WO2014200076A1 - 不定比チタン化合物・炭素複合体及びその製造方法、並びに負極活物質及びリチウムイオン二次電池 - Google Patents
不定比チタン化合物・炭素複合体及びその製造方法、並びに負極活物質及びリチウムイオン二次電池 Download PDFInfo
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- WO2014200076A1 WO2014200076A1 PCT/JP2014/065652 JP2014065652W WO2014200076A1 WO 2014200076 A1 WO2014200076 A1 WO 2014200076A1 JP 2014065652 W JP2014065652 W JP 2014065652W WO 2014200076 A1 WO2014200076 A1 WO 2014200076A1
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- H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
- H01M4/131—Electrodes based on mixed oxides or hydroxides, or on mixtures of oxides or hydroxides, e.g. LiCoOx
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- 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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- 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
- H01M4/583—Carbonaceous material, e.g. graphite-intercalation compounds or CFx
- H01M4/587—Carbonaceous material, e.g. graphite-intercalation compounds or CFx for inserting or intercalating light metals
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- H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
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- H01M10/00—Secondary cells; Manufacture thereof
- H01M10/05—Accumulators with non-aqueous electrolyte
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- H01M10/0525—Rocking-chair batteries, i.e. batteries with lithium insertion or intercalation in both electrodes; Lithium-ion batteries
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- 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 composite containing a non-stoichiometric titanium compound and carbon, a method for producing the same, a negative electrode active material containing the non-stoichiometric titanium compound / carbon composite, and a lithium ion secondary battery using the same.
- lithium ion secondary batteries are widely used mainly in electronic devices such as portable devices. This is because a lithium ion secondary battery has a higher voltage, a larger charge / discharge capacity, and less harmful effects such as a memory effect than a nickel cadmium battery.
- lithium ion secondary batteries have excellent energy density and output density, and are used not only for many portable electronic devices such as notebook computers and mobile phones, but also for hybrid vehicles and electric vehicles. In the future, it is expected to be applied to electric vehicles and power storage power sources.
- lithium ion secondary batteries have room for improvement in terms of safety and high temperature resistance.
- Li 4 Ti 5 O 12 which is a non-combustible metal oxide, has attracted attention as a new negative electrode material that can replace the carbon negative electrode.
- the insertion potential of lithium ions is as high as 1.55 V vs Li / Li + and shows a flat potential, so that metallic lithium does not precipitate, A solid electrolyte interface (SEI) film with poor thermal stability is difficult to form on the electrode surface.
- SEI solid electrolyte interface
- Li 4 Ti 5 O 12 exhibits a considerably good cycle characteristic. Therefore, by using Li 4 Ti 5 O 12 for the negative electrode, a battery that is safer than a battery using a carbon material for the negative electrode can be designed.
- Li 4 Ti 5 O 12 is obtained as a mixture with rutile TiO 2 (hereinafter referred to as “r-TiO 2 ”) or Li 2 TiO 3 , which contributes to a decrease in battery performance during synthesis.
- r-TiO 2 rutile TiO 2
- Li 2 TiO 3 Li 2 TiO 3
- the range in which Li 4 Ti 5 O 12 having a constant stoichiometric composition can be synthesized is very narrow, and it can be obtained as a mixture of r-TiO 2 or Li 2 TiO 3 depending on the ratio of lithium and titanium.
- Non-patent Document 1 Li 4 Ti 5 O 12 exists as a mixture with them in the papers and commercial products published so far.
- the electronic conductivity of Li 4 Ti 5 O 12 is low (10 ⁇ 13 Scm ⁇ 1 ), so that when used as a negative electrode active material, there is a problem that the electric capacity becomes small particularly when discharging at a large current. It was.
- Non-patent Document 2 Li 4 Ti 5 O 12 with a conductive material such as carbon
- Non-patent Document 3 a conductive material
- Patent Document 1 discloses that a solution obtained by adding a predetermined amount of a dicarboxylic acid having 4 or more carbon atoms, a lithium salt, and a titanium alkoxide in the presence of water, and stirring and dissolving is spray-dried.
- the precursor obtained by spraying and drying is calcined at 700 to 900 ° C. for a predetermined time, and is expressed by the chemical formula Li 4 + x Ti 5-x O 12 (where 0 ⁇ x ⁇ 0.3). Describes a technique for producing carbon composites of nonstoichiometric titanium compounds.
- Patent Document 1 discloses that the non-stoichiometric titanium compound produced by the above method has excellent crystallinity, does not contain impurities such as r-TiO 2 , Li 2 TiO 3, and is a carbon composite, so that lithium ions can be obtained. It is described that charge / discharge characteristics can be improved when used as a negative electrode active material for a secondary battery.
- Li 4 Ti 5 O 12 is generally synthesized by a solid-phase reaction method.
- r-TiO which is an impurity phase
- 2 and Li 2 TiO 3 are easily generated.
- the particle size is large and the distribution tends to be wide.
- the electronic conductivity of Li 4 Ti 5 O 12 itself is considerably low, which greatly affects the charge / discharge characteristics, particularly the deterioration of battery characteristics at a high rate.
- Non-Patent Documents 1 to 3 provide materials with high electronic conductivity. However, lithium ion secondary batteries obtained using these materials are satisfactory in terms of their discharge and charge characteristics. The characteristics to be obtained are not obtained.
- Patent Document 1 reports that when a carbon composite of a non-stoichiometric titanium compound obtained using a dicarboxylic acid having 4 or more carbon atoms as a carbon source is used as a negative electrode active material, high charge / discharge characteristics are reported. However, Patent Document 1 does not disclose a structure in which carbon is uniformly distributed on the surface of the non-stoichiometric titanium compound, and variation in surface electronic conductivity among individual particles is large. Further, Patent Document 1 manufactures a non-stoichiometric titanium compound and a carbon composite by firing the precursor, but does not show a method for suppressing particle coarsening due to aggregation during firing.
- the non-stoichiometric titanium compound carbon composite used as the negative electrode active material is suppressed in particle coarsening and excellent in particle dispersibility. Moreover, it is desirable that there is little variation in the electronic conductivity of the particle surface and that the movement of lithium ions between the nonstoichiometric titanium compound and the electrolyte is not inhibited.
