Energy storage element

Abstract

To provide a power storage element with reduced resistance.SOLUTION: A power storage element comprises: a positive electrode; a negative electrode; a separator; and a non-aqueous electrolyte. The positive electrode includes a positive electrode active material layer including a positive electrode active material, the positive electrode active material includes a transition metal composite oxide, a specific surface area of the positive electrode active material is 3.0 m2 / g or more, the positive electrode active material layer indicates a peak of F1 s within the range of 680 eV or more and 683 eV or less in a spectrum by an X-ray photoelectron spectroscopy, indicates a peak of P2p within the range of 130 eV or more and 135 eV or less, and indicates the peak of O1 s within the range of 527 eV or more and 533 eV or less. Relative element concentration of fluorine, phosphate, and oxygen based on the peak of F1 s, the peak of P2p, and the peal of O1 s, in the spectrum are 2.0 atm% or more, 0.5 atm% or more, and 10 atm% or more, respectively.SELECTED DRAWING: None

Description

[Technical Field]

[0001] This invention relates to an energy storage element. [Background technology]

[0002] Non-aqueous electrolyte secondary batteries, such as lithium-ion batteries, are widely used in electronic devices like personal computers and communication terminals, as well as 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] A common form of energy storage element is one in which a positive electrode and a negative electrode are formed by laminating active material layers containing an active material on an electrode substrate.

[0004] A coating is formed on the surface of the active materials of the positive and negative electrodes due to charging and discharging, consisting of decomposition products of non-aqueous electrolytes and the like. For example, the coating formed on the surface of the positive electrode active material is known to suppress the chemical reaction between the positive electrode and the non-aqueous electrolyte, contributing to improved charge-discharge cycle performance (see Patent Document 1). [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2018-018644 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] In addition to the charge-discharge cycle performance mentioned above, output performance is another requirement for energy storage elements. The aforementioned coating can easily inhibit ion movement at the interface between the positive electrode active material and the non-aqueous electrolyte, potentially increasing the resistance of the energy storage element and thus reducing its output performance. Therefore, improvements to the energy storage element are desirable so that the coating is formed with low resistance.

[0007] This invention was made based on the circumstances described above, and aims to provide an energy storage element with reduced resistance. [Means for solving the problem]

[0008] An energy storage element according to one aspect of the present invention comprises a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte, wherein the positive electrode has a positive electrode active material layer containing a positive electrode active material, the positive electrode active material contains a transition metal composite oxide, and the specific surface area of ​​the positive electrode active material layer is 3.0 m². 2 The concentration is 1 / g or more, and the positive electrode active material layer, in a spectrum obtained by X-ray photoelectron spectroscopy, shows an F1s peak in the range of 680 eV to 683 eV, a P2p peak in the range of 130 eV to 135 eV, and an O1s peak in the range of 527 eV to 533 eV, and the relative elemental concentrations of fluorine, phosphorus, and oxygen based on the F1s peak, P2p peak, and O1s peak in the spectrum are 2.0 atm% or more, 0.5 atm% or more, and 10 atm% or more, respectively. [Effects of the Invention]

[0009] In one aspect of the present invention, the energy storage element has reduced resistance. [Brief explanation of the drawing]

[0010] [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]

[0011] First, an overview of the energy storage elements disclosed herein will be provided.

[0012] An energy storage element according to one aspect of the present invention comprises a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte, wherein the positive electrode has a positive electrode active material layer containing a positive electrode active material, the positive electrode active material contains a transition metal composite oxide, and the specific surface area of ​​the positive electrode active material layer is 3.0 m². 2 The concentration is 1 / g or more, and the positive electrode active material layer, in a spectrum obtained by X-ray photoelectron spectroscopy, shows an F1s peak in the range of 680 eV to 683 eV, a P2p peak in the range of 130 eV to 135 eV, and an O1s peak in the range of 527 eV to 533 eV, and the relative elemental concentrations of fluorine, phosphorus, and oxygen based on the F1s peak, P2p peak, and O1s peak in the spectrum are 2.0 atm% or more, 0.5 atm% or more, and 10 atm% or more, respectively.

[0013] The reason for the reduced resistance in the energy storage element is not clear, but the following reason is speculated. The positive electrode active material layer of the energy storage element shows an F1s peak in the range of 680 eV to 683 eV, a P2p peak in the range of 130 eV to 135 eV, and an O1s peak in the range of 527 eV to 533 eV in the spectrum obtained by X-ray photoelectron spectroscopy. In other words, a coating made of a mixture containing lithium fluoride and phosphate is formed on the surface of the positive electrode active material layer. In this way, the surface of the positive electrode active material layer is coated with a mixture containing lithium fluoride and phosphate, and the relative elemental concentrations of fluorine, phosphorus, and oxygen are 2.0 atm%, 0.5 atm%, and 10 atm%, respectively, which increases the ionic conductivity at the interface between the positive electrode active material layer and the non-aqueous electrolyte, thereby reducing the resistance of the positive electrode active material layer. Here, fluorine, phosphorus, and oxygen refer to elements that constitute a compound, not individual elements. Furthermore, the specific surface area of ​​the positive electrode active material layer is 3.0 m². 2Because the concentration is greater than or equal to / g, the contact area between the positive electrode active material layer and the non-aqueous electrolyte increases, which is presumed to form a uniform and dense coating of the mixture, thereby reducing the resistance of the positive electrode active material layer. Due to the above factors, the resistance of the energy storage element is reduced.

[0014] Preferably, the above transition metal composite oxide has an α-NaFeO2 structure and contains nickel, cobalt, and at least one of manganese and aluminum. By having the above transition metal composite oxide have an α-NaFeO2 structure and containing nickel, cobalt, and at least one of manganese and aluminum, the energy density of the positive electrode increases and the output performance is improved.

