JP5314810B1 - Titanium oxide compound - Google Patents

Abstract

In the X-ray diffraction spectrum using a Cu—Kα ray source, the titanium oxide compound according to the present invention has a (200) plane peak intensity Ia, a (004) plane peak intensity Ic, and a (31-3) plane Potassium tetratitanate represented by the general formula K ₂ Ti ₄ O ₉ which satisfies the relationship of Ia>Ib> Ic and 3.3 ≧ Ia / Ic> 2.0 between the peak intensity Ib Is eluted and heat-treated in the range of 300 ° C to 550 ° C.
[Selection] Figure 5

Description

  The present invention relates to a titanium oxide compound and a method for producing the same, and further, potassium tetratitanate used for the synthesis of the titanium oxide compound, a negative electrode using the titanium oxide compound as a negative electrode active material, and using this negative electrode To a lithium ion secondary battery.

  Conventionally, carbon-based materials are generally used for the negative electrode of lithium ion secondary batteries, but in recent years, abnormal heat generation and ignition (so-called thermal runaway) of such lithium ion secondary batteries have been frequently reported. . The thermal runaway described above is considered as one of the causes due to an internal short circuit of the battery. This is because when an internal short circuit of the battery occurs, an excessive inrush current flows toward the negative electrode, which causes heat generation of the negative electrode and other members.

  Possible causes of the internal short circuit include not only external impact but also destruction of the separator due to columnar metallic lithium crystals (dendrites) deposited on the negative electrode surface. In the lithium ion secondary battery using a carbon-based material as the negative electrode, the reason why the metal lithium crystal is likely to precipitate is that the negative electrode potential is as low as 0.08 V (vs. Li).

  On the other hand, as a negative electrode of a lithium ion secondary battery, it is also known to use spinel type lithium titanate (S-LTO) in addition to the above carbon materials. In the lithium ion secondary battery using this negative electrode, since the negative electrode potential is as high as 1.55 V (vs. Li), it is difficult for metal lithium crystals to be deposited on the negative electrode surface, reducing the risk of internal short circuit. On the other hand, there was a defect that the theoretical capacity of the negative electrode was only about 175 mAh / g (the carbon-based material was 372 mAh / g).

  In addition, it has been proposed to use a bronze-type titanium oxide compound as a negative electrode of a lithium ion secondary battery (see, for example, Patent Document 1). In the lithium ion secondary battery using this negative electrode, the negative electrode potential becomes as high as around 1.5 V (vs. Li) as in the case of using S-LTO, so that metal lithium crystals are unlikely to precipitate on the negative electrode surface. Thus, the risk of internal short circuit can be reduced, and the negative electrode theoretical capacity can be increased to 335 mAh / g as compared with the case of using S-LTO.

JP 2008-117625 A

  However, the titanium oxide compound obtained by the conventional manufacturing method has a negative capacity smaller than the above theoretical value when used as a negative electrode of a lithium ion secondary battery, leaving room for further improvement. It was.

In view of the above-mentioned problems, the present invention is characterized by an intermediate having a crystalline structure in which TiO ₆ octahedrons are chained to form a layered structure via a precursor in which TiO ₅ triangular pyramidal bodies are chained to form a layered structure. Titanium oxide that is safer than conventional methods and can be used as a high-capacity negative electrode by synthesizing a titanium oxide compound having a crystal structure in which TiO ₆ octahedrons are chained from this intermediate It aims at providing a compound, its manufacturing method, and a lithium ion secondary battery using the same.

In order to achieve the above object, the titanium oxide compound according to the present invention comprises (200) plane peak intensity Ia, (004) plane peak intensity Ic, and X-ray diffraction spectrum using a Cu-Kα radiation source. Elution of potassium tetratitanate (4T) represented by the general formula K ₂ Ti ₄ O _{9 in} which the relationship of Ia>Ib> Ic is established between the peak intensity Ib of the (31-3) plane And it is set as the structure (1st structure) obtained by heat-processing.

  Note that the titanium oxide compound having the above first structure has the above-mentioned potassium tetratitanate (4T) having a (200) plane peak intensity Ia and (004) in an X-ray diffraction spectrum using a Cu—Kα radiation source. It is preferable to adopt a configuration (second configuration) in which a relationship of 10.0> Ia / Ic> 2.0 is established between the peak intensity Ic of the surface).

Further, in the titanium oxide compound having the first or second structure, the potassium tetratitanate is a part of potassium dititanate (2T) represented by the general formula K ₂ Ti ₂ O _5. After elution and composition conversion, the structure (third structure) obtained by heat treatment may be used.

  The titanium oxide compound having any one of the first to third configurations may have a configuration (fourth configuration) in which the potassium concentration exceeds 0 and is 2.0% by mass or less.

The titanium oxide compound having any one of the first to fourth configurations may have a configuration (fifth configuration) having a specific surface area of 3 or more and 80 m ² / g or less by the BET method. Further, the titanium oxide compound having any one of the first to fifth configurations is composed of bronze-type titanium oxide or bronze-type titanium oxide as a main component and containing a trace amount of anatase-type or hydrated titanium oxide (No. 6 (Configuration).

  The method for producing a titanium oxide compound according to the present invention includes a step of obtaining potassium tetratitanate from potassium dititanate, a step of obtaining a hydrated tetratitanate compound from the potassium tetratitanate, and the hydrated tetratitanate. And a step of obtaining a titanium oxide compound from the titanic acid compound (seventh configuration).

