TW201311547A - Iron lithium phosphate preparation method, electrode active material, and secondary battery - Google Patents

Abstract

A P source such as H3PO4, a divalent Fe compound such as FeSO4 DEG 7H2O, and an oxidizing agent such as H2O2 are mixed at a predetermined proportion to form a mixed aqueous solution. The mixed aqueous solution is dripped onto a buffer solution with a pH of between 1.5 and 9 to form a co-precipitated powder of FePO4 DEG nH2O. FePO4 DEG nH2O reacts with an acidic or neutral lithium salt to form a co-precipitated powder of LiFePO4, and sintering is performed to obtain a sintered powder of LiFePO4. A neutralized solution in which a neutralizer such as an organic acid is mixed with lithium hydroxide may be used instead of the lithium salt. The added molar amount of the neutralizer may be a molar ratio of at least 0.2 based on the molar quantity of the lithium hydroxide when the neutralizer is added to the lithium hydroxide to reach an equivalence point. According to the present invention, iron lithium phosphate suitable for an electrode active material for a secondary battery which has high rapid charging characteristics can be manufactured with ease and by means of a simple manufacturing process.

Description

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

The present invention relates to a method for producing lithium iron phosphate, an electrode active material, and a secondary battery, and more particularly to a method for producing lithium iron phosphate suitable for an electrode active material for a secondary battery, using the lithium iron phosphate The electrode active material and the secondary battery containing the electrode active material in the positive electrode.

With the expansion of the mobile electronic device market such as mobile phones, notebook computers, and digital cameras, a secondary battery having a large energy density and a long life is desired as a wireless power source for such electronic devices.

Further, in response to such a request, a secondary battery using an alkali metal ion such as lithium ion as a charged carrier and utilizing an electrochemical reaction accompanying charge transfer has been developed. In particular, lithium ion secondary batteries having a large energy density are currently widely used.

Among the constituent elements of the secondary battery, the electrode active material directly contributes to a reaction of a battery electrode such as a charging reaction or a discharge reaction, and has a central role of a secondary battery. In other words, the battery electrode reaction is performed at the time of charge and discharge of the battery by applying a voltage to the electrode active material electrically connected to the electrode disposed in the electrolyte and reacting with electrons. Therefore, as described above, the electrode active material has a central role in the secondary battery in the system.

Further, in the lithium ion secondary battery, a lithium-containing transition metal oxide is used as a positive electrode active material, and a carbon material is used as a negative electrode active material, and lithium ions are charged and discharged by insertion reaction and desorption reaction of the electrode active materials. .

Lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), lithium manganate (LiMn ₂ O ₄ ), and the like have been known as lithium-containing transition metal oxides used for the positive electrode active material. Among them, LiCoO ₂ is widely used because it has better charge and discharge characteristics or energy density than LiMn ₂ O ₄ or the like.

However, LiCoO ₂ has a problem of Co having a large resource restriction, a high price, and a high toxicity. Further, since LiCoO ₂ releases a large amount of oxygen at a temperature of about 180 ° C, there is a problem in safety in a lithium ion battery using a combustible organic electrolyte. Therefore, when LiCoO _{2 is} used for an electrode active material, it is suitable for a small-capacitance secondary battery, and when it is used for a secondary battery of high power and large capacitance, there are many problems to be solved.

Therefore, in recent years, as an electrode active material for a lithium ion secondary battery, lithium iron phosphate (LiFePO ₄ ) having an olivine crystal structure has been attracting attention. The LiFePO ₄ contains phosphorus (P) in the constituent elements, and all of the oxygen and phosphorus are strongly covalently bonded. Therefore, it is considered that it does not release oxygen even at a high temperature and is excellent in thermal stability, and is suitable for use as an electrode active material for secondary batteries of high power and large capacitance.

As a method for synthesizing the LiFePO ₄ , a solid phase method, a hydrothermal synthesis method, a coprecipitation method, a sol-gel method, and the like have been known from the prior art, and in particular, the coprecipitation method can industrially synthesize fine particles of about 0.1 μm.

Further, Patent Document 1 proposes a method for producing a lithium iron phosphorus composite oxide carbon composite containing Mn atoms, which comprises the steps of: adding an aqueous solution in which a divalent iron salt and a divalent manganese salt and phosphoric acid are dissolved; a first step of obtaining a coprecipitate containing iron, manganese and phosphorus with a base; a second step of mixing the above-mentioned co-submersible, lithium phosphate and a conductive carbon material; The third step of obtaining a reaction precursor having a specific volume of 1.5 mL/g or less by dry pulverization treatment, and the fourth step of baking the reaction precursor at 500 to 700 °C.

In the patent document 1, a divalent Fe salt such as FeSO ₄ ‧7H ₂ O, a divalent Mn salt such as MnSO ₄ ‧H ₂ O, or an aqueous solution of H ₃ PO ₄ is added to an aqueous solution of sodium hydroxide to synthesize it ( Fe, Mn) ₃ (PO ₄ ) ₂ ‧nH ₂ O co-precipitated powder, and a conductive carbon material such as Li ₃ PO ₄ and carbon black is mixed in the co-sediment, and pulverized to 1.5 mL/g or less, and then 500~ The calcination was carried out at a temperature of 700 ° C, whereby a Li-Fe-P-based composite oxide carbon composite was obtained.

