Negative active material and battery

Abstract

This negative electrode active material is provided with mother particles (10) containing an alkali aluminate phase (11) containing at least one alkali metal element and Al, a silicon phase (12) dispersed in the alkali aluminate phase (11), and a carbon phase (13) dispersed in the alkali aluminate phase (11). In a cross-section of the primary particles of the mother particles (10), the area ratio of the carbon phase (13) is 4% or more and 25% or less.

Description

Technical Field

[0001] This disclosure relates to negative electrode active materials and batteries. Background Technology

[0002] In recent years, lithium-ion batteries and other rechargeable batteries have been widely used in applications requiring high capacity, such as automotive and energy storage. As a key component of the negative electrode, the negative electrode active material is crucial for achieving high battery capacity, leading to various studies on this topic. Among these, silicon-containing materials (Si-containing materials), which boast high theoretical capacity density, are attracting increasing attention.

[0003] Patent Document 1 discloses a negative electrode active material comprising composite particles containing a lithium aluminate phase and a silicon phase dispersed within the lithium aluminate phase. The negative electrode active material disclosed in Patent Document 1, by comprising composite particles (hereinafter sometimes referred to as "LAX particles") containing a silicon phase dispersed within a lithium aluminate phase with excellent alkali resistance, can reduce side reactions during the initial charge-discharge phase of a lithium-ion secondary battery and suppress the initial decrease in charge-discharge efficiency.

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: International Publication No. 2022 / 113500 Summary of the Invention

[0007] The problem the invention aims to solve

[0008] As mentioned above, negative electrode active materials containing LAX particles can suppress the initial reduction in charge and discharge efficiency in the battery.

[0009] However, negative electrode active materials containing LAX particles have issues with charge-discharge durability, requiring improvements in battery cycle characteristics.

[0010] This disclosure provides a negative electrode active material that can improve the cycle characteristics of a battery.

[0011] Solution for solving the problem

[0012] The negative electrode active material disclosed herein has master particles.

[0013] The parent material comprises:

[0014] Alkali aluminate phase containing at least one alkali metal element and Al,

[0015] The silicon phase dispersed within the alkali aluminate phase, and

[0016] The carbon phase dispersed within the alkali aluminate phase,

[0017] In the cross-section of the primary particles of the mother particle, the area ratio of the carbon phase is more than 4% and less than 25%.

[0018] The effects of the invention

[0019] According to the technology disclosed herein, it is possible to provide a negative electrode active material that can improve the cycle characteristics of a battery. Attached Figure Description

[0020] Figure 1 This is a simplified cross-sectional view showing an example of the structure of the master particles possessed by the negative electrode active material of this embodiment.

[0021] Figure 2 It means to include Figure 1 A schematic cross-sectional view of a composite particle consisting of a master particle and a conductive layer covering at least a portion of the surface of the master particle.

[0022] Figure 3 This is a cross-sectional view showing a simplified configuration of an example of the battery in this embodiment.

[0023] Figure 4 This is a cross-sectional view showing a simplified configuration of another example of the battery in this embodiment. Detailed Implementation

[0024] [Insights that form the basis of this disclosure]

[0025] Negative electrode active materials containing composite particles such as LAX particles can reduce side reactions during the initial charging and discharging phases of the battery and suppress the decrease in initial charging and discharging efficiency. These composite particles, like LAX particles, comprise an alkali aluminate phase containing at least one alkali metal element and Al, similar to a lithium aluminate phase, and a silicon phase dispersed within this alkali aluminate phase. The alkali aluminate phase exhibits excellent alkali resistance. Negative electrode active materials containing such composite particles with a silicon phase dispersed within the alkali aluminate phase can reduce side reactions during the initial charging phase of the battery and achieve high initial efficiency.

[0026] However, the negative electrode active material containing the aforementioned composite particles with an alkali aluminate phase exhibits insufficient charge-discharge durability, and its cycle characteristics have room for improvement. The inventors investigated the reasons for this and found that, due to the high melting and softening points of the alkali aluminate phase, a large number of pores are easily generated internally during the sintering process. Furthermore, the presence of these pores makes the particle structure prone to disintegration due to expansion and contraction caused by charge-discharge. The inventors focused on the internal structure of the particles to suppress the disintegration of the particle structure caused by expansion and contraction during charge-discharge, and conducted in-depth research. They discovered that by dispersing the 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. This led to the negative electrode active material of this disclosure, as described below.

[0027] [Implementation of this disclosure]

[0028] The embodiments of this disclosure will now be described in detail with reference to the accompanying drawings. This disclosure is not limited to the following embodiments.

[0029] (Negative electrode active material)

[0030] The negative electrode active material in this embodiment includes master particles. Figure 1 This is a simplified cross-sectional view showing an example of the structure of the master particles possessed by the negative electrode active material of this embodiment.

[0031] The negative electrode active material of this embodiment includes a master particle 10 comprising 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 the primary particles of the master particle 10, the area ratio of the carbon phase 13 is 4% or more and 25% or less.

[0032] In the negative electrode active material of this embodiment, the carbon phase 13 is dispersed within the alkali aluminate phase 11 such that the area ratio in the cross-section of the primary particles of the master particle 10 is 4% or more and 25% or less. This configuration suppresses the disintegration (e.g., cracking and rupture) of the particle structure of the master particle 10 caused by the expansion and contraction of the negative electrode active material during battery charging and discharging. Therefore, the negative electrode active material of this embodiment improves its charging and discharging durability, and consequently, improves the battery's cycle characteristics.

[0033] The alkali aluminate phase 11 exhibits excellent alkali resistance. Therefore, during initial charging, side reactions between the negative electrode active material containing the master particles 10 and alkali metal ions such as Li ions are suppressed, along with the degradation of the negative electrode active material accompanying the side reactions and the resulting decrease in initial capacity. In other words, the negative electrode active material according to this embodiment, in addition to the aforementioned improvement in battery cycle characteristics, can also suppress the decrease in initial charge / discharge efficiency.

[0034] In this specification, "master particles" refers to a particulate structure composed of a material in which silicon phase 12 and carbon phase 13 are dispersed in a matrix of alkali aluminate phase 11. Therefore, master particles do not include a covering layer such as a conductive layer disposed on the surface of the master particles as described later.

[0035] As described above, the specific area ratio of carbon phase 13 in this specification refers to the total area occupied by carbon phase 13 dispersed inside the parent particle 10 when the cross-sectional area of ​​the primary particles of the parent particle 10 is taken as the whole. Therefore, for example, when a conductive layer or similar covering layer is provided on the surface of the parent particle 10 and the covering layer contains carbon, the carbon contained in the covering layer located inside the secondary particles formed by the aggregation of the parent particle 10 is naturally not included in the aforementioned area ratio of carbon phase 13.

[0036] The area proportion of carbon phase 13 in the cross-section of the primary particle 10 can be determined by elemental mapping analysis based on energy-dispersive X-rays (EDX). Details of the method for analyzing carbon phase 13 (i.e., carbon element) in the cross-section of the parent particle based on EDX are described later along with the analysis of other elements.

[0037] Here, when determining the area proportion of carbon phase 13 using EDX-based elemental mapping, for example, when a conductive layer containing carbon is provided on the surface of the parent particle 10, it is necessary to reliably calculate the area of ​​carbon phase 13 dispersed inside the parent particle 10 to prevent the carbon elements contained in the capping layer from being calculated as the area of ​​carbon phase 13 inside the parent particle 10. Therefore, when determining the area proportion of carbon phase 13 in the cross-section of the parent particle 10, the region inside the particle surface at a distance of 200 nm or more is used.

[0038] In the negative electrode active material of this embodiment, the area ratio of the carbon phase 13 in the cross-section of the primary particles of the mother particle 10 can be more than 5% and less than 20%. Therefore, the negative electrode active material of this embodiment can further improve the cycle characteristics of the battery.

[0039] In the negative electrode active material of this embodiment, the master particles 10 may contain other elements besides the alkali aluminate phase 11, silicon phase 12 and carbon phase 13.

[0040] The negative electrode active material of this embodiment may further include a conductive layer covering at least a portion of the surface of the parent particle 10. In other words, the negative electrode active material of this embodiment may include a composite particle 20 comprising the parent particle 10 and a conductive layer 21 covering at least a portion of the surface of the parent particle.

