JP2022079000A - Composite material and negative electrode material for all-solid-state lithium ion secondary battery - Google Patents

Abstract

To provide a composite material capable of giving high coulomb efficiency and good cycle characteristics when used as a negative electrode material for an all-solid-state lithium ion secondary battery, and the negative electrode material for the all-solid-state lithium ion secondary battery.SOLUTION: A composite material contains composite particles C in which metal oxide particles 23 are adhered to a surface of a graphite particle 21 through a carbonaceous layer 22, and the composite particles have, on their surfaces, a part where at least one of the graphite particle and the carbonaceous layer is exposed.SELECTED DRAWING: Figure 2

Description

The present invention relates to a composite material and a negative electrode material for an all-solid-state lithium ion secondary battery, and more particularly to a composite material containing graphite and a metal oxide and a negative electrode material for an all-solid-state lithium ion secondary battery containing the composite material.

In recent years, lithium ion secondary batteries have become widespread as a power source for electronic devices such as notebook computers and mobile phones, and transportation devices such as automobiles. There is an increasing demand for these devices to be smaller, lighter, and thinner. In order to meet these demands, a small, lightweight and large capacity secondary battery is required.

In a general lithium ion secondary battery, a compound containing lithium is used as a positive electrode material, and a carbon material such as graphite or coke is used as a negative electrode material. Further, between the positive electrode and the negative electrode, an electrolytic solution obtained by dissolving a lithium salt such as LiPF ₆ or LiBF ₄ as an electrolyte in an aprotonic solvent having penetrating power such as propylene carbonate or ethylene carbonate, or an electrolytic solution thereof. It is provided with an electrolyte layer made of a polymer gel impregnated with. These configurations are packaged and protected by the battery case packaging material.

In an all-solid-state lithium-ion secondary battery, there is a solid electrolyte layer between the positive electrode layer and the negative electrode layer. That is, the electrolyte is solid as compared with the above-mentioned general lithium ion secondary battery. As the solid electrolyte layer, sulfides such as LiS and _{P2S 5 are} typically used. In the negative electrode, graphite is typically used as the negative electrode active material.

However, the solid electrolyte and graphite are incompatible with each other, and particularly when sulfide is used as the solid electrolyte, the electrical resistance between the solid electrolyte and the negative electrode using graphite becomes particularly high. Further, in an all-solid-state battery, a solid electrolyte may be added to the negative electrode, and it is difficult to disperse graphite and the solid electrolyte with each other, and graphite particles are not uniformly dispersed, which causes an increase in resistance. .. On the other hand, a configuration in which graphite is coated with a layer of a metal oxide is being studied.

For example, Patent Document 1 discloses a negative electrode material for a lithium ion secondary battery in which a polymer compound having a carboxyl group and metal oxide particles are attached to the surface of a negative electrode active material such as a carbon material.

Patent Document 2 discloses a negative electrode active material including a crystalline carbon-based substrate and metal oxide nanoparticles arranged on the surface thereof. As a method for producing a negative electrode active material, it is described that a crystalline carbon-based substrate, a metal oxide, a precursor and a solvent are mixed, and the obtained mixed solution is dried and heat-treated.

Japanese Unexamined Patent Publication No. 2013-157339 Japanese Unexamined Patent Publication No. 2015-115319

However, in the negative electrode material of Patent Document 1, the metal oxide particles do not adhere to the surface of the negative electrode active material with sufficient force, and when charging and discharging are repeated, many metal oxide particles fall off from the negative electrode active material. .. Therefore, the secondary battery manufactured by using this negative electrode material does not have sufficient cycle characteristics. Further, in order to ensure the conductivity of the negative electrode material, it is necessary to provide a portion that exposes the surface of the negative electrode active material, and the range in which the polymer compound can cover the negative electrode active material is limited, and the surface of the negative electrode material is covered. It is difficult to adhere the metal oxide particles without bias.

Further, also in the negative electrode active material of Patent Document 2, the bonding force between the crystalline carbon base material and the metal oxide nanoparticles is not sufficient, and when charging and discharging are repeated, many metal oxide nanoparticles become the base material. It is thought that it will drop out of. Therefore, the secondary battery manufactured by using this negative electrode active material does not have sufficient cycle characteristics.

Therefore, the present invention provides a composite material that can obtain high Coulomb efficiency and good cycle characteristics when used as a negative electrode material for an all-solid-state lithium-ion secondary battery, and a negative electrode material for an all-solid-state lithium-ion secondary battery. The purpose is to do.

The configuration of the present invention for achieving the above object is as follows.

[1] A composite material containing composite particles in which metal oxide particles are attached to the surface of graphite particles via a carbonaceous layer, in which at least one of the graphite particles and the carbonaceous layer is exposed. A composite material characterized by having a portion on the surface.
[2] The composite material according to [1], wherein the composite particles have a portion where the carbonaceous layer is exposed.
[3] The composite material according to [1] or [2], wherein the carbonaceous layer has a graphene structure formed along the surface of graphite particles.
[4] The composite material according to any one of [1] to [3], wherein the carbonaceous layer has an average thickness of 0.1 nm or more and 30 nm or less.
[5] The 50% particle size of the graphite particles in the volume-based cumulative particle size distribution is 2 μm or more, and the average primary particle size calculated from the BET specific surface area of the metal oxide particles is 100 nm or less [5]. 1] The composite material according to any one of [4].
[6] The composite material according to any one of [1] to [5], wherein the content of the metal oxide particles with respect to 100 parts by mass of the graphite particles is 0.1 part by mass or more and 10 parts by mass or less.
[7] Any of [1] to [6], wherein the metal oxide particles contain oxides of at least one of Group 1 to Group 12, aluminum, gallium, indium, thallium, tin, and lead. The composite material described in.
[8] The composite material according to any one of [1] to [7], wherein the metal oxide particles contain titanium oxide.
[9] The composite material according to [8], wherein the content of the anatase crystal phase in the total crystal phase of titanium oxide is 70% by mass or more.
[10] The composite material according to [8], wherein the content of the rutile crystal phase in the total crystal phase of titanium oxide is 70% by mass or more.
[11] A negative electrode material for an all-solid-state lithium ion secondary battery containing the composite material according to any one of [1] to [10].
[12] A negative electrode for an all-solid-state lithium-ion secondary battery containing the composite material according to any one of [1] to [10] and a sulfide solid electrolyte.
[13] A mixing step of mixing graphite particles, an organic compound, and metal oxide particles, and a heat treatment step of heat-treating the mixture obtained in the mixing step at 600 ° C. or higher and 2000 ° C. or lower in a non-oxidizing gas atmosphere. A method for manufacturing a composite material including.
[14] A mixing step of mixing metal oxide particles containing an organic compound and graphite particles, and a mixture obtained in the mixing step are heat-treated at 600 ° C. or higher and 2000 ° C. or lower in a non-oxidizing gas atmosphere. A method for producing a composite material including a heat treatment step.