Moreover, in Non-Patent Document 1, the effect of the morphological characteristics in the electrochemical behavior of the LiFePO ₄ -carbon composite having a high knocking degree by the co-precipitation method is reported.

In Non-Patent Document 1, Fe(NO) ₃ ‧9H ₂ O and H ₃ PO ₄ as a trivalent Fe salt are used as a starting material, and FePO ₄ ‧nH ₂ O is synthesized by a coprecipitation method in an argon atmosphere. The heat treatment was carried out at a temperature of 550 ° C for 10 hours to remove the hydrated water to prepare anhydrous FePO ₄ , after which the anhydrous FePO _{4 was} mixed with Li ₂ CO ₃ as a Li source and sucrose as a C coating source, in Ar-H _{2 .} The environment was calcined at a calcination temperature of 650 to 850 ° C for 15 hours, whereby a LiFePO ₄ -carbon composite was obtained.

In other words, in Non-Patent Document 1, when FePO ₄ ‧ nH ₂ O and Li ₂ CO _{3 are} directly mixed and calcined, there is a enthalpy of oxidation of FePO ₄ during calcination, and FePO ₄ ‧ nH ₂ O is used before calcination The heat treatment is performed to remove the hydrated water.

[Previous Technical Literature] [Patent Literature]

[Patent Document 1] Japanese Patent Laid-Open Publication No. 2005-047751 (Technical Solution 1)

[Non-patent literature]

[Non-Patent Document 1] Sung Woo Oh et al. "The effect of Morphological Properties on the Electrochemical Behavior of High Tap Density C-LiFePO ₄ Prepared via Coprecipitation'', Journal of the Electrochemical Society, Vol. 155, No. 6 , 2008, pp. A 414-420.

However, Patent Document 1 has a problem in that Fe in (Fe,Mn) ₃ (PO ₄ ) ₂ ‧nH ₂ O as a co-precipitated powder is divalent, so it is unstable in the atmosphere and is easily oxidized to trivalent Fe.

Further, in Non-Patent Document 1, the inventors have found that LiFePO ₄ cannot be obtained in a single phase when FePO ₄ ‧ nH ₂ O is directly mixed with Li ₂ CO ₃ and calcined without heat treatment.

Further, it was confirmed that LiFePO _{4 of a} single phase can be synthesized by heat-treating FePO ₄ ‧ nH ₂ O before mixing FePO ₄ ‧ nH ₂ O with Li ₂ CO ₃ .

However, it is considered that the reason why the above heat treatment is required is not only the oxidation of the hydrated water of the FePO ₄ ‧ nH ₂ O co-precipitated powder described in Non-Patent Document 1, but also Li ₂ CO ₃ . That is, in the case where heat treatment does not perform the above-described co-precipitated powder prior to firing, FePO ₄ ‧nH ₂ O in the water of hydration of calcined to function as an oxidizing agent, Li ₂ CO ₃ The CO ₂ produced also play a role as an oxidizing agent. As a result, it is considered that since the reduction of Fe from trivalent to divalent is further suppressed, the trivalent Fe compound remains as a hetero phase, and a single-phase high-purity LiFePO ₄ cannot be obtained.

In other words, in the case where FePO ₄ ‧ nH ₂ O and Li ₂ CO _{3 are} mixed as in Non-Patent Document 1, it is considered that heat treatment is performed before baking to remove hydrated water of FePO ₄ ‧ nH ₂ O, thereby making it possible to The oxidation source is reduced, and thereafter calcination treatment is carried out, whereby single-phase LiFePO ₄ can be synthesized.

In short, as in Non-Patent Document 1, in order to produce LiFePO _{4 by} reacting FePO ₄ ‧ nH ₂ O with Li ₂ CO ₃ , it is necessary to heat-treat the co-precipitated powder of FePO ₄ ‧ nH ₂ O and then to treat it with Li ₂ CO ₃ , followed by calcination treatment.

As described above, the method described in Non-Patent Document 1 requires at least two heat treatments, and there is a problem that the manufacturing steps are complicated.

The present invention has been made in view of such circumstances, and an object thereof is to provide a lithium iron phosphate which can easily produce a lithium iron phosphate suitable for an electrode active material for a secondary battery which has a good rapid charge and discharge characteristic in a simple manufacturing step. A production method, an electrode active material mainly composed of lithium iron phosphate obtained by using the production method, and a secondary battery containing the electrode active material in a positive electrode.

The inventors of the present invention conducted intensive studies to achieve the above object, and as a result, obtained the following knowledge: by reacting iron (III) phosphate with an acidic or neutral lithium salt solution, for example, with a lithium acetate solution, it can be easily manufactured even quickly. Lithium iron phosphate (LiFePO ₄ ) which suppresses the decrease in the retention rate of the charge and discharge capacitor can also be suppressed by charging and discharging.

The present invention is based on such a finding, and the method for producing lithium iron phosphate according to the present invention is characterized in that iron phosphate (III) is mixed with an acidic or neutral lithium salt solution and reacted, followed by calcination to synthesize phosphoric acid. Lithium iron powder.

Further, in the method for producing lithium iron phosphate according to the present invention, it is preferred that the lithium salt solution is lithium acetate.

