JPH10188986A - Manufacture of positive electrode active material for lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide positive electrode active material for a lithium secondary battery with uniform composition and high discharge capacity. SOLUTION: Lithium acetate water solution, manganese acetate water solution and citric acid water solution are mixed together to produce citric acid complex with high Mn<2+> ion concentration, which is dehydrated by heating and then baked to form positive electrode active material of composition formula Lix Mn2 O4 with high discharge capacity. The compounding ratio of the citric acid to the citric acid complex solution is preferably 110% or so, equivalent to or more than the total valence number of lithium ions and manganese ions as anode ions, to provide higher discharge capacity.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium secondary battery, and more particularly, to a method for producing an active material used for a positive electrode of a lithium secondary battery.

[0002]

2. Description of the Related Art Lithium secondary batteries have a high energy density and can be reduced in size and weight, so that they can be used for communication office equipment such as personal computers and mobile phones, or power supplies for electric vehicles in the future. The use as is expected.

[0003] Incidentally, this lithium secondary battery generally uses a composite oxide of lithium (Li) and a transition metal (M) for the positive electrode, and uses metal lithium or a lithium atom contained in carbon for the negative electrode. ing. Discharge is caused by the movement of lithium ions to the positive electrode in the non-aqueous organic electrolyte, and charging is performed by movement to the negative electrode. By repeating such charge and discharge, the characteristics as a secondary battery are exhibited. .

In the battery characteristics of such a lithium secondary battery, a positive electrode material is required to have a high discharge capacity in order to increase the capacity of the battery. Conventionally, lithium cobalt oxide (LiCo
O ₂ ) has been used, but lithium nickel oxide (LiNiO) has been used as an alternative material due to cost and resource problems.
₂ ) and materials such as lithium manganate (LiMn ₂ O ₄ ) have also been developed.

Among them, for example, a solid phase method or a liquid phase method is known as a method for producing LiMn ₂ O ₄ . For example, Japanese Patent Application Laid-Open No.
As shown in JP 39860B, lithium oxide (L
i ₂ O) and manganese dioxide (MnO ₂ ) powder materials are mixed and fired, or, although not shown in this publication, lithium carbonate (Li ₂ CO ₃ ) and manganese carbonate (Mn)
CO ₃ ) powdered raw materials are mixed and heated and fired.

On the other hand, the method using the liquid phase method is disclosed in, for example, Japanese Patent Laid-Open No. 7-142065, in which a lithium hydroxide (LiOH) aqueous solution and manganese acetate (Mn (C
H ₃ OO) ₂ ) aqueous solution and citric acid (C ₃ H ₄ (OH) (C
After mixing each aqueous solution of OOH) ₃ ) and drying by dehydrating and heating to obtain a citrate complex, this is heated and calcined to produce a LiMn ₂ O ₄ cathode active material.

[0007]

However, in the case of using the former solid-phase method, when the firing temperature is low (500 ° C. or less), the discharge capacity as the positive electrode active material decreases, and when the firing temperature is high (700 ° C.). (° C. or more), the discharge capacity is slightly increased, but there is a problem that the material itself is easily deteriorated.

On the other hand, in the case of using the latter liquid phase method, for example, LiM is disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 7-142065.
The n ₂ O ₄ active material can obtain a relatively high discharge capacity as compared with that based on the solid-phase method.
The inventors' experiments confirmed that the discharge capacity of iMn ₂ O ₄ was about 130 mAh / g, which was only about 88% of the theoretical capacity (148 mAh / g).

The present inventors have conducted intensive studies to further improve the discharge capacity. As a result, they have found that it is important to further improve the uniformity of the composition in order to improve the discharge capacity. Therefore, the present inventors have proposed that the state of formation of the precursor in the citric acid complex method in the production process of LiMnO ₄ shown in the above-mentioned Japanese Patent Application Laid-Open No. 7-142065 depends on the type of lithium raw material or the mixing ratio of citric acid. The present invention focuses on the fact that it is changed by changing, and has led to the present invention.

That is, in terms of lithium raw materials, various lithium salts were examined. As a result, when an acidic lithium salt, particularly lithium acetate, was used rather than lithium hydroxide, electron spin resonance ESR (Electron Spin Resonance) was used.
ance) was found to increase the Mn ²⁺ concentration in the complex.

