Cathode active material for power storage element, cathode for power storage element, power storage element, and method of manufacturing cathode active material for power storage element

Abstract

To provide a cathode active material for a power storage element capable of suppressing reduction of a capacity maintenance rate during a charge / discharge cycle of a power storage element, a cathode for a power storage element, and a power storage element having such a cathode active material for a power storage element, and a method of manufacturing such a cathode active material for a power storage element.SOLUTION: A cathode active material for a power storage element includes particles each containing a lithium transition metal compound and a covering layer which covers at least a part of the particles. The covering layer contains a lithium element, a niobium element, and an oxygen element. A spectrum, which is measured by electron energy loss spectroscopy, derived from the covering layer has two peaks in a range from 530 eV or more to 540 eV or less in a K absorption end of oxygen. In the two peaks, a peak intensity on a lower energy side is smaller than a peak intensity on a higher energy side.SELECTED DRAWING: Figure 3

Description

[Technical Field]

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

[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, automobiles, etc. 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 such as lithium 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] As a positive electrode active material for an electricity storage element, a lithium transition metal compound such as a lithium transition metal composite oxide may be used (Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent Publication No. 2020-047383 Summary of the Invention [Problem to be solved by the invention]

[0005] When a lithium transition metal compound is used as a positive electrode active material, a side reaction may occur between the positive electrode active material and the electrolyte, which may result in a decrease in discharge capacity with increasing charge-discharge cycles.

[0006] An object of the present invention is to provide a positive electrode active material for an energy storage element that can suppress a decrease in the capacity retention rate during charge-discharge cycling of the energy storage element, a positive electrode for an energy storage element and an energy storage element that have such a positive electrode active material for an energy storage element, and a method for producing a positive electrode active material for an energy storage element that can suppress a decrease in the capacity retention rate during charge-discharge cycling of the energy storage element. [Means for solving the problem]

[0007] A positive electrode active material for a storage element according to one aspect of the present invention comprises particles containing a lithium transition metal compound and a coating layer that coats at least a portion of the particles, the coating layer containing lithium, niobium, and oxygen, and a spectrum derived from the coating layer measured by electron energy loss spectroscopy has two peaks in the range of 530 eV to 540 eV at the oxygen K absorption edge, and the intensity of the peak on the lower energy side of the two peaks is smaller than the intensity of the peak on the higher energy side.

[0008] A positive electrode for an energy storage device according to another aspect of the present invention includes the positive electrode active material for an energy storage device according to the aspect of the present invention and a sulfide solid electrolyte.

[0009] An energy storage device according to another aspect of the present invention includes the energy storage device positive electrode according to the aspect of the present invention.

[0010] A method for producing a positive electrode active material for a storage element according to another aspect of the present invention includes preparing particles containing a lithium transition metal compound, preparing a coating material containing lithium, niobium, and oxygen, and forming a coating layer using the coating material to coat at least a portion of the particles, wherein the coating material is not heated to 200°C or higher when forming the coating layer. [Effects of the Invention]

[0011] According to any one aspect of the present invention, it is possible to provide a positive electrode active material for an energy storage element that can suppress a decrease in the capacity retention rate during charge-discharge cycling of the energy storage element, a positive electrode for an energy storage element and an energy storage element that have such a positive electrode active material for an energy storage element, and a method for producing such a positive electrode active material for an energy storage element. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a schematic cross-sectional view showing an all-solid-state energy storage element, which is an energy storage element according to one embodiment of the present invention. [Figure 2] FIG. 2 is a schematic diagram showing an electricity storage device configured by assembling a plurality of electricity storage elements. [Figure 3] FIG. 3 is a graph showing a spectrum derived from the coating layer of the positive electrode active material of Example 1, measured by electron energy loss spectroscopy. [Figure 4] FIG. 4 is a graph showing a spectrum derived from the coating layer of the positive electrode active material of Comparative Example 1, measured by electron energy loss spectroscopy. [Figure 5] FIG. 5 is a graph showing a spectrum derived from the coating layer of the positive electrode active material of Comparative Example 4, measured by electron energy loss spectroscopy. DETAILED DESCRIPTION OF THE INVENTION

[0013] First, an outline of the positive electrode active material for an energy storage device, the positive electrode for an energy storage device, the energy storage device, and the method for producing the positive electrode active material for an energy storage device disclosed in this specification will be described.

[0014] [1] A positive electrode active material for a storage device according to one aspect of the present invention comprises particles containing a lithium transition metal compound and a coating layer that coats at least a portion of the particles, the coating layer containing lithium, niobium, and oxygen, and a spectrum derived from the coating layer measured by electron energy loss spectroscopy has two peaks in the range of 530 eV to 540 eV at the K absorption edge of oxygen, and the intensity of the peak on the lower energy side of the two peaks is smaller than the intensity of the peak on the higher energy side.

[0015] The positive electrode active material for an energy storage device described in [1] above can suppress a decrease in the capacity retention rate during charge-discharge cycling of the energy storage device. While the reason for this is unclear, the following reason is presumed. When a positive electrode active material containing a lithium transition metal compound is used in the positive electrode of an energy storage device, a side reaction may occur between the positive electrode active material and the electrolyte. As a result, the capacity retention rate of the energy storage device after charge-discharge cycling may be reduced. In contrast, the positive electrode active material for an energy storage device described in [1] above has a coating layer that coats at least a portion of the particles containing the lithium transition metal compound, thereby suppressing side reactions between the positive electrode active material and the electrolyte. Furthermore, the coating layer contains lithium, niobium, and oxygen, and the spectrum derived from the coating layer measured by electron energy loss spectroscopy exhibits a characteristic shape at the oxygen K-absorption edge. More specifically, this spectrum has two peaks in the range of 530 eV to 540 eV, with the intensity of the lower-energy peak being smaller than the intensity of the higher-energy peak. Therefore, it is considered that the coating layer is formed from a composition containing lithium, niobium, and oxygen, and having an electrochemically stable structure characterized by the bonding state of oxygen. For these reasons, it is presumed that the positive electrode active material for an energy storage device described in [1] above can suppress a decrease in the capacity retention rate during charge-discharge cycles of the energy storage device.

[0016] In the present invention, the term "lithium transition metal compound" means a compound containing lithium element and at least one transition metal element.

[0017] In the present invention, electron energy loss spectroscopy (EELS) measurements are performed in combination with a transmission electron microscope (TEM). The instruments used are a Cs-STEM: HD2700 (Hitachi High-Tech) and an EELS: Enfina (Gatan). More specifically, EELS line measurements are performed on the layer coating particles containing a lithium transition metal compound in the cross section of a positive electrode active material observed using a transmission electron microscope. The highest intensity peak at the oxygen K absorption edge, which is greater than 540.0 eV and less than 550.0 eV and is attributed to the positive electrode active material, is corrected to 545.0 eV. Next, the spectrum near the coating layer is extracted, and the intensities of the peaks originating from the coating layer in the range of 530 eV to 540 eV are compared. The measurement target is either a positive electrode or a positive electrode active material, but a positive electrode active material containing no other components is preferred. When the measurement target is a positive electrode, a measurement sample is prepared by exposing the cross section of the positive electrode using a focused ion beam method. When the measurement target is a positive electrode active material, the positive electrode active material is fixed with a thermosetting resin, and then a cross section of the positive electrode active material is exposed using a focused ion beam method to prepare a measurement sample. The positive electrode active material or positive electrode to be measured is prepared using the following procedure. If a positive electrode active material or positive electrode can be prepared before assembling an energy storage device, it is used as is. When preparing a positive electrode active material or positive electrode from an assembled energy storage device, first, the energy storage device is discharged at a constant current of 0.05 C to the lower limit voltage during normal use. Next, the energy storage device is disassembled and the electrode assembly is removed. When the measurement target is a positive electrode, a portion of the electrode assembly including the positive electrode is removed and used as the measurement sample. When the measurement target is a positive electrode active material, the following operations are further performed. If the removed electrode assembly is observed and confirmed to contain a solid electrolyte, the electrode assembly is immersed in a solvent that only dissolves solid electrolytes, such as ethanol or ion-exchanged water, to remove the solid electrolyte. Next, the surface of the positive electrode is observed, and if it is confirmed that the positive electrode active material layer contains particulate positive electrode active material, the positive electrode is immersed in a solvent such as butyl butyrate that dissolves only the binder, and the positive electrode substrate and the binder are removed from the positive electrode active material layer, and the conductive agent and the positive electrode active material are extracted. Next, decantation is performed using ion-exchanged water or the like as a solvent to separate the conductive agent and the positive electrode active material, and the obtained positive electrode active material is used as a measurement sample.

