JPS6362869B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an electrolytic solution used in a lithium secondary battery.

Batteries using lithium as a negative electrode active material are being researched as small-sized batteries with high energy density, but secondaryization has become a major problem.

As a positive electrode active material that can be secondaryized, V ₂ O ₅ ,
Metal oxides such as TiO ₂ and layered compounds such as TiS ₂ and WS ₂ are known to intercalate with Li, and to date, sulfides of titanium, zirconium, hafnium, niobium, tantalum, and vanadium have been used. , batteries using selenide, telluride (see US Pat. No. 4,089,052), and batteries using niobium selenide (J.
Electrochem Soc.vol.124, No.7, page 968 and
325 (1977)) etc. are disclosed.

However, compared to such research on positive electrode active materials for secondary batteries, research on the charging and discharging characteristics of Li electrodes is not sufficient, and in order to realize Li secondary batteries, charging and discharging efficiency and cycle life are The search for electrolytes with good charge-discharge characteristics has become a serious issue. As an attempt to improve the charging and discharging efficiency of Li electrodes,
There have been attempts to add additives such as nitromethane and SO ₂ to LiClO ₄ /propylene carbonate [Electrochimica.Acta.Vol.22, pp. 75-83 (1977)] and attempts to use LiClO ₄ /methyl acetate [Electrochimica Acta. Vol.22, pages 85-91
Page (1977)], but the efficiency of the charge/discharge characteristics is less than 50% at the 40th cycle, which is not sufficient, and there is a need for an electrolyte system with even better characteristics.

The present invention has been made in view of the current situation, and its purpose is to provide a non-aqueous electrolyte for lithium secondary batteries that has excellent charging and discharging characteristics of Li electrodes.

Therefore, the nonaqueous electrolyte for lithium secondary batteries according to the present invention uses lithium as a negative electrode active material, is electrochemically active toward lithium ions, and performs a reversible electrochemical reaction with lithium ions. In a secondary battery that uses a substance as a positive electrode active material and a non-aqueous electrolyte in which an inorganic or organic salt is dissolved in an organic solvent, a mixture of propylene carbonate and tetrahydrofuran is used as the organic solvent of the non-aqueous electrolyte. It is characterized by the fact that

The charging and discharging efficiency of Li is mainly affected by the physical properties of the lithium surface film, which is a reaction product of activated lithium deposited during charging and a solvent. An attempt has been made in primary batteries to reduce the viscosity by mixing tetrahydrofuran with propylene carbonate and to facilitate the diffusion of the electrolyte within the positive electrode active material.
−19052). However, from this literature, the interaction between lithium and the solvent, that is, the interaction of the mixed solvent on the charging and discharging efficiency of Li, that is, the effect of the mixed solvent on the charging and discharging efficiency of Li, is completely unclear. Furthermore, even if a low viscosity solvent is mixed with propylene carbonate to lower the viscosity, the charge/discharge efficiency of Li does not improve. For example, if tetrahydropyran or dimethylacetamide is mixed with propylene carbonate, On the other hand, the charging/discharging efficiency of Li is 1/10
On the contrary, it decreases from ~3/4. That is, there is no correlation between lowering the viscosity of the electrolytic solution and Li charge/discharge efficiency.

On the other hand, as a result of examining the state of the Li surface, the inventors found that the state of the Li electrode surface before discharge is the same for propylene carbonate and propylene carbonate/tetrahydrofuran mixed systems, but that the state of the Li electrode after charging is the same.
The surface conditions are completely different (the Li surface film is different)
I found out. In other words, it was found that in the propylene carbonate alone system, lithium was precipitated in a dendritic state, whereas in the propylene carbonate/tetrahydrofuran mixed system, lithium was precipitated in the form of particles with good smoothness, forming a smooth film on the surface.

This difference in surface state is the cause of improving the charging and discharging efficiency of Li, as will be clear from the examples described later.

