Negative-electrode active material and battery

Abstract

A negative-electrode active material 10 according to the present disclosure comprises composite particles 1 and a covering layer 4 which covers at least a part of the surface of each composite particle 1, wherein the composite particle 1 comprises an alkali aluminate phase 2 including at least one alkali metal element and Al and a silicon phase 3 dispersed in the alkali aluminate phase 2, and the covering layer 4 comprises a lithium sulfonate compound and a hydrophobic polymer compound.

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-containing materials has been attracting attention as negative electrode active materials with high theoretical capacity densities. Known conventional 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] Furthermore, Patent Document 1 discloses a negative electrode active material for a secondary battery that includes composite particles containing a lithium aluminate phase and a silicon phase dispersed within the lithium aluminate phase. The negative electrode active material disclosed in Patent Document 1 includes composite particles (hereinafter sometimes referred to as "LAX particles") in which a silicon phase is dispersed within a lithium aluminate phase that has excellent alkali resistance, thereby reducing side reactions that occur in the early stages of charge and discharge in a lithium ion secondary battery and suppressing a decrease in the initial charge and discharge efficiency.

[0004] International Publication No. 2022 / 113500

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

[0006] However, negative electrode active materials containing LAX particles have a problem in that the lithium aluminate phase is eroded due to a side reaction between the composite particles and the electrolyte, resulting in degradation of the negative electrode active material. Therefore, there is a demand for improving the cycle characteristics of batteries.

[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 an alkali aluminate phase containing at least one alkali metal element and Al, and a silicon phase dispersed within the alkali aluminate phase, and the coating layer includes a lithium sulfonate compound and a hydrophobic polymer compound.

[0008] According to the technology of the present disclosure, it is possible to provide a negative electrode active material that can improve the cycle characteristics of a battery.

[0009] Fig. 1 is a cross-sectional view showing a schematic configuration of an example of a negative electrode active material according to the present embodiment. Fig. 2 is a cross-sectional view showing a schematic configuration of another example of a negative electrode active material according to the present embodiment. Fig. 3 is a cross-sectional view showing a schematic configuration of an example of a battery according to the present embodiment. Fig. 4 is a cross-sectional view showing a schematic configuration of another example of a battery according to the present embodiment.

[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] (Negative Electrode Active Material) Fig. 1 is a cross-sectional view showing a schematic configuration of one example of a negative electrode active material according to this embodiment. The negative electrode active material 10 includes a composite particle 1 and a coating layer 5 that coats at least a portion of the surface of the composite particle 1. The composite particle 1 includes an alkali aluminate phase 2 and a silicon phase 3 dispersed within the alkali aluminate phase 2. The alkali aluminate phase 2 includes at least one alkali metal element and Al. The coating layer 5 includes a lithium sulfonate compound and a hydrophobic polymer compound.

[0012] By covering at least a portion of the surface of the composite particle 1 with the coating layer 5, contact between the composite particle 1 and the electrolyte is prevented, thereby suppressing a side reaction between the negative electrode active material 10 and the electrolyte. This suppresses erosion of the alkali aluminate phase 2 associated with the side reaction, thereby suppressing deterioration of the negative electrode active material 10 due to the erosion. This therefore improves the cycle characteristics of the battery, and further improves the charge / discharge efficiency.

[0013] By including the lithium sulfonate compound in the coating layer 5, the coating layer 5 has good ionic conductivity, thereby suppressing side reactions in the battery. By including the hydrophobic polymer compound in the coating layer 5, the retention of the lithium sulfonate compound on the surface of the composite particle 1 is improved, and the surface of the composite particle 1 is effectively coated with the lithium sulfonate compound. This significantly suppresses side reactions between the negative electrode active material 10 and the electrolyte. Therefore, the cycle characteristics of the battery and the charge / discharge efficiency are improved.

[0014] The composite particle 1 may contain other elements in addition to the alkali aluminate phase 2 and the silicon phase 3. For example, the composite particle 1 may further contain a conductive layer.

[0015] FIG. 2 is a cross-sectional view showing a schematic configuration of another example of the negative electrode active material according to this embodiment. The negative electrode active material 20 includes a composite particle 6 and a coating layer 5 that coats at least a portion of the surface of the composite particle 6. The composite particle 6 includes an alkali aluminate phase 2 and a silicon phase 3 dispersed within the alkali aluminate phase 2. The composite particle 6 further includes a conductive layer 4 that coats at least a portion of the surface of the particle composed of the alkali aluminate phase 2 and the silicon phase 3. The conductive layer 4 is located between the particle composed of the alkali aluminate phase 2 and the silicon phase 3 and the coating layer 5. Hereinafter, the particle composed of the alkali aluminate phase 2 and the silicon phase 3 will be referred to as the "mother particle." In other words, the conductive layer 4 coats at least a portion of the surface of the mother particle. This configuration improves conductivity.

[0016] Each component of the negative electrode active material of this embodiment will be specifically described below.

[0017] [Composite Particle] The following description of the composite particle can be applied to both the composite particle 1 shown in FIG. 1 and the composite particle 6 shown in FIG.

[0018] The composite particles have, for example, a sea-island structure in which multiple fine silicon phases 3 are dispersed in a matrix of an alkali aluminate phase 2. As a result, the alkali aluminate phase 2 alleviates stress associated with the expansion and contraction of the silicon phase 3 during charge and discharge, suppressing cracking and fracture of the composite particles. Therefore, it is possible to achieve both high capacity and improved cycle characteristics by including silicon. The composite particles include, for example, mother particles composed of the alkali aluminate phase 2 and the silicon phase 3, and a conductive layer 4 covering at least a portion of the surface of the mother particle. Alternatively, the composite particles may consist of only the mother particles.

[0019] The average particle size of the composite particles may be 1 μm or more and 25 μm or less, or 4 μm or more and 15 μm or less. This configuration facilitates alleviating stress caused by volumetric changes in the composite particles during charging and discharging, making it easier to obtain good cycle characteristics. Furthermore, by providing the composite particles with an appropriate surface area, capacity loss due to side reactions with the electrolyte is suppressed.

[0020] The average particle size of the composite particles means the particle size (volume average particle size) at which the volume cumulative value is 50% in the particle size distribution measured by laser diffraction scattering. For example, an "LA-750" manufactured by Horiba, Ltd. can be used as the measuring device. When the surface of the base particle is covered with a conductive layer 4, the thickness of the conductive layer 4 is thin enough not to substantially affect the average particle size of the composite particles, and therefore the average particle size of the composite particles 1 and the average particle size of the composite particles 6 having the conductive layer 4 can be considered to be substantially the same.

[0021] The composite particles can be removed from the battery by, for example, the following method, which is merely an example.

[0022] 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).

[0023] The alkali aluminate phase 2 contains at least one alkali metal element, aluminum (Al), and oxygen (O). The alkali aluminate phase 2 is a phase containing alkali aluminate, which is a composite oxide containing at least one alkali metal element and Al.

[0024] The alkali aluminate phase 2 has good ionic conductivity, and ions such as Li ions can be smoothly absorbed and released by the silicon phase 3 via the alkali aluminate phase 2. In addition, the alkali aluminate phase 2 alleviates the effects of expansion and contraction of the silicon phase 3.

[0025] The alkali aluminate phase 2 has better alkali resistance than the lithium silicate phase in LSX. Therefore, in the negative electrode active material of this embodiment, side reactions with Li ions during initial charging, for example, are suppressed compared to a negative electrode active material using LSX, and deterioration of the negative electrode active material due to side reactions is suppressed.

