Active material composite particles, secondary battery, and method for manufacturing active material composite particles

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 active material composite particles. [Background technology]

[0002] Patent Document 1 discloses a negative electrode active material that expands little with charging, consisting of nonwoven fabric-like particles containing Si fibers, having a predetermined average particle size (D50), and being amorphous. Patent Document 2 discloses a negative electrode active material with a small initial irreversible capacity, comprising 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] Japanese Patent Publication No. 2021-022554 [Patent Document 2] Japanese Patent Publication No. 2021-057216 [Overview of the project] [Problems that the invention aims to solve]

[0004] Conventional active materials containing Si have room for improvement, for example, in terms of their cycling properties under low constraint conditions. [Means for solving the problem]

[0005] This application discloses several embodiments as means for solving the above-mentioned problems. <Aspect 1> Active material composite particles comprising Si and resin, When the cross-section of the composite particle is observed, the area ratio of the resin in the surface layer of the composite particle is higher than the area ratio of the resin in the center of the composite particle. 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 Embodiment 1. <Embodiment 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 Embodiment 1 or 2. <Embodiment 4> The surface layer portion is made of the resin. The active material composite particles of any one of Embodiments 1 to 3. <Embodiment 5> The central portion contains a plurality of Si particles. The active material composite particles of any one of Embodiments 1 to 4. <Embodiment 6> The Si particles are porous. The active material composite particles of Embodiment 5. <Embodiment 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 Embodiment 5 or 6. <Embodiment 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 Embodiments 1 to 7. Secondary battery. <Embodiment 9> At least one of the positive electrode, the electrolyte layer, and the negative electrode contains a solid electrolyte. The secondary battery of Embodiment 8. <Embodiment 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. <Embodiment 11> Spray drying is used to form droplets of the slurry and to perform air-flow drying of the slurry droplets. Manufacturing method according to embodiment 10. [Effects of the Invention]

[0006] The active material composite particles of this disclosure exhibit excellent cycling characteristics under low constraint conditions. [Brief explanation of the drawing]

[0007] [Figure 1A] A schematic example of the cross-sectional shape of an active material composite particle is shown. [Figure 1B] This is a schematic diagram illustrating the "surface layer" and "central part" in the cross-section of an active material composite particle. [Figure 2] This shows a schematic example of a secondary battery configuration. [Figure 3] This shows an example of a manufacturing process for active material composite particles. [Figure 4A] This is an SEM image showing an example of the cross-sectional structure of the active material composite particles according to the example, before pressing. [Figure 4B] This shows the results of the analysis of the image related to Figure 4A. [Figure 5A] This is an SEM image showing an example of the cross-sectional structure of the active material composite particles after pressing, according to the example. [Figure 5B] This shows the results of the analysis of the image related to Figure 5A. [Figure 6] This shows the results of comparing the cycle characteristics of the examples and comparative examples under constraints. [Modes for carrying out the invention]

[0008] 1.Active material composite particles Figure 1A schematically shows the cross-sectional structure of an active material composite particle 1 according to one embodiment. The active material composite particle 1 contains Si and resin. When the cross-section of the composite particle 1 is observed, the area ratio of the resin in the surface layer 1x of the composite particle 1 is higher than the area ratio of the resin in the central part 1y of the composite particle 1.

[0009] 1.1 Surface and central part In this application, the "surface layer" and "center" of the active material composite particle are defined as follows. That is, when a boundary is observed between the resin-rich outermost layer and the layer inside it when a cross-section of the active material composite particle 1 is observed, the outermost layer outside the boundary is considered the surface layer 1x, and the inner layer inside the boundary is considered the center 1y. On the other hand, when a boundary is not observed between the surface layer 1x and the center 1y inside it when a cross-section of the active material composite particle 1 is observed, the surface layer 1x and the center 1y are distinguished as follows. Specifically, the surface layer 1x of the active material composite particle 1 is identified as follows. In other words, as shown in Figure 1B, a two-dimensional image of the cross-section of the active material composite particle is obtained by observing the cross-section of the active material composite particle with a scanning electron microscope (SEM) or transmission electron microscope (TEM), and when the area of ​​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 entire particle is a1+a2, the region X where a1 / (a1+a2) is 0.5 is considered to be the "surface layer of the active material composite particle". The part deeper than the "surface layer of the active material composite particle" identified in this way (the inner part) 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 resin in the surface layer 1x of the composite particle 1 is higher than the area ratio of resin in the central part 1y of the composite particle 1. For example, the active material composite particle 1 may have a resin-rich portion in the surface layer 1x, or the surface layer 1x may be made of resin; that is, the surface layer 1x may be a resin layer 2. In this way, the high ratio of resin in the surface layer 1x of the composite particle 1 means that even when Si expands due to charging, the overall volume change of the composite particle 1 is mitigated by the resin in the surface layer 1x, and the overall shape of the composite particle 1 is easily maintained. Also, because of the high ratio of resin in the surface layer 1x, even when Si expands due to charging, the resin in the surface layer 1x functions as a cushioning material, making it difficult for cracks or gaps to occur in the material surrounding the composite particle 1. Therefore, excellent cycle characteristics are easily exhibited under both high and low constraint pressure.

[0011] The ratio AR1 / AR2, which is the area ratio of resin occupying the surface layer 1x of the active material composite particle 1 and the area ratio of resin occupying the central part 1y of the composite particle 1, is not particularly limited. For example, the ratio AR1 / AR2 may be greater 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. There is no particular upper limit to the ratio AR1 / AR2, and AR2 may be 0%. Furthermore, the value of the area ratio AR1 of resin occupying the surface layer 1x of the composite particle 1 is not particularly limited. For example, when observing a cross-section of the composite particle 1, with the total area of ​​the surface layer 1x being 100 area%, the area ratio AR1 of resin contained in the surface layer 1x may be 70 area% or more and 100 area%, 80 area%, or 90 area%, or 100 area%, or 90 area%, or 100 area%, or Furthermore, the area ratio AR2 of the resin in the central part 1y of the composite particle 1 is not particularly limited. For example, when observing the cross-section of the composite particle 1, if the total area of ​​the central part 1y is taken as 100 area%, the area ratio AR2 of the resin contained in the central part 1y may be 0 area% to 30 area%, 0 area% to 20 area%, or 0 area% to 10 area%.

