JP2003217570A - Electrode for lithium secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrode for a lithium secondary battery that has high discharge capacity and excellent charge and discharge characteristics. <P>SOLUTION: This electrode for a lithium secondary battery comprises a current collector 7 and a positive electrode active material layer that is formed on the current collector 7 by a vacuum evaporator 1 and is made mainly of Fe<SB>3</SB>O<SB>4</SB>and has a diffraction peak in the vicinity of 30° when evaluated by the X-ray diffraction method. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an electrode for a lithium secondary battery, and more particularly to an electrode for a lithium secondary battery in which an active material layer is formed on a current collector.

[0002]

2. Description of the Related Art In recent years, a lithium secondary battery called a lithium ion battery has been widely used as a power source for mobile phones and personal computers. These mobile phones, personal computers, and the like are required to have a higher capacity lithium secondary battery because developments such as higher functionality and smaller size are progressing. However, a lithium secondary battery that has been put into practical use generally has a weight energy density of about 150 Wh / kg, and further higher weight energy density is required.

In the electrodes for lithium secondary batteries which have been put into practical use as described above, a carbon material such as graphite is used for the negative electrode and a lithium-containing oxide such as LiCoO ₂ is used for the positive electrode. There is. Then, as the electrolytic solution, an organic solvent such as a cyclic carbonate in which an electrolyte such as LiPF ₆ is dissolved or a chain carbonate such as dimethyl carbonate is used.

Then, during charging, lithium ions move from the positive electrode to the negative electrode and are stored therein, and during discharging, lithium ions return from the negative electrode to the positive electrode. Therefore, the energy density is determined by the theoretical capacity of the positive electrode, the theoretical capacity of the negative electrode and the battery voltage. For example, the theoretical capacity of a carbon-based material that is commonly used for the negative electrode material is
The largest graphite is 370 mAh / g. The theoretical capacity of LiCoO ₂ generally used for the positive electrode material is about 150 mAh / g. Thus, when comparing the theoretical capacities of the positive and negative electrodes, the capacity of the negative electrode is
Since it is more than double, to increase the energy density,
It is known that it is more effective to increase the theoretical capacity of the positive electrode than that of the negative electrode.

[0005]

However, LiC
In a positive electrode made of a lithium-containing oxide such as oO ₂ ,
When lithium ions are completely extracted from the positive electrode during charging / discharging, the crystal structure of LiCoO ₂ collapses, resulting in a disadvantage that the charging / discharging cycle is significantly reduced.
Therefore, in the conventional positive electrode made of LiCoO ₂ , Li
It is difficult to further increase the theoretical capacity of the positive electrode, because it is necessary to use all the lithium in CoO ₂ without using it and to leave it to some extent. As a result, there is a problem that it is difficult to obtain a lithium secondary battery having a high discharge capacity and excellent charge / discharge characteristics.

Further, the cobalt contained in LiCoO ₂ has a small reserve and is expensive, so that L
There is a need for a positive electrode material that replaces iCoO ₂ .

The present invention has been made to solve the above problems, and one object of the present invention is to:
An object is to provide an electrode for a lithium secondary battery, which has a high discharge capacity and an excellent charge / discharge characteristic.

Another object of the present invention is to use a positive electrode material as an alternative to a lithium-containing oxide such as LiCoO _{2 in} the above lithium secondary battery electrode.

[0009]

In order to achieve the above object, an electrode for a lithium secondary battery according to the first aspect of the present invention comprises a current collector and a raw material from a gas phase or a liquid phase on the current collector. Is formed by the method of supplying Fe, has an oxide containing Fe as a main component, and is 3 when evaluated by an X-ray diffraction method.
And a positive electrode active material layer having a diffraction peak near 0 °. The method of supplying the raw material from the vapor phase or the liquid phase in the present invention is, for example, a sputtering method, a reactive vapor deposition method, a vacuum vapor deposition method, a chemical vapor deposition method (CVD method), a thermal spraying method or a plating method, or a combination thereof. It is a broad concept including.

In the lithium secondary battery electrode according to the first aspect of the present invention, as described above, the main component is an oxide containing Fe, and it is 30 ° when evaluated by an X-ray diffraction method.
By providing the positive electrode active material layer having a diffraction peak in the vicinity, the discharge capacity of the positive electrode is increased, so that a lithium secondary battery having high discharge capacity and excellent charge / discharge cycle characteristics can be obtained. Further, a material different from the conventional lithium-containing oxide such as LiCoO ₂ can be used as the positive electrode material.

