Negative active material and battery

Abstract

This negative electrode active material (10) is provided with composite particles (1) and a coating layer (4) that covers at least a portion of the surface of the composite particles (1), the composite particles (1) containing an alkali aluminate phase (2) containing at least one alkali metal element and Al, and a silicon phase (3) dispersed in the alkali aluminate phase (2), and the coating layer (4) containing a lithium sulfonate compound and a hydrophobic polymer compound.

Description

Technical Field

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

[0002] In recent years, secondary batteries, represented by lithium-ion batteries, have been widely used as power sources for portable electronic devices and electric vehicles. Meanwhile, the use of silicon-containing materials, which are theoretically high-capacity anode active materials, has attracted attention. Previous silicon-containing materials include, for example, materials in which silicon particles are dispersed in a SiO2 phase, and materials 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 secondary batteries, comprising composite particles containing a lithium aluminate phase and a silicon phase dispersed within the lithium aluminate phase. The negative electrode active material disclosed in Patent Document 1, by comprising composite particles (hereinafter sometimes referred to as "LAX particles") containing a silicon phase dispersed within a lithium aluminate phase exhibiting excellent alkali resistance, can reduce initial side reactions during charging and discharging in lithium-ion secondary batteries and suppress the reduction in initial charging and discharging efficiency.

[0004] Existing technical documents

[0005] Patent documents

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

[0007] The problem the invention aims to solve

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

[0009] However, for negative electrode active materials containing LAX particles, there is a problem that the lithium aluminate phase is eroded due to the side reaction between the composite particles and the electrolyte, resulting in the deterioration of the negative electrode active material, which requires improvement of the battery's cycle characteristics.

[0010] Solution for solving the problem

[0011] The negative electrode active material disclosed herein possesses:

[0012] Composite particles, and

[0013] A covering layer that covers at least a portion of the surface of the aforementioned composite particles.

[0014] The aforementioned composite particles comprise: an alkali aluminate phase containing at least one alkali metal element and Al, and a silicon phase dispersed within the aforementioned alkali aluminate phase.

[0015] The aforementioned coating layer comprises lithium sulfonate compound and hydrophobic polymer compound.

[0016] The effects of the invention

[0017] According to the technology disclosed herein, it is possible to provide a negative electrode active material that can improve the cycle characteristics of a battery. Attached Figure Description

[0018] Figure 1 This is a cross-sectional view showing a schematic configuration of an example of the negative electrode active material in this embodiment.

[0019] Figure 2 This is a cross-sectional view showing a schematic configuration of another example of the negative electrode active material in this embodiment.

[0020] Figure 3 This is a cross-sectional view showing a schematic configuration of an example of the battery in this embodiment.

[0021] Figure 4 This is a cross-sectional view showing a schematic configuration of another example of the battery in this embodiment. Detailed Implementation

[0022] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. The present disclosure is not limited to the following embodiments.

[0023] (Negative electrode active material)

[0024] Figure 1 This is a cross-sectional view showing a schematic configuration of an example of the negative electrode active material of this embodiment. The negative electrode active material 10 includes composite particles 1 and a capping layer 5 covering at least a portion of the surface of the composite particles 1. The composite particles 1 comprise an alkali aluminate phase 2 and a silicon phase 3 dispersed within the alkali aluminate phase 2. The alkali aluminate phase 2 comprises at least one alkali metal element and Al. The capping layer 5 comprises a lithium sulfonate compound and a hydrophobic polymer compound.

[0025] Since at least a portion of the surface of the composite particle 1 is covered by the capping layer 5, the contact between the composite particle 1 and the electrolyte is hindered, thus suppressing the side reactions between the negative electrode active material 10 and the electrolyte. Consequently, the erosion of the alkali aluminate phase 2 accompanying the side reactions is suppressed, and therefore the degradation of the negative electrode active material 10 caused by this erosion is suppressed. Therefore, the cycle characteristics of the battery are improved, and consequently, the charge-discharge efficiency is also improved.

[0026] By including a lithium sulfonate compound in the capping layer 5, which exhibits good ion conductivity, side reactions in the battery can be suppressed. By including a hydrophobic polymer compound in the capping layer 5, the retention of the lithium sulfonate compound on the surface of the composite particles 1 is improved, and the surface of the composite particles 1 is effectively covered by the lithium sulfonate compound. Therefore, a significant suppression effect on side reactions between the negative electrode active material 10 and the electrolyte can be achieved. Consequently, the battery's cycle characteristics are improved, and consequently, the charge-discharge efficiency is also improved.

[0027] In addition to the alkali aluminate phase 2 and silicon phase 3, composite particles 1 may also contain other elements. For example, composite particles 1 may also contain a conductive layer.

[0028] Figure 2 This is a cross-sectional view showing a schematic configuration of another example of the negative electrode active material of this embodiment. The negative electrode active material 20 includes composite particles 6 and a capping layer 5 covering at least a portion of the surface of the composite particles 6. The composite particles 6 comprise an alkali aluminate phase 2 and a silicon phase 3 dispersed within the alkali aluminate phase 2. The composite particles 6 also include a conductive layer 4 covering at least a portion of the surface of the particles composed of the alkali aluminate phase 2 and the silicon phase 3. The conductive layer 4 is located between the particles composed of the alkali aluminate phase 2 and the silicon phase 3 and the capping layer 5. Hereinafter, the particles composed of the alkali aluminate phase 2 and the silicon phase 3 will be referred to as "parent particles". That is, the conductive layer 4 covers at least a portion of the surface of the parent particles. According to the above configuration, conductivity is improved.

[0029] The following describes in detail the composition of the negative electrode active material in this embodiment.

[0030] [Composite Particles]

[0031] The following description of the composite particles is applicable to Figure 1 The composite particles 1 and shown Figure 2 Any type of composite particle 6 shown.

[0032] The composite particles, for example, have an island structure in which multiple fine silicon phases 3 are dispersed in a matrix of alkali aluminate phase 2. Thus, the stress caused by the expansion and contraction of the silicon phase 3 during charging and discharging is mitigated by the alkali aluminate phase 2, and cracking and breakage of the composite particles are suppressed. Therefore, by containing silicon, both high capacity and improved cycle characteristics can be achieved. The composite particles, for example, comprise a parent particle composed of alkali aluminate phase 2 and silicon phase 3, and a conductive layer 4 covering at least a portion of the surface of the parent particle. Alternatively, the composite particles may consist only of the parent particle.

[0033] The average particle size of the composite particles can be 1 μm or more and 25 μm or less, or 4 μm or more and 15 μm or less. Based on this configuration, stress caused by volume changes of the composite particles during charging and discharging can be easily mitigated, resulting in good cycling characteristics. Furthermore, by making the surface area of ​​the composite particles appropriately sized, capacity reduction caused by side reactions with the electrolyte can be suppressed.

[0034] The average particle size of the composite particles refers to the particle size at which the cumulative volume value in the particle size distribution measured by laser diffraction scattering is 50% (volume average particle size). The measuring apparatus can be, for example, the "LA-750" manufactured by Horiba Corporation. When the surface of the parent particle is covered by the conductive layer 4, the thickness of the conductive layer 4 is substantially thin enough not to affect the average particle size of the composite particles; therefore, the average particle size of the composite particle 1 and the average particle size of the composite particle 6 having the conductive layer 4 can be considered substantially the same.

[0035] Composite particles can be removed from the battery using methods such as the following. It should be noted that the following method is an example.

[0036] First, the fully discharged battery is disassembled, and the negative electrode is removed. The negative electrode is then cleaned with, for example, anhydrous methyl ethyl carbonate or dimethyl carbonate to remove the anhydrous electrolyte components. The negative electrode compound layer is peeled off from the current collector (e.g., copper foil) serving as the negative electrode core, and the compound layer is pulverized using a mortar and pestle to obtain a sample powder. Next, the sample powder is dried in a dry atmosphere for 1 hour, or for example, immersed in slightly boiling 6M hydrochloric acid for about 10 minutes, to remove elements originating from the composite particles. Then, the sample powder is washed with deionized water, filtered, and dried, for example, at 200°C for 1 hour. It should be noted that a fully discharged state refers to a depth of discharge (DOD) of 90% or more (state of charge (SOC) of 10% or less).

[0037] Alkali aluminate phase 2 contains at least one alkali metal element, aluminum (Al), and oxygen (O). Alkali aluminate phase 2 is a phase containing alkali aluminate, which is a complex oxide containing at least one alkali metal element and Al.

[0038] Alkali aluminate phase 2 exhibits excellent ionic conductivity, facilitating the absorption and release of ions such as Li ions from silicon phase 3. Furthermore, alkali aluminate phase 2 mitigates the effects of expansion and contraction of silicon phase 3.

[0039] The alkali aluminate phase 2 exhibits superior alkali resistance compared to the lithium silicate phase in LSX. Therefore, compared to the negative electrode active material using LSX, the negative electrode active material of this embodiment exhibits, for example, suppressed side reactions with Li ions during initial charging, and the degradation of the negative electrode active material accompanying the side reactions is also suppressed.

