JP6026359B2 - Lithium titanate negative electrode active material - Google Patents

Description

  The present invention relates to a lithium titanate negative electrode active material capable of exhibiting good battery characteristics.

  Conventionally, a highly conductive substance is used as a positive electrode material or a negative electrode material in order to improve battery performance. In recent years, next-generation batteries such as lithium ion batteries have been increasingly used. As a negative electrode material in such batteries, a carbon-based material is mainly used, but for example, titanic acid described in Non-Patent Document 1 The use of lithium compounds is also being studied. In order to employ such a lithium titanate compound as a negative electrode material for a next-generation battery, it is necessary to ensure good battery physical properties by making the particles finer because the conductivity of the lithium titanate compound itself is low.

Hongsen Li et al, Journal of Power Source 221 (2013), p122-127

  However, simply refining the particles of the lithium titanate compound tends to reduce the tap density, and there is a possibility that desired battery characteristics cannot be sufficiently exhibited.

  Therefore, the subject of this invention is providing the lithium titanate negative electrode active material which can express the outstanding battery physical property as a negative electrode material of a lithium ion battery.

Accordingly, the present inventors have made various studies and found that a lithium titanate negative electrode active material obtained by performing a specific treatment has a large tap density and can exhibit excellent battery properties, thereby completing the present invention. I came to let you.

  That is, the present invention provides a lithium titanate negative electrode active material obtained by mixing a lithium titanate compound and conductive carbon and then mixing them while adding compressive force and shear force. .

  Since the lithium titanate negative electrode active material of the present invention is a particle in which a lithium titanate compound and conductive carbon are dispersed extremely uniformly and with reduced voids, the tap density is large and the performance of the lithium ion battery is improved. It is expected to contribute greatly.

2 is a SEM image showing particles of a lithium titanate negative electrode active material obtained in Example 1.

Hereinafter, the present invention will be described in detail.
The lithium titanate negative electrode active material of the present invention is obtained by mixing a lithium titanate compound and conductive carbon and then mixing them while adding compressive force and shearing force. By undergoing such treatment, the voids were reduced by forming particles (hereinafter also referred to as “composite particles”) by firmly aggregating the lithium titanate compound and the conductive carbon while being uniformly dispersed. Composite particles can be obtained. Alternatively, a conductive carbon layer can be formed by attaching to the surface of particles (hereinafter also referred to as “primary particles”) exhibited by a lithium titanate compound while deforming or extending the conductive carbon. It is preferable to use an airtight container provided with an impeller that rotates at a peripheral speed of 25 to 40 m / s for the process of mixing while applying a compressive force and a shearing force. By putting a lithium titanate compound and conductive carbon into such a container and operating the container, it is possible to perform a mixing process while applying compressive force and shearing force. In a closed container equipped with such an impeller, the lithium titanate compound and conductive carbon are uniformly mixed by the rotation of the impeller, and a shearing force is also applied while a compressive force is applied between the impeller and the inner wall of the container. It becomes. The peripheral speed of the impeller is preferably 25 to 40 m / s, more preferably 27 to 35 m / s, from the viewpoint of increasing the tap density of the resulting composite particles.

  In addition, from the viewpoint of increasing the uniformity of the obtained composite particles and shortening the processing time in the closed container equipped with the impeller, before introducing the lithium titanate compound and the conductive carbon into the closed container. These may be mixed in advance.

Examples of the apparatus provided with a closed container that can be mixed while applying such compressive force and shearing force include a high-speed shear mill, a blade-type kneader, and the like. (Manufactured by Hosokawa Micron Corporation) can be preferably used. By using such an apparatus, it is possible to easily perform a mixing process while applying a predetermined compressive force and shearing force, and the lithium titanate negative electrode active material of the present invention can be obtained only by performing such a process. it can.
As the processing conditions for the mixing, the processing temperature is preferably 5 to 80 ° C., more preferably 10 to 50 ° C., and the processing time is preferably 5 to 90 minutes, more preferably 10 to 60 minutes. The treatment atmosphere is not particularly limited, but is preferably an inert gas atmosphere or a reducing gas atmosphere.

