KR20000042002A - Formation and aging method of a secondary lithium battery - Google Patents

Abstract

PURPOSE: A formation and aging method of a secondary lithium battery is provided to improve the productivity of the secondary lithium battery by shortening the processing time. CONSTITUTION: A formation and aging method of a secondary lithium battery comprises the steps of (1) formation : charging and discharging the secondary lithium battery to form a SEI film on the surface of the secondary lithium battery, (2) aging : leaving the SEI film at a predetermined temperature to stabilize the SEI film which is formed in the formation step.

Description

Formation and Aging Method of Lithium Secondary Battery

The present invention relates to a method of forming and aging a lithium secondary battery.

In general, secondary batteries capable of charging and discharging have been actively researched by the development of portable electronic devices such as cellular phones, notebook computers, computer camcorders, and the like.

In particular, the secondary battery is a nickel-cadmium battery, a lead-acid battery, a nickel metal hydride battery, a lithium ion battery, a lithium polymer battery, a metal lithium 2 There are various types of batteries, such as battery cells and air zinc accumulators.

Among the batteries, lithium secondary battery has an operating voltage of 3.6 V and has a service life of about three times that of nickel-cadmium (Ni-Cd) or nickel-hydrogen (Ni-MH) batteries, which are widely used as power sources for electronic devices. It is rapidly evolving in terms of excellent energy density per weight.

The lithium secondary battery may be classified into a liquid electrolyte battery and a polymer electrolyte battery according to the type of electrolyte, and generally, a battery using a liquid electrolyte is called a lithium ion battery and a lithium polymer battery when a polymer electrolyte is used.

In general, in manufacturing a lithium secondary battery, first, a material in which an active material, a binder, and a plasticizer are mixed is applied to a positive electrode current collector and a negative electrode current collector to prepare a positive electrode plate and a negative electrode plate, and laminated on both sides of a separator to form a battery cell of a predetermined shape. After the step of forming the battery cell, the battery cell is inserted into the battery case and sealed to complete the battery pack.

However, lithium secondary batteries are not released immediately after assembly. This is because even if the battery is assembled without any defects through all the above steps, it is impossible to make a good product judgment without performing charge / discharge, which is a characteristic of the secondary battery.

Thus, in order to determine whether the battery is defective or to ensure the stability of the battery performance, in particular, the lifespan, two steps of formation and aging must be performed before the product is shipped.

The formation process is to activate the battery by repeating charging and discharging after battery assembly. In this process, lithium ions from the lithium metal oxide used as the positive electrode during charging are moved and inserted into the carbon (crystalline or amorphous) electrode used as the negative electrode. In this case, lithium reacts at the carbon negative electrode because Li is highly reactive. And compounds such as LiOH are formed, and these form a film on the surface of the carbon negative electrode. Such a film is called a SEI (Solid Electrolyte Interface) film. This SEI film is a nonconductor, and once formed, functions to prevent reaction between lithium ions and other materials in the carbon negative electrode during subsequent charging. In addition, the film functions as a kind of ion tunnel, and performs a function of passing only lithium ions. By such an ion tunnel effect, organic solvents having a high molecular weight, which dissolve and move lithium ions, are inserted together in the carbon negative electrode to prevent the structure of the carbon negative electrode from being collapsed. That is, once the SEI film is formed, lithium ions do not react sideways with the carbon negative electrode or other materials, and thus not only can maintain the amount of lithium ions reversibly, but also organic solvents are inserted together with the lithium ions to form the carbon negative electrode. By preventing the structure from collapsing, the charge and discharge of the lithium secondary battery is reversibly maintained to improve battery life.

Most of the formation of the SEI film takes place during the first filling of the formation process. In the formation process, the first charging curve shows the flatness curve of the dislocation at about 3.4 to 3.7V. This is the section where the SEI film is produced by forming lithium ion into the carbon negative electrode and meeting the carbon negative electrode. In addition, comparing the battery capacity of the first charge and the battery capacity of the first discharge, it can be confirmed that the capacity decrease of about 10% -30% at the time of discharge compared to the time of charging. This reduction in capacity is exactly what the SEI film is produced and consumed.

