Non-aqueous electrolyte energy storage element and method for manufacturing the same

Abstract

To provide a nonaqueous electrolyte electric storage element having a high capacity retention rate after charge-discharge cycles and a suppressed increase in volume, and a method for manufacturing such a non-aqueous electrolyte electric storage element.SOLUTION: A nonaqueous electrolyte electric storage element includes a negative electrode having metallic lithium, and a non-aqueous electrolyte containing a non-aqueous solvent and a cyclic compound containing elemental sulfur, and the content of the fluorinated cyclic carbonate in the non-aqueous solvent is 20 mass% or more, and the content of the cyclic compound containing elemental sulfur in the non-aqueous electrolyte is less than 10 mass%.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a non-aqueous electrolyte energy storage element and a method for manufacturing the same. [Background technology]

[0002] 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 automobiles, due to their high energy density. Generally, these non-aqueous electrolyte secondary batteries consist of a pair of electrodes electrically isolated by a separator and a non-aqueous electrolyte interposed between these electrodes. They are configured to charge and discharge by transferring ions between the two electrodes. In addition to non-aqueous electrolyte secondary batteries, capacitors such as lithium-ion capacitors and electric double-layer capacitors are also widely used as non-aqueous electrolyte energy storage elements. Lithium metal is known as a high-energy-density negative electrode active material used in non-aqueous electrolyte energy storage elements (see Patent Documents 1 and 2). [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2016-100065 [Patent Document 2] Japanese Patent Application Publication No. 7-245099 [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] Non-aqueous electrolyte energy storage elements that use metallic lithium as the negative electrode active material have the disadvantage of being more prone to a decrease in discharge capacity and an increase in volume with each charge-discharge cycle compared to non-aqueous electrolyte energy storage elements that use graphite or the like as the negative electrode active material.

[0005] An object of the present invention is to provide a non-aqueous electrolyte storage element including a negative electrode having metallic lithium, the non-aqueous electrolyte storage element having a high capacity retention rate after charge-discharge cycles and suppressed volume increase, and a method for manufacturing such a non-aqueous electrolyte storage element.

Means for Solving the Problems

[0006] The non-aqueous electrolyte storage element according to one aspect of the present invention includes a negative electrode having metallic lithium and a non-aqueous electrolyte containing a cyclic compound including a non-aqueous solvent and sulfur element. The non-aqueous solvent includes a fluorinated cyclic carbonate and a chain carbonate, the content of the fluorinated cyclic carbonate in the non-aqueous solvent is 20% by volume or more, and the content of the cyclic compound including the sulfur element in the non-aqueous electrolyte is less than 10% by mass.

[0007] The method for manufacturing a non-aqueous electrolyte storage element according to another aspect of the present invention includes preparing a positive electrode, preparing a negative electrode, and preparing a non-aqueous electrolyte containing a cyclic compound including a non-aqueous solvent and sulfur element. The negative electrode has metallic lithium, or the negative electrode has a surface area where metallic lithium can be deposited and the positive electrode has a positive electrode active material including lithium element. The non-aqueous solvent includes a fluorinated cyclic carbonate and a chain carbonate, the content of the fluorinated cyclic carbonate in the non-aqueous solvent is 20% by volume or more, and the content of the cyclic compound including the sulfur element in the non-aqueous electrolyte is less than 10% by mass.

Effects of the Invention

[0008] According to one aspect of the present invention, it is possible to provide a non-aqueous electrolyte storage element including a negative electrode having metallic lithium, the non-aqueous electrolyte storage element having a high capacity retention rate after charge-discharge cycles and suppressed volume increase, and a method for manufacturing such a non-aqueous electrolyte storage element.

Brief Description of the Drawings

[0009] [Figure 1] FIG. 1 is a perspective view showing an embodiment of the non-aqueous electrolyte storage element. [Figure 2] Figure 2 is a schematic diagram showing one embodiment of an energy storage device composed of multiple non-aqueous electrolyte energy storage elements. [Modes for carrying out the invention]

[0010] First, an overview of the non-aqueous electrolyte energy storage element and its manufacturing method disclosed herein will be described.

[0011] A non-aqueous electrolyte energy storage element according to one aspect of the present invention comprises a negative electrode having metallic lithium and a non-aqueous electrolyte containing a non-aqueous solvent and a cyclic compound containing a sulfur element, wherein the non-aqueous solvent contains a fluorinated cyclic carbonate and a chain carbonate, the content of the fluorinated cyclic carbonate in the non-aqueous solvent is 20% by volume or more, and the content of the cyclic compound containing the sulfur element in the non-aqueous electrolyte is less than 10% by mass.

[0012] The non-aqueous electrolyte energy storage element in question is a non-aqueous electrolyte energy storage element equipped with a negative electrode having metallic lithium, and exhibits high capacity retention and suppressed volume increase after charge-discharge cycles. The reason for this is not clear, but the following reason is speculated. When a fluorinated cyclic carbonate is contained in the non-aqueous solvent, a film with reduction resistance to metallic lithium is formed on the negative electrode surface during charge-discharge, thereby increasing the capacity retention rate after discharge cycles. However, the reductive decomposition potential of fluorinated cyclic carbonate is 1V (vs. Li / Li +Because of this, in non-aqueous electrolyte energy storage elements equipped with a negative electrode containing metallic lithium, fluorinated cyclic carbonates continuously decompose with charge-discharge cycles, resulting in a high-resistance coating. Therefore, the effect of fluorinated cyclic carbonates on improving capacity retention is limited. Furthermore, the continuous decomposition of fluorinated cyclic carbonates with charge-discharge cycles also leads to an increase in the volume of the non-aqueous electrolyte energy storage element. On the other hand, cyclic compounds containing sulfur generally decompose at a higher potential than fluorinated cyclic carbonates, forming a highly stable coating. Therefore, by including cyclic compounds containing sulfur in the non-aqueous electrolyte in addition to fluorinated cyclic carbonates, the continuous decomposition of fluorinated cyclic carbonates with charge-discharge cycles is suppressed, increasing the capacity retention after charge-discharge cycles and suppressing volume increase. However, if the content of cyclic compounds containing sulfur is too high, the amount of coating formed increases, the coating becomes highly resistive, and the capacity retention decreases. Based on these findings, it is presumed that a non-aqueous electrolyte energy storage element in which the fluorinated cyclic carbonate content in the non-aqueous solvent is 20% by volume or more, and the non-aqueous electrolyte contains a cyclic compound containing sulfur elements in a content of less than 10% by mass, exhibits high capacity retention and suppressed volume increase after charge-discharge cycles.

[0013] Furthermore, the negative electrode of a non-aqueous electrolyte energy storage element only needs to have metallic lithium in the charged state, but does not need to have metallic lithium in the discharged state. For example, a non-aqueous electrolyte energy storage element may be configured such that metallic lithium is deposited on at least a portion of the negative electrode surface during charging, so that the negative electrode has metallic lithium in the charged state, and the metallic lithium on the negative electrode surface substantially dissolves into the non-aqueous electrolyte as lithium ions during discharge, so that the negative electrode substantially does not have metallic lithium in the discharged state.

[0014] The total content of the fluorinated cyclic carbonate and chain carbonate in the non-aqueous solvent is preferably 90% by volume or more. By increasing the total content of the fluorinated cyclic carbonate and chain carbonate in the non-aqueous solvent in this way, it is possible to further improve the capacity retention rate after charge-discharge cycles.

