JP5886923B1 - Method for producing lithium silicate compound having amorphous structure - Google Patents

Abstract

An olivine-type lithium silicate compound that can be used as a positive electrode active material for obtaining a lithium ion battery capable of maintaining a high discharge capacity satisfactorily. The following formula (A): LiaMSiO4 (A) (wherein M represents one or more metals selected from Fe, Mn, and Zr, and a is 0.3 ≦ a) ≦ 1.) And is supported by carbon on the surface, and lithium is extracted by performing a constant voltage charging process of 4.5 to 5.0 V. An olivine-type lithium silicate compound having: [Selection] Figure 1

Description

  The present invention relates to an olivine-type lithium silicate compound having an amorphous structure capable of exhibiting excellent battery characteristics and a method for producing the same.

  A secondary battery such as a lithium ion battery is a kind of non-aqueous electrolyte battery, and is used in a wide range of fields such as a mobile phone, a digital camera, a notebook PC, a hybrid vehicle, and an electric vehicle. 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.

There are many known positive electrode materials such as lithium cobaltate (LiCoO ₂ ), lithium manganate (LiMnO ₂ ), lithium iron phosphate (LiFePO ₄ ), and lithium iron silicate (Li ₂ FeSiO ₄ ). Yes. Among them, so-called olivine-type lithium silicate compounds such as Li ₂ FeSiO ₄ having an olivine structure are attracting attention as excellent positive electrode materials used for high-capacity lithium ion batteries, and various developments have been made so far. Yes.

  For example, Patent Document 1 discloses a positive electrode active material containing a solid solution compound that is an olivine-type lithium silicate compound containing Co and Mn, and the raw material mixture is solidified through a plurality of firing steps of calcination and main firing. A solid solution compound is obtained by a phase reaction. Further, Patent Document 2 discloses a silicate compound obtained by mixing a raw material, heating and melting it, and then gradually cooling the melt.

JP 2007-335325 A JP 2008-218303 A

However, in the method described in Patent Document 1, not only a long time is required for production, but the resulting compound tends to increase in particle size by firing at a high temperature. In addition, since the compound obtained by the method described in Patent Document 2 is accompanied by crystallization in the cooling process, it tends to cause variation in composition and crystal type and segregation in the crystal, and when pulverizing a massive cooling product. There is a risk that a large amount of distortion is accumulated in the particles.
Therefore, when these compounds are used as a positive electrode material to obtain a battery, there is a high possibility that the discharge capacity after the second cycle cannot be maintained well even if a high discharge capacity can be shown in the first cycle. Further improvement is required.

  Therefore, the subject of this invention is providing the olivine type lithium silicate compound which can be used as a positive electrode active material for obtaining the lithium ion battery which can maintain high discharge capacity favorably.

  Therefore, the present inventors have made various studies, and if the olivine-type lithium silicate compound having an amorphous structure obtained by carrying carbon on the surface and having a specific charge treatment, an excellent discharge is obtained. The present inventors have found that a lithium ion battery capable of exhibiting a capacity retention rate can be obtained, and have completed the present invention.

That is, the present invention provides the following formula (A):
Li _a MSiO ₄ (A)
(In the formula, M represents one or more metals selected from Fe, Mn, and Zr, and a represents a number satisfying 0.3 ≦ a ≦ 1.)
And carbon is supported on the surface,
The present invention provides an olivine-type lithium silicate compound having an amorphous structure in which lithium is extracted by performing a constant voltage charging process of 4.5 to 5.0 V.
The present invention also provides a step (I) of obtaining primary particles of an olivine-type lithium silicate compound by subjecting a lithium compound, a silicate compound, and a metal (M) compound to a hydrothermal reaction.
Step (II) for supporting carbon on the surface of primary particles of the olivine type lithium silicate compound obtained in step (I), and olivine type silicic acid obtained by supporting carbon on the surface obtained in step (II) Step (III) of subjecting the lithium compound to a constant voltage charging treatment of 4.5 to 5.0 V
The manufacturing method of the olivine type lithium silicate compound which has the said amorphous structure provided with this is provided.

