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

Abstract

To provide a discharge method 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 a method of discharging a non-aqueous secondary battery in which lithium-free transition metal sulfide is used as a positive electrode active material. A depth of discharge in a charge and discharge cycle is set to be 70-90%.SELECTED DRAWING: None

Description

[Technical field]

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

[0002] In recent years, the increasing performance of portable electronic devices, hybrid vehicles, etc. has led to a demand for increasingly higher capacity lithium-ion secondary batteries used in them. However, current lithium-ion secondary batteries have insufficient capacity in the positive electrode compared to the negative electrode, and even lithium nickel oxide-based materials, which are said to have relatively high capacity, have a capacity of only about 190 to 220 mAh / g.

[0003] On the other hand, sulfur has a high theoretical capacity of approximately 1670 mAh / g, and is expected to be used as a positive electrode active material, but it is generally known that the capacity of sulfur-based positive electrode active materials decreases with repeated charge-discharge cycles. This is because sulfur dissolves into the organic electrolyte as lithium polysulfide during charging and discharging, so technology to suppress its dissolution into the organic electrolyte is essential.

[0004] Although non-lithium-containing transition metal sulfides (transition metal sulfides that do not contain lithium) have electronic conductivity and are less likely to dissolve in organic electrolytes, this is not sufficient. For example, vanadium sulfide is one of the non-lithium-containing transition metal sulfides, and crystalline vanadium(III) sulfide (V 2 S 3 When vanadium sulfide having a specific composition 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 a 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 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 continuing demand for higher performance lithium ion secondary batteries, and further improvements in the charge-discharge cycle characteristics are required.

[0007] The present invention has been made in consideration of the current state of the prior art described above, and its main object is to provide a discharge method capable of improving the charge-discharge cycle characteristics in a non-aqueous secondary battery using a lithium-free transition metal sulfide as a positive electrode active material. [Means for solving the problem]

[0008] The present inventors have conducted extensive research to achieve the above-mentioned object. As a result, they have found that the charge-discharge cycle characteristics can be further improved by controlling the depth of discharge during the charge-discharge cycle within a certain range, rather than 100%. The present invention has been completed as a result of further research based on such findings. That is, the present invention includes the following configurations.

[0009] Item 1. A method for discharging a non-aqueous secondary battery that uses a lithium-free transition metal sulfide as a positive electrode active material, comprising: A discharge method in which the depth of discharge during the charge / discharge cycle is 70-90%.

[0010] Item 2. The discharge method according to Item 1, wherein the non-lithium-containing transition metal sulfide is at least one selected from the group consisting of vanadium sulfide, molybdenum sulfide, and iron sulfide.

[0011] Item 3. The lithium-free transition metal sulfide is VS 4 If VS 4 +xLi⇔Li x VS 4 3. In the charge / discharge reaction shown below, x is adjusted to be 3.50 to 4.50, assuming that the state of charge is 100% when x=5.0.

[0012] Item 4. The nonaqueous secondary battery further contains an electrolyte solution, 4. The discharging method according to any one of items 1 to 3, wherein the electrolyte solution contains an organic solvent containing a cyclic carbonate compound.

[0013] Item 5. The discharge method according to item 4, wherein the cyclic carbonate compound is at least one selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC) and butylene carbonate.

[0014] Item 6. The discharging method according to item 4 or 5, wherein the content of the cyclic carbonate compound is 80 to 100 volume % and the content of the chain carbonate compound is 0 to 20 volume % relative to 100 volume % of the total amount of the organic solvent.

[0015] Item 7. The discharging method according to item 6, 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.

[0016] Item 8. The discharging method according to any one of Items 4 to 7, wherein the electrolyte further contains a lithium salt.

[0017] Item 9. The discharging method according to item 8, wherein the lithium salt is at least one selected from the group consisting of organic lithium salts having a sulfonyl group, inorganic lithium salts, and organic lithium salts having a boron atom.

[0018] Item 10. 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 Item 10. The method for discharging according to item 8 or 9, wherein the discharge electrode is at least one selected from the group consisting of:

[0019] Item 11. The discharging method according to any one of Items 8 to 10, wherein the concentration of the lithium salt in the electrolyte solution is 0.3 to 2.0 mol / L.

[0020] Item 12. The discharging method according to any one of Items 1 to 11, wherein the nonaqueous secondary battery is a lithium ion secondary battery. Effect of the Invention

[0021] According to the present invention, in a non-aqueous secondary battery using a lithium-free transition metal sulfide as a positive electrode active material, the charge-discharge cycle characteristics can be further improved. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] In this specification, "containing" is a concept that encompasses all of "comprise," "consist essentially of," and "consist only of." In addition, in this specification, when a numerical range is shown as "A to B," it means A or more and B or less.

[0023] 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.

[0024] The discharge method of the present invention is a method for discharging a non-aqueous secondary battery that uses a lithium-free transition metal sulfide as a positive electrode active material, and the depth of discharge during charge-discharge cycles is set to 70 to 90%.

