JP4794833B2 - Positive electrode material for lithium ion secondary battery, method for producing the same, and lithium ion secondary battery - Google Patents

Description

  The present invention relates to a positive electrode material for a lithium ion secondary battery, a method for producing the same, and a lithium ion secondary battery using the positive electrode material.

  Lithium ion secondary batteries are lightweight and have a high energy density, and are therefore widely used in consumer applications, mainly in IT information terminals, for mobile phones, laptop computers, and small batteries for backup power supplies. Today, the demand is growing on a global scale.

  In addition to these small batteries, industrial large batteries are also expected to be used in various fields such as hybrid cars, electric cars, power leveling, power storage, and robots, and research and development are thriving. Has been done.

Under such circumstances, high safety, long life, high output, and low price are required for the positive electrode material as a problem for full-scale commercialization of large industrial batteries. Among them, LiFePO ₄ , which exhibits high safety and excellent cycle performance and can be manufactured at a low price, has attracted attention as an alternative positive electrode material such as LiCoO ₂ or LiMn ₂ O ₄ .

However, LiFePO ₄ has poor conductivity of the material itself, and the lithium insertion / desorption reaction during charging / discharging is slow, so that a high output cannot be obtained when a battery is used. Therefore, mixing with a conductive additive has been studied. However, a positive electrode material that greatly improves the output of the battery has not yet been developed, and the defect of the positive electrode material LiFePO ₄ has not been solved.

To solve this shortcoming,
(1) Making primary particles in LiFePO ₄ secondary particles fine.
(2) Uniformly impart conductivity to the entire fine particles, that is, the positive electrode material having a large outer surface area.
(3) Replacing a part of LiFePO ₄ with a different element to improve conductivity (for example, see Non-Patent Document 1)
Has been proposed as an important countermeasure. Furthermore, in order to increase the energy density of the battery only for the composite of carbon and LiFePO ₄
(4) The necessity of increasing the packing density of the composite of carbon and LiFePO ₄ has been reported by Dahn et al. (For example, see Non-Patent Document 2).

  Regarding (1), for example, a hydrothermal synthesis method and a method for suppressing particle growth (for example, see Patent Document 1) are disclosed. However, the manufacturing method is difficult, and a firing time is required for suppressing particle growth, which is not suitable for a large amount of synthesis.

On the other hand, LiFePO ₄ particles produced without suppressing particle growth are secondary particles in which primary particles are aggregated, and the secondary particles have a large number of spaces (pores) surrounded by the primary particles that have grown. An existing porous body is formed. Therefore, the LiFePO ₄ particles that did not suppress particle growth have very poor conductivity. As a result, a large amount of a conductive additive such as acetylene black and a binder are required when producing the electrode.

When a large amount of conductive additive and binder are added, not only the density of the active material in the electrode is lowered, but also a slurry of LiFePO ₄ particles cannot be formed at the time of electrode preparation, so that an electrode cannot be prepared by coating. . Even if an electrode is forcibly produced, the electrode density is low, and it is easy to peel off and does not constitute a substantial electrode. With such a positive electrode material, a battery with high capacity, high output, and long life has not been obtained.

  As for (2), a method of improving the conductivity by making conductive particles exist around the particles (for example, see Patent Document 2), and a method of firing together with carbon or a carbon precursor (for example, Patent Documents 3 and 4) are disclosed.

However, in many cases, the adhesion with LiFePO ₄ is poor because it is only mixed with a conductive additive. Further, the conductive additive cannot be uniformly dispersed into the particles of the positive electrode material.

A method of using a metal as a conductive aid has also been devised. However, in this method, since the specific gravity of the metal is heavy, in order to give sufficient conductivity to LiFePO ₄ , it must be added in a larger amount than when carbon is used as a conductive aid. Therefore, light elemental carbon is generally most preferable as the conductive additive.

From this point of view, as shown in Patent Document 3, LiFePO ₄ bonded to carbon and complexed is preferable. However, even with the method of Patent Document 3, it is necessary to reduce the primary particles of LiFePO ₄ in order to improve the output of the battery.

That is, it is necessary to perform firing while suppressing particle growth under conditions of 700 ° C. or lower. However, when the firing temperature is 700 ° C. or lower, the conductivity of carbon bonded and combined with LiFePO ₄ is low, and the conductivity of the positive electrode material itself is also low.

  On the other hand, firing at 700 to 900 ° C. is necessary to improve carbon conductivity. However, when the firing temperature is 700 to 900 ° C., particle growth cannot be suppressed as described above. Therefore, in the method of Patent Document 3, it is difficult to improve the output of the battery and improve the conductivity of the electrode at any firing temperature.

As for the method of substituting a part of LiFePO ₄ of (3) with a different element (M), many methods such as Patent Documents 5, 6 and 7 are disclosed. However, the dissimilar element-substituted positive electrode material alone does not have a great effect of improving the conductivity, and a large amount of conductive auxiliary material is required during electrode production.

In addition, there has been no proposal of a LiFe _x M _1-x PO ₄ carbon composite treated at a high temperature of 700 to 900 ° C. in an extremely reducing atmosphere in which carbon coexists as a positive electrode material.

For (4), in order LiFePO ₄ is a porous body, when complexed with the LiFePO ₄ carbon, since carbon is deposited to cover the LiFePO ₄ particles without penetration into the pores of the LiFePO ₄ Contrary to the need stated in the aforementioned report, the composite of LiFePO ₄ and carbon has a low packing density.

Also, when a carbon precursor is mixed with a raw material for producing LiFePO ₄ (hereinafter referred to as a LiFePO ₄ raw material) and baked, a large amount of volatile matter is generated from the raw material and the carbon precursor, and the generated LiFePO ₄ carbon composite is produced. The body is porous, the particle itself has a low bulk density, and the particle shape is not spherical. Therefore, the packing density of the composite obtained by mixing and firing the LiFePO ₄ raw material and the carbon precursor is much lower than that of the above composite in which LiFePO ₄ particles are coated and attached with carbon.

In addition to this, since the composite obtained by mixing and firing the LiFePO ₄ raw material and the carbon precursor is pulverized again after firing, the positive electrode material particles coated with carbon are destroyed and the LiFePO ₄ Since the cross section is exposed and the particles become smaller, bridging occurs between the particles, the packing density is lowered, and the conductivity as the positive electrode material is extremely lowered.

