Negative electrode active material and battery

Abstract

A negative electrode active material 10 according to the present disclosure comprises: silicon-containing particles 11; and a coating layer 12 that covers at least a portion of the surface of the silicon-containing particles 11. The coating layer 12 contains a boron-containing compound and a fluorine-containing hydrophobic polymer.

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, typified by lithium-ion batteries, have been widely used as power sources for electronic devices such as mobile terminals and as power sources for vehicles such as electric vehicles. In recent years, the use of silicon (Si)-containing materials has attracted attention as negative electrode active materials with high theoretical capacity densities. Known silicon-containing materials include, for example, a material in which silicon particles are dispersed in a SiO phase and a material in which silicon particles are dispersed in a lithium silicate phase containing Li, Si, and O.

[0003] Patent Document 1 discloses a negative electrode material including a lithiated silicon oxide material, an inorganic coating layer, and a polymer coating layer. In this negative electrode material, the inorganic coating layer includes a compound containing at least one of phosphorus, aluminum, and boron, and the polymer coating layer includes a hydrophobic polymer. The negative electrode material disclosed in Patent Document 1 uses a lithiated silicon oxide material to improve initial charge / discharge efficiency, and further improves durability and hydrogen generation issues through the inorganic coating layer and the polymer coating layer.

[0004] International Publication No. 2021 / 022821

[0005] However, although the coating layer in the negative electrode material disclosed in Patent Document 1 is effective in improving durability and the problem of hydrogen generation, the provision of the coating layer reduces the initial charge / discharge efficiency of the battery. Thus, the negative electrode material disclosed in Patent Document 1 leaves room for improvement in terms of battery characteristics.

[0006] The present disclosure provides a technology that can suppress a decrease in battery characteristics, specifically a decrease in the initial charge / discharge efficiency of a battery, caused by the provision of a coating layer on a negative electrode active material using a silicon-containing material, while suppressing an increase in the internal resistance of the battery.

[0007] The negative electrode active material of the present disclosure includes: silicon-containing particles; and a coating layer covering at least a portion of the surface of the silicon-containing particles, wherein the coating layer includes a boron-containing compound and a fluorine-containing hydrophobic polymer.

[0008] According to the technology of the present disclosure, for a negative electrode active material using a silicon-containing material, it is possible to suppress the deterioration of battery characteristics, specifically the deterioration of the initial charge / discharge efficiency of the battery, caused by the provision of a coating layer, while also suppressing an increase in the internal resistance of the battery.

[0009] Fig. 1 is a cross-sectional view showing a schematic configuration of an example of a negative electrode active material according to embodiment 1. Fig. 2 is a cross-sectional view showing a schematic configuration of an example of the negative electrode active material of Fig. 1 in which the silicon-containing particles are particles of a composite material containing Si. Fig. 3 is a longitudinal cross-sectional view schematically showing an example of a battery according to embodiment 2.

[0010] 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.

[0011] 1 is a cross-sectional view showing a schematic configuration of an example of a negative electrode active material according to Embodiment 1. A negative electrode active material 10 according to Embodiment 1 includes silicon-containing particles 11 and a coating layer 12 that covers at least a portion of the surface of the silicon-containing particles 11. The coating layer 12 includes a boron-containing compound and a fluorine-containing hydrophobic polymer.

[0012] The inclusion of a fluorine-containing hydrophobic polymer in the coating layer 12 facilitates the formation of a favorable fluorine-atom-derived solid electrolyte interphase (SEI), such as LiF, thereby facilitating the insertion and desorption of lithium ions into and from the negative electrode active material 10 and effectively suppressing side reactions between the negative electrode active material 10 and the battery electrolyte. As a result, the coating layer 12 of the negative electrode active material 10 according to embodiment 1 can suppress the deterioration of battery characteristics, specifically the deterioration of the initial charge / discharge efficiency of the battery, due to the presence of the coating layer. Furthermore, the inclusion of a boron-containing compound in the coating layer 12 can suppress an increase in resistance due to the fluorine-containing hydrophobic polymer. Furthermore, the combination of the boron-containing compound with the fluorine-containing hydrophobic polymer improves the retention of the boron-containing compound on the surface of the silicon-containing particles 11, effectively coating the surface of the silicon-containing particles 11 with the boron-containing compound. That is, the coating layer 12 of the negative electrode active material 10 according to embodiment 1 can achieve excellent coating properties of the boron-containing compound. As a result, the negative electrode active material 10 according to the first embodiment can effectively suppress an increase in resistance due to the provision of the coating layer.

[0013] As described above, the negative electrode active material 10 according to the first embodiment includes the coating layer 12 containing a boron-containing compound and a fluorine-containing hydrophobic polymer, and thereby can suppress a decrease in the initial charge / discharge efficiency of the battery due to the provision of the coating layer, and can also suppress an increase in the internal resistance of the battery.

[0014] Each component of the negative electrode active material 10 according to the first embodiment will be specifically described below.

[0015] (Coating Layer) As described above, the coating layer 12 coats at least a portion of the surface of the silicon-containing particle 11 and contains a boron-containing compound and a fluorine-containing hydrophobic polymer. The coating layer 12 may be a layer containing a mixture of the boron-containing compound and the fluorine-containing hydrophobic polymer, or may be a layer consisting of a mixture of the boron-containing compound and the fluorine-containing hydrophobic polymer. In the coating layer 12, the mixture of the boron-containing compound and the fluorine-containing hydrophobic polymer may be a mixture in which the boron-containing compound and the fluorine-containing hydrophobic polymer are mixed three-dimensionally randomly, or may be a mixture mixed to have an overall uniform composition.

[0016] The boron-containing compound includes at least one selected from the group consisting of, for example, H3BO3, Li2BO7, LiBO2, Na2BO7, NaBO2, K2BO7, KBO2, a hydrate of HBO3, a hydrate of LiBO7, a hydrate of LiBO2, a hydrate of Na2BO7, a hydrate of NaBO2, a hydrate of K2BO7, and a hydrate of KBO2. By including the above compound in the coating layer 12 as a boron-containing compound, the negative electrode active material 10 according to the first embodiment can effectively suppress a decrease in the initial charge / discharge efficiency of the battery and suppress an increase in the internal resistance of the battery.

[0017] The boron-containing compound may contain lithium (Li). When the boron-containing compound is a compound containing Li, the negative electrode active material 10 according to the first embodiment can improve the charge-discharge cycle characteristics of the battery.

[0018] The amount of boron-containing compound attached to the surface of silicon-containing particle 11 (hereinafter sometimes referred to as "amount of boron-containing compound supported") may be 1 mass% or more, or 2 mass% or more, relative to the mass of silicon-containing particle 11. The amount of boron-containing compound supported may be 10 mass% or less. For example, the amount of boron-containing compound supported may be 1 mass% or more and 10 mass% or less, 2 mass% or more and 10 mass% or less, 1 mass% or more and 6 mass% or less, or 2 mass% or more and 6 mass% or less.

[0019] When the amount of the boron-containing compound supported is 1% by mass or more, the surface of the silicon-containing particle 11 can be sufficiently coated with the boron-containing compound, and the effect of the boron-containing compound (i.e., the effect of suppressing the increase in resistance due to the fluorine-containing hydrophobic polymer) can be easily obtained. When the amount of the boron-containing compound supported is 6% by mass or less, a low-resistance coating layer 12 and a low-resistance negative electrode active material 10 can be easily obtained. When forming the coating layer 12 containing the boron-containing compound and the fluorine-containing hydrophobic polymer, for example, the amount of the boron-containing compound in the above range is supported on the surface of the silicon-containing particle 11.

[0020] The fluorine-containing hydrophobic polymer may have good binding properties and heat melting properties. In this case, the fluorine-containing hydrophobic polymer can firmly support the boron-containing compound on the surface of the silicon-containing particles 11, making it easier to obtain the effects of the boron-containing compound (e.g., the effect of reducing the resistance increased by the fluorine-containing hydrophobic polymer). The fluorine-containing hydrophobic polymer is almost insoluble in water.

[0021] The fluorine-containing hydrophobic polymer may include at least one selected from the group consisting of polyvinylidene fluoride (PVDF), polytetrafluoroethylene, perfluoroalkoxyalkane, perfluoroethylene propene copolymer, ethylene tetrafluoroethylene copolymer, polychlorotrifluoroethylene, and ethylene chlorotrifluoroethylene copolymer. The fluorine-containing hydrophobic polymer may include PVDF. When the fluorine-containing hydrophobic polymer is PVDF, a decrease in charge / discharge efficiency can be effectively suppressed. Furthermore, PVDF has good binding properties, allowing the boron-containing compound to be firmly supported on the surface of the silicon-containing particles 11, making it easier to obtain the effects of the boron-containing compound being supported on the surface of the silicon-containing particles 11 (e.g., the effect of reducing the resistance increased by the fluorine-containing hydrophobic polymer). Furthermore, when the fluorine-containing hydrophobic polymer is PVDF, PVDF has a low melting point, which also allows the coating layer 12 to be formed at a low heat treatment temperature.

