Non-aqueous electrolyte and non-aqueous secondary battery

Abstract

Provided is a non-aqueous electrolyte which contains at least one cyclic carbonate or chain carbonate, contains, as a solvent, 5-90 vol% of a mononitrile compound represented by the formula R-CN (in the formula, R is a C1-C4 alkyl group or a C1-C4 halogenated alkyl group), and contains alkali metal ions other than lithium ions.

Description

Non-aqueous electrolyte and non-aqueous secondary battery

[0001] The present invention relates to a non-aqueous electrolyte solution and a non-aqueous secondary battery using the same.

[0002] In recent years, from the perspectives of preserving the global environment, saving resources, and making effective use of energy, attention has been drawn to wind power generation power smoothing systems or late-night power storage systems, distributed home energy storage systems based on solar power generation technology, and energy storage systems for electric vehicles.

[0003] Lithium-ion secondary batteries are being developed as a candidate for use in these energy storage systems, and research is progressing daily to increase their energy density and improve their long-term durability in order to achieve the required performance.

[0004] On the other hand, as demand for lithium-ion batteries expands, there are concerns about the depletion of lithium resources, including lithium carbonate, and the resulting rise in resource prices. Therefore, research into next-generation secondary batteries that can replace lithium-ion batteries is being actively conducted, and various attempts are being made.

[0005] Sodium-ion batteries are attracting particular attention as next-generation secondary batteries. Because sodium is more abundant on Earth than lithium, sodium-ion batteries have the potential to be significantly cheaper than lithium-ion batteries.

[0006] Sodium-ion batteries operate on a principle very similar to that of lithium-ion batteries. They consist of a positive electrode, a negative electrode, and an electrolyte. During charging and discharging, energy is stored or released by absorbing or releasing cations into or from the positive or negative electrode active material.

[0007] In sodium-ion batteries, non-graphitizable carbon (hard carbon) is generally used as the negative electrode active material. Hard carbon reversibly intercalates and deintercalates sodium ions, resulting in a capacity of 200 mAh / g or more. Because hard carbon exhibits capacity even in a low potential range of 0.4 V or less relative to the deposition potential of metallic sodium, the use of hard carbon as the negative electrode active material in sodium-ion secondary batteries can increase the energy density of the battery.

[0008] Although sodium-ion batteries and lithium-ion batteries operate on similar principles, their optimal electrolyte compositions are different. This is thought to be due to differences in the potential at which redox reactions occur, as well as the Lewis acidity and ionic radius of the alkali metal cations, as disclosed in Non-Patent Documents 1 and 2. These differences are expected to affect the decomposition behavior of the electrolyte during the first charge and the composition of the film formed at the negative electrode-electrolyte interface or the positive electrode-electrolyte interface.

[0009] The present inventors have newly discovered that when electrolytes composed of solvents commonly used in lithium-ion batteries are used in sodium-ion batteries, problems not observed in lithium-ion batteries arise. As will be described in the Examples section below, in sodium-ion batteries using electrolytes containing cyclic carbonates and chain carbonates as solvents, continuous side reactions that exceeded the theoretical capacity of the active material were observed during low-temperature, high-current charging. This is presumably due to the continuous occurrence of side reactions at the positive and negative electrodes that did not involve the absorption or release of sodium ions in the active material. Possible side reactions include the deposition of metallic sodium or reductive decomposition of the electrolyte at the negative electrode, and the oxidative decomposition of the electrolyte at the positive electrode.

[0010] In order to prevent such continuous side reactions, it is thought to be effective to include a solvent that is stable over a wide range of potentials in the electrolyte. To date, in order to suppress decomposition of the electrolyte at positive potentials, electrolytes for sodium ion batteries have been proposed that can use, as the positive electrode active material, substances that have low viscosity, high ionic conductivity, are resistant to decomposition even at high potentials, and have charge / discharge characteristics extending to a high positive potential range. For example, Patent Document 1 proposes an electrolyte for sodium ion batteries that contains a nitrile compound such as a chain saturated hydrocarbon dinitrile compound, a chain cyanoether compound, or a cyanoacetate ester, as well as a cyclic carbonate, a cyclic ester, a chain carbonate, or the like.

[0011] Furthermore, Patent Document 2 discloses a nonaqueous electrolyte solution containing acetonitrile, vinylene carbonate, and a predetermined sulfur compound, in which the content of vinylene carbonate is lower than the content of the predetermined sulfur compound, and a nonaqueous secondary battery using the same.

[0012] JP 2010-165674 A International Publication No. 2021 / 049648

[0013] Electrolytes and Interfaces in Sodium-Based Rechargeable Batteries: Recent Advances and Perspectives, Gebrekidan Gebresilassie Eshetu et. al. , ADVANCED ENERGY MATERIALS, 2020, 10, 2000093Electrochemical Na Insertion and Solid Electrolyte Interphase for Hard-Carbon Electrodes and Application to Na-Ion Batteries, Shinichi Komaba et. al. , ADVANCED FUNCTIONAL MATERIALS, 2011, 21, 3859-3867

[0014] In sodium-ion batteries using the sodium-ion battery electrolyte described in Patent Document 1, although the decomposition of the electrolyte at the positive electrode can be suppressed by adjusting the electrolyte composition, it is believed that the reductive decomposition of the electrolyte at the negative electrode surface cannot be suppressed. Therefore, there is room for further improvement in terms of increasing the energy density of batteries. In addition, Patent Document 1 describes that the carbonate compound in the electrolyte plays a role in reducing the viscosity of the highly viscous nitrile compound and increasing the ionic conductivity. If the oxidation stability of the electrolyte can be improved as in Patent Document 1 by using a nitrile compound with a lower viscosity than the carbonate compound, there is room for further improvement of the ionic conductivity of the electrolyte while maintaining high oxidation stability. Furthermore, if the reduction stability of the electrolyte can be simultaneously improved, a material that exhibits capacity at low potential, such as non-graphitizable carbon, can be used as the negative electrode active material, thereby providing a sodium-ion battery that simultaneously exhibits high energy density, high cycle stability, and high output performance.

[0015] Furthermore, Patent Document 2 does not disclose a non-aqueous electrolyte solution or a non-aqueous secondary battery containing alkali metal ions other than lithium ions as cations. As disclosed in Non-Patent Document 1, the optimal electrolyte composition differs between lithium ion batteries and sodium ion batteries, so it is unclear whether a composition similar to the non-aqueous electrolyte solution described in Patent Document 2 is the optimal composition. Furthermore, Non-Patent Document 2 discloses that the inclusion of vinylene carbonate in an electrolyte solution results in a deterioration in the sodium ion absorption / release characteristics of the negative electrode in a sodium ion battery. From this, it is presumed that using an electrolyte solution containing vinylene carbonate as disclosed in Patent Document 2 is not a suitable means for a sodium ion battery.

[0016] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a non-aqueous secondary battery that contains alkali metal ions other than lithium ions, has high energy density, excellent cycle stability, and excellent output characteristics.

[0017] The present inventors conducted extensive research to solve the above-mentioned problems and discovered that nonaqueous secondary batteries using a nonaqueous electrolyte solution containing nitriles and a specific compound operate stably as batteries and exhibit excellent output characteristics, even when a substance that exhibits capacity at low potential is used as the negative electrode active material. This finding led to the completion of the present invention. Specifically, the present invention is as follows: [1] A nonaqueous electrolyte solution, the nonaqueous electrolyte solution containing one or more cyclic carbonates or chain carbonates, the nonaqueous electrolyte solution containing a mononitrile compound represented by the formula: R—CN (wherein R is an alkyl group having 1 to 4 carbon atoms or a halogenated alkyl group having 1 to 4 carbon atoms) as a solvent in a proportion of 5 to 90% by volume, the nonaqueous electrolyte solution containing an alkali metal ion other than lithium ion. [2] The nonaqueous electrolyte solution according to [1], wherein the mononitrile compound is acetonitrile. [3] The nonaqueous electrolyte solution according to [1] or [2], wherein the nonaqueous electrolyte solution contains sodium ions or potassium ions as alkali metal ions. [4] The nonaqueous electrolyte solution according to any one of [1] to [3], wherein the nonaqueous electrolyte solution contains one or more compounds selected from the group consisting of a fluorinated cyclic carbonate, an unsaturated bond-containing cyclic carbonate, and a cyclic sulfur compound. [5] The nonaqueous electrolyte solution according to any one of [1] to [4], wherein the nonaqueous electrolyte solution contains one or more additives selected from fluoroethylene carbonate, ethylene sulfite, and vinylene carbonate. [6] The nonaqueous electrolyte solution according to any one of [1] to [5], wherein the nonaqueous electrolyte solution contains 0.5% by volume to 10% by volume of vinylene carbonate. [7] The nonaqueous electrolyte solution according to any one of [1] to [6], wherein the nonaqueous electrolyte solution contains 0.5 vol% to 10 vol% of vinylene carbonate, and the electrolyte solution contains one or more additives selected from fluoroethylene carbonate and cyclic sulfur compounds, and the volume fraction of the additives in the electrolyte solution is 10 vol% or less.[8] The nonaqueous electrolyte solution according to any one of [1] to [7], wherein, when the volume fractions of the vinylene carbonate and the cyclic sulfur compound in the nonaqueous electrolyte solution are X vol% and Y vol%, respectively, the following relationships are satisfied: 0.5≦X≦10 and 0≦Y≦10. [9] The nonaqueous electrolyte solution according to any one of [1] to [8], wherein, when the volume fractions of the cyclic sulfur compound and the vinylene carbonate in the nonaqueous electrolyte solution are X vol% and Y vol%, respectively, the following relationships are satisfied: 0.5≦X≦10 and 0≦Y<X.

[10] The nonaqueous electrolyte solution according to any one of [1] to [9], wherein the cyclic sulfur compound is ethylene sulfite.

[11] The nonaqueous electrolyte solution according to any one of [1] to

[10] , wherein, when the volume fractions of vinylene carbonate and ethylene sulfite in the nonaqueous electrolyte solution are X vol% and Y vol%, respectively, the following relationships are satisfied: 1.0≦X≦5.0 and 0≦Y<X.

[12] The nonaqueous electrolyte solution according to any one of [1] to

[10] , wherein, when the volume fractions of the unsaturated bond-containing cyclic carbonate and the cyclic sulfur compound in the nonaqueous electrolyte solution are X vol% and Y vol%, respectively, the following relationships are satisfied: 0<X≦Y≦10.

[13] The nonaqueous electrolyte solution according to any one of [1] to

[12] , wherein the ionic conductivity of the nonaqueous electrolyte solution at 25°C is 12 mS / cm or more.

[14] A non-aqueous secondary battery comprising a positive electrode, a negative electrode, and the non-aqueous electrolyte solution according to any one of [1] to

[13] , wherein the positive electrode and the negative electrode each contain a material capable of absorbing and releasing sodium ions or potassium ions.

[15] The non-aqueous secondary battery according to

[14] , wherein the negative electrode current collector of the non-aqueous secondary battery is aluminum.

[16] The negative electrode in the non-aqueous secondary battery has a voltage of 0.4 V vs. Na / Na. ＋

[17] The nonaqueous secondary battery according to

[14] or

[15] , further comprising a material that absorbs sodium ions at a potential lower than 1000 mV.

[17] Raman spectroscopy of the negative electrode of the nonaqueous secondary battery shows a Raman shift of 1250 cm －１ More than 1450cm －１ The maximum value of the Raman intensity below is A, and the Raman shift is 1500 cm－１ More than 1700cm －１ The nonaqueous secondary battery according to any one of

[14] to

[16] , wherein the ratio B / A of A and B is 1.30 or less, where B is the maximum value of the Raman intensity in the following.

[18] The nonaqueous secondary battery according to any one of

[14] to

[17] , wherein the nonaqueous secondary battery contains non-graphitizable carbon as a negative electrode active material.

[19] The nonaqueous secondary battery according to

[18] , wherein the proportion of the non-graphitizable carbon in the weight of the negative electrode active material layer of the nonaqueous secondary battery is 50 mass % or more.