According to the present invention, there are provided a composite material that can obtain high Coulomb efficiency and good cycle characteristics when used as a negative electrode material for an all-solid-state lithium-ion secondary battery, and a negative electrode material for an all-solid-state lithium-ion secondary battery. can do.

It is a schematic diagram which showed an example of the structure of the all-solid-state lithium ion battery 1 which concerns on one Embodiment of this invention. It is a schematic diagram which showed the composition of the particle of the composite material which concerns on one Embodiment of this invention. It is a schematic diagram which showed the composition of the particle of the composite material which concerns on one Embodiment of this invention. 9 is an electron micrograph of the composite material particles produced in Example 48. FIG. 3 is an electron micrograph of the composite material particles produced in Example 53. (The enclosed part is a metal oxide)

Hereinafter, embodiments of the present invention will be described.

In the following description, "50% particle size in the volume-based cumulative particle size distribution" and "D50" are particle sizes that are 50% in the volume-based cumulative particle size distribution obtained by the laser diffraction / scattering method.

<1. All-solid-state lithium-ion secondary battery ＞
FIG. 1 is a schematic view showing an example of the configuration of the all-solid-state lithium ion secondary battery 1 according to the present embodiment. The all-solid-state lithium-ion secondary battery 1 includes a positive electrode layer 11, a solid electrolyte layer 12, and a negative electrode layer 13.

The positive electrode layer 11 has a positive electrode current collector 111 and a positive electrode mixture layer 112. The positive electrode current collector 111 is connected to a positive electrode lead 111a for exchanging and receiving electric charges with an external circuit. The positive electrode current collector 111 is preferably a metal foil, and the metal foil is preferably an aluminum foil.

The positive electrode mixture layer 112 contains a positive electrode active material, and may contain a solid electrolyte, a conductive auxiliary agent, a binder, and the like. Examples of the positive electrode active material include rock salt type layered active materials such as LiCoO ₂ , LiMnO ₂ , LiNiO ₂ , LiVO ₂ , LiNi _1/3 Mn _1/3 Co _1/3 O ₂ , and spinel type active materials such as LiMn ₂ O ₄ . An olivine-type active material such as LiFePO ₄ , LiMnPO ₄ , LiNiPO ₄ , _{LiCuPO 4} and a sulfide active material such as Li 2S can be used. Further, these active substances may be coated with LTO (Lithium Tin Oxide) or the like.

As the solid electrolyte contained in the positive electrode mixture layer 112, the materials listed in the solid electrolyte layer 12 described later can be used, but a material different from the material contained in the solid electrolyte layer 12 may be used. .. The content of the solid electrolyte in the positive electrode mixture layer 112 is preferably 50 parts by mass or more, more preferably 70 parts by mass or more, and more preferably 80 parts by mass or more with respect to 100 parts by mass of the positive electrode active material. Is even more preferable. The content of the solid electrolyte in the positive electrode mixture layer 112 is preferably 200 parts by mass or less, more preferably 150 parts by mass or less, and 125 parts by mass or less with respect to 100 parts by mass of the positive electrode active material. Is even more preferable.

As the conductive auxiliary agent, it is preferable to use a particulate carbonaceous conductive auxiliary agent and a fibrous carbonaceous conductive auxiliary agent. The particulate carbonaceous conductive aids are Denka Black (registered trademark) (manufactured by Electrochemical Industry Co., Ltd.), Ketchen Black (registered trademark) (manufactured by Lion Co., Ltd.), Graphite fine powder SFG series (manufactured by Timcal), Particulate carbon such as graphene can be used. As the fibrous carbonaceous conductive aid, vapor phase carbon fibers "VGCF (registered trademark)" and "VGCF (registered trademark) -H" (manufactured by Showa Denko KK), carbon nanotubes, carbon nanohorns, etc. can be used. can. The vapor phase carbon fiber "VGCF (registered trademark) -H" (manufactured by Showa Denko KK) is most preferable because of its excellent cycle characteristics.

Examples of the binder include polyvinylidene fluoride, polytetrafluoroethylene, polyethylene oxide, polyvinyl acetate, polymethacrylate, polyacrylate, polyacrylonitrile, polyvinyl alcohol, styrene-butadiene rubber, carboxymethyl cellulose and the like.

In the positive electrode mixture layer 112, the content of the binder with respect to 100 parts by mass of the positive electrode active material is preferably 1 part by mass or more and 10 parts by mass or less, and more preferably 1 part by mass or more and 7 parts by mass or less.

The solid electrolyte layer 12 is interposed between the positive electrode layer 11 and the negative electrode layer 13, and serves as a medium for transferring lithium ions between the positive electrode layer 11 and the negative electrode layer 13. The solid electrolyte layer 12 preferably contains at least one selected from the group consisting of a sulfide solid electrolyte and an oxide solid electrolyte, and more preferably contains a sulfide solid electrolyte.

