JPH08236154A - Nonaqueous solvent lithium secondary battery - Google Patents

Abstract

PURPOSE: To provide a lithium secondary battery which has a long charge and discharge life and excellent stability by composing organic solvent of the mixed solution of a specific substance, and specifying the volume mixing ratio of the specific substance. CONSTITUTION: A lithium secondary battery is composed of a negative electrode 1 which is made of lithium or can charge and discharge the lithium, a positive electrode 3 which is mainly made of amorphous V2 O5 , and electrolyte which is prepared by dissolving at least one kind of lithium salt in organic solvent. In the lithium secondary battery, the mixed solvent of ethylene carbonate and propylene carbonate is used as the organic solvent. The volume mixing ratio of the ethylene carbonate is set to 5 to 35%. The nonaqueous lithium secondary battery being excellent in charge and discharge characteristics and safety is obtained, thereby.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium secondary battery having lithium or a negative electrode capable of discharging and charging lithium and a positive electrode, and an electrolyte solution in which a lithium salt is dissolved in a non-aqueous solvent.

[0002]

2. Description of the Related Art Lithium secondary batteries are secondary batteries that have hitherto been commercially available (for example, lead storage batteries, nickel cadmium).
It is expected as a battery with a higher energy density compared to. A small lithium secondary battery such as a coin type is already on the market. In addition, carbon containing lithium ions in the negative electrode,
AA and similar size secondary batteries using cobalt oxide for the positive electrode are commercially available, but the discharge capacity is smaller than nickel cadmium batteries. Therefore, a high energy lithium secondary battery of size AA to size A has not yet been put to practical use. This is mainly due to the following two reasons. The first reason is that the charge / discharge efficiency of lithium in the non-aqueous solvent electrolyte is low. The second reason is a problem that the safety of the battery is reduced as the battery becomes larger. If the lithium secondary battery is upsized, a large amount of gas may be generated from the battery if the battery is mistakenly used as a secondary battery at extremely high temperatures, or in extreme cases there is a risk of ignition or explosion. . The safety problem of the lithium secondary battery is closely related to the combination of the positive electrode, the negative electrode and the electrolytic solution. Lithium can realize high energy batteries, while
The high chemical reactivity of lithium risks making the battery unsafe. The lithium and the non-aqueous solvent react thermodynamically, and the decomposition products of the electrolytic solution are usually solids and gases remaining on the lithium surface. This exothermic reaction is due to the melting point of lithium (1
The battery temperature can be increased up to 80 ° C. When the lithium melts, it reacts violently with the electrolytic solution and the positive electrode, further raising the internal temperature of the battery and accelerating these exothermic reactions. Some reaction products are highly flammable. Further, when the battery is repeatedly charged and discharged, the safety tends to decrease. This is interpreted that the precipitation form of lithium is not flat, the surface area is increased, and the reactivity of charged and discharged lithium is increased. Therefore, when an electrolyte solution with high lithium charge / discharge efficiency is used, on the other hand, the reaction activity of lithium has a high risk, and the electrolyte solution used in a practical battery has a high level of safety and lithium charge / discharge efficiency. Must be a compromise. In other words, there is a danger that the safety of the battery cannot be actually ensured with an electrolyte solution having a high lithium charge / discharge efficiency. of course,
As described above, the charge / discharge performance and safety of the battery are affected by the negative electrode, the positive electrode, the electrolytic solution, and all combinations. In other words, a practical and optimum electrolytic solution suitable for the positive electrode is required. Various electrolytes have been studied so far in order to improve the charge / discharge efficiency of lithium, but almost no mention is made of their safety. At present, it is considered possible to use an electrolytic solution containing 2-methyltetrahydrofuran (hereinafter abbreviated as 2MeTHF) as a solvent component in order to obtain the highest lithium charge / discharge efficiency. For example, it has been reported in an experiment using a lithium half cell (no positive electrode) that an electrolyte solution using a single solvent of 2MeTHF exhibits high charge / discharge efficiency of lithium [US Pat. No. 4,118,550 (1978)]. . In addition, ethylene carbonate (hereinafter abbreviated as EC) is 2MeTH
It has also been reported that when added to F, a lithium battery using V ₂ O ₅ as a positive electrode tends to have a long charge / discharge life [US Pat. No. 4,737,424 (1988)]. Furthermore, for charging and discharging lithium batteries using VO ₂ as a positive electrode, EC
/ Propylene carbonate (hereinafter abbreviated as PC) /
It has been reported that an electrolytic solution using 2MeTHF (12.5 / 12.5 / 75) is effective [US Pat. No. 4,965,150 (1990)]. However, 2M
eTHF has a low flash point of -11 ° C, is volatile (boiling point is about 80 ° C), and easily burns. In addition, it is easily oxidized and easily forms an explosive peroxide. Although a large amount of 2MeTHF improves the charge / discharge efficiency of lithium, it has a serious practical problem of reducing the safety of the battery.

