WO2015046394A1 - 負極活物質、それを用いた負極、及びリチウムイオン二次電池 - Google Patents
負極活物質、それを用いた負極、及びリチウムイオン二次電池 Download PDFInfo
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- 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
- H01M4/362—Composites
- H01M4/364—Composites as mixtures
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M10/00—Secondary cells; Manufacture thereof
- H01M10/05—Accumulators with non-aqueous electrolyte
- H01M10/052—Li-accumulators
- H01M10/0525—Rocking-chair batteries, i.e. batteries with lithium insertion or intercalation in both electrodes; Lithium-ion batteries
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- 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
- H01M4/386—Silicon or alloys based on silicon
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- 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
- H01M4/48—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides
- H01M4/483—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides for non-aqueous cells
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- 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
- H01M4/48—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides
- H01M4/485—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of mixed oxides or hydroxides for inserting or intercalating light metals, e.g. LiTi2O4 or LiTi2OxFy
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M10/00—Secondary cells; Manufacture thereof
- H01M10/05—Accumulators with non-aqueous electrolyte
- H01M10/052—Li-accumulators
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- 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
- 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 relates to a negative electrode active material, a negative electrode using the same, and a lithium ion secondary battery.
- lithium ion secondary batteries are lighter and have a higher capacity than nickel cadmium batteries and nickel metal hydride batteries, they are widely applied as power sources for portable electronic devices. It is also a promising candidate as a power source for use in hybrid vehicles and electric vehicles. With the recent miniaturization and higher functionality of portable electronic devices, further increase in capacity is expected for lithium ion secondary batteries that serve as these power sources.
- the capacity of the lithium ion secondary battery mainly depends on the active material of the electrode.
- graphite is used as the negative electrode active material, but it is necessary to use a higher capacity negative electrode active material in order to meet the above requirements. Therefore, metallic silicon (Si) having a much larger theoretical capacity (4210 mAh / g) than the theoretical capacity of graphite (372 mAh / g) has attracted attention.
- the negative electrode active material using such metal silicon is a mixture of silicon and silicon oxide.
- a mixture of silicon and silicon oxide is considered to have better cycle characteristics than silicon because silicon oxide relieves stress due to expansion and contraction during charge and discharge of silicon. Since the mixture of silicon and silicon oxide is an irreversible reaction in which the oxide is reduced at the first charge, the amount of electricity necessary for the reduction becomes an irreversible capacity. Such an irreversible reaction reduces lithium that contributes to the discharge, and the capacity corresponding to the decrease in lithium becomes the irreversible capacity. Since the amount of lithium contributing to charging / discharging is uniquely determined by the amount of lithium in the positive electrode, an increase in irreversible capacity in the negative electrode further leads to a decrease in capacity of the entire battery.
- Patent Document 1 discloses a method of doping lithium into the negative electrode by forming a film containing lithium on the negative electrode.
- Patent Document 2 discloses a method of doping lithium into the negative electrode by incorporating lithium particles in the negative electrode active material layer.
- Patent Documents 1 and 2 have a problem that highly reactive metallic lithium must be used. Moreover, in the method described in the said patent document 1 and 2, since it did not improve the negative electrode active material itself, it was not improved essentially and was inadequate.
- the present invention has been made in view of the above-described problems of the prior art, and aims to provide an excellent negative electrode active material having high initial charge and discharge efficiency, a negative electrode using the same, and a lithium ion secondary battery. To do.
- the negative electrode active material according to the present invention is a negative electrode active material containing silicon and silicon oxide.
- the negative electrode active material has two phases having different compositions in the primary particles, and one phase is more than the other phase.
- the elemental concentration of silicon is low, and the one phase is a negative electrode active material that is a fibrous phase forming a network structure in a cross section of primary particles of the negative electrode active material.
- the initial charge / discharge efficiency can be remarkably improved.
- the one phase and the other phase are both amorphous.
- the area ratio of the fibrous phase forming the network structure per unit area is preferably 19.6% or more and 51.6% or less.
- the width of the fibrous phase forming the network structure observed in the cross section of the primary particles of the negative electrode active material is preferably 0.29 nm or more and 9.72 nm or less.
- the ratio (D / C) of the element concentration D between the element concentration C of the other phase silicon and the fibrous phase silicon forming the network structure is preferably 0.44 or more and 0.97 or less. .
- the one phase preferably contains a compound represented by Li x SiO y (2 ⁇ x ⁇ 4, 3 ⁇ y ⁇ 4).
