KR101738660B1 - Silicon-based anode active material and lithium battery having the same - Google Patents

Abstract

Disclosed is a silicon-based negative electrode active material. The disclosed silicon-based negative electrode active material is in an amorphous phase and contains an alloy of silicon, titanium, and iron. The atomic ratio of silicon is 80-84 at%, and the silicon-based negative electrode active material has a water wave-like patterns due to ununiformity in concentration of titanium and iron.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a silicon-based anode active material and a lithium battery including the same,

The present invention relates to a negative electrode active material, and more particularly, to a silicon-based negative electrode active material which can be used as an anode material of a lithium battery and a lithium battery including the same.

Conventionally, a lithium metal is used as a negative electrode active material of a lithium battery. However, when a lithium metal is used, a short circuit occurs due to formation of dendrite and there is a danger of explosion. Therefore, a carbonaceous material instead of lithium metal is widely used as an anode active material .

Examples of the carbon-based active material include crystalline carbon such as natural graphite and artificial graphite, and amorphous carbon such as soft carbon and hard carbon. However, although the amorphous carbon has a large capacity, there is a problem that irreversibility is large in the charging and discharging process. Graphite is a typical example of crystalline carbon, and its theoretical limit capacity is 372 mAh / g, which is used as an anode active material because of its high capacity.

In order to develop a next-generation high-capacity lithium battery, it is essential to develop a high-capacity negative electrode active material that exceeds the capacity of graphite. To this end, active materials currently being investigated are the silicon anode active materials. Silicon has a high capacity and a high energy density and can absorb and release more lithium ions than an anode active material using a carbon-based material, thereby making it possible to produce a secondary battery having a high capacity and a high energy density.

However, when the silicon based negative electrode active material is applied to a lithium battery, there is a problem in that the lifetime characteristics of the lithium battery are deteriorated by repeated expansion and contraction in the charging and discharging process.

Thus, there is a continuing need for a silicon-based negative electrode active material capable of improving lifetime characteristics of a lithium battery while maintaining a high capacity characteristic of silicon.

Disclosure of Invention Technical Problem [8] Accordingly, the present invention has been made to solve the above problems occurring in the prior art, and it is an object of the present invention to provide a silicon-based anode active material and a lithium battery including the same, which can improve lifetime characteristics of a lithium battery while maintaining high-

The silicon-based negative electrode active material according to the embodiment for realizing the above-described present invention has an amorphous phase and includes an alloy of silicon, titanium and iron. The element ratio of silicon is 80 at% to 84 at%.

In one embodiment, the elemental ratios of titanium and iron are 8 at% to 10 at%, respectively.

In one embodiment, the silicon-based negative electrode active material is characterized by having a wavy pattern due to concentration irregularities of titanium and iron.

In one embodiment, the alloy of silicon, titanium and iron is formed by sputtering of silicon, titanium and iron.

In one embodiment, the elemental proportion of silicon is between 80 at% and 81 at%.

A lithium battery according to an embodiment of the present invention includes a negative electrode including a negative electrode active material layer. The anode active material layer includes an alloy of silicon, titanium, and iron, and the element ratio of silicon is 80 at% to 84 at%.

In one embodiment, as the repetition of charging and discharging proceeds, a fine protrusion array is formed on the surface of the negative electrode active material layer.

In one embodiment, the average diameter of the fine protrusions of the microprojection array is 1 占 퐉 or less.

According to the present invention, the capacity and durability of a battery using a silicon-based negative electrode active material can be increased.

1 is a cross-sectional view illustrating a change in a cathode when a charge / discharge cycle is repeated using a silicon-based negative electrode active material according to an embodiment of the present invention.
2A and 2B are graphs showing the electric capacity of a lithium battery cell (silicon ratio 78.5 at%, 80.5 at%, 81.5 at%, 82.5 at%, 84 at%) according to the embodiment of the present invention, to be.
3 is a graph showing the electric capacity (discharge) and the coulombic efficiency of a lithium battery cell (silicon ratio 80.5 at%) according to the embodiment of the present invention in accordance with the number of charge / discharge cycles.
4 is a SEM (scanning electron microscope) photograph of the surface shape of a negative electrode active material according to the number of charging / discharging of a lithium battery cell (silicon ratio 80.5 at%) using the embodiment of the present invention.
5 is a TEM (transmission electron microscope) photograph of a cross section of a negative electrode according to the number of times of charging and discharging of a lithium battery cell (silicon ratio 80.5 at%) using the embodiment of the present invention.
6 is an element map using an HAADF (high angle annual dark field) image of an anode active material layer after 50 charge / discharge cycles and an EDS analysis in a lithium battery cell (silicon ratio 80.5 at%) using the embodiment of the present invention.

