Non-aqueous electrolytic solution, composition, power storage element, method for manufacturing non-aqueous electrolytic solution, and method for manufacturing power storage element

Abstract

Provided are: a non-aqueous electrolytic solution for which the amount of impurities can be controlled; a composition; a power storage element; a method for manufacturing the nonaqueous electrolytic solution; and a method for manufacturing the power storage element. The non-aqueous electrolytic solution contains hexafluorophosphate ions. The integrated value of a peak derived from difluorophosphate ions in the phosphorus-31 nuclear magnetic resonance spectrum is less than 0.1% with respect to the integrated value of a peak derived from the hexafluorophosphate ions. The absorbance at a wavelength of 300 nm in an ultraviolet-visible absorption spectrum measured at an optical path length of 10 mm and at 25°C ± 2°C is 2 or more. The method for producing a non-aqueous electrolytic solution comprises: a preparation step for preparing a first non-aqueous electrolytic solution containing hexafluorophosphate ions; and a treatment step for bringing the first non-aqueous electrolytic solution into contact with an oxide-based solid electrolyte that exhibits ion conductivity to obtain a second non-aqueous electrolytic solution.

Description

Nonaqueous electrolyte, composition, storage element, method for producing nonaqueous electrolyte, and method for producing storage element

[0001] The present invention relates to a non-aqueous electrolyte, a composition, an electric storage element, a method for producing a non-aqueous electrolyte, and a method for producing an electric storage element.

[0002] Because the hexafluorophosphate ion has a relatively large ionic radius and a high degree of dissociation, it is used in nonaqueous electrolytes, particularly as a lithium salt, in energy storage devices. The hexafluorophosphate ion reacts with residual moisture in the nonaqueous electrolyte to produce impurities such as difluorophosphate ions. These impurities are known to affect the ionic conductivity of the nonaqueous electrolyte, the interfacial resistance of the electrode, and the low-temperature characteristics. Patent Document 1 discloses a prior art technique in which lithium hexafluorophosphate, which has been vacuum-heated, is dissolved in a nonaqueous solvent prepared by precision distilling a commercially available solvent and then treating it with a molecular sieve, thereby controlling the amount of fluorine-containing impurities in the nonaqueous electrolyte.

[0003] Japanese Patent Application Publication No. 10-144345

[0004] There is a need for a technique to control the amount of impurities in the non-aqueous electrolyte, as in the prior art.

[0005] The present invention has been made to meet this demand, and an object of the present invention is to provide a non-aqueous electrolyte, a composition, an electricity storage element, a method for producing a non-aqueous electrolyte, and a method for producing an electricity storage element, in which the amount of impurities can be controlled.

[0006] A first aspect for achieving this object is a non-aqueous electrolyte solution containing hexafluorophosphate ions, in which the integral value of a peak derived from difluorophosphate ions in a phosphorus-31 nuclear magnetic resonance spectrum is 0.3% or less of the integral value of a peak derived from hexafluorophosphate ions.

[0007] In a second aspect, in the first aspect, the concentration of difluorophosphate ions is less than 250 ppm.

[0008] In a third aspect, in the first or second aspect, the absorbance at a wavelength of 300 nm in an ultraviolet-visible absorption spectrum measured at an optical path length of 10 mm and at 25°C ± 2°C is 2 or more.

[0009] A fourth aspect is a composition comprising the non-aqueous electrolyte solution of any one of the first to third aspects, and an oxide-based solid electrolyte exhibiting ion conductivity in contact with the non-aqueous electrolyte solution.

[0010] A fifth aspect is an electric storage element, which includes the nonaqueous electrolyte solution according to any one of the first to third aspects or the composition according to the fourth aspect.

[0011] A sixth aspect is a method for producing a nonaqueous electrolyte solution, comprising: a preparation step of preparing a first nonaqueous electrolyte solution containing hexafluorophosphate ions; and a treatment step of bringing the first nonaqueous electrolyte solution into contact with an oxide-based solid electrolyte exhibiting ion conductivity to obtain a second nonaqueous electrolyte solution.

[0012] In a seventh aspect, when the second nonaqueous electrolyte solution is analyzed under the same conditions by reversed-phase liquid chromatography mass spectrometry using an aqueous eluent as in the sixth aspect, the area of ​​a peak that appears in a range of retention time shorter than the retention time at which a peak derived from hexafluorophosphate ions appears is larger in the second nonaqueous electrolyte solution than in the first nonaqueous electrolyte solution.

[0013] In an eighth aspect, in the sixth or seventh aspect, the self-diffusion coefficient of cations excluding protons at 25°C ± 2°C measured by pulsed field gradient nuclear magnetic resonance spectroscopy is such that the self-diffusion coefficient in the second nonaqueous electrolyte is 0.9 to 1.1 relative to the self-diffusion coefficient in the first nonaqueous electrolyte.

[0014] In a ninth aspect, in any one of the sixth to eighth aspects, the second nonaqueous electrolyte solution has an absorbance of 2 or more at a wavelength of 300 nm in an ultraviolet-visible absorption spectrum measured with an optical path length of 10 mm at 25°C ± 2°C.

[0015] A tenth aspect is a method for producing an electric storage element, which uses a second nonaqueous electrolyte obtained by the method for producing a nonaqueous electrolyte according to any one of the sixth to ninth aspects.

[0016] In the nonaqueous electrolyte of the present invention, the integral value of the peak derived from difluorophosphate ions in a phosphorus-31 nuclear magnetic resonance spectrum is 0.3% or less of the integral value of the peak derived from hexafluorophosphate ions. By using a nonaqueous electrolyte having this property, it is possible to obtain an energy storage element with a controlled amount of impurities.

[0017] It is a cross-sectional view of the energy storage element according to the first embodiment. It is a diagram schematically showing a garnet-type crystal structure. It is a cross-sectional view of the energy storage element according to the second embodiment.

[0018] Preferred embodiments of the present invention will be described below with reference to the accompanying drawings. FIG. 1 is a schematic cross-sectional view of an energy storage element 10 according to a first embodiment. The energy storage element 10 is an element that converts chemical energy into electrical energy and vice versa. The ions (hereinafter referred to as "charge carriers") that contribute to the energy conversion of the energy storage element 10 are Li. ＋ , Na ＋ , K. ＋ , Mg ２＋ , Cu ＋ , Ag ＋ Examples of cations include:

[0019] Examples of the energy storage element 10 include secondary batteries such as lithium ion batteries, sodium ion batteries, potassium ion batteries, magnesium ion batteries, and calcium ion batteries, and electrochemical capacitors. Examples of electrochemical capacitors include electric double layer capacitors, redox capacitors that utilize redox reactions of electrodes or redox reactions of ions in a non-aqueous electrolyte, and hybrid capacitors that combine electric double layers and redox reactions, or that combine them with secondary battery materials.