[0026] The atomic ratio of O to Al (O / Al) in the alkali aluminate is, for example, 1.6 or more and 4 or less. The atomic ratio of alkali metal (MA) to Al (MA / Al) in the alkali aluminate is, for example, 1 / 5 or more and 5 or less. When these atomic ratios are within the above ranges, the stability and ionic conductivity of the alkali aluminate phase 2 are improved. In this specification, the stability of the alkali aluminate phase 2 includes both the chemical stability (alkali resistance) and thermal stability of the alkali aluminate phase 2.

[0027] The alkali metal element is an element belonging to Group 1 of the periodic table. That is, the alkali aluminate phase 2 contains at least one element selected from the group consisting of Li, Na, K, Rb, Cs, and Fr.

[0028] The alkali metal element may include at least one selected from the group consisting of Li, Na, and K. The alkali metal element may include at least one selected from the group consisting of Li and Na, or may include Li.

[0029] The alkali aluminate phase 2 may contain two types of alkali metal elements. The alkali aluminate phase 2 may contain at least two selected from the group consisting of Li, Na, and K, or may contain Li and Na.

[0030] The alkali aluminate phase 2 may further contain an element M in addition to the alkali metal element, Al, and O. An example of the element M is at least one selected from the group consisting of calcium (Ca), magnesium (Mg), zirconium (Zr), iron (Fe), boron (B), phosphorus (P), and lanthanum (La).

[0031] The element M may form a compound. The compound may be, for example, an oxide of the element M or an aluminate of the element M depending on the type of the element M.

[0032] The element M may be B. That is, the alkali aluminate phase 2 may further contain B. The addition of the element M, for example, improves the stability and ionic conductivity of the alkali aluminate phase 2. Furthermore, the addition of the element M can reduce the porosity of the composite particle 1. This reduces the number of areas that can become the starting points for cracks and fractures when charging and discharging are repeated, thereby suppressing deterioration of the negative electrode active material 10 and further improving the cycle characteristics of the battery.

[0033] The alkali aluminate phase 2 may further contain trace amounts of elements such as chromium (Cr), nickel (Ni), manganese (Mn), copper (Cu), and molybdenum (Mo).

[0034] The alkali aluminate phase 2 may be amorphous. In this case, the influence of the expansion and contraction of the silicon phase 3 can be more effectively alleviated.

[0035] A fine Al2O3 phase with high crystallinity may be dispersed within the alkali aluminate phase 2. The Al2O3 phase is, for example, distributed in the form of islands within the matrix of the alkali aluminate phase 2. In this case, expansion and cracking of the alkali aluminate phase 2 due to expansion and contraction of the silicon phase 3 are more likely to be suppressed, enhancing the effect of improving cycle characteristics. When the Al2O3 phase is present, a peak derived from the Al2O3 phase may be observed near 2θ = 25.4° in the X-ray diffraction pattern of the composite particles obtained by X-ray diffraction measurement. The Al2O3 content in the composite particles is, for example, 10 mass% or less.

[0036] The silicon phase 3 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 3. The silicon phase 3 has a large capacity. Furthermore, the silicon phase 3 also expands and contracts to a large extent as the battery is charged and discharged. However, in the negative electrode active material of the present disclosure, the silicon phase 3 is dispersed within the alkali aluminate phase 2, and therefore the stress caused by the expansion and contraction of the silicon phase 3 is alleviated by the alkali aluminate phase 2.

[0037] The silicon phase 3 may be particulate. The silicon phase 3 is particulate, for example, at least before the first charge. The average particle size of the silicon phase 3 may be 1 nm or more and 1000 nm or less. The average particle size of the silicon phase 3 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 3 may be 400 nm or less, or 100 nm or less. By dispersing the fine silicon phase 3 in the alkali aluminate phase 2 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.

[0038] The average particle size of the silicon phase 3 can be measured using a scanning electron microscope (SEM) image obtained by SEM observation of a cross section of the negative electrode active material in which the cross section of the silicon phase 3 is exposed. Specifically, the average particle size of the silicon phase 3 is determined by averaging the maximum diameters of 100 silicon phases 3 arbitrarily selected from the cross-sectional SEM image of the negative electrode active material.

[0039] The silicon phase 3 may contain crystalline silicon. The silicon phase 3 is composed of, for example, a plurality of crystallites. The crystallite size of the silicon phase 3 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 3 during charge and discharge can be reduced, and the effect of improving cycle characteristics becomes more significant. The crystallite size of the silicon phase 3 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.

[0040] The lower limit of the crystallite size of the silicon phase 3 is not particularly limited, but is, for example, 1 nm. An example of a suitable crystallite size of the silicon phase 3 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 3 is 1 nm or more, for example, the surface area of ​​the silicon phase 3 can be kept small, making it difficult for deterioration of the silicon phase 3, 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 3 uniform, and stress generated in the negative electrode active material is effectively alleviated.

[0041] From the viewpoint of increasing capacity, the content of silicon phase 3 in the composite particles may be 30% by mass or more, 35% by mass or more, or 55% by mass or more. From the viewpoint of improving cycle characteristics, the content of silicon phase 3 in the composite particles 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 3 exposed on the surface of the composite particles without being covered by the alkali aluminate phase 2 is reduced, and side reactions between the electrolyte and the silicon phase 3 are also suppressed. The content of silicon phase 3 in the composite particles 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.

[0042] The content of the silicon phase 3 in the composite particle can be determined by quantifying the amount of Si constituting the silicon phase 3 in the composite particle using Si-NMR, as will be described later.

[0043] The composite particles may be substantially free of lithium silicate and SiO2. The total content of lithium silicate and SiO2 in the composite particles may be, for example, 3 mass% or less.

[0044] The mass ratio (mAl) of Al to the total mass of elements other than oxygen (O) constituting the alkali aluminate phase 2 and the silicon phase 3 may be 10 mass% or more and 47 mass% or less, or may be 11.5 mass% or more and 45.5 mass% or less.

[0045] The mass ratio (mMA) of the alkali metal element to the total mass of elements other than O constituting the alkali aluminate phase 2 and the silicon phase 3 may be 0.7 mass% or more and 13.5 mass% or less, 1.0 mass% or more and 9.5 mass% or less, or 1.5 mass% or more and 3.5 mass% or less.

[0046] The above configuration can improve the stability and ionic conductivity of the alkali aluminate phase 2. The stability mentioned above includes both chemical stability (alkali resistance) and thermal stability.

[0047] The ratio (mMA / mAl) of the mass ratio of alkali metal elements (mMA) to the mass ratio of Al (mAl) may be 0.01 or more and 0.50 or less, or 0.05 or more and 0.25 or less, from the viewpoint of the stability and ionic conductivity of the alkali aluminate phase 2.

[0048] The mass ratio of Si (mSi) to the total mass of elements other than O constituting the alkali aluminate phase 2 and the silicon phase 3 may be 40 mass % or more and 90 mass % or less, or 50.8 mass % or more and 85.5 mass % or less. With the above configuration, the battery can easily achieve both high capacity and good cycle characteristics.

[0049] When the alkali aluminate phase 2 further contains B, the mass ratio (mB) of B to the total mass of elements other than O constituting the alkali aluminate phase 2 and the silicon phase 3 may be 1 mass % or more and 20 mass % or more, or 2 mass % or more and 15 mass % or less. This configuration can make the effect of improving cycle characteristics more significant.