[0040] The atomic ratio of O to Al in the alkali aluminate (O / Al) is, for example, 1.6 or more and 4 or less. Furthermore, the atomic ratio of alkali metal (MA) to Al in the alkali aluminate (MA / Al) is, for example, 1 / 5 or more and 5 or less. When these atomic ratios are within the above ranges, the stability and ionic conductivity of the alkali aluminate phase 2 become better. It should be noted that, in this specification, the stability of the alkali aluminate phase 2 includes both its chemical stability (alkali resistance) and thermal stability.

[0041] Alkali metals are elements belonging to Group 1 of the periodic table. That is, alkali aluminate phase 2 contains at least one element selected from the group consisting of Li, Na, K, Rb, Cs, and Fr.

[0042] Alkali metal elements may include at least one element selected from the group consisting of Li, Na, and K. Alkali metal elements may include at least one element selected from the group consisting of Li and Na, or may include Li.

[0043] Alkali aluminate phase 2 may contain two alkali metal elements. Alkali aluminate phase 2 may contain at least two elements selected from the group consisting of Li, Na, and K, or may contain Li and Na.

[0044] In addition to alkali metals, Al, and O, alkali aluminate phase 2 may also contain element M. Examples of element M 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).

[0045] Element M can form compounds. Depending on the type of element M, these compounds can be, for example, oxides of element M or aluminates of element M.

[0046] Element M can also be B. That is, the alkali aluminate phase 2 can also contain B. By adding element M, the stability and ionic conductivity of the alkali aluminate phase 2 are improved. In addition, by adding element M, the porosity of the composite particles 1 can be reduced. As a result, fewer parts become the starting point for cracking and rupture during repeated charge and discharge, thus suppressing the degradation of the negative electrode active material 10 and further improving the cycle characteristics of the battery.

[0047] Alkali aluminate phase 2 may also contain trace amounts of elements such as chromium (Cr), nickel (Ni), manganese (Mn), copper (Cu) and molybdenum (Mo).

[0048] Alkali aluminate phase 2 can be amorphous. In this case, the effects of expansion and contraction of silicon phase 3 can be mitigated more effectively.

[0049] A highly crystalline, finely crystalline Al₂O₃ phase can be dispersed within the alkali aluminate phase 2. The Al₂O₃ phase, for example, is distributed in an island-like pattern within the matrix of the alkali aluminate phase 2. In this case, the expansion and cracking of the alkali aluminate phase 2, which accompanies the expansion and contraction of the silicon phase 3, is easily suppressed, resulting in improved cycling characteristics. In the presence of the Al₂O₃ phase, a peak originating from the Al₂O₃ phase can be observed near 2θ = 25.4° in the X-ray diffraction pattern of the composite particles obtained by X-ray diffraction analysis. The Al₂O₃ content in the composite particles is, for example, less than 10% by mass.

[0050] Silicon phase 3 is the elemental Si phase, which repeatedly absorbs and releases Li ions during battery charging and discharging. The Faraday reaction involving silicon phase 3 exhibits capacity. Silicon phase 3 has a large capacity. Furthermore, silicon phase 3 expands and contracts significantly during charging and discharging. However, in the negative electrode active material disclosed herein, since silicon phase 3 is dispersed within alkali aluminate phase 2, the stress generated by the expansion and contraction of silicon phase 3 is mitigated by alkali aluminate phase 2.

[0051] The silicon phase 3 can be particulate. For example, the silicon phase 3 is particulate at least before the first charge. The average particle size of the silicon phase 3 can be greater than 1 nm and less than 1000 nm. Alternatively, the average particle size of the silicon phase 3 can be less than 500 nm, less than 200 nm, or less than 50 nm. After the first charge, the average particle size of the silicon phase 3 can be less than 400 nm or less than 100 nm. As described above, by dispersing the fine silicon phase 3 within the alkali aluminate phase 2, 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.

[0052] The average particle size of silicon phase 3 can be determined using a scanning electron microscope (SEM) image of the cross-section of the negative electrode active material exposed through the cross-section of silicon phase 3. Specifically, the average particle size of silicon phase 3 is calculated by averaging the maximum diameter of 100 silicon phase 3 particles randomly selected from the cross-sectional SEM images of the negative electrode active material.

[0053] Silicon phase 3 may contain crystalline silicon. Silicon phase 3 may consist of, for example, multiple crystallites. The crystallite size of silicon phase 3 may be less than 30 nm, less than 20 nm, or less than 15 nm. Based on the above, the volume change caused by the expansion and contraction of silicon phase 3 during charging and discharging can be reduced, and the improvement in cycle characteristics becomes more significant. The crystallite size of silicon phase 3 is calculated by the Scherrer formula from the half-width of the diffraction peaks originating from the Si(111) plane in the X-ray diffraction pattern obtained by X-ray diffraction of Cu-Kα rays.

[0054] The lower limit of the crystallite size of silicon phase 3 is not particularly limited, but 1 nm is an example. A preferred crystallite size for silicon phase 3 is 1 nm or more and 15 nm or less, or 5 nm or more and 11 nm or less. When the crystallite size of silicon phase 3 is 1 nm or more, for example, the surface area of ​​silicon phase 3 can be kept relatively small, thus reducing the likelihood of degradation of silicon phase 3 associated with irreversible capacity formation. When the crystallite size is 15 nm or less, it is easier to homogenize the expansion and contraction of silicon phase 3, effectively mitigating the stress generated in the negative electrode active material.

[0055] From the perspective of high capacity, the content of silicon phase 3 in the composite particles can be 30% by mass or more, 35% by mass or more, or 55% by mass or more. From the perspective of improving cycling characteristics, the content of silicon phase 3 in the composite particles can be 95% by mass or less, 75% by mass or less, or 70% by mass or less. In this case, the amount of silicon phase 3 exposed on the surface of the composite particles that is not covered by the alkali aluminate phase 2 is reduced, and the side reactions between the electrolyte and silicon phase 3 are also suppressed. The content of silicon phase 3 in the composite particles can be 30% by mass or more and 90% by mass or less, or 35% by mass or more and 75% by mass or less.

[0056] The content of silicon phase 3 in the composite particles was determined by quantifying the amount of Si in silicon phase 3 constituting the composite particles using Si-NMR as described later.

[0057] The composite particles may substantially not contain lithium silicate and SiO2. The combined content of lithium silicate and SiO2 in the composite particles may, for example, be less than 3% by mass.

[0058] The mass ratio (mAl) of Al relative to the total mass of elements other than oxygen (O) constituting alkali aluminate phase 2 and silicon phase 3 can be 10% by mass or more and 47% by mass or less, or 11.5% by mass or more and 45.5% by mass or less.

[0059] The mass ratio (mMA) of the alkali metal element relative to the total mass of the elements other than O constituting the alkali aluminate phase 2 and silicon phase 3 can be 0.7% or more and 13.5% or less, 1.0% or more and 9.5% or less, or 1.5% or more and 3.5% or less.

[0060] Based on the above composition, the stability and ionic conductivity of the alkali aluminate phase 2 can be improved. It should be noted that the above stability includes both chemical stability (alkali resistance) and thermal stability.

[0061] From the perspective of the stability and ionic conductivity of alkali aluminate phase 2, the ratio of the mass ratio of alkali metal elements (mMA) to the mass ratio of Al (mAl) (mMA / mAl) can be greater than 0.01 and less than 0.50, or greater than 0.05 and less than 0.25.

[0062] The mass ratio (mSi) of Si relative to the total mass of elements other than O constituting alkali aluminate phase 2 and silicon phase 3 can be 40% by mass and 90% by mass, or 50.8% by mass and 85.5% by mass. Based on the above composition, the battery can easily achieve both high capacity and good cycle characteristics.

[0063] When the alkali aluminate phase 2 also contains B, the mass ratio (mB) of B relative to the total mass of elements other than O constituting the alkali aluminate phase 2 and silicon phase 3 can be 1% by mass or more and 20% by mass or less, or 2% by mass or more and 15% by mass or less. Based on the above composition, the improvement effect on cycle characteristics can become more significant.

[0064] The ratio of the mass ratio of Al (mAl) to the mass ratio of B (mB) (mAl / mB) can be 1.0 or more and 30.0 or less, 1.0 or more and 20.0 or less, or 1.0 or more and 10.0 or less. By making mAl / mB 30.0 or less, the reduction in porosity of composite particles 1 becomes more significant. This suppresses the degradation of particle structure caused by voids within the composite particles, thus effectively improving the cycle characteristics of the battery. By making mAl / mB 1.0 or more, the reduction in initial charge-discharge efficiency can be reduced. Therefore, by keeping mAl / mB within the above range, both improved cycle characteristics and good initial charge-discharge efficiency can be achieved.

