JP7217531B2 - Lithium metal sulfide and method for producing the same - Google Patents

Description

The present invention relates to a lithium metal sulfide and a method for producing the same.

2. Description of the Related Art Due to the recent high performance of portable electronic devices, hybrid vehicles, and the like, secondary batteries (especially lithium-ion secondary batteries) used therein are required to have higher capacities. In particular, high energy density is required for lithium ion secondary batteries for electric vehicles. In current lithium-ion _secondary batteries, the capacity of the positive electrode is lagging behind that of the negative electrode. /g. Therefore, there is a demand for a positive electrode active material with a higher capacity.

On the other hand, sulfur has a high theoretical capacity of about 1670 mAh/g, making it a promising candidate for high-capacity electrode materials. However, since elemental sulfur does not contain lithium, it is necessary to use lithium or an alloy containing lithium for the negative electrode, and there is a disadvantage that the range of choices for the negative electrode is narrow.

On the other hand, since lithium sulfide contains lithium, alloys such as graphite and silicon can be used for the negative electrode. Dangers such as short circuits can be avoided. Sulfides such as lithium sulfide have a high theoretical capacity (1167 mAh/g in the case of lithium sulfide), and are expected as positive electrode active materials for high energy density batteries. However, since lithium sulfide is an insulating material, it is common to mix carbon or the like into the electrode in order to supplement conductivity, which results in a decrease in energy density. In addition, in a battery system using an organic electrolyte, lithium sulfide is eluted into the electrolyte as lithium polysulfide during charging and discharging, decomposing the electrolyte, and moving to the negative electrode to segregate. cannot be used as it is for charging and discharging. Therefore, in order to improve the performance of batteries using lithium sulfide as the positive electrode, it is necessary to design a positive electrode layer that retains eluted lithium polysulfide in the positive electrode, devise an electrolytic solution that protects the negative electrode, and furthermore. Countermeasures such as replacement with solid electrolytes that do not cause elution are required.

As a method of suppressing lithium polysulfide elution, a method of forming bonds with different elements so that sulfur atoms are not liberated during the Li intercalation/deintercalation reaction is conceivable. For that purpose, it is necessary to prepare a compound in which a different element is introduced into lithium sulfide. As the dissimilar element to be added, a transition metal element capable of imparting electrical conductivity to insulating lithium sulfide is suitable. is described. As a result, the elution of sulfur into the electrolytic solution is suppressed, enabling charging and discharging, and the electrical conductivity is also improved.

However, it is generally known that the capacity of sulfur-based positive electrode active materials decreases after repeated charge-discharge cycles. For example, in the case of the material in which Fe is combined with lithium sulfide by current sintering in the above-mentioned Patent Document 1, the bond between S and Fe in the outermost surface exposed to the electrolyte during charging and discharging is weak and unstable. Therefore, when the charge-discharge cycle is repeated, the positive electrode active material is eluted into the electrolyte and the polysulfide ions react with the lithium metal of the negative electrode. As the liquid is decomposed, an insoluble inorganic or organic film is formed on the surface of the positive electrode active material, increasing the resistance and lowering the capacity. Therefore, after 50 charge/discharge cycles, the capacity retention rate is only a few percent.

In order to suppress such cycle deterioration, coating the positive electrode active material with various materials has also been investigated. For example, Patent Document 2 describes coating a positive electrode active material with carbon, Patent Document 3 describes coating a positive electrode active material with a polymer, and Patent Document 4 describes a positive electrode active material. is described to be coated with a solid electrolyte.

WO2015/037598 JP 2006-024415 A JP 2016-197590 A JP 2016-164888 A

However, in the coating methods as in Patent Documents 2 to 4, the material forming the coating layer does not contribute to charging and discharging of lithium ions, so the energy density is reduced accordingly. In addition, as in Patent Documents 2 to 4, when the material is coated with a material having no lithium conductivity, the diffusion of lithium on the surface of the positive electrode active material is hindered and the resistance increases, so the capacity rather decreases.

The present invention has been made in view of the current state of the prior art described above, and its main object is to provide a material that improves charge-discharge cycle characteristics without reducing initial capacity.

