JP2002352801A - Lithium secondary cell, negative electrode material for it, and improving method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increase the energy capacity of the material used as the negative electrode for a lithium secondary cell, and to improve the output characteristics and charge/discharge cycle characteristics. SOLUTION: The negative electrode material for a lithium secondary cell is covered with a metal compound thin-film. The thickness of the thin-film is preferred to be 2-50 nm. The metal compound thin-film may be acquired by either physical deposition method or chemical deposition method, otherwise by vapor-phase deposition method or liquid-phase deposition method. The lithium secondary cell and the negative electrode material for it are thus acquired.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a novel negative electrode material for a lithium secondary battery, a method for improving the same, and a resulting lithium secondary battery.

[0002]

_2. Description of the Related Art A lithium secondary battery combines an anode material capable of releasing lithium such as lithium cobaltate LiCoO ₂ , a negative electrode material capable of inserting and removing lithium such as carbon, and an electrolyte such as lithium perchlorate. It is made and has the feature of having a high energy density.

At present, graphite or non-graphitizable carbon is in practical use as a negative electrode for a lithium secondary battery. However, the diffusion of lithium between carbon layers is not smooth, the output characteristics are not improved, and the energy capacity is only about 300 mAh / g for natural graphite, which is widely used. Has been proposed (see
JP 2001-23638, JP 2000-357506, JP 2000-2646
No. 14, No. 2000-90928, No. 10-214615,
Nos. 11-40158 and 2000-260428), which are not sufficient, and it is desired to further improve these characteristics.

[0004]

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to increase the energy capacity of a material used as a negative electrode for a lithium secondary battery and to improve output characteristics and charge / discharge cycle characteristics. .

[0005]

Means for Solving the Problems The present invention has been studied diligently to achieve the above object, and has found that the above object can be achieved by the following. (1) A negative electrode material for a lithium secondary battery, wherein the surface of the negative electrode material for a lithium secondary battery is coated with a metal compound thin film.

(2) The metal compound thin film has a thickness of 2 to 5
The negative electrode material for a lithium secondary battery according to (1), wherein the thickness is within a range of 00 nm. (3) The negative electrode material for a lithium secondary battery according to (1) or (2), wherein the metal compound thin film is an aluminum oxide thin film. (4) The negative electrode material for a lithium secondary battery according to any one of (1) to (3), wherein the negative electrode material for a lithium secondary battery is graphite or a vanadium-based oxide.

(5) A method for improving a negative electrode material for a lithium secondary battery, which comprises coating the surface of the negative electrode material for a lithium secondary battery with a metal compound thin film. (6) a metal alkoxide treatment method for the metal compound thin film,
(5) The method for improving a negative electrode material for a lithium secondary battery according to (5), which is formed by a method selected from a sol-gel method, a CVD method, a PVD method, and a plating method.

(7) A metal alkoxide is adhered to the surface of the negative electrode material for a lithium secondary battery, and then is thermally decomposed to cover the surface of the negative electrode material for a lithium secondary battery with a metal oxide thin film. A method for improving a negative electrode material for a lithium secondary battery according to the above item. (8) A lithium secondary battery using the negative electrode material for a lithium secondary battery according to any one of (1) to (4).

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The negative electrode material for a lithium secondary battery of the present invention may be any material capable of inserting and removing lithium ions, and typically, graphite, non-graphitizable carbon and other carbon materials are used. It is used, but not limited thereto, and may be a vanadium-based oxide such as MnV ₂ O ₆ . The present inventors have found that MnV ₂ O ₆ is suitable as a negative electrode material for a lithium secondary battery, but its substance and its production method are known. MnV ₂ O ₆ has a higher energy capacity of 600 to 700 mAh / g than carbon, and has a higher energy capacity density per volume because of its higher density than carbon. Low reactivity with organic electrolytes
It has the characteristic that there is little danger such as temperature rise and explosion.

[0010] These negative electrode materials for lithium secondary batteries include:
It has been found that when coated with a metal compound thin film according to the present invention, cycle characteristics are improved, or energy capacity and output characteristics are improved. Especially in natural graphite, the conventional energy capacity is about 300 mAh / g,
When coated with a metal compound thin film according to the present invention, it shows a value almost equal to the theoretical capacity of 372 mAh / g (for example, 370 mAh / g), and the conventional carbon material has an output density of 0.2 C or more (1 C discharges capacity in one hour). However, it was difficult to secure stable cycle characteristics at a current density of 0.5 C. However, when coated with a metal compound thin film according to the present invention, extremely stable cycle characteristics could be exhibited even at an output density of 0.5 C. Vanadium-based oxides such as MnV ₂ O ₆ have twice or more the energy capacity of carbon materials that have been put to practical use (see FIG. 4). There was a problem with the irreversible capacity of the first time as well. According to the present invention, by coating a vanadium-based oxide with a metal compound thin film, the irreversible capacity at the first time can be largely suppressed, and the loss of reversible energy capacity and the deterioration of cycle characteristics after the second time are observed. The effect of not being obtained was obtained.

