Electrolyte for non-aqueous secondary battery and non-aqueous secondary battery using the same

Abstract

To provide an electrolytic solution capable of improving charge and discharge cycle characteristics in a non-aqueous secondary battery in which lithium-free transition metal sulfide is used as a positive electrode active material.SOLUTION: Provided is an electrolytic solution used for a non-aqueous secondary battery in which lithium-free transition metal sulfide is used as a positive electrode active material. The electrolytic solution contains: an organic solvent containing a cyclic carbonate compound; and an additive agent.SELECTED DRAWING: None

Description

[Technical Field]

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

[0002] The recent advances in performance of portable electronic devices, hybrid vehicles, etc. have created a demand for increasingly higher capacity lithium-ion secondary batteries for use in them. However, current lithium-ion secondary batteries do not have a high enough capacity positive electrode compared to the negative electrode, and even lithium nickel oxide-based materials, which are said to have relatively high capacity, only have a capacity of around 190 to 220 mAh / g.

[0003] On the other hand, sulfur has a high theoretical capacity of approximately 1670 mAh / g, making it promising for use as a positive electrode active material. However, it is generally known that sulfur-based positive electrode active materials lose capacity with repeated charge-discharge cycles. This is because sulfur dissolves into the organic electrolyte as lithium polysulfide during charge-discharge, making it essential to develop a technology to suppress its dissolution into the organic electrolyte.

[0004] Although lithium-free transition metal sulfides (transition metal sulfides that do not contain lithium) have electronic conductivity and little elution into organic electrolytes, this is not sufficient. Taking vanadium sulfide as an example of a lithium-free transition metal sulfide, when crystalline vanadium(III) sulfide (V2S3), which is commercially available as a reagent, is used as a positive electrode active material, the reaction with the organic electrolyte cannot be suppressed, and the measured capacity is only about 23 mAh / g for charge capacity and 52 mAh / g for discharge capacity. In contrast, the present inventors have reported that low-crystalline vanadium sulfide having a specific composition exhibits high capacity and excellent charge-discharge cycle characteristics when used as an electrode active material for lithium-ion secondary batteries (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2018 / 181698 Summary of the Invention [Problem to be solved by the invention]

[0006] As described above, the present inventors have developed a material that exhibits high capacity when used as an electrode active material for lithium ion secondary batteries and also has excellent charge-discharge cycle characteristics. However, there is a never-ending demand for higher performance in lithium ion secondary batteries, and further improvements in the charge-discharge cycle characteristics are required.

[0007] The causes of cycle deterioration include the accumulation of by-products due to the reaction between the lithium-free transition metal sulfide and the electrolyte, the reduction of the electrode active material components, etc., and it is thought that suppressing these reactions will lead to improvement of charge-discharge cycle characteristics. For example, a method of suppressing the reaction between the two can be mentioned, such as using a less reactive electrolyte.

[0008] The present invention has been made in view of the current state of the prior art described above, and its main object is to provide an electrolyte solution that can improve the charge-discharge cycle characteristics of a non-aqueous secondary battery that uses a lithium-free transition metal sulfide as a positive electrode active material. [Means for solving the problem]

[0009] The present inventors have conducted extensive research to achieve the above-mentioned object. As a result, they have found that by incorporating an organic solvent containing a cyclic carbonate compound and an additive, the charge-discharge cycle characteristics of a non-aqueous secondary battery using a lithium-free transition metal sulfide as a positive electrode active material can be further improved. The present invention was completed as a result of further research based on this finding. That is, the present invention includes the following configurations.

[0010] Item 1. A non-aqueous secondary battery electrolyte used in a non-aqueous secondary battery that uses a lithium-free transition metal sulfide as a positive electrode active material, The electrolyte solution for a non-aqueous secondary battery contains an organic solvent containing a cyclic carbonate compound and an additive.

[0011] Item 2. The additive is represented by general formula (1):

[0012] [ka]

[0013] [In the formula, R 1 and R 2 are the same or different and represent a hydrogen atom or a halogen atom. Bonds represented by solid and dashed lines represent single or double bonds.] or a compound represented by general formula (2):

[0014] [ka]

[0015] [In the formula, R 3 are the same or different and represent a halogen atom. M represents a counter cation. 2. The nonaqueous electrolyte solution for a secondary battery according to claim 1, wherein the compound is a compound represented by the formula:

[0016] Item 3. The compound represented by the general formula (1) contains at least one selected from the group consisting of vinylene carbonate (VC), fluoroethylene carbonate (FEC), trifluoromethylethylene carbonate, and vinylethylene carbonate; Item 3. The nonaqueous electrolyte solution for a secondary battery according to Item 2, wherein the compound represented by the general formula (2) contains lithium difluoro(oxalato)borate (DFOB).

[0017] Item 4. The nonaqueous electrolyte solution for a secondary battery according to any one of Items 1 to 3, wherein the content of the additive is 0.5 to 20 parts by mass per 100 parts by mass of the organic solvent.

[0018] Item 5. The nonaqueous electrolyte solution for a secondary battery according to any one of Items 1 to 4, wherein the content of the cyclic carbonate compound is 80 to 100% by volume and the content of the chain carbonate compound is 0 to 20% by volume, where the total amount of the organic solvent is 100% by volume.

[0019] Item 6. The nonaqueous electrolyte solution for a secondary battery according to Item 5, wherein the chain carbonate compound is at least one selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and methyl propyl carbonate.

[0020] Item 7. The nonaqueous electrolyte solution for a secondary battery according to any one of Items 1 to 6, wherein the cyclic carbonate compound is at least one selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate.

[0021] Item 8. The nonaqueous electrolyte solution for a secondary battery according to any one of Items 1 to 7, wherein the lithium-free transition metal sulfide is at least one selected from the group consisting of vanadium sulfide, molybdenum sulfide, and iron sulfide.

[0022] Item 9. The nonaqueous electrolyte solution for a secondary battery according to any one of Items 1 to 8, further containing a lithium salt.

[0023] Item 10. The nonaqueous electrolyte solution for a secondary battery according to Item 9, wherein the lithium salt is at least one selected from the group consisting of organic lithium salts having a sulfonyl group and organic lithium salts having a boron atom.

[0024] Item 11. The nonaqueous electrolyte solution for a secondary battery according to Item 9 or 10, wherein the lithium salt is at least one selected from the group consisting of lithium trifluoromethanesulfonate (LiCF3SO3), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(pentafluoroethanesulfonyl)imide (Li(C2F5SO2)2N), lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium hexafluoroarsenate (LiAsF6), lithium perchlorate (LiClO4), lithium bis(oxalato)borate (LiBOB), lithium oxalatodifluoroborate (LiBF2(C2O4)), and lithium bis(malonate)borate (LiB(C3O4H2)2).

[0025] Item 12. The nonaqueous electrolyte solution for a secondary battery according to any one of Items 9 to 11, wherein the concentration of the lithium salt is 0.3 to 2.5 mol / L.