- the negative electrode according to the present invention is a negative electrode containing a binder and the negative electrode active material described above on a current collector, whereby a negative electrode with significantly improved initial charge / discharge efficiency can be obtained.
- the lithium ion secondary battery according to the present invention is a battery in which the initial charge and discharge efficiency is remarkably improved by forming a lithium ion secondary battery having a positive electrode, the negative electrode, a separator disposed therebetween, and an electrolytic solution. Obtainable.
- the present invention it is possible to provide an excellent negative electrode active material having high initial charge / discharge efficiency, a negative electrode using the same, and a lithium ion secondary battery.
- FIG. 1 is a schematic cross-sectional view showing a lithium ion secondary battery according to this embodiment.
- a lithium ion secondary battery 100 is interposed between a positive electrode 10, a negative electrode 20 facing the positive electrode 10, the positive electrode 10 and the negative electrode 20, and the main surface of the positive electrode 10 and the main surface of the negative electrode 20.
- a separator 30 in contact with each other, and an electrolyte solution containing lithium ions.
- the lithium ion secondary battery 100 mainly includes a laminate 30, a case 50 that accommodates the laminate 30 in a sealed state, and a pair of leads 60 and 62 connected to the laminate 30.
- the positive electrode 10 has a positive electrode current collector 12 and a positive electrode active material layer 14 formed on the positive electrode current collector 12.
- the negative electrode 20 includes a negative electrode current collector 22 and a negative electrode active material layer 24 formed on the negative electrode current collector 22.
- the separator 18 is located between the negative electrode active material layer 24 and the positive electrode active material layer 14.
- a metal laminate film can be used for the case 50.
- the positive electrode active material layer 14 is formed on the positive electrode current collector 12.
- the positive electrode active material layer 14 contains at least the following positive electrode active material and a conductive additive.
- the conductive aid include carbon materials such as carbon blacks, metal powders such as copper, nickel, stainless steel, and iron, a mixture of carbon materials and metal powders, and conductive oxides such as ITO.
- the carbon material preferably contains carbon having a tap density of 0.03 to 0.09 g / ml and carbon having a tap density of 0.1 to 0.3 g / ml.
- the positive electrode active material layer may include a binder that binds the active material and the conductive additive.
- the positive electrode active material layer 14 is formed by applying a paint containing an active material, a binder, a solvent, and a conductive additive on the positive electrode current collector 12.
- the positive electrode current collector 12 may be a conductive plate material, and for example, a metal thin plate (metal foil) of aluminum, copper, nickel, or an alloy thereof can be used.
- the following compounds are exemplified as the positive electrode active material.
- Lithium ion occlusion and release, lithium ion desorption and insertion (intercalation), or lithium ion and a counter anion (for example, PF 6 ⁇ ) of the lithium ion are allowed to reversibly proceed. If it is possible, it will not specifically limit, A well-known electrode active material can be used.
- lithium cobaltate LiCoO 2
- lithium nickelate LiNiO 2
- lithium manganese spinel LiMn 2 O 4
- the binder binds the positive electrode active materials and the positive electrode current collector 12 together with the positive electrode active materials.
- the binder is not particularly limited as long as the above-described bonding is possible, and examples thereof include fluorine resins such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE).
- Polyimide resin polyamideimide resin, styrene / butadiene / styrene block copolymer (SBR), cellulose, ethylene / propylene / diene rubber (EPDM) hydrogenated product, styrene / ethylene / butadiene / styrene copolymer, styrene / Thermoplastic elastomeric polymers such as isoprene / styrene block copolymers and hydrogenated products thereof may be used.
- SBR styrene / butadiene / styrene block copolymer
- EPDM ethylene / propylene / diene rubber
- the negative electrode active material layer 24 is formed on the negative electrode current collector 22.
- the negative electrode current collector 22 may be a conductive plate material, and for example, a thin metal plate (metal foil) of aluminum, copper, nickel, stainless steel, or an alloy thereof can be used.
- the negative electrode active material layer 24 is mainly composed of a negative electrode active material, a binder, and a conductive auxiliary agent in an amount as required.
- the negative electrode active material of this embodiment is a negative electrode active material containing silicon and silicon oxide.
- the negative electrode active material has two phases with different compositions inside the primary particles, and one phase is more than the other phase.
- the element concentration of silicon is low, and the one phase is a fibrous phase that forms a network structure in the cross section of the primary particles of the negative electrode active material.