The present inventors have made intensive studies to achieve the above-described object. As a result, when a specific lithium metal sulfide is used as a core material and is coated with a coating layer made of a specific (lithium) metal sulfide, lithium conductivity and electronic conductivity are improved unlike conventional coating methods. It has been found that the charge-discharge cycle characteristics can be improved without lowering the initial capacity because the surface of the sulfide positive electrode is coated with the material having the sulfide. This material can be manufactured by a convenient method. The present invention was completed as a result of further studies based on these findings. That is, the present invention includes the following configurations.
Section 1. All or part of the surface of the core material is covered with a coating layer,
The core material has the general formula (1):
_{LixM1SyCz (} ¹ ₎
[In the formula, M ¹ represents at least one selected from the group consisting of Fe, Ti, V and Nb. x indicates 5.0 to 15.0. y indicates 4.0 to 8.0. z indicates 0 to 5.0. ]
containing a compound represented by
Lithium metal sulfide, wherein the coating layer contains a lithium transition metal sulfide.
Section 2. Item 2. The lithium metal sulfide according to item 1, wherein the coating amount of the coating layer is 1.0 to 15.0 mol %.
Item 3. Item 3. The lithium metal sulfide according to item 1 or 2, which is a reaction-fired product of the compound represented by the general formula (1) and a transition metal compound.
Section 4. Item 3. Item 3, wherein the transition metal constituting the transition metal compound is at least one selected from the group consisting of Ti, V, Zr, Fe, Co, Ni, Cu, Nb, Mo, Hf, Ta and W. of lithium metal sulfide.
Item 5. Item 5. The lithium metal sulfide according to any one of Items 1 to 4, wherein the lithium transition metal sulfide is a crystalline material.
Item 6. The core material contains lithium sulfide (Li ₂ S) as a main phase, and the crystallite size calculated from the half-value width of the diffraction peak based on the (111) plane of Li ₂ S obtained by powder X-ray diffraction is 50 nm. Item 6. The lithium metal sulfide according to any one of items 1 to 5, wherein:
Item 7. The method for producing lithium metal sulfide according to any one of items 1 to 6,
A production method comprising a step of reacting the lithium metal sulfide represented by the general formula (1) with a transition metal compound, followed by firing.
Item 8. A positive electrode active material for a lithium ion battery, containing the lithium metal sulfide according to any one of items 1 to 6.
Item 9. Item 9. A positive electrode for a lithium ion battery, comprising the positive electrode active material according to item 8.
Item 10. Item 10. A lithium ion battery comprising the positive electrode according to item 9.

According to the present invention, charge-discharge cycle characteristics can be improved without lowering the initial capacity.

1 is a mapping diagram of a sample obtained in Example 1 by SEM-EDX measurement. FIG. Upper left is SEM image, upper right is sulfur mapping, lower left is iron mapping, and lower right is titanium mapping. 1 is a mapping diagram of a sample obtained in Example 1, obtained by TEM measurement. FIG. The upper left figure shows the HAADF-TEM image, the upper right figure shows the mapping map of sulfur by STEM-EDX, the lower left figure shows the mapping map of iron by STEM-EDX, and the lower right figure shows the mapping map of titanium by STEM-EDX. Scale bar is 300 nm. 4 is a graph showing the results of a charge/discharge test of lithium secondary batteries using the samples obtained in Examples 1 and 2 and Comparative Examples 1 and 2 as positive electrode active materials.

As used herein, "contain" is a concept that includes both "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.

1. Lithium metal sulfide The lithium metal sulfide of the present invention is a material in which all or part of the surface of the core material is covered with a coating layer.

(1-1) Core material In the lithium metal sulfide of the present invention, the core material has the general formula (1):
_{LixM1SyCz (} ¹ ₎
[In the formula, M ¹ represents at least one selected from the group consisting of Fe, Ti, V and Nb. x indicates 5.0 to 15.0. y indicates 4.0 to 8.0. z indicates 0 to 5.0. ]
Contains a compound represented by.

In the present invention, M ¹ in general formula (1) is preferably Fe, Ti, V, Nb, or the like, from the viewpoint of further suppressing free sulfur by bonding with sulfur and further improving electrical conductivity. Among them, Fe is preferable from the viewpoint of electrical conductivity and cost. These elements can be used alone or in combination of two or more.

Furthermore, regarding the abundance ratio of each element in the core material of the lithium metal sulfide of the present invention, there is an amount of M ¹ that can form an M ¹ -S bond to the extent that free sulfur is not generated, and it is estimated from the amount of Li content. It is preferable that the amount of Li having a large theoretical capacity is present and the amount of M ¹ sufficient to ensure conductivity is present. From this point of view, in the above general formula (1), x is 5.0 to 15.0, preferably 6.0 to 10.0, more preferably 7.0 to 9.0. In general formula (1) above, y is 4.0 to 8.0, preferably 5.0 to 7.0. Furthermore, the core material may contain C, and in the above general formula (1), z is 0 to 5.0, preferably 1.0 to 4.0.

The average particle diameter of such a core material is not particularly limited, but is preferably 0.05 to 500 μm, more preferably 0.1 to 200 μm, from the viewpoint of further improving the charge-discharge cycle characteristics while maintaining the initial capacity by coating with a coating layer described later. is more preferred. The average particle size of the core material is measured by a dry laser diffraction/scattering method. Also, the shape of the core material is not particularly limited, and any shape may be used as long as it does not particularly interfere with normal electrode preparation and charge/discharge tests.

Such a compound represented by the general formula (1) contains lithium sulfide (Li ₂ S) as a main phase, and half of the diffraction peak based on the (111) plane of Li ₂ S obtained by powder X-ray diffraction A material having a crystallite size of 50 nm or less calculated from the value width (hereinafter also referred to as “lithium sulfide composite”) is preferable. In this material, the iron atoms, which are the additive element, are arranged in the lithium sulfide crystal lattice to form Fe-S bonds and do not contain free sulfur. Elution can be further suppressed, and degradation of cycle characteristics due to decomposition of the electrolytic solution and migration or deposition to the negative electrode can be particularly prevented. In addition, since it contains Fe and, if necessary, C, it is endowed with conductivity, and it is a material that exhibits particularly excellent charge-discharge characteristics.

