JP7428431B2 - Method for manufacturing a negative electrode material for a lithium ion secondary battery, method for manufacturing a negative electrode layer for a lithium ion secondary battery, and method for manufacturing a lithium ion secondary battery - Google Patents

Description

The present invention relates to a method for manufacturing a negative electrode material for a lithium ion secondary battery, and a method for manufacturing a negative electrode layer for a lithium ion secondary battery using this negative electrode material for a lithium ion secondary battery, and a lithium ion secondary battery. The present invention relates to a manufacturing method.

Lithium ion secondary batteries are indispensable as power sources for information devices such as personal computers, communication devices such as mobile phones, and electric vehicles.

A lithium ion secondary battery is formed by placing an electrolyte between a positive electrode and a negative electrode, and discharge and charge are performed by lithium ions moving between the positive electrode and the negative electrode.

The currently popular lithium-ion secondary batteries use organic solvents as electrolytes, and as these organic solvents pose a risk of fire or explosion, it is necessary to increase safety by using durable containers. However, there are problems such as restrictions on structure and shape.

On the other hand, all-solid-state lithium-ion secondary batteries that use a solid electrolyte rather than a liquid electrolyte are attracting attention, and research is progressing toward practical application.

Since the electrolyte in all-solid-state lithium-ion secondary batteries is solid, there is less risk of fire or explosion, and there is a greater degree of freedom in structure and shape, making it possible to increase capacity by making it thinner or multi-layered. There are advantages such as: It is also expected that high-speed charging and discharging will become possible (see, for example, Patent Documents 1 and 2).

In the above-mentioned lithium ion secondary batteries, carbon-based materials are used as negative electrode materials not only in conventional lithium ion secondary batteries but also in all-solid lithium ion secondary batteries. In particular, amorphous carbon-coated graphite, which is graphite coated with an amorphous carbon material, has been developed as a negative electrode material that improves electrical properties such as rate characteristics, initial Coulombic efficiency, and specific volume (for example, Patent Document 3- (See 5th grade).

Patent Document 3 uses amorphous carbon-coated graphite coated with an amorphous carbon coating film obtained by firing a thermoplastic resin selected from polyvinyl chloride, polyvinyl alcohol, polyvinylpyrrolidone, etc. as a negative electrode material. (See claim 2-3, [0018]).

In the invention of Cited Document 3, graphite and a thermoplastic resin such as polyvinyl chloride are mixed, the surface of the graphite is coated with the thermoplastic resin, and then the thermoplastic resin coated on the graphite is carbonized by firing. By this method, amorphous carbon-coated graphite is produced by coating graphite with an amorphous carbon coating film (see [0019]-[0020]).

Patent Document 4 uses an amorphous carbon-coated graphite material in which the surface of graphite particles is coated with amorphous carbon as a negative electrode material (see claim 1).

In the invention of Patent Document 4, graphite particles are mixed with an organic compound such as pitch, which is an amorphous carbon precursor, and the surface of the graphite particles is coated with the organic compound such as pitch, and then fired. By carbonizing the organic compound coated on graphite particles, an amorphous carbon-coated graphite material in which graphite particles are coated with amorphous carbon is produced (see [0051]-[0062]). ).

Patent Document 5 uses carbon-coated graphite having graphite particles as a core and a carbon layer covering the graphite particles as a negative electrode material (see claim 1).

In the invention of Patent Document 5, graphite particles are dispersed and mixed in a solution containing a nitrogen-containing polymer compound such as polyacrylonitrile or polyimide, and the solvent is removed to form graphite particles into a nitrogen-containing polymer compound that is a carbon precursor. Carbon-coated graphite, in which graphite particles are coated with a carbon layer, is produced by coating the graphite particles with a carbon layer and then carbonizing the nitrogen-containing polymer compound coated on the graphite particles by firing ([0031] - see [0038]).

Japanese Patent Application Publication No. 2017-107826 JP 2019-40709 Publication Japanese Patent Application Publication No. 2001-229914 JP 2019-87519 Publication Japanese Patent Application Publication No. 2014-139942

As mentioned above, the inventions of Patent Documents 3 to 5 utilize organic materials that serve as carbon precursors, such as thermoplastic resins such as polyvinyl chloride, organic compounds such as pitch, and nitrogen-containing polymer compounds such as polyacrylonitrile, to graphite. By mixing with particles, the surface of graphite particles is coated with an organic material that becomes a carbon precursor, and by firing this and carbonizing the organic material, graphite particles are coated with a layer of amorphous carbon. This method produces coated amorphous carbon-coated graphite.

Here, when using a lithium ion secondary battery for electric vehicles, hybrid vehicles, large-scale power storage, etc., the negative electrode material needs to have higher conductivity. When using amorphous carbon-coated graphite as the negative electrode material as in Patent Documents 3 to 5 above, the carbide of the coated amorphous carbon needs to have a higher carbonization density in order to obtain higher conductivity. It is. However, when the carbon precursor is an organic material such as those disclosed in Patent Documents 3 to 5 above, it is difficult to obtain a high carbonization density because the carbonization yield is low.

In addition, when manufacturing all-solid-state lithium-ion secondary batteries, when molding the negative electrode or stacking the solid electrolyte between the positive and negative electrodes, large uniaxial pressure or linear pressure of several tons is applied. It turns out. At this time, if the negative electrode material is made of graphite particles, graphite has a smaller elastic modulus than the oxides that form the positive electrode, so the graphite particles deform greatly in the uniaxial direction, forming a surface where lithium ions are inserted. Strain occurs in the (002) plane, making it difficult for lithium ions to insert and withdraw, resulting in a decline in input/output characteristics. When graphite particles are coated with carbon as in Patent Documents 3 to 5 above, isostatic pressure is applied to the graphite particles instead of uniaxial pressure, and deformation can be alleviated. However, in order to prevent deformation of graphite particles with the carbon film coated in this way, the carbon film needs to have a high density that can withstand high pressure. It is difficult to obtain a carbon film with high density using such materials, and the effect of preventing deformation of graphite particles cannot be said to be sufficient.