[0011] When the insertion and detachment of lithium due to the charge and discharge of the battery are repeated, the progress of structural destruction of the negative electrode material is considered to be the cause of the deterioration of the cycle characteristics. When coated with a metal compound thin film,
It is considered that the cycle characteristics are improved because the thin film on the surface of the negative electrode material suppresses structural destruction due to repeated insertion and extraction of lithium. In addition, when the surface of the negative electrode material is coated with a metal compound thin film, the metal or metal compound acts on the surface of the negative electrode material, thereby expanding the lithium insertion / desorption opening, facilitating lithium insertion / desorption, and improving energy capacity and output characteristics. It is thought to contribute to the improvement of Also, in the oxide-based negative electrode material, a passivation film is formed and the first irreversible capacity is large, which is a problem. However, when the surface of the oxide-based negative electrode material is coated with a metal compound thin film, the formation of the passive film is suppressed. Therefore, it is considered that most of the initial irreversible capacity can be suppressed. Therefore, the coating with the metal compound thin film of the present invention can be generally applied to negative electrode materials for lithium secondary batteries.

As a metal compound for forming a thin film, a compound of a metal such as aluminum, titanium, zinc, iron and silicon can be widely applied. Any metal compound may be used as long as it is a compound that is stable under the conditions of use of the lithium secondary battery. The type of metal or metal compound to be used may be appropriately selected according to the negative electrode material.

The metal compound thin film of the present invention has a thickness of 2 to 2.
It is preferably within a range of 500 nm, more preferably 2 to 100 nm, furthermore 5 to 50 nm, especially 5 to 5 nm.
A range of 20 nm is preferred. The metal compound thin film does not need to completely cover the surface of the negative electrode material. If the thickness of the metal compound thin film is less than 2 nm, it is difficult to form the thin film as a thin film, and the effect of improving the characteristics of the negative electrode material is small. It is not preferable because the possibility of inhibition increases.

The negative electrode material for a lithium secondary battery to be used is usually used in the form of a powder, and the particle size may be the same as that of the conventional one. Typically, but not limited to, 0.1-5
0 μm, especially in the range of 0.5 to 30 μm. The method of coating the negative electrode material for a lithium secondary battery with a metal oxide thin film is not particularly limited, and includes a metal alkoxide treatment method, a sol-gel method, a CVD method, a PVD method (sputtering and others),
A plating method or the like may be used.

In the metal alkoxide treatment, a negative electrode material for a lithium secondary battery is immersed in a metal alkoxide solution, pulled up, and a metal alkoxide (solution) is attached to the surface of the negative electrode material for a lithium secondary battery. ) To remove the organic components of the metal alkoxide. Aluminum, titanium,
An alkoxide of a metal such as zinc, iron, silicon or the like, for example, ethoxide, metaoxide, propoxide, butoxide and the like can be appropriately selected and used. It is believed that metal alkoxides are heat-treated to form metal compounds, especially metal oxide films when heated in air. Despite the fact that metal oxides are generally insulators, metal alkoxide-treated lithium secondary Although the improvement in conductivity was observed even in the negative electrode material for batteries, although it is not clear, it is considered that the lithium insertion / desorption opening was expanded as described above, while the thickness of the metal compound film such as a metal oxide is thin. So it is not considered a problem.

Experiments have shown that the concentration of the metal alkoxide solution is less relevant. This is presumably because the concentration of the metal alkoxide adhering to the negative electrode material was not so different after the negative electrode material was immersed in the metal alkoxide solution and pulled up. Unless the processing is performed a plurality of times, the concentration of the solution is not so important in the usual one processing. Although not limited, a solution having a concentration of 2 to 30% by weight may be generally used. The solvent to be used is not limited, and a typical solvent such as methylpyrrolidone can be used. For example, an alcohol such as ethanol, propanol and butanol can be easily used. It may be used by mixing with water.

The temperature of the heat treatment is preferably a temperature at which organic components can be completely removed by thermal decomposition of the metal alkoxide. The thermal decomposition temperature also depends on the type of the metal alkoxide. 200 to 500 ° C., more preferably 200 to 3
The temperature is preferably in the range of 00 ° C., but it is possible to obtain a certain effect even by heating at less than 200 ° C. It is sufficient to perform the heat treatment in air, but theoretically, other oxidizing gas or inert gas can be used.

