JP7480579B2 - Nonaqueous electrolyte storage element and method for producing same - Google Patents

Description

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

Non-aqueous electrolyte secondary batteries, such as lithium ion secondary batteries, are widely used in electronic devices such as personal computers and communication terminals, as well as automobiles, due to their high energy density. The 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 ions between the two electrodes. In addition to non-aqueous electrolyte secondary batteries, capacitors such as lithium ion capacitors and electric double layer capacitors are also widely used as non-aqueous electrolyte storage elements.

As a non-aqueous electrolyte storage element, a Li-S battery or the like is known, which uses sulfur as the positive electrode active material. Patent Document 1 describes a Li-S battery that uses a solvent containing fluorine atoms, such as fluoroethylene carbonate.

JP 2013-508927 A

Sulfur has a large theoretical capacity, and nonaqueous electrolyte storage elements that use sulfur as the positive electrode active material are expected to have high energy density. However, nonaqueous electrolyte storage elements that use sulfur as the positive electrode active material have the disadvantage of low capacity retention during charge/discharge cycles.

The present invention was made based on the above circumstances, and its purpose is to provide a nonaqueous electrolyte storage element having a positive electrode containing sulfur as a positive electrode active material, which has a high capacity retention rate during charge/discharge cycles, and a method for manufacturing such a nonaqueous electrolyte storage element.

One aspect of the present invention is a non-aqueous electrolyte storage element that includes a positive electrode containing a positive electrode active material and a non-aqueous electrolyte containing a fluorinated solvent and an additive, in which the positive electrode active material contains sulfur, and the additive is at least one of a sulfur-containing cyclic compound and a boron-containing compound.

Another aspect of the present invention is a method for producing a nonaqueous electrolyte storage element, comprising preparing a positive electrode containing a positive electrode active material, and preparing a nonaqueous electrolyte containing a fluorinated solvent and an additive, the positive electrode active material containing sulfur, and the additive being at least one of a sulfur-containing cyclic compound and a boron-containing compound.

According to one aspect of the present invention, it is possible to provide a nonaqueous electrolyte storage element having a positive electrode containing sulfur as a positive electrode active material, which has a high capacity retention rate during charge/discharge cycles, and a method for manufacturing such a nonaqueous electrolyte storage element.

FIG. 1 is an external perspective view showing 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 formed 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 aspect of the present invention is a nonaqueous electrolyte storage element that includes a positive electrode containing a positive electrode active material and a nonaqueous electrolyte containing a fluorinated solvent and an additive, in which the positive electrode active material contains sulfur, and the additive is at least one of a sulfur-containing cyclic compound and a boron-containing compound.

The nonaqueous electrolyte storage element according to one embodiment of the present invention is a nonaqueous electrolyte storage element having a positive electrode containing sulfur as a positive electrode active material, and has a high capacity retention rate during charge and discharge cycles. The reason for this is unclear, but the following reason is presumed. In a nonaqueous electrolyte storage element having a positive electrode containing sulfur as a positive electrode active material, a coating is formed on the positive electrode surface due to a side reaction between the sulfur in the positive electrode and the nonaqueous electrolyte that occurs during initial charge and discharge. In the nonaqueous electrolyte storage element according to one embodiment of the present invention, the nonaqueous electrolyte contains a specific additive and a fluorinated solvent, which is presumed to improve the coating formed on the positive electrode surface, thereby increasing the capacity retention rate during charge and discharge cycles.

It is preferable that the additive is a compound containing boron, and that the compound containing boron is a salt.

When the additive is such a compound, the nonaqueous electrolyte storage element according to one embodiment of the present invention has a high capacity retention rate during charge/discharge cycles and a large initial discharge capacity.

It is preferable that the additive is a cyclic compound containing sulfur, and that the cyclic compound containing sulfur is a sultone.

When the additive is such a compound, the capacity retention rate during charge-discharge cycles of the nonaqueous electrolyte storage element according to one embodiment of the present invention is improved.

A method for producing a nonaqueous electrolyte storage element according to one aspect of the present invention includes preparing a positive electrode containing a positive electrode active material, and preparing a nonaqueous electrolyte containing a fluorinated solvent and an additive, in which the positive electrode active material contains sulfur, and the additive is at least one of a sulfur-containing cyclic compound and a boron-containing compound.

This manufacturing method makes it possible to manufacture a nonaqueous electrolyte storage element that has a positive electrode containing sulfur as the positive electrode active material and has a high capacity retention rate during charge/discharge cycles.

