KR20060067459A - Silicon containing multi-layered thin film electrodes for lithium secondary battery and preparation thereof - Google Patents

Abstract

The present invention relates to an electrode having a current collector and an electrode active material layer capable of occluding and releasing lithium on one or both surfaces of the current collector, wherein the electrode active material layer comprises: a silicon active material layer; And a metal inert material layer made of lithium and a metal which does not form a solid solution or an intermetallic compound is alternately stacked. In addition, a lithium secondary battery including the electrode is provided.

The multi-layered electrode of the present invention prevents volume change and cracks during the charge / discharge process of the battery, which is a disadvantage of the conventional silicon electrode, thereby improving the structural stability of the electrode and the battery life characteristics as well as the electrochemical characteristics of the battery. By doing so, it is possible to provide a long life and high capacity lithium secondary battery.

Silicon, metal inert material, molybdenum, multilayer thin film, electrolytic copper foil, lithium secondary battery

Description

Multi-layered electrode including silicon for lithium secondary battery and manufacturing method therefor {SILICON CONTAINING MULTI-LAYERED THIN FILM ELECTRODES FOR LITHIUM SECONDARY BATTERY AND PREPARATION THEREOF}

1 is a schematic cross-sectional view of a lithium secondary battery manufactured according to an embodiment of the present invention.

FIG. 2 is a Raman spectroscopy graph of multilayer electrode electrodes prepared in Comparative Example 2, Example 2, Example 4, and Example 5. FIG.

3 is a view comparing cycle characteristics of a lithium secondary battery manufactured using the multilayer thin film electrodes of Examples 1 and 2 and the single silicon thin film electrodes of Comparative Examples 1 and 2, respectively.

4 is a diagram comparing cycle characteristics of a lithium secondary battery manufactured using the multilayer thin film electrodes of Examples 2 and 3 and the single silicon thin film electrode of Comparative Example 2.

FIG. 5 is a diagram comparing discharge capacities per unit weight of a lithium secondary battery manufactured using the multilayer thin film electrodes of Examples 2, 4, and 5, and the single silicon thin film electrodes of Comparative Examples 3 and 4. .

FIG. 6 is a view comparing discharge capacities per unit area of a lithium secondary battery manufactured using the multilayer thin film electrodes of Examples 2, 4, and 5 and the single silicon thin film electrodes of Comparative Examples 3 and 4.                 

7 is a view comparing voltage characteristics of first cycles of the lithium secondary batteries manufactured in Example 11 and Comparative Example 10;

8 is a view comparing cycle characteristics of the lithium secondary batteries manufactured in Example 11, Comparative Example 9, and Comparative Example 10.

The present invention relates to an electrode for a lithium secondary battery having improved life characteristics and electrochemical characteristics, and a method of manufacturing the same.

Lithium secondary batteries are manufactured by reversibly inserting and detaching lithium ions as a positive electrode and a negative electrode, and filling an organic electrolyte or a polymer electrolyte between the positive electrode and the negative electrode. Reduction and oxidation reactions upon desorption produce electrical energy. The lithium secondary battery greatly depends on the charge and discharge voltage, cycle life characteristics and storage characteristics of the battery according to the material of the electrode active material used, and efforts to improve the active material of both electrodes to improve the characteristics of the battery is steadily It's going on.

Currently, carbon-based active materials commercialized as a negative electrode active material of a lithium secondary battery have been studied for surface modification and the like in order to achieve a capacity increase through stability and initial irreversible capacity by a solid electrolyte interface. Examples of the carbon-based active material include graphite, which is typically used by almost all companies except Japan's Sony, and soft carbon containing amorphous carbon which can be converted into graphite according to graphitization, and non-graphite non-graphite It can be divided into hard carbon, which is carbon. Despite the excellent cycle characteristics of graphite carbon, graphite has a theoretical capacity of 372 mAh / g (LiC ₆ composition), which is only about 10% of the metal lithium capacity (3860 mAh / g), in particular lithium per 6 carbon atoms Insertion of one has the disadvantage that the capacity per unit volume is very small. In order to develop a lithium secondary battery with improved high capacity and energy density, a new negative electrode material needs to be developed, and a new high capacity negative electrode material includes a non-carbon material or a composite material.

Among the high capacity negative electrode materials, Li _y M (M = Al, Si, Sn, Pb, Ge, In, etc.), which is a lithium alloy, has a reaction potential different from that of lithium metal in the range of 0.1 to 1.0 V, and the charge capacity is carbon material. It is about 2 to 10 times higher than that. In the case of Li _x Sn (x = 1 to 4.4), the melting point of the Li-rich phase is higher than 400 ° C., which greatly reduces the risk of explosion, thereby improving battery safety. However, mechanical stresses caused by large volume changes during alloy forming / decomposition cause gradual cracking between the particles, and even crushing that leads to loss of contact with the current collector or between the metal particles. It has a problem of deterioration. Therefore, despite the advantages in terms of storage capacity, the lithium alloy system was difficult to commercialize, and even if the morphology or structure of the alloy system was adjusted, the cycle characteristics were still unsatisfactory. As a major approach to overcome this shortcoming, nanoparticle granulation using high-energy milling methods reduces the particle size of the active material or creates multi-phase or intermetallic compounds with materials with small volume changes to reduce volume expansion. Although focused, there are still major problems with irreversible capacity loss and cycle characteristics.

