JP3252433B2 - Method for producing positive electrode active material and non-aqueous electrolyte secondary battery using the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a non-aqueous electrolyte secondary battery,
In particular, the present invention relates to a method for producing a positive electrode active material used for a lithium secondary battery and a nonaqueous electrolyte secondary battery using the same.

[0002]

2. Description of the Related Art In recent years, portable and cordless electronic devices such as AV devices and personal computers have been rapidly advanced, and there has been a demand for a small, lightweight, high energy density secondary battery as a power supply for driving these devices. high. In this respect, non-aqueous secondary batteries, especially lithium secondary batteries, are expected to be batteries having a high voltage and a high energy density.

As a positive electrode active material satisfying this demand, LiCoO ₂ has been proposed as a layered compound capable of intercalating and de-intercalating lithium, and the development of a 4 V class high energy density secondary battery has been advanced. .

[0004] Such LiCoO ₂ is disclosed in
As disclosed in the publication of No. 256371, lithium carbonate Li ₂ Co ₃ and cobalt carbonate CoCO ₃ are produced by using Co / L
It is synthesized by mixing at a molar ratio of 1: 1 and firing at 900 ° C. for 5 hours (typically at 650 to 1000 ° C. for 5 to 20 hours or by repeating and firing). The method is general. In this case,
The chemical reaction formula is shown in (Chem. 1).

[0005]

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[0006]

When such a conventional cathode material is formed as a thin-film electrode plate, a paste material such as an aqueous solution of carboxymethyl cellulose is mixed after mixing the cathode active material with a conductive agent and a binder. It is suspended in a solution to form a paste, applied to a current collector metal foil, dried and rolled to obtain an electrode plate. In this method, the production process is very easy, carboxymethyl cellulose can be easily removed by heat treatment, and water is used as a solvent, which is advantageous in cost.

However, LiCoO ₂ synthesized with Li ₂ CO ₃ and CoCO ₃ as starting materials and a Co / Li molar ratio of 1.0 to 1.1 is extremely basic when suspended in an aqueous solution of carboxymethylcellulose. When applied to an aluminum foil as a current collector, there is a problem that the aluminum surface is corroded and the electrode plate is significantly collapsed.

Further, such a base component forms a film on the electrode plate when the battery is stored at a high temperature (for example, 60 ° C.),
There is a problem that battery characteristics are significantly deteriorated.

Further, such a base component easily absorbs water, and the water mixed in the battery may cause the electrolyte (for example, L
It was easy to cause problems in battery configuration, such as decomposing iPF ₆ ) and deteriorating battery characteristics.

As described above, as a result of exploring the cause of exhibiting basicity, the following has become clear. As shown in Chemical Formula ₁ , oxygen is required for the synthesis reaction of LiCoO ₂ , but carbon dioxide is generated as the reaction proceeds, so that the reaction system becomes an inert atmosphere. For this reason, it was found that oxygen required for the synthesis of LiCoO ₂ was insufficient, the oxidation reaction did not proceed sufficiently, and unreacted lithium carbonate remained, which was eluted into the aqueous solution and exhibited basicity.

The present invention has been made to solve the above problems, and provides a method for producing a positive electrode active material that suppresses carbon dioxide generation during the synthesis of LiCoO ₂ and completes an oxidation reaction, and a battery using the same. It is intended for.

[0012]

In order to solve this problem, the present invention provides a method for producing LiCoO ₂ by using cobalt oxide Co ₃ O ₄ instead of cobalt carbonate as a cobalt source. As shown, LiCoO
The amount of carbon dioxide generated when 1 mole of ₂ is synthesized is
This is one-third that of the case where cobalt carbonate is used, and is less likely to be in an inert atmosphere, which is advantageous for promoting the oxidation reaction.

[0013]

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In order to prevent lithium carbonate from remaining, it is more preferable to set the condition that the Co / Li ratio is larger than 1. On the contrary, if the Co / Li ratio is too large, cobalt oxide remains in the active material. However, when the battery is stored at a high temperature, there are problems such as decomposition of the electrolytic solution. Therefore, it is necessary to minimize the residual amount of cobalt oxide.

