invested to stabilize superionic conduction in such materials at room temperature (RT).We have previously reported stable Cs-and Rb-borohydrides containing the closo -borane anion [B 12 H 12 ] 2− on the A -site of an inversed perovskite structure, [ 11 ] whose RT polymorph had already been reported for Cs [ 12 ] and forming during the decomposition of bi-and trimetallic borohydrides. [ 11 ] We suspected that the incorporation of both complex anions in the same structure could provide means of stabilizing compounds with high symmetry, favoring cationic mobility at RT. Herein, we apply this concept to anion-mixed metal boranes containing mobile species of relevance and present a novel class of solid electrolytes with the compounds Na 3 BH 4 B 12 H 12 and (Li 0.7 Na 0.3 ) 3 BH 4 B 12 H 12 , capable of conducting either Na + or both Na + and Li + ions. Unlike order-disorder-governed dodeca-and decaboranes the cationic mobility is not entropically activated in Na 3 BH 4 B 12 H 12 and (Li 0.7 Na 0.3 ) 3 BH 4 B 12 H 12 , thus requiring no HT phase transition. The conduction pathways in Na 3 BH 4 B 12 H 12 are 2D where Na 2 B 12 H 12 -like slabs are in fact nonconducting, reaching RT conductivity values close to the order of 10 −3 S cm −1 and hence superior to many sulfi de glasses, NASICON and rivaling even β-alumina. [ 13,14 ] (Li 0.7 Na 0.3 ) 3 BH 4 B 12 H 12 , on the other hand, forms 1D channels for mixed cation conduction. The chemical reaction forming the compound at 500 K is accompanied by a dramatic increase in ionic conductivity attaining values higher than 10 −1 S cm −1 in the thermal stability region. Conductivity decreases upon cooling owed to full reversibility of the reaction. Such temperatures are nevertheless within reasonable limits for large-scale facilities, which operate at over 573 K in the case of Na-S power grids, for instance.The Figure S1). Ball milling is also attracting attention in battery research due to its fl exibility and the ease with which it is brought to the industrial scale. It has recently been used to prepare promising Na-based materials. [ 13,15 ] We also recently employed it to scan a wide variety of perovskite-type metal-borohydride systems. [ 12,16 ] Here it is used to "activate" the powders, whereby no reaction takes place even at high energies. (Li 0.7 Na 0.3 ) 3 BH 4 B 12 H 12 forms at 498 K (reaction onset) and dissociates to the precursors upon cooling. Meanwhile, the Na-rich Na 3 BH 4 B 12 H 12 forms above 638 K, and once formed it maintains the same structure in a wide temperature range of 100-653 K. Differential scanning calorimetry was used to follow the thermal events for the evolution of Na 3 BH 4 B 12 H 12 It has been recognized that battery grids are one of the more realistic contenders for future large-scale energy storage. [ 1 ] A paradigm shift is taking place in battery research that is focusing on inexpensive and abundant chemical species as alternative to compensate for the dwindling resources of lithium in terms of charge carrier (electrolyte) [ 2 ] as we...

The renewed interest of mechanochemistry as an ecofriendly synthetic route has inspired original methodologies to probe reactions, with the aim to rationalize unknown mechanisms. Recently, Friščić et al. ( Nat. Chem. 2013 , 5 , 66 - 73 , DOI: 10.1038/nchem.1505 ) monitored the progress of milling reactions by synchrotron X-ray powder diffraction (XRPD). For the first time, it was possible to acquire directly information during a mechanochemical process. This new methodology is still in its early stages, and its development will definitively transform the fundamental understanding of mechanochemistry. A new type of in situ ball mill setup has been developed at the Materials Science beamline (Swiss Light Source, Paul Scherrer Institute, Switzerland). Its particular geometry, described here in detail, results in XRPD data displaying significantly lower background and much sharper Bragg peaks, which in turn allow more sophisticated analysis of mechanochemical processes, extending the limits of the technique.

Three different types of anion packing, i.e., hexagonal close packed (hcp), cubic close packed (ccp), and body centered cubic (bcc), are investigated experimentally and with ab initio calculations in the model system NaBH. Solvent free and water assisted mechanical grinding provide polycrystalline samples for temperature-dependent synchrotron radiation X-ray powder diffraction and electrochemical impedance spectroscopy. It is shown that among the common close packed lattices, the hcp anionic backbone creates very favorable conditions for three-dimensional ionic conduction pathways, comprised of O-O, T-T, and T-O-T (O for octahedral, T for tetrahedral) cation hops. The hcp lattice is stable with respect to ccp and bcc lattices only at higher volumes per formula unit, which is achieved either by cationic substitution with larger cations or partial substitution of hydrogen by iodine on the closo-anion. It is found that the partial cationic substitution of sodium with lithium, potassium, or cesium does not lead to enhanced conductivity due to the obstruction of the conduction pathway by the larger cation located on the octahedral site. Substitution on the closo-anion itself shows remarkable positive effects, the ionic conductivity of NaBHI reaching values of close to 10 S cm at a rather low temperature of 360 K. While the absolute value of σ is comparable to that of NaCBH, the temperature at which it is attained is approximately 20 K lower. The activation energy of 140 meV is determined from the Arrhenius relation and among the lowest ever reported for a Na-conducting solid.