[0018] In the present invention, the term "peak intensity" means the intensity of the peak top in a spectrum obtained by electron energy loss spectroscopy (EELS).

[0019] [2] In the positive electrode active material for a storage battery element according to the above [1], the lithium transition metal compound may be a lithium transition metal composite oxide.

[0020] The positive electrode active material for an electricity storage device according to the above item [2] can further suppress the decrease in the capacity retention rate during charge-discharge cycles of the electricity storage device.

[0021] [3] A positive electrode for an electric storage device according to another aspect of the present invention includes the positive electrode active material for an electric storage device according to [1] or [2] above and a sulfide solid electrolyte.

[0022] The positive electrode for an energy storage device described in [3] above contains the positive electrode active material for an energy storage device described in [1] or [2] above, and therefore can suppress a decrease in the capacity retention rate during charge-discharge cycling of the energy storage device. Furthermore, sulfide solid electrolytes are particularly susceptible to side reactions with lithium transition metal compounds. Therefore, the advantage of the present invention, that is, suppression of side reactions between the positive electrode active material and the electrolyte, can be particularly pronounced.

[0023] [4] An electric storage device according to another aspect of the present invention includes the positive electrode active material for an electric storage device according to [1] or [2] above or the positive electrode for an electric storage device according to [3] above.

[0024] The energy storage device described in [4] above includes the positive electrode active material for an energy storage device described in [1] or [2] above or the positive electrode for an energy storage device described in [3] above, and therefore has a large capacity retention rate during charge-discharge cycles.

[0025] [5] The electricity storage element according to [4] above may be an all-solid-state electricity storage element.

[0026] The electricity storage element described in [5] above is an all-solid-state electricity storage element, and therefore the advantage of the present invention, that is, a large capacity retention rate during charge-discharge cycles, can be particularly significantly obtained.

[0027] [6] Another aspect of the present invention provides a method for producing a positive electrode active material for a storage element, the method comprising: preparing particles containing a lithium transition metal compound; preparing a coating material containing lithium, niobium, and oxygen; and forming a coating layer using the coating material to coat at least a portion of the particles, wherein the coating material is not heated to 200°C or higher when forming the coating layer.

[0028] The method for producing a positive electrode active material for an energy storage device described in [6] above can produce a positive electrode active material for an energy storage device having a coating layer that coats at least a portion of particles containing a lithium transition metal compound. The inventors also discovered that by forming a coating layer on the positive electrode active material for an energy storage device using a coating material containing lithium, niobium, and oxygen without heating the coating material at 200°C or higher, the spectrum derived from the coating layer measured by electron energy loss spectroscopy exhibits a characteristic shape at the oxygen K-absorption edge. More specifically, this spectrum has two peaks in the range of 530 eV to 540 eV, with the intensity of the lower-energy peak being smaller than the intensity of the higher-energy peak. Therefore, the method for producing a positive electrode active material for an energy storage device described in [6] above can produce a positive electrode active material for an energy storage device that can suppress a decrease in the capacity retention rate during charge-discharge cycling of the energy storage device, for reasons similar to those assumed for the positive electrode active material for an energy storage device described in [1] above.

[0029] In the present invention, "not heating the coating material to 200°C or higher" means that the surface temperature of the coating material is kept below 200°C.

[0030] A positive electrode active material for an energy storage device, a method for manufacturing a positive electrode active material for an energy storage device, a positive electrode for an energy storage device, an energy storage device, a method for manufacturing an energy storage device, an energy storage device, and other embodiments according to one embodiment of the present invention will be described in detail below. Note that the lower and upper limits of each numerical range described in the embodiments of the present invention can be combined in any combination.

[0031] [Positive electrode active material for energy storage devices] A positive electrode active material for a storage element according to one embodiment of the present invention (hereinafter also referred to as "positive electrode active material") has particles containing a lithium transition metal compound (hereinafter also referred to as "active material particles") and a coating layer that coats at least a portion of the particles.

[0032] (active material particles) As described above, the active material particles contain a lithium transition metal compound. This lithium transition metal compound is a material capable of absorbing and releasing lithium ions. The lithium transition metal compound is preferably a lithium transition metal composite oxide or a polyanion compound, more preferably a lithium transition metal composite oxide. The active material particles may contain one or more lithium transition metal compounds.

[0033] Examples of the transition metal element contained in the lithium transition metal composite oxide include nickel, cobalt, and manganese. The lithium transition metal composite oxide may also contain a typical metal element such as aluminum. Examples of the lithium transition metal composite oxide include lithium transition metal composite oxides having an α-NaFeO2 crystal structure and lithium transition metal composite oxides having a spinel crystal structure.

[0034] Lithium transition metal composite oxides with an α-NaFeO2 type crystal structure include Li 1+α Ma 1-α02 (Ma is a metal element other than lithium that contains one or more transition metal elements, with 0≦α<1). Ma preferably contains one or more of Ni, Co, and Mn. The total content of Ni, Co, and Mn relative to Ma ((Ni+Co+Mn) / Ma) is preferably 90 mol % or more, more preferably 95 mol % or more, and even more preferably 98 mol % or more. Ma also preferably contains one or more of Ni, Co, and Mn and Al, or preferably contains Ni, Co, Mn, and Al. The total content of Ni, Co, Mn, and Al relative to Ma ((Ni+Co+Mn+Al) / Ma) is preferably 90 mol % or more, more preferably 95 mol % or more, and even more preferably 98 mol % or more.

[0035] In lithium transition metal composite oxides having an α-NaFeO2-type crystal structure, the lower limit of the molar ratio of Ni to the metal element (Ma) other than lithium is preferably 0.4, more preferably 0.4, even more preferably 0.5, even more preferably 0.6, and particularly preferably 0.7 or 0.8. When the molar ratio of Ni is equal to or greater than the lower limit, the energy density of the energy storage device can be easily increased. However, lithium ions in the lithium transition metal composite oxide are likely to be desorbed, and side reactions between the active material particles and the coating layer are likely to occur. Therefore, as described below, by having a coating layer formed from a composition with an electrochemically stable structure, the effect of the present invention, namely, suppressing a decrease in the capacity retention rate during charge-discharge cycling of the energy storage device, is significantly achieved. On the other hand, the upper limit of the molar ratio of Ni is preferably 0.95, more preferably 0.90, even more preferably 0.85, and even more preferably 0.82, from the viewpoint of suppressing a decrease in the capacity retention rate during charge-discharge cycling of the energy storage device.

[0036] In the lithium transition metal composite oxide having an α-NaFeO2-type crystal structure, the lower limit of the molar ratio of Co to the metal element (Ma) other than lithium is preferably 0.05, more preferably 0.075, and in some cases even more preferably 0.10 or 0.15. Meanwhile, the upper limit of the molar ratio of Co is preferably 0.30, more preferably 0.20. When the molar ratio of Co is within the above range, it is possible to easily suppress a decrease in the capacity retention rate during charge-discharge cycling of the energy storage element.

[0037] In the lithium transition metal composite oxide having an α-NaFeO2-type crystal structure, the upper limit of the molar ratio of Mn to the metal element (Ma) other than lithium is preferably 0.60, more preferably 0.55, and even more preferably 0.50. Meanwhile, the lower limit of the molar ratio of Mn may be, for example, 0. When the molar ratio of Mn is within the above range, it is possible to easily suppress a decrease in the capacity retention rate during charge-discharge cycling of the energy storage element.

[0038] In the lithium transition metal composite oxide having an α-NaFeO2-type crystal structure, the lower limit of the molar ratio of Al to the metal element (Ma) other than lithium is preferably 0.005, more preferably 0.01, and in some cases even more preferably 0.02 or 0.05. On the other hand, the upper limit of the molar ratio of Al is preferably 0.10, more preferably 0.08. When the molar ratio of Al is within the above range, it is possible to easily suppress a decrease in the capacity retention rate during charge-discharge cycling of the energy storage element.

[0039] Lithium transition metal composite oxides with spinel-type crystal structures include Li β Examples include those represented by Mb2O4 (Mb is a metal element other than lithium containing one or more transition metal elements, and 0<β≦1.2). Mb preferably contains Mn. The content of Mn relative to Mb (Mn / Mb) is preferably 50 mol % or more, and more preferably 80 mol % or more.

