Nonaqueous electrolyte power storage element and power storage device

Abstract

A nonaqueous electrolyte power storage element according to one aspect of the present invention is provided with an electrode body which has a negative electrode and a positive electrode that contains positive electrode active material particles, wherein: the void index A represented by formula (1) of the positive electrode active material particles is 5 or more; elemental boron is present on at least a part of the surfaces of the positive electrode active material particles; and at least a part of the electrode body is in a pressed state.　(1): A = S1 / S2 (In formula (1), S1 represents the BET specific surface area of the positive electrode active material particles. S2 represents the specific surface area on the assumption that the positive electrode active material particles are spheres which have the average particle diameter of the positive electrode active material particles as the diameter thereof.)

Description

Nonaqueous electrolyte energy storage element and energy storage device

[0001] The present invention relates to a non-aqueous electrolyte electricity storage element and an electricity storage device.

[0002] Due to their high energy density, non-aqueous electrolyte secondary batteries, such as lithium ion secondary batteries, are widely used in electronic devices such as personal computers and communication terminals, as well as in automobiles. Non-aqueous electrolyte secondary batteries generally have 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. In addition to non-aqueous electrolyte secondary batteries, capacitors such as lithium ion capacitors and electric double layer capacitors are also widely used as non-aqueous electrolyte energy storage elements.

[0003] Particulate active materials such as particles of lithium transition metal composite oxides are widely used as positive electrode active materials for non-aqueous electrolyte storage elements. Patent Document 1 describes positive electrode active material particles that are composite oxides containing lithium, manganese, and at least one of nickel and cobalt, and a non-aqueous electrolyte secondary battery in which such positive electrode active material particles are used in the positive electrode.

[0004] JP 2015-153551 A

[0005] It is desirable for a non-aqueous electrolyte electricity storage element to have low resistance in terms of output performance, etc. In a non-aqueous electrolyte electricity storage element, even if the initial resistance is low, the resistance usually increases with the number of charge-discharge cycles.

[0006] An object of the present invention is to provide a nonaqueous electrolyte electricity storage element and electricity storage device that have low resistance after charge / discharge cycles.

[0007] A nonaqueous electrolyte storage element according to one aspect of the present invention includes an electrode assembly having a positive electrode containing positive electrode active material particles and a negative electrode, wherein the positive electrode active material particles have a void index A represented by the following formula (1) of 5 or more, boron element is present on at least a portion of the surface of the positive electrode active material particles, and at least a portion of the electrode assembly is in a compressed state: A=S １ / S ２ ...(1) (In formula (1), S １is the BET specific surface area of ​​the positive electrode active material particles. ２ is the specific surface area when the positive electrode active material particles are assumed to be spheres with a diameter equal to the average particle diameter.)

[0008] An electricity storage device according to another aspect of the present invention includes two or more electricity storage elements and one or more of the nonaqueous electrolyte electricity storage elements.

[0009] According to one aspect of the present invention, it is possible to provide a nonaqueous electrolyte electricity storage element and electricity storage device that have low resistance after charge / discharge cycles.

[0010] Fig. 1 is a perspective view showing one embodiment of a nonaqueous electrolyte electricity storage element, and Fig. 2 is a schematic view showing one embodiment of an electricity storage device formed by assembling a plurality of nonaqueous electrolyte electricity storage elements.

[0011] First, an outline of the nonaqueous electrolyte electricity storage element and electricity storage device disclosed in this specification will be described.

[0012] [1] A nonaqueous electrolyte storage element according to one aspect of the present invention includes an electrode assembly having a positive electrode containing positive electrode active material particles and a negative electrode, wherein the positive electrode active material particles have a porosity index A represented by the following formula (1) of 5 or more, elemental boron is present on at least a portion of the surface of the positive electrode active material particles, and at least a portion of the electrode assembly is in a compressed state: A=S １ / S ２ ...(1) (In formula (1), S １ is the BET specific surface area of ​​the positive electrode active material particles. ２ is the specific surface area when the positive electrode active material particles are assumed to be spheres with a diameter equal to the average particle diameter.)

[0013] The nonaqueous electrolyte storage element described in [1] above has low resistance after charge-discharge cycles. While the reason for this is unclear, the following is presumed. Positive electrode active material particles with a porosity index A of 5 or more, as expressed by the above formula (1), are positive electrode active material particles with a large number of voids. Positive electrode active material particles with a large number of voids have low initial resistance, due in part to the ease with which nonaqueous electrolyte penetrates into the voids. Meanwhile, the electrode assembly expands with repeated charge-discharge cycles, resulting in an increase in resistance. In response, maintaining at least a portion of the electrode assembly in a compressed state can suppress the expansion of the electrode assembly and thus the increase in resistance. However, in the case of an electrode assembly using positive electrode active material particles with a large number of voids, the positive electrode active material particles are prone to cracking due to compression. Cracks in the positive electrode active material particles reduce electronic conductivity between the positive electrode active material particles, resulting in an increase in resistance with repeated charge-discharge cycles. Therefore, by making boron element present on at least a portion of the surface of the positive electrode active material particle having many voids, the boron element protects the grain boundaries of the primary particles on the surface of the positive electrode active material particle, which are the starting points for cracks, and makes cracks less likely to occur. For the reasons described above, in the nonaqueous electrolyte storage element described in the above [1], in which positive electrode active material particles having many voids and having boron element present on at least a portion of the surface are used and at least a portion of the electrode body is in a pressed state, it is presumed that the initial resistance is low and the increase in resistance after charge-discharge cycles is sufficiently suppressed, resulting in low resistance after charge-discharge cycles.

[0014] The "specific surface area when the positive electrode active material particles are assumed to be spheres with a diameter equal to the average particle diameter" is a value determined from the average particle diameter and true density of the positive electrode active material particles. Specifically, the surface area per positive electrode active material particle when the positive electrode active material particles are assumed to be spheres with a diameter equal to the average particle diameter is determined from the average particle diameter of the positive electrode active material particles. Furthermore, the mass per positive electrode active material particle when the positive electrode active material particles are assumed to be spheres with a diameter equal to the average particle diameter is determined from the average particle diameter and true density of the positive electrode active material particles. The "specific surface area when the positive electrode active material particles are assumed to be spheres with a diameter equal to the average particle diameter" is determined by dividing the determined surface area per positive electrode active material particle by the determined mass per positive electrode active material particle.