Further, in terms of the mixing ratio of citric acid, the Mn ²⁺ concentration in the complex can be further increased by setting it to 110% of the total valence of lithium ions and manganese ions, which are cations in the citric acid complex. Found to be higher. This indicates that the ratio of Mn ²⁺ ions in the aqueous solution taken into the complex in a state of independent ions is high.

Namely, the principle of electron spin resonance (ESR), the antiferromagnetic interaction Mn ²⁺ ions close occurs because Mn ²⁺ ions according to the observation is underestimated, Mn ²⁺ The fact that more ions are observed means that Mn ²⁺ ions are uniformly dispersed in the complex without segregation. Therefore, a high Mn ²⁺ ion concentration observed by electron spin resonance (ESR) is considered to indicate that a more uniform complex has been formed. Is expected to be a uniform compound.

The problem to be solved by the present invention is to produce a positive electrode active material for a lithium secondary battery by producing a citric acid complex having a high manganese ion concentration as an intermediate product by a so-called liquid phase method. Finally, it is to produce a positive electrode active material having a uniform composition and a high discharge capacity. Thus, the electric capacity (battery capacity) of the lithium secondary battery is increased.

[0014]

Means for Solving the Problems To solve this problem, a method for producing a positive electrode active material for a lithium secondary battery according to the present invention comprises a method of preparing an aqueous solution of a citric acid complex with an acidic lithium salt, an acidic manganese salt and citric acid. Forming a precursor, dehydrating the citrate complex to produce a precursor, and calcining the precursor to produce Li _x Mn ₂ O ₄ (0 <X <2). It is an abstract.

In this case, examples of the acidic lithium salt include lithium salts of inorganic acids such as hydrochloric acid, diluted sulfuric acid, diluted nitric acid, and phosphoric acid, and lithium salts of organic acids such as acetic acid, oxalic acid, and tartaric acid. Suitable acidic manganese salts include manganese salts of various inorganic acids and manganese salts of various organic acids such as acetic acid.

The compounding ratio of the acidic lithium salt and the acidic manganese salt is the composition ratio of the composite oxide. However, the mixing ratio of citric acid added thereto is such that the sum of the charges of the citric acid (trivalent) is equal to or more than the total valence of lithium ions and manganese ions which are cations in the citric acid complex solution. Preferably, it is preferably about 110%.

Next, in the dehydration step, the target Li _x Mn ₂
This is a step of forming an amorphous citric acid complex as a precursor of O ₄ (0 <X <2), and this step is performed under reduced pressure.
It is desirable to heat and dehydrate at 80 to 90 ° C and further heat to 150 to 300 ° C. If the mixture is heated at a high temperature from the beginning, the complex may be partially decomposed, and if the mixture is heated at a low temperature for a long time, there is a concern that the crystalline material is segregated and the amorphous is damaged.

The final firing step is preferably performed at a high firing temperature within a range where the active material is not decomposed. For example, 600-8
When calcination is performed at a calcination temperature of 00 ° C. for 3 to 8 hours, the crystal grows well, the material has good deterioration resistance, and a large discharge capacity is obtained.

However, by using an acidic lithium salt as a lithium raw material, the acidity becomes higher than the pH of an aqueous solution when an alkali salt such as lithium hydroxide is used, thereby promoting the dissociation of the acidic manganese salt. Therefore, more Mn ²⁺ ions can be present in the aqueous citric acid complex solution. In particular, when lithium acetate is used, the acidity increases, the degree of dissociation of the acidic manganese salt further increases, and the abundance of Mn ²⁺ ions in the aqueous citrate complex solution increases.

The compounding ratio of citric acid is adjusted so that the sum of the charges of citric acid in the complex solution is equal to or greater than the total valence of lithium ions and manganese ions as cations. at the stage where Mn ²⁺ ions to form a citrate complex with lithium ions, excess citric acid was added are all of Mn ²⁺ ions and complex. Therefore, the Mn ²⁺ ions in the solution form a complex with citric acid without excess, so that the Mn ²⁺ ions are uniformly dispersed in the complex, and the fired product obtained therefrom has a uniform composition and a high discharge capacity. Li _x Mn ₂ O ₄ (0 <X <2) is obtained.