[0040] A polyanion compound is a compound composed of a polyanion (i.e., a polyvalent anion) and a cation. The polyanion compound preferably contains a lithium cation and a transition metal cation as the cation. Examples of the polyanion compound include LiFePO4, LiMnPO4, LiMn x Fe 1-x PO4(0 < x < 1), LiNiPO4, LiCoPO4, Li3V2(PO4)3, Li2MnSiO4, Li2CoPO4F, and the like. The surface of the particles of the polyanion compound may be coated with another material (such as the carbon material described later).

[0041] The active material particles may further contain other compounds other than the above lithium transition metal compounds. As the other compounds, various conventionally known cathode active materials can be used. However, the content of the lithium transition metal compound in the active material particles is preferably 90% by mass or more, more preferably 99% by mass or more. Thus, by making the main component of the active material particles be the above lithium transition metal compound, the effects of the present invention can be particularly sufficiently exhibited, and the like.

[0042] The average particle size of the active material particles is, for example, preferably 0.1 μm or more and 20 μm or less, more preferably 0.5 μm or more and 12 μm or less, still more preferably 1 μm or more and 8 μm or less, and even more preferably 2 μm or more and 5 μm or less. By setting the average particle size of the active material particles to be above the above lower limit, the production or handling of the active material particles becomes easy. By setting the average particle size of the active material particles to be below the above upper limit, the electron conductivity of the cathode containing the active material particles is improved, and as a result, the reaction area becomes large, and the output performance and the like can also be enhanced. The "average particle size" means a value at which the volume-based integrated distribution calculated in accordance with JIS-Z-8819-2 (2001) based on the particle size distribution measured by the laser diffraction / scattering method for a dilution obtained by diluting the particles with a solvent conforms to JIS-Z-8825 (2013) and is 50%.

[0043] (Coating layer) The coating layer may coat the entire active material particles or may coat only a portion of the active material particles. The coating layer contains lithium, niobium, and oxygen. The coating layer may coat the active material particles directly, or may coat the active material particles via another layer. From the viewpoint of reliably suppressing side reactions between the lithium transition metal compound contained in the active material particles and the electrolyte, it is preferable that the coating layer coat the active material particles directly.

[0044] The coating layer may contain a compound containing lithium, niobium, oxygen, or a combination thereof. The coating layer may contain one or more of the above compounds. The coating layer may essentially consist of the above compounds. The coating layer "substantially" consisting of the above compounds means that the content of the above compounds in the coating layer (the total content if two or more compounds) is 99% by mass or more. The content of the above compounds in the coating layer may be 99.5% by mass or more, 99.8% by mass or more, or even 100% by mass. In this way, when the coating layer essentially consists of the above compounds, the effects of the present invention can be particularly fully exhibited.

[0045] The lower limit of the content of the coating layer based on the mass of the active material particles is preferably 0.5 mass%, more preferably 0.8 mass%, and even more preferably 1.0 mass%, from the viewpoint of suppressing side reactions between the active material particles and the electrolyte. On the other hand, the upper limit of the content of the coating layer is preferably 3.0 mass%, more preferably 2.5 mass%, and even more preferably 2.0 mass%, from the viewpoint of ensuring the output performance of a storage device using the positive electrode active material. Thus, by having the content of the coating layer within the above range, the effects of the present invention can be particularly fully exhibited.

[0046] The spectrum derived from the coating layer measured by electron energy loss spectroscopy has two peaks in the range of 530 eV to 540 eV at the oxygen K absorption edge. Furthermore, if the lower-energy peak of these two peaks is designated as the first peak and the higher-energy peak is designated as the second peak, the intensity of the first peak is smaller than the intensity of the second peak. When the spectrum derived from the coating layer exhibits such a characteristic shape at the oxygen K absorption edge, it is believed that the coating layer is formed from a composition containing lithium, niobium, and oxygen and having an electrochemically stable structure characterized by the oxygen bonding state. Therefore, the positive electrode active material can suppress a decrease in the capacity retention rate during charge-discharge cycling of the energy storage device. The characteristic shape described above can be easily obtained by forming a coating layer on the positive electrode active material using a coating material containing lithium, niobium, and oxygen without heating the coating material at 200°C or higher. In contrast, when a coating layer of a positive electrode active material is formed by using a coating material containing lithium, niobium, and oxygen and heating the coating material at 200°C or higher, the intensity of the first peak tends to be equal to or greater than the intensity of the second peak, or either the first peak or the second peak tends to disappear. Furthermore, such a coating layer of a positive electrode active material makes it difficult to suppress a decrease in the capacity retention rate during charge-discharge cycling of the energy storage device.

[0047] The peak top position of the first peak is preferably from 530 eV to 536 eV, more preferably from 532 eV to 536 eV, and even more preferably from 533 eV to 536 eV. When the peak top position of the first peak is within the above range, a decrease in the capacity retention rate during charge-discharge cycling of the energy storage element can be easily suppressed.

[0048] The position of the peak top of the second peak is preferably greater than 536 eV and not greater than 540 eV, more preferably greater than 536 eV and not greater than 539 eV, and even more preferably greater than 536 eV and not greater than 538 eV. When the position of the peak top of the second peak is within the above range, a decrease in the capacity retention rate during charge-discharge cycling of the energy storage element can be easily suppressed.

[0049] The lower limit of the ratio of the intensity of the first peak to the intensity of the second peak is preferably 0.5, more preferably 0.6, even more preferably 0.7, and even more preferably 0.75. Meanwhile, the upper limit of the ratio of the intensity of the first peak to the intensity of the second peak is preferably 0.9, more preferably 0.85. When the ratio of the intensity of the first peak to the intensity of the second peak is within the above range, a decrease in the capacity retention rate during charge-discharge cycling of the energy storage element can be easily suppressed.

[0050] The positive electrode active material may be in a particulate form. The average particle size of the positive electrode active material is, for example, preferably 0.1 μm to 20 μm, more preferably 0.5 μm to 12 μm, even more preferably 1 μm to 8 μm, and even more preferably 2 μm to 5 μm. By setting the average particle size of the positive electrode active material to the above lower limit or above, the production and handling of the positive electrode active material becomes easier. By setting the average particle size of the positive electrode active material to the above upper limit or below, the electronic conductivity of the positive electrode containing the positive electrode active material is improved, and the reaction area is increased, resulting in improved output performance, etc. The average particle size of the positive electrode active material can be adjusted by, for example, adjusting the average particle size of the active material particles.

[0051] The positive electrode active material may further contain components other than the active material particles and the coating layer. However, the total content of the active material particles and the coating layer in the positive electrode active material is preferably 90% by mass or more, more preferably 99% by mass or more, and may even be 100% by mass. In this way, when the positive electrode active material is mainly composed of the active material particles and the coating layer, the effects of the present invention can be particularly fully exhibited.

[0052] [Method of manufacturing a positive electrode active material for an energy storage device] A method for producing a positive electrode active material for a storage device according to one embodiment of the present invention includes the steps of preparing particles containing a lithium transition metal compound (hereinafter also referred to as a "first preparation step"), preparing a coating material containing lithium, niobium, and oxygen (hereinafter also referred to as a "second preparation step"), and forming a coating layer using the coating material to cover at least a portion of the particles (hereinafter also referred to as a "forming step"). The above-mentioned positive electrode active material is produced by this production method.

[0053] (1st preparation step) In the first preparation step, particles containing a lithium transition metal compound are prepared. This lithium transition metal compound may be the lithium transition metal compound contained in the active material particles of the positive electrode active material described above. The lithium transition metal compound may be prepared by a known procedure or may be selected from known commercially available lithium transition metal compounds.

[0054] (Second preparation process) In the second preparation step, a coating material containing lithium, niobium, and oxygen is prepared. The coating material may contain, for example, a compound containing lithium, a compound containing niobium, and a compound containing oxygen. In addition, the coating material is preferably prepared as a solution using a dispersion medium, from the viewpoint of facilitating the formation of a coating layer.

[0055] Examples of compounds containing lithium include organic salts of lithium ions and anions having an organic group. Such organic salts are preferably organic salts of lithium ions and anions having a hydrocarbon group, and more preferably lithium alkoxides. Examples of such organic salts include ethoxylithium.

[0056] The compound containing niobium element includes an organic salt of a niobium ion and an anion having an organic group. Such an organic salt is preferably an organic salt of a niobium ion and an anion having a hydrocarbon group, and more preferably a niobium alkoxide. An example of such an organic salt is pentaethoxyniobium.

