Nonaqueous electrolyte storage element and method of using same

Abstract

To provide a non-aqueous electrolyte power storage element which suppresses reduction of coulomb efficiency with a charge / discharge cycle and improves capacity maintainability after the charge / discharge cycle, and a method of application for such a non-aqueous electrolyte power storage element.SOLUTION: A non-aqueous electrolyte power storage element comprises: a cathode including a cathode active material layer containing a lithium transition metal composite oxide; and an anode including metal lithium at least in a state of charge. A mass of the cathode active material layer per unit area is 22 mg / cm2 or more, the lithium transition metal composite oxide is a compound which has an α-NaFeO2 structure and in which a molar ratio of the content of lithium elements with respect to transition metal elements exceeds 1.0, and a cathode potential at a discharge end voltage in normal use is 3.0 V vs.Li / Li+ or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a non-aqueous electrolyte electricity storage element and a method for using the same. [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 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] Metallic lithium is known as a negative electrode active material having a high energy density and used in non-aqueous electrolyte storage elements (see Patent Documents 1 and 2). 0.6 Co 0.2 Mn 0.2 Lithium transition metal composite oxides having an α-NaFeO2 type crystal structure, such as O2 and LiCoO2, are known (see Patent Document 3). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-100065 [Patent Document 2] Japanese Patent Application Publication No. 7-245099 [Patent Document 3] Japanese Patent Application Laid-Open No. 2011-216485 Summary of the Invention [Problem to be solved by the invention]

[0005] One possible way to increase the energy density, capacity density, etc. of a nonaqueous electrolyte storage element is to thicken the active material layer, i.e., increase the mass per unit area of ​​the active material layer. However, in a nonaqueous electrolyte storage element using metallic lithium as the negative electrode active material, if the mass per unit area of ​​the positive electrode active material layer containing a lithium transition metal composite oxide is increased, the coulombic efficiency is likely to decrease with charge-discharge cycles. Furthermore, a nonaqueous electrolyte storage element is desired to have a high capacity retention rate after charge-discharge cycles.

[0006] An object of the present invention is to provide a nonaqueous electrolyte storage element comprising a positive electrode having a positive electrode active material layer containing a lithium transition metal composite oxide and having a large mass per unit area, and a negative electrode having metallic lithium at least in a charged state, wherein the nonaqueous electrolyte storage element is inhibited from decreasing in coulombic efficiency due to charge / discharge cycles and has a high capacity retention rate after charge / discharge cycles, and a method for using such a nonaqueous electrolyte storage element. [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 containing a lithium transition metal composite oxide, and a negative electrode having metallic lithium at least in a charged state, wherein the mass per unit area of ​​the positive electrode active material layer is 22 mg / cm. 2 or more, the lithium transition metal composite oxide has an α-NaFeO2 structure, and is a compound in which the content of lithium element to transition metal element exceeds 1.0 in molar ratio, and the positive electrode potential at the discharge end voltage during normal use is 3.0 V vs. Li / Li + The following is the result.

[0008] A method of using a nonaqueous electrolyte storage element according to another aspect of the present invention comprises: + and discharging the nonaqueous electrolyte electricity storage element until the battery reaches a voltage of 0.1 V. [Effects of the Invention]

[0009] According to one aspect of the present invention, it is possible to provide a nonaqueous electrolyte storage element including a positive electrode having a positive electrode active material layer that contains a lithium transition metal composite oxide and has a large mass per unit area, and a negative electrode that has metallic lithium at least in a charged state, wherein the nonaqueous electrolyte storage element is inhibited from decreasing in coulombic efficiency due to charge-discharge cycles and has a high capacity retention rate after charge-discharge cycles, and a method for using such a nonaqueous electrolyte storage element. [Brief explanation of the drawings]

[0010] [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

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

[0012] A nonaqueous electrolyte storage element according to one aspect of the present invention includes a positive electrode having a positive electrode active material layer containing a lithium transition metal composite oxide, and a negative electrode having metallic lithium at least in a charged state, wherein the mass per unit area of ​​the positive electrode active material layer is 22 mg / cm. 2 or more, the lithium transition metal composite oxide has an α-NaFeO2 structure, and is a compound in which the content of lithium element to transition metal element exceeds 1.0 in molar ratio, and the positive electrode potential at the discharge end voltage during normal use is 3.0 V vs. Li / Li + The following is the result.

