JPH05299122A - Charging method - Google Patents

Abstract

PURPOSE:To reduce self discharging of a lithium secondary battery using carbon as a negative electrode active material by carrying out charging at a temperature higher than the temperature at which the lithium secondary battery is used. CONSTITUTION:The operating temperature of a lithium secondary battery is the temperature at the time of using as well as of preservation, namely, 10 deg.-30 deg.C when normally used, and charging is carried out at no less than 60 deg.C when the temperature at the time of preservation reaches the maximum of 60 deg.C. The charging includes the one at the time of forming a negative electrode such as implanting or holding lithium ion in carbon, besides the one as battery charging. The self discharging ratio is thus reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for charging a lithium secondary battery in which carbon is used as a negative electrode active material.

[0002]

2. Description of the Related Art Due to its layered structure, carbon incorporates various atoms and molecules between layers to form an intercalation compound. Utilization of this property has been studied for application to various batteries, but in particular, research for utilizing it as a negative electrode of a lithium secondary battery has become active in recent years. In the lithium secondary battery, lithium is used for the negative electrode, but when lithium metal is used as it is, there is a problem that the cycle life of charging and discharging is short, and an alloy such as Li-Al may be used as the negative electrode. Many. However, even in this case, the cycle life is not long enough, and when lithium or a lithium alloy is used, the utilization efficiency of lithium must be kept low, and further, in the case of overdischarge, the positive electrode is not used. Therefore, there is a drawback in that the performance deteriorates because lithium invades beyond the limit at which the positive electrode can sufficiently maintain the cycle life.

A carbon electrode using a lithium intercalation compound of graphite can be used as a negative electrode-dominant battery electrode whose battery capacity is determined by the amount of lithium supported on the carbon electrode, and is used for the above-mentioned cycle life, over discharge, etc. Development is proceeding as an electrode that can solve the problem. This carbon electrode greatly changes its suitability as an electrode depending on the manufacturing method of carbon as an active material.
It has been found that the best electrode is produced by vapor-decomposing hydrocarbon such as benzene at about 1000 ° C.

[0004]

The characteristics of the lithium secondary battery include high energy density and low self-discharge as compared with the conventional secondary battery such as NiCad. With regard to self-discharge, it is possible to realize a very low self-discharge rate of 20% / month for Nicad and 1% / month for a lithium secondary battery. This is because when lithium metal or a lithium alloy is used as the negative electrode, lithium is excessively present as an active material. However, when a carbon electrode supporting only the minimum necessary lithium is used as the negative electrode, self-discharge of about 6% / month occurs at room temperature under the present circumstances. It is the only problem. The present invention is
It is an object of the present invention to reduce the self-discharge that occurs when a carbon electrode is used as the negative electrode of a lithium secondary battery.

[0005]

In order to achieve the above object, the present invention is to charge a lithium secondary battery using carbon as a negative electrode active material at a temperature higher than the operating temperature of the lithium secondary battery. There is provided a charging method characterized in that

The carbon in the present invention is carbon having a graphite structure, and lithium is inserted between the layers of the layered structure to form a so-called intercalation compound. When the lithium secondary battery is constructed using a non-aqueous organic solvent, it is preferable to use, as the carbon, one produced by a vapor phase pyrolysis method of hydrocarbons. The hydrocarbons mean hydrocarbons or their compounds.

The operating temperature of the lithium secondary battery refers to the temperature during use and storage, for example, the normal operating temperature is 10
When the storage temperature is 60 ° C. to 30 ° C. and the maximum temperature during storage reaches 60 ° C., charging is performed at 60 ° C. or higher.

The charging of the lithium secondary battery includes, in addition to charging as a so-called battery, charging during negative electrode formation, that is, insertion and loading of lithium ions into carbon.

As a charging method, before assembling the battery, for example, a method of carrying in a gas phase, in which lithium and a carbon electrode are sealed in a container kept at about 400 ° C. and kept for several days, or There are a method of soaking the lithium metal and the carbon electrode in the electrolytic solution and electrically shorting each other for a long time, a method of charging in the electrolytic solution at a predetermined current, and the like, but preferably a constant voltage in the electrolytic solution. , Or constant current charging is better. This method is also used when charging as a battery.

[0010]

As a result of investigating the characteristics of the above self-discharge of the carbon electrode, it was found that most of lithium in carbon, which was thought to be lost by the self-discharge, remained in carbon. A carbon electrode sufficiently supporting lithium has a voltage of 0 Vvs.
Although it has a potential of Li / Li +, the potential gradually increases as lithium is released by discharging the battery.
(See FIG. 1) The figure shows data at a current density of 0.5 mA / cm ² using a counter electrode Li sheet using LiClO ₄ 1M / lPC. The battery used depends on the structure of the battery, but is, for example, 0 to 0.7 Vvs. The potential region of Li / Li + is used. When the battery is charged and the carbon electrode is loaded with lithium, and then stored, and then discharged, the capacity decreases by about 6% after storage for one month at room temperature. The potential of the carbon electrode at this time has risen to 0.7 VvsLi / Li +, but when discharging is further performed to 1.2 VvsLi / Li +, compared to the case where the same is done for the electrode that has not been stored. About 4% extra electricity could be extracted.

