KR20030094633A - Negative electrode for lithium secondary thin battery and manufacturing method thereof - Google Patents

Abstract

PURPOSE: A negative electrode for a lithium secondary thin film battery and its preparation method are provided, to improve the stability by using Si instead of Li and to enhance the charge/discharge performance by suppressing the volume change due to the reaction with Li. CONSTITUTION: The negative electrode comprises a nanocompound matrix of Si and a metal which does not react with Li and has a strong affinity with Si; and a nano-sized silicon dispersed in the matrix. Preferably the metal which does not react with Li is selected from the group consisting of Ni, Co, Fe, Cr, Fe, Hf, Mn, Mo, Nb, Ti, V, Zr, Ta, W and Re. The method comprises the steps of forming a thin film on a substrate with Si and the metal which does not react with Li; and impressing energy the substrate. Preferably the energy impressing is carried out by impressing direct bias, heating, irradiating an ion beam or treating plasma.

Description

A negative electrode for a lithium secondary thin film battery and a manufacturing method therefor {NEGATIVE ELECTRODE FOR LITHIUM SECONDARY THIN BATTERY AND MANUFACTURING METHOD THEREOF}

The present invention relates to a negative electrode for a lithium secondary thin film battery and a manufacturing method thereof, and more particularly, to a negative electrode for a lithium secondary thin film battery which is excellent in cycle life characteristics and can exhibit high capacity.

[Prior art]

Recently, due to the rapid development of semiconductor technology, high integration and ultra-miniaturization of various electronic devices are rapidly progressing, and as a result, mobile phones, notebook computers, digital camcorders, and the like, which are capable of carrying electronic products such as telephones, computers, and video cameras It is quickly being replaced. Small batteries are the key components that enable widespread use of portable electronic devices. Lithium (Li) secondary batteries have a high energy density per weight and volume, and rapidly replace existing Ni-MH and Ni-Cd batteries. It is used as a power source for most commercially available portable electronic devices.

However, currently used lithium secondary batteries are bulk batteries consisting of two electrodes made of an active material in powder form and a liquid electrolyte, and thus are not suitable for microelectronic devices because they are manufactured in the form of independent battery packs. In addition, various additives and liquid electrolytes reduce the capacity of the battery, cause a decrease in charge and discharge life, and act as a factor causing environmental problems.

In order to solve the above problems, the solid-state thin-film lithium secondary micro fabricated in the form of a thin film using a thin film deposition process such as sputtering, chemical vapor deposition (CVD), electron beam deposition, etc. Batteries (hereinafter, micro cells) have been studied.

1 is a cross-sectional view schematically showing the configuration of a general micro battery. As shown in FIG. 1, in the thin film micro cell, a current collector 2, a positive electrode thin film 3, an electrolyte thin film 4, a negative electrode thin film 5, and a protective film 6, encapsulation, are sequentially deposited on a substrate 1. Formed.

In general, alumina (Al ₂ O ₃ ) or silicon oxide (SiO ₂ ) is mainly used as the substrate 1, and glass for electronic parts may be used. Au, Pt, Al, Cr, and the like are used as the current collector 2, and Pt is mainly used.

As the positive electrode active material used in the positive electrode 3, for example, TiS ₂ , WO ₃ and MnO ₂ , lithium metal oxides (Li-MO, M = Co, Mn, Ni) and the like are used. The anode deposition process uses sputtering, electron beam evaporation, sol-gel method and the like. As the electrolyte 4 of the lithium secondary battery, solid oxides or nitrates are mainly used and may be formed of a polymer film. The negative electrode 5 is mainly formed of lithium (Li). The protective film 6 of FIG. 1 is made of a metal film, nitride oxide, or the like which is stable in the air without chemical reaction with ceramic-metal, parylene-metal, lithium.

However, lithium metal used as a negative electrode has a low melting point (about 180 ° C.) and is therefore limited in applications, and in order to handle high activity lithium metal, a device to isolate from moisture and oxygen has to be added and protection inside the battery. The design of the floor is essential and there are many problems with safety.

The present invention is to solve the problems as described above, an object of the present invention is to provide a negative electrode for a lithium secondary thin film micro battery thermally stable and improved safety in a high temperature process by using a thin film of silicon-based negative electrode material as a negative electrode of the micro battery It is.