[0015] A method for manufacturing a non-aqueous electrolyte energy storage element according to another aspect of the present invention comprises preparing a positive electrode, preparing a negative electrode, and preparing a non-aqueous electrolyte containing a non-aqueous solvent and a cyclic compound containing a sulfur element, wherein the negative electrode has metallic lithium, or the negative electrode has a surface region on which metallic lithium can be deposited and the positive electrode has a positive electrode active material containing a lithium element, the non-aqueous solvent contains a fluorinated cyclic carbonate and a chain carbonate, the content of the fluorinated cyclic carbonate in the non-aqueous solvent is 20% by volume or more, and the content of the cyclic compound containing the sulfur element in the non-aqueous electrolyte is less than 10% by mass.

[0016] According to the method for manufacturing a non-aqueous electrolyte energy storage element, it is possible to manufacture a non-aqueous electrolyte energy storage element having a negative electrode containing metallic lithium, which has a high capacity retention rate after charge-discharge cycles and suppresses volume increase.

[0017] This document describes in detail a non-aqueous electrolyte energy storage element, an energy storage device, a method for manufacturing a non-aqueous electrolyte energy storage element, and other embodiments according to one embodiment of the present invention. Note that the names of the components (parts) used in each embodiment may differ from the names of the components (parts) used in the background art.

[0018] <Non-aqueous electrolyte energy storage element> A non-aqueous electrolyte energy storage element (hereinafter also simply referred to as "energy storage element") according to one embodiment of the present invention comprises an electrode body having a positive electrode, a negative electrode, and a separator, a non-aqueous electrolyte, and a container for housing the electrode body and the non-aqueous electrolyte. The electrode body is usually a stacked type in which a plurality of positive electrodes and a plurality of negative electrodes are stacked with a separator in between, or a wound type in which the positive electrode and negative electrode are wound in a stacked state with a separator in between. The non-aqueous electrolyte exists in a state impregnated with the positive electrode, negative electrode, and separator. As an example of a non-aqueous electrolyte energy storage element, a non-aqueous electrolyte secondary battery (hereinafter also simply referred to as "secondary battery") will be described.

[0019] (positive electrode) The positive electrode comprises a positive electrode substrate and a positive electrode active material layer disposed directly on the positive electrode substrate or via an intermediate layer.

[0020] The positive electrode substrate is conductive. Whether or not it is conductive is determined by the volume resistivity measured in accordance with JIS-H-0505 (1975), which is 10 7 The determination is made using Ω·cm as the threshold. The positive electrode substrate material can be a metal such as aluminum, titanium, tantalum, or stainless steel, or an alloy thereof. Among these, aluminum or an aluminum alloy is preferred from the viewpoint of high potential resistance, high conductivity, and cost. Examples of positive electrode substrates include foil, vapor-deposited film, mesh, and porous material, with foil being preferred from the viewpoint of cost. Therefore, aluminum foil or aluminum alloy foil is preferred as the positive electrode substrate. Examples of aluminum or aluminum alloys include A1085, A3003, and A1N30 as specified in JIS-H-4000 (2014) or JIS-H-4160 (2006).

[0021] The average thickness of the positive electrode substrate is preferably 3 μm to 50 μm, more preferably 5 μm to 40 μm, even more preferably 8 μm to 30 μm, and particularly preferably 10 μm to 25 μm. By setting the average thickness of the positive electrode substrate within the above range, it is possible to increase the strength of the positive electrode substrate while increasing the energy density per unit volume of the non-aqueous electrolyte energy storage element.

[0022] 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. For example, it contains a binder and a conductive agent.

[0023] The positive electrode active material layer contains a positive electrode active material. The positive electrode active material layer may contain optional components such as a conductive agent, a binder, a thickener, and a filler as necessary. The positive electrode active material layer is preferably formed from a positive electrode mixture containing a positive electrode active material and other optional components.

[0024] The positive electrode active material can be appropriately selected from known positive electrode active materials. Usually, a material capable of occluding and releasing lithium ions is used as the positive electrode active material. Examples of the positive electrode active material include lithium transition metal composite oxides having an α-NaFeO2-type crystal structure, lithium transition metal composite oxides having a spinel-type crystal structure, polyanion compounds, chalcogen compounds, sulfur, etc. Examples of the lithium transition metal composite oxide having an α-NaFeO2-type crystal structure include Li[Li x Ni (1-x) O2 (0 ≦ x < 0.5), Li[Li x Ni γ Co (1-x-γ) O2 (0 ≦ x < 0.5, 0 < γ < 1), Li[Li x Co (1-x) O2 (0 ≦ x < 0.5), Li[Li x Ni γ Mn (1-x-γ) O2 (0 ≦ x < 0.5, 0 < γ < 1), Li[Li x Ni γ Mn β Co (1-x-γ-β) O2 (0 ≦ x < 0.5, 0 < γ, 0 < β, 0.5 < γ + β < 1), Li[Li x Ni γ Co β Al (1-x-γ-β) O2 (0 ≦ x < 0.5, 0 < γ, 0 < β, 0.5 < γ + β < 1), etc. Examples of the lithium transition metal composite oxide having a spinel-type crystal structure include Li x Mn2O4, Li x Ni γMn (2-γ) Examples include O4. Examples of polyanion compounds include LiFePO4, LiMnPO4, LiNiPO4, LiCoPO4, Li3V2(PO4)3, Li2MnSiO4, Li2CoPO4F, etc. Examples of chalcogen compounds include titanium disulfide, molybdenum disulfide, molybdenum dioxide, etc. Some atoms or polyanions in these materials may be substituted with atoms or anions of other elements. The surfaces of these materials may be coated with other materials. In the positive electrode active material layer, one of these materials may be used alone, or two or more may be used in mixture form.

[0025] As the positive electrode active material, lithium transition metal composite oxides (such as lithium transition metal composite oxides having an α-NaFeO2 type crystal structure or a spinel type crystal structure) are preferred, and lithium transition metal composite oxides having an α-NaFeO2 type crystal structure are more preferred. By using such a positive electrode active material, the capacity density, energy density, etc. of the non-aqueous electrolyte energy storage element can be increased.

[0026] Lithium transition metal composite oxides having an α-NaFeO2 type crystal structure preferably contain nickel or manganese as transition metal elements, and more preferably contain both nickel and manganese. Lithium transition metal composite oxides may further contain other transition metal elements such as cobalt. In lithium transition metal composite oxides having an α-NaFeO2 type crystal structure, the molar ratio of lithium atoms (Li) to transition metal atoms (Me) (Li / Me) may be greater than 0.9, preferably greater than 1.0, more preferably 1.1 or higher, even more preferably 1.2 or higher, and even more preferably 1.3 or higher. By using such lithium transition metal composite oxides, it is possible to increase the discharge capacity, etc. The upper limit of the molar ratio of lithium atoms to transition metal atoms (Li / Me) is preferably 1.6, and more preferably 1.5.

[0027] As lithium transition metal composite oxides having an α-NaFeO2 type crystal structure, compounds represented by the following formula (1) are preferred. Li 1+α Me 1-α O2···(1) In equation (1), Me is a transition metal element containing Ni or Mn. 0 ≤ α < 1.

[0028] In formula (1), Me preferably contains Ni and Mn. Me is preferably composed of substantially two elements, Ni and Mn, or three elements, Ni, Mn, and Co. Me may also contain other transition metal elements.

[0029] In equation (1), the lower limit of the molar ratio of Ni to Me (Ni / Me) is preferably 0.1, and more preferably 0.2. On the other hand, the upper limit of this molar ratio (Ni / Me) is preferably 0.5, and more preferably 0.45. By setting the molar ratio (Ni / Me) within the above range, the energy density and other properties are improved.

[0030] In equation (1), the lower limit of the molar ratio of Mn to Me (Mn / Me) is preferably 0.5, more preferably 0.55, and even more preferably 0.6. On the other hand, the upper limit of this molar ratio (Mn / Me) is preferably 0.75, and more preferably 0.7. By setting the molar ratio (Mn / Me) within the above range, the energy density and other properties are improved.

