Composition, energy storage element, method for producing a non-aqueous electrolyte, and method for producing an energy 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

[Technical Field]

[0001] This invention relates to a non-aqueous electrolyte, a composition, an energy storage element, a method for producing a non-aqueous electrolyte, and a method for producing an energy storage element. [Background technology]

[0002] Hexafluoride phosphate ions have a relatively large ionic radius and a high degree of dissociation, so they are used as electrolytes in non-aqueous electrolytes, especially as lithium salts, in energy storage devices. Hexafluoride phosphate ions react with residual water in non-aqueous electrolytes to produce impurities such as difluorophosphate ions. These impurities are known to affect the ionic conductivity, electrode interfacial resistance, and low-temperature characteristics of non-aqueous electrolytes. Patent document 1 discloses prior art in which lithium hexafluoride phosphate, which has been treated with vacuum heating, is dissolved in a non-aqueous solvent prepared by precision distillation of a commercially available solvent followed by molecular sieving, thereby controlling the amount of fluorine-containing impurities in the non-aqueous electrolyte. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 10-144345 [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] A technique for controlling the amount of impurities in the non-aqueous electrolyte, similar to prior art, is required.

[0005] This invention was made to meet this need and aims to provide a non-aqueous electrolyte, a composition, an energy storage element, a method for producing a non-aqueous electrolyte, and a method for producing an energy storage element, all of which allow for control of the amount of impurities. [Means for solving the problem]

[0006] A first embodiment for achieving this objective is a non-aqueous electrolyte containing hexafluoride phosphate ions, wherein the integral value of the peak originating from the difluorophosphate ions in the phosphorus-31 nuclear magnetic resonance spectrum is 0.3% or less of the integral value of the peak originating from the hexafluoride phosphate ions.

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

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

[0009] The fourth embodiment is a composition comprising a non-aqueous electrolyte according to any of the first to third embodiments, and an ionic conductive oxide-based solid electrolyte in contact with the non-aqueous electrolyte.

[0010] The fifth embodiment is an energy storage element comprising a non-aqueous electrolyte according to any of the first to third embodiments, or a composition according to the fourth embodiment.

[0011] The sixth aspect is a method for producing a non-aqueous electrolyte, comprising a preparation step of preparing a first non-aqueous electrolyte containing hexafluoride phosphate ions, and a processing step of contacting the first non-aqueous electrolyte with an oxide-based solid electrolyte exhibiting ionic conductivity to obtain a second non-aqueous electrolyte.

[0012] In the seventh aspect, in the sixth aspect, when analyzed under the same conditions by reversed-phase liquid chromatography-mass spectrometry using an aqueous eluent, the area of ​​the peak appearing in the range where the retention time is shorter than the retention time at which the peak originating from hexafluoride phosphate ions appears is larger for the second non-aqueous electrolyte than for the first non-aqueous electrolyte.

[0013] The eighth aspect is that, in the sixth or seventh aspect, the self-diffusion coefficient of cations excluding protons at 25°C ± 2°C, as measured by pulsed magnetic field gradient nuclear magnetic resonance spectroscopy, is such that the self-diffusion coefficient in the second non-aqueous electrolyte is 0.9 or more and 1.1 or less compared to the self-diffusion coefficient in the first non-aqueous electrolyte.

[0014] The ninth aspect is that, in any of the sixth to eighth aspects, the second non-aqueous electrolyte has an absorbance of 2 or more at a wavelength of 300 nm in an ultraviolet-visible absorption spectrum measured at a path length of 10 mm and 25°C ± 2°C.

[0015] The tenth aspect is a method for manufacturing an energy storage element, wherein a second non-aqueous electrolyte obtained by the method for manufacturing a non-aqueous electrolyte according to any of the sixth to ninth aspects is used. [Effects of the Invention]

[0016] The non-aqueous electrolyte of the present invention has an integral value of the peak originating from the difluorophosphate ion in the phosphorus-31 nuclear magnetic resonance spectrum that is 0.3% or less compared to the integral value of the peak originating from the hexafluoride ion. By using a non-aqueous electrolyte having this property, it is possible to obtain an energy storage element with controlled impurity levels. [Brief explanation of the drawing]

[0017] [Figure 1] This is a cross-sectional view of the energy storage element in the first embodiment. [Figure 2] This is a schematic diagram showing the crystal structure of a garnet-type crystal. [Figure 3] This is a cross-sectional view of the energy storage element in the second embodiment. [Modes for carrying out the invention]

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

[0019] Examples of energy storage elements 10 include secondary batteries such as lithium-ion batteries, sodium-ion batteries, potassium-ion batteries, magnesium-ion batteries, and calcium-ion batteries, as well as electrochemical capacitors. Examples of electrochemical capacitors include electric double-layer capacitors, redox capacitors that utilize redox reactions of electrodes or ions in a non-aqueous electrolyte, and hybrid capacitors that combine electric double layers and redox reactions, or 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 and 20 contained in the positive electrode 11 and negative electrode 15 and the non-aqueous electrolyte, and allows the charge carrier to pass through but does not conduct electrons. Examples of separators 14 include nonwoven fabrics and porous membranes made of cellulose, polypropylene, polyethylene, polyimide, alumina, etc.

