JPH01294375A - Charging/discharging method for lithium secondary battery - Google Patents

Abstract

PURPOSE:To minimize capacity deterioration of a lithium secondary battery when charged/discharged by charging/discharging the lithium secondary battery in which a specified Li compound is employed as positive active material at a specified upper or lower limit voltages by voltage conversion to Li electrodes. CONSTITUTION:Employed as a negative electrode of a lithium secondary battery is lithium or lithium alloy and as its positive active material is lithium- manganese oxide expressed by Li1-xMnO2 (x is 0-1 and 1-x is expressed such that LiMnO2 is composed, from which Li is electrochemically taken out.). The lithium secondary battery is charged/discharged by voltage conversion to Li electrodes at a upper limit voltage of lower than 4.1V and a lower limit voltage of higher than 1.5V. Extreme oxidation reaction of electrolyte by Li1-xMnO2 is, thus, restricted by the upper limit voltage and its deoxidation is restricted by the lower limit voltage or crystal construction variation of the positive active material is not occurred. Accordingly, capacity deterioration of the lithium secondary battery due to charging/discharging is minimized.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to a method for charging and discharging a lithium secondary battery.

[Conventional technology]

Conventionally, titanium disulfide (TiSz) and molybdenum disulfide (Most) have been used as positive electrode active materials for lithium secondary batteries.
Metal sulfides such as [for example, 25th Battery Symposium Abstracts, P296 (1984)] were used.

However, with these metal sulfide-based positive electrode active materials, the battery voltage is only about 2.5 V even at its highest, and from the viewpoint of obtaining a battery with high energy density, there is a problem that the battery voltage is low.

Therefore, in order to obtain a battery with higher energy density, the present inventors investigated LiMn0. We considered using it as a positive electrode active material and succeeded in increasing the battery voltage to about 4V at its highest.

[Problem to be solved by the invention]

However, it has been found that when the above-mentioned lithium secondary battery is charged and discharged without considering the voltage range, the capacity may significantly deteriorate due to charging and discharging.

The present invention provides LiMn0. The purpose of the present invention is to solve the problem that a lithium secondary battery using lithium oxide as a positive electrode active material causes large capacity deterioration due to charging and discharging, and to provide a charging and discharging method that causes less capacity deterioration.

[Means to solve the problem]

As a result of repeated studies on the method of charging and discharging the lithium secondary battery, the present inventor discovered that capacity deterioration due to charging and discharging can be reduced by charging and discharging within a specific voltage range, and has completed the present invention. It arrived.

That is, the present invention uses lithium or a lithium alloy for the negative electrode, and uses the formula (11% formula% (where χ is 0 to 1, and 1 - A lithium secondary battery using a lithium-manganese oxide as a positive electrode active material (indicates that the lithium-manganese oxide is used after electrochemically removing the The present invention relates to a method for charging and discharging a lithium secondary battery, which is characterized by charging and discharging at a voltage of .5V or higher.

As mentioned above, the upper limit voltage is 4.
The reason why capacity deterioration due to charging and discharging is suppressed by charging and discharging in the range of 1V or less and the lower limit voltage of 1.5cm or more is considered to be as follows.

The main causes of deterioration accompanying charging and discharging of lithium secondary batteries are thought to be due to one of the following.

■Reaction between the positive electrode active material and the organic electrolyte used in the battery ■Structural change in the positive electrode active material In the case of ■ above, the positive electrode active material changes to another substance due to the reaction, or the electrolyte changes onto the positive electrode active material. It is thought that the properties may deteriorate due to decomposition or polymerization.

In the case of (2) above, the crystal structure of the positive electrode active material changes, resulting in a crystal structure in which Li can no longer enter or exit, and the characteristics are considered to deteriorate.

Therefore, in the present invention, the upper limit voltage (charging end voltage) is set to 4.
By setting the voltage to 1v or less, L I l−Z M n
Li in the positive electrode active material can be suppressed by suppressing the significant oxidation reaction of the electrolyte caused by 02 or preventing changes in the crystal structure of the positive electrode active material, and by setting the lower limit voltage (discharge cutoff voltage) to 1.5 V or higher. This is to suppress the significant reduction reaction of the electrolyte caused by the electrolyte, or to prevent changes in the crystal structure of the positive electrode active material.

In other words, if the upper limit voltage becomes too high, the organic solvent with a cyclic structure such as propylene carbonate used as the electrolyte solvent will be oxidized and decomposed by the positive electrode active material, or polymerization will occur, or Li will crystallize. Li is removed from the structure and the positive electrode active material returns to its original state.
It is thought that the crystal structure changes to a different crystal structure from the structure of Mn0z. In addition, if the lower limit voltage becomes too low, too much Li enters the crystal structure of the positive electrode active material, and the Li reacts with propylene carbonate etc., resulting in decomposition of the propylene carbonate, or too much Li enters the crystal structure. As a result, the crystal structure is destroyed and LizO
It is thought that this results in the formation of Mn or other manganese oxides.

