Abstract: Lithium (Li) metal is an attractive anode material with high capacity (3860 mAh g^-1) and low potential (-3.04 V vs. standard hydrogen electrode) that shows highly promising for applications requiring high energy density. However, the low electrochemical potential of Li metal makes it extremely reactive and inevitably forming a native oxidized layer in the ambient environment and repeatedly being consumed when exposed to liquid electrolytes. It is therefore beneficial to replace the poorly controlled native passivation layer with a tailored artificial SEI to improve interface management between Li and electrolyte and enhance the stability of Li metal battery. Here, we use an integrated glovebox-atomic layer deposition (ALD)-X-ray photoelectron spectroscopy (XPS) setup to in-situ investigating the pristine Li surface and the surface composition after Ar, H₂, O₂, N₂ and NH₃ plasma treatment processes. We find that the pristine Li foil is naturally being covered with a native oxidized layer, which is mainly composed of LiOH, Li₂O and Li₂CO₃. These investigated plasmas can efficiently remove the oxidized layer from the Li metal surface, in which metallic Li surface is obtained after Ar or H₂ plasma treatments, where Ar plasma is more efficient. While O₂ plasma treatment produces a Li₂O layer, and N₂ or NH₃ plasma treatment leads to a Li₃N (including a certain amount of LiON) layer on the Li surface. When employing the representative metallic Li (by Ar plasma treatment), Li₂O layer coated Li (by O₂ plasma treatment) and Li₃N layer coated Li (by N₂ plasma treatment) foils as electrodes in symmetric Li metal batteries, the Li₃N coated Li electrode exhibits much higher stability than that of metallic and Li₂O layer coated Li foils. Improved electrochemical performance has also been achieved in LiMn₂O₄ (LMO)||Li full cells using Li anode with Li₃N protective coating layer. Our work reveals the detailed process of surface engineering of Li metal anodes with plasma treatments by in vacuo XPS, which may also be extended to other gas-treatment or plasma-treatment for stabilization of high energy density Li metal anodes and other metal-based anodes.

Key words: Plasma treatment, In vacuo XPS, Lithium metal anode, Metallic Li, Li₂O layer, Li₃N layer