[0050] The ratio (mAl / mB) of the mass ratio of Al (mAl) to the mass ratio of B (mB) may be 1.0 or more and 30.0 or less, 1.0 or more and 20.0 or less, or 1.0 or more and 10.0 or less. When mAl / mB is 30.0 or less, the effect of reducing the porosity of the composite particle 1 can be more significant. This suppresses deterioration of the particle structure caused by voids within the composite particle, thereby effectively improving the cycle characteristics of the battery. When mAl / mB is 1.0 or more, it is possible to reduce the decrease in initial charge / discharge efficiency. Therefore, when mAl / mB is within the above range, it is possible to achieve both improved cycle characteristics and good initial charge / discharge efficiency.

[0051] The conductive layer 4 covers at least a portion of the surface of the base particle composed of the alkali aluminate phase 2 and the silicon phase 3. The conductive layer 4 may cover the entire surface of the base particle. The conductive layer 4 is, for example, a thin film layer containing a conductive material. The inclusion of the conductive layer 4 in the composite particle improves the conductivity.

[0052] The conductive material may be a conductive carbon material. That is, the conductive layer 4 may contain carbon. Examples of the carbon material that can be used include graphite such as natural graphite, artificial graphite, and graphitized mesophase carbon, amorphous carbon, soft carbon, and hard carbon. The carbon material may be amorphous carbon. This makes it easy to form a thin conductive layer 4 that covers the surface of the base particles. Examples of amorphous carbon include carbon black, burned pitch, coke, and activated carbon.

[0053] The thickness of the conductive layer 4 is desirably thin enough not to affect the average particle size of the composite particles. The thickness of the conductive layer 4 may be 1 nm or more and 200 nm or less, or 5 nm or more and 100 nm or less, taking into consideration ensuring conductivity and the diffusibility of ions such as Li ions. The thickness of the conductive layer 4 can be measured by SEM observation or transmission electron microscope (TEM) observation of a cross section of the negative electrode active material in which the cross section of the composite particle is exposed.

[0054] The porosity of the composite particle before the first charge / discharge may be 25% or less. Since the conductive layer 4 does not affect the porosity, the porosity of the composite particle 6 is substantially the same as that of the base particle (composite particle 1). By keeping the porosity of the composite particle at 25% or less, the number of areas that can become the starting point for cracks and breaks when charging and discharging is repeated is reduced. This suppresses deterioration of the negative electrode active material and further improves the cycle characteristics of the battery. The porosity of the composite particle may be 20% or less, or may be 15% or less. The lower limit of the porosity is not particularly limited, but an example is 1%.

[0055] The porosity of the composite particle means the proportion of voids in the cross section of the composite particle. The porosity of the composite particle can be measured using an SEM image of the cross section of the negative electrode active material in which the cross section of the composite particle is exposed. The porosity of the composite particle is calculated by extracting the void area by binarizing the SEM image using image analysis software (e.g., ImageJ), and dividing the total area of ​​the voids by the total area of ​​the particle cross section.

[0056] The porosity of the composite particles can be controlled to some extent by adjusting the firing temperature in the manufacturing process of the composite particles, the compressive force applied to the particles during firing, mMA / mAl, etc., in addition to the addition of B to the alkali aluminate phase 2 described above.

[0057] The composite particle may have a Vickers hardness of 300 HV or more. Since the conductive layer 4 does not affect the Vickers hardness, the Vickers hardness of the composite particle 6 is substantially the same as that of the mother particle (composite particle 1). When the composite particle has a high Vickers hardness, it is easy to suppress the volume change of the silicon phase 3 during charge and discharge, and deterioration of the particle structure can be reduced. As a result, the effect of improving the cycle characteristics becomes more significant. The Vickers hardness of the composite particle is more preferably 350 HV or more, and may be 400 HV or more, or 500 HV or more.

[0058] The Vickers hardness of the composite particles can be measured using a Vickers hardness tester. Specifically, the composite particles from which the coating layer 5 has been removed from the negative electrode active material are embedded in a thermosetting resin and polished with No. 400 abrasive paper to expose the cross section of the composite particles 1. The cross section is then polished to a mirror finish using No. 2000 abrasive paper and buff polishing. The Vickers hardness is measured under conditions of a load of 1 kg and a holding time of 15 seconds. The upper limit of the Vickers hardness of the composite particles 1 is not particularly limited, but is, for example, 1500 HV.

[0059] The content of each element in the composite particles is measured by the following methods. The oxygen content can be measured using an oxygen, nitrogen, and hydrogen analyzer. The Si content can be measured using NMR. The carbon content can be measured using a carbon and sulfur analyzer. The contents of other elements can be measured by inductively coupled plasma optical emission spectroscopy. If measurement by inductively coupled plasma optical emission spectroscopy (ICP) is not possible, measurement can also be performed using energy dispersive X-ray (EDX). If measurement by neither ICP nor EDX is possible, measurement can also be performed by Auger electron spectroscopy (AES). The composition of the alkali aluminate phase 2 can be determined from the content of each element. When measuring from the state of the battery, for example, measurement can be performed by disassembling a fully discharged battery, removing and cleaning the negative electrode, removing the non-aqueous electrolyte component, drying, and then analyzing the cross section of the resulting negative electrode mixture layer using a cross-section polisher.

[0060] <Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP)> A sample of composite particles is completely dissolved in a heated acid solution (a mixed acid of hydrofluoric acid, nitric acid, and sulfuric acid), and the carbon remaining in the solution is filtered off. The resulting filtrate is then analyzed by ICP to measure the spectral intensity of each element. Next, a calibration curve is created using commercially available standard solutions of the elements, and the content of each element contained in the composite particles is calculated.

[0061] <Energy Dispersive X-ray (EDX)> When measurement cannot be performed using ICP, measurement is performed using EDX. Elemental mapping analysis is performed using EDX from a backscattered electron image of a cross section of the negative electrode active material in which the cross section of the composite particle is exposed. The area containing the target element is calculated using image analysis software. The observation magnification is, for example, 2000 to 20,000 times. When measuring from the battery state, 10 composite particles 1 with a maximum particle diameter of 5 μm or more are randomly selected from the cross-sectional image of the backscattered electron image of the negative electrode mixture layer containing the negative electrode active material, and elemental mapping analysis is performed on each of them using EDX. The measured values ​​of the area containing a specific element contained in the 10 particles are averaged to calculate the content of the target element.

[0062] Desirable measurement conditions for cross-sectional SEM-EDX analysis are shown below: Processing equipment: SM-09010 (Cross Section Polisher) manufactured by JEOL Processing conditions: Acceleration voltage 6 kV Current value: 140 μA Vacuum degree: 1×10 -3 Pa to 2 x 10 -3 Pa Measuring device: Hitachi SU-70 electron microscope Acceleration voltage during analysis: 10 kV Field: Free mode Probe current mode: Medium Probe current range: High Anode Ap.: 3 OBJ Ap.: 2 Analysis area: 1 μm square Analysis software: EDAX Genesis CPS: 20500 Lsec: 50 Time constant: 3.2

[0063] <Auger Electron Spectroscopy (AES)> When neither ICP nor EDX can be used for measurement, AES is used for measurement. Qualitative and quantitative analysis of elements is performed using an AES analyzer (e.g., JAMP-9510F, manufactured by JEOL Ltd.) from a backscattered electron image of a cross section of the negative electrode active material in which the cross section of the composite particle is exposed. Measurement conditions may be, for example, an acceleration voltage of 10 kV, a beam current of 10 nA, and an analysis area of ​​20 μmφ. When measuring from the state of the battery, 10 composite particles with a maximum particle diameter of 5 μm or more are randomly selected from a cross-sectional image of a backscattered electron image of the negative electrode mixture layer containing the negative electrode active material, and qualitative and quantitative analysis of elements is performed on each of them using an AES analyzer. The content of a specified element contained in 10 particles is calculated by averaging the contents.

