WO2022181083A1 - 二次電池用負極活物質およびこれを用いた二次電池 - Google Patents
二次電池用負極活物質およびこれを用いた二次電池 Download PDFInfo
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- WO2022181083A1 WO2022181083A1 PCT/JP2022/000625 JP2022000625W WO2022181083A1 WO 2022181083 A1 WO2022181083 A1 WO 2022181083A1 JP 2022000625 W JP2022000625 W JP 2022000625W WO 2022181083 A1 WO2022181083 A1 WO 2022181083A1
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- Prior art keywords
- negative electrode
- composite particles
- phase
- silicon
- secondary battery
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- H01M4/36—Selection of substances as active materials, active masses, active liquids
- H01M4/58—Selection of substances as active materials, active masses, active liquids of inorganic compounds other than oxides or hydroxides, e.g. sulfides, selenides, tellurides, halogenides or LiCoFy; of polyanionic structures, e.g. phosphates, silicates or borates
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- H01M10/052—Li-accumulators
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- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
- H01M4/134—Electrodes based on metals, Si or alloys
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- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
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- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
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- H01M4/36—Selection of substances as active materials, active masses, active liquids
- H01M4/58—Selection of substances as active materials, active masses, active liquids of inorganic compounds other than oxides or hydroxides, e.g. sulfides, selenides, tellurides, halogenides or LiCoFy; of polyanionic structures, e.g. phosphates, silicates or borates
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- H01M2004/026—Electrodes composed of, or comprising, active material characterised by the polarity
- H01M2004/027—Negative electrodes
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- 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 mainly relates to a negative electrode active material for secondary batteries.
- secondary batteries such as non-aqueous electrolyte secondary batteries have high voltage and high energy density, so they are expected to be used as power sources for small consumer applications, power storage devices, and electric vehicles.
- the use of materials containing silicon is expected as a negative electrode active material with a high theoretical capacity density.
- Patent Document 1 a continuous phase containing silicon having a Si—Si bond and having a three-dimensionally continuous foam-like skeleton and silicon having a Si—O bond are included in regions partitioned by the continuous phase
- a silicon-containing material containing an encapsulated and dispersed phase has been proposed as an active material for a secondary battery.
- Patent Document 2 proposes a negative electrode for a non-aqueous electrolyte secondary battery in which a portion containing a high concentration of silicon in a negative electrode active material forms a continuous silicon network in the form of a three-dimensional network.
- one aspect of the present invention includes composite particles that include a lithium silicate phase and a silicon phase dispersed within the lithium silicate phase, the silicon phase forming linear portions within the lithium silicate phase. wherein the maximum diameter D1 and the minimum diameter D2 of the linear portion have a relationship of 3 ⁇ D1 / D2, and the composite particles contain metal Me dispersed in the lithium silicate phase and the metal Me is at least one selected from the group consisting of Fe, Pb, Zn, Sn, Cu, Ni, Cr, Zr, and Ti.
- Another aspect of the present invention relates to a secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the negative electrode contains the negative electrode active material for a secondary battery.
- FIG. 4 is a diagram showing an example of the distribution state of silicon phases obtained by STEM-EELS analysis of the negative electrode active material (composite particles) for a secondary battery according to one embodiment of the present invention.
- FIG. 3 shows EELS spectra of a standard sample of Si and a standard sample of SiO 2 ;
- FIG. 3 is a cross-sectional view schematically showing a negative electrode active material (composite particles) included in a secondary battery before initial charging.
- 1 is a schematic perspective view of a partially cutaway secondary battery according to an embodiment of the present invention; FIG.
- a negative electrode active material for a secondary battery includes composite particles including a lithium silicate phase and a silicon phase dispersed within the lithium silicate phase.
- the silicon phase has linear portions in the lithium silicate phase, and the maximum diameter D 1 (nm) and the minimum diameter D 2 (nm) of the linear portions satisfy the relationship of 3 ⁇ D 1 /D 2 . have.
- the composite particles contain metal Me dispersed within the lithium silicate phase, wherein the metal Me is at least one selected from the group consisting of Fe, Pb, Zn, Sn, Cu, Ni, Cr, Zr, and Ti. .
- the silicon phase may form a network structure within the lithium silicate phase, and the linear portions may be linear portions of the network structure.
- the shape of the silicon phase is not particularly limited.
- the linear shape does not necessarily mean a strict linear shape, and may include a branch shape, a rod shape of various shapes, a constricted shape, and the like.
- the above D 1 /D 2 is the maximum diameter D 1 of the linear portion with respect to the minimum diameter D 2 of the linear portion in the fully discharged state of the secondary battery (the state in which lithium ions are almost released from the silicon phase). ratio: D 1 /D 2 .
- the above-mentioned fully discharged state means a state in which the secondary battery is discharged to a depth of discharge (DOD) of 90% or more. This is the state in which the secondary battery is discharged to the voltage.
- the silicon phase dispersed in the silicate phase can form a network structure as lithium ions are absorbed during charging. As a result, a conductive network of the silicon phase is formed inside the composite particles.
- the mesh structure is composed of thin linear portions L2 .
- the thin linear portion L2 is advantageous in relieving stress caused by the expansion and contraction of the silicon phase during charging and discharging. can be blocked, thereby reducing the conductivity inside the composite particles.
- the ratio of the maximum diameter D1 to the minimum diameter D2 of the linear portions constituting the network structure: D1 / D2 is set to 3 or more, and the thickness of the linear portions constituting the network structure is set to 3 or more.
- the thick linear portion L1 and the thin linear portion L2 coexist.
- a thick linear portion L1 forms a thick electronic conduction path, and a good conductive network is maintained during charging and discharging.
- An electron conduction path is secured between the thin linear portions L2 via the thick linear portion L1. Therefore, the deterioration of the conductivity inside the composite particles due to the interruption of the electronic conduction path of the linear portion L2 due to the deterioration of the silicon phase during charging and discharging is suppressed.
- D 1 /D 2 may be 4 or more, or 5 or more.
- the local stress concentration associated with the expansion and contraction of the thick linear portion L1 during charging and discharging is alleviated, and the grain cracking caused by the stress concentration is reduced. This suppresses deterioration of cycle characteristics due to grain cracking.
- the maximum diameter D1 may be 60 nm or less , or 40 nm or less, from the viewpoint of alleviating local stress concentration accompanying expansion and contraction of the thick linear portion L1 during charging and discharging. From the viewpoint of maintaining a good conductive network, the maximum diameter D1 may be greater than 20 nm and may be 21 nm or more.
- the minimum diameter D2 may be 1 nm or more , or 3 nm or more.
- the minimum diameter D2 may be 15 nm or less, or 10 nm or less, from the viewpoint of relaxation of stress caused by expansion and contraction of the silicon phase during charging and discharging.
- the diameter of the thick linear portion L1 is, for example, in the range of (D2 + 10 ) nm or more and D1 nm or less.
- the diameter of the thin linear portion L 2 is, for example, in the range of D 2 nm or more and less than (D 2 +10) nm.
- the diameter of the linear portion means the minimum value of the diameter passing through the point at which the later-described coefficient b Si is maximum in the linear portion.
- the number of thick linear portions L 1 NL may be, for example, 5 or more and 20 or less.
- the difference between D1 and D2 may be, for example, 15 nm or more, or 15 nm or more and 65 nm or less.
- D2 may be, for example, 2 nm or more and 8 nm or less.