Therefore, in the present invention, we searched for the upper limit voltage at which the oxidation reaction of the electrolytic solution and the crystal structure of the positive electrode active material do not change.
Furthermore, by finding and specifying the lower limit voltage at which the reduction reaction of the electrolytic solution and the crystal structure of the positive electrode active material do not change, we have provided a charging/discharging method that prevents capacity deterioration due to discharge as much as possible. be.

In the lithium secondary battery that is the object of the present invention, lithium or a lithium alloy is used for the negative electrode. Lithium alloys used for such purposes include, for example, lithium-aluminum alloy, lithium-tin alloy, and lithium-zinc alloy. alloy, lithium-lead alloy, lithium-bismuth alloy, lithium-silicon alloy, lithium-antimony alloy, lithium-magnesium alloy, lithium-indium alloy, lithium-gallium alloy, lithium-germanium alloy, lithium-gallium-indium alloy, etc. can give. Furthermore, a lithium alloy containing a small amount of other metals can be used for the negative electrode.

In the present invention, the upper limit voltage (end-of-charge voltage) during charging and discharging is 4.1 or less, and the lower limit voltage (end-of-discharge voltage) is 1.5.
This is a voltage conversion value for the Li electrode, and if lithium (Li) is used for the negative electrode, it can be left as is, but if a lithium alloy is used for the negative electrode, the voltage between the lithium alloy and lithium is It is necessary to subtract the potential difference from the upper and lower voltage limits, respectively. for example,
Lithium-aluminum alloy [lithium 10-4
0 atomic% (atomic%), aluminum 90-60
When using atomic%], the potential of this lithium-aluminum alloy has a potential difference of 0.4 V with respect to lithium, so the above potential difference is subtracted from the upper limit voltage of 4.1 cm converted to the voltage of the Li electrode, and the lower limit voltage of 1 Since the above potential difference is subtracted from .5V, the upper limit voltage during charging and discharging is 3.7V or less,
The lower limit voltage will be set to 1.1■ or more.

Furthermore, as the electrolyte for the lithium secondary battery to which the present invention is directed, those normally used for this type of battery can be used as they are. Examples of electrolytes include 1,
Organic solvents such as 2-dimethoxyethane, 1,2-jethoxyethane, ethylene carbonate, propylene carbonate, γ-butyrolactone, tetrahydrofuran, 1,3-dioxolane, 4-methyl-1,3-dioxolane, etc. alone or in combination of two or more In the solvent, for example, LiCl
0. , LiPFb , LiAsF6, LiBF4, Li
Examples include those prepared by dissolving one or more electrolytes such as B(C6H5)4.

〔Example〕

Next, the present invention will be explained in more detail with reference to Examples.

Example 1 First, LiMn0z was synthesized. The synthesis method is as follows.

LixO and M nz O) were mixed in equivalent amounts, and this mixture was heated to Ar with a small amount of H2 added at A r / Hz (95/
5) LiMn by heating at 550°C in
0. was synthesized.

LiMnO2 synthesized as described above was used as a positive electrode active material, and 10% by weight of a conductive additive was added thereto.
After adding and mixing scaly graphite and 7% by weight polytetrafluoroethylene as a binder, the mixture was
g weighed out and pressure molded at 3 t/al to give a diameter of 9
+aa+, a disc-shaped molded body with a thickness of about 0.3 mm was produced. A battery (model cell) shown in FIG. 1 was produced using the obtained molded body as a positive electrode.

In FIG. 1, part A shows only the essential parts of the battery. In the figure, 1 is a negative electrode. This negative electrode 1 is made by punching a foil-shaped lithium into a circle with a diameter of 13IIIM and attaching it to a nickel wire. It is crimped.

2 is a positive electrode, and this positive electrode 2 is made of a press-molded body in which the LiMnOx synthesized as described above is used as a positive electrode active material, and flaky graphite and polytetrafluoroethylene are added thereto. It is.

