WO2015046693A1

WO2015046693A1 - Method of forming electrode for lithium secondary battery

Info

Publication number: WO2015046693A1
Application number: PCT/KR2014/002963
Authority: WO
Inventors: 윤우영; 염지호
Original assignee: 고려대학교 산학협력단
Priority date: 2013-09-30
Filing date: 2014-04-07
Publication date: 2015-04-02
Also published as: KR20150037163A; KR101527286B1

Abstract

In a method of forming an electrode for a lithium secondary battery, silicon oxide powder and lithium metal powder are mixed to form a mixture, and then the mixture is heated to a temperature equal to or greater than the melting point of the lithium metal powder so as to react liquid lithium metal with silicon oxide powder and form a lithium silicon oxide from the mixture. Thereafter, the lithium silicon oxide and water-based binder are used to form an active material body.

Description

Formation Method of Electrode for Lithium Secondary Battery

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

Lithium secondary battery is a type of secondary battery that is charged and discharged by insertion and desorption of lithium ions in the battery. During charging, lithium ions move from cathode to cathode toward anode. It is inserted into the active material of, on the contrary, during discharge, lithium ions inserted into the negative electrode move toward the positive electrode and are inserted into the active material of the positive electrode. Such lithium secondary batteries have high energy density, large electromotive force, and high capacity, and thus are widely used as power sources for mobile phones and laptops.

The lithium secondary battery is usually 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 detaching lithium ions as described above. A separator prevents physical cell contact between the positive and negative electrodes. Instead, the movement of ions through the separator is free. The electrolyte serves as a path through which ions can move freely between the anode and the cathode.

Meanwhile, carbon for constituting the negative electrode may be used. The carbon material is widely used as a negative electrode material of a lithium ion battery due to its small volume change, excellent reversibility, and relatively low price when intercalating / deintercalating lithium. Carbon materials used as such anode materials include graphite, coke, fiber, pitch, and meso carbon. However, the graphite has a theoretical limit (372 mAh g-1) in charge capacity per unit mass. Therefore, there has been a demand for the development of a new negative electrode material capable of greatly improving operating characteristics such as energy density, reversible capacity, and initial charging efficiency of a lithium ion battery.

Recently, the development of electrodes using silicon has been attracting attention in an attempt to solve the above problems. Silicon has a lot of interest in the field of lithium ion batteries because it has a theoretical capacity per unit mass (approximately 4,200 mAh g-1) that is more than 10 times higher than the charge capacity of graphite electrodes or other various oxide and nitride material electrodes. It is a material. However, when silicon is applied to an electrode of a lithium ion battery, due to the insertion / desorption of lithium into the silicon electrode, a large volume change of 400% or more occurs in the silicon electrode, and thus the application to the actual anode material has many limitations.

In order to solve the problem of the silicon electrode, silicon oxide (SiOx) has been proposed.

The silicon oxide is in the form of an oxygen group attached to the silicon and when the reaction with lithium, in addition to the main reaction between lithium and silicon, other oxides are generated. These ancillary products show improved performance by acting to mitigate the volume expansion of silicon. However, lithium ions used in the reaction of ancillary products are not reversible and thus are not used again during charge and discharge, resulting in loss of lithium ions. When the irreversible reaction occurs, the capacity may be reduced due to the loss of lithium ions, which may deteriorate the characteristics of the lithium secondary battery.

One object of the present invention is to provide a method for forming an electrode for a lithium secondary battery having an improved capacity and lifetime.

In the method of forming an electrode for a lithium secondary battery according to an embodiment of the present invention, silicon oxide powder and lithium metal powder are mixed to form a mixture, and then the mixture is heated to a temperature above the melting point of the lithium metal powder. As a result, the liquid lithium metal and silicon oxide powder are reacted to form lithium silicon oxide from the mixture. Thereafter, an active material body is formed using the lithium silicon oxide and the aqueous binder.

