JP2011076793A - Method of synthesizing olivine-type m lithium silicate, and lithium ion secondary cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To simply synthesize olivine-type M lithium silicate of microscopic particles useful as a cathode active material for a secondary battery, and to improve the performance of a lithium ion secondary cell. <P>SOLUTION: A method of synthesizing olivine-type M lithium silicate (M is a metallic element) used as a cathode material of a lithium ion secondary cell includes a kneading and drying step of wet kneading colloidal silica as a silicate source, a lithium source, and a metal element M source, kneading the resultant kneaded material, and at the same time, drying it; and a thermal processing step of thermally processing a mixed and dried material obtained after the drying under a predetermined atmosphere that should reduce the metallic element M. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method for synthesizing olivine-type lithium M silicate (M is a metal element) and a lithium ion secondary battery, and more particularly to a technique for providing an electrode having a higher capacity than conventional ones.

As a next-generation positive electrode active material for a lithium ion secondary battery, polyanionic LiMPO ₄ (phosphate system) and Li ₂ MSiO ₄ (silicate system) having an olivine type crystal structure have been proposed (M: metal) Elements, such as Fe, Mn, Co, etc.).
These positive electrode active materials have higher thermal stability than conventional oxide-based positive electrode active materials such as LiCoO ₂ , and generation of oxygen gas can be suppressed even when the battery temperature rises abnormally due to a short circuit or the like.
By the way, although LiFePO ₄ is about to enter the practical stage in the phosphate-based positive electrode active material, the silicate-based positive electrode active material is expected to have a larger capacity due to a two-electron reaction, but is still sufficient. The capacity is not obtained. This is probably because the synthesis method and the material design technology are not established, and the conductivity and lithium ion diffusion of the material are low compared to the phosphoric acid positive electrode active material. For this reason, it has been proposed to coat the surface with carbon, but the performance has not yet been brought out sufficiently.

Conventional silicate-based positive electrode active materials are synthesized by a solid-phase reaction, and there is a problem that a long time is required for the reaction. Further, since treatment at high temperature for a long time is accompanied by crystal growth, it is difficult to obtain nano-level small particles. For example, in a typical synthesis as shown in Patent Document 1, an oxide serving as a supply source of each constituent element is mixed for a long time with a ball mill and pre-fired at 650 ° C. for 12 hours, and then 1100 ° C. The main baking for 24 hours is performed twice.
Further, Patent Document 2 proposes a method of synthesizing a silicon source using silicone, lithium silicate, or the like, but it takes several tens of hours for mixing and heat treatment.

JP 2007-335325 A JP 2001-266882 A

By the way, the reason why a long time is required for the mixing and heat treatment process in these methods is that the raw material and precursor of the silicon source are composed of large particles, and if this process condition and time are short, impurities may be mixed. There was a problem that manufacturing management was difficult.
Accordingly, an object of the present invention is to simply synthesize olivine-type M lithium silicate, which is useful as a positive electrode active material for a secondary battery, and to improve the performance of the lithium ion secondary battery.

  In order to achieve the above object, a first aspect of the present invention is a method for synthesizing olivine-type lithium M silicate (M is a metal element) used as a positive electrode material of a lithium ion secondary battery. Colloidal silica, a lithium source and a metal element M source are wet-kneaded, a kneading and drying step of kneading and drying the obtained kneaded product, and the metal element M is added to the mixed desiccant obtained after the drying. And a heat treatment step in which heat treatment is performed in a predetermined atmosphere for reduction.

According to the above configuration, the colloidal silica is used as the silicic acid source, and in the kneading and drying step, the lithium source and the metal element M source are wet-kneaded, and the obtained kneaded material is dried while kneading. At least a part of the metal element M is oxidized, and the metal element M is reduced in the heat treatment step.
As a result, a phase change occurs in each of the oxidation process and the reduction process, and the produced olivine-type lithium M silicate is inhibited from aggregation and particle growth is suppressed. The fine particle state generated due to the particle size can be maintained, and as a result, when configured as a lithium ion secondary battery, the current capacity per unit mass of olivine-type M lithium silicate is increased. It is possible to manufacture a high-performance lithium ion secondary battery.

According to a second aspect of the present invention, in the first aspect, the metal element M constitutes the olivine-type M lithium silicate as a divalent element, and in the kneading and drying step, a part of the metal element M or All is characterized by being oxidized once.
Therefore, since the oxidation and reduction steps are surely performed, it is possible to suppress the particle growth of the generated olivine type lithium M silicate.