[0026] Item 13. The nonaqueous electrolyte solution for a secondary battery according to any one of Items 1 to 12, which is an electrolyte solution for a lithium ion secondary battery.

[0027] Item 14. A non-aqueous secondary battery comprising the non-aqueous electrolyte solution for secondary batteries according to any one of Items 1 to 13.

[0028] Item 15. The nonaqueous secondary battery according to Item 14, which is a lithium ion secondary battery. [Effects of the Invention]

[0029] According to the present invention, it is possible to further improve the charge-discharge cycle characteristics of a non-aqueous secondary battery using a lithium-free transition metal sulfide as a positive electrode active material. DETAILED DESCRIPTION OF THE INVENTION

[0030] In this specification, the term "containing" is a concept that encompasses all of "comprise," "consist essentially of," and "consist only of." Furthermore, in this specification, when a numerical range is expressed as "A to B," it means A or more and B or less.

[0031] In this specification, the concentration (mol / L) of each component means that the desired number of moles is contained per 1 L of organic solvent.

[0032] 1. Electrolyte for non-aqueous secondary batteries The non-aqueous secondary battery electrolyte of the present invention is used in a non-aqueous secondary battery that uses a lithium-free transition metal sulfide as a positive electrode active material, and the electrolyte contains an organic solvent containing a cyclic carbonate compound and an additive.

[0033] (1-1) Lithium-free transition metal sulfides In the present invention, as the transition metal sulfide, lithium-free transition metal sulfides are used because lithium-containing transition metal sulfides must be handled in an inert atmosphere such as an argon gas atmosphere. Such lithium-free transition metal sulfides are not particularly limited as long as they are lithium-free transition metal sulfides used as positive electrode active materials in non-aqueous secondary batteries using the non-aqueous secondary battery electrolyte of the present invention, and are known as positive electrode active materials for lithium-ion secondary batteries. Specific examples include vanadium sulfide (lithium-free vanadium sulfide; WO 2018 / 181698), niobium sulfide and titanium niobium sulfide (lithium-free niobium sulfide and lithium-free titanium niobium sulfide; WO 2015 / 049986), molybdenum sulfide (lithium-free molybdenum sulfide), and iron sulfide (lithium-free iron sulfide). The descriptions of WO 2018 / 181698 and WO 2015 / 049986 are incorporated by reference. These lithium-free transition metal sulfides can be used alone or in combination of two or more. Among these, from the viewpoints of charge / discharge capacity, charge / discharge cycle characteristics, etc., vanadium sulfide (lithium-free vanadium sulfide; WO 2018 / 181698), molybdenum sulfide (lithium-free molybdenum sulfide), iron sulfide (lithium-free iron sulfide), etc. are preferred, and vanadium sulfide (lithium-free vanadium sulfide; WO 2018 / 181698) is more preferred.

[0034] Such lithium-free transition metal sulfides can be either crystalline or low-crystalline (or amorphous) materials. Among them, low-crystalline (or amorphous) materials are preferred because they are particularly excellent in charge / discharge capacity and charge / discharge cycle characteristics, and are likely to inhibit reaction with an organic electrolyte solution if they come into contact with the organic electrolyte solution.

[0035] In the present invention, the lithium-free transition metal sulfide has a composition ratio of sulfur to transition metal (S / M 1) is preferably in a molar ratio of 2.1 to 10, from the viewpoints that it is particularly excellent in charge / discharge capacity, charge / discharge cycle characteristics, etc., is easy to synthesize, and easily inhibits reaction with the organic electrolyte solution when it comes into contact with the organic electrolyte solution.

[0036] More specifically, the non-lithium-containing transition metal sulfide has the general formula (2): M 1 S x (2) [In the formula, M 1 represents a transition metal. x represents 2.1 to 10.] It is preferable that the composition be represented by the formula: 1 When multiple transition metals are contained as sulfur, the composition ratio of sulfur to the total amount of transition metals (S / M 1 ) is preferably in a molar ratio of 2.1 to 10.

[0037] Thus, in the present invention, the lithium-free metal sulfide is a transition metal (M 1 ) element ratio to lithium. Therefore, in the present invention, by using a lithium-free metal sulfide, it is possible to obtain a high charge / discharge capacity and excellent charge / discharge cycle characteristics. In the present invention, the higher the sulfur content (the larger x), the higher the charge / discharge capacity tends to be, and the lower the sulfur content (the smaller x), the less elemental sulfur is contained, and the higher the charge / discharge cycle characteristics tend to be. In the present invention, even if a sulfide with poor charge / discharge cycle characteristics is used, the charge / discharge cycle characteristics can be improved by using an electrolyte solution with the composition described below. Therefore, it is particularly useful to apply a polysulfide, which tends to have a high charge / discharge capacity but insufficient charge / discharge cycle characteristics. Therefore, x is preferably 2.1 to 10, more preferably 3 to 8.

[0038] Hereinafter, vanadium sulfide (lithium-free vanadium sulfide), which is a preferred lithium-free transition metal sulfide, will be described as an example.

[0039] In the present invention, the vanadium sulfide preferably has a crystal structure similar to that of crystalline vanadium (IV) tetrasulfide (VS4) (hereinafter, sometimes referred to as "VS4-type crystal structure").

[0040] More specifically, vanadium sulfide preferably has peaks at 15.4°, 35.3°, and 45.0° within a diffraction angle 2θ range of 10° to 80° in an X-ray diffraction diagram using CuKα radiation, with a tolerance of ±1.0°. In other words, it preferably has peaks in the ranges of 14.4° to 16.4°, 34.3° to 36.3°, and 44.0° to 46.0°.

[0041] In the present invention, the X-ray diffraction pattern is determined by powder X-ray diffraction measurement (θ-2θ method) under the following measurement conditions: Measuring device: D8ADVANCE (BrukerAXS) X-ray source: CuKα40kV / 40mA Measurement conditions: 2θ = 10° to 80°, 0.1° step, scanning speed 0.02° / sec Measured at.

[0042] In the present invention, vanadium sulfide preferably has a peak at the 2θ position described above, but also preferably has a peak at at least one (particularly all) of 54.0° and 56.0° within the diffraction angle 2θ range of 10° to 80°, with an allowance of ±1.0°.

[0043] In the present invention, although the vanadium sulfide has a high sulfur ratio in its average composition, sulfur is preferably hardly present as elemental sulfur, as described below, and is bonded to vanadium to form a low-crystalline sulfide. Thus, in the present invention, by reducing the crystallinity of the vanadium sulfide, more sites for lithium ion insertion and extraction are present, and the structure is more likely to have gaps that can serve as three-dimensional lithium conduction paths. Furthermore, vanadium sulfide has many advantages, such as the tendency for three-dimensional volume change during charge and discharge. Therefore, charge and discharge capacity and charge and discharge cycle characteristics can be further improved. Furthermore, it is preferable that vanadium sulfide (e.g., V2S3) used as a raw material is also almost absent. In this specification, the average composition of the sulfide refers to the elemental ratio of each element that constitutes the entire sulfide.

