JP7714864B2 - Nonaqueous electrolyte storage element and method for manufacturing same - Google Patents

Description

The present invention relates to a non-aqueous electrolyte storage element and a method for manufacturing the same.

Due to their high energy density, non-aqueous electrolyte secondary batteries, such as lithium-ion secondary batteries, are widely used in electronic devices such as personal computers and communication terminals, as well as in automobiles. Non-aqueous electrolyte secondary batteries generally have a pair of electrodes electrically isolated by a separator and a non-aqueous electrolyte sandwiched between the electrodes, and are configured to charge and discharge by transferring 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.

In order to improve the performance of non-aqueous electrolyte storage elements, the addition of various additives to non-aqueous electrolytes has been studied. Patent Document 1 describes a non-aqueous electrolyte secondary battery that uses a non-aqueous electrolyte containing a cyclic sulfate ester.

Japanese Patent Application Laid-Open No. 2006-120460

It is desirable for non-aqueous electrolyte energy storage elements to maintain their initial performance for a long period of time, even after repeated charge-discharge cycles. In particular, a high capacity retention rate and suppressed resistance increase during charge-discharge cycles are required, but it is difficult to achieve both of these goals with conventional non-aqueous electrolyte energy storage elements.

The present invention was made based on the above circumstances, and its purpose is to provide a nonaqueous electrolyte storage element that has a high capacity retention rate during charge/discharge cycles and suppresses resistance increase, as well as a method for manufacturing such a nonaqueous electrolyte storage element.

One aspect of the present invention is a nonaqueous electrolyte storage element comprising a negative electrode containing a negative electrode active material and a nonaqueous electrolyte containing an additive, wherein a coating containing elemental sulfur is formed on the surface of the negative electrode, the additive is a compound containing elemental sulfur, and the concentration of the additive in the nonaqueous electrolyte is 0.2 mass% or less.

Another aspect of the present invention is a method for producing a nonaqueous electrolyte storage element, comprising assembling an uncharged/discharged storage element using a negative electrode containing a negative electrode active material and a nonaqueous electrolyte containing an additive, and initially charging/discharging the uncharged/discharged storage element, wherein the additive is a compound containing elemental sulfur and satisfies the following formula 1:
0.004≦ _mS / _mN ≦0.016...1
In formula 1, m 1 _S is the mass (g) of the additive contained in the non-aqueous electrolyte, and m _{1 N} is the mass (g) of the negative electrode active material contained in the negative electrode.

Another aspect of the present invention is a method for producing a nonaqueous electrolyte storage element, comprising assembling an uncharged/discharged storage element using a negative electrode containing a negative electrode active material and a nonaqueous electrolyte containing an additive, and initially charging/discharging the uncharged/discharged storage element, wherein the additive is a compound containing elemental sulfur and satisfies the following formula 2:
0.010≦m _S /c _N ≦0.048 ...2
In Equation 2, m _S is the mass (g) of the additive contained in the non-aqueous electrolyte, and c _N is the discharge capacity (Ah) of the negative electrode.

One aspect of the present invention provides a nonaqueous electrolyte storage element that has a high capacity retention rate during charge/discharge cycles and suppresses resistance increases, as well as a method for manufacturing such a nonaqueous electrolyte storage element.

FIG. 1 is a perspective view showing the appearance of a nonaqueous electrolyte electricity storage element according to one embodiment of the present invention. FIG. 2 is a schematic diagram showing an electricity storage device constructed by assembling a plurality of nonaqueous electrolyte electricity storage elements according to one embodiment of the present invention.

First, we will provide an overview of the nonaqueous electrolyte storage element and the method for manufacturing the nonaqueous electrolyte storage element disclosed in this specification.

A nonaqueous electrolyte storage element according to one embodiment of the present invention is a nonaqueous electrolyte storage element comprising a negative electrode containing a negative electrode active material and a nonaqueous electrolyte containing an additive, a coating containing elemental sulfur formed on the surface of the negative electrode, the additive being a compound containing elemental sulfur, and the concentration of the additive in the nonaqueous electrolyte being 0.2 mass% or less.

A nonaqueous electrolyte storage element according to one embodiment of the present invention exhibits a high capacity retention rate and suppressed resistance increase during charge-discharge cycles. While the reasons for this are unclear, the following is presumed. In this nonaqueous electrolyte storage element, an additive is contained in the nonaqueous electrolyte. The additive is a compound containing elemental sulfur, and a coating containing elemental sulfur is formed on the surface of the negative electrode, resulting in a high capacity retention rate during charge-discharge cycles. When a nonaqueous electrolyte storage element is manufactured using a nonaqueous electrolyte containing the additive, a portion of the additive decomposes during initial charge-discharge cycles and forms a coating on the surface of the negative electrode. However, the inventors have discovered that if the concentration of the additive remaining in the nonaqueous electrolyte after initial charge-discharge cycles exceeds 0.2% by mass, the remaining additive adversely affects the positive electrode, resulting in an increase in resistance during charge-discharge cycles. Therefore, in this nonaqueous electrolyte storage element, the concentration of the additive in the nonaqueous electrolyte is set to 0.2% by mass or less, which is presumed to suppress undesirable effects, primarily on the positive electrode, and thereby suppress the increase in resistance during charge-discharge cycles.

Note that the concentration of the additive in the nonaqueous electrolyte provided in a nonaqueous electrolyte storage element according to one embodiment of the present invention is not the concentration of the additive in the nonaqueous electrolyte used during production, but the concentration of the additive in the nonaqueous electrolyte provided in the completed nonaqueous electrolyte storage element. As described above, a nonaqueous electrolyte storage element is typically completed by initial charging and discharging after assembly, during which some of the additive is consumed in forming a coating. Therefore, the concentration of the additive in the nonaqueous electrolyte provided in the completed nonaqueous electrolyte storage element is lower than the concentration in the nonaqueous electrolyte used during production. The initial charging and discharging may be performed after assembly of an uncharged and discharged storage element, for example, to check the capacity or to activate a specific positive electrode active material. This refers to the charging and discharging performed before the nonaqueous electrolyte storage element is shipped or used as a completed product.

The concentration of the additive in the nonaqueous electrolyte is preferably 0.01% by mass or more. There is a certain degree of correlation between the concentration of the additive in the nonaqueous electrolyte used in production before the initial charge/discharge and the concentration of the additive remaining in the nonaqueous electrolyte of the completed nonaqueous electrolyte storage element. Therefore, if the concentration of the additive remaining in the nonaqueous electrolyte of the completed nonaqueous electrolyte storage element is 0.01% by mass or more, it can be said that the nonaqueous electrolyte used in production before the initial charge/discharge contained a sufficient concentration of additive to form a coating containing a sufficient amount of elemental sulfur on the surface of the negative electrode. In this case, the capacity retention rate of the nonaqueous electrolyte storage element during charge/discharge cycles is further improved.