[0031] In formula (1), the upper limit of the molar ratio of Co to Me (Co / Me) is preferably 0.3, more preferably 0.2, and even more preferably 0.1. This molar ratio (Co / Me) or the lower limit of this molar ratio (Co / Me) may be 0.

[0032] In equation (1), the molar ratio of Li to Me (Li / Me), i.e., (1+α) / (1-α), is preferably greater than 1.0 (α>0), more preferably 1.1 or greater, even more preferably 1.2 or greater, and even more preferably 1.3 or greater. On the other hand, the upper limit of this molar ratio (Li / Me) is preferably 1.6, and more preferably 1.5. By setting the molar ratio (Li / Me) within the above range, the discharge capacity is increased.

[0033] Lithium transition metal composite oxides having a molar ratio of lithium atoms (Li) to transition metal atoms (Me) greater than 1.0 (Li / Me) are preferably those in which no diffraction peaks are present in the range of 20° to 22° in the X-ray diffraction pattern using CuKα rays. Lithium transition metal composite oxides having a molar ratio of lithium atoms (Li) to transition metal atoms (Me) greater than 1.0 (Li / Me) generally have a positive electrode potential of, for example, 4.5V (vs. Li / Li + The capacitance increases through initial charging up to a certain level. Furthermore, due to the change in crystal structure during such initial charging, the diffraction peak in the range of 20° to 22° that existed before the initial charging disappears. In other words, a lithium transition metal composite oxide in which the molar ratio of lithium atoms (Li) to transition metal atoms (Me) (Li / Me) is greater than 1.0, and in which no diffraction peak exists in the range of 20° to 22° in the above X-ray diffraction pattern, has a large capacitance.

[0034] In this specification, the composition ratio of lithium transition metal composite oxide refers to the composition ratio when the device is fully discharged by the following method. First, the non-aqueous electrolyte energy storage element is charged with a constant current of 0.05C until it reaches the charging termination voltage for normal use, and is brought to a fully charged state. After a 30-minute rest, it is discharged with a constant current of 0.05C until it reaches the lower limit voltage for normal use. The device is disassembled, the positive electrode is removed, and a test battery is assembled with a metallic lithium electrode as the counter electrode. The positive electrode potential is set to 2.0V (vs. Li / Li) with a current value of 10mA per gram of positive electrode mixture. +Constant current discharge is performed until the positive electrode is fully discharged. Pure metallic lithium is used for the metallic lithium electrode here. The device is disassembled again and the positive electrode is removed. The non-aqueous electrolyte adhering to the removed positive electrode is thoroughly washed with dimethyl carbonate, and after drying at room temperature for 24 hours, the lithium transition metal composite oxide of the positive electrode active material is collected. The collected lithium transition metal composite oxide is subjected to measurement. The work from disassembling the non-aqueous electrolyte energy storage element to collecting the lithium transition metal composite oxide is performed in an argon atmosphere with a dew point of -60°C or lower. Note that "normal use" refers to the case where the non-aqueous electrolyte energy storage element is used under the charge and discharge conditions recommended or specified for the non-aqueous electrolyte energy storage element, and if a charger for the non-aqueous electrolyte energy storage element is available, it refers to the case where the charger is used to use the non-aqueous electrolyte energy storage element.

[0035] X-ray diffraction measurements of lithium transition metal composite oxides are performed on lithium transition metal composite oxides that have been brought to a fully discharged state using the method described above. Specifically, the X-ray diffraction measurements are performed by powder X-ray diffraction using an X-ray diffractometer (Rigaku's "MiniFlex II"), with the radiation source being CuKα rays, the tube voltage being 30kV, and the tube current being 15mA. At this time, the diffracted X-rays pass through a 30μm 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 photodetection slit width is 13mm (OPEN), and the scattering slit width is 8mm.

[0036] The positive electrode active material is usually in the form of particles (powder). The average particle size of the positive electrode active material is preferably, for example, 0.1 μm or more and 20 μm or less. Setting the average particle size of the positive electrode active material above the lower limit makes it easier to manufacture or handle the positive electrode active material. Setting the average particle size of the positive electrode active material below the upper limit improves the electronic conductivity of the positive electrode active material layer. When a composite material of the positive electrode active material and other materials is used, the average particle size of the composite material is used as the average particle size of the positive electrode active material. "Average particle size" refers to the value at which the volume-based integrated distribution calculated in accordance with JIS-Z-8819-2 (2001), based on the particle size distribution measured by laser diffraction / scattering method on a dilution of particles diluted with a solvent, in accordance with JIS-Z-8825 (2013), becomes 50%.

[0037] To obtain powder with a predetermined particle size, grinders and classifiers are used. Examples of grinding methods include using mortars, ball mills, sand mills, vibrating ball mills, planetary ball mills, jet mills, counter-jet mills, swirling airflow jet mills, or sieves. Wet grinding, which involves the coexistence of water or organic solvents such as hexane, can also be used during grinding. For classification, sieves and wind classifiers are used as needed, both dry and wet.

[0038] The content of the positive electrode active material in the positive electrode active material layer is preferably 50% to 99% by mass, more preferably 70% to 98% by mass, and even more preferably 80% to 95% by mass. By setting the content of the positive electrode active material within the above range, it is possible to achieve both high energy density and manufacturability in the positive electrode active material layer.

[0039] The conductive agent is not particularly limited as long as it is a conductive material. Examples of such conductive agents include carbonaceous materials, metals, and conductive ceramics. Examples of carbonaceous materials include graphite, non-graphitic carbon, and graphene-based carbon. Examples of non-graphitic carbon include carbon nanofibers, pitch-based carbon fibers, and carbon black. Examples of carbon black include furnace black, acetylene black, and Ketjen black. Examples of graphene-based carbon include graphene, carbon nanotubes (CNTs), and fullerenes. The conductive agent can take the form of powder or fiber. One of these materials may be used alone as the conductive agent, or two or more may be used in mixture form. These materials may also be used in composite form. For example, a composite material of carbon black and CNTs may be used. Among these, carbon black is preferred from the viewpoint of electronic conductivity and coating properties, and acetylene black is particularly preferred.

[0040] The content of the conductive agent in the positive electrode active material layer is preferably 1% by mass or more and 10% by mass or less, and more preferably 3% by mass or more and 9% by mass or less. By setting the content of the conductive agent within the above range, the energy density of the non-aqueous electrolyte energy storage element can be increased.

[0041] Examples of binders include thermoplastic resins such as fluororesins (polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), etc.), polyethylene, polypropylene, polyacrylate, and polyimide; elastomers such as ethylene-propylene-diene rubber (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), and fluororubber; polysaccharide polymers; and polyaramid.

[0042] The binder content in the positive electrode active material layer is preferably 1% by mass or more and 10% by mass or less, and more preferably 3% by mass or more and 9% by mass or less. By keeping the binder content within the above range, the active material can be stably maintained.

[0043] Examples of thickening agents include polysaccharide polymers such as carboxymethylcellulose (CMC) and methylcellulose. If the thickening agent has a functional group that reacts with lithium or the like, this functional group may be deactivated beforehand by methylation or the like. In one embodiment of the present invention, it is preferable that the thickening agent is not contained in the positive electrode active material layer.

[0044] The filler is not particularly limited. Examples of fillers include polyolefins such as polypropylene and polyethylene, inorganic oxides such as silicon dioxide, alumina, titanium dioxide, calcium oxide, strontium oxide, barium oxide, magnesium oxide, and aluminosilicates, hydroxides such as magnesium hydroxide, calcium hydroxide, and aluminum hydroxide, carbonates such as calcium carbonate, sparingly soluble ionic crystals such as calcium fluoride, barium fluoride, and barium sulfate, nitrides such as aluminum nitride and silicon nitride, mineral resource-derived materials such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica, or artificial products thereof. In one embodiment of the present invention, it is preferable that the filler is not contained in the positive electrode active material layer.

