Characterisation and modelling of lithium-ion battery electrolytes

University dissertation from Stockholm : Kemiteknik

Abstract: Rechargeable batteries play an important role as energycarriers in our modern society, being present in wirelessdevices for everyday use such as cellular phones, video camerasand laptops, and also in hybrid electric cars. The batterytechnology dominating the market today is the lithium-ion(Li-ion) battery. Battery developments, in terms of improvedcapacity, performance and safety, are major tasks for bothindustry and academic research. The performance and safety ofthese batteries are greatly influenced by transport andstability properties of the electrolyte; however, both haveproven difficult to characterise properly.The specific aim of this work was to characterise and modelthe electrolytes used in Li-ion batteries. In particular, themass transport in these electrolytes was studied throughcharacterisation and modelling of electrolyte transport in bulkand in porous electrodes. The characterisation methodology assuch was evaluated and different models were tested to find themost suitable. In addition, other properties such aselectrochemical stability and thermal properties were alsostudied.In the study of electrochemical stability it wasdemonstrated that the electrode material influenced thevoltammetric results significantly. The most versatileelectrode for probing the electrolyte stability proved to beplatinum. The method was concluded to be suitable for comparingelectrolytes and the influences of electrolyte components,additives and impurities, which was also demonstrated for a setof liquid and polymer containing electrolytes.A full set of transport properties for two binary polymerelectrolytes, one binary liquid and the corresponding ternarygel were achieved. The transport was studied both in the bulkand in porous electrodes, using different electrochemicaltechniques as well as Raman spectroscopy. In general, theconductivity, the salt and solvent diffusivity decreasedsignificantly when going from liquid to gel, and to polymerelectrolyte. Additionally, low cationic transport numbers wereachieved for the polymer and gel and significant salt activityfactor variations were found. The results were interpreted interms of molecular interactions. It was concluded that both theionic interactions and the influences from segmental mobilitywere significant for the polymer containing electrolytes. Thecharacterisation methods and the understanding were improved bythe use of a numerical modelling using a model based on theconcentrated electrolyte theory. It was concluded thatelectrochemical impedance spectroscopy and Raman spectroscopywere insufficient for determining a full set of transportproperties. It was demonstrated that the transport is veryinfluential on electrochemical impedance as well as batteryperformance.Keywords:lithium battery, electrolyte, mass transport,stability, modelling, characterisation, electrochemical, Ramanspectroscopy, impedance