Session: Physical Chemistry Poster:

Lithium ion electrolytes composed of a lithium salt and organic solvents have become ubiquitous due to their extensive usage inas energy storage devices. However, the understanding of the molecular mechanisms giving rise to the macroscopic properties is still limited. Few studies have been able to unravel the molecular motions and mechanisms behind the dynamics of the lithium ion (Li+) solvation shell because of the complexity of these molecular systems. In this work, solutions of a highly dissociated salt (LiTFSI: lithium bis(trifluoromethanesulfonyl)imide) and a highly associated salt (LiSCN: lithium thiocyanate) in acetonitrile (ACN) were investigated by linear and non-linear spectroscopieswith the assistance in combination withof ab initio molecular dynamics simulations (AIMDS). Li+ is found as free ions iIn the ACN/LiTFSI system, the cations are found to be fully solvatedby solvent molecules.. In contrast On the other hand, Li+ is found to formas contact ion pairs in ACN/LiSCN system, meaning each cation is solvated by three solvent molecules and one thiocyanate ion (SCN-). In addition, iIt is also observed that the dynamics of coordinated ACN in the Li+ first solvation shell has a characteristic time of ~1.6 ps irrespective of the ionic speciation of Li+. Similar characteristic times were deduced from AIMDS and DFT calculations. Furthermore, theAIMDS and DFT calculations revealed that the molecular mechanism behind the dynamics of ~1.6 ps is directly related to the fluctuation in the angle between Li+ and coordinated ACN, which is the angle Li+-N≡C, while other geometrical changes like the distance between Li+ and coordinated solvent play a minor role. In summary, this work discoversdescribes the time scale of the molecular motions of the organic solvents in the Li+ first solvation shell, andmore importantly, unravels the molecular mechanism behind thesuch dynamicals process of such motions through a combination of experimental and theoretical tools.

Xiaobing Chen

Postdoctoral Scholar, University of Texas at Austin

Appearances