Active material composite particle, secondary battery, and method of manufacturing active material composite particle

Abstract

To provide an active material composite particle containing Si that is excellent in cycle characteristic under low restraint.SOLUTION: An active material composite particle contains Si and a resin. Here, in a case where a cross section of the composite particle is observed, an area rate of the resin that occupies a surface layer part of the composite particle is higher than an area rate of the resin that occupies a central part of the composite particle.SELECTED DRAWING: Figure 1A

Description

Technical Field

[0001] This application discloses active material composite particles, a secondary battery, and a method for manufacturing the active material composite particles.

Background Art

[0002] Patent Document 1 discloses a negative electrode active material having a small amount of expansion during charging, which is composed of non-woven particles containing Si fibers, has a predetermined average particle diameter (D50), and is amorphous. Further, Patent Document 2 discloses a negative electrode active material having a small initial irreversible capacity, which has secondary particles of Si-based particles and a fluorinated layer formed on the surface of the secondary particles.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0004] Conventional active materials containing Si have room for improvement, for example, in cycle characteristics under low restraint.

Means for Solving the Problems

[0005] As means for solving the above problems, this application discloses the following multiple aspects. <Aspect 1> Active material composite particles, comprising Si and a resin, when observing a cross-section of the composite particles, the area ratio of the resin in the surface layer portion of the composite particles is higher than the area ratio of the resin in the central portion of the composite particles, Active material composite particles. <Aspect 2> When observing the cross-section of the composite particles, the porosity in the central portion of the composite particles is higher than the porosity in the surface layer portion of the composite particles. The active material composite particles of Aspect 1. <Aspect 3> When observing the cross-section of the composite particles, the area ratio of Si in the central portion of the composite particles is higher than the area ratio of Si in the surface layer portion of the composite particles. The active material composite particles of Aspect 1 or 2. <Aspect 4> The surface layer portion is made of the resin. The active material composite particles of any one of Aspects 1 to 3. <Aspect 5> The central portion contains a plurality of Si particles. The active material composite particles of any one of Aspects 1 to 4. <Aspect 6> The Si particles are porous. The active material composite particles of Aspect 5. <Aspect 7> The composite particles have a surface layer portion containing the resin and a central portion containing a plurality of the Si particles and voids. The active material composite particles of Aspect 5 or 6. <Aspect 8> A secondary battery having a positive electrode, an electrolyte layer, and a negative electrode. The negative electrode contains the active material composite particles of any one of Aspects 1 to 7. Secondary battery. <Aspect 9> At least one of the positive electrode, the electrolyte layer, and the negative electrode contains a solid electrolyte. The secondary battery of Aspect 8. <Aspect 10> Atomizing a slurry containing Si particles, a resin, and a solvent to obtain slurry droplets, and Airflow-drying the slurry droplets in a heated gas to obtain an active material composite containing Si particles and a resin. A method for manufacturing active material composite particles, including the above steps. <Aspect 11> By spray drying, atomization of the slurry and airflow drying of the slurry droplets are performed. The manufacturing method of Mode 10.

Effects of the Invention

[0006] The active material composite particles of the present disclosure are excellent in cycle characteristics under low restraint.

Brief Description of the Drawings

[0007]

Figure 1A

Figure 1B

Figure 2

Figure 3

Figure 4A

Figure 4B

Figure 5A

Figure 5B

Figure 6

Modes for Carrying Out the Invention

[0008] 1. Active material composite particles Fig. 1A schematically shows the cross-sectional structure of active material composite particles 1 according to one embodiment. The active material composite particles 1 contain Si and resin. Here, when observing the cross section of the composite particles 1, the area ratio of the resin in the surface layer part 1x of the composite particles 1 is higher than the area ratio of the resin in the central part 1y of the composite particles 1.

[0009] 1.1 Surface part and central part In the present application, the "surface part" and "central part" of the active material composite particles are defined as follows. That is, when observing the cross-section of the active material composite particle 1, if a boundary is observed between the outermost resin-rich layer and the inner layer, the outermost layer outside the boundary is regarded as the surface part 1x, and the inner layer inside the boundary is regarded as the central part 1y. On the other hand, when observing the cross-section of the active material composite particle 1, if no boundary is observed between the surface part 1x and the central part 1y inside it, the surface part 1x and the central part 1y shall be distinguished as follows. Specifically, the surface part 1x of the active material composite particle 1 is specified as follows. That is, as shown in Fig. 1B, the cross-section of the active material composite particle is observed with a scanning electron microscope (SEM), a transmission electron microscope (TEM), etc. to obtain a two-dimensional image of the cross-section of the active material composite particle. When the area of the region X from the surface of the active material composite particle to a predetermined depth in the two-dimensional image is a1 and the area of the whole particle is a1 + a2, the region X where a1 / (a1 + a2) = 0.5 is regarded as the "surface part of the active material composite particle". The part deeper (inner part) than the "surface part of the active material composite particle" specified in this way can be the "central part of the active material composite particle".

[0010] 1.1.1 Area ratio of resin When observing the cross-section of the active material composite particle 1, the area ratio of the resin in the surface layer portion 1x of the composite particle 1 is higher than the area ratio of the resin in the central portion 1y of the composite particle 1. For example, the active material composite particle 1 may have a resin-rich portion in the surface layer portion 1x, or the surface layer portion 1x may be made of resin, that is, the surface layer portion 1x may be a resin layer 2. Thus, due to the high ratio of resin in the surface layer portion 1x of the composite particle 1, even when Si expands due to charging, the volume change of the entire composite particle 1 is alleviated by the resin in the surface layer portion 1x, and the shape of the entire composite particle 1 is easily maintained. Also, due to the high ratio of resin in the surface layer portion 1x, even when Si expands due to charging, the resin in the surface layer portion 1x functions as a cushioning material, and cracks and gaps are less likely to occur in the materials around the composite particle 1. Therefore, excellent cycle characteristics are likely to be exhibited under both high confinement pressure and low confinement.

[0011] The ratio AR1 / AR2 of the area ratio AR1 of the resin in the surface layer portion 1x of the active material composite particle 1 to the area ratio AR2 of the resin in the central portion 1y of the composite particle 1 is not particularly limited. For example, the ratio AR1 / AR2 may be more than 1.0, 1.5 or more, 2.0 or more, 2.5 or more, 3.0 or more, 3.5 or more, 4.0 or more, 4.5 or more, or 5.0 or more. The upper limit of the ratio AR1 / AR2 is not particularly limited, and AR2 may be 0%. Also, the value of the area ratio AR1 of the resin in the surface layer portion 1x of the composite particle 1 is not particularly limited. For example, when observing the cross-section of the composite particle 1, with the total area of the surface layer portion 1x being 100 area%, the area ratio AR1 of the resin contained in the surface layer portion 1x may be 70 area% or more and 100 area% or less, 80 area% or more and 100 area% or less, or 90 area% or more and 100 area% or less. Further, the area ratio AR2 of the resin in the central portion 1y of the composite particle 1 is not particularly limited. For example, when observing the cross-section of the composite particle 1, with the total area of the central portion 1y being 100 area%, the area ratio AR2 of the resin contained in the central portion 1y may be 0 area% or more and 30 area% or less, 0 area% or more and 20 area% or less, or 0 area% or more and 10 area% or less.

