WO2023074099A1 - 二次電池用複合活物質および二次電池 - Google Patents
二次電池用複合活物質および二次電池 Download PDFInfo
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- WO2023074099A1 WO2023074099A1 PCT/JP2022/031974 JP2022031974W WO2023074099A1 WO 2023074099 A1 WO2023074099 A1 WO 2023074099A1 JP 2022031974 W JP2022031974 W JP 2022031974W WO 2023074099 A1 WO2023074099 A1 WO 2023074099A1
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- active material
- silicon
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- secondary battery
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- 239000001307 helium Substances 0.000 description 1
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- PAZHGORSDKKUPI-UHFFFAOYSA-N lithium metasilicate Chemical compound [Li+].[Li+].[O-][Si]([O-])=O PAZHGORSDKKUPI-UHFFFAOYSA-N 0.000 description 1
- 229910003002 lithium salt Inorganic materials 0.000 description 1
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- 229910052912 lithium silicate Inorganic materials 0.000 description 1
- DMEJJWCBIYKVSB-UHFFFAOYSA-N lithium vanadium Chemical class [Li].[V] DMEJJWCBIYKVSB-UHFFFAOYSA-N 0.000 description 1
- 229910001629 magnesium chloride Inorganic materials 0.000 description 1
- VZCYOOQTPOCHFL-UPHRSURJSA-N maleic acid Chemical compound OC(=O)\C=C/C(O)=O VZCYOOQTPOCHFL-UPHRSURJSA-N 0.000 description 1
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- 229910052976 metal sulfide Inorganic materials 0.000 description 1
- KKQAVHGECIBFRQ-UHFFFAOYSA-N methyl propyl carbonate Chemical compound CCCOC(=O)OC KKQAVHGECIBFRQ-UHFFFAOYSA-N 0.000 description 1
- LVHBHZANLOWSRM-UHFFFAOYSA-N methylenebutanedioic acid Natural products OC(=O)CC(=C)C(O)=O LVHBHZANLOWSRM-UHFFFAOYSA-N 0.000 description 1
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- FOWDZVNRQHPXDO-UHFFFAOYSA-N propyl hydrogen carbonate Chemical compound CCCOC(O)=O FOWDZVNRQHPXDO-UHFFFAOYSA-N 0.000 description 1
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- 239000011269 tar Substances 0.000 description 1
- 125000004213 tert-butoxy group Chemical group [H]C([H])([H])C(O*)(C([H])([H])[H])C([H])([H])[H] 0.000 description 1
- 125000001302 tertiary amino group Chemical group 0.000 description 1
- YLQBMQCUIZJEEH-UHFFFAOYSA-N tetrahydrofuran Natural products C=1C=COC=1 YLQBMQCUIZJEEH-UHFFFAOYSA-N 0.000 description 1
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- OGIDPMRJRNCKJF-UHFFFAOYSA-N titanium oxide Inorganic materials [Ti]=O OGIDPMRJRNCKJF-UHFFFAOYSA-N 0.000 description 1
- ZNOCGWVLWPVKAO-UHFFFAOYSA-N trimethoxy(phenyl)silane Chemical compound CO[Si](OC)(OC)C1=CC=CC=C1 ZNOCGWVLWPVKAO-UHFFFAOYSA-N 0.000 description 1
- BPSIOYPQMFLKFR-UHFFFAOYSA-N trimethoxy-[3-(oxiran-2-ylmethoxy)propyl]silane Chemical compound CO[Si](OC)(OC)CCCOCC1CO1 BPSIOYPQMFLKFR-UHFFFAOYSA-N 0.000 description 1
- 238000001291 vacuum drying Methods 0.000 description 1
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Classifications
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
- C01B32/00—Carbon; Compounds thereof
- C01B32/05—Preparation or purification of carbon not covered by groups C01B32/15, C01B32/20, C01B32/25, C01B32/30
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
- C01B33/00—Silicon; Compounds thereof
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M4/36—Selection of substances as active materials, active masses, active liquids
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M4/36—Selection of substances as active materials, active masses, active liquids
- H01M4/38—Selection of substances as active materials, active masses, active liquids of elements or alloys
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/10—Energy storage using batteries
Definitions
- the present invention relates to composite active materials for secondary batteries and secondary batteries. More specifically, the present invention relates to a composite active material for secondary batteries and a secondary battery containing the composite material for secondary batteries in a negative electrode.
- Non-aqueous electrolyte secondary batteries are used in mobile devices, hybrid vehicles, electric vehicles, household storage batteries, etc., and are required to have well-balanced characteristics such as electrical capacity, safety, and operational stability. ing.
- various lithium ion batteries have been developed and put to practical use, mainly using a lithium intercalation compound as a negative electrode active material, which is capable of intercalating and releasing lithium ions between crystal planes during charging and discharging. has also been transformed.
- batteries with higher capacity and various battery characteristics such as cycle characteristics and discharge rate characteristics are required as power sources for driving these devices. There is a strong need to develop even more improved lithium-ion batteries.
- graphite or the like is mainly used as a negative electrode active material in conventional lithium ion batteries, and the theoretical specific capacity of graphite-based negative electrode materials limits the development of lithium ion batteries with even higher energy densities. Therefore, the development of negative electrode materials using metals such as silicon and tin, which are elements with high theoretical capacity and capable of intercalating and deintercalating lithium ions, or alloys with other elements, is being studied. Among them, attention is paid to silicon having a theoretical capacity ten times or more that of graphite-based negative electrode materials, and silicon-containing active materials. However, silicon-containing active materials are known to have low initial efficiencies.
- Patent Document 1 describes lithium-doped layered silicon obtained by contacting a layered polysilane with a lithium complex solution in which metallic lithium and a polycyclic aromatic compound are dissolved in an organic solvent, and doping the layered polysilane with lithium. .
- Patent Document 2 discloses a silicon oxide containing silicon particles having a crystal size of 1 to 25 nm and MgSiO3 crystals in the silicon oxide, and a silicon containing a carbon coating on the surface.
- a negative electrode active material comprising the compound is disclosed.
- Patent Document 3 describes a negative electrode active material comprising a silicon oxide composite containing silicon oxide and magnesium silicate, and a carbon coating layer containing a carbonaceous material located on the surface of the silicon oxide composite. ing.
- the silicon compound particles contain a Li compound, contain a salt of polyacrylic acid or a salt of carboxymethyl cellulose, and are at least one selected from Mg and Al.
- a negative electrode active material containing a metal salt containing a metal of is also being studied.
- the present inventors have investigated a method for efficiently suppressing the formation of lithium oxide generated during initial charging and discharging, and as a result, a secondary battery in which the initial coulombic efficiency and capacity retention rate of the negative electrode active material are improved.
- a composite active material for That is, the present invention relates to a secondary battery composite active material used in a lithium ion secondary battery and a secondary battery containing the above-mentioned secondary battery composite active material as a negative electrode active material, which is excellent in initial coulombic efficiency and capacity retention rate. It is an object of the present invention to provide a negative electrode active material for a secondary battery that provides a secondary battery with improved performance.
- the present invention has the following aspects.
- Silicon particles having an average particle diameter of 150 nm or less, a matrix phase in which the silicon particles are dispersed, and at least one metal silicate compound selected from the group consisting of Li, K, Na, Ca, Mg and Al. and a composite active material for a secondary battery, comprising the silicate compound in the vicinity of the surface of the silicon particles.
- a composite active material for a secondary battery comprising the silicate compound in the vicinity of the surface of the silicon particles.
- the composite active material for a secondary battery according to [1] wherein the silicate concentration near the surface of the silicon particles is higher than the silicate concentration in the matrix phase.
- a secondary battery comprising the composite active material for a secondary battery according to any one of [1] to [9] in a negative electrode.
- a negative electrode active material for a secondary battery that gives a secondary battery excellent in initial coulombic efficiency and capacity retention rate.
- the composite active material for a secondary battery of the present invention includes silicon particles having an average particle size of 150 nm or less (hereinafter also referred to as “this silicon particle”), and the silicon particles are It has a dispersed matrix phase and a silicate compound of at least one metal selected from the group consisting of Li, K, Na, Ca, Mg and Al, and has the silicate compound near the surface of the silicon particles.
- This silicon oxide film is thought to exist on the surface of the silicon particles, and it is thought that the formation of lithium oxide due to the reaction between lithium and silicon oxide during initial charging occurs in the vicinity of silicon oxide.
- the present active material efficiently generates lithium oxide by allowing a large amount of at least one metal silicate compound selected from the group consisting of Li, K, Na, Ca, Mg and Al to exist in the vicinity of the surface of the silicon particles. can be suppressed. As a result, when this active material is used as a negative electrode active material, it is believed that the initial coulombic efficiency and capacity retention rate of the negative electrode active material are improved.
- the present silicon particles are composed of zero-valent silicon and have an average particle diameter of 150 nm or less.
- the average particle size is a D50 value that can be measured using a laser diffraction particle size analyzer or the like. D50 can be measured by a dynamic light scattering method using a laser particle size analyzer or the like.
- the average particle diameter of the present silicon particles is the particle diameter at which the volume cumulative distribution curve is drawn from the small diameter side in the particle diameter distribution, and the cumulative distribution is 50%.
- Silicon particles having a large size exceeding 300 nm form large lumps, and when the present active material is used as a negative electrode active material, the phenomenon of pulverization is likely to occur during charging and discharging, so the capacity retention rate of the negative electrode active material tends to decrease.
- silicon particles with a small size of less than 10 nm are too fine, the silicon particles tend to agglomerate. Therefore, the dispersibility of the silicon particles in the negative electrode active material may deteriorate.
- the silicon particles are too fine the surface activation energy of the particles tends to increase, and by-products tend to increase on the surface of the silicon particles when the negative electrode active material is baked at a high temperature. These may lead to deterioration in charge/discharge performance.
- the present silicon particles contain the large-sized silicon particles exceeding 300 nm and the small-sized silicon particles less than 10 nm as small as possible.
- the average particle size is preferably 120 nm or less, more preferably 100 nm or less.
- the average particle size is preferably 20 nm or more, more preferably 30 nm or more.
- the present silicon particles can be obtained, for example, by pulverizing a lump of silicon so that the average particle size falls within the above range.
- crushers used for crushing silicon lumps include crushers such as ball mills, bead mills, and jet mills.
- the pulverization may be wet pulverization using an organic solvent, and as the organic solvent, for example, alcohols, ketones, etc. can be preferably used. Group hydrocarbon solvents can also be used.
- the average particle diameter of the silicon particles can be adjusted to the above range.
- the shape of the present silicon particles may be granular, needle-like, or flaky as long as it satisfies the above-mentioned average particle size, but the flaky shape is preferable from the viewpoint of handling.
- the present silicon particles are flakes, it is preferable from the viewpoint of initial coulombic efficiency and capacity retention rate that the crystallite size obtained from the half width of the peak at 28.4 degrees in the X-ray diffraction spectrum is 35 nm or less.
- the crystallite size is more preferably 25 nm or less.
- the present silicon particles preferably have a length in the longitudinal direction of 70 to 300 nm and a thickness of 15 to 70 nm.
- the so-called aspect ratio which is the ratio of thickness to length, is preferably 0.5 or less.
- the morphology of the present silicon particles can be measured by the dynamic light scattering method, but by using the analysis means of a transmission electron microscope (TEM) or a field emission scanning electron microscope (FE-SEM). , samples of said aspect ratio can be more easily and precisely identified.
- the sample can be cut with a focused ion beam (FIB) and the cross section can be observed with FE-SEM, or the sample can be sliced and observed with TEM. can identify the state of the present silicon particles.
- the aspect ratio of the present silicon particles is the result of calculation based on 50 particles in the main portion of the sample within the field of view shown in the TEM image.
- the present silicon particles are dispersed in the matrix phase.
