Power storage element positive electrode, electrode body, power storage element, and power storage device

Abstract

One aspect of the present invention is a power storage element positive electrode that comprises a base material and a positive electrode active material layer that is layered on the base material and includes positive electrode active material particles. The positive electrode active material particles contain first active material particles and second active material particles that have different particle diameters. On a particle size distribution diagram for the positive electrode active material particles on which the horizontal axis is particle diameter (μm) and the vertical axis is frequency (%) by volume, a first peak particle diameter that represents the first active material particles is at least 4.0 μm, a second peak particle diameter that represents the second active material particles is less than the first peak particle diameter, and the ratio of the frequency (%) for the first peak particle diameter to the frequency (%) for the second peak particle diameter is 1.0–4.3. The breaking strength of the first active material particles is at least 20.0 MPa, and the mass per unit area of the positive electrode active material layer is at least 1.0 g / 100 cm2 but no more than 2.0 g / 100 cm2.

Description

Positive electrode for energy storage element, electrode body, energy storage element, and energy storage device

[0001] The present invention relates to a positive electrode for an electric storage element, an electrode body, an electric storage element, and an electric 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. Capacitors such as lithium ion capacitors and electric double layer capacitors are also widely used as energy storage elements other than non-aqueous electrolyte secondary batteries.

[0003] An example of an electrode for an electricity storage element is one having an active material layer obtained by applying a mixture containing active material particles to a metal foil, drying it, and then pressing it (Patent Document 1).

[0004] Re-table 2018 / 047843 publication

[0005] In order to suppress an increase in the resistance of an energy storage element, efforts have been made to improve the adhesion between the electrode substrate (metal foil) and the active material layer, including increasing both the hardness and particle size of the active material particles contained in the active material layer.

[0006] However, when the particle diameter of the active material particles is large, pressing the mixture hard to increase the electrode density can cause cracks, breakage, or the like in the active material layer or the substrate. If cracks or the like occur in the active material layer, the conductive paths between the active materials can be cut off, and the resistance of the energy storage element can increase. Furthermore, if breakage or the like occurs in the substrate, the capacity achieved will be lower than the designed capacity. For this reason, it may be difficult to increase the electrode density while reducing the resistance of the energy storage element.

[0007] The present invention has been made based on the above circumstances, and its object is to provide a positive electrode for a storage element that can reduce the resistance of the storage element and increase the electrode density, as well as an electrode body, a storage element, and a storage device that include such a positive electrode for the storage element.

[0008] A positive electrode for an energy storage element according to one aspect of the present invention includes a substrate and a positive electrode active material layer laminated on the substrate and containing positive electrode active material particles, the positive electrode active material particles including first active material particles and second active material particles having different particle diameters, wherein in a particle size distribution diagram of the positive electrode active material particles with the horizontal axis representing particle diameters [μm] and the vertical axis representing volume-based frequency [%], the first active material particles exhibit a first peak particle diameter of 4.0 μm or more and the second active material particles exhibit a second peak particle diameter smaller than the first peak particle diameter, the first active material particles have a breaking strength of 20.0 MPa or more, the ratio of the frequency [%] of the first peak particle diameter to the frequency [%] of the second peak particle diameter in the particle size distribution diagram is 1.0 or more and 4.3 or less, and the positive electrode active material layer has a mass per unit area of ​​1.0 g / 100 cm ２ 2.0g / 100cm or more ２ The following is the result.

[0009] An electrode assembly according to another aspect of the present invention includes a positive electrode for an energy storage element according to one embodiment of the present invention.

[0010] An energy storage element according to another embodiment of the present invention includes a positive electrode for the energy storage element according to one embodiment of the present invention.

[0011] A power storage device according to another embodiment of the present invention includes one or more power storage elements according to one embodiment of the present invention, and includes two or more power storage elements.

[0012] According to any one embodiment of the present invention, it is possible to provide a positive electrode for an energy storage element that can reduce the resistance of the energy storage element and increase the electrode density, or an electrode body, an energy storage element, or an energy storage device that includes such a positive electrode for the energy storage element.

[0013] Fig. 1 is a perspective view showing an energy storage element having a positive electrode for an energy storage element according to one embodiment of the present invention. Fig. 2 is a schematic diagram showing an energy storage device formed by assembling a plurality of energy storage elements having a positive electrode for an energy storage element according to one embodiment of the present invention. Fig. 3 is a particle size distribution diagram of positive electrode active material particles contained in the energy storage elements of Example 3 and Comparative Example 2.

[0014] First, an outline of the positive electrode for an energy storage element, the electrode body, the energy storage element, and the energy storage device disclosed in this specification will be described.

[0015] [1] A positive electrode for an energy storage device according to one aspect of the present invention includes a substrate and a positive electrode active material layer laminated on the substrate and containing positive electrode active material particles, the positive electrode active material particles including first active material particles and second active material particles having different particle diameters, wherein in a particle size distribution diagram of the positive electrode active material particles with the horizontal axis representing particle diameter [μm] and the vertical axis representing volume-based frequency [%], the first active material particles exhibit a first peak particle diameter of 4.0 μm or more and the second active material particles exhibit a second peak particle diameter smaller than the first peak particle diameter, the first active material particles have a breaking strength of 20.0 MPa or more, the ratio of the frequency [%] of the first peak particle diameter to the frequency [%] of the second peak particle diameter in the particle size distribution diagram is 1.0 or more and 4.3 or less, and the positive electrode active material layer has a mass per unit area of ​​1.0 g / 100 cm ２ 2.0g / 100cm or more ２ The following is the result.

[0016] The positive electrode for an energy storage device described in [1] above can reduce the resistance of the energy storage device and increase the electrode density. While the reason for this is unclear, the following is presumed. To reduce the resistance of the positive electrode, the hardness of the active material particles may be increased so that the active material particles sink into the substrate and improve adhesion between the substrate and the positive electrode active material layer. Furthermore, to effectively improve adhesion between the substrate and the positive electrode active material layer, it is necessary to increase not only the hardness of the active material particles but also their particle size to a certain level. However, when forming the positive electrode active material layer, if a mixture containing large-particle-size active material particles is pressed too hard, the strength of the active material layer stacked on the substrate decreases, which can cause cracks or other problems in the active material layer. If cracks or other problems occur in the active material layer, the conductive paths between the active materials are severed, potentially increasing the resistance of the energy storage device. For this reason, with conventional positive electrodes for energy storage devices, it has been difficult to increase the electrode density while reducing the resistance of the energy storage device. In contrast, the positive electrode for a storage battery device described in [1] includes a positive electrode active material layer containing first active material particles having a peak particle size and fracture strength within the above-mentioned ranges in a particle size distribution diagram. Therefore, the first active material particles sink into the substrate, thereby enhancing adhesion between the substrate and the positive electrode active material layer, thereby reducing the resistance of the storage battery device. Furthermore, the positive electrode active material layer contains, in addition to the first active material particles, second active material particles having a smaller peak particle size than the first active material particles at a certain frequency. Therefore, the first active material particles reduce the resistance of the storage battery device, while the second active material particles penetrate between the first active material particles, thereby increasing the density of the positive electrode active material layer without causing cracks or other problems. Furthermore, because the mass per unit area of ​​the positive electrode active material layer is controlled within the above-mentioned ranges, it is easy to reduce the resistance of the storage battery device and improve the electrode density. This point is explained below.