Moreover, in the inventions of Patent Documents 3 to 5 above, by mixing graphite particles and an organic material, the surface of the graphite particles is coated with a layer of an organic material that becomes a carbon precursor, and this is fired to carbonize the organic material. By this method, amorphous carbon-coated graphite is manufactured by coating graphite particles with a layer of amorphous carbon, but the layer of organic material that becomes the carbon precursor is produced by mixing graphite particles and organic material. Because it is formed by the graphite particles, the adhesion is insufficient because it simply adheres to the surface of the graphite particles. Furthermore, it is difficult to continuously coat the entire surface of the graphite particles with a layer of the organic material due to the poor wettability of the organic material to the graphite particles. Therefore, the amorphous carbon layer formed by firing the organic material layer is only attached to the surface of the graphite particles, and the adhesion is insufficient, and the amorphous carbon layer is divided. However, it is difficult to coat the entire surface of graphite particles with a layer of carbon.

In this way, if the adhesion between the graphite particles and the amorphous carbon layer is insufficient and the amorphous carbon layer is separated, the graphite may be coated with a layer of amorphous carbon and its electrical properties may deteriorate. In addition, the effect of suppressing the deformation of graphite during molding of an all-solid-state lithium ion secondary battery may also become insufficient.

The present invention has been made in view of the above points, and provides a lithium ion secondary battery that can obtain a lithium ion secondary battery with excellent electrical properties, particularly an all-solid lithium ion secondary battery with excellent electrical properties. It is an object of the present invention to provide a method for manufacturing a negative electrode material, a method for manufacturing a negative electrode layer for a lithium ion secondary battery, and a method for manufacturing a lithium ion secondary battery.

The method for producing a negative electrode material for lithium ion secondary batteries according to the present invention involves mixing furans with graphite particles and causing a condensation reaction in the presence of a reaction catalyst, or mixing furans and aldehydes with graphite particles. A process of manufacturing a granular graphite/furan resin composite material in which the surface of graphite particles is coated with a layer of furan resin by carrying out a condensation reaction in the presence of a reaction catalyst; This method is characterized by a step of producing a granular graphite/furan resin composite carbonized material in which the surface of graphite particles is coated with a layer of carbonized furan resin by heating in an oxidizing atmosphere.

Furan resins obtained by condensing furans alone or by condensing furans and aldehydes have a high carbonization yield, so the carbon obtained by carbonizing this furan resin is The layer has a high carbonization density, and by using graphite particles coated with a carbonization layer with a high carbonization density as a negative electrode material, it is possible to form a highly conductive negative electrode layer, which is useful for electric vehicles and large power sources. This makes it possible to create large-capacity lithium-ion secondary batteries for storage purposes, etc. In addition, since the carbon layer obtained by carbonizing furan resin has a high density, it is possible to cover graphite particles with a high-density carbon film, which is useful when molding all-solid-state lithium-ion secondary batteries, etc. This makes it possible to create a lithium ion secondary battery that can withstand the high pressure applied to the graphite particles and suppress deformation of the graphite particles, has excellent input/output characteristics, and is capable of rapid discharging and charging.

The furan resin layer coated on the surface of the graphite particles is formed by the condensation reaction of furans alone on the surface of the graphite particles, or by the condensation reaction of furans and aldehydes. , the adhesion of the furan resin layer to the graphite particles is higher than when a furan resin layer is formed by mixing graphite particles and furan resin, and the furan resin is divided over the entire surface of the graphite particles. It can be coated without needing to be coated. For this reason, the carbon layer obtained by carbonizing furan resin has high adhesion to the graphite particles, and the entire surface of the graphite particles is coated without being separated. It is possible to obtain a high effect of improving the electrical characteristics and of mitigating deformation of graphite particles during molding of an all-solid-state lithium ion secondary battery.

In the present invention, the above condensation reaction may be carried out in a reduced pressure atmosphere.

By performing the reaction in a reduced pressure atmosphere in this way, it is possible to perform the reaction of furans alone or with aldehydes while degassing and removing gas from the surface of the graphite particles, and it is possible to react with furans alone or with aldehydes, and to react with the liquid. By bringing the reaction liquid into contact with the surface of graphite particles, which have poor wettability, it is possible to completely coat the entire surface of the graphite particles with furan resin, which is a condensation reaction product, in close contact with the surface. Therefore, a carbon layer obtained by carbonizing furan resin can be completely formed in close contact with the surface of graphite particles, and the above-mentioned effect of coating graphite particles with a carbon layer and improving electrical properties can be achieved. Moreover, the above-mentioned effect of alleviating deformation of graphite particles during molding of an all-solid-state lithium ion secondary battery can be obtained even more.

The present invention is characterized in that the degree of pressure reduction of the above-mentioned reduced pressure atmosphere is in the range of 0.007 to 0.095 MPa.

By setting the degree of pressure reduction within this range, the reaction rate of the condensation reaction can be maintained at an appropriate level while obtaining a sufficient deaeration effect.

The method for manufacturing a negative electrode layer for a lithium ion secondary battery according to the present invention is characterized in that the negative electrode layer is manufactured using the negative electrode material manufactured by the method described above.

By manufacturing a negative electrode layer for a lithium ion secondary battery using the above negative electrode material, it is possible to obtain a lithium ion secondary battery with a large capacity and capable of rapid discharging and charging using this negative electrode layer. It is.