- the fact that the silicon phase forms a network structure in the lithium silicate phase should be confirmed by elemental mapping by electron energy loss spectroscopy (EELS) using a cross-sectional image of the composite particles by a scanning transmission electron microscope (STEM). can be done.
- EELS electron energy loss spectroscopy
- Said D1 / D2 can be calculated
- STEM Scanning Transmission Electron Microscope
- the negative electrode (negative electrode mixture layer) is processed into a thin film by a CP (cross section polisher) method, an FIB (focused ion beam) method, or the like to obtain a thin film sample (for example, 30 nm thick) for STEM observation of the cross section of the negative electrode.
- a cross section of the negative electrode (composite particles) is observed by STEM using a thin film sample. Observation by STEM is performed at a high magnification (for example, 20,000 to 1,000,000 times).
- Elemental analysis by EELS is performed using an STEM image (dark field image) of the cross section of the composite particles.
- the size (area) of the dark field image is, for example, 0.02 ⁇ m 2 to 2 ⁇ m 2 .
- FIG. 1 shows an example of silicon mapping by STEM-EELS of composite particles after charging and discharging. In FIG. 1, the lighter the color, the higher the silicon concentration. The light-colored portions where the silicon concentration is very high are distributed in a mesh-like manner, which almost corresponds to the distribution of the silicon phase.
- Desirable STEM-EELS measurement conditions are shown below. ⁇ STEM-EELS measurement conditions> Measuring device: JEM-F200 (manufactured by JEOL Ltd.) EELS detector: Quantum ER (manufactured by Gatan) Accelerating voltage: 200 kV Degree of vacuum: 1.0 ⁇ 10 -6 to 8.0 ⁇ 10 -5 Pa Dispersion: 0.050 eV/cH Spot size: 7 Camera length: 40mm Pixel time: 0.1 second
- Silicon phase map creation A silicon phase map is obtained by separating the SiO2 component derived from the silicate phase from the EELS analysis data of the cross section of the composite particles and extracting the Si component derived from the silicon phase.
- the silicon phase is almost a phase of simple Si, and the Si component can be regarded as a component derived from the silicon phase.
- the silicate phase is represented by Li 2 O.xSiO 2 , and the SiO 2 component can be regarded as a component derived from the silicate phase.
- the measured value obtained by the EELS analysis is regarded as the sum of the Si component and the SiO2 component, and fitting is performed based on the following formula to obtain the coefficient b Si and the coefficient b SiO2 .
- b Si +b SiO2 1 is satisfied.
- Actual value b Si x S Si + b SiO2 x S SiO2
- S Si and S SiO2 are the spectral values of standard samples of Si and SiO2 , respectively.
- the spectra of the standard samples of Si and SiO2 are obtained from the data of the EELS spectra of the standard samples of Si and SiO2 ( data shown in FIG. 2).
- a region having a coefficient b Si of 0.14 or more is extracted from the Si elemental map as a silicon phase, and a map of the silicon phase is obtained. From the map of the silicon phase obtained above, it is confirmed that the silicon phase is distributed in a network.
- the thickness of the silicon phase in the thin film sample tends to be large, and the diameter of the linear portion of the network structure confirmed by the map of the silicon phase described later tends to be large.
- the majority of the linear portions forming the network structure have a diameter D 3 (nm) of 20 nm or less. That is, the ratio of the number N1 of linear portions having a diameter D3 of 20 nm or less to the total number N0 of linear portions forming the network structure: N1/ N0 is preferably greater than 1/2 .
- the diameter D3 of the linear portion here means the minimum value of the diameter passing through the point where the coefficient bSi is maximum in any one linear portion.
- the above N 1 /N 0 can be obtained by the following method. Arbitrarily select 10 to 20 linear portions constituting the network structure indicated by the silicon phase map obtained in (3) above, and for each linear portion, obtain the point P3 at which the coefficient b Si is the maximum, The minimum value of the diameter of the linear portion passing through the point P3 is obtained and defined as the diameter D 3 (nm). A ratio of the number N1 of the linear portions having a diameter D3 of 20 nm or less to the total number N0 of the selected linear portions, that is, N1 / N0 is obtained. N 1 /N 0 is determined for each of the STEM images at any five locations on the cross section of the composite particle, and the average value thereof is determined.
- the above NL2 / NL1 can be obtained by the following method.
- a linear portion having a diameter D 3 within a range of (D 2 +10) nm or more and D 1 nm or less is defined as a linear portion L 1 .
- a linear portion having a diameter D3 within a range of D 2 nm or more and less than (D 2 +10) nm is defined as a linear portion L 2 .
- the number NL 1 of linear portions L 1 and the number NL 2 of linear portions L 2 are obtained, and NL 2 /NL 1 is calculated.
- the malleable metal Me is dispersed in the lithium silicate phase. This suppresses the cracking and collapse of the composite particles caused by expansion and contraction of the composite particles during charging and discharging.
- the metal Me may be dispersed in particulate form within the lithium silicate phase.
- Metal Me is at least one selected from the group consisting of Fe, Pb, Zn, Sn, Cu, Ni, Cr, Zr, and Ti. Among them, the metal Me is preferably Fe, Pb, or Cu from the viewpoint of excellent malleability.
- the content of metal Me contained in the composite particles may be 20% by mass or less, or may be 5% by mass or less. From the viewpoint of suppressing particle cracking, the content of metal Me contained in the composite particles may be 0.1% by mass or more, or may be 0.5% by mass or more.
- the content of metal Me (eg, Fe) in the composite particles can be measured, for example, by inductively coupled plasma atomic emission spectrometry (ICP-AES). Specifically, a sample of composite particles is completely dissolved in a heated acid solution (mixed acid of hydrofluoric acid, nitric acid and sulfuric acid), carbon remaining in the solution is removed by filtration, and then the obtained filtrate is It is analyzed by ICP-AES and the spectral intensity of metallic Me is measured. Subsequently, a calibration curve is created using a commercially available standard solution of elements, and the content of metal Me is calculated.
- ICP-AES inductively coupled plasma atomic emission spectrometry
- the composite particles include a lithium silicate phase, a silicon phase dispersed within the lithium silicate phase, and metal Me dispersed within the lithium silicate phase.
- the lithium silicate phase provides composite particles with small irreversible capacity and high capacity. Lithium ions are absorbed into the silicon phase during charging, and lithium ions are released from the silicon phase during discharging. Since the silicon phase is dispersed in the matrix of the lithium silicate phase, contact between the silicon phase and the electrolyte is restricted and side reactions are suppressed. Also, the stress caused by the expansion and contraction of the silicon phase is relieved by the matrix of the lithium silicate phase. Furthermore, the stress caused by the expansion and contraction of the silicon phase is also relieved by the malleable metal Me, thereby relieving the stress on the lithium silicate phase.
- the average particle size (D50) of the composite particles (secondary particles) is, for example, 1 ⁇ m or more and 25 ⁇ m or less, and may be 4 ⁇ m or more and 15 ⁇ m or less.
- the average particle size (D50) means the particle size (volume average particle size) at which the volume integrated value is 50% in the particle size distribution measured by the laser diffraction scattering method.
- "LA-750" manufactured by Horiba, Ltd. can be used as the measuring device.
- the lithium silicate phase can, for example, have a composition represented by the formula: Li 2z SiO 2+z (0 ⁇ z ⁇ 2).
- Lithium silicate is lightweight and has excellent lithium ion conductivity.
