Negative electrode active material and battery

Abstract

According to one embodiment of the present disclosure, an anode active substance contains porous silicon particles and a solid electrolyte, wherein the porous silicon particles have a plurality of pores, the solid electrolyte has a thin film shape and covers at least part of the inner surface of the pores, and the average thickness of said thin film is less than 30 nm.

Description

[Technical Field]

[0001] The present disclosure relates to a negative electrode active material and a battery. [Background technology]

[0002] Patent Document 1 discloses a negative electrode for a lithium ion secondary battery, which contains porous silicon particles having a three-dimensional network structure on at least one surface of a current collector.

[0003] Patent Document 2 discloses a negative electrode layer containing a Si-based active material on the surface of which a coating layer containing a solid electrolyte is formed. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2012-084522 A [Patent Document 2] Patent Publication No. 2020-021674 Summary of the Invention [Problem to be solved by the invention]

[0005] In the prior art, it is desirable to improve the charge-discharge cycling characteristics of batteries that use silicon as the active material. [Means for solving the problem]

[0006] In one embodiment of the present disclosure, the negative electrode active material is Porous silicon particles; a solid electrolyte; Including, the porous silicon particles have a plurality of pores; the solid electrolyte has the form of a thin film covering at least a part of the inner surface of the pore; The average thickness of the thin film is less than 30 nm. [Effects of the Invention]

[0007] According to the present disclosure, the charge-discharge cycle characteristics of a battery using silicon as an active material can be improved. [Brief explanation of the drawings]

[0008] [Figure 1A] FIG. 1A is a cross-sectional view showing a schematic configuration of a negative electrode active material according to Embodiment 1. FIG. [Figure 1B] FIG. 1B is a cross-sectional view showing a schematic configuration of a porous silicon particle according to the first embodiment. [Figure 2] FIG. 2 is a flowchart showing a method for producing the negative electrode active material according to the first embodiment. [Figure 3] FIG. 3 is a cross-sectional view showing a schematic configuration of a battery according to the second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0009] (Findings that formed the basis of this disclosure) In all-solid-state lithium-ion batteries, both electrons and lithium ions must be efficiently supplied to the active material in the electrode. In all-solid-state lithium-ion batteries, the active material is dispersed throughout the electrode. In a typical negative electrode, it is desirable to have both an electron conduction path formed by contact between the active material and the conductive additive and an ion conduction path formed by connecting the solid electrolytes together.

[0010] Silicon particles are often used as anode active materials. Silicon particles can absorb lithium ions by alloying with lithium. Silicon particles can improve the capacity of batteries compared to other active materials such as graphite.

[0011] Silicon particles expand during charging to absorb lithium and contract during discharging to release lithium. As a result, repeated volume changes in the silicon particles due to charge-discharge cycles deteriorate the contact between the silicon particles and the conductive additive and between the silicon particles and the solid electrolyte. This reduces the interface between the silicon particles and the conductive additive and the interface between the silicon particles and the solid electrolyte. This results in a deterioration of battery performance.

[0012] In order to avoid such problems, various proposals have been made to suppress the volume change caused by the expansion and contraction of silicon particles during charging and discharging.

[0013] In Patent Document 1, porous silicon particles having a three-dimensional network structure are used as the negative electrode active material, and the pores in the three-dimensional network structure are secured as expansion spaces during charging. In Patent Document 2, a coating layer containing a solid electrolyte is formed around the Si-based active material by a dry method, thereby generating micropores around the Si-based active material, and these micropores are secured as expansion spaces during charging.

[0014] From the viewpoint of lithium ion transport, it is important that the negative electrode active material and the solid electrolyte have good contact in the negative electrode. However, the porous silicon particles disclosed in Patent Document 1 have an uneven surface. The coated Si-based active material disclosed in Patent Document 2 has micropores around the periphery. Therefore, neither of these negative electrode active materials has good contact with the solid electrolyte. In addition, in these negative electrode active materials, lithium ions move between the surface layer of the silicon particles and the solid electrolyte, so the expansion and contraction rate of the surface layer of the silicon particles tends to be higher than the expansion and contraction rate inside the silicon particles. Therefore, local expansion and contraction may occur in the surface layer of the negative electrode active material. Therefore, repeated charge and discharge cycles tend to deteriorate the battery performance.

[0015] The present inventors have conducted extensive research into techniques for improving the charge-discharge cycle characteristics of batteries, and as a result have arrived at the technique of the present disclosure.

[0016] (Summary of one aspect of the present disclosure) The negative electrode active material according to the first embodiment of the present disclosure comprises: Porous silicon particles; a solid electrolyte; Including, the porous silicon particles have a plurality of pores; the solid electrolyte has the form of a thin film covering at least a part of the inner surface of the pore; The average thickness of the thin film is less than 30 nm.

[0017] According to the above configuration, the solid electrolyte covers at least a portion of the inner surfaces of the pores of the porous silicon particles in the form of a thin film, forming many interfaces, i.e., many ion conduction paths, between the porous silicon particles and the solid electrolyte. This allows lithium ions to be transported deep into the pores of the porous silicon particles, improving the ionic conductivity of the negative electrode active material. Furthermore, because charge / discharge reactions can be smoothly carried out at these many interfaces, localized expansion and contraction of the negative electrode active material is suppressed. This improves the charge / discharge cycle characteristics of the battery.

[0018] Furthermore, with the above-described configuration, the thin film of the solid electrolyte is sufficiently thin, so that the pores are sufficiently maintained in the negative electrode active material, thereby ensuring sufficient space for expansion during charging in the negative electrode active material.

[0019] In the second aspect of the present disclosure, for example, in the negative electrode active material according to the first aspect, the ratio of the pore volume of the negative electrode active material to the pore volume of the porous silicon particles may be 55% or more and less than 99%. This configuration ensures sufficient expansion space during charging in the negative electrode active material, while providing sufficient interfaces for ion transport deep into the porous silicon particles. As a result, the charge-discharge cycle characteristics of the battery can be further improved.

[0020] In a third aspect of the present disclosure, for example, in the negative electrode active material according to the first or second aspect, the ratio of the specific surface area of ​​the negative electrode active material to the specific surface area of ​​the porous silicon particles may be 55% or more and less than 99%. This configuration ensures sufficient expansion space during charging in the negative electrode active material while providing an interface for ion transport deep into the porous silicon particles. Furthermore, since the specific surface area of ​​the negative electrode active material is smaller than that of the porous silicon particles, contact between the negative electrode active material and other solid electrolytes in the negative electrode is easier than when only porous silicon particles are used as the negative electrode active material. As a result, the charge / discharge cycle characteristics of the battery may be further improved.

[0021] In a fourth aspect of the present disclosure, for example, in the negative electrode active material according to any one of the first to third aspects, the ratio of the volume of the solid electrolyte to the volume of the porous silicon particles may be 0.5% or more and less than 15%. This configuration can improve ionic conductivity while suppressing a decrease in capacity density of the negative electrode active material.

[0022] In a fifth aspect of the present disclosure, for example, in the negative electrode active material according to any one of the first to fourth aspects, the solid electrolyte may contain lithium, phosphorus, sulfur, and a halogen. This configuration can improve the ionic conductivity of the negative electrode active material.

[0023] A battery according to a sixth aspect of the present disclosure includes: a negative electrode; A positive electrode and an electrolyte layer disposed between the negative electrode and the positive electrode; Equipped with The negative electrode includes the negative electrode active material according to any one of the first to fifth aspects.

[0024] According to the above configuration, the charge / discharge cycle characteristics of the battery can be improved.

[0025] The negative electrode active material according to the seventh aspect of the present disclosure is Porous silicon particles; a solid electrolyte; Including, the porous silicon particles have a plurality of pores; The solid electrolyte has the form of a thin film that covers at least a portion of the inner surface of the pores.

[0026] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings.

[0027] (Embodiment 1) 1A is a cross-sectional view showing a schematic configuration of a negative electrode active material 1000 in accordance with Embodiment 1. FIG. 1B is a cross-sectional view showing a schematic configuration of a porous silicon particle 100 in accordance with Embodiment 1.

[0028] The negative electrode active material 1000 includes porous silicon particles 100 and a solid electrolyte. The solid electrolyte included in the negative electrode active material 1000 is referred to as a first solid electrolyte 101. The porous silicon particles 100 have a plurality of pores 102. The first solid electrolyte 101 has the shape of a thin film that covers at least a portion of the inner surfaces of the pores 102.

[0029] The porous silicon particles 100 can function as an active material. Because the first solid electrolyte 101 covers at least a portion of the inner surfaces of the pores 102 of the porous silicon particles 100 in the form of a thin film, many interfaces, i.e., many ion conduction paths, are formed between the porous silicon particles 100 and the first solid electrolyte 101. This allows lithium ions to be transported to the interior of the pores 102, improving the ion conductivity of the negative electrode active material 1000. Furthermore, because charge / discharge reactions can be carried out smoothly at these many interfaces, local expansion and contraction of the negative electrode active material 1000 is suppressed. This improves the charge / discharge cycle characteristics of the battery.

[0030] In this disclosure, "at least a portion" means part or all of the range in question.