[0022] The fluorine-containing hydrophobic polymer may include a polymer containing vinylidene fluoride units in addition to PVDF. Examples of polymers containing vinylidene fluoride units include copolymers of vinylidene fluoride with other monomers. Examples of other monomers include hexafluoropropylene (HFP) and tetrafluoroethylene (TFE). Polymers containing vinylidene fluoride units include polyvinylidene fluoride and its modified products, vinylidene fluoride-hexafluoropropylene copolymers, and vinylidene fluoride-chlorotrifluoroethylene copolymers. In polymers containing vinylidene fluoride units, the content of vinylidene fluoride units is, for example, 30 mol% or more, and may be 50 mol% or more.

[0023] The amount of fluorine-containing hydrophobic polymer attached to the surface of silicon-containing particle 11 (hereinafter sometimes referred to as "supported amount of fluorine-containing hydrophobic polymer") may be 1 mass% or more, or 2 mass% or more, relative to the mass of silicon-containing particle 11. The supported amount of fluorine-containing hydrophobic polymer may be 10 mass% or less. For example, the supported amount of fluorine-containing hydrophobic polymer may be 1 mass% or more and 10 mass% or less, 2 mass% or more and 10 mass% or less, 1 mass% or more and 6 mass% or less, or 2 mass% or more and 6 mass% or less.

[0024] When the amount of the fluorine-containing hydrophobic polymer supported is 1 mass % or more, the fluorine-containing hydrophobic polymer can sufficiently improve the retention of the boron-containing compound on the surface of the silicon-containing particles 11. When the amount of the fluorine-containing hydrophobic polymer supported is 6 mass % or less, a coating layer 12 with low resistance is likely to be obtained.

[0025] When the coating layer 12 contains, for example, PVDF as the boron-containing compound and the fluorine-containing hydrophobic polymer, the supported amounts of the boron-containing compound and the fluorine-containing hydrophobic polymer can be determined by the following method.

[0026] The negative electrode active material 10 is washed with N-methyl-2-pyrrolidone (NMP) to dissolve the fluorine-containing hydrophobic polymer, and the difference in mass before and after dissolution is determined as the mass of the fluorine-containing hydrophobic polymer. The remainder that is not dissolved in NMP is then washed with water to dissolve the boron-containing compound. The mass of the boron-containing compound dissolved in water is determined by quantitative analysis such as inductively coupled plasma atomic emission spectroscopy (ICP).

[0027] The remaining mass that is not dissolved in water and NMP is determined as the mass of silicon-containing particles 11. If silicon-containing particles 11 have a conductive layer on their surfaces, the remaining mass that is not dissolved in water and NMP is subjected to quantitative carbon analysis using a carbon-sulfur analyzer or the like. The amount of carbon determined is derived from the carbon material of the conductive layer. The value obtained by subtracting the mass of carbon determined by analysis from the remaining mass that is not dissolved in water and NMP is determined as the mass of silicon-containing particles 11.

[0028] The amount of the fluorine-containing hydrophobic polymer supported is calculated by the formula: (mass of the fluorine-containing hydrophobic polymer / mass of the silicon-containing particles) × 100 using the mass of the fluorine-containing hydrophobic polymer and the mass of the silicon-containing particles 11 calculated above. The amount of the boron-containing compound supported is calculated by the formula: (mass of the boron-containing compound / mass of the silicon-containing particles) × 100 using the mass of the boron-containing compound and the mass of the silicon-containing particles 11 calculated above.

[0029] The mass of the fluorine-containing hydrophobic polymer attached to the surface of the silicon-containing particle may be 0.5 times or more and 2 times or less than the mass of the boron-containing compound attached to the surface of the silicon-containing particle. That is, the amount of the fluorine-containing hydrophobic polymer supported may be 0.5 times or more and 2 times or less than the amount of the boron-containing compound supported. When the amount of the fluorine-containing hydrophobic polymer supported and the amount of the boron-containing compound supported satisfy this quantitative relationship, the effect of improving the retention of the boron-containing compound on the surface of the silicon-containing particle 11 by the fluorine-containing hydrophobic polymer can be sufficiently obtained. Therefore, by further improving the coating property of the boron-containing compound, the initial charge / discharge efficiency can be improved while suppressing the increase in the internal resistance of the battery.

[0030] The thickness of the coating layer 12 is desirably thin enough not to substantially affect the average particle size of the negative electrode active material. From the viewpoint of protecting the silicon-containing particles 11 from the electrolyte, the thickness of the coating layer 12 may be 1 nm or more. From the viewpoint of suppressing an increase in resistance, the thickness of the coating layer 12 may be 300 nm or less. When the silicon-containing particles 11 have a conductive layer, the thickness of the coating layer 12 may be smaller than the thickness of the conductive layer. The thickness of the coating layer 12 can be measured by observing a cross section of the negative electrode active material with a scanning electron microscope (SEM) or a transmission electron microscope (TEM).

[0031] (Silicon-containing particles) In this specification, the silicon-containing particles 11 refer to particles of a material containing Si. Examples of the material containing Si include Si, a Si alloy, a Si compound, and a composite material containing Si.

[0032] The silicon-containing particles 11 may be, for example, particles of a composite material containing Si. The negative electrode active material 10 including the composite material containing Si can improve the initial charge / discharge efficiency of the battery.

[0033] 2 is a cross-sectional view showing a schematic configuration of an example of the anode active material 10 of FIG. 1 in which the silicon-containing particles 11 are particles of a composite material containing Si. In this example of the anode active material 20, the silicon-containing particles 11 include, for example, an ion-conducting phase 13 and a silicon phase 14 dispersed within the ion-conducting phase 13. Hereinafter, silicon-containing particles 11 having such a configuration are referred to as composite particles 11. The stress associated with the expansion and contraction of the silicon phase 14 during charge and discharge is alleviated by the ion-conducting phase 13, thereby suppressing cracking and fracture of the composite particles 11. As a result, it is possible to achieve both high capacity due to the inclusion of silicon and improved charge and discharge cycle characteristics.

[0034] The average particle size of the composite particles 11 may be 1 μm or more and 25 μm or less, or 4 μm or more and 15 μm or less. This configuration facilitates alleviating stress caused by volumetric changes in the composite particles 11 during charging and discharging, making it easier to obtain good charge-discharge cycle characteristics. Furthermore, since the surface area of ​​the composite particles 11 is appropriately sized, capacity reduction due to side reactions with the battery electrolyte is suppressed.

[0035] The average particle size of the composite particles 11 refers to the particle size (volume average particle size) at which the volume cumulative value is 50% in the particle size distribution measured by a laser diffraction scattering method. For example, an "LA-750" manufactured by Horiba Ltd. can be used as the measuring device.

[0036] The composite particles 11 can be removed from the battery by, for example, the following method, which is just an example.

[0037] 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 negative electrode core current collector (e.g., copper foil), and the mixture layer is crushed in a mortar to obtain a sample powder. Next, the sample powder is dried in a dry atmosphere for 1 hour and, for example, immersed in gently boiled 6 M hydrochloric acid for about 10 minutes to remove elements derived from materials other than the composite particles. Next, the sample powder is washed with ion-exchanged water, filtered, and dried for 1 hour at 200 °C, for example. 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).

[0038] The ion-conducting phase 13 may be composed of one phase or multiple phases.

[0039] The ion-conducting phase 13 includes, for example, at least one selected from the group consisting of a lithium-containing phase and a carbon phase. The lithium-containing phase and the carbon phase have good ion conductivity. Therefore, when the ion-conducting phase 13 includes at least one selected from the group consisting of a lithium-containing phase and a carbon phase, the silicon phase 14 more smoothly absorbs and releases ions such as Li ions through the ion-conducting phase 13. This improves battery characteristics such as the initial charge / discharge efficiency and charge / discharge cycle characteristics of the battery.

[0040] Because the carbon phase is conductive, even if voids are formed around the composite particles 11, contact between the composite particles 11 and their surroundings is likely to be maintained. This results in improved battery characteristics, such as the initial charge / discharge efficiency and charge / discharge cycle characteristics of the battery. The carbon phase may be composed of amorphous carbon (i.e., amorphous carbon). The amorphous carbon may be hard carbon, soft carbon, or other. Amorphous carbon generally refers to a carbon material in which the average interplanar spacing d002 of the (002) plane measured by X-ray diffraction exceeds 0.34 nm.

[0041] The lithium-containing phase may include at least one selected from the group consisting of a lithium aluminate phase and a lithium silicate phase. The lithium aluminate phase and the lithium silicate phase have better ionic conductivity. Therefore, when the ion-conducting phase 13 includes at least one selected from the group consisting of a lithium aluminate phase and a lithium silicate phase, the silicon phase 14 more smoothly absorbs and releases ions such as Li ions through the ion-conducting phase 13. This improves battery characteristics such as the initial charge / discharge efficiency and charge / discharge cycle characteristics of the battery. Furthermore, when the ion-conducting phase 13 includes at least one selected from the group consisting of a lithium aluminate phase and a lithium silicate phase, the ion-conducting phase 13 reduces the effects of expansion and contraction of the silicon phase 14.

