Effect of Salt Concentration on Ion Clustering and Transport in Polymer Solid Electrolytes: A Molecular Dynamics Study of PEO–LiTFSI
Nicola Molinari, Jonathan P. Mailoa, and Boris Kozinsky
Chemistry of Materials
Currently available solid polymer electrolytes for Li-ion cells require deeper understanding and significant improvement in ionic transport properties to enable their use in high-power batteries. We use molecular dynamics simulations to model the solid amorphous polymer electrolyte system comprising poly(ethylene) oxide (PEO), lithium, and bis(trifluoromethane)sulfonimide anion (TFSI), exploring effects of high salt concentrations relevant for battery applications. Using statistical analysis of ion distribution and transport, we investigate the significant effect that salt concentration has on ion mobility. At practical salt concentrations, a previously undetected ensemble of Li–TFSI clusters emerges where Li ions have significantly lower coordination by the polymer, and this results in their significantly lower mobility as compared to Li ions coordinated by the polymer. We also find the tendency for cation–anion clusters to be asymmetrical, with the anions in greater number than Li cations, which may further affect the transport properties of this material. The existence of such negatively charged clusters has been recently speculated to explain the experimentally observed negative transference number at high LiTFSI concentrations in PEO. Our methodology enables us to suggest strategies for improvement of transport properties and can be generalized to other polymer–Li–salt combinations.