JP2018166100A - Manufacturing method of negative electrode active material for lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method of a negative electrode active material that can adequately exhibit superior rate characteristics in a lithium ion secondary battery.SOLUTION: A manufacturing method of a negative electrode active material for a lithium ion secondary battery in which many sodium titanate lithium compound oxide nanoparticles represented by a specific formula are arranged at a surface of a carbon tubular body having an average outer diameter being less than or equal to 50 nm comprises the steps of: (I) obtaining slurry (X) by dispersing cellulose nanofibers having an average fiber diameter being less than or equal to 50 nm into a solvent; (II) subjecting the slurry (X) and slurry (Y) prepared from a titanium compound to hydrothermal reaction to obtain a titanium oxide nanoparticle assembly (Z); and (III) burning after mixing a lithium compound and a sodium compound, or a lithium compound, sodium compound and a metal (M) compound with the obtained titanium oxide nanoparticle assembly (Z).SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method for producing a negative electrode active material for a lithium ion secondary battery.

  In recent years, with the reduction in size and weight of electronic devices, lithium ion secondary batteries having high energy density have attracted more and more attention as extremely useful materials. Furthermore, in order to realize ultra-thin and ultra-lightweight lithium-ion secondary batteries, many of them employ aluminum laminate films as exterior materials, but the electrode volume is repeatedly expanded and contracted by charging and discharging. As a result, the battery is bent and the distance between the electrodes is increased, and the battery resistance is likely to increase. This leads to a decrease in battery characteristics.

Under these circumstances, the lithium lithium titanate composite oxide having a crystal structure belonging to the space group Fmmm such as Li ₂ Na ₂ Ti ₆ O ₁₄ has almost no volume change due to charge / discharge, and as a result, the volume change of the electrode is extremely small. In this respect, it is expected as a highly useful negative electrode material. For example, Patent Document 1 discloses a negative electrode active material for a lithium ion secondary battery, which is a mixed phase of a sodium lithium titanate composite oxide phase (Li _{(2 + y)} NaTi ₆ O ₁₄ ) and a titanium oxide phase (TiO ₂ ). Attempts have been made to address the above problems.

By the way, in general, when a lithium ion secondary battery is used, lithium called SEI (solid electrolyte phase) is formed by an electrochemical reaction between the surface of the negative electrode active material and the electrolytic solution (LiPF ₆ ) at the electrolyte / negative electrode active material interface. A film mainly composed of oxygen, fluorine, or phosphorus is formed. Although the presence of such SEI can suppress the decomposition of the electrolyte and facilitate the insertion and removal of lithium ions, the resistance on the surface of the negative electrode active material increases as the thickness of the SEI increases, and the diffusion of lithium ions is inhibited. There is also a risk of being done. In particular, the lithium sodium titanate composite oxide is prone to SEI growth on its surface, and therefore the conductive path tends to deteriorate with repeated charge and discharge, and the cycle characteristics tend to decrease accordingly. Various developments have also been made for improvement.

  For example, Patent Document 2 discloses a method of coating at least a part of the surface of sodium lithium titanate composite oxide particles with carbon derived from sucrose or the like or lithium titanate, and Non-Patent Document 1 discloses A method for coating the surface of a lithium sodium titanate composite oxide with a copper / carbon composite phase is disclosed, and all attempts to improve cycle characteristics.

JP, 2006-171011, A JP, 2006-171071, A

Journal of Power Sources 2591, 2014, p. 177-182

  However, the manufacturing method that combines the synthesis step by the solid phase method and the pulverization step as described in Patent Document 1 described above sufficiently reduces the size of the sodium lithium titanate composite oxide while maintaining high crystallinity. In addition to the use of a sodium lithium titanate composite oxide that is insufficiently miniaturized even in the method described in Patent Document 2 or Non-Patent Document 1, the sodium lithium titanate is difficult to achieve. In consideration of providing a sufficient conductive path to the composite oxide, the investigation is still insufficient. As described above, in any of the techniques described in any of the documents, in order to secure excellent rate characteristics while taking into account the enhancement of the conductive path in the lithium ion secondary battery, further improvement is required.

  Accordingly, an object of the present invention is to provide a negative electrode active material for a lithium ion secondary battery using a lithium sodium titanate composite oxide in order to obtain a negative electrode material that can sufficiently exhibit excellent rate characteristics in a lithium ion secondary battery. It is to provide a manufacturing method.

  Therefore, the present inventors have made various studies, and by using a production method for obtaining a nanoparticle aggregate of titanium oxide by a hydrothermal reaction while using specific cellulose nanofibers, the surface of the carbon tubular body is combined with sodium lithium titanate composite oxidation. A negative electrode active material having a unique shape formed by arranging a large number of particle nanoparticles can be obtained, and by using this as a negative electrode material, it has been found that a lithium ion secondary battery exhibiting high rate characteristics can be realized, The present invention has been completed.