The additive is preferably a compound containing a sulfonyl group structure. When such an additive is used, a good coating is formed on the surface of the negative electrode, and undesirable effects on the positive electrode, etc., are minimized, resulting in a higher capacity retention rate during charge-discharge cycles of the nonaqueous electrolyte storage element and more suppression of resistance increases.

The additive is preferably a sulfate. When such an additive is used, a better coating is formed on the surface of the negative electrode and undesirable effects on the positive electrode, etc., are minimized, resulting in a higher capacity retention rate during charge-discharge cycles of the nonaqueous electrolyte storage element and more suppression of resistance increases.

A nonaqueous electrolyte storage element according to one embodiment of the present invention preferably further comprises a positive electrode containing a polyanionic compound. Although polyanionic compounds can be used as positive electrode active materials with high capacity retention, when the additive is a compound containing elemental sulfur, the additive or its decomposition products remaining in the nonaqueous electrolyte tend to leach transition metals from the polyanionic compound, resulting in an increase in resistance. Therefore, when the nonaqueous electrolyte storage element comprises a positive electrode containing a polyanionic compound, the effect of suppressing the increase in resistance during charge-discharge cycles can be particularly fully achieved.

A method for manufacturing a nonaqueous electrolyte storage element according to one embodiment of the present invention includes assembling an uncharged/discharged storage element using a negative electrode containing a negative electrode active material and a nonaqueous electrolyte containing an additive, and initially charging/discharging the uncharged/discharged storage element, wherein the additive is a compound containing elemental sulfur, and the method (α) for manufacturing a nonaqueous electrolyte storage element satisfies the following formula 1:
0.004≦ _mS / _mN ≦0.016...1
In formula 1, m 1 _S is the mass (g) of the additive contained in the non-aqueous electrolyte, and m _{1 N} is the mass (g) of the negative electrode active material contained in the negative electrode.

Manufacturing method (α) makes it possible to produce a nonaqueous electrolyte storage element that has a high capacity retention rate during charge/discharge cycles and suppresses resistance increase. While the reason for this is unclear, the following reason is presumed: the amount of additive consumed during initial charge/discharge is highly correlated with the mass of the negative electrode active material. By satisfying formula 1 above, manufacturing method (α) produces a nonaqueous electrolyte storage element in which a coating containing elemental sulfur is sufficiently formed on the surface of the negative electrode and a small amount of additive remains in the nonaqueous electrolyte, thereby presumably achieving the above-mentioned effects.

A method for manufacturing a nonaqueous electrolyte storage element according to another embodiment of the present invention includes assembling an uncharged/discharged storage element using a negative electrode containing a negative electrode active material and a nonaqueous electrolyte containing an additive, and initially charging/discharging the uncharged/discharged storage element, wherein the additive is a compound containing elemental sulfur and satisfies the following formula 2:
0.010≦m _S /c _N ≦0.048 ...2
In Equation 2, m _S is the mass (g) of the additive contained in the non-aqueous electrolyte, and c _N is the discharge capacity (Ah) of the negative electrode.

Manufacturing method (β) makes it possible to produce a nonaqueous electrolyte storage element that has a high capacity retention rate during charge/discharge cycles and suppresses resistance increase. While the reason for this is unclear, the following reason is presumed: The amount of additive consumed during initial charge/discharge is highly correlated with the mass of the negative electrode active material described above, as well as the discharge capacity of the negative electrode. Therefore, by satisfying formula 2 above, manufacturing method (β) is presumed to achieve the above-mentioned effects by producing a nonaqueous electrolyte storage element in which a coating containing elemental sulfur is sufficiently formed on the surface of the negative electrode and a small amount of additive remains in the nonaqueous electrolyte.

In the production methods (α) and (β), the positive electrode potential achieved during the initial charge/discharge is preferably 2.0 V (vs. Li/Li ⁺ ) to 4.2 V (vs. Li/ ^{Li +} ). By performing the initial charge/discharge at a positive electrode potential within this range, a coating containing elemental sulfur can be sufficiently formed on the negative electrode surface, and the amount of additive remaining in the nonaqueous electrolyte of the completed nonaqueous electrolyte storage element can be kept within a more appropriate range.

The following describes a nonaqueous electrolyte storage element and a method for manufacturing a nonaqueous electrolyte storage element according to one embodiment of the present invention.

<Non-aqueous electrolyte energy storage element>
A nonaqueous electrolyte storage element according to one embodiment of the present invention has a positive electrode, a negative electrode, and a nonaqueous electrolyte. Hereinafter, a nonaqueous electrolyte secondary battery (hereinafter simply referred to as a "secondary battery") will be described as an example of a nonaqueous electrolyte storage element. The positive electrode and negative electrode are usually stacked or wound alternately with a separator interposed therebetween to form an electrode assembly. This electrode assembly is housed in a container, and the container is filled with a nonaqueous electrolyte. The nonaqueous electrolyte is interposed between the positive electrode and the negative electrode. The container may be a known metal container, a resin container, or the like, that is typically used as a container for secondary batteries.

(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.

The positive electrode substrate is conductive. "Conductive" means that the volume resistivity measured in accordance with JIS-H-0505 (1975) is 10 ⁷ Ω·cm or less, and "non-conductive" means that the volume resistivity exceeds 10 ⁷ Ω·cm. Metals such as aluminum, titanium, tantalum, and stainless steel, or alloys thereof, are used as the material for the positive electrode substrate. Among these, aluminum and aluminum alloys are preferred in terms of the balance between potential resistance, high conductivity, and cost. The positive electrode substrate may be formed in the form of foil, vapor-deposited film, or the like, with foil being preferred in terms of cost. In other words, aluminum foil is preferred as the positive electrode substrate. Examples of aluminum or aluminum alloys include A1085 and A3003 as specified in JIS-H-4000 (2014).

The average thickness of the positive electrode substrate is preferably 3 μm or more and 50 μm or less, more preferably 5 μm or more and 40 μm or less, even more preferably 8 μm or more and 30 μm or less, and particularly preferably 10 μm or more and 25 μm or less. 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 also increasing the energy density per volume of the secondary battery. The "average thickness" of the positive electrode substrate and the negative electrode substrate described below refers to the value obtained by dividing the punched mass when a substrate of a given area is punched out by the true density and punched area of the substrate.

The intermediate layer is a coating layer on the surface of the positive electrode substrate, and by containing conductive particles such as carbon particles, it reduces the contact resistance between the positive electrode substrate and the positive electrode active material layer. There are no particular restrictions on the composition of the intermediate layer, and it can be formed, for example, from a composition containing a resin binder and conductive particles.

The positive electrode active material layer is formed from a so-called positive electrode mixture containing a positive electrode active material. Furthermore, the positive electrode mixture that forms the positive electrode active material layer may contain optional components such as a conductive agent, binder, thickener, and filler as needed.

