Negative electrode active material and battery

Abstract

A negative electrode active material 10 according to the present disclosure comprises: composite particles 11; and a coating layer 12 that covers at least a part of a surface of the composite particles 11. The composite particles 11 include particulate silicon-containing phases 13 and phases 14 of a first hydrophobic polymer present between the silicon-containing phases 13. The coating layer 12 contains an inorganic acid salt and a second 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 (hereinafter sometimes referred to as "LSX").

[0003] Silicon-containing materials tend to be gradually corroded due to side reactions in batteries containing non-aqueous electrolytes. This corrosion causes the silicon-containing material to deteriorate, resulting in a decrease in the charge-discharge cycle performance of the battery. Therefore, as an anode active material that can suppress such deterioration due to side reactions, for example, Patent Document 1 proposes an anode active material having a configuration in which at least a portion of the surface of particles of a silicon-containing material (silicon-containing particles) is covered with a coating layer containing a lithium sulfonate compound and a hydrophobic polymer compound.

[0004] International Publication No. 2023 / 190239

[0005] However, the negative electrode active material disclosed in Patent Document 1 leaves room for improvement in terms of the charge-discharge cycle characteristics of the battery.

[0006] The present disclosure proposes a technique for improving the charge-discharge cycle characteristics of a battery using a silicon-containing material as a negative electrode active material.

[0007] The negative electrode active material of the present disclosure comprises: composite particles; and a coating layer covering at least a portion of the surface of the composite particles, wherein the composite particles include a particulate silicon-containing phase and a phase of a first hydrophobic polymer present between the silicon-containing phases, and the coating layer includes an inorganic acid salt and a second hydrophobic polymer.

[0008] According to the technology of the present disclosure, it is possible to improve the charge-discharge cycle characteristics of a battery using a negative electrode active material that uses a silicon-containing material.

[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 a silicon-containing phase formed 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 a composite particle 11 and a coating layer 12 that covers at least a portion of the surface of the composite particle 11. The composite particle 11 includes a particulate silicon-containing phase 13 and a first hydrophobic polymer phase 14 that is present between the silicon-containing phases 13. The coating layer 12 includes an inorganic acid salt and a second hydrophobic polymer.

[0012] As described above, in the negative electrode active material 10 according to the first embodiment, the composite particles 11 have a configuration in which the first hydrophobic polymer phase 14 is present between the particulate silicon-containing phases 13. In other words, this configuration means that the first hydrophobic polymer is present inside the composite particles 11. Furthermore, the coating layer 12 formed on the surface of the composite particles 11 contains an inorganic acid salt and a second hydrophobic polymer. With these configurations, in the negative electrode active material 10 according to the first embodiment, the coating layer 12 suppresses side reactions with the electrolyte in the battery, and the first hydrophobic polymer phase 14 suppresses particle collapse from the inside of the composite particles 11 during battery charge and discharge. In this way, the negative electrode active material 10 according to the first embodiment can suppress degradation from both the particle interior and particle surface, thereby improving the charge and discharge cycle characteristics of the battery. Furthermore, the negative electrode active material 10 according to the first embodiment can also improve the initial charge and discharge efficiency of the battery due to the above-described configuration.

[0013] In the negative electrode active material 10, the mass ratio of the total content of the first hydrophobic polymer and the second hydrophobic polymer to the content of the inorganic acid salt is, for example, 0.3 or more and 10 or less, or may be 1 or more and 10 or less, or 1 or more and 4 or less. When the inorganic acid salt and the hydrophobic polymer satisfy this relationship, side reactions on the surface of the negative electrode active material 10 can be effectively suppressed, while particle collapse from within the composite particles 11 can also be effectively suppressed. Therefore, the charge / discharge cycle characteristics of the battery can be further improved.

[0014] The content of the inorganic acid salt and the total content of the first hydrophobic polymer and the second hydrophobic polymer in the negative electrode active material 10 are determined by the following method. As an example, a case will be described in which the first hydrophobic polymer and the second hydrophobic polymer are fluorine-containing hydrophobic polymers (e.g., polyvinylidene fluoride (PVDF)).

[0015] 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. This mass of the fluorine-containing hydrophobic polymer is defined as the sum of the content of the first hydrophobic polymer and the content of the second hydrophobic polymer in the negative electrode active material 10. The remainder that is not dissolved in NMP is then washed with water to dissolve the inorganic acid salt. The mass of the inorganic acid salt dissolved in water is determined by quantitative analysis such as ICP emission spectroscopy. This mass of the inorganic acid salt is defined as the content of the inorganic acid salt in the negative electrode active material 10.

[0016] The mass of the remainder that is not dissolved in water and NMP is determined as the mass of the portion excluding the first hydrophobic polymer from the composite particle 11. If the composite particle 11 has a conductive layer on the surface, the remainder that is not dissolved in water and NMP is subjected to a quantitative analysis of carbon using a carbon-sulfur analyzer or the like, and the determined amount of carbon is determined to be derived from the carbon material of the conductive layer.

[0017] Using the content of the inorganic acid salt determined above and the content of the fluorine-containing hydrophobic polymer (the sum of the content of the first hydrophobic polymer and the content of the second hydrophobic polymer), the mass ratio of the sum of the content of the first hydrophobic polymer and the content of the second hydrophobic polymer to the content of the inorganic acid salt ((the sum of the content of the first hydrophobic polymer and the content of the second hydrophobic polymer) / content of the inorganic acid salt) is determined.