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

<Non-aqueous electrolyte electricity storage element>
The 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 the negative electrode are usually stacked or wound alternately with a separator interposed therebetween to form an electrode body. This electrode body 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. In addition, as the container, a known metal container, a resin container, or the like that is usually used as a container for a secondary battery can be used.

(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 has electrical conductivity. Having "electrical conductivity" means that the volume resistivity measured in accordance with JIS-H-0505 (1975) is 10 ⁷ Ω·cm or less, and "non-electrically conductive" means that the volume resistivity is more than 10 ⁷ Ω·cm. As the material of the positive electrode substrate, metals such as aluminum, titanium, tantalum, stainless steel, etc. or alloys thereof are used. Among these, aluminum and aluminum alloys are preferred in terms of the balance between potential resistance, high electrical conductivity, and cost. In addition, examples of the formation form of the positive electrode substrate include foil, vapor deposition film, etc., and foil is preferred in terms of cost. In other words, aluminum foil is preferred as the positive electrode substrate. In addition, examples of aluminum or aluminum alloys include A1085P, A3003P, etc., as specified in JIS-H-4000 (2014).

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 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 contains conductive particles such as carbon particles to reduce the contact resistance between the positive electrode substrate and the positive electrode active material layer. The composition of the intermediate layer is not particularly limited, 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 that contains a positive electrode active material. In addition, the positive electrode mixture that forms the positive electrode active material layer contains optional components such as a conductive agent, a binder, a thickener, and a filler as necessary.

The positive electrode active material layer contains sulfur as a positive electrode active material. That is, the positive electrode contains sulfur as a positive electrode active material. The sulfur as the positive electrode active material may be elemental sulfur or a sulfur compound. Examples of the sulfur compound include metal sulfides such as lithium sulfide, organic disulfide compounds, and organic sulfur compounds such as carbon sulfide compounds. Sulfur has the advantages of a high theoretical capacity and low cost.

The sulfur may be in the form of a complex with a conductive agent (a material having a higher conductivity than sulfur). Examples of such complexes include those in which sulfur is supported on a conductive agent or the like as a carrier, and specifically, examples include a complex of sulfur and porous carbon (sulfur-porous carbon composite: SPC). The lower limit of the sulfur content in this SPC (the mass ratio of sulfur atoms to the mass of SPC) is preferably 50 mass%, more preferably 60 mass%. Meanwhile, the upper limit of this content is preferably 90 mass%, more preferably 80 mass%. By setting the sulfur content in the SPC to the above range, it is possible to achieve both a large electric capacity and good conductivity.

The lower limit of the sulfur content in the positive electrode active material layer (mass ratio of sulfur atoms to the mass of the positive electrode active material layer) is preferably 40 mass%, more preferably 50 mass%. On the other hand, the upper limit of this content is preferably 90 mass%, more preferably 70 mass%. When SPC is used, the lower limit of the SPC content in the positive electrode active material layer is preferably 60 mass%, more preferably 70 mass%, and even more preferably 80 mass%. On the other hand, the upper limit of this content is preferably 95 mass%, more preferably 90 mass%. By setting the sulfur or SPC content within the above range, it is possible to achieve both large electrical capacity and good conductivity, and to increase the energy density, etc.

The positive electrode active material may contain other positive electrode active materials besides sulfur. However, the lower limit of the sulfur (sulfur atom) content in the entire positive electrode active material is preferably 30 mass%, more preferably 50 mass%, and even more preferably 70 mass%. The lower limit of the SPC content in the entire positive electrode active material is preferably 80 mass%, more preferably 90 mass%, and even more preferably 99 mass%.

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 and carbon black. Examples of carbon black include furnace black, acetylene black, and ketjen black. Among these, carbonaceous materials are preferred from the viewpoint of electrical conductivity and coatability. Of these, acetylene black and ketjen black are preferred. The conductive agent may be in the form of a powder, a sheet, or a fiber.

The content of the conductive agent (excluding the conductive agent in the SPC) in the positive electrode active material layer is preferably 1% by mass or more and 40% by mass or less, more preferably 3% by mass or more and 30% by mass or less, even more preferably 5% by mass or more and 20% by mass or less, and in some cases even more preferably 10% by mass or more. By setting the content of the conductive agent in 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 setting the binder content within the above range, the active material can be stably maintained.

Examples of thickeners include polysaccharide polymers such as carboxymethyl cellulose (CMC) and methyl cellulose. In addition, 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 the filler 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 ion crystals such as calcium fluoride, barium fluoride, and barium sulfate, nitrides such as aluminum nitride and silicon nitride, substances derived from mineral resources such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica, or man-made products thereof.

