We perform a joint experimental and computational study of ion transport properties in a systematic set of linear polyethers synthesized via acyclic diene metathesis (ADMET) polymerization. We measure ionic conductivity, σ, and glass transition temperature, Tg, in mixtures of polymer and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt. While Tg is known to be an important factor in the ionic conductivity of polymer electrolytes, recent work indicates that the number and proximity of lithium ion solvation sites in the polymer also play an important role, but this effect has yet to be systematically investigated. Here, adding aliphatic linkers to a poly(ethylene oxide) (PEO) backbone lowers Tg and dilutes the polar groups; both factors influence ionic conductivity. To isolate these effects, we introduce a two-step normalization scheme. In the first step, Vogel-Tammann-Fulcher (VTF) fits are used to calculate a temperature-dependent reduced conductivity, σr(T), which is defined as the conductivity of the electrolyte at a fixed value of T - Tg. In the second step, we compute a nondimensional parameter fexp, defined as the ratio of the reduced molar conductivity of the electrolyte of interest to that of a reference polymer (PEO) at a fixed salt concentration. We find that fexp depends only on oxygen mole fraction, x0, and is to a good approximation independent of temperature and salt concentration. Molecular dynamics simulations are performed on neat polymers to quantify the occurrences of motifs that are similar to those obtained in the vicinity of isolated lithium ions. We show that fexp is a linear function of the simulation-derived metric of connectivity between solvation sites. From the relationship between σr and fexp we derive a universal equation that can be used to predict the conductivity of ether-based polymer electrolytes at any salt concentration and temperature.

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, Tg, 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 Tg 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.

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.