[0020] The energy storage element 10 includes, in order, a positive electrode 11, a separator 14, and a negative electrode 15. The separator 14 is made of a porous material that is durable against the active materials 19, 20 and non-aqueous electrolyte solution contained in the positive electrode 11 and the negative electrode 15, and that allows charge carriers to pass through but does not have electronic conductivity. Examples of the separator 14 include nonwoven fabrics and porous films made of cellulose, polypropylene, polyethylene, polyimide, alumina, etc.

[0021] The positive electrode 11 is formed by stacking a current collector 12 and an active material layer 13. The current collector 12 is a conductive member. Examples of materials for the current collector 12 include a metal selected from Ni, Ti, Fe, and Al, an alloy containing two or more of these elements, stainless steel, and a carbon material.

[0022] The active material layer 13 includes an active material 19. The active material 19 is appropriately selected depending on the type of charge carrier and the electricity storage device. When the electricity storage device is an electrochemical capacitor, a material capable of reversibly supporting anions is used as the active material 19, and examples of such material include carbon-based materials such as porous carbon, natural graphite, artificial graphite, graphitizable carbon (hard carbon), non-graphitizable carbon (soft carbon), and carbon fiber.

[0023] When the power storage element is an ion battery, examples of the active material 19 include metal oxides containing transition metals, sulfur-based active materials, and organic active materials. ＋ In this case, the metal oxide containing a transition metal is exemplified by a metal oxide containing Li and one or more elements selected from Mn, Co, Ni, Fe, Cr, and V. The metal oxide containing a transition metal is LiCoO ２ , LiNi ０．８ Co ０．１５ Al ０．０５ O ２ , LiMn ２ O ４ , LiNiVO ４ , LiNi ０．５ Mn １．５ O ２ , LiNi １／３ Mn １／３ Co １／３ O ２ and LiFePO ４ is exemplified.

[0024] The sulfur-based active material is S, TiS ２ , NiS, FeS ２ , Li ２ S, MoS ３Examples of organic active materials include radical compounds such as 2,2,6,6-tetramethylpiperidinoxyl-4-yl methacrylate and polytetramethylpiperidinoxyl vinyl ether, quinone compounds, radialene compounds, tetraciaquinodimethane, and phenazine oxide.

[0025] The active material layer 13 may contain a conductive additive to reduce the resistance of the active material layer 13. Examples of the conductive additive include carbon black, acetylene black, ketjen black, carbon fiber, Ni, Pt, and Ag.

[0026] In this embodiment, the active material layer 13 includes a composition. The composition includes an oxide-based solid electrolyte 18 and a non-aqueous electrolyte solution. The solid electrolyte 18 has ion conductivity. Examples of the solid electrolyte 18 include oxides having a NASICON structure, oxides having a perovskite structure, and oxides having a garnet structure. The oxides having a NASICON structure include oxides containing at least Li, M (M is one or more elements selected from Ti, Zr, and Ge), and P, such as Li(Al,Ti). ２ (P.O. ４ ) ３ and Li(Al,Ge) ２ (P.O. ４ ) ３ The oxide having a perovskite structure is an oxide containing at least Li, Ti, and La, for example, La ２／３－Ｘ Li ３Ｘ TiO ３ Examples include:

[0027] The solid electrolyte 18 is preferably a composite oxide containing Li, La, and Zr and having a garnet-type crystal structure. －３ This is because the garnet-type crystalline structure has an ionic conductivity of the order of 50 S / cm and is resistant to reduction by metallic lithium. ３ A ２ B ３ O １２ It is expressed as:

[0028] FIG. 2 is a diagram schematically showing a garnet-type crystal structure. In the garnet-type crystal structure, the C site Sc is dodecahedrally coordinated with an oxygen atom Oa, the A site Sa is octahedrally coordinated with an oxygen atom Oa, and the B site Sb is tetrahedrally coordinated with an oxygen atom Oa. In an oxide solid electrolyte, Li can exist in a position that is octahedrally coordinated with an oxygen atom Oa in a normal garnet-type crystal structure, but becomes a void V. The void V is, for example, a position sandwiched between the B site Sb1 and the B site Sb2. The Li present in the void V is octahedrally coordinated with an oxygen atom Oa that constitutes an octahedron including the tetrahedral face Fb1 that forms the B site Sb1 and the tetrahedral face Fb2 that forms the B site Sb2. For example, Li having a garnet-type crystal structure ７ La ３ Zr ２ O １２ In the formula, La can occupy the C site Sc, Zr can occupy the A site Sa, and Li can occupy the B site Sb and the void V.

[0029] The garnet-type crystal structure can be identified by X-ray diffraction. The garnet-type crystal structure is identified by X-ray diffraction file No. 422259 (Li) in the Cambridge Structural Database (CSD). ７ La ３ Zr ２ O １２ ) has an XRD pattern similar to that of No. 422259. Compared to No. 422259, the solid electrolyte may differ in the type of constituent elements, Li concentration, etc., and therefore the diffraction angle and intensity ratio may differ. A typical crystal structure of this type is a cubic system (space group Ia-3d (- indicates an overline indicating a reversal operation), JCPDS: 84-1753).

[0030] Solid electrolytes with a garnet-type crystal structure are typically Li ７ La ３ Zr ２ O １２ The solid electrolyte is Li ７ La ３ Zr ２ O １２A part of the constituent elements may be substituted with another element, or a small amount of another element may be added without substituting the constituent elements. Examples of the other element include at least one element selected from the group consisting of Mg, Al, Si, Ca, Ti, V, Ga, Sr, Y, Nb, Sn, Sb, Ba, Hf, Ta, W, Bi, Rb, and lanthanides (excluding La).