Conventionally, as a technique for manufacturing a carbon electrode, thin film thinning methods using various thin film processes such as chemical vapor deposition (CVD), sputtering, and vacuum deposition have been pursued, but these methods are also used for two-dimensional thin films. If the thickness is thick, the electrochemical properties deteriorate. Theoretically, increasing the thickness of the active material is expected to increase the capacity of the electrode in proportion. However, in the case of the thin film electrode, the diffusion length and internal resistance of the lithium may be increased, and also the stress applied to the film during deposition may be increased. Damage to the active material may occur. The most important is the adhesion between the current collector, which is a substrate and the film, as the film thickness increases. This is because the film is exfoliated from the current collector due to the occurrence of the film due to the accumulated stress due to the mismatch of the lattice constant between the material to be deposited and the substrate material and the volume change of the active material during charging and discharging. As a result, the cycle characteristics of the battery due to the electrical contact is lowered due to the above problems.

For the development of new anode materials, silicon, a non-carbon material, is emerging as a material that can replace the carbon material. Silicon has a very large theoretical capacity of 4000 mAh / g or more, but has a problem in that long life cycle characteristics are deteriorated due to low electrical conductivity and volume change generated during charging and discharging of the battery and deterioration of the electrode.                         

In Korean Patent Laid-Open Publication No. 2002-0045616, an electrode formed by depositing a silicon thin film on a current collector by a CVD method and a thin film formation method of a sputtering method has been proposed. As the components of the current collector are diffused into the silicon thin film alloyed with lithium, adhesion between the active material thin film and the current collector is improved, and excellent current collecting characteristics are exhibited, but the volume change of the silicon layer is not effectively suppressed during charge and discharge.

In addition, Korean Patent Publication No. 2003-0017944 proposes a cathode thin film electrode including a multilayer thin film in which a silicon-metal layer and a silver (Ag) layer in which silicon, which is a carbon-based active material is dispersed, are laminated. However, due to the coexistence of silicon and metal in one layer, the volume change of silicon due to lithium insertion and desorption could not be effectively suppressed. In addition, the stacked silver and lithium form an alloy (Li-Ag) during the charging process. By acting as an irreversible capacity, it did not achieve the high capacity and long life cycle characteristics of the lithium secondary battery.

The present inventors intend to solve the problem of the peeling of the active material resulting from the sudden volume change of the silicon-based material and the resulting degradation of cycle characteristics through the structure of the electrode, the composition of the material and the surface modification method.

Accordingly, the present invention provides a high capacity and long life lithium secondary battery electrode and a method of manufacturing the same by introducing a metal inert material phase that is not alloyed with lithium to mitigate the volume change of silicon, thereby improving the structural stability and electrochemical properties of the electrode. For the purpose of                         

In addition, another object of the present invention is to provide a lithium secondary battery including the electrode.

The present invention provides a current collector and an electrode active material layer capable of occluding and releasing lithium on one or both surfaces of the current collector, the electrode active material layer comprising: a) a silicon active material layer; And b) a metal non-active material layer composed of lithium and a metal which does not form a solid solution or an intermetallic compound is alternately stacked to provide a multi-layered electrode and a lithium secondary battery comprising the same.

In addition, the present invention comprises the steps of a) providing a metal inert material that does not form a solid solution or an intermetallic compound with the current collector, silicon and lithium; b) laminating silicon on the current collector; c) depositing a metal inert material on the silicon layer; And d) repeating the steps b) and c).

Hereinafter, the present invention will be described in detail.

As the active material capable of occluding and releasing lithium in the multilayer electrode of the present invention, all of amorphous, microcrystalline, polycrystalline, and monocrystalline silicon can be used. In particular, silicon in an amorphous state which is an open structure is preferable. The open structure does not have the long range regularity of the lattice, so it can absorb part of the expansion itself upon lattice expansion and provide many diffusion paths of lithium into the electrode. In addition, thin film processes such as sputtering and vacuum deposition are easier to use than silicon lamination.

However, the silicon has a problem that the volume change due to occlusion and release of lithium occurs during charging and discharging of the battery, resulting in a decrease in cycle characteristics of the battery due to deterioration of the electrode.

Accordingly, the present invention is to prepare a negative electrode of a multilayer form in which a metal inert material layer made of lithium and a metal that does not form a solid solution or an intermetallic compound is alternately stacked on a silicon active material layer, so that the active material is not peeled off from the current collector. In addition, it is possible to prevent the irreversible capacity of the battery generated due to the lithium-metal alloy formed during the charging process of the battery can provide a silicon-based electrode having high capacity maintenance and long life characteristics.

The metal constituting the metal inert material layer preferably has a strong affinity with silicon, which is an active material. This is because the structural stability of the electrode due to the interfacial stability between the silicon active material layer and the metal inert material layer can be achieved. Non-limiting examples of metals that do not form a lithium-metal alloy and have a strong affinity with silicon include Mo, Cu, Fe, Co, Ca, Cr, Mg, Mn, Nb, Ni, Ta, Ti, or V. .

In addition, the metal preferably has a large difference in diffusion rate from that of silicon. When the active material and the inactive material form a layer, the microstructure in which the inactive material layer surrounds the silicon layer as the active material is effective in suppressing the volume expansion of silicon generated during charge and discharge. In order to implement this more effectively, it is more preferable that the metal inactive material layer is diffused into the silicon layer to form an intermetallic compound. In general, when the material is deposited at room temperature, the low melting point element is known to have a faster diffusion rate than the high melting point element. Therefore, as the metal of the inert material layer has a lower melting point than silicon having a high melting point (1410 ° C.) desirable.