  Regarding the electrode life, generally, when a large amount of conductive aid such as acetylene black is used, the produced electrode is easily peeled off. In addition, there is a problem in that the surface performance of the electrode increases and reacts with the electrolytic solution to cause a phenomenon that the electrolytic solution decreases (so-called liquid withdrawing phenomenon), resulting in deterioration of cycle performance.

As described above, the conventional composite of LiFePO ₄ and carbon is porous, has a low packing density, and has a low conductivity. For this reason, a large amount of a conductive additive and a binder are still required when producing an electrode. In addition, this conventional composite has a problem that a high-density electrode, that is, a battery having a high capacity, a high output, and a long life cannot be obtained.
Electrochemistry vol. 71 no. 8 p. 717-722, 2003 R. Dahn, Journal of the Electrochemical Society, 149, A1184-A1189, 2002 JP 2003-45430 A (Claims, paragraph numbers [0007] to [0016]) JP 2001-110414 A (claims, paragraph numbers [0010] to [0012]) JP 2003-292308 A (Claims, paragraph numbers [0010] to [0014]) JP 2003-292309 A (claims, paragraph numbers [0010] to [0014]) JP 2001-307726 A (claims, paragraph numbers [0012] to [0013]) JP 2001-307732 A (claims, paragraph numbers [0019] to [0026]) JP-A-2001-85010 (Claims, paragraph numbers [0008] to [0009])

While various studies on the above problems, the present inventors have changed the aggregation state of LiFe _x M _1-x PO ₄ in which LiFePO ₄ or a part of Fe is substituted with a different element to reduce the number of voids in the positive electrode A material can be obtained, and further, a process of combining LiFePO ₄ and carbon with few voids in this way, or a part of LiFePO ₄ with a different element M such as Co, Cr, Mn, Ni, Ti, V, etc. In the process of compounding the substituted LiFe _x M _1-x PO ₄ and carbon, the particles are spheroidized, and the spheroidized particles are used as a positive electrode material. That the electrode can be produced with the adhesive material, and as a result, the active material density in this electrode can be increased, and that this electrode can provide a high-capacity, high-power, long-life lithium ion battery. Tokushi, which resulted in the completion of the present invention.

  Accordingly, an object of the present invention is to provide a positive electrode material for a lithium ion secondary battery, a method for producing the same, and a lithium ion secondary battery using the positive electrode material, which have solved the above problems.

  The present invention for achieving the above object is described below.

[1] LiFePO ₄ and carbon composite, or LiFe _x M _1-x PO _{4 in} which part of Fe is substituted with a different element (0.5 <x <1, M is Co, Cr, Mn, Ni , Ti or V, or a mixed element thereof) and a positive electrode material for a lithium ion secondary battery comprising a composite of carbon, the particle shape of the positive electrode material having an aspect ratio (L / D) of 1 A positive electrode material for a lithium ion secondary battery having a spherical shape of ~ 2.

  [2] The positive electrode material for a lithium ion secondary battery according to [1], wherein the carbon content is 1 to 20% by mass and the average particle size is 5 to 60 μm.

[3] The positive electrode material for a lithium ion secondary battery according to [1], which has a packing density of 1.0 to 1.8 g / cm ³ .

  [4] The positive electrode material for a lithium ion secondary battery according to [1], wherein the volume resistance value at a press pressure of 49 MPa is 0.1 to 20 Ω · cm.

[5] LiFePO ₄ or LiFe _x M _1-x PO _{4 in} which part of Fe is substituted with a different element (0.5 <x <1, M is Co, Cr, Mn, Ni, Ti or V, or These mixed elements are pulverized to 5 μm or less, a carbon precursor is added to the obtained pulverized product, mixed and granulated, and the obtained granulated product is fired at 700 to 900 ° C. A method for producing a positive electrode material for a lithium ion secondary battery.

[6] LiFePO ₄ , or LiFe _x M _1-x PO _{4 in} which part of Fe is substituted with a different element (0.5 <x <1, M is Co, Cr, Mn, Ni, Ti or V, or And these mixed elements are mixed), two or more raw materials for producing the active material LiFePO ₄ , or two or more raw materials for producing the active material LiFe _x M _1-x PO ₄ are mixed, and the resulting mixture is The method for producing a positive electrode material for a lithium ion secondary battery according to [5], which is a fired product obtained by firing at 300 to 900 ° C.

  [7] A lithium ion secondary battery having an electrode that is formed by applying the positive electrode material according to any one of [1] to [6] and a binder onto an aluminum foil and does not include a conductive additive.

  According to the positive electrode material of the present invention, it is possible to produce an electrode with a minimum pressure-sensitive adhesive material without using a conductive aid during electrode production, to increase the active material density in the electrode, and to use this electrode. A high-capacity, high-power, long-life lithium ion secondary battery can be obtained.

  Moreover, according to the manufacturing method of the positive electrode material of this invention, the positive electrode material for lithium ion secondary batteries which can satisfy | fill high capacity | capacitance, high output, and long lifetime simultaneously can be manufactured efficiently and stably.

  Hereinafter, the present invention will be described in more detail.

The positive electrode material for a lithium ion secondary battery of the present invention is a composite of an active material LiFePO ₄ and carbon, or a part of Fe is replaced with a different element (M) such as Co, Cr, Mn, Ni, Ti, V, etc. the active material _{LiFe x M 1-x PO 4} (0.5 <x <1, preferably 0.7 <x <1) consists of a complex of the carbon.

By replacing a part of Fe in the positive electrode material active material LiFePO ₄ with a different element, the conductivity of the positive electrode material can be improved. However, it is difficult to significantly improve conductivity only by the substitution of this different element.

On the other hand, the conductivity of the positive electrode material formed by combining the active material LiFePO ₄ and carbon is dramatically improved. In this active material LiFePO ₄ carbon composite cathode material, the conductivity of the cathode material can be further improved by substituting a part of the active material Fe with a different element. For example, when x = 0.9, the volume resistance value of the positive electrode material particles can be further reduced by an order of magnitude compared to the unsubstituted positive electrode material particles (x = 1).

  The element substituting Fe in this heteroelement-substituted active material is easy to substitute with Fe and has excellent crystallinity after substitution, so that the element number is close to Fe and has a high melting point like Fe, Co, Cr, Mn, Ni, Ti, V and the like are preferable. Moreover, you may substitute Fe with these mixed elements.