The positive electrode active material can be appropriately selected from known positive electrode active materials. A material capable of absorbing and releasing lithium ions is typically used as the positive electrode active material for lithium ion secondary batteries. Examples of the positive electrode active material include lithium transition metal composite oxides having an α-NaFeO _2- type crystal structure, lithium transition metal composite oxides having a spinel-type crystal structure, polyanion compounds, chalcogen compounds, and sulfur. Examples of lithium transition metal composite oxides having α-NaFeO ₂ type crystal structure include Li[Li _x Ni _1-x ]O ₂ (0≦x<0.5), Li[Li _x Ni _γ Co _1-x-γ ]O ₂ (0≦x<0.5, 0<γ<1), Li[Li _x Co _1-x ]O ₂ (0≦x<0.5), Li[Li _x Ni _γ Mn _1-x-γ ]O ₂ (0≦x<0.5, 0<γ<1), Li[Li _x Ni _γ Mn _β Co _1-x-γ-β ]O ₂ (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 ]O ₂ (0≦x<0.5, 0<γ, 0<β, 0.5<γ+β<1). Examples of lithium transition metal composite oxides having a spinel crystal structure include Li _x Mn ₂ O ₄ and Li _x Ni _γ Mn _2-γ O _4. Examples of polyanion compounds include LiFePO ₄ , LiMnPO ₄ , LiNiPO ₄ , LiCoPO ₄ , Li ₃ V ₂ (PO ₄ ) ₃ , Li ₂ MnSiO ₄ , and Li ₂ CoPO ₄ F. Examples of chalcogen compounds include titanium disulfide, molybdenum disulfide, and molybdenum dioxide. Atoms or polyanions in these materials may be partially substituted with atoms or anion species of other elements. The surface of these materials may be coated with another material. In the positive electrode active material layer, these materials may be used alone or in combination of two or more.

As the positive electrode active material, a polyanion compound is preferred because it can effectively suppress the increase in resistance during charge-discharge cycles as described above. Among polyanion compounds, compounds containing phosphate ions (PO ₄ ³⁻ ) (phosphate compounds) are preferred, and lithium iron phosphate is more preferred. Lithium iron phosphate may be LiFePO ₄ or a compound in which a portion of the atoms or polyanions of LiFePO ₄ are substituted with other atoms or other anion species.

The positive electrode active material is usually in the form of particles (powder). The average particle diameter of the positive electrode active material is preferably, for example, 0.1 μm or more and 20 μm or less. Setting the average particle diameter of the positive electrode active material to be equal to or greater than the above-mentioned lower limit makes the positive electrode active material easier to manufacture and handle. Setting the average particle diameter of the positive electrode active material to be equal to or less than the above-mentioned upper limit improves the electronic conductivity of the positive electrode active material layer. When a composite of the positive electrode active material and another material is used, the average particle diameter of the composite is taken as the average particle diameter of the positive electrode active material.

To obtain powders of a specified particle size, mills and classifiers are used. Examples of milling methods include those using a mortar, ball mill, sand mill, vibrating ball mill, planetary ball mill, jet mill, counter jet mill, swirling air jet mill, or sieves. Wet milling, in which water or an organic solvent such as hexane is present, can also be used. Sieves and air classifiers are used as classification methods, both dry and wet, as needed.

The content of the positive electrode active material in the positive electrode active material layer is preferably 50% by mass or more and 99% by mass or less, more preferably 70% by mass or more and 98% by mass or less, and even more preferably 80% by mass or more and 95% by mass or less. By keeping 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.

The conductive agent is not particularly limited as long as it is a material that is conductive. Examples of such conductive agents include carbonaceous materials, metals, and conductive ceramics. Examples of carbonaceous materials include graphite and carbon black. Examples of carbon black include furnace black, acetylene black, and ketjen black. Of these, carbonaceous materials are preferred from the standpoint of conductivity and coatability. Acetylene black and ketjen black are particularly preferred. The conductive agent may be in the form of powder, sheet, fiber, or the like.

The content of the conductive agent in the positive electrode active material layer is preferably 1% by mass or more and 40% by mass or less, more preferably 2% by mass or more and 30% by mass or less, even more preferably 4% by mass or more and 20% by mass or less, and in some cases even more preferably 10% by mass or less. By keeping the content of the conductive agent within the above range, the energy density of the secondary battery can be increased.

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

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 this range, the active material can be stably maintained.

Examples of thickeners include polysaccharide polymers such as carboxymethyl cellulose (CMC) and methyl cellulose. Furthermore, if the thickener has a functional group that reacts with lithium, it is preferable to deactivate this functional group in advance by methylation or the like. The content of the thickener in the positive electrode active material layer is preferably 0.5% by mass or more and 10% by mass or less, and more preferably 2% by mass or more and 5% by mass or less.

The filler is not particularly limited. Examples of fillers include polyolefins such as polypropylene and polyethylene; inorganic oxides such as silicon dioxide, aluminum oxide, titanium dioxide, calcium oxide, strontium oxide, barium oxide, magnesium oxide, and aluminosilicates; hydroxides such as magnesium hydroxide, calcium hydroxide, and aluminum hydroxide; carbonates such as calcium carbonate; sparingly soluble ionic crystals such as calcium fluoride, barium fluoride, and barium sulfate; nitrides such as aluminum nitride and silicon nitride; mineral-derived substances such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica; and synthetic versions of these.

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, Si, 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.

(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 intermediate layer can have the same configuration as the intermediate layer of the positive electrode. A coating containing elemental sulfur is formed on the surface of the negative electrode, specifically on the surface of the negative electrode active material layer. This coating is usually formed by decomposition of an additive, such as a compound containing elemental sulfur, in the non-aqueous electrolyte during initial charge/discharge. That is, the coating contains a component derived from the additive in the non-aqueous electrolyte. The coating may contain elements other than sulfur and may contain a component derived from a component other than the additive in the non-aqueous electrolyte.

The negative electrode substrate can have the same structure as the positive electrode substrate, but is made of a metal such as copper, nickel, stainless steel, or nickel-plated steel, or an alloy thereof, with copper or a copper alloy being preferred. In other words, copper foil is preferred as the negative electrode substrate. Examples of copper foil include rolled copper foil and electrolytic copper foil.

The average thickness of the negative electrode substrate is preferably 2 μm or more and 35 μm or less, more preferably 3 μm or more and 30 μm or less, even more preferably 4 μm or more and 25 μm or less, and particularly preferably 5 μm or more and 20 μm or less. By keeping 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 and per mass of the secondary battery.

The negative electrode active material layer is generally formed from a so-called negative electrode mixture containing a negative electrode active material. The negative electrode mixture that forms the negative electrode active material layer also contains optional components such as a conductive agent, binder, thickener, and filler as needed. The optional components, such as a conductive agent, binder, thickener, and filler, can be the same as those used in the positive electrode active material layer. The negative electrode active material layer may be a layer consisting essentially of a negative electrode active material, such as metallic Li.

