Raw material for negative electrode active material of all-solid lithium ion secondary battery

Abstract

PROBLEM TO BE SOLVED To provide a raw material for a negative electrode active material composed of silicon particles capable of improving initial current density in a negative electrode material having improved cycle characteristics by controlling the particle diameter of the silicon. SOLUTION The silicon used as a raw material for a negative electrode active material of an all-solid-state lithium ion secondary battery has an oxygen concentration of 15-30 mass % and a carbon concentration of 3-15 mass % measured at a depth of 5 nm from the surface, and a weight ratio (C-O / Si-Si) of a ratio of a C-O bond to a ratio of an Si-Si bond exceeds 0.01.

Description

Raw materials for negative electrode active materials in all-solid-state lithium-ion secondary batteries

[0001] The present invention relates to a novel raw material for a negative electrode active material of an all-solid-state lithium ion secondary battery.

[0002] Among various secondary batteries being developed, lithium-ion secondary batteries (LiBs) are seen as promising because they can easily achieve high energy density. Meanwhile, as the applications of LiBs expand, attention is being paid to large-capacity batteries, such as automotive batteries and stationary batteries. Ensuring safety is even more important for large-capacity batteries than for small batteries. Therefore, all-solid-state lithium-ion secondary batteries (all-solid-state LiBs) that use inorganic solid electrolytes are expected to be safer and more effective in utilizing the capacity of the active material, even when enlarged, compared to LiBs that use liquid electrolytes.

[0003] To increase the capacity of all-solid-state LiBs, silicon has been considered as a negative electrode active material to increase the capacity of the negative electrode as one means of increasing the energy density. Silicon has a theoretical capacity density of 4200 mAh / g (volumetric capacity density of 2370 mAh / cm) as a negative electrode active material. 3 ), which has a capacity / weight ratio of about 11 times and a capacity / volume ratio of about 3 times that of carbon, and also has excellent properties such as a low Li-based voltage of 0.2 V (see Patent Document 1). It has also been reported that by controlling the particle size of silicon as the anode material, the cycle characteristics can be improved when this is used as the anode material (see Patent Document 2).

[0004] However, when an anode material using silicon alone is used, it is not possible to sufficiently increase the initial current density (at the start of use) of an all-solid-state LiB when an attempt is made to thicken the anode layer in order to increase the capacity of the all-solid-state LiB, and there is room for improvement in its practical application.

[0005] On the other hand, when used as a silicon anode material, a method is generally known in which a conductive additive that imparts electronic conductivity is used in combination with silicon particles to increase the initial current density. Examples of the combined use of the conductive additive include mixing fine carbon particles such as acetylene black with silicon powder and coating fine carbon particles on silicon particles that make up the silicon powder. The combined use of such a conductive additive can increase the initial current density of the all-solid-state LiB.

[0006] However, the use of the conductive additive increases the amount of carbon present in the anode material, which reduces the relative volume of silicon and reduces the overall capacity of the LiB. Furthermore, the presence of free fine carbon particles in the amorphous silicon during charging and discharging makes it easier for silicon to become finer and more isolated as a result of repeated charging and discharging, which raises concerns about the deterioration of the cycle characteristics of the resulting all-solid-state LiB battery.

[0007] JP 2009-301879 A Japanese Patent No. 7327850

[0008] Therefore, the problem to be solved by the present invention is to provide a raw material for an anode active material that can improve the initial current density while preventing a decrease in the cycle characteristics of a battery when silicon is used as the anode material of an all-solid-state LiB battery and the capacity of the all-solid-state LiB is increased by thickening the anode layer made of the anode material.

[0009] The present inventors conducted extensive research to solve the above-mentioned problems, and found that by using, as a silicon powder used as a raw material for the negative electrode active material of an all-solid-state lithium-ion secondary battery, silicon powder composed of silicon particles having specific oxygen concentrations and carbon concentrations on their surfaces, and with a specific amount of the carbon bonded to the silicon surfaces as C—O bonds, it is possible to improve the initial current density, and furthermore, the carbon bonded to the silicon particle surfaces does not induce isolation of the silicon during subsequent charge and discharge, thereby effectively preventing a decrease in the cycle characteristics of the battery, thereby solving all of the above-mentioned problems, and have thus completed the present invention.

[0010] That is, the present invention provides a silicon powder raw material for a negative electrode active material of an all-solid-state lithium-ion secondary battery, characterized in that silicon particles constituting the silicon powder have an oxygen concentration of 15 to 30 mass %, a carbon concentration of 3 to 15 mass %, and a weight ratio of the proportion of C—O bonds to the proportion of Si—Si bonds (C—O / Si—Si) of more than 0.01, as measured at a depth of 5 nm from the surface by X-ray photoelectron spectroscopy (XPS).

[0011] In the raw material for a negative electrode active material of the present invention, the silicon particles preferably have a weight ratio of C—O bonds to Si—O bonds (C—O / Si—O) of 0.05 to 0.20, and a weight ratio of C—C bonds to C—O bonds (C—C / C—O) of 2.00 to 6.00, as measured at a depth of 5 nm from the surface by X-ray photoelectron spectroscopy (XPS).

[0012] In addition, in the raw material for a negative electrode active material of the present invention, it is preferable that the average particle diameter of the silicon powder is 0.3 to 10.0 μm, as this further improves cycle characteristics. Furthermore, the present invention provides a material for forming an anode of an all-solid-state lithium-ion secondary battery, in which a layer made of the raw material for a negative electrode active material of the all-solid-state lithium-ion secondary battery is formed on a current collector to a thickness of 10 μm or more. In the material for forming an anode of an all-solid-state lithium-ion secondary battery, the layer made of the raw material for a negative electrode active material of the all-solid-state lithium-ion secondary battery is preferably patterned on the current collector to form island-shaped convex portions connected at the valleys, as this improves cycle characteristics with good reproducibility. Furthermore, the present invention also provides an all-solid-state lithium-ion secondary battery having a positive electrode, a negative electrode, and a solid electrolyte layer, characterized in that the material for forming an anode of an all-solid-state lithium-ion secondary battery is disposed as the negative electrode.

