JP5678685B2 - Precursor of positive electrode active material for lithium secondary battery, method for producing the same, and method for producing positive electrode active material for lithium secondary battery - Google Patents

Description

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

In recent years, lithium ion secondary batteries have been widely used as power sources for small electronic devices such as notebook computers, mobile phones, and video cameras. As for this lithium ion secondary battery, since it was reported that lithium cobalt oxide is useful as a positive electrode active material for lithium ion secondary battery, research and development on lithium cobalt oxide has been actively promoted. Many proposals have been made.
However, since Co is unevenly distributed on the earth and is a rare resource, development of, for example, LiNiO ₂ , LiMn ₂ O ₄ , LiFePO _4, etc., is being promoted as new positive electrode active materials replacing lithium cobaltate. .

Among the positive electrode active materials, LiFePO ₄ which is an olivine-type lithium phosphate compound has a large volume density of 3.6 g / cm ³ , generates a high potential of 3.4 V, and has a large theoretical capacity of 170 mAh / g. It has characteristics. In addition to Fe being rich in resources and inexpensive, LiFePO ₄ has a stable polyphosphoric acid skeleton, so it is difficult to release oxygen atoms and is excellent in safety, so a new lithium instead of lithium cobaltate The expectation as a positive electrode active material of a secondary battery is great.
However, since LiFePO ₄ has low electron conductivity, sufficient electrode characteristics cannot be obtained as it is. To improve these disadvantages, the lithium secondary to miniaturization of the particles to several hundred nm, complexation with conductive material such as graphite, the LiFePO ₄ obtained by substituting a part of the Fe in the other metal as the positive electrode active material Secondary batteries have been proposed.

As a general method for producing LiFePO ₄ , ferrous phosphate hydrate and lithium salt, or ammonium nitrate other than the combination of iron oxalate and lithium salt such as ammonium hydrogen phosphate and lithium carbonate which are known from the beginning There has been proposed a method of obtaining LiFePO ₄ having an olivine structure by a solid-phase reaction by using a combination of solid raw materials such as iron and a lithium salt and mixing and firing them (see, for example, Patent Documents 1 and 2). .) In addition to phosphate, a method using metallic iron powder or iron oxide has been proposed as an iron source (see, for example, Patent Documents 3 and 4).
In these production methods, since a separate raw material is used for each component, a relatively high firing temperature is required to obtain a good olivine structure having a uniform composition by sufficiently diffusing the constituent components. .
However, when calcination is performed at a high temperature, the average particle size is coarsened to several μm to several tens of μm, and the particles become very hard as crystals develop. For this reason, there exists a fault that the reactivity is bad and processing, such as a grinding | pulverization, is difficult. As a result, it is difficult to develop the use of LiFePO ₄ as a positive electrode active material for lithium secondary batteries.

On the other hand, as a method for obtaining an olivine structure at a low temperature, a hydrothermal synthesis method in which high pressure is applied has also been proposed (for example, see Patent Document 5). According to this method, a fine olivine structure can be obtained at a low temperature, but not only a special high-pressure vessel is required, but also the apparatus is complicated and is not suitable for industrial production.
Further, as an attempt to obtain a reaction precursor having a uniform composition without using such a high-pressure vessel, a solution spray drying method, stationary drying solidification, and the like have been proposed (for example, see Patent Document 6).
However, these methods are inferior in productivity because a large amount of heat is required to remove water as a medium.

In addition, as an attempt to obtain a reaction precursor having a target composition, an alkaline solution such as sodium hydroxide is mixed in a phosphoric acid aqueous solution containing a lithium salt and an iron salt, and a composite phosphorous oxide of lithium and iron A method for obtaining a coprecipitate is also proposed (see Patent Document 7).
However, in this proposal, detailed reaction conditions have not been clarified, and, for example, when a neutralization reaction is performed using sodium hydroxide, sodium salts generated during the reaction are mixed, resulting in battery characteristics. May adversely affect

JP 2003-292307 A JP 2006-056754 A JP 2008-004317 A JP 2008-159495 A JP 2005-276474 A JP 2006-131485 A JP 2002-117831 A

  In view of the problems of the prior art, the object of the present invention is to provide a suitable uniform fine precursor as a raw material for producing a positive electrode active material for a lithium secondary battery, and to obtain a finer, It is an object to provide a positive electrode active material having excellent positive battery characteristics with few impurities and a method for producing the same.

In order to achieve the above object, the present inventors have conducted extensive research on a positive electrode active material for a LiFePO ₄ lithium secondary battery that is useful as a positive electrode active material for a lithium secondary battery. By reacting a mixed aqueous solution containing phosphoric acid and a lithium aqueous solution containing lithium hydroxide under a specific pH condition, the resulting fine uniform and excellent precursor is heat-treated, and thus excellent. The inventors have found that a positive electrode active material can be obtained, and have completed the present invention.

That is, according to the first aspect of the present invention, the diffraction peak of the (311) plane in the X-ray diffraction is composed of single-phase primary particles of olivine type lithium iron phosphate and secondary particles in which the primary particles are aggregated. To produce a positive electrode active material for a lithium secondary battery having a half-value width of 0.1 to 0.3 ° and an average particle size of the primary particles determined from observation with a scanning electron microscope of 0.05 to 0.5 μm A precursor used as a raw material of
It is obtained by mixing a mixed aqueous solution (a) containing a divalent iron salt and phosphoric acid and a lithium aqueous solution (b) containing lithium hydroxide to crystallize a lithium iron phosphate composition, In addition, a positive electrode active material precursor for a lithium secondary battery is provided in which a diffraction peak derived from the lithium iron phosphate composition is in an undetected state in X-ray diffraction .