Another object of the present invention is to provide a negative electrode for a lithium secondary thin film micro-cell that can provide a lithium secondary thin film battery with increased cycle life characteristics and capacity.

Still another object of the present invention is to provide a lithium secondary thin film battery using the negative electrode.

1 is a cross-sectional view schematically showing a general structure of a thin film battery.

2 is a cross-sectional view schematically showing the structure of a coin-type two-electrode cell.

3 is a graph showing the charge and discharge cycle life characteristics of Example 1 and Comparative Examples 1 to 2 of the present invention.

4 is a graph showing the charge and discharge cycle life characteristics of Example 2 and Comparative Example 3 of the present invention.

5 is a graph showing the charge and discharge cycle life characteristics of Example 3 and Comparative Example 4 of the present invention.

In order to achieve the above object, the present invention provides a method for producing a negative electrode for a lithium secondary thin film battery comprising a step of forming a thin film on a substrate using a metal, and silicon, which does not react with lithium while applying energy. The step of applying the energy may be carried out after the step of forming a thin film.

The present invention also does not react with lithium and has a strong affinity with silicon and a nano compound matrix of Si with metal; And it provides a negative electrode for a lithium secondary thin film battery comprising nano-sized silicon dispersed in the matrix.

Hereinafter, the present invention will be described in more detail.

The present invention relates to a negative electrode material to replace lithium used as a negative electrode in a conventional thin film battery.

To be used as a negative electrode for a thin film battery, the following conditions must be satisfied.

First, the charge and discharge capacity per volume and weight should be large.

Second, the material must be stable even at high temperatures.

Third, there should be no capacity reduction due to the irreversible reaction in the process of detachment after insertion of lithium in the first cycle.

Fourth, there should be no capacity reduction in continuous charge and discharge cycles.

Fifth, it must be chemically and electrically stable at the interface with the electrolyte.

Sixth, it should be excellent in adhesion with the electrolyte and the current collector.

Seventh, the production should be easy at low temperatures.

In the present invention, a silicon (Si) is used as a material to replace lithium by satisfying the above physical properties.

The negative electrode for a lithium secondary thin film battery of the present invention is manufactured by the following method.

A metal which does not react with lithium and has a high affinity with silicon (hereinafter referred to as a "non-reactive metal"), a first substrate having a current collector deposited on the substrate using silicon, and a substrate, a current collector, and an anode in that order A thin film cathode is formed on the second substrate (hereinafter, the first substrate and the second substrate are both referred to as the substrate). In the thin film forming step, a step of applying energy may be performed simultaneously, or a step of applying energy after forming a thin film may be performed.

The obtained thin film cathode is formed of a reaction compound of the unreacted metal and silicon, that is, a matrix formed of a silicide compound and silicon dispersed in the matrix.

In general, the thin film process may be any known thin film process, and is not particularly limited, but typical examples thereof include sputtering processes such as RF sputtering, DC sputtering, ion beam sputtering, ion plating, electron beam deposition, and ion beam auxiliary deposition. , Chemical vapor deposition, plasma chemical vapor deposition, low temperature thin film deposition or plating.

The thin film process is preferably carried out under conditions in which silicon is dispersed in an excessive amount in the thin film cathode. The composition of the silicon in the thin film cathode may be changed by changing the composition of starting materials such as unreacted metal and silicon, adjusting the power applied to the starting material, or controlling the flow rate of the starting material through the reactor used in the thin film process. Can be. As it can be widely understood by those in the art, detailed description thereof will be omitted. As such, since the silicon, which may exhibit high capacity in basic properties, is excessively dispersed in the negative electrode, the battery capacity may be increased.

The energy applying step is performed by applying a direct current bias to the substrate, irradiating an ion beam, and heating or plasma treating the substrate. As described above, according to the present invention, a process of applying energy in the conventional thin film forming process can prevent formation of an aggregate of the non-reactive metal and Si. In addition, a matrix of nanoscale unreacted metals and compounds of silicon is formed on the substrate, and silicon dispersed in the matrix is also dispersed at nanoscale.

Silicon is a material that reacts with lithium as shown in Scheme 1, and has a very high reversible charge / discharge capacity of 4000 mAh / g, and it is a useful material as a negative electrode active material because it satisfies the conditions to be a thin film negative electrode for a thin film battery.