[0013] The raw material for the negative electrode active material of the all-solid-state LiB battery of the present invention has the above-described configuration, and in an all-solid-state LiB battery using the raw material, even when the negative electrode layer is thickened to increase the capacity of the all-solid-state LiB, it is possible to increase the initial current density while suppressing a decrease in cycle characteristics.

[0014] Fig. 1 is a schematic cross-sectional view showing one embodiment of a material for forming an all-solid-state LiB negative electrode according to the present invention. Fig. 2 is a schematic plan view showing one embodiment of a material for forming an all-solid-state LiB negative electrode according to the present invention. Fig. 3 is a plan view showing another embodiment of a material for forming an all-solid-state LiB negative electrode according to the present invention. Fig. 4 is a plan view showing another embodiment of a material for forming an all-solid-state LiB negative electrode according to the present invention. Fig. 5 is a schematic cross-sectional view showing one embodiment of a state before charge and discharge of an all-solid-state LiB according to the present invention.

[0015] In this specification, an all-solid-state LiB refers to a lithium ion secondary battery that does not contain a liquid such as a non-aqueous electrolyte or an ionic liquid as an electrolyte.

[0016] (Raw material for negative electrode active material of all-solid-state LiB) The raw material for negative electrode active material of the all-solid-state LiB battery according to the present invention is silicon powder.

[0017] The crystallinity of the silicon particles constituting the silicon powder is not particularly limited, but silicon powder composed of silicon crystal particles is preferably used. When a battery is assembled using a negative electrode active material made of such silicon powder and charged and discharged, some or all of the silicon crystal particles become amorphous, and then fuse and densify. Here, "crystalline silicon" refers to both polycrystalline and single-crystalline silicon. Therefore, the negative electrode active material particles of the present invention may be polycrystalline silicon particles, single-crystalline silicon particles, or a mixture thereof. Silicon crystals exhibit clear peaks in X-ray diffraction.

[0018] The silicon powder, which is a raw material for a negative electrode active material of the present invention, is most characterized in that the silicon particles constituting the powder have an oxygen concentration of 15 to 30 mass %, a carbon concentration of 3 to 15 mass %, and a weight ratio of the proportion of C—O bonds to the proportion of Si—Si bonds (C—O / Si—Si) of more than 0.01, as measured at a depth of 5 nm from the surface by X-ray photoelectron spectroscopy (XPS).

[0019] Unlike conventional methods that solve the problem of initial charging in all-solid-state LiBs by blending free carbon powder with silicon powder, the raw material for negative electrode active material of the present invention solves the problem by having carbon elements bonded to the surface of silicon via oxygen.

[0020] The presence of the oxygen and carbon in the surface layer of the silicon particles was confirmed by SEM-EDS, and it is presumed that the oxygen forms an oxide layer on the surface of the silicon particles, and the carbon exists in the oxide layer through a C-O bond with the oxygen. Furthermore, within the measurement range, the remainder of the oxygen and carbon is essentially silicon.

[0021] If the oxygen concentration is less than 15 mass%, a sufficient oxide layer is not formed on the silicon surface, making it difficult to form C—O bonds on the surface, while if it exceeds 30 mass%, it causes an increase in the electrical resistance of the silicon surface. The lower limit of the oxygen concentration is preferably 20 mass%, and the upper limit is preferably 25 mass%. The thickness of the oxide layer formed on the silicon surface at these oxygen concentrations is about 1 nm.

[0022] If the carbon concentration is less than 3% by mass, the amount of carbon present on the silicon surface is insufficient, and when the negative electrode layer of the negative electrode material of the all-solid-state LiB battery obtained using this is thickened, the initial current density cannot be increased, and in some cases, there is a problem that charge and discharge does not start. On the other hand, if the carbon concentration exceeds 15% by mass, there is a problem that free carbon increases, and the cycle characteristics of the all-solid-state LiB battery obtained using this deteriorates. The lower limit of the carbon concentration is preferably 5% by mass, and the upper limit is preferably 10% by mass.

[0023] Furthermore, even when the oxygen concentration and carbon concentration satisfy the above ranges, if the weight ratio of the proportion of C-O bonds to Si-Si bonds (C-O / Si-Si) in the above measurement range is 0.01 or less, the initial current density cannot be increased when the anode layer of the anode material of the all-solid-state LiB battery obtained using this is increased in thickness. The higher the C-O / Si-Si weight ratio is within the above carbon concentration range, the better, and it is preferably 0.02 or more, and further preferably 0.03 or more.

[0024] In the raw material for a negative electrode active material of the present invention, the silicon particles preferably have a weight ratio (C-C / C-O) of C-C bonds to C-O bonds, as measured by X-ray photoelectron spectroscopy (XPS) at a depth of 5 nm from the surface, of 2.00 to 6.00, particularly 2.50 to 5.50.

[0025] The CC / CO weight ratio can be taken as an indication of the degree of length of one carbon-containing group.

[0026] In the raw material for a negative electrode active material of the present invention, the silicon particles preferably have a weight ratio (C—O / Si—O) of the proportion of C—O bonds to the proportion of Si—O bonds, measured at a depth of 5 nm from the surface by X-ray photoelectron spectroscopy (XPS), of 0.05 to 0.20, particularly 0.10 to 0.15.

[0027] The CO / Si-O weight ratio can be understood as indicating the degree of the abundance of carbon-containing groups in the oxide layer on the silicon surface.

[0028] A representative example of a method for producing the silicon powder having a specific amount of C—O bonds on the surface of the silicon particles is a method in which coarse silicon powder is pulverized in the presence of an organic solvent, which accompanies a mechanochemical reaction.

[0029] The coarse silicon powder may be obtained by crushing and classifying polycrystalline silicon rods obtained by the Siemens process or single-crystal silicon obtained by the Czochralski process, or by crushing and classifying metallic silicon obtained by a silica reduction method.