[0015] The "BET specific surface area" and "average particle size" of the positive electrode active material particles are values ​​measured for positive electrode active material particles that have been removed from a nonaqueous electrolyte storage element and treated according to the following procedure. First, the nonaqueous electrolyte storage element is discharged at a constant current of 0.05 C to the discharge cut-off voltage during normal use. Here, "normal use" refers to the case where the nonaqueous electrolyte storage element is used under the charge / discharge conditions recommended or specified for the nonaqueous electrolyte storage element. The nonaqueous electrolyte storage element in this state is disassembled, the positive electrode is removed, and a half cell is assembled using metallic lithium as the counter electrode. A discharge current of 10 mA per 1 g of positive electrode active material particles is applied until the positive electrode potential reaches 3.0 V vs. Li / Li. ＋ The positive electrode is adjusted to a fully discharged state by constant current discharge until the positive electrode reaches a fully discharged state. The battery is disassembled again, and the positive electrode is removed. The process from disassembling the nonaqueous electrolyte storage element to removing the positive electrode from the half-cell is carried out in an argon atmosphere with a dew point of −60°C or lower. The removed positive electrode is baked in air at 600°C for 3 hours to separate the positive electrode active material particles from components other than the positive electrode active material particles (e.g., conductive agent, binder, positive electrode substrate, etc.).

[0016] The "BET specific surface area" of the positive electrode active material particles is a value obtained by measurement according to the following procedure. 1.00 g of the positive electrode active material particles is placed in a measurement sample tube and dried under reduced pressure at 120°C for 12 hours. Next, an adsorption isotherm is measured by a nitrogen gas adsorption method using liquid nitrogen within a relative pressure P / P0 (P0 = approximately 770 mmHg) range of 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. The BET specific surface area is calculated from the y-intercept and slope of the line.

[0017] The "average particle size" of the positive electrode active material particles is defined as a value at which the volume-based cumulative distribution calculated in accordance with JIS-Z-8819-2 (2001) is 50% based on the particle size distribution measured by a laser diffraction / scattering method for a diluted solution of particles diluted with a solvent in accordance with JIS-Z-8825 (2013). The same applies to the average particle size of the negative electrode active material described below.

[0018] [2] In the nonaqueous electrolyte storage element according to [1] above, the electrode assembly may further include a separator disposed between the positive electrode and the negative electrode, and the separator may include a layer in contact with the positive electrode and containing a resin as a main component.

[0019] The nonaqueous electrolyte storage element described in [2] above has lower resistance after charge-discharge cycles. While the reason for this is unclear, the following is presumed. The resin-based layer in the separator is softer than, for example, a layer primarily composed of inorganic particles. In the nonaqueous electrolyte storage element described in [2] above, the positive electrode surface is in contact with such a soft layer, making it less likely for cracks to occur in the positive electrode active material particles when at least a portion of the electrode assembly is pressed. Therefore, it is presumed that the nonaqueous electrolyte storage element described in [2] above has lower resistance after charge-discharge cycles.

[0020] "Major component" refers to the component that is contained in the greatest amount by mass.

[0021] [3] The nonaqueous electrolyte storage element according to [1] or [2], wherein the positive electrode active material particles are α-NaFeO ２ The lithium transition metal composite oxide may contain a lithium transition metal composite oxide having a type crystalline structure.

[0022] Generally, α-NaFeO ２ Particles of lithium transition metal composite oxides having a crystalline structure are prone to cracking when pressed. ２ The nonaqueous electrolyte storage element according to the above item [3], which contains a lithium transition metal composite oxide having a crystalline structure, exhibits a particularly remarkable effect of low resistance after charge-discharge cycles.

[0023] [4] In the nonaqueous electrolyte storage element according to any one of [1] to [3] above, at least a portion of the electrode assembly may be pressed at a pressure of 0.5 MPa or more and 4 MPa or less.

[0024] In the nonaqueous electrolyte storage element described in [4] above, at least a portion of the electrode assembly is pressed with an appropriate pressure, and therefore the resistance after charge-discharge cycles is lower.

[0025] The pressure applied to at least a portion of the electrode assembly is measured by the following method. First, the nonaqueous electrolyte storage element is discharged at a constant current to the discharge end voltage during normal use, and then placed in an X-ray computed tomography (CT) device. Scanning is performed along a direction parallel to the thickness direction of the electrode assembly to confirm whether the electrode assembly is in direct or indirect contact with the inner surface of the container. If the electrode assembly is not in direct or indirect contact with the inner surface of the container, the pressure applied to the electrode assembly is set to 0 MPa (the electrode assembly is not in a pressed state). If the electrode assembly is in direct or indirect contact with the inner surface of the container, an X-ray radiographic image of the electrode assembly is taken, and the maximum thickness of the electrode assembly in the stacking direction is measured. The nonaqueous electrolyte storage element is disassembled, the electrode assembly is removed, and the electrode assembly is placed in an autograph. A load is gradually applied to the electrode assembly using the autograph, compressing it to the maximum thickness in the thickness direction of the electrode assembly measured from the X-ray radiographic image. The load measured using the autograph at this time is considered to be the load applied to the electrode assembly. The pressure applied to at least a portion of the electrode body is determined by dividing the load applied to the electrode body by the area of ​​the contact surface between the autograph and the electrode body. Note that, although a load is usually applied to a pair of opposing surfaces of the electrode body by a container, the area of ​​only one of the surfaces to which the load is applied is defined as the area of ​​the surface to which the load is applied.

[0026] [5] An electricity storage device according to another aspect of the present invention includes two or more electricity storage elements, and includes one or more of the nonaqueous electrolyte electricity storage elements according to any one of [1] to [4] above.

[0027] The electricity storage device according to the above [5] includes one or more nonaqueous electrolyte electricity storage elements according to any one of the above [1] to [4], and therefore has low resistance after charge-discharge cycles.

[0028] A nonaqueous electrolyte electricity storage element, an electricity storage device, a method for manufacturing a nonaqueous electrolyte electricity storage element according to one embodiment of the present invention, and other embodiments will be described in detail below. 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.

[0029] <Non-aqueous Electrolyte Storage Element> A non-aqueous electrolyte storage element (hereinafter also simply referred to as "storage element") according to one embodiment of the present invention includes an electrode assembly having a positive electrode, a negative electrode, and a separator, a non-aqueous electrolyte, and a container that accommodates the electrode assembly and the non-aqueous electrolyte. The electrode assembly is typically a stacked type in which multiple positive electrodes and multiple negative electrodes are stacked with separators interposed therebetween, or a wound type in which multiple positive electrodes and multiple negative electrodes are stacked with separators interposed therebetween and wound. That is, the separator is disposed between the positive electrode and the negative electrode. The container may be a known container such as a metal container or a resin container. The container may be box-shaped or may be a container made of a film (e.g., a resin film, a metal-resin composite film, etc.) that is heat-sealed. The non-aqueous electrolyte is present in a state contained in the positive electrode, the negative electrode, and the separator. As an example of a non-aqueous electrolyte storage element, a non-aqueous electrolyte secondary battery (hereinafter also simply referred to as "secondary battery") will be described.

[0030] (Positive Electrode) 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.