[0015] The above positive electrode active material layer has an S2p spectrum in the range of 164 eV to 168 eV. 3 / 2 It is preferable to show a peak. Thus, the inclusion of sulfur in the coating formed on the surface of the positive electrode active material layer tends to improve the chemical stability of the positive electrode active material layer.

[0016] The non-aqueous electrolyte preferably contains an oxalate compound, and the content of the oxalate compound is preferably 0.2% by mass or more and 1.5% by mass or less relative to the total amount of the non-aqueous electrolyte. By containing an oxalate compound in the non-aqueous electrolyte and setting the content of the oxalate compound within the above range, the coating formed on the surface of the positive electrode active material layer becomes thinner and more uniform, thereby further reducing the resistance of the positive electrode active material layer.

[0017] In this invention, the specific surface area of ​​the positive electrode active material layer is determined from the adsorption isotherm using the nitrogen gas adsorption method. Specifically, the specific surface area of ​​the positive electrode active material layer is measured by the following method. First, the energy storage element is discharged with a constant current of 0.05C to the lower limit voltage for normal use. Here, "normal use" refers to the case where the energy storage element is used under the discharge conditions recommended or specified for the energy storage element. The energy storage element is disassembled, the positive electrode is removed, and a test battery is assembled with metallic Li as the counter electrode. The positive electrode potential is 3.0V vs. Li / Li at a current of 10mA per gram of positive electrode active material. + Constant current discharge is performed until the battery reaches a certain state. After that, the test battery is disassembled again and the positive electrode is removed. The electrolyte adhering to the removed positive electrode is thoroughly washed using dimethyl carbonate, and after vacuum drying at room temperature for 24 hours, the positive electrode active material layer is collected and used as the sample to be measured. 1.00 g of the sample to be measured (positive electrode active material layer) powder is placed in a sample tube for measurement and vacuum drying at 120°C for 12 hours to thoroughly remove moisture from the sample. Next, the adsorption isotherm is measured using the nitrogen gas adsorption method with liquid nitrogen in the range of relative pressure P / P0 (P0 = approximately 770 mmHg) from 0 to 1. Five points are extracted from the region of P / P0 = 0.05 to 0.3 of the obtained adsorption isotherm and a BET plot is performed, and the BET specific surface area is calculated from the y-intercept and slope of the line. The disassembly of the energy storage element and the collection of the positive electrode active material layer are performed in an argon atmosphere with a dew point of -60°C or lower.

[0018] In the present invention, the measurement of the positive electrode active material layer using X-ray photoelectron spectroscopy is carried out, for example, as follows. First, the energy storage element is discharged with a constant current of 0.05C to the lower limit voltage during normal use. The energy storage element is disassembled and the positive electrode is removed. The positive electrode is thoroughly washed with dimethyl carbonate and then dried under reduced pressure at room temperature for 24 hours. The dried positive electrode is cut to a predetermined size (for example, 2cm x 2cm square) and used as the sample for measuring the spectrum by X-ray photoelectron spectroscopy. After preparing the sample as described above, the measurement is performed using Shimadzu Corporation's "KRATOS Nova" with the X-ray source set to AlKα rays, the acceleration voltage to 15kV, and the emission current to 10mA. The work from disassembling the energy storage element to preparing the sample for measurement by X-ray photoelectron spectroscopy is carried out in an argon atmosphere with a dew point of -60°C or lower, and the sample is sealed in a transfer vessel and subjected to measurement of the spectrum by X-ray photoelectron spectroscopy without exposure to air.

[0019] In this invention, relative elemental concentrations are determined by quantitative analysis of the entire spectrum obtained by X-ray photoelectron spectroscopy measurement of the positive electrode active material layer. First, the relative elemental concentrations of each element detected in the spectrum obtained by X-ray photoelectron spectroscopy measurement in the range of 0 eV to 1200 eV are calculated. In this case, for example, if a transition metal composite oxide is used as the positive electrode active material and PVdF is used as the binder, the F1s peak in the range of 680 eV to 683 eV may overlap with the peak derived from the coating containing lithium fluoride and the peak derived from PVdF, and the O1s peak in the range of 527 eV to 533 eV may overlap with the peak derived from the coating containing phosphate and the peak derived from the positive electrode active material. In such cases, peak separation is performed and the relative elemental concentrations of fluorine and oxygen derived from the coating are calculated.

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

[0021] <Configuration of 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 contained within 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.

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

[0023] (Positive electrode substrate) The positive electrode substrate is conductive. Whether or not it is conductive is determined by the volume resistivity measured in accordance with JIS-H-0505 (1975), which is 10 7 The determination is made using Ω·cm as the threshold. The positive electrode substrate material can be a metal such as aluminum, titanium, tantalum, or stainless steel, or an alloy thereof. Among these, aluminum or an aluminum alloy is preferred from the viewpoint of high potential resistance, high conductivity, and cost. Examples of positive electrode substrates include foil, vapor-deposited film, mesh, and porous material, with foil being preferred from the viewpoint of cost. Therefore, aluminum foil or aluminum alloy foil is preferred as the positive electrode substrate. Examples of aluminum or aluminum alloys include A1085, A3003, and A1N30 as specified in JIS-H-4000 (2014) or JIS-H4160 (2006).

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

[0025] (Middle class) The intermediate layer is a layer placed between the positive electrode substrate and the positive electrode active material layer. The intermediate layer contains a conductive agent such as carbon particles to reduce the contact resistance between the positive electrode substrate and the positive electrode active material layer. The composition of the intermediate layer is not particularly limited and may include, for example, a binder and a conductive agent.

