Negative electrode active material and battery

Abstract

A negative electrode active material according to the present disclosure comprises base particles 10 which each include: an alkali aluminate phase 11 that contains at least one alkali metal element and Al; silicon phases 12 that are dispersed in the alkali aluminate phase 11; and carbon phases 13 that are dispersed in the alkali aluminate phase 11. In a cross-section of primary particles of the base particles 10, an area ratio occupied by the carbon phases 13 is 4% or more and 25% or less.

Description

Negative electrode active material and battery

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

[0002] In recent years, secondary batteries such as lithium-ion batteries have been widely used in applications requiring high capacity, such as automotive applications and power storage applications. Since the negative electrode active material, which is a main component of the negative electrode, is one of the important factors for increasing the capacity of batteries, various studies on the negative electrode active material have been conducted. Among these, the use of silicon-containing materials (Si-containing materials) has attracted attention as negative electrode active materials with high theoretical capacity density.

[0003] Patent Document 1 discloses an anode active material including composite particles containing a lithium aluminate phase and a silicon phase dispersed within the lithium aluminate phase. The anode active material disclosed in Patent Document 1 includes composite particles (hereinafter sometimes referred to as "LAX particles") in which a silicon phase is dispersed within a lithium aluminate phase having excellent alkali resistance, thereby reducing side reactions that occur in the early stages of charge and discharge in a lithium ion secondary battery and suppressing a decrease in the initial charge and discharge efficiency.

[0004] International Publication No. 2022 / 113500

[0005] As described above, a negative electrode active material containing LAX particles can suppress a decrease in the initial charge / discharge efficiency of a battery.

[0006] However, negative electrode active materials containing LAX particles have problems with durability against charge and discharge, and there is a demand for improving the cycle characteristics of the battery.

[0007] The present disclosure provides a negative electrode active material that can improve the cycle characteristics of a battery.

[0008] The negative electrode active material of the present disclosure comprises a base particle including: an alkali aluminate phase containing at least one alkali metal element and Al; a silicon phase dispersed within the alkali aluminate phase; and a carbon phase dispersed within the alkali aluminate phase, wherein the carbon phase occupies an area ratio of 4% to 25% of a cross section of a primary particle of the base particle.

[0009] According to the technology of the present disclosure, it is possible to provide a negative electrode active material that can improve the cycle characteristics of a battery.

[0010] Fig. 1 is a cross-sectional view showing a schematic configuration of an example of a base particle included in the negative electrode active material according to the present embodiment. Fig. 2 is a cross-sectional view showing a schematic configuration of a composite particle including the base particle shown in Fig. 1 and a conductive layer covering at least a portion of the surface of the base particle. Fig. 3 is a cross-sectional view showing a schematic configuration of an example of a battery according to the present embodiment. Fig. 4 is a cross-sectional view showing a schematic configuration of another example of a battery according to the present embodiment.

[0011] [Findings underlying the present disclosure] A negative electrode active material including composite particles such as LAX particles, which include an alkali aluminate phase, such as a lithium aluminate phase, containing at least one alkali metal element and Al, and a silicon phase dispersed within the alkali aluminate phase, can reduce side reactions during the initial charging and discharging of a battery and suppress a decrease in initial charging and discharging efficiency. The alkali aluminate phase has excellent alkali resistance. A negative electrode active material including composite particles in which a silicon phase is dispersed within such an alkali aluminate phase can reduce side reactions during the initial charging of a battery and achieve high initial efficiency.

[0012] However, negative electrode active materials containing the above composite particles containing an alkali aluminate phase have insufficient durability against charge and discharge, leaving room for improvement in cycle characteristics. The inventors have investigated the cause and found that, due to the high melting point and softening point of the alkali aluminate phase, many voids are likely to form inside the alkali aluminate phase when it is prepared by a sintering process, and the presence of these voids makes the particle structure prone to collapse due to expansion and contraction during charge and discharge. The inventors have conducted extensive research focusing on the internal structure of the particles that can suppress the collapse of the particle structure due to expansion and contraction during charge and discharge, and have found that by dispersing a carbon phase within the particles at a specific area ratio, the durability of the particles can be improved, thereby effectively improving the cycle characteristics of the battery, leading to the idea of ​​the negative electrode active material of the present disclosure, which will be described below.

[0013] [Embodiments of the Present Disclosure] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. The present disclosure is not limited to the following embodiments.

[0014] (Negative Electrode Active Material) The negative electrode active material according to this embodiment includes base particles. Fig. 1 is a cross-sectional view showing a schematic configuration of an example of the base particles included in the negative electrode active material according to this embodiment.

[0015] The base particle 10 included in the negative electrode active material according to this embodiment includes an alkali aluminate phase 11, a silicon phase 12 dispersed within the alkali aluminate phase 11, and a carbon phase 13 dispersed within the alkali aluminate phase 11. In the cross section of a primary particle of the base particle 10, the area ratio occupied by the carbon phase 13 is 4% or more and 25% or less.

[0016] In the negative electrode active material according to this embodiment, the carbon phase 13 is dispersed within the alkali aluminate phase 11 so that the area ratio of the cross section of the primary particle of the base particle 10 is 4% or more and 25% or less. This configuration suppresses the collapse of the particle structure of the base particle 10 (for example, the occurrence of cracks and fractures) caused by the expansion and contraction of the negative electrode active material accompanying the charge and discharge of the battery. This enables the negative electrode active material according to this embodiment to have improved durability against charge and discharge, and as a result, to improve the cycle characteristics of the battery.

[0017] The alkali aluminate phase 11 has excellent alkali resistance. Therefore, the negative electrode active material including the base particles 10 is prevented from undergoing side reactions with alkali metal ions such as Li ions during initial charging, thereby preventing deterioration of the negative electrode active material due to side reactions and a decrease in initial capacity due to the deterioration. That is, the negative electrode active material according to this embodiment not only improves the cycle characteristics of the battery described above, but also prevents a decrease in initial charge / discharge efficiency.

[0018] Here, in this specification, the term "mother particle" refers to a particulate structure made of a material in which a silicon phase 12 and a carbon phase 13 are dispersed in a matrix of an alkali aluminate phase 11. Therefore, for example, a coating layer such as a conductive layer provided on the surface of the mother particle, as will be described later, is not included in the mother particle.

[0019] As described above, the area ratio of the carbon phase 13 specified in this specification is the ratio of the total area occupied by the carbon phase 13 dispersed inside the base particle 10, when the cross-sectional area of ​​the primary particle of the base particle 10 is taken as the total area. Therefore, for example, when a coating layer such as a conductive layer is provided on the surface of the base particle 10 and this coating layer contains carbon, in secondary particles formed by aggregation of the base particles 10, the carbon contained in the coating layer located inside the secondary particles is naturally not included in the area ratio of the carbon phase 13 described above.

[0020] The area ratio of the carbon phase 13 in the cross section of the primary particle of the base particle 10 can be identified by elemental mapping analysis using energy dispersive X-rays (EDX). Details of the method for analyzing the carbon phase 13 (i.e., carbon element) in the cross section of the base particle using EDX will be described later together with the analysis of other elements.

[0021] Here, when determining the area ratio occupied by the carbon phase 13 by elemental mapping using EDX, for example, if a coating layer such as a conductive layer is provided on the surface of the base particle 10 and the coating layer contains carbon, it is necessary to prevent the carbon element contained in the coating layer from being calculated as the area of ​​the carbon phase 13 inside the base particle 10, and to reliably calculate the area of ​​the carbon phase 13 dispersed inside the base particle 10. Therefore, when determining the area ratio occupied by the carbon phase 13 in the cross section of the base particle 10, a region 200 nm or more inward from the particle surface is used.

[0022] In the negative electrode active material according to this embodiment, the area ratio of the carbon phase 13 in the cross section of the primary particle of the base particle 10 may be 5% or more and 20% or less. This enables the negative electrode active material according to this embodiment to further improve the cycle characteristics of the battery.

[0023] In the negative electrode active material according to this embodiment, the base particle 10 may further contain other elements in addition to the alkali aluminate phase 11 , the silicon phase 12 , and the carbon phase 13 .

[0024] The negative electrode active material according to this embodiment may further include a conductive layer that covers at least a portion of the surface of the base particle 10. In other words, the negative electrode active material according to this embodiment may include a composite particle 20 that includes the base particle 10 and a conductive layer 21 that covers at least a portion of the surface of the base particle.

[0025] Fig. 2 is a cross-sectional view showing a schematic configuration of a composite particle 20 including the base particle 10 shown in Fig. 1 and a conductive layer 21 that covers at least a portion of the surface of the base particle 10. A negative electrode active material including a composite particle 20 in which at least a portion of the surface of the base particle 10 is coated with the conductive layer 21, as shown in Fig. 2, can improve conductivity.

