Non-aqueous electrolyte energy storage element

Abstract

A non-aqueous electrolyte electricity storage element is provided which has a high energy density per mass and which has improved high-rate discharge performance and discharge capacity retention rate during charge-discharge cycles at high temperatures. [Solution] A nonaqueous electrolyte storage element according to one aspect of the present invention comprises a positive electrode having a positive electrode active material layer and a negative electrode having a negative electrode active material layer, the positive electrode active material layer being composed primarily of a lithium transition metal composite oxide having an α-NaFeO2-type crystal structure, and the mass per area of ​​the positive electrode active material layer being 3.5 g / 100 cm 2 More than 10.5g / 100cm 2 or less, the content of nickel element relative to metal elements other than lithium element in the lithium transition metal composite oxide is 65 mol % or more, and the negative electrode active material layer is mainly composed of a silicon-based active material.

Description

[Technical Field]

[0001] The present invention relates to a non-aqueous electrolyte electricity storage element. [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, as well as in automobiles. Non-aqueous electrolyte secondary batteries generally have a pair of electrodes electrically isolated by a separator and a non-aqueous electrolyte interposed between the electrodes, and are configured to charge and discharge by transferring charge-transporting ions between the electrodes. In addition to non-aqueous electrolyte secondary batteries, capacitors such as lithium ion capacitors and electric double layer capacitors are also widely used as non-aqueous electrolyte energy storage elements.

[0003] Conventionally, the positive electrode active materials used in non-aqueous electrolyte storage elements include LiCoO2 and LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 Lithium transition metal composite oxides having an α-NaFeO2 type crystal structure, such as O2, have been studied and put to practical use (see Patent Document 1). On the other hand, carbon materials such as graphite are widely used as negative electrode active materials (see Patent Document 2). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-18678 [Patent Document 2] Japanese Patent Application Laid-Open No. 2005-222933 Summary of the Invention [Problem to be solved by the invention]

[0005] In nonaqueous electrolyte energy storage devices, further improvement in energy density per mass is desired. To address this issue, it has been considered to use a silicon-based active material, which has a larger theoretical capacity than carbon materials such as graphite, as the negative electrode active material. However, in nonaqueous electrolyte energy storage devices equipped with a negative electrode using a silicon-based active material, the high-rate discharge performance and discharge capacity retention rate may deteriorate during charge-discharge cycles at high temperatures.

[0006] An object of the present invention is to provide a nonaqueous electrolyte electricity storage element that has a high energy density per mass and has improved high-rate discharge performance and discharge capacity retention rate during charge-discharge cycles at high temperatures. [Means for solving the problem]

[0007] A nonaqueous electrolyte storage element according to one aspect of the present invention includes a positive electrode having a positive electrode active material layer and a negative electrode having a negative electrode active material layer, the positive electrode active material layer being mainly composed of a lithium transition metal composite oxide having an α-NaFeO2-type crystal structure, and the positive electrode active material layer having a mass per area of ​​3.5 g / 100 cm 2 More than 10.5g / 100cm 2 or less, the content of nickel element relative to metal elements other than lithium element in the lithium transition metal composite oxide is 65 mol % or more, and the negative electrode active material layer is mainly composed of a silicon-based active material. [Effects of the Invention]

[0008] According to one aspect of the present invention, it is possible to provide a nonaqueous electrolyte energy storage element that has a high energy density per mass and has improved high-rate discharge performance and discharge capacity retention rate after charge-discharge cycling at high temperatures. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a perspective view showing one embodiment of a nonaqueous electrolyte electricity storage element. [Figure 2] FIG. 2 is a schematic diagram showing one embodiment of an electricity storage device configured by assembling a plurality of nonaqueous electrolyte electricity storage elements. DETAILED DESCRIPTION OF THE INVENTION

[0010] First, an outline of the nonaqueous electrolyte electricity storage element disclosed in this specification will be described.

[0011] [1] A nonaqueous electrolyte storage element according to one aspect of the present invention includes a positive electrode having a positive electrode active material layer and a negative electrode having a negative electrode active material layer, the positive electrode active material layer being composed primarily of a lithium transition metal composite oxide having an α-NaFeO2-type crystal structure, and the positive electrode active material layer having a mass per area of ​​3.5 g / 100 cm 2 More than 10.5g / 100cm 2 or less, the content of nickel element relative to metal elements other than lithium element in the lithium transition metal composite oxide is 65 mol % or more, and the negative electrode active material layer is mainly composed of a silicon-based active material.

[0012] The nonaqueous electrolyte storage element described in [1] above has a high energy density per mass and improved high-rate discharge performance and discharge capacity retention rate after high-temperature charge-discharge cycling. While the reasons for this are unclear, the following are presumed. The silicon-based active material used as the negative electrode active material in this nonaqueous electrolyte storage element has a larger theoretical capacity than carbon materials such as graphite, resulting in a high energy density per mass. This allows the thickness of the negative electrode active material layer to be reduced, suppressing kinetically-induced deposition of metallic lithium in the negative electrode, thereby improving the discharge capacity retention rate after high-temperature charge-discharge cycling. The theoretical capacity of the silicon-based active material is smaller than that of metallic lithium. However, in a nonaqueous electrolyte storage element using a silicon-based active material as the negative electrode active material, increasing the mass per area of ​​the positive electrode active material layer mainly composed of a lithium transition metal composite oxide having an α-NaFeO2-type crystal structure can reduce the opposing area of ​​the positive electrode active material layer and the negative electrode active material layer required to obtain the same amount of energy, thereby reducing the mass of the positive electrode (e.g., positive electrode substrate), etc., and thereby increasing the energy density per mass of the nonaqueous electrolyte storage element. On the other hand, if the mass per area of ​​the positive electrode active material layer is too large, polarization due to the diffusion resistance of charge-transporting ions (lithium ions, etc.) in the thickness direction of the positive electrode active material layer and localized disconnection of electronic conduction paths and ionic conduction paths are likely to occur, resulting in a decrease in the energy density per mass of the nonaqueous electrolyte storage element. In the nonaqueous electrolyte storage element described in [1] above, when the mass per area of ​​the positive electrode active material layer mainly composed of a lithium transition metal composite oxide having an α-NaFeO2-type crystal structure is 3.5 g / 100 cm 2 More than 10.5g / 100cm 2or less, the energy density per mass is high. Furthermore, when the content of nickel relative to the metal elements other than lithium in the lithium transition metal composite oxide is less than 65 mol%, the charging potential of the positive electrode increases when a certain amount of charge-transporting ions (such as lithium ions) is extracted from the positive electrode active material, resulting in a high charging voltage of the nonaqueous electrolyte storage element. This accelerates decomposition of the nonaqueous electrolyte during high-temperature charge-discharge cycling, resulting in increased gas generation and increased deposition of coatings on the positive and negative electrode surfaces, leading to a decrease in the discharge capacity retention rate and high-rate discharge performance after high-temperature charge-discharge cycling. In contrast, in the nonaqueous electrolyte storage element described in [1] above, the content of nickel relative to the metal elements other than lithium in the lithium transition metal composite oxide is 65 mol% or more, resulting in a decrease in the positive electrode charging potential when a certain amount of charge-transporting ions (such as lithium ions) is extracted from the positive electrode active material, resulting in a low charging voltage of the nonaqueous electrolyte storage element. This maintains the energy density per mass and improves the discharge capacity retention rate and high-rate discharge performance after high-temperature charge-discharge cycling. For the reasons described above, it is presumed that the nonaqueous electrolyte storage element described in [1] above has a high energy density per mass and has improved high-rate discharge performance and discharge capacity retention rate after charge-discharge cycling at high temperatures.