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

[0027] The positive electrode active material layer exhibits an F1s peak in the range of 680 eV to 683 eV, a P2p peak in the range of 130 eV to 135 eV, and an O1s peak in the range of 527 eV to 533 eV in the spectrum obtained by X-ray photoelectron spectroscopy. That is, a coating consisting of a mixture containing lithium fluoride and phosphate is formed on the surface of the positive electrode active material layer. The lower limit of the relative elemental concentration of fluorine based on the F1s peak is 2.0 atm%, preferably 2.5 atm%, and more preferably 3.0 atm%. The upper limit of the relative elemental concentration of fluorine based on the F1s peak is preferably 10.0 atm%, and more preferably 8.0 atm%. The relative atomic concentration of fluorine based on the F1s peak is preferably 2.0 atm% to 10.0 atm%, more preferably 2.5 atm% to 10.0 atm%, and even more preferably 3.0 atm% to 8.0 atm%. The lower limit of the relative elemental concentration of phosphorus based on the P2p peak is 0.5 atm%, preferably 0.8 atm%, and more preferably 1.2 atm%. The upper limit of the relative elemental concentration of phosphorus based on the P2p peak is preferably 5.0 atm%, and more preferably 4.0 atm%. The relative elemental concentration of phosphorus based on the P2p peak is preferably 0.5 atm% to 5.0 atm%, more preferably 0.8 atm% to 5.0 atm%, and even more preferably 1.2 atm% to 4.0 atm%. The lower limit of the relative elemental concentration of oxygen based on the O1s peak is 10 atm%, preferably 15 atm%, and more preferably 20 atm%. The upper limit of the relative elemental concentration of oxygen based on the O1s peak is preferably 40 atm%, and more preferably 35 atm%. The relative elemental concentration of oxygen based on the O1s peak is preferably 10 atm% or more and 40 atm%, more preferably 15 atm% or more and 40 atm%, and even more preferably 20 atm% or more and 35 atm%. By setting the relative elemental concentrations of fluorine, phosphorus, and oxygen based on the F1s peak, P2p peak, and O1s peak in the spectrum of the positive electrode active material layer to the above ranges, the resistance of the positive electrode active material layer is reduced.

[0028] In the above-mentioned spectrum, the positive electrode active material layer preferably further shows a peak of S2p in the range of 164 eV or more and 168 eV or less. 3 / 2 Thus, since the coating formed on the surface of the positive electrode active material layer contains sulfur, the chemical stability of the positive electrode active material layer is likely to be improved.

[0029] The lower limit of the specific surface area of the positive electrode active material layer is 3.0 m 2 / g, preferably 3.5 m 2 / g, and more preferably 4.0 m 2 / g. The upper limit of the specific surface area of the positive electrode active material layer is preferably 6.0 m 2 / g, and more preferably 5.5 m 2 / g. By setting the specific surface area of the positive electrode active material layer to be not less than the above lower limit, the contact area between the positive electrode active material layer and the non-aqueous electrolyte increases, and the resistance of the positive electrode active material layer is reduced. Also, by setting the specific surface area of the positive electrode active material layer to be not more than the above upper limit, the charge-discharge cycle performance of the positive electrode active material layer is improved. From these aspects, the specific surface area of the positive electrode active material layer is preferably 3.0 m 2 / g or more and 6.0 m 2 / g or less, more preferably 3.5 m 2 / g or more and 6.0 m 2 / g or less, and even more preferably 4.0 m 2 / g or more and 5.5 m 2 / g or less.

[0030] The density of the positive electrode active material layer is preferably 1.4 g / cm 3 or more and 3.3 g / cm 3 or less, and more preferably 1.8 g / cm 3 or more and 3.0 g / cm 3 or less. Thereby, the non-aqueous electrolyte is likely to uniformly contact the positive electrode active material layer, and the coating formed on the surface of the positive electrode active material layer is likely to be formed uniformly and thinly. The "density" of the positive electrode active material layer is a value obtained from the mass per unit area of the positive electrode active material layer and the average thickness of the positive electrode active material layer.

[0031] The mass per unit area of the positive electrode active material layer is 15 mg / cm 2The following is preferable. This prevents the specific surface area of ​​the positive electrode active material layer from becoming too large, even when a positive electrode active material with a high specific surface area is used, and makes it easier to form the above-mentioned coating on the surface of the positive electrode active material layer uniformly and thinly. The lower limit of the mass per unit area of ​​the positive electrode active material layer is not particularly limited, but is 1 mg / cm². 2 It is preferable.

[0032] The positive electrode active material includes a transition metal composite oxide. Examples of the above transition metal composite oxide include a lithium transition metal composite oxide having an α-NaFeO2 type crystal structure and a lithium transition metal composite oxide having a spinel type crystal structure. Examples of lithium transition metal composite oxide 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, Li x Mn2O4, Li x Ni γ Mn (2-γ) Examples include O4. Furthermore, as a lithium transition metal composite oxide, the compositional formula LiNi x Co γ Mn β M (1-x-γ-β)A compound represented by O2 (0.3 ≤ x < 1, 0 ≤ γ < 0.5, 0 ≤ β < 0.5) may be used. In the above compositional formula, M is B, Al, Si, S, Ti, Mo, W, or a combination thereof. Some atoms in the above lithium transition metal composite oxide may be substituted with atoms of other elements. The surface of the above lithium transition metal composite oxide may be coated with other materials. In the positive electrode active material layer, one type of the above lithium transition metal composite oxide may be used alone, or two or more types may be used in mixture form.

[0033] As the above-mentioned transition metal composite oxide, it is preferable to use a transition metal composite oxide having an α-NaFeO2 structure and containing nickel, cobalt, and at least one of manganese and aluminum. This increases the energy density of the positive electrode of the energy storage element and improves the output performance of the energy storage element.

