JP7697326B2 - Negative electrode layer - Google Patents

Description

This disclosure relates to a negative electrode layer.

In recent years, with the rapid spread of information-related devices and communication devices such as personal computers, video cameras, and mobile phones, the development of batteries to be used as power sources for these devices has become important. In the automobile industry, too, development of high-output, high-capacity batteries for electric vehicles or hybrid vehicles is underway.
Moreover, solid-state batteries have attracted attention because they use a solid electrolyte as the electrolyte between the positive electrode and the negative electrode, instead of an electrolytic solution containing an organic solvent.

Patent Document 1 discloses an all-solid-state battery that contains a fibrous carbon material having a six-membered carbon ring as the conductive material for the negative electrode.

Patent document 2 discloses the use of carbon fiber as a conductive material for the positive electrode of an all-solid-state battery.

Patent document 3 discloses that VGCF (vapor grown carbon fiber) is used as a conductive additive for the positive electrode of an all-solid-state battery.

Patent document 4 discloses the use of a combination of fine fibrous carbon and fibrous carbon having a diameter of 100 nm or more.

Patent document 5 discloses the use of a combination of carbon fiber and particulate carbon.

JP 2019-125481 A JP 2011-159534 A Patent No. 6314563 JP 2010-238575 A JP 2016-058277 A

Conventional solid-state batteries containing carbon fibers have the problem of poor charge-discharge cycle characteristics.

This disclosure has been made in consideration of the above-mentioned circumstances, and its main objective is to provide a negative electrode layer that can improve the charge-discharge cycle characteristics of a solid-state battery.

The negative electrode layer of the present disclosure is a negative electrode layer for a solid-state battery,
The negative electrode layer includes a negative electrode active material, a binder, carbon fibers, and ceramic fibers,
The ceramic fibers are contained in the negative electrode layer at 1.7 to 4.1% by volume.

The negative electrode layer for the disclosed solid-state battery can improve the charge/discharge cycle characteristics of the solid-state battery.

FIG. 1 is a schematic cross-sectional view showing an example of a negative electrode layer according to the present disclosure. FIG. 2 is a graph showing the initial resistivity and the resistance increase rate after 30 cycles for 0 volume % of ceramic fiber added.

The negative electrode layer of the present disclosure is a negative electrode layer for a solid-state battery,
The negative electrode layer includes a negative electrode active material, a binder, carbon fibers, and ceramic fibers,
The ceramic fibers are contained in the negative electrode layer at 1.7 to 4.1% by volume.

In the case of a solid-state battery using an active material that expands and contracts due to charging and discharging, such as a Si-based or C-based material, in the negative electrode layer, the expansion and contraction of the negative electrode active material due to charging and discharging causes peeling at the interface between the negative electrode active material and the solid electrolyte, resulting in an increase in the internal resistance of the solid-state battery and a decrease in the capacity retention rate, etc.
Conventional solid-state batteries using carbon fibers have been unable to prevent delamination at the interface between the negative electrode active material and the solid electrolyte, presumably because carbon fibers are too flexible to exhibit sufficient strength as fibers, and the expansion and contraction of the negative electrode active material disrupts the ionic and electronic conductive paths of the negative electrode.
The researcher found that by adding ceramic fibers with low flexibility to the negative electrode layer, they are arranged linearly in the negative electrode layer compared to carbon fibers, and the fixation by the binder increases, making it difficult for interface peeling to occur due to expansion and contraction accompanying charge and discharge of the negative electrode active material, and improving the discharge capacity retention rate. The researcher also found that when fine ceramic fibers are added, there is a region where the increase in resistance is small in the region of small addition amount, and the discharge capacity retention rate is improved. Normally, when ceramic fillers are added to an electrode, the electronic conduction path in the electrode is obstructed, and the resistance increases in proportion to the amount added, but a small amount of fine ceramic fibers does not obstruct the electronic conduction path of the carbon fiber and improves the strength of the binder. In addition, if the amount of ceramic fibers added is too large, the electronic conduction path and ion conduction path in the electrode are obstructed, and uniform charge and discharge reactions cannot occur in the electrode, local expansion and contraction occurs, the negative electrode active material and solid electrolyte are peeled off, and the discharge capacity retention rate is significantly reduced, so the appropriate amount of addition is important.

In this disclosure, unless otherwise specified, the average particle size of particles is the volume-based median diameter (D50) value measured by laser diffraction/scattering particle size distribution measurement. In addition, in this disclosure, the median diameter (D50) is the diameter (volume average diameter) at which the cumulative volume of particles is half (50%) of the total volume when the particles are arranged in order from smallest to largest particle size.

