Patent Application
20140093779
The present invention is one which relates to a process for producing a negative-electrode active material for lithium-ion secondary battery and to a negative-electrode active material that is produced by this production process, as well as to a process for manufacturing a lithium-ion secondary battery in which this negative-electrode active material is used and to a lithium-ion secondary battery in which this negative-electrode active material is used.
Since lithium-ion secondary batteries have a smaller size and a larger capacity, they have been used widely as a secondary battery for cellular phones, notebook-size personal computers, and so forth. Recently, proposals have also been made for intents and purposes for their use as a battery for electric automobiles, hybrid automobiles, and so on.
A lithium-ion secondary battery possesses an active material, which can insert (or sorb) lithium (Li) into itself and eliminate (or desorb) the same from itself, in the positive electrode and negative electrode, respectively. The lithium-ion secondary battery operates by means of the migrations of lithium ions between the two electrodes.
It has been required to make lithium-ion secondary batteries exhibit higher capacities, and to make them produce higher outputs. In order to upgrade these characteristics, using Li4Ti5O12 with a spinel structure as a negative-electrode active material has been proposed (see Patent Literature No. 1, for instance). However, the capacity of Li4Ti5O12 is not so large, because it is 170 mAh/g approximately. Consequently, negative-electrode active materials, which furthermore enable lithium-ion secondary batteries to exhibit higher capacities, have been desired. Among negative-electrode active materials including titanium (Ti), TiO2 has been getting a great deal of attention recently, because it is inexpensive, and because it is feasible to charge and discharge it at a faster speed.
As for the structures of TiO2, a plurality of them, such as a rutile type and an anatase type, have been known. Among these, anatase-type TiO2 has been said to be better in terms of the reversibility comparatively, and to have higher capacities. However, it has been difficult to manufacture high-capacity and rapidly-chargeable/dischargeable lithium-ion secondary batteries by only using anatase-type TiO2 simply as a negative-electrode active material.
The present invention is one which has been done in view of the circumstances being aforementioned. It is therefore an object to provide a negative-electrode active material for lithium-ion secondary battery, negative-electrode active material which includes Ti and which enables lithium-ion secondary batteries to furthermore exhibit higher capacities and to be chargeable and dischargeable at a faster speed when being used as the negative-electrode active material, and to provide a process for producing the same; as well as to provide a lithium-ion secondary battery that uses the present negative-electrode active material, and to provide a process for manufacturing the same.
The inventors of the present invention found out, as a result of their earnest studies, that lithium-ion secondary batteries, which have higher capacities and which are chargeable and dischargeable at a faster speed, are obtainable by fluorinating anatase-type TiO2 with use of hydrofluoric acid, and then using the thus obtained oxidized titanium fluoride as a negative-electrode active material.
Specifically, a process according to the present invention, which solves the aforementioned assignment, for producing a negative-electrode active material for lithium-ion secondary battery is characterized in that:
the process is equipped with a heating step of obtaining oxidized titanium fluoride by heating a mixed raw material, which includes a mixture of anatase-type TiO2 and hydrofluoric acid, at 70° C. or more; and
the mixed raw material includes hydrogen fluoride (HF) in an amount exceeding 2 mol per the anatase-type TiO2 making 1 mol.
A negative-electrode active material for lithium-ion secondary battery according to the present invention that solves the aforementioned assignment is characterized in that:
the negative-electrode active material includes titanium (Ti), and fluorine (F); and
a content of fluorine (F) per 1-mol titanium (Ti) exceeds 1 mol.
Moreover, another negative-electrode active material for lithium-ion secondary battery according to the present invention that solves the aforementioned assignment is characterized in that:
the negative-electrode active material is produced by the process for producing a negative-electrode active material for lithium-ion secondary battery according to the present invention;
the negative-electrode active material includes titanium (Ti), and fluorine (F); and
a content of fluorine (F) per 1-mol titanium (Ti) exceeds 1 mol.
Moreover, a lithium-ion secondary battery according to the present invention that solves the aforementioned assignment is characterized in that the lithium-ion secondary battery includes one of the negative-electrode active materials for lithium-ion secondary battery according to the present invention in a negative electrode.
