JP5636772B2 - Olivine-type lithium transition metal composite oxide and method for producing the same - Google Patents

Description

  The present invention relates to a positive electrode active material capable of reversibly doping and dedoping lithium and a method for producing the same.

  A lithium transition metal composite oxide used as a positive electrode active material for a non-aqueous electrolyte secondary battery is known to have a high working voltage of 4 V when a secondary battery is constructed, and to obtain a large capacity. For this reason, lithium ion secondary batteries using lithium transition metal composite oxides as positive electrode active materials are often used as power sources for electronic devices such as mobile phones, notebook computers, and digital cameras. In recent years, due to environmental considerations, there is an increasing demand for lithium ion secondary batteries for use with large secondary batteries mounted on electric vehicles, hybrid vehicles, and the like.

  In particular, as a positive electrode active material that is safer and cheaper than lithium transition metal composite oxide (lithium cobaltate) using cobalt as a transition metal, for example, as disclosed in Patent Document 1 and Patent Document 2, 3 An olivine-type lithium iron composite oxide having a voltage of .5V has been attracting attention.

This olivine-type lithium iron composite oxide has an olivine-type crystal structure having a polyanion as a basic skeleton as a positive electrode active material of a lithium ion secondary battery. For example, a compound represented by a composition formula of LiFePO ₄ is known. ing. When these compounds are used as positive electrode active materials for secondary batteries, they have excellent cycle characteristics due to little change in the crystal structure accompanying charge / discharge, and oxygen atoms in the crystals exist stably due to covalent bonds with phosphorus. Therefore, even when the battery is exposed to a high temperature environment, there is a merit that the possibility of oxygen release is small and the safety is excellent.

International Publication WO2005 / 041327 JP 2004-79276 A

  However, while the olivine-type lithium iron composite oxide has the advantages as described above, the problem is that the electrical conductivity is essentially low. For this reason, insertion and desorption of lithium into the crystal hardly proceed, and it is difficult to ensure a sufficient charge / discharge capacity. In order to compensate for the low electrical conductivity, it is necessary to produce an electrode together with a large amount of a conductive agent, which causes a decrease in the volume density of the electrode, that is, a decrease in volume energy density.

  Therefore, as another means for overcoming the low electrical conductivity, as disclosed in Patent Document 2, the size of the olivine-type lithium iron composite oxide particles can be adjusted.

  That is, in order to obtain a positive electrode active material with smooth lithium insertion and desorption and good charge / discharge characteristics, the Li ion diffusion rate is increased by reducing the particle size of the olivine-type lithium iron composite oxide. be able to.

  However, the method for producing an olivine-type lithium iron composite oxide disclosed in Patent Document 2 suppresses particle growth during firing and reduces the particle size, so that the interface is separate from the raw material of the olivine-type lithium iron composite oxide. An activator is added. Since this surfactant disperses the precursor of the positive electrode active material as fine particles (micelles) in the liquid, the particle diameter of the olivine-type lithium iron composite oxide can be reduced. As such a surfactant disclosed in Patent Document 2, for example, Octapole (Octapole, trade name, manufactured by Sanyo Chemical Industries, Ltd.) consisting of isooctylphenol and ethylene oxide (Ethylene Oxide) is used. Such materials that are not normally used in the manufacturing process of the positive electrode active material are mentioned. Therefore, not only the material cost and the manufacturing cost of such a surfactant are required, but also a labor for adjusting and adding the surfactant in the manufacturing process of the positive electrode active material is required.

  Therefore, the present invention has good charge / discharge characteristics in which lithium is smoothly inserted into and desorbed by reducing the particle size of the olivine-type lithium transition metal composite oxide by means different from the prior art. An object is to provide a positive electrode active material and a method for producing the same.