That is, the present invention provides the following formula (1) on the surface of a carbon tubular body having an average outer diameter of 50 nm or less:
Li _a Na _b M _c Ti _d O _e (1)
(In the formula (1), M is K, Be, Mg, Ca, Sr, Ba, Al, Sc, Si, P, S, Fe, Mn, Co, Ni, Zn, V, Cu, Y, Zr, Nb. , Mo, Pb, Ga, Ge, Bi, Ta, Sn, Eu, La, Ce, Nd, W, Ru, or Gd represents one or more, a, b, c, d, and e are 0 <a ≦ 4, 0 <b ≦ 4, 0 ≦ c ≦ 6, 0 <d ≦ 6, 13 ≦ e ≦ 15, and a number satisfying a + b + c × (valence of M) + 4d = 2e)
A method for producing a negative electrode active material for a lithium ion secondary battery comprising a large number of sodium lithium titanate composite oxide nanoparticles represented by the following steps (I) to (III):
(I) A step of obtaining slurry (X) by dispersing cellulose nanofibers having an average fiber diameter of 50 nm or less in a solvent,
(II) subjecting the obtained slurry (X) and the slurry (Y) prepared from the titanium compound to a hydrothermal reaction to obtain a nanoparticle aggregate (Z) of titanium oxide,
(III) Lithium ion secondary comprising a step of mixing the obtained titanium oxide nanoparticle aggregate (Z) with a lithium compound and a sodium compound, or a lithium compound, a sodium compound and a metal (M) compound, followed by firing. The manufacturing method of the negative electrode active material for batteries is provided.

  According to the production method of the present invention, while using specific cellulose nanofibers and also using a titanium oxide nanoparticle aggregate obtained through a hydrothermal reaction, sodium titanate is formed on the surface of a carbon tubular body. A large number of lithium composite oxide nanoparticles can be arranged to give a unique shape, and by using this as a negative electrode active material for lithium ion secondary batteries, it has excellent rate characteristics while effectively increasing the conductive path A lithium ion secondary battery having the above can be realized.

2 is a SEM image showing a negative electrode active material for a lithium ion secondary battery obtained in Example 1. FIG. 2 is a SEM image showing a negative electrode active material for a lithium ion secondary battery obtained in Comparative Example 1.

Hereinafter, the present invention will be described in detail.
In the production method of the present invention, on the surface of a carbon tubular body having an average outer diameter of 50 nm or less, the following formula (1):
Li _a Na _b M _c Ti _d O _e (1)
(In the formula (1), M is K, Be, Mg, Ca, Sr, Ba, Al, Sc, Si, P, S, Fe, Mn, Co, Ni, Zn, V, Cu, Y, Zr, Nb. , Mo, Pb, Ga, Ge, Bi, Ta, Sn, Eu, La, Ce, Nd, W, Ru, or Gd represents one or more, a, b, c, d, and e are 0 <a ≦ 4, 0 <b ≦ 4, 0 ≦ c ≦ 6, 0 <d ≦ 6, 13 ≦ e ≦ 15, and a number satisfying a + b + c × (valence of M) + 4d = 2e)
A method for producing a negative electrode active material for a lithium ion secondary battery comprising a large number of sodium lithium titanate composite oxide nanoparticles represented by the following steps (I) to (III):
(I) A step of obtaining slurry (X) by dispersing cellulose nanofibers having an average fiber diameter of 50 nm or less in a solvent,
(II) subjecting the obtained slurry (X) and the slurry (Y) prepared from the titanium compound to a hydrothermal reaction to obtain a nanoparticle aggregate (Z) of titanium oxide,
(III) The obtained titanium oxide nanoparticle aggregate (Z) is mixed with a lithium compound and a sodium compound, or a lithium compound, a sodium compound and a metal (M) compound, and then fired.

In the production method of the present invention, on the surface of a carbon tubular body having an average outer diameter of 50 nm or less, the following formula (1):
Li _a Na _b M _c Ti _d O _e (1)
(In the formula (1), M is K, Be, Mg, Ca, Sr, Ba, Al, Sc, Si, P, S, Fe, Mn, Co, Ni, Zn, V, Cu, Y, Zr, Nb. , Mo, Pb, Ga, Ge, Bi, Ta, Sn, Eu, La, Ce, Nd, W, Ru, or Gd represents one or more, a, b, c, d, and e are 0 <a ≦ 4, 0 <b ≦ 4, 0 ≦ c ≦ 6, 0 <d ≦ 6, 13 ≦ e ≦ 15, and a number satisfying a + b + c × (valence of M) + 4d = 2e)
This is a production method for obtaining a negative electrode active material for a lithium ion secondary battery in which a large number of sodium lithium titanate composite oxide nanoparticles represented by the formula:

  The carbon tubular body (A) having an average outer diameter of 50 nm or less (hereinafter also referred to as “C-CNF (A)”) has a cellulose nanofiber having an average fiber diameter of 50 nm or less (hereinafter also referred to as “CNF”). It is a structure that has a tubular shape and is made of derived carbon. The negative electrode active material for a lithium ion secondary battery of the present invention (hereinafter also referred to as “active material nanoarray”) has a large number of sodium titanates on the surface of a carbon tubular body (A) composed of carbon derived from CNF. The lithium composite oxide nanoparticles (B) have a structure in which they are arranged, that is, a large number of lithium sodium titanate composite oxide nanoparticles (B) so as to surround the carbon tubular body (A). Has a structure that is continuously arranged, and externally, a large number of sodium lithium titanate composite oxide nanoparticles (B) are linearly continuously arranged, skewer-like or corn-like It has the shape of

  Here, in general, the nanoarray means a shape in which one nanoscale structure is arranged on the other nanoscale structure, and the active material nanoarray obtained by the present invention has a diameter. C-CNF (A) of 50 nm or less, that is, a shape in which one nanoscale structure is arrayed with sodium lithium titanate composite oxide nanoparticles (B), that is, the other nanoscale structure is arranged. ing.

