JPH03266372A - Lithium secondary battery - Google Patents

Abstract

PURPOSE:To improve cycle life by using a mixed solvent consisting of ethylene carbonate, a specified ether compound, and lithium salt as the electrolyte of a lithium secondary battery, and specifying the mixing ratio of ethylene carbonate. CONSTITUTION:As the electrolyte of a lithium battery using a lithium alloy as a negative electrode active material and a transition metal chalcogen compound as a positive electrode active material, those in which lithium salt is dissolved in a mixed solvent of ethylene carbonate and an ether compound represented by the formula I (wherein R1 and R2 are hydrocarbon groups celeoted from CH3, C2H5, and C3H7) are used. The mixing ratio of ethylene carbonate in the mixed solvent is 25-75% by volume. Hence, charge and discharge cycle life in long-time discharge is improved. When the mixing ratio of ethylene carbonate is high, the viscosity is increased, and when it is low, the dielectric constant is reduced. Hence, the deterioration of the battery by long-time discharge is prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to improving the charge/discharge cycle life of lithium secondary batteries.

(Summary of the Invention) The present invention provides a lithium secondary battery using lithium or a lithium alloy as a negative electrode active material and a transition metal chalcogen compound as a positive electrode active material.
Ethylene carbonate and general formula R+-0-+CHzCt
lz-0)-z Rz (However, R1 and R2 are CH3, t,
A lithium salt is dissolved in a mixed solvent of an ether compound (which is any hydrocarbon group selected from US, CJ'r, and R2 and R2 may be the same), and in the mixed solvent. The mixing ratio of ethylene carbonate is 2
By setting the content to 5 to 75% by capacity, it is intended to provide an excellent lithium secondary battery with a long charge/discharge cycle life in long-term rate discharge.

[Conventional technology]

Batteries using lithium as a negative electrode active material are attracting attention as small-sized batteries with high energy density, and lithium-secondary batteries using MnO□, CFx, etc. as positive electrode materials have been put into practical use. In recent years, there has been a strong desire to develop lithium secondary batteries that are expected to have superior characteristics compared to conventional batteries. However, in order to turn a lithium battery into a rechargeable and dischargeable secondary battery, there are many issues that need to be improved, including the positive electrode active material and battery configuration.

The development of electrolytes is also an important issue. As an electrolyte for lithium secondary batteries, lithium salt is used as T-butyrolactone,
Electrolytes for lithium secondary batteries that are dissolved in solvents such as propylene carbonate and 1,2-dimethoxyethane have been used and have shown good properties. High discharge efficiency,
Currently, an excellent electrolytic solution has not yet been developed, as it has many required characteristics such as low reactivity with lithium and little dependence on charging time rate.

For example, as a conventional example, a charge/discharge cycle test was conducted on a lithium secondary battery using an electrolyte solution in which LiPFb was dissolved in a mixed solvent of propylene carbonate and 1,2-dimethoxyethane by changing the discharge time rate of the charge/discharge cycle. , when examining its cycle life, as shown in Figure 2,
It can be seen that repeated long-term discharge at a rate of 5 hours or more has the disadvantage that the deterioration of lithium is significantly accelerated and the cycle life is extremely shortened.

[Problem to be solved by the invention]

An object of the present invention is to provide an excellent lithium secondary battery whose charging and discharging characteristics are less likely to deteriorate even when subjected to charging and discharging cycles in which long-term rate discharge is repeated.

[Means for Solving the Invention] The present invention provides a lithium secondary battery using lithium or a lithium alloy as a negative electrode active material and a transition metal chalcogen compound as a positive electrode active material, in which as an electrolyte,
Ethylene carbonate and general formula R, -0-+CHzCH
z-0←2R2 (However, R,, Rz is C1,, C2H5,
C: Any hydrocarbon group selected from lH7, and R1 and R2 may be the same. Carbonate mixing ratio is 25~
It is a lithium secondary battery with a capacity of 75%.

As the electrolyte used in the present invention, any conventionally known electrolyte can be used, including LiClO4, LiAsF6, t
,1pFt, ,LiB(CJs)4,LiC1,Li
There are Br5LiCF3SO, LiClO4 (h, etc.).

The amount of such lithium salt in the mixed solvent is 0.5 to 1.
It is desirable to use it by dissolving 5 mol/1. Outside this range, the charge/discharge characteristics deteriorate significantly.

As the positive electrode material of the lithium secondary battery of the present invention, Mn0
z,CrzOs,Ti0z,TiS,,v203,L
iMnzOn, LiCo0z, CuO, Mob, ,Mo
Transition metal chalcogen compounds such as S, P2O3, WO, etc. can be used.