FIG. 1 is a schematic cross-sectional view showing an example of a negative electrode layer according to the present disclosure.
1 , the negative electrode layer 100 includes a negative electrode active material 11, a binder 12, a solid electrolyte 13, carbon fibers 14, and ceramic fibers 15. The ceramic fibers 15 are present in the binder 12 present between particles of the negative electrode active material 11 and the solid electrolyte 13.

[Negative electrode layer]
The negative electrode layer of the present disclosure contains at least a negative electrode active material, a binder, carbon fibers, and ceramic fibers, and optionally contains a solid electrolyte and the like.
The thickness of the negative electrode layer is not particularly limited, but may be, for example, 10 to 100 μm.

Examples of the negative electrode active material include graphite, mesocarbon microbeads (MCMB), highly oriented graphite (HOPG), hard carbon, soft carbon, lithium alloy, simple Si, Si alloy, and Li ₄ Ti ₅ O _12. As the lithium alloy and Si alloy, the same ones as those exemplified in the positive electrode active material can be used.
The shape of the negative electrode active material is not particularly limited, and examples thereof include a particulate shape, a plate shape, etc. When the negative electrode active material is particulate, the negative electrode active material may be primary particles or secondary particles.
The content of the negative electrode active material in the negative electrode layer is not particularly limited, but may be, for example, 20% by mass to 90% by mass.

Examples of the binder include acrylonitrile butadiene rubber (ABR), butadiene rubber (BR), polyvinylidene fluoride (PVdF), and styrene butadiene rubber (SBR). The content of the binder in the negative electrode layer is not particularly limited and may be 1 to 5 mass%.

The carbon fiber may be any one or a mixture of two or more selected from the group consisting of vapor grown carbon fiber (VGCF), carbon nanotube (CNT) and carbon nanofiber (CNF).
The length of the carbon fibers is not particularly limited, but may be longer than the length of the ceramic fibers.
The content of carbon fibers in the negative electrode layer is not particularly limited, and may be 1.0 to 20.0 mass %.
The volume percentage of the carbon fibers in the negative electrode layer is not particularly limited, and may be 1.0 to 20.0% by volume.

Ceramic fibers are less flexible than carbon fibers and may be a less reactive material that does not degrade other battery components.
The length of the ceramic fibers is not particularly limited, and may be 200 nm or more, or 400 nm or more, and may be shorter than the length of the carbon fibers.
The diameter of the ceramic fibers is not particularly limited and may be 1 to 10 nm, or may be 2 to 6 nm.
From the viewpoint of strengthening the negative electrode layer, the ceramic fibers in the negative electrode layer need to be arranged linearly. Since the longer the fiber, the higher the probability of bending. From the viewpoint of arranging the ceramic fibers more linearly than the carbon fibers, the ceramic fibers may be shorter and less flexible than the carbon fibers.
The ceramic fibers may be one or a mixture of two or more selected from the group consisting of oxides, carbides, nitrides, and borides, or may be fibrous aluminum oxide.
The aspect ratio of the line length to the line diameter (line length/line diameter) of the ceramic fiber may be 100 or more. The ceramic fiber may be present in a binder present between particles of the negative electrode active material and the solid electrolyte. From the viewpoint of having the ceramic fiber present in the binder, the line diameter may be sufficiently thin compared to the negative electrode active material and the solid electrolyte, and the aspect ratio of the line length to the line diameter (line length/line diameter) may be 100 or more.
The ceramic fibers may be contained in the negative electrode layer in a volume ratio to the carbon fibers of 0.24 or more and 0.59 or less.
The content of the ceramic fibers in the negative electrode layer is not particularly limited, and may be 1.0 to 5.5 mass %, or 1.9 to 4.8 mass %.
The ceramic fibers may be contained in the negative electrode layer in an amount of 1.7 to 4.1% by volume.

