JP2007035358A - Positive electrode active substance, its manufacturing method and lithium ion secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide positive electrode active substance composed of lithium iron phosphoric acid compound demonstrating high capacity and output even when charging/recharging is repeatedly carried out, its manufacturing method, and a lithium ion secondary battery using the positive electrode active substance. <P>SOLUTION: The positive electrode active substance is composed of lithium iron phosphoric acid compound of olivine structure expressed by a general formula Li<SB>1-x</SB>Fe<SB>1-y</SB>M<SB>y</SB>PO<SB>4</SB>. The active substance is composed of deformed particles provided with a plurality of particle-like projections with particle diameters of 1 μm or less. Furthermore, the manufacturing method is for the positive electrode active substance provided with a material dispersing process and an annealing process. In the material dispersing process, material slurry is obtained by having phosphoric acid lithium compound, phosphoric acid iron (II) compound, and a compound containing metallic element M dispersed in a polar solvent. In the annealing process, the material slurry is heated. In addition, the lithium ion secondary battery uses the positive electrode substance composed of deformed particles provided with a plurality of particle-like projections with particle diameters of 1 μm or less on their surfaces. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a positive electrode active material comprising a lithium iron phosphate compound having an olivine structure used in a lithium ion secondary battery, a method for producing the same, and a lithium ion secondary battery using the positive electrode active material.

  Lithium ion secondary batteries using non-aqueous electrolytes not only provide high voltage and high energy density, but can also be reduced in size and weight, so they are already in practical use in information communication equipment such as personal computers and mobile phones. It has become. In recent years, electric vehicles and hybrid electric vehicles have attracted attention due to environmental problems and resource issues, and the lithium ion secondary battery is also used as a power source mounted on electric vehicles and hybrid electric vehicles.

A lithium ion secondary battery is formed by binding a positive electrode active material such as a lithium transition metal composite oxide to a positive electrode current collector and a negative electrode active material such as a carbon material to the negative electrode current collector. The main components are a negative electrode and a nonaqueous electrolytic solution obtained by dissolving an electrolyte such as a lithium salt in a nonaqueous solvent such as an organic solvent. In general, as the positive electrode active material, a compound having a layered structure or spinel structure such as LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ is used, and as the negative electrode active material, for example, graphite or coke is used. As the non-aqueous solvent, for example, a mixed solvent of a chain carbonate and a cyclic carbonate is used, and as the electrolyte, LiPF ₆ is used because of its high conductivity. In addition, plate-like aluminum or the like is used as the positive electrode current collector, and plate-like copper or the like is used as the negative electrode current collector.
For the positive electrode or the negative electrode, for example, a positive electrode active material or a negative electrode active material, a conductive additive, a binder, a dispersant, and the like are mixed to produce an active material paste adjusted to an appropriate viscosity, and this active material paste is used as a current collector. It can be coated and pressed.

As the positive electrode active material, a compound having an olivine structure made of a lithium iron phosphate compound such as LiFePO ₄ has been developed. The lithium iron phosphate compound can be obtained by a so-called solid phase synthesis method in which a mixture comprising a lithium salt, an iron salt, a phosphate compound, and the like is heated.
The lithium iron phosphate compound having such an olivine structure is inferior in theoretical capacity as compared with the compound having the layered structure and the spinel structure described above, but contains Fe as a main component as a transition metal element. Has an advantage. Therefore, lithium iron phosphate compounds are attracting attention as the next generation positive electrode active material.
However, lithium iron phosphate compounds are inherently low in electronic conductivity. For this reason, when the charge / discharge rate is increased, a sufficient capacity cannot be taken out. Therefore, it has been difficult to use as an active material for batteries for electric vehicles and hybrid electric vehicles that require a particularly high charge / discharge rate.

In the past, in order to improve the charge / discharge rate characteristics, a technique has been developed in which a lithium iron phosphate compound is coated or supported with a conductive substance such as carbon or a noble metal (see Patent Documents 1 to 3).
It is also known that it is effective to make the active material particles fine in order to improve the charge / discharge rate characteristics. That is, by making the active material particles fine and increasing the specific surface area, the reaction area of Li insertion / desorption can be increased, and the reaction rate can be improved. Furthermore, the diffusion resistance of the Li in the active material particles is shortened, so that the diffusion resistance can be lowered.

  However, when fine active material particles are used, a large amount of dispersant is required to produce the above active material paste having a viscosity suitable for application to the current collector, and the active material particles and the current collector There was a problem that the adhesiveness with. That is, even if the active material paste containing fine active material particles is applied to a current collector and pressed, the binding force between the active material particles and the current collector cannot be sufficiently improved. Therefore, when charging / discharging is repeated, there is a problem that the adhesion between the active material particles and the current collector is deteriorated, and battery characteristics such as capacity and output are deteriorated.