[0030] The method for pulverizing the silicon coarse powder can be carried out using a pulverizer that applies a shear force sufficient to cause a mechanochemical reaction on the silicon particle surfaces with the organic solvent present during pulverization. Specific examples include pulverization methods using pulverizers such as jet mills, planetary ball mills, vibration mills, hammer mills, and stamp mills. The amount of the organic solvent present is 0.1 to 10 times, preferably 0.2 to 5.0 times, by mass relative to the silicon powder. The pulverization temperature is preferably in the range of 10 to 70°C, preferably 20 to 50°C. The pulverization time is preferably 5 to 120 minutes, preferably 10 to 60 minutes.

[0031] The organic solvent used is appropriately selected from alcohols, aldehydes, ketones, ethers, esters, amides, imides, aliphatic hydrocarbons, alicyclic hydrocarbons, aromatic hydrocarbons, heterocycles, and the like, and examples thereof include methanol, ethanol, normal propyl alcohol, isopropyl alcohol, normal butyl alcohol, isobutyl alcohol, pentyl alcohol, hexyl alcohol, heptyl alcohol, octyl alcohol, 2-ethylhexyl alcohol, benzyl alcohol, ethylene glycol monomethyl ether, diethylene glycol monomethyl ether, triethylene glycol monomethyl ether, ethylene glycol monoisopropyl ether, diethylene glycol monoisopropyl ether, triethylene glycol monoisopropyl ether, ethylene glycol monobutyl ether, diethylene glycol butyl ether, triethylene glycol monobutyl ether, propylene glycol monomethyl ether, dipropylene glycol monomethyl ether, ethyl acetate, normal propyl acetate, isopropyl acetate, normal butyl acetate, acetate, isobutyl acetate, hexyl acetate, ethylene glycol monomethyl ether acetate, diethylene glycol monomethyl ether acetate, triethylene glycol monomethyl ether acetate, ethylene glycol monoisopropyl ether acetate, diethylene glycol monoisopropyl ether acetate, triethylene glycol monoisopropyl ether acetate, ethylene glycol monobutyl ether acetate, diethylene glycol monobutyl ether acetate, triethylene glycol monobutyl ether acetate, propylene glycol monomethyl ether acetate, dipropylene glycol monomethyl ether acetate, hexane, nonane, decane, isodecane, dodecane, isododecane, terpene oil solvents, naphthenic solvents, isoparaffin solvents, cyclohexane, methylcyclohexane, dimethylcyclohexane, ethylcyclohexane, alkylcyclohexane, toluene, xylene, aromatic high-boiling solvents, acetone, methyl ethyl ketone, methyl pentyl ketone, methyl isobutyl ketone, cyclohexanone, terpineol,Examples of suitable organic solvents include dihydroterpineol, dihydroterpineol acetate, NMP (N-methylpyrrolidone), methoxybenzene, diethyl ether, dipropyl ether, and dibutyl ether. These organic solvents may be selected depending on the type of carbon-containing group to be present on the silicon surface. These organic solvents may be used alone or in combination. After the treatment, the free organic solvent may be removed by drying, if necessary.

[0032] The average particle diameter of the silicon powder is preferably 0.3 to 10.0 μm, more preferably 0.5 to 4.5 μm, more preferably 0.5 to 2.0 μm, and particularly preferably 1.0 to 1.5 μm, from the viewpoint of improving the cycle characteristics of the negative electrode active material layer after energization when used in an all-solid-state LiB battery. The average particle diameter refers to the 50% cumulative diameter (D50) measured by a laser scattering method for particle size distribution. (Composition for Forming a Negative Electrode of an All-Solid-State Lithium-Ion Secondary Battery) The raw material for the negative electrode active material is mixed with components constituting a negative electrode to form a composition for forming a negative electrode, and a layer of the negative electrode active material is formed on a negative electrode current collector, thereby obtaining a negative electrode.

[0033] Furthermore, when a battery is assembled using the above-mentioned negative electrode active material and charged and discharged, as described above, some or all of the silicon crystal particles become amorphous, fuse into blocks, and densify in each block. The blocked negative electrode active material layer is dense and adheres closely to the negative electrode current collector, exhibiting excellent electronic conductivity. It has been confirmed that the carbon-containing groups present on the surfaces of the silicon particles constituting the silicon powder remain in a state that does not affect the densification. Therefore, charging at a high current density is possible even after the second charge / discharge.

[0034] The composition for forming an all-solid-state LiB negative electrode contains the above-mentioned negative electrode active material, and the content of the electronic conductivity imparting agent is 2 parts by mass or less per 100 parts by mass of the negative electrode active material particles. In the most preferred embodiment, the composition is substantially free of the electronic conductivity imparting agent. According to the present invention, the proportion of the electronic conductivity imparting agent can be reduced, thereby increasing the relative amount of the active material particles and contributing to an improvement in battery capacity.

[0035] The composition for forming an all-solid-state LiB negative electrode can contain an ion-conductivity imparting agent to improve ion conductivity. The solid electrolyte described below can be used as such an ion-conductivity imparting agent. If the blending amount of the ion-conductivity imparting agent is too large, the relative amount of active material particles decreases, and the cycle characteristics also decrease. Therefore, the content of the ion-conductivity imparting agent in the composition for forming an all-solid-state LiB negative electrode is preferably 30% by mass or less, more preferably 20% by mass or less, and more preferably substantially free of the ion-conductivity imparting agent.

[0036] The composition for forming an all-solid-state LiB negative electrode can contain a binder, a plasticizer, etc., to form the raw material for the negative electrode active material into a layer. These components are preferably used in as small an amount as possible within the range that allows the above-mentioned forming, and the total amount is preferably 20 parts by mass or less, more preferably 15 parts by mass or less, per 100 parts by mass of the negative electrode active material particles. If the amount of binder or plasticizer is too large, the amount of active material in the negative electrode active material layer will relatively decrease, which is undesirable in terms of increasing battery capacity.