As such, the role of the SEI film in the lithium secondary battery is important, and the POM line process for forming the SEI film plays an important role in determining the life of the battery. However, the formation of the SEI film through formation does not necessarily result in a battery with a good lifetime. If the SEI film is not formed uniformly and stably but is roughly formed on the surface of the carbon negative electrode, lithium ions continue to form and discharge the SEI film as the battery is repeatedly charged and discharged in actual battery use, and as a result, a large amount of capacity is continuously reduced. Accordingly, the amount of available lithium ions is also reduced, which shortens the life of the battery. Therefore, the conditions of the formation process should be set to form the SEI film uniformly and stably. To this end, it is possible to consider a method of allowing lithium ions to stably reach the carbon negative electrode by using a low current during formation, and a method of increasing the number of formation steps. However, both methods are disadvantageous in terms of cost and productivity. Therefore, in general, after the formation process, a process called aging is separately performed. The aging process refers to a process of stabilizing the formation of the SEI film by leaving the formed battery at room temperature for a certain period of time.

The conventional aging process uses the method of leaving about 4 weeks at the temperature of 20 degreeC. That is, the battery that has undergone the charge-discharge-charge formation has a voltage of 4.19V-4.20V. When the battery is aged by the conventional method, the battery has a voltage of about 4.12V. The SEI film is additionally formed by stabilization and uniformity by the difference between the two voltages.

The battery, which is processed by the conventional formation and aging method of the lithium secondary battery, has a slight life improvement compared to the battery that does not undergo aging, but this is not satisfactory. In addition, since the aging time (about 4 weeks) is very long, there is a problem that the productivity is lowered.

The present invention has been made to solve the above problems, and an object thereof is to provide a method of forming and aging a lithium secondary battery, the process of which is improved to improve battery life and productivity.

Figure 1 shows the change in OCV over time when aging is performed at three different temperatures.

2A and 2B show changes in OCV and discharge capacity with time at an aging temperature of 60 ° C, respectively.

Figure 3 shows the change in discharge capacity with charge and discharge cycles in a cell in which aging was performed at three different temperatures.

4 shows a comparison of battery life when the batteries having different electrolytes are aged at 20 ° C. and 60 ° C., respectively.

Figure 5 shows the battery life with time when aged at 60 ℃.

Formation and aging method of the lithium secondary battery of the present invention for achieving the above object, the formation step of performing a charge and discharge of the battery assembled so that the SEI film is formed on the negative electrode surface of the battery at least one time, and the formation It comprises an aging step of standing for a certain period of time at a predetermined temperature to uniform and stabilize the SEI film formed in the step,

The aging step is characterized in that it is carried out at a temperature substantially between 40 ℃-60 ℃.

And the aging temperature is preferably 60 degrees.

In addition, although the aging step is different depending on the type of electrolyte and the electrode, it is preferably performed for a time between 1 day and 14 days.

In the formation step, the primary charging, the primary discharge, and the secondary charge may be sequentially performed.

Hereinafter, a preferred embodiment of the formation and aging method of the lithium secondary battery according to the present invention will be described in detail.

First, the battery formation step will be described.

After the assembly is completed, the battery is left to stand at 20 ° C. for 24 hours so as to impregnate the electrolyte well, and then a formation process is performed to generate an SEI film on the surface of the battery negative electrode.

Experimental conditions of the formation process

1) test subjects; Standard capacity 1450mAh, 18650 size lithium ion cylindrical battery.

2) electrolyte of battery; EC: DMC: EMC: LiPF ₃ = 38.81: 28.99: 14.33; 2.4: 15.47 (%)

3) temperature; 20 ℃ constant temperature chamber

4) current; Primary Charge-312.5 mA, Primary Discharge and Secondary Charge-600 mA

5) hours; 12 hours

* Experimental data Item Primary charge Primary discharge Secondary charging Remarks Charge current (mA) 312.5 600 600 Irreversible capacity; 11.2% Capacity (mAh) 1630-1640 1450-1460 1450-1460

As shown in Table 1, it can be seen that the capacity reduction amount (non-reversible capacity), which is the difference between the capacity during the primary charge and the primary discharge capacity, is about 11.2%. This amount of capacity reduction is the irreversible capacity used to form the SEI film on the carbon negative electrode surface as lithium ions migrate to the carbon negative electrode. On the other hand, it can be seen that the secondary charge capacity is the same as the primary discharge capacity. These experimental results show that most of the SEI film is formed during the primary filling.

Secondly, a process of aging the battery in which the formation is completed will be described.