  The potassium tetratitanate according to the present invention is potassium tetratitanate used for the synthesis of a titanium oxide compound, and in the X-ray diffraction spectrum using a Cu—Kα ray source, the peak intensity Ia of the (200) plane. , The (004) plane peak intensity Ic and the (31-3) plane peak intensity Ib are configured to satisfy the relationship of Ia> Ib> Ic (eighth configuration).

  In addition, the potassium tetratitanate having the above-described seventh configuration is between the peak intensity Ia on the (200) plane and the peak intensity Ic on the (004) plane in an X-ray diffraction spectrum using a Cu—Kα ray source. In addition, a configuration in which a relationship of 10.0> Ia / Ic> 2.0 is established (a ninth configuration) is preferable.

  In addition, the negative electrode according to the present invention has a configuration in which the titanium oxide compound having any one of the first to sixth configurations or the titanium oxide compound obtained by the manufacturing method having the seventh configuration is used as the negative electrode active material. (Tenth configuration).

  In addition, the lithium ion secondary battery according to the present invention has a configuration using the negative electrode having the above tenth configuration (an eleventh configuration).

  According to the present invention, it is possible to provide a titanium oxide compound that is highly safe and can be used as a high-capacity negative electrode active material, a method for producing the same, and a lithium ion secondary battery using the same.

Flow chart showing an example of a process for producing a titanium oxide compound from firing method 2T Flow chart showing an example of a process for producing a titanium oxide compound from the melting method 2T Flow chart showing an example of a process for producing a titanium oxide compound from direct synthesis method 4T Table showing the crystal structure of potassium titanate X-ray diffraction spectrum diagram of firing method 4T X-ray diffraction spectrum of melting method 4T X-ray diffraction spectrum of direct synthesis method 4T Table showing intensity ratio of X-ray diffraction peak of potassium tetratitanate Schematic diagram showing the schematic configuration of a lithium ion secondary battery Schematic diagram of coin cell used for battery performance evaluation Table showing initial discharge capacities of Examples 1 to 6, Reference Example 1 and Comparative Example 1

  Hereinafter, embodiments of a titanium oxide compound according to the present invention, a manufacturing method thereof, and a lithium ion secondary battery using the same will be described. Examples of the titanium oxide compound include hydrated titanium oxide, bronze-type titanium oxide, anatase-type titanium oxide, and a mixture of two or more types of rutile-type titanium oxide.

<Method for producing titanium oxide compound>
(Overview)
The method for producing a titanium oxide compound according to the present invention comprises a step of obtaining potassium tetratitanate (K ₂ Ti ₄ O ₉ ) from potassium dititanate (K ₂ Ti ₂ O ₅ ), and a hydration process from potassium tetratitanate. A step of obtaining a titanic acid compound (H ₂ Ti ₄ O ₉ .nH ₂ O) and a step of obtaining a titanium oxide compound from the hydrated tetratitanic acid compound.

  In addition, about the process of obtaining said potassium dititanate, the method of attaching | subjecting the mixture of the titanium compound and potassium compound mixed by the predetermined composition ratio to a baking process, or a predetermined amount of water to the said titanium compound and potassium compound. In addition, a step of obtaining potassium dititanate (baking method) such as a method of mixing, spray-drying and then subjecting to baking treatment may be employed, or a titanium compound and a potassium compound mixed at a predetermined composition ratio A step (melting method) for obtaining potassium dititanate by subjecting the mixture to melting treatment and solidification treatment may be employed.

  In the following, a first production method for synthesizing a titanium oxide compound from potassium dititanate (calcination method 2T) obtained by a firing method via potassium tetratitanate and a hydrated tetratitanate compound, and a melting method Details are divided into two cases of the second production method of synthesizing a titanium oxide compound from the obtained potassium dititanate (melting method 2T) via potassium tetratitanate and a hydrated tetratitanate compound. Explained.

(First manufacturing method)
FIG. 1 is a flowchart showing an example of a schematic process for producing a titanium oxide compound from the firing method 2T. In the first manufacturing method, first, a mixed solution of titanium dioxide (TiO ₂ ) and potassium carbonate (K ₂ CO ₃ ) mixed at a predetermined composition ratio is spray-dried (step S11) and fired (step S12). To synthesize potassium dititanate (K ₂ Ti ₂ O ₅ ; firing method 2T). As shown in FIG. 4, this potassium dititanate has a layered structure in which chains of TiO ₅ triangular double pyramids are stacked, and a space for carrying potassium ions is formed between the layers.

Next, the above-described potassium dititanate is subjected to a depotassification treatment (step S13) and a firing treatment (step S14), and structural conversion from a TiO ₅ triangular pyramidal body to a TiO ₆ octahedron is carried out, so that Potassium titanate (K ₂ Ti ₄ O ₉ ; firing method 4T) is synthesized. In other words, this potassium tetratitanate was obtained by elution of a part of potassium ions of potassium dititanate represented by the general formula K ₂ Ti ₂ O ₅ to convert the composition, followed by firing treatment. is there. As shown in FIG. 4, this potassium tetratitanate has a layered structure in which chains of TiO ₆ octahedrons are stacked, and a space for carrying potassium ions is formed between the layers.

