Negative electrode active material, negative electrode, and battery

Abstract

A negative electrode active material 10 according to the present disclosure is provided with: silicon-containing particles 11; a resin 12 that adheres to at least a part of a surface of the silicon-containing particles 11 and is insoluble in water; and a low-hardness material 13 that adheres to the surface of the silicon-containing particles 11 via the resin 12. The low-hardness material 13 is at least one selected from the group consisting of a mineral with a Mohs hardness of 1 and an inorganic compound with a Mohs hardness of 1.

Description

Anode active material, anode, and battery

[0001] The present disclosure relates to a negative electrode active material, a negative electrode, 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 density.

[0003] However, silicon-containing negative electrode active materials experience a large volume increase upon charging, and negative electrodes containing silicon-containing negative electrode active materials swell upon charging. Conventionally, techniques for suppressing electrode swelling due to repeated charge / discharge cycles have been proposed. For example, Patent Document 1 discloses a composition for a non-aqueous secondary battery functional layer capable of forming an electrode mixture layer, the composition including a specific block copolymer functioning as a binder, non-conductive inorganic particles, a solvent, and electrode active material particles. An electrode mixture layer formed from this composition suppresses swelling due to repeated charge / discharge cycles of the secondary battery and also enhances the charge carrier acceptance of the secondary battery at low temperatures.

[0004] International Publication No. 2019 / 194194

[0005] However, as described above, silicon-containing negative electrode active materials undergo a large volume increase upon charging, and therefore, there is room for improvement in the prior art in terms of effectively reducing swelling of negative electrodes when silicon-containing materials are used as negative electrode active materials.

[0006] The present disclosure provides a technique that can reduce swelling of a negative electrode due to charging when using a silicon-containing material as a negative electrode active material.

[0007] The negative electrode active material of the present disclosure comprises: silicon-containing particles; a water-insoluble resin attached to at least a portion of the surface of the silicon-containing particles; and a low-hardness material attached to the surface of the silicon-containing particles via the resin, wherein the low-hardness material is at least one selected from the group consisting of minerals and inorganic compounds having a Mohs hardness of 1.

[0008] According to the technology of the present disclosure, it is possible to reduce swelling of the negative electrode due to charging when a negative electrode active material using a material containing silicon is used.

[0009] Fig. 1 is a cross-sectional view schematically showing the general configuration of an example of a negative electrode active material according to embodiment 1. Fig. 2 is a cross-sectional view schematically showing the general configuration of a modified example of the negative electrode active material according to embodiment 1. Fig. 3 is a cross-sectional view schematically showing the general configuration of an example in which the silicon-containing particles are particles of a composite material containing Si. Fig. 4 is a cross-sectional view schematically showing the general configuration of an example of a negative electrode according to embodiment 2. Fig. 5 is a longitudinal cross-sectional view schematically showing an example of a battery according to embodiment 3.

[0010] [Findings that Form the Basis of the Present Disclosure] The present inventors investigated the structure of a negative electrode that swells during charging in order to reduce swelling of a negative electrode that uses a silicon-containing negative electrode active material. This investigation confirmed that the porosity of a negative electrode after swelling is higher than that of a negative electrode before swelling. That is, the present inventors found that the voids present in the negative electrode before swelling, i.e., before charging, are not effectively used as space to absorb the swelling of the negative electrode active material, which results in increased swelling during charging.

[0011] Therefore, the present inventors conducted further intensive research and found that by imparting slipperiness to the surface of a silicon-containing material, allowing the silicon-containing material to slide during swelling, the voids that exist before swelling can be effectively used as spaces that absorb the expansion of the negative electrode active material. This led the present inventors to conceive of the negative electrode active material of the present disclosure, which is described below, and which can reduce swelling of the negative electrode.

[0012] 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.

[0013] [Embodiments of the Present Disclosure] (Embodiment 1) Fig. 1 is a cross-sectional view schematically illustrating the general 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, a resin 12 attached to at least a portion of the surface of the silicon-containing particles, and a low-hardness material 13 attached to the surface of the silicon-containing particles 11 via the resin 12. The resin 12 is a water-insoluble resin. The low-hardness material 13 is at least one selected from the group consisting of minerals having a Mohs hardness of 1 and inorganic compounds having a Mohs hardness of 1.

[0014] In this specification, a water-insoluble resin is a resin whose solubility in water is 0.1 g / L or less at room temperature (e.g., 25° C.) For example, when a resin-containing object is stirred in water at room temperature and the supernatant is analyzed, if the resin content in the water is 0.1 g / L or less, the resin is considered to be a water-insoluble resin.

[0015] The negative electrode active material 10 includes a low-hardness material 13 attached to the surface of the silicon-containing particles 11. This configuration provides slipperiness to the surface of the silicon-containing particles 11 due to the low-hardness material 13. Therefore, even if the negative electrode active material 10 expands due to an increase in the volume of the silicon-containing particles 11 during charging, the negative electrode active material 10 can slide into, for example, voids that existed in the negative electrode prior to the expansion of the negative electrode active material 10. In this way, the negative electrode active material 10 is believed to be able to effectively use the voids that existed prior to the expansion as spaces that absorb the expansion of the negative electrode active material 10. Therefore, the negative electrode active material 10 can suppress a significant increase in the thickness of the negative electrode due to the expansion of the negative electrode active material 10 during charging, i.e., can reduce the expansion of the negative electrode due to charging.