[0064] During charging and discharging, a coating may be formed on the surface of the composite particle due to decomposition of the non-aqueous electrolyte, etc. As described above, the composite particle may further include a conductive layer. Therefore, the EDX analysis and AES analysis are performed on a range, for example, 1 μm inward from the peripheral edge of the cross section of the composite particle so as not to include the thin coating and conductive layer in the measurement range.

[0065] <Inert Gas Fusion - Non-Dispersive Infrared Absorption Method> The oxygen content of the composite particles can be measured using an oxygen, nitrogen, and hydrogen analyzer (e.g., EGMA-830 manufactured by Horiba, Ltd.). The sample is placed in a Ni capsule and then placed in a carbon crucible heated at 5.75 kW with Sn and Ni pellets as flux, and the released carbon monoxide gas is detected. A calibration curve is created using a standard sample, YO, and the oxygen content of the sample is calculated.

[0066] <Nuclear Magnetic Resonance Spectroscopy (NMR)> The amount of Si constituting the silicon phase 3 can be quantified using Si-NMR. Desirable measurement conditions for Si-NMR are shown below. Measurement apparatus: Solid-state nuclear magnetic resonance spectrometer (INOVA-400) manufactured by Varian Probe: Varian 7 mm CPMAS-2 MAS: 4.2 kHz MAS speed: 4 kHz Pulse: DD (45° pulse + signal acquisition time 1H decoupled) Repetition time: 1200 sec to 3000 sec Observation width: 100 kHz Observation center: near -100 ppm Signal acquisition time: 0.05 sec Number of accumulations: 560 Sample amount: 207.6 mg

[0067] <High-frequency induction heating furnace combustion-infrared absorption method> The carbon content of the composite particles can be measured using a carbon / sulfur analyzer (for example, EMIA-520 model manufactured by Horiba, Ltd.). A sample is weighed out onto a magnetic board, a combustion improver is added, and the board is inserted into a combustion furnace (carrier gas: oxygen) heated to 1350°C, and the amount of carbon dioxide gas generated during combustion is detected by infrared absorption. A calibration curve is prepared using, for example, carbon steel (carbon content 0.49%) manufactured by Bureau of Analyzed Samples Ltd., and the carbon content of the sample is calculated.

[0068] [Coating Layer 5] The coating layer 5 covers at least a part of the surface of the composite particle. The coating layer 5 may cover the entire surface of the composite particle.

[0069] The coating layer 5 contains a lithium sulfonate compound and a hydrophobic polymer compound.

[0070] 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.

[0071] 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.

[0072]

[0073] 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.

[0074] The lithium sulfonate compound may include at least one selected from the group consisting of lithium methanesulfonate, lithium ethanesulfonate, and lithium propanesulfonate. The lithium sulfonate compound may include lithium methanesulfonate.

[0075] The mass ratio of the lithium sulfonate compound in the coating layer 5 to the composite particles (hereinafter referred to as the "amount of the lithium sulfonate compound supported") may be 1 mass% or more. The amount of the lithium sulfonate compound supported may be 1 mass% or more and 10 mass% or less, 1 mass% or more and 6 mass% or less, or 2 mass% or more and 6 mass% or less.

[0076] When the amount of the lithium sulfonate compound supported is 1% by mass or more, the surface of the composite particle can be sufficiently coated with the lithium sulfonate compound, and the effect of suppressing side reactions caused by the lithium sulfonate compound is likely to be achieved. When the amount of the lithium sulfonate compound supported is 6% by mass or less, a low-resistance coating layer 5 and a low-resistance negative electrode active material are likely to be obtained. When forming a coating layer 5 containing a lithium sulfonate compound and a hydrophobic polymer compound, for example, the lithium sulfonate compound is supported on the surface of the composite particle in an amount within the above range.

[0077] The hydrophobic polymer compound may have good binding properties and heat melting properties. In this case, the hydrophobic polymer compound can firmly support the lithium sulfonate compound on the surface of the composite particles, and the effect of suppressing side reactions caused by the lithium sulfonate compound can be stably obtained. The hydrophobic polymer compound is almost insoluble in water.

[0078] From the viewpoint of stability against the electrolyte, the hydrophobic polymer compound may contain a fluororesin. The fluororesin may contain at least one selected from the group consisting of polyvinylidene fluoride (PVDF), polytetrafluoroethylene, perfluoroalkoxyalkane (PFA), perfluoroethylene propene copolymer, ethylene tetrafluoroethylene copolymer, polychlorotrifluoroethylene, and ethylene chlorotrifluoroethylene copolymer. The fluororesin may contain PVDF. When the hydrophobic polymer compound is PVDF, PVDF has good binding properties and a low melting point, so that the coating layer 5 can be formed at a low heat treatment temperature.

[0079] The hydrophobic polymer compound may include, in addition to polyvinylidene fluoride, 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.

[0080] The mass ratio of the hydrophobic polymer compound in the coating layer 5 to the composite particle (hereinafter referred to as the "amount of hydrophobic polymer compound carried") may be 1 mass% or more. The amount of the hydrophobic polymer compound carried may be 1 mass% or more and 10 mass% or less, 1 mass% or more and 6 mass% or less, or 2 mass% or more and 6 mass% or less.

[0081] When the amount of the hydrophobic polymer compound supported is 1% by mass or more, the hydrophobic polymer compound can sufficiently improve the retention of the lithium sulfonate compound on the surface of the composite particles.When the amount of the hydrophobic polymer compound supported is 6% by mass or less, a low-resistance coating layer 5 and a low-resistance negative electrode active material are easily obtained.

[0082] When the coating layer 5 contains a lithium sulfonate compound and a fluororesin (for example, PVDF) as the hydrophobic polymer compound, the supported amounts of the lithium sulfonate compound and the fluororesin are determined by the following method.

[0083] The negative electrode active material is washed with N-methyl-2-pyrrolidone (NMP) to dissolve the fluororesin, and the difference in mass before and after dissolution is determined as the mass of the fluororesin. The remainder that is not dissolved in NMP is then washed with water to dissolve the lithium sulfonate compound. The mass of the lithium sulfonate compound dissolved in water is determined by quantitative analysis such as ICP emission spectroscopy.

[0084] The mass of the composite particles is determined as the mass of the remaining particles that are not dissolved in water or NMP. When the composite particles have a conductive layer 4, the remaining particles that are not dissolved in water or NMP are subjected to a quantitative analysis of carbon using a carbon-sulfur analyzer or the like, and the determined amount of carbon is derived from the carbon material of the conductive layer 4.

[0085] Using the mass of the fluororesin and the mass of the composite particles determined above, the amount of supported fluororesin is calculated according to the formula: (mass of fluororesin / mass of composite particles) × 100. Using the mass of the lithium sulfonate compound and the mass of the composite particles determined above, the amount of supported lithium sulfonate compound is calculated according to the formula: (mass of lithium sulfonate compound / mass of composite particles) × 100.

[0086] The thickness of the coating layer 5 is desirably thin enough not to substantially affect the average particle size of the negative electrode active material. From the viewpoint of protecting the composite particles from the electrolyte, the thickness of the coating layer 5 may be 1 nm or more. From the viewpoint of suppressing an increase in resistance, the thickness of the coating layer 5 may be 300 nm or less. The thickness of the coating layer 5 may be smaller than the thickness of the conductive layer 4. The thickness of the coating layer 5 can be measured by SEM or TEM observation of a cross section of the negative electrode active material.

