(Invited) Visualizing Live-Formation of Interphase at Atomic Scale

Abstract

The solid-electrolyte-interphase (SEI) in Li-ion battery has been considered one of the most important but least understood and elusive component. In this work, its live-formation was visualized quantitatively at atomistic level with unique combination of in-situ/operando techniques. For the first time, the precise weighing of graphitic anode during its initial lithiation process was realized with electrochemical quartz crystal microbalance, which unequivocally identifies lithium fluoride (LiF) and lithium alkylcarbonates as the main chemical components of the interphase at different potentials. In-situ gas analysis confirmed the preferential reduction of cyclic over acyclic carbonate molecules, making lithium ethylene dicarbonate (LEDC) the major product contributed from solvent. This lithium alkylcarbonate, at least in its nascent form, was revealed to be re-oxidizable, despite general belief that interphase is electrochemically inert and its formation irreversible. Meanwhile, atomic force microscope found that the presence of LEDC on graphitic surface started at edge sites, which does not seem to occur in an obvious staging manner as the intercalation of Li+ does. These atomistic and quantitative observations shed new light on the formation chemistry, mechanism and properties of SEI. Such in-depth understanding will serve as guiding principles for the tailor-design of ideal interphases in future battery chemistries.