- the lithium silicate phase may be an oxide phase containing Li, Si and O, and may contain other elements.
- the atomic ratio of O to Si: O/Si in the lithium silicate phase is greater than 2 and less than 4, for example.
- O/Si is greater than 2 and less than 3.
- the atomic ratio of Li to Si in the lithium silicate phase: Li/Si is greater than 0 and less than 4, for example.
- the lithium silicate phase may contain another element M1 in addition to Li, Si and O.
- Element M1 is, for example, at least one selected from the group consisting of K, Na, Mg, Ca, B, Al, Nb, Ta, La, Y, P, Bi, Sb, Co, Er, F and W. can be Depending on the type of element M1, the ionic conductivity of the silicate phase is improved. Also, the resistance of the silicate phase to the electrolyte is improved.
- the element M1 is preferably La from the viewpoint of improving the initial charge/discharge efficiency.
- the element M1 may form a compound.
- the compound may be, for example, an oxide of the element M1 or a silicate of the element M1, depending on the type of the element M1.
- the content of the element M1 is, for example, 0.3 mol % or more and 3 mol % or less with respect to the total amount of elements other than oxygen.
- the composition of the lithium silicate phase can be analyzed by the following method.
- the composition analysis is desirably performed using the composite particles or the negative electrode mixture layer in the discharged state.
- the content of each element contained in the composite particles can be measured, for example, by ICP-AES. Specifically, a composite particle sample is completely dissolved in a heated acid solution, the carbon remaining in the solution is removed by filtration, and then the obtained filtrate is analyzed by ICP-AES to obtain the spectrum of each element. Measure strength. Subsequently, a calibration curve is created using a commercially available standard solution of each element, and the content of each element is calculated.
- the composite particles may be taken out of the battery by, for example, the following method. Specifically, the battery (completely discharged state) is disassembled, 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 mixture layer is peeled off from the negative electrode current collector and pulverized in a mortar to obtain a sample powder. Next, the sample powder is dried in a dry atmosphere for 1 hour and immersed in weakly boiled 6M hydrochloric acid for 10 minutes to remove alkali metals such as Na and Li that may be contained in the binder and the like. Next, the sample powder is washed with deionized water, separated by filtration, and dried at 200° C. for 1 hour. After that, by heating in an oxygen atmosphere to remove the carbon component, only the composite particles can be isolated.
- the battery completely discharged state
- the negative electrode is taken out
- the negative electrode is washed with anhydrous e
- the lithium silicate phase and silicon phase in the composite particles can be distinguished and quantified by using Si-NMR.
- the Si content obtained by ICP-AES as described above is the sum of the amount of Si constituting the silicon phase and the amount of Si in the lithium silicate phase.
- the amount of Si constituting the silicon phase can be separately quantified using Si-NMR. Therefore, the amount of Si in the lithium silicate phase can be quantified by subtracting the amount of Si constituting the silicon phase from the amount of Si obtained by ICP-AES.
- a mixture containing lithium silicate with a known Si content and silicon particles in a predetermined ratio may be used as a standard substance necessary for quantification.
- Si-NMR measurement conditions Desirable Si-NMR measurement conditions are shown below.
- Measurement device Solid-state nuclear magnetic resonance spectrometer (INOVA-400) manufactured by Varian Probe: Varian 7mm CPMAS-2 MAS: 4.2kHz MAS speed: 4kHz Pulse: DD (45° pulse + signal acquisition time 1H decouple) Repeat time: 1200sec-3000sec Observation width: 100kHz Observation center: Around -100 ppm Signal capture time: 0.05 sec Cumulative count: 560 Sample amount: 207.6 mg
- quantification of each element in the composite particles 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. be.
- AES Auger electron spectroscopy
- LA-ICP-MS laser ablation ICP mass spectrometry
- XPS X-ray photoelectron spectroscopy
- the quantification of each element in the composite particles by SEM-EDX analysis can also be performed by cross-sectional observation of the composite particles in the cross section of the negative electrode mixture layer.
- Cross-sectional observation can be performed, for example, by the following method. First, the battery is disassembled, the negative electrode is taken out, and a cross section of the negative electrode mixture layer is obtained using a cross section polisher (CP). A cross section of the negative electrode mixture layer is observed using a scanning electron microscope (SEM). 10 composite particles having a maximum particle diameter of 5 ⁇ m or more are randomly selected from the cross-sectional image of the backscattered electron image of the negative electrode mixture layer, and elemental mapping analysis is performed on each of the composite particles by energy dispersive X-ray (EDX). The area containing the target element is calculated using image analysis software. The observation magnification is desirably 2000 to 20000 times. It is obtained by averaging the measured values of the area containing the predetermined element of 10 obtained particles.
- mapping analysis by EDX is performed on a region 1 ⁇ m or more inside from the peripheral edge of the cross section of the composite particle so that the measurement range does not include the film or the conductive layer.
- SEM-EDX measurement conditions Processing equipment: SM-09010 (Cross Section Polisher) manufactured by JEOL Processing conditions: acceleration voltage 6 kV Current value: 140 ⁇ A Degree of vacuum: 1 ⁇ 10 -3 to 2 ⁇ 10 -3 Pa Measuring device: Electron microscope SU-70 manufactured by HITACHI Acceleration voltage during analysis: 10 kV Field: Free Mode Probe Current Mode: Medium Probe current range: High Anode Ap.: 3 OBJ App.: 2 Analysis area: 1 ⁇ m square Analysis software: EDAX Genesis CPS: 20500 Lsec: 50 Time constant: 3.2
- the content of the silicon phase in the composite particles is, for example, 30% by mass or more and 80% by mass or less.
- the content of the silicon phase is preferably 40% by mass or more, more preferably 50% by mass or more.
- the silicon phase can be dispersed in particles in the lithium silicate phase of the composite particles.
- the range of particle size (maximum size) of the particulate silicon phase contained in the composite particles may be, for example, 20 nm or more and 200 nm or less.
- the average particle diameter of the particulate silicon phase contained in the composite particles may be 100 nm or less, or 70 nm or less. It may be 50 nm or less.
- the average particle diameter of the silicon phase is obtained by measuring the maximum diameter of arbitrary 100 silicon phases using SEM images or TEM images of the cross section of the composite particles and averaging them.
- At least part of the surface of the composite particles may be covered with a conductive layer.
- a conductive layer By forming a conductive layer on the surface of the composite particles, the conductivity of the composite particles can be dramatically increased.
- a carbon material is preferable as the conductive material forming the conductive layer.
- the carbon material preferably contains at least one selected from the group consisting of carbon compounds and carbonaceous substances.
- the thickness of the conductive layer is substantially thin enough not to affect the average particle size of the composite particles.
- the thickness of the conductive layer is preferably 1 to 200 nm, more preferably 5 to 100 nm, in consideration of ensuring conductivity and diffusibility of lithium ions.
- the thickness of the conductive layer can be measured by cross-sectional observation of the composite particles using SEM or TEM.
- Examples of carbon compounds include compounds containing carbon and hydrogen and compounds containing carbon, hydrogen and oxygen.
- amorphous carbon with low crystallinity, graphite with high crystallinity, or the like can be used.
- Amorphous carbon includes carbon black, coal, coke, charcoal, activated carbon, and the like.
- Examples of graphite include natural graphite, artificial graphite, and graphitized mesophase carbon particles. Among them, amorphous carbon is preferable because it has a low hardness and a large buffering effect on silicon particles whose volume changes due to charging and discharging.
