JPH05326017A - Nonaqueous solvent type lithium secondary battery - Google Patents

Abstract

PURPOSE:To provide a nonaqueous solvent lithium secondary battery which has high conductivity, high charge and discharge efficiency of lithium with less gas generation, is stable against heat as compared with a conventional battery system and thus has no possibility of igniting even in the case the temperature rises extremetly. CONSTITUTION:Regarding a nonaqueous solvent-type secondary battery having a negative pole which can discharge and charge lithium ion, a positive pole which can discharge and charge lithium ion, and an electrolytic solution consisting of a nonaqueous solvent and an ion-dissociative lithium salt dissolved in the solvent, as the nonaqueous solvent, a mixture of three kinds of solvents; ethylene carbonate, propylene carbonate, and sulfurane is used. As a result, a nonaqueous solvent-type lithium secondary battery having excellent charge- discharge characteristics, high safety and reliability is obtained.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a non-aqueous solvent type lithium secondary battery, and more particularly to a non-aqueous solvent type electrolytic solution used in a battery having a negative electrode capable of charging and discharging lithium ions.

[0002]

2. Description of the Related Art As electronic devices are becoming smaller, lighter and more portable, the development of high energy density batteries is required as a power source. As a battery that meets such requirements, development of a high-performance secondary battery capable of charging and discharging lithium ions in the negative electrode is expected. Examples of the negative electrode that can be charged with lithium ions include lithium metal, lithium metal alloys such as lithium and aluminum, or chemical substances that can insert and release lithium ions (for example, various carbon materials, Nb ₂ O ₅ , WO ₃ etc.) has been attempted. In the present specification, a battery capable of charging and discharging these lithium ions is referred to as a lithium secondary battery. The lithium secondary battery is required to have two points: (i) high output energy (capable of high-rate discharge) and (ii) long charge / discharge cycle life. Many positive electrode materials have been investigated to meet these demands. For example,
Crystalline or amorphous V ₂ O ₅ , V ₆ O ₁₃ , Li _x V ₃ O ₈ ,
Metal oxides such as MnO ₂ , LiCoO ₂ , LiNiO ₂ and MoO ₃ , metal sulfides such as MoS ₂ and TiS ₂ , NbSe ₃
And the like, polymer compounds such as polyaniline and polypyrrole, and sulfur compounds such as SO ₂ have been proposed.

However, when a battery using these positive electrodes is manufactured, there are problems that the output energy is extremely reduced when the discharge current is increased, the number of charge and discharge cycles is small, etc., and a high performance lithium secondary battery is realized. It has not been. The main cause of these problems is the choice of electrolyte material.
That is, since lithium reacts violently with water, it is not possible to use an aqueous solution-type electrolyte unlike the conventional nickel-cadmium secondary battery or lead storage (secondary) battery. For this reason, the conductivity of the lithium battery electrolyte using the non-aqueous solvent electrolyte is one to two orders of magnitude lower than that of the aqueous solution, and there is an essential problem that the battery acquisition current value is low.

Further, since lithium is highly reactive, it is considered thermodynamically that almost all non-aqueous solvent type electrolytic solutions will react. The products of this electrolyte / lithium reaction are solids and gases, the solid products are formed as a film on the lithium surface and in the case of primary batteries this film suppresses further electrolyte / Li reactions, In the case of a secondary battery that repeats electrodeposition (charging) and dissolution (discharging), the electrical properties (stable voltage range), electronic conductivity, ionic conductivity, etc., chemical properties (reactivity, etc.), physical properties of this film Properties (such as porosity) and mechanical properties (such as strength) affect the charge / discharge efficiency of Li. Further, the generated gas is an electrical insulator, and there is a problem that it physically hinders the movement of ions or covers the electrode active material and hinders the electrode reaction.
Therefore, it is necessary to use an electrolyte solution material different from that of the primary battery for the electrolyte solution for the lithium secondary battery, and the electrolyte solution for the secondary battery has high conductivity, high charge / discharge efficiency of lithium, and gas generation amount. There are few things that are needed. Further, a lithium battery has a difficulty in thermal stability as compared with a conventional battery system, and there is a risk that the battery may ignite in the worst case when the temperature rises extremely.

