WO2022196753A1

WO2022196753A1 - Electrolyte for non-aqueous secondary battery and non-aqueous secondary battery using same

Info

Publication number: WO2022196753A1
Application number: PCT/JP2022/012172
Authority: WO
Inventors: 一重河野; 友成竹内; 比夏里栄部
Original assignee: 国立研究開発法人産業技術総合研究所
Priority date: 2021-03-19
Filing date: 2022-03-17
Publication date: 2022-09-22
Also published as: JP7498535B2; JPWO2022196753A1

Abstract

The present invention provides an electrolyte capable of improving both charge-discharge cycle characteristics and reliability in a non-aqueous secondary battery that uses a lithium-free transition metal sulfide as a positive-electrode active material. An electrolyte for non-aqueous secondary batteries, in which: the non-aqueous secondary battery uses a lithium-free transition metal sulfide as a positive-electrode active material; and, the electrolyte contains a carbonate-compound-containing organic solvent, a phosphate ester compound, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and an additive.

Description

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

The present invention relates to an electrolytic solution for non-aqueous secondary batteries and non-aqueous secondary batteries using the same.

Due to the recent improvements in the performance of portable electronic devices and hybrid vehicles, there is a growing demand for higher capacity lithium-ion secondary batteries used in these devices. However, in current lithium-ion secondary batteries, the capacity of the positive electrode is insufficient compared to that of the negative electrode, and even lithium nickel oxide-based materials, which are said to have relatively high capacity, have a capacity of about 190mAh/g to 220mAh/g. It's nothing more than

On the other hand, sulfur has a high theoretical capacity of about 1670 mAh/g, and is expected to be used as a positive electrode active material. Are known. This is because lithium polysulfide is eluted into the organic electrolyte during charging and discharging, and a technique for suppressing elution into the organic electrolyte is essential.

Lithium-free transition metal sulfides (lithium-free transition metal sulfides) have electronic conductivity, and although they are less eluted into organic electrolytes, they are not sufficient. Taking vanadium sulfide as an example of a lithium-free transition metal sulfide, when crystalline vanadium sulfide (III) (V ₂ S ₃ ) sold as a reagent is used as a positive electrode active material, Since the reaction with the organic electrolyte cannot be suppressed, the measured capacity is only about 23 mAh/g for charge capacity and about 52 mAh/g for discharge capacity. In contrast, the present inventors have found that a low-crystalline vanadium sulfide having a specific composition exhibits a high capacity when used as an electrode active material for a lithium-ion secondary battery, and has excellent charge-discharge cycle characteristics. have also been reported to be excellent (see, for example, Patent Document 1).

WO2018/181698

As described above, the present inventors have developed a material that exhibits a high capacity when used as an electrode active material for a lithium ion secondary battery and also has excellent charge-discharge cycle characteristics. There is no end to the demand for higher reliability for , and improvements are required in order to achieve compatibility with charge-discharge cycle characteristics.

In order to improve reliability, even if the battery ignites due to an external impact such as an earthquake or traffic accident, or abnormal heating due to a fire, etc., it will lead to improved safety by providing a self-extinguishing function. Conceivable. As a method for suppressing the combustion reaction of the battery, for example, a method of adding a phosphoric acid ester-based material, which has a function of trapping oxygen radicals and blocking combustion-supporting gas, to the electrolytic solution can be used. However, when a phosphate ester-based material is added to a general electrolytic solution, there is a problem that the internal resistance increases and the life characteristics deteriorate when the charge-discharge cycle is repeated.

The present invention has been made in view of the current state of the prior art described above, and the main object thereof is to provide a non-aqueous secondary battery using a lithium-free transition metal sulfide as a positive electrode active material, in which charge-discharge cycle characteristics are improved. It is an object of the present invention to provide an electrolytic solution capable of achieving both the improvement of reliability and the improvement of reliability.

The inventors of the present invention have been earnestly conducting research to achieve the above objectives. As a result, by adding an organic solvent containing lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and a carbonate compound to trimethyl phosphate (TMP) and an additive, a lithium-free transition metal sulfide can be converted into a positive electrode active material. In the non-aqueous secondary battery used as, it was found that the charge-discharge cycle characteristics can be improved.

The present invention was completed as a result of further research based on such knowledge.

That is, the present invention includes the following configurations.

Item 1.
An electrolytic solution for a non-aqueous secondary battery,
The non-aqueous secondary battery is a non-aqueous secondary battery using a lithium-free transition metal sulfide as a positive electrode active material,
The electrolytic solution is
an organic solvent containing a carbonate compound;
a phosphate ester compound;
lithium bis(trifluoromethanesulfonyl)imide (LiTFSI);
containing an additive,
Electrolyte for non-aqueous secondary batteries.

Section 2.
The phosphate ester compound is at least one selected from the group consisting of trimethyl phosphate (TMP), triethyl phosphate (TEP), and tris(2,2,2-trifluoroethyl) phosphate (TFEP). 3. The electrolytic solution for a non-aqueous secondary battery according to item 1 above.

Item 3.
Item 3. The electrolytic solution for a non-aqueous secondary battery according to Item 1 or 2, wherein the additive is at least one selected from the group consisting of vinylene carbonate (VC) and fluoroethylene carbonate (FEC).

Section 4.
4. Any one of the above items 1 to 3, wherein the content of the phosphoric acid ester compound is 0.25 to 1.0 times the content of the lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in terms of molar ratio. Electrolyte solution for non-aqueous secondary batteries according to.

Item 5.
5. The electrolytic solution for a non-aqueous secondary battery according to any one of Items 1 to 4, wherein the content of the additive is 5% to 10% by weight based on the total amount of the electrolytic solution being 100% by weight. .