[0021] The positive electrode 11 consists of a current collector 12 and an active material layer 13 superimposed on each other. The current collector 12 is a conductive material. Examples of materials for the current collector 12 include metals selected from Ni, Ti, Fe, and Al, alloys containing two or more of these elements, stainless steel, and carbon materials.

[0022] The active material layer 13 contains an active material 19. The active material 19 is appropriately selected according to the type of charge carrier or energy storage element. When the energy storage element is an electrochemical capacitor, the active material 19 is a material that can reversibly carry anions, and examples thereof include carbon-based materials such as porous carbon, natural graphite, artificial graphite, easily graphitizable carbon (hard carbon), hardly graphitizable carbon (soft carbon), and carbon fiber.

[0023] When the energy storage element is an ion battery, examples of the active material 19 include metal oxides having transition metals, sulfur-based active materials, and organic-based active materials. When the charge carrier is Li + , examples of the metal oxide having a transition metal include metal oxides containing one or more elements selected from Mn, Co, Ni, Fe, Cr, and V and Li. Examples of the metal oxide having a transition metal include LiCoO2, LiNi 0.8 Co 0.15 Al 0.05 O2, LiMn2O4, LiNiVO4, LiNi 0.5 Mn 1.5 O2, LiNi 1 / 3 Mn 1 / 3 Co 1 / 3 O2, and LiFePO4.

[0024] Examples of the sulfur-based active materials include S, TiS2, NiS, FeS2, Li2S, MoS3, and sulfur-carbon composites. Examples of the organic-based active materials include radical compounds typified by 2,2,6,6-tetramethylpiperidinoxyl-4-yl methacrylate and polytetramethylpiperidinoxyl vinyl ether, quinone compounds, radicalene compounds, tetracyanoquinodimethane, and phenazine oxide.

[0025] In order to lower the resistance of the active material layer 13, a conductive aid may be included in the active material layer 13. Examples of the conductive aid include carbon black, acetylene black, ketjen black, carbon fiber, Ni, Pt, and Ag.

[0026] In this embodiment, the active material layer 13 comprises a composition. The composition comprises an oxide-based solid electrolyte 18 and a non-aqueous electrolyte. The solid electrolyte 18 is ionic conductive. Examples of the solid electrolyte 18 include oxides having a NASICON-type structure, oxides having a perovskite structure, and oxides having a garnet-type structure. Oxides having a NASICON-type structure include oxides containing at least Li, M (where M is one or more elements selected from Ti, Zr, and Ge) and P, such as Li(Al,Ti)2(PO4)3 and Li(Al,Ge)2(PO4)3. Oxides having a perovskite structure include oxides containing at least Li, Ti, and La, such as La 2 / 3-X Li 3X TiO3 is one example.

[0027] The solid electrolyte 18 is preferably a composite oxide having a garnet-type crystalline structure containing Li, La, and Zr. It is strongly basic, and 10 -3 This is because it has an ionic conductivity in the S / cm range and is resistant to reduction by metallic lithium. The garnet-type crystal structure is general formula C3A2B3O 12 It is represented as follows.

[0028] Figure 2 schematically shows a garnet-type crystal structure. In a garnet-type crystal structure, the C site Sc coordinates dodecahedrally with the oxygen atom Oa, the A site Sa coordinates octahedrally with the oxygen atom Oa, and the B site Sb coordinates tetrahedrally with the oxygen atom Oa. In oxide solid electrolytes, Li can be present in the void V, which is the site where the oxygen atom Oa would normally coordinate octahedrally in a typical garnet-type crystal structure. The void V is, for example, the area between B site Sb1 and B site Sb2. The Li present in void V is octahedrally coordinated with the oxygen atom Oa that constitutes an octahedron including the tetrahedral face Fb1 forming B site Sb1 and the tetrahedral face Fb2 forming B site Sb2. For example, Li7La3Zr2O has a garnet-type crystal structure. 12 In this case, La may occupy site C (Sc), Zr may occupy site A (Sa), and Li may occupy site B (Sb) and the void V.

[0029] Garnet-type crystal structures can be identified by X-ray diffraction. The garnet-type crystal structure is found in the Cambridge Structural Database (CSD) X-ray diffraction file No. 422259 (Li7La3Zr2O 12 It has an XRD pattern similar to ). The solid electrolyte may differ from No. 422259 in terms of the types of constituent elements and Li concentration, so the diffraction angle and intensity ratio may differ. A typical crystal structure of this type is cubic (space group Ia-3d (- indicates an overline meaning reversal operation), JCPDS:84-1753).