- the amorphous carbon may be graphitizable carbon (soft carbon) or non-graphitizable carbon (hard carbon).
- Examples of carbon black include acetylene black and ketjen black.
- Step (i) As a raw material for lithium silicate, a raw material mixture containing a Si raw material and a Li raw material in a predetermined ratio is used. Further, the raw material mixture may contain the element M1 described above. The raw material mixture is melted, and the melt is passed through metal rolls to form flakes to produce lithium silicate. After that, the flaked silicate is crystallized by heat treatment in an air atmosphere at a temperature above the glass transition point and below the melting point. Note that the flaked silicate can also be used without being crystallized. It is also possible to produce silicate by solid-phase reaction by firing at a temperature below the melting point without melting the raw material mixture.
- Silicon oxide can be used as the Si raw material.
- Li raw materials that can be used include lithium carbonate, lithium oxide, lithium hydroxide, and lithium hydride. These may be used alone or in combination of two or more.
- Source materials for the element M1 include oxides, hydroxides, carbonate compounds, hydrides, nitrates, sulfates, and the like of each element.
- Step (ii) Next, raw material silicon is mixed with lithium silicate to form a composite.
- composite particles are produced by the following steps (a) to (c).
- step (a) A raw material silicon powder and a lithium silicate powder are mixed at a mass ratio of, for example, 20:80 to 95:5 to obtain a mixed powder.
- the raw material silicon powder for example, a silicon powder having a wide particle size range may be used, or a silicon powder having two peaks in the volume-based particle size distribution may be used.
- the difference between the particle size corresponding to one of the two peaks and the particle size corresponding to the other peak is, for example, 50 ⁇ m or more.
- the raw material silicon powder includes, for example, coarse silicon powder (e.g., average particle size (D50) of 80 ⁇ m or more and 300 ⁇ m or less) and silicon fine powder (e.g., average particle size (D50) of 1 ⁇ m). 20 ⁇ m or less) at a predetermined mass ratio may be used.
- D 1 /D 2 at the formation of the network structure can be adjusted by the particle size distribution of the raw material silicon powder.
- metal Me for example, Fe
- the metal Me powder may be added to the raw material mixture of step (i) instead of being added to the mixed powder of step (a).
- step (b) 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 finely divided. At this time, an organic solvent may be added to the mixture for wet pulverization.
- the organic solvent plays a role in preventing the material to be ground from adhering to the inner wall of the grinding vessel.
- D 1 /D 2 at the time of formation of the network structure may be adjusted by changing the grinding time, rotation speed of the pot, filling amount of balls, and the like.
- organic solvents alcohols, ethers, fatty acids, alkanes, cycloalkanes, silicate esters, metal alkoxides, etc. can be used.
- the raw material silicon and the lithium silicate may be separately pulverized and then mixed. Silicon nanoparticles and amorphous lithium silicate nanoparticles may also be produced and mixed without using a pulverizer.
- a known method such as a vapor phase method (eg, plasma method) or a liquid phase method (eg, liquid phase reduction method) may be used to prepare nanoparticles.
- step (c) the mixture is heated to 600° C. to 1000° C. in, for example, an inert gas atmosphere (eg, an atmosphere of argon, nitrogen, etc.) and pressurized to sinter.
- an inert gas atmosphere eg, an atmosphere of argon, nitrogen, etc.
- a sintering apparatus capable of applying pressure under an inert atmosphere, such as a hot press, can be used.
- the silicate softens and flows to fill the gaps between the silicon particles.
- a dense block-shaped sintered body having a silicate phase as a sea portion and a silicon phase and metal Me as an island portion.
- Composite particles are obtained by pulverizing the obtained sintered body.
- Step (iii) Subsequently, at least part of the surface of the composite particles may be coated with a conductive material to form a conductive layer.
- Methods for coating the surface of the composite particles with a conductive carbon material include a CVD method using a hydrocarbon gas such as acetylene and methane as a raw material, and a method in which coal pitch, petroleum pitch, phenol resin, etc. are mixed with the composite particles and heated in an inert atmosphere.
- a method of carbonizing by heating at 700° C. to 950° C. in for example, an atmosphere of argon, nitrogen, etc.
- carbon black may be attached to the surface of the composite particles.
- Step (iv) A step of washing the composite particles (including those having a conductive layer on the surface) with an acid may be performed.
- an acidic aqueous solution it is possible to dissolve and remove trace amounts of alkaline components that may be generated when the raw material silicon and lithium silicate are composited.
- an aqueous solution of inorganic acids such as hydrochloric acid, hydrofluoric acid, sulfuric acid, nitric acid, phosphoric acid and carbonic acid
- organic acids such as citric acid and acetic acid
- FIG. 3 schematically shows a cross section of a composite particle 20 whose surface is coated with a conductive layer as an example of a negative electrode active material included in a secondary battery before initial charge.
- the composite particle 20 includes a mother particle 23 composed of secondary particles in which a plurality of primary particles 24 are aggregated.
- the mother particles 23 (primary particles 24 ) comprise a lithium silicate phase 21 and silicon phases 22 dispersed within the lithium silicate phase 21 .
- the mother particles 23 have a structure in which fine particulate silicon phases 22 are dispersed in a matrix of a lithium silicate phase 21 .
- carbon (amorphous carbon) derived from the organic solvent used in step (b) may be present.
- the average particle size of the metal Me particles 27 is, for example, 1 nm or more and 100 nm or less.
- the average particle size of the metal Me particles 27 is obtained by measuring the maximum size of arbitrary 100 metal Me particles using SEM images or TEM images of the cross section of the composite particles and averaging them.
- the average particle size of the metal Me particles is, for example, 1 nm or more and 100 nm or less even after the first charge/discharge.
- At least part of the surface of the base particles 23 may be covered with the conductive layer 26 .
- the particulate silicon phases 22 adjacent to each other may be connected to each other to form a silicon phase having a network structure.
- the lithium silicate phase 21 may further contain the element M1.
- a secondary battery according to an embodiment of the present invention includes a positive electrode, a negative electrode, and an electrolyte.
- the negative electrode contains a negative electrode active material containing the above composite particles.
- the negative electrode, positive electrode, and electrolyte included in the secondary battery according to the embodiment of the present invention are described below.
- the negative electrode includes, for example, a negative electrode current collector and a negative electrode mixture layer formed on the surface of the negative electrode current collector and containing a negative electrode active material.
- the negative electrode mixture layer can be formed by applying a negative electrode slurry obtained by dispersing the negative electrode mixture in a dispersion medium on the surface of the negative electrode current collector and drying the slurry. The dried coating film may be rolled if necessary.
- the negative electrode mixture contains the negative electrode active material containing the composite particles as an essential component, and may contain a binder, a conductive agent, a thickener, etc. as optional components. Since the silicon phase of the composite particles can occlude a large amount of lithium ions, a high-capacity negative electrode can be obtained.
- the negative electrode active material may further contain other active materials that electrochemically occlude and release lithium ions.
- a carbon-based active material is preferable. Since the volume of the composite particles expands and contracts with charging and discharging, when the proportion of the composite particles in the negative electrode active material increases, poor contact between the negative electrode active material and the negative electrode current collector tends to occur with charging and discharging. On the other hand, by using the composite particles and the carbon-based active material together, it is possible to achieve excellent cycle characteristics while imparting a high capacity of the silicon phase to the negative electrode.