According to a third aspect of the present invention, in the first or second aspect, in the kneading and drying step, the silicic acid source, the compound serving as the metal element M source, and the organic solution as the carbon source are added and mixed. The solvent is removed while oxidizing the metal element M, and the heat treatment is performed in an inert atmosphere or a reducing atmosphere in the heat treatment step.
According to the above configuration, the olivine-type lithium M silicate raw material powder suppresses the material diffusion with the organic substance as the carbon source before the heat treatment step, thereby suppressing the particle growth and further making the particles finer.

  According to a fourth aspect of the present invention, in a lithium ion secondary battery in which olivine-type lithium M silicate (M is a metal element) is used as a positive electrode material, colloidal silica as a silicate source, a lithium source, and a metal element M Obtained by wet kneading and drying the kneaded product obtained by kneading, and heat-treating the mixed desiccant obtained after the drying in a predetermined atmosphere to reduce the metal element M. A positive electrode active material layer containing olivine-type lithium M silicate is formed on a positive electrode current collector.

  According to the above configuration, the olivine-type M lithium silicate inhibits aggregation and the like, suppresses particle growth, and maintains the fine particle state generated due to the particle size of colloidal silica. Thus, a high-performance lithium ion secondary battery having a large current capacity per unit mass of olivine-type lithium M silicate is obtained.

  ADVANTAGE OF THE INVENTION According to this invention, the fine particle olivine type | mold M lithium silicate useful as a secondary battery positive electrode active material can be synthesize | combined simply, and the performance improvement of a lithium ion secondary battery can be aimed at.

It is an X-ray diffraction pattern measured in relation to the first example. It is a scanning electron microscope (SEM) photograph of powder A concerning the 1st example. It is a scanning electron microscope (SEM) photograph of powder D concerning the 1st comparative example. It is a scanning electron microscope (SEM) photograph of powder E concerning the 2nd comparative example. It is a figure explaining the result of the discharge capacity of the 10th cycle relevant to 1st Example and 2nd Example. It is an X-ray diffraction pattern measured in relation to the second example. It is a scanning electron microscope (SEM) photograph of powder B concerning the 2nd example. It is a scanning electron microscope (SEM) photograph of powder C concerning the 3rd example. It is a scanning electron microscope (SEM) photograph of powder D concerning the 1st comparative example. It is a scanning electron microscope (SEM) photograph of powder E concerning the 2nd comparative example.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[1] First Embodiment As a result of repeated studies, the inventor has focused particularly on the shape of the silicon source when synthesizing the olivine-type M lithium silicate compound. Generally, raw materials and precursors are often silicon dioxide (SiO ₂ ) as a silicon source. Silicon dioxide has a melting point of 1650 ° C., and when the particles are large, a temperature close to the melting point is required to cause a solid-phase reaction.

  However, when synthesized at a high temperature, the lithium M silicate particles produced by the reaction grow or sinter and become large particles. There is also a method of controlling the temperature and taking a long reaction time, but it is not industrially attractive and particle growth is unavoidable. Since lithium M silicate is poor in lithium ion diffusion and conductivity in the solid phase, in order to be used as a practical active material, it is necessary to be small particles in order to shorten the diffusion and conduction distance. Focused on the fact that silicon dioxide particles with the greatest chemical stability in the raw materials and precursors are very small and a few nanometers, the dispersibility of the particles and the sintering temperature are reduced, achieving the above objective I found out that

Colloidal silica is a dispersion of fine silicon dioxide with a diameter of several to several tens of nanometers in water. By synthesizing olivine-type M lithium silicate using this as a silicon source, high purity can be achieved at low temperature in a short time. Fine particles are obtained. When these particles are used as a positive electrode active material, an electrode having a higher capacity than the conventional one can be obtained.
In the general formula Li ₂ MSiO ₄ representing the olivine-type M lithium silicate targeted by the present invention, M represents Mn (manganese), Fe (iron), Co (cobalt), or Ni (nickel).

In synthesizing the general formula Li ₂ MSiO ₄ according to the present invention, as the source of lithium and M (metal elements, especially transition metal elements such as Mn, Fe, Co, Ni, etc.) in the general formula, these oxalate salts or It is preferred to supply in carbonate granules. In addition, since the target M is a divalent metal element, it is better to charge it in a divalent state. However, if it is subject to oxidation in the mixing process due to its stability in air, the firing atmosphere can be reduced. It is necessary to devise such as.
As the Si (silicon) source, colloidal silica in which fine particles of silicon dioxide are dispersed in water is used. In these compositions, each compound is charged in a stoichiometric ratio, and when volatilization occurs during heat treatment, the amount of raw material charged is adjusted so that olivine-type M lithium silicate having a desired composition can be obtained.

