Method for preparing solvent-free LiB ₁₁ H ₁₄ and its use The present invention is directed to a method for preparing anhydrous LiB ₁₁ H ₁₄ metal salts in a solvent with an excess of Li or a Li-base, and to corresponding anhydrous LiB ₁₁ H ₁₄ compounds or compositions with high purity. The present invention further encompasses the use of anhydrous LiB ₁₁ H ₁₄ compounds or compositions for preparing a Li-ion conducting solid state electrolyte for use in lithium batteries and an electrolyte or battery comprising an anhydrous LiB ₁₁ H ₁₄ compound or composition. Rechargeable lithium and sodium all-solid-state batteries will replace the currently used lithium-ion technology in the mid-term future. A reliable, safe and inexpensive solid-state conductor with sufficiently high ionic conductivity is the prerequisite for this technology. Currently, the only Li-ion solid-state electrolyte in the hydroborates family with sufficient ionic conductivity is Li(CB ₉ H ₁₀ ) _0.7 (CB ₁₁ H ₁₂ ) _0.3 (S. Kim., Nat. Commun., 2019, 10, 1081). However, Li(CB ₉ H ₁₀ ) is expensive with a price >120000 €/kg and challenging to synthesize, which hinders the practical application of this material. The CB ₉ H ₁₀ - anion can be replaced in the electrolyte by theB ₁₁ H ₁₄ - anion to obtain a Li-ion solid state electrolyte with similar properties. LiB ₁₁ H ₁₄ is easy to synthesize based on cheap reactants (sodium borohydride 10 €/kg, n-Butyl lithium 10 €/kg, 1-bromopentane 20 €/kg) (G. Dunks., Inorg. Chem, 1981, 20, 1692; S. Payandeh., Chem. Mater.2020, 32, 3, 1101). However, preparing solvent-free LiB ₁₁ H ₁₄ seems impossible. Currently the only commercial supplier (Katchem, Czech Republic) of this compound provides only the hydrate phase, LiB ₁₁ H ₁₄ .(H ₂ O) _n . The removal of the coordinated water in this compound by heat treatment forms LiB ₁₁ H ₁₃ OH and Li ₂ (B ₁₁ H ₁₃ ) ₂ O as additional impurity phases. The objective underlying the present invention is the provision of an anhydrous LiB ₁₁ H ₁₄ com- pound or a composition comprising said compound, the compound optionally having high purity of at least 90 or more percent by weight, optionally with the proviso that it is substantially free of its hydrates, LiB ₁₁ H ₁₃ OH and/or Li(B ₁₁ H ₁₃ ) ₂ O. It is a further objective to provide cost-effective Li-ion conducting solid state electrolytes for use in corresponding lithium batteries. The above objective is solved according to the invention by a method for preparing anhydrous LiB ₁₁ H ₁₄ metal salts, comprising the steps (ia) reacting R _n ZX _y B ₁₁ H ₁₄ , wherein each R is selected independently from the group consisting of hydrogen, phenyl, C _1-6 branched or unbranched alkyl, Z is selected from the group consisting of C, Si, Ge, Sn, N, P, As, Sb, O and S, and X is selected from the group consisting of H, F, Cl, Br and I, and n and y are each independently a number from 1 to 10, in a solvent with an excess of Li or a Li-base, optionally in an inert atmosphere; or (ib) reacting a LiB ₁₁ H ₁₄ (H ₂ O) _m metal salt, wherein m is 0.5 to 10, with Li or a Li-base in a solvent, optionally water; and (ii) isolating LiB ₁₁ H ₁₄ and Li ₂ B ₁₁ H ₁₃ for the product of step (ia) by removal of solvent and addition of water, if water is not already present (since the solvent