- Compounds constituting the matrix phase include silicon dioxide and titanium oxide, but compounds containing silicon, oxygen, and carbon are preferred, and the compounds containing silicon, oxygen, and carbon have a three-dimensional network structure of a silicon-oxygen-carbon skeleton. and free carbons are preferred.
- free carbon is carbon that is not contained in the three-dimensional skeleton of silicon-oxygen-carbon.
- Free carbon includes carbon present as a carbon phase, carbon bonded between carbon phase carbons, and carbon bonded between a silicon-oxygen-carbon skeleton and a carbon phase.
- the silicon-oxygen-carbon skeleton in the matrix phase is chemically It is highly stable and has a composite structure with free carbon, which facilitates the diffusion of lithium ions along with the reduction in electronic transition resistance. Direct contact between the silicon particles and the electrolytic solution is prevented by tightly enveloping the silicon particles in the composite structure of the silicon-oxygen-carbon skeleton and free carbon.
- the silicon particles in the negative electrode play a role as a main component in the expression of charge-discharge performance, while avoiding a chemical reaction between silicon and the electrolyte during charge-discharge.
- the deterioration of the performance of the present silicon particles can be prevented to the maximum.
- the silicon-oxygen-carbon skeleton When the compound constituting the matrix phase has a three-dimensional network structure of silicon-oxygen-carbon skeleton and a structure containing free carbon, the silicon-oxygen-carbon skeleton is transformed into a silicon-oxygen-carbon skeleton by the approach of lithium ions. A change occurs in the electron distribution inside the , and electrostatic bonds and coordinate bonds are formed between the silicon-oxygen-carbon skeleton and lithium ions. Lithium ions are stored in the silicon-oxygen-carbon skeleton by this electrostatic bond and coordinate bond. On the other hand, since the coordination bond energy is relatively low, the desorption reaction of lithium ions easily occurs. In other words, it is considered that the silicon-oxygen-carbon skeleton can reversibly cause intercalation and deintercalation reactions of lithium ions during charging and discharging.
- the matrix phase preferably contains a compound represented by the following formula (1).
- SiOx Cy (1) x represents the molar ratio of oxygen to silicon, and y represents the molar ratio of carbon to silicon.
- x represents the molar ratio of oxygen to silicon
- y represents the molar ratio of carbon to silicon.
- 1 ⁇ x ⁇ 2 is preferable, 1 ⁇ x ⁇ 1.9 is more preferable, and 1 ⁇ x ⁇ 1.8 is more preferred.
- 1 ⁇ y ⁇ 20 is preferable, and 1.2 ⁇ y ⁇ 15 is more preferable, from the viewpoint of the balance between charge/discharge performance and initial coulombic efficiency.
- the compound constituting the matrix phase may contain nitrogen in addition to silicon, oxygen and carbon.
- Nitrogen is a functional group in the raw material used in the manufacturing method of the active material described later, such as phenolic resin, dispersant, polysiloxane compound, other nitrogen compounds, and nitrogen gas used in the firing process. By having the containing atomic group, it can be introduced into the matrix phase. Since the matrix phase contains nitrogen, the charge/discharge performance and the capacity retention rate tend to be excellent when the present active material is used as a negative electrode active material.
- the compound constituting the matrix phase is a compound containing silicon, oxygen, carbon and nitrogen
- the matrix phase preferably contains a compound represented by the following formula (2).
- SiOxCyNz (2) In formula (2), x and y have the same meanings as above, and z represents the molar ratio of nitrogen to silicon.
- the matrix phase contains the compound represented by the formula (2), 1 ⁇ x ⁇ 2, 1 ⁇ y ⁇ 20, 0 ⁇ z ⁇ 0.5 are preferred, and 1 ⁇ x ⁇ 1.9, 1.2 ⁇ y ⁇ 15, and 0 ⁇ z ⁇ 0.4 are more preferred.
- the above x, y and z can be obtained by measuring the mass content of each element and then converting to a molar ratio (atomic number ratio).
- the content of oxygen and carbon can be quantified by using an inorganic elemental analyzer, and the content of silicon can be quantified by using an ICP optical emission spectrometer (ICP-OES).
- ICP-OES ICP optical emission spectrometer
- local analysis of the present active material is performed, and a large number of measurement points for the content ratio data obtained thereby is obtained. It is also possible to analogize the content ratio of the entire active material. Local analysis includes, for example, Energy Dispersive X-ray Spectroscopy (SEM-EDX) and Electron Probe Microanalyzer (EPMA).
- the present active material is a silicate compound of at least one metal selected from the group consisting of Li, K, Na, Ca, Mg and Al (hereinafter also referred to as "present silicate compound"). ) in the vicinity of the surface of the present silicon particles.
- a silicate compound is generally a compound containing an anion having a structure in which one or several silicon atoms are centered and surrounded by electronegative ligands. It is a salt of at least one metal selected from the group consisting of Mg and Al and a compound containing the anion.
- Examples of compounds containing the anion include orthosilicate ion (SiO 4 4- ), metasilicate ion (SiO 3 2- ), pyrosilicate ion (Si 2 O 7 6- ), cyclic silicate ion (Si 3 O 9 6- or Si 6 O 18 12- ) are known.
- the present silicate compound is preferably a silicate compound which is a salt of metasilicate ion and at least one metal selected from the group consisting of Li, K, Na, Ca, Mg and Al. Li or Mg is preferred among the metals.
- the present silicate compound contains at least one metal selected from the group consisting of Li, K, Na, Ca, Mg and Al, and may contain two or more of these metals.
- one silicate ion may have a plurality of kinds of metals, or may be a mixture of silicate compounds having different metals.
- the present silicate compound may contain other metals as long as it contains at least one metal selected from the group consisting of Li, K, Na, Ca, Mg and Al.
- the present silicate compound is preferably a lithium silicate compound or a magnesium silicate compound, more preferably lithium metasilicate (Li 2 SiO 3 ) or magnesium metasilicate (MgSiO 3 ), and particularly preferably magnesium metasilicate (MgSiO 3 ).
- the present active material has the present silicate compound in the vicinity of the surface of the present silicon particles. That is, the present silicate compound may chemically or physically directly adhere to the surface of the present silicon particles, and the present silicate compound may exist in the vicinity of the surface of the present silicon particles.
- the vicinity of the surface of the silicon particles is, for example, within 10 nm, preferably within 5 nm, from the surface.
- the present silicate compound preferably exists in a large amount near the surface of the present silicon particles, and is 50% by mass or more of the total mass of the present silicate compound contained in the present active material. is more preferably present in the vicinity of the surface, and more preferably 60% by mass or more is present in the vicinity of the surface.
- the presence of the silicate compound in the vicinity of the surface of the silicon particles can be confirmed by a high-resolution transmission electron microscope (hereinafter also referred to as "HR-TEM"). Specifically, the sample can be sliced by a focused ion beam (FIB) and confirmed by HR-TEM observation.
- HR-TEM high-resolution transmission electron microscope
- the concentration of the silicate compound (silicate concentration) in the vicinity of the surface of the silicon particles is preferably higher than the concentration (silicate concentration) of the compound forming the matrix phase.
- the vicinity of the surface is as described above, and the concentration of the present silicate compound (silicate concentration) in the vicinity of the surface of the present silicon particles is the concentration of the present silicate compound within the above range.
- the concentration of the present silicate compound near the surface is the mass per unit volume of the present silicate compound within the above range.
- the concentration of the present silicate compound in the vicinity of the surface of the present silicon particles is the area of the lattice structure attributed to the crystallite of the silicate compound per unit area in the observation field of view of 1,000,000 times the HR-TEM, and the area of the entire field of view. and is calculated by the following formula.
- Area ratio area of lattice structure attributed to crystallite/area of all fields of view This area ratio is determined using an average value of five or more different fields of view.
- STEM-EDS Sccanning Transmission Electron Microscope Energy-Dispersive-Spectroscopy
- concentrations of Mg element, Si element, and O element present in the vicinity of silicon particles in a mapping image can be used to detect the concentrations of Mg element, Si element, and O element present in the vicinity of silicon particles in a mapping image.
- concentration of the silicate compound can be determined by combining the Mg element, Si element, and O element.
- the concentration of the present silicate compound near the surface is preferably twice, more preferably three times the concentration of the compound constituting the matrix phase (silicate concentration), and the present silicon particles are coated with the present silicate compound. is more preferred. Further, when the concentrations of the present silicate compound and the compound forming the matrix phase are compared within the range near the surface of the present silicon particles using HR-TEM, it is found that the concentration of the silicate compound crystallites near the surface of the present silicate compound is The area of the lattice structure is preferably twice or more, more preferably three times or more, the area of the crystal lattice structure of the compound forming the matrix phase per unit area of the observation field.
- the magnesium silicate compound is a magnesium silicate compound
- it is preferred that the magnesium silicate compound is a crystalline film covering at least part of the surface of the present silicon particles.
- the coverage is more preferably 50% or more, particularly preferably 80% or more.
- the thickness of the crystalline film is preferably 0.2 nm to 10 nm, more preferably 1 nm to 8 nm. The coverage and the thickness of the crystalline film can be measured by the HR-TEM. The coverage can be obtained from the amount ratio of the silicon particles and the magnesium silicate compound.
- the present active material preferably has silicon dioxide in addition to the present silicate compound in the vicinity of the surface of the present silicon particles from the viewpoint of excellent balance between the initial coulomb efficiency and the capacity retention rate.
- the present active material contains silicon dioxide
- the present silicate compound and silicon dioxide are present near the surface of the present silicon particles.
- the molar ratio of the present silicate compound and silicon dioxide is preferably 0.9 or less, more preferably 0.5 or less, where the total number of moles of the present silicate compound and silicon dioxide is 1.
- the molar ratio of the present silicate compound and silicon dioxide is preferably 0.05 or more, more preferably 0.1 or more, where the total number of moles of the present silicate compound and silicon dioxide is 1.
- the surfaces of the present silicon particles are preferably coated with a silicon dioxide film, which is a silicon oxide film.
- the present silicate compound is preferably present on the surface of the silicon dioxide film.
- the silicate compound is a crystal film of a magnesium silicate compound, it is preferable that the silicon dioxide film covering the silicon particles is further coated with a crystal film of the magnesium silicate compound.
- the average particle size of the silicon particles, the matrix phase, and the active material having the silicate compound near the surface of the silicon particles is preferably 1 ⁇ m to 15 ⁇ m, more preferably 2 ⁇ m to 8 ⁇ m.
- the average particle size is the D50 value as described above, and the measurement method is also the same as described above. If the average particle diameter is too small, when the active material is used as a secondary battery, the amount of solid phase interfacial electrolyte decomposition products (hereinafter also referred to as "SEI") generated during charging and discharging will increase as the specific surface area increases significantly. The increase may reduce the reversible charge/discharge capacity per unit volume. If the average particle diameter is too large, there is a risk of separation from the current collector during electrode film production.
- the specific surface area of the present active material is preferably 1 m 2 /g to 30 m 2 /g, more preferably 2 m 2 /g to 15 m 2 /g.
- the specific surface area is a value determined by the BET method, and can be determined by nitrogen gas adsorption measurement, for example, using a specific surface area measuring device.
- the active material has an average particle diameter of 1 ⁇ m to 15 ⁇ m and a specific surface area of 1 m 2 /g to 30 m 2 /g. Particularly preferably, the active material has an average particle size of 2 ⁇ m to 8 ⁇ m and a specific surface area of 2 m 2 /g to 15 m 2 /g.
- the present active material may contain other necessary third components in addition to the above. Further, the surface of the present active material may be coated with a coating material.
- a coating material a substance that can be expected to have electronic conductivity, lithium ion conductivity, and an effect of suppressing decomposition of the electrolytic solution is preferable.
- the average thickness of the coating layer is preferably 10 nm or more and 300 nm or less.
- the average thickness is more preferably 20 nm or more and 200 nm or less. Since the present active material has a coating layer having the above average thickness, it is possible to protect the present silicon particles exposed on the surface of the present active material. The chemical and thermal stability of the substance is improved. It is possible to further suppress the deterioration of the charge/discharge performance of the secondary battery obtained as a result.