[0087] When the negative electrode active material is subjected to surface analysis by X-ray photoelectron spectroscopy (XPS), a peak derived from the lithium sulfonate compound is observed in the surface portion (including the outermost surface) of the coating layer 5. Specifically, the peak has a binding energy of approximately 165 eV to 170 eV and an intensity (c / s) of 200 to 1000.

[0088] (Method for Producing Negative Electrode Active Material) The negative electrode active material of the present disclosure is produced, for example, by a production method including step I of obtaining composite particles and step II of forming coating layer 5 on the surface of the composite particles.

[0089] [Step I] Step I includes the following first to fourth steps. Step 1: A step of obtaining a raw material alkali aluminate (hereinafter referred to as "raw aluminate"). Step 2: A step of combining the raw aluminate and raw silicon to obtain a composite intermediate in which the raw silicon is dispersed in the raw aluminate. Step 3: A step of heat-treating the composite intermediate to obtain a sintered body containing an alkali aluminate phase 2 and a silicon phase 3 dispersed within the alkali aluminate phase 2. Step 4: A step of pulverizing the sintered body to obtain composite particles 1 (mother particles).

[0090] The first step includes, for example, a step of mixing an aluminum compound, a compound containing an alkali metal element, and, if necessary, a compound containing element M to obtain a mixture, and a step of calcining the mixture to obtain a raw aluminate. The calcination is 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.

[0091] Examples of the aluminum compound include aluminum oxide, aluminum hydroxide, aluminum carbonate, etc. The aluminum compound may be used alone or in combination of two or more.

[0092] Examples of the compound containing an alkali metal element include lithium carbonate, lithium oxide, lithium hydroxide, lithium hydride, sodium carbonate, sodium oxide, sodium hydroxide, sodium hydride, potassium carbonate, potassium oxide, potassium hydroxide, potassium hydride, etc. The compound containing an alkali metal element may be used alone or in combination of two or more.

[0093] The compound containing element M is, for example, a boron compound. Examples of the boron compound include boron oxide, boric acid, borax, sodium tetraborate, etc. One type of boron compound may be used alone, or two or more types may be used in combination.

[0094] In the first step, aluminum compounds that did not react with the alkali metal-containing compound during the preparation of the raw aluminate may remain in the raw aluminate. When the amount of aluminum compound used is large relative to the amount of alkali metal-containing compound, the aluminum compounds are likely to remain. When the aluminum compound remaining in the raw aluminate is Al2O3, an Al2O3 phase dispersed in the alkali aluminate phase 2 may be formed in the composite particles finally obtained.

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

[0096] The raw silicon is, for example, coarse silicon particles having an average particle size of several μm to several tens of μm. The silicon particles may be prepared so that the crystallite size of the silicon phase 3 is 15 nm or less, calculated by Scherrer's formula from the half-width of the diffraction peak derived from the Si(111) plane in the X-ray diffraction pattern of the negative electrode active material or composite particle 1. It is also possible to synthesize silicon nanoparticles and raw aluminate nanoparticles and mix them together without using a pulverizer.

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

[0098] An example of the firing temperature is 450°C or higher and 1000°C or lower. If the firing temperature is within this range, a structure in which minute silicon phases 3 are dispersed within a low-crystalline alkali aluminate phase 2 is easily formed. The raw aluminate is stable at this temperature and hardly reacts with silicon. The firing temperature may be 550°C or higher and 950°C or lower, or may be 650°C or higher and 900°C or lower. In order to reduce the porosity of the composite particles to 25% or lower, firing at a temperature of 650°C or higher is desirable. The firing time is, for example, 1 hour or higher and 10 hours or lower.

[0099] The fourth step is a step of pulverizing the composite particles 1 (base particles) so that they have a desired particle size distribution, for example, so that the median diameter of the base particles is 1 μm or more and 25 μm or less.

[0100] Step I may further include the following fifth step after steps 1 to 4: Fifth step: a step of forming a conductive layer 4 on the surface of a base particle composed of an alkali aluminate phase 2 and a silicon phase 3.

[0101] As described above, a conductive carbon material is desirable as the conductive material that constitutes the conductive layer 4. Examples of methods for coating the surfaces of the base particles with a carbon material include a CVD method using hydrocarbon gases such as acetylene and methane as raw materials, and a method in which coal pitch, petroleum pitch, phenolic resin, etc. is mixed with the base particles and heated to carbonize them. Carbon black may also be attached to the surfaces of the base particles.

[0102] In the fifth step, for example, the mixture of the base particles and the carbon material is heated in an inert atmosphere (e.g., an argon, nitrogen, or the like) at 700° C. or higher and 950° C. or lower to form the conductive layer 4 on the surface of the base particles. In this manner, composite particles 6 having the conductive layer 4 on the surface of the base particles are obtained.

[0103] [Step II] Step II is a step of forming a coating layer 5 on the surface of the composite particle.

[0104] For example, after granulating the composite particles using the composite particles, a lithium sulfonate compound, and a hydrophobic polymer compound, 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 a coating layer 5 is formed on the surface of the composite particles.

[0105] The granulation is carried out, for example, by dispersing the composite particles, powder of the lithium sulfonate compound, and the hydrophobic polymer compound in water, and then spray-drying the dispersion, thereby forming a coating layer, which is a mixed layer of the lithium sulfonate compound and the hydrophobic polymer compound, on the surface of the composite particles.

[0106] The amount of the lithium sulfonate compound added may be 1% by mass or more and 6% by mass or less relative to the composite particles.The amount of the hydrophobic polymer compound added may be 1% by mass or more and 6% by mass or less relative to the composite particles.

[0107] The particle sizes of the lithium sulfonate compound and the hydrophobic polymer compound dispersed in water are preferably smaller than the particle size of the composite particles. In this case, the surfaces of the composite particles are easily covered uniformly with the lithium sulfonate compound and the hydrophobic polymer compound. The average particle size of the lithium sulfonate compound may be 1 μm or more and 100 μm or less, or 1 μm or more and 10 μm or less. The average particle size of the hydrophobic polymer compound may be 200 nm or more and 1 μm or less.

[0108] The method for forming the coating layer 5 on the surface of the composite particle is not limited to the above. For example, the composite particle, a powder of a lithium sulfonate compound, and a powder of a hydrophobic polymer compound may be dry-mixed to obtain a mixture, and then the mixture may be heat-treated to form the coating layer 5 on the surface of the composite particle.

[0109] The mixing process results in an intermediate (mixture) in which the mixture of the lithium sulfonate compound and the hydrophobic polymer compound adheres to the surface of the composite particles. A ball mill method can be used for the dry mixing process. The amounts and particle sizes of the lithium sulfonate compound and the hydrophobic polymer compound added may be the same as those described above.

[0110] The heat treatment is preferably performed at a temperature above the melting point of the hydrophobic polymer compound and below the decomposition temperature of the hydrophobic polymer compound. When polyvinylidene fluoride (PVDF) is used as the hydrophobic polymer compound, the heat treatment temperature may be above the melting point of PVDF (150°C to 170°C) and below the decomposition temperature (340°C), preferably above 200°C to 250°C. The heat treatment may be performed in an inert atmosphere. The heat treatment time is, for example, about 1 hour to 3 hours. The heat treatment liquefies the hydrophobic polymer compound in the mixture, and penetrates and diffuses around the composite particles and the lithium sulfonate compound particles, filling the gaps between the composite particles and the lithium sulfonate compound particles, as well as the gaps between the lithium sulfonate compound particles themselves. This improves the retention of the lithium sulfonate compound on the surface of the composite particles. In this way, a coating layer 5, which is a mixed layer of the lithium sulfonate compound and the hydrophobic polymer compound, is formed. The mixture after the heat treatment is crushed to obtain a negative electrode active material having a desired average particle size.

