Manufacturing Method for Lithium Difluorophosphate Powder, and Lithium Difluorophosphate

Abstract

To provide a manufacturing method with which lithium difluorophosphate powder can be recovered from a lithium difluorophosphate solution. A method for manufacturing lithium difluorophosphate powder is used which includes the steps of precipitating solid lithium difluorophosphate by adding a poor solvent to a solution in which lithium difluorophosphate is dissolved in a main solvent, and obtaining lithium difluorophosphate powder by solid-liquid separation of the solid lithium difluorophosphate from the liquid containing the main solvent and the poor solvent, wherein the relational expression between the octanol/water partition coefficient P.sub.P of the main solvent and the octanol/water partition coefficient P.sub.A of the poor solvent is defined by the following formula (1). (1): P.sub.A≧−4/3×P.sub.P+1.2

Claims

1. A method for manufacturing lithium difluorophosphate powder, comprising the steps of: precipitating solid lithium difluorophosphate by adding a poor solvent to a solution in which lithium difluorophosphate is dissolved in a main solvent; and obtaining the lithium difluorophosphate powder by solid-liquid separation of the solid lithium difluorophosphate from a liquid containing the main solvent and the poor solvent, wherein a relational expression between an octanol/water partition coefficient P.sub.P of the main solvent and an octanol/water partition coefficient P.sub.A of the poor solvent is defined by a formula (1).
P.sub.A≧−4/3×P.sub.P+1.2   (1):

2. The method for manufacturing the lithium difluorophosphate powder according to claim 1, wherein the poor solvent is at least one kind selected from the group consisting of a saturated hydrocarbon-based solvent, an aromatic-based solvent, an ether-based solvent and a carbonic acid ester-based solvent.

3. The method for manufacturing the lithium difluorophosphate powder according to claim 2, wherein the saturated hydrocarbon-based solvent is at least one kind selected from the group consisting of n-hexane, n-heptane and n-octane.

4. The method for manufacturing the lithium difluorophosphate powder according to claim 2, wherein the aromatic-based solvent is at least one kind selected from the group consisting of toluene, xylene and benzene.

5. The method for manufacturing the lithium difluorophosphate powder according to claim 2, wherein the ether-based solvent is at least one kind selected from the group consisting of diisopropyl ether, tetrahydrofuran and diethyl ether.

6. The method for manufacturing the lithium difluorophosphate powder according to claim 2, wherein the carbonic acid ester-based solvent is at least one kind selected from the group consisting of ethyl methyl carbonate, dimethyl carbonate, propylene carbonate and ethylene carbonate.

7. The method for manufacturing the lithium difluorophosphate powder according to claim 1, wherein the main solvent is at least one kind selected from the group consisting of an ester-based solvent, a ketone-based solvent and an alcohol-based solvent.

8. The method for manufacturing the lithium difluorophosphate powder according to claim 7, wherein the ester-based solvent is at least one kind selected from the group consisting of ethyl acetate, methyl acetate and propyl acetate.

9. The method for manufacturing the lithium difluorophosphate powder according to claim 7, wherein the ketone-based solvent is at least one kind selected from the group consisting of acetone, methyl ethyl ketone and cyclohexanone.

10. The method for manufacturing the lithium difluorophosphate powder according to claim 7, wherein the alcohol-based solvent is at least one kind selected from the group consisting of ethanol, isopropanol, methanol and isobutanol.

11. The method for manufacturing the lithium difluorophosphate powder according to claim 1, wherein the octanol/water partition coefficient P.sub.P of the main solvent is −0.40 or greater to 0.80 or less.

12. The method for manufacturing the lithium difluorophosphate powder according to claim 1, wherein the main solvent is ethyl acetate, and the poor solvent is toluene.

13. The method for manufacturing the lithium difluorophosphate powder according to claim 1, an amount of the poor solvent to be added to the solution is 0.1-5 mass times, relative to the solution.