  According to the olivine-type lithium silicate compound of the present invention, in a lithium ion battery obtained by using this as a positive electrode active material, it is possible to effectively suppress a decrease in discharge capacity every time a charge / discharge cycle passes. It is possible to maintain excellent battery characteristics.

The XRD pattern figure of the olivine type lithium silicate compound obtained in Example 1, Example 4, and Comparative Example 1 is shown.

Hereinafter, the present invention will be described in detail.
The olivine-type lithium silicate compound of the present invention has the following formula (A):
Li _a MSiO ₄ (A)
(In the formula, M represents one or more metals selected from Fe, Mn, and Zr, and a represents a number satisfying 0.3 ≦ a ≦ 1.)
And having an amorphous structure in which carbon is supported on the surface and lithium is extracted by performing a constant voltage charging process of 4.5 to 5.0 V.
The olivine-type lithium silicate compound of the present invention is represented by the above formula (A), and specifically includes, for example, the following aspects.
Li ₂ Fe _b Zr _c SiO ₄ (A1)
(In formula (A1), b is b = 1-2c, and c is 0 <c <0.5)
Li ₂ Fe _d Mn _1-d SiO ₄ (A2)
(In formula (A2), d is 0 ≦ d ≦ 1)
Li ₂ Fe _e Mn _f Zr _g SiO ₄ (A3)
(In the formula (A3), e and f are e + f = 1−2 g, and g is 0 <g <0.5)
Especially, the olivine type lithium silicate compound represented by Formula (A3) is preferable from the viewpoint of maintaining excellent battery characteristics.

The olivine-type lithium silicate compound of the present invention can be first obtained as primary particles, for example, by subjecting a lithium compound, a silicate compound, and a metal (M) compound to a hydrothermal reaction (step (I)). . Specifically, for example, as a metal (M) compound, a metal sulfate represented by MSO ₄ (wherein M represents Fe, Mn, or Zr) or (R) ₂ M (wherein R Represents an organic acid residue, M represents an organic acid metal salt represented by Fe, Mn, or Zr), a basic water containing a lithium compound and a silicic acid compound, and optionally containing an antioxidant. The dispersion is reacted hydrothermally.

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. When using metal sulfate (MSO ₄ ) as the metal (M) compound, the concentration of the lithium compound in the basic aqueous dispersion is controlled to an appropriate average particle diameter of the olivine-type lithium silicate compound of the present invention. From the viewpoint of firmly supporting carbon on the surface, 0.30 to 3.00 mol / l is preferable, 1.00 to 2.80 mol / l is more preferable, and 1.50 to 2.80 mol / l is more preferable. .

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. When metal sulfate (MSO ₄ ) is used as the metal (M) compound, the concentration of the silicate compound in the basic aqueous dispersion is controlled while controlling the olivine type lithium silicate compound to an appropriate average particle size. From the viewpoint of firmly supporting carbon on the surface, 0.15 to 1.50 mol / l is preferable, 0.50 to 1.40 mol / l is more preferable, and 0.80 to 1.40 mol / l is even more preferable.

Furthermore, it is preferable to add an antioxidant to the basic aqueous dispersion from the viewpoint of preventing side reactions. As the antioxidant, hydrosulfite sodium (Na ₂ S ₂ O ₄ ), aqueous ammonia, sodium sulfite and the like can be used. When metal sulfate (MSO ₄ ) is used as the metal (M) compound, the antioxidant content in the basic aqueous dispersion suppresses the generation of primary particles of the olivine type lithium silicate compound when added in a large amount. Therefore, an equimolar amount or less with respect to the metal (M) is preferable, and a molar ratio with respect to the metal (M) is more preferably 0.5 or less.

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

Specific examples of the metal sulfate MSO ₄ include FeSO ₄ , MnSO ₄ , or Zr (SO ₄ ) _2, which may be used alone or in combination of two or more. The addition amount of the metal sulfate (MSO ₄ ) is as described above from the viewpoint of firmly supporting carbon on the surface of the obtained olivine-type lithium silicate compound and maintaining it well while exhibiting excellent discharge capacity in the battery. The total amount of the reaction mixture containing both the basic aqueous dispersion and the metal sulfate is preferably 0.15 to 1.50 mol / l, more preferably 0.50 to 1.40 mol / l. The amount of 0.80 to 1.40 mol / l is more preferable.
In this case, the Li content in the reaction mixture is preferably 2 mol or more with respect to M.