[0030] Solid electrolytes with a garnet-type crystal structure are typically Li7La3Zr2O 12 Examples include: The solid electrolyte is Li7La3Zr2O 12 Some of the constituent elements may be substituted with other elements, or trace amounts of other elements may be added without substituting any constituent elements. Examples of other elements 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] Solid electrolytes include, for example, Li6La3Zr 1.5 W 0.5 O 12 Li 6.15 La3Zr 1.75 Ta 0.25 Al 0.2 O 12 Li 6.15 La3Zr 1.75 Ta 0.25 Ga 0.2 O 12 Li 6.25 La3Zr2Ga 0.25 O 12 Li 6.4 La3Zr 1.4 Ta 0.6 O 12 Li 6.5 La3Zr 1.75 Te 0.25 O 12 Li 6.75 La3Zr1.75 Nb 0.25 O 12 Li 6.9 La3Zr 1.675 Ta 0.289 Bi 0.036 O 12 Li 6.46 Ga 0.23 La3Zr 1.85 Y 0.15 O 12 Li 6.8 La 2.95 Ca 0.05 Zr 1.75 Nb 0.25 O 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 Zr2O 12 These are some examples.

[0032] The solid electrolyte is preferably one that contains Mg and at least one of element A (where A is at least one element selected from the group consisting of Ca, Sr, and Ba), and the molar ratio of each element satisfies all of (1) to (3) below, or one that contains both Mg and element A, and the molar ratio of each element satisfies all of (4) to (6) below. 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] Let's return to Figure 1 for explanation. The non-aqueous electrolyte contained in the active material layer 13 is the medium through which the charge carriers move, and is a solution in which an electrolyte is dissolved in a non-aqueous solvent. Examples of electrolytes include compounds consisting of charge carriers (anions) and cations. Non-aqueous solvents are broadly classified into molecular solvents, which consist mostly of molecules, and ionic liquids, which consist of cations and anions. Non-aqueous electrolytes containing non-aqueous solvents can have a wider potential window compared to aqueous electrolytes that use water as the solvent.

[0034] Molecular solvents are preferably aprotic solvents in order to broaden the potential window of the non-aqueous electrolyte. Examples of aprotic solvents include cyclic esters, linear esters, aliphatic carboxylic acid esters, phosphate esters, nitriles, amides, sulfur compounds, ketones, ethers, nitro compounds, fluorescein solvents, and sulfones. Mixtures of these 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, as well as 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 and their derivatives in which hydrogen atoms of hydrocarbons are replaced with fluorine atoms. Examples of sulfones include trimethylene sulfone, tetramethylene sulfone (sulfolane), dimethyl sulfone, ethyl methyl sulfone, and ethyl isopropyl sulfone.

[0038] The reaction in which electrolytes dissolve in a molecular solvent and dissociate into free ions proceeds more readily when the relative permittivity of the solvent is high and when solvation of ions is easy. r A relatively large solvent (ε r A dielectric constant of >20 is preferred. Examples of molecular solvents with a dielectric constant greater than 20 include cyclic esters, nitriles, amides, sulfur compounds, acetone, acetylacetone, nitro compounds, and sulfones. It is naturally possible to mix a solvent with a dielectric constant greater than 20 with a solvent with a dielectric constant of 20 or less in order to adjust the viscosity of the solvent.

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

[0040] The anionic component of the ionic liquid is not particularly limited. The anionic component is BF4 - ,N(SO2F)2 - Inorganic anions such as B(C6H5)4 - CH3SO3 - CF3SO3 - ,N(SO2CF3)2 - ,N(SO2C4F9)2 - Examples of organic anions include the following.

[0041] The ionic liquid may be a solvated ionic liquid. Examples of the solvated ionic liquid include those obtained by dissolving an electrolyte in a sulfone-based solvent such as sulfolane or a sulfolane derivative, or a glyme-based solvent such as tetraglyme.

[0042] The non-aqueous electrolyte contains phosphate hexafluoride ion (PF6 - ). When the charge carrier is Li + , an example of the electrolyte is lithium hexafluorophosphate (LiPF6). The non-aqueous electrolyte may contain other anions than phosphate hexafluoride ion. Examples of the other anions include BF4 - , ClO4 - , N(SO2F)2 - , PO3F - , PO2F2 - .

[0043] The concentration of the electrolyte in the non-aqueous electrolyte is 0.2 mol / dm 3 or more, preferably 0.5 mol / dm 3 or more, and more preferably 1.0 mol / dm 3 or more. As the concentration of the electrolyte increases, the number of solvent molecules coordinated to the charge carrier increases, so the uncoordinated solvent decreases, and coordination by the counter anion (so-called ion association) predominates. This suppresses the reduction decomposition of the non-aqueous electrolyte, while increasing the oxidation potential and widening the potential window. The concentration of the electrolyte is preferably 4.0 mol / dm 3 or less, and more preferably 2.0 mol / dm 3 or less. This is because when the concentration of the electrolyte exceeds 4.0 mol / dm 3 , the ionic conductivity tends to decrease significantly due to an increase in the viscosity of the non-aqueous electrolyte.