In order to achieve the above object, the olivine-type lithium transition metal composite oxide according to the present invention has a composition of Li _x Fe _1-z Mo _z PO ₄ (wherein x and z are 0.9 <x < In the olivine-type lithium transition metal composite oxide, wherein 1.3 and 0 <z <0.3), the molybdenum (Mo) is the olivine-type lithium transition metal using molybdenum (Mo) oxide as a raw material. The olivine-type lithium transition metal composite oxide is dissolved in a composite oxide, and has a crystallite size of 30 nm to 45.9 nm and an average secondary particle size of 4 μm to 7 μm. And

In the method for producing an olivine-type lithium transition metal composite oxide according to the present invention, the composition is Li _x Fe _1-z Mo _z PO ₄ (where x and z are 0.9 <x <1.3, 0 <Z <0.3.) In the method for producing an olivine-type lithium transition metal composite oxide, wherein an iron phosphate compound, a lithium source, a carbon source, an oxide of molybdenum (Mo), a solvent, A step of preparing a slurry containing, a step of pulverizing particles contained in the slurry, a step of spray-drying the pulverized slurry to form a precursor, and the precursor under an inert atmosphere And a heat treatment step.

  INDUSTRIAL APPLICABILITY According to the present invention, a positive electrode active material having smooth charge and discharge characteristics in which insertion and desorption of lithium into and from a crystal can be smoothly performed with relatively little production cost and effort. Moreover, according to the manufacturing method concerning this invention, the positive electrode active material with little deterioration of the electrode plate in a lithium ion secondary battery can be manufactured.

FIG. 1 shows an SEM photograph of positive electrode active material particles when molybdenum is not added to an olivine-type lithium iron composite oxide. FIG. 2 shows an SEM photograph of positive electrode active material particles when 1 mol% of molybdenum is added to Fe in the olivine-type lithium iron composite oxide. FIG. 3 shows an SEM photograph of positive electrode active material particles when 3 mol% of molybdenum is added to Fe in the olivine-type lithium iron composite oxide. FIG. 4 shows an SEM photograph of positive electrode active material particles when 5 mol% of molybdenum is added to Fe in the olivine-type lithium iron composite oxide. FIG. 5 shows an SEM photograph of the positive electrode active material particles when 10 mol% of molybdenum is added to Fe in the olivine-type lithium iron composite oxide.

  The best mode for carrying out the present invention will be described. However, the embodiment shown below exemplifies an olivine type lithium transition metal composite oxide and a manufacturing method thereof for embodying the technical idea of the present invention, and the present invention is an olivine type lithium transition metal composite oxide. The product and its manufacturing method are not limited to the following.

As a result of intensive research, the present inventors have found that the composition of the olivine-type lithium transition metal composite oxide is Li _x (Fe _1-y M1 _y ) _1-z M2 _z PO ₄ (wherein x, y, z are 0.9 <x <1.3, 0 ≦ y <1, 0 <z <0.3, and M1 is at least one metal element selected from the group consisting of Mn, Co and Ni , M2 is at least one metal element selected from the group consisting of Mo, Mg, Zr, Ti, Al, Ce, Cr, Mn, Co, and Ni.) And is an olivine type lithium transition metal composite oxide The product has a crystallite size of 80 nm or less, an average secondary particle size of 2 μm or more and 15 μm or less, and the metal element M2 is included in the composition using an oxide or hydroxide as a raw material. It came to solve the problem.

That is, it has been found that the size of particles having good charge / discharge characteristics can be controlled by dissolving a specific metal element, for example, molybdenum (Mo) in LiFePO ₄ in the olivine-type lithium transition metal composite oxide.

The metal element M2 is molybdenum (Mo), and molybdenum is preferably added in an amount of 0.1 mol% or more and 10 mol% or less in terms of element ratio with respect to iron (Fe) and M1 having the above composition. For example, it is preferable to add molybdenum in an amount of 0.1 mol% or more and 10 mol% or less in terms of element ratio with respect to iron whose composition of the olivine type lithium iron composite oxide is LiFePO ₄ . More preferably, they are 1 mol% or more and 3 mol% or less. This is because the molybdenum content can be adjusted to the target particle size and good charge / discharge characteristics can be obtained when the molybdenum content is in the above range.

Hereinafter, the olivine-type lithium transition metal composite oxide of this embodiment and the production method thereof will be described in detail.
(Olivine type lithium transition metal composite oxide)
First, the olivine type lithium transition metal composite oxide according to the present invention will be described. The composition of the olivine-type lithium transition metal composite oxide according to the present invention is Li _x (Fe _1-y M1 _y ) _1-z M2 _z PO ₄ . However, x, y, and z in the composition formula are 0.9 <x <1.3, 0 ≦ y <1, and 0 <z <0.3. M1 is at least one metal element selected from the group consisting of Mn, Co and Ni, and M2 is composed of Mo, Mg, Zr, Ti, Al, Ce, Cr, Mn, Co and Ni. At least one metal element selected from the group.