[0057] The compound containing oxygen element includes an organic compound having oxygen element, and an organic compound having oxygen element and hydrocarbon group is preferable. Such organic compound includes, for example, alcohols such as ethanol, alkoxide compounds, etc. The compound containing oxygen element may be the dispersion medium described above.

[0058] (Formation process) In the forming step, a coating layer is formed using a coating material to cover at least a part of the particles (active material particles). This coating layer may be the coating layer that the positive electrode active material has.

[0059] The forming step may include a process of spraying the coating material onto the active material particles, a process of fluidizing or rolling the active material particles sprayed with the coating material, and a process of drying the coating material. These processes may be performed using a known coating device. Examples of coating devices include a Wurster fluidized bed coating device, a centrifugal fluidized bed coating device, and a tumbling fluidized bed coating device. Among these, a tumbling fluidized bed coating device is preferred from the viewpoint of forming a uniform coating layer that covers the active material particles.

[0060] Generally, when forming a coating layer on active material particles, the coating material may be heated at 200°C or higher during or after the spraying process to fix the coating material to the surface of the active material particles. In contrast, in the formation process of the present manufacturing method, the coating material is not heated at 200°C or higher. As a result, the spectrum derived from the coating layer measured by electron energy loss spectroscopy exhibits a characteristic shape at the oxygen K absorption edge. More specifically, this spectrum has two peaks in the range of 530 eV to 540 eV, with the intensity of the lower-energy peak being smaller than the intensity of the higher-energy peak. Therefore, the present manufacturing method can produce a positive electrode active material that can suppress a decrease in the capacity retention rate during charge-discharge cycling of an energy storage device.

[0061] In the formation step, the upper limit of the surface temperature of the coating material is preferably 180° C., more preferably 160° C., and may be 140° C. or 120° C. When the surface temperature of the coating material is equal to or lower than the upper limit, a positive electrode active material that can suppress a decrease in the capacity retention rate during charge-discharge cycling of an energy storage element can be easily produced.

[0062] [Positive electrodes for energy storage elements] A positive electrode for a storage device according to one embodiment of the present invention (hereinafter also simply referred to as "positive electrode") has a positive electrode substrate and a positive electrode active material layer laminated on the positive electrode substrate directly or via an intermediate layer. Typically, the positive electrode has a portion where the positive electrode substrate is exposed. This exposed portion of the positive electrode substrate is usually connected to the above-mentioned positive electrode lead. The positive electrode may have a shape such as a sheet, plate, or strip.

[0063] The thickness of the positive electrode is appropriately set depending on the application of the energy storage device, etc. The average thickness of the positive electrode may be, for example, 30 μm or more and 1,000 μm or less. The lower limit of the average thickness of the positive electrode may be 50 μm, 100 μm, or 200 μm. The upper limit of the average thickness of the positive electrode may be 500 μm, 400 μm, 300 μm, 200 μm, or 100 μm. The average thickness of the positive electrode is the average thickness of the portion where the positive electrode active material layer is laminated directly on the positive electrode substrate or via an intermediate layer. In addition, in this specification, "average thickness" means the average value of thicknesses measured at any five positions.

[0064] The positive electrode substrate has electrical conductivity. In this specification, "having electrical conductivity" means that the volume resistivity is 10 -2 The volume resistivity is a value measured in accordance with JIS-H-0505 (1975). On the other hand, in this specification, "not having electrical conductivity" or "having (electrical) insulation" means that the volume resistivity is 10 7 This means that the resistance is Ω·cm or more.

[0065] Examples of materials for the positive electrode substrate include metals such as aluminum, titanium, iron, and alloys thereof (stainless steel, etc.). Among these, aluminum or an aluminum alloy is preferred from the viewpoints of potential resistance, high electronic conductivity, and cost.

[0066] The positive electrode substrate has a shape such as a sheet, plate, or strip. Examples of the positive electrode substrate include foil, vapor-deposited film, mesh, and porous material, and foil is preferred. The positive electrode substrate may be, for example, aluminum foil or aluminum alloy foil.

[0067] The average thickness of the positive electrode substrate may be, for example, 3 μm or more and 50 μm or less. The lower limit of the average thickness of the positive electrode substrate may be 5 μm, 8 μm, 10 μm, or 15 μm. The upper limit of the average thickness of the positive electrode substrate may be 40 μm, 30 μm, 20 μm, or 15 μm.

[0068] The intermediate layer is a layer disposed between the positive electrode substrate and the positive electrode active material layer. The intermediate layer contains, for example, a conductive agent and a binder. When the intermediate layer contains a conductive agent, the contact resistance between the positive electrode substrate and the positive electrode active material layer can be reduced. Examples of the conductive agent and binder used in the intermediate layer include the same conductive agent and binder used in the positive electrode active material layer described below.

[0069] The positive electrode active material layer includes the positive electrode active material according to one embodiment of the present invention described above. The positive electrode active material layer preferably includes a solid electrolyte. The positive electrode active material layer includes optional components such as a conductive agent, a binder, a thickener, and a filler, as needed. The positive electrode active material layer may be formed from a positive electrode mixture including a positive electrode active material and other optional components. As in the all-solid-state energy storage element 1 of FIG. 1, the positive electrode active material layer may be provided on only one side of a positive electrode substrate having a shape such as a sheet. In another embodiment, the positive electrode active material layer may be provided on both sides of the positive electrode substrate.

[0070] The term "solid electrolyte" refers to an electrolyte that maintains a solid state at 25°C under a nitrogen atmosphere. Examples of solid electrolytes include sulfide solid electrolytes, oxide solid electrolytes, dry polymer electrolytes, gel polymer electrolytes, and pseudo-solid electrolytes, with sulfide solid electrolytes being preferred. When the solid electrolyte contained in the positive electrode active material layer is a sulfide solid electrolyte, side reactions are particularly likely to occur between the sulfide solid electrolyte and the lithium transition metal compound contained in the positive electrode active material. This significantly enhances the advantage of the present invention, namely, suppressing side reactions between the positive electrode active material and the electrolyte. The solid electrolyte may be a solid electrolyte other than a sulfide solid electrolyte. The upper limit of the oxygen element content in the solid electrolyte may be 10 mol%, 1 mol%, or 0.1 mol%. The solid electrolyte may be a crystalline solid electrolyte or an amorphous solid electrolyte. A crystalline solid electrolyte refers to a solid electrolyte in which a peak derived from the solid electrolyte is observed in the X-ray diffraction pattern. An amorphous solid electrolyte refers to a solid electrolyte in which a halo pattern is observed in the X-ray diffraction pattern, in which peaks derived from the raw materials are substantially absent. One or more solid electrolytes can be used.

[0071] The sulfide solid electrolyte preferably contains at least sulfur element and further contains lithium element. The sulfide solid electrolyte preferably has lithium ion conductivity. The sulfide solid electrolyte preferably also contains phosphorus element and preferably further contains a halogen element. The sulfide solid electrolyte preferably contains at least one of bromine element and iodine element as the halogen element.

[0072] When the sulfide solid electrolyte is a crystalline solid electrolyte, its crystal structure may be an argyrodite-type crystal structure, a Li3PS4 crystal structure, a Li4P2S6 crystal structure, or a Li7P3S 11 Crystal structure, Li 10 GeP2S 12Examples of sulfide solid electrolytes include a sulfide solid electrolyte having a crystalline structure, such as a crystalline structure, a Thio-LISICON type crystalline structure, an inverse fluorite type crystalline structure, a crystalline structure having diffraction peaks in the ranges of diffraction angle 2θ of 19.9° ± 0.5° and 29.3° ± 0.5° in an X-ray diffraction pattern using CuKα rays, a crystalline structure having diffraction peaks in the ranges of diffraction angle 2θ of 21.0 ± 0.5° and 28.0 ± 0.5° in an X-ray diffraction pattern using CuKα rays, and a crystalline structure having different diffraction peaks in the ranges of diffraction angle 2θ of 17.9° ± 0.5° or 19.1° ± 0.5°, 2θ of 29.1° ± 0.5°, and 2θ of 29.8° ± 0.5°, wherein any of these diffraction peaks is the maximum diffraction peak.