Such a lithium sulfide composite has a main phase composed of lithium sulfide in powder X-ray diffraction measurement. The amount of the lithium sulfide phase present is not particularly limited, and is preferably 90 mol % or more, more preferably 95 mol % or more, based on the entire composite.

Moreover, such a lithium sulfide composite is refined by, for example, a mechanical milling method to have a crystallite size of 50 nm or less. The crystallite size is a value calculated based on Scherrer's formula from the half-value width of the diffraction peak based on the (111) plane, which indicates the highest intensity of the lithium sulfide peak observed as the main phase in powder X-ray diffraction measurement. is.

In such a lithium sulfide composite, the iron atoms are arranged in the lithium sulfide crystal lattice to form Fe-S bonds, forming a metastable phase of iron-containing lithium sulfide, which can be processed by, for example, mechanical milling. The iron-containing lithium sulfide, which is originally a metastable phase, is stabilized by miniaturization into submicron particles.

In addition, the lithium sulfide composite described above may contain a small amount of impurities other than the crystal phase of lithium sulfide as long as it is about 10 mol% or less (especially 5 mol% or less). The effect on charge/discharge characteristics is limited.

Such a lithium sulfide composite can be produced according to the method described in WO2015/037598. The entire description of WO2015/037598 is incorporated by reference. Incidentally, as the lithium metal sulfide described above, the same applies to those containing transition metals other than Fe.

(1-2) Coating Layer In the lithium metal sulfide of the present invention, the coating material constituting the coating layer contains a lithium transition metal sulfide. Lithium transition metal sulfides refer to compounds containing lithium, transition metals and sulfur, among which compounds consisting only of lithium, transition metals and sulfur are preferred. Such a lithium transition metal sulfide is, for example, a reaction baked product of the compound represented by the general formula (1) and a transition metal compound (details of the transition metal compound will be described later), as in the production method described later. and, for example, general formula (2):
_{LipM2Sq (} ² )
[In the formula, M ² is the same as or different from M ¹ and represents a transition metal. p is greater than 0 and less than or equal to 4.0. q indicates 0.5 to 3.0. ]
It can be a lithium transition metal sulfide represented by This (lithium) metal sulfide itself also contributes to charging and discharging.

In the present invention, the transition metal (M ² in the general formula (2)) constituting the transition metal compound is a transition metal from the viewpoint of making it difficult to be eluted by the electrolytic solution and making it a material with excellent lithium conductivity and electron conductivity. and preferably Ti, V, Zr, Fe, Co, Ni, Cu, Nb, Mo, Hf, Ta, W, and the like. Among them, Ti and Mo are preferable from the viewpoint of forming a layered positive electrode active material and having higher stability to an electrolytic solution. These elements can be used alone or in combination of two or more.

Furthermore, regarding the abundance ratio of each element in the lithium metal sulfide coating material of the present invention, p is greater than 0 from the viewpoint of making the material less likely to be eluted by the electrolytic solution and having better lithium conductivity and electron conductivity. It is preferably 4.0 or less, more preferably 0.5 to 3.0, even more preferably 1.0 to 2.0. Moreover, in the above general formula (2), q is preferably 0.5 to 3.0, more preferably 1.0 to 2.0.

A crystalline material is preferable for the coating material that constitutes such a coating layer, from the viewpoint that the crystallinity of the coating material that constitutes the coating layer can be increased and the charge-discharge cycle characteristics can be further improved. Crystallinity is confirmed by X-ray diffraction measurement (XRD).

The coating layer containing the coating material as described above covers all or part of the surface of the core material. In particular, the coating amount is preferably such that the entire surface of the core material is coated in order to uniformly coat and further suppress the elution of polysulfides to further improve the cycle characteristics. On the other hand, from the viewpoint of maintaining the initial capacity, it is preferable that the coating amount is not too large. From such a viewpoint, the coating amount of the coating layer is preferably 1.0 to 15.0 mol%, more preferably 2.0 to 10.0 mol%, still more preferably 3.0 to 8.0 mol%, particularly preferably 3.3 to 7.0 mol%, and 3.5 to 6.0 mol % is most preferred. The coating amount of the coating layer was calculated from the ^ratio of the amount of the transition metal ( ^M2 ) in the lithium metal sulfide measured by SEM ^- EDX to the ^total amount of M1 and ^M2 . The total amount of is calculated as 100 mol%.

In addition, the thickness of the coating layer made of the above coating material is preferably 0.5 to 500 nm, from the viewpoint of maintaining the initial capacity while further suppressing the elution of polysulfides and further improving the cycle characteristics, and 1 to 500 nm. 200 nm is more preferred. In addition, the thickness of the coating layer is measured by TEM-EDX.