  C-CNF (A), which is a carbon tubular body having an average outer diameter of 50 nm or less, is a structure formed by carbonizing CNF having an average fiber diameter of 50 nm or less by the production method of the present invention. CNF is a skeletal component that occupies about 50% of all plant cell walls, and is a lightweight high-strength fiber that can be obtained by defibration of plant fibers constituting such cell walls to nano size. C-CNF (A) formed from carbon derived from CNF exhibits a carbon tubular body that retains the periodic structure of the cellulose molecular chain that constitutes CNF. Therefore, the average outer diameter of C-CNF (A) coincides with the average fiber diameter of CNF.

  The average outer diameter of C-CNF (A) is 50 nm or less, preferably 1 nm to 50 nm, more preferably 1 nm to 20 nm, and still more preferably 1 nm to 10 nm. Moreover, the average length of C-CNF (A) will also correspond with the average length of CNF, and the average length of this C-CNF (A) is preferably 100 nm to 100 μm, more preferably 1 μm to 100 μm. And more preferably 5 μm to 100 μm.

A large number of sodium lithium titanate composite oxide nanoparticles (B) arranged on the surface of C-CNF (A) are particles represented by the above formula (1), and have a crystal structure belonging to the space group Fmmm. For example, Li ₂ Na ₂ Ti ₆ O ₁₄ , Li _1.94 Na ₂ Al _0.02 Ti ₆ O ₁₄ , and Li _1.94 Na ₂ Mg _0.03 Ti ₆ O ₁₄ are included in the orthorhombic oxide. Can be mentioned.

Similarly to C-CNF (A), the sodium lithium titanate composite oxide nanoparticles (B) are nanoscale structures, and specifically, the average particle size is a lithium ion secondary having excellent rate characteristics. From the viewpoint of obtaining a battery, the thickness is preferably 5 nm to 450 nm, more preferably 5 nm to 250 nm, and still more preferably 5 nm to 200 nm.
In addition, the average particle diameter of lithium sodium titanate composite oxide nanoparticles (B) refers to several tens of sodium lithium titanate composite oxide nanoparticles (B) measured by observation with an SEM or TEM electron microscope. Mean value of particle diameter (length of major axis).

  Furthermore, the crystallite size of the sodium lithium titanate composite oxide phase contained in the sodium lithium titanate composite oxide nanoparticles (B) is preferably 5 nm to 5% from the viewpoint of obtaining a lithium ion secondary battery excellent in rate characteristics. It is 300 nm, more preferably 5 nm to 250 nm, still more preferably 5 nm to 200 nm, and its crystallinity is also high. Here, the crystallite diameter of the lithium sodium titanate composite oxide phase is a value obtained by applying Scherrer's formula for an X-ray diffraction profile having a diffraction angle 2θ range of 10 ° to 80 ° by Cu-kα rays. Means.

Further, BET specific surface area of titanium sodium lithium composite oxide nanoparticles (B), from the viewpoint of obtaining a lithium ion secondary battery having excellent rate characteristics, preferably 3m ² / g~40m ² / g, more preferably 5m ² / g~35m ² / g, more preferably from 7m ² / g~30m ² / g. When the BET specific surface area is larger than 40 m ² / g, the volume energy density of the negative electrode may be lowered.

  In the negative electrode active material for a lithium ion secondary battery obtained by the production method of the present invention, the content of C-CNF (A) is preferably 0.1% in a total amount of 100% by mass of the negative electrode active material for a lithium ion secondary battery. It is 5-10 mass%, More preferably, it is 1-7 mass%.

  The manufacturing method of this invention is a manufacturing method for obtaining the said negative electrode active material for lithium ion secondary batteries, and the process (I) with which the manufacturing method of this invention is equipped with the cellulose nanofiber whose average fiber diameter is 50 nm or less In this step, slurry (X) is obtained by dispersing (CNF) in a solvent.

  The average fiber diameter of CNF used in step (I) is 50 nm or less, preferably 20 nm or less, more preferably 10 nm or less. Although there is no restriction | limiting in particular about a lower limit, Usually, it is 1 nm or more. Moreover, the average length of CNF is preferably 100 nm to 100 μm, more preferably 1 μm to 100 μm, and further preferably 5 μm to 100 μm, from the viewpoint of efficiently processing an electrode using an active material nanoarray. is there.

  The solvent used in the step (I) is preferably water from the viewpoint of CNF exhibiting good dispersibility and handling. Further, by using water as a solvent, for example, when the hydrothermal reaction in the step (II) described later is performed, the obtained slurry (X) is used as it is without requiring chemical modification or addition of a dispersant. Step (II) can be passed through.

