WO2013099409A1 - Method for producing iron phosphate, lithium iron phosphate, electrode active material, and secondary battery - Google Patents

Abstract

A mixed aqueous solution, in which a phosphorus source such as H₃PO₄, a divalent Fe compound such as FeSO₄·7H₂O and an oxidant such as H₂O₂ are mixed at a predetermined ratio, is prepared. A pH regulating agent having a pH of 6-9 (preferable a pH of 6.5-7.5) is introduced into the mixed aqueous solution, thereby increasing the pH of the solution to 1.5 or higher within 60 seconds and producing a FePO₄ sediment powder. This FePO₄ sediment powder is filtered, cleaned and dried so as to be formed into a powder, and then synthesized with a lithium compound such as Li(CH₃COO), thereby obtaining LiFePO₄. An electrode active material which is mainly composed of this LiFePO₄ is used for a positive electrode (4) of a secondary battery. Consequently, there are provided: a method for producing iron phosphate, by which iron phosphate in the form of fine particles having a uniform particle size distribution can be easily obtained; lithium iron phosphate which is obtained using the production method; an electrode active material which is mainly composed of this lithium iron phosphate; and a secondary battery which contains the electrode active material in the positive electrode.

Description

Method for producing iron phosphate, lithium iron phosphate, electrode active material, and secondary battery

The present invention relates to a method for producing iron phosphate, lithium iron phosphate, an electrode active material, and a secondary battery, and more specifically, a method for producing iron phosphate as a raw material for lithium iron phosphate, and the production method. The present invention relates to lithium iron phosphate using iron phosphate, an electrode active material mainly composed of this lithium iron phosphate, and a secondary battery including the electrode active material in a positive electrode.

With the expansion of the market for portable electronic devices such as mobile phones, notebook computers, and digital cameras, secondary batteries with high energy density and long life are expected as cordless power sources for these electronic devices.

In order to meet such demands, secondary batteries using an alkali metal ion such as lithium ion as a charge carrier and utilizing an electrochemical reaction accompanying the charge transfer have been developed. In particular, lithium ion secondary batteries having a high energy density are now widely used.

Among the constituent elements of the secondary battery, the electrode active material is a substance that directly contributes to the battery electrode reaction such as the charge reaction and the discharge reaction, and has the central role of the secondary battery. That is, the battery electrode reaction is a reaction that occurs with the transfer of electrons by applying a voltage to an electrode active material that is electrically connected to an electrode disposed in the electrolyte, and proceeds during charging and discharging of the battery. To do. Therefore, as described above, the electrode active material has a central role of the secondary battery in terms of system.

In the lithium ion secondary battery, a lithium-containing transition metal oxide is used as a positive electrode active material, a carbon material is used as a negative electrode active material, and lithium ion insertion reaction and desorption reaction with respect to these electrode active materials. Charging and discharging is performed using

Conventionally known lithium-containing transition metal oxides include lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), lithium manganate (LiMn ₂ O ₄ ), and the like. Among these, LiCoO ₂ is widely adopted because it has better charge / discharge characteristics and energy density than LiMn ₂ O _{4 and the} like.

However, LiCoO ₂ has a problem that it has high resource constraints and is expensive and contains highly toxic Co. Moreover, since LiCoO ₂ releases a large amount of oxygen at a temperature of about 180 ° C., a lithium ion battery using a flammable organic electrolyte has a problem in terms of safety. For this reason, when LiCoO ₂ is used as the electrode active material, it is suitable for a small capacity secondary battery, but there are many problems to be solved when it is used for a high output and large capacity secondary battery.

Therefore, in recent years, lithium iron phosphate (LiFePO ₄ ) having an olivine crystal structure has attracted attention as an electrode active material for lithium ion secondary batteries. This LiFePO ₄ contains phosphorus (P) as a constituent element, and all oxygen is strongly covalently bonded to phosphorus. For this reason, it does not release oxygen even at high temperatures, has excellent thermal stability, and is suitable for application to an electrode active material for secondary batteries with high output and large capacity.

As a method for synthesizing this LiFePO ₄ , conventionally, a solid phase method, a hydrothermal synthesis method, a coprecipitation method, a sol-gel method, and the like are known. It is possible to synthesize.

Non-Patent Document 1 reports the effect of morphological characteristics on the electrochemical behavior of a LiFePO ₄ -carbon composite having a high tap density produced by a coprecipitation method.

In the non-patent document 1, and _H 3 PO ₄ containing the _{Fe (NO) 3 · 9H 2} O and pentavalent P is a trivalent Fe salt was used as a starting material, and using a co-precipitation method FePO ₄ • nH ₂ O is synthesized.

That is, in this non-patent document 1, the reaction temperature is controlled to 50 ° C., NH ₄ OH is dropped into a mixed aqueous solution in which Fe (NO) ₃ .9H ₂ O and H ₃ PO ₄ are dissolved for 24 hours. The reaction vessel was stirred at a stirring speed of 1000 rpm while adjusting the pH to 2.0 M and the pH to 5.0, thereby obtaining FePO ₄ · nH ₂ O precipitated powder (hereinafter referred to as “FePO ₄ precipitated powder”). Yes.

