Negative electrode for secondary battery, method for manufacturing negative electrode for secondary battery, and secondary battery

Abstract

To provide a negative electrode for a secondary battery that has a small change in thickness at charging and discharging and a small resistance.SOLUTION: A negative electrode for a secondary battery has an active material layer. The active material layer contains a sulfide solid electrolyte and a composite particle as an active material. The composite particle contains a plurality of porous silicon particles and a binder. The active material layer has a porosity greater than 15%.SELECTED DRAWING: None

Description

Technical Field

[0001] This application discloses a negative electrode for a secondary battery, a method for manufacturing the negative electrode for a secondary battery, and a secondary battery.

Background Art

[0002] Negative electrodes for secondary batteries containing Si-based active materials are known. For example, Patent Document 1 discloses a negative electrode layer used in an all-solid-state battery, which has a negative electrode active material and a sulfide solid electrolyte, and the negative electrode active material is a composite particle having a plurality of particles containing Si element and a binder. The porosity of the negative electrode layer disclosed in Patent Document 1 is 15% or less.

[0003] Patent Document 2 discloses a method for producing an active material having voids by extracting Li from a LiSi precursor using a Li extraction solvent.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] Conventional negative electrodes for secondary batteries have room for improvement in suppressing changes in the thickness of the negative electrode due to expansion and contraction of the active material during charge and discharge, and reducing the resistance of the negative electrode.

Means for Solving the Problems

[0006] As one of the means for solving the above problems, this application provides a negative electrode for a secondary battery, having an active material layer, wherein the active material layer contains a sulfide solid electrolyte and composite particles as an active material, The composite particles include a plurality of porous silicon particles and a binder, The active material layer has a porosity of more than 15%. are disclosed.

[0007] In the negative electrode for a secondary battery of the present disclosure, when observing the cross-section of the active material layer, more than half of the plurality of the composite particles extracted by the following extraction method may have an aspect ratio of 2.5 or more.

[0008] Extraction method: Observe the cross-section of the active material layer, extract the composite particles contained in the cross-section in descending order of cross-sectional area, and terminate the extraction when the total area of the extracted composite particles exceeds 80% of the total area of all the composite particles contained in the cross-section.

[0009] In the negative electrode for a secondary battery of the present disclosure, the composite particles may contain porous carbon as the binder.

[0010] One of the means for solving the above problems in the present application is A method for manufacturing a negative electrode of a secondary battery, comprising obtaining a LiSi precursor containing Li and Si, removing Li from the LiSi precursor to obtain porous silicon particles, obtaining composite particles containing a plurality of the porous silicon particles and a binder, obtaining an active material composite containing a sulfide solid electrolyte and the composite particles, and pressing the active material composite to obtain an active material layer having a porosity of more than 15%. are disclosed.

[0011] The manufacturing method of the present disclosure may include deforming the composite particles by pressing the active material composite. In this case, when observing the cross-section of the active material layer after pressing, more than half of the plurality of the composite particles extracted by the following extraction method may have an aspect ratio of 2.5 or more.

[0012] Extraction method: Observe the cross-section of the active material layer, extract the composite particles contained in the cross-section in descending order of cross-sectional area, and end the extraction when the total area of the extracted composite particles exceeds 80% of the total area of all the composite particles contained in the cross-section.

[0013] The manufacturing method of the present disclosure may include mixing a plurality of the porous silicon particles and an organic component to obtain an intermediate composite, and carbonizing the organic component of the intermediate composite to obtain the composite particles including a plurality of the porous silicon particles and porous carbon as the binder.

[0014] As one of the means for solving the above problems, the present application discloses a secondary battery having the negative electrode for a secondary battery of the present disclosure.

Effect of the Invention

[0015] The negative electrode for a secondary battery of the present disclosure has a small change in the thickness of the negative electrode accompanying the expansion and contraction of the active material during charge and discharge. Further, the negative electrode for a secondary battery of the present disclosure has a small resistance. The secondary battery including the negative electrode of the present disclosure has a small change in the thickness of the negative electrode during charge and discharge, and thus, for example, has a small change in the restraint pressure during charge and discharge. Further, the secondary battery including the negative electrode of the present disclosure has a small resistance of the negative electrode, and thus, for example, has excellent charge and discharge performance.

Brief Description of the Drawings

[0016]

Figure 1

Figure 2

Figure 3

Figure 4A

Figure 4B

Figure 4C

Embodiments for Carrying Out the Invention

[0017] 1. Negative electrode for secondary battery The negative electrode for a secondary battery of the present disclosure has an active material layer. The active material layer includes a sulfide solid electrolyte and composite particles as an active material. The composite particles include a plurality of porous silicon particles and a binder. The active material layer has a porosity of more than 15%.

[0018] 1.1 Active material layer The active material layer includes a sulfide solid electrolyte and composite particles as an active material. The shape of the active material layer is not particularly limited, and for example, it may be a sheet-shaped active material layer having a substantially flat surface. The thickness of the active material layer is not particularly limited, and for example, it may be 100 nm or more, 1 μm or more, or 10 μm or more, and may also be 2 mm or less, or 1 mm or less.

[0019] 1.1.1 Sulfide solid electrolyte The active material layer contains a sulfide solid electrolyte. The sulfide solid electrolyte may be any sulfide capable of conducting carrier ions in a secondary battery. Specific examples of the sulfide solid electrolyte in the case of constructing a lithium-ion secondary battery include Li2S-P2S5, Li2S-SiS2, LiI-Li2S-SiS2, LiI-Si2S-P2S5, Li2S-P2S5-LiI-LiBr, LiI-Li2S-P2S5, LiI-Li2S-P2O5, LiI-Li3PO4-P2S5, Li2S-P2S5-GeS2, etc. Among them, those containing at least Li, S, and at least one of P, Si, and Ge as constituent elements have high performance, and those containing at least Li, S, and P have particularly high performance. The sulfide solid electrolyte may be amorphous or crystalline. The sulfide solid electrolyte may be, for example, particulate. Only one kind of sulfide solid electrolyte may be used alone, or two or more kinds may be used in combination.

[0020] 1.1.2 Composite Particles The active material layer contains composite particles as the active material. The composite particles contain a plurality of porous silicon particles and a binder. More specifically, the composite particles are formed by binding the plurality of porous silicon particles to each other via the binder.

[0021] 1.1.2.1 Porous Silicon Particles The composite particles contain a plurality of porous silicon particles. The porous silicon particles contain silicon having a plurality of voids. There is no particular limitation on the form of the voids in the porous silicon particles. The porous silicon particles may be particles containing nanoporous silicon. Nanoporous silicon refers to silicon in which there are a plurality of pores having a pore diameter on the order of nanometers (less than 1000 nm, preferably 100 nm or less). The porous silicon particles may contain pores having a diameter of 55 nm or less. Pores having a diameter of 55 nm or less are difficult to be crushed by pressing. That is, the porous silicon particles containing pores having a diameter of 55 nm or less are likely to maintain porosity even after pressing. For example, per 1 g of the porous silicon particles, the pores having a diameter of 55 nm or less may be contained in an amount of 0.21 cc or more, 0.22 cc / g or more, or 0.23 cc / g or more, and may be contained in an amount of 0.30 cc / g or less, 0.28 cc / g or less, or 0.26 cc / g or less. The amount of the pores having a diameter of 55 nm or less contained in the porous silicon particles can be determined, for example, from the pore size distribution by the nitrogen gas adsorption method or the DFT method.

[0022] The porous silicon particles may have a predetermined porosity. The porosity of the porous silicon particles may be, for example, 1% or more, 5% or more, 10% or more, or 20% or more, and may be 80% or less, 70% or less, 60% or less, 50% or less, 40% or less, or 30% or less. The porosity can be determined, for example, by observation using a scanning electron microscope (SEM). It is preferable that the number of samples is large, for example, 100 or more. The porosity can be the average value obtained from these samples.

[0023] However, in the negative electrode for a secondary battery of the present disclosure, it is not necessary to distinguish the voids in the porous silicon particles, the voids in the composite particles, the voids outside the composite particles, etc. In the negative electrode for a secondary battery of the present disclosure, the porosity of the entire active material layer including the voids in the porous silicon particles, the voids in the composite particles, the voids outside the composite particles, etc. may be more than 15%. That is, regardless of the magnitude of the porosity of the porous silicon particles, the magnitude of the porosity of the composite particles, and the magnitude of the porosity outside the composite particles, if the porosity of the entire active material layer is more than 15%, the effect of suppressing the thickness change of the negative electrode during charging can be expected.