The content of CNF in the slurry (X) is preferably 0.01 to 50 parts by mass, more preferably 0. 0 to 50 parts by mass with respect to 100 parts by mass of the solvent in the slurry (X). 05 parts by mass to 40 parts by mass.
Since the obtained slurry (X) maintains a good dispersion state for several days, the slurry (X) can be prepared. When the slurry (X) prepared as appropriate is used in the step (II) to be described later, it is preferable to use a high-concentration slurry (X) (master batch) with an increased CNF content. The content of the cellulose nanofiber in is in terms of carbon atoms, preferably 5 to 50 parts by mass, more preferably 8 to 40 parts by mass, with respect to 100 parts by mass of the solvent in the slurry (X). It is.
In addition, when diluting the high concentration slurry (X) with water to form the slurry (X), the slurry (X) may be used after sufficiently stirring with a disperser (homogenizer). Examples of such a disperser include the above-described disaggregator, beater, low-pressure homogenizer, high-pressure homogenizer, grinder, cutter mill, ball mill, jet mill, short-axis extruder, twin-screw extruder, ultrasonic stirrer, household juicer mixer, and the like. Can be used effectively.

  Moreover, when using the obtained slurry (X) as it is in process (II), content of a cellulose nanofiber is a carbon atom conversion amount with respect to 100 mass parts of solvent in slurry (X), Preferably it is 0. 0.01 parts by mass to 10 parts by mass, and more preferably 0.05 parts by mass to 8 parts by mass.

  The slurry (X) is preferably subjected to a hydrothermal reaction in advance before transferring to the step (II). At this time, it is preferable to stir the slurry (X) before subjecting to a hydrothermal reaction, and it is preferable to use a disperser (homogenizer) for the stirring. Examples of such a disperser include a disaggregator, a beater, a low-pressure homogenizer, a high-pressure homogenizer, a grinder, a cutter mill, a ball mill, a jet mill, a short-axis extruder, a twin-screw extruder, an ultrasonic stirrer, and a home juicer mixer. Can be mentioned. Among these, an ultrasonic stirrer is preferable from the viewpoint of CNF dispersion efficiency.

  When subjecting the slurry (X) to a hydrothermal reaction in the step (I), the temperature during the hydrothermal reaction may be 100 ° C. or higher, and preferably 100 ° C. to 160 ° C. The hydrothermal reaction is preferably performed in a pressure vessel. When the reaction is performed at 100 to 160 ° C, the pressure at this time is preferably 0.1 to 0.7 MPa, and the reaction is performed at 120 to 160 ° C. The pressure in this case is preferably 0.2 MPa to 0.6 MPa. The hydrothermal reaction time is preferably 0.1 hour to 12 hours, more preferably 0.2 hour to 6 hours.

  In the obtained slurry (X), single CNF (single cellulose nanofiber) is well dispersed. The degree of dispersion of CNF in the slurry can be quantitatively evaluated by, for example, light transmittance using a UV / visible light spectroscope or viscosity using an E-type viscometer. Specifically, for example, the slurry (X) obtained by dispersing CNF in a solvent has an E-type viscometer when 2 parts by mass of CNF is dispersed in terms of carbon atoms with respect to 100 parts by mass of the solvent. The dispersion state in which the viscosity at 25 ° C. measured by the above is 500 mPa · s or more is maintained.

In the step (II) of the production method of the present invention, the slurry (X) obtained in the step (I) and the slurry (Y) prepared from the titanium compound are subjected to a hydrothermal reaction to collect nanoparticles of titanium oxide. This is a step of obtaining a body (Z).
The titanium oxide nanoparticle aggregate (Z) obtained in the step (II) is a nanoparticle aggregate in which titanium oxide nanoparticles are supported linearly and continuously on CNF having an average fiber diameter of 50 nm or less. Then, as a precursor of the negative electrode active material for a lithium ion secondary battery of the present invention, it is fired with a lithium compound and a sodium compound in the next step (III) to become a negative electrode active material for a lithium ion secondary battery.

  The titanium compound used in the step (II) includes titanium oxide (anatase type, rutile type, brookite type), titanium complex (titanium glycolate complex, titanium citrate complex, etc.), titanium alkoxide (titanium isopropoxide, etc.), titanium One type or two or more types selected from salts (titanium acetate, titanium sulfate, titanium nitrate, titanyl sulfate, etc.), titanium chloride (titanium tetrachloride, etc.) and the like can be mentioned. Among them, from the viewpoint of efficiently proceeding a hydrothermal reaction for obtaining a nanoparticle aggregate (Z) of titanium oxide, one or two selected from titanium alkoxide, titanium chloride, titanium acetate, titanium sulfate, and titanyl sulfate. More than species are preferred.

  The slurry (Y) is a slurry prepared from the slurry (X) and the titanium compound. If the slurry (X) and the titanium compound, or the slurry (X), the titanium compound and the solvent are mixed and stirred, the slurry (Y) is prepared. Good. As a solvent to be used, water is preferable like the solvent used in the step (I). The content of the titanium compound in the obtained slurry (Y) is preferably 10 to 50 parts by mass, more preferably 15 to 30 parts by mass with respect to 100 parts by mass of the solvent in the slurry (Y). Yes, and more preferably 20 to 30 parts by mass.

Moreover, content of CNF in slurry (Y) is a carbon atom conversion amount with respect to 100 mass parts of solvent in slurry (Y), Preferably it is 0.01 mass part-10 mass parts, More preferably, it is 0. 0.05 parts by mass to 8 parts by mass.
What is necessary is just to adjust suitably the mixing ratio of slurry (X), a titanium compound, and a solvent so that content of the titanium compound and CNF in slurry (Y) may become the above content.

