Negative electrode active material for secondary battery and secondary battery using the same

Abstract

The negative electrode active material for a secondary battery contains a silicon-containing material, and the silicon-containing material contains: a lithium ion conduction phase, silicon particles dispersed in the lithium ion conduction phase, and particles containing vanadium dispersed in the lithium ion conduction phase.

Description

Technical Field

[0001] The present disclosure mainly relates to a negative electrode active material for a secondary battery. Background Art

[0002] In recent years, secondary batteries such as non-aqueous electrolyte secondary batteries have high voltage and high energy density, and thus can be expected as power sources for small consumer applications, power storage devices, and electric vehicles. Since high energy density of the battery is required, as a negative electrode active material having a high theoretical capacity density, a material containing silicon alloyed with lithium (for example, Patent Document 1) can be expected.

[0003] However, a material containing silicon has large expansion and contraction due to charge and discharge, and thus there is a problem that the capacity is likely to decrease if the secondary battery is repeatedly charged and discharged.

[0004] Therefore, Patent Document 2 proposes a negative electrode active material for a non-aqueous electrolyte secondary battery, which includes: Li 2z SiO (2+z) a lithium silicate phase represented by {0 < z < 2}, silicon particles dispersed in the lithium silicate phase, and metal particles dispersed in the lithium silicate phase and having a main component of one or more metals or alloys selected from Fe, Pb, Zn, Sn, Cu, Ni, and Cr.

[0005] Prior Art Documents

[0006] Patent Documents

[0007] Patent Document 1: International Publication No. 2016 / 35290

[0008] Patent Document 2: International Publication No. 2016 / 121320 Summary of the Invention

[0009] According to Patent Document 2, in a non-aqueous electrolyte secondary battery using a silicon material as a negative electrode active material, the initial charge-discharge efficiency can be improved and the cycle life can be extended. However, the effect of extending the cycle life is not sufficient, and there is room for improvement.

[0010] One aspect of the present disclosure relates to a negative electrode active material for a secondary battery, which includes a silicon-containing material, and the silicon-containing material includes: a lithium ion conduction phase, silicon particles dispersed in the lithium ion conduction phase, and particles containing vanadium dispersed in the lithium ion conduction phase.

[0011] Another aspect of the present disclosure relates to a secondary battery, which includes: a positive electrode, a negative electrode, an electrolyte, and a separator interposed between the positive electrode and the negative electrode, and the negative electrode includes the negative electrode active material for a secondary battery described above.

[0012] According to the negative electrode active material of the present disclosure, a secondary battery having excellent charge-discharge cycle characteristics can be obtained. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 FIG. is a cross-sectional view schematically showing a negative electrode active material (silicon-containing material) for a secondary battery according to an embodiment of the present disclosure.

[0014] Figure 2 FIG. is a schematic perspective view of a part of a secondary battery according to an embodiment of the present disclosure cut away. DETAILED DESCRIPTION

[0015] The negative electrode active material for a secondary battery according to an embodiment of the present disclosure contains a silicon-containing material. In addition, the silicon-containing material includes: a lithium ion conduction phase, silicon particles dispersed in the lithium ion conduction phase, and particles containing vanadium dispersed in the lithium ion conduction phase.

[0016] The silicon-containing material has a so-called island structure. The silicon particles (islands) are dispersed in the matrix (sea) of the lithium ion conduction phase and are covered by the lithium ion conduction phase. In the island structure, the contact between the silicon particles and the electrolyte is limited, so side reactions can be suppressed. In addition, the stress generated by the expansion and contraction of the silicon particles is alleviated by the matrix of the lithium ion conduction phase.

[0017] Here, particles containing vanadium are dispersed in the lithium ion conduction phase. Vanadium improves the strength of the particles of the silicon-containing material. It is considered that vanadium forms micro-regions with high hardness in the lithium ion conduction phase. By uniformly distributing the micro-regions with high hardness in the matrix of the lithium ion conduction phase, the expansion and contraction of the silicon-containing material due to charge and discharge can be suppressed, and the occurrence of cracks in the silicon-containing material and the isolation of a part of the silicon-containing material caused by the collapse of the particles can be suppressed. Therefore, the cycle life of the secondary battery is significantly improved.

[0018] It should be noted that by dispersing iron in the lithium ion conduction phase, the expansion of the silicon-containing material due to charge and discharge can also be suppressed, and the occurrence of cracks in the particles of the silicon-containing material and the collapse of the particles can be suppressed. In order to obtain such an effect, a relatively large amount of iron needs to be contained in the lithium ion conduction phase.

[0019] On the other hand, iron tends to react with silicon particles and reduce the capacity. Different from iron, only a trace amount of vanadium contained in the lithium ion conduction phase can exhibit the effects of suppressing the expansion and contraction of the silicon-containing material due to charge and discharge, suppressing the occurrence of cracks in the particles of the silicon-containing material, and suppressing the collapse of the particles.

[0020] The content of vanadium contained in the silicon-containing material can be, for example, 1000 ppm or less, 500 ppm or less, 300 ppm or less, or 100 ppm or less. From the viewpoint of enhancing the effect of increasing the hardness of the lithium-ion conductive phase, the content of vanadium contained in the silicon-containing material is preferably 5 ppm or more, can be 10 ppm or more, or can be 30 ppm or more.