  The positive electrode material of the present invention has a spherical shape with a particle shape having an aspect ratio (L / D) of 1 to 2, preferably 1 to 1.2. When the aspect ratio (L / D) indicating the particle shape of the positive electrode material exceeds 2, the packing density is lowered and the conductivity as the positive electrode material is lowered, which is not preferable.

Packing density obtained by later-described measuring method of the positive electrode material of the present invention is preferably ^{1.0~1.8g / cm 3, 1.4~1.8g / cm} 3 is more preferred. When the packing density of the positive electrode material is less than 1.0 g / cm ³ , the density of an electrode manufactured using the positive electrode material is not preferable. The filling density of the positive electrode material reaches 1.8 g / cm ³ and reaches a saturated state in the filling structure.

  1-20 mass% is preferable and, as for carbon content of the positive electrode material of this invention, 5-10 mass% is still more preferable. By setting the carbon content of the positive electrode material to 1% by mass or more, a highly conductive electrode can be produced without using a conductive aid such as acetylene black, ketjen black, graphite, or vapor phase carbon fiber at the time of producing the electrode.

That is, to form a complex with LiFePO ₄ or LiFe _x M _1-x PO ₄ and carbon, easily viscous slurry only requires minimal amount of binder, high slurry concentration 50-70 wt% it can. Through an ordinary process of applying and drying the slurry on an aluminum foil, a high-density, high-conductivity, high-capacity, high-power, long-life electrode of a lithium ion secondary battery can be produced.

If the carbon content of the cathode material exceeds 20 wt%, the effect is not much to further improve the conductivity of the electrode, the partial carbon content is large, the active material in the positive electrode material LiFePO ₄ or LiFe _x M _1-x Since the ratio of PO ₄ decreases, it is not preferable.

  In general, it is common sense to add a conductive additive when an electrode is produced by applying a positive electrode active material to an aluminum foil or the like. On the other hand, when producing an electrode using the positive electrode material of the present invention, a conductive additive is not necessary.

This is because the positive electrode material of the present invention is a composite in which the active material LiFePO ₄ or LiFe _x M _1-x PO ₄ is composited with carbon having good conductivity, and the composite itself is good. This is because of having good conductivity.

  The average particle diameter of the positive electrode material of the present invention is preferably 5 to 60 μm, and more preferably 5 to 20 μm for forming an electrode of a high-power lithium ion secondary battery.

  The particle size distribution of the positive electrode material of the present invention is preferably a sharp distribution in which 90% or more of all particles are included in the range of the average particle diameter ± 5%. When an electrode is produced using a positive electrode material having a sharp particle size distribution, electric charges are easily applied to the positive electrode material particles, and the volume resistance value is reduced.

  Therefore, the positive electrode material having the sharp particle size distribution is suitable for use in large batteries for electric vehicles and power storage that require high output. Hereinafter, in order to represent the sharpness (particle sharpness) of the particle size distribution of the positive electrode material, a numerical value of ±% indicating a range from the average particle diameter including 90% or more of all the particles is used.

Regarding the positive electrode material of the present invention, the volume resistance value when the pressing pressure is 49 MPa (500 kgf / cm ² ) is preferably 0.1 to 20 Ω · cm, and more preferably 0.1 to 1 Ω · cm.

In order to make the volume resistance value of the positive electrode material less than 0.1 Ω · cm, the carbon content of the positive electrode material needs to exceed 20 mass%. This is not preferable, minute high carbon content as described above, the ratio in the positive electrode material active material LiFePO ₄ or LiFe _x M _1-x PO ₄ is lowered.

  When the volume resistance value of the positive electrode material exceeds 20 Ω · cm, the conductivity of an electrode produced using this positive electrode material is not preferable.

  The positive electrode material of the present invention is not particularly limited as long as its physical properties are within the above range, but can be manufactured by, for example, the following manufacturing method.

First, the raw materials of two or more active materials LiFePO ₄ or the raw materials of two or more active materials LiFe _x M _1-x PO ₄ are mixed, the mixture is fired at 300 to 900 ° C., and the fired product is reduced to 5 μm or less. Grind (first step). Next, a carbon precursor is added to the pulverized product, mixed and granulated (second step). Thereafter, the granulated product is fired at 700 to 900 ° C. (third step), whereby the positive electrode material for a lithium ion secondary battery of the present invention can be produced.

The fired product of the first step has a composition of LiFePO ₄ or LiFe _x M _1-x PO ₄ , and when this is fired, there is no generation of further volatile matter and generation of further voids. If available, commercially available LiFePO ₄ or LiFe _x M _1-x PO ₄ can also be used.

  Next, individual raw materials and processes in the method for producing the positive electrode material of this example will be described in detail.

(First step)
[Active material]
In the production of the positive electrode material of the present invention, usually, two or more active material materials are mixed and then fired to obtain an active material represented by LiFePO ₄ or LiFe _x M _1-x PO ₄ . Examples of the active material raw material include lithium dihydrogen phosphate, iron oxalate dihydrate, manganese carbonate, nickel acetate, cobalt oxalate, chromium oxide, titanium oxide, and vanadium oxide.

[Mixing raw materials]
The active material raw materials are mixed by a mixer such as a planetary mill, a Henschel mixer, a vibration mill, or a ball mill. The degree of mixing is not particularly limited. For example, in the case of a planetary mill, the mixing is preferably performed at 50 to 500 rpm for 1 to 15 hours, more preferably 100 to 200 rpm for 2 to 5 hours.

[Primary firing]
The raw material mixture is fired in an inert gas atmosphere such as nitrogen and argon at 300 to 900 ° C. for 0.5 to 15 hours, preferably at 350 to 900 ° C. for 5 to 10 hours, so that the active material LiFePO ₄ or LiFe _x M _1-x PO ₄ is obtained (primary firing). This primary firing may be performed in two stages, 1 to 5 hours at 300 to 450 ° C. and 1 to 5 hours at 700 to 900 ° C.

By this primary firing, in the second firing in the third step, almost no volatile matter is generated from the primary fired product LiFePO ₄ or LiFe _x M _1-x PO ₄ , and the resulting LiFePO ₄ or LiFe _x M _1-x PO _{4 is obtained.} The carbon / carbon composite has a high packing density.