[0017] The positive electrode for a storage device described in [1] above is typically manufactured by applying a so-called positive electrode mixture to a substrate and then pressing it to form a positive electrode active material layer. This is then wound or stacked with a negative electrode or the like to produce an electrode assembly, which is then used in the storage device. Pressing the positive electrode mixture too hard increases the density of the positive electrode active material layer, but as described above, the strength of the positive electrode active material layer is likely to decrease. Furthermore, if the mass per unit area of ​​the positive electrode active material layer is large, the positive electrode active material layer laminated on the substrate becomes thick, which is likely to cause cracking of the positive electrode active material layer, peeling from the substrate, or fracture of the substrate itself during the process of producing the electrode assembly. Therefore, in order to prevent defective products due to cracking of the positive electrode active material layer or an increase in the resistance of the storage device, if the mass per unit area of ​​the positive electrode active material layer is large, it is necessary to correspondingly reduce the density of the positive electrode active material layer (i.e., to weaken the pressure on the positive electrode mixture). Conversely, when the mass per unit area of ​​the positive electrode active material layer is small, it is possible to increase the density of the positive electrode active material layer (to press the positive electrode mixture more strongly). In other words, by controlling the mass per unit area of ​​the positive electrode active material layer within the above range, it is possible to easily reduce the resistance of the energy storage element and improve the electrode density. For the above reasons, it is presumed that the positive electrode described in [1] above exhibits the effects of reducing the resistance of the energy storage element and increasing the electrode density due to the synergistic effect of the physical properties and frequency in the particle size distribution diagram of the first active material particles and the second active material particles, and the mass per unit area of ​​the positive electrode active material layer.

[0018] In the present invention, the term "particle size distribution diagram" refers to a graph showing the particle size distribution (frequency distribution curve) of particle sizes measured by a laser diffraction / scattering method for a diluted solution of positive electrode active material particles diluted with a solvent in accordance with JIS-Z-8825 (2013). The term "peak particle size" refers to the particle size corresponding to the position showing the maximum value based on the particle size distribution diagram. The term "first peak particle size shown by first active material particles" refers to the particle size at the position showing the maximum value in the portion corresponding to the first active material particles in the particle size distribution diagram. The term "second peak particle size shown by second active material particles" refers to the particle size at the position showing the maximum value in the portion corresponding to the second active material particles in the particle size distribution diagram. Here, the term "peak particle size" refers to the peak particle size of secondary particles, but if the positive electrode active material particles do not constitute secondary particles, it refers to the peak particle size of primary particles. The term "different particle sizes" refers to different peak particle sizes in the particle size distribution diagram.

[0019] In the particle size distribution diagram, when the overlapping portion of the particle size distributions of the first active material particles and the second active material particles is large and the respective peak particle diameters cannot be clearly distinguished, the two particle size distributions may be separated and the peak particle diameters measured using numerical analysis software. Examples of numerical analysis software include MATLAB (registered trademark) (The MathWorks, Inc.) and ORIGIN PRO (registered trademark) (Lightstone, Inc.). Separation of the particle size distributions can be performed by known techniques.

[0020] In the present invention, "breaking strength" is measured in accordance with JIS-Z-8844 (2019). Breaking strength is measured by collecting five first active material particles and using the average value. However, particles whose particle diameters fall within a range where the particle size distributions of the first active material particles and the second active material particles overlap are excluded from selection. The "particle diameter" of each particle used to measure breaking strength is the average value of the minor axis and major axis. The minor axis is the shortest diameter passing through the center of the smallest circumscribing circle of the particle, and the major axis is the diameter passing through the center and perpendicular to the minor axis. If there are two or more shortest diameters, the diameter perpendicular to the shortest diameter is taken as the minor axis.

[0021] The particle size distribution and fracture strength of the positive electrode active material particles are measured on the positive electrode active material particles before charge / discharge when the positive electrode active material particles are available. When a positive electrode containing the positive electrode active material particles is incorporated into an energy storage device, the measurements are performed on the positive electrode active material particles collected from the positive electrode of the energy storage device in a fully discharged state treated according to the following procedure. First, the energy storage device is charged at a constant current of 0.05 C until the end-of-charge voltage for normal use is reached, and then fully charged. After a 30-minute rest, the device is discharged at a constant current of 0.05 C until the end-of-discharge voltage for normal use is reached. The device is disassembled, the positive electrode is removed, and a test battery is assembled using the removed positive electrode as the working electrode and a metallic lithium electrode as the counter electrode. Pure metallic lithium is used for the metallic lithium electrode. The test battery is charged at a current of 10 mA per gram of positive electrode active material until the positive electrode potential reaches 2.0 V (vs. Li / Li ＋ ) to adjust the positive electrode to a fully discharged state. The device is disassembled again and the positive electrode is removed. The electrolyte and other materials adhering to the removed positive electrode are thoroughly washed using dimethyl carbonate, and the device is vacuum dried at room temperature for 24 hours, after which the positive electrode active material particles from the positive electrode active material layer are collected. The collected positive electrode active material particles are then subjected to measurement. The operations from disassembling the energy storage device to collecting the positive electrode active material particles are carried out in an argon atmosphere with a dew point of -60°C or below. "Normal use" refers to the case where the energy storage device is used under the charge and discharge conditions recommended or specified for the energy storage device, and, if equipment for using the energy storage device is available, the energy storage device is used with that equipment.

[0022] The "mass per unit area of ​​the positive electrode active material layer" is the mass (g) of the positive electrode active material layer in the positive electrode multiplied by the area (100 cm) of the region where the positive electrode active material layer is laminated. ２ ) For example, when a positive electrode active material layer is laminated over the entire surface of one side of a sheet-like positive electrode substrate, the "mass per unit area of ​​the positive electrode active material layer" is the value obtained by dividing the total mass of the positive electrode active material layer by the area of ​​one side of the positive electrode substrate. On the other hand, when a positive electrode active material layer is laminated over the entire surface of both sides of a sheet-like positive electrode substrate, the "mass per unit area of ​​the positive electrode active material layer" is the value obtained by dividing the total mass of the positive electrode active material layer by the area of ​​both sides of the positive electrode substrate.

[0023] [2] In the positive electrode for an energy storage element according to the above item [1], the ratio of the first peak particle diameter to the second peak particle diameter may be 6.0 or more. In the positive electrode for an energy storage element according to the above item [2], the ratio of the first peak particle diameter to the second peak particle diameter is in the above range, so that the difference between the first peak particle diameter and the second peak particle diameter is sufficiently large, and therefore the electrode density can be more easily increased.