[0041] Figure 2 It means to include Figure 1 A schematic cross-sectional view of the composite particles 20 comprising the parent particle 10 and a conductive layer 21 covering at least a portion of the surface of the parent particle 10. (See diagram below.) Figure 2 The negative electrode active material of the composite particle 20 shown, in which at least a portion of the surface of the parent particle 10 is covered by the conductive layer 21, can improve conductivity.

[0042] The following describes in detail the composition of the negative electrode active material in this embodiment.

[0043] [Master Particles]

[0044] For example, such as Figure 1 As shown, the parent particle 10 has an island structure in which multiple fine silicon phases 12 and carbon phases 13 (i.e., islands) are dispersed in a matrix (i.e., the sea portion) of alkali aluminate phase 11. The alkali aluminate phase 11 has good ionic conductivity. Therefore, according to this configuration, the adsorption and release of alkali metal ions based on silicon phase 12 can be smoothly achieved with the aid of the alkali aluminate phase 11. According to this configuration, the stress caused by the expansion and contraction of silicon phase 12 during charging and discharging can be mitigated by the alkali aluminate phase 11, and cracking and breakage of the parent particle 10 are suppressed. Therefore, it is possible to achieve both high capacity due to the presence of silicon and improved cycling characteristics.

[0045] The master particles 10 can be contained in the negative electrode active material as primary particles or as secondary particles formed by combining multiple primary particles. The average particle size of the secondary particles in the master particles 10 is, for example, 1 μm or more and 25 μm or less, or 4 μm or more and 15 μm or less. In this case, since the stress caused by the volume change of the master particles 10 during charging and discharging is easily mitigated, the negative electrode active material of this embodiment easily achieves good battery cycle characteristics. The surface area of ​​the master particles 10 is also of an appropriate size, which can suppress capacity reduction caused by side reactions with non-aqueous electrolytes.

[0046] The average particle size of the parent particle 10 refers to the particle size (volume average particle size) that accounts for 50% of the total volume in the particle size distribution determined by laser diffraction scattering. The measuring apparatus can be, for example, the "LA-750" manufactured by Horiba, Ltd. (HORIBA). It should be noted that when the surface of the parent particle 10 is covered by the conductive layer 21, i.e., for the composite particle 20, the thickness of the conductive layer 21 is thin enough not to substantially affect the average particle size of the parent particle 10. Therefore, the average particle size of the composite particle 20 can be considered as the average particle size of the parent particle 10.

[0047] The mother particle 10 can be removed from the battery, for example, by the following method. It should be noted that the following method is an example.

[0048] First, the fully discharged battery is disassembled, and the negative electrode is removed. The negative electrode is then cleaned with, for example, anhydrous methyl ethyl carbonate or dimethyl carbonate to remove the anhydrous electrolyte components. The negative electrode compound layer is peeled off from the current collector (e.g., copper foil) serving as the negative electrode core, and the compound layer is pulverized using a mortar to obtain sample powder. Next, the sample powder is dried in a dry atmosphere for approximately 1 hour and then immersed in, for example, gently boiling 6M hydrochloric acid for approximately 10 minutes to remove elements other than those originating from the parent particle 10. The sample powder is then washed with deionized water, filtered, and dried at, for example, 200°C for 1 hour. It should be noted that a fully discharged state refers to a depth of discharge (DOD) of 90% or more (state of charge (SOC) of 10% or less).

[0049] Alkali aluminate phase 11 contains at least one alkali metal element, aluminum (Al), and oxygen (O). Alkali aluminate phase 11 is a phase containing alkali aluminate, which is a complex oxide containing at least one alkali metal element and Al.

[0050] Alkali aluminate phase 11 has good ionic conductivity, which facilitates the absorption, storage and release of ions such as Li ions based on silicon phase 12.

[0051] 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 can be expressed by the formula: Li u AlO （3+u） / 2 The expression is used to represent the state of u. From the perspectives of ease of production, stability, and ionic conductivity, u in the formula can be, for example, greater than 0 and less than 5, or greater than 0 and less than 1. When u = 1 / 5, it can be represented by LiAl5O8; when u = 1 / 2, it can be represented by Li2Al4O7; when u = 1, it can be represented by LiAlO2; and when u = 5, it can be represented by Li5AlO4.

[0052] When the alkali aluminate phase 11 contains lithium aluminate, the alkali aluminate phase 11 may contain at least one element selected from the group consisting of LiAl5O8, Li2Al4O7, LiAlO2, and Li5AlO4, and may also contain LiAlO2 as the main component. Here, "main component" refers to a component that accounts for 50% or more of the total mass of the alkali aluminate phase 11. The content of LiAlO2 may be 70% or more of the mass.

[0053] The alkali aluminate phase 11 exhibits superior alkali resistance compared to the lithium silicate phase. Therefore, compared to negative electrode active materials using the lithium silicate phase, the negative electrode active material of this embodiment exhibits, for example, suppressed side reactions with Li ions during initial charging, and suppressed degradation of the negative electrode active material accompanying the side reactions.

[0054] The atomic ratio of O to Al in the alkali aluminate (O / Al) is, for example, 1.6 or more and 4 or less. Furthermore, the atomic ratio of alkali metal (MA) to Al in the alkali aluminate (MA / Al) is, for example, 1 / 5 or more and 5 or less. When these atomic ratios are within the above ranges, the stability and ionic conductivity of the alkali aluminate phase 11 become better. It should be noted that, in this specification, the stability of the alkali aluminate phase 11 includes both its chemical stability (alkali resistance) and thermal stability.

[0055] Alkali metals are elements belonging to Group 1 of the periodic table. That is, alkali aluminate phase 11 contains at least one element selected from the group consisting of Li, Na, K, Rb, Cs, and Fr.

[0056] Alkali metal elements may include at least one element selected from the group consisting of Li, Na, and K. Alkali metal elements may include at least one element selected from the group consisting of Li and Na, or may include Li.

[0057] Alkali aluminate phase 11 may contain two alkali metal elements. Alkali aluminate phase 11 may contain at least two elements selected from the group consisting of Li, Na and K, or may contain Li and Na.

[0058] In addition to alkali metal elements, Al and O, alkali aluminate phase 11 may also contain element M.

[0059] As an example of element M, at least one can be selected from the group consisting of calcium (Ca), magnesium (Mg), zirconium (Zr), iron (Fe), boron (B), phosphorus (P), and lanthanum (La). By including the above-exemplified element as element M in the alkali aluminate phase 11, for example, the stability and ionic conductivity of the alkali aluminate phase 11 are further improved. In addition, side reactions caused by the contact between the alkali aluminate phase 31 and the electrolyte are suppressed. Element M is preferably selected from the group consisting of Zr, Fe, P, and B. La can further improve the initial charge and discharge efficiency.

[0060] Element M can also be B. That is, the alkali aluminate phase 11 can also contain B. By adding B, the porosity contained in the master particles 20 can be reduced. As a result, the deterioration of the particle structure caused by the porosity in the master particles 10 can be suppressed, thus further improving the cycle characteristics of the battery.

[0061] Element M can form compounds. Depending on the type of element M, the compound can be, for example, an oxide of element M or an aluminate of element M. In the alkali aluminate phase 11, the content of element M relative to the total amount of elements other than oxygen is, for example, 0.3 mol% or more and 3 mol% or less.

[0062] Alkali aluminate phase 11 may also contain trace amounts of elements such as chromium (Cr), nickel (Ni), manganese (Mn), copper (Cu) and molybdenum (Mo).

[0063] The alkali aluminate phase 11 can be amorphous. In this case, the effects of expansion and contraction of the silicon phase 12 can be mitigated more effectively.

[0064] A highly crystalline, finely crystalline Al2O3 phase can be dispersed within the alkali aluminate phase 11. The Al2O3 phase, for example, is distributed in an island-like pattern within the matrix of the alkali aluminate phase 11. In this case, the expansion and cracking of the alkali aluminate phase 11 accompanying the expansion and contraction of the silicon phase 12 is easily suppressed, resulting in improved cycling characteristics. In the presence of the Al2O3 phase, a peak originating from the Al2O3 phase can be observed near 2θ = 25.4° in the X-ray diffraction pattern of the parent particle 10 obtained by X-ray diffraction. The Al2O3 content in the parent particle 10 is, for example, 10% by mass or less.