Moreover, a process for manufacturing a lithium-ion secondary battery according to the present invention that solves the aforementioned assignment is characterized in that the oxidized titanium fluoride, which is produced by the process for producing a negative-electrode active material for lithium-ion secondary battery according to the present invention, is used as a negative-electrode active material.
Hereinafter, the process for producing a negative-electrode active material for lithium-ion secondary battery according to the present will be simply abbreviated to as a “production process” according to the present invention. Moreover, the negative-electrode active materials for lithium-ion secondary battery according to the present invention will be simply abbreviated to as “negative-electrode active materials” according to the present invention.
In accordance with the production process according to the present invention, it is possible to produce negative-electrode active materials that enable lithium-ion secondary batteries to exhibit higher capacities, and which enable them to be chargeable and dischargeable at a faster speed. Moreover, in accordance with the negative-electrode active materials according to the present invention, it is possible to make lithium-ion secondary batteries exhibit higher capacities, and to make them chargeable and dischargeable at a faster speed. In addition, it is feasible for the lithium-ion secondary battery according to the present invention to have higher capacities, and to be capable of charging and discharging at a faster speed.
In a production process according to the present invention, anatase-type TiO2, and hydrofluoric acid (i.e., an aqueous solution of hydrogen fluoride) are used. Hereinafter, unless otherwise specified, the term, anatase-type TiO2, will be simply abbreviated to as “TiO2.”
As for the anatase-type TiO2, it is allowable to use those which have been produced by known processes. For example, it has been known that the anatase-type TiO2 can be produced by processes which are called a uniform precipitation method and hydrothermal method. The uniform precipitation method is a process for causing a precipitate to generate uniformly in a reaction liquid entirely by means of chemical reactions, and the following have been known: positive-ion emission methods (e.g., oxidation-reduction methods, and complex decomposition methods), negative-ion emission methods (e.g., urea hydrolysis methods, amide hydrolysis methods, ester hydrolysis methods, and oxidation-reduction methods), and the like. Among these, since a urea decomposition method makes it possible to produce fine anatase-type TiO2 with 1 μm-or-less particle diameters, it is suitable for producing the anatase-type TiO2 that makes a raw material of the negative-electrode active material. For reference, urea hydrolysis methods are respectively a process for causing an objective substance to precipitate by heating urea to do the hydrolysis in order to cause ammonia to generate in the reaction liquid, and then gently changing a pH of the reaction liquid. Note that the anatase-type TiO2 being used in the present invention is not at all limited to those which are produced by this process, but it is even permissible that the anatase-type TiO2 can also be those which are produced by any other processes.
Although it is allowable that the hydrofluoric acid can be capable of fluorinating the anatase-type TiO2, a preferable range exists for a concentration of hydrogen fluoride in the mixed raw material as described later. To be concrete, it is permissible that hydrogen fluoride (HF) in the mixed raw material can be included in an amount exceeding 2 mol per TiO2 that makes 1 mol. When the amount of hydrogen fluoride in the mixed raw material falls within this range, TiOF2 can be fluorinated sufficiently, so that it is possible to obtain oxidized titanium fluoride. Note that it is preferable that hydrogen fluoride (HF) in the mixed raw material can make 2 mol or more per TiO2 that makes 1 mol, and it is more preferable that it can make 10 mol or more. Oxidized titanium fluoride being obtained by this process has particulate shapes. It is preferable that particle diameters of particles of oxidized titanium fluoride serving as the negative-electrode active material can be the smaller the better. When particle diameters of the negative-electrode active material are smaller, it is possible to give faster-speed chargeable/dischargeable characteristics to lithium-ion secondary batteries, because the migration distance of Li can be managed to be shorter at the time of charging and discharging them. Preferably, it is advisable that the resulting oxidized titanium fluoride can have fine particulate shapes with particulate diameters of from 1 nm to 50 nm approximately.