[0026] Each component of the negative electrode active material of this embodiment will be specifically described below.

[0027] [Mother Particle] As shown in FIG. 1 , the mother particle 10 has a sea-island structure in which a plurality of fine silicon phases 12 and carbon phases 13 (i.e., islands) are dispersed in a matrix (i.e., sea portion) of an alkali aluminate phase 11. The alkali aluminate phase 11 has good ionic conductivity. Therefore, this structure allows the silicon phase 12 to smoothly absorb and release alkali metal ions via the alkali aluminate phase 11. This structure allows the alkali aluminate phase 11 to alleviate stress associated with the expansion and contraction of the silicon phase 12 during charge and discharge, thereby suppressing cracking and fracture of the mother particle 10. Therefore, the inclusion of silicon can achieve both high capacity and improved cycle characteristics.

[0028] The base particles 10 may be contained in the negative electrode active material in the form of primary particles, or in the form of secondary particles formed by bonding multiple primary particles. The average particle size of the secondary particles of the base particles 10 may be, for example, 1 μm or more and 25 μm or less, or 4 μm or more and 15 μm or less. In this case, stress caused by volumetric changes in the base particles 10 associated with charge and discharge is easily alleviated, making it easier for the negative electrode active material according to this embodiment to achieve good battery cycle characteristics. The surface area of ​​the base particles 10 is also appropriately large, suppressing capacity loss due to side reactions with the nonaqueous electrolyte.

[0029] The average particle size of the base particles 10 refers to the particle size (volume average particle size) at which the volume cumulative value is 50% in the particle size distribution measured by laser diffraction scattering. For example, an "LA-750" manufactured by Horiba, Ltd. can be used as the measuring device. Note that when the surface of the base particle 10 is covered with a conductive layer 21, i.e., in the case of the composite particle 20, the thickness of the conductive layer 21 is thin enough not to substantially affect the average particle size of the base particles 10. Therefore, the average particle size of the composite particle 20 can be regarded as the average particle size of the base particles 10.

[0030] The base particles 10 can be removed from the battery by, for example, the following method, which is just an example.

[0031] First, a fully discharged battery is disassembled to remove the negative electrode. The negative electrode is washed with, for example, anhydrous ethyl methyl carbonate or dimethyl carbonate to remove non-aqueous electrolyte components. The negative electrode mixture layer is peeled off from the current collector (e.g., copper foil) that serves as the negative electrode core, and the mixture layer is pulverized in a mortar to obtain a sample powder. Next, the sample powder is dried in a dry atmosphere for about 1 hour, and then immersed in, for example, gently boiled 6 M hydrochloric acid for about 10 minutes to remove elements derived from materials other than the base particles 10. Next, the sample powder is washed with ion-exchanged water, filtered, and dried, for example, at 200°C for 1 hour. Note that a fully discharged state refers to a state in which the depth of discharge (DOD) is 90% or more (the state of charge (SOC) is 10% or less).

[0032] The alkali aluminate phase 11 contains at least one alkali metal element, aluminum (Al), and oxygen (O). The alkali aluminate phase 11 is a phase containing alkali aluminate, which is a composite oxide containing at least one alkali metal element and Al.

[0033] The alkali aluminate phase 11 has good ionic conductivity, and ions such as Li ions can be smoothly absorbed and released by the silicon phase 12 via the alkali aluminate phase 11 .

[0034] When the alkali metal element contained in the alkali aluminate phase 11 is Li, the alkali aluminate phase 11 contains lithium aluminate. The composition of lithium aluminate is represented by the formula: Li uAlO (3+u) / 2 From the viewpoints of ease of preparation, stability, ionic conductivity, and the like, u in the formula may be, for example, greater than 0 and 5 or less, or greater than 0 and 1 or less. When u = 1 / 5, it can be expressed as LiAl5O8, and when u = 1 / 2, it can be expressed as Li2Al4O7. When u = 1, it can be expressed as LiAlO2, and when u = 5, it can be expressed as Li5AlO4.

[0035] When the alkali aluminate phase 11 contains lithium aluminate, the alkali aluminate phase 11 may contain, for example, at least one selected from the group consisting of LiAlO, LiAlO, LiAlO, and LiAlO, or may contain LiAlO as a major component. Here, the "major component" refers to a component that accounts for 50 mass% or more of the total mass of the alkali aluminate phase 11. The content of LiAlO may be 70 mass% or more.

[0036] The alkali aluminate phase 11 has better alkali resistance than the lithium silicate phase. Therefore, in the negative electrode active material of this embodiment, side reactions with Li ions during initial charging, for example, are suppressed compared to a negative electrode active material using a lithium silicate phase, and deterioration of the negative electrode active material due to the side reactions is suppressed.

[0037] The atomic ratio of O to Al (O / Al) in the alkali aluminate is, for example, 1.6 or more and 4 or less. The atomic ratio of alkali metal (MA) to Al (MA / Al) in the alkali aluminate is, for example, 1 / 5 or more and 5 or less. When each of these atomic ratios is within the above range, the stability and ionic conductivity of the alkali aluminate phase 11 are improved. In this specification, the stability of the alkali aluminate phase 11 includes both the chemical stability (alkali resistance) and thermal stability of the alkali aluminate phase 11.

[0038] The alkali metal element is an element belonging to Group 1 of the periodic table. That is, the alkali aluminate phase 11 contains at least one element selected from the group consisting of Li, Na, K, Rb, Cs, and Fr.

[0039] The alkali metal element may include at least one selected from the group consisting of Li, Na, and K. The alkali metal element may include at least one selected from the group consisting of Li and Na, or may include Li.

[0040] The alkali aluminate phase 11 may contain two types of alkali metal elements. The alkali aluminate phase 11 may contain at least two selected from the group consisting of Li, Na, and K, or may contain Li and Na.

[0041] The alkali aluminate phase 11 may further contain an element M in addition to the alkali metal element, Al, and O.

[0042] An example of the element M is at least one selected from the group consisting of calcium (Ca), magnesium (Mg), zirconium (Zr), iron (Fe), boron (B), phosphorus (P), and lanthanum (La). When the alkali aluminate phase 11 contains one of the above-mentioned elements as the element M, for example, the stability and ionic conductivity of the alkali aluminate phase 11 are further improved. In addition, side reactions caused by contact between the alkali aluminate phase 31 and the electrolyte are suppressed. In particular, it is desirable that the element M contains at least one selected from the group consisting of Zr, Fe, P, and B. La can further improve the initial charge / discharge efficiency.

[0043] The element M may be B. That is, the alkali aluminate phase 11 may further contain B. The addition of B can reduce the number of voids contained in the base particles 20. This suppresses deterioration of the particle structure caused by the voids in the base particles 10, thereby further improving the cycle characteristics of the battery.

[0044] The element M may form a compound. The compound may be, for example, an oxide of the element M or an aluminate of the element M depending on the type of the element M. In the alkali aluminate phase 11, the content of the element M is, for example, 0.3 mol % or more and 3 mol % or less with respect to the total amount of elements other than oxygen.

[0045] The alkali aluminate phase 11 may further contain trace amounts of elements such as chromium (Cr), nickel (Ni), manganese (Mn), copper (Cu), and molybdenum (Mo).

[0046] The alkali aluminate phase 11 may be amorphous, in which case the influence of the expansion and contraction of the silicon phase 12 can be more effectively alleviated.

[0047] A fine Al2O3 phase with high crystallinity may be dispersed within the alkali aluminate phase 11. The Al2O3 phase is distributed, for example, in an island-like manner within the matrix of the alkali aluminate phase 11. In this case, expansion and cracking of the alkali aluminate phase 11 due to expansion and contraction of the silicon phase 12 are likely to be suppressed, enhancing the effect of improving cycle characteristics. When the Al2O3 phase is present, a peak derived from the Al2O3 phase may be observed near 2θ = 25.4° in the X-ray diffraction pattern of the base particle 10 obtained by X-ray diffraction measurement. The content of Al2O3 in the base particle 10 is, for example, 10 mass% or less.

[0048] The silicon phase 12 is a phase of simple silicon, which repeatedly absorbs and releases Li ions as the battery is charged and discharged. The capacity is generated by a Faraday reaction involving the silicon phase 12. The silicon phase 12 has a large capacity. Furthermore, the silicon phase 12 also expands and contracts to a large extent as the battery is charged and discharged. However, in the negative electrode active material of the present disclosure, the silicon phase 12 is dispersed within the alkali aluminate phase 11, and therefore the stress caused by the expansion and contraction of the silicon phase 12 is alleviated by the alkali aluminate phase 11.