[0018] In addition, for the negative electrode active material 10, when the amount of the first hydrophobic polymer added to the part of the composite particle 11 excluding the first hydrophobic polymer, i.e., the amount of the first hydrophobic polymer added to the particle before the first hydrophobic polymer was added and the amount of the second hydrophobic polymer added to prepare the coating layer 12, and the amount of the inorganic acid salt added to prepare the coating layer 12 are known, it is also possible to use these amounts to determine the mass ratio of the total content of the first hydrophobic polymer and the second hydrophobic polymer to the content of the inorganic acid salt.

[0019] The mass ratio of the mass of the inorganic acid salt contained in the coating layer 12 to the mass of the portion of the composite particle 11 excluding the first hydrophobic polymer (hereinafter referred to as the "amount of inorganic acid salt carried") is, for example, 2 mass% or more and 30 mass% or less, or alternatively 3 mass% or more and 17 mass% or less, or 4 mass% or more and 16 mass% or less, or alternatively 5 mass% or more and 15 mass% or less.

[0020] The mass of the portion of composite particle 11 excluding the first hydrophobic polymer and the mass of the inorganic acid salt contained in coating layer 12 can be determined by the method described above. Furthermore, for negative electrode active material 10, when the mass of the portion of composite particle 11 excluding the first hydrophobic polymer, i.e., the mass of the particles before the first hydrophobic polymer is added, and the amount of inorganic acid salt added to the particles are known, it is also possible to determine the amount of inorganic acid salt supported by using the mass of the particles before the first hydrophobic polymer is added and the amount of inorganic acid salt added.

[0021] When the amount of inorganic acid salt supported is 2% by mass or more, the surface of the composite particle 11 can be sufficiently coated with the inorganic acid salt, and the effect of suppressing side reactions caused by the inorganic acid salt can be easily achieved. When the amount of inorganic acid salt supported is 30% 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 inorganic acid salt and the second hydrophobic polymer, for example, the inorganic acid salt is supported on the surface of the composite particle 11 in an amount within the above range.

[0022] The mass ratio (hereinafter referred to as "hydrophobic polymer loading") of the sum of the mass of the first hydrophobic polymer contained in the composite particle 11 and the mass of the second hydrophobic polymer contained in the coating layer 12 to the mass of the portion of the composite particle 11 excluding the first hydrophobic polymer is, for example, 2 mass% or more and 40 mass% or less, and may be 5 mass% or more and 20 mass% or less.

[0023] The mass of the portion of composite particle 11 excluding the first hydrophobic polymer and the total mass of the first hydrophobic polymer contained in composite particle 11 and the second hydrophobic polymer contained in coating layer 12 can be determined by the method described above. Furthermore, for negative electrode active material 10, when the mass of the portion of composite particle 11 excluding the first hydrophobic polymer, i.e., the mass of the particle before the first hydrophobic polymer is added, the amount of the first hydrophobic polymer added to the particle, and the amount of the second hydrophobic polymer added to produce coating layer 12 are known, it is also possible to determine the amount of hydrophobic polymer supported by using the mass of the particle before the first hydrophobic polymer is added and the amounts of the first hydrophobic polymer and the second hydrophobic polymer added.

[0024] Each component of the negative electrode active material 10 according to the first embodiment will be specifically described below.

[0025] (Composite Particle) As described above, the composite particle 11 includes a particulate silicon-containing phase 13 and a first hydrophobic polymer phase 14 present between the silicon-containing phases 13. Hereinafter, the first hydrophobic polymer phase 14 will be referred to as the hydrophobic polymer phase 14.

[0026] The silicon-containing phase 13 is a particulate phase made of a silicon-containing material. In this specification, the silicon-containing material means a material containing Si. Examples of the Si-containing material include Si, a Si alloy, a Si compound, and a composite material containing Si.

[0027] The silicon-containing phase 13 may be a silicon phase. That is, the silicon-containing phase 13 may be a particulate phase of simple silicon. Such a silicon phase is composed of a single crystallite or multiple crystallites. When the silicon-containing phase 13 is a silicon phase, not only can the charge / discharge cycle be improved but also a high capacity can be achieved. The silicon-containing phase 13 may be composed of silicon particles.

[0028] The silicon-containing phase 13 may be formed of a composite material containing Si. The Si-containing composite material may include, for example, an ion-conducting phase and a silicon phase dispersed within the ion-conducting phase. FIG. 2 is a cross-sectional view showing a schematic configuration of an example of the silicon-containing phase 13 formed of a composite material containing Si. As shown in FIG. 2, the silicon-containing phase 13 may include an ion-conducting phase 131 and a silicon phase 132 dispersed within the ion-conducting phase 131. When the silicon-containing phase 13 is formed of a composite material having such a configuration, the stress associated with the expansion and contraction of the silicon phase 132 during charge and discharge is alleviated by the ion-conducting phase 131, thereby suppressing cracking and fracture of the silicon-containing phase 13. Therefore, deterioration of the composite particle 11 from within the particle due to charge and discharge of the battery can be further suppressed, and the charge and discharge cycle characteristics can be further improved.

[0029] The ion conducting phase 131 may include, for example, at least one selected from the group consisting of an aluminate phase, a silicate phase, a silicon oxide phase, and a carbon phase.