[0044] The term "low crystallinity" in the present invention will be explained below. In the present invention, it is preferable that the vanadium sulfide does not have peaks at 2θ=15.4°, 35.3°, and 45.0°, or even if peaks appear, the full width at half maximum of the peaks is 0.8 to 2.0° (particularly 1.0 to 2.0°). This is because the full width at half maximum of an X-ray diffraction peak is inversely proportional to the crystallite size, i.e., a large full width at half maximum of a peak indicates a relatively small crystal grain size. In crystalline vanadium(IV) sulfide (VS4), the full width at half maximum of the peaks at 2θ=15.4°, 35.3°, and 45.0° are all 0.2 to 0.6°. Thus, in the present invention, it is preferable that the vanadium sulfide does not exhibit peaks at 2θ=15.4°, 35.3°, and 45.0°, or even if a peak does appear, the full width at half maximum of the peak is larger than the full width at half maximum of the peak of crystalline vanadium(IV) sulfide (VS4). Thus, in the present invention, low crystallinity tends to increase the number of sites where Li can stably exist, and therefore, when a low-crystalline lithium-free metal sulfide is used as a positive electrode active material, it is easy to improve the charge / discharge capacity and charge / discharge cycle characteristics.

[0045] Furthermore, when a material containing a large amount of elemental sulfur or the like is used as a positive electrode active material, the cyclic carbonate compound contained in the electrolyte solution for a nonaqueous secondary battery of the present invention is likely to react with elemental sulfur. In contrast, in the present invention, for example, when mechanical milling is performed for a sufficient period of time, the above-mentioned vanadium sulfide contains almost no elemental sulfur or the like. Therefore, when used as a positive electrode active material, these problems do not arise even when a cyclic carbonate compound is used, and it is easy to dramatically improve the charge / discharge capacity and charge / discharge cycle characteristics.

[0046] More specifically, the strongest peak of sulfur (S8) is at 2θ=23.0° within a tolerance range of ±1.0°. Therefore, in an X-ray diffraction pattern using CuKα radiation, it is preferable that, within a tolerance range of ±1.0°, there is no peak characteristic of elemental sulfur having a maximum near 2θ=23.0°, or the area of ​​the peak having a maximum near 2θ=23.0° is 0 to 20%, particularly 0.1 to 19%, of the area of ​​the peak having a maximum near 2θ=35.3°. This allows the vanadium sulfide of the present invention to be a material that contains almost no elemental sulfur, thereby reducing the risk of reaction with the electrolyte and further improving the charge / discharge capacity and charge / discharge cycle characteristics.

[0047] In the present invention, the vanadium sulfide preferably does not have peaks at positions near 2θ=25.8° and 27.8°, which are peaks characteristic of elemental sulfur, within a tolerance of ±1.0°, or the area of ​​the peaks having maxima at these positions is 0 to 10%, particularly 0.1 to 8%, of the area of ​​the peak having a maxima at 2θ=35.3°. This allows the vanadium sulfide to be a material that contains almost no elemental sulfur, which reduces the risk of reaction with the electrolyte as described above and further improves the charge / discharge capacity and charge / discharge cycle characteristics.

[0048] Vanadium sulfides that satisfy these conditions preferably have a strong peak at g(r) = 2.4 Å (g(r) is the pair distribution function, which shows peaks at each interatomic distance) in X-ray / neutron atomic pair distribution function analysis (PDF analysis) within a ±0.1 Å tolerance range. However, for sulfides with better charge / discharge capacity and charge / discharge cycle characteristics, a shoulder peak at g(r) = 2.0 Å within a ±0.1 Å tolerance range is more preferable, and a peak at g(r) = 3.3 Å within a ±0.1 Å tolerance range is even more preferable. In other words, vanadium sulfides preferably have not only VS bonds but also SS bonds (disulfide bonds). This is because in VS4, the g(r) derived from the VS bond has peaks at 2.4 Å, and the g(r) derived from the SS bond has peaks at 2.0 and 3.3 Å.

[0049] In the present invention, the vanadium sulfide can be obtained, for example, by a production method using vanadium sulfide and sulfur as raw materials or intermediates, and including a step of subjecting the raw materials or intermediates to a mechanical milling method.

[0050] Mechanical milling is a method of grinding and mixing raw materials while applying mechanical energy. According to this method, the raw materials are subjected to mechanical impact and friction, which causes the vanadium sulfide and sulfur to come into vigorous contact with each other, resulting in fine particles and a reaction of the raw materials. In other words, mixing, grinding, and reaction occur simultaneously. This makes it possible to more reliably react the raw materials without heating them to high temperatures. Mechanical milling can sometimes produce a metastable crystal structure that cannot be obtained by ordinary heat treatment.

[0051] Specifically, the mechanical milling treatment can be carried out by mixing and pulverizing using a mechanical pulverizing device such as a ball mill, a bead mill, a rod mill, a vibration mill, a disk mill, a hammer mill, or a jet mill.

[0052] These raw materials or intermediates can be all mixed together and subjected to mechanical milling, or some of the materials or intermediates can be subjected to mechanical milling first, and then the remaining materials can be added and subjected to mechanical milling.

[0053] In particular, when producing vanadium sulfide with a high sulfur content (the molar ratio of sulfur to vanadium (S / V) is 3.3 or more), crystalline vanadium sulfide may be obtained depending on the charged mass. Therefore, in order to easily obtain low-crystalline vanadium sulfide with excellent charge / discharge capacity and charge / discharge cycle characteristics, it is preferable to first subject the vanadium sulfide and a portion of the sulfur to mechanical milling treatment to obtain the desired low-crystalline sulfide as an intermediate, and then subject the obtained low-crystalline sulfide and the remaining sulfur to mechanical milling treatment.

[0054] As a specific raw material, it is preferable to use crystalline vanadium(III) sulfide (V2S3) as the vanadium sulfide. There are no particular limitations on the vanadium sulfide, and any commercially available vanadium sulfide can be used. In particular, it is preferable to use a high-purity vanadium sulfide. Furthermore, since the vanadium sulfide is mixed and pulverized by mechanical milling, there are no limitations on the particle size of the vanadium sulfide used, and commercially available powdered vanadium sulfide can usually be used.

[0055] Furthermore, as sulfur, elemental sulfur (S8) can be used in an amount necessary to form a sulfide of the desired composition. There is no particular limitation on the sulfur used as a raw material, and any sulfur can be used. In particular, it is preferable to use sulfur of high purity. Furthermore, since the sulfur is mixed and pulverized by mechanical milling, there is no limitation on the particle size of the sulfur used, and commercially available powdered sulfur can usually be used.

[0056] Furthermore, as described above, when the powder is subjected to multiple (particularly two-stage) mechanical milling treatments, the intermediate product is a low-crystalline vanadium sulfide having a desired composition (low-crystalline VS2.5 etc.) can also be used.