[0073] Examples of sulfide solid electrolytes include Li2S-P2S5, Li2S-P2S5-LiI, Li2S-P2S5-LiCl, Li2S-P2S5-LiBr, Li2S-P2S5-Li2O, Li2S-P2S5-Li2O-LiI, Li2S-P2S5-Li3N, Li2S-SiS2, Li2S-SiS2-LiI, Li2S-SiS2-LiBr, Li2S-SiS2-LiCl, Li2S-SiS2-B2S3-LiI, Li2S-SiS2-P2S5-LiI, Li2S-B2S3, and Li2S-P2S5-Z. m S 2n (where m and n are positive numbers, and Z is Ge, Zn, or Ga.), Li2S-GeS2, Li2S-SiS2-Li3PO4, Li2S-SiS2-Li x MO y (where x and y are positive numbers, and M is one of P, Si, Ge, B, Al, Ga, and In.) Li 10 GeP2S 12 etc.

[0074] In the positive electrode active material layer, the solid electrolyte may form a complex with the positive electrode active material, and such a complex may further contain other components (e.g., a conductive agent) in addition to the solid electrolyte and the positive electrode active material.

[0075] When the positive electrode active material layer contains a solid electrolyte, the content of the solid electrolyte in the positive electrode active material layer is preferably 5% by mass or more and 50% by mass or less, and may be 10% by mass or more and 40% by mass or less, or may be 15% by mass or more and 30% by mass or less.

[0076] Conductive agents are usually components made of materials that have electrical conductivity. Even if the volume resistivity of a conductive agent cannot be measured directly, it is possible to measure the volume resistivity by measuring the volume resistivity of the conductive agent when the volume resistivity is 10 -2 Conductive agents are materials known to have a resistivity of Ω·cm or less. Examples of conductive agents include carbon materials, metals, and conductive ceramics. Carbon materials are materials whose primary constituent element is carbon. The primary constituent element refers to the element with the highest content by mass. For example, the carbon content in a carbon material may be 80% by mass or more, 90% by mass or more, 95% by mass, 99% by mass, or 99.9% by mass or more. The carbon material is preferably a carbon material other than an uncarbonized polymer compound. Examples of carbon materials include graphite, non-graphitic carbon, and graphene-based carbon. Examples of non-graphitic carbon include carbon nanofiber, pitch-based carbon fiber, 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 fullerene. Conductive agents may be in the form of powder, fiber, or the like. The conductive agent may be one or more kinds. The conductive agent may be a composite of these materials. For example, a composite material of carbon black and CNT may be used.

[0077] The content of the conductive agent in the positive electrode active material layer is preferably 0.1% by mass to 10% by mass, more preferably 1% by mass to 9% by mass, and even more preferably 3% by mass to 8% by mass. The upper limit of the content of the conductive agent may be 5%, 4%, or 3% by mass. By setting the content of the conductive agent within the above range, it is possible to increase the energy density of the energy storage element.

[0078] Examples of the binder include a water-based binder and an organic solvent-based binder.

[0079] The aqueous binder is a binder that dissolves or disperses in water. The aqueous binder may be a binder that dissolves or disperses at 1 part by mass or more in 100 parts by mass of water at 20°C. When a positive electrode active material layer is formed using a positive electrode mixture paste whose dispersion medium is water or a mixed solvent mainly composed of water, an aqueous binder (a water-soluble or water-dispersible polymer material) can be used. Examples of aqueous binders include polyethylene oxide, polypropylene oxide, polyvinyl alcohol, polyacrylic acid, polymethacrylic acid, polytetrafluoroethylene, styrene-butadiene rubber, polyethylene, polypropylene, nitrile-butadiene rubber, and cellulose.

[0080] The organic solvent-based binder is a binder that dissolves or disperses in an organic solvent (e.g., N-methylpyrrolidone). The organic solvent-based binder may be a binder that dissolves or disperses at 1 part by mass or more in 100 parts by mass of N-methylpyrrolidone at 20°C. When a positive electrode active material layer is formed using a positive electrode mixture paste whose dispersion medium is an organic solvent or a mixed solvent mainly containing an organic solvent, an organic solvent-based binder (a polymer material that is soluble or dispersible in an organic solvent) can be used. Examples of organic solvent-based binders include polyvinylidene fluoride, copolymers of vinylidene fluoride and hexafluoropropylene, copolymers of ethylene and vinyl alcohol, polyacrylonitrile, polyphosphazene, polysiloxane, polyvinyl acetate, polymethyl methacrylate, polystyrene, polycarbonate, polyamide, polyimide, polyamideimide, crosslinked polymers of cellulose and chitosan pyrrolidone carboxylate, chitosan derivatives, and the like.

[0081] The binder may be a fluororesin (polytetrafluoroethylene, polyvinylidene fluoride, etc.), a polyolefin (polyethylene, polypropylene, etc.), an elastomer (ethylene propylene diene rubber, styrene butadiene rubber, fluororubber, etc.), a polysaccharide polymer (cellulose, chitosan derivatives, etc.), etc. One or more types of binders may be used.

[0082] The content of the binder in the positive electrode active material layer is preferably 0.1% by mass to 10% by mass, more preferably 1% by mass to 9% by mass, and more preferably 3% by mass to 8% by mass. The upper limit of the binder content may be 5%, 4%, or 3% by mass. By setting the binder content within the above range, it is possible to stably hold the positive electrode active material. The technology disclosed herein can also be implemented in an embodiment in which the positive electrode active material layer does not contain a binder.

[0083] Examples of thickeners include polysaccharide polymers such as carboxymethyl cellulose 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. The thickener may also function as a binder. One or more types of thickeners may be used. When the positive electrode active material layer contains a thickener, the content of the thickener in the positive electrode active material layer is preferably 0.1% by mass or more and 8% by mass or less, more preferably 5% by mass or less, and even more preferably 2% by mass or less. The technology disclosed herein may also be implemented in an embodiment in which the positive electrode active material layer does not contain a thickener.

[0084] The filler is not particularly limited. The filler may be a component other than the positive electrode active material, solid electrolyte, conductive agent, binder, and thickener, and may be intentionally added. The filler may be added to fill gaps in the positive electrode active material layer, or may be added for other purposes. The filler may be an organic substance such as polyolefin, or an inorganic substance such as an inorganic oxide, hydroxide, or carbonate. One or more fillers may be used. When the positive electrode active material layer contains a filler, the content of the filler in the positive electrode active material layer may be 0.1% by mass or more and 8% by mass or less, typically 5% by mass or less is preferred, and 2% by mass or less is more preferred. The technology disclosed herein may also be implemented in an embodiment in which the positive electrode active material layer does not contain a filler.

[0085] The positive electrode active material layer may further contain other components in addition to the positive electrode active material, solid electrolyte, conductive agent, binder, thickener, and filler. The other components include those unintentionally present in the positive electrode active material layer. The positive electrode active material layer may also contain unintentionally contained impurities as the other components, as long as the effects of the present invention are achieved. The upper limit of the content of the other components in the positive electrode active material layer may be 10% by mass, 5%, 2%, 1%, 0.1%, or 0.01% by mass. The upper limit of the content of the unintentionally contained components in the positive electrode active material layer may be 10% by mass, 5%, 2%, 1%, 0.1%, or 0.01% by mass. The upper limit of the content of the unintentionally contained impurities in the positive electrode active material layer may be 10% by mass, 5%, 2%, 1%, 0.1%, or 0.01% by mass.

[0086] The average thickness of one positive electrode active material layer may be, for example, 5 μm or more and 1,000 μm or less. The lower limit of the average thickness of one positive electrode active material layer may be 10 μm, 20 μm, 30 μm, 40 μm, 50 μm, 60 μm, or 100 μm. The upper limit of the average thickness of one positive electrode active material layer may be 800 μm, 500 μm, 200 μm, 100 μm, 80 μm, 60 μm, or 40 μm. The mass per unit area of ​​one positive electrode active material layer may be, for example, 4 mg / cm. 2 More than 100mg / cm 2 The lower limit of the mass per unit area of ​​one positive electrode active material layer may be 6 mg / cm or less. 2 , 8 mg / cm 2 or 10 mg / cm 2 The upper limit of the mass per unit area of ​​one positive electrode active material layer is 50 mg / cm. 2 , 20 mg / cm 2 , 15 mg / cm 2 , 12 mg / cm 2 or 10 mg / cm 2 may be.

[0087] [Energy storage element] An energy storage element according to one embodiment of the present invention includes the positive electrode, negative electrode, and separator according to one embodiment of the present invention described above, and a container that accommodates these. The separator is a layer that is interposed between the positive electrode and the negative electrode and electrically insulates the positive electrode from the negative electrode, and contains a solid electrolyte that is a non-aqueous electrolyte. The solid electrolyte may also be contained in the positive electrode and the negative electrode. The energy storage element according to one embodiment of the present invention is preferably an all-solid-state energy storage element.