Japanese Patent Application Laid-Open No. 2004-514639 JP 2003-338992 A JP 2001-110414 A

  The present invention has been made in view of such conventional problems, and is capable of exhibiting high capacity even when the charge / discharge rate is increased, and lithium iron capable of maintaining high capacity and output even when repeated charge / discharge is performed. It is an object of the present invention to provide a positive electrode active material comprising a phosphoric acid compound, a method for producing the same, and a lithium ion secondary battery using the positive electrode active material.

The first invention has a general formula Li _1-x Fe _{1- y My} PO ₄ (where −0.2 ≦ x ≦ 0.2, 0 ≦ y ≦ 0.5, M is Li, Ni, Co, A lithium iron phosphate compound having an olivine structure represented by Mn, Mg, Al, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y). In the method for producing a positive electrode active material used for producing a positive electrode for a lithium ion secondary battery by bonding to a metal positive electrode current collector,
A raw material dispersion step in which a lithium phosphate compound, an iron (II) phosphate compound, and a compound containing a metal element M added as necessary are dispersed in a polar solvent to produce a raw material slurry;
Heat treatment step of synthesizing the lithium iron phosphate compound having an olivine structure represented by the above general formula by heating the raw material slurry in a sealed container at a temperature of 120 to 280 ° C. under an inert gas atmosphere,
In the raw material dispersion step, the iron (II) phosphate compound is dispersed in the polar solvent so that the molar concentration of the iron element in the raw material slurry is 3 mol / L or less. (Claim 1).

In the first invention, by performing the raw material dispersion step and the heat treatment step, the lithium iron phosphate compound having an olivine structure represented by the general formula Li _1-x Fe _{1- y My} PO ₄ is used. A positive electrode active material is produced.
Specifically, in the raw material dispersion step, the lithium phosphate compound, the iron (II) phosphate compound, and the phosphate compound containing the metal element M added as necessary are polar solvents. The raw material slurry is prepared by dispersing in a slurry. At this time, the iron phosphate (II) is dispersed in the polar solvent so that the molar concentration of the iron element in the raw material slurry is 3 mol / L or less. In the heat treatment step, the raw material slurry is heated in a sealed container at a temperature of 120 to 280 ° C. in an inert gas atmosphere.

As a result, it is composed of the above lithium iron phosphate compound having the olivine structure represented by the above general formula and has a plurality of irregular shaped particles having a particle size of 1 μm or less on the surface, in other words, as if the particle size is 1 μm or less. Thus, a positive electrode active material having a characteristic structure such as secondary particles in which primary particles are aggregated can be obtained. That is, the step of producing primary particles having a particle size of 1 μm or less and the step of aggregating the primary particles to form secondary particles are not performed, but the raw material dispersion step and the heat treatment step are performed. Thus, the positive electrode active material composed of the irregular shaped particles can be obtained.
The positive electrode active material having such a characteristic structure can have both the characteristics of an active material composed of fine particles and the characteristics of an active material composed of relatively large particles.

  That is, since the positive electrode active material is formed of irregularly shaped particles having a relatively large particle size, the positive electrode current collector can be bitten by the so-called anchor effect when bonded to the positive electrode current collector. Therefore, the positive electrode active material can be adhered to the positive electrode current collector with excellent adhesion. Therefore, in the positive electrode produced using the positive electrode active material, even when charging and discharging are repeated, the adhesion between the positive electrode active material and the positive electrode current collector is hardly lowered, and excellent capacity and output are maintained. can do.

  Moreover, the said positive electrode active material has the said fine particle-shaped convex part with a particle size of 1 micrometer or less. Therefore, the positive electrode active material has a large specific surface area and a large reaction area for Li insertion and desorption, so that the reaction rate can be improved. Further, the diffusion path of Li in the particles of the positive electrode active material is shortened, and the diffusion resistance can be reduced. Therefore, the electrical conductivity of the positive electrode active material can be improved, and the positive electrode active material can exhibit a high capacity even at a high charge / discharge rate.

A second invention is a positive electrode active material used for producing a positive electrode for a lithium ion secondary battery by adhering to a metal positive electrode current collector,
The positive electrode active material has a general formula Li _1-x Fe _{1- y My} PO ₄ (where −0.2 ≦ x ≦ 0.2, 0 ≦ y ≦ 0.5, M represents Li, Ni, Co, A lithium iron phosphate compound having an olivine structure represented by Mn, Mg, Al, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y).
The positive electrode active material is a positive electrode active material characterized in that the positive electrode active material comprises irregularly shaped particles having a plurality of particulate protrusions having a particle diameter of 1 μm or less on the surface.

The most notable point in the second aspect of the invention is that the positive electrode active material is composed of irregularly shaped particles having a plurality of particulate protrusions having a particle size of 1 μm or less on the surface.
Therefore, the positive electrode active material can exhibit an excellent electrical conductivity by making use of the characteristics of the fine particle-shaped convex part having a particle size of 1 μm or less, and further by taking advantage of the characteristics of the irregularly shaped particles having a relatively large particle size. The positive electrode current collector can be bonded with excellent adhesion. Therefore, the positive electrode active material can exhibit a high capacity even when the charge / discharge rate is increased, and can maintain a high capacity and output even when the charge / discharge is repeated.

