KR20040063803A - Negative active material for rechargeable lithium battery and rechargeable lithium battery - Google Patents

Abstract

PURPOSE: A negative electrode active material for a lithium secondary battery and a lithium secondary battery containing the negative electrode active material are provided, to inhibit the pulverization of a negative electrode active material due to volume expansion and contraction in charge/discharge, thereby improving cycle characteristic. CONSTITUTION: The negative electrode active material comprises an ultrafine micropowder which has a diameter of 1-200 nm, a Raman shift of 480-520 cm-1 and a peak half-width of 10-30 cm-1 and contains an element capable of forming an alloy together with lithium. Preferably the ultrafine micropowder is formed by evaporation method under gas atmosphere. Preferably the ultrafine micropowder contains Si and comprises at least one selected from the group consisting of an isolated micropowder, a chain-type micropowder and an aggregated micropowder, and the diameter of the isolated micropowder, the chain-type micropowder and the aggregated micropowder is 1-200 nm.

Description

Negative active material and lithium secondary battery for lithium secondary battery {NEGATIVE ACTIVE MATERIAL FOR RECHARGEABLE LITHIUM BATTERY AND RECHARGEABLE LITHIUM BATTERY}

[Industrial use]

The present invention relates to a negative electrode active material and a lithium secondary battery for a lithium secondary battery.

[Prior art]

The study of increasing the capacity of a lithium secondary battery negative electrode active material has been conducted before the battery system using the negative electrode active material as carbon, and is currently being actively researched mainly on metals or metal materials such as Si, Sn, Al, etc. Could not be reached. The main reason is that powders such as Si, Sn, and Al are alloyed with lithium during charging and discharging, so that the expansion and contraction of the powder occurs, which further causes the micronization of the powder and thus cannot solve the problem of deteriorating the cycle characteristics.

Therefore, in order to solve this problem, it has been studied to use an amorphous or microcrystalline Si foil formed by CVD and sputtering as a negative electrode active material, as described in Japanese Patent Laid-Open No. 2002-83594. In comparison with crystalline Si, amorphous Si had less volume expansion when alloyed with lithium, and thus, even when charging and discharging were repeated, it was found that the Si had good cycle characteristics.

However, in order to obtain a capacity higher than that of the graphite electrode conventionally used, since the Si film thickness must be formed to a considerable thickness, a long time and a cost are required, and as the film becomes thick, the conductivity decreases, and it is difficult to obtain sufficient battery characteristics.

Subsequently, the use of Si powder in which crystalline Si is pulverized by applying a high shearing force by mechanical grinding to distort the crystallites in Si and amorphized and the average particle diameter is made small is considered.

However, the amorphous Si powder produced by the mechanical grinding described above has a relatively wide particle size distribution, and therefore, particles having an average particle diameter of several hundred nm and an increase in particle diameter of about 1 μm are mixed. If such large particle size is present in the amorphous Si powder, particle micronization at the time of super charging is caused intensively in this large particle size, and there exists a problem of the cycling characteristic fall.

There is also a material in which mechanically pulverized Si powder and graphite are combined, but even in this case, mechanically pulverized particles have a wide particle size distribution and particles having a particle diameter of about 1 μm, so that the relative expansion width when the large particles expand As a result, there was a problem that causes deterioration of the cathode.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and an object of the present invention is to provide a negative electrode active material and a lithium secondary battery excellent in cycle characteristics by suppressing the differentiation caused by the volume expansion and contraction of the active material during charge and discharge.

BRIEF DESCRIPTION OF THE DRAWINGS The schematic diagram which shows an example of the ultrafine particle which comprises the negative electrode active material of this embodiment.

2 is a schematic diagram showing an example of ultrafine particles constituting the negative electrode active material of the present embodiment.

3 is a schematic diagram showing an example of ultrafine particles constituting the negative electrode active material of the present embodiment.

In order to achieve the above object, the present invention adopts the following configuration.

The negative active material of the present invention is in the range of particle diameter of more than 1nm, 200nm or less, the Raman shift measured by Raman spectrometry over 480cm ^-1, 520cm ^-1 in the range of or less, the peak half-value width is 10cm ^-1 or more, It is characterized in that it comprises a powder of ultra-fine particles containing an element capable of alloying with lithium in the range of 30 cm ^-1 or less.

In particular, the ultrafine particles preferably contain Si.