  The above lithium and M source compound granules are put into colloidal silica (dispersion) and stirred well with a planetary ball mill or the like, then transferred to another container and evaporated to dryness while stirring. In this state, the dry residue is collected and heat-treated. When M is Fe or the like, it reacts with oxygen in the air and Fe is oxidized from divalent to trivalent. Therefore, it is necessary to use a reducing atmosphere such as nitrogen to which 3% hydrogen is added during the heat treatment. Since the silicon dioxide is a nanoparticle, the heat treatment can be completed in 4 to 8 hours at 600 to 800 ° C., which is considerably lower than the original melting point (1650 ° C.).

The obtained olivine-type lithium M silicate was confirmed to be a single phase with an X-ray diffractometer or the like, and as a result of confirming the size of the particles with a scanning electron microscope, the synthesis conditions were low temperature and short time, It was confirmed to be a particle having a diameter of about several tens to several hundreds of nanometers.
By the way, since the olivine-type M lithium silicate obtained by the synthesis method of this embodiment has poor conductivity, it is mixed with about 5 to 10% of carbon black as a conductive material to form a positive electrode active material mixture. It is desirable to use for production. As a mixing method, a media dispersion method such as a ball mill is suitable.

The positive electrode can be produced by a known electrode production method. For example, the positive electrode active material mixture described above may be a known binder (binder: for example, PVdF; polyvinylidene fluoride, SBR; styrene-butadiene rubber, acrylic; polymethyl methacrylate, etc.) or a solution thereof as necessary. (NMP is used for organic systems, water is used for water systems) and aqueous solutions for thickeners such as CMC (carboxymethylcellulose) are mixed to prepare a slurry, which is then used as a metal current collector foil (current collector) such as an aluminum foil. It can be prepared by coating, drying and rolling.
Moreover, a well-known thing can be used for a negative electrode. As the negative electrode active material, graphite, lithium titanate (LTO) and others can be used, and they can be produced by the same method as that for the positive electrode.

What is used for a well-known nonaqueous electrolyte secondary battery can be used for electrolyte solution. For example: The electrolytic solution usually includes an electrolyte and a solvent.
The solvent constituting the electrolytic solution is not particularly limited as long as it is non-aqueous. For example, carbonates, ethers, ketones, sulfolane compounds, lactones, nitriles, chlorinated hydrocarbons, ethers, amines , Esters, amides, phosphate ester compounds and the like can be used. Listed as representative of these are 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, ethylene carbonate, vinylene carbonate, methyl formate, dimethyl sulfoxide, propylene carbonate, acetonitrile, γ-butyrolactone, dimethylformamide, dimethyl carbonate, diethyl carbonate, sulfolane, ethyl methyl carbonate, 1,4-dioxane, 4-methyl-2-pentanone, 1,3-dioxolane, 4-methyl-1,3-dioxolane, diethyl Use ether, sulfolane, methyl sulfolane, propionitrile, benzonitrile, butyronitrile, valeronitrile, 1,2-dichloroethane, trimethyl phosphate, triethyl phosphate, etc. It can be. These can be used alone or in combination of two or more.

Examples of the electrolyte constituting the electrolytic solution include LiClO ₄ , LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiAsF ₆ , LiB (C ₆ H ₅ ) ₄ , LiCl, LiBr, CH ₃ SO ₃ Li, and CF ₃ SO. ₃ Li, LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ , LiC (SO ₂ CF ₃ ) ₃ , LiN (SO ₃ CF ₃ ) _2, or the like can be used.
In the battery of the present invention, various materials mainly composed of a conventionally known polyolefin-based resin can be used for elements such as a separator, a battery case, and other structural materials. What is necessary is just to assemble the battery of this invention according to a well-known method using said battery element, and can employ | adopt a shape and a size suitably.

[2] Second Embodiment Next, a second embodiment of the present invention will be described.
As in the first embodiment, the inventors of the present invention have repeatedly studied, and as a result, the silicon dioxide particles having the maximum chemical stability in the raw materials and precursors are very small and several nanometers, metal The element M source is oxidized to a divalent or higher value of the target product, undergoes a reduction reaction, the raw material particles are kneaded with an organic solution, the solvent is removed, and the carbon source is attached to the surface of each raw material particle. Therefore, when synthesizing the target product by heat treatment in an inert or reducing atmosphere, it is possible to improve the dispersibility of the particles and suppress the material diffusion during the reaction, and to suppress the particle growth of the reaction product and to apply the carbon coat. It was found that the high-purity fine particles thus obtained were obtained, and that the above object was achieved.