- the ratio of the composite particles to the total of the composite particles and the carbon-based active material is, for example, preferably 0.5 to 15% by mass, more preferably 1 to 5% by mass. This makes it easier to achieve both high capacity and improved cycle characteristics.
- carbon-based active materials examples include graphite, graphitizable carbon (soft carbon), and non-graphitizable carbon (hard carbon). Among them, graphite is preferable because it has excellent charging/discharging stability and low irreversible capacity.
- Graphite means a material having a graphite-type crystal structure, and includes, for example, natural graphite, artificial graphite, graphitized mesophase carbon particles, and the like.
- One of the carbon-based active materials may be used alone, or two or more thereof may be used in combination.
- a non-porous conductive substrate metal foil, etc.
- a porous conductive substrate meh body, net body, punching sheet, etc.
- materials for the negative electrode current collector include stainless steel, nickel, nickel alloys, copper, copper alloys, and the like.
- binders include fluorine resins, polyolefin resins, polyamide resins, polyimide resins, vinyl resins, styrene-butadiene copolymer rubber (SBR), polyacrylic acid and derivatives thereof. These may be used individually by 1 type, and may be used in combination of 2 or more type.
- Examples of conductive agents include carbon black, conductive fibers, carbon fluoride, and organic conductive materials. These may be used individually by 1 type, and may be used in combination of 2 or more type.
- thickeners examples include carboxymethyl cellulose (CMC) and polyvinyl alcohol. These may be used individually by 1 type, and may be used in combination of 2 or more type.
- CMC carboxymethyl cellulose
- polyvinyl alcohol examples include polyvinyl alcohol. These may be used individually by 1 type, and may be used in combination of 2 or more type.
- dispersion media examples include water, alcohol, ether, N-methyl-2-pyrrolidone (NMP), and mixed solvents thereof.
- the positive electrode includes, for example, a positive electrode current collector and a positive electrode mixture layer formed on the surface of the positive electrode current collector.
- the positive electrode mixture layer can be formed by applying a positive electrode slurry in which a positive electrode mixture is dispersed in a dispersion medium to the surface of the positive electrode current collector and drying the slurry. The dried coating film may be rolled if necessary.
- the positive electrode mixture contains a positive electrode active material as an essential component, and may contain a binder, a conductive agent, etc. as optional components.
- a lithium composite metal oxide can be used as the positive electrode active material.
- Lithium composite metal oxides include, for example, Li a CoO 2 , Li a NiO 2 , Li a MnO 2 , Li a Co b Ni 1-b O 2 , Li a Co b M 1-b O c , Li a Ni 1- bMbOc , LiaMn2O4 , LiaMn2 - bMbO4 , LiMePO4 , Li2MePO4F .
- 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).
- a transition element for example, contains at least one selected from the group consisting of Mn, Fe, Co, and Ni.
- 0 ⁇ a ⁇ 1.2, 0 ⁇ b ⁇ 0.9, and 2.0 ⁇ c ⁇ 2.3 0 ⁇ a ⁇ 1.2, 0 ⁇ b ⁇ 0.9, and 2.0 ⁇ c ⁇ 2.3.
- the value a which indicates the molar ratio of lithium, is the value immediately after the preparation of the active material, and increases or decreases due to charging and discharging.
- the binder and conductive agent the same ones as exemplified for the negative electrode can be used.
- the conductive agent graphite such as natural graphite and artificial graphite may be used.
- a conductive substrate conforming to the negative electrode current collector can be used for the positive electrode current collector.
- materials for the positive electrode current collector include stainless steel, aluminum, aluminum alloys, and titanium.
- the electrolyte includes a solvent and a lithium salt dissolved in the solvent.
- the concentration of lithium salt in the electrolyte is, for example, 0.5-2 mol/L.
- the electrolyte may contain known additives.
- Aqueous solvents or non-aqueous solvents are used as solvents.
- non-aqueous solvents include cyclic carbonates, chain carbonates, cyclic carboxylates, chain carboxylates, and the like.
- Cyclic carbonates include propylene carbonate (PC), ethylene carbonate (EC), and the like.
- Chain carbonates include diethyl carbonate (DEC), ethylmethyl carbonate (EMC), dimethyl carbonate (DMC) and the like.
- Cyclic carboxylic acid esters include ⁇ -butyrolactone (GBL) and ⁇ -valerolactone (GVL).
- Chain carboxylic acid esters include methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and propyl propionate.
- the non-aqueous solvent may be used singly or in combination of two or more.
- Lithium salts include, for example, lithium salts of chlorine-containing acids (LiClO4, LiAlCl4 , LiB10Cl10 , etc.), lithium salts of fluorine - containing acids ( LiPF6 , LiBF4 , LiSbF6 , LiAsF6 , LiCF3SO3 ). , LiCF3CO2 , etc.), lithium salts of fluorine-containing acid imides ( LiN ( CF3SO2 ) 2 , LiN ( CF3SO2 ) ( C4F9SO2 ), LiN ( C2F5SO2 ) 2 , etc.), lithium halides (LiCl, LiBr, LiI, etc.) and the like can be used. Lithium salts may be used singly or in combination of two or more.
- Separator It is desirable to interpose a separator between the positive electrode and the negative electrode.
- the separator has high ion permeability and moderate mechanical strength and insulation.
- a microporous thin film, a woven fabric, a nonwoven fabric, or the like can be used as the separator.
- Polyolefins such as polypropylene and polyethylene can be used as the material of the separator, for example.
- a secondary battery there is a structure in which an electrode group in which a positive electrode and a negative electrode are wound with a separator interposed therebetween, and an electrolyte are accommodated in an exterior body.
- another type of electrode group may be applied, such as a laminated electrode group in which a positive electrode and a negative electrode are laminated with a separator interposed therebetween.
- the secondary battery may be of any shape such as cylindrical, square, coin, button, and laminate.
- the battery includes a prismatic battery case 4 with a bottom, and an electrode group 1 and a non-aqueous electrolyte (not shown) housed in 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 negative electrode current collector of the negative electrode is electrically connected to a negative electrode terminal 6 provided on a sealing plate 5 via a negative electrode lead 3 .
- the negative electrode terminal 6 is insulated from the sealing plate 5 by a resin gasket 7 .
- the positive current collector of the positive electrode is electrically connected to the rear surface of the sealing plate 5 via the positive lead 2 . That is, the positive electrode is electrically connected to the battery case 4 which also serves as a positive electrode terminal.
- the peripheral edge of the sealing plate 5 is fitted into the open end of the battery case 4, and the fitted portion is laser-welded.
- the sealing plate 5 has an injection hole for a non-aqueous electrolyte, which is closed by
- Composite particles A1 were pulverized and passed through a 40 ⁇ m mesh, mixed with coal pitch (manufactured by JFE Chemical Co., Ltd., MCP250), and heat-treated at 800 ° C. in an inert atmosphere to flatten the surface of composite particles A1.
- a conductive layer was formed by coating with carbon. The carbon coating amount is about 5% by mass with respect to the total of the composite particles A1 and the conductive layer. Thereafter, a sieve was used to obtain composite particles A1 (average particle size (D50) of 5 ⁇ m) having a conductive layer.
- TK Hibismix manufactured by Primix
- Lithium cobaltate, acetylene black, and polyvinylidene fluoride are mixed at a mass ratio of 95:2.5:2.5, N-methyl-2-pyrrolidone (NMP) is added, and then a mixer (Primex (manufactured by TK Hibismix) was used to prepare a positive electrode slurry.
