The present invention relates to a salt of calcium hexafluorophosphate. Additionally, the present invention relates to a method of making a calcium hexafluorophosphate salt and the use of the calcium hexafluorophosphate salt in an electrolyte in a cell or battery.

Lithium-ion batteries are currently used in a variety of electronic devices. The use of lithium-ion cells has prevailed over other battery technologies due to the ability of a lithium-ion cell to be recharged without a loss of a significant charge capacity in the short term. In addition, the energy density of a lithium-ion battery enables its use in portable products such as laptop computers and mobile phones. Over time however, lithium batteries are known to suffer from loss of charge capacity. Furthermore, issues of thermal runaway and overheating risks have been widely reported.

Many lithium-ion electrolyte systems have been developed and studied using a wide range of lithium salts including LiBF ₄ , L1CIO ₄ , LiNTF ₂ , LiPF ₆ , LiAsF ₆ , and LiSbF ₆ as well as others. LiPF ₆ is the preferred electrolyte salt in lithium-ion cells due to its balance of several properties that no other lithium salt has been found to possess. However, there are concerns over the long term use of lithium cells, given the relatively low abundance of lithium in the Earth's crust and the current high price of lithium relative to other Alkali and Alkaline Earth metals.

In a first aspect, the present invention provides salt of the general formula:

Ca(L) _x (PF ₆ ) ₂

(1)

wherein x represents a number between 0 and 6; and when x is not equal to 0, each L represents a ligand selected from one of the following compounds: an ether or aza macrocyclic; a halomethane or a nitrile of the general formula R-C≡N.

It has been recognised theoretically that alkaline earth metals such as calcium could be used as electrolyte solutions in electrochemical cells and batteries. Calcium is the fifth most Earth- abundant element and therefore has a relatively low cost per ton compared to lithium. In addition, calcium has a higher charge capacity than lithium. Furthermore, the large ionic radius of a Ca ²⁺ ion, and thus lower charge density with respect to Mg ²⁺ and Al ³⁺ , could permit faster solid-state diffusion into electrode materials, in electrodes with appropriately sized voids within the structure, an issue that has limited the construction of efficient Mg-ion batteries so far. However, despite this knowledge calcium has not been widely adopted as an electrolyte or as a material for anodes because of difficulties in forming electrolytes that are stable over a wide voltage range and also compatible with multiple electrodes.

As mentioned above, the lithium hexafluorophosphate salt is the preferred electrolyte salt in lithium-ion cells. However, a barrier for using a calcium hexafluorophosphate based electrolyte in calcium-ion batteries is the fact that the synthesis of an Alkaline Earth metal hexafluorophosphate salt can be costly and more problematic (often resulting in lower purity materials) when compared with the synthesis of a lithium hexafluorophosphate salt. It has been found however that the calcium hexafluorophosphate salt of the present invention can be readily synthesised in an anhydrous solution under relatively mild conditions.

The term salt used throughout the specification is intended to cover complex calcium salts with ligands (L) that fall within the general formula given above. The choice of ligand or mixture of ligands may allow for a more stable reaction mixture in the synthesis of the calcium hexafluorophosphate salt. Each ligand may be independently selected from an ether or aza macrocyclic, a halomethane or a nitrile compound. With a view to simplifying the reaction mixture during synthesis, L may represent a ligand selected from one only of the following compounds: a cyclic crown ether; an aza macrocyclic compound; a halomethane; or a nitrile of the general formula R-C≡N. That is to say that L may comprise one or more cyclic crown ethers, one or more aza macrocyclic compounds, two or more halomethanes, or two or more nitriles of the general formula R-C≡N.