[0025] 1.Non-aqueous secondary battery As described above, the nonaqueous secondary battery employing the discharge method of the present invention uses a lithium-free transition metal sulfide as the positive electrode active material.

[0026] (1-1) Lithium-free transition metal sulfides In the present invention, as the transition metal sulfide, a lithium-free transition metal sulfide is used because those containing lithium need to be handled under 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 a positive electrode active material in a nonaqueous secondary battery employing the discharge method of the present invention and are known as a positive electrode active material for lithium ion secondary batteries. Specifically, vanadium sulfide (lithium-free vanadium sulfide; International Publication No. 2018 / 181698), niobium sulfide and titanium niobium sulfide (lithium-free niobium sulfide and lithium-free titanium niobium sulfide; International Publication No. 2015 / 049986), molybdenum sulfide (lithium-free molybdenum sulfide), iron sulfide (lithium-free iron sulfide), and the like. The descriptions in 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, vanadium sulfide (lithium-free vanadium sulfide; WO 2018 / 181698), molybdenum sulfide (lithium-free molybdenum sulfide), iron sulfide (lithium-free iron sulfide), etc. are preferred from the viewpoints of charge / discharge capacity, charge / discharge cycle characteristics, etc., and vanadium sulfide (lithium-free vanadium sulfide; WO 2018 / 181698) is more preferred.

[0027] Such lithium-free transition metal sulfides may 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 easily inhibited from reacting with an organic electrolyte when in contact with the organic electrolyte.

[0028] 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 of being particularly excellent in charge / discharge capacity, charge / discharge cycle characteristics, etc., being easy to synthesize, and being easily inhibited from reacting with the organic electrolyte solution when it comes into contact with the organic electrolyte solution.

[0029] More specifically, the non-lithium-containing transition metal sulfide has the general formula (1): M 1 S y (1) [In the formula, M 1 represents a transition metal. y represents 2.1 to 10. It is preferable that the composition is 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.

[0030] Thus, in the present invention, the non-lithium-containing metal sulfide is a transition metal (M 1 ) element ratio. Therefore, in the present invention, by using a lithium-free metal sulfide, it is possible to have a high charge / discharge capacity and excellent charge / discharge cycle characteristics. In the present invention, the higher the sulfur content (the larger y), the higher the charge / discharge capacity, and the lower the sulfur content (the smaller y), the less elemental sulfur is contained and the higher the charge / discharge cycle characteristics. 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 a composition described later, so that it is particularly useful to apply a polysulfide that is likely to have a high charge / discharge capacity but insufficient charge / discharge cycle characteristics. For this reason, y is preferably 2.1 to 10, more preferably 3 to 8.

[0031] Hereinafter, vanadium sulfide (lithium-free vanadium sulfide), which is a preferred lithium-free transition metal sulfide, will be described as an example.

[0032] In the present invention, vanadium sulfide is crystalline vanadium(IV) tetrasulfide (VS 4) and preferably has a crystal structure similar to (hereinafter sometimes referred to as "VS 4 type crystal structure").

[0033] More specifically, the vanadium sulfide preferably has peaks at 15.4°, 35.3° and 45.0° within the range of diffraction angle 2θ = 10° to 80° in the X-ray diffraction pattern by CuKα ray, with an allowable range of ±1.0°. That is, it preferably has peaks in the ranges of 14.4° to 16.4°, 34.3° to 36.3° and 44.0° to 46.0°.

[0034] In the present invention, the X-ray diffraction pattern is obtained by the powder X-ray diffraction measurement method (θ-2θ method), and the following measurement conditions: Measuring device: D8 ADVANCE (Bruker AXS) X-ray source: CuKα 40 kV / 40 mA Measurement conditions: 2θ = 10° to 80°, 0.1° step, scanning speed 0.02° / second are used for measurement.

[0035] In the present invention, the vanadium sulfide preferably has peaks at the above-mentioned 2θ positions, but within the range of diffraction angle 2θ = 10° to 80°, it preferably has peaks at at least one position (especially all) of 54.0° and 56.0° with an allowable range of ±1.0°.

[0036] In the present invention, although the vanadium sulfide has a high ratio of sulfur as an average composition, sulfur hardly exists as elemental sulfur as described later, and it is preferable that sulfur combines with vanadium to form a low-crystalline sulfide. Thus, in the present invention, by lowering the crystallinity of the vanadium sulfide, more sites where lithium ions can be inserted and desorbed exist, and it is possible to structurally have gaps that can serve as three-dimensional lithium conduction paths more easily. Also, it has many advantages such as being likely to undergo three-dimensional volume changes during charge and discharge. Therefore, the charge-discharge capacity and charge-discharge cycle characteristics can be further improved. Also, vanadium sulfide (V used as a raw material2 S 3 In this specification, the average composition of the sulfide indicates the element ratio of each element constituting the entire sulfide.