In the general formula Li ₂ MSiO ₄ representing the olivine-type M lithium silicate targeted by the second embodiment, M is a transition metal such as Mn (manganese), Fe (iron), Co (cobalt), or Ni (nickel). ).
In synthesizing the olivine-type M lithium silicate having the general formula Li ₂ MSiO ₄ according to the second embodiment, a lithium source for supplying lithium in the general formula, M (Mn, Fe, Co, Ni) is supplied. The M source is preferably supplied as granules of oxalate or carbonate of Mn, Fe, Co or Ni.

  Moreover, since M of the olivine-type lithium silicate M, which is the target product, is a divalent metal element, it is better to prepare it with a divalent charge, but without changing the valence, that is, without an oxidation reaction or a reduction reaction. Rather than performing a solid-phase reaction, a state in which all of a part of divalent M is once oxidized to trivalent or more is accompanied by a phase change. found.

Further, by covering the raw material powder with an organic substance of a carbon source before the heat treatment, the organic substance undergoes a chemical reaction such as condensation during the heat treatment. As a result of this synergistic effect, the particle growth is further suppressed and fine particles of olivine-type lithium M silicate are easily obtained.
As organic substances of the carbon source, resins soluble in various organic solvents, various organic compounds, pitches, and the like can be used. Water-soluble polymers such as carboxymethyl cellulose, methyl cellulose, and polyvinyl alcohol, various saccharides, and alcohols that are soluble in water. And glycols can be used. These compounds are used as a source of carbon precipitation on the particle surface by reducing M to divalent by heat treatment after vaporizing the solvent.

As heat treatment conditions, most of M must be finally divalent, so the heat treatment atmosphere is an inert atmosphere when the amount of organic matter is sufficient to reduce M to divalent, and when it is not sufficient A reducing atmosphere is required. Nitrogen, argon, or the like can be used as the inert atmosphere, and nitrogen, argon, or the like mixed with a reducing gas such as hydrogen can be used as the reducing atmosphere.
For each Si source, M source, and Li source raw material, the respective raw material compounds are charged in a stoichiometric ratio, and when volatilization occurs during heat treatment, the raw material charging amount is adjusted to obtain a desired composition. An olivine type lithium M silicate is obtained.

The above-mentioned Li and M source compound granules and the carbon source organic solution are put into colloidal silica (dispersion) and stirred well in the air using a planetary ball mill or the like, then transferred to another container while stirring. Evaporate to dryness. At this time, M undergoing oxidation is oxidized to a divalent or higher value. Next, the dry residue is collected and subjected to heat treatment.
Since the heat treatment process requires a process in which the organic substance of the carbon source undergoes a condensation reaction and changes to carbon, it is desirable to temporarily hold at a temperature of 200 to 300 ° C. for 2 to 8 hours.

Since the gas constituting the inert or reducing atmosphere carries the volatile matter of the organic matter, the amount of carbon coating may change depending on the flow rate. Therefore, it is necessary to control the flow rate so that the carbon coating amount does not change.
Since the main baking temperature after holding the temperature is silicon dioxide nanoparticles, the reaction can be completed in 4 to 8 hours at 600 to 800 ° C. which is considerably lower than the original melting point (1650 ° C.).
The obtained olivine-type M lithium silicate was confirmed to be a single phase with an X-ray diffractometer or the like, and then the particle size was confirmed with a scanning electron microscope. Thereby, it was confirmed that the olivine-type lithium M silicate is a particle having a diameter of about several tens to several hundreds of nanometers. This was thought to be because the synthesis conditions were low temperature for a short time.

  The olivine-type M lithium silicate obtained by the synthesis method of the second embodiment is carbon coated and has good conductivity on the particle surface, but stronger olivine-type M lithium silicate interparticle and olivine-type. In order to ensure electrical conductivity between the lithium silicate M particles and the current collector, it is desirable to mix a conductive agent such as carbon black of about 2 to 10% and to provide the positive electrode. Here, the same elements as those in the first embodiment can be used for the production of the positive electrode, the negative electrode, the electrolytic solution (solvent and electrolyte), the separator, the battery case, and other structural materials.

As described above, according to each embodiment, it is possible to simply synthesize olivine-type lithium M silicate, which is useful as a secondary battery positive electrode active material, and to improve the performance of the lithium ion secondary battery.
In the description of the above embodiment, when synthesizing the olivine-type M lithium silicate, a part or all of the metal element M was once oxidized and reduced to cause phase change to reduce the size. However, part or all of the metal element M as a raw material may be a trivalent element, and the phase change may be performed through the reduction process, and the particles may be formed by the phase change in the reduction process. is there.