[0042] Lithium silicate is a silicate containing lithium (Li), silicon (Si), and oxygen (O). Lithium silicate may further contain other elements M1, such as magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), aluminum (Al), boron (B), or rare earth elements (e.g., lanthanum (La)). The atomic ratio O / Si of O to Si in the lithium silicate is, for example, greater than 2 and less than 4. When the atomic ratio O / Si is greater than 2 and less than 4 (where z in the formula described below is 0<z<2), this is advantageous in terms of the stability of the lithium silicate phase and lithium ion conductivity. Preferably, the atomic ratio O / Si is greater than 2 and less than 3. The atomic ratio Li / Si of Li to Si in the lithium silicate is, for example, greater than 0 and less than 4. In this specification, the stability of the lithium aluminate phase includes both the chemical stability (alkali resistance) and the thermal stability of the lithium aluminate phase.

[0043] The composition of lithium silicate is represented by the formula: Li 2z SiO 2+z (0<z<2). From the viewpoints of stability, ease of preparation, lithium ion conductivity, etc., z preferably satisfies the relationship 0<z<1, and more preferably z=1 / 2. Lithium silicate that satisfies z=1 / 2 can be expressed as Li2Si2O5.

[0044] Lithium aluminate is an aluminate containing lithium (Li), aluminum (Al), and oxygen (O). The atomic ratio of O to Al in lithium aluminate, O / Al, is, for example, 1.6 or more and 4 or less. The atomic ratio of Li to Al in lithium aluminate, Li / 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 lithium aluminate phase are improved. In this specification, the stability of the lithium aluminate phase includes both the chemical stability (alkali resistance) and thermal stability of the lithium aluminate phase.

[0045] The lithium aluminate phase may further contain an element M2 in addition to Li, Al, and O. An example of the element M2 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).

[0046] The element M2 may form a compound. The compound may be, for example, an oxide of the element M2 or an aluminate of the element M2 depending on the type of the element M2.

[0047] The element M2 may be B. That is, the lithium aluminate phase may further contain B. The addition of the element M2, for example, improves the stability and ionic conductivity of the lithium aluminate phase. Furthermore, the addition of the element M2 can reduce the porosity of the composite particles 11. This reduces the number of areas that can become the starting points for cracks and fractures when charging and discharging is repeated, thereby suppressing deterioration of the negative electrode active material and further improving the charge and discharge cycle characteristics of the battery.

[0048] The lithium aluminate phase may further contain trace amounts of elements such as chromium (Cr), nickel (Ni), manganese (Mn), copper (Cu), and molybdenum (Mo).

[0049] The lithium aluminate phase may be amorphous, in which case the influence of the expansion and contraction of the silicon phase 14 can be more effectively alleviated.

[0050] A highly crystalline, fine Al2O3 phase may be dispersed within the lithium aluminate phase. The Al2O3 phase is, for example, distributed in an island-like pattern within the lithium aluminate phase matrix. In this case, expansion and cracking of the lithium aluminate phase due to expansion and contraction of the silicon phase 14 are more likely to be suppressed, enhancing the effect of improving charge-discharge 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 composite particle obtained by X-ray diffraction measurement. The Al2O3 content in the composite particle 11 is, for example, 10 mass% or less.

[0051] The silicon phase 14 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 14. The silicon phase 14 has a large capacity. Furthermore, the silicon phase 14 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 14 is dispersed within the ion-conducting phase 13, and therefore the stress caused by the expansion and contraction of the silicon phase 14 is alleviated by the ion-conducting phase 13.

[0052] The silicon phase 14 may be particulate. The silicon phase 14 is particulate, for example, at least before the first charge. The average particle size of the silicon phase 14 may be 1 nm or more and 1000 nm or less. The average particle size of the silicon phase 14 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 14 may be 400 nm or less, or 100 nm or less. By dispersing the fine silicon phase 14 in the ion conductive phase 13 as described above, the volume change of the composite particles during charge and discharge is reduced, and the structural stability of the negative electrode active material is further improved.

[0053] The average particle size of the silicon phases 14 can be measured using an SEM image obtained by SEM observation of a cross section of the negative electrode active material 10 in which the cross sections of the silicon phases 14 are exposed. Specifically, the average particle size of the silicon phases 14 is determined by averaging the maximum diameters of 100 silicon phases 14 arbitrarily selected from the cross-sectional SEM image of the negative electrode active material 10.

[0054] The silicon phase 14 may contain crystalline silicon. The silicon phase 14 is composed of, for example, a plurality of crystallites. The crystallite size of the silicon phase 14 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 14 during charge and discharge can be reduced, and the effect of improving the charge and discharge cycle characteristics becomes more significant. The crystallite size of the silicon phase 14 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.

[0055] The lower limit of the crystallite size of the silicon phase 14 is not particularly limited, but is, for example, 1 nm. An example of a suitable crystallite size of the silicon phase 14 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 14 is 1 nm or more, for example, the surface area of ​​the silicon phase 14 can be kept small, making it difficult for deterioration of the silicon phase 14, 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 14 uniform, and stress generated in the negative electrode active material is effectively alleviated.

[0056] When the ion conductive phase 13 contains a carbon phase, from the viewpoint of increasing capacity and improving charge / discharge cycle characteristics, the content of the silicon 14 phase in the composite particle 11 may be 30% by mass or more and 80% by mass or less, or 40% by mass or more and 70% by mass or less.

[0057] When the ion-conducting phase 13 contains a lithium silicate phase, from the viewpoint of increasing capacity and improving charge-discharge cycle characteristics, the content of the silicon 14 phase in the composite particle 11 may be 30% by mass or more and 90% by mass or less, or 30% by mass or more and 75% by mass or less.

[0058] When the ion conductive phase 13 includes a lithium aluminate phase, from the viewpoint of increasing capacity, the content of the silicon phase 14 in the composite particle 11 may be 30% by mass or more, 35% by mass or more, or 55% by mass or more. From the viewpoint of improving charge / discharge cycle characteristics, the content of the silicon phase 14 in the composite particle 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 14 exposed on the surface of the composite particle 11 without being covered by the lithium aluminate phase is reduced, and side reactions between the electrolyte and the silicon phase 14 are also suppressed. The content of the silicon phase 14 in the composite particle 11 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.

[0059] The content of the silicon phase 14 in the composite particle 11 can be determined by quantifying the amount of Si that constitutes the silicon phase 14 in the composite particle 11 using Si-NMR, as will be described later.

[0060] Composite particle 11 may further include a conductive layer (not shown) that coats at least a portion of the surface of a particle composed of ion-conducting phase 13 and silicon phase 14. This conductive layer is located between the particle composed of ion-conducting phase 13 and silicon phase 14 and coating layer 12. Hereinafter, the particle composed of ion-conducting phase 13 and silicon phase 14 will be referred to as the "mother particle." In other words, the conductive layer coats at least a portion of the surface of the mother particle. This configuration improves the conductivity of composite particle 11.

[0061] The conductive layer may cover the entire surface of the base particle. The conductive layer is, for example, a thin film layer containing a conductive material. When the composite particle 11 includes the conductive layer, the conductivity is improved.

[0062] The conductive material may be a conductive carbon material. That is, the conductive layer 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. This makes it easy to form a thin conductive layer that covers the surface of the base particles. Examples of amorphous carbon include carbon black, burned pitch, coke, and activated carbon.

[0063] The thickness of the conductive layer is desirably thin enough not to affect the average particle size of the composite particles 11. The thickness of the conductive layer 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 can be measured by SEM or TEM observation of a cross section of the negative electrode active material in which the cross section of the composite particles 11 is exposed.

[0064] The porosity of the composite particle 11 before the initial charge / discharge may be 25% or less. Since the conductive layer does not affect the porosity, the porosity of the composite particle 11 and the porosity of the base particle are substantially the same. By keeping the porosity of the composite particle 11 at 25% or less, the number of areas that can become the starting point for cracks and breaks when charging and discharging are repeated is reduced. This suppresses deterioration of the negative electrode active material, further improving the charge / discharge cycle characteristics of the battery. The porosity of the composite particle 11 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%.

[0065] The porosity of the composite particle 11 means the proportion of voids in the cross section of the composite particle 11. The porosity of the composite particle 11 can be measured using an SEM image of a cross section of the negative electrode active material in which the cross section of the composite particle 11 is exposed. The porosity of the composite particle 11 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 sections.

[0066] The porosity of the composite particles 11 can be controlled to some extent by adjusting, for example, the firing temperature in the manufacturing process of the composite particles 11 and the compressive force applied to the particles during firing.

[0067] The composite particle 11 may have a Vickers hardness of 300 HV or more. Since the conductive layer does not affect the Vickers hardness, the Vickers hardness of the composite particle 11 is substantially the same as that of the mother particle. When the composite particle 11 has a high Vickers hardness, the volume change of the silicon phase 14 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 charge and discharge cycle characteristics becomes more pronounced. The Vickers hardness of the composite particle 11 is more preferably 350 HV or more, and may be 400 HV or more, or 500 HV or more.

[0068] The Vickers hardness of the composite particles 11 can be measured using a Vickers hardness tester. Specifically, the composite particles 11, from which the coating layer 12 has been removed from the negative electrode active material, are embedded in a thermosetting resin and polished with #400 abrasive paper to expose the cross section of the composite particles 11. 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 composite particles 11 is not particularly limited, but is, for example, 1500 HV.