[0037] 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 all 0.8 to 2.0° (particularly 1.0 to 2.0°). Note that crystalline vanadium(IV) sulfide (VS 4 In the vanadium sulfide of the present invention, the full width at half maximum of the peaks at 2θ=15.4°, 35.3°, and 45.0° is 0.2 to 0.6°. Thus, in the vanadium sulfide of the present invention, there is no peak at 2θ=15.4°, 35.3°, and 45.0°, or even if a peak appears, the full width at half maximum of the peak is smaller than that of crystalline vanadium(IV) sulfide (VS 4 ) is preferably larger than that of the metal sulfide of the present invention. In this way, since the low crystallinity in the present invention tends to increase the number of sites where Li can exist stably, the use of the metal sulfide of the present invention as a positive electrode active material tends to improve the charge / discharge capacity and the charge / discharge cycle characteristics.

[0038] 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 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 occur even when a cyclic carbonate compound is used, and it is easy to dramatically improve the charge / discharge capacity and charge / discharge cycle characteristics.

[0039] More specifically, sulfur (S 8) is present at 2θ=23.0° within a tolerance range of ±1.0°. From this, it is preferable that, in the X-ray diffraction pattern by CuKα radiation, there is no peak having a maximum at 2θ=23.0°, which is a peak characteristic of elemental sulfur, within a tolerance range of ±1.0°, or the area of ​​the peak having a maximum at 2θ=23.0° is 20% or less (0 to 20%, particularly 0.1 to 19%) of the area of ​​the peak having a maximum at 2θ=35.3°. As a result, in the present invention, the vanadium sulfide can be a material containing almost no elemental sulfur, which reduces the concern of causing the reaction with the electrolyte as described above, and can further improve the charge / discharge capacity and charge / discharge cycle characteristics.

[0040] In the present invention, the vanadium sulfide preferably does not have peaks at 2θ=25.8° and 27.8°, which are peaks characteristic of elemental sulfur, or the area of ​​the peak having a maximum at said positions is 10% or less (0 to 10%, particularly 0.1 to 8%) of the area of ​​the peak having a maximum at 2θ=35.3°, within a tolerance range of ±1.0°. This allows the vanadium sulfide to be a material that contains almost no elemental sulfur, which reduces the concern of causing the above-mentioned reaction with the electrolyte, and further improves the charge / discharge capacity and charge / discharge cycle characteristics.

[0041] Vanadium sulfides that satisfy such conditions preferably have a strong peak at g(r)=2.4 Å within a tolerance of ±0.1 Å in X-ray / neutron atomic pair correlation function analysis (PDF analysis), but for sulfides with better charge / discharge capacity and charge / discharge cycle characteristics, it is more preferable to have a shoulder peak at g(r)=2.0 Å and more preferably a peak at g(r)=3.3 Å. In other words, vanadium sulfides preferably have not only VS bonds but also SS bonds (disulfide bonds).

[0042] In the present invention, the above-mentioned vanadium sulfide can be obtained, for example, by a production method using vanadium sulfide and sulfur as raw materials or intermediates, the production method including a step of subjecting the raw materials or intermediates to a mechanical milling method.

[0043] Mechanical milling is a method of grinding and mixing raw materials while applying mechanical energy. According to this method, the raw materials are ground and mixed by applying mechanical impact and friction, so that vanadium sulfide and sulfur come into vigorous contact with each other and are finely divided, causing a reaction of the raw materials. In other words, mixing, grinding, and reaction occur simultaneously. Therefore, it is possible to make the raw materials react more reliably without heating them to high temperatures. By using mechanical milling, a metastable crystal structure that cannot be obtained by normal heat treatment can be obtained.

[0044] Specifically, the mechanical milling treatment can involve mixed pulverization 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.

[0045] 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.

[0046] In particular, when producing vanadium sulfide with a high sulfur content (the composition ratio (S / V) of sulfur to vanadium is 3.3 or more in molar ratio), 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 vanadium sulfide and a part of sulfur to mechanical milling treatment to obtain a desired low-crystalline sulfide as an intermediate, and then subject the obtained low-crystalline sulfide and the remaining sulfur to mechanical milling treatment.

[0047] As a specific raw material, vanadium sulfide is crystalline vanadium(III) sulfide (V 2 S 3 ) is preferably used. There is no particular limitation on the vanadium sulfide, and any commercially available vanadium sulfide can be used. In particular, it is preferable to use one with high purity. In addition, since the vanadium sulfide is mixed and pulverized by mechanical milling, there is no limitation on the particle size of the vanadium sulfide used, and usually, commercially available powdered vanadium sulfide can be used.

[0048] As for sulfur, the amount of elemental sulfur (S) required to form a sulfide of the desired composition is used. 8 ) can be used. 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 with high purity. In addition, 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.

[0049] Furthermore, as described above, when the mechanical milling process is performed in multiple steps (particularly in two steps), the intermediate product is a low-crystalline vanadium sulfide having a desired composition (low-crystalline VS 2.5 etc.) can also be used.