[0045] The positive electrode active material layer may contain typical nonmetallic elements such as B, N, P, F, Cl, Br, and I, typical metallic elements such as Li, Na, Mg, Al, K, Ca, Zn, Ga, Ge, Sn, Sr, and Ba, and transition metal elements such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Zr, Nb, and W as components other than the positive electrode active material, conductive agent, binder, thickener, and filler.

[0046] (Negative electrode) The negative electrode comprises a negative electrode substrate and a negative electrode active material layer disposed directly on the negative electrode substrate or via an intermediate layer. The configuration of the intermediate layer is not particularly limited and can be selected from, for example, the configurations exemplified in the positive electrode.

[0047] The negative electrode substrate is electrically conductive. Suitable materials for the negative electrode substrate include metals such as copper, nickel, stainless steel, nickel-plated steel, or alloys thereof, as well as carbonaceous materials. Among these, copper or copper alloys are preferred. Examples of negative electrode substrates include foil, vapor-deposited film, mesh, and porous materials, with foil being preferred from a cost perspective. Therefore, copper foil or copper alloy foil is preferred as the negative electrode substrate. Examples of copper foil include rolled copper foil and electrolytic copper foil.

[0048] The average thickness of the negative electrode substrate is preferably 2 μm to 35 μm, more preferably 3 μm to 30 μm, even more preferably 4 μm to 25 μm, and particularly preferably 5 μm to 20 μm. By setting the average thickness of the negative electrode substrate within the above range, it is possible to increase the strength of the negative electrode substrate while increasing the energy density per unit volume of the non-aqueous electrolyte energy storage element.

[0049] The negative electrode active material layer contains metallic lithium. That is, the negative electrode active material layer is a layer containing metallic lithium. Metallic lithium is a component that functions as the negative electrode active material. Metallic lithium may exist as pure metallic lithium consisting substantially only of the element lithium, or as a lithium alloy containing other elements. Examples of lithium alloys include lithium silver alloy, lithium zinc alloy, lithium calcium alloy, lithium aluminum alloy, lithium magnesium alloy, and lithium indium alloy. Lithium alloys may also contain multiple elements other than lithium.

[0050] The negative electrode active material layer may be a layer consisting substantially only of metallic lithium. The metallic lithium content in the negative electrode active material layer may be 90% by mass or more, 99% by mass or more, or 100% by mass.

[0051] The negative electrode active material layer may be pure metallic lithium foil or lithium alloy foil. The negative electrode active material layer may be a non-porous layer (solid layer). Alternatively, the negative electrode active material layer may be a porous layer having particles containing metallic lithium. A negative electrode active material layer that is a porous layer having particles containing metallic lithium may further contain, for example, resin particles, inorganic particles, etc.

[0052] The negative electrode active material layer, i.e., the layer containing metallic lithium, is preferably a layer that is present even in the discharge state, that is, a layer that is present in all states from the charge state to the discharge state. The average thickness of the negative electrode active material layer in the discharge state is preferably 5 μm to 1,000 μm, more preferably 10 μm to 500 μm, and even more preferably 30 μm to 300 μm. The average thickness of the negative electrode active material layer is the average of the thicknesses measured at any five locations. When a negative electrode active material containing metallic lithium is present even in the discharge state, and preferably its average thickness is above the above lower limit, a sufficient amount of metallic lithium is present, which further increases the capacity retention rate after charge-discharge cycles of the non-aqueous electrolyte energy storage element.

[0053] Furthermore, in the case of a non-aqueous electrolyte energy storage element configured such that metallic lithium is deposited on at least a portion of the surface area of ​​the negative electrode during charging, and the metallic lithium of the negative electrode substantially dissolves into the non-aqueous electrolyte as lithium ions during discharge, the negative electrode does not need to substantially have a negative electrode active material layer in the discharge state.

[0054] (Separator) The separator can be appropriately selected from known separators. Examples of separators include a separator consisting only of a base layer, or a separator in which a heat-resistant layer containing heat-resistant particles and a binder is formed on one or both sides of the base layer. Examples of the base layer shape of the separator include woven fabric, nonwoven fabric, and porous resin film. Among these shapes, porous resin film is preferred from the viewpoint of strength, and nonwoven fabric is preferred from the viewpoint of liquid retention of non-aqueous electrolytes. As for the material of the base layer of the separator, polyolefins such as polyethylene and polypropylene are preferred from the viewpoint of shutdown function, and polyimide and aramid are preferred from the viewpoint of oxidative degradation resistance. A composite material of these resins may also be used as the base layer of the separator.

[0055] The heat-resistant particles contained in the heat-resistant layer preferably have a mass loss of 5% or less when heated from room temperature to 500°C in an air atmosphere of 1 atmosphere, and more preferably have a mass loss of 5% or less when heated from room temperature to 800°C. Inorganic compounds are examples of materials with a mass loss of less than the specified amount. 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; covalent crystals such as silicon and diamond; mineral resource-derived materials such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica, or artificial products thereof. These inorganic compounds may be used individually or in combination, or two or more may be used as a mixture. Among these inorganic compounds, silicon dioxide, aluminum oxide, or aluminosilicates are preferred from the viewpoint of safety for energy storage elements.

[0056] The porosity of the separator is preferably 80 volume% or less from the viewpoint of strength, and preferably 20 volume% or more from the viewpoint of discharge performance. Here, "porosity" refers to a volume-based value and means the measurement value obtained using a mercury porosimeter.

[0057] A polymer gel composed of a polymer and a non-aqueous electrolyte may be used as a separator. Examples of polymers include polyacrylonitrile, polyethylene oxide, polypropylene oxide, polymethyl methacrylate, polyvinyl acetate, polyvinylpyrrolidone, and polyvinylidene fluoride. Using a polymer gel has the effect of suppressing leakage. A polymer gel may also be used in combination with a porous resin film or nonwoven fabric as described above as a separator.

[0058] (Non-aqueous electrolytes) Non-aqueous electrolytes contain a non-aqueous solvent and a cyclic compound containing sulfur. The non-aqueous solvent does not contain a cyclic compound containing sulfur. Non-aqueous electrolytes usually also contain a lithium salt. Non-aqueous electrolytes may also be non-aqueous electrolyte solutions.

[0059] Non-aqueous solvents include fluorinated cyclic carbonates and linear carbonates.

[0060] A fluorinated cyclic carbonate is a cyclic carbonate in which at least some of the hydrogen atoms are replaced with fluorine atoms. A cyclic carbonate is a carbonate having a ring structure. In a fluorinated cyclic carbonate, fluorine atoms may be present in the ring structure or in the side chains if side chains are present, but it is preferable that they be present in the ring structure.

[0061] Examples of fluorinated cyclic carbonates include fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), and 4-(2,2,3,3,3-pentafluoropropyl)-1,3-dioxolan-2-one), with FEC and DFEC being preferred, and FEC being more preferred. One or more fluorinated cyclic carbonates can be used.

[0062] The content of fluorinated cyclic carbonate in the non-aqueous solvent is 20% by volume or more. Preferably, the content of fluorinated cyclic carbonate in the non-aqueous solvent is 20% to 70% by volume, more preferably 22% to 60% by volume, even more preferably 24% to 50% by volume, even more preferably 26% to 45% by volume, and even more preferably 28% to 40% by volume. By setting the content of fluorinated cyclic carbonate above the lower limit and below the upper limit, the amount of film formed, the viscosity of the non-aqueous electrolyte, etc., become appropriate, thereby increasing the capacity retention rate after charge-discharge cycles and further suppressing volume increase.

