Process for synthesising a metal hexafluorophosphate

Abstract

The present invention concerns a new continuous process for synthesising metal hexafluorophosphates, and especially sodium and lithium hexafluorophosphate.

Description

[0001] Process for Synthesising a Metal Hexafluorophosphate

[0002] The present invention concerns a new continuous process for synthesising metal hexafluorophosphates, and especially sodium and lithium hexafluorophosphate.

[0003] Sodium-ion batteries are a promising alternative to lithium-ion batteries.

[0004] Conventionally hexafluorophosphates are the chosen salt for sodium and lithium ion batteries. Commercially available sources of these salts often contain insoluble impurities due to hydrolytic byproducts produced during synthesis, in storage or during transport, and so there is a desire to synthesise these salts under anhydrous conditions for immediate use in batteries, if possible. Quid D.M.C et al (Angewandte Chemie, Int. Ed.; Volume 60, Issue 47, November 15, 2021 , pages 24882 to 24887) discloses a synthetic approach for generating sodium hexafluorophosphate (NaPFe) which involves the addition of ammonium hexafluorophosphate (in tetrahydrofuran (THF) solvent) dropwise to sodium metal, under anhydrous conditions, to generate NaPFe. The reaction requires heating to reflux for 6 hours, with stirring for a further sixteen hours at 50°C, followed by filtration and drying under vacuum to provide a yield of 89%. However, there remains a need for sustainable, scalable, rapid, and high yielding routes to generating these salts.

[0005] The Applicant has now conceived of an approach for generating metal hexafluorophosphates, and especially Groupl / alkali metal hexafluorophosphates, and more especially both NaPFe and LiPFe, in a continuous process, with significant advantages over the current batch processes.

[0006] The Applicant has surprisingly shown that NaPFe, and LiPFe, can be rapidly synthesised in high yield by passing ammonium hexafluorophosphate (in THF solvent) through a packed bed of sodium (or lithium) metal, and that this is achieved at room temperature within seconds (ten seconds). This is a phenomenal improvement over the current batch process which takes many hours. This approach can also be extended to other metal hexafluorophosphates, and especially other Group 1 / alkali metals, such as potassium.

[0007] Thus, in a first aspect, the present invention provides a method for generating / synthesising a metal hexafluorophosphate comprising dissolving ammonium hexafluorophosphate in tetrahydrofuran solvent, and flowing the resulting dissolved ammonium hexafluorophosphate in solvent, at a predetermined flow rate, through a packed bed (column), of predetermined dimensions, of metal, or a packed bed comprising metal.

[0008] Continuous flow has been shown by the Applicant to provide a much better alternative to batch synthesis because it can prepare NaPFe far faster and safer than batch synthesis, and importantly the purity of the NaPFe can potentially be continually monitored in line. Moreover, such an approach can provide NaPFe on demand without storage, which may be especially advantageous if used as part of a battery assembly plant, for example: freshly prepared product may comprise less degradation products, and consequently should last longer, and potentially provide an improved energy density.

[0009] The metal may be a Group 1 / alkali metal, such as sodium, lithium, or potassium, and especially sodium or lithium.

[0010] The method of the first aspect may be achieved using organic solvents other than

[0011] THF. Any solvent not reactive to sodium, and which is polar can be exchanged with the THF, especially linear and cyclic ethers. The metal may also be provided through use of metal salts packed into the column, such as metal carbonates, metal nitrates, or metal sulfates.

[0012] In chemical processing, a packed bed is often a hollow tube, pipe, column, or other vessel that is filled with a packing material, providing a high surface area of packing material to react or contact with a solvent. In this case the packing material is, or comprises, metal, such as metal granules, and may be, or comprise, granular sodium or lithium metal, in a solvent dispersion.

[0013] The granular metal may be added to the column, to generate the packed bed, in an appropriate solvent. For example, sodium may be added / packed into the column in paraffin or toluene. For example, the sodium (metal) may be added to the column in a 30 weight% dispersion in toluene. The granule / particle size of the sodium or metal may range from pm to mm in diameter (for example, a granule size of between 1 pm and 1 mm), and may for example be approximately 0.1 mm, or less than 0.1 mm. To prevent conglomeration / agglomeration of the granular metal, additional packing material may, where necessary, be mixed with the granular metal, such as glass beads, or silica beads. This approach may also be used to prevent formation of cavities within the packed bed.