[0111] (Battery) The battery according to this embodiment includes a negative electrode, a positive electrode, and an electrolyte. The negative electrode contains the negative electrode active material according to this embodiment. In the battery according to this embodiment, the negative electrode contains the negative electrode active material according to this embodiment, which improves the cycle characteristics and ultimately improves the initial charge / discharge efficiency.

[0112] 3 is a longitudinal cross-sectional view schematically illustrating an example of a battery according to this embodiment. The battery 100 is a cylindrical battery including a cylindrical battery case, a wound electrode group 24, and an electrolyte (not shown). The electrode group 24 is housed in the battery case and is in contact with the electrolyte.

[0113] 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.

[0114] 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.

[0115] 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.

[0116] 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.

[0117] 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.

[0118] 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.

[0119] 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.

[0120] 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.

[0121] Each component of the battery 100 will be specifically described below.

[0122] 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 may include a positive electrode current collector and a positive electrode mixture layer supported on the surface of the positive electrode current collector.

[0123] The positive electrode mixture layer includes a positive electrode active material. Examples of the positive electrode active material include lithium-containing transition metal oxides, lithium-containing transition metal phosphates, transition metal fluorides, polyanionic materials, fluorinated polyanionic materials, transition metal sulfides, transition metal oxysulfides, and transition metal oxynitrides. In particular, using a lithium-containing transition metal oxide or a lithium-containing transition metal phosphate as the positive electrode active material can reduce the manufacturing cost of the battery and increase the average discharge voltage. Examples of lithium-containing transition metal oxides include lithium cobalt oxide, lithium nickel cobalt aluminum oxide, lithium nickel cobalt manganese oxide, and lithium nickel manganese oxide. Examples of lithium-containing transition metal phosphates include lithium iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate, and lithium nickel phosphate. At least one selected from these positive electrode active materials can be used.

[0124] The positive electrode mixture layer may contain a conductive additive, an ion conductor, and a binder, as needed.

[0125] Conductive additives and ion conductors are used to reduce the resistance of the electrodes. Examples of conductive additives include carbon materials and conductive polymer compounds. Examples of carbon materials include carbon black, graphite, acetylene black, carbon nanotubes, carbon nanofibers, graphene, fullerene, and graphite oxide. Examples of conductive polymer compounds include polyaniline, polypyrrole, and polythiophene. At least one selected from these conductive additives can be used.

[0126] Binders are used to improve the binding properties of the materials that make up the electrode. Examples of binders include polymeric materials such as polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, polytetrafluoroethylene, carboxymethyl cellulose, polyacrylic acid, styrene-butadiene copolymer rubber, polypropylene, polyethylene, and polyimide. At least one selected from these binders can be used.

[0127] The positive electrode current collector is a sheet or film made of a metal material such as aluminum, an aluminum alloy, stainless steel, titanium, or a titanium alloy. The sheet or film may be porous or non-porous. Examples of the sheet or film include metal foil and metal mesh. A carbon material may be applied to the surface of the positive electrode current collector as a conductive auxiliary material.

[0128] The negative electrode 23 includes the negative electrode active material according to this embodiment. The negative electrode 23 includes, for example, a negative electrode current collector and a negative electrode mixture layer carried on the surface of the negative electrode current collector.

[0129] 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.

[0130] The negative electrode mixture layer contains the negative electrode active material according to this embodiment. The negative electrode mixture layer may contain other materials such as a conductive additive, an ion conductor, and a binder, as needed. The conductive additive, ion conductor, and binder materials described above for the positive electrode mixture layer can also be used for the negative electrode mixture layer.

[0131] The electrolyte may contain a non-aqueous solvent and a lithium salt dissolved in the non-aqueous solvent. The concentration of the lithium salt in the electrolyte may be, for example, 0.5 mol / L or more and 2 mol / L or less. By controlling the lithium salt concentration within the above range, an electrolyte having excellent ionic conductivity and appropriate viscosity can be obtained. However, the lithium salt concentration is not limited to the above.

[0132] 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.

[0133] 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.

[0134] 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.

[0135] In the battery of the present disclosure, the electrolyte may be impregnated into a polymer provided as a separator, for example, i.e., the battery of the present disclosure may have a structure in which the electrolyte and the polymer are used in combination.

[0136] The battery of the present disclosure may further include a solid electrolyte as an electrolyte. That is, the battery of the present disclosure may have a hybrid structure in which an electrolytic solution and a solid electrolyte are used in combination. Examples of solid electrolyte materials include halide solid electrolytes, sulfide solid electrolytes, oxide solid electrolytes, and organic polymer solid electrolytes. In this disclosure, a "halide solid electrolyte" refers to a solid electrolyte containing a halogen element as the main component of the anions. A "sulfide solid electrolyte" refers to a solid electrolyte containing sulfur as the main component of the anions. An "oxide solid electrolyte" refers to a solid electrolyte containing oxygen as the main component of the anions. The "main component of the anions" refers to the anion with the largest substance amount among all the anions constituting the solid electrolyte.

[0137] In the present disclosure, as an example of the structure of a battery according to the present embodiment, the configuration example shown in FIG. 3 is described, i.e., a cylindrical nonaqueous electrolyte secondary battery in which a wound electrode group formed by winding a positive electrode and a negative electrode with a separator interposed therebetween and an electrolyte solution are housed in an outer casing. However, the battery according to the present disclosure is not limited to this configuration example. The battery according to the present disclosure may have any shape, such as a prismatic shape, a coin shape, a button shape, or a laminate shape. Furthermore, instead of a wound electrode group in the secondary battery according to the present disclosure, an electrode group of another shape, such as an electrode group formed by stacking a positive electrode and a negative electrode with a separator interposed therebetween, may be used.

[0138] 4 is a cross-sectional view showing a schematic configuration of another example of a battery according to the present embodiment. The battery 200 includes a positive electrode 210, an electrolyte layer 220, and a negative electrode 230. The negative electrode 230 contains the negative electrode active material according to the present embodiment. The electrolyte layer 220 is disposed between the positive electrode 210 and the negative electrode 230. The battery 200 is, for example, an all-solid-state battery.

[0139] The positive electrode 210 includes a positive electrode current collector 211 and a positive electrode mixture layer 212 carried on the surface of the positive electrode current collector 211. The positive electrode mixture layer 212 is located between the positive electrode current collector 211 and the electrolyte layer 220. The materials described above for the positive electrode 21 in the battery 100 can be used for the positive electrode current collector 211 and the positive electrode mixture layer 212.

[0140] The positive electrode mixture layer 212 may contain a solid electrolyte. Examples of the solid electrolyte include the solid electrolytes described above as the electrolyte in the battery 100.

[0141] The electrolyte layer 220 is a layer containing an electrolyte. The electrolyte is, for example, a solid electrolyte. The electrolyte layer 220 may be a solid electrolyte layer. Examples of solid electrolytes include the solid electrolytes described above as the electrolyte in the battery 100.

[0142] The electrolyte layer 220 may further include a binder. As the binder, the materials described above for the positive electrode mixture layer of the battery 100 can also be used for the electrolyte layer 220.

[0143] The negative electrode 230 includes a negative electrode current collector 231 and a negative electrode mixture layer 232 supported by the negative electrode current collector 231. The negative electrode mixture layer 232 contains the negative electrode active material according to this embodiment. The negative electrode mixture layer 232 is located between the negative electrode current collector 231 and the electrolyte layer 220. The materials described above for the negative electrode 23 in the battery 100 can be used for the negative electrode current collector 231 and the negative electrode mixture layer 232.