[Function] According to the research results of the present inventors, a mixture of ethylene carbonate and diethylene glycol diethyl ether is used as an electrolyte for a lithium secondary battery that uses lithium as a negative electrode active material and a transition metal chalcogen compound as a positive electrode active material. By using a lithium salt dissolved in the solvent and setting the mixing ratio of ethylene carbonate in the mixed solvent to 25 to 75% by volume, the charge/discharge cycle life will be short even after repeated long-rate discharges of 5 hours or more. You can obtain a lithium secondary battery that does not require a lithium battery. In addition, the above-mentioned diethylene glycol diethyl ether, czHs-o(CHzC)I
Instead of t-0+-z CzHs, the same series as that,
General formula R+-0-+CHzCHz-0+-z R2 (However, R1 and R2 are CH.

A similar lithium secondary battery can be obtained by using , C2H,, C, I (which is any hydrocarbon group selected from , and R and R2 may be the same).

Although the exact reason why the battery of the present invention shows good results is unknown, the use of the electrolyte changes the lithium precipitation form from resin-like crystals to nearly block-like lithium crystals, which prevents lithium from peeling off from the electrodes. This is thought to be because lithium ions are quickly supplied and the state of lithium precipitation improves because , desorption, and dropout are reduced and the degree of dissociation of the lithium salt is improved.

Furthermore, it is generally believed that by using a solvent with a high dielectric constant as a solvent for an electrolyte solution for a lithium secondary battery, dissociation of the electrolyte salt is promoted and the efficiency of charging and discharging lithium is increased. Ethylene carbonate has an extremely high dielectric constant of 73.0, but has a high melting point of 39°C and is solid at room temperature. By mixing, for example, ethylene glycol diethyl ether with this, a solvent with a high dielectric constant and a low melting point can be obtained. The higher the ratio of ethylene carbonate in this solution, the better the charging and discharging efficiency of lithium tends to be. However, if the ratio of ethylene carbonate in the mixed solvent is higher than 75% by volume, the viscosity of the solution is high and it is difficult to use it as an electrolyte. Also, the ion mobility is poor ← The heavy load characteristics and low temperature characteristics of the battery are deteriorated.

On the other hand, if the ratio is lower than 25% by volume, the dielectric constant will also decrease and the efficiency of charging and discharging lithium will decrease.

〔Example〕

FIG. 1 is a schematic diagram showing the cross-sectional structure of a lithium secondary battery to which the present invention is applied in an example.

Examples and comparative examples of the lithium secondary battery of the present invention will be described below with reference to FIG.

Example 1 First, positive electrode 1 was created as follows.

LiMnz
Obtain O4 and use it as a positive electrode active material, and this LiM
10 parts by weight of graphite as a conductive material and 3 parts by weight of polyvinylidene fluoride as a binder were added to parts by weight of nz0487 and mixed to obtain a positive electrode mixture. Then, this positive electrode mixture was dispersed in a solvent N-methylpyrrolidone to form a slurry (paste).

Next, this positive electrode mixture slurry is uniformly applied to both sides of a strip-shaped aluminum foil serving as a positive electrode current collector, and dried.
Thereafter, compression molding was performed using a roller press machine to produce a strip-shaped positive electrode 1.

Negative electrode 2 was created by using copper foil as a negative electrode current collector and pressing metallic lithium onto this.

Next, using the positive electrode 1 and the negative electrode 2, and further using a pair of separators 3, and stacking them on each other,
A spiral-shaped body was made by winding the material many times.

This wound body was housed in a nickel-plated iron battery can 5. In order to collect current from the positive electrode 1, an aluminum positive electrode lead was attached to the positive electrode 1, led out from the positive electrode 1, and welded to the battery lid 7. Further, in order to collect current from the negative electrode 2, a nickel negative electrode lead was attached to the negative electrode 2, led out from the negative electrode 2, and welded to the battery can 5.

This battery can 5 contains 50% by volume of ethylene carbonate.
An electrolytic solution obtained by dissolving 1 mol/1 LiPh in a mixed solvent consisting of 50% by volume of diethylene glycol diethyl ether was injected.

Next, an insulating plate 4 was placed inside the battery can 5 so as to face the upper and lower surfaces of the wound body. Further, the battery can 5 and the battery lid 7 were caulked together with an insulating sealing gasket 6 interposed therebetween, and the battery lid 7 was sealed. As described above, a cylindrical lithium secondary battery A having a diameter of 13.8 n+m and a height of 42 mm was created.

Comparative Example 1 For comparison, a lithium secondary battery was prepared using a solvent used in lithium secondary batteries.

50% by volume of propylene carbonate as an electrolyte; 1.
A lithium secondary battery B was prepared in the same manner as in Example 1 except that LiPFb was dissolved at 1 mol/l in a mixed solvent consisting of 50% by volume of 2-dimethoxyethane.