Further, the inventors of the present invention have conducted intensive studies, and as a result, it has been found that a neutralization solution is prepared by mixing a lithium hydroxide with a neutralizing agent such as an organic acid, and the neutralization solution is reacted with iron phosphate. It is possible to manufacture lithium iron phosphate which can suppress the decrease in the retention rate of the charge and discharge capacitor even when charging and discharging are performed rapidly.

That is, the method for producing lithium iron phosphate according to the present invention is characterized in that a lithium hydroxide is mixed with a neutralizing agent to prepare a neutralization solution, and iron (III) phosphate is mixed with the neutralization solution and reacted, followed by baking. And synthetic lithium iron phosphate powder.

Further, in the method for producing lithium iron phosphate according to the present invention, it is preferable that the amount of the molar amount of the neutralizing agent in the neutralization solution is 0.2 times or more relative to the molar amount of the neutralizing agent at the time of reaching the neutralization point. .

Further, in the method for producing lithium iron phosphate according to the present invention, it is preferred that the neutralizing agent is an organic acid.

Further, iron (III) phosphate can be synthesized by reacting a phosphorus source with an iron compound containing trivalent iron, and more preferably, an aqueous solution in which a phosphorus source and the above iron compound are dissolved is added dropwise to a buffer having a pH of 1.5 to 9. The contact is made in the solution, whereby the dispersion can be efficiently produced without uneven distribution of the elements of iron and phosphorus. A high-purity iron phosphate in which iron and phosphorus are uniformly or substantially uniformly dispersed.

That is, in the method for producing lithium iron phosphate according to the present invention, it is preferred that the iron phosphate (III) is synthesized by reacting a phosphorus source with an iron compound containing trivalent iron.

Further, in the method for producing lithium iron phosphate according to the present invention, it is preferred that the iron phosphate (III) is formed by bringing a mixed aqueous solution of a phosphorus source and the iron compound into contact with a buffer solution having a pH of 1.5 to 9.

Further, the electrode active material of the present invention is characterized in that it is used as an active material of a secondary battery which is repeatedly charged and discharged by a battery electrode reaction, and the lithium iron phosphate produced by the method described in any of the above is used. main body.

Further, the secondary battery of the present invention is characterized in that it has a positive electrode, a negative electrode, and an electrolyte, and the positive electrode contains the electrode active material.

According to the method for producing lithium iron phosphate of the present invention, after the neutral or acidic lithium salt solution such as iron (III) phosphate and lithium acetate are mixed and reacted, the calcination treatment is carried out to synthesize lithium iron phosphate powder, so that complicated manufacturing is not required. In the step, it is possible to produce lithium iron phosphate which can suppress a decrease in the retention rate of the charge and discharge capacitor even when charging and discharging are performed rapidly by one heat treatment.

Further, a lithium hydroxide is mixed with a neutralizing agent to prepare a neutralization solution, and iron (III) phosphate and a neutralizing solution are mixed and reacted, and then calcined to synthesize a lithium iron phosphate powder, so that it can be used in the same manner as described above. Lithium iron phosphate having a fast charge and discharge property is easily produced by one heat treatment.

In this case, in the neutralization solution, the lithium hydroxide is made by making the amount of the molar amount of the neutralizing agent to be 0.2 times or more relative to the molar amount of the neutralizing agent at the time of reaching the neutralization point. The alkalinity is diluted and the pH is toward the neutral side. mobile. Therefore, it is possible to produce lithium iron phosphate suitable for an electrode active material for a secondary battery having a desired rapid charge and discharge property without causing the iron phosphate to come into contact with a strong alkaline solution to cause a decomposition reaction.

The iron phosphate (III) is synthesized by reacting a phosphorus source with an iron compound containing trivalent iron, and more specifically, a mixed aqueous solution in which a phosphorus source and the iron compound are dissolved and a buffer solution having a pH of 1.5 to 9. By contact, it is possible to efficiently obtain high-purity iron phosphate in which the elements of Fe and P are uniformly or substantially uniformly dispersed without generating Fe(OH) ₃ . Further, the change in the pH value at the time of powder formation by the buffering action of the buffer solution is also reduced, and the iron phosphate powder having fine particles and uniform particle diameter can be obtained.

Further, according to the electrode active material of the present invention, the electrode active material used as the active material of the secondary battery which is repeatedly charged and discharged by the battery electrode reaction is mainly composed of the lithium iron phosphate described above, so that it is possible to obtain safety and high energy density. Electrode active material.

Further, according to the secondary battery of the present invention, since the positive electrode, the negative electrode, and the electrolyte are contained, and the positive electrode contains the electrode active material, a secondary battery having a large capacitance and a high power excellent in safety can be obtained.

Next, the form for carrying out the invention will be described in detail.

In one embodiment (first embodiment) of the method for producing lithium iron phosphate according to the present invention, a P source is reacted with an Fe compound containing trivalent Fe to form iron phosphate (FePO ₄ ‧ nH ₂ O) powder (co-precipitated powder) The above-mentioned co-precipitated powder and an acidic or neutral lithium salt solution are wet-mixed and calcined to form lithium iron phosphate (LiFePO ₄ ) powder (calcined powder).