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

[0013] 1.1.2 Porosity When observing the cross-section of the active material composite particle 1, the porosity in the central part 1y of the composite particle 1 may be higher than the porosity in the surface part 1x of the composite particle 1. That is, the active material composite particle 1 may have voids 3 in the central part 1y. Also, the active material composite particle 1 may or may not have voids 3 in the surface part 1x. In this way, when the porosity in the central part 1y is higher than the porosity in the surface part 1x, even if Si expands due to charging, the expansion of Si can be absorbed by the voids, the volume change of the composite particle 1 as a whole is mitigated, and the shape of the composite particle 1 as a whole is more easily maintained. Furthermore, because the volume change of the composite particle 1 as a whole is mitigated, cracks and gaps are less likely to occur in the material surrounding the composite particle 1. Therefore, even better cycle characteristics are more easily exhibited under both high and low constraint pressure.

[0014] The ratio P1 / P2 of the porosity P1 in the surface layer 1x of the active material composite particle 1 to the porosity P2 in the central part 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 be 0 (P1=0%). Furthermore, the value of the porosity P1 in the surface layer 1x of the composite particle 1 is not particularly limited. For example, when observing a cross-section of composite particle 1, with the total area of ​​the surface layer 1x being 100 area%, the area ratio P1 of voids contained in the surface layer 1x may be 0 area% or more and 10 area%, 0 area% or more and 7 area%, 0 area% or more and 5 area%, 0 area% or more and 3 area%, or 0 area% or more and 1 area%. Furthermore, the void ratio P2 in the central part 1y of composite particle 1 is not particularly limited. For example, when observing a cross-section of composite particle 1, with the total area of ​​the central part 1y being 100 area%, the area ratio P2 of voids contained in the central part 1y may be greater than 0 area% and 50 area%, 10 area% or more and 50 area%, or 20 area% or more and 50 area%.

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

[0016] 1.1.3 Area fraction of Si When observing the cross-section of the active material composite particle 1, the area ratio of Si in the central part 1y of the composite particle 1 may be higher than the area ratio of Si in the surface part 1x of the composite particle 1. In other words, the active material composite particle 1 may have a Si-rich portion in the central part 1y. Furthermore, the central part 1y may contain multiple Si particles 4. In this way, because the area ratio of Si in the central part 1y is higher than the area ratio of Si in the surface part 1x, even if Si expands due to charging, the expansion of Si is less likely to extend to the outside of the composite particle 1, the overall volume change of the composite particle 1 is mitigated, and the overall shape of the composite particle 1 is more easily maintained. In addition, because the overall volume change of the composite particle 1 is mitigated, cracks and gaps are less likely to occur in the material surrounding the composite particle 1. Therefore, even better cycle characteristics are more easily exhibited under both high and low constraint pressure.

[0017] The ratio AR3 / AR4, which is the area ratio of Si in the surface layer 1x of the active material composite particle 1 and the area ratio of Si in the central part 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 be 0 (AR3=0%). Furthermore, the value of the area ratio AR3 of Si in the surface layer 1x of the composite particle 1 is not particularly limited. For example, when observing a cross-section of the composite particle 1, if the total area of ​​the surface layer 1x is taken as 100 area%, the area ratio AR3 of Si contained in the surface layer 1x may be 0 area% or more and 30 area%, 0 area% or more and 20 area%, or 0 area% or more and 10 area%, or 0 area%, or 10 area%, or more. Furthermore, the area ratio AR4 of Si in the central part 1y of the composite particle 1 is not particularly limited. For example, when observing a cross-section of the composite particle 1, if the total area of ​​the central part 1y is taken as 100 area%, the area ratio AR4 of Si contained in the central part 1y may be 50 area% or more but less than 100 area%, 50 area% or more but 90 area%, or 50 area% or more but 80 area%, or 10

[0018] Furthermore, the area ratio AR3 of Si in the surface layer 1x of the active material composite particle 1 and the area ratio AR4 of Si in the central part 1y of the composite particle 1 can be determined by elemental analysis of the cross-section of the composite particle 1 using EDX or the like, and by identifying the region where Si exists in that cross-section.

[0019] 1.2 Si The active material composite particle 1 contains Si. As shown in Figure 1A, in the active material composite particle 1, Si may be included, 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 proportion of Si element in all elements contained in the Si particles may be, for example, 50 mol% or more, 70 mol% or more, or 90 mol% or more. In addition to Si element, the Si particles may contain other elements such as Li element. Other elements include Li element, as well as Sn element, Fe element, Co element, Ni element, Ti element, Cr element, B element, P element, etc. Furthermore, the Si particles may contain impurities such as oxides. The Si particles may be amorphous or crystalline. The crystalline phase contained in the Si particles is not particularly limited.

[0020] When the Si contained in the active material composite particle 1 is in particulate form, 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 10,000 or less, 1,000 or less, or 500 or less.

[0021] When the Si contained in the active material composite particle 1 is in particulate form, 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, or 10 μm or less, 5 μm or less, 3 μm or less, 2 μm or less, or 1 μm or less. 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, or 20 μm or less, 15 μm or less, or 10 μm or less. The average primary particle diameter and average secondary particle diameter can be determined by observation with an electron microscope such as an SEM, and can be determined, for example, as the average value of the maximum Ferret diameter of each of the multiple particles. The number of samples is preferably large, for example 20 or more, but may be 50 or more, or 100 or more. The average primary particle diameter and average secondary particle diameter can be adjusted as appropriate, for example, by appropriately changing the manufacturing conditions of the Si particles or by performing a classification process.