  The mass ratio of the lithium titanate compound and the conductive carbon charged into the closed container is preferably 97: 3 to 85:15, more preferably 95: 5 to 88, from the viewpoint of improving the physical properties of the obtained battery. : 12, and more preferably 93: 7 to 90:10.

  In addition, you may bake the obtained composite particle from a viewpoint of improving battery physical property more. The firing conditions are 400 ° C. or higher, preferably 400 to 800 ° C. for 10 minutes to 3 hours, preferably 0.5 to 1.5 hours under an inert gas atmosphere or reducing conditions.

The lithium titanate compound is preferably primary particles obtained by firing a titanate compound and a lithium compound.
Examples of titanic acid compounds that can be used include hydrous titanium oxide such as titanium oxide, orthotitanic acid, and metatitanic acid. These may be used alone or in combination of two or more. Among these, titanium oxide is preferable from the viewpoint of improving battery physical properties.
Examples of the lithium compound that can be used include lithium oxide and lithium hydroxide. Specific examples include lithium carbonate, lithium hydroxide, lithium nitrate, lithium oxide, lithium oxalate, and lithium acetate. These may be used alone or in combination of two or more. Among these, lithium carbonate is preferable from the viewpoint of improving battery physical properties.

  As a method for obtaining primary particles of the lithium titanate compound by firing the titanate compound and the lithium compound, for example, a method described in Journal of Power Source 221 ((2013), p122-127) can be used.

Specifically, for example, first, the titanate compound and the lithium compound may be mixed and pulverized as powder, or may be mixed and pulverized using an alcohol such as water or ethanol as a solvent. For the mixing / pulverization treatment, for example, a ball mill or a bead mill can be used. Next, the mixture is obtained by drying.
Next, the obtained mixture is fired. The firing conditions are, for example, a firing temperature of 700 to 900 ° C. and a firing time of 8 to 24 hours. Further, when firing, an inert gas atmosphere such as nitrogen or argon may be used, or an oxygen atmosphere or an air atmosphere may be used.
By pulverizing the obtained fired product, a lithium titanate compound as a raw material can be obtained.

  The average particle size X of the lithium titanate compound is preferably 20 to 200 nm, more preferably 20 to 150 nm, from the viewpoint of improving battery physical properties obtained by increasing the uniformity as composite particles. More preferably, it is 20-100 nm. The average particle diameter X means a value measured by a dynamic light scattering particle size analyzer (Nanotrack UPA-EX150, manufactured by Nikkiso Co., Ltd.) after uniformly dispersing the sample with a solvent.

  Carbon black is preferable as the conductive carbon, and specific examples include acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black. Of these, acetylene black and ketjen black are preferred from the viewpoint of imparting good conductivity. The conductive carbon has a hollow shape from the viewpoint of further improving the physical properties of the battery obtained by coating at least a part of the surface of the primary particles of the lithium titanate compound with a layer made of conductive carbon. Or a shape including voids is preferred.

  In addition, the conductive carbon used in the present invention preferably has an average particle size Y that is equal to or less than the average particle size X of the primary particles of the lithium titanate compound. Since the conductive carbon has such an average particle diameter, the conductive carbon is efficiently arranged in the gap between the lithium titanate compound particles and the particles, and the composite particles have high uniformity with reduced voids. Can be obtained.

  The ratio (X / Y) of the average particle diameter X possessed by the lithium titanate compound and the average particle diameter Y possessed by the conductive carbon is a viewpoint in which the lithium titanate compound particles are more efficiently disposed between the particles, And from a viewpoint of aiming at the improvement of the battery physical property obtained, Preferably it is 1-20, More preferably, it is 1.5-10. Moreover, the average particle diameter Y which electroconductive carbon has becomes like this. Preferably it is 10-100 nm, More preferably, it is 10-50 nm. In addition, this average particle diameter Y means the value measured by the method similar to the average particle diameter X of the said lithium titanate compound.