[0065] Silicon phase 12 is the elemental Si phase, which repeatedly absorbs and releases Li ions during battery charging and discharging. Capacity is manifested through the Faraday reaction involving silicon phase 12. Silicon phase 12 has a large capacity. Furthermore, silicon phase 12 expands and contracts significantly during charging and discharging; however, in the negative electrode active material disclosed herein, since silicon phase 12 is dispersed within the alkali aluminate phase 11, the stress caused by the expansion and contraction of silicon phase 12 is mitigated by the alkali aluminate phase 11.

[0066] The silicon phase 12 may contain crystalline silicon. The silicon phase 12 may be composed of multiple microcrystals, for example. The size of the silicon phase 12 microcrystals may be less than 30 nm, less than 20 nm, or less than 15 nm. Based on the above, the volume change caused by the expansion and contraction of the silicon phase 12 during charging and discharging can be reduced, and the improvement effect on cycle characteristics becomes more significant. The size of the silicon phase 12 microcrystals can be calculated based on the half-width of the diffraction peaks originating from the Si(111) plane in the X-ray diffraction pattern obtained by X-ray diffraction using Cu-Kα rays and the Scherrer formula.

[0067] The lower limit of the crystallite size of the silicon phase 12 is not particularly limited, but is 1 nm as an example. A preferred crystallite size of the silicon phase 12 is 1 nm or more and 15 nm or less, or 5 nm or more and 11 nm or less. When the crystallite size of the silicon phase 12 is 1 nm or more, the surface area of ​​the silicon phase 12 can be kept small, thus reducing the likelihood of degradation of the silicon phase 12 associated with irreversible capacity formation. When the crystallite size is 15 nm or less, the expansion and contraction of the silicon phase 12 is more easily homogenized, effectively mitigating the stress generated in the parent particle 10.

[0068] The silicon phase 12 can be granular. For example, the silicon phase 12 is granular at least before the first charge. The average particle size of the silicon phase 12 can be greater than 1 nm and less than 1000 nm. Alternatively, the average particle size of the silicon phase 12 can be less than 500 nm, less than 200 nm, or less than 50 nm. After the first charge, the average particle size of the silicon phase 12 can be less than 400 nm or less than 100 nm. By dispersing the fine silicon phase 12 within the alkali aluminate phase 11 as described above, the volume change of the parent particle 10 during charge and discharge is reduced, and the structural stability of the parent particle 10 is further improved.

[0069] The average particle size of the silicon phase 12 can be determined using SEM images obtained by scanning electron microscopy (SEM) of the parent particle 10 with the cross-section of the silicon phase 12 exposed. Specifically, the average particle size of the silicon phase 12 is calculated by averaging the maximum diameters of 100 silicon phase 12 samples randomly selected from the cross-sectional SEM images of the parent particle 10.

[0070] From the perspective of maximizing capacity, the content of silicon phase 12 in the masterbatch 10 can be 30% by mass or more, 35% by mass or more, or 55% by mass or more. From the perspective of improving cycling characteristics, the content of silicon phase 12 in the masterbatch 10 can 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 the masterbatch 10 that is not covered by the alkali aluminate phase 11 is reduced, and the side reactions between the electrolyte and the silicon phase 12 are also suppressed. The content of silicon phase 12 in the masterbatch 10 can be 30% by mass or more and 90% by mass or less, or 35% by mass or more and 75% by mass or less.

[0071] The content of silicon phase 12 in the mother particle 10 is determined as described below by quantifying the amount of Si constituting silicon phase 12 in the mother particle 10 using Si-NMR.

[0072] Carbon phase 13 is a phase of carbon materials. Examples of carbon materials include natural graphite, artificial graphite, graphitized mesophase carbon, amorphous carbon, soft carbon, and hard carbon. Carbon materials can be amorphous carbon. Examples of amorphous carbon include carbon black, calcined pitch, coke, and activated carbon.

[0073] The carbon phase 13 can be granular. The shape of the granules is not particularly limited, and can include spherical, angular, plate-like, and linear shapes.

[0074] There is no particular limitation on the size of carbon phase 13. When carbon phase 13 is in particulate form, for example, the average particle size of carbon phase 13 can be greater than 1 nm and less than 1000 nm, greater than 10 nm and less than 500 nm, or greater than 10 nm and less than 200 nm.

[0075] The average particle size of the carbon phase 13 can be determined using SEM images obtained by SEM observation of the parent particle 10 with the cross-section of the carbon phase 13 exposed. Specifically, the average particle size of the carbon phase 13 is calculated by averaging the maximum diameters of 100 carbon phase 13 particles arbitrarily selected from the cross-sectional SEM images of the parent particle 10.

[0076] The master particle 10 may substantially not contain lithium silicate and SiO2. The combined content of lithium silicate and SiO2 in the master particle 10 may, for example, be less than 3% by mass.

[0077] The mass ratio of Al in the parent particle 10 to the total mass of elements other than oxygen and carbon (mAl) can be 10% by mass or more and 47% by mass or less, or 11.5% by mass or more and 45.5% by mass or less. The mass ratio of alkali metal elements to the total mass of elements other than oxygen and carbon (mMA) can be 0.7% by mass or more and 13.5% by mass or less, 1.0% by mass or more and 9.5% by mass or less, or 1.5% by mass or more and 3.5% by mass or less. When the mass ratio of Al (mAl) and the mass ratio of alkali metal elements (mMA) are within the above ranges, an aluminate phase with excellent stability and ionic conductivity is easily obtained.

[0078] Based on the above composition, the stability and ionic conductivity of the alkali aluminate phase 11 can be improved. It should be noted that the above stability includes both chemical stability (alkali resistance) and thermal stability.

[0079] From the perspective of the stability, ionic conductivity and reduction of porosity of the alkali aluminate phase 11, the ratio of the mass of alkali metal elements in the parent particle 10 to the total mass of elements other than oxygen and carbon (mMA) to the ratio of the mass of Al elements to the total mass (mAl) (mMA / mAl) can be 0.04 or more and 0.50 or less, or 0.05 or more and 0.25 or less.

[0080] The mass ratio of silicon (Si) in the master particle 10 to the total mass of elements excluding oxygen and carbon (mSi) can be 40% to 90% by mass, or 50.8% to 85.5% by mass. Based on this composition, the battery can easily achieve both high capacity and good cycle characteristics. The mass ratio of Si (mSi) represents the amount of Si constituting the silicon phase 12 in the master particle 10.

[0081] When the alkali aluminate phase 11 also contains B, the mass ratio of B in the parent particle 10 to the total mass of elements other than oxygen and carbon (mB) can be 1% or more and 20% or less, or 2% or more and 15% or less. Based on the above composition, the improvement effect on the cycling characteristics can become more significant.

[0082] The ratio of the mass ratio of Al (mAl) to the mass ratio of B (mB) (mAl / mB) can 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 reduction in porosity of the mother particle 10 becomes more significant. This suppresses the degradation of the particle structure caused by porosity within the mother particle 10, thus effectively improving the cycle characteristics of the battery. When mAl / mB is 1.0 or more, the decrease in initial charge-discharge efficiency is reduced. Therefore, by keeping mAl / mB within the above range, both improved cycle characteristics and good initial charge-discharge efficiency can be achieved.

[0083] like Figure 1 As shown, pores 14 may exist within the parent particle 10. For example, the total pore volume of pores with a diameter of 10 nm or more and 200 nm or less in the parent particle 10, calculated by the BJH (Barrett-Joyner-Halenda) method, is preferably 0.05 cm³. 3 / g or less, more preferably 0.04cm 3 / g or less. This configuration more effectively suppresses the disintegration of the particle structure of the parent particles 10 caused by the expansion and contraction of the negative electrode active material during battery charging and discharging. Therefore, the negative electrode active material of this embodiment can improve its durability against charge and discharge, resulting in further improvement of the battery's cycle characteristics. It should be noted that the lower limit of the total pore volume is not particularly limited, but is preferably 0.001 cm³. 3 / g, as an example, is 0.003cm 3 / g. It should be noted that when a conductive layer 21 is provided on the surface of the parent particle 10, the total pore volume of pores with a diameter of 10 nm or more and 200 nm or less, calculated by the BJH method, in the composite particle 20 containing the parent particle 10 and the conductive layer 21, is preferably 0.05 cm³. 3 / g or less, more preferably 0.04cm 3 / g or less. Therefore, the negative electrode active material of this embodiment can improve the durability against charge and discharge, and as a result, can further improve the cycle characteristics of the battery. It should be noted that, at this time, the lower limit of the total pore volume is not particularly limited, but is preferably 0.001 cm³. 3 / g, as an example, is 0.003cm3 / g.