In addition, the metals described above preferably have high conductivity, ie low resistivity. This is because silicon, which is an active material of the present invention, is a semiconductor having a specific resistance of 2.3 × 10 ⁵ Pa · cm, so that the conductivity of the entire electrode can be improved when a metal having a low specific resistance is introduced.

In view of the above conditions, the most preferred metal constituting the metal inert material layer of the present invention is molybdenum (Mo). Molybdenum has a high affinity with silicon in the inert material metal, a large difference in melting point due to the large melting point difference can occur, and because the lowest resistivity value among the metals except silver, copper and magnesium. In addition, molybdenum silicide, which is a compound of silicon and molybdenum, also has a low specific resistance, which is advantageous as an electrode material.

In addition, as long as the material includes the above-described properties, any material may be used as an inactive material, and may be used alone or in combination of one or more metals.

In the multilayer film type electrode, the thickness of one layer of the silicon active material layer is not particularly limited, but is preferably in the range of 25 to 50 nm. If the thickness is less than 25 nm, the amount of effective silicon that is inserted and desorbed of lithium is insufficient, resulting in low charge and discharge capacity of the battery. If the thickness exceeds 50 nm, the cycle characteristics of the battery are increased due to the increase in lithium reaction distance and the change in volume of silicon. This deterioration problem may occur.

In addition, one layer thickness of the metal inert material layer is suitably in the range of 1 to 10 nm, but is not particularly limited thereto. If it is less than 1 nm, effective silicon volume change cannot be suppressed, and if it exceeds 10 nm, the diffusion distance of lithium increases due to the increase of the thickness of the thin film, and the charge and discharge capacity of the silicon active material and lithium become difficult to react with each other. Decreasing problems may occur.

Although the number of layers of the formed multilayer thin film is not specifically limited, The range of 10-500 layers is suitable. If it is less than 10 layers, sufficient battery capacity cannot be obtained, and if it exceeds 500 layers, a problem may occur in that the capacity retention rate of the battery is lowered. Increasing the number of thin film layers of the electrode increases the fraction of silicon in the element ratio of silicon to metal, resulting in a lack of structural stability of the electrode resulting in a decrease in cycle characteristics of the battery.

An element ratio of silicon to metal constituting the multilayer thin film is suitably 1 to 10: 1, and when it exceeds 10: 1, the same effect as the problem of increasing the number of thin film layers occurs.

In the multilayer film electrode of the present invention, the silicon active material layer thin film and the metal inert material layer thin film are alternately stacked. In particular, it is preferable that the uppermost layer of the multilayer thin film is a metal inert material layer. This is because silicon is effectively prevented from falling off due to cracking and peeling due to the volume change occurring during the reaction with the electrolyte or the charge and discharge in the upper inert material layer. Most preferred is to mitigate the volume change of the silicon active material layer in both metal inert material layers. Each layer laminated may differ in composition, crystallinity, impurities or concentration.

It is preferable that the shape of the multilayer thin film is separated into a columnar shape by a gap formed in the thickness direction, and the bottom portion of the columnar portion is in close contact with the current collector. This is because the mechanical stress caused by the large volume change can be effectively dispersed.

In general, the multilayer electrode active material layer of the present invention is preferably manufactured in a form laminated on a current collector. The electrode active material layer may be prepared alone, which is within the scope of the present invention.

As said collector, if an active material thin film can be formed in favorable adhesive on it, it can be used without a restriction | limiting. Non-limiting examples thereof include copper, nickel, stainless, tungsten or tantalum. Double copper metal foil is preferable, and one or both surfaces thereof may be roughened or surface treated. The surface treatment of the current collector is preferably surface treatment using plasma. This is for removing the oxide on the surface of the current collector, in which the power of the microwave generating plasma is suitably 100 to 500W, particularly preferably 200W. In addition, it is preferable that the surface roughness Ra of an electrical power collector is 0.1-1 micrometer. This is because the rough surface of the substrate can increase the area in contact with the deposited material to improve the adhesion of the thin film. The thickness of the current collector is preferably in the range of 10 to 30㎛, but is not limited thereto. The current collector surface is not particularly limited, but in order to induce stress dispersion, it is preferable that a structure or irregularities are formed in a column shape by a gap formed in the thickness direction. This is for effectively inducing the dispersion of stress.

The multilayer electrode of the present invention can be prepared by a conventional thin film forming method, for example, a) providing a metal inert material capable of occluding and releasing a current collector, silicon and lithium; b) laminating silicon on the current collector; c) depositing a metal inert material on the laminated silicon layer; And d) repeating steps b) and c).

As a method of stacking the silicon or metal inert material, physical vapor deposition (PVD), chemical vapor deposition (PVD), chemical vapor deposition (SPD), vacuum deposition, and electrostatic spray deposition (ESD), CVD), thermal spraying, electroplating or electroless plating and the like can be used without limitation.

For example, the method of laminating the silicon or metal inert material of the present invention may use magnetron sputtering, which is one of physical vapor deposition methods. One embodiment of the method for manufacturing a multilayer film electrode according to the present invention comprises the steps of: a) providing a current collector, a silicon target and a metal inert material target inside a chamber of a thin film device; b) forming the chamber in a vacuum state; And c) after supplying an inert gas to the chamber and generating a plasma, alternately sputtering a silicon target and a metal inert material target to alternately stack a silicon layer and a metal inert material layer.