[0030] The aluminate phase preferably contains an alkali metal element (a Group 1 element other than hydrogen in the long periodic table). Examples of alkali metal elements include lithium (Li), potassium (K), sodium (Na), etc. The aluminate phase may be a lithium aluminate phase containing lithium (Li), aluminum (Al), and oxygen (O). The lithium aluminate phase may further contain other elements in addition to Li, Al, and O. Examples of other elements include at least one selected from the group consisting of calcium (Ca), magnesium (Mg), zirconium (Zr), iron (Fe), boron (B), phosphorus (P), and lanthanum (La).

[0031] The silicate phase preferably contains at least one of an alkali metal element (a Group 1 element other than hydrogen in the long periodic table) and a Group 2 element in the long periodic table. Alkali metal elements include lithium (Li), potassium (K), sodium (Na), etc. Group 2 elements include magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), etc. The silicate phase may be lithium silicate containing lithium (Li), silicon (Si), and oxygen (O). The silicate phase may further contain other elements such as rare earth elements such as lanthanum (La), aluminum (Al), and boron (B).

[0032] The silicon oxide phase is composed of a compound of silicon (Si) and oxygen (O). The main component of the silicon oxide phase (for example, 95 mass % or more and 100 mass % or less) may be silicon dioxide.

[0033] The carbon phase may be composed of, for example, 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. The carbon phase is electrically conductive, and therefore improves battery characteristics such as the initial charge / discharge efficiency and charge / discharge cycle characteristics of the battery.

[0034] The average particle size of the silicon-containing phase 13 is, for example, 3 μm or less, and may be 1.5 μm or less. By appropriately miniaturizing the silicon-containing phase 13, volume change during charge and discharge is reduced, improving structural stability. Furthermore, uniform expansion and contraction of the silicon-containing phase 13 is achieved, suppressing particle cracking, thereby further improving charge and discharge cycle characteristics. The average particle size of the silicon-containing phase 13 is, for example, 1 nm or more.

[0035] The average particle size of the silicon-containing phase 13 can be measured using a scanning electron microscope (SEM) image obtained by SEM observation of a cross section of the negative electrode active material 10 in which the cross section of the silicon-containing phase 13 is exposed. Specifically, the average particle size of the silicon-containing phase 13 is determined by averaging the maximum diameters of 100 silicon-containing phases 13 arbitrarily selected from the cross-sectional SEM image of the negative electrode active material 10.

[0036] The hydrophobic polymer phase 14 is present between the silicon-containing phases 13. That is, the hydrophobic polymer phase 14 is present inside the composite particle 11. This configuration suppresses particle collapse from the inside of the composite particle 11 due to expansion and contraction of the silicon-containing phase 13 that accompanies charging and discharging of the battery.

[0037] The first hydrophobic polymer constituting the hydrophobic polymer phase 14 may have good binding properties and thermal melting properties. In this case, the first hydrophobic polymer can firmly support the silicon-containing phases 13, thereby further suppressing particle collapse from within the composite particles 11 during charge and discharge. The first hydrophobic polymer is almost insoluble in water.

[0038] The first hydrophobic polymer constituting the hydrophobic polymer phase 14 includes, for example, a fluorine-containing hydrophobic polymer. The fluorine-containing hydrophobic polymer has good binding properties and high stability with respect to the electrolyte, and therefore can effectively suppress deterioration of the composite particles 11 due to charge and discharge. Therefore, the charge and discharge cycle characteristics of the battery can be further improved.

[0039] 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 used as the first hydrophobic polymer may include PVDF. PVDF has good binding properties, so it can firmly bind the silicon-containing phases 13 together, thereby more effectively suppressing deterioration of the composite particles 11 associated with charge and discharge. This can further improve the charge and discharge cycle characteristics of the battery. Furthermore, when the fluorine-containing hydrophobic polymer used as the first hydrophobic polymer is PVDF, PVDF has a low melting point, which also provides the effect of allowing the negative electrode active material 10 to be formed at a low heat treatment temperature.

[0040] The fluorine-containing hydrophobic polymer used as the first hydrophobic polymer may include, in addition to PVDF, a polymer containing vinylidene fluoride units. 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.

[0041] The presence of the hydrophobic polymer phase 14 between the silicon-containing phases 13 inside the composite particle 11 can be confirmed by energy dispersive X-ray spectroscopy in combination with a scanning electron microscope (SEM-EDX). Specifically, the location of the hydrophobic polymer phase 14 in the composite particle 11 can be confirmed by performing elemental mapping analysis by EDX on a backscattered electron image (SEM image) of a cross section of the negative electrode active material 10 in which the cross section of the composite particle 11 is exposed.

[0042] Furthermore, the presence of the hydrophobic polymer phase 14 inside the composite particles 11 can be confirmed by performing a surface analysis of the negative electrode active material 10 using X-ray photoelectron spectroscopy (XPS) in addition to SEM-EDX.

[0043] Composite particle 11 may have a configuration in which silicon-containing phase 13 is dispersed within hydrophobic polymer phase 14. That is, composite particle 11 may have a sea-island structure in which the regions of silicon-containing phase 13 are regarded as islands and the regions of hydrophobic polymer phase 14 are regarded as a sea.

[0044] The average particle size of the composite particles 11 may be 3 μm or more and 15 μm or less, 5 μm or more and 14 μm or less, or 6 μm or more and 10 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, by ensuring that the surface area of ​​the composite particles 11 is of an appropriate size, capacity reduction due to side reactions with the electrolyte is suppressed.