[0024] The composition of the porous silicon particles is not particularly limited. The proportion of the Si element in all the elements contained in the porous silicon particles may be, for example, 50 mol% or more, 70 mol% or more, or 90 mol% or more. The porous silicon particles may contain other elements such as Li element in addition to the Si element. Examples of the other elements include, in addition to the Li element, Sn element, Fe element, Co element, Ni element, Ti element, Cr element, B element, P element, etc. Further, the porous silicon particles may contain impurities such as oxides. The porous silicon particles may be amorphous or crystalline. The crystal phase contained in the porous silicon particles is not particularly limited.

[0025] The shape and size of the porous silicon particles are not particularly limited. The average primary particle diameter of the porous silicon particles may be, for example, 30 nm or more, 50 nm or more, 100 nm or more, or 150 nm or more, and may be 10 μm or less, 5 μm or less, 3 μm or less, 2 μm or less, or 1 μm or less. Further, the average secondary particle diameter of the porous silicon particles may be, for example, 100 nm or more, 1 μm or more, or 2 μm or more, and may be 20 μm or less, 15 μm or less, or 10 μm or less. Incidentally, the average primary particle diameter and the average secondary particle diameter can be determined by observation with an electron microscope such as SEM, and are obtained, for example, as the average value of the maximum Feret diameters of each of a plurality of particles. The number of samples is preferably large, for example 20 or more, and may be 50 or more, or 100 or more. The average primary particle diameter and the average secondary particle diameter can be appropriately adjusted, for example, by appropriately changing the manufacturing conditions of the porous silicon particles described later or performing a classification process.

[0026] 1.1.2.2 Binder The composite particles contain a binder. The binder binds the plurality of porous silicon particles to each other. The type of the binder is not particularly limited. The binder may be selected from, for example, a butadiene rubber (BR)-based binder, an isobutylene rubber (IIR)-based binder, an acrylate butadiene rubber (ABR)-based binder, a styrene butadiene rubber (SBR)-based binder, a polyvinylidene fluoride (PVdF)-based binder, a polytetrafluoroethylene (PTFE)-based binder, a polyimide (PI)-based binder, a carboxymethyl cellulose (CMC)-based binder, a polyacrylate-based binder, a polyacrylate ester-based binder, etc. Only one type of binder may be used alone, or two or more types may be used in combination.

[0027] The composite particles may contain porous carbon as a binder. Porous carbon as a binder can be obtained, for example, by carbonizing various organic components. For example, a plurality of porous silicon particles and an organic component are mixed to bond the porous silicon particles together with the organic component, and then the organic component is carbonized by heating, so that composite particles in which a plurality of porous silicon particles are bonded to each other via porous carbon are obtained. The ratio of the porous carbon in the whole binder is not particularly limited. For example, it may be 0% by volume or more, more than 0% by volume, 10% by volume or more, 20% by volume or more, 30% by volume or more, or 40% by volume or more, and may be 100% by volume or less, or 90% by volume or less. When the composite particles contain porous carbon as a binder, the conductivity of the composite particles is improved, and the resistance of the negative electrode tends to be small. Further, even when the porous silicon particles expand or contract, the volume change of the whole composite particles is easily relaxed by the voids of the porous carbon, and the thickness change of the negative electrode is easily suppressed. Whether the composite particles contain porous carbon as a binder or not can be determined by an image obtained by observation with an electron microscope or the like and elemental analysis.

[0028] 1.1.2.3 Ratio of Porous Silicon Particles and Binder Contained in Composite Particles The ratio of the porous silicon particles and the binder contained in the composite particles is not particularly limited as long as it can form the composite particles. For example, the ratio of the binder in the total of the porous silicon particles and the binder may be 1% by mass or more, 5% by mass or more, or 8% by mass or more, and may be 30% by mass or less, 28% by mass or less, 26% by mass or less, 24% by mass or less, or 22% by mass or less. When the ratio of the binder in the total of the porous silicon particles and the binder is 1% by mass or more and 30% by mass or less, it is easier to ensure a larger charge-discharge capacity.

[0029] 1.1.2.4 Number of Porous Silicon Particles Contained in Composite Particles The number of porous silicon particles contained in one composite particle is plural. The number of porous silicon particles contained in the composite particle may be, for example, 3 or more, 5 or more, 10 or more, or 50 or more, and may also be 1000 or less. The number of porous silicon particles contained in the composite particle can be specified, for example, by an image obtained by observation with an electron microscope or the like or by elemental analysis.

[0030] 1.1.2.5 Particle Size of Composite Particles The composite particle can be regarded as a secondary particle in which a plurality of porous silicon particles are aggregated via a binder. The average particle size of the composite particle is not particularly limited. The average particle size of the composite particle may be 100 nm or more, 1 μm or more, 2 μm or more, or 3 μm or more, and may also be 20 μm or less, 15 μm or less, or 10 μm or less. The average particle size of the composite particles contained in the active material layer can be determined by observation with an electron microscope such as SEM, and is, for example, determined as the average value of the maximum Feret diameters of a plurality of composite particles. The number of samples is preferably large, for example 20 or more, and may be 50 or more, or 100 or more. Alternatively, only the composite particles are taken out from the active material layer, and the average particle size (D50, median diameter) of the composite particles measured using a laser diffraction particle size distribution measuring device may be 100 nm or more, 1 μm or more, 2 μm or more, or 3 μm or more, and may also be 20 μm or less, 15 μm or less, or 10 μm or less.

[0031] 1.1.2.6 Shape of Composite Particles As described below, the active material layer containing the sulfide solid electrolyte and the composite particles can be formed by pressing an active material composite material containing the sulfide solid electrolyte and the composite particles. At this time, the composite particles can be crushed in the pressing direction and have an aspect ratio of a predetermined value or more. By pressing the composite particles to have an aspect ratio of a predetermined value or more, the contact resistance within the composite particles, the contact resistance between the composite particles, the contact resistance between the composite particles and other materials, etc. are reduced, and the resistance of the entire negative electrode is more likely to be smaller. Specifically, from the viewpoint of further reducing the resistance of the negative electrode, in the negative electrode for a secondary battery of the present disclosure, when observing the cross section of the active material layer, more than half (50% or more in terms of the number ratio) of the plurality of composite particles extracted by the following extraction method may have an aspect ratio of 2.5 or more.

[0032] Extraction method: Observe the cross section of the active material layer, extract the composite particles contained in the cross section in descending order of cross-sectional area, and end the extraction when the total area of the extracted composite particles exceeds 80% of the total area of all the composite particles contained in the cross section.

[0033] In addition, the above extraction method may be performed by image analysis based on the cross-sectional image of the active material layer obtained by SEM or the like. In the image analysis, after approximating the composite particles contained in the image by an ellipse, the aspect ratio of each composite particle may be specified. The extraction of the composite particles and the specification of the aspect ratio by image analysis of the cross section of the active material layer will be described in more detail in the examples described later.

[0034] 1.1.3 Other Components The active material layer contains at least the above-mentioned sulfide solid electrolyte and composite particles. Further, the active material layer may optionally further contain an electrolyte other than the sulfide solid electrolyte, an active material other than the composite particles, a conductive aid, a binder other than the composite particles, and the like. Furthermore, the active material layer may contain various additives. The content of each of the active material, electrolyte, conductive aid, binder, etc. in the active material layer may be appropriately determined according to the intended battery performance. For example, taking the entire active material layer (total solid content) as 100% by mass, the content of the above composite particles may be 40% by mass or more, 45% by mass or more, 50% by mass or more, or 55% by mass or more, and may also be 99% by mass or less, 95% by mass or less, 90% by mass or less, or 80% by mass or less. Also, taking the entire active material layer (total solid content) as 100% by mass, the content of the sulfide solid electrolyte may be 1% by mass or more, 5% by mass or more, 10% by mass or more, or 20% by mass or more, and may also be 60% by mass or less, 55% by mass or less, 50% by mass or less, or 45% by mass or less.

[0035] 1.1.3.1 Electrolytes Other than Sulfide Solid Electrolytes The electrolytes other than the sulfide solid electrolyte that can be contained in the active material layer may be, for example, solid electrolytes, liquid electrolytes (electrolyte solutions), or combinations thereof. As the solid electrolyte, those known as solid electrolytes for secondary batteries may be used. The solid electrolyte may be an inorganic solid electrolyte or an organic polymer electrolyte. In particular, inorganic solid electrolytes are excellent in ionic conductivity and heat resistance. Examples of inorganic solid electrolytes other than sulfide solid electrolytes include, for example, lithium lanthanum zirconate, LiPON, Li 1+X Al X Ge 2-XExamples of the oxide solid electrolyte include (PO4)3, Li—SiO-based glass, and Li—Al—S—O-based glass. The electrolytic solution may contain, for example, lithium ions as carrier ions. The electrolytic solution may be, for example, a non-aqueous electrolytic solution. The composition of the electrolytic solution may be the same as that known as the composition of the electrolytic solution of a secondary battery. For example, as the electrolytic solution, a solution in which a lithium salt is dissolved in an organic solvent such as a carbonate-based solvent at a predetermined concentration can be used. There is no particular limitation on the type of the lithium salt. The proportion of the sulfide solid electrolyte in the total electrolyte contained in the active material layer may be, for example, 80% by mass or more, 90% by mass or more, or 95% by mass or more, and may be 100% by mass or less.