A third invention is a lithium ion secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte obtained by dissolving an electrolyte in a non-aqueous solvent,
The positive electrode is formed by bonding a positive electrode active material to a metal positive electrode current collector,
In the lithium ion secondary battery, the positive electrode active material of the second invention is adopted as the positive electrode active material.

In the lithium ion secondary battery of the third invention, the positive electrode active material of the second invention is used as the positive electrode active material.
Therefore, the lithium ion secondary battery can exhibit a high capacity even when the charge / discharge rate is increased by taking advantage of the excellent characteristics of the positive electrode active material of the second invention. Furthermore, even if charging / discharging is repeated, the capacity hardly decreases and the resistance hardly increases. Therefore, the lithium ion secondary battery can maintain high capacity and high output even when charging and discharging are repeated.

Next, a preferred embodiment of the present invention will be described.
In the present invention, the positive electrode active material has the general formula Li _1-x Fe _{1- y My} PO ₄ (where -0.2 ≦ x ≦ 0.2, 0 ≦ y ≦ 0.5, M is Li, Lithium iron phosphate having an olivine structure represented by one or more elements selected from Ni, Co, Mn, Mg, Al, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y) Consists of compounds.
In the above general formula, when x and y are out of the above ranges, the conductivity decreases, the lithium iron phosphate compound cannot maintain the olivine structure, the rate characteristics deteriorate, or the capacity decreases. There is a risk of

In the above general formula, the value of y can be set to y = 0. In this case, the above general formula is represented by Li _1-x FePO ₄ . Therefore, the metal element M is an arbitrary component.
Preferably, the range of y in the above general formula is 0 <y ≦ 0.5.
In this case, the metal element M is an essential component (where M is Li, Ni, Co, Mn, Mg, Al, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y One or more elements selected from: As a result, in this case, the electron conductivity of the positive electrode active material can be improved. Further, from the viewpoint of securing a higher capacity, y is more preferably y ≦ 0.25.

In the first invention, the raw material dispersion step and the heat treatment step are performed.
In the raw material dispersion step, a raw material slurry is prepared by dispersing a lithium phosphate compound, an iron (II) phosphate compound, and a compound containing a metal element M added as necessary in a polar solvent.
The lithium phosphate compound, the iron (II) phosphate compound, and the compound containing the metal element M are the target compounds represented by the general formula Li _1-x Fe _{1- y My} PO ₄ It is preferable to mix by the compounding ratio which can be obtained. In this case, it is possible to suppress a side reaction from occurring in the heat treatment step and to prevent the raw material from remaining after the heat treatment step. As a result, the lithium iron phosphate compound having high purity can be synthesized.

In the general formula _{Li 1-x Fe 1-y} M y PO 4, in the case of y = 0, in the raw material dispersion step can be omitted by using a compound containing the above metal element M . That is, the compound containing the metal element M is a selective compound used when producing a lithium iron phosphate compound of y> 0 in the general formula Li _1-x Fe _{1- y My} PO ₄ . is there.
As the compound containing the metal element M, for example, organic acid salts, oxides, carbonates, nitrates, and hydroxides containing the metal element M can be used.

In the raw material dispersion step, the iron (II) phosphate compound is dispersed in the polar solvent so that the molar concentration of iron element in the raw material slurry is 3 mol / L or less.
When the molar concentration of the iron element exceeds 3 mol / L, the lithium iron phosphate compound particles obtained after the heat treatment step become fine and the above-mentioned irregular particle shape cannot be formed. As a result, the adhesion between the positive electrode active material comprising the lithium iron phosphate compound and the positive electrode current collector may be reduced.
Preferably, the molar concentration of the iron element in the raw material slurry in the raw material dispersion step is 0.005 mol / L or more. When the molar concentration of the iron element is less than 0.005 mol / L, both the deformed particles of the lithium iron phosphate compound obtained after the heat treatment step and the particle size of the particle-shaped convex portions are both coarse and fine. There is a possibility that the above-mentioned characteristics of the irregularly shaped particles having the characteristics of the particles and coarse particles cannot be fully exhibited.
More preferably, the molar concentration of the iron element in the raw material slurry is 0.005 to 0.5 mol / L.

  Moreover, as said polar solvent, water, alcohol, these mixed solvents, etc. can be used, for example.

Next, in the heat treatment step, the raw material slurry is heated in a sealed container under an inert gas atmosphere at a temperature of 120 to 280 ° C. to synthesize a lithium iron phosphate compound having an olivine structure represented by the above general formula. To do.
When the heating temperature is less than 120 ° C., the reaction does not proceed sufficiently, and the positive electrode active material comprising a lithium iron phosphate compound having a desired olivine structure may not be obtained. On the other hand, when the temperature exceeds 280 ° C., it is necessary to use a very special pressure vessel or the like in the heat treatment step, which is not practical as a method for synthesizing the active material, and it is difficult to synthesize the positive electrode active material. There is.