A method for forming a metal compound thin film other than the metal alkoxide treatment method is known. Therefore, the metal compound thin film may be formed using a known method by appropriately selecting the type of the metal compound and the thickness of the thin film according to the negative electrode material. The negative electrode material for a lithium secondary battery obtained by coating the metal compound thin film according to the present invention can be used for producing a negative electrode for a lithium secondary battery according to a conventional method. In general, a negative electrode material (active material) obtained by coating a metal compound thin film is mixed with a binder (for example, polyvinylidene fluoride, polytetrafluoroethylene, etc.) and a conductive agent (for example, acetylene black). ,
The negative electrode is formed by molding on an electrode substrate (for example, a copper plate).

The method for producing a lithium secondary battery using this negative electrode may be the same as the conventional method. A lithium secondary battery can be manufactured by selecting appropriate materials such as a negative electrode, an anode, an electrolytic solution, and a diaphragm. The type of the lithium secondary battery is not limited.

[0020]

EXAMPLES (Example 1) Natural graphite (NG) having an average particle size of 2 μm
2: Kansai Thermal Chemical Co., Ltd.) and aluminum triethoxide (A
A mixture of l (OC ₂ H ₅ ) ₃ ) in a weight ratio of 9: 1 was immersed in an ethanol solution, stirred for 3 hours under ultrasonic irradiation, filtered, and dried by heating at 200 ° C. or higher in air. This was used as a new negative electrode active material.

FIG. 1 shows the Raman spectrum of ordinary untreated NG2 and new natural graphite surface-treated with aluminum triethoxide. The G-band (1580 cm ^-1 ) of the surface-treated natural graphite is shifted to a higher wavenumber side and the strength is increased as compared with the untreated one. It is considered that the bond strength of the surface-treated natural graphite is increased as compared with the untreated one. On the other hand corresponds to the randomness of the graphite crystal in the vicinity of 1360 cm ^-1 derived from the vibration mode of A _lg D-band hardly changed in both. Analysis by EDX (energy dispersive X-ray analysis) revealed that aluminum was clearly present in the surface-treated natural graphite. From this, it can be inferred that aluminum exists as a thin oxide film on the graphite surface.

Next, the electrochemical properties of the negative electrode active materials of the natural graphite treated with the oxide film and the untreated natural graphite were compared and studied by a constant current method. The electrode was prepared by dissolving and mixing 80% by weight of active material, 10% by weight of conductive material acetylene black, and 10% by weight of binder (polyvinylidene fluoride PVdF) in a solvent (1-methyl-2-pyrrolidone NMP) and then stretching it on copper foil. To obtain a negative electrode. In the electrochemical measurement, lithium perchlorate LiClO ₄ was dissolved at a concentration of 1 mol / l in a counter electrode of metallic lithium and an electrolytic solution of (ethylene carbonate EC) / (diethyl carbonate DEC) (volume ratio 1: 1). Using
The test was carried out at room temperature in a CR2032 type cell in an argon circulation type glove box.

The chemical diffusion coefficient of lithium was calculated by a current pulse relaxation method. Untreated natural graphite and aluminum ethoxide were thermally decomposed at 240 ° C, and the current density was 0.2
C (70mA / g), 0.5C (175mA / g), 1.0C (350mA / g)
FIG. 2 shows the charge / discharge curves of the first and 50th cycles when g) was used. The treated aluminum oxide film has an apparent energy capacity of about 2 compared to the untreated aluminum oxide film.
The initial capacity was 370 mAh / g, which was close to the theoretical capacity. Also, the cycle characteristics have been improved.

FIG. 3 shows 0.2C, 0.5C, 1.0 of untreated natural graphite and natural graphite treated with aluminum oxide film.
The relationship between the discharge capacity and the number of cycles at the current density of C is shown.
The cycle characteristics and output characteristics have been significantly improved by the aluminum treatment. The chemical diffusion coefficient of lithium in the natural graphite treated with the aluminum oxide film was about 10 ⁻⁸ cm ₂ / s, which is about 100 times larger than that of the untreated natural graphite, confirming the results of FIG.