This is considered as follows. With respect to one active material raw material, the reaction from this active material raw material to LiFePO ₄ or LiFe _x M _1-x PO ₄ proceeds at 90% or more at 350 ° C. Therefore, since the generation of volatile matter and the generation of voids are almost completed in the primary firing of the first step, there is no further generation of volatile matter in the secondary firing of the third step, and no further creation of voids. .

  Regarding the carbon raw material of the other raw material, the carbon precursor of this carbon raw material is characterized by shrinking to high density carbon when heat treated as symbolized by carbon fiber. Therefore, the carbon precursor that is added, mixed, and granulated in the second step is shrunk into high-density carbon in the secondary firing in the third step.

As described above, in the secondary firing in the third step, both the active material raw material and the carbon raw material move toward a high density, and the resulting LiFePO ₄ or LiFe _x M _1-x PO ₄ and carbon composite is The packing density is high.

[Crushing]
The primary fired product is classified into particles as needed after dry pulverization, or is wet pulverized and dried to give particles of 5 μm or less. As the dry pulverizer, general-purpose pulverizers such as a jet mill, a vibration mill, and a centrifugal impact mill can be used. Similarly, the wet pulverizer may be a general pulverizer such as a wet ball mill or bead mill using water as a solvent.

The reason why these various general-purpose pulverizers can be used is that LiFePO ₄ or LiFe _x M _1-x PO _{4 of the} primary fired product is porous and is easily pulverized. The classifier is a general-purpose product as long as the particle size after pulverization classification can be reduced to 5 μm or less, and there is no problem.

  In addition, when the particle diameter after pulverization classification exceeds 5 μm, when the battery performance test is performed using this, the rate performance is extremely lowered. Moreover, when granulating with a carbon precursor in a 2nd process, since it cannot granulate well, it is unpreferable.

In order to increase the packing density of the complex of the LiFePO ₄ or LiFe _x M _1-x PO ₄ and carbon, pulverized primary sintered product as described above, it is necessary to classification if necessary. This is considered as follows.

The LiFePO ₄ particles or LiFe _x M _1-x PO ₄ particles of the primary fired product are secondary particles in which primary particles are aggregated, and the secondary particles are surrounded by the primary particles (pores). Forms a porous body. When the secondary particles are pulverized and classified as necessary, the primary particles are separated. As a result, the space (hole) surrounded by the primary particles disappears. That, LiFePO ₄ or LiFe _x M _1-x PO ₄ wasted space (hole) in itself is not.

As described above, when the composite porous LiFePO ₄ particles or LiFe _x M _1-x PO ₄ particles and carbon in the conventional method, the carbon in the pores of the LiFePO ₄ or LiFe _x M _1-x PO ₄ There is a problem that it adheres so as to cover the LiFePO ₄ or LiFe _x M _1-x PO ₄ particles without penetrating.

On the other hand, since the pulverized product of the primary fired product has no useless space (holes), the above-described conventional problems are eliminated. Therefore, the LiFePO ₄ or LiFe _x M _1-x PO ₄ and carbon composite particles obtained by the production method of the present invention have a high bulk density of the particles themselves and a high packing density required by the measurement method described later. It becomes.

(Second step)
In the second step, LiFePO ₄ or LiFe _x M _1-x PO ₄ obtained in the first step and a carbon precursor are mixed and granulated.

[Carbon precursor]
Examples of carbon precursors include phenol condensation products such as phenol resins, naphthalene condensates such as naphthalene sulfonic acid resins, anthracene condensates, styrene resins, acrylonitrile resins, vinyl resins, imide resins, aramid resins, and cellulose. It is preferable to use carbon-based resin, its monomer and its derivative, coal tar, coal pitch, petroleum tar, petroleum pitch, etc. to generate carbon by pyrolysis.

[Granulation]
And LiFePO ₄ or LiFe _x M _1-x PO ₄ obtained in the first step, mixing the carbon precursor, upon granulation, the addition amount of the carbon precursor is preferably 1 to 20 mass% as a carbon. When the amount of carbon precursor added is less than 1% by mass, granulation cannot be performed, which is not preferable. When the addition amount of the carbon precursor exceeds 20% by mass, the addition effect is not so much, and the proportion of the active material LiFePO ₄ or LiFe _x M _1-x PO _{4 in} the positive electrode material is reduced by the amount of addition of the carbon precursor. This is not preferable.

The carbon precursor is carbonized by firing in the third step, and not only serves as a material for imparting conductivity in the obtained positive electrode material, but also in this granulation, LiFePO ₄ or LiFe _x M _1-x PO ₄ particles It also functions as a binder that granulates spheres.

  For granulation, it is possible to use an ordinary powder granulating device such as a Henschel mixer or a spray dryer, and it is preferable to use different devices depending on the carbon precursor used.

  For example, a carbon precursor that is hardly soluble in a solvent such as styrene resin, acrylonitrile resin, vinyl resin, imide resin, aramid resin, and pitch and softens with temperature is preferably granulated with a Henschel mixer. A carbon precursor that is easily soluble in a solvent such as naphthalene condensate, tar, or cellulose resin is preferably granulated with a spray dryer. Phenol resin and monomer can be granulated using either a spray dryer or a Henschel mixer.

For without the carbon precursor, carbon solids, for example, in the case of granulation by adding acetylene black, that the LiFePO ₄ or _{LiFe x M 1-x PO 4} , are not completely bonded to carbon, the positive electrode It is not preferable because the conductivity of the material is easily impaired and rate performance and cycle performance are deteriorated in the battery test.

Therefore, when carbon is added to impart conductivity, it is particularly preferable to use the carbon precursor having strong adhesion to LiFePO ₄ or LiFe _x M _1-x PO ₄ .

  In addition, according to the granulation method using a normal powder granulator such as a spray dryer or a Henschel mixer, an arbitrary particle size distribution is obtained as compared with a pulverization method using a pulverizer such as a ball mill, a vibration mill or a turbo mill. Spherical shaped particles can be obtained stably and with high efficiency.

  Granulation into the above spherical shape is characterized in that not only the average particle diameter can be arbitrarily set, but also the particle size distribution can be arbitrarily set. From the granulated product having this average particle size and particle size distribution, a positive electrode material having the same average particle size and particle size distribution is obtained by secondary firing in the third step.

