Innovative Electrolytes for Safer Sodium-Ion Batteries

Abstract: The overconsumption of non-renewable/fossil fuels by vehicles and industry has resulted in dangerously high levels of CO2 in the atmosphere the last 40 years. The impact on the environment, climate and public health urge governments to find new technologies to ensure a sustainable development. In this context, the development of greener energy storage technologies such as novel secondary batteries has already had a wide impact. The commercialisation of the first lithium-ion battery (LIB) in 1991 by Sony has revolutionized mobile devices and the LIB now emerges in electric vehicles, but can be even more important for load levelling of renewable energy. Unfortunately, an increase in our lithium consumption coupled with its low abundance in the Earth’s crust raises financial and sustainability concerns, forcing us to think about complementary battery technologies. One of the most appealing alternatives is to use sodium instead of lithium. Chemically these elements are close and these similarities should ease a technological change. Therefore, the sodium-ion battery (SIB) is a concept worth studying - especially for large-scale applications due to the “unlimited” abundance of sodium in the Earth’s crust and the overall low materials cost, anticipated to be 30 times lower than for Li. Electrolytes for SIBs can be based on organic solvents or ionic liquids (IL), or a mixture of both as matrices, all doped with the appropriate sodium salt. Several features and properties of hybrid IL and pure IL-based electrolytes for SIBs are investigated in this thesis; the ionic conductivity of novel electrolytes using a few ILs chosen among the large number available. These studies are complemented by Raman vibrational spectroscopy to understand the interactions within the electrolytes, and the possible operation temperature ranges by differential scanning calorimetry. Moreover, several electrolytes have been analysed to understand the IL contribution to various safety measures; ignition time (IT), flash point (FP), and self-extinguishing time (SET). In addition, a significant part of the work is dedicated to the electrochemical compatibility of the electrolytes with novel SIB electrode materials. The stability and behaviour toward the electrodes are investigated to make possible a fully operative safer SIB in the future, possibly based on hybrid IL or pure IL-based electrolytes.