[0036] In the negative electrode for a secondary battery of the present disclosure, the active material layer may or may not contain a liquid component. The liquid component may be an electrolytic solution that can function as an electrolyte, or may be a component that does not function as an electrolyte (for example, a lubricating component). In the negative electrode for a secondary battery of the present disclosure, a plurality of porous silicon particles are bonded to each other via a binder to form composite particles, and the composite particles and the sulfide solid electrolyte are combined, so that an ion conduction path and a conduction path can be ensured even in the absence of a liquid component.

[0037] 1.1.3.2 Active Materials Other than Composite Particles Examples of the active materials other than the composite particles that can be contained in the active material layer include silicon-based active materials such as Si, Si alloys, and silicon oxides; carbon-based active materials such as graphite and hard carbon; various oxide-based active materials such as lithium titanate; metallic lithium, lithium alloys, and the like. The proportion of the composite particles in the total of the composite particles and the active materials other than the composite particles contained in the active material layer may be, for example, 80% by mass or more, 90% by mass or more, or 95% by mass or more, and may be 100% by mass or less.

[0038] 1.1.3.3 Conductive Auxiliaries Examples of the conductive aids that can be included in the active material layer include carbon materials such as vapor-grown carbon fiber (VGCF), acetylene black (AB), ketjen black (KB), carbon nanotube (CNT), and carbon nanofiber (CNF); and metal materials such as nickel, aluminum, and stainless steel. The conductive aids may be, for example, particulate or fibrous, and their size is not particularly limited. Only one type of conductive aid may be used alone, or two or more types may be used in combination.

[0039] 1.1.3.4 Binders Other than Composite Particles The active material layer may contain a binder separately from the composite particles. The binder may be selected from, for example, butadiene rubber (BR)-based binders, isobutylene rubber (IIR)-based binders, acrylate butadiene rubber (ABR)-based binders, styrene butadiene rubber (SBR)-based binders, polyvinylidene fluoride (PVdF)-based binders, polytetrafluoroethylene (PTFE)-based binders, polyimide (PI)-based binders, carboxymethyl cellulose (CMC)-based binders, polyacrylate-based binders, polyacrylate ester-based binders, etc. Only one type of binder may be used alone, or two or more types may be used in combination. The binder constituting the composite particles and the binder other than the composite particles may be of the same type or different types.

[0040] 1.1.4 Porosity of the Active Material Layer In the negative electrode for a secondary battery of the present disclosure, the active material layer has a porosity of more than 15%. Since the active material layer has such a high porosity, even when the composite particles as the active material expand and contract during charge and discharge, the change in the thickness of the negative electrode is less likely to be large. The upper limit of the porosity of the active material layer is not particularly limited. In the negative electrode for a secondary battery of the present disclosure, even when the porosity of the active material layer is large, the resistance can be reduced by the effects of composite of porous silicon particles and a binder, the effects of combination with a sulfide solid electrolyte, and the like. The porosity of the active material layer may be, for example, 50% or less, 45% or less, 40% or less, 35% or less, 30% or less, 25% or less, or 20% or less. The "porosity" of the active material layer is the ratio of the volume of voids in the active material layer to the whole of the active material layer. The porosity of the active material layer is specified as follows. That is, when the porosity of the active material layer is A, the total volume x obtained by dividing the weight of each material constituting the active material layer by the true density of each material, and the volume y obtained from the actual dimensions of the active material layer, the porosity A can be calculated by A(%) = (1 - x / y) × 100.

[0041] 1.2 Configuration other than the active material layer The negative electrode for a secondary battery of the present disclosure has an active material layer, and further optionally may have a current collector in contact with the active material layer. FIG. 1 schematically shows the configuration of a secondary battery 100 according to an embodiment. As shown in FIG. 1, the negative electrode 30 of the secondary battery 100 may have an active material layer 31 and a current collector 32 in contact with the active material layer 31.

[0042] 1.2.1 Current collector Any generally used current collector for a battery can be adopted. Also, the current collector may be in the form of a foil, a plate, a mesh, a punched metal, a foam, or the like. The current collector may be a metal foil or a metal mesh, or may be a carbon sheet. In particular, a metal foil is excellent in terms of handleability and the like. The current collector may be composed of a plurality of foils or sheets. Examples of the metal constituting the current collector include Cu, Ni, Cr, Au, Pt, Ag, Al, Fe, Ti, Zn, Co, stainless steel, and the like. In particular, from the viewpoint of ensuring reduction resistance, etc., the current collector may contain at least one metal selected from Cu, Ni, and stainless steel. The current collector may have some coating layer on its surface for the purpose of adjusting resistance or the like. Also, the current collector may be one in which the above-mentioned metal is plated or vapor-deposited on a metal foil or a substrate. Further, when the current collector is composed of a plurality of metal foils, there may be some layer between the plurality of metal foils. The thickness of the current collector is not particularly limited. For example, it may be 0.1 μm or more or 1 μm or more, and may be 1 mm or less or 100 μm or less.

[0043] 1.2.2 Other Configurations In addition to the above active material layer and current collector, the negative electrode for a secondary battery of the present disclosure may have a general configuration as a negative electrode of a secondary battery. For example, tabs, terminals, and the like.

[0044] 1.3 Supplementary The negative electrode for a secondary battery of the present disclosure is preferably used, for example, as a negative electrode of a lithium-ion secondary battery. During charge and discharge of a lithium-ion secondary battery, silicon as an active material occludes lithium ions and expands greatly, and also releases lithium ions and contracts greatly. That is, in a conventional lithium-ion secondary battery, during charge and discharge, the expansion and contraction of silicon as an active material are large, the thickness change of the negative electrode is large, and the restraint pressure is likely to change greatly. In particular, in a lithium-ion secondary battery containing a sulfide solid electrolyte, the restraint pressure is likely to become excessively large. On the other hand, by adopting the negative electrode for a secondary battery of the present disclosure in a lithium-ion secondary battery, the thickness change of the negative electrode during charge and discharge can be suppressed to be small, and the change in the restraint pressure can be reduced.

[0045] 1.4 Effects As described above, in the negative electrode for a secondary battery of the present disclosure, the active material layer contains composite particles of porous silicon particles and a binder, and the porosity of the active material layer is greater than 15%. The negative electrode for a secondary battery of the present disclosure has such an active material layer with a large porosity, so that the thickness change during charge and discharge is small. Further, in the negative electrode for a secondary battery of the present disclosure, the porous silicon particles are combined by a binder in the active material layer, and by combining the composite particles with a sulfide solid electrolyte, a conductive path and an ion conduction path are secured, and the resistance of the negative electrode is likely to be small. In particular, when the composite particles have the predetermined aspect ratio described above or when the composite particles contain porous carbon as a binder, the resistance of the negative electrode is more likely to be small.

[0046] 2. Method for manufacturing a negative electrode for a secondary battery The negative electrode for a secondary battery of the present disclosure can be manufactured, for example, as follows. That is, the method for manufacturing a negative electrode for a secondary battery of the present disclosure is obtaining a LiSi precursor containing Li and Si, removing Li from the LiSi precursor to obtain porous silicon particles, obtaining composite particles containing a plurality of the porous silicon particles and a binder, obtaining an active material composite containing a sulfide solid electrolyte and the composite particles, and Pressing the active material composite to obtain an active material layer having a porosity of more than 15%. including.

