JP2778065B2 - Non-aqueous electrolyte secondary battery - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a non-aqueous electrolyte secondary battery.

2. Description of the Related Art Conventionally, a non-aqueous electrolyte battery of this type has a high voltage and a high energy density, and thus has been widely used as a power source for consumer electronic devices. Recently, attempts to convert this battery into a secondary battery are active. At present, a cylindrical battery using manganese dioxide for the positive electrode has also been announced. In order to achieve higher energy density, lithium metal is used for the negative electrode. In this case, the chemical reaction between lithium metal and the electrolyte, that is, the supporting salt, the organic solvent, and the impurities contained therein, and the electrochemical reaction during charging It is required to minimize consumption of the lithium anode due to the thermal decomposition reaction. With the current technology, the above-mentioned consumption is inevitable, so that the capacity of the negative electrode is inevitably 3 to 4 times larger than the capacity of the positive electrode.

Problems to be Solved by the Invention Lithium metal is used for the negative electrode, and charging / discharging reversibility of the negative electrode lithium is one of the major issues in the development of non-aqueous electrolyte secondary batteries that attempt to be reusable by charging. . For that purpose, it is essential to develop or select an optimal supporting salt and an organic solvent which are chemically stable and provide reversibility. Of the support salts that have the greatest impact, lithium arsenate (LiAsF ₆ ), which has the best charge-discharge efficiency, which is an indicator of charge-discharge reversibility, is almost at the same level. Lithium perchlorate (LiCl
O ₄ ), followed by lithium trifluoromethanesulfonate (LiCF ₃ SO ₃ ) and lithium hexafluorophosphate (LiPF ₆ ). On the other hand, LiAsF ₆ and LiClO ₄ are superior in terms of high-temperature storability, and LiPF ₆ and LiCF ₃ SO ₃ belong to lower ranks. Ethylene carbonate (EC) is most preferable as the organic solvent in view of the structural stability of the molecule and the deposition form of the subsequent reaction product. However, considering that it is a solid at room temperature, it is difficult to use ethylene carbonate (EC) with other relatively stable solvents. Most problems can be solved by using a mixed solvent.

Among the above supporting salts, those containing arsenic (AS) further have problems in terms of environmental preservation, and LiClO ₄ has problems in terms of safety after overdischarge and reversal when converted into a battery. hard. The remaining LiCF ₃ SO ₃ is not practical in terms of cost and polarization characteristics based on ionic conductivity and solubility. Finally, improving the high-temperature storage characteristics of LiPF ₆ is an issue for putting batteries to practical use.

An object of the present invention is to solve the above-mentioned problems, and an object of the present invention is to obtain a secondary battery having a non-aqueous electrolyte having excellent charge / discharge efficiency and high-temperature storability.

Means to solve the problem It is to regulate the residual amount of hydrogen fluoride (HF) used in the stable and high-purity production of LiPF ₆ as the above supporting salt, together with EC. When used, the amount
It should be less than 500ppm.

Action HF remaining in LiPF ₆ becomes hydrofluoric acid in the battery due to water remaining in components such as the separator and the positive electrode, and this acid decomposes the organic solvent in the electrolyte. It is thought that this will reduce the From this, by limiting the amount of HF in LiPF ₆ , decomposition of the electrolytic solution is suppressed, and a nonaqueous electrolytic solution secondary battery having excellent high-temperature storage characteristics can be obtained.

Embodiment An embodiment of the present invention will be described below with reference to the drawings.

Example 1 FIG. 1 shows a coin-shaped non-aqueous electrolyte secondary battery used in Examples. In the figure, 1 is a corrosion-resistant stainless steel case, 2 is a sealing plate of the same material, 3 is a current collector made of a 120 mesh stainless steel net spot-welded to the inner surface of the sealing plate, 4 is lithium having a thickness of 80 μm and a diameter of 15.0 mm. Negative electrode 3
With a capacity of 26.5 mAh. Reference numeral 5 denotes a polypropylene microporous film separator. Reference numeral 6 denotes a positive electrode, which is made of chemically synthesized manganese dioxide in the atmosphere.
100 mg of a mixture obtained by adding 10 parts by weight of carbon black and 20 parts by weight of a fluorocarbon resin binder to 70 parts by weight of the substance baked at 0 ° C. for 120 hours and molding the mixture to a diameter of 15 mm and a thickness of 0.9 mm. The capacity is about 22mAh.