In order to satisfy the above conditions, the present invention relates to Li
Using cobalt oxide Co ₃ O ₄ as cobalt sources CoO _2, is obtained by a 1.01 to 1.07 the Co / Li ratio in order to proceed to completion for a further reaction.

Further, in order to reduce the amount of the remaining cobalt oxide Co ₃ O ₄ , the Co / Li ratio is adjusted to 1.02 to 1.
A value of 05 is preferred.

A non-aqueous electrolyte secondary battery using such a positive electrode active material has an X-ray diffraction diagram of LiCoO _{2 at about} 20 degrees in a CuKα ray X-ray diffraction diagram of a lithium-containing cobalt composite oxide. the peak of the surface, in the vicinity of 35 ° Co ₃ of O ₄ (311) peak intensity ratio of the surface Co ₃ O ₄ (31
1) / LiCoO ₂ (003) is 0.005 to 0.06
And furthermore, 0.015 to 0.055
It is preferable to use those in the range of

[0018]

As described above, LiCoO_TwoIn the synthesis of
By using cobalt oxide as a Baltic source,
The amount of carbonized carbon is 1/3 that of the case of using cobalt carbonate
And no longer inhibits the oxidation reaction. Further, Co / L
By setting the i ratio to 1.01 or 1.02 or more, (X
In the line diffraction, Co_ThreeO_Four(311) / LiCoO _Two(0
03) is 0.005 or more, 0.015 or more).
The reaction can be completed and lithium carbonate remains.
LiCoO_TwoCan be synthesized. this
As a result, the electrode absorbs water and reacts with lithium carbonate itself.
Prevents deterioration of battery characteristics such as deterioration of storage characteristics
can do.

Further, when forming the electrode plate, even if the active material is suspended in a paste such as an aqueous solution of carboxymethylcellulose, it does not exhibit basicity, and the electrode plate is prevented from collapsing due to corrosion of the aluminum surface. be able to.

By setting the Co / Li ratio to 1.07 or 1.05 or less (Co ₃ O ₄ (311) / LiCoO ₂ (003) in the X-ray
6 or 0.055 or less), the amount of cobalt oxide remaining in the active material is minimized, and the decomposition of the electrolytic solution during high-temperature storage can be avoided.

Further, by combining the positive electrode using the active material according to the present invention with an appropriate negative electrode, for example, carbon or lithium metal having high charge / discharge efficiency, it has high voltage, high capacity, and excellent charge / discharge cycle characteristics. A non-aqueous electrolyte secondary battery can be realized.

[0022]

An embodiment of the present invention will be described below with reference to the drawings.

A mixture of Li ₂ CO ₃ and Co ₃ O ₄ at the seven ratios shown in Table 1 so that the Co / Li ratio is 0.9-1.1 is obtained at 900 ° C. in air. What was calcined for 5 hours was used as a positive electrode active material.

[0024]

[Table 1]

The Co / Li ratio is 0.9, 1.0, 1.0
LiCoO synthesized as 5,1.1 _TwoX-ray diffraction diagram of
As shown in FIG.

[0026] As apparent from FIG. 1, although Co / Li ratio is not observed a peak of Co ₃ O ₄ is 1 or less, Co ₃ to 31.24 degrees and 36.83 degrees becomes greater than 1
Peaks of the (220) plane and the (311) plane of O ₄ appear.

The peak of the (003) plane of LiCoO ₂ ,
The peak intensity ratio (Co ₃ O ₄ (311) / LiCoO ₂ (003)) of the (311) plane of Co ₃ O _{4 at} around 37 degrees and C
FIG. 2 shows the relationship with the o / Li ratio. The peak intensity ratio is C
When the o / Li ratio becomes 1 or more, it rises almost linearly,
When the / Li ratio is 1.1, it reaches 0073.

Each of the thus synthesized samples was mixed with 100 parts by weight of a positive electrode active material, 3 parts by weight of acetylene black and 7 parts by weight of a fluororesin binder to form a positive electrode mixture, and suspended in an aqueous solution of carboxymethyl cellulose. It became cloudy and became a paste. This paste was applied on both sides of an aluminum foil, dried and rolled to obtain an electrode plate.