- the content of the coating material is 100% by mass of the total amount of the present active material from the viewpoint of improving the chemical stability and thermal stability of the negative electrode active material.
- the total amount of the present active material is the total amount of the present silicon particles, the matrix phase, the present silicate compound and the coating material that constitute the present active material.
- the matrix phase contains nitrogen, it is the total amount including nitrogen.
- the coating material examples include electron conductive substances such as carbon, titanium, and nickel. Among these, from the viewpoint of improving the chemical stability and thermal stability of the negative electrode active material, carbon is preferable, and low-crystalline carbon is more preferable.
- the coating material is low-crystalline carbon, the average thickness of the coating layer is preferably 10 nm or more and 300 nm or less.
- a secondary battery using the present active material as a battery negative electrode exhibits good charge/discharge characteristics.
- a slurry containing the present active material, an organic binder, and, if necessary, other components such as a conductive aid is applied in the form of a thin film onto a current collector copper foil to form a negative electrode.
- a negative electrode can also be produced by adding a carbon material such as graphite to the slurry.
- Carbon materials include natural graphite, artificial graphite, amorphous carbon such as hard carbon or soft carbon, and the like.
- the present active material and a binder that is an organic binder are kneaded together with a solvent using a dispersing device such as a stirrer, ball mill, super sand mill, or pressure kneader to prepare a negative electrode material slurry, which is used as a current collector. It can be obtained by applying it to the body to form a negative electrode layer. It can also be obtained by forming a paste-like negative electrode material slurry into a sheet-like or pellet-like shape and integrating this with a current collector.
- a dispersing device such as a stirrer, ball mill, super sand mill, or pressure kneader
- organic binder examples include styrene-butadiene rubber copolymer (hereinafter also referred to as "SBR"); methyl (meth) acrylate, ethyl (meth) acrylate, butyl (meth) acrylate, (meth) acrylonitrile , and ethylenically unsaturated carboxylic acid esters such as hydroxyethyl (meth)acrylate, and ethylenically unsaturated carboxylic acids such as acrylic acid, methacrylic acid, itaconic acid, fumaric acid, and maleic acid (meth)acrylic copolymerization
- Unsaturated carboxylic acid copolymers such as coalescence; A high molecular compound is mentioned.
- these organic binders can be dispersed or dissolved in water, or dissolved in an organic solvent such as N-methyl-2-pyrrolidone (NMP).
- NMP N-methyl-2-pyrrolidone
- the content ratio of the organic binder in the negative electrode layer of the lithium ion secondary battery negative electrode is preferably 1% by mass to 30% by mass, more preferably 2% by mass to 20% by mass, and 3% by mass. to 15% by mass is more preferable.
- the present active material has high chemical stability and is easy to handle in terms of practical use in that an aqueous binder can also be used.
- the negative electrode material slurry may be mixed with a conductive aid, if necessary.
- conductive aids include carbon black, graphite, acetylene black, oxides and nitrides exhibiting conductivity, and the like.
- the amount of the conductive aid used may be about 1% by mass to 15% by mass with respect to the negative electrode active material of the present invention.
- the material and shape of the current collector for example, copper, nickel, titanium, stainless steel, etc. may be used in the form of a foil, a perforated foil, a mesh, or the like in a strip shape.
- Porous materials such as porous metal (foamed metal) and carbon paper can also be used.
- Examples of the method for applying the negative electrode material slurry to the current collector include a metal mask printing method, an electrostatic coating method, a dip coating method, a spray coating method, a roll coating method, a doctor blade method, a gravure coating method, and a screen printing method. etc. After coating, it is preferable to carry out a rolling treatment using a flat plate press, calendar rolls, or the like, if necessary.
- the negative electrode material slurry can be made into a sheet or pellet form, and this can be integrated with the current collector by, for example, rolling, pressing, or a combination thereof.
- the negative electrode layer formed on the current collector or the negative electrode layer integrated with the current collector is preferably heat-treated according to the organic binder used.
- the organic binder used For example, when a water-based styrene-butadiene rubber copolymer (SBR) or the like is used, heat treatment at 100 to 130° C. is sufficient, and when using an organic binder having a main skeleton of polyimide or polyamideimide, Heat treatment at 150 to 450° C. is preferred.
- SBR styrene-butadiene rubber copolymer
- This heat treatment removes the solvent and hardens the binder to increase the strength, improving the adhesion between particles and between the particles and the current collector.
- these heat treatments are preferably performed in an inert atmosphere such as helium, argon, or nitrogen, or in a vacuum atmosphere in order to prevent the current collector from being oxidized during the treatment.
- the negative electrode using the present active material preferably has an electrode density of 1 g/cm 3 to 1.8 g/cm 3 , more preferably 1.1 g/cm 3 to 1.7 g/cm 3 . More preferably from 0.2 g/cm 3 to 1.6 g/cm 3 .
- the electrode density there is a tendency that the higher the electrode density, the higher the adhesion and the volume capacity density of the electrode.
- the electrode density is too high, the voids in the electrode are reduced, which weakens the effect of suppressing the volume expansion of silicon or the like, and the capacity retention rate may decrease. Therefore, an optimum range of electrode densities is selected.
- the secondary battery of the present invention contains the present active material in the negative electrode.
- a secondary battery having a negative electrode containing the present active material a non-aqueous electrolyte secondary battery and a solid electrolyte secondary battery are preferable, and excellent performance is exhibited particularly when used as a negative electrode of a non-aqueous electrolyte secondary battery. It is.
- a positive electrode and a negative electrode containing the negative electrode active material of the present invention are placed facing each other with a separator interposed therebetween, and an electrolytic solution is injected. It can be configured by
- the positive electrode can be obtained by forming a positive electrode layer on the surface of the current collector in the same manner as the negative electrode.
- the current collector may be a strip-shaped one made of a metal or alloy such as aluminum, titanium, or stainless steel in the form of foil, foil with holes, mesh, or the like.
- the positive electrode material used for the positive electrode layer is not particularly limited.
- a metal compound, a metal oxide, a metal sulfide, or a conductive polymer material capable of doping or intercalating lithium ions should be used.
- lithium cobaltate LiCoO 2
- lithium nickelate LiNiO 2
- lithium manganate LiMnO 2
- lithium manganese spinel LiMn 2 O 4
- lithium vanadium compounds V2O5 , V6O13 , VO2 , MnO2
- TiO2 , MoV2O8 TiS2 , V2S5 , VS2
- olivine-type LiMPO 4 (where M is Co, Ni, Mn or Fe)
- conductive polymers such as polyacetylene, polyaniline, polypyrrole, polythiophene and polyacene, porous carbon, etc.
- the separator for example, a non-woven fabric, cloth, microporous film, or a combination of them can be used, the main component of which is polyolefin such as polyethylene or polypropylene.
- the positive electrode and the negative electrode of the non-aqueous electrolyte secondary battery to be manufactured are structured such that they do not come into direct contact with each other, there is no need to use a separator.
- electrolytes examples include lithium salts such as LiClO 4 , LiPF 6 , LiAsF 6 , LiBF 4 and LiSO 3 CF 3 , ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, fluoroethylene carbonate, cyclopentanone, sulfolane.
- the structure of the secondary battery of the present invention is not particularly limited, but usually, a positive electrode, a negative electrode, and an optional separator are wound into a flat spiral to form a wound electrode plate group. It is common to have a structure in which flat plates are laminated to form a laminated electrode plate group, and these electrode plate groups are enclosed in an outer package.
- the active material is mainly used for the negative electrode, and a simple evaluation is performed using metallic lithium for the counter electrode. for comparison.
- Secondary batteries using this active material are not particularly limited, but are used as paper-type batteries, button-type batteries, coin-type batteries, laminate-type batteries, cylindrical batteries, prismatic batteries, and the like.
- the negative electrode active material of the present invention described above can also be applied to general electrochemical devices having a charging/discharging mechanism of intercalating and deintercalating lithium ions, such as hybrid capacitors and solid lithium secondary batteries.
- the present active material can be produced, for example, by a method including steps 1 to 4 below.
- a method of using a polysiloxane compound and a carbon source resin as the silicon-containing compound serving as the matrix phase is exemplified, but the method is not limited to these methods.
- Step 1 A silicon (zero-valent) slurry pulverized by a wet method is mixed with an aggregate containing a polysiloxane compound and a carbon source resin to obtain a suspension.
- Step 2 A salt of at least one metal selected from the group consisting of Li, K, Na, Ca, Mg and Al is added to the obtained suspension, and after mixing, the solvent is removed to obtain a precursor.
- Step 3 The precursor obtained in step 2 is fired in an inert atmosphere at a maximum temperature of 1000°C to 1180°C to obtain a fired product.
- Step 4 The fired material obtained in Step 3 is pulverized to obtain the present active material.
- Step 1 Silicon (zero valent) slurry
- the wet-ground silicon (zero-valent) slurry used in step 1 can be prepared by using an organic solvent while grinding silicon particles with a wet powder grinder.
- a dispersant may be used to facilitate the grinding of the silicon particles in the organic solvent.
- the wet pulverizer is not particularly limited, and includes roller mills, high-speed rotary pulverizers, container-driven mills, bead mills, and the like. In wet pulverization, it is preferable to pulverize until the silicon particles have the particle size of the present silicon particles.
- Organic solvents used in the wet method include those that do not chemically react with silicon. Examples thereof include ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone and diisobutyl ketone; alcohols such as ethanol, methanol, normal propyl alcohol and isopropyl alcohol; aromatic benzene, toluene and xylene.
- ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone and diisobutyl ketone
- alcohols such as ethanol, methanol, normal propyl alcohol and isopropyl alcohol
- aromatic benzene, toluene and xylene include those that do not chemically react with silicon. Examples thereof include ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone and diisobutyl ketone; alcohols such as ethanol, methanol, normal
- Types of the dispersant include aqueous and non-aqueous dispersants.
- a non-aqueous dispersant is preferably used in order to suppress excessive oxidation of the surface of the present silicon particles.
- Types of non-aqueous dispersants include polymer types such as polyethers, polyalkylene polyamines, polycarboxylic acid partial alkyl esters, low molecular types such as polyhydric alcohol esters and alkylpolyamines, and polyphosphates.
- the concentration of silicon in the silicon (zero-valent) slurry is not particularly limited, but when the solvent and optionally a dispersant are included, the total amount of the dispersant and silicon is 100% by mass, and the amount of silicon is 5% by mass. to 40% by mass, more preferably 10% to 30% by mass.
- the polysiloxane compound used in step 1 is a resin containing at least one of a polycarbosilane structure, a polysilazane structure, a polysilane structure and a polysiloxane structure.
- a resin containing only these structures may be used, or a composite resin having at least one of these structures as a segment and chemically bonded to another polymer segment may be used.
- Forms of composite include graft copolymerization, block copolymerization, random copolymerization, alternating copolymerization, and the like.
- composite resins that have a graft structure in which polysiloxane segments and side chains of polymer segments are chemically bonded
- composite resins that have a block structure in which polysiloxane segments are chemically bonded to the ends of polymer segments. mentioned.
- the polysiloxane segment preferably has a structural unit represented by the following general formula (S-1) and/or the following general formula (S-2).
- the polysiloxane compound more preferably has a carboxy group, an epoxy group, an amino group, or a polyether group at the side chain or end of the siloxane bond (Si--O--Si) main skeleton.
- R 1 represents an optionally substituted aromatic hydrocarbon group, an alkyl group, an epoxy group, a carboxy group, or the like.
- R 2 and R3 represents an alkyl group, a cycloalkyl group, an aryl group or an aralkyl group, an epoxy group, a carboxy group, etc.