2. Method for Producing Lithium Metal Sulfide The method for producing lithium metal sulfide of the present invention is not particularly limited. For example, it can be produced by a method comprising a step of reacting a lithium metal sulfide (nucleus material) represented by the general formula (1) with a transition metal compound (coating material precursor), followed by firing. .

For example, when the above lithium sulfide composite is used as the compound (nucleus material) represented by the general formula (1) and titanium halide is used as the transition metal compound, the lithium sulfide present on the outermost surface of the core material is , the following reaction equation:
2Li ₂ S + TiX ₄ → TiS ₂ (coating material) + 4LiX (by-product)
A covering material is formed. By firing after this, the crystallinity of the coating material is increased to improve the charge-discharge cycle characteristics, and the diffusion of the lithium atoms contained in the core material makes the coating material a material containing lithium, resulting in the initial charge-discharge efficiency. and energy density can be improved. The coating material formed at this time is a material that is less likely to be eluted into the electrolytic solution and has excellent lithium conductivity and electron conductivity, so that the charge/discharge cycle characteristics can be improved while maintaining the initial capacity.

The compound (nucleus material) represented by the general formula (1) can be produced according to the method described in International Publication No. 2015/037598, as described above. The shape of the compound (nucleus) represented by formula (1) is not particularly limited, but it can be adjusted so as to have a desired size in the lithium metal sulfide of the present invention. The average particle size can also be controlled by using a material having a large particle size as the compound (nucleus material) represented by the general formula (1) and pulverizing the material with a mortar or the like.

The transition metal compound (coating material precursor) to be reacted with the compound (nucleus material) represented by the general formula (1) is not particularly limited as long as it is a material from which a lithium transition metal sulfide can be obtained by reaction. Examples include halides, transition metal nitrates, transition metal sulfates, and the like. The transition metals constituting these transition metal compounds can be the same as those constituting the lithium transition metal sulfide to be obtained. Among these, transition metal halides are preferred, and transition metal chlorides are more preferred, from the viewpoint of reactivity with the compound (nucleus material) represented by formula (1), solubility in organic solvents, and the like. Examples of such transition metal compounds ₍ coating material precursors) include _TiCl4 , _VCl4 , _ZrCl4 , FeCl3, _CoCl2 , _NiCl2 , _CuCl4 , _NbCl4 , _MoCl5 , _HfCl4 , _TaCl5 , _WCl4 , _TiBr4 , _VBr4 , _ZrBr4 , _FeBr3 , _CoBr2 , _NiBr2 , CuBr4, _NbBr4 , _MoBr5 , _HfBr4 , _TaBr5 , _WBr4 , _TiI4 , _VI4 , _{ZrI4 , FeI3} , _CoI2 , _NiI2 , _CuI4 , _NbI4 , _MoI5 , _HfI4 , TaI5, WI4, Ti( _NO3 ) ₄ , V( _NO3 ) ₄ , Zr _{( NO3} ) ₄ , Fe _{( NO3} ) ₃ , Co( _NO3 ) ₂ , Ni( _NO3 ) ₂ , Cu( _NO3 ) ₄ , Nb( _NO3 ) ₄ , Mo( _NO3 ) ₅ , Hf( _NO3 ) ₄ , Ta( _NO3 ) ₅ , W( _NO3 ) ₄ , Ti( _SO4 ) ₂ , V( _SO4 ) ₂ , Zr( _SO4 ) ₂ , Fe2( _SO4 ) ₃ , _CoSO4 , _NiSO4 , Cu _{( SO4} ) ₂ , Nb(SO ₄ ) ₂ , Mo ₂ (SO ₄ ) ₅ , Hf(SO ₄ ) ₂ , Ta ₂ (SO ₄ ) ₅ and the like. These transition metal compounds can be used alone or in combination of two or more.

The amount of the transition metal compound to be used is not particularly limited, but it is preferable to adjust the coating amount to the above-mentioned amount. Specifically, the transition metal compound is preferably 2.0 to 50.0% by mass, more preferably 2.5 to 40.0% by mass, when the total amount of the compound (nuclear material) represented by the general formula (1) and the transition metal compound is 100% by mass. is more preferable, 3.0 to 35.0% by mass is more preferable, and 10.0 to 30.0% by mass is particularly preferable.

The reaction between the compound represented by formula (1) (nucleus material) and the transition metal compound is not particularly limited, but is conveniently carried out in a liquid phase. For example, the compound (nucleus material) represented by the general formula (1) can be reacted with the transition metal compound in an organic solvent. Specifically, the compound (nucleus material) represented by the general formula (1) and the transition metal compound can be added simultaneously or sequentially to the organic solvent. In particular, from the viewpoint of coating the compound (nucleus material) represented by general formula (1) more uniformly with the coating material, the compound (nucleus material) represented by general formula (1) is added to an organic solvent. It is preferable to obtain the lithium metal sulfide of the present invention by adding and dispersing, then adding the transition metal compound, stirring as necessary, and subjecting the resulting suspension to solid-liquid separation.

