JP3519466B2 - Method for treating positive electrode material and lithium secondary battery using the same - Google Patents

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 its design, and more particularly to the surface condition of its positive electrode material.

[0002]

2. Description of the Related Art As positive electrode materials for lithium secondary batteries, in addition to titanium disulfide, lithium metal and the like, lithium transition metal oxides such as lithium cobalt oxide and lithium nickel oxide are known.

[0003]

However, although such a lithium transition metal oxide is preferable for increasing the discharge capacity, there is a problem that the capacity deteriorates due to the cycle characteristics due to charge and discharge.

[0004]

In order to solve the above-mentioned problems, the inventors of the present invention have made various investigations. As a result, the lithium transition metal oxide powder was allowed to stand at room temperature in an atmosphere containing CO ₂ gas for a certain period of time. However, it was found that good cycle characteristics can be achieved by using this lithium transition metal oxide powder for the positive electrode. The reason why the cycle characteristics deteriorate when a lithium transition metal oxide such as lithium nickel oxide is used as the positive electrode active material is that the reaction between the electrolytic solution and the positive electrode active material or the decrease in the adhesive strength between the current collector and the active material. Although various factors such as the above can be considered, the cause of the positive electrode active material is the former reaction with the electrolytic solution, and it is considered that the surface state of the positive electrode active material has a great influence on this.

In the XPS measurement of the C1S spectrum, the peak of 285 eV and the peak of 289 eV are CO ₃
It is considered to indicate the presence of a group.

That is, the surface state of a usual lithium transition metal oxide is an OH group in which H ₂ O or the like is adsorbed by oxygen, and in this state, a positive electrode active material of the lithium transition metal oxide showing a strong oxidizing power. Is considered to cause oxidative decomposition of the electrolytic solution and deteriorate cycle characteristics during charge and discharge.

Therefore, by chemically adsorbing a CO ₃ group on the surface of the positive electrode active material, a surface state showing peaks at 285 eV and 289 eV can be obtained by XPS measurement of the C1S spectrum as described above. The layer on which the CO ₃ group is adsorbed suppresses the reaction between the positive electrode active material and the electrolytic solution,
It is considered that good cycle characteristics can be achieved.

Further, the peak ratio of 285 eV and 289 eV changes depending on the amount of CO ₃ groups adsorbed on the surface of the positive electrode active material, and the peak ratio of 285 eV and 289 eV is between 0.4 and 0.9. Is more preferable.

As a method for producing such a positive electrode material showing peaks at 285 eV and 289 eV by XPS measurement of C1S spectrum, a high temperature in an atmosphere containing CO ₂ at the time of synthesizing a lithium transition metal oxide is used. A method of heat treatment may be used, but in order to form the chemisorption layer only on the outermost surface, the method of the present invention in which the lithium transition metal oxide is allowed to stand in an atmosphere containing CO ₂ gas for a certain period of time is preferable.

[0010] That is, in a method of treatment in an atmosphere containing CO ₂ during the former synthesis, first, a chemical reaction occurs between the material and the CO _2, the by-products are produced, such as Li ₂ CO ₃ on the surface of the cathode active material Can be considered.

This is because the processing temperature is from 200 ° C to 900 ° C.
It is thought to be a reaction that occurs due to the treatment at high temperature.

On the other hand, in the latter treatment method, since the treatment is carried out at room temperature, the above reaction does not occur, and by-products are not formed, so that an adsorption layer can be formed by so-called chemical adsorption.

Next, there is a difference in the thickness of the surface layer modified by CO ₂ gas. In the former method, not only the very surface layer is modified, but also the surface layer of the original material of the lithium transition metal oxide at a considerable depth reacts, which makes it impossible to charge and discharge, which adversely affects the battery characteristics. Exert.

On the other hand, in the latter method, CO ₂ gas is chemisorbed, and is limited to an extremely surface layer that can be detected by XPS measurement. Therefore, the inside of the active material remains as the original material of the lithium transition metal oxide, and has almost no effect on the battery characteristics.

In addition, among lithium transition metal oxides, lithium nickel oxide has a particularly large capacity deterioration due to cycling, and the method of the present invention is more effective.

The lithium ion secondary battery is constructed, for example, as follows. For the positive electrode, for example, lithium nickel oxide alone or as a solid solution, or a mixture of small amounts of other oxides can be used.