- NMP N-methyl-2-pyrrolidone
- a mixer Principal (manufactured by TK Hibismix) was used to prepare a positive electrode slurry.
- the positive electrode slurry is applied to both sides of the aluminum foil, the coating film is dried, and then rolled to obtain a positive electrode having positive electrode mixture layers with a density of 3.6 g/cm 3 formed on both sides of the aluminum foil. rice field.
- a non-aqueous electrolyte was prepared by dissolving LiPF 6 at a concentration of 1.0 mol/L in a mixed solvent containing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) at a volume ratio of 3:7.
- EC ethylene carbonate
- EMC ethyl methyl carbonate
- a tab was attached to each electrode, and an electrode group was produced by spirally winding the positive electrode and the negative electrode A1 with the separator interposed therebetween such that the tab was positioned at the outermost periphery. After inserting the electrode group into an outer package made of an aluminum laminate film and vacuum-drying at 105° C. for 2 hours, a non-aqueous electrolyte was injected and the opening of the outer package was sealed to obtain a secondary battery A1. .
- Example 2 In the preparation of the composite particles, the Si powder is mixed with coarse Si powder (average particle size (D50) 500 ⁇ m) and fine Si powder (average particle size (D50) 10 ⁇ m) at a mass ratio of 1:9.
- Composite particles A2, negative electrode A2, and secondary battery A2 were obtained in the same manner as in Example 1, except that the composite particles A2 were used.
- Example 3 In the preparation of the composite particles, the Si powder was mixed with coarse Si powder (average particle size (D50) 150 ⁇ m) and fine Si powder (average particle size (D50) 10 ⁇ m) at a mass ratio of 8:2. A composite particle A3, a negative electrode A3, and a secondary battery A3 were obtained in the same manner as in Example 1, except that the material was used.
- Composite particles B4, negative electrode B4 and secondary battery B4 were prepared in the same manner as in Example 1, except that iron powder was not added to the mixture of Si powder and Li 2 Si 2 O 5 powder in the preparation of the composite particles. got
- the rest period between charging and discharging was 10 minutes.
- the ratio (percentage) of the discharge capacity at the first cycle to the charge capacity at the first cycle was obtained as the initial charge/discharge efficiency (%).
- the ratio (percentage) of the discharge capacity at the 50th cycle to the discharge capacity at the 1st cycle was determined as the capacity retention rate (%) at the 50th cycle.
- Table 1 shows the evaluation results.
- the initial charge/discharge efficiency and the capacity maintenance rate are shown as relative values with the initial charge/discharge efficiency and the capacity maintenance rate of Battery B1 of Comparative Example 1 set to 100, respectively.
- the average particle size of the silicon phase obtained by the method described above was 10 nm for composite particles A1, 15 nm for composite particles A2, and 30 nm for composite particles A3.
- the average particle size of the Fe particles obtained by the method described above was 15 nm for composite particles A1, 20 nm for composite particles A2, and 30 nm for composite particles A3.
- Batteries A1 to A3 exhibited higher initial charge/discharge efficiency and capacity retention rate than batteries B1 to B4.
- battery A1 in which D 1 is 60 nm or less and N 1 /N 0 is more than 1/2 showed particularly high initial charge/discharge efficiency and capacity retention rate.
- the improvement ranges from Battery B2 to Battery B3 when D 1 /D 2 was 2.4 were 9 and 8, respectively.
- the improvement ranges from Battery B4 to Battery A1 were 18 and 33, respectively.
- the extent of improvement from battery B4 to battery A1 was significantly greater than the extent of improvement from battery B2 to battery B3. It was shown that the initial charge/discharge efficiency and the capacity retention rate are significantly improved by dispersing the silicon phase having D 1 /D 2 of 3 or more and the metal Me in the silicate phase.