[0031] The positive electrode substrate has electrical conductivity. Whether or not it has electrical conductivity is determined by whether or not the volume resistivity measured in accordance with JIS-H-0505 (1975) is 10 －２ The resistance is determined using Ω cm as a threshold value. Metals such as aluminum, titanium, tantalum, and stainless steel, or alloys thereof, are used as the material for the positive electrode substrate. Among these, aluminum or aluminum alloys are preferred from the viewpoints of potential resistance, high conductivity, and cost. Examples of the positive electrode substrate include foil, vapor-deposited film, mesh, and porous material, with foil being preferred from the viewpoint of cost. Therefore, aluminum foil or aluminum alloy foil is preferred as the positive electrode substrate. Examples of aluminum or aluminum alloys include A1085, A3003, and A1N30, as specified in JIS-H-4000 (2014) or JIS-H-4160 (2006).

[0032] 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 of the nonaqueous electrolyte storage element.

[0033] 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.

[0034] The positive electrode active material layer contains positive electrode active material particles and, as needed, optional components such as a conductive agent, a binder, a thickener, and a filler.

[0035] The positive electrode active material particles are particles having a porosity index A of 5 or more, as expressed by the following formula (1): A=S １ / S ２ ...(1) (In formula (1), S １ is the BET specific surface area of ​​the positive electrode active material particles. ２ is the specific surface area when the positive electrode active material particles are assumed to be spheres with a diameter equal to the average particle diameter.)

[0036] The lower limit of the void index A is 5, preferably 5.5, and more preferably 6. When the void index A is equal to or greater than the above lower limit, the initial resistance of the nonaqueous electrolyte storage element is reduced, and as a result, the resistance after charge-discharge cycling can also be reduced. The upper limit of the void index A may be 15, or may be 12, 10, 9, or 8. When the void index A is equal to or less than the above upper limit, cracking of the positive electrode active material particles when the electrode body is pressed can be suppressed, and the resistance after charge-discharge cycling can also be reduced. The void index A may be equal to or greater than any of the above lower limits and equal to or less than any of the above upper limits.

[0037] Positive electrode active material particles having a predetermined porosity index A can be produced by a known method. Alternatively, the porosity index A of commercially available positive electrode active material particles can be measured, and positive electrode active material particles having the predetermined porosity index A can be selected and used.

[0038] The positive electrode active material particles are usually secondary particles formed by agglomeration of multiple primary particles. The primary particles may be particles consisting essentially of a positive electrode active material (a material capable of absorbing and releasing charge-transporting ions such as lithium ions). In one embodiment of the present invention, boron element is present on at least a portion of the surface of the positive electrode active material particles, which are secondary particles. The boron element is preferably present on at least a portion of the grain boundaries of the primary particles on the surface of the positive electrode active material particles. The degree of aggregation of the primary particles due to manufacturing conditions, the shapes of the primary particles and secondary particles, the particle size ratio between the primary particle diameter and the secondary particle diameter, etc. affect the porosity index A of the positive electrode active material particles.

[0039] For example, in the case of positive electrode active material particles containing a lithium transition metal composite oxide, positive electrode active material particles having a porosity index A of 5 or greater can be effectively produced by the following precipitation method. A sulfate aqueous solution containing a sulfate containing a transition metal element (e.g., nickel, manganese, cobalt, etc.) is prepared. Ion-exchanged water is then poured into a reaction vessel, and oxygen is removed by nitrogen bubbling or the like. The sulfate aqueous solution is added dropwise to the reaction vessel while stirring the ion-exchanged water in the reaction vessel at a rotation speed of, for example, about 1500 rpm using a paddle blade equipped with a stirring motor. During this process, the pH is controlled to be constantly maintained at, for example, 11.0 (±0.1) by appropriately adding a mixed alkaline solution containing, for example, sodium hydroxide, ammonium, and hydrazine. This addition produces a precipitate, which is a precursor (hydroxide precursor) of the positive electrode active material. High-speed stirring then granulates the precipitate into spherical particles. In this process, increasing the dropping speed makes it easier to obtain a precursor with a high porosity index A (high porosity). When producing positive electrode active material particles having a porosity index A of 5 or more, the dropping speed is set to 2.3 cm ３ / min or more 3.3cm ３ / min or less. However, suitable conditions are appropriately changed depending on the concentration of the sulfate aqueous solution to be dropped, the amount of ion-exchanged water, etc. After the dropwise addition is completed, the liquid containing the precipitate in the reaction tank is further stirred for, for example, one hour, and then allowed to stand. After that, the precipitate is subjected to suction filtration and washed with ion-exchanged water, and then dried in a drying furnace. The dried product is crushed for several minutes to obtain hydroxide precursor particles in order to uniform the particle size. The obtained hydroxide precursor is mixed with a lithium source and calcined to obtain positive electrode active material particles. Calcination can be performed, for example, in an electric furnace under an air flow atmosphere, by raising the temperature to 1000°C over 10 hours and maintaining it for 4 hours. After calcination, the calcined product may be crushed for several minutes to uniform the particle size.

[0040] The method for producing positive electrode active material particles having a porosity index A of 5 or more is not limited to the above method. For example, positive electrode active material particles having a controlled porosity index A can also be obtained by a spray drying method using an aqueous solution containing hydroxide precursor particles. Furthermore, as described below, before calcination, a hydroxide precursor, a lithium source, and a boron source may be mixed and subjected to calcination. The boron-free positive electrode active material particles obtained through calcination may be coated with a compound containing boron.

[0041] BET specific surface area (S １ ) is 0.8m ２ / g or more 6.0m ２ / g or less is preferable, and 1.0m ２ / g or more 4.0m ２ / g or less is more preferable, and 1.2m ２ / g or more 3.0m ２ / g or less is more preferable, and 1.6m ２ / g or more 2.6m ２ / g or less is even more preferable. In some embodiments, the BET specific surface area (S １ ) is 2.0 m ２ / g or more, and ２ / g or less. １By setting the value of ρ in the above range, it is possible to reduce the resistance after charge-discharge cycles and also to make other charge-discharge performances sufficiently excellent.

[0042] The average internal porosity of the positive electrode active material particles is preferably more than 6.0% and not more than 20.0%, and more preferably more than 10.0% and not more than 16.0%. By setting the average internal porosity of the positive electrode active material particles within this range, it is possible to further reduce the resistance after charge-discharge cycles.