The pH of the paste obtained by kneading the carboxymethylcellulose aqueous solution and the positive electrode active material (hereinafter simply referred to as paste) is also shown in Table 1 below.

The negative electrode is a carbon material 10 obtained by heat treating coke.
0 parts by weight, 10 parts by weight of a fluororesin binder is mixed,
It was suspended in an aqueous solution of carboxymethyl cellulose to form a paste. Then, this pace was applied to both surfaces of the copper foil, dried and rolled to obtain an electrode plate.

FIG. 3 is a longitudinal sectional view of the cylindrical battery used in this embodiment. (Battery size: diameter 13.8mm, height 50
mm (AA)) Leads were attached to the positive and negative electrodes, respectively, and spirally wound through a polypropylene separator, and stored in a battery case. As the electrolytic solution, a solution prepared by dissolving LiFP ₆ at a ratio of 1 mol / liter in a mixed solvent of equal volumes of diethyl carbonate and ethylene carbonate was used. The sealed battery was used as a test battery.

In FIG. 3, reference numeral 1 denotes a battery case formed by processing a stainless steel plate having resistance to organic electrolyte, reference numeral 2 denotes a sealing plate provided with a safety valve, and reference numeral 3 denotes an insulating packing. Reference numeral 4 denotes an electrode group, in which a positive electrode and a negative electrode are spirally wound a plurality of times via a separator and housed in a case. A positive electrode lead 5 is pulled out from the positive electrode and connected to the sealing plate 2, and a negative electrode lead 6 is drawn out from the negative electrode and connected to the bottom of the battery case 1. Reference numeral 7 denotes an insulating ring provided on the upper and lower portions of the electrode plate group 4, respectively.

(Comparative Example) Li ₂ CO ₃ and CoCO ₃ having a Co / Li ratio of 0.9 to 1.
As shown in (Table 2), a mixture obtained by mixing seven kinds at a ratio of 1 and calcining at 900 ° C. for 5 hours in air was used as a positive electrode active material. The pH of the paste is also shown in (Table 2).

[0034]

[Table 2]

Using each sample synthesized in this manner as a positive electrode active material, a battery was prepared in the same manner as in the example.

These test batteries were charged and discharged at a current of 100 mA.
h, after 10 cycles of a constant current charge / discharge test under the conditions of a charge termination voltage of 4.1 V and a discharge termination voltage of 3.0 V, a storage test at 60 ° C. for 20 days in a charged state (hereinafter referred to as high-temperature charge storage) ) Was performed to determine the capacity retention of the battery after storage.

FIG. 4 shows the Co / Li ratios of the batteries prepared in the example (○: synthesized from cobalt oxide) and the comparative example (合成: synthesized from cobalt carbonate) and the batteries A to L corresponding thereto.
4 shows the capacity retention ratio (capacity after storage / capacity before storage) of the battery in the high-temperature charge storage test.

As shown in FIG. 4, batteries using LiCoO ₂ synthesized from cobalt carbonate (△: batteries GL) and batteries A and B having a Co / Li ratio of 0.9 to 1.0 are: The capacity retention of the batteries after high-temperature storage is remarkably poor at about 60%. As shown in (Table 1) and (Table 2), these active materials have high basicity with a paste pH of 9 or more. When such a highly basic paste is applied to an aluminum foil serving as a current collector, the aluminum surface is corroded, causing significant collapse of the electrode plate. Thus, it is considered that the collapse of the electrode plate caused the active material to fall off during storage, and the capacity after storage to be reduced.

Further, such a base component absorbs water, and the water mixed in the battery decomposes LiPF ₆ as an electrolyte to generate hydrofluoric acid. It is considered that the storage characteristics deteriorated because the hydrofluoric acid corroded the core material and the case in the battery. Also, a battery F having a Co / Li ratio of 1.1
Contains a large amount of cobalt oxide, as can be seen from the X-ray diffraction diagram of FIG. Since the cobalt oxide acts as a catalyst for decomposition of the electrolytic solution during storage, the capacity retention is significantly reduced.

On the other hand, the batteries C to E have the paste p
As is evident from the fact that H is almost neutral with 7 or 8, the complete reaction of lithium carbonate eliminates the problem of corrosion of the core material and water absorption, and also contains cobalt oxide. Since the amount was small, it showed good storage characteristics.