The positive electrode active material layer may contain typical nonmetallic elements such as B, N, P, F, Cl, Br, and I, typical metallic elements such as Li, Na, Mg, Al, Si, K, Ca, Zn, Ga, Ge, Sn, Sr, and Ba, and transition metallic 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 may have the same structure as the intermediate layer of the positive electrode.

The negative electrode substrate can have the same structure as the positive electrode substrate, but the material used is 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 setting the average thickness of the negative electrode substrate within the above range, it is possible to increase the strength of the negative electrode substrate while increasing the energy density per volume and per mass of the secondary battery.

The negative electrode active material layer is generally formed from a so-called negative electrode mixture that contains 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, a binder, a thickener, and a filler as necessary. The optional components such as the conductive agent, binder, thickener, and filler can be the same as those in the positive electrode active material layer. The negative electrode active material layer may be a layer that is substantially composed of only a negative electrode active material such as metallic Li.

The negative electrode active material layer may contain typical nonmetallic elements such as B, N, P, F, Cl, Br, and I, typical metallic elements such as Li, Na, Mg, Al, K, Ca, Zn, Ga, Ge, Sn, Sr, and Ba, and transition metal elements such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Zr, 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 the negative electrode active material for a lithium ion secondary battery, a material capable of absorbing and releasing lithium ions is usually used. Examples of the negative electrode active material 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). Among these, compounds containing alkali metals or alkali metals are preferred, and compounds containing lithium (metallic lithium) or lithium are 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. When a negative electrode active material that does not contain an alkali metal is used, the positive electrode active material preferably contains a compound that contains an alkali metal in addition to sulfur.

"Graphite" refers to a carbon material having an average lattice spacing (d ₀₀₂ ) of the (002) plane determined by X-ray diffraction before charging and discharging or in a discharged state of 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 obtaining a material with stable physical properties.

"Non-graphitic carbon" refers to a carbon material in which the average lattice spacing (d002) of the ( ₀₀₂ ) plane, as determined by X-ray diffraction before charging and discharging or in a discharged state, is 0.34 nm or more and 0.42 nm or less. Examples of non-graphitic carbon include carbon that is difficult to graphitize and carbon that is easy to graphitize. 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 the carbon material refers to a state in which the open circuit voltage is 0.7 V or more 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. Since 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 more in the single-electrode battery means that sufficient lithium ions that can be absorbed and released during charging and discharging have been released from the carbon material, which is the negative electrode active material.

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 "graphitizable carbon" refers to a carbon material having the above _d002 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, the average particle size may be preferably 1 μm or more and 100 μm or less. When the negative electrode active material is a metal, a semi-metal, a metal oxide, a semi-metal oxide, a titanium-containing oxide, a polyphosphate compound, or the like, the average particle size may be preferably 1 nm or more and 1 μm or less. By setting the average particle size of the negative electrode active material to the above lower limit or more, the negative electrode active material can be easily manufactured or handled. By setting the average particle size of the negative electrode active material to the above upper limit or less, the electronic conductivity of the active material layer is improved. In order to obtain powder with a predetermined particle size, a pulverizer, a classifier, or the like is used. In addition, when the negative electrode active material is an alkali metal or a compound containing an alkali metal, the form may be a foil or a plate.

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 in the above range, it is possible to achieve both high energy density and manufacturability of the negative electrode active material layer. 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 be 100% by mass.

(Separator)
The separator can be appropriately selected from known separators. For example, a separator consisting of only a substrate layer, a separator in which a heat-resistant layer containing heat-resistant particles and a binder is formed on one or both surfaces of the substrate layer, etc. can be used as the separator. Examples of the material of the substrate layer of the separator include woven fabric, nonwoven fabric, and porous resin film. Among these materials, a porous resin film is preferable from the viewpoint of strength, and a nonwoven fabric is preferable from the viewpoint of liquid retention of non-aqueous electrolyte. As the material of the substrate layer of the separator, polyolefins such as polyethylene and polypropylene are preferable from the viewpoint of shutdown function, and polyimide and aramid are preferable from the viewpoint of oxidation decomposition resistance. A material obtained by combining these resins may be used as the substrate layer of the separator.

