A process for the preparation of metal hexafluorophosphate

Abstract

A PROCESS FOR THE PREPARATION OF METAL HEXAFLUOROPHOSPHATE The present invention relates to a mechanical process for the preparation of metal- hexafluorophosphate, wherein the process is devoid of solvent use and purification step and involves less metal use. The present invention also relates to the use of metal-hexafluorophosphate as an electrolyte salt in different batteries.

Description

[0001] P_W0100712

[0002] A PROCESS FOR THE PREPARATION OF METAL HEXAFLUOROPHOSPHATE

[0003] TECHNICAL FIELD OF THE INVENTION

[0004] The present invention relates to a process for the preparation of metal-hexafluorophosphate. More particularly, the present invention relates to a mechanical process for the preparation of metal-hexafluorophosphate, wherein the process is devoid of solvent use and purification step and involves less metal use. The present invention also relates to the use of metal- hexafluorophosphate as an electrolyte salt in different batteries.

[0005] BACKGROUND AND PRIOR ART OF THE INVENTION

[0006] Sodium hexafluorophosphate is a vital component of sodium- ion batteries, which are touted to relieve the tremendous pressure on the most sought-after lithium-ion batteries. The NaPFe is a highly hygroscopic compound, hence absorbs moisture rapidly leading to hydrolyzed products such as NaF. The insoluble NaF affects the battery performance. This can be easily circumvented if the synthesis of NaPFe is simple and doesn’t involve expensive equipment.

[0007] Most of the synthesis of NaPFe involves the use of HF. The synthesis is a multistep process. Recently, NaPFe was synthesized without using HF. In this approach, ammonium hexafluorophosphate was treated with two equivalents of sodium metal in THF solvent. Removal of solvent from the NaPFe turned out to be a challenge. The duration of the reaction is well over a day in a Schlenk line. Therefore, a method to synthesize NaPFe in a short time is essential.

[0008] Darren M. C. Quid et al., Angew. Chem. Int. Ed. 2021, 60, pages 24882-24887 reports a process of sodium hexafluorophosphate (NaPFe) wherein sodium metal is reacted with ammonium hexafluorophophate in presence of solvent THF at reflux for 6 hrs followed by heating at 50 °C for 16 hrs (making reaction time as 22 hrs) to obtain sodium hexafluorophosphate with the yield of 88%. Said Angew. Chem. Int. Ed. paper notes about problem of solvent residues in final compound as below: “Attempts were made to fully remove the residual solvent by drying under vacuum at both ambient temperature and at 160 °C for 72 hours and 24 hours, respectively, but both cases still showed THF solvent signals in the solution- state ’H NMR spectrum.” In batteries, THF is not used as a solvent; hence its P_W0100712 presence is likely to affect the battery performance. Although, the magnitude of the impact of THF is not known, it is unlikely that the battery performance is unaffected.

[0009] WO2015150862A1 discloses synthesis of alkali hexafluorophosphate salt, in non-aqueous medium, which involves multistep synthesis involving solvents and subsequent extraction. The approach is far more complicated than that is reported in the Angew. Chem. Int. Ed. Furthermore, it is desirable to develop a process that uses less metal, no use of solvent during the synthesis and devoid of the purification step.

[0010] So, the present invention discloses a process for the preparation of metal-hexafluorophos- phate comprising one pot, single step of mechanically mixing / grinding and reacting the dry metal particles / powder with ammonium hexafluorophophate.

[0011] OBJECTS OF THE INVENTION

[0012] An object of the present invention is to provide a process for the preparation of metal- hexafluorophosphate, wherein said process is devoid of solvent and the purification step and involves less metal use.

[0013] Another object of the present invention is to provide one pot, single step and cost-effective process for the preparation of metal -hexafluorophosphate.

[0014] SUMMARY OF THE INVENTION

[0015] Accordingly, in order to accomplish objectives, an embodiment of the present invention provides a process for the preparation of metal-hexafluorophosphate (M-PFe).

[0016] The present invention provides a mechanical process for the preparation of metal-hex- afluorophosphate, wherein the process is devoid of purification step and solvent and involves less metal use.