[0063] A chain-like carbonate is a carbonate that does not have a ring structure. Chain-like carbonates may also be those in which at least some of the hydrogen atoms are substituted with halogen atoms or the like.

[0064] Examples of linear carbonates include diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diphenyl carbonate, trifluoroethyl methyl carbonate (TFEMC), and bis(trifluoroethyl) carbonate (FDEC). Among these, DMC, EMC, and TFEMC are preferred, and DMC and TFEMC are more preferred. Using these as linear carbonates increases the capacity retention rate after charge-discharge cycles and further suppresses volume increase. Furthermore, using fluorinated linear carbonates such as TFEMC tends to increase the capacity retention rate after charge-discharge cycles. One or more linear carbonates can be used.

[0065] The content of linear carbonates in the non-aqueous solvent is preferably 20% to 80% by volume, more preferably 30% to 78% by volume, even more preferably 40% to 76% by volume, even more preferably 50% to 74% by volume, and even more preferably 60% to 72% by volume. By setting the content of linear carbonates above the lower limit and below the upper limit, the capacity retention rate after charge-discharge cycles is further increased and the volume increase is further suppressed.

[0066] The non-aqueous solvent may further contain other solvents besides fluorinated cyclic carbonates and linear carbonates. Examples of other solvents include unfluorinated cyclic carbonates, carboxylic acid esters, phosphate esters, linear sulfonic acid esters, ethers, amides, and nitriles. As these other solvents, compounds in which some of the hydrogen atoms are substituted with halogen atoms or the like may be used. However, the total content of fluorinated cyclic carbonates and linear carbonates in the non-aqueous solvent is preferably 90% by volume or more, more preferably 95% by volume or more, and even more preferably 99% by volume or more. In this way, increasing the total content of fluorinated cyclic carbonates and linear carbonates in the non-aqueous solvent can further improve the capacity retention rate after charge-discharge cycles. The total content of fluorinated cyclic carbonates and linear carbonates in the non-aqueous solvent may be 100% by mass or less. Furthermore, in one embodiment of the present invention, the non-aqueous solvent may consist only of carbonates (fluorinated cyclic carbonates, linear carbonates, and other carbonates).

[0067] A cyclic compound containing sulfur is a compound that has a molecular structure including a ring structure and contains sulfur atoms in that molecular structure. In a cyclic compound containing sulfur, the sulfur atoms may be present in the ring structure or, if a side chain is present, in the side chain, but it is preferable that they be present in the ring structure.

[0068] Examples of cyclic compounds containing sulfur include cyclic sulfonic acid esters, cyclic sulfones, cyclic sulfoxides, cyclic sulfites, cyclic sulfates, and cyclic sulfides. One or more hydrogen atoms in these sulfur-containing cyclic compounds may be substituted with halogen atoms or other substituents. One or more sulfur-containing cyclic compounds can be used.

[0069] Among these cyclic compounds containing sulfur, cyclic sulfonic acid esters are preferred. When cyclic sulfonic acid esters are used, the capacity retention rate after charge-discharge cycles tends to be higher and the volume increase is more suppressed. Cyclic compounds containing sulfur preferably contain sulfur and oxygen, and more preferably consist of sulfur, oxygen, carbon, and hydrogen. Furthermore, cyclic compounds containing sulfur preferably are molecular compounds rather than salts.

[0070] Cyclic sulfonic acid esters are compounds having a ring structure in which two carbon atoms are each bonded to a sulfonyloxy group (-S(=O)2O-). Among cyclic sulfonic acid esters, sultones are preferred. When sultones are used, the capacity retention rate after charge-discharge cycles is higher and the volume increase is more suppressed.

[0071] Sultones are cyclic sulfonic acid esters of hydroxysulfonic acid. In other words, sultones are cyclic sulfonic acid esters in which the number of sulfonyloxy groups (-S(=O)2O-) in the ring structure is 1. Examples of sultones include 1,3-propanesultone, 1,4-butanesultone, 1,3-propensultone, 1-methyl-1,3-propanesultone, 1-methyl-1,3-propensultone, 2-methyl-1,3-propanesultone, 2-methyl-1,3-propensultone, 3-methyl-1,3-propanesultone, and 3-methyl-1,3-propensultone, with 1,3-propanesultone and 1,3-propensultone being preferred, and 1,3-propensultone being more preferred. It is also preferable that the sultone is an alkenesultone having an unsaturated double bond between carbon atoms, such as 1,3-propensultone.

[0072] Other examples of cyclic sulfonic acid esters include methylene-methanedisulfonic acid esters and ethylene-methanedisulfonic acid esters.

[0073] A cyclic sulfone is a compound having a ring structure in which two carbon atoms are each bonded to a sulfonyl group (-S(=O)2-). Examples of cyclic sulfones include sulfolane, 3-methylsulfolane, 3-sulfolene, 1,1-dioxothiophene, 3-methyl-2,5-dihydrothiophene-1,1-dioxide, and methyl 3-sulfolene-3-carboxylate.

[0074] A cyclic sulfoxide is a compound having a ring structure in which two carbon atoms are each bonded to a sulfinyl group (-S(=O)-). Examples of cyclic sulfoxides include tetramethylene sulfoxide.

[0075] Cyclic sulfites are compounds that have a ring structure in which two carbon atoms are each bonded to an oxysulfinyloxy group (-OS(=O)-O-). Examples of cyclic sulfites include ethylene sulfite, 1,2-propylene glycol sulfite, trimethylene sulfite, and 1,3-butylene glycol sulfite.

[0076] Cyclic sulfates are compounds that have a ring structure in which two carbon atoms are each bonded to an oxysulfonyloxy group (-OS(=O)2-O-). Examples of cyclic sulfates include ethylene sulfate, 1,3-propylene sulfate, 2,3-propylene sulfate, 4,5-pentene sulfate, 4,4'-bis(2,2-dioxo-1,3,2-dioxathiolane), and 4-methylsulfonyloxymethyl-2,2-dioxo-1,3,2-dioxathiolane.

[0077] A cyclic sulfide is a compound having a ring structure in which two carbon atoms are each bonded to a divalent sulfur (-S-). Examples of cyclic sulfides include tetrahydrothiophene, thiophene, thiane, 1,3-dithiane, 5,6-dihydro-1,4-dithiyne-2,3-dicarboxylic anhydride, and 3,4-thiophenedicarboxylic anhydride.

[0078] Furthermore, the number of ring members in the ring structure of a cyclic compound containing a sulfur element is preferably 3 to 8, and more preferably 4 to 6.

[0079] It is preferable that the reductive decomposition potential of a cyclic compound containing sulfur is nobler than that of a fluorinated cyclic carbonate. For example, it is preferable that the reductive decomposition potential of a cyclic compound containing sulfur is nobler than that of FEC, such that 1V (vs. Li / Li) is nobler. + It is also preferable that it be nobler than ).

[0080] The content of sulfur-containing cyclic compounds in the non-aqueous electrolyte, i.e., the mass of sulfur-containing cyclic compounds relative to the total mass of the non-aqueous electrolyte, is less than 10% by mass. By keeping the content of sulfur-containing cyclic compounds below the above upper limit, the high resistance of the formed film is suppressed, resulting in a high capacity retention rate and suppressed volume increase after charge-discharge cycles. The content of sulfur-containing cyclic compounds in the non-aqueous electrolyte is preferably 0.01% by mass or more and 9% by mass or less, more preferably 0.1% by mass or more and 8% by mass or less, even more preferably 0.5% by mass or more and 8% by mass or less, and even more preferably 1% by mass or more and 7% by mass or less. The lower limit of the content of sulfur-containing cyclic compounds in the non-aqueous electrolyte may more preferably be 2%, 3%, or 4% by mass. By keeping the content of sulfur-containing cyclic compounds above the above lower limit, a sufficient amount of highly stable film derived from sulfur-containing cyclic compounds is formed, further increasing the capacity retention rate and suppressing volume increase after charge-discharge cycles. On the other hand, keeping the content of sulfur-containing cyclic compounds below the above upper limit further suppresses the high resistance of the formed film.