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

The negative electrode active material can be appropriately selected from known negative electrode active materials. For example, as a negative electrode active material for a lithium ion secondary battery, a material capable of absorbing and releasing lithium ions is usually used. Examples of negative electrode active materials include alkali metals such as lithium and sodium; compounds containing alkali metals such as lithium alloys, sodium alloys, and lithium composite oxides; metals or semimetals other than alkali metals such as Si, Ge, and Sn; oxides or semimetals of metals other than alkali metals such as Si oxide, Ti oxide, and Sn oxide; titanium-containing oxides such as Li ₄ Ti ₅ O ₁₂ , LiTiO _{2 , and} TiNb ₂ O ₇ ; polyphosphate compounds; silicon carbide; carbon materials such as graphite and non-graphitic carbon (easily graphitizable carbon or non-graphitizable carbon). Carbon materials are preferred as negative electrode active materials, and graphite is more preferred. In the negative electrode active material layer, one of these materials may be used alone, or two or more may be mixed and used.

"Graphite" refers to a carbon material in which the average lattice spacing (d ₀₀₂ ) of the (002) plane, as determined by X-ray diffraction before charge/discharge or in a discharged state, is 0.33 nm or more and less than 0.34 nm. Examples of graphite include natural graphite and artificial graphite. Artificial graphite is preferred from the viewpoint of being able to obtain a material with stable physical properties.

"Non-graphitic carbon" refers to a carbon material in which the average lattice spacing (d ₀₀₂ ) of the (002) plane, as determined by X-ray diffraction before charge/discharge or in a discharged state, is 0.34 nm or more and 0.42 nm or less. Examples of non-graphitic carbon include non-graphitizable carbon and graphitizable carbon. Examples of non-graphitic carbon include resin-derived materials, petroleum pitch or petroleum pitch-derived materials, petroleum coke or petroleum coke-derived materials, plant-derived materials, and alcohol-derived materials.

Here, the "discharged state" of a carbon material refers to a state in which the open circuit voltage is 0.7 V or higher in a single-electrode battery using a negative electrode containing a carbon material as the negative electrode active material as the working electrode and metallic Li as the counter electrode. Because the potential of the metallic Li counter electrode in the open circuit state is approximately equal to the redox potential of Li, the open circuit voltage in the single-electrode battery is approximately equal to the potential of the negative electrode containing the carbon material relative to the redox potential of Li. In other words, an open circuit voltage of 0.7 V or higher in the single-electrode battery means that the carbon material, which is the negative electrode active material, has sufficiently released lithium ions that can be absorbed and released during charging and discharging.

The term "non-graphitizable carbon" refers to a carbon material having the above d ₀₀₂ of 0.36 nm or more and 0.42 nm or less.

The term "easily graphitizable carbon" refers to a carbon material having the above d ₀₀₂ of 0.34 nm or more and less than 0.36 nm.

When the negative electrode active material is in the form of particles (powder), the average particle size of the negative electrode active material can be, for example, 1 nm or more and 100 μm or less. When the negative electrode active material is, for example, a carbon material, its average particle size may preferably be 1 μm or more and 100 μm or less. When the negative electrode active material is a metal, semi-metal, metal oxide, semi-metal oxide, titanium-containing oxide, polyphosphate compound, etc., its average particle size may preferably be 1 nm or more and 1 μm or less. Setting the average particle size of the negative electrode active material to be above the above lower limit makes it easier to manufacture and handle the negative electrode active material. Setting the average particle size of the negative electrode active material to be below the above upper limit improves the electronic conductivity of the active material layer. A pulverizer, classifier, etc. is used to obtain powder with the specified particle size.

When the negative electrode active material layer is formed from a negative electrode mixture, the content of the negative electrode active material in the negative electrode active material layer is preferably 60% by mass or more and 99% by mass or less, and more preferably 90% by mass or more and 98% by mass or less. By setting the content of the negative electrode active material within this range, both high energy density and manufacturability of the negative electrode active material layer can be achieved. When the negative electrode active material is an alkali metal or a compound containing an alkali metal, the content of the negative electrode active material in the negative electrode active material layer may be 99% by mass or more, or may even be 100% by mass.

The mass of the negative electrode active material in the negative electrode active material layer is preferably, for example, 1 g or more and 5 g or less, and more preferably 1.5 g or more and 3 g or less. When the mass of the negative electrode active material is within this range, the thickness of the sulfur-containing coating formed on the surface of the negative electrode is optimized, thereby further increasing the capacity retention rate during charge/discharge cycles and further suppressing resistance increases.

The mass of the negative electrode active material in the negative electrode active material layer can be determined by the following procedure. The nonaqueous electrolyte storage element is disassembled in a dry room with a dew point of -40°C or lower, and a negative electrode of a predetermined area is removed. The removed negative electrode is washed with dimethyl carbonate (DMC) to rinse away the nonaqueous electrolyte, and then vacuum dried at 100°C for 5 hours. After vacuum drying, the negative electrode active material layer is separated from the negative electrode, and the mass per unit area (A) of the negative electrode active material layer is measured. The separated negative electrode active material layer is subjected to thermogravimetry (TG) to determine the content ratio (B) of the negative electrode active material in the negative electrode active material layer. The opposing area (C) of the positive electrode and negative electrode in the nonaqueous electrolyte storage element is also measured. The mass of the negative electrode active material contained in the negative electrode is calculated by the product of the mass per unit area (A) of the negative electrode active material layer, the content ratio (B) of the negative electrode active material in the negative electrode active material layer, and the opposing area (C).

The discharge capacity of the negative electrode is preferably 0.3 Ah or more and 2.0 Ah or less, and more preferably 0.5 Ah or more and 1.0 Ah or less. When the discharge capacity of the negative electrode is within the above range, the thickness of the sulfur-containing coating formed on the surface of the negative electrode is optimized, thereby further improving the capacity retention rate during charge/discharge cycles and further suppressing increases in resistance.

The discharge capacity of the negative electrode can be determined by the following procedure. A nonaqueous electrolyte storage element is disassembled in a dry room with a dew point of −40° C. or less, a negative electrode of a predetermined area is removed, and the area of the negative electrode active material layer of this negative electrode is measured. A test battery is assembled using the removed negative electrode and a metallic lithium electrode as the counter electrode, and charging and discharging are performed at a test temperature of 25±2° C. under the following conditions:
Discharge (first time): After constant current discharge with a discharge current of 0.12 mA/cm ² and a discharge cut-off voltage of 2.0 V, a rest period of 10 minutes was provided.
Charging: constant current/constant voltage charging was performed with a charging current of 0.12 mA/cm ² and a charge cut-off voltage of 0.02 V, with the charge cut-off condition being that the charging current fell below 0.012 mA/cm ² , followed by a rest period of 10 minutes.
Discharge (second time): A constant current discharge was performed with a discharge current of 0.12 mA/cm ² and a discharge cut-off voltage of 2.0 V, and the quantity of electricity discharged in the second discharge was measured. The quantity of electricity discharged in the second discharge obtained was divided by the area of the negative electrode active material layer of the negative electrode used in the test battery to determine the quantity of electricity discharged (a) per unit area of the negative electrode. In addition, the opposing area (b) of the positive electrode and negative electrode in the nonaqueous electrolyte storage element was measured. The discharge capacity of the negative electrode was calculated by multiplying the quantity of electricity discharged (a) per unit area of the negative electrode by the opposing area (b).