In the present invention, the negative electrode active material for a lithium secondary battery is made of ultrafine particles formed by an evaporation method under a gas atmosphere, and has a particle size distribution range of 1 to 200 nm, and includes ultrafine particles having a maximum particle size of 200 nm. These ultrafine particles have a crystal structure different from Si particles having a particle size larger than the above range due to the size effect, so that even when alloyed with lithium, almost no volume expansion is found and the cycle characteristics are excellent.

In addition, the negative electrode active material for a lithium secondary battery of the present invention is the above-described negative electrode active material for a lithium secondary battery, wherein the ultrafine particles are isolated ultrafine particles in which particles are isolated, chain ultrafine particles in which a plurality of particles are connected in a chain, and bulk ultrafine particles in which a plurality of particles are combined into a block. It is preferable that the particle size of the isolated ultrafine particles, the chain ultrafine particles, and the bulk ultrafine particles is in the range of 1 to 200 nm.

The negative electrode active material for a lithium secondary battery according to the present invention includes at least one or more of isolated ultrafine particles, chain granulated particles, and bulk ultrafine particles, and their particle size distribution range is narrow to 1 to 200 nm and also has a particle size of up to 200 nm, thereby alloying with lithium. Even if the volume expansion hardly occurs, the cycle characteristics are excellent.

In addition, negative active material of the present invention as the above-described negative active material, the Raman shift is measured in the Raman spectrometry over 480cm ^-1, 520cm ^-1 in the range of or less, the peak half-value width of at least 10cm ^-1, 30cm ^It is characterized by the range of ^-1 or less.

With such a negative active material, and the Raman shift is 480cm ^-1 or more, the range of 520cm ^-1 or less, and a peak half-value width of at least 10cm ^-1, 30cm ^-1 or less, because it is mainly an amorphous phase particle, in the case of alloying with lithium Less volume expansion and excellent cycle characteristics.

Next, the lithium secondary battery of the present invention is characterized by including the above-described negative electrode active material for lithium secondary batteries.

According to the lithium secondary battery, since the negative electrode active material is included, expansion of the active material is hardly caused during charging, thereby improving cycle characteristics.

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

The negative electrode active material for a lithium secondary battery of the present invention is formed by an evaporation method in a gas atmosphere, and is a powder of ultra-fine particles made of an element capable of alloying with lithium having a particle diameter of 1 nm or more and 200 nm or less. Elements that can be alloyed with lithium include Si, Pb, Al, Sn, and the like, and Si is particularly preferable.

This negative electrode active material is contained in the negative electrode of a lithium secondary battery. When the lithium secondary battery is charged, lithium moves from the positive electrode to the negative electrode, where lithium and ultrafine particles are alloyed at the negative electrode. The alloyed ultrafine particles hardly cause volume expansion, thereby improving the cycle characteristics of the lithium battery.

Volume expansion does not occur even when the ultrafine particles are alloyed with lithium, because the ultrafine particles have a very small particle size of 1 to 200 nm and a narrow particle size distribution range. It is thought to appear.

There are several forms of the ultrafine particles constituting the negative electrode active material of the present embodiment. That is, as shown in Fig. 1, isolated ultra-fine particles in which particles are isolated, chain ultra-fine particles in which a plurality of particles are chained as shown in Fig. 2, and a plurality of particles as shown in Figs. 3 (a) and 3 (b) are bulky. It includes one of the bulk ultra-fine particles combined with. As shown to FIG. 2 and FIG. 3, the particle | grains which comprise a chain ultrafine particle and a bulk ultrafine particle may differ in size, respectively. The negative electrode active material may contain at least one or more of such isolated ultrafine particles, chain ultrafine particles, and bulk ultrafine particles, and may include all forms.

The particle size of such isolated ultrafine particles, chain ultrafine particles, and bulk ultrafine particles is preferably in the range of 1 to 200 nm. In the chain ultrafine particles, the particle diameter is the length in the direction along the direction in which the particles are connected in a chain, and in the bulk ultrafine particles, the particle diameter is the length along the long side direction in which a plurality of particles are combined into a block.

Such isolated fine particles, chain ultrafine particles, and bulk ultrafine particles have a narrow particle size distribution ranging from 1 to 200 nm, a maximum particle size of 200 nm, and exhibit excellent cycle characteristics with little volume expansion even when alloyed with lithium.