[0047] 2.1 Obtaining a LiSi precursor The LiSi precursor contains Li and Si as constituent elements. The LiSi precursor may be, for example, an alloy of Li and Si. The LiSi precursor may be any material that can form voids and become porous by removing Li. The LiSi precursor may have a crystal phase of Si (diamond type). The crystal phase of Si has typical peaks at positions of 2θ = 28.4°, 47.3°, 56.1°, 69.2°, and 76.4° in XRD measurement using CuKα radiation. These peak positions may shift back and forth within a range of ±0.5° respectively, or within a range of ±0.3°. The LiSi precursor may have a crystal phase of Si (diamond type) as the main phase. The "main phase" refers to the crystal phase to which the peak with the highest intensity belongs in the XRD chart. The LiSi precursor may have a crystal phase of Li 22 Si5 or Li 15 Si4. The crystal phase of Li 22 Si5 has typical peaks at positions of 2θ = 24.8° and 40.8° in XRD measurement using CuKα radiation, and the crystal phase of Li 15 Si4 has typical peaks at positions of 2θ = 20.3°, 26.2°, 39.4°, 41.2°, and 42.9° in XRD measurement using CuKα radiation. These peak positions may shift back and forth within a range of ±0.5° respectively, or within a range of ±0.3°.

[0048] The composition of the LiSi precursor is not particularly limited. The LiSi precursor may contain only Li element and Si element, or may contain other elements in addition to Li element and Si element. The total ratio of Li element and Si element to all elements contained in the LiSi precursor may be, for example, 50 mol% or more, 70 mol% or more, or 90 mol% or more. In the LiSi precursor, the ratio of Li element to the total of Li element and Si element may be, for example, 30 mol% or more, 50 mol% or more, or 80 mol% or more, and may also be 95 mol% or less, or 90 mol% or less.

[0049] The LiSi precursor may be obtained by mixing a raw material containing Li element and a raw material containing Si element. For example, in the manufacturing method of the present disclosure, Si particles and Li metal may be mixed to obtain a LiSi precursor. Examples of the mixing method include a method of mixing Si particles and Li metal using an agate mortar, a method of mixing Si particles and Li metal using a mechanical milling method, etc. The temperature and pressure during mixing are not particularly limited. Heating or cooling may or may not be performed during mixing, and pressurization or depressurization may or may not be performed. The atmosphere during mixing is also not particularly limited, and may be, for example, an inert gas atmosphere such as an Ar atmosphere. The shape and size of the Si particles and Li metal to be mixed are not particularly limited, and may be appropriately selected according to the shape and size of the target porous silicon particles, etc.

[0050] 2.2 Obtaining porous silicon particles In the manufacturing method of the present disclosure, porous silicon particles are obtained by removing Li from the above-described LiSi precursor. In the manufacturing method of the present disclosure, it is not necessary to remove all of the Li from the LiSi precursor, and Li element may remain partially in the porous silicon particles. The method for removing Li from the LiSi precursor is not particularly limited. For example, Li may be extracted from the LiSi precursor. A Li extraction solvent may be used for the extraction of Li. For example, by bringing the LiSi precursor into contact with the Li extraction solvent, the Li extraction solvent reacts with Li, and Li is extracted from the LiSi precursor into the Li extraction solvent. The form of bringing the LiSi precursor into contact with the Li extraction solvent is not particularly limited, and the LiSi precursor may be immersed in the Li extraction solvent, the Li extraction solvent may be sprayed onto the LiSi precursor, the LiSi precursor and the Li extraction solvent may be mixed, or the LiSi precursor may be dispersed in a dispersion medium to form a dispersion, and the dispersion and the Li extraction solvent may be mixed.

[0051] As the Li extraction solvent, for example, alcohols such as methanol, ethanol, and propanol can be used. The Li extraction solvent may contain a co-solvent together with the alcohol. Examples of the co-solvent include the acids described below. Depending on the type of the Li extraction solvent, the Li extraction rate varies, and the pore diameter formed after Li extraction changes. For example, when comparing the cases of using methanol, ethanol, and propanol as the Li extraction solvent, the Li extraction rate is the fastest when using methanol, followed by ethanol, then 1-propanol, and then isopropanol. Also, the pore diameter after Li extraction is most likely to be the largest when using methanol, followed by ethanol, then 1-propanol, and then isopropanol. The type of the Li extraction solvent and the extraction time may be selected according to the target pore diameter. The voids formed in the active material after Li extraction have various pore diameters. Here, by adjusting the pore diameter in the porous silicon particles after Li extraction, the pores are less likely to collapse even when the porous silicon particles are pressurized (for example, when the negative electrode is pressed during battery manufacturing). For example, as described above, by increasing the amount of pores with a diameter of 55 nm or less in the porous silicon particles, the pores are less likely to collapse, and the expansion and contraction of the active material during charge and discharge are more easily suppressed.

[0052] The manufacturing method of the present disclosure may include dispersing a LiSi precursor in a dispersion medium to obtain a dispersion, and mixing the dispersion with a Li extraction solvent to extract Li from the LiSi precursor to form voids. In this case, the pore diameter of the porous silicon particles after Li extraction can be more easily adjusted appropriately. Examples of the dispersion medium include saturated hydrocarbons such as n-heptane, n-octane, n-decane, 2-ethylhexane, and cyclohexane, unsaturated hydrocarbons such as hexene and heptene, aromatic hydrocarbons such as 1,3,5-trimethylbenzene, toluene, xylene, ethylbenzene, propylbenzene, cumene, 1,2,4-trimethylbenzene, and 1,2,3-trimethylbenzene, and ethers such as n-butyl ether, n-hexyl ether, isoamyl ether, diphenyl ether, methyl phenyl ether, and cyclopentyl methyl ether. Among these, one or more selected from n-butyl ether, 1,3,5-trimethylbenzene, and n-heptane are preferable, and particularly 1,3,5-trimethylbenzene is preferable. In addition, it is preferable that the water content in the dispersion medium is low. This is because water reacts with the LiSi precursor. The water content in the dispersion medium may be, for example, 100 ppm or less, 50 ppm or less, 30 ppm or less, or 10 ppm or less. The ratio (solid content ratio) of the LiSi precursor in the dispersion is not particularly limited as long as it is a ratio at which the LiSi precursor can be dispersed in the dispersion medium. The dispersion can be obtained, for example, by mixing the LiSi precursor and the dispersion medium. The mixing means in this case is not particularly limited.

[0053] The temperature and pressure during the extraction of Li from the LiSi precursor using the Li extraction solvent are not particularly limited. Heating or cooling may or may not be performed during Li extraction, and pressurization or depressurization may or may not be performed. The atmosphere and the Li extraction time during Li extraction are also not particularly limited. In addition, the ratio of the LiSi precursor to the Li extraction solvent during Li extraction is not particularly limited. The temperature, pressure, time, ratio, etc. may be adjusted so that the required amount of Li can be extracted from the LiSi precursor.

[0054] The removal of Li from the LiSi precursor may be carried out in one step or in two or more steps. For example, porous silicon particles may be obtained in one step only by reacting the LiSi precursor with a Li extraction solvent, or after the formation of voids using the Li extraction solvent, Li may be extracted again using the Li extraction solvent, or Li may be extracted using an acid, etc., and porous silicon particles may be obtained through an extraction process of two or more steps. By using an acid after using the Li extraction solvent, Li can be more appropriately extracted from the LiSi precursor. Examples of the acid include one or more of acetic acid, formic acid, propionic acid, and oxalic acid. Acetic acid is particularly preferred. The temperature, pressure, time, volume ratio, etc. during Li extraction using an acid are not particularly limited.

[0055] The porous silicon particles after Li extraction may be optionally washed. Thereby, the impurities contained in the porous silicon particles can be reduced. When the concentration of the LiSi precursor in the total of the LiSi precursor and the Li extraction solvent is high, the production efficiency is high, but the amount of impurities generated tends to increase. For example, when the concentration of the LiSi precursor is 3.3 g or more with respect to 1 L of the Li extraction solvent, by washing the active material after Li extraction, both an improvement in production efficiency and a reduction in impurities can be achieved. The washing may be, for example, an acid washing in which an acid is brought into contact with the porous silicon particles. Incidentally, the extraction of the Li element by the above acid may also serve as the acid washing of the porous silicon particles.

[0056] 2.3 Obtaining composite particles In the manufacturing method of the present disclosure, composite particles are obtained from a plurality of porous silicon particles and a binder obtained as described above. The composite particles can be obtained, for example, by mixing the porous silicon particles and the binder. The mixing ratio of the porous silicon particles and the binder is not particularly limited and may be appropriately determined according to the composition of the target composite particles. The mixing may be performed either dry or wet. For example, the porous silicon particles and the binder may be dispersed or dissolved in a dispersion medium to obtain a slurry, and the slurry may be dried to composite the porous silicon particles and the binder. The type of the dispersion medium is not particularly limited, and for example, dimethyl carbonate or the like can be adopted. The drying of the slurry may be performed by known drying means. For example, spray drying or drying by rolling can be mentioned. Specifically, it may be dried using a spray dryer. On the other hand, the generation of the composite particles by a dry method can be performed, for example, by applying compression or physical impact to the porous silicon particles and the binder.