EXAMPLES Examples will be described below to more specifically illustrate the features of the present invention, but the present invention is not limited to the following examples.
[1] Example of First Embodiment First, an example (first example) of the first embodiment will be described.
[1.1] First Example Silicon dioxide having a particle size of several nanometers, in addition to a gas phase synthesis method such as aerosil synthesis by thermal decomposition of silicon tetrachloride, It is synthesized by a liquid phase synthesis method such as hydrolysis of alkoxide and is commercially available as colloidal silica.

As the first embodiment, a product name “Cataloid S-20L” (20% aqueous dispersion as silicon dioxide) available from Catalyst Kasei Kogyo Co., Ltd. is available. A synthesis was attempted. Below, the synthesis example of the olivine type lithium iron silicate whose M is Fe was described.
In order to measure the exact amount of silicon dioxide of “Cataloid S-20L” as the Si source, thermogravimetric analysis was used to confirm that it was 21.32% as silicon dioxide.

Lithium carbonate as a reagent was used as the Li source, and iron (II) oxalate dihydrate was used as the Fe source.
Samples of these elements were weighed so that the molar ratio of Li: Fe: Si was 2: 1: 1, and mixed for 3 minutes (rotation 2000 rpm, revolution 800 rpm) using a planetary ball mill. A kneaded product was obtained. While kneading the kneaded product at 100 ° C. in an inert atmosphere, the water was evaporated to obtain a mixed dried product.

  Subsequently, the mixed dried product was put into a graphite crucible and put into a vacuum substitution furnace, and the inside of the furnace was evacuated and replaced with nitrogen. While introducing nitrogen into the furnace at a flow rate of 100 ml / min, the temperature was raised to 700 ° C. and heat treatment was performed for 4 hours. After completion of the treatment, the mixture was allowed to cool while introducing hydrogen mixed nitrogen, and was taken out after the temperature dropped to 80 ° C. or lower. The powder mass after the heat treatment had a gray to olive color and was in a loosely aggregated state. The lump was subjected to various measurements after passing through a sieve having an opening of 50 μm. This powder was designated as powder A according to the present invention.

[1.2] First Comparative Example As a first comparative example, olivine-type lithium M silicate was synthesized by a method in which a raw material was once melted and then gradually cooled. The Si source was silicon dioxide (particle size: 35 μm), the Li source was lithium carbonate, and the Fe source was iron (II) oxalate dihydrate.
Each sample was put in a platinum crucible, heated to 1500 ° C. at 200 ° C./hr in an argon atmosphere using an electric furnace, held at that temperature for 5 minutes to maintain a molten state, and then the electric furnace power supply was stopped. Then, it was left to cool in the electric furnace as it was (slow cooling).
And it took out after temperature fell to 80 degrees C or less. The powder lump after the heat treatment had a gray to olive color and was hard and baked. After making it fine using a pulverizer, it was passed through a sieve having an opening of 50 μm and subjected to various measurements as powder D.

[1.3] Second Comparative Example Also, olivine-type lithium M silicate was synthesized by a solid phase method. Lithium metasilicate as the starting material and lithium source as the Si source, and iron (II) oxalate dihydrate as the Fe source are weighed in a stoichiometric ratio, mixed using an agate mortar, placed in an alumina crucible, and an electric furnace The mixture was pre-fired at 650 ° C. for 12 hours and then pulverized, followed by firing at 1000 ° C. for 24 hours twice. After completion of the main firing, the product was allowed to cool in the electric furnace (slow cooling) as it was, and was taken out after the temperature dropped to 80 ° C. or lower. The powder lump after the heat treatment had a gray to olive color and was hard and baked. After making it fine using a pulverizer, it was passed through a sieve having an opening of 50 μm, and subjected to various measurements as powder E.

[2] Physical property measurement The obtained powders A, D, and E were subjected to measurement of an X-ray diffraction pattern and shape observation with a scanning electron microscope (SEM).
FIG. 1 is an X-ray diffraction pattern measured for the example of the first embodiment.
As shown in FIG. 1, all powders A, D, and E are composed of a substantially single phase of olivine type lithium iron silicate. Compared with powder A, powder D and powder E are X The line diffraction peak was high and sharp, and it was confirmed that the crystal growth was larger than that of the powder A.

FIG. 2 is a scanning electron microscope (SEM) photograph of powder A according to the first example.
FIG. 3 is a scanning electron microscope (SEM) photograph of the powder D according to the first comparative example.
FIG. 4 is a scanning electron microscope (SEM) photograph of powder E according to the second comparative example.
Here, in FIGS. 2 to 4, the magnification is 5000 times.
As shown in FIG. 2, the powder A is composed of fine particles having a particle diameter of 50 to 200 nanometers, whereas the powder B and the powder C are as shown in FIGS. It was found to be composed of crushed 5-50 μm coarse particles. This is because, in the manufacturing method of the first example according to the present invention, the synthesis conditions are low temperature and short time, whereas in the first comparative example and the second comparative example, the molten state has passed or the high temperature is long. This is presumed to be due to the growth of crystals.