[0031] The solid electrolyte is, for example, Li ６ La ３ Zr １．５ W ０．５ O １２ , Li ６．１５ La ３ Zr １．７５ Ta ０．２５ Al ０．２ O １２ , Li ６．１５ La ３ Zr １．７５ Ta ０．２５ Ga ０．２ O １２ , Li ６．２５ La ３ Zr ２ Ga ０．２５ O １２ , Li ６．４ La ３ Zr １．４ Ta ０．６ O １２ , Li ６．５ La ３ Zr １．７５ Te ０．２５ O １２ , Li ６．７５ La ３ Zr １．７５ Nb ０．２５ O １２ , Li ６．９ La ３ Zr １．６７５ Ta ０．２８９ Bi ０．０３６ O １２ , Li ６．４６ Ga ０．２３ La ３ Zr １．８５ Y ０．１５ O １２ , Li ６．８ La ２．９５ Ca ０．０５ Zr １．７５ Nb ０．２５O １２ , Li ７．０５ La ３．００ Zr １．９５ Gd ０．０５ O １２ , Li ６．２０ Ba ０．３０ La ２．９５ Rb ０．０５ Zr ２ O １２ Examples include:

[0032] The solid electrolyte preferably contains at least one of Mg and element A (A is at least one element selected from the group consisting of Ca, Sr, and Ba), with the molar ratio of each element satisfying all of the following (1) to (3), or contains both Mg and element A, with the molar ratio of each element satisfying all of the following (4) to (6). Element A is preferably Sr, in order to increase the ionic conductivity of the solid electrolyte. (1) 1.33≦Li / (La+A)≦3 (2) 0≦Mg / (La+A)≦0.5 (3) 0≦A / (La+A)≦0.67 (4) 2.0≦Li / (La+A)≦2.6 (5) 0.01≦Mg / (La+A)≦0.14 (6) 0.04≦A / (La+A)≦0.17

[0033] Returning to FIG. 1 , the nonaqueous electrolyte contained in the active material layer 13 is a medium through which charge carriers move, and is a solution in which an electrolyte is dissolved in a nonaqueous solvent. Examples of electrolytes include compounds composed of charge carriers (anions) and cations. Nonaqueous solvents are broadly classified into molecular solvents, which are mostly composed of molecules, and ionic liquids, which are composed of cations and anions. Nonaqueous electrolytes containing nonaqueous solvents can have a wider potential window than aqueous electrolytes, which use water as the solvent.

[0034] The molecular solvent is preferably an aprotic solvent to widen the potential window of the nonaqueous electrolyte. Examples of aprotic solvents include cyclic esters, chain esters, aliphatic carboxylic acid esters, phosphate esters, nitriles, amides, sulfur compounds, ketones, ethers, nitro compounds, fluorous solvents, and sulfones. Mixtures of these solvents are also acceptable.

[0035] Examples of cyclic esters include carbonate esters such as propylene carbonate, ethylene carbonate, butylene carbonate, vinylene carbonate, vinylethylene carbonate, and fluoroethylene carbonate, and lactones such as β-propiolactone, γ-butyrolactone, δ-valerolactone, α-pyrone, and coumarin. Examples of chain esters include carbonate esters such as dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate. Examples of aliphatic carboxylic acid esters include methyl formate, ethyl formate, methyl acetate, ethyl acetate, and ethyl propionate. Examples of phosphate esters include trimethyl phosphate. Examples of nitriles include acetonitrile, propionitrile, butyronitrile, and benzonitrile.

[0036] Examples of amides include formamide, N-methylformamide, dimethylformamide, N-methylacetamide, dimethylacetamide, N-methylpropioamide, hexamethylphosphoramide, and N-methylpyrrolidone. Examples of sulfur compounds include dimethyl sulfoxide, sulfolane, dimethylthioformamide, and N-methylthiopyrrolidone. Examples of ketones include acetone, 4-methyl-2-pentanone, and acetylacetone.

[0037] Examples of ethers include tetrahydrofuran, 2-methyltetrahydrofuran, 2,5-dimethyltetrahydrofuran, monoglyme, diglyme, triglyme, and tetraglyme. Examples of nitro compounds include nitromethane and nitrobenzene. Fluorous solvents are compounds in which the hydrogen atoms of hydrocarbons are substituted with fluorine atoms, and derivatives thereof. Examples of sulfones include trimethylene sulfone, tetramethylene sulfone (sulfolane), dimethyl sulfone, ethyl methyl sulfone, and ethyl isopropyl sulfone.

[0038] The reaction in which an electrolyte dissolves in a molecular solvent and dissociates into free ions proceeds more easily as the relative dielectric constant of the solvent increases and as the solvation of ions becomes more likely. Therefore, the relative dielectric constant ε ｒ The solvent with a relatively large ｒ>20) is preferred. Examples of molecular solvents with a dielectric constant of greater than 20 include cyclic esters, nitriles, amides, sulfur compounds, acetone, acetylacetone, nitro compounds, and sulfones. It is of course possible to mix a solvent with a dielectric constant of greater than 20 with a solvent with a dielectric constant of 20 or less in order to adjust the viscosity of the solvent, etc.

[0039] Ionic liquids are compounds consisting of cations and anions, and are liquid at room temperature and normal pressure. If the solvent of a non-aqueous electrolyte is an ionic liquid, the flame retardancy of the non-aqueous electrolyte can be improved. The ionic liquid preferably contains one or more cation species selected from the group consisting of ammonium, imidazolium, pyrrolidinium, and piperidinium.

[0040] The anion component of the ionic liquid is not particularly limited. ４ － , N(SO ２ F) ２ － inorganic anions such as B(C ６ H ５ ) ４ － , CH ３ SO ３ － , C.F. ３ SO ３ － , N(SO ２ CF ３ ) ２ － , N(SO ２ C ４ F ９ ) ２ － Examples of organic anions include:

[0041] The ionic liquid may be a solvated ionic liquid, such as a sulfone-based solvent such as sulfolane or a sulfolane derivative, or a glyme-based solvent such as tetraglyme, in which an electrolyte is dissolved.

[0042] The non-aqueous electrolyte is a hexafluorophosphate ion (PF ６ － The charge carrier is Li ＋ In this case, the electrolyte is lithium hexafluorophosphate (LiPF６ The non-aqueous electrolyte may contain anions other than hexafluorophosphate ions. The other anions include BF ４ － , ClO ４ － , N(SO ２ F) ２ － , P.O. ３ F － , P.O. ２ F ２ － is exemplified.

[0043] The electrolyte concentration of the non-aqueous electrolyte is 0.2 mol / dm ３ or more, preferably 0.5 mol / dm ３ More preferably, 1.0 mol / dm ３ As the electrolyte concentration increases, the number of solvent molecules coordinated to the charge carrier increases, resulting in fewer uncoordinated solvent molecules, and coordination with counter anions (so-called ionic association) predominates. This suppresses the reductive decomposition of the nonaqueous electrolyte, while increasing the oxidation potential and widening the potential window. The electrolyte concentration is 4.0 mol / dm ３ Preferably, it is 2.0 mol / dm or less, more preferably 2.0 mol / dm ３ The electrolyte concentration is 4.0 mol / dm or less. ３ If the temperature exceeds this range, the ionic conductivity tends to decrease significantly due to an increase in the viscosity of the non-aqueous electrolyte.