  Here, when x is smaller than 0.9, the theoretical capacity of the battery is lowered, which is not preferable. Further, when x is larger than 1.3, the amount of impurities increases, and battery characteristics are deteriorated. Further, more preferable ranges of x and z are 1.0 <x <1.1 and 0 <z <0.1. When the metal element M2 is Mo, Mg, Zr, Ti, Al, Ce, or Cr, z cannot be inserted at the time of solid solution of the oxide or hydroxide of these metal elements. If it is larger, the theoretical capacity of the battery is lowered, which is not preferable.

  The crystallite diameter of the olivine-type lithium transition metal composite oxide according to the present invention is 80 nm or less. When the crystallite diameter exceeds 80 nm, the charge / discharge capacity becomes small, which is not preferable. More preferably, it is 30 nm or more and 55 nm or less. Similarly, when the crystallite size of the olivine type lithium transition metal composite oxide is less than 30 nm, the characteristics are similarly deteriorated, which is not preferable.

  The average secondary particle diameter of the olivine type lithium transition metal composite oxide according to the present invention is 2 μm or more and 15 μm or less. When the average secondary particle diameter is less than 2 μm or exceeds 15 μm, it is not preferable because the workability deteriorates when the positive electrode active material is applied to the electrode plate of the lithium ion secondary battery. Further, when it exceeds 15 μm, the packing density is lowered, which is not preferable. More preferably, the average secondary particle diameter of the olivine type lithium transition metal composite oxide is 4 μm or more and 7 μm or less.

  The oxide, phosphorus oxide or hydroxide of the metal element M2 is preferably present uniformly distributed inside the particles of the olivine type lithium transition metal composite oxide. This is because by improving the mixed state of the metal element M2, the effect of controlling the particle diameter by adding the metal element is further increased.

The carbon content of the olivine-type lithium transition metal composite oxide according to the present invention is 0.2% by weight or more and 10% by weight or less, and is preferably present inside or on the surface of the secondary particles. When the carbon content is less than 0.2% by weight, the electrical resistance increases. Further, when the carbon content exceeds 10% by weight, the discharge capacity per weight is reduced. Preferably, they are 0.5 weight% or more and 5 weight% or less.
(Method for measuring physical properties of olivine type lithium transition metal composite oxide)
The physical properties of the olivine type lithium transition metal composite oxide in this embodiment are measured by the following methods.

  The crystallite diameter is calculated based on the assumption that there is no non-uniform strain using the so-called Scherrer equation from the peak height and full width at half maximum (FWHM) of a specific diffraction angle (plane). Find the average size of a child.

  Note that the X-ray diffraction profile of olivine particles shows a large number of peaks. For example, a peak derived from a lattice plane (031) plane is appropriate as a peak for obtaining a crystallite. This is because, in the case of this peak, since there is a distance from other peaks, it is difficult to receive interference from other peaks, and an accurate value can be obtained.

The average secondary particle diameter is the value of the median diameter (d50) measured by the laser diffraction method, especially the average secondary particle diameter of the olivine-type lithium transition metal composite oxide in the form.
(Method for producing olivine-type lithium transition metal composite oxide)
Next, a method for producing the olivine type lithium transition metal composite oxide according to the present invention will be described. The method for producing an olivine-type lithium transition metal composite oxide according to the present invention includes at least the following steps (1) to (4).
(1) Phosphorus oxide, lithium source, carbon source, and oxide of at least one metal element selected from the group consisting of Mo, Mg, Zr, Mn, Ti, Co, Ni, Al, Ce, and Cr Producing a slurry containing a phosphorus oxide or hydroxide and a solvent;
(2) A step of pulverizing the particles contained in the slurry to refine them,
(3) a step of spray drying the pulverized slurry to form a precursor;
(4) A step of firing the precursor in an inert atmosphere.