Furthermore, the above-mentioned potassium tetratitanate is subjected to a grinding treatment (the grinding treatment may be omitted) (step S15) and a proton exchange treatment (step S16) to obtain a hydrated tetratitanate compound (H ₂ Ti ₄ O). ₉ · nH ₂ O). The hydrated tetratitanate compound is subjected to a heat treatment (step S17) in the range of 200 ° C. to 1000 ° C. (more preferably, 300 ° C. to 550 ° C.) for 0.5 hours to 5 hours, and titanium oxide. A compound is synthesized.

(Second manufacturing method)
FIG. 2 is a flowchart showing an example of a schematic process for producing a titanium oxide compound from the melting method 2T. In the second production method, first, a mixture of titanium dioxide (TiO ₂ ) and potassium carbonate (K ₂ CO ₃ ) mixed at a predetermined composition ratio is subjected to melting treatment (step S21) and solidification treatment (step S22). Then, potassium dititanate (K ₂ Ti ₂ O ₅ ; melting method 2T) is synthesized. As shown in FIG. 4, this potassium dititanate has a layered structure in which chains of TiO ₅ triangular double pyramids are stacked, and a space for carrying potassium ions is formed between the layers.

Next, the above-described potassium dititanate is subjected to a depotassification treatment (step S23) and a firing treatment (step S24), and structural conversion from the TiO ₅ triangular pyramid body to the TiO ₆ octahedron is carried out, so that Potassium titanate (K ₂ Ti ₄ O ₉ ; melting method 4T) is synthesized. In other words, this potassium tetratitanate was obtained by elution of a part of potassium ions of potassium dititanate represented by the general formula K ₂ Ti ₂ O ₅ to convert the composition, followed by firing treatment. is there. As shown in FIG. 4, this potassium tetratitanate has a layered structure in which chains of TiO ₆ octahedrons are stacked, and a space for carrying potassium ions is formed between the layers.

Further, the above-mentioned potassium tetratitanate is subjected to a pulverization process (the pulverization process may be omitted) (step S25) and a proton exchange process (step S26) to obtain a hydrated tetratitanate compound (H ₂ Ti ₄ O). ₉ · nH ₂ O). Then, the hydrated tetratitanate compound is subjected to a heat treatment (step S27) in the range of 200 ° C. to 1000 ° C. (more preferably 300 ° C. to 550 ° C.) to synthesize a titanium oxide compound.

The titanium oxide compound obtained by the first production method and the second production method was constructed with a chain of TiO ₆ octahedrons affected by the basic crystal structure of potassium tetratitanate, which is an intermediate. It has a structure.

Here, the manufacturing method of potassium dititanate (K ₂ Ti ₂ O ₅ ) mentioned in the first manufacturing method and the second manufacturing method is merely an example and is not limited thereto.

(Conventional manufacturing method)
On the other hand, the conventional method for producing a titanium oxide compound is an alkali titanate compound having a layered structure in which a chain of TiO ₆ octahedrons synthesized directly from a mixture of a titanium compound and an alkali compound mixed at a predetermined composition ratio is laminated. A titanium oxide compound is synthesized using (for example, sodium trititanate or potassium tetratitanate).

FIG. 3 is a flowchart showing an example of a schematic process for producing a titanium oxide compound from directly synthesized potassium tetratitanate (direct synthesis method 4T). In the conventional manufacturing method, first, a mixture of titanium dioxide (TiO ₂ ) and potassium carbonate (K ₂ CO ₃ ) mixed at a predetermined composition ratio is subjected to kneading treatment (step S31) and firing treatment (step S32). By attaching, potassium tetratitanate (K ₂ Ti ₄ O ₉ ; direct synthesis method 4T) is directly synthesized without going through potassium dititanate.

Next, the above-mentioned potassium tetratitanate is subjected to a grinding treatment (the grinding treatment may be omitted) (step S35) and a proton exchange treatment (step S36) to obtain a hydrated tetratitanate compound (H ₂ Ti ₄ O ₉ .nH ₂ O) is synthesized. Then, the hydrated tetratitanate compound is subjected to a heat treatment (step S37) in the range of 200 ° C. to 1000 ° C. to synthesize a titanium oxide compound.

  Thus, the biggest difference between the manufacturing method of the present invention (see FIGS. 1 and 2) and the conventional manufacturing method (see FIG. 3) is that potassium dititanate (calcined) is used in the manufacturing process of the titanium oxide compound. Whether or not a step of obtaining potassium tetratitanate from any of the methods 2T and 2T) is included, and due to this difference, tetratitanic acid contained in the production process of the titanium oxide compound Differences described below occur in the crystal structure of potassium (and hence the crystal structure of the titanium oxide compound that is the final product).

<Differences in the structure of potassium tetratitanate>
5 to 7 are X-ray diffraction spectrum diagrams (X-ray source: Cu-Kα) of the firing method 4T, the melting method 4T, and the direct synthesis method 4T, respectively.

  (1) For potassium tetratitanate produced by any of the production methods, an X-ray diffraction peak showing the (200) plane appears in the vicinity of 2θ = 10.008 °. The peak intensity Ia of the (200) plane indicates the degree of crystal growth in the direction (a-axis direction) in which layered surfaces containing alkali metal ions are stacked. Therefore, it can be said that it is desirable that the peak intensity Ia is large in order to increase the capacity of alkali metal ions.

  (2) For potassium tetratitanate produced by any of the production methods, an X-ray diffraction peak showing the (004) plane appears in the vicinity of 2θ = 31.0 °. The peak intensity Ic of the (004) plane indicates the degree of crystal growth in the c-axis direction. The larger the peak intensity Ic, the wider the width of the layered surface containing alkali metal ions (the crystal in the c-axis direction). Is expressed as width). Therefore, in order to increase the capacity of alkali metal ions, the peak intensity Ic must be large to some extent. However, if the peak intensity Ic is too large, the movement distance required for insertion / extraction of alkali metal ions becomes long. It is considered that the mobility of alkali metal ions decreases.