When an organic acid metal salt (R) ₂ M (wherein R represents an organic acid residue and M represents Fe, Mn, or Zr) is used as the metal (M) compound, a lithium compound, silica A basic aqueous dispersion containing an acid compound and an antioxidant and further containing an organic acid metal salt is prepared. In this case, the order of addition of the lithium compound, silicic acid compound, antioxidant and organic acid metal salt is not particularly limited. Moreover, atmospheric conditions may be sufficient. When the organic acid metal salt (R) ₂ M is used as the metal (M) compound, the concentration of the lithium compound in the basic aqueous dispersion (in the total amount of the reaction mixture) is an appropriate average of the olivine type lithium silicate compound. From the viewpoint of firmly supporting carbon on the surface of such a compound while controlling the particle size, and maintaining this well while exhibiting excellent discharge capacity in the battery, 0.30 to 3.00 mol / l is preferable, 1.00 to 2.80 mol / l is more preferable, and 1.50 to 2.80 mol / l is more preferable. Further, the content of the silicate compound in the basic aqueous dispersion (in the total amount of the reaction mixture) is preferably 0.15 to 1.50 mol / l, and preferably 0.50 to 1.40 mol / l from the same viewpoint. Is more preferable, and 0.80 to 1.40 mol / l is more preferable. Furthermore, since the concentration of the antioxidant in the basic aqueous dispersion (in the total amount of the reaction mixture) suppresses the generation of primary particles of the olivine-type lithium silicate compound when added in a large amount, the metal (M) On the other hand, an equimolar amount or less is preferable, and a molar ratio with respect to the metal (M) is more preferably 0.5 or less. 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, Li in the reaction mixture is preferably used in a molar ratio of 2 times or more with respect to the metal, and Li: M is more preferably about 2.5: 1 to 3: 1.

The organic acid represented by an organic acid 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. The concentration of the organic acid metal salt (R) ₂ M in the basic aqueous dispersion (in the total amount of the reaction mixture) allows the carbon to be firmly supported on the surface of the resulting olivine-type lithium silicate compound, resulting in excellent discharge in the battery. From the viewpoint of maintaining the capacity while exhibiting the capacity, 0.15 to 1.50 mol / l is preferable, 0.5 to 1.40 mol / l is more preferable, and 0.80 to 1.40 mol / l is further preferable.

Making the aqueous dispersion basic is important in preventing side reactions and dissolving the silicate compound. Specifically, the pH of the basic aqueous dispersion is 12.0 to 13.5 to prevent side reactions (formation of Fe ₃ O ₄ ), solubility of silicate compounds, and progress of the reaction. Is particularly preferable. The pH of the basic aqueous dispersion may be adjusted by adding a base such as sodium hydroxide, but it is particularly preferable to use Na ₄ SiO ₄ as the silicate compound.

  Hydrothermal reaction should just be 100 degreeC or more, 130-200 degreeC is preferable and 140-180 degreeC is more preferable. The hydrothermal reaction is preferably performed in a pressure-resistant vessel, and the pressure when the reaction is performed at 130 to 200 ° C is 0.3 to 1.5 MPa, and the pressure when the reaction is performed at 140 to 180 ° C is 0.4 to 1.0 MPa. The hydrothermal reaction time is preferably 1 to 24 hours, more preferably 3 to 12 hours.

  By the hydrothermal reaction, an olivine type lithium silicate compound is obtained as primary particles in a high yield, and its crystallinity is also high. The average particle diameter of such particles is preferably 10 to 500 nm, more preferably 10 to 100 nm, from the viewpoint of improving the battery characteristics of the lithium ion battery.