The electrode for a lithium secondary battery according to the second aspect of the present invention is formed by a current collector and a method of supplying a raw material from the gas phase or the liquid phase onto the current collector, and contains Fe ₃ O ₄ . And a positive electrode active material layer containing Fe oxide as a main component.

In the electrode for a lithium secondary battery according to the second aspect of the present invention, as described above, by providing the positive electrode active material layer containing Fe oxide containing Fe ₃ O ₄ as a main component, the discharge of the positive electrode is improved. Since the capacity is increased, a lithium secondary battery having a high discharge capacity and excellent charge / discharge cycle characteristics can be obtained. Further, a material different from the conventional lithium-containing oxide such as LiCoO ₂ can be used as the positive electrode material.

In the lithium secondary battery electrode according to the first or second aspect, the positive electrode active material layer preferably contains a transition metal other than Fe. According to this structure, it is possible to reduce the decrease in the discharge capacity of the positive electrode.
In this case, the positive electrode active material layer has a peak of an intermetallic compound of Fe and a transition metal when evaluated by an X-ray diffraction method,
It is preferable not to have a peak of a composite oxide of Fe and a transition metal. This means that the transition metal is dispersed in the oxide containing Fe, but does not form an intermetallic compound of Fe and the transition metal or a complex oxide of Fe and the transition metal. The transition metals are Ni and Co.
It is preferable that either one of

In the lithium secondary battery electrode according to the first or second aspect, the current collector may be Al or Al alloy.

[0015]

EXAMPLES Examples of the present invention will be specifically described below.

(Examples 1 to 6) [Preparation of Positive Electrode] FIG. 1 shows a vacuum vapor deposition apparatus used for manufacturing electrodes (positive electrodes) for lithium secondary batteries according to Examples 1 to 6 of the present invention. It is a schematic diagram. First, referring to FIG. 1, the vacuum vapor deposition apparatus 1 used in Examples 1 to 6 includes a vacuum chamber 2
1, a rotary holder 3, a crucible 4, an electron gun 5, and a film thickness sensor (crystal vibrating type) 6. The rotation holder 3 and the crucible 4 are installed so as to face each other.
A current collector 7 is arranged on the rotation holder 3. A vapor deposition material 8 is arranged in the crucible 4. Further, the electron gun 5 is installed so that the vapor deposition material 8 in the crucible 4 is irradiated with the electron beam 5a. The film thickness sensor 6 is installed near the current collector 7.

In Examples 1 to 6, the current collector 7 was used under the conditions shown in Table 1 below using the vacuum vapor deposition apparatus 1 as described above.
A thin film was formed on top.

[0018]

[Table 1] Referring to Table 1 above, in Examples 1 to 6, Fe, Co and Ni were used as the vapor deposition material 8. In Example 1, only Fe was used as the combination of the vapor deposition materials 8. In Examples 2 and 3, a mixture of Fe and Co was used.
In Examples 4 and 5, a mixture of Fe and Ni was used. In Example 6, a mixture of Fe, Co, and Ni was used. The ratio of each weight was such that Co and Ni were about 0.1 to 0.3 when Fe was 1.0.

The current collector 7 is about 20 cm × 6.
An Al foil having a size of 0 cm and a thickness of 20 μm was used, and the thickness of the thin film formed on the current collector 7 was 1.0 μm to 1.3 μm. A cylindrical holder having a diameter of 20 cm was used as the rotary holder 3, and a current collector 7 made of Al foil was placed on the cylindrical surface of the cylindrical holder.

As a concrete manufacturing process, first, the inside of the vacuum chamber 2 shown in FIG. 1 was put into a vacuum state. Then, the vapor deposition material 8 was heated by irradiating the electron beam 5a from the electron gun 5 while rotating the current collector 7 made of Al foil arranged on the rotary holder 3 at about 10 rpm. As a result, the evaporated molecules and atoms of the vapor deposition material 8 were deposited on the current collector 7 to form a thin film on the current collector 7. The evaporation rate is measured using the film thickness sensor 6,
It was controlled to be 0.2 nm / s (converted to FeO).

When the vapor deposition material 8 is vaporized,
O ₂ gas (20 sccm) was introduced as an atmosphere gas to form a thin film.

X-ray fluorescence analysis was performed on the thin film formed on the current collector 7 made of Al foil according to Examples 1 to 6 above, and the ratios of Fe, Co and Ni in the thin film were measured. The results are shown in Table 2 below. The ratio of each element is a weight ratio.

