Cathode active material for power storage element, cathode for power storage element and power storage element

Abstract

To provide a cathode active material for a power storage element which includes a lithium transition metal oxide having a spinel crystal structure and is capable of suppressing reduction in a discharge capacity maintenance rate of a power storage element after a charge / discharge cycle, a cathode for a power storage element including such a cathode active material for a power storage element, and a power storage element.SOLUTION: A cathode active material for a power storage element includes a lithium transition metal oxide having a spinel crystal structure and a cover layer which uniformly covers the lithium transition metal oxide, and the cover layer includes niobium elements or phosphor elements. When a ratio of a total mass [mg] of the niobium elements and the phosphor elements with respect to a total mass [g] of the lithium transition metal oxide and the cover layer is defined as A [mg / g] and a BET specific surface area of a portion consisting of the lithium transition metal oxide and the cover layer is defined as B[m2 / g], A / B ranges from 0.4 mg / m2 or more to 9.0 mg / m2 or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] Due to their high energy density, non-aqueous electrolyte secondary batteries, such as lithium-ion non-aqueous electrolyte secondary batteries, are widely used in electronic devices such as personal computers and communication terminals, automobiles, etc. Non-aqueous electrolyte secondary batteries generally include an electrode assembly having a pair of electrodes electrically isolated by a separator, and a non-aqueous electrolyte interposed between the electrodes, and are configured to charge and discharge by transferring charge-transporting ions between the electrodes. Furthermore, as energy storage elements other than non-aqueous electrolyte secondary batteries, capacitors such as lithium-ion capacitors and electric double layer capacitors, as well as energy storage elements using electrolytes other than non-aqueous electrolytes, are also widely used.

[0003] Lithium transition metal oxides having a spinel structure, such as lithium manganate, are known as positive electrode active materials used in electricity storage elements (Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2018-045965 Summary of the Invention [Problem to be solved by the invention]

[0005] When a positive electrode active material containing a lithium transition metal oxide having a spinel crystal structure is used for the positive electrode of an energy storage device, transition metal ions may leach out of the lithium transition metal oxide into the electrolyte of the energy storage device, which can lead to a decrease in discharge capacity with repeated charge and discharge.

[0006] The present invention has been made in light of the above circumstances, and aims to provide a positive electrode active material for an energy storage element, which contains a lithium transition metal oxide having a spinel crystal structure and is capable of suppressing a decrease in the discharge capacity retention rate of the energy storage element after charge-discharge cycles, as well as a positive electrode for an energy storage element and an energy storage element including such a positive electrode active material for an energy storage element. [Means for solving the problem]

[0007] A positive electrode active material for an energy storage element according to one aspect of the present invention comprises a lithium transition metal oxide having a spinel crystal structure and a coating layer uniformly coating the lithium transition metal oxide, the coating layer containing niobium or phosphorus, a ratio of the total mass [mg] of the niobium and phosphorus to the total mass [g] of the lithium transition metal oxide and the coating layer being A [mg / g], and a BET specific surface area of ​​a portion consisting of the lithium transition metal oxide and the coating layer being B [m 2 / g], A / B is 0.4 mg / m 2 More than 9.0mg / m 2 The following is the result.

[0008] A positive electrode for an electric storage device according to another aspect of the present invention includes the positive electrode active material for an electric storage device according to the aspect of the present invention.

[0009] An electric storage device according to another aspect of the present invention includes a positive electrode for the electric storage device according to the aspect of the present invention. [Effects of the Invention]

[0010] According to any one aspect of the present invention, it is possible to provide a positive electrode active material for an energy storage element, which contains a lithium transition metal oxide having a spinel crystal structure and is capable of suppressing a decrease in the discharge capacity retention rate of the energy storage element after charge-discharge cycles, as well as a positive electrode for an energy storage element and an energy storage element including such a positive electrode active material for an energy storage element. [Brief explanation of the drawings]

[0011] [Figure 1]FIG. 1 is a perspective view showing an embodiment of an energy storage device according to the present invention. [Figure 2] FIG. 2 is a schematic diagram showing an embodiment of an electricity storage device configured by assembling a plurality of electricity storage elements according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0012] First, an outline of the positive electrode active material for an electric storage device, the positive electrode for an electric storage device, and the electric storage device disclosed in this specification will be described.

[0013] [1] A positive electrode active material for an energy storage device according to one aspect of the present invention comprises a lithium transition metal oxide having a spinel crystal structure and a coating layer uniformly coating the lithium transition metal oxide, the coating layer containing niobium or phosphorus, a ratio of the total mass [mg] of the niobium and phosphorus to the total mass [g] of the lithium transition metal oxide and the coating layer being A [mg / g], and a BET specific surface area of ​​a portion consisting of the lithium transition metal oxide and the coating layer being B [m 2 / g], A / B is 0.4 mg / m 2 More than 9.0mg / m 2 The following is the result.

[0014] The positive electrode active material for an electric storage device according to the above item [1] can suppress a decrease in the discharge capacity retention rate of the electric storage device after charge-discharge cycling. Although the reason for this is not clear, the following reason is presumed. In the past, when a positive electrode active material containing a lithium transition metal oxide having a spinel crystal structure was used for the positive electrode of an energy storage device, transition metal ions were sometimes eluted from the lithium transition metal oxide into the electrolyte. If the transition metal ions were eluted from the lithium transition metal oxide into the electrolyte in this way, the transition metal ions would be deposited on the surface of the negative electrode of the energy storage device, which could reduce the discharge capacity of the energy storage device. In contrast, the positive electrode active material for an energy storage device described in [1] above has a coating layer containing niobium or phosphorus, thereby suppressing the elution of transition metal ions from the lithium transition metal oxide into the electrolyte. Furthermore, the coating layer uniformly coats the lithium transition metal oxide, and the A / B ratio is within the above range. That is, the coating layer uniformly coats the lithium transition metal oxide with an appropriate coating amount. This prevents contact between the lithium transition metal oxide and the electrolyte, effectively suppressing the elution of transition metal ions from the lithium transition metal oxide. Therefore, the positive electrode active material for an energy storage device described in [1] above is believed to be able to suppress a decrease in the discharge capacity retention rate of an energy storage device after charge-discharge cycles.

[0015] The state of being "uniformly coated with lithium transition metal oxide" means that the surface coverage of the lithium transition metal oxide is 50% or more. For example, when the lithium transition metal oxide forms secondary particles, the "surface of the lithium transition metal oxide" means the surface of the secondary particles. The surface coverage of the lithium transition metal oxide is measured by the following method. The positive electrode active material to be measured is observed by element mapping using a scanning electron microscope-energy dispersive X-ray spectroscopy (SEM-EDX). The image obtained is analyzed using the image analysis software ImageJ to calculate the ratio of the area where niobium or phosphorus is present to the area of ​​the active material, and this is calculated as the surface coverage.