3 is a mixed solvent of propylene carbonate and 1,2-dimethoxyethane with a volume ratio of 1:1 and 0.9 m of LiPFb.
ol/j2 is an electrolytic solution dissolved therein, and 4 is a separator made of polypropylene nonwoven fabric and microporous polypropylene film. 5 is a reference electrode made of a press-molded body containing Lia, +VzOs as an active material, 6 is a polypropylene container, 7 is a current collector made of a platinum expanded band network spot-welded with platinum lead wires, 8 is a nickel lead wire. The theoretical amount of electricity of the positive electrode of this battery is the charging/discharging region Li1-χMnO□ (0≦χ≦1)
15.8mAh, the theoretical amount of electricity of the negative electrode is about 80m
Ah or more, and the amount of electricity at the negative electrode is set to be more than the amount of electricity at the positive electrode.

Next, charge and discharge this battery at a current value of 0.0636 mA.
(0.1 mA/c+fl per unit cross-sectional area of positive electrode) upper limit voltage (end of charge voltage) 4. IV, the lower limit voltage (discharge end voltage) was 1.5V.

Example 2 A lithium secondary battery similar to the battery in Example 1 was charged and discharged under the same conditions as in Example 1 except that the upper limit voltage was set to 3.8 .

Example 3 A lithium secondary battery similar to the battery in Example 1 was charged and discharged under the same conditions as in Example I except that the upper limit voltage was set to 3.5V.

Comparative Example 1 A lithium secondary battery similar to the battery in Example 1 was charged and discharged under the same conditions as in Example 1 except that the upper limit voltage was set to 4.4V.

Example 4 A lithium secondary battery similar to the battery in Example 1 was charged and discharged under the same conditions as in Example 1 except that the lower limit voltage (discharge end voltage) was set to 1.8V.

Comparative Example 2 A lithium secondary battery similar to the battery in Example 1 was charged and discharged under the same conditions as in Example 1, except that the lower limit voltage was set to 1.2V.

Table 1 shows the deterioration rates of battery capacity due to charging and discharging using the charging and discharging methods of Examples 1 to 4 and Comparative Examples 1 and 2. The above deterioration rate is compared to the capacity (C+) at the first charge/discharge cycle, and the capacity after charge/discharge, here, the 20th charge/discharge cycle (C+).
This indicates how much the capacitance (Cgo) has deteriorated, and is calculated from the following equation.

In Table 1, in order to show the capacity deterioration rate at the 20th cycle, first, Examples 1 to 3 and Comparative Example 1, in which the lower limit voltage was fixed at 1.5 V and the upper limit voltage was varied, were compared with the upper limit voltage. Display the upper limit voltage in descending order, and then set the upper limit voltage to 4.
Example 1, Example 4, and Comparative Example 2 in which the lower limit voltage was fixed at 1■ and varied were displayed in order from the one with the highest lower limit voltage.

Table 1 Referring to the results shown in Table 1 above, it can be seen from the capacity deterioration rates of Comparative Example 1 and Examples 1 to 3, in which the lower limit voltage was fixed at 1.5 V and the upper limit voltage was varied. In addition, significant capacity deterioration due to charging and discharging does not occur when the upper limit voltage is set to 4.
This is when charging and discharging was performed with the voltage set to 1V or less.
゛Also, as can be seen from the capacity deterioration rates of Example 1, Example 4, and Comparative Example 2, in which the upper limit voltage was fixed at 4.1 V and the lower limit voltage was varied, there was no significant capacity deterioration due to charging and discharging. This is when charging and discharging is performed with the lower limit voltage set to 1.5V or higher.

As described above, by charging and discharging with the upper limit voltage set to 4.1 V or lower and the lower limit voltage set to 1.5 V or higher, capacity deterioration due to charging and discharging can be suppressed.

In addition, when investigating capacity deterioration due to charging and discharging, we used a model cell because in mounted batteries, the influence of battery components such as the negative electrode appears, making it difficult to accurately identify differences in capacity deterioration due to differences in charging and discharging conditions. Because it will be.

〔Effect of the invention〕

As explained above, in the present invention, L is used as the positive electrode active material.
By charging and discharging a lithium secondary battery using i1-χMnO□ at an upper limit voltage of 4.1v or less and a lower limit voltage of 1.5v or more in terms of voltage with respect to Li electrodes, capacity deterioration due to charging and discharging is reduced. I was able to do that.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view schematically showing an example of a lithium secondary battery according to the present invention. 1...Negative electrode, 2...Positive electrode

Claims (1)

[Claims] (1) Lithium or lithium alloy is used as the negative electrode, and the formula (
I) Li_1_-_xMnO_2(I) (In the formula, x is 0 to 1, and 1-x indicates that LiMnO_2 is first synthesized and Li is electrochemically removed from the LiMnO_2 for use.) A lithium secondary battery using manganese oxide as a positive electrode active material is charged and discharged at an upper limit voltage of 4.1 V or less and a lower limit voltage of 1.5 V or more in terms of voltage with respect to Li electrodes. Charging and discharging method.