JP2013125648A - Positive electrode active material for lithium ion battery, and lithium ion battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a positive electrode active material for a lithium ion battery, containing an olivine-type silicate compound composed of primary particles and secondary particles having a specific size and shape and exhibiting an excellent battery physical property, and a lithium ion battery obtained from the same.SOLUTION: A positive electrode active material for a lithium ion battery contains an olivine-type silicate compound represented by LiMSiO(where M represents one or two kinds or more selected from Fe, Ni, Co, and Mn.) The olivine-type silicate compound has secondary particles having an average grain size of 1-100 μm, and the secondary particles being an aggregation of spherical primary particles having an average grain size of 20-100 nm.

Description

  The present invention relates to a positive electrode active material for a lithium ion battery and a lithium ion battery obtained therefrom.

  Lithium ion batteries are a type of non-aqueous electrolyte battery and are used in a wide range of fields such as mobile phones, digital cameras, notebook PCs, hybrid cars, and electric cars. Lithium ion batteries mainly use lithium metal oxide as a positive electrode material and a carbon material such as graphite as a negative electrode material.

As this positive electrode material, lithium cobaltate (LiCoO ₂ ), lithium manganate (LiMnO ₂ ), lithium iron phosphate (LiFePO ₄ ), lithium iron silicate (Li ₂ FeSiO ₄ ) and the like are known. Among these, LiFePO ₄ and Li ₂ FeSiO ₄ have an olivine structure and are useful as a positive electrode material for a high capacity lithium ion battery. Among these, lithium metal phosphate-based positive electrode materials such as LiFePO ₄ are known in which the particle size, particle size distribution, shape, and the like of the formed particles are controlled in order to further improve the physical properties of the obtained battery ( Patent Documents 1 and 2).

On the other hand, as a method for producing a lithium silicate metal positive electrode material such as Li ₂ FeSiO ₄ , a solid phase method in which a mixture of a Li source, an iron (metal) source and a silicate source is pulverized and fired at 500 to 900 ° C. Is common (Patent Documents 3 and 4). However, in the solid phase method, it is necessary to perform firing and pulverization in an inert gas atmosphere, which requires complicated operations and it is difficult to control the particle size and crystallinity.
On the other hand, Non-Patent Document 1 describes that Li ₂ Mn _1-y Fe _y SiO ₄ (y = 0 to 1) can be obtained by hydrothermal synthesis.

Japanese Patent No. 4190912 JP 2006-32241 A JP 2001-266882 A JP 2002-198050 A

GS Yuasa Technical Report June 2009, Vol. 6, No. 1, p21-26

Under these circumstances, the inventors have found that if the primary particle size constituting the lithium metal silicate-based positive electrode material is made finer, the battery physical properties are further improved. It was also found that even when the three silicate sources were mixed with water and the mixture was subjected to a hydrothermal reaction as it was, the particle diameter of the resulting Li ₂ FeSiO ₄ could not be controlled sufficiently. Therefore, for lithium metal silicate positive electrode materials such as Li ₂ FeSiO _4, those having a controlled particle size of primary particles and secondary particles that can sufficiently improve battery physical properties are not yet known. It is a fact.

  Accordingly, an object of the present invention is to provide a positive electrode active material for a lithium ion battery that contains an olivine-type silicate compound composed of primary particles and secondary particles having a specific particle size and shape, and exhibits excellent battery properties, and obtained therefrom. The object is to provide a lithium ion battery.

  Accordingly, the present inventors have developed a positive electrode for a lithium ion battery that exhibits excellent battery physical properties by using an olivine-type silicate compound having secondary particles whose particle diameter is controlled by agglomerating fine spherical primary particles. The present inventors have found that an active material can be obtained and have completed the present invention.