[0016] Furthermore, because low hardness material 13 is firmly attached to the surface of silicon-containing particle 11 by resin 12, low hardness material 13 is unlikely to fall off from the surface of silicon-containing particle 11. Therefore, even if negative electrode active material 10 is mixed with other materials during preparation of the negative electrode mixture, low hardness material 13 is supported on the surface of silicon-containing particle 11, and the above-described action and effect of low hardness material 13 can be obtained.

[0017] The low-hardness material 13 is preferably located, for example, on the outermost surface of the negative electrode active material 10. This configuration improves the slipperiness of the surface of the negative electrode active material 10, making it easier for the negative electrode active material 10 to move in the negative electrode. Therefore, the negative electrode active material 10 can reduce swelling of the negative electrode due to charging.

[0018] In a modified example of the negative electrode active material 10, the silicon-containing particles 11 may have a conductive layer on at least a portion of their surface. FIG. 2 is a cross-sectional view schematically illustrating a schematic configuration of a modified example of the negative electrode active material according to embodiment 1. As shown in FIG. 2, in the modified negative electrode active material 20, the silicon-containing particles 11 have a conductive layer 11a on at least a portion of their surface in the configuration of the negative electrode active material 10. In other words, in the negative electrode active material 20, the silicon-containing particles 11 are formed of base particles 11b formed of a silicon-containing material and a conductive layer 11a covering at least a portion of the surface of the base particles 11b. This configuration improves the conductivity of the silicon-containing particles 11. In the negative electrode active material 20, the resin 12 is attached to, for example, the conductive layer 11a of the silicon-containing particles 11.

[0019] 2, the conductive layer 11a may be provided on the entire surface of the silicon-containing particle 11. With this configuration, the conductivity of the silicon-containing particle 11 is further improved.

[0020] The conductive layer 11a is, for example, a thin film layer containing a conductive material.

[0021] The conductive material may be a conductive carbon material. That is, the conductive layer 11a may contain carbon. Examples of the carbon material that can be used include graphite such as natural graphite, artificial graphite, and graphitized mesophase carbon, amorphous carbon, soft carbon, and hard carbon. The carbon material may be amorphous carbon. As described above, the conductive layer 11a can be easily formed on the surface of the silicon-containing particle 11. Examples of amorphous carbon include carbon black, burned pitch, coke, and activated carbon.

[0022] The thickness of conductive layer 11a may be 1 nm or more and 200 nm or less, or 5 nm or more and 100 nm or less, in consideration of ensuring conductivity and the diffusibility of ions such as Li ions. The thickness of conductive layer 11a can be measured by observing a cross section of negative electrode active material 20 in which the cross sections of silicon-containing particles 11 are exposed using a scanning electron microscope (SEM) or a transmission electron microscope (TEM).

[0023] The following describes in detail each component of the negative electrode active material according to embodiment 1. Unless otherwise specified, the following description applies to both the negative electrode active material 10 shown in Fig. 1 and the modified negative electrode active material 20 shown in Fig. 2.

[0024] (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.

[0025] The silicon-containing particles 11 may be, for example, particles of a composite material containing Si.

[0026] 3 is a cross-sectional view showing a schematic configuration of an example in which silicon-containing particles 11 are particles of a composite material containing Si. In this example of anode active material 30, silicon-containing particles 11 include, for example, an ion-conducting phase 14 and a silicon phase 15 dispersed within ion-conducting phase 14. Hereinafter, silicon-containing particles 11 having such a configuration are referred to as composite particles 11. Stress associated with the expansion and contraction of silicon phase 15 during charge and discharge is alleviated by ion-conducting phase 14, thereby suppressing cracking and fracture of composite particles 11. Therefore, it is possible to achieve both high capacity due to the inclusion of silicon and improved charge and discharge cycle characteristics.

[0027] 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 volume changes in the composite particles 11 during charging and discharging, making it easier to obtain good charge-discharge cycle characteristics. Furthermore, by ensuring that the surface area of ​​the composite particles 11 is of an appropriate size, capacity reduction due to side reactions with the battery electrolyte is suppressed. Even when the silicon-containing particles 11 are not composite particles but are, for example, Si particles, the average particle size of the silicon-containing particles 11 may be 1 μm or more and 25 μm or less, or 4 μm or more and 15 μm or less.

[0028] The average particle size of the composite particles 11 (silicon-containing 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.

[0029] The composite particles 11 can be removed from the battery by, for example, the following method, which is just an example.

[0030] 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).

[0031] The ion conducting phase 14 may be composed of one phase or multiple phases.

[0032] The ion-conducting phase 14 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 14 includes at least one selected from the group consisting of a lithium-containing phase and a carbon phase, the silicon phase 15 more smoothly absorbs and releases ions such as Li ions through the ion-conducting phase 14. This improves battery characteristics such as the initial charge / discharge efficiency and charge / discharge cycle characteristics of the battery.

[0033] 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.

[0034] 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 14 includes at least one selected from the group consisting of a lithium aluminate phase and a lithium silicate phase, the silicon phase 15 more smoothly absorbs and releases ions such as Li ions through the ion-conducting phase 14. 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 14 includes at least one selected from the group consisting of a lithium aluminate phase and a lithium silicate phase, the ion-conducting phase 14 reduces the effects of expansion and contraction of the silicon phase 15.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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).

[0039] 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.

[0040] 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.

[0041] The lithium aluminate phase may further contain trace amounts of elements such as chromium (Cr), nickel (Ni), manganese (Mn), copper (Cu), and molybdenum (Mo).

[0042] The lithium aluminate phase may be amorphous, in which case the influence of expansion and contraction of the silicon phase 15 can be more effectively alleviated.

[0043] 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 15 are easily 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.

