JP2000331711A - Electrolyte for lithium secondary battery and lithium secondary battery provided therewith - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve stability and service life of a battery by adding the monomer of a conductive polymer electrochemically polymerizable with a specified potential difference to lithium potential to a nonaqueous organic solvent. SOLUTION: This monomer, for example, consists of pyrrole, aniline, or their derivatives, which is electrochemically polymerizable at about 4.0 V to lithium potential, and in a nonaqueous electrolyte battery containing this, a conductive polymer film is formed on the surface of a positive electrode material during its charging. This does not participate in the movement of lithium ions in normal charging and discharging but rather assists its reversible migration, and it forms an immobilized film at a high potential of 4.3 V or higher which is generated in overcharge or penetration to interrupt the flow of heavy current. The addition quantity to the organic solvent is preferably set to less than 2.0 wt.%, to avoid the increase in discharge capacity and non-reversible capacity. Further more, as active materials of positive electrode and negative electrode, a lithium composite oxide and a graphite carbonaceous material, which are absorbable/releasable of lithium ion, are preferably used.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrolyte for a lithium secondary battery and a lithium secondary battery including the same, and more particularly, to a technology of a lithium secondary battery having excellent safety.

[0002]

2. Description of the Related Art In recent years, the use of portable electronic devices has been increasing with the advancement of the advanced electronic industry, which has made it possible to reduce the size and weight of electronic equipment. The need for batteries having a high energy density as a power source for such portable electronic devices has increased, and research on lithium secondary batteries has been actively conducted. Lithium metal or carbon material is used as a negative electrode material of a lithium secondary battery, and lithium metal oxide is used as a positive electrode material. When lithium metal is used for the negative electrode material, there is a risk of explosion due to short-circuiting of the battery due to the formation of dendrites (dendrites). Therefore, a carbon material has been used instead of lithium metal as the negative electrode material. LiMn as positive electrode material
₂ O ₄ , LiMnO ₂ , LiCoO ₂ , LiNiO ₂ , Li
A composite metal oxide such as Ni ₁ -xCoxO ₂ (0 <x <1) is used. Mn-based electrode materials such as LiMn ₂ O ₄ and LiMnO ₂ are advantageous in that they can be easily synthesized, are relatively inexpensive, and have little pollution to the environment, but have disadvantages in that they have a small capacity. In particular, LiMn ₂ O ₄
oO ₂ , the discharge capacity is smaller than other active materials such as LiNiO ₂ , the discharge capacity at the time of high-rate charge and discharge decreases rapidly, and the manganese elution at the time of continuous charge and discharge at high temperature causes There is a problem that the life is rapidly deteriorated. LiCoO ₂ has good electrical conductivity, high battery voltage, and excellent electrode characteristics.
Although it is a typical positive electrode material commercialized and marketed by company Y, it has a disadvantage that it is expensive. LiNi
O ₂ is relatively inexpensive among the above-mentioned positive electrode materials and has the highest battery capacity with the highest discharge capacity, but is difficult to synthesize, and the problem of securing the stability of the battery due to the high discharge capacity and the like is emerging.

[0003] In addition, since the characteristics of a battery appear due to a complex reaction of a positive electrode / electrolyte solution and a negative electrode / electrolyte solution, use of an appropriate electrolyte solution is one of the important factors for improving the performance of the battery. . The conventional electrolyte system is expected to play only the role of a medium that simply moves lithium ions,
It has also worked that way. When using an existing electrolyte, if a large current such as penetration or overcharging flows, there is no way to shut it off, so a thermal runaway phenomenon will be induced, which may turn into a dangerous situation . Therefore, recently, in order to solve such a problem, a method of using an additive for forming a thin passivation film on the surface of the negative electrode by an initial reaction with the negative electrode in an existing electrolytic solution is known.

[0004] However, there is no known electrolyte capable of forming a conductive polymer film on the surface of the positive electrode to block a large current such as overcharge or penetration when the current flows.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to form a conductive polymer film on the surface of a positive electrode, and to prevent the overcurrent from flowing when the overcurrent flows, thereby improving the stability of the battery. An object of the present invention is to provide an electrolyte for a secondary battery.

Another object of the present invention is to provide a lithium secondary battery having improved stability and battery life.