In addition to the negative electrode active material of this embodiment is preferably a Raman shift measured by Raman spectroscopy in the range of more than 480cm ^-1, 520cm ^-1 or less, in addition, peak half-value width is 10cm ^-1 or more, the range of 30cm ^-1 or less desirable. In the case of Si, the crystalline Si has a Raman shift of more than 520 cm ⁻¹ , but in the case of amorphous, the value of the Raman shift is lower than this, and the shape of the peak is also broad. Therefore, the negative electrode active material of this embodiment is the Raman shift range of 480cm ^-1 or more, 520cm ^-1 or less, since the peak half-value width is 10cm ^-1 or more, the range of 30cm ^-1 or less, is mainly an amorphous phase, and a lithium alloy Even in this case, the volume expansion is small and the cycle characteristics are excellent.

Further, the ultrafine particles of the present invention may be used as a negative electrode active material by adhering the surface of the graphite powder to a composite material.

Subsequently, the lithium secondary battery of this embodiment contains at least the negative electrode, the positive electrode, and the electrolyte containing the negative electrode active material.

The negative electrode of a lithium secondary battery can illustrate, for example, that the negative electrode active material which consists of aggregates of ultrafine particles solidifies and shape | molds into a sheet form by the binder which binds ultrafine particles mutually.

In addition, the pellets solidified in the form of columnar, disc, plate or column may be used, without being limited to the sheet-shaped solidified.

The binder may be either organic or inorganic. Any binder may be used as long as it can disperse or dissolve in the solvent together with the ultrafine particles and can also bind the ultrafine particles by removing the solvent. In addition, it is also good to be able to bind the ultra-fine particles by mixing with the ultra-fine particles to perform a solidification molding such as pressure molding. As the binder, a vinyl resin, a cellulose resin, a phenyl resin, a thermoplastic resin, a thermosetting resin, or the like may be used. For example, a resin such as polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose, butylbutadiene rubber, or the like may be used. It can be illustrated.

In the negative electrode of the present invention, carbon black or the like may be added as a conductive assistant in addition to the negative electrode active material and the binder.

Subsequently, a positive electrode active material capable of occluding and releasing lithium, such as an organic disulfide compound and an organic polysulfide compound, such as LiMn ₂ O ₄ , LiCoO ₂ , LiNiO ₂ , LiFeO ₂ , V ₂ O ₅ , TiS, MoS, etc. It can be illustrated to include.

In addition to the positive electrode active material, a binder such as polyvinylidene fluoride and a conductive additive such as carbon black may be added to the positive electrode.

As a specific example of the positive electrode and the negative electrode, the positive electrode or the negative electrode may be applied to a current collector made of metal foil or metal foil and molded into a sheet shape.

Examples of the electrolyte include organic electrolytes in which lithium salts are dissolved in an aprotic solvent.

Examples of aprotic solvents include propylene carbonate, ethylene carbonate, butylene carbonate, benzonitrile, acetonitrile, tetrahydrofuran, 2-methyltetrahydrofuran, ν-butyrolactone, dioxirane, 4-methyldioxirane, N, N-dimethylformamide, dimethylacetamide, dimethyl sulfoxide, dioxane, 1,2-dimethoxyethane, sulfolane, dichloroethane, chlorobenzene, nitroheptane, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, Aprotic solvents such as methyl propyl carbonate, methyl isopropyl carbonate, ethyl butyl carbonate, dipropyl carbonate, diisopropyl carbonate, dibutyl carbonate, diethylene glycol, dimethyl ether, or mixed solvents containing two or more of these solvents Propylene carbonate, ethylene carbonate (EC) and butylene carbonate. Includes and is also preferable to be included in one of dimethyl carbonate (DMC), methylethyl carbonate (MEC), diethyl carbonate (DEC).

LiPF ₆ , LiBF ₄ , LiSbF ₆ , LiAsF ₆ , LiClO ₄ , LiCF ₃ SO ₃ , Li (CF ₃ SO ₂ ) ₂ N, LiC ₄ F ₉ SO ₃ , LiSbF ₆ , LiAlO ₄ , LiAlCl ₄ Or a mixture of one or two or more lithium salts of LiN (C _x F _{2x + 1} SO ₂ ) (CyF _{2y + 1} SO ₂ ) (where x and y are natural water), LiCl, LiI, and the like. In particular, it is preferable to include one of LiPF ₆ and LiBF ₄ .

In addition, what is conventionally known as an organic electrolyte solution of a lithium battery can also be used.

As another example of the electrolyte solution, a so-called polymer electrolyte may be used, such as one in which a lithium salt is mixed with a polymer such as PEO or PVA, or an organic electrolyte solution is impregnated with a polymer having high swellability.

In addition, the lithium secondary battery of this invention is not limited only to including a positive electrode, a negative electrode, and an electrolyte, You may include other members etc. as needed. For example, a separator that isolates the positive electrode and the negative electrode may be included.