As the solid electrolyte contained in the negative electrode layer, any known solid electrolyte that can be used in solid-state batteries can be appropriately used, and examples of such solid electrolyte include oxide-based solid electrolytes and sulfide-based solid electrolytes.
Examples of sulfide-based solid electrolytes include Li ₂ S-P ₂ S ₅ , Li ₂ S-SiS ₂ , LiX-Li ₂ S-SiS ₂ , LiX-Li ₂ S-P ₂ S ₅ , LiX-Li ₂ O-Li ₂ S-P ₂ S ₅ , LiX-Li ₂ S-P ₂ O ₅ , LiX-Li ₃ PO ₄ -P ₂ S ₅ , and Li ₃ PS _4. The above description of "Li ₂ S-P ₂ S ₅ " means a material obtained by using a raw material composition containing Li ₂ S and P ₂ S ₅ , and the same applies to other descriptions. The "X" in the above LiX represents a halogen element. The LiX-containing raw material composition may contain one or more types of LiX. When two or more types of LiX are contained, the mixing ratio of the two or more types is not particularly limited.
The molar ratio of each element in the sulfide-based solid electrolyte can be controlled by adjusting the content of each element in the raw material. The molar ratio and composition of each element in the sulfide-based solid electrolyte can be measured, for example, by ICP emission spectrometry.

The sulfide-based solid electrolyte may be a sulfide glass, a crystallized sulfide glass (glass ceramics), or a crystalline material obtained by subjecting a raw material composition to a solid-phase reaction treatment.
The crystalline state of the sulfide-based solid electrolyte can be confirmed, for example, by subjecting the sulfide-based solid electrolyte to powder X-ray diffraction measurement using CuKα radiation.

The sulfide glass can be obtained by subjecting a raw material composition (e.g., _a mixture of _Li2S and _P2S5 ) to an amorphous process. Examples of the amorphous process include mechanical milling.

Glass ceramics can be obtained, for example, by heat treating a sulfide glass.
The heat treatment temperature may be any temperature higher than the crystallization temperature (Tc) of the sulfide glass observed by thermal analysis, and is usually 195° C. or higher. On the other hand, there is no particular upper limit to the heat treatment temperature.
The crystallization temperature (Tc) of the sulfide glass can be measured by differential thermal analysis (DTA).
The heat treatment time is not particularly limited as long as it is a time that can obtain the desired crystallinity of the glass ceramics, but is, for example, in the range of 1 minute to 24 hours, and particularly, in the range of 1 minute to 10 hours.
The method of heat treatment is not particularly limited, but may be, for example, a method using a baking furnace.

The oxide-based solid electrolyte may be, for example, a substance having a garnet-type crystal structure containing Li, La, A (A is at least one of Zr, Nb, Ta, and Al), and O. The oxide-based solid electrolyte may be, for example, Li _3+x PO _4-x N _x (1≦x≦3).

The solid electrolyte may be in the form of particles from the viewpoint of ease of handling.
The solid electrolyte may be used alone or in combination of two or more kinds. When two or more kinds of solid electrolytes are used, the two or more kinds of solid electrolytes may be mixed.
The proportion of the solid electrolyte in the negative electrode layer is not particularly limited, but may be, for example, within a range of 1 mass % to 80 mass % when the total mass of the negative electrode layer is taken as 100 mass %.

The method for producing the negative electrode layer is, for example, as follows. Particles of the negative electrode active material, particles of the solid electrolyte, carbon fibers, ceramic fibers, and a binder are mixed with an organic solvent (dispersion medium) such as N-methylpyrrolidone to form a slurry (or a paste). The slurry (or paste) for the negative electrode layer may be applied to one surface of a support such as a current collector, and the slurry for the negative electrode layer may be dried to form a negative electrode layer on the support. The method for applying the slurry for the negative electrode layer is not particularly limited, and examples thereof include a doctor blade method, a metal mask printing method, an electrostatic coating method, a dip coating method, a spray coating method, a roll coating method, a gravure coating method, and a screen printing method.
The support can be appropriately selected from those having self-supporting properties and is not particularly limited. For example, metal foils such as Cu and Al can be used.

The negative electrode layer of the present disclosure is for use in a solid-state battery. The solid-state battery may include a positive electrode including a positive electrode layer, a solid electrolyte layer, and a negative electrode including a negative electrode layer.

[Positive electrode]
The positive electrode includes a positive electrode layer and, optionally, a positive electrode current collector.

[Positive electrode layer]
The positive electrode layer contains a positive electrode active material, and may contain, as optional components, a solid electrolyte, a conductive material, a binder, and the like.