[0043] The "average internal porosity" refers to the average value of the internal porosity of any 10 positive electrode active material particles. The internal porosity refers to the ratio of the cross-sectional area of ​​the voids formed inside the positive electrode active material particles to the apparent cross-sectional area of ​​the positive electrode active material particles (i.e., the cross-sectional area including the internal voids). The average internal porosity is determined by the following method. Specifically, the positive electrode or positive electrode active material particles to be measured are fixed with a thermosetting resin. The cross-section of the positive electrode or positive electrode active material particles fixed with the resin is exposed by ion milling to prepare a measurement sample. Using a scanning electron microscope (SEM), the magnification is set so that a predetermined number of positive electrode active material particles (e.g., 10 to 100) can be observed within one field of view, and a cross-sectional SEM image of the positive electrode active material particles is captured. The SEM observation conditions are not particularly limited, but are typically 1000 to 5000 times (e.g., 2000 times). Next, the cross-section of the positive electrode active material particles and the voids within the particles are distinguished (typically by binarization) based on differences in color tone and shading from the acquired SEM image. Then, the internal porosity is calculated by (S2 / S1) x 100, where S1 is the apparent cross-sectional area of ​​the positive electrode active material particles (total area of ​​the particle) and S2 is the area occupied by the voids within the particle. The average internal porosity can be calculated by arithmetically averaging the internal porosities of a predetermined number of particles. The internal porosity and average internal porosity of each positive electrode active material particle can be calculated using general image analysis software. The SEM image for calculating the average internal porosity of the positive electrode active material particles is acquired for a positive electrode or positive electrode active material particles adjusted to a fully discharged state using the same procedure as used to measure the BET specific surface area and average particle size described above.

[0044] The average particle size of the positive electrode active material particles is, for example, preferably 0.1 μm to 20 μm, more preferably 1 μm to 8 μm, even more preferably 2 μm to 6 μm, and even more preferably 3 μm to 5 μm. By setting the average particle size of the positive electrode active material particles to the above lower limit or more, the positive electrode active material particles are easy to manufacture or handle. By setting the average particle size of the positive electrode active material particles to the above upper limit or less, the electronic conductivity of the positive electrode active material layer is improved, and the resistance after charge-discharge cycles is lowered.

[0045] To obtain positive electrode active material particles with a predetermined particle size, a pulverizer, a classifier, or the like is used. Examples of pulverization methods include methods using a mortar, a ball mill, a sand mill, a vibration ball mill, a planetary ball mill, a jet mill, a counter jet mill, a swirling airflow jet mill, or a sieve. Wet pulverization in the presence of water or an organic solvent such as hexane can also be used. As a classification method, a sieve, an air classifier, or the like is used as needed for both dry and wet methods.

[0046] The positive electrode active material (a material capable of absorbing and releasing charge transport ions such as lithium ions) constituting the positive electrode active material particles can be appropriately selected from known positive electrode active materials. As the positive electrode active material for lithium ion secondary batteries, a material capable of absorbing and releasing lithium ions is usually used. Examples of the positive electrode active material include α-NaFeO ２ Examples of suitable lithium transition metal composite oxides include those having a α-type crystal structure, those having a spinel type crystal structure, polyanion compounds, chalcogen compounds, and sulfur. ２ As the lithium transition metal composite oxide having a crystalline structure, for example, Li[Li ｘ Ni （１－ｘ） ]O ２ (0≦x<0.5), Li[Li ｘ Ni γ Co （１－ｘ－γ） ]O ２ (0≦x<0.5, 0<γ<1, 0<1-x-γ), Li[Li ｘ Co （１－ｘ） ]O ２ (0≦x<0.5), Li[Li ｘ Ni γMn （１－ｘ－γ） ]O ２ (0≦x<0.5, 0<γ<1, 0<1-x-γ), Li[Li ｘ Ni γ Mn β Co （１－ｘ－γ－β） ]O ２ (0≦x<0.5, 0<γ, 0<β, 0.5<γ+β<1, 0<1−x−γ−β), Li[Li ｘ Ni γ Co β Al （１－ｘ－γ－β） ]O ２ (0≦x<0.5, 0<γ, 0<β, 0.5<γ+β<1, 0<1−x−γ−β), etc. Examples of lithium transition metal composite oxides having a spinel crystal structure include Li ｘ Mn ２ O ４ , Li ｘ Ni γ Mn （２－γ） O ４ Examples of polyanion compounds include LiFePO ４ , LiMnPO ４ , LiNiPO ４ , LiCoPO ４ , Li ３ V ２ (P.O. ４ ) ３ , Li ２ MnSiO ４ , Li ２ CoPO ４ Examples of chalcogen compounds include titanium disulfide, molybdenum disulfide, molybdenum dioxide, etc. The atoms or polyanions in these substances may be partially substituted with atoms or anion species of other elements.

[0047] The positive electrode active material particles are α-NaFeO ２Preferably, the lithium transition metal composite oxide includes a lithium transition metal oxide having a crystalline structure. The lithium transition metal composite oxide preferably includes nickel, cobalt, and manganese or aluminum, and more preferably includes nickel, cobalt, and manganese. The content of nickel relative to all metal elements other than lithium contained in the lithium transition metal composite oxide is preferably 10 mol% to 80 mol%, and more preferably 30 mol% to 70 mol%. The content of cobalt relative to all metal elements other than lithium contained in the lithium transition metal composite oxide is preferably 5 mol% to 60 mol%, and more preferably 10 mol% to 40 mol%. The content of manganese relative to all metal elements other than lithium contained in the lithium transition metal composite oxide is preferably 5 mol% to 60 mol%, and more preferably 10 mol% to 40 mol%. The total content of nickel, cobalt, and manganese relative to all metal elements other than lithium contained in the lithium transition metal composite oxide is preferably 95 mol% or more and 100 mol% or less, and more preferably 99.0 mol% or more or 99.9 mol% or more. By using such a lithium transition metal composite oxide, it is possible to increase the energy density, etc. Furthermore, when one embodiment of the present invention is applied to a nonaqueous electrolyte energy storage element in which positive electrode active material particles contain such a lithium transition metal composite oxide, the effect of low resistance after charge-discharge cycling is particularly pronounced.

[0048] The elemental composition ratio of the positive electrode active material (a material capable of absorbing and releasing charge-transporting ions) refers to the elemental composition ratio when the positive electrode active material particles are brought into a fully discharged state using the same procedure as before measuring the BET specific surface area and average particle size of the positive electrode active material particles.

[0049] The positive electrode active material (a material capable of absorbing and releasing charge transport ions) may be used singly or in combination of two or more. The positive electrode active material preferably contains 50% by mass or more of a lithium transition metal composite oxide (preferably 70% by mass to 100% by mass, more preferably 80% by mass to 100% by mass, and even more preferably 90% by mass to 100% by mass). The positive electrode active material may essentially consist of only a lithium transition metal composite oxide.

[0050] Elemental boron is present on at least a portion of the surface of the positive electrode active material particles. Preferably, elemental boron is present on at least a portion of the grain boundaries of the primary particles on the surface of the positive electrode active material particles. The presence of elemental boron on at least a portion of the grain boundaries of the primary particles on the surface of the positive electrode active material particles makes the positive electrode active material particles particularly resistant to cracking, resulting in lower resistance after charge-discharge cycles.