- the initial charge / discharge efficiency can be remarkably improved. This is because the fibrous phase that forms a network structure with a low silicon element concentration that does not contribute to charge / discharge during the initial charge prevents the occurrence of cracks due to expansion of the active material by covering the inside of the negative electrode active material, It is speculated that this is due to the reduction of lithium that is used to form this film and becomes inactive at the first charge, while the reaction area of the film can be moderately limited to suppress the formation of the film.
- the one phase and the other phase are both amorphous.
- the strength is high and expansion and contraction can be suppressed.
- the ratio of silicon oxide to silicon in the negative electrode active material is preferably in the range of 1: 1 to 1:10.
- the ratio of silicon oxide to silicon is the above, the volume expansion of silicon accompanying charge / discharge can be suppressed while sufficiently expressing the high discharge capacity of silicon.
- the primary particle diameter of the negative electrode active material containing silicon and silicon oxide is preferably 100 nm or more and 15 ⁇ m or less. Moreover, it is more preferable that the primary particles of the negative electrode active material containing silicon and silicon oxide have a particle diameter of 1 ⁇ m to 8 ⁇ m.
- the primary particle diameter of the negative electrode active material containing silicon and silicon oxide is less than the above range, the formation of the fibrous phase forming the network structure is insufficient, and the reaction area is increased more sufficiently. Therefore, there is a possibility that the best characteristics cannot be obtained in the discharge capacity at a high rate.
- the primary particle diameter of the negative electrode active material containing silicon and silicon oxide exceeds the above range, the increase in the Li diffusion path makes it difficult for the inside of the primary particles to contribute to the charge / discharge reaction, and the cycle characteristics may be deteriorated. is there.
- the area ratio of the fibrous phase forming the network structure per unit area is preferably 19.6% or more and 51.6% or less.
- the area ratio of the fibrous phase forming the network structure per unit area is more preferably 29.4% or more and 45.1% or less.
- the area ratio of the fibrous phase forming the network structure is less than the above range, the formation of the fibrous phase forming the network structure with a low silicon element concentration becomes insufficient, and expansion and contraction can be sufficiently suppressed. Therefore, there is a possibility that the cycle characteristics may be deteriorated. If the fibrous area ratio forming the network structure exceeds the above range, the fibrous phase forming the network structure with a low silicon element concentration increases, so the amount of lithium irreversibly trapped in this layer increases. The initial charge / discharge efficiency may not be able to obtain the highest characteristics.
- the width of the fibrous phase forming the network structure observed in the cross section of the primary particles of the negative electrode active material is preferably 0.29 nm or more and 9.72 nm or less.
- the width of the fibrous phase forming the network structure observed in the cross section of the primary particles of the negative electrode active material is more preferably 2.89 nm or more and 5.17 nm or less.
- the width of the fibrous phase forming the network structure observed in the cross section of the primary particles of the negative electrode active material is less than the above range, the concentration of the fibrous phase forming the network structure with a low silicon element concentration Formation becomes insufficient, expansion and contraction cannot be sufficiently suppressed, and the cycle characteristics may be deteriorated. If the width of the fibrous phase exceeds the above range, the fibrous phase forming a network structure with a low silicon element concentration increases, so the amount of lithium irreversibly trapped in this layer increases, and the initial charge / discharge efficiency May not get the best characteristics.
- the ratio (D / C) of the elemental concentration C of the other phase silicon observed in the cross section of the primary particles of the negative electrode active material to the elemental concentration D of the fibrous phase silicon forming the network structure is 0. More preferably, it is 0.44 or more and 0.97 or less.
- the D / C is more preferably 0.61 or more and 0.85 or less.
- the ratio of the elemental concentration C of the silicon of the other phase and the elemental concentration D of the fibrous phase forming the network structure is less than the above range, the expansion during charging / discharging of the other phase is relatively Since it becomes large, expansion cannot be suppressed and cycle characteristics may deteriorate. If the ratio of the elemental concentration C of the other phase silicon to the elemental concentration D of the one phase silicon exceeds the above range, the contribution to charge / discharge of the one phase is increased, and the cycle characteristics may be deteriorated. There is.
- the one phase contains a compound represented by Li x SiO y (2 ⁇ x ⁇ 4, 3 ⁇ y ⁇ 4).
- the elemental concentrations of silicon in the one phase and the other phase can be measured by performing EELS (Electron Energy Loss Spectroscopy).