In this way, the FePO ₄ ‧nH ₂ O is not decomposed in the solution, and it is possible to produce lithium phosphate which can suppress the decrease in the retention rate of the charge/discharge capacitor even if the charge and discharge are rapidly performed by one heat treatment without complicated manufacturing steps. iron.

For example, in the case where lithium hydroxide (LiOH) is used instead of the lithium salt, the aqueous solution of LiOH is strongly alkaline at a pH of 12 to 13. Thus, when the FePO ₄ ‧nH ₂ O in contact with aqueous LiOH, the FePO ₄ ‧nH ₂ O decomposition reaction occurs one surface side of the reaction with Li, so fine FePO _4, but when the rapid charge-discharge charge-discharge rate is increased to perform Compared with the case where the charge and discharge rate is small, the maintenance rate of the charge and discharge capacitor is lowered, and the rapid charge and discharge characteristics are poor.

Therefore, by reacting an acidic or neutral lithium salt solution with such a strongly alkaline aqueous LiOH solution and FePO ₄ ‧ nH ₂ O, the decomposition reaction of FePO ₄ ‧nH ₂ O is avoided, thereby obtaining a fast charge LiFePO _{4 with} good discharge characteristics.

Further, LiFePO ₄ can be easily produced from FePO ₄ ‧ nH ₂ O by one-time baking treatment without complicating the manufacturing steps.

Here, the lithium salt is not particularly limited as long as it is acidic or neutral. For example, lithium acetate dihydrate having a pH of 7, lithium nitrate, lithium sulfate monohydrate, lithium lactate, lithium oxalate or lithium tartrate can be used. Monohydrate, lithium citrate tetrahydrate, and the like. Among these, it is particularly preferable to use lithium acetate.

Hereinafter, the method for producing LiFePO ₄ described above will be specifically described.

First, a P source is reacted with an Fe compound containing trivalent Fe to form a FePO ₄ ‧ nH ₂ O powder.

The method for producing FePO ₄ ‧nH ₂ O is not particularly limited, but it is preferably produced by bringing a mixed aqueous solution in which a P source and an Fe compound are dissolved and a buffer solution having a pH of 1.5 to 9.

In other words, when a mixed aqueous solution in which a P source and a trivalent Fe compound are dissolved is added dropwise to a buffer solution having a pH of 9 or less and brought into contact with each other, FePO can be produced without generating by-products such as Fe(OH) ₃ . ₄ ‧nH ₂ O. Further, by this, FePO ₄ ‧nH ₂ O in which the elements of Fe and P are uniformly distributed or substantially uniform, fine particles and high in purity can be efficiently produced. Moreover, the pH value is suppressed by the buffering action of the buffer solution, so that the pH value of the FePO ₄ ‧nH ₂ O powder is also reduced, and a spherical FePO ₄ ‧nH ₂ O powder having a uniform particle size can be obtained. .

Further, if the pH of the buffer solution is less than 1.5, the amount of Fe and P which are not precipitated and the amount of Fe which is eluted increases, and the yield of the powder is lowered, which is not preferable.

Next, a method of producing FePO ₄ ‧ nH ₂ O using the buffer solution will be described in detail.

First, a Fe source such as FeSO ₄ ‧7H ₂ O or FeCl ₂ ‧4H ₂ O containing Fe-valent Fe, H ₃ PO ₄ , (NH ₄ )H ₂ PO ₄ , (NH ₄ ) ₂ HPO ₄ and the like An oxidizing agent such as H ₂ O ₂ is mixed so as to have a specific ratio to prepare a mixed aqueous solution. Here, the divalent Fe compound and the P source are mixed in an equal or substantially equal amount in terms of a molar ratio, and the oxidizing agent is obtained by completely oxidizing divalent Fe to trivalent Fe with respect to the divalent Fe compound. Excess (for example, about 1.5 times the molar ratio) is added.

Then, a buffer solution having a pH of 9 or less, preferably 1.5 to 9, is prepared.

Here, the preparation method of the buffer solution is not particularly limited, and for example, acetic acid-ammonium acetate, lactic acid-sodium lactate, glycolic acid-sodium glycolate, and cis-butene are used. A method of preparing a buffer solution by mixing a weak acid-conjugated base such as acid-maleic acid disodium is widely known, and the mixing ratio of the weak acid-conjugate base can be appropriately adjusted to prepare. Further, the composition of the constituent material of the buffer solution is not limited to a weak acid-conjugated base, and may be other components such as a weak acid-strong base.

Then, the buffer solution was stirred at a normal temperature, and while the pH of the buffer solution was monitored, the mixed aqueous solution was added dropwise to the buffer solution to make the contact, thereby obtaining a brown FePO ₄ ‧ nH ₂ O co-precipitated powder.

Further, as the amount of the dropwise addition of the mixed aqueous solution increases, the pH of the buffer solution decreases, preferably, the dropwise addition of the mixed aqueous solution ends before the pH reaches 1.5. When the mixed aqueous solution is continuously added after the pH becomes 1.5 or less, as described above, the precipitated FePO ₄ ‧ nH ₂ O starts to dissolve, and Fe or P elutes, so that the yield of the precipitated powder is lowered.

