KR100291067B1 - Pre-lithiation method of carbon electrodes and its application to assembling lithium secondary batteries - Google Patents

Abstract

PURPOSE: Provided are method for completely treating carbon electrode with lithium which is used for enhancing a capacity and a cycle life of the carbon electrode, and a method for producing lithium secondary cell using the same. CONSTITUTION: The method for completely treating carbon electrode with lithium comprises the steps of (i) treating a carbon electrode with lithium by varying a temperature and ionic conductivity under the state which carbon electrode and lithium metal are connected or contacted with each other by resistance, (ii) stabilizing the treated carbon electrode at the predetermined temperature for the predetermined period to form a stable film on the surface of carbon electrode, thereby enhancing a reversibility in charging/discharging.

Description

Pre-lithiation method of carbon electrode and manufacturing method of lithium secondary battery using same {PRE-LITHIATION METHOD OF CARBON ELECTRODES AND ITS APPLICATION TO ASSEMBLING LITHIUM SECONDARY BATTERIES}

The present invention relates to a method of prelithiation of a carbon electrode and a method of manufacturing a lithium secondary battery using the same. The prelithiation of a carbon electrode is characterized in that the temperature and the ion conductivity of the electrolyte in the state in which the carbon electrode and the lithium metal are directly connected or directly contacted By changing the rate and amount of lithiation of the carbon electrode, and stabilizing the carbon electrode at a constant temperature for a certain time after lithiation to form a stable film on the surface of the carbon electrode, thereby improving the reversibility of the carbon electrode, and lithiated carbon electrode. By manufacturing a lithium secondary battery, by increasing the capacity by preventing the capacity decrease due to irreversible capacity in the carbon electrode appearing during the initial charge, and also by replenishing the amount of lithium consumed due to charge and discharge efficiency problems during charging and discharging All-lithiation method of carbon electrode to improve cycle life and lithium secondary battery using same It relates to a process for producing the same.

Since a conventional lithium secondary battery uses a compound containing lithium, such as LiCoO ₂ and LiMn ₂ O ₄ , as a positive electrode, a battery is manufactured in a state in which lithium is not inserted into a carbon electrode used as a negative electrode. In the case of the carbon electrode, a passivation film is formed on the surface of the carbon electrode during initial charging, which prevents the organic solvent from intercalating between the carbon lattice layers and suppresses decomposition reaction of the organic solvent, thereby stabilizing the carbon structure and reversibility of the carbon electrode. It can be used as a negative electrode for a lithium secondary battery. However, since the film forming reaction is an irreversible reaction, there is also an adverse effect of reducing the capacity of the battery by the consumption of lithium ions. In addition, since the charge and discharge efficiency of the carbon electrode and the positive electrode is not 100% completely, as the number of cycles progresses, consumption of lithium ions occurs, which leads to a decrease in electrode capacity, resulting in a decrease in cycle life. Therefore, when the pre-lithiated carbon electrode is used as a negative electrode as in the present invention, since the film formation reaction during initial charging is performed in advance, not only a lithium secondary battery having a high capacity can be manufactured without a decrease in capacity but also lithium ions appearing as the number of cycles increases. The cycle life can be significantly improved by replenishing the consumption of.

The conventional method of prelithiation of a carbon electrode is a method of preparing an electrode after lithiating a carbon active material by a physicochemical method (K. Zaguib, R. Yazami and M. Broussely, 8th International Meeting on Lithium Batteries, 192 (1996). And electrochemically prelithiation of carbon electrodes (XY Song and K. Kinoshita, J. Electrochem. Soc., 143, L120 (1996), LL Olsen and R. Koksbang, US Pat. 5,595,837 (1997) There are risks such as fire and explosion because the physicochemical method must be carried out at high temperature. In contrast, the electrochemical method has an advantage that can be carried out at room temperature, but there are some difficulties in the process. Looking at the problems of the conventional electrochemical method for differentiation from the present invention has the disadvantage of having to install and control the power supply by adjusting the lithiation reaction rate by adjusting the current using the power supply, there is no stabilization process in the carbon electrode Lithium is not evenly dispersed in the battery characteristics, in particular the cycle life characteristics have a disadvantage that appears poor.