[3] Evaluation of electrode characteristics [3.1] Fabrication of lithium secondary battery Each of powders A, D, and E obtained in the first example, the first comparative example, and the second comparative example was used as a conductive agent. The mixture was further mixed for 5 hours using a ball mill in which acetylene black was mixed to 10% and the inside was replaced with nitrogen.
Further, the mixed powder and polyvinylidene fluoride (PVdF) which is a binder (binder) are mixed at a weight ratio of 95: 5, and N-methyl-2-pyrrolidone (NMP) is added and kneaded sufficiently. A slurry was obtained.

The obtained positive electrode slurry was applied to an aluminum foil current collector with a thickness of 15 μm at a coating amount of 50 g / m ² and dried at 120 ° C. for 30 minutes.
Then, it was rolled to a density of 2.0 g / cc with a roll press, punched into a 2 cm ² disk shape, and used as a positive electrode. Using these positive electrodes, metallic lithium for the negative electrode, and a mixed solvent in which ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 1: 1 in the electrolyte solution, LiPF ₆ was dissolved at a concentration of 1M, and a lithium secondary battery was used. Produced. The production atmosphere was a dew point of −50 ° C. or lower. Each electrode was used by being crimped to a battery case with a current collector. A coin-type lithium secondary battery having a diameter of 25 mm and a thickness of 1.6 mm was formed using the positive electrode, the negative electrode, the electrolyte, and the separator. A battery using each positive electrode active material was designated as a battery BA according to the present invention, and a battery BD and a battery BE as comparative examples.

[3.2] Battery Test of Lithium Secondary Battery A large number of each battery was produced and repeated for 10 cycles from the initial charge / discharge at a current of 0.01 CA. The charging conditions were set as constant current constant voltage charging with a current of 0.01 CA and a voltage of 4.5 V until the current during constant voltage charging dropped to 0.005 CA. The discharge conditions were a constant current discharge with a current of 0.01 CA and a final voltage of 1.5V. All temperatures were 25 ° C.

FIG. 5 is a diagram for explaining the result of the discharge capacity at the 10th cycle related to the embodiment.
In FIG. 5, the capacity was converted per 1 g of olivine type lithium iron silicate.
In FIG. 5, the battery BA using the powder A according to the first example showed the largest capacity. Compared to the battery BA, the capacity of the battery BD and the battery BE is small because the olivine-type lithium iron silicate particles as these positive electrode active materials are coarsened based on the X-ray diffraction measurement and SEM observation results. For this reason, it is presumed that the diffusion of lithium ions inside the particles does not proceed smoothly. Therefore, in order to obtain good positive electrode characteristics, a method of synthesizing an active material having a particle size as small as a nano level is necessary, and this is provided by the present invention.
As described above, according to the first embodiment, it is possible to easily synthesize olivine-type M lithium silicate, which is useful as a secondary battery positive electrode active material, and to improve the performance of the lithium ion secondary battery. .

[4] Examples of the Second Embodiment Next, examples (second example and third example) of the second embodiment will be described.
[4.1] Second Example As a result of repeated studies aimed at obtaining carbon-coated olivine type M lithium silicate having a high purity and a small particle size, the number of particles as a silicon source was determined. A compound in which nanometer silicon dioxide is used and a metal element M that becomes divalent when the olivine-type M lithium silicate is used as an M source, and a part or all of the metal element M becomes trivalent or higher when kneaded before synthesis. It was found that the organic element solution was used as a carbon source, and these were mixed to remove the solvent and then heat-treated in an inert or reducing atmosphere so that the metal element M was made divalent again.

As this 2nd Example, the synthesis | combination of the olivine type M lithium silicate was tried using the brand name "Cataloid S-20L" (20% aqueous dispersion as a silicon dioxide) by a catalyst chemical industry company. Below, the synthesis example of the olivine type lithium iron silicate whose metal element M is Fe is described.
It was confirmed that the exact silicon dioxide content of Cataloid S-20L as the Si source was 21.32%.

  Lithium carbonate as a reagent was used as the Li source, and iron (II) oxalate dihydrate was used as the Fe source. A sample of each of these elements was weighed so that the molar ratio of Li: Fe: Si was 2: 1: 1, and a 5% aqueous solution of polyvinyl alcohol corresponding to a mass of 10% with respect to the measured solid content. Was prepared and premixed, and then mixed for 3 minutes using a planetary ball mill (spinning 2000 rpm, revolution 800 rpm) to obtain a slurry-like kneaded product. The slurry was a high-viscosity fluid exhibiting a green color after kneading, but had the property of gelling in a few minutes. The surface of the slurry turns brown only by leaving it in the air because Fe is oxidized from divalent to trivalent. While kneading the kneaded product in air at 100 ° C., water was evaporated to obtain a mixed dried product. The mixed dried product was oxidized by kneading, and the whole turned brown.

