JP3369937B2 - Purification method of lithium tetrafluoroborate - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for purifying an electrolytic solution of lithium tetrafluoroborate useful as an electrolyte for lithium batteries.

[0002]

2. Description of the Related Art Two known methods of synthesizing lithium tetrafluoroborate (LiBF ₄ ) have been reported: a wet method and an ether method. In the wet method, a hydrous salt (LiBF ₄ .H ₂ O) is produced by the reaction between borofluoric acid and lithium carbonate. Since heating at about 200 ° C. is required to dehydrate this hydrated salt, decomposition of lithium tetrafluoroborate (Li
BF ₄ → LiF + BF ₃ ) occurs and not only the purity is lowered, but also moisture remains, so that it cannot be used for lithium batteries. In the ether method, an anhydrous salt can be obtained by reacting a complex compound of boron trifluoride and methyl ether or ethyl ether with lithium fluoride, but since lithium tetrafluoroborate is hardly soluble in ether,
It is difficult to obtain one that satisfies the quality for lithium batteries,
There are also drawbacks such as the use of dangerous ether.

On the other hand, when an electrolyte such as lithium tetrafluoroborate is used as an electrolytic solution for a lithium battery, it is easily hydrolyzed by water contained in the solvent to form an acidic impurity such as hydrogen fluoride. When using an electrolytic solution containing such an acidic impurity in a lithium battery, it reacts with the positive electrode, the negative electrode, and the solvent, resulting in various problems such as a decrease in discharge capacity of the battery, an increase in internal resistance, and a decrease in cycle life cause. Conventionally, in order to reduce this acidic impurity, removal of acidic components from the electrolyte has been performed by various methods, but since the electrolyte is a crystalline solid, the acidic impurities trapped inside the crystal are completely removed. Difficult to remove. Also, in order to prevent hydrolysis, dehydration of the non-aqueous solvent has also been carried out by various methods, but it is difficult to completely remove this because of the strong interaction between water and the non-aqueous solvent for the battery. The electrolytic solution prepared from this solvent has an increased amount of acidic components due to hydrolysis.

As described above, the conventional methods are not always satisfactory in terms of the purity of lithium tetrafluoroborate obtained and the electrolytic solution using the lithium tetrafluoroborate.

[0005]

[Means for Solving the Problems] The present inventors
As a result of intensive studies in view of the problems of the prior art, a method for obtaining an electrolyte solution containing few impurities by using a halide in a lithium tetrafluoroborate electrolyte solution containing an acidic impurity, etc., has arrived at the present invention. is there.

That is, according to the present invention, various acidic impurities contained in an electrolyte for lithium batteries containing lithium tetrafluoroborate are converted into hydrogen halides by adding halides other than fluorides , An electrolytic solution for a lithium battery, which removes the generated hydrogen halide from the electrolytic solution, and a method for further removing the hydrogen halide utilizes a vapor pressure difference with a solvent due to distillation or inert gas flow. The present invention provides a method for purifying the above.

The purification method of the present invention is carried out by using hydrogen fluoride (HF) derived from lithium salts of a fluorine compound used as an electrolyte and acidic impurities derived from other raw materials and hydrolysis products of the electrolyte (for example, in the case of LiBF ₄ ). , HBF ₄ , HBOxFy, etc.) can obtain an electrolyte solution for a lithium battery which is extremely lower than that of the conventional method. When this is applied to a lithium battery, deterioration of the solvent with time and increase in internal resistance accompanying it , Various problems such as a decrease in battery capacity and a decrease in cycle life are solved, and an extremely good lithium battery can be obtained.

The present invention will be described in detail below. Examples of the acidic impurities that are problematic in the present invention include HBF _{4 and the} like derived from a strong acid as a raw material, HF and HBOF _{2 and the} like generated by hydrolysis and thermal decomposition, etc. Even if the purification is carried out, it is difficult to evaporate the acidic impurities themselves because of their low vapor pressure and because the vapor pressure is further suppressed by the influence of solvation. The neutralization reaction with hydroxides and oxides, which is a general method for removing acidic impurities, also has a problem because water is generated as a by-product after the reaction, which adversely affects the performance of the lithium battery. .

As a result of various investigations by the present inventors in consideration of these problems, HCl, HCl,
It has been found that HBr and HI, which are hydrogen halides, have high vapor pressure, do not solvate with organic solvents generally used in lithium batteries, and can be easily separated by a separation method utilizing vapor pressure difference such as distillation. It is a thing. Utilizing these facts, in the present invention, by reacting a halide containing Cl, Br, and I with the above acidic impurities, H ions harmful to the lithium battery are converted into hydrogen halide and then separated. According to this method, an electrolytic solution in which acidic impurities hardly remain can be obtained by this method.