[20] The positive electrode is a cathode material having a structure represented by the following formula: Na ｘ M ｅ O ２ (wherein x is a number satisfying 0≦x≦1, and M ｅ represents one or more metal elements selected from Fe, Mn, Co, Ni, Ti, Mg, Zn, Te, and Cu), Na ｘ (FeSO ４ ) ２ (wherein x is a number satisfying 0≦x≦1), Na ３ｘ M ４ ２ (P.O. ４ ) ( ３－ｅ） F ３ｅ (In the formula, M ４ is Ti or V, and e is a number satisfying 0≦e≦1), and Na ｘ M １ [M ２ (CN) ６ ] （１－ｙ） ・ ｚ H ２ O (in the formula, M １ and M ２ are Fe, Mn, Co, Ni, Ti, or Cu, x is a number satisfying 0≦x≦1, y is a number satisfying 0≦y≦1.0, and z is a number satisfying 0≦z≦3.0).

[21] The nonaqueous secondary battery according to any one of

[14] to

[19] , wherein the positive electrode contains at least one selected from the group consisting of positive electrode active materials represented by the following formula: Na ｘ MO ２ (wherein x is a number satisfying 0≦x≦1, and M is Fe, Mn, Co, Ni, Ti, Mg, Zn, Te or Cu), Na ｘ M１ ｙ M ２ ｚ O ２ (In the formula, M １ and M ２ are Fe, Mn, Co, Ni, Ti, Mg, Zn, Te, or Cu, x is a number satisfying 0≦x≦1, y is a number satisfying 0.1<y<0.9, z is a number satisfying 0.1<z<0.9, and y and z satisfy 0.95<y+z<1.05), and Na ｘ M １ ｃ M ２ ｄ M ３ （１－ｃ－ｄ） O ２ (In the formula, M １ , M ２ and M ３ are Fe, Mn, Co, Ni, Ti, Mg, Zn, Te, or Cu, x is a number satisfying 0≦x≦1, c is a number satisfying 0.03<c<0.97, and d is a number satisfying 0.03<d<0.97).

[0018] According to the present invention, it is possible to provide a nonaqueous secondary battery that includes a nonaqueous electrolyte solution containing alkali metal ions other than lithium ions, in which continuous side reactions are suppressed when a negative electrode with a high weight energy density is used, and that has excellent cycle stability and output characteristics.

[0019] Hereinafter, an embodiment of the present invention (hereinafter referred to as "the present embodiment") will be described. The present invention is not limited to the present embodiment described below. At least one of the upper limit value and the lower limit value in each numerical range described in this specification may be arbitrarily combined, that is, any numerical range may be formed. At least one of the upper limit value and the lower limit value in each numerical range described in this specification may be replaced with the corresponding value described in the examples.

[0020] <Non-aqueous secondary battery> The non-aqueous secondary battery of this embodiment includes a positive electrode, a negative electrode, and a non-aqueous electrolyte solution containing one or more types of alkali metal ions.

[0021] <Non-aqueous electrolyte> In this embodiment, the term "non-aqueous electrolyte" refers to an electrolyte containing 1% by mass or less of water relative to the total amount of the non-aqueous electrolyte. The non-aqueous electrolyte in this embodiment preferably contains as little water as possible, but may contain a very small amount of water as long as it does not impede the solution of the problems of the present invention. The water content is 300 ppm by mass or less, preferably 200 ppm by mass or less, relative to the total amount of the non-aqueous electrolyte. As long as the non-aqueous electrolyte has a configuration for achieving the problem of the present invention, the other components can be appropriately selected and applied from the constituent materials of known non-aqueous electrolytes used in non-aqueous secondary batteries.

[0022] <Non-aqueous solvent> In this embodiment, the term "non-aqueous solvent" refers to elements remaining in the non-aqueous electrolyte solution excluding alkali metal salts and various additives. When the non-aqueous electrolyte solution contains an electrode protection additive, the term "non-aqueous solvent" refers to elements remaining in the non-aqueous electrolyte solution excluding alkali metal salts and additives other than the electrode protection additive. Examples of non-aqueous solvents include alcohols such as methanol and ethanol; and aprotic solvents. Among these, aprotic solvents are preferred as non-aqueous solvents. The non-aqueous solvent may contain a solvent other than an aprotic solvent, as long as it does not impede the solution of the problems of the present invention.

[0023] The non-aqueous solvent in this embodiment is characterized by containing a mononitrile compound represented by the formula R-CN (wherein R is an alkyl group having 1 to 4 carbon atoms or a halogenated alkyl group having 1 to 4 carbon atoms). When the non-aqueous solvent contains a mononitrile compound represented by the formula R-CN, the oxidation resistance of the non-aqueous electrolyte can be improved and the ionic conductivity of the non-aqueous electrolyte can be significantly increased. Because mononitrile compounds are easily reductively decomposed, when a mononitrile compound is used as the non-aqueous solvent, it is preferable to use another solvent (e.g., an aprotic solvent other than a mononitrile compound) in combination with the mononitrile compound and / or to add a predetermined additive.

[0024] The content of the mononitrile compound is preferably 5% by volume or more and 90% by volume or less, more preferably 10% by volume or more and 80% by volume or less, and even more preferably 15% by volume or more and 70% by volume or less, relative to the total amount of the nonaqueous solvent. If the content of the mononitrile compound is 5% by volume or more, the ionic conductivity of the nonaqueous electrolyte solution is increased, and the output characteristics of the nonaqueous alkali metal ion battery can be improved. If the content of the mononitrile compound is 90% by volume or less, the charge-discharge cycle characteristics are excellent.

[0025] Examples of mononitrile compounds include acetonitrile, propionitrile, butyronitrile, isobutyronitrile, pentanenitrile, 2-methylbutyronitrile, 3-methylbutyronitrile, etc. Among these, acetonitrile is preferred.

[0026] Examples of aprotic solvents other than mononitrile compounds include cyclic carbonates, chain carbonates, lactones, cyclic ethers, chain ethers, short-chain fatty acid esters, ketones, and compounds in which some or all of the H atoms in the above compounds have been substituted with halogen atoms.

[0027] Examples of cyclic carbonates include ethylene carbonate, propylene carbonate, 1,2-butylene carbonate, trans-2,3-butylene carbonate, cis-2,3-butylene carbonate, 1,2-pentylene carbonate, trans-2,3-pentylene carbonate, cis-2,3-pentylene carbonate, vinylene carbonate, 4,5-dimethylvinylene carbonate, and vinylethylene carbonate;

[0028] Lactones include γ-butyrolactone, α-methyl-γ-butyrolactone, γ-valerolactone, γ-caprolactone, δ-valerolactone, δ-caprolactone, and ε-caprolactone;

[0029] Examples of chain carbonates include ethyl methyl carbonate, dimethyl carbonate, diethyl carbonate, methyl propyl carbonate, methyl isopropyl carbonate, dipropyl carbonate, methyl butyl carbonate, dibutyl carbonate, ethyl propyl carbonate, and diisobutyl carbonate;

[0030] Cyclic ethers include, for example, tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, and 1,3-dioxane;

[0031] Examples of short-chain fatty acid esters include methyl acetate, methyl propionate, methyl isobutyrate, methyl butyrate, methyl isovalerate, methyl valerate, methyl pivalate, methyl hydroangelate, methyl caproate, ethyl acetate, ethyl propionate, ethyl isobutyrate, ethyl butyrate, ethyl isovalerate, ethyl valerate, ethyl pivalate, ethyl hydroangelate, ethyl caproate, propyl acetate, propyl propionate, propyl isobutyrate, propyl butyrate, propyl isovalerate, propyl valerate, propyl pivalate, propyl hydroangelate, propyl caproate, isopropyl acetate, isopropyl propionate, isopropyl isobutyrate, isopropyl butyrate, isopropyl isovalerate, isopropyl valerate, and iso pivalate. propyl, isopropyl hydroangelate, isopropyl caproate, butyl acetate, butyl propionate, butyl isobutyrate, butyl butyrate, butyl isovalerate, butyl valerate, butyl pivalate, butyl hydroangelate, butyl caproate, isobutyl acetate, isobutyl propionate, isobutyl isobutyrate, isobutyl butyrate, isobutyl isovalerate, isobutyl valerate, isobutyl pivalate, isobutyl hydroangelate, isobutyl caproate, tert-butyl acetate, tert-butyl propionate, tert-butyl isobutyrate, tert-butyl butyrate, tert-butyl isovalerate, tert-butyl valerate, tert-butyl pivalate, tert-butyl hydroangelate, and tert-butyl caproate;

[0032] Examples of chain ethers include dimethoxyethane, diethyl ether, 1,3-dioxolane, diglyme, triglyme, and tetraglyme;

[0033] Ketones include, for example, acetone, methyl ethyl ketone, and methyl isobutyl ketone;

[0034] Examples of the compound in which some or all of the H atoms of the aprotic solvent have been substituted with halogen atoms include compounds in which the halogen atoms are fluorine.

[0035] In the present embodiment, the aprotic solvent other than the mononitrile compound may be used alone or in combination of two or more.

[0036] In the present embodiment, the nonaqueous solvent preferably includes one or more of a cyclic carbonate, a chain carbonate, and a chain ether in combination with the mononitrile compound, from the viewpoint of improving the stability of the nonaqueous electrolyte solution. Furthermore, in the present embodiment, the nonaqueous solvent preferably includes one or more of a chain ether, a short-chain fatty acid ester, a ketone, and a compound in which some or all of the H atoms in the above compounds are substituted with halogen atoms, from the viewpoint of improving the output characteristics of the battery.

[0037] <Electrolyte> The nonaqueous electrolyte solution according to this embodiment may contain an alkali metal salt as an electrolyte. Examples of the alkali metal salt in this embodiment include MPF ６ , MN(SO ２ C ｍ F ２ｍ＋１ ) ２、 MPO ２ F ２ , MC(SO ２ F) ３ , MC(SO ２ CF ３ ) ３ , MC(SO ２ C ２ F ５ ) ３ , MCF ３ SO ３ , M.C. ４ F ９ SO ３ , MAsF ６ , MB (C ２ O ４ ) ２ , MBF ２ (C ２ O ４ ), MBF ４ {wherein M is one selected from Li, Na, and K, and m is an integer of 0 to 8}. ６The electrolyte preferably contains a hexafluorophosphate represented by the formula: The inclusion of the hexafluorophosphate in the nonaqueous electrolyte forms a passive film on the surface of the positive electrode current collector, thereby suppressing corrosion of the current collector during charge and discharge. The content of the hexafluorophosphate is preferably 0.5% by mass or more and 35% by mass or less, based on the total amount of the nonaqueous electrolyte. When the content of the hexafluorophosphate is 0.5% by mass or more, a stable passive film can be formed on the positive electrode current collector. When the content of the hexafluorophosphate is 35% by mass or less, the generation of hydrogen fluoride resulting from decomposition of the hexafluorophosphate can be suppressed, improving charge and discharge cycle characteristics. In the case of a nonaqueous secondary battery containing sodium ions, the nonaqueous electrolyte preferably contains one or more solutes selected from sodium hexafluorophosphate, sodium bis(fluorosulfonyl)imide, sodium bis(trifluoromethanesulfonyl)imide, and sodium trifluoromethanesulfonate.

[0038] The alkali metal salt concentration in the non-aqueous electrolyte is preferably 0.5 mol / L or more, and more preferably 0.5 mol / L or more and 2.5 mol / L or less, as the total concentration of the alkali metal salts. When the alkali metal salt concentration is 0.5 mol / L or more, sufficient anions are present, making it easy to increase the battery capacity. When the alkali metal salt concentration is 2.5 mol / L or less, it is easy to prevent undissolved alkali metal salts from precipitating in the non-aqueous electrolyte and the viscosity of the non-aqueous electrolyte from becoming too high, making it difficult for the output characteristics to deteriorate and the separator impregnation ability to be impaired. Specifically, in a non-aqueous electrolyte containing alkali metal ions other than lithium ions, when the total concentration of the alkali metal ions is A (mol / L), A preferably satisfies the relationship 0.5≦A≦2.5. In the case of a non-aqueous electrolyte for a sodium-ion battery, the sodium ion concentration in the non-aqueous electrolyte is preferably 0.5 mol / L or more and 2.5 mol / L or less.