- An object of the present invention is to provide a non-stoichiometric titanium compound / carbon composite capable of increasing the charge / discharge capacity of a lithium ion secondary battery, a negative electrode active material for a lithium ion secondary battery using the composite, and a lithium secondary battery.
- the next battery is to provide.
- a further object of the present invention is to provide a non-stoichiometric titanium compound / carbon composite capable of increasing the charge / discharge capacity of a lithium ion secondary battery, which is suppressed in coarsening and has uniform surface characteristics, and the composite. It is providing the negative electrode active material for lithium ion secondary batteries using, and a lithium secondary battery.
- Another object of the present invention is to provide a method for efficiently producing such a non-stoichiometric titanium compound / carbon composite.
- the present inventors have found that when a hydrophilic polymer is used as a carbon source in producing a carbon composite of a nonstoichiometric titanium compound, A hybrid layer with a specific ratio of carbon and carbon is formed, and because of such a hybrid layer, when this non-stoichiometric titanium compound carbon composite is used as an electrode, good battery characteristics can be exhibited. And the present invention has been completed.
- the hybrid layer may exist over the entire surface layer of the composite particle.
- the hybrid layer is preferably formed with a thickness in the range of 100 nm or less.
- the ratio Ti / C of the number of titanium atoms to the number of carbon atoms in the hybrid layer is preferably 1 ⁇ 2 or less.
- the non-stoichiometric titanium compound / carbon composite may have a specific surface area measured by a BET method of 20 to 100 m 2 g ⁇ 1 .
- the average particle diameter of the composite particles constituting the non-stoichiometric titanium compound / carbon composite may be 20 ⁇ m or less.
- a second aspect of the present invention is a method for producing a nonstoichiometric titanium compound / carbon composite represented by the chemical formula Li 4 + x Ti 5-x O 12 (where 0 ⁇ x ⁇ 0.30).
- a dissolution step of adding a predetermined amount of a hydrophilic polymer, a lithium salt, and a titanium alkoxide to an aqueous solvent, and dissolving them while stirring and dispersing;
- a firing step in which the precursor is fired in a furnace at a temperature of 600 ° C. or higher and 900 ° C. or lower for a predetermined time in a reducing atmosphere or in an inert atmosphere; Is a method for producing the non-stoichiometric titanium compound / carbon composite.
- the precursor obtained by spray drying the stock solution is pre-baked for a predetermined time at a temperature of 300 ° C. or higher and lower than 600 ° C. in a reducing atmosphere or an inert atmosphere before the baking step (main baking step).
- a pre-baking step may be provided.
- polyvinyl alcohol may be used as the hydrophilic polymer.
- a third aspect of the present invention is a negative electrode active material containing the non-stoichiometric titanium compound / carbon composite.
- a fourth aspect of the present invention is a lithium secondary battery comprising a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and an electrolyte, wherein the negative electrode is the negative electrode active material A lithium ion secondary battery.
- the non-stoichiometric titanium compound / carbon composite of the present invention there is a hybrid layer containing titanium and carbon at a predetermined atomic ratio on the surface layer of the non-stoichiometric titanium compound.
- the charge / discharge characteristics of the obtained lithium ion secondary battery can be improved.
- the non-stoichiometric titanium compound / carbon composite of the present invention since a mixed layer containing the non-stoichiometric titanium compound and carbon is formed on the surface of the non-stoichiometric titanium compound particles serving as the core, the indefinite ratio during firing The coarsening due to the aggregation of the specific titanium compound particles is suppressed.
- FIG. 4 is a diagram showing the results of TEM-EDX analysis of the core portion of the constituent particles of the non-stoichiometric titanium compound / carbon composite A-1 according to Example 1.
- FIG. 3 is a diagram showing a result of TEM-EDX analysis of a hybrid layer existing in a surface layer portion of constituent particles of the non-stoichiometric titanium compound / carbon composite A-1 according to Example 1.
- 4 is a TEM photograph of constituent particles of non-stoichiometric titanium compound A-3 according to Comparative Example 1.
- 4B is a TEM photograph near the surface layer of the particles shown in FIG. 4A.
- 4 is a TEM photograph of constituent particles of a non-stoichiometric titanium compound / carbon composite A-4 according to Comparative Example 2.
- FIG. 6 is a diagram showing the result of TEM-EDX analysis of the core portion of the constituent particles of the non-stoichiometric titanium compound / carbon composite A-4 according to Comparative Example 2.
- FIG. 6 is a diagram showing the result of TEM-EDX analysis of the surface layer portion of the constituent particles of the non-stoichiometric titanium compound / carbon composite A-4 according to Comparative Example 2.
- FIG. 6 is a diagram showing the result of TEM-EDX analysis of the surface layer portion of the constituent particles of the non-stoichiometric titanium compound / carbon composite A-4 according to Comparative Example 2.
- the non-stoichiometric titanium compound / carbon composite of the present invention is a composite of a non-stoichiometric titanium compound represented by the chemical formula Li 4 + x Ti 5-x O 12 (where 0 ⁇ x ⁇ 0.30) and a carbon-containing substance. Is the body.
- the carbon-containing material may be substantially composed of carbon, but may contain carbon and a compound of carbon and another element.
- the non-stoichiometric titanium compound / carbon composite is a composite having a core part made of non-stoichiometric titanium compound and a hybrid layer (hybrid layer) containing the non-stoichiometric titanium compound and carbon formed on the surface of the core part. It consists of body particles (composite unit particles).
- the non-stoichiometric titanium compound / carbon composite may be composed of a single composite particle or may be composed of an aggregate of a plurality of composite particles.
- the composite particles of the non-stoichiometric titanium compound / carbon composite are mostly composed of non-stoichiometric titanium compounds constituting a core portion (core particle).
- the nonstoichiometric titanium compound is an oxide containing Li and Ti in a non-stoichiometric composition, and is represented by the chemical formula Li 4 + x Ti 5-x O 12 (where 0 ⁇ x ⁇ 0.30). Having a composition.
- the hybrid layer contains a nonstoichiometric titanium compound and carbon.
- Ti / C When the atomic ratio Ti / C is greater than 1/2, conduction of ions or electrons in the carbon-containing coating (hybridized layer) is hindered, resulting in a decrease in electrical conductivity and failure to obtain desired electrical characteristics. There is a case. More preferably, Ti / C is about 1/30 to 1/4.