14. The method for manufacturing the lithium difluorophosphate powder according to claim 1, comprising, after the solid-liquid separation, the steps of: cleaning an obtained solid with cleaning liquid; and removing the cleaning liquid, and drying the solid.

15. Lithium difluorophosphate, wherein 1000 mass ppm or less of toluene is contained.

Description

BRIEF DESCRIPTION OF THE DRAWING(S)

[0020] FIG. 1 is a graph in which the result of a first example 1 is plotted when a horizontal axis represents the octanol/water partition coefficient of a main solvent and a vertical axis represents the octanol/water partition coefficient of a poor solvent.

MODE FOR IMPLEMENTING THE INVENTION

[0021] In the following, the present invention be explained in detail.

[0022] A method for manufacturing lithium difluorophosphate powder of the present invention includes a step of obtaining lithium difluorophosphate powder by solid-liquid separation of solid lithium difluorophosphate from a liquid containing a main solvent and a poor solvent, after the crystal of lithium difluorophosphate is precipitated by adding the poor solvent to a lithium difluorophosphate solution in which lithium difluorophosphate is dissolved in the main solvent. When the poor solvent having a low solubility of lithium difluorophosphate is added to the lithium difluorophosphate solution, the amount of lithium difluorophosphate which can be dissolved in the solution decreases, and lithium difluorophosphate which cannot be dissolved in the solution is precipitated as a crystal. The obtained crystal has a high purity.

[0023] Furthermore, it is characterized that the relational expression between the octanol/water partition coefficient P.sub.P of the main solvent and the octanol/water partition coefficient P.sub.A of the poor solvent is defined by the following formula (1).

P.sub.A≧−4/3×P.sub.P+1.2   (1):

[0024] An octanol/water partition coefficient (Log P.sub.OW, hereinafter, simply also called “partition coefficient”) is a concentration ratio of a chemical substance in two solvent phases of n-octanol and water, when the chemical substance is added to the two solvent phases and becoming an equilibrium state. It is used when the substance is judged if it is hydrophilicity or hydrophobicity and if the value of the partition coefficient is large, the substance has a high hydrophobicity. The octanol/water partition coefficient can be evaluated with a method described in JIS 27260. However, it can be also calculated with a computer.

[0025] When the relational expression between the octanol/water partition coefficient P.sub.P of the main solvent and the octanol/water partition coefficient P.sub.A of the poor solvent satisfies the formula (1), it is possible to recover the lithium difluorophosphate powder with a poor solvent precipitation method.

[0026] The addition of the poor solvent is preferably performed while stirring the solution to make the whole system uniform.

[0027] In addition, a general solid-liquid separation method can be used for the solid-liquid separation, and natural filtration, vacuum filtration, pressure filtration and centrifugal filtration can be also used. In particular, the pressure filtration is used widely and industrially, and it is preferable because filtering speed can be increased.

[0028] In addition, it is preferable that the poor solvent is at least one kind selected from the group consisting of a saturated hydrocarbon-based solvent, an aromatic-based solvent, an ether-based solvent and a carbonic acid ester-based solvent. The octanol/water partition coefficients of these solvents tend to be high, and the relational expression of the formula (1) therefore tends to be satisfied.

[0029] In addition, it is preferable that the saturated hydrocarbon-based solvent is at least one kind selected from the group consisting of n-hexane (3.90), n-heptane (4.66) and n-octane (5.18), the aromatic-based solvent is at least one kind selected from the group consisting of toluene (2.69), xylene (3.12) and benzene (3.12), and that the ether-based solvent is at least one kind selected from the group consisting of diisopropyl ether (1.52), tetrahydrofuran (0.46) and diethyl ether (0.89), because these solvents are generally distributed and their partition coefficients are relatively high. In addition, a number in parentheses after each substance name is a value of the octanol/water partition coefficient of each of the substances.

