WO1999040027A1 - METHOD FOR MAKING HEXAFLUOROPHOSPHATE OF A METAL, M(PF6)n, PARTICULARLY OF LiPF6 - Google Patents

Abstract

The invention concerns a method for making solid M(PF6)n from a M(PF6)n solution or suspension in HF liquid, M being a metal cation having as degree of oxidation n, n being an integer between 1 and 2. The invention is characterised in that it consists in introducing in and/or above said solution or suspension a gas containing gaseous PF5 while evaporating liquid HF, thereby resulting at the end of the evaporation in solid M(PF6)n with low MFn and HF content. The solid M(PF6)n can also be purified under a PF5 stream.

Description

 Process for the manufacture of Hexafluorophosphate of a metal, M (PF6) n, in particular of LiPF6.

The development of rechargeable batteries based on lithium hexafluorophosphate leads to a high demand for product that meets the highest criteria of purity and capable of meeting the most demanding specifications in terms of impurities, in particular humidity and residual acidity . In most processes, lithium hexafluorophosphate, LiPF6, is first obtained in the form of a solution in anhydrous hydrogen fluoride which is a liquid boiling at 20 ° under normal pressure and obtaining the product final is done by evaporation of this solution; for unspecified reasons, and which has been attributed to the inevitable presence of traces of moisture in the reaction medium, this operation is never quantitative.

Most of the methods for obtaining LiPF6 are carried out in two stages: first the fluoridation of PC15 with anhydrous HF then the selective absorption of PF5 produced in a solution of LiF in anhydrous HF, HF in this case playing the role of solvent. The LiPF6 is then recovered by evaporation of the HF. Such a process is described in patent application JP 05 279 003 (Morita).

The presence of LiF, an undesirable impurity of LiPF6, is avoided, either by using an excess of PC15 with respect to LiF as indicated in patent applications JP 05 279 003 (Morita). , JP 04 175 216 (Tochem Products) or JP 6 251 109 (Dai in Kogyo) either by separating the LiPF6 by crystallization under the combined action of a concentration and a lowering of the temperature of the solution according to the processes described in patent applications JP04 175 216, 05 279 003 and 06 056 413 (Central Glass). Whatever the process used, the yield relative to the quantity of LiF used does not exceed 65% (JP 4 108 890, DE 19614503 or JP 6 251 109). To obtain this result, the LiPF6 solution in HF is concentrated in a ratio of 1: 2 to 1: 5 and then cooled to -20 ° C to cause precipitation of part of the LiPF6 present, the crystals then being isolated. by filtration. The yield can be slightly improved by carrying out the filtration at very low temperature, for example between -50 and - 80 ° as indicated in patent application RU 2075435. Such temperature conditions are hardly compatible with an industrial process.

When the LiPF6 is recovered by complete evaporation of the HF, the crude product must undergo a purification in order to remove the LiF which may be present; for this ^• , the raw LiPF6 is dissolved in an aprotic organic solvent and the solution filtered to remove the LiF generally insoluble in this type of solvent. The organic complex LiPF6-solvent is recovered and decomposed at as low a temperature as possible, 30 to 40 ° C under reduced pressure as indicated for example in DE 19614503. Such a process can lead to the presence of organic impurities in the product finished and thus adversely affect its quality.

The aim of the present invention is to provide a high-efficiency industrial process leading to M (PF6) n of high purity, M being a metal of oxidation state n, with n being an integer ranging from 1 to 2 and of preferably equal to 1.