[0031] The average thickness of the thin film of the first solid electrolyte 101 is less than 30 nm. In the negative electrode active material 1000, the thin film of the first solid electrolyte 101 is sufficiently thin, so that the pores 102 are sufficiently maintained in the negative electrode active material 1000. This ensures that the negative electrode active material 1000 has sufficient expansion space during charging. Furthermore, when the average thickness of the thin film of the first solid electrolyte 101 is less than 30 nm, the total amount of solid electrolyte relative to the entire negative electrode can be kept at an appropriate level. This can prevent a decrease in the weight energy density of the negative electrode.

[0032] Other materials such as a conductive additive may be present inside the pores 102. If the first solid electrolyte 101 does not fill all of the pores 102, leaving a space, the space functions as an "expansion space during charging."

[0033] The first solid electrolyte 101 reaches the center of the porous silicon particle 100. For example, when a cross section of a negative electrode active material 1000 arbitrarily selected from the powder of the negative electrode active material 1000 is observed with a scanning electron microscope (SEM) or a transmission electron microscope (TEM), the first solid electrolyte 101 is present in the center of the cross section. The "center of the cross section" is defined, for example, as a range of a distance r / 3 from the center of a circle with the smallest area that surrounds the cross section of the particle of the negative electrode active material 1000, where r is the radius of the circle. For cross-sectional observation, a negative electrode active material 1000 that is relatively large and nearly spherical in shape can be selected.

[0034] The average thickness of the thin film of the first solid electrolyte 101 may be 25 nm or less, or may be 10 nm or less. With the above configuration, it is possible to provide an interface for ion transport sufficiently deep into the porous silicon particles 100 while ensuring expansion space during charging in the negative electrode active material 1000. As a result, the charge / discharge cycle characteristics of the battery can be further improved. There is no particular lower limit to the average thickness of the thin film of the first solid electrolyte 101. The lower limit of the average thickness of the thin film of the first solid electrolyte 101 may be 3 nm or 4 nm.

[0035] In the present disclosure, the term "thin film shape" refers to a state in which first solid electrolyte 101 is formed thinly.

[0036] The average thickness of the thin film of the first solid electrolyte 101 can be determined, for example, by the following method. Specifically, first, the negative electrode active material 1000 is processed to expose a cross section of the negative electrode active material 1000. The processing of the negative electrode active material 1000 can be performed, for example, using a cross-section polisher (registered trademark). The cross-section polisher can form a smooth cross section of the negative electrode active material 1000. Next, the cross section of the negative electrode active material 1000 is observed with a scanning electron microscope (SEM) or a transmission electron microscope (TEM). This results in an SEM image or a TEM image of the cross section of the negative electrode active material 1000. Next, the porous silicon particles 100, the first solid electrolyte 101, and the pores 102 are identified from the obtained SEM image or TEM image. These identifications may be performed based on the contrast of the image or the results of elemental analysis such as energy dispersive X-ray analysis (EDS). Next, the thickness of the thin film of first solid electrolyte 101 is measured at 10 points arbitrarily selected from the SEM or TEM image. The average thickness of the thin film of first solid electrolyte 101 can be determined by averaging these measurement values.

[0037] The shape of the first solid electrolyte 101 itself constituting the thin film is not particularly limited. The thin film of the first solid electrolyte 101 may be formed by an accumulation of fine particles of the first solid electrolyte 101, such as plate-like, needle-like, spherical, or oval-spherical. When the first solid electrolyte 101 has a thin film shape, ion conduction tends to be promoted within the plane of the first solid electrolyte 101. Whether the first solid electrolyte 101 has a thin film shape can be confirmed by observing an SEM or TEM image of a cross section of the negative electrode active material 1000.

[0038] When the thin film of first solid electrolyte 101 is formed by an accumulation of fine particles of first solid electrolyte 101, the median diameter of the fine particles of first solid electrolyte 101 may be 1 nm or more and 20 nm or less, or may be 1 nm or more and 5 nm or less.

[0039] Generally, the term "median diameter" refers to the particle size where the cumulative volume in the volume-based particle size distribution is equal to 50%. The volume-based particle size distribution is measured, for example, by a laser diffraction measurement device.

[0040] The thin film of first solid electrolyte 101 may or may not uniformly cover the inner surfaces of pores 102 of porous silicon particles 100. In other words, there may be a portion of the inner surface of pores 102 of porous silicon particles 100 where first solid electrolyte 101 is not present. When the thin film of first solid electrolyte 101 uniformly covers the inner surfaces of pores 102, the ionic conductivity of negative electrode active material 1000 is further improved.

[0041] The first solid electrolyte 101 may be in the form of a thin film or may be in a form other than a thin film that covers at least a portion of the inner surface of the pores 102 of the porous silicon particles 100. Examples of a form other than a thin film include a layer or a porous form. The thin film or layer of the first solid electrolyte 101 may have a porous structure.

[0042] The first solid electrolyte 101 may cover, in the form of a thin film, at least a part of the outer surface of the porous silicon particle 100, in addition to the inner surfaces of the pores 102 of the porous silicon particle 100. With the above configuration, more interfaces are formed between the porous silicon particle 100 and the first solid electrolyte 101. This can further improve the ionic conductivity of the negative electrode active material 1000.

[0043] The thin film of the first solid electrolyte 101 may or may not uniformly cover the outer surface of the porous silicon particle 100. In other words, there may be a part of the outer surface of the porous silicon particle 100 where the first solid electrolyte 101 is not present. When the thin film of the first solid electrolyte 101 uniformly covers the outer surface of the porous silicon particle 100, the ionic conductivity of the negative electrode active material 1000 is further improved.

[0044] In addition to being in the form of a thin film, first solid electrolyte 101 may also cover at least a part of the outer surface of porous silicon particle 100 in a form other than a thin film.

[0045] The porous silicon particles 100 may contain silicon as a major component, for example, may consist essentially of silicon. In this disclosure, "major component" refers to the component that is contained in the porous silicon particles 100 in the largest amount by mass. "Consisting essentially of silicon" means excluding other components that alter the essential characteristics of the referenced material. However, the porous silicon particles 100 may contain impurities in addition to silicon.

[0046] In the porous silicon particle 100, the plurality of pores 102 may be formed three-dimensionally continuous. At least one of the plurality of pores 102 may penetrate the porous silicon particle 100. In this manner, the porous silicon particle 100 may have a so-called three-dimensional network structure. When the porous silicon particle 100 has a three-dimensional network structure, lithium ions can be transported deeper into the pores 102. This further improves the ionic conductivity of the negative electrode active material 1000.

[0047] The porous silicon particles 100 may be secondary particles containing a plurality of aggregated primary particles. According to the above configuration, porous silicon particles 100 having a plurality of pores 102 therein can be easily produced using silicon microparticles.

[0048] When the porous silicon particle 100 is a secondary particle, a plurality of primary particles may be in contact with each other.

[0049] The shape of the primary particles is not particularly limited, and may be, for example, plate-like, scale-like, needle-like, spherical, or oval-spherical.

[0050] For example, a pore 102 may be formed between two primary particles among the plurality of primary particles. The plurality of pores 102 may be formed three-dimensionally continuous. At least one pore 102 among the plurality of pores 102 may penetrate the porous silicon particle 100. In this way, when the porous silicon particle 100 is a secondary particle, the porous silicon particle 100 may have a three-dimensional network structure.

[0051] The primary particles may contain silicon as a main component, for example, may consist essentially of silicon, although the primary particles may also contain impurities in addition to silicon.

[0052] There may be pores 102 partially filled with first solid electrolyte 101.

[0053] The shortest diameter of the pores 102 of the porous silicon particles 100 is, for example, 1 nm or more and 200 nm or less. When the shortest diameter is 1 nm or more, the first solid electrolyte 101 can be easily introduced into the pores 102. When the shortest diameter is 200 nm or less, the pores 102 can provide the porous silicon particles 100 with sufficient expansion space.

[0054] The lower limit of the shortest diameter of the pores 102 may be 10 nm, and the upper limit of the shortest diameter of the pores 102 may be 100 nm.

[0055] The shortest diameter of the pore 102 can be determined, for example, by the following method. First, an SEM image or TEM image of the cross section of the negative electrode active material 1000 is obtained by the same method as described above. From the obtained SEM image or TEM image, the porous silicon particles 100, the first solid electrolyte 101, and the pores 102 are identified. Next, from the SEM image or TEM image, the center of gravity of the pore 102 is identified. The shortest diameter of the pore 102 among the diameters of the pore 102 passing through the center of gravity can be considered to be the shortest diameter of the pore 102. In the SEM image or TEM image of the cross section of the negative electrode active material 1000, the diameter of the circle with the smallest area surrounding the pore 102 may also be considered to be the shortest diameter of the pore 102.

[0056] The average shortest diameter of the pores 102 may be 1 nm or more and 200 nm or less. The average shortest diameter of the pores 102 can be determined by determining the shortest diameters of any number of pores 102 (for example, five) from an SEM image or TEM image of the cross section of the negative electrode active material 1000 and averaging these values.

[0057] When a plurality of pores 102 are displayed in an SEM image or a TEM image, the largest of the shortest diameters of the displayed plurality of pores 102 may be 1 nm or more and 200 nm or less.

