JP7498535B2 - Electrolyte for non-aqueous secondary battery and non-aqueous secondary battery using the same - Google Patents

Description

The present invention relates to an electrolyte for a non-aqueous secondary battery and a non-aqueous secondary battery using the same.

Due to the recent trend towards higher performance in portable electronic devices and hybrid vehicles, the lithium-ion secondary batteries used in them are required to have ever higher capacities. However, current lithium-ion secondary batteries do not have a high enough positive electrode capacity compared to the negative electrode, and even lithium nickel oxide-based materials, which are said to have a relatively high capacity, only have a capacity of around 190 mAh/g to 220 mAh/g.

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. However, it is generally known that sulfur-based positive electrode active materials lose capacity with repeated charge-discharge cycles. This is because sulfur dissolves into the organic electrolyte as lithium polysulfide during charging and discharging, and technology to suppress its dissolution into the organic electrolyte is essential.

Although non-lithium-containing transition metal sulfides (transition metal sulfides not containing lithium) have electronic conductivity and little elution into organic electrolyte, it is not sufficient. For example, vanadium sulfide is taken as an example of a non-lithium-containing transition metal sulfide. When crystalline vanadium sulfide (III) (V ₂ S ₃ ), which is sold as a reagent, is used as a positive electrode active material, the reaction with the organic electrolyte cannot be suppressed, and the actual measured capacity is only about 23 mAh/g charge capacity and 52 mAh/g discharge capacity. In contrast, the present inventors have reported that low-crystalline vanadium sulfide having a specific composition exhibits high capacity and excellent charge/discharge cycle characteristics when used as an electrode active material for lithium-ion secondary batteries (see, for example, Patent Document 1).

International Publication No. 2018/181698

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, the demand for higher reliability of lithium-ion secondary batteries continues, and improvements are required to achieve both high reliability and excellent charge-discharge cycle characteristics.

To improve reliability, it is believed that safety can be improved by giving the battery a self-extinguishing function in the event of a battery catching fire due to external impact such as an earthquake or traffic accident, or abnormal heating due to a fire. One method for suppressing the combustion reaction in a battery is, for example, adding a phosphate ester-based material to the electrolyte, which has the function of trapping oxygen radicals and blocking combustion-supporting gases. However, when a phosphate ester-based material is added to a general electrolyte, 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 consideration of the current state of the prior art described above, and its main objective is to provide an electrolyte that can achieve both improved 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.

The present inventors have conducted extensive research to achieve the above-mentioned objective. As a result, they have discovered that by adding an organic solvent containing lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and a carbonate compound to trimethyl phosphate (TMP), and an additive, it is possible to improve the charge-discharge cycle characteristics of a non-aqueous secondary battery using a lithium-free transition metal sulfide as a positive electrode active material.

The present invention was completed as a result of further research based on these findings.

That is, the present invention includes the following configuration.

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

Item 2.
Item 2. The nonaqueous secondary battery electrolyte according to item 1, wherein 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).

Item 3.
3. The nonaqueous secondary battery electrolyte 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).

Item 4.
4. The nonaqueous secondary battery electrolyte solution according to any one of items 1 to 3, wherein the content of the phosphate ester compound is 0.25 to 1.0 times the content of the lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in terms of a molar ratio.

Item 5.
5. The nonaqueous secondary battery electrolyte according to any one of items 1 to 4, wherein the content of the additive is 5% by weight to 10% by weight, based on 100% by weight of the total amount of the electrolyte.

Item 6.
6. The nonaqueous secondary battery electrolyte according to any one of 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).

Item 7.
7. The non-aqueous secondary battery electrolyte 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.

Item 8.
8. A non-aqueous secondary battery comprising the non-aqueous secondary battery electrolyte according to any one of items 1 to 7.

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

According to the present invention, the 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.

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

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.

1. Non-aqueous secondary battery electrolyte The present invention provides a non-aqueous secondary battery electrolyte,
The non-aqueous secondary battery is a non-aqueous secondary battery that uses a lithium-free transition metal sulfide as a positive electrode active material,
The electrolyte 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, 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 in which the nonaqueous secondary battery electrolyte of the present invention is used, and are known as a positive electrode active material for lithium ion secondary batteries. Specifically, vanadium sulfide (lithium-free vanadium sulfide: WO 2018/181698), niobium sulfide and titanium niobium sulfide (lithium-free niobium sulfide and lithium-free titanium niobium sulfide: WO 2015/049986), molybdenum sulfide (lithium-free molybdenum sulfide), iron sulfide (lithium-free iron sulfide), and the like. 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 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.