[0021] The content of vanadium contained in the silicon-containing material can be measured, for example, by inductively coupled plasma atomic emission spectrometry (ICP-AES). Specifically, a sample of the silicon-containing material is completely dissolved in a heated acid solution (a mixed acid of hydrofluoric acid, nitric acid, and sulfuric acid), the carbon in the solution residue is filtered off and removed, and then the resulting filtrate is analyzed by ICP-AES to measure the spectral intensity of vanadium. Next, a calibration curve is prepared using a commercially available standard solution of the element, and the content of vanadium is calculated.

[0022] In the vanadium-containing particles, vanadium can also be contained in at least one selected from the group consisting of vanadium alloys and vanadium carbides. That is, the vanadium-containing particles can be vanadium-containing alloys, vanadium carbides, etc. Among them, the Vickers hardness of vanadium carbide is about 2800, and the effect of significantly increasing the hardness of the lithium-ion conductive phase can be obtained.

[0023] The vanadium-containing particles can further contain iron. That is, the vanadium-containing particles can be formed from an alloy containing vanadium and iron, or an alloy containing vanadium carbide and iron. The vanadium-containing alloy can also contain elemental vanadium and / or vanadium carbide in a dispersed state. Since iron has excellent ductility, it is possible to significantly suppress cracking and collapse of the silicon-containing material that may occur during repeated charge and discharge. Therefore, the cycle life can be greatly improved. In addition, when vanadium and iron are used in combination, this unique effect of iron can be fully exhibited even when the amount of iron is reduced.

[0024] The content of iron contained in the silicon-containing material is, for example, 0.01% by mass or more, can be 10% by mass or less, can be 0.1% by mass or more, or can be 3% by mass or less.

[0025] The content of iron contained in the silicon-containing material can be measured by ICP-AES in the same manner as vanadium. Specifically, a sample of the silicon-containing material is completely dissolved in a heated acid solution, the carbon in the solution residue is filtered off and removed, and then the resulting filtrate is analyzed by ICP-AES to measure the spectral intensity of iron. Next, a calibration curve is prepared using a commercially available standard solution of the element, and the content of iron is calculated.

[0026] The average particle diameter of the silicon particles contained in the silicon-containing material can be, for example, 1 nm or more and 1000 nm or less. The average particle diameter of the silicon particles is preferably 500 nm or less, more preferably 200 nm or less, and further preferably 50 nm or less. That is, the silicon particles can be fine particles. The smaller the average particle diameter of the silicon particles, the smaller the volume change during charge and discharge, and the more improved the structural stability. In addition, the expansion and contraction of the silicon particles are made uniform, and particle cracks are suppressed.

[0027] The average particle diameter of the silicon particles is measured by observing the cross-section of the silicon-containing material using SEM or TEM. Specifically, it is obtained by averaging the maximum diameters of any 100 silicon particles.

[0028] The lithium-ion conductive phase can be, for example, at least one selected from the group consisting of a silicon oxide phase, a silicate phase, and a carbon phase. Among them, the silicate phase is suitable for obtaining a silicon-containing material with a small irreversible capacity and a high capacity at the initial stage of charge and discharge.

[0029] The silicon oxide phase contains silica as its main component (for example, 95 to 100% by mass). The composition of the composite material containing the silicon oxide phase and the silicon particles dispersed in the silicon oxide phase can be represented as SiO x SiO x has a structure in which fine particles of silicon are dispersed in amorphous SiO2. The content ratio x of oxygen to silicon is, for example, 0.5 ≤ x < 2.0, and more preferably 0.8 ≤ x ≤ 1.5.

[0030] The carbon phase can be composed of, for example, low-crystalline amorphous carbon (i.e., amorphous carbon). The amorphous carbon can be, for example, hard carbon, soft carbon, or others.

[0031] The silicate phase can contain at least one selected from the group consisting of an alkali metal element and a Group II element. By containing such an element, the irreversible capacity of the silicate phase is more significantly reduced. Among them, from the viewpoint of a small irreversible capacity and a high initial charge-discharge efficiency, a silicate phase containing lithium (hereinafter also referred to as a lithium silicate phase) is preferred. As the alkali metal element and the Group II element, for example, Li, K, Na, Mg, Ca, Sr, Ba, etc. can be used.

[0032] The silicate phase can further contain an element M other than the alkali metal element and the Group II element. Here, the element M can be, for example, at least one selected from the group consisting of B, Al, Zr, Nb, Ta, La, Y, Ti, P, Bi, Zn, Sn, Pb, Sb, Co, Er, F, and W.

[0033] The silicate phase preferably contains the formula Li 2z SiO 2+zLithium silicate as shown in (0 < z < 2). The lithium silicate is light in weight and excellent in lithium ion conductivity. The lithium silicate may be an oxide phase containing Li, Si, and O, and may also contain other elements. The atomic ratio O / Si of O to Si in the lithium silicate phase is, for example, greater than 2 and less than 4. O / Si is preferably greater than 2 and less than 3. The atomic ratio Li / Si of Li to Si in the lithium silicate phase is, for example, greater than 0 and less than 4. z preferably satisfies the relationship 0 < z < 1, and more preferably z = 1 / 2.