[0013] "Major component" refers to the component that is contained in the greatest amount by mass. Mass per area of ​​the positive electrode active material layer (g / 100cm 2 ) is the area of ​​the positive electrode active material layer (100 cm 2) is the mass (g) of the positive electrode active material layer per 100 cm2 of the positive electrode active material layer. The area of ​​the positive electrode active material layer refers to the area of ​​one of a pair of opposing main surfaces (front and back surfaces) of one positive electrode active material layer. That is, for example, when the positive electrode active material layer is provided by coating, the area of ​​the positive electrode active material layer is equal to the area where the positive electrode active material layer is coated. When positive electrode active material layers are provided on both sides of the positive electrode substrate, the area and mass of the positive electrode active material layer refer to the area and mass of one positive electrode active material layer. The area and mass of the negative electrode active material layer described below are also defined in the same way as the area and mass of the positive electrode active material layer. For example, when the positive electrode active material layer is provided on both sides of the positive electrode substrate with a thickness of 10.0 g / 100 cm2, respectively, 2 When the coating amount (solid content equivalent) is 10.0 g / 100 cm, the "mass per area of ​​the positive electrode active material layer" is 10.0 g / 100 cm. 2 The positive electrode active material layer was 10.0 g / 100 cm on one side of the positive electrode substrate. 2 Even if the coating amount (solid content equivalent) is 10.0 g / 100 cm, the "mass per area of ​​the positive electrode active material layer" is 10.0 g / 100 cm. 2 That is, in the case of a positive electrode active material layer provided on one or both sides of a positive electrode substrate, the "mass per area of ​​the positive electrode active material layer" is the mass per area of ​​the positive electrode active material layer per one side. The "mass per area of ​​the negative electrode active material layer" described below is also defined in the same way as the "mass per area of ​​the positive electrode active material layer." The lithium transition metal composite oxide is confirmed to have an α-NaFeO2-type crystal structure by X-ray diffraction measurement. X-ray diffraction measurement of the lithium transition metal composite oxide is performed before charge / discharge or on the lithium transition metal composite oxide that has been fully discharged by the following method. First, the nonaqueous electrolyte storage element is discharged at a constant current of 0.05 C to the lower limit voltage for normal use. The nonaqueous electrolyte storage element in this state is disassembled, the positive electrode is removed, and a half cell is assembled with metallic lithium as the counter electrode. The positive electrode potential is measured at a discharge current of 10 mA per 1 g of positive electrode active material until the positive electrode potential reaches 2.75 V vs. Li / Li. +The positive electrode is then fully discharged by constant current discharge. The device is then disassembled again and the positive electrode is removed. The components (non-aqueous electrolyte, etc.) adhering to the removed positive electrode are thoroughly washed using dimethyl carbonate. After drying under reduced pressure at room temperature for 24 hours, the lithium transition metal composite oxide (positive electrode active material) is extracted. The entire process from disassembly of the non-aqueous electrolyte storage element to extraction of the lithium transition metal composite oxide for measurement is carried out in an argon atmosphere with a dew point of -60°C or below. X-ray diffraction measurements are performed using a powder X-ray diffractometer (Rigaku's "MiniFlex II"), with a CuKα radiation source, a tube voltage of 30 kV, and a tube current of 15 mA. The diffracted X-rays pass through a 30 μm-thick Kβ filter and are detected by a high-speed one-dimensional detector (D / teX Ultra 2). The sampling width is 0.02°, the scan speed is 5° / min, the divergence slit width is 0.625°, the receiving slit width is 13 mm (OPEN), and the scattering slit width is 8 mm.

[0014] [2] In the nonaqueous electrolyte storage element according to [1] above, the positive electrode active material layer may have a porosity of 15% or more and 35% or less.

[0015] The nonaqueous electrolyte storage element described in [2] above can further increase the energy density per mass. The reason for this is presumably that by making the porosity of the positive electrode active material layer relatively low, the mass of the nonaqueous electrolyte filled in the pores of the positive electrode active material layer can be reduced, and local disconnection of the electron conduction path is less likely to occur. On the other hand, when the mass per area is 10.5 g / 100 cm, 2 Conversely, when the porosity is relatively low in a positive electrode active material layer exceeding 100%, the energy density per mass tends to decrease. The reason for this is presumably that the reduction in the porosity of the positive electrode active material layer has a large effect on reducing the diffusion paths of charge-transporting ions (lithium ions, etc.) within the positive electrode active material layer.

[0016] The "porosity of the positive electrode active material layer" refers to a value calculated from the true density of the positive electrode active material layer, which is calculated from the true density of each component constituting the positive electrode active material layer, and the apparent density of the positive electrode active material layer, using the following formula (1): Porosity (%) = 100 - (apparent density / true density) × 100 (1) The apparent density of the positive electrode active material layer is the value obtained by dividing the mass of the positive electrode active material layer by the apparent volume of the positive electrode active material layer. The apparent volume refers to the volume including voids (pores) and can be calculated as the product of the average thickness and area of ​​the positive electrode active material layer. The average thickness of the positive electrode active material layer is the average value of thicknesses measured at any five positions.

[0017] [3] In the nonaqueous electrolyte storage element according to the above [1] or [2], the content of silicon element in the negative electrode active material layer may be 30% by mass or more and 70% by mass or less.

[0018] The nonaqueous electrolyte storage element described in [3] above can further increase the energy density per mass and further improve the discharge capacity retention rate after high-temperature charge-discharge cycling. The reasons for this are presumably that by setting the silicon element content in the negative electrode active material layer to 30% by mass or more, the amount of electricity reversibly charged and discharged by the negative electrode increases, allowing the mass of the negative electrode to be reduced, thereby further increasing the energy density per mass, and that by setting the silicon element content in the negative electrode active material layer to 70% by mass or less, electrical isolation of the silicon-based active material due to charge-discharge cycling can be suppressed.

[0019] A nonaqueous electrolyte electricity storage element, an electricity storage device, a method for manufacturing a nonaqueous electrolyte electricity storage element according to one embodiment of the present invention, and other embodiments will be described in detail below. Note that the names of the components (elementary components) used in each embodiment may differ from the names of the components (elementary components) used in the background art.

[0020] <Non-aqueous electrolyte energy storage element> A nonaqueous electrolyte storage element (hereinafter also simply referred to as "storage element") according to one embodiment of the present invention comprises an electrode assembly having a positive electrode, a negative electrode, and a separator, a nonaqueous electrolyte, and a container that accommodates the electrode assembly and the nonaqueous electrolyte. The electrode assembly is typically a laminated type in which multiple positive electrodes and multiple negative electrodes are stacked with separators interposed therebetween, or a wound type in which positive electrodes and negative electrodes are stacked with separators interposed therebetween and wound. The nonaqueous electrolyte storage element according to one embodiment of the present invention preferably comprises a laminated electrode assembly. A laminated electrode assembly has the advantage of being able to reduce the porosity of the positive electrode active material layer, thereby further increasing the energy density per mass of the nonaqueous electrolyte storage element. Known containers, such as metal containers and resin containers, can be used. From the perspective of further increasing the energy density per mass of the nonaqueous electrolyte storage element, heat-sealed containers made of film (e.g., resin film, metal-resin composite film, etc.) are also preferably used. The nonaqueous electrolyte is present in a state contained in the positive electrode, negative electrode, and separator. As an example of a nonaqueous electrolyte storage element, a nonaqueous electrolyte secondary battery (hereinafter also simply referred to as a "secondary battery") will be described.

[0021] (positive electrode) The positive electrode has a positive electrode substrate and a positive electrode active material layer disposed on the positive electrode substrate directly or via an intermediate layer.

[0022] 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.

[0023] Examples of materials for the positive electrode substrate include metals such as aluminum, titanium, and iron, and alloys thereof (stainless steel, etc.), etc. Among these, aluminum or aluminum alloys are preferred from the viewpoints of potential resistance, high conductivity, and cost.

[0024] 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.

[0025] 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, 12 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. By setting the average thickness of the positive electrode substrate within the above range, the strength of the positive electrode substrate can be increased while increasing the energy density per mass of the nonaqueous electrolyte storage element.

[0026] The intermediate layer is a layer disposed between the positive electrode substrate and the positive electrode active material layer. The intermediate layer contains a conductive agent such as carbon particles to reduce the contact resistance between the positive electrode substrate and the positive electrode active material layer. The configuration of the intermediate layer is not particularly limited, and may contain, for example, a binder and a conductive agent.

[0027] The positive electrode active material layer is mainly composed of a lithium transition metal composite oxide having an α-NaFeO2 crystal structure. The lithium transition metal composite oxide serves as the positive electrode active material. The positive electrode active material layer may contain optional components such as other positive electrode active materials in addition to the lithium transition metal composite oxide, a conductive agent, a binder, a thickener, and a filler, as needed.