[0044] The method for producing the non-aqueous electrolyte includes a preparation step of preparing a first non-aqueous electrolyte containing phosphate hexafluoride ion, and a treatment step of bringing the first non-aqueous electrolyte into contact with the solid electrolyte 18 to obtain a second non-aqueous electrolyte. When residual moisture is present in the first non-aqueous electrolyte, the phosphate hexafluoride ion is hydrolyzed, and further subsequent decomposition reactions proceed, resulting in monofluorophosphate ion (PO3F 2-), difluorophosphate ion (PO2F2 - It is known that impurities such as ) are generated. Impurities in non-aqueous electrolytes affect the ionic conductivity, electrode interfacial resistance, and low-temperature characteristics of the non-aqueous 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 non-aqueous electrolyte during the processing step, the amount of impurities in the second non-aqueous electrolyte is reduced. The mechanism by which the amount of impurities in the second non-aqueous electrolyte is reduced is presumed to be that when the first non-aqueous electrolyte comes into contact with the basic solid electrolyte 18, the impurities contained in the first non-aqueous electrolyte decompose and are adsorbed onto the solid electrolyte 18 as insoluble compounds. The amount of impurities in the second non-aqueous electrolyte can be controlled by a simple operation of bringing the solid electrolyte 18 into contact with the first non-aqueous electrolyte, without the need for special operations such as precise distillation of the non-aqueous solvent.

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

[0047] Difluorophosphate ions are representative of the impurities present in the first non-aqueous electrolyte. Phosphorus-31 nuclear magnetic resonance spectroscopy ( 31 The second non-aqueous electrolyte was determined by P-NMR. 31 When the P-NMR spectrum was measured, the integral value of the peak originating from difluorophosphate ions (chemical shift value around -21 ppm) was less than 0.3% of the area (integral value) of the peak originating from hexafluoride phosphate ions (chemical shift value around -146 ppm). This is because the amount of difluorophosphate ions in the second non-aqueous electrolyte was reduced compared to the first non-aqueous electrolyte.

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

[0049] When reverse-phase liquid chromatography mass analysis using an aqueous eluent is performed on the first and second non-aqueous electrolytes, peaks appear in the retention time range shorter than the retention time at which peaks originating from hexafluoride phosphate ions appear. Since the ions originating from these peaks have shorter retention times than hexafluoride phosphate ions, this indicates that they have a lower affinity for the hydrophobic groups of the reverse-phase column, i.e., higher polarity, compared to hexafluoride phosphate ions. Comparing the area of ​​these peaks, the area of ​​the second non-aqueous electrolyte is larger than that of the first non-aqueous electrolyte, indicating that the proportion of ions with higher polarity than hexafluoride phosphate ions is increased in the second non-aqueous electrolyte compared to the first non-aqueous electrolyte.

[0050] The absorbance of the first non-aqueous electrolyte changes significantly when the solid electrolyte 18 comes into contact with it. The absorbance of the second non-aqueous electrolyte is greater than that of the first non-aqueous electrolyte. Preferably, the second non-aqueous electrolyte has an absorbance of 2 or more at a wavelength of 300 nm in the ultraviolet-visible absorption spectrum measured at a path length of 10 mm and 25°C ± 2°C. It is presumed that an increase in the proportion of ions with higher polarity than hexafluoride phosphate ions affects the absorbance in that wavelength region, or that when organic components contained in the non-aqueous electrolyte undergo some reaction with the solid electrolyte 18, they incorporate difluorophosphate ions, thereby producing polymerization products that show absorption in that wavelength region.

[0051] The self-diffusion coefficient D2 of cations excluding protons contained in the second non-aqueous electrolyte at 25°C ± 2°C, as measured by pulsed magnetic field gradient nuclear magnetic resonance spectroscopy (PFG-NMR), is approximately the same as the self-diffusion coefficient D1 of the same cations contained in the first non-aqueous electrolyte, measured at the same temperature. D2 is in the range of 0.9 to 1.1 times D1. This indicates that the diffusion rate of cations (charge carriers) contained in the second non-aqueous electrolyte is approximately the same as the diffusion rate of cations contained in the first non-aqueous electrolyte.

[0052] The active material layer 13 may contain a binder that binds the active material 19. The binder is not particularly limited as long as it binds the active material 19. Examples of binders include rubbery polymers such as fluorinated resins, polyolefins, polyimides, polyvinylpyrrolidone, polyvinyl alcohol, cellulose ethers, and 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] Preferably, the ratio (vol%) of the volume of the non-aqueous electrolyte to the combined volume of the non-aqueous electrolyte and the active material 19 in the active material layer 13 is 10% to 20%. This is to reduce the interfacial resistance of the active material 19, which is the reaction field for the charge-discharge reaction, while ensuring the equilibrium potential of the positive electrode 11.

[0054] When the active material layer 13 contains a solid electrolyte 18, it is preferable that 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 15% or more and 20% or less. This is to ensure the equilibrium potential of the positive electrode 11 and to ensure the contact interface between the solid electrolyte 18 and the active material 19.

