(188g) Probing the Lithium Metal Anode Surface with First Principles

Batteries which employ a metallic Li anode are among the most promising technologies for next generation battery chemistries, owing to their high theoretical energy capacities (3860 mAh/g vs. 370 mAh/g for graphite anodes in state-of-the-art Li-ion technologies). Nonetheless, the implementation of Li metal anodes has been stifled by several key factors, many of which can be attributed to a lack of understanding of the surface phenomena occurring at the anode. More specifically, the native oxide layer is predicted to be present in any battery employing a Li metal anode. Yet, little is known about the structure of the Li metal/native oxide interface and its effect on the performance of Li metal batteries. In this work, first-principles calculations were used to characterize the atomic and electronic structure of the Li metal/native oxide interface.

Multiple models were generated for the Li/lithium oxide (Li₂O) interface, including crystalline and amorphous structures. With regard to the crystalline structures, the oxygen-terminated Li₂O interface yielded a lower interfacial energy and exhibited a work of adhesion more than 30 times larger than the lithium-terminated structure. These results suggest that the oxygen-terminated Li₂O surface is a more probable model under equilibrium conditions and that Li will more evenly coat an oxygen-terminated Li₂O surface during plating. In addition, a more realistic model of the interface, the amorphous Li/Li₂O model, was created using an ab initio molecular dynamics (AIMD) scheme to sequentially add O₂ molecules to a Li metal surface (mimicking the oxidation process in nature). The electronic structure of this amorphous interface illuminated behaviors that the crystalline interface failed to predict. In particular, considering atomic charges and shifts in Li 1s core-level electron binding energies, the size of the interfacial region is larger than that determined by a visual depiction of the atomic structure alone. Finally, the transport of Li ions through the amorphous interface were quantified with AIMD. To our knowledge, this work represents one of the first detailed analyses of the native oxide layer. These findings provide a set of fundamental limitations on batteries that employ high-capacity anodes based on lithium metal.