A negative electrode for lithium-ion secondary battery according to the present invention includes the negative-electrode active material being described above. This negative electrode has a current collector, and an active-material layer being bound on the current collector. Other than the negative-electrode active material, the active-material layer is able to include known materials, such as conductive additives and binder resins, which constitute negative-electrode materials. It is possible to fabricate the negative electrode in a lithium-ion secondary battery according to the present invention by means of the following steps: coating one, which have been turned into a slurry by adding an organic solvent to these materials and then mixing them one another, onto the negative-electrode active material by such a method as roll coating methods, dip coating methods, doctor blade methods, spray coating methods and curtain coating methods; and then causing the binder resin to cure.
As for the current collector, although it is possible to employ such a configuration as foils and plates, it is not limited especially at all as far as it has a configuration that complies with purposes. As the current collector, copper foils, aluminum foils, and the like, can be used, for instance.
A conductive additive is added in order to enhance the conductivity of electrode. As the conductive additive, it is possible to add one of the following independently, or to combine two or more members of the following to add: carbon black (or CB), acetylene black (or AB) and KETJENBLACK (or KB), namely, carbonaceous fine particles; or gas-phase-method carbon fibers (or vapor grown carbon fibers (or VGCF)), and so forth. Although it is not restrictive especially at all as to an employed amount of the conductive additive, it can be set in general at from 20 to 100 parts by mass with respect to 100 parts by mass of the negative-electrode active material.
The binder resin is used as a binding agent for binding the negative-electrode active material and conductive additive onto the current collector. It is required that the binder resin fasten the negative-electrode active material, and so on, in an amount as small as possible. It is preferable that a compounded amount of the binder resin can be from 0.5 to 50% by mass when a summed amount of the negative-electrode active material, conductive additive and binder resin is taken as 100% by mass. When an amount of the binder resin is less than 0.5% by mass, the formability of the resulting electrode declines; whereas, when it exceeds 50% by mass, the energy density of the resultant electrode becomes lower. Although a type of the binder resin is not restrictive at all, the following can be exemplified: fluorine-based polymers, such as polyvinylidene fluoride (or PVDF) and polytetrafluoroethylene (or PTFE); rubbers, such as stylene-butadiene rubber (or SBR); imide-based polymers, such as polyimide; alkoxysilyl group-containing resins; polyacrylic acids (or PAA); polymethacrylic acids; polyitaconic acids, and the like.
It is possible for a lithium-ion secondary battery according to the present invention using the negative electrode being aforementioned to use a known positive electrode, electrolytic solution, and separator that are not limited especially at all. It is allowable that the positive electrode can be those which are employable in lithium-ion secondary batteries. The positive electrode has a current collector, and a positive-electrode active-material layer being bound on the current collector. It is even permissible that the positive-electrode active-material layer can include a positive-electrode active material, and a binder; and can further include a conductive additive as well. The positive-electrode active material, conductive additive and binder are not limited especially at all, and it is advisable that they can be those which are employable in lithium-ion secondary batteries.
As for the positive-electrode active material, the following can be given: Li“M”O2 (note, however, that “M” is at least one member being selected from the group consisting of Ni, Co and Mn, and that, in a case where “M” includes a plurality of members being selected from this group, a sum of them makes 1); spinel-type lithium manganese oxides being expressed by Li1+xMn2-x-y“M”yO4; LiFePO4; LiMnPO4, and so forth. For the current collector, it is allowable to use those which have been common as a current collector for the positive electrode of lithium-ion secondary batteries. For the conductive additive and binder, it is possible to use the same constituent elements as those which have been set forth in the above-mentioned negative electrode.
Although the electrolytic solution is not limited especially at all, it is preferable to use those in which an Li metallic salt, namely, an electrolyte, has been caused to dissolve in an organic solvent. As the organic solvent, it is possible to use one or more members being selected from nonprotonic organic solvents, such as propylene carbonate (or PC), ethylene carbonate (or EC), dimethyl carbonate (or DMC), diethyl carbonate (or DEC) and ethyl methyl carbonate (EMC). Moreover, as for an electrolyte to be caused to dissolve, it is possible to use Li metallic salts being soluble in organic solvents, such as LiPF6, LiBF4, LiAsF6, LiI, LiClO4 and LiCF3SO3.