First, a current collector, a silicon target and a metal inert material, for example, a molybdenum target, are provided inside a chamber of a thin film device. The current collector may maintain a temperature of room temperature without heating, or may be heated to maintain a specific temperature range. In addition, it is preferable that the collector is rotated. This is to facilitate deposition of the silicon active material layer and the metal inert material layer. The distance between the silicon target and the metal inert material target and the substrate is preferably 4.0 to 6.0 cm and 6.0 to 9.0 cm, respectively.

Preferably the chamber forms a vacuum. This is to improve the linear motion of the atoms, molecules, and ions generated after sputtering. The pressure range in the vacuum state can be used within the pressure range of the general thin film manufacturing method.

When a certain potential is applied, a plasma is formed in which some argon (Ar) atoms are separated into ions and an electron state by glow discharge. When a magnetic field is applied to the plasma, the ions collide with the silicon target while performing a spiral motion. At this time, the molecules in the collided silicon are deposited on the current collector, which is a substrate to be deposited by jumping out of the target through exchange of momentum with the collided ions to form a silicon active material layer.

After a suitable silicon active material layer is deposited, a metal inert material, for example, a molybdenum (Mo) target, is sputtered in the same manner, and a metal inert material layer is deposited on the silicon active material layer by DC power supply. By repeating the above process, the multilayer electrode of the present invention can be produced. The power density at the time of sputtering is not particularly limited, but silicon is preferably carried out under (rf) 5.0 to 7.0 W / cm ² and molybdenum under (dc) 0.5 to 2.0 W / cm ² .

The above-described thin film process technology can easily control the particle size formed in the growth direction of the thin film, it is possible to implement a multilayer thin film structure of the desired form. In order to precisely adjust the deposition of each layer, the thickness of the thin film can be easily adjusted by placing a shutter between the substrate and the target to control the opening and closing time of the shutter.

The multilayer film electrode manufactured by the above method may be used as an electrode for a lithium secondary battery, preferably as a negative electrode.

In addition, the present invention can provide a lithium secondary battery comprising the multilayer film negative electrode, the positive electrode and the electrolyte, the lithium secondary battery of the present invention is a porous method between the positive electrode and the negative electrode in a conventional manner known in the art It can be prepared by adding a separator and adding an electrolyte.

As the cathode active material, a conventional lithium transition metal composite oxide may be used. For example, LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ , LiMnO ₂ , LiNi _1-X Co _X M _Y O ₂ (where M = Al, Ti, Mg, Zr, 0 <X ≤ 1, 0 ≤ Y ≤ 0.2), LiNi _X Co _Y Mn _1-XY O ₂ (where 0 <X ≤ 0.5, 0 <Y _≤ 0.5) and LiM _x M ' _at least one transition metal oxide selected from the group consisting of _y Mn _(2-xy) O ₄ (M, M '= V, Cr, Fe, Co, Ni, Cu, 0 <X ≤ 1, 0 <Y ≤ 1) Can be used. The positive electrode may be prepared by a conventional method known in the art, and for example, the positive electrode slurry may be obtained by mixing the positive electrode active material with a binder or a dispersion medium, and the like, and include a small amount of a conductive agent or a viscosity regulator. It is preferable.

In manufacturing the battery of the present invention, it is preferable to use a porous separator as a separator, and non-limiting examples include a polypropylene-based, polyethylene-based or polyolefin-based porous separator.

The electrolyte of the present invention is a conventional electrolyte consisting of an electrolyte solution and a lithium salt, wherein the lithium salt is 1 from the group consisting of LiClO ₄ , LiCF ₃ SO ₃ , LiPF ₆ , LiBF ₄ , LiAsF _6, and LiN (CF ₃ SO ₂ ) ₂ . More than one species can be selected, and the electrolyte solution is ethylene co (carbonate), propylene carbonate, butylene carbonate, vinylene carbonate, sulfolane, γ-butylarolactone, dimethyl carbonate, diethyl carbonate, 1,2-dimeth One or more selected from the group consisting of oxyethane, tetrahydrofuran and mixtures thereof can be used.

The lithium secondary battery of the present invention is not limited in appearance, but may be cylindrical, square, pouch type, or coin type using a can.

The invention is explained in more detail based on the following examples and experimental examples. However, Examples and Experimental Examples are for illustrating the present invention and are not limited to these.

Preparation Example 1 Surface Treatment of Current Collector

As the current collector, an electrolytic copper foil (surface roughness of 0.5 μm) having a thickness of 18 μm having irregularities on the surface was used. ECR plasma, a high density plasma source, was generated and irradiated with the electrolytic copper foil for 10 minutes at a microwave power of 200 W and a process pressure of 0.5 mTorr. The visual and scanning electron microscope (SEM) observations did not show any difference, but the roughness of the surface roughness of the electrolytic copper foil by the confocal laser scanning microscope (CLSM) was slightly increased.

[Examples 1 to 5. Fabrication of Multilayer Film Electrode]

Example 1. Manufacture of multilayer thin film electrode A1

An electrolytic copper foil having a surface roughness (Ra) value of 0.5 μm and a thickness of 18 μm was used as a current collector, and a mask was applied to the electrodeposited copper foil to maintain a constant area of the cathode electrode. After depositing silicon on the current collector for 1 hour with RF, a silicon thin film layer having a thickness of about 26 nm was manufactured, and molybdenum was generated by plasma with DC on the silicon thin film layer to prepare an inert material layer having a thickness of about 3 nm. The multilayer film electrode was manufactured by depositing about 120 layers alternately with a layer (refer Table 2).