[0034] Furthermore, among the transition metal composite oxides, lithium transition metal composite oxides having an α-NaFeO2 type crystal structure, and compounds represented by the following formula (1), are more preferred. Li 1+α Me 1-α O2···(1) In equation (1), Me is a metal (excluding Li) that contains Ni, Co, and at least one of Mn and Al. 0 ≤ α < 1.

[0035] In formula (1), Me is preferably composed substantially of the three elements Ni, Co, and Mn, or Ni, Co, and Al, and more preferably composed of Ni, Co, and Mn. However, Me may contain other metals.

[0036] From the perspective of achieving higher electrical capacitance, the preferred content (composition ratio) of each constituent element in the compound represented by equation (1) is as follows. Note that the molar ratio is equal to the atomic ratio.

[0037] In formula (1), the lower limit of the molar ratio of Ni to Me (Ni / Me) is preferably 0.1, and may be more preferably 0.2, 0.3, or 0.4. On the other hand, the upper limit of this molar ratio (Ni / Me) is preferably 0.9, and may be more preferably 0.8, 0.7, or 0.6.

[0038] In equation (1), the lower limit of the molar ratio of Co to Me (Co / Me) is preferably 0.01, and may be more preferably 0.1 or 0.2. On the other hand, the upper limit of this molar ratio (Co / Me) is preferably 0.5, and may be more preferably 0.4 or 0.3.

[0039] In formula (1), the lower limit of the molar ratio of Mn to Me (Mn / Me) may be 0, preferably 0.05, and more preferably 0.1 or 0.2. On the other hand, the upper limit of this molar ratio (Mn / Me) is preferably 0.6, and more preferably 0.4 or 0.3.

[0040] In formula (1), the lower limit of the molar ratio of Al to Me (Al / Me) may be 0, preferably 0.01, and more preferably 0.02 or 0.03. On the other hand, the upper limit of this molar ratio (Al / Me) is preferably 0.3, and more preferably 0.2 or 0.1.

[0041] In equation (1), the molar ratio of Li to Me (Li / Me), i.e., (1+α) / (1-α), may be 1 (α=0), and may be greater than 1.0 (α>0). On the other hand, the upper limit of this molar ratio (Li / Me) is preferably 1.6, and may be more preferably 1.4 or 1.2.

[0042] The composition ratio of the lithium transition metal composite oxide mentioned above refers to the composition ratio when the battery is fully discharged using the following method. First, the energy storage element (secondary battery) is charged with a constant current of 0.05C until it reaches the charging termination voltage for normal use, bringing it to a fully charged state. After a 30-minute rest, it is discharged with a constant current of 0.05C until it reaches the lower limit voltage for normal use. The battery is disassembled, the positive electrode is removed and used as the working electrode, and a test battery is assembled with metallic Li as the counter electrode. A current of 10mA per gram of positive electrode active material is applied, and the positive electrode potential is measured at 2.0V vs. Li / Li + Constant current discharge is performed until the positive electrode is fully discharged. The device is disassembled again, and the positive electrode is removed. The non-aqueous electrolyte adhering to the removed positive electrode is thoroughly washed using dimethyl carbonate, and after drying at room temperature for 24 hours, the lithium transition metal composite oxide of the positive electrode active material is collected. The collected lithium transition metal composite oxide is subjected to measurement. The work from disassembling the energy storage element to collecting the lithium transition metal composite oxide is performed in an argon atmosphere with a dew point of -60°C or lower. Here, "normal use" refers to using the energy storage element under the charge and discharge conditions recommended or specified for the element, and if a charger for the energy storage element is available, it refers to using the element with that charger.

[0043] Suitable lithium transition metal composite oxides include, for example, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2, LiLiLi 3 / 5 Co 1 / 5 Mn 1 / 5 O2, LiLiLi 1 / 2 Co 1 / 5 Mn 3 / 10 O2, LiLiLi 1 / 2 Co 3 / 10 Mn 1 / 5 O2, LiLiLi 8 / 10 Co 1 / 10 Mn 1 / 10 O2, LiLiLi 0.8 Co 0.15 Al 0.05 Examples include O2, etc.

[0044] The positive electrode active material may be used alone or as a mixture of two or more types. The content of the above transition metal composite oxide in all positive electrode active materials used is preferably 50% to 100% by mass, more preferably 70% to 100% by mass, and even more preferably 80% to 100% by mass. It is particularly preferable to use a positive electrode active material consisting substantially only of the above transition metal composite oxide.

[0045] It is preferable that at least a portion of the surface of the positive electrode active material is coated with boron or tungsten. Furthermore, it is preferable that the content of boron or tungsten relative to the total amount of positive electrode active material be between 0.2 mol% and 1.5 mol%. By coating at least a portion of the surface of the positive electrode active material with boron or tungsten in this way, and by setting the content of boron or tungsten relative to the total amount of positive electrode active material within the above range, it is easier to control the relative elemental concentrations of fluorine, phosphorus, and oxygen in the coating formed on the surface of the positive electrode active material layer.

[0046] The positive electrode active material is usually in the form of particles (powder). The average particle size of the positive electrode active material is preferably, for example, 0.1 μm or more and 20 μm or less. Setting the average particle size of the positive electrode active material above the lower limit makes it easier to manufacture or handle the positive electrode active material. Setting the average particle size of the positive electrode active material below the upper limit improves the electronic conductivity of the positive electrode active material layer. When a composite material of the positive electrode active material and other materials is used, the average particle size of the composite material is used as the average particle size of the positive electrode active material. "Average particle size" refers to the value at which the volume-based integrated distribution calculated in accordance with JIS-Z-8819-2 (2001), based on the particle size distribution measured by laser diffraction / scattering method on a dilution of particles diluted with a solvent, in accordance with JIS-Z-8825 (2013), becomes 50%.

[0047] 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, counterjet 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.