[0012] Furthermore, the area ratio AR1 of the resin in the surface layer portion 1x of the active material composite particle 1 and the area ratio AR2 of the resin in the central portion 1y of the composite particle 1 can be obtained by performing elemental analysis on the cross-section of the composite particle 1 using EDX or the like and identifying the region where the resin exists in the cross-section.

[0013] 1.1.2 Porosity When observing the cross-section of the active material composite particle 1, the porosity in the central portion 1y of the composite particle 1 may be higher than the porosity in the surface layer portion 1x of the composite particle 1. That is, the active material composite particle 1 may have voids 3 in the central portion 1y. Also, the active material composite particle 1 may or may not have voids 3 in the surface layer portion 1x. In this way, when Si expands due to charging, even if the porosity in the central portion 1y is higher than the porosity in the surface layer portion 1x, the expansion of the Si can be absorbed by the voids, the volume change of the entire composite particle 1 is alleviated, and the shape of the entire composite particle 1 is easily maintained. Further, since the volume change of the entire composite particle 1 is alleviated, cracks and gaps are less likely to occur in the material around the composite particle 1. Therefore, excellent cycle characteristics are more likely to be exhibited under both high and low confinement pressures.

[0014] The ratio P1 / P2 of the porosity P1 in the surface layer portion 1x of the active material composite particle 1 to the porosity P2 in the central portion 1y of the composite particle 1 is not particularly limited. For example, the ratio P1 / P2 may be less than 1.0, 0.9 or less, 0.8 or less, 0.7 or less, 0.6 or less, 0.5 or less, 0.4 or less, 0.3 or less, 0.2 or less, or 0.1 or less. The lower limit of the ratio P1 / P2 is not particularly limited and may even be 0 (P1 = 0%). Also, the value of the porosity P1 in the surface layer portion 1x of the composite particle 1 is not particularly limited. For example, when observing the cross-section of the composite particle 1, with the total area of the surface layer portion 1x being 100 area%, the area ratio P1 of the voids contained in the surface layer portion 1x may be 0 area% or more and 10 area% or less, 0 area% or more and 7 area% or less, 0 area% or more and 5 area% or less, 0 area% or more and 3 area% or less, or 0 area% or more and 1 area% or less. Further, the porosity P2 in the central portion 1y of the composite particle 1 is not particularly limited. For example, when observing the cross-section of the composite particle 1, with the total area of the central portion 1y being 100 area%, the area ratio P2 of the voids contained in the central portion 1y may be more than 0 area% and 50 area% or less, 10 area% or more and 50 area% or less, or 20 area% or more and 50 area% or less.

[0015] Incidentally, the porosity P1 in the surface layer portion 1x of the active material composite particle 1 and the porosity P2 in the central portion 1y of the composite particle 1 can be determined by performing elemental analysis on the cross-section of the composite particle 1 by means of EDX or the like and identifying the regions where voids exist in the cross-section.

[0016] 1.1.3 Area Ratio of Si When observing the cross-section of the active material composite particle 1, the area ratio of Si in the central portion 1y of the composite particle 1 may be higher than the area ratio of Si in the surface layer portion 1x of the composite particle 1. That is, the active material composite particle 1 may have an Si-rich portion in the central portion 1y. Further, the central portion 1y may contain a plurality of Si particles 4. Thus, when Si expands due to charging, since the area ratio of Si in the central portion 1y is higher than the area ratio of Si in the surface layer portion 1x, it is difficult for the expansion of the Si to reach the outside of the composite particle 1, the volume change of the entire composite particle 1 is alleviated, and the shape of the entire composite particle 1 is easily maintained. Also, since the volume change of the entire composite particle 1 is alleviated, cracks and gaps are less likely to occur in the material around the composite particle 1. Therefore, excellent cycle characteristics are more likely to be exhibited under both high confinement pressure and low confinement.

[0017] The ratio AR3 / AR4 of the area ratio AR3 of Si in the surface layer portion 1x of the active material composite particle 1 to the area ratio AR4 of Si in the central portion 1y of the composite particle 1 is not particularly limited. For example, the ratio AR3 / AR4 may be less than 1.0, 0.9 or less, 0.8 or less, 0.7 or less, 0.6 or less, 0.5 or less, 0.4 or less, 0.3 or less, 0.2 or less, or 0.1 or less. The lower limit of the ratio AR3 / AR4 is not particularly limited and may even be 0 (AR3 = 0%). Also, the value of the area ratio AR3 of Si in the surface layer portion 1x of the composite particle 1 is not particularly limited. For example, when observing the cross-section of the composite particle 1, with the total area of the surface layer portion 1x being 100 area%, the area ratio AR3 of Si contained in the surface layer portion 1x may be 0 area% or more and 30 area% or less, 0 area% or more and 20 area% or less, or 0 area% or more and 10 area% or less. Further, the area ratio AR4 of Si in the central portion 1y of the composite particle 1 is not particularly limited. For example, when observing the cross-section of the composite particle 1, with the total area of the central portion 1y being 100 area%, the area ratio AR4 of Si contained in the central portion 1y may be 50 area% or more and less than 100 area%, 50 area% or more and 90 area% or less, or 50 area% or more and 80 area% or less.

[0018] The area ratio AR3 of Si in the surface layer portion 1x of the active material composite particle 1 and the area ratio AR4 of Si in the central portion 1y of the composite particle 1 can be determined by subjecting the cross-section of the composite particle 1 to elemental analysis by EDX or the like and identifying the region where Si is present in the cross-section.

[0019] 1.2 Si The active material composite particle 1 contains Si. As shown in FIG. 1A, in the active material composite particle 1, Si may be contained, for example, as particles. The Si particles may exist as primary particles or as secondary particles. The composition of the Si particles is not particularly limited. The ratio of the Si element to all the elements contained in the Si particles may be, for example, 50 mol% or more, 70 mol% or more, or 90 mol% or more. The Si particles may contain other elements such as Li element in addition to the Si element. Examples of the other elements include Sn element, Fe element, Co element, Ni element, Ti element, Cr element, B element, P element, etc. in addition to the Li element. Further, the Si particles may contain impurities such as oxides. The Si particles may be amorphous or crystalline. The crystal phase contained in the Si particles is not particularly limited.

[0020] When the Si contained in the active material composite particle 1 is in the form of particles, the number of Si particles contained in one active material composite particle 1 is not particularly limited. The number of Si particles may be 2 or more, 5 or more, 10 or more, 50 or more, or 100 or more, and may be 10000 or less, 1000 or less, or 500 or less.

[0021] When the Si contained in the active material composite particle 1 is in the form of particles, its size is not particularly limited. The average primary particle diameter of the Si particles may be, for example, 10 nm or more, 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. Also, the average secondary particle diameter of the Si 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. For example, it is determined 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, may be 50 or more, or may be 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 Si particles or performing a classification process.

[0022] When the Si contained in the active material composite particle 1 is in the form of particles, the Si particles may be porous. For example, when the central portion 1y contains a plurality of Si particles, the fact that the Si particles are porous can reduce the amount of expansion of Si during charging. There is no particular limitation on the form of the voids in the porous Si particles. The porous Si 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 Si particles may contain pores having a diameter of 55 nm or less. The pores having a diameter of 55 nm or less are difficult to be crushed by pressing. That is, the porous Si 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 Si 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 Si particles can be determined, for example, from the pore size distribution by the nitrogen gas adsorption method or the DFT method.

[0023] When the porous Si particles are included in the active material composite particles 1, the porous Si particles may have a predetermined porosity. The porosity of the porous Si 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 of the Si particles can be determined, for example, by observation with 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.

[0024] 1.3 Resin The active material composite particles 1 contain a resin. The resin can have, for example, a function as a binder that binds Si particles to each other. The type of the resin is not particularly limited. As the resin, various binders known as constituent materials of secondary batteries may be employed. For example, it may be selected from 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. In particular, the performance of the PVdF - based binder is high. The PVdF - based binder may be a copolymer having units derived from monomers other than VdF. Only one type of resin may be used alone, or two or more types may be used in combination.

[0025] When the active material composite particles 1 contain Si particles, the mass ratio of the Si particles in the active material composite particles 1 is not particularly limited. For example, the active material composite particles 1 may contain 70% by mass or more and 90% by mass or less of Si particles. The mass ratio of the resin in the active material composite particles 1 is not particularly limited. For example, the active material composite particles 1 may contain 10% by mass or more and 30% by mass or less of the resin. Furthermore, the ratio of the Si particles and the resin contained in the active material composite particles 1 is not particularly limited as long as the composite particles 1 can be formed. For example, the ratio of the resin in the total of the Si particles and the resin 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 resin in the total of the Si particles and the resin is 1% by mass or more and 30% by mass or less, it is easier to secure a larger charge-discharge capacity.

[0026] 1.4 Other Components The active material composite particles 1 may consist only of the above Si and resin (and voids), or may contain other components other than these. Examples of other components include various solid components and liquid components.

[0027] 1.5 Particle Diameter of Active Material Composite Particles The active material composite particle 1 can be regarded as a secondary particle in which a plurality of Si particles are aggregated via a resin. The average particle diameter of the composite particle 1 is not particularly limited. The average particle diameter of the composite particle 1 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 diameter of the composite particle 1 can be determined by observation with an electron microscope such as SEM. For example, it is 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, it may be 20 or more, 50 or more, or 100 or more. Alternatively, only the composite particle 1 is taken out, and the average particle diameter (D50, median diameter) of the composite particle 1 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.

[0028] 1.6 Structure and Shape of Active Material Composite Particles As described above, the active material composite particle 1 may have the surface layer portion 1x containing the resin and the central portion 1y containing the plurality of Si particles and voids. Further, the active material composite particle 1 may have a major axis and a minor axis, for example, in a state before being applied to a secondary battery. The ratio of the major axis to the minor axis (major axis / minor axis) may be, for example, 1.0 or more or 1.1 or more, and may also be 1.3 or less or 1.2 or less. On the other hand, as will be described later, when the active material composite particle 1 is applied to the negative electrode active material layer of a secondary battery, the negative electrode active material layer can be formed by pressing the negative electrode active material composite material containing the active material composite particle 1. At this time, the composite particle 1 can be crushed in the pressing direction and have an aspect ratio of a predetermined value or more. By pressing the composite particle 1 to have an aspect ratio of a predetermined value or more, the contact resistance within the composite particle, the contact resistance between the composite particles, and the contact resistance between the composite particle and other materials are likely to be reduced. Specifically, from the viewpoint that the resistance of the negative electrode described later becomes smaller, when observing the cross-section of the negative electrode active material layer, more than half (50% or more in terms of the number ratio) of the plurality of composite particles 1 extracted by the following extraction method may have an aspect ratio of 1.5 or more.

[0029] 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.

[0030] Note that 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 image analysis, the composite particles contained in the image may be approximated by an ellipse, and the aspect ratio of each composite particle may be specified.

[0031] 2. Secondary battery Fig. 2 schematically shows the configuration of a secondary battery 100 according to an embodiment. As shown in Fig. 2, the secondary battery 100 has a positive electrode 10, an electrolyte layer 20, and a negative electrode 30, and the negative electrode 30 contains the above-described active material composite particles 1. In the secondary battery 100, at least one of the positive electrode 10, the electrolyte layer 20, and the negative electrode 30 may contain a solid electrolyte. Also, in the secondary battery 100, all of the positive electrode 10, the electrolyte layer 20, and the negative electrode 30 may contain a solid electrolyte. Further, the secondary battery 100 may be a solid battery. A solid battery refers to one in which an electrolyte having carrier ion conductivity is mainly composed of a solid electrolyte. However, a liquid component may be contained at the additive level. Alternatively, the secondary battery 100 may be an all-solid battery that substantially does not contain a liquid component.

[0032] 2.1 Positive electrode The positive electrode 10 may be any one that can function properly as the positive electrode of a secondary battery, and its configuration is not particularly limited. As shown in Fig. 2, the positive electrode 10 may include a positive electrode active material layer 11 and a positive electrode current collector 12.

[0033] 2.1.1 Positive electrode active material layer The positive electrode active material layer 11 contains at least a positive electrode active material, and may further optionally contain an electrolyte, a conductive assistant, a binder, and the like. The positive electrode active material layer 11 may also contain various additives. The content of each component 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 (the entire 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, 60% by mass or more, or 70% by mass or more, and may also be 100% by mass or less or 90% by mass or less. The shape of the positive electrode active material layer 11 is not particularly limited, and for example, it may be a sheet-shaped positive electrode active material layer having a substantially flat surface. The thickness of the positive electrode active material layer 11 is not particularly limited, and for example, it may be 0.1 μm or more, 1 μm or more, or 10 μm or more, and may also be 2 mm or less, 1 mm or less, or 500 μm or less.

[0034] As the positive electrode active material, those known as the positive electrode active material of a secondary battery may be used. Among the known active materials, a material having a relatively noble potential (charge-discharge potential) for occluding and releasing a predetermined carrier ion (for example, lithium ion) can be used as the positive electrode active material. The positive electrode active material may be, for example, at least one selected from various lithium-containing compounds, elemental sulfur, sulfur compounds, and the like. The lithium-containing compound as the positive electrode active material is lithium cobaltate, lithium nickelate, Li 1±α Ni 1 / 3 Co 1 / 3 Mn 1 / 3 O 2±δ , lithium manganate, spinel-type lithium compound (Li 1+x Mn 2-x-y M yHetero-element substituted Li-Mn spinel represented by O4 (M is one or more selected from Al, Mg, Co, Fe, Ni, and Zn), lithium titanate, lithium metal phosphate (such as LiMPO4, where M is one or more selected from Fe, Mn, Co, and Ni), and various other lithium-containing oxides may be used. In particular, when the cathode active material contains a lithium-containing oxide that includes, as constituent elements, at least Li, at least one of Ni, Co, and Mn, and O, even higher effects can be expected. Only one type of cathode active material may be used alone, or two or more types may be combined and used.

[0035] The shape of the cathode active material may be a common shape for the cathode active material of a battery. For example, the cathode active material may be in the form of particles. The cathode active material may be solid, hollow, have voids, or be porous. The cathode active material may be primary particles or secondary particles formed by aggregation of a plurality of primary particles. The average particle diameter D50 of the cathode 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. Note that the average particle diameter D50 referred to in this application is the particle diameter (median diameter) at the integrated value of 50% in the volume-based particle size distribution determined by the laser diffraction / scattering method.

[0036] A protective layer containing an ion-conductive oxide may be formed on the surface of the cathode active material. This makes it easier to suppress the reaction between the cathode active material and a sulfide (for example, the sulfide solid electrolyte described later). Examples of the ion-conductive oxide include Li3BO3, LiBO2, Li2CO3, LiAlO2, Li4SiO4, Li2SiO3, Li3PO4, Li2SO4, Li2TiO3, Li4Ti5O 12, such as Li2Ti2O5, Li2ZrO3, LiNbO3, Li2MoO4, Li2WO4, etc. The ion-conductive oxide may be one in which some elements are substituted by doping elements such as P and B. The coverage rate (area ratio) of the protective layer on the surface of the positive electrode active material 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, and may be 100 nm or less or 20 nm or less.

[0037] 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. In particular, when the positive electrode active material layer 11 contains at least a solid electrolyte as the electrolyte, a higher effect is more easily obtained.

[0038] 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 have high ionic conductivity and excellent heat resistance. Examples of inorganic solid electrolytes include lithium lanthanum zirconate, LiPON, Li 1+X Al X Ge 2-X (PO4)3, oxide solid electrolytes such as Li-SiO-based glass, Li-Al-S-O-based glass; sulfide solid electrolytes such as 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 can be exemplified. In particular, sulfide solid electrolytes, especially those containing at least Li, S, and P as constituent elements, have high performance. The solid electrolyte may be amorphous or crystalline. The solid electrolyte may be, for example, particulate. Only one type of solid electrolyte may be used alone, or two or more types may be used in combination.

[0039] The electrolyte may contain a predetermined carrier ion (e.g., lithium ion). The electrolyte may be, for example, a non-aqueous electrolyte. The composition of the electrolyte may be the same as those known as the composition of the electrolyte of a secondary battery. For example, as the electrolyte, a solution in which a lithium salt is dissolved in a carbonate solvent at a predetermined concentration can be used. Examples of the carbonate solvent include fluoroethylene carbonate (FEC), ethylene carbonate (EC), dimethyl carbonate (DMC), and the like. Examples of the lithium salt include LiPF6 and the like.

[0040] Examples of the conductive assistant that may be included in the positive electrode active material layer 11 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 assistant may be, for example, particulate or fibrous, and its size is not particularly limited. Only one kind of conductive assistant may be used alone, or two or more kinds may be used in combination.

[0041] Examples of the binder that may be included in the positive electrode active material layer 11 include butadiene rubber (BR)-based binders, butylene 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, and the like. Only one kind of binder may be used alone, or two or more kinds may be used in combination.

[0042] 2.1.2 Positive Electrode Current Collector As shown in FIG. 2, the positive electrode 10 may include a positive electrode current collector 12 that contacts the above-described positive electrode active material layer 11. Any generally used material for the positive electrode current collector of a battery can be adopted as the positive electrode current collector 12. Further, the positive electrode current collector 12 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 12 may be composed of a metal foil or a metal mesh. In particular, a metal foil is excellent in handleability and the like. The positive electrode current collector 12 may be composed of a plurality of foils. Examples of the metal constituting the positive electrode current collector 12 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 positive electrode current collector 12 may contain Al. The positive electrode current collector 12 may have some coating layer on its surface for the purpose of adjusting resistance or the like. Further, the positive electrode current collector 12 may be a metal foil or a substrate on which the above metal is plated or vapor-deposited. Further, when the positive electrode current collector 12 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 12 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] 2.2 Electrolyte layer The electrolyte layer 20 is disposed between the positive electrode 10 and the negative electrode 30 and can function as a separator. The electrolyte layer 20 contains at least an electrolyte, and further may optionally contain a binder or the like. The electrolyte layer 20 may further contain various additives. The content of each component in the electrolyte layer 20 is not particularly limited and may be appropriately determined according to the intended battery performance. The shape of the electrolyte layer 20 is not particularly limited, and for example, it may be in the form of a sheet having a substantially flat surface. The thickness of the electrolyte layer 20 is not particularly limited, and 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.

[0044] 2.2.1 Electrolyte As the electrolyte contained in the electrolyte layer 20, it may be appropriately selected from those exemplified as the electrolytes that can be contained in the above-described positive electrode active material layer. In particular, the performance of the electrolyte layer 20 containing a solid electrolyte, among which a sulfide solid electrolyte, and among them, a sulfide solid electrolyte containing at least Li, S, and P as constituent elements, is high. When the electrolyte is a solid electrolyte, the solid electrolyte may be amorphous or crystalline. When the electrolyte is a solid electrolyte, the solid electrolyte may be, for example, in the form of particles. The electrolyte may be used alone or in combination of two or more kinds.

[0045] 2.2.2 Binder The binder that can be contained in the electrolyte layer 20 may be appropriately selected from those exemplified as the binders that can be contained in the above-described positive electrode active material layer, for example.

[0046] 2.3 Negative Electrode The negative electrode 30 may contain the above-described active material composite particles 1 and may be any that can function appropriately as the negative electrode of the secondary battery, and its configuration is not particularly limited. As shown in FIG. 2, the negative electrode 30 may include a negative electrode active material layer 31 and a negative electrode current collector 32.

[0047] 2.3.1 Negative Electrode Active Material Layer The negative electrode active material layer 31 contains at least the active material composite particles 1, and may further optionally contain other active materials, electrolytes, conductive aids, binders, etc. The negative electrode active material layer 31 may also contain various additives. The content of each component in the negative electrode active material layer 31 may be appropriately determined according to the target battery performance. For example, taking the entire negative electrode active material layer 31 (the entire solid content) as 100% by mass, the content of the active material composite particles 1 may be 40% by mass or more, 50% by mass or more, 60% by mass or more, or 70% by mass or more, and may also be 100% by mass or less or 90% by mass or less. The shape of the negative electrode active material layer 31 is not particularly limited, and for example, it may be a sheet-shaped negative electrode active material layer having a substantially flat surface. The thickness of the negative electrode active material layer 31 is not particularly limited, and for example, it may be 0.1 μm or more, 1 μm or more, or 10 μm or more, and may also be 2 mm or less, 1 mm or less, or 500 μm or less.

[0048] Regarding the active material composite particles 1, it is as described above. In addition, in the negative electrode active material layer 31, the active material composite particles 1 may be contained in a pressed state. That is, when applying the active material composite particles 1 to the negative electrode active material layer 31 of the secondary battery 100, the negative electrode active material layer 31 can be formed by pressing the negative electrode active material composite material containing the active material composite particles 1. At this time, the composite particles 1 can be crushed in the pressing direction and have an aspect ratio of a predetermined value or more. By pressing the composite particles 1 to have an aspect ratio of a predetermined value or more, the contact resistance within the composite particles 1, the contact resistance between the composite particles 1, and the contact resistance between the composite particles 1 and other materials are likely to be reduced. An example of the aspect ratio of the composite particles 1 in this case is as described above.

[0049] As the negative electrode active material other than the active material composite particles 1, those known as the negative electrode active material of a secondary battery may be used. Among the known active materials, a material having a relatively low potential (charge-discharge potential) for occluding and releasing a predetermined carrier ion (for example, lithium ion) can be used as the negative electrode active material. Only one kind of the other negative electrode active materials may be used alone, or two or more kinds may be used in combination. From the viewpoint of further enhancing the performance of the secondary battery, it is preferable that the proportion of the active material composite particles 1 in the total negative electrode active material contained in the negative electrode active material layer 31 is higher. For example, assuming that the total of the active material composite particles 1 and the other negative electrode active materials is 100% by mass, the active material composite particles 1 may be contained in an amount of 50% by mass or more, 60% by mass or more, 70% by mass or more, 80% by mass or more, 90% by mass or more, or 95% by mass or more. The upper limit is 100% by mass.

[0050] The electrolyte that can be contained in the negative electrode active material layer 31 may be a solid electrolyte, a liquid electrolyte (electrolyte solution), or a combination thereof. In particular, when the negative electrode active material layer 31 contains at least a solid electrolyte as the electrolyte, a higher effect is more easily obtained. The negative electrode active material layer 31 may contain a solid electrolyte, among which a sulfide solid electrolyte, and among them, a sulfide solid electrolyte containing Li, S, and P as constituent elements. Examples of the conductive auxiliary agent that can be contained in the negative electrode active material layer 31 include the above-described carbon material and the above-described metal material. The binder that can be contained in the negative electrode active material layer 31 may be appropriately selected, for example, from those exemplified as the binder that can be contained in the positive electrode active material layer 11 described above.

[0051] 2.3.2 Negative electrode current collector As shown in FIG. 2, the negative electrode 30 may include a negative electrode current collector 32 that contacts the above-described negative electrode active material layer 31. Any generally used material as the negative electrode current collector of the battery can be adopted for the negative electrode current collector 32. Further, the negative electrode current collector 32 may be in the form of a foil, a plate, a mesh, a punched metal, a foam, or the like. The negative electrode current collector 32 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 negative electrode current collector 32 may be composed of a plurality of foils or sheets. Examples of the metal constituting the negative electrode current collector 32 include Cu, Ni, Cr, Au, Pt, Ag, Al, Fe, Ti, Zn, Co, stainless steel, and the like. In particular, from the viewpoints of ensuring reduction resistance and being difficult to alloy with lithium, the negative electrode current collector 32 may contain at least one metal selected from Cu, Ni, and stainless steel. The negative electrode current collector 32 may have some coating layer on its surface for the purpose of adjusting resistance or the like. Further, the negative electrode current collector 32 may be a metal foil or a base material on which the above metal is plated or vapor-deposited. Further, when the negative electrode current collector 32 is composed of a plurality of metal foils, there may be some layer between the plurality of metal foils. The thickness of the negative electrode current collector 32 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.

[0052] 2.4 Other Configurations The secondary battery 100 may be one in which each of the above-described configurations is housed inside the exterior body. Any publicly known exterior body of a battery can be adopted for the exterior body. Further, a plurality of secondary batteries 100 may be arbitrarily electrically connected and arbitrarily stacked to form a battery pack. In this case, the battery pack may be housed inside a publicly known battery case. The secondary battery 100 may further include obvious configurations such as necessary terminals. Examples of the shape of the secondary battery 100 include a coin type, a laminate type, a cylindrical type, and a square type.

[0053] 3. Manufacturing Method of Secondary Battery The secondary battery 100 can be manufactured by applying a known method. For example, it can be manufactured as follows. However, the manufacturing method of the secondary battery 100 is not limited to the following method. For example, each layer may be formed by dry pressing or the like. (1) Dispersing active material composite particles or the like constituting the negative electrode active material layer in a solvent to obtain a negative electrode slurry. The solvent used in this case is not particularly limited, and water or various organic solvents can be used. Then, using a doctor blade or the like, the negative electrode slurry is coated on the surface of the negative electrode current collector or the electrolyte layer described later, and then dried to form a negative electrode active material layer on the surface of the negative electrode current collector or the electrolyte layer, thereby obtaining a negative electrode. Here, the negative electrode active material layer may be press-molded. (2) Dispersing a positive electrode active material or the like constituting the positive electrode active material layer in a solvent to obtain a positive electrode slurry. The solvent used in this case is not particularly limited, and water or various organic solvents can be used. Then, using a doctor blade or the like, the positive electrode slurry is coated on the surface of the positive electrode current collector or the electrolyte layer described later, and then dried to form a positive electrode active material layer on the surface of the positive electrode current collector or the electrolyte layer, thereby obtaining a positive electrode. Here, the positive electrode active material layer may be press-molded. (3) Stacking the layers so that the electrolyte layer is sandwiched between the negative electrode and the positive electrode 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. The electrolyte layer may be, for example, obtained by molding an electrolyte mixture containing an electrolyte and a binder, or may be obtained by press-molding. Here, the laminate may be further press-molded. Other members such as terminals are attached to the laminate as necessary. When an electrolytic solution is used, a separator may be employed for the electrolyte layer. (4) The secondary battery is obtained by housing the laminate in a battery case and sealing it.

[0054] 4. Method for manufacturing active material composite particles The active material composite particles of the present disclosure can be manufactured, for example, in the process shown in FIG. 3. That is, as shown in FIG. 3, the method for manufacturing the active material composite particles of the present disclosure includes atomizing a slurry containing Si particles, a resin, and a solvent to obtain slurry droplets (step S1), and subjecting the slurry droplets to airflow drying in a heated gas to obtain an active material composite containing Si particles and a resin (step S2). In the manufacturing method of the present disclosure, as described later, atomization of the slurry and airflow drying of the slurry droplets may be performed by spray drying.

[0055] 4.1 Step S1 In step S1, a slurry containing Si particles, a resin, and a solvent is atomized to obtain slurry droplets. The Si particles and the resin are as described above. The solvent may be any one that can disperse Si particles and dissolve the resin. For example, various organic solvents such as dimethyl carbonate can be employed.

[0056] The "slurry" in step S1 is a suspension or suspension liquid containing Si particles, a resin, and a solvent, and it may have fluidity to such an extent that it can be atomized. In step S1, the slurry may have fluidity to such an extent that it can be atomized using, for example, a spray nozzle or a rotary atomizer. Incidentally, the slurry may contain some solid components or liquid components in addition to the above-described Si particles, resin, and solvent.

[0057] The "atomization" of the slurry in step S1 means making the slurry containing Si particles, a resin, and a solvent into particles containing Si particles, a resin, and a solvent. In step S1, the method for atomizing the slurry containing Si particles, a resin, and a solvent is not particularly limited. For example, a method of atomizing the slurry by spraying can be mentioned. When spraying the slurry, a spray nozzle may be used. Examples of the method of spraying the slurry using a spray nozzle include, but are not limited to, a pressure nozzle method and a two-fluid nozzle method.

[0058] When spraying the slurry using a spray nozzle, the nozzle diameter is not particularly limited. The nozzle diameter may be, for example, 0.1 mm or more or 1 mm or more, and may also be 10 mm or less or 1 mm or less. Also, the spraying speed of the slurry (the supply speed of the slurry to the spray nozzle) is not particularly limited. The spraying speed may be adjusted according to the viscosity and solid content concentration of the slurry, the nozzle dimensions, etc.

[0059] As a method of atomizing the slurry, in addition to the method of spraying the slurry using a spray nozzle as described above, for example, a method of supplying a slurry containing Si particles, resin, and solvent onto a rotating disk at a constant speed and atomizing it by centrifugal force can also be exemplified. Also in this case, the supply speed of the slurry may be adjusted according to the viscosity and solid content concentration of the slurry, etc. Alternatively, a method of applying a high voltage to the surface of a slurry containing Si particles, resin, and solvent to atomize it can also be adopted.

[0060] In the manufacturing method of the present disclosure, for example, atomization of the slurry (step S1) and the subsequent airflow drying (step S2) may be performed by a spray dryer. The type of the spray dryer is not particularly limited, and examples include the method using the above spray nozzle and the method using a rotating disk.

[0061] A "slurry droplet" is a particle of a slurry containing Si particles, resin, and solvent. The size of the slurry droplet is not particularly limited. The diameter (equivalent spherical diameter) of the slurry droplet may be, for example, 0.5 μm or more or 5 μm or more, and may also be 5000 μm or less or 1000 μm or less. The diameter of the slurry droplet can be measured, for example, using a two-dimensional image obtained by imaging the slurry droplet, or can also be measured using a laser diffraction type particle size distribution analyzer. Alternatively, the droplet diameter can also be estimated from the operating conditions of the apparatus for forming the slurry droplet, etc.

[0062] 4.2 Step S2 In step S2, the slurry droplets are dried by air flow in a heated gas to obtain an active material composite containing Si particles and resin.

[0063] In the manufacturing method of the present disclosure, "air flow drying" means drying while suspending slurry droplets in a high-temperature air flow. "Air flow drying" may include not only drying but also accompanying operations by using a dynamic air flow. By continuously applying hot air to the slurry droplets by air flow drying, a force is continuously applied to the slurry droplets. Utilizing this, for example, step S2 may include disintegrating (crushing) the slurry droplets or granulated bodies by air flow drying. In other words, in the method of the present disclosure, even when granulation of the slurry droplets occurs, the granulated bodies can be disintegrated by air flow drying. Therefore, a slurry with a low solid content can also be used, and it is easy to increase the processing speed. In step S2, the above drying and disintegration may be performed simultaneously or separately. In step S2, a first air flow drying in which drying of the slurry droplets is dominant and a second air flow drying in which disintegration of the granulated bodies is dominant may be performed. Also, step S2 may be repeated.

[0064] In step S2, the temperature of the heated gas may be any temperature at which it is possible to volatilize the solvent from the slurry droplets. For example, it may be 100 °C or higher, 110 °C or higher, 120 °C or higher, or 130 °C or higher, and may also be 180 °C or lower, 170 °C or lower, 160 °C or lower, or 150 °C or lower.

[0065] In step S2, the supply amount (flow rate) of the heated gas can be appropriately set in consideration of the size of the apparatus used, the supply amount of the slurry droplets, etc. For example, the flow rate of the heated gas is 0.10 m 3 / min or more, 0.15 m 3 / min or more, 0.20 m 3 / min or more, 0.25 m 3 / min or more, 0.30 m 3 / min or more, 0.35 m 3 / min or more, 0.40 m 3 / min or more, 0.45 m 3 / min or more, or 0.50 m 3It may be above / minute, and also 5.00 m 3 / minute or less, 4.00 m 3 / minute or less, 3.00 m 3 / minute or less, 2.00 m 3 / minute or less, or 1.00 m 3 It may be / minute or less.

[0066] In step S2, the supply rate (flow rate) of the heating gas can also be appropriately set in consideration of the size of the apparatus used, the supply amount of the slurry droplets, etc. For example, the flow rate of the heating gas may be 1 m / second or more or 5 m / second or more in at least a part of the system, and may be 50 m / second or less or 10 m / second or less.

[0067] In step S2, the treatment time (drying time) by the heating gas can also be appropriately set in consideration of the size of the apparatus used, the supply amount of the slurry droplets, etc. For example, the treatment time may be 5 seconds or less, or 1 second or less.

[0068] In step S2, a heating gas that is substantially inert to the Si particles, resin, and solvent may be used. For example, an oxygen-containing gas such as air, an inert gas such as nitrogen or argon, or dry air with a low dew point can be used. The dew point in that case may be -10°C or less, -50°C or less, or -70°C or less.

[0069] As the apparatus for performing flash drying, for example, a spray dryer can be used, but it is not limited thereto.

[0070] According to the above manufacturing method, for example, slurry droplets containing a plurality of Si particles, a resin, and a solvent are dried by an air current, and the resin can be concentrated on the surface layer of the composite particles as the solvent volatilizes. Also, a plurality of Si particles can be arranged at the center of the composite particles to form voids. As a result, the active material composite particles 1 of the present disclosure are obtained. That is, when observing the cross-section of the composite particles 1, the area ratio of the resin in the surface layer portion 1x of the composite particles 1 is higher than the area ratio of the resin in the central portion 1y of the composite particles 1. Further, when observing the cross-section of the composite particles 1, the porosity in the central portion 1y of the composite particles 1 is likely to be higher than the porosity in the surface layer portion 1x of the composite particles 1, and the area ratio of the Si in the central portion 1y of the composite particles 1 is likely to be higher than the area ratio of the Si in the surface layer portion 1x of the composite particles 1.

[0071] 5. Method for Charging and Discharging a Secondary Battery and Method for Improving Cycle Characteristics of a Secondary Battery When the active material composite particles of the present disclosure are included in the negative electrode of a secondary battery, the cycle characteristics of the secondary battery are likely to be improved. That is, the method for charging and discharging a secondary battery and the method for improving cycle characteristics of a secondary battery of the present disclosure include repeating charging and discharging of the secondary battery, and the secondary battery has a positive electrode, an electrolyte layer, and a negative electrode, and the negative electrode is characterized by including the active material composite particles of the present disclosure.

[0072] 6. Vehicle Having a Secondary Battery As described above, when the active material composite particles of the present disclosure are included in the negative electrode of a secondary battery, improvement in the cycle characteristics of the secondary battery can be expected. A secondary battery having excellent charge and discharge cycle characteristics as such can be suitably used, for example, in at least one type of vehicle selected from a hybrid vehicle (HEV), a plug-in hybrid vehicle (PHEV), and an electric vehicle (BEV). That is, the technology of the present disclosure also has an aspect as a vehicle having a secondary battery, where the secondary battery has a positive electrode, an electrolyte layer, and a negative electrode, and the negative electrode includes the active material composite particles of the present disclosure.

Examples

[0073] Hereinafter, while showing examples, the technology of the present disclosure will be described in more detail, but the technology of the present disclosure is not limited to the following examples.

[0074] 1. Preparation of negative electrode active material 1.1 Comparative example: Preparation of nanoporous Si particles 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 Metals Co., Ltd.) were mixed using 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, 125 ml of ethanol (manufactured by Nacalai Tesque) was added dropwise, and the mixture was reacted for 120 minutes. After the reaction, 50 ml of acetic acid (manufactured by Nacalai Tesque) was further added dropwise, and the mixture was reacted for 60 minutes. After the reaction, the liquid and the solid reactant (negative electrode active material) were separated by suction filtration. The obtained solid reactant was vacuum dried at 120 °C for 2 hours to recover nanoporous Si particles.

[0075] 1.2 Example: Preparation of active material composite particles Si particles (particle size 0.1 μm, manufactured by Kojundo Chemical Laboratory Co., Ltd.) and a PVdF-HFP binder (manufactured by Kuraray Co., Ltd.) were dispersed and dissolved in dimethyl carbonate (manufactured by Nacalai Tesque) at a mass ratio such that Si particles: binder = 100:26.7 to obtain a slurry. This slurry was sprayed into a spray dryer under a nitrogen gas atmosphere at a temperature of 140 °C to form droplets, while performing air drying. After air drying, active material composite particles containing Si particles and the PVdF-HFP binder as a resin were recovered.

[0076] 2. Preparation of solid electrolyte 0.550 g of Li2S (manufactured by Furuchi Chemical), 0.887 g of P2S5 (manufactured by Aldrich), 0.285 g of LiI (manufactured by Nihon Kagaku), and 0.277 g of LiBr (manufactured by Kojundo Chemical) were mixed in an agate mortar for 5 minutes. To the obtained mixture, 4 g of n-heptane (dehydrated grade, manufactured by Kanto Chemical) was added, and mechanical milling was performed for 40 hours using a planetary ball mill to obtain a sulfide solid electrolyte.

[0077] 3. Preparation of the positive electrode composite material LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 The surface of O2 (manufactured by Nichia Chemical Industries, Ltd.) was treated using LiNbO3 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 sulfide solid electrolyte, 0.011 g of a binder (PVdF, manufactured by Kureha), 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.

[0078] 4. Preparation of the negative electrode composite material 1.0 g of the negative electrode active material according to the above comparative example or example, 0.04 g of a conductive assistant (VGCF, manufactured by Showa Denko), 0.776 g of the above sulfide solid electrolyte, 0.02 g of a binder (PVdF, manufactured by Kureha), 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.

[0079] 5. Preparation of the evaluation battery 1 cm 2 0.065 g of the above sulfide solid electrolyte was placed in a ceramic mold of, and pressed at 1 ton / cm 2 to prepare a solid electrolyte layer. 0.018 g of the above positive electrode composite material was placed on one side and pressed at 1 ton / cm 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 / cm 2The negative electrode active material layer was fabricated by pressing. An Al foil was used for the positive electrode current collector and a Cu foil was used for the negative electrode current collector. Thereby, an evaluation battery (all-solid-state battery) was fabricated.

[0080] 6. Evaluation 6.1 Cross-sectional Observation and Image Processing of Active Material Composite Particles Regarding the active material composite particles according to the examples, the cross-sectional structure before pressing and the cross-sectional structure after pressing were observed by SEM, the elements contained in the cross-section of the composite particles were analyzed by EDX, and the cross-sectional structure of the composite particles was specified by image processing. Fig. 4A is a cross-sectional SEM image of the composite particles before pressing, Fig. 4B is the result of image processing of Fig. 4A, Fig. 5A is a cross-sectional SEM image of the composite particles after pressing, and Fig. 5B is the result of image processing of Fig. 5A.