[0069] The content of each element in the composite particle 11 is measured by the following method. The oxygen content can be measured using an oxygen, nitrogen, and hydrogen analyzer. The Si content can be measured using NMR. The carbon amount can be measured using a carbon and sulfur analyzer. The contents of other elements can be measured by inductively coupled plasma optical emission spectroscopy. When measurement by inductively coupled plasma optical emission spectroscopy (ICP) is not possible, measurement can also be performed using energy dispersive X-ray (EDX). When measurement by neither ICP nor EDX is possible, measurement can also be performed by Auger electron spectroscopy (AES). The composition of the ion conductive phase 13 can be determined from the content of each element. When measurement is performed from the state of the battery, for example, measurement can be performed by disassembling a fully discharged battery, removing and cleaning the negative electrode, removing the non-aqueous electrolyte component, drying, and then analyzing the cross section of the resulting negative electrode mixture layer using a cross-section polisher.

[0070] <Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP)> A sample of the composite particles 11 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 removed by filtration. 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 composite particles 11 is calculated.

[0071] <Energy Dispersive X-ray (EDX)> When measurement cannot be performed using ICP, measurement is performed using EDX. Elemental mapping analysis is performed using EDX from a backscattered electron image of a cross section of the negative electrode active material in which the cross section of the composite particle 11 is exposed. The area of ​​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 composite particles 11 with a maximum particle diameter of 5 μm or more are randomly selected from the cross-sectional 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 using EDX. The measured values ​​of the area of ​​the specified element contained in the 10 particles are averaged to calculate the content of the target element.

[0072] 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

[0073] <Auger Electron Spectroscopy (AES)> When measurement cannot be performed by either ICP or EDX, measurement is performed by AES. Qualitative and quantitative analysis of elements is performed using an AES analyzer (e.g., JAMP-9510F, manufactured by JEOL Ltd.) from a backscattered electron image of a cross section of the negative electrode active material in which the cross section of the composite particle 11 is exposed. 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 composite particles 11 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 qualitative and quantitative analysis of elements is performed on each of them using an AES analyzer. The content of a predetermined element contained in 10 particles is calculated by averaging the contents.

[0074] During charging and discharging, a coating may be formed on the surface of the composite particle 11 due to decomposition of the non-aqueous electrolyte, etc. As described above, the composite particle may further include a conductive layer. Therefore, the EDX analysis and AES analysis are performed on a range, for example, 1 μm inward from the peripheral edge of the cross section of the composite particle so as not to include the thin coating and conductive layer in the measurement range.

[0075] <Inert Gas Fusion - Non-Dispersive Infrared Absorption Method> The oxygen content of the composite particles 11 can be measured using an oxygen, nitrogen, and hydrogen analyzer (for example, EGMA-830 manufactured by Horiba, Ltd.). A sample is placed in a Ni capsule, and then placed in a carbon crucible heated at 5.75 kW with Sn pellets and Ni pellets 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.

[0076] <Nuclear Magnetic Resonance Spectroscopy (NMR)> The amount of Si constituting the silicon phase 14 can be quantified using Si-NMR. Desired measurement conditions for Si-NMR are shown below. Measurement apparatus: 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

[0077] <High-frequency induction heating furnace combustion-infrared absorption method> The carbon content of the composite particles 11 can be measured using a carbon / sulfur analyzer (for example, EMIA-520 model manufactured by Horiba, Ltd.). A sample is weighed out and placed on 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.

[0078] (Method for Producing Negative Electrode Active Material) The negative electrode active material of the present disclosure is produced, for example, by a production method including step I of obtaining silicon-containing particles 11 and step II of forming coating layer 12 on the surface of silicon-containing particles 11.

[0079] [Step I] When the silicon-containing particles 11 are the composite particles 11 described above, the composite particles 11 are prepared, for example, as follows.

[0080] When the ion-conducting phase 13 is a carbon phase, the composite particles 11 can be obtained, for example, by pulverizing a mixture of a carbon source and raw silicon while stirring it in a ball mill or the like to form fine particles, then heat-treating the mixture in an inert atmosphere and pulverizing the sintered product obtained by the heat treatment. Examples of the carbon source that can be used include sugars such as carboxymethyl cellulose (CMC) and water-soluble resins such as polyvinylpyrrolidone.

[0081] When the ion-conducting phase 13 is a lithium silicate phase, the composite particle 11 can be produced, for example, by the following steps (iA) and (iiA).

[0082] Step (iA) (Step of Obtaining Lithium Silicate) A raw material mixture containing a Si raw material and a Li raw material in a predetermined ratio is used as the raw material for lithium silicate. The raw material mixture may contain another element M1, if necessary. A mixture of predetermined amounts of the raw materials is melted, and the molten liquid is passed through a metal roll to form flakes, producing lithium silicate. The flaked silicate is then crystallized by heat treatment in an air atmosphere at a temperature above the glass transition point and below the melting point. The flaked silicate can also be used without crystallization. Alternatively, a predetermined amount of the mixture can be fired at a temperature below the melting point without being melted, to produce silicate by a solid-phase reaction.

[0083] Silicon oxide can be used as the Si raw material. For example, lithium carbonate, lithium oxide, lithium hydroxide, lithium hydride, etc. can be used as the Li raw material. These may be used alone or in combination of two or more. Examples of raw materials for element M1 include oxides, hydroxides, carbonate compounds, hydrides, nitrates, sulfates, etc. of each element. Si raw material that has not reacted with the Li raw material may remain in the lithium silicate. The remaining Si raw material is dispersed in the lithium silicate as fine crystals of silicon oxide.

[0084] Step (iiA) (Step of Obtaining Composite Particles Containing a Lithium Silicate Phase) Next, raw material silicon is blended with lithium silicate to form composite particles. For example, the composite particles are produced through the following steps (iiA-a) to (iiA-c).

[0085] Step (iiA-a) First, raw silicon powder and lithium silicate powder are mixed in a mass ratio of, for example, 20:80 to 95:5. The raw silicon powder may be coarse silicon particles having an average particle size of several μm to several tens of μm.

[0086] Step (iiA-b): Next, using a grinding device such as a ball mill, the mixture of raw silicon and lithium silicate is ground and composited while being finely divided. At this time, an organic solvent may be added to the mixture and wet-ground. A predetermined amount of organic solvent may be added to the grinding vessel all at once at the beginning of grinding, or a predetermined amount of organic solvent may be added to the grinding vessel intermittently in multiple batches during the grinding process. The organic solvent serves to prevent the material to be ground from adhering to the inner wall of the grinding vessel.

[0087] As the organic solvent, alcohols, ethers, fatty acids, alkanes, cycloalkanes, silicate esters, metal alkoxides, etc. can be used.

[0088] The raw silicon may be coarse silicon particles having an average particle size of several μm to several tens of μm. The silicon particles finally obtained are preferably controlled so that the crystallite size calculated from the half-width of the diffraction peak attributable to the Si(111) plane in the X-ray diffraction pattern using the Scherrer equation is 5 nm or more and 50 nm or less.

[0089] The raw silicon and lithium silicate may be separately microparticulated and then mixed. Alternatively, silicon nanoparticles and amorphous lithium silicate nanoparticles may be prepared without using a pulverizer and then mixed. The nanoparticles may be prepared by a known method such as a gas phase method (e.g., a plasma method) or a liquid phase method (e.g., a liquid phase reduction method).

[0090] Step (iiA-c): Next, the pulverized material is fired under pressure using a hot press or the like to obtain a sintered body. The firing is carried out, for example, in an inert atmosphere (e.g., an argon, nitrogen, or other atmosphere). The firing temperature is preferably 450°C or higher and 1000°C or lower. Within this temperature range, fine silicon particles are easily dispersed within the silicate phase with low crystallinity. During sintering, the lithium silicate softens and flows to fill the gaps between the silicon particles. As a result, a dense block-shaped sintered body can be obtained, with the silicate phase forming a sea portion and the silicon particles forming islands. The firing temperature is preferably 550°C or higher and 900°C or lower, more preferably 650°C or higher and 850°C or lower. The firing time is, for example, 1 hour or higher and 10 hours or lower.

[0091] The obtained sintered body is pulverized to obtain composite particles 11 containing a lithium silicate phase. By appropriately selecting the pulverization conditions, composite particles 11 having a predetermined average particle size can be obtained. By steps (iA) and (iiA), composite particles 11 having a lithium silicate phase as a matrix and a silicon phase dispersed in the matrix are obtained.

[0092] When the ion-conducting phase 13 is a lithium aluminate phase, the composite particles 11 can be produced, for example, by the following steps (iB) and (iiB).

[0093] Step (iB) (Step of Obtaining Lithium Aluminate) The lithium aluminate production process includes, for example, a step of mixing an Al raw material, a Li raw material, and, if necessary, a compound containing element M2 to obtain a mixture, and a step of calcining the mixture to obtain lithium aluminate. The calcination is performed, 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.

[0094] Examples of Al raw materials include aluminum oxide, aluminum hydroxide, aluminum carbonate, etc. The Al raw materials may be used alone or in combination of two or more.

[0095] Examples of the Li raw material include lithium carbonate, lithium oxide, lithium hydroxide, lithium hydride, etc. The Li raw material may be used alone or in combination of two or more.