[0022] If the Si contained in the active material composite particle 1 is particulate, the Si particles may be porous. For example, if the central part 1y contains multiple Si particles, the porous nature of the Si particles can reduce the amount of Si expansion during charging. There are no particular restrictions on the form of voids in porous Si particles. Porous Si particles may also contain nanoporous silicon. Nanoporous silicon refers to silicon having multiple pores with pore diameters on the order of nanometers (less than 1000 nm, preferably 100 nm or less). Porous Si particles may also contain pores with a diameter of 55 nm or less. Pores with a diameter of 55 nm or less are difficult to crush even by pressing. That is, porous Si particles containing pores with a diameter of 55 nm or less tend to maintain their porous nature even after pressing. For example, per gram of porous Si particles, the amount of pores with a diameter of 55 nm or less may be 0.21 cc or more, 0.22 cc / g or more, or 0.23 cc / g or more, and may also be 0.30 cc / g or less, 0.28 cc / g or less, or 0.26 cc / g or less. The amount of pores with a diameter of 55 nm or less contained in porous Si particles can be determined, for example, from the pore size distribution by nitrogen gas adsorption method or DFT method.

[0023] If the active material composite particle 1 contains porous Si particles, 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 using a scanning electron microscope (SEM). A large number of samples is preferable, for example, 100 or more. The porosity can be the average value obtained from these samples.

[0024] 1.3 Resin The active material composite particle 1 contains a resin. This resin may function, for example, as a binder that binds Si particles together. The type of resin is not particularly limited. Various binders known as constituent materials for secondary batteries may be used as the resin. For example, it may be selected from butadiene rubber (BR) binders, butylene rubber (IIR) binders, acrylate butadiene rubber (ABR) binders, styrene butadiene rubber (SBR) binders, polyvinylidene fluoride (PVdF) binders, polytetrafluoroethylene (PTFE) binders, polyimide (PI) binders, carboxymethylcellulose (CMC) binders, polyacrylate binders, polyacrylic acid ester binders, etc. PVdF binders, in particular, have high performance. The PVdF binder may be a copolymer having units derived from monomers other than VdF. The resin may be used alone or in combination of two or more types.

[0025] When the active material composite particle 1 contains Si particles, the mass ratio of Si particles in the active material composite particle 1 is not particularly limited. For example, the active material composite particle 1 may contain 70% to 90% by mass of Si particles. The mass ratio of resin in the active material composite particle 1 is not particularly limited. For example, the active material composite particle 1 may contain 10% to 30% by mass of resin. Furthermore, the proportion of Si particles and resin contained in the active material composite particle 1 is not particularly limited as long as it is sufficient to form the composite particle 1. For example, the proportion of resin in the total of Si particles and resin may be 1% or more by mass, 5% or more by mass, or 8% or more by mass, and may be 30% or less by mass, 28% or less by mass, 26% or less by mass, 24% or less by mass, or 22% or less by mass. When the proportion of resin in the total of Si particles and resin is 1% or more by mass and 30% or less by mass, it is easier to secure a larger charge / discharge capacity.

[0026] 1.4 Other ingredients The active material composite particle 1 may consist only of the above-mentioned Si and resin (and voids), or it may contain other components. Examples of other components include various solid components and liquid components.

[0027] 1.5 Particle size of active material composite particles The active material composite particle 1 can be considered as a secondary particle formed by the aggregation of multiple Si particles 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 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 a SEM, and can be determined, for example, as the average value of the maximum Ferret diameter of multiple composite particles. The number of samples is preferably large, for example 20 or more, may be 50 or more, or may be 100 or more. Alternatively, the average particle diameter (D50, median diameter) of the composite particle 1, measured using a laser diffraction particle distribution analyzer, may be 100 nm or more, 1 μm or more, 2 μm or more, or 3 μm or more, and may 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 a surface layer 1x containing the resin and a central part 1y containing a plurality of Si particles and voids. Furthermore, the active material composite particle 1 may have a major axis and a minor axis in its state before being applied to a secondary battery, for example. 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, or 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 containing the active material composite particle 1. In this case, the composite particle 1 may be crushed in the pressing direction and have an aspect ratio greater than or equal to a predetermined value. When the composite particle 1 is pressed to the extent that it has an aspect ratio greater than or equal to a predetermined value, the contact resistance within the composite particle, the contact resistance between the composite particles, and the contact resistance between the composite particles and other materials are easily reduced. Specifically, from the viewpoint of further reducing the resistance of the negative electrode, as described later, when observing the cross-section of the negative electrode active material layer, more than half (more than 50% in terms of number) of the multiple composite particles 1 extracted by the extraction method described below may have an aspect ratio of 1.5 or higher.

[0029] Extraction method: The cross-section of the active material layer is observed, and the composite particles contained in the cross-section are extracted in descending order of cross-sectional area. Extraction is terminated when the total area of ​​the extracted composite particles exceeds 80% of the total area of ​​all composite particles contained in the cross-section.

[0030] Furthermore, the above extraction method may be performed by image analysis based on cross-sectional images of the active material layer obtained by SEM or the like. In the image analysis, the composite particles contained in the image may be approximated as ellipses, and the aspect ratio of each composite particle may be determined.

[0031] 2. Secondary battery Figure 2 schematically shows the configuration of a secondary battery 100 according to one embodiment. As shown in Figure 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 active material composite particles 1 described above. 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. Alternatively, the secondary battery 100 may contain a solid electrolyte in all three components: the positive electrode 10, the electrolyte layer 20, and the negative electrode 30. Furthermore, the secondary battery 100 may be a solid-state battery. A solid-state battery is one in which the electrolyte having carrier ion conductivity is mainly composed of a solid electrolyte. However, liquid components may be included at the additive level. Alternatively, the secondary battery 100 may be an all-solid-state battery that substantially does not contain liquid components.