Accordingly, the present invention has been made in view of the above problems, and the lithiated carbon electrode which changes the temperature and ion conductivity of the electrolyte to control the rate and amount of lithiation of the carbon electrode and improves the reversibility of the carbon electrode. The purpose of the present invention is to provide a lithium secondary battery, and to provide a lithium secondary battery manufacturing method using the same and a lithium secondary battery that can increase capacity and improve cycle life.

1 is a graph showing a method of prelithiation of a carbon electrode according to the present invention.

2 is a graph showing a method of prelithiation of a carbon electrode according to the present invention.

3 is a graph showing charge and discharge characteristics of Example 1 and Comparative Example 1 of a lithium secondary battery manufactured by a carbon electrode according to the present invention.

4 is a graph showing the electrode capacity and the life test results of the lithium secondary battery manufactured by the carbon electrode according to the present invention and Comparative Example 1.

In the present invention, in the method of prelithiation of a carbon electrode and a method of manufacturing a lithium secondary battery using the same, the carbon electrode is lithiated by changing the temperature and the ion conductivity of the electrolyte in a state in which the carbon electrode and the lithium metal are connected or connected in direct contact with each other. By controlling the reaction rate to be constant and stabilizing for a certain period of time after lithiation, lithium is evenly dispersed in the carbon electrode, thereby greatly improving the battery capacity and cycle life.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

The method of prelithiation of a carbon electrode according to the present invention can be broadly divided into two methods. First, a method of controlling lithium by adjusting the resistance as shown in FIG. 1 and stabilizing it, and second, by direct contact as shown in FIG. There is a way to stabilize. First, the method of using the resistor of FIG. 1 will be described in detail. Initially, by connecting a high resistor of about 1 to 5 kΩ / g (carbon) to slow the lithiation rate, a stable passivation film may be formed on the surface of the carbon electrode. By connecting a low-resistance of about 50 ~ 500Ω / g (carbon) to increase the reaction rate is to insert lithium ions between the carbon grid. In particular, during the initial film formation reaction, lowering the temperature has the advantage of making the passivation film more dense. After the lithiation reaction is stabilized for about one day at a temperature of 30 ~ 100 ℃ in a state in which the connection is blocked so that the lithiation is evenly made on the electrode active material. Since lithium metal has excellent adhesion and workability, it is compressed and coated on a rolling roller to be used as a lithium electrode. The separator is wrapped thereon to prevent short circuit between the carbon electrode and the lithium electrode and to impregnate the organic solvent electrolyte. An electric current was made to flow. The method of lithiation by direct contact of FIG. 2 is a method of directly contacting a carbon electrode and a lithium metal. If only the contact is performed, the lithiation rate is initially increased, and the lithiation rate gradually decreases with time. However, if the lithiation rate is high initially, the passivation film formation reaction and the lithium insertion reaction into the carbon lattice simultaneously occur, resulting in the destruction of the carbon structure. Therefore, in the present invention, a stable passivation film is formed at an initial stage, and a proper temperature and ionic conductivity of the electrolyte are controlled so that a lithium insertion reaction is performed well later. Specifically, in the first step, lithiation reaction is performed under low temperature (-10-15 ° C.) and low ion conductivity electrolyte (˜10 ^-3 S / cm) so that lithiation reaction occurs gradually so that the passivation film is well formed. In step 2, lithium was reacted under an electrolyte having high temperature and ion conductivity (˜10 ⁻² S / cm) to allow lithium to be inserted into the carbon grid. After the lithiation reaction, the lithium metal was removed and stabilized at a temperature of 30 to 100 ° C. for 12 to 36 hours to allow lithiation to be uniformly applied to the electrode active material.