A method for manufacturing a lithium ion secondary battery according to the present invention is characterized by using the above-mentioned negative electrode layer and including a step of disposing an electrolyte between the negative electrode layer and the positive electrode layer.

By manufacturing a lithium ion secondary battery using the negative electrode layer as described above, a lithium ion secondary battery with a large capacity and capable of rapid charging and discharging can be obtained.

According to the present invention, it is possible to obtain a lithium ion secondary battery with a large capacity and excellent electrical properties such as being capable of rapid discharging and charging, particularly an all-solid lithium ion secondary battery with excellent electrical properties. It is.

FIG. 1 is a schematic diagram showing the structure of an all-solid-state lithium ion secondary battery.

Embodiments of the present invention will be described below.

In the present invention, the negative electrode material is produced by forming a carbonized layer of furan resin on the surface of graphite particles.

The graphite particles mentioned above are not particularly limited, but natural graphite, artificial graphite, Quiche graphite, expanded graphite, etc. can be used. Further, spheroidized graphite or agglomerated graphite obtained by processing these may also be used. In addition to selecting and using one type of graphite particles from these, it is also possible to use a mixture of a plurality of types. Although the particle size of the graphite particles is not particularly limited, it is preferable that the average particle size is about 0.1 to 200 μm. In the present invention, the average particle diameter is a value measured based on a laser diffraction/scattering method.

Further, the above-mentioned furan resin can be obtained by subjecting furans alone to a condensation reaction in the presence of a reaction catalyst, or by subjecting furans and aldehydes to a condensation reaction in the presence of a reaction catalyst.

As the furan, furan compounds such as furfural, 5-methyl-2-furfural, and furfuryl alcohol can be used. These may be used alone or in combination of two or more.

The most suitable aldehyde is formalin, which is in the form of an aqueous solution of formaldehyde, but other forms such as paraformaldehyde, acetaldehyde, benzaldehyde, trioxane, and tetraoxane can also be used, and some or most of other aldehydes can also be used. It is also possible to use a substitute for furfural or furfuryl alcohol.

As a reaction catalyst, oxides, hydroxides, and carbonates of alkali metals such as sodium, potassium, and lithium, or oxides, hydroxides, and carbonates of alkaline earth metals such as calcium, magnesium, and barium may be used. Can be done. Furthermore, hydrochloric acid, phosphoric acid, sulfuric acid, xylene sulfonic acid, p-toluenesulfonic acid, oxalic acid, maleic acid, maleic anhydride, 7-amino-4-hydroxy-2-naphthalenesulfonic acid, etc. can also be used.

In the present invention, first, by mixing furans alone with graphite particles and causing a condensation reaction in the presence of a reaction catalyst, or by mixing furans and aldehydes with graphite particles and causing a condensation reaction in the presence of a reaction catalyst. By this, a layer of furan resin is formed on the surface of the graphite particles. When reacting furans and aldehydes here, the amount of aldehydes blended with respect to the furans is preferably in the range of 0.2 to 2.5 moles of aldehydes per 1 mole of furans, and the amount of the reaction catalyst blended. varies greatly depending on the type of reaction catalyst, but is preferably in the range of 0.05 to 10% by mass based on the furans.

The above reaction is carried out in a sufficient amount of water to stir the reaction system, and the furans are subjected to a self-condensation reaction while the reaction system is stirred, or the furans and aldehydes are subjected to a condensation reaction. . At the beginning of the reaction, the solution in the reaction system is nearly transparent, but as the reaction progresses, it becomes milky and cloudy, and the reaction system becomes viscous mayonnaise-like and fluid with stirring, but as the reaction progresses, it gradually becomes cloudy. , the condensation reaction product of furans containing graphite particles, or the condensation reaction product of furans containing graphite particles and aldehydes, began to separate from the water in the system, and the furan resin produced by the reaction and graphite particles agglomerated. The composite particles suddenly become dispersed throughout the reaction vessel.

Then, when the reaction of the furan resin is further advanced to a desired degree and the stirring is stopped after cooling, the composite particles are precipitated and separated from the water. These composite particles are microscopic water-containing granules that can be easily separated from water by taking them out of the reaction vessel and filtering them, and can be dried to obtain granules. .

The granules obtained as described above are in the form of graphite particles (single graphite particles or secondary particles or composite particles of graphite particles aggregated) coated with a layer made of furan resin. Here, the condensation reaction of furans and the condensation reaction of furans and aldehydes proceeds on the surface of graphite particles, and furan resin is generated on the surface of graphite particles, and the graphite particles and furan resin are mixed. The adhesion between the graphite particles and the furan resin layer formed on the surface thereof is significantly higher than that in the case where a furan resin layer is formed on the surface of the graphite particle. In addition, the condensation reaction of furans and the condensation reaction of furans and aldehydes proceeds over the entire surface of the graphite particles, so it is possible to coat the entire surface of the graphite particles with a layer of furan resin without breaking them. It is. The particle size of this graphite/furan resin composite material is not particularly limited, but it is desirable that the average particle size is in the range of about 1 to 30 μm.

Since the particles of the graphite/furan resin composite material are agglomerated graphite and furan resin, the ratio of graphite particles to furan resin is the same in each particle, and the furan resin is the same as the graphite particles. Since the surface can be coated thinly and uniformly, a graphite/furan resin composite material can be easily obtained even if the amount of furan resin is reduced. Therefore, the coating ratio of the furan resin to the graphite particles can be adjusted arbitrarily, and the amount of the furan resin can be adjusted, for example, from 3 to 45% by mass based on the total amount of the graphite/furan resin composite material (that is, the amount of the graphite particles can be adjusted to 55% by mass). 97% by mass).