  The primary particles of the obtained olivine type lithium silicate compound can be isolated by filtering after washing and then drying. Examples of the drying means include spray drying, box drying, fluidized bed drying, external heat drying, freeze drying, and vacuum drying. Of these, freeze-drying is preferred. Moreover, when drying, it is preferable to use a slurry prepared in advance using primary particles. The content (solid content concentration) of primary particles of the olivine-type lithium silicate compound in such a slurry is preferably 20 to 60% by mass from the viewpoint of obtaining particles capable of favorably supporting carbon on the surface. Preferably it is 30-50 mass%, More preferably, it is 35-45 mass%.

At this time, a small amount of a dispersant or a thickener may be added to the slurry, and as the dispersant, for example, a polycarboxylic acid type, a polyether type, a polyacrylic acid type, or the like can be used.
As the thickener, for example, a base such as sodium hydroxide, lithium hydroxide or ammonia, or an organic acid such as acetic acid or citric acid can be used.

  The dried product thus obtained may be classified with a separator, cyclone, sieve or the like during or after drying, or may be pulverized with a mortar pin mill, roll mill, crusher or the like.

  The average particle size of the obtained olivine-type lithium silicate compound is preferably 3 to 150 μm, more preferably 3 to 100 μm, and further preferably 5 to 50 μm from the viewpoint of favorably supporting carbon on the surface. is there.

  Thereafter, carbon is supported on the surface of the primary particles of the obtained olivine type lithium silicate compound (step (II)). By doing in this way, the electronic conduction area (electron conduction path) of an olivine type lithium silicate compound will increase, and more sufficient electronic conductivity can be ensured and the outstanding battery characteristic can be exhibited.

  Examples of the treatment for supporting carbon include a treatment in which a conductive carbon material is used as a carbon source, a slurry containing the obtained olivine-type lithium silicate compound and the conductive carbon material is prepared, and calcined after granulation. It is done. The slurry may appropriately contain an organic binder and an inorganic binder. By performing such treatment, a carbon thin film can be formed on the surface of the olivine type lithium silicate compound, and more excellent battery characteristics can be exhibited.

  Examples of the conductive carbon material include glucose, polyvinyl alcohol, carboxymethyl cellulose, and carbon black.

Examples of the binder include glucose, polyvinyl alcohol and carboxymethyl cellulose which can also be used as a conductive carbon material, as well as fructose, polyethylene glycol, saccharose, starch, dextrin, citric acid and the like.
Among these, glucose, polyvinyl alcohol, and carboxymethyl cellulose are preferable, and glucose is more preferable because it functions as a carbon source by adjusting the amount used and can also be used as a conductive carbon material.

  Examples of the inorganic binder include scaly silica (silicon dioxide), silica-titania, silicon glass, colloidal silica, silica sol, and alumina sol.

  The amount of the conductive carbon material used in the process of preparing the slurry and firing after granulation is the primary particle 100 of the olivine-type lithium silicate compound in the slurry from the viewpoint of good charge / discharge capacity and economy. It is preferably 0.5 to 20 parts by mass and more preferably 2 to 10 parts by mass with respect to parts by mass.

  Moreover, you may use water or an organic solvent as a solvent, and water is preferable from an economical viewpoint. The content (slurry concentration) of the olivine-type lithium silicate compound (primary particles) and the conductive carbon material in the slurry is preferably 30 to 60% by mass, and more preferably 45 to 55% by mass. The slurry viscosity at 25 ° C. is preferably 3 to 3000 m · Pa, more preferably 10 to 100 m · Pa. Furthermore, the pH of the slurry is preferably adjusted to 10.5 to 11.2.

The granulation treatment is not particularly limited as long as the olivine type lithium silicate compound having a desired particle size can be obtained as secondary particles, but is preferably by spray drying, and in particular, spraying by a spray drying method. The one by drying is optimal. The average particle size of the particles obtained after the granulation treatment is preferably 1 μm to 500 μm, and more preferably 20 μm to 50 μm.
The obtained particles are preferably fired from the viewpoint of firmly supporting carbon on the surface.

  In addition to the above granulation treatment, as a treatment for supporting carbon on the surface of the particles, for example, a method of pulverizing / compositing / mixing a mixture containing an olivine type lithium silicate compound (primary particles) and a conductive carbon material May be used. By performing such treatment, secondary particles in which the primary particles and the conductive carbon material are combined can be formed, and the conductivity can be further increased.

