KR101497824B1 - Electrode for a lithium secondary battery, method of forming the same and lithium secondary battery - Google Patents

Abstract

The anode for a lithium secondary battery includes an active material composed of a silicon nano powder, a carbon-based conductive material and a binder, and a chromium layer covering the surface of the active material. The chromium layer can increase the electrical conductivity and increase the current density. The chromium layer can suppress direct contact between the active material and the electrolytic solution. Therefore, when the anode including the chromium layer is applied to a lithium secondary battery, the lithium secondary battery may have stable cycle characteristics.

Description

TECHNICAL FIELD The present invention relates to an anode for a lithium secondary battery, a method for forming the anode, and a lithium secondary battery. BACKGROUND OF THE INVENTION 1. Field of the Invention [0001]

The present invention relates to an anode for a lithium secondary battery, a method for forming the anode for the lithium secondary battery, and a lithium secondary battery, and more particularly to an anode for a lithium secondary battery constituting a secondary battery capable of charging and discharging, Battery.

Lithium secondary batteries are a type of secondary battery in which charging and discharging are performed by inserting and separating lithium ions in a battery. During charging, lithium ions move from the cathode to the anode, In contrast, during discharging, the lithium ions inserted into the negative electrode move toward the positive electrode and are inserted into the active material of the positive electrode. Such a lithium secondary battery has advantages of high energy density, large electromotive force, and high capacity, and is widely used as a power source for mobile phones and notebook computers.

The lithium secondary battery is generally composed of a negative electrode, a positive electrode, a separator, and an electrolyte. The negative electrode and the positive electrode include a negative electrode active material and a positive electrode active material capable of inserting and desorbing lithium ions as described above. The separator prevents physical cell contact between the anode and cathode. Instead, the movement of the ions through the separator is free. The electrolyte serves as a passage through which ions can move freely between the anode and the cathode.

On the other hand, carbon for constituting the negative electrode may be used. The carbon material is widely used as a negative electrode material for a lithium ion battery because it has less change in volume during intercalation / deintercalation of lithium, is excellent in reversibility, and is relatively inexpensive. Carbon materials used for such cathode materials include graphite, coke, fiber, pitch, and meso carbon. However, the graphite has a theoretical limit (372 mAh g < ^-1 >) to the charge capacity per unit mass. Therefore, there has been a demand for the development of a new anode material capable of greatly improving the operating characteristics such as the energy density, the reversible capacity and the initial charging efficiency of the lithium ion battery.

Recently, development of electrodes using silicon has attracted attention as an attempt to solve the above problems. Silicon is of great interest in the field of lithium ion batteries because it has a theoretical capacity per unit of mass (about 4,200 mAh g <" ¹ >) 10 times higher than the charge capacity of the graphite electrode or various other oxide, Is the material.

When lithium is used as a negative electrode material in the lithium secondary battery, the lithium ion reacts with silicon to form a new compound, and in the opposite direction, a de-alloying reaction occurs. This is different from the intercalation reaction of lithium ions between the interlayer structure of the anode in the case of a lithium secondary battery including a carbon-based anode used as a conventional anode material. However, due to the silicon reaction, the silicon negative electrode material has a large volume change of 400% or more before and after the reaction with the lithium ion, so that there are many restrictions on the application to the actual negative electrode material. That is, when the volume of the anode made of silicon is greatly expanded, a strong mechanical force is applied between the lattices, which can destroy the bond or the structure between the crystal lattices. As a result, the stability and the capacity of the lithium- .

A lot of research has been carried out to reduce the large volume change rate which is a problem of the silicon anode, and one of them uses silicon monoxide (SiOx). Silicon monoxide is a form of silicon with oxygen attached to it, and when it reacts with lithium, a reaction occurs in which other oxides are formed besides the main reaction between lithium and silicon. These ancillary products exhibit improved performance by mitigating the volumetric expansion of silicon.

On the other hand, in order to improve the low electrical conductivity of silicon materials, many studies have been conducted to improve the battery life by synthesizing silicon monoxide with carbon materials. However, there is a problem that the charge capacity of the lithium secondary battery is reduced due to the synthesis of the oxygen group and the carbon material bonded to the silicon atom.

It is an object of the present invention to provide an anode for a lithium secondary battery having an improved capacity and a long life.

Another object of the present invention is to provide a method for manufacturing the anode for a lithium secondary battery.