[0049] The silicon phase 12 may contain crystalline silicon. The silicon phase 12 is composed of, for example, a plurality of crystallites. The crystallite size of the silicon phase 12 may be 30 nm or less, 20 nm or less, or 15 nm or less. As a result, the volume change due to expansion and contraction of the silicon phase 12 during charge and discharge can be reduced, and the effect of improving cycle characteristics becomes more significant. The crystallite size of the silicon phase 12 is calculated by the Scherrer equation from the half-width of the diffraction peak derived from the Si(111) plane in the X-ray diffraction pattern obtained by X-ray diffraction measurement using Cu-Kα radiation.

[0050] The lower limit of the crystallite size of the silicon phase 12 is not particularly limited, but is, for example, 1 nm. An example of a suitable crystallite size of the silicon phase 12 is 1 nm or more and 15 nm or less, or may be 5 nm or more and 11 nm or less. When the crystallite size of the silicon phase 12 is 1 nm or more, for example, the surface area of ​​the silicon phase 12 can be kept small, making it difficult for deterioration of the silicon phase 12, which is accompanied by the generation of irreversible capacity, to occur. When the crystallite size is 15 nm or less, it is easy to make the expansion and contraction of the silicon phase 12 uniform, and the stress generated in the base particle 10 is effectively alleviated.

[0051] The silicon phase 12 may be in a particulate form. The silicon phase 12 is particulate, for example, at least before the first charge. The average particle size of the silicon phase 12 may be 1 nm or more and 1000 nm or less. The average particle size of the silicon phase 12 may be 500 nm or less, 200 nm or less, or 50 nm or less. After the first charge, the average particle size of the silicon phase 12 may be 400 nm or less, or 100 nm or less. By dispersing the fine silicon phase 12 in the alkali aluminate phase 11 as described above, the volume change of the base particles 10 during charge and discharge is reduced, and the structural stability of the base particles 10 is further improved.

[0052] The average particle size of the silicon phase 12 can be measured using a scanning electron microscope (SEM) image obtained by observing the base particle 10 in which the cross section of the silicon phase 12 is exposed. Specifically, the average particle size of the silicon phase 12 is determined by averaging the maximum diameters of 100 silicon phases 12 arbitrarily selected from the cross-sectional SEM image of the base particle 10.

[0053] From the viewpoint of increasing capacity, the content of silicon phase 12 in base particle 10 may be 30% by mass or more, 35% by mass or more, or 55% by mass or more. From the viewpoint of improving cycle characteristics, the content of silicon phase 12 in base particle 10 may be 95% by mass or less, 75% by mass or less, or 70% by mass or less. In this case, the amount of silicon phase 12 exposed on the surface of base particle 10 without being covered by alkali aluminate phase 11 is reduced, and side reactions between the electrolyte and silicon phase 12 are also suppressed. The content of silicon phase 12 in base particle 10 may be 30% by mass or more and 90% by mass or less, or 35% by mass or more and 75% by mass or less.

[0054] The content of the silicon phase 12 in the base particle 10 can be determined by quantifying the amount of Si that constitutes the silicon phase 12 in the base particle 10 using Si-NMR, as will be described later.

[0055] The carbon phase 13 is a phase of a carbon material. Examples of the carbon material include graphite such as natural graphite, artificial graphite, and graphitized mesophase carbon, amorphous carbon, soft carbon, and hard carbon. The carbon material may be amorphous carbon. Examples of amorphous carbon include carbon black, burned pitch, coke, and activated carbon.

[0056] The carbon phase 13 may have a particulate shape. The shape of the particles is not particularly limited, and examples thereof include spherical, angular, plate-like, and linear shapes.

[0057] There is no particular limitation on the size of the carbon phase 13. When the carbon phase 13 is particulate, the average particle size of the carbon phase 13 may be, for example, 1 nm or more and 1000 nm or less, 10 nm or more and 500 nm or less, or 10 nm or more and 200 nm or less.

[0058] The average particle size of the carbon phase 13 can be measured using an SEM image obtained by SEM observation of the base particle 10 in which the cross section of the carbon phase 13 is exposed. Specifically, the average particle size of the carbon phase 13 is determined by averaging the maximum diameters of 100 carbon phases 13 arbitrarily selected from the cross-sectional SEM image of the base particle 10.

[0059] The base particles 10 may be substantially free of lithium silicate and SiO2. The total content of lithium silicate and SiO2 in the base particles 10 may be, for example, 3 mass % or less.

[0060] In the base particle 10, the ratio of the mass of Al element (mAl) to the total mass of elements excluding oxygen element and carbon element may be 10 mass% or more and 47 mass% or less, or 11.5 mass% or more and 45.5 mass% or less. The ratio of the mass of alkali metal element (mMA) to the total mass of elements excluding oxygen element and carbon element may be 0.7 mass% or more and 13.5 mass% or less, 1.0 mass% or more and 9.5 mass% or less, or 1.5 mass% or more and 3.5 mass% or less. When the mass ratio of Al element (mAl) and the mass ratio of alkali metal element (mMA) are within the above ranges, an aluminate phase having excellent stability and ionic conductivity is likely to be obtained.

[0061] The above configuration can improve the stability and ionic conductivity of the alkali aluminate phase 11. The stability mentioned above includes both chemical stability (alkali resistance) and thermal stability.

[0062] In the base particle 10, the ratio (mMA / mAl) of the ratio of the mass of the alkali metal element (mMA) to the total mass of elements excluding oxygen and carbon to the ratio (mAl) of the mass of the Al element to the total mass of elements excluding oxygen and carbon may be 0.04 or more and 0.50 or less, or 0.05 or more and 0.25 or less, from the viewpoints of the stability of the alkali aluminate phase 11, ionic conductivity, and reduction of porosity, etc.

[0063] In the base particle 10, the ratio (mSi) of the mass of silicon (Si) to the total mass of elements excluding oxygen and carbon may be 40% by mass or more and 90% by mass or less, or 50.8% by mass or more and 85.5% by mass or less. This configuration facilitates achieving both high capacity and good cycle characteristics in the battery. The Si mass ratio (mSi) is the amount of Si that constitutes the silicon phase 12 in the base particle 10.

[0064] When the alkali aluminate phase 11 further contains B, the ratio of the mass of B (mB) to the total mass of elements excluding oxygen and carbon in the base particle 10 may be 1 mass % or more and 20 mass % or less, or 2 mass % or more and 15 mass % or less. This configuration can make the effect of improving cycle characteristics more significant.

[0065] The ratio (mAl / mB) of the mass ratio of Al (mAl) to the mass ratio of B (mB) may be 1.0 or more and 30.0 or less, 1.0 or more and 20.0 or less, or 1.0 or more and 10.0 or less. When mAl / mB is 30.0 or less, the effect of reducing voids in the base particles 10 can be more significant. This suppresses deterioration of the particle structure caused by voids in the base particles 10, thereby effectively improving the cycle characteristics of the battery. When mAl / mB is 1.0 or more, it is possible to reduce a decrease in initial charge / discharge efficiency. Therefore, when mAl / mB is within the above range, it is possible to achieve both improved cycle characteristics and good initial charge / discharge efficiency.

[0066] As shown in FIG. 1, voids 14 may exist within the base particle 10. For example, the total void volume of voids having a diameter of 10 nm or more and 200 nm or less in the base particle 10 calculated by the BJH (Barrett-Joyner-Halenda) method is 0.05 cm 3 / g or less, and 3 / g or less. This configuration more effectively suppresses the collapse of the particle structure of the base particles 10 caused by the expansion and contraction of the negative electrode active material accompanying the charge and discharge of the battery. As a result, the negative electrode active material according to this embodiment can improve durability against charge and discharge, and as a result, the cycle characteristics of the battery can be further improved. The lower limit of the total pore volume is not particularly limited, but is preferably 0.001 cm 3 / g is preferable, and an example is 0.003 cm 3When the conductive layer 21 is provided on the surface of the base particle 10, the total pore volume of the pores having a diameter of 10 nm or more and 200 nm or less calculated by the BJH method in the composite particle 20 including the base particle 10 and the conductive layer 21 is 0.05 cm 3 / g or less, and 3 / g or less. This allows the negative electrode active material according to this embodiment to have improved durability against charge and discharge, and as a result, the cycle characteristics of the battery can be further improved. Even in this case, the lower limit of the total pore volume is not particularly limited, but is preferably 0.001 cm 3 / g is preferable, and an example is 0.003 cm 3 / g.