The halides added here include chlorides, bromides, and iodides other than fluorides, and specifically, LiCl, LiBr, LiI, NaCl, N.
_{aBr, NaI, CaCl 2, CaBr} 2, CaI 2, M
_{gCl 2, MgBr 2, MgI 2} , KCl, KBr, K
I, SiCl ₄ , BCl ₃ , PCl ₃ , PCl ₅ , POCl
₃ , inorganic compounds such as PF ₃ Cl ₂ and SCl ₄ , and active Cl and Br such as acetyl chloride, oxalyl chloride and phosgene,
Organic compounds having I can also be used. In the case of fluoride, the interaction between the generated HF and the organic solvent generally used in lithium batteries is strong, as it reacts with acidic impurities,
It is not preferable because it cannot be removed. Furthermore, considering the ease of removal of hydrogen halide generated after the reaction and the remaining in the electrolyte due to dissolution of the halide, HCl with the highest vapor pressure is generated, and the solubility is higher than that of other halides. Most preferred are low chlorides. Further, when an electrolytic solution for high-purity applications in which elements other than Li are disliked is required, lithium halide, preferably lithium chloride is used, or an acidic impurity is removed, and then vapor pressure is used to make excess. Boiling point capable of removing 150
It is better to use a halide having a volatility of ℃ or less. When attempting to remove halides having a boiling point of 150 ° C. or higher by decompression, removal of excess unreacted halide is not sufficient unless the temperature of the electrolytic solution is raised to near the boiling point.
Increasing the temperature here increases the loss of the solvent, which is not economical and the electrolyte (Li
BF ₄ ) is not preferable because the decomposition reaction of the solvent by the electrolyte occurs. Halides having a boiling point of 150 ° C. or less include SiCl ₄ , BCl ₃ , PC
Examples include l ₃ , PCl ₅ , POCl ₃ , PF ₃ Cl ₂ , acetyl chloride, oxalyl chloride, phosgene and the like.

Next, the amount of the halide to be added may be an equimolar amount or more with respect to the acidic impurities, but since the additive may become a new impurity in some cases, the reaction is quantitatively performed. Since it proceeds, it is preferable to add it in an equimolar amount to 1.5 times the amount.

Any method may be used for the reaction between the above halide and the acidic impurities. For example, when the batch reaction is carried out in a reaction tank or when the halide is a solid, the halide is packed in a column. Then, a method of continuously treating by circulating an electrolytic solution containing an acidic impurity can also be carried out.

By the above reaction, acidic impurities are HCl, H
It is converted to hydrogen halide such as Br and HI. This hydrogen halide is removed by a general method utilizing vapor pressure difference in the next step. That is, it is removed by a method of degassing under reduced pressure, a method of bubbling an inert gas through the electrolytic solution and expelling it together with the inert gas.

At this time, it is also effective to apply heat in order to promote desorption of hydrogen halide, but the temperature is in the range of 0 to 150 ° C., preferably 30 to 100.
It is better to remove hydrogen halide within the range of ° C.
When the temperature is 0 ° C or lower, the removal rate of hydrogen halide is slow and not practical, and when the temperature is 150 ° C or higher, the vapor pressure of the solvent is high and the loss of the solvent is large, which is not economical and the electrolyte with high reactivity LiBF ₄ ) is not preferable because the decomposition reaction of the solvent by the electrolyte occurs.

When a volatile halide is used as an additive, the excess halide is removed at the same time in the step of removing hydrogen halide. When a solid halide is used as an additive, it is necessary to perform solid-liquid separation such as filtration in order to remove precipitates of by-products generated by the reaction with excess halide and acidic impurities.

By the above operation, an electrolytic solution for a lithium battery having an extremely low amount of acidic impurities as compared with the conventional method can be obtained.

[0017]

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

Example 1 In a glove box controlled at a dew point of -60 ° C, 94 g of lithium tetrafluoroborate (LiBF ₄ ) was added to diethyl carbonate and ethylene carbonate in a ratio of 1: 1.
(Volume ratio) It melt | dissolved in the mixed solvent and adjusted the volume to 1000 ml. L having a concentration of 1 mol / l thus obtained
When an iBF ₄ / (diethyl carbonate + ethylene carbonate) solvent solution was analyzed by a titration method and an ion chromatography method, 100 ppm of HF was contained as an acidic impurity.