[0028] The lithium transition metal composite oxide having an α-NaFeO2-type crystal structure contains nickel. The lower limit of the content of nickel relative to metal elements other than lithium in the lithium transition metal composite oxide, expressed as a molar ratio of 0.65, 0.70, or 0.75, when the content of metal elements other than lithium is taken as 1.00. A relatively high content of nickel tends to improve the discharge capacity retention rate and high-rate discharge performance after high-temperature charge-discharge cycling of a nonaqueous electrolyte storage element in which a certain amount of charge-transport ions are extracted from the positive electrode active material. The upper limit of the content of nickel, expressed as a molar ratio of 1.00, when the content of metal elements other than lithium is taken as 1.0, may be 0.95, 0.90, or 0.85. The content of nickel may be equal to or greater than any of the above-mentioned lower limits and equal to or less than any of the above-mentioned upper limits.

[0029] The lithium transition metal composite oxide preferably further contains at least one transition metal element other than nickel, preferably at least one of cobalt and manganese, and more preferably cobalt. The lithium transition metal composite oxide also preferably further contains manganese, more preferably nickel, cobalt, and manganese. Use of such a lithium transition metal composite oxide can further increase the energy density per mass of the nonaqueous electrolyte storage element. The lithium transition metal composite oxide may contain only nickel, cobalt, and manganese as transition metal elements. The lithium transition metal composite oxide may further contain other metal elements such as aluminum.

[0030] The lower limit of the content of cobalt relative to metal elements other than lithium in the lithium transition metal composite oxide may be 0.00, 0.10, 0.20, 0.30, or 0.35 in terms of molar ratio when the content of metal elements other than lithium is 1.00. The upper limit of the content of cobalt relative to metal elements other than lithium is 0.35, 0.30, 0.20, 0.10, or 0.00 in terms of molar ratio when the content of metal elements other than lithium is 1.00. The content of cobalt may be equal to or greater than any of the above lower limits and equal to or less than any of the above upper limits.

[0031] The lower limit of the manganese content relative to the metal elements other than lithium in the lithium transition metal composite oxide may be 0.00, 0.10, 0.20, 0.30, or 0.35 in terms of molar ratio when the content of metal elements other than lithium is 1.00. The upper limit of the manganese content may be 0.35, 0.30, 0.20, 0.10, or 0.00 in terms of molar ratio when the content of metal elements other than lithium is 1.00. The manganese content may be equal to or greater than any of the above lower limits and equal to or less than any of the above upper limits.

[0032] The total content of nickel, cobalt, and manganese relative to the metal elements other than lithium in the lithium transition metal composite oxide is preferably 0.9 or more and 1.0 or less, and more preferably 0.99 or more and 1.00 or less, in terms of molar ratio, when the content of the metal elements other than lithium is taken as 1.0.

[0033] The lower limit of the lithium content relative to the metal elements other than lithium in the lithium transition metal composite oxide may be 0.9, 1.0, or 1.05, in terms of a molar ratio when the content of the metal elements other than lithium is taken as 1.0. The upper limit of the lithium content may be 1.6, 1.5, 1.4, 1.3, or 1.2, in terms of a molar ratio when the content of the metal elements other than lithium is taken as 1.0. When the lithium content is below the above upper limit, the amount of gas generated during high-temperature charge / discharge cycles tends to be further reduced, and the discharge capacity of the positive electrode active material increases, thereby increasing the energy density per mass of the nonaqueous electrolyte storage element. The lithium content may be equal to or greater than any of the above lower limits and equal to or less than any of the above upper limits.

[0034] The lithium transition metal composite oxide is preferably a compound represented by the following formula (2). Li 1+α (Ni β M 1-β ) 1-α O2···(2) In formula (2), M is at least one metal element (excluding Li and Ni), 0≦α<1, and 0.65≦β≦1.00.

[0035] In formula (2), M preferably contains at least one transition metal element other than Ni, preferably contains at least one of Co and Mn, and more preferably contains Co. M also preferably contains Mn, and more preferably contains Co and Mn. M may consist of only Co, or may consist of only Mn, or may consist of only Co and Mn. M may further contain another metal element such as Al.

[0036] In the above formula (2), the specific ranges of the molar ratio of Ni to the total amount of Ni and M (Ni+M) (Ni / (Ni+M)), the molar ratio of Co to the total amount of Ni and M (Ni+M) (Co / (Ni+M)), the molar ratio of Mn to the total amount of Ni and M (Ni+M) (Mn / (Ni+M)), the molar ratio of the total amount of Ni, Co, and Mn to the total amount of Ni and M (Ni+M) ((Ni+Co+Mn) / (Ni+M)), and the molar ratio of Li to the total amount of Ni and M (Ni+M) can be the same as the specific ranges of each metal element in the above-mentioned lithium transition metal composite oxide.

[0037] The composition ratio of the lithium transition metal composite oxide refers to the composition ratio before charge / discharge or when fully discharged by the following method. First, the nonaqueous electrolyte storage element is discharged at a constant current of 0.05 C to the lower limit voltage for normal use. The nonaqueous electrolyte storage element in this state is disassembled, the positive electrode is removed, and a half cell is assembled with metallic lithium as the counter electrode. The positive electrode potential is measured at a discharge current of 10 mA per 1 g of positive electrode active material until the positive electrode potential reaches 2.75 V vs. Li / Li. +The positive electrode is then fully discharged by constant current discharge until the positive electrode reaches a fully discharged state. The battery is then disassembled again and the positive electrode is removed. Components (such as non-aqueous electrolyte) adhering to the removed positive electrode are thoroughly washed using dimethyl carbonate. The removed positive electrode is then dried under reduced pressure at room temperature for 24 hours, after which the lithium transition metal composite oxide serving as the positive electrode active material is collected. The collected lithium transition metal composite oxide is then completely dissolved using a microwave decomposition method with an acid capable of dissolving the metal elements contained in the lithium transition metal composite oxide. This solution is then diluted to a certain amount with pure water to obtain a measurement solution. The metal element concentrations in the measurement solution are then measured by ICP emission spectroscopy using a multi-type ICP emission spectroscopy analyzer, ICPE-9820 (Shimadzu Corporation). The element contents in the positive electrode active material are quantified from the obtained concentrations of each element. The concentration of each element in the measurement solution can be calculated using a calibration curve method, in which a calibration curve is created from solutions of known concentrations of each element and the concentration of each element in the measurement solution is determined. The operations from disassembling the nonaqueous electrolyte storage element to extracting the lithium transition metal composite oxide for measurement are carried out in an argon atmosphere with a dew point of −60° C. or lower. Here, normal use refers to the use of the nonaqueous electrolyte storage element under the charge / discharge conditions recommended or specified for the nonaqueous electrolyte storage element.

[0038] The surface of the lithium transition metal composite oxide may be coated with another material, such as a compound containing aluminum, tungsten, boron, zirconium, or niobium.

[0039] The lithium transition metal composite oxide is usually in the form of particles (powder). The average particle size of the lithium transition metal composite oxide is preferably, for example, 0.1 μm or more and 20 μm or less. The lower limit of the average particle size of the lithium transition metal composite oxide is preferably 1 μm, more preferably 4 μm, and even more preferably 8 μm in some cases. By setting the average particle size of the lithium transition metal composite oxide to be equal to or greater than the lower limit, the production and handling of the lithium transition metal composite oxide is facilitated, and the content of the lithium transition metal composite oxide in the positive electrode active material layer can be increased, thereby increasing the energy density per mass of the nonaqueous electrolyte storage element. By setting the average particle size of the lithium transition metal composite oxide to be equal to or less than the upper limit, the conductivity of the positive electrode active material layer is improved. Note that when a composite of the lithium transition metal composite oxide and another material is used, the average particle size of the composite is taken as the average particle size of the lithium transition metal composite oxide. "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 accordance with JIS-Z-8825 (2013) for a diluted solution of particles diluted with a solvent.

[0040] To obtain powders with a predetermined particle size, grinders, classifiers, etc. are used. Grinding methods include, for example, methods using a mortar, ball mill, sand mill, vibration ball mill, planetary ball mill, jet mill, counter jet mill, swirling airflow jet mill, or sieves. Wet grinding in the presence of water or an organic solvent such as hexane can also be used during grinding. As classification methods, sieves, air classifiers, etc. are used as needed for both dry and wet methods.

[0041] The content of the lithium transition metal composite oxide in the positive electrode active material layer is preferably 60% by mass or more and 100% by mass or less, more preferably 80% by mass or more and 99.5% by mass or less, and even more preferably 90% by mass or more and 99% by mass or less. The content of the lithium transition metal composite oxide may be 95% by mass or more, 97% by mass or more, or 98% by mass or more. By setting the content of the lithium transition metal composite oxide within the above range, it is possible to achieve both a high energy density per mass and manufacturability of the nonaqueous electrolyte storage element.