[0084] It should be noted that the total pore volume of the master particles 10 or composite particles 20 can be controlled by adjusting the addition of element M (e.g., B) to the alkali aluminate phase 11, the firing temperature and the compressive force applied to the particles during firing in the manufacturing process of the master particles 10, and the mA / mAl ratio, etc. Furthermore, by filling the pores formed inside the master particles 10 with other materials (hereinafter referred to as filler materials) during the production of the master particles 10, the total pore volume can also be controlled to 0.05 cm³. 3 / g or less. Examples of filler materials that fill the pores include carbon materials and resin materials. For example, when forming the carbon phase 13 of the parent particle 10, if the pores formed inside the parent particle 10 are filled with, for example, carbon material, the parent particle 10 containing pores is mixed with an amount of carbon material selected considering the pore size and pore volume contained in the parent particle 10, and the mixture is heat-treated. This allows carbon material to fill the pores of the parent particle 10. By adjusting the carbon material used and the heat treatment conditions, the total pore volume can be controlled to be less than 0.05 cm³. 3 / g or less.

[0085] The porosity of the mother particle 10 before the initial charge and discharge can be below 25%. By suppressing the porosity of the mother particle 10 to below 25%, the degradation of the negative electrode active material is further suppressed, and the cycle characteristics of the battery are further improved. The porosity of the mother particle 10 can be below 20% or below 15%. There is no particular limitation on the lower limit of porosity; as an example, it is 1%.

[0086] The porosity of the parent particle 10 refers to the proportion of pores in the cross-section of the parent particle 10. The porosity of the parent particle 10 can be determined using a SEM image of the cross-section of the negative electrode active material that exposes the cross-section of the parent particle 10. The porosity of the parent particle 10 is calculated by extracting the pore area by binarizing the SEM image using image analysis software (e.g., ImageJ) and dividing the total area of ​​the pores by the total area of ​​the particle cross-section.

[0087] It should be noted that the porosity of the master particles 10 can be controlled by adjusting the addition of element M (e.g., B) to the above-mentioned alkali aluminate phase 11, the firing temperature and the compressive force applied to the particles during firing in the manufacturing process of the master particles 10, as well as mA / mAl, etc.

[0088] The master particle 10 can have a Vickers hardness of 300 HV or higher. When the master particle 10 has a high Vickers hardness, it is easier to suppress the volume change of the silicon phase 12 during charge and discharge, thus reducing the degradation of the particle structure. As a result, the improvement in cycle characteristics becomes more significant. The Vickers hardness of the master particle 10 is more preferably 350 HV or higher, but can also be 400 HV or higher, or 500 HV or higher.

[0089] The Vickers hardness of the master particle 10 can be measured using a Vickers hardness tester. Specifically, the master particle 10 is embedded in thermosetting resin and polished with 400-grit abrasive paper to expose its cross-section. Then, the cross-section is mirror-finished by polishing with 2000-grit abrasive paper. The Vickers hardness is measured under a load of 1 kg and a holding time of 15 seconds. There is no specific upper limit to the Vickers hardness of the master particle 10; for example, it could be 1500 HV.

[0090] The content of each element in the master particle 10 was determined by the following methods. Oxygen content was determined using an oxygen-nitrogen-hydrogen analyzer. Si content was determined using NMR. Carbon content was determined using a carbon-sulfur analyzer. The content of other elements was determined by inductively coupled plasma atomic emission spectrometry (ICP). If ICP was not feasible, energy-dispersive X-ray diffraction (EDX) was used. If neither ICP nor EDX was feasible, Auger electron spectroscopy (AES) was used. The composition of the alkali aluminate phase 11 was determined based on the content of each element. When determining the state of the battery, the determination could be performed, for example, by disassembling the fully discharged battery, removing the negative electrode, cleaning the negative electrode with, for example, anhydrous ethyl methyl carbonate or dimethyl carbonate to remove the anhydrous electrolyte components, drying it, and then analyzing the cross-section of the negative electrode composite layer obtained using a cross-section polishing instrument. Furthermore, when it is necessary to remove the master particle 10 from the battery as a test sample, as described above, the negative electrode compound layer is removed from the negative electrode removed from the battery, pulverized, and elements other than those originating from the master particle 10 are removed, allowing the master particle to be extracted from the battery. The master particle can be extracted from the battery as a composite particle with a conductive layer on its surface. The elements of the master particle can be determined based on the analysis results of each element in the composite particle. Alternatively, the conductive layer can be removed using appropriate means corresponding to the constituent material of the conductive layer, and the internal master particle portion can be analyzed.

[0091] Inductively Coupled Plasma Emission Spectroscopy (ICP)

[0092] The sample of masterbatch 10 was completely dissolved in a heated acid solution (a mixture of hydrofluoric acid, nitric acid, and sulfuric acid), and the carbon residue in the solution was removed by filtration. The resulting filtrate was then analyzed by ICP to determine the spectral intensity of each element. Next, a standard curve was prepared using commercially available standard solutions of the elements to calculate the content of each element in masterbatch 10.

[0093] Energy-dispersive X-rays (EDX)

[0094] Based on the reflected electron image of the cross-section of the negative electrode active material exposing the cross-section of the parent particle 10, elemental mapping analysis based on EDX is performed. The area containing the target element is calculated using image analysis software. The observation magnification is, for example, 2000x to 20000x. When measuring from the state of the battery, 10 parent particles 10 with a maximum diameter of 5μm or more are randomly selected from the cross-sectional image of the reflected electron image of the negative electrode compound layer containing the negative electrode active material, and elemental mapping analysis based on EDX is performed on each particle. The content of the target element is calculated by averaging the measured values ​​of the area containing the specified element in the 10 particles.

[0095] The following shows the determination conditions for the desired cross-sectional SEM-EDX analysis.

[0096] Processing equipment: JEOL SM-09010 (Cross Section Polisher)

[0097] Processing conditions: Accelerating voltage 6kV

[0098] Current value: 140μA

[0099] Vacuum degree: 1×10 -3 Pa to 2×10 -3 Pa

[0100] Measurement apparatus: HITACHI SU-70 electron microscope

[0101] Accelerating voltage during analysis: 10kV

[0102] Mode: Free Mode

[0103] Probe current mode: Medium

[0104] Probe current range: High

[0105] Anode Ap.: 3

[0106] OBJ Ap.:2

[0107] Analysis area: 1 μm square

[0108] Analysis software: EDAX Genesis

[0109] CPS: 20500

[0110] Lsec: 50

[0111] Time constant: 3.2

[0112] As described above, elemental mapping analysis based on EDX is used as a method to determine the area proportion of carbon phase 13 in the cross-section of the primary particles of the parent particle 10. In this case, in order to accurately determine the area occupied by carbon phase 13 dispersed inside the parent particle 10, the area inside the particle surface at a distance of 200 nm or more is used when determining the area occupied by carbon phase 13.

[0113] Auger electron spectroscopy (AES)

[0114] Based on the reflected electron image of the cross-section of the negative electrode active material exposing the cross-section of the parent particle 10, qualitative and quantitative elemental analysis is performed using an AES analyzer (e.g., JAMP-9510F, manufactured by NEC Corporation). Measurement conditions can be set, for example, an accelerating voltage of 10 kV, a beam current of 10 nA, and an analysis area of ​​20 μm φ. When measuring from the state of the battery, 10 parent particles 10 with a maximum diameter of 5 μm or more are randomly selected from the cross-sectional image of the reflected electron image of the negative electrode binder layer containing the negative electrode active material, and qualitative and quantitative elemental analysis is performed on each using an AES analyzer. The content of the specified elements contained in the 10 particles is averaged to calculate the content.

[0115] It should be noted that during the charging and discharging process, a coating can be formed on the surface of the parent particle 10 through decomposition of the non-aqueous electrolyte, etc. Furthermore, as mentioned above, a conductive layer is sometimes provided on the surface of the parent particle 10. Therefore, not limited to determining the area ratio of the carbon phase 13 in the cross-section of the primary particle of the parent particle 10, EDX and AES analyses are performed on a range extending from, for example, 200 nm or more inward from the peripheral edge of the particle's cross-section, in a manner that excludes the thin coating and conductive layer within the measurement range.