[0055] The vol% content of the solid electrolyte 18, active material 19, and non-aqueous electrolyte can be determined by analyzing a 5000x magnification field of view randomly selected from the cross-section of the active material layer 13 using a scanning electron microscope (SEM) equipped with an energy-dispersive X-ray spectrometer (EDS). The analysis identifies the area of ​​the solid electrolyte 18, the area of ​​the active material 19, and the area of ​​the non-aqueous electrolyte by identifying the elemental distribution and performing image analysis of the contrast of the backscattered electron image. The vol% content of the solid electrolyte 18 is obtained by considering the ratio of the area of ​​the solid electrolyte 18 to the total area of ​​the solid electrolyte 18 and the active material 19 as a volume ratio. Similarly, the vol% content of the non-aqueous electrolyte is obtained by considering the ratio of the area of ​​the non-aqueous electrolyte to the total area of ​​the active material 19 and the non-aqueous electrolyte as a volume ratio.

[0056] The cross-sections of the active material layer 13 used for analysis are polished surfaces, surfaces obtained by irradiation with a focused ion beam (FIB), and surfaces obtained by ion milling. Polished surfaces are, for example, surfaces 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 consists of a current collector 16 and an active material layer 17 superimposed on each other. The current collector 16 is a conductive material. Examples of materials for the current collector 16 include metals selected from Ni, Ti, Fe, Cu, and Si, alloys containing two or more of these elements, stainless steel, and carbon materials.

[0058] The active material layer 17 contains an active material 20. To lower the resistance of the active material layer 17, a conductive additive may be included in the active material layer 17. Examples of conductive additives include carbon black, acetylene black, Ketjen black, carbon fiber, Ni, Pt, and Ag. In this embodiment, the active material layer 17 contains a solid electrolyte 18.

[0059] The active material 20 is not limited to any material 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 can be porous carbon, natural graphite, artificial graphite, easily graphitizable carbon, difficult-to-graphitize carbon, carbon fibers, and other carbon-based materials, as well as Li4Ti5O 12, Si, Si-Li alloys, compounds containing Si and O as constituent elements (hereinafter referred to as "SiO x This is referred to as "lithium alloys" (where 0.5 ≤ X ≤ 1.5). Examples include metallic lithium, Li-Al alloys, Li-Sn alloys, Li-Si alloys, Li-Mg alloys, Li-Si alloys, In-Sb alloys, and Si-Li alloys. x Examples include silicon oxides and structures in which microcrystalline or amorphous silicon is dispersed within an amorphous SiO2 matrix.

[0060] The energy storage element 10 is manufactured, for example, as follows: A solid electrolyte 18, an active material 19, and a conductive additive are mixed, and a solution of a binder dissolved in a solvent is added to make a slurry. After applying the slurry onto the current collector 12, it is dried to obtain a positive electrode sheet.

[0061] A slurry is made by mixing a solid electrolyte 18, an active material 20, and a conductive additive, and then mixing in a solution of the binder dissolved in a solvent. After applying the slurry onto the current collector 16, it is dried to obtain a negative electrode sheet.

[0062] A separator 14 separates the positive electrode sheet and the negative electrode sheet. The positive electrode sheet and the negative electrode sheet are stacked and wound together on a winding machine to produce a cylindrical or rectangular cell. Terminals (not shown) are connected to the current collectors 12 and 16, respectively. A second non-aqueous electrolyte is then filled into a container (not shown) containing the cell, and the container is 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 it contains a second non-aqueous electrolyte in which the amount of impurities such as difluorophosphate ions is controlled, it is expected to have an advantageous effect on ionic conductivity, electrode interfacial resistance, and low-temperature characteristics.

[0064] A second embodiment will be described with reference to Figure 3. In the first embodiment, the separator 14 separating the positive electrode 11 and the negative electrode 15 was described as a nonwoven fabric or porous membrane made of cellulose, polypropylene, or the like. In contrast, in the second embodiment, the separator 22 separating the positive electrode 11 and the negative electrode 15 is described as an energy storage element 21 containing an electrolyte 23. In the second embodiment, the same parts as in the first embodiment are denoted by the same reference numerals and their descriptions are omitted below.

[0065] Figure 3 is a cross-sectional view of the energy storage element 21 in 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 contains an electrolyte 23. Examples of the electrolyte 23 include solid or gel-like electrolytes having ion conductivity. A composition in which the electrolyte 23 and a non-aqueous electrolyte is mixed may be placed in the separator 22.

[0066] Electrolyte 23 includes one or more selected from sulfide-based, oxide-based, hydride-based, halide-based, and organic-based electrolytes. Sulfide-based electrolytes include crystalline thiolysicone-type, Li 10 GeP2S 12 Type, argyrodite type, Li7P3S 11 Examples of oxide-based electrolytes include glass and glass-ceramic systems, such as Li2S-P2S5. Examples of oxide-based electrolytes include oxides with a NASICON-type structure, oxides with a perovskite structure, and oxides with a garnet-type structure.

[0067] Examples of hydride-based electrolytes include solid solutions of LiBH4 with lithium halide compounds (LiI, LiBr, LiCl) and lithium amide (LiNH2). An example of a halide solid electrolyte is Li3YCl6. 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 non-aqueous electrolyte and a solid electrolyte 18 is mixed with an active material 19, and then a solvent containing a binder is added to create a slurry. The slurry is then applied to the current collector 12 and dried to obtain an active material layer 13.