of (ia) could possibly be or comprise water), or isolating LiB ₁₁ H ₁₄ and Li ₂ B ₁₁ H ₁₃ for the product of step (ib) by removal of solvent and addition of water, if water is not already present, and removal of water-insoluble R _n ZX _y B ₁₁ H ₁₄ ; (iii) heat treatment and drying of LiB ₁₁ H ₁₄ and Li ₂ B ₁₁ H ₁₃ from step (ii) under vacuum, or in an inert atmosphere to form LiB ₁₁ H ₁₄ and LiOH; (iv) removal of LiOH, optionally in an inert atmosphere. It was surprisingly found that the anhydrous LiB ₁₁ H ₁₄ compound can be prepared from cost- effective starting materials and without unwanted by-products such as LiB ₁₁ H ₁₃ OH and Li(B ₁₁ H ₁₃ ) ₂ O by the method of the present invention. Furthermore, this method allows for a scalable production of solvent free LiB ₁₁ H ₁₄ as a solid electrolyte component up to industrially relevant amounts. With- out wishing to be bound by theory it seems that the intermediate synthesis and subsequent oxida- tion of Li ₂ B ₁₁ H ₁₃ by heat treatment to LiB ₁₁ H ₁₄ avoids undesirable by-products that would impair the use of LiB ₁₁ H ₁₄ as a solid electrolyte component. The intermediate product mixture of LiB ₁₁ H ₁₄ and Li ₂ B ₁₁ H ₁₃ can be prepared directly from R _n ZX _y B ₁₁ H ₁₄ or by dehydration and partial reduction of a hydrate form of LiB ₁₁ H ₁₄ . For R _n ZX _y B ₁₁ H ₁₄ Z may be optionally selected from the group consisting of C, N and S, X is selected from the group consisting of H, Cl and I, and n and y are each independently a number from 1 to 4. For example, in one embodiment of the present invention R _n ZX _y B ₁₁ H ₁₄ may be (R) ₄ NB ₁₁ H _14. In another embodiment each R in R _n ZX _y B ₁₁ H ₁₄ or (R) ₄ NB ₁₁ H ₁₄ is selected independently from hydrogen, methyl, ethyl or n-butyl. In a further specific embodiment (R) ₄ N in (R) ₄ NB ₁₁ H ₁₄ is (R ¹ )3HN, wherein R ¹ in each case is selected from hydrogen, methyl, ethyl or n-butyl. In alternative embodiments for practicing the present invention R _n ZX _y B ₁₁ H ₁₄ may be selected from the group consisting of (CH ₃ ) ₃ NHB ₁₁ H ₁₄ , (C ₂ H ₅ ) ₃ NHB ₁₁ H ₁₄ , (CH ₃ ) ₃ SIB ₁₁ H ₁₄ , NH ₄ B ₁₁ H ₁₄ and C ₁₆ H ₃₆ NB ₁₁ H ₁₄ (Tetra-n-butylammonium). In another embodiment of the method of the present invention the solvent in step (ia) is a solvent with a boiling point below 150 °C, optionally consisting of or comprising an ether, water, alcohol or ketone, optionally selected from the group consisting of tetrahydrofuran, dioxane, dimethoxyethane, ethanol, methanol, isopropanol, acetone, 2-butanone, butanal and 2-pentanone, optionally the solvent is tetrahydrofuran. The term solvent as used herein is understood to include one or more solvent compounds. The inert atmospheres for use or optional for the steps of the invention may be or comprise, for example, argon, nitrogen, helium or mixtures thereof. In a specific embodiment the reagent in step (ia) is n-butyllithium and the solvent is optionally tetrahydrofuran. For the hydrated LiB ₁₁ H ₁₄ (H ₂ O) _n metal salt for use in step (ib) may be 0.5 to 10, for example, 1 to 3, optionally 1, 2 or 3, for example, n is about 2 or 3. In this regard it is noted that it is hard to determine the exact water content of these hydrated borates and that the knowledge of the exact hydrate water content of the