JPH1092468A - Electrolyte for lithium battery, method for refining it, and lithium battery using it - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide electrolyte for a battery having high purity, from which oxide impurities is removed, by removing generated hydrogen halide from electrolyte after converting oxide impurities into hydrogen halide through a process of adding halide into electrolyte for a lithium battery containing oxide impurities. SOLUTION: Halide containing Cl, Br, I is added into electrolyte for a lithium battery containing oxide impurities and allowed to react with oxide impurities, and H ion harmful to the lithium battery is converted into hydrogen halide. The hydrogen halide HCl, HBr, HI are oxide impurities, have high vapor pressure, not to be solvated to organic solvent to be generally used for the lithium battery, and can be easily separated by a separating method making use of vapor presser difference of distillation. Therefore, electrolyte in which oxide impurities are hardly remain can be provided. If this electrolyte is applied to the lithium battery, problems of deterioration of solvent with time, increase of inner resistance, drop of battery capacity, shortening of the cycle life can be solved.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an electrolyte for a lithium battery, a method for purifying the same, and a lithium battery using the same.

[0002]

2. Description of the Related Art In a lithium battery using lithium metal, lithium alloy, carbon or the like as a negative electrode active material, an organic non-aqueous solution in which an electrolyte is dissolved is generally used as an electrolyte for imparting ionic conductivity. As the organic non-aqueous solvent, carbonates, ethers, carboxylic esters and the like are used, and as the electrolyte, lithium salts of fluorine compounds are mainly used.

[0003] These electrolytes contain various acidic impurities such as hydrogen fluoride. Among them, lithium hexafluorophosphate (LiPF ₆ ) and the like are easily hydrolyzed by water contained in a solvent. There is a problem in that acidic impurities such as hydrogen fluoride, phosphoric acid, and oxyfluorophosphoric acid are generated. When an electrolytic solution containing such an acidic impurity is used in a lithium battery, it reacts with a positive electrode, a negative electrode, and a solvent, causing various problems such as a decrease in battery discharge capacity, an increase in internal resistance, and a decrease in cycle life.

Heretofore, in order to reduce acidic impurities in the electrolytic solution, acidic components have been removed from the electrolyte by various methods. However, since the electrolyte is a crystalline solid,
It is difficult to completely remove the acidic impurities caught inside the crystal. In order to prevent hydrolysis, dehydration of the non-aqueous solvent has also been performed by various methods. However, since the interaction between water and the non-aqueous solvent for a battery is strong, it is difficult to completely remove the non-aqueous solvent. In the electrolyte prepared from this solvent, the amount of acidic components increases due to hydrolysis.

[0005] As described above, all of the conventional methods are not always satisfactory in terms of the purity of the electrolytic solution.

[0006]

[Specific means for solving the problem]
As a result of intensive studies in view of the problems of the prior art, the inventors have found that a high-purity battery electrolyte solution can be obtained by refining by a specific method, and have reached the present invention.

That is, according to the present invention, a halide is added to an electrolyte for a lithium battery containing an acidic impurity,
After converting various acidic impurities into hydrogen halide, a method for purifying an electrolyte for a lithium battery, which comprises removing generated hydrogen halide from the electrolyte, contains hydrogen fluoride as an acidic impurity. In this case, in the lithium battery electrolyte, a halide other than fluoride is added to convert hydrogen fluoride into another hydrogen halide, and then the generated hydrogen halide is removed from the electrolyte. Further, a method of removing the generated hydrogen halide from the electrolytic solution is a method of utilizing a vapor pressure difference with a solvent due to distillation or the flow of an inert gas, and further, the halide is lithium halide or It is a compound having a boiling point of 150 ° C. or less, more preferably, a halide used here is a chloride, and an electrolytic solution used in the above purification method. As the electrolyte, and also contains lithium hexafluorophosphate, further there is provided respectively a lithium battery, which comprises using an electrolytic solution for a lithium cell obtained by the above purification method.

According to the purification method of the present invention, hydrogen fluoride (HF) derived from a lithium salt of a fluorine compound used as an electrolyte and acidic impurities (eg, LiPF ₆ ) derived from other raw materials and hydrolysis products of the electrolyte are used. In the case of H
PF ₆ and HPOxFy) can obtain an extremely low electrolyte for a lithium battery as compared with the conventional method. If this is applied to a lithium battery, the deterioration of the solvent with the lapse of time, the increase in the internal resistance associated therewith, Various problems such as a decrease in battery capacity and a decrease in cycle life are solved, and extremely good results are obtained.