Further, when an aqueous solution such as ammonia water or NaOH is added dropwise to the mixed aqueous solution without using the above buffer solution, the pH value in the vicinity of the dropwise addition is temporarily increased to preferentially form Fe(OH) ₃ , which is not preferable. That is, in this case, by stirring and mixing the aqueous solution, the pH value of the periphery is dropped to form FePO ₄ , but it is difficult for Fe(OH) _{3 which} has been formed to become FePO ₄ . Therefore, the obtained co-precipitated powder is a mixture of FePO ₄ and Fe(OH) ₃ , Fe and P are unevenly dispersed to cause uneven dispersion, the particle size distribution is also largely different, and the shape is not uniform, which is not preferable.

Then, the FePO ₄ ‧nH ₂ O is wet-mixed with an acidic or neutral lithium salt to react, thereby producing LiFePO ₄ .

That is, the FePO ₄ ‧nH ₂ O powder and the lithium salt are weighed in such a manner that the FePO ₄ ‧nH ₂ O powder and the lithium salt are 1:1, and the precipitate is mixed with pure water and poly A polymer dispersant such as a carboxylic acid is put into a ball mill for mixing and pulverization to obtain a mixed powder.

Further, from the viewpoint of securing electron conductivity, it is preferred to add a carbon source such as sucrose to FePO ₄ and coat the surface of LiFePO ₄ with carbon. Further, in order to increase the specific surface area in order to refine LiFePO ₄ , it is preferred to add carbon. That is, it is preferable to coat the surface of LiFePO ₄ with carbon to inhibit grain growth of LiFePO ₄ during firing and to secure a large specific surface area.

Then, after the mixed powder is dried and granulated, heat treatment is performed at a specific temperature (for example, 500 to 700 ° C) for about 5 hours in a specific reducing environment. Further, by this, trivalent Fe is reduced to divalent, and a powder of LiFePO ₄ is obtained.

As described above, in the above embodiment, the phosphorus source is reacted with the iron compound containing trivalent iron to synthesize iron phosphate, and the iron phosphate is mixed with the acidic or neutral lithium salt solution and reacted, followed by calcination to synthesize LiFePO ₄ powder, therefore, FePO _{4 is} not decomposed, and FePO ₄ can be reacted with a lithium salt to obtain LiFePO ₄ powder, and LiFePO _{4 having} excellent rapid charge and discharge characteristics for an electrode active material for a secondary battery can be obtained.

Further, LiFePO ₄ can be easily produced from FePO ₄ ‧ nH ₂ O by one-time baking treatment without complicating the manufacturing steps.

Further, the raw material of the electrode active material mainly composed of LiFePO ₄ is not limited to the resource property of Co, and is inexpensive and easy to obtain, and the electrode active material can realize a large-capacitance, high-power secondary battery which is excellent in safety at low cost. .

Moreover, the present invention is not limited to the above embodiment, and is a second embodiment. It is also preferred to prepare a neutralizing solution by mixing LiOH with a neutralizing agent, and to use the neutralizing solution instead of the above lithium salt.

That is, in the same manner as in the first embodiment, Fe is reacted with an iron compound containing trivalent iron to synthesize FePO ₄ . And the neutralizing agent is mixed with LiOH to prepare a neutralizing solution, and mixing with a neutralizing solution FePO ₄ and after the reaction, the firing process, can be synthesized LiFePO _4.

Here, the neutralizing agent is not particularly limited, and an organic acid such as acetic acid, maleic acid, lactic acid, glycolic acid, malic acid, tartaric acid, citric acid or the like can be preferably used.

In addition, as the neutralization solution, the ratio of the amount of the molar amount of the neutralizing agent to the amount of the molar amount of the neutralizing agent at the time of reaching the neutralization point (hereinafter referred to as "neutralization degree") may be 0.2 or more. . That is, the degree of neutralization referred to herein means the amount of the molar amount of the neutralization point when the neutralizing agent is added to the LiOH, and is expressed by the molar ratio, for example, because acetic acid is A monovalent acid, so in the case of a molar addition to LiOH, the degree of neutralization is 1, and since maleic acid is a divalent acid, 1/2 mol is added to LiOH. The degree of neutralization is 1. Further, the degree of neutralization is preferably one of the neutral values of pH 7, but may not necessarily be one. If it is 0.2 or more, LiFePO _{4 which} suppresses a decrease in the retention ratio of the charge and discharge capacitor during rapid charge and discharge can be obtained.

Among them, if the degree of neutralization is less than 0.2, the influence of lithium hydroxide increases, and the neutralized solution shows strong alkalinity. Therefore, when FePO ₄ ‧ nH ₂ O is brought into contact with the neutralizing solution, FePO _{4 is} decomposed to fail to obtain the desired rapid charge and discharge characteristics.

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

1 is a cross-sectional view showing a button type secondary battery which is one embodiment of the secondary battery of the present invention, and in the present embodiment, an electrode active material mainly composed of LiFePO ₄ is used for the positive electrode active material.