[0057] The mixing ratio of the raw materials can be set to the same ratio as the elemental ratio of vanadium and sulfur in the target vanadium sulfide, since the charging ratio of the raw materials almost directly determines the ratio of each element in the product. For example, the ratio of sulfur to 1 mole of vanadium sulfide is preferably 1.2 moles or more (particularly, 1.2 to 17.0 moles, more preferably 3.0 to 13.0 moles).

[0058] The temperature at which the mechanical milling treatment is carried out is not particularly limited, but is preferably 300°C or lower, more preferably -10 to 200°C, in order to make it difficult for sulfur to volatilize and to make it difficult for the previously reported crystalline phase to be formed.

[0059] The time for the mechanical milling treatment is not particularly limited, and the mechanical milling treatment can be carried out for any time until the target vanadium sulfide is precipitated.

[0060] The atmosphere in which the mechanical milling treatment is carried out is not particularly limited, but an inert gas atmosphere such as a nitrogen gas atmosphere or an argon gas atmosphere can be used.

[0061] For example, the mechanical milling treatment can be carried out within a treatment time range of 0.1 to 100 hours (particularly 15 to 80 hours). Note that this mechanical milling treatment can also be carried out in multiple steps with breaks in between as needed.

[0062] When the mechanical milling process is repeated multiple times, the above conditions can be applied to each mechanical milling process.

[0063] By the mechanical milling treatment described above, the target vanadium sulfide can be obtained as a fine powder.

[0064] (1-2) Organic Solvent As described above, the nonaqueous secondary battery electrolyte of the present invention is a nonaqueous secondary battery electrolyte used in a nonaqueous secondary battery that uses a lithium-free transition metal sulfide as a positive electrode active material. Thus, in the present invention, even though the electrolyte is used in a nonaqueous secondary battery that uses a lithium-free transition metal sulfide, by adding the additive described below, it is possible to suppress the reaction between the carbonate compound and the lithium-free transition metal sulfide and dramatically improve the charge-discharge cycle characteristics.

[0065] The cyclic carbonate compound is not particularly limited as long as it can be used as an organic solvent in the electrolyte of a lithium ion secondary battery, and examples thereof include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, etc. These cyclic carbonate compounds can be used alone or in combination of two or more.

[0066] The content of the cyclic carbonate compound is preferably 80 to 100% by volume, more preferably 85 to 100% by volume, and even more preferably 90 to 100% by volume, based on 100% by volume of the total organic solvent, from the viewpoint of easily suppressing the reaction between the carbonate compound and the lithium-free transition metal sulfide and easily improving charge-discharge cycle characteristics. In the present invention, the organic solvent may be solely the cyclic carbonate compound (the content of the cyclic carbonate compound is 100% by volume), or may be other organic solvents such as a chain carbonate compound in addition to the cyclic carbonate compound (the content of the cyclic carbonate compound is 80 to 99.9% by volume, particularly 85 to 99.8% by volume, or even 90 to 99.5% by volume). However, from the viewpoint of charge-discharge cycle characteristics, it is preferable to use only the cyclic carbonate compound as the organic solvent (the content of the cyclic carbonate compound is 100% by volume).

[0067] The chain carbonate compound is not particularly limited as long as it can be used as an organic solvent in the electrolyte of a lithium ion secondary battery, and examples thereof include dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), methyl propyl carbonate, etc. These chain carbonate compounds can be used alone or in combination of two or more.

[0068] The content of the chain carbonate compound is preferably 0 to 20% by volume, more preferably 0 to 15% by volume, and even more preferably 0 to 10% by volume, of the total amount of the organic solvent as 100% by volume, from the viewpoint of easily suppressing the reaction between the carbonate compound and the lithium-free transition metal sulfide and easily improving the charge-discharge cycle characteristics. As described above, in the present invention, only a cyclic carbonate compound can be used as the organic solvent (the content of the chain carbonate compound is 0% by volume), or a chain carbonate compound can be used (the content of the chain carbonate compound is 0.1 to 20% by volume, particularly 0.2 to 15% by volume, or even 0.5 to 10% by volume). Note that, from the viewpoint of easily suppressing decomposition of the organic solvent, a small content of the chain carbonate compound is preferable, and it is particularly preferable to use only a cyclic carbonate compound as the organic solvent (the content of the chain carbonate compound is 0% by volume).

[0069] In the present invention, the organic solvent constituting the nonaqueous electrolyte solution for secondary batteries may be composed solely of the above-mentioned cyclic carbonate compound and, if necessary, a chain carbonate compound, or may contain, in addition to these, a compound known as an organic solvent in the electrolyte solution for lithium ion secondary batteries.

[0070] Examples of such organic solvents as the third component include cyclic carboxylic acid ester compounds such as γ-butyrolactone, chain carboxylic acid ester compounds such as methyl acetate, methyl propionate, and ethyl acetate, sulfone compounds such as sulfolane and diethyl sulfone, and ether compounds such as tetrahydrofuran, 2-methyltetrahydrofuran, and 1,2-dimethoxyethane. These organic solvents as the third component can be used alone or in combination of two or more.

[0071] When the organic solvent is contained as the third component, the content of the organic solvent as the third component is preferably 0.1 to 10% by volume, and more preferably 0.2 to 5% by volume, based on 100% by volume of the total amount of the organic solvent, from the viewpoint of charge-discharge cycle characteristics.

[0072] (1-3) Additives As described above, in the nonaqueous electrolyte solution for a secondary battery of the present invention, by containing an additive, it is possible to suppress the reaction between the carbonate compound and the non-lithium-containing transition metal sulfide, and dramatically improve the charge-discharge cycle characteristics.

[0073] As such an additive, from the viewpoint of easily suppressing the reaction between the carbonate compound and the lithium-free transition metal sulfide and easily improving the charge-discharge cycle characteristics, a compound represented by the general formula (1):

[0074] [ka]

[0075] [In the formula, R 1 and R 2 are the same or different and represent a hydrogen atom or a halogen atom. Bonds represented by solid and dashed lines represent single or double bonds.] and a compound represented by general formula (2):

[0076] [ka]

[0077] [In the formula, R 3 are the same or different and represent a halogen atom. M represents a counter cation. A compound represented by the following formula is preferred.

[0078] The compound represented by the above general formula (1) is represented by the general formula (1A):

[0079] [ka]

[0080] [In the formula, R 1 and R 2 are the same or different and represent a hydrogen atom or a halogen atom. and a compound represented by general formula (1B):

[0081] [ka]

[0082] [In the formula, R 1 and R 2 are the same or different and represent a hydrogen atom or a halogen atom. and a compound represented by the formula:

[0083] In the general formulas (1), (1A) and (1B), R 1 and R 2 The halogen atom represented by is not particularly limited, and examples thereof include a fluorine atom, a chlorine atom, a bromine atom, an iodine atom, etc. Among these, from the viewpoints of charge / discharge capacity, charge / discharge cycle characteristics, etc., a fluorine atom, a chlorine atom, a bromine atom, etc. are preferred, a fluorine atom, a chlorine atom, etc. are more preferred, and a fluorine atom is even more preferred.