[0030] It is preferable that the carbonic acid ester-based solvent is at least one kind selected from the group consisting of ethyl methyl carbonate (0.75), dimethyl carbonate (0.354), propylene carbonate (−0.48) and ethylene carbonate (0.11), because these solvents can be used for electrolytes for non-aqueous electrolyte batteries, and when these are used for the electrolytes, problems do not occur even if these solvents remain in solid contents.

[0031] It is preferable that the main solvent is at least one kind selected from the group consisting of an ester-based solvent, a ketone-based solvent and an alcohol-based solvent, because the partition coefficients of these solvents tend to be low, and the relational expression of the formula (1) tends to be satisfied.

[0032] It is preferable that the ester-based solvent is at least one kind selected from the group consisting of ethyl acetate (0.73), methyl acetate (0.18) and propyl acetate (1.24), the ketone-based solvent is at least one kind selected from the group consisting of acetone (−0.24), methyl ethyl ketone (0.29) and cyclohexanone (0.81), and that the alcohol-based solvent is at least one kind selected from the group consisting of ethanol (−0.32), isopropyl alcohol (0.05), methanol (−0.82) and isobutanol (0.8), because these solvents are generally distributed and the partition coefficients of these solvents are relatively low.

[0033] In particular, it is preferable that the octanol/water partition coefficient P.sub.P of the main solvent is −0.40 or greater to 0.80 or less. In this range, in the examples, it has been proved that precipitation efficiency is high in a case where the relational expression of the formula (1) is satisfied.

[0034] In particular, it is preferable that the main solvent is ethyl acetate and the poor solvent is toluene. In this combination, the precipitation efficiency is particularly high, as compared with other combinations of the solvents, and furthermore, the lithium difluorophosphate solution is not turned into gel, and a precipitated product can be easily separated by filtration.

[0035] When the poor solvent is added to the lithium difluorophosphate solution, the amount of the poor solvent to be added is preferably 0.1-5 mass times, more preferably 0.2-2 mass times, further preferably 0.4-1 mass times, relative to the mass of the solution. If the amount of the poor solvent is too small, the precipitation efficiency deteriorates, and if the amount of the poor solvent is large, when the precipitated product is separated by filtration, the amount of filtrate increases, and the filtration takes time, and moreover, the amount of waste increases.

[0036] Moreover, it is preferable to include the steps of cleaning an obtained solid with cleaning liquid, removing the cleaning liquid, and drying after the solid-liquid separation. As the cleaning liquid, a saturated hydrocarbon-based solvent, an aromatic-based solvent, an ether-based solvent and a carbonic acid ester-based solvent, which can be used as the poor solvent, can be used. By the cleaning step, an excess solid content can be removed, and by the drying step, the solvent contained in the solid content is volatilized, and it can be removed.

[0037] When lithium difluorophosphate is manufactured by a well-known manufacturing method, there is almost no possibility that reaction is completely proceeded, and only lithium difluorophosphate is produced. In general, a lithium difluorophosphate solution is obtained which contains non-target substances, such as unreacted starting substances, side reaction products and decomposition products under reaction, as impurities. The amount of the impurities is approximately 1-50 mass %, generally 5-25 mass %, relative to the amount of lithium difluorophosphate. By applying the method for manufacturing the lithium difluorophosphate powder of the present invention to the lithium difluorophosphate solution containing those impurities, it is possible to obtain high purity lithium difluorophosphate powder in which the content of the impurities have been reduced, and lithium difluorophosphate can be purified. As a method for manufacturing lithium difluorophosphate, there can be used a method described in a patent document 2, method in which halides other than fluoride, LiPF.sub.6 and water are reacted in a non-aqueous solvent, a method described in a patent document 3, method in which diphosphorus pentoxide, lithium hexafluorophosphate and hydrogen fluoride are reacted, and a method described in a patent document 4, method in which difluorophosphate and lithium chloride are reacted. In addition, the purity of the lithium difluorophosphate powder obtained with the manufacturing method of the present invention is high because crystal precipitation process is carried out. The purity of the lithium difluorophosphate powder is generally 95 mass % or greater, preferably 98 mass % or greater, more preferably 99 mass % or greater.