Next, in the second aspect of the invention, the positive electrode active material is formed of deformed particles having a plurality of particulate convex portions having a particle size of 1 μm or less on the surface. When the particle size of the particulate protrusion exceeds 1 μm, the electron conductivity of the positive electrode active material may be reduced. More preferably, the particle size of the particulate protrusions is 0.8 μm or less, and more preferably 0.7 μm or less.
Further, the irregularly shaped particles preferably have a particle size of 1 μm or more. If the irregularly shaped particles have a particle size of less than 1 μm, the positive electrode active material particles may not be able to exhibit the anchor effect sufficiently. More preferably, the particle diameter of the irregular shaped particles is 2 μm or more. From the viewpoint of reducing the amount of the positive electrode active material contained in the unit volume of the positive electrode when the positive electrode is produced by adhering to the positive electrode current collector when the particle size of the irregular shaped particles is too large. The particle size of the irregular shaped particles is preferably 10 μm or less.
The particle size of the particulate convex portion and the irregular shaped particle can be measured, for example, by observation with a scanning electron microscope (SEM), and can be determined by the longest particle size of the particulate convex portion and the irregular shaped particle. it can.

Moreover, it is preferable that the said positive electrode active material is manufactured by the manufacturing method of the said 1st invention (Claim 3).
In this case, the positive electrode active material composed of the irregularly shaped particles having a plurality of the particulate convex portions on the surface can be easily produced.

Further, it is preferable that at least a part of the surface of the irregularly shaped particle is covered with a conductive substance composed of one or more selected from carbon, noble metals, metals, and conductive polymers.
In this case, the electronic conductivity of the positive electrode active material can be further improved.
More preferably, the conductive material is carbon (Claim 5).
In this case, the conductivity of the positive electrode active material can be improved without significantly increasing the manufacturing cost and weight of the positive electrode active material.

Next, in the third invention, the lithium ion secondary battery moves lithium between the positive electrode and the negative electrode, a separator sandwiched between the positive electrode and the negative electrode, and the positive electrode and the negative electrode. The non-aqueous electrolyte to be used can be configured as a main component.
For example, the positive electrode active material is mixed with a conductive material and a binder and an appropriate solvent is added to form a paste-like positive electrode mixture on the surface of a positive electrode current collector made of, for example, aluminum. And it can compress and form so that an electrode density may be raised as needed. Specifically, for example, an aluminum foil or the like can be used as the positive electrode current collector.

  Further, the conductive material is for ensuring the electrical conductivity of the positive electrode. For example, one or more carbon powder materials such as carbon black, acetylene black, natural graphite, artificial graphite, and cokes are used. Can be used.

The binder serves to bind the active material particles and the conductive material particles. For example, a fluorine-containing resin such as polytetrafluoroethylene, polyvinylidene fluoride, or fluororubber, or a thermoplastic resin such as polypropylene or polyethylene. Etc. can be used. In addition, an aqueous dispersion of a cellulose-based or styrene-butadiene rubber that is an aqueous binder can also be used.
As a solvent for dispersing these active material, conductive material, and binder, for example, an organic solvent such as N-methyl-2-pyrrolidone can be used.

  Next, the negative electrode is prepared by mixing a binder with a negative electrode active material, applying a suitable solvent to form a paste of the negative electrode mixture on the surface of a negative electrode current collector made of, for example, copper, and then drying. It can be formed by pressing. Similarly to the positive electrode, a fluorine-containing resin such as polyvinylidene fluoride can be used as a binder to be mixed with the negative electrode active material, and an organic solvent such as N-methyl-2-pyrrolidone can be used as a solvent. As the negative electrode current collector, specifically, for example, copper foil or the like can be used.

As the negative electrode active material, for example, a carbon material that can occlude and release lithium can be used.
Examples of the carbon material include natural or artificial graphite, mesocarbon microbeads (MCMB), mesophase pitch-based carbon fiber and a mixture thereof, vapor-grown carbonized fiber, organic compound fired body such as phenol resin, coke, Examples thereof include carbon black, pyrolytic carbons, and carbon fibers. These carbon materials can be used alone or in combination of two or more.

  In addition, the separator to be narrowly attached to the positive electrode and the negative electrode separates the positive electrode and the negative electrode and holds the non-aqueous electrolyte. For example, a thin microporous film such as polyethylene or polypropylene can be used.

As the non-aqueous electrolyte, for example, a lithium salt as an electrolyte dissolved in an organic solvent can be used. In this case, the lithium salt is dissociated by dissolving in an organic solvent, and lithium ions are present in the electrolytic solution. Examples of the lithium salt that can be used include LiBF ₄ , LiPF ₆ , LiClO ₄ , LiCF ₃ SO ₃ , LiAsF ₆ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) _{2 and the} like. These lithium salts may be used alone or in combination of two or more thereof.

  As the organic solvent for dissolving the lithium salt, an aprotic organic solvent can be used. As such an organic solvent, for example, a mixed solvent composed of one or more selected from cyclic carbonate, chain carbonate, cyclic ester, cyclic ether, chain ether, and the like can be used.