[0045] The average particle size of the composite particles 11 refers to the particle size at which the volume cumulative value is 50% in the particle size distribution measured by a laser diffraction scattering method (volume average particle size, D50). For example, an "LA-750" manufactured by Horiba Ltd. can be used as the measuring device.

[0046] The composite particles 11 can be removed from the battery by, for example, the following method, which is just an example.

[0047] 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 is a state in which the depth of discharge (DOD) is 90% or more (the state of charge (SOC) is 10% or less).

[0048] (Coating Layer) As described above, the coating layer 12 includes an inorganic acid salt and a second hydrophobic polymer. By coating at least a portion of the surface of the composite particle 11 with the coating layer 12, contact between the composite particle 11 and the electrolyte is prevented, thereby suppressing side reactions between the negative electrode active material 10 and the electrolyte. This suppresses erosion of the composite particle 11 due to side reactions, thereby suppressing deterioration of the negative electrode active material 10 from the surface due to the erosion. This improves the charge / discharge cycle characteristics of the battery and further improves the charge / discharge efficiency. The coating layer 12 may be a layer containing a mixture of the inorganic acid salt and the second hydrophobic polymer, or may be a layer consisting of a mixture of the inorganic acid salt and the second hydrophobic polymer. In the coating layer 12, the mixture of the inorganic acid salt and the second hydrophobic polymer may be a mixture in which the inorganic acid salt and the second hydrophobic polymer are mixed three-dimensionally randomly, or may be a mixture mixed to have an overall uniform composition.

[0049] The coating layer 12 containing the inorganic acid salt has good ionic conductivity, thereby suppressing side reactions in the battery. The coating layer 12 containing the second hydrophobic polymer improves the retention of the inorganic acid salt on the surface of the composite particle 11, and the surface of the composite particle 11 is effectively coated with the inorganic acid salt. This significantly suppresses side reactions between the negative electrode active material 10 and the electrolyte. This improves the charge / discharge cycle characteristics of the battery, as well as the charge / discharge efficiency.

[0050] The inorganic acid salt may be, for example, a lithium sulfonate compound. The lithium sulfonate compound is a lithium salt of a sulfonic acid compound. Here, the sulfonic acid compound is an organic compound having a sulfonic acid group (SO3H). The sulfonic acid compound may be a monosulfonic acid compound or a disulfonic acid compound. By including a lithium sulfonate compound in the coating layer 12, the ionic conductivity of the coating layer 12 is further improved, thereby further suppressing side reactions in the battery.

[0051] The lithium sulfonate compound may be a compound represented by the following general formula (1): In general formula (1), R represents an n-valent hydrocarbon group which may have a substituent, and n is 1 or 2.

[0052]

[0053] The number of carbon atoms in the hydrocarbon group is not particularly limited as long as it is 1 or more. The number of carbon atoms in the hydrocarbon group may be, for example, 20 or less, 10 or less, or 5 or less. The substituent may be, for example, a halogen or a fluorine atom. The hydrocarbon group may be an aliphatic hydrocarbon group. In general formula (1), R may be an n-valent aliphatic hydrocarbon group having 1 to 5 carbon atoms.

[0054] The inorganic acid salt includes, for example, at least one selected from the group consisting of lithium methanesulfonate, lithium ethanesulfonate, and lithium propanesulfonate. The inorganic acid salt may include lithium methanesulfonate.

[0055] The second hydrophobic polymer may have good binding properties and heat melting properties. In this case, the second hydrophobic polymer can firmly support the inorganic acid salt on the surface of the composite particle 11, and the effect of suppressing side reactions caused by the inorganic acid salt can be stably obtained. The second hydrophobic polymer is almost insoluble in water.

[0056] The second hydrophobic polymer includes, for example, a fluorine-containing hydrophobic polymer, which has good binding properties and high stability against electrolytes, and therefore can more firmly support the inorganic acid salt on the surface of the composite particle 11, making it easier to more stably suppress side reactions caused by the inorganic acid salt.

[0057] The fluorine-containing hydrophobic polymer may contain at least one selected from the group consisting of PVDF, polytetrafluoroethylene, perfluoroalkoxyalkane, perfluoroethylenepropene copolymer, ethylenetetrafluoroethylene copolymer, polychlorotrifluoroethylene, and ethylenechlorotrifluoroethylene copolymer. The fluorine-containing hydrophobic polymer used as the second hydrophobic polymer may contain PVDF. PVDF has good binding properties, so the inorganic acid salt can be more firmly supported on the surface of the composite particle 11. This makes it possible to more stably suppress side reactions caused by the inorganic acid salt. Furthermore, when the fluorine-containing hydrophobic polymer used as the second hydrophobic polymer is PVDF, PVDF has a low melting point, which also provides the effect of allowing the negative electrode active material 10 to be formed at a low heat treatment temperature.

[0058] The fluorine-containing hydrophobic polymer used as the second hydrophobic polymer may include, in addition to PVDF, a polymer containing vinylidene fluoride units. 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.

[0059] The hydrophobic polymer used as the second hydrophobic polymer may be the same compound as or a different compound from the hydrophobic polymer used as the first hydrophobic polymer in the composite particle 11. When the first hydrophobic polymer and the second hydrophobic polymer are the same compound, in the step of producing the coating layer 12, the hydrophobic polymer for producing the coating layer 12 can be made to reach the interior of the precursor particle of the composite particle 11, thereby producing the composite particle 11 simultaneously with the coating layer 12. Therefore, the negative electrode active material 10 can be produced efficiently.