(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 form of the substrate layer of the separator include woven fabric, nonwoven fabric, and porous resin film. Among these materials, porous resin film is preferred from the viewpoint of strength, and nonwoven fabric is preferred from the viewpoint of non-aqueous electrolyte retention. As the material for the substrate 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 decomposition resistance. A composite material of these resins may also be used as the substrate layer of the separator.

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 air, and more preferably exhibit a mass loss of 5% or less when heated from room temperature to 800°C in air. Examples of materials exhibiting a mass loss of a predetermined amount or less when heated include inorganic compounds. Examples of inorganic compounds include oxides such as iron oxide, silicon oxide, aluminum oxide, titanium oxide, barium titanate, 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 and barium fluoride; 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 composite, or two or more may be mixed and used. Of these inorganic compounds, silicon oxide, aluminum oxide, or aluminosilicate are preferred from the perspective of secondary battery safety.

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

A polymer gel composed of a polymer and a non-aqueous electrolyte may be used as the 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. The separator may also be a combination of a porous resin film or nonwoven fabric, such as those described above, and a polymer gel.

(Non-aqueous electrolyte)
The non-aqueous electrolyte contains an additive. The non-aqueous electrolyte is usually a non-aqueous electrolytic solution containing a non-aqueous solvent, an electrolyte salt dissolved in the non-aqueous solvent, and the additive.

Additives are compounds containing elemental sulfur. Additives are typically organic compounds containing elemental sulfur. Examples of additives include sulfates, sulfonic acid esters, sulfones, sulfites, sulfoxides, sulfides, and disulfides.

Sulfates refer to compounds containing a structure in which two carbon atoms are each bonded to an oxysulfonyloxy group (—O—S(═O) ₂ —O—). Examples of sulfates include cyclic sulfates and linear sulfates. Specific 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. Specific examples of linear sulfates include dimethyl sulfate and diethyl sulfate.

Sulfonate esters refer to compounds containing a structure in which two carbon atoms are each bonded to a sulfonyloxy group (-S(=O) ₂ O-). Examples of sulfonate esters include cyclic sulfonate esters and chain sulfonate esters. Specific examples of cyclic sulfonate esters include 1,3-propane sultone, 1,4-butane sultone, 1,3-propene sultone, 1-methyl-1,3-propene sultone, 2-methyl-1,3-propene sultone, and 3-methyl-1,3-propene sultone. Specific examples of chain sulfonate esters include methyl methanesulfonate and ethyl methanesulfonate.

Sulfones refer to compounds containing a structure in which two carbon atoms are each bonded to a sulfonyl group (-S(=O) ₂ -). Examples of sulfones include cyclic sulfones and chain sulfones. Specific 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. Specific examples of chain sulfones include dimethyl sulfone, diethyl sulfone, ethyl methyl sulfone, and divinyl sulfone.

Sulfites are compounds containing a structure in which two carbon atoms are each bonded to an oxysulfinyloxy group (-O-S(=O)-O-). Examples of sulfites include cyclic sulfites and linear sulfites. Specific examples of cyclic sulfites include ethylene sulfite, 1,2-propylene glycol sulfite, trimethylene sulfite, and 1,3-butylene glycol sulfite. Specific examples of linear sulfites include dimethyl sulfite.

Sulfoxides are compounds containing a structure in which two carbon atoms are each bonded to a sulfinyl group (-S(=O)-). Sulfoxides include cyclic sulfoxides and chain sulfoxides. Specific examples of cyclic sulfoxides include tetramethylene sulfoxide. Specific examples of chain sulfoxides include dimethyl sulfoxide and diethyl sulfoxide.

Sulfides are compounds containing a structure in which two carbon atoms are each bonded to divalent sulfur (-S-). Examples of sulfides include cyclic sulfides and linear sulfides. Specific examples of cyclic sulfides include tetrahydrothiophene, thiophene, thiane, 1,3-dithiane, 5,6-dihydro-1,4-dithiin-2,3-dicarboxylic anhydride, and 3,4-thiophenedicarboxylic anhydride. Specific examples of linear sulfides include diallyl sulfide, diphenyl sulfide, and thioanisole.

Disulfides are compounds containing a structure in which two carbon atoms are each bonded to a disulfide group (-S-S-). Disulfides include cyclic disulfides and linear disulfides. Specific examples of disulfides include diphenyl disulfide, dipyridinium disulfide, and diallyl disulfide.

The additive is preferably a compound containing a sulfonyl group (—S(═O) ₂ —) structure. Examples of such compounds include the above-mentioned sulfates, sulfonate esters, and sulfones, with sulfates being preferred.

Among sulfates, 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 are preferred, with ethylene sulfate, 1,3-propylene sulfate, 2,3-propylene sulfate, 4,5-pentene sulfate, and 4,4'-bis(2,2-dioxo-1,3,2-dioxathiolane) being more preferred. These compounds have similar sulfur atom contents per molecular weight or sulfonyl group structure contents per molecular weight. Therefore, when using any of these compounds, similar effects can be expected as long as the concentrations in the nonaqueous electrolyte are similar.

As the additive, a compound having a cyclic structure is preferred, a compound containing a sulfur atom in the cyclic structure is more preferred, and a compound containing a sulfonyl group structure in the cyclic structure is even more preferred. The additive preferably contains elemental sulfur and elemental oxygen, and more preferably is composed of the elements sulfur, oxygen, carbon, and hydrogen. The additive preferably has two sulfonyl groups or two oxysulfonyloxy groups in one molecule. Furthermore, the additive is preferably a molecular compound that is not an ionic compound. Such compounds decompose efficiently during initial charge and discharge, and even if they remain in the non-aqueous electrolyte, they have relatively little adverse effect on the positive electrode, etc., and therefore can further increase the capacity retention rate during the charge and discharge cycles of the secondary battery and further suppress resistance increases.

The upper limit of the additive concentration in the non-aqueous electrolyte is 0.2% by mass, preferably 0.17% by mass, more preferably 0.14% by mass, even more preferably 0.12% by mass, and even more preferably 0.08% by mass or 0.05% by mass or less. By setting the additive concentration at or below this upper limit, an increase in resistance during charge/discharge cycling of the secondary battery is suppressed. On the other hand, the lower limit of the additive concentration may be, for example, 0.001% by mass, but is preferably 0.01% by mass, and more preferably 0.05% by mass. By setting the additive concentration at or above this lower limit, the capacity retention rate during charge/discharge cycling of the secondary battery is increased. The additive concentration may be within a range between any of the above lower limits and any of the above upper limits.

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, ethers, amides, and nitriles. Non-aqueous solvents in which some of the hydrogen atoms have been substituted with halogens may also be used.

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. Of these, EC is preferred.

Examples of chain carbonates include diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diphenyl carbonate, trifluoroethyl methyl carbonate (TFEMC), bis(trifluoroethyl) carbonate, etc. Among these, DMC and EMC are preferred.