Then, the FePO ₄ precipitate powder under an argon atmosphere, and heat-treated at a temperature of 550 ° C. 10 hours, after the water of hydration desorbed to produce a FePO ₄ anhydride, comprising the FePO ₄ anhydride Li source Li ₂ CO ₃ and sucrose as a C coating source are mixed and fired in an Ar—H ₂ atmosphere at a firing temperature of 650 to 850 ° C. for 15 hours, thereby obtaining a LiFePO ₄ -carbon composite.

LiFePO ₄ is a material suitable as a positive electrode material of a lithium ion battery as described above. Further, the rate characteristics of the lithium ion battery greatly depend on the particle size of LiFePO ₄ , and the better the fineness and uniformity of the particle size, the larger the charge / discharge capacity, and the higher the capacity maintenance rate during high rate charge / discharge. It is possible.

Then, the particle size and particle size distribution of the LiFePO _4, since it largely depends on the particle size and particle size distribution of the FePO ₄ precipitate powder as a precursor, the control of particle size and particle size distribution of the FePO ₄ precipitate powder, very It becomes important.

However, the particle size and particle size distribution of the FePO ₄ precipitated powder are easily influenced by synthesis conditions such as the dropping rate of the pH adjusting agent, the stirring method and stirring rate of the mixed aqueous solution, and the reaction temperature. Therefore, in order to synthesize the desired FePO ₄ precipitated powder described above, it is necessary to optimize these synthesis conditions and strictly manage the synthesis conditions.

In particular, as in Non-Patent Document 1, when NH ₄ OH is dropped into a mixed aqueous solution in which Fe ³⁺ and P ⁵⁺ are dissolved, the pH around the dropping temporarily increases. Fe (OH) ₃ is preferentially generated over FePO ₄ . And once produced Fe (OH) ₃ is not easily changed to FePO _4, and thus the obtained precipitated powder becomes a mixture of FePO ₄ and Fe (OH) _3, and Fe and P are not uniformly dispersed. Dispersion unevenness occurs, the particle size distribution varies widely, and the shape is uneven.

Thus, conventionally, there has been a situation in which a method for producing FePO ₄ having excellent fine particle size and particle size uniformity cannot be found.

The present invention has been made in view of such circumstances, a method of manufacturing a iron phosphate which can be obtained fine and iron phosphate having a uniform particle size distribution (FePO ₄₎ easily, using this production method An object of the present invention is to provide lithium iron phosphate (LiFePO ₄ ) obtained in this manner, an electrode active material mainly composed of this lithium iron phosphate, and a secondary battery including the electrode active material in a positive electrode.

The present inventor put a pH adjuster into a mixed aqueous solution in which an iron compound containing phosphorus (P) source and iron (hereinafter referred to as “Fe compound”) was dissolved, and conducted earnest research. When the pH is less than a certain threshold, the precipitation yield is low, and the precipitated iron phosphate is chemically unstable, and the precipitated powder tends to grow and become coarse, but when the pH exceeds the above threshold, The present inventors have found that the precipitation yield of iron phosphate can be improved rapidly, and a fine powdered iron phosphate powder having good uniformity in particle size can be obtained.

Therefore, by removing the low pH region below the threshold value in a short time and setting the mixed aqueous solution to a pH higher than the threshold value at an early stage, it is possible to obtain iron phosphate precipitated powder with fine particles and good particle size uniformity. it is conceivable that.

The present invention has been made on the basis of such knowledge, and the method for producing iron phosphate according to the present invention comprises introducing a pH adjuster into a mixed aqueous solution in which a phosphorus source and an Fe compound are dissolved. It is characterized in that iron phosphate is produced by reaching a predetermined pH over time.

Further, in the method for producing iron phosphate of the present invention, the pH adjuster preferably has a pH of 6 to 9, and more preferably a pH of 6.5 to 7.5.

In the method for producing iron phosphate of the present invention, the predetermined pH is preferably 1.5 or more.

Furthermore, in the method for producing iron phosphate of the present invention, the short time is preferably within 60 seconds.

In the method for producing iron phosphate of the present invention, the pH adjuster is selected from ammonium acetate, sodium lactate, sodium hydrogen tartrate, ammonium hydrogen tartrate, disodium maleate, ammonium chloride, and ammonium formate. It is preferable to include at least one kind.

Moreover, in the method for producing iron phosphate of the present invention, it is preferable that the pH adjuster is in a liquid state.

Thus, the pH adjusting agent can be poured into the mixed aqueous solution at once, and the mixed aqueous solution can be efficiently reached the predetermined pH in a short time.

Furthermore, in the method for producing iron phosphate of the present invention, the Fe compound is preferably produced by oxidizing at least one of iron (II) sulfate and iron (II) chloride in the mixed aqueous solution.

In the method for producing iron phosphate of the present invention, the oxidation treatment is preferably performed using an oxidizing agent containing hydrogen peroxide.

Moreover, the lithium iron phosphate according to the present invention is characterized in that iron phosphate and lithium compound produced by any one of the production methods described above are synthesized.