[0024] [3] In the positive electrode for an energy storage device according to [1] or [2] above, the first active material particles and the second active material particles may contain a lithium transition metal compound having a polyanion structure. Active material particles containing a lithium transition metal compound having a polyanion structure tend to have low adhesion to a substrate. Therefore, the positive electrode for an energy storage device according to [3] above, in which the first active material particles and the second active material particles contain a lithium transition metal compound having a polyanion structure, significantly achieves the advantages of the present invention, namely, reducing the resistance of the energy storage device and increasing the electrode density.

[0025] [4] An electrode assembly according to another aspect of the present invention has a positive electrode for an energy storage element according to any one of [1] to [3] above.

[0026] The electrode body described in [4] above has a positive electrode for a storage element described in any one of [1] to [3] above, and therefore can reduce the resistance of the storage element and increase the electrode density of the positive electrode.

[0027] [5] An energy storage device according to another aspect of the present invention includes the positive electrode for the energy storage device according to any one of [1] to [3] above or the electrode body according to [4] above.

[0028] The storage element described in [5] above has a positive electrode for the storage element described in any one of [1] to [3] above or an electrode body described in [4] above, and therefore the resistance of the storage element can be reduced and the electrode density of the positive electrode can be increased.

[0029] [6] A power storage device according to another embodiment of the present invention includes one or more of the power storage elements described in [5] above, and includes two or more power storage elements.

[0030] The electricity storage device according to [6] above includes one or more of the electricity storage elements according to [5] above, and therefore the resistance of the electricity storage device can be reduced and the electrode density of the positive electrode can be increased.

[0031] Hereinafter, a positive electrode for an energy storage element, an electrode body, an energy storage element, a manufacturing method for an energy storage element, an energy storage device, and other embodiments according to one embodiment of the present invention will be described in detail. Note that the names of the components (components) used in each embodiment may differ from the names of the components (components) used in the background art. Furthermore, when both a lower limit and an upper limit are specified for the value of various parameters such as physical properties, the value of the parameter can be within a range that combines any of the lower limits and any of the upper limits (however, the lower limit is smaller than the upper limit).

[0032] [Positive electrode for energy storage device] A positive electrode for an energy storage device according to one embodiment of the present invention includes a substrate and a positive electrode active material layer laminated directly on the substrate. The positive electrode active material layer contains positive electrode active material particles. Hereinafter, the positive electrode for an energy storage device will also be referred to simply as the "positive electrode," and the substrate of the positive electrode will also be referred to as the "positive electrode substrate."

[0033] In the positive electrode, the positive electrode active material particles and the positive electrode active material layer have the composition and physical properties described below, so that the resistance of the electricity storage element can be reduced and the electrode density can be increased.

[0034] 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).

[0035] 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 and also increase the energy density per volume of the energy storage element.

[0036] The positive electrode active material layer contains positive electrode active material particles. The positive electrode active material layer can be formed from a so-called positive electrode mixture containing the positive electrode active material particles. The positive electrode active material layer contains optional components such as a conductive agent, a thickener, and a filler as needed.

[0037] [Positive Electrode Active Material Particles] The positive electrode active material particles contain first and second active material particles with different particle diameters. The particle diameter of the first active material particles is larger than the particle diameter of the second active material particles. That is, in a particle size distribution diagram of the positive electrode active material particles, with the horizontal axis representing particle diameter [μm] and the vertical axis representing volume-based frequency [%], the first peak particle diameter of the first active material particles (hereinafter also referred to as "first particle diameter") is larger than the second peak particle diameter of the second active material particles (hereinafter also referred to as "second particle diameter"). The first particle diameter usually coincides with the peak particle diameter of the first active material particles shown in a particle size distribution diagram of the first active material particles alone. Similarly, the second particle diameter usually coincides with the peak particle diameter of the second active material particles shown in a particle size distribution diagram of the second active material particles alone. By including first and second active material particles with different particle diameters in the positive electrode active material particles, the second active material particles enter between the first active material particles, thereby increasing the density of the positive electrode active material layer.

[0038] The first active material particles and the second active material particles may be single particles each consisting of a plurality of primary particles that do not aggregate, but are preferably secondary particles formed by aggregation of a plurality of primary particles. The first active material particles and the second active material particles are, for example, secondary particles of a lithium transition metal compound having a polyanion structure.

[0039] In the particle size distribution curve of the particle size distribution diagram of the positive electrode active material particles, it is preferable to have an inflection point between the peak shown by the first active material particles and the peak shown by the second active material particles, and more preferably to have a valley portion or a flat portion. By having an inflection point between the peak shown by the first active material particles and the peak shown by the second active material particles in the particle size distribution curve of the particle size distribution diagram, the differences in particle diameters and frequencies in the particle size distribution diagram between the first active material particles and the second active material particles are within appropriate ranges, and therefore the resistance of the energy storage device can be easily reduced.

[0040] The lower limit of the first particle diameter is 4.0 μm, preferably 4.2 μm, more preferably 4.4 μm, even more preferably 4.6 μm, and even more preferably 4.8 μm. Meanwhile, the upper limit of the first particle diameter is preferably 25.0 μm, more preferably 20.0 μm, even more preferably 15.0 μm, and even more preferably 10.0 μm. When the first particle diameter is equal to or greater than the lower limit, the resistance of the energy storage element can be reduced. Furthermore, when the first particle diameter is equal to or less than the upper limit, the density of the positive electrode active material layer can be easily increased.

[0041] The upper limit of the second particle diameter is preferably 2.0 μm, more preferably 1.5 μm, even more preferably 1.2 μm, and even more preferably 1.0 μm. When the second particle diameter is equal to or less than the upper limit, the density of the positive electrode active material layer can be easily increased. On the other hand, from the viewpoint of manufacturability, the lower limit of the second particle diameter is preferably 0.4 μm, more preferably 0.5 μm, more preferably 0.6 μm, and even more preferably 0.7 μm.

[0042] The lower limit of the ratio of the first particle diameter to the second particle diameter is preferably 6.0, more preferably 6.5, and may be 7.0 or 7.5. The lower limit of the ratio may be 8.0, 10.0, 20.0, or 30.0. When the ratio is equal to or greater than the lower limit, the density of the positive electrode active material layer can be easily increased. On the other hand, the upper limit of the ratio is not particularly limited, but may be, for example, 50.0 or 45.0.

[0043] To obtain the positive electrode active material particles (first active material particles and second active material particles) with a desired 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 during pulverization. As a classification method, a sieve, an air classifier, or the like is used as needed for both dry and wet methods.

[0044] The lower limit of the breaking strength of the first active material particles is 20.0 MPa, preferably 30.0 MPa, more preferably 40.0 MPa, and even more preferably 50.0 MPa. When the breaking strength of the first active material particles is equal to or greater than the lower limit, the resistance of the energy storage device can be reduced. On the other hand, from the viewpoint of manufacturability, the upper limit of the breaking strength of the first active material particles is preferably 200.0 MPa, more preferably 150.0 MPa, and even more preferably 100.0 MPa. The breaking strength of the first active material particles can be controlled by manufacturing conditions such as the type of positive electrode active material and the carbon material coating its surface, as well as the manufacturing method of the positive electrode active material (hydrothermal method, solid-phase method, etc.), firing conditions, and conditions of the method of forming the positive electrode active material particles (spray drying method, etc.).