[0039] In the nonaqueous secondary battery of this embodiment, the alkali metal ions used as the cations are preferably one or more selected from sodium (Na) ions and potassium (K) ions. As alkali metal elements of Group 1 elements, in addition to the above-mentioned Na, K, and lithium (Li), rubidium (Rb) and cesium (Cs) may also be used. However, these elements cause large volume changes when inserted into and removed from the carbon material of the negative electrode, so their use is not preferred. If the alkali metal element used is Na or K, not only can ions be stably inserted into and removed from the carbon material of the negative electrode, but the solvation state of the ions in the nonaqueous electrolyte is equivalent, so similar results are likely to be obtained.

[0040] <Additive for Protecting Electrode> The additive for protecting an electrode is not particularly limited as long as it does not impede the solution of the problems of the present invention, and may substantially overlap with a substance that serves as a solvent for dissolving an alkali metal salt (i.e., the non-aqueous solvent described above). The additive for protecting an electrode is preferably a substance that contributes to improving the performance of the non-aqueous electrolyte solution and the non-aqueous secondary battery in this embodiment, but also includes substances that are not directly involved in the electrochemical reaction.

[0041] Specific examples of the electrode protection additive include 4-fluoro-1,3-dioxolane-2-one, 4,4-difluoro-1,3-dioxolane-2-one, cis-4,5-difluoro-1,3-dioxolane-2-one, trans-4,5-difluoro-1,3-dioxolane-2-one, 4,4,5-trifluoro-1,3-dioxolane-2-one, and 4,4,5,5-tetrafluoro-1,3-dioxolane-2-one. fluorinated cyclic carbonates typified by vinylene carbonate, 4,5-dimethylvinylene carbonate, and vinylethylene carbonate; unsaturated bond-containing cyclic carbonates typified by γ-butyrolactone, γ-valerolactone, γ-caprolactone, δ-valerolactone, δ-caprolactone, and ε-caprolactone; lactones typified by 1,4-dioxane; cyclic ethers typified by ethylene sulfite, propylene sulfite, butylene sulfite, pentene sulfite, sulfolane, 3-sulfolene, 3-methyl sulfolane, 1,3-propane sultone, 1,4-butane sultone, 1-propene-1,3-sultone, and tetramethylene sulfoxide; chain acid anhydrides typified by acetic anhydride, propionic anhydride, and benzoic anhydride; cyclic acid anhydrides typified by malonic anhydride, succinic anhydride, glutaric anhydride, maleic anhydride, phthalic anhydride, 1,2-cyclohexanedicarboxylic anhydride, 2,3-naphthalenedicarboxylic anhydride, and naphthalene-1,4,5,8-tetracarboxylic dianhydride; and mixed acid anhydrides having a structure formed by dehydration condensation of different types of acids, such as two different types of carboxylic acids or a carboxylic acid and a sulfonic acid. These may be used alone or in combination of two or more types.

[0042] In this embodiment, among the electrode protection additives listed above, it is preferable to use one or more of fluorinated cyclic carbonates, unsaturated bond-containing cyclic carbonates, and cyclic sulfur compounds from the viewpoint of forming a good solid electrolyte coating (SEI) at the electrode-electrolyte interface. Among fluorinated cyclic carbonates, 4-fluoro-1,3-dioxolan-2-one, among unsaturated bond-containing cyclic carbonates, vinylene carbonate, and among cyclic sulfur compounds, are particularly preferable.

[0043] The content of the electrode protection additive in the non-aqueous electrolyte solution in this embodiment is not particularly limited, but the content of the electrode protection additive relative to the total amount of the non-aqueous solvent is preferably 0.1 to 30% by volume, more preferably 0.3 to 15% by volume, even more preferably 0.5 to 10% by volume, and particularly preferably 0.5 to 7% by volume.

[0044] In this embodiment, the higher the content of the electrode protection additive, the more the deterioration of the nonaqueous electrolyte solution is suppressed. However, the lower the content of the electrode protection additive, the more the excessive SEI formation can be suppressed, and the output characteristics of the nonaqueous secondary battery are improved. Therefore, by adjusting the content of the electrode protection additive within the above-mentioned range, the excellent performance based on the high ionic conductivity of the nonaqueous electrolyte solution tends to be maximized without impairing the basic functions of the nonaqueous secondary battery. By preparing a nonaqueous electrolyte solution with such a composition, the cycle performance of the nonaqueous secondary battery, high output performance in a low-temperature environment, and all other battery characteristics tend to be further improved.

[0045] In this embodiment, for example, with respect to the content of an unsaturated bond-containing cyclic carbonate such as vinylene carbonate, and a cyclic sulfur compound such as ethylene sulfite in a non-aqueous electrolyte, when the volume fraction of the electrode protection additive relative to the total amount of the non-aqueous solvent is X vol% and Y vol%, respectively, it is preferable to satisfy 0.5≦X≦10 and 0≦Y≦10, more preferably satisfy 0.5≦X≦10 and 0≦Y<X, and even more preferably satisfy 1.0≦X≦5.0 and 0≦Y<X. By adjusting the content of each electrode protection additive within the above-mentioned range, it is possible to suppress reductive decomposition of the mononitrile compound and suppress the formation of excessive SEI at the negative electrode-electrolyte interface, thereby improving output characteristics. From the same viewpoint, the content of vinylene carbonate in the non-aqueous electrolyte is preferably 0.5% by volume or more and 10% by volume or less, and / or the volume fraction of one or more additives selected from fluoroethylene carbonate and cyclic sulfur compounds in the non-aqueous electrolyte is preferably 10% by volume or less. On the other hand, the higher the content of the cyclic sulfur compound, the more likely it is that low-temperature output characteristics will improve. In order to obtain low-temperature characteristics, it is preferable that the cyclic sulfur compound be contained in the non-aqueous electrolyte in an amount equal to or greater than that of the unsaturated bond-containing cyclic carbonate. Specifically, when the volume fractions of the unsaturated bond-containing cyclic carbonate and the cyclic sulfur compound relative to the total amount of the non-aqueous solvent are X% by volume and Y% by volume, respectively, the content of the electrode protection additive relative to the total amount of the non-aqueous solvent preferably satisfies 0<X≦Y≦10, more preferably 0.2≦X≦Y≦10, even more preferably 0.5≦X≦Y≦10, and particularly preferably 1≦X≦Y≦8. By adjusting the content of each electrode protection additive within the above-mentioned range, it is possible to suppress the reductive decomposition of the mononitrile compound and also to suppress the formation of excessive SEI at the negative electrode-electrolyte interface, thereby improving the output characteristics. In addition, the low-resistance SEI derived from the cyclic sulfur compound improves the low-temperature characteristics.

[0046] <Optional Additives> In this embodiment, for the purpose of improving the charge-discharge cycle characteristics, high-temperature storage properties, and safety (for example, preventing overcharge) of the nonaqueous secondary battery, for example, sulfonic acid esters, diphenyl disulfide, cyclohexylbenzene, biphenyl, fluorobenzene, tert-butylbenzene, phosphate esters [ethyl diethyl phosphonoacetate (EDPA): (C ２ H ５ O) ２ (P=O)-CH ２ (C=O)OC ２ H ５ , tris(trifluoroethyl) phosphate (TFEP): (CF ３ CH ２ O) ３ P═O, triphenyl phosphate (TPP): (C ６ H ５ O) ３ P=O:(CH ２ =CHCH ２ O) ３ It is also possible to appropriately contain optional additives selected from compounds such as cyclic compounds containing nitrogen with no steric hindrance around the unshared electron pair (e.g., pyridine, 1-methyl-1H-benzotriazole, 1-methylpyrazole, caffeine), and derivatives of these compounds. Phosphate esters are particularly effective because they have the effect of suppressing side reactions during storage.

[0047] The content of other optional additives in this embodiment is calculated as a mass percentage relative to the total mass of all components constituting the non-aqueous electrolyte solution. There are no particular restrictions on the content of other optional additives, but it is preferably in the range of 0.01 mass% to 10 mass% of the total amount of the non-aqueous electrolyte solution, more preferably 0.02 mass% to 5 mass% and even more preferably 0.05 mass% to 3 mass%. By adjusting the content of other optional additives within the above range, it tends to be possible to add even better battery characteristics without impairing the basic functions of a non-aqueous secondary battery.

[0048] <Physical Properties of Electrolyte> -Ionic Conductivity- The ionic conductivity of the nonaqueous electrolyte according to this embodiment at 25°C is preferably 12 mS / cm or more, more preferably 12 mS / cm or more and 40 mS / cm or less. When the ionic conductivity of the nonaqueous electrolyte at 25°C is 12 mS / cm or more, sufficient alkali metal ion conduction occurs within the electrode active material layer, enabling large current charging and discharging. When the ionic conductivity of the nonaqueous electrolyte at 25°C is 40 mS / cm or less, it is presumed that the content of the mononitrile compound is within a suitable range, thereby improving the cycle stability of the battery. The ionic conductivity of the electrolyte can be controlled, for example, by adjusting the viscosity and / or polarity of the nonaqueous solvent. More specifically, the ionic conductivity of the electrolyte can be controlled to a high level by mixing a low-viscosity nonaqueous solvent with a highly polar nonaqueous solvent. Furthermore, the ionic conductivity of the electrolyte can also be controlled to a high level by using a nonaqueous solvent that is low in viscosity and highly polar. The ionic conductivity of the electrolyte solution can be measured in accordance with the method described in "(Measurement of ionic conductivity of non-aqueous electrolyte solution)" in the Examples section below.

[0049] <Positive Electrode and Positive Electrode Current Collector> The positive electrode has a positive electrode current collector and a positive electrode active material layer provided on one or both surfaces of the positive electrode current collector.

[0050] The positive electrode active material layer contains a positive electrode active material, and may further contain optional components such as a conductive filler, a binder, and a dispersion stabilizer, as necessary.

[0051] The positive electrode active material layer preferably contains, as the positive electrode active material, a material capable of absorbing and releasing alkali metal ions present mainly in the electrolyte, and more preferably a material capable of absorbing and releasing sodium ions or potassium ions. Use of such a material is preferred because it tends to be possible to obtain a high voltage and a high energy density.

[0052] The positive electrode active material includes a transition metal oxide capable of absorbing and releasing alkali metal ions. There are no particular limitations on the transition metal oxide used as the positive electrode active material. Examples of the transition metal oxide include oxides containing at least one element selected from the group consisting of cobalt, nickel, manganese, iron, vanadium, and chromium. When the alkali metal ion is a sodium ion, examples of the transition metal oxide include oxides represented by the following formula: Na ｘ MO ２ (wherein x is a number satisfying 0≦x≦1, and M is Fe, Mn, Co, Ni, Ti, Mg, Zn, Te or Cu), Na ｘ M １ ｙ M ２ ｚ O ２ (In the formula, M １ and M ２ are Fe, Mn, Co, Ni, Ti, Mg, Zn, Te, or Cu, x is a number satisfying 0≦x≦1, y is a number satisfying 0.1<y<0.9, z is a number satisfying 0.1<z<0.9, and y and z satisfy 0.95<y+z<1.05), Na ｘ M １ ｃ M ２ ｄ M ３ （１－ｃ－ｄ） O ２ (In the formula, M １ , M ２ and M ３ are Fe, Mn, Co, Ni, Ti, Mg, Zn, Te, or Cu, x is a number satisfying 0≦x≦1, c is a number satisfying 0.03<c<0.97, and d is a number satisfying 0.03<d<0.97), Na ｘ M １ O ２ (wherein x is a number satisfying 0≦x≦1, and M １ is Fe, Mn, Co, Ni, Ti, Mg, Zn, Te or Cu), Na ｘ M e O ２ (wherein x is a number satisfying 0≦x≦1, and M ｅrepresents one or more metal elements selected from Fe, Mn, Co, Ni, Ti, Mg, Zn, Te, and Cu), Na ２ｘ Fe ７ P.O. ４ (wherein x is a number satisfying 0≦x≦1), Na ｘ FePO ４ (wherein x is a number satisfying 0≦x≦1), Na ｘ (FeSO ４ ) ２ (wherein x is a number satisfying 0≦x≦1), Na ３ｘ M ４ ２ (P.O. ４ ) ( ３－ｅ） F ３ｅ (In the formula, M ４ is Ti or V, x is a number satisfying 0≦x≦1, and e is a number satisfying 0≦e≦1), Na ｘ M １ [M ２ (CN) ６ ] （１－ｙ） ・ ｚ H ２ O (in the formula, M １ and M ２ are Fe, Mn, Co, Ni, Ti, or Cu, x is a number that satisfies 0≦x≦1, y is a number that satisfies 0≦y≦1.0, and z is a number that satisfies 0≦z≦3.0).