[0050] The mixing ratio of the raw materials can be the same as the element ratio of vanadium and sulfur in the target vanadium sulfide, since the ratio of the raw materials is almost the same as the ratio of each element in the product. For example, it is preferable to use 1.2 moles or more (particularly 1.2 to 17.0 moles, more preferably 3.0 to 13.0 moles) of sulfur per mole of vanadium sulfide.

[0051] 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 crystal phase to be formed.

[0052] 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 desired vanadium sulfide is precipitated.

[0053] The atmosphere in which the mechanical milling process 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.

[0054] 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, if necessary.

[0055] When the mechanical milling process is repeated multiple times, the above conditions can be applied to each mechanical milling process.

[0056] By the above-mentioned mechanical milling treatment, the target vanadium sulfide can be obtained as a fine powder.

[0057] (1-2) Electrolyte The electrolyte constituting the nonaqueous secondary battery employing the discharging method of the present invention preferably contains an organic solvent containing a cyclic carbonate compound and an additive.

[0058] Organic solvents As described above, the discharge method of the present invention is adopted for a non-aqueous secondary battery using a lithium-free transition metal sulfide as a positive electrode active material. In this way, in the present invention, even though a non-aqueous secondary battery using a lithium-free transition metal sulfide is used, the discharge method described below is adopted to suppress the reaction between the carbonate compound and the lithium-free transition metal sulfide, and the charge / discharge cycle characteristics can be dramatically improved.

[0059] 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 kinds.

[0060] 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 the total amount of the organic solvent being 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. In the present invention, only the cyclic carbonate compound can be used as the organic solvent (the content of the cyclic carbonate compound is 100% by volume), or other organic solvents such as chain carbonate compounds can be used 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, and even more preferably 90 to 99.5% by volume). However, from the viewpoint of the 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).

[0061] 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 kinds.

[0062] 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, based on the total amount of the organic solvent being 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, and further 0.5 to 10% by volume). In addition, from the viewpoint of easily suppressing the decomposition of the organic solvent, it is preferable that the content of the chain carbonate compound is small, and it is particularly preferable that only a cyclic carbonate compound is used as the organic solvent (the content of the chain carbonate compound is 0% by volume).

[0063] In the present invention, the organic solvent constituting the nonaqueous secondary battery electrolyte may be composed only 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 of a lithium ion secondary battery.

[0064] Examples of such organic solvents as the third component include cyclic carboxylate compounds such as γ-butyrolactone, chain carboxylate compounds such as methyl acetate, methyl propionate, ethyl acetate, sulfone compounds such as sulfolane, diethylsulfone, ether compounds such as tetrahydrofuran, 2-methyltetrahydrofuran, 1,2-dimethoxyethane, etc. These organic solvents as the third component can be used alone or in combination of two or more kinds.

[0065] 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 volume %, and more preferably 0.2 to 5 volume %, based on 100 volume % of the total amount of the organic solvent, from the viewpoint of charge / discharge cycle characteristics.

[0066] Additives As described above, by containing an additive in the electrolyte, the reaction between the carbonate compound and the non-lithium-containing transition metal sulfide can be easily suppressed, and the charge-discharge cycle characteristics can be easily improved.

[0067] As such an additive, from the viewpoint of easily suppressing the reaction between the carbonate compound and the non-lithium-containing transition metal sulfide and easily improving the charge-discharge cycle characteristics, a compound represented by the general formula (1) or (2):

[0068] [ka]

[0069] [In the formula, R 1 and R 2 R may be the same or different and represents a hydrogen atom or a halogen atom. 3 are the same or different and each represents a halogen atom. Y represents a carbon atom or a sulfur atom. M represents a counter cation. n represents 1 or 2. Bonds represented by solid and dashed lines represent single or double bonds. However, when Y is a carbon atom, n represents 1, and when Y is a sulfur atom, n represents 2.] Preferred is a compound represented by the following formula:

[0070] The compound represented by the above general formula (1) is represented by the general formula (1A):

[0071] [ka]

[0072] [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 (1B):

[0073] [ka]

[0074] [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 the formula:

[0075] The compound represented by general formula (1A) is represented by general formula (1A1):

[0076] [ka]

[0077] [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 (1A2):

[0078] [ka]

[0079] [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:

[0080] The compound represented by general formula (1B) is represented by general formula (1B1):

[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 general formula (1B2):

[0083] [ka]

[0084] [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:

[0085] In the general formulae (1), (1A), (1A1), (1A2), (1B), (1B1) and (1B2), 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 viewpoint 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.

[0086] In the general formula (1), Y represents a carbon atom or a sulfur atom, and n represents 1 when Y is a carbon atom, and n represents 2 when Y is a sulfur atom.

[0087] In the general formula (2), R 3 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 viewpoint 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.

[0088] In the 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, the lithium ion is preferred from the viewpoints of charge / discharge capacity, charge / discharge cycle characteristics, etc.

