriately selected from known non-aqueous solvents. Examples of non-aqueous solvents include cyclic carbonates, linear carbonates, carboxylic acid esters, phosphate esters, sulfonic acid 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.

[0080] 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.

[0081] 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.

[0082] 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.

[0083] The electrolyte salt can be appropriately selected from known electrolyte salts. Examples of electrolyte salts include lithium salts, sodium salts, potassium salts, magnesium salts, and onium salts. Among these, lithium salts are preferred.

[0084] Examples of lithium salts include inorganic lithium salts such as LiPF6, LiPO2F2, LiBF4, LiClO4, and LiN(SO2F)2; lithium oxalate salts such as lithium bis(oxalate)borate (LiBOB), lithium difluorooxalate borate (LiFOB), and lithium bis(oxalate)difluorophosphate (LiFOP); and lithium salts having halogenated hydrocarbon groups such as LiSO3CF3, LiN(SO2CF3)2, LiN(SO2C2F5)2, LiN(SO2CF3)(SO2C4F9), LiC(SO2CF3)3, and LiC(SO2C2F5)3. Among these, inorganic lithium salts are preferred, and LiPF6 is more preferred.

[0085] The electrolyte salt content in the non-aqueous electrolyte 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 3More than 1.5mol / dm 3 The following is particularly preferable. By setting the electrolyte salt content within the above range, the ionic conductivity of the non-aqueous electrolyte can be increased.

[0086] Non-aqueous electrolytes may contain additives in addition to the non-aqueous solvent and electrolyte salt. Examples of additives include halogenated carbonate esters 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, alkylbiphenyl, terphenyl, partially hydrogenated terphenyl, cyclohexylbenzene, t-butylbenzene, t-amylbenzene, diphenyl ether, and dibenzofuran; partially halogenated compounds of the aforementioned 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, methylvinylene carbonate, and ethyl vinylene carbonate. - Bonate, succinic anhydride, glutaric anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, cyclohexanedicarboxylic acid anhydride; ethylene sulfite, propylene sulfite, dimethyl sulfite, methyl methanesulfonate, busulfan, methyl toluenesulfonate, dimethyl sulfate, ethylene sulfate, sulfolane, dimethyl sulfone, diethyl sulfone, dimethyl sulfoxide, diethyl sulfoxide, tetramethylene sulfoxide, diphenyl sulfide, 4,4'-bis(2,2-dioxo-1) Examples include 3,2-dioxathiolane, 4-methylsulfonyloxymethyl-2,2-dioxo-1,3,2-dioxathiolane, thioanisole, diphenyl disulfide, dipyridinium disulfide, 1,3-propensultone, 1,3-propanesultone, 1,4-butanesultone, 1,4-butensultone, perfluorooctane, tristrimethylsilyl borate, tristrimethylsilyl phosphate, tetrakithtrimethylsilyl titanate, lithium monofluorophosphate, lithium difluorophosphate, etc. These additives may be used individually or in combination of two or more.

[0087] The additive content in the non-aqueous electrolyte is preferably 0.01% to 10% by mass relative to the total mass of the non-aqueous electrolyte, more preferably 0.1% to 7% by mass, even more preferably 0.2% to 5% by mass, and particularly preferably 0.3% to 3% by mass. By setting the additive content within the above range, it is possible to improve the capacity retention performance or cycle performance after high-temperature storage, and to further improve safety.

[0088] For the non-aqueous electrolyte, a solid electrolyte may be used, or a non-aqueous electrolyte and a solid electrolyte may be used in combination.

[0089] The solid electrolyte can be selected from any material that has ionic conductivity, such as lithium, sodium, and calcium, and is solid at room temperature (e.g., 15°C to 25°C). Examples of solid electrolytes include sulfide solid electrolytes, oxide solid electrolytes, oxynitride solid electrolytes, and polymer solid electrolytes.

[0090] Examples of sulfide solid electrolytes include Li2S-P2S5, LiI-Li2S-P2S5, and Li 10 Ge-P2S 12 These are some examples.

[0091] 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.

[0092] 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.

[0093] [Configuration of the energy storage system] The energy storage element of this embodiment can be mounted as an energy storage device in which multiple energy storage elements 1 are assembled, such as in power supplies for automobiles 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 storage power supplies. 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 device.

[0094] An energy storage device according to one embodiment of the present invention is an energy storage device comprising two or more energy storage elements and one or more energy storage elements according to one embodiment of the present invention. Figure 2 shows an example of an energy storage device 30 which is further composed of energy storage units 20, each composed of two or more electrically connected energy storage elements 1. The energy storage device 30 may include a busbar (not shown) for electrically connecting two or more energy storage elements 1, a busbar (not shown) for electrically connecting 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) for monitoring the state of one or more energy storage elements.

