Synthesis in Molten Salts and Characterization of Li₆B₁₈(Li₂O) Nanoparticles

Lithium borides have been synthesized exclusively through classical solid-state chemistry processes that lead to bulk materials. Indeed, due to the lack of reactivity of the solid boron precursors usually employed and to the high covalent connectivity in such solids, high temperatures and long reaction times are necessary to obtain lithium borides. These conditions result in extensive crystal growth. Here we present the synthesis of nanoparticles of a lithium boride bearing tunnel-like cavities templated by neutral Li₂O species, which have been reported to be labile. To reach this goal, a liquid-phase synthesis in inorganic molten salts has been developed. The Li₆B₁₈(Li₂O) nanoparticles have been characterized by scanning and transmission electronic microscopy (SEM and TEM), X-ray diffraction (XRD), and Raman spectroscopy. We provide an in-depth structural characterization by using ¹H, ⁷Li, and ¹¹B solid-state nuclear magnetic resonance (NMR) coupled with DFT modeling to provide the first assignment of ⁷Li and ¹¹B solid-state NMR signals in lithium borides. We then assess the nanoparticle morphology oriented along the direction of the cavities. This feature shows similarities with structurally related hexagonal tungsten bronzes and could therefore affect the electrochemical and ion exchange properties.