  The mixed dried product was put into a graphite crucible and put into a vacuum substitution furnace. After the inside of the furnace was evacuated, the mixture was replaced with nitrogen mixed with 3% hydrogen. While introducing hydrogen mixed nitrogen into the furnace at a flow rate of 100 ml / min, the temperature was maintained at 250 ° C. for 4 hours, and then heat treatment was performed at 700 ° C. for 4 hours. After completion of the treatment, the mixture was allowed to cool with nitrogen introduced, and was taken out after the temperature dropped to 80 ° C. or lower. The powder mass after the heat treatment was black and loosely aggregated. The lump was subjected to various measurements after passing through a sieve having an opening of 50 μm. This powder was designated as Powder B according to the second example. The amount of coated carbon was determined by thermogravimetric analysis and found to be 2.0%.

[4.2] Third Example As the second example, cataloid S-20L as a Si source, lithium carbonate as a reagent as a Li source, and Fe source as iron (II) oxalate dihydrate and iron oxide ( III) was used by mixing at a molar ratio of 1: 1. A sample of each of these elements was weighed so that the molar ratio of Li: Fe: Si was 2: 1: 1, and a 5% aqueous solution of polyvinyl alcohol corresponding to a mass of 10% with respect to the measured solid content. Was prepared and premixed, and then mixed for 3 minutes using a planetary ball mill (spinning 2000 rpm, revolution 800 rpm) to obtain a slurry-like kneaded product.
The slurry was a highly viscous fluid having a greenish brown color immediately after kneading, but had a property of gelling in a few minutes. When the slurry is left in the air, the surface turns brown, because the divalent Fe is oxidized and the proportion of trivalent Fe increases. Water was evaporated while kneading the kneaded product in air at 100 ° C., and a mixed desiccant product having a brown color was obtained by oxidation during kneading.

  The mixed dried product was put into a graphite crucible and put into a vacuum substitution furnace. After the inside of the furnace was evacuated, the mixture was replaced with nitrogen mixed with 3% hydrogen. While introducing hydrogen mixed nitrogen into the furnace at a flow rate of 100 ml / min, the temperature was maintained at 250 ° C. for 4 hours, and then heat treatment was performed at 700 ° C. for 4 hours. After completion of the treatment, the mixture was allowed to cool with nitrogen introduced, and was taken out after the temperature dropped to 80 ° C. or lower. The powder mass after the heat treatment was black and loosely aggregated. The lump was subjected to various measurements after passing through a sieve having an opening of 50 μm. This powder was designated as Powder C according to the third example. The amount of coated carbon was determined by thermogravimetric analysis and found to be 2.0%.

[5] Measurement of physical properties For the obtained powders B and C, measurement of an X-ray diffraction pattern and shape observation by a scanning electron microscope (SEM) were performed. For comparison, powder D, which is the first comparative example of the first embodiment, and powder E, which is the second comparative example, were also performed. The SEM magnification was 10,000 times.
FIG. 6 is an X-ray diffraction pattern measured in relation to the second embodiment.
As shown in FIG. 6, all the powders B to E are composed of a substantially single phase of olivine type lithium iron silicate, but the powders D and E are X compared with the powders B and C. The line diffraction peak was high and sharp, and it was confirmed that crystals were grown larger than powders B and C.
Powder B and powder C were coated with 2% carbon coating, but no diffraction peak appeared because the coated carbon was estimated to be amorphous.

FIG. 7 is a scanning electron microscope (SEM) photograph of powder B according to the second example.
FIG. 8 is a scanning electron microscope (SEM) photograph of powder C according to the third example.
FIG. 9 is a scanning electron microscope (SEM) photograph of powder D according to the first comparative example.
FIG. 10 is a scanning electron microscope (SEM) photograph of powder E according to the second comparative example.

  As shown in FIGS. 7 to 10, the powder B has fine particles with a particle size of 50 to 200 nm (nanometers), and the powder E has some relatively large particles with a size of about 1 μm. It was found that the powder D and the powder E were composed of crushed coarse particles of several tens of μm. It is presumed that this is because the crystal growth was suppressed in the method of the present invention, whereas in the comparative example, the crystal grew greatly because it was in a molten state or was at a high temperature for a long time.

