Pseudosolid Electrolytes and their Application in Next-Generation Sodium-ion Storage Devices

Author(s): DeBlock, Ryan | Advisor(s): Dunn, Bruce | Abstract: Sodium-ion batteries are quickly becoming a promising, earth-abundant alternative to lithium-ion technology. To address ongoing safety issues associated with both lithium- and sodium-ion batteries, we apply sol-gel processing to create nonflammable, pseudosolid electrolytes. An interconnected, porous silica scaffold encapsulates an ionic liquid creating an “ionogel” electrolyte with high liquid content. These electrolytes retain the conductivity the ionic liquid (~1 mS cm–1), but are macroscopically solid. In Chapter 2, we demonstrate the synthesis of compliant, organically-modified ionogel electrolytes for sodium-ion batteries and demonstrate their use in energy storage devices. When tested with common electrode materials (such as activated carbon, sodium vanadium phosphate, sodium titanium phosphate) these ionogel electrolytes enable at least 100 cycles with minimal capacity fade and high Coulombic efficiency.To enable the use of sodium metal as a high-capacity electrode, we modify the pseudosolid electrolyte synthesis to incorporate tetraglyme as a low vapor pressure alternative to ionic liquid. In Chapter 3, we demonstrate that these pseudosolid electrolytes have both wide electrochemical stability windows (up to 4.5 V vs Na|Na+) and ionic conductivity values near 1 mS cm–1. We observe extremely low overpotentials (~50 mV) to sodium metal plating/stripping onto carbon-coated aluminum over hundreds of cycles. When paired with sodium vanadium fluorophosphate (NVOPF) cathode, devices with glyme electrolyte deliver theoretical capacity of 125 mAh g–1 at a rate of 0.5C and are among the most energy-dense solid-state, sodium-ion storage devices to date.Typical battery electrodes are crystalline compounds with layered or tunnel structures which allow facile ion transport. In contrast, amorphous materials are explored seldomly and have widely varying properties due to their variable nature. In Chapter 4 we explore the synthesis amorphous vanadium dioxide (a–VO2) and its capability as a sodium-ion storage material. Pristine a–VO2 powder delivers nearly 300 mAh g–1 and, when grown on graphite foam (presented in Appendix A), sustains 160 mAh g–1 at an extremely fast rate of 60C.