Effect of Zwitterionic Molecules on Ionic Association and Transport in Single-Ion Conducting Polymers: A Molecular Simulation Study

We investigated the zwitterionic effect on the Li⁺ association and transport in the single-ion conducting polymers using molecular dynamics simulations. Solid polymer electrolytes play an essential role in the development of lithium ion batteries with high performance. One central task is to develop polymer electrolytes with high ionic conductivity and high Li⁺ transference number. Single-ion conducting polymers show a promising role in resolving the issue of low Li⁺ transference number. However, they suffer from low ionic conductivity due to the strong ion-ion association. Zwitterionic molecules possess both negatively and positively charged groups allowing them to associate with both cations and anions. This ability enables them to dissociate ion-ion associations. Here we investigated the ionic association and transport in poly(styrene trifluoromethanesulphonylimide of lithium)/ethylene carbonate (P(STFSILi)/EC) systems with and without zwitterionic molecules at 1M Li⁺ concentration using molecular dynamics simulations. The ionic association is analyzed using radial distribution functions between Li⁺, anionic groups, carbonate and zwitterionic molecules, and the related association numbers and residence times. The ionic transport is analyzed using the diffusion coefficient of ions and molecules. The simulation results illustrate that zwitterionic molecules could free Li⁺ from ion-ion associations and enhance Li⁺ diffusion. Furthermore, we investigate the relationship between zwitterionic effect and the charged groups. These simulation results provide molecular principles for designing high-performance solid polymer electrolytes.