JP5052145B2 - Method for producing lithium ion secondary battery - Google Patents

Description

The present invention relates to a method for manufacturing a lithium ion secondary battery. In particular, a method for manufacturing a lithium ion secondary battery using a carbon material in the positive electrode.

Patent Document 1 discloses a lithium ion secondary battery having a positive electrode including a carbon material, a positive electrode active material, and a conductive material, a negative electrode, and an electrolytic solution. In order to increase the output of the lithium ion secondary battery, it is known that it is advantageous to increase the electric double layer capacity. In order to increase the electric double layer capacity, it is also known that it is advantageous to increase the specific surface area of the carbon material used for the positive electrode. Therefore, it is common technical knowledge that in a lithium ion secondary battery, it is advantageous to use a carbon material having a large specific surface area in order to increase the output. In Patent Document 1, by using a carbon material having a specific surface area of 500 m ² / g or more (preferably described as 1500 m ² / g or more), a large electric double layer capacity is obtained to increase output. ing.

JP 2002-260634 A

However, studies by the present inventors have revealed that the use of a carbon material having a large specific surface area has another adverse effect.
If water is mixed in the electrolyte solution of the lithium ion secondary battery, the electrolyte solution and water react to reduce the battery capacity. A carbon material having a large specific surface area has many pores. The pores are easy to adsorb water. That is, when a carbon material having a large specific surface area is used, water is mixed in the lithium ion secondary battery, and the battery capacity of the lithium ion secondary battery is reduced. Further, when the electrolyte enters the pores, the electrolyte may be decomposed inside the pores.
When a carbon material having a large specific surface area is used as a positive electrode material for a lithium ion secondary battery, good output characteristics can be obtained. However, when observed over a long period of time, the battery capacity is greatly reduced.
In the present invention aims to provide that the method of manufacturing a lithium ion secondary batteries simultaneously satisfy both of maintaining the battery capacity for a long time with good output characteristics.

A lithium ion secondary battery disclosed in this specification includes a positive electrode having a carbon material having a specific surface area of 30 to 500 m ² / g and an interlayer distance of 002 plane of 0.35 to 0.38 nm, a negative electrode, An electrolyte containing at least a lithium compound is provided, and anions generated by ionization of the lithium compound are trapped between the layers of the carbon material. As used herein, “interlayer distance between 002 planes” refers to the surface spacing of the 002 planes measured by X-ray diffraction.

According to the above lithium ion secondary battery, it is possible to simultaneously achieve both good output characteristics and maintaining battery capacity over a long period of time. That is, since a carbon material having a specific surface area of 30 to 500 m ² / g is used for the positive electrode, it is possible to prevent water from being mixed into the lithium ion secondary battery. Further, the lithium compound is ionized in the electrolytic solution. When the interlayer distance of the 002 plane of the carbon material is within the range of 0.35 to 0.38 nm, the anion is trapped between the layers in such a manner as to expand the interlayer of the carbon material. That is, the specific surface area of the carbon material can be increased by trapping the anion between the layers of the carbon material. The battery capacity of the lithium ion secondary battery can be increased. In addition, when the specific surface area of a carbon material is smaller than 30 m < ² > / g, an anion cannot be capture | acquired favorably. If the specific surface area of the carbon material is larger than 500 m ² / g, water is likely to be mixed into the lithium ion secondary battery.

In the lithium ion secondary battery disclosed in this specification , the lithium compound is preferably LiPF ₆ .
According to the above lithium ion secondary battery, LiPF ₆ is ionized into Li ⁺ (cation) and PF ₆ ⁻ (anion) in the electrolytic solution. The anion (PF ₆ ⁻ ) generated by ionization tends to be trapped between the layers in such a manner as to expand the layers of the carbon material having an interplanar distance of 0.35 to 0.38 nm.

The lithium ion secondary battery disclosed in this specification has favorable output characteristics and can maintain the battery capacity over a long period of time, and thus can be used for various applications. For example, it can be used for automobiles such as electric cars and hybrid cars.

