Diffusional Motion of Solvated Ions in Super-Concentrated Electrolytes
Authors: Han Kee Sung , Murugesan Vijayakumar , Ma Lin , Xu Kang C. , Mueller Karl T 
Published: February 2020

The evolution of solvation structure and dynamics of aqueous LiTFSI based electrolytes with varying salt concentration and competing divalent (Zn2+, Mg2+ and Ca2+) and ammonium based (Me3EtN+) cations were studied using 7Li, 19F and 1H nuclear magnetic resonance (NMR) spectroscopy and pulsed-field gradient (PFG) NMR methods. The NMR peak intensity analysis (Figure 1(a)), reveals that more than half of Li+, TFSI- and H2O molecules are invisible in liquid NMR spectra, likely due to the loss of mobility (broader NMR line width) associated with formation of bigger solvate clusters at higher salt concentrations. The 7Li, 19F and 1H chemical shift values (Figure 1(b)), were shifted to the higher field (more negative chemical shift values) with salt concentration, indicating the interaction between Li+ cations and TFSI- anions became stronger, while the hydrogen bonding network between H2O molecules became weaker resulting from the localization of water clusters in the solutions. The mobility of the remaining species were analyzed through the PFG-NMR based diffusion measurements (Figure 1(c)). It showed that Li+ cation diffuses relatively faster than TFSI- anion (D
H2O>D
Li>D
TFSI) for these aqueous electrolytes, and differ from non-aqueous electrolyte solutions which traditionally register higher anion diffusion rates (D
anion > D
Li) [1-4]. The D
H2O/D
TFSI diffusion ratio showed that the relative mobility of water molecules increases with LiTFSI concentration and by addition of multivalent (Zn2+, Mg2+ and Ca2+) cation salt. The D
Li/D
TFSI diffusion ratio reaches the maximum value of ~3.4 for the 20 m LiTFSI/H2O solution, and the addition of competing cations tends to minimally affect the lithium diffusion. The different behavior between D
H2O/D
TFSI and D
Li/D
TFSI values suggest that the localized water molecules diffuse freely in the higher concentration electrolytes, while the diffusion motions of Li+ cation and TFSI- anion depend strongly on the total composition of the competing ions in the solution. The structural and diffusional properties of high concentration electrolytes will be discussed based on diffusion and chemical shift analysis.

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Figure 1