[0016] In measuring the coverage rate, the positive electrode active material to be measured is used as is if it is possible to prepare the positive electrode active material before assembling the energy storage element. When preparing the positive electrode active material to be measured from an assembled energy storage element, it is sampled from a fully discharged positive electrode using the following method. First, the energy storage element is charged at a constant current of 0.05 C until it reaches the end-of-charge voltage in normal use, and then fully charged. After a 10-minute rest, it is discharged at a constant current of 0.05 C until it reaches the end-of-discharge voltage (lower limit voltage) in normal use. The battery is then disassembled, the positive electrode is removed, and a test battery is assembled using a metallic lithium electrode as the counter electrode. A current of 10 mA per 1 g of positive electrode mixture is applied, and the positive electrode potential is measured at 3.5 V vs. Li / Li.+ The positive electrode is then fully discharged by constant current discharge until the positive electrode reaches a fully discharged state. The device is then disassembled again, and the positive electrode is removed. The electrolyte and other materials adhering to the removed positive electrode are thoroughly washed using dimethyl carbonate. The removed positive electrode is then dried under reduced pressure at room temperature for one day and night, and a powder (powder of the positive electrode active material layer) containing the positive electrode active material and optional components such as a conductive agent is collected. After removing optional components from the powder using air classification or other methods, the powder is washed and filtered using a solvent in which the binder or thickener is soluble, if necessary, to remove the binder or thickener. Finally, the resulting positive electrode active material is heated and dried to collect the positive electrode active material to be measured. The process from disassembling the storage element to collecting the positive electrode active material is carried out in an argon atmosphere with a dew point of -60°C or below, as necessary. "Normal use" refers to the use of the storage element under the recommended or specified charging and discharging conditions for the storage element. Regarding charging conditions, for example, if a charger for the storage element is provided, the charger is used to operate the storage element.

[0017] In the present invention, a portion of the coating layer may be present inside the secondary particles of the lithium transition metal oxide (for example, at the grain boundaries of the lithium transition metal oxide). In other words, the coating layer may have a portion that covers the surface of the lithium transition metal oxide and a portion that is present inside the lithium transition metal oxide. When a portion of the coating layer is present inside the secondary particles of the lithium transition metal oxide, the "total mass of elemental niobium and elemental phosphorus" means the total mass of elemental niobium and elemental phosphorus present on the surface of the secondary particles of the lithium transition metal oxide and elemental niobium and elemental phosphorus present inside the secondary particles of the lithium transition metal oxide.

[0018] In the present invention, the total mass of the niobium and phosphorus elements can be determined by inductively coupled plasma (ICP) optical emission spectroscopy. The total mass of the niobium and phosphorus elements is measured by the following procedure. First, a cathode active material to be measured is prepared using the procedure described in the above-mentioned coverage measurement. Using a microwave decomposition method, the lithium transition metal oxide and coating layer contained in the cathode active material are completely dissolved in an acid capable of dissolving the lithium transition metal oxide and coating layer. Next, this solution is diluted to a certain amount with pure water to obtain a measurement solution. Then, using a multi-type ICP optical emission spectrometer "ICPE-9820" (manufactured by Shimadzu Corporation), the concentrations of the niobium and phosphorus elements contained in the measurement solution are measured by ICP optical emission spectroscopy. The total mass of the niobium and phosphorus elements is determined from the obtained concentrations of the niobium and phosphorus elements. In addition, in calculating the concentrations of niobium element and phosphorus element in the measurement solution, for example, a calibration curve method can be used in which a calibration curve is created from the area of ​​the peaks when solutions of niobium element and phosphorus element with known concentrations are measured, and the concentrations of niobium element and phosphorus element contained in the measurement solution are determined.

[0019] The BET specific surface area of ​​the portion consisting of the lithium transition metal oxide and the coating layer is determined by measuring the pore size distribution using a nitrogen adsorption method with a mixture of the lithium transition metal oxide coated with the coating layer. This measurement can be performed using an "autosorb iQ" manufactured by Quantachrome. Five points are extracted from the region of P / P0 = 0.06 to 0.3 of the resulting adsorption isotherm, and a BET plot is performed. The BET specific surface area is calculated from the y-intercept and slope of the resulting line. The portion consisting of the lithium transition metal oxide and the coating layer used to measure the BET specific surface area is obtained by removing optional components, if necessary, from the powder of the positive electrode active material layer prepared using the procedure described for measuring the coverage rate.

[0020] [2] The positive electrode active material for an electricity storage device according to [1] above may be particles having a ratio of an average particle size to an average primary particle size of the lithium transition metal oxide of 5 or less.

[0021] The positive electrode active material for an energy storage element described in [2] above is made of particles having a ratio of the average particle size to the average primary particle size of the lithium transition metal oxide of 5 or less, and therefore is less likely to undergo changes in surface shape due to cracks between primary particles, etc. This makes it easier to maintain the coating layer uniform on the surface of the lithium transition metal oxide, and further suppresses the elution of transition metal ions.

[0022] The "average primary particle diameter" of a lithium transition metal oxide is the average value of the primary particle diameters of any 50 primary particles constituting the lithium transition metal oxide observed under a scanning electron microscope (SEM). Primary particles are particles for which no grain boundaries are observed externally under the SEM. The primary particle diameter of a primary particle is determined as follows: The shortest diameter passing through the center of the smallest circumscribing circle of the primary particle is defined as the minor diameter, and the diameter passing through the center and perpendicular to the minor diameter is defined as the major diameter. The particle diameter is defined as the average of the major and minor diameters. If there are two or more shortest diameters, the longest diameter perpendicular to the minor diameter is defined as the minor diameter.

[0023] The "average particle size" of lithium transition metal oxides is the value at which the volume-based cumulative distribution reaches 50% (D50: median diameter) as calculated in accordance with JIS-Z-8819-2 (2001) based on the particle size distribution measured by laser diffraction / scattering in a diluted solution of a lithium transition metal oxide measurement sample diluted with a solvent in accordance with JIS-Z-8815 (2013). The average particle size of lithium transition metal oxides based on the above measurement has been confirmed to be nearly identical to the average particle size (the average particle size in the case of secondary particles) measured by extracting 50 particles from an SEM image of the lithium transition metal oxide particles, excluding extremely large and small particles. The particle size of each particle based on this SEM image measurement is calculated as follows: The shortest diameter passing through the center of the smallest circumscribing circle of each particle is defined as the minor diameter, and the diameter passing through the center and perpendicular to the minor diameter is defined as the major diameter. The average of the major and minor diameters is defined as the particle size of each particle. When there are two or more shortest diameters, the longest diameter that intersects at right angles is taken as the shortest diameter. The average particle size is measured in the same manner as above in the cases described later in this specification.

[0024] The average primary particle size and the average particle size are measured using a positive electrode active material prepared by the procedure described in the measurement of the coverage ratio as a measurement sample. Note that, in the present invention, the change in the average primary particle size and the average particle size due to the coating layer is extremely small, so the positive electrode active material can be used as the measurement sample.

[0025] [3] A positive electrode for an electric storage device according to another aspect of the present invention includes the positive electrode active material for an electric storage device according to the above [1] or [2]. The positive electrode for an electric storage device according to the above [3] includes the positive electrode active material for an electric storage device according to the above [1] or [2], and therefore can suppress a decrease in the discharge capacity retention rate of the electric storage device after charge-discharge cycles.

[0026] [4] An electric storage device according to another aspect of the present invention includes the positive electrode for the electric storage device according to the above [3]. The electric storage device according to the above [4] includes the positive electrode for the electric storage device according to the above [3], which includes the positive electrode active material for the electric storage device according to the above [1] or [2], and therefore can suppress a decrease in the discharge capacity retention rate after charge-discharge cycles.