[0034] When the lithium ion conductive phase is a silicate phase, from the viewpoint of improving high capacity and cycle characteristics, the content of silicon particles in the silicon-containing material is desirably, for example, 30% by mass or more and 80% by mass or less. By setting the content of silicon particles to 30% by mass or more, the proportion of the silicate phase becomes smaller, and it is easy to improve the initial charge-discharge efficiency. By setting the content of silicon particles to 80% by mass or less, it is easy to reduce the degree of expansion and contraction of the silicon-containing material during charge and discharge. The content of silicon particles in the silicon-containing material is preferably 40% by mass or more, and more preferably 50% by mass or more.

[0035] The composition of the silicate phase can be analyzed by the following method. The analysis of the composition is desirably carried out using the silicon-containing material or the negative electrode composite material layer in the discharged state. In addition, from the viewpoint of eliminating the influence of decomposition products of the electrolyte, it is desirable to analyze a sample of the silicon-containing material in the battery before or at the initial stage of charge-discharge cycling.

[0036] The contents of B, Na, K, and Al contained in the silicate layer can be determined, for example, by quantitative analysis according to JIS R3105 (1995) (Method for analysis of borosilicate glass). In addition, the Ca content can be determined by quantitative analysis according to JIS R3101 (1995) (Method for analysis of soda-lime glass).

[0037] The content of each element contained in the silicon-containing material can be measured by ICP-AES in the same manner as vanadium. Specifically, the sample of the silicon-containing material is completely dissolved in a heated acid solution, the carbon in the solution residue is filtered and removed, and then the resulting filtrate is analyzed by ICP-AES to measure the spectral intensity of each element. Then, a calibration curve is made using commercially available standard solutions of each element, and the content of each element is calculated.

[0038] When analyzing the composition of the silicate phase, a silicon-containing material can be taken out of the battery using the following method, for example. Specifically, the battery is disassembled and the negative electrode is taken out, and the negative electrode is washed with anhydrous ethyl methyl carbonate or dimethyl carbonate to remove the electrolyte. Next, the negative electrode composite layer is peeled off from the negative electrode current collector and pulverized with a mortar to obtain a sample powder. Next, the sample powder is dried in a dry atmosphere for 1 hour, immersed in slightly boiling 6M hydrochloric acid for 10 minutes to remove alkali metals such as Na and Li that may be contained in the binder, etc. Next, the sample powder is washed with ion-exchanged water, filtered, and dried at 200 °C for 1 hour. Then, by heating in an oxygen atmosphere and removing the carbon component, it is possible to isolate only the particles of the silicon-containing material.

[0039] The silicon-containing material may contain a silicate phase, a silicon oxide phase, silicon particles, etc. By using Si-NMR, these can be distinguished and quantified. As described above, the Si content obtained by ICP-AES is the total of the Si amount constituting the silicon particles, the Si amount in the lithium-ion conducting phase, and the Si amount in the silicon oxide phase. On the other hand, the Si amount constituting the silicon particles and the Si amount in the silicon oxide phase can be quantified separately using Si-NMR. Therefore, by subtracting the Si amount constituting the silicon particles and the Si amount in the silicon oxide phase from the Si content obtained by ICP-AES, it is possible to quantify the Si amount in the silicate phase. It should be noted that as the reference material required for quantification, a mixture containing a silicate and silicon particles with a known Si content in a specified ratio may be used.

[0040] The following shows the desired measurement conditions for Si-NMR.

[0041] <Si-NMR Measurement Conditions>

[0042] Measuring device: Solid nuclear magnetic resonance spectrometer (INOVA-400) manufactured by Varian

[0043] Probe: Varian 7mm CPMAS-2

[0044] MAS: 4.2 kHz

[0045] MAS speed: 4 kHz

[0046] Pulse: DD (45° pulse + signal acquisition time 1H decoupling)

[0047] Repetition time: 1200 seconds to 3000 seconds

[0048] Observation width: 100 kHz

[0049] Observation center: Around -100 ppm

[0050] Signal acquisition time: 0.05 seconds

[0051] Cumulative count: 560

[0052] Sample amount: 207.6 mg

[0053] In addition, the quantification of each element in the silicon-containing material can be performed by SEM-EDX analysis, Auger electron spectroscopy (AES), laser ablation ICP mass spectrometry (LA-ICP-MS), X-ray photoelectron spectroscopy (XPS), etc.

[0054] For example, the quantification of each element in the silicon-containing material based on SEM-EDX analysis can also be performed by observing the cross-section of the silicon-containing material in the cross-section of the negative electrode composite material layer. The cross-section observation can be performed, for example, by the following method. First, disassemble the battery, take out the negative electrode, and use a cross-section polisher (CP) to obtain the cross-section of the negative electrode composite material layer. Observe the cross-section of the negative electrode composite material layer using a scanning electron microscope (SEM). From the cross-sectional image of the backscattered electron image of the negative electrode composite material layer, randomly select 10 silicon-containing materials with a maximum diameter of 5 μm or more for each particle, and perform elemental mapping analysis based on energy-dispersive X-ray (EDX) for each. Use image analysis software to calculate the area containing the target element. The observation magnification is preferably 2000 to 20000 times. The measured values of the area containing 10 specified elements of the obtained particles are averaged to obtain.

[0055] It should be noted that during the charge and discharge process, a coating film is formed on the surface of the silicon-containing material due to the decomposition of the electrolyte, etc. In addition, the silicon-containing material sometimes has a conductive layer on its surface. Therefore, the EDX-based mapping analysis is performed for the region 1 μm or more inside from the peripheral edge of the cross-section of the silicon-containing material so that the measurement range does not include the coating film and the conductive layer.