According to the second invention of the present invention, there is provided a positive electrode active material precursor for a lithium secondary battery, wherein the sodium content is 0.5% by mass or less in the first invention. .

On the other hand, according to the third aspect of the present invention, a mixed aqueous solution (a) containing a divalent iron salt and phosphoric acid and a lithium aqueous solution (b) containing lithium hydroxide are mixed to obtain a reaction aqueous solution. (C) is formed, and the reaction aqueous solution (c) after mixing with the lithium aqueous solution (b) is controlled so as to be in the range of pH 8 to 9, whereby the lithium iron phosphate composition is crystallized. A positive electrode active for a lithium secondary battery according to the first or second invention, characterized by comprising a crystallization step for precipitation, and a drying step for washing the lithium iron phosphate composition in a non-oxidizing atmosphere after washing with water A method for producing a material precursor is provided.
According to a fourth invention of the present invention, in the third invention, an alkali metal hydroxide is further added to the reaction aqueous solution (c) for the pH control. A method for producing a positive electrode active material precursor for a secondary battery is provided.
Furthermore, according to a fifth invention of the present invention, in the third or fourth invention, the divalent iron salt is ferrous sulfate hydrate (FeSO ₄ .7H ₂ O). A method for producing a positive electrode active material precursor for a lithium secondary battery is provided.

According to the sixth aspect of the present invention, the positive electrode active material precursor for lithium secondary battery according to the first or second aspect of the invention is mixed with the conductive carbonaceous material product to obtain a mixture. There is provided a method for producing a positive electrode active material for a lithium secondary battery, characterized by heating at 350 to 700 ° C. in an active or reducing atmosphere.
Further, according to a seventh invention of the present invention, there is provided a method for producing a positive electrode active material for a lithium secondary battery, characterized in that, in the sixth invention, the precursor is dry-type or wet-ground before the heat treatment. Provided.

As described above, the present invention relates to a positive electrode active material for a lithium secondary battery, a positive electrode active material precursor for a lithium secondary battery, and the like. Preferred embodiments include the following.
(1) In the third invention, the reaction temperature during crystallization in the crystallization step is 5 to 80 ° C., and the atmosphere during crystallization is an inert gas atmosphere such as nitrogen. To produce a positive electrode active material precursor for a lithium secondary battery.
( 2 ) In the third invention, in the drying step, the non-oxidizing atmosphere is an inert atmosphere or a vacuum atmosphere, and the drying temperature is 35 to 250 ° C. A method for producing a positive electrode active material precursor.
( 3 ) A method for producing a positive electrode active material for a lithium secondary battery, wherein the heat treatment time in the sixth invention is 2 to 20 hours.
( 4 ) A secondary battery using, as a positive electrode, a positive electrode active material for a lithium secondary battery obtained from the method for producing a positive electrode active material for a lithium secondary battery according to the sixth or seventh invention.

The positive electrode active material for a lithium secondary battery of the present invention is composed of uniform fine olivine-type lithium iron phosphate (LiFePO ₄ ) particles, and a battery composed of a positive electrode using the positive electrode active material is a theoretical discharge of LiFePO ₄ . It shows a value close to the capacity and exhibits excellent characteristics.
Moreover, the positive electrode active material precursor for a lithium secondary battery of the present invention has low crystallinity, and the positive electrode active material can be easily obtained by heat treatment.
Furthermore, the method for producing a positive electrode active material for a lithium secondary battery according to the present invention can easily obtain the positive electrode active material in a high yield without using a toxic substance, and its industrial value is extremely large.

  Hereinafter, the present invention will be described in detail for each item.

1. Positive electrode active material for lithium secondary battery The positive electrode active material for lithium secondary battery of the present invention (hereinafter sometimes simply referred to as the positive electrode active material of the present invention) is a single olivine type lithium iron phosphate (LiFePO ₄ ). Phase primary particles and secondary particles in which the primary particles are aggregated, and the diffraction peak half-value width (or full width at half maximum) of the (311) plane in X-ray diffraction is 0.1 to 0.3 °, and scanning type The average particle diameter of the primary particles obtained from observation with an electron microscope is 0.05 to 0.5 μm.

By having both the diffraction peak half width and the average particle size of the primary particles, the lithium iron phosphate particles have sufficient crystallinity and become fine particles, which improves the reactivity with the battery electrolyte. Therefore, it has excellent characteristics as a positive electrode active material for a lithium secondary battery, particularly a battery capacity.
On the other hand, even when the average particle diameter is in the above range, when the diffraction peak half-value width exceeds 0.3 °, the crystallinity of the lithium iron phosphate particles is not sufficient, and is excellent as a positive electrode active material. Characteristics cannot be obtained. Even if the half-width of the diffraction peak is in the above range, when the average particle diameter exceeds 0.5 μm, the lithium iron phosphate particles are coarse even if the crystallinity is sufficient, and thus excellent. Characteristics cannot be obtained.
In addition, the form of the positive electrode active material of the present invention may be aggregate particles (secondary particles) having an average particle diameter of 1 to 60 μm formed by aggregation of the primary particles.