In the purification method of the present invention, a typical electrolyte contained in the electrolytic solution to be purified is LiP
F ₆ (lithium hexafluorophosphate), but other examples include LiBF ₄ , LiSbF ₆ , LiAs
F ₆ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC
The present invention is applicable to various lithium salts of strong acids containing acidic impurities other than those described herein, such as 10 ₄ .

In the present invention, the solvent for the electrolytic solution to be used is not particularly limited, but a solvent such as a carbonate ester, a carboxylate ester, an ether and a nitrile is used. Specifically, there are ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, dimethoxyethane, methyl acetate, ethyl acetate, diethyl ether, isopropyl ether, acetonitrile and the like.

The acidic impurities which are problematic here include, for example, HPF ₆ , HBF ₄ , and HSbF derived from the strong acid of the raw material.
₆ , HAsF ₆ , HCF ₃ SO ₃ , HN (CF ₃ SO ₂ ) ₂ ,
HClO ₄ or the like and hydrolysis and H generated by thermal decomposition or the like
F, HPO ₂ F ₂ , HBOF ₂ , HSbO ₂ F ₂ , H ₂ PO ₃
F, H ₂ SO ₄ and the like.

These acidic impurities are contained in the solid electrolyte, and cannot be sufficiently removed only by refining the electrolyte. In addition, even when the acidic impurities are dissolved in an organic solvent and subjected to general purification such as distillation, the vapor pressure of these acidic impurities themselves is low, and the vapor pressure is further suppressed by the effect of solvation, so that evaporation is performed. It is difficult to let. The neutralization reaction with hydroxides and oxides, which is a common 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. .

In consideration of these problems, the inventors of the present invention have made various studies, and as a result, even with the same acidic impurities, HCl,
HBr and HI have a high vapor pressure and are not solvated with organic solvents commonly used in lithium batteries, and can be easily separated by a separation method using a vapor pressure difference such as distillation. Was. Utilizing these facts, in the present invention, H ions harmful to a lithium battery are converted into hydrogen halide by reacting the above-mentioned acidic impurities with a halide containing Cl, Br, I, and then separated. According to this method, an electrolytic solution in which almost no acidic impurities remain can be obtained.

Examples of the halide to be added include chlorides, bromides and iodides other than fluorides. Specifically, LiCl, LiBr, LiI, NaCl, NCl
_{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 B such as acetyl chloride, oxalyl chloride and phosgene.
Organic compounds having r and I can also be used. Fluoride is not preferred because it reacts with acidic impurities and generates strong interaction between HF and an organic solvent generally used for lithium batteries, and cannot be removed. Furthermore, considering the ease of removal of hydrogen halide generated after the reaction and remaining in the electrolytic solution due to dissolution of the halide, HCl having the highest vapor pressure is generated, and the solubility is higher than that of other halides. Is the most preferred. Also, L
When an electrolytic solution for high-purity applications that dislikes the incorporation of elements other than i is required, use lithium halide, preferably lithium chloride, or after removing acidic impurities,
Boiling point 1 capable of removing excess using vapor pressure
It is better to use a halide having a volatility of 50 ° C. or less. For halides with a boiling point of 150 ° C or higher, even if removal under reduced pressure is attempted, the removal of excess unreacted halide is not sufficient without raising the temperature of the electrolytic solution to near the boiling point. As a result, the loss of the solvent increases, so that it is not economical, and the highly reactive electrolytes (for example, LiPF ₆ , LiBF ₄ , LiSbF
₆ , LiAsF ₆ , etc.) are not preferred because the decomposition reaction of the solvent by the electrolyte occurs.

The amount of the additive is
The amount may be equal to or more than the equimolar amount, but since the additive may become a new impurity, and the reaction proceeds quantitatively, it is preferable to add the equimolar amount to 1.5 times the amount.

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

According to the above reaction, acidic impurities include HCl, H
It is converted to hydrogen halide such as Br and HI. This hydrogen halide is removed by a general method utilizing a 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 electrolyte, and expelling the inert gas together with the inert gas.

At this time, it is effective to add heat to promote the elimination of hydrogen halide, but the temperature is 0 ° C. to 150 ° C., preferably 30 ° C. to 100 ° C.
To remove the hydrogen halide. Below 0 ° C, the rate of hydrogen halide removal is slow and impractical,
At 150 ° C. or higher, the vapor pressure of the solvent increases and the loss of the solvent increases, so that it is not economical and a highly reactive electrolyte (for example, LiPF ₆ , LiBF ₄ , LiSbF ₆ ,
LiAsF ₆ , etc.) is not preferred because the decomposition reaction of the solvent by the electrolyte occurs.