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.

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

In the present invention, the composition ratio of sulfur to transition metal (S/M ¹ ) of the lithium-free transition metal sulfide is preferably 2.1 to 10 in molar ratio, 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 an organic electrolyte solution if it comes into contact with the organic electrolyte solution.

More specifically, the non-lithium-containing transition metal sulfide has the general formula (2):
^M1S _x (2)
[In the formula, ^M1 represents a transition metal, and x represents 2.1 to 10.]
It is preferable that the composition has the following formula: When M ¹ contains a plurality of transition metals, the composition ratio of the total amount of sulfur and the transition metals (S/M ¹ ) is 2.1 to 10 in molar ratio.

Thus, in the present invention, the lithium-free metal sulfide has a high elemental ratio of sulfur to the transition metal (M ¹ ). Therefore, by using the lithium-free metal sulfide in the present invention, 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 x), the higher the charge/discharge capacity tends to be, and the lower the sulfur content (the smaller x), the less likely it is to contain elemental sulfur, and the higher the charge/discharge cycle characteristics tend to be.

In the present invention, even if a sulfide with poor charge-discharge cycle characteristics is used, the charge-discharge cycle characteristics can be improved by using an electrolyte having the composition described below, so it is particularly useful to apply polysulfides that tend to have high charge-discharge capacity but insufficient charge-discharge cycle characteristics. For this reason, x is preferably 2.1 to 10, and more preferably 3 to 8.

Below, we will explain using vanadium sulfide (lithium-free vanadium sulfide), which is a preferred lithium-free transition metal sulfide, as an example.

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

More specifically, vanadium sulfide preferably has peaks at 15.4°, 35.3°, and 45.0°, with a tolerance of ±1.0°, within the range of diffraction angles 2θ = 10° to 80° in an X-ray diffraction diagram using CuKα radiation. In other words, it is preferable for the vanadium sulfide to have 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 powder X-ray diffraction measurement (θ-2θ method) under the following measurement conditions:
Measuring device: D8ADVANCE (BrukerAXS)
X-ray source: CuKα 40kV/40mA
Measurement conditions: 2θ = 10° to 80°, 0.1° step, scan speed 0.02°/sec.

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

In the present invention, although vanadium sulfide is a sulfide having a high ratio of sulfur in its average composition, sulfur is hardly present as elemental sulfur as described below, and it is preferable that sulfur is combined with vanadium to form a low-crystalline sulfide.

Thus, in the present invention, vanadium sulfide has a low crystallinity, so that there are more sites where lithium ions can be inserted and removed, and the structure can easily have gaps that can be three-dimensional lithium conductive paths. In addition, it has many advantages, such as being easy to undergo three-dimensional volume changes during charging and discharging. Therefore, the charge/discharge capacity and charge/discharge cycle characteristics can be further improved. In addition, it is preferable that there is almost no _vanadium sulfide ( _V2S3 , etc.) used as a raw material.

In this specification, the average composition of a sulfide refers to the elemental ratio of each element that constitutes the entire sulfide.

The term "low crystallinity" in this invention is explained below.

In the present invention, the low-crystalline vanadium sulfide preferably has no peaks at 2θ=15.4°, 35.3°, and 45.0°, or if any peaks appear, the full width at half maximum of the peaks is preferably 0.8° to 2.0° (particularly 1.0° to 2.0°). Note that in crystalline vanadium(IV) sulfide (VS ₄ ), the full width at half maximum of the peaks at 2θ=15.4°, 35.3°, and 45.0° is preferably 0.2° to 0.6°.

Thus, in the present invention, it is preferred that the low-crystalline vanadium sulfide has no peaks at 2θ=15.4°, 35.3°, and 45.0°, or that even if a peak does appear, the full width at half maximum of the peak is larger than that of crystalline vanadium(IV) sulfide ( _VS4 ).

In this way, in the present invention, the low crystallinity makes it easier to increase the number of sites where Li can exist stably, so using a low-crystalline lithium-free metal sulfide as a positive electrode active material makes it easier to improve charge/discharge capacity and charge/discharge cycle characteristics.