[0044] The method for producing a non-aqueous electrolyte solution includes a preparation step of preparing a first non-aqueous electrolyte solution containing hexafluorophosphate ions, and a treatment step of bringing the first non-aqueous electrolyte solution into contact with a solid electrolyte 18 to obtain a second non-aqueous electrolyte solution. If residual moisture is present in the first non-aqueous electrolyte solution, the hexafluorophosphate ions are hydrolyzed, and a subsequent decomposition reaction proceeds to produce monofluorophosphate ions (PO ３ F ２－ ), difluorophosphate ion (PO ２ F ２ －It is known that impurities such as ammonium nitrate, ammonium nitrate, and ammonium nitrate are generated in non-aqueous electrolytes. These impurities affect the ionic conductivity of the non-aqueous electrolyte, the interfacial resistance of the electrodes, and the low-temperature characteristics of the electrolyte. Therefore, it is necessary to control the amount of impurities in the non-aqueous electrolyte.

[0045] When the solid electrolyte 18 comes into contact with the first nonaqueous electrolyte in the treatment process, the amount of impurities in the second nonaqueous electrolyte is reduced. The mechanism by which the amount of impurities in the second nonaqueous electrolyte is reduced is believed to be that when the first nonaqueous electrolyte comes into contact with the basic solid electrolyte 18, the impurities contained in the first nonaqueous electrolyte decompose and are adsorbed to the solid electrolyte 18 as insoluble compounds. The amount of impurities in the second nonaqueous electrolyte can be controlled by the simple operation of contacting the solid electrolyte 18 with the first nonaqueous electrolyte, without requiring special operations such as precision distillation of the nonaqueous solvent.

[0046] After contacting the solid electrolyte 18 with the first nonaqueous electrolyte solution, both the solid electrolyte 18 and the second nonaqueous electrolyte solution may be placed in the active material layer 13, or a second nonaqueous electrolyte solution in which fine particles of the solid electrolyte 18 are suspended in the first nonaqueous electrolyte solution may be used. Alternatively, after contacting the solid electrolyte 18 with the first nonaqueous electrolyte solution, the solid electrolyte 18 and the second nonaqueous electrolyte solution may be separated, and only the second nonaqueous electrolyte solution may be supplied to the active material layer 13. The second nonaqueous electrolyte solution may also be used when manufacturing an energy storage element other than the energy storage element 10. Examples of means for separating the solid electrolyte 18 from the second nonaqueous electrolyte solution include filtration, sedimentation, and centrifugation.

[0047] Difluorophosphate ions represent impurities contained in the first non-aqueous electrolyte. ３１ P-NMR) of the second non-aqueous electrolyte ３１ When the P-NMR spectrum was measured, the area (integral value) of the peak derived from difluorophosphate ions (chemical shift value: approximately −21 ppm) was 0.3% or less relative to the area (integral value) of the peak derived from hexafluorophosphate ions (chemical shift value: approximately −146 ppm). This is because the amount of difluorophosphate ions in the second nonaqueous electrolyte solution is reduced compared to the first nonaqueous electrolyte solution.

[0048] A known amount of the standard substance was added to the second non-aqueous electrolyte. ３１By measuring the P-NMR spectrum, the absolute amount of difluorophosphate ions in the second nonaqueous electrolyte can be quantified. The concentration of difluorophosphate ions in the second nonaqueous electrolyte is preferably less than 250 wtppm in order to control the amount of impurities.

[0049] When reversed-phase liquid chromatography mass analysis was performed on the first and second nonaqueous electrolytes using an aqueous eluent, a peak appeared in a range with a shorter retention time than the peak derived from hexafluorophosphate ion. Because the ions derived from this peak have a shorter retention time than hexafluorophosphate ion, this indicates a lower affinity for the hydrophobic group of the reversed-phase column, i.e., a higher polarity, compared to hexafluorophosphate ion. Comparing the areas of these peaks, the second nonaqueous electrolyte has a larger area than the first nonaqueous electrolyte, indicating that the second nonaqueous electrolyte contains a higher proportion of ions with higher polarity than hexafluorophosphate ion compared to the first nonaqueous electrolyte.

[0050] The absorbance changes significantly when the solid electrolyte 18 comes into contact with the first nonaqueous electrolyte. The absorbance of the second nonaqueous electrolyte is greater than that of the first nonaqueous electrolyte. The second nonaqueous electrolyte preferably has an absorbance of 2 or greater at a wavelength of 300 nm in an ultraviolet-visible absorption spectrum measured with an optical path length of 10 mm at 25°C ± 2°C. It is presumed that an increase in the proportion of ions more polar than hexafluorophosphate ions affects the absorbance in that wavelength region, or that when an organic component contained in the nonaqueous electrolyte reacts with the solid electrolyte 18 in some way, it incorporates difluorophosphate ions to produce a polymerization product that exhibits absorption in that wavelength region.

[0051] The self-diffusion coefficient D of cations excluding protons contained in the second non-aqueous electrolyte at 25°C ± 2°C measured by pulsed field gradient nuclear magnetic resonance spectroscopy (PFG-NMR) ２ is the self-diffusion coefficient D of the same cation contained in the first nonaqueous electrolyte measured at the same temperature. １ is almost the same as D ２ is D １This indicates that the diffusion rate of the cations (charge carriers) contained in the second nonaqueous electrolyte is approximately the same as the diffusion rate of the cations contained in the first nonaqueous electrolyte.

[0052] The active material layer 13 may contain a binder that binds the active material 19. There are no particular limitations on the binder as long as it binds the active material 19. Examples of the binder include fluorinated resins, polyolefins, polyimides, polyvinylpyrrolidone, polyvinyl alcohol, cellulose ethers, and rubber-like polymers such as styrene butadiene rubber. Examples of fluorinated resins include vinylidene fluoride polymers, polychlorotrifluoroethylene, polyvinyl fluoride, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymers, tetrafluoroethylene-hexafluoropropylene copolymers, ethylene-tetrafluoroethylene copolymers, and ethylene-chlorotrifluoroethylene copolymers.