[0060] 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 10. From the viewpoint of protecting the composite 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 composite particles 11 include 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 SEM observation or transmission electron microscope (TEM) observation of a cross section of the negative electrode active material 10.

[0061] (Method for Producing Negative Electrode Active Material) The negative electrode active material 10 of the present disclosure is produced by a production method including, for example, step I of preparing precursor particles of composite particles 11, step II of supplying a liquid-phase first hydrophobic polymer to the precursor particles to form a hydrophobic polymer phase inside the precursor particles, thereby obtaining composite particles 11, and step III of forming a coating layer 12 on the surface of the composite particles 11. When the same compound is used for the first hydrophobic polymer and the second hydrophobic polymer, steps II and III may be performed simultaneously. That is, the negative electrode active material 10 can be produced by supplying a material for the coating layer 12 containing a hydrophobic polymer to the surface of precursor particles of the composite particles 11, allowing the hydrophobic polymer to reach the interior of the precursor particles and forming a hydrophobic polymer phase, while also forming the coating layer 12.

[0062] [Step I] Preparing precursor particles of the composite particles 11. For example, when the silicon-containing phase 13 of the composite particles 11 is a silicon phase, commercially available silicon may be used as the raw silicon.

[0063] [Step II] Step II is a step of obtaining composite particles 11. For example, a liquid-phase first hydrophobic polymer is supplied to the surface of precursor particles as a material for the hydrophobic polymer phase, and the first hydrophobic polymer reaches the interior of the precursor particles to form a hydrophobic polymer phase. Specifically, for example, a dispersion containing the precursor particles and the first hydrophobic polymer is spray-dried to obtain a granulated powder. As a result, the first hydrophobic polymer enters the interior of the precursor particles, and composite particles 11 in which a hydrophobic polymer phase is formed are obtained.

[0064] [Step III] Step III is a step of forming a coating layer 12 on the surface of the composite particle 11 .

[0065] For example, after granulating the composite particles 11, the inorganic acid salt, and the second 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.

[0066] For example, a granulated powder is obtained by spray-drying a dispersion of composite particles 11, a powder of an inorganic acid salt, and a second hydrophobic polymer. The obtained granulated powder is classified to obtain a negative electrode active material 10 in which a coating layer 12 is formed on the surface of the composite particles 11. As another example, a powder of an inorganic acid salt and a liquid-phase second hydrophobic polymer are added to the composite particles 11 and mixed, and the inorganic acid salt and the second hydrophobic polymer are attached to the surface of the composite particles 11 to obtain an intermediate (mixture). The coating layer 12 may be formed on the surface of the composite particles 11 by heat-treating this intermediate.

[0067] It is also possible to carry out Step II and Step III simultaneously. For example, when the same compound is used for the first hydrophobic polymer and the second hydrophobic polymer, precursor particles, a powder of an inorganic acid salt, and a hydrophobic polymer (the first hydrophobic polymer and the second hydrophobic polymer) may be added and mixed to simultaneously form a hydrophobic polymer phase inside the precursor particles and a coating layer 12 on the surface of the precursor particles. Specifically, for example, a granulated powder is obtained by spray-drying a dispersion of the precursor particles, a powder of an inorganic acid salt, and a hydrophobic polymer. The obtained granulated powder is classified to obtain a negative electrode active material 10 in which a hydrophobic polymer phase is formed inside the composite particles 11 and a coating layer 12 is formed on the surface of the composite particles 11.

[0068] (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. This configuration improves the cycle characteristics of the battery according to embodiment 2, and thus improves the initial charge / discharge efficiency.

[0069] 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.

[0070] 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.

[0071] 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.

[0072] 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.

[0073] 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.

[0074] 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.

[0075] 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.

[0076] 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.

[0077] 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.

[0078] Each component of the battery 100 will be specifically described below.

[0079] 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.

[0080] 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.

[0081] 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.

[0082] 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.

[0083] 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.

[0084] 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.

[0085] 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.

[0086] 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.

[0087] 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.

[0088] 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.

[0089] 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.

[0090] 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.

[0091] 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.

[0092] 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.

[0093] 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.

[0094] (Other Embodiments) (Additional Notes) The above description of the embodiments discloses the following techniques.

[0095] (Technology 1) A negative electrode active material comprising: composite particles; and a coating layer covering at least a portion of a surface of the composite particles, wherein the composite particles include a particulate silicon-containing phase and a phase of a first hydrophobic polymer present between the silicon-containing phases, and the coating layer includes an inorganic acid salt and a second hydrophobic polymer.

[0096] With this configuration, the negative electrode active material according to Technology 1 can improve the charge / discharge cycle characteristics of the battery.

[0097] (Technology 2) The negative electrode active material according to Technology 1, wherein the silicon-containing phase has an average particle size of 3 μm or less.

[0098] With this configuration, the negative electrode active material according to Technique 2 can further improve the charge / discharge cycle characteristics of the battery.

[0099] (Technology 3) The negative electrode active material according to Technology 1 or 2, wherein a mass ratio of the total content of the first hydrophobic polymer and the second hydrophobic polymer to the content of the inorganic acid salt is 0.3 or more and 10 or less.

[0100] With this configuration, the negative electrode active material according to Technology 3 can further improve the charge / discharge cycle characteristics of the battery.

