JPH11111267A - Manufacture of lithium ton secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To form a stable SET on a negative electrode surface, reduce irreversible capacity, and improve a cycle characteristic by performing first time charge at least under one condition among charge in a cooling atmosphere and low electric current charge, and forming an SEI coating film having specific capacitance on the negative electrode surface. SOLUTION: When a charging time temperature is lowered from the relationship between the charging time temperature and irreversible capacity, the irreversible capacity can be reduced. Therefore, it is effective to perform first time charge in a cooling atmosphere, that is, at a low temperature not more than 20 deg.C or perform charge by a low electric current not more than 1 C. The irreversible capacity sharply reduces when capacitance by an SET is not more than 0.4 mF/cm<2> from the relationship between the capacitance of the SET and the irreversible capacity. Then, an SEI coating film having capacitance not more than 0.4 mF/cm<2> is formed on a nagative electrode surface, a quantity of the SET formed on the negative electrode surface reduces, and the irreversible capacity is reduced, and since the generating SET is stable, a cycle characteristic can also be improved.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention
The present invention relates to an improvement in a method for manufacturing a secondary battery.

[0002]

2. Description of the Related Art SEI (Solid Electrolyte) is usually provided on the surface of a negative electrode of a lithium ion secondary battery.
(Interface) is formed. This SEI
Is mainly formed on the surface of the negative electrode during the first charging. Since this SEI is a lithium compound such as Li ₂ O, Li ₂ CO ₃ , and LiF, the lithium ions consumed here cannot contribute to the charge capacity, and the irreversible capacity at the time of the first charge, ie, the charge capacity and the discharge capacity. Will increase. This irreversible capacity increases as the amount of SEI formed increases. Therefore, it is desirable to minimize the amount of SEI formed on the negative electrode surface.

FIG. 5 shows a charge / discharge current (1
/ 3) The relationship between the number of times of charging and discharging and the irreversible capacity when charging and discharging are repeated at a constant current of C is shown. As shown in FIG. 5, the irreversible capacity is highest during the first charge / discharge. This is because, as described above, the amount of SEI formed on the negative electrode surface during the first charging is the largest.

[0004]

However, in the conventional method for manufacturing a lithium ion secondary battery, the first charging is performed at room temperature (20 to 25 ° C.). Also, in order to shorten the initial charging time, the charging current is also (1/3) C
It is often performed at about 1C. Here, 1C represents a current value that can be fully charged in one hour, that is, one hour, and (1
/ 3) C represents a current value that can be fully charged at a 1/3 hour rate, that is, 1/3 hour. As described above, when the initial charge is performed at normal temperature and at a high current value of 1 C or more, the amount of SEI formed on the negative electrode surface increases and the S
The EI becomes porous. For this reason, there is a problem that SEI is also formed at the time of the second and subsequent charging, and the irreversible capacity at the second and subsequent charging increases. In addition, there is also a problem that the thickness of the SEI is increased and the electric resistance of the negative electrode is increased. Further, in the case of SEI formed on the negative electrode surface when charged at normal temperature and at a high current value, lithium in the SEI is also discharged during discharging of the secondary battery, and SEI is decomposed to obtain stable SEI. Can not. For this reason, there is also a problem that SEI is formed on the surface of the negative electrode every time the battery is charged, and the cycle characteristics deteriorate.

The present invention has been made in view of the above-mentioned conventional problems, and has as its object the purpose of forming a stable SEI on the surface of a negative electrode, reducing the irreversible capacity, and improving the cycle characteristics. An object of the present invention is to provide a method for manufacturing a battery.

[0006]

In order to achieve the above-mentioned object, the present invention provides an initial charging method comprising: charging in a cooling atmosphere;
C is performed under at least one of low-current charging conditions of not more than C, and an SE having an electrostatic capacity of 0.4 mF / cm ² or less is formed on the negative electrode surface.
It is characterized by forming an I film.

[0007]

Embodiments of the present invention (hereinafter referred to as embodiments) will be described below.

The present inventors have reduced the amount of SEI formed on the surface of the negative electrode during the first charge,
Investigations were made to make the EI stable and to reduce the SEI formed on the negative electrode surface after the second cycle. As a result, it has been found that it is effective to perform the initial charging in a cooled atmosphere, that is, at a low temperature, or to charge the battery at a low current of 1 C or less.

The lithium ion secondary batteries used in the above study are as follows. A mixture of natural graphite: PVDF = 9: 1 and NMP as a solvent was used to prepare a paste, and then the paste was applied to a copper foil and dried.
A negative electrode of 8 mg / cm ² was obtained. After combining this negative electrode with a lithium foil as a counter electrode via a separator, PC: EC: DE containing 1 mol / l-LiBF ₄
It was immersed in an electrolytic solution of C = 2: 3: 5, and the charge / discharge characteristics of the negative electrode were examined.