- the negative electrode active material for secondary batteries according to the present invention is useful in secondary batteries that serve as main power sources for mobile communication devices, portable electronic devices, and the like. While the invention has been described in terms of presently preferred embodiments, such disclosure is not to be construed in a limiting sense. Various alterations and modifications will no doubt become apparent to those skilled in the art to which the invention pertains after reading the above disclosure. Therefore, the appended claims are to be interpreted as covering all variations and modifications without departing from the true spirit and scope of the invention.
Abstract
Description
本発明の新規な特徴を添付の請求の範囲に記述するが、本発明は、構成および内容の両方に関し、本発明の他の目的および特徴と併せ、図面を照合した以下の詳細な説明によりさらによく理解されるであろう。
本発明の実施形態に係る二次電池用負極活物質は、リチウムシリケート相と、リチウムシリケート相内に分散するシリコン相と、を含む複合粒子を含む。シリコン相は、リチウムシリケート相内において線状部分を有しており、当該線状部分の最大径D1(nm)および最小径D2(nm)は、3≦D1/D2の関係を有する。複合粒子は、リチウムシリケート相内に分散する金属Meを含み、金属Meは、Fe、Pb、Zn、Sn、Cu、Ni、Cr、Zr、およびTiからなる群より選択される少なくとも1種である。シリコン相は、リチウムシリケート相内において網目構造を形成していてもよく、線状部分は、当該網目構造の線状部分であってもよい。以下では、シリコン相が網目構造を形成している場合を例にとって説明するが、シリコン相の形状は特に限定されない。また、線状とは、必ずしも厳密な線状を意味せず、枝状、様々な形状のロッド状、括れ状なども包含し得る。
(1)走査型透過電子顕微鏡(STEM)による複合粒子の断面の撮影
電池(完全放電状態)を分解し、負極を取り出す。当該負極(負極合材層)をCP(クロスセクションポリッシャー)法、FIB(集束イオンビーム)法等により薄膜状に加工し、負極断面のSTEM観察用の薄膜試料(例えば、厚み30nm)を得る。薄膜試料を用いてSTEMにより負極(複合粒子)の断面を観察する。STEMによる観察は、高倍率(例えば、2万倍~100万倍)で行う。
複合粒子の断面のSTEMによる画像(暗視野像)を用いて、EELSによる元素分析を行う。暗視野像の大きさ(面積)は、例えば、0.02μm2~2μm2である。ここで、充放電後における複合粒子のSTEM-EELSによるシリコンのマッピングの一例を図1に示す。図1中、色が薄いほどシリコン濃度が高いことを示す。色が薄いシリコン濃度が非常に高い部分は網目状に分布しており、ほぼシリコン相の分布に対応している。
<STEM-EELS測定条件>
測定装置:JEM-F200(日本電子社製)
EELS検出器:Quantum ER(Gatan社製)
加速電圧:200kV
真空度:1.0×10-6~8.0×10-5Pa
Dispersion:0.050eV/cH
Spot size:7
カメラ長:40mm
Pixel time:0.1秒
複合粒子の断面のEELS分析データからシリケート相に由来するSiO2成分を分離し、シリコン相に由来するSi成分を抽出することで、シリコン相のマップを得る。シリコン相はほぼSi単体の相であり、Si成分はシリコン相に由来する成分とみなすことができる。シリケート相はLi2O・xSiO2で表され、SiO2成分はシリケート相に由来する成分とみなすことができる。
実測値=bSi×SSi+bSiO2×SSiO2
上記のシリコン相のマップが示す網目構造の線状部分において係数bSiが最大(例えば、0.234)である点P1を求め、点P1を通る線状部分の径の最小値を求め、これを最大径D1(nm)とする。点P1が複数存在する場合、それぞれ最大径D1を求め、その中で最も大きい値を選出する。
上記のシリコン相のマップが示す網目構造の骨格において係数bSiが0.150である点P2を求め、点P2を通る線状部分の径の最小値を求め、これを最小径D2(nm)とする。点P2が複数存在する場合、それぞれ最小径D2を求め、その中で最も小さい値を選出する。係数bSiが0.15未満の領域では、上記マップにおいてドット状に分布するシリコン相(網目構造を構成する線状部分以外の部分)が含まれ得る。
上記(3)で得られるシリコン相のマップが示す網目構造を構成する線状部分の10~20個を任意に選出し、各線状部分について、それぞれ係数bSiが最大である点P3を求め、点P3を通る線状部分の径の最小値を求め、これを径D3(nm)とする。選出した線状部分の総数N0に対する、径D3が20nm以下の線状部分の数N1の割合、すなわち、N1/N0を求める。複合粒子の断面の任意の5箇所のSTEM像に対して、それぞれN1/N0を求め、それらの平均値を求める。
上記(3)で得られるシリコン相のマップが示す網目構造を構成する線状部分の20~50個を任意に選出し、各線状部分について、それぞれ係数bSiが最大である点P3を求め、点P3を通る線状部分の径の最小値を求め、これを径D3(nm)とする。径D3が、(D2+10)nm以上、D1nm以下の範囲内である線状部分を、線状部分L1とする。径D3が、D2nm以上、(D2+10)nm未満の範囲内である線状部分を、線状部分L2とする。線状部分L1の数NL1および線状部分L2の数NL2を求め、NL2/NL1を算出する。
<Si-NMR測定条件>
測定装置:バリアン社製、固体核磁気共鳴スペクトル測定装置(INOVA‐400)
プローブ:Varian 7mm CPMAS-2
MAS:4.2kHz
MAS速度:4kHz
パルス:DD(45°パルス+シグナル取込時間1Hデカップル)
繰り返し時間:1200sec~3000sec
観測幅:100kHz
観測中心:-100ppm付近
シグナル取込時間:0.05sec
積算回数:560
試料量:207.6mg
<SEM-EDX測定条件>
加工装置:JEOL製、SM-09010(Cross Section Polisher)
加工条件:加速電圧6kV
電流値:140μA
真空度:1×10-3~2×10-3Pa
測定装置:電子顕微鏡HITACHI製SU-70
分析時加速電圧:10kV
フィールド:フリーモード
プローブ電流モード:Medium
プローブ電流範囲:High
アノード Ap.:3
OBJ Ap.:2
分析エリア:1μm四方
分析ソフト:EDAX Genesis
CPS:20500
Lsec:50
時定数:3.2
工程(i)
リチウムシリケートの原料には、Si原料と、Li原料とを所定の割合で含む原料混合物を用いる。また、原料混合物に、上述の元素M1を含ませてもよい。原料混合物を溶解し、融液を金属ロールに通してフレーク化してリチウムシリケートを作製する。その後フレーク化したシリケートを大気雰囲気で、ガラス転移点以上、融点以下の温度で熱処理により結晶化させる。なお、フレーク化したシリケートは結晶化させずに使用することも可能である。原料混合物を溶解せずに、融点以下の温度で焼成して固相反応によりシリケートを製造することも可能である。
次に、リチウムシリケートに原料シリコンを配合して複合化を行う。例えば、以下の工程(a)~(c)により複合粒子が作製される。
原料シリコンの粉末とリチウムシリケートの粉末とを、例えば、20:80~95:5の質量比で混合し、混合粉末を得る。
次に、ボールミルのような粉砕装置を用いて、原料シリコンとリチウムシリケートの混合物を微粒子化しながら粉砕および複合化する。このとき、混合物に有機溶媒を添加して、湿式粉砕してもよい。有機溶媒は、粉砕対象物の粉砕容器の内壁への付着を防ぐ役割を果たす。例えば、ボールミルの場合、粉砕時間、ポットの回転数、ボールの充填量等を変えることで、網目構造の形成時のD1/D2を調節してもよい。
次に、混合物を、例えば不活性ガス雰囲気(例えばアルゴン、窒素等の雰囲気)中で600℃~1000℃に加熱された状態で加圧し、焼結させる。焼結には、ホットプレス等、不活性雰囲気下で加圧できる焼結装置を用い得る。焼結時、シリケートが軟化し、シリコン粒子間の隙間を埋めるように流動する。その結果、シリケート相を海部とし、シリコン相および金属Meを島部とする緻密なブロック状の焼結体を得ることができる。得られた焼結体を粉砕すれば、複合粒子が得られる。
引き続き、複合粒子の表面の少なくとも一部を導電性材料で被覆して導電層を形成してもよい。導電性炭素材料で複合粒子の表面を被覆する方法としては、アセチレン、メタン等の炭化水素ガスを原料に用いるCVD法、石炭ピッチ、石油ピッチ、フェノール樹脂等を複合粒子と混合し、不活性雰囲気(例えば、アルゴン、窒素等の雰囲気)中で、700℃~950℃で加熱して炭化させる方法等が例示できる。また、カーボンブラックを複合粒子の表面に付着させてもよい。
