Probing the Surface Chemistry of Lithium Nitridation

Chemical synthesis of Li₃N through lithium nitridation has potential to advance rechargeable battery and nitrogen fixation technology. However, studies of the conditions for forming Li₃N on the lithium surface via nitrogen gas exposure report contradictory findings, such as the spontaneous reaction of Li with pure N₂, the impossibility of forming Li₃N through pure Li and N₂ interaction, the requirement of trace H₂O 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 Li₃N was detected. At higher pressures, surface Li₃N rapidly reacts with trace CO₂. 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 O₂ gas, CO₂ gas, or H₂O vapor, surface Li₃N formation is inhibited. These results suggest that nitrogen gas can diffuse through the oxidized lithium metal surface to react with subsurface metallic lithium.