[0042] The positive electrode active material layer may further contain a positive electrode active material other than the lithium transition metal composite oxide having an α-NaFeO2 crystal structure. As the other positive electrode active material, various conventionally known positive electrode active materials can be used. However, the content of the lithium transition metal composite oxide relative to the total positive electrode active material contained in the positive electrode active material layer is preferably 90 mass% or more, more preferably 99 mass% or more, and even more preferably 100 mass%. Thus, by using the lithium transition metal composite oxide as the main positive electrode active material, it is possible to further increase the energy density per mass of the nonaqueous electrolyte storage element.

[0043] The content of all the positive electrode active materials in the positive electrode active material layer is preferably 60% by mass or more and 100% by mass or less, more preferably 80% by mass or more and 99.5% by mass or less, and even more preferably 90% by mass or more and 99% by mass or less. The content of all the positive electrode active materials may be 95% by mass or more, 97% by mass or more, or 98% by mass or more. By setting the content of all the positive electrode active materials within the above range, it is possible to achieve both a high energy density per mass of the nonaqueous electrolyte storage element and manufacturability.

[0044] 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 (CNTs), and fullerenes. The conductive agent may be in the form of powder or fiber. As the conductive agent, one of these materials may be used alone, or two or more may be mixed. These materials may also be used in combination. For example, a composite of carbon black and CNT may be used. Among these, carbon black and CNT are preferred from the viewpoints of electronic conductivity and coatability, and a mixture of acetylene black and CNT is more preferred.

[0045] 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 0.3% by mass to 9% by mass. The content of the conductive agent may be 5% by mass or less, 3% by mass or less, 2% by mass or less, or 1% by mass or less. By setting the content of the conductive agent within the above range, the energy density per mass of the nonaqueous electrolyte storage element can be increased.

[0046] 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.

[0047] The content of the binder in the positive electrode active material layer is preferably 0.5% by mass to 10% by mass, more preferably 1% by mass to 9% by mass. The content of the binder may be 5% by mass or less, 3% by mass or less, or 2% by mass or less. By setting the content of the binder within the above range, it is possible to stably maintain the lithium transition metal composite oxide having an α-NaFeO2 crystal structure.

[0048] Examples of thickeners include polysaccharide polymers such as carboxymethyl cellulose (CMC) and methyl cellulose. When the thickener has a functional group that reacts with lithium or the like, the functional group may be deactivated in advance by methylation or the like. In one embodiment of the present invention, the positive electrode active material layer may not contain a thickener.

[0049] The filler is not particularly limited. Examples of the filler include polyolefins such as polypropylene and polyethylene, inorganic oxides such as silicon dioxide, alumina, titanium dioxide, calcium oxide, strontium oxide, barium oxide, magnesium oxide, and aluminosilicates, hydroxides such as magnesium hydroxide, calcium hydroxide, and aluminum hydroxide, carbonates such as calcium carbonate, sparingly soluble ionic crystals such as calcium fluoride, barium fluoride, and barium sulfate, nitrides such as aluminum nitride and silicon nitride, mineral-derived substances such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica, and artificial products thereof. In one embodiment of the present invention, the positive electrode active material layer may not contain a filler.

[0050] The positive electrode active material layer may contain typical non-metallic elements such as B, N, P, F, Cl, Br, and I; typical metallic elements such as Li, Na, Mg, Al, K, Ca, Zn, Ga, Ge, Sn, Sr, and Ba; and transition metal elements such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Zr, Nb, and W as components other than the lithium transition metal composite oxide having an α-NaFeO2-type crystal structure, other positive electrode active materials, conductive agents, binders, thickeners, and fillers.

[0051] The lower limit of the mass per area of ​​the positive electrode active material layer is 3.5 g / 100 cm 2 and 4.0g / 100cm 2 is preferred, 4.5g / 100cm 2 or 5.0g / 100cm 2 On the other hand, the upper limit of the mass per area of ​​the positive electrode active material layer is 10.5 g / 100 cm. 2 and 10.0g / 100cm 2 is preferred, and 9.0 g / 100 cm 2 or 8.0g / 100cm 2 In some cases, it is more preferable that the mass per area of ​​the positive electrode active material layer is within the above range. By setting the mass per area of ​​the positive electrode active material layer within the above range, the energy density per mass of the nonaqueous electrolyte storage element can be increased. In addition, in order to make it easier to handle the positive electrode during production, the upper limit of the mass per area of ​​the positive electrode active material layer is set to 7.0 g / 100 cm. 2 is preferred, and 6.0 g / 100 cm 2 The mass per area of ​​the positive electrode active material layer may be equal to or greater than any of the above-mentioned lower limits and equal to or less than any of the above-mentioned upper limits.

[0052] The upper limit of the porosity of the positive electrode active material layer is preferably 35%, more preferably 30%, even more preferably 28%, and even more preferably 25%. The lower limit of the porosity of the positive electrode active material layer is preferably 15%, more preferably 18%, and even more preferably 20%. By setting the porosity of the positive electrode active material layer within the above range, the energy density per mass of the nonaqueous electrolyte storage element can be further increased. The porosity of the positive electrode active material layer may be equal to or greater than any of the above lower limits and equal to or less than any of the above upper limits.

[0053] The porosity of the positive electrode active material layer can be adjusted by the average particle size of the lithium transition metal composite oxide, the content of the components constituting the positive electrode active material layer, the strength of the press used when producing the positive electrode, and the like.

[0054] (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.

[0055] The negative electrode substrate is conductive. Metals such as copper, nickel, stainless steel, nickel-plated steel, and aluminum, alloys thereof, and carbonaceous materials are used as the material for the negative electrode substrate. Among these, copper or copper alloys are preferred. Examples of the negative electrode substrate include foils, vapor-deposited films, meshes, and porous materials, with foils being preferred from the viewpoint of cost. Therefore, copper foil or copper alloy foil is preferred as the negative electrode substrate. Examples of copper foil include rolled copper foil and electrolytic copper foil.

[0056] The average thickness of the negative electrode substrate is preferably 2 μm to 35 μm, more preferably 3 μm to 30 μm, even more preferably 4 μm to 25 μm, and particularly preferably 5 μm to 20 μm. By setting the average thickness of the negative electrode substrate within the above range, the strength of the negative electrode substrate can be increased while also increasing the energy density per mass of the nonaqueous electrolyte storage element.

[0057] The negative electrode active material layer is mainly composed of a silicon-based active material. The silicon-based active material is the negative electrode active material. The negative electrode active material layer may contain optional components such as other negative electrode active materials besides the silicon-based active material, a conductive agent, a binder, a thickener, and a filler, as needed. The optional components such as the conductive agent, binder, thickener, and filler can be selected from the materials exemplified for the positive electrode above.

[0058] 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 silicon-based active material, other negative electrode active materials, conductive agents, binders, thickeners, and fillers.

[0059] A silicon-based active material is an active material containing silicon element. Examples of the silicon-based active material include a simple substance of silicon element or a compound containing silicon element. Examples of the compound containing silicon element include silicon oxide (SiO x : 0 < x < 2, preferably 0.8 ≤ x ≤ 1.2), silicon nitride, silicon carbide, metal silicon compound, etc. Examples of the metal silicon compound include a compound containing aluminum element, tin element, zinc element, nickel element, copper element, titanium element, vanadium element, magnesium element, etc. and silicon element. In addition, the silicon-based active material may be a composite material such as a SiO / Si / SiO2 composite material. As the silicon-based active material, one in which an element serving as a charge transport ion is pre-doped can also be used. That is, for example, the silicon-based active material may further contain lithium element. One kind or two or more kinds of the silicon-based active materials can be mixed and used. Among the silicon-based active materials, silicon oxide is preferred.

[0060] The silicon-based active material may have a surface coated with a conductive material such as a carbon material. By using the silicon-based active material in such a form, the conductivity of the negative electrode active material layer can be enhanced. When the silicon-based active material is in the form of particles or the like coated with a conductive material, the mass ratio of the conductive material to the total amount of the silicon-based active material and the conductive material coating it is, for example, preferably 1% by mass or more and 10% by mass or less, and more preferably 2% by mass or more and 5% by mass or less.

