We explore a novel ether aided superconcentrated
ionic liquid electrolyte; a combination of ionic liquid, <i>N</i>-propyl-<i>N</i>-methylpyrrolidinium
bis(fluorosulfonyl)imide (C<sub>3</sub>mpyrFSI) and ether solvent, <i>1,2</i> dimethoxy ethane (DME) with 3.2
mol/kg LiFSI salt, which offers an alternative ion-transport mechanism and
improves the overall fluidity of the electrolyte. The molecular dynamics (MD)
study reveals that the coordination environment of lithium in the ether aided
ionic liquid system offers a coexistence of both the ether DME and FSI anion
simultaneously and the absence of ‘free’, uncoordinated DME solvent. These
structures lead to very fast kinetics and improved current density for lithium
deposition-dissolution processes. Hence the electrolyte is used in a lithium
metal battery against a high mass loading (~12 mg/cm<sup>2</sup>) LFP cathode
which was cycled at a relatively high current rate of 1mA/cm<sup>2</sup> for
350 cycles without capacity fading and offered an overall coulombic efficiency
of >99.8 %. Additionally, the rate
performance demonstrated that this electrolyte is capable of passing current
density as high as 7mA/cm<sup>2</sup> without any electrolytic decomposition
and offers a superior capacity retention. We have also demonstrated an ‘anode
free’ LFP-Cu cell which was cycled over 50 cycles and achieved an average
coulombic efficiency of 98.74%. The coordination chemistry and
(electro)chemical understanding as well as the excellent cycling stability
collectively leads toward a breakthrough in realizing the practical applicability
of this ether aided ionic liquid electrolytes in lithium metal battery
applications, while delivering high energy density in a prototype cell.
An ultrafast rechargeable lithium metal battery