Several electrolytic solutions for lithium secondary batteries have been proposed, but the above problems have not been solved. For example, an electrolytic solution in which LiAsF ₆ is dissolved in a mixed solvent of ethylene carbonate and propylene carbonate shows high charge / discharge efficiency and conductivity of lithium, but when the charge / discharge is repeated, a large amount of gas is generated due to reductive decomposition of the electrolytic solution. There is a problem. This generated gas is propylene and ethylene, and its flash point is as low as -100 ° C or lower. An electrolyte solution in which LiAsF ₆ is dissolved in sulfolane is known as an electrolyte solution that generates less gas during charge and discharge, but has the drawback that the conductivity of the electrolyte solution is extremely low and the charge and discharge efficiency of lithium is also low. .. Therefore, development of an electrolytic solution for a lithium secondary battery having high conductivity and high safety is desired.

[0006]

Features of the Invention and Differences from Prior Art A battery according to the present invention is a negative electrode capable of discharging and charging lithium ions, a positive electrode capable of discharging and charging lithium ions, and an ion dissociable lithium salt dissolved in a non-aqueous solvent. In the non-aqueous solvent-based secondary battery having the electrolytic solution, as the non-aqueous solvent,
It is characterized by using a mixture of three kinds of solvents of (1) ethylene carbonate, (2) propylene carbonate and (3) sulfolane.

The present invention will be described in more detail.

The lithium secondary battery according to the present invention is a battery using an electrolytic solution in which an ion dissociative lithium salt is dissolved in a non-aqueous solvent, and (1) ethylene carbonate and (2) propylene carbonate are used as the non-aqueous solvent. (3) It is characterized by using a mixture of three solvents of sulfolane.

The mixed solvent of ethylene carbonate and propylene carbonate has advantages in terms of performance such as high conductivity and high charge / discharge efficiency of lithium, but has a drawback of generating too much gas as the number of charge / discharge cycles of the battery increases. Have. On the other hand, sulfolane has less gas generation, is less likely to be oxidized and decomposed, and has a high flash point of 160 ° C. and is excellent in safety. Is small and the charge / discharge cycle life is short. None of these three solvents are volatile and their flash points are 100
It has a practical advantage that it is as high as ℃ or more, it is cheap, and it is easy to handle during battery production. In the present invention, by mixing these three kinds of solvents, it is possible to eliminate the drawbacks of the three kinds of solvents, combine the advantages of the three kinds of solvents, and realize an electrolytic solution having high performance and safety.

In the mixed solvent of three kinds of ethylene carbonate, sulfolane and propylene carbonate used in the present invention, the volume mixing ratio of ethylene carbonate and sulfolane is respectively maximum at 1/3 in the mixed solvent (that is, propylene carbonate (The minimum volume mixing ratio is 1/3). Further, it is more preferable that the content of ethylene carbonate is the smallest. this is,
Ethylene carbonate has the highest melting point of the solvent (36
This is because the low temperature performance of 0 ° C. or less becomes a practical problem when the content of ethylene carbonate and sulfolane is high (28 ° C.), and then the sulfolane is high (28 ° C.). This is because there is no problem in low temperature performance), and ethylene carbonate has a problem that the amount of gas generated during charging and discharging is the largest. Therefore, by setting the volume mixing ratio of the three kinds of solvents as described above, it is possible to realize the electrolytic solution for a lithium secondary battery, which is an object of the present invention and is excellent in both performance and safety.

The lithium salt used in the electrolytic solution is not particularly limited as long as it can be used in a lithium battery. For example, LiAsF ₆ , LiPF ₆ , LiSb.
_{F 6, LiCF 3 SO 3,} LiN (CF 3 SO 2) 2, LiC
(CF ₃ SO ₂ ) ₃ , LiClO ₄ , LiBF ₄ , LiAl
Cl _{4 and the} like can be used, and these lithium salts can be used alone or as a mixture of two or more kinds in a concentration range of 0.5 to 2.0 mol / l.

The lithium secondary battery of the present invention is a rechargeable battery that uses a chemical substance for electrochemically performing a reversible redox reaction in the negative electrode and the positive electrode, and uses an ion conductive electrolyte between both electrodes. This battery is equipped with a separator and suppresses direct contact between both electrodes, and takes out electrical energy to an external circuit. A lithium secondary battery uses a chemical substance capable of discharging lithium ions as a negative electrode, uses a chemical substance that undergoes a reversible electrochemical reaction with lithium ions as a positive electrode active material, and dissolves a lithium salt in a non-aqueous solvent. Is a lithium ion conductive electrolyte solution. For example, as the negative electrode, lithium metal, a lithium metal alloy such as lithium and aluminum, or a chemical substance capable of inserting and releasing lithium ions (for example, various carbon materials, N 2
b ₂ O ₅ , WO ₃ etc.) can be used, and LixCoO ₂ (0 ≦ x ≦ 1), Li _x NiO ₂ (0
≦ x ≦ 1), Li _x Mn ₂ O ₄ (0 ≦ x ≦ 1), crystalline or amorphous V ₂ O ₅ , Li _x V ₃ O ₈ (0 ≦ x ≦ 1), T
iS ₂ , NbSe ₃ or the like can be used.