Then, an electron conduction aid such as flaky graphite or acetylene black, and a binder such as polyvinylidene fluoride (hereinafter abbreviated as PVDF) or polytetrafluoroethylene are added thereto. It is prepared by mixing and molding the obtained positive electrode mixture by an appropriate means. Usually, N-methylpyrrolidone (hereinafter, NM
An electrode obtained by applying a coating liquid in which an active material, an electron conduction aid and PVDF are dissolved in (abbreviated as P) onto a metal foil such as Al or stainless steel, and drying and pressing is used. In particular, the effect was remarkable when PVDF was used as the binder.

Lithium metal or a lithium-containing compound is used for the negative electrode. Examples of the lithium-containing compound include lithium alloys and others. Examples of the lithium alloy include lithium-aluminum, lithium-lead, lithium-indium, lithium-gallium, lithium-indium-gallium and the like. Examples of lithium-containing compounds other than lithium alloys include carbon materials having a turbostratic structure, graphite, tungsten oxide, lithium iron composite oxide, and the like.
Some of these do not contain lithium at the time of production, but when they act as a negative electrode, they are in a state of containing lithium by chemical means or electrochemical means.

The weight ratio of the positive electrode and the negative electrode depends on the materials used. For example, in a system using lithium cobalt oxide for the positive electrode and graphite for the negative electrode, the charge / discharge capacity of the battery becomes a large value at 1.9 or less, In a system in which lithium nickel oxide is used for the positive electrode and graphite is used for the negative electrode, it is desirable to set 2.0 to 3.0 in order to obtain a large capacity battery in view of the difference in the diffusion coefficient of the material.

Further, the positive electrode material and the negative electrode material each have a difference in charge capacity and discharge capacity called retention. For example, in the case of lithium nickel oxide, a certain proportion of the lithium that is lost after the first charge of the battery does not return to the active material in a normal discharge state, and does not return until it is discharged to nearly 1.5V. .

It is considered that this is a phenomenon that a certain proportion of lithium in the lithium charged in the first charging of the battery is taken into the carbon material. Therefore, in order to increase the capacity of the battery, It is preferable that the retention ratio (%) of the positive electrode and the negative electrode (ratio of irreversible capacity that cannot be charged / discharged to capacity that can be charged / discharged) be matched in order to obtain a battery having a large capacity.

Even with the same graphite material, for example, artificial graphite and natural graphite have different retention ratios (%).
Therefore, it is also an effective method to mix two or more kinds of carbon materials having different retention rates (%) and adjust the retention rate (%) of the lithium transition metal oxide. On the contrary, there is also a method in which lithium manganese oxide or the like is mixed with lithium nickel oxide or the like to adjust the retention of the negative electrode.

Examples of the electrolytic solution include 1,2-dimethoxyethane, 1,2-diethoxyethane, propylene carbonate, ethylene carbonate, γ-butyrolactone, tetrahydrofuran, 1,3-dioxolane, diethylene carbonate, dimethyl carbonate and ethylmethyl. It is possible to use a single solvent such as carbonate or a mixed solvent of two or more kinds. If the amount of the electrolytic solution is large, it may cause leakage or the like, and if it is small, the electrode cannot penetrate into the electrode and the load characteristics are deteriorated. From this point, 0.1 to 0.5% by weight is preferable in terms of weight% with respect to the total amount of the positive electrode and the negative electrode active material.

As the electrolyte, for example, LiCF ₃ SO ₃ ,
LiC ₄ F ₉ SO ₃ , LiClO ₄ , LiPF ₆ , LiBF ₄
An organic electrolyte solution in which one or more electrolytes are dissolved is used.

As the separator, one having sufficient strength and capable of holding a large amount of electrolytic solution is preferable. From this point, 10
Separator made of polypropylene or polyethylene having an opening ratio of 30 to 70% of 50 to 50 μm is preferable.

The electron conduction aid mixed with the positive electrode material is, for example, 1 to 9% by weight, particularly 2 to 6% by weight of flake graphite, and the binder is, for example, 1 to 5% by weight, particularly 1 to 5% by weight of PVDF. It is preferable to mix 3% by weight. Similarly, for the negative electrode, it is preferable to mix the binder in an amount of 5 to 20% by weight.

Although an applicator was used for coating the electrodes this time, a reverse roll or die coating is preferable for coating several tens of microns. Also, a multilayer coating in which coating liquids having different compositions are applied in layers is possible. The positive electrode material may be changed, and lithium cobalt oxide may be used as a lower layer, lithium nickel oxide may be used as an upper layer, and PVDF as a binder and polyurethane as a binder may be stacked.