The ether or aza macrocyclic can comprise typical cyclic crown ethers selected from one of the following: [12]-crown-4, [15]-crown-5, [18]-crown-6, [24]-crown-8. The cyclic crown ether may be used to sequester, or partly sequester the calcium cation. Furthermore, the aza macrocyclic compound may be cyclen or cyclam. The use of a multidentate ligand can be favourable since the calcium cation remains in solution but has a lowered reactivity and could also inhibit the decomposition of the PF ₆ ion during synthesis; and plating of calcium onto an electrode surface if the salt is used in an electrolyte in a calcium-ion cell. These ether or aza macrocyclic compounds can be used in combination with ether or nitrile based solvents without hindering the desired synthesis of the resultant calcium salt. In terms of the general formula for the nitrile, and when x is equal to 6, each R may represent an organic group independently selected from the following: methyl, ethyl, propyl, butyl, 'butyl, pentyl, ethylene, propylene, butylene, pentylene, toluene, naphthalene, or phenyl. A sterically bulky ligand could prevent the solvation of the calcium cation. Therefore for the general formula, R may preferably represents a group that would provide a nitrile that is considered to have low sterically hindrance.

Each L may be the same nitrile. This renders the synthesis of the salt more straightforward since the same nitrile solution can be used in both the activation and the treatment steps. For the salt, L may be acetonitrile, which is the least sterically hindered nitrile. As an added advantage, the use of acetonitrile provides good solvation of the calcium cation, as well as low manufacturing expense since desolvation under high vacuum can be more easily achieved than with other solvents. This desolvated salt could then be re-solvated with, for instance, an ether (such as THF, diethyl ether) or another donor solvent.

The halomethane may be a chlorinated methane, such as CH ₂ CI ₂ , CHCI ₃ , CCI4. The chloromethanes represent stable and cost effective dry solvents for the synthesis. Dichloromethane (CH ₂ CI ₂ ) is particularly suited as a ligand and solvent for the synthesis of the magnesium salt due to its low boiling point and solvating characteristics.

A single crystal obtained from the diffusion of Et ₂ 0 in to a CH ₃ CN solution of the salt of the present invention may have the general formula (Cac l5-crown-5) ₄ (PF ₆ )8(CH3CN)2.

In a second aspect, the present invention provides a method of making a salt of the general formula:

Ca(L _y ) _x (PF ₆ ) ₂

(11)

wherein x represent a number between 0 and 6, when x is not equal to zero, L _y represents a ligand independently selected from any one of the following compounds: an ether or aza macrocyclic; a halomethane or a nitrile of the general formula R-C≡N; and L _y comprises a mixture of compounds Li and L2; the method comprising: providing Ca metal, washing and activating the Ca metal in a first dry solution comprising a first compound (Li), treating the solution of activated Ca metal and first compound Li with NOPF ₆ in a second dry solution comprising a second compound (L ₂ ), removing the residual solvent, and recrystallizing the remaining solid to form the salt of Formula (ii).

The residual solvent can be removed by evaporation, for example, under vacuum or by heating.

In a third aspect, the present invention provides an electrolyte comprising a salt in accordance with the above Formula (i) or Formula (ii). The electrolyte may comprise the salt as an additive to a conventional electrolyte, or the salt may be used in a pure solution to form, with an appropriate solvent, an electrolyte by itself.

In a fourth aspect, the present invention provides a cell or battery with an electrolyte comprising a salt in accordance with the above Formula (i) or Formula (ii). The salts of the present invention do not suffer from some of the same disadvantages observed with the use of lithium salts in electrochemical cells or batteries.

When using the salt of the present invention in an electrolyte in calcium-ion cell or battery, the salt of the present invention may be useful in terms of reducing or limiting the corrosion of cell components.

In order that the present invention may be more readily understood, an embodiment of the invention will now be described, by way of example, with reference to the accompanying Figures, in which:

Figure 1 is an X-ray crystal structure of a salt crystal of the present invention selected from bulk solution;

Figure 2 is an X-ray crystal structure of another salt crystal falling outside the scope of the present invention;

Figure 3 is a ^X H NMR spectrum of a salt of the present invention; Figure 4 is a ¹⁹ F NMR spectrum of a salt of the present invention; and Figure 5 is a ³¹ P NMR spectrum of a salt of the present invention. The present invention will now be illustrated with reference to the following examples. Example 1 - Synthesis of (Cacl5-crown-5)(PF ₆ ) ₂