Item 6.
6. Any one of the above items 1 to 5, wherein the carbonate compound is at least one selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and propylene carbonate (PC). 2. The electrolytic solution for non-aqueous secondary batteries according to item 1.

Item 7.
7. The electrolysis for a non-aqueous secondary battery according to any one of items 1 to 6, wherein the lithium-free transition metal sulfide is at least one selected from the group consisting of vanadium sulfide and molybdenum sulfide. liquid.

Item 8.
A non-aqueous secondary battery comprising the electrolytic solution for a non-aqueous secondary battery according to any one of Items 1 to 7 above.

Item 9.
9. The nonaqueous secondary battery according to claim 8, which is a lithium ion secondary battery.

According to the present invention, charge-discharge cycle characteristics can be further improved in a non-aqueous secondary battery using a lithium-free transition metal sulfide as a positive electrode active material.

As used herein, "contain" is a concept that includes all of "comprise", "consist essentially of", and "consist of". Further, in this specification, when a numerical range is indicated by "A to B", it means A or more and B or less.

In this specification, the concentration (mol/L) of each component means that it contains the desired number of moles per 1 L of the organic solvent.

1. Electrolyte solution for non-aqueous secondary battery The present invention is an electrolyte solution for non-aqueous secondary battery,
The non-aqueous secondary battery is a non-aqueous secondary battery using a lithium-free transition metal sulfide as a positive electrode active material,
The electrolytic solution is
an organic solvent containing a carbonate compound;
a phosphate ester compound;
lithium bis(trifluoromethanesulfonyl)imide (LiTFSI);
and additives.

(1-1) Lithium-free transition metal sulfide In the present invention, transition metal sulfides containing lithium must be handled in an inert atmosphere such as an argon gas atmosphere. Containing transition metal sulfides are used. Examples of such lithium-free transition metal sulfides include lithium-free transition metal sulfides used as positive electrode active materials in non-aqueous secondary batteries in which the electrolyte for non-aqueous secondary batteries of the present invention is used. and is not particularly limited as long as it is a lithium-free transition metal sulfide 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 sulfides (lithium-free molybdenum sulfides), iron sulfides (lithium-free iron sulfides), and the like. Note that the descriptions of International Publication No. 2018/181698 and International Publication No. 2015/049986 are incorporated by reference.

These lithium-free transition metal sulfides can be used alone or in combination of two or more. Among them, from the viewpoint of charge-discharge capacity, charge-discharge cycle characteristics, etc., vanadium sulfide (lithium-free vanadium sulfide: International Publication No. 2018/181698), molybdenum sulfide (lithium-free molybdenum sulfide), Iron sulfide (lithium-free iron sulfide) and the like are preferable, and vanadium sulfide (lithium-free vanadium sulfide: International Publication No. 2018/181698) is more preferable.

In the present invention, the non-lithium-containing transition metal sulfide is preferably at least one selected from the group consisting of vanadium sulfide and molybdenum sulfide.

Both crystalline materials and low-crystalline materials (or amorphous materials) can be used for such lithium-free transition metal sulfides. Among them, particularly excellent charge-discharge capacity, charge-discharge cycle characteristics, etc., and from the viewpoint of easily suppressing the reaction with the organic electrolyte when it comes into contact with the organic electrolyte, low-crystalline materials (or amorphous materials) is preferred.

In the present invention, as the lithium-free transition metal sulfide, the composition ratio (S/M ¹ ) of sulfur and transition metal is particularly excellent in charge-discharge capacity, charge-discharge cycle characteristics, etc. A molar ratio of 2.1 to 10 is preferable from the viewpoint of easily suppressing the reaction with the organic electrolyte when in contact with the liquid.

More specifically, non-lithium containing transition metal sulfides have the general formula (2):
M1S _x ( ² )
[In the formula, M ¹ represents a transition metal. x indicates 2.1-10. ]
It is preferable to have a composition represented by When a plurality of transition metals are contained as M ¹ , the composition ratio (S/M ¹ ) of sulfur to the total amount of transition metals is preferably 2.1-10 in terms of molar ratio.

Thus, in the present invention, the lithium-free metal sulfide has a high elemental ratio of sulfur to transition metal (M ¹ ). Therefore, in the present invention, by using a lithium-free metal sulfide, a high charge-discharge capacity and excellent charge-discharge cycle characteristics can be obtained. In the present invention, the higher the sulfur content (the larger x), the higher the charge-discharge capacity, and the lower the sulfur content (the smaller x), the less elemental sulfur is contained. charge-discharge cycle characteristics tend to be high.

In the present invention, even if a sulfide having poor charge-discharge cycle characteristics is used, the charge-discharge cycle characteristics can be improved by using an electrolytic solution having a composition described later, so the charge-discharge capacity tends to increase. Use of polysulfides, which tend to have insufficient charge-discharge cycle characteristics, is particularly useful. Therefore, x is preferably 2.1-10, more preferably 3-8.

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

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

More specifically, vanadium sulfide is 15.4 °, 35.3 °, and 45.0 ° with a tolerance of ± 1.0 ° in the range of diffraction angles 2θ = 10 ° to 80 ° in the X-ray diffraction diagram using CuKα rays. It is preferable to have a peak at . 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°.

In the present invention, the X-ray diffraction pattern is obtained by a powder X-ray diffraction measurement method (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° steps, scanning speed 0.02°/sec.

In the present invention, the vanadium sulfide preferably has a peak at the 2θ position described above, but within the diffraction angle 2θ = 10 ° to 80 °, the tolerance range of ±1.0 °, 54.0 ° and 56.0 ° It is preferable to have a peak at at least one location (especially all).