There is no particular limitation on the type of the positive electrode active material, and any material that can be used as an active material for a solid-state battery can be used. Examples of the positive electrode active material include metallic lithium (Li), lithium alloys, LiCoO ₂ , LiNi _0.8 Co _0.15 Al _0.05 O ₂ , LiNi _x Co _1-x O ₂ (0<x<1), LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , LiMnO ₂ , Li-Mn spinel substituted with a different element, lithium titanate, lithium metal phosphate, LiCoN, Li ₂ SiO ₃ , and Li ₄ SiO ₄ , transition metal oxides, TiS ₂ , Si, SiO ₂ , Si alloys, and lithium storage intermetallic compounds. Examples of the Li-Mn spinel _{substituted with} different elements _{include LiMn1.5Ni0.5O4, LiMn1.5Al0.5O4 , LiMn1.5Mg0.5O4 , LiMn1.5Co0.5O4 , LiMn1.5Fe0.5O4 , and LiMn1.5Zn0.5O4 .} Examples of _{the lithium} titanate include _Li4Ti5O12 . Examples of the _lithium metal phosphate _{include LiFePO4} , _LiMnPO4 , _LiCoPO4 , and _{LiNiPO4 . Examples} of _the transition _metal oxide include _V2O5 and _MoO3 . Examples of lithium-storing intermetallic compounds include Mg ₂ Sn, Mg ₂ Ge, Mg ₂ Sb, and Cu ₃ Sb.
Examples of lithium alloys include Li-Au, Li-Mg, Li-Sn, Li-Si, Li-Al, Li-B, Li-C, Li-Ca, Li-Ga, Li-Ge, Li-As, Li-Se, Li-Ru, Li-Rh, Li-Pd, Li-Ag, Li-Cd, Li-In, Li-Sb, Li-Ir, Li-Pt, Li-Hg, Li-Pb, Li-Bi, Li-Zn, Li-Tl, Li-Te, and Li-At. Examples of Si alloys include alloys with metals such as Li, and may also be alloys with at least one metal selected from the group consisting of Sn, Ge, and Al.
The shape of the positive electrode active material is not particularly limited, and may be particulate. When the positive electrode active material is particulate, the positive electrode active material may be primary particles or secondary particles.
A coating layer containing a Li ion conductive oxide may be formed on the surface of the positive electrode active material, because this can suppress the reaction between the positive electrode active material and the solid electrolyte.
Examples of Li ion conductive oxides include LiNbO ₃ , Li ₄ Ti ₅ O ₁₂ , and Li ₃ PO ₄ . The thickness of the coating layer is, for example, 0.1 nm or more, and may be 1 nm or more. On the other hand, the thickness of the coating layer is, for example, 100 nm or less, and may be 20 nm or less. The coverage of the coating layer on the surface of the positive electrode active material is, for example, 70% or more, and may be 90% or more.

As the solid electrolyte, the same ones as those exemplified in the negative electrode layer can be exemplified.
The content of the solid electrolyte in the positive electrode layer is not particularly limited, but may be, for example, within a range of 1 mass % to 80 mass % when the total mass of the positive electrode layer is taken as 100 mass %.

The conductive material may be a known material, such as a carbon material and metal particles. The carbon material may be at least one selected from the group consisting of acetylene black, furnace black, VGCF, carbon nanotubes, and carbon nanofibers. Among them, from the viewpoint of electronic conductivity, at least one selected from the group consisting of VGCF, carbon nanotubes, and carbon nanofibers may be used. The metal particles may be particles of Ni, Cu, Fe, SUS, etc.
The content of the conductive material in the positive electrode layer is not particularly limited.

Examples of binders include those exemplified in the negative electrode layer.

There are no particular limitations on the thickness of the positive electrode layer.

The positive electrode layer can be formed by a conventionally known method.
For example, a positive electrode active material and, if necessary, other components are put into a solvent and stirred to prepare a positive electrode layer slurry, and the positive electrode layer slurry is applied onto one side of a support such as a positive electrode current collector and dried to obtain a positive electrode layer.
Examples of the solvent include butyl acetate, butyl butyrate, mesitylene, tetralin, heptane, and N-methyl-2-pyrrolidone (NMP).
The method for applying the positive electrode layer slurry onto one surface of a support such as a positive electrode current collector is not particularly limited, and may be, for example, the same method as the method for applying the negative electrode layer slurry.

As another method for forming the positive electrode layer, the positive electrode layer may be formed by pressure molding a powder of a positive electrode mixture containing a positive electrode active material and other components as necessary. When the powder of the positive electrode mixture is pressure molded, a press pressure of about 1 MPa to 2000 MPa is usually applied.
The method of applying pressure is not particularly limited, but examples thereof include a method of applying pressure using a plate press, a roll press, or the like.