[0044] The silicon phase 15 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 15. The silicon phase 15 has a large capacity. Furthermore, the silicon phase 15 also undergoes a large degree of expansion and contraction as the battery is charged and discharged. However, in the negative electrode active material of the present disclosure, the silicon phase 15 is dispersed within the ion conductive phase 14, and therefore the stress caused by the expansion and contraction of the silicon phase 15 is alleviated by the ion conductive phase 14.

[0045] The silicon phase 15 may be particulate. The silicon phase 15 is particulate, for example, at least before the first charge. The average particle size of the silicon phase 15 may be 1 nm or more and 1000 nm or less. The average particle size of the silicon phase 15 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 15 may be 400 nm or less, or 100 nm or less. By dispersing the fine silicon phase 15 in the ion conductive phase 14 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.

[0046] The average particle size of the silicon phases 15 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 15 are exposed. Specifically, the average particle size of the silicon phases 15 is determined by averaging the maximum diameters of 100 silicon phases 15 arbitrarily selected from the cross-sectional SEM image of the negative electrode active material 10.

[0047] The silicon phase 15 may contain crystalline silicon. The silicon phase 15 is composed of, for example, a plurality of crystallites. The crystallite size of the silicon phase 15 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 15 accompanying 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 15 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.

[0048] The lower limit of the crystallite size of the silicon phase 15 is not particularly limited, but is, for example, 1 nm. An example of a suitable crystallite size of the silicon phase 15 is 1 nm or more and 15 nm or less, and may be 5 nm or more and 11 nm or less. When the crystallite size of the silicon phase 15 is 1 nm or more, for example, the surface area of ​​the silicon phase 15 can be kept small, making it difficult for deterioration of the silicon phase 15, 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 15 uniform, and the stress generated in the negative electrode active material is effectively alleviated.

[0049] When the ion-conducting phase 14 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.

[0050] When the ion-conducting phase 14 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.

[0051] When the ion conductive phase 14 includes a lithium aluminate phase, from the viewpoint of increasing capacity, the content of the silicon phase 15 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 15 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 15 exposed on the surface of the composite particle without being covered by the lithium aluminate phase is reduced, and side reactions between the electrolyte and the silicon phase 15 are also suppressed. The content of the silicon phase 15 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.

[0052] The content of the silicon phase 15 in the composite particle 11 can be determined by quantifying the amount of Si that constitutes the silicon phase 15 in the composite particle 11 using Si-NMR, as will be described later.

[0053] Composite particle 11 may further include a conductive layer (not shown) that covers at least a portion of the surface of the particle composed of ion-conducting phase 14 and silicon phase 15. This conductive layer corresponds to conductive layer 11a shown in Fig. 2, and the particle composed of ion-conducting phase 14 and silicon phase 15 corresponds to mother particle 11b. This configuration improves the conductivity of composite particle 11.

[0054] (Low Hardness Material) As described above, the low hardness material 13 is at least one selected from the group consisting of minerals having a Mohs hardness of 1 and inorganic compounds having a Mohs hardness of 1. The minerals and inorganic compounds used as the low hardness material 13 are very soft materials having a Mohs hardness of 1. By having such low hardness material 13 attached to the surfaces of the silicon-containing particles 11, the negative electrode active material according to embodiment 1 can be easily moved within the negative electrode, and a significant increase in the thickness of the negative electrode due to swelling of the negative electrode active material during charging can be suppressed.

[0055] An example of a mineral with a Mohs hardness of 1 is talc. Materials known as steatite, soapstone, French chalk, and lava can also be used as talc.

[0056] An example of an inorganic compound with a Mohs hardness of 1 is molybdenum disulfide.

[0057] Low-hardness material 13 may contain at least one selected from the group consisting of talc and molybdenum disulfide, which provides good slipperiness to the surfaces of silicon-containing particles 11, thereby more effectively reducing swelling of the negative electrode.

[0058] The low-hardness material 13 is, for example, in the form of particles. The particles of the low-hardness material 13 have a smaller particle size than the silicon-containing particles 11.

[0059] The ratio of the mass of the low-hardness material 13 to the mass of the silicon-containing particles 11 (hereinafter referred to as the "low-hardness material ratio") may be, for example, 0.1% by mass or more and 20% by mass or less, or 0.5% by mass or more and 10% by mass or less. By setting the low-hardness material ratio to 0.1% by mass or more, preferably 0.5% by mass or more, sufficient slipperiness is imparted to the surface of the silicon-containing particles 11, thereby further reducing swelling of the negative electrode. Furthermore, by setting the low-hardness material ratio to 20% by mass or less, preferably 10% by mass or less, the increase in resistance of the negative electrode active material due to the presence of the low-hardness material 13 on the surface of the silicon-containing particles 11 can be reduced, thereby reducing the increase in internal resistance of the battery. In other words, by setting the low-hardness material ratio within the above range, it is possible to minimize the increase in resistance while reducing swelling of the negative electrode.

[0060] Here, the crystal structure of the low-hardness material can be identified by powder X-ray diffraction, and elements can be identified and quantified by elemental analysis such as X-ray fluorescence analysis (XRF) and energy dispersive X-ray spectroscopy (SEM-EDX). In the case of SEM-EDX, it is desirable to analyze at least 10 particles. Both XRF and SEM-EDX can be analyzed more accurately by using a calibration curve with an internal standard method. The content of the target element is calculated by averaging the number of analyzed particles and the measurement results of the amount of the analyzed element. The mass of the silicon-containing particles 11 and the mass of the low-hardness material 13 are calculated from the obtained element content. The low-hardness material ratio is calculated using the obtained mass of the silicon-containing particles 11 and the mass of the low-hardness material 13.