Hereinafter, preferred embodiments of the present invention will be described in more detail with reference to the drawings. The present invention is capable of various modifications and various forms, and specific embodiments are illustrated in the drawings and described in detail in the text. It should be understood, however, that the invention is not intended to be limited to the particular forms disclosed, but includes all modifications, equivalents, and alternatives falling within the spirit and scope of the invention. Like reference numerals are used for like elements in describing each drawing. In the accompanying drawings, the dimensions of the structures are enlarged to illustrate the present invention in order to clarify the present invention. The terms first, second, etc. may be used to describe various components, but the components should not be limited by the terms. The terms are used only for the purpose of distinguishing one component from another. For example, without departing from the scope of the present invention, the first component may be referred to as a second component, and similarly, the second component may also be referred to as a first component. The singular expressions include plural expressions unless the context clearly dictates otherwise.

In this application, the terms "comprises", "having", and the like are used to specify that a feature, a number, a step, an operation, an element, a part or a combination thereof is described in the specification, But do not preclude the presence or addition of one or more other features, integers, steps, operations, components, parts, or combinations thereof.

A silicon-based negative active material according to an embodiment of the present invention includes a silicon alloy. Specifically, the negative electrode active material includes an alloy of silicon (Si), titanium (Ti), and iron (Fe).

Silicon can serve to store and release lithium ions when the negative electrode active material is used in a battery. Titanium and iron may serve to extend the lifetime of the negative electrode active material.

The negative electrode active material is preferably amorphous. When the negative electrode active material is crystallized by heat treatment or the like, performance deterioration due to charging and discharging is significant. Therefore, the life of the battery can be reduced.

In the silicon alloy, the element ratio of silicon is preferably 80 to 84 at%. When the element ratio of the silicon is less than 80 at%, the specific capacity sharply decreases. More preferably, the element ratio of the silicon is 80 to 81 at%. When the element ratio of silicon exceeds 81 at%, the width of the capacitance which decreases with the progress of charge / discharge sharply increases.

For example, the silicon alloy may comprise 80 to 84 at% of silicon, 8 to 10 at% of titanium and 8 to 10 at% of iron, more preferably 80 to 81 at% of silicon, 8 to 10% at% and iron from 8 to 10 at%.

The negative electrode active material forms a ripple-shaped pattern inside by electrochemical activity (charging and discharging). For example, when charging and discharging are conducted using the negative electrode active material as an electrode, segregation of inactive materials (titanium and iron) from the active material (silicon) occurs, and a nonuniform distribution of such materials (I.e., a portion having a relatively high ratio and a portion having a relatively low ratio) can form a wave-like pattern.

The separation of the active material and the inactive material may be performed in the process of repeatedly performing the lithium ion-silicon bond (lithium suicide formation) and separation in the charging and discharging process of the lithium battery.

1 is a cross-sectional view illustrating a change in a cathode when a charge / discharge cycle is repeated using a silicon-based negative electrode active material according to an embodiment of the present invention.

Referring to FIG. 1, (a) a silicon-based negative electrode active material 10 is laminated to form a negative electrode. The negative electrode active material includes silicon, titanium and iron, and has an amorphous phase. The negative electrode may be formed through sputtering or the like so that an amorphous phase negative electrode active material can be laminated. (b) When the lithium battery including the negative electrode is charged and discharged, the wavy pattern 15 appears due to the separation of the inactive material. (c) When the lithium battery is further charged and discharged, fine protrusions are formed. A solid electrolyte layer 20 may be formed on the surface of the microprojections. The solid electrolyte layer 20 is a surface layer formed by the reaction between the negative electrode active material and the electrolyte.

The structure having such a wave-like pattern suppresses the loss of active material during the charging and discharging process of the electrode and forms minute protrusions (bumps) on the surface of the electrode, thereby suppressing the volume expansion of the electrode and increasing the durability of the battery . The size and shape of the fine protrusions vary according to charge / discharge of the electrode, but the average diameter of the fine protrusions can be reduced to 1 μm or less.

The silicon-based negative electrode active material may be used as a negative electrode of a lithium battery. The silicon-based negative electrode active material may be laminated on a collector including copper, gold, silver, nickel, carbon bodies, and the like.

A lithium battery according to an embodiment of the present invention may include an anode, a cathode, a separator disposed between the anode and the cathode, and an electrolyte. The lithium battery may include a lithium ion secondary battery, a lithium polymer secondary battery, or a lithium ion polymer secondary battery.