B ₁₁ H ₁₄ (H ₂ O) _n metal salt, optionally LiB ₁₁ H ₁₄ (H ₂ O) _n , is not relevant for practicing the method of the present invention. In another embodiment the reagent for use in step (ib) is n-butyllithium or LiOH. For example, the solvent in step (ib) may comprise or consist of and is selected from the group of solvents consisting of THF, water, esters, optionally cyclic esters, ketones and alcohols, optionally selected from the group consisting of tetrahydrofuran, dioxane, dimethoxyethane, ethanol, metha- nol, isopropanol, acetone, 2-butanone, butanal, and 2-pentanone, optionally the solvent in step (ib) is tetrahydrofuran or water. Most conveniently and cheap, the solvent for step (ib) may be or comprise water. Once the mixture of LiB ₁₁ H ₁₄ and Li ₂ B ₁₁ H ₁₃ is obtained by one of steps (ia) or (ib), these two compounds are isolated together in step (ii). LiB ₁₁ H ₁₄ and Li ₂ B ₁₁ H ₁₃ for the product of step (ia) are isolated by removal of solvent and addition of water, if water is not already present. LiB ₁₁ H ₁₄ and Li ₂ B ₁₁ H ₁₃ for the product of step (ib) are isolated by removal of solvent and addition of water, if water is not already present, and removal of water-insoluble starting material R _n ZX _y B ₁₁ H ₁₄ , optionally by centrifugation and/or filtration, followed by the removal of the water. Once the mixture of LiB ₁₁ H ₁₄ and Li ₂ B ₁₁ H ₁₃ is isolated, i.e. free of substantial amounts of solvents, other salts, remaining starting material R _n ZX _y B ₁₁ H ₁₄ , etc., that might interfere, and the isolated mixture is dissolved in water, it is subjected to a heat treatment and drying under vacuum or, alternatively, in an inert atmosphere to form LiB ₁₁ H ₁₄ and LiOH from Li ₂ B ₁₁ H ₁₃ + H ₂ O. The formation of LiOH drives the reaction toward LiB ₁₁ H ₁₄ production. For example, the heat treatment may be done under vacuum at a pressure range of 10 to 10 ^-3 mbar, optionally 10 ^-2 to 10 ^-3 mbar, and at a temperature in the range of 50 to 150 or 60 to 120 °C optionally 90 to 100°C. It is noted that the skilled person will routinely choose a vacuum pressure and temperature that will avoid product decomposition. The removal of LiOH from LiB ₁₁ H ₁₄ in step (iv) may, for example, be done by addition of a suitable solvent that does not dissolve LiOH but the LiB ₁₁ H ₁₄ product, optionally THF or other solvents such as, e.g. isopropanol or ethanol, followed by, for example, filtration and/or centrifugation of solid LiOH, and subsequent optional removal of said solvent. In one embodiment of the method of the present invention steps (ia) and (iv) use THF or a THF-comprising solvent and/or step (ib) uses water or a water-comprising solvent. The method of the present invention for the first time makes available an anhydrous LiB ₁₁ H ₁₄ compound or a composition comprising an anhydrous LiB ₁₁ H ₁₄ compound with high purity, optio- nally an or comprising an anhydrous LiB ₁₁ H ₁₄ compound in a purity of more than 90, 95, 97, 98 or 99 % by weight, optionally with the proviso that it is substantially free of its hydrates, LiB ₁₁ H ₁₃ OH and/or Li(B ₁₁ H ₁₃ ) ₂ O. The term substantially free, as understood in the context of the present invention, is meant to indicate impurities of less than 10 %, optionally less than 5%, optionally