Oligomerization of lithium ions in water-in-salt electrolytes

Water-in-salts (WiS) have recently emerged as promising electrolytes for energy storage applications, ranging from aqueous batteries to supercapacitors. Here, ab initio molecular dynamics is used to study the structure of a 21 m LiTFSI WiS. The simulation reveals a new feature, in which the lithium ions form oligomer-like nanochains that involve up to 10 ions. Despite the strong Coulombic interaction between them, the ions in the chains are found at a distance of 2.5 Angstroms. They display a drastically different solvation shell compared to the isolated ions, in which they share on average two water molecules. The nanochains have a highly transient character due to the low free energy barrier for forming/breaking them. Providing new insights into the nanostructure of WiS electrolytes, our work calls for re-evaluating our current knowledge of highly concentrated electrolytes and the impact of the modification of solvation of active species on their electrochemical performances.