[0065] The conductive layer 4 covers at least a portion of the surface of the parent particle, which is composed of the alkali aluminate phase 2 and the silicon phase 3. The conductive layer 4 may cover the entire surface of the parent particle. The conductive layer 4 is, for example, a thin film layer containing a conductive material. By including the conductive layer 4 in the composite particle, conductivity is improved.

[0066] The conductive material can also be a conductive carbon material. That is, the conductive layer 4 can contain carbon. As a carbon material, natural graphite, artificial graphite, graphitized mesophase carbon, amorphous carbon, soft carbon, and hard carbon can be used. The carbon material can be amorphous carbon. Based on the above, a thin conductive layer 4 covering the surface of the master particles can be easily formed. Examples of amorphous carbon include carbon black, calcined pitch, coke, and activated carbon.

[0067] The thickness of the conductive layer 4 is preferably thin enough not to affect the average particle size of the composite particles. Considering the assurance of conductivity and the diffusion of ions such as Li ions, the thickness of the conductive layer 4 can be greater than 1 nm and less than 200 nm, or greater than 5 nm and less than 100 nm. The thickness of the conductive layer 4 can be measured by SEM observation or transmission electron microscopy (TEM) observation of the cross-section of the negative electrode active material exposed in the cross-section of the composite particles.

[0068] The porosity of the composite particles before the initial charge and discharge can be below 25%. It should be noted that since the conductive layer 4 does not affect the porosity, the porosity of the composite particle 6 is essentially the same as that of the parent particle (composite particle 1). By suppressing the porosity of the composite particles to below 25%, the portion that becomes the starting point for cracking and breakage during repeated charge and discharge is reduced. Therefore, the degradation of the negative electrode active material is suppressed, and the cycle characteristics of the battery are further improved. The porosity of the composite particles can be below 20% or below 15%. There is no particular limitation on the lower limit of the porosity; as an example, it is 1%.

[0069] The porosity of composite particles refers to the proportion of voids in the cross-section of the composite particle. The porosity of composite particles can be determined using a SEM image of the cross-section of the composite particle containing the exposed negative electrode active material. The porosity is calculated by binarizing the SEM image using image analysis software (e.g., ImageJ) to extract the void regions and dividing the total void area by the total area of ​​the particle cross-section.

[0070] It should be noted that, in addition to adding B to the above-mentioned alkali aluminate phase 2, the porosity of the composite particles can also be controlled to some extent by adjusting the firing temperature, the compressive force applied to the particles during firing, and mA / mAl in the manufacturing process of the composite particles.

[0071] The composite particles can possess a Vickers hardness of 300 HV or higher. It should be noted that since the conductive layer 4 does not affect the Vickers hardness, the Vickers hardness of the composite particle 6 is essentially the same as that of the parent particle (composite particle 1). With high Vickers hardness in the composite particles, the volume change of the silicon phase 3 during charge and discharge is easily suppressed, reducing particle structure degradation. As a result, the improvement in cycle performance becomes more significant. More preferably, the Vickers hardness of the composite particles is 350 HV or higher, but it can also be 400 HV or higher, or 500 HV or higher.

[0072] The Vickers hardness of the composite particles can be determined using a Vickers hardness tester. Specifically, composite particles with the capping layer 5 removed from the negative electrode active material are embedded in thermosetting resin and polished with 400-grit sandpaper to expose the cross-section of composite particle 1. The cross-section is then mirror-polished using 2000-grit abrasive paper. The Vickers hardness is measured under a load of 1 kg and a holding time of 15 seconds. There is no specific upper limit to the Vickers hardness of composite particle 1; for example, it could be 1500 HV.

[0073] The content of each element in the composite particles was determined by the following methods: Oxygen content was determined using an oxygen-nitrogen-hydrogen analyzer. Si content was determined using NMR. Carbon content was determined using a carbon-sulfur analyzer. The content of other elements was determined using inductively coupled plasma atomic emission spectrometry (ICP). If ICP cannot be used, energy-dispersive X-ray diffraction (EDX) can be used. If neither ICP nor EDX can be used, Auger electron spectroscopy (AES) can be used. The composition of the alkali aluminate phase 2 can be determined from the content of each element. When determining based on the battery state, for example, the content can be determined by disassembling a fully discharged battery, removing the negative electrode, cleaning it, removing the non-aqueous electrolyte components, drying it, and then analyzing the cross-section of the negative electrode composite layer obtained using a cross-section polishing instrument.

[0074] <Inductively Coupled Plasma Emission Spectrometry (ICP)>

[0075] The composite particle sample was completely dissolved in a heated acid solution (a mixture of hydrofluoric acid, nitric acid, and sulfuric acid), and the carbon residue in the solution was removed by filtration. The filtrate was then analyzed by ICP to determine the spectral intensity of each element. Next, a standard curve was constructed using commercially available standard solutions of the elements to calculate the content of each element in the composite particles.

[0076] <Energy Dispersive X-ray (EDX)>

[0077] When ICP is not feasible, EDX is used for determination. Elemental mapping analysis is performed using EDX based on the reflected electron image of the cross-section of the negative electrode active material exposed in the cross-section of the composite particle. The area containing the target element is calculated using image analysis software. The observation magnification is, for example, 2000x to 20000x. When measuring according to the battery state, 10 composite particles with a maximum diameter of 5 μm or more are randomly selected from the cross-sectional image of the reflected electron image of the negative electrode composite layer containing the negative electrode active material, and elemental mapping analysis is performed on each particle using EDX. The content of the target element is calculated by averaging the measured area containing the specified element in the 10 particles.

[0078] The following shows the preferred measurement conditions for cross-sectional SEM-EDX analysis.

[0079] Processing equipment: JEOL SM-09010 (Cross Section Polisher)

[0080] Processing conditions: Accelerating voltage 6kV

[0081] Current value: 140μA

[0082] Vacuum degree: 1×10 -3 Pa to 2×10 -3 Pa

[0083] Measurement apparatus: HITACHI SU-70 electron microscope

[0084] Accelerating voltage during analysis: 10kV

[0085] Field: Free Mode

[0086] Probe current mode: Medium

[0087] Probe current range: High

[0088] Anode Ap.: 3

[0089] OBJ Ap.:2

[0090] Analysis area: 1 μm square

[0091] Analysis software: EDAX Genesis

[0092] CPS: 20500

[0093] Lsec: 50

[0094] Time constant: 3.2

[0095] Auger Electron Spectroscopy (AES)

[0096] When ICP and EDX methods fail to detect the elemental composition, AES is used. Based on the reflected electron image of the cross-section of the composite particle revealing the negative electrode active material, qualitative and quantitative elemental analysis is performed using an AES analyzer (e.g., JAMP-9510F, manufactured by NEC Corporation). Measurement conditions can be set, for example, an accelerating voltage of 10 kV, a beam current of 10 nA, and an analysis area of ​​20 μm φ. When measuring according to the battery's state, 10 composite particles with a maximum diameter of 5 μm or more are randomly selected from the cross-sectional image of the reflected electron image of the negative electrode composite layer containing the negative electrode active material, and qualitative and quantitative elemental analysis is performed on each particle using an AES analyzer. The content of the specified element contained in the 10 particles is averaged to calculate the element content.

[0097] It should be noted that during the charging and discharging process, a coating can be formed on the surface of the composite particles through decomposition of non-aqueous electrolytes, etc. Furthermore, as mentioned above, the composite particles sometimes also possess a conductive layer. Therefore, EDX and AES analyses are performed on a range extending from, for example, 1 μm inside the periphery of the composite particle's cross-section, ensuring that the thin coating and conductive layer are not included in the measurement range.

[0098] <Inactive Gas Melting-Non-dispersive Infrared Absorption Method>

[0099] The oxygen content in the composite particles can be determined using an oxygen-nitrogen-hydrogen analyzer (e.g., the EGMA-830 model manufactured by Horiba Corporation). The sample is placed in a Ni capsule and, together with Sn and Ni particles used as flux, is added to a carbon crucible heated to 5.75 kW. The released carbon monoxide gas is then detected. A standard curve is prepared using the standard sample Y₂O₃, and the oxygen content of the sample is calculated.

[0100] Nuclear Magnetic Resonance Spectroscopy (NMR)

[0101] The amount of Si constituting silicon phase 3 can be quantified using Si-NMR. Preferred Si-NMR measurement conditions are shown below.