Here, the above condensation reaction is desirably carried out in a reduced pressure atmosphere (pressure lower than atmospheric pressure) by reducing the pressure inside the reaction vessel. Although the pressure reduction can be carried out from the beginning of the reaction or from the middle of the reaction, it is preferable to create a reduced pressure atmosphere from the beginning of the reaction. Further, this reaction can be carried out under boiling reflux of the water contained in the reaction solution, but it can also be carried out below the boiling temperature.

By performing the reaction in a reduced pressure atmosphere in this way, the air adhering to the surface of the graphite particles and the gas in the macropores and micropores inside the graphite particles can be degassed and removed. It becomes possible to bring the reaction liquid into direct contact with the surface of graphite particles, which have poor wettability with the graphite particles. As a result, the condensation reaction of furans alone or the condensation reaction of furans and aldehydes proceeds with the reaction liquid in contact with the entire surface of the graphite particles, and the condensation reaction product is spread over the entire surface of the graphite particles. It is possible to obtain graphite/furan resin composite particles in which the furan resin is formed in close contact with the graphite particles and the entire surface of the graphite particles is completely covered with a layer of the furan resin. In addition, monomers such as unreacted free furans are easily removed from the furan resin that coats the surface of the graphite particles by reducing the pressure, and the amount of unreacted monomers remaining in the furan resin is reduced, resulting in stable quality. Graphite/furan resin composite particles can be obtained.

The above-mentioned reduced pressure atmosphere is preferably set in a range of about 0.007 to 0.095 MPa (0.07 to 9.69 kgf/cm ² ). The higher the degree of vacuum, the higher the deaeration effect, and the easier it is to produce graphite/furan resin composite particles that exclude air from the surface of graphite particles and are completely coated with furan resin. When the reaction is carried out under boiling reflux of water, the boiling temperature depends on the degree of reduced pressure; in a reduced pressure atmosphere of less than 0.007 MPa, the boiling temperature of water is approximately 40°C or less, and it is impossible to raise the temperature higher than this. Therefore, in order to advance the reaction, a large amount of reaction catalyst is required to be added, which is unfavorable from an economic point of view. On the other hand, in a reduced pressure atmosphere exceeding 0.095 MPa, the degassing effect is insufficient and the boiling temperature of water is approximately 97.5°C or higher, so the reaction rate becomes faster and the degassing cannot catch up with the reaction. In this respect as well, the deaeration effect becomes insufficient.

In producing the graphite/furan resin composite material as described above, the condensation reaction of the furan resin is sustained until the produced furan resin becomes insoluble and infusible, and then stopped, resulting in a completely cured furan resin. It is possible to obtain a resin-coated graphite/furan resin composite material.

In addition, in order to obtain a cured graphite/furan resin composite material, in the process of coating the surface of graphite particles with a layer of furan resin, a condensation reaction is carried out until the furan resin produced becomes insoluble and infusible. In addition to making it last longer, in the process of coating the surface of the graphite particles with a layer of furan resin, the condensation reaction of the furan resin is stopped in a state where the resin has thermosetting properties, so that it remains uncured and thermosetting. After preparing graphite/furan resin composite particles made of furan resin, the furan resin may be completely cured by heat-treating the particles to obtain a cured graphite/furan resin composite material. The conditions for the heat treatment in the step of curing the furan resin are not particularly limited, but are preferably set at a temperature of about 80 to 350° C. and a heating condition of about 1 to 100 hours.

After obtaining the graphite/furan resin composite material with hardened furan resin as described above, the graphite/furan resin composite material is heat treated in a non-oxidizing atmosphere to carbonize the furan resin coated on the surface of the graphite particles. . The non-oxidizing atmosphere only needs to be an atmosphere in which the furan resin is not oxidized, and can be set to an inert gas atmosphere such as argon, helium, or nitrogen gas. The conditions for the heat treatment are preferably set at 400 to 3000° C. for about 1 to 300 hours in order to sinter and carbonize the furan resin. In this way, it is possible to obtain a granular graphite/furan resin composite carbonized material in which the surfaces of graphite particles are coated with a layer of amorphous carbon in which furan resin is carbonized. Although the particle size of this graphite/furan resin composite carbonized material is not particularly limited, it is desirable that the average particle size is in the range of about 1 to 30 μm.

When heat-treated in a non-oxidizing atmosphere as described above, the furan resin undergoes thermal decomposition by being exposed to heat, and the decomposition products that become low molecular weight substances volatilize, leaving behind the voids. Therefore, a large number of pores are formed in the carbon layer formed in the graphite/furan resin composite carbonized material, similar to activated carbon, and the graphite/furan resin composite carbonized material obtained by the present invention It can be said that this material is more suitable for use as a negative electrode material for lithium ion secondary batteries.

Next, a lithium ion secondary battery will be explained. A lithium ion battery is formed by disposing an electrolyte between a positive electrode and a negative electrode. For example, an all-solid lithium ion secondary battery is shown in FIG. 1. It is formed by laminating a positive electrode layer 1, a negative electrode layer 2, and a solid electrolyte layer 3, and further laminating a positive electrode current collecting layer 4 that collects current from the positive electrode layer 1 and a negative electrode current collecting layer 5 that collects current from the negative electrode layer 2. Ru. This all-solid-state lithium ion secondary battery can be manufactured, for example, by the method described in JP-A-2016-207570, and will be described below with reference to the description in this publication.