[0053] The average particle diameter of the positive electrode active material is preferably 0.1 to 20 μm. If the average particle diameter of the positive electrode active material is 0.1 μm or more, the density of the active material layer is high, and therefore the capacity per electrode volume tends to be high. If the average particle diameter of the positive electrode active material is small, durability may be reduced, but if the average particle diameter is 0.1 μm or more, durability is less likely to be reduced. If the average particle diameter of the positive electrode active material is 20 μm or less, it tends to be more suitable for high-speed charging and discharging. The average particle diameter of the positive electrode active material is more preferably 0.5 to 15 μm, and even more preferably 1 to 10 μm.

[0054] The average particle diameter of the active material in this embodiment refers to the particle diameter at the point where the cumulative curve is 50% when the particle size distribution is measured using a particle size distribution analyzer and the total volume is taken as 100% (i.e., the 50% diameter (median diameter)). This average particle diameter can be measured using a commercially available laser diffraction particle size distribution analyzer.

[0055] The content of the positive electrode active material in the positive electrode active material layer is preferably 70% by mass or more and 99% by mass or less, based on the total mass of the positive electrode active material layer. The lower limit of the content of the positive electrode active material is more preferably 80% by mass or more, and even more preferably 85% by mass or more. The upper limit of the content of the positive electrode active material is even more preferably 98% by mass or less. By having the content of the positive electrode active material in the positive electrode active material layer be 70% by mass or more and 99% by mass or less, favorable charge / discharge characteristics are exhibited.

[0056] (Positive electrode current collector) In this embodiment, the material constituting the positive electrode current collector is preferably a material that has high electronic conductivity and is not easily deteriorated by elution into the electrolytic solution, reaction with the electrolyte or ions, etc. In this embodiment, the positive electrode current collector is preferably a metal foil, more preferably an aluminum foil.

[0057] The metal foil may be a flat metal foil having no irregularities or through holes, or may be a metal foil having irregularities that has been subjected to embossing, chemical etching, electrolytic deposition, blasting, or the like, or may be a metal foil having through holes such as expanded metal, punched metal, or etched foil.

[0058] The thickness of the positive electrode current collector is not limited as long as it can sufficiently maintain the shape and strength of the positive electrode, and is preferably, for example, 1 to 100 μm.

[0059] (Optional Components) The positive electrode active material layer in this embodiment may further contain optional components such as a conductive filler, a binder, and a dispersion stabilizer, as needed.

[0060] Examples of the conductive filler that can be used include acetylene black, ketjen black, vapor-grown carbon fiber, graphite, carbon nanotubes, graphene, and mixtures thereof. The amount of the conductive filler is preferably more than 0 parts by mass and not more than 20 parts by mass, more preferably more than 0 parts by mass and not more than 15 parts by mass, and even more preferably 1 part by mass or more and not more than 10 parts by mass, relative to 100 parts by mass of the positive electrode active material. When the amount of the conductive filler mixed is 20 parts by mass or less, the content of the positive electrode active material in the positive electrode active material layer increases, making it easier to ensure the energy density per volume of the positive electrode active material layer.

[0061] Examples of binders that can be used include PVdF (polyvinylidene fluoride), PTFE (polytetrafluoroethylene), polyimide, latex, styrene-butadiene copolymer, fluororubber, and acrylic copolymer. The amount of binder is preferably 0 to 20 parts by mass, more preferably 1 to 15 parts by mass, and even more preferably 1 to 10 parts by mass, per 100 parts by mass of the positive electrode active material. If the amount of binder is 0 parts by mass or more, sufficient electrode strength is likely to be achieved. On the other hand, if the amount of binder is 20 parts by mass or less, the ingress and egress of ions into and diffusion from the positive electrode active material are less likely to be hindered, and therefore, high input / output characteristics are likely to be achieved.

[0062] Examples of the dispersion stabilizer that can be used include PVP (polyvinylpyrrolidone), PVA (polyvinyl alcohol), and cellulose derivatives. The amount of binder is preferably more than 0 parts by mass and not more than 10 parts by mass per 100 parts by mass of the positive electrode active material. If the amount of dispersion stabilizer is 10 parts by mass or less, the ingress and egress of ions into and diffusion from the positive electrode active material are less likely to be hindered, and therefore, high input / output characteristics are more likely to be exhibited.

[0063] (Physical Properties of Positive Electrode Active Material Layer) - Thickness - The thickness of the positive electrode active material layer is preferably 20 μm or more and 500 μm or less, more preferably 25 μm or more and 300 μm or less, and even more preferably 30 μm or more and 200 μm or less per side of the positive electrode current collector. If the thickness of the positive electrode active material layer is 20 μm or more, sufficient charge / discharge capacity is easily achieved. If the thickness of the positive electrode active material layer is 500 μm or less, the ion diffusion resistance within the electrode can be maintained low. Therefore, sufficient output characteristics are easily obtained. In addition, the cell volume can be reduced, making it easy to increase the energy density. Note that when the positive electrode current collector has through holes and / or irregularities, the thickness of the positive electrode active material layer refers to the average thickness per side of the portion of the positive electrode current collector that does not have through holes and / or irregularities.

[0064] -Bulk density- The bulk density of the positive electrode active material layer is preferably 1.5 g / cm ３ More preferably, 2.0 g / cm ３ 4.5g / cm or more ３ The bulk density of the positive electrode active material layer is 1.5 g / cm or less. ３ If the bulk density of the positive electrode active material layer is 4.5 g / cm or more, a high energy density is easily achieved, and therefore, it is easy to achieve miniaturization of the electricity storage element. ３ If the thickness is equal to or less than this, the electrolyte solution will diffuse sufficiently in the pores in the positive electrode active material layer, and high output characteristics will be easily obtained.

[0065] (Production of Positive Electrode) The positive electrode has a positive electrode active material layer on one or both surfaces of a positive electrode current collector. Typically, the positive electrode active material layer is fixed to one or both surfaces of the positive electrode current collector.

[0066] In this embodiment, the positive electrode can be manufactured by known manufacturing techniques for electrodes in non-aqueous alkali metal ion batteries.

[0067] For example, a positive electrode active material and other optional components used as needed are dispersed or dissolved in water or an organic solvent (e.g., N-methyl-2-pyrrolidone, etc.) to prepare a slurry coating liquid, and this coating liquid is applied to one or both sides of a positive electrode current collector to form a coating film, which is then dried to obtain a positive electrode.

[0068] The obtained positive electrode may be pressed to adjust the thickness and / or bulk density of the positive electrode active material layer. Alternatively, a method is possible in which the positive electrode active material and other optional components used as needed are dry-mixed without using a solvent, the resulting mixture is press-molded, and then attached to a positive electrode current collector using a conductive adhesive.

[0069] <Negative Electrode> The negative electrode in this embodiment has a negative electrode current collector and a negative electrode active material layer provided on one or both surfaces of the negative electrode current collector.

[0070] (Negative Electrode Current Collector) In this embodiment, the material constituting the negative electrode current collector is preferably a material that has high electronic conductivity and is resistant to degradation due to leaching into the electrolyte and reaction with the electrolyte or ions. The negative electrode current collector in this embodiment is preferably a metal foil. Examples of such metal foils include aluminum foil, copper foil, nickel foil, and stainless steel foil. In the nonaqueous alkali metal ion battery of this embodiment, when the alkali metal ion is sodium ion or potassium ion, the negative electrode current collector is preferably made of aluminum, and aluminum foil is more preferred, from the viewpoint of reducing the battery weight. Furthermore, when copper foil is used as the negative electrode current collector, the mononitrile compound causes trace amounts of copper, a transition metal, to leach into the electrolyte. The leached copper precipitates during charging, which can cause a decrease in battery performance. From the viewpoint of suppressing leaching, when the nonaqueous electrolyte contains a mononitrile compound as a solvent, aluminum foil is more preferred for the negative electrode current collector than copper foil.

[0071] The metal foil may be a flat metal foil having no irregularities or through holes, or may be a metal foil having irregularities that has been subjected to embossing, chemical etching, electrolytic deposition, blasting, or the like, or may be a metal foil having through holes such as expanded metal, punched metal, or etched foil.

[0072] The thickness of the negative electrode current collector is not limited as long as it can sufficiently maintain the shape and strength of the negative electrode, and is preferably, for example, 1 to 100 μm.

[0073] (Negative Electrode Active Material) As the negative electrode active material, a material capable of absorbing and releasing alkali metal ions can be used, and a material capable of absorbing and releasing sodium ions or potassium ions is preferably used. In the nonaqueous secondary battery according to this embodiment, the negative electrode active material preferably contains, specifically, a carbon material as the first negative electrode active material, and preferably contains, as the second negative electrode active material, one or more selected from the group consisting of silicon, silicon compounds, phosphorus, phosphorus compounds, tin, and tin compounds.

[0074] The second negative electrode active material can form an alloy with alkali metal ions, and is therefore also called an "alloy-based active material."

[0075] (First Negative Electrode Active Material: Carbon Material) The negative electrode active material preferably contains a carbon material as the first negative electrode active material.

[0076] Examples of carbon materials include amorphous or microcrystalline carbon materials, nanocarbons, crystalline carbon materials, etc. Examples of amorphous or microcrystalline carbon materials include non-graphitizable carbon materials and graphitizable carbon materials; examples of nanocarbons include carbon nanoparticles, fullerenes, graphene, etc.; and examples of crystalline carbon materials include graphite, etc.

[0077] Examples of graphite include artificial graphite, natural graphite, graphitized mesophase carbon microspheres, graphite whiskers, and composite carbon materials thereof.

[0078] Among these carbon materials, from the viewpoint of increasing the capacity of a non-aqueous alkali metal ion battery, graphite is preferred when the alkali metal ion is potassium ion, and one or more selected from artificial graphite, natural graphite, graphitized mesophase carbon microspheres, graphite whiskers, and composite carbon materials thereof are preferably used. When the alkali metal ion is sodium ion, a non-graphitizable carbon material is preferably used. Also, from the viewpoint of increasing the capacity of a non-aqueous secondary battery, the proportion of non-graphitizable carbon (hard carbon) in the weight of the negative electrode active material layer is preferably 50 mass% or more. When hard carbon is used as the negative electrode active material of a sodium ion secondary battery, from the viewpoint of increasing the voltage of the battery, the negative electrode has a resistance of 0.4 V vs. Na / Na ＋ It is preferable that the material contains a material that absorbs sodium ions at a potential lower than the potential of the material.

[0079] (Second Negative Electrode Active Material: Alloy-Based Active Material) In addition to the first negative electrode active material described above, the negative electrode active material preferably contains, as a second negative electrode active material, one or more selected from the group consisting of silicon, a silicon compound, phosphorus, a phosphorus compound, tin, and a tin compound. In particular, when the alkali metal ion is potassium ion, the negative electrode active material preferably contains, as the second negative electrode active material, at least one of silicon and a silicon compound, and when the alkali metal ion is sodium ion, it preferably contains at least one of phosphorus and a phosphorus compound. Among these, the silicon compound is preferably silicon oxide, and is represented by the formula SiO ｘ {wherein x satisfies 0.01≦x≦2.} The tin compound is preferably a tin oxide, and more preferably tin dioxide (SnO ２ ) is more preferred.

[0080] The second negative electrode active material may be in the form of a composite material, in which the second negative electrode active material is composited with carbon or a carbonaceous material.

[0081] In the alloy-based active material that is the second negative electrode active material, the charge-discharge reaction of alkali metal ions proceeds on the high potential side and with gradual potential fluctuations, compared to, for example, graphite or a non-graphitizable carbon material that is the first negative electrode active material.

[0082] (Use ratio of first negative electrode active material and second negative electrode active material) In the present embodiment, the ratio of the first negative electrode active material is preferably 70 parts by mass or more and 99 parts by mass or less, more preferably 80 parts by mass or more and 98 parts by mass or less, and even more preferably 85 parts by mass or more and 97 parts by mass or less, based on the total mass of the negative electrode active material layer.