The negative electrode active material for lithium secondary battery of this embodiment can be manufactured by the evaporation method in gas atmosphere. In the evaporation method under a gas atmosphere, an inert gas is introduced into a vacuum vessel, and various materials are heated and evaporated or sublimed in an inert gas atmosphere to collide the vapor molecules obtained with the inert gas molecules, and gradually cool them to aggregate the molecules to form fine particle powder. And the fine particle powder is recovered.

In the manufacturing method of the negative electrode active material of this embodiment, in order to remove water vapor, an inert gas is thrown into the vacuum container depressurized about 1X10 <^-3> Pa ~ 1X10 <^-4> Pa, and the back pressure is 1 In the inert gas atmosphere set at X 10 ^-4 Pa to 5 X 10 ⁵ Pa, the silicon incoat, the silicon powder, and the like are arc discharged to heat the silicon to evaporate the silicon, and the obtained silicon evaporation molecules are collided with the inert gas molecules and gradually cooled. As the molecules aggregate, ultrafine particles are formed and the fine particles are recovered to obtain a powder.

As an inert gas to be put into the vacuum container, a gas which is not reactive with silicon such as N ₂ gas can be selected in addition to an inert gas such as argon or helium.

As the silicon heating means, a means such as heater heating, induction heating, laser heating, resistance heating or electron gun heating can be used in addition to the arc discharge. Usually, the heating temperature in the gas evaporation method is set to about 100 to 200 degreeC higher than the melting point of the material to be heated. If the temperature is low, the particles are difficult to evaporate. If the temperature is high, the rapid freezing degree is lowered, thereby making it impossible to obtain an amorphous material. In the case of silicone, about 1550-1700 degreeC is preferable.

In the inert gas atmosphere, the silicon molecules are gradually cooled to agglomerate to form ultra-fine particles, so that the silicon molecules aggregate randomly to form mainly amorphous tissues. Accordingly, the range of particle size is less than 1nm, 200nm, Raman shift is in a range of more than 480cm ^-1, 520cm ^-1 or less, the peak half width is ^-1 or more, the range of ultra-fine powder of less than 10cm 30cm ^-1 is obtained .

Hereinafter, preferred examples and comparative examples of the present invention are described. However, the following examples are only one preferred embodiment of the present invention and the present invention is not limited to the following examples.

(Manufacture of Anode Active Material)

(Example 1)

The inside of the vacuum vessel was made a helium atmosphere of 5 X 10 ⁴ Pa, and the silicon powder previously installed in the vacuum vessel was heated to 1700 ° C by the arc heating method to generate silicon vapor. The resulting silicon vapor was cooled and agglomerated in a helium atmosphere and finally attached to the inner surface of the vacuum vessel as ultrafine particles. This process was performed continuously for about 4 hours, and Si ultrafine particle powder was manufactured. This powder was used as the negative electrode active material of Example 1.

As a result of measuring the particle size of the ultrafine particles in the obtained powder with an electron microscope, it was in the range of 10 nm to 200 nm. In addition, the presence of isolated ultrafine particles as shown in FIG. 1, chain ultrafine particles as shown in FIG. 2, and bulk ultrafine particles as shown in FIG. 3 were confirmed by electron microscopic observation. In addition, a peak was confirmed in the results of measuring the near 500cm ^-1 Raman shift by Raman spectroscopy was also a peak half-value width is 15cm ^-1.

(Comparative Example 1)

Using a silicon powder having an average particle diameter of 1 µm, the powder was ground for about 24 hours using a bead mill using zirconia beads having a diameter of 0.5 mm to obtain a powder. This powder was used as the negative electrode active material of Comparative Example 1.

As a result of confirming the particle size of the powder by an electron microscope, the average particle diameter was about 250 nm. However, the particle | grains of about 0.9 micrometer in particle size were also contained. In addition, the Raman shift was measured by Raman spectroscopy, the peak was confirmed around 490cm ^-1 and the peak half width was 40cm ^-1 .

(Comparative Example 2)

The silicon powder with an average particle diameter of 1 micrometer was used as the negative electrode active material of Comparative Example 2. As a result of measuring a Raman shift of the powder, a peak was confirmed at around 520cm ^-1, the peak had a full width at half maximum 9cm ^-1.