Examples of the sulfide solid electrolyte include sulfide glass, sulfide glass ceramics, and Thio-LISION type sulfide. More specifically, for example, Li ₂ SP ₂ S ₅ , Li ₂ SP ₂ S ₅ -LiI, Li ₂ SP ₂ S ₅ -LiCl, Li ₂ SP ₂ S ₅ -LiBr, Li ₂ S-P ₂ S ₅ -Li ₂ O, Li ₂ S-P ₂ S ₅ -Li ₂ O-Li I, Li ₂ S-SiS ₂ , Li ₂ S-SiS ₂ -Li I, 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 SB ₂ S ₃ , Li ₂ SP ₂ S ₅ -Z _m S _n (where m and n are positive numbers. Z is any of Ge, Zn, Ga), Li ₂ S-GeS ₂ , Li ₂ S-SiS _2- Li ₃ PO ₄ , Li ₂ S-SiS ₂ -Li _x MO _y (where x, y are positive numbers, M is any of P, Si, Ge, B, Al, Ga, In), Li ₁₀ GeP ₂ S ₁₂ , Li _3.25 Ge _0.25 P _0.75 S ₄ , 30Li ₂ S ・ 26B ₂ S 3.44LiI, 63Li ₂ S ・ 36SiS 2.1Li ₃ PO ₄ , _{57Li 2 S}・_{38SiS 2 ・}5Li ₄ SiO ₄ , 70Li ₂ S / 30P ₂ S ₅ , 50LiS _{2.50GeS 2 ,} Li ₇ P ₃ S ₁₁ , Li _3.25 P _0.95 S ₄ , Li ₃ PS ₄ , Li ₂ S / P ₂ S _3. P ₂ S ₅ and the like can be mentioned. Further, the sulfide solid electrolyte material may be amorphous, crystalline, or glass ceramics.

The negative electrode layer 13 has a negative electrode current collector 131 and a negative electrode mixture layer 132. The negative electrode current collector 131 is connected to a negative electrode lead 131a for exchanging and receiving electric charges with an external circuit. The negative electrode current collector 131 is preferably a metal foil, and the metal foil is preferably a copper foil or an aluminum foil.

The negative electrode mixture layer 132 contains a negative electrode active material, and may contain a solid electrolyte, a binder, a conductive auxiliary agent, and the like. As the negative electrode active material, a composite material described later is used.

As the solid electrolyte contained in the negative electrode mixture layer 132, the material mentioned in the solid electrolyte layer 12 can be used, but a material different from the material contained in the solid electrolyte layer 12 may be used. The content of the solid electrolyte in the negative electrode mixture layer 132 is preferably 50 parts by mass or more, more preferably 70 parts by mass or more, and more preferably 80 parts by mass or more with respect to 100 parts by mass of the negative electrode active material. Is even more preferable. The content of the solid electrolyte in the negative electrode mixture layer 132 is preferably 200 parts by mass or less, more preferably 150 parts by mass or less, and 125 parts by mass or less with respect to 100 parts by mass of the negative electrode active material. Is even more preferable.

As the conductive auxiliary agent contained in the negative electrode mixture layer 132, the materials listed in the conductive auxiliary agent contained in the positive electrode mixture layer 112 can be used, but the conductive auxiliary agent contained in the positive electrode mixture layer 112 can be used. Different materials may be used. The content of the conductive auxiliary agent in the negative electrode mixture layer 132 is preferably 3 parts by mass or more, and more preferably 4 parts by mass or more with respect to 100 parts by mass of the negative electrode active material. The content of the conductive auxiliary agent in the negative electrode mixture layer 132 is preferably 10 parts by mass or less, and more preferably 8 parts by mass or less with respect to 100 parts by mass of the negative electrode active material.

As the binder, for example, the materials mentioned in the description of the positive electrode mixture layer 112 may be used, but the binder is not limited thereto. In the negative electrode mixture layer 132, the content of the binder with respect to 100 parts by mass of the negative electrode active material is preferably 1 part by mass or more and 10 parts by mass or less, and more preferably 1 part by mass or more and 7 parts by mass or less.

<2. Composite material>
FIG. 2 is a schematic diagram showing the configuration of composite particles C contained in the composite material according to the present embodiment. The composite particle C has a structure in which the metal oxide particles 23 are attached to the surface of the graphite particles 21 via the carbonaceous layer 22. According to this structure, the metal oxide particles 23 having a high affinity with the solid electrolyte provide sufficient bonding force between the composite particles C and the solid electrolyte, and between the negative electrode layer 13 and the solid electrolyte layer 12. The bonding force of is also improved. At this time, as shown in FIG. 3, the metal oxide particles 23 may be partially embedded in the carbonaceous layer 22. Further, when a mixture of the composite material and the solid electrolyte is used as the negative electrode mixture layer 132, the composite particles C and the particles constituting the solid electrolyte are well dispersed with each other.

The composite particle C has a portion on the surface where at least one of the graphite particles 21 and the carbonaceous layer 22 is exposed. The composite particle C preferably has a portion where the carbonaceous layer 22 is exposed. According to this configuration, the graphite particles 21 or the carbonaceous layer 22 can be in direct contact with the solid electrolyte or the adjacent composite particles C, the movement of lithium ions can be facilitated between them, and good conductivity is obtained. You can secure sex. Therefore, good rate characteristics and high coulombic efficiency can be obtained as a secondary battery. Note that FIG. 2 shows, as an example of such a configuration, a configuration having a portion where only the carbonaceous layer 22 is exposed.

It can be confirmed by a transmission electron microscope (TEM) that at least one of the graphite particles 21 and the carbonaceous layer 22 has an exposed portion (a portion to which the metal oxide particles 23 are not attached) on the surface. can.