That is, the present invention contains an olivine-type silicate compound represented by Li ₂ MSiO ₄ (wherein M represents one or more selected from Fe, Ni, Co, or Mn),
Provided is a positive electrode active material for a lithium ion battery, characterized in that the olivine-type silicate compound has secondary particles having an average particle diameter of 1 to 100 μm formed by agglomerating spherical primary particles having an average particle diameter of 20 to 100 nm. It is.
Moreover, this invention provides the lithium ion battery which has a positive electrode containing the said positive electrode active material for lithium ion batteries.

  According to the positive electrode active material for a lithium ion battery of the present invention, a lithium ion battery having a high capacity and excellent charge / discharge characteristics can be obtained.

The SEM image of the spherical primary particle which comprises the olivine type | mold silicate compound obtained in Example 1 is shown. The SEM image of the secondary particle which comprises the olivine type | mold silicate compound obtained in Example 1 is shown. The TEM image of a part of secondary particle shown in FIG. 2 is shown. The charging / discharging curve of the battery using the positive electrode active material for lithium ion batteries obtained in Example 1 is shown.

Hereinafter, the present invention will be described in detail.
The olivine-type silicate compound used in the present invention is represented by Li ₂ MSiO ₄ (wherein M represents one or more selected from Fe, Ni, Co, or Mn). Specific examples of the olivine silicate compound include Li ₂ FeSiO ₄ , Li ₂ NiSiO ₄ , Li ₂ CoSiO ₄ , Li ₂ MnSiO ₄ , Li ₂ (Fe) _m (Mn) _1-m SiO ₄ (0 <m < 1). Among these, Li ₂ FeSiO ₄ and Li ₂ MnSiO ₄ are preferable from the viewpoint of raw material cost, and Li ₂ FeSiO ₄ is more preferable.

The olivine-type silicate compound has secondary particles formed by agglomerating spherical primary particles. The spherical primary particles are spherical particles composed of a plurality of crystallites which are fine crystals that can be regarded as single crystals. Here, the spherical shape means the shape of particles grown almost equally in the three-dimensional direction, and is not particularly limited to a true sphere, but includes a shape that approximates a sphere. Therefore, plate shape and flat shape are not included. The average particle diameter of the spherical primary particles is 20 to 100 nm, preferably 30 to 70 nm, and more preferably 30 to 50 nm.
In addition, the average particle diameter of a primary particle means the value calculated | required by SEM observation. Specifically, in the 1100 × 800 nm ² field of view, it means the average of the values of the particle sizes obtained by randomly extracting 20 primary particles.

  As described above, since the spherical primary particles are very fine particles, it is easy to occlude / release lithium ions in the olivine-type silicate compound crystal, which contributes to increasing the battery capacity as a positive electrode material. In addition, when obtaining the positive electrode active material, when the secondary particles obtained by aggregating the primary particles are supported on carbon and fired, the carbon forms a uniform carbon layer on the surface of the primary particles and the secondary particles, so that the space between the particles is moderate. By bonding the particles to each other, the gap between particles can be reduced, so that the conductivity can be sufficiently increased.

  The thickness of the carbon layer formed on the surfaces of the primary particles and the secondary particles is preferably 0.1 to 3 nm, more preferably 1 to 2.5 nm.

  The spherical primary particles are composed of a plurality of crystallites which are fine crystals regarded as single crystals, and the crystallites preferably have a particle size of 20 to 50 nm, more preferably 20 to 30 nm. The crystallite particle size can be determined from the half-value width of the diffraction angle measured by X-ray diffraction using the Debye-Scherrer equation.

The average particle diameter of secondary particles formed by agglomerating a plurality of fine spherical primary particles is 1 to 100 μm, preferably 5 to 50 μm, and more preferably 5 to 20 μm. Since the olivine-type silicate compound has the secondary particles, it exhibits excellent coating properties, so that a smoother and more uniform electrode layer can be formed.
In addition, the average particle diameter of secondary particles means the value calculated | required by SEM observation similarly to a primary particle.