In the present invention, vanadium sulfide is a sulfide with a high sulfur ratio as an average composition, but sulfur hardly exists as elemental sulfur as described later, and is combined with vanadium to have low crystallinity. preferably forms a sulfide of

Thus, in the present invention, vanadium sulfide has a lower crystallinity, so that there are more sites where lithium ions can be inserted and detached, and gaps that can be three-dimensionally conductive paths for lithium can be made structurally easier to have. In addition, it has many advantages such as easy three-dimensional volume change during charging and discharging. Therefore, the charge/discharge capacity and charge/discharge cycle characteristics can be further improved. Moreover, it is preferable that vanadium sulfide (V ₂ S ₃ etc.) used as a raw material is almost absent.

In this specification, the average composition of sulfide indicates the element ratio of each element that constitutes the entire sulfide.

"Low crystallinity" in the present invention will be described below.

In the present invention, the low-crystalline vanadium sulfide has no peaks 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 0.8 ° to 2.0 °. (especially 1.0° to 2.0°) is preferred. In crystalline vanadium sulfide (IV) (VS ₄ ), the full widths 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, the low-crystalline vanadium sulfide has no peaks 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 Large compared to vanadium (IV) (VS ₄ ) is preferred.

Thus, in the present invention, due to the low crystallinity, the number of sites where Li can stably exist tends to increase. And it is easy to improve charge-discharge cycle characteristics.

In addition, when a material containing a large amount of elemental sulfur or the like is used as the positive electrode active material, the cyclic carbonate compound contained in the electrolyte for non-aqueous secondary batteries of the present invention tends to react with elemental sulfur. Then, in the present invention, for example, when the mechanical milling treatment is performed for a sufficient time, the vanadium sulfide described above hardly contains elemental sulfur or the like, so when used as a positive electrode active material, cyclic carbonate Even when a compound is used, these problems do not occur, and the charge/discharge capacity and charge/discharge cycle characteristics are likely to be dramatically improved.

More specifically, the strongest peak for sulfur (S ₈ ) is at 2θ=23.0° with a tolerance of ±1.0°. From this, in the X-ray diffraction diagram with CuKα rays, it is possible to have a peak with a maximum at 2θ = 23.0°, which is a peak characteristic of elemental sulfur, within an allowable range of ± 1.0°. is preferably 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, vanadium sulfide can be made a material that hardly contains elemental sulfur, and the concern of causing a reaction with the electrolyte solution as described above is further reduced, and the charge-discharge capacity and charge-discharge cycle characteristics can be further improved.

In the present invention, vanadium sulfide also has no peaks at 2θ = 25.8° and 27.8°, which are peaks characteristic of elemental sulfur, within a tolerance of ±1.0°. The area of the peak having the maximum at 2θ=35.3° is preferably 10% or less (0% to 10%, particularly 0.1% to 8%) of the area of the peak having the maximum at 2θ=35.3°. As a result, vanadium sulfide can be made into a material that contains almost no elemental sulfur, which further reduces the concern of causing a reaction with the electrolyte solution as described above, and further improves charge-discharge capacity and charge-discharge cycle characteristics. be able to.

Vanadium sulfide that satisfies these conditions has a strong peak at g(r) = 2.4 Å with a tolerance of ±0.1 Å in X-ray/neutron atom pair correlation function analysis (PDF analysis). However, 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 also at g(r) = 3.3 Å. Having a peak is more preferable. In other words, vanadium sulfide preferably has not only V-S bonds but also S-S bonds (disulfide bonds).

In the present invention, the vanadium sulfide described above can be obtained, for example, by using vanadium sulfide and sulfur as raw materials or intermediates and using a production method comprising a step of subjecting them to a mechanical milling method.

Mechanical milling is a method of grinding and mixing raw materials while applying mechanical energy. According to this method, vanadium sulfide and Sulfur contacts violently and becomes fine, causing reaction of raw materials. In other words, at this time, mixing, grinding and reaction occur simultaneously. Therefore, it is possible to react the raw materials more reliably without heating the raw materials to a high temperature. A metastable crystal structure, which cannot be obtained by ordinary heat treatment, may be obtained by using mechanical milling treatment.

Specifically, as the mechanical milling treatment, for example, mixed grinding can be performed using a mechanical grinding device such as a ball mill, bead mill, rod mill, vibration mill, disk mill, hammer mill, jet mill, or the like.

All of these raw materials or intermediates can be mixed at the same time and subjected to mechanical milling. Some of the materials or intermediates are first subjected to mechanical milling, and then the remaining materials are added to mechanical milling. can also be provided to

In particular, when producing vanadium sulfide with a high sulfur content (composition ratio (S/V) of sulfur and vanadium is 3.3 or more in terms of molar ratio), crystalline vanadium sulfide may can be obtained. Therefore, in order to easily obtain low-crystalline vanadium sulfide having excellent charge-discharge capacity and charge-discharge cycle characteristics, first, vanadium sulfide and a part of sulfur are subjected to mechanical milling to obtain the desired intermediate. After obtaining the low crystalline sulfide, it is preferable to subject the obtained low crystalline sulfide and the remaining sulfur to a mechanical milling treatment.

As a specific raw material, it is preferable to use crystalline vanadium sulfide (III) (V ₂ S ₃ ) as vanadium sulfide. Vanadium sulfide is not particularly limited, and any commercially available vanadium sulfide can be used. In particular, it is preferable to use a highly pure one. In addition, since the vanadium sulfide is mixed and pulverized by mechanical milling, the particle size of the vanadium sulfide used is not limited, and powdery vanadium sulfide that is commercially available can be used.