[0144] The negative electrode mixture layer 232 may contain a solid electrolyte. Examples of the solid electrolyte include the solid electrolytes described above as the electrolyte in the battery 100.

[0145] The battery 200 can be configured as a battery of various shapes, such as a coin type, a cylindrical type, a square type, a sheet type, a button type, a flat type, or a laminated type.

[0146] (Other Embodiments) (Additional Notes) The above description of the embodiments discloses the following techniques.

[0147] (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 an alkali aluminate phase containing at least one alkali metal element and Al, and a silicon phase dispersed within the alkali aluminate phase, and the coating layer includes a lithium sulfonate compound and a hydrophobic polymer compound.

[0148] This configuration can improve the cycle characteristics of the battery.

[0149] (Technology 2) The lithium sulfonate compound is represented by the general formula (1): In the general formula (1), R is an n-valent aliphatic hydrocarbon group having 1 to 5 carbon atoms, and n is 1 or 2. According to this configuration, the cycle characteristics of the battery can be improved.

[0150] (Technology 3) The negative electrode active material according to Technology 1 or 2, wherein the lithium sulfonate compound includes at least one selected from the group consisting of lithium methanesulfonate, lithium ethanesulfonate, and lithium propanesulfonate. This configuration can improve the cycle characteristics of the battery.

[0151] (Technology 4) The negative electrode active material according to any one of Technologies 1 to 3, wherein a mass ratio of the lithium sulfonate compound in the coating layer relative to the composite particles is 1 mass % or more. With this configuration, side reactions between the composite particles and an electrolyte are further suppressed, and the cycle characteristics and initial charge / discharge efficiency of the battery can be improved.

[0152] (Technology 5) The negative electrode active material according to any one of Technologies 1 to 4, wherein a mass ratio of the hydrophobic polymer compound in the coating layer relative to the composite particles is 1 mass % or more. With this configuration, the effect of improving the retention of the lithium sulfonate compound on the surface of the composite particles can be sufficiently obtained, thereby improving the cycle characteristics and initial charge / discharge efficiency of the battery.

[0153] (Technology 6) The negative electrode active material according to any one of Technologies 1 to 5, wherein the hydrophobic polymer compound contains a fluororesin. With this configuration, the cycle characteristics of the battery can be improved.

[0154] (Technology 7) The negative electrode active material according to Technology 6, wherein the fluororesin contains at least one selected from the group consisting of polyvinylidene fluoride, polytetrafluoroethylene, perfluoroalkoxyalkane, perfluoroethylene propene copolymer, ethylene tetrafluoroethylene copolymer, polychlorotrifluoroethylene, and ethylene chlorotrifluoroethylene copolymer. With this configuration, the cycle characteristics of the battery can be improved.

[0155] (Technology 8) The negative electrode active material according to any one of Technologies 1 to 7, wherein the alkali aluminate phase further contains B. This configuration improves the stability and ionic conductivity of the alkali aluminate phase. Furthermore, the porosity of the composite particles can be reduced, thereby suppressing deterioration of the particle structure due to voids. This improves the cycle characteristics of the battery.

[0156] (Technology 9) The negative electrode active material according to Technology 8, wherein when a mass ratio of Al to a total mass of elements other than O constituting the alkali aluminate phase and the silicon phase is mAl and a mass ratio of B to a total mass of elements other than O constituting the alkali aluminate phase and the silicon phase is mB, the ratio (mAl / mB) is 1.0 or more and 30.0 or less. With this configuration, the effect of reducing the porosity of the composite particles can be more significantly obtained, thereby improving the cycle characteristics and initial charge / discharge efficiency of the battery.

[0157] (Technology 10) The negative electrode active material according to any one of Technologies 1 to 9, wherein the alkali metal element includes at least one selected from the group consisting of Li, Na, and K. With this configuration, the cycle characteristics of the battery can be improved.

[0158] (Technology 11) The negative electrode active material according to any one of Technologies 1 to 10, wherein the composite particles have a porosity of 25% or less before the first charge. This configuration suppresses deterioration of the particle structure caused by voids, thereby improving the cycle characteristics of the battery.

[0159] (Technology 12) The negative electrode active material according to any one of Technologies 1 to 11, wherein the composite particles include base particles composed of the alkali aluminate phase and the silicon phase, and conductive layers covering at least a portion of the surfaces of the base particles, the conductive layers containing carbon. With this configuration, the conductivity of the composite particles is improved, and battery characteristics can be improved.

[0160] (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.

[0161] This configuration provides the effect of improving cycle characteristics.

[0162] 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.

[0163] <Preparation of Negative Electrode Material Particles> (Example 1) [Preparation of Composite Particles] Al2O3, Li2CO3, and BO3 were mixed in a mass ratio of Al2O3:Li2CO3:BO3 = 55:25:20. The mixture was fired in air at 750°C for 10 hours. The fired product was pulverized to obtain raw aluminate with an average particle size of 10 µm.

[0164] Raw material aluminate (average particle size 10 μm) and raw material silicon (3N, average particle size 10 μm) were mixed in a mass ratio of 40:60. The resulting mixture was filled into a pot (SUS, volume: 500 mL) of a planetary ball mill (Fritsch, P-5). Next, 24 SUS balls (diameter 20 mm) were placed in the pot, the lid was closed, and the mixture was milled at 200 rpm for 50 hours in an inert atmosphere. This resulted in a powdery mixture.

[0165] The powder mixture obtained in the inert atmosphere was taken out and sintered at 800°C for 4 hours in an inert atmosphere while applying a pressure of 200 MPa using a hot press, thereby obtaining a sintered body.

[0166] The obtained sintered body was pulverized and passed through a 40 μm mesh to obtain base particles (LAX particles) in which a silicon phase was dispersed in an alkali aluminate phase containing predetermined amounts of Li and B.

[0167] The obtained base particles were mixed with coal pitch (MCP250, manufactured by JFE Chemical Corporation). The amount of coal pitch mixed was 5 parts by mass per 100 parts by mass of the base particles. The mixture was fired at 800°C for 5 hours in an inert atmosphere, forming a conductive layer on the surface of the base particles. Then, composite particles with an average particle size of 5 μm, which were LAX particles with a conductive layer, were obtained using a sieve.

[0168] [Formation of Coating Layer] Composite particles, lithium methanesulfonate (MSL) powder, and polyvinylidene fluoride (PVDF) dispersion were dispersed in water. At this time, 2 parts by mass of MSL was used per 100 parts by mass of composite particles, and 2 parts by mass of PVDF was used per 100 parts by mass of composite particles. The dispersion was then spray-dried to obtain a granulated powder.

[0169] The obtained granulated powder was crushed in a mill (IKA MultiDrive basic, manufactured by IKA) and then classified using an ultrasonic oscillator (manufactured by Artech Ultrasonic Systems), thereby forming a coating layer, which was a composite layer of MSL and PVDF, on the surface of the composite particles.

[0170] In this manner, the particles of Example 1 were prepared. The average particle size of the particles of Example 1 was 7 μm.

[0171] (Example 2) Particles of Example 2 were produced in the same manner as in Example 1, except that the amount of MSL and the amount of PVDF used to form the coating layer were each changed to 4 parts by mass per 100 parts by mass of the composite particles.

[0172] (Example 3) The particles of Example 3 were produced in the same manner as in Example 1, except that the amount of MSL and the amount of PVDF used to form the coating layer were each changed to 6 parts by mass per 100 parts by mass of the composite particles.