[0102] Measurement apparatus: Varian Corporation, solid-state nuclear magnetic resonance spectroscopy apparatus (INOVA-400)

[0103] Probe: Varian 7mm CPMAS-2

[0104] MAS: 4.2kHz

[0105] MAS speed: 4kHz

[0106] Pulse: DD (45° pulse + 1H signal acquisition time for decoupling)

[0107] Repeat time: 1200 to 3000 seconds

[0108] Observation width: 100kHz

[0109] Observation center: around -100ppm

[0110] Signal acquisition time: 0.05 seconds

[0111] Total number of times: 560

[0112] Sample volume: 207.6 mg

[0113] <High-Frequency Induction Heating Furnace Combustion - Infrared Absorption Method>

[0114] The carbon content in the composite particles can be determined using a carbon-sulfur analysis device (e.g., the EMIA-520 model manufactured by Horiba Corporation). A sample is weighed onto a magnetic plate, a combustion accelerant is added, and the sample is inserted into a combustion furnace heated to 1350°C (carrier gas: oxygen). The amount of carbon dioxide gas produced during combustion is detected by infrared absorption. A standard curve is prepared, for example, using carbon steel (0.49% carbon content) manufactured by Bureau of Analyzed Samples Ltd., and the carbon content of the sample is calculated.

[0115] [Overlay 5]

[0116] The capping layer 5 covers at least a portion of the surface of the composite particles. The capping layer 5 may cover the entire surface of the composite particles.

[0117] The capping layer 5 contains lithium sulfonate compound and hydrophobic polymer compound.

[0118] Lithium sulfonates are lithium salts of sulfonic acid compounds. Here, sulfonic acid compounds are organic compounds containing a sulfonic acid group (SO3H). Sulfonic acid compounds can be monosulfonic or disulfonic.

[0119] Lithium sulfonate compounds can be represented by the following general formula (1). In general formula (1), R is an n-valent hydrocarbon group that may have substituents, and n is 1 or 2.

[0120]

[0121] There are no particular restrictions on the number of carbon atoms in the hydrocarbon group as long as it is 1 or more. The number of carbon atoms in the hydrocarbon group can be, for example, 20 or less, 10 or less, or 5 or less. The substituent can be, for example, a halogen or a fluorine atom. The hydrocarbon group can be an aliphatic hydrocarbon group. In general formula (1), for R, in general formula (1), R can be an n-valent aliphatic hydrocarbon group with 1 to 5 carbon atoms.

[0122] Lithium sulfonate compounds may include at least one lithium compound selected from the group consisting of lithium methanesulfonate, lithium ethanesulfonate, and lithium propanesulfonate. Lithium sulfonate compounds may include lithium methanesulfonate.

[0123] The mass ratio of lithium sulfonate compound in the capping layer 5 to the composite particles (hereinafter referred to as "loading of lithium sulfonate compound") can be 1% by mass or more. The loading of lithium sulfonate compound can be 1% by mass or more and 10% by mass or less, 1% by mass or more and 6% by mass or less, or 2% by mass or more and 6% by mass or less.

[0124] When the loading of lithium sulfonate compound is 1% by mass or more, the surface of the composite particles can be sufficiently covered by the lithium sulfonate compound, and the inhibitory effect of the lithium sulfonate compound on side reactions can be easily obtained. When the loading of lithium sulfonate compound is 6% by mass or less, a low-resistivity capping layer 5 and a low-resistivity negative electrode active material can be easily obtained. When forming a capping layer 5 comprising lithium sulfonate compound and hydrophobic polymer compound, for example, the lithium sulfonate compound in the above-mentioned range is loaded onto the surface of the composite particles.

[0125] Hydrophobic polymer compounds can exhibit good adhesion and thermal melting properties. In this case, hydrophobic polymer compounds can be used to firmly load lithium sulfonate compounds onto the surface of composite particles, easily and stably achieving the inhibitory effect of lithium sulfonate compounds on side reactions. Hydrophobic polymer compounds are almost insoluble in water.

[0126] From the viewpoint of electrolyte stability, the hydrophobic polymer compound may include a fluoropolymer. The fluoropolymer may include at least one selected from the group consisting of polyvinylidene fluoride (PVDF), polytetrafluoroethylene, perfluoroalkoxyalkane (PFA), perfluoroethylene-propylene copolymer, ethylene-tetrafluoroethylene copolymer, polychlorotrifluoroethylene, and ethylene-chlorotrifluoroethylene copolymer. The fluoropolymer may include PVDF. When the hydrophobic polymer compound is PVDF, PVDF has good adhesion and a low melting point, thus enabling the formation of a coating layer 5 at low heat treatment temperatures.

[0127] Hydrophobic polymer compounds, besides polyvinylidene fluoride (PVDF), can also include polymers containing PVDF units. Examples of polymers containing PVDF units include copolymers of PVDF with other monomers. Examples of other monomers include hexafluoropropylene (HFP) and tetrafluoroethylene (TFE). Examples of polymers containing PVDF units include polyvinylidene fluoride and its modified forms, PVDF-hexafluoropropylene copolymers, and PVDF-trifluorochloroethylene copolymers. In polymers containing PVDF units, the content of PVDF units is, for example, 30 mol% or more, and may also be 50 mol% or more.

[0128] The mass ratio of the hydrophobic polymer compound in the capping layer 5 to the composite particles (hereinafter referred to as "the loading of the hydrophobic polymer compound") can be 1% by mass or more. The loading of the hydrophobic polymer compound can be 1% by mass or more and 10% by mass or less, 1% by mass or more and 6% by mass or less, or 2% by mass or more and 6% by mass or less.

[0129] When the loading of the hydrophobic polymer compound is 1% by mass or more, the improved retention of lithium sulfonate compound on the surface of the composite particles due to the hydrophobic polymer compound can be fully obtained. When the loading of the hydrophobic polymer compound is 6% by mass or less, a low-resistance capping layer 5 and a low-resistance negative electrode active material can be easily obtained.

[0130] When the capping layer 5 contains a lithium sulfonate compound and a fluoropolymer (e.g., PVDF) as a hydrophobic polymer compound, the loading amounts of the lithium sulfonate compound and the fluoropolymer are determined by the following method.

[0131] The negative electrode active material was cleaned with N-methyl-2-pyrrolidone (NMP) to dissolve the fluoropolymer resin. The mass difference before and after dissolution was taken as the mass of the fluoropolymer resin. Then, the residue undissolved in NMP was washed with water to dissolve the lithium sulfonate compound. The mass of the lithium sulfonate compound dissolved in water was determined by quantitative analysis, such as ICP emission spectroscopy.

[0132] The mass of the residue insoluble in water and NMP is determined as the mass of the composite particles. It should be noted that, when the composite particles have a conductive layer 4, quantitative analysis of the carbon in the residue insoluble in water and NMP is performed using a carbon-sulfur analysis device, and the determined carbon content originates from the carbon material of the conductive layer 4.

[0133] Using the mass of the fluororesin and the mass of the composite particles obtained above, the loading of the fluororesin is calculated using the mathematical formula: (mass of fluororesin / mass of composite particles) × 100. Similarly, using the mass of the lithium sulfonate compound and the mass of the composite particles obtained above, the loading of the lithium sulfonate compound is calculated using the mathematical formula: (mass of lithium sulfonate compound / mass of composite particles) × 100.

[0134] The thickness of the capping layer 5 is preferably 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 influence of the electrolyte, the thickness of the capping layer 5 can be 1 nm or more. From the viewpoint of suppressing the increase in resistance, the thickness of the capping layer 5 can be 300 nm or less. The thickness of the capping layer 5 can be less than the thickness of the conductive layer 4. The thickness of the capping layer 5 can be measured by SEM or TEM observation of the cross-section of the negative electrode active material.

[0135] During surface analysis of the negative electrode active material using X-ray photoelectron spectroscopy (XPS), a peak originating from lithium sulfonate compounds was observed in the surface portion (including the outermost surface) of the capping layer 5. Specifically, this peak is characterized by a binding energy of approximately 165 eV to 170 eV and an intensity (c / s) of 200 to 1000.

[0136] (Method for manufacturing negative electrode active material)

[0137] The negative electrode active material disclosed herein is manufactured, for example, by a manufacturing method including step I of obtaining composite particles and step II of forming a coating layer 5 on the surface of the composite particles.

[0138] [Process I]

[0139] Process I includes processes 1 through 4 as described below.

[0140] Step 1: The process of obtaining alkali aluminate (hereinafter referred to as "raw aluminate") as a raw material.

[0141] Step 2: The process of combining raw aluminate and raw silicon to obtain a composite intermediate in which raw silicon is dispersed in the raw aluminate.

[0142] Step 3: Heat treatment of the composite intermediate to obtain a sintered body containing alkali aluminate phase 2 and silicon phase 3 dispersed in alkali aluminate phase 2.

[0143] Step 4: The process of crushing the sintered body to obtain composite particles 1 (master particles).

[0144] The first step includes, for example, a step of mixing an aluminum compound, a compound containing an alkali metal element, and a compound containing element M as needed to obtain a mixture; and a step of calcining the mixture to obtain a raw aluminate. Calcination is carried out, for example, in an oxidizing atmosphere. The calcination temperature can be above 400°C and below 1200°C, or above 700°C and below 1100°C.

[0145] Examples of aluminum compounds include aluminum oxide, aluminum hydroxide, and aluminum carbonate. Aluminum compounds can be used alone or in combination of two or more.