  Here, examples of the cyclic carbonate include ethylene carbonate, propylene carbonate, butylene carbonate, and vinylene carbonate. Examples of the chain carbonate include dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate. Examples of the upper cyclic ester carbonate include gamma butyrolactone and gamma valerolactone. Examples of the cyclic ether include tetrahydrofuran and 2-methyltetrahydrofuran. Examples of the chain ether include dimethoxyethane and ethylene glycol dimethyl ether. As the organic solvent, any one of these can be used alone, or two or more kinds can be mixed and used.

Examples of the shape of the lithium ion secondary battery include a cylindrical shape, a stacked shape, a coin shape, and a square shape. As the battery case that accommodates the positive electrode, the negative electrode, the nonaqueous electrolytic solution, and the like, those corresponding to these shapes can be used.
In the lithium ion secondary battery, for example, an electrode body formed by sandwiching the separator between the positive electrode and the negative electrode is housed in a battery case having a predetermined shape, and the positive electrode current collector and the negative electrode current collector are accommodated. The body can be electrically connected to the positive electrode external terminal and the negative electrode external terminal via lead wires, and the electrode body is impregnated with the nonaqueous electrolyte solution, and the battery case is sealed.

Example 1
Next, an embodiment of the present invention will be described with reference to FIGS.
In this example, a positive electrode active material according to an example of the present invention is produced, and a lithium secondary battery is produced using the positive electrode active material.

The positive electrode active material of this example is used for producing a positive electrode for a lithium ion secondary battery by being adhered to a metal positive electrode current collector. The positive electrode active material is made of LiFePO _{4 having} an olivine structure. The positive electrode active material is formed of irregularly shaped particles having a plurality of particulate protrusions having a particle size of 1 μm or less on the surface (see FIG. 1).

In producing the positive electrode active material of this example, a raw material dispersion step and a heat treatment step are performed.
In the raw material dispersion step, a raw material slurry is prepared by dispersing a lithium phosphate compound and an iron (II) phosphate compound in a polar solvent. At this time, the iron (II) phosphate compound is dispersed in the polar solvent so that the molar concentration of the iron element is 3 mol / L or less. In the heat treatment step, the raw slurry is heated in a sealed container at a temperature of 120 to 280 ° C. in an inert gas atmosphere to synthesize the lithium iron phosphate compound having the olivine structure represented by the general formula.

Hereinafter, the method for producing the positive electrode active material of this example will be described in detail.
First, a lithium phosphate compound (Li ₃ PO ₄ ) and an iron (II) phosphate compound (Fe ₃ (PO ₄ ) ₂ · 8H ₂ O) are dispersed in 100 ml of sufficiently deoxygenated deionized water. A raw material slurry was prepared (raw material dispersion step). At this time, it was dispersed so that the concentrations of Li and Fe were both 0.15 mol / L. Next, the raw slurry was introduced into a polyethylene pot together with 5 mmφ zirconia balls, filled with an inert gas, and mixed with a ball mill for 24 hours. Thereafter, the raw slurry was transferred to an autoclave, filled with an inert gas, and heated at a temperature of 180 ° C. for 24 hours (heat treatment step). Thereafter, it was sufficiently washed to obtain a positive electrode active material composed of a lithium iron phosphate compound (LiFePO ₄ ) having an olivine structure.

  Next, the obtained positive electrode active material was observed with a scanning electron microscope (SEM). An SEM photograph of the positive electrode active material is shown in FIG. As known from FIG. 1, the positive electrode active material was formed of irregularly shaped particles having a plurality of particulate protrusions on the surface. The particle size of the particle-shaped convex part and irregular-shaped particle | grains of the positive electrode active material of this example was measured by the microscope observation by SEM. The results are shown in Table 1 below.

  Further, in this example, the positive electrode active material composed of lithium iron phosphate obtained as described above is further dispersed in deionized water, and the weight ratio of lithium iron phosphate and carbon in the final product is The mixture was stirred with sucrose so as to be 95: 5 and dried to obtain lithium iron phosphate coated with sucrose. Next, this was heated in an inert gas atmosphere at a temperature of 800 ° C. for 1 hour to obtain a positive electrode active material composed of lithium iron phosphate coated with carbon. This is designated as Sample E1.

Next, a lithium ion secondary battery is manufactured using the positive electrode active material of the sample E1.
As shown in FIG. 2, the lithium ion secondary battery 1 of this example includes a positive electrode 2, a negative electrode 3, a separator 4, a gasket 59, a battery case 6, and the like. The battery case 6 is a 18650 type cylindrical case, and includes a cap 63 and an outer can 65. In the battery case 6, a sheet-like positive electrode 2 and a negative electrode 3 are arranged in a wound state together with a separator 4 sandwiched between the positive electrode 2 and the negative electrode 3.
A gasket 59 is disposed inside the cap 63 of the battery case 6, and a non-aqueous electrolyte is injected into the battery case 6.