The total area ratio R (area%) of the portion where the graphite particles 21 are exposed and the portion where the carbonaceous layer 22 is exposed on the surface of the composite particles C is obtained by the following method. (1) The cross section of the composite material is observed with a transmission electron microscope (TEM), and one composite particle C1 is randomly extracted from the composite particles C. (2) In the extracted composite particles _C1 , the total length of the portions where the metal oxide particles 23 are attached to the outer periphery is LA 1 (that is, the portions where the metal oxide particles 23 are attached are the composite particles C1). If there are a plurality of them on the outer periphery of the outer circumference, the total length of those portions) and the total length of the portions to which the metal oxide particles 23 are not attached to the outer periphery _LB 1 (that is, the metal oxide particles 23 are attached). When there are a plurality of non-existing portions on the outer periphery of the composite particle C1, the total length of those portions) is measured. It should be noted that _LA 1 + _LB 1 is the length of the outer circumference of the composite particle C1.

(3) Repeat (1) and (2) 50 times. That is, for 50 composite particles C1 to C50 randomly extracted from the composite particles _C observed by TEM, the total length of the portion to which the metal oxide particles 23 are attached is LA 1 to _LA . The total lengths of 50 and the portion to which the metal oxide particles 23 are not attached are measured from _LB 1 to _LB 50. It should be noted that none of the randomly extracted composite particles C1 to C50 overlap with each other. (4) Add the measured values of _LA 1 to _LA 50, that is, calculate _LA 1 + _LA 2 + _LA 3 + ... + _LA 48 + _LA 49 + _LA 50, and use this value as _SA . do. The respective values of _LB 1 to _LB 50 measured are added, that is, _LB 1 + _LB 2 + _LB 3 + ... + _LB 48 + _LB 49 + _LB 50 is calculated, and this value is defined as _SB . (5) Calculate 100 × SB / ( _SA + _SB ) as the area ratio _R (area%).

The area ratio R is preferably 30 area% or more, more preferably 40 area% or more, and further preferably 50 area% or more. This is to ensure conductivity between the composite particles C and / or between the composite particles C and the solid electrolyte particles.

Further, the area ratio R is preferably 90 area% or less, more preferably 80 area% or less, and further preferably 70 area% or less. This is because the portion coated with the metal oxide particles 23 improves the affinity between the composite particles C and the solid electrolyte particles.

The area ratio R can be adjusted, for example, by adjusting the size and content of the metal oxide particles 23 with respect to the graphite particles 21, the shape of the metal oxide particles, and the like.

[2-1. Graphite particles 21]
The graphite particles 21 may be either artificial graphite or natural graphite, but artificial graphite is preferable. This is because good cycle characteristics can be obtained. Further, it is possible to appropriately control the shape, the ratio between the basal surface and the edge surface, the crystallite size, the structure of the optical structure, and the like according to the specifications of the battery. A more preferable example of the graphite particles 21 is SCMG (registered trademark, Showa Denko KK). Further, the graphite particles 21 are not limited to those made of only graphite, and graphite particles coated with amorphous carbon or the like may be used.

The 50% particle size D50 in the volume-based cumulative particle size distribution of the graphite particles 21 is preferably 2 μm or more, more preferably 3 μm or more, and further preferably 5 μm or more. By using the composite material according to the present embodiment as a negative electrode material of a lithium ion secondary battery, in order to secure good cycle characteristics, to improve the dispersibility of graphite particles in the composite material, and by handling fine particles. This is to suppress an increase in battery manufacturing cost.

The D50 of the graphite particles 21 is preferably 20 μm or less, more preferably 12 μm or less, and even more preferably 7 μm or less. This is to improve the input / output characteristics of the negative electrode using the composite material by increasing the surface area.

[2-2. Carbonaceous layer 22]
The carbonaceous layer 22 is a layer that covers the surface of the graphite particles 21. The carbonaceous layer 22 may cover the entire surface of the graphite particles, or may partially cover the surface of the graphite particles. The structure of the carbonaceous layer 22 is not particularly limited, but an amorphous or graphene structure is preferable. This is to improve the conductivity of the surface of the composite particle C. The graphene structure is a structure in which carbon atoms are continuous as a honeycomb-like surface.

It is more preferable that the carbonaceous layer 22 has a graphene structure formed along the surface of the graphite particles 21. The graphene structure formed along the surface of the graphite particles 21 is a structure in which a honeycomb-shaped surface is formed along the surface of the graphite particles 21. This is because the conductivity of the surface of the composite particle C is further improved, and the chemical stability and mechanical strength of the carbonaceous layer 22 are further improved. When the carbonaceous layer 22 has a graphene structure, the carbonaceous layer 22 may be composed of one graphene layer, or may be a stack of a plurality of graphene layers.

The average thickness t of the carbonaceous layer 22 is a value obtained by the following method. (1) One composite particle C1 is randomly extracted from the composite particles C observed by a transmission electron microscope (TEM). (2) In the extracted composite particles C1, one place is randomly selected from the portion where the carbonaceous layer 22 is formed, and the thickness t1 of the carbonaceous layer 22 in the selected portion is measured. The thickness t1 is obtained as follows. The intersection x1 between the line perpendicular to the surface of the graphite particles 21 and the surface of the graphite particles 21 and the outer periphery of the vertical line and the carbonaceous layer 22 (when the carbonaceous layer 22 is covered with the metal oxide particles 23, the carbonaceous material). The boundary between the layer 22 and the metal oxide particles 23 is the outer periphery of the carbonaceous layer 22), and the intersection x2 is obtained. The distance between the obtained intersections x1 and x2 is the thickness t1.

(3) Repeat (1) and (2) 50 times. That is, the thicknesses t1 to t50 of the carbonaceous layer 22 measured in the 50 composite particles C1 to C50 randomly extracted from the composite particles C observed by a transmission electron microscope (TEM) are measured. It should be noted that none of the randomly extracted composite particles C1 to C50 overlap with each other. (4) The average value of the obtained values t1 to t50 is defined as the average thickness t of the carbonaceous layer 22.

The average thickness t of the carbonaceous layer 22 is preferably 0.1 nm or more, more preferably 1.0 nm or more, and further preferably 2.0 nm or more. In order to attach the metal oxide particles 23 to the graphite particles 21 with a sufficient amount and force, to improve the conductivity of the composite particles C, and to secure the chemical stability and mechanical strength of the carbonaceous layer 22. Is.