In addition, when a material containing a large amount of elemental sulfur, etc. is used as the positive electrode active material, the cyclic carbonate compound contained in the electrolyte for a non-aqueous 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, etc., so 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.

More specifically, the strongest peak of 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 diagram 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 made into 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.

In the present invention, it is also preferable that the vanadium sulfide does not have peaks at 2θ = 25.8° and 27.8°, which are peaks characteristic of elemental sulfur, or that the area of the peaks having maximums at these 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 concerns about the above-mentioned reaction with the electrolyte and further improves the charge/discharge capacity and charge/discharge cycle characteristics.

Vanadium sulfides that satisfy these 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 even more preferable to have a peak at g(r) = 3.3 Å. In other words, it is preferable for vanadium sulfides to have not only V-S bonds but also S-S bonds (disulfide bonds).

In the present invention, the above-mentioned vanadium sulfide can be obtained, for example, by a manufacturing method using vanadium sulfide and sulfur as raw materials or intermediates, and including 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, the raw materials are ground and mixed by applying mechanical impact and friction, which causes the vanadium sulfide and sulfur to come into vigorous contact and break down into fine particles, resulting in a reaction of the raw materials. In other words, mixing, grinding, and reaction occur simultaneously. This makes it possible to react the raw materials more reliably without heating them to high temperatures. Using mechanical milling can sometimes produce metastable crystal structures that cannot be obtained by ordinary heat treatment.

As a mechanical milling process, specifically, mixed grinding can be performed using a mechanical grinding device such as a ball mill, bead mill, rod mill, vibrating mill, disk mill, hammer mill, or jet mill.

All of these raw materials or intermediates can be mixed together at the same time 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.

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

As a specific _raw material, it is preferable to use crystalline vanadium (III) sulfide ( _V2S3 ) as vanadium sulfide. 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 a high-purity vanadium sulfide. 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.

As for sulfur, elemental sulfur (S ₈ ) can be used in an amount necessary to form a sulfide of the desired composition. There is no particular limitation on the sulfur used as a raw material, and any sulfur can be used. In particular, it is preferable to use sulfur of high purity. 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.

Furthermore, as described above, when subjecting the material to multiple (particularly two-stage) mechanical milling treatments, a low-crystalline vanadium sulfide of a desired composition (such as low-crystalline VS _2.5 ) can also be used as the intermediate.

The mixing ratio of the raw materials can be the same as the elemental ratio of vanadium and sulfur in the target vanadium sulfide, since the ratio of the raw materials charged 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 of sulfur (particularly 1.2 moles to 17.0 moles, and even more preferably 3.0 moles to 13.0 moles) per mole of vanadium sulfide.

There are no particular restrictions on the temperature when performing mechanical milling, but a temperature of 300°C or less is preferable, and -10°C to 200°C is even more preferable, in order to prevent sulfur from volatilizing and to prevent the formation of the previously reported crystalline phase.

There is no particular limitation on the time for mechanical milling, and mechanical milling can be performed for any length of time until the desired vanadium sulfide is precipitated.

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

For example, the mechanical milling process can be carried out within a processing time range of 0.1 to 100 hours (particularly 15 to 80 hours). Note that this mechanical milling process can also be carried out in multiple steps with breaks in between, if necessary.

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

The above-mentioned mechanical milling process makes it possible to obtain the desired vanadium sulfide as a fine powder.

(1-2) Organic Solvent The nonaqueous secondary battery electrolyte 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 non-aqueous secondary battery electrolyte of the present invention is an electrolyte for non-aqueous secondary batteries that uses a lithium-free transition metal sulfide as a positive electrode active material. Thus, in the present invention, even though the electrolyte is used in a non-aqueous secondary battery that uses a lithium-free transition metal sulfide, the addition of the additive described below can suppress the reaction between the carbonate compound and the lithium-free transition metal sulfide, dramatically improving the charge/discharge cycle characteristics.

Carbonate compounds The carbonate compounds are not particularly limited as long as they can be used as organic solvents in the electrolyte of lithium ion secondary batteries. For example, chain carbonate compounds include dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), etc. Cyclic carbonate compounds include propylene carbonate (PC), etc. 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), ethyl methyl carbonate (EMC), and propylene carbonate (PC).

In the present invention, the organic solvent constituting the electrolyte for non-aqueous secondary batteries may consist solely of the carbonate compounds described above, or may contain compounds known as organic solvents in the electrolytes of lithium ion secondary batteries.

