JPH04171674A - Nonaqueous-electrolyte secondary battery - Google Patents

Abstract

PURPOSE:To improve the cycle characteristic by containing cyclic carbonate and chain carbonate in the solvent component of a nonaqueous electrolyte. CONSTITUTION:A positive electrode mix using lithium-cobalt composite oxide as a main active material is molded in a can to form a positive electrode 6, and it is fixed to a grid 7 made of titanium. The positive electrode 6 and a negative electrode 4 are impregnated with an electrolyte, then they are coupled via a separator 5 and caulked together with a gasket 8 made of polypropylene and sealed. Cyclic carbonate and chain carbonate are contained in the solvent component of the nonaqueous electrolyte. The decomposition of the electrolyte and the breakdown of the layer structure of a negative electrode carbon material by the electrolyte are suppressed. The cycle characteristic is improved.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a non-aqueous electrolyte secondary battery, and particularly to improvement of its cycle characteristics.

Conventional technology Conventionally, this type of non-aqueous electrolyte battery has been widely used as a power source for consumer electronic devices because it has high voltage, high energy density, and has excellent reliability such as storage performance and leakage resistance. There is.

Recently, there have been many attempts to convert this battery into a secondary battery. Alloys, carbon materials, and conductive polymers that can repeatedly release and store lithium ions as negative electrodes for secondary batteries.
Metallic lithium and other materials are being considered. In addition, transition metal oxides and chalcogen compounds having a layered or tunnel-type crystal structure and having a reactivity capable of accommodating lithium ions eluted from the negative electrode are being considered for the positive electrode. Furthermore, during the charging and discharging process of secondary batteries, lithium ions move between the positive and negative electrodes via an electrolyte, and propylene carbonate is often used in primary batteries as a solvent for the electrolyte. This is because propylene carbonate dissolves the supporting salt well, is stable against lithium, and has excellent discharge characteristics.

For example, it is used in primary batteries such as lithium/manganese dioxide and lithium/copper oxide batteries.

Problems to be Solved by the Invention As described above, propylene carbonate is an excellent solvent for primary batteries, but when used alone as a solvent for secondary batteries that use a carbon material for the negative electrode, it decomposes during charging. Good cycle characteristics cannot be obtained. Several reports have been made regarding the cause of the decomposition, but the details are unknown.

On the other hand, when other cyclic carbonates are considered as a solvent for the secondary battery, for example, ethylene carbonate has a high freezing point of 36.4° C., so the freezing point of the electrolyte becomes high even when considering the freezing point drop due to solute dissolution. That is, -2
At temperatures as low as 0°C, the electrolyte becomes solid and the battery does not operate. 4. Therefore, it is difficult to use it alone as an electrolyte for secondary batteries.

Furthermore, when similar studies are attempted for chain carbonates, the decomposition of the solvent and solidification at low temperatures observed with cyclic carbonates are alleviated. However, with repeated charging and discharging, the internal impedance of the battery increases significantly. As a result, the polarization of the battery increases, resulting in a decrease in battery voltage during discharge, resulting in a decrease in discharge capacity. This is because the molecules of chain carbonate are smaller than the molecules of cyclic carbonate, so that they are easily incorporated into the negative electrode carbon layer together with lithium ions during charging. This expands the interlayer distance of the negative electrode carbon. Therefore, it is thought that as lithium ions are repeatedly stored and released through charging and discharging, the layered structure of the negative electrode carbon progresses and the internal impedance of the battery increases.

The present invention aims to solve the above problems and improve cycle characteristics.

Means for Solving the Problems The present invention includes a non-aqueous electrolytic solution containing a cyclic carbonate and a chain carbonate as a solvent component.

Function In the present invention, by including cyclic carbonate and chain carbonate in the solvent component of the nonaqueous electrolyte, decomposition of the electrolyte and destruction of the layer structure of the negative electrode carbon material by the electrolyte can be suppressed, and cycle characteristics can be improved. be.

Example Examples of the present invention will be described below.

FIG. 1 is a sectional view of a coin-shaped non-aqueous electrolyte secondary battery used in an example. In the figure, 1 is a case made of corrosion-resistant stainless steel.
2 is a sealing plate made of the same material, 3 is a nickel grid spot welded to the inner surface of the sealing plate 2, and 4 is a negative electrode active material mainly composed of carbon molded inside the can and fixed to the nickel grid 3. has been done. 5 is a three-dimensional pore structure (
polyolefins (polypropylene, spongy)
A separator made of a microporous film of polyethylene or a copolymer thereof. Reference numeral 6 denotes a positive electrode, which is formed by molding a positive electrode mixture containing lithium cobalt composite oxide (LiCOO2) as the main active material in a can, and is fixed to a grid 7 made of titanium. After the positive electrode 6 and the negative electrode 4 are impregnated with an electrolyte, they are coupled via a seven-layer generator 5,
It was caulked and sealed together with the polypropylene gasket No. 8.

[Example 1] As a solvent for the electrolytic solution, dimethyl carbonate (hereinafter referred to as DMC), which is a chain carbonate, and diethyl carbonate (hereinafter referred to as DEC) were used alone (solvents ① and ①), and ethylene carbonate (hereinafter referred to as DEC) was used. ), DMC, and EC,! : DEC volume ratio 50
The above coin batteries were prototyped using four types of solvents (referred to as solvents ■ and ■) mixed at 50%. Lithium perchlorate was used as the solute in the electrolytic solution, and the concentration was adjusted to 1 mol/l.

