Influence of initial surface condition of lithium metal anodes on surface modification with HF

Surface modification of as-received lithium foils was carried out using acid-base reactions of the native surface films on lithium metal with HF. Two types of as-received lithium foils covered with different native films were used as samples for this surface modification. One was a lithium foil having a very thin native surface film and the other one had a thicker native surface film. The surface condition of the lithium metal was analysed by X-ray photoelectron spectroscopy before and after the surface modification using HF, and the coulombic efficiency was measured electrochemically. The thickness of the surface film on the modified lithium foils was related to the Li2O layer thickness in the native film on the as-received lithium foils. The modified lithium foil which had the thinner native surface film provided more uniform deposition of lithium and a higher coulombic efficiency during charge and discharge cycles when propylene carbonate electrolyte with 1.0 m LiPF6 was used as the electrolyte. These results show that the initial condition of the native surface film plays an important role in surface modification with HF.