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

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

(1-3) Additives The nonaqueous secondary battery electrolyte of the present invention has
an organic solvent containing a carbonate compound;
A phosphate ester compound,
Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI),
and additives.

As described above, in the non-aqueous secondary battery electrolyte of the present invention, by containing an additive, it is possible to suppress the reaction between the carbonate compound and the non-lithium-containing transition metal sulfide, and dramatically improve the charge-discharge cycle characteristics.

As such an additive, vinylene carbonate (VC) or fluoroethylene carbonate (FEC) is preferable 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.

These 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 additive content is increased.

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

From the viewpoints of charge/discharge capacity, charge/discharge cycle characteristics, energy density, etc., the content of the above-mentioned additives is preferably 2.5 to 20.0 parts by mass, more preferably 2.5 to 15.0 parts by mass, and even more preferably 2.5 to 10.0 parts by mass, per 100 parts by mass of organic solvent. Even when only one type of additive is used, such as fluoroethylene carbonate (FEC), trifluoromethylethylene carbonate, vinylethylene carbonate, etc., it is easier to improve the charge/discharge cycle characteristics by increasing the amount of additive, so 2.5 to 10.0 parts by mass is preferred per 100 parts by mass of organic solvent. In addition, when two or more types of additives are used, it is easier to improve the charge/discharge cycle characteristics and the energy density even if the content is increased, so the total content of the additives is preferably 2.5 to 20.0 parts by mass, more preferably 2.5 to 15.0 parts by mass, and even more preferably 5.0 to 10.0 parts by mass, per 100 parts by mass of organic solvent.

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

The nonaqueous secondary battery electrolyte of the present invention further contains lithium bis(trifluoromethanesulfonyl)imide (LiTFSI: Li( _CF3SO2 ) _2N ) 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 withstand charging at _a higher voltage and further improve the charge/discharge cycle characteristics.

In addition, 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 nonaqueous secondary batteries, and examples thereof include organic lithium salts having a perfluoroalkanesulfonyl group, such as lithium bis( _{pentafluoroethanesulfonyl} )imide (Li( _C2F5SO2 ) _2N , etc.). These organic lithium salts having a sulfonyl group may be used alone or in combination of _two or more.

From the viewpoint of the above-mentioned charge-discharge cycle characteristics, the lithium salt is preferably an organic lithium salt having a sulfonyl group, rather than an inorganic lithium salt (LiPF ₆ , LiBF ₄ , etc.).

In addition, an organic lithium salt having a boron atom 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 nonaqueous secondary battery of the present invention uses a lithium-free metal sulfide as the positive electrode active material, the effect of reactivity with sulfur on charge-discharge cycle characteristics is taken into consideration.

As the lithium salt mentioned above, by using LiTFSI as the lithium salt, the charge-discharge cycle characteristics are good.

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

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

(1-5) Phosphate Ester Compound The nonaqueous electrolyte for a secondary battery of the present invention is
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 at least one selected from the group consisting of trimethyl phosphate (TMP), triethyl phosphate (TEP), and tris(2,2,2-trifluoroethyl) phosphate (TFEP).

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

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

(1-6) Others The nonaqueous electrolyte solution for a secondary battery of the present invention may contain other components, such as other additives, in addition to those described above, within a range that does not impair the effects of the present invention (e.g., 0.01 mol/L to 0.2 mol/L, particularly 0.02 mol/L 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.), etc. 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 includes the above-mentioned electrolyte for non-aqueous secondary batteries. Other configurations and structures may be the same as those used in conventionally known non-aqueous secondary batteries. In general, the non-aqueous secondary battery of the present invention may include a positive electrode, a negative electrode, and a separator in addition to the above-mentioned electrolyte for non-aqueous secondary batteries.

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

This positive electrode mixture layer can be produced by adding a binder to the positive electrode active material and a conductive assistant added as necessary, dispersing this 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), applying this to the surface (one or both sides) of a positive electrode current collector made of metal foil or the like, drying to form the positive electrode mixture layer, and processing as necessary.

The above-mentioned lithium-free metal sulfide is used as the positive electrode active material. Details of the lithium-free metal sulfide are as described above.