The manufacturing method of the positive electrode active material for lithium ion batteries of this invention is as follows.
First, an olivine type silicate compound represented by Li ₂ MSiO ₄ is subjected to a hydrothermal reaction to form primary particles. In the hydrothermal reaction, it is preferable to prepare a basic aqueous dispersion containing a lithium compound, a silicate compound and an antioxidant using a transition metal (M) source.

As the transition metal (M) source, for example, transition metal sulfate represented by MSO ₄ (wherein M represents Fe, Ni, Co or Mn) or (R) ₂ M (wherein R is organic) Represents an acid residue, M represents an organic acid transition metal salt represented by Fe, Ni, Co or Mn, or (R) ₂ M (wherein R represents an organic acid residue, and M represents Fe , Ni, Co or Mn)).

Specific examples of the transition metal sulfate MSO ₄ include FeSO ₄ , NiSO ₄ , CoSO _{4, and} MnSO ₄ , and these may be used alone or in combination of two or more. Of these, FeSO ₄ and MnSO ₄ are more preferable, and FeSO ₄ is more preferable. When using a transition metal sulfate, a basic aqueous dispersion containing a lithium compound, a silicate compound and an antioxidant is prepared in advance separately from the transition metal sulfate from the viewpoint of suppressing side reactions. preferable. In this case, the aqueous dispersion and the transition metal sulfate are mixed and subjected to a hydrothermal reaction. The amount of transition metal sulfate added is preferably 0.15 to 1.50 mol / l in the reaction mixture, and more preferably 0.50 to 0.75 mol / l. In this case, the content of Si and Li in the reaction mixture is preferably 2 mol or more with respect to M.

The organic acid represented by an organic acid transition metal salt (R) of ₂ M R, preferably an organic acid having 1 to 20 carbon atoms, more preferably an organic acid having 2 to 12 carbon atoms. More specific organic acids include dicarboxylic acids such as oxalic acid and fumaric acid, hydroxycarboxylic acids such as lactic acid, and fatty acids such as acetic acid. When using an organic acid transition metal salt, it is preferable to prepare a basic aqueous dispersion containing a lithium compound, a silicic acid compound and an antioxidant, and further containing an organic acid transition metal salt. Usually, an organic acid salt is a raw material used in a solid phase method, but side reactions can be suppressed by using it in a hydrothermal reaction. In this case, Si and Li in the reaction mixture are preferably used in a molar ratio of 2 or more with respect to the transition metal, and Si: Li: M is from 1: 1: 2.5 to 1: 1: 3. The degree is more preferable.

Examples of the lithium compound include lithium hydroxide (for example, LiOH.H ₂ O), lithium carbonate (Li ₂ CO ₃ ), lithium sulfate, and lithium acetate, and lithium hydroxide and lithium carbonate are particularly preferable. The concentration of the lithium compound in the aqueous dispersion is preferably 0.30 to 3.00 mol / l, more preferably 1.00 to 1.50 mol / l.

The silicic acid compound is not particularly limited as long as it is a reactive silica compound, and amorphous silica and Na ₄ SiO ₄ (for example, Na ₄ SiO ₄ .H ₂ O) are preferable. Of these, the use of Na ₄ SiO ₄ is more preferable because the aqueous dispersion becomes basic. The concentration of the silicate compound in the aqueous dispersion is preferably 0.15 to 1.50 mol / l, more preferably 0.50 to 0.75 mol / l.

Examples of the antioxidant include sodium hydrosulfite (Na ₂ S ₂ O ₄ ), aqueous ammonia, sodium sulfite and the like. The content of the antioxidant in the aqueous dispersion is preferably equal to or less than the transition metal (M) because the formation of the olivine-type silicate compound is suppressed when added in a large amount. The molar ratio is more preferably 0.5 or less.