  Examples of the conductive carbon material used in the pulverization treatment include the same conductive carbon materials that can be used in the granulation treatment. Of these, carbon black is preferable, and acetylene black and ketjen black are more preferable. The addition amount of the conductive carbon material in the pulverization treatment is 0.5 to 20 mass in terms of carbon atom with respect to 100 mass parts of olivine type lithium silicate compound (primary particles) from the viewpoint of good discharge capacity and economy. Part is preferable, and 2 to 10 parts by mass is more preferable.

  The mixture containing the olivine-type lithium silicate compound (primary particles) and the conductive carbon material is pulverized / composited / mixed in a dry manner. At this time, a small amount of diethylene glycol, ethanol or the like may be added as an auxiliary agent.

  As an apparatus for pulverizing / combining / mixing, an ordinary ball mill may be used, but a planetary ball mill (manufactured by Fritsch) capable of rotating and revolving is preferred, and nobilta (manufactured by Hosokawa Micron), multipurpose mixer (manufactured by Nippon Coke Industries, Ltd.). ), Or a hybridizer (manufactured by Nara Machinery Co., Ltd.), etc., as long as it can perform a mechanochemical action / combination treatment on the workpiece.

  Examples of the container of the device used in the planetary ball mill include steel, stainless steel, and nylon, and the inner wall includes alumina brick, magnetic material, natural silica, rubber, urethane, and the like. As the ball, alumina sphere, natural silica, iron ball, zirconia ball or the like is used. The size of the ball is preferably 0.1 mm to 20 mm, more preferably 0.5 mm to 5 mm. It is preferable that the filling amount of the ball is a ratio in which the filling volume of the ball occupies 5 to 50% with respect to the internal volume of the mill to be used.

  Mixing using a planetary ball mill is performed, for example, under conditions of revolution of 50 to 800 rpm and rotation of 100 to 1,600 rpm, preferably 5 minutes to 24 hours, more preferably 0.5 to 6 hours, and further preferably 1 to 3 hours.

  The secondary particles subjected to the treatment for supporting carbon on the surface of the olivine-type lithium silicate compound (primary particles) as described above are preferably 10 minutes at 500 to 800 ° C. under an inert gas atmosphere or under reducing conditions. It is preferable to bake at ˜24 hours, more preferably at 600 to 700 ° C. for 0.5 to 3 hours. By such treatment, secondary particles in which carbon is further firmly supported on the surface of the primary particles can be obtained. The apparatus used for firing is not particularly limited as long as the firing atmosphere and temperature can be adjusted, and any of a batch system, a continuous system, and a heating system (indirect or direct) can be used. . Examples of such an apparatus include tubular electric furnaces such as an external heat kiln and a roller hearth.

  The average particle size of the olivine-type lithium silicate compound (secondary particles) in which carbon is further firmly supported on the surface is preferably 1 to 100 μm, more preferably 5 to 50 μm. The tap density is preferably 0.5 g / ml or more, more preferably 0.7 g / ml or more.

The olivine-type lithium silicate compound in which carbon is firmly supported on the surface is further subjected to a constant voltage charging treatment of 4.5 to 5.0 V (step (III)) to extract lithium. Thereby, the obtained olivine-type lithium silicate compound has an amorphous structure, and it becomes possible to maintain excellent discharge capacity. The constant voltage charging treatment may be carried out at any stage as long as it is applied to the olivine type lithium silicate compound having carbon supported on the surface obtained as described above. After producing a positive electrode for a lithium ion battery as an active material, it is preferable to construct a lithium ion battery and apply it to the obtained battery.
The olivine type lithium silicate compound having an amorphous structure can be confirmed by an XRD pattern diagram as shown in FIG. 1, for example.