[0023]

[Table 2] Referring to Table 2 above, the thin film formed in Example 1 does not contain Co and Ni. The thin film formed in Example 2 contained Co at a ratio of 3.0%. In the thin film formed in Example 3, Co
Was included at a rate of 4.6%. The thin film formed in Example 4 contained Ni at a rate of 4.4%. In the thin film formed in Example 5, Ni was 6.
It was included at a rate of 7%. The thin film formed in Example 6 contained Co in an amount of 4.3% and Ni in an amount of 6.2%.

After forming the thin film, the thin films formed in Examples 1 to 6 were annealed at 600 ° C. for 2 hours in the atmosphere.

Then, the ratios of Fe, Co and Ni in the thin films formed in Examples 1 to 6 were measured in the same manner as described above. As a result, Fe and Co in the as-deposited state before annealing and after annealing were measured. It was found that there was no change in the ratio with Ni and Ni.

Next, X-ray diffraction was performed on the as-deposited state of the thin films formed in Examples 1 to 6 before annealing and after the annealing, and their crystallinity was evaluated.

FIG. 2 is a waveform diagram showing the evaluation results by the X-ray diffraction method of the thin film in the as-deposited state before the annealing treatment and the state after the annealing treatment according to Examples 1 to 6 of the present invention. The horizontal axis of FIG. 2 represents an angle (2θ) with the direction of the incident line of X-rays, and the vertical axis represents an arbitrary unit (a.
u. ) Is taken. Further, the as-deposited state indicates a state before the annealing process, and the annealing state indicates a state after the annealing process. In the figure, "○" is Fe ₂ O ₃ , "□" is Fe ₃ O ₄ , and "△" is Al (current collector).
The peak positions that are considered to correspond respectively to are shown.

Referring to FIG. 2, the thin films formed in Examples 1 to 6 were annealed at 30 °.
It was found that the peaks considered to be Fe ₃ O _{4 in the} vicinity decrease and the peaks considered to be Fe ₂ O ₃ increase.

Further, in all the thin films according to Examples 1 to 6, peaks considered to be Fe ₂ O ₃ or Fe ₃ O ₄ are seen, so that the crystalline thin films mixed with Fe ₂ O ₃ or Fe ₃ O ₄ are present. Is considered to be.

Further, in the thin films according to Examples 2 to 6, Fe
No clear peaks, which are considered to be an intermetallic compound of Co and Ni or a complex oxide of Fe and Co or Ni, were observed. As a result, in the thin films according to Examples 2 to 6, Co and Ni exist in the thin film without forming an intermetallic compound of Fe and Co or Ni or a complex oxide of Fe and Co or Ni. It is thought that

The positive electrode according to Examples 1 to 6 was produced by cutting the current collector 7 and the thin film made of Al foil according to Examples 1 to 6 formed as described above into 20 mm square pieces.

Comparative Example 1 As Comparative Example 1, a pellet electrode was prepared using Fe ₂ O ₃ powder. Specifically, Fe
40 parts by weight of ₂ O ₃ powder, 40 parts by weight of acetylene black as a conductive material, and 20 parts by weight of polytetrafluoroethylene as a binder were mixed. Then, the mixture was pressure-molded to have a diameter of 16 mm and a thickness of 0.1 mm, and vacuum-dried at 110 ° C. to prepare a positive electrode of Comparative Example 1.

[Preparation of Electrolyte Solution] LiPF ₆ was dissolved in an equal volume mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) to prepare an electrolyte solution having a concentration of 1 mol / liter.

[Preparation of Beaker Cell (Tripolar Cell)] A beaker cell was prepared by using the positive electrode composed of the thin film after the as-deposited state and the annealing treatment formed in Examples 1 to 6 and the positive electrode according to Comparative Example 1 as working electrodes. (Tripolar cell) was produced. As the counter electrode and the reference electrode, molded lithium metal was used. The electrolytic solution prepared as described above was used as the electrolytic solution.

Next, Example 1 produced as described above
In order to investigate the battery characteristics of the beaker cells according to ~ 6 and Comparative Example 1, the following charge / discharge cycle test was performed.