In one embodiment of the present invention, in order to heat the mixture, a process of placing the mixture in the crucible and directly heating the crucible may be performed.

In one embodiment of the present invention, in order to heat the mixture, a process of placing the mixture in the crucible and heating the crucible in a bath.

In one embodiment of the present invention, a process of performing a milling process for the mixture may be additionally performed. Here, the milling process may be carried out for 12-24 hours at a speed between 140 and 200 rpm.

According to the method for forming an electrode for a lithium secondary battery according to the present invention, a mixture of lithium metal powder and silicon oxide powder is heated to react with a liquid lithium powder and silicon oxide powder to form lithium silicon oxide and lithium silicon oxide By using as an electrode active material body, irreversible reaction does not occur during charge and discharge, only the reversible reaction may occur during movement between the electrodes of lithium ions.

Therefore, when the electrode including the lithium silicon oxide is applied to the lithium secondary battery, the lithium secondary battery may have a stable cycle characteristics.

1 is a voltage-capacity graph showing cycle characteristics during axial discharge of a lithium secondary battery including an electrode made of silicon oxide and an electrode including lithium silicon oxide, respectively.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. As the inventive concept allows for various changes and numerous embodiments, particular embodiments will be illustrated in the drawings and described in detail in the text. However, this is not intended to limit the present invention to the specific disclosed form, it should be understood to include all modifications, equivalents, and substitutes included in the spirit and scope of the present invention. In the accompanying drawings, the size and amount of the objects are shown to be enlarged or reduced than actual for clarity of the invention.

Terms such as first and second 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 the second component, and similarly, the second component may also be referred to as the first component.

The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting of the present invention. Singular expressions include plural expressions unless the context clearly indicates otherwise. In this application, the terms "comprise" or "include" are intended to indicate that there is a feature, step, function, component, or combination thereof described on the specification, and other features, steps, functions, components Or it does not exclude in advance the possibility of the presence or addition of them in combination.

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. Terms such as those defined in the commonly used dictionaries should be construed as having meanings consistent with the meanings in the context of the related art and shall not be construed in ideal or excessively formal meanings unless expressly defined in this application. Do not.

In the method for forming an electrode for a lithium secondary battery according to an embodiment of the present invention, first, a silicon oxide powder and a lithium metal powder are mixed to form a mixture.

The lithium metal powder may be formed through a droplet emulsion technique (DET) process. That is, the silicone oil is heated to a temperature above the break point of lithium. Thereafter, lithium foil is charged into the silicone oil to melt the lithium. At this time, the impeller pulverizes the molten lithium by the turbulent energy generated by the high speed rotation by using the rotational force of the motor, and at the same time, the lithium metal and the silicone oil are mixed to form an emulsified solvent. Thereafter, the emulsified solvent may be layered using hexane to obtain lithium powder.

In order to uniformly mix the silicon oxide powder and the lithium metal powder, a homogenizer or a stirrer may be used.

In this case, as the lithium metal powder is used, generation of by-products due to side reactions may be suppressed as compared with the case where lithium compounds such as lithium chloride and lithium hydroxide are used.

In addition, as the lithium metal in the powder state is used, the reaction surface area may be increased during the reaction with the silicon oxide, thereby improving reactivity as compared with the bulk lithium metal. That is, the lithium powder and the silicon oxide powder may easily react at a temperature of 190 to 210 ° C. close to the melting point of the lithium.

Furthermore, as the mixture is formed by using the lithium powder and the silicon oxide powder, the lithium powder and the silicon oxide powder may be uniformly mixed at a predetermined ratio in the mixing process as compared with the lithium in the bulk state.

Subsequently, the liquid lithium metal and silicon oxide react with each other by heating the mixture to a temperature higher than the melting point of the lithium metal powder. This forms lithium silicon oxide from the mixture. Examples of the lithium silicon oxide include lithium silicate (Li ₄ SiO ₄ ).