- the fibrous phase forming the network structure can be confirmed by observing the cross section of the negative electrode active material with a STEM. Moreover, whether it is amorphous can be confirmed by electron beam diffraction of the negative electrode active material cross section.
- the area ratio of the one phase per unit area in the cross section of the primary particles of the negative electrode active material can be measured by the following procedure.
- a cross section of the negative electrode active material is photographed using STEM.
- a square region of 100 nm ⁇ 100 nm is arbitrarily selected in the cross section of one particle of the arbitrarily selected negative electrode active material, and the area of the fibrous phase forming the network structure in the region is measured.
- Area ratio (area of fibrous phase forming network structure) / (area of 100 nm ⁇ 100 nm square).
- the above operation is carried out for 10 arbitrary particles and 100 arbitrary particles in the same particle. What averaged the area ratio obtained by said method is made into the area ratio of said one phase per unit area in the cross section of a negative electrode active material.
- the average distance of the width of the fibrous phase forming the network structure was measured using a STEM image obtained by photographing a cross section of the negative electrode active material using a STEM.
- FIG. 3 shows a schematic diagram of a cross section of the negative electrode active material.
- the width 201 of the fibrous phase forming the network structure was determined by averaging 8 arbitrary 100 particles within one particle and averaging them. In addition, in order to obtain any eight locations in one particle, first, the intersection is located at the approximate center of the particle, and four lines that divide the particle into eight are drawn. The angle between adjacent lines is 45 degrees. Using this line, the width of the fibrous phase forming the network structure traversed by the line was taken as the value of the width of the fibrous phase.
- the binder and the conductive additive used for the negative electrode active material layer the same materials as those used for the positive electrode 10 described above can be used. Further, the content of the binder and the conductive auxiliary agent is the same as the content in the positive electrode 10 described above, except for the case where the volume change of the negative electrode active material and the adhesion with the foil must be taken into account. Adopt it.
- the electrodes 10 and 20 can be produced by a commonly used method. For example, it can manufacture by apply
- N-methyl-2-pyrrolidone N, N-dimethylformamide and the like can be used.
- the coating method is not particularly limited, and a method usually employed when producing an electrode can be used. Examples thereof include a slit die coating method and a doctor blade method.
- the method for removing the solvent in the paint applied on the current collectors 12 and 22 is not particularly limited, and the current collectors 12 and 22 applied with the paint are dried, for example, in an atmosphere of 80 ° C. to 150 ° C. Just do it.
- the electrodes on which the active material layers 14 and 24 are formed in this way may be pressed by a roll press device or the like, if necessary.
- the linear pressure of the roll press can be, for example, 10 to 50 kgf / cm.
- the negative electrode active material in the present embodiment can be produced as follows. For example, a negative electrode active material having a ratio of amorphous silicon (Si) to silicon oxide (SiO 2 ) of 5: 1 is heat-treated in a vacuum at 350 ° C. and rapidly cooled, so that the active material can be obtained from the difference in thermal expansion coefficient. When cracks are generated inside and baked in an oxygen atmosphere, the crack part is oxidized, the amount of oxygen on the crack surface increases, and a phase composed of silicon and silicon oxide with a low elemental concentration of silicon is formed. Can be obtained by sintering again at 350 ° C. in vacuum.
- a lithium-containing solution may be impregnated with the negative electrode active material and doped with lithium.
- the separator is not particularly limited as long as it is stable with respect to the electrolytic solution and has excellent liquid retention, but generally includes a porous sheet of polyolefin such as polyethylene and polypropylene, or a nonwoven fabric.
- the electrolyte is contained in the positive electrode active material layer 14, the negative electrode active material layer 24, and the separator 18.
- the electrolyte is not particularly limited, and for example, in the present embodiment, an electrolytic solution containing a lithium salt (electrolyte aqueous solution, electrolyte solution using an organic solvent) can be used.
- the electrolyte aqueous solution is preferably an electrolyte solution (non-aqueous electrolyte solution) using an organic solvent because the electrochemical decomposition voltage is low, so that the withstand voltage during charging is limited to a low level.
- a lithium salt dissolved in a non-aqueous solvent is preferably used as the electrolytic solution.
- the lithium salt used as an electrolyte of a lithium ion secondary battery can be used.
- inorganic acid anion salts such as LiPF 6 and LiBF 4
- organic acid anion salts such as LiCF 3 SO 3 and (CF 3 SO 2 ) 2 NLi, and the like can be used.