[0021]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described specifically with reference to examples. (Example 1) Lithium acetate dihydrate (LiCOOH
13.263 g of 2H ₂ O) was dissolved in 50 ml of water to prepare a lithium acetate aqueous solution. Then, manganese acetate
Tetrahydrate (Mn (CH ₃ COO) ₂ .4H ₂ O) 63.7
23 g was dissolved in 200 ml of water to prepare a manganese acetate aqueous solution.

Next, citric acid (C ₃ H ₄ (OH) (COO)
H) ₃ ) 46.122 g was dissolved in 150 ml of water to prepare a citric acid aqueous solution, to which a manganese acetate aqueous solution was added, and further, a lithium acetate aqueous solution was added. At this time, the molar ratio of lithium ion, manganese ion, and citrate ion was 1.00: 2.00: 1.83. Therefore, the mixing ratio of citric acid in this aqueous solution is about 110% of lithium ions and manganese ions. The pH of the mixed solution at this time was 3.0.

Next, this mixed solution was heated to 80 to 90 ° C. in a rotary evaporator under a reduced pressure atmosphere to be dehydrated to obtain a solid. The solid is further heated to 250 ° C., and Li _x Mn ₂ O ₄ (0 <X <2
A citric acid complex as a precursor of 0) was obtained.

When the number of Mn ²⁺ ions in this precursor was quantified by an electron spin resonance (ESR) apparatus, 1.63 × 10 ²¹ Mn ²⁺ ions per unit mass were detected. Since the total weight of the precursor was 63.1 g, 10.3 × 1
It was found that there were 0 ²² Mn ²⁺ ions.

10 g of this precursor was placed in an alumina crucible and calcined at 400 ° C. for 4 hours in an air atmosphere. The rate of temperature rise was 100 ° C./hour and the rate of temperature drop was 100 ° C./hour.
After crushing, press-molding is performed under an oxygen atmosphere.
Calcination was performed at 8 ° C. for 8 hours to obtain a positive electrode active material. And this positive electrode active material, conductive material (carbon) and binder (fluoro rubber)
Was dispersed in n-methylpyrrolidone at a weight ratio of 90: 5: 5 and applied to an A1 plate. This was hot-pressed at 200 ° C., and then punched out into φ15 mm to obtain a positive electrode.

On the other hand, a lithium metal plate was used for the negative electrode. One mole of lithium phosphide (LiPF ₆ ) was mixed as a supporting electrolyte in an organic solvent in which polycarbonate (PC) and dimethoxyethane (DME) were mixed in equal amounts as an electrolyte.
To prepare a lithium secondary battery using the _{M-LiPF 6 / PC + DME} . The separator of this lithium ion battery uses a polyethylene porous membrane as a polyolefin-based material.

Next, the charge / discharge current density of the lithium secondary battery thus manufactured was set to 1 mA / cm ² ,
As a result of charging and discharging under the conditions of a charge upper limit voltage of 4.3 V and a discharge lower limit voltage of 3.0 V, the discharge capacity of the positive electrode was 1
36 mAh / g was obtained.

(Example 2) An aqueous solution of lithium acetate, an aqueous solution of manganese acetate, and an aqueous solution of citric acid were prepared in the same manner as in Example 1, and these aqueous solutions were mixed at a molar ratio of lithium ion, manganese ion and citrate ion of 1 .00: 2.
00: 1.67. In this case, the mixing ratio of citric acid to lithium ions and manganese ions in the mixed aqueous solution is substantially equal. PH of mixed solution
Was 3.0.

This mixed solution was dehydrated and heated under the same conditions as in Example 1, and Li _x Mn ₂ O ₄ (0 <X
<2) at which the Mn ²⁺ ion number in the precursor was also quantified by ESR, Mn ²⁺ ion count of the total precursor was 9.5 × 10 ²² pieces. Then, the precursor was baked under the same conditions as in Example 1, and a lithium secondary battery was similarly manufactured and subjected to a charge / discharge test.
The discharge capacity of the positive electrode was 132 mAh / g.

Example 3 An aqueous solution of lithium acetate, an aqueous solution of manganese acetate, and an aqueous solution of citric acid were prepared in the same manner as in Examples 1 and 2, and these aqueous solutions were mixed. The molar ratio of manganese ions to citrate ions is now 1.00: 2.00:
It mixed so that it might be set to 2.00. In this case, the mixing ratio of citric acid to lithium ions and manganese ions in the mixed aqueous solution is approximately 120%. The pH of the mixed solution is
3.0.