[0101] (Technology 4) The negative electrode active material according to any one of Technologies 1 to 3, wherein a mass ratio of a mass of the inorganic acid salt contained in the coating layer to a mass of a portion excluding the first hydrophobic polymer from the composite particle is 2 mass% or more and 30 mass% or less.

[0102] With this configuration, the negative electrode active material according to Technology 4 can further improve the charge / discharge cycle characteristics of the battery.

[0103] (Technology 5) The negative electrode active material according to any one of Technologies 1 to 4, wherein a mass ratio of a total mass of the first hydrophobic polymer contained in the composite particle and a mass of the second hydrophobic polymer contained in the coating layer to a mass of a portion of the composite particle excluding the first hydrophobic polymer is 2 mass% or more and 40 mass% or less.

[0104] With this configuration, the negative electrode active material according to Technique 5 can further improve the charge / discharge cycle characteristics of the battery.

[0105] (Technology 6) The negative electrode active material according to any one of Technologies 1 to 5, wherein the inorganic acid salt includes at least one selected from the group consisting of lithium methanesulfonate, lithium ethanesulfonate, and lithium propanesulfonate.

[0106] With this configuration, the negative electrode active material according to Technology 6 can further improve the charge / discharge cycle characteristics of the battery.

[0107] (Technology 7) The negative electrode active material according to any one of Technologies 1 to 6, wherein at least one selected from the group consisting of the first hydrophobic polymer and the second hydrophobic polymer includes a fluorine-containing hydrophobic polymer.

[0108] With this configuration, the negative electrode active material according to Technology 7 can further improve the charge / discharge cycle characteristics of the battery.

[0109] (Technology 8) The negative electrode active material according to Technology 7, 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.

[0110] With this configuration, the negative electrode active material according to Technology 8 can further improve the charge / discharge cycle characteristics of the battery.

[0111] (Technology 9) The negative electrode active material according to any one of Technologies 1 to 8, wherein the silicon-containing phase is a silicon phase.

[0112] With this configuration, the negative electrode active material according to Technology 9 can achieve a high capacity in addition to improving the charge / discharge cycle.

[0113] (Technology 10) The negative electrode active material according to any one of Technologies 1 to 9, wherein the silicon-containing phase is formed of a composite material, and the composite material includes an ion-conducting phase and a silicon phase dispersed in the ion-conducting phase.

[0114] With this configuration, the negative electrode active material according to Technique 10 can further improve the charge / discharge cycle characteristics of the battery.

[0115] (Technology 11) The negative electrode active material according to Technology 10, wherein the ion-conducting phase includes at least one selected from the group consisting of an aluminate phase, a silicate phase, a silicon oxide phase, and a carbon phase.

[0116] With this configuration, the negative electrode active material according to Technique 11 can further improve the charge / discharge cycle characteristics of the battery.

[0117] (Technology 12) The negative electrode active material according to any one of Techniques 1 to 11, wherein the composite particles have an average particle size of 3 μm or more and 15 μm or less.

[0118] With this configuration, the negative electrode active material according to Technique 12 can further improve the charge / discharge cycle characteristics of the battery.

[0119] (Technology 13) A battery comprising: a negative electrode containing the negative electrode active material according to any one of Technologies 1 to 12; a positive electrode; and an electrolyte.

[0120] With this configuration, the battery according to Technique 13 can improve the charge / discharge cycle characteristics.

[0121] 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.

[0122] <Preparation of Negative Electrode Active Material> [Example 1] Commercially available silicon particles (average particle size (D50): 1 μm) were prepared as precursor particles for composite particles. These silicon particles, lithium methanesulfonate (MSL) as an inorganic acid salt, and PVDF as a hydrophobic polymer were dispersed in water to prepare a dispersion. The amount of MSL added to the precursor particles was 10 mass%, and the amount of PVDF added to the precursor particles was 10 mass%. The dispersion was then spray-dried to obtain a granulated powder. The spray-drying apparatus used was a "Mini Spray Dryer S-300" manufactured by Nihon Buchi Co., Ltd. The obtained granulated powder was classified. By the above method, PVDF was penetrated into the interior of the precursor particles (silicon particles) to form a hydrophobic polymer phase, and a coating layer containing MSL and PVDF was also formed. This resulted in the negative electrode active material of Example 1. Note that in this example, the precursor particles correspond to the portion of the composite particles in the completed negative electrode active material excluding the first hydrophobic polymer. Therefore, the amount of MSL added to the precursor particles corresponds to the mass ratio of the inorganic acid salt contained in the coating layer to the mass of the composite particles excluding the first hydrophobic polymer, and the amount of PVDF added to the precursor particles corresponds to the sum of the mass of the first hydrophobic polymer contained in the composite particles and the mass of the second hydrophobic polymer contained in the coating layer to the mass of the composite particles excluding the first hydrophobic polymer.

[0123] In this manner, the negative electrode active material of Example 1 was prepared. Surface analysis of the prepared negative electrode active material by XPS confirmed that PVDF was present not only on the surface (i.e., the coating layer) but also inside the particles. Furthermore, a cross section of the negative electrode active material was prepared using a JEOL SM-09010 (Cross Section Polisher), and elemental mapping analysis of the cross section was performed by SEM-EDX. This confirmed that PVDF was present between silicon phases (silicon-containing phases).