- aprotic high dielectric constant solvents such as ethylene carbonate and propylene carbonate
- aprotic low viscosity such as acetic acid esters and propionic acid esters such as dimethyl carbonate and ethyl methyl carbonate, etc.
- a solvent is mentioned. It is desirable to use these aprotic high dielectric constant solvents and aprotic low viscosity solvents in combination at an appropriate mixing ratio.
- ionic liquids using imidazolium, ammonium, and pyridinium type cations can be used.
- the counter anion is not particularly limited, and examples thereof include BF 4 ⁇ , PF 6 ⁇ , (CF 3 SO 2 ) 2 N ⁇ and the like.
- the ionic liquid can be used by mixing with the organic solvent described above.
- the concentration of the lithium salt in the electrolytic solution is preferably 0.5 to 2.0 M from the viewpoint of electrical conductivity.
- the conductivity of the electrolyte at 25 ° C. is preferably 0.01 S / m or more, and is adjusted by the type of electrolyte salt or its concentration.
- additives may be added to the electrolytic solution of the present embodiment as necessary.
- additives include vinylene carbonate and methyl vinylene carbonate for the purpose of improving cycle life, biphenyl and alkyl biphenyl for the purpose of preventing overcharge, various carbonate compounds for the purpose of deoxidation and dehydration, Carboxylic anhydride, various nitrogen-containing and sulfur-containing compounds can be mentioned.
- the case 50 seals the laminated body 30 and the electrolytic solution therein.
- the case 50 is not particularly limited as long as it can suppress leakage of the electrolytic solution to the outside and entry of moisture and the like into the lithium ion secondary battery 100 from the outside.
- a metal laminate film in which a metal foil 52 is coated with a polymer film 54 from both sides can be used as the case 50.
- an aluminum foil can be used as the metal foil 52 and a film such as polypropylene can be used as the polymer film 54.
- the material of the outer polymer film 54 is preferably a polymer having a high melting point, such as polyethylene terephthalate (PET) or polyamide, and the material of the inner polymer film 54 is polyethylene (PE) or polypropylene (PP). Etc. are preferred.
- the leads 60 and 62 are made of a conductive material such as aluminum. Then, the leads 60 and 62 are welded to the negative electrode current collector 22 and the positive electrode current collector 12 by a known method, respectively, and a separator is provided between the positive electrode active material layer 14 of the positive electrode 10 and the negative electrode active material layer 24 of the negative electrode 20. 18 may be inserted into the case 50 together with the electrolytic solution with the 18 interposed therebetween, and the entrance of the case 50 may be sealed.
- the lithium ion secondary battery is not limited to the shape shown in FIG. 1, but a coin type in which an electrode punched into a coin shape and a separator are stacked, or a cylinder in which an electrode sheet and a separator are wound in a spiral shape. It may be a type or the like.
- Example 1 (Preparation of negative electrode active material) A negative electrode active material made of silicon and silicon oxide having a ratio of amorphous silicon (Si) to silicon oxide (SiO 2 ) of 5: 1 is heat-treated in vacuum at 350 ° C. Due to the difference, a crack is generated in the active material and fired in an oxygen atmosphere (0.5 atm), so that the crack part is oxidized, the amount of oxygen on the crack surface is increased, and the silicon element concentration is reduced to SiOx. Sintering again at 350 ° C. in vacuum has two phases with different compositions inside the negative electrode active material, and one phase has a lower elemental concentration of silicon than the other phase. A negative electrode active material having a structure composed of a fibrous phase forming a network structure in the cross section of the primary particles of the active material was obtained.
- FIG. 1 A STEM image of the obtained negative electrode active material is shown in FIG.
- the active material has two phases with different compositions inside the primary particle, and one of the phases is a fibrous phase forming a network structure in the cross section. It can be confirmed that it spreads evenly.
- the area ratio of the fibrous phase forming the network structure per unit area of the obtained negative electrode active material and the average distance of the width of the fibrous phase were measured. The results are shown in Table 1.
- Electron beam diffraction of the fibrous phase forming the network structure of the obtained negative electrode active material cross section and the other phase was performed. As a result, it was confirmed that both the fibrous phase and the other phase were amorphous.
- (Preparation of negative electrode) 83 parts by mass of the negative electrode active material prepared by the above method, 2 parts by mass of acetylene black, 15 parts by mass of polyamideimide, and 82 parts by mass of N-methylpyrrolidone were mixed to prepare a slurry for forming an active material layer. This slurry was applied to one surface of a copper foil having a thickness of 14 ⁇ m so that the amount of active material applied was 2.0 mg / cm 2 and dried at 100 ° C. to form an active material layer. Then, the negative electrode was pressure-molded by a roll press and heat-treated at 350 ° C. for 3 hours in a vacuum to obtain a negative electrode having an active material layer thickness of 19 ⁇ m.