[0027] Hereinafter, a positive electrode active material for an energy storage element, a positive electrode for an energy storage element, an energy storage element, an energy storage device, a method for manufacturing an energy storage element, and other embodiments according to one embodiment of the present invention will be described in detail. Note that the names of the components (elementary components) used in each embodiment may differ from the names of the components (elementary components) used in the background art.

[0028] [Positive electrode active material for energy storage devices] A positive electrode active material for an energy storage element according to one embodiment of the present invention (hereinafter also simply referred to as "positive electrode active material") comprises a lithium transition metal oxide having a spinel-type crystal structure and a coating layer that uniformly coats the lithium transition metal oxide.

[0029] (Lithium transition metal oxides) Lithium transition metal oxides having a spinel crystal structure are usually particulate. A typical example of a lithium transition metal oxide having a spinel crystal structure is a spinel-type lithium manganese oxide. The spinel-type lithium manganese oxide contains lithium, manganese, and oxygen, and may further contain one or more other elements. Examples of other elements include nickel, titanium, vanadium, chromium, iron, copper, zinc, boron, phosphorus, magnesium, aluminum, calcium, zirconium, molybdenum, tungsten, and halogens. The total content of lithium, manganese, and oxygen in the spinel-type lithium manganese oxide may be 80 mol% or more, 90 mol% or more, or 99 mol% or more. The composition ratio (content ratio of each element) of the spinel-type lithium manganese oxide refers to the composition ratio before charge / discharge or when the positive electrode is fully discharged using the above-described method. In this positive electrode active material, for example, manganese ions as transition metal ions can be prevented from eluting into the electrolyte.

[0030] As the spinel-type lithium manganese oxide, for example, a compound represented by the following formula (1) is preferred. Li 1+a Mn 2-a-b A b O4···(1) In formula (1), A is at least one element selected from the group consisting of Ni, Ti, V, Cr, Fe, Cu, Zn, B, P, Mg, Al, Ca, Zr, Mo, and W. a and b satisfy 0≦a≦0.2 and 0≦b≦0.6, respectively.

[0031] In formula (1), a may be 0 or more and 0.1 or less, or 0 or more and 0.05 or less. b may be 0 or more and 0.5 or less, or 0 or more and 0.3 or less, or 0 or more and 0.1 or less, or 0 or more and 0.05 or less. The spinel-type lithium manganese oxide may be, for example, LiMn2O4.

[0032] The lithium transition metal oxide having a spinel crystal structure preferably has a ratio of its average particle size to its average primary particle size of 5 or less, more preferably 4 or less, even more preferably 3 or less, and even more preferably 2.5 or less. When the ratio of the average particle size to the average primary particle size is equal to or less than the above-mentioned upper limit, changes in the surface shape due to cracks between primary particles or the like are unlikely to occur. Therefore, a uniform coating layer is easily maintained on the surface of the lithium transition metal oxide, and elution of transition metal ions is further suppressed. The lower limit of the ratio of the average particle size to the average primary particle size of the lithium transition metal oxide may be 1. Note that, due to differences in the methods for measuring the average primary particle size and the average particle size, the lower limit of the ratio of the average particle size to the average primary particle size may be less than 1, for example, 0.9. The ratio of the average particle size to the average primary particle size of the spinel-type lithium manganese oxide may be equal to or greater than any of the above-mentioned lower limits and equal to or less than any of the above-mentioned upper limits. The ratio of the average particle size to the average primary particle size can be adjusted by increasing the calcination temperature or the calcination time in the production process of the lithium transition metal oxide to grow the crystals of multiple primary particles and increase the primary particle size, or by crushing the secondary particles.

[0033] The lithium transition metal oxide having a spinel-type crystal structure may be a particle (single particle) made up of primary particles, a secondary particle formed by aggregation of multiple primary particles, or a mixed particle of single particles and secondary particles.

[0034] (covering layer) The coating layer contains niobium or phosphorus. The coating layer may contain a compound containing niobium or a compound containing phosphorus, or may contain a compound containing at least one of niobium and phosphorus. The compound contained in the coating layer is preferably a compound containing phosphorus. However, the above-mentioned compounds contained in the coating layer do not include the compounds exemplified as lithium transition metal oxides having a spinel crystal structure. The coating layer may contain only one type of the above-mentioned compound, or may contain two or more types.

[0035] The compound containing niobium element preferably contains oxygen element from the viewpoint of suppressing elution of transition metal ions, and preferably contains lithium element from the viewpoint of facilitating the movement of lithium ions which are charge transport ions. The compound containing niobium element may be, for example, niobium oxides such as NbO and Nb2O5, or may be lithium niobate.

[0036] The compound containing phosphorus element preferably contains oxygen element from the viewpoint of suppressing elution of transition metal ions, and preferably contains lithium element from the viewpoint of facilitating the movement of lithium ions which are charge transport ions. The compound containing phosphorus element may be, for example, phosphorus oxides such as P2O5, or may be lithium phosphate. The compound containing phosphorus element may be a compound represented by Li x P y O z (0 < x < 1, 0 < y < 1, 0 < z < 1).

[0037] The total content of the compound containing niobium element and the compound containing phosphorus element in the coating layer may be 60% by mass or more, may be 80% by mass or more, may be 90% by mass or more, may be 95% by mass or more, or may be 99% by mass or more. That is, the coating layer may consist essentially of the compound containing niobium element and the compound containing phosphorus element.

[0038] The content of the compound containing niobium element in the coating layer may be 60% by mass or more, may be 80% by mass or more, may be 90% by mass or more, may be 95% by mass or more, or may be 99% by mass or more. That is, the coating layer may consist essentially of the compound containing niobium element.

[0039] The content of the compound containing phosphorus element in the coating layer may be 60% by mass or more, may be 80% by mass or more, may be 90% by mass or more, may be 95% by mass or more, or may be 99% by mass or more. That is, the coating layer may consist essentially of the compound containing phosphorus element.

[0040] The coating layer uniformly coats the lithium transition metal oxide having a spinel crystal structure. More specifically, the coating layer uniformly coats the surface of secondary particles of the lithium transition metal oxide having a spinel crystal structure. When the lithium transition metal oxide is a single particle, the coating layer uniformly coats the surface of the single particle. The lower limit of the coverage of the coating layer on the surface of the lithium transition metal oxide is 50%, preferably 60%, more preferably 70%, even more preferably 80%, and even more preferably 90%, from the viewpoint of suppressing the elution of transition metal ions from the lithium transition metal oxide into the electrolyte. The upper limit of the coverage may be 100%. The coating layer can be formed, for example, by treating the lithium transition metal oxide with a solution containing niobium or phosphorus and then calcining the resulting solution. The coverage of the coating layer on the surface of the lithium transition metal oxide can also be increased by spray coating using a tumbling fluidization device manufactured by Powrex Corporation or by forming a film by a sol-gel method.