The organic solvent that can be used at this time is preferably one that can disperse the compound represented by the general formula (1) (nucleus material) without reacting or dissolving it, and dissolves the transition metal compound. Specific examples include heptane, limonene, ethylhexanol, and the like. These organic solvents are preferably selected appropriately according to the compound (nucleus material) represented by the general formula (1) and the transition metal compound to be used. In addition, these organic solvents can be used alone, or two or more of them can be used in combination.

Moreover, the method of solid-liquid separation is not particularly limited, and known methods such as filtration and centrifugation can be employed.

The temperature of the above reaction is not particularly limited, and can be usually 0 to 100°C, particularly 10 to 50°C. The reaction time is not particularly limited as long as the coating layer can be sufficiently formed, and is preferably 5 minutes to 24 hours, more preferably 10 minutes to 12 hours. In addition, the atmosphere for the above reaction is not particularly limited, and can usually be an inert atmosphere such as a nitrogen gas atmosphere or an argon gas atmosphere.

After that, a coating layer can be formed on the compound (nucleus material) represented by the general formula (1) by performing a usual post-treatment such as washing and drying, if necessary.

In this way, when the compound (nucleus material) represented by the general formula (1) is coated with the coating material, the coating material does not contain lithium (p is 0 in the general formula (2)). , an amorphous material is likely to be obtained. In the present invention, while the coating material is a crystalline material, the sintering treatment is performed to make the material containing lithium (a material in which p is not 0 in the general formula (2)) by diffusion of the lithium atoms contained in the core material. In this way, by firing, the crystallinity of the coating material that constitutes the coating layer is improved, thereby further improving the charge-discharge cycle characteristics, and by using a material containing lithium, the initial charge-discharge efficiency and energy density are also improved. can be improved.

Although the firing temperature is not particularly limited, it is preferably 200 to 1000°C, more preferably 300 to 800°C, from the viewpoint of further improving the charge-discharge cycle characteristics by further increasing the crystallinity. The baking time is not particularly limited as long as the coating material can be a crystalline material, and is preferably 5 minutes to 48 hours, more preferably 10 minutes to 24 hours. In addition, the atmosphere for the above reaction is not particularly limited, and can usually be an inert atmosphere such as a nitrogen gas atmosphere or an argon gas atmosphere.

Thereafter, the lithium metal sulfide of the present invention can be obtained by subjecting the product to post-treatment such as washing and drying as required. From the viewpoint of removing by-products, it is effective to use an organic solvent for washing, and washing using an organic solvent used as an electrolytic solution, a known higher alcohol, or the like can be applied. For example, dimethyl carbonate (DMC), ethylhexanol and the like are preferably used.

3. Lithium Ion Batteries The lithium metal sulfide of the present invention can be effectively used as a positive electrode active material for lithium ion batteries such as lithium primary batteries, lithium ion secondary batteries, metal lithium secondary batteries, etc., by taking advantage of the excellent properties described above. can. In particular, since the lithium metal sulfide of the present invention is a material containing lithium in its structure, it is a material that can be charged and discharged starting from charging, and has excellent cycle characteristics. It is useful as a positive electrode active material for lithium ion batteries. The lithium ion battery using the lithium metal sulfide of the present invention as a positive electrode active material is characterized by excellent charge-discharge cycle characteristics because it is difficult to dissolve in the electrolytic solution. It is preferably used for a non-aqueous electrolyte lithium ion battery (especially a non-aqueous electrolyte lithium ion secondary battery) using a liquid.

The structure of the non-aqueous electrolyte lithium ion battery can be the same as that of known lithium ion batteries, except that the lithium metal sulfide of the present invention is used as a positive electrode active material. For example, the basic structure can be the same as that of known non-aqueous electrolyte lithium ion batteries, except that the lithium metal sulfide described above is used as the positive electrode active material.

As for the positive electrode, the lithium metal sulfide of the present invention is used as a positive electrode active material, and for example, a positive electrode mixture prepared by mixing the lithium metal sulfide of the present invention and, if necessary, a conductive aid and a binder is used. It can be supported on a positive electrode current collector such as Al, Ni, Ti, Au, Ag, Cu, stainless steel, and carbon cloth. Examples of conductive aids include carbon materials (graphite, coke, carbon black, activated carbon, etc.), conductive fibers (needle-shaped carbon; carbon nanotubes; vapor-grown carbon, pitch (by-products such as petroleum, coal, coal tar, etc.). Carbon fiber manufactured from acrylic fiber), corrosion-resistant metals (titanium, gold, etc.), carbides (SiC, WC, etc.), nitrides _{( Si3N4} , BN, etc.) ) etc. can also be used. As the binder, known materials can be used without particular limitation, and examples thereof include styrene-butadiene rubber, carboxymethyl cellulose, polyvinylidene fluoride (PVDF), polyimide and the like. Binders having reactive groups that are crosslinked by heating, ultraviolet irradiation, or the like can also be preferably used. Examples of reactive groups include vinylene groups, hydroxy groups, epoxy groups, allyl groups, carbonyl groups, and reactive groups obtained by substituting a part of the carbon atoms of the above reactive groups with different elements. There are no particular restrictions on the shape of the positive electrode current collector, and examples include metal foil (thickness 10-100 μm), perforated metal foil (thickness 10-100 μm, hole diameter 0.1-10 mm), expanded metal, foamed metal plate, glassy carbon plate, and the like. be done.