[0038] In addition, there is a case where the lithium difluorophosphate powder manufactured by applying the present method contains the used poor solvent within a range of 50 mass ppm or greater to 1000 mass ppm or less.

EXAMPLES

Example 1

[0039] Lithium difluorophosphate was added to 10 g of a main solvent until saturated at a room temperature (25° C.). A predetermined amount of a poor solvent was added to this solution. One night later, the mixed liquid was pressure-filtered (pressure: 0.4 MPaG, filter: polytetrafluoroethylene porous membrane having a pore diameter of 0.5 μm), and the amount of lithium difluorophosphate powder in a residue was determined by determining the amount of lithium difluorophosphate in filtrate.

[0040] A value obtained by dividing the amount of the lithium difluorophosphate powder in the residue by the amount of lithium difluorophosphate contained in the saturated solution before adding the poor solvent was set as precipitation efficiency. In addition, the solution after adding the poor solvent was visually confirmed, and one which was capable of being stirred but which had a high viscosity was expressed as jelly, and one having a high viscosity which was not capable of being stirred was expressed as gel.

[0041] The kinds of the main solvent and the poor solvent which were used, and the amount of the poor solvent to be added to the solution were changed and shown in the following Table 1 and Table 2. In addition, EtOAc is ethyl acetate, IPE is diisopropyl ether, THF is tetrahydrofuran, EMC is ethyl methyl carbonate, DMC is dimethyl carbonate, PC is propylene carbonate and EC is ethylene carbonate, in the tables. Each of their partition coefficients is shown in Table 3.

TABLE-US-00001 TABLE 1 Precipitation Poor solvent efficiency Main solvent Poor solvent Addition Poor solvent (vs saturated (partition (partition amount concentration solution) coefficient) coefficient) [mass times] [mass %] [%] Note EtOAc Toluene 0.2 19.8% 68.9% (0.73) (2.69) 0.4 33.1% 82.3% 0.8 49.8% 86.1% Hexane 0.1 11.0% 58.4% Gel or jelly immediately after (3.9)  mixing .fwdarw. Cloudy liquid, one night later 0.2 19.8% 74.5 Gel or jelly immediately after mixing .fwdarw. Cloudy liquid, one night later 0.6 42.6% 85.9% Gel or jelly immediately after mixing .fwdarw. Cloudy liquid, one night later IPE 0.1 11.0% 51.0% (1.52) 0.3 27.1% 77.1% Gel or jelly immediately after mixing .fwdarw. Cloudy liquid, one night later 0.8 49.7% 86.0% Gel or jelly immediately after mixing .fwdarw. Cloudy liquid, one night later THF 0.2 19.8% 33.1% (0.46) 0.6 42.6% 68.9% 1.0 55.3% 65.0% EMC 1.0 55.3% 25.0% No precipitation immediately (0.75) after mixing (one night later, slight precipitation) DMC 1.0 55.3% 22.4% No precipitation immediately  (0.354) after mixing (one night later, slight precipitation) PC 1.0 55.3% 0.0% (−0.48)  EC 1.0 55.3% 0.0% (0.11) Acetone Toluene 0.1 13.4% 23.3% (−0.24)  (2.69) 0.3 23.6% 45.5% 0.8 48.2% 71.2% Hexane 0.1 13.4% 44.4% (3.9)  0.3 23.6% 63.2% Jelly .fwdarw. Jelly, one night later 0.6 43.6% 77.2% Jelly .fwdarw. Jelly, one night later IPE 0.1 13.4% 34.8% (1.52) 0.3 23.6% 55.8% Jelly .fwdarw. Jelly, one night later 0.6 43.6% 73.6% Jelly .fwdarw. Jelly, one night later THF 0.1 13.4% 0.0% (0.46) 0.3 23.6% 0.0% 0.6 43.6% 17.3% EMC 0.6 43.6% 0.0% (0.75) DMC 0.6 43.6% 0.0%  (0.354) PC 0.6 43.6% 0.0% (−0.48)  EC 0.6 43.6% 0.0% (0.11)