Moreover, the electrode active material according to the present invention is an electrode active material used as an active material of a secondary battery that repeats charging and discharging by a battery electrode reaction, and is characterized by being mainly composed of the above-described lithium iron phosphate. Yes.

Further, the secondary battery according to the present invention has a positive electrode, a negative electrode, and an electrolyte, and the positive electrode is formed of the electrode active material described above.

According to the above iron phosphate production method, a pH adjuster having a pH of preferably 6 to 9, more preferably 6.5 to 7.3 is preferably added to a mixed aqueous solution in which a phosphorus source and an Fe compound are dissolved. Since iron phosphate is produced by reaching a predetermined pH (preferably 1.5 or more) in a time (preferably within 60 seconds), iron phosphate having fine particles and good uniformity in particle size can be obtained.

That is, the mixed aqueous solution exhibits a low pH of 1 or less before the pH adjusting agent is added. In this low pH region, the precipitation yield is low, and the produced FePO ₄ is chemically unstable and the nucleation rate is slow. Therefore, grain growth is likely to occur, and large grains are formed in the FePO ₄ precipitated powder. Easy to mix particles of diameter.

Therefore, in the method for producing iron phosphate, a pH adjusting agent is added to the mixed aqueous solution and allowed to reach a predetermined pH in a short time, so that the precipitation yield is low and the chemically unstable state is short. Thus, it is possible to promote the synthesis reaction and thereby obtain iron phosphate having fine particles and good uniformity in particle size.

Moreover, according to the said lithium iron phosphate, since the iron phosphate manufactured by the said manufacturing method and a lithium compound are synthesize | combined, the highly purified lithium phosphate suitable for the electrode active material for secondary batteries is obtained. be able to.

Further, according to the electrode active material of the present invention, the electrode active material used as the active material of the secondary battery that repeats charging and discharging by the battery electrode reaction is mainly composed of the lithium iron phosphate, so that it is safe and high energy. An electrode active material having a density can be obtained.

In addition, according to the secondary battery of the present invention, the secondary battery has a positive electrode, a negative electrode, and an electrolyte, and the positive electrode is formed of the electrode active material. A battery can be obtained.

It is sectional drawing which shows one Embodiment of the coin-type battery as a secondary battery which concerns on this invention. _{2 is} a SEM image of FePO _{4 in} an example sample of Example 1. FIG. _{2 is} a SEM image of FePO ₄ in a comparative example sample of Example 1. _{2 is} a SEM image of LiFePO _{4 in} an example sample of Example 1. FIG. _{2 is} a SEM image of LiFePO ₄ in a comparative example sample of Example 1. FIG.

Next, an embodiment for carrying out the present invention will be described in detail.

In the iron phosphate according to the present invention, a pH adjuster is introduced into a mixed aqueous solution in which a phosphorus (P) source and an Fe compound are dissolved, and a predetermined pH is reached in a short time to produce FePO ₄ . Thus, high-purity FePO ₄ having fine particles and good uniformity in particle size can be produced with high efficiency.

That is, the mixed aqueous solution in which the P source such as H ₃ PO ₄ and the Fe compound are dissolved has a pH of 1 or less. However, when a pH adjuster is added to this mixed aqueous solution, the pH of the mixed aqueous solution is 1.3. When the temperature rises to the extent, production of FePO ₄ is started. In a low pH region where the precipitation yield is low, for example, in a region where the pH is less than 1.5, FePO ₄ is chemically unstable, and the nucleation rate of the precipitate is slow. The resulting precipitated powder tends to increase in particle size.

Since the precipitated powder generated in the pH region where the precipitation yield is low as described above is easy to grow and coarsen, if the time in the low pH region becomes long, the coarsened particles in the synthesized FePO ₄ precipitated powder. Are present, the particle size distribution is widened, and the variation in particle size is increased.

However, the precipitation yield of FePO ₄ is dramatically improved as the pH is increased. When the pH is 1.5, the precipitation yield is about 90%, and when the pH is increased to 1.8, the precipitation yield is decreased. About 98%.

Therefore, in this embodiment, the low pH region with a low precipitation yield is removed early to reach a predetermined pH in a short time, thereby obtaining FePO ₄ having fine particles and good uniformity in particle size.

Here, the predetermined pH is not particularly limited as long as FePO ₄ having fine particles and good uniformity in particle size can be obtained with a high precipitation yield, but the pH reaches 1.5 as described above. Then, since a precipitation yield improves dramatically to 90%, 1.5 or more are preferable, More preferably, it is 2.0.

In addition, as to the time until the pH is reached after the pH adjusting agent is added, as the time in the low pH region becomes longer as described above, the time during which the precipitated FePO ₄ is chemically unstable becomes longer. Therefore, a time as short as possible is desirable, and it is preferably within 60 seconds.

Furthermore, the pH adjusting agent may be either liquid or powdery, but from the viewpoint of quickly reaching the predetermined pH in a short time, a liquid in which a predetermined amount of the pH adjusting agent can be poured into the mixed aqueous solution at a stretch is preferable.