[0058] The fact that first solid electrolyte 101 covers at least a portion of the inner surface of pores 102 in the form of a thin film can be confirmed by SEM or TEM observation of the cross section of the above-mentioned negative electrode active material 1000. Furthermore, by using a gas adsorption method using nitrogen or mercury intrusion porosimetry, it can be confirmed that first solid electrolyte 101 is present inside pores 102 based on the pore diameters of porous silicon particles 100 and negative electrode active material 1000.

[0059] In the gas adsorption method using nitrogen gas, the data of the adsorption isotherm obtained for a sample having pores can be converted by the BJH (Barrett-Joyner-Halenda) method to obtain a pore distribution in which the pore volume is specified for each diameter D. The pore distribution is, for example, a graph showing the relationship between the pore diameter D and the Log differential pore volume.

[0060] In mercury intrusion porosimetry, high-pressure mercury is first injected into a sample with pores. The pore distribution can be determined from the relationship between the pressure applied to the mercury and the amount of mercury injected into the sample. Specifically, the diameter D of the pores in the sample into which mercury has been injected can be determined from the following relational expression (I). In relational expression (I), γ is the surface tension of mercury, θ is the contact angle between the mercury and the wall surface of the sample, and P is the pressure applied to the mercury.

[0061] D=-4γcosθ÷P (I)

[0062] The pressure P is changed in stages, and the amount of injected mercury is measured for each pressure P. The amount of injected mercury can be regarded as the cumulative value of the volume of pores up to a diameter D corresponding to a specific pressure P. This makes it possible to obtain a pore distribution in which the pore volume is specified for each diameter D. The pore distribution is, for example, a graph showing the relationship between the pore diameter D and the Log differential pore volume.

[0063] In this embodiment, for example, the pore size distribution can be obtained for each of the negative electrode active material 1000 having the first solid electrolyte 101 present inside the pores 102 and the porous silicon particles 100 having no first solid electrolyte 101 present inside the pores 102 by the BJH method of gas adsorption measurement using nitrogen or mercury intrusion porosimetry. As the porous silicon particles 100 having no first solid electrolyte 101 present inside the pores 102, porous silicon particles 100 before the first solid electrolyte 101 is introduced into the pores 102 can be used. Porous silicon particles 100 obtained by removing the first solid electrolyte 101 from the negative electrode active material 1000 may also be used. The first solid electrolyte 101 can be removed from the negative electrode active material 1000 using, for example, a solvent or the like.

[0064] Based on the pore distribution of the negative electrode active material 1000 and the pore distribution of the porous silicon particles 100, it can be determined that the first solid electrolyte 101 is present inside the pores 102 in the negative electrode active material 1000. Specifically, for example, the Log differential pore volume at a specific diameter D is determined for each of the pore distributions of the negative electrode active material 1000 and the porous silicon particles 100. When the Log differential pore volume of the negative electrode active material 1000 at the specific diameter D is smaller than the Log differential pore volume of the porous silicon particles 100, it can be determined that the first solid electrolyte 101 is located inside the pores 102 of the porous silicon particles 100. Furthermore, when the diameter D at the peak of the pore distribution of the negative electrode active material 1000 is smaller than the diameter D at the peak of the pore distribution of the porous silicon particles 100, it can also be determined that the first solid electrolyte 101 is located inside the pores 102 of the porous silicon particles 100.

[0065] When the porous silicon particles 100 have a plurality of pores 102, the average pore diameter S of the porous silicon particles 100 determined by the BJH method of gas adsorption measurement using nitrogen or mercury intrusion porosimetry is not particularly limited. The average pore diameter S of the porous silicon particles 100 determined by the BJH method of gas adsorption measurement using nitrogen or mercury intrusion porosimetry is, for example, 1 nm or more and 200 nm or less. The lower limit of the average pore diameter S may be 10 nm. The upper limit of the average pore diameter S may be 100 nm.

[0066] The average pore diameter S of the porous silicon particles 100 can be determined, for example, by the following method. First, for porous silicon particles 100 in which the first solid electrolyte 101 is not present inside the pores 102, a pore distribution showing the relationship between the pore diameter D and the Log differential pore volume is obtained by the BJH method of gas adsorption measurement using nitrogen or mercury intrusion porosimetry as described above. Next, the peak of the pore distribution of the porous silicon particles 100 is identified. The diameter D at the peak of the pore distribution can be considered to be the average pore diameter S. The diameter D at the peak of the pore distribution corresponds to the mode diameter of the pores.

[0067] The shape of the porous silicon particles 100 is not particularly limited. The shape of the porous silicon particles 100 is, for example, spherical or oval spherical. The shape of the porous silicon particles 100 may be needle-like, plate-like, or the like. When the porous silicon particles 100 are secondary particles, the surfaces of the porous silicon particles 100 may have an uneven shape, such as a plate-like shape, that is due to the primary particles.

[0068] The median diameter of the porous silicon particles 100 is not particularly limited and is, for example, 50 nm or more and 30 μm or less. Porous silicon particles 100 with a median diameter of 50 nm or more are easy to handle and are therefore suitable for producing the negative electrode active material 1000. Porous silicon particles 100 with a median diameter of 30 μm or less allow the first solid electrolyte 101 to be easily introduced into the pores 102. The median diameter of the porous silicon particles 100 may be 200 nm or more and 10 μm or less.

[0069] As used herein, ratios may be expressed as percentages.

[0070] The ratio of the pore volume of the negative electrode active material 1000 to the pore volume of the porous silicon particles 100 may be 55% or more and less than 99%. With the above configuration, it is possible to ensure expansion space in the negative electrode active material 1000 during charging, while providing a sufficient interface for ion transport all the way to the inside of the porous silicon particles 100. As a result, the charge / discharge cycle characteristics of the battery can be further improved.

[0071] The ratio of the pore volume of the negative electrode active material 1000 to the pore volume of the porous silicon particles 100 may be 70% or more and 98% or less, or may be 80% or more and 95% or less.

[0072] In the present disclosure, the term "pore volume" refers to the cumulative pore volume in the pore distribution obtained by the BJH method of gas adsorption measurement using nitrogen or mercury intrusion porosimetry as described above.

[0073] The pore volumes of the porous silicon particles 100 and the negative electrode active material 1000 can be determined from the pore distribution obtained by the BJH method of gas adsorption measurement using nitrogen or mercury intrusion porosimetry as described above.

[0074] The pore volume of the porous silicon particle 100 is not particularly limited. The pore volume of the porous silicon particle 100 is, for example, 5×10 -8 m 3 / g or more. Pore volume is 5×10 -8 m 3 / g or more, the inner surfaces of the pores 102 can be covered with a sufficient amount of the first solid electrolyte 101. The larger the pore volume of the porous silicon particle 100, the larger the area of ​​the inner surfaces of the pores 102 that can be covered with the first solid electrolyte 101. There is no particular upper limit to the pore volume of the porous silicon particle 100. The upper limit of the pore volume of the porous silicon particle 100 is 5×10 -6 m 3 / g.

[0075] There is no particular limitation on the pore volume of the negative electrode active material 1000. The pore volume of the negative electrode active material 1000 is, for example, 4×10 -8 m 3 / g or more. The upper limit of the pore volume of the negative electrode active material 1000 is not particularly limited. The upper limit of the pore volume of the negative electrode active material 1000 is 4×10 -6 m 2 / g.

[0076] The ratio of the specific surface area of ​​the negative electrode active material 1000 to the specific surface area of ​​the porous silicon particles 100 may be 55% or more and less than 99%. With the above configuration, it is possible to provide a sufficient interface for ion transport all the way to the interior of the porous silicon particles 100 while ensuring expansion space in the negative electrode active material 1000 during charging. Furthermore, since the specific surface area of ​​the negative electrode active material 1000 is smaller than that of the porous silicon particles 100, the negative electrode active material 1000 and other solid electrolytes are more likely to come into contact with each other in the negative electrode than when only the porous silicon particles 100 are used as the negative electrode active material. As a result, the charge / discharge cycle characteristics of the battery can be further improved.

[0077] The ratio of the specific surface area of ​​the negative electrode active material 1000 to the specific surface area of ​​the porous silicon particles 100 may be 60% or more and 95% or less, or may be 70% or more and 90% or less.

[0078] The specific surface area of ​​each of the porous silicon particles 100 and the negative electrode active material 1000 can be determined, for example, by converting data on an adsorption isotherm obtained by a gas adsorption method using nitrogen using the BET (Brunauer-Emmett-Teller) method.

[0079] The specific surface area of ​​the porous silicon particles 100 is not particularly limited. The specific surface area of ​​the porous silicon particles 100 is, for example, 10 m 2 / g or more. The specific surface area is 10m 2 / g or more, the inner surfaces of the pores 102 can be covered with a sufficient amount of the first solid electrolyte 101. The larger the specific surface area of ​​the porous silicon particles 100, the larger the area of ​​the inner surfaces of the pores 102 that can be covered with the first solid electrolyte 101. There is no particular upper limit to the specific surface area of ​​the porous silicon particles 100. The upper limit of the specific surface area of ​​the porous silicon particles 100 is 500 m 2 / g.

[0080] The specific surface area of ​​the negative electrode active material 1000 is not particularly limited. The specific surface area of ​​the negative electrode active material 1000 is, for example, 10 m 2 / g or more. The upper limit of the specific surface area of ​​the negative electrode active material 1000 is not particularly limited. The upper limit of the specific surface area of ​​the negative electrode active material 1000 is 500 m 2 / g.