When a volatile halide is used as an additive, excess halide is simultaneously removed in the step of removing hydrogen halide. When a solid halide is used as an additive, it is necessary to perform a solid-liquid separation such as filtration to remove a precipitate of a by-product generated by a reaction with an excess of the halide and an acidic impurity.

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

[0021]

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

Example 1 In a glove box controlled at a dew point of -60 ° C, 152 g of lithium hexafluorophosphate (LiPF ₆ ) was mixed with diethyl carbonate and ethylene carbonate at a ratio of 1: 1.
(Volume ratio) It was dissolved in a mixed solvent to adjust the volume to 1000 ml. The thus obtained L having a concentration of 1 mol / l
When the iPF ₆ / (diethyl carbonate + ethylene carbonate) solution was analyzed by a titration method and an ion chromatography method, it was found that 100 ppm of HF was contained as an acidic impurity.

0.3 g of lithium chloride was added to the above solution and stirred at room temperature for 12 hours. The solution was then transferred to a PTFE container equipped with a nitrogen blowing nozzle,
Nitrogen gas was passed through the solution at 4 ° C. for 4 hours. The exhaust gas at this time was sampled and analyzed by IR. As a result, it was confirmed that HCl and diethyl carbonate were contained in nitrogen.

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

The thus obtained concentration of 1 mol / l
LiPF₆/ (Diethyl carbonate + ethylene car
Carbonate (1: 1))
When measured at 25 ° C. using a conductivity meter of the formula, 7.8 was obtained.
mS / cm and LiPF ₆Simply ethylene carbonate
Dissolved in a mixed solvent of sodium carbonate and diethyl carbonate
Was equivalent to In addition, IR, NMR, gas chromatography
Chromatography, liquid chromatography
The solution was examined, but the presence of decomposition products was confirmed.
Did not.

Next, a test cell was prepared using this solution, and the performance as an electrolyte was evaluated by a charge / discharge test.
Specifically, 5 parts by weight of polyvinylidene fluoride (PVDF) is mixed as a binder with 95 parts by weight of natural graphite powder, and N, N-dimethylformamide is further added.
A slurry was formed. This slurry was applied on a nickel mesh and dried at 150 ° C. for 12 hours to obtain a test negative electrode body. In addition, lithium cobaltate 8
10 parts by weight of graphite powder and 5 parts by weight of PVDF were mixed with 5 parts by weight, and N, N-dimethylformamide was further added to form a slurry. This slurry was applied on an aluminum foil and dried at 150 ° C. for 12 hours to obtain a positive electrode for testing. A test cell was assembled using the negative electrode body and the positive electrode body with the electrolytic solution of this example using a polypropylene nonwoven fabric as a separator.
Subsequently, a constant current charge / discharge test was performed under the following conditions. Both charging and discharging were performed at a current density of 0.35 mA / cm ² , charging was performed up to 4.2 V, and discharging was performed up to 2.5 V. The charge / discharge cycle was repeated to observe changes in discharge capacity. As a result, the charge / discharge efficiency was almost 100%. When charge / discharge was repeated 500 cycles, the discharge capacity did not change at all. This test cell is used for accelerated deterioration test.
After storage at 60 ° C. for 3 months, a charge / discharge test was conducted again under the same conditions as above, and the discharge capacity was about 96% of the initial value. Was not seen.

Example 2 In a glove box controlled at a dew point of -60 ° C., 304 g of lithium hexafluorophosphate (LiPF ₆ ) was dissolved in propylene carbonate, and the volume was 1000 ml.
Was prepared. The thus obtained concentration of 2 mol / l
The LiPF ₆ / propylene carbonate solution was analyzed by titration to find that HF was 130 pP as an acidic impurity.
pm.

0.7 g of lithium bromide was added to the above solution and stirred at room temperature for 12 hours. The solution is then brought to temperature 6
The mixture was degassed under reduced pressure at 0 ° C. and a pressure of 10 torr for 7 hours. After filtering off the precipitate of lithium fluoride generated by the above reaction,
The acidic impurity concentration of this solution was measured and found to be below the lower limit of quantification (10 ppm).

Example 3 In a glove box controlled at a dew point of -60 ° C., 304 g of lithium hexafluorophosphate (LiPF ₆ ) was dissolved in propylene carbonate containing 30 ppm of water, and the volume was adjusted to 1000 ml. . The thus obtained LiPF ₆ / propylene carbonate solution having a concentration of 2 mol / l was analyzed by a titration method and an ion chromatography method.
ppm and 50 ppm of HPO ₂ F ₂ .