[0081] Examples of lithium salts include inorganic lithium salts such as LiPF6, LiPO2F2, LiBF4, LiClO4, and LiN(SO2F)2; lithium oxalate salts such as lithium bis(oxalate)borate (LiBOB), lithium difluorooxalate borate (LiFOB), and lithium bis(oxalate)difluorophosphate (LiFOP); and lithium salts having halogenated hydrocarbon groups such as LiSO3CF3, LiN(SO2CF3)2, LiN(SO2C2F5)2, LiN(SO2CF3)(SO2C4F9), LiC(SO2CF3)3, and LiC(SO2C2F5)3. Among these, inorganic lithium salts are preferred, and LiPF6 is more preferred.

[0082] The lithium salt content in the non-aqueous solution is 0.1 mol / dm³ at 20°C and 1 atm. 3 More than 2.5mol / dm 3 Preferably, it is 0.3 mol / dm³ 3 More than 2.0mol / dm 3It is more preferable that it be less than or equal to 0.5 mol / dm 3 More than 1.7mol / dm 3 It is even more preferable that the following is the case: 0.7 mol / dm 3 More than 1.5mol / dm 3 The following conditions are particularly preferable. By setting the lithium salt content within the above range, the ionic conductivity of the non-aqueous electrolyte can be increased. Note that "non-aqueous solution" refers to the state in which the lithium salt is dissolved in the non-aqueous solvent, and means the state before the inclusion of cyclic compounds containing sulfur elements and additives.

[0083] Non-aqueous electrolytes may contain other additives. Other additives in non-aqueous electrolytes refer to components other than non-aqueous solvents, cyclic compounds containing sulfur elements, and lithium salts. Examples of other additives include aromatic compounds such as biphenyl, alkylbiphenyl, terphenyl, partially hydrogenated terphenyl, cyclohexylbenzene, t-butylbenzene, t-amylbenzene, diphenyl ether, and dibenzofuran; partially halides of the aforementioned aromatic compounds such as 2-fluorobiphenyl, o-cyclohexylfluorobenzene, and p-cyclohexylfluorobenzene; halogenated anisole compounds such as 2,4-difluoroanisole, 2,5-difluoroanisole, 2,6-difluoroanisole, and 3,5-difluoroanisole; and succinic anhydride. Examples include glutaric anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, cyclohexanedicarboxylic acid anhydride; dimethyl sulfite, methyl methanesulfonate, busulfan, methyl toluenesulfonate, dimethyl sulfate, dimethyl sulfone, diethyl sulfone, dimethyl sulfoxide, diethyl sulfoxide, diphenyl sulfide, thioanisole, diphenyl disulfide, dipyridinium disulfide, perfluorooctane, tristrimethylsilyl borate, tristrimethylsilyl phosphate, tetrakithrimethylsilyl titanate, etc. These other additives may be used individually or in combination of two or more.

[0084] The content of other additives in the non-aqueous electrolyte is preferably 0.01% to 10% by mass, more preferably 0.1% to 7% by mass, even more preferably 0.2% to 5% by mass, and particularly preferably 0.3% to 3% by mass, relative to the total mass of the non-aqueous electrolyte. By setting the content of other additives within the above range, it is possible to improve the capacity retention performance or cycle performance after high-temperature storage, or to further improve safety.

[0085] Non-aqueous electrolytes may be used in combination with non-aqueous electrolytes and solid electrolytes.

[0086] In non-aqueous electrolyte energy storage elements, the lower limit of the positive electrode potential at the charging termination voltage during normal use is 4.2V (vs.Li / Li + It is preferable that the voltage is 4.3V (vs. Li / Li + It is more preferable that it be 4.4V (vs. Li / Li + It is even more preferable that it be 4.5V (vs. Li / Li + It is even more preferable that it be 4.6V (vs. Li / Li + It is even more preferable that the positive electrode potential at the charging termination voltage during normal use is set to be above the above lower limit, thereby increasing energy density and voltage, and also increasing discharge capacity. Furthermore, even when the positive electrode potential at the charging termination voltage during normal use is set to a high level, the non-aqueous electrolyte energy storage element maintains a high capacity retention rate after charge-discharge cycles and suppresses volume increase.

[0087] The upper limit of the positive electrode potential at the charging termination voltage during normal use of the non-aqueous electrolyte energy storage element is, for example, 5.0V (vs. Li / Li + ) may also be 4.8V (vs.Li / Li + ) may also be 4.7V (vs.Li / Li + ) is also acceptable.

[0088] The shape of the non-aqueous electrolyte energy storage element in this embodiment is not particularly limited, and examples include cylindrical batteries, prismatic batteries, flat batteries, coin-type batteries, button-type batteries, and the like.

[0089] Figure 1 shows a non-aqueous electrolyte energy storage element 1 as an example of a rectangular battery. Note that the figure is a transparent view of the inside of the container. An electrode body 2 having a positive electrode and a negative electrode wound around a separator is housed in a rectangular container 3. The positive electrode is electrically connected to the positive electrode terminal 4 via a positive electrode lead 41. The negative electrode is electrically connected to the negative electrode terminal 5 via a negative electrode lead 51.

[0090] <Energy storage device> The non-aqueous electrolyte energy storage element of this embodiment can be mounted as an energy storage unit (battery module) composed of multiple non-aqueous electrolyte energy storage elements 1 in power supplies for automobiles such as electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs), power supplies for electronic devices such as personal computers and communication terminals, or power storage power supplies. In this case, it is sufficient that the technology of the present invention is applied to at least one non-aqueous electrolyte energy storage element included in the energy storage unit.

[0091] Figure 2 shows an example of a power storage device 30 which is formed by further assembling power storage units 20, each of which is an assembly of two or more electrically connected non-aqueous electrolyte energy storage elements 1. The power storage device 30 may include busbars (not shown) that electrically connect two or more non-aqueous electrolyte energy storage elements 1, busbars (not shown) that electrically connect two or more power storage units 20, etc. The power storage unit 20 or the power storage device 30 may include a condition monitoring device (not shown) that monitors the state of one or more non-aqueous electrolyte energy storage elements.

[0092] <Method for manufacturing a non-aqueous electrolyte energy storage element> The method for manufacturing a non-aqueous electrolyte energy storage element of this embodiment comprises preparing a positive electrode, preparing a negative electrode, and preparing a non-aqueous electrolyte containing a non-aqueous solvent and a cyclic compound containing a sulfur element, wherein the negative electrode has metallic lithium, or the negative electrode has a surface region on which metallic lithium can be deposited and the positive electrode has a positive electrode active material containing a lithium element, the non-aqueous solvent contains fluorinated cyclic carbonate and chain carbonate, the content of the fluorinated cyclic carbonate in the non-aqueous solvent is 20% by volume or more, and the content of the cyclic compound containing the sulfur element in the non-aqueous electrolyte is less than 10% by mass.

[0093] Preparing a positive electrode may also mean fabricating a positive electrode. A positive electrode can be fabricated, for example, by applying a paste-like positive electrode mixture (positive electrode mixture paste) directly to a positive electrode substrate or via an intermediate layer, and then drying it to form a positive electrode active material layer. Specific examples and preferred examples of the prepared positive electrode are similar to the positive electrode provided in a non-aqueous electrolyte energy storage element according to one embodiment of the present invention.