The pH value of the pH adjuster is not particularly limited, but a pH adjuster having a pH of 6 to 9 is preferably used, and more preferably 6.5 to 7.5.

That is, since the precipitated powder of FePO ₄ is an amorphous hydrate (FePO ₄ .nH ₂ O), chemical decomposition is likely to occur when it comes into contact with a strong alkaline substance having a pH of 9 or more in the mixed aqueous solution. . That is, when a strong alkaline substance such as NH ₄ OH (pH: about 11) having a pH exceeding 9 is used as a pH adjuster as in Non-Patent Document 1, Fe (OH) ₃ is mixed in the precipitated powder, There is a risk of segregation of Fe and P in FePO ₄ .

Therefore, the pH of the pH adjuster is preferably 6 to 9, and more preferably 6.5 to 7.5.

As such a pH adjuster, ammonium acetate, sodium lactate, sodium hydrogen tartrate, ammonium hydrogen tartrate, disodium maleate, ammonium chloride, ammonium formate and the like can be preferably used.

Specifically, the FePO ₄ can be produced by the following method.

First, an Fe compound containing divalent Fe such as FeSO ₄ .7H ₂ O and FeCl ₂ .4H ₂ O (hereinafter referred to as “divalent Fe compound”), H ₃ PO ₄ , (NH ₄ ) H Prepare a P source such as ₂ PO ₄ , (NH ₄ ) ₂ HPO ₄ , and an oxidizing agent such as H ₂ O ₂ , and mix them so that they are in a predetermined ratio to prepare a mixed aqueous solution having a pH of 1 or less. . Here, the divalent Fe compound and the P source are mixed so that the molar ratio is equal to or substantially equal, and the oxidizing agent is such that the divalent Fe is completely oxidized to Fe in the trivalent state. It is preferable to contain an excess of the divalent Fe compound (for example, about 1.5 times in molar ratio).

Next, a pH adjuster having a pH of preferably 6 to 9, more preferably 6.5 to 7.5 is prepared.

Next, when the entire amount of the pH adjuster is charged into the mixed aqueous solution, a predetermined amount of the pH adjuster is weighed so that the predetermined pH is obtained.

And the whole quantity of this pH adjuster is rapidly thrown into the said mixed aqueous solution. In this case, the powdery pH adjusting agent may be dissolved in an aqueous solution and then charged into the mixed aqueous solution, or the powdered pH adjusting agent may be directly charged into the mixed aqueous solution. Then, the pH of the mixed aqueous solution suddenly increases in a short time (for example, within 60 seconds) after the pH adjusting agent is added, reaches a predetermined pH (for example, 1.5), and may cause grain growth. Thus, the coarsening of the particles can be suppressed, and a brown FePO ₄ precipitated powder having fine particles and good uniformity in particle size can be obtained.

Then, this FePO ₄ precipitated powder is filtered, washed, and dried, thereby obtaining FePO ₄ .

High-purity lithium iron phosphate (LiFePO ₄ ) can be obtained using FePO ₄ thus produced.

That is, these FePO ₄ and lithium compound are weighed so that the molar ratio of FePO ₄ and lithium compound is 1: 1, and this weighed product is ball milled together with pure water and a polymer dispersant such as polycarboxylic acid. And mixed and pulverized to obtain a slurry-like mixed powder.

Here, the lithium compound is not particularly limited, and for example, CH ₃ COOLi · 2H ₂ O, LiOH · H ₂ O, or the like can be used.

Also, from the viewpoint of ensuring electronic conductivity, it is preferable to add a carbon source such as sucrose to the mixed powder and coat the surface of the mixed powder with carbon.

Next, the mixed powder is dried and granulated, and then heat-treated at a predetermined temperature (for example, 500 to 700 ° C.) for about 5 hours in a predetermined reducing atmosphere. Thereby, trivalent Fe is reduced to divalent, and LiFePO ₄ is obtained.

The LiFePO ₄ thus obtained has high purity, fine particles and good particle size uniformity, and can be suitably used as an electrode active material for a secondary battery. In addition, it is possible to realize a high-capacity, high-output secondary battery that is inexpensive and easily available, and that is excellent in safety, at a low cost, without the resource restrictions such as Co.

Next, a secondary battery using the electrode active material will be described in detail.

FIG. 1 is a cross-sectional view showing a coin-type secondary battery as an embodiment of a secondary battery according to the present invention. In this embodiment, an electrode active material mainly composed of LiFePO ₄ is used as a positive electrode active material. Used for substances.

The battery can 1 has a positive electrode case 2 and a negative electrode case 3, and both the positive electrode case 2 and the negative electrode case 3 are formed in a disk-like thin plate shape. Moreover, the positive electrode 4 which formed the electrode active material in the sheet form is distribute | arranged to the center of the bottom part of the positive electrode case 2 which comprises a positive electrode electrical power collector. A separator 5 formed of a porous film such as polypropylene is laminated on the positive electrode 4, and a negative electrode 6 is further laminated on the separator 5. As the negative electrode 6, for example, one obtained by superimposing a lithium metal foil on Cu or one obtained by applying a lithium storage material such as graphite or hard carbon to the metal foil can be used. A negative electrode current collector 7 made of Cu or the like is laminated on the negative electrode 6, and a metal spring 8 is placed on the negative electrode current collector 7. Further, the electrolyte 9 is filled in the internal space, and the negative electrode case 3 is fixed to the positive electrode case 2 against the urging force of the metal spring 8 and is sealed through the gasket 10.