[0017] In an aspect of an embodiment, the present invention provides a mechanical process for the preparation of metal-hexafluorophosphate comprising a one pot, single step of mechanical mixing / grinding and reacting the dry metal particles / powder with hexafluorophophate component.

[0018] In an aspect, the present invention provides a one-pot, single step mechanical process for the preparation of metal-hexafluorophosphate, wherein the process comprises of: adding dry metal component / powder and hexafluorophophate component, followed by mechanical mixing / grinding to obtain metal-hexafluorophosphate; and then optionally, purifying the metal-hexafluorophosphate for further use. If the side products, which are ammonia and P_W0100712 hydrogen, are trapped in the metal hexafluorophosphate, a high vacuum is applied to remove these gases.

[0019] Another embodiment of the present invention also provides the use of metal-hexafluoro- phosphate as electrolyte salt in different batteries.

[0020] In yet another aspect of an embodiment, the present invention provides an energy storage device comprising a negative electrode material layer, a positive electrode material layer, a separator, and the metal hexafluorophosphate(M-PF6) as an electrolyte.

[0021] DETAILED DESCRIPTION OF THE DRAWINGS

[0022] FIG. 1 depicts the a)xH, b)19F, c)31P, and d)23Na NMR spectra of NaPFeprepared as per the examples described herein where a) 1H sample is prepared in CD3CN at 298 K, b)19F sample is prepared in CD3CN at 298 K, c)31P sample is prepared in acetonitrile at 298 K and d)23Na sample is prepared in CD3CN at 298 K.

[0023] FIG. 2 depicts the APCI mass spectra of NaPFeprepared as per the examples described herein.

[0024] FIG. 3 depicts the Deconvoluted XPS spectra of Nals, FIs, P2p for NaPFe as per the examples described herein.

[0025] FIG.4 depicts the Cyclic Voltammogram of sodium ion cell using NVP cathode with 1.0 M NaPFr, prepared as per the examples described herein.

[0026] FIG.5 depicts the long-term cycling of sodium ion cell with NVP cathode using 1.0 M NaPF6at 1.0 C.

[0027] FIG. 6 depicts the long-term cycling of sodium ion cell with NVP cathode using 1.0 M NaPF6at 2.0 C.

[0028] FIG. 7 depicts Rate performance of sodium ion cell with NVP cathode using 1.0 M NaPFe at various C rates.

[0029] DETAILED DESCRPITION OF THE INVENTION

[0030] The invention will now be described in detail in connection with certain preferred and optional embodiments, so that various aspects thereof may be more fully understood and appreciated.

[0031] Embodiments of the present invention relate to a process for the preparation of metal-hex - afluorophosphate (M-PFe). P_W0100712

[0032] The present invention provides a mechanical process for the preparation of metal-hex- afluorophosphate, wherein the process is devoid of purification step and solvent and involves less metal use.

[0033] The present invention also provides the use of metal-hexafluorophosphate as an electrolyte salt in different batteries.

[0034] In an embodiment, the present invention provides a mechanical process for the preparation of metal-hexafluorophosphate comprising a one pot, single step of mechanical mix- ing / grinding and reacting the dry metal particles / powder with hexafluorophophate component.

[0035] In an embodiment, the present invention provides a one -pot, single step mechanical process for the preparation of metal-hexafluorophosphate, wherein the process comprises of: adding dry metal component / powder and hexafluorophophate component in a ratio ranging from 0.1:2.0 to 2.0:0.1, followed by mechanical mixing / grinding for a period of 25-45 min at a temperature in the range of 25-35°C to obtain metal-hexafluorophosphate.

[0036] In an aspect of an embodiment, optionally high vacuum can be applied to remove gases trapped during the synthesis of NaPF6 such as ammonia and hydrogen to purify NaPF6 for further use.

[0037] In an embodiment of the present invention, the metal component of the metal hexafluoro- phosphate (M-PFe) is selected from but not limited to lithium (Li), sodium (Na), potassium (K), and the like.

[0038] In an embodiment of the present invention, the hexafluorophosphate (PFe) component is ammonium hexafluorophophate.