Also, as sulfur, it is possible to use elemental sulfur (S ₈ ) in an amount necessary to form a sulfide having the desired composition. The sulfur used as a raw material is also not particularly limited, and any sulfur can be used. In particular, it is preferable to use a highly pure one. In addition, since sulfur is mixed and pulverized by mechanical milling, the particle size of the sulfur used is not limited, and powdered sulfur commercially available can be used.

Furthermore, as described above, when subjected to multiple (especially two-stage) mechanical milling treatments, as an intermediate, a low-crystalline vanadium sulfide (low-crystalline VS _2.5 , etc.) having a desired composition can also be used. .

Regarding the mixing ratio of raw materials, the ratio of each element in the product is almost the same as the ratio of the raw materials, so it can be the same ratio as the elemental ratio of vanadium and sulfur in the target vanadium sulfide. For example, sulfur is preferably 1.2 mol or more (especially 1.2 mol to 17.0 mol, more preferably 3.0 mol to 13.0 mol) per 1 mol of vanadium sulfide.

The temperature at which the mechanical milling is performed is not particularly limited, and is preferably 300° C. or less, more preferably -10° C. to 200° C., in order to make it difficult for sulfur to volatilize and to make it difficult to generate the previously reported crystal phase. .

The time of the mechanical milling treatment is not particularly limited, and the mechanical milling treatment can be performed for an arbitrary time until the desired vanadium sulfide precipitates.

Although there are no particular restrictions on the atmosphere in which the mechanical milling process is performed, an inert gas atmosphere such as a nitrogen gas atmosphere or an argon gas atmosphere can be used.

For example, mechanical milling can be performed within the processing time range of 0.1 hours to 100 hours (especially 15 hours to 80 hours). It should be noted that this mechanical milling process can also be performed by dividing it into a plurality of times with intervening breaks on the way, if necessary.

When the mechanical milling process is repeated multiple times, the above conditions can be applied to the mechanical milling process in each step.

The target vanadium sulfide can be obtained as a fine powder by the mechanical milling treatment described above.

(1-2) Organic solvent The electrolytic solution for non-aqueous secondary batteries of the present invention is
an organic solvent containing a carbonate compound;
a phosphate ester compound;
lithium bis(trifluoromethanesulfonyl)imide (LiTFSI);
and additives.

As described above, the electrolyte solution for non-aqueous secondary batteries of the present invention is an electrolyte solution for non-aqueous secondary batteries that uses a lithium-free transition metal sulfide as a positive electrode active material. As described above, in the present invention, although the lithium-free transition metal sulfide is used in a non-aqueous secondary battery using the lithium-free transition metal sulfide, by adding the additive described later, the carbonate compound and the lithium-free transition metal sulfide It is possible to suppress the reaction with substances and dramatically improve the charge-discharge cycle characteristics.

Carbonate compound The 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. , ethyl methyl carbonate (EMC), and the like. Cyclic carbonate compounds include propylene carbonate (PC) and the like. These carbonate compounds can be used alone or in combination of two or more.

In the present invention, the carbonate compound is preferably at least one selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), ethylmethyl carbonate (EMC), and propylene carbonate (PC).

In the present invention, the organic solvent constituting the electrolytic solution for non-aqueous secondary batteries may be composed of only the above-described carbonate compound. It is also possible to include compounds known as

Examples of such an organic solvent 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; 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.

When the organic solvent as the third component is included, the content of the organic solvent as the third component is 0.1% by volume to 10% by volume, with the total amount of the organic solvent being 100% by volume, from the viewpoint of charge-discharge cycle characteristics. %, more preferably 0.2% to 5% by volume.

(1-3) Additives The electrolytic solution for non-aqueous secondary batteries of the present invention contains:
an organic solvent containing a carbonate compound;
a phosphate ester compound;
lithium bis(trifluoromethanesulfonyl)imide (LiTFSI);
and additives.

As described above, the electrolytic solution for a non-aqueous secondary battery of the present invention contains an additive to suppress the reaction between the carbonate compound and the lithium-free transition metal sulfide, thereby dramatically improving the charge-discharge cycle characteristics. can be improved to

Such additives are vinylene carbonate (VC), or fluoro Ethylene carbonate (FEC) is preferred.

These additives can be used alone or in combination of two or more. By using a combination of two or more additives, it is possible to improve charge-discharge cycle characteristics even when the additive content is increased.

In the present invention, the additive is preferably at least one selected from the group consisting of vinylene carbonate (VC) and fluoroethylene carbonate (FEC).

From the viewpoint of charge/discharge capacity, charge/discharge cycle characteristics, energy density, etc., the content of the above additives is preferably 2.5 parts by mass to 20.0 parts by mass, and 2.5 parts by mass to 15.0 parts by mass with respect to 100 parts by mass of the organic solvent. parts is more preferred, and 2.5 to 10.0 parts by mass is even more preferred. Even if only one type of additive is used, if only one type such as fluoroethylene carbonate (FEC), trifluoromethylethylene carbonate, vinylethylene carbonate, etc. is used, it is better to charge and discharge with more additives. The amount is preferably 2.5 parts by mass to 10.0 parts by mass with respect to 100 parts by mass of the organic solvent, since it is easy to improve cycle characteristics. In addition, when using two or more additives, even if the content is increased, it is easy to improve the charge-discharge cycle characteristics and the energy density, so the total content of the additives is 100 parts by mass of the organic solvent. 2.5 parts by mass to 20.0 parts by mass is preferable, 2.5 parts by mass to 15.0 parts by mass is more preferable, and 5.0 parts by mass to 10.0 parts by mass is even more preferable.