Both materials containing lithium and materials not containing lithium can be used for the negative electrode. For example, graphite (artificial graphite, natural graphite, etc.), graphitizable carbon (soft carbon), etc., lithium metal or alloys of lithium and aluminum, etc., phosphorus, tin, silicon and alloys containing these, SiO etc. can also be used. Lithium metal is preferred from the standpoint of capacity. These negative electrode active materials can also be supported on a negative electrode current collector made of Al, Ni, Ti, Au, Ag, Cu, stainless steel, carbon, etc., using a conductive agent, a binder, etc., as necessary. can. Examples of conductive aids include carbon materials (graphite, coke, carbon black, activated carbon, etc.), conductive fibers (needle-shaped carbon; carbon nanotubes; vapor-grown carbon, pitch (by-products such as petroleum, coal, coal tar, etc.). Carbon fiber manufactured from acrylic fiber), corrosion-resistant metals (titanium, gold, etc.), carbides (SiC, WC, etc.), nitrides _{( Si3N4} , BN, etc.) ) etc. can also be used. As the binder, known materials can be used without particular limitation, and examples thereof include styrene-butadiene rubber, carboxymethyl cellulose, polyvinylidene fluoride (PVDF), polyimide and the like. Binders having reactive groups that are crosslinked by heating, ultraviolet irradiation, or the like can also be preferably used. Examples of reactive groups include vinylene groups, hydroxy groups, epoxy groups, allyl groups, carbonyl groups, and reactive groups obtained by substituting a part of the carbon atoms of the above reactive groups with different elements. There are no particular restrictions on the shape of the negative electrode current collector, such as metal foil (thickness 10-100 μm), perforated metal foil (thickness 10-100 μm, hole diameter 0.1-10 mm), expanded metal, foamed metal plate, glassy carbon plate, etc. be done.

The separator is made of, for example, polyolefin resin such as polyethylene and polypropylene, fluororesin, nylon, aromatic aramid, inorganic glass, and the like, and can be used in the form of porous membrane, nonwoven fabric, woven fabric, or the like.

The non-aqueous electrolyte is not particularly limited, and a known organic solvent and electrolyte salt can be used in combination. As the solvent for the non-aqueous electrolyte, a solvent known as a solvent for non-aqueous secondary batteries such as carbonate, ether, nitrile, sulfur-containing compound, ionic liquid, and solvated ionic liquid can be used.

The shape of the non-aqueous electrolyte lithium ion battery is also not particularly limited, and may be cylindrical, square, or the like.

EXAMPLES The present invention will be specifically described below with reference to examples and comparative examples. However, it goes without saying that the present invention is not limited only to the examples shown below.

Synthesis Example 1: Synthesis of Lithium Sulfide Complex (Nuclear Material) Commercially available lithium sulfide (Li ₂ S) (average particle size: about 16 μm) and iron sulfide (FeS) (average particle size: about 6 μm) were mixed at a molar ratio of 4:4. The mixture was weighed in a glove box (dew point -80°C) in an argon gas atmosphere so as to be 1, thoroughly mixed in a mortar, and filled in a graphite mold with an inner diameter of 15 mm.

Next, the graphite shaped material filled with raw materials was placed in an electric sintering machine. The current-carrying part including the graphite mold material and the electrode part is housed in a vacuum chamber, and the chamber is filled with high-purity argon gas (oxygen concentration: about 0.2ppm) to atmospheric pressure after degassing to a vacuum (about 20Pa). bottom.

After that, a pulse current of about 600 A (pulse width of 2.5 milliseconds, period of 28.6 Hz) was applied while pressurizing the material filled in the graphite mold at about 30 MPa. The vicinity of the graphite mold was heated at a heating rate of about 200°C/min, reaching 600°C 3 minutes after the start of pulse current application. Immediately after that, the current application and pressurization were stopped, and the sample was allowed to cool naturally.

After cooling to room temperature, the graphite jig was transferred to a glove box having an argon gas atmosphere with a dew point of −80° C., and the reaction product of lithium sulfide and iron sulfide was taken out from the mold and pulverized in a mortar. Acetylene black (AB) powder was mixed with this so that the weight ratio of Li ₂ S-FeS: AB = 9: 1. Using a vibration cup mill (model MC-4A) manufactured by Seisakusho, it was processed by mechanical milling for 8 hours to obtain Li ₂ S-FeS.

This Synthesis Example 1 is the same as Example 5 of WO 2015/037598, and the obtained material has the same properties. The obtained material contained 48.4 atomic % of Li, 6.1 atomic % of Fe, 30.3 atomic % of S, and 15.2 atomic % of C, and represented by a composition formula of Li ₈ FeS ₅ C _2.5 .