Scheme 1

Si + 4.4Li = Li _4.4 Si

However, when the silicon particle size is large, as the charge and discharge cycle proceeds, the insertion and desorption of lithium is repeated, and the silicon particles are repeatedly expanded and contracted in volume. May be generated to reduce the charge / discharge capacity. In the thin film formation process, a crystal structure of the deposited deposit is formed in an amorphous state in order to manufacture a thin film anode cell. Thus, the amorphous crystal structure inevitably causes many dangling bonds. The dangling bond has a high probability of fixing lithium ions and acting as a trap, thereby increasing the irreversible capacity of lithium during battery charging and discharging, thereby reducing battery charging and discharging capacity. When the substrate is cooled during the formation of the thin film, the deposited deposits may form a cluster, which may also cause a problem of reducing charge and discharge capacity. However, in the present invention, instead of the cooling process, as a process of applying energy during deposition, the deposited deposit may form a complete crystal structure and form finely dispersed silicon particles in a nano size, thereby reducing the capacity problem. I can solve it.

The non-reactive metal is a metal having excellent affinity with silicon while not reacting with lithium, and representative examples thereof include Ni, Co, Fe, Cr, Hf, Mn, Mo, Nb, Ti, V, Zr, Ta, W and Any one selected from the group consisting of Re can be used.

As the substrate, the current collector and the positive electrode active material, all materials generally used in a lithium secondary thin film battery may be used. As a representative example, alumina or silicon oxide may be used as the substrate, and as the current collector, Au, Pt, Al, Cr, and the like may be used, and the positive electrode active material may be TiS ₂ , WO ₃ , MnO ₂ , lithium metal oxide (Li-MO, M = Co, Mn, Ni), or the like.

The negative electrode for a lithium secondary thin film battery produced by the above production method includes a matrix containing a reaction compound of the unreacted metal and silicon, and silicon dispersed in the matrix. Since this negative electrode contains a reaction mixture of the unreacted metal and silicon and a silicon having a phase shift temperature relatively higher than that of a conventional lithium metal, the battery including the negative electrode can be used at a high temperature (180 ° C. or higher).

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.

(Example 1)

Using sputtering targets, silicon and zirconium were simultaneously sputtered and deposited on a 0.2 mm thick, 12 mm diameter Cu disk substrate, and a direct current bias was applied to the substrate. The manufacturing conditions at this time are shown in Table 1 below.

(Comparative Examples 1 and 2)

The deposition process was performed in the same manner as in Example 1 except that the substrate cooling process was performed instead of the DC bias application process. Preparation conditions are shown in Table 1 below.

Example 1 (Si-Zr Silicide) Comparative Example 1 (Si) Comparative Example 2 (Si-Zr) Initial vacuum 1 × 10 ⁶ Torr 1 × 10 ⁶ Torr 1 × 10 ⁶ Torr Deposition atmosphere Ar Ar Ar Gas flow rate 20 sccm 20 sccm 20 sccm Working pressure 5 mTorr 5 mTorr 5 mTorr Sputtering Target / Deposition Method Si, Zr / Simultaneous Deposition Si Si, Zr / Simultaneous Deposition Sputtering power 180 W 180 W 180 W Board Uncooled / Bias-100V Water cooling Water cooling Furtherance Si _0.8 Zr _0.2 Si Si _0.8 Zr _0.2 Characteristic Nanocomposites of Si and Zr Silicide Phases Si single phase Amorphous Mixture of Si and Zr

As shown in Table 1, while the nanocomposite of Si phase and Zr silicide phase is formed by the method of Example 1, Si and Zr are the method of Comparative Example 2 in which the same composition is obtained by simultaneously depositing Si and Zr. It can be seen that an amorphous mixture of is formed. When the amorphous mixture is formed in this way, during the initial Li insertion reaction, a dangling bond is formed in which Li ⁺ is trapped, so that the initial irreversible reaction is large, and Si is unlikely to be finely dispersed and aggregated. There is a problem that the cycle life characteristics can be reduced.