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

[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 active material layer is preferably 1% by mass or more and 10% by mass or less, and more preferably 3% by mass or more and 9% by mass or less. By setting the content of the conductive agent within the above range, the energy density of the secondary battery 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 active material layer is preferably 1% by mass or more and 10% by mass or less, and more preferably 3% by mass or more and 9% by mass or less. By keeping the binder content within the above range, the positive electrode active material can be stably maintained.

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

[0056] The positive electrode can be manufactured, for example, by applying a positive electrode mixture paste directly to a positive electrode substrate or via an intermediate layer, and then drying it. After drying, pressing may be performed as needed. The positive electrode mixture paste contains the positive electrode active material and optional components such as a conductive agent and a binder, which constitute the positive electrode active material layer. The positive electrode mixture paste usually also contains a dispersion medium. The specific surface area of ​​the positive electrode active material layer can be adjusted by the specific surface area of ​​the positive electrode active material and optional components such as the conductive agent, the composition of the positive electrode active material layer, the mixing process of the positive electrode mixture paste, the pressing process of the positive electrode plate, etc.

[0057] [Negative electrode] The negative electrode comprises a negative electrode substrate and a negative electrode active material layer disposed directly on the negative electrode substrate or via an intermediate layer. The configuration of the intermediate layer is not particularly limited and can be selected from, for example, the configurations exemplified in the positive electrode.

[0058] 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.

[0059] 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.

[0060] The negative electrode active material layer contains a negative electrode active material. The negative electrode active material layer may contain optional components such as conductive agents, binders, thickeners, and fillers as needed. These optional components can be selected from the materials exemplified above for the positive electrode.

[0061] The negative electrode active material layer may contain typical nonmetallic elements such as B, N, P, F, Cl, Br, and I, typical metallic elements such as Li, Na, Mg, Al, K, Ca, Zn, Ga, Ge, Sn, Sr, and Ba, and transition metallic elements such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Zr, Ta, Hf, Nb, and W as components other than the negative electrode active material, conductive agent, binder, thickener, and filler.

[0062] The negative electrode active material can be appropriately selected from known negative electrode active materials. For lithium-ion secondary batteries, materials capable of intercalating and releasing lithium ions are typically used as negative electrode active materials. Examples of negative electrode active materials include: metallic Li; metals or metalloids such as Si and Sn; metal oxides or metalloid oxides such as Si oxide, Ti oxide, and Sn oxide; and Li4Ti5O 12 LiTiO 2、 Examples of materials include titanium-containing oxides such as TiNb2O7; polyphosphate compounds; silicon carbide; and carbon materials such as graphite and non-graphitizable carbon (easily graphitizable carbon or poorly graphitizable carbon). Among these materials, graphite and non-graphitizable carbon are preferred. In the negative electrode active material layer, one of these materials may be used alone, or two or more may be used in mixture form.

[0063] "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.

[0064] "Non-graphite carbon" refers to the average lattice plane spacing (d) of the (002) plane, which is determined by X-ray diffraction before charging or during the discharge state. 002 This refers to carbon materials with a nautical radius of 0.34 nm or more and 0.42 nm or less. Non-graphitized carbons include poorly graphitizable carbons and easily graphitizable carbons. Examples of non-graphitized carbons include resin-derived materials, petroleum pitch or materials derived from petroleum pitch, petroleum coke or materials derived from petroleum coke, plant-derived materials, and alcohol-derived materials.

[0065] 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.

[0066] "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.

[0067] "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.

[0068] 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 active material 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.

[0069] The content of the negative electrode active material in the negative electrode active material layer is preferably 60% by mass or more and 99% by mass or less, and more preferably 90% by mass or more and 98% by mass or less. By setting the content of the negative electrode active material within the above range, it is possible to achieve both high energy density and manufacturability in the negative electrode active material layer.

[0070] [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.

[0071] The heat-resistant particles contained in the heat-resistant layer preferably have a mass reduction 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 reduction of 5% or less when heated from room temperature to 800°C. Inorganic compounds are examples of materials with a mass reduction of less than or equal to the specified amount. Examples of inorganic compounds include oxides such as iron oxide, silicon oxide, aluminum oxide, titanium oxide, barium titanate, 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 and barium fluoride; 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.

[0072] The permeability of the separator is preferably 100 seconds / 100 ml or less. By keeping the permeability of the separator below the above upper limit, the non-aqueous electrolyte can be more easily distributed uniformly within the pores of the separator, and the coating formed on the surface of the positive electrode active material layer can be more easily formed uniformly and thinly. From the viewpoint of ensuring strength and insulation, the permeability of the separator is preferably 40 seconds / 100 ml or more.

[0073] 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.

[0074] 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.

[0075] [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. Preferably, the non-aqueous electrolyte contains a compound containing fluorine, a compound containing phosphorus, and a compound containing oxygen. Fluorine, phosphorus, and oxygen may be contained in one compound, or they may each be contained in two or more compounds.

[0076] 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.

[0077] 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.

[0078] 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.

[0079] 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.

[0080] 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.

[0081] Examples of lithium salts include inorganic lithium salts such as LiPF6, LiPO2F2, LiBF4, LiClO4, and LiN(SO2F)2; lithium salts that are oxalate compounds such as lithium bis(oxalate) borate (LiBOB), lithium difluorooxalate borate (LiFOB), lithium difluorobis(oxalate) phosphate (LiFOP), and lithium tetrafluorooxalate phosphate; 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, it is preferable to use a lithium salt containing fluorine in order to set the relative elemental concentration of fluorine in the positive electrode active material layer within a predetermined range, more preferable to use an inorganic lithium salt containing fluorine, and even more preferable to use LiPF6.

[0082] 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 3 More than 1.5mol / dm 3 The following is particularly preferable. By setting the electrolyte salt content within the above range, the ionic conductivity of the non-aqueous electrolyte can be increased.