[0069] A non-aqueous electrolyte and electrolyte 23 are mixed with a solvent containing a dissolved binder to create a separator slurry. The separator slurry is then applied to the active material layer 13 and dried to obtain a positive electrode sheet.

[0070] A mixture of a non-aqueous electrolyte and a solid electrolyte 18 is mixed with the active material 20, and then a solvent containing a dissolved binder is added to create a slurry. The slurry is applied to the current collector 16 and then dried to obtain the active material layer 17. A separator slurry is applied to the active material layer 17 and then dried to obtain the negative electrode sheet.

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

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

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

[0074] (Preparation of solid electrolytes) Li 6.95 Mg 0.15 La 2.75 Sr 0.25 Zr2.0 O 12 Li2CO3, MgO, La(OH)3, SrCO3, and ZrO2 were weighed accordingly. Li2CO3 was kept in excess by approximately 15 mol% elementally, considering the volatilization of Li during calcination. The weighed raw materials and ethanol were placed in a nylon pot along with zirconia balls and ground and mixed in a ball mill for 15 hours. After drying the slurry removed from the pot, it was calcined at 1100°C for 15 hours on an MgO plate. The resulting powder was ground, placed in an MgO pod, and further calcined at 1100°C for 4 hours. The resulting powder was ground in a glove box under an argon atmosphere to obtain an oxide (hereinafter referred to as "LLZ").

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

[0076] Using a dry jet mill (Aisin Nanotechnologies Corporation, NanoJetmizer® NJ-50 model), the LLZ was pulverized by passing it through the jet mill three times under conditions of a nitrogen atmosphere with a nozzle pressure of 2.0 MPa and a processing rate of 480 g / hr, thereby obtaining the first solid electrolyte. The first solid electrolyte was stored in an argon atmosphere with a dew point of -70°C.

[0077] A powdered second solid electrolyte was obtained by wet grinding using LLZ. The second solid electrolyte was stored in an argon atmosphere with a dew point of -70°C.

[0078] LLZTO(Li 6.5 La3Zr 1.5 Ta 0.5 O 12 A third solid electrolyte in powder form was obtained by preparing ). LATP(Li 1.4 Al 0.4 Ti 1.6 (PO4)3) was prepared to obtain a fourth solid electrolyte in powder form.

[0079] (Example 1) Ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate are mixed in a 1:1:1 (volume ratio), and then 1 wt% vinylene carbonate is added to this non-aqueous solvent, to which the electrolyte LiPF6 is added at a concentration of 1 mol / dm³. 3 The first non-aqueous electrolyte was prepared by mixing the two components in such a manner. The first non-aqueous electrolyte and the first solid electrolyte were placed in a test tube in a ratio of 5:1 (by mass), and the mixture was intermittently shaken for 1 hour to obtain the non-aqueous electrolyte of Example 1, in which a portion of the first solid electrolyte was suspended in the second non-aqueous electrolyte and coexisted in the test tube.

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

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

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

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

[0084] (Comparative example) The first non-aqueous electrolyte was used as the non-aqueous electrolyte in the comparative example.

[0085] (Measurement of total ion conductivity) The total ionic conductivity of the non-aqueous electrolytes in Examples 1-5 and the Comparative Example was measured by AC impedance spectroscopy. The measurement temperature was 25°C. The total ionic conductivity of the non-aqueous electrolytes in Examples 1-5 and the Comparative Example was the same. Therefore, it was assumed that the concentration of hexafluoride phosphate ions in the non-aqueous electrolytes in Examples 1-5 and the Comparative Example was constant.

[0086] ( 31 (Measurement of P-NMR spectrum) The non-aqueous electrolytes in Examples 1-5 and the Comparative Example were each placed in a 5 mm outer diameter sample tube under an argon atmosphere, and a known amount of standard substance was added to the non-aqueous electrolyte. The NMR spectrum of the phosphorus-31 nucleus was then measured using a nuclear magnetic resonance spectrometer (JEOL ECZ700R). 31 The P-NMR spectrum was measured. In Example 1, the non-aqueous electrolyte was prepared by shaking a test tube containing the first solid electrolyte, allowing it to stand for 24 hours, and then placing the supernatant liquid, after the first solid electrolyte had settled, into 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 around the chemical shift value of -144 pmm in the P-NMR spectrum originates from the hexafluoride phosphate ion, and the peak around the chemical shift value of -19 pmm originates from the difluorophosphate ion. The ratio of the integral value of the peak originating from the difluorophosphate ion to the integral value of the peak originating from the hexafluoride ion (PO2F2) - The proportions were calculated for each. Furthermore, the concentration of difluorophosphate ions (wt ppm) was determined from the integrated signals of the standard substance and the difluorophosphate ions. The measurement results are shown in Table 1.

[0088] [Table 1]

[0089] According to Table 1, the ratio of the integral value of the peak derived from the difluorophosphate ion to the integral value of the peak derived from the hexafluoride ion (PO2F2) - The percentage was 0.3% or less in Examples 1-5 compared to 0.4% in the comparative example. The concentration of difluorophosphate ions was less than 250 wt ppm in Examples 1-5 compared to 353 wt ppm in the comparative example. It is presumed that the difluorophosphate ions in the non-aqueous electrolyte in Examples 1-5 were removed by adsorption or decomposition by the solid electrolyte.