[0053] The ratio (vol %) of the volume of the nonaqueous electrolyte to the combined volume of the nonaqueous electrolyte and active material 19 contained in active material layer 13 is preferably 10% or more and 20% or less. This is to ensure the equilibrium potential of positive electrode 11 while reducing the interfacial resistance of active material 19, which is the reaction field for charge / discharge reactions.

[0054] When the active material layer 13 contains the solid electrolyte 18, the ratio (vol %) of the volume of the solid electrolyte 18 to the combined volume of the solid electrolyte 18 and the active material 19 is preferably 15% or more and 20% or less. This is to ensure the equilibrium potential of the positive electrode 11 and the contact interface between the solid electrolyte 18 and the active material 19.

[0055] The contents (vol%) of the solid electrolyte 18, active material 19, and non-aqueous electrolyte solution can be determined by analyzing a randomly selected cross-section of the active material layer 13 at a magnification of 5000 times using a scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS). The analysis involves identifying element distribution and performing image analysis of the contrast of a backscattered electron image to determine the areas of the solid electrolyte 18, active material 19, and non-aqueous electrolyte solution. The content (vol%) of the solid electrolyte 18 is determined by considering the ratio of the area of ​​the solid electrolyte 18 to the total area of ​​the solid electrolyte 18 and active material 19 as the volume ratio. The content (vol%) of the non-aqueous electrolyte solution is also determined by considering the ratio of the area of ​​the non-aqueous electrolyte solution to the total area of ​​the active material 19 and non-aqueous electrolyte solution as the volume ratio.

[0056] The cross section of the active material layer 13 used for analysis is a polished surface, a surface obtained by irradiating with a focused ion beam (FIB), or a surface obtained by ion milling. The polished surface is, for example, a surface obtained by freezing the active material layer 13 or by embedding and solidifying the active material layer 13 in a tetrafunctional epoxy resin or the like and then polishing it.

[0057] The negative electrode 15 is formed by stacking a current collector 16 and an active material layer 17. The current collector 16 is a conductive member. Examples of the material for the current collector 16 include a metal selected from Ni, Ti, Fe, Cu, and Si, an alloy containing two or more of these elements, stainless steel, and a carbon material.

[0058] The active material layer 17 includes an active material 20. To reduce the resistance of the active material layer 17, the active material layer 17 may include a conductive additive. Examples of the conductive additive include carbon black, acetylene black, ketjen black, carbon fiber, Ni, Pt, and Ag. In this embodiment, the active material layer 17 includes a solid electrolyte 18.

[0059] There is no limitation on the material of the active material 20 as long as it can absorb and release charge carriers. The active material 20 is appropriately selected depending on the type of charge carrier. The active material 20 may be a carbon-based material such as porous carbon, natural graphite, artificial graphite, graphitizable carbon, non-graphitizable carbon, or carbon fiber; Li ４ Ti ５ O １２, Si, Si-Li alloys, compounds containing Si and O as constituent elements (hereinafter referred to as "SiO ｘ " where 0.5≦x≦1.5), examples include metallic lithium, lithium alloys such as Li-Al alloy, Li-Sn alloy, Li-Si alloy, Li-Mg alloy, and Li-Si alloy, In-Sb alloy, and Si-Li alloy. SiO ｘ is an oxide of Si, amorphous SiO ２ Examples include those having a structure in which microcrystalline or amorphous Si is dispersed in a matrix.

[0060] The energy storage element 10 is manufactured, for example, as follows: The solid electrolyte 18, the active material 19, and the conductive additive are mixed, and then a solution in which a binder is dissolved in a solvent is mixed to form a slurry. The slurry is then applied onto the current collector 12 and dried to obtain a positive electrode sheet.

[0061] The solid electrolyte 18, the active material 20, and the conductive additive are mixed, and then mixed with a solution in which a binder is dissolved in a solvent to form a slurry. The slurry is applied onto the current collector 16 and then dried to obtain a negative electrode sheet.

[0062] A separator 14 separating the positive electrode sheet from the negative electrode sheet, the positive electrode sheet, and the negative electrode sheet are stacked and wound up using a winding machine to produce a cylindrical or rectangular cell. Terminals (not shown) are connected to the current collectors 12 and 16, respectively, and a container (not shown) containing the cell is filled with a second nonaqueous electrolyte solution. The container is then sealed to obtain an energy storage element 10 including a positive electrode 11, a separator 14, and a negative electrode 15.

[0063] According to the energy storage element 10, since the second nonaqueous electrolyte solution contains a controlled amount of impurities such as difluorophosphate ions, it is expected to have advantageous effects on ion conductivity, electrode interface resistance, low-temperature characteristics, and the like.

[0064] A second embodiment will be described with reference to Fig. 3. In the first embodiment, the separator 14 separating the positive electrode 11 and the negative electrode 15 is a nonwoven fabric or porous film made of cellulose, polypropylene, or the like. In contrast, in the second embodiment, an energy storage element 21 will be described in which the separator 22 separating the positive electrode 11 and the negative electrode 15 contains an electrolyte 23. In the second embodiment, the same parts as in the first embodiment are designated by the same reference numerals, and the following description will be omitted.

[0065] 3 is a cross-sectional view of an energy storage element 21 according to the second embodiment. The energy storage element 21 includes, in order, a positive electrode 11, a separator 22, and a negative electrode 15. The separator 22 includes an electrolyte 23. The electrolyte 23 may be, for example, a solid or gel electrolyte having ion conductivity. A composition in which the electrolyte 23 and a non-aqueous electrolytic solution are mixed may be disposed in the separator 22.

[0066] The electrolyte 23 includes at least one selected from sulfide-based, oxide-based, hydride-based, halide-based, and organic-based electrolytes. The sulfide-based electrolyte includes crystalline thiolithium-based, Li １０ GeP ２ S １２ type, argyrodite type, Li ７ P ３ S １１ Type, Li ２ S-P ２ S ５ Examples of oxide electrolytes include oxides having a NASICON structure, oxides having a perovskite structure, and oxides having a garnet structure.

[0067] The hydride electrolyte is LiBH ４ and lithium halide compounds (LiI, LiBr, LiCl) and lithium amide (LiNH ２ ) is an example of a solid solution. ３ YCl ６ Examples of organic solid electrolytes include polyethylene oxide, polypropylene oxide, and polyacrylonitrile.

[0068] The energy storage element 21 is manufactured, for example, as follows: A mixture of a nonaqueous electrolyte solution and a solid electrolyte 18 is mixed with the active material 19, and then a solvent in which a binder is dissolved is mixed to form a slurry. The slurry is then applied onto the current collector 12 and dried to obtain the active material layer 13.