The average thickness t of the carbonaceous layer 22 is preferably 30 nm or less, more preferably 20 nm or less, and further preferably 10 nm or less. This is because it is possible to prevent the size of the composite particles C from becoming larger than necessary, and to improve the cycle characteristics of the secondary battery.

[2-3. Metal oxide particles 23]
The average primary particle diameter (nm) of the metal oxide particles 23 is 6000 / (S _BET × ρ) (ρ: metal oxide density [g / cm) based on the BET specific surface area _SBET [m ² / g]. It is a value obtained by ³ ]). The average primary particle diameter of the metal oxide particles 23 is preferably 100 nm or less, more preferably 55 nm or less, further preferably 10 nm or less, and particularly preferably 7 nm or less. This is because the specific surface area of the metal oxide particles 23 is increased and the contact area with the solid electrolyte is increased, and as a result, good affinity with the solid electrolyte can be obtained even if the content of the metal oxide particles 23 is small. be. The density of titanium oxide is 4.0 g / cm ³ , the density of copper (II) oxide is 6.3 g / cm ³ , and the density of aluminum oxide Al ₂ O ₃ of γ-type crystal is 4.0 g / cm ³ . ..

The average primary particle diameter of the metal oxide particles 23 is preferably 1 nm or more, and more preferably 2 nm or more. This is to expose the graphite particles 21 while improving the affinity between the composite particles C and the solid electrolyte.

The metal oxide particles 23 are not particularly limited, but preferably contain oxides of at least one of Group 1 to Group 12, aluminum, gallium, indium, thallium, tin, and lead, Group 3 to Group 12. It is more preferable to contain at least one of the oxides of the metal of the above, and it is further preferable to contain titanium (IV) oxide. Hereinafter, "titanium oxide" refers to titanium oxide (IV), that is, TiO ₂ unless otherwise specified.

When the metal oxide particles 23 contain titanium oxide, the crystal type of titanium oxide contained in the metal oxide particles 23 includes anatase type, rutile type, and brookite type, but is not particularly limited. The content of any of the crystal phases in the total crystal phase of the metal oxide particles 23 is preferably 70% by mass or more, more preferably 80% by mass or more, and further preferably 90% by mass or more. preferable. For example, when the titanium oxide contained in the metal oxide particles 23 contains an anatase crystal phase as a main component, the content of the anatase crystal phase in the total crystal phase of the titanium oxide is preferably 70% by mass or more. , 80% by mass or more, more preferably 90% by mass or more. The same applies to the case where the titanium oxide contained in the metal oxide particles 23 contains a rutile crystal phase as a main component and the case where the brookite crystal phase is a main component.

The content of the metal oxide particles 23 with respect to 100 parts by mass of the graphite particles 21 in the composite material is preferably 0.1 part by mass or more, more preferably 0.3 parts by mass or more, and 0. It is more preferably 1.5 parts by mass or more. This is to improve the affinity between the composite material and the solid electrolyte and reduce the electrical resistance between them.

The content of the metal oxide particles 23 with respect to 100 parts by mass of the graphite particles 21 in the composite material is preferably 10 parts by mass or less, more preferably 5.0 parts by mass or less, and 3.0 parts by mass. It is more preferably less than or equal to parts by mass. To expose the graphite particles 21 or the carbonaceous layer 22 in the composite particles C to improve the conductivity between the composite particles C or between the composite particles C and the solid electrolyte, and to facilitate the movement of lithium ions. Is.

<3. Manufacturing method of composite material ＞
Hereinafter, embodiments of the method for producing a composite material of the present invention will be exemplified, but the composite material of the present invention is not limited to that obtained by the following production method 1 or 2.

[3-1. Manufacturing method 1]
In an example of the method for producing a composite material according to the present invention, graphite particles and an organic compound serving as a precursor of the carbonaceous layer 22 (hereinafter, “organic compound” is a precursor of the carbonaceous layer 22 unless otherwise specified. It includes a mixing step of mixing the organic compound) and the metal oxide particles, and a heat treatment step of heat-treating the mixture obtained in the mixing step.

The organic compound has a role of adhering the metal oxide particles to the graphite particles before the heat treatment step, forms a carbonaceous layer 22 after the heat treatment step, and adheres the graphite particles 21 and the metal oxide particles 23 more firmly. .. The organic compound preferably has a high residual carbon content, and when mixed in a liquid medium, it preferably has high solubility in the liquid medium. Examples of the organic compound having a high residual coal ratio include petroleum pitch, coal pitch, phenol resin and the like. Examples of the organic compound soluble in water include polyvinyl alcohol, acrylic acid, salicylic acid, phthalic acid, isophthalic acid, terephthalic acid, salicylic acid, citric acid, tartaric acid, malic acid and the like.

The amount of the organic compound added to 100 parts by mass of the graphite particles is preferably 0.1 part by mass or more, more preferably 0.5 part by mass or more, and further preferably 1.0 part by mass or more. .. This is because the graphite particles are sufficiently covered with the organic compound.

The amount of the organic compound added to 100 parts by mass of the graphite particles is preferably 25 parts by mass or less, more preferably 18 parts by mass or less, and further preferably 10 parts by mass or less. This is to increase the insertion and desorption capacities of lithium ions in the produced composite material.

The preferred range of the metal element contained in the metal oxide particles and the average primary particle diameter is as described above. A preferable example of the metal oxide particles used as a raw material for this production method is titanium oxide described in JP-A-2017-114700. The average primary particle size of this titanium oxide calculated from the BET specific surface area (200 m ² / g or more) is as small as 7.5 nm or less, and the bulk density is 0.2 to 0.8 g / ml, which is a range suitable for handling. Is.