  As described above, the average particle size of the positive electrode material of the present invention is preferably 5 to 60 μm, and more preferably 5 to 20 μm for forming an electrode of a high output lithium ion secondary battery. In addition, the particle size distribution of the positive electrode material of the present invention is preferably a sharp distribution in which 90% or more of all particles are included in the range of the average particle diameter ± 5%.

  In order to set the average particle size of the positive electrode material to the above-mentioned conditions, the average particle size of the granulated product is preferably set to 1 to 60 μm, and 1 to form an electrode of a high-power lithium ion secondary battery. It is particularly preferable to set it to 20 μm. In addition, in order to make the particle size distribution of the positive electrode material the above-mentioned condition, the particle size distribution of the granulated product should be set to a sharp one in which 90% or more of all particles are included in the range of the average particle diameter ± 5%. Is preferred.

  In addition, when the particle size sharpness is shown by a particle size ratio (D30 / D70) of 30% particle size and 70% particle size, the particle size sharpness of the positive electrode material and the granulated product of the present invention is preferably 0.5 or more, 0.6 or more is more preferable, and 0.75 or more is particularly preferable.

(Third step)
[Secondary firing]
In the third step, the LiFePO ₄ or LiFe _x M _1-x PO ₄ obtained in the second step, heat treating the granulated product of carbon precursor (secondary firing).

  As the firing furnace, it is preferable to use a general-purpose furnace that handles raw materials that generally become carbon after firing, such as a fixed bed furnace, a rotary kiln, a fluidized furnace, and a stirring furnace. Among these general-purpose furnaces, a rotary kiln, a fluid type furnace, and a stirring furnace are particularly preferable from the viewpoint of uniformly processing raw materials.

  The firing atmosphere may be an inert atmosphere such as a nitrogen atmosphere or an argon atmosphere.

In the conventional method for producing the positive electrode material LiFePO ₄ or LiFe _x M _1-x PO ₄ , when the firing temperature is 700 ° C. or higher, the porosity of the particles is increased by the growth of LiFePO ₄ crystal particles or LiFe _x M _1-x PO ₄ crystal particles. There were problems of quality improvement, bulk density reduction, and packing density reduction, and the firing temperature could not be raised to 700 ° C or higher.

According to the production method of the present invention, the object in the secondary firing in the third step is a mixed granulated product of LiFePO ₄ or LiFe _x M _1-x PO ₄ obtained in the second step and a carbon precursor. In addition, since the object in the mixed granulation in the second step is a pulverized product of the carbon precursor and the primary fired product in the first step, the fired secondary product can be set to 700 ° C. or higher. The particles do not become porous, and a positive electrode material having a high bulk density and a high packing density can be obtained.

From the above, secondary firing temperature in the firing may be at least 700 ° C., the carbon precursor Among the more that 700 ° C. exerts conductive as carbon, and highly crystalline LiFePO ₄ or LiFe _x 700 to 900 ° C. are preferred M _1-x PO ₄ is obtained, more preferably 750 to 900 ° C., particularly preferably from 790 to 900 ° C..

If the firing temperature exceeds 900 ° C., LiFePO ₄ or LiFe _x M _1-x PO ₄ in complex with the resulting LiFePO ₄ or LiFe _x M _1-x PO ₄ and carbon, their thermal stability is poor. Moreover, it is not preferable because it is reduced and crystallinity is lowered.

When the firing temperature is less than 700 ° C., the resistance value of carbon in the resulting LiFePO ₄ or LiFe _x M _1-x PO ₄ and carbon composite increases by an order of magnitude or more compared to that of the firing temperature of 750 to 900 ° C. Therefore, it is not preferable.

Due to the secondary firing in the third step, the granulated product of LiFePO ₄ or LiFe _x M _1-x PO ₄ and carbon that has been spheroidized by the granulation in the second step is fired and combined in the same particle shape. The

Since this secondary fired product does not need to be pulverized for particle size adjustment, it has a spherical shape having good conductivity without causing a decrease in conductivity such as exposure of the LiFePO ₄ cross section or LiFe _x M _1-x PO ₄ cross section. LiFePO ₄ or LiFe _x M _1-x PO ₄ and complexes with carbon.

Object in this firing, rather than the mixture granulated product of LiFePO ₄ or LiFe _x M _1-x PO ₄ and carbon precursor obtained in the second step, directly a carbon precursor in the raw active material used in the first step When the body is mixed and granulated, during firing, the active material raw material emits a decomposition gas to reduce the thermal weight, so that the resulting composite is a positive electrode material that is porous and has a low packing density.

  Hereinafter, the present invention will be described more specifically with reference to examples and comparative examples. Moreover, each physical property value of the negative electrode material in these Examples and Comparative Examples was measured by the following methods.

(1) Average particle size and particle size distribution Measurement was performed using a laser diffraction particle size distribution analyzer SALD-1000 manufactured by Shimadzu Corporation.

(2) Aspect ratio (L / D)
The measurement was performed using a scanning electron microscope (SEM) JSM5300 (trade name) manufactured by JEOL Ltd.

(3) Carbon content Measured by JIS M 8813 Cefield high temperature method.

(4) Packing density A sample was put into a 10 mL glass graduated cylinder and tapped, and when the volume of the sample ceased to change, the sample volume was measured, and the value obtained by dividing the sample weight by the sample volume was taken as the packing density.

(5) Volume resistance The resistance of the sample was measured under the condition of 49 MPa (500 kgf / cm ² ) using an impedance measuring device manufactured by Hosen, and the volume resistance was calculated.

(6) Crystal analysis by XRD method Measured with an X-ray diffractometer X'pert pro (trade name) manufactured by Philips.

[Example 1]
As active material raw materials, 1 mol of lithium dihydrogen phosphate and 1 mol of iron oxalate dihydrate were used, and these were mixed in a planetary mill at 200 rpm for 12 hours. 100 g of this raw material mixture was baked for the first time in a small electric furnace in a nitrogen atmosphere at 350 ° C. for 5 hr and 800 ° C. for 1 hr to obtain a primary baked product as an active material. This primary fired product was LiFePO ₄ having high crystallinity as measured by the XRD method.