The solid electrolyte layer can be formed by containing a sulfide solid electrolyte material. As the sulfide solid electrolyte material, materials having Li ion conductivity can be used, such as LiI-Li ₂ S-SiS ₂ , LiI-Li ₂ SP ₂ O ₅ , LiI-Li ₃ PO ₄ -P ₂ S ₅ , Li ₂ SP ₂ S ₅ , LiI-Li ₃ PS ₄ , LiI-LiBr-Li ₃ PS ₄ , Li ₃ PS ₄ , Li ₂ SP ₂ S ₅ , Li ₂ SP ₂ S ₅ - LiI, Li ₂ S-P ₂ S ₅ -Li ₂ O, Li ₂ S-P ₂ S ₅ -Li ₂ O-LiI, Li ₂ S-SiS ₂ , Li ₂ S-SiS ₂ -LiI, Li ₂ S- SiS ₂ -LiBr, Li ₂ S-SiS ₂ -LiCl, Li ₂ S-SiS ₂ -B ₂ S ₃ -LiI, Li ₂ S-SiS ₂ -P ₂ S ₅ -LiI, Li ₂ S-B ₂ S ₃ , Li ₂ S-P ₂ S ₅ -Z _m S _n (m and n are positive numbers. Z is either Ge, Zn, or Ga.), Li ₂ S-GeS ₂ , Li ₂ S-SiS ₂ - Li ₃ PO ₄ , Li ₂ S-SiS ₂ -Li _x MO _y (x, y are positive numbers, M is any one of P, Si, Ge, B, Al, Ga, In), and the like. Note that the description "Li ₂ SP ₂ S ₅ " means a sulfide solid electrolyte material made using a raw material composition containing Li ₂ S and P ₂ S ₅ , and the same applies to other descriptions.

The sulfide solid electrolyte material may have a Li ₃ PS ₄ skeleton, a Li ₄ P ₂ S ₇ skeleton, or a Li ₄ P ₂ S ₆ skeleton. Examples of the sulfide solid electrolyte material having a Li ₃ PS ₄ skeleton include LiI-Li ₃ PS ₄ , LiI-LiBr-Li ₃ PS ₄ , and Li ₃ PS ₄ . Moreover, as a sulfide solid electrolyte material having a Li ₄ P ₂ S ₇ skeleton, Li ₇ P ₃ S ₁₁ can be mentioned. Furthermore, as the sulfide solid electrolyte material, LGPS or the like represented by Li _(4-x) Ge _(1-x) PxS ₄ (x satisfies 0<x<1) can also be used.

Further, the sulfide solid electrolyte material is preferably a sulfide solid electrolyte material containing P element, and the sulfide solid electrolyte material is more preferably a material containing Li ₂ SP ₂ S ₅ as a main component. preferable. Furthermore, the sulfide solid electrolyte material may contain halogen (F, Cl, Br, I).

Here, when the sulfide solid electrolyte material is the above-mentioned Li ₂ SP ₂ S ₅ system, the ratio of Li ₂ S and P ₂ S ₅ is a molar ratio of Li ₂ S:P ₂ S ₅ = 50. :50 to 100:0, especially Li ₂ S:P ₂ S ₅ =70:30 to 80:20.

Further, the sulfide solid electrolyte material may be sulfide glass, crystallized sulfide glass, or a crystalline material obtained by a solid phase method. Here, sulfide glass can be obtained, for example, by performing mechanical milling (ball mill, etc.) on a raw material composition, and crystallized sulfide glass can be obtained, for example, by processing sulfide glass at a temperature higher than the crystallization temperature. It can be obtained by heat treatment. Further, the ionic conductivity (for example, Li ion conductivity) of the sulfide solid electrolyte material at room temperature (25° C.) is preferably 1×10 ⁻⁵ S/cm or more, for example, 1×10 ⁻⁴ S /cm or more is more preferable. This ionic conductivity can be measured by an alternating current impedance method.

In addition to the above-mentioned materials, the solid electrolyte layer may contain a binder for forming the solid electrolyte layer into a sheet (layer). Examples of the binder include fluorine-containing binders such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), butadiene rubber (BR), and acrylate-butadiene rubber (ABR). It will be done. The content of the sulfide solid electrolyte material in the solid electrolyte layer is, for example, preferably in the range of 10 to 100% by mass, more preferably in the range of 50 to 100% by mass. The thickness of the solid electrolyte layer varies depending on the configuration of the intended all-solid lithium ion secondary battery, but is preferably within the range of 0.1 to 1000 μm, and preferably within the range of 0.1 to 300 μm. is more preferable.

In the present invention, the negative electrode layer preferably contains a solid electrolyte material in addition to the above-described negative electrode material. As the solid electrolyte material, the above-mentioned sulfide solid electrolyte material can be used. The content of the solid electrolyte material in the negative electrode layer is preferably in the range of 1 to 60% by mass, more preferably in the range of 10 to 50% by mass.

In addition to the above-mentioned negative electrode materials and solid electrolyte materials, the negative electrode layer may contain carbon materials such as acetylene black, Ketjen black, and carbon fiber (VGCF), conductive agents such as nickel, aluminum, and SUS, and the aforementioned binders. It can contain adhesives and the like. The content of the negative electrode material in the negative electrode layer is preferably in the range of 40 to 99% by mass. The thickness of the negative electrode layer varies depending on the structure of the intended all-solid-state lithium ion secondary battery, but is preferably in the range of 1 to 1000 μm.

Next, the positive electrode layer is formed of a positive electrode material, and the positive electrode layer may contain the above-described solid electrolyte material, conductive agent, binder, etc. as necessary.

As the positive electrode material, oxide active materials, sulfide active materials, etc. can be used. Examples of the oxide active material include rock salt layered active materials such as LiCoO ₂ , LiMnO ₂ , LiNiO ₂ , LiVO ₂ , LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , LiMn ₂ O ₄ , LiNi _0. Examples include spinel-type active materials such as ₅ Mn _1.5 O ₄ , olivine-type active materials such as LiFePO ₄ and LiMnPO ₄ , and Si-containing active materials such as Li ₂ FeSiO ₄ and Li ₂ MnSiO ₄ . Further, examples of oxide active materials other than those mentioned above include Li ₄ Ti ₅ O1 ₂ .