  The slurry (Y) is preferably agitated in advance before being subjected to a hydrothermal reaction, and a disperser (homogenizer) is preferably used for the agitation. Examples of such a disperser include the above-described disaggregator, beater, low-pressure homogenizer, high-pressure homogenizer, grinder, cutter mill, ball mill, jet mill, short-axis extruder, twin-screw extruder, ultrasonic stirrer, household juicer mixer, and the like. Can be used effectively.

  The temperature during the hydrothermal reaction in the step (II) may be 100 ° C. or higher, and preferably 130 ° C. to 180 ° C. The hydrothermal reaction is preferably performed in a pressure vessel. When the reaction is performed at 130 ° C to 180 ° C, the pressure at this time is preferably 0.3 MPa to 0.9 MPa, and the reaction is performed at 140 ° C to 160 ° C. The pressure in carrying out is preferably 0.3 MPa to 0.6 MPa. The hydrothermal reaction time is preferably 0.5 hours to 24 hours, more preferably 0.5 hours to 15 hours.

  In the step (III) to be described later, the slurry (Y) after the hydrothermal reaction can be used as it is as the titanium oxide nanoparticle aggregate (Z), but after the slurry (Y) is filtered. A nanoparticle aggregate (Z) of titanium oxide obtained by washing may be used. Such washing is performed with water after filtering the slurry (Y) after the hydrothermal reaction. The amount of water used for the washing is preferably 5 to 100 parts by mass with respect to 1 part by mass of the residue after filtration. In this case, the nanoparticle aggregate (Z) of titanium oxide is obtained as a cake (Y ′), and the cake (Y ′) is the same as the slurry (Y) when the slurry after the hydrothermal reaction is used as it is. Further, it can be used as a nanoparticle aggregate (Z) of titanium oxide.

  In the step (III) of the production method of the present invention, the titanium oxide nanoparticle aggregate (Z) obtained in the step (II) is added to a lithium compound and a sodium compound, or a lithium compound, a sodium compound and a metal (M). In this step, the compounds are mixed and then baked. The type of these lithium compounds, sodium compounds and metal (M) compounds, and whether or not they are used are determined by the composition of sodium titanate lithium composite oxide nanoparticles (B) formed on the surface of the carbon tubular body, which is the final object. What is necessary is just to determine suitably according to.

  Examples of the lithium compound used in step (III) include one or more selected from hydroxides, carbonates, and organic acid salts. More specifically, examples include lithium hydroxide, lithium carbonate, lithium acetate, and lithium oxalate. Especially, lithium acetate is more preferable from a viewpoint of making a calcination reaction advance efficiently.

  Examples of the sodium compound used in step (III) include one or more selected from hydroxides, carbonates and organic acid salts. More specifically, for example, sodium hydroxide, sodium bicarbonate, sodium carbonate, sodium acetate, sodium oxalate and the like can be mentioned. Especially, sodium acetate is more preferable from a viewpoint of making a calcination reaction advance efficiently.

  The average particle diameters of these lithium compounds and sodium compounds are 400 nm or less, more preferably 300 nm or less, more preferably 250 nm or less, from the viewpoint of efficiently proceeding the firing reaction and efficiently obtaining the active material nanoarray. It is.

The mixing ratio of the lithium compound in the step (III) is a molar ratio (Ti: Li) between the amount of titanium in the nanoparticle aggregate (Z) of titanium oxide and the amount of lithium, preferably 3: 0.95-3. : 1.05, more preferably 3: 0.97 to 3: 1.03, and still more preferably 3: 0.99 to 3: 1.01.
The mixing ratio of the sodium compound in the step (III) is a molar ratio (Ti: Na) between the amount of titanium and the amount of sodium in the nanoparticle aggregate (Z) of titanium oxide, preferably 3: 0.95. To 3: 1.05, more preferably 3: 0.97 to 3: 1.03, and even more preferably 3: 0.99 to 3: 1.01.

Furthermore, in the step (III), a metal (M) compound that is a compound other than a lithium compound, a sodium compound, and a titanium compound may be used as necessary. This metal (M) is preferably K, Be, Mg, Ca, Sr, Ba, Al, Sc, Si, P, S, Fe, Mn, Co, Ni, Zn, V, Cu, Y, Zr, Nb One, two or more metals selected from Mo, Pb, Ga, Ge, Bi, Ta, Sn, Eu, La, Ce, Nd, W, Ru or Gd, more preferably Mg, Cu, Al , Nb, more preferably Mg, Al, Nb.
Examples of such metal (M) compounds include halides, sulfates, organic acid salts, hydroxides, sulfides, oxides and hydrates thereof. Among these, from the viewpoint of allowing the firing reaction to proceed efficiently, one or more selected from sulfates and organic acid salts are preferred.

  Mixing these lithium compounds and sodium compounds, or mixing lithium compounds, sodium compounds, and metal (M) compounds gives a homogeneous mixture with nanoparticle aggregates (Z) of titanium oxide, which are fibrous particle aggregates. From the viewpoint, wet mixing is preferable. Therefore, the slurry (Y) or cake (Y ′) obtained in the above step (II) can be used as it is as the titanium oxide nanoparticle aggregate (Z) used in the step (III).