Still another object of the present invention is to provide a lithium secondary battery including an anode for a lithium secondary battery having a capacity and a life.

An anode for a lithium secondary battery according to an embodiment of the present invention includes an active material composed of a silicon oxide powder, a graphite, and a binder, and a chromium layer covering a surface of the active material. Here, the silicon oxide powder may be a silicon mono-oxide. The binder may also include polyvinylidene fluoride (PVDF).

In one embodiment of the present invention, the active material may include a silicon oxide thin film and a graphite thin film formed on the silicon oxide thin film.

In the method of forming an anode for a lithium secondary battery according to an embodiment of the present invention, an active material composed of silicon oxide powder, graphite, and a binder is formed. Thereafter, a chromium layer is formed on the surface of the active material. Here, the chromium layer may be formed through an ion beam sputtering process.

In one embodiment of the present invention, the active material may be formed by mixing the silicon oxide powder and the graphite to form a composite, and adding a binder to the composite.

A lithium secondary battery according to an embodiment of the present invention includes a cathode including a cathode active material, an anode active material disposed to face the anode, the anode active material including silicon oxide powder, graphite, and a binder, and a chromium layer And an electrolyte layer interposed between the anode and the cathode.

An anode for a lithium secondary battery according to the present invention includes a chromium layer covering a surface of an active material. Here, the chromium layer has relatively high electrical conductivity and current density. Accordingly, when the anode for a lithium secondary battery having the chromium layer is applied to a lithium secondary battery, the lithium secondary battery may have an improved charging capacity.

In addition, since the chromium layer has mechanical characteristics capable of suppressing corrosion, it is possible to suppress the decomposition and reaction of the electrolyte by suppressing direct contact with the active material and the electrolyte in the anode. Therefore, when the anode including the lithium layer is applied to a lithium secondary battery, the lithium secondary battery may have stable cycle characteristics.

FIG. 1 is an image of an anode for a lithium secondary battery according to an embodiment of the present invention measured by EPMA.
2 is an electron micrograph of the anode for a lithium secondary battery of FIG. 1;
3 is a graph showing the cycle characteristics with respect to the charging capacity of the anode after coating the anode and the chromium layer before coating the active material with the chromium layer.
4 is a graph showing the cycle characteristics when the current density changes with respect to the charging capacity of the anode after coating the anode and the chromium layer before coating the active material with the chromium layer.
FIG. 5 is a graph showing the cycle characteristics of the anode on the anode after the anode and the chromium layer are coated before coating the active material with the chromium layer. FIG.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying 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. In the accompanying drawings, the sizes and the quantities of objects are shown enlarged or reduced from the actual size for the sake of clarity of 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 terminology used in this application is used only to describe a specific embodiment and is not intended to limit the invention. The singular expressions include plural expressions unless the context clearly dictates otherwise. In the present application, the terms "comprise", "comprising" and the like are intended to specify that there is a stated feature, step, function, element, or combination thereof, Quot; or " an " or < / RTI > combinations thereof.

On the other hand, unless otherwise defined, all terms used herein, including technical or scientific terms, have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Terms such as those defined in commonly used dictionaries are to be interpreted as having a meaning consistent with the contextual meaning of the related art and are to be interpreted as either ideal or overly formal in the sense of the present application Do not.

Anode for lithium secondary battery

An anode for a lithium secondary battery according to an embodiment of the present invention includes an active material and a chromium layer.

The active material is composed of silicon oxide powder, graphite, and a binder.

The silicon oxide powder may be made of, for example, silicon mono-oxide. When the active material includes silicon monoxide, the products generated when the silicon and lithium react with each other may alleviate volumetric expansion of silicon, thereby having improved axial discharge performance.

The graphite has a crystalline structure. For example, the graphite has a hexagonal crystal structure. As the graphite is included in the active material, the lifetime of the anode for the lithium secondary battery can be prolonged and the electrical conductivity of the active material can be improved.

 The binder interconnects the silicon oxide powder and the graphite. An example of the binder is polyvinylidene fluoride (PVDF).

In one embodiment of the present invention, the active material may have a multi-layer structure including a silicon oxide thin film made of silicon oxide and a graphite thin film formed on the silicon oxide thin film.

The chromium layer covers the surface of the active material. The chromium layer has relatively good electrical conductivity and electrical conductivity. Accordingly, the anode for a lithium secondary battery including the chrome layer may have a reduced impedance.