Next, an example of a method for manufacturing the secondary battery will be described in detail.

First, LiFePO ₄ as a main component of the electrode active material is formed into an electrode shape. For example, LiFePO ₄ is mixed with a conductive additive and a binder, a solvent is added to form a slurry, the slurry is applied on the positive electrode current collector by an arbitrary coating method, and dried to form the positive electrode 4. Form.

Here, the conductive auxiliary agent is not particularly limited, for example, carbonaceous fine particles such as graphite, carbon black, and acetylene black, vapor grown carbon fibers, carbon nanotubes, carbon fibers such as carbon nanohorns, polyaniline, Conductive polymers such as polypyrrole, polythiophene, polyacetylene, and polyacene can be used. Further, two or more kinds of conductive assistants can be mixed and used. The content of the conductive auxiliary agent in the positive electrode 4 is preferably 10 to 80% by weight.

Also, the binder is not particularly limited, and various resins such as polyethylene, polyvinylidene fluoride, polyhexafluoropropylene, polytetrafluoroethylene, polyethylene oxide, carboxymethylcellulose, and the like can be used.

Further, the solvent is not particularly limited, and examples thereof include basic solvents such as dimethyl sulfoxide, dimethylformamide, N-methyl-2-pyrrolidone, propylene carbonate, diethyl carbonate, dimethyl carbonate, and γ-butyrolactone, acetonitrile, Nonaqueous solvents such as tetrahydrofuran, nitrobenzene, and acetone, and protic solvents such as methanol and ethanol can be used.

Also, the type of solvent, the compounding ratio between the organic compound and the solvent, the type of additive and the amount of the additive, etc. can be arbitrarily set in consideration of the required characteristics and productivity of the secondary battery.

Next, the positive electrode 4 is impregnated in the electrolyte 9 so that the positive electrode 4 is impregnated with the electrolyte 9, and then the positive electrode 4 is placed on the positive electrode current collector at the bottom center of the positive electrode case 2. Next, the separator 5 impregnated with the electrolyte 9 is laminated on the positive electrode 4, the negative electrode 6 and the negative electrode current collector 7 are sequentially laminated, and then the electrolyte 9 is injected into the internal space. Then, a metal spring 8 is placed on the negative electrode current collector 9 and a gasket 10 is arranged on the periphery, and the negative electrode case 3 is fixed to the positive electrode case 2 by a caulking machine or the like, and the outer casing is sealed. A type secondary battery is produced.

Incidentally, the electrolyte 9, performs the charge carrier transport between being interposed both electrodes between the anode 6 which is a counter electrode of the positive electrode 4 and the positive electrode 4, as such a electrolyte 9, 10 at room temperature ^- Those having an electric conductivity of ⁵ to 10 ⁻¹ S / cm can be used. For example, an electrolytic solution in which an electrolyte salt is dissolved in an organic solvent can be used.

Here, as the electrolyte salt, for example, LiPF ₆ , LiClO ₄ , LiBF ₄ , LiCF ₃ SO ₃ , Li (CF ₃ SO ₂ ) ₂ , Li (C ₂ F ₅ SO ₂ ) ₂ N, Li (CF ₃ SO ₂ ) ₃ C, Li (C ₂ F ₅ SO ₂ ) ₃ C, or the like can be used.

As the organic solvent, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, γ-butyrolactone, tetrahydrofuran, dioxolane, sulfolane, dimethylformamide, dimethylacetamide, N-methyl-2-pyrrolidone, etc. are used. be able to.

Thus, according to the present embodiment, it is possible to realize a secondary battery having a large capacity, high output, and excellent safety at low cost.

In addition, this invention is not limited to the said embodiment, A various deformation | transformation is possible in the range which does not deviate from a summary. For example, in the above embodiment, in the process of manufacturing FePO ₄ , a divalent Fe compound and an oxidizing agent are mixed to oxidize divalent Fe to trivalent Fe to obtain a trivalent Fe compound. However, the oxidation treatment method is not particularly limited. Further, without oxidizing the divalent Fe compound to obtain a trivalent Fe compound, may be used trivalent Fe compounds from the beginning, as a Fe compound in this case, for example, FeCl 3 _· 6H ₂ O or the like can be used.

In the above embodiment, the coin-type secondary battery has been described. However, the battery shape is not particularly limited, and can be applied to a cylindrical type, a square type, a sheet type, and the like. Also, the exterior method is not particularly limited, and a metal case, mold resin, aluminum laminate film, or the like may be used.

Next, specific examples of the present invention will be described.