[0013] The nonaqueous electrolyte storage element includes a positive electrode having a positive electrode active material layer containing a lithium transition metal composite oxide and having a large mass per unit area, and a negative electrode having metallic lithium at least in a charged state. The element suppresses the decrease in Coulombic efficiency associated with charge-discharge cycles and exhibits a high capacity retention rate after charge-discharge cycles. While the reason for this is unclear, the following is presumed. In general, in nonaqueous electrolyte storage elements having a negative electrode containing metallic lithium as the negative electrode active material, metallic lithium may deposit in a dendritic form on the negative electrode surface during charging (hereinafter, dendritic metallic lithium is referred to as "dendrite"). If these dendrites grow and penetrate the separator to come into contact with the positive electrode, a short circuit occurs, resulting in a decrease in Coulombic efficiency. It is believed that dendrite growth is promoted by the deposition of transition metal elements eluted from the lithium transition metal composite oxide in the positive electrode active material layer on the negative electrode surface. However, a compound having an α-NaFeO2 structure and in which the molar ratio of lithium to transition metal exceeds 1.0 (hereinafter also referred to as "lithium-excess active material") is less likely to cause elution of the transition metal element, even among lithium transition metal composite oxides. For this reason, it is presumed that a nonaqueous electrolyte energy storage element using this lithium-excess active material will suppress the growth of dendrites accompanying charge-discharge cycles, and as a result, will suppress a decrease in Coulomb efficiency. Furthermore, among lithium transition metal composite oxides, lithium-excess active materials are less likely to undergo particle cracking and changes in crystal structure accompanying charge-discharge cycles. For this reason, it is presumed that a nonaqueous electrolyte energy storage element using this lithium-excess active material will have a high capacity retention rate after charge-discharge cycles. Furthermore, a nonaqueous electrolyte energy storage element having a positive electrode containing a lithium-excess active material was tested at a positive electrode potential of 3.0 V vs. Li / Li + When used in a range more noble than Li, the capacity density per unit area decreases, which suppresses the decrease in coulomb efficiency, but the capacity retention rate after charge-discharge cycling decreases. This is because the reaction potential of the lithium-excess active material changes due to changes in the crystalline structure of the lithium-excess active material caused by charge-discharge cycling, and the positive electrode potential becomes 3.0 V vs. Li / Li. +This is thought to be because the discharge reaction does not occur sufficiently unless the device is used within the range below. In this non-aqueous electrolyte storage element, the positive electrode potential at the discharge end voltage during normal use is 3.0 V vs. Li / Li. + or less, and it is presumed that the capacity retention rate after charge-discharge cycling is high because the discharge reaction occurs sufficiently even when the crystal structure of the lithium-excess active material changes with the charge-discharge cycle and the reaction potential changes.

[0014] The negative electrode provided in the nonaqueous electrolyte storage element only needs to contain metallic lithium at least in a charged state, and need not contain metallic lithium in a discharged state. For example, the nonaqueous electrolyte storage element may be configured such that metallic lithium is deposited on at least a partial region of the negative electrode surface during charging, so that the negative electrode contains metallic lithium in a charged state, and that substantially all of the metallic lithium on the negative electrode surface is eluted into the nonaqueous electrolyte as lithium ions during discharge, so that the negative electrode does not substantially contain metallic lithium in a discharged state.

[0015] Mass per unit area of ​​the positive electrode active material layer (mg / cm 2 )" refers to the positive electrode active material layer of 1 cm 2 For example, when a positive electrode active material layer is provided on each side of the positive electrode substrate, the "mass per unit area of ​​the positive electrode active material layer (mg / cm)" is 2 When a positive electrode active material layer is provided on only one surface of the positive electrode substrate, the term "mass of one positive electrode active material layer" refers to the mass of that single positive electrode active material layer.

[0016] The composition ratio of the lithium transition metal composite oxide refers to the composition ratio when the battery is fully discharged using the following method. First, the nonaqueous electrolyte storage element is charged at a constant current of 0.05 C until it reaches the end-of-charge voltage in normal use, and then fully charged. After a 30-minute rest, it is discharged at a constant current of 0.05 C until it reaches the end-of-charge voltage (lower limit voltage) in normal use. The battery is then disassembled, the positive electrode is removed, and a test battery is assembled using a metallic lithium electrode as the counter electrode. The positive electrode potential is measured at a current of 10 mA per 1 g of positive electrode mixture until the positive electrode potential reaches 2.0 V vs. Li / Li.+ The positive electrode is then fully discharged by constant current discharge until the electrode temperature reaches 100°C. Pure metallic lithium is used for the metallic lithium electrode. The device is then 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 the device is dried overnight at room temperature, after which the lithium transition metal composite oxide is extracted. The extracted lithium transition metal composite oxide is then subjected to measurement. The entire process from disassembling the non-aqueous electrolyte storage element to extracting the lithium transition metal composite oxide is carried out in an argon atmosphere with a dew point of -60°C or below.

[0017] "During normal use" refers to a case where the nonaqueous electrolyte storage element is used under the charging and discharging conditions recommended or specified for the nonaqueous electrolyte storage element. The charging conditions refer to a case where the nonaqueous electrolyte storage element is used using a charger, if one is provided for the nonaqueous electrolyte storage element. The discharging conditions are determined, for example, by the settings of the electrical equipment in which the nonaqueous electrolyte storage element is used.

[0018] A method of using a nonaqueous electrolyte storage element according to another aspect of the present invention comprises: + and discharging the nonaqueous electrolyte electricity storage element until the battery reaches a voltage of 0.1 V.

[0019] According to the method for using the nonaqueous electrolyte storage element, the nonaqueous electrolyte storage element is provided with a positive electrode having a positive electrode active material layer that contains a lithium transition metal composite oxide and has a large mass per unit area, and a negative electrode that has metallic lithium at least in a charged state, and the decrease in coulombic efficiency that accompanies charge-discharge cycles is suppressed, and the capacity retention rate after charge-discharge cycles is high.

[0020] A nonaqueous electrolyte electricity storage element according to one embodiment of the present invention, a method for using the same, an electrical device, an electricity storage device, a method for manufacturing a nonaqueous electrolyte electricity storage element, 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.

[0021] <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 stacked type in which a plurality of positive electrodes and a plurality of negative electrodes are stacked with separators interposed therebetween, or a wound type in which a positive electrode and a negative electrode are stacked with separators interposed therebetween and wound. The nonaqueous electrolyte is present in a state of being impregnated in the positive electrode, the negative electrode, and the separator. As an example of a nonaqueous electrolyte storage element, a nonaqueous electrolyte secondary battery (hereinafter also simply referred to as "secondary battery") will be described.

[0022] (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.