The structure of the battery is, for example, nickel-plated iron,
An explosion-proof vent is provided in a rectangular or cylindrical container such as stainless steel to prevent the battery from bursting when gas is generated inside.

The charger controls the maximum current with a constant voltage,
Use what is charged. Further, as the battery pack, in order to ensure reliability, a battery pack provided with a protection circuit such as a fuse when an excessive current flows is usually used.

[0030]

EXAMPLES Next, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to only those examples.

Example 1 As a negative electrode material, artificial graphite synthesized at 2800 ° C. was used. In addition, as the positive electrode material, lithium hydroxide (LiOH.H ₂ O) and nickel oxide (Ш) are used.
Lithium nickel oxide synthesized by heat treating (Ni ₂ O ₃ ) (normally expressed as LiNiO ₂ ,
(The ratio of the values slightly deviates from the stoichiometric composition).

The above synthesis was carried out as follows.

Li / hydroxide of lithium hydroxide and nickel oxide
After weighing so as to have a ratio of Ni = 1 / 1.05 (molar ratio), they were pulverized and mixed in an agate mortar. This was preheated in an oxygen (O ₂ ) stream at 500 ° C. for 2 hours and then heated at 700 ° C. for 20 hours to be fired.

Lithium nickel oxide synthesized by heat treatment as described above was used as a positive electrode active material. Since the synthesized lithium nickel oxide is weak against moisture, handling such as pulverization was performed in an Ar gas atmosphere.

The obtained lithium nickel oxide powder was left to stand in an atmosphere of 100% CO ₂ gas for 60 minutes.

Next, a sheet electrode was prepared by using the respective materials as follows. In addition, the weight ratio of the positive electrode and the negative electrode was set to positive electrode / negative electrode = 2.0 in terms of the weight ratio of the active material.

6% by weight of flake graphite as an electron conduction aid and 3% by weight of PVDF as a binder were mixed with the above positive electrode material. For mixing, pre-dissolve PVDF in NMP,
A positive electrode material and scaly graphite were added to this, and NMP was added to prepare a coating liquid having a viscosity adjusted.

This coating solution was placed on an Al foil having a thickness of 20 μm, and the coating solution was rubbed off with an applicator having a constant gap, and dried. Similarly, the back surface of the Al foil was also applied and vacuum dried. This electrode was pressed and cut into a width of 28 mm to produce a positive electrode sheet-shaped electrode.

Similarly, the negative electrode also contains P as a binder for artificial graphite.
10% by weight of VDF was mixed, NMP was added to adjust the viscosity, and the composition was applied on a Cu foil having a thickness of 18 μm to prepare a sheet electrode.

The electrolytic solution is ethylene carbonate (EC).
An organic electrolytic solution was used in which 1 mol / l of LiPF ₆ was dissolved in a mixed solution of 1 mol / l of ethyl methyl carbonate (EMC) (volume ratio 1: 1).

Using the above battery constituent materials, R5 type (1
A prototype having a shape of 4.95 mm diameter and 39.7 mm length) was produced. A cross-sectional view of a prototype lithium-ion secondary battery is shown in FIG.

A strip-shaped tab of Al or Ni was resistance-welded to the exposed portion of the Al foil or Cu foil at the end of each electrode, and a polyethylene separator having a thickness of 25 μm was sandwiched and wound.

The Ni tab of the negative electrode was welded to the bottom of the can through the insulating ring, the insulating ring was inserted into the upper part of the can, and after grooved, the sealing body and the Al tab of the positive electrode were bonded by a welding machine. The battery was dried with a vacuum dryer, and 2 cc of the electrolytic solution was injected into the glove box in a dry atmosphere, and then the cell was sealed.

In FIG. 1, 1 is a positive electrode and 2 is a negative electrode. 3 is a separator made of a microporous polyethylene film, 4 is a sealing body made by welding a positive electrode tab made of stainless steel, and 5 is a negative electrode can made of stainless steel.

The negative electrode tab is welded to 5.

Comparative Example 1 A battery was prepared in the same manner as in Example 1 except that the positive electrode material was not treated with CO ₂ gas.