[0067] The total pore volume of the base particle 10 or the composite particle 20 can be controlled by adding an element M (for example, B) to the alkali aluminate phase 11, adjusting the firing temperature in the manufacturing process of the base particle 10, the compressive force applied to the particles during firing, mMA / mAl, etc. Also, the total pore volume can be reduced to 0.05 cm or less by filling the pores formed inside the base particle 10 during the production of the base particle 10 with another material (hereinafter referred to as a filling material). 3 / g or less. Examples of the filler material filled in the pores include carbon materials and resin materials. For example, when filling the pores formed inside the base particle 10 with a carbon material during the formation of the carbon phase 13 of the base particle 10, the pores of the base particle 10 may be filled with the carbon material by, for example, mixing the base particle 10 containing the pores with an amount of carbon material selected in consideration of the pore size and pore volume contained in the base particle 10, and then heat-treating the mixture. By adjusting the carbon material used and the heat-treatment conditions, etc., the total pore volume can be reduced to 0.05 cm. 3 / g or less.

[0068] The porosity of the base particles 10 before the initial charge / discharge may be 25% or less. By keeping the porosity of the base particles 10 at 25% or less, deterioration of the negative electrode active material is further suppressed, and the cycle characteristics of the battery are further improved. The porosity of the base particles 10 may be 20% or less, or may be 15% or less. The lower limit of the porosity is not particularly limited, but an example is 1%.

[0069] The porosity of the base particle 10 means the proportion of voids in the cross section of the base particle 10. The porosity of the base particle 10 can be measured using an SEM image of a cross section of the negative electrode active material in which the cross section of the base particle 10 is exposed. The porosity of the base particle 10 is calculated by extracting void regions by binarizing the SEM image using image analysis software (e.g., ImageJ), and dividing the total area of ​​the voids by the total area of ​​the particle cross section.

[0070] The porosity of the base particle 10 can be controlled by adding an element M (e.g., B) to the alkali aluminate phase 11 described above, adjusting the firing temperature in the manufacturing process of the base particle 10, the compressive force applied to the particle during firing, and mMA / mAl, etc.

[0071] The base particles 10 may have a Vickers hardness of 300 HV or more. When the base particles 10 have a high Vickers hardness, the volume change of the silicon phase 12 during charge and discharge can be easily suppressed, and deterioration of the particle structure can be reduced. As a result, the effect of improving the cycle characteristics becomes more significant. The Vickers hardness of the base particles 10 is more preferably 350 HV or more, and may be 400 HV or more, or 500 HV or more.

[0072] The Vickers hardness of the base particles 10 can be measured using a Vickers hardness tester. Specifically, the base particles 10 are embedded in a thermosetting resin and polished with #400 abrasive paper to expose the cross section of the base particles 10. The cross section is then polished to a mirror finish with #2000 abrasive paper and buffed. The Vickers hardness is measured under conditions of a load of 1 kg and a holding time of 15 seconds. The upper limit of the Vickers hardness of the base particles 10 is not particularly limited, but is, for example, 1500 HV.

[0073] The content of each element in the base particle 10 is measured by the following methods. The oxygen content can be measured using an oxygen, nitrogen, and hydrogen analyzer. The Si content can be measured using NMR. The carbon content can be measured using a carbon and sulfur analyzer. The contents of other elements can be measured by inductively coupled plasma optical emission spectroscopy (ICP). If measurement by ICP is not possible, measurement can also be performed using energy dispersive X-ray (EDX). If measurement by neither ICP nor EDX is possible, measurement can also be performed by Auger electron spectroscopy (AES). The composition of the alkali aluminate phase 11 can be determined from the content of each element. When measuring from the battery state, for example, a fully discharged battery is disassembled, the negative electrode is removed, and the negative electrode is washed with, for example, anhydrous ethyl methyl carbonate or dimethyl carbonate to remove non-aqueous electrolyte components and dried, and then the cross section of the resulting negative electrode mixture layer is analyzed using a cross-section polisher. Furthermore, when it is necessary to remove the base particles 10 from the battery as a measurement sample, as described above, the base particles can be removed from the battery by, for example, removing the negative electrode mixture layer from the negative electrode removed from the battery, pulverizing it, and removing elements derived from other than the base particles 10. The base particles may be removed from the battery in the form of composite particles on the surface of which a conductive layer is provided, and each element in the base particles may be determined based on the analysis results of each element in the composite particles, or the conductive layer may be removed using an appropriate means depending on the constituent material of the conductive layer, and the internal portion of the base particle may be analyzed.

[0074] <Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP)> A sample of the base particles 10 is completely dissolved in a heated acid solution (a mixed acid of hydrofluoric acid, nitric acid, and sulfuric acid), and the carbon remaining in the solution is filtered off. The resulting filtrate is then analyzed by ICP to measure the spectral intensity of each element. Next, a calibration curve is created using commercially available standard solutions of the elements, and the content of each element contained in the base particles 10 is calculated.

[0075] <Energy Dispersive X-ray (EDX)> Elemental mapping analysis is performed by EDX from a backscattered electron image of a cross section of the negative electrode active material in which the cross section of the base particle 10 is exposed. The area containing the target element is calculated using image analysis software. The observation magnification is, for example, 2000 to 20,000 times. When measuring from the battery state, 10 base particles 10 with a maximum particle diameter of 5 μm or more are randomly selected from the cross section image of the backscattered electron image of the negative electrode mixture layer containing the negative electrode active material, and elemental mapping analysis is performed on each of them by EDX. The measured values ​​of the area containing a predetermined element contained in the 10 particles are averaged to calculate the content of the target element.

[0076] Desirable measurement conditions for cross-sectional SEM-EDX analysis are shown below: Processing equipment: SM-09010 (Cross Section Polisher) manufactured by JEOL Processing conditions: Acceleration voltage 6 kV Current value: 140 μA Vacuum degree: 1×10 -3 Pa to 2 x 10 -3 Pa Measuring device: Hitachi SU-70 electron microscope Acceleration voltage during analysis: 10 kV Field: Free mode Probe current mode: Medium Probe current range: High Anode Ap.: 3 OBJ Ap.: 2 Analysis area: 1 μm square Analysis software: EDAX Genesis CPS: 20500 Lsec: 50 Time constant: 3.2

[0077] As described above, elemental mapping analysis by EDX is used as a method for determining the area ratio occupied by the carbon phase 13 in the cross section of the primary particle of the base particle 10. In this case, in order to accurately determine the area occupied by the carbon phase 13 dispersed inside the base particle 10, a region 200 nm or more inward from the particle surface is used when determining the area occupied by the carbon phase 13.

[0078] Auger Electron Spectroscopy (AES) From a backscattered electron image of a cross section of the negative electrode active material in which the cross section of the base particle 10 is exposed, a qualitative and quantitative analysis of elements is performed using an AES analyzer (e.g., JAMP-9510F manufactured by JEOL Ltd.). Measurement conditions may be, for example, an acceleration voltage of 10 kV, a beam current of 10 nA, and an analysis area of ​​20 μmφ. When measuring from the state of the battery, 10 base particles 10 having a maximum particle diameter of 5 μm or more are randomly selected from a cross-sectional image of a backscattered electron image of the negative electrode mixture layer containing the negative electrode active material, and a qualitative and quantitative analysis of elements is performed on each of them using an AES analyzer. The content of a predetermined element contained in the 10 particles is calculated by averaging the contents.

[0079] During the charge / discharge process, a coating may be formed on the surface of the base particle 10 due to decomposition of the non-aqueous electrolyte, etc. Furthermore, as described above, a conductive layer may be provided on the surface of the base particle 10. Therefore, not only when determining the area ratio of the carbon phase 13 in the cross section of the primary particle of the base particle 10, but also when EDX analysis and AES analysis are performed on a range, for example, 200 nm or more inward from the peripheral edge of the cross section of the particle so as not to include thin coatings and conductive layers in the measurement range.

[0080] <Inert Gas Fusion - Non-Dispersive Infrared Absorption Method> The oxygen content of the base particles 10 can be measured using an oxygen, nitrogen, and hydrogen analyzer (for example, the EGMA-830 model manufactured by Horiba, Ltd.). A sample is placed in a Ni capsule, and then placed in a carbon crucible heated at 5.75 kW along with Sn and Ni pellets that serve as flux, and the released carbon monoxide gas is detected. A calibration curve is created using a standard sample, YO, and the oxygen content of the sample is calculated.