Further, when the present invention is applied to a secondary battery including the anode and the electrolyte containing the chromium layer, the chromium layer suppresses contact between the active material and the electrolyte, so that decomposition and reaction of the electrolyte can be suppressed. Therefore, when the anode including the chromium layer is applied to a lithium secondary battery, the lithium secondary battery may have stable cycle characteristics.

Method for forming anode for lithium secondary battery

In the method for forming an anode for a lithium secondary battery according to an embodiment of the present invention, an active material composed of a silicon oxide powder, a graphite, and a binder is formed. The silicon oxide powder, the graphite and the binder may be adjusted to a weight ratio of 4.5: 4.5: 1, for example. The silicon oxide powder may include silicon nano-oxide.

More specifically, the graphite is mixed with the silicon monoxide at a ratio of 1: 1 by mass. Here, the mixing process may include, for example, a ball mill process. The ball milling process may be conducted at a rotational speed of 1,200 rpm for 30 minutes to form a composite. Subsequently, the composite and a binder such as polyvinylidene fluoride (PVDF) are mixed with a solvent to form a slurry. Thereafter, the slurry is cast on a copper foil, and then the cast material is dried in a vacuum oven to form an active material. Here, the silicon oxide powder, the carbon-based conductive material, and the binder contained in the active material may be adjusted to a mass ratio of 4.5: 4.5: 1.

Next, a chromium layer is coated on the surface of the active material. Here, the chromium layer may be formed through an ion beam sputtering process.

In the ion beam sputtering process, an ohmic beam having independently controllable energy and flux density is inclinedly incident on a target made of chromium in a vacuum maintained chamber to release chromium ions from the surface of the target. The released chromium ions adhere to the surface of the active material. Accordingly, the thickness of the chromium layer can be easily controlled through the ion beam sputtering process.

Lithium secondary battery

A lithium secondary battery according to an embodiment of the present invention includes a cathode, a cathode, and an electrolyte layer.

The positive electrode includes a positive electrode active material capable of inserting and desorbing lithium ions. Such a cathode active material may be a lithiated cathode containing lithium used for a cell reaction such as LiCoO2, LiMnO2, LiNiO2, LiCrO2, LiMn2O4 and the like. In addition, the cathode active material contained in the anode portion is environmentally friendly, and does not use a rare metal such as cobalt (Co). Instead, it contains iron rich in reserves, so that the cost of raw materials is very low, (Lithium Iron Phosphate, LiFePO4). ≪ / RTI >

The negative electrode is disposed to face the positive electrode. The negative electrode includes an active material composed of silicon oxide powder, graphite and a binder, and a chromium layer covering the surface of the active material.

The active material is composed of silicon oxide powder, graphite, and a binder. The silicon oxide powder may be made of, for example, silicon mono-oxide. When the active material includes silicon monoxide, the products generated when the silicon monoxide and lithium react with each other may alleviate the volumetric expansion of silicon, thereby having improved axial discharge performance.

The graphite has a crystalline structure. For example, the graphite has a hexagonal crystal structure. As the graphite is included in the active material, the lifetime of the anode for the lithium secondary battery can be prolonged and the electrical conductivity of the active material can be improved.

 The binder interconnects the silicon oxide powder and the graphite. An example of the binder is polyvinylidene fluoride (PVDF).

The chromium layer covers the surface of the active material. The chrome layer may have relatively good electrical conductivity and current density.

Further, when the present invention is applied to a secondary battery including the anode and the electrolyte containing the chromium layer, the chromium layer suppresses contact between the active material and the electrolyte, so that decomposition and reaction of the electrolyte can be suppressed. Therefore, when the anode including the chromium layer is applied to a lithium secondary battery, the lithium secondary battery may have stable cycle characteristics.

The electrolyte layer is interposed between the positive electrode and the negative electrode. The electrolyte layer includes an electrolytic solution. Examples of the electrolytic solution may be a non-aqueous organic solvent, and a lithium salt may be included therein. As the non-aqueous organic solvent, a cyclic or acyclic carbonate, an aliphatic carboxylic acid ester or the like may be used, or a mixture of two or more thereof may be used.