[0037] Examples of binders include thermosetting resins such as thermosetting polyimide, phenolic resin, epoxy resin, urea resin, melamine resin, unsaturated polyester resin, and polyurethane; water-soluble polymers such as cellulose derivatives such as carboxymethyl cellulose, hydroxypropyl methyl cellulose, hydroxypropyl cellulose, ethyl cellulose, hydroxyethyl cellulose, and hydroxymethyl cellulose, and polyvinyl alcohol; polycarbonate resins such as polypropylene carbonate; polyvinylidene fluoride; styrene-butadiene copolymers (so-called SBR rubber-based), styrene-propylene copolymers, and styrene-ethylene-propylene copolymers (so-called SES-based and SEPS-based).

[0038] The all-solid-state LiB negative electrode composition may contain a dispersion medium for forming island-shaped protrusions made of the all-solid-state LiB negative electrode composition described later on the negative electrode current collector. The dispersion medium may be appropriately selected from alcohols, aldehydes, ketones, ethers, esters, amides, imides, aliphatic hydrocarbons, alicyclic hydrocarbons, aromatic hydrocarbons, heterocycles, and the like. Examples of the dispersion medium include methanol, ethanol, normal propyl alcohol, isopropyl alcohol, normal butyl alcohol, isobutyl alcohol, pentyl alcohol, hexyl alcohol, heptyl alcohol, octyl alcohol, 2-ethylhexyl alcohol, benzyl alcohol, ethylene glycol monomethyl ether, diethylene glycol monomethyl ether, triethylene glycol monomethyl ether, ethylene glycol monoisopropyl ether, diethylene glycol monoisopropyl ether, triethylene glycol monoisopropyl ether, ethylene glycol monobutyl ether, diethylene glycol butyl ether, triethylene glycol monobutyl ether, propylene glycol monomethyl ether, dipropylene glycol monomethyl ether, and ethyl acetate. , normal propyl acetate, isopropyl acetate, normal butyl acetate, isobutyl acetate, hexyl acetate, ethylene glycol monomethyl ether acetate, diethylene glycol monomethyl ether acetate, triethylene glycol monomethyl ether acetate, ethylene glycol monoisopropyl ether acetate, diethylene glycol monoisopropyl ether acetate, triethylene glycol monoisopropyl ether acetate, ethylene glycol monobutyl ether acetate, diethylene glycol monobutyl ether acetate, triethylene glycol monobutyl ether acetate, propylene glycol monomethyl ether acetate, dipropylene glycol monomethyl ether acetate, hexane, nonane, decane, isodecane, dodecane, isododecane, terpene oil solvents, naphthenic solvents, isoparaffin solvents, cyclohexane, methylcyclohexane, dimethylcyclohexane, ethylcyclohexane, alkylcyclohexane,Examples of the dispersion medium include toluene, xylene, aromatic high-boiling solvents, acetone, methyl ethyl ketone, methyl pentyl ketone, methyl isobutyl ketone, cyclohexanone, terpineol, dihydroterpineol, dihydroterpineol acetate, NMP (N-methylpyrrolidone), methoxybenzene, diethyl ether, dipropyl ether, and dibutyl ether. These may be used alone or in combination. The amount of the dispersion medium used may be determined appropriately so that the viscosity of the paste containing the all-solid-state LiB negative electrode composition is optimal for the method of forming the negative electrode active material layer. The dispersion medium is removed by drying after coating the paste containing the all-solid-state LiB negative electrode composition.

[0039] (All-solid-state LiB anode-forming material) An anode active material layer made of the all-solid-state LiB anode-forming composition is formed on an anode current collector to obtain an all-solid-state LiB anode-forming material. The anode-forming material is one of the components constituting a battery and functions as an anode itself, but when an LiB is assembled and charged / discharged, the silicon crystals become amorphous, resulting in a crystal structure different from that of the anode in the battery after charging / discharging.

[0040] The packing rate of silicon, the active material in the negative electrode active material layer, is preferably in the range of 35 to 55% by volume, more preferably 40 to 50% by volume. Because the packing rate of the active material is low and voids exist in the negative electrode active material layer, lithium ions are absorbed into the silicon active material during charge and discharge, making it possible to mitigate volume expansion during amorphization (lithium-silicon alloying). The packing rate of the active material can be adjusted by the particle size or particle size distribution of the silicon particles, the amount of binder added, and the like. The packing rate of the active material in the negative electrode active material layer refers to the ratio of the volume of silicon to the volume of the negative electrode.

[0041] When using the anode active material raw material made of the specific silicon powder of the present invention, even if the thickness of the anode active material layer is increased to increase the battery capacity, it is possible to set a high current density at the initial stage of charging. Therefore, in the present invention, it is possible to form the anode active material layer with a thickness of 10 μm or more, particularly 15 μm or more, and even 20 μm or more. The upper limit of the thickness is not particularly limited, but is generally about 200 μm, particularly about 100 μm.

[0042] In an embodiment in which island-shaped convex portions are formed in the negative electrode active material layer described below, the above thickness refers to the thickness of the connection portion in the groove portion when the area ratio of the groove portion is 10% or more of the total area of ​​the negative electrode active material layer, and refers to the thickness of the convex portion when the area ratio of the groove portion is less than 10%.

[0043] Copper foil, nickel foil, or SUS foil is generally used as the negative electrode current collector, but other conductive metal foils may also be used. The negative electrode current collector may be electrolytic copper with a rust-proofing treatment applied to the surface. The thickness of the negative electrode current collector is not particularly limited, but from the viewpoint of battery miniaturization and handling, a thickness of 3 μm to 100 μm is usually used, and when a roll-to-roll manufacturing method is used, a thickness of 5 μm to 50 μm is preferably used. The shape of the negative electrode current collector may be a non-perforated sheet, or a perforated sheet such as a two-dimensional mesh, a three-dimensional net, or a punched metal. The surface of the negative electrode current collector may be subjected to a known surface treatment, such as mechanical surface processing, etching, chemical conversion treatment, anodizing, wash primer, corona discharge, or glow discharge.