[0095] [Manufacturing method for energy storage elements] The energy storage element can be manufactured by known methods, except that the positive electrode described above is used as the positive electrode. The method for manufacturing the energy storage element of this embodiment can be appropriately selected from known methods. The method for manufacturing the energy storage element comprises, 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 comprises preparing the positive electrode and the negative electrode, and forming the electrode body by stacking or winding the positive electrode and the negative electrode via a separator.

[0096] 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.

[0097] <Other Embodiments> It should be noted that 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. Furthermore, a part of the configuration of one embodiment may be deleted. Also, well-known technology may be added to the configuration of one embodiment.

[0098] 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.

[0099] 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 composite layer of the positive electrode or the negative electrode. [Examples]

[0100] The present invention will be described in more detail below with reference to examples. The present invention is not limited to the following examples.

[0101] [Examples 1 to 10 and Comparative Examples 1 to 4] (Fabrication of positive electrode active material) First, 750cm 3 2dm 3 Add 1 mol / dm³ to the reaction vessel. 3 While adding the FeSO4 aqueous solution dropwise at a constant rate, maintain the pH at the values ​​shown in Table 1 at 4 mol / dm³ 3 NaOH aqueous solution and 0.5 mol / dm 3 NH3 aqueous solution and 0.5 mol / dm 3An 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 solid-phase mixed with LiH2PO4 and sucrose powder. Then, the resulting mixture was calcined under a nitrogen atmosphere to prepare a positive electrode active material in which the entire surface of the positive electrode active material LiFePO4 represented by formula 1 above was coated with carbon.

[0102] Table 1 shows the pH during precursor preparation for the positive electrode active material (LiFePO4) in Examples 1 to 10 and Comparative Examples 1 to 4, and the ratio of the full width at half maximum (FWHM) of the charged state peak to the discharged state peak in the peak corresponding to the (131) plane observed by powder X-ray diffraction using CuKα rays. The FWHM of the above peaks was measured based on the method described above.

[0103] (Fabrication of the positive electrode) N-methylpyrrolidone (NMP) was used as the dispersion medium, the positive electrode active materials from Examples 1 to 10 and Comparative Examples 1 to 4 were used as the positive electrode active materials, acetylene black (AB) was used as the conductive agent, and polyvinylidene fluoride (PVdF) was used as the binder. The positive electrode active materials, conductive agent, binder, and dispersion medium were mixed. At that time, the solid content mass ratio of positive electrode active material:conductive agent:binder was set to 80:12.5:7.5 for Example 1, 87.5:7.5:5 for Example 4, and 85:10:5 for the other examples and comparative examples. An appropriate amount of NMP was added to the obtained mixture to adjust the viscosity and prepare a positive electrode mixture paste. Next, the above positive electrode mixture paste was applied to both sides of an aluminum foil, which was the positive electrode substrate, leaving uncoated areas (areas 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 amount of positive electrode mixture paste to be applied is 10 mg / cm² in terms of solid content. 2 In this way, the positive electrodes for Examples 1 to 10 and Comparative Examples 1 to 4 were obtained.

[0104] The peak differential pore volume of the positive electrode mixture layer was measured by the method described above. The peak differential pore volume of the positive electrode mixture layer is shown in Table 1.

[0105] (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 a dispersant were mixed. At that time, the solid content mass ratio of the negative electrode active material:binder:thickener was set to 97:2:1. An appropriate amount of water was added to the obtained mixture 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), and dried to form a negative electrode mixture layer. Thereafter, roll pressing was performed to fabricate the negative electrode.

[0106] (Preparation of the non-aqueous electrolyte) LiPF6 was dissolved in a mixed solvent obtained by mixing EC and EMC at a volume ratio of 3:7 at a concentration of 1 mol / dm 3 to prepare a non-aqueous electrolyte.

[0107] (Fabrication of the energy storage device) Next, the positive electrode and the negative electrode were laminated through a separator composed of a polyethylene microporous membrane substrate and an inorganic layer formed on the polyethylene microporous membrane substrate to fabricate an electrode body. The inorganic layer was arranged on the surface facing the positive electrode. This electrode body was housed in a rectangular aluminum container, and positive and negative terminals were attached. After injecting the non-aqueous electrolyte into the interior of this rectangular container, it was sealed to obtain the energy storage devices of the examples and comparative examples.