That is, it was found that about 4% of about 6% of lithium considered to be lost has moved to the high potential region. Further, it was found that when the temperature of the electrode was raised to 60 ° C. and electric discharge was performed, the remaining amount of electricity corresponding to about 2% could be extracted. From the above, the cause of self-discharge, which was as large as about 6%, was due to insufficient charging.
There is an empty site that takes in lithium in carbon,
And the potential when lithium enters this part is 0.7
Vvs. It is considered that it is higher than Li / Li +, and further, lithium that has entered this portion is difficult to take out at room temperature.

Therefore, if this empty part is eliminated,
There is no lithium that migrates to this site during storage and no lithium that cannot be taken out. Therefore, even if the battery is not charged by raising the temperature, the self-discharge can be reduced to some extent by performing the charging for a sufficient time.

Further, it is considered that the migration of lithium to the vacant site becomes faster with the temperature, and the number of vacant sites that can be transferred further increases. Therefore, by charging at a temperature equal to or higher than the use temperature, it becomes possible to fill all empty sites that can be transferred at the use temperature with lithium. In addition, charging in an electrolytic solution increases the charging speed and increases the amount that can be charged at a constant current value.

It is considered that the same thing occurs when other alkali metals are used by inserting them into carbon, and this method is widely used for batteries utilizing the putting in and out of alkali metals with respect to carbon. It is thought to be possible.

[0015]

【Example】

Example 1 A carbon electrode as a negative electrode was manufactured as follows using a manufacturing apparatus having a structure in which a cylindrical quartz reaction tube was surrounded by an infrared heater.

First, a nickel plate having a size of 14 × 50 × 0.05 thickness (mm) is prepared, and this is placed as a substrate in the quartz reaction tube. Next, a mixed gas of propane and argon is supplied as a raw material gas into the quartz reaction tube, and the inside of the quartz reaction tube is heated by an infrared heater so that the substrate temperature becomes 1000 ° C. Propane and argon were mixed at a volume ratio of 1: 1. As a result, a carbon film having a thickness of 128 μm was also formed on the substrate.

The obtained carbon film was subjected to X-ray diffraction using CuKα ray, and was found to have graphite (00
The peak corresponding to the 2) plane was 26.4 °, and the half-width thereof was 0.58 °. From this, it was found that this carbon film had a graphite structure, the interplanar spacing in the C-axis direction was 3.372Å, and the crystallite size in the C-axis direction was 144Å. Moreover, when the argon laser Raman spectrum of the surface was measured, it was 1580 cm ⁻¹ and 1
Peak was measured 360 cm ^-1, the ratio of the peak intensity of 1360 cm ^-1 to the peak intensity of 1580 cm ^-1 was 0.9.

Without peeling off this carbon film from the nickel plate,
The working electrode 1 is used as it is, the counter electrode 2 is a lithium plate, and the reference electrode 3 is
In a propylene carbonate solution 4 containing 1 mol / l of lithium perchlorate in a three-pole method using lithium pieces as the anode, 0 V vs. Charging was performed until it became Li / Li + (see FIG. 2). Liquid temperature at this time is 80 ℃
Kept on. At this time, the amount of electricity required for charging was 41.0 mA.
It was h. Next, the system of this electrode (FIG. 2) was returned to room temperature, and discharge was repeated 5 times with a potential range of 0 to 0.7 Vv.s.Li/Li+. The amount of electricity discharged for the first time is 30.0 mA
h, and 27.0 mAh after the second time. The charge and discharge efficiency after the second time was 100%. When the electrode that had been charged for the fifth time was left as it was for 1 month and then discharged again, an electric quantity of 26.5 mAh could be extracted. From this, it was found that the self-discharge rate was 1.7% / month.

When an electrode manufactured by the same method is tested in exactly the same manner except that the liquid temperature is kept at room temperature by the first charge,
The self-discharge rate was 7.0% / month. In addition, the time required for charging was about 20 times that of the example.