[0051] The boron element may exist in the form of a compound. Examples of compounds containing boron element include boron oxide (B ２ O ３ ), boric acid (H ３ BO ３ Examples of suitable boron-containing compounds include inorganic compounds such as boron and oxygen, boron, oxygen, and hydrogen, and other elements. The boron-containing compound may be present on the surface of the positive electrode active material particles, or may form a coating layer.

[0052] In one embodiment of the present invention, the positive electrode active material particles may be composed of a lithium transition metal composite oxide and a compound containing boron. The positive electrode active material particles may be, for example, primary particles of the lithium transition metal composite oxide aggregate to form secondary particles, and the compound containing boron may be present on the surfaces of the secondary particles and at least in part of the grain boundaries of the primary particles on the surfaces of the secondary particles.

[0053] Examples of methods for making boron present on at least a portion of the surface of positive electrode active material particles include a method of coating the particle surface of a positive electrode active material (a material capable of absorbing and releasing charge-transporting ions) with a compound containing boron. After coating with the compound containing boron, calcination or the like may be performed. Another method includes adding a boron source during synthesis of the positive electrode active material and calcining the resulting material to support the compound containing boron on the surface of the positive electrode active material particles. Commercially available products may be used as positive electrode active material particles having boron present on at least a portion of their surface.

[0054] The content of boron element in the positive electrode active material particles is preferably 0.05 mol % to 3 mol %, more preferably 0.2 mol % to 2 mol %, based on the content of all metal elements other than lithium contained in the positive electrode active material particles. By having the content of boron element in the above range, cracking of the positive electrode active material particles can be more sufficiently suppressed, and the resistance after charge-discharge cycles can be further reduced.

[0055] The contents of boron and all metal elements other than lithium contained in the positive electrode active material particles are determined by inductively coupled plasma atomic emission spectroscopy (ICP). Specifically, the positive electrode active material particles are removed from the nonaqueous electrolyte storage device and treated using the same procedure as when measuring the "BET specific surface area" and "average particle size" of the positive electrode active material particles. The positive electrode active material particles are then completely dissolved using a microwave decomposition method using an acid capable of dissolving the boron element and all metal elements contained in the positive electrode active material particles. This solution is then diluted to a certain amount with pure water to obtain a measurement solution. The concentrations of boron and all metal elements in the measurement solution are then measured by ICP atomic emission spectroscopy using a multi-type ICP atomic emission spectrometer ICPE-9820 (manufactured by Shimadzu Corporation). The element contents in the positive electrode active material particles are quantified from the obtained concentrations of each element. The calibration curve method is used to calculate the concentration of each element in the measurement solution. Furthermore, the presence of boron element on at least a part of the surface of the positive electrode active material particles can be confirmed by observing the surface of the positive electrode active material particles with a scanning electron microscope-energy dispersive X-ray analyzer (SEM-EDX), an electron probe microanalyzer (EPMA), or the like.

[0056] The content of the positive electrode active material particles 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 particles within the above range, both high energy density and manufacturability of the positive electrode active material layer can be achieved.

[0057] The conductive agent is not particularly limited as long as it is a material having electrical conductivity. 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 (CNT), 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 CNT may be used. Among these, carbon black is preferred from the viewpoints of electronic conductivity and coatability, and acetylene black is particularly preferred.

[0058] 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 in this range, the energy density of the nonaqueous electrolyte storage element can be increased.

[0059] 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.

[0060] The content of the binder in the positive electrode active material layer is preferably 1% by mass to 10% by mass, more preferably 2% by mass to 9% by mass, so that the positive electrode active material particles can be stably held.

[0061] 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. In one embodiment of the present invention, the positive electrode active material layer may not contain a thickener.

[0062] The filler is not particularly limited. Examples of the filler include polyolefins such as polypropylene and polyethylene, inorganic oxides such as silicon dioxide, alumina, titanium dioxide, calcium oxide, strontium oxide, barium oxide, 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. In one embodiment of the present invention, the positive electrode active material layer may not contain a filler.

[0063] 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 metallic 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 particles, conductive agent, binder, thickener, and filler.

[0064] 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, alloys thereof, and carbonaceous materials 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 foils, vapor-deposited films, meshes, and porous materials, with foils 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 or more and 35 μm or less, more preferably 3 μm or more and 30 μm or less, even more preferably 4 μm or more and 25 μm or less, and particularly preferably 5 μm or more and 20 μm or less. By setting the average thickness of the negative electrode substrate within the above range, it is possible to increase the strength of the negative electrode substrate and to increase the energy density of the nonaqueous electrolyte 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 a lithium ion secondary battery. Examples of the negative electrode active material include metallic lithium; metals or semimetals such as Si and Sn; metal oxides or semimetal oxides such as Si oxide, Ti oxide, and Sn oxide; Li ４ Ti ５ O １２ , LiTiO ２ , TiNb ２ O ７Examples of the material include titanium-containing oxides such as titanium dioxide, polyphosphate compounds, silicon carbide, and carbon materials such as graphite and non-graphitic carbon (easily graphitizable carbon or non-graphitizable carbon). Among these materials, graphite and non-graphitic 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 combination.

[0070] "Graphite" refers to a graphite material 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 a carbon material that has an average lattice spacing (d ００２ ) is 0.34 nm or more and 0.42 nm or less. Examples of non-graphitizable carbon include non-graphitizable carbon and graphitizable carbon. Examples of non-graphitizable 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" of the carbon material refers to a state in which the carbon material, which is the negative electrode active material, is discharged so that lithium ions that can be absorbed and released during charging and discharging are sufficiently released from the carbon material. For example, this refers to a state in which the open circuit voltage of a half cell using a negative electrode containing a carbon material as the negative electrode active material as the working electrode and metallic lithium as the counter electrode is 0.7 V or higher.

[0073] "Non-graphitizable carbon" refers to the above-mentioned d ００２ The carbon material has a particle size of 0.36 nm or more and 0.42 nm or less.

[0074] "Graphitizable carbon" refers to the above-mentioned d ００２ The term "carbon material" refers to a carbon material having 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 greater than the above lower limit, the production or handling of the negative electrode active material becomes easier. By setting the average particle size of the negative electrode active material to be equal to or less than the above upper limit, the electronic conductivity of the negative electrode active material layer is improved. To obtain powder with a predetermined particle size, a pulverizer, a classifier, or the like is used. The pulverization method and classification method can be selected from, for example, the methods exemplified for the positive electrode. When the negative electrode active material is a metal such as metallic lithium, 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] (Separator) The separator can be appropriately selected from known separators. For example, a separator consisting only of a layer mainly composed of a resin (hereinafter also referred to as a "resin layer"), or a separator in which a layer mainly composed of inorganic particles (hereinafter also referred to as an "inorganic layer") is formed on one or both sides of the resin layer, can be used. The inorganic layer may contain a binder or the like as a component other than the inorganic particles.