However, as shown in the examples below, when a mixed solvent electrolyte of propylene carbonate and tetrahydrofuran is used in a primary battery, the volume mixing ratio of tetrahydrofuran to propylene carbonate is in the range of 3:7 to 2:8. shows good characteristics.

However, as shown in the examples below, the optimum conditions for use in secondary batteries are a volumetric mixing ratio of tetrahydrofuran to propylene carbonate of 8:2 to 3:7; However, the charging and discharging characteristics are not good. This is because, as explained above, the characteristics of the electrolyte used in primary batteries and secondary batteries are different, and especially in the case of secondary batteries, fine control of the Li surface film composition is required.

According to the non-aqueous electrolyte for lithium batteries according to the present invention, since a mixture of propylene carbonate and tetrahydrofuran is used as the organic solvent of the non-aqueous electrolyte, the charge/discharge efficiency of the formed lithium battery is significantly improved.

The present invention will be explained in more detail.

As mentioned above, the organic solvent of the electrolytic solution according to the present invention is propylene carbonate and tetrahydrofuran, but the solute dissolved therein can be any solute conventionally used in this type of battery. For example, LiClO ₄ , LiBF ₄ , LiAsF ₆ ,
Inorganic salts such as LiPF ₆ , LiAlCl ₄ , and CF ₃ SO ₃ Li,
Organic salts such as CF ₃ COOLi can be used.

These solutes are preferably added to the organic solvent.
0.5-2.5N is dissolved. This is because if the amount of solute dissolved in propylene carbonate is less than 0.5N, the charge/discharge characteristics will be significantly reduced, and if it exceeds 2.5N, the solute will not dissolve. Furthermore, if the concentration of the solute dissolved in tetrahydrofuran is less than 0.5N, the charge-discharge characteristics will deteriorate, and if it exceeds 2.5N, the viscosity will increase, and the charge-discharge characteristics will also deteriorate. Most preferably, each is around 2N.

The mixing ratio of propylene carbonite and tetrahydrofuran is 0.5 to 2.5N of propylene carbonite dissolved in the solute and 0.5 to 2.5N of the solute dissolved in the propylene carbonite and tetrahydrofuran.
The volume ratio of 2.5N dissolved tetrahydrofuran is preferably 2:8 to 7:3, most preferably 4:
It is around 6. If the mixing ratio of propylene carbonite in which the solute is dissolved is less than 2:8, the meaning of mixing the two becomes weak, and the charge-discharge characteristics approach that of tetrahydrofuran alone, and if it is more than 7:3, On the contrary, the charging and discharging characteristics approach those of propylene carbonite alone, and the charging and discharging characteristics deteriorate in any case.

Examples of the present invention will be described below.

Example 1 Propylene carbonate was obtained by distilling a commercially available product under reduced pressure (degree of vacuum: 4 mmHg, distillation temperature: 109°C), adding molecular sieve, and drying under vacuum heat.
Mixed with LiClO ₄ and stored in an argon dry box.

Tetrahydrofuran was distilled under normal pressure under a stream of argon gas (distillation temperature 65°C), then molecular sieves were added and vacuum heat dried.
The electrolyte was mixed with LiClO ₄ and stored in an argon dry box.

The mixed solvent is propylene carbonate/ _LiClO4 formed as above and tetrahydrofuran/
LiClO ₄ was mixed, molecular sieve was added to this mixed solvent electrolyte, and the mixture was stored in an argon dry box.

P _t electrode is the working electrode, Li is the counter electrode, and Li is the reference electrode.
The charge and discharge characteristics of the Li electrode were measured by assembling a battery using the Pt electrode and depositing Li on the Pt electrode. The electrolyte contains 2NLiClO ₄ /propylene carbonate and
2NLiClO ₄ /tetrahydrofuran at a volume ratio of 4:
The mixture in step 3 was used. The electrical conductivity of this mixed electrolyte system was 9×10 ⁻³ Ω ⁻¹ cm ⁻¹ .