[0069] A solvent in which a binder is dissolved is mixed with a mixture of the nonaqueous electrolytic solution and the electrolyte 23 to prepare a separator slurry. The separator slurry is applied onto the active material layer 13 and then dried to obtain a positive electrode sheet.

[0070] A mixture of a nonaqueous electrolyte solution and a solid electrolyte 18 is mixed with an active material 20, and then a solvent in which a binder is dissolved is mixed to form a slurry. The slurry is applied onto a current collector 16 and then dried to obtain an active material layer 17. A separator slurry is applied onto the active material layer 17 and then dried to obtain a negative electrode sheet.

[0071] After cutting the positive electrode sheet and the negative electrode sheet into a predetermined shape, the positive electrode sheet and the negative electrode sheet are stacked and pressed together to form a cell so that a separator 22 is formed between the positive electrode 11 and the negative electrode 15. Terminals (not shown) are connected to the current collectors 12 and 16, respectively, and the resulting product is sealed in a container (not shown), thereby obtaining an energy storage element 21 including the positive electrode 11, the separator 22, and the negative electrode 15.

[0072] The energy storage element 21 in the second embodiment contains a non-aqueous electrolyte in the positive electrode 11, the negative electrode 15, and the separator 22, and is therefore expected to have advantageous properties in terms of ionic conductivity, electrode interfacial resistance, and low-temperature characteristics, similar to the energy storage element 10 in the first embodiment.

[0073] The present invention will be explained in more detail with reference to examples, but the present invention is not limited to these examples.

[0074] (Preparation of solid electrolyte) Li ６．９５ Mg ０．１５ La ２．７５ Sr ０．２５ Zr ２．０ O １２ So that Li ２ CO ３ , MgO, La(OH) ３ , SrCO ３ , ZrO２ was weighed. ２ CO ３ Considering the volatilization of Li during firing, the amount of LLZ was set to about 15 mol% excess in elemental terms. The weighed raw materials and ethanol were placed in a nylon pot together with zirconia balls and milled and mixed for 15 hours in a ball mill. The slurry removed from the pot was dried and then fired on an MgO plate at 1100°C for 15 hours. The fired powder was pulverized, placed in an MgO scabbard, and further fired at 1100°C for 4 hours. The fired powder was pulverized in a glove box under an argon atmosphere to obtain an oxide (hereinafter referred to as "LLZ").

[0075] The crystal structure of LLZ was confirmed to be garnet-type by powder X-ray diffraction. The median diameter (D50) of the particle size distribution of LLZ measured by laser diffraction / scattering was 74 μm.

[0076] Using a dry jet mill (Nano Jetmizer (registered trademark) NJ-50 model, manufactured by Aisin Nano Technologies Co., Ltd.), the LLZ was pulverized by passing it through the jet mill three times under conditions of a nozzle source pressure of 2.0 MPa, a nitrogen atmosphere, and a throughput of 480 g / Hr, to obtain a first solid electrolyte. The first solid electrolyte was stored in an argon atmosphere with a dew point of -70°C.

[0077] The second solid electrolyte was obtained in powder form by wet-pulverizing the LLZ, and was stored in an argon atmosphere with a dew point of −70° C.

[0078] LLZTO (Li ６．５ La ３ Zr １．５ Ta ０．５ O １２ ) was prepared to obtain a powdery third solid electrolyte. １．４ Al ０．４ Ti １．６ (P.O. ４ ) ３ ) was prepared to obtain a fourth powdery solid electrolyte.

[0079] Example 1 Ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate were mixed in a volume ratio of 1:1:1, and 1 wt % of vinylene carbonate was further mixed to form a non-aqueous solvent. ６ is 1 mol / dm ３ The first nonaqueous electrolyte solution and the first solid electrolyte were placed in a test tube in a mass ratio of 5:1, and the mixture was shaken intermittently for 1 hour to obtain the nonaqueous electrolyte solution of Example 1 in which the first solid electrolyte was partially suspended in the second nonaqueous electrolyte solution and coexisted in the test tube.

[0080] Example 2 The nonaqueous electrolyte solution of Example 2 was obtained in the same manner as in Example 1, except that the first nonaqueous electrolyte solution and the first solid electrolyte were placed in a test tube in a mass ratio of 5:1, and the mixture was shaken intermittently for 1 hour, and then allowed to stand for 24 hours to separate the first solid electrolyte by sedimentation, and the supernatant was collected as a second nonaqueous electrolyte solution.

[0081] Example 3 The nonaqueous electrolyte solution of Example 3 was obtained in the same manner as in Example 1, except that the first nonaqueous electrolyte solution and the second solid electrolyte were placed in a test tube in a mass ratio of 5:1, and the mixture was shaken intermittently for 1 hour, and then allowed to stand for 24 hours to separate the second solid electrolyte by sedimentation, and the supernatant was collected as the second nonaqueous electrolyte solution.

[0082] Example 4 The nonaqueous electrolyte solution of Example 4 was obtained in the same manner as in Example 1, except that the first nonaqueous electrolyte solution and the third solid electrolyte were placed in a test tube in a mass ratio of 5:1, and the mixture was shaken intermittently for 1 hour, and then allowed to stand for 24 hours to separate the third solid electrolyte by sedimentation, and the supernatant was collected as the second nonaqueous electrolyte solution.

[0083] Example 5 The nonaqueous electrolyte solution of Example 5 was obtained in the same manner as in Example 1, except that the first nonaqueous electrolyte solution and the fourth solid electrolyte were placed in a test tube in a mass ratio of 5:1, and the mixture was shaken intermittently for 1 hour, and then allowed to stand for 24 hours to separate the fourth solid electrolyte by sedimentation, and the supernatant was collected as a second nonaqueous electrolyte solution.

[0084] Comparative Example The first nonaqueous electrolyte was used as a nonaqueous electrolyte in a comparative example.

[0085] (Measurement of total ionic conductivity) The total ionic conductivity of the nonaqueous electrolyte in Examples 1 to 5 and the comparative example was measured by an AC impedance method. The measurement temperature was 25°C. The total ionic conductivity of the nonaqueous electrolyte in Examples 1 to 5 and the comparative example was the same. Therefore, it was assumed that the concentration of hexafluorophosphate ions in the nonaqueous electrolyte in Examples 1 to 5 and the comparative example was constant.