For example, it is possible to employ a solution in which an Li metallic salt, such as LiClO4, LiPF6, LiBF4 or LiCF3SO3, has been caused to dissolve in an organic solvent, such as ethylene carbonate, dimethyl carbonate, propylene carbonate or diethyl carbonate, in a concentration of from 0.5 mol/L to 1.7 mol/L approximately.
The separator is not limited especially at all as far as it can be those which can be employed for lithium-ion secondary batteries. The separator is one which separates a positive electrode from a negative electrode or vice versa, and retains an electrolytic solution therein. It is possible to use thin microporous films, such as polyethylene and polypropylene, therefor.
The lithium-ion secondary battery according to the present invention is not limited especially at all in terms of the configuration, so that it is possible to employ a variety of configurations, such as cylindrical types, laminated types and coin types. Even in a case where any one of the configurations is adopted, a battery is made as follows: the separators are interposed between the positive electrodes and the negative electrodes, thereby making electrode assemblies; and then these electrode assemblies are sealed hermetically in a battery case along with the electrolytic solution after connecting intervals to and from the positive-electrode terminals and negative-electrode terminals, which lead to the outside from the resulting positive-electrode current collectors and negative-electrode current collectors, with use of leads for collecting current, and the like.
Hereinafter, the present invention will be explained in more detail while giving a specific example.
A mixture, in which distilled water, titanium tetrachloride, urea, ammonium sulfate and ethanol had been mixed one another in a ratio of 4:0.99:1:0.01:4 by mass, was stirred in an ice bath for 2 hours.
The post-stirring mixture was subjected to a hydrothermal treatment at 120° C. for 5 hours. After the hydrothermal treatment, the resulting solid contents were washed with distilled water, and were then dried at 80° C. for 12 hours, thereby obtaining anatase-type TiO2. The obtained anatase-type TiO2 was nanometer-size particles whose average particle diameter was about 30 nm approximately.
The TiO2 nanometer-size particles being obtained by the above-mentioned step, and a 46%-by-mass HF solution were mixed one another in a ratio of TiO2:HF=1:10 by mol, thereby obtaining a mixed raw material. This mixed raw material was stirred at 80° C. for 24 hours. The post-stirring mixed raw material was heated at 80° C. under reduced pressure (i.e., a heating step). This heating step was carried out until volatile components, such as the solvents (e.g., water), had volatilized. Note that, in a case where the heating step is carried out at 80° C. or less (at 70° C., for instance), the heating time can be made longer. By means of the heating step, TiO2 and HF in the mixed raw material reacted with each other so that oxidized titanium fluoride generated. It is believed that this oxidized titanium fluoride could possibly be TiOF2 as described below. Thereafter, the solid contents were washed with distilled water, and then particulate-shaped oxidized titanium fluoride was obtained by drying them at 80° C. for 12 hours. This particulate-shaped oxidized titanium fluoride was also nanometer-size particles whose particle diameters were from about 5 nm to 40 nm approximately. The oxidized titanium fluoride being obtained by this step was a negative-electrode active material according to an example.
80 parts by mass of the negative-electrode active material being obtained by the above-mentioned steps, 7.5 parts by mass of CB (e.g., “Super-P,” a product of TIMCAL GRAPHITE AND CARBON Corporation) serving as a conductive additive, and graphite (e.g., “KS6”) serving as another conductive additive, and 5 parts by mass of PAA serving as a binder were mixed one another, thereby preparing a negative-electrode mixture material. This negative-electrode mixture material was coated onto a surface of an aluminum foil with 20 μm in thickness so as to make a thickness of 50 μm, and was then dried at 120° C. for 8 hours. After the drying, the aluminum foil with the negative-electrode mixture material coated was punched out to a predetermined configuration, thereby obtaining a negative electrode having a 10×10 mm-squared shape and a thickness of 70 μm.