[0007]

In order to achieve the above-mentioned object, the present invention relates to a method for preparing a monomer containing a conductive polymer which can be electrochemically polymerized at about 4 V with respect to a lithium potential in a non-aqueous organic solvent. A non-aqueous electrolyte solution is prepared by adding a body and applied to a lithium secondary battery.

Hereinafter, the present invention will be described in more detail.

The electrolyte for a lithium secondary battery according to the present invention is prepared by adding a monomer of a conductive polymer which can be electrochemically polymerized at a potential of about 4.0 V to a non-aqueous solvent. You. Examples of conductive polymer monomers that can be used in the present invention include pyrrole,
There is aniline, or derivatives thereof. These compounds are preferably used in an amount of less than 2.0% by weight. When added in an amount of 2.0% by weight or more, the discharge capacity and the irreversible capacity increase, which causes a problem that the performance of the battery is reduced. As the non-aqueous solvent, an organic solvent such as a cyclic or chain carbonate may be used, and two or more may be used in combination. Specific examples of these include ethylene carbonate (EC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC).

The electrolyte of the lithium secondary battery includes lithium hexafluorophosphate (lithium).
hexafluorophosphate) (LiPF
₆ ), lithium tetrafluoroborate (lithhiu)
m tetrafluoroborate) (LiBF
₄ ) Lithium hexafluoroarsenate (lith
ium hexafluorosenate) (L
iAsF ₆ ), lithium perchlorate (lithiu)
m perchlorate (LiClO ₄ ), lithium trifluoromethanesulfonate (lithium)
trifluoromethanesulfonat
e) One of (CF ₃ SO ₃ Li), or a mixture of two or more thereof, can be added as a supporting electrolyte salt.

The lithium secondary battery of the present invention comprises the electrolyte for a lithium secondary battery of the present invention described above, a negative electrode plate and a positive electrode plate. The negative electrode plate is made of a resin binder and a graphite-based carbon material capable of absorbing / releasing lithium ions as a negative electrode active material. The negative active material, d ₀₀₂ interlayer distance (interplanar
distance) is 3.35 to 3.38 °, and X
Crystallite size by X-ray diffraction
ze) Lc is at least 20 nm or more and 700 ° C.
The above has an exothermic peak. The negative electrode active material used in the present invention is mesophase.
Spherical particles are used, which is a graphite-based carbon material produced by the steps of carbonization and graphitization. In addition, fiber type mesophase pitch (mesophase pit)
Also, a fibrous graphite-based carbon material prepared by using a carbon fiber and a carbonization step and a graphitization step can be used as the negative electrode active material, and both artificial graphite and natural graphite can be used.

The positive electrode plate absorbs lithium ions /
A releasable lithium composite oxide is used. LiCoO ₂ to these specific _{examples, LiNi 1-xy Co x M} y O
₂ (0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ x + y ≦ 1, M is A
l, metals such as Sr, Mg, La), LiMnO ₂ , L
iMn ₂ O ₄ and the like.

When a non-aqueous electrolyte containing a monomer of a conductive polymer is applied between the positive electrode and the negative electrode plate, a conductive polymer film is formed on the surface of the positive electrode active material during charging. The conductive polymer film thus formed does not participate in the movement of lithium ions during normal charge / discharge, but becomes ineffective due to loss of conductivity at a high potential of 4.3 V or more generated during overcharge or penetration. It has the properties of a passivation film. As a result, a large current flow can be cut off, and a thermal runaway phenomenon due to overcharging or penetration can be prevented. In addition, since the conductive polymer film is formed on the surface of the positive electrode, lithium ions can be more reversibly inserted and removed, and the life of the battery can be extended.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, preferred embodiments will be presented for understanding the present invention. However, the following examples are provided for easier understanding of the present invention, and the present invention is not limited to the following examples.

Ethylene carbonate / ethyl methyl carbonate / diethyl carbonate (EC / EMC / DE
An electrolytic solution for a lithium secondary battery was manufactured while changing the amount of pyrrole added to a non-aqueous organic solvent in which C) was mixed at 3: 3: 1. _{LiNi 1-xy Co x Sr y} O 2 (0 ≦ x ≦ 1,
0 ≦ y ≦ 1, 0 ≦ x + y ≦ 1) A positive electrode is composed of an active material and a negative electrode is composed of graphite (KMFC: product of Kawasaki Steel Corporation), and the above-mentioned electrolyte for lithium secondary batteries is applied to 18650 lithium. A secondary battery was manufactured. Using the manufactured lithium secondary battery, the cycle life, capacity (initial discharge capacity and irreversible capacity), and stability of the battery were measured and evaluated, and are shown in Table 1 below.