The thickness of the positive electrode layer varies depending on the structure of the intended all-solid-state lithium ion secondary battery, but is preferably in the range of 1 to 1000 μm.

Furthermore, examples of the material of the positive electrode current collector that collects current in the positive electrode layer include SUS, aluminum, nickel, iron, titanium, and carbon. Further, examples of the material of the negative electrode current collector that collects current in the negative electrode layer include SUS, copper, nickel, and carbon. Further, the all-solid-state lithium ion secondary battery may be formed with an outer shell such as a battery case or an exterior body.

In the above embodiment, an example was shown in which an all-solid-state lithium ion secondary battery was manufactured using the negative electrode material of the present invention, but the present invention can also be applied to a negative electrode material of a lithium ion secondary battery whose electrolyte is a liquid. You can also do it.

Next, the present invention will be specifically explained using examples.

(Example 1)
A reaction vessel equipped with a stirring device was charged with 470 parts by mass of furfuryl alcohol, 420 parts by mass of formalin with a concentration of 37% by mass, 13 parts by mass of an aqueous phosphoric acid solution with a concentration of 85% by mass as a reaction catalyst, and 3500 parts by mass of water, Furthermore, 1,500 parts by mass of flaky graphite with an average particle size of 6 μm was charged as graphite particles, stirring was started, and a vacuum pump was activated to reduce the pressure in the reaction vessel so that the boiling temperature of water reached 85°C. The pressure was reduced to 0.60 kgf/cm ² (0.06 MPa). Then, heating was started, and it took about 60 minutes to boil and reflux, and the condensation reaction was continued for 240 minutes.

Next, after stopping heating and cooling to 50° C., the vacuum was released, stirring was stopped, and the contents of the flask were discharged into a beaker. After this was allowed to stand for 4 days, the supernatant liquid was removed by a decanting method, and the contents in the beaker were discharged into a stainless steel vat. The contents were spread thinly and air-dried for 3 days, then placed in a dryer at 100° C. and heated and dried for 10 hours to obtain a granular graphite/furan resin composite material.

In this graphite/furan resin composite material, the furan resin on the outer periphery was completely cured to be insoluble and infusible, and the furan resin content was 11% by mass. In addition, when the average particle diameter of this graphite/furan resin composite material was measured using a laser diffraction particle size distribution analyzer "LA-920" manufactured by Horiba, Ltd., it was found to be _D50 (when the cumulative mass reaches 50%). The particle size) was 8 μm.

Next, this graphite/furan resin composite material was heated to 1000° C. at a temperature increase rate of 100° C./hour in a nitrogen atmosphere, and then heat-treated at 1000° C. for 3 hours to sinter it. By carbonizing the furan resin by firing in this manner, a granular graphite/furan resin composite carbon material in which graphite particles were coated with an amorphous carbon layer was obtained. The yield of this graphite/furan resin composite carbon material was 95%. Further, the average particle size of this graphite/furan resin composite carbon material was D ₅₀ =8 μm. This graphite/furan resin composite carbon material was used as a negative electrode material.

(Example 2)
Except for charging 2000 parts by mass of graphite particles, a granular graphite/furan resin composite material was prepared by condensation reaction of furfuryl alcohol and formalin in the same manner as in Example 1, and further baked in the same manner as in Example 1. A granular graphite/furan resin composite carbon material was obtained. The yield of this graphite/furan resin composite carbon material was 96%. Further, the average particle size of this graphite/furan resin composite carbon material was D ₅₀ =8 μm. This graphite/furan resin composite carbon material was used as a negative electrode material.

(Example 3)
A granular graphite/furan resin composite material was prepared by subjecting furfural and formalin to a condensation reaction under the same conditions as in Example 1, except that 470 parts by mass of furfural was charged instead of 470 parts by mass of furfuryl alcohol. Further, this graphite/furan resin composite material was fired in the same manner as in Example 1 to obtain a granular graphite/furan resin composite carbon material. The yield of this graphite/furan resin composite carbon material was 95%. Further, the average particle size of this graphite/furan resin composite carbon material was D ₅₀ =8 μm.

(Example 4)
A granular graphite/furan resin composite material was prepared by condensation reaction of furfural and formalin in the same manner as in Example 3, except that 2000 parts by mass of graphite particles were charged, and then granular graphite was prepared by firing in the same manner as in Example 1. A graphite/furan resin composite carbon material was obtained. The yield of this graphite/furan resin composite carbon material was 96%. Further, the average particle size of this graphite/furan resin composite carbon material was D ₅₀ =8 μm. This graphite/furan resin composite carbon material was used as a negative electrode material.

(Example 5)
A reaction vessel equipped with a stirring device was charged with 470 parts by mass of furfuryl alcohol, 15 parts by mass of an 85% by mass phosphoric acid aqueous solution as a reaction catalyst, and 3500 parts by mass of water, and scales with an average particle size of 6 μm as graphite particles. After charging 1,500 parts by mass of graphite and starting stirring, a vacuum pump was activated to increase the pressure inside the reaction vessel to 0.60 kgf/cm ² (0.06 MPa) so that the boiling temperature of water was 85°C. The pressure was reduced to Then, heating was started, and it took about 60 minutes to boil and reflux, and the condensation reaction was continued for 240 minutes.

The rest was carried out in the same manner as in Example 1 by cooling, discharging, removing the supernatant liquid, and drying to obtain a granular graphite/furan resin composite material. The furan resin on the outer periphery of this graphite/furan resin composite material was completely cured to be insoluble and infusible.