To the above solution, 1.0 g of lithium chloride was added, and the mixture was stirred at room temperature for 12 hours. The solution is then brought to temperature 6
The mixture was degassed under reduced pressure at 0 ° C. and a pressure of 10 torr for 7 hours. After filtering off the precipitate of lithium fluoride generated by the above reaction,
The acidic impurity concentration of this solution was measured and found to be below the lower limit of quantification (10 ppm).

Example 4 In a glove box controlled at a dew point of −60 ° C., 152 g of lithium hexafluorophosphate (LiPF ₆ ) was dissolved in dimethyl carbonate to adjust the volume to 1,000 ml. When the thus obtained LiPF ₆ / dimethyl carbonate solution having a concentration of 1 mol / l was analyzed by a titration method, it was found that 90 ppm of HF was contained as an acidic impurity.

The above 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 nitrogen blowing nozzle, and nitrogen gas was passed through the solution at 40 ° C. for 5 hours. When the concentration of the acidic impurities in the obtained solution was measured, it was lower than 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. The thus obtained L having a concentration of 1 mol / l
When the iBF ₄ / diethyl carbonate solution was analyzed by a titration method, it was found that 110 ppm of HF was contained as an acidic impurity.

To the above solution, 1.1 g of acetyl chloride was added, and the mixture was stirred at room temperature for 12 hours. Next, the solution was transferred to a PTFE container equipped with a nitrogen blowing nozzle,
Nitrogen gas was passed through the solution at 4 ° C. for 4 hours to remove HCl and excess acetyl chloride. When the concentration of the acidic impurities in the obtained solution was measured, it was lower than the lower limit of quantification (10 ppm).

Example 6 In a glove box controlled at a dew point of -60 ° C, 152 g of lithium hexafluorophosphate (LiPF ₆ ) was dissolved in diethyl carbonate to adjust the volume to 1000 ml. When the thus obtained LiPF ₆ / diethyl carbonate solution having a concentration of 1 mol / l was analyzed by a titration method, it was found that acidic impurities were 100 ppm in terms of HF.
Was included.

0.4 g of phosphorus trichloride was added to the above solution, followed by stirring at room temperature for 12 hours. Next, the solution was transferred to a PTFE container equipped with a nitrogen blowing nozzle,
A nitrogen gas was passed through the solution for 4 hours to remove HCl and excess phosphorus trichloride. When the concentration of the acidic impurities in the obtained solution was measured, it was lower than the lower limit of quantification (10 ppm).

Example 7 In a glove box controlled at a dew point of -60 ° C., 152 g of lithium hexafluorophosphate (LiPF ₆ ) was dissolved in propylene carbonate, and the volume was 1000 ml.
Was prepared. The thus obtained concentration of 1 mol / l
The LiPF ₆ / propylene carbonate solution was analyzed by titration to find that acidic impurities were 100 p
pm.

2.0 g of calcium chloride was added to the above solution, and the mixture was stirred at room temperature for 12 hours. The solution was then transferred to a PTFE container equipped with a nitrogen blowing nozzle,
Nitrogen gas was passed through the solution at 0 ° C. for 4 hours to remove HCl. Next, the excess calcium chloride and the precipitate of calcium fluoride generated by the reaction were filtered off, and the concentration of acidic impurities in this solution was measured.
m).

Example 8 In a glove box controlled at a dew point of -60 ° C., 304 g of lithium hexafluorophosphate (LiPF ₆ ) was dissolved in propylene carbonate, and the volume was 1000 ml.
Was prepared. The thus obtained concentration of 2 mol / l
HF was added to the LiPF ₆ / propylene carbonate solution of Example 2 to adjust the HF concentration to 2% by weight.

70 g of lithium chloride was added to the above solution, and the mixture was stirred at room temperature for 12 hours. The solution is then brought to a temperature of 60
The mixture was degassed under reduced pressure at a pressure of 10 torr for 7 hours. After the precipitate of lithium fluoride generated by the above reaction was filtered off, the concentration of acidic impurities in this solution was measured, and it was lower than the lower limit of quantification (10 ppm).

Comparative Example In a glove box controlled at a dew point of −60 ° C., 152 g of lithium hexafluorophosphate (LiPF ₆ ) was mixed with diethyl carbonate and ethylene carbonate in a ratio of 1: 1.
(Volume ratio) It was dissolved in a mixed solvent to adjust the volume to 1000 ml. The thus obtained L having a concentration of 1 mol / l
Analysis of the iPF ₆ / (diethyl carbonate + ethylene carbonate) solution revealed that 70 ppm of HF and 30 ppm of HPO ₂ F ₂ were contained as acidic impurities (the concentration was determined by titration).

The above solution was transferred to a PTFE container equipped with a nitrogen blowing nozzle, and nitrogen gas was passed through the 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 HF and HPO of this solution were added.
_{When the 2} F ₂ concentration was measured, it was 70 ppm and 30 ppm, respectively.
ppm was the same as before operation. That is, HF could not be separated by a separation method utilizing a difference in vapor pressure.

The thus obtained concentration of 1 mol / l
The ionic conductivity of the LiPF ₆ / (diethyl carbonate + ethylene carbonate (1: 1)) electrolyte was measured at 25 ° C. using an AC bipolar conductivity meter, and was 7.8.
mS / cm.

Next, a test cell was prepared using this solution, and the performance as an electrolyte was evaluated by a charge / discharge test.
Specifically, 5 parts by weight of polyvinylidene fluoride (PVDF) is mixed as a binder with 95 parts by weight of natural graphite powder, and N, N-dimethylformamide is further added.
A slurry was formed. This slurry was applied on a nickel mesh and dried at 150 ° C. for 12 hours to obtain a test negative electrode body. In addition, lithium cobaltate 8
10 parts by weight of graphite powder and 5 parts by weight of PVDF were mixed with 5 parts by weight, and N, N-dimethylformamide was further added to form a slurry. This slurry was applied on an aluminum foil and dried at 150 ° C. for 12 hours to obtain a positive electrode for testing. A test cell was assembled by using the negative electrode body and the positive electrode body with the electrolytic solution of this comparative example using a polypropylene nonwoven fabric as a separator.
Subsequently, a constant current charge / discharge test was performed under the following conditions. Both charging and discharging were performed at a current density of 0.35 mA / cm ² , charging was performed up to 4.2 V, and discharging was performed up to 2.5 V. The charge / discharge cycle was repeated to observe changes in discharge capacity. As a result, the charge / discharge efficiency was almost 100%. When charge / discharge was repeated 500 cycles, the discharge capacity did not change at all. This test cell is used for accelerated deterioration test.
After storing at 60 ° C. for 3 months, the charge / discharge test was performed again under the same conditions as above, and the discharge capacity was about 88% of the initial value. Was seen.

[0046]

According to the purification method of the present invention, 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, For LiPF ₆ ,
HPF ₆ and HPOxFy) can obtain an electrolyte solution for lithium batteries that is extremely low as compared with the conventional method. If this is applied to lithium batteries, the deterioration of the solvent over time, the increase in internal resistance associated therewith, Various problems such as a decrease in battery capacity and a decrease in cycle life are solved, and extremely good results are obtained.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Tadayuki Kawashima 5253 Oki Obe, Oji, Ube, Yamaguchi Prefecture Inside the Chemical Research Laboratory of Central Glass Co., Ltd. Inside the Chemical Laboratory Co., Ltd. (72) Inventor Hiromi Sasaki 5253 Oki Obe, Ube City, Yamaguchi Prefecture Central Glass Co., Ltd.

Claims (8)

[Claims] 1. A method for removing a generated hydrogen halide from an electrolytic solution after converting an acidic impurity into a hydrogen halide by adding a halide to an electrolytic solution for a lithium battery containing the acidic impurity. A method for purifying an electrolyte for a lithium battery. 2. A halogen other than fluoride is added to an electrolyte for lithium batteries containing hydrogen fluoride as an impurity to convert hydrogen fluoride into another hydrogen halide, and then generate halogen. A method for purifying an electrolyte for a lithium battery, comprising removing hydrogen hydride from the electrolyte. 3. A method for removing generated hydrogen halide from an electrolytic solution according to claim 1 or 2, wherein the method uses a vapor pressure difference with a solvent due to distillation or a flow of an inert gas. A method for purifying an electrolyte for a lithium battery. 4. A method for purifying an electrolyte for a lithium battery, wherein the halide according to claim 1 is lithium halide. 5. A method for purifying an electrolyte for a lithium battery, wherein the halide according to claim 1 is a compound having a boiling point of 150 ° C. or lower. 6. A method for purifying an electrolytic solution for a lithium battery, wherein the halide according to claim 1 is a chloride. 7. An electrolyte for a lithium battery, wherein the electrolyte for a lithium battery obtained by the purification method according to claim 1 contains lithium hexafluorophosphate as an electrolyte. 8. A lithium battery using the electrolyte solution for a lithium battery according to claim 7.