On the other hand, when the half width of the diffraction peak is less than 0.1 °, the lithium iron phosphate particles are coarse, and when the average particle size is less than 0.05 μm, sufficient crystals of the lithium iron phosphate particles are obtained. In any case, excellent characteristics as a positive electrode active material cannot be obtained.
Here, the half width of the diffraction peak is measured by powder X-ray diffraction using Cu-Kα rays using a powder X-ray diffraction (XRD) apparatus, and the crystallinity of the (311) plane which is a crystal plane is evaluated. Yes. Its technical significance is as follows.
In general, the XRD diffraction peak has a deeper relationship with crystallinity, and the higher the crystallinity (regular atomic arrangement), the higher the diffraction peak and the narrower the width. On the other hand, the diffraction peak is also affected by the size of the crystallite, and when the crystallite becomes fine, the diffraction peak is low and wide as in the state of poor crystallinity. Furthermore, coarse particles are often polycrystalline, but fine primary particles according to the present invention are close to single crystals, and the size of the primary particles can be evaluated by the size of the crystallites. In the present invention, the crystallinity and the size of the crystallite are evaluated by the diffraction peak on the (311) plane where a large diffraction peak appears in LiFePO ₄ . As a result, when the half-width of the diffraction peak and the average primary particle diameter are within the above ranges, primary particles having fineness and excellent crystallinity can be obtained.

Further, the surface of the primary particles is coated with a conductive carbonaceous material of 3 to 10% by mass, preferably 3 to 8% by mass with respect to the total amount of the positive electrode active material for a lithium secondary battery. Preferably it is.
Since the lithium iron phosphate particles constituting the positive electrode active material of the present invention have low conductivity, coating with a conductive carbonaceous material in the above range ensures sufficient conductivity between the particles, Excellent properties can be obtained as an active material. When the coating amount of the conductive carbonaceous material is less than 3% by mass, sufficient conductivity is not ensured. On the other hand, when the coating amount of the conductive carbonaceous material exceeds 10% by mass, preferably over 8% by mass, the positive electrode active Since the amount of lithium iron phosphate particles per volume of material decreases, the battery capacity per volume of battery decreases, which is disadvantageous.
Examples of the conductive carbonaceous material (or carbon material) include natural graphite such as scaly graphite, scaly graphite and earthy graphite, and graphite such as artificial graphite, carbon black, acetylene black, ketjen black, channel black, Examples thereof include carbon blacks such as furnace black, lamp black and thermal black, carbon fibers, and the like, and these can be used alone or in combination of two or more. Among these, those having fine ketjen black are particularly preferable because they can be easily obtained industrially.
In addition, the coating layer made of the conductive carbonaceous material is prepared by mixing, for example, an organic material containing carbon such as sucrose with the raw material compound (precursor) in addition to the conductive carbonaceous material, and firing the mixture. In obtaining the positive electrode active material, the organic material can be burned to produce conductive carbon, and the carbon can be formed by coating the carbon on the positive electrode active material particles.

  The content of sodium (Na) as an impurity of the positive electrode active material of the present invention is preferably as small as possible, preferably 0.5% by mass or less, and more preferably 0.1% by mass or less. When sodium content exceeds 0.5 mass%, it may become sodium phosphate in a positive electrode active material, and the performance of the battery using this positive electrode active material may be reduced.

The BET specific surface area of the positive electrode active material is preferably 10 to 100 m ² / g, more preferably 30 to 70 m ² / g. When the BET specific surface area is less than 10 m ² / g, when used in a battery, a sufficient reaction area with the electrolytic solution cannot be obtained and the capacity is not increased. Moreover, when the BET specific surface area exceeds 100 m ² / g, the bulk density becomes too high, which is disadvantageous as a battery material whose volume is limited.

The positive electrode active material of the present invention can be suitably used as a positive electrode active material of a lithium secondary battery comprising a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte containing a lithium salt. High capacity and safety.
The positive electrode active material is used in combination with other known lithium cobalt composite oxide, lithium nickel composite oxide, lithium metal composite oxide such as lithium manganese composite oxide, and so on. The safety of the secondary battery can be further improved. In this case, the other lithium metal composite oxide used in combination is not particularly limited, and a general one can be used.

2. Positive electrode active material precursor for lithium secondary battery The positive electrode active material precursor for lithium secondary battery of the present invention (hereinafter sometimes simply referred to as the precursor of the present invention) is used as a raw material for the positive electrode active material. A mixed aqueous solution (a) containing a divalent iron salt and phosphoric acid and a lithium aqueous solution (b) containing lithium hydroxide are mixed to crystallize a lithium iron phosphate composition. And the diffraction peak derived from the lithium iron phosphate composition is in an undetected state in X-ray diffraction.

In the precursor of the present invention, it is important that the average particle size of primary particles is 0.05 μm or less, and that a diffraction peak derived from the lithium iron phosphate composition is not detected in X-ray diffraction. With significant significance.
That is, the precursor is in an amorphous state, excellent in reactivity, and easily crystallized by firing at a low temperature to produce the fine lithium iron phosphate particles. Here, the state where the diffraction peak is not detected means that the peak is in a broad state and the diffraction angle is not specified, that is, a state where it is not mechanically detected by the X-ray diffraction apparatus. On the other hand, a precursor with high crystallinity that can detect the diffraction peak has low reactivity and requires firing at a high temperature to produce lithium iron phosphate particles, and the particles also become coarse. .

Although the olivine structure is not formed and the precursor has an amorphous structure, the crystalline peak of Fe ₃ (PO ₄ ) ₂ or Li ₃ PO ₄ may be weakly observed in some cases. is there. However, even in this case, each phase is very finely dispersed, so that the reactivity is high.