Then, this graphite/furan resin composite material was fired in the same manner as in Example 1 to obtain a granular graphite/furan resin composite carbon material. The yield of this graphite/furan resin composite carbon material was 96%. Further, the average particle size of this graphite/furan resin composite carbon material was D ₅₀ =8 μm. This graphite/furan resin composite carbon material was used as a negative electrode material.

(Comparative example 1)
The graphite particles used in Examples 1 to 4 were used as negative electrode materials without being coated with furan resin or subjected to firing treatment.

(Comparative example 2)
A 2-liter four-neck flask was charged with 500 g of furfuryl alcohol, 80 g of 92% by mass paraformaldehyde, and 750 g of water, and further charged with 5 g of 7-amino-4-hydroxy-2-naphthalenesulfonic acid as a reaction catalyst. Then, the stirring speed of a propeller type stirrer installed in the flask was set to 8 m/min, the temperature was raised to 85° C. over about 60 minutes, and the furfuryl alcohol was allowed to undergo a condensation reaction for 120 minutes. The pH of the reaction solution in the flask was 2.7.

The solution in the flask was initially nearly transparent, but gradually became cloudy and milky. After 120 minutes had elapsed, the contents of the flask were poured out onto a stainless steel vat covered with a polyethylene sheet and cooled. Immediately after spreading on a stainless steel vat, by pouring out the reaction solution such as water from the stainless vat, the product, which has a block shape but contains water and becomes a rice cake that can be dented when pressed, is separated from the reaction solution. When the temperature was measured, it was 40°C.

Next, this water-containing product is placed in a -10°C freezer to freeze, and the frozen solid water-containing product is passed through a knife-edge crusher equipped with a screen having 3 mm diameter round holes. Shattered. Then, this pulverized product was introduced into a fluidized bed dryer and dried for 6 hours by blowing air at a flowing air temperature of 25°C to obtain a solid furan resin with a particle size of 0.5 to 2.5 mm and a water content of 0.8% by mass. 400 g of particles were obtained. The melting point of this resin was 85°C.

This furan resin was pulverized using a pin mill to obtain a powdered furan resin having an average particle size of 25 μm. Then, 11 parts by mass of this powdered furan resin was added to 90 parts by mass of flaky graphite with an average particle diameter of 6 μm, and 10 parts by mass of tetrahydrofuran was further added, and the mixture was mixed in a mortar for 15 minutes to mix the furan resin into tetrahydrofuran. Furan resin was coated on the surface of graphite while it was being melted. Next, this was dispensed onto a vat, spread thinly, and dried for 3 days to evaporate the tetrahydrofuran, thereby obtaining a granular graphite/furan resin composite material.

The graphite/furan resin composite material prepared as described above was fired in the same manner as in Example 1 to obtain a granular graphite/furan resin composite carbon material. The yield of this graphite/furan resin composite carbon material was 95%. Moreover, the average particle diameter of this graphite/furan resin composite carbon material was D ₅₀ =7.5 μm. This graphite/furan resin composite carbon material was used as a negative electrode material.

(Comparative example 3)
50 parts by mass of powdered polyvinyl chloride was blended with 100 parts by mass of the same graphite particles as in Example 1, and dry mixed for 30 minutes at room temperature using an aluminum ball mill to coat the graphite particles with polyvinyl chloride. Next, this was heated to 1000° C. at a rate of 100° C./hour in a nitrogen atmosphere, and was heat-treated at 1000° C. for 3 hours to sinter it. By carbonizing the polyvinyl chloride by firing in this manner, a granular graphite/polyvinyl chloride composite carbon material in which graphite particles were coated with an amorphous carbon layer was obtained. This graphite/polyvinyl chloride composite carbon material was used as a negative electrode material.

The negative electrode materials of Examples 1 to 5 and Comparative Examples 1 to 3 described above were evaluated for electrical conductivity. Since it is difficult to mold in the form of particles, 5 parts by mass of SEBS (styrene thermoplastic elastomer) as a binder is added to 95 parts by mass of the negative electrode material and mixed, and this is dissolved in anisole to obtain a solid content concentration of 53% by mass. A slurry was prepared. Next, this slurry was applied to the surface of a polyimide sheet using a doctor blade with a 200 μm gap, and this was pressed using a sheet press machine at 10 tons for 1 minute, punched into 9 mmφ, and then pressed using a 10 mmφ mold at 3 tons for 1 minute. A test piece was prepared by molding. Using this test piece, volume resistivity was measured using a low resistivity measuring device (Loresta AP "MCP-T400" manufactured by Mitsubishi Yuka Co., Ltd.). The results are shown in Table 1.

A comparison of Examples 1 to 5 and Comparative Example 1 in Table 1 confirms that conductivity is improved by forming a carbonized layer of furan resin on graphite particles. It is also confirmed that Examples 1 to 5, in which graphite particles are coated with a carbonized layer of furan resin, have higher conductivity than Comparative Example 3, in which graphite particles are coated with a carbonized layer of polyvinyl chloride.

The negative electrode materials of Examples 1 to 5 and Comparative Examples 1 to 3 described above were evaluated for particle uniaxial compression fracture strength. The evaluation of the unconfined compressive fracture strength was carried out by measuring the compressive fracture force using a microparticle crushing force measuring device ("NS-A series" manufactured by Nano Seeds Co., Ltd.). The measurement results are shown in Table 2.

As seen in Table 2, Examples 1 to 5 have higher compressive breaking force than Comparative Examples 1 to 3, and are characterized by the fact that the condensation reaction of furans or the condensation reaction of furans and aldehydes occurs on the surface of graphite particles. It was confirmed that the effect of forming a layer of furan resin on the adhesion and covering properties of the carbonized layer of furan resin was confirmed.