The heat-resistant particles contained in the heat-resistant layer preferably have a mass loss of 5% or less when heated from room temperature to 500°C in the atmosphere, and more preferably have a mass loss of 5% or less when heated from room temperature to 800°C in the atmosphere. Examples of materials that have 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 aluminosilicate; hydroxides such as magnesium hydroxide, calcium hydroxide, and aluminum hydroxide; nitrides such as aluminum nitride and silicon nitride; carbonates such as calcium carbonate; sulfates such as barium sulfate; sparingly soluble ion crystals such as calcium fluoride and barium fluoride; covalent crystals such as silicon and diamond; mineral resource-derived substances such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica, or 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 mixed and used. Among these inorganic compounds, silicon oxide, aluminum oxide, or aluminosilicates are preferred from the viewpoint of the safety of the secondary battery.

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

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

(Non-aqueous electrolyte)
The non-aqueous electrolyte contains a fluorinated solvent and an additive. The additive is at least one of a cyclic compound containing sulfur and a compound containing boron. The non-aqueous electrolyte is usually a non-aqueous electrolytic solution containing a non-aqueous solvent containing the fluorinated solvent, an electrolyte salt dissolved in the non-aqueous solvent, and the additive. When the additive is a salt, an electrolyte salt may or may not be contained in addition to the additive.

A fluorinated solvent is a solvent having fluorine atoms. The fluorinated solvent may be a non-aqueous solvent having a hydrocarbon group in which some or all of the hydrogen atoms in the hydrocarbon group are replaced with fluorine atoms. By using a fluorinated solvent, the oxidation resistance of the non-aqueous electrolyte is improved, and the capacity retention rate in the charge/discharge cycle of the non-aqueous electrolyte storage element is increased, and the discharge capacity is also increased. Examples of the fluorinated solvent include fluorinated carbonates, fluorinated carboxylates, fluorinated phosphates, and fluorinated ethers. One or more types of fluorinated solvents can be used.

Among fluorinated solvents, fluorinated carbonates are preferred, and it is more preferred to use a combination of fluorinated cyclic carbonates and fluorinated chain carbonates. By using a fluorinated cyclic carbonate, it is possible to promote dissociation of the electrolyte salt and improve the ionic conductivity of the non-aqueous electrolyte. By using a fluorinated chain carbonate, it is possible to keep the viscosity of the non-aqueous electrolyte low. When a fluorinated cyclic carbonate and a fluorinated chain carbonate are used in combination, the volume ratio of the fluorinated cyclic carbonate to the fluorinated chain carbonate (fluorinated cyclic carbonate:fluorinated chain carbonate) is, for example, preferably in the range of 5:95 to 80:20, more preferably in the range of 20:80 to 70:30, even more preferably in the range of 40:60 to 60:40, and even more preferably in the range of 45:55 to 55:45.

The lower limit of the content of the fluorinated carbonate in the fluorinated solvent is preferably 50% by volume, more preferably 70% by volume, and even more preferably 90% by volume or 99% by volume. The upper limit of the content of the fluorinated carbonate in the fluorinated solvent may be 100% by volume.

Examples of fluorinated cyclic carbonates include fluorinated ethylene carbonates such as fluoroethylene carbonate (FEC) and difluoroethylene carbonate, fluorinated propylene carbonate, and fluorinated butylene carbonate. Among these, fluorinated ethylene carbonate is preferred, and FEC is more preferred. FEC has high oxidation resistance and is highly effective in suppressing side reactions (such as oxidative decomposition of non-aqueous solvents) that may occur during charging and discharging of secondary batteries.

Examples of fluorinated chain carbonates include 2,2,2-trifluoroethyl methyl carbonate and bis(2,2,2 trifluoroethyl) carbonate.

Examples of fluorinated carboxylates include methyl 3,3,3-trifluoropropionate and 2,2,2-trifluoroethyl acetate.

Fluorinated phosphate esters include tris(2,2-difluoroethyl) phosphate and tris(2,2,2-trifluoroethyl) phosphate.

Fluorinated ethers include 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, methyl heptafluoropropyl ether, methyl nonafluorobutyl ether, etc.

The non-aqueous solvent may contain a non-aqueous solvent other than a fluorinated solvent. Examples of such non-aqueous solvents include carbonates, carboxylates, phosphates, ethers, and the like other than fluorinated solvents.

The lower limit of the content of the fluorinated solvent relative to the total nonaqueous solvent is preferably 50% by volume, more preferably 70% by volume, even more preferably 90% by volume, and even more preferably 99% by volume. It is particularly preferable that the content of the fluorinated solvent relative to the total nonaqueous solvent is 100% by volume. By making the nonaqueous solvent essentially consist of only the fluorinated solvent, it is possible to further increase the oxidation resistance of the nonaqueous electrolyte.

