Toward Reliable Quantification of Lithium Cycling Efficiency for Rechargeable Lithium Metal Batteries

Lithium metal anode (LMA) has been deemed as an essential ingredient for building high-energy rechargeable lithium metal batteries (LMBs), thus coping with the challenges faced by today's battery technology. Lithium cycling efficiency (LCE) is one of the most dominant parameters in the course of electrolyte optimization and anode engineering. Herein, we delve into the quantitative understanding of LCE in two popular liquid electrolytes with lithium bis(trifluoromethanesulfonyl)imide and lithium bis(fluorosulfonyl)imide as conducting salts. Systematic investigations using both the standard Aurbach test and long-term cycling protocols are performed, with the implementation of statistical approaches based on relatively large dataset (e.g., 12 replicates). Our results demonstrate that the Coulombic efficiency values obtained with the Aurbach test are good indicators for the electrolytes with distinctive compatibility with LMAs. However, the accidental errors within the Li°||Cu° experiments bring great challenges in accurately capturing the effective LCEs of different electrolytes, thus calling for meticulous statistical treatment of the cycling data. A cumulative LCE plot inspired by the Walden relation is suggested to better visualize the evolution of LCE values during continuous cycles. This work offers a possible solution toward a reliable and quantitative treatment of LCEs, thus guiding future design strategies for improving the lifespan of rechargeable LMBs.