[0084] In general formula (2), R 3The halogen atom represented by is not particularly limited, and examples thereof include a fluorine atom, a chlorine atom, a bromine atom, an iodine atom, etc. Among these, from the viewpoints of charge / discharge capacity, charge / discharge cycle characteristics, etc., a fluorine atom, a chlorine atom, a bromine atom, etc. are preferred, a fluorine atom, a chlorine atom, etc. are more preferred, and a fluorine atom is even more preferred.

[0085] In general formula (2), the counter cation represented by M is not particularly limited, and examples thereof include alkali metal ions such as lithium ion, sodium ion, potassium ion, etc. Among these, lithium ion is preferred from the viewpoints of charge / discharge capacity, charge / discharge cycle characteristics, etc.

[0086] Examples of additives that satisfy the above conditions include compounds represented by general formula (1), such as vinylene carbonate (VC), fluoroethylene carbonate (FEC), trifluoromethylethylene carbonate, and vinylethylene carbonate, and compounds represented by general formula (2), such as lithium difluoro(oxalato)borate (DFOB). These compounds represented by general formula (1) may be used alone or in combination of two or more. The compounds represented by general formula (2) may be used alone or in combination of two or more.

[0087] The additives are not particularly limited, but are preferably compounds represented by general formula (1) from the viewpoint of charge / discharge capacity, charge / discharge cycle characteristics, and the like.

[0088] The above-mentioned additives can be used alone or in combination of two or more. When two or more additives are used in combination, even if an additive that is preferably added in a small amount, such as vinylene carbonate (VC), is used, the charge-discharge cycle characteristics can be improved even if the total content of the additives is increased.

[0089] The content of the additives is preferably 0.5 to 20.0 parts by mass, more preferably 0.7 to 15.0 parts by mass, and even more preferably 1.0 to 10.0 parts by mass, relative to 100 parts by mass of the organic solvent, from the viewpoints of charge / discharge capacity, charge / discharge cycle characteristics, energy density, etc. However, when vinylene carbonate (VC) or a compound represented by general formula (2) is used alone as the additive, a small amount is more likely to improve the charge / discharge cycle characteristics, so the content is preferably 0.5 to 5.0 parts by mass, more preferably 0.7 to 3.0 parts by mass, and even more preferably 1.0 to 2.0 parts by mass, relative to 100 parts by mass of the organic solvent. Even when only one additive is used, such as fluoroethylene carbonate (FEC), trifluoromethylethylene carbonate, or vinylethylene carbonate, increasing the amount of additive tends to improve charge-discharge cycle performance, so the amount is preferably 0.5 to 20.0 parts by mass, more preferably 0.7 to 15.0 parts by mass, and even more preferably 1.0 to 10.0 parts by mass, relative to 100 parts by mass of the organic solvent. When two or more additives are used, even when an additive that is preferably added in small amounts, such as vinylene carbonate (VC), is used, increasing the total amount of additives tends to improve charge-discharge cycle performance and energy density, so the total amount of additives is preferably 1.0 to 20.0 parts by mass, more preferably 1.5 to 15.0 parts by mass, and even more preferably 2.0 to 10.0 parts by mass, relative to 100 parts by mass of the organic solvent.

[0090] (1-4) Lithium salt The nonaqueous secondary battery electrolyte solution of the present invention preferably further contains a lithium salt. The lithium salt is not particularly limited, and examples thereof include organic lithium salts having a sulfonyl group and organic lithium salts having a boron atom. From the viewpoint of the charge-discharge cycle characteristics described above, when a lithium salt is contained, organic lithium salts having a sulfonyl group and organic lithium salts having a boron atom are preferred over inorganic lithium salts (e.g., LiPF6, LiBF4).

[0091] The organic lithium salt having a sulfonyl group is not particularly limited as long as it is one that has conventionally been used in electrolytes for nonaqueous secondary batteries, and examples thereof include lithium trifluoromethanesulfonate (LiCF3SO3); organic lithium salts having a perfluoroalkanesulfonyl group (lithium bis(trifluoromethanesulfonyl)imide (LiTFSI; Li(CF3SO2)2N), lithium bis(pentafluoroethanesulfonyl)imide (Li(C2F5SO2)2N, etc.). Among these, from the viewpoints of withstanding charging at higher voltages and further improving charge-discharge cycle characteristics, organic lithium salts having a perfluoroalkanesulfonyl group are preferred, and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI; Li(CF3SO2)2N) is more preferred. These organic lithium salts having a sulfonyl group may be used alone or in combination of two or more.

[0092] The organic lithium salt containing a boron atom is not particularly limited as long as it is one that has conventionally been used in electrolytes for nonaqueous secondary batteries, and examples thereof include lithium bis(oxalate)borate (LiBOB; LiB(CO)), lithium oxalate difluoroborate (LiBF(CO)), and lithium bis(malonate)borate (LiB(COH)). Of these, lithium bis(oxalate)borate (LiBOB; LiB(CO)) is preferred from the viewpoint of withstanding higher voltage charging and further improving charge-discharge cycle characteristics. These organic lithium salts containing a boron atom may be used alone or in combination of two or more.

[0093] As the lithium salt, an organic lithium salt having a sulfonyl group is preferred, and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI; Li(CFSO)N) is more preferred, from the viewpoint of charge-discharge cycle characteristics, in consideration of the effect of reactivity with sulfur on charge-discharge cycle characteristics, since the nonaqueous secondary battery of the present invention uses a lithium-free metal sulfide as the positive electrode active material.

[0094] In the nonaqueous electrolyte solution for a secondary battery of the present invention, the concentration of the lithium salt is not particularly limited, but is preferably 0.3 to 2.5 mol / L, more preferably 1.0 to 2.0 mol / L, from the viewpoint of charge-discharge cycle characteristics.

[0095] (1-5) Other The nonaqueous secondary battery electrolyte solution of the present invention may contain components other than those described above, such as other additives, as long as the components do not impair the effects of the present invention (e.g., 0.01 to 0.2 mol / L, particularly 0.02 to 0.1 mol / L). Examples of such other additives include tetrabutylammonium hexafluorophosphate, tetrabutylammonium perchlorate, tetramethylammonium tetrafluoroborate, tetramethylammonium chloride, tetraethylammonium chloride, tetrabutylammonium chloride, tetramethylammonium bromide, tetraethylammonium bromide, tetrabutylammonium bromide, biphenyl, and trialkyl phosphates (e.g., trimethyl phosphate). These other additives may be used alone or in combination of two or more.

[0096] The non-aqueous electrolyte solution for a secondary battery of the present invention is usually in a liquid state, but a gel electrolyte gelled with a gelling agent made of a polymer or the like can also be used.

[0097] 2.Non-aqueous secondary battery The nonaqueous secondary battery of the present invention includes the above-described nonaqueous secondary battery electrolyte. Other configurations and structures may be those employed in conventionally known nonaqueous secondary batteries. In general, the nonaqueous secondary battery of the present invention includes, in addition to the above-described nonaqueous secondary battery electrolyte, a positive electrode, a negative electrode, and a separator.