The present invention is, that Kyosu Hisage a method for producing a lithium ion secondary battery. The manufacturing method includes a positive electrode having a carbon material having a specific surface area of 30 to 500 m ² / g and an interlayer distance of 002 plane of 0.35 to 0.38 nm, a negative electrode, and an electrolytic solution containing at least a lithium compound. It has the process of accommodating in a container and forming a battery structure, and the process of capture | acquiring between the layers of the carbon material which forms the positive electrode the anion which the lithium compound ionized and ionized.

According to the above manufacturing method, a lithium ion secondary battery having a large battery capacity and capable of maintaining the battery capacity over a long period of time can be manufactured. Since a carbon material having a small specific surface area (30 to 500 m ² / g) is used, it is difficult for water to be mixed into the battery structure in the step of forming the battery structure. That is, even when a lithium ion secondary battery is used for a long period of time, the phenomenon that the electrolytic solution is decomposed can be suppressed. In addition, the specific surface area of the carbon material forming the positive electrode while remarkably suppressing water mixed in the lithium ion secondary battery in order to capture the anion between the carbon material layers while the battery structure is formed. Can be increased.

In the production method of the present invention, during the first charge, the battery structure is charged with a voltage equal to or higher than the operating voltage of the lithium ion secondary battery, and the anion generated by the ionization of the lithium compound is trapped between the carbon material layers. The

  According to the above production method, the anion generated by the ionization of the lithium compound can be reliably captured between the layers of the carbon material. It is difficult to determine whether anions are trapped between carbon material layers. However, by measuring the current flowing through the battery structure when a voltage is applied, it can be confirmed electrochemically that anions have been trapped between the carbon material layers.

In the production method of the present invention shall be the 4.2~4.4V the potential at the time of the initial charge of the lithium metal. Here, “potential with respect to lithium metal” means a potential difference between the positive electrode and the lithium metal. In this specification, the potential difference between the positive electrode and the lithium metal is referred to as “potential with respect to the lithium metal”, and the potential difference between the positive electrode and the negative electrode is simply referred to as “voltage”. When the material constituting the positive electrode and / or the material constituting the negative electrode changes, the potential difference between the positive electrode and the negative electrode changes. That is, even if the same positive electrode is used, when the material constituting the negative electrode changes, the voltage between the positive electrode and the negative electrode changes. The characteristics of the lithium ion secondary battery characterized by the positive electrode cannot be expressed in terms of voltage. Therefore, in this specification, in order to accurately express the characteristics of the lithium ion secondary battery, “potential with respect to lithium metal” and “voltage” are expressed separately.

  When an excessively high voltage is applied to the battery structure, anions generated by ionization of the lithium compound are not only trapped between the layers of the carbon material, but the carbon material may be decomposed. In addition, the electrolytic solution may be decomposed by a high voltage. By setting the voltage at the time of the first charge to the above range, it is possible to suppress the decomposition of the carbon material and the decomposition of the electrolytic solution and increase the battery capacity of the lithium ion secondary battery.

  According to the present invention, the battery capacity of a lithium ion secondary battery can be increased. In addition, the battery capacity can be maintained over a long period of time.

The main features of the examples are listed.
(First feature)
Easily graphitized carbon (coke, mesocarbon spherules, mesophase pitch-based carbon fiber, pyrolytic vapor-grown carbon fiber, etc.) is fired at 600-1400 ° C. and activated with potassium hydroxide. Produce carbon materials to be used.
(Second feature)
A solvent having a ratio of ethylene carbonate (EC) to ethyl methyl carbonate (EMC) of 3: 7 and a solute of lithium hexafluorophosphate (LiPF ₆ ) (also referred to as a supporting salt) is mixed to produce an electrolytic solution.