[0083] From the viewpoint of increasing the capacity of the nonaqueous alkali metal ion battery, it is particularly preferable to use graphite as the first negative electrode active material and to set the concentration of the graphite contained in the negative electrode active material layer to the above-mentioned ratio based on the total mass of the negative electrode active material layer.

[0084] In this embodiment, the proportion of the second negative electrode active material is preferably 0 parts by mass or more and 30 parts by mass or less based on the total mass of the negative electrode active material layer.

[0085] When the second negative electrode active material is in the form of a composite material of an alloy-based material and carbon or a carbonaceous material, the proportion of the second negative electrode active material is calculated based on the mass excluding the carbon or carbonaceous material.

[0086] In this embodiment, it is preferable that the first negative electrode active material is contained in the above proportion (e.g., 70 parts by mass or more and 99 parts by mass or less) and the second negative electrode active material is contained in the above proportion (e.g., 0 parts by mass or more and 30 parts by mass or less) based on the total mass of the negative electrode active material layer. If the second negative electrode active material is 0 parts by mass or more, it is easy to make the negative electrode thin, thereby making it easy to increase the energy density of the non-aqueous alkali metal ion battery. In addition, it is easy to suppress a sudden voltage drop at the end of discharge of the non-aqueous alkali metal ion battery. If the second negative electrode active material is 30 parts by mass or less, it is possible to reduce the irreversible capacity of the negative electrode during the initial charge and discharge. Therefore, it is easy to increase the energy density of the non-aqueous alkali metal ion battery.

[0087] In this embodiment, the total content of the first and second negative electrode active materials in the negative electrode active material layer of the negative electrode is preferably 70 parts by mass or more, more preferably 80 parts by mass or more, based on the total mass of the negative electrode active layer.

[0088] (Average particle diameter of negative electrode active material) The average particle diameter of the negative electrode active material is preferably 0.1 μm or more and 20 μm or less, with the lower limit being more preferably 0.5 μm or more, and even more preferably 1 μm or more, and the upper limit being more preferably 18 μm or less, and even more preferably 15 μm or less. If the average particle diameter of the negative electrode active material is 0.1 μm or more, the contact area with the nonaqueous electrolyte increases, making it easier to reduce the resistance of the nonaqueous alkali metal ion battery. If the average particle diameter of the negative electrode active material is 20 μm or less, it is easier to thin the negative electrode active material layer, making it easier to improve the energy density of the nonaqueous alkali metal ion battery.

[0089] The average particle size of the negative electrode active material can be measured in the same manner as the average particle size of the positive electrode active material.

[0090] The average particle size of the negative electrode active material can be adjusted by pulverizing the material using a wet or dry jet mill with a built-in classifier, an agitation ball mill, etc. The pulverizer is equipped with a centrifugal classifier, and the fine particles pulverized in an inert gas environment such as nitrogen or argon can be collected with a cyclone or a dust collector.

[0091] (Optional Components) The negative electrode active material layer in this embodiment may contain optional components such as a conductive filler, a binder, and a dispersion stabilizer, as needed.

[0092] Examples of the conductive filler include acetylene black, ketjen black, vapor-grown carbon fiber, etc. The amount of the conductive filler is preferably 0.1 parts by mass or more and 30 parts by mass or less, more preferably 0.1 parts by mass or more and 20 parts by mass or less, and even more preferably 0.1 parts by mass or more and 15 parts by mass or less, relative to 100 parts by mass of the negative electrode active material.

[0093] Examples of binders that can be used include PVdF (polyvinylidene fluoride), PTFE (polytetrafluoroethylene), polyimide, latex, styrene-butadiene copolymer, fluororubber, acrylic copolymer, polyacrylic acid, and polyglutamic acid. The amount of binder is preferably 1 part by mass or more and 20 parts by mass or less, more preferably 1.2 parts by mass or more and 15 parts by mass or less, and even more preferably 2 parts by mass or more and 10 parts by mass or less, relative to 100 parts by mass of the negative electrode active material. When the amount of binder is 1 part by mass or more, sufficient electrode strength is easily achieved. When the amount of binder is 20 parts by mass or less, the movement of alkali metal ions such as sodium ions, potassium ions, and lithium ions into and out of the negative electrode active material is less likely to be hindered, and therefore, high input / output characteristics are easily achieved.

[0094] Examples of the dispersion stabilizer that can be used include PVP (polyvinylpyrrolidone), PVA (polyvinyl alcohol), and cellulose derivatives. The amount of the dispersion stabilizer is preferably 0 parts by mass or more and 10 parts by mass or less relative to 100 parts by mass of the negative electrode active material. When the amount of the dispersion stabilizer is 10 parts by mass or less, the movement of alkali metal ions such as sodium ions, potassium ions, and lithium ions into and out of the negative electrode active material is less likely to be inhibited, and high input / output characteristics are more likely to be exhibited.

[0095] (Physical Properties of Negative Electrode Active Material Layer) —Thickness— The thickness of the negative electrode active material layer is preferably 5 μm or more and 500 μm or less per side of the negative electrode current collector. The lower limit of the thickness of the negative electrode active material layer is more preferably 10 μm or more, and even more preferably 20 μm or more. The upper limit of the thickness of the negative electrode active material layer is more preferably 300 μm or less, and even more preferably 200 μm or less. When the thickness of the negative electrode active material layer is 5 μm or more, streaks and the like are unlikely to occur when the negative electrode active material layer is applied, and therefore, excellent coatability is likely to be achieved. When the thickness of the negative electrode active material layer is 500 μm or less, cracks are unlikely to occur in the active material layer, and therefore, a decrease in yield is easily prevented. Note that, when the negative electrode current collector has through holes and / or irregularities, the thickness of the negative electrode active material layer refers to the average thickness of the negative electrode active material layer per side in a portion of the negative electrode current collector that does not have through holes and / or irregularities.

[0096] -Bulk density- The bulk density of the negative electrode active material layer is preferably 0.70 g / cm ３ 2.00g / cm or more ３ or less, more preferably 0.80 g / cm ３ 1.70g / cm or more ３ More preferably, 0.90 g / cm ３ 1.65g / cm or more ３ The bulk density of the negative electrode active material layer is 0.70 g / cm or less. ３ If the bulk density of the negative electrode active material layer is 2.00 g / cm or more, sufficient strength can be easily maintained and electrical conductivity between the negative electrode active materials can be easily exhibited. ３ If the thickness is equal to or less than this, pores through which ions can be sufficiently diffused can be easily secured in the negative electrode active material layer.

[0097] -Spectroscopic Characteristics- In this embodiment, when the electrolyte contains 0.5 mol / L or more and 2.5 mol / L or less of sodium ions as alkali metal ions, it is preferable to use a negative electrode containing a non-graphitizable carbon material as a main component from the viewpoint of increasing the energy density of the battery. Whether or not a non-graphitizable carbon material is contained as the main negative electrode active material is determined by Raman spectroscopy. When Raman spectroscopy was performed on a negative electrode containing a carbon material, a Raman shift of 1250 cm －１ 1450cm or more －１ and Raman shift 1500 cm －１ More than 1700cm －１ The following peaks are observed. These peaks are derived from the structure of the carbon material and are called the D band and G band. Whether or not the negative electrode contains a non-graphitizable carbon material can be determined from the peak intensity ratio of the D band and the G band. Raman shift 1250 cm －１ 1450cm or more －１ The maximum value of the Raman intensity below is A, and the Raman shift is 1500 cm －１ More than 1700cm －１ When the maximum value of the Raman intensity below is B, if the ratio B / A of A and B is 1.30 or less, it is determined that the negative electrode contains a non-graphitizable carbon material as the main active material component.

[0098] (Production of Negative Electrode) The negative electrode has a negative electrode active material layer on one or both sides of a negative electrode current collector. Typically, the negative electrode active material layer is fixed to one or both sides of the negative electrode current collector. Such a negative electrode can be produced in the same manner as the production of a positive electrode, except that the constituent components are appropriately changed depending on the configuration of the negative electrode in the resulting nonaqueous alkali metal ion battery.

[0099] In this embodiment, the anode active material layer in which the contents of the first anode active material and the second anode active material are adjusted is produced, for example, through a step of selecting a desired material as the first anode active material, a step of selecting a desired material as the second anode active material, and a step of producing a anode using the material selected as the first anode active material and the material selected as the second anode active material in predetermined amounts, respectively, while assuming the content ratios in the resulting anode active material layer.

[0100] <<Electrode Stack or Electrode Wound Body>> A positive electrode and a negative electrode are generally stacked or wound with a separator interposed therebetween, thereby forming an electrode body (electrode stack or electrode wound body) having a positive electrode, a negative electrode, and a separator.

[0101] <Separator> The separator may be a microporous polyethylene film, a microporous polypropylene film, or a nonwoven cellulose paper. A film composed of organic or inorganic fine particles may be laminated on one or both sides of the separator. The separator may also contain organic or inorganic fine particles.

[0102] The thickness of the separator is preferably 5 μm or more and 25 μm or less. A separator thickness of 5 μm or more tends to reduce self-discharge due to internal micro-short circuits. A separator thickness of 25 μm or less tends to improve the output characteristics of the nonaqueous alkali metal ion battery.

[0103] The thickness of the film made of organic or inorganic fine particles is preferably 0.1 μm or more and 10 μm or less. When the thickness of the film made of organic or inorganic fine particles is 0.1 μm or more, self-discharge due to internal micro-short circuits tends to be reduced. When the thickness of the film made of organic or inorganic fine particles is 10 μm or less, the output characteristics of the nonaqueous alkali metal ion battery tend to be improved.

[0104] The separator may contain an organic polymer that swells upon penetration of the non-aqueous electrolyte solution. Alternatively, an organic polymer may be used alone as a separator. The organic polymer preferably has good affinity with the non-aqueous electrolyte solution and gels upon penetration and swelling of the electrolyte solution. Suitable organic polymers include polyethylene oxide, polyacrylonitrile, polyvinylidene fluoride, polymethyl methacrylate, and mixtures thereof, which tend to exhibit high lithium ion conductivity upon gelation.

[0105] The organic polymer can contain the electrolyte solution within the organic polymer, which is preferable from the viewpoint of safety because it has the effect of preventing the electrolyte solution from leaking out of the nonaqueous alkali metal ion battery when the exterior body is damaged.

[0106] One aspect of this embodiment is an electricity storage module including the nonaqueous secondary battery. Such an electricity storage module is fabricated by a conventional method. The electricity storage module can be configured, for example, by connecting multiple nonaqueous secondary batteries in series or in parallel.

[0107] This energy storage module is preferably incorporated into at least one system selected from the group consisting of a power regeneration assist system, a power load leveling system, an uninterruptible power supply system, a contactless power supply system, an energy harvesting system, an electric vehicle, a plug-in hybrid vehicle, a hybrid vehicle, an electric motorcycle, and a rapid charging system.

[0108] <<Method for Producing Nonaqueous Secondary Battery>> The nonaqueous secondary battery of this embodiment can be produced using the nonaqueous electrolyte solution, positive electrode, separator, and negative electrode described above, for example, by the following method: (1) housing an electrode assembly consisting of a positive electrode, a negative electrode, and a separator in an outer casing (cell assembly); (2) injecting a nonaqueous electrolyte solution into the outer casing (electrolyte injection); and (3) applying a voltage between the positive electrode and the negative electrode at a temperature of 20°C to 80°C to decompose the nonaqueous electrolyte solution and additives and form a solid electrolyte coating (SEI) on the surfaces of the positive electrode and the negative electrode (conditioning).

[0109] <Cell Assembly> An electrode assembly is constructed using the above-described positive electrode, negative electrode, and separator. In assembling the electrode assembly, a positive electrode terminal and a negative electrode terminal are connected to a laminate formed by stacking the positive electrode and negative electrode cut into sheet shapes with a separator interposed therebetween to produce an electrode laminate. Alternatively, a positive electrode terminal and a negative electrode terminal are connected to a wound body formed by winding the positive electrode and negative electrode with a separator interposed therebetween to produce an electrode wound body. The shape of the electrode wound body may be cylindrical or flat.