[0061] (Resin) As described above, the resin 12 is a water-insoluble resin that is attached to at least a portion of the surface of the silicon-containing particles.

[0062] The resin 12 may include, for example, at least one selected from the group consisting of styrene butadiene rubber (SBR), acrylic resin, polyolefin resin, and polyvinylidene fluoride (PVDF). These resins have good binding properties, allowing the low-hardness material 13 to firmly adhere to the silicon-containing particles 11. Therefore, even when the negative electrode active material is kneaded with a binder and a solvent to prepare a slurry used in forming the negative electrode mixture layer, applying shear stress to the surface of the negative electrode active material, the low-hardness material 13 can be supported on the surface of the silicon-containing particles 11 by the resin 12. This allows the negative electrode active material according to embodiment 1 to more reliably reduce swelling of the negative electrode. To effectively reduce swelling of the negative electrode, the resin 12 may include SBR.

[0063] The ratio of the mass of the resin 12 to the mass of the low-hardness material 13 (hereinafter referred to as the "resin ratio") may be, for example, 0.5% by mass or more and 5% by mass or less. A resin ratio of 0.5% by mass or more allows the low-hardness material 13 to be firmly attached to the silicon-containing particles 11, so that the negative electrode active material 10 can more reliably reduce swelling of the negative electrode. Furthermore, a resin ratio of 5% by mass or less can reduce the increase in resistance of the negative electrode active material 10 due to the presence of the resin 12 on the surface of the silicon-containing particles 11, thereby reducing the increase in internal resistance of the battery. In other words, by keeping the resin ratio within the above range, it is possible to minimize the increase in resistance while reducing swelling of the negative electrode. To further reduce the increase in resistance, the resin ratio may be 3% by mass or less.

[0064] Here, the resin ratio is determined by first analyzing the negative electrode active material by TOF-SIMS, or by extracting the resin component with a solvent in which the resin is soluble (for example, NMP or GBL) and quantitatively analyzing it by NMR, GC / MAS, or the like, to determine the mass of resin 12 in negative electrode active material 10. The resin ratio is determined using the mass of low-hardness material 13 and the mass of the obtained resin 12.

[0065] (Method for manufacturing negative electrode active material) The negative electrode active material of the present disclosure is manufactured by a manufacturing method including, for example, step I of obtaining silicon-containing particles 11 and step II of adhering resin 12 and low-hardness material 13 to the surface of silicon-containing particles 11.

[0066] [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.

[0067] When the ion-conducting phase 14 is a carbon phase, the composite particles 11 can be obtained by, for example, pulverizing a mixture of a carbon source and raw silicon while stirring it in a ball mill or the like to form fine particles, 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.

[0068] When the ion-conducting phase 14 is a lithium silicate phase, the composite particle 11 can be produced, for example, by the following steps (iA) and (iiA).

[0069] 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 lithium 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 lithium 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 lithium silicate by a solid-phase reaction.

[0070] 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.

[0071] 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).

[0072] 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.

[0073] 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.

[0074] As the organic solvent, alcohols, ethers, fatty acids, alkanes, cycloalkanes, silicate esters, metal alkoxides, etc. can be used.

[0075] 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.

[0076] 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).

[0077] 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.

[0078] 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.

[0079] When the ion-conducting phase 14 is a lithium aluminate phase, the composite particle 11 can be produced, for example, by the following steps (iB) and (iiB).

[0080] 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.

[0081] 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.

[0082] 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.

[0083] 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.

[0084] 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.

[0085] 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).

[0086] 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.

[0087] 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.

[0088] As the organic solvent, alcohols, ethers, fatty acids, alkanes, cycloalkanes, silicate esters, metal alkoxides, etc. can be used.

[0089] 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.

[0090] 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).

[0091] 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.

[0092] 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.

[0093] When the composite particles 11 have the conductive layer, 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 conductive layer is formed on the surfaces of the base particles by heating a mixture of the base particles and the carbon material in an inert atmosphere (e.g., an argon, nitrogen, or other atmosphere) at 700°C or higher and 950°C or lower. In this manner, composite particles 11 having a conductive layer 11a on the surface of the base particle 11b, as shown in FIG. 2, are obtained.

[0094] In addition, as step I for obtaining silicon-containing particles 11, a method for producing composite particles 11 as silicon-containing particles 11 has been described above, but commercially available silicon-containing particles such as Si particles may also be prepared as silicon-containing particles 11.

[0095] [Step II] Step II is a step of attaching resin 12 and low-hardness material 13 to the surfaces of silicon-containing particles 11 .

[0096] There are no particular limitations on the method for adhering resin 12 and low-hardness material 13 to the surfaces of silicon-containing particles 11. As an example, the following method can be used.

[0097] First, a slurry containing resin 12 and low-hardness material 13 is prepared. For example, when SBR is used as resin 12 and talc is used as low-hardness material 13, the slurry can be prepared by mixing SBR latex and talc. The mass ratio of resin 12 to low-hardness material 13 in the slurry can be determined taking into consideration the mass ratio of resin 12 to low-hardness material 13 in the target negative electrode active material.

[0098] The slurry is added to silicon-containing particles 11, such as composite particles 11, obtained in step I, and mixed. The mixture is gradually dried while mixing to obtain a mixture in a near-powder state. The mixture is then dried to obtain a powder. This results in a powder in which low-hardness material 13 is adhered to the surfaces of silicon-containing particles 11 by resin 12. This powder can be used as the negative electrode active material according to embodiment 1.