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

Examples of the lithium salt include inorganic lithium salts such as LiPF ₆ , LiPO ₂ F ₂ and LiClO ₄ , and organic lithium salts such as lithium imide salts. One or more kinds of lithium salts can be used.

As the lithium salt, an organic lithium salt is preferred, and a lithium imide salt is more preferred. The lithium imide salt includes not only lithium imide salts having a structure in which two carbonyl groups are bonded to a nitrogen atom, but also those having a structure in which two sulfonyl groups are bonded to a nitrogen atom and those having a structure in which two phosphonyl groups are bonded to a nitrogen atom.

The lithium imide salt is
LiN(SO ₂ F) ₂ (lithium bis(fluorosulfonyl)imide: LiFSI), LiN(SO ₂ CF ₃ ) ₂ (lithium bis(trifluoromethanesulfonyl)imide: LiTFSI), LiN(SO ₂ C ₂ F ₅ ) ₂ (lithium bis(pentafluoroethanesulfonyl)imide: LiBETI), LiN(SO ₂ C ₄ F ₉ ) ₂ (lithium bis(nonafluorobutanesulfonyl)imide), CF ₃ -SO ₂ -N-SO ₂ -N-SO ₂ CF ₃ Li ₂ , FSO ₂ -N-SO ₂ -C ₄ F ₉ Li, CF ₃ -SO ₂ -N-SO ₂ -CF ₂ -SO ₂ -N-SO ₂ -CF ₃ Li ₂ , CF Lithium sulfonylimide salts such as ₃ -SO ₂ -N-SO ₂ -CF ₂ -SO ₃ Li ₂ , CF ₃ -SO ₂ -N-SO ₂ -CF ₂ -SO ₂ -C(-SO ₂ CF ₃ ) ₂ Li ₂ ;
Examples of the lithium phosphonylimide salt include lithium _bis (difluorophosphonyl)imide (LiDFPI) and the like _.

The lithium imide salt preferably has a fluorine atom, specifically, for example, a fluorosulfonyl group, a difluorophosphonyl group, a fluoroalkyl group, or the like. Among the lithium imide salts, lithium sulfonylimide salts are preferred, LiTFSI and LiFSI are more preferred, and LiTFSI is even more preferred.

The content of the electrolyte salt in the non-aqueous electrolyte is preferably 0.1 mol/dm ³ or more and 2.5 mol/dm ³ or less, more preferably 0.3 mol/dm ³ or more and 2.0 mol/dm ³ or less, even more preferably 0.5 mol/dm ³ or more and 1.7 mol/dm ³ or less, and particularly preferably 0.7 mol/dm ³ or more and 1.5 mol/dm ³ or less. By setting the content of the electrolyte salt in the above range, the ionic conductivity of the non-aqueous electrolyte can be increased.

The additive is at least one of a sulfur-containing cyclic compound and a boron-containing compound. The use of such an additive increases the capacity retention rate of the nonaqueous electrolyte storage element during charge-discharge cycles.

The sulfur-containing cyclic compound is preferably a compound containing sulfur in the ring structure. Examples of the sulfur-containing cyclic compound include cyclic sulfonate esters, cyclic sulfones, cyclic sulfoxides, cyclic sulfites, cyclic sulfates, and cyclic sulfides. One or more hydrogen atoms of these sulfur-containing cyclic compounds may be substituted with halogens or other substituents.

Among these sulfur-containing cyclic compounds, cyclic sulfonate esters are preferred. When cyclic sulfonate esters are used, the capacity retention rate during charge/discharge cycles of the nonaqueous electrolyte storage element tends to be higher. The sulfur-containing cyclic compound preferably contains sulfur and oxygen, and more preferably is composed of sulfur, oxygen, carbon, and hydrogen. In addition, the sulfur-containing cyclic compound is preferably a molecular compound that is not a salt.

A cyclic sulfonate ester refers to a compound having a ring structure containing a structure in which two carbon atoms are each bonded to a sulfonyloxy group (-S(=O) ₂ O-). Among the cyclic sulfonate esters, sultones are preferred. When sultones are used, the capacity retention rate of the nonaqueous electrolyte storage element during the charge-discharge cycle is further increased.

The term "sultones" refers to cyclic sulfonate esters of hydroxysulfonic acid. That is, sultones refer to cyclic sulfonate esters in which the number of sulfonyloxy groups (-S(=O) ₂ O-) present in the ring structure is one. Examples of sultones 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. Of these, 1,3-propane sultone and 1,3-propene sultone are preferred, and 1,3-propene sultone is more preferred.