- Alkyl groups include, for example, methyl group, ethyl group, propyl group, isopropyl group, butyl group, isobutyl group, sec-butyl group, tert-butyl group, pentyl group, isopentyl group, neopentyl group, tert-pentyl group, 1 -methylbutyl group, 2-methylbutyl group, 1,2-dimethylpropyl group, 1-ethylpropyl group, hexyl group, isohesyl group, 1-methylpentyl group, 2-methylpentyl group, 3-methylpentyl group, 1,1 -dimethylbutyl group, 1,2-dimethylbutyl group, 2,2-dimethylbutyl group, 1-ethylbutyl group, 1,1,2-trimethylpropyl group, 1,2,2-trimethylpropyl group, 1-ethyl- 2-methylpropyl group, 1-ethyl-1-methylpropyl group
- aryl groups include phenyl, naphthyl, 2-methylphenyl, 3-methylphenyl, 4-methylphenyl, 4-vinylphenyl, and 3-isopropylphenyl groups.
- the aralkyl group includes, for example, a benzyl group, a diphenylmethyl group, a naphthylmethyl group and the like.
- polymer segments other than the polysiloxane segment possessed by the polysiloxane compound include vinyl polymer segments such as acrylic polymers, fluoroolefin polymers, vinyl ester polymers, aromatic vinyl polymers, and polyolefin polymers, Examples include polymer segments such as polyurethane polymer segments, polyester polymer segments, and polyether polymer segments. Among them, a vinyl polymer segment is preferred.
- the polysiloxane compound may be a composite resin in which polysiloxane segments and polymer segments are bonded in a structure represented by the following structural formula (S-3), or may have a three-dimensional network-like polysiloxane structure.
- the carbon atom is the carbon atom that constitutes the polymer segment, and the two silicon atoms are the silicon atoms that constitute the polysiloxane segment
- the polysiloxane segment of the polysiloxane compound may have a functional group capable of reacting by heating, such as a polymerizable double bond, in the polysiloxane segment.
- a functional group capable of reacting by heating such as a polymerizable double bond
- the cross-linking reaction proceeds and the polysiloxane compound is solidified, thereby facilitating the thermal decomposition treatment.
- polymerizable double bonds examples include vinyl groups and (meth)acryloyl groups. Two or more polymerizable double bonds are preferably present in the polysiloxane segment, more preferably 3 to 200, and even more preferably 3 to 50. In addition, by using a composite resin having two or more polymerizable double bonds as the polysiloxane compound, the cross-linking reaction can be facilitated.
- the polysiloxane segment may have silanol groups and/or hydrolyzable silyl groups.
- Hydrolyzable groups in hydrolyzable silyl groups include, for example, halogen atoms, alkoxy groups, substituted alkoxy groups, acyloxy groups, phenoxy groups, mercapto groups, amino groups, amido groups, aminooxy groups, iminooxy groups, alkenyloxy and the like, and the hydrolyzable silyl group becomes a silanol group by hydrolysis of these groups.
- a hydrolytic condensation reaction proceeds between the hydroxyl group in the silanol group and the hydrolyzable group in the hydrolyzable silyl group, thereby obtaining a solid polysiloxane compound. can.
- a silanol group as used in the present invention is a silicon-containing group having a hydroxyl group directly bonded to a silicon atom.
- the hydrolyzable silyl group referred to in the present invention is a silicon-containing group having a hydrolyzable group directly bonded to a silicon atom, specifically, for example, a group represented by the following general formula (S-4) is mentioned.
- R4 represents a monovalent organic group such as an alkyl group, an aryl group or an aralkyl group
- R5 represents a halogen atom, an alkoxy group, an acyloxy group, an allyloxy group, a mercapto group, an amino group, an amido group, an aminooxy group, iminooxy group or alkenyloxy group
- b is an integer of 0 to 2.
- Alkyl groups include, for example, methyl group, ethyl group, propyl group, isopropyl group, butyl group, isobutyl group, sec-butyl group, tert-butyl group, pentyl group, isopentyl group, neopentyl group, tert-pentyl group, 1 -methylbutyl group, 2-methylbutyl group, 1,2-dimethylpropyl group, 1-ethylpropyl group, hexyl group, isohexyl group, 1-methylpentyl group, 2-methylpentyl group, 3-methylpentyl group, 1,1 -dimethylbutyl group, 1,2-dimethylbutyl group, 2,2-dimethylbutyl group, 1-ethylbutyl group, 1,1,2-trimethylpropyl group, 1,2,2-trimethylpropyl group, 1-ethyl- 2-methylpropyl group, 1-ethyl-1-methylpropyl group
- aryl groups include phenyl, naphthyl, 2-methylphenyl, 3-methylphenyl, 4-methylphenyl, 4-vinylphenyl, and 3-isopropylphenyl groups.
- the aralkyl group includes, for example, a benzyl group, a diphenylmethyl group, a naphthylmethyl group and the like.
- the halogen atom includes, for example, fluorine atom, chlorine atom, bromine atom, iodine atom and the like.
- alkoxy groups include methoxy, ethoxy, propoxy, isopropoxy, butoxy, sec-butoxy, and tert-butoxy groups.
- acyloxy groups include formyloxy, acetoxy, propanoyloxy, butanoyloxy, pivaloyloxy, pentanoyloxy, phenylacetoxy, acetoacetoxy, benzoyloxy, and naphthoyloxy groups. mentioned.
- allyloxy groups include phenyloxy groups and naphthyloxy groups.
- alkenyloxy groups include vinyloxy, allyloxy, 1-propenyloxy, isopropenyloxy, 2-butenyloxy, 3-butenyloxy, 2-petenyloxy, 3-methyl-3-butenyloxy, 2 -hexenyloxy group and the like.
- polysiloxane segments having structural units represented by general formula (S-1) and/or general formula (S-2) include those having the following structures.
- the polymer segment may have various functional groups as necessary to the extent that the effects of the present invention are not impaired.
- Such functional groups include, for example, carboxyl group, blocked carboxyl group, carboxylic anhydride group, tertiary amino group, hydroxyl group, blocked hydroxyl group, cyclocarbonate group, epoxy group, carbonyl group, primary amide group, secondary An amide group, a carbamate group, a functional group represented by the following structural formula (S-5), and the like can be used.
- polymer segment may have polymerizable double bonds such as vinyl groups and (meth)acryloyl groups.
- the polysiloxane compound is preferably produced, for example, by the methods shown in (1) to (3) below.
- a polymer segment containing a silanol group and/or a hydrolyzable silyl group is prepared in advance, and the polymer segment and the silanol group and/or the hydrolyzable silyl group are and a method of mixing with a silane compound having a polymerizable double bond and carrying out a hydrolytic condensation reaction.
- a polymer segment containing a silanol group and/or a hydrolyzable silyl group is prepared in advance.
- Polysiloxane is also prepared in advance by subjecting a silane compound having both a silanol group and/or a hydrolyzable silyl group and a polymerizable double bond to a hydrolytic condensation reaction. Then, a method of mixing the polymer segment and polysiloxane and performing a hydrolytic condensation reaction.
- a polysiloxane compound is obtained by the method described above.
- Examples of the polysiloxane compound include the Ceranate (registered trademark) series (organic/inorganic hybrid type coating resin; manufactured by DIC Corporation) and the Compoceran SQ series (silsesquioxane type hybrid; manufactured by Arakawa Chemical Industries, Ltd.). .
- the carbon source resin used in step 1 is preferably a synthetic resin or a natural chemical raw material that has good miscibility with the polysiloxane compound, is carbonized by high-temperature baking in an inert atmosphere, and has an aromatic functional group. .
- Synthetic resins include thermoplastic resins such as polyvinyl alcohol and polyacrylic acid, and thermosetting resins such as phenol resin and furan resin.
- Natural chemical raw materials include heavy oils, especially tar pitches such as coal tar, light tar oil, medium tar oil, heavy tar oil, naphthalene oil, anthracene oil, coal tar pitch, pitch oil, mesophase pitch, and oxygen-crosslinked petroleum pitch. , heavy oil, etc., but the use of phenolic resin is more preferable from the viewpoint of inexpensive availability and removal of impurities.
- the carbon source resin is preferably a resin containing an aromatic hydrocarbon moiety
- the resin containing an aromatic hydrocarbon moiety is a phenolic resin, an epoxy resin, or a thermosetting resin.
- the phenol resin is preferably a resol type.
- phenolic resins include the Sumilite Resin series (resol-type phenolic resin, manufactured by Sumitomo Bakelite Co., Ltd.).
- the salt of at least one metal selected from the group consisting of Li, K, Na, Ca, Mg and Al (hereinafter referred to as the "present metal salt") used in step 2 includes fluorides of these metals, Halides such as chlorides and bromides, hydroxides, carbonates and the like can be mentioned.
- the present metal salt may be a salt of two or more metals, one salt may contain multiple metals, or a mixture of salts containing different metals.
- the amount of the present metal salt added when adding the present metal salt to the suspension obtained in step 1 is not particularly limited, but the molar ratio of the amount of the metal salt compound added to the total number of moles of the silicon particles but preferably 0.01 to 0.4.
- a method of mixing includes a method of dissolving the metal salt in an organic solvent, adding the metal salt to the suspension, and mixing when the metal salt is soluble in the organic solvent. If it is insoluble in an organic solvent, the metal salt particles may be dispersed in an organic solvent and then added to the suspension and mixed.
- the metal salt is preferably nanoparticles having an average particle size of 100 nm or less from the viewpoint of improving the dispersion effect.
- Alcohols, ketones and the like can be suitably used as the organic solvent, but aromatic hydrocarbon solvents such as toluene, xylene, naphthalene and methylnaphthalene can also be used.
- the suspension obtained in the step 1 and the present metal salt are uniformly mixed and stirred, and the precursor of the present active material (hereinafter also referred to as "precursor") is obtained through desolvation and drying. can get.
- the assembly containing the polysiloxane compound and the carbon source resin is preferably in a state in which the polysiloxane compound and the carbon source resin are uniformly mixed. Said mixing is carried out using a device having the function of dispersing and mixing. For example, a stirrer, an ultrasonic mixer, a premix disperser and the like can be used.
- a dryer, a reduced-pressure dryer, a spray dryer, or the like can be used for solvent removal and drying for the purpose of distilling off the organic solvent.
- the precursor contains 3% to 50% by mass of the present silicon particles which are silicon (zero valent), 15% to 85% by mass of the solid content of the polysiloxane compound, and 3% to 70% by mass of the solid content of the carbon source resin.
- the solid content of the silicon particles is preferably 8% to 40% by mass
- the solid content of the polysiloxane compound is 20% to 70% by mass
- the solid content of the carbon source resin is 3% to 60% by mass. It is more preferable to contain % by mass.
- the molecules of the present metal salt and the present silicon particles can be sufficiently brought into contact with each other.
- silicon oxide is present on the surface or in the periphery of the silicon particles
- the metal salt molecules and the silicon particles are brought into sufficient contact under conditions that cause a solid-phase reaction between the metal salt molecules and the silicon particles.
- the present silicate compound can be present near the surface of the silicon particles. In order to make the concentration of the present silicate compound in the vicinity of the surface of the present silicon particles higher than the concentration of the compound constituting the matrix phase, it is important to improve the contact state between the present metal salt and the present silicon particles. .
- the molecules of the present metal salt can be attached to the vicinity of the surface of the present silicon particles.
- the molecular structure of the organic additive is not particularly limited as long as it can physically or chemically bond with the dispersant present on the surface of the silicon particles.
- the physical or chemical bond includes electrostatic action, hydrogen bond, intermolecular Van der Waals force, ionic bond, covalent bond and the like.
- the molecules of the present metal salt undergo a solid phase reaction with the silicon oxide on the surface of the present silicon particles, whereby the surfaces of the present silicon particles can be coated with the present silicate compound.
- step 3 the precursor obtained in step 2 is calcined in an inert atmosphere at a maximum temperature of 900° C. to 1200° C. to completely decompose the thermally decomposable organic components, and other
- the main component is made into a sintered product suitable for the present active material by precisely controlling the sintering conditions. Specifically, the raw material polysiloxane compound and carbon source resin are converted into a silicon-oxygen-carbon skeleton and free carbon by the energy of the high-temperature treatment.