[0057] As described above, in the negative electrode for a secondary battery of the present disclosure, the composite particles may contain porous carbon as a binder. Thereby, the resistance of the negative electrode can be further reduced. From this viewpoint, the manufacturing method of the present disclosure includes obtaining an intermediate composite by mixing a plurality of porous silicon particles and an organic component, and carbonizing the organic component of the intermediate composite to obtain composite particles including a plurality of porous silicon particles and porous carbon as a binder. The organic component may be any component that can become porous carbon by carbonization. For example, when the organic component is carbonized, those in which some elements are separated from the organic component to form voids can be mentioned. Further, in the intermediate composite, it is preferable that a plurality of porous silicon particles are bonded to each other via an organic component. That is, the organic component is preferably one that becomes porous carbon by carbonization and has a function of binding porous silicon particles to each other. Specific examples of such an organic component include, for example, butadiene rubber (BR), isobutylene rubber (IIR), acrylate butadiene rubber (ABR), styrene butadiene rubber (SBR), polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), carboxymethyl cellulose (CMC), polyacrylate, polyacrylate ester, and the like. For example, when a binding fluorine-based organic compound such as PVdF or PTFE is used as the organic component, when the fluorine-based organic compound is carbonized, H and F are volatilized and removed as hydrofluoric acid or the like to form voids, and porous carbon having small pores can be obtained.

[0058] 2.4 Obtaining the active material composite In the manufacturing method of the present disclosure, an active material composite is obtained by the composite particles and the sulfide solid electrolyte obtained as described above. For example, an active material composite is obtained by mixing the composite particles with the sulfide solid electrolyte. The active material composite may contain components constituting the active material layer, and as described above, in addition to the composite particles and the sulfide solid electrolyte, it may contain optional components such as a conductive agent. The active material composite can be obtained by mixing these components by a known method. The mixing may be performed dry or wet. That is, the active material composite may consist only of solids such as composite particles and sulfide solid electrolyte, or the solids such as composite particles and sulfide solid electrolyte may be dispersed in a dispersion to form a slurry state. There are no particular restrictions on the mixing conditions of the active material composite.

[0059] 2.5 Obtaining the active material layer When obtaining the active material composite, if a slurry containing the active material composite is obtained by wet mixing, for example, the slurry is applied to the surface of the current collector or the electrolyte layer described later and dried to laminate the active material composite on the surface of the current collector or the electrolyte layer, and the active material composite is pressed, an active material layer can be formed through these steps. Alternatively, the active material layer may be formed by dry molding (for example, powder pressing molding) of the active material composite. In any case, in the manufacturing method of the present disclosure, the active material layer is obtained by pressing the active material composite obtained as described above.

[0060] Here, the porosity in the active material layer can change depending on the magnitude of the pressure applied to the active material composite. Also, even if the pressure applied to the active material composite is the same, the porosity of the active material layer can change depending on the composition of the active material composite. For example, the fluidity of the active material composite is different when the active material composite contains a liquid and when it consists only of a solid. Also, the fluidity of the active material composite varies depending on the size and shape of the particles contained in the active material composite. Due to such differences in fluidity, the porosity of the active material after pressing can change. In the manufacturing method of the present disclosure, the magnitude of the pressure applied to the active material composite may be adjusted according to the form and composition of the active material composite so that the porosity of the active material layer exceeds 15%. The porosity of the active material layer after pressing may be, for example, 50% or less, 45% or less, 40% or less, 35% or less, 30% or less, 25% or less, or 20% or less. The means for pressing the active material composite is not particularly limited. For example, various pressing means such as CIP, HIP, roll press, uniaxial press, and die press can be employed.

[0061] As described above, in the negative electrode for a secondary battery of the present disclosure, by pressing the composite particles to have a predetermined aspect ratio, the contact resistance within the composite particles, the contact resistance between the composite particles, and the contact resistance between the composite particles and other materials can be reduced, and the resistance of the entire negative electrode can be made small. From this perspective, the manufacturing method of the present disclosure may include deforming the composite particles by pressing the active material composite, and when observing the cross-section of the active material layer after pressing, more than half of the plurality of composite particles extracted by the following extraction method may have an aspect ratio of 2.5 or more, that is, the magnitude of the pressure applied to the active material composite and the like may be adjusted so that the composite particles have the target aspect ratio.

[0062] Extraction method: Observe the cross-section of the active material layer, extract the composite particles contained in the cross-section in descending order of cross-sectional area, and end the extraction when the total area of the extracted composite particles exceeds 80% of the total area of all the composite particles contained in the cross-section.

[0063] 3. Secondary battery The secondary battery of the present disclosure has the negative electrode for a secondary battery of the present disclosure. FIG. 1 shows the configuration of a secondary battery 100 according to an embodiment. As shown in FIG. 1, the secondary battery 100 has a positive electrode 10, an electrolyte layer 20, and a negative electrode 30, and the negative electrode 30 is the negative electrode for a secondary battery of the present disclosure.

[0064] 3.1 Positive electrode In the secondary battery 100, the positive electrode 10 may adopt, for example, the following configuration. As shown in FIG. 1, the positive electrode 10 according to an embodiment may include a positive electrode active material layer 11 and a positive electrode current collector 12.

[0065] 3.1.1 Positive electrode active material layer The positive electrode active material layer 11 contains a positive electrode active material, and may further optionally contain an electrolyte, a conductive assistant, a binder, etc. Further, the positive electrode active material layer 11 may contain various other additives. The content of each of the positive electrode active material, electrolyte, conductive assistant, binder, etc. in the positive electrode active material layer 11 may be appropriately determined according to the intended battery performance. For example, taking the entire positive electrode active material layer 11 (total solid content) as 100% by mass, the content of the positive electrode active material may be 40% by mass or more, 50% by mass or more, or 60% by mass or more, and may also be less than 100% by mass or 90% by mass or less. The shape of the positive electrode active material layer 11 is not particularly limited, and may be, for example, a sheet-like shape having a substantially flat surface. The thickness of the positive electrode active material layer 11 is not particularly limited, and may be, for example, 0.1 μm or more, 1 μm or more, 10 μm or more, or 30 μm or more, and may also be 2 mm or less, 1 mm or less, 500 μm or less, or 100 μm or less.

[0066] As the positive electrode active material, those known as positive electrode active materials for secondary batteries may be used. Among the known active materials, a material that exhibits a potential (charge-discharge potential) for occluding and releasing a predetermined ion that is nobler than that of the negative electrode active material described later can be used as the positive electrode active material. For example, when constructing a lithium-ion secondary battery, various lithium-containing composite oxides such as lithium cobaltate, lithium nickelate, lithium manganate, lithium manganese nickel cobaltate, and spinel-based lithium compounds may be used as the positive electrode active material, or sulfur-based active materials such as elemental sulfur and sulfur compounds may be used. The positive electrode active material may be used alone as only one type, or two or more types may be combined and used. The positive electrode active material may be, for example, in the form of particles, and its size is not particularly limited. The particles of the positive electrode active material may be solid particles, hollow particles, or particles having voids. The particles of the positive electrode active material may be primary particles or secondary particles in which a plurality of primary particles are aggregated. The average particle diameter of the particles of the positive electrode active material may be, for example, 1 nm or more, 5 nm or more, or 10 nm or more, and may also be 500 μm or less, 100 μm or less, 50 μm or less, or 30 μm or less.

[0067] The surface of the positive electrode active material may be coated with a protective layer containing an ion-conductive oxide. That is, the positive electrode active material layer 11 may contain a composite including the above positive electrode active material and a protective layer provided on its surface. Thereby, the reaction between the positive electrode active material and a sulfide (for example, a sulfide solid electrolyte, etc.) is likely to be suppressed. When the secondary battery is a lithium-ion secondary battery, examples of the ion-conductive oxide for coating and protecting the surface of the positive electrode active material include Li3BO3, LiBO2, Li2CO3, LiAlO2, Li4SiO4, Li2SiO3, Li3PO4, Li2SO4, Li2TiO3, Li4Ti5O 12, such as Li2Ti2O5, Li2ZrO3, LiNbO3, Li2MoO4, and Li2WO4. The coverage rate (area ratio) of the protective layer may be, for example, 70% or more, 80% or more, or 90% or more. The thickness of the protective layer may be, for example, 0.1 nm or more or 1 nm or more, or 100 nm or less or 20 nm or less.

[0068] The electrolyte that can be included in the positive electrode active material layer 11 may be a solid electrolyte, a liquid electrolyte (electrolyte solution), or a combination thereof. The electrolyte may be the same as or different from that in the negative electrode for the secondary battery of the present disclosure. In particular, when the positive electrode contains a sulfide solid electrolyte, a higher effect can be expected. The conductive assistant and binder that can be included in the positive electrode active material layer 11 may also be the same as or different from those in the negative electrode for the secondary battery of the present disclosure.

