Abstract:The prerequisite for widespread use of hydrogen as an energy carrier is the development of new materials that can safely store it at high gravimetric and volumetric densities. Metal borohydrides M(BH 4 ) n (n is the valence of metal M), in particular, have high hydrogen density, and are therefore regarded as one such potential hydrogen storage material. For fuel cell vehicles, the goal for on-board storage systems is to achieve reversible store at high density but moderate temperature and hydrogen pressure. To this end, a large amount of effort has been devoted to improvements in their thermodynamic and kinetic aspects. This review provides an overview of recent research activity on various M(BH 4 ) n , with a focus on the fundamental dehydrogenation and rehydrogenation properties and on providing guidance for material design in terms of tailoring thermodynamics and promoting kinetics for hydrogen storage.

Magnesium batteries are considered promising solutions for future energy storage beyond the lithium-ion battery era. However, the development of magnesium batteries is hindered by the lack of suitable electrolytes. Here we present solid Mg 2+ electrolytes based on ammine magnesium borohydride composites, Mg(BH 4 ) 2 •xNH 3 , which have conductivities ca. three orders of magnitude higher than the parent compounds (x = 1, 2, 3, and 6). A nanocomposite formed by the Mg(BH 4 ) 2 •xNH 3 composite and MgO nanoparticles exhibits outstanding Mg 2+ conductivity of the order of 10 −5 S cm −1 at room temperature and around 10 −3 S cm −1 at moderate temperature (ca. 70 °C), with an activation energy for Mg 2+ conduction of E a ∼108 kJ/mol (1.12 eV) and high thermal stability (T dec = 120 °C). Characterization using solid-state nuclear magnetic resonance, powder X-ray diffraction, and transmission electron microscopy reveals that the high Mg 2+ conductivity is attributed to amorphization of Mg(BH 4 ) 2 •xNH 3 resulting in a highly dynamic state. This nanocomposite is compatible with a Mg metal anode and allows stable Mg plating/stripping (at least 100 cycles) in a symmetric cell. The results represent a major advancement of solid-state multivalent ion conductors here demonstrated for Mg 2+ .

Light weight and cheap electrolytes with fast multi-valent ion conductivity can pave the way for future high-energy density solid-state batteries, beyond the lithium-ion battery.

Experimental and theoretical studies on Mg(BH4)2 were carried out from the viewpoint of the formation of the intermediate compound MgB12H12 with B12H12 cluster. The full dehydriding and partial rehydriding reactions of Mg(BH4)2 occurred according to the following multistep reaction: Mg(BH4)2 -->1/6MgB12H12 + 5/6MgH2 + 13/6H2 <--> MgH2 + 2B + 3H2 <--> Mg + 2B + 4H2. The dehydriding reaction of Mg(BH4)2 starts at approximately 520 K, and 14.4 mass% of hydrogen is released upon heating to 800 K. Furthermore, 6.1 mass% of hydrogen can be rehydrided through the formation of MgB12H12. The mechanism for the formation of MgB12H12 under the present rehydriding condition is also discussed.

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storage materials. [4] As SSEs for lithiumion batteries, they offer multiple advantages including the natural abundance of their constituent elements, their light weight, negligible electronic conduction, and low grain boundary resistance. [5] The prototypical example is LiBH 4 , whose Li + conductivity increases abruptly to >1 × 10 −3 S cm −1 above 100 °C due to a structural phase transition. [6] This superionic high-temperature phase can be stabilized at room temperature by incorporation of lithium halides (LiBH 4 -LiX, X = Cl, Br, I). Among these materials, the solid solution with lithium iodide displays the highest conductivity at room temperature of >10 −5 S cm −1 . [7] Conductivities reaching 10 −4 S cm −1 near room temperature have also been reported for two compounds of the lithium amide-borohydride Li(BH 4 ) 1−x (NH 2 ) x system, namely, for the cubic α phase (x = 3/4) and for the trigonal β phase (x = 1/2). [4,8] Here, we report the discovery of a transition to even higher Li + conductivities of up to 6.4 × 10 −3 S cm −1 near room temperature (40 °C) in BH 4 -rich lithium amide-borohydride (x = 2/3). We discuss the conduction mechanism in light of latent heat absorbed/released during the transition upon heating/cooling, respectively. We further demonstrate an allsolid-state Li 4 Ti 5 O 12 -based half-cell (employing the BH 4 -rich electrolyte) with excellent rate capability and cycling stability, comparable to a reference cell with standard liquid electrolyte representing an important step toward an all-solid-state amideborohydride-based battery. Results and DiscussionLi(BH 4 ) 1−x (NH 2 ) x powders, employing LiBH 4 and LiNH 2 precursors in amounts equivalent to x = 2/3, were prepared via reactive ball milling for 45 min and subsequent heat treatment at 120 °C for 12 h (see the Experimental Section for details). A reference sample with x = 3/4 (cubic α phase) and intermediate compositions were also synthesized.Ionic conductivities were determined for pellets pressed from the powders via temperature-dependent impedance spectroscopy. For low to intermediate conductivities, Nyquist plots take the typical form composed of a single semicircle and the electrode polarization tail (see Figure S1 in the Supporting Information for exemplary Nyquist plots). Conductivities as a High ionic conductivity of up to 6.4 × 10 −3 S cm −1 near room temperature (40 °C) in lithium amide-borohydrides is reported, comparable to values of liquid organic electrolytes commonly employed in lithium-ion batteries. Density functional theory is applied coupled with X-ray diffraction, calorimetry, and nuclear magnetic resonance experiments to shed light on the conduction mechanism. A Li 4 Ti 5 O 12 half-cell battery incorporating the lithium amide-borohydride electrolyte exhibits good rate performance up to 3.5 mA cm −2 (5 C) and stable cycling over 400 cycles at 1 C at 40 °C, indicating high bulk and interfacial stability. The results demonstrate the potential of lithium amide-borohydrides as solid-state electrolytes for high-power...

A new hemiammine borohydride, LiBH4·1/2NH3, has been presented, which exhibits fast Li-ion conduction promoted by ammonia.

Hydrogen storage properties and polymorphism in KB3H8. The order–disorder polymorphic transition results in disordered B3H8− anions, facilitating cation mobility.