  The constant voltage charging process is a constant voltage of 4.5 to 5.0 V, preferably a constant voltage of 4.55 to 4.9 V, and preferably 4.55 to 4.8 V. It is more preferable that the voltage be constant. At this time, it is preferable to carry out at a constant current of 0.01 to 1 CA, and preferably at a constant current of 0.05 to 0.5 CA. Thereby, an olivine-type lithium silicate compound having an amorphous structure from which lithium is effectively extracted can be obtained. The proportion of lithium effectively extracted can be determined from the chemical composition formula, and is preferably 60% or more, more preferably, based on lithium present in the olivine-type lithium silicate compound obtained immediately after the hydrothermal reaction. Is 65 to 85%.

  The lithium ion battery to which the positive electrode for a lithium ion battery containing the olivine-type lithium silicate compound of the present invention as a positive electrode active material can be applied is not particularly limited as long as the positive electrode, the negative electrode, the electrolytic solution, and the separator are essential components.

  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]
37.5 mL of ultrapure water was added to and mixed with 4.20 g of LiOH.H ₂ O and 13.98 g of Na ₄ SiO ₄ .nH ₂ O. To this aqueous dispersion, 3.92 g of FeSO ₄ .7H ₂ O, 7.93 MnSO ₄ .5H ₂ O and 0.53 g of Zr (SO ₄ ) ₂ .4H ₂ O were added and mixed (pH = about 13). ).
The obtained mixed solution was put into an autoclave and subjected to a hydrothermal reaction at 150 ° C. for 12 hours. The pressure in the autoclave was 0.4 MPa. The produced crystal was filtered, and then washed with 12 parts by mass of water with respect to 1 part by mass of the crystal. The washed crystal was freeze-dried at −50 ° C. for 12 hours to obtain primary particles of an olivine type lithium silicate compound (Li ₂ Fe _0.09 Mn _0.85 Zr _0.03 SiO ₄ , average particle size: 40 nm).

  Then, 8.4 g of primary particles of the obtained olivine type lithium silicate compound, 2.3 g of glucose (corresponding to 10 parts by mass with respect to 100 parts by mass of primary particles of the olivine type lithium silicate compound), and 10 mL of water. An olivine type silica whose surface is uniformly coated with carbon by pulverizing and mixing for 1 hour at 400 rpm in a planetary ball mill (medium 1 mmΦ), and firing at 650 ° C. for 1 hour in an argon hydrogen atmosphere (hydrogen concentration 3%). An acid lithium compound was obtained.

Using the obtained olivine-type lithium silicate compound as a positive electrode active material, a positive electrode of a lithium ion secondary battery was produced. Specifically, the obtained olivine-type lithium silicate compound, ketjen black, and polyvinylidene fluoride were mixed at a blending ratio of 75:15:10, and N-methyl-2-pyrrolidone was added to the mixture. The mixture was 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 ion secondary battery (CR-2032).

Next, using the obtained coin-type lithium ion secondary battery, constant current and constant voltage charging was performed once at a current of 0.1 CA (33 mA / g) and a voltage of 4.5 liters in an environment of 30 ° C. . The lithium silicate compound contained in the battery after the constant current and constant voltage charge treatment was calculated from the charge capacity. As a result, Li _0.8 Fe _0.28 Mn _0.66 Zr _0.03 SiO ₄ was confirmed and lithium was effectively extracted. (Ratio of lithium extracted based on lithium present in the olivine-type lithium silicate compound obtained immediately after the hydrothermal reaction = 60%).
An XRD pattern diagram of such a lithium silicate compound is shown in FIG.

[Example 2]
Using the coin-type lithium ion secondary battery obtained in Example 1, a constant current and constant voltage charge was performed once at a voltage of 4.6 mm at a current of 0.1 CA (33 mA / g) in an environment of 30 ° C. did. Next, in the same manner as in Example 1, the composition of the lithium silicate compound contained in the battery after the constant current constant voltage charging treatment was calculated. As a result, Li _0.66 Fe _0.28 Mn _0.66 Zr _0.03 SiO ₄ was obtained, and lithium was effectively used. It was confirmed that the lithium was extracted (ratio of lithium extracted based on lithium present in the olivine-type lithium silicate compound obtained immediately after the hydrothermal reaction = 67%).