[0081] <Nuclear Magnetic Resonance Spectroscopy (NMR)> The amount of Si constituting the silicon phase 12 in the base particle 10 can be quantified using Si-NMR. Desired measurement conditions for Si-NMR are shown below. Measurement device: Solid-state nuclear magnetic resonance spectrometer (INOVA-400) manufactured by Varian Probe: Varian 7 mm CPMAS-2 MAS: 4.2 kHz MAS speed: 4 kHz Pulse: DD (45° pulse + signal acquisition time 1H decoupled) Repetition time: 1200 sec to 3000 sec Observation width: 100 kHz Observation center: near -100 ppm Signal acquisition time: 0.05 sec Number of accumulations: 560 Sample amount: 207.6 mg

[0082] <High-Frequency Induction Heating Furnace Combustion-Infrared Absorption Method> The carbon content contained in the base particles 10, i.e., the content of the carbon phase 13 in the base particles, can be measured using a carbon / sulfur analyzer (for example, EMIA-520 model manufactured by Horiba, Ltd.). A sample is weighed out onto a magnetic board, a combustion improver is added, and the board is inserted into a combustion furnace (carrier gas: oxygen) heated to 1350°C, and the amount of carbon dioxide gas generated during combustion is detected by infrared absorption. A calibration curve is created using, for example, carbon steel (carbon content 0.49%) manufactured by Bureau of Analyzed Samples Ltd., and the carbon content of the sample is calculated.

[0083] An example of an XRD pattern obtained by X-ray diffraction (XRD) measurement of a base particle will be described. Here, the XRD pattern of a base particle having a configuration in which the alkali metal element constituting the alkali aluminate phase is lithium (i.e., the alkali aluminate phase is a lithium aluminate phase) and the base particle does not contain a carbon phase 13 will be described. In such a base particle, a peak derived from the lithium aluminate phase is observed near 2θ=x°. x° is at least one selected from the group consisting of 19.4°, 22.3°, 31.9°, 34.3°, and 37.5°. Cu Kα rays are used as the X-rays for the XRD measurement. In this specification, "near x°" means, for example, within the range of x±1°.

[0084] [Conductive Layer] The conductive layer 21 covers at least a portion of the surface of the base particle. The conductive layer 21 may cover the entire surface of the base particle. The conductive layer 21 is, for example, a thin film layer containing a conductive material. When the negative electrode active material according to this embodiment further includes the conductive layer 21 that covers at least a portion of the surface of the base particle 10, the conductivity of the negative electrode active material is improved.

[0085] The conductive material may be a conductive carbon material. That is, the conductive layer 21 may contain carbon. Examples of the carbon material that can be used include graphite such as natural graphite, artificial graphite, and graphitized mesophase carbon, amorphous carbon, soft carbon, and hard carbon. The carbon material may be amorphous carbon. As described above, it is easy to form a thin conductive layer 21 that covers the surface of the base particles. Examples of amorphous carbon include carbon black, burned pitch, coke, and activated carbon.

[0086] The thickness of the conductive layer 21 is desirably thin enough not to affect the average particle size of the composite particles 20, i.e., thin enough not to significantly increase the average particle size of the composite particles 20 relative to the average particle size of the base particles. The thickness of the conductive layer 21 may be 1 nm or more and 200 nm or less, or 5 nm or more and 100 nm or less, taking into consideration ensuring conductivity and the diffusibility of ions such as Li ions. The thickness of the conductive layer 21 can be measured by SEM observation or transmission electron microscope (TEM) observation of a cross section of the negative electrode active material in which the cross section of the composite particle 20 is exposed.

[0087] (Method for Producing Negative Electrode Active Material) The negative electrode active material of the present disclosure includes, for example, producing base particles 10. The base particles 10 are produced, for example, by a production method including the following first to fifth steps.

[0088] Step 1: A step of obtaining a raw material alkali aluminate (hereinafter referred to as "raw aluminate"). Step 2: A step of combining raw aluminate and raw silicon to obtain a composite intermediate in which raw silicon is dispersed in the raw aluminate. Step 3: A step of heat-treating the composite intermediate to obtain a sintered body containing an alkali aluminate phase 11 and a silicon phase 12 dispersed within the alkali aluminate phase 11. Step 4: A step of pulverizing the sintered body to obtain a precursor of the base particle 10. Step 5: A step of filling pores present inside the precursor of the base particle 10 with a carbon material to form a carbon phase 13 and obtain the base particle 10.

[0089] [First Step] The first step includes, for example, a step of mixing an aluminum compound, a compound containing an alkali metal element, and, if necessary, a compound containing element M to obtain a mixture, and a step of calcining the mixture to obtain a raw aluminate. The calcination is carried out, for example, in an oxidizing atmosphere. The calcination temperature may be 400°C or higher and 1200°C or lower, or 700°C or higher and 1100°C or lower.

[0090] Examples of the aluminum compound include aluminum oxide, aluminum hydroxide, aluminum carbonate, etc. The aluminum compound may be used alone or in combination of two or more.

[0091] Examples of the compound containing an alkali metal element include lithium carbonate, lithium oxide, lithium hydroxide, lithium hydride, sodium carbonate, sodium oxide, sodium hydroxide, sodium hydride, potassium carbonate, potassium oxide, potassium hydroxide, potassium hydride, etc. The compound containing an alkali metal element may be used alone or in combination of two or more.

[0092] The compound containing element M is, for example, a boron compound. Examples of the boron compound include boron oxide, boric acid, borax, sodium tetraborate, etc. One type of boron compound may be used alone, or two or more types may be used in combination.

[0093] The ratio (mMA / mAl) may be, for example, 0.04 to 0.50, or 0.05 to 0.25, as described above. The mass ratio of alkali metal elements (mMA) may be, for example, 1.0 mass% to 9.5 mass% as described above.

[0094] In the first step, aluminum compounds that did not react with the alkali metal-containing compound during the preparation of the raw aluminate may remain in the raw aluminate. When the amount of aluminum compound used is large relative to the amount of alkali metal-containing compound, the aluminum compounds are likely to remain. When the aluminum compound remaining in the raw aluminate is Al2O3, an Al2O3 phase dispersed in the alkali aluminate phase 11 may be formed in the finally obtained mother particles.

[0095] [Step 2] In Step 2, for example, a mixture of raw aluminate and raw silicon is pulverized while applying shear force to the mixture to obtain a finely divided composite intermediate. One example is a method in which raw aluminate and raw silicon are mixed in a predetermined mass ratio and the mixture is pulverized using a pulverizer such as a ball mill.

[0096] The raw silicon is, for example, coarse silicon particles having an average particle size of several μm to several tens of μm. The silicon particles may be prepared so that the crystallite size of the silicon phase 12 is 15 nm or less, calculated by Scherrer's formula from the half-width of the diffraction peak derived from the Si(111) plane in the X-ray diffraction pattern of the negative electrode active material or the base particles. Alternatively, silicon nanoparticles and raw aluminate nanoparticles may be synthesized and then mixed without using a pulverizer.

[0097] [Step 3] In step 3, for example, the composite intermediate is sintered while applying pressure to the microparticulated composite intermediate using a hot press or the like to obtain a sintered body. The pressure applied to the composite intermediate is, for example, 100 MPa or more, and may be 100 MPa or more and 300 MPa or less. The higher the pressure in step 3, the more likely the voids in the composite particles are to be reduced. The sintering of the composite intermediate is preferably carried out in an inert atmosphere (e.g., an argon, nitrogen, or other atmosphere). The sintering conditions in step 3 also affect the crystallites of the silicon phase 12; generally, the higher the sintering temperature, the larger the crystallite size.

[0098] An example of the firing temperature is 450°C or higher and 1000°C or lower. If the firing temperature is within this range, a structure in which minute silicon phases 12 are dispersed within a low-crystalline alkali aluminate phase 11 is easily formed. The raw aluminate is stable at this temperature and hardly reacts with silicon. The firing temperature may be 550°C or higher and 950°C or lower, or may be 650°C or higher and 900°C or lower. In order to reduce voids in the base particles 10, firing at a temperature of 650°C or higher is desirable. The firing time is, for example, 1 hour or higher and 10 hours or lower.

[0099] [Fourth Step] The fourth step is a step of pulverizing the base particles 10 so that they have a desired particle size distribution. For example, the base particles 10 are pulverized so that the median diameter of the base particles 10 is 1 μm or more and 25 μm or less. This gives precursors of the base particles 10.

[0100] [Step 5] Step 5 is a step of filling pores present inside the precursor of the base particle 10 obtained in Step 4 with a carbon material to form a carbon phase 13, thereby obtaining the base particle 10. The sintered body formed in Step 3 has pores inside. Therefore, the precursor of the base particle 10 obtained by pulverizing the sintered body in Step 4 also has pores inside. In Step 5, the pores are filled with a carbon material to form the carbon phase 13, thereby obtaining the base particle 10. One example of a method for filling the pores of the precursor of the base particle 10 with a carbon material involves mixing the precursor of the base particle 10 containing pores with an amount of carbon material selected in consideration of the pore size and pore volume contained in the precursor of the base particle 10, firing the mixture, and filling the pores of the base particle 10 with the carbon material. For example, by adjusting the amount of carbon material used, it is possible to produce base particles 10 in which the area ratio of the carbon phase 13 in the cross section of the primary particle is 4% or more and 25% or less. The carbon material used to form the carbon phase 13 is, for example, coal pitch, petroleum pitch, phenol resin, or the like.