[0014] To ensure a quantitative reaction, parameters of the method of the first aspect, such as the dimensions of the packed bed / column (predetermined dimensions), and the flow rate (predetermined flow rate), may be optimised. In one embodiment, the predetermined dimensions of the packed bed / column are length 5 cm, or 10 cm, and inner diameter 6.6 mm. In one embodiment, the predetermined flow rate is 0.25 ml / min. The Applicant has shown that such predetermined dimensions and predetermined flow rate provide for a quantitative reaction, generating pure product (metal hexafluorophosphate). These parameters achieve small scale synthesis at a rate of 0.5 g per hour, but parameters can be accordingly scaled up and modified for larger scale synthesis.

[0015] The concentration of ammonium hexafluorophosphate in tetrahydrofuran (THF) solvent may be any appropriate concentration, and may for example be a concentration of 2 M in THF. The column, pipe or tube for the packed bed may be any suitable column, pipe or tube of suitable dimensions or internal volume, as known to the skilled person, for undertaking continuous flow chemistry. Indeed the method of the first aspect may be scaled up to an industrial scale process, with the size, and number, of the packed beds (columns) being increased accordingly.

[0016] The method of the first aspect may comprise flowing the resulting dissolved ammonium hexafluorophosphate in solvent through multiple packed beds (columns) of metal, in series or in parallel, or separately controlled by a valve, or similar, to flow through subsequent packed beds (columns).

[0017] Apparatus to undertake the method may utilise a pump to flow the solvent through the metal packed bed at the predetermined flow rate, or multiple packed beds, such as through use of a valve. The quality of the final product may also be analysed with FTIR or NMR, which can also be in-line within the apparatus.

[0018] The resulting metal hexafluorophosphate may be separated from the solvent by evaporation.

[0019] Preparation of the packed bed may be achieved through any appropriate methodology, and for sodium, may comprise a sodium dispersion in toluene being added to the column. Examples

[0020] Having regard to Figure 1 , is the equation for generating sodium hexafluorophosphate from ammonium hexafluorophosphate (in THF solvent) and sodium metal in a packed bed through a rapid (ten seconds) continuous process, at room / ambient temperature (approx. 20°C).

[0021] Having regard to Figure 2 and Figure 3, flow reactions were performed on a Vapourtec R-series Flow System; consisting of an R2 pumping unit fitted with a 10 mL ceramic pump head; an R4 reactor platform; a custom degasser; Diba Omnifit column; and custom inline evaporator. Solutions of ammonium hexafluorophosphate (~2.0 M in THF) were prepared. All connections and transfer lines were made using PFA tubing (I.D. = 0.5 mm, O.D. = 1.6 mm). The packed-bed reactor prepared was installed on the Vapourtec R-series using a custom glass housing fitted with a pt100 temperature sensor. The system was dried and pressure-tested by flushing with dry THF (flow rate: 1.00 mL / min; run-time: 20 min) prior to the addition of the NH4PF6 solution (~2.0 M in THF; flow rate: 0.25 mL / min). In-line analysis was conducted using either FT-IR (Mettler-Toledo FlowlR with Di-comp flow-cell) or NMR (Magritek 43 MHz inline NMR with flow-cell attachment). Under these conditions and flow rate, 0.5 g of product was synthesised per hour, even in this small scale apparatus. Under these conditions, with column / packed bed dimensions of length 10 cm, and inner diameter 6.6 mm, and flow rate of 0.25 ml / min, the material exiting the column is pure, and the reaction is quantitative.

[0022] Having regard to Figure 3, the process flow is divided into separate components: dosing, reactor, and workup. The NH4PF6 stock solution (1) is provided in a Schlenk tube sealed with a rubber seal to ensure dry conditions. After leaving the reactor, the formed ammonia is removed using a degasser (2). The stock solution is flowed through the packed bed of metal (3), where the reaction occurs. The progress is monitored by IR or NMR (4) (process analytical technology, PAT). The pure product NaPFe is collected as a solution in THF in a second Schlenk tube (5) connected to a vacuum manifold (6), allowing the subsequent removal of the solvent.