The purpose of the aging step is to make the SEI film formed on the carbon negative electrode uniform by the formation and to be stable, and to take a method of leaving the battery under a predetermined temperature for a predetermined time without applying a current.

During aging, the amount of diffusion of ions varies with temperature, and the diffusion coefficient theoretically follows the following equation (1). Therefore, it can be seen that as the temperature increases, the movement of ions increases, thereby aging is activated.

---------(One)

D; Diffusion coefficient

R; Gas constant

T; Absolute temperature

F; Paradigm constant

v1, v2; Number of cations and anions

Z1; Valence of the cation

λ ⁰ _1, λ ⁰ ₂ ; Limiting equivalent conductance of cations and anions

To determine the effect of temperature on aging, measure the change in the amount of OCV in the battery over time when aged at low temperature (-20 ° C), room temperature (20 ° C), and high temperature (60 ° C). It was. The results of this experiment are shown in FIG.

Referring to the figure, the initial OCV before performing aging is 4.19V for all temperatures. At low temperature (-20 ° C) aging, OCV dropped to 4.18V after 7 days, resulting in self discharge of about 0.01V. At a commercial temperature (20 ° C.), a self discharge of about 0.03 V occurred with a final voltage of 4.16 V, and a self discharge of about 0.1 V occurred with a final voltage of 4.10 V for high temperature (60 ° C.) aging.

The amount of reduction of each OCV as described above contributes to the formation of the SEI film. Therefore, since the movement of ions is minute at low temperature (-20 degreeC), it can be said that formation of SEI film is minute by this. On the other hand, since the decrease in OCV slightly increased at room temperature (20 ° C.), the amount of SEI film formed would increase accordingly. On the other hand, at a high temperature (60 ° C.), the OCV drops rapidly to 4.13 V from the first day of the aging step, and decreases to 4.10 V on the third day and stabilizes there. Thus, a correspondingly large amount of SEI film would have been formed.

In conclusion, as the aging temperature increases, the amount of ions moving increases to form the SEI film. In order to prove this fact, when the aging process was performed under the same conditions (60 ° C.), the amount of change in OCV and the amount of change in battery capacity were measured over time, and the results are shown in FIGS. 2A and 2B. Figure 2a is for OCV, Figure 2b is for battery capacity. As can be seen in the figure, it can be seen that the reduction curve of the cell capacity during aging almost coincides with the reduction curve of OCV. Thus, it can be concluded that ion migration, which causes a reduction in OCV upon aging, is eventually used to produce SEI films, leading to reduced capacity of the cell.

As described above, lithium ions migrated during the aging process are consumed to produce the SEI film as occurs in the first charge of the formation, resulting in irreversible capacity. As the irreversible capacity is greater, more SEI films are formed. As shown in FIG. 1, it can be seen that the most SEI films are formed at 60 ° C. at which OCV is most reduced.

However, what is important to the life of the battery is not the amount of SEI film produced, but the stability and uniformity of the film. That is, the SEI film should be produced stably and uniformly to prevent further side reactions, thereby exhibiting reversible charging and discharging efficiency without consuming lithium ions. If a large amount of SEI film is simply produced without stability and uniformity, lithium ions may continue to react with the non-uniformly formed film, which may adversely affect the lifetime.

In order to confirm the uniformity of the SEI film whose production amount increases with increasing temperature, a life test of the battery was performed. A method of directly measuring the state of formation of the film may be considered, but a battery life test is a more effective and easier method. This is because the stability and uniformity of the film are directly related to the life of the battery.

To this end, the discharge capacity was measured while charging and discharging the battery aged at each other temperature (-20 ° C, 20 ° C, 60 ° C) for 7 days. This experimental result is shown in FIG. 3, where the horizontal axis can be treated as the life of the battery as the ratio of the measured discharge capacity to the original discharge capacity. As shown, when the battery undergoes an aging test at −20 ° C. (curve c), it exhibits a discharge capacity of about 75.69% at 300 cycles, and about 79.42% for a battery aged at 20 ° C. (curve b). And a battery aged at 60 ° C. (curve a) shows a lifetime of about 86.97%. As can be clearly seen from the above results, as the aging temperature is increased, the battery life is much better, which means that lithium ions not only produce a lot of SEI film when aging at high temperature, It can be said to make it more uniform and stable.

Combining the experimental results as above, the following conclusions can be obtained.