- the firing is carried out according to a firing program defined by the rate of temperature increase, retention time at a constant temperature, etc.
- the maximum attainable temperature is the maximum temperature to be set, and strongly affects the structure and performance of the fired composite active material for secondary batteries.
- the maximum temperature is set to 900° C. to 1200° C., the fine structure of the active material for secondary batteries having the chemical bonding state of silicon and carbon can be precisely controlled, and the silicon Since the oxidation of the particles can also be avoided, better charge-discharge characteristics can be obtained.
- the calcination method is not particularly limited, but a reaction apparatus having a heating function may be used in an inert atmosphere, and continuous or batchwise processing is possible.
- a fluidized bed reactor, a rotary furnace, a vertical moving bed reactor, a tunnel furnace, a batch furnace, a rotary kiln, or the like can be appropriately selected as the firing apparatus according to the purpose.
- Step 4 is a step of pulverizing the baked product obtained in Step 3 and classifying as necessary to obtain the present active material.
- the pulverization may be carried out in one step until the target particle size is obtained, or may be carried out in several steps.
- an active material of about 10 ⁇ m from a sintered product of lumps or agglomerates of 10 mm or more it is roughly pulverized with a jaw crusher, a roll crusher, etc., and after making particles of about 1 mm, a glow mill, a ball mill, etc. is used to about 100 ⁇ m. and pulverized to about 10 ⁇ m with a bead mill, jet mill, or the like.
- Particles produced by pulverization may contain coarse particles, and classification is performed to remove them or to adjust the particle size distribution by removing fine powder.
- the classifier to be used may be a wind classifier, a wet classifier, or the like depending on the purpose, but when removing coarse particles, the classification method through a sieve is preferable because the purpose can be reliably achieved.
- the pulverization step can be omitted when the precursor mixture is controlled to have a shape near the target particle size by spray drying or the like before firing, and firing is performed in that shape.
- the surface of the present silicon particles can be oxidized to form a silicon dioxide film in the process of solvent removal and drying in step 2 and in the process of step 3.
- the present active material when used as a negative electrode active material for a secondary battery, the secondary battery is excellent in initial coulombic efficiency and capacity retention rate.
- the present active material can be used as a negative electrode by the method described above to form a secondary battery having the negative electrode.
- the present active material and the secondary battery including the present active material in the negative electrode have been described above, the present invention is not limited to the configurations of the above embodiments. In the configuration of the present embodiment and the secondary battery containing the present active material in the negative electrode, any other configuration may be added, or any configuration that exhibits the same function may be substituted. good.
- the present invention will be described in detail below with reference to Examples, but the present invention is not limited to these.
- the negative electrode is composed mainly of the silicon-containing active material of the present invention, and the counter electrode is metallic lithium. This is to clearly compare the cycle characteristics.
- Synthesis Example 1 Preparation of Silicon Particles Zirconia beads with a particle size of 0.1 mm to 0.2 mm and 100 ml of methyl ethyl ketone solvent (MEK) were put into a container of a small bead mill apparatus of 150 ml at a filling rate of 60%. After that, silicon powder (commercially available) with an average particle size of 5 ⁇ m and a cationic dispersant liquid (BYK-Chemie Japan Co., Ltd.: BYK145) are added, and wet milling is performed under the conditions shown in Table 1 to obtain a solid. A dark brown liquid silicon slurry having a concentration of 30% by mass was obtained. The morphology and size of the pulverized silicon products were confirmed by TEM observation, and as shown in Table 1, Si1, Si2, Si3, Si4 and Si5, respectively.
- MEK methyl ethyl ketone solvent
- Synthesis Example 2 Preparation of polysiloxane compound (synthesis of condensate (a1) of methyltrimethoxysilane) 1,421 parts by mass of methyltrimethoxysilane (hereinafter abbreviated as "MTMS”) was charged into a reaction vessel equipped with a stirrer, thermometer, dropping funnel, condenser and nitrogen gas inlet, and heated to 60°C. heated up. Then, a mixture of 0.17 parts by mass of iso-propyl acid phosphate ("Phoslex A-3" manufactured by SC Organic Chemical Co., Ltd.) and 207 parts by mass of deionized water was dropped into the reaction vessel over 5 minutes. , and stirred at a temperature of 80° C.
- MTMS methyltrimethoxysilane
- the effective ingredient is the value obtained by dividing the theoretical yield (parts by mass) when all the methoxy groups of the silane monomer such as MTMS are condensed by the actual yield (parts by mass) after the condensation reaction. Theoretical yield when all methoxy groups are condensed (parts by mass)/Actual yield after condensation reaction (parts by mass)].
- MMA methyl methacrylate
- BMA butyl methacrylate
- BA butyric acid
- MPTS methacryloyloxypropyltrimethoxysilane
- BuOH butylperoxy-2-ethylhexanoate
- Example 1 The polysiloxane resin having an average molecular weight of 3500 (curable resin composition (1)) prepared in Synthesis Example 2 and the phenolic resin having an average molecular weight of 3000 were added at a resin solid weight ratio of 45/55, and the product after high temperature baking was obtained.
- the silicon slurry of Si3 obtained in Synthesis Example 1 and an appropriate amount of methyl ethyl ketone solvent were added so that the content of silicon particles in the product was 50% by mass, and the mixture was thoroughly mixed in a stirrer. As a result, a mixed suspension of silicon particle-containing resin having a solids concentration of 10% by mass was obtained.
- Active material composite particles were produced after pulverizing with a planetary ball mill.
- the silicon crystallite size was 21 nm.
- a slurry was prepared by mixing 80 parts by mass of the active material composite particles obtained above, 10 parts by mass of acetylene black as a conductive aid, and 10 parts by mass of a mixture of CMC and SBR as a binder.
- the obtained slurry was formed into a film on a copper foil.
- a coin-type lithium ion battery was produced as a half cell using a Li metal foil as a counter electrode.
- a secondary battery charge/discharge test device manufactured by Hokuto Co., Ltd.
- charge/discharge characteristics of the half-cells produced were evaluated.
- the cutoff voltage range was 0.005 to 1.5V.
- the charge/discharge measurement results were an initial discharge capacity of 1545 mAh/g and an initial coulombic efficiency of 85%.
- a single-layer sheet using LiCoO 2 as a positive electrode active material and aluminum foil as a current collector was used to prepare a positive electrode film, and graphite powder was used at a discharge capacity design value of 450 mAh / g. and the active material powder were mixed to prepare a negative electrode film.
- a non-aqueous electrolyte solution prepared by dissolving lithium hexafluorophosphate in a mixture of ethylene carbonate and diethyl carbonate in a volume ratio of 1/1 at a concentration of 1 mol/L was used as the non-aqueous electrolyte, and polyethylene having a thickness of 30 ⁇ m was used as the separator.
- a laminate-type lithium-ion secondary battery was fabricated using a microporous film made by A laminated lithium ion secondary battery is charged at room temperature at a constant current of 1.2 mA (0.25c based on the positive electrode) until the voltage of the test cell reaches 4.2 V. After reaching 4.2 V, Charging was performed by decreasing the current so as to keep the cell voltage at 4.2 V, and the discharge capacity was determined. The capacity retention rate after 100 cycles at 45°C was 91%. Table 2 shows the results.
- Examples 2 to 5 Mg 2+ /Si (molar ratio) with respect to the amount of silicon particles is respectively 4.0/100 (Example 2), 5.5/100 (Example 3), 7.5/100 (Example 4), MgCl 2 as a raw material was added to the mixed suspension of the silicon particle-containing resin so as to have a ratio of 12/100 (Example 5).
- Other conditions were the same as in Example 1 to obtain active material composite particles.
- a secondary battery using a negative electrode active material containing the obtained active material composite particles was evaluated. Table 2 shows the results.
- Example 6 Active material composite particles were obtained by changing the precursor preparation conditions and the like to the same conditions as in Example 5 and setting the sintering temperature to 1180°C. A secondary battery using a negative electrode active material containing the obtained active material composite particles was evaluated. Table 2 shows the results.
- Examples 7-8 LiCl was used as the raw material of the silicate compound instead of MgCl2 .
- LiCl was added to the silicon particle-containing resin mixture suspension with the Li + /Si (molar ratio) of 10/100 (Example 7) and 24/100 (Example 8) relative to the amount of silicon particles, respectively,
- the firing temperature was 1000°C.
- Other conditions were the same as in Example 1 to obtain active material composite particles.
- a secondary battery using a negative electrode active material containing the obtained active material composite particles was evaluated. Table 2 shows the results.
- Examples 9 to 11 A polysiloxane resin having an average molecular weight of 3500 (curable resin composition (2)) prepared in the same manner as in Synthesis Example 2 and a phenolic resin having an average molecular weight of 3000 were mixed in a resin solid weight ratio of 10/90 (Example 9). , 20/80 (Example 10) and 80/20 (Example 11).
- the amount of MgCl 2 added was such that the Mg 2+ /Si (molar ratio) with respect to the amount of silicon particles was 7.5/100. added to the turbidity.
- Other conditions were the same as in Example 1 to obtain active material composite particles.
- a secondary battery using a negative electrode active material containing the obtained active material composite particles was evaluated. Table 2 shows the results.
- Examples 12-14 The silicon particles in the mixed suspension of the silicon particle-containing resin described in Example 1 were added to the silicon particles Si4 (Example 12), Si2 (Example 13), and Si1 (Example 14) obtained in Synthesis Example 1. , and other conditions were the same as in Example 1 to obtain active material composite particles. A secondary battery using a negative electrode active material containing the obtained active material composite particles was evaluated. Table 2 shows the results.
- Example 15 The same procedure as in Example 4 was repeated except that the amount of MgCl 2 added was such that Mg 2+ /Si (molar ratio) with respect to the amount of silicon particles was 2.0/100. Active material composite particles were obtained. A secondary battery using a negative electrode active material containing the obtained active material composite particles was evaluated. Table 2 shows the results.
- Example 16 Using commercially available silicon particles (manufactured by Alfa Aesar, 50 nm) and pitch (carbon source), a mixed suspension of silicon particle-containing resin was prepared so that the silicon particle content was 50% by weight after high temperature firing. Other conditions were the same as in Example 1 to obtain active material composite particles. A secondary battery using a negative electrode active material containing the obtained active material composite particles was evaluated. Table 2 shows the results.
- Comparative example 1 After preparing a precursor dried product in the same manner as in Example 1 except that no Li + or Mg 2+ compound was added as a silicate compound, the dried precursor was fired in a nitrogen atmosphere at 1100° C. for 4 hours to obtain an active material powder. Obtained.
- the active material powder had a D50 of about 5.4 ⁇ m and a specific surface area of 4.1 m 2 /g.
- the charge-discharge measurement results show a half-cell charge-discharge capacity of 1,935 mAh/g, a discharge capacity of 1,587 mAh/g, and an initial efficiency of 80.1%. %Met.
- Comparative example 2 A precursor was prepared by using the silicon particles Si5 of Synthesis Example 1 in the mixed suspension of the silicon particle-containing resin. Other conditions were the same as in Example 1 to obtain active material composite particles. A secondary battery using a negative electrode active material containing the obtained active material composite particles was evaluated. Table 2 shows the results.
- each evaluation method is as follows. D50: Measured using a laser diffraction particle size distribution analyzer (Mastersizer 3000, manufactured by Malvern Panalytical). Specific surface area: Measured by BET method from nitrogen adsorption measurement using a specific surface area measuring device (BELSORP-mini manufactured by BEL JAPAN). 29 Si-NMR: JNM-ECA600 manufactured by JEOL RESONANCE was used.