In the step (III), the order of addition of the slurry (Y) or the cake (Y ′), the lithium compound, the sodium compound, and the metal (M) compound, or a solvent that may be used as necessary is not particularly limited. From the viewpoint of enhancing the dispersibility of each component in the suspension obtained in (III), slurry (Y) or cake (Y ′), or slurry in which slurry (Y) or cake (Y ′) and a solvent are mixed in advance. (Ya) and a lithium compound, a sodium compound and a solvent, or a slurry (Yb) in which a lithium compound, a sodium compound, a metal (M) compound and a solvent are mixed, and then the slurry (Ya) and the slurry (Yb) are mixed. It is preferable to do this.
Also in the step (III), it is preferable to use water as a solvent. In the step (III), it is possible to effectively perform the mixing of the slurry (Ya) and the slurry (Yb) and the stirring of the obtained suspension to improve the mixing state of the suspension. Therefore, the amount of solvent used in step (III) may be determined by appropriately judging from the mixed state of the suspension.

  The mixing time of the suspension is preferably 0.2 hours to 5 hours, more preferably 0.3 hours to 3 hours, from the viewpoint of mixing with sufficient stirring. Moreover, the temperature at the time of mixing becomes like this. Preferably it is 50 degrees C or less, More preferably, it is 30 degrees C or less. As a mixing device, it is necessary to prevent the decomposition of the titanium oxide nanoparticle aggregate (Z) in such mixing. For example, the mixing of the suspension such as a device equipped with a magnetic stirrer or a rotor blade is performed. Usual equipment that can be used can be used.

  Next, the obtained suspension is dehydrated or dried to obtain a raw material for firing the active material nanoarray. That is, in step (III), it is preferable to perform dehydration or drying before firing. Examples of the dehydrating means include a suction filter, a filter press, and a centrifugal filter. Especially, it is preferable to use a suction filter or a filter press from a viewpoint of obtaining solid content efficiently. Examples of the drying means include spray drying, vacuum drying, freeze drying, and the like. Among these, spray drying is preferable from the viewpoint of obtaining a uniformly baked raw material without pulverization. When vacuum drying or freeze-drying is selected as the drying means, the liquid phase content is reduced as much as possible by subjecting it to solid-liquid separation using a filter press, a centrifugal filter or the like prior to drying. It is good.

Next, in the step (III), the above-obtained firing raw material may be fired, and it is preferably performed in an inert gas atmosphere or under reducing conditions. As a result, titanium oxide, lithium compound and sodium compound are reacted to form sodium lithium titanate composite oxide nanoparticles (B), and CNF contained in the titanium nanoparticle aggregate (Z) is carbonized. C-CNF (A) having high conductivity can be formed.
Here, the sodium lithium titanate composite oxide nanoparticle (B) uses titanium oxide contained in the titanium nanoparticle aggregate (Z) as a site for the generation reaction, so that the sodium lithium titanate composite oxide is formed around the CNF. At the same time as the product nanoparticles (B) are formed, such CNF forms C-CNF (A), so that it finally surrounds the periphery of C-CNF (A) on the surface of C-CNF (A). An active material nanoarray in which a large number of sodium lithium titanate composite oxide nanoparticles (B) are arranged can be obtained.

  Firing in step (III) is performed from the viewpoint of preventing enlargement of the obtained sodium lithium titanate composite oxide nanoparticles (B) and unnecessary sintering of the sodium lithium titanate composite oxide particles (B). The firing temperature is preferably 400 ° C to 800 ° C, more preferably 500 ° C to 700 ° C. The firing time is preferably 0.5 hours to 24 hours, and more preferably 6 hours to 18 hours. The apparatus used for firing is not particularly limited as long as the firing atmosphere and temperature can be adjusted, and any of batch, continuous, and heating (indirect or direct) systems can be used. . Examples of such an apparatus include firing furnaces such as an external heat kiln and a roller hearth.

  A method for producing a lithium ion secondary battery using the negative electrode active material for a lithium ion secondary battery of the present invention thus obtained is not particularly limited, and a known method can be used. For example, the negative electrode active material for a lithium ion secondary battery is mixed with additives such as a binder and a solvent to obtain a coating solution. At this time, if necessary, a conductive additive may be further added and mixed. The binder is not particularly limited, and any known agent can be used. Specific examples include polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl chloride, and ethylene propylene diene polymer. Moreover, it does not specifically limit as this conductive support agent, A well-known agent can be used. Specific examples include acetylene black, ketjen black, natural graphite, artificial graphite, and fibrous carbon. Next, this coating solution is applied onto a negative electrode current collector such as an aluminum foil or a copper foil, and dried to obtain a negative electrode.

  The negative electrode active material for lithium ion secondary batteries of the present invention is useful in that it exhibits extremely excellent rate characteristics as a negative electrode for lithium ion secondary batteries. The lithium ion secondary battery to which such a negative electrode can be applied is not particularly limited as long as it has a positive electrode, a negative electrode, an electrolytic solution, a separator, or a positive electrode, a negative electrode, and a solid electrolyte.

  Here, as for the positive electrode, as long as lithium ions can be released during charging and occluded during discharging, the material structure is not particularly limited, and a known material structure can be used. For example, a positive electrode made of various polyanion-type positive electrode active materials obtained by hydrothermal reaction of raw materials can be suitably used.

  The electrolytic solution is obtained by dissolving a supporting salt in an organic solvent. The organic solvent is not particularly limited as long as it is an organic solvent usually used for an electrolyte solution of a lithium ion secondary battery. For example, carbonates, halogenated hydrocarbons, ethers, ketones, nitriles, lactones An oxolane compound or the like can be used.