[0124] [Example 2] A negative electrode active material was formed in the same manner as in Example 1, except that the amounts of MSL and PVDF added to the precursor particles were changed as shown in Table 1. Thus, the negative electrode active material of Example 2 was produced. Surface analysis of the produced negative electrode active material was performed by XPS in the same manner as in Example 1, and it was confirmed that PVDF was present not only on the surface (i.e., the coating layer) but also inside the particles. Furthermore, element mapping analysis was performed by SEM-EDX in the same manner as in Example 1 on a cross section of the negative electrode active material, and it was also confirmed that PVDF was present between silicon phases (silicon-containing phases).

[0125] [Example 3] A negative electrode active material was formed in the same manner as in Example 1, except that the amounts of MSL and PVDF added to the precursor particles were changed as shown in Table 1. Thus, the negative electrode active material of Example 3 was produced. Surface analysis of the produced negative electrode active material was performed by XPS in the same manner as in Example 1, and it was confirmed that PVDF was present not only on the surface (i.e., the coating layer) but also inside the particles. Furthermore, element mapping analysis was performed by SEM-EDX in the same manner as in Example 1 on a cross section of the negative electrode active material, and it was also confirmed that PVDF was present between silicon phases (silicon-containing phases).

[0126] [Example 4] A negative electrode active material was formed in the same manner as in Example 1, except that the amounts of MSL and PVDF added to the precursor particles were changed as shown in Table 1. Thus, the negative electrode active material of Example 4 was produced. Surface analysis of the produced negative electrode active material was performed by XPS in the same manner as in Example 1, and it was confirmed that PVDF was present not only on the surface (i.e., the coating layer) but also inside the particles. Furthermore, elemental mapping analysis was performed by SEM-EDX in the same manner as in Example 1 on a cross section of the negative electrode active material, and it was also confirmed that PVDF was present between silicon phases (silicon-containing phases).

[0127] Example 5 A negative electrode active material was formed in the same manner as in Example 1, except that the amounts of MSL and PVDF added to the precursor particles were changed as shown in Table 1. Thus, the negative electrode active material of Example 5 was produced. Surface analysis of the produced negative electrode active material was performed by XPS in the same manner as in Example 1, and it was confirmed that PVDF was present not only on the surface (i.e., the coating layer) but also inside the particles. Furthermore, elemental mapping analysis of a cross section of the negative electrode active material was performed by SEM-EDX in the same manner as in Example 1, and it was also confirmed that PVDF was present between silicon phases (silicon-containing phases).

[0128] Example 6 A negative electrode active material was formed in the same manner as in Example 1, except that commercially available silicon particles (average particle size (D50): 0.4 μm) were prepared as precursor particles for the composite particles, and the amounts of MSL and PVDF added to the precursor particles were changed as shown in Table 1. In this way, the negative electrode active material of Example 6 was produced. The produced negative electrode active material was subjected to surface analysis by XPS in the same manner as in Example 1, and it was confirmed that PVDF was present not only on the surface (i.e., the coating layer) but also inside the particles. Furthermore, element mapping analysis by SEM-EDX was performed on a cross section of the negative electrode active material in the same manner as in Example 1, and it was also confirmed that PVDF was present between silicon phases (silicon-containing phases).

[0129] Comparative Example 1 Precursor particles were prepared in the same manner as in Example 1. A hydrophobic polymer phase and a coating layer were not formed, that is, the precursor particles were used as they were as the negative electrode active material.

[0130] Comparative Example 2 Precursor particles similar to those in Example 1 were prepared. PVDF was not added, and only MSL was added so that the amount of MSL added relative to the precursor particles was the amount shown in Table 1. Except for these points, a coating layer was formed in the same manner as in Example 1. This produced the negative electrode active material of Comparative Example 2.

[0131] Comparative Example 3 Precursor particles similar to those in Example 1 were prepared. No MSL was added, and only PVDF was added so that the amount of PVDF added relative to the precursor particles was the amount shown in Table 1. Except for these points, a hydrophobic polymer phase and a coating layer were formed in the same manner as in Example 1. This produced the negative electrode active material of Comparative Example 3.

[0132] Comparative Example 4 Precursor particles were prepared in the same manner as in Example 6. A hydrophobic polymer phase and a coating layer were not formed, that is, the precursor particles were used as they were as the negative electrode active material.

[0133] Comparative Example 5 Precursor particles similar to those in Example 6 were prepared. These precursor particles and MSL were dispersed in water to prepare a dispersion. The amount of MSL added relative to the precursor particles was 15% by mass. This dispersion was spray-dried using the same apparatus as in Example 1 to obtain a granulated powder. Next, the obtained granulated powder and powdered PVDF were mixed so that the amount of PVDF added relative to the precursor particles was 15% by mass. The resulting mixture was fired in an inert atmosphere at a temperature equal to or higher than the melting point of PVDF, thereby preparing the negative electrode active material of Comparative Example 5. Surface analysis of the prepared negative electrode active material was performed using XPS in the same manner as in Example 1, and it was confirmed that PVDF was present only on the surface (i.e., the coating layer) and not inside the particles. Furthermore, elemental mapping analysis of a cross section of the negative electrode active material using SEM-EDX in the same manner as in Example 1 also confirmed that PVDF was not present between the silicon phase (silicon-containing phase).

[0134] <Fabrication of Batteries> Using the negative electrode active materials of Examples 1 to 6 and Comparative Examples 1 to 5, batteries of Examples 1 to 6 and Comparative Examples 1 to 5 were fabricated as follows.