The battery can 1 has a positive electrode cap 2 and a negative electrode cap 3, and both of the positive electrode cap 2 and the negative electrode cap 3 are formed in a disk-shaped thin plate shape. Further, a positive electrode 4 in which an electrode active material is formed into a sheet shape is disposed at the center of the bottom of the positive electrode cap 2 constituting the positive electrode current collector. Further, a separator 5 formed of a porous film of polypropylene or the like is laminated on the positive electrode 4, and a negative electrode 6 is laminated on the separator 5. As the negative electrode 6, for example, a metal foil in which lithium is superposed on Cu or a lithium-containing material such as graphite or hard carbon may be applied to the metal foil. Further, 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 cap 3 is fixed to the positive electrode cap 2 against the ability of the metal spring 8, and is sealed via the gasket 10.

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

First, LiFePO ₄ which is a main body of the electrode active material is formed into an electrode shape. For example, LiFePO _{4 is} mixed with a conductive auxiliary agent and a binder, and a solvent is added to prepare a slurry, and the slurry is applied onto a positive electrode current collector by any coating method and dried to form a positive electrode 4 . .

Here, the conductive auxiliary agent is not particularly limited, and for example, carbonaceous fine particles such as graphite, carbon black, and acetylene black, carbon fibers such as vapor deposited carbon fibers (VGCF), carbon nanotubes, and carbon nanohorns can be used. A conductive polymer such as aniline, polypyrrole, polythiophene, polyacetylene or polyacene. Also Two or more kinds of conductive assistants can be used in combination. Further, the content of the conductive auxiliary agent in the positive electrode 4 is preferably from 10 to 80% by weight.

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

Further, the solvent is not particularly limited, and for example, dimethyl hydrazine, dimethylformamide, N-methyl-2-pyrrolidone, propylene carbonate, diethyl carbonate, dimethyl carbonate, or the like can be used. An alkaline solvent such as γ-butyrolactone, a nonaqueous solvent such as acetonitrile, tetrahydrofuran, nitrobenzene or acetone, or a protic solvent such as methanol or ethanol.

In addition, the type of the solvent, the mixing ratio of the organic compound and the solvent, the type of the additive, and the amount of the additive can be arbitrarily set in consideration of the required characteristics of the secondary battery, productivity, and the like.

Then, the positive electrode 4 is immersed in the electrolyte 9 to allow the electrolyte 9 to permeate into the positive electrode 4, and then the positive electrode 4 is placed on the positive electrode current collector at the center of the bottom of the positive electrode cap 2. Then, the separator 5 impregnated with the above electrolyte 9 is laminated on the positive electrode 4, and 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. Further, the metal spring 8 is placed on the negative electrode current collector 7, and the gasket 10 is placed on the periphery, and the negative electrode cap 3 is fixed to the positive electrode cap 2 by a caulking machine or the like, and sealed and sealed, thereby producing a button. Secondary battery.

Further, the electrolyte 9 is interposed between the positive electrode 4 and the counter electrode 6, that is, the negative electrode 6, and carries the charged carrier between the electrodes. As such an electrolyte 9, it can be used for 10 ^-5 to 10 ^-1 S at room temperature. For the conductivity of /cm, for example, an electrolyte obtained by dissolving an electrolyte salt 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 ) may be used.} SO ₂ ) ₃ C, Li(C ₂ F ₅ SO ₂ ) ₃ C, and the like.

Further, as the organic solvent, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, γ-butyrolactone, tetrahydrofuran, dioxolane, and cyclobutane may be used. , dimethylformamide, dimethylacetamide, N-methyl-2-pyrrolidone, and the like.

As described above, according to the present embodiment, since the above-described fine particles and high-purity LiFePO _{4 are} used as the main body of the positive electrode active material, it is possible to realize a secondary battery having high capacitance, high power, and excellent safety at low cost.

Furthermore, the present invention is not limited to the above embodiment. For example, in the above embodiment, a divalent Fe compound is oxidized to trivalent Fe to obtain a trivalent Fe compound by mixing a divalent Fe compound and an oxidizing agent in the production process of FePO ₄ , but it may be used from the beginning. The trivalent Fe compound is added dropwise to the buffer solution to bring the contacts into contact, and in the case of the Fe compound, for example, FeCl ₃ ‧6H ₂ O or the like can be used.

Further, although the button type secondary battery has been described in the above embodiment, the shape of the battery is not particularly limited, and a cylindrical shape, an angular shape, a sheet shape, or the like can be used. Further, the encapsulation method is not particularly limited, and a metal case, a molding resin, an aluminum laminate film, or the like can be used.

Next, embodiments of the invention will be specifically described.

[Example 1] [Production of sample]

FeSO ₄ ‧7H ₂ O was dissolved in pure water, and H ₃ PO ₄ (85% aqueous solution) as a P source and H ₂ O ₂ (30% aqueous solution) as an oxidizing agent were added thereto to prepare a mixed aqueous solution. Here, FeSO ₄ ‧7H ₂ O, H ₃ PO ₄ and H ₂ O ₂ were blended in such a manner that the molar ratio was 1:1:1.5.

Then, pure water was added to acetic acid, and ammonium acetate was dissolved therein, thereby preparing a buffer solution. Further, the molar ratio of acetic acid to ammonium acetate was 1:1, and the concentrations of acetic acid and ammonium acetate were both set to 0.5 mol/L. The pH of the buffer solution was measured and found to be 4.6.

