First Principles Hierarchical Selection and Testing of Anion Receptors for High Specific Energy Lithium-Fluoride Batteries

摘要

New anion receptors have been designed, modeled, synthesized, and tested at the battery cell level for use in dual ion intercalating batteries. In particular, a promising new class of cyclic fluorophenyl boronic acid anhydride (fluorophenyl boroxine) anion receptors was identified. The binding energetics of these compounds to fluoride can be readily varied by tailoring the degree of fluorination on the phenyl moieties or by substituting alternative fluorinated moieties. To the best of our knowledge, this is the first time these compounds have been identified as anion receptors. Battery cells have been fabricated and tested using several of these as-prepared, unpurified anion receptors, with specific discharge capacities on the order of 50-80 mAh/g at discharge voltages near 5V. Further improvements in specific capacity and Coulombic efficiency are expected by incorporating purification methods following anion receptor synthesis. 19F NMR studies of solutions of these fluorophenyl boroxines in propylene carbonate with and without LiF strongly suggest that these compounds can bind fluoride at several non- equivalent boron sites with varying binding energies, as predicted by the modeling studies. 7Li NMR indicates that the diffusion coefficient of Li in these solutions is comparable to other well-known anion receptors such as tris (pentaflurophenyl) borane. 11B studies are underway to elucidate the nature of the boron-fluoride binding for these compounds in solution. Ab initio modeling coupled with experimental studies suggest a trend in anion receptor binding energy and practical cathode specific capacity, which indicates weaker binding energies should provide more favorable cathode specific capacities. As such modeling and experimental work was re-focused on weaker rather than stronger binding compounds.