[0013]

EXAMPLE Lithium metal was used as a negative electrode, vanadium pentoxide (V ₂ O ₅ ) was used as a positive electrode, and 1 mol / l was used as an electrolytic solution.
LiAsF ₆ of the present invention is a mixed solvent of ethylene carbonate (hereinafter abbreviated as EC), propylene carbonate (hereinafter abbreviated as PC) and sulfolane (hereinafter abbreviated as SL) of the present invention (volume mixing ratio, 1: 1: Two AA batteries were prepared using the one dissolved in 1) (these batteries are referred to as "Battery A" and "Battery B"). The conductivity of this electrolytic solution at 25 ° C. is 4.4 × 10 −3 S.
/ Cm, which is about twice as high as that of an electrolyte solution prepared by dissolving 1 mol / l of LiAsF ₆ in a solvent of sulfolane alone.

Further, as a comparative example for showing the effect of the present invention, four identical AA batteries except for the electrolytic solution were prepared. The four batteries of these comparative examples are referred to as battery C, battery D, battery E and battery F, respectively. The electrolytes of batteries C and D were 1 mol / l of LiAsF ₆ in EC and P.
What was dissolved in the mixed solvent of C (volume mixing ratio: 1: 1) was used. Electrolyte solution of battery E and battery F is 1 mol / l
It is used those obtained by dissolving the LiAsF _6. All of these produced AA batteries have a safety valve at the bottom of the battery can that opens when the internal pressure of the battery increases.

First, three batteries, battery A, battery C and battery E, were discharged at 60 mA (the lower limit voltage of discharge was 1.8 V) and then charged at 60 mA (the upper limit voltage of charge was 3.5 V). After repeating the cycle 50 times, a high temperature storage test was performed. When the batteries were left in a sealed constant temperature bath at 170 ° C., the safety valves of the batteries A and E did not open, and no gas generation was observed. Further, no voltage drop was observed and no change was observed in the appearance. However, the safety valve of the battery C was opened, and the gas accumulated in the battery was released. From these results, the electrolytic solution of the present invention does not generate gas even when it is heated to near the melting point of lithium (180 ° C.), and exhibits excellent performance in terms of safety and reliability as compared with conventional electrolytic solutions. It is clear.

Next, the batteries B, D and F were discharged at 600 mA (the discharge lower limit voltage was 1.8 V).
After that, charge at 100 mA (upper limit charging voltage is 3.5 V)
The cycle was defined as one cycle, and this charge / discharge cycle was continued until the discharge capacity of the battery reached the first 50%.
The results are shown in Table 1. In Table 1, as an index showing the charge / discharge cycle life, the FOM (Figure of
Merit) is used, and the cycle life of the battery B and the battery D is represented by a relative ratio when the FOM value of the battery F is 1. This FOM is an index representing the cycle life of the battery, and the larger the value of this FOM, the longer the cycle life of the battery. In addition, Table 1 shows the first discharge capacity. The discharge capacity is also expressed as a relative value when the value of the battery F is 1. As can be seen from the results shown in Table 1, the battery using the electrolytic solution of the present invention has excellent discharge capacity and charge / discharge cycle life as compared with the battery using the electrolytic solution according to the prior art of the comparative example. showed that.

FOM = (cumulative discharge capacity of charge / discharge cycle) / (discharge capacity of lithium charged in battery) = Σ (discharge capacity per cycle) / (discharge capacity of lithium charged in battery)

[0018]

EC: ethylene carbonate, PC: propylene carbonate, SL: sulfolane

[0020]

As is clear from the above description, by using the electrolytic solution of the present invention, a non-aqueous solvent electrolytic solution type lithium secondary battery having excellent charge / discharge characteristics, high safety and high reliability can be obtained. realizable.

Claims (1)

[Claims] 1. A negative electrode capable of discharging and charging lithium ions and a positive electrode capable of discharging and charging lithium ions,
In a non-aqueous solvent type secondary battery having an electrolytic solution in which an ion dissociative lithium salt is dissolved in a non-aqueous solvent, the non-aqueous solvent includes (1) ethylene carbonate, (2) propylene carbonate and (3) sulfolane. A non-aqueous solvent type lithium secondary battery characterized by using a mixture of different solvents.