[0069] 3.1.2 Positive Electrode Current Collector As shown in FIG. 1, the positive electrode 10 may include a positive electrode current collector 12 that contacts the above-described positive electrode active material layer 11. Any of the commonly used ones as the positive electrode current collector of the battery can be adopted for the positive electrode current collector 12. The positive electrode current collector may be in the form of a foil, a plate, a mesh, a punched metal, a foam, or the like. The positive electrode current collector may be a metal foil or a metal mesh. In particular, the metal foil is excellent in handleability and the like. The positive electrode current collector may be composed of a plurality of foils or sheets. Examples of the metal constituting the positive electrode current collector include Cu, Ni, Cr, Au, Pt, Ag, Al, Fe, Ti, Zn, Co, stainless steel, and the like. In particular, from the viewpoint of ensuring oxidation resistance, etc., the current collector may contain Al. The positive electrode current collector may have some coating layer on its surface for the purpose of adjusting the resistance or the like. Also, the positive electrode current collector may be one in which the above-mentioned metal is plated or vapor-deposited on a metal foil or a substrate. Further, when the positive electrode current collector is composed of a plurality of metal foils, there may be some layer between the plurality of metal foils. The thickness of the positive electrode current collector is not particularly limited. For example, it may be 0.1 μm or more or 1 μm or more, and may be 1 mm or less or 100 μm or less.

[0070] In addition to the above configuration, the positive electrode 10 may have a general configuration as a positive electrode of a secondary battery. For example, tabs, terminals, etc. The positive electrode 10 can be manufactured by applying a known method. For example, the positive electrode active material layer 11 can be easily formed by molding a positive electrode composite material containing the above various components in a dry or wet manner. The positive electrode active material layer 11 may be molded together with the positive electrode current collector 12, or may be molded separately from the positive electrode current collector 12.

[0071] 3.2 Electrolyte layer The electrolyte layer 20 contains at least an electrolyte. The electrolyte layer 20 may contain a solid electrolyte, and may further optionally contain a binder or the like. The contents of the solid electrolyte and the binder or the like in the electrolyte layer 20 are not particularly limited. Alternatively, the electrolyte layer 20 may contain an electrolytic solution, and may further have a separator or the like for holding the electrolytic solution and preventing contact between the positive electrode active material layer 11 and the negative electrode active material layer 31. The thickness of the electrolyte layer 20 is not particularly limited. For example, it may be 0.1 μm or more or 1 μm or more, and may be 2 mm or less or 1 mm or less.

[0072] As the electrolyte contained in the electrolyte layer 20, it may be appropriately selected from those exemplified as the electrolyte that can be contained in the negative electrode for the secondary battery of the present disclosure. In particular, the electrolyte layer 20 preferably contains a sulfide solid electrolyte. Also, regarding the binder that can be contained in the electrolyte layer 20, it may be appropriately selected from those exemplified as the binder that can be contained in the negative electrode for the secondary battery of the present disclosure. Each of the electrolyte and the binder may be used alone as only one kind, or two or more kinds may be used in combination.

[0073] When the electrolyte layer 20 contains a liquid component such as an electrolytic solution, the liquid component may be held in the gaps of the solid electrolyte or may be held by a separator. The separator may be a separator commonly used in secondary batteries. For example, those made of resins such as polyethylene (PE), polypropylene (PP), polyester, and polyamide can be mentioned. The separator may have a single-layer structure or a multi-layer structure. Examples of the multi-layer structure separator include a 2-layer structure separator of PE / PP, or a 3-layer structure separator of PP / PE / PP or PE / PP / PE. The separator may be made of a non-woven fabric such as a cellulose non-woven fabric, a resin non-woven fabric, or a glass fiber non-woven fabric.

[0074] 3.3 Negative Electrode The negative electrode 30 is the negative electrode for a secondary battery of the present disclosure described above. Details of the negative electrode for a secondary battery of the present disclosure are as already described.

[0075] 3.4 Supplementary The secondary battery may be an all-solid battery that substantially does not contain a liquid electrolyte, or may contain a liquid component together with a sulfide solid electrolyte. The secondary battery only needs to have at least each of the above-described configurations, and may have other members in addition thereto. The members described below are an example of other members that the secondary battery may have.

[0076] The secondary battery may be one in which each of the above-described configurations is housed inside an exterior body. More specifically, the portion excluding a tab or a terminal for taking out electric power from the secondary battery to the outside may be housed inside the exterior body. Any known exterior body for a battery can be adopted as the exterior body. For example, a laminate film may be used as the exterior body. Also, a plurality of secondary batteries may be electrically connected and optionally stacked to form a battery pack. In this case, the battery pack may be housed inside a known battery case. Examples of the shape of the secondary battery include a coin type, a laminate type, a cylindrical type, and a rectangular type.

[0077] In the secondary battery, each of the above-described configurations may be sealed with a resin. For example, at least the side surfaces (the surfaces along the lamination direction of each layer) of the positive electrode, the electrolyte layer, and the negative electrode may be sealed with a resin. Thereby, it becomes easier to suppress the entry of moisture into each layer. As the sealing resin, known curable resins or thermoplastic resins can be adopted.

[0078] The secondary battery may or may not have a restraint member for restraining each of the above-described configurations in the thickness direction (the direction along the lamination direction of each layer). By applying a restraint pressure by the restraint member, the internal resistance of the battery is likely to be reduced. The restraint pressure by the restraint member is not particularly limited.

[0079] The secondary battery can be manufactured by applying known methods. For example, it can be manufactured as follows. However, the manufacturing method of the secondary battery is not limited to the following method. For example, the secondary battery may be manufactured through dry forming such as pressure molding. (1) Prepare a negative electrode composite material that constitutes the negative electrode active material layer. The negative electrode composite material may be in the form of a slurry dispersed in a solvent. The solvent used in this case is not particularly limited, and various organic solvents can be used. The slurry is applied to the surface of the negative electrode current collector or the electrolyte layer using a doctor blade or the like, and then dried to form the negative electrode active material layer. (2) Prepare a positive electrode composite material that constitutes the positive electrode active material layer. The positive electrode composite material may be in the form of a slurry dispersed in a solvent. The solvent used in this case is not particularly limited, and various organic solvents can be used. The slurry is applied to the surface of the positive electrode current collector or the electrolyte layer using a doctor blade or the like, and then dried to form the positive electrode active material layer. (3) Sandwich the electrolyte layer between the negative electrode active material layer and the positive electrode active material layer to obtain a laminate having a negative electrode current collector, a negative electrode active material layer, an electrolyte layer, a positive electrode active material layer, and a positive electrode current collector in this order. Other members such as terminals are attached to the laminate as necessary. (4) The secondary battery is obtained by housing the laminate in a battery case and sealing it. When obtaining a battery containing an electrolytic solution, the electrolytic solution may be sealed together with the laminate.

[0080] 3.5 Effects As described above, since the thickness change of the negative electrode during charge and discharge of the secondary battery of the present disclosure is small, for example, it is difficult for the change in the restraint pressure due to the thickness change of the negative electrode to become large. In addition, since the resistance of the negative electrode is small, it is likely to have excellent charge and discharge performance. In particular, when the composite particles in the negative electrode have the above-described predetermined aspect ratio, or when the composite particles contain porous carbon as a binder, the resistance of the negative electrode becomes even smaller, and it is likely to have more excellent performance.

[0081] As described above, one embodiment of the negative electrode for a secondary battery of the present disclosure has been described. However, the negative electrode for a secondary battery of the present disclosure can be variously modified other than the above embodiments without departing from the gist thereof.

Examples

[0082] Hereinafter, while showing examples, the technology of the present disclosure will be described in more detail. However, the technology of the present disclosure is not limited to the following examples. In the following examples, the case where an all-solid-state battery containing no liquid component is constructed using the negative electrode for a secondary battery of the present disclosure will be illustrated. However, the presence of liquid is considered not to substantially affect the problem-solving mechanism and effects by the technology of the present disclosure. That is, the technology of the present disclosure is applicable also to secondary batteries containing a liquid component.

[0083] 1. Reference Example 1 Using the following procedure, porous silicon particles were produced, composite particles were produced using the porous silicon particles and a binder, a negative electrode active material composite was produced using the composite particles and a sulfide solid electrolyte, a negative electrode was produced using the negative electrode active material composite, and an all-solid-state battery was produced using the negative electrode.