[0039] In an embodiment of the present invention, the mixing is effectuated for 25 to 45 min. Preferably, 30 min.

[0040] In an embodiment of the present invention, the ratio of the ammonium hexafluorophophate to metal component ranges from 0.1:2.0 to 2.0:0.1. Preferably, l(ammonium hexafluorophosphate): 1.2 (metal).

[0041] In an embodiment of the present invention, the mechanical mixing / grinding is effectuated by using mortar and pestle, ball milling or any other suitable dry powder mixing known in the prior art.

[0042] In an embodiment, the present invention provides a process for the production of metal- hexafluorophosphate without the need of any solvent instead it covers simple dry mixing of materials to obtain sodium hexafluorophosphate with yield of up to 95%. P_W0100712

[0043] In an embodiment of the present invention, the mechanical process for the preparation of metal-hexafluorophosphate can be performed economically because of the use of inexpensive raw materials. Further, said process can improve the yield and purity while lowering the cost. In an embodiment of the present invention, the M-PFe may have purity from 98 to 99.999%, and specifically 99.9 to 99.999%. In some embodiments, the M-PFe is NaPFe (sodium hexafluorophosphate).

[0044] In a particularly useful embodiment, the present invention provides a one-pot, single step mechanical process of preparation of NaPFe, wherein said process comprises of adding dry sodium powder and ammonium hexafluorophophate, followed by mechanical mix- ing / grinding to obtain NaPFe.

[0045] In an aspect of an embodiment, optionally high vacuum can be applied to remove gases trapped during the synthesis of NaPFe such as ammonia and hydrogen to purify NaPFe for further use.

[0046] In another embodiment, the present invention provides an energy storage device comprising a negative electrode material layer, a positive electrode material layer, a separator, and the M-PFe as an electrolyte solution.

[0047] In an aspect of an embodiment, the metal-hexafluorophosphate (M-PFe) as an electrolyte solution is prepared by mixing said metal-hexafluorophosphate (M-PFe) in a solvent; wherein the solvent can be single solvent or mixture of solvents with pre-defined ratios as per the requirement and need.

[0048] In an aspect of an embodiment of the present invention, the negative electrode material layer is sodium vanadium phosphate.

[0049] In another aspect of an embodiment of the present invention, the positive electrode material layer is sodium metal.

[0050] In further aspect of an embodiment of the present invention, the metal-hexafluorophosphate (M-PFe) electrolyte can be used in a solution or a solid-state electrolyte.

[0051] In yet another aspect of an embodiment of the present invention, the M-PFe electrolyte solution may be prepared by using organic solvent selected from but not limited to an acyclic or cyclic carbonate ester, a lactone, acyclic or cyclic ether, or a mixture thereof. The acyclic carbonate ester may be dimethyl carbonate, diethyl carbonate (DEC), or methyl ethyl carbonate, and the cyclic carbonate ester may be ethylene carbonate (EC), propylene carbonate, or butylene carbonate. The lactone may be gamma-butyrolactone or gammavalerolactone. The ether may be acyclic ether such as dimethoxy ethane, diethyl ether, or P_W0100712 cyclic ether such as tetrahydrofuran, methyl terahydrofuran or dioxane. In preferred embodiments, the M-PFe electrolyte solution may be prepared by using ethylene carbonate (EC) and diethyl carbonate (DEC) mixture in the ratio of 1: 1 (v / v).

[0052] In further aspect of an embodiment the separator may be a porous insulator selected from but not limited to a film laminate containing polyethylene, polypropylene, or combinations thereof (e.g. celgard membrane), nonwoven fabric containing cellulose, polyester, or polypropylene. Preferably, a celgard membrane.

[0053] In another embodiment of the present invention, the M-PFe can be used as electrolyte in rechargeable energy- storage devices including but not limited to batteries.

[0054] In an embodiment of the present invention, the battery may be selected from but not limited to an alkali ion battery, for example, a lithium battery, a sodium battery, a potassium battery, and the like.

