JPH0675401B2 - Method for preparing non-aqueous electrolyte - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial field of application> The present invention relates to lithium hexafluorophosphate (LiPF ₆ ).
The present invention relates to a method for preparing a non-aqueous electrolytic solution in a non-aqueous electrolytic solution battery configured by using a non-aqueous electrolytic solution containing substantially as a solute.

<Prior Art> A non-aqueous electrolyte battery is provided with a negative electrode whose active material is an alkali metal or alkaline earth metal such as lithium or sodium, and a positive electrode mixture whose active material is manganese dioxide or fluorocarbon. Further, as the electrolytic solution, propylene carbonate, γ-butyrolactone, dimethoxyethane, tetrahydrofuran and other non-aqueous organic solvents, alkali metal salts such as lithium perchlorate or lithium tetrafluoroborate or lithium hexafluorophosphate as a solute It employs a configuration that uses a mixed and dissolved non-aqueous electrolyte.

Among the solutes used in the above non-aqueous electrolyte, lithium hexafluorophosphate (LiPF ₆ ) has a large ionic conductivity, so an electrolyte using this can improve battery performance, and lithium perchlorate (LiPF ₆ ) Compared with LiClO ₄ ), it is characterized by higher safety without danger of explosion and is known as a solute of electrolyte for non-aqueous electrolyte secondary batteries.

<Problems to be Solved by the Invention> However, the above-mentioned lithium hexafluorophosphate is extremely unstable in a crystalline state, gradually decomposes at room temperature as shown by the following formula, and even if a trace amount of water is present, It has the drawback of being hydrolyzed as in the formula.

(When water does not exist) LiPF ₆ → LiF + PF ₅ (when water exists) LiPF ₆ + H ₂ O → LiF + POF ₃ ＋ 2HF POF ₃ ＋ 3H ₂ O → H ₃ PO ₄ ＋ 3HF Lithium hexafluorophosphate (LiPF ₆ ) Is stable (up to about 80 ° C) when dissolved in a non-aqueous solvent solution.

Therefore, when it is stored, or when it is mixed with an organic solvent and dissolved, it is necessary to store under an inert atmosphere or the like, and it is necessary to perform the work while suppressing the temperature rise due to the heat of dissolution during mixing, Strict control is required for its storage and handling. Further, it is practically difficult to completely suppress the above decomposition even with such management, and there is a problem that the decomposition product causes the solvent in the electrolytic solution to polymerize, resulting in deterioration of battery performance.

<Means for Solving Problems> The method for preparing a non-aqueous electrolytic solution according to the present invention is a method for preparing a non-aqueous electrolytic solution used in a non-aqueous electrolytic solution, wherein hexafluorophosphoric acid is used as a non-aqueous organic solvent. The gist is that potassium is dissolved, and then the potassium ion in the organic solvent is replaced with lithium ion by electrolysis using lithium as an anode.

<Operation> By using the above means, a non-aqueous electrolytic solution having substantially the same composition as that obtained by dissolving lithium hexafluorophosphate in a crystalline state in an organic solvent can be obtained. And in this case, since the crystals of potassium hexafluorophosphate are stable at room temperature and do not cause hydrolysis, the above-mentioned decomposition as in the case of using the crystals of lithium hexafluorophosphate does not occur, and the production and handling are performed. It becomes very easy and the deterioration of battery performance due to the above decomposition products can be suppressed.

<Example> 20 ml of a solvent in which 1,3-dioxolane and 1,2-dimethoxyethane purified by distillation were mixed at a volume ratio of 1: 1 (water content 30 mg /)
, Potassium hexafluorophosphate (KPF ₆ ) 2.95 g (0.8
(Molar concentration) was dissolved to make an organic electrolyte. Put this organic electrolyte as an electrolyte in a Teflon container, and
Using a lithium plate of mm × 15mm as an anode and a nickel plate of the same size as a cathode, 30mA
Electrolysis was performed for 26 hours at a current of (current density 8 mA / cm ² ). As the electrolysis progresses, lithium ions Li ⁺ are eluted from the lithium plate into the electrolytic solution, while potassium, which has a higher reduction potential than lithium, is preferentially electrodeposited on the nickel plate. A reaction occurred in which the ion K ⁺ was replaced by the lithium ion Li ⁺ . In addition, since a considerable amount of lithium is codeposited on the nickel plate at the end of electrolysis,
In order to sufficiently perform the substitution, it is necessary to apply a larger amount of electricity than the theoretical value to carry out the electrolysis. In the case of this embodiment, electricity of 180% of the logical value was applied.

