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Ion Correlations in Concentrated Liquid Battery Electrolytes

Figure: Tabita Pothmann

Figure 1: Examples for concentrated liquid electrolytes: (top) LiTFSI/sulfolane mixtures with molar ratios up to 1:2; (bottom) solvate ionic liquid consisting of [Li+/tetraglyme] complex cations and FSI- anions.

Summary:

For the development of safer lithium ion batteries (LIBs), alternative liquid electrolytes with high salt concentrations are of great interest. In these concentrated electrolytes, most solvent molecules are bound in the solvation spheres of the ions, and only few “free” solvent molecules exist. This leads to a low vapor pressure and low flammability of these electrolytes. The high ion concentrations and the low number of “free” solvent molecules result in strong ion-ion interactions and in strong directional correlations between the ionic movements, see Fig. 2.

Figure: Tabita Pothmann
Figure 2: In diluted electrolytes the ionic movements are uncorrelated (left). In concentrated electrolytes the ionic movements are either positively or negatively correlated (right).

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 the diffusion current of the anions cancel exactly. It can be shown that the Li+ ion transport under these conditions is strongly influenced by the ion correlations sketched above. Both positive and negative correlations reduce the stationary Li+ current.

Figure: Tabita Pothmann

Figure 3: Cation/anion correlations in equilibrium exert a strong influence on the migration and diffusion of the ions during stationary battery charging/discharging.

By measuring different electrolyte transport parameters,  such as ionic conductivity (σion), Li+-transference number under anion-blocking conditions (tLi+) and salt diffusion coefficient (Dsalt), we obtain information about the ion correlations in concentrated electrolytes. To this end, we use different experimental methods, such as electrochemical impedance spectroscopy and concentration cell measurements.

Hightlighted Publications:

  • T. Pothmann, M. Middendorf, C. Gerken, P. Nürnberg, M. Schönhoff, B. Roling, 'Overdetermination method for accurate dynamic ion correlations in highly concentrated electrolytes', Faraday discussions (2024).  doi:10.1039/d4fd00034j  
  • S. Pfeifer, F. Ackermann, F. Sälzer, M. Schönhoff, B. Roling, 'Quantification of cation–cation, anion–anion and cation–anion correlations in Li salt/glyme mixtures by combining very-low-frequency impedance spectroscopy with diffusion and electrophoretic NMR', Phys. Chem. Chem. Phys. 23 (2021), 628-640. doi: 10.1039/D0CP06147F
  • N. M. Vargas-Barbosa, B. Roling, 'Dynamic Ion Correlations in Solid and Liquid Electrolytes: How Do They Affect Charge and Mass Transport?', ChemElectroChem 7 (2020), 367-385. doi: 10.1002/celc.201901627
  • 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
  • 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