[0083] 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); salts of oxalate compounds such as lithium bis(oxalate) borate (LiBOB), lithium difluorooxalate borate (LiFOB), lithium difluorobis(oxalate) phosphate (LiFOP), and lithium tetrafluorooxalate phosphate; imide salts such as lithium bis(fluorosulfonyl)imide (LiFSI); biphenyls, alkylbiphenyls, and terphenyls. Aromatic compounds such as partially hydrogenated terphenyl, cyclohexylbenzene, t-butylbenzene, t-amylbenzene, diphenyl ether, and dibenzofuran; partially halides 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, methyl vinylene 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, propanesultone, propensultone, butanesultone, methyl methanesulfonate, busulfan, methyl toluenesulfonate, dimethyl sulfate, ethylene sulfate, sulfolane, dimethyl sulfone, diethylsulfone, dimethyl sulfoxide, diethylsulfoxide, tetramethylene sulfoxide, diphenyl sulfide, 4,4'-bis( Examples include 2,2-dioxo-1,3,2-dioxathiolane, 4-methylsulfonyloxymethyl-2,2-dioxo-1,3,2-dioxathiolane, thioanisole, diphenyl disulfide, dipyridinium disulfide, 1,3-propensultone (PRS), 1,3-propanesultone, 1,4-butanesultone, 1,4-butensultone, perfluorooctane, tristrimethylsilyl borate, tristrimethylsilyl phosphate, tetrakithtrimethylsilyl titanate, lithium monofluorophosphate, and lithium difluorophosphate.These additives may be used individually or in combination of two or more.

[0084] The non-aqueous electrolyte preferably contains an oxalate compound among the above additives, and the content of the oxalate compound is preferably 0.2% by mass or more and 1.5% by mass or less, and more preferably 0.2% by mass or more and 1.0% by mass or less, relative to the total amount of the non-aqueous electrolyte. Examples of the oxalate compound include lithium bis(oxalate) borate (LiBOB), lithium difluorooxalate borate (LiFOB), lithium difluorobis(oxalate) phosphate (LiFOP), lithium tetrafluorooxalate phosphate, etc. In this way, by containing an oxalate compound in the non-aqueous electrolyte and setting the content of the oxalate compound within the above range, the coating formed on the surface of the positive electrode active material layer is likely to be thin and uniform.

[0085] The non-aqueous electrolyte preferably contains a sulfur-containing compound among the above additives, and more preferably contains 1,3-propensultone (PRS). By including a sulfur-containing compound in this way, the coating is more likely to be formed to contain sulfur, and the chemical stability of the positive electrode active material layer is easily improved.

[0086] The additive content in the non-aqueous electrolyte is preferably 0.01% to 10% by mass, 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, relative to the total amount of the non-aqueous electrolyte. By setting the additive content within the above range, it is possible to improve the capacity retention performance or charge / discharge cycle performance after high-temperature storage, and to further enhance safety.

[0087] The shape of the energy storage element in this embodiment is not particularly limited, and examples include cylindrical batteries, prismatic batteries, flat batteries, coin-type batteries, button-type batteries, and the like. Figure 1 shows an example of a rectangular battery, specifically a storage element 1 (non-aqueous electrolyte storage element). 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.

[0088] <Configuration of the energy storage device> The energy storage element of this embodiment can be mounted as an energy storage unit (battery module) composed of multiple energy storage elements 1 in power supplies for vehicles such as electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs), power supplies for electronic devices such as personal computers and communication terminals, or power 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 unit. Figure 2 shows an example of a power storage device 30 which is formed by further assembling power storage units 20, each of which is an assembly of two or more electrically connected power storage elements 1. The power storage device 30 may include busbars (not shown) that electrically connect two or more power storage elements 1, busbars (not shown) that electrically connect two or more power storage units 20, etc. The power storage unit 20 or the power storage device 30 may include a condition monitoring device (not shown) that monitors the state of one or more power storage elements.

[0089] <Manufacturing method for energy storage elements> The method for manufacturing the energy storage element of this embodiment can be appropriately selected from known methods. The manufacturing method includes, for example, preparing an electrode body, preparing a non-aqueous electrolyte, and housing the electrode body and the non-aqueous electrolyte in a container. Preparing the electrode body includes preparing a positive electrode and a negative electrode, and forming the electrode body by stacking or winding the positive electrode and the negative electrode via a separator.

[0090] 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.

[0091] <Other Embodiments> Furthermore, the energy storage element of the present invention is not limited to the embodiments described above, and various modifications may be made without departing from the spirit of the present invention. For example, the configuration of one embodiment may be added to the configuration of another embodiment, and a part of the configuration of one embodiment may be replaced with the configuration of another embodiment or with well-known technology. In addition, a part of the configuration of one embodiment may be deleted. Also, well-known technology may be added to the configuration of one embodiment.

[0092] 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.

[0093] In the above embodiment, an electrode body in which a positive electrode and a negative electrode are stacked with a separator in between has been described, but the electrode body does not need to have a separator. For example, the positive electrode and the negative electrode may be in direct contact with each other, with a non-conductive layer formed on the active material layer of either the positive electrode or the negative electrode. [Examples]

[0094] The present invention will be described in more detail below with reference to examples. The present invention is not limited to the following examples.

[0095] [Example 1] (Fabrication of the positive electrode) As the positive electrode active material, LiNi has an α-NaFeO2 type crystal structure and an average particle size of 4 μm. 1 / 3 Co 1 / 3 Mn 1 / 3O2 particles were used. A positive electrode mixture paste containing the above positive electrode active material: polyvinylidene fluoride (PVDF): acetylene black (AB) in a ratio of 93:4:3 by mass (in terms of solids) and using N-methylpyrrolidone (NMP) as a dispersion medium was prepared. This positive electrode mixture paste was applied to both sides of an aluminum foil serving as a positive electrode substrate, and dried and pressed to form a positive electrode active material layer, thereby obtaining a positive electrode. The mass per unit area of the positive electrode active material layer of the obtained positive electrode was 10 mg / cm 2 and the BET specific surface area was 4.14 m 2 / g.