FIG. 1 shows the relationship between the temperature during charging and the irreversible capacity. In FIG. 1, the charge / discharge current is (1 /
3) The total irreversible capacity when charging / discharging at a constant current of C, performing 2 cycles of charging and discharging at each temperature, and further charging and discharging at 20 ° C. for 3 cycles is shown on the vertical axis. FIG. 1 shows that the irreversible capacity can be reduced by lowering the temperature during charging.

FIG. 2 shows the relationship between the magnitude of the charging current and the irreversible capacity. In FIG. 2, the total irreversible capacity when the charging temperature is 20 ° C., the charging current in the first two cycles is changed, and then the charging and discharging in the three cycles with a constant current of (１ ／) C is vertical. Shown on the axis. FIG.
As can be seen from the graph, the lower the value of the charging current in the first two cycles, the lower the irreversible capacity. From the results shown in FIG. 2, it can be seen that it is desirable to charge with a low current of 1 C or less as the initial charging current.

Next, the temperature conditions at the time of the first charge were examined in further detail. FIG. 3 shows the relationship between the temperature condition of the first charge and the capacitance of the SEI formed on the negative electrode. It is considered that the more SEI formed on the surface of the negative electrode, the higher the capacitance thereof. The capacitance was measured by impedance measurement and determined by the Cole-Cole plot method.

FIG. 4 shows the relationship between the above-mentioned SEI capacitance and irreversible capacitance. As can be seen from FIG. 4, the irreversible capacitance is significantly reduced when the capacitance based on SEI is 0.4 mF / cm ² or less. FIG. 3 shows that this 0.4 mF / cm ² capacitance is the SEI capacitance generated when the temperature at the time of the first charge is set to 20 ° C. As described above, it is desirable that the initial charging be performed in a cooling atmosphere of 20 ° C. or less.

In order to confirm the effect of performing the initial charging at a low temperature, the charging current is set to (1/3) C and the discharging current is set to (1/3) C at a temperature of -20.degree. went. Thereafter, charge and discharge of (1/3) C at 20 ° C. is further performed for 3 times.
The irreversible capacity after the cycle was performed was 42 mAh / g, and a decrease in the irreversible capacity was confirmed. In addition, in order to investigate the stability of SEI formed on the negative electrode surface, a lithium ion secondary battery in which initial charging was performed at 20 ° C. and a charging current of (１ ／) C was used. ) 2 with charging current of C
After performing cycle charge and discharge, at a temperature of 20 ° C., (1/1 /
3) Using a lithium ion secondary battery according to the manufacturing method of the present invention in which charge / discharge was performed for 3 cycles with a charge / discharge current of C, these two types of lithium ion secondary batteries were left in a discharged state for 5 days. . Then, again at 20 ° C. (1/3) C
, And the irreversible capacity for a total of three cycles was measured. As a result, in the conventional example in which the first charging was performed at the above-mentioned temperature of 20 ° C. and the charging current of (１ ／) C, the irreversible capacity was 16 m.
Ah / g was 8 mAh / g for the one manufactured by the manufacturing method according to the present invention. As described above, the reason that the irreversible capacity of the lithium ion secondary battery manufactured by the manufacturing method according to the present invention is lower than that of the lithium ion secondary battery is considered to be due to the generation of stable SEI, and the cycle characteristics can be improved. all right.

As described above, when manufacturing a lithium ion secondary battery, a cooling atmosphere of 20 ° C. or less
It has been found that performing the initial charging at the following low current is effective for reducing the amount of SEI formed on the negative electrode surface and stabilizing the generated SEI.

[0016]

As described above, according to the present invention,
The amount of SEI formed on the negative electrode surface is reduced, the irreversible capacity can be reduced, and the generated SEI is stable, so that the cycle characteristics can be improved.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between a temperature at the time of first charging and an irreversible capacity.

FIG. 2 is a diagram showing a relationship between charging current and irreversible capacity at the time of initial charging.

Fig. 3 Temperature at first charging and SE generated at that time
FIG. 3 is a diagram illustrating a relationship between I and capacitance.

FIG. 4 is a diagram showing the relationship between the capacitance of SEI and the irreversible capacitance.

FIG. 5 is a diagram showing the relationship between the number of times of charge and discharge and the irreversible capacity in a lithium ion secondary battery.

Claims (1)

[Claims] 1. An initial charge is performed by charging in a cooling atmosphere and
C is performed under at least one of low-current charging conditions of not more than C, and an SE having an electrostatic capacity of 0.4 mF / cm ² or less is formed on the negative electrode surface.
A method for producing a lithium ion secondary battery, comprising forming an I film.