[0089] Examples of additives that satisfy the above conditions include compounds represented by general formula (1A), such as vinylene carbonate (VC), fluoroethylene carbonate (FEC), trifluoromethylethylene carbonate, and vinylethylene carbonate; compounds represented by general formula (1B), such as 1,3,2-dioxathiolane 2,2-dioxide (DOTL), and 3-sulfolene; and compounds represented by general formula (2), such as lithium difluoro(oxalato)borate (DFOB). These additives may be used alone or in combination of two or more.

[0090] The additives are not particularly limited, but from the viewpoints of charge / discharge capacity, charge / discharge cycle characteristics, and the like, the compounds represented by general formula (1A) and the compounds represented by general formula (2) are preferred, and the compounds represented by general formula (1A) are more preferred.

[0091] The above-mentioned additives can be used alone or in combination of two or more. By using two or more additives in combination, it is possible to improve the charge-discharge cycle characteristics even if the content of the additives is increased.

[0092] When two or more kinds of additives are used in combination, from the viewpoints of charge / discharge capacity, charge / discharge cycle characteristics, energy density, and the like, it is preferable to use a compound represented by general formula (1A) or general formula (2) in combination with a compound represented by general formula (1B), and it is preferable to use a compound represented by general formula (2) in combination with a compound represented by general formula (1B).

[0093] The content of the additive 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 only one compound represented by general formula (2) is used as the additive, the amount 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, since a small amount of the additive is more likely to improve the charge / discharge cycle characteristics. In addition, even when only one kind of additive is used, when only one kind of additive is used, such as fluoroethylene carbonate (FEC), trifluoromethylethylene carbonate, vinylethylene carbonate, etc., the charge-discharge cycle characteristics are easily improved by increasing the amount of additive, 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 organic solvent. In addition, when two or more kinds of additives are used, the charge-discharge cycle characteristics and energy density are easily improved even if the content is increased, so the total content 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 organic solvent.

[0094] Lithium salts The electrolyte preferably further contains a lithium salt. The lithium salt is not particularly limited, and examples thereof include organic lithium salts having a sulfonyl group, inorganic lithium salts, and organic lithium salts having a boron atom.

[0095] The organic lithium salt having a sulfonyl group is not particularly limited as long as it is one that has been conventionally used in electrolytes for non-aqueous secondary batteries. For example, lithium trifluoromethanesulfonate (LiCF 3 SO 3 ); organic lithium salts with perfluoroalkanesulfonyl groups (lithium bis(trifluoromethanesulfonyl)imide (LiTFSI; Li(CF 3 SO2 ) 2 N), lithium bis(pentafluoroethanesulfonyl)imide (Li(C 2 F 5 SO 2 ) 2 Among them, from the viewpoint of withstanding charging at a higher voltage and further improving the charge-discharge cycle characteristics, an organic lithium salt having a perfluoroalkanesulfonyl group is preferred, and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI; Li(CF 3 SO 2 ) 2 N) is more preferable. These organic lithium salts having a sulfonyl group may be used alone or in combination of two or more.

[0096] The inorganic lithium salt is not particularly limited as long as it is one that has been conventionally used in electrolytes for non-aqueous secondary batteries. For example, lithium hexafluorophosphate (LiPF 6 ), lithium tetrafluoroborate (LiBF 4 ), lithium hexafluoroarsenate (LiAsF 6 ), lithium perchlorate (LiClO 4 Among them, lithium hexafluorophosphate (LiPF 6 ), lithium tetrafluoroborate (LiBF 4 ), and lithium hexafluorophosphate (LiPF 6 These inorganic lithium salts may be used alone or in combination of two or more.

[0097] The organic lithium salt having a boron atom is not particularly limited as long as it is one that has been conventionally used in electrolytes for non-aqueous secondary batteries. For example, lithium bis(oxalato)borate (LiBOB; LiB(C 2 O 4 ) 2 ), lithium oxalate difluoroborate (LiBF 2 (C 2O 4 )), lithium bis(malonate)borate (LiB(C 3 O 4 H 2 ) 2 Among them, lithium bis(oxalato)borate (LiBOB; LiB(C)) is preferred from the viewpoint of enduring charging at a higher voltage and further improving the charge-discharge cycle characteristics. 2 O 4 ) 2 These organic lithium salts having a boron atom may be used alone or in combination of two or more.

[0098] As the lithium salt, since the nonaqueous secondary battery of the present invention uses a lithium-free metal sulfide as the positive electrode active material, taking into consideration the effect of reactivity with sulfur on the charge-discharge cycle characteristics, an organic lithium salt having a sulfonyl group is preferred, and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI; Li(CF 3 SO 2 ) 2 N) is more preferred.

[0099] In the nonaqueous secondary battery electrolyte of the present invention, the concentration of the lithium salt is not particularly limited, and is preferably 0.3 to 2.0 mol / L, more preferably 1.0 to 2.0 mol / L, from the viewpoint of charge / discharge cycle characteristics.