The mixing step is not particularly limited, and an example thereof includes a method in which graphite particles and metal oxide particles are mixed in a liquid medium in which an organic compound is dissolved, and then the liquid medium is removed. The liquid medium is not particularly limited, but a liquid medium capable of dissolving an organic compound is preferable. The solid content after removing the liquid medium may be appropriately crushed.

Another example of the mixing step is a method of mixing graphite particles, an organic compound, and metal oxide particles without using a liquid medium. In this case, the organic compound of graphite particles and metal oxides is preferably a viscous substance such as petroleum pitch. This is because the metal oxide particles can be adhered to the surface of the graphite particles before the heat treatment step.

The heat treatment step is a step of carbonizing the organic compound to form the carbonaceous layer 22. Further, when the organic compound in the mixing step is a solid, the organic compound can be softened and the metal oxide particles can be attached to the graphite particles via the softened organic compound. It is preferably performed in a non-oxidizing gas atmosphere, and more preferably in an inert gas atmosphere. This is to prevent the graphite particles and the organic compound from being oxidized by the atmospheric gas during the heat treatment. Examples of the inert gas include nitrogen gas and argon gas.

The heat treatment temperature in the heat treatment step is preferably 600 ° C. or higher, more preferably 900 ° C. or higher, and even more preferably 1000 ° C. or higher. This is to sufficiently promote carbonization of organic compounds, suppress residual hydrogen and oxygen, and improve battery characteristics. Further, in order to suppress graphitization and maintain good charge / discharge rate characteristics, the heat treatment temperature is preferably 2000 ° C. or lower, more preferably 1500 ° C. or lower, and even more preferably 1200 ° C. or lower.

In the heat treatment step, the rate of temperature rise up to the heat treatment temperature is preferably 200 ° C./h or less, more preferably 150 ° C./h or less, and even more preferably 100 ° C./h or less.

The heat treatment time is not particularly limited as long as carbonization has progressed sufficiently, but is preferably 10 minutes or longer, more preferably 30 minutes or longer, and even more preferably 50 minutes or longer. Here, the heat treatment time refers to a predetermined temperature, that is, a time during which the state of ± 20 ° C. is maintained with respect to the heat treatment temperature, and the heat treatment time includes the heating time by feedback control for keeping the device warm. Is done.

After the heat treatment step, the composite material may be crushed if necessary. In the composite particles C in the composite material according to the present invention, the metal oxide particles 23 are firmly fixed to the graphite particles 21 via the carbonaceous layer 22, so that the metal is oxidized from the graphite particles 21 by crushing. The dropout of the object particles 23 is sufficiently suppressed.

[3-2. Manufacturing method 2]
Another example of the method for producing a composite material according to the present invention is a mixing step of mixing metal oxide particles containing an organic compound and graphite particles, and a heat treatment step of heat-treating the mixture obtained in the mixing step. include.

As the metal oxide particles containing the organic compound, particles in which a part or the whole of the surface of the metal compound particles is covered with the organic compound are preferable. An example of a method for producing such particles is to disperse a metal oxide in a solution in which an organic compound is dissolved to remove a solvent. The mixture of the organic compound and the metal oxide particles may be pulverized if necessary. Further, as another example of the method for producing the particles, there is also a method of mixing the organic compound and the metal oxide particles without using a liquid medium.

The conditions of the heat treatment step, such as the atmospheric gas, the heat treatment temperature, the temperature rising time, and the heat treatment time, are the same as those in the manufacturing method 1.

Hereinafter, examples and comparative examples of the present invention will be described, but these do not limit the technical scope of the present invention.

<1. Manufacture of composite materials>
Under the conditions shown in Tables 1 and 2, the composite materials of each example were prepared as described below.

Table 3 shows the physical characteristics of the obtained composite material.

[1-1. material〕
The details of the raw materials shown in Table 1 are as follows.

(Graphite particles)
[SCMG] SCMG (registered trademark), an artificial graphite manufactured by Showa Denko KK, is used. This artificial graphite has a D50 of 6.0 μm and a BET specific surface area of 5.9 m ² / g.
[SCMG-2] SCMG (registered trademark) -2, an artificial graphite manufactured by Showa Denko KK, is used. This artificial graphite has a D50 of 11.8 μm and a BET specific surface area of 2.6 m ² / g.
[Natural graphite] Chinese scale-like natural graphite is used. This natural graphite has a D50 of 26.8 μm and a BET specific surface area of 9.1 m ² / g.

(Organic compound)
Use one of petroleum pitch, citric acid, tartaric acid, malic acid, salicylic acid, and terephthalic acid.

(Metal oxide particles)
The following metal oxide particles are used.
[Titanium Oxide A] The product obtained by the production method of Example 1 of JP-A-2017-114700. The content of the anatase crystal phase in the whole crystal phase was 100% by mass, and the average primary particle diameter obtained from the BET specific surface area was 3.83 nm (BET specific surface area 392 m ² / g).
[Titanium oxide B] Manufactured by IoLiTec, anatase type, purity 99.5% by mass, average primary particle diameter 20 nm.
[Titanium oxide C] Made by IoLiTec, rutile type, purity 99.5% by mass, average primary particle diameter 20 nm.
[Aluminum oxide]: manufactured by IoLiTec, Al ₂ O ₃ , γ type, average primary particle diameter 5 nm.
[Copper (II) Oxide]: Manufactured by IoLiTech, with an average primary particle diameter of 50 nm.

[1-2. Mixing process]
Using the raw materials of the types and amounts shown in Table 1, a mixture containing graphite particles, an organic compound and metal oxide particles is obtained by the following steps A to D.

