Atomistic Modelling and ReaxFF Parameter Optimization for Ionic Liquid Electrolyte

Abstract

The optimization of organic carbonate-based electrolytes, such as ethylene carbonate (EC) and propylene carbonate (PC), was a pivotal enabler for graphite anode materials in the mid-1990s and remains at the heart of modern Li-ion battery (LIB) technology. With battery R&D publications growing 4.5 times faster than general literature between 2010 and 2017 (Li et al., 2018), current research prioritizes electrode and electrolyte improvements to enhance energy capacity, cycling rates, and safety. However, future advancements rely heavily on the digitalization of materials science. Recent industry roadmaps indicate a critical global need for integrating multi-sourced, multi-fidelity data streams—combining experimental and computational data—to holistically analyze cell performance and safety (Batteries Europe Secretariat, 2023). In this framework, this thesis investigates an organic liquid electrolyte with an ionic liquid additive using atomistic and molecular simulations. Initial molecular topology, equilibration, and thermalization were established using Generalized Amber Force Field (GAFF) parameters. Subsequently, the Reactive Force Field (ReaxFF) was employed to simulate the reactive electrolyte environment. To optimize ReaxFF parameters for this specific system, Plane-wave Density Functional Theory (DFT) electronic calculations were performed to derive energy baselines. A custom-developed Python library was created to generate a comprehensive training dataset, comprising bond lengths, 3-body angles, 4-body dihedrals, partial atomic charges, interatomic forces, and reaction enthalpies. Molecular dynamics simulations revealed that the ionic liquid additive improves electrolyte properties by altering the solvation structure and acting as a Li-salt stabilizer. Furthermore, the weak cation-anion ligand interactions introduced by the additive were found to enhance Li-ion diffusion.