Effects of zwitterionic molecules on ionic association in ethylene oxide-based electrolytes

This work investigates the effect of zwitterionic molecules on ionic association in ethylene oxide (EO)-based electrolytes using molecular dynamics simulations. Zwitterionic molecules can associate with cations and anions because they possess both positively and negatively charged groups. This unique feature can be leveraged to develop electrolytes with high ionic conductivity if we understand how zwitterionic molecules influence ionic associations. We investigate the ionic associations in the electrolytes composed of oligo(ethylene oxide) (EO) (EO_x, x = 2, 3, 4, and 5), LiTFSI and zwitterionic molecules containing cationic imidazole group and anionic sulfonate group using molecular dynamics simulations. The analyzed properties include the radial distribution functions between Li⁺, [TFSI]^-, EO_x and zwitterionic molecules, the structures and dynamics of Li⁺-[TFSI]^-, Li⁺- EO_x and Li⁺-zwitterion associations, and the diffusion coefficients of Li⁺, [TFSI]^-, EO_x and zwitterionic molecules. The simulation results show two distinct effects of zwitterionic molecules on ionic associations in the electrolytes. First, they could release Li⁺ from the trapping effect of EO_x chains and accelerate Li⁺ transport. Second, they can associate with Li⁺ themselves and slow down the Li⁺ transport. The competition between these two effects relates to the length of the EO_x chains. Our simulations suggest that zwitterionic molecules could help manipulate the ionic conductivity of polyethylene oxide electrolytes.