The precursor is obtained by mixing a mixed aqueous solution (a) containing a divalent iron salt and phosphoric acid and a lithium aqueous solution (b) containing lithium hydroxide and neutralizing and crystallization. By neutralizing crystallization, the components constituting the precursor are uniformly mixed, and the crystallinity and particle diameter can be easily controlled.
Therefore, the precursor is different from, for example, a precursor in which iron phosphate and lithium phosphate are mixed, has the target composition of the positive electrode active material, and is excellent in uniformity and reactivity of the components. An olivine structure can be obtained at a heat treatment temperature lower than the firing temperature of the precursor, and coarsening and sintering of lithium iron phosphate particles can be suppressed.

  Since the precursor of the present invention is used as a raw material for producing a positive electrode material for a lithium battery that requires high purity, high purity is required. In particular, since the sodium content of the positive electrode active material is preferably 0.5% by mass or less, the sodium content of the precursor of the present invention is also preferably 0.5% by mass or less. It is more preferable that the amount is not more than mass%. When the sodium content of the precursor exceeds 0.5% by mass, a step of removing sodium is required in the process of generating the positive electrode active material from the precursor, and when the step of removing sodium is not employed, The sodium content of the positive electrode active material may exceed 0.5% by mass.

3. Method for Producing Positive Electrode Active Material Precursor for Lithium Secondary Battery A method for producing a precursor of the present invention comprises a mixed aqueous solution (a) containing a divalent iron salt and phosphoric acid, and a lithium aqueous solution containing lithium hydroxide ( b) to form a reaction aqueous solution (c), and the lithium aqueous solution (b) causes the reaction aqueous solution (c) after mixing to be controlled so as to be in the range of pH 8-9. Thus, the method comprises a crystallization step of crystallizing a lithium iron phosphate composition, and a drying step of drying the lithium iron phosphate composition in a non-oxidizing atmosphere after washing with water.
Hereinafter, it demonstrates for every process.

(1) Crystallization Step Since the precursor of the present invention has a fine structure and low crystallinity, it is optimal to use a neutralization crystallization method, and is obtained by controlling the crystallization conditions. .
In the crystallization step, the target composition of the positive electrode active material is obtained by mixing a mixed aqueous solution (a) containing a divalent iron salt and phosphoric acid and a lithium aqueous solution (b) containing lithium hydroxide. And a reaction aqueous solution (c) in which the components are uniformly mixed can be formed.
Here, first, a mixed aqueous solution (a) containing a divalent iron salt and phosphoric acid is prepared. When the divalent iron salt or phosphoric acid and the lithium aqueous solution (b) are mixed first, a reaction is performed. Since a product is formed, even if it is mixed with an unmixed divalent iron salt or phosphoric acid after that, a precursor that is uniformly mixed in components cannot be obtained.

Specifically, first, phosphoric acid is used in a molar ratio with respect to iron atoms in the divalent iron salt in accordance with the composition of the target lithium iron phosphate (LiFePO ₄ ), preferably 0.9 to 1.1. More preferably, the mixed aqueous solution (a) is prepared by dissolving a divalent iron salt and phosphoric acid so as to be 0.95 to 1.05.
Here, the concentration of the mixed aqueous solution is not particularly limited as long as it can dissolve the divalent iron salt and phosphoric acid, but the concentration of the divalent iron salt should be 0.1 mol / L or more. Is preferable, and it is more preferable to set it as 0.5-1.0 mol / L.
On the other hand, in the lithium aqueous solution (b), the lithium content is adjusted so that the component of the reaction aqueous solution (c) after the mixing has a composition of LiFePO ₄ , but the molar amount relative to the iron atom in the divalent iron salt is adjusted. The ratio is preferably 0.8 to 1.2, more preferably 0.95 to 1.1.

Examples of the divalent iron salt include ferrous sulfate, ferrous chloride, iron acetate, iron oxalate, and the like, and at least one of them or a mixture thereof can be used. May be hydrated or anhydrous. Among these, ferrous sulfate heptahydrate (FeSO ₄ · 7H ₂ O) is particularly preferable because it is easily available industrially and is inexpensive.

Thus, in the crystallization process, by adjusting the composition, a precursor having a uniform composition distribution of the target product, LiFePO ₄ , is obtained, which is a major feature of the precursor manufacturing method of the present invention. is there. For this reason, it is not necessary to add a raw material and perform a solid-phase reaction at the time of baking for component adjustment in a post process, and the baking temperature can be lowered.
The mixed aqueous solution (a) is an acidic solution containing iron and phosphoric acid and is neutralized by mixing with the lithium aqueous solution (b) containing lithium to form the reaction aqueous solution (c). The precursor is generated by the crystallization reaction.
At the time of the crystallization, the reaction aqueous solution (c) is controlled in the range of pH 8-9 by adding lithium hydroxide or an aqueous lithium solution (b). When the reaction is carried out in an acidic or neutral region of less than pH 8, lithium is difficult to precipitate, while in a highly alkaline region of more than pH 9, Fe is hydroxideized, and a uniformly mixed lithium iron phosphate composition is obtained. I can't get it. By reacting the reaction aqueous solution (c) in the range of pH 8 to 9, lithium and Fe crystallize as a phosphate, and a lithium iron phosphate composition having a target composition of LiFePO ₄ can be obtained.

  The pH is controlled by adding lithium hydroxide or an aqueous lithium solution (b), but other inorganic alkalis can be used in combination for auxiliary pH adjustment. As the inorganic alkali, sodium hydroxide is preferred because it is industrially easily available and inexpensive. However, when only lithium hydroxide or an aqueous lithium solution (b) is used, the resulting precursor can be made substantially free of sodium, but when sodium hydroxide is used, Sodium may remain in the precursor. For this reason, it is preferable that the amount of sodium hydroxide used for the adjustment of the pH is set to a minimum so as to be within the range of the pH.