[Preparation of Example Sample]
(Production of FePO ₄ )
FeSO ₄ · 7H ₂ O was dissolved in water, and a mixed aqueous solution was prepared by adding H ₃ PO ₄ (85% aqueous solution) and H ₂ O ₂ (30% aqueous solution) as a P source. Here, FeSO ₄ · 7H ₂ O, H ₃ PO ₄ , and H ₂ O ₂ were mixed so as to have a molar ratio of 1: 1: 1.5.

By adding H ₂ O ₂ , Fe ²⁺ was oxidized to Fe ³⁺ , and the mixed aqueous solution changed color from blue-green to dark brown. When pH meters were inserted into a plurality of locations in the reaction vessel containing the mixed aqueous solution and the pH of the mixed aqueous solution was measured, the pH was 1.0 at all measurement locations.

Next, powdered ammonium acetate was prepared as a pH adjuster. Next, a predetermined amount of ammonium acetate was weighed so that the pH was 2.0 when all of the weighed ammonium acetate was put into the mixed aqueous solution. And this ammonium acetate was dissolved in water, and ammonium acetate aqueous solution was produced.

Next, while stirring the mixed aqueous solution, a predetermined amount of aqueous ammonium acetate solution was added in about 10 seconds. Then, immediately after the start of the addition of the aqueous ammonium acetate solution, the production of brown FePO ₄ precipitated powder started. After 15 seconds from the start of the input, the pH became 2.0 at all the measurement points of the mixed aqueous solution.

Thereafter, the FePO ₄ precipitated powder was filtered and washed, and then dried and powdered to obtain FePO ₄ .

(Preparation of LiFePO ₄ )
The FePO ₄ in in _CH 3 COOLi · _2H 2 O (lithium dihydrate acid) and the molar ratio of 1: formulated to be 1, further LiFePO _4: the 7 parts by weight per 100 parts by weight As described above, sucrose as a carbon source was weighed, pure water and a polycarboxylic acid polymer dispersant were added thereto, and the mixture was pulverized using a ball mill to obtain a slurry-like mixed powder.

Next, this mixed powder was dried with a spray dryer, granulated, adjusted to a reducing atmosphere having an oxygen partial pressure of 10 ⁻²⁰ MPa using a mixed gas of H ₂ —N ₂ , and maintained at a temperature of 700 ° C. for 5 hours. Then, heat treatment was performed to obtain LiFePO ₄ .

(Production of secondary battery)
LiFePO ₄ produced as described above, acetylene black as a conductive auxiliary agent, and polyvinylidene fluoride as a binder were prepared. Then, these LiFePO ₄ , acetylene black, and polyvinylidene fluoride are weighed and mixed so that the weight ratio is 88: 6: 6, and this is dispersed in N-methyl-2-pyrrolidone as a solvent. A slurry was prepared.

Next, this slurry was applied on an aluminum foil having a thickness of 20 μm so as to be 6 mg / cm ² , dried at a temperature of 140 ° C., and then pressed at a pressure of 98 MPa, whereby an electrode sheet was produced, and the diameter was further increased. The positive electrode was punched to 12 mm.

Next, this positive electrode was impregnated with an electrolytic solution, and the electrolytic solution was infiltrated into voids in the positive electrode. As the electrolytic solution, an ethylene carbonate / diethyl carbonate mixed solution, which is an organic solvent containing LiPF ₆ (electrolyte salt) having a molar concentration of 1.0 mol / L, was used. In addition, the mixing ratio of ethylene carbonate and diethyl carbonate was ethylene carbonate: diethyl carbonate = 3: 7 by volume%.

Next, this positive electrode was placed on a positive electrode current collector, and a separator having a thickness of 20 μm made of a polypropylene porous film impregnated with the electrolytic solution was laminated on the positive electrode. The negative electrode to which was attached was laminated on the separator.

Next, after laminating a negative electrode current collector made of Cu on the negative electrode, injecting an electrolyte into the internal space, and then placing a metal spring on the negative electrode current collector and arranging a gasket on the periphery The negative electrode case was joined to the positive electrode case and sealed with a caulking machine, thereby producing a secondary battery having a diameter of 20 mm and a thickness of 3.2 mm having LiFPO ₄ as the positive electrode active material and metallic lithium as the negative electrode active material. .

[Production of Comparative Sample]
Example sample, except that the ammonium acetate aqueous solution charging speed was slowed and the time for reaching the pH of 2.0 at all measurement locations after the start of the ammonium acetate aqueous solution solution was 300 seconds (5 minutes) FePO ₄ , LiFePO ₄ , and a secondary battery were produced by the same method and procedure as in Example 1.

(Sample evaluation)
Using a scanning electron microscope (hereinafter referred to as “SEM”), FePO ₄ produced from the example sample and the comparative example sample was observed.

FIG. 2 is an SEM image of FePO _{4 in} the example sample, and FIG. 3 is an SEM image of FePO ₄ in the comparative example sample.

