Ion Correlations in Concentrated Liquid Electrolytes
Figure 1: Examples for concentrated liquid electrolytes like ionic liquids (IL) or solvate ionic liquids (SIL).
For the development of safer lithium ion batteries (LIBs), alternative electrolytes with low vapor pressure and flammability are of great interest. Examples are ionic liquid / lithium salt mixtures and solvate ionic liquids, see Fig. 1. These are highly concentrated electrolytes, in which cations and anions do not move independently, but the movements of distinct ions are characterized by strong directional correlations, see Fig. 2.
Figure 2: Ion correlations in diluted and concentrated electrolytes.
During the stationary charging and discharging of batteries, the Li+ ions move under anion-blocking conditions. In this case, a salt concentration gradient in the electrolyte builds up, such that the migration current and diffusion current of the anions cancel exactly. It can be shown that the Li+ ion transport under these conditions is strongly influenced by ion correlations sketched above. Both positive and negative correlations reduce the stationary Li+ current.
Figure 3: Different correlations of anions and cations influencing the ions’ motion of migration and diffusion, respectively.
By measuring different electrolyte transport parameters, like ionic conductivity (σion), Li+-transference number under anion-blocking conditions (tLi+) and salt diffusion coefficient (Dsalt), we characterize the ion correlation parameters of different concentrated electrolytes.
- F. Wohde, M. Balabajew, B. Roling, 'Li+ Transference Numbers in Liquid Electrolytes obtained by Very-low-frequency Impedance Spectroscopy at variable Electrode Distances', J. Electrochem. Soc. 163, 5 (2016) A714-A721. doi: 10.1149/2.0811605jes
- D. Dong, F. Sälzer, B. Roling, D. Bedrov 'How efficient is Li+ ion transport in solvate ionic liquids under anion-blocking conditions in a battery?', Phys. Chem. Chem. Phys. 20 (2018) 29174-29183. doi: 10.1039/c8cp06214e