[0032] 2.1 Positive electrode The positive electrode 10 only needs to be capable of functioning appropriately as the positive electrode of a secondary battery, and its configuration is not particularly limited. As shown in Figure 2, the positive electrode 10 may comprise a positive electrode active material layer 11 and a positive electrode current collector 12.

[0033] 2.1.1 Cathode active material layer The positive electrode active material layer 11 contains at least positive electrode active material and may optionally contain an electrolyte, conductive additive, binder, etc. The positive electrode active material layer 11 may also contain various other additives. The content of each component in the positive electrode active material layer 11 can be appropriately determined according to the desired battery performance. For example, with the entire positive electrode active material layer 11 (total solid content) as 100% by mass, the content of 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, or 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-like 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, or 2 mm or less, 1 mm or less, or 500 μm or less.

[0034] As the positive electrode active material, any known positive electrode active material for secondary batteries may be used. Among the known active materials, a material whose potential for intercalating and releasing a predetermined carrier ion (e.g., lithium ions) (charge / discharge potential) is relatively noble can be used as the positive electrode active material. The positive electrode active material may be at least one selected from, for example, various lithium-containing compounds, elemental sulfur and sulfur compounds. Lithium-containing compounds as positive electrode active materials include lithium cobaltate, lithium nickelate, and Li 1±α Ni 1 / 3 Co 1 / 3 Mn 1 / 3 O 2±δ Lithium manganate, spinel-type lithium compounds (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), etc., lithium titanate, lithium metal phosphate (such as LiMPO4, etc., M is one or more selected from Fe, Mn, Co, and Ni), and various lithium-containing oxides may be used. In particular, when the positive electrode active material contains a lithium-containing oxide containing at least Li, at least one of Ni, Co, and Mn, and O as constituent elements, a higher effect can be expected. The positive electrode active material may be used alone as one kind, or two or more kinds may be combined and used.

[0035] The shape of the positive electrode active material may be a general shape as the positive electrode active material of the battery. The positive electrode active material may be, for example, particulate. The positive electrode active material may be solid, hollow, have voids, or be porous. 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 D50 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. In addition, the average particle diameter D50 referred to in the present application is the particle diameter (median diameter) at the integrated value 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 positive electrode active material. Thereby, the reaction between the positive electrode active material and a sulfide (for example, the sulfide solid electrolyte described later) etc. is likely to be suppressed. Examples of the ion-conductive oxide include Li3BO3, LiBO2, Li2CO3, LiAlO2, Li4SiO4, Li2SiO3, Li3PO4, Li2SO4, Li2TiO3, Li4Ti5O 12Examples include Li2Ti2O5, Li2ZrO3, LiNbO3, Li2MoO4, and Li2WO4. Ion-conducting oxides may have some elements 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, 100 nm or less, or 20 nm or less.

[0037] The electrolyte contained in the positive electrode active material layer 11 may be a solid electrolyte, a liquid electrolyte (electrolyte solution), or a combination thereof. In particular, a higher effect is more likely to be obtained when the positive electrode active material layer 11 contains at least a solid electrolyte.

[0038] The solid electrolyte can be any known solid electrolyte used in secondary batteries. 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, and Li. 1+X Al X Ge 2-X Examples of oxide solid electrolytes include (PO4)3, Li-SiO glass, and Li-Al-SO glass; and 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, and Li2S-P2S5-GeS2. In particular, sulfide solid electrolytes, especially those containing at least Li, S, and P as constituent elements, exhibit high performance. The solid electrolyte may be amorphous or crystalline. The solid electrolyte may also be particulate, for example. 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 that known for the electrolytes of secondary batteries. For example, a solution of lithium salt dissolved at a predetermined concentration in a carbonate-based solvent can be used as the electrolyte. Examples of carbonate-based solvents include fluoroethylene carbonate (FEC), ethylene carbonate (EC), and dimethyl carbonate (DMC). Examples of lithium salts include LiPF6.

[0040] Examples of conductive additives that may be included in the positive electrode active material layer 11 include carbon materials such as vapor-processed carbon fiber (VGCF), acetylene black (AB), Ketjenblack (KB), carbon nanotubes (CNT), and carbon nanofibers (CNF); and metallic materials such as nickel, aluminum, and stainless steel. The conductive additive may be in the form of parts or fibers, and its size is not particularly limited. One type of conductive additive may be used alone, or two or more types may be used in combination.

[0041] Examples of binders that may be included in the positive electrode active material layer 11 include butadiene rubber (BR) binders, butylene rubber (IIR) binders, acrylate butadiene rubber (ABR) binders, styrene butadiene rubber (SBR) binders, polyvinylidene fluoride (PVdF) binders, polytetrafluoroethylene (PTFE) binders, and polyimide (PI) binders. A single binder may be used alone, or two or more binders may be used in combination.

[0042] 2.1.2 Positive electrode current collector As shown in Figure 2, the positive electrode 10 may include a positive electrode current collector 12 that contacts the positive electrode active material layer 11. The positive electrode current collector 12 can be any of the types commonly used as positive electrode current collectors for batteries. The positive electrode current collector 12 may be in the form of foil, plate, mesh, perforated metal, or foam. The positive electrode current collector 12 may be made of metal foil or metal mesh. Metal foil is particularly easy to handle. The positive electrode current collector 12 may consist of multiple foils. Examples of metals that make up the positive electrode current collector 12 include Cu, Ni, Cr, Au, Pt, Ag, Al, Fe, Ti, Zn, Co, stainless steel, etc. In particular, the positive electrode current collector 12 may contain Al from the viewpoint of ensuring oxidation resistance. The positive electrode current collector 12 may have some kind of coating layer on its surface for the purpose of adjusting resistance, etc. Furthermore, the positive electrode current collector 12 may be a metal foil or a substrate on which the above-mentioned metal is plated or deposited. Also, if the positive electrode current collector 12 consists of multiple metal foils, there may be some kind of layer between the multiple 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, or 1 mm or less or 100 μm or less.