[0023] Preparation of the packed bed can be achieved by any appropriate method. In a first embodiment, the Applicant employed washed sodium in paraffin, dispersed with glass beads and / or silica (Cellite®). Sodium (25-35 wt % dispersion in paraffin) was mixed with celite® (3:1 ratio) into a Diba OmniFit adjustable column (length: 10 cm; inner diameter: 6.6 mm). The column was then flushed with THF for 30 minutes at a flow rate of 2.5 mL / min to remove the paraffin wax. The column was then adjusted to the resulting sodium / celite mixture volume. Care should be taken to avoid over- tightening the adjustable column and compacting the material.

[0024] In a second embodiment, a dispersion of sodium in toluene was used. A commercial sodium dispersion (30% w / w) in toluene (diameter <0.1 mm) was charged into an oven-dried Diba Omnifit adjustable column (length: 10 cm; inner diameter: 6.6 mm) using a 10 mL syringe until half of the column was filled. The column was then closed by adjusting the PTFE endpiece to give a total packed height of 5 cm. Care should be taken to avoid removing all solvent and compressing the dispersion. Over- tightening of the column may result in sodium re-aggregating and blocking the column.

[0025] As a comparison, the continuous flow process was compared to the batch process.

[0026] A Schlenk tube was charged with ammonium hexafluorophosphate (10.0 g, 61 mmol, 1 .0 equiv.) and dissolved in THF (30 mL). In a separate Schlenk tube freshly cut sodium metal (2.8 g, 122 mmol, 2.0 equiv.) was washed with hexane and dried in vacuo. THF (10 mL) was added to the metallic sodium and the NH4PFe-solution was added dropwise. The reaction was stirred at ambient temperature for 1 hour, followed by reflux for six hours, and finally 16 hours at 50 °C. The solution was then filtered using a filter cannula and the solvent was removed in vacuo over 16 hours to give NaPFe as a white powder. The batch process thus requires hours (24 h) to achieve a reasonable yield of product, as opposed to the continuous flow process, with product being synthesised in seconds (10 s) to minutes (20 min).

[0027] The Applicant has thus built a small-scale lab set-up which can produce batterygrade NaPFe (on this small scale) at a rate of 12g per day, as compared to only 4 g per day in batch.

[0028] The approach detailed above has also been used to efficiently generate LiPFe by flowing ammonium hexafluorophosphate through a column comprising Li granules.

[0029] All reactions were carried out under a dry atmosphere of dinitrogen using standard Schlenk and glove box techniques. All reagents were purchased from Sigma Aldrich and used without further purification. Anhydrous THF was distilled over sodium wire / benzophenone.

Claims

Claims1 . Method for generating / synthesising a metal hexafluorophosphate comprising dissolving ammonium hexafluorophosphate in tetrahydrofuran solvent, and flowing the resulting dissolved ammonium hexafluorophosphate in solvent, at a predetermined flow rate, through a packed bed, of predetermined dimensions, of the metal.

2. A method according to Claim 1 , wherein the metal is a Group 1 / alkali metal.

3. A method according to Claim 1 or Claim 2, wherein the metal is sodium.

4. Method according to Claims 1 to Claim 3, wherein the concentration of ammonium hexafluorophosphate in tetrahydrofuran solvent is 2 M.

5. Method according to Claims 1 to 4, wherein the packed bed has predetermined dimensions of length 5 cm, and inner diameter 6.6 mm.

6. Method according to Claims 1 to 5, wherein the predetermined flow rate is 0.25 ml / min.

7. Method according to Claim 1 to 6, wherein the metal comprises granules of metal, with a granule size of between 1 pm and 1 mm.

8. Method according to Claim 7, wherein the granule size is less than 0.1 mm.

9. Method according to Claims 1 to 8, wherein the resulting metal hexafluorophosphate is separated from the THF solvent by evaporation.

Images