- the hybrid layer is preferably formed over the entire surface layer of the composite particle.
- the hybrid layer exists over the entire surface layer, particularly preferably without substantial loss, the interior of the composite can be protected by the hybrid layer, and the stability of the composite can be improved. . In particular, coarsening due to aggregation of non-stoichiometric titanium compounds is suppressed.
- the presence or absence of the hybrid layer can be confirmed by observation of the composite with a transmission electron microscope (TEM) and energy dispersive X-ray analysis (EDX analysis).
- TEM transmission electron microscope
- EDX analysis energy dispersive X-ray analysis
- the hybrid layer is particularly preferably formed substantially uniformly over the entire surface of the composite particle.
- substantially uniform means that the thickness of the hybrid layer is formed within a range that is within ⁇ 20% of the average thickness.
- the thickness of the hybrid layer may be preferably 100 nm or less from the viewpoint of improving the charge / discharge capacity of a lithium secondary battery containing a nonstoichiometric titanium compound / carbon composite as a negative electrode active material.
- the thickness of the hybrid layer is preferably 1 nm or more.
- the thickness of the hybrid layer can be measured by observation of the composite particles with a transmission electron microscope.
- the average particle diameter of the composite particles constituting the non-stoichiometric titanium compound / carbon composite may be, for example, 20 ⁇ m or less, preferably 10 ⁇ m or less, and more preferably 5 ⁇ m or less.
- the average particle diameter described here is obtained by sampling a plurality of particles randomly in an observation with a transmission electron microscope and obtaining an average value of the maximum diameter. It may be.
- the specific surface area of the non-stoichiometric titanium compound / carbon composite may be 20 to 100 m 2 g ⁇ 1 as a specific surface area measured by the BET method.
- the specific surface area may be 30-50 m 2 g ⁇ 1 .
- the method described in the Example mentioned later can be used as a measuring method of the specific surface area by BET method.
- the non-stoichiometric titanium compound / carbon composite can be produced by the following production method.
- the method for producing the non-stoichiometric titanium compound / carbon composite according to the present invention is as follows: A dissolution step of adding a predetermined amount of a hydrophilic polymer, a lithium salt, and a titanium alkoxide to an aqueous solvent, and dissolving them while stirring and dispersing; A precursor forming step of obtaining a precursor by spray drying the stock solution obtained in the dissolving step; A firing step of firing the precursor in a reducing atmosphere or an inert atmosphere at 600 to 900 ° C.
- the blending ratio of the hydrophilic polymer, the lithium salt, and the titanium alkoxide is not particularly limited, but the lithium salt is 1 to 50 parts by mass and the hydrophilic polymer is 1 to 100 parts by mass with respect to 100 parts by mass of the titanium alkoxide. It is preferable that it is 30 mass parts.
- the precursor is obtained by spray-drying the stock solution, the precursor is overheated under appropriate conditions, and compared with the amorphous film non-stoichiometric titanium compound obtained by the sputtering method.
- a carbon composite of a non-stoichiometric titanium compound having a titanium / carbon hybrid layer on the surface layer of the composite particle having a non-stoichiometric titanium compound composed of a highly crystalline single phase in the inner layer (core portion) of the composite particle is obtained. be able to.
- dissolution process In the dissolution process, a predetermined amount of hydrophilic polymer, lithium salt, and titanium alkoxide are added to an aqueous solvent, and these are stirred and dispersed to prepare a stock solution for forming a precursor. To do. In the dissolution step, a first solution in which lithium salt, titanium alkoxide is dissolved, and a second solution in which a hydrophilic polymer solution is dissolved are prepared, and then the first solution and the second solution are mixed. May be.
- the aqueous solvent is not particularly limited as long as it is a solvent that can be dissolved in water.
- various organic acids, inorganic acids, aprotic polar solvents (DMSO), and the like can be used. Among them, water is preferable from the viewpoint of handleability.
- lithium salt lithium carbonate, lithium hydroxide and the like can be used, and among them, lithium carbonate is preferable.
- titanium alkoxide tetramethoxytitanium, tetraethoxytitanium, tetraisopropoxytitanium, titanium dioxide or the like can be used, and among them, tetraisopropoxytitanium is preferable.
- the composition of the non-stoichiometric titanium compound can be adjusted by setting the Li / Ti ratio of the lithium salt and the titanium alkoxide to the ratio of the target composition.
- Patent Document 1 reports that a non-stoichiometric titanium compound having excellent crystallinity and consisting of a single phase can be obtained by spray-drying an aqueous solution containing a lithium salt and a titanium alkoxide and firing it under predetermined conditions. .
- a hydrophilic polymer is used as the carbon source.
- a hydrophilic polymer By using a hydrophilic polymer, the stability of the composite is improved, and the covering state of the titanium / carbon hybrid layer formed in the carbon composite of the obtained nonstoichiometric titanium compound is made uniform.
- a lithium ion secondary battery using the composite as a negative electrode active material stably exhibits a high capacity.
- Patent Document 1 there is no report of a composite in which the particle surface of a nonstoichiometric titanium compound is coated with a hybrid layer containing the nonstoichiometric titanium compound and carbon.
- hydrophilic polymer examples include various polymers having a hydrophilic group such as a hydroxyl group (—OH), a carboxyl group (—COOH), an amino group (—NH 2 ), and the molecular weight thereof is, for example, a viscosity average molecular weight. Usually, it is 1000 or more.
- examples of the hydrophilic polymer include polyvinyl alcohol, polyacrylic acid and salts thereof, polyamide, polyacrylamide, cellulose derivatives, polyalkylene oxide, gelatin, and starch.
- water-soluble polymers are preferable, and for example, carboxymethyl cellulose, methyl cellulose, ethyl cellulose, polyvinyl alcohol, polyacrylic acid and salts thereof, polyacrylamide, polyethylene oxide, and the like are preferable. From the viewpoint of excellent handleability, it is particularly desirable to use polyvinyl alcohol.
- the viscosity average degree of polymerization of the polyvinyl alcohol used in the present invention is measured according to JIS K6726.