[0078] When the separator has a resin layer, the resin layer is preferably in contact with the positive electrode. Specifically, the separator is preferably disposed between the positive electrode and the negative electrode so that the surface of the resin layer of the separator is in direct contact with the surface of the positive electrode active material layer of the positive electrode. In such a case, the resistance of the nonaqueous electrolyte storage element after charge-discharge cycles is lower. Examples of nonaqueous electrolyte storage elements of this type include nonaqueous electrolyte storage elements using a separator consisting only of a resin layer, and nonaqueous electrolyte storage elements using a separator with an inorganic layer formed on one side of the resin layer, with the separator disposed so that the resin layer faces the positive electrode and the inorganic layer faces the negative electrode.

[0079] The resin layer in the separator may be a substrate layer. Examples of the shape of the separator resin layer 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 nonaqueous electrolyte retention. As the material for the separator resin layer, 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 decomposition resistance. Polyolefins are preferred because they are softer than other resins and can further suppress cracking of the positive electrode active material particles. A composite material of these resins may be used as the resin layer. The resin layer may contain inorganic particles, etc.

[0080] The resin content in the resin layer is preferably 70% by mass to 100% by mass, more preferably 90% by mass to 100% by mass, and even more preferably 99% by mass to 100% by mass.

[0081] The inorganic particles contained in the inorganic layer or the like preferably exhibit a mass loss of 5% or less when heated from room temperature to 500°C under an air atmosphere at 1 atmosphere, and more preferably exhibit a mass loss of 5% or less when heated from room temperature to 800°C. Examples of inorganic compounds constituting the inorganic particles 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 resource-derived substances such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica, or artificial products thereof. As the inorganic compound, these substances may be used singly or in the form of a complex, or two or more may be mixed and used. Among these inorganic compounds, silicon oxide, aluminum oxide, or aluminosilicate is preferred from the viewpoint of safety of the nonaqueous electrolyte storage element.

[0082] The porosity of the separator is preferably 80% by volume or less from the viewpoint of strength, and is preferably 20% by volume or more from the viewpoint of discharge performance. Here, "porosity" refers to a volume-based value measured with a mercury porosimeter.

[0083] The separator may be a polymer gel composed of a polymer and a non-aqueous 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.

[0084] (Non-aqueous electrolyte) The non-aqueous electrolyte can be appropriately selected from known non-aqueous electrolytes. The non-aqueous electrolyte may be a non-aqueous electrolyte solution. The non-aqueous electrolyte solution contains a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent.

[0085] 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.

[0086] 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. Of these, EC is preferred.

[0087] 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.

[0088] As the non-aqueous solvent, it is preferable to use at least one of a cyclic carbonate and a chain carbonate, 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.

[0089] 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.

[0090] The lithium salt is LiPF ６ , LiPO ２ F ２ , LiBF ４ , LiClO ４ , LiN(SO ２ F) ２ inorganic lithium salts such as lithium oxalate salts, lithium bis(oxalate)borate (LiBOB), lithium difluorooxalateborate (LiFOB), and lithium bis(oxalate)difluorophosphate (LiFOP); ３ CF ３ , LiN(SO ２ CF ３ ) ２ , LiN(SO ２ C ２ F ５ ) ２ , LiN(SO ２ CF ３ ) (SO ２ C ４ F ９ ), LiC(SO ２ CF ３ ) ３ , LiC(SO ２ C ２ F ５ ) ３ Among these, inorganic lithium salts are preferred, and LiPF ６ is more preferred.

[0091] The content of the electrolyte salt in the non-aqueous electrolyte solution is 0.1 mol / dm at 20°C and 1 atmosphere. ３ 2.5mol / dm or more ３ It is preferably 0.3 mol / dm or less. ３ 2.0mol / dm or more ３ It is more preferable that it is 0.5 mol / dm or less. ３ More than 1.7mol / dm ３It is more preferable that it is 0.7 mol / dm or less. ３ 1.5mol / dm or more ３ By setting the content of the electrolyte salt within the above range, the ionic conductivity of the non-aqueous electrolyte can be increased.

[0092] The non-aqueous electrolyte may contain additives in addition to the non-aqueous solvent and the electrolyte salt. Examples of the additives include halogenated carbonates such as fluoroethylene carbonate (FEC) and difluoroethylene carbonate (DFEC); oxalates such as lithium bis(oxalate)borate (LiBOB), lithium difluorooxalateborate (LiFOB), and lithium bis(oxalate)difluorophosphate (LiFOP); imide salts such as lithium bis(fluorosulfonyl)imide (LiFSI); biphenyl, alkylbiphenyl, terphenyl, partially hydrogenated terphenyl, and silyl. Aromatic compounds such as 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.

[0093] 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, based on the total mass of the non-aqueous electrolyte. 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.

[0094] The non-aqueous electrolyte may be a solid electrolyte, or a non-aqueous electrolytic solution and a solid electrolyte may be used in combination.

[0095] 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.

[0096] Examples of sulfide solid electrolytes include Li ２ S-P ２ S ５ , LiI-Li ２ S-P ２ S ５ , Li １０ Ge-P ２ S １２ etc.

[0097] (Pressing of electrode body, etc.) At least a portion of the electrode body is in a pressed state within the container. For example, the electrode body is in a pressed state at least in part in the thickness direction (stacking direction) of each layer. For example, in the case of a flat, wound electrode body, the flat portion may be pressed from both sides. A portion of the electrode body (for example, a pair of curved surface portions in a flat, wound electrode body) may not be in a pressed state. Furthermore, only a portion of the stacked electrode body and a portion of the flat portion of the flat, wound electrode body, for example, only the central portion, may be in a pressed state. By having at least a portion of the electrode body in a pressed state, the nonaqueous electrolyte storage element has low resistance after charge / discharge cycles.

[0098] Pressing (applying a load) to at least a portion of the electrode assembly can be performed, for example, by a pressurizing member that pressurizes the container from the outside. The pressurizing member may be a restraining member that restrains the shape of the container. The pressurizing member (restraining member) is arranged, for example, via the container, to sandwich and press at least a portion of the electrode assembly from both sides in the thickness direction. The pressed surface of the electrode assembly is in contact with the inner surface of the container directly or via another member. Therefore, by pressurizing the container, at least a portion of the electrode assembly is pressed. Examples of the pressurizing member (restraining member) include a restraining band and a metal frame. For example, the metal frame may be configured so that the load can be adjusted using bolts or the like. That is, in one embodiment of the present invention, the nonaqueous electrolyte storage element may further include a pressurizing member. Alternatively, multiple nonaqueous electrolyte storage elements may be arranged in the thickness direction of the electrode assembly and fixed using a frame or the like while being pressed from both ends in the thickness direction. Alternatively, at least a portion of the electrode assembly may be pressed by a method that does not use a pressurizing member, such as reducing the pressure inside the container.