[0081] The porosity of the porous silicon particles 100 is not particularly limited. The porosity of the porous silicon particles 100 may be, for example, 5% or more. When the porosity is 5% or more, the inner surfaces of the pores 102 can be covered with a sufficient amount of the first solid electrolyte 101. The upper limit of the porosity of the porous silicon particles 100 is not particularly limited. The upper limit of the porosity of the porous silicon particles 100 is, for example, 50%. When the porosity is 50% or less, the porous silicon particles 100 tend to have sufficiently high strength.

[0082] In this disclosure, the "porosity of the porous silicon particle 100" refers to the ratio of the total volume of the pores 102 to the volume of the porous silicon particle 100 including the pores 102.

[0083] The porosity of the porous silicon particles 100 can be measured by, for example, mercury intrusion porosimetry. The porosity of the porous silicon particles 100 can also be calculated from the pore volume determined by the BJH method of gas adsorption measurement using nitrogen.

[0084] The ratio of the volume of the first solid electrolyte 101 to the volume of the porous silicon particles 100 may be 0.5% or more and less than 15%. With this configuration, the ionic conductivity of the negative electrode active material 1000 can be improved while suppressing a decrease in the capacity density.

[0085] The ratio of the volume of the first solid electrolyte 101 to the volume of the porous silicon particles 100 may be 1% or more and 12% or less, or may be 2% or more and 7% or less.

[0086] The ratio of the volume of the first solid electrolyte 101 to the volume of the porous silicon particles 100 can be calculated from the amounts of the materials charged, or by the method described below. The first solid electrolyte 101 is extracted from the negative electrode active material 1000 using a solvent in which the first solid electrolyte 101 is soluble (e.g., a polar solvent such as ethanol). This removes the first solid electrolyte 101 from the negative electrode active material 1000, allowing only the porous silicon particles 100 to be extracted. The extracted porous silicon particles 100 are dried and their mass is measured. The mass of the first solid electrolyte 101 can be determined by subtracting the mass of the porous silicon particles 100 from the mass of the negative electrode active material 1000. The mass of the first solid electrolyte 101 can also be determined by quantitatively analyzing the solvent used to extract the first solid electrolyte 101 using ICP analysis or the like. The volume of the porous silicon particles 100 can be calculated from the mass of the porous silicon particles 100 and the true density of the porous silicon particles 100. The volume of the porous silicon particles 100 may be determined by subtracting the volume of the first solid electrolyte 101 from the volume of the negative electrode active material 1000. The volume of the negative electrode active material 1000 can be calculated from the mass of the negative electrode active material 1000 and the true density of the negative electrode active material 1000. The volume of the first solid electrolyte 101 can be calculated from the mass of the first solid electrolyte 101 and the true density of the first solid electrolyte 101. The true densities of the porous silicon particles 100, the first solid electrolyte 101, and the negative electrode active material 1000 can be measured by, for example, a pycnometer method.

[0087] When the negative electrode active material 1000 is contained in an electrode, the negative electrode active material 1000 can be extracted, for example, by the following method: An electrode containing the negative electrode active material 1000 is dispersed in a solvent in which the first solid electrolyte 101 is insoluble. By centrifuging the resulting dispersion medium, only the negative electrode active material 1000 can be extracted due to the difference in particle density.

[0088] First solid electrolyte 101 has, for example, lithium ion conductivity and includes, for example, at least one selected from the group consisting of inorganic solid electrolytes and organic solid electrolytes.

[0089] The first solid electrolyte 101 may contain a sulfide solid electrolyte. The sulfide solid electrolyte has excellent reduction stability and is therefore suitable for combination with the porous silicon particles 100, which are a low-potential negative electrode material.

[0090] The first solid electrolyte 101 may contain lithium, phosphorus, sulfur, and a halogen. According to the above configuration, the ionic conductivity of the negative electrode active material 1000 can be improved.

[0091] The first solid electrolyte 101 may be represented by, for example, the following composition formula (1).

[0092] Li α PS β X γ ...Equation (1)

[0093] In formula (1), α, β, and γ satisfy the following conditions: 5.5≦α≦6.5, 4.5≦β≦5.5, and 0.5≦γ≦1.5. X contains at least one selected from the group consisting of F, Cl, Br, and I. X may contain at least one selected from the group consisting of Cl and Br. X may contain Cl. First solid electrolyte 101 may be Li6PS5X.

[0094] The solid electrolyte represented by composition formula (1) has, for example, an argyrodite-type crystal structure. That is, the first solid electrolyte 101 may have an argyrodite-type crystal structure. The negative electrode active material 1000 containing such a first solid electrolyte 101 tends to have high ionic conductivity.

[0095] Examples of sulfide solid electrolytes other than those represented by the composition formula (1) include Li2S-P2S5, Li2S-SiS2, Li2S-B2S3, Li2S-GeS2, Li 3.25 Ge 0.25 P 0.75 S4, Li 10 GeP2S 12 These include LiX, LiO, MO q , Li p MO q The element X in "LiX" is at least one selected from the group consisting of F, Cl, Br, and I. q " and "Li p MO q "The element M is at least one selected from the group consisting of P, Si, Ge, B, Al, Ga, In, Fe, and Zn. q " and "Li p MO q " p and q in this expression are independent natural numbers.

[0096] First solid electrolyte 101 may include at least one selected from the group consisting of oxide solid electrolytes, polymer solid electrolytes, and complex hydride solid electrolytes.

[0097] Examples of oxide solid electrolytes include NASICON-type solid electrolytes, such as LiTi2(PO4)3 and its elemental substitution products, (LaLi)TiO3-based perovskite-type solid electrolytes, and Li 14 ZnGeO 16 , Li4SiO4, LiGeO4 and their element-substituted LISICON-type solid electrolytes, Li7La3Zr2O 12Garnet-type solid electrolytes, such as those substituted with LiN and its element, LiN and its H-substituted compounds, LiPO4 and its N-substituted compounds, and glass or glass ceramics containing a base material containing Li-BO compounds such as LiBO2 and LiBO3 to which a material such as LiSO4 or LiCO3 has been added can be used.

[0098] As the polymer solid electrolyte, for example, a compound of a polymer compound and a lithium salt can be used. The polymer compound may have an ethylene oxide structure. By having an ethylene oxide structure, the polymer compound can contain a large amount of lithium salt, thereby further increasing ionic conductivity. As the lithium salt, LiPF6, LiBF4, LiSbF6, LiAsF6, LiSO3CF3, LiN(SO2CF3)2, LiN(SO2C2F5)2, LiN(SO2CF3)(SO2C4F9), LiC(SO2CF3)3, etc. can be used. As the lithium salt, one lithium salt selected from these may be used alone, or a mixture of two or more lithium salts selected from these may be used.

[0099] Examples of the complex hydride solid electrolyte that can be used include LiBH4-LiI and LiBH4-P2S5.

[0100] The negative electrode active material 1000 may or may not further contain a solid electrolyte other than the first solid electrolyte 101. The solid electrolyte other than the first solid electrolyte 101 may be in contact with the outer surface of the porous silicon particles 100. The outer surface of the negative electrode active material 1000 may be coated with a solid electrolyte other than the first solid electrolyte 101.

[0101] <Method of manufacturing negative electrode active material> Next, a method for producing the above-mentioned negative electrode active material 1000 will be described. The negative electrode active material 1000 can be produced, for example, by the following method. Figure 2 is a flowchart showing the method for producing the negative electrode active material 1000 according to the first embodiment.

[0102] A porous silicon particle 100 is prepared, which has a plurality of pores 102. The porous silicon particle 100 may be a secondary particle formed by agglomeration of a plurality of primary particles.

[0103] A raw material powder for the first solid electrolyte 101 is prepared. The raw material powder for the first solid electrolyte 101 is a raw material powder containing lithium, phosphorus, sulfur, and a halogen. The raw material powder containing lithium, phosphorus, sulfur, and a halogen may contain, for example, a raw material powder containing lithium and sulfur, a raw material powder containing phosphorus and sulfur, and a raw material powder containing lithium and a halogen. An example of the raw material powder containing lithium and sulfur is Li2S. An example of the raw material powder containing phosphorus and sulfur is P2S5. An example of the raw material powder containing lithium and a halogen is LiBr, LiCl, and LiI.

[0104] Next, a solution L containing the first solid electrolyte 101 is prepared (step S11). Specifically, the raw material powders are weighed out in a stoichiometric ratio and mixed. The resulting mixed powder is stirred with a solvent to obtain the solution L. The concentration of the raw material powder of the first solid electrolyte 101 in the solution L is not particularly limited, and is, for example, 1% by mass or more and 20% by mass or less.

[0105] Next, the porous silicon particles 100 are brought into contact with the solution L (step S12). At the time of step S12, the first solid electrolyte 101 has not been generated in the solution L. In other words, the solution L does not contain particles of the first solid electrolyte 101. Therefore, when the porous silicon particles 100 are brought into contact with the solution L, the solution L can easily permeate into the pores 102 of the porous silicon particles 100. More specifically, the solution L easily permeates into the pores 102 of the porous silicon particles 100 due to capillary action. As a result, the solution L is introduced into the pores 102.

