Structure and Dynamics of the Lithium-Ion Solvation Shell in Ureas

Lithium-ion batteries have become ubiquitous to modern life because of their use in the energy storage needs of our daily lives. In past several decades, much effort has been put into studying the molecular structure of electrolytes composed of organic carbonates. However, other solvents with similar properties but better thermal stabilities, such as tertiary amides, have not received the same level of scrutiny. In this work, solutions of lithium salts in ureas, tertiary amides with the structure RR'N-CO-NR″R‴, with different sizes and connectivity are studied. Ureas present an interesting case study because unlike organic carbonates, the amide bond is planar and has restricted conformational change. In addition, ureas cannot bind the lithium ion through their nitrogen atoms. By using steady-state and time-resolved infrared spectroscopies and ab-initio computational methods, detailed descriptions of the changes to the lithium-ion solvation structure as a result of the urea structure were derived for three ureas bearing a strong resemblance to commonly used organic carbonates. These results show that the solvation shell of ureas has a tetrahedral structure similar to that of other organic solvents. Although the structure of the amide bonds in these ureas is similar to that of carbonate molecules, the atomic connectivity differs. In addition, the dynamics of the cation solvation shell formed by ureas shows a picosecond motion, which is attributed to deformation of the tetrahedral structure. Our investigations also indicate that the deformation dynamics is controlled directly by the size of the urea because of the rigidity of the amide bond in these molecules. Overall, this work shows that ureas share similarity with their organic carbonate analogues, but the rigid urea structure provides an easier framework for interpreting the vibrational observations in terms of the solvent molecular structure.