- Examples 2 to 22 The negative electrode actives of Examples 2 to 22 were the same as Example 1 except that the cooling rate of the heat treatment was 200 ° C./min or 800 ° C./min and the hydrogen pressure was changed in the range of 0.2 atm to 2.5 atm. Obtained material.
- the obtained negative electrode active material As a result of evaluating the obtained negative electrode active material by STEM observation, it has two phases with different compositions inside, and one phase is a fibrous phase that forms a network structure in the cross section of the primary particles of the negative electrode active material. It was confirmed to consist of Moreover, as a result of conducting the electron beam diffraction of the cross section of the obtained negative electrode active material, it was confirmed that the two phases having different compositions were both amorphous as in Example 1.
- Example 2 Similar to Example 1, using STEM and EELS, the area ratio of the fibrous phase forming the network structure per unit area, the width of the fibrous phase, the element concentration C of silicon in the other phase, and the network structure The ratio (D / C) of the fibrous phase forming the silicon to the element concentration D of silicon was measured.
- negative electrode active material negative electrodes of Examples 2 to 22 and lithium ion secondary batteries for evaluation were produced in the same manner as in Example 1.
- Comparative Example 1 The negative electrode of Comparative Example 1 and the lithium ion secondary battery for evaluation were obtained in the same manner as in Example 1, except that the active material was not subjected to a treatment such as vacuum heat treatment and a negative electrode active material that did not form a network structure was used. Produced.
- the primary particles have two phases having different compositions, and one phase has a lower elemental concentration of silicon than the other phase, and the one phase has a network structure in the cross section of the primary particles.
- the negative electrode active material composed of a fibrous phase forming a high initial charge / discharge efficiency at a higher rate than when a negative electrode active material not forming a network structure was used.
- the active material of the present invention can provide a lithium ion secondary battery with sufficiently high initial charge and discharge efficiency, and can be used widely for devices using lithium ion secondary batteries, and is beneficial.