[0099] (Embodiment 2) The negative electrode according to Embodiment 2 includes a negative electrode mixture layer containing an active material. The active material includes the negative electrode active material according to Embodiment 1. As described in Embodiment 1, the negative electrode active material according to Embodiment 1 can reduce swelling of the negative electrode due to charging. Therefore, the negative electrode according to Embodiment 2 can reduce swelling due to charging.

[0100] Fig. 4 is a cross-sectional view showing a schematic configuration of an example of a negative electrode according to embodiment 2. As shown in Fig. 4, a negative electrode 40 according to embodiment 2 includes, for example, a negative electrode mixture layer 41 and a negative electrode current collector 42. The negative electrode mixture layer 41 is disposed on the negative electrode current collector 42 and contains the negative electrode active material according to embodiment 1.

[0101] The negative electrode mixture layer 41 may further contain another negative electrode active material, such as a carbon material, in addition to the negative electrode active material according to embodiment 1. By including a carbon material as the negative electrode active material, a conductive path in the negative electrode active material can be ensured even when the negative electrode active material according to embodiment 1 expands and contracts with charge and discharge, thereby further suppressing capacity degradation associated with charge and discharge cycles. Furthermore, by including a carbon material, it is possible to easily control the porosity in the negative electrode mixture layer 41, thereby enabling control of the porosity in consideration of the permeability of the electrolyte. The carbon material functioning as the negative electrode active material is, for example, at least one selected from the group consisting of graphite, soft carbon, and hard carbon.

[0102] The negative electrode mixture layer 41 further includes, for example, 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.

[0103] The binder may contain SBR and PAA, and in this case, the ratio of the total mass of SBR and PAA to the mass of the active material contained in the negative electrode mixture layer 41 may be, for example, 3 mass % or more and 10 mass % or less. This allows the negative electrode 40 according to the second embodiment to further reduce swelling due to charging.

[0104] In order to reduce the resistance of the negative electrode 40, the negative electrode mixture layer 41 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.

[0105] The negative electrode current collector 42 may be a sheet or film made of a metal material such as stainless steel, nickel, copper, or an alloy thereof. The sheet or film may be porous or non-porous. Examples of the sheet or film include metal foil and metal mesh. A carbon material such as carbon may be applied to the surface of the negative electrode current collector 42 as a conductive auxiliary material.

[0106] (Embodiment 3) A battery according to embodiment 3 includes a positive electrode, a negative electrode, and an electrolyte. The negative electrode is the negative electrode according to embodiment 2. With this configuration, the battery according to embodiment 3 can reduce swelling of the negative electrode due to charging.

[0107] 5 is a longitudinal cross-sectional view schematically illustrating an example of a battery according to embodiment 3. Battery 100 is a cylindrical battery including a cylindrical battery case, a wound electrode group 54, and an electrolyte (not shown). The electrode group 54 is housed in the battery case and is in contact with the electrolyte.

[0108] The battery case is composed of a case body 55, which is a cylindrical metal container with a bottom, and a sealing body 56 that seals the opening of the case body 55. A gasket 67 is disposed between the case body 55 and the sealing body 56. The gasket 67 ensures the airtightness of the battery case. Within the case body 55, insulating plates 57 and 58 are disposed on both ends of the electrode group 54 in the direction of the winding axis of the electrode group 54.

[0109] Case body 55 has, for example, a step 61. Step 61 can be formed by partially pressing the side wall of case body 55 from the outside. Step 61 may be formed in an annular shape on the side wall of case body 55 along the circumferential direction of an imaginary circle defined by case body 55. In this case, sealing body 56 is supported by, for example, the surface of step 61 on the opening side.

[0110] Sealing body 56 includes a filter 62, a lower valve body 63, an insulating member 64, an upper valve body 65, and a cap 66. These components are stacked in this order in sealing body 56. Sealing body 56 is attached to the opening of case body 55 so that cap 66 is located on the outside of case body 55 and filter 62 is located on the inside of case body 55.

[0111] Each of the above-mentioned members constituting the sealing body 56 has, for example, a disk or ring shape. The above-mentioned members, except for the insulating member 64, are electrically connected to each other.

[0112] The electrode group 54 has a positive electrode 51, a separator 52, and a negative electrode 53. The positive electrode 51, the separator 52, and the negative electrode 53 are all strip-shaped. The width direction of the strip-shaped positive electrode 51 and the negative electrode 53 is, for example, parallel to the winding axis of the electrode group 54. The separator 52 is disposed between the positive electrode 51 and the negative electrode 53. The positive electrode 51 and the negative electrode 53 are spirally wound with the separator 52 interposed between these electrodes.

[0113] When observing the cross section of the battery 100 in a direction perpendicular to the winding axis of the electrode group 54, the positive electrodes 51 and negative electrodes 53 are stacked alternately in the radial direction of an imaginary circle defined by the case body 55, with a separator 52 interposed between these electrodes.

[0114] The positive electrode 51 is electrically connected to a cap 66, which also serves as a positive electrode terminal, via a positive electrode lead 59. One end of the positive electrode lead 59 is connected, for example, to near the center of the positive electrode 51 in the longitudinal direction of the positive electrode 51. The positive electrode lead 59 passes through a through-hole formed in the insulating plate 57 and extends from the positive electrode 51 to the filter 62. The other end of the positive electrode lead 59 is welded, for example, to the surface of the filter 62 on the electrode group 54 side.