[0025] (Example _{2) Mn (CH 3 COO)} 2 4H 2 O ( 99.9%)
And V ₂ O ₅ (purity 99.9%) were separately dissolved in water in a stoichiometric ratio, mixed and stirred to form a clear solution, and then mixed with an appropriate amount (2.5% of metal) for uniform mixing. PVA (molecular weight 1500)
~ 1800 polyvinyl alcohol) solution. Although the solution was colored, it was heated at about 80 ° C. with gentle stirring to evaporate the water and turn into a brown viscous gel. During this time, the mixture was gently stirred to maintain uniformity. Then, the mixture was heated at about 300 ° C. for 2 hours in the air to be oxidatively decomposed to produce a powder precursor. After pulverized, it was calcined at 450 ° C. for 24 hours to obtain the target MnV ₂ O ₆ . It was confirmed by X-ray diffraction and the like that this substance was MnV ₂ O ₆ .

The vanadium-based oxide MnV ₂ O ₆ and aluminum triethoxide (Al (C ₂ H ₅ O) ₃ ), which were homogeneously mixed with the polymer and synthesized by the sol-gel method, had a weight ratio of 9: 1. The resultant is immersed in an ethanol solution, stirred for 3 hours under ultrasonic irradiation, filtered, and dried by heating at 200 ° C. or more in air. This was used as a new negative electrode active material. This MnV ₂ O ₆
The results obtained by examining the insertion / desorption characteristics of lithium using as a negative electrode active material are shown by the solid lines in FIG. Energy capacity 600-70
It is recognized as large as 0 mAh / g.

The irreversible capacity in the first charge / discharge is because Li is consumed extra for the formation of the passivation film. It is known that the passivation film is formed by the decomposition of the electrolytic solution on the surface of the active material, but it is not known clearly. However, it can be considered that there is no change in the bulk structure including the magnetic properties when the passivation film is formed, and the magnetic properties were examined. XANES (X-ray absorption near-end structure) Measurement showed that Mn was +2 and V was +5. The Bohr magneton number measured by magnetic susceptibility is 5.86, which is in good agreement with the theoretical value 5.92 of Mn ²⁺ , and the result of magnetic susceptibility measurement is reliable. The charge guarantee when inserting Li is made by V since the valence of Mn does not decrease any more. At this time, the valence of V changes from +5 to +4, +3, and +2, and the magnetic susceptibility also changes accordingly. However, the magnetic susceptibility does not change when the passive film is formed. Magnetic susceptibility measurements can distinguish between the formation of a passive film and the Li insertion reaction. From the measurement of the magnetic susceptibility, it can be seen that a passivation film is formed at a plateau near 1.0 to 0.7 V.
As described above, in order to control the irreversible capacity, it is necessary to suppress the formation of a passive film, and many surface treatments have been performed.

Referring to FIG. 4, the surface treatment reduced the plateau around 1-0.7 V of the charge / discharge curve (broken line), which means that the formation of the passive film was suppressed. That is, the irreversible capacity has decreased. In order to investigate the cause, we measured the effective impedance to see the reaction at the interface. FIG. 5 shows the results. It is known that a semicircle in the frequency band of 1-100 HZ represents charge transfer resistance. Referring to FIG. 5, since this semicircle is smaller, it can be said that the charge transfer resistance has been reduced. Also, the semicircle on the high frequency side represents the bulk resistance, and since the semicircle is smaller, it can be said that the resistance similarly decreased. On the low frequency side, there is a change in the straight line indicating the diffusion in the bulk, and it can be said that the surface treatment also affected the diffusion in the bulk.

The surface treatment causes the passivated alumina to cover a part of the surface of the active material, prevent the formation of a passivated film there, and reduce the charge transfer resistance. Thereby, the irreversible capacity was reduced.

[0030]

When a conventional carbon material is used as a negative electrode active material of a lithium secondary battery, there is an advantage that the operating potential is low and the Coulomb efficiency is high with respect to lithium of the counter electrode, but lithium diffusion between carbon layers is difficult. It has been pointed out that when discharged at a high energy density, it is not easy to cause structural destruction leading to cycle degradation.Therefore, it is required to improve the high energy density charge / discharge output characteristics, the energy capacity, and the cycleability of the carbon anode. Was. In the present invention, by forming a thin metal oxide film on the surface of the carbon material, a part of the end face of the carbon layer is opened more to facilitate the insertion / desorption of lithium by charging / discharging. The output characteristics were equivalent to those of 0.1C in carbon and improved about 5 times. In addition, structural destruction due to expansion and contraction between layers during charge / discharge cycles was suppressed, and 95% of the initial capacity was maintained even in the 100th cycle. Furthermore, when natural graphite is surface-treated by this method, the initial discharge capacity is about 300 mAh / g in the prior art, whereas 370 mAh / g, which is close to the theoretical energy capacity of 372 mAh / g.
showed that. Thus, it became clear that the surface treatment of carbon with a metal alkoxide can significantly improve the electrochemical properties.