When transition metal sulfate MSO ₄ (wherein M represents Fe, Ni, Co, or Mn) is used as the transition metal source, a lithium compound, silica, and silica are separated from the transition metal sulfate from the viewpoint of suppressing side reactions. It is preferable to prepare in advance a basic aqueous dispersion containing an acid compound and an antioxidant. In this case, the aqueous dispersion and the transition metal sulfate are mixed and subjected to a hydrothermal reaction. In preparing the aqueous dispersion, the order of addition of the lithium compound, the silicate compound and the antioxidant is not particularly limited, and these three components may be added to water.

The aqueous dispersion is preferably basic in terms of preventing side reactions and dissolving the silicate compound. Specifically, the pH of the aqueous dispersion is preferably 12.0 to 13.5. The pH of the aqueous dispersion may be adjusted by adding a base such as sodium hydroxide, but it is preferable to use Na ₄ SiO ₄ as the silicate compound.

  The hydrothermal reaction should just be 100 degreeC or more, 130-180 degreeC is preferable and 140-160 degreeC is more preferable. The hydrothermal reaction is preferably performed in a pressure vessel. When the reaction is performed at 130 to 180 ° C, the pressure at this time is 0.3 to 0.9 MPa, and the pressure when the reaction is performed at 140 to 160 ° C is 0. 3 to 0.4 MPa. The hydrothermal reaction time is preferably 1 to 24 hours, more preferably 3 to 12 hours.

By the hydrothermal reaction, Li ₂ MSiO ₄ (M is the same as described above) is obtained in a high yield, and its crystallinity is also high. After the hydrothermal reaction, it is preferable to obtain primary particles by collecting and washing the produced LiMSiO ₄ by filtration. The washing is preferably carried out with water using a filtration device having a cake washing function, and then primary particles are obtained by drying. As the drying means, freeze drying or vacuum drying can be used.

  Next, it is preferable that secondary particles are obtained by preparing a slurry containing the obtained primary particles and granulating the slurry. In order to obtain secondary particles having a controlled particle size, the content of primary particles in the slurry, and the viscosity and pH of the slurry may be appropriately adjusted.

Furthermore, you may make this slurry contain an organic binder, an inorganic binder, and a conductive carbon material suitably.
Examples of the organic binder include glucose, fructose, polyethylene glycol, polyvinyl alcohol, carboxymethyl cellulose, saccharose, starch, dextrin, and citric acid. Of these, glucose, polyvinyl alcohol, and carboxymethylcellulose are preferred because they can function as a carbon source by adjusting the amount used. Examples of the inorganic binder include scaly silica (silicon dioxide), silica-titania, silicon glass, colloidal silica, and the like.

  When an inorganic binder is used as the binder, it is preferable to use a conductive carbon material in combination. Examples of the conductive carbon material include carbon black, and among them, acetylene black and ketjen black are preferable. The amount of the conductive carbon material used is preferably 0.01 to 20 parts by mass and preferably 0.1 to 10 parts by mass with respect to 100 parts by mass of the primary particles from the viewpoint of good charge / discharge capacity and economy.

  In the slurry, water or an organic solvent may be used as a solvent.

The granulation is preferably performed by spray drying, and spray drying by a spray drying method is optimal. The obtained secondary particles can then be used as a positive electrode active material for a secondary battery by firing. 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. By such treatment, a positive electrode active material in which carbon is supported on the surface of Li ₂ MSiO ₄ can be obtained.

  The obtained positive electrode active material for lithium ion batteries is excellent in terms of charge / discharge capacity, and a very useful secondary battery can be obtained. The secondary battery to which the positive electrode active material for a lithium ion battery obtained by the production method of the present invention can be applied may be a lithium ion secondary battery, and has a positive electrode, a negative electrode, an electrolytic solution, and a separator as essential components. If it does not specifically limit.

  Here, as long as lithium ions can be occluded at the time of charging and released at the time of discharging, the material structure is not particularly limited, and a known material structure can be used. For example, a carbon material such as lithium metal, graphite, or amorphous carbon. It is preferable to use an electrode formed of an intercalating material capable of electrochemically inserting and extracting lithium, particularly a carbon material.