[0115] The negative electrode 53 is electrically connected to the case body 55, which also serves as a negative electrode terminal, via a negative electrode lead 60. One end of the negative electrode lead 60 is connected to, for example, an end of the negative electrode 53 in the longitudinal direction of the negative electrode 53. The other end of the negative electrode lead 60 is welded to, for example, the inner bottom surface of the case body 55.

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

[0117] The positive electrode 51 includes a material having the property of absorbing and releasing metal ions (e.g., lithium ions). The positive electrode 51 includes, for example, a positive electrode active material. The positive electrode 51 includes, for example, a positive electrode current collector and a positive electrode mixture layer supported on the surface of the positive electrode current collector.

[0118] The positive electrode 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.

[0119] 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.

[0120] The positive electrode mixture layer may further contain a binder. As the binder, the materials described as binders usable for the negative electrode mixture layer in the second embodiment can also be used for the positive electrode mixture layer.

[0121] The positive electrode mixture layer may further contain a conductive additive. As the conductive additive, the materials described in the second embodiment as conductive additives that can be used in the negative electrode mixture layer can also be used in the positive electrode mixture layer.

[0122] The negative electrode 53 is the negative electrode 40 according to the second embodiment.

[0123] 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.

[0124] 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.

[0125] 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.

[0126] It is usually desirable to interpose a separator between the positive electrode and the negative electrode. The separator 52 has high ion permeability and adequate mechanical strength and insulating properties. The separator 52 can be made of a microporous thin film, a woven fabric, a nonwoven fabric, or the like. The separator 52 can be made of a polymer, for example. The polymer can be a polyolefin such as polypropylene or polyethylene.

[0127] In the battery according to the third embodiment, the electrolyte may be impregnated into a polymer provided as a separator, for example. That is, the battery according to the third embodiment may have a structure in which the electrolyte and the polymer are used in combination.

[0128] The battery according to the third 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 anion component. The term "sulfide solid electrolyte" refers to a solid electrolyte containing sulfur as the main anion component. The term "oxide solid electrolyte" refers to a solid electrolyte containing oxygen as the main anion component. The term "main anion component" refers to the anion with the largest mass among all anions constituting the solid electrolyte.

[0129] As an example of the structure of the battery according to the third embodiment, the configuration example shown in FIG. 5 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 third 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 third 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.

[0130] (Other Embodiments) (Additional Notes) The above description of the embodiments discloses the following techniques.

[0131] (Technology 1) A negative electrode active material comprising: silicon-containing particles; a water-insoluble resin attached to at least a portion of the surface of the silicon-containing particles; and a low-hardness material attached to the surface of the silicon-containing particles via the resin, wherein the low-hardness material is at least one selected from the group consisting of minerals and inorganic compounds having a Mohs hardness of 1.

[0132] This configuration can reduce swelling of the negative electrode due to charging.

[0133] (Technology 2) The negative electrode active material according to Technology 1, wherein the silicon-containing particles have a conductive layer on at least a part of the surface.

[0134] This configuration can improve the electrical conductivity of the silicon-containing particles.

[0135] (Technology 3) The negative electrode active material according to Technology 1 or 2, wherein the low hardness material includes at least one selected from the group consisting of talc and molybdenum disulfide.

[0136] This configuration makes it possible to more effectively reduce swelling of the negative electrode due to charging.

[0137] (Technology 4) The negative electrode active material according to any one of Technologies 1 to 3, wherein a ratio of the mass of the low hardness material to the mass of the silicon-containing particles is 0.5 mass % or more and 10 mass % or less.

[0138] This configuration can further reduce swelling of the negative electrode due to charging.

[0139] (Technology 5) The negative electrode active material according to any one of Technologies 1 to 4, wherein the resin includes at least one selected from the group consisting of styrene-butadiene rubber, an acrylic resin, a polyolefin resin, and polyvinylidene fluoride.

[0140] This configuration can more reliably reduce swelling of the negative electrode due to charging.

[0141] (Technology 6) The negative electrode active material according to any one of Technologies 1 to 5, wherein a ratio of the mass of the resin to the mass of the low hardness material is 0.5 mass % or more and 5 mass % or less.

[0142] This configuration makes it possible to suppress the increase in resistance and reduce swelling of the negative electrode.

[0143] (Technology 7) A negative electrode comprising a negative electrode mixture layer containing an active material, wherein the active material contains the negative electrode active material according to any one of Technologies 1 to 6.

[0144] With this configuration, the negative electrode according to Technique 7 can reduce swelling due to charging.

[0145] (Technology 8) The negative electrode according to Technology 7, wherein the negative electrode mixture layer further includes a binder, the binder includes styrene-butadiene rubber and polyacrylic acid, and a ratio of a total mass of the styrene-butadiene rubber and the polyacrylic acid to a mass of the active material is 3 mass% or more and 10 mass% or less.

[0146] This configuration can further reduce swelling due to charging.

[0147] (Technology 9) A battery comprising: a negative electrode containing the negative electrode active material according to Technology 7 or 8; a positive electrode; and an electrolyte.

[0148] With this configuration, the battery according to Technique 9 can reduce swelling of the negative electrode due to charging.

[0149] 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.

[0150] <Preparation of Negative Electrode Active Material> [Examples 1 to 13] (Preparation of Composite Particles) Lithium carbonate (Li2CO3) as a Li raw material and silicon dioxide (SiO2) as a Si raw material were mixed so that the atomic ratio: Si / Li was 1.05 to obtain a mixture. The mixture was fired at 800°C for 10 hours in an inert gas atmosphere to obtain lithium silicate (Li2SiO5). The obtained lithium silicate was pulverized to an average particle size of 10 μm.

