Effects of LiBF4 Addition on the Lithium-Ion Conductivity of LiBH4.

Complex hydrides, such as LiBH4, are a promising class of ion conductors for all-solid-state batteries, but their application is constrained by low ion mobility at room temperature. Mixing with halides or complex hydride anions, i.e., other complex hydrides, is an effective approach to improving the ionic conductivity. In the present study, we report on the reaction of LiBH4 with LiBF4, resulting in the formation of conductive composites consisting of LiBH4, LiF and lithium closo-borates. It is believed that the in-situ formation of closo-borate related species gives rise to highly conductive interfaces in the decomposed LiBH4 matrix. As a result, the ionic conductivity is improved by orders of magnitude with respect to the Li-ion conductivity of the LiBH4, up to 0.9 × 10-5 S cm-1 at 30 °C. The insights gained in this work show that the incorporation of a second compound is a versatile method to improve the ionic conductivity of complex metal hydrides, opening novel synthesis pathways not limited to conventional substituents.

A Unique Double-Layered Carbon Nanobowl-Confined Lithium Borohydride for Highly Reversible Hydrogen Storage.

Poor reversibility and high desorption temperature restricts the practical use of lithium borohydride (LiBH4 ) as an advanced hydrogen store. Herein, a LiBH4 composite confined in unique double-layered carbon nanobowls prepared by a facile melt infiltration process is demonstrated, thanks to powerful capillary effect under 100 bar of H2 pressure. The gradual formation of double-layered carbon nanobowls is witnessed by transmission electron microscopy (TEM) observation. Benefiting from the nanoconfinement effect and catalytic function of carbon, this composite releases hydrogen from 225 °C and peaks at 353 °C, with a hydrogen release amount up to 10.9 wt%. The peak temperature of dehydriding is lowered by 112 °C compared with bulk LiBH4 . More importantly, the composite readily desorbs and absorbs ≈8.5 wt% of H2 at 300 °C and 100 bar H2 , showing a significant reversibility of hydrogen storage. Such a high reversible capacity has not ever been observed under the identical conditions. The usable volumetric energy density reaches as high as 82.4 g L-1 with considerable dehydriding kinetics. The findings provide insights in the design and development of nanosized complex hydrides for on-board applications.

Li2 NH-LiBH4 : a Complex Hydride with Near Ambient Hydrogen Adsorption and Fast Lithium Ion Conduction.

Complex hydrides have played important roles in energy storage area. Here a complex hydride made of Li2 NH and LiBH4 was synthesized, which has a structure tentatively indexed using an orthorhombic cell with a space group of Pna21 and lattice parameters of a=10.121, b=6.997, and c=11.457 Å. The Li2 NH-LiBH4 sample (in a molar ratio of 1:1) shows excellent hydrogenation kinetics, starting to absorb H2 at 310 K, which is more than 100 K lower than that of pristine Li2 NH. Furthermore, the Li+ ion conductivity of the Li2 NH-LiBH4 sample is about 1.0×10-5  S cm-1 at room temperature, and is higher than that of either Li2 NH or LiBH4 at 373 K. Those unique properties of the Li2 NH-LiBH4 complex render it a promising candidate for hydrogen storage and Li ion conduction.

Synthesis of Nickel and Cobalt Ferrite-Doped Graphene as Efficient Catalysts for Improving the Hydrogen Storage Kinetics of Lithium Borohydride.

Featuring a high hydrogen storage content of up to 20 wt%, complex metal borohydrides remain promising solid state hydrogen storage materials, with the real prospect of reversible behavior for a zero-emission economy. However, the thermodynamic barriers and sluggish kinetics are still barriers to overcome. In this context, nanoconfinement has provided a reliable method to improve the behavior of hydrogen storage materials. The present work describes the thermodynamic and kinetic enhancements of LiBH4 nanoconfined in MFe2O4 (M=Co, Ni) ferrite-catalyzed graphene host. Composites of LiBH4-catalysts were prepared by melt infiltration and investigated by X-ray diffraction, TEM, STEM-EDS and TPD. The role of ferrite additives, metal precursor treatment (Ar, Ar/H2) and the effect on hydrogen storage parameters are discussed. The thermodynamic parameters for the most promising composite LiBH4-graphene-NiFe2O4 (Ar) were investigated by Kissinger plot method, revealing an EA = 127 kJ/mol, significantly lower than that of neat LiBH4 (170 kJ/mol). The reversible H2 content of LiBH4-graphene-NiFe2O4 (Ar) after 5 a/d cycles was ~6.14 wt%, in line with DOE's target of 5.5 wt% storage capacity, while exhibiting the lowest desorption temperature peak of 349 °C. The composites with catalysts treated in Ar have lower desorption temperature due to better catalyst dispersion than using H2/Ar.