More specifically, one of the aims of the present invention is to provide a process making it possible to evaporate, in an almost quantitative manner, HF from solutions or suspensions of LiPF6 in anhydrous liquid HF. Another object is to substantially reduce the residual content of HF and MFn present in solid M (PF6) n, and particularly in solid LiPF6. LiPF6 in solution or suspension in an ionizing solvent such as anhydrous HF behaves as a pair of ions, a Li + cation and a complex anion PF6- which can be considered as the salt of Lewis acid PF5 and of strong base F-,

It appears from the data in the literature (A.F. Clifford and S. Kongpricha, J. Inorg. Nucl. Chem 20, p 147-154, 1961) that PF5 is a relatively strong acid in liquid HF medium; consequently its salt, the anion PF6- should not give rise to any phenomenon of solvolysis according to the reaction:

PF6 ~ + HF O PF5 + HF2 ^"

However, the tests at the origin of the present invention surprisingly show that this solvolysis does take place and that the evaporation of the HF by an inert gas causes the decomposition of LiPF6 and entrainment of the PF5 formed. LiPF6 + HF O PF5 in solution. + Li + HF2 ~ with PF5 in solution => PF5 gazt (2)

This decomposition by solvolysis of LiPF6 is detrimental for several reasons, because it: a) causes the formation of solid LiF remaining in the state of impurity in the crystals of LiPF6, b) decreases the yield, c) delays and makes more difficult elimination of the last traces of HF as a result of the formation first of (Li ⁺ HF2 ~) in solution, then of LiF, HF to dryness.

Lithium acid fluoride is a compound sensitive to temperature, traces of humidity and already exhibiting a vapor pressure of 1.89 mm Hg of HF at 25 ° C (Jander, Spandau and CC Addison Chemistry in Anhydrous Prototropic Inorganic Solvents Vol. II Part 1: Inorganic Chemistry in liquid Hydrogen cyanide and Liquid Hydrogen Fluoride p. 221 Pergamon Press, Oxford, 1971). Unless using selective dissolution processes using organic solvents as described in patent application DE 19614503, it is very difficult to separate LiF, HF from LiPF6, the thermal dissociation of the LiF complex, HF occurring in the same temperature range as the decomposition of LiPF6.

It has been found and it is one of the objects of the present invention that the decomposition of LiPF6, during the evaporation of 'its solutions in liquid HF, could be reduced and even eliminated by carrying out the evaporation under pressure partial sufficient of gaseous PF5.

Thus, the present invention provides a method for manufacturing solid M (PF6) n from a solution or a suspension of M (PF6) n in liquid HF, M being a metal cation of oxidation state n, with n whole number ranging from 1 to 2, characterized in that one introduces into and / or above this solution or suspension a gas containing gaseous PF5 while carrying out the evaporation of the liquid HF, which leads at the end of the evaporation to solid M (PF6) n having a low content of MFn and HF.

Advantageously, the evaporation of the HF is obtained by heating said solution or suspension.

Advantageously, the heating is carried out by electromagnetic induction.

A simple means of achieving this evaporation may consist in placing the solution to be evaporated in a sealed container made of stainless steel or in fluorinated resins and in sweeping the apparatus with a gas stream composed of a dry inert gas for example, nitrogen, helium, and PF5; the PF5 content being between 3 and 100% and preferably between 4 and 15 mol% in the gas stream.

Under these conditions, it is possible to obtain, with a quantitative yield and an excellent productivity, a LiPF6 practically free of LiF in a notable quantity and therefore easier to get rid of the last traces of HF and LiF. The PF5 used, to carry out the evaporation of the solution, can of course be recycled by a suitable device. Another object of this invention is therefore to obtain a crude lithium hexafluorophosphate free of LiF and much easier to "dry" than the products obtained according to currently known methods. By the expression "dry" is meant the elimination of the liquid HF.

Another object is the use of partial pressure of PF5 to carry out the final drying and remove the last traces of HF. Such a method has the additional advantage of transforming the residual traces of LiF into LiPF6.

The present invention therefore therefore also provides a process in which the solid M (PF6) n obtained with low MFn and HF content is scanned with a gas containing PF5 while heating the solid M (PF6) n to a temperature t sufficient for qu '' it releases its traces of residual HF and that its traces of MFn combines with the PF5 of the gas to give M (PF6) n particularly pure solid, then in that finally the M (PF6) n solid always under a gas containing PF5.