[0044] When a negative electrode active material layer containing silicon crystals undergoes charging and discharging, the silicon crystal particles, which are the negative electrode active material, become amorphous and fuse into blocks, densifying them. However, because the blocking of the negative electrode active material layer occurs spontaneously during charging and discharging, the size, shape, and gaps between the blocks tend to become nonuniform. When the size, shape, and gaps between the blocks are nonuniform, adjacent blocks may come into contact with each other due to expansion and contraction during charging and discharging, generating compressive forces that can cause the blocks to crack. This can result in the negative electrode active material becoming more granular and isolated, potentially leading to a decrease in capacity. Furthermore, many of the grooves formed between the blocks reach the negative electrode current collector. The penetration of solid electrolyte into the interface between the blocks and the negative electrode current collector creates gaps, promoting block delamination and silicon isolation.

[0045] In order to solve the above problem, we have thoroughly investigated means for controlling the size, shape, and spacing of the spontaneously generated blocks. As a result, we have found that it is preferable to form the negative electrode active material layer of the all-solid-state LiB negative electrode forming material before charging and discharging into a pattern of island-shaped convex portions with spacing between them, and to form a connection layer that covers the bottom of the grooves formed between the island-shaped convex portions.

[0046] That is, as shown in a cross-sectional view in FIG. 1 and a plan view in FIG. 2 , an all-solid-state LiB anode-forming material 10 of this embodiment has an anode active material layer 2 made of an all-solid-state LiB anode-forming composition on an anode current collector 1, and the anode active material layer 2 has island-shaped protrusions 11 formed in a pattern at intervals, and grooves 12 formed between the island-shaped protrusions have connection layers 13 made of the all-solid-state lithium-ion secondary battery anode-forming composition formed on the bottom surfaces of the island-shaped protrusions so as to be continuous with the island-shaped protrusions.

[0047] The shape of the island-shaped protrusions 11 is not particularly limited. While Figures 1 and 2 show the case where the island-shaped protrusions 11 are truncated quadrangular pyramids, they may be cylindrical (Figure 3), elliptical cylindrical, or polygonal prisms such as triangular, quadrangular, pentagonal, or hexagonal prisms. They may also be truncated cones or elliptical cones. They may also be truncated polygonal pyramids such as triangular, quadrangular, pentagonal, or hexagonal pyramids (Figure 4). They may also be cones, elliptical cones, or polygonal pyramids, or combinations thereof. Furthermore, when an all-solid-state LiB is constructed, the island-shaped protrusions 11 may be covered with the solid electrolyte, and the solid electrolyte may also penetrate into the grooves 12.

[0048] Referring to FIG. 1 , the dimensions of each portion of the negative electrode active material layer 2 formed on the negative electrode current collector are described. The width (W) of the island-shaped protrusions 11 is preferably in the range of 10 μm to 100 μm, particularly 15 to 50 μm, and even more preferably 15 to 30 μm. By setting the width of the island-shaped protrusions 11 within this range, the blocks can be formed in a state in which the outer shape of the island-shaped protrusions is substantially maintained even during charge and discharge of the all-solid-state LiB after formation, thereby enabling stable performance. Furthermore, if the width of the island-shaped protrusions 11 is formed within the above range and is larger than the average size of blocks naturally formed during charge and discharge of the all-solid-state LiB, specifically, a width of approximately 15 to 25 μm, cracks may occur within the blocks originating from the island-shaped protrusions. However, these cracks do not reach the negative electrode current collector, and the effect of forming the blocks more uniformly remains unchanged compared to when the negative electrode active material layer is formed over the entire surface of the negative electrode current collector.

[0049] The size of the island-shaped convex portions is 100 to 10,000 μm. 2 , especially 225 to 2500 μm 2 , and further 225 to 900 μm 2 It is preferable that the range is set as follows: Specifying the size of the island-shaped convex portion by area is particularly effective when the shape to be patterned is complex.

[0050] The height (H) of the island-shaped protrusions 11 is preferably set to a height corresponding to the thickness of the layer made of the raw material for the negative electrode active material. The height (H) of the island-shaped protrusions 11 refers to the distance from the surface of the current collector to the highest point of the island-shaped protrusions 11. Furthermore, the spacing (P) between the island-shaped protrusions 11 is preferably in the range of 10 to 50%, particularly 15 to 30%, of the width (W) of the island-shaped protrusions 11, in order to suppress the effects of expansion and contraction between the island-shaped protrusions during charge and discharge after the all-solid-state LiB is formed. Furthermore, the thickness (t) of the connection layer 13 only needs to be smaller than the height of the island-shaped protrusions, and is preferably in the range of 1 to 50%, particularly 10 to 30%, of the height of the island-shaped protrusions, and is patterned so as not to exceed 15 μm. This is preferable because blocking in such portions is unlikely to occur during charge and discharge after the all-solid-state LiB is formed, and exposure of the negative electrode current collector to the bottom of the grooves 12 is reliably prevented.

[0051] The width (W) of an island-shaped protrusion refers to the length measured from the rising portion of the island-shaped protrusion as shown in Figure 1. Furthermore, the above length refers to the distance between opposing sides in the case of a rectangle having opposing sides, the diameter in the case of a circle, and the equivalent diameter in the case of a polygon having an odd number of sides. The spacing (P) between the island-shaped protrusions 11 refers to the distance between the rising portions of opposing island-shaped protrusions. The thickness (t) of the connection layer 13 refers to the average thickness of the negative electrode active material layer present between the rising portions.

[0052] In addition, when the side-to-side length and diameter vary depending on the measurement location, it is preferable that each measured value falls within the above-mentioned range. When the island-shaped protrusion and the connection layer are connected by a gentle curve, the width of the island-shaped protrusion 11 can be measured by taking the part of the rising portion that exceeds 50% of the height of the island-shaped protrusion as the rising portion. An angle of about 60 to 90 degrees is preferable, as this allows a large amount of silicon to be secured per island-shaped protrusion.

[0053] To form a negative electrode active material layer made of an all-solid-state LiB negative electrode composition on a negative electrode current collector, a method is recommended in which the all-solid-state LiB negative electrode composition is made into a paste with a solvent, a pattern is formed on the negative electrode current collector, and then the solvent is dried. The application method is not particularly limited as long as it can form a pattern of island-shaped convex portions described below. Examples include screen printing and 3D printing. Drying can be performed at a temperature sufficient to volatilize the solvent used.