[0099] The pressure applied to the electrode body when at least a portion of the electrode body is pressed may be, for example, 0.1 MPa to 10 MPa, preferably 0.5 MPa to 4 MPa, and more preferably 0.6 MPa to 3.3 MPa. When at least a portion of the electrode body is pressed with a pressure in this range, the resistance of the nonaqueous electrolyte storage element after charge-discharge cycles becomes lower.

[0100] The shape of the nonaqueous electrolyte storage element of this embodiment is not particularly limited, and examples thereof include a square battery, a flat battery, a coin battery, and a button battery.

[0101] Figure 1 shows a nonaqueous electrolyte storage element 1 as an example of a prismatic battery. The 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.

[0102] <Electricity Storage Device> The nonaqueous electrolyte energy storage element of the present embodiment can be mounted as an energy storage unit (battery module) comprising a plurality of nonaqueous electrolyte energy storage elements in an automobile power source such as an electric vehicle (EV), a hybrid electric vehicle (HEV), a plug-in hybrid electric vehicle (PHEV), etc., 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 nonaqueous electrolyte energy storage element included in the energy storage unit.

[0103] An electricity storage device according to one embodiment of the present invention includes two or more electricity storage elements, and includes one or more nonaqueous electrolyte electricity storage elements according to one embodiment of the present invention described above (hereinafter referred to as the "second embodiment"). It is sufficient that the technology according to one embodiment of the present invention is applied to at least one nonaqueous electrolyte electricity storage element included in the electricity storage device according to the second embodiment, and the electricity storage device may include one nonaqueous electrolyte electricity storage element according to one embodiment of the present invention described above and one or more electricity storage elements not according to one embodiment of the present invention described above, or may include two or more nonaqueous electrolyte electricity storage elements according to one embodiment of the present invention described above.

[0104] 2 shows an example of an energy storage device 30 according to a second embodiment, in which energy storage units 20, each of which is an assembly of two or more electrically connected nonaqueous electrolyte energy storage elements 1, are further assembled. The energy storage device 30 may include a bus bar (not shown) that electrically connects two or more nonaqueous electrolyte energy storage elements 1, a bus bar (not shown) that electrically connects two or more energy storage units 20, and the like. The energy storage unit 20 or the energy storage device 30 may include a state monitoring device (not shown) that monitors the state of one or more nonaqueous electrolyte energy storage elements. The energy storage unit 20 or the energy storage device 30 may further include a pressure member for indirectly pressing at least a portion of the electrode body of the nonaqueous electrolyte energy storage element.

[0105] <Method for Manufacturing Nonaqueous Electrolyte Storage Element> The method for manufacturing the nonaqueous electrolyte storage element of this embodiment can be appropriately selected from known methods. The manufacturing method includes, for example, preparing an electrode assembly, preparing a nonaqueous electrolyte, housing the electrode assembly and the nonaqueous electrolyte in a container, and placing at least a portion of the electrode assembly in a pressed state within the 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.

[0106] The method for placing the non-aqueous electrolyte in the container can be appropriately selected from known methods. For example, when a non-aqueous electrolyte solution is used as the non-aqueous electrolyte, the non-aqueous electrolyte solution may be poured into the container through an inlet formed in the container, and then the inlet may be sealed.

[0107] As described above, at least a part of the electrode body can be pressed inside the container by providing a pressure member that applies pressure to the container from the outside.

[0108] <Other Embodiments> The nonaqueous electrolyte storage element of the present invention is not limited to the above-described embodiments, and various modifications may be made without departing from the spirit of the present invention. For example, the configuration of one embodiment can be added to the configuration of another embodiment, or 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.

[0109] In the above embodiment, the nonaqueous electrolyte storage element is used as a chargeable and dischargeable nonaqueous electrolyte secondary battery (e.g., a lithium ion secondary battery), but the type, shape, size, capacity, etc. of the nonaqueous electrolyte storage element are arbitrary. The present invention can also be applied to various secondary batteries.

[0110] In the above embodiment, the electrode assembly in which the positive electrode and the negative electrode are stacked with a separator interposed therebetween has been described, but the electrode assembly may not include a separator. For example, the positive electrode and the negative electrode may be in direct contact with each other in a state in which a non-conductive layer is formed on the active material layer of the positive electrode or the negative electrode.

[0111] 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.

[0112] Example 1 (Preparation of Positive Electrode) α-NaFeO ２ Lithium transition metal composite oxide having a LiNi type crystal structure １／２ Co ３／１０ Mn １／５ O ２ Secondary particles of boric acid (H ３ BO ３ ) the above LiNi １／２ Co ３／１０ Mn １／５ O ２ The secondary particles were mixed to prepare a suspension, and the suspension was then dried and fired to obtain positive electrode active material particles having elemental boron present on at least a portion of the surface. The content of elemental boron in the positive electrode active material particles was 0.5 mol% relative to the content of metal elements other than lithium contained in the positive electrode active material. The obtained positive electrode active material particles had an average particle size of 3.5 μm and a BET specific surface area (S １ ) is 2.4m ２ / g, and the specific surface area (S ２ ) is 0.36 m ２ / g, void index A was 6.7, and average internal porosity was 12.5%. A positive electrode mixture paste was prepared using the positive electrode active material particles, 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 particles, AB, and PVDF was 93:4.5:2.5 (solid content equivalent). The positive electrode mixture paste was applied to both sides of aluminum foil as a positive electrode substrate and dried. Then, roll pressing was performed to obtain a positive electrode.

[0113] (Preparation of Negative Electrode) A negative electrode mixture paste was prepared using graphite as a 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, SBR, and CMC was 98:1:1 (solid content equivalent). The negative electrode mixture paste was applied to both sides of copper foil as a negative electrode substrate and dried. Then, a roll press was performed to obtain a negative electrode.

[0114] (Non-aqueous electrolyte) A 1.2 mol / dm ３ At a concentration of LiPF ６ was dissolved to obtain a non-aqueous electrolyte.

[0115] (Separator) The separator used was a separator having an inorganic layer formed of inorganic particles and a binder provided on one surface of a polyolefin microporous film (resin layer).

[0116] (Assembly of non-aqueous electrolyte storage element) An electrode body was produced by stacking the positive electrode and the negative electrode with the separator interposed therebetween. The separator was positioned so that the inorganic layer of the separator was in contact with the positive electrode. The electrode body was housed in a container made of a metal resin composite film, the non-aqueous electrolyte was injected into the interior, and the opening was sealed by thermal welding. The container was then pressed in the thickness direction with a pressure of 0.6 MPa using a pressure member, thereby obtaining a non-aqueous electrolyte storage element of Example 1 in which the flat portion of the electrode body was pressed. Note that, because a container made of a metal resin composite film was used, it can be determined that the pressure applied to the container and the pressure applied to the electrode body were substantially equal.