(1-4) Lithium salt The electrolyte for non-aqueous secondary batteries of the present invention is
an organic solvent containing a carbonate compound;
a phosphate ester compound;
lithium bis(trifluoromethanesulfonyl)imide (LiTFSI);
and additives.

The electrolytic solution for non-aqueous secondary batteries of the present invention further contains lithium bis(trifluoromethanesulfonyl)imide (LiTFSI: Li(CF ₃ SO ₂ ) ₂ N) as a lithium salt. This lithium salt is an organic lithium salt having a sulfonyl group (perfluoroalkanesulfonyl group). By using LiTFSI as the lithium salt, it is possible to endure charging at a higher voltage and further improve charge-discharge cycle characteristics.

In addition, the organic lithium salt having a sulfonyl group is not particularly limited as long as it is conventionally used in electrolyte solutions for non-aqueous secondary batteries. (Pentafluoroethanesulfonyl) imide (Li(C ₂ F ₅ SO ₂ ) ₂ N, etc.), etc. These organic lithium salts having a sulfonyl group may be used singly or in combination of two or more. may be used.

From the viewpoint of the charge-discharge cycle characteristics described above, the lithium salt is preferably an organic lithium salt having a sulfonyl group rather than an inorganic lithium salt ( _LiPF6 , _LiBF4 , etc.).

In addition, an organic lithium salt having boron atoms may be added. From the viewpoint of charge-discharge cycle characteristics, the lithium salt is preferably an organic lithium salt having a boron atom.

Since the non-aqueous secondary battery of the present invention uses a lithium-free metal sulfide as a positive electrode active material, the influence of reactivity with sulfur on charge-discharge cycle characteristics is taken into consideration.

By using LiTFSI as the lithium salt, the charge-discharge cycle characteristics are good.

In the non-aqueous secondary battery electrolyte solution of the present invention, the concentration of the lithium salt described above is preferably 2 to 4 times the molar ratio of the chain carbonate from the viewpoint of charge-discharge cycle characteristics and internal resistance. 2 to 3 times is more preferable.

In the present invention, the content of phosphate ester (preferably trimethyl phosphate (TMP)) is preferably 0.25 times to the content of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in terms of molar ratio. 1.0 times.

(1-5) Phosphate ester compound The electrolytic solution for non-aqueous secondary batteries of the present invention comprises
an organic solvent containing a carbonate compound;
a phosphate ester compound;
lithium bis(trifluoromethanesulfonyl)imide (LiTFSI);
and additives.

In the present invention, the phosphate ester compound is preferably selected from the group consisting of trimethyl phosphate (TMP), triethyl phosphate (TEP), and tris(2,2,2-trifluoroethyl) phosphate (TFEP). at least one

In the present invention, a phosphate compound, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and an additive are contained in an organic solvent containing a carbonate compound so that a lithium-free transition metal sulfide can be used as a positive electrode active material. In the non-aqueous secondary battery used as the substance, charge-discharge cycle characteristics can be improved.

(1-6) Others In the electrolytic solution for non-aqueous secondary batteries of the present invention, the range that does not impair the effects of the present invention (for example, 0.01 mol / L to 0.2 mol / L, particularly 0.02 mol / L to 0.1 mol / In the case of L), components other than those mentioned above, such as other additives, can also be included.

Such other additives include, for example, tetrabutylammonium hexafluorophosphate, tetrabutylammonium perchlorate, tetramethylammonium tetrafluoroborate, tetramethylammonium chloride, tetraethylammonium chloride, tetrabutylammonium chloride, tetrabromide methylammonium, tetraethylammonium bromide, tetrabutylammonium bromide, biphenyl, trialkyl phosphate (trimethyl phosphate etc.) and the like. These other additives may be used alone or in combination of two or more.

2. Non-Aqueous Secondary Battery The non-aqueous secondary battery of the present invention comprises the above electrolyte for non-aqueous secondary batteries. For other configurations and structures, configurations and structures employed in conventionally known non-aqueous secondary batteries can be applied. Generally, the non-aqueous secondary battery of the present invention can comprise a positive electrode, a negative electrode and a separator in addition to the electrolyte for non-aqueous secondary batteries.

(2-1) Positive Electrode As the positive electrode, 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 can be adopted.

This positive electrode mixture layer is prepared by adding a binder to a positive electrode active material and a conductive aid added as necessary, and dispersing this in an organic solvent to prepare a positive electrode mixture layer forming paste (in this case, The binder may be dissolved or dispersed in an organic solvent in advance), applied to the surface (one side or both sides) of a positive electrode current collector made of metal foil or the like, and dried to form a positive electrode mixture layer, It can be manufactured through a process of processing as necessary.

The above lithium-free metal sulfide is used as the positive electrode active material. The details of the lithium-free metal sulfide follow those explained above.

As a conductive aid, graphite; carbon black (acetylene black, ketjen black, etc.); amorphous carbon materials such as carbon materials with amorphous carbon generated on the surface, as in ordinary non-aqueous secondary batteries. fibrous carbon (vapor-grown carbon fiber, carbon fiber obtained by carbonizing pitch after spinning, etc.); carbon nanotube (various multilayer or single-wall carbon nanotubes); As the conductive aid for the positive electrode, one may be used alone, or two or more may be used in combination.

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

The organic solvent used in producing the positive electrode mixture is not particularly limited, and examples thereof include N-methylpyrrolidone (NMP). can be done.