In the formation process, since a current is directly applied to the battery, no matter how low a current is charged, the ions move faster than the ions move during aging. In other words, the charge is reduced at low current, but the non-uniform and unstable SEI film is formed on the surface of the negative electrode. The aging process following this formation process moves the ions at a slower rate (approximately 0.1 V drop for 7 days at 60 ° C) than when a low current is applied, whereby the surface of the carbon negative electrode where the SEI film is not properly formed It reacts with the uneven surface of to form a uniform and stable film. However, the lower the temperature of the aging, the slower the movement speed of the ions, and the slower the reaction rate of the ions. As a result, a sufficient amount of ions cannot effectively react on the surface of the carbon negative electrode. Therefore, when the aging temperature is increased, the reaction amount of the ions increases correspondingly, it can be said that the SEI film is formed effectively. Thus, as can be seen in the figure, the best results are obtained for 60 ° C aging.

In the case of aging at a temperature of 60 ° C. or higher, it appears that theoretically better battery life can be obtained, but in practice there is a risk of evaporating low boiling organic solvents used as electrolyte solution. This results in an increase in the internal pressure of the battery, which destroys the rupture plate of the battery, which may render the battery unusable, or may cause a decrease in battery performance due to a change in electrolyte composition due to a temperature rise.

Therefore, due to the above problems it can be said that the aging temperature between 40 ℃-60 ℃ is suitable.

4 compares battery life by aging batteries having different electrolytes at 20 ° C. and 60 ° C., respectively. As shown, in most of the electrolytes commonly used, it can be seen that a battery aged at 60 ° C. has a lifespan that is increased from a minimum of 50 cycles to a maximum of 200 cycles compared to a battery aged at 20 ° C.

In the following, the influence of the aging time on the battery life is considered.

In the case of aging at a temperature of 60 ° C. from above, it was found that the life of the battery was greatly improved. However, the change in battery life with time as well as the effect of temperature is also very important in terms of productivity. In other words, it is important to adopt the optimum aging time with the best battery life. To this end, the formed cells were aged at 60 ° C. for different periods of time, and then the change in battery life with each aging time was measured.

5 shows the results of the experiment. The charge and discharge of the formation were all performed at a current of 1 C at 20 ° C., and the discharge capacity (horizontal axis) of the battery showing the life was measured after the charge and discharge were performed for 300 cycles. In the figure, the straight line d indicates that the battery under the same conditions is aged at 20 ° C, and its discharge capacity is approximately 80.16%. In the case of aging at 60 degrees as shown, it has a lifespan greater than 20 ° C. for all the aging times, especially the discharge capacity of 89.7%, which is the highest lifespan when aged for 14 days, from then on. Little by little the life is reduced.

Therefore, the aging time is preferably from 1 day to a maximum of 14 days. The battery life is the highest when aged for 14 days, but it does not differ significantly compared to the aging time below. Therefore, when selecting the aging period, it is best considering not only the battery life but also the relationship with the productivity of the battery. The condition of the

As described above, the formation and aging method of the lithium secondary battery according to the present invention has the following advantages.

First, the battery life can be improved by at least 50 cycles up to 200 cycles.

Second, since the conventional aging period of about 4 weeks is shortened from 1 day to 2 weeks, the process time is reduced and productivity can be greatly improved.

Third, the battery that has passed the high temperature aging is advantageous in terms of stability.

Although the present invention has been described with reference to the embodiments illustrated in the accompanying drawings, this is merely exemplary, and a person skilled in the art may understand that various modifications and equivalent embodiments are possible. will be. Therefore, the true scope of protection of the present invention should be defined by the appended claims.

Claims (4)

A formation step of charging and discharging the assembled battery at least once so that the SEI film is formed on the negative electrode surface of the battery; In the forming and aging method of a lithium secondary battery comprising a; aging step of leaving for a predetermined period of time at a predetermined temperature to uniformly stabilize the SEI film formed in the formation step, Wherein said aging step is performed at a temperature substantially between 40 ° C. and 60 ° C. 18. The method of claim 1, The aging temperature is a formation and aging method of a lithium secondary battery, characterized in that substantially 60 degrees. The method of claim 1, Wherein the aging step is performed for substantially between 1 day and 14 days. The method of claim 1, The formation step is a method of forming and aging a lithium secondary battery, characterized in that the first charge, the first discharge, the second charge is performed sequentially.