[0098] (2-1) Positive electrode The positive electrode may have a configuration in which a positive electrode mixture layer containing a positive electrode active material, a binder, etc. is formed on one or both sides of a positive electrode current collector.

[0099] This positive electrode mixture layer can be produced through a process in which a binder is added to a positive electrode active material and an optional conductive additive, and the resulting mixture is dispersed in an organic solvent to prepare a paste for forming the positive electrode mixture layer (in this case, the binder may be dissolved or dispersed in the organic solvent in advance), which is then applied to the surface (one or both sides) of a positive electrode current collector made of metal foil or the like, dried to form the positive electrode mixture layer, and processed as necessary.

[0100] The positive electrode active material is the above-described lithium-free metal sulfide, and the details of the lithium-free metal sulfide are the same as those described above.

[0101] As in ordinary nonaqueous secondary batteries, the conductive additive may be graphite, carbon black (acetylene black, ketjen black, etc.), amorphous carbon materials such as carbon materials having amorphous carbon formed on the surface, fibrous carbon (vapor-grown carbon fiber, carbon fiber obtained by spinning pitch and then carbonizing it, etc.), carbon nanotubes (various multi-layer or single-layer carbon nanotubes), etc. The conductive additive for the positive electrode may be used alone or in combination of two or more.

[0102] Examples of binders include polyvinylidene fluoride (PVDF), polytetrafluoroethylene, polyacrylic acid, styrene butadiene rubber, polyimide, polyvinyl alcohol, and water-soluble carboxymethyl cellulose.

[0103] The organic solvent used in producing the positive electrode mixture is not particularly limited, and examples thereof include N-methylpyrrolidone (NMP), which can be made into a paste by using the organic solvent together with the positive electrode active material, a binder, and the like.

[0104] The positive electrode mixture layer preferably contains, for example, about 70 to 95% by weight of the above-mentioned positive electrode active material and about 1 to 30% by weight of the binder. When a conductive additive is used, the positive electrode mixture layer preferably contains about 50 to 90% by weight of the above-mentioned positive electrode active material, about 1 to 20% by weight of the binder, and about 1 to 40% by weight of the conductive additive. Furthermore, the thickness of the positive electrode mixture layer is preferably about 1 to 100 μm per side of the current collector.

[0105] The positive electrode current collector may be, for example, a foil, punched metal, expanded metal, or mesh made of aluminum, stainless steel, nickel, titanium, or an alloy thereof, and typically, an aluminum foil having a thickness of about 10 to 30 μm is preferably used.

[0106] (2-2) Negative electrode The negative electrode may have a configuration in which a negative electrode mixture layer containing a negative electrode active material, a binder, etc. is formed on one or both sides of a negative electrode current collector.

[0107] This negative electrode mixture layer can be produced by mixing a negative electrode active material, a conductive additive added as needed, and a binder, forming the mixture into a sheet, and then pressing the sheet onto the surface (one or both sides) of a negative electrode current collector made of metal foil or the like.

[0108] The negative electrode active material is not particularly limited, and examples thereof include graphite (natural graphite, artificial graphite, etc.), non-sinterable carbon, lithium metal, tin, silicon, and alloys containing these, and SiO. Preferably, lithium metal, lithium alloys, etc. can be used in metal lithium primary batteries and metal lithium secondary batteries, and materials that can be doped and dedoped with lithium ions (graphite (natural graphite, artificial graphite, etc.), non-sinterable carbon, etc.) can be used as the active material in lithium ion secondary batteries. These negative electrode active materials may be used alone or in combination of two or more.

[0109] As in the case of ordinary nonaqueous secondary batteries, the conductive additive may be graphite; carbon black (acetylene black, ketjen black, etc.); amorphous carbon materials such as carbon materials having amorphous carbon formed on the surface; fibrous carbon (vapor-grown carbon fiber, carbon fiber obtained by spinning pitch and then carbonizing it, etc.); carbon nanotubes (various multi-layer or single-layer carbon nanotubes), etc. The conductive additive for the negative electrode may be used alone or in combination of two or more kinds, or may not be used if the negative electrode active material has high conductivity.

[0110] Examples of binders include polyvinylidene fluoride (PVDF), polytetrafluoroethylene, polyacrylic acid, styrene butadiene rubber, polyimide, polyvinyl alcohol, and water-soluble carboxymethyl cellulose.

[0111] The negative electrode mixture layer preferably contains, for example, about 70 to 95% by weight of the above-mentioned negative electrode active material and about 1 to 30% by weight of a binder. When a conductive additive is used, the negative electrode mixture layer preferably contains about 50 to 90% by weight of the above-mentioned negative electrode active material, about 1 to 20% by weight of a binder, and about 1 to 40% by weight of a conductive additive. Furthermore, the thickness of the negative electrode mixture layer is preferably about 1 to 100 μm per side of the current collector.

[0112] The negative electrode current collector may be, for example, a foil made of aluminum, copper, stainless steel, nickel, titanium, or an alloy thereof, a punched metal, an expanded metal, a mesh, a net, or the like, and typically, a copper foil having a thickness of about 5 to 30 μm is preferably used.

[0113] (2-3) Separator The positive electrode and negative electrode described above can be used in the form of, for example, a laminated electrode body in which they are laminated with a separator interposed therebetween, or in the form of a wound electrode body in which this is further wound in a spiral shape.

[0114] The separator should have sufficient strength and be able to retain a large amount of electrolyte. From this perspective, a microporous film or nonwoven fabric having a thickness of 10 to 50 μm and an opening ratio of 30 to 70% and containing one or more of polyethylene, polypropylene, ethylene-propylene copolymer, etc. is preferred.

[0115] The nonaqueous secondary battery of the present invention may be in the form of a tubular (rectangular or cylindrical) shape using a stainless steel can, an aluminum can, or the like as an outer can. A soft package battery may also be used, in which the outer casing is a laminate film integrated with a metal foil. [Example]

[0116] The present invention will be described in detail below based on examples, but it goes without saying that the present invention is not limited to the following examples.

[0117] Synthesis Example 1: Synthesis of vanadium sulfide (positive electrode active material) Commercially available vanadium(III) sulfide (VS; manufactured by Kojundo Chemical Laboratory Co., Ltd.) and sulfur (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) were weighed in a molar ratio of 1:6 in an argon gas glove box (dew point -80°C) and sealed in a glass tube under vacuum. The vacuum-sealed sample was calcined in a tube furnace at 400°C for 5 hours. The calcined sample was then calcined in vacuum at 200°C for 8 hours to desulfurize the excess sulfur and synthesize crystalline vanadium sulfide VS4 (c-VS4).

[0118] Next, the obtained crystalline VS4 (c-VS4) was subjected to mechanical milling (ball diameter 4 mm, rotation speed 270 rpm) in a ball mill (Fritsch PL-7) in an argon gas glove box (dew point -80°C) for 40 hours to synthesize low-crystalline vanadium sulfide VS4 (a-VS4), which was used as the positive electrode active material. Powder XRD analysis of the obtained a-VS4 showed no clear peaks other than the very small peak of the very small impurity VO, indicating that it was completely amorphous.