Regarding the composition of the positive electrode mixture layer, for example, it is preferable that the positive electrode active material is about 70% to 95% by weight and the binder is about 1% to 30% by weight. In addition, when using a conductive aid, the positive electrode active material is about 50% to 90% by weight, the binder is about 1% to 20% by weight, and the conductive aid is 1% to 40% by weight. % is preferable.

The thickness of the positive electrode mixture layer is preferably about 1 μm to 100 μm per side of the current collector.

As the positive electrode current collector, for example, aluminum foil, stainless steel, nickel, titanium or alloys thereof, punched metal, expanded metal, mesh, etc. can be used. Foil is preferably used.

(2-2) Negative Electrode As the negative electrode, a structure 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 can be adopted.

This negative electrode mixture layer is formed by mixing a binder with a negative electrode active material and a conductive aid that is added as necessary and forming it into a sheet, which is formed on the surface (one side) of a negative electrode current collector made of a metal foil or the like. or both sides).

The negative electrode active material is not particularly limited, and for example, graphite (natural graphite, artificial graphite, etc.), difficult-to-sinter carbon, lithium metal, tin, silicon, alloys containing these, SiO, etc. can be used. Preferably, lithium metal, lithium alloys, etc. can be used for metallic lithium primary batteries and metallic lithium secondary batteries, and materials capable of doping and dedoping lithium ions (graphite (natural graphite, artificial graphite, etc.), difficult-to-sinter carbon, etc.), etc. can be used as the active material. These negative electrode active materials may be used alone or in combination of two or more.

As a conductive aid, graphite; carbon black (acetylene black, ketjen black, etc.); amorphous carbon materials such as carbon materials with amorphous carbon generated on the surface, as in ordinary non-aqueous secondary batteries. fibrous carbon (vapor-grown carbon fiber, carbon fiber obtained by carbonizing pitch after spinning, etc.); carbon nanotube (various multilayer or single-wall carbon nanotubes); As the conductive aid for the negative electrode, it may be used alone, or two or more of them may be used in combination. If the negative electrode active material has high conductivity, it may not be used.

Regarding the composition of the negative electrode mixture layer, for example, it is preferable that the negative electrode active material is about 70% to 95% by weight and the binder is about 1% to 30% by weight. In addition, when using a conductive aid, the negative electrode active material is about 50% to 90% by weight, the binder is about 1% to 20% by weight, and the conductive aid is 1% to 40% by weight. % is preferred.

The thickness of the negative electrode mixture layer is preferably about 1 μm to 100 μm per side of the current collector.

As the negative electrode current collector, for example, a foil, punched metal, expanded metal, mesh, mesh, etc. made of aluminum, copper, stainless steel, nickel, titanium, or alloys thereof can be used, and the thickness is usually 5 μm to 5 μm. A copper foil of about 30 μm is preferably used.

(2-3) Separator The positive electrode and the negative electrode described above can be used, for example, in the form of a laminated electrode body in which a separator is interposed and laminated, or in the form of a wound electrode body in which this is spirally wound.

As the separator, it is preferable to use a separator that has sufficient strength and can hold a large amount of electrolytic solution. From this point of view, polyethylene, polypropylene, ethylene-propylene with a thickness of 10 μm to 50 μm and an aperture ratio of 30% to 70%. Microporous films, non-woven fabrics, etc. containing one or more copolymers are preferred.

In addition, as the form of the non-aqueous secondary battery of the present invention, a cylindrical shape (square cylindrical shape, cylindrical shape, etc.) using a stainless steel can, an aluminum can, or the like as an outer can can be adopted. Also, a soft package battery in which a laminated film integrated with a metal foil is used as an outer package can be employed.

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

Synthesis Example 1: Synthesis of vanadium sulfide (positive electrode active material) Commercially available vanadium sulfide (III) (V ₂ S ₃ : manufactured by Kojundo Chemical Laboratory Co., Ltd.) and sulfur (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.) were weighed in a glove box (dew point -80°C) in an argon gas atmosphere so that the molar ratio was 1:6, and sealed in a glass tube in vacuum.

The vacuum-sealed sample was sintered in a tubular furnace at 400°C for 5 hours. The calcined sample was calcined in vacuum at 200°C for 8 hours to desulfurize surplus sulfur and synthesize crystalline vanadium sulfide VS ₄ (c-VS ₄ ).

Next, the obtained crystalline VS ₄ (c-VS ₄ ) was mechanically milled (ball diameter of 4 mm and rotation speed of 270 rpm), a low-crystalline vanadium sulfide VS ₄ (a-VS ₄ ) was synthesized and used as a positive electrode active material.

The powder XRD measurement of the obtained a-VS ₄ showed no clear peaks other than the minimum peak of V ₂ O ₃ , which is an extremely small amount of impurity, and it was found to be completely amorphous.

Synthesis example 2: Synthesis of molybdenum sulfide (positive electrode active material) Zhang, M. Xue, and J. Chen, ACS Appl. Mater. Interface, 9, 38606-38611 (2017).).

Commercially available ammonium molybdate tetrahydrate ((NH4) _6Mo7O24.4H2O : manufactured by FUJIFILM Wako Pure _Chemical _Industries , Ltd.) and hydroxylamine chloride ₍ _NH2OH.HCl : FUJIFILM Wako Pure Chemical _Industries , Ltd.) Co., Ltd.) was weighed into a volumetric flask so that the weight ratio was 4:3, and a mixture of ammonium sulfide ((NH ₄ ) ₂ S: Fuji Film Wako Pure Chemical Co., Ltd.) and ion-exchanged water was dripped. After that, the mixture was held at 50° C. for 1 hour and then at 90° C. for 4 hours to obtain a precipitate.