[0016]

[Table 1]

The criteria for the safety evaluation in Table 1 above are as follows. L0: good, L1: liquid leakage, L2: flash, L3: flame, L
4: smoke, L5: ignition, L6: burst.

As shown in Table 1 above, in Examples 1 to 4 in which pyrrole, a monomer of the conductive polymer, was added, not only safety but also cycle life was excellent. . In comparison, it can be seen that Comparative Example 1 has significantly reduced safety characteristics. FIG. 1 shows the cycle characteristics as a relationship between the number of charge / discharge cycles and the capacity according to the pyrrole content in the results in Table 1 above. In addition, the safety was evaluated under various test conditions using a lithium secondary battery including the electrolyte solution of Example 2 having a pyrrole content of 0.5% by weight, and the results were shown in Table 2 below. Shown in In the table below, OCV indicates an open-circuit voltage, which means that the battery was charged to this voltage in order to make the battery usable.

[0019]

[Table 2]

As shown in Table 2 above, all the lithium secondary batteries of Example 2 of the present invention showed good results even under poor conditions such as penetration and overcharging. Therefore, it can be seen that the lithium secondary battery constituted according to the present invention has very excellent safety.

[0021]

The electrolytic solution containing the monomer of the conductive polymer produced according to the present invention forms a conductive polymer film on the surface of the positive electrode during charging, and causes overcharging or penetration. When a current flows, the present invention has an effect of cutting off the current and improving the stability of the battery. The lithium secondary battery including the electrolytic solution according to the present invention has the property of a passivation film by losing conductivity at a high potential generated at the time of overcharging or penetration, and causes a thermal runaway phenomenon due to overcharging or penetration. It also has the effect of the invention that can be prevented. In addition, the conductive polymer film formed on the surface of the positive electrode allows lithium ions to enter and exit more reversibly, thereby improving the life of the battery.

Simple modifications or alterations of the present invention can be easily implemented by those having ordinary skill in the art, and all such alterations and modifications are considered to be included in the scope of the present invention. it can.

[Brief description of the drawings]

FIG. 1 is a graph showing cycle characteristics, which is a relationship between the number of charge / discharge cycles and capacity according to the pyrrole content of a lithium secondary battery.

Claims (8)

[Claims] An electrolyte for a lithium secondary battery comprising a non-aqueous organic solvent and a monomer of a conductive polymer which can be electrochemically polymerized at a potential of about 4.0 V with respect to a lithium potential. 2. The electrolyte according to claim 1, wherein the conductive polymer monomer is selected from the group consisting of pyrrole, aniline, and derivatives thereof. 3. The content of the monomer of the conductive polymer is:
2. The composition according to claim 1, wherein the content of the organic solvent is less than 2.0% by weight.
Or the electrolyte solution for a lithium secondary battery according to 2. 4. A support selected from the group consisting of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium perchlorate, and a mixture containing one or more of lithium trifluoromethanesulfonate. The electrolytic solution for a lithium secondary battery according to any one of claims 1 to 3, further comprising an electrolytic salt. 5. The lithium secondary battery electrolyte according to claim 1, a resin binder, and a graphite-based carbon material capable of absorbing and releasing lithium ions as a negative electrode active material. A lithium secondary battery comprising: a negative electrode plate configured as described above; and a positive electrode plate formed of a lithium composite oxide capable of absorbing and releasing lithium ions as a positive electrode active material. 6. The negative electrode active material, wherein the d ₀₀₂ interlayer distance is 3.35 to 3.38 °, the crystallite diameter Lc by X-ray diffraction is 20 nm or more, and it has an exothermic peak at 700 ° C. or more. The lithium secondary battery according to claim 5. 7. The lithium secondary battery according to claim 5, wherein the negative electrode active material is a graphite-based carbon material manufactured from mesophase spherical particles through a carbonization step and a graphitization step in this order. 8. The lithium secondary battery according to claim 5, wherein the negative electrode active material is a fibrous graphite-based carbon material manufactured from a fibrous mesophase pitch through a carbonization step and a graphitization step.