[0081] As shown in Figs. 4A and B, the active material composite particles according to the examples contain a plurality of nanoporous Si particles and a binder. When observing the cross-section thereof, a large amount of binder exists in the surface layer portion of the composite particles (the black part in Fig. 4B). On the other hand, it can be seen that almost no binder exists in the central portion of the composite particles, and a plurality of nanoporous Si particles and voids exist (the gray part and the white part in Fig. 4B). That is, when observing the cross-section of the active material composite particles according to the examples, (I) the area ratio of the resin occupying the surface layer portion of the composite particles was higher than the area ratio of the resin occupying the central portion of the composite particles. Also, (II) the porosity of the central portion of the composite particles was higher than the porosity of the surface layer portion of the composite particles, and (III) the area ratio of Si occupying the central portion of the composite particles was higher than the area ratio of Si occupying the surface layer portion of the composite particles.

[0082] In the central part of the images of FIGS. 5A and B, there are flat active material composite particles. As shown in FIGS. 5A and B, the active material composite particles according to the examples are crushed in the pressing direction by pressing, and the aspect ratio is large. As is clear from FIG. 5B, the flat active material composite particles have an outermost layer (black part) made of a binder, and further, there are voids (white part) inside (central part) the outermost layer, and it can be seen that a plurality of Si particles (gray part) exist together with the voids. That is, from FIGS. 5A and B, it can be seen that the active material composite particles according to the examples maintain the structures (I) to (III) even after pressing.

[0083] As shown in FIGS. 4A and B, and FIGS. 5A and B, the active material composite particles according to the examples are clearly divided into a binder layer as the outermost layer and a central part inside the binder layer, which has a plurality of nanoporous Si particles and voids. For 14 composite particles after pressing and 7 composite particles before pressing, based on the cross-sectional SEM images, the area ratio of the outermost layer in the entire cross-section of the composite particle ([area of the outermost layer] / [area of the entire cross-section of the composite particle]), the area ratio of Si contained in the central part in the entire cross-section of the composite particle ([area of Si in the central part] / [area of the entire cross-section of the composite particle]), and the area ratio of voids contained in the central part in the entire cross-section of the composite particle ([area of voids in the central part] / [area of the entire cross-section of the composite particle]) were specified. The results are shown in Table 1 below.

[0084]

Table 1

[0085] From the results shown in Table 1, it can be seen that although the area ratios of the active material composite particles according to the examples change significantly before and after pressing, the structures (I) to (III) are maintained throughout before and after pressing.

[0086] 6.2 Endurance Test of the Evaluation Battery For the evaluation battery in the low restraint state (restraint pressure: 0.2 MPa) or high restraint state (restraint pressure: 5.0 MPa), after CC / CV charging at 0.3 mA up to 4.35 V, CC / CV discharge was performed at 0.3 mA down to 2.5 V. This was repeated 5 times. Then, the following DC-IR measurement was carried out to obtain the initial resistance value.

[0087] DC-IR measurement: 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.

[0088] Regarding the above evaluation battery, further, after CC charging at 1.4 mA up to 4.1 V, CC discharge at 1.4 mA down to 3.1 V was repeated 500 cycles with one cycle being defined as such. Then, DC-IR measurement was carried out in the same manner as in the initial case to obtain the resistance value after 500 cycles. The resistance increase rate was obtained from the following formula. [Resistance increase rate] = ((Resistance value after 500 cycles) / (Initial resistance value) - 1)×100

[0089] The results are shown in Figure 6. As shown in Figure 6, for the battery according to the comparative example, the resistance increase rate was high under both low restraint pressure and high restraint pressure, and particularly high under low restraint pressure. In contrast, for the battery according to the example, the resistance increase rate could be suppressed low both under low restraint pressure and high restraint pressure. This is presumed to be due to the following mechanism.

[0090] As described above, for the active material composite particles according to the example, the area ratio of the resin in the surface layer part is higher than the area ratio of the resin in the central part. Thus, due to the high ratio of the resin in the surface layer part of the composite particles, even when Si expands due to charging, the volume change of the composite particles as a whole is relaxed by the resin in the surface layer part, and it is considered that the shape of the composite particles as a whole is maintained. Also, due to the high ratio of the resin in the surface layer part, even when Si expands due to charging, the resin in the surface layer part functions as a cushioning material, and it is considered that cracks and gaps were less likely to occur in the battery materials around the composite particles. Thereby, it is considered that excellent cycle characteristics were exhibited both under high restraint pressure and low restraint.

[0091] Alternatively, as described above, in the active material composite particles according to the embodiment, the porosity in the central portion is higher than the porosity in the surface layer portion. Thus, since the porosity in the central portion is higher than the porosity in the surface layer portion, even when Si expands due to charging, the expansion of the Si can be absorbed by the voids, and it is considered that the volume change of the entire composite particle 1 is alleviated and the shape of the entire composite particle 1 is maintained. Further, it is considered that since the volume change of the entire composite particle is alleviated, cracks and gaps are less likely to occur in the battery material around the composite particle. Thereby, it is considered that excellent cycle characteristics are exhibited under both high confinement pressure and low confinement.

[0092] Alternatively, as described above, in the active material composite particles according to the embodiment, the area ratio of Si in the central portion is higher than the area ratio of Si in the surface layer portion. Thus, since the area ratio of Si in the central portion is higher than the area ratio of Si in the surface layer portion, even when Si expands due to charging, the expansion of the Si hardly reaches the outside of the composite particle, and it is considered that the volume change of the entire composite particle is alleviated and the shape of the entire composite particle is maintained. Further, it is considered that since the volume change of the entire composite particle is alleviated, cracks and gaps are less likely to occur in the battery material around the composite particle. Thereby, it is considered that excellent cycle characteristics are exhibited under both high confinement pressure and low confinement.

[0093] In the above embodiment, the case where nanoporous Si particles are used as Si constituting the active material composite particles and a PVdF-based binder is used as the resin constituting the active material composite particles is exemplified, but the technology of the present disclosure is not limited to this form.

[0094] As described above, in the case of active material composite particles containing Si and resin, when observing the cross section of the composite particles, those in which the area ratio of the resin in the surface layer portion of the composite particles is higher than the area ratio of the resin in the central portion of the composite particles can be said to have excellent cycle characteristics under low confinement.

Explanation of Reference Numerals

[0095] 1 Active material composite particle 2 Resin layer 3 Void 4 Si particle 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. An active material composite particle comprising a plurality of Si particles, a resin, and voids, when observing a cross-section of the composite particle, an area ratio of the resin in a surface layer portion of the composite particle is higher than an area ratio of the resin in a central portion of the composite particle, and the central portion includes the voids, an active material composite particle.

2. The active material composite particle according to claim 1, wherein when observing a cross-section of the composite particle, an area ratio of the Si in the central portion of the composite particle is higher than an area ratio of the Si in the surface layer portion of the composite particle.

3. The active material composite particle according to claim 1, wherein the Si particles are porous.

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