[0061] The silicon-based active material is usually particles (powder). The average particle size of the silicon-based active material can be, for example, 1 nm or more and 100 μm or less, and may be 1 μm or more and 30 μm or less, or may be 5 μm or more and 25 μm or less. By setting the average particle size of the silicon-based active material to be not less than the above lower limit, the production or handling of the silicon-based active material becomes easy. By setting the average particle size of the silicon-based active material to be not more than the above upper limit, the conductivity of the negative electrode active material layer is improved. In order to obtain the silicon-based active material with a predetermined particle size, a pulverizer, a classifier, etc. are used. The pulverization method and the classification method can be selected, for example, from the methods exemplified for the positive electrode above.

[0062] The silicon content in the negative electrode active material layer is preferably 30% by mass to 70% by mass, more preferably 35% by mass to 65% by mass, and even more preferably 40% by mass to 60% by mass. By setting the silicon content within this range, the amount of electricity reversibly charged and discharged by the negative electrode increases, allowing the mass of the negative electrode to be reduced, thereby further increasing the energy density per mass of the nonaqueous electrolyte storage element. In addition, electrical isolation of the silicon-based active material due to charge-discharge cycles can be suppressed.

[0063] The negative electrode active material layer may further contain a negative electrode active material other than the silicon-based active material. Examples of the other negative electrode active material include various conventionally known negative electrode active materials, such as carbon materials (graphite, non-graphitic carbon, etc.). However, the content of the silicon-based active material relative to the total negative electrode active materials contained in the negative electrode active material layer is preferably 90% by mass or more, more preferably 99% by mass or more, and even more preferably 100% by mass. Thus, by primarily using a silicon-based active material as the negative electrode active material, it is possible to further increase the energy density per mass of the nonaqueous electrolyte storage element and further reduce gas generation associated with charge-discharge cycles at high temperatures.

[0064] The content of all negative electrode active materials in the negative electrode active material layer is preferably 60% by mass to 99% by mass, more preferably 70% by mass to 98% by mass, and even more preferably 80% by mass to 97% by mass. The content of all the negative electrode active materials may be 85% by mass to 95% by mass, or may be 88% by mass to 92% by mass. By setting the content of all the negative electrode active materials within the above range, both a high energy density per mass and manufacturability of the nonaqueous electrolyte storage element can be achieved.

[0065] The conductive agent in the negative electrode active material layer may be the same as the conductive agent in the positive electrode active material layer. In the negative electrode active material layer, it is preferable to use CNT as the conductive agent, and CNT may be used alone.

[0066] The content of the conductive agent in the negative electrode active material layer is preferably 0.1% by mass to 10% by mass, more preferably 0.3% by mass to 9% by mass. The content of the conductive agent may be 5% by mass or less, 3% by mass or less, 2% by mass or less, or 1% by mass or less. By setting the content of the conductive agent within the above range, the energy density per mass of the nonaqueous electrolyte storage element can be increased.

[0067] The binder in the negative electrode active material layer can be the same as that in the positive electrode active material layer. Among these, it is preferable to use styrene butadiene rubber (SBR) as the binder. The content of the binder in the negative electrode active material layer is preferably 1% by mass or more and 15% by mass or less, more preferably 3% by mass or more and 12% by mass or less, and even more preferably 4% by mass or more and 10% by mass or less. By setting the content of the binder in the negative electrode active material layer within the above range, it is possible to stably hold the silicon-based active material, etc.

[0068] As a combination of the binder and the conductive agent in the negative electrode active material layer, it is preferable to use styrene butadiene rubber (SBR) as the binder and CNT as the conductive agent.

[0069] The content of the thickener in the negative electrode active material layer is preferably 0.3% by mass or more and 4% by mass or less, and more preferably 0.5% by mass or more and 2% by mass or less.

[0070] In one embodiment of the present invention, the negative electrode active material layer may not contain a filler.

[0071] The lower limit of the mass per area of ​​the negative electrode active material layer is 0.5 g / 100 cm 2 is preferred, and 0.6g / 100cm 2 is more preferable, and 0.7 g / 100 cm 2 , 0.8g / 100cm 2 , 0.9g / 100cm 2 or 1.0g / 100cm 2 On the other hand, the upper limit of the mass per area of ​​the negative electrode active material layer is 2.0 g / 100 cm. 2is preferred, and 1.8g / 100cm 2 is more preferable, and 1.7 g / 100 cm 2 , 1.6g / 100cm 2 , 1.4g / 100cm 2 , 1.2g / 100cm 2 or 1.0g / 100cm 2 is even more preferable in some cases. By setting the mass per area of ​​the negative electrode active material layer within the above range, the energy density per mass of the nonaqueous electrolyte storage element can be further increased. The mass per area of ​​the negative electrode active material layer may be equal to or greater than any of the above-mentioned lower limits and equal to or less than any of the above-mentioned upper limits.

[0072] (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. The substrate layer may contain heat-resistant particles.

[0073] The heat-resistant particles contained in the heat-resistant layer or base material layer preferably exhibit a mass loss of 5% or less when heated from room temperature to 500°C in an air atmosphere at 1 atmosphere pressure, and more preferably exhibit a mass loss of 5% or less when heated from room temperature to 800°C. Examples of materials exhibiting a mass loss of a predetermined value or less include inorganic compounds. Examples of inorganic compounds include oxides such as iron oxide, silicon oxide, aluminum oxide, titanium oxide, zirconium oxide, calcium oxide, strontium oxide, barium oxide, magnesium oxide, and aluminosilicates; nitrides such as aluminum nitride and silicon nitride; carbonates such as calcium carbonate; sulfates such as barium sulfate; sparingly soluble ionic crystals such as calcium fluoride, barium fluoride, and barium titanate; covalently bonded crystals such as silicon and diamond; and 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 safety of the nonaqueous electrolyte electricity storage element.

[0074] The porosity of the separator is preferably 80% by volume or less from the viewpoint of strength, and 20% by volume or more from the viewpoint of discharge performance. Here, "porosity" refers to a volume-based value measured with a mercury porosimeter.

[0075] 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.

[0076] (non-aqueous electrolyte) The nonaqueous electrolyte can be appropriately selected from known nonaqueous electrolytes. The nonaqueous electrolyte may be a nonaqueous electrolytic solution. The nonaqueous electrolytic solution contains a nonaqueous solvent and an electrolyte salt dissolved in the nonaqueous solvent.

[0077] 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.

[0078] Examples of cyclic carbonates include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinylene carbonate (VC), vinylethylene carbonate (VEC), chloroethylene carbonate, fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), styrene carbonate, 1-phenylvinylene carbonate, and 1,2-diphenylvinylene carbonate.

[0079] As the cyclic carbonate, a fluorinated cyclic carbonate is preferred. Because fluorinated cyclic carbonates have high oxidation resistance, they are suitable as nonaqueous solvents for nonaqueous electrolyte energy storage elements that use a lithium transition metal composite oxide having an α-NaFeO2 crystal structure and in which the positive electrode reaches a high potential. Furthermore, when the nonaqueous electrolyte contains a fluorinated cyclic carbonate, a good coating is formed on the surface of the negative electrode active material, resulting in good charge / discharge performance. Meanwhile, as described above, fluorinated cyclic carbonates are nonaqueous solvents that are prone to gas generation during charge / discharge cycles at high temperatures. Therefore, when one embodiment of the present invention is a nonaqueous electrolyte energy storage element that includes a nonaqueous electrolyte containing a fluorinated cyclic carbonate, the amount of gas generation during charge / discharge cycles at high temperatures is reduced, and the decrease in discharge capacity retention rate due to gas generation is significantly suppressed.

[0080] The fluorinated cyclic carbonate refers to a compound in which some or all of the hydrogen atoms of a cyclic carbonate have been substituted with fluorine atoms. Examples of the fluorinated cyclic carbonate include the above-mentioned fluorinated ethylene carbonates such as FEC and DFEC, fluorinated propylene carbonates such as 3,3,3-trifluoropropylene carbonate, and fluorinated butylene carbonate. Among these, fluorinated ethylene carbonate is preferred, and FEC is more preferred, from the viewpoint of oxidation resistance and the like.

[0081] The content of the fluorinated cyclic carbonate relative to the nonaqueous solvent is preferably 1% by volume to 70% by volume, more preferably 5% by volume to 60% by volume, even more preferably 10% by volume to 50% by volume, and even more preferably 15% by volume to 40% by volume. By setting the content of the fluorinated cyclic carbonate to the above-mentioned lower limit or higher, the above-mentioned effects of the fluorinated cyclic carbonate are particularly fully exhibited. On the other hand, by setting the content of the fluorinated cyclic carbonate to the above-mentioned upper limit or lower, the amount of gas generated during charge-discharge cycles at high temperatures can be further reduced, and a decrease in the discharge capacity retention rate due to gas generation can be suppressed.