For the counter electrode (i.e., the positive electrode), a metallic lithium foil whose shape was a 2×2 mm square and thickness was 70 μm was used. For the electrolytic solution, a mixture solution was used which comprised: a mixed solvent including EC (i.e., ethylene carbonate), DMC (i.e., dimethyl carbonate) and EMC (i.e., ethyl methyl carbonate) in a ratio of EC:DMC:EMC=1:1:1 by volume; and “LiBETI” (i.e., a lithium imide electrolyte (e.g., Li(C2F5SO2)2N)) being dissolved in the mixed solvent in a concentration making 1 mol/L. Note that the “LiBETI” was added in order to prevent alloying from occurring between aluminum within the aluminum foil and lithium.
With use of the above-mentioned positive electrode and negative electrode, a laminated square cell was manufactured. Specifically, a rectangle-shaped sheet, which comprised a polypropylene resin and whose shape was a 40×40 mm square and thickness was 30 μm, was interposed or held between the positive electrode and the negative electrode as a separator to make a polar-plate subassembly. This polar-plate subassembly was covered with laminated films in which two pieces made a pair. After sealing the laminated films at the three sides, the above-mentioned electrolytic solution was then injected into the laminated films which had been turned into a bag shape. Thereafter, the remaining one side was sealed, thereby obtaining a laminated cell whose four sides were sealed air-tightly and in which the polar-plate subassembly and electrolytic solution were closed hermetically. Note that the positive electrode and negative electrode were provided with a tab being electrically connectable to the outside, respectively, and that the tabs extended out partially to the outside of the resulting laminated cell. A laminated-cell-shaped (or bipolar pouched cellular) lithium-ion secondary battery was obtained by those steps as above.
A lithium-ion secondary battery was manufactured in the same manner as the present example, except that TiO2, which was identical with that was used in the present example, was used as the negative-electrode active material.
A production process according to Comparative Example No. 2 was the same process as that in the present example, except for a HF concentration in the mixed raw material. Specifically, in Comparative Example No. 2, the mixed raw material was obtained by mixing the same TiO2 nanometer-size particles as those in the present example and a 46%-by-mass HF solution one another in such a ratio as TiO2:HF=1:2 by mol when synthesizing oxidized hydrogen titanium, namely, a negative-electrode active material. Other than this, the production process was identical with that in the present example. Moreover, a lithium-ion secondary battery according to Comparative Example No. 2 was one which was identical with that according to the present example, except for the negative-electrode active material.
A charging/discharging test was carried for lithium-ion secondary batteries according to the above described present example and Comparative Example Nos. 1 through 2. The charging/discharging conditions were as follows: a constant current (e.g., 20 mA/g); voltage range of from 0.25 to 3.0 V; and 30° C. measurement temperature. The term, “charging,” specifies the direction in which an active material in an electrode to be evaluated sorbs Li; whereas the term, “discharging,” specifies the other direction in which the active material in the electrode to be evaluated desorbs Li. Results of the charging/discharging test are illustrated in
As illustrated in
However, the charged and discharged capacities of the lithium-ion secondary battery according to the present example were large enough to be 400 mAh/g approximately even in the second-round cycle or later. Therefore, it is possible to say that the lithium-ion secondary battery according to the present example had higher capacities. And, it is possible to say that the negative-electrode active material according to the present example can turn lithium-ion secondary batteries into higher-capacity ones.