As in conventional non-aqueous secondary batteries, the conductive additives that can be used include graphite, carbon black (acetylene black, ketjen black, etc.), amorphous carbon materials such as carbon materials with amorphous carbon formed on the surface, fibrous carbon (vapor-grown carbon fiber, carbon fiber obtained by spinning pitch and then carbonizing it, etc.), and carbon nanotubes (various multi-layer or single-layer carbon nanotubes). The conductive additives for the positive electrode may be used alone or in combination of two or more.

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

There are no particular limitations on the organic solvent used in producing the positive electrode mixture, but examples include N-methylpyrrolidone (NMP), which can be used in combination with the positive electrode active material, a binder, etc. to form a paste.

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 additive is used, it is preferable that the above positive electrode active material is about 50% to 90% by weight, the binder is about 1% to 20% by weight, and the conductive additive is about 1% to 40% by weight.

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

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

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

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

The negative electrode active material is not particularly limited, and examples thereof include graphite (natural graphite, artificial graphite, etc.), sinter-resistant 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.), sinter-resistant 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.

As in conventional non-aqueous secondary batteries, the conductive additives that can be used include graphite, carbon black (acetylene black, ketjen black, etc.), amorphous carbon materials such as carbon materials with amorphous carbon formed on the surface, fibrous carbon (vapor-grown carbon fiber, carbon fiber obtained by spinning pitch and then carbonizing it, etc.), and carbon nanotubes (various multi-layer or single-layer carbon nanotubes). The conductive additives for the negative electrode may be used alone or in combination of two or more kinds, or may not be used if the conductivity of the negative electrode active material is high.

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

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. Furthermore, when a conductive assistant is used, it is preferable that the above-mentioned negative electrode active material is about 50% to 90% by weight, the binder is about 1% to 20% by weight, and the conductive assistant is about 1% to 40% by weight.

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

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, copper foil with a thickness of about 5 μm to 30 μm is preferably used.

(2-3) 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.

A separator should be strong enough and capable of retaining a large amount of electrolyte. From this viewpoint, a microporous film or nonwoven fabric having a thickness of 10 μm to 50 μm and an opening ratio of 30% to 70%, containing one or more of polyethylene, polypropylene, ethylene-propylene copolymer, etc., is preferred.

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

The present invention will now be described in detail with reference to the following examples, but 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(III) sulfide ( _V2S3 : manufactured by _Kojundo Chemical Laboratory Co., Ltd.) and sulfur (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) were weighed in a glove box in an argon gas atmosphere (dew point -80°C) so that the molar ratio was 1:6, and the mixture was sealed in a glass tube in a vacuum.

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

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 ball mill (Fritsch PL-7) in an argon gas atmosphere glove box (dew point -80°C) to synthesize low-crystalline vanadium sulfide VS ₄ (a-VS ₄ ), which was used as the positive electrode active material.

Powder XRD measurement of the obtained a- _VS4 showed that no clear peaks were observed other than the very small peak of _V2O3 , which is a very small amount of _impurity , and that the a-VS4 was completely amorphous.

Synthesis Example 2: Synthesis of molybdenum sulfide (positive electrode active material) Molybdenum sulfide was synthesized according to a method similar to that described in a previous report (X. Wang, K. Du, C. Wang, L. Ma, B. Zhao, J. Yang, M. Li, X. Zhang, M. Xue, and J. Chen, ACS Appl. Mater. Interface, 9, 38606-38611 (2017)).

Commercially available ammonium _molybdate tetrahydrate (( _{NH4 )6Mo7O24.4H2O :} Fujifilm Wako Pure Chemical Industries, Ltd.) and _{hydroxylamine} chloride ( _NH2OH.HCl : Fujifilm Wako Pure Chemical Industries, Ltd.) were weighed into a measuring flask in a weight ratio of 4:3, and a mixture of ammonium sulfide (( _NH4 ) _2S : Fujifilm Wako Pure Chemical Industries, Ltd.) and ion-exchanged water was added dropwise. The mixture was then kept 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. The dried sample was then heated in an electric furnace in an Ar atmosphere at 220 °C for 1 hour to synthesize amorphous MoS _5.7 .

Explanation of 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 mass %/LiTFSI:TMP:EMC (molar ratio of 1:TMP 1:solvent 1) TMP and LiTFSI were added to an EMC solvent to give a concentration of (1:1:1 molar ratio), and 10 parts by mass of FEC was added per 100 parts by mass of the mixed electrolyte solution to obtain a nonaqueous electrolyte solution for a secondary battery of Example 1.

