Enhanced ion transport in an ether aided super concentrated ionic liquid electrolyte for long-life practical lithium metal battery applications

Abstract

We explore a novel ether aided superconcentrated ionic liquid electrolyte: a combination of ionic liquid, N-propyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide (C₃mpyrFSI) and ether solvent, 1,2 dimethoxy ethane (DME) with 3.2 mol kg⁻¹ 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⁻²) LFP cathode which was cycled at a relatively high current rate of 1 mA cm⁻² 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 7 mA cm⁻² 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 98.36%. 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 electrolyte in lithium metal battery applications, while delivering high energy density in a prototype cell.