[0045] In the particle size distribution diagram of the positive electrode active material particles, the lower limit of the ratio of the frequency [%] of the first particle diameter to the frequency [%] of the second particle diameter is 1.0, preferably 1.5, more preferably 2.0, even more preferably 2.5, even more preferably 3.0, and particularly preferably 3.5 or 4.0. On the other hand, the upper limit of the ratio is 4.3, preferably 4.2, and more preferably 4.1. The upper limit of the ratio may be 4.0, 3.5, 3.0, 2.5, 2.0, or 1.5. When the ratio is equal to or greater than the lower limit, the frequency of the first active material particles relative to the second active material particles in the particle size distribution diagram falls within a suitable range, thereby sufficiently reducing the resistance of the energy storage device. Furthermore, when the ratio is equal to or less than the upper limit, the frequency of the second active material particles in the particle size distribution diagram falls within a suitable range, thereby increasing the density of the positive electrode active material layer without causing cracks or the like. The ratio of the frequency [%] of the first particle diameter to the frequency [%] of the second particle diameter can be controlled by adjusting the mixing ratio of the first active material particles and the second active material particles, for example.

[0046] The first active material particles and the second active material particles contain a positive electrode active material, such as a lithium transition metal compound having a polyanion structure, a layered α-NaFeO ２ Examples of suitable positive electrode active materials include lithium transition metal composite oxides having a crystalline structure and other known positive electrode active materials. Among these, lithium transition metal compounds having a polyanion structure (hereinafter also referred to as "polyanion compounds") are preferred. The inclusion of a polyanion compound in the first active material particles and the second active material particles can increase the discharge capacity of the energy storage device. On the other hand, active material particles containing a polyanion compound tend to have poor adhesion to the substrate. This significantly enhances the advantages of the present invention, namely, reduced resistance of the energy storage device and increased electrode density. When the first active material particles and the second active material particles contain a polyanion compound, the surfaces of the particles may be coated with a carbon material from the viewpoint of electronic conductivity, etc.

[0047] The polyanion compound includes an oxo acid anion (PO ４ ３－ , S.O. ４ ２－ , SiO４ ４－ , B.O. ３ ３－ , V.O. ４ ３－ The oxo acid anion may be a compound containing a condensed anion (P ２ O ７ ４－ , P ３ O １０ ５－ The polyanion compound may have an olivine-type crystal structure. The polyanion compound typically contains a lithium element and a transition metal element, and may further contain other elements (for example, halogen elements, etc.). The transition metal elements contained in the polyanion compound are preferably iron, manganese, nickel, and cobalt, and more preferably iron. The oxo acid anion contained in the polyanion compound is preferably a phosphate anion (PO ４ ３－ ) is preferred.

[0048] The polyanion compound is preferably a compound represented by the following formula (1): Li ａ M ｂ (A.O. ｃ ) ｄ X ｅ ... (1) In formula (1), M is at least one transition metal element. A is at least one selected from B, Al, Si, P, S, Cl, Ti, V, Cr, Mo, and W. X is at least one halogen element. a, b, c, d, and e are numbers satisfying 0<a≦3, 0<b≦2, 2≦c≦4, 1≦d≦3, and 0≦e≦1. a, b, c, d, and e may all be integers or decimals.

[0049] As M in formula (1), any one of Fe, Mn, Ni, and Co, or a combination of any two of them, is preferable. As M, Fe, Mn, or a combination thereof is more preferable, and Fe is more preferable. Furthermore, the content of Fe in M ​​is preferably 50 mol% or more, more preferably 70 mol% or more, 90 mol% or more, or 99 mol% or more. As A, P is preferable. As X, F is preferable. In one embodiment, a = 1, b = 1, c = 4, d = 1, and e = 0 may be preferable.

[0050] Specific examples of polyanion compounds include LiFePO ４ , LiCoPO ４ , LiFe ０．５ Co ０．５ P.O. ４ , LiMnPO ４ , LiNiPO ４ , LiMn ０．５ Fe ０．５ P.O. ４ , LiCrPO ４ , LiFeVO ４ , Li ２ FeSiO ４ , Li ２ Fe ２ (SO ４ ) ３ , LiFeBO ３ , LiFePO ３．９ F ０．２ , Li ３ V ２ (P.O. ４ ) ３ , Li ２ MnSiO ４ , Li ２ CoPO ４ F. Atoms or polyanions in these polyanion compounds may be partially substituted with other atoms or anion species. The polyanion compounds may be used singly or in combination of two or more.

[0051] The first active material particles and the second active material particles each preferably contain a polyanionic compound as a main component. "Containing a polyanionic compound as a main component" means that the content of the polyanionic compound is 50% by mass or more. The content of the polyanionic compound contained in each of the first active material particles and the second active material particles is preferably 70% by mass or more, more preferably 90% by mass or more, and even more preferably 99% by mass or more. By using a polyanionic compound as a main component of the first active material particles and the second active material particles, the advantages of the present invention, such as reduced resistance of the energy storage element and increased electrode density, can be significantly achieved.

[0052] In one embodiment of the present invention, the first active material particles and the second active material particles are mainly composed of a polyanionic compound and have a coating layer of a carbon material on their surfaces. A portion of the carbon material may be present inside the particles. The first active material particles and the second active material particles may have portions that are not coated with the carbon material (e.g., portions where the polyanionic compound is exposed).

[0053] The first active material particles and the second active material particles have a coating layer of a carbon material on their surfaces, which allows the first active material particles and the second active material particles to exhibit sufficient electron conductivity between the particles. The carbon material is, for example, an inorganic material having a carbon element content of 80% by mass or more and 100% by mass or less. The carbon element content in the carbon material may be 90% by mass or more, or may be 95% by mass or more. Elements other than carbon that may be contained in the carbon material include oxygen, hydrogen, and nitrogen. Examples of the carbon material include graphite and non-graphitic carbon.

[0054] The content of the carbon material in each of the first active material particles and the second active material particles is preferably 0.1% by mass or more and 20% by mass or less, more preferably 0.2% by mass or more and 10% by mass or less, even more preferably 0.3% by mass or more and 5% by mass or less, and even more preferably 0.5% by mass or more and 2% by mass or less. When the content of the carbon material in each of the first active material particles and the second active material particles is equal to or greater than the above-mentioned lower limit, it is possible to improve electronic conductivity, etc. When the content of the carbon material in each of the first active material particles and the second active material particles is equal to or less than the above-mentioned upper limit, it is possible to increase the content of the polyanion compound, and it is possible to increase the discharge capacity per volume of a positive electrode using the first active material particles and the second active material particles, etc.

