(719e) Elucidating the Solvation Structure and Dynamics of Lithium Polysulfides Using Coupled High Throughput Simulations and Experiments

Fundamental molecular level understanding of functional properties of liquid solutions provides an important basis for designing optimized electrolytes for numerous applications. In particular, exhaustive knowledge of solvation structure, stability and transport properties is critical for developing stable electrolytes for fast charging and high energy density next-generation energy storage systems. Here we report the correlation between solubility, solvation structure, and translational dynamics of a lithium salt (Li-TFSI) and polysulfides species using well-benchmarked classical molecular dynamics simulations combined with nuclear magnetic resonance (NMR). It is observed that the polysulfide chain length has a significant effect on the ion-ion and ion-solvent interaction as well as on the diffusion coefficient of the ionic species in solution. In particular, extensive cluster formation is observed in lower order polysulfides (S_x2-; x≤4), whereas the longer polysulfides (S_x²-; x>4) show high solubility and slow dynamics in the solution.¹ It is observed that optimal solvent/salt ratio is essential to control the solubility and conductivity as the addition of Li salt increases the solubility but decreases the mobility of the ionic species. We also observed that counter anion (such as TFSI, TfO, FSI & TDI) and solvent interaction strength with Li⁺ is critical in controlling polysulfide solubility.²Also, high mobility of counter anions can cause faster capacity loss due to enhanced polysulfide solubility and SEI layer formation. This work provides a coupled theoretical and experimental study of bulk solvation structure and transport properties of multi-component electrolyte systems, yielding design metrics for developing optimal electrolytes with improved stability and solubility.

References

1. Rajput, N. N.; Murugesan, V.; Shin, Y.; Han, K. S.; Lau, K. C.; Chen, J.; Liu, J.; Curtiss, L. A.; Mueller, K. T.; Persson, K. A., Elucidating the Solvation Structure and Dynamics of Lithium Polysulfides resulting from Competitive Salt and Solvent Interactions. Chem. Mater. 2017.

2. Vijayakumar, M.; Govind, N.; Walter, E.; Burton, S. D.; Shukla, A.; Devaraj, A.; Xiao, J.; Liu, J.; Wang, C.; Karim, A., Molecular structure and stability of dissolved lithium polysulfide species. Phys. Chem. Chem. Phys. 2014, 16 (22), 10923-10932.