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 H-1, Li-6,Li-7, and B-11 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 B-11 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 degrees 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.