[0055] The positive electrode active material particles may contain active material particles other than the first active material particles and second active material particles described above. Known positive electrode active material particles may be used as the other active material particles. However, the total content of the first active material particles and the second active material particles in all the positive electrode active material particles is preferably 90% by mass or more, more preferably 99% by mass or more, and may be substantially 100% by mass. That is, the positive electrode active material particles may be substantially composed of the first active material particles and the second active material particles.

[0056] [Method for manufacturing positive electrode active material particles] In one embodiment of the present invention, the positive electrode active material particles contain first active material particles and second active material particles. The positive electrode active material particles are obtained by mixing the first active material particles and the second active material particles. The first active material particles and the second active material particles can be selected from, for example, known positive electrode active material particles that are commercially available. The first active material particles and the second active material particles can also be manufactured by the method described below. However, the first active material particles and the second active material particles contained in the positive electrode active material particles are not limited to those manufactured by the manufacturing method described below.

[0057] When the first active material particles are mainly composed of a polyanion compound, the first active material particles are produced by a method using a hydroxide precursor, a lithium source, and a carbon source, and adjusting the pH of the reaction solution using ammonia water or the like when producing the hydroxide precursor, thereby easily obtaining first active material particles having the desired breaking strength.

[0058] First, a hydroxide precursor is obtained by a precipitation reaction between transition metal ions and hydroxide ions in water. Specifically, for example, a transition metal salt aqueous solution and a sodium hydroxide aqueous solution are dropped into water to obtain a hydroxide precursor (a transition metal hydroxide). The transition metal salt may be any water-soluble salt containing the transition metal element that constitutes the desired lithium transition metal compound, such as iron sulfate, iron chloride, cobalt sulfate, manganese sulfate, or nickel sulfate. Alternatively, a potassium hydroxide aqueous solution may be used instead of the sodium hydroxide aqueous solution. When the transition metal salt aqueous solution and the sodium hydroxide aqueous solution are dropped into water, an ammonia aqueous solution or the like is further dropped into the reaction solution to maintain the pH of the water (reaction solution) into which these aqueous solutions are dropped within a predetermined range. The pH of the reaction solution is preferably in the range of 8.5 to 11.6. If the pH of the reaction solution is outside the above range, or if the pH of the reaction solution is within the above range but an aqueous ammonia solution or the like is not added dropwise to the reaction solution, the amount of change in particle size of the first active material particles when pressed tends to be large, that is, the breaking strength of the first active material particles tends to be small. ３ 1mol / dm or more ３ The pH of the reaction solution can be adjusted to about 100% or less. The pH of the reaction solution can be adjusted by adjusting the concentrations, amounts, etc. of the aqueous ammonia solution and the aqueous sodium hydroxide solution to be added dropwise. Another alkaline aqueous solution such as an aqueous hydrazine solution may be added dropwise together with the aqueous ammonia solution. The pH of the reaction solution can also be adjusted by the amount, etc., of the other alkaline aqueous solution added.

[0059] Next, the obtained hydroxide precursor, a lithium source, and a carbon source are mixed and baked in an inert atmosphere (for example, a nitrogen atmosphere), thereby obtaining first active material particles containing the polyanion compound according to one embodiment of the present invention as a main component. ２ P.O. ４ , Li ３ P.O. ４ , LiHSO ４ Compounds having a polyanion structure such as those described above and containing lithium element can be suitably used. Other lithium sources that can be used include LiOH, lithium halide, etc. When the lithium source used is not a compound having a polyanion structure, a compound having a polyanion structure is further mixed and then calcined. Examples of compounds having a polyanion structure include NH ４ H ２ P.O. ４ , (NH ４ ) ３ P.O. ４ , (NH ４ ) ２ HPO ４ , (NH ４ ) ２ SO ４ , N.H. ４ VO ３ Suitable examples of the carbon source include organic substances such as sucrose, lactose, maltose, sucrose, polyvinyl alcohol, and ascorbic acid. The baking temperature can be, for example, 500°C or higher and 800°C or lower.

[0060] When the second active material particles have a polyanion compound as their main component, the second active material particles can be obtained by adjusting the particle size of active material particles obtained by the same manufacturing method as that for the first active material particles having a polyanion compound as their main component, using the grinding method described above as appropriate.

[0061] [Others] The content of the positive electrode active material particles in the positive electrode active material layer is preferably 70% by mass to 99% by mass, more preferably 80% by mass to 98% by mass, and even more preferably 90% by mass to 95% by mass. By setting the content of the positive electrode active material particles within the above range, the resistance of the energy storage element can be easily reduced and the electrode density can be easily increased.

[0062] The lower limit of the mass per unit area of ​​the positive electrode active material layer is 1.0 g / 100 cm ２ The lower limit is 1.2 g / 100 cm ２ , 1.4g / 100cm ２ , or 1.6 g / 100 cm ２ On the other hand, the upper limit of the mass per unit area of ​​the positive electrode active material layer is 2.0 g / 100 cm. ２ or less, 1.8 g / 100 cm ２ The upper limit is preferably 1.6 g / 100 cm ２ , 1.4g / 100cm ２ , or 1.2 g / 100 cm ２ When the mass per unit area of ​​the positive electrode active material layer is within the above range, the resistance of the electricity storage element can be easily reduced and the electrode density can be easily improved.

[0063] 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.

[0064] The content of the conductive agent in the positive electrode active material layer is preferably 1% by mass or more and 8% by mass or less, and more preferably 3% by mass or more and 5% by mass or less. By setting the content of the conductive agent within the above range, the energy density of the nonaqueous electrolyte storage element can be increased. Note that when the first active material particles and the second active material particles have coating layers made of a carbon material on their surfaces, the coating layers of the first active material particles and the second active material particles are not included in the conductive agent.

[0065] 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.

[0066] The content of the binder in the positive electrode active material layer is preferably 1% by mass to 8% by mass, more preferably 2% by mass to 5% by mass, by which the positive electrode active material can be stably maintained.

[0067] Examples of thickeners include polysaccharide polymers such as carboxymethyl cellulose (CMC) and methyl cellulose. When the thickener has a functional group that reacts with lithium or the like, the functional group may be deactivated in advance by methylation or the like. When a thickener is used, the content of the thickener in the positive electrode active material layer is preferably 5% by mass or less, and more preferably 1% by mass or less. The technology disclosed herein can be preferably implemented in an embodiment in which the positive electrode active material layer does not contain a thickener.

[0068] The filler is not particularly limited. Examples of fillers include polyolefins such as polypropylene and polyethylene, inorganic oxides such as silicon dioxide, alumina, titanium dioxide, calcium oxide, strontium oxide, barium oxide, magnesium oxide, and aluminosilicates, hydroxides such as magnesium hydroxide, calcium hydroxide, and aluminum hydroxide, carbonates such as calcium carbonate, sparingly soluble ionic crystals such as calcium fluoride, barium fluoride, and barium sulfate, nitrides such as aluminum nitride and silicon nitride, mineral-derived substances such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica, and artificial products thereof. When a filler is used, the content of the filler in the positive electrode active material layer is preferably 5% by mass or less, and even 1% by mass or less. The technology disclosed herein can be preferably implemented in an embodiment in which the positive electrode active material layer does not contain a filler.