From the X-ray diffraction pattern of the obtained sample, a peak derived from lithium sulfide was recognized as the main phase, and a peak of FeS was recognized as a very small amount of impurities. The abundance ratio (mol%) of FeS estimated by Rietveld analysis was about 5%. The crystallite size estimated from the half width of the diffraction peak based on the (111) plane of lithium sulfide was about 29 nm. Furthermore, the average particle diameter measured by a dry laser diffraction/scattering method was about 7 μm.

Example 1
Li ₂ S-FeS and the coating material precursor (TiCl ₄ ) were weighed so that the coating concentration (25.7% by mass) shown in Table 1 was obtained, dispersed in 40 mL of heptane, and stirred for 1 hour. The coating concentration is given by the following formula [1]:
Coating concentration (% by mass) = (mass of coating material precursor) / (mass of coating material precursor + mass of Li2S _- FeS) x 100 [1]
was calculated using

Solids were separated by filtration and washed 10 times with 1 mL of dimethyl carbonate (DMC). After drying at room temperature for 12 hours or more, it was baked at 400° C. for 10 hours to obtain a coated positive electrode active material. All of the above operations were performed in a glove box in an argon atmosphere.

[Coating amount]
SEM-EDX measurement was performed in order to calculate the coating amount of the coated positive electrode active material obtained in the above examples. The content of Ti that makes up the coating layer when the total amount of Fe that makes up the core material and Ti that makes up the coating layer is taken as 100 mol% from EDX measurement after the coated positive electrode active material is spread on the carbon tape. The amount of coating was estimated by The amount of coverage is given by the following formula [2]:
Coating amount (mol%) = (Ti mol%) / (Fe mol% + Ti mol%) x 100 [2]
Calculated according to Moreover, the thickness of the coating layer was measured by TEM-EDX.

FIG. 1 shows a mapping diagram of sulfur, iron and titanium obtained by SEM-EDX measurement for the sample of Example 1. FIG. Further, FIG. 2 shows a mapping diagram of sulfur, iron and titanium obtained by TEM-EDX measurement for the sample of Example 1. As shown in FIG. As a result, since sulfur and iron are detected at the same position, it can be seen that the metal polysulfide active material composed of sulfur and iron was normally synthesized by firing. Titanium was also detected in the same way, and it can be seen that a coating layer containing Ti was uniformly formed on the entire surface of the metal polysulfide on the microscale and nanoscale by the coating treatment.

In addition, as a result of elemental analysis of iron, lithium, and titanium by ICP-AES and calculating the molar ratio, iron: lithium = 1: 8 (molar ratio) before coating, but iron: lithium after coating = 1:8.7 (molar ratio), indicating a significant increase in the ratio of lithium to iron. Since iron exists only in the core part and the composition ratio of the core part does not change significantly during firing, the significant increase in the lithium ratio suggests that lithium-containing metal sulfides were formed on the surface of the core material. ing. From the TEM-EDX image, it is inferred that a layer containing titanium is formed on the surface of the coated active material, and that the coating layer contains titanium and lithium. The ratio of titanium to excess lithium is titanium:lithium = 4.1:4.6, which is close to the ratio of the stable compound LiTiS ₂ , suggesting that a similar substance is formed on the surface.

[Charging and discharging test]
Using a mortar, the coated positive electrode active material (1.8 mg), conductive agent (Ketjenblack, KB), and binder (polytetrafluoroethylene, PTFE) were mixed so that the mass ratio was 51.4: 34.3: 14.3 (mass%). and mixed together to produce a positive electrode. A positive electrode, a separator (polypropylene), and lithium metal are layered in order from the bottom of a stainless steel cell (HS flat cell, manufactured by Hosen), and an electrolytic solution (1.0M LiPF ₆ / ethylene carbonate (EC) + propylene carbonate (PC )= 1:1% by volume) was injected into the cell by 300 μL to caulk the cell. After standing for 8 hours, the charge/discharge test was repeated for 100 cycles under the conditions of 2.6-1.0V vs Li/Li ⁺ at a current value corresponding to 0.11C (1C = 986mAh/g). From the results of the charge-discharge cycle test, the following formula [3]:
Cycle capacity retention rate (%) = discharge capacity after 100 cycles / initial discharge capacity x 100 [3]
The cycle capacity retention rate was calculated based on. Also, the initial discharge capacity was evaluated as the initial capacity.