The type of the supporting salt is not particularly limited, but an inorganic salt selected from LiPF ₆ , LiBF ₄ , LiClO ₄ and LiAsF ₆ , a derivative of the inorganic salt, LiSO ₃ CF ₃ , LiC (SO ₃ CF ₃ ) ₂ and LiN (SO ₃ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ and LiN (SO ₂ CF ₃ ) (SO ₂ C ₄ F ₉ ), and organic salt derivatives It is preferable that it is at least 1 type of these.

  The separator plays a role of electrically insulating the positive electrode and the negative electrode and holding the electrolytic solution. For example, a porous synthetic resin film, particularly a polyolefin polymer (polyethylene, polypropylene) porous film may be used.

The solid electrolyte electrically insulates the positive electrode and the negative electrode and exhibits high lithium ion conductivity. For example, La _0.51 Li _0.34 TiO _2.94 , Li _1.3 Al _0.3 Ti _1.7 (PO ₄ ) ₃ , Li ₇ La ₃ Zr ₂ O ₁₂ , 50Li ₄ SiO ₄ .50Li ₃ BO ₃ , Li _2.9 PO _3.3 N _0.46 , Li _3.6 Si _0.6 P _0.4 O ₄ , Li _1.07 Al _0.69 Ti _1.46 (PO ₄ ) ₃ , Li _1.5 Al _0.5 Ge _1.5 (PO ₄ ) ₃ , Li ₁₀ GeP ₂ S ₁₂ , Li _3.25 Ge _0.25 P _0.75 S ₄ , 30Li ₂ S · 26B ₂ S ₃ · 44LiI, 63Li ₂ S · 36SiS ₂ · 1Li ₃ PO ₄ , 57Li ₂ S · 38SiS ₂ · 5Li ₄ SiO ₄ , 70Li ₂ S · 30P ₂ S ₅ , 50Li ₂ S · 50GeS ₂ , Li ₇ P ₃ S ₁₁ , Li _3.25 P _0.95 S ₄ may be used.

  The shape of the lithium ion secondary battery having the above configuration is not particularly limited, and may be various shapes such as a coin shape, a cylindrical shape, a square shape, and an indefinite shape sealed in a laminate outer package. .

  EXAMPLES Hereinafter, although this invention is demonstrated concretely based on an Example, this invention is not limited to these Examples.

[Example 1]
50 mL of water was mixed with 12.5 g of cellulose nanofiber (TMa-10002 manufactured by Sugino Machine Co., Ltd., average fiber diameter 10 nm, average fiber length 10 μm, water content 98 mass%) to prepare slurry A, and then slurry A was charged into the autoclave. Then, a hydrothermal reaction at 140 ° C. and 0.4 MPa was performed for 1 hour to obtain slurry B.
The viscosity at 25 ° C. of slurry B (CNF concentration (converted to carbon atom weight) of 0.4 mass%) was 200 mPa · s as measured using an E-type viscometer (DVM-E2, manufactured by Tokimec).
After the slurry B obtained was mixed with 17.05 g (60 mmol) of [(CH ₃ ) ₂ CHO] ₄ Ti to form a slurry C, the slurry C was put into an autoclave and water at 140 ° C. and 0.4 MPa was added. A thermal reaction was performed for 3 hours to obtain a slurry D containing 8% by mass of the titanium oxide nanoparticle aggregate (Z). The average particle diameter of titanium oxide in the obtained nanoparticle aggregate (Z) of titanium oxide was 30 nm.

The obtained slurry D was mixed with 2.03 g (20 mmol) of CH ₃ COOLi · 2H ₂ O and 1.64 g (20 mmol) of CH ₃ COONa to make slurry E, and then mixed for 1 hour using a ball mill to form a slurry. E was obtained. Next, the obtained slurry E was filtered with an evaporator, and the obtained fired raw material F was fired at 600 ° C. for 10 hours in a nitrogen atmosphere to form an active material nanoarray (lithium sodium titanate on the surface of a carbon tubular body having an average outer diameter of 10 nm). Negative electrode active material A for lithium ion secondary battery (a large number of composite oxide nanoparticles are arranged) (average particle size of sodium lithium titanate composite oxide nanoparticles 40 nm, BET specific surface area 38 m ² / g, carbon content 2.5 Mass%).
The SEM photograph of the negative electrode active material A for lithium ion secondary batteries is shown in FIG.

[Comparative Example 1]
A planetary ball mill (P-5, manufactured by Fritsch) was used with 30 mL of ethanol using 2.03 g (20 mmol) of CH ₃ COOLi · 2H ₂ O, 1.64 g (20 mmol) of CH ₃ COONa and 4.79 g (60 mmol) of TiO _2. After mixing at 25 ° C. for 15 hours, the mixture was dried to obtain a mixture G. The obtained mixture G was fired at 900 ° C. for 10 hours in an air atmosphere to obtain sodium lithium titanate composite oxide particles H. The average particle diameter of the obtained sodium lithium titanate composite oxide particles H was 500 nm.
2 g of the obtained sodium lithium titanate composite oxide particles H were collected, and 0.25 g (5% by mass in terms of carbon) of glucose was added and mixed therewith to obtain a firing raw material I. Next, the obtained firing raw material I is fired at 700 ° C. for 1 hour in a nitrogen atmosphere, and is composed of sodium titanate lithium composite oxide particles coated with glucose-derived carbon on the surface, for a lithium ion secondary battery having a substantially spherical shape. Negative electrode active material B (average particle diameter of lithium sodium titanate composite oxide particles 500 nm, BET specific surface area 11 m ² / g, carbon content 2.5 mass%) was obtained.
An SEM photograph of the negative electrode active material B for a lithium ion secondary battery is shown in FIG.