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 promotes dissociation of the electrolyte salt, improving the ionic conductivity of the non-aqueous electrolyte. The use of a chain carbonate reduces the viscosity of the non-aqueous electrolyte. When a cyclic carbonate and a chain carbonate are used in combination, the volume ratio of the cyclic carbonate to the chain carbonate (cyclic carbonate:chain carbonate) is preferably in the range of 5:95 to 50:50, and more preferably in the range of 10:90 to 30:70.

The electrolyte salt can be appropriately selected from known electrolyte salts. Examples of electrolyte salts include lithium salts, sodium salts, potassium salts, magnesium salts, and onium salts. Of these, lithium salts are preferred.

Examples of lithium salts include inorganic lithium salts such as _LiPF6 , _LiBF4 , _LiClO4 , _{LiPO2F2 ,} and LiN( _SO2F ) _{2 ,} 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.

The content of the electrolyte salt in the non-aqueous electrolyte is preferably 0.1 mol/dm3 or ^more and 2.5 mol/dm3 ^{or less} , ^more preferably 0.3 mol/dm3 or ^more and 2.0 mol/dm3 or less, even more preferably 0.5 mol/dm3 or ^more and 1.7 mol/dm3 or ^less , and particularly preferably 0.7 mol/ ^dm3 or more and 1.5 mol/dm3 or ^less . By setting the content of the electrolyte salt within the above range, the ionic conductivity of the non-aqueous electrolyte can be increased.

The nonaqueous electrolyte may contain additives other than the sulfur-containing compounds described above. Examples of such 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 above aromatic compounds such as 2-fluorobiphenyl, o-cyclohexylfluorobenzene, and p-cyclohexylfluorobenzene; halogenated anisole compounds such as 2,4-difluoroanisole, 2,5-difluoroanisole, 2,6-difluoroanisole, and 3,5-difluoroanisole; succinic anhydride, glutaric anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, and cyclohexanedicarboxylic anhydride; perfluorooctane, tristrimethylsilyl phosphate, and tetrakistrimethylsilyl titanate. These additives may be used alone or in combination.

The content of the other additives contained in the non-aqueous electrolyte is preferably 0.01% by mass or more and 10% by mass or less, more preferably 0.1% by mass or more and 7% by mass or less, even more preferably 0.2% by mass or more and 5% by mass or less, even more preferably 0.3% by mass or more and 3% by mass or less, and may be 2% by mass or less or 1% by mass or less, relative to the total mass of the non-aqueous electrolyte. Setting the content of the other additives within the above ranges can improve the capacity retention performance or charge/discharge cycle performance of the secondary battery after high-temperature storage, and can further improve safety.

The nonaqueous electrolyte may be a combination of a nonaqueous electrolytic solution and a solid electrolyte. The solid electrolyte may be selected from any material that has lithium ion conductivity and is solid at temperatures below 60°C. Examples of solid electrolytes include sulfide solid electrolytes, oxide solid electrolytes, oxynitride solid electrolytes, and polymer solid electrolytes.

The shape of the nonaqueous electrolyte storage element of this embodiment is not particularly limited, and examples include cylindrical batteries, laminated film batteries, prismatic batteries, flat batteries, coin batteries, and button batteries.

Figure 1 shows a nonaqueous electrolyte storage element 1 as an example of a prismatic battery. Note that this figure is a see-through view of the inside of the case. 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.

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

Figure 2 shows an example of an energy storage device 30 that further assembles energy storage units 20, each of which is an assembly of two or more electrically connected nonaqueous electrolyte energy storage elements 1. The energy storage device 30 may include a bus bar (not shown) that electrically connects two or more nonaqueous electrolyte energy storage elements 1, a bus bar (not shown) that electrically connects two or more energy storage units 20, and the like. The energy storage unit 20 or the energy storage device 30 may also include a status monitoring device (not shown) that monitors the status of one or more nonaqueous electrolyte energy storage elements.

<Method (α) for manufacturing non-aqueous electrolyte electricity storage element>
A method for producing a nonaqueous electrolyte storage element according to one embodiment of the present invention includes assembling an uncharged/discharged storage element using a negative electrode containing a negative electrode active material and a nonaqueous electrolyte containing an additive, and initially charging/discharging the uncharged/discharged storage element, wherein the additive is a compound containing elemental sulfur and satisfies the following formula 1:
0.004≦ _mS / _mN ≦0.016...1
In formula 1, m 1 _S is the mass (g) of the additive contained in the non-aqueous electrolyte, and m _{1 N} is the mass (g) of the negative electrode active material contained in the negative electrode.

Specifically, assembling an uncharged/discharged energy storage element may include preparing a positive electrode containing a positive electrode active material, preparing a negative electrode containing a negative electrode active material, preparing a non-aqueous electrolyte containing an additive, forming an alternately stacked electrode assembly by stacking or winding the positive electrode and negative electrode with a separator interposed therebetween, placing the positive electrode and negative electrode (electrode assembly) in a container, and injecting the non-aqueous electrolyte into the container. After injection, the injection port can be sealed to obtain an uncharged/discharged energy storage element.

The positive electrode used to assemble the above-mentioned uncharged/discharged energy storage element may be the same as the positive electrode provided in the nonaqueous electrolyte energy storage element according to one embodiment of the present invention described above. The negative electrode used to assemble the above-mentioned uncharged/discharged energy storage element may be the same as the negative electrode provided in the nonaqueous electrolyte energy storage element according to one embodiment of the present invention described above, except that the negative electrode does not typically have a coating containing elemental sulfur formed on its surface.

The nonaqueous electrolyte used to assemble the uncharged/discharged energy storage element may be the same as the nonaqueous electrolyte provided in the nonaqueous electrolyte energy storage element according to one embodiment of the present invention, except that the concentration of the additive is not limited. However, in the manufacturing method (α), the relationship between the nonaqueous electrolyte used and the negative electrode satisfies the above formula 1. m _S (the mass of the additive contained in the nonaqueous electrolyte) in formula 1 is the product of the amount (mass) of the nonaqueous electrolyte injected and the concentration of the additive in the injected nonaqueous electrolyte. The concentration of the additive in the injected nonaqueous electrolyte may be, for example, 0.1% by mass or more and 2% by mass or less, or 0.2% by mass or more and 1% by mass or less.

The lower limit of m _S /m _N may preferably be 0.006, and the upper limit of _{m S} /m _N may preferably be 0.015 or 0.010.

The resulting uncharged/discharged energy storage element is subjected to one or more charge/discharge cycles as an initial charge/discharge. During this cycle, a portion of the additive in the nonaqueous electrolyte decomposes, forming a coating containing elemental sulfur on the negative electrode surface. The positive electrode potential achieved during the initial charge/discharge cycle is preferably 2.0 V (vs. Li/Li ⁺ ) to 4.2 V (vs. Li/Li ⁺ ), and more preferably 3.0 V (vs. Li/Li ⁺ ) to 3.7 V (vs. Li/Li ⁺ ).