Moreover, the positive electrode 2 contains the sample E1 produced as mentioned above as a positive electrode active material, and the negative electrode 3 contains fibrous graphite as a negative electrode active material.
The positive electrode 2 and the negative electrode 3 are respectively provided with a positive electrode current collecting lead 23 and a negative electrode current collecting lead 33 by welding. The positive electrode current collector lead 23 is connected to the positive electrode current collector tab 235 disposed on the cap 63 side by welding. Further, the negative electrode current collecting lead 33 is connected by welding to a negative electrode current collecting tab 335 disposed on the bottom of the outer can 65.

Further, the nonaqueous electrolytic solution is prepared by dissolving LiPF ₆ in a mixed organic solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) so as to be 1 mol / L and injected into the battery case 6. .

Hereinafter, a method for manufacturing the lithium ion secondary battery of this example will be described.
First, 85 wt% of the positive electrode active material of Sample E1, 10 wt% of carbon black as a conductive material, and 5 wt% of polyvinylidene fluoride as a binder were mixed, and further N-methyl-2-pyrrolidone as a dispersing material was mixed. An appropriate amount of was added and dispersed to obtain a slurry-like positive electrode mixture. This positive electrode mixture was applied to both sides of a positive electrode current collector made of an aluminum foil having a thickness of 20 μm and dried. Then, it densified with the roll press, cut out in the shape of 52 mm width x 450 mm length, and produced the sheet-like positive electrode 2. The amount of positive electrode active material deposited was about 7 mg / cm ² per side.

On the other hand, in producing the negative electrode 3, first, fibrous graphite as a negative electrode active material and polyvinylidene fluoride as a binder are mixed, and an appropriate amount of N-methyl-2-pyrrolidone is added and dispersed as a dispersing agent. Thus, a slurry-like negative electrode mixture was obtained. The mixing ratio of the negative electrode active material and the binder was, by weight, negative electrode active material: binder = 95: 5.
Next, the negative electrode mixture obtained as described above was applied to both surfaces of a negative electrode current collector made of a copper foil having a thickness of 10 μm and dried. Then, it densified with the roll press, cut out into the shape of 54 mm width x 500 mm length, and produced the sheet-like negative electrode 3. Incidentally, the adhesion amount of the negative electrode active material, per side, was 5 mg / cm ² or so.

  Next, as shown in FIG. 2, a positive electrode current collecting lead 23 and a negative electrode current collecting lead 33 were welded to the sheet-like positive electrode 2 and negative electrode 3 obtained as described above, respectively. The positive electrode 2 and the negative electrode 3 were wound in a state where a polyethylene separator 4 having a width of 56 mm and a thickness of 25 μm was sandwiched between them, and a roll-shaped electrode body was produced.

  Subsequently, the roll-shaped electrode body was inserted into an 18650-type cylindrical battery case 6 including an outer can 65 and a cap 63. At this time, the positive electrode current collecting lead 23 is welded to the positive electrode current collecting tab 235 disposed on the cap 63 side of the battery case 6 and the negative electrode current collecting lead 335 is disposed on the negative electrode current collecting tab 335 disposed on the bottom of the outer can 65. 33 was connected by welding.

  Next, the battery case 6 was impregnated with the nonaqueous electrolyte solution (sample E1) prepared as described above. A gasket 59 is disposed inside the cap 63, and the cap 63 is disposed in the opening of the outer can 65. Subsequently, the battery case 6 was sealed by caulking the cap 63, and the lithium ion secondary battery 1 was manufactured. This was designated as battery E1.

(Example 2)
In this example, a positive electrode active material made of deformed particles made of LiFe _0.5 Mn _0.5 PO ₄ having an olivine structure and having a plurality of particulate protrusions on the surface was prepared.
Specifically, first, a lithium phosphate compound, an iron (II) phosphate compound, and manganese acetate were dispersed in 100 ml of deionized water to prepare a raw material slurry. At this time, the Fe concentration was 0.075 mol / L, and Fe: Mn: P was dispersed so that the molar ratio was 1: 1: 2.
Subsequently, in the same manner as in Example 1, the raw slurry was subjected to ball mill mixing and a heat treatment step, and sufficiently washed to obtain a positive electrode active material composed of a lithium iron phosphate compound (LiFe _0.5 Mn _0.5 PO ₄ ) having an olivine structure. .

  Next, the obtained positive electrode active material was observed with a scanning electron microscope (SEM). As a result, the positive electrode active material of this example was composed of irregularly shaped particles having a plurality of particulate protrusions on the surface, as in Example 1. The particle size of the particle-shaped convex part and irregular-shaped particle | grains of the positive electrode active material of this example was measured by the microscope observation by SEM. The results are shown in Table 1 below.

Further, in this example, in the same manner as in Example 1, the positive electrode active material made of LiFe _0.5 Mn _0.5 PO ₄ obtained as described above was dispersed in deionized water, and phosphoric acid in the final product was dispersed. LiFe _0.5 Mn _0.5 coated with carbon was stirred with sucrose so that the weight ratio of iron lithium to carbon was 95: 5, dried and then heated in an inert gas atmosphere at a temperature of 800 ° C. for 1 hour. A positive electrode active material made of PO ₄ was obtained. This is designated as Sample E2.