[0041] When the ratio of the total mass [mg] of niobium and phosphorus to the total mass [g] of the lithium transition metal oxide having a spinel crystal structure and the coating layer is defined as A [mg / g], the lower limit of the ratio A is preferably 0.4 mg / g, more preferably 0.8 mg / g, and even more preferably 1.0 mg / g. On the other hand, the upper limit of the ratio A is preferably 10.0 mg / g, more preferably 5.0 mg / g, and even more preferably 3.0 mg / g. When the ratio A is equal to or greater than the lower limit, leaching of transition metal ions from the lithium transition metal oxide into the electrolyte can be easily suppressed. Furthermore, when the ratio A is equal to or less than the upper limit, the lithium transition metal oxide is not excessively coated with niobium or phosphorus, thereby suppressing an increase in the resistance of the energy storage device. The ratio A may be equal to or greater than any of the lower limits and equal to or less than any of the upper limits.

[0042] A portion of the coating layer may be present inside the secondary particles of the lithium transition metal oxide (for example, at the grain boundaries of the lithium transition metal oxide). In this case, the portion of the coating layer preferably contains a compound containing niobium or a compound containing phosphorus. By having a portion of the coating layer also present inside the secondary particles of the lithium transition metal oxide, the elution of transition metal ions from the lithium transition metal oxide can be further suppressed.

[0043] The total content of the lithium transition metal oxide having a spinel crystal structure and the coating layer in the positive electrode active material may be 60% by mass or more, 80% by mass or more, 90% by mass or more, 95% by mass or more, or 99% by mass or more.

[0044] (physical properties, etc.) The average particle size of the positive electrode active material is preferably, for example, 0.1 μm or more and 20 μm or less. By setting the average particle size of the positive electrode active material to the above lower limit or more, the production and handling of the positive electrode active material becomes easier. By setting the average particle size of the positive electrode active material to the above upper limit or less, the electronic conductivity of the positive electrode active material improves. To obtain a positive electrode active material with a predetermined average particle size, a pulverizer, a classifier, or the like is used. Examples of pulverization methods include a mortar, a ball mill, a sand mill, a vibration ball mill, a planetary ball mill, a jet mill, a counter jet mill, a swirling airflow jet mill, or a sieve. Wet pulverization in the presence of water or an organic solvent such as hexane can also be used during pulverization. As a classification method, a sieve, an air classifier, or the like can be used in both dry and wet methods as needed. To obtain a positive electrode active material with a predetermined average particle size, the firing temperature of the precursor during the production of the positive electrode active material may be controlled. For example, by increasing the baking temperature, the growth of particles of the positive electrode active material is promoted, and therefore the average particle size of the positive electrode active material can be increased.

[0045] In the positive electrode active material, the lower limit of the BET specific surface area B of the portion consisting of the lithium transition metal oxide having a spinel crystal structure and the coating layer is 0.1 m 2 / g is preferred, and 0.4m 2 / g is more preferred, and 0.6m 2 / g is more preferred, and 0.8m 2 On the other hand, the upper limit of the BET specific surface area B of the portion consisting of the lithium transition metal oxide and the coating layer is 6.0 m 2 / g is preferred, 4.0m 2 / g is more preferred, and 2.0m 2 / g is more preferred, and 1.5m 2 / g is even more preferable. When the BET specific surface area B is equal to or greater than the lower limit, the output performance of the energy storage element can be easily maintained. Furthermore, when the BET specific surface area B is equal to or less than the upper limit, elution of transition metal ions from the lithium transition metal oxide into the electrolyte can be easily suppressed. The BET specific surface area B may be equal to or greater than any of the lower limits and equal to or less than any of the upper limits. The BET specific surface area B can be adjusted by the average particle size of the lithium transition metal oxide, the firing temperature when forming the coating layer, etc.

[0046] In the positive electrode active material, the ratio of the total mass [mg] of niobium element and phosphorus element to the total mass [g] of the lithium transition metal oxide having a spinel crystal structure and the coating layer is defined as A [mg / g], and the BET specific surface area of ​​the portion consisting of the lithium transition metal oxide and the coating layer is defined as B [m 2 / g], the lower limit of A / B is 0.4 mg / m 2 and 0.6 mg / m 2 is preferred, and 0.8 mg / m 2 More preferably, 1.0 mg / m 2 The lower limit of the A / B ratio is more preferably 1.5 mg / m 2 or 2.0 mg / m 2 In some cases, the upper limit of the A / B ratio is 9.0 mg / m 2 and 5.0 mg / m 2 is preferred, and 4.0 mg / m 2 more preferably 3.8 mg / m 2 The upper limit of the A / B ratio is preferably 3.0 mg / m 2is even more preferable in some cases. When the A / B ratio is equal to or greater than the lower limit, it is possible to easily prevent transition metal ions from leaching out of the lithium transition metal oxide into the electrolyte. Furthermore, when the A / B ratio is equal to or less than the upper limit, the lithium transition metal oxide is not excessively coated with niobium or phosphorus, and the movement of lithium ions, which are charge-transporting ions, is not inhibited, thereby preventing an increase in resistance in the energy storage device. The A / B ratio may be equal to or greater than any of the lower limits and equal to or less than any of the upper limits.

[0047] [Positive electrode for energy storage element] A positive electrode for an energy storage device according to one embodiment of the present invention (hereinafter also simply referred to as "positive electrode") includes a positive electrode active material according to one embodiment of the present invention.

[0048] The positive electrode has a positive electrode substrate and a positive electrode active material layer disposed on the positive electrode substrate directly or via an intermediate layer.

[0049] The positive electrode substrate is electrically conductive. Whether or not it has "electrical conductivity" is determined by whether the volume resistivity measured in accordance with JIS-H-0505 (1975) is 10 -2 The threshold value is Ω·cm. The material of the positive electrode substrate is a metal such as aluminum, titanium, tantalum, or stainless steel, or an alloy of these. Among these, aluminum or an aluminum alloy is preferred from the viewpoints of potential resistance, high conductivity, and cost. Positive electrode substrates include foils, vapor-deposited films, meshes, and porous materials, 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).

[0050] 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, the strength of the positive electrode substrate can be increased while increasing the energy density per volume of the energy storage element.

[0051] The intermediate layer is a layer disposed 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 configuration of the intermediate layer is not particularly limited, and may contain, for example, a binder and a conductive agent.

[0052] The positive electrode active material layer contains the positive electrode active material described above. The positive electrode active material layer may contain other positive electrode active materials other than the positive electrode active material described above, as well as optional components such as a conductive agent, a binder, a thickener, and a filler, as needed.

[0053] The content of the positive electrode active material in the positive electrode active material layer is preferably 50% by mass to 99% by mass, more preferably 70% by mass to 98% by mass, and even more preferably 80% by mass to 95% by mass. By setting the content of the positive electrode active material within the above range, it is possible to suppress a decrease in the discharge capacity retention rate of the energy storage element after charge-discharge cycling and to achieve both manufacturability.

[0054] The positive electrode active material layer may further contain a positive electrode active material other than the positive electrode active material according to one embodiment of the present invention. As the other positive electrode active material, various conventionally known positive electrode active materials can be used. However, the content of the positive electrode active material according to one embodiment of the present invention relative to all the positive electrode active materials contained in the positive electrode active material layer (the total of the positive electrode active material according to one embodiment of the present invention and the other positive electrode active materials) is preferably 90 mass% or more, more preferably 99 mass% or more, and even more preferably 100 mass%. By thus constructing the positive electrode active material substantially only from the positive electrode active material according to one embodiment of the present invention, it is possible to easily suppress a decrease in the discharge capacity retention rate of the energy storage device after charge / discharge cycling.