[0110] The positive electrode terminal and the negative electrode terminal can be connected by, for example, resistance welding, ultrasonic welding, laser welding, or the like.

[0111] <Storage in Exterior Body> The exterior body may be a metal can, a laminated packaging material, etc. As the metal can, an aluminum can is preferred from the viewpoint of weight reduction.

[0112] <Injection of Electrolyte Solution, Fabrication of Non-Aqueous Alkali Metal Ion Battery> After assembly, a non-aqueous electrolyte solution is injected into the electrode body housed in the exterior housing. After the injection, it is desirable to further impregnate the positive electrode, negative electrode, and separator so that they are sufficiently immersed in the non-aqueous electrolyte solution. After the injection of the electrolyte solution, the electrode stack is sealed while reducing the pressure while the exterior housing is open, thereby producing a non-aqueous secondary battery.

[0113] <Conditioning> Next, a conditioning step is carried out at a temperature of 20°C or higher and 80°C or lower.

[0114] In this embodiment, it is preferable to apply a voltage to the positive electrode and the negative electrode to decompose the solvent and additives in the non-aqueous electrolyte solution, thereby forming an SEI on the surfaces of the positive electrode and the negative electrode.

[0115] Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments and can be modified in various ways without departing from the spirit and scope of the present invention.

[0116] Examples and comparative examples of the present invention will be described below, but the present invention is not limited to these examples and comparative examples.

[0117] <Production and Evaluation of Positive Electrodes: P1 and P2> Na(Mn ０．４ Fe ０．３ Ni ０．３ ) O ２ 92.5 parts by mass of the powder, 4.2 parts by mass of acetylene black as a conductive filler, 3.3 parts by mass of PVdF (polyvinylidene fluoride) as a binder, and NMP (N-methylpyrrolidone) as an organic solvent were mixed to obtain a positive electrode slurry with a solids concentration of 45% by mass. The obtained positive electrode slurry was applied to one side of a 15 μm thick aluminum foil as a positive electrode current collector, dried, and pressed to obtain a positive electrode P1. The thickness of the positive electrode active material layer of P1 was 55 μm per side, and the basis weight of the positive electrode active material layer per side was 152 g / m ２ The bulk density of the positive electrode active material layer was 2.80 g / cm ３ It was.

[0118] Na as the positive electrode active material ０．６７ (Mn ０．５ Fe ０．５ ) O ２90 parts by mass of the powder, 5 parts by mass of acetylene black as a conductive filler, 5 parts by mass of PVdF (polyvinylidene fluoride) as a binder, and NMP (N-methylpyrrolidone) as an organic solvent were mixed to obtain a positive electrode slurry with a solids concentration of 51% by mass. The obtained positive electrode slurry was applied to one side of a 17 μm thick aluminum foil as a positive electrode current collector, dried, and pressed to obtain positive electrode P2. The thickness of the positive electrode active material layer of P2 was 44 μm, and the basis weight of the positive electrode active material layer was 114 g / m ２ The bulk density of the positive electrode active material layer is 2.60 g / cm ３ It was.

[0119] <Production and Evaluation of Negative Electrodes: N1 and N2> 94 parts by mass of non-graphitizable carbon as the negative electrode active material, 3 parts by mass of carboxymethyl cellulose as a dispersant, 3 parts by mass of styrene-butadiene copolymer as a binder, and water as a dispersion medium were mixed to obtain a negative electrode slurry with a solids concentration of 40% by mass. The obtained negative electrode slurry was applied to one side of a 12 μm thick aluminum foil as a negative electrode current collector, dried, and pressed to obtain negative electrode N1. The thickness of the negative electrode active material layer of N1 was 86.6 μm, and the basis weight of the negative electrode active material layer was 81.7 g / m ２ and the bulk density is 0.94 g / cm ３ It was.

[0120] 90 parts by mass of non-graphitizable carbon as the negative electrode active material, 5 parts by mass of acetylene black as the conductive filler, 5 parts by mass of PVdF (polyvinylidene fluoride) as the binder, and NMP (N-methylpyrrolidone) as the organic solvent were mixed to obtain a negative electrode slurry with a solids concentration of 34% by mass. The obtained negative electrode slurry was applied to one side of a 10 μm thick copper foil as a negative electrode current collector, dried, and pressed to obtain negative electrode N2. The thickness of the negative electrode active material layer of N2 was 35 μm, and the basis weight of the negative electrode active material layer was 31.6 g / m ２ and the bulk density is 0.90 g / cm ３ It was.

[0121] <Preparation of Non-Aqueous Electrolyte Solution 1> Various non-aqueous solvents and various additives were mixed in an inert atmosphere to give predetermined concentrations, and various alkali metal salts were further added to give predetermined concentrations to prepare non-aqueous electrolyte solutions.

[0122] The prepared electrolyte solutions are listed in Tables 1 and 2. The abbreviations have the following meanings: EC: ethylene carbonate, MEC: methyl ethyl carbonate, AcN: acetonitrile, FEC: fluoroethylene carbonate, VC: vinylene carbonate, ES: ethylene sulfite, NaPF6: sodium hexafluorophosphate, NaFSI: sodium bis(fluorosulfonyl)imide, KFSI: potassium bis(fluorosulfonyl)imide.

[0123]

[0124]

[0125] (Measurement of ionic conductivity of non-aqueous electrolyte) A non-aqueous electrolyte was prepared in a polypropylene container, and an ionic conductivity measuring cell "CT-57101B" (trade name) manufactured by DKK TOA Corporation connected to an ionic conductivity meter "CM-30R" (trade name) manufactured by DKK TOA Corporation was inserted into the container containing the non-aqueous electrolyte, and the ionic conductivity of the non-aqueous electrolyte at 25° C. was measured. The ionic conductivities of the non-aqueous electrolytes are shown in Table 3.

[0126]

[0127] The ionic conductivity of a non-aqueous electrolyte solution tends to increase as the content of the mononitrile compound increases. As will be described later in this embodiment, when an electrolyte solution having an ionic conductivity of 12 mS / cm or more at 25°C is used, the cycle characteristics and output characteristics tend to improve.

[0128] <Production of non-aqueous secondary battery 1> The positive electrode was placed on a 2 cm ２ The negative electrode was cut into a disk with an area of ​​2.05 cm ２ A 15 μm thick polyolefin microporous membrane was used as the separator, and the area was 4.5 cm. ２The negative electrode was placed in a SUS304 case with the negative electrode active material layer facing upward. A separator was stacked on top of the negative electrode so that the centers overlapped. 400 μL of nonaqueous electrolyte was poured, and the positive electrode was stacked on top of the separator with the positive electrode active material layer facing downward and the center overlapping the separator. The top of the case containing the negative electrode, separator, and positive electrode was then sealed with an aluminum lid. The case was held at 25°C for 24 hours to impregnate the positive electrode, separator, and negative electrode with the nonaqueous electrolyte, thereby obtaining a nonaqueous secondary battery. Nonaqueous secondary batteries of each example and comparative example were assembled using the combinations of positive electrode, negative electrode, and electrolyte shown in Table 4.

[0129]

[0130] (Charge-Discharge Evaluation 1 of Nonaqueous Secondary Battery) The nonaqueous secondary battery obtained as described above was subjected to the following four-stage charge-discharge evaluation. (1) Initial charge treatment (2) Cycle characteristic evaluation (3) Output characteristic evaluation (4) Side reaction resistance evaluation Charging and discharging were performed using a charge-discharge device ACD-M01A (product name) manufactured by Asuka Electronics Co., Ltd. and a programmable thermostatic chamber IN804 (product name) manufactured by Yamato Scientific Co., Ltd. Here, 1 C refers to the current value at which a fully charged battery is expected to be discharged at a constant current until discharge is complete in 1 hour.

[0131] (Initial charge-discharge treatment) The ambient temperature of the nonaqueous secondary battery was set to 25° C., and the battery was charged at a constant current equivalent to 0.05 C until it reached 3.95 V, and then charged at a constant voltage of 3.95 V until the current decayed to 0.01 C. The battery was then discharged to 1.5 V at a constant current equivalent to 0.20 C.

[0132] (Evaluation of cycle characteristics) Next, the ambient temperature of the nonaqueous secondary battery was maintained at 25°C, and the battery was charged at a constant current equivalent to 0.33 C until it reached 3.95 V. Then, it was charged at a constant voltage of 3.95 V until the current decayed to 0.05 C. The battery was then discharged to 1.5 V at a constant current equivalent to 0.33 C. Thereafter, 10 cycles of charge and discharge were performed in the same manner as above. The discharge capacity at a constant current equivalent to 0.33 C in the first cycle was defined as A (mAh), and the discharge capacity in the tenth cycle was defined as B (mAh). The cycle capacity retention rate C (%) was calculated by C = B / A × 100.

[0133] (Evaluation of Output Characteristics) The ambient temperature of the nonaqueous secondary battery was set to 25° C. The battery was charged at a constant current equivalent to 0.33 C until it reached 3.95 V, and then charged at a constant voltage of 3.95 V until the current attenuated to 0.05 C. The battery was then discharged at a constant current equivalent to 10 C until it reached 1.5 V. The discharge capacity when discharged at a constant current equivalent to 10 C was defined as D (mAh), and the output capacity retention rate E was calculated by E = D / A × 100 (%).

[0134] (Side Reaction Resistance Evaluation) The ambient temperature of the nonaqueous secondary battery was set to 0°C, and the nonaqueous secondary battery was left standing for 3 hours. The nonaqueous secondary battery was charged to 3.95 V at a constant current equivalent to 1 C. If a side reaction occurs at the positive or negative electrode, capacity resulting from the side reaction is generated before the upper limit voltage of 3.95 V is reached, and the time required for charging may exceed the theoretical charging time of 60 minutes. Therefore, whether the charging time to reach 3.95 V exceeded 60 minutes was used as an evaluation index for the side reaction resistance of the electrolyte. Table 5 shows the results of each test. In the side reaction resistance evaluation, samples with a charging time of less than 60 minutes were judged to have not experienced a continuous side reaction and were assigned an "a" as the evaluation result. Samples with a charging time of 60 minutes or more were judged to have experienced a continuous side reaction and were assigned a "b" as the evaluation result.

[0135]

[0136] In Comparative Example 1, an electrolyte solution containing a cyclic carbonate and a chain carbonate as a solvent was used. In Comparative Example 1, the charging time at 0°C and 1C exceeded the theoretical maximum of 60 minutes. This is thought to be because, apart from the reaction in which sodium ions are absorbed or released into the positive and negative electrode active materials during charging, a side reaction, such as decomposition of the electrolyte, continuously occurred in each of the positive and negative electrodes.

[0137] One factor contributing to this continuous side reaction is the high positive electrode potential in sodium-ion batteries. As described in Non-Patent Document 1, the deposition potential of metallic sodium is approximately 0.3 V higher than that of metallic lithium. Therefore, assuming that the negative electrode potential is equal to the deposition potential of the alkali metal, when comparing the positive electrode potentials of a sodium-ion battery and a lithium-ion battery with the same cell voltage, the positive electrode potential of the sodium-ion battery is approximately 0.3 V higher. Therefore, compared to a lithium-ion battery with the same cell voltage, oxidative decomposition of the electrolyte on the positive electrode side is more likely to occur in sodium-ion batteries. For side reactions to occur continuously, reactions must occur at both the positive and negative electrodes, and electrons must move between the positive and negative electrodes. It is widely known that metallic lithium deposition reactions occur at the negative electrode-electrolyte interface during low-temperature charging in lithium-ion batteries. However, it is not known that lithium-ion batteries can achieve continuous charge capacities exceeding their theoretical capacity during low-temperature, high-speed charging. This is thought to be because in lithium-ion batteries, side reactions sufficient to compensate for the electrons transferred due to side reactions such as the deposition of metallic lithium on the negative electrode and the reductive decomposition of the electrolyte do not occur on the positive electrode side. On the other hand, in the sodium-ion battery shown in Comparative Example 1, the positive electrode potential is high, so side reactions such as the oxidative decomposition of the electrolyte also occur on the positive electrode side, which is thought to be why continuous charge capacity is exhibited.