- the degree of polymerization of the PVA of the present invention is 200 to 5000.
- the viscosity average degree of polymerization is preferably 300 to 4500, more preferably 500 to 3500.
- the saponification degree of the polyvinyl alcohol used in the present invention is measured according to JIS K6726.
- the degree of saponification of polyvinyl alcohol is 50 to 99.99 mol%.
- the saponification degree is preferably 60 to 99.8 mol%, more preferably 70 to 96.50 mol%.
- the polyvinyl alcohol used in the present invention may be formed together with other modified units (copolymerizable monomers) as desired.
- copolymerizable monomers include ⁇ -olefins such as ethylene, propylene, 1-butene, isobutene, and 1-hexene; fumaric acid, maleic acid, itaconic acid, maleic anhydride, itaconic anhydride, and the like.
- the proportion of the modified unit relative to the vinyl alcohol unit varies depending on the purpose and use of the vinyl alcohol unit, but is usually a proportion based on all monomers used for copolymerization, and the proportion of the modified unit is 20 mol%. Hereinafter, it is preferably 10 mol% or less.
- the precursor is formed by spray drying the stock solution obtained in the dissolution step.
- Spray drying of the stock solution can be appropriately set according to the required shape of the precursor, but the stock solution is preferably spray-dried by using a spray dryer.
- the spray drying conditions include an inlet temperature of about 100 to 200 ° C., an outlet temperature of about 60 to 150 ° C., an injection pressure of about 50 to 150 kPa, and a hot air volume of 0.5 to 1.3 m 3 / min.
- the flow rate may be about 100 to 800 mL / h.
- the precursor obtained in the precursor forming step is fired in a reducing atmosphere or an inert atmosphere at a temperature of 600 ° C. or higher and 900 ° C. or lower for a predetermined time (eg, 6 to 48 hours).
- treatment under a reducing atmosphere means a mixed gas of argon / hydrogen
- under an inert atmosphere means treatment with air substituted with nitrogen or argon.
- the precursor obtained by spray drying may be used as it is in the firing step (main firing step), but in a reducing atmosphere or an inert atmosphere at a temperature in the range of 300 ° C. or more and less than 600 ° C.
- Pre-baking is preferably performed by heating for a predetermined time (for example, 1 to 6 hours, preferably 2 to 5 hours).
- Pre-firing can improve the charge / discharge efficiency provided by the resulting composite.
- non-stoichiometric titanium represented by the chemical formula Li 4 + x Ti 5-x O 12 (where 0 ⁇ x ⁇ 0.30) A complex of the compound and the carbon-containing material can be obtained.
- the lithium ion secondary battery which concerns on one Embodiment of this invention is equipped with the positive electrode, the negative electrode, the separator arrange
- the positive electrode may include at least a positive electrode current collector and a positive electrode active material
- the negative electrode may include at least a negative electrode current collector and a negative electrode active material.
- the lithium ion secondary battery according to the present invention includes the above-described non - stoichiometric titanium compound represented by the chemical formula Li 4 + x Ti 5 ⁇ x O 12 (where 0 ⁇ x ⁇ 0.30) and a carbon-containing material. This composite (non-stoichiometric titanium compound / carbon composite) is included as a negative electrode active material.
- the non-stoichiometric titanium compound / carbon composite of the present invention is required to stably maintain a high voltage over a long period of time, and to have a large charge / discharge capacity and safety, particularly as in a lithium ion secondary battery. It is also suitable for applications that require water resistance, oxidation resistance, handleability, safety and stability.
- FIG. 1 is a schematic sectional view of a lithium ion secondary battery of a coin type cell.
- the lithium ion secondary battery 1 includes a positive electrode composed of a positive electrode current collector layer 14 and a positive electrode active material layer 17, a negative electrode active material layer 16, and a negative electrode current collector inside a positive electrode can 11 having a gasket 18.
- a separator 15 holding an electrolytic solution as an electrolyte layer is provided between the negative electrode composed of the layer 13 and the positive electrode and the negative electrode.
- the positive electrode can 11 is covered with a negative electrode terminal 12, and the peripheral portions of the positive electrode can 11 and the negative electrode terminal 12 are sealed by caulking through an insulating gasket 18.
- the negative electrode active material layer 16 contains the non-stoichiometric titanium compound / carbon composite.
- the lithium ion secondary battery of this invention is not limited to the form of such a battery.
- a lithium ion secondary battery using a thin film solid electrolyte, a solution electrolyte, a gel electrolyte, a polymer electrolyte, or the like as the electrolyte may be used.
- Each of the positive electrode and the negative electrode may contain a binder, a conductive additive, a solvent, and the like as necessary.
- a binder for example, polyvinylidene difluoride, polyvinylidene fluoride, and polyacrylic acid (PAA) can be used, and among these, polyvinylidene difluoride is particularly preferable.
- acetylene black As the conductive assistant, acetylene black, graphite or the like can be used, and among them, acetylene black is particularly preferable.
- N-methyl-2-pyrrolidinone, N-ethyl-2-pyrrolidone, N-butyl-2-pyrrolidone, water and the like can be used, and among them, N-ethyl-2-pyrrolidinone is particularly preferable.
- the negative electrode current collector and the positive electrode current collector are coated with a metal foil such as copper, nickel, and stainless steel, a conductive polymer film such as polyaniline and polypyrrole, a carbon sheet, or a conductive polymer film.
- a metal foil or carbon sheet coated or coated can be used.
- the positive electrode active materials are spinel type lithium-manganese-nickel oxide (LiMn 1.5 N i0.5 O 4 ), lithium-cobalt oxide (LiCoO 2 ), lithium-nickel oxide (LiNiO 2 ), lithium-nickel Cobalt-manganese oxide (LiNi 1 / 3Mn 1 / 3Co 1 / 3O 2 ), lithium iron phosphate (LiFePO 4 ) can be used, and these may be used alone or in combination .
- methyl carbonate, propylene carbonate, dimethoxyethane, or the like can be used in addition to ethylene carbonate and dimethyl carbonate as a solvent.
- LiPF 4 or the like can be used in addition to LiPF 6 .
- TEM transmission electron microscope
- powder particles (1 g) were dispersed in ethanol (100 mL), ultrasonic treatment was performed at room temperature for 30 minutes, dropped onto a grid and dried at room temperature, and used as a sample for TEM observation.