[0023] The positive electrode substrate is electrically conductive. Whether or not it has "electrical conductivity" is determined by whether the volume resistivity measured in accordance with JIS-H-0505 (1975) is 10 7 The threshold value is Ω·cm. The material of the positive electrode substrate is a metal such as aluminum, titanium, tantalum, or stainless steel, or an alloy of these. Among these, aluminum or an aluminum alloy is preferred from the viewpoints of potential resistance, high conductivity, and cost. Positive electrode substrates include foils, vapor-deposited films, meshes, and porous materials, 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).

[0024] The average thickness of the positive electrode substrate is preferably 3 μm to 50 μm, more preferably 5 μm to 40 μm, even more preferably 8 μm to 30 μm, and particularly preferably 10 μm to 25 μm. By setting the average thickness of the positive electrode substrate within the above range, the strength of the positive electrode substrate can be increased while increasing the energy density per volume of the nonaqueous electrolyte storage element.

[0025] 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.

[0026] The positive electrode active material layer contains a lithium transition metal composite oxide. The lithium transition metal composite oxide is a component that functions as a positive electrode active material. The positive electrode active material layer contains optional components such as a conductive agent, a binder, a thickener, and a filler as needed. The positive electrode active material layer is preferably formed from a positive electrode mixture containing the lithium transition metal composite oxide and other optional components.

[0027] The lithium transition metal composite oxide is a compound (lithium-excess active material) having an α-NaFeO2 structure and a lithium to transition metal molar ratio of more than 1.0. The use of such a lithium-excess active material as the positive electrode active material can increase the capacity density, energy density, etc. of the nonaqueous electrolyte energy storage element, suppress the decrease in coulombic efficiency due to charge-discharge cycles, and increase the capacity retention rate after charge-discharge cycles.

[0028] The lithium-excess active material preferably contains nickel or manganese as a transition metal element, and more preferably contains both nickel and manganese. The molar ratio ((Ni+Mn) / Me) of the sum of nickel (Ni) and manganese (Mn) to the transition metal element (Me) in the lithium-excess active material is preferably 0.5 or more, more preferably 0.8 or more, and more preferably 0.9 or more. The lithium transition metal composite oxide may further contain another transition metal element such as cobalt. The lithium-excess active material has a molar ratio (Li / Me) of lithium (Li) to the transition metal element (Me) greater than 1.0, preferably 1.1 or more, more preferably 1.2 or more, and even more preferably 1.3 or more. The upper limit of the lithium-to-transition metal molar ratio (Li / Me) is preferably 1.6, more preferably 1.5. The lithium-excess active material may contain elements other than lithium, transition metal, and oxygen (e.g., aluminum, etc.).

[0029] The lithium-excess active material is preferably a compound represented by the following formula (1). Li 1+α Me 1-α O2···(1) In formula (1), Me is a transition metal element including Ni or Mn, and 0<α<1.

[0030] In formula (1), Me preferably contains Ni and Mn. Me preferably consists essentially of two elements, Ni and Mn, or three elements, Ni, Mn, and Co. Me may contain other transition metal elements.

[0031] In formula (1), the lower limit of the molar ratio of Ni to Me (Ni / Me) is preferably 0.1, more preferably 0.2. On the other hand, the upper limit of this molar ratio (Ni / Me) is preferably 0.5, more preferably 0.45. By setting the molar ratio (Ni / Me) within the above range, the energy density and other properties are improved.

[0032] In formula (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, more preferably 0.7. By setting the molar ratio (Mn / Me) within the above range, the energy density and other properties are improved.

[0033] 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, while the lower limit of this molar ratio (Co / Me) may be 0.

[0034] In formula (1), the molar ratio of Li to Me (Li / Me), i.e., (1+α) / (1-α), is greater than 1.0 (α>0), preferably 1.1 or greater, 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, more preferably 1.5.

[0035] The lithium-excess active material is preferably one that does not have a diffraction peak in the range of 20° to 22° in an X-ray diffraction pattern using CuKα radiation. Generally, the lithium-excess active material is used when the positive electrode potential is, for example, 4.5 V vs. Li / Li + The discharge capacity increases as a result of the initial charge up to this point. Furthermore, due to the change in the crystal structure during the initial charge, the diffraction peak in the range of 20° to 22° that existed before the initial charge disappears. In other words, a lithium-excess active material that does not have a diffraction peak in the range of 20° to 22° in the X-ray diffraction pattern has a large discharge capacity.

[0036] X-ray diffraction measurements of lithium transition metal composite oxides were performed on the lithium transition metal composite oxides in a fully discharged state using the method described above for determining the composition ratio of the lithium transition metal composite oxides. Specifically, X-ray diffraction measurements were performed by powder X-ray diffraction using an X-ray diffractometer (Rigaku's "MiniFlex II") with a CuKα source, a tube voltage of 30 kV, and a tube current of 15 mA. The diffracted X-rays passed through a 30 μm-thick Kβ filter and were detected by a high-speed one-dimensional detector (D / teX Ultra 2). The sampling width was 0.02°, the scan speed was 5° / min, the divergence slit width was 0.625°, the receiving slit width was 13 mm (open), and the scattering slit width was 8 mm.

[0037] The content of the lithium-excess active material in the positive electrode active material layer is preferably 50% by mass to 99% by mass, more preferably 70% by mass to 98% by mass, and even more preferably 80% by mass to 95% by mass. By setting the content of the lithium-excess active material within the above range, both high energy density and manufacturability of the positive electrode active material layer can be achieved.