[0043] 2.2 Electrolyte layer The electrolyte layer 20 is positioned 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 may optionally contain a binder or the like. The electrolyte layer 20 may also contain various additives. The content of each component in the electrolyte layer 20 is not particularly limited and can be appropriately determined according to the desired battery performance. The shape of the electrolyte layer 20 is not particularly limited and may, for example, be a sheet with a substantially flat surface. The thickness of the electrolyte layer 20 is not particularly limited and may be, for example, 0.1 μm or more or 1 μm or more, or 2 mm or less or 1 mm or less.

[0044] 2.2.1 Electrolytes The electrolyte included in the electrolyte layer 20 may be appropriately selected from among the examples of electrolytes that can be included in the positive electrode active material layer described above. In particular, electrolyte layers 20 containing solid electrolytes, especially sulfide solid electrolytes, and among those, sulfide solid electrolytes containing at least Li, S, and P as constituent elements, exhibit high performance. When the electrolyte is a solid electrolyte, it may be amorphous or crystalline. When the electrolyte is a solid electrolyte, it may be, for example, particulate. Only one type of electrolyte may be used alone, or two or more types may be used in combination.

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

[0046] 2.3 Negative electrode The negative electrode 30 only needs to contain the above-mentioned active material composite particles 1 and be capable of functioning appropriately as the negative electrode of a secondary battery, and its configuration is not particularly limited. As shown in Figure 2, the negative electrode 30 may comprise 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 active material composite particles 1, and may optionally contain other active materials, electrolytes, conductive additives, 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 can be appropriately determined according to the desired battery performance. For example, with the entire negative electrode active material layer 31 (total solid content) as 100% by mass, the content of 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, or 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-like 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, or 2 mm or less, 1 mm or less, or 500 μm or less.

[0048] The active material composite particles 1 are as described above. Furthermore, the active material composite particles 1 may be included in the negative electrode active material layer 31 in a pressed state. That is, when the active material composite particles 1 are applied 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 mixture containing the active material composite particles 1. In this case, the composite particles 1 may be crushed in the pressing direction and have an aspect ratio greater than or equal to a predetermined value. By pressing the composite particles 1 to the extent that they have an aspect ratio greater than or equal to a predetermined value, 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 easily reduced. An example of the aspect ratio of the composite particles 1 in this case is as described above.

[0049] Other than the active material composite particle 1, any negative electrode active material known as a negative electrode active material for secondary batteries may be used. Among the known active materials, a material whose charge / discharge potential (potential for intercalating and releasing a predetermined carrier ion (e.g., lithium ions) is relatively low can be used as the negative electrode active material. The other negative electrode active materials may be used individually or in combination of two or more types. From the viewpoint of further improving the performance of the secondary battery, it is preferable that the proportion of active material composite particle 1 in the total negative electrode active material contained in the negative electrode active material layer 31 be high. For example, if the total of active material composite particle 1 and other negative electrode active materials is 100% by mass, the active material composite particle 1 may be present in amounts of 50% by mass or more, 60% by mass or more and 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 may be included in the negative electrode active material layer 31 may be a solid electrolyte, a liquid electrolyte (electrolyte solution), or a combination thereof. In particular, a higher effect is more likely to be obtained when the negative electrode active material layer 31 contains at least a solid electrolyte. The negative electrode active material layer 31 may contain a solid electrolyte, especially a sulfide solid electrolyte, and moreover, a sulfide solid electrolyte containing Li, S, and P as constituent elements. Examples of conductive additives that may be included in the negative electrode active material layer 31 include the carbon materials and metal materials mentioned above. The binder that may be included in the negative electrode active material layer 31 may be appropriately selected from, for example, the binders exemplified above that may be included in the positive electrode active material layer 11.

[0051] 2.3.2 Negative electrode current collector As shown in Figure 2, the negative electrode 30 may include a negative electrode current collector 32 that contacts the negative electrode active material layer 31. Any type of negative electrode current collector commonly used in batteries can be used for the negative electrode current collector 32. The negative electrode current collector 32 may be in the form of foil, plate, mesh, perforated metal, or foam. The negative electrode current collector 32 may be a metal foil or metal mesh, or a carbon sheet. Metal foil is particularly advantageous in terms of handling. The negative electrode current collector 32 may consist of multiple foils or sheets. Examples of metals that make up the negative electrode current collector 32 include Cu, Ni, Cr, Au, Pt, Ag, Al, Fe, Ti, Zn, Co, stainless steel, etc. In particular, from the viewpoint of ensuring reduction resistance and being less prone to alloying 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 kind of coating layer on its surface for purposes such as adjusting resistance. Alternatively, the negative electrode current collector 32 may be a metal foil or a substrate on which the above-mentioned metal is plated or deposited. Furthermore, if the negative electrode current collector 32 consists of multiple metal foils, there may be some kind of layer between the multiple 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, 1 μm or more, 1 mm or less, or 100 μm or less.

[0052] 2.4 Other Configurations The secondary battery 100 may have all of the above components housed inside an outer casing. Any known battery casing can be used. Furthermore, multiple secondary batteries 100 may be electrically connected and stacked as desired to form a battery pack. In this case, the battery pack may be housed inside a known battery case. The secondary battery 100 may also have other obvious components such as necessary terminals. Examples of shapes for the secondary battery 100 include coin-type, laminate-type, cylindrical, and prismatic types.