By undergoing these steps, manufacturing method (α) forms a coating containing elemental sulfur on the surface of the negative electrode and produces a nonaqueous electrolyte storage element with a small amount of residual additive in the nonaqueous electrolyte. Therefore, the resulting nonaqueous electrolyte storage element has a high capacity retention rate during charge-discharge cycles and suppressed resistance increase. Specific and preferred examples of nonaqueous electrolyte storage elements produced by manufacturing method (α) include the nonaqueous electrolyte storage element according to one embodiment of the present invention described above.

<Method for manufacturing nonaqueous electrolyte electricity storage element (β)>
A method for manufacturing a nonaqueous electrolyte storage element according to another embodiment of the present invention is a method (β) for manufacturing a nonaqueous electrolyte storage element, comprising assembling an uncharged/discharged storage element using a negative electrode containing a negative electrode active material and a nonaqueous electrolyte containing an additive, and initially charging/discharging the uncharged/discharged storage element, wherein the additive is a compound containing elemental sulfur and satisfies the following formula 2:
0.010≦m _S /c _N ≦0.048 ...2
In Equation 2, m _S is the mass (g) of the additive contained in the non-aqueous electrolyte, and c _N is the discharge capacity (Ah) of the negative electrode.

The production method (β) does not necessarily satisfy the above formula 1, and is similar to the above production method (α) except that it satisfies the above formula 2. Therefore, for specific and suitable examples of the production method (β), reference can be made to the above example of the production method (α). c _N (discharge capacity of the negative electrode) in formula 2 is adjusted by the amount, type, etc. of the negative electrode active material.

The lower limit of m _S /c _N may preferably be 0.020. The upper limit of _{m S} /c _N may preferably be 0.047, and more preferably 0.045 or 0.035.

In manufacturing method (β), by going through these steps, a coating containing elemental sulfur is formed on the surface of the negative electrode, and a nonaqueous electrolyte storage element is obtained in which the amount of residual additive in the nonaqueous electrolyte is small. Therefore, the resulting nonaqueous electrolyte storage element has a high capacity retention rate during charge-discharge cycles and suppressed resistance increase. Specific and preferred examples of nonaqueous electrolyte storage elements obtained by manufacturing method (β) include the nonaqueous electrolyte storage element according to one embodiment of the present invention described above.

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

In the above embodiment, the nonaqueous electrolyte storage element is described as being used as a chargeable and dischargeable nonaqueous electrolyte secondary battery (e.g., a lithium-ion secondary battery), but the type, shape, dimensions, capacity, etc. of the nonaqueous electrolyte storage element are arbitrary. The nonaqueous electrolyte storage element of the present invention can also be used in various nonaqueous electrolyte secondary batteries, electric double layer capacitors, lithium-ion capacitors, and other capacitors.

The present invention will be explained in more detail below using examples, but the present invention is not limited to the following examples.

[Example 1]
(Preparation of Positive Electrode)
A positive electrode mixture paste containing lithium iron phosphate as the positive electrode active material, polyvinylidene fluoride (PVDF) as the binder, and acetylene black as the conductive agent, and N-methylpyrrolidone (NMP) as the dispersion medium, was prepared. The ratio of the positive electrode active material to the binder and the conductive agent was 90:5:5 by mass. The positive electrode mixture paste was applied to the surface of aluminum foil as the positive electrode substrate, dried, and then compressed to a predetermined thickness to form a positive electrode active material layer, thereby obtaining a positive electrode.

(Preparation of negative electrode)
A negative electrode mixture paste containing graphite as the negative electrode active material, styrene butadiene rubber (SBR) as the binder, and carboxymethyl cellulose (CMC) as the thickener, with water as the dispersion medium, was prepared. The ratio of the negative electrode active material to the binder and the thickener was 97:1:2 by mass. The negative electrode mixture paste was applied to the surface of copper foil as the negative electrode substrate, dried, and then compressed to a predetermined thickness to form a negative electrode active material layer, thereby obtaining a negative electrode.

(Preparation of non-aqueous electrolyte)
Lithium hexafluorophosphate (LiPF ₆ ) was mixed as an electrolyte salt into a non-aqueous solvent prepared by mixing ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate in a volume ratio of 20:35:45, so that the content was 0.9 mol/dm ^3. To this mixed solution, 4,4′-bis(2,2-dioxo-1,3,2-dioxathiolane) (additive A) as an additive, 0.3% by mass of vinylene carbonate, and 1.0% by mass of lithium difluorophosphate were added to prepare a non-aqueous electrolyte.

(Assembly of non-aqueous electrolyte electricity storage element)
The positive electrode, negative electrode, and polyethylene separator were stacked and wound to prepare a wound electrode assembly. The wound electrode assembly was then placed in a container. The non-aqueous electrolyte was then poured into the container to obtain a non-charged/discharged energy storage element. The mass (amount) of the poured non-aqueous electrolyte was 3.34 g.

(Initial charge/discharge)
The obtained uncharged and discharged energy storage element was subjected to constant current constant voltage (CCCV) charging in a thermostatic chamber at 25°C under conditions of a charging current of 1 C and a charge cut-off voltage of 3.6 V for a total charge time of 3 hours. Next, constant current (CC) discharging was performed under conditions of a discharge current of 1 C and a discharge cut-off voltage of 2.0 V. After the above initial charge and discharge, a completed nonaqueous electrolyte energy storage element of Example 1 was obtained. The amount of electricity during discharge in this initial charge and discharge was taken as the initial discharge capacity.

The completed nonaqueous electrolyte element of Example 1 was disassembled, and the concentration of Additive A (4,4'-bis(2,2-dioxo-1,3,2-dioxathiolane)) in the nonaqueous electrolyte was measured and found to be 0.12% by mass. In addition, the opposing area of the positive electrode and negative electrode in the nonaqueous electrolyte storage element of Example 1 was 436.1 cm ² , the mass of the negative electrode active material was 2.242 g, and the discharge capacity of the negative electrode was 0.772 Ah.

[Examples 2 to 4, Comparative Examples 1 to 3]
Each of the nonaqueous electrolyte storage elements of Examples 2 to 4 and Comparative Examples 1 to 3 was produced in the same manner as in Example 1, except that the concentration of Additive A (4,4'-bis(2,2-dioxo-1,3,2-dioxathiolane)) in the nonaqueous electrolyte used in the production and the mass (amount injected) of the nonaqueous electrolyte injected into the container were as shown in Table 1. Each of the completed nonaqueous electrolyte storage elements of Examples 2 to 4 and Comparative Examples 1 to 3 obtained was disassembled, and the concentration of Additive A in the nonaqueous electrolyte was measured. The measurement results are shown in Table 1.

Table 1 also shows the mass (mS) of additive A in each nonaqueous electrolyte used in the production of each example and comparative example, the ratio (mS _/mN ) of the mass of additive A in the nonaqueous electrolyte to the mass of the negative electrode active material contained in the negative electrode, and the ratio ( _mS / _cN ) of the mass of additive A in _the nonaqueous electrolyte to the discharge capacity of the negative _electrode .