[0055] In an embodiment, the NaPFe used to fabricate battery is without purification and battery performance data confirms that the purification of metal hexaflurophosphate, specifically NaPFr, is not needed for the application. One may choose to purify if an application demands a higher purity.

[0056] In another embodiment, the present invention provides process of preparation of metal- hexafluorophosphate via flow through synthesis (instead of mechanical mixing).

[0057] In a nutshell, the present invention relates to a process for the preparation of metal-hex- afluorophosphate with higher yields, less side products and impurities, and without the need of solvent. Specifically, the process comprises one pot, single step of mechanical mix- ing / grinding and reacting the dry metal particles / powder with ammonium hexafluoro- phophate for 25 to 45 minutes optionallyfollowed by purification to obtain metal-hex- afluorophosphate wherein the metal is sodium, lithium or potassium, and wherein the mechanical mixing is done by using mortar and pestle, ball milling or any other suitable dry powder mixing can be covered. More specifically, the ammonium hexafluorophosphate and sodium metal is mixed in ratio of 1: 1.2, respectively. Again more specifically, the process was carried out at room temperature (25-30°C) and under atmospheric pressure.

[0058] While the foregoing description discloses various embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope of the invention. The invention is not limited to the described embodiments, versions or examples, which are included to enable a person having ordinary skill in the art to make P_W0100712 and use the invention when combined with information and knowledge available to the person having ordinary skill in the art.

[0059] EXAMPLES

[0060] The following examples, which include preferred embodiments, will serve to illustrate the practice of this invention, it being understood that the particulars shown are by way of example and for purpose of illustrative discussion of preferred embodiments of the invention.

[0061] Experimental Section

[0062] Materials: Sodium was purchased from Molychem and the ammonium hexafluorophosphate was purchased from Sigma. Porcelain Mortar and pestle was used. The Lithium metal was purchased from Global Nanotech.

[0063] Example 1: Synthesis of NaPFe

[0064] One gram of ammonium hexafluorophosphate was added with 141 mg of sodium metal in a mortar. The setup was in a glove bag purged with argon. The mixture was grounded with pestle for about 30 min. By this time, the product was formed by evolution of NH3 and H2.

[0065] Characterization of NaPFe:

[0066] The non-purified NaPFe was studied usingXH,19F,31P,23Na NMR. The NMR spectra confirm the formation of NaPFe (FIG. 1).

[0067] The mass of NaPFe was determined by Atmospheric Pressure Chemical Ionisation (APCI) Method. The data confirm the presence of pure NaPFe (FIG. 2).

[0068] The NaPFe was also subjected to XPS analysis. The binding energy at 1073 eV confirms the presence of Na+ion. The presence of PFe“ is confirmed by fluorine peaks at 684 and 688 eV and phosphorous peak at 138 eV (FIG. 3).

[0069] Example 2: Synthesis of LiPFe

[0070] One gram of ammonium hexafluorophosphate was added to 43 mg of Lithium metal in a mortar. The setup was in the glove bag purged with Argon. The mixture was grounded with a pestle for about 30 min. The product was formed by the evolution of NH3 and H2.

[0071] Example 3: Fabrication of a battery using NaPF6 P_W0100712

[0072] Sodium ion batteries were fabricated using NaPFe as electrolyte. Sodium vanadium phosphate (NVP) blended with carbon particles and PVDF was used as one electrode while the sodium foil was used as another electrode. Celgard membrane was the separator. The NaPFe synthesized by us was used as electrolyte in EC:DEC (1: 1) solvent. The concentration of the electrolyte is 1 M. Using these components, coin cells were fabricated. The cells were subjected to cyclic voltammetry experiment (FIG. 4). The CV was recorded between 1 V and 4.5 V vs Na / Na+. The two reduction peaks around 3.25 V and 1.6 V are attributed to V4+to V3+and from V3+to V2+respectively. Similarly, the oxidation peaks at 1.75 V and 3.6 V are attributed to the V2+to V3+and V3+to V4+respectively.