As shown in FIG. 3, in the FePO ₄ sample of the comparative example, coarse particles are mixed in the fine particles. This is because the time until the pH of the mixed aqueous solution reaches 2.0 (hereinafter referred to as “arrival time”) is as long as 300 seconds after the start of the addition of the aqueous ammonium acetate solution, and a chemically unstable low pH region is obtained. It cannot be removed in a short time, and thus the precipitated FePO ₄ partially grows, and it seems that coarse particles are mixed in FePO ₄ .

On the other hand, the FePO ₄ of the example sample had a short arrival time of 15 seconds, and as shown in FIG. 2, the presence of coarse particles was not observed, and it was confirmed that the particle size uniformity was also good. . That is, the time until the pH at which the precipitation yield of FePO ₄ is increased to reach 2.0 (arrival time) is as short as 15 seconds after addition of the aqueous ammonium acetate solution. It seems that the generation of enlarged particles could be suppressed. That is, it is considered that the nucleation rate of FePO ₄ is high in the pH range where the precipitation yield is high, but most of the FePO ₄ is synthesized in the pH range of 1.5 to 2.0. It seems that FePO ₄ having a uniform particle size distribution was obtained.

Next, for each LiFePO ₄ of the example sample and the comparative example sample, an X-ray diffraction spectrum was measured using an X-ray diffractometer, and the constituent phases were identified. As a result, it was confirmed to be a single phase of LiFePO ₄ . .

Next, each LiFePO ₄ of the example sample and the comparative example sample was observed using SEM.

FIG. 4 is an SEM image of LiFePO _{4 in} the example sample, and FIG. 5 is an SEM image of LiFePO ₄ in the comparative example sample.

In the comparative sample, coarse particles are mixed in FePO _{4 as} a precursor. Therefore, as shown in FIG. 5, coarse particles are also mixed in LiFePO _{4 of the} comparative sample.

On the other hand, since FePO _{4 of the} example sample was fine and the uniformity of particle size was good, as shown in FIG. 4, it was found that LiFePO _{4 of the} example sample was also fine and uniform in particle size. .

Then, for each LiFePO ₄ of example samples and comparative example samples, measured the amount of carbon in the CS meter, the specific surface area was measured by further BET method.

In addition, the secondary battery manufactured as described above was charged in a constant temperature bath at 25 ° C. with a voltage range of 2.0 to 4.2 V and charge / discharge rates of 0.2 C and 5 C (1 C is charged in 1 hour). Or the amount of current until the discharge is completed). That is, the battery was charged until the voltage reached 4.2 V at each charging rate of 0.2 C and 5 C, and then discharged until the voltage reached 2.0 V at each discharging rate of 0.2 C and 5 C. .

And each capacity density of charge and discharge in each charge and discharge rate, and capacity maintenance rate were calculated. Here, the capacity retention rate was determined as a ratio of the charge / discharge capacity density when the charge / discharge rate was 5C, based on the charge / discharge capacity density of 0.2C.

Table 1 shows the arrival times and measurement results of the samples of Examples and Comparative Examples.

Since the comparative example sample has a small specific surface area of 14.7 m ² / g, the charge capacity is as low as 145.6 mAh / g, the discharge capacity is as low as 144.8 mAh / g, the charge capacity maintenance rate is also 85.2%, and the discharge capacity. The maintenance rate was as low as 79.8%. This is because the coarse particles are mixed in LiFePO ₄ , and thus the diffusion rate of Li ^{+ in} the particles is slow, and therefore, it is difficult to insert and desorb Li ^{+ at} the center of the particles with large diameter particles. Therefore, it seems that the deterioration of battery characteristics was caused.

In contrast, the example sample has a large specific surface area of 20.9 m ² / g, fine LiFePO ₄ particles, and good particle size uniformity, so that the charge capacity is 155.1 mAh / g and the discharge capacity is 154.4 mAh. / G, high charge capacity retention rate of 95.8%, and discharge capacity retention rate of 92.2%.

Thus, in the example sample, FePO ₄ was fine and uniform in particle size, and thus LiFePO ₄ was also fine and uniform in particle size, so that it was confirmed that good rate characteristics could be obtained. .

FePO ₄ , LiFePO ₄ , and Sample Nos. 1 to 5 in the same manner and procedure as in Example 1 except that sodium lactate, ammonium hydrogen tartrate, disodium maleate, ammonium chloride, and ammonium formate were used as pH adjusters. And the secondary battery was produced sequentially.

The specific surface area, discharge capacity, and discharge capacity retention rate were measured for each of the sample numbers 1 to 5 in the same manner and procedure as in Example 1.

Table 2 shows the types and pH of pH adjusting agents, and measurement results.

As is apparent from Table 2, LiFePO ₄ having a fine particle size and good particle size uniformity can be obtained with a pH of 6.8 to 7.3 and a specific surface area of 19.8 to 20.9 m ² / g. It was. As a result, it was found that a high discharge capacity of 152 mAh / g or more and a good discharge capacity maintenance ratio of 90% or more can be obtained.

FePO ₄ , LiFePO ₄ , and secondary batteries of sample numbers 11 to 19 were sequentially prepared in the same manner and procedure as in Example 1 except that the concentration of the aqueous ammonium acetate solution was changed variously as a pH adjuster.