The mixed solution was dehydrated and heated under the same conditions as in Examples 1 and 2, and the number of Mn ²⁺ ions in all the precursors determined by ESR was 10.2 × 10 ²² . Further, the precursor was fired under the same conditions as in Examples 1 and 2 to prepare a lithium secondary battery and a charge / discharge test was performed. As a result, the discharge capacity of the positive electrode was 134 mAh /
g.

Next, as a comparative example, lithium carbonate (Li ₂
CO ₃ ) and manganese carbonate (MnCO ₃ ) to produce a positive electrode active material by a solid phase reaction method, and alkaline lithium hydroxide (LiOH) as a lithium raw material,
A case where a positive electrode active material is prepared by mixing an aqueous solution of manganese acetate and citric acid is shown below as a comparative example.

Comparative Example 1 A positive electrode active material was obtained from lithium carbonate and manganese carbonate as raw materials by a usual solid-phase reaction method. The firing conditions are 800 ° C. for 8 hours, and the rate of temperature rise is 1 hour / hour.
The temperature was 50 ° C. and the cooling rate was 100 ° C./hour. At this time, the discharge capacity at the positive electrode of the lithium secondary battery prototype was 1
It was 20 mAh / g.

(Comparative Example 2) Alkaline lithium hydroxide was used as a lithium raw material, and this was added to a mixed solution of a manganese acetate aqueous solution and a citric acid aqueous solution. Thus, the molar ratio of lithium ion, manganese ion and citrate ion was 1.00: 2.00: 1.67. The mixing ratio of citric acid to cations in the mixed solution at this time is
It is about 100%. The pH of the mixed solution at that time
Was 4.0.

In a quantitative measurement by ESR of a precursor obtained by heating the mixed solution by dehydration, the number of Mn ²⁺ ions in all the precursors was 8.5 × 10 ²² . Further, in a charge / discharge test using a lithium secondary battery, the discharge capacity of the positive electrode was 130 mAh / g.

(Comparative Example 3) As in Comparative Example 2, lithium hydroxide, lithium acetate and citric acid were used. At this time, the molar ratio of lithium ion, manganese ion and citrate ion was used. But 1.00: 2.00:
It was blended to give 1.83. At this time, the mixing ratio of citric acid to cation in the mixed solution was about 110
%. The pH of the mixed solution was 4.0.

Also in this case, the number of Mn ²⁺ ions in all the precursors was 9.1 × 10 ²² by quantitative measurement by ESR of the precursor obtained by dehydration heating of the mixed solution. Further, in a charge / discharge test using a lithium secondary battery, the discharge capacity of the positive electrode was 130 mAh / g.

FIG. 1 summarizes the above results and shows the relationship between the number of Mn ²⁺ ions in all the precursors and the discharge capacity of the positive electrode. The horizontal axis shows the number of Mn ²⁺ ions in all the precursors, and the vertical axis shows the discharge capacity of the positive electrode.

FIG. 1 shows that all of the positive electrodes (Examples 1 to 3) produced by the production method according to the present invention have a high discharge capacity (> 130 mAh / g). Comparative Example 1
Is 120 mAh / g, that of Comparative Examples 2 and 3 is 1
It was about 30 mAh / g. The extremely low value of Comparative Example 1 is considered to be due to a problem in the dispersibility (uniformity) of the lithium and manganese compositions because it was produced by the solid phase method. It is.

More specifically, FIG. 1 shows that Mn ²⁺
There is a correlation between the number and the discharge capacity of the positive electrode, and Mn in the precursor
It was confirmed that the discharge capacity increased as the ²⁺ number increased. The reason that the Mn ²⁺ ion concentration in the precursor in the case of the production method according to the present invention (Examples 1 to 3) is higher than that in Comparative Examples 2 and 3 is that in the present invention, an acidic lithium acetate is used as a lithium raw material. Therefore, it is considered that the acidity in the mixed aqueous solution of manganese acetate and citric acid was increased, and this promoted the dissociation of manganese acetate, so that more Mn ²⁺ ions could be present in the solution. On the other hand, in Comparative Examples 2 and 3, the dissociation of manganese acetate was suppressed because alkaline lithium hydroxide was used as the lithium raw material, and it is considered that a large amount of Mn ²⁺ ions could not be present.