- a JEM-2100F manufactured by JEOL was used as the apparatus used for the observation.
- the TEM photograph was taken under the conditions of an acceleration voltage of 200 kV and a magnification of 30000 to 100000 times depending on the observation object of the sample.
- the thickness of the hybrid layer was measured at five measurement points randomly selected from the hybrid layer, and the average value thereof was calculated as the thickness of the hybrid layer.
- the average particle size of the composite particles is calculated by measuring the widest width of three randomly selected non-stoichiometric titanium compound / carbon composite particles as the particle size, and calculating the average value as the average particle size. did.
- C ratio of particle surface layer (TEM-EDX)> The Ti: C ratio of the particle surface layer was measured by TEM-EDX analysis.
- the apparatus used for EDX analysis is JED-2300 manufactured by JEOL Ltd. At least 10 measurement points on the particle surface layer were analyzed to calculate the Ti: C ratio.
- the 20 nm ⁇ 20 nm area of the particle surface layer was irradiated with an electron beam, an EDX spectrum was collected, and a Ti: C ratio was calculated.
- Ti: C inside the particle (core) was analyzed by analyzing in the same manner as the surface layer except that the electron beam was irradiated toward the central part of the composite particle and calculated from at least three measurement points. .
- R2032 coin type cell was produced as the lithium ion secondary battery 1 which concerns on embodiment of this invention.
- the electrode was produced as follows.
- the negative electrode active material obtained in Examples or Comparative Examples, a binder (polyvinylidene difluoride), and a conductive additive (acetylene black) are mixed at a weight ratio of 88: 6: 6 (wt%), N-methyl-2-pyrrolidinone was added as a solvent and kneaded to form a slurry. This was apply
- a lithium metal secondary foil was used using a metallic lithium foil for the counter electrode, 1 moldm-3LiPF 6 / ethylene carbonate + dimethyl carbonate (mixing ratio: 30/70 vol%) for the electrolyte, and Celgard (registered trademark) # 2325 for the separator.
- a battery was produced.
- the lithium ion secondary battery was manufactured in a glove box substituted with argon.
- the negative electrode active material according to the present invention corresponds to the positive electrode in the evaluation cell, but is referred to as a negative electrode active material here for convenience.
- the present Example showed about the case where electrolyte solution is used, you may make it use other electrolyte salt.
- examples of other electrolytes that can be used include ion conductive ceramics, ion conductive glass, and ion crystalline inorganic solid electrolytes.
- Example 1 Synthesis of non-stoichiometric titanium compound / carbon composite A-1> Synthesis of a carbon composite obtained by subjecting a non-stoichiometric titanium compound Li 4 + x Ti 5-x O 12 (where 0 ⁇ x ⁇ 0.30) to carbon composite treatment as a pretreatment, and The lithium ion secondary battery used as the material was manufactured as follows.
- the non-stoichiometric titanium compound / carbon composite is expressed as “Li 4 + x Ti 5-x O 12 / C”.
- Li 4 + x Ti 5-x O 12 / C (where 0 ⁇ x ⁇ 0.30) was carried out as follows. First, 3.18 g of lithium carbonate and 29.31 g of tetraisopropoxytitanium are added to distilled water (300 ml) so that a non-stoichiometric titanium compound having a Li / Ti ratio in the above range is obtained, and the solution is stirred. It was. By this blending, a nonstoichiometric titanium compound having a composition of Li 4.2 Ti 4.8 O 12 is obtained.
- PVA-1 polyvinyl alcohol having a viscosity average polymerization degree of 1700 and a saponification degree of 98.5 mol% was added to distilled water (100 ml), and the mixture was stirred and dissolved. Added and mixed with stirring. Next, the obtained Li / Ti solution was spray-dried using a spray dryer to obtain a precursor. At this time, the spray drying conditions were an inlet temperature: 160 ° C., an outlet temperature: 100 ° C., an injection pressure: 100 kPa, a hot air amount: 0.70 m 3 / min, and a flow rate: 400 mL / h.
- FIG. 2A shows a TEM photograph of the composite particles constituting A-1
- FIG. 2B shows a TEM photograph of the vicinity of the composite particle surface layer part
- TEM used for composition analysis of the composite layer formed on the particle core and the particle surface layer.
- Carbon composite Li 4 + x Ti 5 ⁇ x O 12 / C of the obtained non-stoichiometric titanium compound (however, 0 ⁇ x ⁇ 0.30 range), average particle diameter of composite particles, surface layer state of particles (Coating state and Ti: C) are shown in Table 1.
- Example 2 ⁇ Synthesis of A-2 and battery using it> Similar to Example 1 except that 1.87 g of polyvinyl alcohol (PVA-2) having a viscosity average polymerization degree of 1700 and a saponification degree of 88.5 mol% was used instead of PVA-1 used in Example 1 above.
- PVA-2 polyvinyl alcohol
- Table 1 shows the specific surface area of the obtained nonstoichiometric titanium compound / carbon composite A-2, the average particle diameter of the composite particles, and the surface layer state (covering state and Ti: C) of the particles.
- Table 1 also shows the results of evaluating the charge / discharge characteristics using the obtained carbon composite of the non-stoichiometric titanium compound as the negative electrode active material.
- FIG. 4A shows a TEM photograph of the particles constituting A-3
- FIG. 4B shows a TEM photograph near the surface layer of the particles.
- Table 1 shows the specific surface area of the obtained nonstoichiometric titanium compound A-3, the average particle diameter of the nonstoichiometric titanium compound particles, and the surface layer state of the particles.
- Table 1 also shows the results of evaluating the charge / discharge characteristics using the obtained non-stoichiometric titanium compound A-3 as the negative electrode active material.
- Example 2 (Comparative Example 2) ⁇ Synthesis of A-4 and battery using the same>
- the carbon composite A-4 of the nonstoichiometric titanium compound was prepared under the same conditions except that 2.43 g of sucrose (reagent special grade manufactured by Wako Pure Chemical Industries, Ltd.) was used instead of PVA-1.
- FIG. 5 shows a TEM photograph of the particles constituting A-4
- FIGS. 6A and 6B show the results of TEM-EDX analysis used for the composition analysis of the core portion of the particles and the hybrid layer.