[0055] The conductive agent is not particularly limited as long as it is a material having electrical conductivity. Examples of such conductive agents include carbon materials, metals, and conductive ceramics. Examples of carbon 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 may be in the form of powder or fiber. As the conductive agent, one of these materials may be used alone, or two or more may be mixed. These materials may also be used in combination. For example, a composite of carbon black and CNTs may be used. Among these, carbon black is preferred from the viewpoints of electronic conductivity and coatability, and acetylene black is particularly preferred.

[0056] The content of the conductive agent in the positive electrode active material layer is preferably 1% by mass to 10% by mass, more preferably 3% by mass to 9% by mass. By setting the content of the conductive agent within this range, the energy density of the energy storage element can be increased.

[0057] 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.

[0058] The content of the binder in the positive electrode active material layer is preferably 1% by mass to 10% by mass, more preferably 3% by mass to 9% by mass. By setting the binder content within this range, the positive electrode active material can be stably maintained.

[0059] Examples of thickeners include polysaccharide polymers such as carboxymethyl cellulose (CMC) and methyl cellulose. When the thickener has a functional group that reacts with lithium or the like, the functional group may be deactivated in advance by methylation or the like. When a thickener is used, the content of the thickener in the positive electrode active material layer can be, for example, 0.1% by mass or more and 8% by mass or less, and can also be 5% by mass or less, or 1% by mass or less. The technology disclosed herein can be preferably implemented in an embodiment in which the positive electrode active material layer does not contain a thickener.

[0060] 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-derived substances such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica; and artificial products thereof. When a filler is used, the content of the filler in the positive electrode active material layer can be, for example, 0.1% by mass or more and 8% by mass or less, and can also be 5% by mass or less, or 1% by mass or less. The technology disclosed herein can be preferably implemented in an embodiment in which the positive electrode active material layer does not contain a filler.

[0061] The positive electrode active material layer may contain typical non-metallic 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 according to one embodiment of the present invention, other positive electrode active materials, conductive agents, binders, thickeners, and fillers.

[0062] [Energy storage element] An energy storage element according to one embodiment of the present invention includes an electrode assembly having a positive electrode, a negative electrode, and a separator, an electrolyte, and a container that accommodates the electrode assembly and the electrolyte. The electrode assembly is typically a stacked type in which a plurality of positive electrodes and a plurality of negative electrodes are stacked with separators interposed therebetween, or a wound type in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween and wound. The electrolyte is present in a state contained in the positive electrode, the negative electrode, and the separator. The electrolyte may be a nonaqueous electrolyte. As an example of an energy storage element, a nonaqueous electrolyte secondary battery (hereinafter simply referred to as a "secondary battery") in which the electrolyte is a nonaqueous electrolyte will be described.

[0063] [Positive electrode] The positive electrode provided in the energy storage element is the positive electrode according to one embodiment of the present invention described above.

[0064] [Negative electrode] The negative electrode has a negative electrode substrate and a negative electrode active material layer disposed on the negative electrode substrate directly or via an intermediate layer. The configuration of the intermediate layer is not particularly limited and can be selected from the configurations exemplified for the positive electrode above, for example.

[0065] The negative electrode substrate is conductive. Metals such as copper, nickel, stainless steel, nickel-plated steel, and aluminum, or alloys thereof, carbon materials, and the like are used as the material for the negative electrode substrate. Among these, copper or copper alloys are preferred. Examples of the negative electrode substrate include foil, vapor-deposited film, mesh, and porous material, with foil being preferred from the viewpoint of cost. 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.

[0066] 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, the strength of the negative electrode substrate can be increased while increasing the energy density per volume of the energy storage element.

[0067] The negative electrode active material layer contains a negative electrode active material. The negative electrode active material layer contains optional components such as a conductive agent, a binder, a thickener, and a filler, as needed. The optional components such as the conductive agent, the binder, the thickener, and the filler can be selected from the materials exemplified for the positive electrode above.

[0068] The negative electrode active material layer may contain typical non-metallic 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, Ta, Hf, Nb, and W as components other than the negative electrode active material, conductive agent, binder, thickener, and filler.

[0069] The negative electrode active material can be appropriately selected from known negative electrode active materials. A material capable of absorbing and releasing lithium ions is usually used as the negative electrode active material for lithium ion secondary batteries. Examples of the negative electrode active material include metallic Li; metals or semimetals such as Si and Sn; metal oxides or semimetal oxides such as Si oxide, Ti oxide, and Sn oxide; and Li4Ti5O 12 , LiTiO 2、 Examples of the material include titanium-containing oxides such as TiNbO, polyphosphate compounds, silicon carbide, and carbon materials such as graphite and non-graphitizable carbon (easily graphitizable carbon or non-graphitizable carbon). In the negative electrode active material layer, one of these materials may be used alone, or two or more may be used in combination.

[0070] "Graphite" refers to graphite that has an average lattice spacing (d 002 ) is 0.33 nm or more and less than 0.34 nm. Examples of graphite include natural graphite and artificial graphite. Artificial graphite is preferred from the viewpoint of being able to obtain a material with stable physical properties.

[0071] "Non-graphitic carbon" refers to the carbon that has an average lattice spacing (d 002) is 0.34 nm or more and 0.42 nm or less. Non-graphitic carbon includes non-graphitizable carbon and graphitizable carbon. Examples of non-graphitic carbon include resin-derived materials, petroleum pitch or petroleum pitch-derived materials, petroleum coke or petroleum coke-derived materials, plant-derived materials, and alcohol-derived materials.

[0072] Here, the "discharged state" refers to a state in which the negative electrode active material, a carbonaceous material, is discharged so that lithium ions capable of being absorbed and desorbed during charging and discharging are sufficiently released. For example, this refers to a state in which the open circuit voltage of a half cell using a negative electrode containing a carbonaceous material as a negative electrode active material as a working electrode and metallic Li as a counter electrode is 0.7 V or higher.

[0073] "Non-graphitizable carbon" means the above d 002 This refers to carbon materials with a particle size of 0.36 nm or more and 0.42 nm or less.

[0074] "Graphitizable carbon" means the above d 002 This refers to carbon materials with a particle size of 0.34 nm or more and less than 0.36 nm.

[0075] The negative electrode active material is usually in the form of particles (powder). The average particle size of the negative electrode active material can be, for example, 1 nm or more and 100 μm or less. When the negative electrode active material is a carbon material, a titanium-containing oxide, or a polyphosphate compound, the average particle size may be 1 μm or more and 100 μm or less. When the negative electrode active material is Si, Sn, Si oxide, Sn oxide, or the like, the average particle size may be 1 nm or more and 1 μm or less. By setting the average particle size of the negative electrode active material to be equal to or more than the above lower limit, the negative electrode active material can be easily produced or handled. By setting the average particle size of the negative electrode active material to be equal to or less than the above upper limit, the electronic conductivity of the negative electrode active material layer is improved. The method for obtaining powder with a predetermined particle size can be selected from the methods exemplified for the positive electrode active material. When the negative electrode active material is a metal such as metallic Li, the negative electrode active material layer may be in the form of a foil.