  (3) For potassium tetratitanate produced by any of the production methods, X indicating (31-3) plane (h = 3, k = 1, l = -3) is near 2θ = 33.7 °. A line diffraction peak appears. This (31-3) plane is a plane that intersects any of the a-axis, b-axis, and c-axis, and its peak intensity Ib is higher than the crystal growth degree in the a-axis direction and the crystal growth degree in the c-axis direction. This peak is highly dependent on the degree of crystal growth in the b-axis direction. The larger the peak intensity Ib, the longer the length of the layered surface containing the alkali metal ions (the crystal size in the b-axis direction is expressed as the length). Therefore, in order to increase the capacity of the alkali metal ions, the peak intensity Ib must be large to some extent. However, if the peak intensity Ib is too large, the movement distance required for insertion / extraction of alkali metal ions becomes long. It is considered that the mobility of alkali metal ions decreases.

  Here, a structure for securing a high accommodation degree of alkali metal ions and further improving insertion / detachment will be considered. First, as a first condition, it is considered that a layer structure capable of accommodating alkali metal ions needs to be sufficiently formed in order to ensure a high degree of accommodation. In addition, in order to improve the insertion / removal property, it is preferable that the width / length of the layered surface be smaller than the layer thickness (the crystal size in the a-axis direction is expressed as thickness). For this reason, it is considered desirable to satisfy the relationship of Ia> Ic, Ia> Ib.

  As a second condition, in order to further improve the insertion / detachment property, the peak intensity Ic indicating the degree of crystal growth in the c-axis direction is highly dependent on the degree of crystal growth in the b-axis direction. It is considered desirable to satisfy a smaller relationship, that is, satisfy the relationship of Ib> Ic.

c-axis direction does not have a planar layered structure for crystal growth while shifted by one side portion of TiO ₆ octahedron every four chain structure of TiO ₆ octahedron. On the other hand, since the b-axis direction has a planar layered structure, the mobility of alkali metal ions is considered to be lower in the c-axis direction than in the b-axis direction. Therefore, it is considered that suppressing the crystal growth in the c-axis direction and reducing the width are effective in improving the mobility of alkali metal ions.

  On the other hand, if the width of the layered surface that accommodates the alkali metal ions is too small, the alkali metal ions are not sufficiently accommodated (unstable). Conversely, if the width is too large, the movement distance of the alkali metal ions increases. Therefore, as the third condition, it is desirable to satisfy a relationship that satisfies 10.0> Ia / Ic> 2.0 (more preferably, a relationship that satisfies 5.0> Ia / Ic> 2.0). it is conceivable that.

  FIG. 8 is a table showing the intensity ratio of X-ray diffraction peaks obtained for each potassium tetratitanate of each production method. In FIG. 8, for each of the firing method 4T, the melting method 4T, and the direct synthesis method 4T, the peak intensities Ic and Ib when the peak intensity Ia is 100, and the peak intensities Ia and Ic The ratio (Ia / Ic) is shown. In addition, about the baking method 4T, the measurement result for 3 times was shown.

As shown in FIG. 8, the firing method 4T and the melting method 4T satisfy all the first to third conditions, but the direct synthesis method 4T does not satisfy the second and third conditions. Here, the titanium oxide compound as the final product is constructed with a chain of TiO ₆ octahedrons affected by the basic crystal structure of the intermediate potassium tetratitanate as described above. Has a crystal structure. The titanium oxide compound produced by using the firing method 4T or the melting method 4T has a characteristic of potassium tetratitanate having the characteristics shown above when the crystal structure is changed by heat treatment from the basic crystal structure of potassium tetratitanate. It is presumed that the crystal structure is influenced and the crystal structure is preferable for insertion / extraction of lithium ions, that is, a crystal structure having a high lithium ion capacity and mobility. Therefore, the titanium oxide compound manufactured using the firing method 4T or the melting method 4T has higher lithium ion capacity and mobility than the titanium oxide compound manufactured using the direct synthesis method 4T. It is considered that the capacity of a lithium ion secondary battery using this as a negative electrode can be increased.

<Application to lithium ion secondary batteries>
FIG. 9 is a schematic diagram showing a schematic configuration of a lithium ion secondary battery. The lithium ion secondary battery 10 of this configuration example includes a positive electrode 11, a negative electrode 12, a nonaqueous electrolyte 13, and a separator 14.

  For example, the positive electrode 11 has a structure in which a positive electrode mixture layer is provided on one or both surfaces of a positive electrode current collector. The positive electrode mixture layer includes, for example, a positive electrode material capable of inserting and extracting lithium as a positive electrode active material, and a conductive agent such as carbon black or graphite and a binder such as polyvinylidene fluoride as necessary. It is composed with an agent.

  The negative electrode 12 has, for example, a structure in which a negative electrode mixture layer is provided on one side or both sides of a negative electrode current collector. The negative electrode mixture layer may contain another negative electrode active material or a conductive agent in addition to the negative electrode material (that is, the titanium oxide compound described above) according to the present embodiment.

