context: As the initial SEI formed on copper modifies the nucleation and growth of lithium metal in a way that gives rise to a columnar microstructure, understanding the formation of this layer and its resulting properties is imperative. XPS results show that the initial SEI formed by galvanostatically bringing copper working electrodes to 0 V vs. Li/Li+ at 0.5 mA cm−2 in LP30 electrolyte with and without added HF are quite similar in chemical composition (Table S1†). Both SEIs contain primarily LiF, LiOH, and organic species (Fig. 4), indicating that the nanostructure of this layer rather than its chemistry is likely key to modifying lithium nucleation and growth. The thickness of this initial SEI layer is roughly 2–3 nm based both on the charge passed (∼3 nm, assuming fully dense and uniform LiF (Fig. 3a)) and the Ar ion sputter rate (∼2 nm, calibrated using SiO2, Fig. S3†). The presence of a smaller peak around 687.6 eV in the F1s spectrum for the as-received sample in Fig. 4a corresponds to PF6− and could be due to a more porous SEI that traps electrolyte. This is another indication of different SEI structures with and without an HF additive. Alternatively, this could be residue from incomplete electrolyte removal during rinsing.
answer: LP30 