(Evaluation of carbon materials)
Experiments were conducted on conditions for capturing anions generated by ionization of lithium compounds between carbon material layers having a specific surface area of 400 m ² / g and a 002 plane interlayer distance of 0.36 nm.
A positive electrode made of the carbon material and a negative electrode made of lithium metal were immersed in an electrolytic solution in which a solvent having EC and EMC of 3: 7 and a support salt of LiPF ₆ were mixed. A voltage was applied between the positive electrode and the negative electrode, and the magnitude of the current flowing between the two electrodes was measured. The relationship between the applied voltage and the measured current is shown in FIG. The vertical axis in FIG. 1 indicates the value of current flowing between both electrodes per 1 g of carbon material (unit: μA / g), and the horizontal axis indicates the voltage applied between both electrodes (unit: V). Yes. Here, since lithium metal is used for the negative electrode, “potential with respect to lithium metal” and “voltage” are the same. In addition, the vertical axis | shaft of FIG. 1 has shown the value which pulled the magnitude | size of the electric current which flows when the lithium ion secondary battery manufactured using the said carbon material is normally used. That is, it shows the magnitude of the current that flows in excess of the current that flows during normal use (hereinafter referred to as the activation current).

As is clear from FIG. 1, it can be confirmed that the activation current starts to flow when the voltage between both electrodes exceeds 4.1V. From the fact that the activation current flows, it can be inferred that the structure of the carbon material of the positive electrode has changed compared to before applying a voltage exceeding 4.1 V between both electrodes. The magnitude of the activation current has a first peak when the voltage is around 4.4V. A substantially constant current value is maintained when the voltage is between 4.4 and 4.5V. When the voltage exceeds 4.5V, the current value begins to increase again, and a second peak is observed around 4.6V.
Up to a voltage of about 4.5 V, it is considered that an activation current flows due to the anion (PF ₆ ⁻ ) generated by ionization of LiPF ₆ being trapped between the layers of the carbon material. When the voltage exceeds 4.5 V, it is considered that not only an anion is trapped between the layers of the carbon material, but also a current flows due to the collapse of the structure of the carbon material. That is, it is estimated that an anion can be trapped between the layers of the carbon material by applying a voltage of 4.1 to 4.5 V to the lithium metal between both electrodes.

Examples will be described below. In the embodiment, a battery structure having a working voltage of 4.2 V with respect to lithium metal was prepared, and a voltage of 4.2 to 4.5 V was applied to the battery structure with respect to lithium metal. A secondary battery was manufactured.
Example 1
The manufacturing method of the lithium ion secondary battery of a present Example is demonstrated. First, the manufacturing method of a positive electrode is demonstrated.
18% by mass of carbon material, 80% by mass of lithium nickelate (LiNiO ₂ ), 1% by mass of polytetrafluoroethylene (PTFE), and 1% by mass of carboxymethylcellulose (CMC) are weighed and mixed, and water is added to the mixture. Was added to make a paste. Here, a carbon material having a specific surface area of 400 m ² / g and an interlayer distance of 002 plane of 0.36 nm was used as the carbon material.
Next, an aluminum current collector was prepared, and a paste was applied to both sides of the current collector, and then the paste was dried to complete a positive electrode.
Next, the manufacturing method of a negative electrode is demonstrated.
Powdered graphite and a binder were mixed to form a paste. Next, a copper current collector was prepared, and a paste was applied to both sides of the current collector, and then the paste was dried to complete a negative electrode.
The positive electrode and the negative electrode were opposed to each other through the porous resin film to complete the electrode body. Next, after the electrode body and the electrolytic solution were accommodated in a container, the container was sealed to form a battery structure. As the solvent of the electrolytic solution, a solvent having a volume ratio of EC and EMC of 3: 7 was used, and as the supporting salt of the electrolytic solution, LiPF ₆ was mixed at 1 mol / L with respect to the solvent.

  The battery was charged at a constant current until the voltage of the battery structure was 4.1 V (4.2 V with respect to lithium metal), and then charged at a constant voltage of 4.1 V for 2.5 hours. Thereafter, the voltage was set to 4.3 V (4.4 V with respect to lithium metal), and constant voltage charging was performed at a voltage of 4.3 V for 15 minutes to complete a lithium ion secondary battery.

