Probing the surface chemistry of lithium nitridation

Abstract

Chemical synthesis of Li<inf>3</inf>N through lithium nitridation has potential to advance rechargeable battery and nitrogen fixation technology. However, studies of the conditions for forming Li<inf>3</inf>N on the lithium surface via nitrogen gas exposure report contradictory findings, such as the spontaneous reaction of Li with pure N<inf>2</inf>, the impossibility of forming Li<inf>3</inf>N through pure Li and N<inf>2</inf>interaction, the requirement of trace H<inf>2</inf>O to catalyze the reaction, and evidence to the contrary. In this study, ambient pressure X-ray photoelectron spectroscopy (APXPS) was applied to evaluate the in situ chemical evolution of the lithium metal surface under nitrogen gas up to 800 mTorr. At pressures ≤10 mTorr, no Li<inf>3</inf>N was detected. At higher pressures, surface Li<inf>3</inf>N rapidly reacts with trace CO<inf>2</inf>. Additionally, because metallic lithium is readily oxidized by trace gases, the atomic nitrogen concentration of the lithium surface remains below 2%. When nitridation follows oxidation by O<inf>2</inf>gas, CO<inf>2</inf>gas, or H<inf>2</inf>O vapor, surface Li<inf>3</inf>N formation is inhibited. These results suggest that nitrogen gas can diffuse through the oxidized lithium metal surface to react with subsurface metallic lithium.