Molecular Insights into Lithium-Ion Coordination and Morphology in Carbonate Polymer Electrolytes

Research of solid-state polymer electrolytes (SPEs) has accelerated due to their promise to address critical barriers in lithium-ion and lithium-metal battery commercialization through their superior thermal stability, reduced flammability, and mitigation of dendrite formation. In this study, we employ all-atom molecular dynamics simulations to investigate the Li+ solvation structure, ion diffusion, and phase morphology in mixtures of ethylene carbonate (EC) and dimethyl carbonate (DMC) across various salt concentrations. Our findings indicate that low salt concentrations diminish ionic interactions and enhance ion mobility, whereas elevated salt levels facilitate ion clustering and reduce ion mobility. Based on these findings, solid polymer electrolytes were designed using EC and DMC moieties. Polymers incorporating DMC exhibit greater backbone flexibility and lower glass transition temperatures than their EC-based counterparts resulting in an ion transport enhancement. The study also examines mixed-branch copolymer systems and polymer blend systems as alternative approaches for tuning mechanical and ionic transport properties. Both direct copolymerization and physical blending of single-branch polymers allow fine-tuning of mechanical and electrochemical properties. Notably, at elevated salt concentrations, Li+ ions act as compatibilizers that reduce phase separation. These findings contribute to a fundamental understanding of the relationships among the polymer structure, salt concentration, and ion transport in carbonate-based polymer electrolytes.