In the measurement, Li was first deposited on the Pt electrode for 1 minute at a constant current of 5 mA/cm ² , and after charging, the Li deposited on the Pt electrode was deposited on the Pt electrode at a constant current of 5 mA/cm ² for 1 minute at 5 mA/cm ^{2 .}
A cycle test was conducted in which the battery was discharged as ions.
The charge/discharge efficiency was determined from the potential change of the Pt electrode and calculated from the ratio of the amount of electricity required to discharge Li deposited on the Pt electrode as Li ⁺ ions and the amount of electricity charged to deposit Li on the Pt electrode.

FIG. 1 is a diagram showing the relationship between charge/discharge efficiency and cycle number, in which a is the case when the above electrolyte is used, and d and e are 2NLiClO ₄ /propylene carbonate and 2NLiClO ₄ / The charging and discharging characteristics when using an electrolyte containing only tetrahydrofuran are shown as a reference example. As can be seen from FIG. 1, the charge/discharge cycle life of the mixed system a was clearly improved dramatically compared to the single systems d and e.
The charge/discharge efficiency after 400 cycles is 72.5 at the 500th cycle.
%, it showed extremely good values of 53.5% at the 1000th time and 49% at the 2000th time.

Example 2 The charging and discharging characteristics of the Li electrode were determined in the same manner as in Example 1, except that a mixture of 1.25NLiClO ₄ /propylene carbonate and 1.25NLiClO ₄ /tetrahydrofuran at a volume ratio of 4:6 was used as the electrolyte. Measurements were made. The conductivity of this mixed electrolyte system is 9.8×
It was 10 ^-3 Ω ^-1 cm ^-1 . Charging/discharging efficiency and number of cycles when performing a Li electrode charge/discharge cycle test using an electrolyte containing a mixture of 1.25NLiClO ₄ /propylene carbonate and 1.25NLiClO ₄ /tetrahydrofuran at a volume ratio of 4:6 in combination with a Li electrode. The relationship is shown by b in FIG. As can be seen from b, d, and e in FIG. 1, the charge/discharge cycle life could be improved by using the electrolyte b as compared to the single systems d and e.

Example 3 The charging and discharging characteristics of Li electrodes were determined in the same manner as in Example 1, except that a mixture of 1.25NLiClO ₄ /propylene carbonate and 2NLiClO ₄ /tetrahydrofuran at a volume ratio of 2:3 was used as the electrolyte. Measurement was carried out. The conductivity of this mixed electrolyte system is 10.0×
It was 10 ^-3 Ω ^-1 cm ^-1 . By combining an electrolyte containing a mixture of 1.25NLiClO ₄ /propylene carbonate and 1.25NLiClO ₄ /tetrahydrofuran with a Li electrode,
Figure 1c shows the relationship between the charge and discharge efficiency and the number of cycles when a Li electrode was subjected to a charge and discharge cycle test.
As can be seen from c, d, and e in Figure 1, the independent system d, e
By using the above electrolyte c, the charge/discharge cycle life could be improved compared to the above.

As is clear from the above explanation, according to the present invention, by using an inorganic salt as a solute and a mixed solvent of propylene carbonate and tetrahydrofuran as a solvent, the charge and discharge characteristics of the Li electrode are good and high. It is possible to realize a nonaqueous electrolyte for lithium secondary batteries that has Li ⁺ ion conductivity.

Example 4 1.5NLiAsF ₆ or 1NLiPF ₆ as electrolyte
The charging and discharging characteristics of the Li electrode were measured in the same manner as in Example 1, except that a mixture of propylene carbonate (PC) and tetrahydrofuran (THF) (1/1) was used. Figure 2 shows the results. As can be seen from Fig. 2, by using the electrolyte of the present invention (Fig. 2 a, b), PC
Compared to the single system (Fig. 2 c, d), the charge/discharge cycle life could be improved.