Interaction of solid KBH 4 with liquid Al(BH 4 ) 3 at room temperature yields a solid bimetallic borohydride KAl(BH 4 ) 4 . According to the synchrotron X-ray powder diffraction, its crystal structure (space group Fddd, a = 9.7405(3), b = 12.4500(4), and c = 14.6975(4) Å) contains a substantially distorted tetrahedral [Al(BH 4 ) 4 ]− anion, where the borohydride groups are coordinated to aluminum atoms via edges. The η -coordination of BH 4− is confirmed by the infrared and Raman spectroscopies. The title compound is the first aluminum-based borohydride complex not stabilized by halide anions or by bulky organic cations. It is not isostructural to bimetallic chlorides, where more regular tetrahedral AlCl 4 − anions are present. Instead, it is isomorphic to the LT phase of TbAsO 4 and can be also viewed as consisting of two interpenetrated dia-type nets where BH 4 ligand is bridging Al and K cations. Variable temperature X-ray powder diffraction, TGA, DSC, and TGA-MS data reveal a single step of decomposition at 160°C, with an evolution of hydrogen and some amount of diborane. Aluminum borohydride is not released in significant amounts; however, some crystalline KBH 4 forms upon decomposition. The higher decomposition temperature than in chloride-substituted Li−Al (70°C) and Na−Al (

The compounds, Li3MZn5(BH4)15, M = Mg and Mn, represent the first trimetallic borohydrides and are also new cationic solid solutions. These materials were prepared by mechanochemical synthesis from LiBH4, MCl2 or M(BH4)2, and ZnCl2. The compounds are isostructural, and their crystal structure was characterized by in situ synchrotron radiation powder X-ray and neutron diffraction and DFT calculations. While diffraction provides an average view of the structure as hexagonal (a = 15.371(3), c = 8.586(2) Å, space group P63/mcm for Mg-compound at room temperature), the DFT optimization of locally ordered models suggests a related ortho-hexagonal cell. Ordered models that maximize Mg-Mg separation have the lowest DFT energy, suggesting that the hexagonal structure seen by diffraction is a superposition of three such orthorhombic structures in three orientations along the hexagonal c-axis. No conclusion about the coherent length of the orthorhombic structure can be however done. The framework in Li3MZn5(BH4)15 is of a new type. It contains channels built from face-sharing (BH4)6 octahedra. While X-ray and neutron powder diffraction preferentially localize lithium in the center of the octahedra, thus resulting in two weakly interconnected frameworks of a new type, the DFT calculations clearly favor lithium inside the shared triangular faces, leading to two interpenetrated mco-nets (mco-c type) with the basic tile being built from three tfa tiles, which is the framework type of the related bimetallic LiZn2(BH4)5. The new borohydrides Li3MZn5(BH4)15 are potentially interesting as solid-state electrolytes, if the lithium mobility within the octahedral channels is improved by disordering the site via heterovalent substitution. From a hydrogen storage point of view, their application seems to be limited as the compounds decompose to three known metal borohydrides.

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In this work, a new type of addition reaction between La(BH 4 ) 3 and LiX, X = Cl, Br, I, is used to synthesize LiLa(BH 4 ) 3 Cl and two new compounds LiLa(BH 4 ) 3 X, X = Br, I. This method increases the amounts of LiLa(BH 4 ) 3 X and the sample purity. The highest Li-ion conductivity is observed for LiLa(BH 4 ) 3 Br, 7.74 × 10 −5 S/cm at room temperature (RT) and 1.8 × 10 −3 S/cm at 140 °C with an activation energy of 0.272 eV. Topological analysis suggests a new lithium ion conduction pathway with two new different types of bottleneck windows. The sizes of these windows reveal an opposite size change with increasing lattice parameter, that is, increasing size of the halide ion in the structure. Thus, we conclude that the sizes of both windows are important for the lithium ion conduction in LiLa(BH 4 ) 3 X compounds. The lithium ion conductivity is measured over one to three heating cycles and with different contacts (gold or carbon) between the electrodes and the electrolyte. Moreover, 11 B MAS NMR is used to verify the contents of the samples, whereas thermogravimetric analysis shows 4.8 and 3.6 wt % of hydrogen release for LiLa(BH 4 ) 3 Cl and LiLa(BH 4 ) 3 Br in the temperature range RT to 400 °C.

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