0.3 g of lithium chloride was added to the above solvent solution, and the mixture was stirred at room temperature for 12 hours. Next, this solvent solution was transferred to a polytetrafluoroethylene (PTFE) container equipped with a nozzle for blowing nitrogen, and nitrogen gas was passed through the solvent solution at 50 ° C. for 4 hours. The exhaust gas at this time was sampled and analyzed by IR.
It was confirmed that Cl and diethyl carbonate were contained.

Diethyl carbonate corresponding to the amount lost by the above operation was added, and the precipitate of lithium fluoride generated by the reaction was filtered off. When the HF concentration of this solution was measured, it was below the lower limit of quantification (10 ppm).

Example 2 188 g of lithium tetrafluoroborate (LiBF ₄ ) was dissolved in propylene carbonate in a glove box controlled at a dew point of −60 ° C., and the volume was 1000 ml.
Was prepared. The concentration thus obtained is 2 mol / l
When a LiBF ₄ / propylene carbonate solvent solution of HF was analyzed by a titration method, HF was found to be 13 as an acidic impurity.
It was contained at 0 ppm.

0.7 g of lithium bromide was added to the above solvent solution and stirred at room temperature for 12 hours. Next, this solvent solution was degassed under reduced pressure at a temperature of 60 ° C. and a pressure of 10 torr for 7 hours. After the precipitation of lithium fluoride generated by the above reaction was filtered off, the concentration of acidic impurities in this solvent solution was measured and found to be below the lower limit of quantification (10 ppm).

Example 3 In a glove box controlled to a dew point of -60 ° C, 94 g of lithium tetrafluoroborate (LiBF ₄ ) was dissolved in dimethyl carbonate to adjust the volume to 1000 ml. L having a concentration of 1 mol / l thus obtained
Analysis of the iBF ₄ / dimethyl carbonate solvent solution by titration revealed that HF was 90 ppm as an acidic impurity.
Was included.

The above solvent solution was introduced into a 60 cm column packed with granular lithium chloride at a flow rate of 10 ml / min. The solution after passing through the column was transferred to a PTFE container equipped with a nozzle for blowing nitrogen, and nitrogen gas was passed through the solvent solution at 40 ° C. for 5 hours. When the acidic impurity concentration of the obtained solvent solution was measured, the lower limit of quantification (10 ppm)
It was below.

Example 4 In a glove box controlled at a dew point of -60 ° C, 94 g of lithium tetrafluoroborate (LiBF ₄ ) was dissolved in diethyl carbonate to adjust the volume to 1000 ml. L having a concentration of 1 mol / l thus obtained
Analysis of the iBF ₄ / diethyl carbonate solvent solution by titration revealed that HF was 110 pp as an acidic impurity.
m was included.

1.1 g of acetyl chloride was added to the above solvent solution and stirred at room temperature for 12 hours. Next, this solvent solution was transferred to a PTFE container equipped with a nozzle for blowing nitrogen, and nitrogen gas was circulated in the solvent solution at 70 ° C. for 4 hours to generate H 2.
Cl and excess acetyl chloride were removed. When the acidic impurity concentration of the obtained solvent solution was measured, it was below the lower limit of quantification (10 ppm).

Example 5 In a glove box controlled at a dew point of -60 ° C, 94 g of lithium tetrafluoroborate (LiBF ₄ ) was dissolved in diethyl carbonate to adjust the volume to 1000 ml. L having a concentration of 1 mol / l thus obtained
Analysis of the iBF ₄ / diethyl carbonate solvent solution by titration showed that acidic impurities were 100 pp in terms of HF.
m was included.

0.4 g of phosphorus trichloride was added to the above solvent solution and stirred at room temperature for 12 hours. Next, this solvent solution was transferred to a PTFE container equipped with a nozzle for blowing nitrogen, and nitrogen gas was circulated in the solvent solution at 70 ° C. for 4 hours to generate H 2.
Cl and excess phosphorus trichloride were removed. When the concentration of acidic impurities in the obtained solvent solution was measured, the lower limit of quantification (1
0 ppm) or less.

Example 6 In a glove box controlled at a dew point of -60 ° C, 94 g of lithium tetrafluoroborate (LiBF ₄ ) was dissolved in propylene carbonate to adjust the volume to 1000 ml. The thus-obtained LiBF ₄ / propylene carbonate solvent solution having a concentration of 1 mol / l was analyzed by a titration method, and it was found that acidic impurities were 100 in terms of HF.
It was contained in ppm.