[0151] The crushed lithium silicate and raw silicon (3N, average particle size 10 μm) were mixed in a mass ratio of 40:60. The mixture was charged 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 crushed at 200 rpm for 50 hours in an inert atmosphere.

[0152] The powder mixture was then removed from the inert atmosphere and fired at 600°C for 4 hours under pressure from a hot press to obtain a sintered body of the mixture. The sintered body of the mixture was then crushed and passed through a 40 µm mesh to obtain composite particles (silicate phase-containing composite particles). Then, composite particles with an average particle size of 10 µm were obtained using a sieve.

[0153] The silicon phase had a crystallite size of 15 nm. The silicon phase was particulate and had an average particle size of 20 nm. The silicate phase was mainly composed of Li2Si2O5, and the content of the silicon phase in the silicate phase-containing composite particles was 60 mass%.

[0154] (Preparation of Negative Electrode Active Material) SBR latex (TRD105A, manufactured by ENEOS Materials Corporation) and talc (Nano Ace D-1000, manufactured by Nippon Talc Co., Ltd.) were mixed so that the ratio of the mass of SBR to the mass of talc (resin ratio) was the value shown in Table 1, to prepare a slurry. This slurry was added to the composite particles prepared by the above method and mixed so that the ratio of the mass of talc to the mass of the composite particles (low hardness material ratio) was the value shown in Table 1, and this was gradually dried while being mixed in a mortar to obtain a mixture in a state close to powder. Thereafter, the mixture was dried to obtain a powder. This powder was used as the negative electrode active material.

[0155] (Fabrication of Negative Electrode) Graphite and the negative electrode active material obtained by the above method were mixed in a mass ratio of graphite:negative electrode active material = 7:3 to obtain a mixture. This mixture was used as the negative electrode active material (hereinafter referred to as the negative electrode active material mixture). A mixed binder of SBR (TRD 105A, manufactured by ENEOS Materials Corporation), PAA (manufactured by Toagosei Co., Ltd.), and CMC (Cerogen BSH-6, manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.) was used as the binder. The negative electrode active material mixture and the binder were mixed, and water was added as a dispersion medium and mixed, to prepare a negative electrode slurry. The total mass ratio of SBR and PAA to the negative electrode active material mixture is shown in Table 1. CMC was added so that its mass ratio to the negative electrode active material mixture was 1 mass%. Table 2 shows the respective mass ratios of SBR and PAA to the negative electrode active material mixture. Next, the negative electrode slurry was applied to both sides of a copper foil negative electrode current collector, the coating was dried, and then rolled. The negative electrode was cut to a predetermined size to produce a negative electrode in which a negative electrode mixture layer was formed on both sides of the negative electrode current collector. At this time, a negative electrode current collector exposed portion was provided in a part of the negative electrode.

[0156] (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.

[0157] (Preparation of non-aqueous electrolyte) A non-aqueous electrolyte was prepared by dissolving lithium hexafluorophosphate (LiPF) at a concentration of 1.2 mol / L in a mixed solvent prepared by mixing ethylene carbonate (EC), fluoroethylene carbonate (FEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of EC:FEC:EMC:DMC=10:10:5:75.

[0158] (Fabrication of a 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 the electrode assembly was then housed in an exterior body made of an aluminum laminate sheet. After a non-aqueous electrolyte was poured into the exterior body, the opening of the exterior body was sealed to obtain a battery.

[0159] [Comparative Example 1] In the preparation of a negative electrode, the composite particles prepared in Examples 1 to 13 were used as they were as a negative electrode active material. That is, the composite particles were used as a negative electrode active material without adhering talc to the surface of the composite particles. A negative electrode was prepared in the same manner as in Examples 1 to 13 except for this point.

[0160] The fabrication of the positive electrode, the preparation of the nonaqueous electrolyte, and the fabrication of the nonaqueous electrolyte secondary battery were the same as in Examples 1 to 13.

[0161] Comparative Example 2 In the preparation of the negative electrode active material, silica having a Mohs hardness of 7 was used instead of talc. Except for this, the negative electrode active material was prepared in the same manner as in Examples 1 to 13.

[0162] The fabrication of the negative electrode, the fabrication of the positive electrode, the preparation of the non-aqueous electrolyte, and the fabrication of the non-aqueous electrolyte secondary battery were the same as in Examples 1 to 13.

[0163] [Comparative Example 3] In the preparation of a negative electrode, the composite particles prepared in Examples 1 to 13 were used as they were as the negative electrode active material. That is, talc was not attached to the surface of the composite particles, and they were used as the negative electrode active material. In addition, talc was added to the negative electrode mixture slurry. As shown in Table 1, the proportion of talc added to the negative electrode mixture slurry was 1 mass % relative to the negative electrode active material mixture.

[0164] The fabrication of the positive electrode, the preparation of the nonaqueous electrolyte, and the fabrication of the nonaqueous electrolyte secondary battery were the same as in Examples 1 to 13.

[0165] <Battery Evaluation> The expansion rate of the negative electrode and the internal resistance of the battery were evaluated for the batteries of Examples 1 to 13 and Comparative Examples 1 to 3 by the following methods.