[0108] (Output retention rate after storage in a high-temperature environment) (1) Output performance tests were conducted for each of the above energy storage elements at a State of Charge (SOC) of 50%. Constant current charging was performed at a charging current of 0.1C until the voltage reached 3.6V in a 25°C environment, followed by constant voltage charging at 3.6V. The charging termination condition was when the charging current reached 0.02C. After a 10-minute rest period following charging, constant current discharge was performed at a discharge current of 0.1C until the voltage reached 2.0V in a 25°C environment, and the "0.1C discharge capacity in a 25°C environment" was measured. This amount of electricity was set as SOC 100%, and half of this "0.1C discharge capacity in a 25°C environment" was set as the "initial SOC 50%". Next, constant current charging was performed at a charging current of 0.1C from a fully discharged state to charge the initial SOC 50% amount. Subsequently, the battery was discharged at a discharge current of 0.1C for 30 seconds, followed by a 10-minute rest period, and then supplemental charging at a charging current of 0.1C for 30 seconds. Next, 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 until the SOC reached 50%. The voltage at 10 seconds after the start of each discharge and the discharge current at that time were plotted to create a VI characteristic. 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 50%" 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, each of the above energy storage elements was charged with a constant current of 0.1C to 3.6V, and then charged with a constant voltage of 3.6V. The charging termination condition was set to when the charging current became 0.02C. After charging to 100% SOC in this manner, the elements were left to stand in a constant temperature bath at 85°C for 14 days. After 14 days, each of the above energy storage elements was left to stand in a 25°C environment for 3 hours, and then discharged with a constant current of 0.1C to 2.0V, and the amount of discharged electricity was calculated. Next, half of that amount of electricity was set as "50% SOC after storage in a high-temperature environment," and constant current charging was performed so that the amount of electricity equivalent to 50% SOC after storage in a high-temperature environment was charged with a charging current of 0.1C. After that, the VI characteristics were plotted using the same method as above, and the "output at 50% SOC after storage in a high-temperature environment" was determined. (3) The output maintenance rate (%) after storage in the high-temperature environment was calculated by dividing the output at SOC 50% after storage in the high-temperature environment by the output at the initial SOC 50% and then multiplying by 100. The results are shown in Table 1.

[0109]

Table 1

[0110] As shown in Table 1 above, Examples 1 to 10 containing a positive electrode active material in which at least a part of the surface of the positive electrode active material represented by Formula 1 above is coated with carbon, and having a half-value width ratio of the peak in the charged state to the peak in the discharged state of 0.85 or more and 1.13 or less at the peak corresponding to the (131) plane observed by the powder X-ray diffraction method using CuKα radiation, are found to have a higher output maintenance rate after storage in a high-temperature environment and a lower increase in resistance after storage in a high-temperature environment compared with Comparative Examples 1 to 4.

[0111] Also, from the results of Examples 1 to 4, when the peak differential pore volume of the positive electrode binder layer is in the range of 5×10 -3 cm 3 / (g·nm) or more and 10×10 -3 cm 3 / (g·nm) or less, a good effect of reducing the increase in resistance is obtained, and particularly when it is in the range of 5×10 -3 cm 3 / (g·nm) or more and 8×10 -3 cm 3 / (g·nm) or less, a better effect of reducing the increase in resistance is obtained.

[0112] From the above results, it was shown that the positive electrode active material can reduce the increase in resistance after storage of the power storage element in a high-temperature environment. The positive electrode is suitable for a positive electrode for a power storage element used as a power source for electronic devices such as personal computers and communication terminals, and automobiles.

Explanation of Reference Numerals

[0113] 1 Power 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. At least a portion of the surface is coated with carbon, In the peak corresponding to the (131) plane observed by powder X-ray diffraction using CuKα rays, the half-width ratio of the charging state peak to the discharge state peak is 0.85 or more and 1.13 or less. A positive electrode active material for an energy storage element, which is a compound represented by the following formula 1. LiFe ｘ Mn （１－ｘ） PO ４ (0≦x≦1) ・・・1

2. The positive electrode active material for an energy storage element according to claim 1, wherein the above-mentioned full width at half maximum ratio is 0.85 or more and 1.10 or less.

3. A positive electrode for a storage element comprising a positive electrode mixture layer containing the positive electrode active material described in claim 1 or claim 2.

4. The peak differential pore volume of the above cathode mixture layer is 5 × 10 －３ cm ３ / (g・nm) or more 8×10 －３ cm ３ A positive electrode for an energy storage element according to claim 3, wherein the value is less than or equal to / (g・nm).

5. A storage element comprising the positive electrode described in claim 3 or claim 4.

6. A power storage device comprising two or more energy storage elements, and one or more of the energy storage elements described in claim 5.

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