In a Schlenk flask, Ca (>95% purchased from Sigma Aldrich) was suspended in freshly distilled CH ₃ CN along with 15-crown-5 (Sigma Aldrich) and stirred at room temperature under a nitrogen atmosphere. In a separate Schlenk flask, NOPF ₆ (purchased from ACROS Organics) was dissolved in freshly distilled CH ₃ CN under a nitrogen atmosphere. The NOPF ₆ solution was then added slowly to the Ca granules using a dry syringe. The reaction mixture was stirred at room temperature for ca. 3 days. The off-white solution was then filtered through a cannula using a glass fibre filter and dried in vacuo. The resulting solid was dissolved in a minimum amount of dry CH ₃ CN, layered with dry Et^O, and left undisturbed for several days during which colourless crystals formed. The supernatant was then decanted to isolate the colourless crystals of (Cacl5- crown-5)(PF ₆ ) ₂ in 46% yield.

CH ₃ CN

Ca + 2 NOPF ₆ + 15-crown-5 *~ (15-crown-5)Ca(PF ₆ ) ₂ + 2 NO _(g) room temperature

Example 2 - Characterization of (Cacl5-crown-5)(PF ₆ ) ₂

A single crystal obtained from the diffusion of Et^O in to a CH ₃ CN solution of a compound of the general formula (Cacl5-crown-5)(PF ₆ )2 as shown in Figure 1 (specifically [Cac(15-crown- 5)(CH ₃ CN) ₃ ] ²⁺ [Cac(l 5-crown-5) ₂ ] ²⁺ (PF ₆ ) ₄ ). X-ray analysis was carried out on data collected with a Bruker D8 Quest CCD diffractometer and confirmed the complex to be the desired salt (Figure 1). Bulk purity of the (Cacl5-crown-5)(PF ₆ )2 salt crystal was confirmed by elemental analysis (C, H, and N). Elemental microanalytical data were obtained from the University of Cambridge, Department of Chemistry microanalytical service. Analysis calculated for C64Hi32Ca4F48N ₄ 024P8 [(Cac 15-crown-5) ₄ (PF ₆ ) ₈ (CH ₃ CN)2 · (2 CH ₃ CN, 4 Et ₂ 0)]: C, 28.9; H, 5.0; N, 2.1; found: C, 28.5; H, 4.9; N, 2.3.

Figure 2 shows the X-ray crystal stuture of another crystal obtained from the diffusion of Et ₂ 0 in to a CH ₃ CN solution. X-ray analysis was carried out on data collected with a Bruker D8 Quest CCD diffractometer and confirmed the complex to be an undesired salt (Figure 2). The structure relates to [(Cac l5-crown- 5) ₄ (SiF ₆ )2(CH3CN) ₂ ] ⁴⁺ (PF ₆ )4, where the two brid ging ions are SiFg . The elemental analysis (C, H, and N) of this crystal was obtained using elemental microanalytical data obtained from the University of Cambridge, Department of Chemistry microanalytical service. Analysis calculated for C ₃₆ H ₆₉ Ca ₂ F ₂₄ N ₃ 0i ₅ P ₄ : C, 29.95; H, 4.82; N, 2.91 ; found: C, 29.81 ; H, 4.83; N, 2.48]

The ¾ ¹³ C, ¹⁹ F and ³¹ P NMR spectra of the bulk white crystalline powder of (Cac l5-crown- 5)(PF ₆ )2 are shown in Figures 3 to 5, respectively. Notably, the ¹⁹ F and ³¹ P NMR spectra exhibited a doublet and heptet, respectively, characteristic of the PF ₆ anion. NMR spectra were recorded at 298.0 K on a Bruker 500 MHz AVIII HD Smart Probe Spectrometer ( ^l at 500 MHz, ³¹ P 202 MHz, ¹⁹ F 471 MHz) or a Bruker 400 MHz AVIII HD Smart Probe spectrometer (*H at 400 MHz, ³¹ P 162 MHz, ¹⁹ F 376 MHz) unless otherwise specified. Chemical shifts (δ, ppm) are given relative to residual solvent signals for ^X H, to external 85% H ₃ PO4 for ³¹ P and to CCI ₃ F for ¹⁹ F.