That is, when an active material body made of silicon oxide is used as an electrode of a lithium secondary battery, an irreversible reaction may occur during axial discharge of the lithium secondary battery, and thus may be formed on an irreversible material including lithium. The irreversible material is already formed in the lithium secondary battery electrode made of the lithium silicon oxide corresponding to the irreversible material before the axial discharge of the lithium secondary battery. Therefore, when the electrode including the lithium silicon oxide is used in the lithium secondary battery, the irreversible reaction is suppressed during the axial discharge of the lithium secondary battery so that lithium ions are not consumed. As a result, the lithium secondary battery to which the electrode including the lithium silicon oxide is applied may have improved cycle characteristics, that is, improved lifespan.

In one embodiment of the present invention, the lithium metal powder and silicon oxide powder is charged to the crucible, the crucible is mounted in a vacuum oven or a furnace to directly melt the crucible at the melting point of the lithium metal (180.54 ° C.). By heating to the above temperature, the lithium metal powder can be melted and the liquid lithium metal and the silicon oxide powder can be reacted to form lithium silicon oxide. The heating process may, for example, proceed for 24 hours or more.

In one embodiment of the present invention, the lithium metal powder and the silicon oxide powder is charged to the crucible, the crucible is mounted in a silicon oil to heat the crucible in a hot water bath to melt the lithium metal powder, and the liquid lithium metal And reacting the silicon oxide powder to form lithium silicon oxide. The silicone oil may be maintained at a temperature sufficient to melt the lithium metal powder at 200 ° C. or higher. The heating process may, for example, proceed for 24 hours or more.

In one embodiment of the present invention, a milling process may be additionally performed on the mixture. Through the milling process, the mixture is synthesized so that the liquid lithium metal and silicon oxide can react more easily in a subsequent heating process. The milling process can for example proceed at a speed between 140 and 200 rpm. In addition, the milling process may be performed for about 12 hours to 24 hours. The milling process can be performed using, for example, a ball mill apparatus or a planetary mill apparatus.

Thereafter, an active material body formed of the lithium silicon oxide and the aqueous binder is formed. The lithium silicon oxide, the conductive material and the binder may be adjusted to a weight ratio of 70: 25: 5, for example. An example of the aqueous binder may be sodium carboxymethyl cellulose (CMC).

When forming an active material body using the lithium silicon oxide, it is not necessary to use a non-aqueous binder to solve the stability problem caused by the reaction of the lithium particles with water. Therefore, when the active material body is formed of lithium silicon oxide, an aqueous binder may be applied. In addition, when the aqueous binder is used, the effect of binding the lithium silicon oxide particles in a smaller amount compared to the non-aqueous binder may be excellent. As a result, the ratio of the aqueous binder in the active material body can be reduced, so that the properties of the active material body can be improved.

On the other hand, when forming the active material body consisting of the lithium silicon oxide and the aqueous binder, the carbon-based conductive agent is additionally mixed to improve the electrical conductivity of the active material body.

Fabrication of Lithium Secondary Battery

Lithium secondary battery according to an embodiment of the present invention includes a positive electrode, a negative electrode and an electrolyte layer.

The positive electrode includes a positive electrode active material capable of inserting and detaching lithium ions. The cathode active material may be a lithium-containing transition metal oxide (lithiated cathode) used in a battery reaction such as LiCoO ₂ , LiMnO ₂ , LiNiO ₂ , LiCrO ₂ , LiMn ₂ O ₄ , and the like. In addition, the positive electrode active material included in the positive electrode portion is environmentally friendly, does not use rare metals such as cobalt (Co), and instead contains iron having abundant reserves, thus the raw material is very inexpensive and greatly contributes to battery capacity. Lithium iron phosphate (Lithium Iron Phosphate, LiFePO ₄ ) It can be.

The cathode is disposed to face the anode. The negative electrode includes a negative electrode active material composed of lithium silicon oxide and an aqueous binder.