[0146] Examples of compounds containing alkali metals include lithium carbonate, lithium oxide, lithium hydroxide, lithium hydride, sodium carbonate, sodium oxide, sodium hydroxide, sodium hydride, potassium carbonate, potassium oxide, potassium hydroxide, and potassium hydride. Compounds containing alkali metals can be used alone or in combination of two or more.

[0147] Compounds containing element M are, for example, boron compounds. Examples of boron compounds include boron oxide, boric acid, borax, and sodium tetraborate. A single boron compound can be used alone, or two or more compounds can be used in combination.

[0148] In the first step, aluminum compounds that did not react with compounds containing alkali metals during the preparation of the raw aluminate may remain in the raw aluminate. When the amount of aluminum compound used is relatively large compared to the amount of compounds containing alkali metals, aluminum compound residue is more likely to remain. If the aluminum compound remaining in the raw aluminate is Al2O3, an Al2O3 phase can be formed dispersed within the alkali aluminate phase 2 in the final composite particles.

[0149] In the second step, for example, a mixture of raw aluminate and raw silicon is pulverized while being subjected to shear force to obtain a micronized composite intermediate. As an example, a method can be described in which raw aluminate and raw silicon are mixed in a specified mass ratio and the mixture is micronized using a pulverizing device such as a ball mill.

[0150] The raw material silicon is, for example, coarse silicon particles with an average particle size of several μm to tens of μm. The silicon particles can be prepared such that the crystallite size of the silicon phase 3, calculated using the Scherrer formula based on the half-width of the diffraction peaks of the Si(111) plane derived from the X-ray diffraction pattern of the negative electrode active material or composite particle 1, is less than 15 nm. It should be noted that silicon nanoparticles and raw material aluminate nanoparticles can also be synthesized without using a pulverizing device and then mixed together.

[0151] In the third step, for example, the micronized composite intermediate is sintered to obtain a sintered body while being pressurized by hot pressing or the like. The pressure applied to the composite intermediate is, for example, 100 MPa or more, and can be 100 MPa or more but less than 300 MPa. 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 inactive atmosphere (e.g., argon, nitrogen, etc.). The sintering conditions in the third step also affect the crystallites of silicon phase 3; generally, the higher the sintering temperature, the larger the crystallite size.

[0152] An example firing temperature is 450°C or higher and 1000°C or lower. If the firing temperature is within this range, it is easy to form a structure in which a small silicon phase 3 is dispersed within a low-crystallinity alkali aluminate phase 2. The raw aluminate is stable at this temperature and hardly reacts with silicon. The firing temperature can be 550°C or higher and 950°C or lower, or 650°C or higher and 900°C or lower. To reduce the porosity of the composite particles to below 25%, firing at a temperature of 650°C or higher is preferred. The firing time is, for example, 1 hour or more and 10 hours or less.

[0153] The fourth step is to pulverize the composite particles 1 (master particles) into a desired particle size distribution. For example, the median particle size of the pulverized master particles is 1 μm or more and 25 μm or less.

[0154] Step I may further include a fifth step as described below after steps 1 to 4.

[0155] Step 5: The process of forming a conductive layer 4 on the surface of the master particles composed of alkali aluminate phase 2 and silicon phase 3.

[0156] As described above, the conductive material constituting the conductive layer 4 is preferably a conductive carbon material. Examples of methods for covering the surface of the master particles with carbon material include CVD methods using hydrocarbon gases such as acetylene and methane as raw materials, and methods such as mixing coal tar pitch, petroleum asphalt, and phenolic resin with the master particles and heating to carbonize them. Alternatively, carbon black can be deposited onto the surface of the master particles.

[0157] In the fifth step, for example, a conductive layer 4 is formed on the surface of the parent particle by heating a mixture of the parent particle and the carbon material in an inactive atmosphere (such as argon, nitrogen, etc.) at a temperature of 700°C or higher and 950°C or lower. As described above, a composite particle 6 having a conductive layer 4 on the surface of the parent particle is obtained.

[0158] [Process II]

[0159] Process II is the process of forming a coating layer 5 on the surface of the composite particles.

[0160] For example, after granulating the composite particles using composite particles, lithium sulfonate compounds, and hydrophobic polymer compounds, the granulated powder is crushed and graded to the desired size, thereby obtaining a negative electrode active material with a desired average particle size having a covering layer 5 formed on the surface of the composite particles.

[0161] Granulation is performed, for example, by dispersing composite particles, lithium sulfonate compound powder, and hydrophobic polymer compound in water, followed by spray drying of the dispersion. This forms a coating layer on the surface of the composite particles, which is a mixture of lithium sulfonate compound and hydrophobic polymer compound.

[0162] The amount of lithium sulfonate compound added relative to the composite particles can be 1% by mass or more and 6% by mass or less. The amount of hydrophobic polymer compound added relative to the composite particles can be 1% by mass or more and 6% by mass or less.

[0163] 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 surface of the composite particles can be easily and uniformly covered by the lithium sulfonate compound and the hydrophobic polymer compound. The average particle size of the lithium sulfonate compound can 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 can be 200 nm or more and 1 μm or less.

[0164] The method for forming the coating layer 5 on the surface of the composite particles is not limited to the method described above. For example, the composite particles, the powder of lithium sulfonate compound, and the powder of hydrophobic polymer compound can be dry-mixed to obtain a mixture, and then the mixture can be heat-treated to form the coating layer 5 on the surface of the composite particles.

[0165] Through a mixing process, an intermediate (mixture) is obtained, in which a mixture of lithium sulfonate compound and hydrophobic polymer compound is adhered to the surface of composite particles. Dry mixing can be achieved using ball milling. The amounts and particle sizes of the lithium sulfonate compound and hydrophobic polymer compound added can be the same as described above.

[0166] The heat treatment is preferably performed at a temperature above the melting point of the hydrophobic polymer compound and below its decomposition temperature. When using polyvinylidene fluoride (PVDF) as the hydrophobic polymer compound, the heat treatment temperature can be above the melting point of PVDF (150°C to 170°C) and below its decomposition temperature (340°C), preferably above 200°C and below 250°C. The heat treatment can be performed in an inactive atmosphere. The heat treatment time is, for example, about 1 to 3 hours. Through heat treatment, the hydrophobic polymer compound in the mixture liquefies and permeates and diffuses around the composite particles and 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. Thus, a capping layer 5 is formed as a mixed layer of lithium sulfonate compound and hydrophobic polymer compound. By crushing the heat-treated mixture, a negative electrode active material with a desired average particle size can be obtained.

[0167] (Battery)

[0168] The battery of this embodiment includes a negative electrode, a positive electrode, and an electrolyte. The negative electrode contains the negative electrode active material of this embodiment. By including the negative electrode active material of this embodiment in the negative electrode, the cycle characteristics of the battery of this embodiment are improved, thereby improving the initial charge-discharge efficiency.

[0169] Figure 3 This is a schematic longitudinal cross-sectional view illustrating an example of the battery according to this embodiment. The battery 100 is a cylindrical battery comprising a cylindrical battery casing, a wound electrode assembly 24, and an electrolyte (not shown). The electrode assembly 24 is housed within the battery casing and is in contact with the electrolyte.

[0170] The battery casing consists of a bottomed cylindrical metal container body 25 and a sealing body 26 that seals the opening of the body 25. A gasket 37 is disposed between the body 25 and the sealing body 26. The gasket 37 ensures the airtightness of the battery casing. Inside the body 25, insulating plates 27 and 28 are respectively disposed at both ends of the electrode assembly 24 in the winding axis direction of the electrode assembly 24.

[0171] The housing body 25, for example, has a stepped portion 31. The stepped portion 31 can be formed by partially pressing the sidewall of the housing body 25 from the outside. The stepped portion 31 can also be formed in a ring shape along the circumference of an imaginary circle defined by the housing body 25 on the sidewall of the housing body 25. In this case, the sealing body 26 is supported, for example, by the surface on the opening side of the stepped portion 31.

[0172] The sealing body 26 includes a perforated metal plate 32, a lower valve body 33, an insulating member 34, an upper valve body 35, and a cover 36. These components are stacked in this order within the sealing body 26. The sealing body 26 is installed at the opening of the housing body 25 with the cover 36 located outside the housing body 25 and the perforated metal plate 32 located inside the housing body 25.

[0173] The components constituting the sealing body 26 are, for example, circular or ring-shaped. All of these components, except for the insulating component 34, are electrically connected to each other.

[0174] Electrode assembly 24 has a positive electrode 21, a separator 22, and a negative electrode 23. The positive electrode 21, separator 22, and negative electrode 23 are all strip-shaped. The width direction of the strip-shaped positive electrode 21 and negative electrode 23 is, for example, parallel to the winding axis of electrode assembly 24. The separator 22 is disposed between the positive electrode 21 and the negative electrode 23. The positive electrode 21 and negative electrode 23 are wound into a spiral shape with the separator 22 spaced between them.