[0090] Comparing Examples 1-4 and Example 5, Example 1-4 has a higher PO2F2 than Example 5. - The proportion was small, and the concentration of difluorophosphate ions was low. It was confirmed that solid electrolytes with a garnet-type crystal structure containing Li, La, and Zr have a greater effect in removing difluorophosphate ions compared to LATP.

[0091] Comparing Examples 1-3 and Example 4, Example 1-3 has a higher PO2F2 than Example 4. - The proportion was small, and the concentration of difluorophosphate ions was low. It was confirmed that solid electrolytes with a garnet-type crystal structure containing Mg and Sr in addition to Li, La, and Zr have a greater effect in removing difluorophosphate ions compared to solid electrolytes with 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 spectra of the non-aqueous electrolytes in Examples 1 and 2 and the Comparative Example was measured using a UV-Vis spectrophotometer (Evolution201, Thermo Fisher Scientific Co., Ltd.). The cell containing the non-aqueous electrolyte was made of 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 a non-aqueous electrolyte, a quartz glass cell without the non-aqueous electrolyte was used as a blank sample, and the absorbance of that cell at a wavelength of 300 nm was corrected to zero.

[0094] (Chromatogram measurement) The non-aqueous electrolytes in Examples 1 and 2 and the comparative example were analyzed using a liquid chromatography-mass spectrometer (Water Synapt G2 HDMS). Each non-aqueous electrolyte was diluted 100 times by volume with pure water to prepare the 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, reversed phase) Eluent: Gradient of 10 mM ammonium acetate aqueous solution and acetonitrile / tetrahydrofuran (8:2) (Composition ratio is based on Electrochemistry, 85(11), 721-727 (2017)). Column temperature: 40℃ Sample injection volume: 5 μL Ionization method: ESI Measurement mode: Negative ion mode

[0096] In the total ion chromatogram, the peak around a retention time of 2 minutes originates from hexafluoride phosphate ions. The area of ​​the peak appearing around 1.75 minutes, which is shorter than the retention time at which the peak originating from hexafluoride phosphate ions appears, was found to be larger in the non-aqueous electrolyte in the examples than in the non-aqueous electrolyte in the comparative example. Since this is the result of analysis using a reversed-phase column, the peak around a retention time of 1.75 minutes originates from an ion that has a lower affinity for the hydrophobic groups of the reversed-phase column, i.e., an ion with higher polarity, compared to hexafluoride phosphate ions. Therefore, it is considered that the proportion of components with higher polarity than hexafluoride phosphate ions increased in the non-aqueous electrolyte in the examples compared to the non-aqueous electrolyte in the comparative example. The ions with higher polarity than hexafluoride phosphate ions are presumed to be reaction products between the first non-aqueous electrolyte and the solid electrolyte.

[0097] Analysis of the difluorophosphate ion mask chromatogram revealed that the non-aqueous electrolyte in the comparative example had a total peak area (count) of 10⁵, with a peak originating from difluorophosphate ions observed around a retention time of 1.5 minutes. On the other hand, the non-aqueous electrolyte in the example had a total peak area (count) of 16, and only noise-like peaks were observed. Therefore, it is considered that difluorophosphate ions disappeared from the non-aqueous electrolyte in the example.

[0098] (Measurement of self-diffusion coefficient) The self-diffusion coefficients of Li ions contained in the non-aqueous electrolytes in Examples 1 and 2 and the Comparative Example at 25°C were measured using pulsed magnetic field gradient nuclear magnetic resonance spectroscopy with a nuclear magnetic resonance spectrometer (JNM-ECA600II, JEOL RESONANCE Co., Ltd.). The non-aqueous electrolyte was placed in a symmetrical microsample tube with an outer diameter of 5 mm to a height of 5 mm from the bottom surface of the outer tube, and then sealed with the 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 lithium-7 component (Li ions) was measured at 233.25 MHz using a stimulated echo pulse series. The magnetic field gradient pulse width, diffusion time, recovery time after the magnetic field gradient pulse, and number of integrations were adjusted according to the observed signal conditions.

[0099] After measuring the self-diffusion coefficient, the ratio (D2 / D1) of the self-diffusion coefficient D2 of the Li ions in the non-aqueous electrolyte in Examples 1 and 2 to the self-diffusion coefficient D1 of the non-aqueous electrolyte in the comparative example was determined. Table 2 is a summary of the measurement results.

[0100] [Table 2]

[0101] As shown in Table 2, D2 / D1 was 1.05, and the self-diffusion coefficient of Li ions remained almost unchanged between Examples 1 and 2 and the Comparative Example. If any component in the non-aqueous electrolyte were to undergo a significant decomposition reaction with 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 non-aqueous electrolyte. However, since the diffusion rate of Li ions (charge carriers) in the non-aqueous electrolytes of Examples 1 and 2 is estimated to be almost the same as the diffusion rate of Li ions in the non-aqueous electrolyte of the Comparative Example, it was found that the concentration of difluorophosphate could be controlled in the non-aqueous electrolytes of the Examples without significant changes in the diffusion behavior of Li ions.