Example 2: FEC 10 mass %/LiTFSI:TMP:EMC (1:TMP 0.5:solvent 1.5 molar ratio) TMP and LiTFSI were added to an EMC medium to give a concentration of (1:0.5:1.5 molar ratio), and 10 parts by mass of FEC was added per 100 parts by mass of the mixed electrolyte solution to obtain a nonaqueous secondary battery electrolyte solution 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 an EMC solvent to give a concentration of (1:0.25:1.75 molar ratio), and 10 parts by mass of FEC was added per 100 parts by mass of the mixed electrolyte solution to obtain a nonaqueous secondary battery electrolyte solution of Example 3.

Example 4: VC 5 mass %/LiTFSI:TMP:EMC (1:TMP 0.5:solvent 1.5 molar ratio) TMP and LiTFSI were added to an EMC solvent to give a concentration of (1:0.5:1.5 molar ratio), and VC was further added in an amount of 5 parts by mass per 100 parts by mass of the mixed electrolyte solution to obtain a nonaqueous electrolyte solution for a secondary battery of Example 4.

Example 5: FEC 10% by mass/LiTFSI:TMP:DEC (molar ratio of 1:TMP, 1:solvent) TMP and LiTFSI were added to a DEC solvent to give a concentration of (1:1:1 molar ratio), and 10 parts by mass of FEC was added per 100 parts by mass of the mixed electrolyte solution to obtain a nonaqueous secondary battery electrolyte solution of Example 5.

Example 6: FEC 10 mass %/LiTFSI:TMP:PC (molar ratio of 1:TMP 1:solvent 1) TMP and LiTFSI were added to a PC solvent to give a concentration of (1:1:1 molar ratio), and 10 parts by mass of FEC was added per 100 parts by mass of the mixed electrolyte solution to obtain a nonaqueous secondary battery electrolyte solution of Example 6.

Example 7: FEC 10% by mass/LiTFSI:TMP:DMC (molar ratio of 1:TMP 1:solvent 1) TMP and LiTFSI were added to a DMC solvent to give a concentration of (1:1:1 molar ratio), and 10 parts by mass of FEC was added per 100 parts by mass of the mixed electrolyte solution to obtain a nonaqueous secondary battery electrolyte solution of Example 7.

Example 8: FEC 10% by mass/LiTFSI:TMP:EMC (1:TMP 0.5:solvent 1.5 molar ratio) TMP and LiTFSI were added to an EMC solvent to give a concentration of (1:0.5:1.5 molar ratio), and 10 parts by mass of FEC was added per 100 parts by mass of the mixed electrolyte solution to obtain a nonaqueous secondary battery electrolyte solution of Example 8.

Comparative Example 1: No additive/LiTFSI:TMP (1:TMP2:solvent molar ratio 0)
LiTFSI was added to TMP so as to give a concentration of (2:1 molar ratio) to obtain a nonaqueous secondary battery electrolyte solution of Comparative Example 1.

Comparative Example 2: FEC 10% by mass/LiTFSI:TMP (1:2:0 molar ratio of TMP:solvent) (solvent does not contain carbonate compound)
LiTFSI was added to TMP to give a concentration of (2:1 molar ratio), and 10 parts by mass of FEC was further added per 100 parts by mass of the mixed electrolyte solution to obtain a nonaqueous electrolyte solution for a secondary battery of Comparative Example 2.

Comparative Example 3: No additive/LiTFSI:TMP:EMC (1:TMP1::solvent 1 molar ratio) TMP and LiTFSI were added to an EMC solvent to give a concentration of (1:1:1 molar ratio) to obtain a nonaqueous secondary battery electrolyte solution of Comparative Example 3.

Comparative Example 4: No additive/LiTFSI:TMP:DEC (1:TMP 1:solvent 1 molar ratio) TMP and LiTFSI were added to a DEC solvent to give a concentration of (1:1:1 molar ratio) to obtain a nonaqueous secondary battery electrolyte solution of Comparative Example 4.

Comparative Example 5: No additive/LiTFSI:TMP:EMC (1:TMP 0.5:solvent 1.5 molar ratio) TMP and LiTFSI were added to the EMC solvent to give a concentration of (1:0.5:1.5 molar ratio) to obtain a nonaqueous secondary battery electrolyte solution of Comparative Example 5.