[0054] Another example is a method in which the composition for forming an all-solid-state LiB negative electrode is applied to the entire surface of the negative electrode current collector, and then a mold such as a mesh is pressed against the negative electrode active material layer in a dry or semi-dry state, thereby forming island-shaped protrusions 11 and grooves 12 in a mesh pattern as shown in Fig. 5. In this case, the width of the island-shaped protrusions can be adjusted by the mesh opening spacing, the spacing between the island-shaped protrusions can be adjusted by the mesh thickness, and the depth of the pressing can be adjusted to the depth of the island-shaped protrusions and the thickness of the connection layer.

[0055] As described above, when an LiB is assembled using an all-solid-state LiB negative electrode forming material having island-shaped protrusions 11 formed in a predetermined pattern on an anode current collector and then charged and discharged, the shape of the island-shaped protrusions 11 is generally maintained, although complete control is difficult. Therefore, the size and shape of each block derived from the island-shaped protrusions 11 and the grooves between the blocks are uniform, suppressing contact between adjacent blocks even with repeated expansion and contraction due to charge and discharge. As a result, the anode active material is less likely to become granular or isolated, preventing capacity loss. Furthermore, the presence of the connection layer 13 prevents the anode current collector 1 from being exposed at the bottom of the grooves 12, even after charge and discharge. When the negative electrode current collector 1 is exposed at the bottom of the groove 12, the solid electrolyte penetrates into the interface between the negative electrode current collector 1 and the block through the exposed portion, creating a gap and isolating the negative electrode active material, but such isolation can be effectively prevented by providing the connection 13. Furthermore, by providing the connection 13, the thickness of the layer made of the negative electrode active material can be partially reduced, further promoting the effect of increasing the initial current density obtained by using the negative electrode active material of the present invention.

[0056] The all-solid-state LiB anode forming material has the above-mentioned anode active material layer 2 on the anode current collector 1, and a solid electrolyte layer may be further formed on the anode active material layer. The solid electrolyte is not particularly limited, but examples include commonly used sulfide-based solid electrolytes and oxide-based solid electrolytes. Sulfide-based solid electrolytes are advantageous due to their high lithium ion conductivity. Oxide-based solid electrolytes are relatively chemically stable and advantageous in terms of high voltage resistance. When an oxide-based solid electrolyte is used for the solid electrolyte layer, a commonly used ion conductor may be used in combination as needed to improve lithium ion conductivity. Sulfide-based solid electrolytes contain, for example, lithium, phosphorus, and sulfur, and may further contain elements such as O, Al, B, Si, Ge, and I. Specifically, amorphous Li3PS4, amorphous 40LiI.60Li3PS4 (mol %), β-Li3PS4, α-Li3PS4, and Li7P3S 11 Crystals, etc., are used. Argyrodite-based solid electrolytes may also be used.

[0057] Such sulfide-based solid electrolytes can be obtained by known methods, such as preparing lithium sulfide (LiS) and diphosphorus pentasulfide (P2S5) as starting materials, mixing LiS and P2S5 in a molar ratio of about 50:50 to 80:20, melting the mixture, and rapidly cooling it, or mechanically milling it, or by known wet methods such as the suspension method, solution method, and sol-gel method.

[0058] The sulfide-based solid electrolyte obtained by the above method is amorphous. It can be used in this amorphous state, but it can also be heat-treated to become a crystalline sulfide-based solid electrolyte. Crystallization is expected to improve lithium ion conductivity.

[0059] The oxide-based solid electrolyte is, for example, Li 5+X La3(Zr X , A 2-X ) O 12 (wherein A is one or more elements selected from the group consisting of Sc, Ti, C, Y, Nb, Hf, Ta, Al, Si, Ga, Ge, and Sn, and X is 1.4≦X≦2), Li 1+X Al XTi 2-X (PO4)3 (where 0≦X≦1), Li 3X La 2 / 3-X TiO3 (where x is 0≦x≦2 / 3), etc. These have high ionic conductivity at room temperature and high electrochemical stability.

[0060] From the viewpoint of electrochemical stability, the oxide-based solid electrolyte may contain insulating particles such as silica (SiO2) particles, γ-alumina (Al2O3) particles, ceria (CeO2) particles, zirconia (ZrO2) particles, etc. Other known metal oxide particles may also be used.

[0061] The solid electrolyte preferably has a Young's modulus (25°C) of 10 to 70 GPa, more preferably 15 to 30 GPa, because when the negative electrode active material is densified into blocks, the solid electrolyte can easily fill the gaps that form between the blocks, thereby maintaining high ionic conductivity. Examples of solid electrolytes having a Young's modulus (25°C) of 10 to 70 GPa include amorphous Li3PS4, LiX-Li3PS4 (X = I, Br, Cl)-based glass, β-Li3PS4, α-Li3PS4, and Li7PS 11 Crystal, Li 10 GeP2S 12 and argyrodite-based crystals such as Li6PS5X (X=I, Br, Cl).

[0062] The thickness of the solid electrolyte layer is preferably 500 nm to 1000 μm, and more preferably 1 μm to 500 μm. If the thickness is 500 nm or more, a solid battery with stable performance can be produced without chipping or cracking. If the thickness is 1000 μm or less, a solid battery with sufficiently low resistance can be produced.

[0063] When a battery is assembled using the above-mentioned material for forming an all-solid-state LiB negative electrode and is charged and discharged, as described above, some or all of the silicon crystal particles, which are the negative electrode active material, become amorphous and densify into blocks while substantially maintaining the pattern of the island-shaped protrusions, thereby exhibiting the unique effects of the present invention described above.

