Phase Behavior and Ion Dynamics of Nanoconfined LiBH₄ in Silica

The increasing demand for high capacity yet safe storage of renewable energy calls for the development of all-solid-state batteries. A major hurdle in this development is the identification of new suitable types of solid-state electrolytes. Nanoconfined lithium borohydride is a solid-state electrolyte candidate due to its high lithium-ion mobility at ambient temperatures. The origin of the high lithium-ion mobility is not fully understood, however. We studied nanocomposites of lithium borohydride and nanoporous silica Santa Barbara Amorphous-15 (SBA-15) with different pore sizes, using ¹H, ^6,7Li, and ¹¹B solid-state NMR at various temperatures, to get in-depth insights into the phase behavior and ion dynamics of lithium borohydride in the silica pores. The results allow us to formulate a detailed dynamic model for lithium borohydride confined in SBA-15; bulklike LiBH₄ is separated from the pore walls by an amorphous, highly dynamic LiBH₄ fraction displaying both Li⁺ and BH₄ ^- diffusion even at ambient temperatures. As shown by ¹¹B temperature-jump exchange NMR, this dynamic fraction increases as a function of temperature. Li⁺ exchange between the bulklike and "dynamic" LiBH₄ fraction is slow at ambient temperatures, but at elevated temperatures (≥90 °C), above the phase transition of the bulklike fraction, lithium ions rapidly diffuse through both LiBH₄ fractions and exchange between these confined fractions at rates approaching the megahertz time scale.