[0106] There are no particular limitations on the method for bringing the porous silicon particles 100 into contact with the solution L. For example, the porous silicon particles 100 may be brought into contact with the solution L by kneading the porous silicon particles 100 and the solution L. When the porous silicon particles 100 are brought into contact with the solution L, the ratio of the volume of the raw material powder of the first solid electrolyte 101 to the total volume of the porous silicon particles 100 and the volume of the raw material powder of the first solid electrolyte 101 is not particularly limited, and is, for example, more than 0.1% and less than 15%.

[0107] Next, the solvent is removed from the solution L impregnated in the porous silicon particles 100 (step S13). As a result, the first solid electrolyte 101 is precipitated inside the pores 102. The solvent is removed, for example, by volatilizing the solvent. As an example, the solvent of the solution L may be volatilized while the porous silicon particles 100 and the solution L are being mixed. The solvent of the solution L that has penetrated inside the porous silicon particles 100 is less likely to volatilize than the solvent of the solution L that is present outside the porous silicon particles 100. Therefore, when the solvent contained in the solution L is volatilized, the first solid electrolyte 101 tends to be concentrated inside the pores 102 of the porous silicon particles 100. When the solvent of the solution L is volatilized, the first solid electrolyte 101 is less likely to precipitate outside the porous silicon particles 100, and is supported on the porous silicon particles 100 by the pores 102.

[0108] After removing the solvent, the porous silicon particles 100 are heated (step S14). This results in an anode active material 1000 in which the first solid electrolyte 101 is present in the form of a thin film on at least a portion of the inner surfaces of the pores 102 of the porous silicon particles 100. The thin film of the first solid electrolyte 101 can also be formed on at least a portion of the outer surfaces of the porous silicon particles 100. The conditions for the heat treatment can be appropriately set depending on the composition of the first solid electrolyte 101, etc. The temperature for the heat treatment is not particularly limited and is, for example, 100°C or higher. The time for the heat treatment is not particularly limited and is, for example, 1 hour or longer. The heat treatment may be performed under a reduced pressure atmosphere or a vacuum atmosphere. The heat treatment tends to improve the crystallinity of the first solid electrolyte 101.

[0109] The method for producing the negative electrode active material 1000 is not limited to the method shown in the flowchart of Fig. 2. For example, the negative electrode active material 1000 may be produced by using the first solid electrolyte 101 instead of the raw material powder of the first solid electrolyte 101, and by using a dispersion liquid of the first solid electrolyte 101 instead of the solution L.

[0110] There is no particular limitation on the method for producing porous silicon particles 100 having a plurality of pores 102. Porous silicon particles 100 can be produced, for example, by removing metals other than silicon from a precursor made of an alloy of silicon and a metal such as lithium, for example by elution, followed by washing and drying.

[0111] (Embodiment 2) The following describes the second embodiment. Explanations that overlap with the first embodiment will be omitted where appropriate.

[0112] FIG. 3 is a cross-sectional view showing a schematic configuration of a battery 2000 according to the second embodiment.

[0113] Battery 2000 includes negative electrode 201, positive electrode 203, and electrolyte layer 202 disposed between negative electrode 201 and positive electrode 203. Negative electrode 201 includes negative electrode active material 1000 of the first embodiment.

[0114] According to the above configuration, the negative electrode 201 contains the negative electrode active material 1000, and thus the charge-discharge cycle characteristics of the battery 2000 can be improved.

[0115] The negative electrode 201 includes, for example, a negative electrode active material layer containing the negative electrode active material 1000, and a negative electrode current collector. The negative electrode active material layer is disposed between the negative electrode current collector and the electrolyte layer 202.

[0116] When producing the battery 2000, a negative electrode active material layer may be produced by compression molding a negative electrode material containing the negative electrode active material 1000. The porous silicon particles 100 contained in the negative electrode active material 1000 have high hardness. Therefore, even after compression molding, the negative electrode active material 1000 tends to maintain the pores 102. In other words, in the battery 2000 using the negative electrode active material 1000, the particle shape of the negative electrode active material 1000 is maintained in the negative electrode 201.

[0117] By determining the shortest diameter of the pores 102 of the negative electrode active material 1000 contained in the negative electrode 201, the structure of the negative electrode active material 1000 in the negative electrode 201 can be understood. The shortest diameter of the pores 102 of the negative electrode active material 1000 contained in the negative electrode 201 can be determined, for example, by the following method. First, the negative electrode 201 is processed to expose a cross section of the negative electrode 201. Next, an SEM image or TEM image of the cross section of the negative electrode 201 is obtained. Next, the negative electrode active material 1000 is identified from the obtained SEM image or TEM image, and further, the porous silicon particles 100, the first solid electrolyte 101, and the pores 102 are identified. Next, the center of gravity of the pores 102 is identified from the SEM image or TEM image. The shortest diameter of the pores 102 passing through the center of gravity can be considered to be the shortest diameter of the pores 102. In an SEM or TEM image of a cross section of negative electrode 201, the diameter of the circle with the smallest area that surrounds pore 102 may be considered to be the shortest diameter of pore 102.

[0118] The average shortest diameter of the pores 102 of the negative electrode active material 1000 contained in the negative electrode 201 can be determined by determining the shortest diameters of any number of pores 102 (for example, five) from an SEM image or TEM image of the cross section of the negative electrode 201 and averaging these values.

[0119] The negative electrode 201 may further include a solid electrolyte. The solid electrolyte that may be included in the negative electrode 201 is referred to as a second solid electrolyte 130. The second solid electrolyte 130, for example, fills spaces between a plurality of negative electrode active materials 1000 in the negative electrode 201. The second solid electrolyte 130 may have a particle shape. A large number of particles of the second solid electrolyte 130 may be compressed and bonded to each other, thereby forming an ion conduction path.

[0120] The second solid electrolyte 130 may or may not be in contact with the first solid electrolyte 101 contained in the negative electrode active material 1000. When the second solid electrolyte 130 is in contact with the first solid electrolyte 101, ion conduction into the negative electrode active material 1000 can occur efficiently.

[0121] The second solid electrolyte 130 has lithium ion conductivity. The second solid electrolyte 130 includes, for example, at least one selected from the group consisting of inorganic solid electrolytes and organic solid electrolytes. The second solid electrolyte 130 may include at least one selected from the group consisting of sulfide solid electrolytes, oxide solid electrolytes, halide solid electrolytes, polymer solid electrolytes, and complex hydride solid electrolytes. As the sulfide solid electrolyte, oxide solid electrolyte, polymer solid electrolyte, and complex hydride solid electrolyte, those described for the first solid electrolyte 101 in the first embodiment can be used. Specific examples of halide solid electrolytes will be described later in the description of the electrolyte layer 202.

[0122] The second solid electrolyte 130 is preferably made of a soft material to achieve a good dispersion state with the negative electrode active material 1000. In this respect, the second solid electrolyte 130 is preferably at least one selected from the group consisting of sulfide solid electrolytes and halide solid electrolytes.

[0123] The second solid electrolyte 130 may include a sulfide solid electrolyte.

[0124] The composition of the second solid electrolyte 130 may be the same as or different from the composition of the first solid electrolyte 101. As an example, the composition of the first solid electrolyte 101 may be adjusted so that the first solid electrolyte 101 can be easily introduced into the minute pores 102 of the porous silicon particles 100. The composition of the second solid electrolyte 130 may be adjusted so that the second solid electrolyte 130 has high ionic conductivity.

[0125] There are no particular limitations on the shape of the second solid electrolyte 130. The shape of the second solid electrolyte 130 may be needle-like, spherical, oval-spherical, scale-like, etc. The shape of the second solid electrolyte 130 may also be particulate.

[0126] When the second solid electrolyte 130 is particulate (e.g., spherical), the median diameter of the second solid electrolyte 130 may be 0.3 μm or more and 100 μm or less. When the median diameter is 0.3 μm or more, the number of contact interfaces between particles of the second solid electrolyte 130 does not increase too much, and an increase in ionic resistance inside the negative electrode 201 can be suppressed. This enables the battery 2000 to operate at high power.

[0127] When the median diameter of second solid electrolyte 130 is 100 μm or less, negative electrode active material 1000 and second solid electrolyte 130 tend to form a good dispersion state in negative electrode 201. This makes it easier to increase the capacity of battery 2000.

[0128] The median diameter of the second solid electrolyte 130 may be smaller than the median diameter of the negative electrode active material 1000. This allows the negative electrode active material 1000 and the second solid electrolyte 130 to form a better dispersed state in the negative electrode 201.

[0129] The negative electrode 201 may further contain an active material other than the negative electrode active material 1000. The shape of the other active material is not particularly limited. The shape of the other active material may be acicular, spherical, oval spherical, or the like. The shape of the other active material may be particulate.

[0130] The median diameter of the other active material may be 0.1 μm or more and 100 μm or less.

[0131] When the median diameter of the other active material is 0.1 μm or more, the other active material and second solid electrolyte 130 tend to form a well-dispersed state in negative electrode 201. As a result, the charging characteristics of battery 2000 are improved.

[0132] When the median diameter of the other active material is 100 μm or less, the diffusion rate of lithium within the active material is sufficiently ensured, which allows the battery 2000 to operate at high power.