[0064] Generally, when silicon is used as the negative electrode active material, the theoretical maximum charge capacity of an all-solid-state LiB is approximately 3600 mAhg -1 However, the practical range of charge and discharge is about 1000 to 3000mAhg. -1 That is, it is presumed that the reason why the all-solid-state LiB exhibits battery performance with good cycle characteristics stably for a long period of time is that the blocks generated by charge and discharge are uniformly controlled by the island-shaped protrusions 11, and that grooves reaching the negative electrode current collector 1 are unlikely to be formed due to the interaction with the connection layer 13, and further that the volume change of the generated blocks is small.

[0065] (All-Solid-State LiB) As shown in FIG. 6, the all-solid-state LiB of the present invention has a positive electrode current collector 5, a positive electrode active material layer 4, a negative electrode active material layer 2, a negative electrode current collector 1, and a solid electrolyte layer 3, and the negative electrode is formed using an all-solid-state LiB negative electrode-forming material. When an LiB is assembled using the all-solid-state LiB negative electrode-forming material and charged / discharged, some or all of the silicon crystals contained in the all-solid-state LiB negative electrode-forming material become amorphous. Furthermore, as a result of the charge / discharge, some or all of the silicon forms an alloy with lithium. That is, after the LiB is assembled and charged / discharged, the crystal structure of the negative electrode active material layer differs from the silicon crystals before assembly and is partially or entirely composed of amorphous silicon. In this specification, the negative electrode active material after charge / discharge may be simply referred to as "amorphous silicon," but the amorphous silicon may contain silicon crystals or may be alloyed with lithium.

[0066] When the battery is charged, the silicon particles in the anode layer expand in volume by absorbing lithium, causing the silicon particles to fuse together and become amorphous. The tiny voids between the silicon particles are pushed out, forming dense blocks of island-like protrusions isolated by grooves (depending on the size of the island-like protrusions, grooves may form within the block that do not reach the anode current collector). This limits the increase in thickness of the anode layer itself to about 1.5 times, even if the silicon particles themselves expand by about 300% during charging. Furthermore, during discharge, the silicon attempts to return to its original volume due to the release of lithium, but the solid electrolyte, which is relatively softer than the silicon blocks, is drawn into the voids around the blocks, resulting in almost no reduction in the thickness of the anode layer itself.

[0067] The solid electrolyte layer 3 is made of the solid electrolyte.

[0068] In the all-solid-state LiB of the present invention, the configuration other than the negative electrode can be the same as that of a known all-solid-state LiB and is not particularly limited. The positive electrode is composed of a positive electrode active material layer 4 and a positive electrode current collector 5, and known positive electrode active materials and current collectors may be used.

[0069] The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples.

[0070] In the examples and comparative examples, the average particle size of the material powder, surface element analysis, and evaluation of the obtained battery (half cell) were carried out by the following methods.

[0071] (1) Average Particle Diameter of Material Powder The average particle diameter of the material powder was determined from the laser diffraction / scattered light intensity using LA-950S2 (manufactured by Horiba, Ltd.).

[0072] (2) Surface Analysis of Raw Negative Electrode Active Material The surface element bonds of the material powder were quantitatively determined from the X-ray intensity measured using an X-ray electron spectrometer VersaProbe III (manufactured by ULVAC-PHI).

[0073] The measurement was performed using Al-Kα as the light source, setting the photoelectron take-off angle to 45 deg, and setting the measurement range to a depth of 5 nm from the surface.

[0074] (3) Battery Materials (Counter Electrode) Lithium (Li) foil: 0.1 mm thick (manufactured by Honjo Metals Co., Ltd.) Indium (In) foil: 0.127 mm thick (manufactured by Aldrich Chemicals) (Negative Electrode) Negative Electrode Current Collector: CF-T7F-35 (manufactured by Fukuda Metal Foil & Powder Co., Ltd.) Negative Electrode Active Material Layer: A mixture of 90 parts by mass of a raw material for a negative electrode active material and 10 parts by mass of a thermosetting polyimide resin (DreamBond (trade name) manufactured by IST Co., Ltd.) (Solid Electrolyte) a-40LiI.60Li3PS3 (prepared by mechanical milling) (All-Solid-State LiB Negative Electrode Forming Material) 360 mg of silicon powder and a polyimide solution (27.2 wt % NMP solution) in an amount to give a solid content of 40 mg were mixed, and NMP (N-methylpyrrolidone) was further added to obtain a composition for forming an all-solid-state LiB negative electrode. The composition was stirred for 2 hours (rotation 1056 rpm, revolution 1600 rpm) and degassed for 6 minutes (rotation 290 rpm, revolution 1360 rpm) to obtain a coating liquid.

[0075] The resulting coating solution was applied to a negative electrode current collector using a doctor blade (feed rate: 1.0 mm / sec). After drying at room temperature for more than half a day, the polyimide was cured by heating under vacuum with a heater (250°C, 30 minutes), yielding a negative electrode current collector having a negative electrode active material layer composed of the negative electrode-forming material. The thickness of the negative electrode active material layer after drying was adjusted to the thickness shown in Table 1 by adjusting the blade gap. (Production of Half Cell) A negative electrode current collector having a negative electrode active material layer was punched out to 9 mm diameter and placed in an insulating die with the negative electrode active material layer facing up. 65 mg of solid electrolyte powder was uniformly loaded onto the negative electrode active material layer, and uniaxial pressing was performed at a molding pressure of 560 MPa. After the upper punch was temporarily removed, a counter electrode consisting of In and Li metal foils punched out to 6 mm diameter and stacked in the order In / Li / In was placed on top of the solid electrolyte layer, and uniaxial pressing was performed again at a pressure of approximately 50 MPa to produce an all-solid-state half cell. The assembly of the half-cell was carried out in an argon atmosphere in a glove box that was sealed off from the outside air in order to eliminate the influence of oxygen, nitrogen, moisture, and the like.

[0076] (4) Initial charging characteristics The charge-discharge test of the battery using the half cell prepared in (3) was carried out using HJ1001SD8 (manufactured by Meiden Hokuto Co., Ltd.) at a measurement temperature of 25°C and a current density of 0.2 mA cm -2 A constant current density test was conducted at a current density of 0.2 mA cm. In the first charge, the cutoff voltage relative to the counter electrode was set to −0.62 V as the termination condition. Evaluation was based on the following criteria: A: Current density 0.2 mA cm -2 F: Current density 0.2 mA cm -2 In the above charging, the cut-off voltage was set to 0.62 V, and the voltage fell below this value.

