Assessing Coulombic Efficiency in Lithium Metal Anodes

Although lithium metal and anode-free rechargeable batteries (LMBs and AFBs) are phenomenal energy storage systems, the formation of lithium deposits with high surfaces during repeated plating-stripping cycles has hindered their practical applications. Recently, extensive efforts have been made to prevent the growth of high-surface lithium deposition, e.g., electrolyte modification, artificial coating deposition, lithiophilic current collectors, composite lithium metal electrodes, etc. In most of these approaches, Coulombic efficiency (CE) has been used as a quantifiable indicator for the reversibility of the LMBs and AFBs. The interpretation and validation of research results, however, are challenging since the measurement of CE is affected by several parameters related to battery assembly and testing. This study aims to unveil the interplay of several potentially overlooked parameters regulating the CE, such as stripping cutoff voltage, electrolyte quantity, precycling to form a solid electrode interphase (SEI), and electrode surface modification, by applying two alternative electrochemical methods. The hidden aspects of nucleation overpotential revealed by studying these parameters, as well as their influence on the composition and stability of the SEI, are discussed. Overall, this work provides an insightful understanding of the methods and parameters used for assessing the performance of LMBs and AFBs.