  In this example, a lithium ion secondary battery (battery E2) was produced using the positive electrode active material of sample E2. The battery E2 of this example was produced in the same manner as in Example 1 except that the sample E2 was used as the positive electrode active material.

(Comparative Example 1)
In this example, a positive electrode active material made of a fine olivine-structure lithium iron phosphate compound (LiFePO ₄ ) having a particle size of 1 μm or less is produced by a so-called solid phase synthesis method.
Specifically, first, divalent iron oxalate, lithium carbonate, ammonium dihydrogen phosphate, and carbon black were prepared as starting materials. Next, iron oxalate, lithium carbonate, and ammonium dihydrogen phosphate are blended in a molar ratio such that Li: Fe: P is 1.2: 1: 1, and carbon black is described later. Were mixed so that the weight ratio of LiFePO ₄ and carbon black obtained after firing was 95: 5. Subsequently, the obtained mixture was baked at 650 ° C. in an inert gas atmosphere for 24 hours to obtain a positive electrode active material (sample C1) made of carbon-coated LiFePO ₄ .

  Next, the obtained positive electrode active material was observed with a scanning electron microscope (SEM). An SEM photograph of the positive electrode active material is shown in FIG. As is known from FIG. 3, the positive electrode active material (sample C1) of this example is composed of fine particles, and a structure composed of irregularly shaped particles such as sample E1 and sample E2 cannot be confirmed. The particle diameter of the positive electrode active material of this example was measured by microscopic observation with SEM. The results are shown in Table 1 below.

  In this example, a lithium ion secondary battery (battery C1) was produced using the positive electrode active material of the sample C1. The battery C1 of this example was produced in the same manner as in Example 1 except that the sample C1 was used as the positive electrode active material.

(Comparative Example 2)
In this example, a positive electrode active material made of a lithium iron phosphate compound (LiFe _0.5 Mn _0.5 PO ₄ ) having a fine olivine structure with a particle size of 1 μm or less is prepared by a so-called solid phase synthesis method.
Specifically, lithium carbonate, divalent iron oxalate, manganese carbonate, ammonium dihydrogen phosphate, and carbon black were prepared as starting materials. Next, a ratio such that Li: Fe: Mn: P is 1.2: 0.5: 0.5: 1 in a molar ratio of lithium carbonate, iron oxalate, manganese carbonate, and ammonium dihydrogen phosphate. Further, carbon black was mixed so that the weight ratio of LiFe _0.5 Mn _0.5 PO ₄ obtained after firing described later and carbon black was 95: 5. Next, the obtained mixture was baked at 650 ° C. in an inert gas atmosphere for 24 hours to obtain a positive electrode active material (sample C2) made of carbon-coated LiFe _0.5 Mn _0.5 PO ₄ .

  Next, the obtained positive electrode active material was observed with a scanning electron microscope (SEM). The positive electrode active material (sample C2) of this example is composed of fine particles like the sample C1, and a structure composed of irregular shaped particles such as the sample E1 and the sample E2 cannot be confirmed. The particle diameter of the positive electrode active material of this example was measured by microscopic observation with SEM. The results are shown in Table 1 below.

  In this example, a lithium ion secondary battery (battery C2) was produced using the positive electrode active material of the sample C2. Battery C2 of this example was produced in the same manner as in Example 1 except that sample C2 was used as the positive electrode active material.

(Experimental example)
Next, charge / discharge rate characteristics of the four types of lithium ion secondary batteries (battery E1, battery E2, battery C1, and battery C2) produced in Example 1, Example 2, Comparative Example 1, and Comparative Example 2 were used. And a charge / discharge cycle test to evaluate the capacity retention rate and the resistance increase rate.

“Charge / Discharge Rate Characteristics”
The charge / discharge rate characteristics were evaluated by measuring the discharge capacity (mAh / g) when discharging was performed at different discharge current values.
That is, first, each battery (battery E1, battery E2, battery C1, and battery C2) was charged with a current of 0.2 C under the condition of a temperature of 20 ° C. Charging was performed at a constant current and a constant voltage up to a charging upper limit voltage of 4.1 V, and the battery capacity was adjusted to 100% (SOC = 100%). Next, discharging was performed at constant currents of 2C, 1C, and 0.1C, and the discharge capacity at that time was measured. The results are shown in Table 1. The current 1C is a current value when charging the battery to SOC 100% over 1 hour.

"Charge / discharge cycle test"
Each battery (battery E1, battery E2, battery C1, and battery C2) is subjected to a constant current with a current density of 2.0 mA / cm ² under a temperature condition of 60 ° C., which is regarded as the upper limit of the actual use temperature range of the battery. Charging / discharging which charged to the charging upper limit voltage 4.1V and then discharged to the discharge lower limit voltage 3.0V with the constant current of 2.0 mA / cm < ² > of current density was made into 1 cycle, and this cycle was performed 100 times in total.