The element ratio of silicon to molybdenum quantified by ICP (Inductively Coupled Plazma) analysis was 6.2: 1. The area of the prepared multilayer thin film negative electrode was 1 cm ² , and the thin film formation conditions are shown in Table 1 below.

Example 2. Manufacture of a multilayer thin film electrode A2

A multilayer film type electrode was prepared in the same manner as in Example 1, except that an electrolytic copper foil having a surface roughness value of 0.5 µm surface treated with a plasma of Preparation Example 1 was used as a current collector (see Table 2).

Example 3. Manufacture of a multilayer thin film electrode A3

A multilayer film electrode was manufactured in the same manner as in Example 1, except that an electrolytic copper foil having a surface roughness value of 0.5 μm that was treated with a plasma of Preparation Example 1 and an element ratio of silicon and molybdenum were used as 2.0: 1. (See Table 2).

Example 4. Manufacture of multilayer thin film electrode A4

The same as in Example 1 except that the surface roughness value of the surface treated with the plasma of Preparation Example 1 using an elemental ratio of 6.5: 1 using an electrolytic copper foil and silicon and molybdenum were deposited in 180 layers. To prepare a multilayer electrode (see Table 2).

Example 5. Manufacture of multilayer thin film electrode A5

The same as in Example 1 except that the surface roughness of the surface treated with plasma of Preparation Example 1 was used as the electrolytic copper foil and the element ratio of silicon and molybdenum as 6.7: 1 and the number of laminated layers was deposited in 240 layers. To prepare a multilayer electrode (see Table 2).

[Comparative Examples 1 to 5. Fabrication of Silicon Thin Film Electrode]

Comparative Example 1. Fabrication of silicon thin film electrode (B1)

Except for using only silicon, the silicon thin film electrode was prepared in the same manner as in Example 1 (see Table 2).

Comparative Example 2. Manufacture of Silicon Thin Film Electrode (B2)

A silicon thin film electrode was prepared in the same manner as in Example 1, except that only electrolytic copper foil and silicon having a surface roughness value of 0.5 μm that were surface treated with plasma of Preparation Example 1 were used (see Table 2).

Comparative Example 3. Manufacture of Silicon Thin Film Electrode (B3)

Using only the electrolytic copper foil and silicon having a surface roughness value of 0.5 µm surface treated with a plasma of Preparation Example 1, except that the same amount as the amount of silicon deposited in Example 4 was deposited, the same as in Example 1 Thin film electrodes were prepared (see Table 2).

Comparative Example 4. Manufacture of Silicon Thin Film Electrode (B4)

Only electrolytic copper foil and silicon having a surface roughness value of 0.5 µm surface treated with plasma of Preparation Example 1 were used, and the same procedure as in Example 1 was carried out except that the same amount of silicon deposited in Example 5 was deposited. Thin film electrodes were prepared (see Table 2).

variable Thin film formation conditions target silicon molybdenum Sputtering Power Density (rf) 6.4 W / cm ² (dc) 1.0 W / cm ² Target-substrate distance 5.0cm 7.5cm Substrate temperature Room temperature (no heating) Exhaust pressure 3.0 × 10 ^-6 Torr or less Process pressure 5 mTorr Board rotation Gas flow rate Argon gas 20 sccm

pellicle Element ratio (Si: Mo) Number of floors Thickness (㎛) Plasma treatment Example 1 (A1) Multilayer thin film 6.2: 1 120 3.50 X Example 2 (A2) Multilayer thin film 6.2: 1 120 3.50 O Example 3 (A3) Multilayer thin film 2.0: 1 120 4.34 O Example 4 (A4) Multilayer thin film 6.5: 1 180 5.04 O Example 5 (A5) Multilayer thin film 6.7: 1 240 6.58 O Comparative Example 1 (B1) Silicon thin film 1.0: 0 One 3.11 X Comparative Example 2 (B2) Silicon thin film 1.0: 0 One 3.11 O Comparative Example 3 (B3) Silicon thin film 1.0: 0 One 4.50 O Comparative Example 4 (B4) Silicon thin film 1.0: 0 One 5.89 O

Examples 6 to 10. Preparation of Half Battery

The positive electrode was each of the multilayer electrode electrodes prepared in Examples 1 to 5, the negative electrode was lithium metal (Aldrich), the separator was a porous polyethylene membrane (Celgard 2300), and the electrolyte was dissolved in EC / DEC (1M LiPF ₆ ). 1) Coin battery was prepared using a liquid electrolyte (Merck) (see Fig. 1). Each cell was assembled in a glove box filled with argon with a moisture content of 0.1 ppm or less to protect lithium metal and electrolyte from moisture.

[Comparative Examples 5 to 9 half cell production]

Comparative Examples 5 to 8. Half Battery Preparation

A coin-type half cell was prepared in the same manner as in Example 6, except that each single silicon thin film electrode manufactured in Comparative Examples 1 to 4 was used as the positive electrode (see FIG. 1).

Comparative Example 9. Preparation of Half Battery (D2)

LiCoO ₂ powder having an average particle diameter of 10 µm, a binder (polyvinylidene fluoride: PVdF) and a conductive agent (carbon black) are uniformly mixed, and 5 g of N-methylpyrrolidone (NMP) is added as a solvent to obtain a uniform state. A slurry was prepared, which was coated on one side of an aluminum foil having a thickness of 18 μm using the doctor blade method, dried in a vacuum oven at 100 ° C. to remove moisture, and used as an anode. A negative electrode and other configurations were used in the same manner as the half cell of Example 6 to produce a coin-type battery (see FIG. 1).