- Table 1 shows the average particle diameter, specific surface area, and surface layer state (covered state and Ti: C) of the carbon composite A-4 particles of non-stoichiometric titanium compound obtained.
- Table 1 also shows the results of evaluation of charge / discharge characteristics using the obtained carbon composite A-4 of non-stoichiometric titanium compound as the negative electrode active material.
- Example 1 In both Example 1 and Comparative Example 2, a single phase was observed in the core portion occupying most of the particles with the same contrast as in Comparative Example 1 in which no carbon source was used, and the core was substantially a non-stoichiometric titanium compound. It was confirmed to consist of
- the outer layer portion is composed of a hybrid layer containing carbon and a non-stoichiometric titanium compound constituting the core.
- the nonstoichiometric titanium compound in which no carbon source is introduced as a pretreatment does not have a mixed layer of titanium and carbon in its surface layer. And in the lithium ion secondary battery using this non-stoichiometric titanium compound, sufficient capacity is not exhibited especially at a high rate such as 10C. Further, as shown in Comparative Example 2, even if a carbon source is introduced, if the carbon source is not a hydrophilic polymer, the composite particle surface layer has a specific ratio of Ti / C ratio. Since the hybrid layer is not formed, the capacity at 10 C is not sufficiently developed as in Comparative Example 1.
- a hybrid layer was formed on the surface layer of the non-stoichiometric titanium compound by forming a non-stoichiometric titanium compound / carbon composite using a hydrophilic polymer as a carbon source.
- the hybrid layer of the composites of Examples 1 and 2 was formed substantially uniformly with a thickness of about 20 nm on the surface layer.
- the non-stoichiometric titanium compound / carbon composite of the present invention can be used as an electrode active material, and particularly useful as an electrode active material for a lithium ion secondary battery.
- Lithium ion secondary batteries using this can be used not only in the same usage pattern as batteries normally used as a power source for general devices, but also can be mounted on IC cards, small medical devices, etc.
- As a thin and small lithium-ion secondary battery as a lithium-ion secondary battery for mobile devices such as mobile phones, notebook computers, digital cameras, and portable games, and for larger devices, for hybrid vehicles, It can be used as a lithium ion secondary battery for automobiles and the like.
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Abstract
Description
リチウムイオン二次電池を安定した品質で供給するためには、負極活物質として用いられる不定比チタン化合物炭素複合体は、粒子の粗大化が抑制され、粒子の分散性にすぐれることが好ましい。また、粒子表面の電子電導度のばらつきが少ないとともに、不定比チタン化合物と電解質の間でのリチウムイオンの移動も阻害されないことが望ましい。
前記不定比チタン化合物からなるコア部分と、前記コア部分の表面上に形成された、不定比チタン化合物と炭素を含有する混成層(外層部)を有する、少なくとも一個の複合体粒子(複合体ユニット粒子)からなり、
前記混成層中のチタン原子数と炭素原子数との比が、Ti/C=1/50以上の範囲である、不定比チタン化合物・炭素複合体である。
水性溶媒に対して、所定量の親水性高分子と、リチウム塩と、チタニウムアルコキシドとを加え、これらを攪拌して分散させながら溶解する溶解工程と、
前記溶解工程で得られた原液を噴霧乾燥させることにより前駆体を得る前駆体形成工程と、
前記前駆体を還元雰囲気中でまたは不活性雰囲気中で、600℃以上900℃以下の温度で所定時間、炉で焼成する焼成工程と、
