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The single-crystal-to-single-crystal phase transition is determined using X-ray crystallography on LiBF4, resolving a longstanding ambiguity in the existence of a high-temperature polymorph of LiBF4. LiBF4 possesses an endothermic phase change at 28.2 °C with ΔH = 1180 J mol-1 and ΔS = 3.92 J mol-1K-1 based on DSC. Single-crystal X-ray diffraction shows that the low temperature phase collected at 200K is a twinned trigonal P system with a twin law indicating reflection through the 110 plane. The same crystal collected above the phase transition temperature at 313 K is a C-centered orthorhombic system describable as the superposition of the two low-temperature twin geometries undergoing interconversion. The geome-tries of the high- and low-temperature phases are consistent with the calorimetry experiments, and with previous NMR find-ings indicating BF4 geometric reorientations above 300 K. 

Alternative solid-electrolytes are the next key step in advancing lithium batteries with better thermal and chemical stability. A soft-solid electrolyte (Adpn)2LiPF6 (Adpn = adiponitrile) is synthesized and characterized, which exhibits high thermal and electrochemical stability and good ionic conductivity, overcoming several limitations of conventional organic and ceramic materials. The surface of the electrolyte possesses a liquid nano-layer of Adpn that links grains for a facile ionic conduction without high pressure/temperature treatments. Further, the material can quickly self-heal if fractured and provides liquid-like conduction paths via the grain boundaries. A significantly high ion conductivity (~ 10-4 S/cm) and lithium-ion transference number (0.54) are obtained due to weak interactions between “hard” (charge-dense) Li+ ions and “soft” (electronically polarizable) -C≡N group of Adpn. Molecular simulations predict that Li+ ions migrate at the co-crystal grain boundaries with a (preferentially) lower Ea and within the interstitial regions between the co-crystals with higher Ea, where the bulk conductivity comprises a smaller but extant contribution. These cocrystals establish a special concept of crystal design to increase the thermal stability of LiPF6 by separating ions in Adpn solvent matrix, and also exhibit a unique mechanism of ion-conduction via low-resistance grain-boundaries, which is contrasting to ceramics or gel-electrolytes.