This primary baked product was pulverized and classified to 5 μm or less, and 20 g was mixed with 50 g of a naphthalenesulfonic acid resin solution (solid content 20 mass% in the resin solution, carbon content 10 mass%) to obtain a liquid material. This liquid material was granulated by using a small spray dryer equipped with an atomizer nozzle and spouting the liquid material at a supply rate of 5 cm ³ / min into 120 ° C. dry air while rotating the nozzle at 3000 rpm by an air flow. .

  After the granulated material was collected, it was fired a second time in a small electric furnace under conditions of 800 ° C. and 2 hours under a nitrogen atmosphere to obtain a secondary fired product, and various physical properties were measured using this as a positive electrode material sample. This sample was spherical as shown in the scanning electron microscope (SEM) photographs of FIGS. 2 and 3, and as a result of measurement by the XRD method, the intensity coincided with the XRD pattern of the primary fired product as shown in FIG.

For the above sample, the carbon content was 10% by mass, the packing density was 1.41 g / cm ³ , the average particle size was 14 μm, the aspect ratio (L / D) was 1.1, and the particle size sharpness (D30 / D70) was 0. 79, volume resistance was 7 Ω · cm.

  About the said sample, evaluation of the electrode was implemented with the coin-type battery. The coin-type battery was produced by the following procedure.

A polyvinylidene fluoride (PVDF) 8 mass% N-methylpyrrolidone (NMP) solution was added to and mixed with the sample as a binder to obtain a mixture of the sample 95 mass% -PVDF 5 mass%. This mixture was applied onto an aluminum foil, dried at 80 ° C., cut into an electrode size of 1.6 mmφ, and pressed at a press pressure of 78 MPa (0.8 tonf / cm ² ) to produce an electrode.

This electrode was used as a positive electrode, the counter electrode in the glove box was made of metal Li, and a C2023 type coin battery was manufactured using an electrolytic solution of EC / DMC = 1/2 (volume ratio) and LiPF ₆ 1 mol / L.

The initial evaluation of the electrodes was performed under the conditions of charge 0.2 mA / cm ² , 4.3 V, discharge 0.2 mA / cm ² , 2.0 V, and rest for 10 minutes.

  The rate performance of the electrode in charging was 1 C at 120 mAh, discharging was performed at a constant 0.5 C, and charging was performed under the condition that the charging was changed in the range of 1 to 15 C. The rate performance of the electrode in discharging was performed under conditions opposite to the charge rate performance. FIG. 4 shows the evaluation result of the charge rate performance, and FIG. 5 shows the evaluation result of the discharge rate performance.

The cycle performance of the electrode was evaluated by charging and discharging at 10C. The transition of this cycle performance is shown in FIG. Measurement conditions for electrode cycle performance are:
Electrolyte: EC / DMC = 1/2 (volume ratio)
Electrolyte: LiPF ₆ 1 mol / L, 25 ° C.
End-of-charge voltage: 4.2V
End-of-charge voltage: 1.5V
Rest time: 10 minutes. The rate condition in each cycle is 1 to 50 cycles .... 3C (3mA / cm ² )
50 to 100 cycles ··· 6C (6mA / cm ² )
100 to 150 cycles ··· 9C (9mA / cm ²⁾
150 to 200 cycles ·· 12C (12mA / cm ²⁾
It was as follows.

  Tables 1 and 2 show the measurement results of the physical properties other than the electrode rate performance and the electrode cycle performance for the above samples.

[Example 2]
As Example 1, except that 1 mol of lithium dihydrogen phosphate, 0.9 mol of iron oxalate dihydrate, and 0.1 mol of manganese carbonate were used as active material raw materials, and these were mixed at 200 rpm for 3 hours by a planetary mill. A sample of the positive electrode material was prepared.

About the obtained sample, the carbon content is 10% by mass, the packing density is 1.42 g / cm ³ , the average particle size is 12 μm, the aspect ratio (L / D) is 1.2, and the particle size sharpness (D30 / D70). Was 0.81 and the volume resistance was 2 Ω · cm. Further, there was no change in crystallinity between the primary fired product and the secondary fired product. Tables 1 and 2 show the measurement results of various physical properties of the sample.

[Example 3]
A sample of the positive electrode material was prepared in the same manner as in Example 2, except that 100 g of the active material mixture prepared in Example 2 was baked for the first time in a small electric furnace in a nitrogen atmosphere at 350 ° C. for 5 hours.

About the obtained sample, the carbon content is 10% by mass, the packing density is 1.48 g / cm ³ , the average particle size is 18 μm, the aspect ratio (L / D) is 1.1, and the particle size sharpness (D30 / D70). Was 0.75 and the volume resistance was 11 Ω · cm. Further, there was no change in crystallinity between the primary fired product and the secondary fired product. Tables 1 and 2 show the measurement results of various physical properties of the sample.

[Example 4]
As Example 1, except that 1 mol of lithium dihydrogen phosphate, 0.9 mol of iron oxalate dihydrate, and 0.1 mol of nickel acetate were used as active material raw materials, and these were mixed by a planetary mill at 200 rpm for 3 hr. A sample of the positive electrode material was prepared.

About the obtained sample, the carbon content is 10% by mass, the packing density is 1.49 g / cm ³ , the average particle size is 19 μm, the aspect ratio (L / D) is 1.1, and the particle size sharpness (D30 / D70). Was 0.82 and the volume resistance was 0.7 Ω · cm. Further, there was no change in crystallinity between the primary fired product and the secondary fired product. Tables 1 and 2 show the measurement results of various physical properties of the sample.

[Example 5]
Except for adding and mixing 10% by mass of phenol resin as carbon content to 200 g of the pulverized and classified product of the primary fired product (LiFe _0.9 Mn _0.1 PO ₄ ) prepared in Example 2 and mixing and granulating at 5000 rpm for 2 hours using a Henschel mixer. A sample of the positive electrode material was prepared in the same manner as in Example 2.

About the obtained sample, the carbon content is 10% by mass, the packing density is 1.55 g / cm ³ , the average particle size is 25 μm, the aspect ratio (L / D) is 1.1, and the particle size sharpness (D30 / D70). Was 0.76 and the volume resistance was 6 Ω · cm. Further, there was no change in crystallinity between the primary fired product and the secondary fired product. Tables 1 and 2 show the measurement results of various physical properties of the sample.