Then, while stirring the buffer solution at normal temperature, the mixed aqueous solution was added dropwise to the buffer solution to prepare a precipitated powder. Further, as the amount of the dropwise addition of the mixed aqueous solution increases, the pH of the buffer solution decreases, and when the pH becomes 2.0, the dropwise addition of the mixed aqueous solution to the buffer solution is ended.

Then, the obtained precipitated powder was filtered, washed with a large amount of water, heated to a temperature of 120 ° C, and dried to prepare a brown FePO ₄ ‧2H ₂ O powder.

Then, the FePO ₄ ‧2H ₂ O powder was blended with CH ₃ COOLi‧2H ₂ O (lithium acetate dihydrate) in a molar ratio of 1:1, and pure water and polycarboxylate were added thereto. The molecular dispersant was mixed and pulverized using a ball mill to obtain a slurry. Then, after the obtained slurry is dried by a spray dryer, granulation is carried out, and the oxygen partial pressure is adjusted to a reduction environment of 10 ^-20 MPa using a mixed gas of H ₂ -H ₂ O, and at a temperature of 600 ° C. The samples of the examples were prepared by heat treatment for 5 hours.

Further, a comparative sample was produced by the same method and procedure as in the example sample except that LiOH‧H ₂ O was used instead of CH ₃ COOLi‧2H ₂ O.

[Production of secondary battery]

A secondary battery was fabricated using each of the samples of the examples and the comparative examples.

First, LiFePO ₄ , acetylene black as a conductive additive, and polyvinylidene fluoride as a binder are prepared, and the LiFePO ₄ , acetylene black, and polyvinylidene fluoride are 88:6:6 by weight ratio. The mixture was weighed and mixed, and dispersed in N-methyl-2-pyrrolidone as a solvent to prepare a slurry, and the slurry was applied to an aluminum foil having a thickness of 20 μm so as to be 6 mg/cm ² . After drying at a temperature of 140 ° C, it was pressed at a pressure of 98 MPa to prepare an electrode sheet, and further punched to a diameter of 12 mm to prepare a positive electrode having a positive electrode active material mainly composed of LiFePO ₄ .

Then, the positive electrode is immersed in an electrolytic solution to allow the electrolytic solution to permeate into the voids in the positive electrode. As the electrolytic solution, a mixed solution of ethylene carbonate/diethyl carbonate containing an organic solvent of LiPF ₆ (electrolyte salt) having a molar concentration of 1.0 mol/L was used. Further, the mixing ratio of ethylene carbonate and diethyl carbonate was 5% by volume of ethylene carbonate: diethyl carbonate = 3:7.

Then, the positive electrode is placed on the positive electrode current collector, and a separator having a thickness of 20 μm containing the polypropylene porous film impregnated with the electrolytic solution is laminated on the positive electrode, and then the double-sided paste of the copper foil is attached. A negative electrode coated with lithium is laminated on the separator. Further, after the negative electrode current collector made of Cu is laminated on the negative electrode, the electrolytic solution is injected into the internal space, and then the metal spring is placed on the negative electrode current collector, and the gasket is placed on the periphery. The negative electrode cap was joined to the positive electrode cap, and sealed by a caulking machine, thereby producing a secondary battery having LiFPO ₄ as a positive electrode active material and having metallic lithium as a negative electrode active material having a diameter of 20 mm and a thickness of 3.2 mm.

[Evaluation of sample]

In a 25 ° C thermostat, the voltage range is set to 2.0 to 4.2 V, with 0.2 C, 1 C, 3 C, and 5 C (1 C is the amount of current that is charged or discharged within 1 hour). The charge and discharge rate causes the secondary battery to be charged and discharged. That is, it was charged at a respective charging rate until the voltage was 4.2 V, and then discharged to 2.0 V at the same discharge rate.

Further, each capacitance density and capacitance retention rate of charge and discharge at each charge and discharge rate are obtained. Here, the capacity retention ratio is a ratio of the charge-discharge capacitance density at a charge and discharge rate of 1 C, 3 C, and 5 C based on the charge-discharge capacitance density of 0.2 C at which the charge and discharge rate is the smallest.

Table 1 shows the measurement results of the sample of the example, and Table 2 shows the measurement results of the sample of the comparative example.

According to Table 2, it can be clarified that the comparative sample sample has a charge and discharge rate of 5 C, and the charge and discharge rate is lower than that of the case where the charge and discharge rate is 0.2 C. Dropped to 92.0%, the discharge capacity retention rate dropped to 86.2%, both of which dropped significantly.

On the other hand, in the case of the example, as shown in Table 1, even when the charge and discharge rate was 5 C, the charge capacity retention rate was 93.2% as compared with the case where the charge and discharge rate was 0.2 C. The discharge capacity retention rate was 90.9%, and the control capacitor retention ratio was decreased as compared with the comparative example sample. It is understood that a good charge and discharge capacitance can be obtained even when a large current is applied and a large current is quickly charged and a large current is released. Density and fast charge and discharge characteristics are obtained.

[Embodiment 2]

Samples Nos. 1 to 6 were prepared in the same manner as in Example 1 except that various lithium salts (pH values of 7) shown in Table 3 were used instead of CH ₃ COOLi‧2H ₂ O. A secondary battery was fabricated for each sample.