Other cyclic sulfonate esters include methylene-methane disulfonate ester, ethylene-methane disulfonate ester, etc.

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

A cyclic sulfoxide is a compound that has a ring structure containing two carbon atoms each bonded to a sulfinyl group (-S(=O)-). Examples of cyclic sulfoxides include tetramethylene sulfoxide.

Cyclic sulfite refers to a compound having a ring structure in which two carbon atoms are each bonded to an oxysulfinyloxy group (-O-S(=O)-O-). Examples of cyclic sulfite include ethylene sulfite, 1,2-propylene glycol sulfite, trimethylene sulfite, and 1,3-butylene glycol sulfite.

A cyclic sulfate is a compound having a ring structure containing a structure in which two carbon atoms are each bonded to an oxysulfonyloxy group (-O-S(=O) ₂ -O-). Examples of cyclic sulfates include ethylene sulfate, 1,3-propylene sulfate, 2,3-propylene sulfate, 4,5-pentene sulfate, 4,4'-bis(2,2-dioxo-1,3,2-dioxathiolane), 4-methylsulfonyloxymethyl-2,2-dioxo-1,3,2-dioxathiolane, and the like.

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

The number of ring members in the ring structure of the sulfur-containing cyclic compound is preferably 3 to 8, more preferably 4 to 6.

Compounds containing boron include salts and molecular compounds, with salts being preferred. By using a salt containing boron, the capacity retention rate during the charge-discharge cycle of the nonaqueous electrolyte storage element is increased, and the initial discharge capacity is also increased.

In salts containing boron, the anion usually contains boron. In addition, the salt containing boron is preferably an alkali metal salt, and more preferably a lithium salt.

Examples of salts containing boron include lithium tetrafluoroborate, lithium difluoro(oxalato)borate, lithium bis(oxalato)borate, lithium dicyano(oxalato)borate, lithium tetracyanoborate, lithium cyanofluoro(oxalato)borate, lithium trifluoromethylcyano(succinato)borate, lithium tetrakis(trifluoroacetate)borate, lithium trifluorotrifluoromethylborate, lithium tetraborate, lithium trilithium borate, lithium metaborate, lithium tetrahydroxyborate, potassium tetrafluoroborate, sodium tetrafluoroborate, etc.

Molecular compounds containing boron other than salts include borate esters, boronic acids, boronate esters, diboronic acids, diboronic esters, etc. Specific examples include trimethyl borate, tris(2-cyanoethyl) borate, tris(trimethylsilyl) borate, tris(2,2,2-trifluoroethyl) borate, 2,4,6-trimethoxyboroxine, (3-methyl-2,4-pentanedionato)(oxalato)borate, 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane, bis[(pinacolato)boryl]methane, 1,4-benzenediboronic acid bis(pinacol), 2-ethyl-4,4,5,5-tetramethyl-1,3,2-dioxaborolane, etc.

The boron-containing compound is preferably a compound (complex) having an oxalato group as a ligand. Examples of such compounds include lithium difluoro(oxalato)borate, lithium bis(oxalato)borate, lithium dicyano(oxalato)borate, (3-methyl-2,4-pentanedionato)(oxalato)borate, and the like, among the compounds mentioned above.

The content of the additive (at least one of a sulfur-containing cyclic compound and a boron-containing compound) is preferably 0.01% by mass to 10% by mass, more preferably 0.1% by mass to 5% by mass, even more preferably 0.3% by mass to 3% by mass, and particularly preferably 0.5% by mass to 2% by mass. By setting the content of the additive within the above range, it is possible to further increase the capacity retention rate during the charge/discharge cycle of the nonaqueous electrolyte storage element and to further increase the initial discharge capacity.

The non-aqueous electrolyte may contain other additives in addition to the additives listed above. Examples of other 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; dimethyl sulfite, dimethyl sulfate, dimethyl sulfone, diethyl sulfone, dimethyl sulfoxide, diethyl sulfoxide, diphenyl sulfide, thioanisole, diphenyl disulfide, dipyridinium disulfide, perfluorooctane, tristrimethylsilyl phosphate, and tetrakistrimethylsilyl titanate. These additives may be used alone or in combination of two or more.

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, and particularly preferably 0.3% by mass or more and 3% by mass or less, based on the total mass of the non-aqueous electrolyte. By setting the content of the other additives within the above range, it is possible to improve the capacity retention performance or charge/discharge cycle performance after high-temperature storage of the non-aqueous electrolyte storage element, and to further improve safety.