An electrode manufactured in the same manner as in Example 1 was charged at room temperature with a current of 0.3 mA. The carbon film thickness at this time was 130 μm, and the amount of electricity charged was 37.0 m.
It was Ah. Next, this electrode was applied with 0 to 0.
Charge and discharge was performed 5 times with a potential range of 7 Vv.s.Li/Li+.
The discharge electricity amount of the first time was 30.1 mAh, the second and subsequent times were 27.4 mAh, and the charge / discharge efficiency was 99%. After completion of the fifth charge, the electrode was left as it was for one month and then discharged again, and an electric quantity of 26.1 mAh could be extracted. From this, it was found that the self-discharge rate was 4.8% / month. 7.0% when charged at initial 3mA
/ The self-discharge rate decreased compared to the month.

In this embodiment, charging was performed by setting the temperature of the electrolytic solution to 80 ° C. However, in the lithium secondary battery using the propylene carbonate-based electrolytic solution as in this example, the operating temperature and the electrolytic solution were used. Considering the nature of, it is preferable to charge at a temperature of about 60 to 80 ° C.

Example 2 An electrode manufactured under the same conditions as in Example 1 was charged under the same conditions as in Example 1. The carbon film thickness of the electrode at this time is 140
μm, and the amount of electricity charged was 45.1 mAh. After storing this electrode at 80 ° C for 1 day, the temperature of the electrode system (Fig. 2) was lowered to room temperature, and 0 to 0.7 Vv.s.
The discharge was repeated 5 times with a potential width of / Li +. The amount of electricity discharged for the first time is 31.9 mAh, and 2 for the second time and thereafter.
It was 9.7 mAh, and the charge / discharge efficiency was 100%.
When the electrode that had been charged for the fifth time was left as it was for 1 month and then discharged again, an electric quantity of 29.4 mAh could be extracted. From this, the self-discharge rate is 1.
It was found to be 0% / month, which was less than in Example 1.

Example 3 In Example 2, when the sample was stored at 80 ° C. for 1 day, but stored at room temperature for 1 day, the self-discharge rate was 1.4% / month, which was smaller than that in Example 1. Greater than Example 2.

Example 4 The same method as in Example 2, that is, after charging at 80 ° C., then at 80 ° C.
It was stored for 1 day at room temperature, and the electrode (charged at this point) that had been discharged 5 times at room temperature was used in a positive electrode of vanadium pentoxide. It was assembled as a battery. The electrolytic solution was propylene carbonate containing 1M LiClO ₄ , and stainless steel was used for the container. After over-discharging this battery, 3mA at 80 ℃
It was charged at a current of 1 and then stored at room temperature for 1 month. The self-discharge rate after this was measured and found to be 1.0% / month.
By the way, when the battery was charged with a current of 3 mA at room temperature after over-discharging, a self-discharge rate of 10% / month was measured. From these, it was found that the self-discharge rate can be suppressed to a low level by charging the battery at a high temperature even when it is overdischarged after being assembled as a battery.

Example 5 A charger having a charging / discharging function and a battery heating function was prepared. It has been found that the use of this charger as a charger for a lithium battery using a carbon electrode as the negative electrode has the following advantages. First, the overcharged lithium secondary battery is charged with a normal charger that does not have a heating capacity, and then left for 1 month to have a capacity of 8
%, But when the temperature was raised to 80 ° C. using the charger and charging was performed, the battery capacity decreased only by 1.4% even if left for one month. Further, when the charged lithium secondary battery was used after being stored for half a year, the battery capacity was decreased by 7.0%. When this battery was used immediately after being charged by a normal charger, the same discharge capacity as the charged capacity was obtained, but the capacity remained 7.0% lower than the initial capacity. .. This battery is 80
When the battery was warmed to ℃, discharged, and subsequently charged, the battery capacity returned to that before storage. Thus, it was found that the charger is a very advantageous charger for a lithium secondary battery having a carbon electrode as a negative electrode.

[0026]

By using the charging method according to the present invention, it was possible to reduce the self-discharge rate of the carbon electrode, which was conventionally the minimum self-discharge rate of 6% / month, to 1% / month.

[Brief description of drawings]

FIG. 1 is a diagram showing a discharge curve of a carbon electrode.

FIG. 2 is an explanatory diagram of a carbon electrode charging / discharging device.

[Explanation of symbols]

 1 Working electrode 2 Counter electrode 3 Reference electrode 4 Propylene carbonate solution

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shigeo Nakajima 22-22 Nagaike-cho, Abeno-ku, Osaka-shi, Osaka Within Sharp Corporation (72) Inventor Hiroshi Wada 22-22 Nagaike-cho, Abeno-ku, Osaka-shi, Osaka Within the corporation

Claims (2)

[Claims] 1. A charging method for a lithium secondary battery in which carbon is used as a negative electrode active material, wherein the charging is performed at a temperature higher than the operating temperature of the lithium secondary battery. 2. The carbon is produced by a vapor phase pyrolysis method of hydrocarbons.
Charging method.