[0100] others The electrolyte may contain components other than those mentioned above, such as other additives, within a range that does not impair the effects of the present invention (for example, 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, trialkylphosphate (trimethylphosphate, etc.), and the like. These other additives may be used alone or in combination of two or more.

[0101] The electrolytic solution 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.

[0102] (1-3) Positive electrode, negative electrode and separator The non-aqueous secondary battery employing the discharge method of the present invention preferably comprises the above-mentioned electrolytic solution. Other configurations and structures may be those employed in conventionally known non-aqueous secondary batteries. In general, the non-aqueous secondary battery may comprise a positive electrode, a negative electrode, and a separator in addition to the above-mentioned electrolytic solution.

[0103] positive electrode The positive electrode may have a configuration in which a positive electrode mixture layer containing a positive electrode active material, a binder, and the like is formed on one or both sides of a positive electrode current collector.

[0104] This positive electrode mixture layer can be produced through a process in which a binder is added to a positive electrode active material and a conductive assistant added as necessary, and the resultant 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 side or both sides) of a positive electrode current collector made of a metal foil or the like, dried to form a positive electrode mixture layer, and processed as necessary.

[0105] The above-mentioned lithium-free metal sulfide is used as the positive electrode active material. The details of the lithium-free metal sulfide are the same as those described above.

[0106] As in the case of ordinary non-aqueous secondary batteries, the conductive assistant can 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 assistant for the positive electrode can be used alone or in combination of two or more kinds.

[0107] Examples of the binder include polyvinylidene fluoride (PVDF), polytetrafluoroethylene, polyacrylic acid, styrene butadiene rubber, polyimide, polyvinyl alcohol, and water-soluble carboxymethyl cellulose.

[0108] 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.

[0109] The composition of the positive electrode mixture layer is preferably, for example, about 70 to 95% by weight of the above positive electrode active material and about 1 to 30% by weight of the binder. When a conductive assistant is used, the composition is preferably about 50 to 90% by weight of the above positive electrode active material, about 1 to 20% by weight of the binder, and about 1 to 40% by weight of the conductive assistant. Furthermore, the thickness of the positive electrode mixture layer is preferably about 1 to 100 μm per side of the current collector.

[0110] 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.

[0111] negative electrode The negative electrode may have a configuration in which a negative electrode mixture layer containing a negative electrode active material, a binder, and the like is formed on one or both sides of a negative electrode current collector.

[0112] This negative electrode mixture layer can be produced by mixing the negative electrode active material, a conductive assistant which is added as necessary, and a binder, forming the mixture into a sheet, and then pressing the sheet onto the surface (one side or both sides) of a negative electrode current collector made of a metal foil or the like.

[0113] 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, SiO, etc. Preferably, lithium metal, lithium alloys, etc. can be used in metal lithium primary batteries and metal lithium secondary batteries, and materials capable of doping and dedoping lithium ions (graphite (natural graphite, artificial graphite, etc.), non-sinterable carbon, etc.) can be used as active materials in lithium ion secondary batteries. These negative electrode active materials may be used alone or in combination of two or more.

[0114] As in the case of ordinary non-aqueous secondary batteries, the conductive assistant can 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 assistant for the negative electrode can be used alone or in combination of two or more kinds, or can be omitted if the negative electrode active material has high conductivity.

[0115] Examples of the binder include polyvinylidene fluoride (PVDF), polytetrafluoroethylene, polyacrylic acid, styrene butadiene rubber, polyimide, polyvinyl alcohol, and water-soluble carboxymethyl cellulose.

[0116] The composition of the negative electrode mixture layer is preferably, for example, about 70 to 95% by weight of the above-mentioned negative electrode active material and about 1 to 30% by weight of the binder. When a conductive assistant is used, the composition is preferably about 50 to 90% by weight of the above-mentioned negative electrode active material, about 1 to 20% by weight of the binder, and about 1 to 40% by weight of the conductive assistant. Furthermore, the thickness of the negative electrode mixture layer is preferably about 1 to 100 μm per side of the current collector.

[0117] The negative electrode current collector may be, for example, a foil, punched metal, expanded metal, mesh, or net made of aluminum, copper, stainless steel, nickel, titanium, or an alloy thereof, and typically, a copper foil having a thickness of about 5 to 30 μm is preferably used.

[0118] Separator The above-mentioned positive and negative electrodes can be used in the form of, for example, a laminated electrode body in which the electrodes are laminated with a separator interposed therebetween, or in the form of a wound electrode body in which the laminated electrode body is further wound in a spiral shape.

[0119] The separator should have sufficient strength and be capable of retaining a large amount of electrolyte. From this viewpoint, 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.

[0120] The nonaqueous secondary battery may be in the form of a tube (such as a rectangular tube or a cylindrical tube) using a stainless steel can or an aluminum can as an exterior can, or a soft package battery may be used in which a laminate film integrated with a metal foil is used as the exterior.