[0056] The following shows the measurement conditions for the desired cross-section SEM-EDX analysis.

[0057] <SEM-EDX measurement conditions>

[0058] Processing device: JEOL, SM-09010 (Cross Section Polisher)

[0059] Processing conditions: Acceleration voltage 6 kV

[0060] Current value: 140 μA

[0061] Vacuum degree: 1×10 -3 ~2×10 -3 Pa

[0062] Measurement device: Electron microscope HITACHI SU-70

[0063] Accelerating voltage during analysis: 10 kV

[0064] Field: Free mode

[0065] Probe current mode: Medium

[0066] Probe current range: High

[0067] Anode Ap.: 3

[0068] OBJ Ap.: 2

[0069] Analysis area: 1 μm square

[0070] Analysis software: EDAX Genesis

[0071] CPS: 20500

[0072] Lsec: 50

[0073] Time constant: 3.2

[0074] The crystallite size of the silicon particles dispersed in the lithium ion conductive phase is preferably 30 nm or less. When the crystallite size of the silicon particles is 30 nm or less, the volume change amount caused by the expansion and contraction of the silicon particles during charge and discharge can be reduced, and the cycle characteristics can be further improved. The crystallite size of the silicon particles is more preferably 2 nm or more and 30 nm or less, and still more preferably 2 nm or more and 20 nm or less. When the crystallite size of the silicon particles is 20 nm or less, the expansion and contraction of the silicon particles can be made uniform, the fine cracks of the particles caused by the expansion and contraction of the silicon particles during charge and discharge can be reduced, and the cycle characteristics can be improved.

[0075] The crystallite size of the silicon particles is calculated by the Scherrer formula using the half-value width of the diffraction peak attributed to the Si(111) plane in the X-ray diffraction (XRD) pattern of the silicon particles.

[0076] At least a part of the surface of the silicon-containing material can also be covered with a conductive layer. By forming a conductive layer on the surface of the silicon-containing material, the conductivity of the silicon-containing material can be significantly improved. As the conductive material constituting the conductive layer, a carbon material is preferred. The carbon material preferably contains at least one selected from the group consisting of carbon compounds and carbon-containing substances.

[0077] The thickness of the conductive layer is preferably thin to the extent that it does not substantially affect the average particle size of the silicon-containing material. Considering the ensuring of conductivity and the diffusibility of lithium ions, the thickness of the conductive layer is preferably 1 to 200 nm, and more preferably 5 to 100 nm. The thickness of the conductive layer can be measured by observing the cross section of the silicon-containing material using SEM or TEM (transmission electron microscope).

[0078] As carbon compounds, examples include compounds containing carbon and hydrogen, and compounds containing carbon, hydrogen, and oxygen. As carbon-containing substances, amorphous carbon with low crystallinity, highly crystalline graphite, etc. can be used. As amorphous carbon, carbon black, coal, coke, charcoal, activated carbon, etc. can be cited. As graphite, natural graphite, artificial graphite, graphitized mesophase carbon particles, etc. can be cited. Among them, from the viewpoints of low hardness and large buffering effect on silicon particles that change in volume due to charge and discharge, amorphous carbon is preferred. The amorphous carbon can be easily graphitizable carbon (soft carbon) or hardly graphitizable carbon (hard carbon). As carbon black, acetylene black, Ketjen black, etc. can be cited.

[0079] Next, an example of the manufacturing method of the silicon-containing material will be described in detail. Here, the case where silicon particles are dispersed in lithium silicate will be described.

[0080] Step (i)

[0081] As raw materials for lithium silicate, a raw material mixture containing Si raw material and Li raw material in a specified ratio is used. A vanadium raw material (such as vanadium carbide, ferrovanadium alloy, etc.) is added to the raw material mixture. In addition, the above-mentioned alkali metal element, Group II element, and / or element M may be included in the raw material mixture. The raw material mixture is dissolved, and the melt is passed through a metal roll and made into a thin sheet to produce lithium silicate. Then, the thinned silicate is heat-treated at a temperature above the glass transition point and below the melting point in an atmospheric atmosphere to crystallize it. It should be noted that the thinned silicate can also be used without crystallization. The raw material mixture can also be calcined at a temperature below the melting point without being dissolved, and the silicate can be produced by a solid-phase reaction.

[0082] As the Si raw material, silicon oxide can be used. As the Li raw material, for example, lithium carbonate, lithium oxide, lithium hydroxide, lithium hydride, etc. can be used. These can be used alone or in combination of two or more. As raw materials for the alkali metal element, Group II element, and element M, oxides, hydroxides, carbonate compounds, hydrides, nitrates, sulfates, etc. of each element can be cited.

[0083] Step (ii)

[0084] Next, the raw material silicon is compounded with lithium silicate. For example, a silicon-containing material as composite particles of lithium silicate and silicon particles (hereinafter also referred to as silicate composite particles) can be produced through the following steps (a) to (c).

[0085] Step (a)

[0086] For example, the powder of the raw material silicon and the powder of lithium silicate are mixed at a mass ratio of 20:80 to 95:5. As the raw material silicon, coarse particles of silicon with an average particle size of several μm to several tens of μm can be used.