[0069] 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.

[0070] [Electrode Assembly] The electrode assembly according to one embodiment of the present invention is an electrode assembly configured by stacking the positive electrode and negative electrode according to one embodiment of the present invention with a separator interposed therebetween. As the electrode assembly, for example, an electrode assembly having a known structure, such as a wound electrode assembly or a stacked electrode assembly, can be used. The electrode assembly is used in, for example, a storage element such as a non-aqueous electrolyte secondary battery. Because the electrode assembly according to one embodiment of the present invention has the positive electrode according to one embodiment of the present invention, the resistance of the storage element can be reduced and the electrode density of the positive electrode can be increased.

[0071] A wound electrode body has a structure in which a positive electrode and a negative electrode are wound in an insulated state. The wound electrode body may be cylindrical (columnar) or flat. The electrode body 2 provided in the energy storage device 1 of FIG. 1 is a flat wound electrode body. The wound electrode body can be produced, for example, by the following procedure. First, a positive electrode, a separator, and a negative electrode, each formed in a strip shape, are stacked in this order to obtain a laminate. This laminate is then wound to obtain a wound electrode body. During production of a wound electrode body, the positive electrode active material layer is prone to cracking and peeling from the substrate at curved portions of the positive electrode, etc. Therefore, the advantages of using such a positive electrode, such as reduced resistance of the energy storage device, can be significantly obtained.

[0072] A laminated electrode assembly has a structure in which one or more positive electrodes and one or more negative electrodes are stacked in an insulated state. For example, a laminated electrode assembly can be obtained by stacking a positive electrode, a separator, and a negative electrode, each of which is formed into a rectangular shape, in this order.

[0073] As the electrode body, for example, one having a structure in which at least one of the positive electrode and the negative electrode is folded in an accordion-like manner and stacked can also be used.

[0074] [Positive Electrode] In an electrode assembly according to one embodiment of the present invention, the positive electrode according to one embodiment of the present invention described above is used as the positive electrode.

[0075] [Negative electrode] The negative electrode has a negative electrode substrate and a negative electrode active material layer disposed on the negative electrode substrate directly or via an intermediate layer. The configuration of the intermediate layer is not particularly limited and can be selected from the configurations exemplified for the positive electrode above, for example.

[0076] The negative electrode substrate is conductive. Metals such as copper, nickel, stainless steel, nickel-plated steel, and aluminum, or alloys thereof, carbon materials, and the like are used as the material for the negative electrode substrate. Among these, copper or copper alloys are preferred. Examples of the negative electrode substrate include foil, vapor-deposited film, mesh, and porous material, with foil being preferred from the viewpoint of cost. Therefore, copper foil or copper alloy foil is preferred as the negative electrode substrate. Examples of copper foil include rolled copper foil and electrolytic copper foil.

[0077] 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 and also increase the energy density per volume of the energy storage element.

[0078] 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.

[0079] 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.

[0080] 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 metal Li; 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.

[0081] "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.

[0082] "Non-graphitic carbon" refers to a carbon having 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.

[0083] Here, the "discharged state" refers to a state in which the battery is discharged so that lithium ions capable of being absorbed and desorbed during charging and discharging are sufficiently released from the carbon material serving as the negative electrode active 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 Li as the counter electrode is 0.7 V or higher.

[0084] "Non-graphitizable carbon" refers to the above-mentioned d ００２ The term "carbon material" refers to a carbon material having a particle size of 0.36 nm or more and 0.42 nm or less.

[0085] "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.

[0086] The negative electrode active material is usually a particle (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-mentioned 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-mentioned upper limit, the electronic conductivity of the negative electrode active material layer is improved. Here, the term "average particle size" refers to the 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 laser diffraction / scattering in a diluted solution obtained by diluting particles with a solvent in accordance with JIS-Z-8825 (2013). In order to obtain powder with a predetermined particle size, a pulverizer, a classifier, etc. are used. When the negative electrode active material is a metal such as metallic Li, the negative electrode active material layer may be in the form of a foil.

[0087] 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.

[0088] [Separator] The separator can be appropriately selected from known separators. Examples of separators that can be used include separators consisting of only a substrate layer and separators in which a heat-resistant layer containing heat-resistant particles and a binder is formed on one or both surfaces of the substrate layer. Examples of the shape of the substrate layer of the separator include woven fabric, nonwoven fabric, and porous resin film. Among these shapes, porous resin films are preferred from the viewpoint of strength, and nonwoven fabrics are preferred from the viewpoint of non-aqueous electrolyte retention. Materials for the substrate layer of the separator are preferably polyolefins such as polyethylene and polypropylene from the viewpoint of shutdown function, and polyimide and aramid from the viewpoint of oxidative decomposition resistance. A composite material of these resins may also be used for the substrate layer of the separator.

[0089] The heat-resistant particles contained in the heat-resistant layer preferably exhibit a mass loss of 5% or less when heated from room temperature to 500°C 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 materials exhibiting a mass loss of a predetermined value or less include inorganic compounds. Examples of inorganic compounds include oxides such as iron oxide, silicon oxide, aluminum oxide, titanium oxide, zirconium oxide, calcium oxide, strontium oxide, barium oxide, magnesium oxide, and aluminosilicate; nitrides such as aluminum nitride and silicon nitride; carbonates such as calcium carbonate; sulfates such as barium sulfate; sparingly soluble ionic crystals such as calcium fluoride, barium fluoride, and barium titanate; covalently bonded crystals such as silicon and diamond; mineral-derived substances such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica, as well as artificial products thereof. As the inorganic compound, these substances may be used alone or in the form of a complex, or two or more of them may be used in combination. Among these inorganic compounds, silicon oxide, aluminum oxide, or aluminosilicate is preferred from the viewpoint of the safety of the energy storage device.

[0090] 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.

[0091] 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.

[0092] [Electricity storage element] An electric storage element according to one embodiment of the present invention includes an electrode assembly having a positive electrode, a negative electrode, and a separator according to one embodiment of the present invention, an electrolyte, and a container that accommodates the electrode assembly and the electrolyte. The electrode assembly is typically a stacked type in which a plurality of positive electrodes and a plurality of negative electrodes are stacked with separators interposed therebetween, or a wound type in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween and wound. The electrolyte is present in a state contained in the positive electrode, the negative electrode, and the separator. The electrolyte may be a nonaqueous electrolyte. As an example of an electric storage element, a nonaqueous electrolyte secondary battery (hereinafter also simply referred to as a "secondary battery") in which the electrolyte is a nonaqueous electrolyte will be described.

[0093] The electric storage element according to one embodiment of the present invention has a positive electrode according to one embodiment of the present invention, and therefore has low resistance and can increase the electrode density of the positive electrode. In the electric storage element according to one embodiment of the present invention, the electrode assembly uses the positive electrode according to one embodiment of the present invention described above, the negative electrode described above, and a separator.