[0038] The positive electrode active material layer may further contain a positive electrode active material other than the lithium-excess active material. Examples of the other positive electrode active material include lithium transition metal composite oxides having an α-NaFeO2 crystal structure other than the lithium-excess active material, lithium transition metal composite oxides having a spinel crystal structure, polyanion compounds, chalcogen compounds, and sulfur. However, the content of the lithium-excess active material relative to the total positive electrode active material contained in the positive electrode active material layer is preferably 60% by mass or more, more preferably 90% by mass or more, even more preferably 99% by mass or more, and even more preferably 100% by mass. By using substantially only the lithium-excess active material as the positive electrode active material, the decrease in Coulombic efficiency associated with charge-discharge cycling of the nonaqueous electrolyte storage element can be further suppressed and the capacity retention rate after charge-discharge cycling can be further increased.

[0039] Positive electrode active materials, such as lithium-excess active materials, are usually particles (powder). The average particle size of the positive electrode active material is preferably, for example, 0.1 μm to 20 μm. By setting the average particle size of the positive electrode active material to the above lower limit or above, the positive electrode active material becomes easier to manufacture and handle. By setting the average particle size of the positive electrode active material to the above upper limit or below, the electronic conductivity of the positive electrode active material layer is improved. When a composite of the positive electrode active material and another material is used, the average particle size of the composite is taken as the average particle size of the positive electrode active material. "Average particle size" refers to the value at which the volume-based cumulative distribution calculated in accordance with JIS-Z-8819-2 (2001) is 50% based on the particle size distribution measured by laser diffraction / scattering in a diluted solution obtained by diluting particles with a solvent in accordance with JIS-Z-8825 (2013).

[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 positive electrode active material in the positive electrode active material layer is preferably 50% by mass to 99% by mass, more preferably 70% by mass to 98% by mass, and even more preferably 80% by mass to 95% by mass. By setting the content of the positive electrode active material within this range, both high energy density and manufacturability of the positive electrode active material layer can be achieved.

[0042] 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 CNTs may be used. Among these, carbon black is preferred from the viewpoints of electronic conductivity and coatability, and acetylene black is particularly preferred.

[0043] The content of the conductive agent in the positive electrode active material layer is preferably 1% by mass to 10% by mass, more preferably 3% by mass to 9% by mass. By setting the content of the conductive agent within this range, the energy density of the nonaqueous electrolyte storage element can be increased.

[0044] 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.

[0045] The binder content in the positive electrode active material layer is preferably 1% by mass to 10% by mass, more preferably 3% by mass to 9% by mass. By setting the binder content within this range, the active material can be stably held.

[0046] 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, it may be preferable that the thickener is not contained in the positive electrode active material layer.

[0047] 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 aluminosilicate, 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, it may be preferable that the positive electrode active material layer does not contain a filler.

[0048] 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 positive electrode active material, conductive agent, binder, thickener, and filler.

[0049] The lower limit of the mass per unit area of ​​the positive electrode active material layer is 22 mg / cm 2 and 24 mg / cm 2 is preferred, and 26 mg / cm 2 , 28 mg / cm 2 , 30 mg / cm 2 or 32 mg / cm 2In this way, by setting the mass per unit area of ​​the positive electrode active material layer to the above lower limit or more, the energy density, capacity density, etc. of the nonaqueous electrolyte storage element can be increased. On the other hand, the upper limit of the mass per unit area of ​​the positive electrode active material layer is, for example, 50 mg / cm. 2 45 mg / cm 2 , 40 mg / cm 2 , 35 mg / cm 2 or 30 mg / cm 2 The mass per unit area 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.

[0050] (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.

[0051] The negative electrode substrate is conductive. Metals such as copper, nickel, stainless steel, and nickel-plated steel, alloys thereof, and carbonaceous materials are used as the material of the negative electrode substrate. Among these, copper or copper alloys are preferred. Examples of the negative electrode substrate include foil, vapor-deposited film, mesh, and porous material, with foil being preferred from the viewpoint of cost. Therefore, copper foil or copper alloy foil is preferred as the negative electrode substrate. Examples of copper foil include rolled copper foil and electrolytic copper foil.

[0052] 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 volume of the nonaqueous electrolyte storage element.

[0053] The negative electrode active material layer contains metallic lithium at least in a charged state. That is, the negative electrode active material layer is a layer containing metallic lithium at least in a charged state. Metallic lithium is a component that functions as a negative electrode active material. Metallic lithium may exist as pure metallic lithium consisting essentially of lithium element alone, or may exist as a lithium alloy containing other elements. Examples of lithium alloys include lithium-silver alloys, lithium-zinc alloys, lithium-calcium alloys, lithium-aluminum alloys, lithium-magnesium alloys, and lithium-indium alloys. The lithium alloy may contain multiple elements other than lithium element.

[0054] The negative electrode active material layer may be a layer consisting essentially of metallic lithium. The content of metallic lithium in the negative electrode active material layer may be 90% by mass or more, 99% by mass or more, or even 100% by mass.

[0055] The negative electrode active material layer may be a pure metallic lithium foil or a 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 containing particles containing metallic lithium. The negative electrode active material layer, which is a porous layer containing particles containing metallic lithium, may further contain, for example, resin particles, inorganic particles, etc.

[0056] The negative electrode active material layer, i.e., the layer containing metallic lithium, is preferably a layer that exists even in a discharged state, i.e., a layer that exists in all states from a charged state to a discharged state. The average thickness of the negative electrode active material layer in a discharged 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 thicknesses measured at any five locations. When the negative electrode active material containing metallic lithium exists even in a discharged state, and preferably when its average thickness is equal to or greater than the above-mentioned lower limit, a sufficient amount of metallic lithium is present, thereby further increasing the capacity retention rate of the nonaqueous electrolyte storage element after charge-discharge cycling.