[0101] The base particles 10 can be produced by the first to fifth steps described above.

[0102] When a conductive layer 21 is formed on the surface of the base particle 10, the following sixth step is further carried out. Sixth step: a step of forming a conductive layer 21 on the surface of the base particle 10 including the alkali aluminate phase 11, the silicon phase 12, and the carbon phase 13.

[0103] As described above, a conductive carbon material is desirable as the conductive material that constitutes the conductive layer 21. Examples of methods for coating the surfaces of the base particles with a carbon material include a CVD method using hydrocarbon gases such as acetylene and methane as raw materials, and a method in which coal pitch, petroleum pitch, phenolic resin, etc. is mixed with the base particles and heated to carbonize them. Carbon black may also be attached to the surfaces of the base particles.

[0104] In the sixth step, for example, the mixture of the base particles 10 and the carbon material is heated in an inert atmosphere (e.g., an argon, nitrogen, or the like) at 700° C. or higher and 950° C. or lower to form the conductive layer 21 on the surface of the base particles 10. In this manner, the composite particles 20 having the conductive layer 21 on the surface of the base particles 10 are obtained.

[0105] When forming the conductive layer 21 on the surface of the base particle 10 and the carbon material constituting the carbon phase 13 is the same as the carbon material serving as the conductive material constituting the conductive layer 21, the fifth step and the sixth step may be performed simultaneously. For example, in the fifth step, the amount of the carbon material used, the heat treatment conditions, and the like may be adjusted so that the carbon phase 13, whose area ratio in the cross section of the primary particle of the base particle 10 is 4% or more and 25% or less, and the conductive layer 21 can be formed simultaneously.

[0106] (Battery) The battery according to this embodiment includes a negative electrode, a positive electrode, and an electrolyte. The negative electrode contains the negative electrode active material according to this embodiment. In the battery according to this embodiment, the negative electrode contains the negative electrode active material according to this embodiment, which improves the cycle characteristics and ultimately improves the initial charge / discharge efficiency.

[0107] 3 is a longitudinal cross-sectional view schematically illustrating an example of a battery according to this embodiment. The battery 100 is a cylindrical battery including a cylindrical battery case, a wound electrode group 34, and an electrolyte (not shown). The electrode group 34 is housed in the battery case and is in contact with the electrolyte.

[0108] The battery case is composed of a case body 35, which is a cylindrical metal container with a bottom, and a sealing body 36 that seals the opening of the case body 35. A gasket 47 is disposed between the case body 35 and the sealing body 36. The gasket 47 ensures that the battery case is airtight. Within the case body 35, insulating plates 37 and 38 are disposed on both ends of the electrode group 34 in the direction of the winding axis of the electrode group 34.

[0109] The case body 35 has, for example, a step portion 41. The step portion 41 can be formed by partially pressing the side wall of the case body 35 from the outside. The step portion 41 may be formed in an annular shape on the side wall of the case body 35 along the circumferential direction of an imaginary circle defined by the case body 35. In this case, the sealing body 36 is supported by, for example, the surface of the step portion 41 on the opening side.

[0110] The sealing body 36 includes a filter 42, a lower valve body 43, an insulating member 44, an upper valve body 45, and a cap 46. These components are stacked in this order in the sealing body 36. The sealing body 36 is attached to the opening of the case body 35 so that the cap 46 is located outside the case body 35 and the filter 42 is located inside the case body 35.

[0111] Each of the above-mentioned members constituting the sealing body 36 has, for example, a disk or ring shape. The above-mentioned members, except for the insulating member 44, are electrically connected to one another.

[0112] The electrode group 34 has a positive electrode 31, a separator 32, and a negative electrode 33. The positive electrode 31, the separator 32, and the negative electrode 33 are all strip-shaped. The width direction of the strip-shaped positive electrode 31 and the negative electrode 33 is, for example, parallel to the winding axis of the electrode group 34. The separator 32 is disposed between the positive electrode 31 and the negative electrode 33. The positive electrode 31 and the negative electrode 33 are spirally wound with the separator 32 interposed between these electrodes.

[0113] When observing the cross section of the battery 100 in a direction perpendicular to the winding axis of the electrode group 34, the positive electrodes 31 and negative electrodes 33 are stacked alternately in the radial direction of the imaginary circle defined by the case body 35, with the separator 32 interposed between these electrodes.

[0114] The positive electrode 31 is electrically connected to a cap 46, which also serves as a positive electrode terminal, via a positive electrode lead 39. One end of the positive electrode lead 39 is connected, for example, to near the center of the positive electrode 31 in the longitudinal direction of the positive electrode 31. The positive electrode lead 39 extends from the positive electrode 31 to the filter 42 through a through-hole formed in the insulating plate 37. The other end of the positive electrode lead 39 is welded, for example, to the surface of the filter 42 on the electrode group 34 side.

[0115] The negative electrode 33 is electrically connected to the case body 35, which also serves as a negative electrode terminal, via a negative electrode lead 40. One end of the negative electrode lead 40 is connected to, for example, an end of the negative electrode 33 in the longitudinal direction of the negative electrode 33. The other end of the negative electrode lead 40 is welded to, for example, the inner bottom surface of the case body 35.

[0116] Each component of the battery 100 will be specifically described below.

[0117] The positive electrode 31 includes a material having the property of absorbing and releasing metal ions (e.g., lithium ions). The positive electrode 31 includes, for example, a positive electrode active material. The positive electrode 31 may include a positive electrode current collector and a positive electrode mixture layer supported on the surface of the positive electrode current collector.

[0118] The positive electrode mixture layer includes a positive electrode active material. Examples of the positive electrode active material include lithium-containing transition metal oxides, lithium-containing transition metal phosphates, transition metal fluorides, polyanionic materials, fluorinated polyanionic materials, transition metal sulfides, transition metal oxysulfides, and transition metal oxynitrides. In particular, using a lithium-containing transition metal oxide or a lithium-containing transition metal phosphate as the positive electrode active material can reduce the manufacturing cost of the battery and increase the average discharge voltage. Examples of lithium-containing transition metal oxides include lithium cobalt oxide, lithium nickel cobalt aluminum oxide, lithium nickel cobalt manganese oxide, and lithium nickel manganese oxide. Examples of lithium-containing transition metal phosphates include lithium iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate, and lithium nickel phosphate. At least one selected from these positive electrode active materials can be used.

[0119] The positive electrode mixture layer may contain a conductive additive, an ion conductor, and a binder, as needed.

[0120] Conductive additives and ion conductors are used to reduce the resistance of the electrodes. Examples of conductive additives include carbon materials and conductive polymer compounds. Examples of carbon materials include carbon black, graphite, acetylene black, carbon nanotubes, carbon nanofibers, graphene, fullerene, and graphite oxide. Examples of conductive polymer compounds include polyaniline, polypyrrole, and polythiophene. At least one selected from these conductive additives can be used.

[0121] Binders are used to improve the binding properties of the materials that make up the electrode. Examples of binders include polymeric materials such as polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, polytetrafluoroethylene, carboxymethyl cellulose, polyacrylic acid, styrene-butadiene copolymer rubber, polypropylene, polyethylene, and polyimide. At least one selected from these binders can be used.

[0122] The positive electrode current collector is a sheet or film made of a metal material such as aluminum, an aluminum alloy, stainless steel, titanium, or a titanium alloy. The sheet or film may be porous or non-porous. Examples of the sheet or film include metal foil and metal mesh. A carbon material may be applied to the surface of the positive electrode current collector as a conductive auxiliary material.

[0123] The negative electrode 33 includes the negative electrode active material according to this embodiment. The negative electrode 33 includes, for example, a negative electrode current collector and a negative electrode mixture layer carried on the surface of the negative electrode current collector.

[0124] The negative electrode current collector is, for example, a foil made of a metal material such as stainless steel, nickel, a nickel alloy, copper, or a copper alloy.

[0125] The negative electrode mixture layer contains the negative electrode active material according to this embodiment. The negative electrode mixture layer may contain other materials such as a conductive additive, an ion conductor, and a binder, as needed. The conductive additive, ion conductor, and binder materials described above for the positive electrode mixture layer can also be used for the negative electrode mixture layer.