Using the negative electrode active material for lithium ion secondary batteries obtained in Example 1 and Comparative Example 1, a negative electrode for a lithium ion secondary battery was produced.
Specifically, a negative electrode active material (J) for a lithium ion secondary battery, acetylene black (conductive agent, K), and polyvinylidene fluoride (binding agent, L), J: K: L (mass ratio) = 80 Was mixed at 10:10, and N-methyl-2-pyrrolidone was added thereto and kneaded sufficiently to prepare a negative electrode slurry.
The obtained negative electrode slurry was applied to a current collector made of a copper foil having a thickness of 10 μm using a coating machine, and vacuum dried at 80 ° C. for 12 hours. Then, it pressed with the roll press heated at 80 degreeC, and it punched in the disk shape of (phi) 14mm, and set it as the negative electrode.

Next, a coin-type lithium ion secondary battery was constructed using the negative electrode. Lithium foil was used for the positive electrode. As the electrolytic solution, a solution obtained by dissolving LIPF ₆ at a concentration of 1 mol / L in a mixed solvent in which ethylene carbonate: ethyl methyl carbonate (volume ratio) = 3: 7 was used. Polypropylene was used for the separator.
These battery components were assembled and housed in a conventional manner under an atmosphere having a dew point of −50 ° C. or lower to produce a coin-type lithium ion secondary battery (CR-2032).

A charge / discharge test at a constant current density was performed using the manufactured lithium ion secondary battery, and a charge / discharge capacity was measured. The charging conditions at this time were constant current charging with a current density of 90 mA / g and a voltage of 3.0 V, and the discharging conditions were constant current discharging with a current density of 90 mA / g and a final voltage of 1.0 V. All temperatures were 30 ° C. Furthermore, the discharge capacity when the current density during discharge was 281 mA / g under the same charge / discharge conditions was also determined, and the ratio of the discharge capacity of 90 mA / g and 281 mA / g was determined by the following equation (2).
Ratio of discharge capacity (%) = (discharge capacity of 281 mA / g) / (discharge capacity of 90 mA / g) × 100 (2)
The results are shown in Table 1.

  From the above results, it can be seen that the active material nanoarray of the present invention having a unique shape exhibits excellent rate characteristics.

Claims (6)

On the surface of the carbon tubular body having an average outer diameter of 50 nm or less, the following formula (1):
Li _a Na _b M _c Ti _d O _e (1)
(In the formula (1), M is K, Be, Mg, Ca, Sr, Ba, Al, Sc, Si, P, S, Fe, Mn, Co, Ni, Zn, V, Cu, Y, Zr, Nb. , Mo, Pb, Ga, Ge, Bi, Ta, Sn, Eu, La, Ce, Nd, W, Ru, or Gd represents one or more, a, b, c, d, and e are 0 <a ≦ 4, 0 <b ≦ 4, 0 ≦ c ≦ 6, 0 <d ≦ 6, 13 ≦ e ≦ 15, and a number satisfying a + b + c × (valence of M) + 4d = 2e)
A method for producing a negative electrode active material for a lithium ion secondary battery comprising a large number of sodium lithium titanate composite oxide nanoparticles represented by the following steps (I) to (III):
(I) A step of obtaining slurry (X) by dispersing cellulose nanofibers having an average fiber diameter of 50 nm or less in a solvent,
(II) subjecting the obtained slurry (X) and the slurry (Y) prepared from the titanium compound to a hydrothermal reaction to obtain a nanoparticle aggregate (Z) of titanium oxide,
(III) Lithium ion secondary comprising a step of mixing the obtained titanium oxide nanoparticle aggregate (Z) with a lithium compound and a sodium compound, or a lithium compound, a sodium compound and a metal (M) compound, followed by firing. A method for producing a negative electrode active material for a battery.   2. The negative electrode active material for a lithium ion secondary battery according to claim 1, wherein the content of the titanium compound in the slurry (Y) prepared in the step (II) is 10 parts by mass to 50 parts by mass with respect to 100 parts by mass of the solvent. Manufacturing method.   The lithium compound secondary battery according to claim 1 or 2, wherein the titanium compound used in step (II) is one or more selected from titanium alkoxide, titanium chloride, titanium acetate, titanium sulfate, and titanyl sulfate. A method for producing a negative electrode active material.   The negative electrode for a lithium ion secondary battery according to any one of claims 1 to 3, wherein an average particle diameter of the lithium sodium titanate composite oxide nanoparticles arranged on the surface of the carbon tubular body is 5 nm to 450 nm. A method for producing an active material.   The manufacturing method of the negative electrode active material for lithium ion secondary batteries of any one of Claims 1-4 which performs baking in process (III) under inert gas atmosphere or reducing conditions.   Content of the carbon tubular body in the negative electrode active material for lithium ion secondary batteries is 0.5 mass%-10 mass%, The negative electrode for lithium ion secondary batteries of any one of Claims 1-5. A method for producing an active material.