[0086] ( ３１ Measurement of P-NMR spectrum) The non-aqueous electrolytes of Examples 1 to 5 and Comparative Example were placed in a solution sample tube with an outer diameter of 5 mm under an argon atmosphere, and a known amount of a standard substance was added to the non-aqueous electrolyte. The NMR spectrum of the phosphorus 31 nucleus ( ３１ The P-NMR spectrum was measured. For the nonaqueous electrolyte in Example 1, a test tube containing the first solid electrolyte was shaken and then allowed to stand for 24 hours, and the supernatant liquid from which the first solid electrolyte was separated by sedimentation was placed in a sample tube. The observation frequency was 700 MHz for protons, the measurement method was single pulse measurement, and the measurement temperature was 25°C.

[0087] ３１ The peak at a chemical shift value of about −144 pmm in the P-NMR spectrum is derived from hexafluorophosphate ions, and the peak at a chemical shift value of about −19 pmm is derived from difluorophosphate ions. The ratio of the integral value of the peak derived from difluorophosphate ions to the integral value of the peak derived from hexafluorophosphate ions (PO ２ F ２ － The concentration of difluorophosphate ion (wt ppm) was calculated from the integral values ​​of the signals of the standard substance and the difluorophosphate ion. The measurement results are shown in Table 1.

[0088]

[0089] According to Table 1, the ratio of the integral value of the peak derived from difluorophosphate ion to the integral value of the peak derived from hexafluorophosphate ion (PO ２ F ２ － ​The concentration of difluorophosphate ions (percentage of difluorophosphate ions) was 0.4% in the Comparative Example, whereas it was 0.3% or less in Examples 1-5. The concentration of difluorophosphate ions was 353 wtppm in the Comparative Example, whereas it was less than 250 wtppm in Examples 1-5. It is presumed that the difluorophosphate ions were removed from the nonaqueous electrolyte in Examples 1-5 by being adsorbed or decomposed by the solid electrolyte.

[0090] Comparing Examples 1-4 and 5, Examples 1-4 have a higher PO content than Example 5. ２ F ２ － It was confirmed that the solid electrolyte having a garnet-type crystal structure containing Li, La, and Zr has a greater effect of removing difluorophosphate ions than LATP.

[0091] Comparing Examples 1-3 and 4, Examples 1-3 have a higher PO content than Example 4. ２ F ２ － It was confirmed that the solid electrolyte having a garnet-type crystal structure containing Mg and Sr in addition to Li, La, and Zr has a greater effect of removing difluorophosphate ions than the solid electrolyte having a garnet-type crystal structure containing Ta in addition to Li, La, and Zr.

[0092] (Measurement of absorbance) The absorbance at a measurement wavelength of 300 nm of the ultraviolet-visible absorption spectrum of the nonaqueous electrolyte solutions in Examples 1 and 2 and Comparative Example was measured using an ultraviolet-visible spectrophotometer (Evolution 201, manufactured by Thermo Fisher Scientific K.K.). The material of the cell containing the nonaqueous electrolyte solution was quartz glass, and the optical path length of the cell was 10 mm. The measurement temperature was 25°C.

[0093] When measuring the absorbance of the non-aqueous electrolyte, a quartz glass cell containing no non-aqueous electrolyte was used as a blank sample, and correction was performed so that the absorbance of this cell at a wavelength of 300 nm was zero.

[0094] (Chromatogram Measurement) The nonaqueous electrolyte solutions in Examples 1 and 2 and Comparative Example were analyzed using a liquid chromatography mass spectrometer (Water Synapt G2 HDMS). Each nonaqueous electrolyte solution was diluted 100 times by volume with pure water to prepare a sample. The main analytical conditions were as follows:

[0095] Column: ACQUITY UPLC BEH C18 (particle size 1.7 μm, inner diameter 2.1 mm, length 50 mm, reverse phase) Eluent: Gradient of 10 mM ammonium acetate aqueous solution and acetonitrile / tetrahydrofuran (8:2) (The composition ratio was based on Electrochemistry, 85(11), 721-727(2017)). Column temperature: 40°C Sample injection volume: 5 μL Ionization method: ESI Measurement mode: Negative ion mode

[0096] In the total ion chromatogram, the peak near a retention time of 2 minutes is derived from hexafluorophosphate ions. It was found that the area of ​​the peak appearing near 1.75 minutes, a retention time shorter than the retention time at which the peak derived from hexafluorophosphate ions appears, was larger in the nonaqueous electrolyte of the Example than in the nonaqueous electrolyte of the Comparative Example. Because this is the result of analysis using a reversed-phase column, the peak near a retention time of 1.75 minutes is derived from ions that have a lower affinity for the hydrophobic group of the reversed-phase column, i.e., a higher polarity, compared to hexafluorophosphate ions. Therefore, it is believed that the nonaqueous electrolyte of the Example has an increased proportion of components more polar than hexafluorophosphate ions, compared to the nonaqueous electrolyte of the Comparative Example. It is presumed that the ions more polar than hexafluorophosphate ions are reaction products between the first nonaqueous electrolyte and the solid electrolyte.

[0097] From the analysis results of the mass chromatogram of difluorophosphate ion, the nonaqueous electrolyte of the comparative example had a total peak area (count number) of 105, and a peak derived from difluorophosphate ion was observed around a retention time of 1.5 minutes. On the other hand, the nonaqueous electrolyte of the example had a total peak area (count number) of 16, and only noise-like peaks were observed. Therefore, it is believed that difluorophosphate ion disappeared from the nonaqueous electrolyte of the example.

[0098] (Measurement of Self-Diffusion Coefficient) The self-diffusion coefficient of Li ions contained in the non-aqueous electrolytes in Examples 1 and 2 and Comparative Example at 25°C was measured using pulsed magnetic field gradient nuclear magnetic resonance spectroscopy with a nuclear magnetic resonance spectrometer (JNM-ECA600II, manufactured by JEOL RESONANCE Co., Ltd.). The non-aqueous electrolyte was placed in a symmetrical microsample tube with an outer diameter of 5 mm, at a height of 5 mm from the bottom of the outer tube, and then sealed with an inner tube. Using a diffusion measurement probe, the sample was not rotated, and the magnetic field gradient was appropriately set within the range of 0.1-13.5 T / m. The self-diffusion coefficient of the component containing lithium 7 (Li ions) was measured at 233.25 MHz using a stimulated echo pulse sequence. The magnetic field gradient pulse width, diffusion time, recovery time after the magnetic field gradient pulse, and number of integrations were adjusted depending on the conditions of the signal being observed.