(Example 2)
The battery structure of Example 1 was formed, and a voltage was applied to the battery structure. The present embodiment is different from the first embodiment only in the method of applying a voltage to the battery structure. In this example, the battery structure was charged at a constant current until the voltage of the battery structure reached 4.1 V (4.2 V with respect to lithium metal), and then charged at a constant voltage of 4.1 V for 2.5 hours. Thereafter, the voltage was set to 4.2 V (4.3 V with respect to lithium metal), and constant voltage charging was performed at a voltage of 4.2 V for 15 minutes to complete a lithium ion secondary battery.

(Example 3)
The battery structure of Example 1 was formed, and a voltage was applied to the battery structure. The present embodiment is different from the first embodiment only in the method of applying a voltage to the battery structure. In this example, the battery structure was charged at a constant current until the voltage of the battery structure reached 4.1 V (4.2 V with respect to lithium metal), and then charged at a voltage of 4.1 V for 2.5 hours at a constant voltage. A secondary battery was completed.

(Comparative Example 1)
Weigh and mix 18% by mass of acetylene black having a specific surface area of 400 m ² / g and an interlayer distance of 002 plane of 0.33 nm, 80% by mass of LiNiO ₂ , 1% by mass of PTFE, and 1% by mass of CMC, Water was added to the mixture to make a paste. Next, an aluminum current collector was prepared, and a paste was applied to both sides of the current collector, and then the paste was dried to complete a positive electrode. Subsequent steps were performed in the same manner as in Example 1 to form a battery structure of this comparative example.
The battery is charged at a constant current until the voltage of the battery structure reaches 4.1V (4.2V with respect to lithium metal), and then charged at a constant voltage of 4.1V for 2.5 hours to complete a lithium ion secondary battery. I let you.

(Comparative Example 2)
The battery structure of Example 1 was formed, and a voltage was applied to the battery structure. This comparative example differs from Example 1 only in the method of applying a voltage to the battery structure. In this comparative example, the battery structure was charged at a constant current until the voltage of the battery structure reached 4.1 V (4.2 V with respect to lithium metal), and then charged at a constant voltage of 4.1 V for 2.5 hours. Thereafter, the voltage was set to 4.4 V (4.5 V relative to lithium metal), and constant voltage charging was performed at a voltage of 4.4 V for 15 minutes to complete a lithium ion secondary battery.

(Discharge test)
About the lithium ion secondary battery of Examples 1-3 and Comparative Examples 1 and 2, the discharge test was implemented. In the discharge test, each lithium ion secondary battery was charged with a constant current until it reached 4.1 V, and then charged with a constant voltage at a voltage of 4.1 V for 2.5 hours. Next, the battery was discharged at a constant current until the voltage reached 3.0V. The capacity at the time of discharge was measured to obtain the discharge capacity. The results are shown in Table 1.

As is clear from Table 1, the lithium ion secondary batteries of Examples 1, 2, and 3 have a larger discharge capacity than the lithium ion secondary battery of Comparative Example 1 (good output characteristics are obtained). I understand). That is, by adding a carbon material having a specific surface area of 400 m ² / g and a d002 plane spacing of 0.35 to 0.38 nm to the positive electrode, anions are trapped in the positive electrode carbon material. It is shown that the charge is suppressed from being consumed by a film (also referred to as SEI) formed on the surface of the negative electrode during the first charge, and the irreversible capacity of the battery is reduced. In particular, the battery capacity (discharge capacity) of the lithium ion secondary battery is increased by charging at a voltage equal to or higher than the voltage (4.2 V) at which the lithium ion secondary battery is used at the first charge. That is, the anion (PF ₆ ⁻ ) generated by the ionization of the lithium compound (LiPF ₆ ) is trapped between the layers of the carbon material, thereby increasing the battery capacity of the lithium ion secondary battery. . As shown in Comparative Example 2, when a voltage of 4.4 V (4.5 V with respect to lithium metal) is applied to the battery structure during the first charge, the battery capacity of the lithium ion secondary battery is reduced. End up. When a voltage of 4.4 V is applied to the battery structure, more anions are trapped between the layers of the carbon material, which indicates that the structure of the carbon material collapses as described above.