  The raw material of the olivine-type lithium transition metal composite oxide according to the present invention comprises a phosphorus oxide, a lithium source, a carbon source, and Mo, Mg, Zr, Mn, Ti, Co, Ni, Al, Ce, and Cr. A raw material containing an oxide, phosphorus oxide or hydroxide of at least one metal element selected from the group. The raw material phosphorus oxide may be phosphorus oxide (II) or phosphorus oxide (III). Examples of the phosphor oxide include iron (II) phosphate, iron (III) phosphate, manganese (II) phosphate, manganese (III) phosphate, and cobalt (II) phosphate. Moreover, it is preferable that the average secondary particle diameter is 5 micrometers or more and 10 micrometers or less considering the workability | operativity in a manufacturing process.

  The raw material phosphorus oxide is preferably washed before mixing with other raw materials in order to remove impurities. A dispersion medium is added to the washed phosphorous oxide. As this dispersion medium, an organic solvent such as water, acetone or ethanol can be used. Among these, water is preferable because it is easy to handle and inexpensive.

  The reason for adding the metal elements such as Mo, Ti, and Mg as oxides, phosphorus oxides, or hydroxides is, for example, that when they are added as chlorides of those metal elements, the remaining chloride ions cause lithium ion secondary This is for avoiding the possibility that the electrode plate of the battery may deteriorate. The metal element oxide, phosphorus oxide or hydroxide can be used as a material having a relatively small influence on the electrode plate of the lithium ion secondary battery.

  As a raw material lithium source, any material containing lithium can be used. For example, lithium phosphate, lithium dihydrogen phosphate, lithium carbonate, lithium acetate, or lithium hydroxide, and mixtures thereof. Among these, lithium carbonate is preferable in view of easy handling and environmental safety.

  As the carbon source of the raw material, saccharides such as glucose, sucrose and lactose, and organic compounds such as glycerin, ascorbic acid, lauric acid and stearic acid can be used. Of these, sucrose is preferable as a carbon source because it is easy to handle. These carbon sources can be used not only as a carbon source for imparting conductivity to the olivine-type lithium transition metal composite oxide, but also as a carbon source for reducing a metal element in the raw material.

  Each raw material was weighed so that the stoichiometric ratio of the composition of the target olivine-type lithium transition metal composite oxide was obtained, and then the phosphorous oxide and the dispersion medium were further mixed with a lithium source, a carbon source, Mo, A slurry is formed by containing an oxide, a phosphorous oxide or a hydroxide of at least one metal element selected from the group consisting of Mg, Zr, Mn, Ti, Co, Ni, Al, Ce and Cr.

  Since the above-mentioned raw materials are usually supplied as particulate raw materials, the method of pulverization treatment for finely pulverizing and mixing these particulate raw materials can be selected from wet pulverization mixing and dry pulverization mixing. . The wet pulverization mixing is a pulverization method in which an object to be pulverized is placed in a dispersion medium (for example, water) and is rotated on a roller stand using a medium of about 1 mm, and can be pulverized more finely than dry pulverization mixing.

  In the step of spray drying the pulverized slurry to form a precursor, spray drying is a method in which the slurry to be dried is sprayed in a shower shape and dried by blowing hot air on the sprayed slurry. Thereby, the secondary particle (spherical shape) which is an aggregate | assembly of a primary particle can be formed as a precursor.

  In the step of firing the precursor under an inert atmosphere, the inert atmosphere is preferably nitrogen or a reducing atmosphere containing hydrogen or ammonia, and more preferably an atmosphere containing hydrogen and nitrogen. The firing temperature is preferably 500 ° C. or higher and 800 ° C. or lower, and more preferably 600 ° C. or higher and 700 ° C. or lower.

  Examples according to the present invention will be described in detail below. Needless to say, the present invention is not limited to the following examples.

<Example 1>
179 g as Fe ₃ (PO ₄ ) ₂ (1.5 mol as Fe), lithium phosphate (Li ₃ PO ₄ ), 27 g of sucrose (0.325 g with respect to 1 g of Fe), and molybdenum trioxide 7 g (3 mol% with respect to Fe) and 1700 ml of pure water as a dispersion medium are mixed to form a slurry. The slurry is put into a ball mill having a capacity of 5000 ml, and is pulverized for 20 hours using alumina balls to be refined.

