This dissertation aims to develop, characterize, and fundamentally understand a new class of materials termed "molecular ionic composites" (MICs). MICs show promise as next-generation solid electrolytes for batteries. MICs form when mixing a rigid polyanion with purely ionic fluids, and they behave mechanically as a solid but contain a high density of ions that move nearly as in a neat liquid. Specifically, prototypical MICs are based on solutions of the rigid-rod polyelectrolyte poly(2,2'-disulfonyl-4,4'-benzideneterephthalamide) (PBDT), which forms a double helix, combined with imidazolium-based ionic liquids (ILs). The IL comprises 75-97 wt% of the final solid, even though the Young's modulus can reach ~ 2 GPa at 80 wt% IL. We propose that these properties are driven by a biphasic internal structure in MICs corresponding to IL-rich "puddles" (an interconnected liquid phase) and PBDT-IL associated "bundles" where IL ions form the collective electrostatic associations that cause the MICs to be a solid. Through this dissertation I will discuss a wide variety of MICs that have been created through the use of two different formation processes, the "ingot" method and the "solvent casting" method, which allow for the use of many different ionic fluid sources to further tune MIC properties. The following chapters build to the fundamental knowledge and our current understanding of the wide variety of materials that can be created from PBDT and IL. / Doctor of Philosophy / Battery electrolytes, biosensors, and hydrogels all depend on new materials for next-generation applications. For these new materials to be used characterization on the interactions, morphological restrictions, and/or what unique internal structures used to generate their properties must be performed. Through This analysis using common polymeric characterization techniques these materials can be further optimized. This dissertation highlights a new class of materials termed "molecular ionic composites" (MICs) which are formed from a rigid double helical polymer, poly(2,2'-disulfonyl-4,4'-benzideneterephthalamide) (PBDT), and fluids composed entirely of ions, including ionic liquids (ILs). These composite systems feature a unique combination of properties including high thermal stability, mechanical stability, and excellent ionic conductivity, all of which are highly tunable through the amount of PBDT incorporated or the fluid ion types. Chapters 3, 4, 5, and 6 present fundamental investigations of MICs to determine how tunable they are, the processes by which they form, and the various ways we can fabricate them. Chapter 7 describes the creation of another impressive material formed from PBDT-low-polymer-content hydrogels. These studies are intended to provide deeper understanding of the behaviors of these unique materials and how they may be used in the future.

Among the myraid energy storage technologies, polymer electrolytes have been widely employed in diverse applications such as fuel cell membranes, battery separators, mechanical actuators, reverse-osmosis membranes and solar cells. The polymer electrolytes used for these applications usually require a combination of properties, including anisotropic orientation, tunable modulus, high ionic conductivity, light weight, high thermal stability and low cost. These critical properties have motivated researchers to find next-generation polymer electrolytes, for example ion gels. This dissertation aims to develop and characterize a new class of ion gel electrolytes based on ionic liquids and a rigid-rod polyelectrolyte. The rigid-rod polyelectrolyte poly (2,2'-disulfonyl-4,4'-benzidine terephthalamide) (PBDT) is a water-miscible system and forms a liquid crystal phase above a critical concentration. The diverse properties and broad applications of this rigid-rod polyelectrolyte may originate from the double helical conformation of PBDT molecular chains. We primarily develop an ionic liquid-based polymer gel electrolyte that possesses the following exceptional combination of properties: transport anisotropy up to 3.5×, high ionic conductivity (up to 8 mS cm⁻¹), widely tunable modulus (0.03 – 3 GPa) and high thermal stability (up to 300°C). This unique platform that combines ionic liquid and polyelectrolyte is essential to develop more advanced materials for broader applications. After we obtain the ion gels, we then mainly focus on modifying and then applying them in Li-metal batteries. As a next generation of Li batteries, the Li-metal battery offers higher energy capacity compared to the current Li-ion battery, thus satisfying our requirements in developing longer-lasting batteries for portable devices and even electric vehicles. However, Li dendrite growth on the Li metal anode has limited the pratical application of Li-metal batteries. This unexpected Li dendrite growth can be suppressed by developing polymer separators with high modulus (~ Gpa), while maintaining enough ionic conductivity (~ 1 mS/cm). Here, we describe an advanced solid-state electrolyte based on a sulfonated aramid rigid-rod polymer, an ionic liquid (IL), and a lithium salt, showing promise to make a breakthrough. This unique fabrication platform can be a milestone in discovering next-generation electrolyte materials. / Ph. D. / Among the myraid energy storage technologies, polymer-based electrolytes have been widely employed in diverse applications such as fuel cell membranes, battery electrolytes, “artificial muscle” mechanical actuators, reverse-osmosis membranes and solar cells. The materials used for each of these applications usually require a specific combination of properties, which include anisotropic orientation, tunable mechanical stiffness (modulus), high ionic conductivity, light weight, high thermal stability and low cost. These critical properties have motivated researchers to find next-generation polymer-based electrolytes, for example “ion gels” that consist of a polymer combined with ionic liquids or salts. This thesis describes development of an ion gel that possesses the following exceptional combination of properties: high ionic conductivity (up to 8 mS cm^-1), widely tunable modulus (0.03 ‒ 3 GPa), ion transport anisotropy up to 3.5×, and high thermal stability (up to 300°C). Thus, this unprecedented material shows liquid-like ion motions inside a matrix with solid-like stiffness, and in a material that can withstand extreme temperatures and will not burn. After obtaining these ion gels, we are then mainly focusing on modifying them for application in safe and high density Li-metal batteries. As a next generation of Li batteries, the Li-metal battery offers higher energy capacity compared to the current Liion battery, thus satisfying our requirements in developing longer-lasting batteries for portable devices and even electric vehicles. However, Li dendrite growth on the Li metal anode has limited the pratical application of Li-metal batteries. This unexpected Li dendrite growth can be supressed by developing polymer electrolytes with high modulus (~ GPa), while maintaining sufficient ionic conductivity (~ 1 mS/cm) for efficient battery operation. In short, this thesis describes an advanced solid-state electrolyte based on a kevlar-like (sulfonated aramid) rigid-rod polymer, an ionic liquid (IL), and a lithium salt, showing promise to make a breakthrough and enable practical Li-metal batteries. Furthermore, the unique fabrication platform for these ion gels represents a new paradigm for discovering next-generation electrolyte materials for a wide variety of applications.