The solvent of the electrolytic solution was a mixed solvent of the same volume of ethylene carbonate (EC) and propylene carbonate (PC) shown above. LiPF ₆ was used as the supporting salt, and the amount of residual hydrogen fluoride (HF) was , As shown in Table 1, 1000,750,500,2
A concentration of 50,100 ppm was used, and the concentration was usually used as 1 mol / concentration.

For comparison, an electrolytic solution having the same composition and concentration using LiAsF ₆ as a supporting salt was prepared. After injecting 100 μl of the above electrolyte into the sealing plate, the separators 5 and 6 and the positive electrode were placed, and the container was sealed with a gasket 7 by caulking. These batteries have a diameter
20mm, total height 1.6mm, Table 1 shows the battery number and electrolyte content
It was shown to. These batteries were pre-discharged at 1 mA for 5 hours to clean the surface of the negative electrode lithium, and then stored at 70 ° C. for 40 days. Next, a sine-wave AC voltage having a frequency of 10 KHz to 1 Hz of ± 5 mV is applied to these open-circuit voltages, and a complex plane analysis of the battery is performed from the response current. The impedance at 1 KHz, the charge transfer resistance, and the interface The capacitance and contact resistance were measured. FIGS. 2A and 2B show the impedance value and the charge transfer resistance value indicating the degree of influence on the lithium negative electrode as a result of the reaction occurring in the battery, respectively. In the figure, the battery C from a comparison of the LiAsF ₆ excellent in storability, D, E
That is, it is understood that the condition for storage stabilization is that the amount of residual HF in LiPF ₆ is 500 ppm or less.

Example 2 A battery was prototyped using the same materials and the same conditions as in Example 1. However, the electrolyte solution is the same volume mixed solvent of ethylene carbonate and dimethoxyethane (DME), and the supporting salt is LiPF ₆ having the same amount of residual HF of 1000, 750, 500, 250 and 100 ppm. / Concentration adjusted. Again, LiAsF ₆ for comparison
An electrolytic solution having the same composition and concentration as above was used (Table 2).

After the same pre-discharge conditions as in Example 1, the storage characteristics were 60
Measured after storage at 60 ° C for 60 days. FIGS. 3A and 3B show the impedance value and the charge transfer resistance value. From the figure, it can be seen from the comparison with LiAsF ₆ having excellent storage properties that the amount of residual HF in the batteries C ′, D ′, E ′, that is, LiPF ₆ is desirably 500 ppm or less.

Effects of the Invention As described above, according to the present invention, in a non-aqueous electrolyte secondary battery, lithium hexafluorophosphate is used in an electrolyte containing a solvent containing ethylene carbonate and lithium hexafluorophosphate as a supporting salt. By setting the amount of residual hydrogen fluoride contained in the steel to 500 ppm or less, an effect excellent in high-temperature storage characteristics can be obtained.

In the examples, manganese dioxide was used for the positive electrode.
Other materials may be used. Although PC and DME were used as the solvent for the electrolytic solution, other solvents may be used.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a coin-type battery according to an embodiment of the present invention, and FIGS. 2A and 2B and FIGS. FIG. 1 ... case, 2 ... sealing plate, 2 ... lithium negative electrode, 5
... Positive electrode.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-62-219475 (JP, A) JP-A-59-81870 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) H01M 10/40 H01M 6/16

Claims (1)

(57) [Claims] 1. A secondary battery comprising a negative electrode containing a light metal as an active material, a non-aqueous electrolyte, and a positive electrode having charge / discharge reversibility, wherein the non-aqueous electrolyte is a solvent containing ethylene carbonate. A non-aqueous electrolyte secondary battery characterized by using lithium hexafluorophosphate having an amount of residual hydrogen fluoride of 500 ppm or less as a supporting salt.