Example 11. Preparation of Lithium Secondary Battery (C1)

A positive electrode was prepared and used in the same manner as in Comparative Example 9, and a negative electrode was manufactured in the same manner as in Example 6 using the 240-layer multilayer electrode, separator and electrolyte prepared in Example 5, to prepare a lithium secondary battery.

Comparative Example 10. Fabrication of Lithium Secondary Battery (D1)

A lithium secondary battery was manufactured in the same manner as in Example 11, except that graphite used as a negative electrode was used.

Experimental Example 1. Analysis of multilayer thin film

In order to confirm the structural characteristics and crystallinity of the resulting multilayer thin film, the following experiment was conducted.

Raman Spectroscopy Analysis is an analytical method for measuring scattering of the lattice, and can measure mechanical properties and crystallinity, that is, magnitude and crystallinity of latent stress. As a sample, multilayer thin film electrodes of Examples 1 to 5 were used, and a single silicon film electrode of Comparative Example 1 was used as a control.

As a result of decomposing and analyzing each component through the peak corresponding to the silicon region in the peak of the TO (transverse optical) phonon mode, the silicon layers deposited in Examples 1 to 5 were deposited in the amorphous region as in the silicon single thin film of Comparative Example 1. It was found that the peak of the 480cm ^-1 near to, 520cm ^-1 of the peak corresponding to the crystalline region was not observed (see Fig. 2). It could be confirmed that the silicon thin film layer thus prepared was amorphous (see FIG. 2).

The results of ICP analysis on the thickness, the number of layers, and the element ratio of the multilayer thin film were the same as in Table 2 (see Table 2).

Experimental Example 2 Electrochemical Characteristic Test of Lithium Secondary Battery

The electrochemical properties of the half cells of Examples 6 to 10 and Comparative Examples 5 to 8 prepared according to the present invention were evaluated as follows.

2-1. Surface treatment comparison experiment of current collector

Half cells of Examples 6 (A1) and 7 (A2), Comparative Examples 5 (B1) and 6 (B2) were used. Each battery was charged and discharged at a constant current density of 0.42 mA / cm ² at a voltage range of 0.01 to 1.0 V at 25 ° C., and a charger / discharger (WBCS 3000, WON A TECH) was used for the measurement.

One cycle charge and discharge of the battery was defined as being charged at a constant current density of 0.42 mA / cm ² until the potential reached 0.01 V and then discharged until it reached 1.0 V. In the half cell of the present invention, charging means that a reduction reaction occurs in a multilayer thin film electrode, thereby inserting lithium, and discharge means that an oxidation reaction occurs to remove lithium. The charge capacity and the discharge capacity in each cycle were measured to obtain the efficiency and capacity retention rate as follows, which is shown in Table 3 below.

Initial Efficiency (%) = (Discharge Capacity of First Cycle / Charge Capacity of First Cycle) × 100

Capacity maintenance rate R _100/1 (%) = (discharge capacity of the first cycle / discharge capacity of the first cycle) x 100

As a result, the half cells of Example 6 (A1) and 7 (A2) using the multilayer thin film electrode of the present invention showed better efficiency than the cells of Comparative Examples 5 (B1) and 6 (B2) using a single silicon thin film electrode. gave. In particular, the half cell of Example 7 (A2) using the current collector surface-treated with plasma had a higher initial discharge capacity and higher charge / discharge efficiency than the battery of Comparative Example 6 (B2), which is a single silicon thin film electrode. It was confirmed that the remarkable improvement (see Table 3 and Figure 3). This is because molybdenum, which is an inert material, suppresses the volume change of silicon, which is an active material during insertion and desorption of lithium, and effectively prevents detachment of the active material layer due to the use of a surface-treated current collector.

As a result, the multilayer electrode of the present invention was confirmed that the high capacity and long life cycle characteristics of the electrode were improved due to the use of the molybdenum inert material layer and the current collector surface-treated with plasma.

Example 6 (A1) Example 7 (A2) Comparative Example 5 (B1) Comparative Example 6 (B2) Foil Multilayer thin film Multilayer thin film Silicon thin film Silicon thin film Element ratio (Si: Mo) 6.2: 1 6.2: 1 1.0: 0 1.0: 0 Number of floors (layers) / thickness (㎛) 120 / 3.50 120 / 3.50 1 / 3.11 1 / 3.11 Plasma treatment X O X O 1 cycle Charge / discharge capacity (mAh / g) 2920/2557 3000/2653 3190/2366 2840/2424 efficiency(%) 87.4 88.4 74.3 85.2 20 cycles Discharge capacity 2500 2576 1150 2424 efficiency(%) 99.2 99.4 98.6 99.1 50 cycles Discharge capacity 2443 2538 755 2061 efficiency(%) 99.2 99.4 98.6 99.1 100 cycles Discharge capacity 2290 2405 460 881 Capacity maintenance rate R _100/1 (%) 89.6 90.7 19.4 36.3

2-2. Comparative experiment of element ratio of silicon and molybdenum

The following experiments were performed on the half cells of Example 7 (A2), Example 8 (A3) and Comparative Example 6 (B2) prepared according to the present invention.

Each battery was charged and discharged at a constant current density of 0.55 mA / cm ² in a voltage range of 25 ° C. and 0.01 to 1.0 V. The charge capacity and discharge capacity in each cycle were measured to obtain efficiency and capacity retention rate, which are shown in Table 4 below.