[0121] 2.Discharge method In current lithium-ion secondary batteries, LiCOO is used as the positive electrode active material. 2 , Li(Ni,Mn,Co)O 2 and the like, and have the general formula: LiMO 2 ⇔ Li 1-x MO2 + xLi (Charge / discharge range 0≦x<1) [In the formula, M represents at least one transition metal.] The electrochemical reaction is expressed as follows. Here, the closer the x value is to 1, the greater the risk of decomposition and fire, so it is normal to strictly control charging. That is, in an overcharged state where lithium ions are excessively extracted, oxygen desorption accompanied by rapid heat generation can cause thermal runaway and, in the worst case, fire. Therefore, especially for consumer applications, it is normal to strictly control the charging side, such as by using a double file-safe mechanism such as a gas release mechanism valve and a voltage monitoring circuit. On the other hand, during discharge, since excessive lithium ions are not inserted into the metal oxide even in overdischarge, precise control is not performed, and the control is only adjusted to the required operating voltage of the electronic device that supplies electricity. In actual consumer applications, charging and discharging are actually performed at about x = 0.5, and the effective capacity value in that case is about 150 to 180 mAh / g. In terms of aiming for future high-capacity innovative batteries, this capacity value is low compared to the required value, and cannot meet the 500 Wh / kg required for future innovative storage batteries. For this reason, the above-mentioned lithium-free transition metal sulfide or the like is required.

[0122] Lithium-free transition metal sulfides are LiMO 2 Unlike the system of general formula: MS y + xLi ⇔ Li x MS y (Charge / discharge range 0≦x≦5~10) The electrochemical reaction is expressed as follows:

[0123] As a specific example of a lithium-free transition metal sulfide, VS 4 For example, the general formula: VS 4 + xLi ⇔ Li x VS 4 (Charge / discharge range 0≦x≦5) The charge and discharge reaction is represented by the following:

[0124] The above LiMO2 Unlike the LiMO, the charge / discharge reaction also occurs in the x>1 region, so the effective capacity is high at 750mAh / g or more, making it suitable for innovative storage batteries. 2 Unlike the metal oxide type, the initial state is fully charged (no lithium), so there is no structural instability due to overcharging (thermal runaway does not occur), and overcharging does not lead to thermal runaway or fire. Therefore, there is no need for the strict control during charging that is required for metal oxide types.

[0125] On the other hand, during discharge, in an overdischarge state where an excessive amount of lithium ions are inserted, the reduction of the electrode active material proceeds, and in part the reduction proceeds to a metallic state, which causes a significant problem in the reversibility of the structure during the next charging stage, and the extremely fine metal particles have high surface activity, which causes the decomposition of the electrolyte, etc., to proceed, significantly reducing the cycle life of the battery. In the present invention, as a result of extensive research, it has become clear that the capacity retention rate can be maintained high by controlling the discharge depth to a certain specific region.

[0126] That is, in the present invention, the battery is not completely discharged, but the discharge is stopped and switched to charging when the battery is discharged to a certain extent. In a nonaqueous secondary battery using a lithium-free transition metal sulfide as a positive electrode active material as in the present invention, if the depth of discharge is too small or too large, the charge-discharge cycle characteristics deteriorate. In other words, the charge-discharge cycle characteristics do not improve as the depth of discharge becomes smaller. For this reason, specifically, the depth of discharge in the charge-discharge cycle is 70 to 90%, preferably 71 to 87%, and more preferably 72 to 80%.

[0127] To explain the above-mentioned depth of discharge in more detail, the lithium-free transition metal sulfide is 4 If VS 4 +xLi→Li x VS 4 In the discharge reaction represented by the following formula, assuming that the depth of discharge is 100% when x=5.0, it is preferable to adjust x to be 3.50 to 4.50, particularly 3.55 to 4.35, and further particularly 3.60 to 4.00.

[0128] The specific discharge control method is not particularly limited. For example, a plurality of test electrochemical cells (lithium secondary batteries) with the same configuration and the same type are prepared. Using one cell, a test is conducted according to the assumed discharge reaction, and the discharge capacity at the end state of the discharge is taken as the capacity at 100% depth of discharge. Based on this value, in the electrochemical cell that actually controls the depth of discharge, while monitoring the capacity of the cell, the depth of discharge is controlled by discharging until the set capacity value of the depth of discharge is reached.

[0129] The present invention controls the depth of discharge during charge-discharge cycles. However, there is no particular limitation on the depth of charge during charge-discharge cycles. In order to make the charge-discharge cycle characteristics easy to improve while maximizing the capacity of the non-aqueous secondary battery, 70 to 100% is preferable, 80 to 100% is more preferable, and 90 to 100% is even more preferable.

[0130] Also, there is no particular limitation on the charge-discharge rate during charge-discharge cycles. It is not particularly limited as long as it is a current density that makes it easy to improve the charge-discharge cycle characteristics while maximizing the capacity of the non-aqueous secondary battery. For example, a range of about 0.05 to 5C can be used.

Examples

[0131] Hereinafter, the present invention will be described in detail based on examples. Needless to say, the present invention is not limited to the following examples.