[0094] [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.

[0095] 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.

[0096] 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.

[0097] 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.

[0098] It is preferable to use a cyclic carbonate or a chain carbonate as the non-aqueous solvent, and it is more preferable to use a cyclic carbonate and a chain carbonate in combination. 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.

[0099] 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.

[0100] 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 ) (SO２ 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.

[0101] 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 ３ It 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.

[0102] 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.

[0103] 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.

[0104] The non-aqueous electrolyte may be a solid electrolyte, or a non-aqueous electrolytic solution and a solid electrolyte may be used in combination.

[0105] 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.

[0106] As the sulfide solid electrolyte, in the case of a lithium ion secondary battery, for example, Li ２ S-P ２ S ５ , LiI-Li ２ S-P ２ S ５ , Li １０ Ge-P ２ S １２ etc.

[0107] The shape of the storage element is not particularly limited, and examples thereof include cylindrical batteries, prismatic batteries, flat batteries, coin batteries, and button batteries.

[0108] FIG. 1 shows an energy storage element 1 as an example of a prismatic battery. Note that this figure is a see-through view of the inside of the container. An electrode assembly 2 having a positive electrode and a negative electrode wound with a separator sandwiched between them is housed in a prismatic container 3. The positive electrode is electrically connected to a positive electrode terminal 4 via a positive electrode lead 41. The negative electrode is electrically connected to a negative electrode terminal 5 via a negative electrode lead 51.

[0109] [Method for Manufacturing Energy Storage Element] The method for manufacturing the energy storage element of this embodiment can be appropriately selected from known methods. The manufacturing method includes, for example, preparing a positive electrode having positive electrode active material particles manufactured by the above-mentioned method, preparing an electrode assembly, preparing an electrolyte, and housing the electrode assembly and the electrolyte in a container. Preparing the electrode assembly includes preparing a negative electrode, and forming the electrode assembly by stacking or winding the positive electrode and the negative electrode with a separator interposed therebetween.

[0110] The method for placing the electrolyte in the container can be appropriately selected from known methods. For example, when a non-aqueous electrolyte solution is used as the electrolyte, the non-aqueous electrolyte solution may be poured into the container through an inlet formed in the container, and the inlet may then be sealed.

[0111] [Electricity Storage Device] The energy storage element of the present embodiment can be mounted as an energy storage unit (battery module) configured by assembling a plurality of energy storage elements in an automobile power source such as an electric vehicle (EV), a hybrid electric vehicle (HEV), or a plug-in hybrid electric vehicle (PHEV), a power source for electronic devices such as a personal computer or a communication terminal, or a power storage power source, etc. In this case, it is sufficient that the technology of the present invention is applied to at least one energy storage element included in the energy storage unit.

[0112] An energy storage device according to one embodiment of the present invention includes two or more energy storage elements, and includes one or more energy 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 energy storage element included in the energy storage device according to the second embodiment, and the energy storage device may include one energy storage element according to one embodiment of the present invention described above and one or more energy storage elements not according to one embodiment of the present invention described above, or may include two or more energy storage elements according to one embodiment of the present invention described above.

[0113] 2 shows an example of a power storage device 30 according to a second embodiment in which power storage units 20, each of which is a collection of two or more electrically connected power storage elements 1, are further assembled. The power storage device 30 may include a bus bar (not shown) that electrically connects two or more power storage elements 1, a bus bar (not shown) that electrically connects two or more power storage units 20, etc. The power storage unit 20 or the power storage device 30 may include a status monitoring device (not shown) that monitors the status of one or more power storage elements.

[0114] [Other Embodiments] The positive electrode for an energy storage device, the electrode body for an energy storage device, and the energy storage device of the present invention are not limited to the above-described embodiments, and various modifications may be made without departing from the spirit of the present invention. For example, the configuration of one embodiment can be added to the configuration of another embodiment, and part of the configuration of one embodiment can be replaced with the configuration of another embodiment or well-known technology. Furthermore, part of the configuration of one embodiment can be deleted. Also, well-known technology can be added to the configuration of one embodiment.

[0115] In the above embodiment, the case where the energy storage element is used as a chargeable and dischargeable non-aqueous electrolyte secondary battery (e.g., a lithium ion secondary battery) has been described, but the energy storage element may be of any type, shape, size, capacity, etc. The present invention can also be applied to various secondary batteries, electric double layer capacitors, lithium ion capacitors, and other capacitors.

[0116] 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.

[0117] 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.

[0118] [Example 1] (Preparation of first active material particles) LiFePO 4 , a lithium transition metal compound having a polyanion structure, was prepared. ４First active material particles were prepared by coating the particles with a carbon material. The carbon material content in the first active material particles was 1.0 mass%. The peak particle diameter (first peak particle diameter) in the particle size distribution diagram obtained by the above method for the first active material particles was 4.5 μm. The breaking strength of the first active material particles measured by the above method was 55.4 MPa.

[0119] (Preparation of Positive Electrode Active Material Particles) Next, LiFePO 4 , a lithium transition metal compound having a polyanion structure, was prepared. ４ Second active material particles were prepared by coating the particles with a carbon material. The carbon material content of the second active material particles was 1.0 mass%. The peak particle diameter (second peak particle diameter) in the particle size distribution diagram obtained by the above method for the second active material particles was 0.67 μm. That is, the ratio of the first peak particle diameter to the second peak particle diameter was 6.7. The first active material particles and the second active material particles were then mixed to obtain positive electrode active material particles. In the particle size distribution diagram obtained by the above method for the positive electrode active material particles, the ratio of the frequency of the first peak particle diameter to the frequency of the second peak particle diameter was 2.1. Note that the first active material particles and second active material particles used in the examples and comparative examples were all selected from commercially available positive electrode active material particles.

[0120] (Preparation of Positive Electrode) A positive electrode mixture paste was prepared using the obtained 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 90:5:5 in terms of solid content. This positive electrode mixture paste was applied to an aluminum foil as a positive electrode substrate, dried, and roll-pressed to form a positive electrode active material layer (positive electrode active material mixture), thereby obtaining a positive electrode. The mass per unit area of ​​the positive electrode active material layer was 1.0 g / 100 cm. ２ The limiting active material density of the positive electrode active material layer was 2.09 g / cm ３ The "limit active material density" was calculated by multiplying the maximum density of the positive electrode active material layer obtained by roll pressing by the mass ratio of the positive electrode active material particles in terms of solid content in the positive electrode active material layer.

[0121] (Preparation of Negative Electrode) A negative electrode mixture paste was prepared by mixing graphite as a negative electrode active material, acetylene black (AB) as a conductive agent, styrene-butadiene rubber (SBR) as a binder, carboxymethyl cellulose (CMC) as a thickener, and water as a dispersion medium. The mass ratio of graphite to AB to SBR to CMC was 97.1:1.0:0.8:1.0 in terms of solid content. This negative electrode mixture paste was applied to copper foil as a negative electrode substrate, dried, and roll-pressed to form a negative electrode active material layer, thereby obtaining a negative electrode.