On the other hand, in many reported lithium secondary batteries, a mixed solvent of EC and PC, which is a mixed solvent of ester and ester, has been proposed as an electrolytic solution solvent. For example, a lithium secondary battery using MoS ₂ or MnO ₂ as a positive electrode may be cited as an example [Journal of Power Sources, Vol. 24,
195-206 (1988)]. EC and PC have high boiling points (238 and 241 ° C, respectively) and high flash points (160 and 132 ° C). It is known that in the EC / PC mixed solvent electrolyte, the larger the amount of EC mixed, the higher the lithium charge / discharge efficiency and the higher the conductivity of the electrolyte. However, since the melting point of EC is as high as 36 ° C, -20 ° C
Considering the use of batteries up to a certain degree, the EC mixing amount is practically limited. Therefore, the lithium secondary battery using the EC / PC mixed electrolytic solution has a maximum EC mixing amount of 50% by volume, that is, EC / PC (volume mixing ratio, 1:
1) is used. By using this mixing ratio, the maximum lithium charge / discharge efficiency is practically realized. However, in the case of a lithium secondary battery using an amorphous V ₂ O ₅ positive electrode, it is a special case, and as a result of examining the battery performance and safety, it was found that the optimum E
The present inventors have found that the C / PC mixing ratio is different from the general result described above.

[0004]

SUMMARY OF THE INVENTION The main object of the present invention is to provide an electrolytic solution for realizing a lithium secondary battery having a long charge / discharge life and good safety. .

[0005]

The present invention is summarized as follows. The present invention relates to a lithium secondary battery, which comprises lithium or a negative electrode capable of discharging and charging lithium, and mainly amorphous V ₂ O _5. And a lithium secondary battery using an electrolyte solution in which at least one lithium salt is dissolved in an organic solvent, the organic solvent is a mixed solvent of EC and PC, and the volume mixing ratio of the EC is It is characterized by being 5 to 35%.

The present invention will be described in more detail below. An electrolyte for a lithium secondary battery is required to have high lithium charge / discharge efficiency. As described above, lithium reacts with the electrolytic solution to form a solid film on the surface of lithium. The charge / discharge performance of lithium is affected by various properties of this film. For example, the production rate of the membrane, the ionic conductivity of the membrane,
Properties such as electronic conductivity, porosity, mechanical strength, and flexibility influence. In the general case where the positive electrode does not affect the reaction between the electrolytic solution and lithium (for example, MoS ₂ or MnO described above).
₂ ) and the like, in an EC / PC mixed solvent, the larger the amount of EC mixed, the higher the lithium charge / discharge performance, and the longer the charge / discharge cycle life of the battery. However, when amorphous V ₂ O ₅ was used for the positive electrode, a vanadium compound was confirmed on the lithium negative electrode with repeated charging and discharging. The reason for this is V
It is presumed that ₂ O ₅ or its lithium compound was partially dissolved in the electrolytic solution and reacted with the lithium negative electrode. It was also found that the amount of vanadium on the negative electrode increased as the amount of EC increased. Furthermore, from EC / PC mixed system to EC
It was also found that the charging / discharging cycle performance was deteriorated as compared with the EC / PC mixed system when the above was excluded (that is, PC alone). That is,
A lithium secondary battery using amorphous V ₂ O ₅ as a positive electrode is special, and when an EC / PC mixed electrolyte is used, the optimum value of the solvent mixing ratio is generally well known 1:
We have found that it has to be optimized instead of 1.

The present invention is capable of producing lithium batteries of various shapes and sizes. FIG. 1 shows an example of a battery structure. In FIG. 1, reference numeral 1 is a negative electrode, 2 is a separator, 3 is a positive electrode, 4 is a separator, 5 is a battery can, 6 is a positive electrode tab, 7 is a negative electrode tab, 8 is a safety valve, 9 is a battery cap, and 10 is an insulator ring. Means

The lithium salt used in the present invention is, for example,
_{LiPF 6, LiAsF 6, LiSbF 6} , LiClO
₄ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , L
iC (CF ₃ SO ₂ ) ₃ , LiCF ₃ CO ₂ , LiC ₄
At least 1 selected from F ₉ SO ₂ , LiBF ₄ , LiAlCl ₄ , LiBr, and LiB (C ₆ H ₅ ) _4.
A variety of compounds can be used, the concentration of which is 0.5-
It can be used in the range of 2.0 mol / liter (M). If the solute concentration is out of this range, the charge / discharge performance of lithium will be significantly deteriorated.