As described above, the method for producing an active material for a non-aqueous electrolyte secondary battery according to the present embodiment uses lithium carbonate Li ₂ CO _3.
And cobalt oxide Co ₃ O ₄ at a Co / Li molar ratio of 1.0
It is desirable to mix and bake them at 1 to 1.07, more preferably 1.02 to 1.05.

As described above, the positive electrode active material in the non-aqueous electrolyte secondary battery has a LiCoO ₂ peak observed at about 2θ19 ° and a CoCo observed at about 37 ° in the X-ray diffraction diagram by CuKα ray. The peak intensity ratio of ₃ O ₄ is 0.005 to 0.06, more preferably 0.015
LiCoO ₂ 〜0.055 is used.

In this embodiment, Li is used as the electrolyte.
PF ₆ was used, but LiClO ₄ , LiBF ₄ , LiAsF
Similar effects were observed with electrolytes such as ₆ .

As for the non-aqueous solvent, cyclic esters such as propylene carbonate and butylene carbonate other than diethyl carbonate and ethylene carbonate, cyclic ethers such as tetrahydrofuran and 4-methyldioxolan, and chain-like compounds such as dimethoxyethane are also shown in this embodiment. Even when a binary solvent mixture using ether or the like is used, a chain ester such as methyl formate, methyl acetate, ethyl acetate, methyl propionate or ethyl propionate, or a chain ether such as dimethoxyethane is used. The same effect was obtained with a ternary or more multi-system solvent to which the above-mentioned components were added.

Similarly, as for the negative electrode, a carbon material is used for the negative electrode in the present embodiment, but a chargeable / dischargeable negative electrode such as lithium metal or a lithium alloy may be used.

[0046]

As is apparent from the above description of the embodiments, according to the present invention, the mixing ratio of lithium carbonate and cobalt oxide is defined in the range of 1.01 to 1.07 by the molar ratio of Co / Li. By doing so, it is possible to obtain a nonaqueous electrolyte secondary battery having a simple electrode plate configuration and excellent in high-temperature storage characteristics.

[Brief description of the drawings]

FIG. 1 shows LiCo as a positive electrode active material according to one embodiment of the present invention.
X-ray diffraction diagram of O ₂

FIG. 2 is a mixture ratio of cobalt oxide and lithium carbonate (Co
/ Li ratio) and the integrated intensity ratio Co of the peak of Co ₃ O ₄ by X-ray diffraction of the synthesized LiCoO ₂ with CuKα radiation.
Diagram showing the relationship of ₃ O ₄ / LiCoO ₂

FIG. 3 is a longitudinal sectional view of a cylindrical battery using the positive electrode active material.

FIG. 4 shows a mixture ratio of cobalt oxide and lithium carbonate (Co /
FIG. 4 is a diagram showing a relationship between the Li ratio) and the capacity retention of the battery after storage at a high temperature.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Battery case 2 Sealing plate 3 Insulation packing 4 Electrode group 5 Positive electrode lead 6 Negative electrode lead 7 Insulation ring

Continuation of the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) H01M 4/02-4/04 H01M 4/58 H01M 10/40 C01G 51/00

Claims (2)

(57) [Claims] 1. A method for producing a composite oxide mainly composed of lithium and cobalt, which is synthesized by mixing lithium carbonate and cobalt oxide Co ₃ O ₄ and firing the mixture, wherein the lithium carbonate and the cobalt oxide are mixed. Is mixed at a molar ratio of Co / Li in the range of 1.01 to 1.07 and calcined. 2. A rechargeable negative electrode, a non-aqueous electrolyte, and a rechargeable negative electrode.
A positive electrode using a cobalt-containing cobalt composite oxide as an active material;
A non-aqueous electrolyte secondary battery comprising:
The X-ray diffraction diagram of the cobalt composite oxide with CuKα radiation
And LiCoO observed when 2θ is around 19 degrees_TwoNo
And Co around 37 degrees_ThreeO _FourPeak intensity
Ratio Co_ThreeO_Four/ LiCoO_TwoIs 0.005 to 0.06
Non-aqueous electrolyte secondary battery.