Crystal Structures and Phase Behavior of the Cyclic Carbonates Fluoroethylene Carbonate, Ethylene Carbonate, and Vinylene Carbonate down to 86 K Using Powder Diffraction Data

Understanding the behavior of organic electrolytes of lithium-ion batteries is ubiquitous to help overcome the adverse impact of their freezing in cold conditions on impacting the reversibility and durability of lithium-ion batteries. Fundamental studies of the popular, yet corrosive solvent/additive fluoroethylene carbonate (FEC) appear to be few in number outside electrochemical characterization. This powder diffraction study forms what we believe to be the first to specifically probe the nature of its crystalline solid state. The crystal structure was determined ab initio with simulated annealing of powder diffraction data supported by Density Functional Theory calculations. Phase and thermal expansion behavior between 90 and 275 K were studied. Comparison powder diffraction data were obtained from the related ethylene carbonate (EC) and vinylene carbonate (VC). No solid–solid phase transitions were observed in the temperature range studied for any of the samples. FEC was found to form a three-dimensional network structure comprising hydrogen-bonded dimers as opposed to the more layered nature of the EC and VC crystal structures. The weak attractive and repulsive intermolecular interactions in the final crystal structures were examined using the Non-Covalent Interaction method and Bader partial charges.

Low temperature in situ diffraction techniques have yielded valuable insights into the crystalline structures of EC, VC, and FEC. A detailed analysis of Van der Waals interactions, dipole–dipole interactions and the factors that promote these interactions as the impact of thermal expansion on intermolecular distances was conducted in real space using the Non-Covalent Interaction (NCI) index. These interactions play a crucial role in determining the structure and thermodynamics of the crystalline phase.