(357al) Effect of Zwitterionic Molecules on Ionic Solvation and Transport in Electrolytes
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The transport of ions in electrolytes is governed mainly by their ionic interactions. Zwitterionic (ZW) molecules can affect electrolytes' lithium-ion (Li+) conductivity by adjusting Li+ solvation. In addition, many newly developed batteries are using cations like sodium (Na+) and potassium (K+). However, the mechanism of ZW ability to tune ionic solvation and transport remains unclear. To address this issue, we investigate the dissociation effect of three ZW molecules (MPC, CB, and SB) on Li+, Na+ and K+ ions in ethylene oxide (EO)-based electrolytes using molecular dynamics simulations (MPC: 2-methacryloyloxyethyl phosphorylcholine, SB: sulfobetaine ethylimidazole, CB: carboxybetaine ethylimidazole). We simulate model systems of LiTFSI/EO10 ,NaTFSI/EO10 electrolytes and KTFSI/EO10 electrolytes at a cation:O(EO10) molar ratio of 1:6 and with addition of ZW at a ZW:cation molar ratio of 1:5 (NaTFSI: sodium bis(trifluoromethylsulfonyl)-imide, and KTFSI: potassium bis(trifluoromethylsulfonyl)-imide). The simulation results unravel the difference in the ionic association and transport of Li+, Na+ and K+ and how the effect of ZW structures depends on cationic size using radial distribution functions, association number, association lifetime, dissociation percentage, and diffusion coefficients. K+ with a larger cationic size has weaker associations with [TFSI]-, EO10 and ZW molecules, thus presenting higher diffusion than Na+, whose mobility is faster than that of Li+. Adding ZW molecules dissociates the cation-EO10 and cation-[TFSI]- associations, which could help transport cations. MPC and SB present the highest and lowest dissociation ability among the three ZW structures. The dissociation ability of ZW molecules correlates with the strength of their associations with cations. MPC and SB also have the strongest and weakest associations with cations. However, these strong cation-ZW associations can slow down cationic transport. The competition of these two effects results in enhancing or decreasing cationic diffusion. MPC reduces the K+ diffusion by 40% while CB and SB have a similar decrease of 25%. Diffusion of Na+ remains unvaried with the presence of MPC and enhanced by 20% with CB addition and decreased by 20% with adding SB. Diffusion of Li+ is not affected by MPC and CB, while decreased by 20% with SB addition. Our study suggests the important role of ZW structures and cationic size on the ionic dissociation effect of ZW.