- Battery characteristics evaluation Battery characteristics are measured using a secondary battery charge-discharge test device (manufactured by Hokuto Denko Co., Ltd.), room temperature 25 ° C., cutoff voltage range from 0.005 to 1.5 V, charge / discharge rate is 0 The charging/discharging characteristics were evaluated under conditions of constant current/constant voltage charging/constant current discharging at 0.2 C (after 4 cycles) and 1 C (1 to 3 times). At the time of switching between charging and discharging, the battery was left in an open circuit for 30 minutes.
- Initial coulombic efficiency and cycle characteristics (in the present application, refer to capacity retention rate at 100 cycles) were obtained as follows.
- the present active material when used as a negative electrode active material, the initial coulomb efficiency is 83% or more and the capacity retention rate is 85% or more, which is generally high.
- Excellent for A secondary battery containing the present active material as a negative electrode active material has excellent battery characteristics.
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Abstract
Description
さらに近年、各種電子機器・通信機器の小型化およびハイブリッド自動車等の急速な普及に伴い、これら機器等の駆動電源として、より高容量であり、かつサイクル特性や放電レート特性等の各種電池特性が更に向上したリチウムイオン電池の開発が強く求められている。
なかでも黒鉛系負極材料の理論容量より10倍以上の理論容量を有するケイ素や、ケイ素含有活物質が注目されている。しかしながらケイ素含有活物質は初期効率が低いことが知られている。
例えば特許文献1には層状ポリシランに金属リチウムと多環芳香族化合物とを有機溶剤に溶解したリチウム錯体溶液を接触させ、前記層状ポリシランにリチウムをドープする方法によるリチウムドープ層状シリコンが記載されている。
さらにはLiとMgの両方を含むケイ酸化合物として、特許文献4ではケイ素化合物粒子がLi化合物を含有し、ポリアクリル酸の塩またはカルボキシメチルセルロースの塩を含み、MgまたはAlから選ばれる少なくとも1種の金属を含む金属塩を含む負極活物質についても検討されている。
前記文献1から4に記載の方法は、ケイ素含有活物質に存在する酸化ケイ素にLiまたはMgを含有させることで初期段階でのリチウムと酸化ケイ素との反応を抑制し、初期の充放電効率の低下を抑制しようとするものである。
またLiおよびMgのシリケートは、強い吸水性を有するのが一般的性状であるため、負極製膜用水系スラリーの調製に対して負極活物質の凝集が起こりやすくなる。
その結果、前記特許文献1から4の負極活物質を用いたリチウム二次電池は初期のクーロン効率および容量維持率の改良効果は不十分であった。
即ち本発明は、リチウムイオン二次電池に用いられる二次電池用複合活物質および前記二次電池用複合活物質を負極活物質として含む二次電池に関し、初期のクーロン効率および容量維持率に優れた二次電池を与える二次電池用負極活物質を提供することを目的とする。
[1] 平均粒径が150nm以下のシリコン粒子、前記シリコン粒子が分散したマトリクス相、およびLi、K、Na、Ca、MgおよびAlからなる群から選ばれる少なくとも1種の金属のシリケート化合物を有し、前記シリコン粒子の表面近傍に前記シリケート化合物を有する二次電池用複合活物質。
[2] 前記シリコン粒子の表面近傍におけるシリケート濃度が、前記マトリクス相におけるシリケート濃度より高濃度である前記[1]に記載の二次電池用複合活物質。
[3] 前記シリコン粒子の表面近傍に、さらに二酸化ケイ素を有する前記[1]または[2]に記載の二次電池用複合活物質。
[4] 前記二酸化ケイ素のモル比が前記シリケート化合物と前記二酸化ケイ素の合モル数を1として、0.9以下である前記[3]に記載の二次電池用複合活物質。
[5] 前記シリケート化合物がマグネシウムシリケート化合物である前記[1]から[4]のいずれかに記載の二次電池用複合活物質。
[6] 前記シリコン粒子の表面近傍の前記マグネシウムシリケート化合物の厚みが0.2nmから10nmである結晶質膜である前記[5]に記載の二次電池用複合活物質。
[7] 前記マトリクス相は少なくともSiOxCyNz(ただし、1<x<2、1<y<20、0<z<0.5)で表される化合物を含む前記[1]から[6]のいずれかに記載の二次電池用複合活物質。
[8] 前記シリコン粒子がフレーク状結晶体であり、X線回折スペクトルにおける2θが28.4度のピークから得られる結晶子サイズが25nm以下である前記[1]から[7]のいずれかに記載の二次電池用複合活物質。
[9] 平均粒径が1μmから15μm、比表面積が1m2/gから30m2/gである前記[1]から[8]のいずれかに記載の二次電池用複合活物質。
[10] 前記[1]から[9]のいずれかに記載の二次電池用複合活物質を負極に含む二次電池。
シリコン粒子表面上に酸化ケイ素膜が存在すると考えられ、初期充電時のリチウムと酸化ケイ素との反応によるリチウム酸化物の生成は酸化ケイ素の近傍で起こると考えられる。本活物質はLi、K、Na、Ca、MgおよびAlからなる群から選ばれる少なくとも1種の金属のシリケート化合物をシリコン粒子の表面近傍に多く存在させることで効率的にリチウム酸化物の生成を抑制することができると考えられる。その結果、本活物質を負極活物質として用いた場合、負極活物質の初期のクーロン効率および容量維持率が改良されると考えられる。
ここで平均粒径はレーザー回折式粒度分析計などを用いて測定することができるD50の値である。D50は、レーザー粒度分析計などを用い動的光散乱法により測定することができる。本シリコン粒子の平均粒径は、粒子径分布において、小径側から体積累積分布曲線を描いた場合に、累積50%となるときの粒子径である。
したがって、本シリコン粒子は300nmを超える大サイズのシリコン粒子および10nm未満の小サイズのシリコン粒子の含有割合が出来るだけ小さいことが好ましい。
前記の観点から、平均粒径は120nm以下が好ましく、100nm以下がより好ましい。また平均粒径は20nm以上が好ましく、30nm以上がより好ましい。
シリコンの塊の粉砕に用いる粉砕機としては、ボールミル、ビーズミル、ジェットミルなどの粉砕機が例示できる。また、粉砕は有機溶剤を用いた湿式粉砕であってもよく、有機溶剤としては、例えば、アルコール類、ケトン類などを好適に用いることができるが、トルエン、キシレン、ナフタレン、メチルナフタレンなどの芳香族炭化水素系溶剤も用いることができる。
得られたシリコンの粒子を、ビーズ粒径、配合率、回転数または粉砕時間などのビーズミルの条件を制御し、分級等することで本シリコン粒子の平均粒径を前記範囲とすることができる。
本シリコン粒子の形態は、動的光散乱法で平均粒径の測定が可能であるが、透過型電子顕微鏡(TEM)や電界放出型走査電子顕微鏡(FE-SEM)の解析手段を用いることで、前記アスペクト比のサンプルをより容易かつ精密に同定することができる。本発明の二次電池用材料を含有する負極活物質の場合は、サンプルを集束イオンビーム(FIB)で切断して断面をFE-SEM観察することができ、またはサンプルをスライス加工してTEM観察により本シリコン粒子の状態を同定することができる。
なお前記本シリコン粒子のアスペクト比は、TEM画像に映る視野内のサンプルの主要部分50粒子をベースにした計算結果である。
SiOxCy (1)
式(1)中、xはケイ素に対する酸素のモル比、yはケイ素に対する炭素のモル比を表す。
本活物質を二次電池に用いた場合、充放電性能と容量維持率とのバランスが優位になるという観点から、1≦x<2が好ましく、1≦x≦1.9がより好ましく、1≦x≦1.8がさらに好ましい。
また、本活物質を二次電池に用いた場合、充放電性能と初回クーロン効率のバランスとの観点から、1≦y≦20が好ましく、1.2≦y≦15がより好ましい。
マトリクス相を構成する化合物がケイ素、酸素、炭素および窒素を含む化合物の場合、マトリクス相は下記式(2)で表される化合物を含有するのが好ましい。
SiOxCyNz (2)
式(2)中、xおよびyは前記と同じ意味であり、zはケイ素に対する窒素のモル比を表す。
マトリクス相が前記式(2)で表される化合物を含む場合、本活物質を二次電池に用いた際の充放電性能や容量維持率の観点から、1≦x≦2、1≦y≦20、0<z≦0.5が好ましく、1≦x≦1.9、1.2≦y≦15、0<z≦0.4がより好ましい。
なお、前記x、yおよびzの測定は前記記載方法によって実施することが好ましいが、本活物質の局所的な分析を行い、それにより得られた含有比データの測定点数を多く取得して、本活物質全体の含有比を類推することでも可能である。局所的な分析としては、例えばエネルギー分散型X線分光法(SEM-EDX)や電子線プローブマイクロアナライザ(EPMA)が挙げられる。
シリケート化合物は一般に1個または数個のケイ素原子を中心とし、電気陰性な配位子がこれを取り囲んだ構造を持つアニオンを含む化合物であるが、本シリケート化合物はLi、K、Na、Ca、MgおよびAlからなる群から選ばれる少なくとも1種の金属と前記アニオンを含む化合物との塩である。
前記アニオンを含む化合物としてはオルトケイ酸イオン(SiO4 4-)、メタケイ酸イオン(SiO3 2-)、ピロケイ酸イオン(Si2O7 6-)、環状ケイ酸イオン(Si3O9 6-またはSi6O18 12-)等のケイ酸イオンが知られている。本シリケート化合物はメタケイ酸イオンとLi、K、Na、Ca、MgおよびAlからなる群から選ばれる少なくとも1種の金属との塩であるシリケート化合物が好ましい。前記金属の中ではLiまたはMgが好ましい。
本シリケート化合物はLi、K、Na、Ca、MgおよびAlからなる群から選ばれる少なくとも1種の金属を有しており、これら金属の2種以上を有していてもよい。2種以上の金属を有する場合、一つのケイ酸イオンが複数種の金属を有していてもよいし、異なる金属を有するシリケート化合物の混合物であってもよい。また本シリケート化合物はLi、K、Na、Ca、MgおよびAlからなる群から選ばれる少なくとも1種の金属を有する限り、他の金属を有してもよい。
本シリケート化合物はリチウムシリケート化合物またはマグネシウムシリケート化合物が好ましく、メタケイ酸リチウム(Li2SiO3)またはメタケイ酸マグネシウム(MgSiO3)がより好ましく、メタケイ酸マグネシウム(MgSiO3)が特に好ましい。
本シリケート化合物は、効率的にリチウム酸化物の生成を抑制する観点から、本シリコン粒子の表面近傍に多く存在するのが好ましく、本活物質に含まれる本シリケート化合物の全質量の50質量%以上が表面近傍に存在するのがより好ましく、60質量%以上が表面近傍に存在するのがより好ましい。