[0053] 3. Manufacturing method of secondary batteries The secondary battery 100 can be manufactured by applying known methods. For example, it can be manufactured as follows. However, the manufacturing method of the secondary battery 100 is not limited to the following method, and each layer may be formed by, for example, dry molding. (1) A negative electrode slurry is obtained by dispersing the active material composite particles, etc., constituting the negative electrode active material layer in a solvent. 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 onto 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, thus forming the negative electrode. The negative electrode active material layer may be press-formed. (2) A positive electrode slurry is obtained by dispersing the positive electrode active material and other materials constituting the positive electrode active material layer in a solvent. 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 onto 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 forming the positive electrode. The positive electrode active material layer may be press-formed. (3) The layers are stacked 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 obtained, for example, by molding an electrolyte mixture containing an electrolyte and a binder, or by press molding. The laminate may also be press molded. Other members such as terminals are attached to the laminate as needed. When an electrolyte solution is used, a separator may be used in the electrolyte layer. (4) A 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, by the process shown in Figure 3. That is, as shown in Figure 3, the method for manufacturing the active material composite particles of the present disclosure includes: forming a slurry containing Si particles, a resin, and a solvent into droplets to obtain slurry droplets (step S1); and air-drying the slurry droplets 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 will be described later, the formation of the slurry into droplets and the air-drying of the slurry droplets may be performed by spray drying.

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

[0056] In step S1, the "slurry" refers to a suspension or aerosol containing Si particles, resin, and solvent, provided it has sufficient fluidity to form droplets. In step S1, the slurry may have sufficient fluidity to form droplets using, for example, a spray nozzle or rotary atomizer. In addition to the Si particles, resin, and solvent described above, the slurry may also contain some solid or liquid components.

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

[0058] When spraying 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, or 10 mm or less, or 1 mm or less. Furthermore, the slurry spraying speed (the rate at which slurry is supplied to the spray nozzle) is not particularly limited. The spraying speed may be adjusted according to the viscosity of the slurry, the solid content concentration, the nozzle dimensions, etc.

[0059] As for methods of forming droplets from a slurry, in addition to the method of spraying the slurry using a spray nozzle as described above, another example is a method in which a slurry containing Si particles, resin, and solvent is supplied at a constant speed onto a rotating disk and dropletized by centrifugal force. In this case as well, the supply speed of the slurry should be adjusted according to the viscosity and solid content concentration of the slurry. Alternatively, a method can be employed in which a high voltage is applied to the surface of the slurry containing Si particles, resin, and solvent to form droplets.

[0060] In the manufacturing method disclosed herein, for example, a spray dryer may be used to perform droplet formation of the slurry (step S1) and air-flow drying (step S2) described later. The type of spray dryer is not particularly limited and examples include a method using the spray nozzle described above, or a method using a rotating disc.

[0061] A "slurry droplet" is a particle of 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, or 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 it can be measured using a laser diffraction particle size analyzer. Alternatively, the droplet diameter can be estimated from the operating conditions of the apparatus that forms the slurry droplet.

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

[0063] In the manufacturing method of this disclosure, "airflow drying" means drying slurry droplets while they are suspended in a high-temperature airflow. "Airflow drying" may include not only drying but also incidental operations using dynamic airflow. By continuously applying hot air to the slurry droplets through airflow drying, a force is continuously applied to the slurry droplets. Taking advantage of this, for example, step S2 may include disintegrating (crushing) the slurry droplets or granules by airflow drying. In other words, in the method of this disclosure, even if granulation of slurry droplets occurs, the granules can be crushed by airflow drying. Therefore, slurries with low solid content can be used, and the processing speed can be easily increased. In step S2, the drying and crushing described above may be performed simultaneously or separately. In step S2, a first airflow drying in which drying of slurry droplets is prioritized and a second airflow drying in which crushing of granules is prioritized may be performed. Step S2 may also be repeated.

[0064] In step S2, the temperature of the heated gas should be such that the solvent can be evaporated 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 it 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 amount (flow rate) of heated gas supplied can be appropriately set considering the size of the equipment used, the amount of slurry droplets supplied, etc. For example, the flow rate of heated gas may be 0.10 m³. 3 / min or more, 0.15m 3 / min or more, 0.20m 3 / min or more, 0.25m 3 / min or more, 0.30m 3 / min or more, 0.35m 3 / min or more, 0.40m 3 / min or more, 0.45m 3 / min or more, or 0.50m 3It may be more than / min, and also 5.00m 3 / min or less, 4.00m 3 / min or less, 3.00m 3 / min or less, 2.00m 3 Less than / min, or 1.00m 3 It can be less than / minute.

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

[0067] In step S2, the processing time (drying time) with heated gas can be appropriately set considering the size of the equipment used, the amount of slurry droplets supplied, etc. For example, the processing time may be 5 seconds or less, or 1 second or less.

[0068] In step S2, a heated 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. In that case, the dew point may be -10°C or lower, -50°C or lower, or -70°C or lower.

[0069] For example, a spray dryer can be used as the device for air-flow drying, but it is not limited to this.

[0070] According to the above manufacturing method, for example, a slurry droplet containing a plurality of Si particles, a resin, and a solvent is air-dried, and as the solvent volatilizes, the resin can be concentrated on the surface of the composite particles, and a plurality of Si particles can be arranged in the center of the composite particles and voids can be formed. As a result, the active material composite particle 1 of this disclosure is obtained. That is, when the cross-section of the composite particle 1 is observed, the area ratio of the resin in the surface portion 1x of the composite particle 1 is higher than the area ratio of the resin in the center portion 1y of the composite particle 1. Also, when the cross-section of the composite particle 1 is observed, the void ratio in the center portion 1y of the composite particle 1 tends to be higher than the void ratio in the surface portion 1x of the composite particle 1, and the area ratio of the Si in the center portion 1y of the composite particle 1 tends to be higher than the area ratio of the Si in the surface portion 1x of the composite particle 1.

[0071] 5. Method for charging and discharging secondary batteries and method for improving the cycle characteristics of secondary batteries When the active material composite particles of this disclosure are included in the negative electrode of a secondary battery, the cycle characteristics of the secondary battery are easily improved. That is, the charging and discharging method of a secondary battery and the method for improving the cycle characteristics of a secondary battery of this disclosure include repeatedly charging and discharging the secondary battery, wherein the secondary battery has a positive electrode, an electrolyte layer, and a negative electrode, and the negative electrode contains the active material composite particles of this disclosure.