[0132] Synthesis Example 1: Synthesis of vanadium sulfide (positive electrode active material) Commercially available vanadium(III) sulfide (V 2 S 3 ; manufactured by High Purity Chemical Research Institute Co., Ltd.) and sulfur (manufactured by Fujifilm Wako Pure Chemical Corporation) were weighed in a glove box under an argon gas atmosphere (dew point -80°C) so that the molar ratio was 1:6, and then sealed in a glass tube under vacuum. The vacuum-sealed sample was fired in a tubular furnace at 400°C for 5 hours. The fired sample was fired at 200°C for 8 hours under vacuum to desulfurize the excess sulfur and obtain crystalline vanadium sulfide VS 4(c-VS 4 ) was synthesized.

[0133] Next, the obtained crystalline VS 4 (c-VS 4 ) was subjected to mechanical milling (ball diameter 4 mm, rotation speed 270 rpm) for 40 hours in a glove box with an argon gas atmosphere (dew point -80°C) using a ball mill (Fritsch PL-7) to obtain low-crystalline vanadium sulfide VS 4 (a-VS 4 ) was synthesized and used as the positive electrode active material.

[0134] Synthesis Example 2: Preparation of electrolyte 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 fluoroethylene carbonate (FEC) was added in an amount of 1.0 part by mass per 100 parts by mass of the mixed solvent, and 1,3,2-dioxathiolane 2,2-dioxide (DOTL) was added in an amount of 1.0 part by mass per 100 parts by mass of the mixed solvent, to obtain nonaqueous secondary battery electrolyte solutions used in the examples and comparative examples.

[0135] Preparation of test electrochemical cells (lithium secondary batteries) VS obtained in Synthesis Example 1 4 A test electrochemical cell (lithium secondary battery) was produced by the following method using the powder as a positive electrode active material and the nonaqueous secondary battery electrolyte obtained in Synthesis Example 2. The test electrochemical cell was produced by the following method using the VS 4 To 10 mg of powder, 1 mg of Ketjen Black and 1 mg of polytetrafluoroethylene (PTFE) as a binder were added, mixed in a mortar for 8 minutes, and then attached to an aluminum mesh to produce the battery. Lithium metal was used as the counter electrode (negative electrode). Polypropylene was used as the separator.

[0136] Examples 1 to 5 and Comparative Examples 1 to 3 A charge / discharge test was carried out under the following conditions using the prepared test electrochemical cell (lithium secondary battery). At 25°C, the charge / discharge rate was 0.1C (1C=747mA / g), the charge depth was 100%, and the voltage range for the discharge depth was set to 69-100%, with a rest time of 10 minutes between cycles. Constant current charge / discharge measurements were carried out for 100 cycles. Specifically, the cell capacity during discharge was monitored, and a discharge test was carried out up to the capacity of the set discharge depth. The discharge conditions (discharge depth (%)) and the discharge reaction VS corresponding to that discharge depth were measured. 4 +xLi→Li x VS 4 The results of the charge-discharge cycle characteristics (capacity retention rate) are shown in Table 1. As shown in Table 1, it was confirmed that the decrease in the capacity retention rate can be suppressed and the battery life can be extended by performing the charge-discharge cycle with the depth of discharge appropriately controlled according to the present invention.

[0137] [Table 1] [Industrial Applicability]

[0138] The nonaqueous secondary battery electrolyte solution of the present invention and the nonaqueous secondary battery using the electrolyte solution can be used in various known applications, such as notebook computers, mobile phones, electric vehicles, load leveling power sources, and natural energy storage power sources.

Claims

1. A method for discharging a non-aqueous secondary battery that uses a lithium-free transition metal sulfide as a positive electrode active material, comprising the steps of: The lithium-free transition metal sulfide is VS ４ and VS ４ +xLi→Li ｘ VS ４ In the discharge reaction shown by the formula: x = 5.0 is set as 100% depth of discharge, and x is adjusted to be 3.50 to 4.

50. A discharge method in which the depth of discharge during the charge / discharge cycle is 70 to 90%.

2. the nonaqueous secondary battery further contains an electrolyte solution, The discharging method according to claim 1 , wherein the electrolyte contains an organic solvent containing a cyclic carbonate compound.

3. The discharge method according to claim 2, wherein the cyclic carbonate compound is at least one selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC) and butylene carbonate.

4. The total amount of the organic solvent is 100% by volume, and 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. The discharge method according to claim 2 or 3.

5. The discharge method according to claim 4, 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.

6. The discharge method according to any one of claims 2 to 5, wherein the electrolyte further contains a lithium salt.

7. 7. The discharge method 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, inorganic lithium salts, 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 method according to claim 6, wherein the conductive material is at least one selected from the group consisting of:

9. The discharge method according to any one of claims 6 to 8, wherein the concentration of the lithium salt in the electrolyte is 0.3 to 2.0 mol / L.

10. The discharge method according to any one of claims 1 to 9, wherein the nonaqueous secondary battery is a lithium ion secondary battery.

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