[0119] Example 1: VC 1.0% by mass / EC+PC (50:50) / LiTFSI 1.0M Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) was added to a mixed solvent of ethylene carbonate (EC) and propylene carbonate (PC) (EC:PC=50:50 (volume ratio)) to a concentration of 1.0 mol / L (volume based on the solvent), and further, 1.0 part by mass of vinylene carbonate (VC) was added relative to 100 parts by mass of the mixed solvent, thereby obtaining a nonaqueous electrolyte solution for a secondary battery of Example 1.

[0120] Example 2: VC 2.0% by mass / EC+PC (50:50) / LiTFSI 1.0M Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) was added to a mixed solvent of ethylene carbonate (EC) and propylene carbonate (PC) (EC:PC=50:50 (volume ratio)) to a concentration of 1.0 mol / L (volume based on the solvent), and further, 2.0 parts by mass of vinylene carbonate (VC) was added relative to 100 parts by mass of the mixed solvent, thereby obtaining a nonaqueous electrolyte solution for a secondary battery of Example 2.

[0121] Example 3: VC 5.0% by mass / EC+PC (50:50) / LiTFSI 1.0M Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) was added to a mixed solvent of ethylene carbonate (EC) and propylene carbonate (PC) (EC:PC=50:50 (volume ratio)) to a concentration of 1.0 mol / L (volume based on the solvent), and further, 5.0 parts by mass of vinylene carbonate (VC) was added relative to 100 parts by mass of the mixed solvent, thereby obtaining a nonaqueous electrolyte solution for a secondary battery of Example 3.

[0122] Example 4: FEC 3.0% by mass / EC+PC (50:50) / LiTFSI 1.0M Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) was added to a mixed solvent of ethylene carbonate (EC) and propylene carbonate (PC) (EC:PC=50:50 (volume ratio)) to a concentration of 1.0 mol / L (volume based on the solvent), and 3.0 parts by mass of fluoroethylene carbonate (FEC) was further added relative to 100 parts by mass of the mixed solvent, thereby obtaining a nonaqueous electrolyte solution for a secondary battery of Example 4.

[0123] Example 5: FEC 1.0 mass% / EC + PC (50:50) / LiTFSI 1.0MLithium bis(trifluoromethanesulfonyl)imide (LiTFSI) was added to a mixed solvent of ethylene carbonate (EC) and propylene carbonate (PC) (EC:PC=50:50 (volume ratio)) to a concentration of 1.0 mol / L (volume based on the solvent), and 1.0 part by mass of fluoroethylene carbonate (FEC) was further added relative to 100 parts by mass of the mixed solvent, thereby obtaining a nonaqueous electrolyte solution for a secondary battery of Example 5.

[0124] Example 6: FEC 5.0% by mass / EC+PC (50:50) / LiTFSI 1.0M Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) was added to a mixed solvent of ethylene carbonate (EC) and propylene carbonate (PC) (EC:PC=50:50 (volume ratio)) to a concentration of 1.0 mol / L (volume based on the solvent), and 5.0 parts by mass of fluoroethylene carbonate (FEC) was further added relative to 100 parts by mass of the mixed solvent to obtain a nonaqueous secondary battery electrolyte solution of Example 6.

[0125] Example 7: DFOB 2.0% by mass / EC+PC (50:50) / LiTFSI 1.0M Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) was added to a mixed solvent of ethylene carbonate (EC) and propylene carbonate (PC) (EC:PC=50:50 (volume ratio)) to a concentration of 1.0 mol / L (volume based on the solvent), and further, 2.0 parts by mass of lithium difluoro(oxalato)borate (DFOB) was added relative to 100 parts by mass of the mixed solvent, thereby obtaining a nonaqueous electrolyte solution for a secondary battery of Example 7.

[0126] Example 8: FEC 10.0 mass% / EC + PC (50:50) / LiTFSI 1.0M Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) was added to a mixed solvent of ethylene carbonate (EC) and propylene carbonate (PC) (EC:PC = 50:50 (volume ratio)) to a concentration of 1.0 mol / L (volume based on the solvent), and 10.0 parts by mass of fluoroethylene carbonate (FEC) was further added relative to 100 parts by mass of the mixed solvent, thereby obtaining a nonaqueous electrolyte solution for a secondary battery of Example 6.

[0127] Example 9: FEC 10.0 mass% / EC + PC (50:50) / LiTFSI 2.0MLithium bis(trifluoromethanesulfonyl)imide (LiTFSI) was added to a mixed solvent of ethylene carbonate (EC) and propylene carbonate (PC) (EC:PC = 50:50 (volume ratio)) to a concentration of 2.0 mol / L (volume based on the solvent), and 10.0 parts by mass of fluoroethylene carbonate (FEC) was further added relative to 100 parts by mass of the mixed solvent to obtain a nonaqueous secondary battery electrolyte solution of Example 9.

[0128] Example 10: FEC 5.0% by mass / EC+PC (50:50) / LiTFSI 2.0M Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) was added to a mixed solvent of ethylene carbonate (EC) and propylene carbonate (PC) (EC:PC = 50:50 (volume ratio)) to a concentration of 2.0 mol / L (volume based on the solvent), and 5.0 parts by mass of fluoroethylene carbonate (FEC) was further added relative to 100 parts by mass of the mixed solvent to obtain a nonaqueous secondary battery electrolyte solution of Example 10.

[0129] Example 11: FEC 3.0% by mass / EC+PC (50:50) / LiTFSI 2.0M Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) was added to a mixed solvent of ethylene carbonate (EC) and propylene carbonate (PC) (EC:PC = 50:50 (volume ratio)) to a concentration of 2.0 mol / L (volume based on the solvent), and 3.0 parts by mass of fluoroethylene carbonate (FEC) was further added relative to 100 parts by mass of the mixed solvent to obtain a nonaqueous secondary battery electrolyte solution of Example 11.

[0130] Example 12: FEC 8.0% by mass / EC+PC (50:50) / LiTFSI 1.0M Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) was added to a mixed solvent of ethylene carbonate (EC) and propylene carbonate (PC) (EC:PC = 50:50 (volume ratio)) to a concentration of 1.0 mol / L (volume based on the solvent), and 8.0 parts by mass of fluoroethylene carbonate (FEC) was further added relative to 100 parts by mass of the mixed solvent, thereby obtaining a nonaqueous secondary battery electrolyte solution of Example 12.

[0131] Example 13: FEC 1.0 mass% / EC + PC (10:90) / LiTFSI 1.0MLithium bis(trifluoromethanesulfonyl)imide (LiTFSI) was added to a mixed solvent of ethylene carbonate (EC) and propylene carbonate (PC) (EC:PC = 10:90 (volume ratio)) to a concentration of 1.0 mol / L (volume based on the solvent), and 1.0 part by mass of fluoroethylene carbonate (FEC) was further added relative to 100 parts by mass of the mixed solvent, thereby obtaining a nonaqueous secondary battery electrolyte solution of Example 13.