[0084] 1.1 Production of nanoporous Si 0.65 g of Si particles (particle size 0.5 μm, manufactured by Kojundo Chemical Laboratory Co., Ltd.) and 0.60 g of Li metal (manufactured by Honjo Metal Co., Ltd.) were mixed in an agate mortar under an Ar atmosphere to obtain a LiSi precursor. In a glass reactor under an Ar atmosphere, 1.0 g of the LiSi precursor and 125 ml of a dispersion medium (1,3,5-trimethylbenzene, manufactured by Nacalai Tesque) were mixed using an ultrasonic homogenizer (UH-50, manufactured by SMT). The LiSi precursor dispersion obtained after mixing was cooled to 0 °C, and 125 ml of ethanol (manufactured by Nacalai Tesque) as a Li extraction solvent was added dropwise and reacted for 120 minutes. After the reaction, 50 ml of acetic acid (manufactured by Nacalai Tesque) was further added dropwise and reacted for 60 minutes. After the reaction, the liquid and solid reactants were separated by suction filtration. The obtained solid reactant was vacuum dried at 120 °C for 2 hours to recover porous silicon particles containing a plurality of pores with a diameter of 55 nm or less (nanoporous Si).

[0085] 1.2 Preparation of Composite Particles The above nanoporous Si (primary particles) and a PVDF-HFP-based binder (manufactured by Kuraray Co., Ltd.) were dispersed and partially dissolved in dimethyl carbonate (manufactured by Nacalai Tesque) so that the ratio of primary particles: binder was 100:13.3 (mass ratio) to obtain a slurry. This slurry was sprayed into a spray dryer under a nitrogen gas atmosphere at 140 °C and dried to obtain composite particles containing nanoporous Si and a binder.

[0086] 1.3 Synthesis of Sulfide Solid Electrolyte 0.550 g of Li2S (manufactured by Fluorochem), 0.887 g of P2S5 (manufactured by Aldrich), 0.285 g of LiI (manufactured by Nippo Chemical Industry), and 0.277 g of LiBr (manufactured by Kojundo Chemical Laboratory Co., Ltd.) were mixed in an agate mortar for 5 minutes. 4 g of n-heptane (dehydrated grade, manufactured by Kanto Chemical) was added to the obtained mixture, and the mixture was mechanically milled for 40 hours using a planetary ball mill to obtain a sulfide solid electrolyte.

[0087] 1.4 Preparation of Anode Composite Material 1.0 g of the above composite particles, 0.04 g of a conductive assistant (VGCF, manufactured by Showa Denko), 0.776 g of the above solid electrolyte, 0.02 g of a binder (PVdF, manufactured by Kreha), and 1.7 g of butyl butyrate (manufactured by Kishida Chemical) were mixed using an ultrasonic homogenizer (UH-50, manufactured by SMT) to obtain a negative electrode composite material.

[0088] 1.5 Preparation of the positive electrode composite material LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 LiNbO3 was used to perform surface treatment on LiNiCoMnO2 (manufactured by Nichia Chemical Industries) to obtain a positive electrode active material. 1.5 g of this positive electrode active material, 0.023 g of a conductive assistant (VGCF, manufactured by Showa Denko), 0.239 g of the above solid electrolyte, 0.011 g of a binder (PVdF, manufactured by Kreha), and 0.8 g of butyl butyrate (manufactured by Kishida Chemical) were mixed using an ultrasonic homogenizer (UH-50, manufactured by SMT) to obtain a positive electrode composite material.

[0089] 1.6 Preparation of the evaluation battery 1 cm 2 0.065 g of the above solid electrolyte was placed in a ceramic mold of 1 ton / cm² 2 and pressed to prepare a separator layer (solid electrolyte layer). 0.018 g of the above positive electrode composite material was placed on one side and pressed at 1 ton / cm² 2 to prepare a positive electrode active material layer. 0.0054 g of the above negative electrode composite material was placed on the side opposite to the positive electrode active material layer and pressed at 4 ton / cm² 2 to prepare a negative electrode active material layer. An aluminum foil as a positive electrode current collector was laminated on the above positive electrode active material layer, and a copper foil as a negative electrode current collector was laminated on the above negative electrode active material layer to prepare an evaluation battery.

[0090] 1.7 Calculation of the porosity and filling rate in the negative electrode active material layer Let the porosity of the negative electrode active material layer be A, the total volume obtained by dividing the weight of each material constituting the negative electrode active material layer by the true density of each material be x, and the volume obtained from the dimensions (area × thickness) of the negative electrode active material layer be y. The porosity A of the negative electrode active material layer was calculated by A(%) = (1 - x / y) × 100. The filling rate (100 - A(%)) in the negative electrode active material layer was calculated from the porosity A.

[0091] 1.8 Measurement of the amount of increase in the confinement pressure For the evaluation battery, after charging at 0.245 mA up to 4.55 V by CC / CV, discharge was performed at 0.245 mA down to 3.0 V by CC / CV. In the first charge, the confinement pressure of the battery was monitored, and the confinement pressure at 4.55 V was measured.

[0092] 1.9 Resistance measurement After charging at 0.3 mA up to 4.35 V by CC / CV, discharge was performed at 0.3 mA down to 2.5 V by CC / CV. This was repeated 5 times. Then, after adjusting the voltage to 3.7 V, the resistance value was obtained from the voltage drop when a current of 10 mA was passed for 5 seconds.

[0093] 2. Reference Example 2 Except that the above-mentioned nanoporous Si (primary particles) and a PVDF-HFP-based binder were dispersed and dissolved in dimethyl carbonate (Nacalai Tesque) so that the ratio (mass ratio) of primary particles: binder was 100:26.7 to obtain a slurry, Reference Composite particles were prepared in the same manner as in Example 1. Using the prepared composite particles, Reference An evaluation battery was prepared in the same manner as in Example 1, and the porosity and filling rate in the negative electrode active material layer were calculated, the amount of increase in the confinement pressure was measured, and the resistance measurement was performed.

[0094] 3. Example 3 The above nanoporous Si (primary particles) and a PVDF-HFP-based binder were dispersed and dissolved in dimethyl carbonate (Nacalai Tesque) so that the ratio of primary particles:binder was 100:40 (mass ratio) to obtain a slurry. The slurry was sprayed into a spray dryer under a nitrogen gas atmosphere at 140°C for drying. Further, the dried particles were heat-treated at 550°C for 1 hour in a nitrogen atmosphere to carbonize the PVDF-HFP-based binder, resulting in porous carbon. Thereby, composite particles containing nanoporous Si and porous carbon as a binder were obtained. Incidentally, the mass ratio of the porous carbon as a binder after the carbonization treatment was such that nanoporous Si:porous carbon = 100:12. Using the composite particles, Reference An evaluation battery was fabricated in the same manner as in Example 1, and the porosity and packing ratio in the negative electrode active material layer were calculated, the increase in the restraint pressure was measured, and the resistance was measured.

[0095] 4. Comparative Example 1 An evaluation battery was fabricated in the same manner as in Example 1, except that Si particles (particle size 0.5 μm, manufactured by Kojundo Chemical Laboratory Co., Ltd.) were used instead of the composite particles, and the porosity and packing ratio in the negative electrode active material layer were calculated, the increase in the restraint pressure was measured, and the resistance was measured. Reference An evaluation battery was fabricated in the same manner as in Example 1, except that Si particles (particle size 0.5 μm, manufactured by Kojundo Chemical Laboratory Co., Ltd.) were used instead of the composite particles, and the porosity and packing ratio in the negative electrode active material layer were calculated, the increase in the restraint pressure was measured, and the resistance was measured.

[0096] 5. Comparative Example 2 Composite particles were fabricated in the same manner as in Example 1, except that the Si particles of Comparative Example 1 and a PVDF-HFP-based binder were dispersed and dissolved in dimethyl carbonate (Nacalai Tesque) so that the ratio of primary particles:binder was 100:13.3 (mass ratio) to obtain a slurry. Using the fabricated composite particles, Reference An evaluation battery was fabricated in the same manner as in Example 1, and the porosity and packing ratio in the negative electrode active material layer were calculated, the increase in the restraint pressure was measured, and the resistance was measured. Reference An evaluation battery was fabricated in the same manner as in Example 1, and the porosity and packing ratio in the negative electrode active material layer were calculated, the increase in the restraint pressure was measured, and the resistance was measured.

[0097] 6. Comparative Example 3 An evaluation battery was fabricated in the same manner as in Example 1, except that the above nanoporous Si was used instead of the composite particles. ReferenceIn the same manner as in Example 1, an evaluation battery was fabricated, and the porosity and filling rate in the negative electrode active material layer were calculated, the increase amount of the restraint pressure was measured, and the resistance measurement was performed.