An example of the aqueous binder may be sodium carboxymethyl cellulose (CMC).

When applied to a secondary battery including an electrode including the lithium silicon oxide, irreversible reaction of generating lithium silicate during axial discharge may be suppressed. Therefore, when the electrode including the lithium silicon oxide is applied to the lithium secondary battery, the lithium secondary battery may have a stable cycle characteristics.

The electrolyte layer is interposed between the positive electrode and the negative electrode. The electrolyte layer includes an electrolyte solution. Examples of the electrolyte may be a non-aqueous organic solvent, which may include a lithium salt. As said non-aqueous organic solvent, cyclic or acyclic carbonate, an aliphatic carboxylic acid ester, etc. can use what is single or 2 or more types are mixed.

Evaluation of the electrode for lithium secondary batteries

Silicon oxide powder and lithium metal powder were mixed to form a mixture. At this time, the lithium metal powder was formed by a DiT process. In addition, a homogenizer was used to form a mixture in which silicon oxide powder and lithium metal powder were uniformly mixed. After charging the mixture to the crucible, the crucible was heat-treated at 200 ° C. for 24 hours in a vacuum oven to form lithium silicon oxide. The lithium silicon oxide, the conductive material, and the aqueous binder were formed with an active material body at a mass ratio of 70: 25: 5 to prepare an electrode for a lithium secondary battery. In addition, the electrolyte for the lithium secondary battery as a negative electrode, a lithium cobalt oxide (LiCoO ₂ ) material as a positive electrode, a polypropylene material as a separator, an electrolyte solution containing 1 mol of LiPF ₆ and EC and DEC in a ratio of 1: 1 A coin cell was produced. The coin cell was charged and discharged once between 0 and 1.5 V at a constant current of 0.1 C.

Referring to FIG. 1, when the silicon monooxide powder is used, the capacity reduction rate of the initial charge amount and the discharge amount is 40.3%, but the lithium secondary battery employing an electrode including lithium silicon oxide in reaction with lithium powder has a capacity of about 12%. A decrease can be seen. That is, it can be seen that the initial efficiency of the lithium secondary battery having the electrode including the lithium silicon oxide has 88%.

That is, in the case of general silicon monooxide, lithium reversible and irreversible reaction of lithium inserted during charging of the battery occur together, and lithium ions involved in the 40.3% capacity corresponding to the irreversible reaction no longer affect the charge and discharge of the battery. On the other hand, in the case of a lithium secondary battery including an electrode made of lithium silicon oxide in which an irreversible material is formed prior to charging and discharging, lithium ions inserted during charging do not participate in the irreversible reaction, thereby suppressing generation of the irreversible material. You can see that.

The method of manufacturing an electrode for a lithium secondary battery according to embodiments of the present invention may be applied to form an electrode of a lithium secondary battery.

Claims

Mixing the silicon oxide powder and the lithium metal powder to form a mixture;

Reacting the liquid lithium metal and silicon oxide powder by heating the mixture to a temperature above the melting point of the lithium metal powder to form lithium silicon oxide from the mixture; And

Method of manufacturing an electrode for a lithium secondary battery comprising the step of forming an active material using the lithium silicon oxide and the aqueous binder.
The method of claim 1, wherein heating the mixture comprises:

Placing the mixture into the crucible; And

Method for producing a lithium secondary battery electrode, characterized in that it comprises the step of heating the crucible directly.
The method of claim 1, wherein heating the mixture comprises:

Placing the mixture into the crucible; And

Method for manufacturing a lithium secondary battery electrode, characterized in that it comprises the step of heating the crucible in a bath.
The method of manufacturing an electrode for a lithium secondary battery according to claim 1, further comprising performing a milling process on the mixture.
The method of claim 4, wherein the milling process is performed at a speed of 140 to 200 rpm for 12 to 24 hours.