  The mixing method of the mixed aqueous solution (a) and the lithium aqueous solution (b) is not particularly limited. For example, the single jet method in which the other raw material aqueous solution is added to one raw material aqueous solution, or both aqueous solutions. A double jet method in which can be simultaneously added to the reaction system can be used. When the single jet method is used, the reaction aqueous solution (c) after mixing may be adjusted to have a pH of 8-9. On the other hand, in the double jet method, both the aqueous solutions are added simultaneously into a reaction system whose pH has been adjusted in advance. However, the flow rate of one aqueous solution is kept constant and gradually introduced into the reaction system. A method of adjusting the flow rate is preferable in order to obtain a stable quality.

In the case of the single jet method, a mixed aqueous solution (a) in which iron and phosphorus coexist in the reaction system in which lithium is present and in the reaction system where lithium is present, that is, in the lithium aqueous solution (b) at the same ratio as the composition of LiFePO _4. The phosphoric acid dissociates uniformly and reacts with the lithium ions and iron ions coexisting in a predetermined ratio with the surroundings, so that a precursor having an average composition of LiFePO ₄ that is the target composition is uniformly obtained. Arise. Such a precursor is considered to be highly reactive due to its extremely uniform composition.
Also in the double jet method, it is considered that the same reaction occurs when the two aqueous solutions come into contact with each other in a reaction aqueous solution controlled to a constant pH. In the double jet method, the pH during mixing can always be controlled within the above pH range, and there is no possibility of crystallization of different phases such as Fe ₃ (PO ₄ ) ₂ or Li ₃ PO ₄ , and mixing is performed uniformly. Since the obtained lithium iron phosphate composition is obtained, it is preferable. In order to always control the pH during mixing to the above pH range, it is preferable to control the pH of the aqueous solution before mixing to 8 to 9 in advance, and pH control is performed by adding lithium hydroxide. Is preferred.

The reaction temperature at the time of the crystallization is not particularly limited, and is usually 5 to 80 ° C, preferably 15 to 40 ° C. If the reaction temperature is less than 5 ° C., the solubility of iron in the reaction aqueous solution is low, so that a foreign phase such as iron hydroxide (Fe (OH) ₂ ) may crystallize. On the other hand, when the reaction temperature exceeds 80 ° C., iron is oxidized and a foreign phase such as iron oxide may be crystallized.
The atmosphere for the crystallization may be an air atmosphere, but iron is likely to be oxidized, and a foreign phase such as iron oxide (FeO) may be crystallized. For this reason, it is preferable to set it as a non-oxidizing atmosphere, and it is more preferable to react in inert gas atmosphere, such as nitrogen.
As an apparatus used for the crystallization step, a reaction vessel with a stirrer is preferable in order to cause the reaction to occur uniformly, and in order to control the atmosphere during crystallization, it is preferable to have a sealed structure. preferable.
After completion of the crystallization reaction, the lithium iron phosphate composition is recovered by solid-liquid separation by filtration, centrifugation, or the like.

(2) Drying step In the drying step, the lithium iron phosphate composition obtained in the crystallization step is sufficiently washed with water and then dried in order to remove impurities. In particular, when sodium hydroxide is used for pH adjustment in the crystallization step, it is preferable to sufficiently wash with water until the sodium content is 0.5% by mass or less.
Here, since the lithium iron phosphate composition has a fine and amorphous structure, sodium can be easily removed by washing with water.

The lithium iron phosphate composition obtained in the crystallization step is easily oxidized during drying, and when iron is oxidized from divalent, it becomes difficult to obtain LiFePO ₄ by a heat treatment in a subsequent step.
For this reason, drying after washing is performed in a non-oxidizing atmosphere. Although it will not specifically limit if it is in a non-oxidizing atmosphere, It is preferable to carry out in an inert atmosphere or a vacuum atmosphere.
Moreover, the drying temperature should just be the range which can suppress oxidation, it is preferable to set it as 250 degrees C or less, and it is more preferable to set it as 100 degrees C or less. On the other hand, if it is less than 35 ° C., it takes time to dry, which is not preferable.

On the other hand, in the temperature range where oxidation can be suppressed, drying in an air atmosphere is also possible. In the case of an air atmosphere, the drying temperature is preferably 35 to 50 ° C, more preferably 40 to 50 ° C. If it is less than 35 ° C., drying takes time. On the other hand, if it exceeds 50 ° C., oxidation of divalent iron may occur.
The apparatus used in the drying step can be either a stationary type or a stirring type drying apparatus such as a fluidized bed type. However, in order to suppress iron oxidation, it is preferable to use a drying apparatus capable of controlling the atmosphere.

4). Method for producing positive electrode active material for lithium secondary battery The method for producing a positive electrode active material according to the present invention comprises mixing the above precursor with a conductive carbonaceous material product to form a mixture, and then in an inert or reducing atmosphere. In this, heat treatment is performed by heating at 350 to 700 ° C.