[0098] 7. Comparative Example 4 Except that the above-mentioned nanoporous Si (primary particles) and the PVDF-HFP-based binder were dispersed and dissolved in dimethyl carbonate (Nacalai Tesque) so that the ratio (mass ratio) of primary particles: binder was 100:40 to obtain a slurry, Reference Composite particles were fabricated in the same manner as in Example 1. Using the fabricated composite particles, Reference An evaluation battery was fabricated in the same manner as in Example 1, and the porosity and filling rate in the negative electrode active material layer were calculated, the increase amount of the restraint pressure was measured, and the resistance measurement was performed.

[0099] 8. Comparative Example 5 Except that the above-mentioned nanoporous Si (primary particles) and the PVDF-HFP-based binder were dispersed and dissolved in dimethyl carbonate (Nacalai Tesque) so that the ratio (mass ratio) of primary particles: binder was 100:100 to obtain a slurry, Reference Composite particles were fabricated in the same manner as in Example 1. Using the fabricated composite particles, Reference An evaluation battery was fabricated in the same manner as in Example 1, and the porosity and filling rate in the negative electrode active material layer were calculated, the increase amount of the restraint pressure was measured, and the resistance measurement was performed.

[0100] 9. Comparative Example 6 Except that the above-mentioned nanoporous Si (primary particles) and the PVDF-HFP-based binder were dispersed and dissolved in dimethyl carbonate (Nacalai Tesque) so that the ratio (mass ratio) of primary particles: binder was 100:200 to obtain a slurry, Reference Composite particles were fabricated in the same manner as in Example 1. Using the fabricated composite particles, Reference An evaluation battery was fabricated in the same manner as in Example 1, and the porosity and filling rate in the negative electrode active material layer were calculated, the increase amount of the restraint pressure was measured, and the resistance measurement was performed.

[0101] 10. Comparative Example 7 Except that the above-mentioned nanoporous Si (primary particles) and the PVDF-HFP-based binder were dispersed and dissolved in dimethyl carbonate (Nacalai Tesque) so that the ratio (mass ratio) of primary particles: binder was 100:300 to obtain a slurry, Reference Composite particles were produced in the same manner as in Example 1. Using the produced composite particles, Reference An evaluation battery was produced in the same manner as in Example 1, and the porosity and filling rate in the negative electrode active material layer were calculated, the increase in confinement pressure was measured, and the resistance was measured.

[0102] 11. Porosity and filling rate, increase in confinement pressure, and resistance value In Table 1 below, Reference Examples 1 to 2. Examples 3 and Comparative Examples 1 to 7 show the porosity and filling rate, increase in confinement pressure, and resistance value for each battery. In Table 1 below, the increase in confinement pressure in the battery according to Comparative Example 2 is set to 1.00, Reference Examples 1 to 2. Examples 3, the increase in confinement pressure in the batteries according to Comparative Examples 1, 3 to 7 was relatively evaluated. Also, in FIG. 2, Reference Examples 1 to 2. Examples 3 and Comparative Examples 1 to 7 show the relationship between the filling rate and the resistance of the battery. Further, in FIG. 3, Reference Examples 1 to 2. Examples 3 and Comparative Examples 1 to 7 show the relationship between the filling rate and the increase in confinement pressure of the battery.

[0103]

Table 1

[0104] From the results shown in Table 1, FIG. 2, and FIG. 3, (1) the composite particles as the active material contained in the active material layer contain a plurality of nanoporous Si (porous silicon particles) and a binder, and (2) the active material layer has a porosity of more than 15%. Both of these conditions are satisfied for Examples 1 to Reference Examples 1 to 2. ExamplesIt can be seen that 3 can significantly reduce the amount of increase in the confinement pressure (i.e., the change in the thickness of the negative electrode is kept small), and has a low resistance. When the porosity of the active material layer exceeds 15.0%, even when the active material expands, the increase in volume due to the expansion of the active material is alleviated by the voids, and it is considered that the change in the thickness of the negative electrode is kept small. Also, even when the porosity of the active material layer exceeds 15.0%, it is considered that the ion conduction path and the conductive path are secured and the resistance is reduced to a certain extent by the composite particle formation of the nanoporous Si together with the binder. In particular, the composite particles contain an organic component as a binder Reference In Examples 1 and 2, the amount of increase in the confinement pressure can be more significantly reduced, and in Example 3 where the composite particles contain porous carbon as a binder, the resistance can be more significantly reduced. On the other hand, in Comparative Examples 1 to 7 that do not satisfy at least one of the above (1) and (2), it is difficult to achieve both a reduction in the amount of increase in the confinement pressure and a reduction in the resistance. Note that in Comparative Examples 4 to 7, since the amount of the binder contained in the composite particles is excessively large, the amount of silicon functioning as the active material is relatively excessively small, and there is a risk that the volume energy density of the battery will decrease.

[0105] 12. Measurement of the aspect ratio of the composite particles Reference For Examples 1 and 2, the aspect ratio of the composite particles in the active material layer was measured. Specifically, the cross-section of the negative electrode was observed by SEM to obtain a cross-sectional image (Fig. 4A). In the observation field of view, it was assumed that 50 or more composite particles were observed. Image analysis was performed on the obtained cross-sectional image, and it was binarized into the part of the composite particles (black) and the part other than the composite particles (white) (Fig. 4B). In the cross-sectional image, the part of the composite particles and the other part were distinguished by elemental analysis or the like. In the binarized image, the part of the composite particles (black) was approximated by an ellipse (Fig. 4C). Here, ImageJ Fiji was used as the image analysis software for the elliptical approximation. For each of the elliptically approximated composite particles, its aspect ratio (major axis / minor axis) was measured.

[0106] On the other hand, in the binarized cross-sectional image, the composite particles included in the cross-section were extracted in descending order of cross-sectional area. Extraction was terminated when the total area of the extracted composite particles exceeded 80% of the total area of all the composite particles included in the cross-sectional image. Among the plurality of extracted composite particles, the ratio of those having an aspect ratio of 2.5 or more was specified.

[0107] As a result, Reference For Example 1, 57.1% (number ratio) of the plurality of composite particles extracted by the above method had an aspect ratio of 2.5 or more. Also, Reference For Example 2, 50.4% (number ratio) of the plurality of composite particles extracted by the above method had an aspect ratio of 2.5 or more. As confirmed by the present inventor, when pressing the active material composite material, the composite particles are deformed and the aspect ratio increases. In other words, Reference In Examples 1 and 2, by pressing the composite particles to have a predetermined aspect ratio, the contact resistance within the composite particles, the contact resistance between the composite particles, and the contact resistance between the composite particles and other materials can be reduced, and it is considered that the resistance of the entire negative electrode can be decreased. Reference From the results of Examples 1 and 2, when observing the cross-section of the active material layer, by having more than half of the plurality of composite particles extracted by the following extraction method have an aspect ratio of 2.5 or more, it is considered that the increase amount of the restraint pressure can be suppressed to a small level and the resistance can be further reduced.

[0108] Extraction method: Observe the cross-section of the active material layer, extract the composite particles included in the cross-section in descending order of cross-sectional area, and terminate the extraction when the total area of the extracted composite particles exceeds 80% of the total area of all the composite particles included in the cross-section.

Explanation of Signs

[0109] 10 Positive electrode 11 Positive electrode active material layer 12 Positive electrode current collector 20 Electrolyte layer 30 Negative electrode 31 Negative electrode active material layer 32 Negative electrode current collector 100 Secondary battery

Claims

1. A method for manufacturing a negative electrode for a secondary battery, comprising: obtaining a LiSi precursor containing Li and Si; removing Li from the LiSi precursor to obtain porous silicon particles; mixing a plurality of the porous silicon particles and an organic component to obtain an intermediate composite; carbonizing the organic component of the intermediate composite to obtain composite particles containing a plurality of the porous silicon particles and porous carbon as a binder; obtaining an active material composite material containing a sulfide solid electrolyte and the composite particles; and pressing the active material composite material to obtain an active material layer having a porosity of more than 15%. The manufacturing method comprising the above steps.

2. The method according to claim 1, further comprising deforming the composite particles by pressing the active material composite material, wherein when observing a cross-section of the active material layer after pressing, more than half of the plurality of the composite particles extracted by the following extraction method have an aspect ratio of 2.5 or more. Extraction method: Observe the cross-section of the active material layer, extract the composite particles contained in the cross-section in descending order of cross-sectional area, and terminate the extraction when the total area of the extracted composite particles exceeds 80% of the total area of all the composite particles contained in the cross-section. ​

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