less than 3, 2 or 1 %-by weight impurities, e.g. of LiB ₁₁ H ₁₄ hydrates, LiB ₁₁ H ₁₃ OH and/or Li(B ₁₁ H ₁₃ ) ₂ O. Therefore and in a further aspect, the present invention pertains to an anhydrous LiB ₁₁ H ₁₄ compound or a composition comprising an anhydrous LiB ₁₁ H ₁₄ compound, optionally with the proviso that it is substantially free of its hydrates, LiB ₁₁ H ₁₃ OH and/or Li(B ₁₁ H ₁₃ ) ₂ O. In a specific embodiment the anhydrous LiB ₁₁ H ₁₄ compound is prepared by a method according to method of the invention as described herein. These compounds and compositions have utility for preparing Li-ion conducting solid state electrolytes and lithium batteries. After synthesis of pure LiB ₁₁ H ₁₄ , anion mixing as a known strategy can be used to introduce disorder in the structure and improve ionic conductivity at low temperatures (S. Payandeh. Chem. Mater.2020, 32, 3, 1101; L. Duchêne, Chem. Commun.2017, 53, 4195). For this purpose, LiB ₁₁ H ₁₄ can be mixed with LiHa (Ha = F, Cl, Br, I), Li _n C _z B _x H _y or Li _n B _x H _(y-w) Ha _w (n, z, x, y, w = 1-20) , for example, such as LiBH4, LiCB ₉ H ₁₀ , LiCB ₁₁ H ₁₂ , Li ₂ B ₁₂ H ₁₂ , Li ₂ B ₁₀ H ₁₀ , Li ₂ B ₆ H ₆ I ₆ , Li ₂ B ₁₁ H ₁₃ forming novel ionic conductors with ionic conductivities in the range of 10 ^-4 to 10 ^-3 S/cm at 25 °C. The electrolyte can then mixed with a cathode active material by physical mixing or impregnation through solution processing (A.Gigante, ChemSusChem, 2019, 4, 4832; L. Duchêne, Energy Environ. Sci, 2017, 10, 2609; L. Duchêne, Energy Storage Mater.2020, 26, 543; L. Duchêne, Energy Storage Mater.2020, 25, 782). Consequently, the present invention relates in a further aspect to the use of anhydrous LiB ₁₁ H ₁₄ compound or compositions thereof for preparing a Li-ion conducting solid state electrolyte for use in lithium batteries. In one embodiment, the invention is directed to an electrolyte and/or battery comprising an anhydrous LiB ₁₁ H ₁₄ compound or a composition of the invention, optionally comprising an electrolyte Li _x+y (B ₁₁ H ₁₄ ) _x (CB ₁₁ H ₁₂ ) _y , Li _x+2y (B ₁₁ H ₁₄ ) _x (B ₁₂ H ₁₂ ) _Y , Li _x+y (B ₁₁ H ₁₄ )(CB ₉ H ₁₀ ) _y wherein X is 1 to 9, optionally about 1 to 2, and Y is 1 to 9, optionally about 1 to 2. In the following the present invention will be further illustrated by way of representative examples, none of which are to be interpreted as limiting the scope of the invention beyond the cited claims. Figures Fig.1 shows a structure model of the [B ₁₁ H ₁₄ ]- anion. Fig.2 shows ¹¹ B NMR spectra: The bottom reference spectrum is from the starting material (CH ₃ ) ₃ NHB ₁₁ H ₁₄ and shows the characteristic ¹¹ B NMR pattern, i.e. a triplet with fixed position and intensity ratios. The middle and top spectra are from the final products obtained by methods 1 (ia) and 2 (ib), respectively. Fig.3 shows structure models and corresponding ¹¹ B NMR spectra: The bottom reference spectrum is of the starting materials ((CH ₃ ) ₃ NHB ₁₁ H ₁₄ in method 1 or LiB ₁₁ H ₁₄ (H ₂ O) _n in method 2) and features the characteristic ¹¹ B NMR pattern, i.e. a triplet with a fixed position and intensity ratios. The middle ¹¹ B NMR pattern was recorded after step (ii) (isolation of products from steps (ia) and ib)) and shows the characteristic resonances of the [B ₁₁ H ₁₄ ]- and [B ₁₁ H ₁₃ ] ^2- (in dashed boxes) anion. The top ¹¹ B NMR spectrum is of the final anhydrous product Li[B ₁₁ H ₁₄ ]. Fig.4 shows the X-ray diffraction pattern of the product (Yobs, black) and a fit resulting from a Rietveld refinement based on the structural model shown in the inset (Y _cal , white dashed line). The black dotted line (Y _obs – Y _cal ) shows the difference between the observed and the calculated pattern. The inset shows the structural model used: Li atoms are depicted as big black spheres, B atoms in black and H atoms as small grey spheres. The calculated pattern matched the observed. Fig.5 is a graph showing the ionic conductivities of pure anhydrous LiB ₁₁ H ₁₄ and other Li- hydroborates LiCB ₁₁ H ₁₂ , and Li ₂ B ₁₂ H ₁₂ , and mixtures thereof demonstrating a significant increase in conductivity for compositions comprising anhydrous LiB ₁₁ H ₁₄ . LiB ₁₁ H ₁₄ (dotted line), LiCB ₁₁ H ₁₂ (dashed line), Li ₂ B ₁₂ H ₁₂ (dashed dotted line), and of the mixes systems Li ₂ (B ₁₁ H ₁₄ )(CB ₁₁ H ₁₂ ) (triangles) and Li3(B ₁₁ H ₁₄ )(B ₁₂ 1H ₁₂ ) (circles). The horizontal lines mark the conductivity values of 10 ^-4 and 10 ^-3 S/cm, respectively. Fig.6 is a graph depicting the battery cycling of a Li│Li ₂ (B ₁₁ H ₁₄ )(CB ₁₁ H ₁₂ )│TiS ₂ all-solid-state cell. The cell shows a coulombic efficiency close to 100% and a moderate capacity fading at C/10 and C/5. Examples Example 1 - Method for preparing LiB ₁₁ H ₁₄ from (CH ₃ ) ₃ NHB ₁₁ H ₁₄ (CH ₃ ) ₃ NHB ₁₁ H ₁₄ (400 mg, 2.07 mmol) and 15 ml THF were transferred to a round bottom glass flask and sealed in the glovebox. n-butyllithium (BuLi, 2.3 ml, 3.72 mmol) was added under argon flow outside the glovebox and the suspension was stirred at room temperature for 1-2 h under ar- gon flow and a mixture of Li ₂ B ₁₁ H ₁₃ and LiB ₁₁ H ₁₄ formed (step ia). Subsequently, the reaction was transferred to a rotary evaporator and the solvent was removed using a rotary evaporator at 60 °C. 20 mL of high purity water (Milli-Q) was added to the remaining solid and the solution was centri- fuged at 4000 rpm for 10 minutes to separate the possibly unreacted (CH ₃ ) ₃ NHB ₁₁ H ₁₄ which is not soluble in water (step ii). The solution was then transferred to a rotary evaporator and the water was removed at 80 °C followed by 2 h heat treatment at 90 °C under vacuum using a Schlenk line. This heating in aqueous environment also reduces Li ₂ B ₁₁ H ₁₃ is to LiB ₁₁ H ₁₄ and LiOH (step iii). To separate the LiB ₁₁ H ₁₄ , 20-40 mL of THF were added (step iv) to the powder in the glovebox which dissolved only the desired product. The suspension was transferred to falcon tubes in the glovebox, sealed and centrifuged at 4000 rpm for 10 minutes to remove the LiOH. The resulting clear solution containing only LiB ₁₁ H ₁₄ was transferred from falcon tubes to a glass vial inside the glovebox and THF was removed using a rotary evaporator at 80 °C. The final coordinating THF was removed by annealing the powder at 100 °C for 16 h under vacuum using a schlenk line (step iv). Example 2 - Method for preparing LiB ₁₁ H ₁₄ from LiB ₁₁ H ₁₄ (H ₂ O) _n 500 mg of purchased LiB ₁₁ H ₁₄ (H ₂ O) _n (2.84 mmol for n = 2), 18.94 mg LiOH (0.79 mmol) and 50 mL Milli-Q water were transferred to a round bottom glass flask and