[0057] In addition, in the case of a nonaqueous electrolyte storage element configured such that metallic lithium is precipitated on at least a portion of the surface region of the negative electrode during charging and that substantially all of the metallic lithium of the negative electrode is eluted into the nonaqueous electrolyte as lithium ions during discharging, the negative electrode may not substantially have a negative electrode active material layer in a discharged state.

[0058] (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.

[0059] The heat-resistant particles contained in the heat-resistant layer preferably exhibit a mass loss of 5% or less when heated from room temperature to 500°C 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 aluminosilicate; nitrides such as aluminum nitride and silicon nitride; carbonates such as calcium carbonate; sulfates such as barium sulfate; sparingly soluble ionic crystals such as calcium fluoride, barium fluoride, and barium titanate; covalently bonded crystals such as silicon and diamond; mineral-derived substances such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica, as well as artificial products thereof. As the inorganic compound, these substances may be used alone or in the form of a complex, or two or more of them may be used in combination. Among these inorganic compounds, silicon oxide, aluminum oxide, or aluminosilicate is preferred from the viewpoint of the safety of the energy storage device.

[0060] The porosity of the separator is preferably 80% by volume or less from the viewpoint of strength, and is preferably 20% by volume or more from the viewpoint of discharge performance. Here, "porosity" refers to a volume-based value measured with a mercury porosimeter.

[0061] 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.

[0062] (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.

[0063] 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.

[0064] 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. Preferred cyclic carbonates are fluorinated cyclic carbonates such as FEC and DFEC, with FEC being more preferred. The use of a fluorinated cyclic carbonate can further increase the capacity retention rate after charge-discharge cycling.

[0065] Examples of the chain carbonate include diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diphenyl carbonate, trifluoroethyl methyl carbonate (TFEMC), bis(trifluoroethyl) carbonate (FDEC), etc. As the chain carbonate, fluorinated chain carbonates such as trifluoroethyl methyl carbonate (TFEMC) and bis(trifluoroethyl) carbonate (FDEC) are preferred. By using a fluorinated chain carbonate, it is possible to further increase the capacity retention rate after charge-discharge cycling.

[0066] It is preferable to use a cyclic carbonate or a chain carbonate as the non-aqueous solvent, and it is more preferable to use a combination of a cyclic carbonate and a chain carbonate. The use of a cyclic carbonate can promote dissociation of the electrolyte salt and improve the ionic conductivity of the non-aqueous electrolyte. The use of a chain carbonate can reduce the viscosity of the non-aqueous electrolyte. When a cyclic carbonate and a chain carbonate are used in combination, the volume ratio of the cyclic carbonate to the chain carbonate (cyclic carbonate:chain carbonate) is preferably in the range of, for example, 5:95 to 50:50.

[0067] The content of carbonate (total of cyclic carbonate and chain carbonate) relative to the total non-aqueous solvent is preferably 50% by volume or more and 100% by volume or less, more preferably 80% by volume or more, and in some cases even more preferably 90% by volume or more, 95% by volume or more, or 99% by volume or more.

[0068] The non-aqueous solvent preferably contains a fluorinated solvent. The content of the fluorinated solvent relative to the total non-aqueous solvent is preferably 60% by volume or more, more preferably 90% by volume or more, even more preferably 99% by volume or more, and particularly preferably 100% by volume. The fluorinated solvent refers to a solvent (non-aqueous solvent) having a fluorine atom in the molecule, such as a fluorinated carbonate (fluorinated chain carbonate or fluorinated cyclic carbonate). By including a fluorinated solvent in the non-aqueous solvent, preferably at a content above the lower limit, the capacity retention rate after charge / discharge cycles can be further increased.

[0069] As the electrolyte salt, a lithium salt is usually used. Examples of the lithium salt 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 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.

[0070] 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 1.5mol / 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.

[0071] The non-aqueous electrolyte may contain additives in addition to the non-aqueous solvent and the electrolyte salt. Examples of the additives include aromatic compounds such as biphenyl, alkylbiphenyl, terphenyl, partially hydrogenated terphenyl, cyclohexylbenzene, t-butylbenzene, t-amylbenzene, diphenyl ether, and dibenzofuran; partial halides of the 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, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, and cyclohexanedicarboxylic anhydride; ethylene sulfite, propylene sulfite, and sulfur dioxide. Examples of the additives include dimethyl ether, 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, and lithium difluorophosphate. These additives may be used alone or in combination of two or more.

[0072] The content of the additive contained in the non-aqueous electrolyte is preferably 0.01% by mass to 10% by mass, more preferably 0.1% by mass to 7% by mass, even more preferably 0.2% by mass to 5% by mass, and particularly preferably 0.3% by mass to 3% by mass. By setting the content of the additive within the above range, it is possible to improve the capacity retention performance or cycle performance after high-temperature storage, and further improve safety.

[0073] The non-aqueous electrolyte may be a combination of a non-aqueous electrolytic solution and a solid electrolyte.

[0074] (positive electrode potential) In this non-aqueous electrolyte storage element, the positive electrode potential at the discharge end voltage during normal use is 3.0 V vs. Li / Li + is less than 2.8V vs. Li / Li + It is more preferable that it is 2.6V vs. Li / Li or less. + Below, 2.4V vs.Li / Li + Below, 2.2V vs.Li / Li + or less than 2.0V vs. Li / Li + In some cases, it may be even more preferable that the positive electrode potential at the discharge cut-off voltage during normal use is set to the above-mentioned value or less. By setting the positive electrode potential at the discharge cut-off voltage during normal use to the above-mentioned value or less, the capacity retention rate after charge / discharge cycling can be increased. Furthermore, by setting the positive electrode potential at the discharge cut-off voltage during normal use to the above-mentioned value or less, the discharge capacity can also be increased.