TABLE-US-00002 TABLE 2 Poor solvent Poor Addition Main solvent solvent amount Poor solvent Precipitation efficiency (partition (partition [mass concentration (vs saturated solution) coefficient) coefficient) times] [mass %] [%] Note IPA Toluene 0.2 20.9% 14.4% Gel, one night later (0.05) (2.69) 0.4 34.6% 35.6% Gel, one night later 0.7 48.0% 52.9% Gel, one night later Hexane 0.2 20.9% 36.5% Gel, one night later (3.9)  0.4 34.6% 55.4% Gel, one night later 0.7 48.0% 70.7% Gel, one night later IPE 0.2 20.9% 1.6% (1.52) 0.4 34.6% 27.1% 0.7 48.0% 48.7% THF 0.2 20.9% 0.0% (0.46) 0.4 34.6% 0.0% 0.7 48.0% 0.0% EMC 0.9 54.3% 0.0% (0.75) DMC 0.9 54.3% 0.0%  (0.354) PC 0.9 54.3% 0.0% (−0.48)  EC 0.9 54.3% 0.0% (0.11) Ethanol Toluene 0.2 25.8% 3.6% (−0.32)  (2.69) 0.4 41.0% 10.2% 0.7 54.9% 20.2% Partial separation into two layers Hexane 0.2 25.8% 6.8% Separation into two layers (3.9)  0.4 41.0% 29.0% Separation into two layers 0.7 54.9% 85.0% Separation into two layers IPE 0.2 25.8% 0.0% (1.52) 0.4 41.0% 0.0% 0.7 54.9% 0.0% THF 0.2 25.8% 0.0% (0.46) 0.4 41.0% 0.0% 0.7 54.9% 0.0% EMC 1.0 63.5% 0.0% (0.75) DMC 1.0 63.5% 0.0%  (0.354) PC 1.0 63.5% 0.0% (−0.48)  EC 1.0 63.5% 0.0% (0.11)

TABLE-US-00003 TABLE 3 Name Log P.sub.OW (partition coefficient) AcOEt (ethyl acetate) 0.73 Acetone −0.24 IPA (isopropyl alcohol) 0.05 Ethanol −0.32 Hexane (n-hexane) 3.9 Toluene 2.69 IPE (diisopropyl ether) 1.52 THF (tetrahydrofuran) 0.46 EMC (ethyl methyl carbonate) 0.75 DMC (dimethyl carbonate) 0.354 PC (propylene carbonate) −0.48 EC (ethylene carbonate) 0.11

[0042] In addition, in FIG. 1, a horizontal axis represents the partition coefficient of the main solvent and a vertical axis represents the partition coefficient of the poor solvent. As to the maximum value of the precipitation efficiency of each combination of the main solvents and the poor solvents, less than 20% is expressed as “×”, 20% or greater to less than 45% is expressed as “Δ”, 45% or greater to less than 70% is expressed as “∘” and 70% or greater is expressed as “”, and results were plotted in FIG. 1. A line in FIG. 1 is a line in a case where the relational expression between the octanol/water partition coefficient P.sub.P of the main. solvent and the octanol/water partition coefficient P.sub.A of the poor solvent is defined by the following formula.

P.sub.A≧−4/3×P.sub.P+1.2

[0043] As shown in FIG. 1, in a region above the line showing the relational expression between P.sub.P and P.sub.A, the maximum precipitation efficiency exceeds 20%, and it is understood that the precipitation efficiency is relatively high.

[0044] In particular, in a case where ethyl acetate was used as the main solvent and toluene was used as the poor solvent, the precipitation efficiency was high, and gelation and separation in the solution did not occur. It can be therefore used for industrially manufacturing lithium difluorophosphate powder, and is particularly preferable.