The solid M (PF6) n to be purified according to the above process can also be obtained from any process giving a solid M (PF6) n containing traces of HF and / or MFn. Preferably, M is lithium, n being equal to 1. Preferably, when M is lithium, said temperature t is from 90 to 130 ° C.

Advantageously, the gas comprises from 3 to 100 mol% of gaseous PF5, preferably from 4 to 15 mol% of gaseous PF5.

The present invention already described above will be better understood using the following examples given by way of illustration. Comparative example 1

4.31 g of commercial LiPF6 crystals (Aldrich 98%) containing 5000 ppm of POF2 (OH) and 6000 ppm of HF are placed in a 100 ml stainless steel autoclave, previously passive with anhydrous HF, purged and dried; the autoclave has two gas phase intake pipes as well as a stirring device by magnetic drive making it possible to place the assembly on a precision balance - in order to follow the weight during evaporation; the outputs are connected to the other fixed parts of the assembly by flexible turns in stainless steel tubes with a diameter of 1 mm.

After closing the lid, 1 • autoclave is removed from the glove box in which the handling of LiPF6 is carried out and connected: on the one hand to a metal circuit comprising an anhydrous HF inlet, an inlet of previously dried nitrogen on a 3 A molecular sieve and a vacuum connection, on the other hand to a vent by means of a gas sample taking followed by an HF and humidity trap.

After careful purging of the connection circuit, 50.26 g of HF containing 600 ppm of water are transferred through one of the two pipes, and by cold trapping. A solution of LiPF6 in anhydrous HF is thus obtained after stirring, which is swept with a stream of dry nitrogen at a rate of 7.5 l / h and at room temperature (21 ° C). The evaporation of the HF is followed quantitatively by the change in weight and qualitatively by the IR analysis of the gases leaving the autoclave. It can thus be seen that, alongside the HF, there is a significant training of PF5 characterized by its infrared absorption bands in the regions from 900 to 1050 cm ⁻¹ and 1540-1900 cm ⁻¹ .

After 2 h 30 min, the weight of the autoclave stabilizes and no more HF is detected in the purging nitrogen. Evaporation is stopped and after opening the autoclave in a glove box, it is found that the crystals of LiPF6 recovered recover a weight loss of 1.2 g corresponding to the PF5 eliminated according to equation (2). Since the LiF formed remains with the LiPF6 crystals, the weight loss observed corresponds in fact to the disappearance of 1.2 * 152/126 = 1.485 g of LiPF6, ie a decomposition rate of 34.2%.

Comparative example 2

Test 1 is repeated with a commercial LiPF6 of very high purity 99.96% (Morita) containing only 200 ppm of POF2 (OH), 100 ppm of POF3 and 100 ppm of HF. A solution of 4.94 g of this product in 50.82 g of HF at 300 ppm of water is prepared and evaporated according to the same procedure as above.

At the end of the test, only 3.67 g of crystals are collected, ie a decomposition rate of 31% taking into account the LiF formed. As before, we observe the formation and training of PF5. The product obtained still contains 1.7% of HF.

The decomposition of lithium hexafluorophosphate during evaporation of the solvent is therefore not linked to the purity of the product.

Comparative example 3

A solution of 5.68 g of 98% purity LiPF6 was prepared in 49.9 g of HF containing 300 ppm of water. At the outlet of the autoclave, there is a polyvinylidene fluoride (PVDF) absorber containing 600 ml of water to trap PF5, transform it into less volatile hydrolysis products and estimate the quantity of PF5 retained after determination of phosphorus by inductive plasma spectrometry.

The HF is evaporated by sweeping with a stream of dry nitrogen to constant weight; after opening the autoclave, 4.46 g of crystals are recovered, ie a decomposition rate of 26.1% taking into account the formation of LiF. The estimated decomposition rate based on the amount of phosphorus dosed in the absorbers is 33.8%. 8

Comparative example 4

A solution of 5.6 lg of this same sample of LiPF6 in 44.56 g of the same anhydrous HF is prepared, in which 2000 ppm additional water has been incorporated voluntarily, and the evaporation of the solution thus obtained is started as in previous example.