[0116] <Inactive Gas Melting-Non-Dispersive Infrared Absorption Method>

[0117] The oxygen content in the masterbatch 10 can be determined using an oxygen-nitrogen-hydrogen analyzer (e.g., the EGMA-830 model manufactured by Horiba, Ltd.). The sample is placed in a Ni capsule and, together with Sn and Ni particles used as flux, is added to a carbon crucible heated to 5.75 kW. The released carbon monoxide gas is then detected. A standard curve is prepared using the standard sample Y₂O₃, and the oxygen content of the sample is calculated.

[0118] Nuclear Magnetic Resonance Spectroscopy (NMR)

[0119] The amount of Si in the silicon phase 12 constituting the parent particle 10 can be quantified using Si-NMR. The desired Si-NMR measurement conditions are shown below.

[0120] Measurement apparatus: Varian Corporation, solid-state nuclear magnetic resonance spectroscopy apparatus (INOVA-400)

[0121] Probe: Varian 7mm CPMAS-2

[0122] MAS: 4.2kHz

[0123] MAS speed: 4kHz

[0124] Pulse: DD (45° pulse + 1H signal acquisition time for decoupling)

[0125] Repeat time: 1200 seconds ~ 3000 seconds

[0126] Observation width: 100kHz

[0127] Observation center: around -100ppm

[0128] Signal acquisition time: 0.05 seconds

[0129] Total number of times: 560

[0130] Sample volume: 207.6 mg

[0131] <High-Frequency Induction Heating Furnace Combustion - Infrared Absorption Method>

[0132] The carbon content in the masterbatch 10, i.e., the content of carbon phase 13 in the masterbatch, can be determined using a carbon-sulfur analysis apparatus (e.g., the EMIA-520 model manufactured by Horiba, Ltd.). A sample is weighed onto a magnetic plate, a combustion accelerant is added, and the sample is inserted into a combustion furnace heated to 1350°C (carrier gas: oxygen). The amount of carbon dioxide gas produced during combustion is detected by infrared absorption. For example, a calibration curve can be prepared using carbon steel (0.49% carbon content) manufactured by Bureau of Analyzed Samples Ltd., and the carbon content of the sample can be calculated.

[0133] An example of the XRD pattern obtained by X-ray diffraction (XRD) of a parent particle is described. Here, the XRD pattern of a parent particle having lithium as the alkali metal element constituting the alkali aluminate phase (i.e., the alkali aluminate phase is the lithium aluminate phase) and not containing carbon phase 13 is described. In such a parent particle, a peak originating 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°. The X-rays used for XRD measurement are Cu Kα rays. It should be noted that in this specification, "near x°" refers to, for example, a range of x ± 1°.

[0134] [Conductive layer]

[0135] The conductive layer 21 covers at least a portion of the surface of the parent particle. The conductive layer 21 may cover the entire surface of the parent particle. The conductive layer 21 is, for example, a thin film layer containing a conductive material. In this embodiment, the conductivity of the negative electrode active material is improved by further comprising the conductive layer 21 covering at least a portion of the surface of the parent particle 10.

[0136] The conductive material can also be a conductive carbon material. That is, the conductive layer 21 can also contain carbon. As a carbon material, natural graphite, artificial graphite, graphitized mesophase carbon, amorphous carbon, soft carbon, and hard carbon can be used. The carbon material can be amorphous carbon. Based on the above, a thin conductive layer 21 covering the surface of the parent particles can be easily formed. Examples of amorphous carbon include carbon black, calcined pitch, coke, and activated carbon.

[0137] The thickness of the conductive layer 21 is preferably such that it does not affect the average particle size of the composite particles 20, that is, the average particle size of the composite particles 20 does not increase significantly relative to the average particle size of the parent particles. Considering the assurance of conductivity and the diffusion of ions such as Li ions, the thickness of the conductive layer 21 can be 1 nm or more and 200 nm or less, or 5 nm or more and 100 nm or less. The thickness of the conductive layer 21 can be measured by SEM observation or transmission electron microscopy (TEM) observation of the cross-section of the negative electrode active material exposing the cross-section of the composite particles 20.

[0138] (Method for manufacturing negative electrode active material)

[0139] The negative electrode active material disclosed herein includes, for example, a master particle 10. The master particle 10 is manufactured, for example, by a manufacturing method including the first to fifth steps described below.

[0140] The first step: the process of obtaining alkali aluminate (hereinafter referred to as "raw aluminate") as a raw material.

[0141] The second step is to composite the raw material aluminate and the raw material silicon to obtain a composite intermediate in which the raw material silicon is dispersed in the raw material aluminate.

[0142] The third step is to heat-treat 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.

[0143] The fourth step is to crush the sintered body to obtain the precursor of the master particles 10.

[0144] The fifth step is to fill the pores inside the precursor of the master particle 10 with carbon material to form a carbon phase 13, thereby obtaining the master particle 10.

[0145] [First Process]

[0146] The first step includes, for example, the process of mixing an aluminum compound, a compound containing an alkali metal element, and a compound containing element M as needed to obtain a mixture; and the process of calcining the mixture to obtain a raw aluminate. Calcination is carried out, for example, in an oxidizing atmosphere. The calcination temperature can be above 400°C and below 1200°C, or above 700°C and below 1100°C.

[0147] Examples of aluminum compounds include aluminum oxide, aluminum hydroxide, and aluminum carbonate. Aluminum compounds can be used alone or in combination of two or more.

[0148] Examples of compounds containing alkali metals include lithium carbonate, lithium oxide, lithium hydroxide, lithium hydride, sodium carbonate, sodium oxide, sodium hydroxide, sodium hydride, potassium carbonate, potassium oxide, potassium hydroxide, and potassium hydride. Compounds containing alkali metals can be used alone or in combination of two or more.

[0149] Compounds containing element M are, for example, boron compounds. Examples of boron compounds include boron oxide, boric acid, borax, and sodium tetraborate. A single boron compound can be used alone, or two or more compounds can be used in combination.

[0150] Furthermore, the ratio (mMA / mAl) can, for example, be 0.04 or more and 0.50 or less, or 0.05 or more and 0.25 or less, as described above. The mass ratio of alkali metal elements (mMA) can, for example, be 1.0% by mass or more and 9.5% by mass or less, as described above.

[0151] In the first process, aluminum compounds that did not react with compounds containing alkali metals 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 compounds, aluminum compound residue is more likely to remain. If the aluminum compound remaining in the raw aluminate is Al2O3, it is possible that an Al2O3 phase dispersed within the alkali aluminate phase 11 will form in the final masterbatch.

[0152] [Second Process]

[0153] In the second step, for example, a micronized composite intermediate is obtained by pulverizing a mixture of raw aluminate and raw silicon while applying shear force. As an example, a method can be described as follows: raw aluminate and raw silicon are mixed in a specified mass ratio, and the mixture is micronized using a pulverizing device such as a ball mill.

[0154] The raw material silicon is, for example, coarse silicon particles with an average particle size of several μm to tens of μm. The silicon particles can be prepared by taking the half-width of the diffraction peaks originating from the Si(111) plane from the X-ray diffraction pattern of the negative electrode active material or the parent particle and calculating the crystallite size of silicon phase 12 based on the Scherrer formula to be less than 15 nm. It should be noted that silicon nanoparticles and raw material aluminate nanoparticles can also be synthesized and mixed without the use of a pulverizing device.

[0155] [Third Process]

[0156] In the third step, the composite intermediate, after being micronized, is sintered while being subjected to pressure, such as through hot pressing, to obtain a sintered body. The pressure applied to the composite intermediate is, for example, 100 MPa or more, and can be between 100 MPa and 300 MPa. The higher the pressure in the third step, the more likely the porosity of the composite particles is to decrease. It is desirable to sinter the composite intermediate in an inactive atmosphere (such as argon, nitrogen, etc.). The sintering conditions in the third step also affect the crystallites of silicon phase 12; generally, the higher the sintering temperature, the larger the crystallite size.

[0157] An example firing temperature is 450°C or higher and 1000°C or lower. If the firing temperature is within this range, it is easy to form a structure in which a small silicon phase 12 is dispersed within a low-crystallinity alkali aluminate phase 11. The raw aluminate is stable at this temperature and hardly reacts with silicon. The firing temperature can be 550°C or higher and 950°C or lower, or 650°C or higher and 900°C or lower. To reduce the porosity of the master particles 10, firing at a temperature of 650°C or higher is preferred. The firing time is, for example, 1 hour or more and 10 hours or less.