[0072] 6. Vehicles equipped with secondary batteries As described above, when the active material composite particles of this disclosure are included in the negative electrode of a secondary battery, an improvement in the cycle characteristics of the secondary battery can be expected. A secondary battery with such excellent charge-discharge cycle characteristics can be suitably used in, for example, at least one type of vehicle selected from hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and battery electric vehicles (BEVs). In other words, the technology of this disclosure also has an aspect as a vehicle having a secondary battery, wherein the secondary battery has a positive electrode, an electrolyte layer, and a negative electrode, and the negative electrode contains the active material composite particles of this disclosure. [Examples]

[0073] The technology of this disclosure will be described in more detail below with reference to examples, but the technology of this disclosure is not limited to the following examples.

[0074] 1. Preparation of the negative electrode active material 1.1 Comparative Example: Preparation of Nanoporous Si Particles A LiSi precursor was obtained by mixing 0.65 g of Si particles (particle size 0.5 μm, manufactured by Kojunsei Kagaku Co., Ltd.) and 0.60 g of Li metal (manufactured by Honjo Kinzoku) in an agate mortar under an Ar atmosphere. In a glass reactor under an Ar atmosphere, 1.0 g of the LiSi precursor and 125 ml of dispersion medium (1,3,5-trimethylbenzene, manufactured by Nacalai Tesque) were mixed using an ultrasonic homogenizer (UH-50, manufactured by SMT Co., Ltd.). The resulting LiSi precursor dispersion was cooled to 0°C, and 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 added dropwise, and the mixture was reacted for another 60 minutes. After the reaction, the liquid and solid reactants (negative electrode active material) were separated by suction filtration. The obtained solid reactants were vacuum-dried at 120°C for 2 hours to recover nanoporous Si particles.

[0075] 1.2 Examples: Preparation of active material composite particles Si particles (particle size 0.1 μm, manufactured by Kojunkagaku Co., Ltd.) and PVdF-HFP binder (manufactured by Kureha Corporation) were dispersed and dissolved in dimethyl carbonate (manufactured by Nacalai Tesque Corporation) in a mass ratio of 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 and air-dried while being converted into droplets. After air-drying, active material composite particles containing Si particles and PVdF-HFP binder as a resin were recovered.

[0076] 2. Preparation of solid electrolytes 0.550 g of Li2S (Furuuchi Chemical), 0.887 g of P2S5 (Aldrich), 0.285 g of LiI (Nippo Chemical), and 0.277 g of LiBr (High Purity Chemical) were mixed in an agate mortar for 5 minutes. 4 g of n-heptane (dehydrated grade, Kanto Chemical) was added to the resulting mixture, and the mixture was mechanically milled for 40 hours using a planetary ball mill to obtain a sulfide solid electrolyte.

[0077] 3. Preparation of the positive electrode composite material LiRing 1 / 3 Co 1 / 3 Mn 1 / 3 A positive electrode active material was obtained by surface-treating O2 (manufactured by Nichia Corporation) with LiNbO3. 1.5 g of this positive electrode active material, 0.023 g of a conductive additive (VGCF, manufactured by Showa Denko Corporation), 0.239 g of the above-mentioned sulfide solid electrolyte, 0.011 g of a binder (PVdF, manufactured by Kureha Corporation), and 0.8 g of butyl butyrate (manufactured by Kishida Chemical Co., Ltd.) were mixed using an ultrasonic homogenizer (UH-50, manufactured by SMT Corporation) to obtain a positive electrode composite material.

[0078] 4. Preparation of the negative electrode composite A negative electrode composite was obtained by mixing 1.0 g of the negative electrode active material according to the above comparative example or example, 0.04 g of a conductive additive (VGCF, manufactured by Showa Denko Corporation), 0.776 g of the above sulfide solid electrolyte, 0.02 g of a binder (PVdF, manufactured by Kureha Corporation), and 1.7 g of butyl butyrate (manufactured by Kishida Chemical Co., Ltd.) using an ultrasonic homogenizer (UH-50, manufactured by SMT Corporation).

[0079] 5. Preparation of evaluation batteries 1cm 2 Place 0.065g of the above sulfide solid electrolyte into the ceramic mold, and fill with 1 ton / cm³. 2 A solid electrolyte layer was fabricated by pressing. 0.018g of the above positive electrode composite material was placed on one side, and the pressure was increased to 1 ton / cm². 2 The positive electrode active material layer was fabricated by pressing. 0.0054g of the above negative electrode composite material was placed on the opposite side of the positive electrode active material layer at a rate of 4 tons / cm². 2The negative electrode active material layer was fabricated by pressing. Al foil was used as the positive electrode current collector, and Cu foil as the negative electrode current collector. This allowed for the fabrication of an evaluation battery (all-solid-state battery).

[0080] 6. Evaluation 6.1 Cross-sectional observation and image processing of active material composite particles The cross-sectional structure of the active material composite particles according to the example was observed using SEM before and after pressing, the elements contained in the cross-section of the composite particles were analyzed using EDX, and the cross-sectional structure of the composite particles was identified by image processing. Figure 4A is a cross-sectional SEM image of the composite particles before pressing, and Figure 4B is the result of image processing of Figure 4A. Figure 5A is a cross-sectional SEM image of the composite particles after pressing, and Figure 5B is the result of image processing of Figure 5A.

[0081] As shown in Figures 4A and 4B, the active material composite particles according to the example contain multiple nanoporous Si particles and a binder. When the cross-section is observed, a large amount of binder is present in the surface layer of the composite particle (black area in Figure 4B). On the other hand, the center of the composite particle contains almost no binder, and multiple nanoporous Si particles and voids are present (gray and white areas in Figure 4B). In other words, when the cross-section of the active material composite particles according to the example is observed, (I) the area ratio of resin in the surface layer of the composite particle is higher than the area ratio of resin in the center of the composite particle. Also, (II) the void ratio in the center of the composite particle is higher than the void ratio in the surface layer of the composite particle, and (III) the area ratio of Si in the center of the composite particle is higher than the area ratio of Si in the surface layer of the composite particle.