[evaluation]
(Charge-discharge cycle test)
A charge-discharge cycle test was performed on each of the obtained nonaqueous electrolyte storage elements as follows. Constant-current, constant-voltage charging was performed at 60°C with a charging current of 1.0 C and a cut-off voltage of 3.6 V. The charge was terminated when the current value decayed to 0.1 C. Subsequently, constant-current discharging was performed with a discharging current of 1.0 C and a cut-off voltage of 2.0 V. A 10-minute rest period was provided after each charge and discharge. This charge-discharge cycle was repeated 100 times.

(DCR increase rate)
The direct current resistance (DCR) of each nonaqueous electrolyte storage element was measured before and after the charge-discharge cycle test as follows. A constant current charge of 1 C was performed at 25°C to bring the SOC (State of Charge) to 50%, and then the element was discharged for 30 seconds at 25°C with a discharge current of 0.2 C, 0.5 C, or 1.0 C. The relationship between the current and the voltage 10 seconds after the start of discharge at each discharge current was plotted, and the DCR was calculated from the slope of the line obtained from the three plots. For each nonaqueous electrolyte storage element, the rate of increase in DCR after the charge-discharge cycle test relative to the initial DCR (before the charge-discharge cycle test) was calculated. The measurement results are shown in Table 1.

(Capacity maintenance rate)
After the charge-discharge cycle test, each nonaqueous electrolyte storage element was charged and discharged under the same conditions as the initial charge-discharge cycle test, and the discharge capacity after the charge-discharge cycle test was measured. The discharge capacity after the charge-discharge cycle test relative to the initial discharge capacity was defined as the capacity retention rate. The capacity retention rates of each nonaqueous electrolyte storage element are shown in Table 1 as relative values, with the capacity retention rate of the nonaqueous electrolyte storage element of Comparative Example 1 set as the reference (100%).

As shown in Table 1, the nonaqueous electrolyte storage elements of Comparative Examples 1 and 2, in which the concentration of Additive A in the nonaqueous electrolyte of the completed nonaqueous electrolyte storage element was high, exhibited a significant increase in DCR after the charge-discharge cycle test. On the other hand, the nonaqueous electrolyte storage element of Comparative Example 3, in which Additive A was not added to the nonaqueous electrolyte, exhibited a low capacity retention rate during the charge-discharge cycle. In contrast, the nonaqueous electrolyte storage elements of Examples 1 to 4, in which the concentration of Additive A in the nonaqueous electrolyte of the completed nonaqueous electrolyte storage element was 0.2 mass% or less, exhibited a high capacity retention rate during the charge-discharge cycle and suppressed an increase in DCR.

Furthermore, from the results of the Examples and Comparative Examples, it can be seen that the concentration of additive A in the nonaqueous electrolyte in the completed nonaqueous electrolyte storage element can be adjusted by setting the ratio (m _S /m _N ) of the mass of additive A in the nonaqueous electrolyte used in the production to the mass of the negative electrode active material contained in the negative electrode, and the ratio (m _S /c _N ) of the mass of additive A in the nonaqueous electrolyte used in the production to the discharge capacity of the negative electrode, within predetermined ranges.

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.

REFERENCE SIGNS LIST 1 nonaqueous electrolyte storage element 2 electrode body 3 container 4 positive electrode terminal 41 positive electrode lead 5 negative electrode terminal 51 negative electrode lead 20 storage unit 30 storage device

Claims (6)

a positive electrode containing a polyanionic compound;
a negative electrode containing a negative electrode active material; and a non-aqueous electrolyte containing an additive,
a coating containing elemental sulfur formed on the surface of the negative electrode,
The additive is a cyclic sulfate, and the concentration of the additive in the nonaqueous electrolyte is 0.2 mass % or less. a negative electrode containing a negative electrode active material; and a non-aqueous electrolyte containing an additive,
a coating containing elemental sulfur formed on the surface of the negative electrode,
The additive is a cyclic sulfate, and the concentration of the additive in the nonaqueous electrolyte is 0.05% by mass or more and 0.2% by mass or less. assembling an uncharged/discharged electricity storage element using a negative electrode containing a negative electrode active material and a non-aqueous electrolyte containing an additive; and initially charging and discharging the uncharged/discharged electricity storage element;
the negative electrode active material is a carbon material,
the additive is a cyclic sulfate,
Satisfies the following formula 1:
The method for producing a nonaqueous electrolyte storage element, wherein the positive electrode potential at the end-of-charge voltage of the initial charge/discharge is 3.0 V (vs. Li/Li ⁺ ) or more and 4.2 V (vs. Li/Li ⁺ ) or less .
0.004≦ _mS / _mN ≦0.016...1
In Formula 1, m 1 _S is the mass (g) of the additive contained in the non-aqueous electrolyte in the uncharged/discharged electricity storage element , and m 1 _N is the mass (g) of the negative electrode active material contained in the negative electrode. assembling an uncharged/discharged electricity storage element using a negative electrode containing a negative electrode active material and a non-aqueous electrolyte containing an additive; and initially charging and discharging the uncharged/discharged electricity storage element;
the negative electrode active material is a carbon material,
the additive is a cyclic sulfate,
Satisfies the following formula 1:
The method for producing a nonaqueous electrolyte electricity storage element includes the step of carrying out the initial charge/discharge for confirming the capacity .
0.004≦ _mS / _mN ≦0.016...1
In Formula 1, m 1 _S is the mass (g) of the additive contained in the non-aqueous electrolyte in the uncharged/discharged electricity storage element , and m 1 _N is the mass (g) of the negative electrode active material contained in the negative electrode. assembling an uncharged/discharged electricity storage element using a negative electrode containing a negative electrode active material and a non-aqueous electrolyte containing an additive; and initially charging and discharging the uncharged/discharged electricity storage element;
the additive is a cyclic sulfate,
Satisfies the following formula 2,
The method for producing a nonaqueous electrolyte storage element, wherein the positive electrode potential at the end-of-charge voltage of the initial charge/discharge is 3.0 V (vs. Li/Li ⁺ ) or more and 4.2 V (vs. Li/Li ⁺ ) or less .
0.010≦m _S /c _N ≦0.048 ...2
In Equation 2, m 1 _S is the mass (g) of the additive contained in the non-aqueous electrolyte in the uncharged/discharged electricity storage element, and c _{1 N} is the discharge capacity (Ah) of the negative electrode. assembling an uncharged/discharged electricity storage element using a negative electrode containing a negative electrode active material and a non-aqueous electrolyte containing an additive; and initially charging and discharging the uncharged/discharged electricity storage element;
the additive is a cyclic sulfate,
Satisfies the following formula 2,
The method for producing a nonaqueous electrolyte electricity storage element includes the step of carrying out the initial charge/discharge for confirming the capacity .
0.010≦m _S /c _N ≦0.048 ...2
In Equation 2, m 1 _S is the mass (g) of the additive contained in the non-aqueous electrolyte in the uncharged/discharged electricity storage element, and c _{1 N} is the discharge capacity (Ah) of the negative electrode.