  The pulverized slurry is spray-dried using a spray dryer to obtain a heat-treated product as a precursor. Thereafter, the heat-treated product is baked at 700 ° C. for 10 hours in a nitrogen gas atmosphere to obtain a baked product.

Using an X-ray diffractometer, phase identification of the obtained fired product was performed. As an X-ray, the CuKα ray (wavelength: λ = 1.54 nm) was analyzed, and as a result, olivine-type lithium iron composite oxide was confirmed, and no impurity peak was observed. The obtained fired product has a composition of LiFe _0.97 Mo _0.03 PO ₄ , a crystallite size of 384 mm (38.4 nm), a carbon content of 2.4 wt%, and an average secondary particle size of 4.9 μm. Met.

In the method for producing the olivine-type lithium iron composite oxide of this example, by adding molybdenum oxide to the olivine-type lithium iron composite oxide, the particle size was reduced, and the charge / discharge characteristics were improved. This is considered to be because molybdenum oxide suppressed grain growth.
<Example 2>
An olivine-type lithium iron composite oxide was produced in the same manner as in Example 1 except that the addition amount of molybdenum was changed to 1 mol% with respect to Fe. The fired product obtained has a composition of LiFe _0.99 Mo _0.01 PO ₄ , a crystallite size of 459 mm (45.9 nm), a carbon content of 2.4 wt%, and an average secondary particle size of 5.0 μm. Met.
<Example 3>
226 g of FePO ₄ (1.5 mol as Fe), lithium carbonate (Li ₂ CO ₃ ), 27 g of sucrose (0.325 g with respect to 1 g of Fe), and 7 g of molybdenum trioxide (with respect to Fe) 3 mol%) and 1650 ml of pure water as a dispersion medium are mixed to form a slurry. The slurry is put into a ball mill having a capacity of 5000 ml, and is pulverized for 20 hours using alumina balls to be refined.

The pulverized slurry is spray-dried using a spray dryer to obtain a heat-treated product as a precursor. Thereafter, the heat-treated product is baked at 700 ° C. for 10 hours in a nitrogen gas atmosphere to obtain a baked product.
<Example 4>
179 g as Fe ₃ (PO ₄ ) ₂ (1.5 mol as Fe), lithium phosphate (Li ₃ PO ₄ ), 27 g of sucrose (0.325 g with respect to 1 g of Fe), and magnesium hydroxide A slurry is prepared by mixing 8.8 g (3 mol% with respect to Fe) and 1700 ml of pure water as a dispersion medium. The slurry is put into a tal having a capacity of 5000 ml, and pulverized for 20 hours using alumina balls to be refined.

The pulverized slurry is spray-dried using a spray dryer to obtain a heat-treated product as a precursor. Thereafter, the heat-treated product is baked at 700 ° C. for 10 hours in a nitrogen gas atmosphere to obtain a baked product.
<Example 5>
179 g as Fe ₃ (PO ₄ ) ₂ (1.5 mol as Fe), lithium phosphate (Li ₃ PO ₄ ), 27 g of sucrose (0.325 g with respect to 1 g of Fe), and 12 of titanium oxide 0.1 g (3 mol% with respect to Fe) and 1700 ml of pure water as a dispersion medium are mixed to form a slurry. The slurry is put into a ball mill having a capacity of 5000 ml, and is pulverized for 20 hours using alumina balls to be refined.

The pulverized slurry is spray-dried using a spray dryer to obtain a heat-treated product as a precursor. Thereafter, the heat-treated product is baked at 700 ° C. for 10 hours in a nitrogen gas atmosphere to obtain a baked product.
<Comparative Example 1>
An olivine-type lithium iron composite oxide was produced in the same manner as in Example 1 except that no molybdenum was added. The obtained fired product had a crystallite size of 474 mm (47.4 nm), a carbon content of 2.4% by weight, and an average secondary particle size of 5.0 μm.
<Comparative example 2>
In order to make it easy to confirm the effect of suppressing particle growth by dissolving molybdenum in LiFePO ₄ , the amount of molybdenum added to Fe is 0 mol%, 1 mol as in <Comparative Example 1> except that sucrose is not added. %, 3 mol%, 5 mol%, and 10 mol%, 5 kinds of olivine type lithium iron composite oxides were produced.