[6] Evaluation of electrode characteristics [6.1] Fabrication of lithium secondary battery Powders B and C obtained in the second and third examples had a total carbon content of 10% as a conductive agent. Acetylene black and polyvinylidene fluoride (PVdF), which is a binder (binder), are mixed at a weight ratio of 95: 5, and N-methyl-2-pyrrolidone (NMP) is added and kneaded sufficiently. Got. The positive electrode slurry was applied to an aluminum foil current collector with a thickness of 15 μm at a coating amount of 50 g / m ² and dried at 120 ° C. for 30 minutes. Then, it was rolled to a density of 2.0 g / cc with a roll press, punched into a 2 cm ² disk shape, and used as a positive electrode.

Using these positive electrodes, metallic lithium for the negative electrode, and a mixed solvent in which ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 1: 1 in the electrolyte solution, LiPF ₆ was dissolved at a concentration of 1M, and a lithium secondary battery was used. Produced. The production atmosphere was a dew point of −50 ° C. or lower. Each electrode was used by being crimped to a battery case with a current collector. A coin-type lithium secondary battery having a diameter of 25 mm and a thickness of 1.6 mm was formed using the positive electrode, the negative electrode, the electrolyte, and the separator. Batteries using the respective positive electrode active materials were designated as the battery BB and the battery BC according to the second and third examples of the present invention, respectively.

[6.2] Battery Test of Lithium Secondary Battery A large number of each battery was produced and repeated for 10 cycles from the initial charge / discharge at a current of 0.01 CA. The charging conditions were set as constant current constant voltage charging with a current of 0.01 CA and a voltage of 4.5 V until the current during constant voltage charging dropped to 0.005 CA. The discharge conditions were a constant current discharge with a current of 0.01 CA and a final voltage of 1.5V. All temperatures were 25 ° C. The results of the first charge / discharge are shown in FIG. The capacity was converted per 1 g of olivine type lithium iron silicate.

Here, referring to FIG. 5 again, the result of the discharge capacity at the 10th cycle related to the second embodiment and the third embodiment will be described.
As shown in FIG. 5, the batteries BB and BC according to the second and third examples showed a larger capacity than the battery BA according to the first example. This is presumed that the powder B and the powder C used for the battery BB and the battery BC were made of fine particles and coated with carbon on the surface, so that a smooth electrochemical reaction was possible. The The voltage of the battery BE is lower than that of the battery BD because the difference in the lithium ion diffusivity in the particles due to the different raw materials is considered.

Therefore, in order to obtain good positive electrode characteristics, a method for synthesizing an active material having a particle size as small as a nano level and having a surface coated with carbon is required, and this is provided by the present invention.
As described above, according to the second and third examples according to the second embodiment, the olivine-type silicic acid M having finer particles useful as the secondary battery positive electrode active material by the carbon source. Lithium can be easily synthesized, and the performance of the lithium ion secondary battery can be improved.

  BA, BB, BC batteries

Claims (4)

In a synthesis method of olivine-type M lithium silicate (M is a metal element) used as a positive electrode material of a lithium ion secondary battery,
A kneading and drying step in which colloidal silica as a silicic acid source, a lithium source and a metal element M source are wet-kneaded, and the obtained kneaded product is dried while kneading;
A heat treatment step of heat-treating the mixed desiccant obtained after the drying in a predetermined atmosphere to reduce the metal element M;
A method for synthesizing olivine-type M lithium silicate, comprising: In the synthesis | combining method of the olivine type | mold M lithium silicate of Claim 1,
The metal element M source constitutes the olivine-type lithium M silicate as a divalent compound,
In the kneading and drying step, a part or all of the metal element M is once oxidized to oxidize the olivine-type lithium M silicate. In the synthesis | combining method of the olivine type M lithium silicate of Claim 1 or Claim 2,
In the kneading and drying step, the organic acid solution as the silicic acid source, the compound serving as the metal element M source and the carbon source is added, and these are mixed to remove the solvent while oxidizing the metal element M,
In the heat treatment step, the heat treatment is performed in an inert atmosphere or a reducing atmosphere.
A method of synthesizing olivine-type M lithium silicate, which is characterized by the above. In a lithium ion secondary battery in which olivine-type M lithium silicate (M is a metal element) is used as a positive electrode material,
Colloidal silica as a silicic acid source, a lithium source and a metal element M source are wet-kneaded, and the obtained kneaded product is dried while kneaded, and the metal element M is added to the mixed dried product obtained after the drying. A lithium ion secondary battery, wherein a positive electrode active material layer containing olivine-type lithium M silicate obtained by heat treatment in a predetermined atmosphere to reduce the amount of lithium is formed on a positive electrode current collector.