Then, the secondary battery was charged to 4.2 V in a 25 ° C thermostatic chamber, and then discharged at a discharge rate of 0.2 C and 5 C until the voltage was 2.0 V, and the discharge was performed at a discharge rate of 0.2 C and 5 C. The capacitance density and the discharge capacity retention rate at a discharge rate of 5 C were calculated based on the discharge capacity density at a discharge rate of 0.2 C.

Table 3 shows the lithium salt and discharge capacity retention ratios used in sample numbers 1 to 6.

As is clear from Table 3, the discharge capacity retention rate is 88.0 to 89.5%, and the discharge capacity retention ratio is compared with the case where CH ₃ COOLi‧2H ₂ O is used as the lithium salt (Example 1, Table 1). When it is lower than the case where LiOH‧H ₂ O is used (Examples 1 and 2), the discharge capacity retention rate is improved, and it is understood that the rapid discharge characteristics can be improved by using a lithium salt.

[Example 3]

After LiOH‧H ₂ O was dissolved in water and made alkaline, a specific amount of each of the organic acids shown in Table 4 was added as a neutralizing agent to prepare a neutralizing solution. Namely, various organic acids were added to LiOH‧H ₂ O in such a manner as to have a degree of neutralization as shown in Table 4, and a neutralizing solution having a specific degree of neutralization was prepared.

Then, samples of sample Nos. 11 to 40 were produced in the same manner as in Example 1 except that the neutralized solution was reacted with iron phosphate.

Then, in the same manner as in Example 2, the secondary battery was charged to 4.2 V in a 25 ° C thermostatic chamber, and then discharged at a discharge rate of 0.2 C and 5 C until the voltage was 2.0 V, and was obtained at 0.2 C and The discharge capacitance density at a discharge rate of 5 C was calculated based on the discharge capacitance density at a discharge rate of 0.2 C, and the discharge capacity retention rate at a discharge rate of 5 C was calculated.

Table 4 shows the neutralizing agent, the degree of neutralization, and the discharge capacity retention ratio used in sample numbers 11 to 40. Further, in Table 4, the discharge capacity retention ratio of the comparative sample prepared in Example 1 was again revealed for comparison.

As is clear from Table 4, even if the neutralization solution is not an acidic or neutral solution, if the degree of neutralization is at least 0.2, the discharge capacity retention rate is 86.9% or more, and LiOH‧H ₂ O alone is used. The discharge capacity retention ratio was improved as compared with the comparative sample sample in which FePO _{4 was} reacted.

Further, it is also known that when the same neutralizing agent is added to LiOH‧H ₂ O, the degree of neutralization increases, that is, as the neutralizing solution moves from the alkaline to the neutral side, the discharge capacity retention rate improve.

[Industrial availability]

The lithium iron phosphate which is excellent in the rapid charge and discharge characteristics of the electrode active material for secondary batteries can be easily produced by a simple manufacturing step.

1‧‧‧Battery cans

2‧‧‧ positive electrode cover

3‧‧‧Negative cover

4‧‧‧ positive

5‧‧‧Parts

6‧‧‧negative

7‧‧‧Negative current collector

8‧‧‧Metal spring

9‧‧‧ Electrolytes

10‧‧‧shims

Fig. 1 is a cross-sectional view showing an embodiment of a button battery as a secondary battery of the present invention.

Claims (11)

A method for producing lithium iron phosphate, characterized in that iron phosphate (III) is mixed with an acidic or neutral lithium salt solution and reacted, followed by calcination to synthesize lithium iron phosphate powder. The method for producing lithium iron phosphate according to claim 1, wherein the lithium salt solution is a lithium acetate solution. A method for producing lithium iron phosphate, characterized in that a lithium hydroxide is mixed with a neutralizing agent to prepare a neutralization solution, and iron (III) phosphate is mixed with the neutralization solution and reacted, followed by calcination to synthesize phosphoric acid. Lithium iron powder. The method for producing lithium iron phosphate according to claim 3, wherein the amount of the molar amount of the neutralizing agent in the neutralization solution is 0.2 times or more relative to the molar amount of the neutralizing agent at the time of reaching the neutralization point. The method for producing lithium iron phosphate according to claim 3 or 4, wherein the neutralizing agent is an organic acid. The method for producing lithium iron phosphate according to any one of claims 1 to 4, wherein the iron (III) phosphate is synthesized by reacting a phosphorus source with an iron compound containing trivalent iron. The method for producing lithium iron phosphate according to claim 5, wherein the iron (III) phosphate is synthesized by reacting a phosphorus source with an iron compound containing trivalent iron. The method for producing lithium iron phosphate according to claim 6, wherein the iron (III) phosphate is produced by bringing a mixed aqueous solution of the phosphorus source and the iron compound into contact with a buffer solution having a pH of 1.5 to 9. The method for producing lithium iron phosphate according to claim 7, wherein the above iron (III) phosphate It is produced by bringing a mixed aqueous solution of a phosphorus source and the above iron compound into contact with a buffer solution having a pH of 1.5 to 9. An electrode active material which is used as an active material of a secondary battery which is repeatedly charged and discharged by a battery electrode reaction, and which is produced by the method of any one of claims 1 to 9. Lithium iron phosphate is the main body. A secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, and the above positive electrode comprises the electrode active material of claim 10.