Novel electrolytes for emerging sodium energy storage application

Abstract

The increasing energy demand along with the growing understanding of the environmental consequences of the use of fossil fuels, have created a need for the development of new and advanced sustainable energy sources. One aspect of this need that arises from the intermittency of many sources is large scale electrical energy storage. Current high energy density electrochemical energy storage technologies rely on electrolytes based on flammable solvents, which are typically volatile organic compounds (VOCs) that result in major safety problems when applied to many novel applications. Using ionic liquids as electrolytes has been explored, as they potentially offer a solution to the safety problem of organic solvents, such as negligible vapor pressure and non flammability. On the other hand, while Li-ion batteries remain an important energy storage technology, there are concerns about the long-term availability and cost of lithium. Alternative electrochemical systems to lithium-based technologies are being investigated to ensure power storage devices are as low-cost and efficient as possible. Sodium-based technologies are promising alternative due to sodium’s high abundance, low cost, low atomic mass, and relatively high (negative) electrochemical reduction potential. This thesis concentrates on three types of sodium-based ionic liquid electrolytes of relevance to emerging sodium energy storage applications. In the first section, it describes the preparation of ionogel electrolytes. In this system, we aimed to investigate the effect of the formation of a silica network on the ionic liquid properties. We found that the ionic conductivity of 3 wt.% silica ionogels is close to that of the pure IL and the Tg does not vary significantly as silica content increased. This shows that the formation of the silica network does not affect the dynamic properties of the IL. In the second section of this thesis, sodium-based ionic liquid electrolytes are prepared and compared with lithium-based ionic liquid electrolytes. The sodium electrolytes possess high ionic conductivity, though marginally lower than that of equivalent lithium systems. Deposition and dissolution of sodium metal was observed through cyclic voltammetry analysis. In order to improve the mechanical properties of these liquid electrolytes, two types of gel electrolytes were investigated: (i) silica gel electrolytes and (ii) PMMA-gel electrolytes. With the former facile plating and stripping of sodium metal was observed through cyclic voltammetry. The ionic conductivity of both gel systems slightly decreased as the physical properties changed from a liquid to gel. However, the Tg was not significantly affected, hence the motional dynamics of liquid electrolytes are not notably affected in the transition to the gel state in these electrolytes. These findings show that sodium-based ionic liquid electrolytes can be a promising candidate for secondary sodium battery applications. In the final section of this thesis polyelectrolyte systems were developed that were designed to be single ion conductors, by tethering the anion to the polymer backbone; such systems are often referred to as ionomers. Two types of ionomers were investigated. The hypothesis guiding the design of these systems was that anionic centers on the polymer that are only weakly associated with the corresponding counterion, would allow decoupling of the cation motion from the bulk dynamics of the material. We found that the ionic conductivity strongly decoupled from the Tg of the backbone, particularly for compositions below 50% Na+ for both systems of ionomers. Characterization showed the Tg of the ionomers did not vary significantly as the amount of Na+ varied, while the conductivity increased with decreasing Na+ content, indicating conductivity increasingly decoupled from Tg. On the other hand, phase separation was clearly observed by SEM and Raman spectroscopy. The introduction of plasticizer significantly increased the ionic conductivity by several orders of magnitude. The effect of different types of ammonium counter-cations on the conductivity of ionomers was also investigated. We observed a decreasing Tg with increasing bulkiness of the quaternary ammonium cation, and an increasing degree of decoupling from Tg within these systems.