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Abstract
Description
図1は、本実施形態に係るリチウムイオン二次電池を示す模式断面図である。図1に示すように、リチウムイオン二次電池100は、正極10と、正極10に対向する負極20と、正極10及び負極20の間に介在し、正極10の主面及び負極20の主面にそれぞれに接触するセパレータ18と、を備える積層体30と、リチウムイオンを含む電解液を備える。
正極活物質層14は、正極集電体12上に形成される。正極活物質層14は、少なくとも下記の正極活物質と導電助剤とを含有する。導電助剤としては、カーボンブラック類等の炭素材料、銅、ニッケル、ステンレス、鉄等の金属粉、炭素材料及び金属粉の混合物、ITOのような導電性酸化物が挙げられる。炭素材料は、タップ密度が0.03~0.09g/mlであるカーボンと、タップ密度が0.1~0.3g/mlであるカーボンと、を含むことが好ましい。
正極活物質層は活物質及び導電助剤を結着するバインダーを含んでもよい。正極活物質層14は、活物質と、バインダーと、溶媒と、導電助剤と、を含む塗料を正極集電体12上に塗布する工程によって形成される。
正極集電体12は、導電性の板材であればよく、例えば、アルミニウム、銅、ニッケル又はそれらの合金の金属薄板(金属箔)を用いることができる。
本実施形態に係るリチウムイオン二次電池では正極活物質として下記のような化合物が挙げられる。リチウムイオンの吸蔵及び放出、リチウムイオンの脱離及び挿入(インターカレーション)、又は、リチウムイオンと該リチウムイオンのカウンターアニオン(例えば、PF6 -)とのドープ及び脱ドープを可逆的に進行させることが可能であれば特に限定されず、公知の電極活物質を使用できる。
バインダーは、正極活物質同士を結合すると共に、正極活物質と正極集電体12とを結合している。バインダーは、上述の結合が可能なものであればよく、例えば、ポリフッ化ビニリデン(PVDF)、ポリテトラフルオロエチレン(PTFE)等のフッ素樹脂が挙げられる。また、ポリイミド樹脂、ポリアミドイミド樹脂、スチレン・ブタジエン・スチレンブロック共重合体(SBR)、セルロース、エチレン・プロピレン・ジエンゴム(EPDM)その水素添加物、スチレン・エチレン・ブタジエン・スチレン共重合体、スチレン・イソプレン・スチレンブロック共重合体、その水素添加物等の熱可塑性エラストマー状高分子を用いてもよい。
負極活物質層24は、負極集電体22上に形成される。負極集電体22は、導電性の板材であればよく、例えば、アルミニウム、銅、ニッケル、ステンレス又はそれらの合金の金属薄板(金属箔)を用いることができる。負極活物質層24は、負極活物質、バインダー、及び、必要に応じた量の導電助剤から主に構成されるものである。
本実施形態の負極活物質は、シリコンと酸化シリコンを含有する負極活物質において、前記負極活物質はその一次粒子内部に組成の異なる2つの相を有し、一方の相は他方の相よりもシリコンの元素濃度が低く、前記一方の相は、前記負極活物質の一次粒子の断面において網目構造を形成する繊維状の相であることを特徴としている。
本実施形態における負極活物質は、以下のようにして作製することができる。例えば、非晶質のシリコン(Si)と酸化シリコン(SiO2)の比が5:1の負極活物質を真空中、350℃で熱処理し、急冷することで、熱膨張率の違いから活物質内にクラックを発生させ、酸素雰囲気中で焼成することにより、クラック部が酸化し、クラック表面の酸素量が増加し、シリコンの元素濃度が低い組成の、シリコンと酸化シリコンからなる相となり、これを再び真空中350℃で焼結することで得ることが出来る。
電解質は、正極活物質層14、負極活物質層24、及び、セパレータ18の内部に含有させるものである。電解質としては、特に限定されず、例えば、本実施形態では、リチウム塩を含む電解液(電解質水溶液、有機溶媒を使用する電解質溶液)を使用することができる。ただし、電解質水溶液は電気化学的に分解電圧が低いことにより、充電時の耐用電圧が低く制限されるので、有機溶媒を使用する電解液(非水電解質溶液)であることが好ましい。電解液としては、リチウム塩を非水溶媒(有機溶媒)に溶解したものが好適に使用される。リチウム塩としては特に限定されず、リチウムイオン二次電池の電解質として用いられるリチウム塩を用いることができる。例えば、リチウム塩としては、LiPF6、LiBF4、等の無機酸陰イオン塩、LiCF3SO3、(CF3SO2)2NLi等の有機酸陰イオン塩等を用いることができる。
ケース50は、その内部に積層体30及び電解液を密封するものである。ケース50は、電解液の外部への漏出や、外部からのリチウムイオン二次電池100内部への水分等の侵入等を抑止できる物であれば特に限定されない。例えば、ケース50として、図1に示すように、金属箔52を高分子膜54で両側からコーティングした金属ラミネートフィルムを利用できる。金属箔52としては例えばアルミ箔を、高分子膜54としてはポリプロピレン等の膜を利用できる。例えば、外側の高分子膜54の材料としては融点の高い高分子、例えば、ポリエチレンテレフタレート(PET)、ポリアミド等が好ましく、内側の高分子膜54の材料としてはポリエチレン(PE)、ポリプロピレン(PP)等が好ましい。
リード60,62は、アルミ等の導電材料から形成されている。