[0096] Examples of raw materials for element M2 include oxides, hydroxides, carbonates, hydrides, nitrates, sulfates, etc. One type of raw material for element M2 may be used alone, or two or more types may be used in combination.

[0097] During the lithium aluminate production process, the Al raw material that did not react with the Li raw material may remain in the lithium aluminate. When the amount of Al raw material used is large relative to the Li raw material, Al compounds are likely to remain. When the Al compound remaining in the lithium aluminate is Al2O3, an Al2O3 phase dispersed within the lithium aluminate phase may be formed in the finally obtained composite particles.

[0098] Step (iiB) (Step of Obtaining Composite Particles Containing a Lithium Aluminate Phase) Next, raw material silicon is blended with lithium aluminate to form composite particles. For example, the composite particles are produced through the following steps (iiB-a) to (iiB-c).

[0099] Step (iiB-a) First, raw silicon powder and lithium aluminate powder are mixed in a mass ratio of, for example, 20:80 to 95:5. The raw silicon powder may be coarse silicon particles having an average particle size of several μm to several tens of μm.

[0100] Step (iiB-b): Next, using a grinding device such as a ball mill, the mixture of raw silicon and lithium aluminate is ground and composited while being finely divided. At this time, an organic solvent may be added to the mixture and wet-ground. A predetermined amount of organic solvent may be added to the grinding vessel all at once at the beginning of grinding, or a predetermined amount of organic solvent may be added to the grinding vessel intermittently in multiple batches during the grinding process. The organic solvent serves to prevent the material to be ground from adhering to the inner wall of the grinding vessel.

[0101] As the organic solvent, alcohols, ethers, fatty acids, alkanes, cycloalkanes, silicate esters, metal alkoxides, etc. can be used.

[0102] The raw silicon may be coarse silicon particles having an average particle size of several μm to several tens of μm. The silicon particles finally obtained are preferably controlled so that the crystallite size calculated from the half-width of the diffraction peak attributable to the Si(111) plane in the X-ray diffraction pattern using the Scherrer equation is 5 nm or more and 50 nm or less.

[0103] Alternatively, the raw silicon and lithium aluminate may be separately microparticulated and then mixed. Alternatively, silicon nanoparticles and amorphous lithium silicate nanoparticles may be prepared and then mixed without using a pulverizer. The nanoparticles may be prepared by known methods such as a gas-phase method (e.g., a plasma method) or a liquid-phase method (e.g., a liquid-phase reduction method).

[0104] Step (iiB-c): Next, the pulverized material is fired under pressure using a hot press or the like to obtain a sintered body. The firing is carried out, for example, in an inert atmosphere (e.g., an argon, nitrogen, or other atmosphere). The firing temperature is preferably 450°C or higher and 1000°C or lower. Within this temperature range, fine silicon particles are easily dispersed within the silicate phase with low crystallinity. During sintering, the lithium aluminate softens and flows to fill the gaps between the silicon particles. As a result, a dense block-shaped sintered body can be obtained, with the lithium aluminate phase forming a sea portion and silicon particles forming islands. The firing temperature is preferably 550°C or higher and 950°C or lower, more preferably 650°C or higher and 900°C or lower. The firing time is, for example, 1 hour or higher and 10 hours or lower.

[0105] The obtained sintered body is pulverized to obtain composite particles 11 containing a lithium aluminate phase. By appropriately selecting the pulverization conditions, composite particles 11 having a predetermined average particle size can be obtained. By steps (iB) and (iiB), composite particles 11 having a lithium aluminate phase as a matrix and a silicon phase dispersed in the matrix are obtained.

[0106] When the composite particles 11 have the conductive layer described above, the conductive material constituting the conductive layer is preferably a conductive carbon material, as described above. Examples of methods for coating the surfaces of the base particles with a carbon material include a CVD method using a hydrocarbon gas such as acetylene or methane as a raw material, and a method in which coal pitch, petroleum pitch, phenolic resin, or the like is mixed with the base particles and heated to carbonize them. Carbon black may also be attached to the surfaces of the base particles. For example, a mixture of the base particles and the carbon material is heated at 700°C or higher and 950°C or lower in an inert atmosphere (e.g., an argon, nitrogen, or other atmosphere) to form a conductive layer 4 on the surfaces of the base particles. In this manner, composite particles 11 having a conductive layer on the surfaces of the base particles are obtained.

[0107] [Step II] Step II is a step of forming a coating layer 12 on the surface of the composite particle 11 .

[0108] For example, after granulating the composite particles 11, the boron-containing compound, and the fluorine-containing hydrophobic polymer, the granulated powder is crushed and classified to a desired size, thereby obtaining a negative electrode active material having a desired average particle size in which the coating layer 12 is formed on the surface of the composite particles 11.

[0109] The granulation is carried out, for example, by dispersing the composite particles 11, the powder of the boron-containing compound, and the fluorine-containing hydrophobic polymer in water, and then spray-drying the dispersion, thereby forming a coating layer 12, which is a mixed layer of the boron-containing compound and the fluorine-containing hydrophobic polymer, on the surface of the composite particles 11.

[0110] The amount of the boron-containing compound added may be 1% by mass or more and 6% by mass or less with respect to the composite particle 11. The amount of the fluorine-containing hydrophobic polymer added may be 1% by mass or more and 6% by mass or less with respect to the composite particle 11.

[0111] The particle sizes of the boron-containing compound and the fluorine-containing hydrophobic polymer dispersed in water are preferably smaller than the particle size of the composite particles 11. In this case, the surfaces of the composite particles 11 are easily covered uniformly with the boron-containing compound and the hydrophobic polymer. The average particle size of the boron-containing compound may be 1 μm or more and 100 μm or less, or 1 μm or more and 10 μm or less. The average particle size of the fluorine-containing hydrophobic polymer may be 200 nm or more and 1 μm or less.

[0112] The method for forming the coating layer 12 on the surface of the composite particle 11 is not limited to the above. For example, the composite particle, a powder of a boron-containing compound, and a powder of a fluorine-containing hydrophobic polymer may be dry-mixed to obtain a mixture, and then the mixture may be heat-treated to form the coating layer 12 on the surface of the composite particle 11.

[0113] The mixing process produces an intermediate (mixture) in which the mixture of the boron-containing compound and the fluorine-containing hydrophobic polymer adheres to the surface of the composite particles 11. A ball mill method can be used for the dry mixing process. The amounts and particle sizes of the boron-containing compound and the fluorine-containing hydrophobic polymer added may be the same as those described above.

[0114] The heat treatment is preferably carried out at a temperature above the melting point of the fluorine-containing hydrophobic polymer and below the decomposition temperature of the fluorine-containing hydrophobic polymer. When PVDF is used as the fluorine-containing hydrophobic polymer, the heat treatment temperature may be above the melting point (150°C to 170°C) of PVDF and below the decomposition temperature (340°C), preferably, for example, 200°C to 250°C. The heat treatment may be carried out in an inert atmosphere. The heat treatment time is, for example, about 1 hour to 3 hours. The heat treatment liquefies the fluorine-containing hydrophobic polymer in the mixture, and penetrates and diffuses around the composite particles 11 and the particles of the boron-containing compound, filling the gaps between the composite particles 11 and the particles of the boron-containing compound, as well as the gaps between the particles of the boron-containing compound. This improves the retention of the boron-containing compound on the surface of the composite particles 11. In this way, a coating layer 12, which is a mixed layer of the boron-containing compound and the fluorine-containing hydrophobic polymer, is formed. By crushing the mixture after the heat treatment, a negative electrode active material having a desired average particle size can be obtained.

[0115] (Embodiment 2) A battery according to Embodiment 2 includes a positive electrode, a negative electrode, and an electrolyte. The negative electrode contains the negative electrode active material according to Embodiment 1. With this configuration, the battery according to Embodiment 2 can suppress a decrease in initial charge / discharge efficiency and a rise in internal resistance.

[0116] 3 is a longitudinal cross-sectional view schematically illustrating an example of a battery according to embodiment 2. The battery 100 is a cylindrical battery including a cylindrical battery case, a wound electrode group 24, and an electrolyte (not shown). The electrode group 24 is housed in the battery case and is in contact with the electrolyte.

[0117] The battery case is composed of a case body 25, which is a cylindrical metal container with a bottom, and a sealing body 26 that seals the opening of the case body 25. A gasket 37 is disposed between the case body 25 and the sealing body 26. The gasket 37 ensures the airtightness of the battery case. Within the case body 25, insulating plates 27 and 28 are disposed on both ends of the electrode group 24 in the direction of the winding axis of the electrode group 24, respectively.

[0118] Case body 25 has, for example, a step portion 31. Step portion 31 can be formed by partially pressing the side wall of case body 25 from the outside. Step portion 31 may be formed in an annular shape on the side wall of case body 25 along the circumferential direction of an imaginary circle defined by case body 25. In this case, sealing body 26 is supported by, for example, the surface of step portion 31 on the opening side.

[0119] Sealing body 26 includes a filter 32, a lower valve body 33, an insulating member 34, an upper valve body 35, and a cap 36. These components are stacked in this order in sealing body 26. Sealing body 26 is attached to the opening of case body 25 so that cap 36 is located on the outside of case body 25 and filter 32 is located on the inside of case body 25.