Test example 1: Charge/discharge test (after 100 cycles)
The nonaqueous secondary battery electrolyte solutions obtained in Examples 1 to 7 and Comparative Examples 1 to 4 were used together with the _VS4 powder obtained in Synthesis Example 1 as a positive electrode active material.

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

A test electrochemical cell (lithium secondary battery) was prepared using the following method, and 100 constant-current charge/discharge measurements were performed at 25°C, with a charge/discharge rate of 0.1C (1C=747mAh/g), a voltage range of 2.6V to 1.9V, and a rest period of 10 minutes between cycles.

The test electrochemical cell was prepared as follows: first, a working electrode (positive electrode) was prepared by adding 1 mg of Ketjen black and 1 mg of polytetrafluoroethylene (PTFE) as a binder to 10 mg of the _VS4 powder obtained in Synthesis Example 1, mixing them in a mortar for 8 minutes, and then attaching them to an aluminum mesh.

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

Polypropylene was used as the separator.

The results of the charge/discharge cycle characteristics (capacity retention at 100 cycles) are shown in Table 1.

The capacity retention rate is the ratio of the capacity measured after 100 cycles, with the capacity at the start of the cycle test (first cycle) set at 100. A higher capacity retention rate indicates better battery life characteristics.

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

After charging to 2.6V, discharge at 0.05C for 10 seconds and measure the voltage difference between when discharge begins and after 10 seconds of discharge. Then charge at a constant current of 0.05C up to 2.6V. After a 10-minute break, discharge at 0.1C for 10 seconds and measure the voltage difference before and after discharge in the same way. Then charge at a constant current of 0.05C up to 2.6V. After a 10-minute break, discharge at 0.2C for 10 seconds and measure the voltage difference before and after discharge.

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

The results of the internal resistance characteristics after 100 cycles are shown in Table 1.

There is no particular threshold value for determining capacity maintenance rate.

When VS4 was used as the active material, Examples 1 to 7 of the present invention showed a capacity retention rate that was 35% or higher than that of Comparative Examples 1 to 4. Examples 1 to 7 of the present invention suggest that it is possible to realize a battery system with high energy efficiency and excellent life characteristics.

Even when MoS5.7 is used as the active material, Example 8 to which the present invention is applied shows a capacity retention rate that is 38% or more higher than the results of Comparative Example 5. 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.

There is no particular threshold value for internal resistance.

When VS4 was used as the active material at the stage before the charge/discharge cycle, Examples 1 to 7 of the present invention had a resistance that was 25% or more lower than 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 the electrical energy used for charging is not wasted due to resistance heating or the like.

Even when MoS5.7 is used as the active material, Example 8 to which the present invention is applied has an internal resistance that 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, similar to the case where VS4 is used.

The nonaqueous secondary battery electrolyte of the present invention and the nonaqueous secondary battery using the electrolyte can be used for various known applications. Specific examples include notebook computers, mobile phones, electric vehicles, load leveling power sources, and natural energy storage power sources.

Claims (9)

A non-aqueous secondary battery electrolyte,
The non-aqueous secondary battery is a non-aqueous secondary battery that uses a lithium-free transition metal sulfide as a positive electrode active material,
The electrolyte solution is
an organic solvent containing a carbonate compound;
A phosphate ester compound,
Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI),
and an additive,
Electrolyte for non-aqueous secondary batteries. The electrolyte for a non-aqueous secondary battery according to claim 1, wherein 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 electrolyte for a non-aqueous secondary battery according to claim 1 or 2, wherein the additive is at least one selected from the group consisting of vinylene carbonate (VC) and fluoroethylene carbonate (FEC). The nonaqueous secondary battery electrolyte solution according to any one of claims 1 to 3, wherein the content of the phosphate ester compound is 0.25 to 1.0 times the content of the lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in terms of a molar ratio. The electrolyte for a non-aqueous secondary battery according to any one of claims 1 to 4, wherein the content of the additive is 5% by weight to 10% by weight, with the total amount of the electrolyte being 100% by weight. The nonaqueous secondary battery electrolyte solution according to 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). The non-aqueous secondary battery electrolyte according to any one of claims 1 to 6, wherein the non-lithium-containing transition metal sulfide is at least one selected from the group consisting of vanadium sulfide and molybdenum sulfide. A non-aqueous secondary battery comprising the non-aqueous secondary battery electrolyte solution according to any one of claims 1 to 7. The nonaqueous secondary battery described in claim 8, which is a lithium ion secondary battery.