Moreover, as illustrated in
On the contrary, the capacities of the lithium-ion secondary battery according to Comparative Example No. 1 were 200 mAh/g approximately, and the capacities of the lithium-ion secondary battery according to Comparative Example No. 2 were 350 mAh/g approximately. Compared with the capacities of the lithium-ion secondary battery according to Comparative Example No. 1, the capacities of the lithium-ion secondary batteries according to the present example and Comparative Example No. 2 were large. From this result, it is understood that, not using simple anatase-type TiO2 as a negative-electrode active material, but using oxidized titanium fluoride, which is made by fluorinating anatase-type TiO2, as a negative-electrode active material makes it possible to turn lithium-ion secondary batteries into higher-capacity ones. Moreover, the lithium-ion secondary battery according to the present example, whose mixed raw material included TiO2 and hydrogen fluoride (HF) in a ratio of 1:10 by mol, had far higher capacities than had the lithium-ion secondary battery according to Comparative Example No. 2, whose mixed raw material included TiO2 and hydrogen fluoride (HF) in a ratio of 1:2 by mol. It is understood that lithium-ion secondary batteries can be turned into higher-capacity ones by using those which include hydrogen fluoride (HF) in an amount exceeding 2 mol per 1-mol TiO2 as the mixed raw material. Moreover, it is understood that those which include hydrogen fluoride (HF) in amount of 5 mol or more per 1-mol TiO2 can preferably be used as the mixed raw material, and that those which include hydrogen fluoride (HF) in amount of 10 mol or more per 1-mol TiO2 can more preferably be used as the mixed raw material. Note that, when an amount of hydrogen fluoride in the mixed raw material is too little (e.g., from 0.01 to 2 mol), it is believed that it is less likely to turn lithium-ion secondary batteries into higher-capacity ones, because TiO2 cannot be fluorinated sufficiently and thereby oxidized titanium fluoride and TiO2 are put in a state in which they are intermingled with each other.
Rate characteristics of the lithium-ion secondary batteries according to the present example and Comparative Example No. 1 were measured. Specifically, repetitive charging and discharging operations were carried out while changing the electric current to be flowed as follows when let 1C=400 mA/g: 0.05C (i.e., 20 mA/g); 0.125C (i.e., 50 mA/g); 0.25C (i.e., 100 mA/g); 0.5C (i.e., 200 mA/g); 1C (i.e., 400 mA/g); 2C (i.e., 800 mA/g); 3C (i.e., 1,200 mA/g); 4C (i.e., 1,600 mA/g); 6C (i.e., 2,400 mA/g); and 8C (i.e., 3,200 mA/g). The cut-off voltage on this occasion was from 0.25 to 3.0 V. The temperature was 30° C. Results of the rate-characteristic test are illustrated in
The lithium-ion secondary battery according to the present example had such a high capacity as 300 mAh/g even in a case where the C rate was such a considerably large rate as 8C. From this result, it is understood that the negative-electrode active material according to the present example enabled lithium-ion secondary batteries to be charge and discharge at a faster speed. On the contrary, the lithium-ion secondary battery according to Comparative Example No. 1 exhibited slightly small capacities, compared with those of the lithium-ion secondary battery according to the present example, even at identical electric-current densities. From this result, it is understood that using oxidized titanium fluoride, not simple titanium oxide, for a negative-electrode active material can make the rate characteristics of lithium-ion secondary batteries upgradable.
The negative-electrode active material (i.e., oxidized titanium fluoride) according to the present example was analyzed using a scanning electron microscope (or SEM), a transmission electron microscope (or TEM), and X-ray diffraction (or XRD).
An image of the negative-electrode active material according to the present example was taken by means of an SEM at a magnification of 20,000 times. An acceleration voltage on this occasion was 15 kV, and platinum was used for the coating. The resulting SEM image of the negative-electrode active material according to the present example is illustrated in
As illustrated in
The negative-electrode active material according to the present example was subjected to an XRD analysis. An X-ray diffraction measurement was carried with use of the CuKα ray by means of a powder X-ray diffractometer (e.g., a product of MAC Science Corporation, Model Number: MO6XCE) that served as an X-ray diffraction apparatus on this occasion. The measurement conditions were as follows: 40-kV voltage; 100-mA current; 4-degree/minute scanning rate; 0.02-degree sampling; one-time cumulated number; and the measurement range of from 15 to 80 degrees by diffraction angle (2θ). Results of the XRD are illustrated in
Note that, as a result of subjecting the anatase-type TiO2, which had been used as a raw material for the negative-electrode active material in the present example, to an XRD analysis and then referring the resulting analyzed results to that of the JCPDS card (i.e., #21-1272), this anatase-type TiO2, that is, a synthesized substance which was obtained by means of a uniform precipitation method and hydrothermal method, was ascertained to belong to the “anatase-type TiO2.”
