Systematic Computational and Experimental Investigation of Lithium-Ion Transport Mechanisms in Polyester-Based Polymer Electrolytes

Understanding the mechanisms of lithium-ion transport in polymers is crucial for the design of polymer electrolytes. We combine modular synthesis, electrochemical characterization, and molecular simulation to investigate lithium-ion transport in a new family of polyester-based polymers and in poly(ethylene oxide) (PEO). Theoretical predictions of glass-transition temperatures and ionic conductivities in the polymers agree well with experimental measurements. Interestingly, both the experiments and simulations indicate that the ionic conductivity of PEO, relative to the polyesters, is far higher than would be expected from its relative glass-transition temperature. The simulations reveal that diffusion of the lithium cations in the polyesters proceeds via a different mechanism than in PEO, and analysis of the distribution of available cation solvation sites in the various polymers provides a novel and intuitive way to explain the experimentally observed ionic conductivities. This work provides a platform for the evaluation and prediction of ionic conductivities in polymer electrolyte materials

Effect of monomer structure on ionic conductivity in a systematic set of polyester electrolytes

Polymer electrolytes may enable the next generation of lithium ion batteries with improved energy density and safety. Predicting the performance of new ion-conducting polymers is difficult because ion transport depends on a variety of interconnected factors which are affected by monomer structure: interactions between the polymer chains and the salt, extent of dissociation of the salt, and dynamics in the vicinity of ions. In an attempt to unravel these factors, we have conducted a systematic study of the dependence of monomer structure on ionic conductivity, σ, and glass transition temperature, T_g, using electrolytes composed of aliphatic polyesters and lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) salt. The properties of these electrolytes were compared to those of poly(ethylene oxide) (PEO), a standard polymer electrolyte for lithium batteries. We define a new measure of salt concentration, ρ, the number of lithium ions per unit length of the monomer backbone. This measure enables collapse of the dependence of both the σ and T_g on salt concentration for all polymers (polyesters and PEO). Analysis based on the Vogel–Tammann–Fulcher (VTF) equation reveals the effect of different oxygen atoms on ion transport. The VTF fits were used to factor out the effect of segmental motion in order to clarify the relationship between molecular structure and ionic conductivity. While the conductivity of the newly-developed polyesters was lower than that of PEO, our study provides new insight into the relationship between ion transport and monomer structure in polymer electrolytes