[0126] The electrolyte may contain a non-aqueous solvent and a lithium salt dissolved in the non-aqueous solvent. The concentration of the lithium salt in the electrolyte may be, for example, 0.5 mol / L or more and 2 mol / L or less. By controlling the lithium salt concentration within the above range, an electrolyte having excellent ionic conductivity and appropriate viscosity can be obtained. However, the lithium salt concentration is not limited to the above.

[0127] The non-aqueous solvent may be a cyclic carbonate, a chain carbonate, a cyclic ether, a chain ether, a nitrile, an amide, etc. One selected from these solvents may be used, or two or more may be used in combination.

[0128] Examples of lithium salts that can be used include lithium hexafluorophosphate (LiPF), lithium tetrafluoroborate (LiBF), lithium perchlorate (LiClO), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bisperfluoroethylsulfonylimide (LiN(SO2C2F5)2), LiAsF6, LiCF3SO3, and lithium difluoro(oxalato)borate. One selected from these electrolyte salts may be used, or two or more may be used in combination.

[0129] It is usually desirable to interpose a separator between the positive electrode and the negative electrode. The separator 32 has high ion permeability and adequate mechanical strength and insulating properties. The separator 32 can be made of a microporous thin film, a woven fabric, a nonwoven fabric, or the like. The separator 32 can be made of a polymer, for example. The polymer can be a polyolefin such as polypropylene or polyethylene.

[0130] In the battery of the present disclosure, the electrolyte may be impregnated into a polymer provided as a separator, for example, i.e., the battery of the present disclosure may have a structure in which the electrolyte and the polymer are used in combination.

[0131] The battery of the present disclosure may further include a solid electrolyte as an electrolyte. That is, the battery of the present disclosure may have a hybrid structure in which an electrolytic solution and a solid electrolyte are used in combination. Examples of solid electrolyte materials include halide solid electrolytes, sulfide solid electrolytes, oxide solid electrolytes, and organic polymer solid electrolytes. In this disclosure, a "halide solid electrolyte" refers to a solid electrolyte containing a halogen element as the main component of the anions. A "sulfide solid electrolyte" refers to a solid electrolyte containing sulfur as the main component of the anions. An "oxide solid electrolyte" refers to a solid electrolyte containing oxygen as the main component of the anions. The "main component of the anions" refers to the anion with the largest substance amount among all the anions constituting the solid electrolyte.

[0132] In the present disclosure, as an example of the structure of a battery according to the present embodiment, the configuration example shown in FIG. 3 is described, i.e., a cylindrical nonaqueous electrolyte secondary battery in which a wound electrode group formed by winding a positive electrode and a negative electrode with a separator interposed therebetween and an electrolyte solution are housed in an outer casing. However, the battery according to the present disclosure is not limited to this configuration example. The battery according to the present disclosure may have any shape, such as a prismatic shape, a coin shape, a button shape, or a laminate shape. Furthermore, instead of a wound electrode group in the secondary battery according to the present disclosure, an electrode group of another shape, such as an electrode group formed by stacking a positive electrode and a negative electrode with a separator interposed therebetween, may be used.

[0133] 4 is a cross-sectional view showing a schematic configuration of another example of a battery according to the present embodiment. The battery 200 includes a positive electrode 210, an electrolyte layer 220, and a negative electrode 230. The negative electrode 230 contains the negative electrode active material according to the present embodiment. The electrolyte layer 220 is disposed between the positive electrode 210 and the negative electrode 230. The battery 200 is, for example, an all-solid-state battery.

[0134] The positive electrode 210 includes a positive electrode current collector 211 and a positive electrode mixture layer 212 carried on the surface of the positive electrode current collector 211. The positive electrode mixture layer 212 is located between the positive electrode current collector 211 and the electrolyte layer 220. The materials described above for the positive electrode 31 in the battery 100 can be used for the positive electrode current collector 211 and the positive electrode mixture layer 212.

[0135] The positive electrode mixture layer 212 may contain a solid electrolyte. Examples of the solid electrolyte include the solid electrolytes described above as the electrolyte in the battery 100.

[0136] The electrolyte layer 220 is a layer containing an electrolyte. The electrolyte is, for example, a solid electrolyte. The electrolyte layer 220 may be a solid electrolyte layer. Examples of solid electrolytes include the solid electrolytes described above as the electrolyte in the battery 100.

[0137] The electrolyte layer 220 may further include a binder. As the binder, the materials described above for the positive electrode mixture layer of the battery 100 can also be used for the electrolyte layer 220.

[0138] The negative electrode 230 includes a negative electrode current collector 231 and a negative electrode mixture layer 232 supported by the negative electrode current collector 231. The negative electrode mixture layer 232 contains the negative electrode active material according to this embodiment. The negative electrode mixture layer 232 is located between the negative electrode current collector 231 and the electrolyte layer 220. The materials described above for the negative electrode 33 in the battery 100 can be used for the negative electrode current collector 231 and the negative electrode mixture layer 232.

[0139] The negative electrode mixture layer 232 may contain a solid electrolyte. Examples of the solid electrolyte include the solid electrolytes described above as the electrolyte in the battery 100.

[0140] The battery 200 can be configured as a battery of various shapes, such as a coin type, a cylindrical type, a square type, a sheet type, a button type, a flat type, or a laminated type.

[0141] (Other Embodiments) (Additional Notes) The above description of the embodiments discloses the following techniques.

[0142] (Technology 1) A negative electrode active material comprising: a base particle including an alkali aluminate phase containing at least one alkali metal element and Al; a silicon phase dispersed within the alkali aluminate phase; and a carbon phase dispersed within the alkali aluminate phase, wherein an area ratio of the carbon phase to a cross section of a primary particle of the base particle is 4% or more and 25% or less.

[0143] With this configuration, the negative electrode active material according to Technology 1 can improve durability against charge and discharge, and as a result, the cycle characteristics of the battery can be improved.

[0144] (Technology 2) The negative electrode active material according to Technology 1, wherein the area ratio is 5% or more and 20% or less.

[0145] With this configuration, the negative electrode active material according to Technique 2 can further improve durability against charge and discharge, and as a result, the cycle characteristics of the battery can further be improved.

[0146] (Technology 3) The negative electrode active material according to Technology 1 or 2, wherein the alkali aluminate phase further contains B.

[0147] This configuration reduces the number of pores contained in the base particles, which can further improve the cycle characteristics of the battery.

[0148] (Technology 4) The negative electrode active material according to Technology 3, wherein, when the ratio of the mass of Al element to the total mass of elements excluding oxygen element and carbon element in the base particle is mAl, and the ratio of the mass of B element to the total mass of elements excluding oxygen element and carbon element in the base particle is mB, the ratio (mAl / mB) is 1.0 or more and 30.0 or less.

[0149] This configuration can more significantly reduce the voids in the base particles due to B. This suppresses deterioration of the particle structure caused by the voids in the base particles, thereby effectively improving the cycle characteristics of the battery.

[0150] (Technology 5) The negative electrode active material according to any one of Technologies 1 to 4, wherein the alkali metal element includes at least one selected from the group consisting of Li, Na, and K.

[0151] With this configuration, the negative electrode active material according to Technology 5 can further improve the cycle characteristics of the battery.

[0152] (Technology 6) The negative electrode active material according to any one of Technologies 1 to 5, further comprising a conductive layer that covers at least a part of the surface of the base particles.

[0153] This configuration improves the conductivity of the negative electrode active material, thereby improving the battery characteristics.

[0154] (Technology 7) A battery comprising: a negative electrode containing the negative electrode active material according to any one of Technologies 1 to 6; a positive electrode; and an electrolyte.

[0155] With this configuration, a battery with improved cycle characteristics can be obtained.

[0156] The present disclosure will be described in more detail below using examples. The following examples are merely examples of embodiments, and are not intended to limit the scope of the present disclosure.

[0157] Example 1 Preparation of Base Particles (First Step) AlO, LiCO, and BO were mixed and fired in air at 750°C for 10 hours to obtain a raw aluminate. In the first step, the mixing ratio of AlO, LiCO, and BO was adjusted so that the constituent elements of the lithium aluminate phase, which is an alkali aluminate phase, had the element ratio shown in Table 1. The raw aluminate was then pulverized to an average particle size of 10 µm.

[0158] (Second Step) Raw silicon (3N, average particle size 10 μm) was mixed with the raw aluminate (average particle size 10 μm) obtained in the first step. In the second step, the mixing ratio of the raw silicon and raw aluminate was adjusted so that the element ratios of the elements constituting the base particles were as shown in Table 1. The mixture was charged into a pot (SUS, volume: 500 mL) of a planetary ball mill (Fritsch, P-5), 24 SUS balls (diameter 20 mm) were placed in the pot, the lid was closed, and the mixture was pulverized at 200 rpm for 50 hours in an inert atmosphere.