stirred for 30 min at RT until full dissolution. Water and LiOH were removed using the same procedure as described for Example 1 (steps iii, iv). Example 3 – Proof of high purity anhydrous LiB ₁₁ H ₁₄ by ¹¹ B NMR and X-ray diffraction The starting material (CH ₃ ) ₃ NHB ₁₁ H ₁₄ and the anhydrous LiB ₁₁ H ₁₄ products resulting from both options (ia) and (ib) of the above-described method were analyzed by NMR. The characteristic triplet in the starting material as well as in the pure products is demonstrated in Fig.2. The intermediate borate product mix isolated in step (ii) was identified by ¹¹ B NMR patterns showing the characteristic resonances of the [B ₁₁ H ₁₄ ]- and [B ₁₁ H ₁₃ ] ^2- anions (see Fig.3). An NMR only proofs the chemical structure of the anion. For further proof of the crystal structure of the anhydrous LiB ₁₁ H ₁₄ products the X-ray diffraction pattern of the product was recorded. Powder X-ray diffraction (PXRD) data were collected in symmetric θ-2θ mode in the 2θ range of 5−90° using Cu-Kα-radiation in transmission geometry. A focusing mirror was used to remove the Kβ-radiation. The crystal structure was solved using global optimization methods and was confirmed by density functional theory (DFT) calculations. In addition, the Rietveld refinement of the PXRD pattern of the synthesized sample, shows a perfect match with the calculated PXRD pattern of the anhydrous LiB ₁₁ H ₁₄ confirming the stoichiometry. Fig.4 proves and shows the crystal structure of an anhydrous LiB ₁₁ H ₁₄ for the first time. Example 4 – Determination of ionic conductivity and mixed hydroborates It was demonstrated that the synthesized anhydrous LiB ₁₁ H ₁₄ exhibits a superior Li-ionic conductivity when compared to other Li-hydroborates such as LiCB ₁₁ H ₁₂ and Li ₂ B ₁₂ H ₁₂ . Mixing anhydrous LiB ₁₁ H ₁₄ with the aforementioned hydroborates, especially with LiCB ₁₁ H ₁₂ increases the conductivity significantly (see Fig.5). LiCB ₁₁ H ₁₂ and Li ₂ B ₁₂ H ₁₂ are commercially available. Ball milling as a mechano-chemical process was used for mixing the powders. Therefore, pre-grinded physical mixtures of the constituent powders were ball milled (BM) in different molar ratios for 15 minutes in 5 repetitions using stainless steel vials and balls (5 mm) with balls to sample ratio of 15-40 to 1. Example 5 – Determination of ionic conductivity and mixed hydroborates The cathode active material TiS ₂ and the electrolyte powders were weighed in a 2:3 mass ratio and mixed by using mortar and pestle for 15 min. The mixture was used as the composite positive electrode. First, 60-100 mg of the electrolyte powder was placed into a 12-mm-diameter die and uniaxially pressed at 60-100 MPa. Secondly, 2 to 4mg of the composite mixture was added on the electrolyte and uniaxially pressed at 150-400 MPa to form a two layer pellet consisting of the electrolyte and cathode. Finally, Li metal was used as anode and placed on the opposite side of the pellet. The battery was assembled in a Swagelok cell and was cycled at 60 °C in the voltage ranges of 1.6 – 2.5 V (vs. Li ⁺ /Li). The battery cycling of the so-produced Li│Li ₂ (B ₁₁ H ₁₄ )(CB ₁₁ H ₁₂ )│TiS ₂ all-solid- state cell demonstrated coulombic efficiency close to 100% and a moderate capacity fading at cycles C/10 and C/5.