"Capacity maintenance rate"
Before and after the charge / discharge cycle test, when the discharge capacity before the charge / discharge cycle test was discharge capacity A and the discharge capacity after the charge / discharge cycle test was discharge capacity B, the capacity retention rate was calculated by the following equation (a). The results are shown in Table 1.
Capacity maintenance ratio (%) = discharge capacity B / discharge capacity A × 100 (a)

"Resistance increase rate"
Each battery is adjusted to 50% of the battery capacity (SOC = 50%), and the current of 0.5A, 1A, 2A, 3A, 5A is passed under the condition of a temperature of 20 ° C, and the battery voltage after 10 seconds is measured. did. The applied current and voltage were linearly approximated, and IV resistance was obtained from the slope.
The IV resistance increase rate can be calculated by the following equation (b), where IV resistance after the charge / discharge cycle test is resistance Y and IV resistance before the charge / discharge cycle test is resistance X. The results are shown in Table 1.
Resistance increase rate (%) = (resistance Y−resistance X) × 100 / resistance X (b)

  As is known from Table 1, when the battery E1 and the battery C1 containing the positive electrode active material having the same composition and the battery E2 and the battery C2 are respectively compared, even if they are discharged at discharge currents of 0.1C, 1C, and 2C, respectively. It can be seen that the discharge capacity is almost the same. That is, the battery E1 and the battery E2 containing positive electrode active materials (sample E1 and sample E2) made of irregularly shaped particles having a plurality of particulate protrusions having a particle diameter of 1 μm or less on the surface are made of fine particles having a particle diameter of 1 μm or less. Thus, excellent charge / discharge rate characteristics similar to those of the battery C1 and the battery C2 containing the positive electrode active materials (sample C1 and sample C2) can be exhibited.

Further, when comparing the battery E1 and the battery C1 having the same composition of the positive electrode active material, and the battery E2 and the battery C2, the battery E1 is compared with the battery C1, and the battery E2 is compared with the battery C2. It can be seen that the resistance increase rate is further reduced.
Therefore, the battery E1 and the battery E2 can maintain a higher capacity than the battery C1 and the battery C2 and exhibit a high output even when charging and discharging are repeated.

The scanning electron microscope (SEM) photograph about the positive electrode active material (sample E1) concerning Example 1. FIG. BRIEF DESCRIPTION OF THE DRAWINGS FIG. The scanning electron microscope (SEM) photograph about the positive electrode active material (sample C1) concerning the comparative example 1. FIG.

Explanation of symbols

1 Lithium ion secondary battery 2 Positive electrode 3 Negative electrode 4 Separator

Claims (6)

Formula _{Li 1-x Fe 1-y} M y PO 4 ( where, -0.2 ≦ x ≦ 0.2,0 ≦ y ≦ 0.5, M is Li, Ni, Co, Mn, Mg, Al, A positive electrode current collector made of a metal, comprising a lithium iron phosphate compound having an olivine structure represented by one or more elements selected from Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y) In the method for producing a positive electrode active material used for producing a positive electrode for a lithium ion secondary battery by bonding to a body,
A raw material dispersion step in which a lithium phosphate compound, an iron (II) phosphate compound, and a compound containing a metal element M added as necessary are dispersed in a polar solvent to produce a raw material slurry;
Heat treatment step of synthesizing the lithium iron phosphate compound having an olivine structure represented by the above general formula by heating the raw material slurry in a sealed container at a temperature of 120 to 280 ° C. under an inert gas atmosphere,
In the raw material dispersion step, the iron (II) phosphate compound is dispersed in the polar solvent so that the molar concentration of the iron element in the raw material slurry is 3 mol / L or less. Manufacturing method. A positive electrode active material used for producing a positive electrode for a lithium ion secondary battery by being adhered to a metal positive electrode current collector,
The positive electrode active material has a general formula Li _1-x Fe _{1- y My} PO ₄ (where −0.2 ≦ x ≦ 0.2, 0 ≦ y ≦ 0.5, M represents Li, Ni, Co, A lithium iron phosphate compound having an olivine structure represented by Mn, Mg, Al, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y).
The positive electrode active material is formed of irregular shaped particles having a plurality of particulate convex portions having a particle size of 1 μm or less on the surface.   The positive electrode active material according to claim 2, wherein the positive electrode active material is produced by the production method according to claim 1.   4. The surface of the irregularly shaped particle according to claim 2, wherein at least a part of the surface of the irregularly shaped particle is coated with a conductive substance made of at least one selected from carbon, a noble metal, a metal, and a conductive polymer. Positive electrode active material.   The positive electrode active material according to claim 4, wherein the conductive material is carbon. A lithium ion secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte obtained by dissolving an electrolyte in a non-aqueous solvent,
The positive electrode is formed by bonding a positive electrode active material to a metal positive electrode current collector,
A lithium ion secondary battery in which the positive electrode active material according to any one of claims 2 to 5 is adopted as the positive electrode active material.