Further, among the products of the present invention (Examples 1 to 3), the value of the discharge capacity was highest in Example 1 and then in Examples 3 and 2 in this order. From the results, it can be said that when preparing a mixed aqueous solution of lithium acetate, manganese acetate, and citric acid, the mixing ratio of citric acid should be larger than the total valency of lithium ions and manganese ions.

FIG. 2 shows the products of the present invention (Examples 1 to 3).
In comparison between 3) and the comparative product (Comparative Examples 2 and 3), the relationship between the ratio of the citrate valence to the cation valence and the Mn ²⁺ number in the precursor by ESR was shown. As can be seen from FIG. 2, even when comparing the comparative products (Comparative Examples 2 and 3), if the mixing ratio of citric acid to cation increases, the number of Mn ²⁺ in the precursor increases. In this case, even if the ratio of the citrate ion valence is the same as that of the comparative product, M
The number of n ²⁺ was further increased, which seems to indicate a high value of the discharge capacity as explained in FIG.

Also, when comparing the products of the present invention (Examples 1 to 3), the mixing ratio of citric acid was particularly so set as to be about 110% with respect to the total valency of lithium ions and manganese ions. A peak effect was observed in the case of Example 1, then Example 2 (mixing ratio 120%) showed a high value, and Example 3 (mixing ratio 100%) with the lowest mixing ratio showed the lowest value. .

By increasing the amount of citric acid with respect to the total valency of lithium ions and manganese ions, Mn ²⁺ ions were added excessively at the stage of forming a citric acid complex with lithium ions. citric acid to form a more Mn ²⁺ ions complexed be uniformly Mn ²⁺ ions dispersed in together in the complex and that according to the liquid phase method, the calcined product obtained therefrom is uniform in composition It is considered that the sample showed a high discharge capacity.

The present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the present invention. For example, although lithium acetate and manganese acetate were shown as acidic lithium salts and acidic manganese salts,
Similar effects can be obtained by using other organic acid salts or inorganic acid salts. As for the acid manganese salt, other transition metal acid salts such as an acid cobalt salt and an acid nickel salt can also be applied based on the development history of this lithium secondary battery.

[0046]

According to the present invention, a citric acid complex having a high Mn ²⁺ ion concentration is produced from a mixed aqueous solution of an acidic lithium salt, an acidic manganese salt, and citric acid, which is dehydrated, heated, and calcined. Thus, a positive electrode active material having a uniform composition and a high discharge capacity can be obtained. Further, by setting the mixing ratio of citric acid in the citric acid complex solution to be equal to or more than the total valence of lithium ions and manganese ions as cations, preferably about 110%, higher discharge capacity can be obtained. can get.

Therefore, the positive electrode active material for a lithium secondary battery obtained by the present invention can be used as a power supply battery for communication office equipment such as personal computers and mobile phones, which are expected to spread in the future, or as a power supply for electric vehicles. It can sufficiently meet the needs for higher performance.

[Brief description of the drawings]

FIG. 1 shows a product of the present invention (Examples 1 to 3) and a comparative product (Comparative Example 1)
FIG. 3 is a diagram showing the relationship between the number of Mn ²⁺ ions in all precursors and the discharge capacity of the positive electrode in comparison with the above (3).

FIG. 2 shows the ratio of citrate valence to cation valence and Mn ²⁺ in the precursor by ESR in comparison between the product of the present invention (Examples 1 to 3) and the comparative product (Comparative Examples 2 and 3). It is a figure showing the relation with number.

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Miho Hatanaka 41 Toyoda Central Research Institute, Inc. 41, Yokomichi, Toyota Central Research Laboratory Co., Ltd. (72) Inventor Jun Sugiyama, 41, Chuchu Yokomichi, Oji, Nagakute-cho, Aichi-gun, Aichi Prefecture, Japan Toyota Central Research Laboratory Co., Ltd. (72) Inventor Tatsumi Hioki Aichi 41 Toyota Chuo R & D Co., Ltd., 41, Nagakute-cho, Nagakute-cho, Aichi-gun, Japan (72) Inventor Hisao Kojima 1-1-1, Showa-cho, Kariya-shi, Aichi, Japan Denso Corporation

Claims (1)

[Claims] 1. A step of producing an aqueous solution of a citric acid complex with an acidic lithium salt, an acidic manganese salt and citric acid, a step of dehydrating the citric acid complex to produce a precursor, and calcining the precursor. A method of producing Li _x Mn ₂ O ₄ (0 <X <2) by using the following method.