複合粒子(表面に導電層を有する場合を含む。)を酸で洗浄する工程を行ってもよい。例えば、酸性水溶液で複合粒子を洗浄することで、原料シリコンとリチウムシリケートとを複合化させる際に生じ得る微量のアルカリ成分を溶解させ、除去することができる。酸性水溶液としては、塩酸、フッ化水素酸、硫酸、硝酸、リン酸、炭酸等の無機酸の水溶液や、クエン酸、酢酸等の有機酸の水溶液を用いることができる。
負極は、例えば、負極集電体と、負極集電体の表面に形成され、かつ負極活物質を含む負極合材層とを具備する。負極合材層は、負極合材を分散媒に分散させた負極スラリーを、負極集電体の表面に塗布し、乾燥させることにより形成できる。乾燥後の塗膜を、必要により圧延してもよい。
正極は、例えば、正極集電体と、正極集電体の表面に形成された正極合材層とを具備する。正極合材層は、正極合材を分散媒に分散させた正極スラリーを、正極集電体の表面に塗布し、乾燥させることにより形成できる。乾燥後の塗膜を、必要により圧延してもよい。
電解質は、溶媒と、溶媒に溶解したリチウム塩とを含む。電解質におけるリチウム塩の濃度は、例えば、0.5~2mol/Lである。電解質は、公知の添加剤を含有してもよい。
正極と負極との間には、セパレータを介在させることが望ましい。セパレータは、イオン透過度が高く、適度な機械的強度および絶縁性を備えている。セパレータとしては、微多孔薄膜、織布、不織布等を用いることができる。セパレータの材質としては、例えば、ポリプロピレン、ポリエチレン等のポリオレフィンが用いられ得る。
[リチウムシリケートの合成]
炭酸リチウムと二酸化ケイ素とを、Li2CO3:SiO2=34:66のモル比となるように混合し、混合物を大気雰囲気中で750℃、5時間焼成し、リチウムシリケート(Li2Si2O5)を得た。粉砕によりリチウムシリケート粉末(平均粒径(D50)10μm)を得た。
不活性雰囲気中で、Si粉末とLi2Si2O5粉末(平均粒径(D50)10μm)とを、58:42の質量比で混合し、更に、鉄粉末(平均粒径(D50)100μm)を所定量添加し、遊星ボールミル(フリッチュ製、P-5)のポット(SUS製、容積:500mL)に充填した。Si粉末には、Siの粗粉末(平均粒径(D50)150μm)と、Siの微粉末(平均粒径(D50)10μm)とを、1:9の質量比で混合したものを用いた。
導電層を有する複合粒子A1と、黒鉛とを、5:95の質量比で含む混合物と、カルボキシメチルセルロースのナトリウム塩(CMC-Na)と、スチレン-ブタジエンゴム(SBR)とを、97.5:1.0:1.5の質量比で混合し、水を添加した後、混合機(プライミクス製、T.K.ハイビスミックス)を用いて攪拌し、負極スラリーを調製した。次に、銅箔の両面に負極スラリーを塗布し、塗膜を乾燥させた後、圧延して、銅箔の両面に密度1.6g/cm3の負極合材層が形成された負極A1を得た。
コバルト酸リチウムと、アセチレンブラックと、ポリフッ化ビニリデンとを、95:2.5:2.5の質量比で混合し、N-メチル-2-ピロリドン(NMP)を添加した後、混合機(プライミクス社製、T.K.ハイビスミックス)を用いて攪拌し、正極スラリーを調製した。次に、アルミニウム箔の両面に正極スラリーを塗布し、塗膜を乾燥させた後、圧延して、アルミニウム箔の両面に密度3.6g/cm3の正極合材層が形成された正極を得た。
エチレンカーボネート(EC)とエチルメチルカーボネート(EMC)とを3:7の体積比で含む混合溶媒にLiPF6を濃度1.0mol/Lで溶解して非水電解液を調製した。
各電極にタブをそれぞれ取り付け、タブが最外周部に位置するように、セパレータを介して正極および負極A1を渦巻き状に巻回することにより電極群を作製した。電極群をアルミニウムラミネートフィルム製の外装体内に挿入し、105℃で2時間真空乾燥した後、非水電解液を注入し、外装体の開口部を封止して、二次電池A1を得た。
複合粒子の調製において、Si粉末に、Siの粗粉末(平均粒径(D50)500μm)と、Siの微粉末(平均粒径(D50)が10μm)とを、1:9の質量比で混合したものを用いた以外は、実施例1と同様の方法で、複合粒子A2、負極A2および二次電池A2を得た。
複合粒子の調製において、Si粉末に、Siの粗粉末(平均粒径(D50)150μm)と、Siの微粉末(平均粒径(D50)10μm)とを、8:2の質量比で混合したものを用いた以外は、実施例1と同様の方法で、複合粒子A3、負極A3および二次電池A3を得た。
複合粒子の調製において、Si粉末に、Siの微粉末(平均粒径(D50)10μm)のみを用いた。Si粉末とLi2Si2O5粉末との混合物に鉄粉末を添加しなかった。上記以外は、実施例1と同様の方法で、複合粒子B1、負極B1および二次電池B1を得た。
複合粒子の調製において、Si粉末に、Siの微粉末(平均粒径(D50)10μm)のみを用いた。ボールミルによる粉砕処理の時間を40時間とした。Si粉末とLi2Si2O5粉末との混合物に鉄粉末を添加しなかった。上記以外は、実施例1と同様の方法で、複合粒子B2、負極B2および二次電池B2を得た。
複合粒子の調製において、Si粉末に、Siの微粉末(平均粒径(D50)10μm)のみをを用いた。ボールミルによる粉砕処理の時間を40時間とした。上記以外は、実施例1と同様の方法で、複合粒子B3、負極B3および二次電池B3を得た。
複合粒子の調製において、Si粉末とLi2Si2O5粉末との混合物に鉄粉末を添加しなかった以外は、実施例1と同様の方法で、複合粒子B4、負極B4および二次電池B4を得た。
実施例および比較例の各電池について、以下の条件で充放電サイクル試験を行った。
<充電>
25℃で、1It(800mA)の電流で電圧が4.2Vになるまで定電流充電を行い、その後、4.2Vの電圧で電流が1/20It(40mA)になるまで定電圧充電を行った。
25℃で、1It(800mA)の電流で電圧が2.75Vになるまで定電流放電を行った。
各複合粒子についてXRD分析を行った結果、いずれの複合粒子のXRDパターンにおいても、Si、Li2Si2O5に由来するピークが確認された。
実施例および比較例の充放電サイクル試験後の各電池(完全放電状態)を分解し、負極を取り出し、既述の方法により、シリコン相が網目構造を形成していることを確認した。さらに、網目構造を構成する線状部分の最大径D1、網目構造を構成する線状部分の最小径D2に対する最大径D1の比:D1/D2、網目構造を構成する線状部分の総数N0に対する、径(上述の径D3)が20nm以下の線状部分の数N1の比:N1/N0を求めた。分析結果を表1に示す。
本発明を現時点での好ましい実施態様に関して説明したが、そのような開示を限定的に解釈してはならない。種々の変形および改変は、上記開示を読むことによって本発明に属する技術分野における当業者には間違いなく明らかになるであろう。したがって、添付の請求の範囲は、本発明の真の精神および範囲から逸脱することなく、すべての変形および改変を包含する、と解釈されるべきものである。
Claims (6)
- リチウムシリケート相と、前記リチウムシリケート相内に分散するシリコン相と、を含む複合粒子を含み、
前記シリコン相は、前記リチウムシリケート相内において線状部分を有しており、
前記線状部分の最大径D1および最小径D2は、3≦D1/D2の関係を有し、
前記複合粒子は、前記リチウムシリケート相内に分散する金属Meを含み、
前記金属Meは、Fe、Pb、Zn、Sn、Cu、Ni、Cr、Zr、およびTiからなる群より選択される少なくとも1種である、二次電池用負極活物質。 - 前記最大径D1は、60nm以下である、請求項1に記載の二次電池用負極活物質。
- 前記線状部分の過半数が、20nm以下の径を有する、請求項1または2に記載の二次電池用負極活物質。
- 前記線状部分は、線状部分L1および線状部分L2を含み、
前記線状部分L1の径は、(D2+10)nm以上、D1nm以下であり、
前記線状部分L2の径は、D2nm以上、(D2+10)nm未満であり、
前記最大径D1と前記最小径D2との差は、10nm超であり、
前記線状部分L1の数NL1に対する、前記線状部分L2の数NL2の比:NL2/NL1は、5以上、20以下である、請求項1または2に記載の二次電池用負極活物質。 - 前記金属Meの粒子の平均粒径は、1nm以上、100nm以下である、請求項1~4のいずれか1項に記載の二次電池用負極活物質。
- 正極と、負極と、電解質と、を備え、
前記負極は、請求項1~5のいずれか1項に記載の二次電池用負極活物質を含む、二次電池。
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WO2018179969A1 (ja) * | 2017-03-29 | 2018-10-04 | パナソニックIpマネジメント株式会社 | 非水電解質二次電池用負極材料および非水電解質二次電池 |
WO2019087771A1 (ja) * | 2017-10-31 | 2019-05-09 | パナソニックIpマネジメント株式会社 | 非水電解質二次電池用負極活物質及び非水電解質二次電池 |
WO2020066576A1 (ja) * | 2018-09-26 | 2020-04-02 | パナソニックIpマネジメント株式会社 | 非水電解質二次電池用負極及び非水電解質二次電池 |
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JP2014010890A (ja) | 2012-06-27 | 2014-01-20 | Toyota Industries Corp | 珪素含有材料および珪素含有材料を含む二次電池用活物質 |
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