[Positive electrode current collector]
The positive electrode current collector may be a known metal that can be used as a current collector for a solid-state battery. Examples of such metals include metal materials containing one or more elements selected from the group consisting of Cu, Ni, Al, V, Au, Pt, Mg, Fe, Ti, Co, Cr, Zn, Ge, and In. Examples of the positive electrode current collector include SUS, aluminum, nickel, iron, titanium, and carbon.
The shape of the positive electrode current collector is not particularly limited, and may be in various shapes such as a foil shape, a mesh shape, etc. The thickness of the positive electrode current collector varies depending on the shape, but may be, for example, in the range of 1 μm to 50 μm, or in the range of 5 μm to 20 μm.

[Negative electrode]
The negative electrode comprises a negative electrode layer of the present disclosure and optionally a negative electrode current collector.

[Negative electrode current collector]
The material of the negative electrode current collector may be a material that does not alloy with Li, and examples thereof include SUS, copper, and nickel. Examples of the shape of the negative electrode current collector include foil and plate. The planar shape of the negative electrode current collector is not particularly limited, and examples thereof include a circle, an ellipse, a rectangle, and any polygonal shape. The thickness of the negative electrode current collector varies depending on the shape, and may be, for example, in the range of 1 μm to 50 μm, or in the range of 5 μm to 20 μm.

[Solid electrolyte layer]
The solid electrolyte layer includes at least a solid electrolyte.
As the solid electrolyte, the same ones as those exemplified in the negative electrode layer can be exemplified.
The proportion of the solid electrolyte in the solid electrolyte layer is not particularly limited, but is, for example, 50% by mass or more, may be in the range of 60% by mass or more and 100% by mass or less, may be in the range of 70% by mass or more and 100% by mass or less, or may be 100% by mass.

The solid electrolyte layer may contain a binder from the viewpoint of expressing plasticity. Examples of such binders include the materials exemplified as the binders used in the negative electrode layer. However, in order to facilitate high output, the binder may be contained in the solid electrolyte layer in an amount of 5 mass% or less from the viewpoint of preventing excessive aggregation of the solid electrolyte and enabling the formation of a solid electrolyte layer having a uniformly dispersed solid electrolyte.

The thickness of the solid electrolyte layer is not particularly limited, and is usually 0.1 μm or more and 1 mm or less.
The method for forming the solid electrolyte layer includes a method of pressing a powder of a solid electrolyte material containing a solid electrolyte, etc. When pressing the powder of the solid electrolyte material, a press pressure of about 1 MPa or more and 2000 MPa or less is usually applied.
The method of applying pressure is not particularly limited, but may be any of the pressure applying methods exemplified in the formation of the positive electrode layer.

The solid-state battery may optionally include an exterior body that houses a laminate including a positive electrode collector, a positive electrode layer, a solid electrolyte layer, a negative electrode layer, and a negative electrode collector in this order, and a restraining member.
The material of the exterior body is not particularly limited as long as it is stable to the electrolyte, and examples of the material include polypropylene, polyethylene, and resins such as acrylic resin.
The restraining member may be any known member usable as a restraining member for a solid-state battery, as long as it can apply a restraining pressure in the stacking direction to the stack. For example, a restraining member having a plate-shaped portion that sandwiches both surfaces of the stack, a rod-shaped portion that connects the two plate-shaped portions, and an adjustment portion that is connected to the rod-shaped portion and adjusts the restraining pressure by a screw structure or the like can be used. The adjustment portion can apply a desired restraining pressure to the stack.
The restraining pressure is not particularly limited, but may be, for example, 0.1 MPa or more, 1 MPa or more, or 5 MPa or more. By increasing the restraining pressure, there is an advantage that the contact between the layers is easily improved. On the other hand, the restraining pressure may be, for example, 100 MPa or less, 50 MPa or less, or 20 MPa or less. If the restraining pressure is too high, the restraining member is required to have high rigidity, and the restraining member may become large.

The solid-state battery may have only one of the laminates, or may be formed by stacking a plurality of the laminates.
The solid-state battery may be a primary battery or a secondary battery, but may be a secondary battery. The secondary battery can be repeatedly charged and discharged. The secondary battery is useful, for example, as an in-vehicle battery. The solid-state battery may be a solid-state lithium secondary battery or a solid-state lithium ion secondary battery.
Examples of the shape of the solid-state battery include a coin type, a laminate type, a cylindrical type, and a square type.
In the present disclosure, a solid-state battery may be any battery that uses a solid electrolyte instead of an electrolytic solution containing an organic solvent as the electrolyte of an electrolyte layer interposed between a positive electrode and a negative electrode, and may be an all-solid-state battery in which the positive electrode, the negative electrode, and the electrolyte layer are all made of solid materials.