The following are examples and comparative examples in which a carbon electrode and a lithium secondary battery were manufactured using the manufacturing method of the present invention and performance tests were performed, whereby the present invention may be more clearly understood.

Preferred Example for Comparative Example

Example 1

The carbon cathode is Gr. 6g, 0.3g of AB, 0.4g of PVdF are mixed with an appropriate amount of NMP and acetone, and then cast on a copper foil, dried and rolled to obtain an electrode when a suitable viscosity is obtained. As shown in FIG. 1, a separator was placed on both sides of the electrode, and lithium metal was applied to the outside thereof. Then, a 1M LiPF ₆ -dissolved EC-DEC solution was wetted, and a high resistance of about 2.5 kΩ / g (carbon) was connected. Lithiate. After that, the resistor is replaced with a low resistor of 250Ω / g (carbon) and lithiated for about 3 hours. The lithiated carbon electrode is left to stabilize at 50 ° C. for one day. The LiCoO ₂ positive electrode is mixed with a composition of 5.7 g of LiCoO ₂ , 0.6 g of AB, and 0.4 g of PVdF in an appropriate amount of NMP and acetone, and then cast on an aluminum sheet, dried, and rolled to obtain an electrode when an appropriate viscosity is obtained. Lithium secondary battery is composed by stacking lithiated carbon anode, PP separator and LiCoO ₂ positive electrode, injecting EC-DEC solution in which 1M LiPF ₆ is dissolved, and then charging / discharging rate C / 3 based on positive electrode capacity and cycle. The lifetime was investigated.

Comparative Example 1

The carbon cathode is Gr. 6g, 0.3g of AB, 0.4g of PVdF are mixed with an appropriate amount of NMP and acetone, and then cast on a copper foil, dried and rolled to obtain an electrode when a suitable viscosity is obtained. The LiCoO ₂ positive electrode is mixed with a composition of 5.7 g of LiCoO ₂ , 0.6 g of AB, and 0.4 g of PVdF in an appropriate amount of NMP and acetone, and then cast on an aluminum sheet, dried, and rolled to obtain an electrode when an appropriate viscosity is obtained. Lithium secondary battery is composed by stacking carbon negative electrode, PP separator, LiCoO ₂ positive electrode and injecting EC-DEC solution in which 1M LiPF ₆ is dissolved. Investigate electrode capacity and cycle life based on positive electrode with layer discharge rate C / 3. It was.

Example 2

The carbon cathode is Gr. 6g, 0.3g of AB, 0.4g of PVdF are mixed with an appropriate amount of NMP and acetone, and then cast on a copper foil, dried and rolled to obtain an electrode when a suitable viscosity is obtained. As shown in FIG. 2, lithium metal was placed on both sides of the electrode, and the ambient temperature was 0 ° C., followed by soaking the EC-DEC solution in which 1M LiPF ₆ was dissolved. Thereafter, the mixture is lithiated for about 3 hours at an ambient temperature of 25 ° C. The lithiated carbon electrode is left to stabilize at 50 ° C. for one day. The LiCoO ₂ positive electrode is mixed with a composition of 5.7 g of LiCoO ₂ , 0.6 g of AB, and 0.4 g of PVdF to a suitable NMP and acetone, and then cast on an aluminum sheet, dried, and rolled to obtain an electrode when a suitable viscosity is obtained. Lithium secondary battery is composed by stacking lithiated carbon anode, PP separator and LiCoO ₂ positive electrode, injecting EC-DEC solution in which 1M LiPF ₆ is dissolved, and then charging / discharging rate C / 3 based on positive electrode capacity and cycle. The lifetime was investigated.