As the non-aqueous electrolyte, a non-aqueous electrolyte solution and a solid electrolyte may be used in combination. The solid electrolyte may be selected from any material that has lithium ion conductivity and is solid at 60°C or less. Examples of the solid electrolyte 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, square batteries, flat batteries, coin batteries, button batteries, etc.

Figure 1 shows a nonaqueous electrolyte storage element 1 as an example of a square battery. Note that this figure is a see-through view of the inside of the case. An electrode body 2 having a positive electrode and a negative electrode wound with a separator sandwiched between them is housed in a square 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 storage element of the present embodiment can be mounted as an electricity storage unit (battery module) constituted by assembling a plurality of nonaqueous electrolyte storage elements 1 in automobile 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 storage element included in the electricity storage unit.

Figure 2 shows an example of an energy storage device 30 in which an energy storage unit 20, which is an assembly of two or more electrically connected nonaqueous electrolyte energy storage elements 1, is further assembled. 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 include a status monitoring device (not shown) that monitors the status of one or more nonaqueous electrolyte energy storage elements.

<Method of Manufacturing Nonaqueous Electrolyte Storage Element>
A method for producing a nonaqueous electrolyte storage element according to one embodiment of the present invention includes preparing a positive electrode containing a positive electrode active material, and preparing a nonaqueous electrolyte containing a fluorinated solvent and an additive, wherein the positive electrode active material contains sulfur, and the additive is at least one of a sulfur-containing cyclic compound and a boron-containing compound.

Preparing a positive electrode containing a positive electrode active material may mean fabricating a positive electrode containing a positive electrode active material. The positive electrode can be fabricated, for example, by applying a positive electrode mixture paste to a positive electrode substrate directly or via an intermediate layer, and drying the paste. The positive electrode mixture paste contains each component constituting the positive electrode mixture, such as a positive electrode active material containing sulfur, and optional components such as a conductive agent and a binder. The positive electrode mixture paste may further contain a dispersion medium. Specific examples and preferred examples of the prepared positive electrode are the same as the positive electrode provided in the nonaqueous electrolyte storage element according to one embodiment of the present invention described above.

Preparing a nonaqueous electrolyte containing a fluorinated solvent and an additive may be preparing a nonaqueous electrolyte containing a fluorinated solvent and an additive. The preparation of the nonaqueous electrolyte can be performed, for example, by mixing a fluorinated solvent, an additive, and other components such as an electrolyte salt. Specific examples and preferred examples of the nonaqueous electrolyte to be prepared are the same as the nonaqueous electrolyte provided in the nonaqueous electrolyte storage element according to one embodiment of the present invention described above.

The method for manufacturing the nonaqueous electrolyte storage element may include, in addition to preparing the positive electrode and the nonaqueous electrolyte described above, preparing a negative electrode, forming an electrode body in which the positive electrode and the negative electrode are alternately stacked by stacking or rolling them with a separator interposed therebetween, housing the positive electrode and the negative electrode (electrode body) in a container, and injecting the nonaqueous electrolyte into the container. After injection, the injection port is sealed to obtain a nonaqueous electrolyte storage element.

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

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

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.

The additives used in the examples and comparative examples are shown below.
Additive A: Lithium bis(oxalato)borate (LiBOB)
Additive B: Lithium difluoro(oxalato)borate (LiDFOB)
Additive C: Lithium tetrafluoroborate (LiBF ₄ )
Additive D: (3-methyl-2,4-pentanedionato)(oxalato)borate (MOAB)
Additive E: 1,3-propene sultone (PRS)
Additive F: Methylenemethane disulfonic acid ester (MMDS)
Additive a: vinylene carbonate (VC)
Additive b: Lithium difluorophosphate (LiDFP)
Additive c: Tris(2,2,2-trifluoroethyl)phosphate (TFEP)

[Example 1]
(Preparation of non-aqueous electrolyte)
Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) was dissolved in a mixed solvent (volume ratio 50:50) of fluoroethylene carbonate (FEC) and 2,2,2-trifluoroethyl methyl carbonate (TFEMC) at a concentration of 1.0 mol/ ^dm3 . Additive A (lithium bis(oxalato)borate (LiBOB)) was added to this solution at a concentration of 1 mass % to prepare a nonaqueous electrolyte.

(Preparation of Positive Electrode)
Sulfur and porous carbon (CNovel) (manufactured by Toyo Tanso Co., Ltd.) were mixed in a mass ratio of 72:28. This mixture was placed in a sealed electric furnace. After argon flow for 1 hour, the temperature was increased to 150°C at a rate of 5°C/min, and held for 5 hours, and then allowed to cool to 80°C, the temperature at which sulfur solidifies. Thereafter, the temperature was increased again to 300°C at a rate of 5°C/min, and held for 2 hours for heat treatment to produce a sulfur-porous carbon composite (SPC). The porous carbon used in this example had an average pore diameter of 5 nm, a pore volume of 1.7 mLg ^-1 , and a specific surface area of 1500 m ² g ^-1 .

