The development of solid polymer electrolytes for lithium battery applications is a challenge of profound technological significance. We have established a collaboration with the aim of understanding and designing improved polymer electrolytes that combines theoretical modeling, polymer synthesis, and experimental characterization. By studying diverse polymer chemistries, we have discovered that ion-solvation-site connectivity is an important feature of polymer electrolytes that is necessary for high lithium-ion conductance. We are employing this insight into search for improved polymer electrolytes, with promising early-stage results.

Networks assembled by reversible association of telechelic polymers constitute a common class of soft materials. Various mechanisms of chain migration in associative networks have been proposed; yet there remains little quantitative experimental data to discriminate among them. Proposed mechanisms for chain migration include multichain aggregate diffusion as well as single-chain mechanisms such as "walking" and "hopping", wherein diffusion is achieved by either partial ("walking") or complete ("hopping") disengagement of the associated chain segments. Here, we provide evidence that hopping can dominate the effective diffusion of chains in associative networks due to a strong entropic penalty for bridge formation imposed by local network structure; chains become conformationally restricted upon association with two or more spatially separated binding sites. This restriction decreases the effective binding strength of chains with multiple associative domains, thereby increasing the probability that a chain will hop. For telechelic chains this manifests as binding asymmetry, wherein the first association is effectively stronger than the second. We derive a simple thermodynamic model that predicts the fraction of chains that are free to hop as a function of tunable molecular and network properties. A large set of self-diffusivity measurements on a series of model associative polymers finds good agreement with this model.

Mixtures of block copolymers and lithium salts are promising candidates for lithium battery electrolytes. Structural changes that occur during the order-to-disorder transition (ODT) in a diblock copolymer/salt mixture were characterized by small-angle X-ray scattering (SAXS). In salt-free block copolymers, the ODT is sharp, and the domain size of the ordered phase decreases with increasing temperature. In contrast, the ODT of the diblock copolymer/salt mixture examined here occurs gradually over an 11 °C temperature window, and the domain size of the ordered phase is a nonmonotonic function of temperature. We present an approach to estimate the fraction of the ordered phase in the 11 °C window where ordered and disordered phases coexist. The domain spacing of the ordered phase increases with increasing temperature in the coexistence window. Both findings are consistent with the selective partitioning of salt into the ordered domains, as predicted by Nakamura et al. (ACS Macro Lett. 2013, 2, 478-481). © 2014 American Chemical Society.

Block copolymer/lithium salt mixtures are promising materials for lithium battery electrolytes. The growth of ordered lamellar grains after a block copolymer electrolyte was quenched from the disordered state to the ordered state was studied by depolarized light scattering. Three quench depths below the order-to-disorder transition temperature were studied: 6, 12, and 24 °C. Regardless of quench depth, elongated ellipsoidal grains with aspect ratios between six and eight were formed during the initial stage of order formation. This was followed by a rapid reduction in aspect ratio; at long times, isotropic grains with aspect ratios in the vicinity of unity were obtained. Unusual grain growth kinetics were observed at all quench depths: (1) The average grain volume decreased with time after the early stage of grain growth. To our knowledge, a decrease in grain size has never been observed before in any quenched block copolymer system. (2) The volume fraction occupied by ordered grains of the shallowest quenched sample (quench depth of 6 °C) was significantly less than unity even after waiting times approaching 400 min. This is consistent with recent theoretical and experimental work indicating the presence of a coexistence window between ordered and disordered phases due to the partitioning of the salt into the ordered domains. At quench depths of 12 and 24 °C, which are outside the coexistence window, the grain volume fraction increases monotonically with time, and ordered grains occupy the entire sample at long times. © 2014 American Chemical Society.

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.