The tap density of the lithium titanate negative electrode active material of the present invention is preferably 0.8 to 2.5 g / cm ³ , more preferably 1.0 to 2.5 g / cm ³ , and still more preferably 1. 2 to 2.5 g / cm ³ . Therefore, the lithium titanate negative electrode active material of the present invention is a particle in which voids are extremely reduced, and these lithium titanate compound and conductive carbon are dispersed very uniformly. If used, it is possible to easily improve the physical properties of the battery obtained. The tap density of the lithium titanate negative electrode active material is the powder volume V when no change in volume is recognized by mechanically tapping a measuring container containing a powder sample m (g) of known weight. (Cm ³ ) is read, and the value calculated using the formula m / V is averaged.

  Moreover, the lithium titanate compound and the conductive carbon in the lithium titanate negative electrode active material of the present invention are primary components of the lithium titanate compound from the viewpoint of enhancing the uniformity and dispersibility thereof and the physical properties of the obtained battery. It is preferable that at least a part of the surface of the particle is covered with a layer made of conductive carbon. The layer made of conductive carbon may cover at least a part of the surface of the primary particles, or may cover almost the entire surface of the primary particles. Thereby, the primary particles of the lithium titanate compound and the conductive carbon can be prevented from aggregating, and the lithium titanate negative electrode active material which is a composite particle in which the conductive carbon is more densely and uniformly dispersed is obtained. As a result, the conductivity can be further increased.

  The thickness of the layer made of conductive carbon is preferably 0.1 to 5.0 nm, more preferably 0.5 to 3.0 nm.

  The average particle size Z of the lithium titanate negative electrode active material of the present invention is 5 to 50 μm, preferably 5 from the viewpoint of weight reduction while maintaining excellent battery properties in the obtained lithium ion battery. It is -30 micrometers, More preferably, it is 5-20 micrometers. Thus, since the lithium titanate negative electrode active material of the present invention is a fine composite particle containing a uniformly dispersed lithium titanate compound and conductive carbon, a negative electrode is formed using this. Thus, a lithium ion battery having excellent battery properties can be obtained. In addition, this average particle diameter Z means the value measured by the method similar to the said average particle diameter X and Y.

  In addition, the lithium titanate negative electrode active material of the present invention may be doped with an atom other than a lithium atom, a titanium atom, or an oxygen atom as long as the effects of the present invention are not impaired. The atoms that can be doped include Be, Mg, Ca, Sc, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Al, Si, Ga, Ge, Y, Sn, Ce, Eu, La, and W. , Ru and the like.

  The method for producing a lithium ion battery using the thus obtained lithium titanate negative electrode active material of the present invention is not particularly limited, and any known method can be used. For example, the lithium titanate negative electrode active material is mixed with additives such as a binder and a solvent to obtain a coating liquid. 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, Any well-known agent can be used. Specific examples include acetylene black, ketjen black, natural graphite, artificial graphite, and fibrous carbon. Subsequently, this coating liquid is apply | coated on negative electrode collectors, such as aluminum foil, and it is made to dry and is set as a negative electrode.

  The lithium titanate negative electrode active material of the present invention is useful in that it exhibits a very excellent discharge capacity as a negative electrode of a lithium ion battery. A lithium 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, and a separator as essential components.

  Here, as for the positive electrode, as long as lithium ions can be released during charging and occluded during discharging, the material configuration is not particularly limited, and a known material configuration can be used. For example, it is preferable to suitably use various olivine compounds obtained by hydrothermal reaction of the raw materials.

  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 that is 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.

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

[Production Example 1]
In accordance with the method described in Journal of Power Source 221 ((2013), p122-127) using titanium oxide and lithium carbonate, mixing, pulverization, and baking treatment (baking temperature of 800 ° C., baking time of 10 hours) are performed. Primary particles (average particle size 100 nm) of the lithium acid compound were obtained.