In the method for producing a solid-state battery of the present disclosure, for example, first, a solid electrolyte layer is formed by pressure-molding a powder of a solid electrolyte material. Then, a positive electrode layer is obtained by pressure-molding a powder of a positive electrode material including a positive electrode active material onto one surface of the solid electrolyte layer. Then, a negative electrode layer is obtained by pressure-molding a powder of the negative electrode material onto one surface of a negative electrode current collector. An anode current collector-negative electrode layer laminate is attached to the surface of the solid electrolyte layer opposite to the surface on which the positive electrode layer is formed, so that the negative electrode layer is in contact with the solid electrolyte layer. Then, a positive electrode current collector is attached to the surface of the positive electrode layer opposite to the solid electrolyte layer. This may form a solid-state battery of the present disclosure.
In this case, the pressing pressure when the powder of the negative electrode material, the powder of the solid electrolyte material, and the powder of the positive electrode material are press-molded is usually about 1 MPa or more and 2000 MPa or less.
The method of applying pressure is not particularly limited, but may be any of the pressure applying methods exemplified in the formation of the positive electrode layer.

Example 1
[Preparation of Positive Electrode]
A positive electrode active material (LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , average particle size 10 μm) coated with LiNbO ₃ using a rolling fluidized granulation coating device, a sulfide-based solid electrolyte (10 LiI.15 LiBr.75 (0.75 Li ₂ S.0.25 _{P 2} S ₅ ) (mol%), average particle size 0.5 μm), a conductive material (VGCF-H), and a binder (PVDF) were weighed out so that the weight ratio of the positive electrode active material:sulfide-based solid electrolyte:conductive material:binder was 85.4:12.7:1.3:0.6, and mixed with a dispersion medium (diisobutyl ketone). The obtained mixture was dispersed using an ultrasonic homogenizer (UH-50, manufactured by SMT Co., Ltd.) to obtain a slurry for the positive electrode layer. The obtained slurry for the positive electrode layer was applied to a positive electrode current collector (Al foil, thickness 15 μm) by a blade coating method using an applicator, and dried for 30 minutes at 100° C. Thereafter, a positive electrode having a positive electrode layer and a positive electrode current collector was obtained by punching out the positive electrode layer and the positive electrode current collector to a size of 1 ^cm2 .
[Preparation of negative electrode]
A negative electrode active material (Si particles, average particle size 2.5 μm), a sulfide-based solid electrolyte (10LiI·15LiBr·75(0.75Li ₂ S·0.25P ₂ S ₅ )(mol%), average particle size 0.5 μm), carbon fiber (VGCF-H), binder (SBR), and ceramic fiber (aluminum oxide manufactured by Sigma-Aldrich (product number: 551643)) were weighed out in a weight ratio of negative electrode active material: sulfide-based solid electrolyte: carbon fiber: binder: aluminum oxide = 45.7: 43.3: 6.4: 1.4: 2.7, and mixed with a dispersion medium (diisobutyl ketone). The obtained mixture was dispersed with a thin film swirling high-speed mixer (Filmix 30-L type, manufactured by Primix Corporation) to obtain a slurry for the negative electrode layer. The obtained slurry for the negative electrode layer was applied to a negative electrode current collector (Ni foil, thickness 22 μm) by a blade coating method using an applicator, and dried at 100 ° C. for 30 minutes. The applicator gap (gap) at this time was adjusted so that the ratio of the positive electrode capacity/negative electrode capacity of the negative electrode active material weight per 1 cm ² was 3 when the positive electrode active material capacity was 207 mAh / g and the negative electrode active material capacity was 3579 mAh / g. Then, by punching out to a size of 1 cm ² , a negative electrode having a negative electrode layer and a negative electrode current collector was obtained. The ceramic fiber was 2 vol % when the volume of the negative electrode layer was 100 vol %. The ceramic fiber was contained in the negative electrode layer at a volume ratio of 0.28 to the carbon fiber.
[Preparation of solid electrolyte layer]
A sulfide-based solid electrolyte (10LiI.15LiBr.75( _{0.75Li2S.0.25P2S5} ) (mol%), average particle size 2.0 μm) and a binder (SBR) were weighed out so that the _weight ratio of sulfide-based solid electrolyte:binder was 99.6:0.4, _and mixed with a dispersion medium (diisobutyl ketone). The obtained mixture was dispersed with an ultrasonic homogenizer (UH-50, manufactured by SMT Co., Ltd.) to obtain a slurry for a solid electrolyte layer. The obtained slurry for a solid electrolyte layer was applied to a support (Al foil, thickness 15 μm) by a blade coating method using an applicator, and dried at 100°C for 30 minutes. Thereafter, a solid electrolyte layer having an Al foil was obtained by punching out to a size of 1 ^cm2 .
[Fabrication of solid-state battery]
The obtained solid electrolyte layer was placed opposite the positive electrode, and the solid electrolyte layer and the positive electrode layer were overlapped, and pressed at a linear pressure of 1.6 t/cm by a roll press method, and then the Al foil was peeled off from the solid electrolyte layer to transfer the solid electrolyte layer onto the positive electrode layer. The solid electrolyte layer transferred onto the positive electrode layer was placed opposite the negative electrode, and the solid electrolyte layer and the negative electrode layer were overlapped, and pressed at a surface pressure of 5.0 t/ ^cm2 by a uniaxial press, and then a current collecting tab was placed on the positive and negative electrode current collecting foils, and laminated and sealed to obtain a solid battery.