Example 2
The same operation as in Example 1 except that the electrolyte in the charge-discharge test was 1M lithium bis(trifluoromethanesulfonyl)imide (LiTFSA) / ethylene carbonate (EC) + ethyl methyl carbonate (EMC) = 3: 7% by volume. and measured the cycle capacity retention rate. The cycle capacity retention rate is given by the following formula [4]:
Cycle capacity retention rate (%) = discharge capacity after 48 cycles / initial discharge capacity x 100 [4]
was calculated using

Example 3
An experiment was conducted in the same manner as in Example 1 except that the coating concentration was adjusted to 14.7% by mass, and the coating amount and cycle capacity retention rate were measured. The coating concentration is given by the following formula [1]:
Coating concentration (% by mass) = (mass of coating material precursor) / (mass of coating material precursor + mass of Li2S _- FeS) x 100 [1]
was calculated using

Example 4
An experiment was conducted in the same manner as in Example 1 except that the coating concentration was adjusted to 34.3% by mass, and the coating amount and cycle capacity retention rate were measured. The coating concentration is given by the following formula [1]:
Coating concentration (% by mass) = (mass of coating material precursor) / (mass of coating material precursor + mass of Li2S _- FeS) x 100 [1]
was calculated using

Example 5
In the charge-discharge test, the electrolyte was 0.6M lithium bis(trifluoromethanesulfonyl)imide (LiTFSA) + 0.4M lithium bisoxalate borate (LiBOB) / ethylene carbonate (EC) + ethylmethyl carbonate (EMC) = 4: 6 volumes. The experiment was performed in the same manner as in Example 1, except that it was changed to %.

Example 6
Except for changing the electrolyte to 1.5M LiPF ₆ + 0.5M lithium bis(trifluoromethanesulfonyl)imide (LiTFSA)/ethylene carbonate (EC) + ethyl methyl carbonate (EMC) = 3: 7% by volume in the charge/discharge test. was performed in the same manner as in Example 1.

Example 7
In the charge/discharge test, the same operation as in Example 1 was performed except that the electrolytic solution was changed to 1.0 M LiBF ₄ /ethylene carbonate (EC) + ethyl methyl carbonate (EMC) = 3:7% by volume.

Example 8
In the charge/discharge test, the same operation as in Example 1 was performed except that the electrolytic solution was changed to 1.0M LiPF ₆ /propylene carbonate (PC).

Example 9
In the charge-discharge test, Example 1 except that the electrolyte was changed to 1.0 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSA)/dimethyl ether (DME) + 1,3-dioxolane (DOL) = 1: 1% by volume. experimented with the same procedure.

Comparative example 1
An experiment was performed in the same manner as in Example 1, except that the active material was not coated. That is, using the sample obtained in Synthesis Example 1, a charge/discharge test was performed in the same manner as in Example 1.

Comparative example 2
An experiment was conducted in the same manner as in Example 2, except that the active material was not coated. That is, using the sample obtained in Synthesis Example 1, a charge/discharge test was performed in the same manner as in Example 2.

The results of a 50-cycle charge/discharge test for the samples obtained in Examples 1 and 2 and Comparative Examples 1 and 2 are shown in FIG. Table 1 shows the coating amount and the capacity retention rate after 100 cycles or 48 cycles for the samples obtained in Examples 1 and 2 and Comparative Examples 1 and 2. In Comparative Examples 1 and 2, which were not coated, the capacity retention rates were 18% and 16%, respectively, but were improved to 60% and 65%, respectively, by coating. This is probably because a coating layer having high stability against the electrolyte was formed on the surface of the active material, and as a result, decomposition of the electrolyte on the surface of the active material was suppressed. It also shows that the coating layer of the present invention contributes to charging and discharging of lithium ions. Moreover, it is presumed that the same results as in Examples 1 and 2 are obtained in Examples 3 to 9 as well.

Claims (10)

All or part of the surface of the core material is covered with a coating layer,
The core material has the general formula (1):
_{LixM1SyCz (} ¹ ₎
[In the formula, M ¹ represents at least one selected from the group consisting of Fe, Ti, V and Nb. x indicates 5-15. y indicates 4.0 to 8.0. z indicates 0 to 5.0. ]
containing a compound represented by
Lithium metal sulfide, wherein the coating layer contains a lithium transition metal sulfide. 2. The lithium metal sulfide according to claim 1, wherein the coating amount of said coating layer is 1 to 15 mol %. 3. The lithium metal sulfide according to claim 1, which is a reaction-fired product of the compound represented by the general formula (1) and a transition metal compound. 4. According to claim 3, wherein the transition metal constituting the transition metal compound is at least one selected from the group consisting of Ti, V, Zr, Fe, Co, Ni, Cu, Nb, Mo, Hf, Ta and W. Lithium metal sulfide as described. Lithium metal sulfide according to any one of claims 1 to 4, wherein the lithium transition metal sulfide is a crystalline material. The core material contains lithium sulfide (Li ₂ S) as a main phase, and the crystallite size calculated from the half-value width of the diffraction peak based on the (111) plane of Li ₂ S obtained by powder X-ray diffraction is 50 nm. The lithium metal sulfide according to any one of claims 1 to 5, wherein: The method for producing lithium metal sulfide according to any one of claims 1 to 6,
A production method comprising a step of reacting the lithium metal sulfide represented by the general formula (1) with a transition metal compound, followed by firing. A positive electrode active material for lithium ion batteries, comprising the lithium metal sulfide according to any one of claims 1 to 6. A positive electrode for a lithium ion battery, comprising the positive electrode active material according to claim 8 . A lithium ion battery comprising the positive electrode according to claim 9 .

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