そして、公知の方法により、リード60、62を負極集電体22、正極集電体12にそれぞれ溶接し、正極10の正極活物質層14と負極20の負極活物質層24との間にセパレータ18を挟んだ状態で、電解液と共にケース50内に挿入し、ケース50の入り口をシールすればよい。
(負極活物質の作製)
非晶質のシリコン(Si)と酸化シリコン(SiO2)の比が5:1のシリコンと酸化シリコンからなる負極活物質を真空中、350℃で熱処理し、急冷することで、熱膨張率の違いから活物質内にクラックを発生させ、酸素雰囲気中(0.5atm)で焼成することにより、クラック部が酸化、クラック表面の酸素量が増加し、シリコンの元素濃度の低いSiOxとなり、これを再び真空中350℃で焼結することで負極活物質内部に組成の異なる2つの相を有し、一方の相は他方の相よりもシリコンの元素濃度が低く、前記一方の相は、前記負極活物質の一次粒子の断面において網目構造を形成する繊維状の相からなる構造体を持つ負極活物質を得た。
得られた負極活物質のSTEM像を図2に示す。STEM像にて明らかな様に活物質の一次粒子内部に組成の異なる2つの相を有し、一方の相は断面において網目構造を形成する繊維状の相であり、負極活物質の一次粒子内に均一に広がっていることが確認できる。得られた負極活物質の単位面積あたりの網目構造を形成する繊維状の相の面積比率、繊維状の相の幅の平均距離を測定した。結果を表1に示す。
得られた負極活物質断面の網目構造を形成する繊維状の相と、他方の相の電子線回折を行った。その結果、繊維状の相と他方の相は、共に非晶質であることが確認できた。
得られた負極活物質断面の網目構造を形成する繊維状の相と、他方の相のEELS測定を行い、他方の相のシリコンの元素濃度Cと前記網目構造を形成する繊維状の一方の相のシリコンの元素濃度Dとの比(D/C)を測定した。結果を表1に示す。
上記の方法で作製した負極活物質83質量部、アセチレンブラック2質量部、ポリアミドイミド15質量部、及びN-メチルピロリドン82質量部を混合し、活物質層形成用のスラリーを調製した。このスラリーを、厚さ14μmの銅箔の一面に、活物質の塗布量が2.0mg/cm2となるように塗布し、100℃で乾燥することで活物質層を形成した。その後、ロールプレスにより負極を加圧成形し、真空中、350℃で3時間熱処理することで、活物質層の厚さが19μmである負極を得た。
上記で作製したリチウム金属粉末分散負極と、銅箔にリチウム金属箔を貼り付けたものを対極とし、それらの間にポリエチレン微多孔膜からなるセパレータを挟んで、アルミラミネートパックに入れ、このアルミラミネートパックに、電解液として1MのLiPF6溶液(溶媒:EC/DEC=3/7(体積比))を注入した後、真空シールし、評価用のリチウムイオン二次電池を作製した。
熱処理の冷却速度を200℃/min、または800℃/minとし、水素圧を0.2atmから2.5atmの範囲で変更したこと以外は実施例1と同様にして実施例2~22の負極活物質を得た。得られた負極活物質をSTEM観察にて評価した結果、内部に組成の異なる2つの相を有し、一方の相は、負極活物質の一次粒子の断面において網目構造を形成する繊維状の相からなることが確認された。また、得られた負極活物質の断面の電子線回折を行った結果、実施例1と同様に組成の異なる2つの相は共に非晶質であることを確認した。
上記活物質に対し真空熱処理等の処理を施さず、網目構造を形成しない負極活物質を用いたこと以外は実施例1と同様にして、比較例1の負極及び評価用リチウムイオン二次電池を作製した。
実施例及び比較例で作製した評価用リチウムイオン二次電池について、二次電池充放電試験装置(北斗電工株式会社製)を用い、電圧範囲を0.005Vから2.5Vまでとし、1C=1600mAh/gとしたときの0.05Cでの電流値で充放電を行った。これにより、初期充電容量、初期放電容量及び初期充放電効率を求めた。なお、初期充放電効率(%)は、初期充電容量に対する初期放電容量の割合(100×初期放電容量/初期充電容量)である。結果を表1に示す。
Claims (8)
- シリコンと酸化シリコンを含有する負極活物質において、前記負極活物質はその一次粒子内部に組成の異なる2つの相を有し、一方の相は他方の相よりもシリコンの元素濃度が低く、前記一方の相は、前記一次粒子の断面において網目構造を形成する繊維状の相であることを特徴とする負極活物質。
- 前記一方の相と前記他方の相は、共に非晶質であることを特徴とする請求項1に記載の負極活物質。
- 前記一次粒子断面において、単位面積あたりの網目構造を形成する繊維状の相の面積比率が19.6%以上51.6%以下であることを特徴とする請求項1に記載の負極活物質。
- 前記一次粒子の断面において観察される網目構造を形成する繊維状の相の幅は、0.29nm以上9.72nm以下であることを特徴とする請求項1に記載の負極活物質。
- 前記他方の相のシリコンの元素濃度Cと網目構造を形成する繊維状の相のシリコンの元素濃度Dとの比(D/C)が、0.44以上0.97以下であることを特徴とする請求項1に記載の負極活物質。
- 前記一方の相は、LixSiOy(2≦x≦4,3≦y≦4)で表される化合物を含有していることを特徴とする請求項1に記載の負極活物質。
- 集電箔上にバインダーと、請求項1乃至6に記載の負極活物質と、を含有することを特徴とする負極。
- 正極と、請求項7記載の負極と、その間に配置されるセパレータと、電解液とを有することを特徴とするリチウムイオン二次電池。
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