By means of XRD (e.g., ex-situ XRD), structural changes of the negative-electrode active material in the lithium-ion secondary battery according to the present example were analyzed at the time of the first-round discharging, and at the time of the first-round charging. Results of the XRD at the time of the first-round discharging are illustrated in
As illustrated in
As illustrated in
In addition, as illustrated in
By means of XPS (e.g., XPS in ex-situ mode by “5600,” a product of ULVAC-PHI), structural changes of the negative-electrode active material in the lithium-ion secondary battery according to the present example were analyzed at the time of the first-round discharging, at the time of the first-round charging, at the time of the second-round discharging, and at the time of the second-round charging. To be concrete, “Ti2p” spectra on the negative electrode resulting from the XPS were analyzed. Results of the XPS analysis at the time of the first-round discharging are illustrated in
As illustrated in
As being mentioned above, from the results of the XRD (e.g.,
When applying this result to one of the discharging curves of the lithium-ion secondary battery, the Ti and F in the negative-electrode active material exist in the state of TiIVOF2+Li++e− in Initial Period “1” of the discharging (i.e., in the vicinity of from 3 V to 1 V, and in the vicinity of from 0 to 100 mAh/g), as illustrated in
Note that, in oxidized titanium fluoride (e.g., TiOF2), 2-mol fluorine (F) is included with respect to 1-mol titanium (Ti). Since the molar ratios of the Ti and F do not change after charging and discharging, it is possible to say 2-mol fluorine (F) is included with respect to 1-mol titanium (Ti) in the negative-electrode active material according to the present example. In other words, it is believed that the negative-electrode active material according to the present example gave the above-described better charged and discharged capacities to the lithium-ion secondary battery and enabled it to charge and discharge at a faster speed because of including 2-mol fluorine (F) with respect to 1-mol titanium (Ti). Note that, as far as fluorine (F) is included in an amount exceeding 1 mol with respect to 1-mol titanium (Ti), it is possible to say that at least a part of the resulting negative-electrode active material acts as the above-mentioned negative-electrode active material. In other words, it is advisable that, in the negative-electrode active material according to the present invention, fluorine (F) can be included in an amount exceeding 1 mol with respect to 1-mol titanium (Ti).
The negative-electrode active material according to the present example was subjected to a TEM analysis in the respective stages at the time of the first-round discharging. Crystal structures of the negative-electrode active material in the respective stages at the time of the first-round discharging were observed by means of this analysis.
Specifically, images of the negative-electrode active were taken by means of an electron diffraction method in the following respective stages at the time of the first-round discharging: Initial Period “A” of the discharging at 3 V; Intermediate Period “B” of the discharging at 0.93 V; and Terminal Period “C” of the discharging at 0.25V. Moreover, images of the negative-electrode active material in Terminal Period “C” of the discharging were taken by mean of a TEM. In addition, the negative-electrode active material in Terminal Period “C” of the discharging was subjected to an elemental analysis by means of EDX. The resulting electron-diffraction images are illustrated in
As illustrated at the top in
As illustrated in
Note that it is allowable that the amorphous layer can be formed on at least a part of the surfaces of the oxide particles, so that such a case might possibly arise where it is formed on a part of the surfaces of the oxide particles alone. Moreover, in the negative-electrode active material according to the present invention, it is even permissible that all of the oxide particles do not necessarily possess the amorphous layer.
For reference, it is believed that TiOF2 in the negative-electrode active material possessed the crystal structure that is illustrated in
| Number | Date | Country | Kind |
|---|---|---|---|
| 2011-103765 | May 2011 | JP | national |
This application is a Continuation In Part of International Application No. PCT/JP2012/002902 filed Apr. 27, 2012, claiming priority based on Japanese Patent Application No. 2011-103765 filed May 6, 2011, the contents of all of which are incorporated herein by reference in their entirety.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP2012/002902 | Apr 2012 | US |
| Child | 14072197 | US |
