Thermodynamics of Miscible Polymer Electrolytes

There is a growing need for improvements in renewable energy sources and in energy storage devices as the effects of global warming become more acute. Conventional lithium-ion batteries are composed of a lithium-graphite composite anode, a liquid electrolyte and a transition metal oxide cathode. Replacing the lithium-graphite anode with a lithium metal anode would greatly increase the energy density of these batteries, enabling higher range electric vehicles and significant improvements in consumer electronics. However, lithium metal anodes are incompatible with conventional liquid electrolytes, prone to dendrites and pose significant safety hazards. There has been significant research into replacing conventional liquid electrolytes with polymer electrolytes, which are significantly less flammable than liquid electrolytes, and have a higher modulus thereby suppressing dendrite growth. However, current polymer electrolytes cannot match the ion transport characteristics of conventional liquid electrolytes. To address this, researchers have attempted to combine various polymeric components with lithium salt to create an electrolyte that is both highly conductive and mechanically rigid. The thermodynamics of conventional polymer electrolytes are still poorly understood. In this Dissertation we study the effect of added salt on the thermodynamic properties of block copolymers and polymer blends comprised of poly(ethylene oxide) (PEO) and poly(methylmethacrylate) (PMMA). This Dissertation represents the first comprehensive study of the thermodynamics of a miscible polymer electrolyte system./nIn Chapter 2, we synthesize a series of PEO-PMMA block copolymers and analyze the effect of added lithium bis(trifluoromethane) sulfonimide (LiTFSI) salt on the phase behavior utilizing small angle X-ray scattering (SAXS). We calculate thermodynamics interaction parameters for this system and find that the effective thermodynamic interaction parameter, χeff, varies nonmonotonically with respect to salt concentration. We shed light upon the complex phase separation of PEO-PMMA/LiTFSI block copolymer electrolytes, which deviates from conventional block copolymer electrolytes./nIn Chapter 3, we prepare a series of PEO/PMMA/LiTFSI blend electrolytes and analyze the phase behavior of these blends via small angle neutron scattering. We find that both blend composition and salt concentration have a significant effect on polymer blend electrolyte phase behavior. We extract thermodynamic interaction parameters from the collected scattering data and build a thermodynamic model to predict blend phase behavior. We find that our model is in good agreement with our experimental data./nIn Chapter 4, we expand on our characterization of PEO/PMMA/LiTFSI blend electrolyte phase behavior by using light scattering to augment our previous phase characterization work. We create a comprehensive phase diagram of PEO/PMMA/LiTFSI polymer blends. This phase diagram presents some of the first experimental evidence of multiple immiscible windows in polymer blend electrolytes. We utilize our previously developed thermodynamic model to create a simulated phase diagram and find good agreement between theory and experiment./nThis work provides new insights into polymer-salt interactions and the underlying thermodynamics of polymer electrolytes. The goal of this Dissertation is to further analyze the complex thermodynamics of polymer electrolytes to enable design of future polymer electrolytes for lithium metal batteries.

Advisor: Balsara, Nitash P