(Step A)
Graphite particles, organic compounds and metal oxide particles are mixed at 25 ° C. Mixing is carried out, for example, using a V-type mixer (VM-10, manufactured by Dalton Corporation) for 10 minutes to obtain a mixture.
(Step B)
Graphite particles and metal oxide particles are added to an aqueous solution of an organic compound, and the graphite particles and the metal oxide particles are dispersed in the aqueous solution. Then, the dispersion is placed in a vacuum dryer and dried at 120 ° C. until all the water is removed to obtain a mixture. Since petroleum pitch is not water-soluble, there is no example of using petroleum pitch as an organic compound in this step.
(Process C)
The organic compound and the metal oxide particles are mixed at 25 ° C. Mixing is performed for 10 minutes using, for example, a V-type mixer (VM-10, manufactured by Dalton Corporation). The obtained mixture of the organic compound and the metal oxide particles is mixed with the graphite particles at 25 ° C. Mixing is performed for 10 minutes using, for example, a V-type mixer (VM-10, manufactured by Dalton Corporation).
(Step D)
Metal oxide particles are added to an aqueous solution of an organic compound, and the metal oxide particles are dispersed in the aqueous solution. Then, the dispersion is placed in a vacuum dryer and dried at 120 ° C. until the water content is exhausted to obtain a mixture of the organic compound and the metal oxide. The obtained mixture of the organic compound and the metal oxide particles is mixed with the graphite particles at 25 ° C. Mixing is performed for 10 minutes using, for example, a V-type mixer (VM-10, manufactured by Dalton Corporation). Since petroleum pitch is not water-soluble, there is no example of using petroleum pitch as an organic compound in this step.

[1-3. Heat treatment process]
The mixture obtained in step A or B is placed in an electric tube furnace, heated at the rate shown in Table 1 under a nitrogen gas atmosphere, and held at the heat treatment temperature shown in Table 1 for 1 hour. Then, the inside of the furnace is cooled to 25 ° C., and the produced composite material is recovered.

<2. Evaluation of composite materials>
[2-1. Observation with a transmission electron microscope (TEM)]
The composite material is dispersed in ethanol, collected by a microgrid mesh, and measured under the following conditions.
Transmission electron microscope device: Hitachi H-9500
Acceleration voltage: 300kV
Observation magnification: 30,000 times

Observe the particles of the composite material with a transmission electron microscope (TEM) and confirm that the metal oxide particles are attached to the graphite particles via the carbonaceous layer. Also, confirm that the composite particles have a portion where the graphite particles or the carbonaceous layer is exposed.

Further, from the obtained TEM observation image, the area ratio R (area%) of the portion where at least one of the graphite particles 21 and the carbonaceous layer 23 occupying the surface of the composite particles C is exposed according to the above method, and The average thickness t (nm) of the carbonaceous layer 22 is obtained.

[2-2.50% Particle Diameter (D50)]
Using a Malvern Mastersizer 2000 (registered trademark) as a laser diffraction type particle size distribution measuring device, put a 5 mg sample in a container, add 10 g of water containing 0.04% by mass of a surfactant, and add 10 g for more than 5 minutes. Measurement was performed after the sound treatment, and a 50% particle size (D50) in the volume-based cumulative particle size distribution was obtained.

[2-3. BET specific surface area]
Using NOVA2200e manufactured by Quantachrome as a BET specific surface area measuring device, a 3 g sample was placed in a sample cell (9 mm × 135 mm), dried at 300 ° C. for 1 hour under vacuum conditions, and then measured. N ₂ was used as the gas for measuring the BET specific surface area.

<3. Battery production>
Hereinafter, a method for manufacturing a battery using the composite materials obtained in Examples and Comparative Examples will be described. Regarding each configuration of the battery manufactured here, those corresponding to the configuration with the reference reference numeral shown in FIG. 1 will be described with reference reference numerals of the corresponding configuration.

[3-1. Preparation of solid electrolyte layer 12]
Under an argon gas atmosphere, the starting materials Li ₂ S (manufactured by Nippon Kagaku Co., Ltd.) and P ₂ S ₅ (manufactured by Sigma Aldrich Japan GK) are weighed and mixed at a molar ratio of 75:25 to form a planetary ball mill (a planetary ball mill (manufactured by Sigma Aldrich Japan GK). By mechanical milling (rotation speed 400 rpm) for 20 hours using P-5 type, manufactured by Fritsch Japan Co., Ltd. and zirconia balls (10 mm φ7, 3 mm φ10), Li ₃ PS ₄ with a D50 of 0.3 μm Obtain an amorphous solid electrolyte.

The obtained amorphous solid electrolyte is press-molded by a uniaxial press molding machine using a polyethylene die having an inner diameter of 10 mmφ and a punch made of SUS to prepare a solid electrolyte layer 12 as a sheet having a thickness of 960 μm. ..

[3-2. Preparation of negative electrode mixture layer 132]
48.5% by mass of the composite material produced in the examples, 48.5% by mass of the solid electrolyte (Li ₃ PS ₄ , D50: 8 μm), and 3% by mass of VGCF-H (manufactured by Showa Denko KK, registered trademark). And mix. The mixture is homogenized by milling at 100 rpm for 1 hour using a planetary ball mill. The homogenized mixture is press-molded at 400 MPa by a uniaxial press molding machine using a polyethylene die having an inner diameter of 10 mmφ and a punch made of SUS to prepare a negative electrode mixture layer 132 as a sheet having a thickness of 65 μm.

[3-3. Preparation of positive electrode mixture layer 112]
Positive electrode active material LiCoO ₂ (manufactured by Nippon Kagaku Kogyo Co., Ltd., D50: 10 μm) 55% by mass, solid electrolyte (Li ₃ PS ₄ , D50: 8 μm) 40% by mass, and VGCF-H (manufactured by Showa Denko KK). , Registered trademark) Mix with 5% by mass. The mixture is homogenized by milling at 100 rpm for 1 hour using a planetary ball mill. The homogenized mixture is press-molded at 400 MPa by a uniaxial press molding machine using a polyethylene die having an inner diameter of 10 mmφ and a punch made of SUS to prepare a positive electrode mixture layer 112 as a sheet having a thickness of 65 μm.