[0087] Step (b)

[0088] Next, using a pulverizing device such as a ball mill, the mixture of raw material silicon and lithium silicate is pulverized and compounded while being made into fine particles. At this time, an organic solvent can also be added to the mixture for wet pulverization. The organic solvent functions to prevent the object to be pulverized from adhering to the inner wall of the pulverization container.

[0089] As the organic solvent, alcohols, ethers, fatty acids, alkanes, cycloalkanes, silicate esters, metal alkoxides, etc. can be used.

[0090] It should be noted that the raw material silicon and lithium silicate can be separately made into fine particles and then mixed. In addition, a pulverizing device may not be used, and silicon nanoparticles and amorphous lithium silicate nanoparticles may be produced and mixed. For the production of the nanoparticles, known methods such as a gas phase method (e.g., plasma method) and a liquid phase method (e.g., liquid phase reduction method) can be used.

[0091] Process (c)

[0092] Next, for example, in an inert gas atmosphere (e.g., an atmosphere of argon, nitrogen, etc.), the mixture is pressurized and sintered while being heated to 600°C to 1000°C. Sintering can be performed using a sintering device capable of pressurization in an inert atmosphere such as hot pressing. During sintering, the silicate softens and flows to fill the gaps between the silicon particles. As a result, a dense massive sintered body in which the silicate phase is the sea part and the silicon particles are the island parts can be obtained. If the obtained sintered body is pulverized, silicate composite particles can be obtained.

[0093] Process (iii)

[0094] Next, at least a part of the surface of the composite particles can be covered with a conductive material to form a conductive layer. As a method of covering the surface of the composite particles with a conductive carbon material, methods such as a CVD method using hydrocarbon gases such as acetylene and methane as raw materials; a method of mixing coal tar pitch, petroleum pitch, phenolic resin, etc. with the composite particles and heating them to carbonize in an inert atmosphere (e.g., an atmosphere of argon, nitrogen, etc.) at 700°C to 950°C can be exemplified. In addition, carbon black can also be attached to the surface of the composite particles.

[0095] Process (iv)

[0096] A process of cleaning the composite particles (including the case where the surface has a conductive layer) with an acid can be performed. For example, by cleaning the composite particles with an acidic aqueous solution, trace amounts of alkali components that may be generated during the compounding of the raw material silicon and lithium silicate can be dissolved and removed. As the acidic aqueous solution, aqueous solutions of inorganic acids such as hydrochloric acid, hydrofluoric acid, sulfuric acid, nitric acid, phosphoric acid, and carbonic acid, and aqueous solutions of organic acids such as citric acid and acetic acid can be used.

[0097] Figure 1Schematically shown is a cross-section of a silicate composite particle 10 covered with a conductive layer as an example of a silicon-containing material.

[0098] The silicate composite particle (base particle) 13 includes: a lithium silicate phase 11 and silicon particles 12 dispersed in the lithium silicate phase 11. The silicate composite particle (base particle) 13 has a sea-island structure in which fine silicon particles 12 are dispersed in the matrix of the lithium silicate phase 11. Particles 15 containing vanadium are also dispersed in the matrix of the lithium silicate phase 11. The particles 15 containing vanadium generally have an average particle size smaller than that of the silicon particles 12. The surface of the base particle 13 is covered with a conductive layer 14.

[0099] The average particle size of the particles 15 containing vanadium is measured by observing the cross-section of the silicon-containing material using SEM or TEM. Specifically, it is obtained by averaging the maximum diameters of any 100 particles containing vanadium.

[0100] A silica phase may be dispersed in the lithium silicate phase 11. The content of SiO2 in the silicate composite particle (base particle) 13 measured by Si-NMR is, for example, preferably 30% by mass or less, more preferably less than 7% by mass.

[0101] The silicate composite particle (base particle) 13 may further contain other components in addition to the above. For example, carbon materials, oxides such as ZrO2, and reinforcing materials such as carbides may be contained in an amount of less than 10% by mass relative to the base particle 13.

[0102] Next, a secondary battery according to an embodiment of the present disclosure includes: a positive electrode, a negative electrode, an electrolyte, and a separator interposed between the positive electrode and the negative electrode. The negative electrode includes a negative electrode composite material layer, and the negative electrode composite material layer includes a negative electrode active material containing the above-described silicon-containing material. Hereinafter, the negative electrode, positive electrode, electrolyte, and separator included in the secondary battery according to the embodiment of the present disclosure will be described.

[0103] [Negative Electrode]

[0104] The negative electrode includes, for example, a negative electrode current collector and a negative electrode composite material layer formed on the surface of the negative electrode current collector and containing a negative electrode active material. The negative electrode composite material layer can be formed by coating a negative electrode slurry in which a negative electrode composite material is dispersed on the surface of the negative electrode current collector and drying it. The dried coating film can also be calendered as needed.

[0105] The negative electrode composite material may contain a negative electrode active material containing the above-described silicon-containing material as an essential component, and may contain a binder, a conductive agent, a thickener, etc. as optional components. Since the silicon particles of the silicon-containing material can absorb and store a large amount of lithium ions, a negative electrode with a high capacity can be obtained.

