New Electrolyte Materials for Lithium Batteries and Fuel Cells - an ab initio and vibrational spectroscopy study

Doctoral thesis, 2006

In order to reduce our fossil based energy consumption there is an urge for new energy storage and conversion devices such as lithium batteries and fuel cells for more efficient energy use. The heart of these devices is an ion conducting electrolyte which needs to provide high performance at low cost. This thesis focuses on new electrolyte materials; the first part concerns lithium salts with prospects to increase the performance and safety of lithium batteries, and the second part deals with protic ionic liquids (pIL’s), a new electrolyte material for low temperature fuel cells. By combining high accuracy quantum mechanical calculations with vibrational spectroscopy, an increased understanding of the properties of these new materials was obtained. This knowledge is important in the development of improved materials. Two promising types of binary lithium salts were investigated; lithium salts incorporating one (azole type) and two (orthoborate type) ring structures with pending electron drawing groups for charge delocalisation. The calculated lithium binding energies of both these types of salts are 10-15% lower than for the lithium salt used in batteries today. From the calculated and characterised salt structures new improved salts have been proposed for synthesis at collaborating laboratories, completing our feedback cycle. An oligomeric lithium salt with single ion conducting properties was investigated to pinpoint structural features limiting the lithium ion transport in these special systems. pIL’s are non-volatile low temperature melting salts with high proton conductivity, formed by proton transfer from an acid to a base, that can swell polymers even at elevated temperatures. pIL’s thus has the potential to replace both the role of water and the ionomer part in polymer electrolyte membranes and improve the fuel cell efficiency considerably. By calculating molecular level properties an increased understanding of important macroscopic properties for fuel cell performance such as melting temperature, fluidity and conductivity was obtained.