As the non-aqueous electrolyte 13, in addition to a liquid non-aqueous electrolyte prepared by dissolving an electrolyte (lithium salt) in an organic solvent, a gel-like non-aqueous electrolyte in which a liquid electrolyte and a polymer material are combined can be used. . Examples of the electrolyte include LiClO ₄ , LiPF ₆ , LiBF _{4, and the} like, and any one of these or a mixture of two or more thereof can be used. Examples of the organic solvent include propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, and 1,2-dimethoxyethane, and any one or two or more of these can be used in combination. .

  The separator 14 separates the positive electrode 11 and the negative electrode 12 and allows lithium ions to pass through while preventing a short circuit of current due to contact between both electrodes. The separator 14 is made of, for example, a porous film made of a synthetic resin made of polytetrafluoroethane, polypropylene, or polyethylene, or a porous film made of an inorganic material such as a ceramic nonwoven fabric. It is good also as a structure which laminated | stacked 2 or more types of porous membranes.

  As described above, as the negative electrode 12 of the lithium ion secondary battery 10, not the carbon-based material but the negative electrode material in the present embodiment (the above-described titanium oxide compound manufactured using the firing method 4T or the melting method 4T). With the configuration used, the safety of the lithium ion secondary battery 10 can be enhanced by the triple mechanism shown below.

First, there is a merit that an internal short circuit due to dendrite precipitation is less likely to occur. In the case of a carbon negative electrode, the negative electrode potential is low and a dendrite precipitation reaction (Li ⁺ + e ⁻ → Li) tends to occur. On the other hand, in the case of a titanium oxide compound negative electrode, the negative electrode potential is high and does not reach the dendrite precipitation potential.

  Second, there is a merit that heat generation during an internal short circuit can be suppressed. The titanium oxide compound changes to insulating properties when lithium ions are completely released. Therefore, the surface of the titanium oxide compound at the short-circuited portion is insulated to suppress the progress of the discharge reaction. That is, in a general lithium ion secondary battery using a carbon negative electrode, rapid discharge (heat generation) occurs at the time of an internal short circuit, whereas in a lithium ion secondary battery using a titanium oxide compound negative electrode, discharge is slow. Proceeds and does not heat up.

  Third, there is a merit that the thermal stability is high. Titanium oxide compounds are extremely unlikely to cause thermal runaway when triggered by an electrolyte solution. In addition, since titanium oxide compounds do not burn unlike carbon, the risk of thermal runaway to ignition is very low.

  Furthermore, if it is the structure using the negative electrode material (The above-mentioned titanium oxide compound manufactured using the baking method 4T or the fusion | melting method 4T) in this Embodiment, it will be lithium from S-LTO and the titanium oxide compound of a conventional manufacturing method. The capacity of the ion secondary battery 10 can be increased.

  Hereinafter, examples of the present invention will be described in more detail, but the present invention is not limited to the following examples. That is, with respect to the parts to which known general techniques such as various processing methods and pulverization methods described below can be applied, the contents thereof are appropriately changed without being limited to the following examples. It goes without saying that it is possible.

<Example 1 [Synthesis from Firing Method 2T]>
(1-1) Synthesis Method of Potassium Dititanate (2T) 26.2 parts by weight of titanium oxide (TiO ₂ ) was mixed and stirred with respect to 100 parts by weight of water. Thereafter, 23.8 parts by weight of potassium carbonate (K ₂ CO ₃ ) was added and further stirred. The mixed solution was spray dried at 200 ° C. and heat treated at 800 ° C. for 3 hours to synthesize potassium dititanate (K ₂ Ti ₂ O ₅ ).

(1-2) Method for synthesizing potassium tetratitanate (4T) After immersing the potassium dititanate obtained in (1-1) in water, it is depotassified by stirring with a stirrer, followed by dehydration and drying. Thereafter, heat treatment was performed at 850 ° C. for 2 hours to synthesize potassium tetratitanate (K ₂ Ti ₄ O ₉ ). The X-ray diffraction spectrum of this synthesized product is as shown in FIG. 5, and the relationship between the peak intensities Ia, Ib and Ic is as shown in D1 of FIG. In addition, the potassium concentration of this composite was 19.3 mass% as a result of the compositional analysis by fluorescent X-rays.

(1-3) Method for synthesizing hydrated tetratitanate compound (hydrated 4T) The potassium tetratitanate obtained in (1-2) was pulverized in a ball mill for 0.5 hour, and then this pulverized product was reduced to 0. Potassium was removed by charging and stirring in a 5 M sulfuric acid solution. A hydrated tetratitanate compound (H ₂ Ti ₄ O ₉ .nH ₂ O) was synthesized by removing these supernatants and then dehydrating them. In addition, the potassium concentration of this composite was 1.23 mass% as a result of the compositional analysis by fluorescent X-rays.

(1-4) Synthesis method of titanium oxide compound The titanium oxide compound was synthesized by heat-treating the hydrated tetratitanate compound obtained in (1-3) at 350 ° C. As a result of compositional analysis by fluorescent X-ray, the potassium concentration of this synthesized product was 1.15% by mass, and the BET specific surface area was 18.6 m ² / g. Further, the X-ray diffraction spectrum (X-ray source: Cu-Kα) of this compound showed bronze-type titanium oxide having a tunnel structure.

<Example 2 [Synthesis from Firing Method 2T]>
(2-1) Synthesis Method of Potassium Dititanate (2T) Potassium dititanate was synthesized in the same step as (1-1).