[0094] Preparing a negative electrode may also mean manufacturing a negative electrode. The negative electrode to be prepared is a negative electrode having metallic lithium, or a negative electrode having a surface region on which metallic lithium can be deposited during charging. A negative electrode having metallic lithium can be manufactured by laminating a negative electrode active material layer containing metallic lithium directly onto a negative electrode substrate or via an intermediate layer, and then pressing or otherwise processing it. The negative electrode active material layer containing metallic lithium may be pure metallic lithium foil or lithium alloy foil. Specific and preferred examples of the negative electrode having metallic lithium to be prepared include specific and preferred examples of a negative electrode provided in a non-aqueous electrolyte energy storage element according to one embodiment of the present invention.

[0095] The negative electrode having a surface region on which metallic lithium can be deposited during charging may, for example, be a negative electrode consisting only of a negative electrode substrate. When preparing a negative electrode having a surface region on which metallic lithium can be deposited during charging, a positive electrode having a positive electrode active material containing lithium is prepared in advance.

[0096] Preparing a non-aqueous electrolyte may be equivalent to preparing a non-aqueous electrolyte. The preparation of a non-aqueous electrolyte can be carried out by mixing fluorinated cyclic carbonates, linear carbonates, cyclic compounds containing sulfur elements, lithium salts, etc. Specific examples and preferred examples of the prepared non-aqueous electrolyte are similar to those of the non-aqueous electrolyte provided in the non-aqueous electrolyte energy storage element according to one embodiment of the present invention.

[0097] The method for manufacturing the non-aqueous electrolyte energy storage element may further include initial charging and discharging of an uncharged energy storage element assembled using a positive electrode, a negative electrode, and a non-aqueous electrolyte. For example, if the positive electrode active material of the non-aqueous electrolyte energy storage element is a lithium transition metal composite oxide in which the molar ratio of lithium atoms (Li) to transition metal atoms (Me) (Li / Me) is greater than 1.0, the capacity will increase after undergoing initial charging and discharging. The number of charge and discharge cycles in the initial charging and discharging may be one, two, or three or more. If the positive electrode active material of the non-aqueous electrolyte energy storage element is a lithium transition metal composite oxide in which the molar ratio of lithium atoms (Li) to transition metal atoms (Me) (Li / Me) is greater than 1.0, the positive electrode potential (positive electrode target potential) at the charging termination voltage during initial charging and discharging is 4.5V (vs. Li / Li + ) Above 4.7V (vs. Li / Li + It is preferable that it be less than or equal to the following:

[0098] <Other Embodiments> Furthermore, the energy storage element of the present invention is not limited to the embodiments described above, and various modifications may be made without departing from the spirit of the present invention. For example, the configuration of one embodiment may be added to the configuration of another embodiment, and a part of the configuration of one embodiment may be replaced with the configuration of another embodiment or with well-known technology. In addition, a part of the configuration of one embodiment may be deleted. Also, well-known technology may be added to the configuration of one embodiment.

[0099] In the above embodiment, the case in which the non-aqueous electrolyte energy storage element is used as a rechargeable non-aqueous electrolyte secondary battery (e.g., a lithium secondary battery) was described, but the type, shape, dimensions, capacity, etc. of the non-aqueous electrolyte energy storage element are arbitrary. The present invention can also be applied to various secondary batteries, electric double-layer capacitors, or capacitors such as lithium-ion capacitors. Furthermore, the non-aqueous electrolyte energy storage element of the present invention can also be applied to lithium-air batteries.

[0100] In the above embodiment, an electrode body in which a positive electrode and a negative electrode are stacked with a separator in between has been described, but the electrode body does not need to have a separator. For example, the positive electrode and the negative electrode may be in direct contact with each other, with a non-conductive layer formed on the active material layer of either the positive electrode or the negative electrode. [Examples]

[0101] 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.

[0102] [Example 1] (Fabrication of the positive electrode) As the positive electrode active material, it has an α-NaFeO2 type crystal structure, Li 1+α Me 1-α A lithium transition metal composite oxide represented as O2 (where Me is a transition metal) was used. Here, the molar ratio of Li to Me (Li / Me) was 1.33, and Me consisted of Ni and Mn, with Ni:Mn present in a molar ratio of 1:2.

[0103] A positive electrode mixture paste was prepared using N-methylpyrrolidone (NMP) as the dispersion medium, containing the above-mentioned positive electrode active material, acetylene black (AB) as a conductive agent, and polyvinylidene fluoride (PVDF) as a binder in a solid content mass ratio of 92.5:4.5:3.0. The positive electrode mixture paste was applied to one side of an aluminum foil with an average thickness of 15 μm, which served as the positive electrode substrate. After drying, it was pressed, resulting in a mass of 26 mg / 100 cm² per unit area. 2 A positive electrode was fabricated in which a positive electrode active material layer having a positive electrode mixture was arranged.

[0104] (Fabrication of the negative electrode) A negative electrode was fabricated by laminating a pure metallic lithium foil with an average thickness of 100 μm onto one side of a copper foil with an average thickness of 8 μm, which served as the negative electrode substrate, and then pressing it to create a negative electrode with a negative electrode active material layer.

[0105] (Preparation of non-aqueous electrolytes) A non-aqueous solvent was prepared by mixing fluoroethylene carbonate (FEC), a fluorinated cyclic carbonate, and dimethyl carbonate (DMC), a chain-like carbonate, in a volume ratio of 30:70, to which 1.0 mol / dm³ of the lithium salt LiPF6 was added. 3 A non-aqueous solution was prepared by dissolving the substance in the specified concentration. Furthermore, 1,3-propensultone (PRS), a cyclic compound containing sulfur, was dissolved in this non-aqueous solution at a concentration of 2% by mass to prepare a non-aqueous electrolyte.

[0106] (Preparing the separator) A microporous membrane made of polyolefin was prepared as a separator.

[0107] (Fabrication of non-aqueous electrolyte energy storage elements) An electrode body was fabricated by stacking the positive electrode and the negative electrode via the above separator. The electrode body was placed in a container made of a metal-resin composite film, the above non-aqueous electrolyte was injected into the container, and then the container was sealed by heat welding to obtain the non-aqueous electrolyte energy storage element of Example 1.

[0108] [Examples 2 to 6, Comparative Examples 1 to 4, and Reference Examples 1 to 3] Except for the type and volume ratio of the non-aqueous solvent used in the preparation of the non-aqueous electrolyte, the content of the sulfur-containing cyclic compound (PRS), and the type of negative electrode active material being as shown in Table 1, non-aqueous electrolyte energy storage elements for Examples 2 to 6, Comparative Examples 1 to 4, and Reference Examples 1 to 3 were obtained in the same manner as for Example 1. In Table 1, EC represents ethylene carbonate.

[0109] In each of the non-aqueous electrolyte energy storage elements in Reference Examples 1 to 3, a negative electrode was used in which the negative electrode active material, fabricated using the following procedure, was graphite. A negative electrode mixture paste was prepared by mixing graphite (Gr), the negative electrode active material; styrene-butadiene rubber (SBR), the binder; carboxymethylcellulose (CMC), the thickener; and water, the dispersion medium. The mass ratio of Gr, SBR, and CMC was 96:2:2 (on a solid content basis). The negative electrode mixture paste was applied to both sides of a copper foil with an average thickness of 8 μm, which served as the negative electrode substrate, and dried. Subsequently, a roll press was performed to obtain the negative electrode.

[0110] (Initial charge / discharge) For each of the non-aqueous electrolyte energy storage elements obtained in Examples 1 to 6, Comparative Examples 1 to 4, and Reference Examples 1 to 3, initial charging and discharging were performed at 25°C according to the following procedure. Constant current and constant voltage charging was performed with a charging current of 0.1C and a charging termination voltage of 4.6V. The charging termination condition was set to when the charging current became 0.05C. After that, a 10-minute rest period was observed. Subsequently, constant current discharge was performed with a discharge current of 0.1C and a discharge termination voltage of 2.0V.