At the end of the test, 4.21 g of crystals are recovered which are dissolved in acetonitrile to check their purity by RM. The product obtained contains by weight only 90.1% of LiPF6 next to 0.83% of POF2 (OH), 1.57% of HF and 7.5% of insoluble materials, mainly LiF.

The decomposition rate calculated according to the quantity of phosphorus dosed in the absorber amounts to 33.9% against 30.4% for the estimation from the change in weight. This example therefore shows that an addition of 2000 ppm of water to the solution does not appreciably modify the rate of decomposition of LiPF6.

Comparative example 5 The procedure is as above except that the evaporation is carried out at low temperature as recommended in certain patent applications. For this, repeat Comparative Example 3 by dissolving 5.16 g of 98% LiPF6 in 50.22 g of HF at 300 ppm of humidity. The solution is cooled to -14 ° and the HF is evaporated under a stream of dry nitrogen first at a rate of 10 l / h for 2 h 30 min then 20 l / h for 7 h 30 h.

After opening the autoclave, 4.45 g of crystals are recovered, which corresponds to a decomposition rate of 16.8% against 18% according to the amount of phosphorus retained in the absorber.

¹⁹ F NMR analysis of these crystals indicates a purity of 97.3% in LiPF6 with 0.7% POF2 (OH) and 2% HF. The insoluble fraction is of the order of 3.5% relative to the crude product.

The complete evaporation of the solution at low temperature therefore decreases, but does not eliminate, the decomposition of LiPF6. In addition, by doing so, the duration of the operation is considerably increased.

Comparative Example 6 In this example, the LiPF6 is separated by crystallization after concentration of the solution up to beyond the solubility limit of LiPF6 in HF.

For this, we use a crystallizer composed of 2 bowls with a capacity of 150 ml, one in poly (trifluorochlorethylene) PTFCE (chlorofluorinated resin), the other in stainless steel, hermetically joined by their opening and separated by a filter in polyvinylidene fluoride (PVDF) fabric.

The stainless steel bowl has in its upper part two outlet pipes making it possible to connect it on one side to a PVDF absorber, on the other, by means of flexible metal fittings to the gas rail described in comparative example 1. The solution of LiPF6 in HF can be introduced into the poly (trifluorochlorethylene) bowl or produced in situ as in the present example from 19.40 g of LiPF6 at 98% purity and 69.2 g of HF at 300 ppm of humidity, the transfer of HF being by cold trapping.

The principle is to concentrate the solution in the PTFCE bowl in the lower position until reaching a volume making it possible to precipitate the LiPF6 without insolubilizing the LiF possibly formed, then to return the assembly. The residual solution then passes into the stainless steel bowl while the crystals are retained on the PVDF filter. In this example, the inversion was carried out after evaporation of 38.6% by weight of the solution. Evaporation is then continued until constant weight.

After opening the assembly in a glove box, we collect: 10

- on the filter, 3.41 g of very pure LiPF6 crystals (99.916% by weight of LiPF6) which contain only 20 ppm of POF2 (OH), 90 ppm of POF3 and 550 ppm of HF.

- in the stainless steel bowl: 13.31 g of raw LiPF6. The dosage of phosphorus retained in the absorber indicates a decomposition rate of 17.1% against 14.6% according to the corrected weight variation.

This example therefore shows that obtaining a very pure product can be carried out according to known methods only at the cost of a considerable loss of yield and that crystallization by concentration and cooling of the solution inevitably leads to a decomposition of the LiPF6.

Example 1

5.68 g of high purity LiPF6 are placed in the autoclave used for Comparative Examples 1 to 5, and dissolved in 52.74 g of HF at 300 ppm of humidity.

A mixture of pure PF5 and dry nitrogen at 2 mol% of PF5 is also produced, which is then used after homogenization, to sweep the autoclave and evaporate the LiPF6 solution to dryness at room temperature.