[0175] When observing the cross-section of the battery 100 in a direction perpendicular to the winding axis of the electrode assembly 24, the positive electrode 21 and the negative electrode 23 are alternately stacked in the radial direction of an imaginary circle defined by the housing body 25, with spacers 22 between them.

[0176] The positive electrode 21 is electrically connected to the cover 36, which also serves as the positive terminal, via a positive electrode lead 29. One end of the positive electrode lead 29 is connected, for example, near the center of the positive electrode 21 along its length. The positive electrode lead 29 extends from the positive electrode 21 to the perforated metal plate 32 through a through hole formed in the insulating plate 27. The other end of the positive electrode lead 29 is, for example, soldered to the side of the electrode assembly 24 of the perforated metal plate 32.

[0177] The negative electrode 23 is electrically connected to the housing body 25, which also serves as the negative terminal, via the negative electrode lead 30. One end of the negative electrode lead 30 is connected, for example, to the end of the negative electrode 23 along its length. The other end of the negative electrode lead 30 is, for example, soldered to the inner bottom surface of the housing body 25.

[0178] The following is a detailed explanation of the components of battery 100.

[0179] The positive electrode 21 contains a material with the property of absorbing and releasing metal ions (e.g., lithium ions). The positive electrode 21 may, for example, contain a positive electrode active material. The positive electrode 21 may have a positive electrode current collector and a positive electrode flux layer loaded on the surface of the positive electrode current collector.

[0180] The positive electrode mixture layer contains a positive electrode active material. Examples of positive electrode active materials include lithium-containing transition metal oxides, lithium-containing transition metal phosphates, transition metal fluorides, polyanionic materials, fluorinated polyanionic materials, transition metal sulfides, transition metal oxysulfides, and transition metal oxynitrides. In particular, using lithium-containing transition metal oxides or lithium-containing transition metal phosphates as positive electrode active materials can reduce battery manufacturing costs and increase the average discharge voltage. Examples of lithium-containing transition metal oxides include lithium cobalt oxide, lithium nickel cobalt aluminum oxide, lithium nickel cobalt manganese oxide, and lithium nickel manganese oxide. Examples of lithium-containing transition metal phosphates include lithium iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate, and lithium nickel phosphate. At least one of these positive electrode active materials can be used.

[0181] The positive electrode layer may contain conductive additives, ionic conductors, and binders as needed.

[0182] Conductive additives and ionic conductors are used to reduce the resistance of electrodes. Examples of conductive additives include carbon materials and conductive polymers. Examples of carbon materials include carbon black, graphite, acetylene black, carbon nanotubes, carbon nanofibers, graphene, fullerene, and graphite oxide. Examples of conductive polymers include polyaniline, polypyrrole, and polythiophene. At least one of these conductive additives may be used.

[0183] Adhesives are used to improve the adhesion of materials constituting electrodes. Examples of adhesives include polyvinylidene fluoride (PVDF), PVDF-hexafluoropropylene copolymer, PVDF-tetrafluoroethylene copolymer, polytetrafluoroethylene (PTFE), carboxymethyl cellulose, polyacrylic acid, styrene-butadiene copolymer rubber, polypropylene, polyethylene, and polyimide. At least one adhesive selected from these can be used.

[0184] The positive current collector is a sheet or film made of metallic materials such as aluminum, aluminum alloys, stainless steel, titanium, or titanium alloys. The sheet or film can be porous or non-porous. Metal foils, metal meshes, etc., are used as sheets or films. Carbon materials can be coated onto the surface of the positive current collector as an auxiliary material for conductivity.

[0185] The negative electrode 23 contains the negative electrode active material of this embodiment. For example, the negative electrode 23 has a negative electrode current collector and a negative electrode additive layer loaded on the surface of the negative electrode current collector.

[0186] The negative current collector is a foil made of metal materials such as stainless steel, nickel, nickel alloy, copper, and copper alloy.

[0187] The negative electrode binder layer contains the negative electrode active material of this embodiment. The negative electrode binder layer may contain other materials such as conductive additives, ionic conductors, and binders, as needed. The materials described above for the positive electrode binder layer may also be used as conductive additives, ionic conductors, and binders in the negative electrode binder layer.

[0188] The electrolyte may contain a non-aqueous solvent and a lithium salt dissolved in the non-aqueous solvent. The concentration of the lithium salt in the electrolyte may, for example, be 0.5 mol / L or more and 2 mol / L or less. By controlling the lithium salt concentration within the above range, an electrolyte with excellent ionic conductivity and moderate viscosity can be obtained. However, the lithium salt concentration is not limited to the above.

[0189] As a non-aqueous solvent, cyclic carbonates, linear carbonates, cyclic ethers, linear ethers, nitriles, amides, etc., can be used. One of these solvents can be used, or two or more can be used in combination.

[0190] As lithium salts, lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bisperfluoroethylsulfonylimide (LiN(SO2C2F5)2), LiAsF6, LiCF3SO3, and lithium difluoro(oxalate-containing)borate, etc., can be used. One of these electrolyte salts can be used, or two or more can be used in combination.

[0191] Typically, a separator is desired between the positive and negative electrodes. The separator 22 has high ion permeability and possesses moderate mechanical strength and insulation. Microporous membranes, woven fabrics, and nonwoven fabrics can be used as the separator 22. The material used for the separator 22 can be, for example, a polymer. Polymers can be, for example, polyolefins such as polypropylene and polyethylene.

[0192] In the battery of this disclosure, the electrolyte may, for example, be impregnated with a polymer that serves as a separator. That is, the battery of this disclosure may have a structure that combines an electrolyte and a polymer.

[0193] The battery disclosed herein may further include a solid electrolyte as the electrolyte. That is, the battery of this disclosure may have a hybrid structure that combines an electrolyte and a solid electrolyte. Examples of solid electrolyte materials are halide solid electrolytes, sulfide solid electrolytes, oxide solid electrolytes, or organic polymer solid electrolytes. In this disclosure, "halide solid electrolyte" refers to a solid electrolyte containing a halogen element as the main component of the anions. "Sulfide solid electrolyte" refers to a solid electrolyte containing sulfur as the main component of the anions. "Oxide solid electrolyte" refers to a solid electrolyte containing oxygen as the main component of the anions. The main component of the anions refers to the anion with the largest molar mass among all the anions constituting the solid electrolyte.

[0194] In this disclosure, an example of the battery structure of this embodiment is described. Figure 3 The illustrated configuration example shows a cylindrical non-aqueous electrolyte secondary battery formed by housing a wound electrode assembly (with the positive and negative electrodes separated by a separator) and an electrolyte within an outer casing. However, the battery of this disclosure is not limited to this configuration example. The battery of this disclosure can be any shape, such as square, coin-shaped, button-shaped, or laminated. Furthermore, instead of a wound electrode assembly, other types of electrode assemblies, such as an electrode assembly formed by stacking the positive and negative electrodes separated by a separator, can be used in the secondary battery of this disclosure.

[0195] Figure 4 This is a cross-sectional view showing a schematic configuration of another example of the battery according to this embodiment. The battery 200 includes a positive electrode 210, an electrolyte layer 220, and a negative electrode 230. The negative electrode 230 contains the negative electrode active material of this embodiment. The electrolyte layer 220 is disposed between the positive electrode 210 and the negative electrode 230. The battery 200 is, for example, an all-solid-state battery.

[0196] The positive electrode 210 includes a positive current collector 211 and a positive electrode flux layer 212 loaded on the surface of the positive current collector 211. The positive electrode flux layer 212 is located between the positive current collector 211 and the electrolyte layer 220. The positive current collector 211 and the positive electrode flux layer 212 can use the material of the positive electrode 21 of the battery 100 described above.

[0197] The positive electrode layer 212 may contain a solid electrolyte. Examples of solid electrolytes include the solid electrolyte described above used as the electrolyte in battery 100.

[0198] Electrolyte layer 220 is a layer containing an electrolyte. This electrolyte is, for example, a solid electrolyte. Electrolyte layer 220 can be a solid electrolyte layer. Examples of solid electrolytes include the solid electrolyte described above used as the electrolyte in battery 100.

[0199] The electrolyte layer 220 may also include a binder. The electrolyte layer 220 may also use the material of the positive electrode binder layer of the battery 100 described above as a binder.

[0200] The negative electrode 230 includes a negative electrode current collector 231 and a negative electrode binder layer 232 supported by the negative electrode current collector 231. The negative electrode binder layer 232 contains the negative electrode active material of this embodiment. The negative electrode binder layer 232 is located between the negative electrode current collector 231 and the electrolyte layer 220. The negative electrode current collector 231 and the negative electrode binder layer 232 can use the material of the negative electrode 23 of the battery 100 described above.

[0201] The negative electrode layer 232 may contain a solid electrolyte. Examples of solid electrolytes include the solid electrolyte described above used as the electrolyte in battery 100.

[0202] Battery 200 can be configured into various shapes such as coin type, cylindrical type, square type, sheet type, button type, flat type, and stacked type.