The test conditions were a constant current of 1mA and a voltage of 4.2■ at the end of charging.
The voltage at the end of discharge is set to 3. Ov, and charging and discharging were repeated 100 cycles. The results are shown in FIGS. 2 and 3.

The battery numbers are assigned to correspond to the solvent numbers.

FIG. 2 shows the change in discharge capacity due to repeated charging and discharging.

From Figure 2, compared to the chain carbonate single system of ■ and ■,
, ■ The mixed system with EC shows little decrease in discharge capacity due to repeated charging and discharging. This is because the average discharge voltage in cases ① and ① decreased more with repeated charging and discharging.

Next, FIG. 3 shows the internal resistance of the battery measured after each charge, and its change with repeated charging and discharging.

Figure 3 shows that in the chain carbonate single systems of ■ and ■, a significant increase in internal resistance is observed as charging and discharging are repeated, but no such remarkable increase is observed in the mixed systems with EC, as shown in ■ and ■. Ta. It is thought that this difference in internal resistance causes the difference between ■, ■ and ■, ■ in FIG.

From these results, it is considered that the intrusion of chain carbonate into the interlayers of negative electrode carbon can be suppressed by the presence of cyclic carbonate. Furthermore, the reason why the internal resistance of (2) is smaller than that of (2) is that the molecules of DEC are larger than those of DMC and are less likely to enter between the layers of negative electrode carbon, making it difficult to cause destruction of the negative electrode structure.

[Example 2] Propylene carbonate (hereinafter referred to as PC) was used as a solvent for the electrolytic solution.
The above-mentioned coin batteries were trial-manufactured using three types of solvents: 1) alone (designated as solvent ①), PC and DMC, and a mixture of PC and DEC at a volume ratio of 50:50 (designated as solvents ① and ①). Conditions for adjusting the electrolytic solution and test conditions are as in Example 1.
The same is true. The results are shown in FIGS. 4 and 5.

The battery numbers are assigned to correspond to the solvent numbers.

FIG. 4 is similar to FIG. 2, and FIG. 5 is similar to FIG. 3, respectively.

From FIG. 4, it is seen that PC (■) alone has a low capacity and a short cycle life. In comparison, the mixed systems with chain carbonate (2) and (2) have a relatively large capacity and a long cycle life. Moreover, as shown in FIG. 5, the internal resistance of the battery (■) increased significantly as charging and discharging were repeated, while no such significant increase was observed in the batteries (2) and (4). This is considered to be because in the battery (2), a reaction between the negative electrode carbon and the PC occurred as described above, and a part of the charging capacity was consumed by the decomposition of the PC. As a result, the discharge capacity became smaller, and it is thought that the liquid dried up due to the decomposition of the PC, causing an increase in internal resistance. 1) It was found that the phenomena observed with C alone can be improved by using a mixed solvent with a chain carbonate. Also, ■ and ■ in Example 1
It was found that the insertion reaction of the chain carbonate into the interlayer of the negative electrode carbon was suppressed by the PC since the properties shown in the cases ① and ② were better than those observed in the above. In other words, it has been found that when PC and chain carbonates, which exhibit poor cycle characteristics when used alone, are used in combination, they exhibit better cycle characteristics due to their interaction.

From the above results, it was found that including a cyclic carbonate and a chain carbonate in the solvent component of the nonaqueous electrolyte according to the present invention has a great effect on improving cycle characteristics.

In the examples, a lithium cobalt composite oxide was used as the positive electrode active material, but other lithium-containing compounds such as a lithium manganese composite oxide may be used.

= 9- Moreover, although propylene carbonate and ethylene carbonate are two examples of cyclic carbonates as solvent components of the electrolytic solution, other cyclic carbonates such as butylene carbonate may be used, or a mixture of two or more types may be used. The chain carbonate may also be dipropyl carbonate, methyl ethyl carbonate, or a mixture of two or more thereof. Further, a mixed solvent of cyclic carbonate, chain carbonate, and one or more other solvents such as lactones such as γ-butyrolactone, ethers such as 1,2-dimethoxyethane, etc. may also be used.

Effects of the Invention As described above, the present invention can provide a non-aqueous electrolyte secondary battery with excellent cycle characteristics.

[Brief explanation of drawings]

Figure 1 is a sectional view showing a typical structure of the coin-shaped battery according to the present invention, Figure 2 is a cycle characteristic diagram of discharge capacity in Example 1, and Figure 3 is the inside of the battery measured after charging in Example 1. FIG. 4 is a cycle characteristic diagram of the discharge capacity of Example 2, and FIG. 5 is a cycle characteristic diagram of the internal resistance of the battery measured after charging in Example 2. DESCRIPTION OF SYMBOLS 1... Positive electrode case, 2... Negative electrode sealing plate, 3... Negative electrode current collector, 4... Negative electrode, 5... Separator, 6...
- Positive electrode, 7... Positive electrode current collector, 8... Gasket.

Claims (3)

[Claims] (1) A negative electrode made of a carbon material capable of intercalating and releasing lithium ions, a nonaqueous electrolyte, and a positive electrode made of a lithium-containing compound, and the solvent of the nonaqueous electrolyte contains a cyclic carbonate and a chain carbonate. A non-aqueous electrolyte secondary battery featuring: (2) The nonaqueous electrolyte secondary battery according to claim 1, wherein the cyclic carbonate that is a solvent component of the electrolyte contains ethylene carbonate. (3) The non-aqueous electrolyte secondary battery according to claim 1, wherein the chain carbonate as a solvent component of the electrolytic solution contains at least one of dimethyl carbonate and diethyl carbonate.