[0073] The ionic conductivity of the electrolyte was determined using the batteries with symmetric sodium cells. We varied the electrolyte concentration. The batteries were subjected to impedance spectroscopy. The impedance spectra were recorded between 0.1 Hz to 1.0 MHz at open circuit potential. Nyquist plots with a semicircle and a linear line at low frequency were obtained for all the batteries (FIG. 5). The calculated ionic conductivity is 1.0 mS / cm (0.8 M), 0.87 mS / cm (1 M), 0.45 mS / cm (2M). This is comparable to the commercially available purified compounds.

[0074] The battery performance of coin cells was measured between 1.0 V and 4.2 V at various C rates. At 1.0 C rate, the initial specific capacity is 150 mA h / g, which decreased to 90 mA h / g at 500 charge-discharge cycle with coulombic efficiency of 97%.

[0075] At 2 C rate, the initial specific capacity was 125 mA h / g which decreased to 100 mA h / g at 150 charge-discharge cycle (FIG. 6).

[0076] Further, the rate performance was carried out at various C rates from 0.1 C to 10.0 C. The specific capacity values are 166 mA h / g (0.1 C), 159 mA h / g (0.2 C), 147 mA h / g (0.5 C), 140 mA h / g (1.0 C), 131 mA h / g (2.0 C), 124 mA h / g (3.0 C), 109 mA h / g (5.0 C) and 68 mA h / g (10.0 C) respectively (FIG. 7).

[0077] The aforesaid battery data confirms that the purification of metal hexaflurophosphate, spe- cificallyNaPFe is not needed for the application. One may choose to purify if an application demands a higher purity.

[0078] Advantages and applications of the present invention:

[0079] • The present invention provides a process of production of metal- hexafluorophosphate where metal is sodium, lithium or potassium, with higher yields. P_W0100712

[0080] • The present invention provides a process of production of metal- hexafluorophosphate where metal is sodium, lithium or potassium, with lesser side products and impurities.

[0081] • The present invention provides a process of production of metal- hexafluorophosphate where metal is sodium, lithium or potassium, without the need of solvent.

[0082] • The present invention provides a process of production of metal- hexafluorophosphate where metal is sodium, lithium or potassium, within a very short time • The present invention provides a process of production of metal- hexafluorophosphate where metal is sodium, lithium or potassium, without purification step.

Claims

P_W0100712We Claim:

1. A one-pot, single step process for the preparation of a metal-hexafluorophosphate, wherein said process comprises of: adding a dry metal component / powder and a hexafluorophosphate component in a ratio in the range of 0.1: 2.0 to 2.0:0.1, followed by mechanical mixing / grinding for a period of 25-45 min to obtain the metal-hexafluorophosphate; and then optionally, purifying the metal-hexafluorophosphate.

2. The process as claimed in claim 1, wherein the mechanical mixing is done by using mortar and pestle, ball milling or by dry powder mixing.

3. The process as claimed in claim 1, wherein the metal component is selected from lithium (Li), sodium (Na), and potassium (K).

4. The process as claimed in claim 1, wherein the hexafluorophosphate component is ammonium hexafluorophosphate.

5. The process as claimed in claim 1, wherein the metal-hexafluorophosphate is selected from sodium hexafluoroposphate (NaPFe) and lithium hexafluorophosphate (LiPFe).

6. The process as claimed in claim 1, wherein the metal-hexafluorophosphate is applicable in an energy storage device, and wherein said energy storage device comprises of a negative electrode material layer, a separator, metal- hexafluorophosphate as an electrolyte without purification, a positive electrode material layer.

7. The process as claimed in claim 6, wherein the metal hexafluorophosphate electrolyte can be used in a solution or in a solid-state electrolyte.

8. The process as claimed in claim 7, wherein the metal hexafluorophosphate electrolyte solution is prepared by using ethylene carbonate (EC) and diethyl carbonate (DEC) solvent mixture in the ratio of 1: 1.

9. The process as claimed in claim 6, wherein the negative electrode material layer is sodium metal, and the positive electrode material layer is sodium vanadium phosphate.

10. The process as claimed in claim 6, wherein the separator is a celgard membrane.

11. The process as claimed in claim 1, wherein the process is devoid of solvent and purification.

Images