[0088] An all-solid-state energy storage element 1 shown in FIG. 1 , which is one embodiment of the present invention, is a secondary battery in which a positive electrode 2 and a negative electrode 3 are arranged with a separator 4 interposed therebetween. The positive electrode 2 has a positive electrode substrate 5 and a positive electrode active material layer 6, with the positive electrode substrate 5 being the outermost layer of the positive electrode 2. The negative electrode 3 has a negative electrode substrate 7 and a negative electrode active material layer 8, with the negative electrode substrate 7 being the outermost layer of the negative electrode 2. In the all-solid-state energy storage element 1 shown in FIG. 1 , the negative electrode active material layer 8, the separator 4, the positive electrode active material layer 6, and the positive electrode substrate 5 are stacked in this order on the negative electrode substrate 7. An intermediate layer may be provided between the positive electrode substrate 5 and the positive electrode active material layer 6. Similarly, an intermediate layer may be provided between the negative electrode substrate 7 and the negative electrode active material layer 8. The all-solid-state energy storage element 1 may further include other components such as a container. Other components such as a container are omitted from the all-solid-state energy storage element 1 shown in FIG. 1 .

[0089] The energy storage element according to one embodiment of the present invention may further include, for example, a positive electrode lead, a positive electrode external terminal, a negative electrode lead, and a negative electrode external terminal. The positive electrode lead and the negative electrode lead are housed in a container. The positive electrode external terminal and the negative electrode external terminal are provided outside the container. The positive electrode is electrically connected to the positive electrode external terminal via the positive electrode lead. The negative electrode is electrically connected to the negative electrode external terminal via the negative electrode lead.

[0090] Hereinafter, main components constituting an energy storage element according to one embodiment of the present invention will be described in detail, focusing on the case where the energy storage element is an all-solid-state energy storage element.

[0091] The positive electrode is the positive electrode for an energy storage element according to one embodiment of the present invention described above.

[0092] As described above, the negative electrode has a negative electrode substrate and a negative electrode active material layer laminated on the negative electrode substrate directly or via an intermediate layer. Usually, the negative electrode has a portion where the negative electrode substrate is exposed. This exposed portion of the negative electrode substrate is usually connected to the above-mentioned negative electrode lead. The negative electrode may have a shape such as a sheet, plate, or strip.

[0093] The thickness of the negative electrode is appropriately set depending on the application of the energy storage device, etc. The average thickness of the negative electrode may be, for example, 30 μm or more and 1,000 μm or less. The lower limit of the average thickness of the negative electrode may be 50 μm, 100 μm, or 200 μm. The upper limit of the average thickness of the negative electrode may be 500 μm, 400 μm, 300 μm, 200 μm, or 100 μm. The average thickness of the negative electrode is the average thickness of the portion where the negative electrode active material layer is laminated directly on the negative electrode substrate or via an intermediate layer.

[0094] The negative electrode substrate is conductive. Examples of materials for the negative electrode substrate include metals such as copper, nickel, iron, and alloys thereof (such as stainless steel), and carbon materials. Among these, copper or copper alloys are preferred.

[0095] The negative electrode substrate has a shape such as a sheet, plate, or strip. Examples of the form of the negative electrode substrate include foil, vapor-deposited film, mesh, and porous material, and foil is preferred. The negative electrode substrate may be, for example, copper foil or copper alloy foil.

[0096] The average thickness of the negative electrode substrate may be, for example, 2 μm or more and 35 μm or less. The lower limit of the average thickness of the negative electrode substrate may be 3 μm, 4 μm, 5 μm, or 10 μm. The upper limit of the average thickness of the negative electrode substrate may be 30 μm, 20 μm, 15 μm, or 10 μm.

[0097] The structure of the intermediate layer of the negative electrode is not particularly limited, and can be selected from the structures exemplified for the intermediate layer of the positive electrode, for example.

[0098] The negative electrode active material layer contains a negative electrode active material. The negative electrode active material layer may contain optional components such as a solid electrolyte, a conductive agent, a binder, a thickener, and a filler, as necessary. The optional components such as the solid electrolyte, the conductive agent, the binder, the thickener, and the filler may be selected from the materials exemplified for the positive electrode above. The negative electrode active material layer may be formed from a negative electrode mixture containing a negative electrode active material and other optional components. As in the all-solid-state energy storage element 1 of FIG. 1, the negative electrode active material layer may be provided on only one side of a negative electrode substrate having a shape such as a sheet. In another embodiment, the negative electrode active material layer may be provided on both sides of the negative electrode substrate.

[0099] The negative electrode active material can be a known negative electrode active material. A material capable of absorbing and releasing lithium ions is typically used as the negative electrode active material for lithium ion secondary batteries. Examples of the negative electrode active material include metallic lithium; metals or semimetals such as silicon and tin; metal oxides or semimetal oxides such as silicon oxide, titanium oxide, and tin oxide; and Li4Ti5O. 12 , LiTiO 2、 Examples of the negative electrode active material include titanium-containing oxides such as TiNbO; polyphosphate compounds; silicon carbide; and carbon materials such as graphite and non-graphitic carbon. The surface of the graphite may be coated with other materials such as non-graphitic carbon. One or more negative electrode active materials may be used.

[0100] "Graphite" refers to a graphite material that has an average lattice spacing (d 002 ) is a carbon material with a particle size of 0.33 nm or more and less than 0.34 nm. Graphite includes natural graphite and artificial graphite.

[0101] "Non-graphitic carbon" refers to carbon that has an average lattice spacing (d 002 ) refers to a carbon material in which the particle size is 0.34 nm or more and 0.42 nm or less. Non-graphitizable carbon includes non-graphitizable carbon and graphitizable carbon. "Non-graphitizable carbon" refers to a carbon material in which the particle size is 0.34 nm or more and 0.42 nm or less. 002The term "easily graphitizable carbon" refers to a carbon material having a particle size of 0.36 nm or more and 0.42 nm or less. 002 This refers to carbon materials with a particle size of 0.34 nm or more and less than 0.36 nm.

[0102] Here, the "discharged state" of a carbon material refers to a state in which the carbon material, which is a 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 a negative electrode active material as a working electrode and metallic lithium as a counter electrode is 0.7 V or higher.

[0103] The negative electrode active material may be in a particulate form. The average particle size of the negative electrode active material may 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, a polyphosphate compound, or the like, 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.

[0104] The content of the negative electrode active material in the negative electrode active material layer is, for example, 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.

[0105] 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. The metallic lithium may exist as pure metallic lithium consisting essentially of elemental lithium, or may exist as a lithium alloy containing other metal elements. When the negative electrode active material is a metal such as metallic lithium, the content of elemental lithium in the negative electrode active material layer may be 90% by mass or more, 99% by mass or more, or even 100% by mass.

[0106] When the negative electrode active material layer contains a conductive agent, the content of the conductive agent in the negative 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. The content of the conductive agent in the negative electrode active material layer may be 5% by mass or less, or may be 2% by mass or less. The technology disclosed herein may also be implemented in an embodiment in which the negative electrode active material layer does not contain a conductive agent.

[0107] When the negative electrode active material layer contains a solid electrolyte, the content of the solid electrolyte is preferably 5% by mass or more and 90% by mass or less, may be 10% by mass or more and 70% by mass or less, or may be 20% by mass or more and 50% by mass or less.

[0108] When the negative electrode active material layer contains a binder, the content of the binder in the negative electrode active material layer is preferably 0.1% by mass to 10% by mass, more preferably 0.5% by mass to 8% by mass. The content of the binder in the negative electrode active material layer may be 5% by mass or less, or may be 2% by mass or less. The technology disclosed herein may also be implemented in an embodiment in which the negative electrode active material layer does not contain a binder.

[0109] When the negative electrode active material layer contains a thickener, the content of the thickener in the negative electrode active material layer is preferably 0.1% by mass or more and 10% by mass or less, and more preferably 0.5% by mass or more and 8% by mass or less. The content of the thickener in the negative electrode active material layer may be 5% by mass or less, or may be 2% by mass or less. The technology disclosed herein may also be implemented in an embodiment in which the negative electrode active material layer does not contain a thickener.