[0106] The negative electrode active material may further contain other active material materials that electrochemically store and release lithium ions. As other active material materials, for example, carbon-based active materials are preferably used. Since the volume of the silicon-containing material expands and contracts during charge and discharge, if the ratio thereof in the negative electrode active material increases, the contact between the negative electrode active material and the negative electrode current collector is likely to deteriorate during charge and discharge. On the other hand, by using the silicon-containing material and the carbon-based active material in combination, it is possible to impart a high capacity of silicon particles to the negative electrode and to achieve excellent cycle characteristics. The proportion of the silicon-containing material in the total of the silicon-containing material and the carbon-based active material is, for example, preferably 0.5 to 15% by mass, more preferably 1 to 5% by mass. Thereby, it is easy to balance high capacity and improvement of cycle characteristics.

[0107] As the carbon-based active material, for example, graphite, graphitizable carbon (soft carbon), non-graphitizable carbon (hard carbon), etc. can be exemplified. Among them, graphite with excellent charge-discharge stability and less irreversible capacity is preferably used. Graphite refers to a material having a graphite-type crystal structure, and includes, for example, natural graphite, artificial graphite, graphitized mesophase carbon particles, etc. The carbon-based active material can be used alone as one kind, or two or more kinds can be used in combination.

[0108] As the negative electrode current collector, a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a grid body, a net body, a punched sheet, etc.) is used. As the material of the negative electrode current collector, stainless steel, nickel, nickel alloy, copper, copper alloy, etc. can be exemplified.

[0109] Binders can be exemplified by fluororesins, polyolefin resins, polyamide resins, polyimide resins, vinyl resins, styrene-butadiene rubber (SBR), polyacrylic acid and its derivatives, etc. These can be used alone as one kind, or two or more kinds can be used in combination.

[0110] As the conductive agent, carbon black, conductive fibers, carbon fluoride, organic conductive materials, etc. can be exemplified. These can be used alone as one kind, or two or more kinds can be used in combination.

[0111] As the thickener, carboxymethyl cellulose (CMC), polyvinyl alcohol, etc. can be listed. These can be used alone as one kind, or two or more kinds can be used in combination.

[0112] As the dispersion medium, water, alcohol, ether, N-methyl-2-pyrrolidone (NMP), or a mixed solvent thereof, etc. can be exemplified.

[0113] [Positive Electrode]

[0114] The positive electrode includes, for example, a positive electrode current collector and a positive electrode composite material layer formed on the surface of the positive electrode current collector. The positive electrode composite material layer can be formed by coating a positive electrode paste in which a positive electrode composite material is dispersed in a dispersion medium on the surface of the positive electrode current collector and drying it. The dried coating film can also be calendered as needed.

[0115] The positive electrode composite material may contain a positive electrode active material as an essential component, and may contain a binder, a conductive agent, etc. as optional components.

[0116] As the positive electrode active material, a lithium composite metal oxide can be used. As the lithium composite metal oxide, for example, Li a CoO2, Li a NiO2, Li a MnO2, Li a Co b Ni 1-b O2, Li a Co b M 1-b O c 、Li a Ni 1-b M b O c 、Li a Mn2O4, Li a Mn 2- b M b O4, LiMePO4, Li2MePO4F. Here, M is at least one selected from the group consisting of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B. Me contains at least a transition element (for example, contains at least one selected from the group consisting of Mn, Fe, Co, and Ni). Here, 0 ≤ a ≤ 1.2, 0 ≤ b ≤ 0.9, 2.0 ≤ c ≤ 2.3. It should be noted that the value of a representing the molar ratio of lithium is the value after the active material is just made, and it increases or decreases according to charge and discharge.

[0117] As the binder and the conductive agent, the same ones as those exemplified for the negative electrode can be used. As the conductive agent, graphite such as natural graphite and artificial graphite can be used.

[0118] The positive electrode current collector can use a conductive substrate based on the negative electrode current collector. As the material of the positive electrode current collector, for example, stainless steel, aluminum, aluminum alloy, titanium, etc. can be exemplified.

[0119] [Electrolyte]

[0120] The electrolyte contains a solvent and a lithium salt dissolved in the solvent. The concentration of the lithium salt in the electrolyte is, for example, 0.5 to 2 mol / L. The electrolyte can also contain known additives.

[0121] The solvent used is an aqueous solvent or a non-aqueous solvent. As the non-aqueous solvent, for example, cyclic carbonates, linear carbonates, cyclic carboxylates, etc. are used. As the cyclic carbonate, propylene carbonate (PC), ethylene carbonate (EC), etc. can be cited. As the linear carbonate, diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), etc. can be cited. As the cyclic carboxylate, γ-butyrolactone (GBL), γ-valerolactone (GVL), etc. can be cited. The non-aqueous solvent can be used alone or in combination of two or more.

[0122] As the lithium salt, for example, lithium salts containing perchloric acid (LiClO4, LiAlCl4, LiB 10 Cl 10 etc.), lithium salts containing fluoric acid (LiPF6, LiBF4, LiSbF6, LiAsF6, LiCF3SO3, LiCF3CO2, etc.), lithium salts containing fluorimide (LiN(CF3SO2)2, LiN(CF3SO2)(C4F9SO2), LiN(C2F5SO2)2, etc.), lithium halides (LiCl, LiBr, LiI, etc.), etc. The lithium salt can be used alone or in combination of two or more.