本シリコン粒子の表面近傍に本シリケート化合物が存在していることの確認は高分解能透過型電子顕微鏡(以下、「HR-TEM」とも記す。)にて確認することができる。詳しくはサンプルを集束イオンビーム(FIB)でスライス加工してHR-TEM観察により確認することができる。
面積比=結晶子に帰属される格子構造の面積/全視野の面積
この面積比は5か所以上の異なる視野の平均値を用いて求められる。
また、STEM―EDS(Scanning Transmission Electron Microscope Energy-Dispersive-Spectroscopy)を用いてシリコン粒子近傍に存在するMg元素、Si元素、O元素の濃度をマッピング像で検出することができる。Mg元素、Si元素、O元素を合わせてシリケート化合物の濃度を判断できる。
また、HR-TEMを用いて本シリコン粒子の表面近傍の範囲内で本シリケート化合物と前記マトリクス相を構成する化合物との濃度を比較した場合、本シリケート化合物の表面近傍にあるシリケート化合物結晶子の格子構造の面積は、観察視野の単位面積あたり、前記マトリクス相を構成する化合物の結晶格子構造の面積の2倍以上が好ましく、3倍以上がより好ましく。
本シリケート化合物がマグネシウムシリケート化合物の場合、マグネシウムシリケート化合物が結晶質膜であり本シリコン粒子の表面の少なくとも一部を被覆しているのが好ましい。マグネシウムシリケート化合物が本シリコン粒子の表面の少なくとも一部を被覆している場合、被覆率は50%以上がより好ましく、80%以上が特に好ましい。マグネシウムシリケート化合物が本シリコン粒子の少なくとも一部を被覆している場合、結晶質膜の厚みは0.2nmから10nmが好ましく、1nmから8nmがより好ましい。前記被覆率および結晶質膜の厚みは前記HR-TEMにより測定することができる。被覆率はシリコン粒子とマグネシウムシリケート化合物との量比から求めることができる。
本活物質が二酸化ケイ素を有する場合、本シリコン粒子の表面近傍に本シリケート化合物と二酸化ケイ素が存在している。本シリケート化合物と二酸化ケイ素とのモル比は、本シリケート化合物と二酸化ケイ素の合計モル数を1として、二酸化ケイ素のモル比は0.9以下が好ましく、0.5以下がより好ましい。また本シリケート化合物と二酸化ケイ素とのモル比は、本シリケート化合物と二酸化ケイ素の合計モル数を1として、二酸化ケイ素のモル比は0.05以上が好ましく、0.1以上がより好ましい。
平均粒径が小さすぎると、比表面積の大幅な上昇につれ、本活物質を二次電池とした時、充放電時に固相界面電解質分解物(以下、「SEI」とも記す。)の生成量が増えることで単位体積当たりの可逆充放電容量が低下することがある。平均粒径が大きすぎると、電極膜作製時に集電体から剥離するおそれがある。
また本活物質は被覆材により表面が被覆されていてもよい。被覆材としては、電子伝導性、リチウムイオン伝導性、電解液の分解抑制効果が期待出来る物質が好ましい。
また本活物質の表面が前記被覆材により被覆されている場合、負極活物質の化学安定性や熱安定性の改善の観点から、被覆材の含有量は前記本活物質の全量を100質量%として、1から30質量%が好ましく、3から25質量%がより好ましい。なお本活物質の全量とは、本活物質を構成する本シリコン粒子、マトリクス相、本シリケート化合物および前記被覆材の合計量である。マトリクス相が窒素を含む場合は、窒素も含む合計量である。
被覆材が低結晶性炭素の場合、被覆層の平均厚みは10nm以上300nm以下、または、低結晶性炭素の含有量は本活物質の全量を100質量%として、1から30質量%が好ましい。
具体的には、本活物質と有機結着剤と、必要に応じてその他の導電助剤などの成分を含んで構成されるスラリーを集電体銅箔上へ薄膜状に塗付して負極とすることができる。また、前記のスラリーに黒鉛など炭素材料を加えて負極を作製することもできる。
炭素材料としては、天然黒鉛、人工黒鉛、ハードカーボンまたはソフトカーボンのような非晶質炭素などが挙げられる。
かかる範囲において、本活物質は、化学安定性が高く、水性バインダーも採用することができる点で、実用化面においても取り扱い容易である。
工程1: 湿式法粉砕したケイ素(0価)スラリーを、ポリシロキサン化合物と炭素源樹脂を含む集合体と混合させ、懸濁液を得る。
工程2:得られた懸濁液にLi、K、Na、Ca、MgおよびAlからなる群から選ばれる少なくとも1種の金属の塩を添加、混合後、脱溶媒して前駆体を得る。
工程3: 前記工程2で得られた前駆体を不活性雰囲気中、最高到達温度1000℃から1180℃の温度範囲内で焼成して焼成物を得る。
工程4: 前記工程3で得られた焼成物を粉砕して本活物質を得る。
<工程1>
(ケイ素(0価)スラリー)
工程1で用いる湿式法粉砕したケイ素(0価)スラリーの調製は、有機溶媒を用い、シリコン粒子を湿式粉末粉砕装置にて粉砕しながら行うことができる。有機溶媒においてシリコン粒子の粉砕を促進させるために分散剤を用いても良い。湿式粉砕装置としては、特に限定されるものでなく、ローラーミル、高速回転粉砕機、容器駆動型ミル、ビーズミルなどが挙げられる。
湿式粉砕ではシリコン粒子が本シリコン粒子の粒径となるまで粉砕するのが好ましい。
工程1で用いるポリシロキサン化合物としては、ポリカルボシラン構造、ポリシラザン構造、ポリシラン構造およびポリシロキサン構造を少なくとも1つ含む樹脂である。これらの構造のみを含む樹脂であっても良く、これら構造の少なくとも一つをセグメントとして有し、他の重合体セグメントと化学的に結合した複合型樹脂でも良い。複合化の形態はグラフト共重合、ブロック共重合、ランダム共重合、交互共重合などがある。例えば、ポリシロキサンセグメントと重合体セグメントの側鎖に化学的に結合したグラフト構造を有する複合樹脂があり、重合体セグメントの末端にポリシロキサンセグメントが化学的に結合したブロック構造を有する複合樹脂等が挙げられる。
上述方法によりポリシロキサン化合物が得られる。
ポリシロキサン化合物としては、例えば、セラネート(登録商標)シリーズ(有機・無機ハイブリッド型コーティング樹脂;DIC株式会社製)やコンポセランSQシリーズ(シルセスキオキサン型ハイブリッド;荒川化学工業株式会社製)が挙げられる。
前記工程1で用いる炭素源樹脂は、ポリシロキサン化合物との混和性が良く、また、不活性雰囲気中、高温焼成により炭化され、芳香族官能基を有する合成樹脂や天然化学原料を用いることが好ましい。
フェノール樹脂としては、例えばスミライトレジンシリーズ(レゾール型フェノール樹脂,住友ベークライト株式会社製)が挙げられる。
前記工程2で用いるLi、K、Na、Ca、MgおよびAlからなる群から選ばれる少なくとも1種の金属の塩(以下、「本金属塩」と記す。)としては、これら金属のフッ化物、塩化物、臭化物等のハロゲン化物、水酸化物、炭酸塩等が挙げられる。
本金属塩は2種以上の金属の塩でもよく、一つの塩が複数種の金属を有していてもよいし、異なる金属を有する塩の混合物であってもよい。
本金属塩を前記工程1で得られた懸濁液に添加する際の本金属塩の添加量は、特に制限がないが、シリコン粒子全量のモル数に対して金属塩化合物添加量のモル比が、0.01から0.4が好ましい。
混合の方法は、有機溶媒に可溶の場合に前記金属塩を有機溶媒に溶かして前記懸濁液に加えて混合する方法が挙げられる。有機溶媒に不溶の場合、本金属塩の粒子を有機溶媒に分散してから前記懸濁液に加えて混合すればよい。前記金属塩は、分散効果向上の観点から平均粒径が100nm以下のナノ粒子が好ましい。前記有機溶媒は、アルコール類、ケトン類などを好適に用いることができるが、トルエン、キシレン、ナフタレン、メチルナフタレンなどの芳香族炭化水素系溶剤も用いることができる。
前記工程1で得られた懸濁液と前記本金属塩とを均一に混合し、攪拌した後、脱溶媒と乾燥を経て本活物質の前駆体(以下、「前駆体」とも記す。)が得られる。ポリシロキサン化合物と炭素源樹脂を含む集合体は、ポリシロキサン化合物と炭素源樹脂とが均一に混合した状態であることが好ましい。前記混合は分散および混合の機能を有する装置を用いて行われる。例えば、攪拌機、超音波ミキサー、プリミックス分散機などが挙げられる。有機溶媒を溜去することを目的とする脱溶剤と乾燥の作業では、乾燥機、減圧乾燥機、噴霧乾燥機などを用いることができる。
また、有機添加物を用いて前記本金属塩の分子を表面修飾することで、本シリコン粒子の表面付近に付着させることができる。有機添加物の分子構造は、特に制限はないが、本シリコン粒子の表面上に存在している分散剤との物理的または化学的な結合ができれば良い。前記物理的または化学的結合は、静電作用、水素結合、分子間ファンデルワールス力、イオン結合、共有結合などが挙げられる。高温焼成の時、前記本金属塩の分子が本シリコン粒子の表面の酸化ケイ素と固相反応することにより、本シリコン粒子の表面を本シリケート化合物で被覆することができる。
工程3は、前記工程2で得られた前駆体を不活性雰囲気中、最高到達温度900℃から1200℃の温度範囲内で焼成することで、熱分解可能な有機成分を完全分解させ、その他の主成分を焼成条件の精密制御により本活物質に適した焼成物とする工程である。具体的にいうと、原料のポリシロキサン化合物および炭素源樹脂が高温処理のエネルギーによってケイ素-酸素-炭素骨格とフリー炭素に転化される。
工程4は、前記工程3で得られた焼成物を粉砕し、必要に応じて分級することで本活物質を得る工程である。粉砕は、目的とする粒径まで一段で行っても良いし、数段に分けて行っても良い。例えば10mm以上の塊または凝集粒子の焼成物から、10μm程度の活物質を作製する場合はジョークラッシャー、ロールクラッシャー等で粗粉砕を行い、1mm程度の粒子にした後、グローミル、ボールミル等で100μm程度とし、ビーズミル、ジェットミル等で10μm程度まで粉砕する。粉砕で作製した粒子には粗大粒子が含まれる場合がありそれを取り除くため、または、微粉を取り除いて粒度分布を調整する場合は分級を行う。使用する分級機は風力分級機、湿式分級機等目的に応じて使い分けるが、粗大粒子を取り除く場合、篩を通す分級方式が確実に目的を達成できるために好ましい。なお、焼成前に前駆体混合物を噴霧乾燥等により目標粒子径付近の形状に制御し、その形状で焼成を行った場合は、粉砕工程を省くことも可能である。
本活物質は前記方法により負極として用い、前記負極を有する二次電池とすることができる。
以上、本活物質、本活物質を負極に含む二次電池に関して説明したが、本発明は前記の実施形態の構成に限定されない。
本活物質および本活物質を負極に含む二次電池は前記実施形態の構成において、他の任意の構成を追加してもよいし、同様の機能を発揮する任意の構成と置換されていてもよい。
なお、本発明の実施例で用いるハーフセルは、負極に本発明のケイ素含有活物質を主体とする構成とし、対極に金属リチウムを用いた簡易評価を行っているが、これはより活物質自体のサイクル特性を明確に比較するためである。
150mlの小型ビーズミル装置の容器中に60%の充填率で粒径が0.1mmから0.2mmのジルコニアビーズおよび100mlのメチルエチルケトン溶媒(MEK)を入れた。その後、平均粒径が5μmのシリコン粉体(市販品)とカチオン性分散剤液(ビックケミー・ジャパン株式会社:BYK145)を入れ、表1に記載の条件下にてビーズミル湿式粉砕を行い、固形物濃度が30質量%の濃い褐色液体状のシリコンスラリーを得た。TEM観察でシリコン粉砕品の形態およびサイズを確認し、表1に示したように、それぞれをSi1、Si2、Si3、Si4およびSi5とした。
(メチルトリメトキシシランの縮合物(a1)の合成)
攪拌機、温度計、滴下ロート、冷却管および窒素ガス導入口を備えた反応容器に、1、421質量部のメチルトリメトキシシラン(以下、「MTMS」と略記する。)を仕込んで、60℃まで昇温した。次いで、前記反応容器中に0.17質量部のiso-プロピルアシッドホスフェート(SC有機化学株式会社製「Phoslex A-3」)と207質量部の脱イオン水との混合物を5分間で滴下した後、80℃の温度で4時間撹拌して加水分解縮合反応させた。