As the negative electrode used in the lithium secondary battery of the present invention, lithium or a negative electrode capable of discharging and charging lithium, that is, a negative electrode capable of occluding and releasing lithium within its concept, capable of charging and discharging lithium. There is. Examples of these negative electrodes include metallic lithium, alloys capable of charging and discharging lithium, and compounds capable of inserting and extracting lithium. As an alloy that can charge and discharge lithium,
For example, Li-Al alloy, Li-Sn, etc. are mentioned.
Examples of the compound capable of inserting and extracting lithium ions include various carbon materials and compounds capable of inserting lithium ions such as tungsten oxide and niobium oxide. A preferable negative electrode has an electrode potential as close as possible to lithium metal and is capable of inserting and releasing as many lithium ions as possible, and as a result, a battery having a higher energy density is superior in performance. It will be.

The positive electrode active material used in the lithium secondary battery of the present invention is mainly composed of amorphous V ₂ O ₅ . In this case, a small amount of a compound called a network former can be added in order to stabilize the amorphous state during repeated charge and discharge, and in particular, V ₂ O ₅ —P ₂ O ₅ and the molar ratio are 95: 5 shows good performance.

[0011]

EXAMPLES The present invention will now be described in more detail with reference to examples, but the present invention is not limited to these examples.

Example 1 Amorphous V ₂ O ₅ -P ₂ O ₅ (molar ratio 95:
5) was used and lithium was used as the negative electrode active material to produce the cylindrical battery shown in FIG. In order to compare and examine the charge / discharge characteristics of the batteries, the following eight types of electrolytic solutions [electrolytic solutions (A) to (H)] were used. Electrolyte _{(A): 1M LiAsF 6 -PC} , electrolyte _{(B): 1M LiAsF 6 -EC} / PC ( volume mixing ratio 10:90), electrolyte _{(C): 1M LiAsF 6 -EC} / PC ( volume Mixing ratio, 30:70), electrolyte (D): 1M LiAsF ₆ -EC / PC (5
0:50), the electrolyte _{(E): 1M LiAsF 6 -EC} / PC ( volume mixing ratio 3:97), electrolyte _{(F): 1M LiAsF 6 -EC} / PC ( volume mixing ratio, 5:95) , Electrolytic solution (G): 1M LiAsF ₆ -EC / PC (volume mixing ratio, 35:65), electrolytic solution (H): 1M LiAsF ₆ -EC / PC (volume mixing ratio, 40:60). The electrolytic solutions (A), (D), (E) and (H) are reference examples for showing the effects of the present invention, and the electrolytic solution (D) is generally considered to exhibit the best cycle life. Has been.
The electrolytic solution (B), the electrolytic solution (C), the electrolytic solution (F) and the electrolytic solution (G) are the electrolytic solution of the battery of the present invention. In the battery charge / discharge test, the discharge current is 600 mA and the charge current is 100 mA.
Then, the end-of-charge voltage is 3.5V, the end-of-charge voltage is 1.8V
I went in. The charge and discharge life of a battery is defined by the following formula (Equation 1): FOM [Figure of Merit (Figure
of Merit)]. The larger this FOM value is,
The charge / discharge cycle performance of the battery is excellent.

[0013]

[Formula 1] FOM = [cumulative discharge capacity] / [discharge capacity of charged lithium in battery]

When evaluating FOM, the initial discharge capacity of 5
The number of charge / discharge cycles at which the discharge capacity of the battery decreased to 0% was defined as the charge / discharge life of the battery. The values of these FOMs are
In this specification, FOM-R was used for comparison. FOM-R
Is the value of the electrolytic solution (D) represented by the following formula (Equation 2) is 1
Is the relative value of FOM.

[0015]

[Formula 2] FOM-R = [FOM of the electrolytic solution of interest
Value] / [FOM value of electrolyte (D)]

As shown in FIG. 2, the electrolytic solution of the present invention (EC
The volume mixing ratio of 5 to 35%) has a longer charge / discharge cycle life of the battery than the electrolytic solutions (A), (D), (E) and (H) of the comparative examples. , It shows excellent performance.

Example 2 In order to investigate the thermal stability of a battery, the electrolytic solution (B) of the present invention was used.
A heating test was performed on the battery prepared in the same manner as in Example 1 by using the electrolyte solution (C). The heating test of the battery was carried out on the battery having the above-mentioned electrolytic solution of the present invention [electrolytic solution (B) and (C)]. The cell was heated to 130 ° C and maintained at 130 ° C for 2 hours. The results are shown in Table 1. A battery in which lithium is used for the negative electrode, the same electrolytic solution as the above electrolytic solution (D) is used, and MoS ₂ having a battery structure substantially similar to the battery used in this example is used for the positive electrode, and M
A report of the same experiment was carried out on a battery using nO ₂ as the positive electrode [Published by The Electrochemical Society, published in The Electrochemical Society, 178th E. C. S. Extended Abstracts of 178th ECS
Meeting, Phenix, Arizona), p. 20 (1991)]
Have reported that MoS ₂ and MnO ₂ batteries ignite or violently smoke during this heating test.
On the other hand, in the battery of the present invention, as shown in Table 1,
No dangerous phenomena such as abnormal temperature rise, voltage drop, safety valve opening, explosion and ignition were observed. From these results, it is understood that the battery of the present invention has excellent practical safety.