[0110] The filler in the negative electrode active material layer may be a component other than the negative electrode active material, solid electrolyte, conductive agent, binder, and thickener, and may be an intentionally contained component. The filler may be contained as a component to fill gaps in the negative electrode active material layer, or may be contained for other purposes. When the negative electrode active material layer contains a filler, the content of the filler in the negative electrode active material layer can be 0.1% by mass or more and 8% by mass or less, and typically 5% by mass or less is preferred, and 2% by mass or less is more preferred. The technology disclosed herein may also be implemented in an embodiment in which the negative electrode active material layer does not contain a filler.

[0111] The negative electrode active material layer may further contain components other than the negative electrode active material, solid electrolyte, conductive agent, binder, thickener, and filler. These other components include those unintentionally present in the negative electrode active material layer. The negative electrode active material layer may also contain unintentionally present impurities as the other components, as long as the effects of the present invention are achieved. The upper limit of the content of the other components in the negative electrode active material layer may be 10% by mass, 5%, 2%, 1%, 0.1%, or 0.01% by mass. The upper limit of the content of the unintentionally present components in the negative electrode active material layer may be 10% by mass, 5%, 2%, 1%, 0.1%, or 0.01% by mass. The upper limit of the content of the unintentionally present impurities in the negative electrode active material layer may be 10% by mass, 5%, 2%, 1%, 0.1%, or 0.01% by mass.

[0112] The thickness of the negative electrode active material layer is appropriately set depending on the type of negative electrode active material, the application of the energy storage device, and the like. The average thickness of one negative electrode active material layer may be, for example, 5 μm or more and 1,000 μm or less. The lower limit of the average thickness of one negative electrode active material layer may be 10 μm, 20 μm, 30 μm, 40 μm, 50 μm, 60 μm, or 100 μm. The upper limit of the average thickness of one negative electrode active material layer may be 800 μm, 500 μm, 200 μm, 100 μm, 80 μm, 60 μm, or 40 μm. The mass per unit area of ​​one negative electrode active material layer is, for example, 2 mg / cm. 2 More than 50mg / cm2 The lower limit of the mass per unit area of ​​one negative electrode active material layer may be 3 mg / cm or less. 2 , 4 mg / cm 2 , 5 mg / cm 2 or 6 mg / cm 2 The upper limit of the mass per unit area of ​​one negative electrode active material layer is 30 mg / cm. 2 , 20 mg / cm 2 , 15 mg / cm 2 , 12 mg / cm 2 or 10 mg / cm 2 may be.

[0113] (isolation layer) The separator layer usually contains a solid electrolyte. The content of the solid electrolyte in the separator layer is preferably 70% by mass or more and 100% by mass or less. The content of the solid electrolyte in the separator layer may be 90% by mass or more, 99% by mass or more, or 100% by mass.

[0114] The separator layer may contain optional components such as additives (e.g., phosphate compounds such as LiPO, oxides, and halogen compounds), binders, thickeners, fillers, etc. The optional components such as binders, thickeners, and fillers can be selected from the materials exemplified for the positive electrode active material layer.

[0115] The average thickness of the separator is preferably 1 μm or more and 100 μm or less, more preferably 2 μm or more and 50 μm or less, and even more preferably 3 μm or more and 20 μm or less. By setting the average thickness of the separator to be equal to or greater than the lower limit, it is possible to insulate the positive electrode and the negative electrode with high reliability. By setting the average thickness of the separator to be equal to or less than the upper limit, it is possible to increase the energy density of the energy storage element.

[0116] (container) The container accommodates the positive electrode, the negative electrode, etc. in its internal space. Metallic materials such as aluminum and stainless steel, resin materials, etc. are used as the container material, and metallic materials are preferred from the viewpoint of strength, etc. Composite materials of metallic materials and resin materials, etc. may also be used.

[0117] The shape of the container is not particularly limited, and may be cylindrical, rectangular (square), disk-like, etc. The container may also be in the shape of a sheet formed from a metal resin composite film.

[0118] (shape, use, etc. of the energy storage element) The shape of the energy storage element according to one embodiment of the present invention is not particularly limited, and the energy storage element may be, for example, a cylindrical battery, a prismatic battery, a flat battery, a coin battery, a button battery, or the like.

[0119] The use of the energy storage element according to one embodiment of the present invention is not particularly limited, and the energy storage element can be used, for example, as a power source for automobiles such as electric vehicles, hybrid vehicles, and plug-in hybrid vehicles, a power source for electronic devices such as personal computers and communication terminals, a power source for power storage, and the like.

[0120] The energy storage element of the present invention may be used singly or in multiples. When the required output and required voltage are small, the energy storage element may be used singly. On the other hand, when at least one of the required output and required voltage is large, the energy storage element may be used as an energy storage device combined with other energy storage elements. In an energy storage device in which multiple energy storage elements are combined, at least one energy storage element included in the energy storage device may be an energy storage element according to one embodiment of the present invention. The energy storage device will be described in detail later.

[0121] In an energy storage element according to one embodiment of the present invention, for example, the container may be constrained so as to maintain a constant thickness, or may not be constrained in this manner. Furthermore, the container may be constrained so as to apply a constant load to the container. When the container is constrained, expansion of the container due to charge / discharge cycles, etc., may be suppressed, and deterioration of charge / discharge performance may be suppressed. When the container is constrained, a load may or may not be applied to the positive and negative electrodes in the container. For example, a constraining member that performs such constraining may be provided in the energy storage element or the energy storage device.

[0122] [Method of manufacturing an energy storage element] The energy storage device according to one embodiment of the present invention can be manufactured by a known method, for example, a method for manufacturing the energy storage device includes preparing a positive electrode mixture, preparing a separator material, preparing a negative electrode mixture, and stacking the positive electrode, separator, and negative electrode.

[0123] Preparing a positive electrode mixture may mean producing a positive electrode mixture. The method for preparing the positive electrode mixture is not particularly limited and can be appropriately selected depending on the purpose. For example, the positive electrode mixture can be prepared by mixing a positive electrode active material and a solid electrolyte using a mechanical milling method or the like. It is also possible to produce a composite of the positive electrode active material and the solid electrolyte in advance, and then mix the obtained composite with other components.

[0124] Preparing an isolation layer material may mean fabricating an isolation layer material. A solid electrolyte as an isolation layer material can be fabricated by a conventionally known method. For example, it can be obtained by processing a predetermined material by mechanical milling. The isolation layer material may also be fabricated by heating predetermined materials to a melting temperature or higher by melt-quenching, melt-mixing the two at a predetermined ratio, and then quenching. Other methods for fabricating an isolation layer material include, for example, a solid-phase method in which the material is sintered under reduced pressure, a liquid-phase method such as solution deposition, a vapor-phase method (PLD), and sintering in an argon atmosphere after processing by mechanical milling.

[0125] Preparing the negative electrode mixture may be preparing a negative electrode mixture. The specific method for preparing the negative electrode mixture is the same as that for the positive electrode mixture. When a metal such as metallic lithium is used as the negative electrode active material, a metal foil that will become the negative electrode active material layer may be prepared instead of preparing the negative electrode mixture.

[0126] By stacking a positive electrode, a separator, and a negative electrode, for example, a positive electrode having a positive electrode substrate and a positive electrode active material layer, a separator, and a negative electrode having a negative electrode substrate and a negative electrode active material layer are stacked. In this process, the positive electrode, the separator, and the negative electrode may be formed sequentially in this order, or vice versa, and the order of forming each layer is not particularly important. For example, the positive electrode is formed by pressure molding a positive electrode substrate and a positive electrode mixture, the separator is formed by pressure molding an separator material, and the negative electrode is formed by pressure molding a negative electrode substrate and a negative electrode mixture. The positive electrode, the separator, and the negative electrode may be stacked by pressure molding the positive electrode substrate, the positive electrode mixture, the separator material, the negative electrode mixture, and the negative electrode substrate all at once. The positive electrode and the negative electrode may be formed in advance, and then pressure molded and stacked with the separator.

[0127] [Electricity storage device] 2 includes a plurality of power storage units 20. Each power storage unit 20 includes a plurality of electrically connected all-solid-state power storage elements 1. The power storage device 30 may include a bus bar (not shown) that electrically connects the plurality of all-solid-state power storage elements 1, a bus bar (not shown) that electrically connects the plurality of power storage units 20, etc. The power storage unit 20 or the power storage device 30 may include a state monitoring device (not shown) that monitors the state of one or more all-solid-state power storage elements 1.

[0128] [Other embodiments] The energy storage device 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.

[0129] In the above embodiment, the electric storage element is used as a chargeable and dischargeable non-aqueous electrolyte secondary battery, but the electric 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.