[0096] (Fabrication of negative electrode) Graphite was used as the negative electrode active material. A negative electrode mixture paste containing the above negative electrode active material: SBR: CMC in a ratio of 98:1:1 by mass (in terms of solids) and using water as a dispersion medium was prepared. This negative electrode mixture paste was applied to both sides of a copper foil serving as a negative electrode substrate, and dried and pressed to obtain a negative electrode. The mass per unit area of the negative electrode active material layer of the obtained negative electrode was 5.5 g / cm 2 .

[0097] (Preparation of non-aqueous electrolyte) LiPF6 as an electrolyte salt was dissolved in a non-aqueous solvent in which EC: DMC: EMC were mixed at a volume ratio of 30:40:30 at a concentration of 1.2 mol / dm 3 . Further, as additives, vinylene carbonate (VC) and lithium bis(oxalato)borate (LiBOB) were added so that the concentrations were each 0.5% by mass based on the total amount of the non-aqueous electrolyte, thereby obtaining a non-aqueous electrolyte.

[0098] (Fabrication of energy storage device) A polyolefin microporous membrane having a heat-resistant layer formed on the surface was used as a separator. Through this separator, the above positive electrode and the above negative electrode were laminated and wound such that the heat-resistant layer faced the positive electrode, thereby fabricating a wound electrode body. This electrode body was housed in an aluminum container, the above non-aqueous electrolyte was injected therein, and then sealed.

[0099] [Examples 2, 3 and Comparative Example 5] Examples 2, 3, and Comparative Example 5 used the type of positive electrode active material, the BET specific surface area of ​​the positive electrode active material layer, and the non-aqueous electrolyte additive as shown in Table 1, and the positive electrode active material was coated. The coating was performed before preparing the positive electrode mixture paste, and in Example 2, a tungsten compound was used, while in Examples 3 and Comparative Example 5, a boron compound was used, so that at least a portion of the positive electrode active material was coated. Except as described above, the energy storage elements of Examples 2, 3, and Comparative Example 5 were obtained in the same manner as in Example 1.

[0100] [Comparative Examples 1 to 4] Comparative Examples 1 to 4 were obtained in the same manner as in Example 1, except that the type of positive electrode active material, the BET specific surface area of ​​the positive electrode active material layer, and the additives to the non-aqueous electrolyte were as shown in Table 1.

[0101] (Initial charge / discharge test) Each of the obtained energy storage elements was charged with a constant current of 0.1C to 4.1V at a temperature of 25°C, and then charged with a constant voltage at 4.1V. The charging termination condition was when the charging current became 0.01C. After a 10-minute rest period, a constant current discharge was performed with a current of 0.2C to 2.5V, followed by a 10-minute rest period. At a temperature of 25°C, the elements were charged with a constant current of 0.2C to 4.1V, and then charged with a constant voltage at 4.1V. The charging termination condition was when the charging current became 0.01C. After a 10-minute rest period, a constant current discharge was performed with a current of 0.2C to 2.5V. The capacity of the second discharge was defined as the "initial discharge capacity".

[0102] (Measurement by X-ray photoelectron spectroscopy) Each energy storage element, after the initial charge and discharge described above, was further discharged at a constant current of 0.05C up to 2.5V. Then, it was disassembled in an argon atmosphere with a dew point of -60°C or lower to remove the positive electrode, which was then washed with dimethyl carbonate and dried under reduced pressure at room temperature. The obtained positive electrode was sealed in a transfer vessel in an argon atmosphere, and X-ray photoelectron spectroscopy (XPS) measurements were performed on the surface of the positive electrode active material layer under the conditions described above. Table 1 shows the relative elemental concentrations of fluorine, phosphorus, and oxygen based on the F1s peak in the range of 680eV to 683eV, the P2p peak in the range of 130eV to 135eV, and the O1s peak in the range of 527eV to 533eV in the obtained X-ray photoelectron spectroscopy (XPS) spectra, labeled as "Relative elemental concentrations by XPS measurement."

[0103] (DC resistance test) The low-temperature DC resistance (DCR) of each energy storage element was evaluated after the initial charge-discharge test described above. In a constant-temperature bath at 25°C, 50% of the initial discharge capacity was charged with a constant current of 0.1C. Under these conditions, the State of Charge (SOC) of each energy storage element was set to 50%. Next, each energy storage element was stored in a constant-temperature bath at -10°C for 4 hours, and then discharged for 10 seconds each with currents of 0.2C, 0.4C, and 0.6C. After each discharge, constant-current charging was performed with a current of 0.1C to adjust the SOC to 50%. The DC resistance value, corresponding to the slope, was determined from the current-voltage performance graph obtained by plotting the voltage 10 seconds after the start of discharge on the vertical axis and the discharge current on the horizontal axis. The DC resistance values ​​of each energy storage element are shown in Table 1.

[0104] [Table 1]

[0105] As shown in Table 1 above, the positive electrode active material is lithium transition metal composite oxide LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 It consists of O2 particles, and the positive electrode active material layer is 3.0 m 2The power storage element of Example 1 having a specific surface area of / g or more, with the relative element concentrations of fluorine, phosphorus, and oxygen being 2.0 atm% or more, 0.5 atm% or more, and 10 atm% or more respectively, has a cathode active material of lithium transition metal composite oxide LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 particles, and it can be seen that the DC resistance value is reduced compared to the power storage element of Comparative Example 1 where the relative element concentrations of fluorine, phosphorus, and oxygen do not satisfy the above conditions.