In order to evaluate the electrochemical properties of the thin film anodes prepared by the method of Example 1 and Comparative Examples 1 to 2, a coin type two-electrode cell (coin type cell) shown in FIG. 2 was prepared. As shown in FIG. 2, in the two-electrode cell, a gasket 12 electrically insulating the cathode side and the anode side is disposed inside the lower cell 11, and the lithium electrode is sequentially disposed on the upper side of the lower cell 11. Electrolyte layer 14 consisting of a separator film made of polypropylene / polyethylene / polypropylene sufficiently wetted with an electrode 13, an ethylene carbonate / diethyl carbonate solution in which 1 mol of LiPF ₆ was dissociated, Example 1 and comparison The working electrode 15 which is a thin film cathode manufactured by the method of Example 1 is formed. Thereafter, a spacer 16 made of copper (Cu) is disposed on the upper side of the working electrode 15 to compress the electrode and the electrolyte layer, and finally, the upper cell 17 is covered and compressed.

The two-electrode cell was charged and discharged at a speed of 30 uA, and the results are shown in FIG. 3. As shown in FIG. 3, it can be seen that the charge / discharge cycle characteristics of the two-electrode cell using the thin film cathode of Example 1 were much better than those of Comparative Examples 1 to 2. In other words, these results indicate that the Si-Zr silicide nanocomposite cathode of Example 1 significantly improved the charge / discharge cycle characteristics.

(Example 2)

Silicon and nickel were simultaneously deposited on a Cu disk substrate having a thickness of 0.2 mm and a diameter of 12 mm using an electron beam evaporation method, and at the same time, an Ar ion beam was irradiated onto the substrate to prepare a cathode for a lithium secondary thin film battery. The production conditions at this time are shown in Table 2 below.

Example 2 (ion beam irradiated Si-Ni) Comparative Example 3 (Si-Ni without ion beam irradiation) Initial vacuum 9.0 × 10 ^-7 Torr 9.0 × 10 ^-7 Torr Deposition atmosphere Ar Gas inflow 3 sccm Working pressure 1.5 × 10 ^-4 Torr 1.8 × 10 ^-6 Torr E-beam source / deposition method Si, Ni / Co-Deposition Si, Ni / Co-Deposition E-beam power Si: -5.7KV, 140mA Ni: -5.7KV, 55mA Si: -5.7KV, 140mA Ni: -5.7KV, 55mA Ion beam power 150 eV 20 mA Board Cooling Do not cool Do not cool Furtherance Si _0.8 Ni _0.2 Si _0.8 Ni _0.2 Characteristic Nanocomposites of Si and Ni Silicide Phases Nanocomposites of Si and Ni

As shown in Table 2, it can be seen that the nanocomposite of Si phase and Ni silicide phase is formed by the method of Example 2.

A coin battery was manufactured in the same manner as in Example 1 using the thin film anodes prepared by the methods of Example 2 and Comparative Example 3, and the charge and discharge cycle characteristics were measured using the battery. The results are shown in FIG. As can be seen in Figure 4, it can be seen that the charge and discharge cycle characteristics of the coin battery manufactured using the thin film anode of Example 2 is significantly superior to Comparative Example 3. This result indicates that the Si-Ni silicide nanocomposite negative electrode of Example 2 significantly improved the charge / discharge cycle characteristics.

(Comparative Example 4)

Pure Fe and Si (-325 mesh) are mixed uniformly in a mortar in a 1: 2 molar ratio, and the mixture is charged with a ball into a stainless vial and argon (Ar) for 1 hour. During milling.

The mixed powder thus obtained was pelleted with 5 tons in a mold of 3 cm in order to make a more uniform phase, which was then maintained at 1000 ° C. for 2 hours at a temperature rising rate of 10 ° C./min in an argon atmosphere to form a FeSi ₂ compound.

The crystalline silicon FeSi ₂ compound prepared was milled again under the same conditions to synthesize a nano crystalline FeSi ₂ silicon compound.

Electrochemical Characterization

The electrode was prepared by mixing 75: 15: 10% by weight of polyvinylidene fluoride as a binder and carbon black as the silicon mixture and the conductive material prepared above. A coin cell of type 2016 was prepared using Li metal foil as a counter electrode and 1 M LiPF ₆ / ethylene carbonate: diethyl carbonate (50: 50% by volume) as an electrolyte, and the electric current was 0 to 2V at a constant current of 0.2 mA / cm ² . The chemical properties were evaluated.

