Coupled Ion Transport in Concentrated PEO-LiTFSI Polymer Electrolytes

Abstract

Understanding how microscopic mechanisms govern macroscopic transport properties is important for development of improved electrolytes for Li-ion batteries. The archetypal polymer electrolyte PEO–LiTFSI has been investigated for more than three decades, but the fundamental ion transport mechanisms are still elusive. Molecular Dynamics (MD) simulations enable us to determine transport properties by directly probing particle movements. Both transport properties and microscopic interactions that govern them can be studied simultaneously. In this work, ionic conductivity and transport numbers of PEO–LiTFSI electrolytes are computed as a function of salt concentration and PEO chain length. The values are obtained using the Nernst–Einstein approximation for dilute or ideal systems, in addition we determine the Onsager coefficients that take into account ionic correlations. We observe significant differences between the two methods, indicating non-ideality. The motion of Li and TFSI is anticorrelated, causing super-ionicity. We discuss the relevance of the frame of reference. The static and dynamic properties of Li-ion coordination environments are analyzed. The distributions of cation-solvent and cation–anion residence times are investigated and indicate that the TFSI facilitate Li transport and Li jumps in the polymer network. Finally, the thermodynamic factors are computed and used to quantify the non-ideality of the systems.