2.0 g of calcium chloride in the above solvent solution
Was added and stirred at room temperature for 12 hours. Next, this solvent solution was transferred to a PTFE container equipped with a nozzle for blowing nitrogen, and nitrogen gas was passed through the solvent solution at 70 ° C. for 4 hours,
HCl was removed. Next, after filtering out excess calcium chloride and the precipitate of calcium fluoride generated by the reaction, the concentration of acidic impurities in this solvent solution was measured,
It was below the lower limit of quantification (10 ppm).

Comparative Example 1 In a glove box controlled at a dew point of -60 ° C, 94 g of lithium tetrafluoroborate (LiBF ₄ ) was mixed with diethyl carbonate and ethylene carbonate in a ratio of 1: 1.
(Volume ratio) It melt | dissolved in the mixed solvent and adjusted the volume to 1000 ml. L having a concentration of 1 mol / l thus obtained
When an iBF ₄ / (diethyl carbonate + ethylene carbonate) solvent solution was analyzed, H
It contained 100 ppm of F (concentration determined by titration method).

The above solvent solution was transferred to a PTFE container equipped with a nozzle for blowing nitrogen, and nitrogen gas was passed through the solvent solution at 50 ° C. for 4 hours. When the exhaust gas at this time was sampled and analyzed by IR, only diethyl carbonate was contained in nitrogen, and HF was not confirmed.

Diethyl carbonate corresponding to the amount lost by the above operation was added, and the HF concentration of this solvent solution was measured. As a result, it was 100 ppm, which was the same as before the operation. That is, HF could not be separated by the separation method using the vapor pressure difference.

Example 7 Lithium tetrafluoroborate obtained in Example 1 /
(Diethyl carbonate: ethylene carbonate = 1:
1) A test cell was prepared using the solution, and the performance as an electrolytic solution was evaluated by a charge / discharge test. Specifically, 95 parts by weight of natural graphite powder is mixed with 5 parts by weight of polyvinylidene fluoride (PVDF) as a binder, and N, N
Dimethylformamide was added to make a slurry.
Apply this slurry on a nickel mesh and apply 150
A negative electrode body for testing was obtained by drying at 12 ° C. for 12 hours. Further, 85 parts by weight of lithium cobalt oxide was mixed with 10 parts by weight of graphite powder and 5 parts by weight of PVDF, and further,
N, N-dimethylformamide was added to make a slurry. Apply this slurry on aluminum foil and
It was dried at 50 ° C. for 12 hours to obtain a test positive electrode body. Using polypropylene non-woven fabric as a separator,
A test cell was assembled using the lithium tetrafluoroborate / (diethyl carbonate: ethylene carbonate = 1: 1) solution as an electrolytic solution and the negative electrode body and the positive electrode body. Subsequently, a constant current charge / discharge test was carried out under the following conditions. Current density 0.35 for both charging and discharging
Performed at mA / cm ² , charged to 4.2V and discharged to 2.5V
Then, this charge / discharge cycle was repeated to observe the change in discharge capacity. As a result, the charging / discharging efficiency is almost 100%.
When the charging and discharging were repeated 100 times, the discharge capacity did not change at all.

[0035]

According to the purification method of the present invention, hydrogen fluoride (HF) derived from the lithium salt of a fluorine compound used as an electrolyte and acidic impurities (HBF) derived from other raw materials and hydrolysis products of the electrolyte are used. ₄ and HBOxFy
It is possible to obtain an electrolyte solution for lithium batteries that is much lower than the conventional method, and if this is applied to lithium batteries, deterioration of the solvent over time, accompanying increase in internal resistance, and decrease in battery capacity. , Various problems such as a decrease in cycle life are solved, and an extremely good lithium battery can be obtained.

Continuation of the front page (72) Inventor Atsushi Tokunaga 5253 Oki, Ube, Yamaguchi Prefecture Ube Central U.S. Chemical Laboratory (72) Inventor Mitsuo Takahata 5253 Oki Obe, Ube City Yamaguchi Central Glass Co., Ltd. In the laboratory (56) Reference JP-A-10-92468 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) H01M 6/16 C01B 35/06 H01M 10/40

Claims (2)

(57) [Claims] 1. A lithium battery electrolyte containing an acidic impurity is added with a halide other than a fluoride to convert the acidic impurity into hydrogen halide, and the generated hydrogen halide is removed from the electrolyte. A method for purifying an electrolyte solution for a lithium battery containing lithium tetrafluoroborate, which comprises removing the electrolyte solution. 2. The method for removing the generated hydrogen halide according to claim 1, wherein the method comprises utilizing a vapor pressure difference with a solvent by distillation or flowing an inert gas, and containing tetrafluoroborate. Method for purifying electrolyte for lithium battery.