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Ionic liquid electrolytes for lithium metal batteries
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posted on 08.02.2017by Lane, George Hamilton
This thesis explores the use of ionic liquids as electrolytes for rechargeable lithium metal batteries. The phenomenon of internal cell short circuiting, a result of dendritic lithium plating, is described along with some practices that ameliorate the problem. The ionic liquid type, charging method and additives are all found to affect short circuit behaviour. Short circuiting is found to be particularly troublesome for a N(SO2CF3)2- based electrolyte, which is remedied by using pulsed charging and an appropriate additive. More promisingly, electrolytes based on N(CN)2- are found to not suffer from short circuiting under the same conditions.
Additives are an important part of electrolyte formulation, and their benefits and limitations are investigated from the viewpoints of interfacial stability and the longevity of their effects. Organic solvents used in typical molecule based lithium battery electrolytes, the alkyl carbonates, are found to improve interfacial stability when used as additives in ionic liquid based electrolytes. However, this does not guarantee a favourable lithium deposit morphology during the charging process.
The chemical compositions of the surface films formed on lithium metal by reaction with different electrolytes are investigated using Fourier transform infrared spectroscopy. Evidence is presented that a cyclic ether type cation reacts at the lithium surface to form an alkoxide species in a surface process analogous to the reaction of a molecular ether with lithium metal. For N(SO2F)2- type ionic liquids, the SO2 group appears to be preserved in the surface film, however the S-F group is not. Electrolytes based on N(CN)2- appear to form a thick surface film that is chemically very similar to the ionic liquid. Although rate performance yet requires improvement, in terms of maximising battery cycle life, the N(CN)2- based electrolyte appears to be a promising alternative to the more exotic N(SO2CF3)2- and N(SO2F)2- based ionic liquids.