[Example 2]
A negative electrode active material (Si particles, average particle size 2.5 μm), a sulfide-based solid electrolyte (10LiI·15LiBr·75(0.75Li ₂ S·0.25P ₂ S ₅ )(mol%), average particle size 0.5 μm), carbon fiber (VGCF-H), binder (SBR) and ceramic fiber (aluminum oxide manufactured by Sigma-Aldrich (product number: 551643)) were weighed out in a weight ratio of negative electrode active material: sulfide-based solid electrolyte: carbon fiber: binder: aluminum oxide = 45.1: 42.8: 6.3: 1.4: 4.1, and mixed with a dispersion medium (diisobutyl ketone). The obtained mixture was dispersed with a thin film swirling high-speed mixer (Filmix 30-L type, manufactured by Primix Corporation) to obtain a slurry for the negative electrode layer. The other preparation methods were the same as those in Example 1. The ceramic fibers accounted for 3% by volume when the volume of the negative electrode layer was taken as 100% by volume. The ceramic fibers were contained in the negative electrode layer in a volume ratio of 0.42 to the carbon fibers.

[Example 3]
Anode active material (Si particles, average particle size 2.5 μm), sulfide-based solid electrolyte (10LiI·15LiBr·75(0.75Li ₂ S·0.25P ₂ S ₅ )(mol%), average particle size 0.5 μm), carbon fiber (VGCF-H), binder (SBR) and ceramic fiber (Sigma-Aldrich aluminum oxide (product number: 551643)) were weighed out in a weight ratio of anode active material:sulfide-based solid electrolyte:carbon fiber:binder:aluminum oxide=44.4:42.1:6.2:1.3:5.5, and mixed with a dispersion medium (diisobutyl ketone). The obtained mixture was dispersed with a thin film swirling high-speed mixer (Filmix 30-L type, manufactured by Primix Corporation) to obtain a slurry for the anode layer. The other preparation methods were the same as those in Example 1. The volume ratio of the ceramic fibers to the carbon fibers in the negative electrode layer was 4.1% by volume, based on the volume of the negative electrode layer being 100% by volume. The volume ratio of the ceramic fibers to the carbon fibers in the negative electrode layer was 0.59.

[Comparative Example 1]
A negative electrode active material (Si particles, average particle size 2.5 μm), a sulfide-based solid electrolyte (10LiI.15LiBr.75(0.75Li ₂ S.0.25P ₂ S ₅ )(mol%), average particle size 0.5 μm), carbon fiber (VGCF-H), and a binder (SBR) were weighed out so that the weight ratio of the negative electrode active material:sulfide-based solid electrolyte:carbon fiber:binder was 47.0:44.6:6.6:1.4, and mixed with a dispersion medium (diisobutyl ketone). That is, no ceramic fiber was added. The obtained mixture was dispersed with a thin film swirling high-speed mixer (Filmix 30-L type, manufactured by Primix Corporation) to obtain a slurry for the negative electrode layer. The other preparation methods were the same as those in Example 1.