[0158] [Fourth Process]

[0159] The fourth step is to pulverize the parent particles 10 until they have the desired particle size distribution. For example, the parent particles 10 are pulverized to have a median particle size of 1 μm or more and 25 μm or less. Thus, the precursor of the parent particles 10 is obtained.

[0160] [Fifth Process]

[0161] The fifth step involves filling the pores within the precursor of the master particle 10 obtained in the fourth step with carbon material to form a carbon phase 13, thus obtaining the master particle 10. In the sintered body formed in the third step, pores exist internally. Therefore, the precursor of the master particle 10 obtained in the fourth step by pulverizing the sintered body also contains pores. In the fifth step, carbon material is filled into these pores to form a carbon phase 13, resulting in the master particle 10. In one example of the method for filling the pores of the precursor of the master particle 10 with carbon material of an amount selected considering the pore size and pore volume contained in the precursor of the master particle 10, and the mixture is fired to fill the pores of the master particle 10 with carbon material. For example, by adjusting the amount of carbon material used, it is possible to produce master 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. Examples of carbon materials used to form the carbon phase 13 include coal tar pitch, petroleum tar pitch, and phenolic resin.

[0162] Through the first to fifth processes described above, master particles 10 can be manufactured.

[0163] If a conductive layer 21 is formed on the surface of the parent particle 10, the following sixth step is further performed.

[0164] The sixth step: forming a conductive layer 21 on the surface of the master particles 10 containing the alkali aluminate phase 11, silicon phase 12 and carbon phase 13.

[0165] As described above, the conductive material constituting the conductive layer 21 is preferably a conductive carbon material. Examples of methods for covering the surface of the master particles with carbon material include: CVD methods using hydrocarbon gases such as acetylene and methane as raw materials; and methods involving mixing and heating master particles with coal tar pitch, petroleum tar pitch, phenolic resin, etc., to carbonize them. Alternatively, carbon black can be deposited onto the surface of the master particles.

[0166] In the sixth step, for example, a conductive layer 21 is formed on the surface of the parent particle 10 by heating a mixture of the parent particle 10 and the carbon material in an inactive atmosphere (such as argon, nitrogen, etc.) at a temperature of 700°C or higher and 950°C or lower. By operating as described above, a composite particle 20 having a conductive layer 21 on the surface of the parent particle 10 is obtained.

[0167] When a conductive layer 21 is formed on the surface of the parent particle 10, and when the carbon material constituting the carbon phase 13 and the carbon material constituting the conductive layer 21 are the same, the fifth and sixth processes can be performed simultaneously. For example, in the fifth process, the amount of carbon material used and the heat treatment conditions can be adjusted so that the conductive layer 21 and the carbon phase 13 having an area ratio of 4% or more and 25% or less in the cross-section of the primary particles of the parent particle 10 can be formed simultaneously.

[0168] (Battery)

[0169] The battery of this embodiment includes a negative electrode, a positive electrode, and an electrolyte. The negative electrode contains the negative electrode active material of this embodiment. By including the negative electrode active material of this embodiment in the negative electrode, the battery of this embodiment improves cycle characteristics and, consequently, improves initial charge-discharge efficiency.

[0170] Figure 3 This is a schematic longitudinal cross-sectional view illustrating an example of the battery according to this embodiment. The battery 100 is a cylindrical battery comprising a cylindrical battery casing, a wound electrode assembly 34, and an electrolyte (not shown). The electrode assembly 34 is housed within the battery casing and is in contact with the electrolyte.

[0171] The battery casing consists of a bottomed cylindrical metal container, namely the casing body 35, and a sealing body 36 that seals the opening of the casing body 35. A gasket 47 is disposed between the casing body 35 and the sealing body 36. The gasket 47 ensures the airtightness of the battery casing. Inside the casing body 35, insulating plates 37 and 38 are respectively disposed at both ends of the electrode assembly 34 in the winding axis direction of the electrode assembly 34.

[0172] The housing body 35, for example, has a stepped portion 41. The stepped portion 41 can be formed by partially stamping the side wall of the housing body 35 from the outside. The stepped portion 41 can also be formed in an annular shape along the circumference of an imaginary circle defined by the housing body 35 on the side wall of the housing body 35. In this case, the sealing body 36 is supported, for example, by the surface on the opening side of the stepped portion 41.

[0173] The sealing body 36 includes a partially open metal plate 42, a lower valve body 43, an insulating member 44, an upper valve body 45, and a cover 46. These components are stacked in this order within the sealing body 36. The sealing body 36 is installed at the opening of the housing body 35 such that the cover 46 is located on the outside of the housing body 35, and the partially open metal plate 42 is located on the inside of the housing body 35.

[0174] The components constituting the sealing body 36 are, for example, circular or ring-shaped. All of these components, except for the insulating component 44, are electrically connected to each other.

[0175] The electrode assembly 34 has a positive electrode 31, a separator 32, and a negative electrode 33. The positive electrode 31, separator 32, and negative electrode 33 are all strip-shaped. The width direction of the strip-shaped positive electrode 31 and negative electrode 33 is, for example, parallel to the winding axis of the electrode assembly 34. The separator 32 is disposed between the positive electrode 31 and the negative electrode 33. The positive electrode 31 and negative electrode 33 are wound into a spiral shape with the separator 32 spaced between them.

[0176] When observing the cross-section of the battery 100 in a direction perpendicular to the winding axis of the electrode assembly 34, the positive electrode 31 and the negative electrode 33 are alternately stacked in the radial direction of an imaginary circle defined by the housing body 35, with spacers 32 between them.

[0177] The positive electrode 31 is electrically connected to the cover 46, which also serves as the positive terminal, via a positive electrode lead 39. One end of the positive electrode lead 39 is connected, for example, near the center of the positive electrode 31 along its length. The positive electrode lead 39 extends from the positive electrode 31 to the partially open metal plate 42 through a through hole formed in the insulating plate 37. The other end of the positive electrode lead 39 is, for example, soldered to the side of the electrode assembly 34 of the partially open metal plate 42.

[0178] The negative electrode 33 is electrically connected to the housing body 35, which also serves as the negative terminal, via a negative electrode lead 40. One end of the negative electrode lead 40 is connected, for example, to the end of the negative electrode 33 along its length. The other end of the negative electrode lead 40 is, for example, soldered to the inner bottom surface of the housing body 35.

[0179] The following is a detailed explanation of the components of battery 100.

[0180] The positive electrode 31 contains a material with the property of absorbing and releasing metal ions (e.g., lithium ions). The positive electrode 31 may, for example, contain a positive electrode active material. The positive electrode 31 may have a positive electrode current collector and a positive electrode flux layer loaded on the surface of the positive electrode current collector.

[0181] The positive electrode mixture layer contains a positive electrode active material. Examples of positive electrode active materials 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 lithium-containing transition metal oxides or lithium-containing transition metal phosphates as positive electrode active materials can reduce battery manufacturing costs 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 of these positive electrode active materials can be used.

[0182] The positive electrode layer may contain conductive additives, ionic conductors, and binders as needed.

[0183] Conductive additives and ionic conductors are used to reduce the resistance of electrodes. Examples of conductive additives include carbon materials and conductive polymers. Examples of carbon materials include carbon black, graphite, acetylene black, carbon nanotubes, carbon nanofibers, graphene, fullerene, and graphite oxide. Examples of conductive polymers include polyaniline, polypyrrole, and polythiophene. At least one of these conductive additives may be used.

[0184] Adhesives are used to improve the adhesion of materials constituting electrodes. Examples of adhesives include polyvinylidene fluoride (PVDF), PVDF-hexafluoropropylene copolymer, PVDF-tetrafluoroethylene copolymer, polytetrafluoroethylene (PTFE), carboxymethyl cellulose, polyacrylic acid, styrene-butadiene copolymer rubber, polypropylene, polyethylene, and polyimide. At least one adhesive selected from these materials may be used.

[0185] The positive current collector is a sheet or thin film made of metallic materials such as aluminum, aluminum alloys, stainless steel, titanium, or titanium alloys. The sheet or film can be porous or non-porous. Metal foils, metal meshes, etc., are used as sheets or films. Carbon materials can be coated onto the surface of the positive current collector as an auxiliary material for conductivity.

[0186] The negative electrode 33 contains the negative electrode active material of this embodiment. For example, the negative electrode 33 has a negative electrode current collector and a negative electrode additive layer loaded on the surface of the negative electrode current collector.

