Molecular Dynamics Study of a Dual-Cation Ionomer Electrolyte

Abstract

The poly(N₁₂₂₂ )_x Li_1-x [AMPS] ionomer system (AMPS=2-acrylamido-2-methylpropane sulfonic acid) with dual cations has previously shown decoupled Li ion dynamics from polymer segmental motions, characterized by the glass-transition temperature, which can result in a conductive electrolyte material whilst retaining an appropriate modulus (i.e. stiffness) so that it can suppress dendrite formation, thereby improving safety when used in lithium-metal batteries. To understand this ion dynamics behavior, molecular dynamics techniques have been used in this work to simulate structure and dynamics in these materials. These simulations confirm that the Li ion transport is decoupled from the polymer particularly at intermediate N₁₂₂₂⁺ concentrations. At 50 mol % N₁₂₂₂⁺ concentration, the polymer backbone is more rigid than for higher N₁₂₂₂⁺ concentrations, but with increasing temperature Li ion dynamics are more significant than polymer or quaternary ammonium cation motions. Herein we suggest an ion-hopping mechanism for Li⁺ , arising from structural rearrangement of ionic clusters that could explain its decoupled behavior. Higher temperatures favor an aggregated ionic structure as well as enhancing these hopping motions. The simulations discussed here provide an atomic-level understanding of ion dynamics that could contribute to designing an improved ionomer with fast ion transport and mechanical robustness.

Keywords: ion dynamics; ionomer; lithium-ion batteries; molecular dynamics simulations; polymer electrolyte.