[0082] In the center of the images in Figures 5A and 5B, flattened active material composite particles are present. As shown in Figures 5A and 5B, the active material composite particles according to the example are crushed in the direction of pressing by the press, resulting in a larger aspect ratio. As is clear from Figure 5B, the flattened active material composite particles have an outermost layer (black part) made of binder, and further inside (in the center) from this outermost layer, there are voids (white parts), and multiple Si particles (gray parts) are present along with these voids. In other words, from Figures 5A and 5B, it can be seen that the active material composite particles according to the example maintain the structure described in (I) to (III) above even after pressing.

[0083] As shown in Figures 4A and 4B, and Figures 5A and 5B, the active material composite particles according to the example are clearly divided into a binder layer as the outermost layer and a central part inside the binder layer that contains multiple nanoporous Si particles and voids. Based on cross-sectional SEM images of 14 composite particles after pressing and 7 composite particles before pressing, the area ratio of the outermost layer to the total cross-section of the composite particle ([area of ​​the outermost layer] / [total cross-sectional area of ​​the composite particle]), the area ratio of Si contained in the central part to the total cross-section of the composite particle ([area of ​​Si in the central part] / [total cross-sectional area of ​​the composite particle]), and the area ratio of voids contained in the central part to the total cross-section of the composite particle ([area of ​​voids in the central part] / [total cross-sectional area of ​​the composite particle]) were determined. The results are shown in Table 1 below.

[0084] [Table 1]

[0085] The results shown in Table 1 indicate that, although the area ratios of the active material composite particles in the examples change significantly before and after pressing, the structures described in (I) to (III) above are maintained throughout the process.

[0086] 6.2 Durability testing of evaluation batteries For evaluation batteries in a low-constrained state (constraining pressure: 0.2 MPa) or high-constrained state (constraining pressure: 5.0 MPa), CC / CV charging was performed at 0.3 mA to 4.35 V, followed by CC / CV discharge at 0.3 mA to 2.5 V. This was repeated five times. Afterwards, the initial resistance values ​​were determined by the following DC-IR measurements.

[0087] DC-IR measurement: After adjusting the voltage to 3.7V, the resistance value was determined from the voltage drop when a current of 10mA was passed through for 5 seconds.

[0088] For the evaluation battery described above, a cycle of CC charging at 1.4mA to 4.1V followed by CC discharge at 1.4mA to 3.1V was repeated 500 times. Afterward, DC-IR measurements were performed as in the initial case to determine the resistance value after 500 cycles. The resistance increase rate was calculated using 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, the battery according to the comparative example showed a high resistance increase rate under both low and high constraint pressures, and in particular, a high resistance increase rate under low constraint pressure. In contrast, the battery according to the example was able to keep the resistance increase rate low under both low and high constraint pressures. This is presumed to be due to the following mechanism.

[0090] As described above, in the active material composite particles of the embodiment, the area ratio of resin in the surface layer is higher than the area ratio of resin in the center. This high proportion of resin in the surface layer of the composite particles is thought to mitigate the overall volume change of the composite particles even when Si expands due to charging, thus maintaining the overall shape of the composite particles. Furthermore, the high proportion of resin in the surface layer is thought to have allowed the resin to function as a cushioning material even when Si expands due to charging, preventing cracks and gaps from forming in the battery material surrounding the composite particles. As a result, excellent cycle characteristics are exhibited under both high and low constraint pressure conditions.

[0091] Alternatively, as described above, the porosity in the central part of the active material composite particles in the embodiment is higher than the porosity in the surface part. Because the porosity in the central part is higher than that in the surface part, even when Si expands due to charging, the expansion of the Si can be absorbed by the voids, mitigating the overall volume change of the composite particle 1 and maintaining its overall shape. Furthermore, because the overall volume change of the composite particle is mitigated, cracks and gaps are less likely to occur in the battery material surrounding the composite particle. As a result, excellent cycle characteristics are exhibited under both high and low confinement pressure conditions.

[0092] Alternatively, as described above, in the active material composite particles according to the example, the area ratio of Si in the center is higher than the area ratio of Si in the surface layer. In this way, because the area ratio of Si in the center is higher than that of Si in the surface layer, even if Si expands due to charging, the expansion of Si is less likely to extend to the outside of the composite particle, the overall volume change of the composite particle is mitigated, and the overall shape of the composite particle is maintained. Furthermore, because the overall volume change of the composite particle is mitigated, it is thought that cracks and gaps are less likely to occur in the battery material surrounding the composite particle. As a result, it is thought that excellent cycle characteristics were exhibited under both high and low constraint pressure.

[0093] In the above embodiments, examples were given in which nanoporous Si particles were used as the Si constituting the active material composite particles and a PVdF-based binder was used as the resin constituting the active material composite particles. However, the technology of this disclosure is not limited to these embodiments.

[0094] As described above, an active material composite particle containing Si and resin, in which, when the cross-section of the composite particle is observed, the area ratio of the resin in the surface layer of the composite particle is higher than the area ratio of the resin in the center of the composite particle, can be said to have excellent cycle characteristics under low constraints. [Explanation of symbols]

[0095] 1. Living matter composite particles 2 Resin layer 3 gaps 4 Si particles 10 Positive electrode 11 Positive active material layer 12 Positive current collector 20 Electrolyte layer 30 Negative electrode 31 Negative active material layer 32 Negative current collector 100 rechargeable batteries

Claims

1. Active material composite particles comprising a plurality of Si particles, resin, and voids, When the cross-section of the composite particle is observed, the area ratio of the resin in the surface layer of the composite particle is higher than the area ratio of the resin in the center of the composite particle, The central part includes the void, Active material composite particles.

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

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

Images