[Charge / Discharge Cycle Test] The charge / discharge conditions in the charge / discharge cycle test are as follows: charge current 1.0 mA
Then, it was charged until the charging voltage became 3.0V. Then 1
At the second cycle, the discharge current is 1.0 mA and the discharge voltage is 0.5.
Discharge until it reaches V, then discharge current 0.5mA
Then, the battery was discharged until the discharge voltage became 0.5V. After the second cycle, discharge was performed at a discharge current of 0.5 mA until the discharge voltage reached 0.5V. Then, with respect to the beaker cells of Examples 1 to 6 and Comparative Example 1, the discharge capacity and the capacity retention rate were obtained. The result of this charge / discharge cycle test is shown in FIG.
And shown in FIG.

FIG. 3 shows 1 to 3 in the charge / discharge cycle test.
It is a battery characteristic view showing discharge capacity at the 5th cycle. FIG. 4 is a battery characteristic diagram showing the capacity retention rates of the first to fifth cycles in the charge / discharge cycle test. 3 and 4, (a) shows the as-deposited state before annealing, and (b) shows the state after annealing. Further, the capacity retention rate in FIG. 4 is based on the maximum capacity measured in the charge / discharge cycle test in each beaker cell. Specifically, Example 5
Except for (b), the capacity of the first cycle is 100%, and in Example 5 (b), the capacity of the second cycle is 100%.
I am trying.

Referring to FIG. 3, in the beaker cells according to Examples 1 to 6 described above, the beaker cells according to Examples 1 (a) to 6 (a) in the as-deposited state in which the annealing treatment was not performed were annealed. Examples 1 (b) to 6 in which
It was found that the discharge capacity after the cycle evaluation was larger and the decrease in the discharge capacity was smaller than that of the beaker cell according to (b). As a result, by performing the annealing treatment as shown in FIG. 2, the peak that is considered to be Fe ₃ O ₄ at around 30 ° is decreased and the peak that is considered to be Fe ₂ O ₃ is increased. Therefore, it can be said that the Fe ₃ O ₄ -based crystal component showing a peak at around 30 ° is effective for increasing and maintaining the discharge capacity. As a result, Fe ₃
It is considered that the discharge capacity of the positive electrode can be increased by providing a positive electrode active material layer containing O ₄ as a main component and having a diffraction peak near 30 ° when evaluated by an X-ray diffraction method. As a result, a lithium secondary battery having high discharge capacity and excellent charge / discharge cycle characteristics can be obtained.

In addition, a beaker cell according to Comparative Example 1 in which a pellet electrode was prepared using Fe ₂ O ₃ powder and Examples 1 (a) to 6 in an as-deposited state without annealing treatment.
Even when compared with the beaker cell according to (a), the beaker cells according to Examples 1 (a) to 6 (a) have a larger discharge capacity after cycle evaluation and a decrease in discharge capacity, as described above. Turned out to be small.

Next, referring to FIG. 4, it was found that the beaker cell according to Example 5 (a) in the as-deposited state in which Ni was added and which was not subjected to the annealing treatment showed a small decrease in discharge capacity. did. However, the beaker cell of Example 4 (a) resulted in a large decrease in discharge capacity as compared with the beaker cell of Example 5 (a), despite the addition of Ni. This is because the ratio of Ni in the thin film was 4.4% in Example 4 and 6.7% in Example 5, as shown in Table 2 above. Ni in thin film
It is considered that this is because the ratio of From this,
It is considered that the Fe ₃ O ₄ component is increased by adding Ni, which is a transition metal other than Fe, at a certain ratio or more when forming the thin film.

Further, as shown in FIG. 4, the beaker cell according to Example 6 (a) in which Ni and Co were added and which was not subjected to the annealing treatment had a small decrease in discharge capacity. I understood. As a result, even if Co, which is a transition metal other than Fe, is added at the time of forming a thin film, Fe
_It is considered that the ₃ O ₄ component increases.

It should be understood that the embodiments disclosed this time are illustrative in all points and not restrictive. The scope of the present invention is shown not by the above description of the embodiments but by the scope of claims for patent, and includes meanings equivalent to the scope of claims for patent and all modifications within the scope.

For example, in Examples 1 to 6 above, as an example of the method of supplying the raw material from the gas phase or the liquid phase onto the current collector, an example of forming the positive electrode active material layer by using the vacuum deposition method was shown. However, the present invention is not limited to this, and the positive electrode active material layer may be formed by using a sputtering method, a reactive vapor deposition method, a chemical vapor deposition method (CVD method), a thermal spraying method, a plating method, or a combination thereof. Good.