An example of a positive electrode active material usable in the positive electrode is a lithium-containing transition metal oxide. For example, the cathode active material may be LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , Li (Ni _a Co _b Mn _c ) O ₂ where 0 <a <1, 0 <b <<1, a + b + c = 1), LiNi 1-Y Co Y O 2, LiCo 1-Y Mn Y O 2, LiNi 1-Y Mn Y O 2 ( where, 0 = Y <1), Li (Ni _a Co _b Mn _c ) O ₄ (0 <a <2, 0 <b <2, 0 <c <2, a + b + c = 2), LiMn _{2 -z} Ni _z O ₄ , LiMn _{2- Z} Co _Z O ₄ (where 0 <Z <2), LiCoPO ₄ , and LiFePO ₄ can be used.

The electrolyte may be an electrolytic solution containing a solvent and an electrolyte salt.

The nonaqueous solvent is not particularly limited as long as it is used as a nonaqueous solvent for a nonaqueous electrolyte solution, and a cyclic carbonate, a linear carbonate, a lactone, an ether, an ester, or a ketone can be used.

Examples of the cyclic carbonates include ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC). Examples of the linear carbonate include diethyl carbonate (DEC), dimethyl carbonate (DMC) (DPC), ethyl methyl carbonate (EMC), and methyl propyl carbonate (MPC). Examples of the lactone include gamma butyrolactone (GBL), and examples of the ether include dibutyl ether, tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane . Examples of the esters include n-methyl acetate, n-ethyl acetate, methyl propionate, methyl pivalate and the like, and the ketone includes polymethyl vinyl ketone. These non-aqueous solvents may be used alone or in combination of two or more.

The electrolyte salt is not particularly limited as long as it is usually used as an electrolyte salt for a non-aqueous electrolyte. For example, the electrolyte salt is, A ⁺ B ^- A salt of the structure, such as, A ⁺ comprises a Li ^+, Na ^+, an alkali metal cation or an ion composed of a combination thereof, such as K ⁺ B ^- is _{^{PF 6 -, BF 4 -,}} Cl -, Br -, I -, ClO 4 -, ASF6 -, CH 3 CO 2 -, CF 3 SO 3 -, N (CF 3 SO 2) 2 -, C (CF 2 SO ₂ ) ₃ ^- , or an ion consisting of a combination of these. In particular, a lithium salt is preferable. These electrolyte salts may be used alone or in combination of two or more.

The separation membrane is not particularly limited, but a porous separation membrane is preferably used, and examples thereof include a polypropylene-based, polyethylene-based, or polyolefin-based porous separation membrane.

Hereinafter, the silicon-based anode active material of the present invention will be described with reference to specific examples and experiments.

Example-Preparation of Lithium Battery Cell

The surface of the copper film having a thickness of about 18 mu m was subjected to argon (Ar) plasma treatment, and a tantalum layer (adhesive layer) was formed thereon to a thickness of about 50 nm. Silicon, titanium and iron were laminated on the adhesive layer using a radio-frequency (RF) magnetron sputtering system to form a silicon alloy film. The thickness of the silicon alloy film was 200 nm, and a plurality of electrode samples were prepared with different element ratios.

Each of the electrode samples was used as a working electrode to prepare a lithium battery cell. Specifically, a lithium film was used as an opposite electrode, and a monolator polypropylene separator (Celgard 2400) was used as a separator. As the electrolyte, 1 M of LiPF ₆ dissolved in a mixed solvent of fluoroethylene carbonate (FEC) and diethyl carbonate (DEC) in a volume ratio of 1: 1 was used.

Electrical characteristics and changes in the negative electrode active material were measured / observed for each of the lithium battery cells while repeating charging and discharging at 0.5 Ag ^-1 in the range of 50 mV and 2.5 V (vs Li / Li ⁺ ).

2 is a graph showing the electric capacity of a lithium battery cell (silicon ratio 78.5 at%, 80.5 at%, 81.5 at%, 82.5 at%, 84 at%) using the embodiment of the present invention in accordance with the number of charge and discharge. When the silicon ratio was 78.5 at%, the elemental ratios of titanium and iron were 11.0 at% and 10.5 at%, respectively. When the silicon ratio was 80.5 at%, the elemental ratios of titanium and iron were 10.0 at% and 9.5 at% And the silicon ratio was 81.5 at%, the elemental ratios of titanium and iron were 9.5 at% and 9.0 at%, respectively, and when the silicon ratio was 82.5 at%, the elemental ratios of titanium and iron were 9.0 at% and 8.5 at%, and when the silicon ratio was 84.0 at%, the elemental ratios of titanium and iron were 8.0 at% and 8.0 at%, respectively.

Referring to FIG. 2, when the silicon ratio is less than 80 at% (78.5 at%), it is confirmed that the electric capacity is rapidly reduced. In addition, when the silicon ratio exceeds 81 at%, it can be confirmed that as the number of charging / discharging increases, specifically, the capacity decreases as the number of times of charging / discharging increases. On the other hand, when the silicon ratio is 80 to 81 at% (80.5 at%), it can be confirmed that the electric capacity is kept high.