[0075] The positive electrode potential at the discharge end voltage during normal use of the nonaqueous electrolyte storage element is, for example, 1.0 V vs. Li / Li + It may be 1.5V vs. Li / Li or more. + It may be 2.0V vs. Li / Li or more. + It may be more than that.

[0076] 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.

[0077] 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.

[0078] <How to use the nonaqueous electrolyte energy storage element> A method of using the nonaqueous electrolyte storage element according to one embodiment of the present invention is to use the nonaqueous electrolyte storage element according to one embodiment of the present invention in a battery at a positive electrode potential of 3.0 V vs. Li / Li. + Discharging until:

[0079] In this method, the nonaqueous electrolyte storage element according to the embodiment of the present invention is used. + It is recommended to discharge the battery until it reaches 2.6V vs. Li / Li. + Below, 2.4V vs.Li / Li + Below, 2.2V vs.Li / Li + or less than 2.0V vs. Li / Li + It may be more preferable to discharge the battery until the positive electrode potential reaches, for example, 1.0 V vs. Li / Li. + It may be 1.5V vs. Li / Li or more. + It may be 2.0V vs. Li / Li or more. + According to the method for using the nonaqueous electrolyte electricity storage element, the decrease in coulombic efficiency accompanying charge-discharge cycling of the nonaqueous electrolyte electricity storage element is suppressed, and the capacity retention rate after charge-discharge cycling is high.

[0080] <Electrical equipment> Electrical devices including the nonaqueous electrolyte electricity storage element according to one embodiment of the present invention include, but are not limited to, electronic devices such as personal computers and communication terminals, home appliances, automobiles such as electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs), and other industrial electrical devices.

[0081] When the electrical equipment is in use, the positive electrode potential at the discharge end voltage of the nonaqueous electrolyte storage element is 3.0 V vs. Li / Li + It is preferable that the positive electrode potential is set to 3.0 V vs. Li / Li. + It is preferable that the nonaqueous electrolyte storage element is configured so that it can be used if it has a remaining amount of electricity until the discharge end voltage reaches 0. A nonaqueous electrolyte storage element provided in the electrical equipment configured in this manner suppresses the decrease in coulombic efficiency that occurs with charge / discharge cycles and also has a high capacity retention rate after charge / discharge cycles. The preferred range of the positive electrode potential at the discharge end voltage of the nonaqueous electrolyte storage element provided in the electrical equipment is the same as the preferred range of the positive electrode potential at the discharge end voltage during normal use of the nonaqueous electrolyte storage element according to one embodiment of the present invention described above.

[0082] <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 1 in an automobile power source such as an electric vehicle (EV), a hybrid electric vehicle (HEV), or a plug-in hybrid electric vehicle (PHEV), a power source for electronic devices such as a personal computer or a communication terminal, or a power storage power source, etc. In this case, the technology of the present invention may be applied to at least one nonaqueous electrolyte energy storage element included in the energy storage unit.

[0083] 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 two or more nonaqueous electrolyte electricity storage elements 1, a bus bar (not shown) that electrically connects 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 one or more nonaqueous electrolyte electricity storage elements.

[0084] <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.

[0085] Preparing a positive electrode may also mean fabricating a positive electrode. The positive electrode can be fabricated, for example, by applying a paste-like positive electrode mixture (positive electrode mixture paste) to a positive electrode substrate directly or via an intermediate layer, and then drying the mixture to form a positive electrode active material layer. Specific examples and preferred examples of the prepared positive electrode are the same as those of the positive electrode included in the nonaqueous electrolyte storage element according to one embodiment of the present invention.

[0086] Preparing a negative electrode may also mean fabricating a negative electrode. The negative electrode to be prepared is typically a negative electrode containing metallic lithium, or a negative electrode having a surface region on which metallic lithium can be deposited during charging. A negative electrode containing metallic lithium can be fabricated by laminating a negative electrode active material layer containing metallic lithium directly or via an intermediate layer onto a negative electrode substrate, and then pressing the laminate. The negative electrode active material layer containing metallic lithium may be a pure metallic lithium foil or a lithium alloy foil. Specific and preferred examples of the prepared negative electrode containing metallic lithium include specific and preferred examples of the negative electrode provided in the nonaqueous electrolyte energy storage element according to one embodiment of the present invention.

[0087] The negative electrode having a surface region on which metallic lithium can be deposited during charging may be, for example, a negative electrode consisting of only 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 element is prepared in advance.

[0088] The method for producing the nonaqueous electrolyte energy storage element may further include initially charging and discharging an uncharged energy storage element assembled using a positive electrode, a negative electrode, and a nonaqueous electrolyte. The positive electrode potential (positive electrode potential) at the end-of-charge voltage in the initial charge and discharge is 4.5 V vs. Li / Li + More than 4.7V vs.Li / Li + Since the nonaqueous electrolyte storage element uses a lithium-excess active material as the positive electrode active material, the positive electrode potential is preferably 4.5 V vs. Li / Li or less. + The discharge capacity increases by undergoing the initial charge up to this point. The number of charge / discharge cycles in the initial charge / discharge may be one or two, or may be three or more.