(2-2) Method for synthesizing potassium tetratitanate (4T) After immersing the potassium dititanate obtained in (2-1) in water, acid was added and the mixture was stirred with a stirrer to remove potassium. . After removing these supernatants, dehydration and drying were followed by heat treatment at 850 ° C. for 2 hours to synthesize potassium tetratitanate. The relationship between the X-ray diffraction peak intensities Ia, Ib, and Ic of this composite is as shown by D2 in FIG. In addition, the potassium concentration of this composite was 19.2 mass% as a result of the compositional analysis by fluorescent X-rays.

(2-3) Method for synthesizing hydrated tetratitanate compound (hydrated 4T) Except for using potassium tetratitanate obtained in (2-2), hydrated tetratitanate compound in the same step as (1-3) A titanic acid compound was synthesized. In addition, the potassium concentration of this synthesized product was 1.30% by mass as a result of compositional analysis by fluorescent X-rays.

(2-4) Method for synthesizing titanium oxide compound A titanium oxide compound was synthesized in the same step as (1-4) except that the hydrated tetratitanate compound obtained in (2-3) was used. As a result of compositional analysis by fluorescent X-ray, the potassium concentration of this synthesized product was 1.23% by mass, and the BET specific surface area was 19.0 m ² / g. Further, the X-ray diffraction spectrum (X-ray source: Cu-Kα) of this compound showed bronze-type titanium oxide having a tunnel structure.

<Example 3 [Synthesis from Firing Method 2T]>
(3-1) Synthesis Method of Potassium Dititanate (2T) Potassium dititanate was synthesized in the same step as (1-1).

(3-2) Synthesis method of potassium tetratitanate (4T) Potassium tetratitanate was synthesized in the same process as (1-2).

(3-3) Method for synthesizing hydrated tetratitanate compound (hydrated 4T) Other than changing the concentration of the sulfuric acid solution to 1.0 M using potassium tetratitanate obtained in (3-2) ( A hydrated tetratitanate compound was synthesized in the same process as 1-3). The potassium concentration of this synthesized product was 0.15% by mass as a result of compositional analysis by fluorescent X-ray.

(3-4) Synthesis Method of Titanium Oxide Compound A titanium oxide compound was synthesized in the same step as (1-4) except that the hydrated tetratitanate compound obtained in (3-3) was used. As a result of compositional analysis by fluorescent X-ray, the potassium concentration of this synthesized product was 0.08% by mass, and the BET specific surface area was 19.6 m ² / g. Further, the X-ray diffraction spectrum (X-ray source: Cu-Kα) of this compound showed bronze-type titanium oxide having a tunnel structure.

<Example 4 [Synthesis from Firing Method 2T]>
(4-1) Synthesis Method of Potassium Dititanate (2T) Potassium dititanate was synthesized in the same step as (1-1).

(4-2) Method of synthesizing potassium tetratitanate (4T) Potassium tetratitanate was synthesized in the same step as (1-2).

(4-3) Method for synthesizing hydrated tetratitanate compound (hydrated 4T) Except for changing the concentration of the sulfuric acid solution to 0.05 M using potassium tetratitanate obtained in (4-2) ( A hydrated tetratitanate compound was synthesized in the same process as 1-3). In addition, the potassium concentration of this synthesized product was 1.88% by mass as a result of compositional analysis by fluorescent X-ray.

(4-4) Synthesis Method of Titanium Oxide Compound A titanium oxide compound was synthesized in the same step as (1-4) except that the hydrated tetratitanate compound obtained in (4-3) was used. As a result of compositional analysis by fluorescent X-ray, the potassium concentration of this synthesized product was 1.78% by mass, and the BET specific surface area was 17.9 m ² / g. Further, the X-ray diffraction spectrum (X-ray source: Cu-Kα) of this compound showed bronze-type titanium oxide having a tunnel structure.

<Example 5 [Synthesis from Firing Method 2T]>
(5-1) Synthesis Method of Potassium Dititanate (2T) Potassium dititanate was synthesized in the same step as (1-1).

(5-2) Synthesis method of potassium tetratitanate (4T) Potassium tetratitanate was synthesized in the same step as (2-2).

(5-3) Potassium tetratitanate obtained by the synthesis method (5-2) of hydrated tetratitanate compound (hydrated 4T) was used except that the pulverization time was extended to 2.5 hours (1- A hydrated tetratitanate compound was synthesized in the same step as 3). The potassium concentration of this synthesized product was 0.52% by mass as a result of compositional analysis by fluorescent X-rays.

(5-4) Synthesis Method of Titanium Oxide Compound A titanium oxide compound was synthesized in the same step as (1-4) except that the hydrated tetratitanate compound obtained in (5-3) was used. As a result of compositional analysis by fluorescent X-ray, the potassium concentration of this synthesized product was 0.44% by mass, and the BET specific surface area was 38.4 m ² / g. Further, the X-ray diffraction spectrum (X-ray source: Cu-Kα) of this compound showed bronze-type titanium oxide having a tunnel structure.

<Example 6 [Synthesis from Firing Method 2T]>
(6-1) Synthesis Method of Potassium Dititanate (2T) Potassium dititanate was synthesized in the same step as (1-1).

(6-2) Method for synthesizing potassium tetratitanate (4T) Potassium tetratitanate was synthesized in the same step as (2-2).

(1-3) except that (6-3) potassium tetratitanate obtained by the synthesis method (6-2) of hydrated tetratitanate compound (hydrated 4T) was not performed. A hydrated tetratitanate compound was synthesized in the same process. In addition, the potassium concentration of this synthesized product was 1.62% by mass as a result of compositional analysis by fluorescent X-ray.