Example 3

The carbon cathode is Gr. 6g, 0.3g of AB, 0.4g of PVdF are mixed with an appropriate amount of NMP and acetone, and then cast on a copper foil, dried and rolled to obtain an electrode when a suitable viscosity is obtained. As shown in FIG. 2, lithium metal was placed on both sides of the electrode, and the ambient temperature was 10 ° C., and the resultant solution was lithiated for 2 hours with an EC-DEC solution in which 0.1M LiPF ₆ was dissolved. Thereafter, the mixture was further infused with an EC-DEC solution in which 1M LiPF ₆ was dissolved at an atmospheric temperature of 25 ° C. and then lithiated for about 3 hours. The lithiated carbon electrode is left to stabilize at 50 ° C. for one day. The LiCoO ₂ positive electrode is mixed with a composition of 5.7 g of LiCoO ₂ , 0.6 g of AB, and 0.4 g of PVdF in an appropriate amount of NMP and acetone, and then cast on an aluminum sheet, dried, and rolled to obtain an electrode when an appropriate viscosity is obtained. Lithium secondary battery is composed by stacking lithiated carbon anode, PP separator and LiCoO ₂ positive electrode, injecting EC-DEC solution in which 1M LiPF ₆ is dissolved, and then charging / discharging rate C / 3 based on positive electrode capacity and cycle. The lifetime was investigated.

The charge and discharge characteristics of the lithium secondary battery (Example 1, Comparative Example 1) prepared according to the above method was as shown in Figure 3, the battery of the present invention does not show an initial irreversible reaction, the charge and discharge efficiency is almost 100 %, The electrode capacity was also large. In contrast, Comparative Example 1, a conventional method, showed an irreversible reaction, resulting in low layer discharge efficiency and electrode capacity. 4 shows the electrode capacity (based on LiCoO ₂ active material) and the cycle characteristics of the lithium secondary batteries manufactured according to the above method, the batteries produced by the method of the present invention showed excellent electrode capacity and cycle life characteristics, but the comparison The battery prepared by the method of Example 1 showed a slightly poor electrode capacity and cycle life characteristics.

According to the present invention, the reaction rate at which the carbon electrode is lithiated is constantly controlled by changing the temperature and the ionic conductivity of the electrolyte in a state in which the carbon electrode and the lithium metal are connected with resistance or in direct contact with each other. Stabilization over time allows lithium to be evenly dispersed in the carbon electrode, thereby significantly improving battery capacity and cycle life.

Claims (8)

On the surface of the carbon electrode, the carbon electrode is lithiated by changing the temperature and the ionic conductivity of the electrolyte while the carbon electrode and the lithium metal are connected by resistance or in direct contact with each other, and the lithiated carbon electrode is stabilized at a constant temperature for a predetermined time. A method of prelithiation of a carbon electrode for a lithium secondary battery, characterized by forming a stable film to improve reversibility during charging and discharging. The method of claim 1, wherein the prelithiation connected by the resistor is performed by connecting a high resistance of 1 to 5 kΩ / g (carbon) in the first step, and a low resistance of 50 to 500 Ω / g (carbon) in the second step. A method of prelithiation of a carbon electrode, characterized in that the connection is carried out. The method of claim 1, wherein the direct contact lithiation is carried out under a low temperature of -10 to 15 ° C and / or a low ion conductivity of -10-3 S / cm in the first step, and in the second step at room temperature and- A method of prelithiation of a carbon electrode, which is carried out under a 10-2 S / cm electrolyte for lithium ion batteries. The method of claim 1, wherein the stabilization is performed under a constant temperature between 30 and 100 ° C and a constant time between 12 and 36 hours. The method of claim 1, wherein the lithiation is carried out in two stages, wherein the first stage is carried out for 1 to 5 hours and the second stage is carried out for 2 to 10 hours. The method of claim 1, wherein the carbon electrode is made of an active material such as graphite, coke, hard carbon, and the like to be prelithiated. A lithium secondary battery comprising a carbon negative electrode for a lithium secondary battery manufactured by the method according to any one of claims 1 to 6, a positive electrode and an electrolyte containing a material capable of removing lithium as a positive electrode active material. 8. The method of claim 7, wherein the material capable of intercalating lithium is LiCoO2 or LiMn2O4.