[0123] [Separator]

[0124] It is desirable to sandwich a separator between the positive electrode and the negative electrode. The separator has a high ion permeability and has appropriate mechanical strength and insulation. As the separator, a microporous film, a woven fabric, a non-woven fabric, etc. can be used. As the material of the separator, for example, polyolefins such as polypropylene and polyethylene can be used.

[0125] As an example of the structure of the secondary battery, a structure in which an electrode group formed by winding a positive electrode and a negative electrode with a separator interposed therebetween and an electrolyte are housed in a housing can be cited. Or, instead of the wound electrode group, other forms of electrode groups such as a laminated electrode group formed by laminating a positive electrode and a negative electrode with a separator interposed therebetween can be applied. The secondary battery can be in any form such as cylindrical, square, coin-shaped, button-shaped, laminated, etc.

[0126] Figure 2 It is a schematic perspective view in which a part of a square secondary battery according to an embodiment of the present disclosure is cut off.

[0127] The battery includes: a bottomed square battery case 4, an electrode group 1 housed in the battery case 4, an electrolyte, and a sealing plate 5 for sealing the opening of the battery case 4. The electrode group 1 has a long strip-shaped negative electrode, a long strip-shaped positive electrode, and a separator interposed therebetween. The sealing plate 5 has a liquid injection port blocked by a sealing plug 8 and a negative terminal 6 insulated from the sealing plate 5 through a gasket 7.

[0128] One end of the negative electrode lead 3 is mounted on the negative electrode current collector by welding or the like. One end of the positive electrode lead 2 is mounted on the positive electrode current collector by welding or the like. The other end of the negative electrode lead 3 is electrically connected to the negative terminal 6. The other end of the positive electrode lead 2 is electrically connected to the sealing plate 5.

[0129] The present disclosure will be specifically described based on examples and comparative examples, and the present invention is not limited to the following examples.

[0130] <Example 1>

[0131] [Synthesis of lithium silicate]

[0132] Lithium carbonate and silicon dioxide were mixed so as to have a molar ratio of Li2CO3:SiO2 = 34:66, and the mixture was melted at 1500 °C for 5 hours in an inert gas atmosphere. The melt was formed into flakes by a metal roll, and then heat-treated at 750 °C for 5 hours to obtain lithium silicate (Li2Si2O5). The lithium silicate was pulverized to an average particle size of 10 μm.

[0133] [Preparation of silicon-containing material]

[0134] In an inert atmosphere, Si powder (3N, average particle size 10 μm) and Li2Si2O5 powder (average particle size 10 μm) were mixed at a mass ratio of 58:42. Further, a predetermined amount of an alloy particle of vanadium and iron containing elemental vanadium and vanadium carbide in a dispersed state was added, and the mixture was filled into a pot (made of SUS, volume: 500 mL) of a planetary ball mill (manufactured by Fritsch Co., Ltd., P-5). Twenty-four SUS balls (diameter 20 mm) were added to the pot, the lid was covered, and the mixture was pulverized at 200 rpm for 50 hours. Then, the powder was taken out in an inert atmosphere and heat-treated at 800 °C for 4 hours in an inert atmosphere to obtain a silicate composite particle containing 30 ppm of vanadium and 1.3% by mass of iron as the silicon-containing material A1.

[0135] The silicon-containing material A1 was pulverized, passed through a 40-μm sieve, and mixed with coal tar pitch (manufactured by JFE Chemical Corporation, MCP250), and heat-treated at 800 °C in an inert atmosphere. Thus, the surface of the silicon-containing material A1 was covered with carbon to form a conductive layer. The coverage amount of carbon was about 5% by mass based on the total of the silicon-containing material A1 and the conductive layer. Then, using a sieve, a silicon-containing material A1 having an average particle size of 5 μm with a conductive layer was obtained.

[0136] [Analysis of silicon-containing material]

[0137] The cross-section of silicon-containing material A1 was observed by TEM, and the average particle size of the Si particles was found to be less than 50 nm. The particle cross-section of silicon-containing material A1 was observed by SEM, and it was confirmed that a powder containing Si particles and vanadium carbide was roughly uniformly dispersed within the silicate phase.

[0138] Peaks derived from Si and Li2Si2O5 were confirmed in the XRD pattern of silicon-containing material A1. No peak of SiO2 was observed at 2θ = 25°.

[0139] Silicon-containing material A1 was measured by Si-NMR, and the content of SiO2 was found to be below the detection limit.

[0140] The contents of vanadium and iron were measured by ICP-AES.

[0141] [Fabrication of the negative electrode]

[0142] A mixture containing silicon-containing material A1 with a conductive layer and graphite in a mass ratio of 5:95, sodium carboxymethyl cellulose (CMC-Na), and styrene-butadiene rubber (SBR) were mixed in a mass ratio of 97.5:1.0:1.5. After adding water, stirring was carried out using a mixer (manufactured by PRIMIX Corporation, T.K.HIVIS MIX.) to prepare a negative electrode slurry. Subsequently, the negative electrode slurry was coated on both sides of a copper foil, and after drying the coating film, rolling was performed to obtain a negative electrode A2 with a negative electrode composite material layer having a density of 1.6 g / cm 3 formed on both sides of the copper foil.

