Effect of organic cations in locally concentrated ionic liquid electrolytes on the electrochemical performance of lithium metal batteries

Organic cations are essential components of locally concentrated ionic liquid electrolytes (LCILEs), but receive little attention. Herein, we demonstrate their significant influence on the electrochemical performance of lithium metal batteries via a comparison study of two LCILEs employing either 1-butyl-1-methylpyrrolidinium cation (Pyr(14) (+)) or 1-ethyl-3-methylimidazolium cation (Emim(+)). It is demonstrated that the structure of the organic cation in LCILEs has only a limited effect on the Li+ - bis(fluorosulfonyl)imide anion (FSI-) coordination. Nonetheless, the coordination of FSI- with the organic cations is different. The less coordination of FSIto Emim+ than to Pyr(14)(+) results in the lower viscosity and faster Li+ transport in the Emim(+)-based electrolyte (EmiBE) than the Pyr(14)(+)-based electrolyte (PyrBE). Additionally, the chemical composition of the solid-electrolyte interphase (SEI) formed on lithium metal is affected by the organic cations. A more stable SEI growing in the presence of Emim+ leads to a higher lithium plating/stripping Coulombic efficiency (99.2%). As a result, Li/EmiBE/LiNi0.8Mn0.1Co0.1O2 cells exhibit a capacity of 185 mAh g(-1) at 1C discharge (2 mA cm-2) and capacity retention of 96% after 200 cycles. Under the same conditions, PyrBE-based cells show only 34 mAh g(-1) capacity with 39.6% retention.

Automated summary

Organic cations significantly influence the electrochemical performance of lithium metal batteries by affecting the coordination of FSI- and the formation of solid-electrolyte interphase (SEI). Two different LCILEs with organic cations Pyr(14)(+) and Emim(+) show differences in viscosity, Li+ transport rate, and lithium plating/stripping efficiency, resulting in improved capacity and retention in Li/EmiBE/LiNi0.8Mn0.1Co0.1O2 cells compared to PyrBE-based cells.