[0187] The negative current collector is a foil made of metal materials such as stainless steel, nickel, nickel alloy, copper, and copper alloy.

[0188] The negative electrode binder layer contains the negative electrode active material of this embodiment. The negative electrode binder layer may contain other materials such as conductive additives, ionic conductors, and binders, as needed. The materials described above for the positive electrode binder layer may also be used as conductive additives, ionic conductors, and binders in the negative electrode binder layer.

[0189] 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, for example, be 0.5 mol / L or more and 2 mol / L or less. By controlling the lithium salt concentration within the above range, an electrolyte with excellent ionic conductivity and moderate viscosity can be obtained. However, the lithium salt concentration is not limited to the above.

[0190] As a non-aqueous solvent, cyclic carbonates, linear carbonates, cyclic ethers, linear ethers, nitriles, amides, etc., can be used. One of these solvents can be used, or two or more can be used in combination.

[0191] As lithium salts, lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bisperfluoroethylsulfonylimide (LiN(SO2C2F5)2), LiAsF6, LiCF3SO3, and lithium difluoro(oxalate-containing)borate, etc., can be used. One of these electrolyte salts can be used, or two or more can be used in combination.

[0192] Typically, a separator is desired between the positive and negative electrodes. The separator 32 has high ion permeability and possesses moderate mechanical strength and insulation. Microporous membranes, woven fabrics, and nonwoven fabrics can be used as the separator 32. The material used for the separator 32 can be, for example, a polymer. Polymers can be, for example, polyolefins such as polypropylene and polyethylene.

[0193] In the battery of this disclosure, the electrolyte may, for example, be impregnated in a polymer that serves as a separator. That is, the battery of this disclosure may have a structure that combines an electrolyte and a polymer.

[0194] The battery disclosed herein may further include a solid electrolyte as the electrolyte. That is, the battery of this disclosure may have a hybrid structure that combines an electrolyte and a solid electrolyte. Examples of solid electrolyte materials are halide solid electrolytes, sulfide solid electrolytes, oxide solid electrolytes, or organic polymer solid electrolytes. In this disclosure, "halide solid electrolyte" refers to a solid electrolyte containing a halogen element as the main component of the anions. "Sulfide solid electrolyte" refers to a solid electrolyte containing sulfur as the main component of the anions. "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 molar mass among all the anions constituting the solid electrolyte.

[0195] In this disclosure, an example of the battery structure of this embodiment is described. Figure 3 The illustrated configuration example shows a cylindrical non-aqueous electrolyte secondary battery consisting of a wound electrode assembly with the positive and negative electrodes separated by a separator and an electrolyte housed in an outer casing. However, the battery of this disclosure is not limited to this configuration example. The battery of this disclosure can be any shape, such as square, coin-shaped, button-shaped, or laminated. Furthermore, instead of a wound electrode assembly, other types of electrode assemblies, such as an electrode assembly with the positive and negative electrodes stacked together separated by a separator, can be used in the secondary battery of this disclosure.

[0196] Figure 4This is a simplified cross-sectional view showing another example of the battery configuration of this 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 of this 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.

[0197] The positive electrode 210 includes a positive current collector 211 and a positive electrode flux layer 212 loaded on the surface of the positive current collector 211. The positive electrode flux layer 212 is located between the positive current collector 211 and the electrolyte layer 220. The positive current collector 211 and the positive electrode flux layer 212 can use the material of the positive electrode 31 of the battery 100 described above.

[0198] The positive electrode layer 212 may contain a solid electrolyte. Examples of solid electrolytes include the solid electrolyte described above as the electrolyte in the battery 100.

[0199] Electrolyte layer 220 is a layer containing an electrolyte. This electrolyte is, for example, a solid electrolyte. Electrolyte layer 220 can be a solid electrolyte layer. Examples of solid electrolytes include the solid electrolyte described above as the electrolyte in battery 100.

[0200] The electrolyte layer 220 may also include a binder. The electrolyte layer 220 may also use the material of the positive electrode binder layer of the battery 100 described above as a binder.

[0201] The negative electrode 230 includes a negative electrode current collector 231 and a negative electrode flux layer 232 supported by the negative electrode current collector 231. The negative electrode flux layer 232 contains the negative electrode active material of this embodiment. The negative electrode flux layer 232 is located between the negative electrode current collector 231 and the electrolyte layer 220. The negative electrode current collector 231 and the negative electrode flux layer 232 can use the material of the negative electrode 33 of the battery 100 described above.

[0202] The negative electrode layer 232 may contain a solid electrolyte. Examples of solid electrolytes include the solid electrolyte described above as the electrolyte in the battery 100.

[0203] Battery 200 can be configured into various shapes such as coin type, cylindrical type, square type, sheet type, button type, flat type, and stacked type.

[0204] (Other implementation methods)

[0205] (Postscript)

[0206] Based on the description of the above embodiments, the following technology is disclosed.

[0207] (Technology 1)

[0208] A negative electrode active material, which has master particles,

[0209] The parent material comprises:

[0210] Alkali aluminate phase containing at least one alkali metal element and Al,

[0211] The silicon phase dispersed within the alkali aluminate phase, and

[0212] The carbon phase dispersed within the alkali aluminate phase,

[0213] In the cross-section of the primary particles of the mother particle, the area ratio of the carbon phase is more than 4% and less than 25%.

[0214] According to this configuration, the negative electrode active material of Technology 1 can improve the durability of charging and discharging, thereby improving the cycle characteristics of the battery.

[0215] (Technology 2)

[0216] According to the negative electrode active material of technology 1, the area ratio is 5% or more and 20% or less.

[0217] According to this configuration, the negative electrode active material of Technology 2 can further improve the durability against charge and discharge, and as a result, can further improve the cycle characteristics of the battery.

[0218] (Technology 3)

[0219] According to the negative electrode active material of technique 1 or 2, the alkali aluminate phase further comprises B.

[0220] According to this composition, the porosity contained in the master particles is reduced, thus the cycle characteristics of the battery can be further improved.

[0221] (Technology 4)

[0222] According to the negative electrode active material described in Technique 3, wherein...

[0223] In the parent particle, the ratio of the mass of Al to the total mass of all elements except oxygen and carbon is mAl.

[0224] In the parent particle, the mass ratio of element B to the total mass of elements excluding oxygen and carbon is mB.

[0225] At this point, the ratio mA1 / mB is above 1.0 and below 30.0.

[0226] According to this configuration, the porosity reduction effect of the B-based master particles becomes more significant. Therefore, the degradation of the particle structure caused by porosity within the master particles can be suppressed, thus effectively improving the battery's cycle characteristics.

[0227] (Technology 5)

[0228] According to any one of the techniques 1 to 4, the negative electrode active material contains at least one alkali metal element selected from the group consisting of Li, Na and K.

[0229] According to this configuration, the negative electrode active material of technology 5 can further improve the cycle characteristics of the battery.

[0230] (Technology 6)

[0231] The negative electrode active material according to any one of techniques 1 to 5 further comprises a conductive layer covering at least a portion of the surface of the parent particle.

[0232] Based on this composition, the conductivity of the negative electrode active material is improved, which can improve battery characteristics.

[0233] (Technology 7)

[0234] A battery that has the following features:

[0235] A negative electrode comprising the negative electrode active material described in any one of techniques 1 to 6,

[0236] Positive electrode, and

[0237] Electrolytes.

[0238] With this configuration, a battery with improved cycle characteristics can be obtained.

[0239] Example

[0240] The present disclosure will now be described in more detail using examples. These examples are merely illustrative and are not intended to limit the scope of the disclosure.

[0241] <Example 1>

[0242] [Preparation of Master Particles]

[0243] (First process)

[0244] Al₂O₃, Li₂CO₃, and B₂O₃ were mixed and calcined in air at 750°C for 10 hours to obtain the raw aluminate. In the first step, the mixing ratio of Al₂O₃, Li₂CO₃, and B₂O₃ was adjusted so that the constituent elements of the lithium aluminate phase, which is the alkali aluminate phase, were in the elemental ratios shown in Table 1. Furthermore, the raw aluminate was pulverized to achieve an average particle size of 10 μm.

[0245] (Second process)

[0246] The 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 raw silicon to raw aluminate was adjusted so that the elements constituting the master particles were in the element ratios shown in Table 1. The mixture was filled into the jar (SUS, volume: 500 mL) of a planetary ball mill (Fritsch, P-5), and 24 SUS balls (20 mm in diameter) were placed in the jar. The jar was then covered, and the mixture was pulverized at 200 rpm for 50 hours in an inactive atmosphere.