[0138] In Example 1, an electrolyte solution containing a mononitrile compound as a solvent was used. Unlike Comparative Example 1, the charging time at 0°C and 1C in Example 1 was less than the theoretical maximum of 60 minutes. This is thought to be because the inclusion of a mononitrile compound in the electrolyte solution improved the oxidative stability of the electrolyte solution, suppressing continuous side reactions on the positive electrode side. Meanwhile, in the results shown in Example 1, unlike Comparative Example 1, the cycle capacity retention rate C was less than 50%, which is thought to be due to the gradual reductive decomposition of the mononitrile compound due to repeated charge and discharge. In other words, although the inclusion of a mononitrile compound in the solvent suggested the suppression of continuous side reactions in the electrolyte solution, it also suggests that there is room for improvement in cycle stability.

[0139] In Example 2, an electrolyte solution containing a mononitrile compound and a specific additive was used. Unlike Comparative Example 1 or Example 1, in Example 2, the cycle capacity retention rate C was stable at 95% or more, and the 0°C 1C charge time F was less than 60 minutes, suggesting that continuous side reactions were suppressed. One possible reason for this is that the use of the specific compound as an additive formed a good SEI at the anode-electrolyte interface, suppressing reductive decomposition of the mononitrile compound at the anode.

[0140] Furthermore, a comparison between Example 2 and Comparative Example 1 shows that the use of a mononitrile compound significantly improves the output capacity retention rate E. This result suggests that the use of a mononitrile compound improves the output characteristics.

[0141] The electrolyte solution in Example 3 contains a higher proportion of the mononitrile compound than the electrolyte solution in Example 2. A comparison between Examples 2 and 3 shows that the output capacity retention rate E tends to be higher as the content of the mononitrile compound increases.

[0142] Furthermore, Examples 4 to 7 suggest that high cycle stability and high output characteristics can be obtained when a mononitrile compound is contained and at least one of a fluorinated cyclic carbonate, an unsaturated bond-containing cyclic carbonate, and a cyclic sulfur compound is also contained. Furthermore, Examples 5 and 1 show that when a mononitrile compound and vinylene carbonate are simultaneously contained in the electrolyte solution, the cycle characteristics tend to be improved compared to when vinylene carbonate is not contained. This is the opposite trend to the results inferred from Non-Patent Document 2.

[0143] The electrolyte in Example 5 contains an unsaturated bond-containing cyclic carbonate, while the electrolyte in Example 4 contains the same volume fraction of the unsaturated bond-containing cyclic carbonate and the cyclic sulfur compound. A comparison between Examples 4 and 5 reveals that when the volume fraction of the cyclic sulfur compound in the electrolyte is lower than the volume fraction of the unsaturated bond-containing cyclic carbonate, a significantly higher output capacity retention rate E is observed. This is the exact opposite trend to the test results for the lithium-ion battery disclosed in Patent Document 2. One possible reason for this effect is the difference in the negative electrode potential between lithium-ion batteries and sodium-ion batteries. As described in Non-Patent Document 1, the deposition potential of metallic sodium is approximately 0.3 V higher than the deposition potential of metallic lithium. Therefore, the negative electrode potential in sodium-ion batteries is higher than the potential of the graphite negative electrode in lithium-ion batteries. In the present invention, the fluorinated cyclic carbonate, the unsaturated bond-containing cyclic carbonate, and the cyclic sulfur compound are added to form an SEI by reductive decomposition at the negative electrode-electrolyte interface, thereby suppressing the reductive decomposition of the mononitrile compound. Since the negative electrode potential in this example is higher than that of a graphite negative electrode in a lithium ion battery, it is thought that an additive composition similar to that of a lithium ion battery would cause excessive formation of SEI in the negative electrode, leaving room for improvement in output characteristics. Therefore, it is thought that the lower content of the cyclic sulfur compound in Example 5 compared to Example 4 suppressed excessive SEI formation, and the output capacity retention rate E was improved.

[0144] <Preparation of Non-Aqueous Electrolyte Solution 2> Various non-aqueous solvents and various additives were mixed in an inert atmosphere to give predetermined concentrations, and various alkali metal salts were further added to give predetermined concentrations, thereby preparing non-aqueous electrolyte solutions.

[0145] The prepared electrolyte solutions are listed in Table 6. The abbreviations have the following meanings: EC: ethylene carbonate, MEC: methyl ethyl carbonate, AcN: acetonitrile, MP: methyl propionate, VC: vinylene carbonate, ES: ethylene sulfite, NaPF6: sodium hexafluorophosphate, NaFSI: sodium bis(fluorosulfonyl)imide.

[0146]

[0147] <Production of non-aqueous secondary battery 2> The positive electrode was placed on a 2 cm ２ The negative electrode was cut into a disk with an area of ​​2.05 cm ２ A 12 μm thick polyolefin microporous membrane was used as the separator, and the area was 4.5 cm. ２ The negative electrode was placed in a SUS304 case with the active material layer facing upward. A separator was placed on top of the negative electrode, with the center overlapping. 400 μL of nonaqueous electrolyte was poured, and the positive electrode was placed on top of the separator, with the positive electrode active material layer facing downward and the center overlapping the separator. The top of the case containing the negative electrode, separator, and positive electrode was then sealed with an aluminum lid. The case was held at 25°C for 24 hours to impregnate the positive electrode, separator, and negative electrode with the nonaqueous electrolyte, yielding a nonaqueous secondary battery. Using P1 as the positive electrode, N1 as the negative electrode, and the electrolyte solutions listed in Tables 1 and 6, respectively, the nonaqueous secondary batteries of Examples 8 to 19 and Comparative Example 1 shown in Table 4 were assembled using the positive electrode, negative electrode, and electrolyte combinations shown in Table 7.

[0148]

[0149] (Charge-Discharge Evaluation 2 of Nonaqueous Secondary Battery) The nonaqueous secondary battery obtained as described above was subjected to the following three-stage charge-discharge evaluation. (1) Initial charge treatment (2) Cycle characteristic evaluation (3) Output characteristic evaluation Charging and discharging were performed using a charge-discharge device ACD-M01A (product name) manufactured by Asuka Electronics Co., Ltd. and a programmable thermostatic chamber IN804 (product name) manufactured by Yamato Scientific Co., Ltd. Here, 1 C refers to the current value at which a fully charged battery is expected to be discharged at a constant current until discharge is complete within one hour.

[0150] (Initial charge-discharge treatment) The ambient temperature of the nonaqueous secondary battery was set to 25° C., and the battery was charged at a constant current equivalent to 0.05 C until it reached 3.95 V, and then charged at a constant voltage of 3.95 V until the current decayed to 0.01 C. The battery was then discharged to 1.5 V at a constant current equivalent to 0.20 C.

[0151] (Evaluation of cycle characteristics) Next, the ambient temperature of the nonaqueous secondary battery was maintained at 25°C, and the battery was charged at a constant current equivalent to 0.33 C until it reached 3.95 V. Then, it was charged at a constant voltage of 3.95 V until the current decayed to 0.05 C. The battery was then discharged to 1.5 V at a constant current equivalent to 0.33 C. Thereafter, 10 cycles of charge and discharge were performed in the same manner as above. The discharge capacity at a constant current equivalent to 0.33 C in the first cycle was defined as A (mAh), and the discharge capacity in the tenth cycle was defined as B (mAh). The cycle capacity retention rate C (%) was calculated by C = B / A × 100.

[0152] (Evaluation of Output Characteristics) The ambient temperature of the nonaqueous secondary battery was set to 25° C. The battery was charged at a constant current equivalent to 0.33 C until it reached 3.95 V, and then charged at a constant voltage of 3.95 V until the current attenuated to 0.05 C. The battery was then discharged at a constant current equivalent to 10 C until it reached 1.5 V. The discharge capacity when discharged at a constant current equivalent to 10 C was defined as D (mAh), and the output capacity retention rate E was calculated by E = D / A × 100 (%).

[0153] The results are shown in Table 8.

[0154]

[0155] A comparison of Examples 8 to 19 with Comparative Example 1 reveals that a high output capacity retention rate is achieved when the non-aqueous electrolyte solution contains a mononitrile compound. In Examples 8 to 19, particularly when the volume fractions of the vinylene carbonate and cyclic sulfur compound in the non-aqueous electrolyte solution are X vol % and Y vol %, respectively, and the ranges of 0.5≦X≦10 and 0≦Y≦X are satisfied, a high cycle capacity retention rate and output capacity retention rate are achieved. Furthermore, a high output capacity retention rate is observed when the ranges of 1.0≦X≦5 and 0≦Y≦X are satisfied. Furthermore, a short-chain fatty acid ester can be used together with the mononitrile compound as the non-aqueous solvent, and in this case, an even higher output capacity retention rate is achieved.

[0156] <Preparation of Non-Aqueous Electrolyte Solution 3> Various non-aqueous solvents and various additives were mixed in an inert atmosphere so as to have predetermined concentrations, and various alkali metal salts were further added thereto so as to have predetermined concentrations, thereby preparing non-aqueous electrolyte solutions.

[0157] The prepared electrolyte solutions are listed in Table 9. E1 from Table 1 and E19 and E20 from Table 6 are extracted and listed again in Table 9. Abbreviations have the following meanings: EC: ethylene carbonate, MEC: methyl ethyl carbonate, AcN: acetonitrile, VC: vinylene carbonate, ES: ethylene sulfite, NaPF6: sodium hexafluorophosphate, NaFSI: sodium bis(fluorosulfonyl)imide.

[0158]

[0159] <Production of non-aqueous secondary battery 3> The positive electrode was placed on a 2 cm ２ The negative electrode was cut into a disk with an area of ​​2.05 cm ２ A 12 μm thick polyolefin microporous membrane was used as the separator, and the area was 4.5 cm. ２The negative electrode was placed in a SUS304 case with the active material layer facing upward. A separator was stacked on top of the negative electrode so that the centers overlapped. 400 μL of non-aqueous electrolyte was poured, and the positive electrode was stacked on top of the separator with the positive electrode active material layer facing downward and the center overlapping the separator. The top of the case containing the negative electrode, separator, and positive electrode was then sealed with an aluminum lid. The case was kept at 25°C for 24 hours to impregnate the positive electrode, separator, and negative electrode with the non-aqueous electrolyte, thereby obtaining a non-aqueous secondary battery. A non-aqueous secondary battery was assembled using P1 as the positive electrode and N1 as the negative electrode.

[0160] (Charge-Discharge Evaluation 3 of Nonaqueous Secondary Battery) The nonaqueous secondary battery obtained as described above was subjected to the following two-stage charge-discharge evaluation. (1) Initial charge treatment (2) Low-temperature characteristic evaluation Charging and discharging were performed using a charge-discharge device ACD-M01A (product name) manufactured by Asuka Electronics Co., Ltd. and a programmable thermostatic chamber IN804 (product name) manufactured by Yamato Scientific Co., Ltd. Here, 1 C refers to the current value at which a fully charged battery is expected to be discharged at a constant current until discharge is complete within one hour.

[0161] (Initial charge-discharge treatment) The ambient temperature of the nonaqueous secondary battery was set to 25° C., and the battery was charged at a constant current equivalent to 0.05 C until it reached 3.95 V, and then charged at a constant voltage of 3.95 V until the current decayed to 0.01 C. The battery was then discharged to 1.5 V at a constant current equivalent to 0.20 C.

[0162] (Low-Temperature Characteristic Evaluation) Next, the ambient temperature of the nonaqueous secondary battery was kept at 25°C, and the battery was charged at a constant current equivalent to 0.33 C until it reached 3.95 V, and then charged at a constant voltage of 3.95 V until the current decayed to 0.05 C. The battery was then discharged to 1.5 V at a constant current equivalent to 1 C. The discharge capacity at this time was defined as F (mAh). The ambient temperature of the nonaqueous secondary battery was then kept at 25°C, and the battery was charged at a constant current equivalent to 0.33 C until it reached 3.95 V, and then charged again at a constant voltage of 3.95 V until the current decayed to 0.05 C. The ambient temperature of the nonaqueous secondary battery was then kept at -30°C and allowed to stand for 3 hours. After that, the battery was discharged to 1.5 V at a constant current equivalent to 1 C while maintaining the ambient temperature of the nonaqueous secondary battery at -30°C. The discharge capacity at this time was defined as G (mAh). The low-temperature capacity retention rate H was calculated as H=G / F×100(%). The discharge characteristics in a low-temperature environment were evaluated based on the magnitude of the H value.