As a result, the half cells of Example 7 (A2) and 8 (A3) using the multilayer thin film electrode of the present invention initially had similar charge and discharge capacity and efficiency as those of Comparative Example 6 (B2) using a single silicon thin film electrode. However, as the cycle progressed, it showed excellent charge and discharge capacity and efficiency, and 100 cycles showed a markedly improved capacity retention rate. In particular, the battery of Example 7 (A2) using a multilayer thin film having an element ratio of silicon of an active material and molybdenum of an inert material of 6.2: 1 showed a good capacity retention of 94.31% even after 100 cycles (Table 4 And FIG. 4).                     

Further, in the multilayer electrode of the present invention, the battery of Example 8 (A3) having a thickness of one layer of molybdenum was less charged than the battery of Example 7 (A2) having a thickness of about 3 nm of one layer of molybdenum. Showed discharge capacity. This indicates that the lithium and the silicon active material to be inserted and detached are difficult to react with each other, and the diffusion distance of lithium increases due to the increase in the thickness of the thin film due to the large amount of molybdenum.

As a result, it was confirmed that the element ratio of silicon and molybdenum and the thickness of the inactive material layer in the multilayer electrode of the present invention are important factors of the charge and discharge capacity of the lithium secondary battery. The optimum condition of the electrode was confirmed.

Example 7 (A2) Example 8 (A3) Comparative Example 6 (B2) pellicle Multilayer thin film Multilayer thin film Silicon thin film Element ratio (Si: Mo) 6.2: 1 2.0: 1 1.0: 0 Number of floors (layers) / thickness (㎛) 120 / 3.50 120 / 4.34 1 / 3.11 Plasma treatment O O O 1 cycle Charge / discharge capacity (mAh / g) 3070/2691 2748/2347 3090/2672 efficiency(%) 87.4 85.8 86.7 20 cycles Discharge capacity 2672 2252 2630 efficiency(%) 99.4 99.5 99.5 50 cycles Discharge capacity 2634 2309 1880 efficiency(%) 99.5 99.6 97.7 100 cycles Discharge capacity 2538 2042 657 Capacity maintenance rate R _100/1 (%) 94.31 87.0 24.6

2-3. Experiment on the comparison of the number of layers

The following experiments were carried out on the half cells of Example 7 (A2), Example 9 (A4), Example 10 (A5), Comparative Example 7 (B3) and Comparative Example 8 (B4) prepared according to the present invention. Was carried out.                     

Each battery was charged and discharged at a constant current density of 0.55 mA / cm ² at a voltage range of 25 ° C. and 0.01 to 1.0 V. The charge capacity and discharge capacity in each cycle were measured to obtain efficiency and capacity retention rate, which are shown in Table 5 below.

As a result, it was confirmed that the capacity retention rate of the Example 7 (A2), 9 (A4) and 10 (A5) batteries decreased as the number of layers of the multilayer thin film increased. In particular, in the case of the batteries of Comparative Examples 7 (B3) and 8 (B4), it was sharply reduced (see Tables 5, 5 and 6). The single silicon thin film electrodes B3 and B4 used in Comparative Examples 7 and 8 had 4.50 µm and 5.89 µm, respectively, the same silicon thickness as that of the A4 and A5 multilayer electrode, and was increased as the thickness of silicon increased. The increase in reaction distance and the change in silicon volume during charging and discharging of the battery result in a lack of stability of the electrode and a decrease in capacity and long life cycle characteristics of the battery.

For reference, the ICP analysis result shows that the amount of silicon and molybdenum decreased by 5.4% and 11.9%, respectively, compared to the 120-layer multilayer electrode A2 in the case of 240-layer multilayer electrode A5. As the number increases, the fraction of silicon in the element ratio of silicon to molybdenum increases. For example, when three layers are laminated with Mo and Si, Mo 1, Si 2, and Mo 1 are arranged in the lowermost layer so that Mo and Si are 1: 1. If 5 layers are formed by stacking Si 2 and Mo 1, Si: Mo is increased to 4: 3, and Si: Mo is increased to 6: 4 when 7 layers are stacked by adding Si 2 and Mo 1 again. . Therefore, in the multi-layered electrode of the present invention in which the molybdenum layer should always be present on the uppermost layer, as the number of thin film layers having the same element ratio increases, the fraction of the inner layer except for the uppermost layer is constant but the absolute amount of silicon increases so that the fraction including the uppermost layer is increased. Done.

For the above reason, as the number of layers increases, the number of layers increases the reaction distance of lithium due to the lack of structural stability of the electrode as well as the lack of molybdenum to suppress the volume change of silicon. It was confirmed that the long life cycle characteristics of the battery is reduced.

Example 7 (A2) Example 9 (A4) Example 10 (A5) Comparative Example 7 (B3) Comparative Example 8 (B4) pellicle Multilayer thin film Multilayer thin film Multilayer thin film Silicon thin film Silicon thin film Element ratio (Si: Mo) 6.2: 1 6.5: 1 6.7: 1 1.0: 0 1.0: 0 Number of floors (layers) / thickness (㎛) 120 / 3.50 180 / 5.04 240 / 6.58 1 / 4.50 1 / 5.89 Plasma treatment O O O O O 1 cycle Charge / discharge capacity (mAh / g) 3070/2691 3206/2718 2947/2553 2860/2520 2836/2472 efficiency(%) 87.4 84.7 86.8 88.1 87.1 20 cycles Discharge capacity 2672 2361 2270 2177 644 efficiency(%) 99.4 99.1 99.0 98.3 95.4 50 cycles Discharge capacity 2634 1913 2010 598 188 Capacity Retention R _50/1 (%) 99.5 70.4 78.8 23.7 7.6

2-4. Comparative experiment with commercialized electrodes

The half cell (D2) of Comparative Example 9 using LiCoO ₂ and lithium metal as the positive electrode and the negative electrode, respectively, exhibited a reversible capacity of 2.35 mAh / cm ² under the conditions of 3.0 to 4.2 V and 0.82 mA / cm ² (Table 6). Reference). The battery of Example 11 (C1) using a 240-layer multilayer thin film electrode capable of maintaining a capacity higher than the above capacity even at 100 cycles as a cathode and the battery of Comparative Example 10 (D1) using a commercially available graphite electrode were as follows. The experiment was conducted.