[Example 1]
93.0 g of primary particles of the lithium titanate compound obtained in Production Example 1 and 7.0 g of Ketjen Black (manufactured by Lion Corporation, average particle size of 30 nm) were mixed in advance to obtain a mixture. Fine particle composite apparatus Nobilta (manufactured by Hosokawa Micron Corporation) was charged and mixed at 25 to 35 ° C. for 30 minutes to obtain composite particles A. The obtained composite particles A had an average particle size of 20 μm and a tap density of 1.45 g / cm ³ .
The SEM image of the obtained composite particle A is shown in FIG.

[Comparative Example 1]
Add 2.63 g of primary particles of the lithium titanate compound obtained in Production Example 1, 0.09 g of Ketjen Black (Lion, average particle size 30 nm) and a dispersion stabilizer (carboxymethylcellulose, manufactured by Daicel Finechem) 30 g of water was put into a zirconia pot of a planetary ball mill (Planet Ball Mill P-5, manufactured by Fritsch) together with zirconia balls, mixed at 25 to 70 ° C. for 240 minutes, and dried to obtain composite particles B It was. The obtained composite particles B had an average particle size of 2 μm and a tap density of 0.93 g / cm ³ .

[Test Example 1]
Using the composite particles obtained in Example 1 and Comparative Example 1, a negative electrode of a lithium ion secondary battery was produced. The composite obtained in Example 1 and Comparative Example 1, ketjen black (conductive agent), and polyvinylidene fluoride (binding agent) were mixed at a weight ratio of 80:10:10, and this was mixed with N-methyl. -2-Pyrrolidone was added and sufficiently kneaded to prepare a negative electrode slurry. The negative electrode slurry was applied to a current collector made of an aluminum foil having a thickness of 20 μm using a coating machine, and vacuum dried at 80 ° C. for 12 hours. Thereafter, it was punched into a disk shape of φ14 mm and pressed at 16 MPa for 2 minutes using a hand press to obtain a 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 and ethyl methyl carbonate were mixed at a volume ratio of 1: 1 was used. As the separator, a known one such as a polymer porous film such as polypropylene was used. These battery components were assembled and housed in a conventional manner in an atmosphere with a dew point of −50 ° C. or lower to produce a coin-type lithium 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 of 0.1 CA (17.5 mA / g) and a voltage of 2.5 V, and the discharging conditions were constant current discharging with a current of 0.1 CA and a final voltage of 1.0 V. All temperatures were 30 ° C.

  From the above results, the composite particle A obtained in Example 1 is extremely uniform and has reduced voids as compared to the composite particle B obtained in Comparative Example 1. Therefore, the tap density It can be seen that the secondary battery using the same exhibits excellent battery properties. On the other hand, although the composite particle B obtained in Comparative Example 1 is a fine particle, it does not contain a sufficient amount of conductive carbon and has many voids, so that the tap density is reduced, and the battery physical properties are reduced. This is thought to have caused a decline.

Claims (4)

After mixing the lithium titanate compound and ketjen black , a process of mixing while further applying compressive force and shearing force is performed at 10-50 ° C. in an airtight container equipped with an impeller rotating at a peripheral speed of 25-40 m / s . The manufacturing method of the lithium titanate negative electrode active material for lithium ion batteries whose tap density is 1.2-2.5 g / cm < ³ > performed for 10 to 60 minutes at temperature. The method for producing a lithium titanate negative electrode active material for a lithium ion battery according to claim 1 , wherein the lithium titanate compound is a fired product of a titanate compound and a lithium compound. The method for producing a lithium titanate negative electrode active material for a lithium ion battery according to claim 2 , wherein the lithium compound is lithium carbonate. The method for producing a lithium titanate negative electrode active material for a lithium ion battery according to any one of claims 1 to 3 , wherein a mass ratio of the lithium titanate compound and ketjen black is 97: 3 to 85:15.