Then, for each of the samples Nos. 11 to 19, the pH reached was measured using a pH meter. In addition, all reached the predetermined pH within 20 seconds.

The specific surface area, discharge capacity, and discharge capacity retention rate were measured by the same method and procedure as in Example 1.

Table 3 shows the reached pH and the measurement results.

Sample No. 11 had a specific pH of as low as 16.2 m ² / g, a discharge capacity of 147 mAh / g, and a discharge capacity retention rate of 82.8% because the ultimate pH was slightly low at 1.4. That is, although the capacity characteristics and rate characteristics were good compared to the comparative example sample of Example 1 (see Table 1), it was slightly lower than that of sample numbers 12-19.

In contrast, sample Nos. 12 to 19 have an ultimate pH of 1.5 or more, so that the specific surface area is as large as 18.5 to 21.4 m ² / g, and the desired LiFePO ₄ can be obtained. As a result, 150 mAh It was found that a high discharge capacity of at least / g and a good discharge capacity maintenance rate of at least 88% can be obtained.

FePO ₄ , LiFePO ₄ , and secondary batteries of sample numbers 21 to 25 were sequentially prepared in the same manner and procedure as in Example 1 except that the charging rate of the aqueous ammonium acetate solution as the pH adjuster was changed.

The specific surface area, discharge capacity, and discharge capacity retention rate were measured for each of the samples Nos. 21 to 25 in the same manner and procedure as in Example 1.

Table 4 shows arrival times and measurement results.

Sample No. 25 had a specific surface area as small as 15.0 m ² / g because the arrival time was slightly long as 75 seconds. Therefore, the discharge capacity was 145.3 mAh / g and the discharge capacity maintenance rate was 80.2%. Although the capacity characteristics and rate characteristics were good compared to the comparative example sample of Example 1 (see Table 1), it was slightly lower than that of sample numbers 21 to 24.

On the other hand, sample numbers 21 to 24 have an arrival time of 60 seconds or less, so that the specific surface area is as high as 18.20 m ² / g or more, the discharge capacity is as high as 150 mAh / g or more, and the discharge capacity retention rate is high. Also, good results were obtained with 85% or more.

FePO ₄ , LiFePO ₄ , and secondary battery of sample number 31 were sequentially prepared in the same manner and procedure as in Example 1 except that ammonium acetate powder was used instead of the ammonium acetate aqueous solution as a pH adjuster.

Then, the pH of the sample No. 31 was measured with a pH meter, and the specific surface area, discharge capacity, and discharge capacity retention rate were measured by the same method and procedure as in Example 1.

Table 5 shows arrival times and measurement results.

As is apparent from Table 5, even when a powdery pH adjuster is used, the arrival time is 20 seconds and within the range of the present invention, so the specific surface area is as large as 20.2 m ² / g, and the discharge capacity is The result was 152.8 mAh / g, the discharge capacity retention rate was 91.8%, and good results were obtained.

High-purity FePO ₄ having fine particles and good uniformity in particle size can be obtained with high efficiency from the P source and Fe compound. By using LiFePO ₄ obtained from FePO ₄ as the positive electrode active material of the secondary battery, a secondary battery having a high charge / discharge capacity and excellent rate characteristics during charge / discharge can be obtained.

4 Positive electrode 6 Negative electrode 9 Electrolyte

Claims (12)

1. Production of iron phosphate, characterized in that a pH adjusting agent is introduced into a mixed aqueous solution in which a phosphorus source and an iron compound containing iron are dissolved, and reaches a predetermined pH in a short time to produce iron phosphate. Method.
2. The method for producing iron phosphate according to claim 1, wherein the pH adjuster has a pH of 6 to 9.
3. The method for producing iron phosphate according to claim 2, wherein the pH adjuster has a pH of 6.5 to 7.5.
4. The method for producing iron phosphate according to any one of claims 1 to 3, wherein the predetermined pH is 1.5 or more.
5. The method for producing iron phosphate according to any one of claims 1 to 4, wherein the short time is within 60 seconds.
6. The pH adjuster comprises at least one selected from ammonium acetate, sodium lactate, sodium hydrogen tartrate, ammonium hydrogen tartrate, disodium maleate, ammonium chloride, and ammonium formate. The manufacturing method of the iron phosphate in any one of Claims 5 thru | or 5.
7. The method for producing iron phosphate according to any one of claims 1 to 6, wherein the pH adjuster is liquid.
8. The said iron compound is produced | generated by oxidizing at least one of iron sulfate (II) and iron chloride (II) in the said mixed aqueous solution. Of manufacturing iron phosphate.
9. The method for producing iron phosphate according to claim 8, wherein the oxidation treatment is performed using an oxidizing agent containing hydrogen peroxide.
10. A lithium iron phosphate obtained by synthesizing an iron phosphate produced by the production method according to any one of claims 1 to 9 and a lithium compound.
11. An electrode active material used as an active material of a secondary battery that repeats charging and discharging by a battery electrode reaction,
    An electrode active material comprising mainly lithium iron phosphate according to claim 10.
12. A secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode contains the electrode active material according to claim 11.

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