(Example 3)

Pure Fe and Si (-325 mesh) were uniformly mixed in a mortar at a ratio of 1: 2.7 mol, then charged into a stainless vial with a ball and milled in an argon (Ar) atmosphere for 1 hour.

The mixed powder thus obtained was pelleted at 5 tons in a mold of 3 cm in order to make a more uniform phase, and then maintained at 1000 ° C. for 2 hours at a temperature rising rate of 10 ° C./min in an argon atmosphere to give FeSi _2.7 (FeSi ₂ + Si _0.7 , silicon). Excess complex).

The crystalline silicon composite thus obtained was milled again under the above conditions to synthesize a nanocrystalline FeSi _2.7 silicon composite.

Electrochemical Characterization

The electrode was prepared by mixing 75: 15: 10% by weight of polyvinylidene fluoride as a binder and carbon black as the silicon mixture and the conductive material prepared above. A coin cell of type 2016 was prepared using this electrode, Li metal foil as a counter electrode, and 1 M LiPF ₆ / ethylene carbonate: diethyl carbonate (50: 50% by volume) as an electrolyte, and produced a constant current of 0.2 mA / cm ² at a constant current of 0.2 mA / cm ² . The electrochemical properties were evaluated in the 2V range.

Charging and discharging cycle life characteristics of the coin cells of Examples 3 and 4 were measured, and the results are shown in FIG. 5. As shown in FIG. 5, both of Examples 3 and 4 had excellent charge / discharge cycle characteristics, but it can be seen that the charge and discharge capacity of the battery using the negative electrode of excessive silicon 4 was increased compared to Example 3.

Since the thin film battery using the negative electrode of the present invention can be used even at a high temperature (particularly 180 ° C or higher), the application of the battery is less restricted. In addition, in the present invention, since silicon having high activity instead of lithium is used as a negative electrode material, very improved stability can be achieved, and the design of the protective film is very easy and thus the cost is reduced. In addition, the volume change caused by the reaction of the silicon thin film anode of the present invention with lithium is suppressed, thereby making it possible to manufacture a negative electrode for a thin film battery which improves the charge / discharge performance of the silicon electrode.

Claims (12)

Forming a thin film on a substrate using a metal that does not react with lithium while applying energy, and silicon The manufacturing method of the negative electrode for lithium secondary thin film batteries containing a process. The method of claim 1, wherein the applying of energy is performed by a step selected from the group consisting of applying a direct current bias to the substrate, heating the substrate, ion beam irradiation, and plasma treatment. . The method of claim 1, wherein the forming of the thin film is selected from the group consisting of sputtering, electron beam deposition, ion beam assisted deposition, chemical vapor deposition, and plating. The method of claim 1, wherein the forming of the thin film is carried out under conditions in which excess silicon may be present in the thin film. According to claim 1, wherein the metal does not react with lithium is selected from the group consisting of Ni, Co, Fe, Cr, Fe, Hf, Mn, Mo, Nb, Ti, V, Zr, Ta, W and Re Phosphorus manufacturing method. A thin film is formed on the substrate using a metal that does not react with lithium and silicon; Applying energy to the substrate on which the thin film is formed The manufacturing method of the negative electrode for lithium secondary thin film batteries containing a process. The method of claim 6, wherein the applying of energy is performed by a step selected from the group consisting of applying a direct current bias to the substrate, heating the substrate, an ion beam irradiation process, and a plasma treatment process. . The method of claim 6, wherein the forming of the thin film is selected from the group consisting of sputtering, electron beam deposition, ion beam assisted deposition, chemical vapor deposition, and plating. The method of claim 6, wherein the metal that does not react with lithium is selected from the group consisting of Ni, Co, Fe, Cr, Fe, Hf, Mn, Mo, Nb, Ti, V, Zr, Ta, W and Re Phosphorus manufacturing method. The method of claim 6, wherein the forming of the thin film is performed under conditions in which excess silicon may be present in the thin film. A nano-compound matrix of metal and Si that does not react with lithium and has a strong affinity with silicon; And Nano-sized silicon dispersed in the matrix A negative electrode for a lithium secondary thin film battery comprising a. The method of claim 11, wherein the metal that does not react with lithium is selected from the group consisting of Ni, Co, Fe, Cr, Fe, Hf, Mn, Mo, Nb, Ti, V, Zr, Ta, W and Re Cathode for phosphorus lithium secondary thin film battery.