Ionic relaxation in ‘Solvent-in-Salt’ electrolytes*

Solvent-in-salt (SIS) systems are one of the promising materials for the next generation of energy storage applications. The ion dynamic is significantly different in these systems from that of ionic liquids and diluted salt solutions. In this study we analyze ion dynamics of two salts Li-TFSI and Li-FSI in highly concentrated aqueous and acetonitrile solutions. We present the comprehensive study of these systems using the PFG-NMR, FTIR, QENS and MD simulations, with major focus on Broadband dielectric spectroscopy (BDS) covering the range up to 50 GHz. The estimated from the BDS data the conductivity relaxation time defines the characteristic crossover time between individual jumps of charge carriers and normal diffusion regime resulting in dc-conductivity. Analysis revealed that normal charge diffusion sets in on the distance ~1-2 Å, comparable to the average distance between the ions. Based on the idea of momentum conservation the distinct ion correlations were estimated and analyzed, and the results suggest that dependence of anion-cation correlations on solvent content. Presented analysis demonstrate that FSI based system shows faster dynamics with shorter onset of normal diffusion regime than TFSI based system resulting in higher conductivity.