The precipitate was collected by filtration and dried under an Ar gas atmosphere for 12 hours. Amorphous MoS _5.7 was synthesized by heat-treating the dried sample at 220°C for 1 hour in an electric furnace in an Ar atmosphere.

Abbreviations EMC: Ethyl methyl carbonate TMP: Trimethyl phosphate LiTFSI: Lithium bis(trifluoromethanesulfonyl)imide FEC: Fluoroethylene carbonate VC: Vinylene carbonate DEC: Diethyl carbonate PC: Propylene carbonate DMC: Dimethyl carbonate

Example 1: FEC 10% by mass/LiTFSI:TMP:EMC (1:TMP1:solvent 1 molar ratio) TMP and LiTFSI were added to the EMC solvent so that the concentration was (1:1:1 molar ratio), and further , 10 parts by mass of FEC was added to 100 parts by mass of the mixed electrolyte to obtain an electrolyte for a non-aqueous secondary battery of Example 1.

Example 2: FEC 10% by mass/LiTFSI:TMP:EMC (1:TMP0.5:solvent 1.5 molar ratio) TMP and LiTFSI were added to the EMC medium so that the concentration was (1:0.5:1.5 molar ratio). Further, 10 parts by mass of FEC was added to 100 parts by mass of the mixed electrolyte to obtain an electrolyte for a non-aqueous secondary battery of Example 2.

Example 3: FEC 10% by mass/LiTFSI:TMP:EMC (1:TMP 0.25:solvent 1.75 molar ratio) TMP and LiTFSI were added to the EMC solvent so that the concentration was (1:0.25:1.75 molar ratio). Further, 10 parts by mass of FEC was added to 100 parts by mass of the mixed electrolyte to obtain an electrolyte for a non-aqueous secondary battery of Example 3.

Example 4: VC5% by mass/LiTFSI:TMP:EMC (1:TMP0.5:solvent 1.5 molar ratio) TMP and LiTFSI were added to the EMC solvent so that the concentration was (1:0.5:1.5 molar ratio). Further, 5 parts by mass of VC was added to 100 parts by mass of the mixed electrolyte to obtain an electrolyte for a non-aqueous secondary battery of Example 4.

Example 5: FEC 10% by mass/LiTFSI:TMP:DEC (1:TMP1:solvent 1 molar ratio) TMP and LiTFSI were added to the DEC solvent so that the concentration was (1:1:1 molar ratio), and , 10 parts by mass of FEC was added to 100 parts by mass of the mixed electrolyte to obtain an electrolyte for a non-aqueous secondary battery of Example 5.

Example 6: FEC 10% by mass/LiTFSI:TMP:PC (1:TMP1:solvent 1 molar ratio) TMP and LiTFSI were added to the PC solvent so that the concentration was (1:1:1 molar ratio), and , 10 parts by mass of FEC was added to 100 parts by mass of the mixed electrolyte to obtain an electrolyte for a non-aqueous secondary battery of Example 6.

Example 7: FEC 10% by mass/LiTFSI:TMP:DMC (1:TMP1:solvent 1 molar ratio) TMP and LiTFSI were added to the DMC solvent so that the concentration was (1:1:1 molar ratio), and , 10 parts by mass of FEC was added to 100 parts by mass of the mixed electrolyte to obtain an electrolyte for a non-aqueous secondary battery of Example 7.

Example 8: FEC 10% by mass/LiTFSI:TMP:EMC (1:TMP0.5:solvent 1.5 molar ratio) TMP and LiTFSI were added to the EMC solvent so that the concentration was (1:0.5:1.5 molar ratio). Further, 10 parts by mass of FEC was added to 100 parts by mass of the mixed electrolyte to obtain an electrolyte for a non-aqueous secondary battery of Example 8.

Comparative Example 1: No additive / LiTFSI: TMP (1: TMP2: 0 molar ratio of solvent)
LiTFSI was added to TMP at a concentration of (2:1 molar ratio) to obtain an electrolytic solution for a non-aqueous secondary battery of Comparative Example 1.

Comparative Example 2: 10% by mass of FEC/LiTFSI:TMP (1:TMP2:0 molar ratio of solvent) (solvent does not contain a carbonate compound)
LiTFSI was added to TMP so that the concentration was (2:1 molar ratio), and 10 parts by mass of FEC was added to 100 parts by mass of the mixed electrolyte. I got the liquid.

Comparative Example 3: No additive/LiTFSI:TMP:EMC (1:TMP1::solvent 1 molar ratio) TMP and LiTFSI were added to the EMC solvent so that the concentration was (1:1:1 molar ratio), An electrolytic solution for a non-aqueous secondary battery of Comparative Example 3 was obtained.

Comparative Example 4: No additive/LiTFSI:TMP:DEC (1:TMP1:solvent 1 molar ratio) TMP and LiTFSI were added to the DEC solvent so that the concentration was (1:1:1 molar ratio) and compared An electrolytic solution for a non-aqueous secondary battery of Example 4 was obtained.

Comparative Example 5: No additive/LiTFSI:TMP:EMC (1:TMP0.5:solvent 1.5 molar ratio) TMP and LiTFSI were added to the EMC solvent so that the concentration was (1:0.5:1.5 molar ratio). , an electrolytic solution for a non-aqueous secondary battery of Comparative Example 5 was obtained.

Test example 1: Charge/discharge test (after 100 cycles)
The VS4 powder obtained in Synthesis Example 1 was used as the positive electrode active material in the nonaqueous secondary battery electrolyte solutions obtained in Examples 1 to 7 and Comparative Examples 1 to ₄ .