[Comparative Example 1]
The primary baked product (LiFePO ₄ ) produced in Example 1 was used as a positive electrode material sample, 80% by mass was mixed with 10% by mass of the conductive auxiliary material acetylene black, and the binder PVDF was 10% by mass. A C2023 type coin battery was produced in the same manner as in Example 1 except that the electrode was produced by wearing.

  The physical property measurement results for the sample are shown in Tables 1 and 2, and FIGS. This sample is irregular as shown in the scanning electron microscope (SEM) photographs of FIGS. 7 and 8, and the particle size distribution varies widely.

[Comparative Example 2]
The primary fired product (LiFe _0.9 Mn _0.1 PO ₄ ) produced in Example 2 was used as a positive electrode material sample, and 80% by mass was mixed with 10% by mass of the conductive auxiliary material acetylene black, and the binder PVDF10 A C2023 type coin battery was produced in the same manner as in Example 2 except that the electrode was produced by binding at mass%.

  The physical property measurement results for the sample are shown in Tables 1 and 2, and FIGS.

[Comparative Example 3]
The pulverized and classified product of the primary fired product (LiFe _0.9 Mn _0.1 PO ₄ ) produced in Example 2 was used as a positive electrode material sample, and 80% by mass was mixed with 10% by mass of the conductive auxiliary material acetylene black, and the binder PVDF10 A C2023 type coin battery was produced in the same manner as in Example 2 except that the electrode was produced by binding at mass%.

  Tables 1 and 2 show the measurement results of various physical properties of the sample.

[Comparative Example 4]
Although 10% by mass of naphthalene sulfonic acid resin as a carbon content was added to 200 g of the unfired product of the primary fired product (LiFe _0.9 Mn _0.1 PO ₄ ) prepared in Example 2, no granulation treatment was performed. That is, a sample of the positive electrode material was prepared in the same manner as in Example 2 except that the pulverization treatment was not performed in the first step and the granulation treatment was not performed in the second step.

About the obtained sample, the carbon content is 10% by mass, the packing density is 1.18 g / cm ³ , the average particle size is 26 μm, the aspect ratio (L / D) is 2.3, and the particle size sharpness (D30 / D70). Was 0.48 and the volume resistance was 19 Ω · cm. Tables 1 and 2 show the measurement results of various physical properties of the sample.

  In addition, since it was difficult to apply the mixture of 95% by mass of the sample to 5% by mass of PVDF on the aluminum foil at the time of producing the coin-type battery, the mixture of the sample of 90% by mass to 10% by mass of PVDF was placed on the aluminum foil. It was applied to.

[Comparative Example 5]
200 g of acetylene black as a carbon component was added to and mixed with 200 g of the active material raw material used in Example 2 and mixed and granulated at 200 rpm for 3 hours using a planetary mill. The fired product of this granulated product was pulverized and all particles were 45 μm. A primary fired product was produced in the same manner as in Example 2 except for the following. This fired product was used as a positive electrode material sample.

About the obtained sample, the carbon content is 10% by mass, the packing density is 0.8 g / cm ³ , the average particle size is 2 μm, the aspect ratio (L / D) is 2.2, and the particle size sharpness (D30 / D70). Was 0.21 and the volume resistance was 79 Ω · cm or more. The physical property measurement results for the sample are shown in Tables 1 and 2, and FIGS.

  In addition, since it was difficult to apply the mixture of 95% by mass of the sample to 5% by mass of PVDF on the aluminum foil at the time of producing the coin-type battery, the mixture of the sample of 90% by mass to 10% by mass of PVDF was placed on the aluminum foil. It was applied to.

[Comparative Example 6]
200 g of a phenol resin was added to and mixed with 200 g of the active material raw material used in Example 2 and mixed and granulated with a Henschel mixer at 5000 rpm for 2 hours, and the fired product of this granulated product was pulverized and all particles were 45 μm. A primary fired product was produced in the same manner as in Example 2 except for the following. This fired product was used as a positive electrode material sample.

About the obtained sample, the carbon content is 10% by mass, the packing density is 0.8 g / cm ³ , the average particle size is 3 μm, the aspect ratio (L / D) is 2.4, and the particle size sharpness (D30 / D70). Was 0.19 and the volume resistance was 37 Ω · cm. The physical property measurement results for the sample are shown in Tables 1 and 2, and FIGS.

  In addition, since it was difficult to apply the mixture of 95% by mass of the sample to 5% by mass of PVDF on the aluminum foil at the time of producing the coin-type battery, the mixture of the sample of 90% by mass to 10% by mass of PVDF was placed on the aluminum foil. It was applied to.

［実施例１〜５及び比較例１〜６についての考察］
図１に示すＸＲＤチャートから、実施例１で作製した炭素未被覆活物質(ＬｉＦｅＰＯ₄)と炭素被覆活物質(ＬｉＦｅＰＯ₄／Ｃ)、並びに、実施例２で作製した炭素未被覆活物質(ＬｉＦｅ_0.9Ｍｎ_0.1ＰＯ₄)と炭素被覆活物質(ＬｉＦｅ_0.9Ｍｎ_0.1ＰＯ₄／Ｃ)は、何れも同等の高い結晶性を示しており、炭素被覆処理されても活物質の結晶性の劣化は認められない。

[Discussion about Examples 1-5 and Comparative Examples 1-6]
From the XRD chart shown in FIG. 1, the carbon uncoated active material (LiFePO ₄ ) and the carbon coated active material (LiFePO ₄ / C) prepared in Example 1, and the carbon uncoated active material (LiFePO ₄ ) prepared in Example 2 are used. _0.9 Mn _0.1 PO ₄ ) and carbon-covered active material (LiFe _0.9 Mn _0.1 PO ₄ / C) both show the same high crystallinity, and even when carbon-coated, the deterioration of the crystallinity of the active material is recognized. I can't.

From the SEM photograph shown in FIG. 2, it can be seen that the carbon-coated active material (LiFePO ₄ / C) produced in Example 1 is a uniform and uniform spherical particle size.

From the SEM photograph obtained by further enlarging FIG. 2 shown in FIG. 3, the carbon-coated active material (LiFePO ₄ / C) produced in Example 1 is not only uniform and uniform spherical particles, but also the particles include It can be seen that there are no large voids.

  From the graph showing the charge rate performance in FIG. 4, comparing Example 1 with Comparative Example 1, it can be seen that high rate performance can be obtained without adding a conductive additive during electrode production.