[0106] Also, as shown in Table 1 above, the power storage elements of Example 2 and Example 3 having a cathode active material of lithium transition metal composite oxide LiNi 0.5 Co 0.3 Mn 0.2 O2 particles, and with the relative element concentrations of fluorine, phosphorus, and oxygen satisfying the above conditions, have a lower DC resistance value compared to the power storage element of Comparative Example 2 where the relative element concentrations of fluorine, phosphorus, and oxygen do not satisfy the above conditions. 0.5 Co 0.3 Mn 0.2 O2 particles, and it can be seen that the DC resistance value is reduced compared to the power storage element of Comparative Example 2 where the relative element concentrations of fluorine, phosphorus, and oxygen do not satisfy the above conditions.

[0107] Furthermore, as shown in Table 1 above, the power storage elements of Example 2 and Example 3 having a cathode active material of lithium transition metal composite oxide LiNi 0.5 Co 0.3 Mn 0.2 O2 particles, and with the specific surface area satisfying the above conditions, have a lower DC resistance value compared to the power storage elements of Comparative Example 3, Comparative Example 4, and Comparative Example 5 where the specific surface area does not satisfy the above conditions. It should be noted that in the comparison between Example 2 and Example 3 and Comparative Example 3, Comparative Example 4, and Comparative Example 5, the influence due to the difference in the composition of the lithium transition metal composite oxide is small, and it is interpreted that the large specific surface area of the cathode active material layer is the factor contributing to the reduction of the DC resistance value. 0.6 Co 0.2 Mn 0.2 O2 particles, and it can be seen that the DC resistance value is reduced compared to the power storage elements of Comparative Example 3, Comparative Example 4, and Comparative Example 5 where the specific surface area does not satisfy the above conditions.

[0108] Although not shown in Table 1, the positive electrode active material is lithium transition metal composite oxide LiNi 0.6 Co 0.2 Mn 0.2 Even in energy storage elements containing lithium transition metal composite oxides with a high molar ratio of Ni to transition metal, such as O2 particles, the DC resistance can be reduced if the relative elemental concentrations of fluorine, phosphorus, and oxygen satisfy the above conditions.

[0109] The results above demonstrate that the resistance of the energy storage element is reduced. [Explanation of Symbols]

[0110] 1. Energy storage element 2 Electrode body 3 containers 4 Positive terminal 41 Positive lead 5 Negative terminal 51 Negative lead 20 Energy storage units 30 Energy storage devices

Claims

1. It comprises a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte, The above positive electrode has a positive electrode active material layer containing a positive electrode active material, The above positive electrode active material includes a transition metal composite oxide, The specific surface area of ​​the above positive electrode active material layer is 3.0 m². ２ / g or more 6.0m ２ / g or less, The above positive electrode active material layer, in the spectrum obtained by X-ray photoelectron spectroscopy, shows an F1s peak in the range of 680 eV to 683 eV, a P2p peak in the range of 130 eV to 135 eV, and an O1s peak in the range of 527 eV to 533 eV. A storage element having relative elemental concentrations of fluorine, phosphorus, and oxygen based on the F1s peak, P2p peak, and O1s peak in the above spectrum of 2.0 atm%, 0.5 atm%, and 10 atm%, respectively (excluding cases where the non-aqueous electrolyte contains a compound of the following formula (1)). 【Chemistry 1】 (In formula (1) above, R f represents a saturated aliphatic hydrocarbon group bonded to an aromatic ring, and is a substituent having 1 to 6 carbon atoms, in which at least one hydrogen atom is replaced by a fluorine atom. q represents the number of hydrocarbon groups bonded to the aromatic ring and is an integer between 2 and 4. The structures of the R f groups bonded to the aromatic ring may be identical or different.)

2. It comprises a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte, The above positive electrode has a positive electrode active material layer containing a positive electrode active material, The above positive electrode active material includes a transition metal composite oxide, At least a portion of the surface of the above positive electrode active material is coated with boron or tungsten. The specific surface area of ​​the above positive electrode active material layer is 3.0 m². ２ / g or more, The above positive electrode active material layer, in the spectrum obtained by X-ray photoelectron spectroscopy, shows an F1s peak in the range of 680 eV to 683 eV, a P2p peak in the range of 130 eV to 135 eV, and an O1s peak in the range of 527 eV to 533 eV. A storage element having relative elemental concentrations of fluorine, phosphorus, and oxygen based on the F1s peak, P2p peak, and O1s peak in the above spectrum of 2.0 atm%, 0.5 atm%, and 10 atm%, respectively (excluding cases where the non-aqueous electrolyte contains a compound of the following formula (1)). 【Chemistry 1】 (In formula (1) above, R f represents a saturated aliphatic hydrocarbon group bonded to an aromatic ring, and is a substituent having 1 to 6 carbon atoms, in which at least one hydrogen atom is replaced by a fluorine atom. q represents the number of hydrocarbon groups bonded to the aromatic ring and is an integer between 2 and 4. The structures of the R f groups bonded to the aromatic ring may be identical or different.)

3. The above transition metal composite oxide is α-NaFeO ２ An energy storage element according to claim 1 or claim 2, having a structure and comprising nickel, cobalt, and at least one of manganese and aluminum.

4. The above positive electrode active material layer has S2p ​​in the spectrum between 164 eV and 168 eV. ３／２ A storage element according to claim 1, claim 2, or claim 3 that exhibits a peak.

5. The above non-aqueous electrolyte contains an oxalate compound, The energy storage element according to any one of claims 1 to 4, wherein the content of the oxalate compound is 0.2% by mass or more and 1.5% by mass or less relative to the total amount of the non-aqueous electrolyte.

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