Lithium iron phosphate (LiFePO ₄ ) has low conductivity, and in order to make it suitable as a positive electrode active material of a lithium secondary battery, the surface of the LiFePO ₄ particles is made of a conductive carbonaceous material (or conductive carbon). Material).
For this reason, before the said heat processing, the said precursor is mixed with a conductive carbonaceous material product (it means the substance which produces | generates a conductive carbonaceous material). Examples of the conductive carbonaceous material (or carbon material) product to be used include, as described above, natural graphite, graphite such as artificial graphite, carbon blacks such as acetylene black and ketjen black, carbon fiber, sucrose, Examples include ascorbic acid and other organic compounds that generate carbonaceous matter by decomposition, and these can be used alone or in combination of two or more. Among them, acetylene black and sucrose are particularly preferable because they can be easily formed with a conductive carbon material and are easily available industrially.
Further, the amount of carbon atoms contained in the conductive carbon material tends to be smaller than that of the conductive carbon material product by heat treatment. For this reason, it is preferable to make the compounding quantity of an electroconductive carbon material product increase 40-100% by mass ratio with respect to the carbon content contained after heat processing, and it is more preferable to increase 50-100%.

  The mixing is preferably sufficiently performed dry using a blender or the like so that the precursor and the conductive carbonaceous material product are uniformly mixed.

The precursor is preferably dry-type or wet-ground before the heat treatment, that is, before and after the mixing. By pulverizing, the positive electrode active material obtained by the subsequent heat treatment becomes finer and the surface of the fine LiFePO ₄ particles can be uniformly coated with the conductive carbon material. In this case, the discharge capacity can be improved. In order to uniformly coat with the conductive carbon material, it is more preferable to perform pulverization after the mixing or to perform mixing and pulverization simultaneously.
The pulverization is preferably performed mainly by the shearing and frictional action of the medium by putting the precursor or mixture and a pulverizing medium such as balls and beads in a container and causing the media to collide with each other. By mixing using a grinding medium, mixing and grinding can be performed simultaneously.
As the pulverizer that can be used, a pulverizer having a shearing force such as a rolling ball mill, a vibration mill, a planetary mill, or a medium stirring mill is preferable. As such an apparatus, a commercially available apparatus can be used.

It is preferable that the particle size of the pulverization medium is 1 to 15 mm because pulverization can be sufficiently performed. Moreover, it is preferable that the material of the medium is zirconia or alumina ceramic because it has high hardness, high wear resistance, and metal contamination of the material to be crushed can be prevented.
Further, the pulverization medium contains the pulverization medium in a container with a space volume of 50 to 90%, and appropriately manages the shearing force and frictional force due to the fluid medium. It is preferable to process.

  Furthermore, in the method for producing a positive electrode active material of the present invention, if necessary, the mixture is pressure-molded before the heat treatment to further increase the contact area between the precursor and the conductive carbonaceous material product. In addition, the discharge capacity and cycle characteristics can be further improved. As the press molding machine for pressure molding, a hand press, a tableting machine, a briquette machine, a roller compactor, and the like can be suitably used. it can.

Next, the mixture of the precursor and the conductive carbonaceous material product is heat-treated. The heat treatment temperature is 350 to 700 ° C, preferably 450 to 650 ° C.
In the method for producing a positive electrode active material of the present invention, by setting the heat treatment temperature within the above range, the lithium secondary battery using the obtained positive electrode active material can improve the discharge capacity and the charge cycle characteristics. When the heat treatment temperature is less than 350 ° C., the crystal structure is not sufficiently developed. On the other hand, when the heat treatment temperature exceeds 700 ° C., the sintering proceeds and particle growth occurs, resulting in coarse particles.

  The heat treatment time is preferably 2 to 20 hours, and more preferably 5 to 10 hours. The heat treatment is performed either in an inert gas atmosphere such as nitrogen or argon or in a reducing atmosphere such as hydrogen or carbon monoxide in order to suppress iron oxidation, but in terms of safety during operation, nitrogen, It is preferable to carry out in an inert gas atmosphere such as argon gas. Moreover, these heat processing can be performed in multiple steps as needed.

After the above heat treatment, a positive electrode active material in which the surface of fine LiFePO ₄ particles is uniformly coated with a conductive carbon material can be obtained by appropriately cooling and further pulverizing or classifying as necessary.
In order to suppress iron oxidation, the cooling is preferably performed in the inert gas atmosphere or the reducing atmosphere.

  EXAMPLES The present invention will be described in more detail below with reference to examples of the present invention, but the present invention is not limited to these examples. The raw materials used in the examples were commercially available reagent grades, and the samples were analyzed and evaluated by the following methods.

(1) Analysis of elemental metals:
Analysis was performed by ICP emission analysis using an ICP emission analyzer (Varian, 725ES).
(2) X-ray diffraction:
Using a powder X-ray diffractometer (manufactured by PANALYTICAL, X'Pert PRO MRD), measurement was performed by powder X-ray diffraction using Cu-Kα rays.
(3) Average particle size:
It was determined by measuring the particle size of 50 primary particles by electron microscope observation and averaging the number.
(4) Carbon (C) content:
It measured using the carbon analyzer (made by LECO).

(Precursor synthesis)
[Example 1]
278 g (1 mol) of ferrous sulfate heptahydrate (FeSO ₄ .7H ₂ O) and 115 g (1 mol) of 80% by mass phosphoric acid (H ₃ PO ₄ ) are dissolved in 1 L of water, and a mixed aqueous solution (a) Was made. On the other hand, 24 g (1 mol) of lithium hydroxide was dissolved in 1 L of water to prepare an aqueous lithium solution (b).
Pure water is placed in a reaction vessel installed in a nitrogen atmosphere, lithium hydroxide is added to adjust the pH to 8.5, and then the mixed aqueous solution (a) is added dropwise at a rate of 20 ml / min. By adding the lithium aqueous solution (b) so as to maintain pH 8.5, the reaction was carried out in the reaction aqueous solution (c) to obtain a crystallized product of a lithium iron phosphate composition.
Next, the lithium iron phosphate composition was recovered by filtration, and this was carefully washed with 2 L of pure water. Furthermore, the lithium iron phosphate composition after washing was dried in a vacuum at 60 ° C. for 12 hours to obtain 155 g of a precursor (yield 98%).
The molar ratio of Li, Fe, and P of the obtained precursor was 1.0: 1.0: 1.0. Moreover, when the obtained precursor was analyzed by X-ray diffraction, a broad peak was observed, but it was difficult to identify the substance indicated by the peak.