[0133] The median diameter of the other active material may be larger than the median diameter of the second solid electrolyte 130. This allows the other active material and the second solid electrolyte 130 to form a good dispersed state.

[0134] The other active materials include materials that have the property of absorbing and releasing metal ions (e.g., lithium ions). Examples of the other active materials that can be used include metal materials, carbon materials, oxides, nitrides, tin compounds, and silicon compounds. The metal materials may be simple metals or alloys. Examples of metal materials include lithium metal and lithium alloys. Examples of carbon materials include natural graphite, coke, partially graphitized carbon, carbon fiber, spherical carbon, artificial graphite, and amorphous carbon. From the viewpoint of capacity density, silicon (Si), tin (Sn), silicon compounds, and tin compounds are preferably used. The other active materials may include a single active material or multiple active materials having different compositions.

[0135] The negative electrode active material 1000 and the second solid electrolyte 130 may be in contact with each other, as shown in Figure 3. The negative electrode 201 may include a plurality of negative electrode active materials 1000 and a plurality of second solid electrolytes 130.

[0136] In the negative electrode 201, the content of the second solid electrolyte 130 and the content of the negative electrode active material 1000 may be the same as or different from each other.

[0137] When the total amount of the negative electrode 201 is taken as 100 mass %, the content of the negative electrode active material 1000 may be 40 mass % or more and 90 mass % or less, or 40 mass % or more and 80 mass % or less. By appropriately adjusting the content of the negative electrode active material 1000, the negative electrode active material 1000 and the second solid electrolyte 130 are likely to be well dispersed in the negative electrode 201.

[0138] The mass ratio "w1:100-w1" of the active material to the second solid electrolyte 130 in the negative electrode 201 may satisfy 40≦w1≦90, or 40≦w1≦80. When 40≦w1 is satisfied, the energy density of the battery 2000 is sufficiently ensured. Furthermore, when w1≦90 is satisfied, the battery 2000 can operate at high output. Note that the term "active material" refers to the negative electrode active material 1000 as well as other active materials other than the negative electrode active material 1000.

[0139] The thickness of the negative electrode 201 may be 10 μm or more and 500 μm or less. When the thickness of the negative electrode 201 is 10 μm or more, the energy density of the battery 2000 is sufficiently ensured. When the thickness of the negative electrode 201 is 500 μm or less, the battery 2000 can operate at high power.

[0140] The electrolyte layer 202 is a layer containing an electrolyte. The electrolyte is, for example, a solid electrolyte. That is, the electrolyte layer 202 may be a solid electrolyte layer.

[0141] The solid electrolyte that can be included in electrolyte layer 202 is referred to as a third solid electrolyte. The third solid electrolyte may include at least one selected from the group consisting of a sulfide solid electrolyte, an oxide solid electrolyte, a halide solid electrolyte, a polymer solid electrolyte, and a complex hydride solid electrolyte. As the sulfide solid electrolyte, the oxide solid electrolyte, the polymer solid electrolyte, and the complex hydride solid electrolyte, those described for first solid electrolyte 101 in the first embodiment can be used.

[0142] The third solid electrolyte may include a sulfide solid electrolyte.

[0143] The third solid electrolyte may include at least one selected from the group consisting of an oxide solid electrolyte, a polymer solid electrolyte, and a complex hydride solid electrolyte.

[0144] The third solid electrolyte may include a halide solid electrolyte.

[0145] The halide solid electrolyte is represented by, for example, the following composition formula (2).

[0146] Li α M β X γ ...Equation (2)

[0147] In composition formula (2), α, β, and γ are each independently a value greater than 0. M includes at least one element selected from the group consisting of metal elements and metalloid elements other than Li. X includes at least one element selected from the group consisting of F, Cl, Br, and I.

[0148] In this disclosure, "metalloid elements" refer to B, Si, Ge, As, Sb, and Te. "Metal elements" refer to all elements in Groups 1 to 12 of the periodic table excluding hydrogen, and all elements in Groups 13 to 16 of the periodic table excluding B, Si, Ge, As, Sb, Te, C, N, P, O, S, and Se. In other words, "metalloid elements" or "metal elements" refer to a group of elements that can become cations when forming inorganic compounds with halogen elements.

[0149] Specifically, examples of halide solid electrolytes that can be used include Li3YX6, Li2MgX4, Li2FeX4, Li(Al,Ga,In)X4, and Li3(Al,Ga,In)X6. In this disclosure, when an element in a formula is represented as "(Al,Ga,In)", this notation indicates at least one element selected from the group of elements in parentheses. In other words, "(Al,Ga,In)" is synonymous with "at least one element selected from the group consisting of Al, Ga, and In." The same applies to other elements.

[0150] The halide solid electrolyte exhibits high ionic conductivity. Therefore, the above configuration can improve the power density of the battery 2000. Furthermore, the thermal stability of the battery 2000 can be improved, and the generation of harmful gases such as hydrogen sulfide can be suppressed.

[0151] In composition formula (2), M may contain Y (=yttrium). That is, the halide solid electrolyte contained in electrolyte layer 202 may contain Y as a metal element. With the above configuration, the ionic conductivity of the halide solid electrolyte can be further improved.

[0152] The halide solid electrolyte containing Y may be a compound represented by the following composition formula (3).

[0153] Li a Me b Y c X16...Formula (3)

[0154] In the composition formula (3), a + mb + 3c = 6 and c > 0 are satisfied. Me includes at least one element selected from the group consisting of metal elements and metalloid elements excluding Li and Y. m is the valence of the element Me. X1 includes at least one element selected from the group consisting of F, Cl, Br, and I. This configuration can further improve the ionic conductivity of the halide solid electrolyte. This can further improve the power density of the battery 2000.

[0155] Me may include, for example, at least one selected from the group consisting of Mg, Ca, Sr, Ba, Zn, Sc, Al, Ga, Bi, Zr, Hf, Ti, Sn, Ta, and Nb. This configuration can further improve the ionic conductivity of the halide solid electrolyte. This can further improve the power density of the battery 2000.

[0156] Specific examples of Y-containing halide solid electrolytes include Li3YF6, Li3YCl6, Li3YBr6, Li3YI6, Li3YBrCl5, Li3YBr3Cl3, Li3YBr5Cl, Li3YBr5I, Li3YBr3I3, Li3YBrI5, Li3YClI5, Li3YCl3I3, Li3YCl5I, Li3YBr2Cl2I2, Li3YBrCl4I, Li 2.7 Y 1.1 Cl6, Li 2.5 Y 0.5 Zr 0.5 Cl6, Li 2.5 Y 0.3 Zr 0.7 Cl6, etc. can be used. According to the above configuration, the output density of the battery 2000 can be further improved.

[0157] The electrolyte layer 202 may contain only one solid electrolyte selected from the group of solid electrolytes described above, or may contain two or more solid electrolytes selected from the group of solid electrolytes described above. The multiple solid electrolytes have different compositions. For example, the electrolyte layer 202 may contain a halide solid electrolyte and a sulfide solid electrolyte.

[0158] The thickness of the electrolyte layer 202 may be 1 μm or more and 300 μm or less. When the thickness of the electrolyte layer 202 is 1 μm or more, the negative electrode 201 and the positive electrode 203 are less likely to short-circuit. When the thickness of the electrolyte layer 202 is 300 μm or less, the battery 2000 can operate at high power.

[0159] The positive electrode 203 contributes to the operation of the battery 2000 as a counter electrode to the negative electrode 201 .

[0160] The positive electrode 203 may include a material having the property of absorbing and releasing metal ions (e.g., lithium ions). The positive electrode 203 includes, for example, a positive electrode active material. Examples of the positive electrode active material that can be used include metal composite oxides, transition metal fluorides, polyanionic materials, fluorinated polyanionic materials, transition metal sulfides, transition metal oxysulfides, and transition metal oxynitrides. In particular, when a lithium-containing transition metal oxide is used as the positive electrode active material, the manufacturing cost can be reduced and the average discharge voltage can be increased.

[0161] The positive electrode 203 includes, for example, a positive electrode active material layer containing a positive electrode active material, and a positive electrode current collector. The positive electrode active material layer is disposed between the positive electrode current collector and the electrolyte layer 202.

[0162] The metal composite oxide selected as the positive electrode active material may contain Li and at least one selected from the group consisting of Mn, Co, Ni, and Al. This configuration can further improve the energy density of the battery 2000. Examples of such materials include Li(Ni,Co,Al)O2, Li(Ni,Co,Mn)O2, and LiCoO2. For example, the positive electrode active material may be Li(Ni,Co,Mn)O2.

[0163] The positive electrode 203 may include an electrolyte, for example, a solid electrolyte. According to the above configuration, the lithium ion conductivity inside the positive electrode 203 is improved, and the battery 2000 can operate at high power. As the solid electrolyte contained in the positive electrode 203, the material exemplified as the third solid electrolyte in the electrolyte layer 202 may be used.

[0164] The shape of the positive electrode active material is not particularly limited. The positive electrode active material may be acicular, spherical, oval spherical, etc. The positive electrode active material may be particulate.