[0203] (Other implementation methods)

[0204] (Postscript)

[0205] Based on the description of the above embodiments, the following technology is disclosed.

[0206] (Technology 1)

[0207] A negative electrode active material, which possesses:

[0208] Composite particles, and

[0209] A covering layer that covers at least a portion of the surface of the aforementioned composite particles.

[0210] The aforementioned composite particles comprise: an alkali aluminate phase containing at least one alkali metal element and Al, and a silicon phase dispersed within the aforementioned alkali aluminate phase.

[0211] The aforementioned coating layer comprises lithium sulfonate compound and hydrophobic polymer compound.

[0212] This configuration can improve the cycle characteristics of the battery.

[0213] (Technology 2)

[0214] According to the negative electrode active material described in Technique 1, wherein...

[0215] The above-mentioned lithium sulfonate compounds include compounds represented by general formula (1),

[0216]

[0217] In general formula (1), R is an n-valent aliphatic hydrocarbon group with 1 to 5 carbon atoms, where n is 1 or 2. This configuration improves the cycle characteristics of the battery.

[0218] (Technology 3)

[0219] According to the negative electrode active material of technique 1 or 2, the lithium sulfonate compound comprises at least one selected from the group consisting of lithium methanesulfonate, lithium ethanesulfonate, and lithium propanesulfonate. With this configuration, the cycle characteristics of the battery can be improved.

[0220] (Technology 4)

[0221] According to any one of techniques 1 to 3, in the negative electrode active material, the lithium sulfonate compound in the aforementioned coating layer has a mass ratio of 1% or more relative to the aforementioned composite particles. With this configuration, side reactions between the composite particles and the electrolyte can be further suppressed, improving the battery's cycle characteristics and initial charge / discharge efficiency.

[0222] (Technology 5)

[0223] According to any one of techniques 1 to 4, in the negative electrode active material, the hydrophobic polymer compound in the aforementioned coating layer has a mass ratio of 1% or more relative to the aforementioned composite particles. With this configuration, the retention of lithium sulfonate compounds on the surface of the composite particles can be sufficiently improved, thereby improving the battery's cycle characteristics and initial charge / discharge efficiency.

[0224] (Technology 6)

[0225] According to any one of techniques 1 to 5, the negative electrode active material comprises a fluoropolymer compound. This configuration improves the cycle characteristics of the battery.

[0226] (Technology 7)

[0227] According to the negative electrode active material of Technology 6, the fluororesin comprises at least one selected from the group consisting of polyvinylidene fluoride, polytetrafluoroethylene, perfluoroalkoxyalkane, perfluoroethylene-propylene copolymer, ethylene-tetrafluoroethylene copolymer, polychlorotrifluoroethylene, and ethylene-chlorotrifluoroethylene copolymer. With this configuration, the cycle characteristics of the battery can be improved.

[0228] (Technology 8)

[0229] According to any one of techniques 1 to 7, the negative electrode active material further comprises B in the aforementioned alkali aluminate phase. With this configuration, the stability and ionic conductivity of the alkali aluminate phase are improved. Furthermore, since the porosity of the composite particles can be reduced, the deterioration of the particle structure caused by porosity can be suppressed. Therefore, the cycle characteristics of the battery can be improved.

[0230] (Technology 9)

[0231] According to the negative electrode active material of Technology 8, when the mass ratio of Al to the total mass of elements other than O constituting the alkali aluminate phase and the silicon phase is mAl, and the mass ratio of B to the 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, a more significant reduction in the porosity of the composite particles can be achieved, thus improving the cycle characteristics and initial charge / discharge efficiency of the battery.

[0232] (Technology 10)

[0233] According to any one of techniques 1 to 9, the negative electrode active material comprises at least one alkali metal element selected from the group consisting of Li, Na, and K. With this configuration, the cycle characteristics of the battery can be improved.

[0234] (Technology 11)

[0235] According to any one of techniques 1 to 10, the negative electrode active material has a porosity of 25% or less before the first charge. With this configuration, the deterioration of the particle structure due to porosity can be suppressed. Therefore, the cycle characteristics of the battery can be improved.

[0236] (Technology 12)

[0237] According to any one of techniques 1 to 11, the negative electrode active material comprises: a master particle composed of the alkali aluminate phase and the silicon phase, and a conductive layer covering at least a portion of the surface of the master particle, wherein the conductive layer comprises carbon. With this configuration, the conductivity of the composite particle is improved, thereby improving battery characteristics.

[0238] (Technology 13)

[0239] A battery that has the following features:

[0240] A negative electrode comprising the negative electrode active material described in any one of techniques 1 to 12,

[0241] Positive electrode, and

[0242] Electrolytes.

[0243] With this configuration, the cycle characteristics can be improved.

[0244] Example

[0245] The present disclosure will now be described in more detail using examples. These examples are merely illustrative and are not intended to limit the scope of the disclosure.

[0246] <Fabrication of Negative Electrode Material Particles>

[0247] (Example 1)

[0248] [Preparation of Composite Particles]

[0249] Al₂O₃, Li₂CO₃, and B₂O₃ were mixed in a mass ratio of Al₂O₃:Li₂CO₃:B₂O₃ = 55:25:20. The mixture was then calcined in air at 750°C for 10 hours. The calcined material was pulverized to obtain a raw aluminate with an average particle size of 10 μm.

[0250] Aluminate (average particle size 10 μm) and silicon (3N, average particle size 10 μm) were mixed at a mass ratio of 40:60. The resulting mixture was then packed into a jar (SUS, volume: 500 mL) of a planetary ball mill (Fritsch P-5). Next, 24 SUS balls (20 mm in diameter) were placed in the jar, the lid was closed, and the mixture was pulverized at 200 rpm for 50 hours in an inert atmosphere. This yielded a powdered mixture.

[0251] The obtained powdered mixture is removed in an inert atmosphere and then sintered at 800°C for 4 hours under a pressure of 200 MPa using a hot press in the same inert atmosphere. This yields a sintered body.

[0252] The resulting sintered body was pulverized and passed through a 40 μm sieve. As described above, master particles (LAX particles) containing a silicon phase dispersed in an alkali aluminate phase containing specified amounts of Li and B were obtained.

[0253] The obtained masterbatch was mixed with coal tar pitch (manufactured by JFE Chemicals, MCP250). At this point, the amount of coal tar pitch mixed was 5 parts by mass relative to 100 parts by mass of masterbatch. A conductive layer was formed on the surface of the masterbatch by calcining the mixture at 800°C for 5 hours in an inert atmosphere. Then, using a sieve, composite particles with an average particle size of 5 μm were obtained as LAX particles possessing the conductive layer.

[0254] [Formation of the covering layer]

[0255] The composite particles, lithium methanesulfonate (MSL) powder, and polyvinylidene fluoride (PVDF) dispersion were dispersed in water. At this point, MSL was set at 2 parts by mass relative to 100 parts by mass of the composite particles, and PVDF was set at 2 parts by mass relative to 100 parts by mass of the composite particles. Then, the dispersion was spray-dried to obtain granulated powder.

[0256] The obtained granulated powder is crushed using a mill (IKA MultiDrive basic) and then classified using an ultrasonic oscillator (Artech Ultrasonic Systems), thereby forming a coating layer on the surface of the composite particles as a composite layer of MSL and PVDF.

[0257] As described above, particles of Example 1 were prepared. The average particle size of the particles of Example 1 was 7 μm.

[0258] (Example 2)

[0259] The amount of MSL and PVDF used in the formation of the coating layer were changed to 4 parts by mass relative to 100 parts by mass of the composite particles, and the particles of Example 2 were prepared in the same manner as in Example 1.

[0260] (Example 3)

[0261] The amount of MSL and PVDF used in the formation of the capping layer were changed to 6 parts by mass relative to 100 parts by mass of the composite particles, and the particles of Example 3 were prepared in the same manner as in Example 1.

[0262] (Example 4)

[0263] The amount of MSL and PVDF used in the formation of the coating layer were changed to 8 parts by mass relative to 100 parts by mass of the composite particles, and the particles of Example 4 were prepared in the same manner as in Example 1.

[0264] (Comparative Example 1)

[0265] No covering layer was formed on the surface of the composite particles. Otherwise, the particles of Comparative Example 1 were prepared in the same manner as in Example 1. That is, the particles of Comparative Example 1 were composite particles having the conductive layer of LAX particles as described in Example 1.

[0266] (Comparative Example 2)

[0267] [Preparation of Composite Particles]

[0268] Li₂CO₃ and SiO₂ were mixed at an atomic ratio of 1.05 (Si / Li) to obtain a mixture. The mixture was calcined at 800°C for 10 hours in an inert gas atmosphere to obtain lithium silicate (Li₂Si₂O₅). Subsequently, the calcined material was pulverized to obtain lithium silicate with an average particle size of 10 μm.