[Example 3]
Using the coin-type lithium ion secondary battery obtained in Example 1, a constant current and constant voltage charge was performed once at a current of 0.1 CA (33 mA / g) and a voltage of 4.7 mm under an environment of 30 ° C. did. Next, in the same manner as in Example 1, the composition of the lithium silicate compound contained in the battery after the constant current constant voltage charging treatment was calculated. As a result, Li _0.55 Fe _0.28 Mn _0.66 Zr _0.03 SiO ₄ was obtained, and lithium was effectively used. It was confirmed that the lithium was extracted (ratio of lithium extracted based on lithium present in the olivine-type lithium silicate compound obtained immediately after the hydrothermal reaction = 72.5%).

[Example 4]
Using the coin-type lithium ion secondary battery obtained in Example 1, constant current / constant voltage charging was performed once at a current of 0.1 CA (33 mA / g) and a voltage of 4.8 kg in an environment of 30 ° C. did. Next, in the same manner as in Example 1, the composition of the lithium silicate compound contained in the battery after the constant current constant voltage charging treatment was calculated. As a result, Li _0.41 Fe _0.28 Mn _0.66 Zr _0.03 SiO ₄ was obtained, and lithium was effectively used. It was confirmed that the lithium was extracted (ratio of lithium extracted based on lithium present in the olivine-type lithium silicate compound obtained immediately after the hydrothermal reaction = 79.5%).
An XRD pattern diagram of such a lithium silicate compound is shown in FIG.

[Comparative Example 1]
Using the coin-type lithium ion secondary battery obtained in Example 1, the composition of the lithium silicate compound contained in the battery was calculated in the same manner as in Example 1 without performing constant current and constant voltage charging. Li ₂ Fe _0.28 Mn _0.66 Zr _0.03 SiO ₄ (ratio of extracted lithium on the basis of lithium present in the olivine-type lithium silicate compound obtained immediately after the hydrothermal reaction = 0%).
An XRD pattern diagram of such a lithium silicate compound is shown in FIG.

<Evaluation of battery characteristics>
About the battery of Examples 1-4 which performed constant current constant voltage charge, all the charged electricity was discharged beforehand. Next, using the batteries obtained in Examples 1 to 4 and Comparative Example 1, constant current and constant voltage charging with a voltage of 4.3 で was performed at a current of 0.1 CA (33 mA / g), and finally at a current of 0.1 CA. A constant current discharge with a voltage of 1.5∨ was performed. Based on the discharge capacity at the first cycle, the capacity retention rate (%) was obtained for the discharge capacity at the second to fourth cycles. In addition, all the temperatures of this test were performed at 30 degreeC.
The results are shown in Table 1.

  From the above results, since the lithium silicate compounds used in Examples 1 to 4 have an amorphous structure in which lithium is satisfactorily extracted, in the obtained lithium secondary battery, the first cycle It can be seen that the discharge capacity is high, and the capacity retention rate after the second cycle is also high.

Claims (3)

By subjecting a lithium compound, a silicate compound, and a metal (M) compound to a hydrothermal reaction, the following formula (A):
Li _a MSiO ₄ (A)
(In the formula, M represents one or more metals selected from Fe, Mn, and Zr, and a represents a number satisfying 0.3 ≦ a ≦ 1.)
Step (I) of obtaining primary particles of an olivine type lithium silicate compound represented by
Step (II) for supporting carbon on the surface of primary particles of the olivine type lithium silicate compound obtained in step (I), and olivine type silicic acid obtained by supporting carbon on the surface obtained in step (II) A step of subjecting the lithium compound to a constant current / constant voltage charging treatment of 4.55 to 5.0 V once to extract 65 to 85% of lithium in the olivine type lithium silicate compound (III)
A method for producing a lithium silicate compound having an amorphous structure. A constant current constant voltage charging process in step (III), carried out at a constant current of 0.01～1CA, method for producing a lithium silicate compound having an amorphous structure according to claim 1.   The non-conductive carbon material according to claim 1 or 2, wherein in the step (II), 0.5 to 20 parts by mass of a conductive carbon material in terms of carbon atom is used with respect to 100 parts by mass of primary particles of the olivine type lithium silicate compound. A method for producing a lithium silicate compound having a crystalline structure.