[0018]

[Table 1] Table 1 ───────────────────────────────── Results of electrolyte solution heating test ───── ──────────────────────────── (B) The safety valve does not open, there is no voltage drop, there is no ignition, and there is no explosion. (C) Safety valve did not open, no voltage drop, no ignition, no rupture. ──────────────────────────────────

Example 3 A battery using the electrolytic solutions (B) and (C) charged and discharged 50 times under the same charge and discharge conditions as in Example 1, a heating test at 130 ° C., and a battery at 21 ° C. and 55 ° C. The external short circuit test was conducted. The results are shown in Tables 2 and 3.

[0020]

[Table 2] Table 2 ───────────────────────────────── Electrolyte heating test results ───── ──────────────────────────── (B) The safety valve does not open, there is no voltage drop, there is no ignition, and there is no explosion. (C) Safety valve did not open, no voltage drop, no ignition, no rupture. ──────────────────────────────────

[0021]

[Table 3] Table 3 ───────────────────────────── Electrolyte Results of external short circuit test at 21 ℃ and 55 ℃ ── ─────────────────────────── (B) Safety valve does not open, does not ignite, and does not burst. (C) The safety valve did not open, did not ignite, and did not burst. ─────────────────────────────

[0022]

As is clear from the above description, according to the present invention, a non-aqueous solvent type lithium secondary battery excellent in charge / discharge characteristics and safety can be realized.

[Brief description of drawings]

FIG. 1 is a diagram showing a structure of a cylindrical battery as an example of the present invention.

FIG. 2 is a diagram showing the relationship between the charge / discharge life of a battery and the composition of an electrolytic solution.

[Explanation of symbols]

1: Negative electrode, 2: Separator, 3: Positive electrode, 4: Separator, 5: Battery can, 6: Positive electrode tab, 7: Negative electrode tab, 8: Safety valve, 9: Battery cap, 10: Insulator ring

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Junichi Yamaki 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Corporation

Claims (5)

[Claims] 1. A negative electrode capable of discharging and charging lithium or lithium, and a positive electrode mainly composed of amorphous V ₂ O ₅ .
In a lithium secondary battery using an electrolytic solution in which at least one kind of lithium salt is dissolved in an organic solvent, the organic solvent is a mixed solvent of ethylene carbonate and propylene carbonate, and the volume mixing ratio of the ethylene carbonate is 5 Lithium secondary battery characterized by being -35%. 2. The lithium secondary battery according to claim 1, wherein the positive electrode is amorphous V ₂ O ₅ —P ₂ O ₅ . 3. The lithium secondary battery according to claim 1, wherein the negative electrode is metallic lithium, an alloy capable of charging and discharging lithium, or a compound capable of inserting and extracting lithium. 4. The lithium salt is LiPF ₆ , LiAsF.
₆ , LiSbF ₆ , LiClO ₄ , LiCF ₃ SO ₃ ,
LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ S
O ₂ ) ₃ , LiCF ₃ CO ₂ , LiC ₄ F ₉ SO ₂ , L
iBF ₄ , LiAlCl ₄ , LiBr or LiB
The lithium secondary battery according to claim 1, wherein at least one compound selected from (C ₆ H ₅ ) ₄ is used. 5. Lithium is used as a negative electrode, amorphous V ₂ O ₅ —P ₂ O ₅ is used as a positive electrode active material, and LiP is used as an electrolytic solution.
F ₆ , LiAsF ₆ , LiSbF ₆ , LiClO ₄ , L
_{iCF 3 SO 3, LiN (CF} 3 SO 2) 2, LiC
(CF ₃ SO ₂ ) ₃ , LiCF ₃ CO ₂ , LiC ₄ F ₉
At least one lithium salt selected from SO ₂ , LiBF ₄ , LiAlCl ₄ , LiBr, or LiB (C ₆ H ₅ ) ₄ at a concentration of 0.5 to 2.0 M and a volume mixing ratio of ethylene carbonate of 5 to 5%. The lithium secondary battery according to any one of claims 1 to 4, wherein a lithium secondary battery dissolved in a mixed solvent of 35% ethylene carbonate and propylene carbonate is used.