(Manufacture of lithium secondary battery)

70 parts by weight of the negative electrode active materials of Example 1 and Comparative Examples 1 and 2, 20 parts by weight of graphite powder having an average particle diameter of 2 μm and 10 parts by weight of polyvinylidene fluoride were mixed with a conductive material, and N-methylpyrrolidone was added thereto. Agitated to prepare a slurry. Subsequently, the slurry was applied onto a copper foil having a thickness of 14 µm and dried, and then rolled to prepare a cathode having a thickness of 80 µm. The prepared negative electrode was cut into a circle having a diameter of 13 mm, using porous polypropylene separator and metal lithium as counter electrode, and 1 mol of LiPF ₆ in a mixed solvent of EC: DMC: DEC = 3: 3: 1 by volume ratio. The coin-type lithium secondary battery was prepared by pouring the electrolyte solution added at the / L concentration.

(Physical Properties of Anode Active Material)

The negative electrode active material of Example 1 has a particle size in the range of 10 nm to 200 nm, while the negative electrode active material of Comparative Example 1 has particles having a particle size of about 0.9 μm, which is larger than that of Example 1 at an average particle diameter of about 250 nm, and which is not present in Example 1 It was. This difference is considered to be due to the difference in the manufacturing method of Example 1 and Comparative Example 1. That is, since the negative electrode active material of Example 1 was prepared by agglomeration of the silicon vapor generated once, the particle size was small and the particle size was similarly obtained, but the negative electrode active material of Comparative Example 1 was prepared by mechanically grinding a powder having a particle diameter of about 1 μm. Therefore, it is thought that particle size is comparatively large and particle size range is obtained widely.

In addition, the negative electrode active material of Example 1 in the Raman shift is 500cm ^-1, the half width 15cm ^-1, is estimated to be amorphous. On the other hand, the negative electrode active material of Comparative Example 2 was found to have high crystallinity as compared to the 9cm ^-1 Raman peak at 520cm ^-1, the half width, the first embodiment. In addition, the negative electrode active material of Comparative Example 1 was found to have a Raman shift lower crystallinity than as a 40cm ^-1 490cm ^-1, the half width, the first embodiment. This is considered to be because silicon crystals are greatly distorted by mechanical grinding.

(Characteristics of Lithium Secondary Battery)

In Table 1, the discharge capacity retention rate at 10 cycles with respect to the discharge capacity at 1 cycle and discharge capacity at 1 cycle is shown.

Discharge Capacity (mAh / g) Discharge retention rate (%) Example 1 1750 90 Comparative Example 1 1870 68 Comparative Example 2 2350 20

As shown in Table 1, Comparative Examples 1 and 2 showed higher initial discharge capacities than Example 1, but Example 1 was found to be higher than Comparative Examples 1 and 2 in terms of discharge retention. This difference in discharge retention is considered to be for the following reason.

That is, the negative electrode active material of Example 1 has a relatively low crystallinity, so the volume expansion of ultrafine particles during charging is small, and it is composed of ultrafine particles of 200 nm or less, and has a specific characteristic different from that of Si having high crystallinity by the so-called size effect, and mechanical grinding It is considered to be due to the fact that it does not contain particles made of a product, there is no deterioration due to expansion and contraction of the particles, and silicon vapor is formed by agglomeration and the silicon atom arrangement is different from Si having high crystallinity.

As described above, as described in detail, the negative electrode active material for a lithium secondary battery of the present invention is made of ultra-fine particle powder formed by an evaporation method in a gas atmosphere, and has a particle size distribution range of 1 to 200 nm and a ultra-fine particle having a particle size of up to 200 nm. Include. Such ultrafine particles hardly undergone volume expansion even when alloyed with lithium, and have excellent cycle characteristics.

Claims (5)

Particle size is in the range of 1nm or more, 200nm or less, the Raman shift range of 480cm ^-1 measured by Raman spectrometry over, 520cm ^-1 or less, peak half-value width is 10cm ^-1 or more, the range of the lithium 30cm ^-1 or less and The negative electrode active material for lithium secondary batteries containing the powder of the ultrafine particle containing the alloying element. The negative active material of claim 1, wherein the ultrafine particles are formed by an evaporation method under a gas atmosphere. The negative active material of claim 1, wherein the ultrafine particles contain Si. The ultrafine particles of claim 1, wherein the ultrafine particles include at least one of isolated ultrafine particles in which particles are isolated, chain ultrafine particles in which a plurality of particles are connected in a chain, and massive ultrafine particles in which a plurality of particles are combined in a bulk form. And the negative electrode active material for lithium secondary battery whose particle size of a bulk ultrafine particle is 1-200 nm. The lithium secondary battery containing the negative electrode active material for lithium secondary batteries in any one of Claims 1-4.