Lithium Superionic Conduction in BH4 -Substituted Thiophosphate Solid Electrolytes.

Compared with conventional liquid electrolytes, solid electrolytes can better improve the safety properties and achieve high-energy-density Li-ion batteries. Sulfide-based solid electrolytes have attracted significant attention owing to their high ionic conductivities, which are comparable to those of their liquid counterparts. Among them, Li thiophosphates, including Li-argyrodites, are widely studied. In this study, Li thiophosphate solid electrolytes containing BH4 - anions are prepared via a simple and fast milling method even without heat treatment. The synthesized materials exhibit a high ionic conductivity of up to 11 mS cm-1 at 25 °C, which is much higher than reported values. To elucidate the mechanism behind, the thiophosphate local structure, whose effect on the ionic conductivity remains unclear to date, is investigated. Raman and solid-state NMR spectroscopies are performed to identify the thiophosphate local structure in the sulfide samples. Based on the analysis results, the ratios of the different thiophosphate units in the prepared electrolyte samples are determined. It is found that the thiophosphate local structure can be varied by changing the amount of LiBH4 and the milling conditions, which significantly impact the ionic conductivity. The all-solid-state cell with the prepared solid electrolyte exhibits superior cycle and rate performances.

Li-Ion Conductivity Enhancement of LiBH4·xNH3 with In Situ Formed Li2O Nanoparticles.

Interfacial engineering is an efficient approach to improve the ionic conductivity of solid-state electrolytes. In the present study, we report the enhancement of in situ formed nanocrystalline Li2O on the thermal stability and electrochemical properties of amide lithium borohydride, LiBH4·xNH3 (x = 0.67-0.8). LiBH4·xNH3-Li2O composites with different amounts of Li2O are prepared by a one-step synthesis process by ball milling the mixture of LiBH4, LiNH2, and LiOH in molar ratios of 1:n:n (n = 1, 2, 3, 4). Owing to the strong interfacial effect with nanocrystalline Li2O, LiBH4·xNH3 is converted to the amorphous state in the presence of 78 wt % Li2O at n = 4. Consequently, the ionic conductivity of LiBH4·xNH3 at 20 °C is improved by orders of magnitude up to 5.4 × 10-4 S cm-1, the NH3 desorption temperature is increased by more than 20 °C, and the electrochemical window is widened from 0.5 to 3.8 V.

Enhanced Low-Temperature Hydrogen Storage in Nanoporous Ni-Based Alloy Supported LiBH4.

To reveal the synergistic effect of nanoconfinement and metallic catalysis on the hydrogen storage properties of LiBH4, the nanoporous Ni-based alloy (np-Ni) was prepared herein by dealloying of the Mn70Ni30 alloy in (NH4)2SO4 solution, and then LiBH4 was loaded into np-Ni to construct the LiBH4/np-Ni hydrogen storage system using wet impregnation. It was found that dehydrogenation of the LiBH4/np-Ni (1:5) system starts at around 70°C and ends before 400°C, with ~11.9 wt.% of hydrogen desorbed. The apparent dehydrogenation activation energy for the LiBH4/np-Ni (1:5) system was remarkable decreased to about 11.4 kJ/mol. After rehydrogenation at 450°C under 8 MPa hydrogen pressure, ~8.2 wt.% of hydrogen can be released from about 60°C upon second dehydrogenation. These obtained results would provide an efficient strategy for improving the hydrogen storage properties of other metal borohydrides.

Li-Ion Diffusion in Nanoconfined LiBH4-LiI/Al2O3: From 2D Bulk Transport to 3D Long-Range Interfacial Dynamics.

Solid electrolytes based on LiBH4 receive much attention because of their high ionic conductivity, electrochemical robustness, and low interfacial resistance against Li metal. The highly conductive hexagonal modification of LiBH4 can be stabilized via the incorporation of LiI. If the resulting LiBH4-LiI is confined to the nanopores of an oxide, such as Al2O3, interface-engineered LiBH4-LiI/Al2O3 is obtained that revealed promising properties as a solid electrolyte. The underlying principles of Li+ conduction in such a nanocomposite are, however, far from being understood completely. Here, we used broadband conductivity spectroscopy and 1H, 6Li, 7Li, 11B, and 27Al nuclear magnetic resonance (NMR) to study structural and dynamic features of nanoconfined LiBH4-LiI/Al2O3. In particular, diffusion-induced 1H, 7Li, and 11B NMR spin-lattice relaxation measurements and 7Li-pulsed field gradient (PFG) NMR experiments were used to extract activation energies and diffusion coefficients. 27Al magic angle spinning NMR revealed surface interactions of LiBH4-LiI with pentacoordinated Al sites, and two-component 1H NMR line shapes clearly revealed heterogeneous dynamic processes. These results show that interfacial regions have a determining influence on overall ionic transport (0.1 mS cm-1 at 293 K). Importantly, electrical relaxation in the LiBH4-LiI regions turned out to be fully homogenous. This view is supported by 7Li NMR results, which can be interpreted with an overall (averaged) spin ensemble subjected to uniform dipolar magnetic and quadrupolar electric interactions. Finally, broadband conductivity spectroscopy gives strong evidence for 2D ionic transport in the LiBH4-LiI bulk regions which we observed over a dynamic range of 8 orders of magnitude. Macroscopic diffusion coefficients from PFG NMR agree with those estimated from measurements of ionic conductivity and nuclear spin relaxation. The resulting 3D ionic transport in nanoconfined LiBH4-LiI/Al2O3 is characterized by an activation energy of 0.43 eV.

Improved Dehydrogenation Properties of LiBH4 Using Catalytic Nickel- and Cobalt-based Mesoporous Oxide Nanorods.

Lithium borohydride (LiBH4 ) with a theoretical hydrogen storage capacity of 18.5 wt % has attracted intense interest as a high-density hydrogen storage material. However, high dehydrogenation temperatures and limited kinetics restrict its practical applications. In this study, mesoporous nickel- and cobalt-based oxide nanorods (NiCo2 O4 , Co3 O4 and NiO) were synthesized in a controlled manner by using a hydrothermal method and then mixed with LiBH4 by ball milling. It is found that the dehydrogenation properties of LiBH4 are remarkably enhanced by doping the as-synthesized metal oxide nanorods. When the mass ratio of LiBH4 and oxides is 1:1, the NiCo2 O4 nanorods display the best catalytic performance owing to the mesoporous rod-like structure and synergistic effect of nickel and cobalt active species. The initial hydrogen desorption temperature of the LiBH4 -NiCo2 O4 composite decreases to 80 °C, which is 220 °C lower than that of pure LiBH4 , and 16.1 wt % H2 is released at 500 °C for the LiBH4 -NiCo2 O4 composite. Meanwhile, the composite also exhibits superior dehydrogenation kinetics, which liberates 5.7 wt % H2 within 60 s and a total of 12 wt % H2 after 5 h at 400 °C. In comparison, pure LiBH4 releases only 5.3 wt % H2 under the same conditions.

Enhanced Hydrogen Storage Properties and Reversibility of LiBH4 Confined in Two-Dimensional Ti3C2.

LiBH4 is of particular interest as one of the most promising materials for solid-state hydrogen storage. Herein, LiBH4 is confined into a novel two-dimensional layered Ti3C2 MXene through a facile impregnation method for the first time to improve its hydrogen storage performance. The initial desorption temperature of LiBH4 is significantly reduced, and the de-/rehydrogenation kinetics are remarkably enhanced. It is found that the initial desorption temperature of LiBH4@2Ti3C2 hybrid decreases to 172.6 °C and releases 9.6 wt % hydrogen at 380 °C within 1 h, whereas pristine LiBH4 only releases 3.2 wt % hydrogen under identical conditions. More importantly, the dehydrogenated products can partially rehydrogenate at 300 °C and under 95 bar H2. The nanoconfined effect caused by unique layered structure of Ti3C2 can hinder the particles growth and agglomeration of LiBH4. Meanwhile, Ti3C2 could possess superior effect to destabilize LiBH4. The synergetic effect of destabilization and nanoconfinement contributes to the remarkably lowered desorption temperature and improved de-/rehydrogenation kinetics.

Tritium-Labeled Compounds XII. Note on the Synthesis of d-Glucose-2-t and d-Mannose-2-t.

d-Glucose-2-t and d-mannose-2-t have been synthesized from sodium 2-keto-d-gluconate (sodium d-arabino-hexulosonate). Reduction of the salt with lithium borohydride-t produced the epimeric, 2-labeled aldonic acids in almost equal proportions. The acids, separated by carrier techniques, were lactonized, and the lactones were reduced with sodium amalgam to the corresponding sugars. The specificity of the labeling was established by converting each sugar into d-arabino-hexulose phenylosazone and determining that the osazone was substantially tritium-free.