を少なくとも備える前記不定比チタン化合物・炭素複合体の製造方法である。
本発明の不定比チタン化合物・炭素複合体は、化学式Li4+xTi5-xO12(但し、0<x<0.30の範囲)で示される不定比チタン化合物と、炭素含有物質との複合体である。炭素含有物質は、実質的に炭素からなるものであってもよいが、炭素および炭素と他の元素の化合物を含むものであってもよい。
前記不定比チタン化合物・炭素複合体は、不定比チタン化合物からなるコア部分と、前記コア部分の表面上に形成された、不定比チタン化合物と炭素を含有する混成層(ハイブリッド層)を有する複合体粒子(複合体ユニット粒子)からなる。不定比チタン化合物・炭素複合体は、複合体粒子単体からなるものであってもよく、複数の複合体粒子の凝集体(aggregate)からなるものであってもよい。
混成層中のチタン原子数と炭素原子数との比(atomic ratio)は、Ti/C=1/50以上の範囲である。混成層におけるチタン原子数と炭素原子数との比が、Ti/C=1/50以上である場合、詳細なメカニズムは定かではないが、不定比チタン化合物にリチウムイオンが入ったり(充電)、出たりする(放電)際に、抵抗が少なくなることにより、効率良い充放電を行うことができるのではないかと推測される。
混成層中のチタン原子数と炭素原子数との比は、Ti/C=1/2以下であることが好ましい。原子数比Ti/Cが1/2より大きい場合、炭素を含有する被膜(混成層)中でのイオンあるいは電子の伝導が妨げられ、電気伝導度が低下して所望の電気特性が得られない場合がある。
より好ましくは、Ti/Cは1/30~1/4程度である。
なお、混成層の有無は、複合体の透過型電子顕微鏡(TEM)観察および、エネルギー分散型X線分析(EDX分析)により確認することが可能である。
また混成層の厚みは、1nm以上であることが好ましい。
混成層の厚みは、複合体粒子の透過型電子顕微鏡観察により、計測できる。
前記不定比チタン化合物・炭素複合体は、以下の製造方法により製造することが可能である。
本発明にかかる不定比チタン化合物・炭素複合体の製造方法は、
水性溶媒に対して、所定量の親水性高分子と、リチウム塩と、チタニウムアルコキシドとを加え、これらを攪拌して分散させながら溶解する溶解工程と、
前記溶解工程で得られた原液を噴霧乾燥させることにより前駆体を得る前駆体形成工程と、
前記前駆体を、還元雰囲気中または不活性雰囲気中、600~900℃で所定時間焼成する焼成工程と、
を少なくとも備えている方法である。
前記の親水性高分子と、リチウム塩と、チタニウムアルコキシドの配合比率は特に制限はないが、チタニウムアルコキシドの100質量部に対して、リチウム塩が1~50質量部、親水性高分子が1~30質量部であることが好ましい。
溶解工程では、水性溶媒に対して、所定量の親水性高分子と、リチウム塩と、チタニウムアルコキシドとを加え、これらを攪拌して分散させながら溶解し、前駆体を形成するための原液を調製する。溶解工程においては、リチウム塩と、チタニウムアルコキシドを溶解した第一の溶液と、親水性高分子の溶液を溶解した第二の溶液を作成し、その後、第一の溶液と第二の溶液を混合してもよい。
これらのうち、水溶性高分子が好ましく、例えば、カルボキシメチルセルロース、メチルセルロース、エチルセルロース、ポリビニルアルコール、ポリアクリル酸およびその塩、ポリアクリルアミド、ポリエチレンオキサイドなどが好ましい。
取扱い性に優れる観点から、特にポリビニルアルコールを使用することが望ましい。
前駆体形成工程では、溶解工程で得られた原液を噴霧乾燥させることにより前駆体を形成する。原液の噴霧乾燥は、求められる前駆体の形状に応じて適宜設定することが可能であるが、好ましくは、スプレードライヤーを用いることにより原液を噴霧乾燥する。
噴霧乾燥の際、例えば、スプレードライ条件としては、入口温度が100~200℃程度、出口温度が60~150℃程度、噴射圧力50~150kPa程度、熱風量0.5~1.3m3/min程度、流量100~800mL/h程度であってもよい。
焼成工程では、前駆体形成工程で得られた前駆体を、還元雰囲気中または不活性雰囲気中、600℃以上、900℃以下の温度で所定時間(例えば、6~48時間)焼成する。
このとき、還元雰囲気下とはアルゴン/水素の混合気体、不活性雰囲気下とは窒素又はアルゴンにより置換された空気で処理することを示す。
この焼成工程(本焼成工程、または予備焼成工程及び本焼成工程)を経ることにより、化学式Li4+xTi5-xO12(但し、0<x<0.30の範囲)で示される不定比チタン化合物と炭素含有物質との複合体を得ることができる。
本発明の一実施形態に係るリチウムイオン二次電池は、正極と、負極と、前記正極と前記負極との間に配置されたセパレータと、電解質とを備えている。
正極は、正極集電体および正極活物質を少なくとも備えていてもよく、負極は、負極集電体および負極活物質を少なくとも備えていてもよい。
ここで、本発明のリチウムイオン二次電池は、前述した、化学式Li4+xTi5-xO12(但し、0<x<0.30の範囲)で示される不定比チタン化合物と炭素含有物質との複合体(不定比チタン化合物・炭素複合体)を負極活物質として含んでいる。
図1は、コインタイプセルのリチウムイオン二次電池の概略断面図を示す。リチウムイオン二次電池1は、ガスケット18を備えた正極缶11の内部に、正極集電体層14および正極活物質層17とで構成された正極と、負極活物質層16および負極集電体層13とで構成された負極と、前記正極と負極との間に、電解質層として電解液を保持したセパレータ15を備えている。そして、この正極缶11は、負極端子12により覆われ、正極缶11及び負極端子12の周縁部は、絶縁ガスケット18を介してかしめられることにより密閉されている。負極活物質層16には、前記不定比チタン化合物・炭素複合体が含まれている。
なお、本発明のリチウムイオン二次電池は、このような電池の形態に限定されない。例えば、電解質として、薄膜固体電解質、溶液状の電解質、ゲル状電解質、ポリマー電解質等を用いたリチウムイオン二次電池であってもよい。
正極および負極は、それぞれ必要に応じて、結着剤、導電助剤、溶剤などを含んでいてもよい。
結着剤としては、例えば、ポリビニリデンジフルオリド、ポリフッ化ビニリデン、ポリアクリル酸(PAA)を用いることができ、その中でも、特にポリビニリデンジフルオリドが好ましい。
電解質塩としては、LiPF6の他、LiBF4等を用いることができる。
不定比チタン化合物・炭素複合体の比表面積は、窒素吸着測定(Brunaur Emmett Teller法)を用いて評価した。測定には高純度ガス/蒸気吸着装置(日本ベル(株)製、BELSORP-mini)を用いた。前処理として、試料を80℃で12時間減圧乾燥させた後、さらにBELSORP-miniに付属の前処理を行った。測定は、吸着質をN2として、吸着温度77Kで行った。
不定比チタン化合物・炭素複合体を構成する複合体粒子の平均粒子径と粒子表層状態の評価には、透過型電子顕微鏡(TEM)を用いて評価した。前処理として、エタノール(100mL)中に粉末粒子(1g)を分散させ、超音波処理を室温で30分処理して、グリッドに滴下して室温で乾燥させ、TEM観察用試料とした。観察に用いた装置は日本電子製のJEM-2100Fを用いた。TEM写真撮影時の条件は加速電圧200kV、倍率は、試料の観察対象に応じて、30000~100000倍で観察を行った。