(6-4) Method for synthesizing titanium oxide compound A titanium oxide compound was synthesized in the same step as (1-4) except that the hydrated tetratitanate compound obtained in (6-3) was used. As a result of compositional analysis by fluorescent X-ray, the potassium concentration of this synthesized product was 1.54% by mass, and the BET specific surface area was 12.1 m ² / g. Further, the X-ray diffraction spectrum (X-ray source: Cu-Kα) of this compound showed bronze-type titanium oxide having a tunnel structure.

<Reference Example 1 [Synthesis from Melting Method 2T]>
(7-1) Synthesis Method of Potassium Dititanate (2T) Potassium carbonate (K ₂ CO ₃ ) and titanium oxide (TiO ₂ ) are mixed so that the molar ratio of K ₂ O: TiO ₂ is 1: 2. After mixing and dissolving at 1100 ° C., potassium dititanate was synthesized by cooling.

(7-2) Method for synthesizing potassium tetratitanate (4T) Potassium dititanate obtained in (7-1) is immersed in water and stirred with a stirrer for depotassification, after dehydration and drying Then, potassium tetratitanate was synthesized by heat treatment at 850 ° C. for 2 hours. The X-ray diffraction spectrum of this synthesized product is as shown in FIG. 6, and the relationship between the peak intensities Ia, Ib, and Ic is as shown by D4 in FIG. In addition, the potassium concentration of this synthesized product was 18.9% by mass as a result of compositional analysis by fluorescent X-ray.

(7-3) Method for synthesizing hydrated tetratitanate compound (hydrated 4T) (1-3, except that the pulverization treatment was not performed using potassium tetratitanate obtained in (7-2)). The hydrated tetratitanate compound was synthesized in the same process as in (1). In addition, the potassium concentration of this synthesized product was 1.54% by mass as a result of composition analysis by fluorescent X-ray.

(7-4) Synthesis method of titanium oxide compound A titanium oxide compound was synthesized in the same step as (1-4) except that the hydrated tetratitanate compound obtained in (7-3) was used. As a result of compositional analysis by fluorescent X-ray, the potassium concentration of this synthesized product was 1.48% by mass, and the BET specific surface area was 14.3 m ² / g. Further, the X-ray diffraction spectrum (X-ray source: Cu-Kα) of this compound showed bronze-type titanium oxide having a tunnel structure.

<Comparative Example 1 [Synthesis from Direct Synthesis Method 4T]>
(8-1) Synthesis Method of Potassium Tetratitanate (4T) Potassium carbonate (K ₂ CO ₃ ) and titanium oxide (TiO ₂ ) are mixed so that the molar ratio of K ₂ O: TiO ₂ is 1: 4. Mix in mortar. Next, water was added and mixed until it became a clay. This was dried and then heat treated at 950 ° C. for 2 hours to synthesize potassium tetratitanate. The X-ray diffraction spectrum of this synthesized product is as shown in FIG. 7, and the relationship between the peak intensities Ia, Ib, and Ic is as D5 in FIG. In addition, the potassium concentration of this composite was 19.6 mass% as a result of the compositional analysis by fluorescent X-rays.

(8-2) Method for synthesizing hydrated tetratitanate compound (hydrated 4T) Except for using potassium tetratitanate obtained in (8-1), hydrated tetratitanate compound in the same step as (1-3). A titanic acid compound was synthesized. In addition, the potassium concentration of this synthesized product was 1.16% by mass as a result of compositional analysis by fluorescent X-ray.

(8-3) Synthesis Method of Titanium Oxide Compound A titanium oxide compound was synthesized in the same step as (1-4) except that the hydrated tetratitanate compound obtained in (8-2) was used. As a result of compositional analysis by fluorescent X-ray, the potassium concentration of this synthesized product was 0.98% by mass, and the BET specific surface area was 22.6 m ² / g. Further, the X-ray diffraction spectrum (X-ray source: Cu-Kα) of this compound showed bronze-type titanium oxide having a tunnel structure.

<Analyzer>
The analyzers used in Examples 1 to 6, Reference Example 1 and Comparative Example 1 are as follows.
X-ray diffractometer: Measurement by Rigaku Corporation, Ultima4, Cu-Kα ray Fluorescence X-ray analyzer: Rigaku Corporation, RIX1000

<Production of electrode>
Each electrode was produced using the titanium oxide compound synthesize | combined in Examples 1-6, the reference example 1, and the comparative example 1 as an active material. Specifically, first, 10 parts by weight of polyvinylidene fluoride is dissolved in N-methyl-2-pyrrolidone, and then 10 parts by weight of conductive carbon as a conductive auxiliary, Examples 1 to 6, Reference Example 1, and A coating material was prepared by adding 80 parts by weight of the titanium oxide compound obtained in Comparative Example 1 and kneading with a rotation and revolution stirrer. This paint was applied on an aluminum foil, then vacuum dried at 150 ° C., pressed, and then punched into a circular shape.

<Assembly of cell>
A coin-type cell 20 shown in FIG. 10 was assembled using each of the electrodes prepared above. The coin cell 20 has an electrode 21, a counter electrode 22, a nonaqueous electrolyte 23, and a separator 24 sandwiched between an upper case 25a and a lower case 25b, and the periphery of the upper case 25a and the lower case 25b is sealed with a gasket 26. It was made.