[0269] Lithium silicate (average particle size 10 μm) and raw silicon (3N, average particle size 10 μm) were mixed at a mass ratio of 40:60. The mixture was then filled into a jar (SUS, volume: 500 mL) of a planetary ball mill (Fritsch P-5). Next, 24 SUS balls (20 mm in diameter) were placed in the jar, the lid was closed, and the mixture was pulverized at 200 rpm for 50 hours in an inactive atmosphere.

[0270] The powdered mixture is taken out in an inert atmosphere and then sintered at 600°C for 4 hours under pressure using a hot press in the same inert atmosphere. This yields a sintered body.

[0271] The sintered mixture obtained was pulverized and passed through a 40 μm sieve. Then, using a sieve, master particles (LSX particles) with an average particle size of 10 μm were obtained, in which a silicon phase was dispersed within the lithium silicate phase.

[0272] A conductive layer was formed on the surface of the obtained parent particles in the same manner as in Example 1. Then, using a sieve, composite particles with an average particle size of 5 μm were obtained as LSX particles having the conductive layer. This composite particle was used as the particle of Comparative Example 2.

[0273] (See Example 1 for reference)

[0274] [Preparation of Composite Particles]

[0275] Composite particles with LSX particles having a conductive layer were prepared in the same manner as in Comparative Example 2.

[0276] [Formation of the covering layer]

[0277] Similar to Example 1, a capping layer, which is a composite layer of MSL and PVDF, is formed on the surface of the composite particles.

[0278] As described above, particles similar to those in Reference Example 1 were produced.

[0279] <Battery Making>

[0280] Using the particles from Examples 1 to 4, Comparative Examples 1 to 2, and Reference Example 1, batteries from Examples 1 to 4, Comparative Examples 1 to 2, and Reference Example 1 were prepared as follows.

[0281] [Making the negative electrode]

[0282] Particles and graphite were mixed at a mass ratio of 10:90 to obtain a mixture. This mixture was used as the negative electrode active material. Water was added to a negative electrode slurry prepared by mixing the negative electrode active material, carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) at a mass ratio of negative electrode active material:CMC:SBR=97.5:1:1.5, and the mixture was stirred. Next, the negative electrode slurry was coated on both sides of a copper foil serving as the negative electrode current collector, and the coating was dried and calendered. It was then cut to a specified size to create a negative electrode with negative electrode slurry layers formed on both sides of the negative electrode current collector. At this point, an exposed portion of the negative electrode current collector was formed on a portion of the negative electrode.

[0283] [The production of the positive electrode]

[0284] A suitable amount of N-methyl-2-pyrrolidone (NMP) was added to a positive electrode mixture prepared by mixing positive electrode active material, acetylene black, and PVDF in a mass ratio of positive electrode active material: acetylene black: PVDF = 95:2.5:2.5, and then stirring to prepare a positive electrode slurry. The positive electrode active material used was LiNi... 0.88 Co 0.09 Al 0.03 O2 represents a lithium transition metal composite oxide. Next, a positive electrode paste is coated on both sides of an aluminum foil serving as the positive electrode current collector, and the coating is dried and then rolled. It is cut to a specified size to form a positive electrode with a positive electrode paste layer formed on both sides of the positive electrode current collector. At this time, a portion of the positive electrode current collector is provided to expose the positive electrode.

[0285] [Preparation of non-aqueous electrolytes]

[0286] A non-aqueous electrolyte was prepared by dissolving lithium hexafluorophosphate (LiPF6) at a concentration of 1 mol / L in a mixed solvent consisting of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of EC:EMC:DMC = 20:5:75.

[0287] [Construction of a non-aqueous electrolyte secondary battery]

[0288] An aluminum positive electrode lead is installed on the exposed portion of the positive current collector at the positive electrode, and a nickel negative electrode lead is installed on the exposed portion of the negative current collector at the negative electrode. The positive and negative electrodes are wound into a spiral shape with a polyolefin separator in between, and then pressed radially to create a flat, wound electrode body. This electrode body is housed in an outer casing made of aluminum laminate, a non-aqueous electrolyte is injected, and the opening of the outer casing is sealed to obtain the battery.

[0289] [Battery Review]

[0290] For the batteries of Examples 1 to 4, Comparative Examples 1 to 2 and Reference Example 1, the initial charge-discharge efficiency and cycle characteristics were evaluated by the following method.

[0291] (Initial charge / discharge efficiency)

[0292] For the batteries of Examples 1 to 4, Comparative Examples 1 to 2, and Reference Example 1, at an ambient temperature of 25°C, they were charged at a constant current of 0.3C until the voltage reached 4.2V, and then charged at a constant voltage of 4.2V until the current reached 0.02C. Then, they were discharged at a constant current of 0.5C until the voltage reached 2.5V. The initial charge-discharge efficiency of the batteries of Examples 1 to 4, Comparative Examples 1 to 2, and Reference Example 1 was determined through this charge-discharge process. Using the obtained initial charge-discharge efficiency, the amount of side reaction was calculated using the following formula. Here, E represents the initial charge-discharge efficiency (%) of Examples 1 to 4, Comparative Examples 1 to 2, and Reference Example 1, and E0 represents the initial charge-discharge efficiency (%) of Comparative Example 1. The results are shown in Table 1.

[0293] Side reaction amount = {(100-E) / (100-E0)} × 100

[0294] (Cyclic characteristics)

[0295] For the batteries of Examples 1 to 4, Comparative Examples 1 to 2, and Reference Example 1, at an ambient temperature of 25°C, they were charged at a constant current of 0.3C until the voltage reached 4.2V, and then charged at a constant voltage of 4.2V until the current reached 0.02C. Then, they were discharged at a constant current of 0.5C until the voltage reached 2.5V. This charge-discharge cycle was considered as one cycle, and 300 cycles were performed. The discharge capacity retention rate was calculated using the following formula.

[0296] Discharge capacity retention rate = (Discharge capacity at 300th cycle / Discharge capacity at 1st cycle) × 100

[0297] Using the calculated discharge capacity retention rate, the cycle degradation rate was determined by the following formula. Here, R represents the discharge capacity retention rate of Examples 1 to 4, Comparative Examples 1 to 2, and Reference Example 1, and R0 represents the discharge capacity retention rate of Comparative Example 1. The results are shown in Table 1.

[0298] Cyclic degradation rate = {(100-R) / (100-R0)} × 100

[0299] [Table 1]

[0300]

[0301] (Inspection)

[0302] The batteries of Examples 1 to 4 exhibited lower cycle degradation rates compared to the batteries of Comparative Examples 1 to 2 and Reference Example 1. That is, the batteries of Examples 1 to 4 demonstrated superior cycle characteristics compared to the batteries of Comparative Examples 1 to 2 and Reference Example 1. Furthermore, the batteries of Examples 1 to 4 exhibited lower amounts of side reactions compared to the battery of Comparative Example 1. In other words, the batteries of Examples 1 to 4 displayed higher initial charge-discharge efficiency than the battery of Comparative Example 1. Additionally, the batteries of Examples 2 to 4 exhibited lower amounts of side reactions compared to the battery of Comparative Example 2.

[0303] Industrial availability

[0304] The technology disclosed herein 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 that covers at least a part of a surface of the composite particle, the composite particle comprising: an alkali aluminate phase containing at least one alkali metal element and Al, and a silicon phase dispersed in the alkali aluminate phase, the coating layer comprising a lithium sulfonate compound and a hydrophobic polymer compound.

2. The negative electrode active material according to claim 1, wherein the lithium sulfonate compound comprises a compound represented by general formula (1), in general formula (1), R is an aliphatic hydrocarbon group having a carbon number of 1 to 5 and a valence of n, and n is 1 or 2.

3. The negative electrode active material according to claim 2, wherein the lithium sulfonate compound comprises 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 the lithium sulfonate compound in the coating layer with respect to the composite particle is 1 mass% or more.

5. The negative electrode active material according to claim 1, wherein a mass ratio of the hydrophobic polymer compound in the coating layer with respect to the composite particle 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, perfluoroalkoxy alkane, perfluoroethylene-propylene copolymer, ethylene-tetrafluoroethylene copolymer, polytrifluorochloroethylene, and ethylene-trifluorochloroethylene copolymer.

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

9. The negative electrode active material according to claim 8, wherein a mass ratio of Al with respect to a total mass of elements other than O that constitute the alkali aluminate phase and the silicon phase is mA1, a mass ratio of B with respect to the total mass of the elements other than O that constitute the alkali aluminate phase and the silicon phase is mB, and a ratio mA1 / 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 comprises at least one selected from the group consisting of Li, Na, and K.

11. The negative electrode active material according to claim 1, wherein a porosity of the composite particle before initial charging is 25% or less.

12. The negative electrode active material according to claim 1, wherein the composite particle comprises: a parent particle composed of the alkali aluminate phase and the silicon phase, and a conductive layer that covers at least a part of a surface of the parent particle, the conductive layer comprising carbon.

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

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