Further, in Examples 1 to 6 described above, the thin film was formed without heating the current collector (substrate), but in the present invention, it is preferable to form the thin film at a temperature as low as possible. As a result, Fe ₃ O ₄ is increased, and as a result, the charge / discharge characteristics can be improved.

In the first to sixth embodiments, the Al foil is used as the current collector (substrate), but the present invention is not limited to this, and an Al alloy may be used. In this case, it is preferable to use a thin foil.

Further, in the above-mentioned Examples 1 to 6, the lithium metal molded as the negative electrode was used in the charge / discharge cycle test, but the present invention is not limited to this, and a material in which lithium is previously occluded is used as the negative electrode. It may be used as. For example, carbon-based materials and alloys in which lithium is stored in advance are included. For this alloy,
Examples include alloys of lithium with silicon, aluminum, tin, germanium, indium and magnesium.

Further, in the above Examples 1 to 6, LiPF ₆ is dissolved in a mixed solvent of ethylene carbonate, which is a cyclic carbonate, and dimethyl carbonate, which is a chain carbonate, as an electrolytic solution during the charge / discharge cycle test. However, the present invention is not limited to this, and a mixed solvent of a cyclic carbonate such as propylene carbonate or butylene carbonate and a chain carbonate such as methyl ethyl carbonate or diethyl carbonate may be used. Alternatively, a mixed solvent of a cyclic carbonate and an ether solvent such as 1,2-dimethoxyethane or 1,2-diethoxyethane may be used. Moreover, as a solute of the electrolytic solution, LiBF ₄ , LiCF ₃ S
O ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ S
O ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ), Li
_{C (CF 3 SO 2) 3} , LiC (C 2 F 5 SO 2) 3 and the like, and may be a mixture thereof. In addition, as an electrolyte, in a polymer electrolyte such as polyethylene oxide, polyacrylonitrile, polyvinylidene fluoride,
A gel polymer electrolyte impregnated with an electrolytic solution may be used.

Further, in Examples 2 to 6 described above, F was contained in the thin film.
Although Ni or Co which is a transition metal other than e is added, the present invention is not limited to this, and a transition metal other than Fe may be added to the thin film.

[0049]

As described above, according to the present invention, it is possible to provide a lithium secondary battery having a high discharge capacity and excellent charge / discharge characteristics.

[Brief description of drawings]

FIG. 1 is a schematic view showing a vacuum vapor deposition apparatus used for manufacturing electrodes (positive electrodes) for lithium secondary batteries according to Examples 1 to 6 of the present invention.

FIG. 2 is a waveform diagram showing the evaluation results of the thin films formed according to Examples 1 to 6 of the present invention by an X-ray diffraction method.

FIG. 3 is a battery characteristic diagram showing discharge capacities at 1st to 5th cycles in a charge / discharge cycle test.

FIG. 4 is a battery characteristic diagram showing the capacity retention rate in the first to fifth cycles in the charge / discharge cycle test.

[Explanation of symbols]

1 Vacuum deposition equipment 7 Current collector

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Hisaki Tarui             2-5-3 Keihan Hondori, Moriguchi City, Osaka Prefecture             Within Yo Denki Co., Ltd. F-term (reference) 5H029 AJ02 AJ03 AK02 AL12 AM03                       AM05 AM07 CJ24 CJ28 DJ07                       DJ17 HJ02 HJ03 HJ13                 5H050 AA02 AA08 BA17 CA02 CB12                       DA04 FA02 FA19 GA24 HA02                       HA03

Claims (5)

[Claims] 1. A current collector, which is formed by a method of supplying a raw material from the gas phase or a liquid phase onto the current collector, contains an oxide containing Fe as a main component, and is evaluated by an X-ray diffraction method. In some cases, a positive electrode active material layer having a diffraction peak near 30 °, and an electrode for a lithium secondary battery. 2. A current collector, Method of supplying raw material from the gas phase or the liquid phase onto the current collector
Formed by Fe ₃O_FourFe oxide containing as a main component
A positive electrode active material layer for
very. 3. The electrode for a lithium secondary battery according to claim 1, wherein the positive electrode active material layer contains a transition metal other than Fe. 4. The positive electrode active material layer has a peak of an intermetallic compound of Fe and the transition metal and a peak of a complex oxide of Fe and the transition metal when evaluated by an X-ray diffraction method. The electrode for a lithium secondary battery according to claim 3, which does not include: 5. The electrode for a lithium secondary battery according to claim 3, wherein the transition metal is one of Ni and Co.