[0089] <Other embodiments> The energy storage device of the present invention is not limited to the above-described embodiments, and various modifications may be made without departing from the spirit of the present invention. For example, the configuration of one embodiment can be added to the configuration of another embodiment, and part of the configuration of one embodiment can be replaced with the configuration of another embodiment or well-known technology. Furthermore, part of the configuration of one embodiment can be deleted. Also, well-known technology can be added to the configuration of one embodiment.

[0090] In the above embodiment, the nonaqueous electrolyte storage element is used as a chargeable and dischargeable nonaqueous electrolyte secondary battery (e.g., a lithium secondary battery), but the nonaqueous electrolyte storage element may be of any type, shape, size, capacity, etc. The present invention can also be applied to various secondary batteries, electric double layer capacitors, lithium ion capacitors, and other capacitors.

[0091] 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]

[0092] 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.

[0093] [Example 1] (Preparation of positive electrode) The positive electrode active material has an α-NaFeO2 type crystal structure and Li 1+α Me 1-α A lithium transition metal composite oxide (Li-rich) represented by O2 (Me is a transition metal element) was used. Here, the molar ratio of Li to Me, Li / Me, was 1.33, and Me was composed of Ni and Mn, with a molar ratio of Ni:Mn=1:2.

[0094] A positive electrode mixture paste was prepared using N-methylpyrrolidone (NMP) as a dispersion medium, containing the above-mentioned positive electrode active material (Li-rich), acetylene black (AB) as a conductive agent, and polyvinylidene fluoride (PVDF) as a binder in a mass ratio of 92.5:4.5:3.0 as solids. The positive electrode mixture paste was applied to one side of an aluminum foil with an average thickness of 15 μm as a positive electrode substrate, dried, and pressed to obtain a mass per unit area of ​​27 mg / cm. 2 A positive electrode was fabricated in which a positive electrode active material layer having the above formula was disposed.

[0095] (Preparation of negative electrode) A pure metallic lithium foil with an average thickness of 100 μm was laminated on one side of a copper foil with an average thickness of 8 μm, which was the negative electrode substrate, and then pressed to prepare a negative electrode with a negative electrode active material layer disposed thereon.

[0096] (Preparation of non-aqueous electrolyte) A non-aqueous solvent was prepared by mixing fluoroethylene carbonate (FEC), a fluorinated cyclic carbonate, and 2,2,2-trifluoroethyl methyl carbonate (TFEMC), a fluorinated chain carbonate, in a volume ratio of 30:70. The lithium salt LiPF6 was added at a concentration of 1.0 mol / dm 3 A non-aqueous solution was prepared by dissolving 1,3-propene sultone in an amount of 2 mass % in this non-aqueous solution. A non-aqueous electrolyte was prepared by dissolving 1,3-propene sultone in an amount of 2 mass % in this non-aqueous solution.

[0097] (Preparing the separator) A microporous polyolefin film was prepared as a separator.

[0098] (Fabrication of non-aqueous electrolyte energy storage element) The positive electrode and the negative electrode were stacked with the separator interposed therebetween to prepare an electrode assembly, which was then housed in a container made of a metal resin composite film, the nonaqueous electrolyte was poured into the container, and the container was then sealed by thermal welding to obtain the nonaqueous electrolyte storage element of Example 1.

[0099] [Examples 2 and 3, Comparative Examples 1 to 5, and Reference Examples 1 and 2] The nonaqueous electrolyte storage elements of Examples 2 and 3, Comparative Examples 1 to 5, and Reference Examples 1 and 2 were obtained in the same manner as in Example 1, except that the type of positive electrode active material and the mass per unit area of ​​the positive electrode active material layer were as shown in Table 1. Note that NCM622, NCM811, and LCO in the column for positive electrode active material in Table 1 are the following compounds each having an α-NaFeO2-type crystal structure. NCM622:LiNi 0.6 Co 0.2 Mn 0.2 O2 NCM811:LiNi 0.8 Co 0.1 Mn 0.1 O2 LCO: LiCoO2

[0100] [Reference examples 3 and 4] The nonaqueous electrolyte storage elements of Reference Examples 3 and 4 were obtained in the same manner as in Example 1, except that the mass per unit area of ​​the positive electrode active material layer was set as shown in Table 1 and a negative electrode prepared by the following procedure was used as the negative electrode. (Preparation of negative electrode used in Reference Example 3) A negative electrode mixture paste containing graphite (Gr), styrene butadiene rubber (SBR), and carboxymethyl cellulose (CMC) in a mass ratio of 96.7:2.1:1.2 (solid content equivalent) was prepared using water as a dispersion medium. This negative electrode mixture paste was applied to a copper foil with an average thickness of 10 μm as a negative electrode substrate, dried, and then roll-pressed to obtain a mass per unit area of ​​21 mg / cm. 2 Thus, a negative electrode having a negative electrode active material layer of the above composition was obtained. (Preparation of negative electrode used in Reference Example 4) A negative electrode mixture paste containing graphite (Gr), styrene butadiene rubber (SBR), and carboxymethyl cellulose (CMC) in a mass ratio of 96.7:2.1:1.2 (solid content equivalent) was prepared using water as a dispersion medium. This negative electrode mixture paste was applied to a copper foil with an average thickness of 10 μm as a negative electrode substrate, dried, and then roll-pressed to obtain a mass per unit area of ​​11 mg / cm. 2 Thus, a negative electrode having a negative electrode active material layer of the above composition was obtained.