[0102] In the comparative example, the absorbance of the non-aqueous electrolyte was 0.1 or less, whereas in Examples 1 and 2, the absorbance of the non-aqueous electrolyte was 3 or more. It is presumed that the difluorophosphate ion reacted with the solid electrolyte to produce an ion with higher polarity than the hexafluoride phosphate ion, and that the reaction product affected the absorbance in that wavelength region, or that when the organic components contained in the non-aqueous electrolyte reacted with the solid electrolyte in some way, they incorporated the difluorophosphate ion, producing a polymerization product that shows absorption in that wavelength region.

[0103] The examples revealed that difluorophosphate ions can be removed by simple operations such as temporarily contacting the first non-aqueous electrolyte with an oxide-based solid electrolyte or by allowing the first non-aqueous electrolyte and the solid electrolyte to coexist. It was confirmed that this operation does not adversely affect the hexafluoride phosphate ions contained in the non-aqueous electrolyte or the ionic conductivity of the non-aqueous electrolyte. This operation is expected to not only be useful in controlling the amount of difluorophosphate ions contained in the non-aqueous electrolyte, but also to enable the use of components that are unstable to difluorophosphate ions, and to reduce hydrolysis of the non-aqueous electrolyte and subsequent decomposition reactions. This is expected to improve the rate characteristics and cycle characteristics of the energy storage element.

[0104] Although the present invention has been described above based on embodiments, it can be easily inferred that the present invention is not limited in any way to the above embodiments, and that various improvements and modifications are possible without departing from the spirit of the present invention.

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

[0106] In the embodiment, the case in which the active material layers 13 and 17 include a solid electrolyte 18 has been described, but the invention is not necessarily limited to this. It is certainly possible to omit at least one of the solid electrolytes 18 in the active material layers 13 and 17.

[0107] Although not explained in the embodiment, it is of course possible to place a protective layer 29 between the active material layer 17 and the separators 14 and 22, or between the current collector 16 and the active material layer 17. Placing a protective layer 29 between the active material layer 17 and the separators 14 and 22 can reduce short circuits caused by dendrites. Placing a protective layer 29 between the current collector 16 and the active material layer 17 can reduce deterioration of the current collector 16. [Explanation of symbols]

[0108] 10,21 Energy storage element 18 Solid electrolyte

Claims

1. A non-aqueous electrolyte containing hexafluoride phosphate ions, The non-aqueous electrolyte contains a basic oxide-based solid electrolyte that exhibits ionic conductivity, The non-aqueous electrolyte is a composition in which the integral value of the peak derived from the difluorophosphate ion in the phosphorus-31 nuclear magnetic resonance spectrum is 0.3% or less compared to the integral value of the peak derived from the hexafluoride phosphate ion.

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

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

4. A storage element comprising the composition according to any one of claims 1 to 3.

5. A preparation step to prepare a first non-aqueous electrolyte containing hexafluoride phosphate ions, The process includes contacting a basic oxide-based solid electrolyte exhibiting ionic conductivity with the first non-aqueous electrolyte to obtain a second non-aqueous electrolyte in which the concentration of difluorophosphate ions is reduced compared to the concentration of difluorophosphate ions in the first non-aqueous electrolyte, A method for producing a non-aqueous electrolyte, wherein the second non-aqueous electrolyte contains hexafluoride phosphate ions, and the integral value of the peak derived from the difluorophosphate ions in the phosphorus-31 nuclear magnetic resonance spectrum is 0.3% or less of the integral value of the peak derived from the hexafluoride phosphate ions.

6. The method for producing a non-aqueous electrolyte according to claim 5, wherein, when analyzed under the same conditions by reverse-phase liquid chromatography-mass spectrometry using an aqueous eluent, the area of ​​the peak appearing in a range with a retention time shorter than the retention time at which the peak originating from hexafluoride phosphate ions appears is greater for the second non-aqueous electrolyte than for the first non-aqueous electrolyte.

7. A method for producing a non-aqueous electrolyte according to claim 5 or 6, wherein the self-diffusion coefficient of cations excluding protons at 25°C ± 2°C, as measured by pulsed magnetic field gradient nuclear magnetic resonance spectroscopy, is such that the self-diffusion coefficient of the second non-aqueous electrolyte is 0.9 or more and 1.1 or less relative to the self-diffusion coefficient of the first non-aqueous electrolyte.

8. The method for producing a non-aqueous electrolyte according to claim 5 or 6, wherein the second non-aqueous electrolyte has an absorbance of 2 or more at a wavelength of 300 nm in an ultraviolet-visible absorption spectrum measured at an optical path length of 10 mm and 25°C ± 2°C.

9. A method for manufacturing an energy storage element using the second non-aqueous electrolyte obtained by the method for manufacturing a non-aqueous electrolyte according to claim 5 or 6.

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