Insights into the Lithium Nucleation and Plating/Stripping Behavior in Ionic Liquid-Based Electrolytes

Rechargeable lithium-metal batteries (LMBs) are anticipated to enable enhanced energy densities, which can be maximized when minimizing the amount of excess lithium in the cell down to zero, also referred to as "zero excess" LMBs. In this case, the only source of lithium is the positive electrode active material─just like in lithium-ion batteries. However, this requires the fully reversible deposition of metallic lithium, i.e., the Coulombic efficiency (CE) approaching 100%. Herein, the lithium plating from ionic liquid-based electrolytes, composed of N-butyl-N-methyl pyrrolidinium bis(fluorosulfonyl)imide (PYR₁₄FSI) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) as the conducting salt, on nickel current collectors is investigated via a comprehensive set of electrochemical techniques coupled with operando and in situ atomic force microscopy and ex situ X-ray photoelectron spectroscopy. The investigation involves the use of fluoroethylene carbonate (FEC) as an electrolyte additive. The results show that an elevated LiTFSI concentration leads to a lower overpotential for the lithium nucleation and a more homogeneous deposition. The incorporation of FEC results in a further lowered overpotential and a stabilized solid electrolyte interphase, enabling a substantially enhanced CE.