前記の加水分解縮合反応によって得られた縮合物を、温度40から60℃および40から1.3kPaの減圧下で蒸留した。なお、「40から1.3kPaの減圧下」とは、メタノールの留去開始時の減圧条件が40kPaで、最終的に1.3kPaとなるまで減圧することを意味する。以下の記載においても同様である。前記反応過程で生成したメタノールおよび水を除去することによって、数平均分子量1,000から5,000のMTMSの縮合物(a1)を含有する液を1、000質量部得た。得られた液の有効成分は70質量%であった。
なお、前記有効成分とは、MTMS等のシランモノマーのメトキシ基が全て縮合反応した場合の理論収量(質量部)を、縮合反応後の実収量(質量部)で除した値、〔シランモノマーのメトキシ基が全て縮合反応した場合の理論収量(質量部)/縮合反応後の実収量(質量部)〕、により算出したものである。
撹拌機、温度計、滴下ロート、冷却管および窒素ガス導入口を備えた反応容器に、150質量部のブタノール(以下、「BuOH」とも記す。)、105質量部のフェニルトリメトキシシラン(以下、「PTMS」とも記す。)、277質量部のジメチルジメトキシシラン(以下、「DMDMS」とも記す。)を仕込んで80℃まで昇温した。
次いで、同温度で21質量部のメチルメタアクリレート(以下、「MMA」とも記す。)、4質量部のブチルメタアクリレート(以下、「BMA」とも記す。)、3質量部の酪酸(以下、「BA」とも記す。)、2質量部のメタクリロイルオキシプロピルトリメトキシシラン(以下、「MPTS」とも記す。)、3質量部のBuOHおよび0.6質量部のブチルペルオキシ-2-エチルヘキサノエート(以下、「TBPEH」とも記す。)を含有する混合物を、前記反応容器中へ6時間で滴下した。滴下終了後、更に同温度で20時間反応させて加水分解性シリル基を有する数平均分子量が10,000のビニル重合体(a2-1)の有機溶剤溶液を得た。
次いで、この液に472質量部の合成例2で得られたMTMSの縮合物(a1)、80質量部の脱イオン水を添加し、同温度で10時間撹拌して加水分解縮合反応させ、合成例1と同様の条件で蒸留することによって生成したメタノールおよび水を除去した。次いで、250質量部のBuOHを添加し、不揮発分が60.1質量%の硬化性樹脂組成物(1)を1、000質量部得た。
撹拌機、温度計、滴下ロート、冷却管および窒素ガス導入口を備えた反応容器に、150質量部のBuOH、249質量部のPTMS、263質量部のDMDMSを仕込んで80℃まで昇温した。
次いで、同温度で18質量部のMMA、14質量部のBMA、7質量部のBA、1質量部のアクリル酸(以下、「AA」とも記す。)、2質量部のMPTS、6質量部のBuOHおよび0.9質量部のTBPEHを含有する混合物を、前記反応容器中へ5時間で滴下した。滴下終了後、更に同温度で10時間反応させて加水分解性シリル基を有する数平均分子量が20、100のビニル重合体(a2-2)の有機溶剤溶液を得た。
次いで、この液に76質量部の3-グリシドキシプロピルトリメトキシシラン、231質量部の合成例2で得られたMTMSの縮合物(a1)、56質量部の脱イオン水を添加し、同温度で15時間撹拌して加水分解縮合反応させ、合成例1と同様の条件で蒸留することによって生成したメタノールおよび水を除去した。次いで、250質量部のBuOHを添加し、不揮発分が60.0質量%の硬化性樹脂組成物(2)を1,000質量部得た。
前記合成例2で作製した平均分子量3500のポリシロキサン樹脂(硬化性樹脂組成物(1))および平均分子量3000のフェノール樹脂を45/55の樹脂固形物の重量比で加え、高温焼成後の生成物中のシリコン粒子含有量が50質量%となるように合成例1で得られたSi3のシリコンスラリーと、適量のメチルエチルケトン溶媒を添加し、撹拌機中にて十分に混合した。その結果、固形物濃度が10質量%のシリコン粒子含有樹脂の混合懸濁液を得た。シリコン粒子の量に対してMg2+/Si(モル比)=2.5/100となるようにMgCl2を原料として前記シリコン粒子含有樹脂の混合懸濁液に加え、充分に混合後に120℃のオイルバース中、窒素フロー条件下にて脱溶媒を行った。
得られた活物質複合粒子のD50は約5.5μmであり、比表面積が3.6m2/gであった。Cu-Kα線による粉末X線回折(XRD)の測定結果によりシリコン(111)結晶面に帰属する回折ピーク(2θ=28.4度)の半値幅に基づき、シェラー式により求めた。シリコン結晶子サイズは21nmであった。
29Si-NMR(JEOL RESONANCE社製)スペクトル測定から、MgSiO3構造体に帰属の-85ppm付近のピークとSiO2構造体に帰属の-110ppm付近のピークの面積比が10/90であった。HR-TEM(High―Resolution Transmission Electron Microscopy)観察結果からシリコン粒子表面上にMgSiO3層の厚みは約0.8nmであり、シリコン粒子から離れたマトリクス相中にMgSiO3と見られる結晶体の格子構造が存在しないことが判明した。また、エネルギー分散型X線分析(Energy dispersive X―ray spectroscopy、EDS)の結果からシリコン粒子の添加量を除いた後のマトリクス相の元素組成比がSiO1.7C5.6N0.1であった。
フルセルの評価は、正極材料としてLiCoO2を正極活物質、集電体としてアルミ箔を用いた単層シートを用いて、正極膜を作製し、450mAh/gの放電容量設計値にて黒鉛粉体と活物質粉末を混合して負極膜を作製した。非水電解質には六フッ化リン酸リチウムをエチレンカーボネートとジエチルカーボネートを体積比で1/1の混合液に1mol/Lの濃度で溶解した非水電解質溶液を用い、セパレータに厚さ30μmのポリエチレン製微多孔質フィルムを用いたラミ型リチウムイオン二次電池を作製した。ラミ型リチウムイオン二次電池を室温下、テストセルの電圧が4.2Vに達するまで1.2mA(正極基準で0.25c)の定電流で充電を行い、4.2Vに達した後は、セル電圧を4.2Vに保つように電流を減少させて充電を行い、放電容量を求めた。45℃で100サイクル後の容量維持率が91%であった。結果を表2に示した。
シリコン粒子の量に対してMg2+/Si(モル比)をそれぞれ4.0/100(実施例2)、5.5/100(実施例3)、7.5/100(実施例4)、12/100(実施例5)となるようにMgCl2を原料として前記シリコン粒子含有樹脂の混合懸濁液に加えた。その他の条件は実施例1と同様にして活物質複合粒子を得た。得られた活物質複合粒子を含む負極活物質を用いた二次電池の評価を行った。結果を表2に示した。
前駆体作製条件などを実施例5と同様に、焼成温度を1180℃にして活物質複合粒子を得た。得られた活物質複合粒子を含む負極活物質を用いた二次電池の評価を行った。結果を表2に示した。
MgCl2の代わりにLiClをシリケート化合物の原料として用いた。シリコン粒子の量に対してLi+/Si(モル比)をそれぞれ10/100(実施例7)、24/100(実施例8)にしてLiClを前記シリコン粒子含有樹脂混合懸濁液に加え、焼成温度は1000℃にした。その他の条件は実施例1と同様にして活物質複合粒子を得た。得られた活物質複合粒子を含む負極活物質を用いた二次電池の評価を行った。結果を表2に示した。
前記合成例2と同様にして作製した平均分子量3500のポリシロキサン樹脂(硬化性樹脂組成物(2))および平均分子量3000のフェノール樹脂を樹脂固形物の重量比として10/90(実施例9)、20/80(実施例10)、80/20(実施例11)とした。MgCl2添加量は、実施例4と同様にしてシリコン粒子の量に対してMg2+/Si(モル比)を7.5/100となるようにMgCl2原料を前記シリコン粒子含有樹脂の混合懸濁液に加えた。その他の条件は実施例1と同様にして、活物質複合粒子を得た。得られた活物質複合粒子を含む負極活物質を用いた二次電池の評価を行った。結果を表2に示した。
実施例1に記載のシリコン粒子含有樹脂の混合懸濁液のシリコン粒子を、合成例1で得られたシリコン粒子Si4(実施例12)、Si2(実施例13)、およびSi1(実施例14)に替え、その他の条件は実施例1と同様にして、活物質複合粒子を得た。得られた活物質複合粒子を含む負極活物質を用いた二次電池の評価を行った。結果を表2に示した。
MgCl2の添加量をシリコン粒子の量に対してMg2+/Si(モル比)が2.0/100となるようにした以外は実施例4と同様にして。活物質複合粒子を得た。得られた活物質複合粒子を含む負極活物質を用いた二次電池の評価を行った。結果を表2に示した。
市販品シリコン粒子(Alfa Aesar社製、50nm)とピッチ(炭素源)を用い、高温焼成後にシリコン粒子含有量が50重量%になるようにシリコン粒子含有樹脂の混合懸濁液を調製した。その他の条件は、実施例1と同様にして、活物質複合粒子を得た。得られた活物質複合粒子を含む負極活物質を用いた二次電池の評価を行った。結果を表2に示した。
シリケート化合物としてLi+またはMg2+の化合物を添加せず、その他の条件は実施例1と同様に前駆体乾燥物を調製後、窒素雰囲気下、1100℃にて4時間焼成して活物質粉末を得た。活物質粉末のD50は約5.4μmであり、比表面積は4.1m2/gであった。充放電測定結果は、ハーフセルにて充放電容量が1935mAh/g、放電容量が1587mAh/g、初回効率が80.1%であり、45℃でのフルセルでの100サイクル後の容量維持率が90%であった。
前記シリコン粒子含有樹脂の混合懸濁液に合成例1のシリコン粒子Si5を用いて前駆体を作製した。その他の条件は実施例1と同様にして、活物質複合粒子を得た。得られた活物質複合粒子を含む負極活物質を用いた二次電池の評価を行った。結果を表2に示した。
表2中、各評価方法は以下のとおりである。
D50:レーザー回折式粒度分布測定装置(マルバーン・パナリティカル社製、マスターサイザー3000)を用いて測定した。
比表面積:比表面積測定装置(BELJAPAN社製、BELSORP-mini)を用いて窒素吸着測定より、BET法で測定した。29Si-NMR:JEOL RESONANCE社製、JNM-ECA600を用いた。
Claims (10)
- 平均粒径が150nm以下のシリコン粒子、前記シリコン粒子が分散したマトリクス相、およびLi、K、Na、Ca、MgおよびAlからなる群から選ばれる少なくとも1種の金属のシリケート化合物を有し、
前記シリコン粒子の表面近傍に前記シリケート化合物を有する二次電池用複合活物質。 - 前記シリコン粒子の表面近傍におけるシリケート濃度が、前記マトリクス相におけるシリケート濃度より高濃度である請求項1に記載の二次電池用複合活物質。
- 前記シリコン粒子の表面近傍に、さらに二酸化ケイ素を有する請求項1または2に記載の二次電池用複合活物質。
- 前記二酸化ケイ素のモル比が、前記シリケート化合物と前記二酸化ケイ素の合計モル数を1として、0.9以下である請求項3に記載の二次電池用複合活物質。
- 前記シリケート化合物が、マグネシウムシリケート化合物である請求項1または2に記載の二次電池用複合活物質。
- 前記シリコン粒子の表面近傍の前記マグネシウムシリケート化合物の厚みが0.2nmから10nmである結晶質膜である請求項5に記載の二次電池用複合活物質。
- 前記マトリクス相は少なくともSiOxCyNz(ただし、1≦x≦2、1≦y≦20、0≦z≦0.5)で表される化合物を含む請求項1または2に記載の二次電池用複合活物質。
- 前記シリコン粒子がフレーク状結晶体であり、X線回折スペクトルにおける2θが28.4°のピークから得られる結晶子サイズが25nm以下である請求項1または2に記載の二次電池用複合活物質。
- 平均粒径が1μmから15μm、比表面積が1m2/gから30m2/gである請求項1または2に記載の二次電池用複合活物質。
- 請求項1または2に記載の二次電池用複合活物質を負極に含む二次電池。
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