[0101] (Initial charge / discharge) Each of the obtained nonaqueous electrolyte storage elements was initially charged and discharged at 25°C in the following manner. Constant-current, constant-voltage charging was performed with a charging current of 0.1 C and a charge cut-off voltage of 4.6 V (4.5 V for Reference Examples 3 and 4). Charging was terminated until the charging current reached 0.05 C. A 10-minute rest period was then provided. Subsequently, constant-current discharge was performed with a discharge current of 0.1 C and a discharge cut-off voltage shown in Table 1. A 10-minute rest period was then provided. This charge and discharge cycle was repeated for two cycles, and the discharged quantity of electricity in the second cycle was designated as the initial discharge capacity. The initial discharge capacity of each of the obtained nonaqueous electrolyte storage elements is shown in Table 1. The initial discharge capacity shown in Table 1 is a value per mass of the positive electrode active material. In addition, in each of the nonaqueous electrolyte storage elements of Examples 1 to 3, Comparative Examples 1 to 5, and Reference Examples 1 and 2, metallic lithium is used as the negative electrode active material, so the positive electrode potential at the end-of-discharge voltage is substantially equal to the end-of-discharge voltage.

[0102] (Charge-discharge cycle test) After the initial charge and discharge, the nonaqueous electrolyte storage elements of Examples 1 to 3, Comparative Examples 1 to 5, and Reference Examples 1, 3, and 4 were subjected to a charge-discharge cycle test at 25°C as follows: Constant-current, constant-voltage charging was performed with a charge current of 0.2 C and a charge cut-off voltage of 4.6 V (4.5 V for Reference Examples 3 and 4). Charging was terminated until the charge current reached 0.05 C. Subsequently, constant-current discharging was performed with a discharge 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. This charge-discharge cycle was repeated until the coulombic efficiency reached 90% or less, or for a maximum of 200 cycles (150 cycles for Reference Example 1). Furthermore, the nonaqueous electrolyte electricity storage element of Reference Example 2 was subjected to a charge-discharge cycle test in the same manner as above, except that the charge current and discharge current were set to 0.33C. Table 1 shows the number of cycles (times) required for the coulomb efficiency to drop to 90% or less in the charge-discharge cycle test. In addition, the percentage of the discharge capacity at the 50th cycle to the discharge capacity at the 1st cycle in the charge-discharge cycle test was calculated and taken as the capacity retention rate (%). The obtained capacity retention rates (%) are shown in Table 1.

[0103] [Table 1]

[0104] The results in Table 1 confirm the following: As in each of the nonaqueous electrolyte storage elements of Reference Examples 1 and 2, the mass per unit area of ​​the positive electrode active material layer was 22 mg / cm 2When the negative electrode active material was graphite, as in the nonaqueous electrolyte storage elements of Reference Examples 3 and 4, the negative electrode active material was graphite, and the Coulomb efficiency did not decrease substantially with charge-discharge cycles, regardless of the mass per unit area of ​​the positive electrode active material layer, and the capacity retention rate after charge-discharge cycles was also high. On the other hand, as in the nonaqueous electrolyte storage elements of Comparative Examples 1 to 3, the negative electrode active material contained metallic lithium at least in a charged state, the positive electrode active material was a compound in which the content of lithium element relative to the transition metal element was 1.0 in molar ratio, and the mass per unit area of ​​the positive electrode active material layer was 22 mg / cm. 2 In the above cases, the coulomb efficiency significantly decreased with the charge-discharge cycle. In addition, as in the nonaqueous electrolyte storage elements of Comparative Examples 4 and 5, the negative electrode active material contained metallic lithium at least in the charged state, and the positive electrode active material layer contained a lithium-excess active material (Li-rich), but the mass per unit area of ​​the positive electrode active material layer was 22 mg / cm. 2 or more, and the positive electrode potential at the discharge end voltage is 3.0 V vs. Li / Li + When the temperature was higher than 1000 ℃, the coulomb efficiency did not decrease substantially, but the capacity retention rate after charge / discharge cycling was low. In contrast to these, the negative electrode active material contains metallic lithium at least in a charged state, the positive electrode active material layer contains a lithium-excess active material (Li-rich), and the positive electrode potential at the end of discharge is 3.0 V vs. Li / Li + In each of the nonaqueous electrolyte storage elements of Examples 1 to 3 described below, the negative electrode active material contains metallic lithium at least in a charged state, and the mass per unit area of ​​the positive electrode active material layer is 22 mg / cm 2 Despite the above, the decrease in coulomb efficiency due to charge-discharge cycles was suppressed, and the capacity retention rate after charge-discharge cycles was high. Furthermore, when the nonaqueous electrolyte storage elements of Reference Examples 3 and 4, in which the negative electrode active material is graphite, are compared with the nonaqueous electrolyte storage elements of Examples 1 to 3, in which the negative electrode active material contains metallic lithium at least in the charged state, it is found that the nonaqueous electrolyte storage elements of Examples 1 to 3 have a larger initial discharge capacity. [Industrial Applicability]

[0105] 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]

[0106] 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 containing a lithium transition metal composite oxide; a negative electrode having metallic lithium at least in a charged state; Equipped with The mass per unit area of ​​the positive electrode active material layer is 22 mg / cm ２ or more and 50 mg / cm 2 or less, The lithium transition metal composite oxide is α-NaFeO ２ a compound having a structure in which the molar ratio of lithium element to transition metal element exceeds 1.0, The positive electrode potential at the discharge end voltage during normal use is 3.0 V vs. Li / Li ＋ A non-aqueous electrolyte electricity storage element as follows:

2. Positive electrode potential: 3.0 V vs. Li / Li ＋ 2. A method for using the nonaqueous electrolyte storage element according to claim 1, comprising discharging the element until the following occurs:

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