[0130] For example, the energy storage element according to the present invention may include layers other than the positive electrode, separator, and negative electrode. The present invention can also be applied to an energy storage element including a bipolar electrode. The energy storage element according to the present invention may also include a liquid. Examples of such an energy storage element include a nonaqueous electrolyte energy storage element in which voids in the positive electrode active material layer 6, separator 4, negative electrode active material layer 8, etc. in the all-solid-state energy storage element 1 described above are filled with a nonaqueous electrolyte solution containing an ionic liquid or the like. [Example]

[0131] [Example 1] The active material particles have an α-NaFeO2 type crystal structure, and the molar ratios of Ni, Co, and Al to the metal element other than lithium (Ma) are 0.8, 0.15, and 0.05, respectively. 0.8 Co 0.15 Al 0.05 Lithium transition metal composite oxide particles represented by the formula O2 were used. The coating process was carried out by spraying a coating solution containing ethoxylithium, pentaethoxyniobium, and ethanol onto the active material particles while fluidizing them with air heated to 100°C. The coating process was carried out using a tumbling fluidized coating device (Powrex Corporation, "FD-MD-micro"). The coating layer content was 1.5% by mass based on the mass of the active material particles. Figure 3 shows the spectrum derived from the coating layer measured by electron energy loss spectroscopy. Figure 3 confirms that the intensity of the first peak A on the low-energy side, observed in the range from 530 eV to 540 eV at the K absorption edge, is smaller than the intensity of the second peak B on the high-energy side, observed in the range from 530 eV to 540 eV.

[0132] [Examples 2 and 3 and Comparative Examples 1 to 6] The molar ratio of each metal element to the metal element (Ma) other than lithium element in the coating solution of the active material particles was made the same as in Table 1, and positive electrode active materials of Examples 2 and 3 and Comparative Examples 1 to 6 were obtained in the same procedure as in Example 1, except that they were heated at the temperature shown in Table 1 after the coating treatment. Note that "-" described in the column of heating temperature in Table 1 means that heating was not performed at 200 °C or higher. Also, "A < B" described in the column of peak intensity in Table 1 means that the intensity of the first peak A is smaller than the intensity of the second peak B, and "A > B" means that the intensity of the first peak A is larger than the intensity of the second peak B. "-" described in the column of peak intensity in Table 1 means that measurement by electron energy loss spectroscopy was not performed. Spectra derived from the coating layer measured by electron energy loss spectroscopy for Comparative Example 1 and Comparative Example 4 are shown in FIGS. 4 and 5, respectively. In FIGS. 4 and 5, it can be confirmed that the intensity of the first peak A is larger than the intensity of the second peak B.

[0133] [Fabrication of Positive Electrode] Positive electrodes were fabricated using the above-described positive electrode active materials of the examples and comparative examples. The positive electrode active material, an argyrodite-type sulfide solid electrolyte represented by Li6PS5Cl, and fibrous carbon as a conductive agent were weighed at a predetermined mass ratio. Next, a fluororesin as a binder and butyl butyrate as a solvent were mixed with this mixture so that the solid content ratio became 70% by mass, and the kneaded product using a hybrid mixer was used as a positive electrode mixture. The obtained positive electrode mixture was applied onto an aluminum foil (average thickness: 20 μm) as a positive electrode substrate using a YBA type baker applicator so that the basis weight was 15 mg·cm -2 above to 25 mg·cm -2 below. This was dried in a dryer set at 100 °C for 10 minutes under normal pressure and then for 10 minutes under reduced pressure to form a positive electrode active material layer on the positive electrode substrate. This was punched out into a circle with a diameter of 10 mm to obtain positive electrodes of Examples 1 to 3 and Comparative Examples 1 to 6.

[0134] [Fabrication of Energy Storage Element] Energy storage elements (all-solid-state energy storage elements) were fabricated using the positive electrodes of the above-described Examples and Comparative Examples. 80 mg of an argyrodite-type sulfide solid electrolyte represented by Li6PS5Cl was inserted into a ceramic powder compactor with an inner diameter of 10 mm as the isolation layer material, and the material was pressure-molded using a uniaxial press at a pressure of 50 MPa for several seconds to form an isolation layer. After releasing the pressure, the fabricated positive electrode was placed on one side of the isolation layer and pressure-molded using a uniaxial press at 400 MPa for 5 minutes. After releasing the pressure, indium foil and lithium foil as the negative electrode, and copper foil as the negative electrode substrate, were placed on the side opposite the bonding surface of the positive electrode and bonded using a uniaxial press at a pressure of 50 MPa for several seconds. The resulting structure was removed from the powder compactor to obtain the energy storage elements (all-solid-state energy storage elements) of Examples 1 to 3 and Comparative Examples 1 to 6. The fabrication of the positive electrodes and energy storage elements was carried out in a glove box in an argon atmosphere with a dew point of −70°C or below.

[0135] (Charge-discharge cycle test) Each of the all-solid-state energy storage elements of the Examples and Comparative Examples was subjected to a charge-discharge cycle test at 25° C. in the following manner. Constant current / constant voltage charging was performed with a charging current of 0.1 C and a charge cut-off voltage of 3.75 V. Charging was terminated when the charging current reached 0.025 C. After that, a 10-minute rest period was allowed. Subsequently, constant current discharging was performed with a discharging current of 0.1 C and a discharge cut-off voltage of 2.25 V. After that, a 10-minute rest period was allowed. These charging and discharging processes constitute one cycle, and three cycles were repeated. Next, constant current / constant voltage charging was performed with a charging current of 0.2 C and a charge cut-off voltage of 3.75 V. The charge was terminated when the charging current reached 0.05 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 2.25 V. A 10-minute rest period was then provided. These charge and discharge steps constitute one cycle, and were repeated 50 times. The discharge capacity at the 50th cycle was divided by the discharge capacity at the first cycle to determine the discharge capacity retention rate after 50 cycles. The discharge capacity retention rate of each all-solid-state energy storage element is shown in Table 1.

[0136] [Table 1]

[0137] As shown in Table 1, for Examples 1 and Comparative Examples 1 to 4 where the composition of the active material particles is equal, in Example 1 where the coating layer was not heated at 200°C or higher, the relationship of the peak intensities was A < B, and the discharge capacity retention rate was 98% or higher. On the other hand, in Comparative Examples 1 and 4 where the coating layer was heated at 200°C or higher, the relationship of the peak intensities was A > B, and the discharge capacity retention rate was less than 98%. For Comparative Examples 2 and 3 as well, since they were heated at a temperature between the heating temperature of Comparative Example 1 and the heating temperature of Comparative Example 4, it is considered that the relationship of the peak intensities is A > B. Also, for Comparative Examples 1 to 4, the discharge capacity retention rate tended to decrease as the heating temperature increased. This is considered to be because the structure showing a characteristic shape at the K absorption edge of oxygen in the spectrum of the coating layer decreased as the heating temperature increased. Similarly, for Example 2 and Comparative Example 5, and Example 3 and Comparative Example 6, the discharge capacity retention rate decreased when the coating layer was heated at 200°C or higher. It is considered that this is because a coating layer having a similar structure is formed regardless of the composition of the active material.

Industrial Applicability

[0138] The present invention can be applied to power storage elements used as power sources for electronic devices such as personal computers and communication terminals, and automobiles.

Explanation of Reference Numerals

[0139] 1 Power storage element (all-solid-state power storage element) 2 Positive electrode 3 Negative electrode 4 Separator 5 Positive electrode substrate 6 Positive electrode active material layer 7 Negative electrode substrate <0​​​​​

Claims

1. A lithium transition metal compound-containing particle and a coating layer that coats at least a portion of the particle, the coating layer contains lithium, niobium, and oxygen, a spectrum derived from the coating layer measured by electron energy loss spectroscopy has two peaks in the range of 530 eV or more and 540 eV or less at the oxygen K absorption edge; A positive electrode active material for a storage element, wherein the intensity of the lower energy peak of the two peaks is smaller than the intensity of the higher energy peak.

2. 2. The positive electrode active material for a storage device according to claim 1, wherein the lithium transition metal compound is a lithium transition metal composite oxide.

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

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

5. The energy storage element according to claim 4, which is an all-solid-state energy storage element.

6. providing particles comprising a lithium transition metal compound; providing a coating material containing lithium, niobium, and oxygen; forming a coating layer using the coating material to coat at least a portion of the particles; and The method for producing a positive electrode active material for a storage cell, wherein the coating material is not heated to 200° C. or higher when forming the coating layer.

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