[0159] (Step 3) The powder mixture obtained in Step 2 was taken out into an inert atmosphere, and fired in the inert atmosphere at 700°C for 4 hours while applying a pressure of 200 MPa using a hot press machine, thereby obtaining a sintered body of the mixture.

[0160] (Fourth Step) The sintered body obtained in the third step was pulverized and passed through a 40 μm mesh to obtain a precursor of mother particles in which a silicon phase was dispersed in a lithium aluminate phase, which was an alkali aluminate phase.

[0161] (Step 5) Coal pitch (MCP250, manufactured by JFE Chemical Corporation) was mixed with the precursor of the base particles obtained in Step 4. The mixture was fired at 800°C for 5 hours in an inert atmosphere to fill the pores inside the precursor of the base particles with a carbon material using coal pitch, and to form a conductive layer containing a conductive carbon material on the surface of the base particles. That is, in Step 5, the amount of coal pitch added was adjusted in consideration of the formation of a conductive layer and a carbon phase in the base particles. As shown in Table 1, the amount of coal pitch added was 20 mass% relative to the total mass of the base particles and coal pitch. Then, composite particles having a conductive layer and an average particle size of 5 μm were obtained using a sieve.

[0162] The contents of Li, Al, B, and Si relative to the total mass of elements excluding oxygen and carbon in the base particles, and the area ratio of the carbon phase in the cross section of the primary particles of the base particles were determined, and the measurement results are shown in Table 1. The contents of Li, Al, B, and Si were determined from the feed ratio of the raw materials. When the contents of each element in the obtained base particles were measured using ICP and Si-NMR, the results were almost the same as the contents of each element determined from the feed ratio. Table 1 shows the contents of each element determined from the feed ratio. The area ratio of the carbon phase in the cross section of the primary particles of the base particles was measured by elemental mapping analysis using EDX under the measurement conditions exemplified above.

[0163] [Preparation of Negative Electrode] The above-mentioned base particles, the surfaces of which were coated with a conductive layer, and graphite were mixed in a mass ratio of 5:95 to obtain a mixture. This mixture was used as the negative electrode active material. The negative electrode active material, a Na salt of carboxymethyl cellulose (CMC), and styrene butadiene rubber (SBR) were mixed in a mass ratio of negative electrode active material:Na salt of CMC:SBR=97.5:1:1.5. Water was added to the negative electrode mixture, and the mixture was stirred to prepare a negative electrode slurry. Next, the negative electrode slurry was applied to both sides of copper foil, which served as a negative electrode current collector. The coating film was dried and then rolled to form a negative electrode slurry having a density of 1.5 g / cm on both sides of the copper foil. 3 A negative electrode having the negative electrode mixture layer formed thereon was fabricated.

[0164] [Preparation of Positive Electrode] A positive electrode active material, acetylene black, and PVDF were mixed in a mass ratio of positive electrode active material:acetylene black:PVDF = 95:2.5:2.5. An appropriate amount of N-methyl-2-pyrrolidone (NMP) was added to the positive electrode mixture, and the mixture was stirred to prepare a positive electrode slurry. Lithium cobalt oxide was used as the positive electrode active material. Next, the positive electrode slurry was applied to both sides of an aluminum foil serving as a positive electrode current collector, the coating film was dried, and then rolled to form a positive electrode slurry having a density of 3.6 g / cm on both sides of the aluminum foil. 3 A positive electrode having the positive electrode mixture layer formed thereon was fabricated.

[0165] [Preparation of Non-Aqueous Electrolyte] A non-aqueous electrolyte was prepared by dissolving lithium hexafluorophosphate (LiPF) at a concentration of 1.0 mol / L in a mixed solvent prepared by mixing ethylene carbonate (EC) and diethyl carbonate (DEC) at a volume ratio of EC:DEC = 3:7 (25°C).

[0166] [Fabrication of Non-Aqueous Electrolyte Secondary Battery] A wound electrode assembly was fabricated by winding a separator between a positive electrode and a negative electrode, which were attached with leads. The electrode assembly was inserted into an exterior case made of an aluminum laminate film, and vacuum dried at 105°C for 2 hours. After that, a non-aqueous electrolyte was poured into the exterior case, and the opening of the exterior case was sealed to obtain a non-aqueous electrolyte secondary battery.

[0167] Example 2 In the preparation of base particles, the firing temperature was changed to 800°C in the third step, and the amount of coal pitch added in the fifth step was changed to 10 mass % with respect to the total mass of the base particles and coal pitch, as shown in Table 1. Except for these changes, base particles were prepared in the same manner as in Example 1, and a nonaqueous electrolyte secondary battery was fabricated.

[0168] Example 3 In the preparation of base particles, the firing temperature in the third step was changed to 800° C. Other than this, base particles were prepared in the same manner as in Example 1, and a non-aqueous electrolyte secondary battery was fabricated.

[0169] Reference Example 1 In the preparation of base particles, the amount of coal pitch added in the fifth step was changed to 2 mass % relative to the total mass of the base particles and coal pitch, as shown in Table 1. Other than this, base particles were prepared in the same manner as in Example 1, and a nonaqueous electrolyte secondary battery was fabricated.

[0170] Reference Example 2 In the preparation of base particles, the amount of coal pitch added in the fifth step was changed to 5 mass % relative to the total mass of the base particles and coal pitch, as shown in Table 1. Other than this, base particles were prepared in the same manner as in Example 1, and a nonaqueous electrolyte secondary battery was fabricated.

[0171] Reference Example 3 In the preparation of base particles, the firing temperature was changed to 800°C in the third step, and the amount of coal pitch added in the fifth step was changed to 2 mass % with respect to the total mass of the base particles and coal pitch, as shown in Table 1. Except for these changes, base particles were prepared in the same manner as in Example 1, and a nonaqueous electrolyte secondary battery was fabricated.

[0172] The batteries of each example and comparative example were evaluated for capacity retention as an evaluation of cycle characteristics by the following method. The evaluation results are shown in Table 1.

[0173] [Capacity Retention Rate (Evaluation of Cycle Characteristics)] Each battery to be evaluated was subjected to 200 cycles of charge and discharge as described below, and the capacity retention rate was calculated using the following formula: Capacity retention rate (%) = (discharge capacity at 200th cycle / discharge capacity at 1st cycle) x 100

[0174] <Charging> The battery to be evaluated was charged at a constant current of 1 It (800 mA) at 25° C. until the voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current reached 1 / 20 It (40 mA).

[0175] <Discharge> After a 10-minute rest, constant-current discharge was carried out at 25° C. with a current of 1 It (800 mA) until the voltage reached 2.75 V.

[0176]

[0177] (Discussion) As shown in Table 1, the batteries of Examples 1 to 3, which used as the negative electrode active material base particles in which the area ratio occupied by the carbon phase 13 was 4% or more and 25% or less in the cross section of the primary particle of the base particle 10, had a higher capacity retention rate at 200 cycles than the batteries of Reference Examples 1 to 3, which used as the negative electrode active material base particles in which the area ratio occupied by the carbon phase 13 was less than 4%. In other words, the negative electrode active materials of Examples 1 to 3 were able to improve the cycle characteristics of the battery.

[0178] The technology of the present disclosure is useful for batteries such as lithium ion secondary batteries.

Claims

1. A negative electrode active material comprising a base particle including an alkali aluminate phase containing at least one alkali metal element and Al, a silicon phase dispersed within the alkali aluminate phase, and a carbon phase dispersed within the alkali aluminate phase, wherein an area ratio of the carbon phase in a cross section of a primary particle of the base particle is 4% or more and 25% or less.

2. The negative electrode active material according to claim 1, wherein the area ratio is 5% or more and 20% or less.

3. The negative electrode active material according to claim 1, wherein the alkali aluminate phase further contains B.

4. The negative electrode active material according to claim 3, wherein when in the base particle, a ratio of the mass of Al element to the total mass of elements excluding oxygen element and carbon element is mAl, and in the base particle, a ratio of the mass of B element to the total mass of elements excluding oxygen element and carbon element is mB, the ratio (mAl / mB) is 1.0 or more and 30.0 or less.

5. The negative electrode active material according to claim 1, wherein the alkali metal element includes at least one selected from the group consisting of Li, Na, and K.

6. The negative electrode active material according to claim 1, further comprising a conductive layer covering at least a portion of the surface of the base particles.

7. A battery comprising: a negative electrode containing the negative electrode active material according to any one of claims 1 to 6; a positive electrode; and an electrolyte.

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