  From the graph showing the discharge rate performance of FIG. 5, even if Example 1 and Comparative Example 1 are compared, high rate performance can be obtained without adding a conductive additive during electrode preparation as in FIG. I understand.

  From the graph showing the cycle performance shown in FIG. 6, the electrode produced in Example 1 shows good cycle performance although the charge / discharge capacity slightly decreases as the number of cycles increases. In contrast, the electrode produced in Comparative Example 1 deteriorated in a short period of 50 cycles, the electrode produced in Comparative Example 5 produced 2 cycles, and the electrode produced in Comparative Example 6 produced 100 cycles.

  From the results shown in Table 1, all of the positive electrode materials produced in Examples 1 to 5 are materials having a high packing density and a low volume resistance. On the other hand, it can be seen that all of the positive electrode materials produced in Comparative Examples 1 to 6 have a low packing density and a high volume resistance.

  From the results shown in Table 2, it can be seen that the electrodes produced in Examples 1 to 5 are excellent electrodes having high binding properties and high electrode properties such as electrode density, charge / discharge capacity, and initial efficiency. On the other hand, the electrodes produced in Comparative Examples 1 to 6 are extremely easy to peel off, and it is understood that at least one of the electrode physical properties is low.

A positive electrode is produced from the positive electrode material of the present invention, Li is used as a negative electrode, a mixed solvent (EC / 2DMC) of ethylene carbonate and dimethyl carbonate in a volume ratio of 1: 2 is used as an electrolytic solution, and LiPF ₆ is used as an electrolytic solution. The coin-type lithium ion secondary battery (Example 1) produced using the results shown in Table 1 and the results shown in FIGS. 4 and 5 are compared with the battery produced using the conventional positive electrode material (Comparative Example 5). The charge / discharge capacity of the initial evaluation is about 1.2 times, and the cycle performance in charge / discharge at 10 C is 5 times or more.

  The positive electrode materials produced in Comparative Examples 1 to 6 are all porous and have a low packing density. In the positive electrode materials of Comparative Examples 1 to 3, 10% by mass of acetylene black, which is a conductive additive, is added at the time of electrode preparation, and the binding property to the electrode is low. Therefore, a large amount of binder PVDF is used.

  In the positive electrode materials of Comparative Examples 4 to 6, the conductive auxiliary material acetylene black was not added at the time of electrode preparation, but the carbon precursor was added to the active material raw material instead of the active material that was the pulverized product of the primary fired product, After mixing and granulating, it is fired, and the binding property to the electrode is low like the positive electrode materials of Comparative Examples 1 to 3. Therefore, a large amount of binder PVDF is used.

  Since the positive electrode material produced in Examples 1-5 has high binding property to an electrode with respect to the positive electrode material produced in these comparative examples 1-6, the compounding quantity of PVDF of a binder may be small. As a result, the positive electrode materials of Examples 1 to 5 have an active material density that is 1.3 to 1.9 times higher than the positive electrode materials of Comparative Examples 1 to 6. Therefore, according to the positive electrode material of the present invention, a high-capacity electrode can be produced.

Crystal by XRD method about carbon uncoated active material (LiFePO ₄ , LiFe _0.9 Mn _0.1 PO ₄ ) and carbon coated active material (LiFePO ₄ / C, LiFe _0.9 Mn _0.1 PO ₄ / C) prepared in Examples 1 and 2 It is an XRD chart which shows the measurement result of property. 2 is a drawing-substitute scanning electron microscope (SEM) photograph of the carbon-coated active material (LiFePO ₄ / C) produced in Example 1. FIG. 3 is an enlarged scanning electron microscope (SEM) photograph substituting for a drawing of the carbon-coated active material (LiFePO ₄ / C) produced in Example 1. FIG. It is a graph which is a test result of the battery performance test done about the electrode produced in Example 1 and Comparative Examples 1, 5, and 6, and shows charge rate performance. It is a graph which is a test result of the battery performance test done about the electrode produced in Example 1 and Comparative Examples 1, 5, and 6, and shows discharge rate performance. It is a graph which is a test result of the battery performance test done about the electrode produced in Example 1 and Comparative Examples 1, 5, and 6, and shows cycle performance. FIG. 2 is a drawing-substitute scanning electron microscope (SEM) photograph of an unground product of a primary fired product (LiFePO ₄ ) produced in Example 1 used as a positive electrode material in Comparative Example 1. FIG. It is a drawing-substitute enlarged scanning electron microscope (SEM) photograph for the unground product of the primary fired product (LiFePO ₄ ) produced in Example 1 used as the positive electrode material in Comparative Example 1.

Claims (2)

LiFePO ₄ or LiFe _x M _1-x PO _{4 in} which a part of Fe is substituted with a different element (0.5 <x <1, M is Co, Cr, Mn, Ni, Ti or V, or these shows a mixing element.) was ground to 5μm or less, was added to the obtained carbon precursor to grind them were mixed, granulated using a modification resulting mixture Henschel mixer or a spray dryer, to obtain The granulated product is fired at 700 to 900 ° C. , the particle shape is spherical with an aspect ratio (L / D) of 1 to 2, the average particle size is 5 to 60 μm, and 30% particles The particle size sharpness indicated by the particle size ratio (D30 / D70) between the diameter and the 70% particle size is 0.5 or more, and the sample is put into a 10 mL glass graduated cylinder and tapped to change the sample volume. Measure the sample volume when it is gone, The amount of a 1.0~1.8g / cm ³ bulk density is represented by a value obtained by dividing the sample volume, the lithium ion secondary volume resistance value when the press pressure 49MPa is 0.1 to 20 · cm A method for producing a positive electrode material for a battery. LiFePO ₄ or LiFe _x M _1-x PO _{4 in} which a part of Fe is substituted with a different element (0.5 <x <1, M is Co, Cr, Mn, Ni, Ti or V, or these 2 represents two or more raw materials for producing the active material LiFePO ₄ , or two or more raw materials for producing the active material LiFe _x M _1-x PO ₄ , and the resulting mixture is 300 to 900 LiFePO ₄ obtained by firing at 0 ° C. or LiFe _x M _1-x PO _{4 in} which a part of Fe is substituted with a different element (0.5 <x <1, M is Co, Cr, Mn, Ni , Ti or V, or a mixed element thereof.) The method for producing a positive electrode material for a lithium ion secondary battery according to claim 1.

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