[0120] Each of the above-mentioned members constituting the sealing body 26 has, for example, a disk or ring shape. The above-mentioned members, except for the insulating member 34, are electrically connected to one another.

[0121] The electrode group 24 has a positive electrode 21, a separator 22, and a negative electrode 23. The positive electrode 21, the separator 22, and the negative electrode 23 are all strip-shaped. The width direction of the strip-shaped positive electrode 21 and the negative electrode 23 is, for example, parallel to the winding axis of the electrode group 24. The separator 22 is disposed between the positive electrode 21 and the negative electrode 23. The positive electrode 21 and the negative electrode 23 are spirally wound with the separator 22 interposed between these electrodes.

[0122] When observing the cross section of the battery 100 in a direction perpendicular to the winding axis of the electrode group 24, the positive electrodes 21 and negative electrodes 23 are stacked alternately in the radial direction of an imaginary circle defined by the case body 25, with a separator 22 interposed between these electrodes.

[0123] The positive electrode 21 is electrically connected to a cap 36, which also serves as a positive electrode terminal, via a positive electrode lead 29. One end of the positive electrode lead 29 is connected, for example, to near the center of the positive electrode 21 in the longitudinal direction of the positive electrode 21. The positive electrode lead 29 passes through a through-hole formed in the insulating plate 27 and extends from the positive electrode 21 to the filter 32. The other end of the positive electrode lead 29 is welded, for example, to the surface of the filter 32 on the electrode group 24 side.

[0124] The negative electrode 23 is electrically connected to the case body 25, which also serves as a negative electrode terminal, via a negative electrode lead 30. One end of the negative electrode lead 30 is connected to, for example, an end of the negative electrode 23 in the longitudinal direction of the negative electrode 23. The other end of the negative electrode lead 30 is welded to, for example, the inner bottom surface of the case body 25.

[0125] Each component of the battery 100 will be specifically described below.

[0126] The positive electrode 21 includes a material having the property of absorbing and releasing metal ions (e.g., lithium ions). The positive electrode 21 includes, for example, a positive electrode active material. The positive electrode 21 includes, for example, a positive electrode current collector and a positive electrode mixture layer supported on the surface of the positive electrode current collector.

[0127] The positive electrode current collector can be, for example, a sheet or film made of a metal material such as aluminum, stainless steel, titanium, or an alloy thereof. Aluminum and its alloys are suitable as materials for the positive electrode current collector because they are inexpensive and easy to form into thin films. The sheet or film may be porous or non-porous. Metal foil, metal mesh, or the like may be used as the sheet or film. A carbon material such as carbon may be applied to the surface of the positive electrode current collector as a conductive auxiliary material.

[0128] The positive electrode mixture layer includes a positive electrode active material. The positive electrode active material can be a material capable of absorbing and releasing metal ions (e.g., lithium ions). 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 battery manufacturing costs and increase 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.

[0129] The positive electrode mixture layer may further contain a binder. Examples of the binder include fluorine-containing resins such as PVDF, polyacrylonitrile (PAN), polyimide, acrylic resin, polyolefin, and SBR. These resins may also be used in combination with carboxymethyl cellulose (CMC) or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), or the like.

[0130] The positive electrode mixture layer may further contain a conductive additive. Examples of the conductive additive include a carbon material and a conductive polymer compound. Examples of the carbon material include carbon black, graphite, acetylene black, carbon nanotubes, carbon nanofibers, graphene, fullerene, and graphite oxide. Examples of the conductive polymer compound include polyaniline, polypyrrole, and polythiophene.

[0131] The negative electrode 23 includes a material having the property of absorbing and releasing metal ions (e.g., lithium ions). The negative electrode 23 is a negative electrode including the negative electrode active material 10 according to embodiment 1. The negative electrode 23 includes, for example, a negative electrode current collector and a negative electrode mixture layer supported on the surface of the negative electrode current collector.

[0132] 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.

[0133] The negative electrode mixture layer contains the negative electrode active material 10 according to embodiment 1. The negative electrode mixture layer may contain other materials such as a binder and a conductive additive, as necessary. As the binder and conductive additive, the materials described above for the positive electrode mixture layer can also be used for the negative electrode mixture layer.

[0134] The electrolyte solution used as 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 solution 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 solution having excellent ionic conductivity and appropriate viscosity can be obtained. However, the lithium salt concentration is not limited to the above.

[0135] 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.

[0136] 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.

[0137] It is usually desirable to interpose a separator between the positive electrode and the negative electrode. The separator 22 has high ion permeability and adequate mechanical strength and insulating properties. The separator 22 can be made of a microporous thin film, a woven fabric, a nonwoven fabric, or the like. The separator 22 can be made of a polymer, for example. The polymer can be a polyolefin such as polypropylene or polyethylene.

[0138] In the battery according to the second embodiment, the electrolyte may be impregnated into a polymer provided as a separator, for example, i.e., the battery according to the second embodiment may have a structure in which the electrolyte and the polymer are used in combination.

[0139] The battery according to the second embodiment may further include a solid electrolyte as the electrolyte. That is, the battery according to 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 the present disclosure, the term "halide solid electrolyte" refers to a solid electrolyte containing a halogen element as the main component of the anions. The term "sulfide solid electrolyte" refers to a solid electrolyte containing sulfur as the main component of the anions. The term "oxide solid electrolyte" refers to a solid electrolyte containing oxygen as the main component of the anions. The term "main component of the anions" refers to the anion with the largest mass among all the anions constituting the solid electrolyte.

[0140] As an example of the structure of the battery according to the second 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 second embodiment may have any shape, such as a prismatic shape, a coin shape, a button shape, or a laminate shape. Furthermore, instead of the wound electrode group in the battery according to the second embodiment, 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.

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

[0142] (Technology 1) A negative electrode active material comprising: silicon-containing particles; and a coating layer covering at least a portion of a surface of the silicon-containing particles, wherein the coating layer includes a boron-containing compound and a fluorine-containing hydrophobic polymer.

[0143] With this configuration, the negative electrode active material according to Technology 1 can suppress a decrease in battery characteristics due to the provision of a coating layer, specifically a decrease in the initial charge / discharge efficiency of the battery, while suppressing an increase in the internal resistance of the battery.

[0144] (Technology 2) The negative electrode active material according to Technology 1, wherein the boron-containing compound includes at least one selected from the group consisting of H3BO3, Li2BO7, LiBO2, Na2BO7, NaBO2, KBO7, KBO2, a hydrate of H3BO3, a hydrate of LiBO7, a hydrate of LiBO2, a hydrate of Na2BO7, a hydrate of NaBO2, a hydrate of K2BO7, and a hydrate of KBO2.

[0145] With this configuration, the negative electrode active material according to Technology 2 can more effectively suppress a decrease in the initial charge / discharge efficiency of the battery, and also suppress an increase in the internal resistance of the battery.

[0146] (Technology 3) The negative electrode active material according to Technology 1 or 2, wherein the boron-containing compound contains lithium.

[0147] With this configuration, the negative electrode active material according to Technology 3 can improve the charge-discharge cycle characteristics of the battery.

[0148] (Technology 4) The negative electrode active material according to any one of Technologies 1 to 3, wherein the fluorine-containing hydrophobic polymer includes at least one selected from the group consisting of polyvinylidene fluoride, polytetrafluoroethylene, perfluoroalkoxyalkane, perfluoroethylene propene copolymer, ethylene tetrafluoroethylene copolymer, polychlorotrifluoroethylene, and ethylene chlorotrifluoroethylene copolymer.

[0149] With this configuration, the negative electrode active material according to Technology 4 can more effectively suppress a decrease in the initial charge / discharge efficiency of the battery, and also suppress an increase in the internal resistance of the battery.

[0150] (Technology 5) The negative electrode active material according to Technology 4, wherein the fluorine-containing hydrophobic polymer contains polyvinylidene fluoride.

[0151] With this configuration, the negative electrode active material according to Technology 5 can more effectively suppress a decrease in the initial charge / discharge efficiency of the battery, and also suppress an increase in the internal resistance of the battery.

[0152] (Technology 6) The negative electrode active material according to any one of Technologies 1 to 5, wherein the silicon-containing particles include an ion-conducting phase and a silicon phase dispersed within the ion-conducting phase.

[0153] With this configuration, the negative electrode active material according to Technology 6 can achieve both a high capacity due to the inclusion of silicon and improved charge-discharge cycle characteristics.

[0154] (Technology 7) The negative electrode active material according to Technology 6, wherein the ion-conducting phase includes at least one selected from the group consisting of a lithium-containing phase and a carbon phase.

[0155] With this configuration, the negative electrode active material according to Technology 7 can improve battery characteristics such as the initial charge / discharge efficiency and charge / discharge cycle characteristics of the battery.

[0156] (Technology 8) The negative electrode active material according to Technology 7, wherein the lithium-containing phase includes at least one selected from the group consisting of a lithium aluminate phase and a lithium silicate phase.

[0157] With this configuration, the negative electrode active material according to Technology 8 can improve battery characteristics such as the initial charge / discharge efficiency and charge / discharge cycle characteristics of the battery.

[0158] (Technology 9) The negative electrode active material according to any one of Technologies 1 to 8, wherein the amount of the boron-containing compound attached to the surface of the silicon-containing particle is 1 mass % or more with respect to the mass of the silicon-containing particle.

[0159] With this configuration, the negative electrode active material according to Technology No. 9 can more effectively suppress a decrease in the initial charge / discharge efficiency of the battery, and also suppress an increase in the internal resistance of the battery.

[0160] (Technology 10) The negative electrode active material according to any one of Technologies 1 to 9, wherein the amount of the fluorine-containing hydrophobic polymer attached to the surface of the silicon-containing particle is 1 mass % or more with respect to the mass of the silicon-containing particle.

[0161] With this configuration, the negative electrode active material according to Technique 10 can more effectively suppress a decrease in the initial charge / discharge efficiency of the battery, and also suppress an increase in the internal resistance of the battery.

[0162] (Technology 11) The negative electrode active material according to any one of Technologies 1 to 10, wherein a mass of the fluorine-containing hydrophobic polymer attached to the surface of the silicon-containing particle is 0.5 to 2 times a mass of the boron-containing compound attached to the surface of the silicon-containing particle.

[0163] With this configuration, the negative electrode active material according to Technology 11 can more effectively suppress a decrease in the initial charge / discharge efficiency of the battery, and also suppress an increase in the internal resistance of the battery.

[0164] (Technology 12) A battery comprising: a negative electrode containing the negative electrode active material according to any one of technologies 1 to 11; a positive electrode; and an electrolyte.

[0165] With this configuration, the battery according to Technique 12 can suppress a decrease in initial charge / discharge efficiency and also suppress an increase in internal resistance.

[0166] 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.

[0167] <Preparation of Negative Electrode Active Material> [Example 1] (Preparation of Composite Particles) Coal pitch (MCP250, manufactured by JFE Chemical Corporation) as a carbon source and raw material silicon (3N, average particle size 10 μm) were mixed in a mass ratio of 50:50. The mixture was charged into a pot (SUS, volume: 500 mL) of a planetary ball mill (P-5, manufactured by Fritsch), 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 to obtain a composite of a silicon phase and a carbon source.

[0168] Next, the composite of the silicon phase and the carbon source was fired in an inert gas atmosphere to carbonize the carbon source, and a sintered product was obtained in which the silicon phase was dispersed in the carbon phase containing amorphous carbon. The sintered product was then pulverized using a jet mill to obtain Si-C composite particles with an average particle size of 10 μm.

[0169] The crystallite size of the silicon phase was 15 nm. The silicon phase was particulate, with an average particle size of 20 nm. The content of the silicon phase in the Si—C composite particles was 60 mass %.

[0170] (Formation of coating layer) Composite particles, H3BO3 as a boron-containing compound, and PVDF as a fluorine-containing hydrophobic polymer were dispersed in water. The H3BO3 and PVDF were mixed so that the amounts were 2% by mass and 2% by mass, respectively, relative to the composite particles. The dispersion was then spray-dried to obtain a granulated powder. The spray-drying device used was a "Mini Spray Dryer S-300" manufactured by Nippon Buchi Co., Ltd. The obtained granulated powder was classified to form a coating layer on the surface of the Si-C composite particles.

[0171] In this manner, the particles of Example 1 were prepared.

[0172] Example 2 (Preparation of Composite Particles) Si—C composite particles were prepared in the same manner as in Example 1.

[0173] (Formation of Coating Layer) A coating layer was formed on the surface of the Si-C composite particles in the same manner as in Example 1, except that Li2B4O7 was used as the boron-containing compound.

[0174] Example 3 (Preparation of Composite Particles) Si—C composite particles were prepared in the same manner as in Example 1.

[0175] (Formation of Coating Layer) A coating layer was formed on the surface of the Si-C composite particles in the same manner as in Example 1, except that LiBO2 was used as the boron-containing compound.

[0176] Example 4 (Preparation of Composite Particles) AlO, LiCO, and BO were mixed in a mass ratio of AlO:LiCO:BO = 55:25:20. The mixture was calcined in air at 750°C for 10 hours. The calcined product was pulverized to obtain raw aluminate with an average particle size of 10 µm.

[0177] Raw aluminate (average particle size 10 μm) and raw silicon (3N, average particle size 10 μm) were mixed at a mass ratio of raw aluminate:raw silicon = 40:60. The resulting mixture was filled into a pot (SUS, volume: 500 mL) of a planetary ball mill (Fritsch, P-5). Next, 24 SUS balls (diameter 20 mm) were placed in the pot, the lid was closed, and the mixture was milled at 200 rpm in an inert atmosphere for 50 hours. This resulted in a powdery mixture.

[0178] The powder mixture obtained in the inert atmosphere was taken out and sintered at 800°C for 4 hours in an inert atmosphere while applying a pressure of 200 MPa using a hot press, thereby obtaining a sintered body.

[0179] The obtained sintered body was pulverized and passed through a 40 μm mesh to obtain base particles (LAX particles) in which a silicon phase was dispersed in an alkali aluminate phase containing predetermined amounts of Li and B.

[0180] The obtained base particles were mixed with coal pitch (MCP250, manufactured by JFE Chemical Corporation). The amount of coal pitch mixed was 5 parts by mass per 100 parts by mass of the base particles. The mixture was fired at 800°C for 5 hours in an inert atmosphere, forming a conductive layer on the surface of the base particles. Then, using a sieve, composite particles with an average particle size of 5 μm, which were LAX composite particles with a conductive layer, were obtained.

[0181] (Formation of Coating Layer) A coating layer was formed on the surface of the LAX composite particles in the same manner as in Example 1.

[0182] Example 5 (Preparation of Composite Particles) LAX composite particles were prepared in the same manner as in Example 4.

[0183] (Formation of Coating Layer) A coating layer was formed on the surface of the LAX composite particles in the same manner as in Example 2.

[0184] Example 6 (Preparation of Composite Particles) LAX composite particles were prepared in the same manner as in Example 4.

[0185] (Formation of Coating Layer) A coating layer was formed on the surface of the LAX composite particles in the same manner as in Example 3.

[0186] Comparative Example 1 (Preparation of Composite Particles) Si—C composite particles were prepared in the same manner as in Example 1.

[0187] (Formation of Coating Layer) A coating layer was not formed, that is, the Si—C composite particles were used as they were as the negative electrode active material.

[0188] Comparative Example 2 (Preparation of Composite Particles) Si—C composite particles were prepared in the same manner as in Example 1.

[0189] (Formation of Coating Layer) A coating layer was formed on the surface of the Si-C composite particles in the same manner as in Example 1, except that PVDF was not added and H3BO3 was mixed at 5 mass % with respect to the composite particles.

[0190] Comparative Example 3 (Preparation of Composite Particles) Si—C composite particles were prepared in the same manner as in Example 1.

[0191] (Formation of Coating Layer) A coating layer was formed on the surface of the Si-C composite particles in the same manner as in Example 1, except that PVDF was not added and Li2B4O7 was mixed in an amount of 5 mass % relative to the composite particles.

[0192] Comparative Example 4 (Preparation of Composite Particles) Si—C composite particles were prepared in the same manner as in Example 1.

[0193] (Formation of Coating Layer) A coating layer was formed on the surface of the Si-C composite particles in the same manner as in Example 1, except that PVDF was not added and LiBO2 was mixed in an amount of 5 mass % relative to the composite particles.

[0194] Comparative Example 5 (Preparation of Composite Particles) LAX composite particles were prepared in the same manner as in Example 4.

[0195] (Formation of Coating Layer) No coating layer was formed, i.e., the LAX composite particles were used as they were as the negative electrode active material.

[0196] Comparative Example 6 (Preparation of Composite Particles) LAX composite particles were prepared in the same manner as in Example 4.

[0197] (Formation of Coating Layer) A coating layer was formed on the surface of the LAX composite particles in the same manner as in Example 4, except that PVDF was not added and H3BO3 was mixed at 5 mass % with respect to the composite particles.

[0198] Comparative Example 7 (Preparation of Composite Particles) LAX composite particles were prepared in the same manner as in Example 4.

[0199] (Formation of Coating Layer) A coating layer was formed on the surface of the LAX composite particles in the same manner as in Example 4, except that PVDF was not added and Li2B4O7 was mixed in an amount of 5 mass % relative to the composite particles.

[0200] Comparative Example 8 (Preparation of Composite Particles) LAX composite particles were prepared in the same manner as in Example 4.

[0201] (Formation of Coating Layer) A coating layer was formed on the surface of the LAX composite particles in the same manner as in Example 4, except that PVDF was not added and LiBO2 was mixed in an amount of 5 mass % relative to the composite particles.

[0202] <Fabrication of Batteries> Using the negative electrode active materials of Examples 1 to 6 and Comparative Examples 1 to 8, batteries of Examples 1 to 6 and Comparative Examples 1 to 8 were fabricated as follows.

[0203] [Fabrication of Negative Electrode] The negative electrode active material prepared in the Examples or Comparative Examples was mixed with graphite in a mass ratio of negative electrode active material:graphite = 6:94 to obtain a mixture. This mixture was used as the negative electrode active material of the negative electrode (hereinafter referred to as the negative electrode active material mixture). This negative electrode active material mixture, carboxymethyl cellulose (CMC), and styrene butadiene rubber (SBR) were mixed in a mass ratio of negative electrode active material mixture: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 serving as a negative electrode current collector, and the coating was dried and then rolled. The negative electrode was cut to a predetermined size to produce a negative electrode with a negative electrode mixture layer formed on both sides of the negative electrode current collector. At this time, a negative electrode current collector exposed portion was provided in a portion of the negative electrode.

[0204] [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, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added to the positive electrode mixture, followed by stirring to prepare a positive electrode slurry. 0.88 Co 0.09 Al 0.03 A lithium transition metal composite oxide represented by O2 was used. Next, the positive electrode slurry was applied to both sides of an aluminum foil serving as a positive electrode current collector, and the coating was dried and then rolled. The resulting product was cut to a predetermined size to produce a positive electrode in which a positive electrode mixture layer was formed on both sides of the positive electrode current collector. At this time, a positive electrode current collector exposed portion was provided on a part of the positive electrode.

[0205] [Preparation of Non-Aqueous Electrolyte] A non-aqueous electrolyte was prepared by dissolving lithium hexafluorophosphate (LiPF) at a concentration of 1 mol / L in a mixed solvent prepared by mixing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of EC:EMC:DMC=20:5:75.

[0206] [Fabrication of Non-Aqueous Electrolyte Secondary Battery] An aluminum positive electrode lead was attached to the exposed portion of the positive electrode current collector of the positive electrode, and a nickel negative electrode lead was attached to the exposed portion of the negative electrode current collector of the negative electrode. The positive electrode and negative electrode were spirally wound with a polyolefin separator interposed therebetween and then pressed radially to produce a flat wound electrode assembly. This electrode assembly was housed in an exterior body made of an aluminum laminate sheet, and after injecting a non-aqueous electrolyte, the opening of the exterior body was sealed to obtain a battery.

[0207] <Battery Evaluation> The batteries of Examples 1 to 6 and Comparative Examples 1 to 8 were evaluated for initial charge / discharge efficiency, internal resistance of the battery, and charge / discharge cycle characteristics by the following methods.

[0208] (Initial charge / discharge efficiency) The batteries of Examples 1 to 6 and Comparative Examples 1 to 8 were subjected to constant current charging at a current of 0.3 C at an ambient temperature of 25° C. until the voltage reached 4.2 V, and then constant voltage charging at a voltage of 4.2 V until the current reached 0.02 C. Thereafter, constant current discharging was performed at a current of 0.5 C until the voltage reached 2.5 V. The initial charge / discharge efficiency of the batteries of Examples 1 to 6 and Comparative Examples 1 to 8 was determined by this charge / discharge.

[0209] The results are shown in Table 1. In Table 1, for Examples 1 to 3 and Comparative Examples 1 to 4, the relative values ​​are shown when the charge / discharge efficiency of Comparative Example 1 is taken as the reference (100%). Furthermore, for Examples 4 to 6 and Comparative Examples 5 to 8, the relative values ​​are shown when the charge / discharge efficiency of Comparative Example 5 is taken as the reference (100%).

[0210] (Charge-Discharge Cycle Characteristics) The batteries of Examples 1 to 6 and Comparative Examples 1 to 8 were subjected to constant-current charging at a current of 0.3 C at an ambient temperature of 25° C. until the voltage reached 4.2 V, and then constant-voltage charging at a voltage of 4.2 V until the current reached 0.02 C. Thereafter, constant-current discharging was performed at a current of 0.5 C until the voltage reached 2.5 V. The above charge-discharge cycle was counted as one cycle, and 350 cycles were performed, and the discharge capacity retention rate was calculated using the following formula: Discharge capacity retention rate = (discharge capacity at 350th cycle / discharge capacity at 1st cycle) × 100

[0211] The obtained capacity retention rates were defined as charge-discharge cycle characteristics. The results are shown in Table 1. In Table 1, for Examples 1 to 3 and Comparative Examples 1 to 4, relative values ​​are shown when the capacity retention rate of Comparative Example 1 is set as the reference (100%). Furthermore, for Examples 4 to 6 and Comparative Examples 5 to 8, relative values ​​are shown when the capacity retention rate of Comparative Example 5 is set as the reference (100%).

[0212] (Measurement of Internal Resistance) The internal resistance of the batteries of Examples 1 to 6 and Comparative Examples 1 to 8 was measured. Initial charge / discharge was performed under the following conditions. (Charge / Discharge Conditions) Initial charge / discharge conditions Constant current charging was performed at a current of 0.5 It (625 mA) until the battery voltage reached 4.2 V. Furthermore, constant voltage charging was performed at a voltage of 4.2 V until the current value reached 0.02 It (25 mA). Then, constant current discharging was performed at a current of 0.5 It (625 mA) until the battery voltage reached 2.5 V.

[0213] Next, charging and discharging were carried out under the following conditions, and then the initial internal resistance of the battery was measured by the following method. The results are shown in Table 1.

[0214] (Charge / Discharge Conditions) At a temperature of 25° C., constant current charging was performed at a current of 0.3 It (375 mA) until the battery voltage reached 3.79 V. Further, constant voltage charging was performed at a constant voltage of 3.79 V until the current value reached 0.02 It (25 mA). After a 2-hour pause, the battery was discharged at a current of 0.2 It (250 mA) for 10 seconds.

[0215] After charging the test cells of each example and comparative example under the above charging conditions, AC impedance measurements were performed in the range of 10 mHz to 100 kHz, and a Cole-Cole plot was created. The internal resistance of the battery was calculated from the size of the approximate semicircle that appeared in the resulting Cole-Cole plot.

[0216] The value obtained by the above method was taken as the internal resistance. The results are shown in Table 1. In Table 1, for Examples 1 to 3 and Comparative Examples 1 to 4, the relative values ​​are shown when the internal resistance of Comparative Example 1 is taken as the reference (100%). Furthermore, for Examples 4 to 6 and Comparative Examples 5 to 8, the relative values ​​are shown when the internal resistance of Comparative Example 5 is taken as the reference (100%).

[0217]

[0218] (Discussion) As shown in Table 1, the batteries of the Examples, in which the negative electrode active material had a coating layer containing both a boron-containing compound and a fluorine-containing hydrophobic polymer, were able to suppress a decrease in the initial charge / discharge efficiency of the battery and also suppress an increase in the internal resistance of the battery. On the other hand, the batteries of Comparative Examples 2 to 4 and 6 to 8, in which the coating layer contained only the boron-containing compound, had initial charge / discharge efficiencies comparable to those of the Examples, but had higher internal resistances than the batteries of the Examples.

[0219] The technology of the present disclosure is useful for batteries such as lithium ion secondary batteries.

Claims

1. A negative electrode active material comprising: silicon-containing particles; and a coating layer covering at least a portion of the surface of the silicon-containing particles, wherein the coating layer includes a boron-containing compound and a fluorine-containing hydrophobic polymer.

2. The negative electrode active material according to claim 1, wherein the boron-containing compound comprises at least one selected from the group consisting of H3BO3, Li2BO7, LiBO2, Na2BO7, NaBO2, KBO7, KBO2, a hydrate of HBO3, a hydrate of LiBO7, a hydrate of LiBO2, a hydrate of NaBO7, a hydrate of NaBO2, a hydrate of KBO7, and a hydrate of KBO2.

3. The negative electrode active material according to claim 1, wherein the boron-containing compound contains lithium.

4. The negative electrode active material according to claim 1, wherein the fluorine-containing hydrophobic polymer comprises at least one selected from the group consisting of polyvinylidene fluoride, polytetrafluoroethylene, perfluoroalkoxyalkane, perfluoroethylene propene copolymer, ethylene tetrafluoroethylene copolymer, polychlorotrifluoroethylene, and ethylene chlorotrifluoroethylene copolymer.

5. The negative electrode active material according to claim 4, wherein the fluorine-containing hydrophobic polymer includes polyvinylidene fluoride.

6. The negative electrode active material according to claim 1, wherein the silicon-containing particles include an ion-conducting phase and a silicon phase dispersed within the ion-conducting phase.

7. The negative electrode active material according to claim 6, wherein the ion-conducting phase includes at least one selected from the group consisting of a lithium-containing phase and a carbon phase.

8. The negative electrode active material according to claim 7, wherein the lithium-containing phase includes at least one selected from the group consisting of a lithium aluminate phase and a lithium silicate phase.

9. The negative electrode active material according to claim 1, wherein the amount of the boron-containing compound attached to the surface of the silicon-containing particle is 1 mass % or more relative to the mass of the silicon-containing particle.

10. The negative electrode active material according to claim 1, wherein the amount of the fluorine-containing hydrophobic polymer attached to the surface of the silicon-containing particle is 1 mass % or more relative to the mass of the silicon-containing particle.

11. The negative electrode active material according to claim 1, wherein the mass of the fluorine-containing hydrophobic polymer attached to the surface of the silicon-containing particle is 0.5 to 2 times the mass of the boron-containing compound attached to the surface of the silicon-containing particle.

12. A battery comprising: a negative electrode containing the negative electrode active material according to any one of claims 1 to 11; a positive electrode; and an electrolyte.

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