The cycle characteristics of the Example 11 (C1), Comparative Example 9 (D2) and 10 (D1) batteries were confirmed, and as a result, the battery of Comparative Example 9 using lithium metal retained its initial charge / discharge efficiency, discharge capacity, and capacity of 100 cycles. In terms of the rate, all of the cells of Example 11 and Comparative Example 10, which are full cells, were shown to be superior (see Table 6 and FIG. 8). This is because lithium metal has excellent potential and capacity.

As a result of charge and discharge of the full-cell Example 11 (C1) and Comparative Example 10 (D1) batteries at a constant current density of 0.82 mA / cm ² at a voltage range of 25 ° C. and a voltage range of 2.75 to 4.2 V, 240 layers of multilayer electrode were obtained. The cell of Example 11 using showed a low discharge capacity due to the low charge / discharge efficiency of the first cycle despite the high charge capacity (see Table 6 and FIG. 7). However, when looking at the thickness of the negative electrode active material used in Example 11 (C1) and Comparative Example 10 (D1), respectively, since the graphite is 55㎛ compared to the multilayer thin film of 6.58㎛, the difference of about 1/8 do. Considering this, it was confirmed that the multilayer electrode of the present invention has a higher energy density per unit volume and a higher capacity per unit weight than commercially available graphite, and it can be seen that it can replace the existing graphite carbon material. . In addition, it was confirmed that the lithium secondary battery using this as a negative electrode can significantly improve high capacity and long life characteristics.

Example 11 (C1) Comparative Example 10 (D1) Comparative Example 9 (D2) Battery Configuration Conditions Cell Full Cell Full Cell Half cell anode LiCoO ₂ LiCoO ₂ LiCoO ₂ cathode Multilayer Thin Film (240 Layers) black smoke Lithium metal 1 cycle Charge / discharge capacity (mAh / g) 160.9 / 124.3 153.8 / 132.5 144.7 / 136.1 efficiency(%) 77.1 86.0 94.0 20 cycles Discharge capacity 119.5 125.4 136.1 efficiency(%) 99.6 99.9 99.1 50 cycles Discharge capacity 111.9 120.7 133.7 efficiency(%) 99.9 100 99.2 100 cycles Discharge capacity 106.1 117.9 129.0 Capacity maintenance rate R _100/1 (%) 85.4 89.0 94.8

The multi-layered electrode of the present invention improves the structural stability of the electrode and the cycle characteristics of the battery by preventing the volume change during the charge and discharge process of the battery, which is a disadvantage of the conventional silicon electrode, and the resulting crack, thereby replacing the existing graphite carbon material Can be used.

Claims (13)

In an electrode having a current collector and an electrode active material layer capable of occluding and releasing lithium on one or both surfaces of the current collector, the electrode active material layer is a) silicon active material layer; And b) a metal inert layer consisting of lithium and a metal that does not form a solid solution or intermetallic compound The multilayer film type electrode, which is alternately stacked. According to claim 1, wherein the metal constituting the metal inert material layer is at least one metal selected from the group consisting of Mo, Cu, Fe, Co, Ca, Cr, Mg, Mn, Nb, Ni, Ta, Ti and V Phosphorus electrode. The electrode of claim 2, wherein the metal is Mo. The electrode of claim 1, wherein the layer thickness of the silicon active material layer is in a range of 25 to 50 nm. The electrode of claim 1, wherein the layer thickness of the metal inert material layer is in a range of 1 to 10 nm. The electrode according to claim 1, wherein the number of layers of the multilayer film is in a range of 10 to 500 layers. The electrode of claim 1, wherein an element ratio of silicon to metal inert material constituting the multilayer film is 1 to 10: 1. The electrode of claim 1, wherein the uppermost layer of the multilayer film is a metal inert material layer. The electrode of claim 1, wherein the current collector is an electrolytic copper foil having a surface roughness Ra in the range of 0.1 to 1 μm. The electrode of claim 1, wherein the current collector is surface treated using a plasma. a) the cathode of any one of claims 1 to 10; b) a positive electrode comprising a positive electrode active material capable of occluding and releasing lithium ions; c) a separator existing between the anode and the cathode; And d) an electrolyte; Lithium secondary battery comprising a. a) providing a metal inert material that does not form a solid solution or an intermetallic compound with the current collector, silicon and lithium; b) laminating silicon on the current collector; c) depositing a metal inert material on the silicon layer; And d) repeating steps b) and c) Method of manufacturing a multilayer film electrode comprising a. The method of claim 12, wherein a current collector, a silicon target and a metal inert material target are provided in a chamber of the thin film device, the chamber is formed in a vacuum state, and an inert gas is supplied to generate a plasma. And sputtering alternately the active material target to alternately stack a silicon layer and a metal inert material layer.

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