[0166] (Measurement of Negative Electrode Expansion Coefficient During Charge) The expansion coefficient during charge was determined for the negative electrodes of each Example and Comparative Example. A battery was fabricated using the fabricated negative electrode, and the expansion coefficient of the negative electrode during charge was measured using the battery by the following method. (1) A battery was fabricated using the fabricated negative electrode, a positive electrode, and an electrolyte. (2) The battery was charged at 0.1 C in a temperature environment of 25°C until the cell voltage reached 4.2 V, then discharged at 0.1 C until the cell voltage reached 2.5 V, and charged again to determine the thickness (Ta) of the negative electrode in the charged state (fully charged state). The thickness (Ta) of the negative electrode in the charged state was determined by disassembling the battery, removing the negative electrode, and measuring the thickness of the negative electrode. (3) The thickness (Tb) of the negative electrode before charge was determined by measuring the thickness of the negative electrode when the battery was fabricated. (4) The thickness increase rate (Ta×100 / Tb−100) of the negative electrode in the charged state relative to the thickness of the negative electrode before charging was calculated and used as the expansion rate (%) of the negative electrode during charging.

[0167] Here, the thickness of the negative electrode was measured with a digital micrometer manufactured by NiKON Corporation. Specifically, the thickness was measured at five arbitrary points, and the average value calculated from the five measured values ​​was used as the thickness.

[0168] The results are shown in Table 1. Table 1 shows relative values ​​when the negative electrode expansion rate of Comparative Example 1 is taken as the reference (100%).

[0169] (Measurement of Internal Resistance) The internal resistance of the batteries of each Example and Comparative Example was measured. Initial charging and discharging were carried out under the following conditions.

[0170] Initial charge / discharge conditions: Constant current charging was performed at a current of 0.1 It (30 mA) until the battery voltage reached 4.2 V. Then, constant current discharging was performed at a current of 0.1 It (30 mA) until the battery voltage reached 2.5 V.

[0171] Next, the batteries were charged and discharged under the following conditions, and the initial DC internal resistance (DCIR) was measured using the following formula (a). The results are shown in Table 1.

[0172] Charge / Discharge Conditions: The battery was charged at a constant current of 0.3 It (90 mA) at a temperature of 25° C. until the SOC reached 50%. After a 2-hour pause, the battery was discharged at a current of 0.5 It (150 mA) for 10 seconds.

[0173] DCIR calculation formula: DCIR (mΩ) = (voltage immediately before discharge starts - voltage 10 seconds after discharge starts) / (discharge current) ... (a)

[0174] The results are shown in Table 1. Table 1 shows relative values ​​when the DCIR value of Comparative Example 1 is taken as the reference (100%).

[0175]

[0176]

[0177] (Discussion) As shown in Table 1, the negative electrodes using the negative electrode active materials of Examples 1 to 13, in which a low-hardness material (here, talc) was attached to the surface of the silicon-containing particles via a resin (here, SBR), exhibited a reduced expansion rate compared to the negative electrode using the negative electrode active material of Comparative Example 1, in which a low-hardness material was not attached to the surface of the silicon-containing particles. That is, the negative electrode active material of the present disclosure was able to reduce swelling of the negative electrode during charging. Furthermore, the internal resistance of the batteries using the negative electrode active materials of Examples 1 to 13 was comparable to that of the negative electrode active material of Comparative Example 1, in which a low-hardness material was not provided, and there was no problem with an increase in internal resistance.

[0178] In the negative electrode active material of Comparative Example 2, in which silica having a Mohs hardness of 7 was attached to the surface of the silicon-containing particles, no reduction in swelling of the negative electrode during charging was confirmed. Furthermore, in the negative electrode of Comparative Example 3, in which talc was mixed into the negative electrode mixture rather than attached to the surface of the silicon-containing particles, no reduction in swelling of the negative electrode during charging was confirmed. These results also confirm that a negative electrode active material in which a low-hardness material having a Mohs hardness of 1 was attached to the surface of the silicon-containing particles via a resin can reduce swelling of the negative electrode.

[0179] In the above examples, talc was used as the low-hardness material, but the inventors have also confirmed that swelling of the negative electrode can be similarly reduced when a low-hardness material (e.g., molybdenum disulfide) having a Mohs hardness comparable to that of talc is used.

[0180] 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; a water-insoluble resin attached to at least a portion of the surface of the silicon-containing particles; and a low-hardness material attached to the surface of the silicon-containing particles via the resin, wherein the low-hardness material is at least one selected from the group consisting of minerals and inorganic compounds having a Mohs hardness of 1.

2. The negative electrode active material according to claim 1, wherein the silicon-containing particles have a conductive layer on at least a portion of the surface thereof.

3. The negative electrode active material according to claim 1, wherein the low hardness material includes at least one selected from the group consisting of talc and molybdenum disulfide.

4. The negative electrode active material according to claim 1, wherein the ratio of the mass of the low hardness material to the mass of the silicon-containing particles is 0.5 mass % or more and 10 mass % or less.

5. The negative electrode active material according to claim 1, wherein the resin includes at least one selected from the group consisting of styrene-butadiene rubber, acrylic resin, polyolefin resin, and polyvinylidene fluoride.

6. The negative electrode active material according to claim 1, wherein the ratio of the mass of the resin to the mass of the low-hardness material is 0.5 mass % or more and 5 mass % or less.

7. A negative electrode comprising a negative electrode mixture layer containing an active material, wherein the active material contains the negative electrode active material according to any one of claims 1 to 6.

8. The negative electrode according to claim 7, wherein the negative electrode mixture layer further contains a binder, the binder contains styrene-butadiene rubber and polyacrylic acid, and the ratio of the total mass of the styrene-butadiene rubber and the polyacrylic acid to the mass of the active material is 3 mass % or more and 10 mass % or less.

9. A battery comprising: the negative electrode according to claim 7; a positive electrode; and an electrolyte.

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