(Output test)
For the lithium ion secondary batteries of Examples 1, 2, 3 and Comparative Examples 1, 2, a short output test was performed. In the output test, each lithium ion secondary battery was charged at a constant current in an ambient temperature (25 ° C.) atmosphere to 4.1 V, and then charged at a constant voltage of 4.1 V for 2.5 hours. Subsequently, the maximum output (W) capable of discharging for 2 seconds in an atmosphere of −30 ° C. was measured. The larger the output, the better the output characteristics of the lithium ion secondary battery in a low temperature environment. In other words, the higher the output, the more the lithium ion secondary battery has power in a low temperature environment.
As shown in Examples 1, 2, and 3, an output value larger than that of Comparative Example 1 is obtained by applying a voltage of 4.2 to 4.4 V to lithium metal in the battery structure. . That is, it can be seen that the output of the lithium ion secondary battery at a low temperature is increased. As shown in Comparative Example 2, when a voltage of 4.4 V (4.5 V with respect to lithium metal) is applied during the first charge, the output characteristics of the lithium ion secondary battery are reduced. This indicates that when a voltage of 4.4 V is applied to the battery structure, more anions are trapped between the layers of the carbon material, and the structure of the carbon material collapses.

From the results of the discharge capacity test and the output test, the voltage at which the lithium ion secondary battery is used is 4.2 V with respect to the lithium metal, and the voltage at the first charge is 4.2 to 4.4 with respect to the lithium metal. By being 4V, both having a large battery capacity and maintaining the battery capacity over a long period of time can be satisfied simultaneously. Furthermore, a lithium ion secondary battery having power in a low temperature environment can be obtained.
In particular, the voltage at the first charge is preferably 4.3 to 4.4 V with respect to lithium metal. As shown in Example 1 (application of a voltage of 4.4 V to lithium metal) and Example 2 (application of a voltage of 4.3 V to lithium metal), the discharge capacity of the lithium ion secondary battery And a lithium ion secondary battery with significantly improved output characteristics can be manufactured.

Specific examples of the present invention have been described in detail above, but these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes of the specific examples illustrated above.
In the above embodiment, the supporting salt of an electrolyte solution is shown for the case of LiPF _6. However, LiClO ₄ , LiBF ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) _2, etc. can be used as the supporting salt in addition to LiPF ₆ . That is, the supporting salt may be selected so that the electrolytic solution contains a lithium compound.
Moreover, in the said Example, it has shown about the case where the ratio of EC and EMC is 3: 7 as the solvent of electrolyte solution. However, the solvent is not limited to the above examples. For example, propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), etc. can be used.
The technical elements described in this specification or the drawings exhibit technical usefulness alone or in various combinations, and are not limited to the combinations described in the claims at the time of filing. In addition, the technology exemplified in this specification or the drawings can achieve a plurality of objects at the same time, and has technical usefulness by achieving one of the objects.

The current-voltage relationship between a carbon material and a metal electrode is shown.

Claims (1)

A method for producing an ion secondary battery having a working voltage of 3.0 to 4.2 V with respect to lithium metal,
An electrolytic solution containing a carbon material having a specific surface area of 30 to 500 m ² / g and a 002 plane interlayer distance of 0.35 to 0.38 nm, a negative electrode, and at least a lithium compound is contained in a container. Forming a battery structure;
Capturing the anion generated by ionization of the lithium compound between the layers of the carbon material forming the positive electrode ,
The first time charged by charging a battery structure at a voltage of 4.2~4.4V versus lithium metal, a lithium ion secondary you characterized by capturing the anion between layers of the carbon material Battery manufacturing method.

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