[3-4. Assembly of all-solid-state lithium-ion secondary battery 1]
The negative electrode mixture layer 132, the solid electrolyte layer 12, and the positive electrode mixture layer 112 are laminated in this order in a die made of polyethylene having an inner diameter of 10 mmφ, and punches made of SUS are punched from both sides of the negative electrode mixture layer 132 side and the positive electrode mixture layer 112 side. The negative electrode mixture layer 132, the solid electrolyte layer 12, and the positive electrode mixture layer 112 are joined together to obtain a laminated body. The laminated body obtained here is referred to as a laminated body A.

The obtained laminated body A is once taken out from the die, and the negative electrode lead 131a, the copper foil (negative electrode current collector 131), and the negative electrode mixture layer 132 are directed downward in the above-mentioned die, and the laminated body A and aluminum. The foil (positive electrode current collector 111) and the positive electrode lead 111a are stacked in this order, sandwiched from both sides of the negative electrode lead 131a side and the positive electrode lead 111a side with a SUS punch at a pressure of 80 MPa, and the negative electrode lead 131a, the copper foil, and the laminate A are sandwiched. , Aluminum foil, and positive electrode lead 111a are joined to obtain an all-solid-state lithium-ion secondary battery 1.

<4. Battery evaluation>
All of the following battery evaluations are performed in the air at 25 ° C.
[4-1. Measurement of Coulomb Efficiency]
The all-solid-state lithium-ion secondary battery 1 manufactured as described above is constantly charged at 1.25 mA (0.05 C) from the rest potential to 4.2 V. Subsequently, constant voltage charging for 40 hours is performed at a constant voltage of 4.2 V. The charge capacity (mAh) by constant voltage charging is defined as the initial charge capacity Qc1.

Next, constant current discharge is performed at 1.25 mA (0.05 C) until the voltage reaches 2.75 V. The discharge capacity (mAh) due to constant current discharge is defined as the initial discharge capacity Qd1. The value obtained by dividing the initial discharge capacity Qd1 (mAh) by the mass of the composite material in the negative electrode layer is defined as the initial discharge capacity density (mAh / g).

Further, the ratio of the initial discharge capacity Qd1 to the initial charge capacity Qc1 is expressed as a percentage, and 100 × Qd1 / Qc1 is defined as the coulomb efficiency (%).

[4-2. Evaluation of rate characteristics]
After charging in the same procedure as above, the discharge capacity Q _2.5 d [mAh] measured by constant current discharge at 2.5 mA (0.1 C) until it reaches 2.75 V is measured. After charging in the same procedure as above, the discharge capacity Q ₇₅ d [mAh] measured by constant current discharge at 75 mA (3.0 C) until it reaches 2.75 V is measured. Let 100 × Q ₇₅ d / Q _2.5 d be the rate characteristic (%).

[4-3. Evaluation of cycle characteristics]
Charging is performed with a constant current charge of 5.0 mA (0.2 C) until it reaches 4.2 V, and then at a constant voltage of 4.2 V, a constant voltage until the current value decreases to 1.25 mA (0.05 C). Charge. The discharge is a constant current discharge of 25 mA (1.0 C) until the voltage reaches 2.75 V.

These charges and discharges are performed 500 times, and the discharge capacity Qd500 at the 500th time is 100 × Qd500 / Qd1 as the cycle maintenance rate (%).

1: All-solid-state lithium ion secondary battery 11: Positive electrode layer 111: Positive electrode current collector 111a: Positive electrode lead 112: Positive electrode mixture layer 12: Solid electrolyte layer 13: Negative electrode layer 131: Negative electrode current collector 131a: Negative electrode lead 132 : Negative electrode mixture layer C: Composite particles 21: Graphite particles 22: Carbonaceous layer 23: Metal oxide particles

Claims (14)

A composite material containing composite particles in which metal oxide particles are attached to the surface of graphite particles via a carbonaceous layer, and the composite particles have a surface on a portion where at least one of the graphite particles and the carbonaceous layer is exposed. A composite material characterized by having in. The composite material according to claim 1, wherein the composite particle has a portion where the carbonaceous layer is exposed. The composite material according to claim 1 or 2, wherein the carbonaceous layer has a graphene structure formed along the surface of graphite particles. The composite material according to any one of claims 1 to 3, wherein the average thickness of the carbonaceous layer is 0.1 nm or more and 30 nm or less. The 50% particle size of the graphite particles in the volume-based cumulative particle size distribution is 2 μm or more, and the average primary particle size calculated from the BET specific surface area of the metal oxide particles is 100 nm or less. The composite material according to any one of 4. The composite material according to any one of claims 1 to 5, wherein the content of the metal oxide particles with respect to 100 parts by mass of the graphite particles is 0.1 parts by mass or more and 10 parts by mass or less. The one according to any one of claims 1 to 6, wherein the metal oxide particles contain an oxide of at least one of groups 1 to 12, aluminum, gallium, indium, thallium, tin, and lead. Composite material. The composite material according to any one of claims 1 to 7, wherein the metal oxide particles contain titanium oxide. The composite material according to claim 8, wherein the content of the anatase crystal phase in the total crystal phase of titanium oxide is 70% by mass or more. The composite material according to claim 8, wherein the content of the rutile crystal phase in the total crystal phase of titanium oxide is 70% by mass or more. A negative electrode material for an all-solid-state lithium ion secondary battery containing the composite material according to any one of claims 1 to 10. A negative electrode for an all-solid-state lithium-ion secondary battery containing the composite material according to any one of claims 1 to 10 and a sulfide solid electrolyte. A composite including a mixing step of mixing graphite particles, an organic compound, and metal oxide particles, and a heat treatment step of heat-treating the mixture obtained in the mixing step at 600 ° C. or higher and 2000 ° C. or lower in a non-oxidizing gas atmosphere. Material manufacturing method. A mixing step of mixing metal oxide particles containing an organic compound and graphite particles, and a heat treatment step of heat-treating the mixture obtained in the mixing step at 600 ° C. or higher and 2000 ° C. or lower in a non-oxidizing gas atmosphere. Method of manufacturing composite material including.

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