In general, an oxide negative electrode enables a large energy capacity, but has a problem that the irreversible capacity is large in the first charge / discharge. Referring to the charge and discharge curves of untreated and aluminized MnV ₂ O ₆ in FIG.
It is recognized that a passivation film was formed at around 1.0 to 0.7 V. In order to suppress such irreversible capacity, it is necessary to suppress generation of a passive film. According to the present invention, as shown in FIG. 4, the plateau around 1.0 to 0.7 V is reduced by forming the metal oxide film, and the formation of the passive film is suppressed. That is, it is recognized that the irreversible capacity has decreased. In order to investigate the cause, we measured the effective impedance to see the reaction at the interface. Referring to FIG.
It is known that the semicircle in the frequency band of １００100 Hz represents the charge transfer resistance, but it can be seen that the charge transfer resistance has decreased because the semicircle has become smaller. Further, the semicircle on the high frequency side represents the bulk resistance, and since the semicircle is smaller, it can be seen that the bulk resistance has also been reduced. On the low frequency side, a change was seen in the straight line indicating the diffusion in the bulk, indicating that the surface treatment also affected the diffusion in the bulk. That is, the passivation metal oxide covers a part of the active material surface by the surface treatment of the present invention,
It is recognized that the irreversible capacity was reduced by reducing the charge transfer resistance by preventing the formation of the passivation film there.

As described above, forming the metal compound thin film on the surface of the negative electrode material for a lithium secondary battery according to the present invention can improve the characteristics such as the energy capacity, output characteristics, and cycle characteristics of the negative electrode material for a lithium secondary battery. This is an effective method for reducing the irreversible capacity for the first time.

[Brief description of the drawings]

FIG. 1 shows Raman spectra of ordinary untreated natural graphite and natural graphite surface-treated with aluminum triethoxide.

Fig. 2 Current densities of untreated natural graphite and aluminum ethoxide-treated natural graphite as negative electrodes
0.2C (70mA / g), 0.5C (175mA / g), 1.0C (350mA
/ G) shows the charge and discharge curves at the first and 50th cycles.

FIG. 3 shows the relationship between the discharge capacity and the number of cycles at current densities of 0.2 C, 0.5 C, and 1.0 C for untreated natural graphite and natural graphite treated with an aluminum oxide coating.

FIG. 4 shows a comparison of lithium charge / discharge cycles for MnV ₂ O ₆ and aluminized MnV ₂ O ₆ samples.

FIG. 5 shows a comparison of impedance for MnV ₂ O ₆ and aluminized MnV ₂ O ₆ samples.

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Yoshihiro Kaduma 2-12-1, Ookayama, Meguro-ku, Tokyo F-term inside Tokyo Institute of Technology 5H029 AJ03 AJ05 AL02 AL03 AL07 AM03 AM05 AM07 CJ24 DJ12 EJ05 HJ04 5H050 AA07 AA08 BA17 CB02 CB03 CB08 DA09 EA12 EA24 FA12 FA18 GA22 GA24 HA04

Claims (8)

[Claims] 1. A negative electrode material for a lithium secondary battery, comprising a metal compound thin film coated on the surface of a negative electrode material for a lithium secondary battery. 2. The metal compound thin film having a thickness of 2 to 500.
The negative electrode material for a lithium secondary battery according to claim 1, wherein the thickness is in the range of nm. 3. The negative electrode material for a lithium secondary battery according to claim 1, wherein the metal compound thin film is an aluminum oxide thin film. 4. The negative electrode material for a lithium secondary battery is graphite or a vanadium-based oxide.
Or the negative electrode material for lithium secondary batteries according to 3. 5. A method for improving a negative electrode material for a lithium secondary battery, wherein the surface of the negative electrode material for a lithium secondary battery is coated with a metal compound thin film. 6. The method for improving a negative electrode material for a lithium secondary battery according to claim 5, wherein the metal compound thin film is formed by a method selected from a metal alkoxide treatment method, a sol-gel method, a CVD method, a PVD method, and a plating method. 7. The negative electrode material for a lithium secondary battery according to claim 5 or 6, wherein a metal alkoxide is adhered to the surface of the negative electrode material for a lithium secondary battery, and then thermally decomposed to cover the surface of the negative electrode material for a lithium secondary battery with a metal oxide thin film. The method for improving a negative electrode material for a lithium secondary battery according to the above. 8. A lithium secondary battery using the negative electrode material for a lithium secondary battery according to claim 1. Description: