Ion Transport in Super-Concentrated Aqueous Electrolytes for Lithium-Ion Batteries

Water-in-salt electrolytes (WiSEs) are a safer alternative to conventional organic electrolytes in battery applications because of their nonflammable nature. The electrochemical performance of these concentrated aqueous solutions of Li electrolytes critically depends on their high electrical conductivity at saturation. Still, the underlying molecular mechanism of Li-ion transport has not yet been clearly elucidated. To better understand this, we investigate four types of predominant atomic interactions and dynamics involving Li ions, anionic oxygen atoms (O-T), and atoms of water molecules (O-W) in the WiSE made of lithium bis(trifluoromethanesulfonyl)-imide with molecular dynamics simulation and theoretical analysis. We present the distribution of atomic composition in the first solvation shells of these atoms and water molecules, thermodynamic stabilities of the contact atom pairs, their lifetimes based on the reactive flux time correlation function and the transition state theory analyses, and the correlation of the Li-ion mobility with the local solvation environment and its dynamics. We find that Li ions follow heterogeneous trajectories on the sub-nanosecond time scale consisting of distinctive water-rich and anion-rich segments, switching between a vehicle-type and a hopping-type mechanism in respective regions. The Li center dot center dot center dot O-W contact pair is slightly more stable than Li center dot center dot center dot O-T at saturation, and this subtle balance appears responsible for the fast Li-ion transport in this class of WiSE.