[0082] Examples of chain carbonates include diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diphenyl carbonate, 2,2,2-trifluoroethyl methyl carbonate (TFEMC), and bis(trifluoroethyl) carbonate. Among these, DEC, EMC, and TFEMC are preferred. The chain carbonate may be a fluorinated chain carbonate or a non-fluorinated chain carbonate (for example, an unsubstituted chain carbonate in which hydrogen atoms are not substituted with halogen or the like).

[0083] As the non-aqueous solvent, it is preferable to use at least one of a cyclic carbonate and a chain carbonate, and it is more preferable to use a combination of a cyclic carbonate and a chain carbonate. The use of a cyclic carbonate can promote dissociation of the electrolyte salt and improve the ionic conductivity of the non-aqueous electrolyte. The use of a chain carbonate can reduce the viscosity of the non-aqueous electrolyte. When a cyclic carbonate and a chain carbonate are used in combination, the volume ratio of the cyclic carbonate to the chain carbonate (cyclic carbonate:chain carbonate) is preferably in the range of 5:95 to 70:30, and more preferably in the range of 15:85 to 40:60. The content of carbonate (the total of the cyclic carbonate and the chain carbonate) in the non-aqueous solvent is preferably 90% by volume or more and 100% by volume or less, and more preferably 99% by volume or more and 100% by volume or less.

[0084] The electrolyte salt can usually be appropriately selected from known lithium salts.

[0085] Examples of lithium salts include inorganic lithium salts such as LiPF6, LiPO2F2, LiBF4, LiClO4, and LiN(SO2F)2, lithium oxalates such as lithium bis(oxalate)borate (LiBOB), lithium difluorooxalateborate (LiFOB), and lithium bis(oxalate)difluorophosphate (LiFOP), and lithium salts having a halogenated hydrocarbon group such as LiSO3CF3, LiN(SO2CF3)2, LiN(SO2C2F5)2, LiN(SO2CF3)(SO2C4F9), LiC(SO2CF3)3, and LiC(SO2C2F5)3. Among these, inorganic lithium salts are preferred, and LiPF6 is more preferred.

[0086] The content of electrolyte salt in the non-aqueous electrolyte is 0.1 mol / dm at 20°C and 1 atmosphere. 3 More than 2.5mol / dm 3 It is preferable that the value is 0.3 mol / dm or less. 3 More than 2.0mol / dm 3 It is more preferable that it is 0.5 mol / dm or less.3 More than 1.7mol / dm 3 It is more preferable that it is 0.7 mol / dm or less. 3 More than 1.6mol / dm 3 By setting the content of the electrolyte salt within the above range, the ionic conductivity of the non-aqueous electrolyte can be increased.

[0087] The non-aqueous electrolyte may contain additives in addition to the non-aqueous solvent and the electrolyte salt. Examples of additives include oxalates such as lithium bis(oxalate)borate (LiBOB), lithium difluorooxalateborate (LiFOB), and lithium bis(oxalate)difluorophosphate (LiFOP); imide salts such as lithium bis(fluorosulfonyl)imide (LiFSI); aromatic compounds such as biphenyl, alkylbiphenyl, terphenyl, partially hydrogenated terphenyl, cyclohexylbenzene, t-butylbenzene, t-amylbenzene, diphenyl ether, and dibenzofuran; partial halides of the above aromatic compounds such as 2-fluorobiphenyl, o-cyclohexylfluorobenzene, and p-cyclohexylfluorobenzene; halogenated anisole compounds such as 2,4-difluoroanisole, 2,5-difluoroanisole, 2,6-difluoroanisole, and 3,5-difluoroanisole; vinylene carbonate, methylvinylene carbonate, ethylvinylene carbonate, succinic anhydride, glutaric anhydride, and 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.

[0088] The content of the additive contained in the non-aqueous electrolyte is preferably 0.01% by mass to 20% by mass, more preferably 0.1% by mass to 10% by mass, even more preferably 0.2% by mass to 7% by mass, and particularly preferably 0.3% by mass to 5% 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 capacity retention or cycle performance after high-temperature storage, and further improve safety.

[0089] The amount (volume) of the nonaqueous electrolyte relative to the total pore volume of the positive electrode, negative electrode, and separator is preferably 1.0 to 1.5 times, more preferably 1.0 to 1.2 times, and even more preferably 1.0 to 1.1 times. By setting the amount of nonaqueous electrolyte within the above range, it is possible to further increase the energy density per mass of the nonaqueous electrolyte storage element. The amount of nonaqueous electrolyte and the total pore volumes of the positive electrode, negative electrode, and separator can be determined by the following method.

[0090] (amount of non-aqueous electrolyte) The mass W1 of the nonaqueous electrolyte storage element is measured. The nonaqueous electrolyte storage element is disassembled, and each component is washed with DMC and then dried under reduced pressure at room temperature for 24 hours. The mass W2 of all components after drying is measured. Note that "all components" refers to all components constituting the nonaqueous electrolyte storage element other than the nonaqueous electrolyte. The mass W3 of the nonaqueous electrolyte is calculated from the difference (W1 - W2) between the mass W1 of the nonaqueous electrolyte storage element and the mass W2 of all components after drying. The amount (volume) of the nonaqueous electrolyte is calculated from the mass W3 of the nonaqueous electrolyte and the density of the nonaqueous electrolyte. Note that the density of the nonaqueous electrolyte is a value at 20°C.

[0091] (Total pore volume of positive electrode) The positive electrode to be measured is prepared according to the following procedure. The nonaqueous electrolyte storage element is discharged at a constant current of 0.05 C until the discharge end voltage during normal use is reached. The nonaqueous electrolyte storage element in this discharged state is disassembled, and the positive electrode is removed and washed with DMC, followed by drying under reduced pressure at room temperature for 24 hours. The total pore volume of a positive electrode can be calculated as the sum of the total pore volumes of the components constituting the positive electrode. For example, when a positive electrode is composed of a non-porous, i.e., pore-free, positive electrode substrate and a positive electrode active material layer, the total pore volume of the positive electrode is equal to the total pore volume of the positive electrode active material layer. The total pore volume of the positive electrode active material layer is calculated from the difference (V1 - V2) between the apparent volume V1 of the positive electrode active material layer and the sum V2 of the actual volumes of the materials constituting the positive electrode active material layer. The sum V2 of the actual volumes of the materials constituting the positive electrode active material layer can be calculated from the content of each material constituting the positive electrode active material layer in the positive electrode active material layer and the true density of each material constituting the positive electrode active material layer.

[0092] (Total pore volume of negative electrode) The total pore volume of the negative electrode can be determined in the same manner as the total pore volume of the positive electrode.

[0093] (total pore volume of separator) The nonaqueous electrolyte storage element is disassembled, and the removed separator is washed with DMC and then dried under reduced pressure at room temperature for 24 hours. The length, average thickness, and width of the dried separator are measured, and the apparent volume (volume including pores) of the separator is calculated. In addition, the volumetric porosity of the dried separator is measured by mercury intrusion porosimetry. The total pore volume of the separator is calculated as the product of the apparent volume and the porosity of the separator.

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

[0095] The solid electrolyte can be selected from any material that has lithium ion conductivity and is solid at room temperature (e.g., 15° C. to 25° C.) Examples of the solid electrolyte include sulfide solid electrolytes, oxide solid electrolytes, nitride solid electrolytes, and polymer solid electrolytes.

[0096] Examples of sulfide solid electrolytes include Li2S-P2S5, LiI-Li2S-P2S5, and Li 10 GeP2S 12 etc.

[0097] (shape, etc.) The shape of the nonaqueous electrolyte storage element of this embodiment is not particularly limited, and examples thereof include cylindrical batteries, prismatic batteries, flat batteries, coin batteries, and button batteries.

[0098] FIG. 1 shows a nonaqueous electrolyte storage element 1 as an example of a prismatic battery. The figure is a see-through view of the inside of the container. An electrode assembly 2 having a positive electrode and a negative electrode wound with a separator sandwiched between them is housed in a prismatic container 3. The positive electrode is electrically connected to a positive electrode terminal 4 via a positive electrode lead 41. The negative electrode is electrically connected to a negative electrode terminal 5 via a negative electrode lead 51.