The batteries shown in Example 1 and Comparative Example 1 were charged at a charging current of 75 mA.
Constant current charging was performed for 9 hours with an upper limit voltage of 3.9V, and then a final voltage of 2.9V was set at 11Ω. A 9-hour rate charge 2-hour rate discharge cycle test in which the battery was discharged to Ov, and a constant current charge was performed for 9 hours with a charging current of 75 mA and an upper limit voltage of 3.9 V, and then a constant current charge of 50 mA was carried out.
End voltage in Ω2. 9 hour rate charge 10 to discharge to Ov
Two types of time rate discharge cycle tests were conducted. In both cases, the discharge capacity is 50% of the discharge capacity at the 10th cycle.
The cycle life is defined as the point where the deterioration reaches %, and the results are shown in Table 1.

As shown in Table 1, in the case of the 9-hour rate charge and 2-hour rate discharge cycle test, both Battery A of the present invention and Battery B of the comparative example using the solvent used in conventional lithium-secondary batteries. , but in the case of a 9-hour rate charge and 10-hour rate discharge cycle test, battery A of the present invention shows the same cycle life as battery B of the comparative example, which uses the electrolyte solution used in conventional lithium-secondary batteries. It shows very good characteristics compared to .

Next, the electrolyte was adjusted by changing the mixing ratio of ethylene carbonate and diethylene glycol diethyl ether, and a battery was created.

Example 2 An electrolytic solution obtained by dissolving 1 mol/l of LiPF+ in a mixed solvent consisting of 25% by volume of ethylene carbonate and 1% by volume of diethylene glycol diethyl ether was used, and the other conditions were the same as in Example 1. Next battery C was created.

Example 3 An electrolytic solution obtained by dissolving 1 mol/l of LiPF in a mixed solvent consisting of 75% by volume of ethylene carbonate and 25% by volume of diethylene glycol diethyl ether was used, and the other conditions were the same as in Example 1. I created a battery.

Comparative Example 2 Diethylene glycol diethyl ether 1 as electrolyte
1s+ol/LiPF6 in a solvent consisting of 00% by volume
A lithium secondary battery E was prepared in the same manner as in Example 1 except that the material obtained by melting I was used.

Comparative Example 3 A lithium secondary battery F was produced in the same manner as in Example 1 except that an electrolyte obtained by dissolving 1IIIol/l of LiPFb in a solvent consisting of 100% by volume of ethylene carbonate was used.

The batteries of Examples 2 and 3 and Comparative Examples 2 and 3 were also subjected to constant current charging for 9 hours with a charging current of 75 mA and an upper limit voltage of 3.9 V, and then a final voltage of 2.9 V at 11 Ω. 9-hour rate charging 2-hour rate discharging cycle test and charging current 75 to discharge to Ov
Constant current charging was performed for 9 hours at +wA with an upper limit voltage of 3.9V, and then with a final voltage of 2.9V at 50Ω. Two types of tests were conducted: 9-hour rate charging and 10-hour rate discharging cycle test in which the battery was discharged to Ov. In each case, the cycle life was defined as the point at which the discharge capacity deteriorated to 50% of the discharge capacity at the 10th cycle, and the results are shown in Table 2.

Table 2 From Tables 1 and 2, when a charge/discharge cycle test with repeated discharge at a rate of 5 hours or more was performed, ethylene carbonate alone or diethylene glycol diethyl ether alone was used as the solvent for the electrolyte of a lithium secondary battery. Batteries E and B using Batteries have a short cycle life, but the mixing ratio of ethylene carbonate as the electrolyte solvent is 25% by volume or more.75
It can be seen that batteries A, C, and D using a mixed solvent of ethylene carbonate and diethylene glycol diethyl ether adjusted to a volume percent or less exhibited a cycle life of 100 cycles or more.

[Effects of the Invention] According to the present invention, it is possible to provide a lithium secondary battery with excellent charge-discharge cycle characteristics and less deterioration even after repeated long-term rate discharges.

[Brief explanation of drawings]

Figure 1 is a schematic diagram showing the cross-sectional structure of a lithium secondary battery applied to an example of the present invention, and Figure 2 is an electrolytic solution in which LiPFb is dissolved in a mixed solvent of propylene carbonate and 1,2-dimethoxyethane. 3 shows the discharge time rate dependence of the cycle life measured for a lithium secondary battery according to the prior art. In the symbols used in FIG. 1, 1. −−−−−−−
---Cross-sectional structure and schematic diagram of separator battery

Claims (1)

[Claims] 1. In a lithium secondary battery using lithium or a lithium alloy as a negative electrode active material and a transition metal chalcogen compound as a positive electrode active material, ethylene carbonate and the general formula R_1-OCH_2CH_2- are used as an electrolyte. O ■_2R_2 (in the formula, R_1 and R_2 are CH_3, C_2H_5, C_3H
Any hydrocarbon group selected from _7, R
_1 and R_2 may be the same) A lithium salt is dissolved in a mixed solvent of an ether compound represented by
A lithium secondary battery characterized by a capacity of 5%. 2. The lithium secondary battery according to claim 1, wherein R_1 and R_2 are C_2H_5.