A metal lithium foil was used for the counter electrode 22. As the non-aqueous electrolyte 23, a solution obtained by dissolving 1 mol / L of LiPF ₆ in ethylene carbonate: dimethyl carbonate 1: 1 v / v% was used. A polypropylene porous membrane was used for the separator 24.

<Battery evaluation method and results>
Here, in the coin-type cell as described above, since metallic lithium is used for the counter electrode, the potential of each electrode is noble with respect to the counter electrode. Therefore, the direction of insertion / extraction of lithium ions by charging / discharging is opposite to that when each electrode is used as the negative electrode of a lithium ion secondary battery. However, in the following, for convenience, the direction in which lithium ions are desorbed from each electrode is expressed as discharging, and the direction in which lithium ions are inserted into each electrode is expressed as charging.

  Using the coin-type cell 20 described above, charge / discharge was performed at a charge / discharge rate of 0.2 C and a potential range of 1.0 to 3.0 V on the basis of metallic lithium at room temperature. FIG. 11 is a table showing the initial discharge capacities of Examples 1 to 6, Reference Example 1, and Comparative Example 1.

As is clear from comparison between Examples 1 to 6, Reference Example 1 and Comparative Example 1, the titanium oxide compound produced using the firing method 4T or the melting method 4T is produced using the direct synthesis method 4T. Compared to a titanium oxide compound, the capacity of lithium ions can be increased and the mobility can be increased to improve the discharge capacity. Therefore, when the titanium oxide compound of the present invention is used as a negative electrode active material, a lithium ion secondary battery provided by using a lithium-containing composite oxide such as LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ , LiFePO _{4 for} the positive electrode It was proved that it is possible to contribute to the improvement of the charge / discharge characteristics.

<Residual potassium content>
In addition, about the potassium concentration which remains in a titanium oxide compound, it is desirable to restrain to 2.0 mass% or less, and 0.1-1.5 mass% is more desirable. Usually, the fact that potassium remains in the titanium oxide compound means that the substitution site where the lithium ion should originally be accommodated is taken away by the potassium ion, which is considered to cause a decrease in battery performance. Has a larger ionic radius than that of lithium ions (Pauling ion radius: 133 pm potassium (pm is picometer), 60 pm lithium ion), so that a certain amount exists in the crystal structure. It is presumed that it has the effect of retaining the structure and has the effect of facilitating the movement of lithium ions having a small ionic radius.

<BET specific surface area>
Further, the titanium oxide compound desirably has a specific surface area of 3 or more and 80 m ² / g or less (more preferably 5 to ²⁰ m ² / g) by the BET method. This is because if the specific surface area is too large (including the case where the average particle size is too small), the reaction activity between the titanium oxide compound and the non-aqueous electrolyte becomes too high, and the battery life is considered to be shortened.

  Although the embodiment of the present invention has been described above, the configuration of the present invention is not limited to this, and various modifications can be made without departing from the gist of the present invention. That is, the above-described embodiment should be considered as illustrative in all points and not restrictive, and the technical scope of the present invention is shown by the claims, and It should be understood that all modifications that come within the meaning and range of equivalency of the claims are embraced.

  The titanium oxide compound according to the present invention can be used, for example, as a negative electrode of a lithium ion secondary battery.

DESCRIPTION OF SYMBOLS 10 Lithium ion secondary battery 11 Positive electrode 12 Negative electrode 13, 23 Nonaqueous electrolyte 14, 24 Separator 20 Coin cell 21 Electrode 22 Counter electrode 25a, 25b Upper case, lower case 26 Gasket

Claims (6)

A titanium oxide compound,
In an X-ray diffraction spectrum using a Cu-Kα radiation source,
Between the peak intensity Ia of the (200) plane, the peak intensity Ic of the (004) plane, and the peak intensity Ib of the (31-3) plane,
Elution of potassium tetratitanate represented by the general formula K ₂ Ti ₄ O ₉ , which satisfies the relationship of Ia>Ib> Ic and 3.3 ≧ Ia / Ic> 2.0, is performed at 300 ° C. to 550 ° C. A titanium oxide compound obtained by heat treatment in the range of ° C. The potassium tetratitanate is
In an X-ray diffraction spectrum using a Cu-Kα radiation source,
Between the peak intensity Ia of the (200) plane, the peak intensity Ic of the (004) plane, and the peak intensity Ib of the (31-3) plane,
It is obtained by eluting potassium tetratitanate represented by the general formula K ₂ Ti ₄ O ₉ , which satisfies the relation of Ia / Ib ≧ 2.2, and heat-treating in the range of 300 ° C. to 550 ° C. The titanium oxide compound according to claim 1. The potassium tetratitanate is
In an X-ray diffraction spectrum using a Cu-Kα radiation source,
Between the peak intensity Ia of the (200) plane, the peak intensity Ic of the (004) plane, and the peak intensity Ib of the (31-3) plane,
The relation of 1.5 ≧ Ib / Ic is established. The titanium oxide compound according to claim 1, wherein a relationship of 1.5 ≧ Ib / Ic is established.   The titanium oxide compound according to any one of claims 1 to 3, wherein the titanium oxide compound is a bronze-type titanium oxide or a titanium oxide mainly composed of a bronze-type titanium oxide. Potassium remains and the residual potassium concentration is 2.0 mass% or less. The titanium oxide compound according to any one of claims 1 to 4, wherein: The specific surface area in BET method is 3 or more and 80 m < ² > / g or less. The titanium oxide compound in any one of Claims 1-5 characterized by the above-mentioned.

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