[0122] (Non-aqueous electrolyte) A 0.9 mol / dm ３ At a concentration of LiPF ６ The above was dissolved in water, and 0.3 mass % of vinylene carbonate (VC) was added as an additive to prepare a solution, which was used as a non-aqueous electrolyte.

[0123] (Separator) A polyethylene microporous film was used as the separator.

[0124] (Assembly of Energy Storage Element) The positive electrode, negative electrode, and separator were stacked to prepare an electrode assembly. The obtained electrode assembly was placed in a container, and the non-aqueous electrolyte was then poured into the container, followed by sealing, to obtain the energy storage element of Example 1.

[0125] [Comparative Examples 1 and 5] A storage battery element of Comparative Example 1 was obtained using the same procedure as in Example 1, except that positive electrode active material particles containing only the second active material particles of Example 1 were used. Also, a storage battery element of Comparative Example 5 was obtained using the same procedure as in Example 1, except that positive electrode active material particles containing only the first active material particles having the first peak particle size shown in Table 1 were used. Table 1 shows the limit active material densities of Comparative Examples 1 and 5.

[0126] [Comparative Examples 2 to 4, 6 to 9, and Examples 2 to 6] First active material particles having the first peak particle size and fracture strength shown in Table 1 were prepared, and second active material particles having a ratio of the first peak particle size to the second peak particle size shown in Table 1 were prepared. The first active material particles and second active material particles were mixed so that the ratio of the frequency at the first peak particle size to the frequency at the second peak particle size in a particle size distribution diagram was as shown in Table 1. Furthermore, a positive electrode active material layer was formed so that the mass per unit area of ​​the positive electrode active material layer was as shown in Table 1. The same procedure as in Example 1 was used to obtain the energy storage elements of Comparative Examples 2 to 4, 6 to 8, and Examples 2 to 6. Note that for Example 9, a positive electrode was also prepared using the same procedure as in Example 1, except that the positive electrode mixture paste was applied and dried to the values ​​shown in Table 1. However, cracks occurred in the positive electrode active material layer when roll-pressed, and therefore the production of the energy storage element was discontinued. Table 1 shows the limiting active material density for each Example and Comparative Example. In Table 1, "-" means that the limiting active material density was not measured. Furthermore, particle size distribution diagrams for Comparative Example 2 and Example 3 are shown in Figure 3. For Example 3, an inflection point was confirmed in the particle size distribution curve of the particle size distribution diagram between the second peak particle size of 0.61 μm and the first peak particle size of 4.9 μm.

[0127] [Evaluation] (Initial Charge / Discharge) Each of the obtained energy storage elements was initially charged and discharged at 25°C in the following manner. Constant-current / constant-voltage charging was performed with a charging current of 0.2 C and a charge cut-off voltage of 3.5 V. The charge was terminated until the charging current reached 0.01 C. A 10-minute rest period was then provided. Subsequently, constant-current discharging was performed with a discharging current of 0.2 C and a discharge cut-off voltage of 2.0 V. For each energy storage element after the initial charge / discharge, the 1 kHz alternating current resistance (ACR) was measured at 25°C using a 1 kHz alternating current impedance meter 10 minutes or more after the end of discharge, and this was taken as the initial ACR. The initial ACR is shown in Table 1.

[0128] (Charge-Discharge Cycle Test) After measuring the initial ACR for each energy storage element, a charge-discharge cycle test was performed at 60°C as follows. Constant-current, constant-voltage charging was performed with a charging current of 1 C and a charge cut-off voltage of 3.5 V. The charge was terminated until the charging current reached 0.01 C. Thereafter, constant-current discharging was performed with a discharging current of 1 C and a discharge cut-off voltage of 2.0 V. A 10-minute rest period was provided after each charge and discharge. This charge-discharge cycle was repeated 150 times. After the charge-discharge cycle, the alternating current resistance (ACR) at 1 kHz was measured using a 1 kHz AC impedance meter at 25°C for at least 3 hours after discharge, and this was recorded as the post-cycle ACR. The post-cycle ACR is shown in Table 1.

[0129]

[0130] From Table 1, Comparative Example 1, in which no first active material particles were present, or Comparative Example 2, in which the ratio of the frequency at the first peak particle size to the frequency at the second peak particle size was less than 1.0, had a large initial ACR or post-cycling ACR. Comparative Examples 3 and 4, in which the ratio of the frequency at the first peak particle size to the frequency at the second peak particle size was more than 4.3, or Comparative Example 5, in which no second active material particles were present, had a small critical active material density. Comparative Example 6, in which the first peak particle size was less than 4.0 μm, had a large initial ACR and post-cycling ACR. Comparative Example 7, in which the breaking strength of the first active material particles was less than 20.0 MPa, had a large post-cycling ACR. The mass per unit area of ​​the positive electrode active material layer was 2.0 g / 100 cm. ２ In contrast to these, Comparative Examples 8 and 9, in which the first peak particle size was 4.0 μm or more and the second peak particle size was smaller than the first peak particle size, the breaking strength of the first active material particles was 20.0 MPa or more, the ratio of the frequency at the first peak particle size to the frequency at the second peak particle size was 1.0 to 4.3, and the mass per unit area of ​​the positive electrode active material layer was 1.0 g / 100 cm or less. ２ 2.0g / 100cm or more ２In the following Examples 1 to 6, the limiting active material density was large, and the initial ACR and the ACR after cycling were small.

[0131] The present invention can be applied to electric storage elements used as power sources for electronic devices such as personal computers and communication terminals, and automobiles.

[0132] REFERENCE SIGNS LIST 1 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 cathode active material layer laminated on the substrate and containing cathode active material particles, wherein the cathode active material particles contain first and second active material particles having different particle diameters, wherein in a particle size distribution diagram of the cathode active material particles, the horizontal axis represents particle diameter [μm] and the vertical axis represents volume-based frequency [%], the first active material particles exhibit a first peak particle diameter of 4.0 μm or more and the second active material particles exhibit a second peak particle diameter smaller than the first peak particle diameter, the first active material particles have a breaking strength of 20.0 MPa or more, the ratio of the frequency [%] of the first peak particle diameter to the frequency [%] of the second peak particle diameter in the particle size distribution diagram is 1.0 or more and 4.3 or less, and the cathode active material layer has a mass per unit area of ​​1.0 g / 100 cm. ２ 2.0g / 100cm or more ２ A positive electrode for a storage element, which is:

2. The positive electrode for an electric storage element according to claim 1, wherein the ratio of said first peak particle size to said second peak particle size is 6.0 or more.

3. The positive electrode for a storage element according to claim 1 or 2, wherein the first active material particles and the second active material particles contain a lithium transition metal compound having a polyanion structure.

4. An electrode body having a positive electrode for a storage element according to claim 1 or 2.

5. An electric storage element having the positive electrode for an electric storage element according to claim 1 or 2.

6. An energy storage device comprising one or more energy storage elements according to claim 5, and two or more energy storage elements.

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