[Example 2]
In the same manner as in Example 1, a mixed aqueous solution (a) and a lithium aqueous solution (b) were prepared.
Next, the mixed aqueous solution (a) is dropped into the lithium aqueous solution (b) at a rate of 20 ml / min to form a reaction aqueous solution (c). When the pH reaches 8, 1 mol% aqueous sodium hydroxide solution is added. The dropwise addition was performed, and the addition of the mixed aqueous solution (a) was completed while maintaining the pH of the aqueous reaction solution (c) at 8.1, whereby a crystallized product of the lithium iron phosphate composition was obtained.
Next, the lithium iron phosphate composition was recovered by filtration, and this was carefully washed with 2 L of pure water. Furthermore, the lithium iron phosphate composition after washing was dried in a vacuum at 60 ° C. for 12 hours to obtain 153 g of a precursor (yield 98%).
The molar ratio of Li, Fe, and P of the obtained precursor was 0.98: 1.0: 1.0. Moreover, when the obtained dried product was analyzed by X-ray diffraction, a broad peak was observed, but it was difficult to identify the substance indicated by the peak. In addition, when the sodium content of the said precursor was measured by the atomic absorption method, it was confirmed that it is 0.1 mass% or less.

[Comparative Example 1]
A precursor was obtained in the same manner as in Example 1 except that the pH of the aqueous reaction solution (c) during crystallization was controlled to 11.6.
When the obtained crystallized product was analyzed by X-ray diffraction, a peak derived from Fe oxide was detected.

[Comparative Example 2]
124 g of precursor was obtained in the same manner as in Example 2 except that the aqueous solution of sodium hydroxide was not added dropwise and the pH of the aqueous reaction solution (c) during crystallization was controlled to 6.5. .
When the obtained precursor was analyzed, Li was not contained below the detection limit (<0.1% by mass), and the molar ratio of Fe to P was 59.8: 40.2. It became about 3: 2. From the X-ray diffraction, it was confirmed that the precursor was Fe ₃ (PO ₄ ) ₂ · n (H ₂ O).
In addition, for this, an attempt was made to synthesize a positive electrode active material by mixing with Li ₃ PO ₄ according to a conventional method (Comparative Examples 5 and 6).

(Synthesis of positive electrode active material)
[Example 3]
300 g of the precursor obtained in Example 1, 30 g of Ketjen Black (trade name ECP, manufactured by Ketjen Black International Co., Ltd.) having an average particle size of 0.05 μm and 200 g of zirconia balls (particle size of 5 mm) are placed in a 2 L plastic container. By mixing with a shaker mixer, mixing and pulverization were performed simultaneously to obtain a mixture.
Next, the obtained mixture is heat-treated at 500 ° C. for 5 hours in a nitrogen-2% by volume hydrogen atmosphere and cooled in the atmosphere to obtain a positive electrode active material made of LiFePO ₄ coated with ketjen black. And evaluated. Crushing and classification were not performed.
Table 1 shows the main physical properties of the obtained positive electrode active material.

[Example 4]
A positive electrode active material made of carbon-coated LiFePO ₄ was prepared in the same manner as in Example 3 except that 500 g of the precursor obtained in Example 1 and 100 g of sucrose and 200 g of zirconia balls (particle size: 5 mm) were mixed. Acquired and evaluated. Crushing and classification were not performed.
Table 1 shows the main physical properties of the obtained positive electrode active material.

[Comparative Example 3]
In the same manner as in Example 3, except that 500 g of the precursor obtained in Example 1 and 100 g of sucrose and 200 g of zirconia balls (particle size: 5 mm) were mixed, and the heat treatment temperature was 800 ° C. A positive electrode active material made of LiFePO ₄ coated with TiO ₂ was obtained and evaluated. Crushing and classification were not performed.
Table 1 shows the main physical properties of the obtained positive electrode active material.

[Comparative Example 4]
Coated with carbon in the same manner as in Example 3 except that 500 g of the precursor obtained in Example 1 and 100 g of sucrose and 200 g of zirconia balls (particle size: 5 mm) were mixed, and the heat treatment temperature was 325 ° C. A positive electrode active material made of LiFePO ₄ was obtained and evaluated.
X-ray diffraction, in addition to _{LiFePO 4, Fe 3 (PO 4} ) 2 and _Li 3 PO ₄ is the detection, single-phase LiFePO ₄ is an object compound is not obtained. Crushing and classification were not performed.
Table 1 shows the main physical properties of the obtained positive electrode active material.

[Comparative Example 5]
180 g of Fe ₃ (PO ₄ ) ₂ obtained in Comparative Example 2, 58 g of Li ₃ PO ₄ (manufactured by High Purity Chemical) and 50 g of zirconia balls (particle size 5 mm) were placed in a 500 cc plastic container and mixed, and heat treatment A positive electrode active material made of LiFePO ₄ was obtained in the same manner as in Example 3 except that the temperature was 700 ° C.