[0099] The lower limit of the energy density per mass of the nonaqueous electrolyte storage element of this embodiment during normal use is preferably 350 Wh / kg, more preferably 380 Wh / kg, and even more preferably 400 Wh / kg. The nonaqueous electrolyte storage element is preferably used in an embodiment having such a high energy density. The upper limit of the energy density may be 600 Wh / kg, 500 Wh / kg, or 450 Wh / kg. The energy density may be equal to or greater than any of the lower limits and equal to or less than any of the upper limits. When specifying the energy density and the discharge capacity described below, if the discharge current during normal use is not determined, the value is taken to be the value when the discharge current is 0.1 C.

[0100] The discharge capacity per area of ​​the positive electrode active material layer during normal use of the nonaqueous electrolyte storage element of this embodiment is 5 mAh / cm 2 More than 14mAh / cm 2 Less than 6mAh / cm is preferred 2 More than 13mAh / cm 2 The following is more preferable: When the discharge capacity per area of ​​the positive electrode active material layer is within the above range, the energy density per mass of the nonaqueous electrolyte storage element can be further increased.

[0101] The lower limit of the end-of-charge voltage during normal use of the nonaqueous electrolyte storage element of this embodiment is preferably 4.10 V, more preferably 4.25 V, and even more preferably 4.30 V. When the end-of-charge voltage is equal to or greater than the above lower limit, the energy density per mass of the nonaqueous electrolyte storage element can be further increased. The upper limit of the end-of-charge voltage is preferably 4.60 V, more preferably 4.40 V, and may be 4.35 V or 4.30 V. The end-of-charge voltage may be equal to or greater than any of the above lower limits and equal to or less than any of the above upper limits.

[0102] <Electricity storage device> The nonaqueous electrolyte energy storage element of this embodiment can be mounted as an energy storage unit (battery module) comprising a plurality of nonaqueous electrolyte energy storage elements in automotive power sources such as electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs), power sources for electronic devices such as personal computers and communication terminals, or power storage power sources, etc. In this case, the technology of the present invention may be applied to at least one nonaqueous electrolyte energy storage element included in the energy storage unit.

[0103] 2 shows an example of an electricity storage device 30 in which electricity storage units 20, each of which is an assembly of two or more electrically connected nonaqueous electrolyte electricity storage elements 1, are further assembled. The electricity storage device 30 may include a bus bar (not shown) that electrically connects the two or more nonaqueous electrolyte electricity storage elements, a bus bar (not shown) that electrically connects the two or more electricity storage units 20, etc. The electricity storage unit 20 or the electricity storage device 30 may include a status monitoring device (not shown) that monitors the status of the one or more nonaqueous electrolyte electricity storage elements.

[0104] <Method of manufacturing nonaqueous electrolyte energy storage element> The method for manufacturing the nonaqueous electrolyte storage element of this embodiment can be appropriately selected from known methods. The manufacturing method includes, for example, preparing an electrode assembly, preparing a nonaqueous electrolyte, and housing the electrode assembly and the nonaqueous electrolyte in a container. Preparing the electrode assembly includes preparing a positive electrode and a negative electrode, and forming the electrode assembly by stacking or winding the positive electrode and the negative electrode with a separator interposed therebetween.

[0105] The method for placing the non-aqueous electrolyte in the container can be appropriately selected from known methods. For example, when a non-aqueous electrolyte solution is used as the non-aqueous electrolyte, the non-aqueous electrolyte solution may be poured into the container through an inlet formed in the container, and then the inlet may be sealed.

[0106] <Other embodiments> The nonaqueous electrolyte storage element of the present invention is not limited to the above-described embodiments, and various modifications may be made without departing from the spirit of the present invention. For example, the configuration of one embodiment may be added to the configuration of another embodiment, or part of the configuration of one embodiment may be replaced with the configuration of another embodiment or well-known technology. Furthermore, part of the configuration of one embodiment may be deleted. Also, well-known technology may be added to the configuration of one embodiment.

[0107] In the above embodiment, the nonaqueous electrolyte storage element is used as a chargeable and dischargeable nonaqueous electrolyte secondary battery (lithium ion secondary battery), but the type, shape, size, capacity, etc. of the nonaqueous electrolyte storage element are arbitrary. The present invention can also be applied to various secondary batteries.

[0108] 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. [Example]

[0109] 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.

[0110] The positive electrode active materials and negative electrode active materials used in the examples and comparative examples are shown below. (Cathode active material) NCM622: LiNi with α-NaFeO2 type crystal structure 0.6 Co 0.2 Mn 0.2 O2 NCM811: LiNi with α-NaFeO2 type crystal structure 0.8 Co 0.1 Mn 0.1 O2 (Negative electrode active material) SiO: Silicon oxide (SiO), a silicon-based active material Gr: Graphite

[0111] [Example 1] (Preparation of positive electrode) A positive electrode mixture paste containing NCM811 as a positive electrode active material, acetylene black (AB) and single-walled carbon nanotubes (SWCNT) as conductive agents, and polyvinylidene fluoride (PVDF) as a binder, with N-methylpyrrolidone (NMP) as a dispersion medium, was prepared. The mass ratio of the positive electrode active material, AB, SWCNT, and binder was 98.52:0.40:0.08:1.00 in terms of solid content. The positive electrode mixture paste was applied to both sides of an aluminum foil with an average thickness of 15 μm as a positive electrode substrate, dried, and pressed to form a positive electrode active material layer. This resulted in a positive electrode in which a positive electrode active material layer was laminated on the positive electrode substrate. The mass per area of ​​the positive electrode active material layer was 4.5 g / 100 cm. 2 The porosity was 25%.

[0112] (Preparation of negative electrode) Silicon oxide (SiO) doped with lithium was used as the negative electrode active material. A negative electrode mixture paste was prepared containing the above negative electrode active material, single-walled carbon nanotubes (SWCNT) as a conductive agent, styrene butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener, with water as a dispersion medium. The mass ratio of the negative electrode active material, conductive agent, binder, and thickener was 89.7:0.3:8.8:1.2 in terms of solid content. The negative electrode mixture paste was applied to one or both sides of a copper foil with an average thickness of 6 μm as a negative electrode substrate, and then dried to form a negative electrode active material layer. This resulted in a negative electrode in which a negative electrode active material layer was laminated on one or both sides of the negative electrode substrate. The mass per area of ​​the negative electrode active material layer was 0.88 g / 100 cm. 2 The silicon content in the negative electrode active material layer was 48.3 mass %.

[0113] (Preparation of non-aqueous electrolyte) A non-aqueous solvent consisting of a mixture of fluoroethylene carbonate (FEC) and diethyl carbonate (DEC) in a volume ratio of 15:85 was added to the lithium salt LiPF6 at a concentration of 1.5 mol / dm 3 Into the obtained solution, 1,3-propene sultone (PRS) was dissolved at a content of 2 mass % to prepare a non-aqueous electrolyte.

[0114] (Assembly of non-aqueous electrolyte energy storage element) An electrode assembly was produced by stacking the positive electrode and the negative electrode with a polyethylene microporous membrane separator interposed therebetween. The two outermost layers of the electrode assembly were negative electrodes, each having a negative electrode substrate with a negative electrode active material layer stacked on one side thereof, with the surface of the negative electrode substrate without the negative electrode active material layer facing outward. The electrode assembly was placed in a container made of a metal-resin composite film, and the nonaqueous electrolyte was poured into the container, which was then sealed by heat welding to produce the nonaqueous electrolyte storage element of Example 1. In the nonaqueous electrolyte storage element of Example 1, the amount of lithium ions extracted from the positive electrode active material was 236 mAh / g.

[0115] [Comparative Examples 1 to 6] Each of the nonaqueous electrolyte storage elements of Comparative Examples 1 to 6 was obtained in the same manner as in Example 1, except that the type of positive electrode active material, the mass per area of ​​the positive electrode active material layer, the type of negative electrode active material, the mass per area of ​​the negative electrode active material layer, the amount of lithium ions extracted from the positive electrode active material, and the nonaqueous solvent composition of the nonaqueous electrolyte were as shown in Table 1. When graphite (Gr) was used as the negative electrode active material, a negative electrode prepared by the following method was used. A negative electrode mixture paste was prepared containing graphite (Gr) as the negative electrode active material, SBR as the binder, and CMC as the thickener, with water as the dispersion medium. The mass ratio of the negative electrode active material to the binder and the thickener was 96.7:2.1:1.2 in terms of solid content. The negative electrode mixture paste was applied to one or both sides of a copper foil with an average thickness of 6 μm as a negative electrode substrate, dried, and pressed to form a negative electrode active material layer. This resulted in a negative electrode in which a negative electrode active material layer was laminated on a negative electrode substrate.