[Example 4]
A negative electrode active material (Si particles, average particle size 2.5 μm), a sulfide-based solid electrolyte (10LiI·15LiBr·75(0.75Li ₂ S·0.25P ₂ S ₅ )(mol%), average particle size 0.5 μm), carbon fiber (VGCF-H), binder (SBR) and ceramic fiber (aluminum oxide manufactured by Sigma-Aldrich (product number: 551643)) were weighed out in a weight ratio of negative electrode active material: sulfide-based solid electrolyte: carbon fiber: binder: aluminum oxide = 45.9: 43.5: 6.4: 1.4: 2.3, and mixed with a dispersion medium (diisobutyl ketone). The obtained mixture was dispersed with a thin film swirling high-speed mixer (Filmix 30-L type, manufactured by Primix Corporation) to obtain a slurry for the negative electrode layer. The other preparation methods were the same as those in Example 1. The amount of the ceramic fibers was 1.7% by volume when the volume of the negative electrode layer was taken as 100% by volume. The volume ratio of the ceramic fibers to the carbon fibers in the negative electrode layer was 0.24.

[Comparative Example 2]
Anode active material (Si particles, average particle size 2.5 μm), sulfide-based solid electrolyte (10LiI·15LiBr·75(0.75Li ₂ S·0.25P ₂ S ₅ )(mol%), average particle size 0.5 μm), carbon fiber (VGCF-H), binder (SBR) and ceramic fiber (Sigma-Aldrich aluminum oxide (product number: 551643)) were weighed out in a weight ratio of anode active material:sulfide-based solid electrolyte:carbon fiber:binder:aluminum oxide=44.0:41.7:6.2:1.3:6.4, and mixed with a dispersion medium (diisobutyl ketone). The obtained mixture was dispersed with a thin film swirling high-speed mixer (Filmix 30-L type, manufactured by Primix Corporation) to obtain a slurry for the anode layer. The other preparation methods were the same as those in Example 1. The volume ratio of the ceramic fibers to the carbon fibers in the negative electrode layer was 4.7% by volume, based on the volume of the negative electrode layer being 100% by volume. The volume ratio of the ceramic fibers to the carbon fibers in the negative electrode layer was 0.68.

[Charge/Discharge Evaluation]
The solid-state batteries prepared in Examples 1 to 4 and Comparative Examples 1 and 2 were charged at a constant current of 0.364 mAh to 4.05 V in a thermostatic chamber at 25° C., then charged at a constant voltage of 4.05 V until the current value reached 0.036 mAh, discharged at a constant current of 0.364 mAh to 2.5 V, and then discharged at a constant voltage of 2.5 V until the current value reached 0.036 mAh. This was repeated three times.
The cycle characteristics were evaluated by performing constant current charging at 3.64 mAh up to 4.05 V in a thermostatic bath at 60° C., constant voltage charging at 4.05 V until the current value reached 1.21 mAh, constant current discharging at 3.64 mAh up to 2.5 V, and constant voltage discharging at 2.5 V until the current value reached 1.21 mAh. This was repeated 30 times to evaluate the discharge capacity retention rate after 30 cycles. The discharge capacity retention rate after 30 cycles was calculated from the following formula. The results are shown in Table 1. In addition, the initial resistivity and the resistance increase rate after 30 cycles for the addition of 0 volume (vol)% ceramic fiber were calculated. The results are shown in FIG. 2.
Discharge capacity retention rate after 30 cycles (%)=(discharge capacity after 30 cycles/discharge capacity at 1st cycle)×100

FIG. 2 is a graph showing the initial resistivity and the resistance increase rate after 30 cycles for 0 volume % of ceramic fiber added.
As shown in FIG. 2, the initial resistivities of Examples 1 to 4 and Comparative Example 2 are higher than those of Comparative Example 1, in which no ceramic fibers were added, but the resistance increase rate after 30 cycles of Examples 1 to 4 is reduced by the addition of ceramic fibers.
As shown in Table 1, the discharge capacity retention rate is improved by adding ceramic fibers in Examples 1 to 4, compared to Comparative Example 1, in which no ceramic fibers were added. From this, it is considered that the difference in the discharge capacity retention rate becomes more noticeable when the number of cycles is further increased.
Therefore, it was demonstrated that the use of a negative electrode layer containing 1.7 to 4.1 volume % of ceramic fibers in a solid-state battery can improve the charge-discharge cycle characteristics of the solid-state battery.

11: negative electrode active material 12: binder 13: solid electrolyte 14: carbon fiber 15: ceramic fiber 100: negative electrode layer

Claims (1)

1. A negative electrode layer for a solid-state battery, comprising:
The negative electrode layer includes a negative electrode active material, a binder, carbon fibers, and ceramic fibers,
The ceramic fibers are contained in the negative electrode layer at 3 to 4.1% by volume ,
The negative electrode layer , wherein the ceramic fibers are contained in the negative electrode layer in a volume ratio to the carbon fibers of 0.42 to 0.59 .

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