[0076] The content of the negative electrode active material in the negative electrode active material layer is preferably 60% by mass to 99% by mass, more preferably 90% by mass to 98% by mass. By setting the content of the negative electrode active material within this range, both high energy density and manufacturability of the negative electrode active material layer can be achieved.

[0077] The content of the conductive agent in the negative electrode active material layer can be, for example, 0.1% by mass to 5% by mass, or 3% by mass or less, or 1% by mass or less, or the negative electrode active material layer may not contain a conductive agent.

[0078] The binder content in the negative electrode active material layer is preferably 0.5% by mass to 5% by mass, more preferably 0.8% by mass to 5% by mass. By setting the binder content within this range, the negative electrode active material and the like can be stably held.

[0079] The content of the thickener in the negative electrode active material layer is, for example, preferably from 0.1% by mass to 6% by mass, and more preferably from 0.5% by mass to 3% by mass.

[0080] The content of the filler in the negative electrode active material layer may be, for example, 0.1% by mass to 8% by mass, or 0.5% by mass to 5% by mass, 3% by mass or less, or 1% by mass or less, or the negative electrode active material layer may not contain a filler.

[0081] [Separator] The separator can be appropriately selected from known separators. Examples of separators that can be used include separators consisting of only a substrate layer and separators in which a heat-resistant layer containing heat-resistant particles and a binder is formed on one or both surfaces of the substrate layer. Examples of the shape of the substrate layer of the separator include woven fabric, nonwoven fabric, and porous resin film. Among these shapes, porous resin films are preferred from the viewpoint of strength, and nonwoven fabrics are preferred from the viewpoint of non-aqueous electrolyte retention. Materials for the substrate layer of the separator are preferably polyolefins such as polyethylene and polypropylene from the viewpoint of shutdown function, and polyimide and aramid from the viewpoint of oxidative decomposition resistance. A composite material of these resins may also be used for the substrate layer of the separator.

[0082] The heat-resistant particles contained in the heat-resistant layer preferably exhibit a mass loss of 5% or less when heated from room temperature to 500°C in an air atmosphere at 1 atmosphere pressure, and more preferably exhibit a mass loss of 5% or less when heated from room temperature to 800°C. Examples of materials exhibiting a mass loss of a predetermined value or less include inorganic compounds. Examples of inorganic compounds include oxides such as iron oxide, silicon oxide, aluminum oxide, titanium oxide, zirconium oxide, calcium oxide, strontium oxide, barium oxide, magnesium oxide, and aluminosilicate; nitrides such as aluminum nitride and silicon nitride; carbonates such as calcium carbonate; sulfates such as barium sulfate; sparingly soluble ionic crystals such as calcium fluoride, barium fluoride, and barium titanate; covalently bonded crystals such as silicon and diamond; mineral-derived substances such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica, as well as artificial products thereof. As the inorganic compound, these substances may be used alone or in the form of a complex, or two or more of them may be used in combination. Among these inorganic compounds, silicon oxide, aluminum oxide, or aluminosilicate is preferred from the viewpoint of the safety of the energy storage device.

[0083] The porosity of the separator is preferably 80% by volume or less from the viewpoint of strength, and 20% by volume or more from the viewpoint of discharge performance. Here, "porosity" refers to a volume-based value measured with a mercury porosimeter.

[0084] The separator may be a polymer gel composed of a polymer and an electrolyte. Examples of polymers include polyacrylonitrile, polyethylene oxide, polypropylene oxide, polymethyl methacrylate, polyvinyl acetate, polyvinylpyrrolidone, and polyvinylidene fluoride. The use of a polymer gel has the effect of suppressing leakage. The separator may be a combination of the porous resin film or nonwoven fabric described above and a polymer gel.

[0085] [Non-aqueous electrolyte] The nonaqueous electrolyte can be appropriately selected from known nonaqueous electrolytes. The nonaqueous electrolyte may be a nonaqueous electrolyte solution. The nonaqueous electrolyte solution includes a nonaqueous solvent and an electrolyte salt dissolved in the nonaqueous solvent.

[0086] The non-aqueous solvent can be appropriately selected from known non-aqueous solvents. Examples of non-aqueous solvents include cyclic carbonates, chain carbonates, carboxylic acid esters, phosphate esters, sulfonic acid esters, ethers, amides, and nitriles. Non-aqueous solvents in which some of the hydrogen atoms contained in these compounds have been substituted with halogens may also be used.

[0087] 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, 1,2-diphenylvinylene carbonate, etc. Among these, EC is preferred.

[0088] Examples of the chain carbonate include diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diphenyl carbonate, trifluoroethyl methyl carbonate, bis(trifluoroethyl) carbonate, etc. Among these, EMC is preferred.

[0089] It is preferable to use a cyclic carbonate or a chain carbonate as the non-aqueous solvent, and it is more preferable to use a combination of a cyclic carbonate and a chain carbonate. The use of a cyclic carbonate can promote dissociation of the electrolyte salt and improve the ionic conductivity of the non-aqueous electrolyte. The use of a chain carbonate can reduce the viscosity of the non-aqueous electrolyte. When a cyclic carbonate and a chain carbonate are used in combination, the volume ratio of the cyclic carbonate to the chain carbonate (cyclic carbonate:chain carbonate) is preferably in the range of, for example, 5:95 to 50:50.

[0090] The electrolyte salt can be appropriately selected from known electrolyte salts, such as lithium salts, sodium salts, potassium salts, magnesium salts, and onium salts. Of these, lithium salts are preferred.

[0091] Examples of lithium salts include inorganic lithium salts such as LiPF6, LiPO2F2, LiBF4, LiClO4, and LiN(SO2F)2, lithium oxalates such as lithium bis(oxalate)borate (LiBOB), lithium difluorooxalateborate (LiFOB), and lithium bis(oxalate)difluorophosphate (LiFOP), and lithium salts having a halogenated hydrocarbon group such as LiSO3CF3, LiN(SO2CF3)2, LiN(SO2C2F5)2, LiN(SO2CF3)(SO2C4F9), LiC(SO2CF3)3, and LiC(SO2C2F5)3. Among these, inorganic lithium salts are preferred, and LiPF6 is more preferred.

[0092] The content of electrolyte salt in the non-aqueous electrolyte is 0.1 mol / dm at 20°C and 1 atmosphere. 3More than 2.5mol / dm 3 It is preferable that the value is 0.3 mol / dm or less. 3 More than 2.0mol / dm 3 It is more preferable that it is 0.5 mol / dm or less. 3 More than 1.7mol / dm 3 It is more preferable that it is 0.7 mol / dm or less. 3 More than 1.5mol / dm 3 By setting the content of the electrolyte salt within the above range, the ionic conductivity of the non-aqueous electrolyte can be increased.