(Examples 6 to 10, Comparative Examples 4 to 7)
Lithium sulfide (Li ₂ S), diphosphorus pentasulfide (P ₂ S ₅ ) and lithium iodide (LiI), Li ₂ S and P ₂ S ₅ are 75:25 (75Li ₂ S-25P ₂ S ₅ ) It was weighed so that the molar ratio of LiI was 10 mol %. By mixing these, a sulfide solid electrolyte material was obtained. The composition of this sulfide solid electrolyte material is 10LiI-90 (0.75Li ₂ S-0.25P ₂ S ₅ ). This sulfide solid electrolyte material was crushed into fine particles, and crystallized by heating at 180° C. for 2 hours.

Then, the crystallized sulfide solid electrolyte material and ABR as a binder were mixed at a volume ratio of 98:2 to obtain a material for forming a solid electrolyte layer.

In addition, 36 parts by mass of the negative electrode materials of Examples 1 to 5 and Comparative Examples 1 to 3, 25 parts by mass of the crystallized sulfide solid electrolyte material, and 0.5 parts by mass of PVDF as a binder were weighed. A slurry prepared by mixing the above was applied to the surface of the Cu foil and dried. Then, the negative electrode layer forming material formed on the Cu foil was scraped off and collected.

Furthermore, using LiNi _1/3 Co _1/3 Mn _1/3 O ₂ as a positive electrode material, 52 parts by mass of this positive electrode material, 17 parts by mass of the above crystallized sulfide solid electrolyte material, and gas as a conductive material. We weighed 1 part by mass of phase grown carbon fiber (VGCF), 15 parts by mass of dehydrated heptane, and 0.4 parts by mass of PVDF as a binder, and mixed these together to obtain a slurry, which was coated on Al foil and dried. . Then, the positive electrode layer forming material formed on the Al foil was scraped off and collected.

Next, 100 mg of the material for forming the solid electrolyte layer was weighed, placed in a ceramic case, and pressed at 4.3 tons for 1 minute to obtain the solid electrolyte layer 3. Further, 100 mg of the material for forming the positive electrode layer was weighed, put into a splittable pellet jig with a diameter of 10 mm, and pressed at 4 tons for 1 minute to obtain the positive electrode layer 1. Further, 80 mg of the material for forming the negative electrode layer was weighed, placed in a pellet jig in the same manner, and pressed for 1 minute at 4 tons to obtain the negative electrode layer 2. Then, the positive electrode layer 1, solid electrolyte layer 3, and negative electrode layer 2 were assembled in the ceramic case and restrained with a pressure of 6N, and Al foil was placed as the positive electrode current collector 4, Cu foil was placed as the negative electrode current collector 5, All-solid-state lithium ion secondary batteries for evaluation of Examples 5 to 8 and Comparative Examples 4 to 7 were obtained.

The charging and discharging characteristics of the all-solid-state lithium ion secondary battery for evaluation produced as described above were evaluated. In order to evaluate the charge/discharge characteristics, a charge/discharge measurement device (HJ-SD8 manufactured by Hokuto Denko Co., Ltd.) was used in a thermostatic chamber set at 25°C, with an applied current of 0.05 mA and a cutoff condition of 0.6. A test was conducted under the conditions of ~0.9V and 10 charge/discharge cycles to determine the second discharge specific capacity and initial coulombic efficiency (initial discharge efficiency). In addition, in order to determine the rate characteristics, after conducting a charge/discharge evaluation for 10 cycles with an applied current of 0.05 mA, the current was changed to 0.1 mA and a further 10 cycle evaluation of discharging and charging was performed, and the discharge specific capacity at the 11th cycle was calculated. It was calculated by dividing by the discharge specific capacity at the 10th cycle. The results are shown in Table 3.

As seen in Table 3, all the all-solid-state lithium ion secondary batteries for evaluation in Examples 6 to 10 had higher charge-discharge characteristics than the all-solid-state lithium-ion secondary batteries for evaluation in Comparative Examples 4 to 6. was confirmed.

1 Positive electrode layer 2 Negative electrode layer 3 Solid electrolyte layer 4 Positive electrode current collecting layer 5 Negative electrode current collecting layer

Claims (6)

A process of manufacturing a granular graphite/furan resin composite material in which the surface of graphite particles is coated with a layer of furan resin by mixing furans with graphite particles and causing a condensation reaction in the presence of a reaction catalyst; A step of manufacturing a granular graphite/furan resin composite carbonized material in which the surface of graphite particles is coated with a layer of carbonized furan resin by heating the graphite/furan resin composite material in a non-oxidizing atmosphere. A method for producing a negative electrode material for a lithium ion secondary battery, characterized by: A step of producing a granular graphite/furan resin composite material in which the surface of graphite particles is coated with a layer of furan resin by mixing furans and aldehydes with graphite particles and causing a condensation reaction in the presence of a reaction catalyst. , a step of producing a granular graphite/furan resin composite carbonized material in which the surface of graphite particles is coated with a layer of carbonized furan resin by heating the granular graphite/furan resin composite material in a non-oxidizing atmosphere; A method for producing a negative electrode material for a lithium ion secondary battery, comprising: 3. The method for producing a negative electrode material for a lithium ion secondary battery according to claim 1, wherein the condensation reaction is carried out in a reduced pressure atmosphere. 4. The method for producing a negative electrode material for a lithium ion secondary battery according to claim 3, wherein the reduced pressure atmosphere is in a range of 0.007 to 0.095 MPa. 5. A method for manufacturing a negative electrode layer for a lithium ion secondary battery, comprising manufacturing the negative electrode layer using a negative electrode material manufactured by the method according to any one of claims 1 to 4. A method for manufacturing a lithium ion secondary battery, comprising the step of using the negative electrode layer according to claim 5 and arranging an electrolyte layer between the negative electrode layer and the positive electrode layer.

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