[0099] After measuring the self-diffusion coefficient, the self-diffusion coefficient D of the Li ion contained in the nonaqueous electrolyte in the comparative example １ The self-diffusion coefficient D of the Li ions contained in the nonaqueous electrolyte in Examples 1 and 2 ２ The ratio (D ２ / D １ ) was obtained. Table 2 shows the measurement results.

[0100]

[0101] As shown in Table 2, ２ / D １ The self-diffusion coefficient of Li ions was 1.05, and there was almost no difference between Examples 1 and 2 and the Comparative Example. If any of the components contained in the nonaqueous electrolyte solution were to undergo a significant decomposition reaction in the oxide-based solid electrolyte, it is expected that the diffusion behavior of Li ions would also change due to undesirable changes in the composition and physical properties of the nonaqueous electrolyte solution. However, since the diffusion rate of Li ions (charge carriers) contained in the nonaqueous electrolyte solutions of Examples 1 and 2 is estimated to be approximately the same as the diffusion rate of Li ions contained in the nonaqueous electrolyte solution of the Comparative Example, it was found that the nonaqueous electrolyte solutions of the Examples allow for control of the difluorophosphate concentration without significant changes in the diffusion behavior of Li ions.

[0102] The absorbance of the nonaqueous electrolyte in the Comparative Example was 0.1 or less, whereas the absorbance of the nonaqueous electrolyte in Examples 1 and 2 was 3 or more. It is presumed that the difluorophosphate ion reacted with the solid electrolyte to generate an ion more polar than the hexafluorophosphate ion, and that this reaction product affected the absorbance in the wavelength region, or that an organic component contained in the nonaqueous electrolyte reacted in some way with the solid electrolyte to incorporate the difluorophosphate ion and produce a polymerization product that exhibits absorption in the wavelength region.

[0103] According to the examples, it has been revealed that difluorophosphate ions can be removed by a simple procedure of temporarily contacting a first nonaqueous electrolyte with an oxide-based solid electrolyte or allowing the first nonaqueous electrolyte to coexist with a solid electrolyte. It has been confirmed that this procedure does not adversely affect the hexafluorophosphate ions contained in the nonaqueous electrolyte or the ionic conductivity of the nonaqueous electrolyte. This procedure is not only useful for controlling the amount of difluorophosphate ions contained in the nonaqueous electrolyte, but is also expected to be useful, for example, for enabling the use of components that are unstable to difluorophosphate ions and for reducing hydrolysis and subsequent decomposition reactions of the nonaqueous electrolyte. This is also expected to improve the rate characteristics and cycle characteristics of the energy storage device.

[0104] The present invention has been described above based on the embodiments, but the present invention is not limited to the above embodiments, and it can be easily inferred that various improvements and modifications are possible within the scope of the present invention.

[0105] In the embodiment, the energy storage element 10 has been described as including a positive electrode 11 having an active material layer 13 provided on one side of a current collector 12, and an negative electrode 15 having an active material layer 17 provided on one side of a current collector 16, but this is not necessarily limited to this. For example, it is of course possible to apply each element in the embodiment to an energy storage element including electrode layers (so-called bipolar electrodes) in which an active material layer 13 and an active material layer 17 are provided on both sides of a current collector 12. If bipolar electrodes and separators 14 are alternately stacked and housed in a case (not shown), an energy storage element with a so-called bipolar structure can be obtained.

[0106] In the embodiment, the active material layers 13 and 17 each contain the solid electrolyte 18, but this is not necessarily limited to this. It is of course possible to omit the solid electrolyte 18 from at least one of the active material layers 13 and 17.

[0107] Although not described in the embodiment, it is of course possible to dispose a protective layer 29 between the active material layer 17 and the separator 14, 22, or between the current collector 16 and the active material layer 17. Disposing a protective layer 29 between the active material layer 17 and the separator 14, 22 can reduce short circuits caused by dendrites. Disposing a protective layer 29 between the current collector 16 and the active material layer 17 can reduce deterioration of the current collector 16.

[0108] 10, 21 Storage element 18 Solid electrolyte

Claims

1. A non-aqueous electrolyte containing a hexafluorophosphate ion, wherein the integral value of a peak derived from a difluorophosphate ion in a phosphorus-31 nuclear magnetic resonance spectrum is 0.3% or less of the integral value of a peak derived from a hexafluorophosphate ion.

2. The non-aqueous electrolyte according to claim 1, wherein the concentration of difluorophosphate ions is less than 250 ppm.

3. The non-aqueous electrolyte according to claim 1, which has an absorbance of 2 or more at a wavelength of 300 nm in an ultraviolet-visible absorption spectrum measured at 25° C.±2° C. with an optical path length of 10 mm.

4. A composition comprising the non-aqueous electrolyte according to any one of claims 1 to 3, and an oxide-based solid electrolyte exhibiting ion conductivity in contact with the non-aqueous electrolyte.

5. An electric storage element comprising the nonaqueous electrolyte according to any one of claims 1 to 3.

6. An electric storage element comprising the composition according to claim 4.

7. A method for producing a non-aqueous electrolyte, comprising: a preparation step of preparing a first non-aqueous electrolyte containing hexafluorophosphate ions; and a treatment step of bringing the first non-aqueous electrolyte into contact with an oxide-based solid electrolyte exhibiting ion conductivity to obtain a second non-aqueous electrolyte.

8. A method for producing a non-aqueous electrolyte according to claim 7, wherein when analyzed under the same conditions by reverse phase liquid chromatography mass spectrometry using an aqueous eluent, the area of ​​a peak appearing in a range of retention times shorter than the retention time attributable to a peak originating from hexafluorophosphate ion is larger for the second non-aqueous electrolyte than for the first non-aqueous electrolyte.

9. A method for producing a non-aqueous electrolyte solution as described in claim 7 or 8, wherein the self-diffusion coefficient at 25°C ± 2°C of cations excluding protons measured by pulsed magnetic field gradient nuclear magnetic resonance spectroscopy is 0.9 to 1.1 relative to the self-diffusion coefficient in the first non-aqueous electrolyte solution.

10. A method for producing a non-aqueous electrolyte solution as described in claim 7 or 8, wherein the second non-aqueous electrolyte solution has an absorbance of 2 or more at a wavelength of 300 nm in an ultraviolet-visible absorption spectrum measured with an optical path length of 10 mm and at 25°C ± 2°C.

11. A method for producing an electric storage element, which uses the second nonaqueous electrolyte obtained by the method for producing a nonaqueous electrolyte according to claim 7 or 8.

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