The MoS5.7 powder obtained in Synthesis Example 2 was used as the positive electrode active material in the non-aqueous secondary battery electrolytes obtained in Example 8 and Comparative Example 5.

An electrochemical cell (lithium secondary battery) for testing was produced by the following method, and the charge/discharge rate was 0.1C (1C = 747mAh/g) at 25°C, and the voltage was within the range of 2.6V to 1.9V. Galvanostatic charge/discharge measurements were performed for 100 cycles with a rest time of 10 minutes.

As a method for producing an electrochemical cell for testing, first, the working electrode (positive electrode) was prepared by adding ₁₀ mg of VS4 powder obtained in Synthesis Example 1, 1 mg of Ketjenblack, and polytetrafluoroethylene ( 1 mg of PTFE) was added, mixed in a mortar for 8 minutes, and then attached to an aluminum mesh.

Lithium metal was used as the counter electrode (negative electrode).

　Polypropylene was used as the separator.

Table 1 shows the results of charge-discharge cycle characteristics (capacity retention rate at 100 cycles).

The capacity retention rate is the ratio of the capacity measured after 100 cycles when the capacity at the start of the cycle test (first cycle) is taken as 100. The higher the capacity retention rate, the better the life characteristics as a battery. indicates that

Test example 2: Internal resistance evaluation (after 100 cycles)
Using the electrochemical cell subjected to 100 cycles of constant-current charge-discharge measurement described in Test Example 1, the internal resistance was evaluated by the method shown below.

　After charging to 2.6V, discharge at 0.05C for 10 seconds, and measure the voltage difference between the voltage at the start of discharge and the voltage after 10 seconds of discharge. After that, constant current charging was performed at 0.05C to 2.6V. After resting for 10 minutes, discharge is performed at 0.1C for 10 seconds, and the voltage difference before and after the discharge is similarly measured. After that, it is charged at a constant current of 0.05C to 2.6V. After resting for 10 minutes, discharge at 0.2C for 10 seconds, and measure the voltage difference before and after the discharge.

The slope calculated from the graph of the current during discharge and the voltage difference measured 10 seconds later was defined as the internal resistance after 100 cycles.

Table 1 shows the results of internal resistance characteristics after 100 cycles.

　The capacity maintenance rate is not determined based on a particular threshold.

When VS4 is used as an active material, Examples 1 to 7 of the present invention exhibit a higher capacity retention rate of 35% or more than Comparative Examples 1 to 4. Examples 1 to 7 of the present invention suggest that a battery system with high energy efficiency and excellent life characteristics can be realized.

Even when MoS5.7 is used as the active material, Example 8 to which the present invention is applied exhibits a value of capacity retention rate higher than that of Comparative Example 5 by 38% or more. Example 8 to which the present invention is applied suggests that a battery system with high energy efficiency and excellent life characteristics can be realized.

　The internal resistance is not determined based on a particular threshold.

When VS4 is used as the active material in the stage before the charge/discharge cycle, the resistance of Examples 1 to 7 of the present invention is 25% or more lower than that of Comparative Examples 1 to 4. Examples 1 to 7 of the present invention suggest that a battery system with high energy efficiency can be realized because electric energy used for charging is not wasted due to resistance heating or the like.

Even when MoS5.7 is used as the active material, the resistance of Example 8 to which the present invention is applied is 40% or more lower than the internal resistance shown in Comparative Example 5. Example 8 to which the present invention is applied suggests that a battery system with high energy efficiency can be realized as in the case of using VS4.

The electrolytic solution for non-aqueous secondary batteries of the present invention and non-aqueous secondary batteries using the same can be used for various known applications. Specific examples include laptop computers, mobile phones, electric vehicles, power sources for load leveling, and natural energy storage power sources.

Claims

An electrolytic solution for a non-aqueous secondary battery,
The non-aqueous secondary battery is a non-aqueous secondary battery using a lithium-free transition metal sulfide as a positive electrode active material,
The electrolytic solution is
an organic solvent containing a carbonate compound;
a phosphate ester compound;
lithium bis(trifluoromethanesulfonyl)imide (LiTFSI);
containing an additive,
Electrolyte for non-aqueous secondary batteries.
The phosphate ester compound is at least one selected from the group consisting of trimethyl phosphate (TMP), triethyl phosphate (TEP), and tris(2,2,2-trifluoroethyl) phosphate (TFEP). , the electrolytic solution for a non-aqueous secondary battery according to claim 1.
3. The electrolytic solution for non-aqueous secondary batteries according to claim 1 or 2, wherein said additive is at least one selected from the group consisting of vinylene carbonate (VC) and fluoroethylene carbonate (FEC).
4. The content of the phosphoric acid ester compound is 0.25 to 1.0 times the content of the lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in molar ratio, according to any one of claims 1 to 3. Electrolyte solution for non-aqueous secondary batteries according to.
5. The electrolytic solution for a non-aqueous secondary battery according to claim 1, wherein the content of said additive is 5% by weight to 10% by weight, with the total amount of said electrolytic solution being 100% by weight. .
Any one of claims 1 to 5, wherein the carbonate compound is at least one selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and propylene carbonate (PC). 2. The electrolytic solution for non-aqueous secondary batteries according to item 1.
The non-aqueous secondary battery electrolyte according to any one of claims 1 to 6, wherein the lithium-free transition metal sulfide is at least one selected from the group consisting of vanadium sulfide and molybdenum sulfide. liquid.
A nonaqueous secondary battery comprising the electrolyte for nonaqueous secondary batteries according to any one of claims 1 to 7.
The nonaqueous secondary battery according to claim 8, which is a lithium ion secondary battery.