[0247] (Third process)

[0248] The powdered mixture obtained in the second step is taken out in an inactive atmosphere and fired at 700°C for 4 hours while applying a pressure of 200 MPa using a hot press, thereby obtaining a sintered body of the mixture.

[0249] (Fourth process)

[0250] The sintered body obtained in the third step is crushed and passed through a 40μm sieve to obtain a precursor containing silicon phase dispersed in lithium aluminate phase, which is an alkali aluminate phase.

[0251] (Fifth process)

[0252] Coal tar pitch (manufactured by JFE Chemicals, MCP250) was mixed with the precursor of the masterbatch obtained in the fourth step. This mixture was calcined at 800°C for 5 hours in an inert atmosphere, filling the pores inside the precursor of the masterbatch with carbon material through the coal tar pitch, and forming a conductive layer containing conductive carbon material on the surface of the masterbatch. That is, in the fifth step, the amount of coal tar pitch added was adjusted to take into account the formation of the conductive layer and the carbon phase in the masterbatch. As shown in Table 1, the amount of coal tar pitch added was set to 20% by mass relative to the total mass of the masterbatch and coal tar pitch. Then, using a sieve, composite particles with an average particle size of 5 μm and a conductive layer were obtained.

[0253] In the masterbatch, the content of Li, Al, B, and Si relative to the total mass of elements other than oxygen and carbon, as well as the area ratio of the carbon phase in the cross-section of the primary particles, were determined, and the results are shown in Table 1. The content of Li, Al, B, and Si was determined by the feed ratio of the raw materials. It should be noted that for the obtained masterbatch, the content of each element was determined using ICP and Si-NMR, and the results were approximately the same as those obtained by the feed ratio. Table 1 shows the content of each element determined by the feed ratio. The area ratio of the carbon phase in the cross-section of the primary particles of the masterbatch was determined by elemental mapping analysis using EDX under the measurement conditions illustrated above.

[0254] [Making the negative electrode]

[0255] The aforementioned master particles, whose surfaces are covered with a conductive layer, are mixed with graphite at a mass ratio of 5:95 to obtain a mixture. This mixture is used as the negative electrode active material. Water is added to a negative electrode mixture prepared by mixing the negative electrode active material, sodium salt of carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) at a mass ratio of negative electrode active material: sodium salt of CMC: SBR = 97.5:1:1.5, and the mixture is stirred to prepare a negative electrode slurry. Next, the negative electrode slurry is coated on both sides of a copper foil, which serves as the negative electrode current collector. After the coating is dried, it is calendered to form a layer with a density of 1.5 g / cm³ on both sides of the copper foil. 3 The negative electrode of the negative electrode compound layer.

[0256] [The production of the positive electrode]

[0257] A suitable amount of N-methyl-2-pyrrolidone (NMP) was added to a positive electrode mixture prepared by mixing positive electrode active material, acetylene black, and PVDF in a mass ratio of positive electrode active material:acetylene black:PVDF = 95:2.5:2.5, and then stirring to prepare a positive electrode slurry. Lithium cobalt oxide was used as the positive electrode active material. Next, the positive electrode slurry was coated on both sides of an aluminum foil serving as the positive electrode current collector. After the coating was dried, it was calendered to form a slurry with a density of 3.6 g / cm³ on both sides of the aluminum foil. 3 The positive electrode of the positive electrode compound layer.

[0258] [Preparation of non-aqueous electrolytes]

[0259] A non-aqueous electrolyte was prepared by dissolving lithium hexafluorophosphate (LiPF6) at a concentration of 1.0 mol / L in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of EC:DEC=3:7 (25°C).

[0260] [Construction of a non-aqueous electrolyte secondary battery]

[0261] The positive and negative electrodes, with leads attached, are wound together through a separator to create a wound electrode body. The electrode body is then inserted into an outer packaging body made of aluminum laminate film, vacuum dried at 105°C for 2 hours, and then a non-aqueous electrolyte is injected. The opening of the outer packaging body is then sealed to obtain a non-aqueous electrolyte secondary battery.

[0262] <Example 2>

[0263] In the preparation of the masterbatch, the firing temperature in the third step is changed to 800°C, and the amount of coal tar pitch added in the fifth step is changed to 10% by mass relative to the total mass of the masterbatch and coal tar pitch, as shown in Table 1. Otherwise, the masterbatch is prepared in the same manner as in Example 1 to produce a non-aqueous electrolyte secondary battery.

[0264] <Example 3>

[0265] In the preparation of the masterbatch, the firing temperature in the third step is changed to 800°C. Otherwise, the masterbatch is prepared in the same manner as in Example 1 to fabricate a non-aqueous electrolyte secondary battery.

[0266] <Reference Example 1>

[0267] In the preparation of the masterbatch, the amount of coal tar pitch added in the fifth step, as shown in Table 1, was changed to 2% by mass relative to the total mass of the masterbatch and coal tar pitch. Otherwise, the masterbatch was prepared in the same manner as in Example 1 to produce a non-aqueous electrolyte secondary battery.

[0268] <Reference Example 2>

[0269] In the preparation of the masterbatch, the amount of coal tar pitch added in the fifth step was changed to 5% by mass relative to the total mass of the masterbatch and coal tar pitch, as shown in Table 1. Otherwise, the masterbatch was prepared in the same manner as in Example 1 to produce a non-aqueous electrolyte secondary battery.

[0270] <Reference Example 3>

[0271] In the preparation of the masterbatch, the firing temperature in the third step is changed to 800°C, and the amount of coal tar pitch added in the fifth step is changed to 2% by mass relative to the total mass of the masterbatch and coal tar pitch, as shown in Table 1. Otherwise, the masterbatch is prepared in the same manner as in Example 1 to produce a non-aqueous electrolyte secondary battery.

[0272] For the batteries of each embodiment and comparative example, the capacity retention rate was evaluated as an assessment of cycle characteristics using the following method. The evaluation results are shown in Table 1.

[0273] [Capacity maintenance (evaluation of cycling performance)]

[0274] For each battery in the evaluation, perform 200 cycles of the following charge-discharge cycle and calculate the capacity retention rate using the following formula.

[0275] Capacity retention rate (%) = (Discharge capacity at 200th cycle ÷ Discharge capacity at 1st cycle) × 100

[0276] <Charging>

[0277] The battery of the evaluation object was charged at 25°C with a constant current of 1 It (800mA) until the voltage reached 4.2V, and then charged at 4.2V with a constant voltage until the current reached 1 / 20 It (40mA).

[0278] <Discharge>

[0279] After a 10-minute pause, a constant current discharge of 1 It (800 mA) was applied at 25°C until the voltage reached 2.75V.

[0280] [Table 1]

[0281]

[0282] (Inspection)

[0283] As shown in Table 1, the batteries of Examples 1-3, which used master particles with a carbon phase 13 area ratio of 4% to 25% in the cross-section of the primary particles of master particle 10 as negative electrode active materials, exhibited higher capacity retention after 200 cycles compared to the batteries of Reference Examples 1-3, which used master particles with a carbon phase 13 area ratio of less than 4% as negative electrode active materials. In other words, the negative electrode active materials of Examples 1-3 can improve the cycle characteristics of the batteries.

[0284] Industrial availability

[0285] The technology disclosed herein is useful for batteries such as lithium-ion secondary batteries.

Claims

1. A negative electrode active material, comprising master particles, The parent material comprises: Alkali aluminate phase containing at least one alkali metal element and Al, The silicon phase dispersed within the alkali aluminate phase, and The carbon phase dispersed within the alkali aluminate phase, In the cross-section of the primary particles of the mother particle, the area ratio of the carbon phase is more than 4% and less than 25%.

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

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

4. The negative electrode active material according to claim 3, wherein, In the parent particle, the ratio of the mass of Al to the total mass of all elements except oxygen and carbon is mAl. In the parent particle, the mass ratio of element B to the total mass of elements excluding oxygen and carbon is mB. At this point, the ratio mA1 / mB is above 1.0 and below 30.

0.

5. The negative electrode active material according to claim 1, wherein, The alkali metal element includes at least one element 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 parent particle.

7. A battery comprising: A negative electrode comprising the negative electrode active material according to any one of claims 1 to 6, Positive electrode, and Electrolytes.

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