After stabilization of the weight, the autoclave is opened in the glove box and 5.00 g of crystals are collected, ie a decomposition rate of only 11.9% according to the corrected weight loss, and 14.4% d 'after the weight of phosphorus dosed in the absorbent solution.

This result should be compared to Comparative Example 2 for which a decomposition rate of 31% was observed.

Example 2

The previous example is repeated by dissolving 5.59 g of the same batch of LiPF6 in 52.84 g of HF at 300 ppm of water.

Evaporation is carried out this time with a dry N2 / PF5 mixture at 5 mol% of PF5 for 3 h 30 at a flow rate of 6

1 / h. Then swept with nitrogen for 4 h 30 at a rate of 10 1 / h. 11

When the autoclave was opened, 5.55 g of LiPF6 crystals of good purity were found, 99.75% containing only 0.20% of HF. The determination of phosphorus in the absorbent solution confirms, with the accuracy of the analyzes, the absence of formation of PF5 during the dry evaporation of the solution.

It can be seen that the yield is quantitative and the decomposition rate practically zero when the solutions of LiPF6 in anhydrous HF are evaporated under a partial pressure of 0.05 bars (5 kPa) of PF5, obtained by sweeping the solution using an N2 / PF5 mixture at 5 mol% of PF5.

This example also shows that in the presence of PF5 and therefore in the absence of decomposition of LiPF6 into PF5 and LiF, a LiPF6 which is easier to dry is obtained. The residual HF rates obtainable according to this process, 2000 ppm, ^• is about 10 times lower than that typically observed ° n at a dry evaporation without PF5 as in Comparative Example 5. On the other hand, it is at least as good as that observed on a product isolated by crystallization with an obviously reduced yield as shown in Comparative Example 6.

Example 3,500 mg of the crystallized product obtained in Example 2 are transferred under a dry atmosphere to a small 316 L stainless steel reactor.

The reactor is then swept by a stream of dry helium containing 3 mol% of PF5 for a time sufficient to purge the entire apparatus, then immersed for 15 min still under sweep in an oil bath previously heated to 130 °.

After cooling, and purging of the assembly by a current of dry helium, the reactor is opened in a glove box and the solid sampled for control by NMR,

Its composition:

LiPF6: 99.95% 12

HF: 30 ppm

POF2 (OH), P0F3: 400 ppm shows that such a treatment makes it possible to considerably reduce the residual HF content which thus goes from 2000 to 30 ppm, and makes it possible to obtain a LiPF6 of excellent purity.

Claims

13 Claims 1. Method for manufacturing solid M (PF6) n from a solution or suspension of M (PF6) n in liquid HF, M being a metal cation of oxidation state n, with n whole number being 1 to 2, characterized in that a gas containing PF5 gas is introduced into and / or above this solution or suspension while evaporating the liquid HF, which leads to the end of the evaporation at -from the solid M (PF6) n having a low content of MFn and HF. 2. Method according to claim 1, characterized in that the evaporation of the HF is obtained by heating said solution or suspension. 3. Method according to claim 2, characterized in that the heating is carried out by electromagnetic induction. 4. Method according to one of claims 1 to 3, characterized in that the solid M (PF6) n obtained with low MFn and HF content is scanned with a gas containing PF5 while heating the M (PF6) n solid at a temperature t sufficient for it to give off its traces of residual HF and for its traces of MFn to combine with the PF5 of the gas to give M (PF6) n a particularly pure solid, then in that finally the M (PF6) n solid always under a gas containing PF5. 5. Method according to one of claims 1 to 4, characterized in that M is lithium, n being equal to 1. 6. Method according to claims 4 and 5, characterized in that the temperature t is from 90 to 130 ° C 7. Method according to one of claims 1 to 6, characterized in that the gas comprises from 3 to 100 mol% of gaseous PF5, preferably from 4 to 15 mol% of gaseous PF5.