[0093] The non-aqueous electrolyte may contain additives in addition to the non-aqueous solvent and the electrolyte salt. Examples of additives include halogenated carbonates such as fluoroethylene carbonate (FEC) and difluoroethylene carbonate (DFEC); oxalates such as lithium bis(oxalate)borate (LiBOB), lithium difluorooxalateborate (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; partial halides of the above aromatic compounds such as 2-fluorobiphenyl, o-cyclohexylfluorobenzene, and p-cyclohexylfluorobenzene; halogenated anisole compounds such as 2,4-difluoroanisole, 2,5-difluoroanisole, 2,6-difluoroanisole, and 3,5-difluoroanisole; vinylene carbonate, methylvinylene carbonate, ethylvinylene carbonate, and the like. Carbonate, succinic anhydride, glutaric anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, cyclohexanedicarboxylic 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 ,3,2-dioxathiolane), 4-methylsulfonyloxymethyl-2,2-dioxo-1,3,2-dioxathiolane, thioanisole, diphenyl disulfide, dipyridinium disulfide, 1,3-propene sultone, 1,3-propane sultone, 1,4-butane sultone, 1,4-butene sultone, perfluorooctane, tristrimethylsilyl borate, tristrimethylsilyl phosphate, tetrakistrimethylsilyl titanate, lithium monofluorophosphate, lithium difluorophosphate, etc. These additives may be used alone or in combination of two or more.

[0094] The content of the additive contained in the non-aqueous electrolyte is preferably 0.01% by mass to 10% by mass, more preferably 0.1% by mass to 7% by mass, even more preferably 0.2% by mass to 5% by mass, and particularly preferably 0.3% by mass to 3% by mass. By setting the content of the additive within the above range, it is possible to improve the capacity retention performance or cycle performance after high-temperature storage, and further improve safety.

[0095] The non-aqueous electrolyte may be a solid electrolyte, or a non-aqueous electrolytic solution and a solid electrolyte may be used in combination.

[0096] The solid electrolyte can be selected from any material that has ionic conductivity of lithium, sodium, calcium, etc., and is solid at room temperature (e.g., 15° C. to 25° C.) Examples of the solid electrolyte include sulfide solid electrolytes, oxide solid electrolytes, nitride solid electrolytes, and polymer solid electrolytes.

[0097] As the sulfide solid electrolyte, for example, Li2S-P2S5, LiI-Li2S-P2S5, Li 10 Ge-P2S 12 etc.

[0098] The shape of the energy storage element of this embodiment is not particularly limited, and examples thereof include a cylindrical battery, a prismatic battery, a flat battery, a coin battery, and a button battery.

[0099] FIG. 1 shows an energy storage element 1 as an example of a prismatic battery. Note that this figure is a see-through view of the inside of the container. An electrode assembly 2 having a positive electrode and a negative electrode wound with a separator sandwiched between them is housed in a prismatic container 3. The positive electrode is electrically connected to a positive electrode terminal 4 via a positive electrode lead 41. The negative electrode is electrically connected to a negative electrode terminal 5 via a negative electrode lead 51.

[0100] [Electricity storage device] The energy storage element of this embodiment can be mounted as an energy storage unit (battery module) comprising a collection of a plurality of energy storage elements in an automobile power source such as an electric vehicle (EV), a hybrid electric vehicle (HEV), or a plug-in hybrid electric vehicle (PHEV), a power source for electronic devices such as a personal computer or a communication terminal, or a power source for power storage, etc. In this case, the technology of the present invention may be applied to at least one energy storage element included in the energy storage unit.

[0101] 2 shows an example of a power storage device 30 in which power storage units 20, each of which is an assembly of two or more electrically connected power storage elements 1, are further assembled. The power storage device 30 may include a bus bar (not shown) that electrically connects two or more power storage elements 1, a bus bar (not shown) that electrically connects two or more power storage units 20, etc. The power storage units 20 or the power storage device 30 may include a status monitoring device (not shown) that monitors the status of one or more power storage elements.

[0102] [Method of manufacturing an energy storage element] 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 assembly, preparing an electrolyte, and housing the electrode assembly and the electrolyte in a container. Preparing the electrode assembly includes preparing a positive electrode and a negative electrode, and forming the electrode assembly by stacking or winding the positive electrode and the negative electrode with a separator interposed therebetween.

[0103] The method for placing the electrolyte in the container can be appropriately selected from known methods. For example, when a non-aqueous electrolyte solution is used as the electrolyte, the non-aqueous electrolyte solution may be poured into the container through an inlet formed in the container, and the inlet may then be sealed.

[0104] [Other embodiments] The positive electrode active material, positive electrode, and energy storage device of the present invention are not limited to the above-described embodiments, and various modifications may be made without departing from the spirit of the present invention. For example, the configuration of one embodiment can be added to the configuration of another embodiment, and part of the configuration of one embodiment can be replaced with the configuration of another embodiment or well-known technology. Furthermore, part of the configuration of one embodiment can be deleted. Also, well-known technology can be added to the configuration of one embodiment.

[0105] In the above embodiment, the case where the energy storage element is used as a chargeable and dischargeable non-aqueous electrolyte secondary battery (for example, a lithium ion secondary battery) has been described, but the energy storage element may be of any type, shape, size, capacity, etc. The present invention can also be applied to various secondary batteries, electric double layer capacitors, lithium ion capacitors, and other capacitors. [Example]

[0106] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0107] [Example 1] (Preparation of positive electrode) Lithium manganate (LiMn2O4) was prepared as a lithium transition metal oxide and subjected to a coating treatment using a tumbling fluidization device manufactured by Powrex. The coating treatment was performed by diluting a mixed solution of lithium ethoxide and niobium (V) ethoxide with absolute ethanol and spraying it onto the lithium manganate. The preliminary drying temperature was 100°C. Then, main baking was performed at 350°C in a nitrogen atmosphere to obtain positive electrode active material particles in which a coating layer made of lithium niobate uniformly coated the surface of the lithium manganate. The coverage rate of the coating layer was 80% or more. The lithium manganate particles were particles in which the ratio of the average particle size to the average primary particle size was 5 or less, and this also applies to the lithium manganate described in the following Examples and Comparative Examples. The ratio A of the mass [mg] of the coating element to the mass [g] of the positive electrode active material particles was 3.2 [mg / g], and the BET specific surface area B of the positive electrode active material particles was 0.9 [m 2 / g].

[0108] A positive electrode mixture paste was prepared using the positive electrode active material, acetylene black (AB) as a conductive agent, polyvinylidene fluoride (PVDF) as a binder, and N-methylpyrrolidone (NMP) as a dispersion medium. The mass ratio of the positive electrode active material, AB, and PVDF was 90:5:5 in terms of solid content. This positive electrode mixture paste was applied to both sides of aluminum foil as a positive electrode substrate and dried. A roll press was performed to obtain a positive electrode in which positive electrode active material layers were laminated on both sides of the positive electrode substrate.

[0109] (Preparation of negative electrode) A negative electrode mixture paste was prepared by mixing graphite as the negative electrode active material, styrene-butadiene rubber (SBR) as a binder, carboxymethyl cellulose (CMC) as a thickener, and water as a dispersion medium. The mass ratio of graphite to SBR to CMC was 97.8:1.0:1.2 in terms of solid content. This negative electrode mixture paste was applied to both sides of copper foil as the negative electrode substrate and dried. Then, roll pressing was performed to obtain a negative electrode in which negative electrode active material layers were laminated on both sides of the negative electrode substrate.