[0111] (Initial capacity verification test) Next, initial capacity verification tests were performed on each non-aqueous electrolyte energy storage element of Examples 1 to 6 and Comparative Examples 1 to 4 at 25°C in the following manner: Constant current constant voltage charging was performed with a charging current of 0.1C and a charging termination voltage of 4.6V. The charging termination condition was when the charging current became 0.05C. After that, a 10-minute rest period was observed. Subsequently, constant current discharge was performed with a discharge current of 0.1C and a discharge termination voltage of 2.0V. After that, a 10-minute rest period was observed. Furthermore, for each of the non-aqueous electrolyte energy storage elements in Reference Examples 1 to 3, initial capacity verification tests were conducted in the same manner as described above, except that the charging termination voltage was set to 4.5V. The discharge capacities of each obtained non-aqueous electrolyte energy storage element are shown in Table 1 as the initial discharge capacities.

[0112] (Charge / discharge cycle test: 150 cycles) After initial capacity verification tests, each non-aqueous electrolyte energy storage element from Examples 1 to 6 and Comparative Examples 1 to 4 underwent a 150-cycle charge-discharge test at 25°C under the following procedure: Constant current and constant voltage charging was performed with a charging current of 0.2C and a termination voltage of 4.6V. The charging termination condition was when the charging current reached 0.05C. Subsequently, constant current discharge was performed with a discharge current of 0.1C and a termination voltage of 2.0V. A 10-minute rest period was provided after both charging and discharging. This charge-discharge cycle was performed 150 times. Furthermore, for each of the non-aqueous electrolyte energy storage elements in Reference Examples 1 to 3, charge-discharge cycle tests were conducted in the same manner as described above, except that the charge termination voltage was set to 4.5V. The discharge capacity at cycle 150 was divided by the discharge capacity at cycle 1 to determine the capacity retention rate (%) after 150 cycles. The results are shown in Table 1. Furthermore, the volume of each non-aqueous electrolyte energy storage element was measured before and after the charge-discharge cycle test, and the volume increase (cm³) after the charge-discharge cycle test was measured. 3 The result was calculated. The results are shown in Table 1.

[0113] [Table 1]

[0114] As shown in Table 1, the non-aqueous electrolyte energy storage elements in Examples 1 to 6, which used a negative electrode having metallic lithium as the negative electrode active material, had a fluorinated cyclic carbonate content of 20% by volume or more in the non-aqueous solvent, and contained a cyclic compound containing sulfur in a content of less than 10% by mass in the non-aqueous electrolyte, showed higher capacity retention rates after charge-discharge cycles than the non-aqueous electrolyte energy storage elements in Comparative Examples 1 to 4. Furthermore, among the comparative examples, the non-aqueous electrolyte energy storage elements in Examples 1 to 6 also showed suppressed volume increase after charge-discharge cycles compared to the non-aqueous electrolyte energy storage element of Comparative Example 1, which did not contain a cyclic compound containing sulfur in the non-aqueous electrolyte, and the non-aqueous electrolyte energy storage element of Comparative Example 4, which contained a fluorinated cyclic carbonate in a non-aqueous solvent of less than 20% by volume. On the other hand, in the non-aqueous electrolyte energy storage elements of Reference Examples 1 to 3, in which graphite was used as the negative electrode active material, the capacity retention rate after charge-discharge cycles was high and the volume increase was suppressed, regardless of the composition of the non-aqueous electrolyte. However, the initial discharge capacity was smaller compared to the non-aqueous electrolyte energy storage elements of Examples 1 to 6.

[0115] [Example 7] A non-aqueous electrolyte energy storage element of Example 7 was obtained in the same manner as in Example 1, except that the type and volume ratio of the non-aqueous solvent used in the preparation of the non-aqueous electrolyte were as shown in Table 2. The fluoroether in Table 2 is 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether. Following the same procedure as in Example 1, initial charge-discharge and initial capacity verification tests were also performed on the non-aqueous electrolyte energy storage element of Example 7.

[0116] (Charge / discharge cycle test: 50 cycles) After the initial capacity verification test, the non-aqueous electrolyte energy storage element of Example 7 underwent a 50-cycle charge-discharge test at 25°C. This charge-discharge test was conducted in the same manner as the 150-cycle charge-discharge test of Example 1 described above, except for the number of cycles. The capacity retention rate (%) after 50 cycles was calculated by dividing the discharge capacity at the 50th cycle by the discharge capacity at the 1st cycle. Similarly, for each non-aqueous electrolyte energy storage element from Examples 1 to 6, the capacity retention rate (%) after 50 cycles was calculated from the discharge capacities at the 1st and 50th cycles of the 150-cycle charge-discharge test described above. The results are shown in Table 2.

[0117] [Table 2]

[0118] As shown in Table 2, among the non-aqueous electrolyte energy storage elements of Examples 1 to 7, in which the fluorinated cyclic carbonate content in the non-aqueous solvent is 20% by volume or more and the non-aqueous electrolyte contains a cyclic compound containing sulfur element in an amount of less than 10% by mass, it can be seen that the non-aqueous electrolyte energy storage elements of Examples 1 to 6, in which the total content of fluorinated cyclic carbonate and chain carbonate in the non-aqueous solvent is 90% by volume or more, exhibit particularly high capacity retention rates after charge-discharge cycles. [Industrial applicability]

[0119] This invention can be applied to non-aqueous electrolyte energy storage elements used as power sources for electronic devices such as personal computers and communication terminals, as well as automobiles and the like. [Explanation of symbols]

[0120] 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 units 30 Energy storage devices

Claims

1. A negative electrode having metallic lithium, Non-aqueous solvent and non-aqueous electrolyte containing cyclic sulfonic acid ester Equipped with, The above non-aqueous solvent includes fluorinated cyclic carbonates and linear carbonates. The above chain-like carbonate is dimethyl carbonate. The content of the fluorinated cyclic carbonate in the above non-aqueous solvent is 20% by volume or more and 70% by volume or less. The content of the chain carbonate in the above non-aqueous solvent is 20% by volume or more and 80% by volume or less. A non-aqueous electrolyte energy storage element wherein the content of the cyclic sulfonic acid ester in the non-aqueous electrolyte is 3% by mass or more and less than 10% by mass. However, this excludes cases where the above non-aqueous electrolyte contains a cyclic imide salt.

2. The non-aqueous electrolyte energy storage element according to claim 1, wherein the total content of the fluorinated cyclic carbonate and the chain carbonate in the non-aqueous solvent is 90% by volume or more.

3. Preparing the positive electrode, Prepare the negative electrode, To prepare a non-aqueous electrolyte containing a non-aqueous solvent and a cyclic sulfonic acid ester, Equipped with, The negative electrode has metallic lithium, or the negative electrode has a surface region on which metallic lithium can be deposited, and the positive electrode has a positive electrode active material containing the element lithium. The above non-aqueous solvent includes fluorinated cyclic carbonates and linear carbonates. The above chain-like carbonate is dimethyl carbonate. The content of the fluorinated cyclic carbonate in the above non-aqueous solvent is 20% by volume or more and 70% by volume or less. The content of the chain carbonate in the above non-aqueous solvent is 20% by volume or more and 80% by volume or less. A method for producing a non-aqueous electrolyte energy storage element, wherein the content of the cyclic sulfonic acid ester in the non-aqueous electrolyte is 3% by mass or more and less than 10% by mass. However, this excludes cases where the above non-aqueous electrolyte contains a cyclic imide salt.

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