粒子表層のTi:C比はTEM-EDX分析により計測した。EDX分析に用いた装置は日本電子(株)製JED-2300である。粒子表層の少なくとも10箇所の測定点を分析しTi:C比を算出した。EDX分析に際しては粒子表層の20nm×20nmエリアに電子線を照射し、EDXスペクトルを収集し、Ti:C比を算出した。
また、粒子内部(コア)のTi:Cは、複合体粒子の中心部分に向けて電子線を照射し、少なくとも3箇所の測定点から算出する以外は、表層と同様に分析することにより行った。
図1に示すように、本発明の実施形態に係るリチウムイオン二次電池1として、R2032コインタイプセルを作製した。電極の作製は次のように行った。実施例または比較例で得られた負極活物質と、結着剤(ポリビニリデンジフルオリド)と、導電助剤(アセチレンブラック)とを88:6:6(wt%)の重量比で混合し、溶剤としてN-メチル-2-ピロリジノンを加え混練後、スラリー状にした。これを負極集電体であるアルミ箔上に塗布して、ロールプレス機を用いて室温でプレスした。
なお、本実施例は電解液を用いる場合について示したが、ほかの電解質塩を用いるようにしてもよい。他の電解質としては、イオン伝導性セラミックス、イオン伝導性ガラス、イオン結晶性の無機固体電解質等を用いることができる。
上述の<充放電特性評価用のコインタイプセルの作製方法>に従って得られたコインタイプセルの充放電評価を行った。評価条件は、電圧範囲1.2~3.0V、測定温度25℃において、各電流密度0.1C、3C、10C(1C=175mA/g)における容量を測定した。
<不定比チタン化合物・炭素複合体A-1の合成>
不定比チタン化合物Li4+xTi5-xO12(但し、0<x<0.30の範囲)において、前処理として炭素複合処理を施すことにより得られる炭素複合体の合成、及びこれを負極活物質として用いたリチウムイオン二次電池は、以下のようにして製造した。なお、不定比チタン化合物・炭素複合体を「Li4+xTi5-xO12/C」と表記する。
<A-2の合成とそれを用いた電池>
上述の実施例1で使用したPVA-1に代えて、粘度平均重合度1700、けん化度88.5モル%のポリビニルアルコール(PVA-2)1.87gを用いた以外は、実施例1と同様の条件で、不定比チタン化合物の炭素複合体A-2を作製した。
表1に得られた不定比チタン化合物・炭素複合体A-2の比表面積、複合体粒子の平均粒子径、粒子の表層状態(被覆状態とTi:C)を示す。
また、得られた不定比チタン化合物の炭素複合体を負極活物質として使用し、充放電特性を評価した結果についても表1に示す。
<A-3の合成とそれを用いた電池>
上述の実施例1において、PVA-1を添加しないこと以外は同じ条件で不定比チタン化合物A-3を作製した。A-3を構成する粒子のTEM写真を図4Aに、粒子の表層付近のTEM写真を図4Bに示す。
表1に、得られた不定比チタン化合物A-3の比表面積、不定比チタン化合物粒子の平均粒子径、および粒子の表層状態を示す。
また、得られた不定比チタン化合物A-3を負極活物質として使用し、充放電特性を評価した結果についても表1に示す。
<A-4の合成とそれを用いた電池>
上述の実施例1において、PVA-1に代えてスクロース2.43g(和光純薬工業(株)製試薬特級)を用いた以外は同じ条件で、不定比チタン化合物の炭素複合体A-4を作製した。A-4を構成する粒子のTEM写真を図5に、粒子のコア部分とハイブリッド層の組成分析に用いたTEM-EDX解析の結果を図6A、6Bに示す。
表1に得られた不定比チタン化合物の炭素複合体A-4粒子の平均粒子径、比表面積、および粒子の表層状態(被覆状態とTi:C)を示す。
また、得られた不定比チタン化合物の炭素複合体A-4を負極活物質として使用し、充放電特性を評価した結果についても表1に示す。
また、比較例2に示すように、たとえ炭素源を導入していても、その炭素源が、親水性高分子ではない場合、複合体の粒子表層に、特定の割合のTi/C比を有する混成層が形成されないため、比較例1と同様に10C時の容量は充分に発現しない。
これを用いたリチウムイオン二次電池は、一般的なデバイスの電源として通常使用される電池と同じような使用形態で用いることができるだけでなく、例えば、ICカード、医療用小型機器等に搭載可能な薄型・小型のリチウムイオン二次電池として、また携帯電話、ノートパソコン、デジタルカメラ、携帯型ゲーム等のモバイル機器へのリチウムイオン二次電池として、さらに大型の機器としては、ハイブリッド車両用、電気自動車などへのリチウムイオン二次電池として利用することができる。
11 正極缶
12 負極端子
13 負極集電体層
14 正極集電体層
15 電解液を保持したセパレータ
16 負極活物質層
17 正極活物質層
18 ガスケット
Claims (10)
- 化学式Li4+xTi5-xO12(但し、0<x<0.30の範囲)で示される不定比チタン化合物と炭素含有物質との複合体であって、
前記不定比チタン化合物からなるコア部分と、前記コア部分の表面上に形成された、不定比チタン化合物と炭素を含有する混成層を有する、少なくとも一個の複合体粒子からなり、
前記混成層中のチタン原子数と炭素原子数との比が、Ti/C=1/50以上の範囲である、不定比チタン化合物・炭素複合体。 - 混成層が、複合体粒子の表層全体にわたって存在している、請求項1記載の不定比チタン化合物・炭素複合体。
- 混成層が、100nm以下の範囲の厚みで形成されている請求項1または2に記載の不定比チタン化合物・炭素複合体。
- 不定比チタン化合物・炭素複合体は、BET法で測定される比表面積が20~100m2g-1である、請求項1~3のいずれか一項に記載の不定比チタン化合物・炭素複合体。
- 前記複合体粒子の平均粒子径は20μm以下である、請求項1~4のいずれか一項に記載の不定比チタン化合物・炭素複合体。
- 化学式Li4+xTi5-xO12(但し、0<x<0.30の範囲)で示される不定比チタン化合物・炭素複合体の製造方法であって、
水性溶媒に対して、親水性高分子と、リチウム塩と、チタニウムアルコキシドとを加え、これらを攪拌して分散させながら溶解する溶解工程と、
前記溶解工程で得られた原液を噴霧乾燥させることにより前駆体を得る前駆体形成工程と、
前記前駆体を還元雰囲気中でまたは不活性雰囲気中で、600℃以上900℃以下の温度で焼成する焼成工程と、
を少なくとも備える請求項1~5のいずれか一項に記載の不定比チタン化合物・炭素複合体の製造方法。 - 請求項6記載の方法において、前記焼成工程の前に、前記前駆体を還元雰囲気中または不活性雰囲気中において、300℃以上600℃未満の温度で焼成する工程をさらに備える、不定比チタン化合物・炭素複合体の製造方法。
- 請求項6または7記載の方法において、前記親水性高分子として、ポリビニルアルコールを用いる、不定比チタン化合物・炭素複合体の製造方法。
- 請求項1~5のいずれか一項に記載の不定比チタン化合物・炭素複合体を含む、負極活物質。
- 正極と、負極と、前記正極と前記負極との間に配置されたセパレータと、電解質とを備えるリチウム二次電池であって、前記負極は、請求項9に記載の負極活物質を含む、リチウムイオン二次電池。
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