[Comparative Example 6]
180 g of Fe ₃ (PO ₄ ) ₂ obtained in Comparative Example 2 and 58 g of Li ₃ PO ₄ (manufactured by High Purity Chemical), 24 g of Ketjen Black and 50 g of zirconia balls (particle size 5 mm) are placed in a 500 cc plastic container. A positive electrode active material composed of LiFePO ₄ coated with ketjen black was obtained and evaluated in the same manner as in Example 3 except for mixing.
From the X-ray diffraction, when Ketjen Black was added, the raw materials Fe ₃ (PO ₄ ) ₂ and Li ₃ PO ₄ were detected, and a single phase of the target compound LiFePO ₄ was not obtained.
It is considered that this is because if the carbon conductive material is added by the conventional manufacturing method, the reaction of the raw material during the heat treatment is hindered.

(Battery evaluation)
The positive electrode active materials obtained in Examples 3 to 4 and Comparative Examples 3 to 6 prepared as described above were vacuum dried, and 50% by mass of the positive electrode active material, 33% by mass of graphite powder, and polytetra Fluoroethylene (PTFE) 17% by mass was mixed to form a positive electrode agent and pressed to produce a positive electrode film (positive electrode plate).
Using this positive electrode plate, a coin-type cell 2032 was produced in a glove box.
At this time, a metal lithium foil was used for the negative electrode, and an electrolytic solution in which 1 mol of LiClO ₄ was dissolved in 1 liter of an equal volume mixture of ethylene carbonate and diethyl carbonate in a volume ratio of 1: 1 was used.
The produced lithium secondary battery was operated at room temperature at a charge of 0.2 mA / cm ² , 60 minutes of rest, 4.5 V, discharge of 0.2 mA / cm ² , 1.5 V, and the initial discharge capacity and discharge after 10 cycles. The capacity was measured.
The obtained battery evaluation results are summarized in Table 2.

From the results of Table 2, the positive electrode active material of the present invention showed a value close to the theoretical discharge capacity (170 mAh / g) of LiFePO ₄ , and a lithium secondary battery having an extremely high discharge capacity was obtained.
On the other hand, in Comparative Example 3 in which the positive electrode active material particles are coarsened by the high-temperature heat treatment, the discharge capacity is low because of low conductivity.
In Comparative Example 4 where the heat treatment temperature is low, the capacity is remarkably low, and it is considered that a good crystal structure is not obtained.
_Further, by using Fe 2 _{(PO 4) 3} and _Li 2 PO _4, the battery capacity of Comparative Example 5 without addition of carbon-based material obtained by the conventional method, low.
Further, in Comparative Example 6, in addition to Comparative Example 6 in which ketjen black was added during mixing, the olivine single-phase structure was not achieved, and the battery capacity was low.

The positive electrode active material for a lithium secondary battery of the present invention is composed of uniform fine olivine-type lithium iron phosphate (LiFePO ₄ ) particles, and a battery composed of a positive electrode using the positive electrode active material of the present invention is made of LiFePO ₄ . Since it shows a value close to the theoretical discharge capacity and exhibits excellent characteristics, the industrial applicability is high.
Moreover, the manufacturing method of the positive electrode active material for lithium secondary batteries of this invention can obtain the said positive electrode active material with a high yield easily, without using a toxic substance, The industrial value is very large.

Claims (7)

It consists of single-phase primary particles of olivine type lithium iron phosphate and secondary particles in which the primary particles are aggregated, and the diffraction peak half-value width of (311) plane in X-ray diffraction is 0.1 to 0.3 ° and A precursor used as a raw material for producing a positive electrode active material for a lithium secondary battery having an average particle diameter of 0.05 to 0.5 μm as determined from observation with a scanning electron microscope ,
It is obtained by mixing a mixed aqueous solution (a) containing a divalent iron salt and phosphoric acid and a lithium aqueous solution (b) containing lithium hydroxide to crystallize a lithium iron phosphate composition, In addition, a positive electrode active material precursor for a lithium secondary battery, wherein a diffraction peak derived from the lithium iron phosphate composition is undetected in X-ray diffraction. The positive electrode active material precursor for a lithium secondary battery according to claim 1 , wherein the sodium content is 0.5 mass% or less. A mixed aqueous solution (a) containing a divalent iron salt and phosphoric acid and a lithium aqueous solution (b) containing lithium hydroxide are mixed to form a reaction aqueous solution (c), and the lithium aqueous solution (b) The crystallization step of crystallizing the lithium iron phosphate composition by controlling the aqueous reaction solution (c) after mixing so as to be in the range of pH 8 to 9 and reacting, and the lithium iron phosphate composition The method for producing a positive electrode active material precursor for a lithium secondary battery according to claim 1 or 2 , comprising a drying step of washing the product with water and drying in a non-oxidizing atmosphere. The method for producing a positive electrode active material precursor for a lithium secondary battery according to claim 3 , wherein an alkali metal hydroxide is further added to the aqueous reaction solution (c) for the pH control. Method for producing a lithium secondary battery positive electrode active material precursor according to claim 3 or 4 wherein the divalent iron salt characterized in that it is a ferrous sulfate hydrate (FeSO 4 · 7H _{2 O)} . The positive electrode active material precursor for a lithium secondary battery according to claim 1 or 2 is mixed with a conductive carbonaceous material product to form a mixture, and then heated at 350 to 700 ° C in an inert or reducing atmosphere. And producing a positive electrode active material for a lithium secondary battery. The method for producing a positive electrode active material for a lithium secondary battery according to claim 6 , wherein the precursor is dry-type or wet-pulverized before the heat treatment.