[0173] (Example 4) The particles of Example 4 were produced in the same manner as in Example 1, except that the amount of MSL and the amount of PVDF used to form the coating layer were each changed to 8 parts by mass per 100 parts by mass of the composite particles.

[0174] Comparative Example 1 Particles of Comparative Example 1 were produced in the same manner as in Example 1, except that no coating layer was formed on the surface of the composite particles. That is, the particles of Comparative Example 1 were composite particles that were LAX particles provided with a conductive layer in Example 1.

[0175] Comparative Example 2 [Preparation of Composite Particles] Li2CO3 and SiO2 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 fired product was then pulverized to obtain lithium silicate with an average particle size of 10 μm.

[0176] Lithium silicate (average particle size 10 μm) 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 milled at 200 rpm for 50 hours in an inert atmosphere.

[0177] The powder mixture was taken out in an inert atmosphere, and then fired at 600° C. for 4 hours in an inert atmosphere while applying pressure using a hot press, thereby obtaining a sintered body.

[0178] The sintered body of the obtained mixture was crushed and passed through a 40 μm mesh, followed by sieving to obtain base particles (LSX particles) with an average particle size of 10 μm in which a silicon phase was dispersed within a lithium silicate phase.

[0179] A conductive layer was formed on the surface of the obtained base particles in the same manner as in Example 1. Then, using a sieve, composite particles having an average particle size of 5 μm, which were LSX particles with a conductive layer, were obtained. These composite particles were designated as particles of Comparative Example 2.

[0180] Reference Example 1 [Preparation of Composite Particles] Composite particles, which were LSX particles provided with a conductive layer, were prepared in the same manner as in Comparative Example 2.

[0181] [Formation of Coating Layer] In the same manner as in Example 1, a coating layer, which was a composite layer of MSL and PVDF, was formed on the surface of the composite particles.

[0182] In this manner, the particles of Reference Example 1 were prepared.

[0183] <Fabrication of Battery> Using the particles of Examples 1 to 4, Comparative Examples 1 and 2, and Reference Example 1, batteries of Examples 1 to 4, Comparative Examples 1 and 2, and Reference Example 1 were fabricated as follows.

[0184] [Preparation of Negative Electrode] The particles and graphite were mixed in a mass ratio of 10:90 to obtain a mixture. This mixture was used as the negative electrode active material. A negative electrode active material, carboxymethyl cellulose (CMC), and styrene butadiene rubber (SBR) were mixed in a mass ratio of negative electrode active material: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 prepare 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 on a portion of the negative electrode.

[0185] [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.

[0186] [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.

[0187] [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.

[0188] [Battery Evaluation] The batteries of Examples 1 to 4, Comparative Examples 1 and 2, and Reference Example 1 were evaluated for initial charge-discharge efficiency and cycle characteristics by the following methods.

[0189] (Initial Charge-Discharge Efficiency) The batteries of Examples 1 to 4, Comparative Examples 1 and 2, and Reference Example 1 were charged at a constant current of 0.3 C at an ambient temperature of 25°C until the voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current reached 0.02 C. Subsequently, constant current discharge was performed at a current of 0.5 C until the voltage reached 2.5 V. The initial charge-discharge efficiencies of the batteries of Examples 1 to 4, Comparative Examples 1 and 2, and Reference Example 1 were calculated using the initial charge-discharge efficiencies obtained using the following formula: Here, E in the formula is the value (%) of the initial charge-discharge efficiency of each of Examples 1 to 4, Comparative Examples 1 and 2, and Reference Example 1, and E0 is the value (%) of the initial charge-discharge efficiency of Comparative Example 1. The results are shown in Table 1. Amount of Side Reaction = {(100 - E) / (100 - E0)} x 100

[0190] (Cycle Characteristics) The batteries of Examples 1 to 4, Comparative Examples 1 and 2, and Reference Example 1 were subjected to constant current charging at an ambient temperature of 25°C at a current of 0.3 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 300 cycles were performed, and the discharge capacity retention rate was calculated using the following formula: Discharge capacity retention rate = (discharge capacity at 300th cycle / discharge capacity at 1st cycle) × 100

[0191] The calculated discharge capacity retention rates were used to calculate the cycle deterioration rate according to the following formula. In the formula, R is the value of the discharge capacity retention rate for each of Examples 1 to 4, Comparative Examples 1 and 2, and Reference Example 1, and R0 is the value of the discharge capacity retention rate for Comparative Example 1. The results are shown in Table 1. Cycle deterioration rate = {(100 - R) / (100 - R0)} x 100

[0192]

[0193] (Discussion) The batteries of Examples 1 to 4 had a smaller cycle deterioration rate than the batteries of Comparative Examples 1 and 2 and Reference Example 1. That is, the batteries of Examples 1 to 4 had better cycle characteristics than the batteries of Comparative Examples 1 and 2 and Reference Example 1. Furthermore, the batteries of Examples 1 to 4 had a smaller amount of side reactions than the battery of Comparative Example 1. That is, the batteries of Examples 1 to 4 showed higher initial charge / discharge efficiency than the battery of Comparative Example 1. Furthermore, the batteries of Examples 2 to 4 had a smaller amount of side reactions than the battery of Comparative Example 2.

[0194] The technology of the present disclosure is useful for batteries such as lithium ion secondary batteries.

Claims

1. A negative electrode active material comprising: a composite particle; and a coating layer covering at least a portion of a surface of the composite particle, wherein the composite particle includes an alkali aluminate phase containing at least one alkali metal element and Al, and a silicon phase dispersed within the alkali aluminate phase, and the coating layer includes a lithium sulfonate compound and a hydrophobic polymer compound.

2. The lithium sulfonate compound has the general formula (1): In the general formula (1), R is an n-valent aliphatic hydrocarbon group having 1 to 5 carbon atoms, and n is 1 or 2. The negative electrode active material according to claim 1 , 3. The negative electrode active material according to claim 2, wherein the lithium sulfonate compound includes at least one selected from the group consisting of lithium methanesulfonate, lithium ethanesulfonate, and lithium propanesulfonate.

4. The negative electrode active material according to claim 1, wherein a mass ratio of said lithium sulfonate compound in said coating layer to said composite particles is 1 mass % or more.

5. The negative electrode active material according to claim 1, wherein a mass ratio of said hydrophobic polymer compound in said coating layer to said composite particles is 1 mass % or more.

6. The negative electrode active material according to claim 1, wherein the hydrophobic polymer compound comprises a fluororesin.

7. The negative electrode active material according to claim 6, wherein the fluororesin comprises at least one selected from the group consisting of polyvinylidene fluoride, polytetrafluoroethylene, perfluoroalkoxyalkane, perfluoroethylenepropene copolymer, ethylenetetrafluoroethylene copolymer, polychlorotrifluoroethylene, and ethylenechlorotrifluoroethylene copolymer.

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

9. The negative electrode active material according to claim 8, wherein when a mass ratio of Al to a total mass of elements other than O constituting the alkali aluminate phase and the silicon phase is mAl, and a mass ratio of B to a total mass of elements other than O constituting the alkali aluminate phase and the silicon phase is mB, the ratio (mAl / mB) is 1.0 or more and 30.0 or less.

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

11. The negative electrode active material according to claim 1, wherein the composite particles have a porosity of 25% or less before the first charge.

12. The negative electrode active material according to claim 1, wherein the composite particle includes a base particle constituted by the alkali aluminate phase and the silicon phase, and a conductive layer covering at least a portion of the surface of the base particle, the conductive layer including carbon.

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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