UC Santa Barbara

The efficient and safe storage of electrochemical energy is critical for emerging technologies such as electric vehicles and portable electronic devices. Practical requirements for next-generation secondary Lithium-ion batteries include higher energy densities and charge-discharge rates, which hinge on new cathode materials and electrolytes with higher ionic conductivities (σ). Further, higher energy densities can only be reached if Lithium-ion cells are operated over a wide potential range that exceeds the electrochemical stability window of current organic solvent-based electrolytes. Next generation Lithium-ion cells require the development of more stable and nonflammable electrolytes. Solid polymer electrolytes (SPEs) have attracted interest due to their stability and mechanical robustness, but it remains challenging to attain SPEs with both high ionic conductivity and lithium selectivity.This thesis explored the limitations that govern the design of SPEs based on metal-ligand coordination. In such SPEs, the metal-ligand bond between the polymer-bound ligand and the free cation of an added salt promotes dissociation of the salt, enabling conductivity in the solid state. These interactions simultaneously act as reversible crosslinks, promoting salt dissociation and enabling conductivity in the solid state. Oscillatory rheology was used to probe the timescale of these crosslinking interactions. A scaling relation was established which demonstrated the inverse relationship between bond lifetime and ionic conductivity, suggesting a hierarchical conduction mechanism that involves an interplay of polymer segmental motion and the dissociation of metal-ligand bond lifetimes. Overall, this relationship suggests a limitation for homogeneous conduction whereby ion conductivity is inherently limited by the sluggish relaxation dynamics of polymers. This problematic coupling of polymer relaxation and ion dynamics was overcome by designing a prearranged pathway of free volume elements along which an ion can opportunistically hop, much like in superionic ceramic electrolytes. Polymers with a zwitterionic functionality tethered to each monomer were designed to leverage this conduction mechanism. These polymers self-assemble into superionically conductive domains, permitting the decoupling of ion motion and polymer segmental rearrangement. Although crystalline domains are conventionally detrimental to ion conduction in SPEs, I demonstrated that these electrolytes displayed excellent lithium conductivity and selectivity. I proceeded to investigate design rules for these materials – finding that large ion size is critical in promoting fast, activated ion transport.