[0110] (electrolyte) A 1.0 mol / dm 3 The electrolyte was obtained by dissolving LiPF6 in the solution at a concentration of

[0111] (Fabrication of energy storage element) The positive electrode and the negative electrode were stacked with a polyolefin microporous membrane separator interposed therebetween to prepare an electrode assembly, which was then housed in a container made of a metal resin composite film, the electrolyte was poured into the container, and the container was sealed by heat welding to obtain the energy storage element of Example 1.

[0112] [Comparative Example 1] The ratio A [mg / g] of the mass [mg] of the coating element to the mass [g] of the positive electrode active material particles, and the BET specific surface area B [m 2 / g] was adjusted as shown in Table 1, the same procedure as in Example 1 was used to obtain a storage element of Comparative Example 1. The BET specific surface area of ​​the positive electrode active material particles was adjusted by controlling the firing temperature when synthesizing the positive electrode active material, and this also applies to the following. In Table 1, LMO means lithium manganese oxide (LiMn2O4), and Li x P y O z means a compound containing lithium, phosphorus, and oxygen. "Island-like" means that the coverage of the coating layer is less than 50%.

[0113] Comparative Example 2 Lithium manganese oxide (LiMn2O4) was prepared as a lithium transition metal oxide, and lithium niobate (LiNbO3) powder was added to a positive electrode mixture paste made by mixing lithium manganese oxide with a conductive additive and a binder. This positive electrode mixture paste was then coated on both sides of an aluminum foil as a positive electrode substrate and dried. The ratio A of the mass [mg] of the coating element to the mass [g] of the positive electrode active material particles and the BET specific surface area B [m 2 / g] are as shown in Table 1. The coverage of the coating layer was 20% or less. A roll press was performed to obtain a positive electrode in which positive electrode active material layers were laminated on both sides of the positive electrode substrate. Using this positive electrode, a storage element of Comparative Example 2 was obtained by the same procedure as in Example 1.

[0114] [Examples 2 to 4] The energy storage devices of Examples 2 to 4 were obtained in the same manner as in Comparative Example 2, except that a phosphonic acid (H3PO3) solution was added to the positive electrode mixture paste. The amounts of phosphonic acid added in Examples 2 to 4 were 0.1 mass%, 0.3 mass%, and 0.5 mass% based on the positive electrode active material particles, respectively. The ratio A of the mass [mg] of the coating element to the mass [g] of the positive electrode active material particles, and the BET specific surface area B [m 2 / g] was as shown in Table 1.

[0115] Comparative Example 3 The energy storage element of Comparative Example 3 was obtained in the same manner as in Comparative Example 2, except that trilithium phosphate (Li3PO4) powder was added to the positive electrode mixture paste. The ratio A of the mass [mg] of the coating element to the mass [g] of the positive electrode active material particles and the BET specific surface area B [m 2 / g] was as shown in Table 1.

[0116] Comparative Example 4 The energy storage element of Comparative Example 4 was obtained using the same procedure as in Example 1, except that lithium manganese oxide (LiMn2O4) was prepared as the lithium transition metal oxide and the positive electrode active material particles were obtained without coating the lithium transition metal oxide with a coating layer.

[0117] Table 1 shows the mass ratio A [mg / g] of the coating element for each energy storage element and the BET specific surface area B [m 2 / g], and the resulting A / B [mg / m 2 ] is shown.

[0118] [evaluation] (1)Initial charge / discharge Each of the obtained energy storage elements was initially charged and discharged at 25°C as follows. Constant-current, constant-voltage charging was performed with a charging current of 0.2 C and a cut-off voltage of 4.1 V. Charging was terminated until the charging current reached 0.01 C. A 10-minute rest period was then provided. Subsequently, constant-current discharging was performed with a discharging current of 0.2 C and a cut-off voltage of 2.75 V.

[0119] (2) Initial capacity confirmation test Next, an initial capacity confirmation test was performed on each energy storage device at 25°C as follows. Constant current / constant voltage charging was performed with a charging current of 1.0 C and a cut-off voltage of 4.1 V. The charging was terminated until the charging current reached 0.01 C. A 10-minute rest period was then provided. Subsequently, constant current discharging was performed with a discharging current of 1.0 C and a cut-off voltage of 2.75 V. The discharge capacity at this time was defined as the "initial discharge capacity." The fully charged state based on the initial discharge capacity was defined as 100% SOC (State of Charge).

[0120] (3) Charge / discharge cycle test After the initial capacity confirmation test, each of the energy storage elements was subjected to a charge-discharge cycle test at 45° C. in the following manner. The battery was charged at a constant current of 1.0 C until the SOC reached 100%, then discharged at a constant current of 1.0 C until the SOC reached 0%, and this charge / discharge cycle was repeated 150 times.

[0121] (4) Capacity confirmation test after charge / discharge cycle test After the charge-discharge cycle test, a capacity confirmation test after the charge-discharge cycle test was conducted in the same manner as the initial capacity confirmation test. The discharge capacity at this time was designated as the "discharge capacity after the charge-discharge cycle test." The percentage of the discharge capacity after the charge-discharge cycle test to the initial discharge capacity for each energy storage element was calculated as the "discharge capacity retention rate." Table 1 shows the relative values ​​of the discharge capacity retention rate with Comparative Example 4 as the reference.

[0122] [Table 1]

[0123] As shown in Table 1, in Comparative Example 2, A / B was 9.0 mg / m 2 In Comparative Example 1, the coating state was uniform, but A / B was 9.0 mg / m or more, and the coating state was island-like, so the discharge capacity retention rate was equivalent to that of Comparative Example 4, which had no coating layer. 2 In contrast, in Examples 1 to 4, A / B was 0.4 mg / m 2 More than 9.0mg / m 2 or less, and since the coating state was uniform, the discharge capacity retention rate was higher than that of Comparative Examples 1 to 4. It is presumed that in Examples 1 to 4, the elution of manganese ions from the lithium transition metal oxide was suppressed. [Industrial Applicability]

[0124] The present invention can be applied to electric storage elements used as power sources for electronic devices such as personal computers and communication terminals, and automobiles. [Explanation of symbols]

[0125] 1. Energy storage element 2 Electrode body 3 containers 4 Positive terminal 41 Positive lead 5 Negative terminal 51 Negative lead 20 Energy storage unit 30 Energy storage device

Claims

1. a lithium transition metal oxide having a spinel-type crystal structure; a coating layer that uniformly coats the lithium transition metal oxide; and the coating layer contains niobium or phosphorus, The ratio of the total mass [mg] of the niobium element and the phosphorus element to the total mass [g] of the lithium transition metal oxide and the coating layer is defined as A [mg / g], and the BET specific surface area of ​​the portion consisting of the lithium transition metal oxide and the coating layer is defined as B [m ２ / g], A / B is 0.4 mg / m ２ 9.0mg / m or more ２ A positive electrode active material for a storage element, which is:

2. 2. The positive electrode active material for an electric storage device according to claim 1, wherein the ratio of the average particle size to the average primary particle size of the lithium transition metal oxide is 5 or less.

3. A positive electrode for an electric storage device, comprising the positive electrode active material for an electric storage device according to claim 1 or 2.

4. An electric storage element comprising the positive electrode for an electric storage element according to claim 3 .

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