[0077] (5) Cycle Characteristics For the batteries that were initially chargeable in (4), charge and discharge were further repeated, and the change in charge and discharge capacity was monitored for each cycle. The cycle performance was evaluated based on the number of cycles at which the charge and discharge capacity retention rate became less than 90% according to the following criteria: A: Current density 0.2 mA cm -2 F: Current density 0.2 mA cm -2 The above did not achieve 100 cycles.

[0078] Examples 1 to 4: 200 parts by mass of a 1:1 mixed solution of ethanol and isopropyl alcohol as an organic solvent was added to 100 parts by mass of coarse silicon powder made of polycrystalline silicon and having an average particle size of 26 μm, and the mixture was then pulverized in a jet mill to obtain silicon powders having the average particle sizes (D50) shown in Table 1. Table 1 also shows the oxygen concentration, carbon concentration, and ratio of various element bonds of the obtained silicon powders measured at a depth of 5 nm from the surface by X-ray photoelectron spectroscopy (XPS).

[0079] The obtained silicon powder was used as a raw material for the negative electrode active material of an all-solid-state LiB to prepare a material for forming an all-solid-state LiB negative electrode, and further, an all-solid-state LiB half cell incorporating this was prepared, and the initial charge characteristics and cycle characteristics were evaluated. The results are also shown in Table 1.

[0080] Comparative Example 1 Silicon powder was obtained in the same manner as in Example 2, except that no organic solvent was used when pulverizing the silicon coarse powder.

[0081] The obtained silicon powder was used as a raw material for the negative electrode active material of an all-solid-state LiB, and a half cell of the all-solid-state LiB was produced in the same manner as in Example 1, and the initial charge characteristics and cycle characteristics were evaluated. The results are also shown in Table 1.

[0082] Comparative Example 2 Silicon powder was obtained in the same manner as in Example 2, except that pure water was used instead of the organic solvent when crushing the coarse silicon powder.

[0083] The obtained silicon powder was used as a raw material for the negative electrode active material of an all-solid-state LiB, and a half cell of the all-solid-state LiB was produced in the same manner as in Example 1, and the initial charge characteristics and cycle characteristics were evaluated. The results are also shown in Table 1.

[0084]

[0085] Example 5: Using the all-solid-state LiB anode active material obtained in Example 2, the anode active material layer was dried at room temperature, heated to 80°C, and pressed against a Ni mesh with a square pattern for 30 minutes to obtain a cathode active material having prismatic island-shaped protrusions with a thickness of 50 μm. The mesh pattern was a square with a side length of 30 μm. A pattern of grooves (groove ratio: 64%) with a width of approximately 20 μm and a depth of approximately 35 μm (active material layer thickness: 15 μm) was transferred to the active material layer.

[0086] A half-cell for evaluation was prepared using the above-mentioned negative electrode material. The behavior of the resulting battery during initial charging and cycle performance were evaluated.

[0087] As a result, the current density was 0.2 mA cm 2 The battery was capable of being charged and discharged at the initial charge of 100 cycles, and the cycle characteristics were 500 cycles or more (no deterioration in charge / discharge capacity after 500 cycles).

[0088] DESCRIPTION OF SYMBOLS 1... Negative electrode current collector 2... Negative electrode active material layer 3... Solid electrolyte layer 4... Positive electrode active material layer 5... Positive electrode current collector 10... All-solid-state lithium ion secondary battery negative electrode forming material 11... Negative electrode active material layer (island-shaped convex portion) 12... Groove portion 13... Connection layer 20... All-solid-state lithium ion secondary battery

Claims

1. A silicon powder, comprising silicon particles constituting the silicon powder, the silicon particles having an oxygen concentration of 15 to 30 mass % and a carbon concentration of 3 to 15 mass %, as measured at a depth of 5 nm from the surface by X-ray photoelectron spectroscopy (XPS), and a weight ratio of a proportion of C-O bonds to a proportion of Si-Si bonds (C-O / Si-Si) exceeding 0.

01.

2. The raw material for a negative electrode active material of an all-solid-state lithium ion secondary battery according to claim 1, wherein the silicon particles have a weight ratio (C-C / C-O) of a proportion of C-C bonds to a proportion of C-O bonds, as measured at a depth of 5 nm from the surface by X-ray photoelectron spectroscopy (XPS), of 2.00 to 6.

00.

3. The raw material for a negative electrode active material of an all-solid-state lithium ion secondary battery according to claim 1, wherein the silicon particles have a weight ratio (C-O / Si-O) of a proportion of C-O bonds to a proportion of Si-O bonds, as measured at a depth of 5 nm from the surface by X-ray photoelectron spectroscopy (XPS), of 0.05 to 0.

20.

4. The raw material for a negative electrode active material of an all-solid-state lithium ion secondary battery according to claim 1, wherein the silicon powder has an average particle size of 0.3 to 10.0 μm.

5. A material for forming an anode of an all-solid-state lithium ion secondary battery, in which an anode active material layer made of a composition containing the raw material for the anode active material of the all-solid-state lithium ion secondary battery according to claim 1 is formed on a current collector to a thickness of 10 μm or more.

6. A material for forming an anode of an all-solid-state lithium ion secondary battery according to claim 5, wherein a layer made of a composition containing a raw material for a negative electrode active material of the all-solid-state lithium ion secondary battery is patterned to form island-shaped convex portions at intervals, and a connection layer made of a composition containing a raw material for a negative electrode active material of the all-solid-state lithium ion secondary battery is formed on a bottom surface of a groove portion formed between the island-shaped convex portions so as to be continuous with the island-shaped convex portions.

7. An all-solid-state lithium ion secondary battery having a positive electrode, a negative electrode, and a solid electrolyte layer, characterized in that the material for forming an all-solid-state lithium ion secondary battery negative electrode according to claim 5 or 6 is disposed as the negative electrode.

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