[0116] [evaluation] (Chemical) The assembled nonaqueous electrolyte storage elements of Example 1 and Comparative Examples 1 to 6 were subjected to the following chemical formation. Constant-current charging was performed at 25°C with a charging current of 0.05C for a charging time of 6 hours. This was followed by a rest period of 48 hours. Next, constant-current / constant-voltage charging was performed with a charging current of 0.05C and a charge cut-off voltage listed in Table 1. The charge cut-off condition was when the charging current decayed to 0.04C. Next, constant-current discharging was performed with a discharging current of 0.1C and a discharge cut-off voltage listed in Table 1. A 10-minute rest period was provided after each charge and discharge. (Initial charge / discharge) The nonaqueous electrolyte storage elements of Example 1 and Comparative Examples 1 to 6 after chemical formation were subjected to the following initial charge / discharge. Constant-current, constant-voltage charging was performed at 25°C with a charging current of 0.2 C and a charge cut-off voltage shown in Table 1. The charge cut-off condition was when the charging current decayed to 0.05 C. Subsequently, constant-current discharging was performed with a discharging current of 0.1 C and a discharge cut-off voltage shown in Table 1. A 10-minute rest period was provided after each charge and discharge.

[0117] (Energy density per mass of nonaqueous electrolyte energy storage element) The energy density (energy density per mass of the nonaqueous electrolyte electricity storage element) was calculated based on the discharge capacity in the initial charge / discharge. The results are shown in Table 1.

[0118] (Capacity confirmation test at high temperature) For each of the nonaqueous electrolyte storage elements of Example 1 and Comparative Examples 1 to 4 and 6 that had undergone the above initial charge / discharge, a high-temperature capacity confirmation test was conducted as follows. Constant-current, constant-voltage charging was performed at 45°C with a charging current of 0.2 C and a charge cut-off voltage listed in Table 1. The charge cut-off condition was when the charging current decayed to 0.05 C. Subsequently, constant-current discharging was performed with a discharging current of 0.1 C and a discharge cut-off voltage listed in Table 1. A 10-minute rest period was provided after each charge and discharge. (High temperature charge / discharge cycle test) For each of the nonaqueous electrolyte storage elements of Example 1 and Comparative Examples 1 to 4 and 6 that underwent the high-temperature capacity confirmation test, a high-temperature charge-discharge cycle test was conducted as follows. Constant-current, constant-voltage charging was performed at 45°C with a charge current of 0.2 C and a charge cut-off voltage listed in Table 1. The charge cut-off condition was when the charge current decayed to 0.05 C. Subsequently, constant-current discharging was performed with a discharge current of 0.33 C and a discharge cut-off voltage listed in Table 1. A 10-minute rest period was provided after each charge and discharge. This charge-discharge cycle was repeated 200 times. (Capacity confirmation test after 200 cycles at high temperature) For each of the nonaqueous electrolyte storage elements of Example 1 and Comparative Examples 1 to 4 and 6 that underwent the high-temperature charge-discharge cycle test, a high-temperature capacity confirmation test was conducted as follows. Constant-current, constant-voltage charging was performed at 45°C with a charging current of 0.2 C and a cut-off voltage shown in Table 1. The charge cut-off condition was when the charging current decayed to 0.05 C. Subsequently, constant-current discharging was performed with a discharging current of 0.1 C and a cut-off voltage shown in Table 1. A 10-minute rest period was provided after each charge and discharge.

[0119] (gas generation amount) The amount of gas generated in each nonaqueous electrolyte storage element during the high-temperature charge-discharge cycle test was determined. Specifically, the volume of the nonaqueous electrolyte storage element at 25°C before the high-temperature capacity confirmation test and after 200 cycles at high temperature was measured by Archimedes' method, and the increase in volume after 200 cycles relative to the volume before the high-temperature charge-discharge cycle test was determined, and this increase in volume was taken as the amount of gas generated. The results are shown in Table 1. Note that "-" in the table means that the measurement was not performed.

[0120] (Discharge capacity maintenance rate) For each nonaqueous electrolyte storage element, the percentage of the discharge capacity in the capacity confirmation test at high temperature after 200 cycles to the discharge capacity in the capacity confirmation test at high temperature before the charge-discharge cycle test at high temperature was calculated as the discharge capacity retention rate. The results are shown in Table 1. In the table, "-" means that the measurement was not performed.

[0121] (High rate discharge performance: discharge capacity ratio) The discharge capacity at the 200th cycle in the charge-discharge cycle test at high temperature was defined as "0.33 C discharge capacity." The discharge capacity in the capacity confirmation test after 200 cycles at high temperature was defined as "0.1 C discharge capacity." From the 0.33C discharge capacity and the 0.1C discharge capacity, the percentage of the 0.33C discharge capacity relative to the 0.1C discharge capacity was calculated as a discharge capacity ratio, which is an index of high-rate discharge performance. The results are shown in Table 1. Note that "-" in the table means that the measurement was not performed.

[0122] [Table 1]

[0123] The results in Table 1 confirm the following: A battery was equipped with a positive electrode having a positive electrode active material layer and a negative electrode having a negative electrode active material layer, the positive electrode active material layer mainly comprising a lithium transition metal composite oxide having an α-NaFeO2 crystal structure, and the mass per area of ​​the positive electrode active material layer was 3.5 g / 100 cm 2 More than 10.5g / 100cm 2The nonaqueous electrolyte storage element of Example 1, in which the content of nickel relative to metal elements other than lithium in the lithium transition metal composite oxide was 65 mol % or more and the negative electrode active material layer was composed mainly of a silicon-based active material, exhibited an increased energy density per mass of the nonaqueous electrolyte storage element when the amount of lithium ions extracted from the positive electrode active material was set to a certain amount or more, and also exhibited high discharge capacity retention and high-rate discharge performance after a high-temperature charge-discharge cycle test. On the other hand, Comparative Example 1, in which the content of nickel relative to metal elements other than lithium in the lithium transition metal composite oxide was less than 65 mol %, exhibited poor discharge capacity retention and high-rate discharge performance after a high-temperature charge-discharge cycle test. Furthermore, Comparative Examples 2, 3, and 6, in which the amount of lithium ions extracted from the positive electrode active material was set to a certain amount or less, exhibited low energy densities per mass of the nonaqueous electrolyte storage element. This is presumably due to a reduced capacity development of the positive electrode. Comparative Example 5, in which graphite was used as the negative electrode active material, and Comparative Example 6, in which the mass per area of ​​the positive electrode active material layer was 3.5 g / 100 cm 2 In Comparative Example 4, where the energy density per unit mass of the nonaqueous electrolyte storage element was less than 100%, the energy density per unit mass of the nonaqueous electrolyte storage element was low. From the above results, it can be said that the nonaqueous electrolyte storage elements of the examples have high energy density per mass, high discharge capacity retention after high-temperature charge-discharge cycle testing, and high-rate discharge performance. Therefore, such nonaqueous electrolyte storage elements are useful as nonaqueous electrolyte storage elements suitable for various applications. [Industrial Applicability]

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

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

Claims

1. a positive electrode having a positive electrode active material layer and a negative electrode having a negative electrode active material layer; The positive electrode active material layer is α-NaFeO ２ The main component is a lithium transition metal composite oxide having a crystalline structure, The mass per area of ​​the positive electrode active material layer is 3.5 g / 100 cm ２ More than 10.5g / 100cm ２ is as follows: the content of nickel element relative to metal elements other than lithium element in the lithium transition metal composite oxide is 65 mol% or more; The nonaqueous electrolyte electricity storage element, wherein the negative electrode active material layer is mainly composed of a silicon-based active material.

2. 2. The nonaqueous electrolyte storage element according to claim 1, wherein the porosity of the positive electrode active material layer is 15% or more and 35% or less.

3. 3. The nonaqueous electrolyte storage element according to claim 1, wherein the content of silicon element in the negative electrode active material layer is 30% by mass or more and 70% by mass or less.

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