[0165] The median diameter of the positive electrode active material may be 0.1 μm or more and 100 μm or less. When the median diameter of the positive electrode active material is 0.1 μm or more, the positive electrode active material and the solid electrolyte can be well dispersed in the positive electrode 203. This improves the charge capacity of the battery 2000. When the median diameter of the positive electrode active material is 100 μm or less, the lithium diffusion rate within the positive electrode active material is sufficiently ensured. This enables the battery 2000 to operate at high power.

[0166] The median diameter of the positive electrode active material may be larger than the median diameter of the solid electrolyte contained in the positive electrode 203. This allows the positive electrode active material and the solid electrolyte to be well dispersed in the positive electrode 203.

[0167] The mass ratio "w2:100-w2" of the positive electrode active material to the solid electrolyte contained in the positive electrode 203 may satisfy 40≦w2≦90. When 40≦w2 is satisfied, the energy density of the battery 2000 is sufficiently ensured. Furthermore, when w2≦90 is satisfied, the battery 2000 can operate at high output.

[0168] The thickness of the positive electrode 203 may be 10 μm or more and 500 μm or less. When the thickness of the positive electrode 203 is 10 μm or more, the energy density of the battery 2000 is sufficiently ensured. When the thickness of the positive electrode 203 is 500 μm or less, the battery 2000 can operate at high output.

[0169] The positive electrode active material may be coated with a coating material to reduce the interfacial resistance with the solid electrolyte. A material with low electronic conductivity may be used as the coating material. Examples of the coating material include the sulfide solid electrolyte, oxide solid electrolyte, halide solid electrolyte, polymer solid electrolyte, and complex hydride solid electrolyte described above.

[0170] The coating material may be an oxide solid electrolyte.

[0171] Oxide solid electrolytes that can be used as coating materials include Li-Nb-O compounds such as LiNbO3, Li-BO compounds such as LiBO2 and Li3BO3, Li-Al-O compounds such as LiAlO2, Li-Si-O compounds such as Li4SiO4, Li2SO4, and Li4Ti5O. 12 Examples of oxide solid electrolytes include Li-Ti-O compounds such as Li2ZrO3, Li-Zr-O compounds such as Li2MoO3, Li-VO compounds such as LiV2O5, and Li-WO compounds such as Li2WO4. Oxide solid electrolytes have high ionic conductivity. Oxide solid electrolytes have excellent high potential stability. Therefore, by using an oxide solid electrolyte as a coating material, the charge / discharge efficiency of the battery 2000 can be further improved.

[0172] At least one of the negative electrode 201, the electrolyte layer 202, and the positive electrode 203 may contain a binder to improve adhesion between particles. The binder is used, for example, to improve the binding properties of the materials constituting the electrodes. Examples of binders include polyvinylidene fluoride, polytetrafluoroethylene, polyethylene, polypropylene, aramid resin, polyamide, polyimide, polyamideimide, polyacrylonitrile, polyacrylic acid, polymethyl acrylate, polyethyl acrylate, polyhexyl acrylate, polymethacrylic acid, polymethyl methacrylate, polyethyl methacrylate, polyhexyl methacrylate, polyvinyl acetate, polyvinylpyrrolidone, polyether, polyethersulfone, hexafluoropolypropylene, styrene-butadiene rubber, and carboxymethyl cellulose. The binder may be a copolymer of two or more materials selected from tetrafluoroethylene, hexafluoroethylene, hexafluoropropylene, perfluoroalkyl vinyl ether, vinylidene fluoride, chlorotrifluoroethylene, ethylene, propylene, pentafluoropropylene, fluoromethyl vinyl ether, acrylic acid, and hexadiene. Alternatively, a mixture of two or more materials selected from these may be used as the binder.

[0173] At least one of the negative electrode 201 and the positive electrode 203 may contain a conductive additive 140 for the purpose of improving electronic conductivity. Examples of the conductive additive 140 include graphites such as natural graphite or artificial graphite, carbon blacks such as acetylene black and ketjen black, conductive fibers such as carbon fiber or metal fiber, metal powders such as carbon fluoride and aluminum, conductive whiskers such as zinc oxide or potassium titanate, conductive metal oxides such as titanium oxide, and conductive polymer compounds such as polyaniline, polypyrrole, and polythiophene. Using a carbon-based conductive additive 140 can reduce costs.

[0174] 3, the negative electrode 201 contains a conductive additive 140. However, the conductive additive 140 is not an essential element.

[0175] The shape of the battery 2000 may be, for example, a coin type, a cylindrical type, a square type, a sheet type, a button type, a flat type, or a laminate type.

[0176] <Battery manufacturing method> The battery 2000 using the negative electrode active material 1000 can be manufactured, for example, by the following method (dry method).

[0177] A powder of a solid electrolyte is poured into a ceramic mold. The solid electrolyte powder is compressed to form an electrolyte layer 202. A powder of a negative electrode material is poured onto one side of the electrolyte layer 202. The powder of the negative electrode material is compressed to form a negative electrode active material layer on the electrolyte layer 202. The negative electrode material includes a negative electrode active material 1000 and a second solid electrolyte 130. A powder of a positive electrode material is poured onto the other side of the electrolyte layer 202. The powder of the positive electrode material is compressed to form a positive electrode active material layer. This results in a power generating element comprising a negative electrode active material layer, an electrolyte layer 202, and a positive electrode active material layer.

[0178] Current collectors are placed above and below the power generating element, and current collecting leads are attached to the current collectors, thereby obtaining a battery 2000.

[0179] The battery 2000 using the anode active material 1000 can also be manufactured by a wet process. In the wet process, for example, an anode slurry containing the anode active material 1000 and the second solid electrolyte 130 is applied to a current collector to form a coating film. Next, the coating film is pressed through a roll or flat press heated to a temperature of 120°C or higher. This results in a anode active material layer. The electrolyte layer 202 and the cathode active material layer are fabricated in a similar manner. Next, the anode active material layer, the electrolyte layer 202, and the cathode active material layer are laminated in this order. This results in a power generating element. [Example]

[0180] Hereinafter, the present disclosure will be described in detail using examples and comparative examples. The following examples are merely illustrative, and the present disclosure is not limited to the following examples.

[0181] Example 1 [Preparation of porous silicon particles] Under an argon atmosphere, 0.65 g of silicon microparticles (Kojundo Chemical Laboratory, particle size 5 μm) and 0.60 g of metallic Li (Honjo Metals) were mixed in an agate mortar to obtain a LiSi precursor. In a glass reactor under an argon atmosphere, 1.0 g of the LiSi precursor was reacted with 250 mL of ethanol (Nacalai Tesque, Inc.) at 0°C for 120 minutes. The first liquid and first solid reactant were then separated by suction filtration. In an air-conditioned glass reactor, 0.5 g of the resulting first solid reactant was reacted with 50 mL of acetic acid (Nacalai Tesque, Inc.) for 60 minutes. The second liquid and second solid reactant were then separated by suction filtration. The second solid reactant was vacuum-dried at 100°C for 2 hours to obtain porous silicon particles with a three-dimensional network structure. The median diameter of the porous silicon particles was 0.5 μm. The average pore diameter of the porous silicon particles, determined by the BJH method of gas adsorption measurement using nitrogen, was 50 nm.

[0182] [Preparation of solution L] As raw material powders for the first solid electrolyte, powders of Li2S, P2S5, and LiCl were prepared.

[0183] Li2S, P2S5, and ultra-dehydrated tetrahydrofuran (THF) were mixed in an argon glove box with a dew point of -60°C or less. The molar ratio of Li2S to P2S5 was 3:1. The THF did not contain any stabilizers. The resulting mixture was stirred overnight to obtain a THF suspension containing Li3PS4.

[0184] In an argon glove box, Li2S and LiCl were dissolved in ultra-dehydrated ethanol (EtOH) to obtain an EtOH solution. The molar ratio of Li2S to LiCl was 1:1.

[0185] The THF suspension and the EtOH solution were mixed in an argon glove box to obtain Solution L. In Solution L, the molar ratio of Li2S, P2S5, and LiCl was 5:1:2. The concentrations of Li2S, P2S5, and LiCl in Solution L were 4.5 mass%.

[0186] [Preparation of negative electrode active material] In an argon glove box, porous silicon particles and solution L were weighed out so that the volume ratio of porous silicon particles to Li2S, P2S5, and LiCl was 97.5:2.5. Next, these were mixed in an agate mortar to bring the porous silicon particles and solution L into contact. Furthermore, while they were kneaded, the solvent contained in solution L was volatilized. Note that the volume ratio of porous silicon particles to Li2S, P2S5, and LiCl can be considered to be the volume ratio of porous silicon particles to Li6PS5Cl.

[0187] Next, the obtained composite was heat-treated at 150°C for 2 hours in a vacuum atmosphere to obtain a negative electrode active material. The negative electrode active material contained Li6PS5Cl as the first solid electrolyte.

[0188] [Preparation of sulfide solid electrolyte A] Li2S and P2S5 were weighed in an argon glove box with a dew point below -60°C. The molar ratio of Li2S to P2S5 was 75:25. These were ground and mixed in an agate mortar to obtain a mixture. Next, the mixture was milled for 10 hours at 510 rpm using a planetary ball mill (Fritsch, Model P-7) to obtain a glassy solid electrolyte. The glassy solid electrolyte was then heat-treated at 270°C for 2 hours in an inert atmosphere. This resulted in the production of glass-ceramic sulfide solid electrolyte A, Li2S-P2S5.