[0143] [Fabrication of the positive electrode]

[0144] Lithium cobalt oxide, acetylene black, and polyvinylidene fluoride were mixed in a mass ratio of 95:2.5:2.5. After adding N-methyl-2-pyrrolidone (NMP), stirring was carried out using a mixer (manufactured by PRIMIX Corporation, T.K.HIVIS MIX.) to prepare a positive electrode slurry. Subsequently, the positive electrode slurry was coated on both sides of an aluminum foil, and after drying the coating film, rolling was performed to obtain a positive electrode with a positive electrode composite material layer having a density of 3.6 g / cm 3 formed on both sides of the aluminum foil.

[0145] [Preparation of the non-aqueous electrolyte]

[0146] LiPF6 was dissolved in a mixed solvent containing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 3:7 at a concentration of 1.0 mol / L to prepare a non-aqueous electrolyte.

[0147] [Fabrication of the secondary battery]

[0148] The electrode group was prepared by installing a tab on each electrode, and winding the positive electrode and the negative electrode into a spiral shape with the tab located at the outermost periphery through a separator. The electrode group was inserted into an outer casing made of an aluminum laminate film, vacuum dried at 105° C. for 2 hours, and then injected with a nonaqueous electrolyte, and the opening of the outer casing was sealed to obtain a secondary battery A1.

[0149] <Comparative Example 1>

[0150] No vanadium carbide was added to the mixture of Si powder and Li2Si2O5 powder, but iron was added to synthesize silicate composite particles containing 1.7% by mass of iron (silicon-containing material R1). Except for this, the negative electrode R1 and the secondary battery R1 were obtained by the same method as in Example 1.

[0151] [evaluate]

[0152] (Cycle life)

[0153] Each battery of Example 1 and Comparative Example 1 was charged and discharged repeatedly under the following conditions to evaluate the cycle life.

[0154] <Charging>

[0155] At 25° C., constant current charging was performed at a current of 1 It (800 mA) until the voltage reached 4.2 V, and then constant voltage charging was performed at a voltage of 4.2 V until the current reached 1 / 20 It (40 mA).

[0156] <Discharge>

[0157] At 25°C, constant current discharge was performed at a current of 1 It (800 mA) until the voltage reached 2.75V.

[0158] The pause period between charge and discharge was 10 minutes. The number of cycles until the discharge capacity reached 80% of the first cycle was measured and was taken as the cycle life. The results are shown in Table 1.

[0159] [Table 1]

[0160] battery V Fe cycle life A 1 30 ppm 1.3 mass % 359 R 1 - 1.7 mass % 333

[0161] From Table 1, it can be understood that, compared with the case where iron is dispersed in the lithium ion conductive phase, the cycle life is significantly improved by dispersing the particles containing vanadium in the lithium ion conductive phase.

[0162] Industrial Applicability

[0163] The secondary battery negative electrode of the present disclosure is useful in secondary batteries that serve as a main power source for mobile communication devices, portable electronic devices, and the like.

[0164] Description of Reference Numerals

[0165] 1 Electrode group

[0166] 2 Positive electrode lead

[0167] 3 Negative electrode lead

[0168] 4 Battery housing

[0169] 5 Sealing plate

[0170] 6 Negative terminal

[0171] 7 Gasket

[0172] 8 Sealing plug

[0173] 10 Silicate composite particles with a conductive layer

[0174] 11 Lithium silicate phase

[0175] 12 Silicon particles

[0176] 13 Silicate composite particles (base particles)

[0177] 14 Conductive layer

[0178] 15 Particles containing vanadium

Claims

1. A negative electrode active material for a secondary battery, which contains a silicon-containing material, The silicon-containing material includes: a lithium ion conduction phase, silicon particles dispersed in the lithium ion conduction phase, and particles containing vanadium dispersed in the lithium ion conduction phase, The content of vanadium contained in the silicon-containing material is 1000 ppm or less, The particles containing vanadium include at least one selected from the group consisting of vanadium alloys and vanadium carbides.

2. The negative electrode active material for a secondary battery according to claim 1, wherein, The vanadium-containing particles contain iron.

3. The negative electrode active material for a secondary battery according to claim 2, wherein, The content of the iron contained in the silicon-containing material is 0.01% by mass or more and 10% by mass or less.

4. The negative electrode active material for a secondary battery according to any one of claims 1 to 3, wherein, The average particle size of the silicon particles is 1 nm or more and 1000 nm or less.

5. The negative electrode active material for a secondary battery according to any one of claims 1 to 3, wherein, The lithium ion conductive phase is at least one selected from the group consisting of a silicon oxide phase, a silicate phase, and a carbon phase.

6. The negative electrode active material for a secondary battery according to claim 5, wherein, The lithium ion conductive phase contains the silicate phase. The silicate phase contains at least one selected from the group consisting of an alkali metal element and a Group II element.

7. The negative electrode active material for a secondary battery according to claim 6, wherein, The silicate phase further contains element M. Element M is at least one selected from the group consisting of B, Al, Zr, Nb, Ta, La, Y, Ti, P, Bi, Zn, Sn, Pb, Sb, Co, Er, F, and W.

8. The negative electrode active material for a secondary battery according to claim 5, wherein, The silicate phase contains lithium silicate of the formula Li 2z SiO 2+z as shown, where 0 < z < 2.

9. A secondary battery, which includes a positive electrode, a negative electrode, an electrolyte, and a separator interposed between the positive electrode and the negative electrode, The negative electrode contains the negative electrode active material for a secondary battery according to any one of claims 1 to 8.

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