Example 5 FIG. 3 is a schematic cross-sectional view of a battery cell for measuring the characteristics of a button-type battery, which is a specific example of the present invention.
Ni-plated brass container (hereinafter referred to as the container); 2 is a lithium negative electrode; 3 is a porous polypropylene diaphragm (hereinafter referred to as the diaphragm); 4 is a felt made of carbon fiber (hereinafter referred to as the felt);
6 indicates a positive electrode mixture, 6 indicates a Teflon container, and 7 indicates a Ni lead wire. A positive electrode mixture 5 was inserted into a concave chamber with a diameter of 26 mm in a container 1, a felt 4 for impregnating an electrolytic solution was placed on top of the positive electrode mixture 5, a lithium negative electrode 2 was placed with a diaphragm 3 in between, and the container 6 made of Teflon was tightened. The lithium negative electrode 2 was a disk with a diameter of 20 mm, and the felt 4 and the diaphragm 3 were also disk-shaped.

Positive electrode mixture 5 consists of 0.1 g of V ₂ O ₅ and acetylene black.
It was formed by mixing 0.1 g.

Propylene carbonate (hereinafter abbreviated as PC) and tetrahydrofuran (hereinafter abbreviated as PC) are used as electrolytes.
1 mol/LiClO ₄ in a mixed solvent of (abbreviated as THF)
A dissolved solution was used. When the battery thus created was subjected to constant current discharge at 3 mA/cm ² , the relationship between the discharge capacity per ₅ weight of V ₂ O and the solvent mixture ratio until the battery's final discharge voltage reached 1 V was as follows: It looked like Figure 4. As can be seen from Figure 4, in order to increase the discharge capacity, that is, for primary batteries, the optimum conditions for using the above electrolyte are that the volumetric mixing ratio of THF to PC is 3:7.
~2:8 is desirable. In addition, for another battery made as above, the current was 150A at a constant current of 3mA/ ^cm2 .
When we conducted a charge/discharge cycle test that repeated discharging and charging at a constant capacity of h/Kg, we found that the final discharge voltage was
The relationship between the number of charge/discharge cycles until the voltage reached 1V and the solvent mixture ratio was as shown in Figure 5. against PC
Particularly excellent charging and discharging characteristics were exhibited in the THF volume mixing ratio range of 8:2 to 3:7, and the conditions for using the above electrolyte for secondary batteries are significantly different from those for primary batteries. I understand. Therefore, when using the electrolyte of the present invention in a secondary battery, PC
The optimum volumetric mixing ratio of THF to 8:2 to 3:7.

If it exceeds this range, excellent charge-discharge characteristics cannot be obtained.

[Brief explanation of drawings]

Figures 1 and 2 are diagrams showing the relationship between the charge/discharge efficiency of a lithium electrode and the number of cycles in an example of the present invention, and Figure 3 is a schematic cross-sectional diagram of a battery mole for measuring the characteristics of a button-type battery. , Figure 4 shows the discharge capacity and
Diagram showing the relationship between the volumetric mixing ratio of PC and THF, 5th
The figure shows the relationship between the number of charge/discharge cycles and the volumetric mixing ratio of PC and THF. 1... Container, 2... Lithium negative electrode, 3... Diaphragm, 4... Felt, 5... Positive electrode mixture, 6... Teflon container, 7... Ni lead wire.

Claims (1)

[Claims] 1. Lithium is used as a negative electrode active material, a substance that is electrochemically active towards lithium ions and undergoes a reversible electrochemical reaction with lithium ions is used as a positive electrode active material, and an inorganic or organic salt is dissolved in an organic solvent. In a secondary battery using a nonaqueous electrolyte, a mixture of propylene carbonate and tetrahydrofuran is used as the organic solvent of the nonaqueous electrolyte, and the volume mixing ratio of tetrahydrofuran to propylene carbonate is 8:2 to 3:7. A non-aqueous electrolyte for lithium secondary batteries featuring: