DE19824984A1 - Process for the crystallization of high-purity LiPF¶6¶ from organic solvents - Google Patents

Abstract

A method for producing highly pure, easily flowable LiPF6 by crystallization of organic solvents, whereby LiPF6 is crystallized from a solution in an aprotic organic solvent 1, a second inert aprotic solvent 2 is added to this solution and the first solvent 1 is substantially distilled off. The invention also relates to the use of LiPF6 in the production of lithium batteries.

Description

The invention relates to a process for the crystallization of high-purity LiPF ₆ from organic aprotic solvents, whereby a free-flowing product with a very low residual content of organic carbon is obtained, and its use for the production of lithium ion batteries.

LiPF ₆ can be used as a conductive salt for electrolytes in primary and secondary elements. In particular, it is used in rechargeable lithium ion batteries. The electrolytes are non-aqueous solutions of LiPF ₆ in organic media, e.g. B. in diethyl carbonate, dimethyl carbonate, ethylene carbonate or propylene carbonate and others, or mixtures of the solvents mentioned.

Very high demands are made of the purity of the LiPF ₆ for this application. In particular, it is necessary that the LiPF ₆ contain a very low content of free hydrogen fluoride, a low water content and only very small amounts of foreign metal ions. Contamination with organic carbon compounds must also be avoided.

Various methods are known for producing and cleaning LiPF ₆ . A known method is the implementation of PCl ₅ or PF ₅ with LiF in anhydrous, liquid hydrogen fluoride, such as. B. disclosed in RU 2075435, JP 251109/85 or DE-A-8 12 247.

A suitable as an industrial process crystallization process for the isolation of LiPF ₆ from hydrogen fluoride is disclosed by JP 268005/97. Here, a solution of LiPF ₆ in hydrogen fluoride is cooled in a pressure vessel by reducing the pressure in the boiler by pumping out gaseous HF. The evaporation of the liquid hydrogen fluoride slowly removes heat from the system and the salt crystallizes on cooling. The pumped hydrogen fluoride is condensed again and recovered.

A disadvantage of the production of LiPF ₆ from hydrogen fluoride is, however, that it is difficult to completely remove free hydrogen fluoride. In addition, the handling of hydrogen fluoride is dangerous and the resulting expenses for safety devices make hydrogen fluoride processes expensive.

Methods have also been disclosed that avoid handling anhydrous liquid hydrogen fluoride. So it has been suggested to make the production of LiPF ₆ directly in solvents that can also be used as battery electrolytes. CA 2,193,119 A discloses the production of LiPF ₆ by introducing PF ₅ gas into a suspension of LiF in alkyl carbonates. In order to be able to use the LiPF ₆ solutions obtained directly as an electrolyte, however, a very pure PF ₅ gas is required, which in particular must be free of Cl- and O-containing compounds. Providing such a PF ₅ is very complex. In addition, further steps for cleaning the raw LiPF ₆ electrolyte are required.

US Pat. No. 3,607,020 teaches the reaction of PF ₅ with LiF in diethyl ether. To crystallize the LiPF ₆ from the ether phase, PF _{5 is used} in a 50-100% excess. The overstoichiometric PF ₅ portion forms a complex with diethyl ether in which LiPF6 is insoluble. However, the use of PF ₅ in excess is disadvantageous. Since PF _{5 is} expensive, it must be returned in any case. However, recycling is complex and increases the cost of the process. The preparation of LiPF ₆ in diethyl ether also discloses DE-A-196 32 543. In this document, the reaction of LiF with PCl ₅ to LiPF ₆ in an organic solvent is described. The LiPF ₆ is purified by evaporation followed by recrystallization from diethyl ether. A disadvantage of the crystallization of LiPF ₆ from ethers, however, is that ethers form complexes with LiPF ₆ and residues of the solvent stubbornly adhere. The LiPF6 obtained thus contains residues of organic carbon (total organic carbon, TOC). However, LiPF ₆ suitable for use in lithium batteries must contain very little TOC. In addition, LiPF ₆ made from diethyl ether is a very fine powder with low bulk density and very poor pourability, which makes product handling very difficult.

It was therefore an object of the present invention to provide a process in which highly pure, free-flowing LiPF ₆ with a low TOC content can be crystallized from organic solvents and which can be used for the production of lithium ion batteries.

Accordingly, a process for the crystallization of high-purity LiPF ₆ from organic solvents was found, which comprises the following steps AC:

• A) dissolving LiPF ₆ are soluble in an aprotic solvent 1, in which more than 5 g of LiPF _6/100 g solvent 1;
• B) addition of an aprotic solvent 2 in which less than 5 g _LiPF6 / 100 g solvent 2 are soluble, which has a higher boiling point than the solvent 1, and which is mixed bar at room temperature or on heating with solvent 1;
• C) Distilling off solvent 1 from the solution of LiPF ₆ in solvents 1 and 2, LiPF ₆ crystallizing out.

The process steps A, B, C can be carried out in succession be performed, or steps B and C can be carried out simultaneously be performed.

The crude LiPF ₆ as starting material can be prepared by the methods known to the person skilled in the art. Production from PF ₅ and LiF in suitable organic solvents is preferred.

Aprotic organic solvents in which LiPF _{6 is} readily soluble are used as solvent 1 in the sense of this invention. The solubility is at least 5 g of _LiPF6 / 100 g solution medium 1, preferably at least 10 g _LiPF6 / 100 g solvent. Carbonic acid esters (ROCOOR ') can be used, where R and R' are straight-chain or branched alkyl radicals having up to 6 carbon atoms. R and R 'can also be connected together to form a ring. Examples are dimethyl carbonate, diethyl carbonate, propylene carbonate, ethylene carbonate, methyl ethyl carbonate. Furthermore, carboxylic acid esters such as methyl formate or methyl acetate or ethyl acetate or carboxylates such as acetonitrile can be used. Linear or cyclic ethers with 1 oxygen atom which are capable of complexing with LiPF ₆ , such as, for example, dimethyl ether, diethyl ether, methyl tert-butyl ether (MTBE), or THF, are particularly suitable. Ether with 2 oxygen atoms can also be used.

Diethyl ether is very particularly preferred. Mixtures of two too or more of the above solvents can be used.  

Aprotic organic solvents are used as solvent 2, in which LiPF _{6 is} poorly soluble and which cannot form complexes with LiPF ₆ . The solubility of LiPF ₆ in the solvent 2 is less than 5 g of LiPF _6/100 g solvent medium 2, preferably less than 1 g of LiPF _6/100 g. Aliphatic or aromatic hydrocarbons, e.g. B. pentane, isopentane, cyclopentane, hexane, cyclohexane, methylhexane, heptane, octane, decane, dodecane, benzene, xylene, toluene or halogenated hydrocarbons such as chloroform, chlorobenzene, methylene chloride or carbon tetrachloride. The boiling point of solvent 2 is above that of the selected solvent 1. Particularly preferred are aliphatic hydrocarbons with 6 carbon atoms, such as. B. n-hexane or cyclohexane, as well as chloroform and toluene. Mixtures of two or more of the abovementioned solvents can also be used. Solvent 1 and solvent 2 are preferably selected so that they are completely miscible with one another in the temperature range of interest, in particular in the heat.

The crystallization process according to the invention is carried out by dissolving crude LiPF ₆ in solvent 1. The concentration of this solution can be varied within wide limits. It is only limited by the saturation concentration of the LiPF6. High concentrations between 5-50% by weight of LiPF ₆ with respect to solvent 1 are preferred, particularly preferably 10-40% by weight. The dissolving can take place both at room temperature and at elevated temperatures. The process according to the invention also comprises synthesizing LiPF ₆ in solvent 1 and using this solution directly for the crystallization process without first isolating solid LiPF ₆ .

In the process according to the invention, the solution of LiPF ₆ in solvent 1, advantageously with heating, is then mixed with solvent 2, the boiling point of which must be higher than that of solvent 1, and solvent 1 is distilled off. This can be done, for example, by heating solution 1 to the boiling point, adding solvent 2, and distilling off solvent 1. Another variant is to continuously add solvent 2, while solvent 2 is already distilled off. If the boiling points of solvents 1 and 2 are very close to one another, a column can be used to prevent solvent 2 from being distilled off, or a mixture of solvent 1 and solvent 2 is distilled off, with more solvent 2 being used accordingly.

The temperature when adding solvent 2 should be as high as possible so that the solvent mixture remains single-phase. It is preferred to use 1 gear at the boiling point of solvent. Temperatures that are too low can lead to the formation of two phases and crystallize LiPF ₆ as a complex from the solvent 1 phase, which can lead to poorly pourable LiPF ₆ with a high TOC content.

However, the process according to the invention also includes solvent combinations in which solvent 1 and solvent 2 are not completely miscible with one another. Here, too, a LiPF ₆ with a low TOC content crystallizes out. It is advantageous - e.g. B. to avoid caking - while stirring the mixture well. An example of such a suitable solvent combination is diethyl ether / n-hexane.

The distillation of solvent 2 and the crystallization of LiPF ₆ is usually carried out under normal pressure. Depending on the type of solvent used, it is also possible to work under excess pressure or under reduced pressure.

In the process according to the invention, LiPF ₆ largely crystallizes out already in the heat - as a rule in the form of small cuboid prisms. It is separated and dried by methods known per se, such as filtration, pressure filtration, centrifuging or decanting. The LiPF _{6 obtained} is characterized by high crystal yields of over 80-90% and has good free-flowing properties. Compared to material crystallized directly from diethyl ether, it has significantly higher bulk densities. Due to the improved flow properties and high bulk densities, handling the product in industrial production is significantly simplified. The LiPF ₆ produced by the process according to the invention always contains small amounts of TOC of at least 10 ppm. The TOC content is not greater than 1000 ppm.

Due to its low TOC content, the LiPF ₆ produced by the process according to the invention is excellently suitable for use as a conductive salt in lithium ion batteries.

The following experimental examples serve for further explanation of the invention, without thereby changing the scope of the invention is restricted.  

The LiPF ₆ starting material was prepared by reacting PF ₅ and LiF in diethyl ether. PF ₅ was obtained by reacting CaF ₂ with PCl ₅ in an autoclave.

example 1

20 g of LiPF ₆ were dissolved in 80 g of diethyl ether at 30 ° C. The solution was heated to boiling and a total of 120 g of cyclohexane was added dropwise in 15 minutes. At the end of the addition of cyclohexane, some precipitation occurred and the temperature was approximately 50 ° C. About 120 g of a cyclohexane / diethyl ether mixture were then distilled off with stirring, the bottom temperature rising to about 70.degree. The mixture was then left to cool overnight to room temperature with slow stirring, filtered through a pressure filter, washed with cyclohexane and dried in a stream of nitrogen and in vacuo at 6 mbar for 2 h.

19.2 g (96%) of LiPF ₆ crystals with a TOC content of 500 ppm and a bulk density of 1.5 kg / l resulted.

Comparative Example 1

20 g of LiPF ₆ were dissolved in 80 g of diethyl ether at 30 ° C. The solution was heated to boiling and a total of 120 g of cyclohexane was added dropwise in 15 minutes. At the end of the addition of cyclohexane, some precipitation occurred and the temperature was approximately 50 ° C.

The mixture was then brought to room temperature with slow stirring cooled, left overnight, over a pressure filter filtered, washed with cyclohexane and in a stream of nitrogen and in Vacuum dried at 6 mbar for 2 h (same conditions as Example 1).

The LiPF ₆ crystallized in a needle shape. When the filter cake was dried with nitrogen, the needles fell apart easily.

The result was 12.8 g (64%) of LiPF ₆ in the form of a fine powder with a TOC content of 4000 ppm and a bulk density of 0.5 kg / l.

Example 2

25 g of LiPF ₆ were dissolved in 50 g of diethyl ether. 65 g of n-hexane were added dropwise over the course of 60 min, and the ether was distilled off at the same time. The temperature in the bottom was 62 ° C. at the end.

The mixture was then cooled to 10 ° C. with stirring, via a Filtered pressure filter, washed with hexane and in a stream of nitrogen and dried in vacuo at 6 mbar for 2 h.

The result was 24.1 g (96%) of LiPF ₆ crystals (cubic prisms) with a TOC content of 500 ppm and a bulk density of 1.6 kg / l.

Comparative Example 2

40 g of LiPF ₆ were dissolved in 74 g of diethyl ether while warm (reflux). The boiling temperature of the solution was approx. 46 ° C. The solution was now cooled. At 15 ° C the solution crystallized suddenly.

The crystal mash was filtered through a pressure filter using Washed hexane and in a stream of nitrogen and in vacuo for 2 h 6 mbar dried. Needles were present during crystallization formed to fine when drying the filter cake Powder crumbled.

The fine powdery product had a TOC content of 13000 ppm and a bulk density of 0.4 kg / l.

Comparative Example 3

23 g of LiPF ₆ 6 were dissolved in 52 g of diethyl ether at 35 ° C. The solution was cooled to -20 ° C. The precipitated material was filtered through a pressure filter, washed with hexane and dried in a nitrogen stream and in vacuo for 2 hours at 6 mbar.

The result was 2 g (9%) of LiPF ₆ crystals with a TOC content of 2400 ppm and a bulk density of 0.5 kg / l.

Example 4

5.2 g (0.2 mol) of LiF were dispersed in 100 g (140 ml) of diethyl ether. PF ₅ was produced by reacting CaF ₂ with PCl ₅ in a molar ratio of approx. 10 l at 300-350 ° C under autogenous pressure in high yields and using a cold trap cooled with acetone / dry ice to remove POF ₃ and partially fluorinated PCl ₅ Implementation products passed into the dispersion with stirring. A total of approximately 0.22 mol of PF _{5 was} introduced. The ethereal dispersion was kept at 10-20 ° C. by cooling. The mixture was then heated briefly to the reflux temperature, giving a largely clear solution. After removing small amounts of precipitate by filtration, the solution was heated to boiling, a total of 100 g of cyclohexane was added dropwise in 15 min and the procedure was as in Example 1.

A total of 29 g of LiPF ₆ (95% yield based on LiF) were obtained. The analysis revealed the following data:

TOC: 500 ppm
Chloride: 30 ppm
LiF: approx.0.03%
LiPO ₂ F ₂ : <0.4%
Na, Ca, Fe, Al: each <5 ppm
free acid (HF): 80 ppm

Claims (8)

1. A process for the crystallization of high-purity LiPF ₆ from organic solvents, which comprises the following stages AC:

• A) Prepare are soluble to a solution of LiPF ₆ in an aprotic solvent 1, in which more than 5 g of LiPF _6/100 g solvent 1;
• B) addition of an aprotic solvent 2 in which less than 5 g _LiPF6 / 100 g solvent 2 are soluble, which has a higher boiling point than the solvent 1, and which is miscible at room temperature or on heating with solvent 1;
• C) Distilling off solvent 1 from the solution of LiPF ₆ in solvents 1 and 2, LiPF ₆ crystallizing out.

2. The method according to claim 1, wherein the implementation takes place according to one of the following sequences:

• a) Level A - Level B - Level C
• b) Level A - Levels B and C at the same time

3. The method according to claim 1 and 2, characterized in that that solvent 1 is an ether containing 1 oxygen atom, a carbonic acid ester, a carboxylic acid ester or acetonitrile. 4. The method according to claim 3, characterized in that it with solvent 1 to diethyl ether, dimethyl ether or Tetrahydrofuran acts. 5. The method according to claim 3, characterized in that it Solvent 1 is diethyl ether. 6. The method according to claim 1 and 2, characterized in that solvent 2 is aliphatic carbon hydrogen, cyclohexane, toluene or chloroform.   7. Use of the LiPF ₆ produced according to claims 1 to ₆ for the production of Li-ion batteries. 8. LiPF ₆ with an organic carbon content of between 0.001 and 0.1% by weight, producible by a process according to claims 1 to 6.