No abstract

Lithium borohydride (LiBH 4 ) is a promising material for hydrogen storage, with a gravimetric hydrogen content of 18.5%. However, the thermodynamics and kinetics of its hydrogen release and uptake need to be improved before it can meet the requirements for mobile applications. In this study, we investigate the confinement of LiBH 4 in ordered mesoporous SiO 2 and its effect on the hydrogen sorption properties. We demonstrate that, only under hydrogen pressure, melt infiltration is an effective method for the synthesis of LiBH 4 /SBA-15 nanocomposites. Our work clearly shows that formation of lithium silicates from LiBH 4 and SiO 2 can effectively be suppressed by hydrogen. Thus, under hydrogen pressure, LiBH 4 can fully fill the mesopores of SBA-15 while the long-range order of the mesopores is maintained. The confined LiBH 4 has enhanced hydrogen desorption properties, with desorption starting at 150 °C. However, upon dehydrogenation, SiO 2 and decomposition products of LiBH 4 react to form Li 2 SiO 3 and Li 4 SiO 4 , leading to irreversible hydrogen loss.

NaBH 4 is an interesting hydrogen storage material for mobile applications due to its high hydrogen content of 10.8 wt%. However, its practical use is hampered by the high temperatures (above 500 C) required to release the hydrogen and by the non reversibility of the hydrogen sorption. In this study, we show that upon heating to 600 C, bulk NaBH 4 decomposed into Na and Na 2 B 12 H 12 , releasing the expected 8.1wt% of hydrogen. Nanosizing and confinement of NaBH 4 in porous carbon resulted in much faster hydrogen desorption kinetics. The onset of hydrogen release was reduced from 470 C for the bulk to below 250 C for the nanocomposites. Furthermore, the dehydrogenated nanocomposites were partially rehydrogenated to form NaBH 4 , with the absorption of about 43% of the initial hydrogen capacity at relatively mild conditions (60 bar H 2 and 325 C). Reversibility in this system was limited due to partial loss of Na during dehydrogenation. The dehydrogenated boron compounds were almost fully rehydrogenated to NaBH 4 (98%) when extra Na was added to the nanocomposites. To the best of our knowledge, this is the first time that reversibility for NaBH 4 has been demonstrated.

The reversible hydrogen capacity of LiBH 4 was improved by a combination of Ni addition, nanosizing and confinement of the active phase in a nanoporous carbon scaffold.

In this work we characterize all-solid-state lithium-sulfur batteries based on nano-confined LiBH 4 in mesoporous silica as solid electrolytes. The nano-confined LiBH 4 has fast ionic lithium conductivity at room temperature, 0.1 mScm −1 , negligible electronic conductivity and its cationic transport number (t + = 0.96), close to unity, demonstrates a purely cationic conductor. The electrolyte has an excellent stability against lithium metal. The behavior of the batteries is studied by cyclic voltammetry and repeated charge/discharge cycles in galvanostatic conditions. The batteries show very good performance, delivering high capacities versus sulfur mass, typically 1220 mAhg −1 after 40 cycles at moderate temperature (55 • During the past decade, the quest for promising next generation energy storage systems has led significant attention to secondary batteries with high specific energy such as lithium/sulfur (Li/S) batteries (1675 mAhg −1 sulfur at 2.15 V), 1-6 which have 3 to 5 times higher energy densities than commercial state-of-art Li-ion batteries, e.g. 7 The inexpensive, abundant and environmentally benign nature of sulfur makes this battery more appealing for large scale application purposes (e.g. transportation, portable and residential applications) than other metal-ion battery systems. However, there are still numerous scientific and technical challenges for practical application. During discharge in Li/S batteries, lithium ions move spontaneously through the electrolyte from the negative electrode, typically lithium metal, silicon or tin-based compounds, to the positive sulfur electrode and S is ultimately reduced to form Li 2 S, while electrons flow through the external circuit. During charge, Li 2 S is oxidized back to S and Li + by applying an external voltage. The overall electrochemical reaction is:However, the electrochemical reduction of sulfur in Li/S battery occurs through the formation of a series of intermediate lithium polysulfides, Li 2 S x (2 ≤ x ≤ 8). 8,9 These intermediates are soluble in most liquid organic solvents/electrolytes and shuttling between the sulfur cathode and Li anode results in fast self-discharge during storage and low coulombic efficiencies during charging. Therefore, a grand challenge for Li/S batteries is to suppress this mechanism, for example by encapsulation or coating of the sulfur electrode, 10-14 use of impermeable membranes, 15 and/or the use of suitable electrolytes that minimize the solubility and diffusivity of the polysulfides. [16][17][18] Another possibility is to use fast ion-conducting solids, i.e. solid electrolytes with an ionic conductivity (σ ∼ 10 −4 to 10 −1 Scm −1 ) comparable to that of standard liquid electrolytes (e.g. σ ∼10−2 Scm −1 for 1.0 M LiFP 6 in organic solvent). Nanoconfinement of LiBH 4 in nanoporous carbon scaffolds has been reported by various groups to improve the BH 4 − rotational diffusivity and lithium mobility at room temperature.46-49 However, due to their high electronic conductivity, nano-scaffolds of carbon are not ver...