Evaluation of anode for lithium secondary battery

The silicon monoxide was subjected to a ball milling process at 1200 rpm for 30 minutes to mix graphite at the same mass ratio to form a composite. The composite was used as a binder to form an active material at a mass ratio of 9: 1 with polyvinylidene fluoride (PVDF). A chromium layer was formed on the active material through an ion beam sputtering process on the active material to prepare an anode for a lithium secondary battery. Further, the lithium secondary cell electrode as the negative electrode, lithium cobalt oxide (LiCoO2) the substance as the cathode, poly propylene material as the distribution plate, containing the LiPF _6, and the EC and EMC 1: coin in the electrolyte are mixed in the ratio of 1 Cells were fabricated.

FIG. 1 is an image of an anode for a lithium secondary battery according to an embodiment of the present invention measured by EPMA.

Referring to FIG. 1, it can be seen that a chromium layer is uniformly formed on the upper surface of the silicon monoxide layer in which the graphite is synthesized.

2 is an electron micrograph of the anode for a lithium secondary battery of FIG. 1;

Referring to FIG. 2, a carbon graphite layer is formed on a silicon monoxide layer, and a chromium layer is formed on the graphite layer. The chromium layer is uniformly formed to a thickness of about 40 nm.

3 is an electron micrograph of the anode for a lithium secondary battery of FIG. 1;

Referring to FIG. 3, graphite having a crystalline structure was coated on the surface of the silicon monoxide to form a graphite thin film, and a chromium layer was formed on the graphite thin film.

3 is a graph showing the cycle characteristics with respect to the charging capacity of the anode after coating the anode and the chromium layer before coating the active material with the chromium layer.

Referring to FIG. 3, cyclic characteristics are shown through electrochemical analysis, and characteristics for a total of 100 cycles are compared. At this time, the current speed was adjusted to 0.1 C-rate.

It can be seen that the cathode in which the chromium layer is formed has a higher charging capacity than the anode in which the chromium layer is formed, as in the present invention. In particular, with respect to the charge retention rate up to 100 cycles, the negative electrode having the chromium layer had a retention rate of 75%, while the negative electrode made of the active material having no chromium layer showed 69% Can be confirmed to have improved cycle characteristics.

4 is a graph showing the cycle characteristics when the current density changes with respect to the charging capacity of the anode after coating the anode and the chromium layer before coating the active material with the chromium layer.

Referring to FIG. 4, the capacitance measurement was measured while varying the current rate every five cycles in order to know the capacitance change with respect to the speed other than 0.1 C-rate. That is, in the measurement condition that the current rate is changed every 5 cycles at a rate of 0.2 C-rate, 0.5 C-rate, and 1.0 C-rate in addition to the 0.1 C-rate, the anode coated with the chromium layer It can be seen that it has excellent charge capacity. This is because the chromium layer has a relatively high electrical conductivity.

FIG. 5 is a graph showing the cycle characteristics of the anode on the anode after the anode and the chromium layer are coated before coating the active material with the chromium layer. FIG.

Comparing the impedance analysis values of the anode coated with the chromium layer and the anode coated with the chromium layer after 1 cycle, 5 cycles, and 10 cycles, it was confirmed that the anode coated with the chromium layer had a relatively low impedance (electric resistance) . It can be seen that the anode coated with the chromium layer proceeds more stable reaction.

Claims (8)

An active material composed of a silicon oxide powder, a graphite and a binder; And
And a chromium layer covering the surface of the active material. The anode for a lithium secondary battery according to claim 1, wherein the silicon oxide powder is made of silicon mono-oxide. The anode for a lithium secondary battery according to claim 1, wherein the binder comprises polyvinylidene fluoride (PVDF). The anode for a lithium secondary battery according to claim 1, wherein the active material comprises a silicon oxide thin film and a graphite thin film formed on the silicon oxide thin film. Forming an active material composed of a silicon oxide powder, a graphite and a binder;
And forming a chromium layer on the surface of the active material. 6. The method of claim 5, wherein the step of coating the chromium layer is performed by an ion beam sputtering process. 6. The method of claim 5, wherein forming the active material comprises:
Mixing the silicon oxide powder and the graphite to form a composite; And
And adding a binder to the composite. ≪ RTI ID = 0.0 > 11. < / RTI > A cathode comprising a cathode active material;
A negative electrode disposed to face the positive electrode and including a negative electrode active material composed of silicon oxide powder, graphite, and a binder, and a chromium layer covering a surface of the active material; And
And an electrolyte layer interposed between the positive electrode and the negative electrode.

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