The SEI on Lithium Metal Anodes: From Understanding to Enhancement

Abstract

Lithium metal batteries (LMB) have risen back into focus of battery research in resent years, due to the high theoretical specific capacity of the lithium metal anodes (3860 mAh g-1).[1] However, the utilization of lithium metal as an anode material requires an in depth understanding of the solid electrolyte interphase (SEI) on lithium metal electrodes, since its formation, composition and electrochemical characteristics significantly impact important LMB lifetime parameters, e.g. lithium stripping/plating behaviour, dendrite growth and in operando electrolyte consumption.[2] Lithium metal electrode performance is influenced by a variety of different, however often overlooked factors, e.g. the native passivation layer, inevitably present on each lithium metal electrode surface, which has only recently gathered more attention as an influencing factor in respect to SEI characteristics.[3] The presented research utilizes the preformation of an uncontaminated SEI on the lithium metal electrode surface to circumvent the influence of native lithium metal passivation. A systematic study of different preformed SEIs resulted in a better understanding of the influence by different electrolyte components on relevant SEI characteristics, namely, selected conducting salts (LiFSI, LiPF6) and functional additives (VC, FEC). Analyses of the SEIs electrochemical properties, composition, morphology and dynamics were conducted via stripping/plating experiments as well as in operando EIS, SEM (incl. cryogenic cross section and post mortem) and XPS (post mortem) measurements. Finally, careful tailoring of the preformed SEI was shown to have an enhancing effect on NMC811||Li cells, increasing the LMB lifetime by up to 25%. Furthermore, the application of a tailored preformed SEI has also shown a positive impact on the performance of solid state LMBs.The presented results highlight the importance of an in depth understanding regarding the cause and effect of varying electrolyte components on the SEIs characteristics. This understanding will ultimately have a significant impact on the research towards enhanced and reliable LMB performance and future applicability.