Example 2 As a negative electrode material, artificial graphite synthesized at 2800 ° C. was used. Further, the positive electrode material includes lithium hydroxide (LiOH.H ₂ O) and cobalt oxide (Co ₃
A lithium cobalt oxide (usually referred to as LiCoO ₂ ) synthesized by heat treatment with O ₄ ) was used. In addition,
The weight ratio of the positive electrode and the negative electrode is the weight ratio of the active material: positive electrode / negative electrode =
It was set to 1.8. Except for the above, in the same manner as in Example 1,
A battery was made.

Comparative Example 2 A battery was prepared in the same manner as in Example 2 except that the positive electrode material was not treated with CO ₂ gas.

Next, the batteries of Examples 1 and 2 and Comparative Examples 1 and 2 were charged and discharged.

Charging was carried out at a constant voltage of 4.2 V and a current limit of 1 C was set. The discharge was performed up to 2.75V. When the charging / discharging current was expressed by C, charging / discharging was performed with R5 type at 560 mA of 1C.

Table 1 shows the capacities of the above Examples and Comparative Examples after each cycle.

[0052]

[Table 1]

As shown in Table 1, in a graphite / lithium nickel oxide battery, Example 1 showed less capacity deterioration due to charge / discharge than Comparative Example 1, and Comparative Example 1 showed capacity deterioration after 20 cycles. It got bigger.

FIG. 2 shows the XPS measurement of the lithium nickel oxide treated with CO ₂ gas in the example. 285 eV and 2
It can be seen that it has a peak at 89 eV. The ratio of each peak was 0.42. In Comparative Example 1, such a peak was not seen.

Further, as shown in Table 1, in the battery of graphite / lithium cobalt oxide system, Example 2 has less capacity deterioration due to charge / discharge as compared with Comparative Example 2, and Comparative Example 2 has capacity after 50 cycles. Deterioration has increased.

FIG. 3 shows the XPS measurement of the lithium cobalt oxide treated with CO ₂ gas in the example. 285 eV and 2
It can be seen that the positive electrode active material has a peak at 89 eV.
The ratio of each peak was 0.70. In Comparative Example 2, such a peak was not seen.

The measurement conditions of XPS are ESCA LB m
It was performed with Mg Kα 12 kV-10 mA at ark2.

[0058]

As described above, in the lithium secondary battery using the lithium transition metal oxide powder for the positive electrode,
It has been clarified that the cycle characteristics can be improved by allowing the lithium transition metal oxide powder to stand at room temperature in an atmosphere containing CO ₂ gas for a certain period of time.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing an example of a lithium secondary battery according to the present invention.

FIG. 2 is a diagram schematically showing an XPS measurement spectrum of the positive electrode active material used in Example 1.

FIG. 3 is a diagram schematically showing an XPS spectrum of the positive electrode active material used in Example 2.

[Explanation of symbols]

1 positive electrode 2 Negative electrode 3 separator 4 Positive electrode cap 5 negative electrode can

Continuation of front page (56) Reference JP-A-5-182667 (JP, A) JP-A-6-150975 (JP, A) JP-A-4-329268 (JP, A) JP-A-6-140077 (JP , A) JP 6-124700 (JP, A) JP 6-338323 (JP, A) JP 59-134567 (JP, A) JP 7-249431 (JP, A) JP 8-162164 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) H01M 4/02 H01M 4/04 H01M 4/58 H01M 10/40

Claims (5)

(57) [Claims] 1. A lithium secondary battery using a lithium transition metal oxide as a positive electrode, wherein the lithium transition metal oxide powder is left at room temperature in an atmosphere containing CO ₂ gas. A method for treating a positive electrode material for a battery. 2. A lithium secondary battery comprising a positive electrode prepared by using a lithium transition metal oxide powder left at room temperature in an atmosphere containing CO ₂ gas. 3. A lithium secondary battery using a lithium transition metal oxide as a positive electrode, wherein the lithium transition metal oxide is 285 eV and 289 in XPS measurement of C1s spectrum.
A lithium secondary battery comprising an extremely surface layer having a peak ratio of eV of 0.4 to 0.9 and an interior which is still the original lithium transition metal oxide. 4. A lithium secondary battery using a lithium transition metal oxide as a positive electrode, wherein the lithium transition metal oxide powder is left at room temperature in an atmosphere containing CO ₂ gas to obtain XPS of C1s spectrum. 285e by measurement
A lithium secondary battery comprising an extremely surface layer having a peak ratio of V to 289 eV of 0.4 to 0.9, and an interior that remains the original lithium transition metal oxide. 5. The lithium secondary battery according to claim 2, wherein the lithium transition metal oxide is lithium nickel oxide.