A positive electrode mixture paste containing the sulfur-porous carbon composite (SPC) obtained above, acetylene black as a conductive agent, CMC as a thickener, and SBR as a binder in a mass ratio of 80:10:3.6:6.4, with water as a dispersion medium, was applied to an aluminum positive electrode substrate and dried to produce a positive electrode.

(Preparation of secondary battery)
A sheet of metallic lithium was prepared as the negative electrode. A polyethylene microporous film was prepared as the separator. The positive electrode, negative electrode, separator, and non-aqueous electrolyte were used to obtain a secondary battery as the non-aqueous electrolyte storage element of Example 1.

[Examples 2 to 6 and Comparative Examples 1 to 4]
Each of the secondary batteries of Examples 2 to 6 and Comparative Examples 1 to 4 was obtained in the same manner as in Example 1, except that each additive listed in Table 1 was added instead of Additive A, or no additive was added. In Table 1, "-" indicates that no additive was added.

[evaluation]
(Charge-discharge cycle test)
Each secondary battery obtained was discharged at a constant current of 0.1 C to 1 V at 25° C. After discharge, it was charged at a constant current of 0.2 C to 3 V at 25° C. These discharge and charge steps constitute one cycle, and this cycle was repeated 80 times. After discharge and charge, a 10-minute pause was provided at 25° C. Discharge, charge, and pause were all performed in a thermostatic chamber at 25° C.
Table 1 shows the discharged electric quantity in the first cycle, the discharged electric quantity in the 80th cycle, and the ratio of the discharged electric quantity in the 80th cycle to the discharged electric quantity in the first cycle as the capacity maintenance rate.

As shown in Table 1, in the secondary batteries of Examples 1 to 6, the addition of a specific additive to the non-aqueous electrolyte increases the capacity retention rate compared to the secondary battery of Comparative Example 1 to which no additive was added. On the other hand, in the secondary batteries of Comparative Examples 2 to 4, the capacity retention rate is conversely decreased compared to the secondary battery of Comparative Example 1 to which no additive was added. It can be seen that the capacity retention rate is increased only when at least one of a cyclic compound containing sulfur and a compound containing boron is used as an additive.

Among the examples, the secondary batteries of Examples 1 to 3, which used additives A to C, which were salts containing boron, as additives, also had a large discharge quantity of electricity in the first cycle. In addition, the secondary battery of Example 5, which used additive E, which was a sultone, as an additive, had a particularly high capacity retention rate.

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 Non-aqueous 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 including a positive electrode active material;
a non-aqueous electrolyte containing a fluorinated solvent and an additive;
The positive electrode active material contains sulfur,
The additive is at least one of a sulfur-containing cyclic compound and a boron-containing compound,
The positive electrode active material has a sulfur content of 50 mass % or more . a positive electrode having a positive electrode active material layer containing a positive electrode active material;
a non-aqueous electrolyte containing a fluorinated solvent and an additive;
The positive electrode active material contains sulfur,
The additive is at least one of a sulfur-containing cyclic compound and a boron-containing compound,
the positive electrode active material layer has a sulfur content of 40% by mass or more and 90% by mass or less; a positive electrode including a positive electrode active material;
a non-aqueous electrolyte containing a fluorinated solvent and an additive;
The positive electrode active material contains sulfur,
The additive is at least one of a sulfur-containing cyclic compound and a boron-containing compound,
The nonaqueous electrolyte electricity storage element , wherein the sulfur contained in the positive electrode active material is present as a complex with porous carbon . the additive is the boron-containing compound,
4. The nonaqueous electrolyte storage element according to claim 1, wherein the compound containing boron is a salt. the additive is the sulfur-containing cyclic compound,
4. The nonaqueous electrolyte storage element according to claim 1, wherein the sulfur-containing cyclic compound is a sultone. Providing a positive electrode including a positive electrode active material;
providing a non-aqueous electrolyte comprising a fluorinated solvent and an additive;
The positive electrode active material contains sulfur,
The additive is at least one of a sulfur-containing cyclic compound and a boron-containing compound,
The method for producing a nonaqueous electrolyte storage element , wherein the positive electrode active material has a sulfur content of 50 mass % or more.

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