[0189] [Preparation of cathode material B] As the positive electrode active material, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (manufactured by Nichia Corporation) was used as the positive electrode active material. The surface of the positive electrode active material was coated with LiNbO3. 1.5 g of this positive electrode active material, 0.023 g of a conductive additive (VGCF, manufactured by Showa Denko K.K.), 0.239 g of sulfide solid electrolyte A, 0.011 g of a binder (PVdF, manufactured by Kureha Corporation), and 0.8 g of a solvent (butyl butyrate, manufactured by Kishida Chemical Co., Ltd.) were weighed and mixed using an ultrasonic homogenizer (UH-50, manufactured by SMT Corporation). This produced positive electrode material B. "VGCF" is a registered trademark of Showa Denko K.K.

[0190] [Preparation of negative electrode material C] In an argon glove box with a dew point of -60°C or less, 1.02 g of the negative electrode active material, 0.100 g of a conductive additive (VGCF, manufactured by Showa Denko K.K.), 0.920 g of sulfide solid electrolyte A, 0.03 g of a binder (PVdF, manufactured by Kureha Corporation), and 2.0 g of a solvent (butyl butyrate, manufactured by Kishida Chemical Co., Ltd.) were weighed and mixed using an ultrasonic homogenizer (UH-50, manufactured by SMT Corporation). This produced negative electrode material C.

[0191] [Secondary battery production] 1cm 2 0.065 g of sulfide solid electrolyte A was weighed and placed in a ceramic mold. 2 The electrolyte layer was formed by applying pressure at 1000 kJ / cm 2 .

[0192] 0.030 g of positive electrode material B was weighed and placed on one side of the electrolyte layer. 2 A positive electrode active material layer was prepared by applying pressure at a pressure of 4 ton / cm. 0.030 g of negative electrode material C was weighed and placed on the other side of the electrolyte layer. 2 A negative electrode active material layer was produced by pressing at a pressure of 1000 kJ / cm.sup.2 at ...

[0193] Aluminum foil was placed on the positive electrode active material layer side of the power generating element as a positive electrode current collector, and a current collecting lead was attached. Copper foil was placed on the negative electrode active material layer side of the power generating element as a negative electrode current collector, and a current collecting lead was attached. This produced a battery of Example 1.

[0194] Example 2 In the process of preparing the negative electrode active material, the porous silicon particles and solution L were weighed out so that the volume ratio of porous silicon particles:LiS, P2S5, and LiCl was 95.0:5.0. Except for this, the negative electrode active material and battery of Example 2 were obtained in the same manner as in Example 1.

[0195] Example 3 In the process of preparing the negative electrode active material, the porous silicon particles and solution L were weighed out so that the volume ratio of porous silicon particles:LiS, P2S5, and LiCl was 90.0:10.0. Except for this, the negative electrode active material and battery of Example 3 were obtained in the same manner as in Example 1.

[0196] Comparative Example 1 The porous silicon particles were used as the negative electrode active material without being combined with the first solid electrolyte. That is, the volume ratio of the porous silicon particles to Li2S, P2S5, and LiCl was porous silicon particles:Li2S, P2S5, and LiCl=100:0. Other than this, the battery of Comparative Example 1 was obtained in the same manner as in Example 1.

[0197] Comparative Example 2 In the process of preparing the negative electrode active material, the porous silicon particles and solution L were weighed out so that the volume ratio of porous silicon particles:LiS, P2S5, and LiCl was 85.0:15.0. Except for this, the negative electrode active material and battery of Comparative Example 2 were obtained in the same manner as in Example 1.

[0198] [SEM observation of the cross section of the negative electrode active material and measurement of the average thickness of the thin film] The cross section of the negative electrode active material of Example 1 was observed using an SEM (SU-70, manufactured by Hitachi High-Technologies Corporation), and elemental mapping of S, P, and Cl was performed using EDS. The porous silicon particles contained in the negative electrode active material had a three-dimensional network structure, and S, P, and Cl originating from the first solid electrolyte Li6PS5Cl were observed in the form of a thin film along the three-dimensional network structure. The distribution of S, P, and Cl was uniform, confirming that Li6PS5Cl had precipitated along the three-dimensional network structure. The average thickness of the Li6PS5Cl thin film measured using a cross-sectional TEM image was 5 nm. The average thickness of the first solid electrolyte thin film was also measured using the same method for the negative electrode active materials of Examples 2 and 3 and Comparative Example 2. The results are shown in Table 1.

[0199] [Measurement of pore volume and specific surface area of ​​negative electrode active material by nitrogen adsorption measurement] For the negative electrode active materials of the Examples and Comparative Examples, the pore volume and specific surface area were determined by a gas adsorption method using nitrogen gas. Specifically, adsorption isotherm data was obtained using a gas adsorption measurement device (BELLSORP MAX, manufactured by Microtrack-Bell) using nitrogen gas. From the obtained adsorption isotherm data, the pore volume at a relative pressure of 0.99 was measured. From the measured pore volume values, the ratio of the pore volume of the negative electrode active materials of Examples 1 to 3 and Comparative Example 2 to the pore volume of the porous silicon particles (corresponding to the negative electrode active material of Comparative Example 1) was calculated. The obtained adsorption isotherm data was converted using the BET method to calculate the specific surface area. From the calculated specific surface area values, the ratio of the specific surface area of ​​the negative electrode active materials of Examples 1 to 3 and Comparative Example 2 to the specific surface area of ​​the porous silicon particles (corresponding to the negative electrode active material of Comparative Example 1) was calculated. The results are shown in Table 1.

[0200] [Charging test] Next, a charging test was carried out on each of the batteries of the example and comparative example under the following conditions.

[0201] First, the battery was placed in a thermostatic chamber at 25°C. While applying a pressure of 5 MPa to the battery using a pressure jig, the battery was subjected to constant current charge and discharge. The charge end voltage was 4.05 V. The discharge end voltage was 2.5 V. Constant current charge and discharge were initially performed at a 0.3 C rate and then at a 1 C rate (1-hour rate) relative to the theoretical capacity of the battery. Based on the obtained results, the discharge capacity ratio at a 0.3 C rate, the discharge capacity ratio at the 100th cycle after 100 cycles of charge and discharge at a 1 C rate, and the discharge capacity retention rate were calculated. The results are shown in Table 1. The discharge capacity ratio at a 0.3 C rate and the discharge capacity ratio at the 100th cycle in Table 1 are values ​​normalized by setting the respective values ​​for the battery of Comparative Example 1 to 100. The discharge capacity retention rate at the 100th cycle in Table 1 is a value when the discharge capacity at the first cycle (before the cycle test) is set to 100.

[0202] [Table 1]

[0203] ≪Consideration≫ In the batteries of Examples 1 to 3, the 0.3 C rate discharge capacity before the cycle test, the 1 C rate discharge capacity at the 100th cycle, and the discharge capacity retention rate were all improved compared to the battery of Comparative Example 1. This is thought to be because in Examples 1 to 3, ion conduction was promoted in the negative electrode active material even to the interior of the porous silicon particles, and as a result, the charge / discharge reaction occurred smoothly throughout the entire porous silicon particle.

[0204] In the battery of Comparative Example 2, the 0.3C rate discharge capacity before the cycle test, the discharge capacity ratio at 1C rate after the 100th cycle, and the discharge capacity retention rate were all lower than those of the batteries of Examples 1 to 3. This is thought to be because in Comparative Example 2, the average thickness of the thin film of the first solid electrolyte in the negative electrode active material was 30 nm or more, so that the first solid electrolyte excessively entered the pores of the porous silicon particles, making it difficult for the expansion and contraction required for high capacity to occur. [Industrial Applicability]

[0205] The battery of the present disclosure can be used, for example, as an all-solid-state lithium secondary battery. [Explanation of symbols]

[0206] 100 porous silicon particles 101 First solid electrolyte 102 pores 130 Second solid electrolyte 140 Conductive additives 201 Negative electrode 202 Electrolyte layer 203 Positive electrode 1000 negative electrode active material 2000 batteries

Claims

1. Porous silicon particles; a solid electrolyte; Including, the porous silicon particles have a plurality of pores; the solid electrolyte has the form of a thin film covering at least a part of the inner surface of the pore; The average thickness of the thin film is less than 30 nm. Negative electrode active material.

2. a ratio of the pore volume of the negative electrode active material to the pore volume of the porous silicon particles is 55% or more and less than 99%; The negative electrode active material according to claim 1 .

3. a ratio of the specific surface area of ​​the negative electrode active material to the specific surface area of ​​the porous silicon particles is 55% or more and less than 99%; The negative electrode active material according to claim 1 or 2.

4. the ratio of the volume of the solid electrolyte to the volume of the porous silicon particles is 0.5% or more and less than 15%; The negative electrode active material according to claim 1 .

5. The solid electrolyte comprises lithium, phosphorus, sulfur, and a halogen. The negative electrode active material according to claim 1 .

6. The average thickness of the thin film is 3 nm or more. The negative electrode active material according to claim 1 .

7. a negative electrode; A positive electrode and an electrolyte layer disposed between the negative electrode and the positive electrode; Equipped with The negative electrode comprises the negative electrode active material according to claim 1 . battery.

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