[0117] [Examples 2 to 5 and Comparative Examples 1 to 6] The nonaqueous electrolyte storage elements of Examples 2 to 5 and Comparative Examples 1 to 6 were obtained in the same manner as in Example 1, except that the positive electrode active material particles and separators shown in Table 1 were used, and that the electrode body was pressed at a pressure shown in Table 1 or not pressed at all. The positive electrode active material particles were all made of α-NaFeO ２ Lithium transition metal composite oxide having a LiNi type crystal structure １／２Co ３／１０ Mn １／５ O ２ The secondary particles were used, and some of the positive electrode active material particles were made to have elemental boron present on their surfaces in the same manner as in Example 1. In addition, a polyolefin microporous membrane (a separator consisting of only a resin layer) was used as the separator having no inorganic layer. That is, in Example 4, a nonaqueous electrolyte storage element was produced in which the resin layer of the separator was in contact with the positive electrode.

[0118] [Evaluation] (1) Initial Charge / Discharge Each of the obtained nonaqueous electrolyte storage elements was initially charged / discharged under the following conditions. In a thermostatic chamber at 25°C, constant-current charging was performed with a charging current of 1.0 C and a cut-off voltage of 4.1 V, followed by constant-voltage charging at 4.1 V. The charging was terminated until the total charging time reached 3 hours. A 10-minute rest period was then provided. Constant-current discharging was performed with a discharging current of 1.0 C and a cut-off voltage of 3.0 V. (2) Initial DC Resistance Each of the nonaqueous electrolyte storage elements after the initial charge / discharge was stored in a thermostatic chamber at 25°C for 3 hours, and then subjected to constant-current charging at a current of 1.0 C at 25°C to bring the SOC to 50%. Subsequently, the elements were discharged at a constant current of 0.2 C, 0.5 C, or 1.0 C at 25°C for 30 seconds. After each discharge, constant-current charging was performed at a current of 0.2 C to bring the SOC to 50%. The relationship between the current and the voltage 10 seconds after the start of discharge for each discharge was plotted, and the DC resistance of each nonaqueous electrolyte storage element was calculated from the slope of the line obtained from the three plots. The initial DC resistance of each nonaqueous electrolyte storage element is shown in Table 1 as a relative value, with the initial DC resistance of the nonaqueous electrolyte storage element of Comparative Example 2 set to 100%. (3) Charge / Discharge Cycle Test Next, each nonaqueous electrolyte storage element was subjected to constant-current discharge at a current of 1.0 C in a thermostatic chamber at 25°C to set the SOC to 0%, and then constant-current charging at a current of 0.5 C to an SOC of 50%. Subsequently, the element was stored in a thermostatic chamber at 55°C for 4 hours. Subsequently, constant-current charging at a current of 4 C to a voltage corresponding to an SOC of 80%, and then constant-current discharging at a current of 4 C to a voltage corresponding to an SOC of 20%. The above charge and discharge cycle was repeated for 2150 hours without rest periods between the charge and discharge. (4) DC Resistance After Charge-Discharge Cycle Test The DC resistance of each nonaqueous electrolyte energy storage element after the charge-discharge cycle test was then determined using the same procedure as that used to determine the initial DC resistance. The DC resistance of each nonaqueous electrolyte energy storage element after the charge-discharge cycle test is shown in Table 1 as a relative value, with the DC resistance of the nonaqueous electrolyte energy storage element of Comparative Example 2 after the charge-discharge cycle test being set at 100%.

[0119]

[0120] The nonaqueous electrolyte storage element of Comparative Example 1 had a void index A of less than 5, and used positive electrode active material particles that did not have elemental boron on at least a portion of their surfaces. Therefore, at least a portion of the electrode assembly was not compressed, and the DC resistance after the charge-discharge cycle test was high. The nonaqueous electrolyte storage element of Comparative Example 2, in which at least a portion of the electrode assembly was compressed, and the nonaqueous electrolyte storage element of Comparative Example 3, in which at least a portion of the electrode assembly was compressed and further boron was present on at least a portion of the surface of the positive electrode active material particles, also did not have a sufficiently low DC resistance after the charge-discharge cycle test. The nonaqueous electrolyte storage element of Comparative Example 4 had a void index A of 5 or more, and used positive electrode active material particles that did not have elemental boron on at least a portion of their surfaces. Therefore, at least a portion of the electrode assembly was not compressed, and the DC resistance after the charge-discharge cycle test was very high. The nonaqueous electrolyte storage element of Comparative Example 5, in which at least a portion of the electrode body was compressed compared to the nonaqueous electrolyte storage element of Comparative Example 4, and the nonaqueous electrolyte storage element of Comparative Example 6, in which boron element was present on at least a portion of the surface of the positive electrode active material particles, also did not have a sufficiently low DC resistance after the charge-discharge cycle test. In contrast, the nonaqueous electrolyte storage elements of Examples 1 to 5, in which positive electrode active material particles having a void index A of 5 or more and having boron element present on at least a portion of the surface thereof were used and in which at least a portion of the electrode body was compressed, had a DC resistance (relative value) of 95% or less after the charge-discharge cycle test, which was a sufficiently low value.

[0121] The present invention can be applied to nonaqueous electrolyte electricity storage elements used as power sources for electronic devices such as personal computers and communication terminals, and automobiles.

[0122] REFERENCE SIGNS LIST 1 nonaqueous electrolyte energy storage element 2 electrode body 3 container 4 positive electrode terminal 41 positive electrode lead 5 negative electrode terminal 51 negative electrode lead 20 energy storage unit 30 energy storage device

Claims

1. A nonaqueous electrolyte storage element comprising an electrode assembly having a positive electrode containing positive electrode active material particles and a negative electrode, the positive electrode active material particles have a void index A represented by the following formula (1) of 5 or more, boron element is present on at least a part of the surface of the positive electrode active material particles, and at least a part of the electrode assembly is in a compressed state. A=S １ / S ２ ...(1) (In formula (1), S １ is the BET specific surface area of ​​the positive electrode active material particles. ２ is the specific surface area when the positive electrode active material particles are assumed to be spheres with a diameter equal to the average particle diameter.) 2. The nonaqueous electrolyte storage element according to claim 1, wherein the electrode body further has a separator disposed between the positive electrode and the negative electrode, the separator having a layer in contact with the positive electrode and containing a resin as a main component.

3. The positive electrode active material particles are α-NaFeO ２ 3. The nonaqueous electrolyte storage element according to claim 1, comprising a lithium transition metal composite oxide having a type crystalline structure.

4. The nonaqueous electrolyte storage element according to claim 1 or 2, wherein at least a portion of the electrode body is pressed at a pressure of 0.5 MPa or more and 4 MPa or less.

5. An electricity storage device comprising two or more electricity storage elements, and comprising at least one nonaqueous electrolyte electricity storage element according to claim 1 or 2.

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