Then, a positive electrode 1 obtained by mixing titanium disulfide TiS ₂ , graphite and PTFE powder in a weight ratio of 4: 1: 0.4 and press-molded into a disk shape is passed through a positive electrode current collector 2 made of a stainless net. SU
The present invention having a diameter of 20 mm and a height of 1.6 mm shown in FIG. 1 is obtained by pressing the inner surface of an S304 positive electrode can 3 and injecting 120 μ of the above-mentioned substituted electrolytic solution into the positive electrode 1 to absorb the liquid. I assembled the batteries. In the figure, 4 is a separator made of propylene non-woven fabric, 5 is a lithium negative electrode, 6 is a negative electrode terminal made of SUS304, and 7 is a sealing gasket made of polypropylene.

On the other hand, 2.43 g of commercially available lithium hexafluorophosphate (0.8 molar concentration: sealed and stored in a Teflon container in an argon atmosphere) was dissolved in 20 ml of the same mixed solvent as above to prepare a non-aqueous electrolytic solution. A conventional battery was made under the same conditions as the battery of the present invention except that it was used.

Each of the battery of the present invention and the conventional battery was stored in an amount of 10 cells at an environmental temperature of 60 ° C., and changes in internal resistance (R _AC ) with time (average value of 10 cells each) were examined. The results are shown in Fig. 2, and the internal resistance of the battery of the present invention was 3 even after storage for 30 days.
In contrast, the conventional battery showed an internal resistance that was more than four times that of the initial value after 30 days, even though it increased by about 30%.

In the conventional battery, such a significant increase in internal resistance is due to the fact that some of the crystals of lithium hexafluorophosphate used as a solute in the non-aqueous electrolyte decompose when stored or dissolve in a solvent. Due to the action of the product, a part of the solvent is polymerized during storage to increase the viscosity of the electrolytic solution, and the decomposition product reacts with lithium of the negative electrode to form an insoluble film on the surface of the lithium negative electrode. It seems to be the main cause. In the battery of the present invention, since the degree of increase in the viscosity of the electrolytic solution and the degree of formation of the coating film are small, the degree of increase in internal resistance is small and the storage characteristics are considered to be excellent.

Needless to say, the present invention can be applied to primary and secondary non-aqueous electrolyte batteries.

<Effects of the Invention> According to the method for preparing a non-aqueous electrolyte of the present invention configured as described above, lithium hexafluorophosphate (LiPF ₆ )
In addition to the ease of handling in the preparation of the non-aqueous electrolytic solution using substantially as a solute, the manufacture of the battery can be facilitated, and the production of the decomposition products can be significantly suppressed to improve the battery performance. Further, since the substitution treatment of potassium ions to lithium ions is performed by electrolysis, a trace amount of water and impurities in the solution are removed by a side reaction at the time of metal deposition, so that the purity of the non-aqueous electrolyte solution is enhanced.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of the battery of the example, and FIG. 2 is a graph showing the storage characteristics of the battery of the present invention and the conventional battery. 1 ... Positive electrode, 4 ... Separator, 5 ... Lithium negative electrode.

Claims (1)

[Claims] 1. When preparing a non-aqueous electrolyte for use in a non-aqueous electrolyte battery, potassium hexafluorophosphate is dissolved in a non-aqueous organic solvent, and then potassium ions in the organic solvent are replaced with lithium. A method for preparing a non-aqueous electrolytic solution, which comprises subjecting a lithium ion to electrolysis to form an anode.