The tremendous advances in science and engineering related to lithium-ion batteries have made such batteries the most popular power source for portable electronic devices. Furthermore, lithium-ion batteries have been proposed for new applications in powering electric and hybrid electric vehicles.
Electrolytes are a ubiquitous and indispensable component of lithium-ion batteries. Because the electrolyte is sandwiched between positive and negative electrodes, the electrolyte is in close interaction with both electrodes. The interfaces between the electrolyte and the two electrodes often dictate the performance of the cells. In particular, the interface between the anode and the is a crucial factor affecting cell performance. The interface is a thin passivation layer, also called SEI (solid electrolyte interface), which is formed during the first charging process and prevents further reaction of the electrolytes on the anode surface. For fuel cells utilizing carbon anodes, the formation process is potential dependant and stepwise, and is determined by the reactive components of the electrolytes that participate in the formation reactions. Therefore, the SEI layer can be tuned to afford better cell performance through the use of various additives.
State-of-the-art electrolytes for lithium-ion batteries include lithium hexafluorophosphate (LiPF6) as solute and mixtures of cyclic carbonates and linear carbonates as solvents. Ethylene carbonate (EC) is a cyclic carbonate typically used in electrolytes for the formation of SEI at the surface of the negative electrode. However, in many cases the SEI protection from conventional electrolytes with simple formulations such as LiPF6 in admixtures with EC and linear carbonates is insufficient in lithium ion batteries where the negative electrode materials are carbonaceous materials. For instance, when cycling under elevated temperature, the capacities of lithium-ion batteries can fade very quickly. Another issue occurs when using carbonaceous anodes. Batteries that employ either inexpensive natural graphite (a kind of graphite carbon) or hard carbon (a kind of amorphous non-graphite carbon), exhibit a larger initial discharge capacity loss and quickly lose capacity in subsequent cycles.
On the other hand, EC has a high melting point, at about 36-38° C., which limits the performance of lithium ion batteries containing EC-based electrolytes in low temperature applications. Thus propylene carbonate (PC) which has a structure similar to that of EC has been considered to fully or partially replace EC because PC remains in the liquid state over a wide temperature window from −55° C. to 240° C. However, LiPF6—PC based electrolytes are not compatible with graphite electrodes in lithium ion batteries due to the exfoliation of graphite structure by PC intercalation.