3 is a graph showing the electric capacity (discharge) and the coulombic efficiency of a lithium battery cell (silicon ratio 80.5 at%) according to the embodiment of the present invention in accordance with the number of charge / discharge cycles.

Referring to FIG. 3, the electric capacity is maintained at about 1550 mAhg ^-1 until 200 times, then gradually decreased from 500 times to about 1400 mAhg ^-1 , which is maintained without abrupt change up to 1000 times, Is maintained close to 99% up to 1000 times. This is superior to a conventional silicon electrode having a nanocrystal structure.

4 is a SEM (scanning electron microscope) photograph of the surface shape of a negative electrode active material according to the number of charging / discharging of a lithium battery cell (silicon ratio 80.5 at%) using the embodiment of the present invention. Specifically, FIG. 5A is a photograph after one charge and discharge, FIG. 5B is a photograph after 5 charge and discharge, FIG. 5C is a photograph after 10 charge and discharge, (F) is a photograph after 100 cycles of charging and discharging, (g) is a photograph after 300 cycles of charging and discharging, (h) is a photograph after charging and discharging It is a photograph after 500 times.

Referring to FIG. 4, as the number of times of charging and discharging increases, a crack grows on the surface, and it is confirmed that an array of fine protrusions is finally formed.

5 is a TEM (transmission electron microscope) photograph of a cross section of a negative electrode according to the number of times of charging and discharging of a lithium battery cell (silicon ratio 80.5 at%) using the embodiment of the present invention. Specifically, (a) is a photograph after one cycle of charge / discharge, (b) is a photograph after 10 cycles of charge / discharge, and (c) is a photograph after 25 cycles of charge and discharge. The upper right corner of each picture is an enlarged view of the anode active material layer. In Fig. 5, the black region under the negative electrode active material layer is a Ta adhesive layer, and the lower portion is a copper current collector.

Referring to FIG. 5, it can be confirmed that the amorphous state of the negative electrode active material layer is maintained even when charging / discharging proceeds. Referring to an enlarged photograph, as the charge / discharge progresses, a wavy pattern Area) is displayed. In the above enlarged photograph, the dark region and the bright region forming the wavy pattern are both amorphous. In the dark region, they are close to the amorphous structure of the metal. In the bright region, they are close to the amorphous structure of glass or semiconductor.

6 is an element map using an HAADF (high angle annual dark field) image of an anode active material layer after 50 charge / discharge cycles and an EDS analysis in a lithium battery cell (silicon ratio 80.5 at%) using the embodiment of the present invention. (A) is an HAADF image of a negative active material layer, (b) is an elemental map of silicon in (a), (c) is an elemental map of titanium in (a) Is an elemental map of iron.

Referring to FIG. 6, no change in the element concentration (density) of silicon is observed over the region where the wavy pattern appears, while a change in the concentration of titanium and iron is observed.

Referring to FIGS. 5 and 6, it can be expected that the wavy pattern appearing in the negative electrode active material of the lithium battery cell according to the embodiment of the present invention is caused by the concentration irregularity of titanium and iron.

Referring to these experimental results, it can be seen that a high concentration region of titanium and iron, which occurs in the amorphous phase of the silicon alloy as the charge and discharge progresses, prevents the volume deformation of the negative electrode active material, Able to know.

It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the present invention as defined by the following claims. You will understand.

The present invention can be used for a battery such as a lithium battery.

Claims (10)

And an amorphous phase, and an alloy of silicon, titanium and iron, wherein the element ratio of silicon is 80 at% to 81 at%, and as the charge and discharge proceeds, a wavy pattern due to concentration irregularity of titanium and iron Lt; RTI ID = 0.0 &gt; a &lt; / RTI &gt; silicon-based anode active material. The silicon-based negative electrode active material according to claim 1, wherein the element ratio of titanium and iron is 9 at% to 10 at%, respectively. delete The silicon-based negative electrode active material according to claim 1, wherein the alloy of silicon, titanium and iron is formed by sputtering of silicon, titanium and iron. delete And an amorphous phase, and an alloy of silicon, titanium and iron, wherein the element ratio of silicon is 80 at% to 81 at%, and as the charge and discharge proceeds, a wavy pattern due to concentration irregularity of titanium and iron A lithium battery comprising a negative electrode comprising a negative active material layer. delete 7. The lithium battery according to claim 6, wherein a fine protrusion array is formed on the surface of the negative electrode active material layer as charging and discharging are repeated. The lithium battery according to claim 8, wherein the fine protrusions have an average diameter of 1 탆 or less. delete

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