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Role of inorganic surface layer on solid electrolyte interphase evolution at Li-metal anodes.

Lithium metal is an ideal anode for rechargeable lithium battery technology. However, the extreme reactivity of Li-metal with electrolytes leads to solid electrolyte interphase (SEI) layers that often impede Li+ transport across interfaces. The challenge is to predict the chemical, structural and topographical heterogeneity of SEI layers arising from a multitude of interfacial constituents. Traditionally the pathways and products of electrolyte decomposition processes were analyzed with the basic and simplifying presumption of an initial pristine Li-metal surface. However, ubiquitous inorganic passivation layers on Li-metal can reduce electronic charge transfer to the electrolyte and significantly alter the SEI layer evolution. In this study, we analyzed the effect of nanometric Li2O, LiOH and Li2CO3 as surface passivation layers on the interfacial reactivity of Li-metal using ab initio molecular dynamics (AIMD) calculations and X-ray photoelectron spectroscopy (XPS) measurements. These nanometric layers impede the electronic charge transfer to the electrolyte and thereby provide some degree of passivation (compared to pristine lithium metal) by altering the redox based decomposition process. The Li2O, LiOH and Li2CO3 layers admit varying levels of electron transfer from a Li-metal slab and subsequent storage of the electronic charges within their structures. As a result, their ability to transfer electrons to the electrolyte molecules, as well as the extent of decomposition of bis(trifluoromethanesulfonyl)imide (TFSI) anions, is significantly reduced compared to similar processes on pristine Li-metal. The XPS experiments revealed that when Li2O is the major component on the altered surface, LiF phases formed to a greater extent. The presence of a dominant LiOH layer, however, results in enhanced sulfur decomposition processes. From AIMD studies, these observations can be explained based on the calculated quantities of electronic charge transfer found for each of the passivating films.