Example 2

[0045] Poor solvent addition in which the combination of ethyl acetate and toluene had been used, combination which was made clear that it was the most preferable combination of the main solvent and the poor solvent in Example 1, was applied to a manufacturing step for lithium difluorophosphate powder.

[0046] [Synthesis]

[0047] Lithium difluorophosphate was synthesized, as follows, with reference to a method described in a patent document 2 for manufacturing lithium difluorophosphate.

[0048] 8.8 kg (57.9 mol) of LiPF.sub.6 was dissolved in 32.2 kg of ethyl acetate, and 7.9 kg (186.8 mol) of lithium chloride was added, following which it was stirred at 35° C. while adding 1.7 kg (97.0 mol) of water. This reaction liquid was depressurized at 35° C., 11 kg of a solvent was distilled off, and hydrogen chloride as a by-product was removed. The free acid of this solution was analyzed by a titration method. It was 4700 ppm converted as HF.

[0049] Next, the reaction liquid obtained in this way was analyzed with .sup.19F-NMR. It was confirmed that this reaction liquid contained 1.5 kg (9.6 mol) of LiPF.sub.6, 5.2 kg (47.9 mol) of lithium difluorophosphate and 0.05 kg (0.4 mol) of LiHPO.sub.3F, and reaction proceeded almost quantitatively.

[0050] As to this reaction liquid, pressure filtration (pressure: 0.2 MPaG, filter: polytetrafluoroethylene membrane having a pore diameter of 0.5 μm) was performed, and lithium fluoride as a by-product generated by the reaction was separated by the filtration, and a solution containing 20.2% of lithium difluorophosphate was obtained.

[0051] [Poor Solvent Precipitation Method]

[0052] 38.1 kg (1.55 times with respect to a lithium difluorophosphate solution, in mass ratio) of toluene was added to 24.6 kg of the ethyl acetate solution containing 20.2% of lithium difluorophosphate, which was obtained by the above-mentioned synthesis, and it was stirred for a hundred minutes, following which solids were precipitated. However, it was not turned into gel, and pressure filtration (pressure: 0.2 MPaG, filter: polytetrafluoroethylene membrane having a pore diameter of 0.5 μm) was therefore performed, and a residue was obtained. The filtration was completed without occurrence of clogging, etc., because gelation did not occur, and filtration performance was 5.97 kg per hour.

[0053] The residue was cleaned with ethyl methyl carbonate and dried for eight hours at 100° C., and powder was obtained.

[0054] The obtained powder was lithium difluorophosphate having a purity of 99.7 mass %. The amount of each impurity was shown in the folio table. In addition, phosphorus-atom-based recovery rate was 94.5%.

TABLE-US-00004 TABLE 4 Acid concentration Chlorine Solvent concentration Composition in solid content in solid content concentration in solid content (mass %) (HF conversion) in solid content (mass ppm) LiPO.sub.2F.sub.2 LiPF.sub.6 LiHPO.sub.3F LiF (mass ppm) (mass ppm) EtOAc EMC Toluene 99.71 0.00 0.00 0.17 448 16 500 600 100

Comparative Example 1

[0055] When an ethyl acetate solution containing lithium difluorophosphate and impurities, which had been obtained with the same method as that of Example 2, was cooled down to 5° C., it became a supersaturated state, and solids were not precipitated. The same experiment was conducted again, and solids were precipitated. However, gelation occurred at the moment when the solids were precipitated, and consequently, filtration became difficult.

[0056] As the above, when a poor solvent precipitation method, in which toluene as the poor solvent was added, was applied to the ethyl acetate solution of lithium difluorophosphate containing impurities, such as unreacted LiPF.sub.6 and LiHPO.sub.3F as a reaction by-product, lithium difluorophosphate powder having high purity was obtained at a high recovery rate without gelation.