  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.
[Example 1]
LiOH.H ₂ O 4.20 g (0.1 mol), Na ₄ SiO ₄ .nH ₂ O 6.99 g (0.025 mol), Na ₂ S ₂ O ₄ 4.35 g (0.025 mol) and ultrapure water 75 cm ³ was added and mixed (the pH at this time was about 12.5). To this aqueous dispersion, 6.95 g (0.025 mol) of FeSO ₄ .7H ₂ O was added and mixed. The obtained mixed liquid was put into an autoclave and subjected to a hydrothermal reaction at 150 ° C. for 16 hours. The reaction solution was filtered and then lyophilized to obtain spherical primary particles having an average particle size of 50 nm. An SEM image of such spherical primary particles is shown in FIG.
In addition, from FIG. 1, it was confirmed that the particle size of a crystallite is 26 nm.

  Next, 45 g of glucose (60% aqueous solution) and 100 g of ultrapure water were added to 100 g of the obtained primary particles to prepare a slurry. The content of primary particles of the obtained slurry was 50 wt%. And after granulating the obtained slurry using a spray dryer (micro mist dryer equipped with a four-fluid nozzle: manufactured by Fujisaki Electric Co., Ltd.), it was calcined at 600 ° C. for 1 hour in a reducing atmosphere to obtain an average particle size. Secondary particles having a diameter of 15 μm were prepared. FIG. 2 shows an SEM image of such secondary particles, and FIG. 3 shows a TEM image of a part of the secondary particles. In addition, from FIG. 3, it was confirmed that the thickness of the carbon layer formed in the surface of a primary particle and a secondary particle is 2.5 nm.

[Charge / discharge test]
Using the secondary particles obtained in Example 1, a positive electrode of a lithium ion secondary battery was produced. The secondary particles obtained in Example 1, ketjen black (conductive agent), and polyvinylidene fluoride (binding agent) were mixed at a weight ratio of 75:15:10, and this was mixed with N-methyl-2- Pyrrolidone was added and sufficiently kneaded to prepare a positive electrode slurry. The positive 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 positive electrode.

Next, a coin-type lithium ion secondary battery was constructed using the positive electrode. A lithium foil punched to φ15 mm was used for the negative 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).

  Using the manufactured lithium ion secondary battery, charging / discharging at a constant current density was performed 4 cycles. The charging conditions at this time were constant current and constant voltage charging with a current of 0.1 CA (33 mA / g) and a voltage of 4.5 V, and the discharging conditions were constant current discharging with a current of 0.1 CA and a final voltage of 1.5 V. All temperatures were 30 ° C. FIG. 4 shows a charge / discharge curve of the fourth cycle of the battery constructed with the positive electrode material of Example 1.

  1-3, very fine and spherical primary particles are obtained, and these particles are joined together by agglomerated carbon and have an average particle diameter in the range of 1 to 100 μm. It can be seen that secondary particles are formed. Further, FIG. 4 shows that the lithium ion battery using the positive electrode active material obtained in Example 1 has excellent battery properties.

Claims (4)

Containing an olivine-type silicate compound represented by Li ₂ MSiO ₄ (wherein M represents one or more selected from Fe, Ni, Co or Mn),
A positive electrode active material for a lithium ion battery, wherein the olivine-type silicate compound has secondary particles having an average particle diameter of 1 to 100 µm formed by agglomerating spherical primary particles having an average particle diameter of 20 to 100 nm.   The positive electrode active material for a lithium ion battery according to claim 1, wherein a carbon layer having a thickness of 1 to 3 nm is formed on the surfaces of the spherical primary particles and secondary particles. The positive electrode active material for a lithium ion battery according to claim 1, wherein Li ₂ MSiO ₄ is Li ₂ FeSiO ₄ .   The lithium ion battery which has a positive electrode containing the positive electrode active material for lithium ion batteries of Claim 3.