[0163] The results are shown in Table 10. In the table, X represents the volume fraction of the unsaturated bond-containing cyclic carbonate in the solvent, and Y represents the volume fraction of the cyclic sulfur compound in the solvent in the electrolyte solution of each Example or Comparative Example.

[0164]

[0165] A comparison of Comparative Example 3 with Examples 20 to 25 shows that the low-temperature capacity retention rate is dramatically improved when the solvent contains a mononitrile compound. Furthermore, a comparison of Examples 20 and 21 with Example 22, and Examples 23 and 24 with Example 25 shows that the low-temperature capacity retention rate is further improved, i.e., the low-temperature characteristics tend to improve, when 0<X≦Y≦10 is satisfied.

[0166] <Production and Evaluation of Positive Electrode: P3> Na as the positive electrode active material ３ V ２ (P.O. ４ ) ３90 parts by weight of the powder, 5 parts by weight of acetylene black as a conductive filler, 5 parts by weight of PVdF (polyvinylidene fluoride) as a binder, and NMP (N-methylpyrrolidone) as an organic solvent were mixed to obtain a positive electrode slurry with a solids concentration of 46.4% by weight. The obtained positive electrode slurry was applied to one side of a 17 μm thick aluminum foil as a positive electrode current collector, dried, pressed, and vacuum-dried for 10 hours while heating to 150 °C, thereby obtaining positive electrode P3. The thickness of the positive electrode active material layer of P3 was 57 μm per side, and the basis weight of the positive electrode active material layer per side was 88 g / m ２ The bulk density of the positive electrode active material layer was 1.54 g / cm ３ In addition, Na used as the positive electrode active material ３ V ２ (P.O. ４ ) ３ For the above, a non-carbon coated sample was used.

[0167] <Preparation of Non-Aqueous Electrolyte Solution 4> Various non-aqueous solvents and various additives were mixed in an inert atmosphere to give predetermined concentrations, and various alkali metal salts were further added to give predetermined concentrations, thereby preparing non-aqueous electrolyte solutions.

[0168] The prepared electrolyte solutions are listed in Table 11. The abbreviations have the following meanings: EC: ethylene carbonate, MEC: methyl ethyl carbonate, AcN: acetonitrile, VC: vinylene carbonate, FEC: fluoroethylene carbonate, ES: ethylene sulfite, NaPF6: sodium hexafluorophosphate.

[0169]

[0170] <Production of Non-Aqueous Secondary Battery 4> In a dry atmosphere with a dew point of -80°C or less, the positive electrode P3 was cut into a 17 mm diameter disk, and the negative electrode, Na metal foil, was cut into a 17.5 mm diameter disk. A 12 μm thick polyolefin microporous membrane was used as the separator and cut into a 24 mm diameter disk. The Na metal foil was placed in a SUS304 case with the active material layer facing upward. A 24 mm diameter, 100 μm thick glass nonwoven fabric and the above-mentioned separator were stacked on top of the Na metal foil, in order, with their centers overlapping. 400 μL of non-aqueous electrolyte was poured, and the positive electrode was stacked on top of the separator with the positive electrode active material layer facing downward and with the separator center overlapping. The top of the case containing the Na metal foil, glass nonwoven fabric, separator, and positive electrode was then sealed with an aluminum lid. The positive electrode, separator, nonwoven glass fabric, and negative electrode were impregnated with the nonaqueous electrolyte solution by keeping the mixture at 25° C. for 24 hours, thereby obtaining a nonaqueous secondary battery.

[0171] (Charge / Discharge Evaluation 4 of Nonaqueous Secondary Battery) The nonaqueous secondary battery obtained as described above was subjected to the following one-stage charge / discharge evaluation. (1) Cycle Characteristics Evaluation Charge / discharge was performed using a charge / discharge device ACD-M01A (product name) manufactured by Asuka Electronics Co., Ltd. and a programmable thermostatic chamber IN804 (product name) manufactured by Yamato Scientific Co., Ltd. Here, 1 C refers to the current value at which a fully charged battery is discharged at a constant current and expected to be discharged to completion in 1 hour. For the positive electrode P3, a current value equivalent to 107 mA per 1 g of active material was set to 1 C.

[0172] (Evaluation of cycle characteristics) The ambient temperature of the nonaqueous secondary battery was set to 25°C, and the battery was charged at a constant current equivalent to 0.1 C until it reached 3.8 V. Then, the battery was charged at a constant voltage of 3.8 V until the current decayed to 0.05 C. The battery was then discharged at a constant current equivalent to 0.10 C to 2.4 V. Similar charge / discharge cycles were repeated for a total of 10 cycles. Table 12 shows the discharge capacity (mAh / g) per gram of positive electrode active material at the first and tenth cycles.

[0173]

[0174] Vinylene carbonate, fluoroethylene carbonate, and ethylene sulfite are included in the unsaturated bond-containing cyclic carbonate, fluorinated cyclic carbonate, and cyclic sulfur compound, respectively. Table 12 shows that when a mononitrile compound is used as a non-aqueous solvent, the inclusion of one or more of the unsaturated bond-containing cyclic carbonate, fluorinated cyclic carbonate, and cyclic sulfur compound results in particularly excellent cycle characteristics. This is presumably because, when one or more of the above compounds are contained in the electrolyte, a passive film is formed on the surface of the negative electrode (Na metal) during the initial charge / discharge, suppressing decomposition of the electrolyte.

[0175] The nonaqueous secondary battery of the present invention is expected to be used, for example, as a consumer storage battery such as a mobile battery for mobile phones and portable devices, an automotive storage battery for hybrid vehicles, plug-in hybrid vehicles, electric vehicles, and the like, as well as an industrial storage battery for power tools, drones, electric motorcycles, and the like, and further as a residential power storage system.

Claims

1. A non-aqueous electrolyte solution comprising one or more cyclic carbonates or chain carbonates, a mononitrile compound represented by the formula R-CN (wherein R is an alkyl group having 1 to 4 carbon atoms or a halogenated alkyl group having 1 to 4 carbon atoms) in a solvent in an amount of 5 to 90% by volume, and containing alkali metal ions other than lithium ions.

2. The nonaqueous electrolyte according to claim 1, wherein the mononitrile compound is acetonitrile.

3. The non-aqueous electrolyte according to claim 1 or 2, wherein the non-aqueous electrolyte contains sodium ions or potassium ions as the alkali metal ions.

4. The non-aqueous electrolyte according to claim 1 or 2, wherein the non-aqueous electrolyte contains one or more compounds selected from the group consisting of fluorinated cyclic carbonates, unsaturated bond-containing cyclic carbonates, and cyclic sulfur compounds.

5. The non-aqueous electrolyte according to claim 1 or 2, wherein the non-aqueous electrolyte contains one or more additives selected from the group consisting of fluoroethylene carbonate, ethylene sulfite, and vinylene carbonate.

6. The non-aqueous electrolyte according to claim 1 or 2, wherein the non-aqueous electrolyte contains 0.5% by volume or more and 10% by volume or less of vinylene carbonate.

7. The nonaqueous electrolyte solution according to claim 1 or 2, wherein the nonaqueous electrolyte solution contains 0.5 volume % or more and 10 volume % or less of vinylene carbonate, the nonaqueous electrolyte solution contains one or more additives selected from fluoroethylene carbonate and cyclic sulfur compounds, and the volume fraction of the additives in the nonaqueous electrolyte solution is 10 volume % or less.

8. The nonaqueous electrolyte solution according to claim 1 or 2, wherein, when the volume fractions of the vinylene carbonate and the cyclic sulfur compound in the nonaqueous electrolyte solution are X vol % and Y vol %, respectively, the relationships 0.5≦X≦10 and 0≦Y≦10 are satisfied.

9. The nonaqueous electrolyte solution according to claim 1 or 2, wherein, when the volume fractions of the vinylene carbonate and the cyclic sulfur compound in the nonaqueous electrolyte solution are X vol % and Y vol %, respectively, the relationships 0.5≦X≦10 and 0≦Y<X are satisfied.

10. The non-aqueous electrolyte according to claim 1 or 2, wherein the non-aqueous electrolyte contains ethylene sulfite as the cyclic sulfur compound.

11. The nonaqueous electrolyte solution according to claim 1 or 2, wherein, when the volume fractions of vinylene carbonate and ethylene sulfite in the nonaqueous electrolyte solution are X vol % and Y vol %, respectively, the relationships 1.0≦X≦5.0 and 0≦Y<X are satisfied.

12. The nonaqueous electrolyte solution according to claim 1 or 2, wherein, when the volume fractions of the unsaturated bond-containing cyclic carbonate and the cyclic sulfur compound in the nonaqueous electrolyte solution are X vol % and Y vol %, respectively, the relationship 0<X≦Y≦10 is satisfied.

13. The non-aqueous electrolyte solution according to claim 1 or 2, wherein the ionic conductivity of the non-aqueous electrolyte solution at 25°C is 12 mS / cm or more.

14. A non-aqueous secondary battery comprising a positive electrode, a negative electrode, and the non-aqueous electrolyte solution according to claim 1 or 2, wherein the positive electrode and the negative electrode each contain a material capable of absorbing and releasing sodium ions or potassium ions.

15. The nonaqueous secondary battery according to claim 14, wherein the negative electrode current collector of the nonaqueous secondary battery is aluminum.

16. The negative electrode in the nonaqueous secondary battery has a voltage of 0.4 V vs. Na / Na ＋ 15. The nonaqueous secondary battery according to claim 14, further comprising a material that absorbs sodium ions at a potential lower than the potential of the nonaqueous secondary battery.

17. Raman spectroscopy of the negative electrode of the nonaqueous secondary battery shows a Raman shift of 1250 cm －１ More than 1450cm －１ The maximum value of the Raman intensity below is A, and the Raman shift is 1500 cm －１ More than 1700cm －１ 15. The nonaqueous secondary battery according to claim 14, wherein when B is the maximum value of the Raman intensity in the following range, the ratio B / A of A and B is 1.30 or less.

18. The non-aqueous secondary battery according to claim 14, wherein the negative electrode active material of the non-aqueous secondary battery contains non-graphitizable carbon.

19. The non-aqueous secondary battery according to claim 18, wherein the proportion of said non-graphitizable carbon in the weight of the negative electrode active material layer of said non-aqueous secondary battery is 50 mass % or more.

20. The positive electrode is a cathode of the following formula: Na ｘ M ｅ O ２ (wherein x is a number satisfying 0≦x≦1, and M ｅ represents one or more metal elements selected from Fe, Mn, Co, Ni, Ti, Mg, Zn, Te, and Cu), Na ｘ (FeSO ４ ) ２ (wherein x is a number satisfying 0≦x≦1), Na ３ｘ M ４ ２ (P.O. ４ ) ( ３－ｅ） F ３ｅ (In the formula, M ４ is Ti or V, and e is a number satisfying 0≦e≦1), and Na ｘ M １ [M ２ (CN) ６ ] （１－ｙ） ・ ｚ H ２ O (in the formula, M １ and M ２ are Fe, Mn, Co, Ni, Ti, or Cu, respectively, x is a number satisfying 0≦x≦1, y is a number satisfying 0≦y≦1.0, and z is a number satisfying 0≦z≦3.

0.

21. The positive electrode is a compound represented by the following formula: Na ｘ MO ２ (wherein x is a number satisfying 0≦x≦1, and M is Fe, Mn, Co, Ni, Ti, Mg, Zn, Te or Cu), Na ｘ M １ ｙ M ２ ｚ O ２ (In the formula, M １ and M ２ are Fe, Mn, Co, Ni, Ti, Mg, Zn, Te, or Cu, x is a number satisfying 0≦x≦1, y is a number satisfying 0.1<y<0.9, z is a number satisfying 0.1<z<0.9, and y and z satisfy 0.95<y+z<1.05), and Na ｘ M １ ｃ M ２ ｄ M ３ （１－ｃ－ｄ） O ２ (In the formula, M １ , M ２ and M ３ where x is a number satisfying 0.03<c<0.97, and d is a number satisfying 0.03<d<0.97).

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