[0132] Example 14: FEC5.0 mass%+VC5.0 mass% / EC+PC(50:50) / LiTFSI1.0M Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) was added to a mixed solvent of ethylene carbonate (EC) and propylene carbonate (PC) (EC:PC = 50:50 (volume ratio)) to a concentration of 1.0 mol / L (volume based on the solvent), and further, 5.0 parts by mass of fluoroethylene carbonate (FEC) and 5.0 parts by mass of vinylene carbonate (VC) were added per 100 parts by mass of the mixed solvent to obtain a nonaqueous secondary battery electrolyte solution of Example 14.

[0133] Comparative Example 1: No additive / EC+PC (50:50) / LiTFSI 1.0M Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) was added to a mixed solvent of ethylene carbonate (EC) and propylene carbonate (PC) (EC:PC=50:50 (volume ratio)) to a concentration of 1.0 mol / L (volume based on the solvent), to obtain a nonaqueous secondary battery electrolyte solution of Comparative Example 1.

[0134] Comparative Example 2: No additive / EC+PC (10:90) Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) was added to a mixed solvent of ethylene carbonate (EC) and propylene carbonate (PC) (EC:PC=10:90 (volume ratio)) to a concentration of 1.0 mol / L (volume based on the solvent), to obtain a nonaqueous secondary battery electrolyte solution of Comparative Example 2.

[0135] Test example 1: Charge / discharge test Using the VS4 powder obtained in Synthesis Example 1 as a positive electrode active material and the nonaqueous secondary battery electrolyte solutions obtained in Examples 1 to 13 and Comparative Examples 1 and 2, test electrochemical cells (lithium secondary batteries) were produced by the following method, and constant-current charge / discharge measurements were performed for 100 cycles at 25°C, a charge / discharge rate of 0.1 C (1 C = 747 mAh / g), a voltage range of 2.6 to 1.5 V, and a rest time of 10 minutes between cycles.

[0136] The test electrochemical cell was prepared by first adding 1 mg of Ketjen Black and 1 mg of polytetrafluoroethylene (PTFE) as a binder to 10 mg of the VS4 powder obtained in Synthesis Example 1, mixing them in a mortar for 8 minutes, and then attaching them to an aluminum mesh to form a working electrode (positive electrode). Lithium metal was used as the counter electrode (negative electrode). Polypropylene was used as the separator. The results of the charge-discharge cycle characteristics (capacity retention rate at 100 cycles) are shown in Table 1. The capacity retention rate is the ratio of the capacity measured after 100 cycles, with the capacity at the start of the cycle test (first cycle) set to 100. A higher capacity retention rate indicates better battery life characteristics.

[0137] [Table 1]

[0138] There is no particular threshold for determining the capacity retention rate, but if an average charge / discharge efficiency of at least 99.5% in each cycle is considered to be a good lifespan characteristic, then after 100 cycles, it is 0.995 100 =0.606, and all of the Examples showed a capacity retention rate of more than 60% after 100 cycles, while all of the Comparative Examples showed a capacity retention rate of less than 60%. Therefore, the Examples of the present invention have excellent life characteristics. [Industrial Applicability]

[0139] The nonaqueous secondary battery electrolyte solution of the present invention and the nonaqueous secondary battery using the same can be used in a variety of known applications, including, for example, notebook computers, mobile phones, electric vehicles, load-leveling power sources, and natural energy storage power sources.

Claims

1. A non-aqueous secondary battery electrolyte solution used in a non-aqueous secondary battery that uses a lithium-free transition metal sulfide as a positive electrode active material, The electrolytic solution contains an organic solvent containing a cyclic carbonate compound and an additive, The lithium-free transition metal sulfide is at least one selected from the group consisting of vanadium sulfide and molybdenum sulfide, The additive is represented by the general formula (1): 【Chemistry 1】 [In the formula, R 1 and R 2 are the same or different and represent a hydrogen atom or a halogen atom. Bonds represented by solid and dashed lines represent single or double bonds.] or a compound represented by general formula (2): 【Chemistry 2】 [In the formula, R 3 s are the same or different and each represents a halogen atom; and M represents a counter cation.] is a compound represented by The content of the additive is 0.5 to 20 parts by mass relative to 100 parts by mass of the organic solvent. Electrolyte for non-aqueous secondary batteries.

2. the compound represented by the general formula (1) contains at least one selected from the group consisting of vinylene carbonate (VC), fluoroethylene carbonate (FEC), trifluoromethylethylene carbonate, and vinylethylene carbonate; 2. The nonaqueous electrolyte solution for a secondary battery according to claim 1, wherein the compound represented by the general formula (2) contains lithium difluoro(oxalato)borate (DFOB).

3. 3. The nonaqueous electrolyte solution for a secondary battery according to claim 1, wherein the content of the cyclic carbonate compound is 80 to 100% by volume and the content of the chain carbonate compound is 0 to 20% by volume, where the total amount of the organic solvent is 100% by volume.

4. 4. The nonaqueous secondary battery electrolyte according to claim 3, wherein the chain carbonate compound is at least one selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and methyl propyl carbonate.

5. 5. The non-aqueous secondary battery electrolyte solution according to claim 1, wherein the cyclic carbonate compound is at least one selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate.

6. The nonaqueous electrolyte solution for a secondary battery according to any one of claims 1 to 5, further comprising a lithium salt.

7. 7. The nonaqueous electrolyte solution for a secondary battery according to claim 6, wherein the lithium salt is at least one selected from the group consisting of organic lithium salts having a sulfonyl group and organic lithium salts having a boron atom.

8. The lithium salt is lithium trifluoromethanesulfonate (LiCF ３ SO ３ ), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(pentafluoroethanesulfonyl)imide (Li(C ２ F ５ SO ２ ) ２ N), lithium hexafluorophosphate (LiPF ６ ), lithium tetrafluoroborate (LiBF ４ ), lithium hexafluoroarsenate (LiAsF ６ ), lithium perchlorate (LiClO ４ ), lithium bis(oxalato)borate (LiBOB), lithium oxalatodifluoroborate (LiBF ２ (C ２ O ４ )), and lithium bis(malonate)borate (LiB(C ３ O ４ H ２ ) ２ 8. The nonaqueous electrolyte solution for a secondary battery according to claim 6, wherein the nonaqueous electrolyte solution for a secondary battery is at least one selected from the group consisting of:

9. 9. The nonaqueous electrolyte solution for a secondary battery according to claim 6, wherein the concentration of the lithium salt is 0.3 to 2.5 mol / L.

10. The nonaqueous secondary battery electrolyte according to any one of claims 1 to 9, which is an electrolyte for a lithium ion secondary battery.

11. A non-aqueous secondary battery comprising the non-aqueous secondary battery electrolyte solution according to any one of claims 1 to 10.

12. The nonaqueous secondary battery according to claim 11, which is a lithium ion secondary battery.

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