Topology dependent energy storage mechanisms

Abstract

Electrically and ionically conducting porous frameworks present an alternative to the kinetically hindered active materials currently used in electrochemical energy storage systems. The materials used in electrodes greatly influence the energy density, rate capability, and environmental footprint of batteries. Low cost and green materials are needed to electrify grid storage and fast charging materials are needed to electrify vehicles. Materials with high porosity and conductivity are rare but ideal for the storage of more abundant metals than lithium, such as sodium and potassium. When porosity and conductivity coexist, electrons and ions can diffuse through the material with the efficiency necessary for electrochemical storage. However, converting chemical energy to electrical energy is a complicated process and it is difficult to characterize directly. It largely takes place at the interface between the solid electrode and liquid electrolyte which is inherently variable. This interaction can be simplified and understood by employing highly ordered materials. In this work, novel materials including zeolite-templated carbons (ZTCs) and metal-organic frameworks (MOFs) were synthesized and characterized to elucidate their topologies and compositions. Compositionally similar MOFs with different topologies (crystalline arrangements of their building blocks) showed the role of pore dimensionality on conductive pathways. ZTCs were fabricated into anodes and cathodes and electrochemically cycled with methodologically varied electrolytes. In one study, the framework was held constant and the ion properties (size, shape, oxidative stability, and desolvation kinetics) were changed. In the next study, the topology of layered 2D and cubic 3D ZTCs were compared. These studies show that ion composition matters more than size or shape and that layered materials are poor ionic conductors but superior electron conductors. The materials used in these studies provide insight into the nature of ionic and electronic pathways that electrochemical energy storage systems depend on.