  Using an X-ray diffractometer, phase identification of the obtained fired product was performed. As X-rays, CuKα rays (wavelength: λ = 1.54 nm) were analyzed, and as a result, olivine-type lithium iron composite oxides were confirmed. In addition, SEM photographs of the positive electrode active material composed of the above five kinds of olivine type lithium iron composite oxides were taken.

FIG. 1 shows a SEM photograph of a positive electrode active material (LiFePO ₄ ) when molybdenum is not added to a positive electrode active material made of an olivine-type lithium iron composite oxide.

FIG. 2 shows the positive electrode active material (LiFe _0.99 Mo _0.01 PO ₄ ) produced when the amount of molybdenum added is 1 mol% with respect to Fe in the positive electrode active material made of olivine-type lithium iron composite oxide. An SEM photograph is shown.

FIG. 3 shows the positive electrode active material (LiFe _0.97 Mo _0.03 PO ₄ ) when the amount of molybdenum added is 3 mol% with respect to Fe in the positive electrode active material made of olivine-type lithium iron composite oxide. An SEM photograph is shown.

FIG. 4 shows the positive electrode active material (LiFe _0.95 Mo _0.05 PO ₄ ) when the amount of molybdenum added is 5 mol% with respect to Fe in the positive electrode active material made of olivine-type lithium iron composite oxide. An SEM photograph is shown.

FIG. 5 shows the positive electrode active material (LiFe _0.90 Mo _0.10 PO ₄ ) when the amount of molybdenum added is 10 mol% with respect to Fe in the positive electrode active material made of olivine-type lithium iron composite oxide. An SEM photograph is shown.

  [Table 1] below shows measurement results for crystallite diameter, lattice constant, charge capacity / discharge capacity, and average secondary particle diameter in <Example 1>, <Example 2>, and <Comparative Example 1>. Indicates.

  From FIG. 1 to FIG. 5 and [Table 1], the sintering of the particles sintered by firing without the addition of sucrose is suppressed as the amount of molybdenum added increases, and the crystallite size and particle size are increased. You can see that it is getting smaller.

  The positive electrode active material of the present invention or the positive electrode active material obtained by the production method of the present invention is used as a positive electrode active material of a secondary battery, for example, various portable devices including a mobile phone, electric vehicles, and hybrid electric vehicles. Is possible.

Claims (5)

An olivine-type lithium transition metal having a composition of Li _x Fe _1-z Mo _z PO ₄ (wherein x and z are 0.9 <x <1.3 and 0 <z <0.3). In complex oxide,
The molybdenum (Mo) is solid-dissolved in the olivine type lithium transition metal composite oxide using a molybdenum (Mo) oxide as a raw material,
The olivine type lithium transition metal composite oxide has a crystallite size of 30 nm to 45.9 nm and an average secondary particle size of 4 μm to 7 μm. .   The olivine type lithium transition according to claim 1, wherein molybdenum (Mo) is added in an amount of 0.1 mol% or more and 10 mol% or less in terms of element ratio with respect to the total amount of iron (Fe) and molybdenum (Mo) having the above composition. Metal complex oxide. An olivine-type lithium transition metal having a composition of Li _x Fe _1-z Mo _z PO ₄ (wherein x and z are 0.9 <x <1.3 and 0 <z <0.3). In the method for producing a composite oxide,
Adjusting a slurry containing an iron phosphate compound, a lithium source, a carbon source, an oxide of molybdenum (Mo), and a solvent;
Crushing particles contained in the slurry;
Spray drying the pulverized slurry to form a precursor;
Heat-treating the precursor under an inert atmosphere;
A process for producing an olivine-type lithium transition metal composite oxide, comprising:   The method for producing an olivine-type lithium transition metal composite oxide according to claim 3, wherein the temperature of the heat treatment is 500 ° C or higher and 800 ° C or lower.   The olivine type lithium transition according to claim 3 or 4, wherein molybdenum (Mo) is added in an element ratio of 0.1 mol% or more and 10 mol% or less with respect to the total amount of iron (Fe) and molybdenum (Mo) having the above composition. A method for producing a metal composite oxide.

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