[0135] [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 = 10:90 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 on a portion of the negative electrode.

[0136] [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.

[0137] [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.

[0138] [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.

[0139] <Battery Evaluation> The batteries of Examples 1 to 6 and Comparative Examples 1 to 5 were evaluated for initial charge-discharge efficiency and charge-discharge cycle characteristics by the following methods.

[0140] (Initial charge / discharge efficiency) The batteries of Examples 1 to 6 and Comparative Examples 1 to 5 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.

[0141] The results are shown in Table 1. In Table 1, for Examples 1 to 5 and Comparative Examples 1 to 3, the relative values ​​are shown when the charge / discharge efficiency of Comparative Example 1 is taken as the reference (100%). Furthermore, for Example 6 and Comparative Examples 4 and 5, the relative values ​​are shown when the charge / discharge efficiency of Comparative Example 4 is taken as the reference (100%).

[0142] (Charge-Discharge Cycle Characteristics) The batteries of Examples 1 to 6 and Comparative Examples 1 to 5 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 100 cycles were performed, and the discharge capacity retention rate was calculated using the following formula: Discharge capacity retention rate = (discharge capacity at 100th cycle / discharge capacity at 1st cycle) × 100

[0143] 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 5 and Comparative Examples 1 to 3, relative values ​​are shown when the capacity retention rate of Comparative Example 1 is set as the reference (100%). Furthermore, for Example 6 and Comparative Examples 4 and 5, relative values ​​are shown when the capacity retention rate of Comparative Example 4 is set as the reference (100%).

[0144]

[0145] (Discussion) As shown in Table 1, the negative electrode active materials of Examples 1 to 5, in which the hydrophobic polymer PVDF was present not only in the coating layer but also between the silicon phases inside the composite particles, exhibited improved battery capacity retention (charge-discharge cycle characteristics) compared to the negative electrode active materials of Comparative Examples 1 to 3, which did not satisfy such a configuration. Similarly, the negative electrode active material of Example 6 exhibited improved battery capacity retention (charge-discharge cycle characteristics) compared to the negative electrode active materials of Comparative Examples 4 and 5. Furthermore, the negative electrode active materials of Examples 1 to 5 also exhibited improved initial charge-discharge efficiency compared to the reference negative electrode active material of Comparative Example 1. Furthermore, when the negative electrode active material of Example 1 was compared to the negative electrode active material of Comparative Example 2, which was provided with a coating layer (a coating layer formed only of MSL) and had the same MSL addition amount of 10 mass%, Example 1 exhibited improved initial charge-discharge efficiency and significantly improved capacity retention. The negative electrode active material of Example 6 also had improved initial charge-discharge efficiency compared to Comparative Example 4, which was used as a reference, and to Comparative Example 5, in which PVDF was contained in the coating layer but not inside the composite particles. That is, the negative electrode active material of Example 6 had improved charge-discharge cycle characteristics and initial charge-discharge efficiency compared to the negative electrode active materials of Comparative Examples 4 and 5, which had the same average particle size of the composite particles.

[0146] The technology of the present disclosure is useful for batteries such as lithium ion secondary batteries.

Claims

1. A negative electrode active material comprising: composite particles; and a coating layer covering at least a portion of the surface of the composite particles, wherein the composite particles include a particulate silicon-containing phase and a phase of a first hydrophobic polymer present between the silicon-containing phases, and the coating layer includes an inorganic acid salt and a second hydrophobic polymer.

2. The negative electrode active material according to claim 1, wherein the silicon-containing phase has an average particle size of 3 μm or less.

3. The negative electrode active material according to claim 1, wherein a mass ratio of the total content of the first hydrophobic polymer and the second hydrophobic polymer to the content of the inorganic acid salt is 0.3 or more and 10 or less.

4. The negative electrode active material according to claim 1, wherein a mass ratio of the mass of the inorganic acid salt contained in the coating layer to the mass of the portion of the composite particle excluding the first hydrophobic polymer is 2 mass % or more and 30 mass % or less.

5. The negative electrode active material according to claim 1, wherein the mass ratio of the sum of the mass of the first hydrophobic polymer contained in the composite particle and the mass of the second hydrophobic polymer contained in the coating layer to the mass of the portion of the composite particle excluding the first hydrophobic polymer is 2 mass% or more and 40 mass% or less.

6. The negative electrode active material according to claim 1, wherein the inorganic acid salt includes at least one selected from the group consisting of lithium methanesulfonate, lithium ethanesulfonate, and lithium propanesulfonate.

7. The negative electrode active material according to claim 1, wherein at least one selected from the group consisting of the first hydrophobic polymer and the second hydrophobic polymer includes a fluorine-containing hydrophobic polymer.

8. The negative electrode active material according to claim 7, 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.

9. The negative electrode active material according to claim 1, wherein the silicon-containing phase is a silicon phase.

10. The negative electrode active material according to claim 1, wherein the silicon-containing phase is formed of a composite material, and the composite material includes an ion-conducting phase and a silicon phase dispersed within the ion-conducting phase.

11. The negative electrode active material according to claim 10, wherein the ion-conducting phase includes at least one selected from the group consisting of an aluminate phase, a silicate phase, a silicon oxide phase, and a carbon phase.

12. The negative electrode active material according to claim 1, wherein the composite particles have an average particle size of 3 μm or more and 15 μm or less.

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

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